Organometallics 2009, 28, 3485–3491
3485
Cobaloximes with Bis(thiophenyl)glyoxime: Synthesis and Structure-Property Relationship Study Gargi Dutta, Kamlesh Kumar, and B. D. Gupta* Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016, India ReceiVed January 28, 2009
Alkyl and non-alkyl cobaloximes with bis(thiophenyl)glyoxime, X/RCo(dSPhgH)2Py (X ) Cl, R ) Me, Et, Pr, Bu, Bn) have been synthesized and characterized for the first time. The X-ray structures of the complexes ClCo(dSPhgH)2Py, EtCo(dSPhgH)2Py, and BuCo(dSPhgH)2Py are reported. The orientation of SPh groups with respect to the dioxime plane varies with the steric bulk of the axial ligand and affects the NMR chemical shifts. The cis and trans influence has been studied by 1H NMR, 13C NMR, and X-ray diffraction. The steric cis influence of the equatorial thiodioxime affects the Co-C bond reactivity in their cobaloxime complexes. The molecular oxygen insertion into the Co-C bond and steric cis influence are related to each, and both follow the same order, dmestgH . dpgH > chgH > dSPhgH g dmgH > gH. A cyclic voltammetry study shows that the reductions, Co(III)/Co(II) and Co(II)/Co(I), are easier in ClCo(dSPhgH)2Py as compared to other chlorocobaloximes (gH, dmgH, dpgH, mestgH). Introduction Cobaloximes have been extensively studied and reviewed over the past four decades.1 [Cobaloximes have the general formula RCo(L)2B, where R is an organic group σ-bonded to cobalt. B is an axial base trans to the organic group, and L is a monoanionic dioxime ligand (e.g., glyoxime (gH), dimethylglyoxime (dmgH), 1,2-cyclohexanedione dioxime (chgH), diphenylglyoxime (dpgH), dimesitylglyoxime (dmestgH), and dithiophenylglyoxime (dSPhgH).] More than 1500 complexes and >150 crystal structures have been reported. Since the Co-C bond cleavage is the key step involved in B12-dependent enzymatic or cobaloxime-mediated reactions,2-5 the strength of the Co-C bond as a function of steric and electronic factors with a wide range of axial ligands in cobaloximes and related * Corresponding author. Tel: +91-512-2597046. Fax: +91-512-2597436. E-mail:
[email protected]. (1) (a) Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.; Randaccio, L.; Summers, M. F.; Toscano, P. J. Coord. Chem. ReV. 1985, 63, 1, and references therein. (b) Randaccio, L.; Bresciani-Pahor, N.; Zangrando, E.; Marzilli, L. G. Chem. Soc. ReV. 1989, 18, 225. (c) Randaccio, L. Comments Inorg. Chem. 1999, 21, 327. (d) Gupta, B. D.; Yamuna, R.; Singh, V.; Tiwari, U. Organometallics 2003, 22, 226. (e) Gupta, B. D.; Qanungo, K.; Barcley, T.; Cordes, W. J. Organomet. Chem. 1998, 560, 155. (f) Gupta, B. D.; Qanungo, K.; Yamuna, R.; Pandey, A.; Tiwari, U.; Vijaikanth, V.; Singh, V.; Barcley, T.; Cordes, W. J. Organomet. Chem. 2000, 608, 106. (g) Gupta, B. D.; Roy, S. Inorg. Chim. Acta 1988, 146, 209. (h) Gupta, B. D.; Mandal, D. Organometallics 2006, 25, 3305. (i) Gupta, B. D.; Roy, S. Tetrahedron Lett. 1985, 26, 3609. (2) Golding, B. T.; Kemp, T. J.; Sell, C. S.; Sellars, P. J.; Watson, W. P. J. Chem. Soc., Perkin Trans. 2 1978, 839. (3) (a) Samsel, E. G.; Kochi, J. K. J. Am. Chem. Soc. 1986, 108, 4790. (b) Atkins, M. P.; Golding, B. T.; Sellers, P. J. J. Chem. Soc., Chem. Commun. 1978, 108, 954. (4) (a) Dodd, D.; Johnson, M. D.; Steeples, I. P.; McKenzie, E. D. J. Am. Chem. Soc. 1976, 98, 6399. (b) Cooksey, C. J.; Dodd, D.; Johnson, M. D.; Lockman, B. L. J. Chem. Soc., Dalton Trans. 1978, 1814. (c) McKenzie, E. D. Inorg. Chim. Acta 1978, 29, 107. (5) (a) Gupta, B. D.; Dixit, V.; Das, I. J. Organomet. Chem. 1999, 572, 49. (b) Gupta, B. D.; Kumar, M.; Roy, S. Inorg. Chem. 1989, 28, 11. (c) Gupta, B. D.; Roy, S. Tetrahedron Lett. 1984, 3255. (d) Roy, S.; Das, I.; Bhanuprakash, K.; Gupta, B. D. Tetrahedron 1994, 50, 1847. (e) Gupta, B. D.; Kumar, M. Inorg. Chim. Acta 1988, 149, 223. (f) Gupta, B. D.; Das, I.; Dixit, V. J. Chem. Res. 1992, 306. (g) Roy, M.; Kumar, M; Gupta, B. D. Inorg. Chim. Acta 1986, 114, 87. (h) Gupta, B. D.; Vijaikanth, V. J. Organomet. Chem. 2004, 689, 1102.
