Inorganica Chimica Acta 388 (2012) 168–174

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Inorganica Chimica Acta

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Electronic effects on the reactivity of copper mono-bipyridine complexes

Martin Moore a, D. Andrew Knight b,⇑, Dan Zabetakis a, Jeffrey R. Deschamps a, Walter J. Dressick a,Eddie L. Chang a, Bianca Lascano a, Rafaela Nita b, Scott A. Trammell a,⇑a Center for Bio/Molecular Science and Engineering, Code 6900, US Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375, USAb Chemistry Department, Florida Institute of Technology, 150 West University Boulevard, Melbourne, FL 32901, USA

a r t i c l e i n f o

Article history:Received 11 December 2011Received in revised form 5 March 2012Accepted 8 March 2012Available online 17 March 2012

Keywords:Copper mono-bipyridine complexesHydrolysisMethyl parathionX-ray crystal structures

0020-1693/$ - see front matter Published by Elsevierhttp://dx.doi.org/10.1016/j.ica.2012.03.020

⇑ Corresponding authors. Tel.: +1 321 674 8175;Knight), tel.: +1 202 404 6063; fax: +1 202 767 9594

E-mail addresses: aknight@fit.edu (D.A. Knight),(S.A. Trammell).

a b s t r a c t

Electronic effects on the synthesis, structure and reactivity of copper 2,20-bipyridine (bpy) complexes offormula [Cu(4,40-R2-2,20-bipyridine)]Cl2, where R = Cl, CH3, OCH3, and [Cu(4,40-R2-2,20-bipyridine)](NO3)2,where R = NO2, Cl, are reported. Pure complexes were formed, as shown by elemental analysis, for monobpy Cu complexes when bpy ligands with groups R = Cl, CH3 or OCH3 were pre-dissolved in chloroformand added to methanol solution of CuCl2 in a 1:1 molar ratio (compounds 1–3). Mono bpy Cu complexeswere obtained when bpy ligands with groups R = Cl and NO2 were pre-dissolved in THF and added tomethanol solution of Cu(NO3)2 in a 1:1 molar ratio (compounds 4 and 5). Under the same conditions,for R = OCH3 the bis bipyridine complex [Cu(4,40-(OCH3)2-2,20-bipyridine)2(NO3)](NO3) (6) was formed.Suitable crystals of 4 and 6�CH3OH for X-ray analysis were formed by the slow evaporation of solvent.The main electronic effect for the catalytic hydrolysis of methyl parathion by Cu(4,40-R2-2,2-bipyri-dine)2+

(aq), where R = Cl, H and CH3, was a shift in the acid dissociation constant of coordinated waterand a small increase in the second order rate constant, k2, measured at pH 9.1.

Published by Elsevier B.V.

1. Introduction

Catalytic reactions, particularly those involving hydrolyticmechanisms, are of interest as a viable approach towards thedevelopment of materials suitable for the hydrolysis of toxicpesticides and chemical warfare agents. We are interested inunderstanding the means by which metal-based catalysts enhancereactions directed towards organophosphorus compound neutral-ization. Catalysts can be finely tuned, both sterically and electron-ically, through a judicious selection of ligands, neutral molecules,or ions bound either covalently or electrostatically to the catalyti-cally active metal ion center.

The role of metal ions and metal–ion complexes in the hydro-lytic catalysis of phosphate esters has long been known and themechanism of these reactions extensively investigated [1–8] andreviewed [9–14]. The general consensus is that the substrate firstcoordinates to a metal center, replacing a metal-bound water mol-ecule. Hydrolysis then occurs by intramolecular attack by a secondcoordinated water. An example with Cu(bpy)2+

(aq) as the metalcomplex ion is shown in Scheme 1.

B.V.

fax: +1 321 674 8951 (D.A.(S.A. Trammell).scott.trammell@nrl.navy.mil

While recognizing the wide variation in metal (mono andbimetallic centers) and ligands used in this field, we focused onCu catalysts based on mono-bipyridine complexes for this work.Martell and co-workers showed early on that 2,20-bipyridine(bpy) Cu(II) complexes are efficient in hydrolyzing selected phosp-horofluoridates, a class of compounds closely related to phosphateesters [2]. Since then, Morrow and Trogler have reported compre-hensively on the hydrolysis of other phosphate esters withCu(bpy)2+

(aq) [6,7].Structural variations of Cu catalysts based on mono-bipyridine

or mono-phenanthroline complexes have been reported to en-hance rates of hydrolysis. For example, the hydrolysis ofadenylyl(30,50)adenosine is dramatically increased by Cu(2,9-dimethyl-1,10-phenanthroline)2+

(aq) compared to Cu(bpy)2+(aq)

[4]. Also, the addition of positive charged groups on the bipyridineligand increases the hydrolytic catalysis of diphosphate esters[1,3,15]. Nevertheless, to the best of our knowledge, a systematicstudy to investigate electronic effects on catalytic hydrolysis usingmono bipyridine Cu complexes in which donor and acceptorgroups are varied at the 4 and 40 positions on the bpy ligand hasnot been performed. Here we present the synthesis, structureand reactivity of a series of copper mono-bipyridine complexesof formula Cu(II)(4,40-R2-2,20-bipyridine)Cl2 (where R = Cl, CH3,OCH3) and formula Cu(II)(4,40-R2-2,20-bipyridine)(NO3)2 (whereR = NO2, Cl) and report on the electronic effects for catalytic hydro-lysis of methyl parathion.

