Indian Journal of Chemistry Vol. 42A, September 2003, pp. 2290-2299

Synthesis and formation of dinuclear mixed-valent complexes of ruthenium and osmium bridged by 2-(2-pyrimidyl)benzimidazolate

Masa-aki Haga*, Masahide Ishizuya , Tomokazu Kanesugi, Tomonu Yu taka. Daisuke Suki yu lllu, Joerg Fees & Wolfgang Kuim

Department of A pplied Chemistry. Faculty of Science & Engineeri ng. Chuo Uni vers ity. 1- 13-27 Ka,uga. Bunkyo-ku , Tokyo 11 2-8551, Japan, & Institut fuer A norganische Chemie. Un i versi taet Stuttgart .

Pfaffenwaldring 55. 0 -70550 Stuttgart . Germany

E-mai l: mhaga@apchem.chem.chuo-u.ac.j p

Received 13 February 2003

Mononuclear Ru/Os complexes, [M (bpyh(Hbimpm)f+. where Hbimpm = 2-(2-pyrimidy l)benzimidazolc. behaves a ~ a monobasic ac id to form the conjugate base, IM(bpyl2(bimpm) 12+ where bimpm = 2-(2-pyr imidyl)benzimidazolale. Nov.: 1 dinuclear Ru/Os complexes bridged by 2-(2-pyrimidy l)benzimiuazo late (bi mpm) have been prepared from the reacti on Ill"

M (bpyhCl2 w i th a deprotonated mononulcear comp lex, [ 1(bpyh( bimpm)t(M = Ru and Os). T he MLCT hand for the di nuclear Ru complexes, [Ru(bpyhCL)Ru(bpyhl"+, is shi fted to longer wavelcngths in the order of L = bibcnzim idazo lat.: (bibzim) > bibzirn > bpm. Furthermore, the Kcurn value of a mixed-va lent complex. calculated from the poten tial di fference between the first and second elcc tron-transfer step. for (he Ru complex is larger than that for the Os analogue. The mixed­valent complexes which were generated electrochemica ll y show the interva lence charge transfer band at 1830 nm for Ru and 1300 nm for Os. respecti vely. T he degree of electronic coupling between two meta l centers is ca lculated as 650 cm·1 for Ru and 360 cm·1 fo r Os, respecti vely . By comparing the K enOl and Hab va lues w ith those of the analogous bridged ligand system slich as bpm and bibzim, a synergetic contr ibution of both electron exchange and hole exchange mechanisms i, operat ive for metal-metal communicati ons in the present bimpm -bridged mixed-valent complexes.

Introduction M ixed-valent states in li gand-bridged diruthenium

complexes have received much attention in recent years in connection with the design of molecular electronic dev ices2

-9

. The metal-metal interaction through a bridging ligand in dinuclear complexes strong ly depends on the nature of that bridging ligand which can control the strength of the electronic coupling and the distance of charge transfer I0

-23

.

Therefore, the des ign of new bridging ligands is one of the key issues in reali ;7. ing molecular electronic dev ices. The most w idely studied class of bridging li gands contains N-heterocycli c compounds such as pyridy l, pyrazine and pyrimidy l groups in mono-, bi-

d 'd f h' 24-29 F I 44' an tn entate as Ions . or examp e, , -bi pyridi ne, 2,3 -bi s(2-pyridyl)pyrazine (dpp )26, and

2 ") ' b' . 'd ' (b )1130-13 h ,_ - Ipynml Ine pm --- . eac can connect two metal ions, mediating metal-metal electronic interac ti on through their n:-system. For the purpose of des ign ing new bi s-bidentate bridging ligands, the selecti on of the coordinating fragments is important. The n:-donorln:-acceptor properti es of the bridging ligand and/or the metal-metal distance can be controlled by choosing the appropr iate combination of components. Pyridine-, pyrazine- and pyrimidine-

containing ligands have relati vely low- lyi ng 71-orbital~ and there fore act as good acceptors. In contra"l. benzimidazole-containing ligands are poorer IT­

acceptors and better n:-donors. [n our prev ious study. we have reported M (U)-M(lII ) mixed-va lent complexes (M = Ru and Os) bri dged by 2.2'­bibenzimidazolate (bibzim). 13 in which the anionic bibzim bridging ligand can stabili ze the mi xed-va lent state by reducing the positi ve charge on the complex. [n addition, the metal-metal interacti on takes place th rough an interaction involving the HOMO or the bridging li gand. On the other hand, th e corresponding mixed-valent complexes bridged by 2,2'-bipyril11i C\ine (bpm) are not stable on a preparati ve scale30

In the present study, we describe homodinuclear Rli and Os complexes wi th a novel bri dging li gand. 2-(2-r y rimidy l)benzimidazolate, for a compari son w ith analogous 2,2' -bibenzi midazo late- and 2,2'-bipyrimi­dine-bridged systems (Scheme 1).