complexes with different chelates has been systematically investigated.1,6-8 The influence of the equatorial ligands on the axial ligands (cis influence) in Costa’s model and iminates (cobalt complexes with a tetradentate Schiff base) has also been reported.1b,c However the similar investigation in dioximates is little studied since the majority of the reported complexes have dmgH as the equatorial ligand. Our work on cobaloximes with different dioximes has shown that the effect of dioxime on the Co-C bond (cis influence) far exceeds the trans influence of the axial base.9 In recent years, the description, spectroscopic data, structure-property relationship, and their correlation to Co-C bonds have been most emphasized.9a,10 The overall evidence from literature strongly suggests that many of the chemical properties related to the axial fragment such as geometry, kinetics, and spectroscopic behavior are significantly affected by a change in the equatorial ligand (cis effect and cis influence).1e,f,6,8,9b,11 The dimesitylglyoxime (dmestgH) complexes have the maximum cobalt anisotropy and the highest steric cis influence among the commonly studied dioximes. The overall cis influence follows the order dmestgH > dpgH > dmgH > chgH > gH.9a Since the cis influence affects the Co-C bond (6) (a) Yohannes, P. G.; Bresciani-Pahor, N.; Randaccio, L.; Zangrando, E.; Marzilli, L. G. Inorg. Chem. 1988, 27, 4738. (b) Summers, M. F.; Marzilli, L. G.; Bresciani-Pahor, N.; Randaccio, L. J. Am. Chem. Soc. 1984, 106, 4478. (7) (a) Marzilli, L. G.; Gerli, A.; Calafat, A. M. Inorg. Chem. 1992, 31, 4617. (b) Hirota, S.; Polson, S. M.; Puckett, J. M., Jr.; Moore, S. J.; Mitchell, M. B.; Marzilli, L. G. Inorg. Chem. 1996, 35, 5646. (c) Polson, S. M.; Cini, R.; Pifferi, C.; Marzilli, L. G. Inorg. Chem. 1997, 36, 314. (8) (a) Randaccio, L.; Furlan, M.; Geremia, S.; Slouf, M.; Srnova, I.; Toffoli, D. Inorg. Chem. 2000, 39, 3403. (b) Randaccio, L.; Geremia, S.; Nardin, G.; Slouf, M.; Srnova, I. Inorg. Chem. 1999, 38, 4087. (9) (a) Mandal, D.; Gupta, B. D. Organometallics 2005, 24, 1501, and references therein. (b) Gupta, B. D.; Qanungo, K. J. Organomet. Chem. 1997, 543, 125. (c) Gupta, B. D.; Vijaikanth, V.; Singh, V. J. Organomet. Chem. 1998, 570, 1. (d) Gupta, B. D.; Roy, M.; Das, I. J. Organomet. Chem. 1990, 397, 219. (e) Gupta, B. D.; Tiwari, U.; Barcley, T.; Cordes, W. J. Organomet. Chem. 2001, 629, 83. (f) Gupta, B. D.; Singh, V.; Yamuna, R. Organometallics 2003, 22, 2670. (10) (a) Mandal, D.; Gupta, B. D. Organometallics 2007, 26, 658. (b) Xin, Z.; Han, D.; Li, Y.; Chen, H. Inorg. Chim. Acta 2006, 359, 1121. (c) Drago, R. S. J. Organomet. Chem. 1996, 512, 61. (11) Gilaberte, J. M.; Lopez, C.; Alvarez, S.; Font-Bardia, M.; Solans, X. New J. Chem. 1993, 17, 193.
10.1021/om900065k CCC: $40.75 2009 American Chemical Society Publication on Web 05/05/2009
3486 Organometallics, Vol. 28, No. 12, 2009
strength and the insertion of oxygen involves Co-C bond cleavage in these complexes, both processes are related to each other and follow the same order.9c,d,12 Hence there has been a sustained interest in the synthesis of organocobaloximes with new or modified equatorial ligands. Bis(thiophenyl)glyoxime has been used as the equatorial ligand in the present study. These are particularly selected because their cobaloximes become the ideal systems for studying the structure-property relationship. All the previously studied cobaloximes had dioximes with carbon substituents. The goal is to see how the SPh substituent on the dioxime affects the cobalt anisotropy and the ring current in the metallabicycle and in turn affects the Co-C bond strength. It is a priori difficult to predict whether the SPh group is electron releasing due to the lone pair on S or electron withdrawing due to the higher electronegativity of S as compared to its carbon analogue. The study will also allow us to make direct comparison with the already reported work on gH, dmgH, chgH, dpgH, and dmestgH complexes. Since the steric bulk of the dioxime affects the Co-C bond, it would be of interest to study the conformation of the SPh group on the dioxime, which, in principle, can adopt the up-up, down-down, up-down, and down-up orientation with respect to the dioxime plane, depending upon the steric bulk of the axial ligands. We have, therefore, undertaken the present study on RCo(dSPhgH)2Py (R ) Cl, Me, Et, Pr, Bu, benzyl, 1-6). All the complexes are new and have been synthesized for the first time. The complexes 1, 3, and 5 have also been characterized by X-ray diffraction.