Scheme 1. The general mechanism of hydrolytic catalysis of phosphate esters by Cu(bpy)2+(aq).

M. Moore et al. / Inorganica Chimica Acta 388 (2012) 168–174 169

2. Experimental

2.1. Materials

Ethanol, methanol, 2-morpholinoethanesulfonic acid (MES), 3-(N-morpholino)propanesulfonic acid (MOPS), 2-(cyclohexylamine)-ethanesulfonic acid (CHES), copper(II) nitrate, copper(II) chloride,2,20-bipyridine and 4,40-dimethyl-2,20-bipyridine were purchasedfrom Sigma–Aldrich. 4,40-Dimethoxy-2,20-bipyridine, 4,40-dinitro-2,20-bipyridine and 4,40-dichloro-2,20-bipyridine were purchasedfrom Carbosynth Limited. Methyl parathion (MeP) was purchasedfrom Supelco. Cu(bpy)(NO3)2 was prepared following a publishedprocedure [16].

2.2. Synthesis

Cu(4,40-(Cl)2-2,20-bipyridine)Cl2 (1) 67 mg (0.50 mmol) of CuCl2

was dissolved in 5 mL of methanol and added to a 10 mL solutionof CHCl3 containing 112 mg (0.500 mmol) of 4,40-dichloro-2,20-bipyridine. The solution was stirred for 30 min and left to standfor 2 h. A green solid was collected by vacuum filtration and thenallowed to air dry, recovering 100 mg (56% yield) of a green pow-der. Anal. Calc. for CuC10H6N2Cl4: C, 33.41; H, 1.68; N, 7.79. Found:C, 33.33; H, 1.60; N, 7.75%. IR (KBr) mmax/cm�1: 3448w-br, 3092w,3059w, 3038m, 1593s, 1550s, 1467m, 1401s, 1296w, 1234w,1107w, 1023w, 902vw, 860s, 832m, 750w, 716w, 531w, 421w.

Cu(4,40-(CH3)2-2,20-bipyridine)Cl2 (2) synthetic procedure wasthe same as for compound 1. A green solid was collected by vacuumfiltration and then allowed to air dry, recovering 129 mg (81% yield)of a green powder. Anal. Calc. for CuC12H12N2Cl2�0.25H2O: C, 44.60;H, 3.90; N, 8.67. Found: C, 44.67; H, 3.76; N, 8.69%. IR (KBr) mmax/cm�1: 3447w-br, 3114w, 3079w, 2980w, 1616vs, 1559m, 1489s,1445s-br, 1380w, 1303m, 1289m, 1249w, 1222 (w), 1202w,1117w, 1079w, 1043m, 1028s, 923m, 903m, 847s, 830s, 741w,559m, 519s, 425m.

Cu(4,40-(OCH3)2-2,20-bipyridine)Cl2 (3) synthetic procedure wasthe same as for compound 1. A green solid was collected by vac-uum filtration and then allowed to air dry, recovering 84 mg(48% yield) of a green powder. Anal. Calc. for CuC12H12N2O2Cl2: C,41.10; H, 3.45; N, 7.99. Found: C, 41.03; H, 3.40; N, 7.95%. IR(KBr) mmax/cm�1: 3449m-br, 3206w, 1611vs, 1558m, 1497s,1468m, 1440m, 1421m, 1333m, 1312m, 1283m, 1273m, 1259s,1232m, 1042s, 1023s, 1000s, 921w, 884m, 869w, 847w, 827m,739w, 583w, 531w, 437w.

Cu(4,40-(NO2)2-2,20-bipyridine)(NO3)2 (4) 123 mg (0.500 mmol)of 4,40-dinitro-2,20-bipyridine was dissolved in a 5 mL solution ofTHF which was then added dropwise to a solution containing94 mg (0.50 mmol) of Cu(NO3)2 dissolved in 5 mL of methanol.The solution was stirred overnight and partially evaporated. A bluesolid was collected by vacuum filtration and then allowed to airdry, recovering 130 mg (60% yield) of blue microcrystals. Anal. Calc.for CuC10H6N6O10: C, 27.69; H, 1.39; N, 19.38. Found: C, 27.89; H,

1.34; N, 19.01%. IR (KBr) mmax/cm�1: 3447m-br, 3109s, 3066w-sh,1611s, 1541vs, 1473vs-br, 1384vs-br, 1296vs-br, 1240s, 1128w,1024s, 1004s, 895m, 861m, 808m, 795s, 742s, 708s, 658w, 421w.

