Chapter V Transformations involving M(Hmal) (M = Co, Zn...

Chapter V

Transformations involving M(Hmal)2(H2O)4

(M = Co, Zn) with some pyridine related compounds

Abstract: Having observed some unprecedented transformations involving M(II)bis-

hydrogen maleates M(Hmal)2(H2O)4 with pyridine we were interested in probing

whether other pyridine related compounds can bring about similar transformation

and generate related products. Presented in this chapter are our trials on

M(Hmal)2(H2O)4 with various picolines, 4,4’-bipyridine, 2,2’ bipyridine and 1,10-

phenanthroline. While 3- and 4-picolines react with M(Hmal)2(H2O)4 in a similar

manner as that of pyridine to produce the maleate-fumarate transformation leading

to M(II) fumarate derivatives and corresponding picolinium succinate zwitterions

the 2-picoline is found to be inert towards such a reaction. We were able to get

crystal structures of one of the chiral zwitterion products (19) done by derivatizing it

with Ca2+ ion. Our attempts to generate a dimeric zwitterion by affecting similar

transformation as above using 4,4’-bipyridine did not, however, yield any such

chiral product but only brought about the anticipated maleate-fumarate

transformation which, then, was found to be not part of the M(II) ion but was getting

converted into a fumarate salt of 4,4’-bipyridine. Unlike in the case with pyridines

the M(II) maleate was getting formed in this case instead of metal(II)fumarates.

Similarly our trials with 2,2;-bipyridine and 1,10-phenanthroline gave unusual

products instead of the anticipated cis-trans isomerisation. The novel features of the

resulting products are evident from structures 20, 21 and 22. The reaction features

and structural aspects of the products obtained are discussed in detail.

…M(Hmal)2(H2O)4 (M = Co, Zn) with some pyridine related compounds 149

5.1 Introduction

As described in Chapter IV it has been unequivocally established that

pyridine reacts with various M(II)-bis-hydrogenmaleates (M = Mn, Co, Ni,

Cu, Zn) to form a chiral pyridyl succinic acid zwitterion and a fumarate

derivative of the metal. In all these cases transformation of maleate to fumarate

is observed at the coordination environment of the metal ion. As an extension

of the above work and also to see whether similar reaction would be possible

with other pyridines we have tried to react all the three isomers of methyl

pyridines (4-, 3- and 2- picolines) with Co(II)-bis-hydrogenmaleate. An

additional interest in this connection was to generate novel chiral zwitterions

containing substituted pyridines which could be of some potential applications.

As expected both 4- and 3-picolines produced reactions exactly similar to that

of pyridine. Conversion of maleate to fumarate in these reactions was again

confirmed from spectral and X-ray diffraction methods. Similarly both these

isomers (4- and 3-picolines) produced the corresponding chiral zwitterions also

as in the case of pyridine. However, our repeated experiments showed that 2-

picoline does not react with Co(II)-bis-hydrogenmaleate under similar or any

other rigorous experimental conditions. In this chapter we intend to present in

detail some of the interesting observations we have made in this aspect. The

products are characterized by elemental analysis, spectral studies (FTIR, UV-

vis., 1H NMR,

13C NMR), PXRD and single crystal X-ray diffraction analyses.

Motivated by the above transformations we also thought of generating a

possible dichiral product by reacting the bipyridine analogue 4,4’-bipyridine

(4-bipy) with Co(II)-bis-hydrogenmaleate in similar condition. Surprisingly no

chiral product of the type 16 is seen to form in this case. Out of the two

products formed in this reaction, the metal containing compound was found to

150 Chapter V

contain clearly a maleate moiety and a 4,4’-bipyridine unit and never a

fumarate moiety, unlike in earlier cases. However, quite interestingly, the

second product is found to be an adduct of fumaric acid and 4,4’-bipyridine,

instead of the expected zwitterion. We have also made some attempts to see

how other metal(II)-bis-hydrogenmaleates react with picolines and 4,4’-

bipyridine. Presented in this chapter are also the nature of the reactions and the

type of products that are formed in the above cases. We have also tried to look

at the nature of reaction with a few other pyridine derivatives like 2,2’-

bipyridine and 1,10-phenanthroline which are analogous to 4,4’-bipyridine.

Some of the products of these reactions could be characterized by single

crystal X-ray diffraction studies and are also included in this section. In

addition, reactions of metal(II)-bis-hydrogenmaleates with simple alkyl amines

were also attempted which were found to give yet another type of

unprecedented transformation which we would be discussing in detail in

succeeding chapter.

5.2 Experimental

5.2.1 Materials

Picolines (98%) were purchased from Merck (Germany) and used as

received. Maleic acid, fumaric acid, sodium carbonate, metal(II) carbonates

and metal(II) chlorides were E.Merck (India) Limited products. 4,4’-

bipyridine, 2,2’-bipyridine, 1,10-phenanthroline and other amines were from

Merck, Germany. All chemicals were used as received.

5.2.2 Analytical Methods

Elemental analysis (C, H and N) were performed using Elementar Vario

EL III elemental analyzer. Infra red spectra (4000-400 cm-1

) were measured on


a Shimadzu FTIR-8400S spectrophotometer, where KBr was used as the

dispersal medium. TG analyses were carried out on a Shimadzu DTG-60

simultaneous DTA-TG apparatus. 1H NMR and

13C NMR spectra were

recorded on a Bruker Advance DPX-300 MHz spectrometer. Luminescence

studies were done on a Perkin Elmer LS 45 fluorescence spectrophotometer.

Single crystal X-ray diffraction data were collected at 293 ± 2K on a Siemen’s

Smart-CCD diffractometer. Powder XRDs were recorded using Rigaku Ultima

X-ray diffractometer.

5.2.3 Experimental procedure and reaction products

As discussed in the introduction section the focus given in the present

chapter is for looking at the nature of transformation reactions that occur when

[M(Hmal)2(H2O)4] react with picolines and several other pyridine derivatives.

We were able to find out the nature of transformations in each case by

separating the reaction products and structurally characterizing them. Given

below are the experimental procedure and reaction products in each case.

Reaction of [Co(Hmal)2(H2O)4], 2 with picolines

An aqueous solution of [Co(Hmal)2(H2O)4] was refluxed with picolines

(4-, 3- and 2-) for one day with occasional addition of picolines. 3-Picoline

yielded a pink compound after a few seconds whereas 4- and 2-picolines gave

only clear solutions. The solutions were filtered and the clear filtrates were

kept in open air for crystallization. The pink compound formed with 3-

picoline, 22 was washed several times with water and dried in air. The light

pink filtrate, obtained after separating 22, on concentration produced a white

crystalline powder, 23 which was filtered, washed with methanol and dried.

The remaining solution was found to be light pink in colour which yielded on

152 Chapter V

evaporation some minor quantity of Co(II)-fumarate and fumaric acid. In the

case of 4-picoline the reaction with [Co(Hmal)2(H2O)4] yielded a clear solution

when refluxed but gave white crystalline powder 25 after slow evaporation (2

days). This was separated by filtration and washed with methanol. The

remaining solution after slow evaporation produced pink powder 26. It was

filtered washed and dried. Our attempts with 2-picoline, however, met with

failure as no reaction was seen to be occurring with [Co(Hmal)2(H2O)4] even

after refluxing for long hours.

We have tried to isolate good quality crystals of 23 for doing its single

crystal XRD analysis, but all attempts produced only crystalline powder.

However, we were able to generate good quality crystals of its calcium salt, the

preparative method of which was as follows. The crystalline compound 23 was

dissolved in water by heating while stirring. CaCO3 in excess quantity was

added to this solution and then filtered. The clear solution yielded colourless

crystals, 24 after a few days.

Reaction of [Co(Hmal)2 (H2O)4], 2 with 4,4’-bipyridine

An aqueous solution (10mL) of 2 (1mmol, 0.361g) was refluxed with a

methanolic solution (10mL) of 4,4’-bipyridine (2mmol, 0.312g). A pink

compound with a silky shining appearance was formed after a few seconds.

The mixture was refluxed for 24h and filtered. The pink compound 27 was

washed several times with water and dried. The filtrate yielded colourless, thin,

needle shaped crystals, 28 after 2 days.

Reaction of [Zn(Hmal)2(H2O)4], 5 with 4,4’-bipyridine

We carried out reaction of 4,4’-bipyridine with the Zn analogue

[Zn(Hmal)2(H2O)4] in the same way as above. A white crystalline product, 29


formed initially which was filtered and dried. The clear filtrate obtained was

kept for slow evaporation which then yielded thin needle like crystals after 2

days. This was filtered, washed carefully with methanol and dried.