Materials and Methods 2-Cyanopyri midi ne (TC[), tetrae thy lal11 l11on i lim

hydrox ide ( 10% water) (TC[), and ruthcni um trichloride trihydrate (N.E. Chemcat, Tokyo) were used without further purificati on. Acetonitrile was

HAGA ef ClI.: DINUCLEAR MIXED V ALENT COMPLEXES OF Ru & Os 229 1

"S(l" t.(" ,,:M , ...--. '

~ II

Scheme 1

purified twice by distillation over P20 S. Tetra-l1-butylammonium tetrafluoroborate (TBAB, Nacalai) was recrystallized from ethanol-water (4: 1 v/v) and dried in vacuo. All other supplied , chemicals were of standard reagent grade quality .

The compounds, Ru(bpyhCh'2H20 and Os(bpyhCI2·2H201.34 were synthesized according to literature methods.

Synthesis of the bridging ligand, 2-(benzimidazol-2-y/)pyrimidine (Hbilllpm)

This compound has been already reported in the literature. However, we used the modified synthetic procedure as follows42 : A solid mixture of 2-cyanopyrimidine (1.00 g, 9.5 mM) and well-ground 0-

phenylenediamine (1.03 g, 9.5 mM) was heated at 160°C for 2 h. During the period of heating, the solid mixture became a homogeneous solution at around lOO°C and further lise of the temperature to 160°C led to solidification. The resulting solid was recrystallized from ethanol. Yield : 1.04 g (56%). Mp 289-290°C. IH NMR (DMSO-d6): 87.24(t, J = 7.6 Hz, IH) , 7.31(t, J = 7.2 Hz, IH), 7.57(t, J = 8.0 Hz, I H) , 7.60(t, J = 4.8 Hz, IH), 7.74(d, J = 7.6 Hz, 1 H), 9.01(d, J = 4.8 Hz, 2H) and 13.25 (br, IH).

Synthesis of the mononuclear complexes (Ru(bpy h( Hbilllpm )}( PF6)2-Ru(bpyhCh'2H20 (0.21

g, 0.40 mM) was dissolved by heating (-I h) in ethanol-water ( I : 1 v/v, 40 cm\ 2-(Benzimidazol-2-yl)py rimidine (0. 18 g, 0.45 mM) was added to the resulting solution and the mixture was further heated for 5 h. During thi s period, the colour of the solution changed to red . The resulting solution was cooled to room temperature and evaporated to a half volume. After filtration, a saturated solution of NH4PF6 (5 cm3

in H20) was added dropwise until precipitation was complete. The precipitate was collected and dried. Puri fication was performed by recrystallization from methanol-water (l: 1 v/v). Yield: 0 .31 g (84 %). Anal. Calcd. for RUCJIH24NgP2FI 2AH20: C, 38.32; H, 3.32;

, 11.53. Found: C , 38.56; H, 3 .65; N, 11 .25 .

{Os(bpyh(Hbimpm)}(PF6Jz-The microwave reactor used in the present study was purchased from Shikoku Keisoku Ltd and provides a magnetic sti rrer and a connection for a reflux condenser. The irradiat ion power of the reactor is 650 W at multimode. A mixture of Os(bpyh Ch (0.1 g, 0.17 mmo'l) and the Hbimpm ligand (0.03 g, 0.17 mmol) in ethylene glycol (30 ml) was intermittently heated by the microwave oven for 10 min . During heati ng the colour of the reaction mixture changed to brown . On cooling to room temperature , water (30 cm") was added to the reaction mixture, followed by filtrati on. To the filtrate 2M HCI (0.5 cm3

) and then a saturated solution of NH4PFo were added to complete precipitation. The precipitate was collected and dri ed in vacuo. Purification was performed by SP Sephadex LH-20 column chromatography, using an acetonitrile/ methanol mixture (l : 1 v/v) as an eluent. The desired complex was obtained by evaporation of the so lvent. Yield: 0.15 g (93 %). Anal. Calcd. for OSC3I H24NgP2FI2·3 H20: C, 35 .71 ; H, 2.90; N. 10.75 . Found: C, 35.92; H, 3.30; N, 11 .05.