Cu(4,40-Cl2-2,20-bipyridine)(NO3)2 (5) synthetic procedure wasthe same as for compound 4. A blue solid was collected by vacuumfiltration and then allowed to air dry, recovering 98 mg (47% yield)of blue microcrystals. Anal. Calc. for CuC10H6N4O6Cl2�0.75H2O: C,28.18; H, 1.77; N, 13.15. Found: C, 28.50; H, 1.89; N, 12.73%. IR(KBr) mmax/cm�1: 3085w, 3033w, 1763w, 1684w, 1587s, 1552s,1464m, 1396vs-br, 1295m, 1233m, 1137w, 1107m, 1016m, 890s,859vs, 841vs, 750s, 716s, 619w, 534m, 511m, 419m.

[Cu(4,40-(OCH3)2-2,20-bipyridine)2](NO3)2 (6) synthetic proce-dure was similar to compound 4. A blue solid was collected by vac-uum filtration and then allowed to air dry, recovering 95 mg (30%yield) of blue microcrystals. Anal. Calc. for CuC24H24N6O10�CH3OH:C, 46.05; H, 4.33; N, 12.89. Found: C, 45.92; H, 3.88; N, 13.4%. IR(KBr) mmax/cm�1: 3446m-br, 3066w, 1611s, 1561m, 1500m,1473w, 1384vs, 1340m-sh), 1311m-sh, 1259m, 1230m, 1040s,900w, 838w, 433w, 741w, 664w, 581w.

2.3. Instrumentation

IR spectra were recorded on a Bruker Tensor 27 IR instrument asKBr pellets between 4000 and 400 cm�1. Thermogravimetric analy-sis was conducted on a TGA 2950HR instrument using a 5 �C/minsample heating rate in the range of 28–800 �C, and with N2 as the car-rier gas. UV–Vis measurements were made with an Agilent 8453diode array spectrometer and referenced against a solvent blank.

2.4. Kinetics

Time-based measurements were made on an Agilent 8453diode array spectrophotometer with a water thermostattable 8-po-sition cuvette holder driven by the manufacturer’s UV–Vis Chem-Station software in kinetics mode. To vary the concentration ofMeP and Cu(LL)2+

(aq), small (5–30 lL) aliquots from concentratedstock solutions of Cu(LL)2+

(aq) in DI water and MeP in MeOH werediluted to 3 mL with the appropriate volume of buffer, cappedand rapidly mixed in 1 cm path length PMMA cuvettes. The reac-tions were monitored at 400 nm to measure the product (4-nitro-phenolate) formation as a function of time at 25 �C in water with5% methanol (v/v) buffered (0.02 M) using MES for pH between 5and 7, MOPs for pH between 7.5 and 9, and CHES for pH 9–10.The reactions were not monitored past 5% completion and the ini-tial rate data was obtained using the kinetic software package.From the initial rate data, observed first-order rate constants, kobs,were calculated by converting to concentration units using theextinction coefficient (18700 M�1 cm�1) for 4-nitrophenolate cor-rected for pH using a pKa of 7.16 for 4-nitrophenol [6,7] and divid-ing by the initial MeP concentration. Non-linear least squares curvefitting was accomplished using Solver in Excel and the regressionstatistics reported using the Excel macro Solvstat [17].

170 M. Moore et al. / Inorganica Chimica Acta 388 (2012) 168–174

2.5. Single crystal X-ray diffraction analysis

Single-crystal X-ray diffraction data on compounds 4, and 6were collected using Mo Ka radiation (k = 0.71073 Å) and a BrukerAPEX 2 CCD area detector. Crystals were prepared for data collec-tion by coating with high viscosity microscope oil (Paratone-N,Hampton Research). The oil-coated crystal was placed on a Micro-Mesh mount (MiTeGen, Ithaca, NY) and transferred immediately tothe diffractometer. Data was collected at 23 �C and �170 �C forcompounds 4 and 6, respectively. Corrections were applied for Lor-entz, polarization, and absorption effects.

The structures were solved by direct methods and refined byfull-matrix least squares on F2 values using the programs foundin the SHELXTL suite (Bruker, SHELXTL v6.10, 2000, Bruker AXS Inc.,Madison, WI). Parameters refined included atomic coordinatesand anisotropic thermal parameters for all non-hydrogen atoms.Hydrogen atoms on carbons were included using a riding model(coordinate shifts of C applied to H atoms) with C–H distance fixedbased on the type of carbon atom.