Experimental Section Cobalt chloride hexahydrate (SD fine, India), glyoxime (Caution! it is highly flammable and explosive when dry) (Alfa Aesar, Lancaster), iodomethane, and ethyl and benzyl bromide (Aldrich Chemical Company) were purchased and used as received. Silica gel (100-200 mesh) and distilled solvents were used in all reactions and chromatographic separations. A Julabo UC-20 low-temperature refrigerated circulator was used to maintain the desired temperature. Cyclic voltammetry measurements were carried out using a BAS Epsilon electrochemical workstation with a platinum working electrode, a Ag/AgCl reference electrode (3 M KCl), and a platinum-wire counter electrode. All measurements were performed in 0.1 M nBu4NPF6 in dichloromethane (dry) at a concentration of 1 mM of each complex. 1 H and 13C spectra were recorded on a JEOL JNM Lambda 400 FT NMR spectrometer (400 MHz for 1H and 100 MHz for 13C) in CDCl3 solution with TMS as internal standard. NMR data are reported in ppm. Elemental analysis was carried out at IIT Kanpur. (12) (a) Bhuyan, M.; Laskar, M.; Mandal, D.; Gupta, B. D. Organometallics 2007, 26, 3559. (b) Dutta, G.; Laskar, M.; Gupta, B. D. Organometallics 2008, 27, 3338. (13) (a) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV. (b) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Dutta et al. X-ray Crystal Structure Determination and Refinements. A slow evaporation of solvent from the solution of complexes 1, 3, and 5 (CH2Cl2/CH3CN/MeOH for 1 and CH2Cl2/MeOH for 3, 5) in the refrigerator resulted in the formation of orange crystals. Single-crystal X-ray data were collected using graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) on a Bruker SMART APEX CCD diffractometer at 100 K. The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were taken from the International Tables for X-ray Crystallography.13a The data integration and reduction were processed with SAINT14 software. An empirical absorption correction was applied to the collected reflections with SADABS15 using XPREP.16 All the structures were solved by the direct method using SIR-9717 and were refined on F2 by the full-matrix leastsquares technique using the SHELXL-9713b program package. All non-hydrogen atoms were refined anisotropically in all the structure. The hydrogen atoms of the OH group of oxime were located on difference Fourier maps and were constrained to those difference Fourier map positions. The hydrogen atom positions or thermal parameters were not refined but were included in the structure factor calculations. The pertinent crystal data and refinement parameters of 1, 3, and 5 are compiled in Table 1. The CIF files for the compounds have been deposited with the Cambridge Crystallographic Data Centre (CCDC numbers for 1, 3, 5 are 715421715423). Copies of the data can be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EX, UK (fax +44-1223-336033; e-mail
[email protected], or www: http://www.ccdc.cam.ac.uk/). Dichloroglyoxime. We always got much lower yield than the reported when we prepared dichloroglyoxime using the literature procedure.18 The following modified procedure works well and afforded a better yield. Glyoxime (5.0 g) in 100 mL of water produced a suspension. Concentrated HCl (25 mL) was added with stirring, which resulted in a clear solution after 5-7 min. The solution can be warmed on a heating mantle for 5 min if the clear solution is not obtained. The solution was transferred to the specially designed glass vessel that has an outer jacket and was cooled to 0 °C by Julabo refrigerator circulator. First a slow stream of chlorine gas was bubbled into the glyoxime solution for exactly 15 min. This was followed by a medium stream (the yield drops if chlorine gas is bubbled at a fast pace) of chlorine gas for exactly 2 h. Although the product started to appear after 20-25 min, the white powdery precipitate formed after 2 h was kept aside at room temperature for 10 h or overnight for the precipitate to settle. It was filtered off, washed with water (three times), and air-dried. Yield ) 4.23 g (48%; average yield based on 10 experiments). Dithiophenylglyoxime (dSPhgH). Dichloroglyoxime (0.10 g, 0.636 mmol) was dissolved in 5-8 mL of methanol with stirring. Thiophenol (0.13 mL, 1.334 mmol) was added dropwise, and the stirring was continued for 10 min. The temperature of the solution was brought down to 0 °C by using an ice bath. Potassium carbonate (0.09 g, 0.636 mmol) was added, the ice bath was removed, and the solution was stirred for 10-12 min to dissolve potassium carbonate. Two pellets of potassium hydroxide were added, and the stirring was continued for an additional 20-25 min. The solution was filtered to remove the precipitate if any. The filtrate was left in a conical flask overnight for slow evaporation of the solvent. (14) SAINT+, 6.02 ed.; Bruker AXS: Madison, WI, 1999. (15) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (16) XPREP, 5.1 ed.; Siemens Industrial Automation Inc.: Madison, WI, 1995. (17) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (18) (a) Ungnade, H. E.; Kissinger, L. W. Tetrahedron 1963, 19, 143. (b) Ramprasad, D.; Busch, D. H. U.S. Patent 4,680,037, July 14, 1987.