Synthesis of the deprotonated complexes {Ru(bpyh(bimpl1l)}(PF6)-To a hot solution of

[Ru(bpyh(Hbimpm)](PF(i)2 (0.2 g, 0.22 mM) in

CH3CN-methanol (1 : lv/v, 20 cm3) was added

tetraethylammonium hydroxide (3 cm}, 10 % in water). The solution was cooled to room temperature and "the solvent was evaporated to 5 cm). Addition of diethylether to the solution led to a brown precipitate which was an ana lytically pure product. Yield: 0.1 6 ~ (98 %). Anal. Calcd. for RUC3IH23 NgPFdH20 : C~. 50.52; H, 3.32; N, 12.74. Found: C, 50.25; H, 3.40; , 12.51.

[Os(bpyh(bimpm)}(PF6)-This complex was synthesized in a similar manner to [Ru(bpy):,­(Hbimpm)](PF6)2' except that [Os(bpyh(Hbimpl11 ) 1-(PF6)2was used instead of [Ru(bpyh( Hbimpm)J(PF(')2 '

A black-brown microcrystalline solid was obtained. Yield : 70%. Anal. Calcd. for OSC3IH2J NgPF(, '3H 20: C. 45.87; H, 3.02; N, 11.57 . Found: C, 46.0 I ; H. 3.30: N, 11.26.

Synthesis of dinuclear complexes {Ru(bpyh(bil11pm)Ru(bpyh}(PFr,h - Ru(bpY)2C I2'

2H20 (0.11 g, 0.22 mM) was dissolved in 40 cm3 o f ethanol-water ( I : J v/v) at 80' C under nitrogen. So lid [Ru(bpyhCHbimpm)](PF(i) (0.17 g , 0.22 mM) was added and heating was continued for 4 h. durin!! which time the colour of solution turned to red-brown~

2292 INDIAN J CHEM, SEC A, SEPTEMBER 2003

After being cooled to room temperature the solution was filtered to remove insoluble impurities, and a saturated NH4PF6 aqueous solution was added to the filtrate until complete precipitation occurred. The precipitate was collected and purified by column chromatography on alumina with CH3CN-methanol. The main band second of the desired dinuclear complex was collected. The eluate was evaporated to obtain the product. Yield: 0.14 g (42%). IH NMR

(CD}CN): 8 5.93 (dd, J = 6.3 Hz, 2H), 6.86 (dd, J = 6,3 Hz, 2H), 7.03 (t, J = 5.7 Hz, IH), 7.5 (m, 8H), 7.69 (d, J = 5.7 Hz, 2H), 8.0 (m, 8H), 8.1 (m, 8H), 8.4 (m, 8H). ESI-Mass spectrum: mlz = 342.042 (M -3PF6)3+). Anal. Calcd. for RU2CSIH39NI 2P3FI8·3H20 : C, 40.54; H, 3.00; N, 11.12. Found: C, 40.75; H, 3.20; N, 11.01.

(Os(bpyh(bimpm)Os(bpyhHPF6h - This complex was prepared in a similar manner as the corresponding Ru complex above, except that [Os(bpyh(bimpm)](PF6h was used instead of [Ru(bpyh(bimpm)](PF6). A black-brown microcry­stalline solid was obtained. Yield: 62%. ESI-Mass spectrum : Inlz = 400.474 (M-3PF6)3+). Anal. Calcd. for OS2CSIH39N I2P3FldH20: C, 36.10; H, 2.68; N, 9.95 . Found: C, 35.93 ; H, 2.70; N, 10.03 .