Reaction of [Zn(Hmal)2(H2O)4], 5 with 2,2’-bipyridine

A methanolic solution of 2,2’-bipyridine was added to a hot aqueous

solution of Zn(II)-bis-hydrogenmaleate (1:1 molar ratio). The clear solution

thus formed was kept for slow evaporation. Colourless crystals, 30 suitable for

single crystal X-ray diffraction analysis were obtained after two days. The

reaction was almost quantitative and there was no secondary product.

Reaction of [Zn(Hmal)2(H2O)4], 5 with 1,10-phenanthroline

It was found that 1,10-phenanthroline did not react with Zn(II)-bis-

hydrogenmaleate unlike in the case of 2,2’-bipyridine during our trials. So we

modified the experimental conditions slightly. Instead of treating 1,10-

phenanthroline directly with Zn(II)-bis-hydrogen maleate, we first made a

Zn(II) phenanthroline complex and then allowed this to react with maleic acid.

The experimental procedure was as follows. A solution of 1,10-phenanthroline

in methanol was added slowly to an aqueous solution of Zn(II) acetate with

occasional stirring. A white precipitate formed was found to be dissolved on

adding solid maleic acid. The final molar ratio of the reactants maintained was

1:1:2. The solution was filtered and kept in open air. Colourless crystalline

compound, 31 was formed after a few days. The crystalline product was

washed first with water and then with methanol. Solubility of the compound in

water was very low and hence it was recrystallised from water: methanol (1:1)

mixture to get good quality crystals suitable for single crystal analysis.

154 Chapter V

5.3 Results and Discussion

We have explored the nature of reaction of [Co(Hmal)2(H2O)4] with all

the three picolines in solution condition rather than through solid-vapour

interaction to facilitate more intimate reaction. As indicated in the experimental

section the nature of reaction and the type of products formed can be

summarized as follows. The authenticity of all the products could be verified

by chemical and spectral analyses. Given in Table 5.1 is the analytical data for

all the products obtained which confirm the molecular composition of each

derivative.

Table 5.1 Elemental analytical data of compounds derived from Co(II)-

bis-hydrogenmaleates and various picolines

Compound (Emp.formula) Formula

weight

Elemental content (%)

Found (calcd.) Colour

(solubility in

water) C H N

[Co(fum)(3-pic)3(H2O)]·4H2O

CoC22H33N3O9, 22 544

48.3

(48.5)

5.86

(6.06)

7.5

(7.7)

pink powder

(insoluble)

3-picolinium succinate zwitterion

C10H11NO4, 23 209

56.8

(57.3)

4.9

(5.26)

6.38

(6.69)

white powder

(sparingly

soluble)

[Ca(3-pic.zwitterion)2(H2O)2].4H2O

CaC20H32N2O14, 24 564.56

42.7

(42.5)

5.82

(5.66)

5.01

(4.95)

colourless

crystals

(soluble)

4-picolinium succinate zwitterion

C10H11NO4, 25 209

56.85

(57.3)

6.1

(5.26)

6.66

(6.69)

white powder

(sparingly

soluble)

[Co(fum)(4-pic)(H2O)3]

CoC10H15NO7 , 26 319.9

45.68

(45)

4.83

(4.68)

4.76

(4.37)

pink powder

(sparingly

soluble)


We have also established the identity of all these compounds by

analysing their FTIR spectra. Some of the significant vibration peaks are

summarised in Table 5.2. In all the metal containing compounds, 22, 24,

and 26, the carboxylate group of the fumarate ion is coordinated to the

metal ion in an η1 mode. This is clear from the νas(COO), νs(COO) and the

Δν values82

in each case. The comparatively low Δν values observed for

some of the products are essentially due to extensive H-bonding in those

compounds. Presence of free carboxylate ion in the zwitterions 23 and 25

is evident from the low Δν values (127 and 159 cm-1

). The νas(COO) and

νs(COO) values of non-bonded carboxylate groups in these two

compounds are also lower comparing to that of the coordinated –COO

group. The presence of non-deprotonated –COOH moiety in the

zwitterions is indicated by the peaks around 1700 cm-1

. The higher

νOH(H2O) values seen in some of the compounds indicate the presence of

non-coordinated water and low frequency peaks seen in some others

suggest the presence of coordinated water. Absence of the ν(C=C)

specific peak at about 1600 cm-1

in the spectra of 23, 24 and 25 shows

that addition has taken place in the double bond of the maleic acid when it

is converted to the zwitterions. Peak corresponding to the CH in plane

symmetric deformation on C=C (at 1311 cm-1

) which is often observed in

maleate compounds is found to be absent in 22 and 26. This is a further

indication that these compounds are not maleate derivatives.

156 Chapter V

Table 5.2 IR spectral data of compounds derived from Metal(II)-bis-

hydrogenmaleates and picolines (in cm-1

)

22 23 24 25 26

νOH(H2O) 3140 --- 3402-

3232 --- 3186

ν (COOH) --- 1704 --- 1674 ---

νas(COO) 1550 1504 1566 1519 1550

νs(COO) 1380 1377 1380 1360 1388

Δν 170 127 186 159 162

ν (C=C) 1595 --- --- --- 1620

ν(C-H) aromatic 3085 3085 3066 3062 3062

ν(C-H) aliphatic 2931 2958

2923

2966

2935

2985

2966

2943

2923

ν(C-C), ν(C-N) ring

stretch

1481

1438

1504

1469

1508

1434

1519

1446

1504

1446

Ring deformation of

pyridine

651

(in plane)

435

(out of

plane)

648

(in plane)

439

(out of

plane)

667

644

(in plane)

455

(out of

plane)

678

(in plane)

447

(out of

plane)

5.3.1 Transformation reactions involving [Co(Hmal)2(H2O)4] and 3-

picoline

We have attempted detailed characterization of all the products

formed by the interaction of both 3- and 4-picolines. In the case of

3-picoline the pink product obtained is seen to be a 3-picoline adduct of

Co(II)-fumarate, 22. Even though on comparison this product formed

appear to be different from that obtained through pyridine reaction, we

could exclusively prove that the Co(II) containing product, 22, has a

fumarate moiety rather than a maleate moiety indicating that a facile cis


to trans conversion has occurred in this case. The authenticity of

[Co(fum)(3-pic)3(H2O)]·4H2O 22 could also be verified by directly

synthesising the picoline adduct by reacting freshly prepared

Co(II)-fumarate with 3-picoline separately and comparing its FTIR and

PXRD with that of 22. Given in Fig. 5.1 are the FTIR spectral trace of

22 and that of the 3-picoline adduct we have prepared separately from

Co(II)-fumarate. There is clearly a one-to-one match in them indicating

the authenticity of 22 as [Co(fum)(3-pic)3(H2O)]·4H2O which obviously

is formed through cis to trans isomerisation during the reaction. Further

to this we were also able to confirm the identity and phase purity of this

product by comparing the PXRDs of 22 and also of the as-made sample

(Fig. 5.2).

Fig. 5.1 (a) FTIR spectra of 16 and (b) of the compound directly

prepared from Co(II)-fumarate

158 Chapter V

Fig. 5.2 PXRD spectra of 22 (red) and that of the compound directly

prepared (black).

Encouraged by the cis to trans isomerisation happened with 3-

picoline (just as in the case of pyridine) we were eager to know whether

the second product formed is a chiral zwitterion akin to the

pyridylsuccinate moiety seen in the case of pyridine reaction. Both the

elemental and IR spectral data of 23 are consistent with the formation of a

chiral zwitterion in the case of 3-picoline also. We could confirm this by

looking at both 1H and

13C NMR of the product 23. Reproduced in Fig.