Cobaloximes with Bis(thiophenyl)glyoxime
Organometallics, Vol. 28, No. 12, 2009 3487
Table 1. Crystal Data and Structure Refinement Details for 1, 3, and 5 [ClCo(dSPhgH)2Py] · CH2Cl2 empirical formula fw temp (K) cryst syst space group unit cell dimens a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z F(calc) (mg/m3) µ(Mo KR) (mm-1) F(000) cryst size (mm3) index ranges no. of reflns collected no. of indep reflns GOOF on F2 final R indices (I > 2σ(I)) R indices (all data) data/restraints/params
EtCo(dSPhgH)2Py
BuCo(dSPhgH)2Py
C67H56Cl4Co2N10O8S8 1645.44 100(2) triclinic P1j
C35H32CoN5O4S4 773.83 100(2) orthorhombic Pca21
C37H36CoN5O4S4 801.92 298(2) monoclinic Cc
11.074(5) 12.615(5) 25.496(5) 99.706(5) 99.115(5) 90.263(5) 3465(2) 2 1.577 0.938 1684 0.35 × 0.30 × 0.25 -10 e h e 13, -15 e k e 15, -30 e l e 26 18 725 12 678 1.034 R1 ) 0.0735 wR2 ) 0.1944 R1 ) 0.1033 wR2 ) 0.2278 12 678/0/892
23.069(5) 8.306(5) 18.064(5) 90.000(5) 90.000(5) 90.000(5) 3461(2) 4 1.485 0.785 1600 0.30 × 0.25 × 0.20 -30 e h e 24, -11 e k e 10, -24 e l e 21 21 612 8163 1.195 R1 ) 0.0623 wR2 ) 0.1520 R1 ) 0.0810 wR2 ) 0.1835 8163/1/442
25.458(5) 10.961(5) 14.499(5) 90.000(5) 118.331(5) 90.000(5) 3561(2) 4 1.496 0.765 1664 0.30 × 0.25 × 0.20 -26 e h e 33, -14 e k e 14, -19 e l e 14 11 388 5634 1.047 R1 ) 0.0802 wR2 ) 0.1967 R1 ) 0.0917 wR2 ) 0.2295 5634/2/446
Table 5. Selected Bond Lengths and Angles and Structural Data for 1, 3, and 5
Table 2. Elemental Analysis Data for 1-6 % found (calculated) no. 1 2 3 4 5 6
formula
C
C33H27ClCoN5O4S4 C34H30CoN5O4S4 C35H32CoN5O4S4 C36H34CoN5O4S4 C37H36CoN5O4S4 C40H34CoN5O4S4
H
50.58(50.81) 54.03(53.76) 53.99 (54.34) 55.02(54.90) 55.09(55.43) 57.89(57.49)
N
3.48(3.46) 3.92(3.94) 4.10(4.13) 4.34(4.31) 4.46(4.49) 4.04(4.06)
9.02(8.97) 9.16(9.21) 9.01(9.04) 8.93(8.88) 8.68(8.73) 8.35(8.37)
Table 3. 1H NMR Data (ppm) for 1-6 Py
compd no.
R
1 2 3 4 5 6
7.95 8.25 8.21 8.22 8.23 8.13
γ
O-H · · · O
7.80 7.83 7.82 7.82 7.81 7.81
17.87 18.15 18.05 18.07 18.10 18.11
β
Table 4.
aromatic proton Co-CH2 7.09-7.24 6.88-7.28 6.87-7.26 6.88-7.26 6.89-7.26 6.76-7.22
1.17 2.01 1.84 1.87 3.13
other proton
0.357 0.942, 0.721 1.16, 0.904, 0.755
13
C NMR Data (ppm) for 1-6
no.
CdN
PyR
1
145.20
150.95
2
142.28
149.98
3 5
142.02 142.07
149.95 149.88
6
142.30
150.20
others 139.32, 127.82, 138.20, 127.58, 132.05, 132.00, 125.65, 132.17, 127.35,
131.11, 130.21, 129.19, 126.23 132.10, 130.31, 129.07, 125.80, 29.79 130.54, 128.88, 127.49 130.55, 128.90, 127.52, 32.80, 23.57, 13.76 130.44, 128.78, 128.25, 125.53
The solid crystalline compound appeared as colorless needles, which were washed with dichloromethane. Yield ) 0.132 g (69%). The same yield was obtained when the reaction was scaled up five times. ClCo(dSPhgH)2Py (1). In a typical reaction CoCl2 · 6H2O (0.100 g, 0.420 mmol) was added to a stirred solution of dithiophenylglyoxime (0.255gm, 0.840 mmol) in ethanol (20 mL). The stirring was continued for 10-15 min, during which the solution turned bluish-green. It was heated for 8-10 min with a hot air gun, and 0.0631 mL (0.783 mmol) of pyridine was added dropwise, which
Co-C/Co-Cl Co-N C-Co-N/Cl-Co-N d (Å) R (deg) τ (deg)
1
3
5
2.231 (17) 1.976 (5) 178.53 (16) -0.020 4.12 72.710
2.024 (6) 2.078 (5) 174.1 (2) -0.015 4.81 75.368
2.011 (9) 2.123 (7) 174.1 (4) +0.015 3.18 65.038
turned the solution green. Air was passed through the solution for about half an hour to get the desired product. The solid product was filtered, washed with water and dried over P2O5 overnight. Then it was purified on a silica gel column using dichloromethane. Yield ) 0.238 g (73%). RCo(dSPhgH)2Py (2-6): General Procedure. A solution of 1 (0.100 g, 0.128 mmol) in about 25 mL of ethanol was purged with N2 for 10 min and was cooled to 0 °C with stirring. The solution turned blue after the addition of a few drops of aqueous NaOH followed by sodium borohydride (0.012 g, 0.320 mmol in 0.5 mL of water). The color of the solution immediately turned orange on the addition of alkyl halide (0.192 mmol in 1 mL of ethanol). The stirring was continued at 0 °C for 1 h. The volume of the reaction mixture was reduced to 5 mL by evaporation on a Rotovap and then poured into 20 mL of water containing a few drops of pyridine. The resulting orange precipitate was filtered, washed with water, and dried. The product was purified on the silica gel column using dichloromethane. Yield ) 53-80%. CHN analysis of 1-6 is given in Table 2.