Physico/measurements Electronic absorption spectra were obtained on a

Hitachi U-3210 spectrophotometer from 200 to 850 nm and a Hitachi 3400 spectrophotometer from 800 to 2500 nm. NMR spectra were measured with a 270 MHz JEOL spectrometer. The mass spectra were measured on a Shimadzu QP1000EX spectrometer for organi c li gands and Micromass LCT electrospray mass spectrometer equipped with electrospray interface (ESl) system for the li gands and Ru complexes .

Electrochemical measurements were made at 200 e with an ALS/CH Model 660A electrochemical analyzer. The working electrode was a g lassy-carbon or platinum disk electrode and the auxiliary electrode was a pl atinum wire. The reference electrode was Ag/Ag 0 3 (0.0 1 M in 0 . 1 M TBAB CH3C N), abbreviated as Ag/Ag+. The £ \12 value for the ferrocen ium/ferrocene (Fc+/Fc) couple is +0.09 V vs Ag/Ag+. Spectroe!ectrochemistry was performed by ll s ing a pl atinum minigrid (80 mesh) working electrode in a thin-layer cell (optical path length 0.05 cm). The ce ll was placed inlo the spectrophotometer, and the absorption change was monitored during the e lectro lysis. Flow e lectrolysi s was performed with the

same flow-through cell as reported previously . pH measurements were made with a TOA model HM-20E pH meter standardized with buffers of pH 4 .0 I and 6.89. A 50% acetonitrile-buffer mixture was employed because of the limited solubility of the present complexes in pure aqueous solution . The readings of the pH meter in this mixture are referred to as "apparent" pH unless stated otherwise. Spectrophotometric iitrations were performed in an acetonitrilelbuffer (1 : 1 vlv) solution, as described previously. Buffer systems and pH ranges employed were as follows : Hcl04-NaCI04 , pH 0-2; Robinson­Britton buffer, pH 2-11.

The geometrical structures of the bridging li gands Hbimpm and its deprotonated form, bbbpy, were optimized using an STO-3G basis set within the Hartree-Fock approximation (HF/STO-3G). All calculations were performed by using the quantum chemistry software SpartanPro software TM.

Results and Discussion

Preparation of complexes The mononuclear complexes [M(bpyh(Hbimpm )]­

(PF6h (M = Ru, Os) are easily prepared by the reaction of [M(bpyhCIz] with Hbimpm ligand. In case of the Os complex, the microwave-assisted preparation leads to a remarkable reduction in reaction time from hours to mi nutes. The mononuclear complex [M(bpyh(Hbimpm)]( PF6)~

possesses the property of a monobasic ac id . Proton di ssociation gave the corresponding conjugate base [M(bpyhCbimpm)](PF6), which can ac t as a bidentate ligand . The complexes [M (bpyh (bimpm)l(PF6 )

reacted with M (bpyhCl2 to form the homod inuclear complexes [M (bpy)z(bimpm)M(bpy)z](PF(,)3 (M = Ru , Os) . The mononuclear and dinuclear complexes were characterized by 1 H NM R and electrospray mass spectra. A typical isotope pattern of ES I-MS spectra for [Ru(bpyhCbimpm)Ru(bpY)2](PF6h is shown in Fig. ] . The observed isotope di stribution pattern was reproduced by the simul ati on o f the expected chemical formul ae for the corresponding dinuclear complex.

The dinuclear compl exes are stable even under ac idic conditi on. This non-reactivity is in sharp contrast to the behav iour of dinuclear complexes bridged by bibenzimidazolate (bibzim) whi ch are easi ly cleaved to [M(bpYh(S)f+ and [M(bpYMH~bibzim)f+ by the addilion of protons\',

HAGA et al.: DINUCLEAR MIXED VALENT COMPLEXES OF Ru & Os 2293

Calc. 3G.Cl5II2 S41. Ru

Obs.

341.038Z SoG.701e

34O.~

340.03158 ~~

J4Ue11

338.11818 S43.7U78

ul v " \J IJ v ... \J v~

338 140 341 342 343 344 304S

Fig. I-Observed and calculated isotope patterns of ESI mass spectra for the [Ru(bpyh(bimpm)Ru(bpyh]3+ion in CH3CN.

