3292 Inorg. Chem. 1988, 27, 3292-3291 Contribution from the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032, India Trivalent Nickel. The Quinone Oximate Family: Synthesis and Redox Regulation of Isomerism and Ligand Redistribution Debashis Ray and Animesh Chakravorty* Received February zyxwvutsrqpo 4, 1988 The anionic N 3 0 3tris chelates Ni"(RQ)<, derived from 1,2-quinone 2-oximes (HRQ, I), occur as equilibrium mixtures offac (2x1) and mer (2b) isomers in acetonitrile solution and display the reversible nickel(II1)-nickel(I1) couples fac-Ni(RQ),-fac- Ni(RQ)< and mer-Ni(RQ),-mer-Ni(RQ)< ('H NMR and voltammetry). The former couple has a higher (by -0.12 V) formal potential. These potentials are also sensitive to the substituent R, and the range 0.3-0.7 V vs SCE is covered by the present complexes. Thefac-Ni(RQ)s isomer is spontaneously transformed into the mer form in solution. In the case R zyxw = Me, the rate constant is estimated to be 0.02 s-l at 258 K. Due to this isomerization pure Ni(RQ), could be isolated only in the mer form via coulometric oxidation of Ni(RQ),-. The dark-colored Ni(RQ), chelates behave as one-electron paramagnets and in frozen solution (77 K) display rhombic EPR spectra (g = 2.04,2.14,2.19) characterizing the mer geometry. Upon reduction, mer-Ni(RQ), is reconverted into the equilibrium mixture offac- and mer-Ni(RQ)<. At 258 K, the mer -+ fac isomerization of Ni(RQ)< is slow on the cyclic voltammetric time scale and the mer-Ni(RQ),-mer-Ni(RQ),- couple alone is observable. Mixed complexes of type Ni(RQ),L, undergo quasi-reversible cyclic voltammetric oxidation to Ni111(RQ)2L2+ (L = two pyridine ligands or one 2,2'-bipyridine ligand). The oxidized complex is unstable and reacts spontaneously with the parent nickel(I1) complex, affording mer-Ni(RQ), via ligand redistribution. By coulometric oxidation of Ni(BuQ),(bpy) at 263 K, a measurable concentration of Ni(B~Q)~(bpy)+ can be produced in dichloromethane solution, and in the frozen state this affords an EPR spectrum characteristic of axial compression (g,, = 2.169, zyxwvutsrq g, = 2.083). Five-line nitrogen hyperfine (a, = 15 G) is observed in the g, region, and a structure (5) having bpy on the xy plane is suggested. The formal nickel(II1)-nickel(I1) potentials (0.49-0.80 V vs SCE) of Ni(MeQ)2L2 are found to correlate linearly with the donor strength (PIC,) of L (L = pyridine, 4-methylpyridine, 4-aminopyridine, and pyrazole). Introduction High oxidation levels of nickel are of current interest.'" The trivalent state occurs in bacterial hydrogena~es.~.' Biological donor atoms usually come from the group N, 0, S. The synthesis, characterization, and redox potency of model nickel(II1) complexes (a) Nag, K.; Chakravorty, A. Coord. Chem. Rev. 1980,33,87-147. (b) Chakravorty, A. Isr. J. Chem. 1985, 25, 99-105. (c) Chakravorty, A. Comments Inorg. Chem. 1985, 4, 1-16. Bhattacharya, S.; Mukherjee, R. N.; Chakravorty, A. Inorg. Chem. 1986, 25, 3448-3452. Recent contributions to higher oxidation states of nickel were extensively cited in this reference. Subsequent progress in nickel(II1) coordination chemistry is referenced Amine, cyanide, peptide, and oxime complexes: (a) Yamashita, M.; Murase, I. Inorg. Chim. Acta 1985, 97, L43-L44. (b) Shen, W.; Lunsford, J. H. Inorg. Chim. Acta 1985, 102, 199-203. (c) Pappen- hagen, T. L.; Margerum, D. W. J. Am. Chem. SOC. 1985, 107, 4576-4577. (d) Kirvan, G. E.; Margerum, D. W. Inorg. Chem. 1985, 24, 3245-3253. (e) Subak, E. J., Jr.; Loyola, V. M.; Margerum, D. W. Inorg. Chem. 1985,24,4350-4356. (f) Pappenhagen, T. L.; Kennedy, W. R.; Bowers, C. P.; Margerum, D. W. Inorg. Chem. 1985, 24, 4356-4362. (g) Lappin, A. G.; Martone, D. P.; Osvath, P. Inorg. Chem. 1985, 24, 4187-4191. Macrocyclic complexes: (a) Fairbank, M. G.; Norman, P. R.; McAuley, A. Inora. Chem. 1985.24. 