Results and Discussion Synthesis. Dithiophenylglyoxime was prepared from dichloroglyoxime and thiophenol in the presence of K2CO3 following the literature procedure.19 We have synthesized six new complexes, X/RCo(dSPhgH)2Py (1-6), by a general procedure used for the preparation of dmgH complexes reported earlier. (19) Nicolaides, D. N.; Kouimtzis, A. Chim. Chronika 1974, 3, 63.
3488 Organometallics, Vol. 28, No. 12, 2009
Dutta et al.
Table 6. CV Data for ClCo(dioxime)2Py in CH2Cl2 and TBAPF6 at 0.2 V/s at 25 °C CoIII/CoII
a
CoII/CoI
CoIV/CoIII
compound
Epc (V)a
Epc (V)b
E1/2 (V)a
E1/2 (V)b
E1/2 (V)a
E1/2 (V)b
ClCo(gH)2Py ClCo(dmgH)2Py ClCo(dpgH)2Py ClCo(dmestgH)2Py ClCo(dSPhgH)2Py
-0.39(irrev) -0.66(irrev) -0.50(irrev) -0.68(irrev) -0.25(irrev)
-0.81 -1.08 -0.92 -1.10 -0.67
-0.66(113) -1.12(200) -0.85(105) -1.01(120) -0.47(69)
-1.08 -1.54 -1.27 -1.43 -0.90
1.20(190) 1.22(200) 1.33(120) 1.40(83)
0.77 0.79 0.904 0.97
vs Ag/AgCl. b vs Fc/Fc+. Fc/Fc+ (E1/2 ) 0.4269). Table 7. CV Data for 1, 3, and 6 in CH2Cl2 and TBAPF6 at 0.2 V/s at 25 °C CoIII/CoII
CoII/CoI
CoIV/CoIII
no.
Epc (V)a
Epc (V)b
ipc (µA)
E1/2 (V)a
E1/2(V)b
ipc (µA)
E1/2 (V)a
E1/2 (V)b
ipc
ipa
1 3 6
-0.253 -1.269 -1.166
-0.679 -1.695 -1.592
1.07 8.36 2.74
-0.475(69)
-0.901
2.50
1.401(83) 1.155(97) 1.150(75)
0.975 0.729 0.724
1.03 6.17 1.37
4.53 7.87 2.24
a
vs Ag/AgCl. b vs Fc/Fc+. Fc/Fc+ (E1/2 ) 0.4269).
Figure 1. Molecular structure of 1.
Figure 3. Molecular structure of 5.
Figure 2. Molecular structure of 3. Table 8. Pseudo-First-Order Rate Constant (kobs) for Oxygen Insertion in PhCH2Co(dioxime)2Py complex
kobs (s-1)
PhCH2Co(gH)2Py PhCH2Co(dmgH)2Py PhCH2Co(dSPhgH)2Py PhCH2Co(dpgH)2Py PhCH2Co(dmestgH)2Py
7.1 × 10-4 1.2 × 10-3 1.29 × 10-3 4.8 × 10-3 5.0 × 10-2
The synthesis of RCo(dSPhgH)2Py involves the oxidative alkylation of CoI with the corresponding halide. Ethanol is a better solvent than methanol, and a large excess of solvent is essential during the generation of cobaloxime(I); otherwise the precipitation occurs after the addition of NaBH4. Spectroscopy. The free bis(thiophenyl)glyoxime20 and its complexes 1-6 have been characterized by 1H and 13C NMR,
Figure 4. Cyclic voltammogram of 1 in CH2Cl2 with 0.1 M (nBu4NPF6) as supporting electrolyte at 0.2 V s-1 at 25 °C.
and in addition 1, 3, and 5 have also been characterized by X-ray. The NMR assignments are consistent with the related cobaloxime complexes with dmgH, gH, and dpgH. PyR and CdN occur close to each other, and the assignment is confirmed by DEPT. The free dithiophenylglyoxime is not completely soluble in CDCl3, and a drop of DMSO-d6 is necessary to dissolve it. The cis and trans influence study includes the investigation of all possible steric and electronic effects of the dioxime on
Cobaloximes with Bis(thiophenyl)glyoxime
Organometallics, Vol. 28, No. 12, 2009 3489
Figure 5. Cyclic voltammograms of 3 and 6 in CH2Cl2 with 0.1 M (nBu4NPF6) as supporting electrolyte at 0.2 V s-1 at 25 °C. Table 9. Pseudo-First-Order Rate Constant (kobs) for Oxygen Insertion in RCo(dioxime)2Py complex
kobs (s-1)
EtCo(dmgH)2Py EtCo(dpgH)2Py EtCo(dSPhgH)2Py MeCo(dmestgH)2Py n BuCo(dmestgH)2Py
6.45 × 10-4 6.95 × 10-4 5.95 × 10-4 2.9 × 10-4 4.5 × 10-4
the axial ligands and vice versa. Generally the chemical shift of PyR and the R-carbon bonded to Co is studied since these are affected most by the dioxime. Three factors simultaneously acting on a particular atom to determine the NMR chemical shift of the coordinated ligand from the free ligand are (a) cobalt anisotropy (cobalt anisotropy is the total field effect of the CoC4N4 system; the field effect is the combination of the inductive effect of cobalt and the effect of donation through Cofdioxime and Cofaxial ligand and back-donation) (leads to deshielding of all the ligand atoms through the σ backbone and it decreases with the distance from the metal), (b) longrange effect of magnetic anisotropy such as the ring current arising from either the metallabicycle, the axial ligand pyridine, or the SPh group, and (c) metal to ligand back-bonding, which may alter the ring current and/or the cobalt anisotropy. A comparison of ∆δ1H(PyR) (∆δ1H(PyR) ) δ1Hcomplex δ1H free Py) in RCo(dioxime)2Py [dioxime ) gH, dmgH, chgH, dpgH, dmestgH, dSPhgH] shows an upfield shift of 0.30-0.44 ppm in 2-6 compared to the shift in other dioxime complexes (Supporting Information, Table S1). This is unexpected and unusually quite high. One should not jump to the conclusion and attribute this to the high ring current of the metallabicycle in 2-6 since the upfield shift can also occur due to throughspace interaction of the phenyl ring of the SPh group with the PyR proton. Its contribution, if any, can be confirmed from X-ray and 13C NMR study. The PyR proton is quite close (3.732 and 3.436 Å in 3 and 5) to the dioxime SPh group (see X-ray details later) and is highly shielded by its ring current.1b This is further confirmed by 1H (20) dSPhgH, 1H NMR: O-H · · · O ) 11.38 ppm, aromatic proton ) 7.14-7.28 ppm. 13C NMR: CdN ) 145.35, other carbons ) 134.47, 128.51, 128.37, 128.10.