Absorption spectra of mononuclear and dinuclear complexes

The metal-to-ligand charge transfer (MLCT) bands for the mononuclear complexes [M(bpyh(Hbimpm)](PF6h (M = Ru, Os) are observed at 454 nm or 480 and 650 nm, respectively. The absorption spectra of the compounds [M(bpyh(Hbimpm)](PF6)2 show a strong pH

dependence. As the pH of a solution of [Ru(bpyh(Hbimpm)](PF6)2 is raised, the MLCT band at 454 nm shifts to 478 nm with three isosbestic points at 475, 407 and 327 nm, as shown in Fig. 2. The Ru complex is stable at all pH values examined. The pKa value for this acid-base equilibrium is calcu lated to be 6.96 from the analysis of titration curves for the absorbance vs pH values.

[ct-ro] 2+ [ct-OO] + + H+

"R~(bPY)2 "R~(bPY)2 ... (1)

1.0

GI 0.5

u c::: O·goo III 400 500 600 .a ... 0 (II .a ~ 1.0

Os

0.5

0.0 300 400 500 600 800

Wavelength, nm

Fig. 2-pH Dependence of the absorpt ion spectra for [M(bpyh(Hbimpm)](PF6)2 {(a) M = Ru and (b) M = O~ ill

CH3CN-buffer ( I : 1 v/v) mixture at the pH region of 2.28 < pH < 9.64.

Similarly, [Os(bpyhCHbimpm)](PF6h reveals a p H dependence on absorption spectra throughout the pH range of 3-10, from which a pKa value of 6.51 is obtained. Thus, the pKa value of [Os(bpyh(Hbimpm)](PF6h is less than that of the Ru analogue.

The Ru dinuclear complex [Ru(bpyh(bimpm)­Ru(bpyh ](PF6h exhibits a new weak band at 570 nm in addition to the strong band at 458 nm. A sim ilar band at longer wavelength was observed for the Os dinuclear complex.

Electrochemistry of the complexes The redox potentials of the complexes are collected

in Table 1. The mononuclear complex. [Ru(bpyh(Hbimpm)]2+ reveals a one-elect ron oxidation process at +0.85 V vs Fc+/Fc and three OIlC­

electron reduction steps at -1.30, - 1.87, and -2 .10 V. It has been reported that the electrode potenti als o f [Ru(bpY)3]2+ are +0.87 V vs Fc+/Fc for the Ru(lI/lll ) oxidation and -1.72, -1.91, and -2. 17 V for the bpy ligand reductions. Comparing the two systems. the first reduction process of [Ru(bpyh( Hbimpm)]2+ can be assigned to the reduction of coordinated Hbimpm while the second and third reduction steps correspond to successive reduction processes of each coordinated bpy ligand as shown in Scheme 2.

The deprotonation of the complex [Ru(bpYh (Hbimpm)]2+ induces a 0.37 V negative sh ift for the Ru(Il/lJI) oxidation potential. Similar negati ve shi fts of oxidation potentials as caused by N-H

2294 INDIAN J CHEM, SEC A, SEPTEMBER 2003

Table I - Spectroscopic and electrochemical data and pKa values of mononuclear complexes [M(bpy)~(L)f+

L MLCT Am,. nm

H

C)-<\X) 457

H

O-{X) 45S·

CriJ N N

"Ref.39; bRef.40: eNot reported ; dRef. 30 and 33.

-1 e - t [R u lI(bpY)2(Hb im pm )]2+

+ 1e -t [R ull(bpYh(Hbimpm-W

+1e -t [Ru lI(bpy) (bpy -) (Hbimpm -)] 0

+1e-t

[R ull(bpy -)2(H bim pm -if

Scheme 2

M =Ru EII2

M(II1III) V vs Fc/Fc+

0.S5

0.7S'

deprotonation have been reported for several metal complexes containing imidazole or benzimidazole I· d 2 J.35-40 19an s .

The Os analogue, [Os(bpyh(Hbimpm)f+, exhibits a si mi I:lr redox behaviour (Table \ ), however, the Os( l1l1l1) couple is 0.45 V lower in potential than the Ru(lI/lIl) one. This difference between Ru and Os complexes is consistent with observations for other Rll /Os sys tems reported previously .