5.3 is the 1H NMR of 23 which could be completely assigned on the basis

of the structure indicated for the compound. The chemical shift values are

Hδ (300 MHz, D2O) 2.55 (3H, s, CH3), 3.15 (1H, dd, CH2), 3.35 (1H, dd,

CH2), 5.5 (1H, dd, CH), 7.93 (1H, t, H4), 8.36 (1H, d, H3), 8.67 (1H, d,

H5) , 8.7 (1H, s, H1). The three doublets of doublets is a clear indication

of the zwitterion nature of 23. We were also able to confirm the structure

of the product through its 13

C NMR which is given in Fig. 5.4. The

experimental spectra match very well with the simulated one. The δ


values are Cδ (300 MHz, D2O) 18.6 (CH3), 41 (CH2), 74.1 (CH), 127.9

(C4), 140.4 (C2), 142.4 (C1), 144.7 (C3), 147.4 (C5), 173 (COOH), 176

ppm (COO-). Melting point of the 3-picolinium succinate zwitterion, 23 is

found to be 184 ºC and it is thermally stable up to 185ºC. A 100% mass

loss is observed as a single stage in the temperature range 185ºC - 220ºC.

a

N

CH3

+

3

COOH

COO

_

1

24

5

b c

Fig. 5.3 (a) The continuous 1H NMR spectrum of 23, (b) the expanded

spectrum illustrating the three doublets of doublets and (c)

structure of the zwitterion

160 Chapter V

N

CH3

+

3

COOH

COO

_

1

24

5

Fig. 5.4 13

C NMR spectrum of 23 3-picolinium succinate zwitterion, 23

Attempt to further confirm the structure of 23 was also made by single

crystal X-ray diffraction studies. Since no quality crystals were obtained even

after repeated trials we derivatised the zwitterion 23 by converting into its Ca

salt as given in section 5.2.3. The Ca salt of 23 could then be crystallized

easily. Presented in Fig. 5.5 is the molecular structure of the compound

obtained through single crystal X-ray diffraction studies which clearly

indicates that compound 23 is a zwitterion. We have included the structural

features of 24 in Chapter X in detail along with some other zwitterions and

metal-free acid-base adducts.

Fig. 5.5 ORTEP of [Ca(3-pic.zwitterion)2(H2O)2]4H2O, 24 with atom

label, which reveals the zwitterion nature of 23.


5.3.2 Transformation reactions involving [Co(Hmal)2(H2O)4] and 4-

picoline

As in the case of 3-picoline, its isomeric compound 4-picoline also is

found to be capable of effecting the cis to trans isomerisation (maleate to

fumarate) at the metal centre. Chemical analysis (Table 5.1) and FTIR

spectral data (Table 5.2) indicate that the pink compound, 26, obtained from

the reaction of [Co(Hmal)2(H2O)4] with 4-picoline is a 4-picoline adduct of

Co(II)-fumarate having the composition [Co(fum)(4-pic)(H2O)3]. Identity of

26 as a fumarate derivative could also be proved from the direct synthesis of

this compound from Co(II)-fumarate and 4-picoline. Given in Fig. 5.6 is the

FTIR spectra of 26 and that of the compound prepared directly by refluxing

Co(II)-fumarate with 4-picoline. Similarity of the two spectra is a clear

indication that the Co(II) derivative is a fumarate and not a maleate

compound and hence a cis to trans conversion must have occurred in the

case of [Co(Hmal)2(H2O)4] during its reaction with 4-picoline also.

Fig. 5.6 (a) FTIR spectra of [Co(fum)(4-pic)(H2O)3, 26 and (b) that of

the compound directly prepared from Co(II)-fumarate.

Since a maleate to fumarate transformation is operative in this

reaction also we anticipated that the metal-free compound formed must be

162 Chapter V

a 4-picolinium succinate zwitterion, 25 as in the case of 3-picoline. The

CHN analysis (Table 5.1) and the IR spectral data (Table 5.2) were

consistent with the zwitterion nature of this product. We could confirm

the identity of 25 as the expected zwitterion from both the 1H and

13C

NMR of the product. Given in Fig. 5.7 is the 1H NMR spectrum of 25

which has the characteristic three sets of doublets of doublets confirming

the anticipated zwitterion nature. The chemical shift values are Hδ (300

MHz, D2O), 2. 65 (3H, s .CH3), 3.18 (1H, dd, CH2), 3.4 (1H, dd, CH2),

5.49 (1H, dd, CH), 7.86 (2H, d, Hm), 8.66 (2H, d, Ho).

a

b c

Fig. 5.7 (a) The complete 1H NMR spectrum (b) the expanded regions

of the spectrum showing the doublets of doublets and (c) the

structure of the zwitterion 25.


The structure of 25 has been further confirmed from its 13

C NMR

spectrum which is found to be identical to the simulated spectrum for the

structure. Given in Fig. 5.8 is the 13

C NMR spectrum of 25 recorded in

D2O solution with a 300 MHz instrument. The chemical shift values are

Cδ; 21.8 (CH3), 39.7 (CH2), 72.4 (CH), 128.6 (C2&C4), 143.6 (C1&C5),

161.2 (C3), 172.4 (COOH), 175.7 ppm (COO-). It is also interesting to see

that both 3- and 4-picolinium succinate zwitterions have identical PXRD

patters which show the identical crystal structure (iso-structural nature) of

the two zwitterions. Given in Fig. 5.9 are the PXRDs of the two

compounds. Melting point of 4-picolinium succinate zwitterion, 25 is

found to be 196ºC and it is thermally stable up to 200ºC. However, the

compound begins to decompose at 200ºC and ends at 220ºC with 100%

mass loss in a single stage. The decomposition is seen to be highly

endothermic in nature.

Fig. 5.8 13

C NMR spectra of 25 4-picolinium succinate zwitterion, 25

164 Chapter V

a b

Fig. 5.9 (a) PXRD spectra of 3-picolinium succinate zwitterion, 23 and

(b) that of 4-picolinium succinate zwitterion, 25 illustrating

the identical nature of the two zwitterions.

5.3.3 Transformation reactions involving 4,4’-bipyridne with

[Co(Hmal)2(H2O)4], 2 and [Zn(Hmal)2(H2O)4], 5

Having observed a facile cis to trans isomerisation along with the

formation of novel chiral zwitterions by the interaction of pyridine,

3-picoline and 4-picoline, we thought it would be worth investigating

the reaction with other pyridine-like compounds. As indicated the

inertness of 2-picoline towards the isomerisation and zwitterion

formation is essentially due to steric reasons. However, when we

consider the case of 4,4’-bipyridine (which can be taken as a molecule

formed through two pyridyl moieties connected through ‘4’ positions) it

can be viewed as a highly favourable molecule that can participate in the

isomerisation reaction as there are no steric constraints which blocks

both of the nucleophilic N from taking part in the reaction. Yet another

interesting factor of considering 4,4’-bipyridine for the transformation

reaction is the propensity of both pyridyl moieties taking part

simultaneously in the reaction to generate a much more interesting

zwitterion (than 16) which can possibly have two chiral centers.


Discussed briefly below are the observations we have made during the

reaction of [Co(Hmal)2(H2O)4] and [Zn(Hmal)2(H2O)4] with 4,4’-

bipyridine and also the nature of products obtained.

As mentioned in the experimental section 5.2, [Co(Hmal)2(H2O)4]

produces an immediate precipitation of a pink compound, 27 on

refluxing with 4,4’-bipyridine. The clear filtrate yielded colourless

crystals, 28. On chemical analysis it is seen that this pink product is a N-

containing Co(II) compound presumably having 4,4’-bipyridine as one

of the constituents. Given in Table 5.3 are the elemental analysis data of

various products formed from the reaction of [M(Hmal)2(H2O)4] with

various bipyridine derivatives including 4,4-bipyridine. To see whether

compound 27 is a fumarate derivative, as in the earlier cases of pyridine

and picolines, we recorded its FTIR spectrum and compared it with that

of the previous Co(II)-fumarate derivatives. But to our surprise the

FTIR spectrum did not give any indication of a fumarate moiety in the

product 27 but showed, interestingly, characteristics of a maleate

function in the compound. The relevant vibrational peaks of this product

are presented in Table 5.4 along with the prominent peaks of other

bipyridine derivatives formed from the respective reactions.