2.5.1. Compound 4The 0.381 � 0.161 � 0.092 mm3 crystal was monoclinic in space

group P21 with unit cell dimensions a = 10.893(7) Å, b = 8.554(5) Å,c = 15.626(9) Å, and b = 96.187(11)�. Data were 99.7% complete to28.97� h (approximately 0.73 Å) with an average redundancy of0.97. It should be noted that the data crystal was twinned and onlynon-overlapped reflections from the strongest twin componentwere used in the refinement.

2.5.2. Compound 6The 0.169 � 0.089 � 0.084 mm3 crystal was triclinic in space

group P�1 with unit cell dimensions a = 7.6311(12) Å, b =11.5984(12) Å, c = 16.577(2) Å, a = 104.141(5)�, b = 99.434(6)�,and c = 102.362(6)�. Data were 98.6% complete to 28.31� h(approximately 0.75 Å) with an average redundancy of 2.22.

3. Results and discussion

3.1. Synthesis

Previous reports show that mono bipyridine Cu complexes canbe prepared in methanol or ethanol by the slow evaporation of sol-vent and precipitation of the resulting product as crystalline mate-rial [16,18–20]. We modified this method since some of thebipyridine ligands of interest to us were only sparingly soluble inmethanol or ethanol. In addition, the ligands with donor groupstended to give poor elemental analysis high in carbon, hydrogenand nitrogen, suggesting mixtures of mono and bis complexes. Thiswas confirmed when reaction of a 1:1 molar ratio of 4,40-(OCH3)2-2,20-bpy with Cu(NO3)2 in methanol, with slow evaporation of sol-vent, gave suitable crystals for X-ray structural determination forcompound 6 showing two coordinated bpy ligands. We obtainedgood elemental analysis for mono bpy Cu complexes when bpy li-gands with groups R = Cl, CH3 or OCH3 were pre-dissolved in chlo-roform and then added to a methanol solution of CuCl2 in a 1:1molar ratio (compounds 1–3). Mono bpy Cu complexes with goodelemental analysis were obtainable when bpy ligands with groupsR = Cl and NO2 were pre-dissolved in THF and added to methanolsolution of Cu(NO3)2 in a 1:1 molar ratio (compounds 4 and 5).

3.2. Infra-red spectroscopy

Infra-red spectra of complexes 1–6 were recorded as KBr pellets.Spectra for all complexes contain absorptions at 1541–1550 cm�1

which have been assigned to the C–N stretching frequencies of the

bipyridine ligands [21]. Absorbances for the nitro groups attachedto the bipyridine rings in complex 4 could not be unambiguouslyassigned due to the presence of interfering bands in this region (typ-ically 1300–1600 cm�1). The aromatic C–O stretching frequency ofcomplexes 3 and 6 occurs at 1259 cm�1 [22]. Complexes 4 and 5 con-tain metal bound nitrate anion and exhibit stretching frequenciesdue to this ligand coordinated in the expected unidentate bondingmode, very similar to those reported for Cu(2,20dipyridylbenzyl-amine)(NO3)2 (m(NO3): 1475, 1384, 1302, 1020 cm�1) [23]. Cu–Nstretching frequencies typically occur in the far infra-red regionand were not determined in the present study [24].

3.3. Thermal gravimetric analysis

To further characterize the new complexes in the solid state, thethermal decomposition of copper bipyridine complexes 1–6 wasexamined using thermogravimetry. The decomposition of 1 beginsat approximately 250 �C and occurs in two steps with the rapid ini-tial loss of 71% of total mass corresponding to removal of the or-ganic 4,40-dichloro-2,20-bipyridine ligand. The second step occursbetween 343 and 460 �C and corresponds to loss of the remainingmass due to CuCl2. This is in contrast to the reported thermaldecomposition of [Cu(bpy)2Cl2] in which copper metal remainsafter decomposition [21]. The 4,40-dimethyl-2,2-bipyridine com-plex 2 behaved in a similar manner losing 55% of total mass be-tween 300 and 340 �C. Further decomposition (340–800 �C)occurred and corresponds to slow loss of the coordinated chlorideligands (27% of total mass), with the remaining mass (18%) due tocopper metal. The 4,40-dimethoxy-2,20-bipyridine complex 3undergoes an initial decomposition at approximately 300 �C, losing18% of the total mass corresponding to either loss of chloride ordecomposition of the organic ligand framework and removal ofthe methoxy substituents from the bipyridine rings. As coordi-nated chloride ligands are known to dissociate at higher tempera-tures, we tentatively suggest that the organic ligand is initiallyundergoing decomposition with scission of aryl–oxygen bonds, fol-lowed by complete ligand loss from the copper center.