For the dinuclear Ru co mplexes, two successive, well separated one-electron oxidation processes are observed at +0.61 and + 1.00 V vs Fc+/Fc. Fig. 3 shows a typi cal cyclic voltammogram of dinuclear

pKa

6.96

6.S'

5.74b

10.51

MLCT Am .. nrn

4S0

635sh 570sh 455

M=Os EI/2

(M(I/IIII) V vs FcfFc+

0.40

pKa

6.51

5.0Sh

9.59

1.4 1.2 1.0 0.8 0.2 V vs Fc+/Fc

Fig. 3- Cyc1ic voltammogram of di nuclear [Ru(bpyh(bimpm)Ru(bpYh 13+ in CH3CN

[Ru(bpyh(bimpm)Ru(bpY)2]3+ in CH3CN . Th is voltammogram indicates that the mixed- valent Ru( II )­Ru(IIJ) complex is formed after one-electron oxidation (Scheme 3).

The compropotionation constant K colTl of a mixed­valent complex is evaluated from the potential difference between the first and second electron ­transfer step, which gives a measure of the stabi lity of the mixed-valent complex. The K com va lue is obtained as 4.0 x 106

, calculated from the potent ial difference (~E= E2 - E, ) of 0.39 V for the present c1irutheniulll complex.

Kco lll

Ru(1\)-Ru(ll)+Ru(UI)-Ru(1IJ) ~ 2Ru(lI )-Ru(lII )

HAGA el 01.: or UCLEAR MIXED VALENT COMPLEXES OF Ru & Os

5+

Scheme 3

Similarly, the dinuclear Os complex reveals two successive oxidation processes at +0.28 and +0.56 V vs Fc+/Fc. The potential diffe rence and K colI/ value for

that complex are 0.28 V and 5 .5 x 104, smaller than

that for the corresponding d iruthenium analogue31 . This is contrary to the results for most o ther RU2 and OS2 complexes reported so far, except for bibzim l3 or 1 ,2-diacy Ihydrazido-bridged dinucJear complexes41 . These results are explained by the difference of the

orbital interact ion between metal dn and bridging

li gand n or n* orbita ls; the reversed effect on using ruthenium or osmi um arises depending o n whether the

bridging ligand has a n-donor o r an-acceptor nature6.

Since the present bimpm-bridging ligand has a n­donor property , the stro nger n-accepto r capacity of Ru(llI ) vs OsO Il ) leads to larger K com va lues for the Ru dinuclear complexes re lative to the cOITesponding Os analogues.

Spectroe /ectrocli elJlistry of the d ilLuclear complexes Spectroelectrochemical measurements of the

dinuclear co mplexes were performed by the use of a th in-layer cell. Figure 4 shows the absorpti on spectra of [Ru (bpyhCbimpm)Ru(bpyh]3+ in CH3CN under the spectroelectroche mical condition by controlling the appl ied potential s. Spectroelectrochemical oxidation of l Ru(bpyh(bimpm)Ru (bpY)2]3+ at 0.8 V vs Fc+lFc leads to a decrease of the MLCT absorpti on bands at 458 and 570 nm and to the appearance of new a weak band around 730 nm as a sho ulder, which corresponds to the li gand-to-metal charge trans fer(LMCT) band. In addit ion , a charac te ri st ic broad band at 1830 nm appeared which is ass ig ned to an interva lence charge transfer (IT) band l3 . After the second oxidation at + 1.2 V vs Fc+/Fc, a complete loss of the MLCT and IT bands and the appearance of an LMCT band at 727 nm are observed.

Successive ox idatio n of [Os(bpyhCbimpm)-Os(bpYh] '+ in C H3CN was spectroe lectroche mically carri ed o ut at +0.5 V and +0.75 V vs Fc+/Fc. Figure 5 shows the near-infrared absorption spectra of [Os(bpY)c( bimpm)Os(bpyh ]'l+ ( n = 3-5). New broad bands around 1300 nm and 2000 nm (IT) are observed

a

'" o '. :ii 0·gO!:-:0:---4:-!0~0-...:....~:::-.:..:o..:..:,~-=='=70~0:==~800 -_ .. ------

.rJ

~ 0.20,---------------, .rJ c:(

0.15

0.10

b

1400 1600 1800 2000 2200 Wavelength, nm

Fig. 4--UV-vis (a) and near-infrared (b) absorption speclra of [Ru(bpyh(bimpm)Ru(bpyh 13+ in CH,CN containing 0. 1 AI TBABF4 under the spcctroelectrochelllical condition al 0 V(ori ginal) (dashed line), +0.8 V (solid linc). and + 1.2 V \ ~ Fc+/Fc (broken line)