NMR of MeCo(dSEtgH)2Py.21 Here SEt lacks ring current and, as expected, no shielding of the PyR proton occurs and δ1HPyR is found to be almost identical with MeCo(dmgH)2Py. Since 13C operates through-bond, the through-space interaction is minimized in the 13C chemical shift. Hence the δ13CPyR carbon should be similar to the other cobaloxime complexes where such through-space interaction with PyR does not occur. This is indeed so, and the chemical shift value is almost identical in RCo(dmgH)2Py and RCo(dSPhgH)2Py (Supporting Information, Table S2). It is clear that the observed high upfield shift [∆δ1H(PyR)] in 2-6 occurs due to the through-space interaction and not because of the high ring current of the Co(dioxime)2. The extent of electron density (i.e., total effect of cobalt anisotropy and ring current) on Co(dioxime)2 for different dioximes (keeping R/X constant) can be understood by comparing ∆δ(13CCdN)22 in R/XCo(dioxime)2Py. In general, the δ13CCdN in free dioxime shifts upfield on coordination to cobalt. This upfield shift [∆δ(13CCdN)] is much smaller in 1-6 as compared to the corresponding gH, dmgH, and dpgH analogues but is comparable to the mesitylglyoxime complexes (Supporting Information, Table S3). The order based on the upfield shift value follows gH > dmgH > dpgH > chgH > dSPhgH > dmestgH. So the charge density on CdN in the thiodioxime complexes is much lower than the other dioxime complexes. This may further mean that the cobalt anisotropy of the dSPhgH complex is close to dmestgH complexes, and this is possible only if the overall effect of SPh is taken as an electronwithdrawing group. This effect shows up in the CV study also. The variation in the R/X group also affects δ13CCdN (compare 1 and 2) and is similar to our earlier observations; for example the upfield shift value in 1 is much smaller, as expected, than 2 because of the higher cobalt anisotropy in the chloro complex. In tune with our earlier work, the trans influence is monitored through the coordination shift (∆δ ) δ complex - δ free base) of the γ-proton only. The chemical shift of the Pyγ proton is a net result of the interplay of cobalt anisotropy and the trans effect of the R/X group. The ∆δ Pyγ value of 0.4 ppm in 1-6 is larger than the corresponding dmgH and chgH analogues but resembles the dpgH complexes (Supporting Information, Table S4). This means that the cobalt anisotropy in 1-6 is high like in dpgH, which leads to more electron withdrawal from pyridine and causes deshielding of the γ-proton. The same observation was made on the basis δ13CCdN data also. The cis influence of the dioxime on the R-carbon bonded to cobalt is measured by its chemical shift; for example Co-CH2R in 2-6 is downfield shifted from the corresponding dmgH complex but is highly upfield shifted with respect to the dpgH or dmestgH complexes (Supporting Information, Table S5); that is, the chemical shift of Co-CH2 in 2-6 lies between the corresponding dmgH and dpgH complexes. The same pattern shows up in the molecular oxygen insertion rate data into these complexes (see later). Correlations. The coordination shift of the CdN group of the dioximato moiety is much lower in 1-6 as compared to the corresponding value in the gH, dmgH, dpgH, and dmestgH complexes with identical R and B groups. The correlation of the ∆δ(13C, CdNdSPhgH) in 1-6 is fairly linear with the other dioximes. (21) MeCo(dSEtgH)2Py: PyR ) 8.60 ppm, Pyβ ) 7.37 ppm, Pyγ ) 7.77 ppm, SEt group(-CH2) ) 3.26, 3.10 ppm, SEt group(Me) ) 1.10 ppm, H-1 ) 1.25 ppm, O-H · · · O ) 18.50 ppm. (22) Instead of δ (13C, CdN), the ∆δ (13C, CdN) value has been taken. This is to avoid the direct effect of the substituent on the δ(CdN) value. ∆δ(13C, CdN) represents the field effect.