The CH in-plane symmetric deformation on C=C appears as an intense

sharp peak at 1311cm-1

which is a clear indication of the maleate moiety in

the metal containing compound, 27. The νs(COO) specific absorptions

appear as doublets in this compound suggesting that each carboxylate group

is involved in different type of coordination modes. Among the two Δν

values seen the high Δν value (173 cm-1

) indicates η1

coordination and the

low value (158 cm-1

) suggests mono-atomic (O) bridging. The relatively low

166 Chapter V

Table 5.3 Elemental analytical data of compounds derived from

metal(II)-bis-hydrogen maleates and various bipyridines

Compound

(Emp.formula)

Formula

weight

Elemental content (%)

Found (Calcd.) Colour

(solubility

in water) C H N

Co(mal)(4-bipy)0.5H2O

CoC14H11N2O4.5 - 27 337.9

49.04

(49.7)

3.03

(3.25)

8.19

8.28

pink

powder

(insoluble)

Co(mal)(4-bipy)0.5H2O

(directly prepared) 337.9

49.35

(49.7)

3.15

(3.25)

8.29

8.28

pink

powder

(insoluble)

[Co(fum)(4,4’-bipy)]8H2O

(directly prepared) 490

35.15

35.59

5.22

5.5

5.7

5.93

light pink

powder

(insoluble)

Fum:4-bipy adduct

C14H12N2O4 - 28 272

62.22

(61.8)

4.81

(4.41)

10.22

(10.29)

white

crystals

(sparingly

soluble)

Zn(mal)(4-bipy)2H2O

ZnC14H14N2O5 - 29 371.4

44.88

(45.2)

3.02

(3.76)

7.37

(7.53)

white

powder

(insoluble)

Zn(Hmal)2(2-bipy)H2O

ZnC18H16N2O9 - 30 469.4

46.15

(46.0)

3.36

(3.4)

6.00

(5.96)

light pink

crystals

(soluble)

[Zn2(mal)2(phen)2(H2O)2]2H2O

ZnC32H28N4O12 -31 791.32

47.35

(48.5)

4.09

(3.54)

7.55

(7.07)

colourless

crystals

(sparingly

soluble)

[Zn2(mal)2(2-bipy)2(H2O)2]2H2O

Zn2C28H28N4O12 - 32 743.28

45.5

(45.2)

3.82

(3.76)

7.55

(7.53)

colourless

crystals

(sparingly

soluble)


Δν value of 173 cm-1

which has been assigned for the η1 coordination is

due to H-bonding involving the carboxylate oxygen with H2O.82

Presence

of intense sharp peaks at 1614, 1431 and 1011 cm-1

in the IR spectrum of

27 indicates that 4,4’-bipyridine is coordinated to Co2+

ion through both

of its pyridyl nitrogens.111-112

A broad peak centered at 3420 cm-1

is

indicative of H-bonded lattice H2O molecule. In order to further verify the

nature of the carboxylate group in 27 we have compared the chemical

composition and FTIR spectra of this compound with those of the directly

and separately prepared adduct of 4,4’-bipyridine and Co(II)-mono-maleate

(Tables 5.3 and 5.4). Both the spectra are found to be identical in nature

(Table 5.4) indicating that the carboxylate moiety in 27 is in fact a

maleate entity and not a fumarate moiety. Moreover the FTIR spectrum of

the 4,4’-bipyridine adduct prepared directly from Co(II)-fumarate is found

to be not matching with that of 27 (Table 5.4). This also is an evidence

that cis to trans isomerisation has not taken place at the metal center in

the reaction between 4,4’-bipyridine and [Co(Hmal)2(H2O)4] unlike in the

case of pyridine and picolines. The FTIR spectra of 27 and those of the

compounds prepared directly from Co(II)-mono-maleate/Co(II)-fumarate

with 4,4’-bipyridine are overlaid in Fig. 5.10 for comparison. TGA curve

of 27 shows three distinct stages of decomposition. The first part shows a

slight downward slope in the beginning stage itself and the mass-loss

corresponds to 0.5 equivalent of H2O. The anhydrous sample is then seen

to be stable up to 300°C which later decomposes with the loss of

4,4’-bipyridine (second step). This is followed by the dissociation of

maleate moiety (third step) at 370 °C. All these data are consistent with

the composition Co(mal)(4-bipy).0.5H2O for 27.

168 Chapter V

Table 5.4 IR spectral data of compounds derived from Metal(II)-bis-

hydrogenmaleates and various bipyridines along with that of

the compounds directly prepared (in cm-1

)

27

Co(

mal

)(4-

bipy

). 0.5

H2O

(dire

ctly

pre

pare

d)

[Co(

fum

)(4-

bipy

)]. 8

H2O

(dire

ctly

pre

pare

d)

28

Fum

.4-b

ipy

addu

ct

(dire

ctly

pre

pare

d)

29 30 31 32

ν OH(H2O) 3420 3425 3367 --- --- 3525-3410

3402 3417-3200

3354-3150

ν(COOH) --- --- --- 1701 1701 --- 1680 --- ---

νas(COO) 1589 1589 1539 1581 1582 1573 1573 1566 1573

νs(COO) 1431

1416

1430

1415 1407 1407 1407

1415

1380 1384 1385 1388

Δν 173

158

174

159 132 174 175

193

158 189 181 185

CH symmetric deformation on C=C (in plane)

1311 1311 --- --- --- 1303 --- 1319 1303

ν(C-H) aromatic

3093

3074

3039

3090

3074

3040

3105-3039

3101

3066

3101

3066

3074

3047

3105

3062

3076

3053

3024

3090

3040

ν(C-H) aliphatic 3001 3002 3105-3039

3008

2974

2931

3004

2943

2966

2923

2854

2993

2927

2854

2950

2925

ν(C-C), ν(C-N) ring stretch

1488

1045

1488

1045

1492

1070

1492

1040 1488 1492 1494 1485

Ring deformation of pyridine

632 631 628

459

628

459

640

420

640

412 630

632

430

pyridine ring vibrations

1614

1431

1011

1610

1432

1010

1601

1431

1010

1608

1415

1014

1442

1442

1415

1022


Fig. 5.10 (a) FTIR spectra of Co(mal)(4-bipy)0.5H2O, 27 (b) that of the

compound obtained from the direct reaction of [Co(mal)(H2O)2]H2O,

7 and 4,4’-bipyridine (c) that of the compound obtained from the

direct interaction of [Co(fum)(H2O)4], 12 with 4,4’-bipyridine).

In order to further confirm the identity and phase purity of 27 we

have compard its PXRD with that of the adduct directly prepared from

Co(II)-mono-maleate and 4,4’-bipyridine. The overlaid PXRDs are given

in Fig. 5.11 which are found to have similar patterns.

170 Chapter V

Fig. 5.11 (a) PXRD patterns of compound 27 and (b) that of the

compound obtained from the direct reaction of

[Co(mal)(H2O)2]H2O, 7 and 4,4’-bipyridine

We could not isolate good quality crystals of 27, suitable for single

crystal XRD analysis. But similarity seen in the elemental analysis, IR,

UV-vis, PXRD and TGA data of 27 with that of the directly prepared

compound from Co(II)-mono-maleate, 7 with 4,4’-bipyridine shows that

the compound 27 is an adduct of 4,4’-bipyridine and Co(II)-mono-maleate

with composition Co(mal)(4-bipy)0.5H2O. Shi et al110

has reported a

Mn(II) adduct, Mn(mal)(4-bipy)0.5H2O, which has the same composition

as that of compound 27 and having spectral and PXRD data matching

with 27. Clearly 27 is iso-structural to the above reported compound and

can be expected to have a structure similar to that shown in Fig. 5.12.


(a) (b)

Fig. 5.12 (a) View of 1D chain and (b) layered structure of

Mn(mal)(4-bipy)0.5H2O.110

We were eager to see whether the metal-free compound 28 was a di-

chiral zwitterion as we expected or a simple salt/adduct formed between

maleic acid and 4,4’-bipyridine, or even a salt of fumaric acid (which would

have formed through cis-trans isomerisation of Co(Hmal)2(H2O)4) with 4,4’-

bipyridine. The chemical analysis data (Table 5.3) indicated that compound

28 is a 1:1 adduct of 4,4’-bipyridine and dicarboxylic acid. In order to see the

nature of the dicarboxylic acid (whether it is maleic acid or fumaric acid) in

28 we recorded its FTIR spectrum and found that the spectrum contained

some features of a fumarate moiety. Absence of the signature peak at

1311cm-1

and also other maleate specific absorptions in the metal-free

compound, 28 is a clear indication that this compound is not a maleate

derivative. The nature of 28 could, however, be confirmed by the direct

synthesis of the compound from both fumaric and maleic acids and 4,4’-

bipyridine. The FTIR spectrum of 28 is seen to be exactly similar to that of

the compound directly prepared from fumaric acid and 4,4’-bipyridine but is

clearly different from the IR spectrum of the compound prepared from

maleic acid and 4,4’-bipyridine. In order to further confirm the nature of the

172 Chapter V

metal-free compound 28 we recorded the 1H NMR spectra of the above three

organic compounds which are presented in Fig. 5.13. The spectrum clearly

indicates that 28 is not a zwitterion (absence of doublets of doublets) as in the

case of pyridine and picolines but an acid-base salt. On comparison of the

three spectra (Fig. 5.13) it is evident that 28 is a fumaric acid: 4,4’-bipyridine

adduct/salt and not a corresponding maleic acid salt.