1.6

1.4 '\ \

1.2 ' \

a

.. '."'---'-- ""\,

g 0 .4 L_---L_"--='·.::·--:::r:·-·-=~====~=--·r.:·-=· ~-~. -::'=- -::r-~-'=-~' 3o=,j

1l 300 400 500 600 700 800

~ 0.08r-----------------, .rJ c:(

Wavelength, nm

b

Fig. 5- UV-v is (a) and ncar-infrared (b) absorpti on , p~ct ra or rOs(bpY)2(billlpm)Os(bpY)cl n

+ ( n = 3-5) in CH ,CN conlaining 0. 1 M TBABF4 under the specl roelec lJ'Ochelllica l condili on ;11 ()

V(origi nal) (dashed line). +0.8 V (solid line). and + 1.2 \' " Fc+/Fc (broken line)

2296 I DIAN J CI-IEM, SEC A. SEPTEMBER 2003

after the first oxidation at +0.5 V, the second oxidation at +0.75 V leads to the complete loss of this band at 1300 nm. Two new bands at 1780 and 2330 nm develop in the progress of the second oxidation . These two near-infrared transitions can be assigned as clrr-drr transitions by comparison with the near­infrared spectra of analogous Os complexes 13.

Met(//-metal in teraction ill mixed-valence comp7exes The degree of electronic coupling between the

metal centers. HAB, can be evaluated from the posit ion. bandwidth, and intensity of the IT band In Figs 4 and 5 through the fo llowing approx imation:

[ ]

112 r l _ _, t . ~\i ? \i . . _ I

H Ail = 2.0) x I0 - m.~ 1/ - Il ';~"X J(ln cm ) ll1~r,

where tm"x is the ext inction coefficient (in M l,m' I),

v,,"" is the waven umber of the IT absorption maximum (in Cln ·

I), ~V II: is the bandwidth at half­

hei e: hr (in em, I), and r is the distance between the Ille~al sites (il~ A). For the bimpm-bridged systems, a vaiue r = 5.5 A was estimated from molecular models. The data for the IT bands of lM(bpyh(bimpm)M­(bpyU'''' (M = Ru. Os) are collected in Table 2, togt'lher with the data of relevant bridgi ng systems. The calculated HAl) va lues for the complexes lM(bpYh (bimpm)M(bpYh]4~ are 650 em' l for Ru and

360 cm-I for Os, respectively. Thus, the H Ail v;.due~ for the bimpm-bridging systems are a litt le sma ll er than those of Ru/Os bibzim bridged systems.

COlllparison of bridging ligands ill dil/uc/ear RIII O.\" mixed- valence complexes

Table 2 summari zes spec troscopic ~Iild

electrochemical propert ies of the dinuclcar Ru and 0, complexes with di fferent bidentate bridgi ng li gands. together with the characteri sti cs of intervalclKt' charge transfer bands. The MLCT band for the dinuclear Ru complexes, [Ru(bpy H L)Ru (bpy):- l"+. i., shifted to longer wavelengths in the order of L = l'ibzim> bibzim > bpm . In order to clarify the rllic of bridging ligand", ah i ll l//o molec ular orbital calculations of bridging ligands bitnpm. bibzim. and bpm were performed, using the softwarc Spart ~II~I)rn

software fiv1• The orbital energies and the C()IllPO~ili()n

of HOMO and LUMO 0rbital s and their neigh hours are illustrated in Fig. 6. The HOMO/LUMO cllc rgic <, increase along the sequence bprn < bimpm < bib7illl. There arc two possible exchange mechanislll ~ for metal-metal communications In ligand- bridged mixed-valent complexes: electron exchange aild ho lt­exchange mechanisms6 (see Scheme 4). 111 [he electron exchange mechanism. the metal-illCI:d interaction occ urs via a Inixing hct\\'('~n filled meta l dn orbitals alld low-lying 1I MOs the briJging li g;!llcl. On the other hand, the hole exchange mechanism i~

T"I' lc :! - Speclruscop ic and c leclroc hemi('~1 dala of dinuclear [M 2(bpY)4(L) 1'1+ and paralllekrs for lhe mixed-vak!1l stale in CII C, '

L.