Complexes 4–6 are nitrate salts and exhibit similar decomposi-tion profiles. Complexes 4 and 5 begin to decompose at approxi-mately 210 and 253 �C, respectively with a 52% and 36% decreasein mass corresponding to dissociation of the 4,40disubstitutedbipyridine ligands. A gradual loss of 21% of the total mass then oc-curs for complex 4 between 350 and 800 �C, accounting for re-moval of the nitrate ligands. Complex 5 loses a single NO3

- (16%mass loss) between 253 and 351 �C. Complex 6 begins to decom-pose at 220 �C with 31% decrease in mass due loss of one bipyri-dine ligand, with the remaining mass lost between 425 and 800 �C.

3.4. Crystal structure analysis

3.4.1. [Cu(4,40-(NO2)2-2,20-bipyridine)(NO3)2] (4)Compound 4 crystallizes as a mononuclear structure and con-

tains a copper(II) ion coordinated to one chelating 4,40-dinitro-2,20-bipyridine ligand and two nitrato ligands coordinated in anasymmetric bidentate fashion. The coordination geometry aroundthe copper(II) ion is best described as distorted or elongated octahe-dral in which two oxygen atoms of separate nitrato anions and twonitrogen atoms of the bipyridine ring form the basal plane and theapical sites are occupied by different nitrate oxygen atoms. Cu–Nand Cu–O bond lengths for 4 are similar to those reported for the re-lated complexes, Cu(5,50-(CH3)2-2,20-bipyridine)(NO3)2 (Cu–N:1.969(3), 1.966(3) Å; Cu–O: 1.979(3), 1.994(3), 2.484(4),2.465(4) Å) [19] and Cu(4,40-(C(CH3)3)2-2,20-bipyridine)(NO3)2

(Cu–N: 1.965(2), 1.980(3) Å; Cu–O: 1.976(3), 1.994(3), 2.452,2.511 Å) [15]. The dihedral angle between the planes of the two pyr-idine rings is 11.16(8.8)� and stacking of adjacent pyridine rings re-

Fig. 1. ORTEP representation of Cu(4,40-(NO2)2-2,20-bipyridine)(NO3)2 (4). Thermalellipsoids are shown at the 50% probability level.

Table 1Selected bond lengths (Å) and angles (�) for 4.a

Cu(1)–N(12) 2.010(5) Cu(1)–N(1) 2.015(6)Cu(10)–N(120) 1.984(6) Cu(10)–N(10) 1.997(6)Cu(1)–O(19) 1.944(6) Cu(1)–O(23)#1 2.323(5)Cu(1)–O(22) 1.972(5) Cu(10)–O(220) 1.954(5)Cu(10)–O(190) 2.012(6) Cu(10)–O(200)#3 2.438(5)O(23)–Cu(1)#2 2.323(5) O(200)–Cu(10)#4 2.439(5)O(19)–Cu(1)–O(22) 92.1(3) O(19)–Cu(1)–N(12) 168.9(3)O(22)–Cu(1)–N(12) 94.7(2) O(19)–Cu(1)–N(1) 95.5(3)O(22)–Cu(1)–N(1) 162.4(2) N(12)–Cu(1)–N(1) 80.5(2)O(19)–Cu(1)–O(23)#1 82.0(3) O(22)–Cu(1)–O(23)#1 78.6(2)N(12)–Cu(1)–O(23)#1 90.8(2) N(1)–Cu(1)–O(23)#1 118.2(2)O(220)–Cu(10)–N(120) 90.6(2) O(220)–Cu(10)–N(10) 167.5(2)N(120)–Cu(10)–N(10) 81.8(2) O(220)–Cu(10)–O(190) 88.9(2)N(120)–Cu(10)–O(190) 162.7(2) N(10)–Cu(10)–O(190) 95.5(2)O(220)–Cu(10)–O(200)#3 109.7(2) N(120)–Cu(10)–O(200)#3 119.2(2)N(10)–Cu(10)–O(200)#3 82.7(2) O(190)–Cu(10)–O(200)#3 77.09(19)

a Symmetry transformations used to generate equivalent atoms: #1 �x, y + 1/2,�z; #2 �x, y � 1/2, �z; #3 �x + 1, y � 1/2, �z + 1; #4 �x + 1, y + 1/2, �z + 1.

Fig. 2. ORTEP representation of [Cu(4,40-(OCH3)2-2,20-bipyridine)2(NO3)](NO3)�CH3OH (6�CH3OH). Thermal ellipsoids are shown at the 50% probability level.

Table 2Selected bond lengths (Å) and angles (�) for 6�CH3OH.