Fig. 5.13 (a) 1H NMR spectra of 28 (b) fumaric acid: 4,4’-bipyridine

adduct and (c) maleic acid : 4,4’-bipyridine adduct.

The observed chemical shift values of the fumaric acid adduct 28 are Hδ

(300 MHz, D2O) 6.66(2H, s, olefinic H), 8.18(4H, d, meta H of bipy), 8.86(4H,

d, ortho H of bipy) which are exactly matching with that of the compound

directly prepared from fumaric acid. As can be seen from the figure these values

are quite different from that of the corresponding maleic acid derivative formed

by its reaction with 4,4’-bipyridine [Hδ (300 MHz, D2O) 6.26(4H, s, olefinic H),

8.4(4H, d, meta H of bipy), 8.92(4H, d, ortho H of bipy)].


We have also established the identity of 28 as a fumaric acid adduct

and not a maleic acid adduct by comparing its PXRDs with those formed

through the direct reactions of fumaric acid and maleic acid with 4,4’-

bipyridine. Given in Fig. 5.14 are the overlaid PXRDs of these three

products. The patterns of 28 and that of the compound directly prepared

from fumaric acid and 4,4’-bipyridine are exactly identical which again

shows that 28 is a fumarate salt and not a maleate salt. The PXRD

patterns of the compound derived from maleic acid and 4-bipyridine are

seen to be entirely different.

Fig. 5.14 (a) PXRD patterns of 28, (b) fumaric acid-4,4’-bipyridine adduct

and (c) maleic acid-4,4’-bipyridine adduct

174 Chapter V

On going through literature113-114

we could find that maleic acid

forms a 2:1 adduct with 4,4’-bipyridine whereas fumaric acid forms a 1:1

adduct, which again confirms the nature of 28 (also see our results in

Chapter X). The crystal structure of these adducts have been reported and

are given in Fig. 5.15. The colorless compound 28 is thus a 1:1 adduct of

fumaric acid and 4,4’-bipyridine which contains two neutral components,

linked into chains by O-H…N hydrogen bonds.

Fig. 5.15 (a) Crystal structure of 28 and (b) crystal structure of maleic acid:4-

bipyridine.113

The above results on the reaction of Co(Hmal)2(H2O)4 with 4,4’-

bipyridine clearly shows that even though there is no formation of di-

chiral zwitterion or transformation of maleate to fumarate at the metal

centre leading to a cobalt(II)-fumarate derivative there is still a maleate to

fumarate conversion as the organic product formed by 4,4’-bipyridine

with Hmal unit in this reaction is a 4,4’-bipyridine salt of fumaric acid

and not a 4,4’-bipyridine salt of maleic acid.

To see whether the nature of the reaction and products will change

with different metal ions in [M(Hmal)2(H2O)4] we have carried out the

reaction with [Zn(Hmal)2(H2O)4] in a similar experimental condition. But in


this case also it has been found that the metal containing derivative is a 4,4’-

bipyridine adduct, 29 of Zn(II)-maleate and the organic product is a fumaric

acid salt as in the case of [Co(Hmal)2(H2O)4]. We, therefore, feel that 4,4’-

bipyridine is incapable of forming any di-chiral zwitterions or bringing about

maleate to fumarate transformation at the metal centre of M(Hmal)2(H2O)4

as in the case of pyridine and picolines but is, however, capable of affecting

the transformation to generate fumaric acid which then combines with 4,4’-

bipyridine to form metal-free fumaric acid : 4,4’-bipy adduct.

5.3.4 Transformation reactions involving [Zn(Hmal)2(H2O)4], 5 and

2,2’-bipyridne

From the investigations we have carried out so far and discussed in

Chapter IV and section 5.3 we could observe some unprecedented

transformations operating when [Co(Hmal)2(H2O)4 is made to react with

pyridine, various picolines and 4,4’-bipyridine. The nature of producs

formed in the above cases is seen to be different in each case. For

example while pyridine, 3- and 4-picolines react with [Co(Hmal)2(H2O)4

to form novel chiral zwitterions by insertion of pyridine in the C=C π-

bond, 2-picoline is seen to be chemically inert towards such a reaction.

Apart from facilitating the formation of the chiral zwitterions (pyridinium

succinate and picolinium succinate) as pure and separate products

pyridine, 3- and 4-picolines bring about the unprecedented cis to trans

isomerisation (from maleate to fumarate) generating Co(fum)(H2O)4.

What we have observed during the reaction is the formation of Co(II)-

fumarate instead of Co(II)-maleate from the Co(II)-bis-hydrogenmaleate

on reacting with the above pyridine derivatives. The type of zwitterions

formed in almost quantitative yield in all the three cases is also seen to be

similar. We have attributed the inertness of 2-picoline towards such a

176 Chapter V

reaction to the steric influence of the –CH3 group at the 2-position. In

contrast 4,4’-bipyridine (which contains two pyridine functions reacts

with Co(Hmal)2(H2O)4 in a quite different manner. Instead of producing

a maleate to fumarate transformation at the metal centre forming a metal

fumarate derivative the reaction facilitates a clear cis to trans conversion

in the metal-free product. Thus the metal containing compound remains as

a metal-maleate (1:1) derivative while the metal-free product is seen to be

a fumaric acid : 4,4’-bipyridine adduct instead of the expected zwitterion.

On observing these clear differences in the nature of reaction

products obtained during the reaction of [Co(Hmal)2(H2O)4] with 4,4’-

bipyridine and pyridine/picolines we were eager to know what would be the

nature of the reaction when 2,2’-bipyridine (which is an isomer of 4,4’-

bipyridine) is made to react with [Co(Hmal)2(H2O)4]. Even though we have

carried out the reaction under various experimental conditions we could not

separate any chemically pure products as the solid products formed were

found to be always a mixture. But we were able to separate good quality

crystals, 30 on reacting 2,2-bipyridine with [Zn(Hmal)2(H2O)4], which is

iso-structural to the Co(II) derivative, 2. As described in the experimental

section the reaction produced only one type of compound and there was no

secondary product. The formation of only a single product during the

reaction clearly suggests that the nature of reaction of 2,2’-bipyridine with

[Zn(Hmal)2(H2O)4], is quite different from the previous cases. Chemical

analysis data (Table 5.3) indicate that the crystalline compound 30 contains

2,2’-bipyridine and almost one molecular unit of [Zn(Hmal)2(H2O)4] and

presumably with a composition Zn(Hmal)2(2-bipy)·H2O. To see the nature

of 30 we have recorded its FTIR spectrum (Table 5.4) and to our surprise


we could observe the vibration peak (1680 cm-1

) highly characteristic of

uncoordinated –COOH group which was absent in any of the previous

metal containing products obtained in the transformation reactions. In

addition the spectra also contained strong -COO- specific vibrations at 1573

(νasCOO-) and 1384 (νsCOO

-) with Δν value of 189 cm

-1 which indicates

that there are additional –COO- groups which are coordinated to the metal

ion in an η1

manner. The spectrum also contained several maleate specific

absorptions. In order to get an idea about the structure and stability of this

compound we have also carried out the TG analysis in N2 atmosphere at a

heating rate of 10°C. Compound 30 is found to be stable up to 120°C.

Thermogram shows two distinct stages of mass loss. In the first step one

molecule of water and one molecule of 2,2’-bipyridine are lost. Second step

corresponds to the weight loss of two maleate units. These experimental

evidences tentatively indicated that the compound could be [Zn(Hmal)2(2-

bipy)H2O]. The 1H NMR spectrum of 30 which was taken for further

confirmation was also consistent with the above composition. The chemical

shift values are Hδ (400 MHz, D2O) 4.83(2H, s, H2O), 6.84(2H, d, H

adjacent to COOH), 8.3(2H, d, H adjacent to COO-), 9(8H, multiplet,

pyridyl H). To get a conclusive evidence of the expected structure we

carried out the single crystal X-ray diffraction anlysis of this compound.

The single crystal XRD studies show that compound 30 is a molecular

adduct of 2,2’-bipyridine and Zn(II)-bis-hydrogenmaleate in which two

carboxyl groups remain uncoordinated. Given in Fig. 5.16 is the ORTEP of

30 which clearly shows that 2,2’-bipyridine has just coordinated to Zn2+

ion

of [Zn(Hmal)2(H2O)4], 5, without cleaving the Hmal units from the metal

ion. The crystal data and structure parameters of [Zn(Hmal)2(2-bipy)H2O],

30 along with that of other two Zn derivatives (31 and 32) are given in

178 Chapter V

Table 5.5. Selected bond lengths and angles are presented in Table 5.6. Zn

has a highly distorted trigonal bipyramid coordination environment in

which N1 and O9 occupy the axial positions (O9 - Zn - N1 = 161.10(8) Ǻ).