,t>.1

~~ ~~~~

'M

,"/'\" erH)) 'M

,M,

c~ 'M

ML.CT

Am"X 11m 4Sg

570

50S

5GO 606

M=Ru £ 112

M(lI /III ) V vs Fc/Fc+

0.6 1 1. 00

0.43 0 .72

1. 16 l.35

"Ref.13

6E V

0.39

0.29

0.17

IT Am,,, 11m

1830

1950

2000

H j\1l

<.:111 .,

650

740"

MLCT

A'l',tl, 11 m

480 700sh

518 ('79~h

732sh

C.NO I reported

~1""'Os

LI I!

(M( lllIll) V V $ Fe/Fe'

0.24 0.52

O. 06 0.24

6£ IT V 1"111.1'\

JlJ1l

0.2 X 1300 2UOO

O.IS 12:!0 :!OSO

11 111

em 360

82()

0.5

0.4

0.3

0.2

0.1

~. ,

~.2

~. 3

HAGA et al.: DINUCLEAR MIXED V ALENT COMPLEXES OF Ru & Os

-- 0.357 LUK> 0.342

bibzim

-- 0.554

LlJt.() 0.509

.390

HOMO 0.119

. , 0.102

--0.220

LUK> 0.175

~ --- --~MCi~o19 -- ----W-'" -------------------- --------

~.029 ~. .. . '.

. ~. . ~ " . .

.4&4

HOMO ~.289

--- ~.3'9

Fig. 6-Energy levels and mo lecular orbitals of HOMO and LUMO of ligands, bpm , bimpm and bibzim.

Electron exchange mechanism

L1t·(LUMO) ........--. . . ........--. " . ,

Ru(lI)d It :' ... Ru(lIl)d 1t :' ...

-tt-<"'it-''+- =:> ~,+\.#-~ L1t(HOMO)

Hole exchange mechanism

Scheme 4

2'297

2298 INDIAN J CHEM, SEC A, SEPTEMBER 2003

predominant for the mlxmg between partially empty

metal dn orbita ls and filled MOs of the bridging li gand . In the present series of mixed-valent co mplexes , the increase of Kmm values in the order of bpm < bibzim < bimpm is not in accord with that of e ither HOMO or LUMO orbital energies. The elec tro n den sity distribution in HOMO and LUMO orbital s of the bimpm ligand shows an interesting feature . i.e., the electron density of the HOMO is concentrated on the benzimidazolate moiety, whereas that of the LUMO is shifted to the pyrimidine ring (see Fig. 6). Therefore, in the case of bimpm bridged sys tem, a sy nergetic contribution of both Ru(lI)dn­

ligand nand Ru (lIJ)-li gand n orbital interactions may be invoked. Thi s synergetic effect in the bimpm­bridged systems may also play an important role for the stability of [M(bpyh(bimpm)M(bpyh]"+ species towards the reaction with acid.

Conclusion ovel bridging bis-bidentate ligand, 2-(2-

pyrimidyl)benzimidazolate, show a moderate n­accepti ng and n-donating property. This synergetic contribution can stabilize the mixed-valent state of M(bpY)2(bimpm)M(bpyh]4+, which gives the rich near-infrared absorption spectra. On the other hand, dinuclear mixed-valent Ru complexes bridged by 2.2' -bi pyri midine, wh ich has a strong n-accepting property, have been reported to be not stable enough to get a re liable intervalence charge transfer band33

.

2,2 /-Bibenzimidazolate reveals a good bridging ligand

with a strong n-donor property for mediating the metal-metal interaction through a hole exchange mechanism. While the bibenzimidazolate bridged sys tem is easily collapsed by the addition of acid, 2-(2-pyrim idyl)benzimidazolate bridged dinuclear system has a resistivity for acids. Therefore, the energy matching between metal dn orbitals and ligand nand n* orbital plays the pivotal role in bis-bidentate bridged system.

Acknowledgement M H gratefully acknowledges financial support

from the Institute of Science and Engineering at Chuo University, the Ministry of Education, Science, Sports and Culture for a Grant-in-Aid for Scientific Research ( o. 12440188), and also a support from the Promotion and Mutual Aid Corporation for Private Schools of Japan. W K thanks the DFG and FC) for continued support.

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