Cu(1)–N(10) 1.9624(12) Cu(1)–N(12) 1.9722(12)Cu(1)–N(120) 2.0281(13) Cu(1)–N(1) 2.0903(12)Cu(1)–O(21) 2.116(2)N(10)–Cu(1)–N(12) 178.64(8) N(10)–Cu(1)–N(120) 80.44(7)N(12)–Cu(1)–N(120) 100.46(7) N(10)–Cu(1)–N(1) 100.11(7)N(12)–Cu(1)–N(1) 80.22(6) N(120)–Cu(1)–N(1) 125.07(7)N(10)–Cu(1)–O(21) 89.88(8) N(12)–Cu(1)–O(21) 88.77(8)N(120)–Cu(1)–O(21) 138.05(8) N(1)–Cu(1)–O(21) 96.74(8)

M. Moore et al. / Inorganica Chimica Acta 388 (2012) 168–174 171

sults in stabilization of the crystal lattice. The structure is shown inFig. 1 and the selected bond lengths and angles are listed in Table 1.

3.4.2. [Cu(4,40-(OCH3)2-2,20-bipyridine)2(NO3)](NO3)�CH3OH (6)Compound 6 crystallizes as a mononuclear dication and con-

tains a copper(II) ion coordinated to two chelating 4,40-(OCH3)2-2,20-bipyridine ligands and a nitrato ligand. A non-coordinated ni-trate anion balances the charge of the monocation and a methanolsolvate molecule is also present in the unit cell. The coordinationgeometry around the copper(II) ion can be described as distortedtrigonal bipyramidal with a s value =0.68 (the geometric parame-ter s is an index of the degree of trigonality: (b � a/60)) [25]. Theapical positions are occupied by nitrogen atoms from two separatebipyridine ligands. N(10)–Cu(1)–N(12) is nearly linear. A trigonalplane is formed by the two remaining nitrogen atoms and an oxy-gen atom from the coordinated nitrato ligand. Bond lengths andangles are similar to those reported for the related compounds,[Cu(5,50-(CH3)2-2,20-bipyridine)2(NO3)](NO3) [19], [Cu(5,50-(CH3)2-2,20-bipyridine)2Br]Br [19] and [Cu(2,20-bipyridine)2Br]Br [26].The dihedral angle between the planes of the two pyridine ringsis 2.59(1.5)�. Pi-stacking of the bipyridine aromatic rings, and ahydrogen bond between the methanol OH group (O1S) and an oxy-gen from the non-coordinated nitrate anion (O1S� � �O13 = 2.924 Å),assist crystal packing. The structure is shown in Fig. 2 and the se-lected bond lengths and angles are listed in Table 2.

3.5. Reactivity

We used methyl parathion (MeP), an organophosphorus insec-ticide, as the substrate for investigating the electronic effects of

substituents on the reactivity for our series of Cu(II) catalysts be-cause of its widespread use as model substrate for the study of cat-alytic hydrolysis of thiophosphate esters. Thus, comparisons ofcatalytic activity can be made with previously reported catalyticmetal complexes if so desired.

3.5.1. Rate lawsIn previous reports for the hydrolysis of phosphate esters, Cu

catalysts were formed in situ from 1:1 or 1:2 ratios of Cu2+(aq)

and bpy in solution, where it was shown through equilibrium cal-culations and experiment that the concentrations of free Cu2+

(aq)

and the bis complex, Cu(bpy)22+, make negligible contributions to

the rate law. The most active form of the Cu catalyst was shownto be one in which one coordinated water has been de-protonatedto form an effective nucleophile for attack on a bound substrate(Fig. 3) [6,7]. The reaction order was reported to be 1st order insubstrate and 1st order in catalyst at low concentrations. At cata-lyst concentrations greater than 5 � 10�5 M, the reaction order de-creases below 1 due to the formation of the inactive hydroxy-bridged dimer [6,7]. At high substrate concentrations under satura-tion kinetic conditions, the Michaelis–Menten formalism has beenused to analyze the rate dependence (Eq. (1)).

Rate ¼ kcat½Catalyst�½Substrate�=ðKM þ ½Substrate�Þ ð1Þ

For Cu(bpy)aq2+, the rate of hydrolysis for 4-nitrophenyl ethyl

phosphate becomes independent of its concentration at high ratiosof substrate to catalyst and the reported Michaelis–Menten param-eters at 75 �C are KM = 47 mM and kcat = 5.6 � 10�4 s�1 [6]. Michae-lis–Menten parameters at 25 �C for the hydrolysis of MeP in a

Fig. 3. The hydrolysis of methyl parathion using different Cu(LL)2+(aq) catalysts where LL = 4,40-R2-2,2-bipyridine and R = Cl, H or CH3 and OCH3.

172 M. Moore et al. / Inorganica Chimica Acta 388 (2012) 168–174

water/methanol mixture (15% MeOH at pH 8) have been reportedto be KM = 4.4 mM and kcat = 7.6 � 10�3 s�1 using a Cu(vbpy)aq

2+

catalyst, where vbpy = 4-vinyl-40-methyl-2,20-bipyridine (mono-mer) [27].