The hydrogen maleate (Hmal) unit is planar due to the intra-molecular H-

bond between the O atom of the COO- and the H atom of free COOH

present in the Hmal moiety as shown. Intermolecular hydrogen bonds (Fig.

5.17) between the H atoms of the coordinated water molecule and the O

atom of the COOH group along with a possible π-π stacking lead to an

overall 3-dimensional structure (Fig. 5.18). It is worth mentioning that this

is the only case where we could obtain an adduct of M(II)-bis-

hydrogenmaleate and a Lewis base, and the reaction was almost

quantitative also. In all other cases one of the Hmal units is seen to be

getting cleaved out from the metal centre along with some transformation

(either zwitterion formation or conversion to fumarate).

Fig. 5.16 ORTEP showing the molecular structure of [Zn(Hmal)2(2-

bipy)H2O], 30 along with atom labeling.


Fig. 5.17 Inter-moleuclar H-bondings in 30.

Fig. 5.18 Packing features of 30 indicating possible π-π interactions.

180 Chapter V

Table 5.5 Crystal data and structure refinement parameters for

compound 30, 31 and 32

30 31 32

Empirical formula Zn C18 H16 N2 O9 Zn2C32H28 N4 O12 Zn2C28 H28 N4 O12

Formula weight 469.70 791.32 743.28

Temperature 293(2) K 293(2) K 293(2) K

Wavelength 0.71073Ǻ 0.71073Ǻ 0.71073Ǻ

Crystal system, space group

Triclinic, P-1 Triclinic, P-1 Monoclinic, C2/c

a./Å 7.9210(16) 8.6591(3) 14.454(3)

b /Å 9.834(2) 9.3047(3) 10.782(2)

c /Å 13.449(3) 10.4867(4) 19.523(4)

α / 92.22(3) 79.017(2) 90

/ 104.74(3) 86.675(2) 103.291(10)

γ / 109.89(3) 72.367(2) 90

Volume 943.6(3)Ǻ3 790.47(5) 2961.0(10) Ǻ3

Z

Calculated density

2 1

1.662

4

1.667 Mg/m3

Absorption coefficient

1.358mm-1 1.591 1.692 mm-1

F(000) 480 404 1520

Theta range for data collection

2.22 to 25.99 deg. 1.98 to 34.37 2.14 to 37.71

Reflections collected / unique

21794 / 5992 [R(int) =

0.0250] 21304 / 5916 [R(int)

= 0.0236]

Goodness-of-fit on F2 1.086 1.054 1.045

Final R indices [I>2sigma(I)]

R1= 0.0248, wR2 = 0.0660

R1 = 0.0258, wR2

= 0.0714

R1 = 0.0311, wR2

= 0.0755

R indices (all data) R1 = 0.0289, wR2 =

0.0679 R1 = 0.0323, wR2 = 0.0745

R1 = 0.0432, wR2 = 0.0797

Refinement method Full-matrix least-

squares on F2

Full-matrix least-squares on F2

Full-matrix least-squares on F2


Table 5.6 Selected Bond lengths [Å] and angles [°] for [Zn(Hmal)2(2-

bipy)H2O, 30

Zn - O1 1.9631(17) O1 - Zn - O5 97.03(6)

Zn - O5 2.0008(14) O1 - Zn - O9 95.72(8)

Zn - O9 2.0923(18) O5 - Zn - O9 87.54(7)

Zn - N2 2.0954(17) O1- Zn - N2 120.15(7)

Zn - N1 2.1490(17) O5 - Zn - N2 142.42(6)

O1- C1 1.256(2) O9 - Zn - N2 93.57(7)

O2 - C1 1.247(2) O1 - Zn - N1 103.17(7)

O3 - C4 1.218(3) O5 - Zn - N1 90.36(6)

O4 - C4 1.298(3) O9 - Zn - N1 161.10(8)

N2 - Zn - N1 76.83(6)

We have also measured the PL property of [Zn(Hmal)2(2-bipy)H2O],

30 anticipating interesting photoluminescence behviour. The PL emission

along with its excitation spectrum is given in Fig. 5.19. Upon excitation at

331 nm, a strong blue fluorescent peak at 459 nm was observed in compound

30. This emission is neither metal-to-ligand charge transfer (MLCT) nor

ligand-to-metal charge transfer (LMCT) in nature since Zn2+

ions are difficult

to oxidize or to reduce due to their d10

configuration. The emission can

probably be assigned to the intra-ligand (π-π*) fluorescent emission of

2,2’-bipyridine. We have also measured the emission spectrum of free

2,2’-bipyridine molecule which shows one emission at 370 nm, indicating

that the 2,2’-bipyridine ligand has no emission in the visible region but when

it is bound to the Zn center, the blue luminescence is observed.117

Compared

with the emission of the free maleic acid (Fig. 5.19a) and the metal salt 5

(Fig. 5.19b) a clear red shift has taken place in 30 (Fig. 5.19c). The energy

182 Chapter V

separation between the excited state and ground state as evident from the

emission energy can be attributed to the increase in the ligand conformational

rigidity due to its coordination to Zn ions resulting in a decrease in the non-

radiative decay of the intra-ligand excited states.115

250 300 350 400 450 500

0

2

4

6

8

10

12

14

16

437318

397351

Inte

nsity (

AU

)

Wave Length (nm)

200 250 300 350 400 450 500 550

0

5

10

15

20

437

399352318

Arb

itra

ry U

nit

Wave Length (nm)

(a) (b)

200 250 300 350 400 450 500 550 600 650

0

10

20

30

40

50

60

459331

Arb

itra

ry U

nits

Wave Length (nm)

(c)

Fig. 5.19 Luminescence spectra of (a) maleic acid (b) [Zn(Hmal)2(H2O)4], 5

and (c) [Zn(Hmal)2(2-bipy)H2O], 30 (Red colored line indicates

excitation spectra)


5.3.5 Transformation reactions involving [Zn(Hmal)2(H2O)4], 5 and

1,10-phenanthroline

Having found diverse reaction features for various pyridine

derivatives (including 2,2’-bipyridine) towards M(Hmal)2(H2O)4 we

thought of looking at the reaction of 1,10-phenanthroline (N-donor Lewis

base very much similar to 2,2’-bipyridine) in some detail. Our trials with

Zn(II)-bis-hydrogenmaleate, 5 , 1,10-phenanthroline is seen to be inert in

almost all experimental conditions which is in contrast to the reaction of

2,2’-bipyridine. But instead of treating 1,10-phenanthroline directly with

Zn(II)-bis-hydrogen maleate, we first made a Zn(II) phenanthroline

complex (1:1 adduct). The solid Zn-phen complex formed initially in the

form of suspension was treated with 2 equivalents of maleic acid during

which a clear solution was obtained. The solution was filtered and then

kept in open air to get crystalline product 31. Solubility of the compound

in water was very low and hence it was recrystallised from water-

methanol mixture (1:1) to get good quality crystals, 31 suitable for single

crystal analysis. Elemental analysis (Table 5.3) indicates that compound

31 contains two molecules of 1,10-phenanthroline and two maleate units

having a composition equivalent to [Zn2(mal)2(phen)2(H2O)2]2H2O. The

νas(COO-), νs(COO

-) and the Δν values (1566, 1385 and 181 cm

-1

respectively) observed in the FTIR spectrum (Table 5.4) clearly indicates

η1

coordinated carboxylate group and absence of any free –COOH groups

as in 30. The ν C=N value (1580 cm-1

)108

and the out of plane hydrogen

deformation vibrations at 850 and 725 cm-1

are clear indication of

coordinated phenanthroline.109

A very broad peak in the 3417-3200 cm-1

range shows that there are H-bonded lattice and coordinated H2Os in

compound 31. These observations together with thermal analysis results

184 Chapter V

indicate that the structure of compound 31 is possibly as indicated above.

We were lucky to get good quality single crystals of the compound for

X-ray diffraction studies. The crystal data are presented in Table 5.5.

Given in Fig. 5.20 is the molecular structure of 31 along with atom label.

It is clear from the ORTEP that compound 31 is a dimer in which two

maleate dianions bridge two Zn2+

ions forming a 14 member ring.