3490 Organometallics, Vol. 28, No. 12, 2009 ∆δ(13C, CdNdSPhgH) ) 1.92(14) + 1.12(4)[∆δ(13C, CdNdpgH)] (r ) 0.99, esd ) 0.08) 2
∆δ(13C, CdNdSPhgH) ) 4.49(71) + 1.46(15)[∆δ(13C, CdNdpgH)] (r2 ) 0.96, esd ) 0.27) ∆δ(13C, CdNdSPhgH) ) 5.63(35) + 1.16(5)[∆δ(13C, CdNdpgH)] (r2 ) 0.99, esd ) 0.11)
Structural Studies. The geometry of the central cobalt atom is distorted octahedral with four nitrogen atoms of the dioxime in the equatorial plane and pyridine and Cl (or Et, Bu) are axially coordinated. The deviation of the central cobalt atom (d) from the mean equatorial N4 plane is -0.020, -0.015, and +0.015 Å, respectively, for 1, 3, and 5. Previous structural studies on cobaloximes have shown that changing the axial R group in RCo(dioxime)2B causes only a small distortion on the Co(dioxime)2 moiety.1b The emphasis of the structural study has mainly been focused on four parameters: (i) Co-C and Co-Npy bond lengths, (ii) the puckering of the dioxime ligand, i.e, butterfly bending angle (R),23 (iii) deviation of the cobalt atom from the mean equatorial N4 plane (d), and (iv) the torsion angle24 between the axial base pyridine and the dioxime ligand. R and d are the guiding factors in defining the geometrical deformation in the Co(dioxime)2 moiety. A large amount of the reported structural data in RCo(dmgH)2Py complexes further reveals that R seems to be more influenced by the bulk of the R group than the d, and the variations obtained in d and R fall in a fairly wide range [R from -12.3° [R ) CH2C(Me)(COOEt)2] to +6.3° (R ) CHdCH2) and d from -0.03 Å [R ) CH2C(Me)(COOEt)2 to +0.04 Å (R ) Me)]. Therefore, the bending of the two dioxime units is the most significant distortion in these compounds. Unfortunately, there is a lack of enough data in the literature to see if a change in the dioxime moiety, keeping the same axial ligands, has any influence on these parameters. The structural data on the thiodioxime complexes are, therefore, significant. A comparison of R and d value in MeCo(dioxime)2Py shows a range of +2° to +7.3° and +0.03 to -0.017, respectively, as we move from gH to dmestgH (Supporting Information, Table S6). The distortion imposed by thiodioxime is similar to the dpgH ligand. When the equatorial ligand (chel) is a dianion of a tetradentate Schiff base such as salen, acacen, and salophen in RCo(chel)Py, it imposes a much larger deviation in the R value as compared to the dioxime complexes (Supporting Information, Table S7). The Co-Cl or Co-C bond distances [2.231, 2.024, 2.011 Å] and Co-NPy bond distances [1.976, 2.078, 2.123 Å] in 1, 3, and 5 do not differ significantly from the reported values for the corresponding (R/X)Co(dioxime)2Py complexes. The electronic nature of R influences the Co-NPy bond length, and more electron-donating R increases the length.1a This is what is observed here also (compare the data between 3 and 5 in Table 5). A further comparison with the RCo(chel)Py complexes shows that while the Co-C distances are almost same, the Co-NPy distance is much larger in the Schiff base complexes. This can be due to two principal aspects of the equatorial (23) (a) The dihedral angle (butterfly bending angle, R) is the angle between two dioxime planes of each cobaloxime unit. (b) The positive sign of R and d indicates bending toward R and displacement toward the base and vice versa. (24) The torsion angle (τ) is the angle between two virtual planes that bisects the cobaloxime plane. Each plane is formed considering the middle point of the C-C bond of two oxime units that passes through cobalt and pyridine nitrogen.
Dutta et al.
ligands. First, the different electronic nature of the chelate macrocycle could change the metal-orbital mix involved in the axial ligand bonding (electronic cis influence), and second, the more flexible macrocycle may bend toward the axial ligand, thus increasing the axial bond length.1b Interaction between the Axial Ligands and the Equatorial Dioxime Ligand Side Groups. The present results are reminiscent of the earlier work by Randaccio et al., who have studied, in detail, the interactions of axial ligands with the equatorial ligand bridging the side groups in diphenylborylated cobaloximes by NMR and X-ray.25-27 They observed that the phenyl groups on BPh2 assumed different fast interconverting conformations in solution, depending on the interaction between the BPh, phenyls, and the axial ligands. Their result suggested that the steric effects (steric trans influence) played an important role in this process. Although our systems differ from Randaccio because the substituent SPh is on the dioxime imine carbon rather than on the bridging OHO, as was the case in Randaccio’s work, nevertheless the orientation of SPh group with respect to the dioxime plane does affect the structure and the NMR. In the present work the conformation of SPh with respect to the dioxime plane varies with the steric bulk of the axial ligand; for example the up-up, down-down conformation in 3 changes to an up-down, up-down conformation in 5. The conformation changes further in 1 as shown below. One of the SPh phenyl groups having a down orientation is close to the pyridine R-proton and shows the C-H(PyR) · · · π interaction with distances of 4.32, 3.732, and 3.436 Å in 1, 3, and 5, respectively. This interaction has led to the upfield shift of the PyR proton in the 1H NMR.