At low substrate concentration, where [Substrate]� KM, therate law simplifies to give the form:

Rate ¼ ðkcat=KMÞ½Catalyst�½Substrate� where k2 ¼ kcat=KM ð2Þ

For our analysis, we have measured the kobs at low substrateconcentration both as a function of pH and total catalyst concen-tration, in which we used the intact synthesized Cu complexes sol-vated as aqua species. The hydrolysis reaction is shown in Fig. 3,where LL represents the different bpy ligands used: LL = 4,40-R2-2,20-bipyridine and R = Cl, H, CH3, OCH3. The copper complexeswith R = NO2 proved to be unstable in water, as demonstrated bythe observation that 2 mM solutions of compound 4 in DI waterproduced a white precipitate within minutes of preparation. Byextraction into CDCl3, proton NMR identified the precipitate asthe free ligand, 4,40-(NO2)2-2,2-bipyridine. All other catalysts werestable under these conditions. Reactions were monitored spectro-photometrically at 400 nm to measure the product (4-nitropheno-late) formation as a function of time at 25 �C in buffered solutionsat an ionic strength of 0.1 M (NaCl) with 5% MeOH to facilitate thesolubility of methyl parathion. The initial reaction rates were ob-tained by following the reaction up to 5% completion.

3.5.2. pH dependenceSince electronic effects at the 4 and 40 position would most

likely shift the acid/base equilibrium of coordinated water, we

tested our series of Cu catalysts to observe how the pH profilewould change for the rate data of the hydrolysis of MeP versuspH. As seen in Fig. 4A for the hydrolysis of methyl parathion usingour Cu(LL)2+

(aq) catalysts, the rate is near zero at pH 5.5 and beginsto increase above pH 6. The rate law from the reaction scheme inFig. 3 is easily derived [28], and the experimental rate constant,kobs, can be fit to Eq. (3), in which KAH is the acid dissociation con-stant for one of the coordinated water molecules, kAH is the exper-imental rate constant for the Cu(LL)(OH2)2

2+-mediated hydrolysisof the phosphate ester, and kA is the experimental rate constantfor Cu(LL)(OH2)(OH)+-mediated hydrolysis of the phosphate ester.The color coded lines in Fig. 4A are the non-linear regression anal-ysis for each catalyst and the fitted parameters are listed in Table 3.

kobs ¼ ðkAH½Hþ� þ kAKAHÞ=ð½Hþ� þ KAHÞ ð3Þ

The calculated fits for each catalyst begin to saturate above pH9. Measuring experimental data at pH >9 proved to be problematicsince appropriate buffers such as CHES interfered with the reac-tion, most likely due to its secondary amine coordinating to the ac-tive site of the copper.

One possible explanation for the unsuccessful attempt to useCHES is that MeP most likely has a weak interaction with the Cusite, allowing CHES to effectively block its access or displace boundMeP readily. In contrast, phosphate diesters, like disodium 4-nitro-phenyl phosphate and bis(4-nitrophenyl) phosphate, apparentlybind Cu(II) more strongly so that only a slight hindrance usingCHES was reported [6]. In addition the hydrolysis of diethyl 4-nitrophenyl phosphate (a triester) was not measured past pH 8and was reported to be inhibited by product formation [7], demon-

Fig. 4. (A) The dependence of kobs vs. pH for the hydrolysis of MeP for the series of catalysts of formula Cu(LL)X2 where LL = 4,40-R2-2,20-bipyridine and R = Cl, X = Cl� (greensquares), R = H, X = NO3

� (blue diamonds), R = CH3, X = Cl� (red circles), and R = OCH3, X = Cl� (black triangles) with background hydrolysis rates for MeP (orange line with X).The colored lines are non-linear least squares regression fits to Eq. (3) for each catalyst, respectively. Ionic strength = 0.1 M using NaCl in 5% MeOH (v/v). Buffers = 20 mM MESfor pH values between 5 and 7, and MOPs for pH’s between 7 and 9. [Cu(LL)2+

(aq)] = 1 � 10�5 M and [MeP] = 2 � 10�4 M. (B) The dependence of kobs vs. [Cutot] at pH 9.1 with[MeP] = 2 � 10�4 M. The colored lines are non-linear least squares regression fits to Eq. (4) for each catalyst, respectively. The inset is kobs vs. [Cutot]1/2. (For interpretation ofthe references to color in this figure legend, the reader is referred to the web version of this article.)

Table 3Kinetics of the hydrolysis of MeP using Cu(4,40-R2-2,20-bipyridine)2+

(aq)a.