Formation of a polymer is prevented by the coordination saturation of

Zn2+

ion by chelating phenanthroline moiety and one water molecule.

There are also two non-coordinated water molecules per molecule. Zn2+

ion has overall distorted trigonal bipyramidal geometry. The atoms O1,

O3 and N2 make the triangular plane and the apices are occupied by N1

and O5. The higher C(13)-O(1) bond length compared to that of C(13)-

O(2) shows that the carboxylate group of maleate ion is in η1

coordination. Eventhough compound 31 is a molecular species strong

inter-molecular H-bonding between the lattice water and the non-

coordinated oxygen of the maleate moiety and π-π interactions facilitate

an overall 3-dimensinal network (see Fig. 5.21 and 5.22). Selected bond

lengths, bond angles and H-bonds are given in Table 5.7 and 5.8.


Fig. 5.20 ORTEP of [Zn2(mal)2(phen)2(H2O)2]2H2O, 31 with atom

label (35% anisotropic ellipsoid)

Fig. 5.21 Intermolecular hydrogen bonds among the lattice water and

the non-coordinated oxygen of the maleate moiety

186 Chapter V

Fig. 5.22 Possible π-π stacking of the phenanthroline units

Table 5.7 Selected Bond lengths [Å] and angles [°] for 31

___________________________________________________________

N(1)-Zn(1) 2.1447(10) O(3)-Zn(1)-O(1) 107.16(4)

N(2)-Zn(1) 2.1131(9) O(3)-Zn(1)-O(5) 93.62(4)

O(1)-Zn(1) 2.0218(9) O(1)-Zn(1)-O(5) 93.42(4)

O(3)-Zn(1) 1.9727(8) O(3)-Zn(1)-N(2) 140.22(4)

O(5)-Zn(1) 2.1104(10) O(3)-Zn(1)-N(2) 140.22(4)

C(13)-O(1) 1.2702(14) O(1)-Zn(1)-N(2) 112.02(4)

C(13)-O(2) 1.2378(14) O(5)-Zn(1)-N(2) 90.72(4)

O(3)-Zn(1)-N(1) 95.18(4)

O(1)-Zn(1)-N(1) 90.75(4)

O(5)-Zn(1)-N(1) 168.70(4)

N(2)-Zn(1)-N(1) 77.98(4)

___________________________________________________________

Symmetry transformations used to generate equivalent atoms:

#1 -x+1,-y,-z+1


Table 5.8 Hydrogen bonds for [Zn2(mal)2(phen)2(H2O)2]2H2O, 31 [Å]

and angles [°]

___________________________________________________________

D-H...A d(D-H) d(H...A) d(D...A) <(DHA)

O(5)-H(5A)...O(6)#2 0.853(9) 1.886(11) 2.7239(14) 167(2)

O(6)-H(6A)...O(1)#3 0.828(9) 2.068(9) 2.8940(13) 175.8(18)

O(6)-H(6B)...O(4)#4 0.826(9) 1.970(9) 2.7912(16) 172.7(18)

O(5)-H(5B)...O(2) 0.852(9) 1.771(10) 2.6130(15) 169(2)

O(5)-H(5B)...O(1) 0.852(9) 2.59(2) 3.0083(13) 111.3(17)

_________________________________________________________________


#1 -x+1,-y,-z+1 #2 x,y-1,z #3 -x+1,-y+1,-z+1

#4 -x,-y+1,-z+1

Anticipating interesting photoluminescence behavior we have

recorded the fluorescence spectrum of 31 both in solid and solution states.

In both cases luminescence spectra of the compound was obtained by

excitation at 270 nm. A strong fluorescent peak at 390 nm which appears

somewhat symmetric was obtained in solution (methanol) state

(Fig. 5.23a and 5.23b). The nature of emission is seen to be the same in

solid state also (Fig. 5.23c). This emission is neither metal-to-ligand

charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in

nature since Zn2+

ions are difficult to oxidize or to reduce due to their d10

configuration. The emission can probably be assigned to the intra-ligand

(π*→ π) fluorescent emission of 1, 10-phenanthroline. The free 1,10-

phenanthroline exhibits two emission peaks at 420 and 445 nm. The

hypsochromic shifts of the emission peaks of the compound, with respect

188 Chapter V

to the 1,10-phenanthroline, indicate that the chelation of the ligand to the

Zn2+

ion, increases the ligand conformational rigidity and thereby reduces

the loss of energy by radiationless decay of the intraligand emission

excited state.116

It is also interesting to see that the emission frequency of

compound 31 is same in both solid state and in methanol solution. This

shows that the dimeric nature of the compound is retained in the solution

phase also.

250 300

0

1

2

3

4

270

Inte

nsit

y (

AU

)

nm

250 300 350 400 450 500-50

0

50

100

150

200

250

300

350390

Inte

ns

ity (

AU

)

nm

a b

350 400 450 500

0

200

400

600

800

1000 390

420445

Inte

ns

ity (

AU

)

nm

c

Fig. 5.23 (a) Absorption spectrum, (b) Emission spectrum of 31 (in methanol)

and (c) Emission spectra of 31 (black) and phen (red) in solid state


5.3.6 [Zn(Hmal)2(2-bipy)H2O], 30 as a ‘metallo-ligand’

Since compound 30 contains two free -COOH groups we can

consider it as a ‘dicarboxylic acid’ but having metal ion as part of the

spacer unit. In principle this metal-containing dicarboxylic acid should act

analogous (like maleic acid or fumaric acid) to simple dicarboxylic acids

towards suitable metal salts and produce polymeric coordination polymers

which then will have two types of metal constituents. One can then

consider this as a ‘metallo-ligand’ and utilize it to generate a polymeric

compound by reacting with metal salts of one’s choice. With this in view

we tried to react the bipyridine containing metallo-ligand 30 with ZnCO3.

For this an aqueous solution of 30 was treated with ZnCO3 under mild

heating and stirring. Solution was filtered and the clear filtrate yielded

colourless crystals, 32 suitable for single crystal X-ray diffraction. 32 was

characterized by elemental analysis (Table 5.1), spectral and single crystal

X-ray analyses and found to be a dimer with the molecular formula

[Zn2(mal)2(2-bipy)2 (H2O)2]2H2O similar to compound 31. Contrary to

our expectation of the formation of the polymer -Zn-mal-Zn-mal- with

2,2’-bipyridine coordinated to the alternate Zn atoms, compound 32 is a

dimer in which two Zn(II) ions are linked by two maleate units to form a

14 member ring (as in 31). The molecular structure of the compound is

given in Fig 5.24 along with atom numbering. It can be clearly seen that

the formation of polymeric type compound (interconnected through

maleate units) is prevented by the coordination saturation of Zn2+

ion by

strongly chelating 2,2’-bipyridine molecule and hence the chain extension

beyond dimeric level gets terminated. In fact this compound 32 is

molecularly analogous to 31. Zn2+

ion in the compound has distorted

190 Chapter V

trigonal bipyramidal geometry. The O(2), O(4) and N(1) atoms make

the triangular plane and the apices are occupied by N(2) and O(5).

N(2)-Zn(1)-O(5) angle is 170.27(4)Å, Zn-O distance is about 2Å and Zn-

N distance is about 2.1Å. Fig. 5.25 clearly illustrates the trigonal

bipyramidal coordination environment and Fig. 5.26 shows the packing

feature of 32. Selected bond lengths and bond angles are presented in

Table 5.9 and H-bonds in Table 5.10. Even though [Zn2(mal)2(2-bipy)2

(H2O)2]2H2O is not a polymeric compound extensive intra- and inter-

molecular H-bondings along with strong π-π interations make the

compound an overall 3-dimensional network. Fig. 5.27 and 5.28 clearly

demenstrates these interactions.

Fig. 5.24 ORTEP representation of 32 with atom label (35% anisotropic

ellipsoid). The asymmetric unit joins with its centre of inversion

(Symm: ½ -x

, ½ -y

, -z) to form a dimer. Only atoms of

asymmetric units are labeled.


Fig. 5.25 The dimeric unit of 32 showing the trigonal bipyramid

coordination.

Fig. 5.26 Packing features of [Zn2(mal)2(2-bipy)2 (H2O)2]2H2O, 32

along with H-bonds.

192 Chapter V

Fig. 5.27 H-bonds among the lattice water, coordinated water and the

carboxylate oxygens of 32

Fig. 5.28 Packing features of 32 showing possible π-π stacking of the

molecular species.