CV Study. Very little work has been reported on the CV studies in cobaloximes, and hence there is a lack of sufficient data to correlate and generalize the electrochemical behavior of these complexes. One of the major problems associated is the change in coordination number of cobalt during the reduction/oxidation process.28 Most of the reported CV work has been done in (X/R)Co(dmgH)2B to understand the effect of base B. The effect of dioxime on the CV value has virtually not been studied. This may be due to the lack of cobaloximes with the dioximes other than dmgH. In general, the inorganic cobaloximes give a better cyclic voltammogram than the organocobaloximes. Three types of redox couples, CoIII/CoII, CoII/CoI, and CoIV/ CoIII, in the cyclic voltammogram of (X/R)Co(dioxime)2B are (25) Asaro, F.; Naradin, G.; Pellizer, G.; Perissini, S.; Randaccio, L.; Siega, P.; Tauzher, G.; Tavagnacco, C. J. Organomet. Chem. 2000, 601, 114. (26) (a) Dreos, R.; Geremia, S.; Nardin, G.; Randaccio, L.; Tauzher, G.; Vuano, S. Inorg. Chim. Acta 1998, 272, 74. (b) Dreos, R.; Tauzher, G.; Vuano, S.; Asaro, F.; Pellizer, G.; Nardin, G.; Randaccio, L.; Geremia, S. J. Organomet. Chem. 1995, 505, 135. (27) Asaro, F.; Dreos, R.; Geremia, S.; Nardin, G.; Pellizer, G.; Randaccio, L.; Tauzher, G.; Vuano, S. J. Organomet. Chem. 1997, 548, 211. (28) (a) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L. J. Am. Chem. Soc. 1981, 103, 5558. (b) Alexander, V.; Ramanujum, V. V. Inorg. Chim. Acta 1989, 156, 125.
Cobaloximes with Bis(thiophenyl)glyoxime
expected, and the CV of both oxidative and reductive halves can be rationalized on the basis of cobalt anisotropy; the higher the cobalt anisotropy, the lower the reduction potential and the higher the oxidation potential.9a,29 The cyclic voltammogram of 1 shows one irreversible peak at -0.253 V (irrev) and one reversible peak (-0.475 V) in the reductive half corresponding to CoIII/CoII and CoII/CoI, respectively. In the oxidation half only one reversible wave corresponding to CoIV/CoIII (1.40 V) is observed. The CoII/CoI moiety should be affected only by the dioxime moiety since there is no axial ligation in CoI. A comparison with the other chlorocobaloximes (gH, dmgH, dpgH, dmestgH) (Table 6) shows that 1 is the most easily reduced among these. It points to high cobalt anisotropy in 1, which is possible only if SPh acts as an electronwithdrawing group. The NMR data also gave the same information. The cyclic voltammogram of 3 and 6, on the other hand, shows only one irreversible peak at -1.26 and -1.16 V, respectively, in the reduction side, corresponding to CoIII/CoII, and one reversible peak at 1.15 V in the oxidation side, corresponding to CoIV/CoIII; 6 is easier to reduce compared to 3 because benzyl is a poorer electron donor than Et. Due to enhanced σ donation by the R group, the CoIII state in 3 and 6 is substantially stabilized. In other words, the CoIII/CoII redox process in 3 and 6 is considerably cathodically shifted. Due to this, the CoII/CoI response is further cathodically shifted and therefore is not observed even down to -1.6 V. Rate Studies. The insertion of molecular oxygen into the Co-C bond has been used to test the reactivity of the Co-C bond in cobaloximes. Since the cleavage of the Co-C bond is the primary step in the oxygen insertion and the effect of dioxime (cis influence) is felt most on the Co-C bond, both are related to each other. In the earlier work we found a good correlation between these two factors; both followed the same order in the benzyl cobaloximes: dmestgH . dpgH > chgH > dmgH > gH.9c,d,12 If this correlation is a general feature of all (29) Yamuna, R.; Gupta, B. D.; Mandal, D. Organometallics 2006, 25, 706.
Organometallics, Vol. 28, No. 12, 2009 3491
cobaloximes, then the oxygen insertion data in the present thiodioximes complexes should fit in well and strengthen the concept further. The rate studies are carried out following the procedures outlined in our earlier paper.12 The reactions follow a pseudofirst-order kinetics, and the rate data in 3 and 6 support our previous observations; for example the benzyl complexes have a higher rate of insertion, as expected, than the alkyl complexes. The rate lies between the dmgH and dpgH complexes in the benzyl complexes (Table 8). This is as per our expectation, arrived on the basis of NMR. The overall rate follows the order dmestgH . dpgH > dSPhgH g dmgH > gH. The generality of this correlation, however, is difficult to test in the alkyl cobaloximes since the pseudo-first-order kinetics of oxygen insertion is slow and does not vary much with the alkyl group or with the dioxime. Nevertheless it is useful information (Table 9).
Conclusion The steric cis influence affects the Co-C bond stability/ reactivity, and a good correlation between the rate of oxygen insertion and the steric cis influence has been found in the alkyl and benzyl cobaloximes with dithiophenyloxime as the equatorial ligand. Both follow the same order: dmestgH . dpgH > dSPhgH g dmgH > gH. This supports our earlier results. As the substituent on S controls the orientation of the SR group and may alter the cobalt anisotropy and/or the ring current of the metallabicycle, which, in turn, will affect the Co-C bond stability, more systems should be studied to answer this question.
Acknowledgment. This work has been supported by a grant from DST, New Delhi, India. Supporting Information Available: Molecular structure comparison data, 1H and 13C NMR table, comparison table of NMR, and CIF files for X-ray crystal structures of 1, 3, and 5. This material is available free of charge via the Internet at http://pubs.acs.org. OM900065K