R Hammett parametersd pKab kAH (s�1b) kA (s�1b) Kd (Mc) k2 (M�1 s�1c)

Cle 0.23 7.32 ± 0.01 0 1.89 ± 0.02 � 10�5 9.2 ± 4 � 10�5 3.0 ± 0.01Hf 0 7.52 ± 0.01 0.02 � 10�5 2.68 ± 0.02 � 10�5 4.8 ± 1 � 10�5 3.3 ± 0.01CH3

e �0.17 7.89 ± 0.03 0.18 � 10�5 4.32 ± 0.05 � 10�5 1.6 ± 1 � 10�5 7.0 ± 0.01OCH3

e �0.27 7.38 ± 0.01 0.09 � 10�5 1.60 ± 0.02 � 10�5 13 ± 4 � 10�5 2.2 ± 0.01

a Conditions listed in Fig. 4.b Calculated from Eq. (3) by non-linear regression analysis.c Calculated from Eq. (4) by non-linear regression analysis from data measured at pH 9.1.d Values are from Ref. [30].e As the chloride salt.f As the nitrate salt.

M. Moore et al. / Inorganica Chimica Acta 388 (2012) 168–174 173

strating that the phosphate tri-esters bind poorly to Cu(II) catalystsand their catalytic hydrolyses are easily inhibited.

3.5.3. Catalyst concentration dependenceAt pH 9.1, where the concentration of Cu(LL)(OH2)2

2+is negligi-ble and the concentration of Cu(LL)(OH2)(OH)+ predominates, wemeasured the dependence of kobs on the total copper concentra-tion, [Cutot]. The results (inset of Fig. 4B) show a linear trend withrespect to the square root of total concentration of copper in solu-tion, which suggests that the catalytic form of the Cu complex,Cu(LL)(OH2)(OH)+, is in equilibrium with the inactive hydroxy-bridged dimer, i.e., [Cu(LL)(OH)2]2

2+. Using non-linear regressionanalysis, data for kobs versus [Cutot] was fit to Eq. (4), which givesa kinetically derived formation constant, Kf, for an inactive dimer[29] (Fig. 4B). The results of the regression are shown in Fig. 4Band listed in Table 3 as a dissociation constant Kd where Kd = 1/Kf

along with k2 for each catalyst.

kobs ¼ k2ð�1=2þ ð1=4þ 2K f ½Cutot�Þ1=2Þ=2K f ð4Þ

To summarize, the overall electronic effect for the hydrolysis ofMeP by our series of Cu(II) complexes, i.e., Cu(4,40-R2-2,20-bipyri-dine)2+

(aq) (where R = Cl, H, and CH3), shows an increase in pKa

and k2 and a decrease in Kd with decreasing Hammett parametervalues, with a noticeable deviation when R = OCH3.

The different pKa and Ka values of each complex suggests otherequilibria between Cu2+ and the ligands may be affected as well. Infact, one explanation of the instability of compound 4 in water mayreflect the strong electron withdrawing nature of the nitro groupson the bpy ligand which would decrease the chelating ability of theligand to shift the Cu(II) complex equilibrium to favor free Cu2+

(aq).Unfortunately, a more complete account of the changes in equilib-

rium for each of our new Cu(II) complexes was not possible bypotentiometric titration due to complications of precipitation atthe higher Cu(II) complexes concentrations required. In an attemptto identify copper containing species present under catalytic con-ditions (in the absence of MeP), ESI-MS spectra were recorded forcomplexes. However the presence of MOPS buffer resulted in spec-tral interference and no well-defined copper complexes could beclearly identified. Therefore a strict interpretation of the electroniceffects on the hydrolysis of MeP should be regarded with suspectsince the electronic effects on copper speciation in solution hasnot been resolved.

4. Conclusion

Previous reports in which changes in ligand structure dramati-cally change the catalytic activity for Cu(II) complexes includecooperative effects by adding positive charged groups at the bpy li-gand [1,3,15], which in turn attracts a negatively charge substrate,or steric effects using 2,9-dimethyl-1,10-phenanthroline [4], whichminimizes the formation of the non-catalytic dimer. In this work,we have synthesized a series of Cu(II) complexes and shown howthe electronic effects provided by donor and acceptor groups atthe 4 and 40 positions on the bpy ligand change the catalytic hydro-lysis of MeP.

Acknowledgments

The authors wish to thank the ‘‘Historically Black Colleges/Minority Institutions/Tribal Colleges and Universities (HBCU/MI/TCU) Summer Intern Program’’ at NRL for support of Bianca Lascan-o. This work received support from the Defense Threat Reduction

174 M. Moore et al. / Inorganica Chimica Acta 388 (2012) 168–174

Agency-Joint Science and Technology Office for Chemical and Bio-logical Defense (MIPR #B102405M and B112542M).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.ica.2012.03.020.

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