Table 5.9 Selected Bond lengths [Å] and angles [°] for 32

C(1)-N(1) 1.3311(19) N(1)-C(1)-C(2) 122.37(16)

C(1)-C(2) 1.383(2) N(1)-C(1)-H(1) 118.8

C(1)-H(1) 0.9300 N(1)-C(5)-C(4) 121.15(14)

C(5)-N(1) 1.3460(18) N(1)-C(5)-C(6) 115.82(12)

C(5)-C(6) 1.480(2) C(4)-C(5)-C(6) 123.01(13)

C(6)-N(2) 1.3357(17) N(2)-C(10)-C(9) 122.19(16)

C(6)-C(7) 1.390(2) N(2)-C(10)-H(10) 118.9

C(10)-N(2) 1.332(2) O(3)-C(11)-O(4) 124.25(13)

C(11)-O(3) 1.2381(17) O(3)-C(11)-C(12) 119.06(12)

C(11)-O(4) 1.2648(17) O(4)-C(11)-C(12) 116.69(12)

C(11)-C(12) 1.4880(19) O(1)-C(14)-O(2) 126.44(13)

C(12)-C(13) 1.3222(19) C(1)-N(1)-Zn(1) 124.87(10)

C(12)-H(12) 0.9300 C(5)-N(1)-Zn(1) 115.71(9)

C(14)-O(1) 1.2392(17) O(4)-Zn(1)-O(2) 108.83(4)

C(14)-O(2) 1.2675(16) O(4)-Zn(1)-N(1) 135.03(4)

N(1)-Zn(1) 2.1007(12) O(2)-Zn(1)-N(1) 115.27(4)

N(2)-Zn(1) 2 1187(12) O(4)-Zn(1)-N(2) 93.00(5)

O(2)-Zn(1) 2.0182(10) O(2)-Zn(1)-N(2) 92.20(4)

O(4)-Zn(1) 1.9686(10) N(1)-Zn(1)-N(2) 77.33(5)

O(5)-Zn(1) 2.1229(12) O(4)-Zn(1)-O(5) 92.34(5)

O(5)-H(5A) 0.841(14) O(2)-Zn(1)-O(5) 93.77(4)

O(6)-H(6B) 0.831(15) N(1)-Zn(1)-O(5) 93.15(5)

N(2)-Zn(1)-O(5) 170.27(4)


#1 -x+1/2,-y+1/2,-z

194 Chapter V

Table 5.10 H- bonds in 32 [Å] and angles [°]

___________________________________________________________

D-H...A d(D-H) d(H...A) d(D...A) <(DHA)

O(5)-H(5A)...O(1) 0.841(14) 1.791(15) 2.6101(17) 164(2)

O(6)-H(6B)...O(3)#2 0.831(15) 1.941(15) 2.7688(18) 174(2)

O(6)-H(6A)...O(2)#1 0.816(15) 2.283(15) 3.0987(17) 177(2)

O(5)-H(5B)...O(6) 0.825(15) 1.933(16) 2.7453(17) 168(2)

___________________________________________________________ Symmetry transformations used to generate equivalent atoms:

#1 -x+1/2,-y+1/2,-z #2 -x,-y,-z

Based on our observations on the reaction of the metal(II)-bis-

hydrogenmaleates [Co(Hmal)2(H2O)4] and [Zn(Hmal)2(H2O)4] with

various amines like pyridine, picolines, 4,4’-bipyridine, 2,2’-bipyridine

and 1,10-phenanthroline the overall transformation reactions involved can

be summarized as follows:

[Co(Hmal)2(H2O)4] + pyridine → pyridiniumsuccinate zwitterion, 16 +

[Co(fum)(H2O)4]n, 17

[Co(Hmal)2(H2O)4] + 3-picoline → 3-picoliniumsuccinate zwitterion, 23 +

Co(fum)(3-pic)3(H2O)]·4H2O, 22

[Co(Hmal)2(H2O)4] + 4-picoline → 4-picoliniumsuccinate zwitterion, 25 +

[Co(fum)(4-pic)(H2O)3], 26

[Co(Hmal)2(H2O)4] + 2-picoline → no reaction

[Co(Hmal)2(H2O)4] + 4,4’-bipyridine → Co(mal)(4-bipy).0.5H2O, 27 +

Fum:4-bipy adduct, 28


[Zn(Hmal)2(H2O)4] + 2,2’-bipyridine → Zn(Hmal)2(2-bipy)H2O, 30

[Zn(Hmal)2(H2O)4]+1,10-phenenthroline→[Zn2(mal)2(phen)2(H2O)2] 2H2O, 31

5.4 Summary and conclusion

The work embodied in this chapter forms an extension of our

investigation on the nature of reaction of various [M(Hmal)2(H2O)4] with

pyridine that has shown the unprecedented maleate to fumarate

transformation from one of the Hmal moieties and the formation of a

novel chiral zwitterion from the second Hmal unit. In this context we have

considered a variety of pyridine derivatives in this chapter. It has been

well established that as in the case of pyridine, methyl pyridines (4- and

3- picolines) also react with M(II)-bis-hydrogenmaleates leading to the

maleate–fumarate transformation at the metal centre and the formation of

chiral zwitter ions. Due to steric factors 2-picoline does not react with

M(II)-bis-hydrogenmaleate at all. Even though pyridine/picolines and

4,4’-bipyridine/2,2’-bipyridine/1,10-phenanthroline are closely related N-

donor ligands, they react with molecular M(II)-bis-hydrogenmaleates in an

entirely different manner. While Pyridine/picolines transform molecular

M(II)-bis-hydrogenmaleates to one-dimensional polymeric M(II)-fumarate

derivatives (17, 22, 26) with the simultaneous generation of pyridinium

succinate zwitterions (16, 23, 25), 4,4’-bipyridine converts molecular

M(II)-bis-hydrogenmaleate to a three-dimensional polymeric adduct of

M(II)-maleate and 4,4’ bipyridine (27) with the concomitant formation of a

fumaric acid:4,4’-bipyridine adduct (28), instead of a zwitterion. It is

interesting to note that in both reactions maleate to fumarate conversion has

taken place. But in the first case (pyridine/picolines) the fumarate moiety is

attached to the metal ion while in the second case (4,4’-bipyridine) the

196 Chapter V

fumarate unit forms an adduct with the base. It is also interesting to see that

only pyridine/picolines form the zwitterion while 4,4’-bipyridine which has

two pyridine groups yields no zwitterion. The closely related 2,2’-

bipyridine does not produce maleate-fumarate transformation at all but

forms a unique molecular adduct with M(II)-bis-hydrogenmaleate. The two

free carboxyl groups of this compound makes it an interesting ‘metallo-

ligand’ which we could utilise to generate novel M(II)-maleate compounds.

It has been observed that unlike in the case of 2,2’-bipyridine, 1,10-

phenanthroline does not form an adduct with Zn(II)-bis-hydrogenmaleate.

But by a ‘soft solution route’, Zn2+

ions, maleic acid and 1,10-

phenanthroline can be made to undergo a self assembly to form a dimeric

compound, [Zn2(mal)2(phen)2(H2O)2]2H2O, (31) which is very much

similar to its 2,2’-bipyridine analogue [Zn2(mal)2(2-bipy)2(H2O)2]2H2O,

(32) i.e. two Zn ions are bridged by two maleate dianions and

coordination saturation is achieved by one phenanthroline and one water.

Extensive hydrogen bonding between the carboxylate oxygens and the

water molecules (both coordinated and non-coordinated) and also the π-π

interactions of the phenanthroline molecules favours a three dimensional

metal-organic coordination network. The luminescence study reveals that

the compounds 31 and 32 are strongly fluorescent and also establishes the

fact that rigidity enhances fluorescence. We found that the formation of

seemingly simpler molecular species [M(Hmal)2(H2O)4] and reaction of

such metal(II)-bis-hydrogenmaleates with various pyridine derivatives

bringing about facile cis to trans isomerisation from maleate to fumarate

and formation of unprecedented chiral zwitterions are quite novel and

unique. The nature of transformation and products formed are seen to

depend on the type of pyridines that has been employed. In contrast to


pyridine, picolines and 4,4’-bipyridine the chelating type bipyridines

(2,2’-bipyridine and 1,10-phenanthroline) do not seem to facilitate any

such transformation. However both these bipyridines generate new

monomeric and dimeric molecular species (30, 31 and 32). We have also

considered employing this monomeric species (30) as a ‘metallo-ligand’

and demonstrated the formation of a novel dimeric product by simple

chemical manipulations.

Chapter V Transformations involving M(Hmal) (M = Co, Zn...

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