2639-2644. (b) Chavan. M. Y.: Meade. T. J.; Busih, D. H.; Kuwana, T. Inorg. Chem. 1986,'25, 314-321. (c) Fabbrizzi, L.; Kaden, T. A,; Perotti, A,; Seghi, B.; Siegfried, L. Inorg. Chem. 1986, 25, 321-327. (d) Wieghardt, K.; Walz, W.; Nuber, B.; Weiss, J.; Ozarowski, A,; Stratemeier, H.; Reinen, D. Inorg. Chem. 1986, 25, 1650-1654. Phosphine, arsine, and thiolate complexes: (a) Higgins, zyxwvuts S. J.; Levason, W.; Feiters, M. C.; Steel, A. T. J. Chem. SOC., Dalton Trans. 1986, 317-322. (b) Yamamura, T.; Miyamae, H.; Katayama, Y.; Sasaki, Y. Chem. Lett. 1985, 269-272. (c) Yamamura, T. Chem. Lett. 1985, 801-804. (d) Harbron, S. K.; Higgins, S. J.; Hope, E. G.; Kemmitt, T.; Levason, W. Inorg. Chim. Acta 1987, 130,43-47. Aryl complexes: (a) Grove, D. M.; van Koten, G.; Zoet, R.; Murrall, N. W.; Welch, A. J. J. Am. Chem. SOC. 1983, 105, 1379-1380. (b) Grove, D. M.; van Koten, G.; Mul, W. P.; Van der Zeijden, A. A. H.; Terheijden, J.; Zoutberg, M. C.; Stam, C. H. Organometallics 1986, 5, 322-326. (a) Albracht, S. P. J.; Graf, E. C.; Thauer, R. K. FEBS Lett. 1982,140, 31 1-313. (b) Lancaster, J. R., Jr. Science (Washington, D.C.) 1982, 216, 1324-1325. (c) Thomson, A. J. Nature (London) 1982, 298, 602-603. (d) Scott, R. A,; Wallin, S. A,; Czechowski, M.; DerVarta- nian, D. V.; LeGall, J.; Peck, H. D., Jr.; Moura, I. J. Am. Chem. SOC. 1984, 106,6864-6865. (e) Lindahl, P. A.; Kojima, N.; Hausinger, R. P.; Fox, J . A,; Teo, B. K.; Walsh, C. T.; Orme-Johnson, W. H. J. Am. Chem. SOC. 1984, 106, 3062-3064. (f) Eidsness, M. K.; Sullivan, R. J.; Schwartz, J. R.; Hartzell, P. L.; Wolfe, R. S.; Flank, A.-M.; Cramer, S. P.; Scott, R. A. J. Am. Chern. SOC. 1986, 108, 3120-3121. (g) Cramer, S. P.; Eidsness, M. K.; Pan, W.-H.; Morton, T. A.; Ragsdale, S. W.; DerVartanian, D. V.; Ljungdahl, L. G.; Scott, R. A. Inorg. Chem. 1987, 26, 2477-2479 and references therein. in the coordination environments of one or more of these atoms have concerned US.^,*,^ The present work stems from our search for high oxidation states of nickel coordinated to ligands of the 1,2-quinone family. The quinones themselves have not been ef- fective, but their monooximes (HRQ, see below) are found to act as good bidentate N,O chelators toward trivalent nickel. Herein we report the synthesis of the tris chelates Ni111(RQ)3 by elec- trooxidation of Ni"(RQ),-. The complexes have afforded an unique opportunity for voltammetric and spectroscopic exami- nation of geometrical isomerism and isomer preferences of the two oxidation states of nickel in a N303 environment. We also describe a redox-driven ligand redistribution reaction that furnishes Ni(RQ), following electrooxidation of Ni"(RQ),(N,N) to Ni111(RQ)2(N,N)+, where N,N represents amine coordination. The effects of geometric structure, substituents, and ligands on the nickel(II1)-nickel(I1) reduction potential in Ni(RQ), and Ni(RQ)2(N,N)+ are noted. Results and Discussion A. Nickel(I1) Tris Chelates, Ni(RQ),-. a. Synthesis and Characterization. Four HRQ ligands (1) were used in the present work. The anions Ni(CIQ),- and Ni(BzQ),- are already 'NOH R = 4-Me, HMeQ 4-But, HBuQ 4-Cl, HClQ 3,4- benzo,HBzQ 1 known.I0J1 We have also prepared Ni(MeQ),- and Ni(BuQ),- with the help of the reaction of eq 1 carried out in ethanol. These Ni(RQ), + HRQ + OH- - Ni(RQ),- + H20 (1) - (8) Singh, A. N.; Chakravorty, A. Inorg. Chem. 1980, 19, 969-971. (9) Ray, D.; Pal, S.; Chakravorty, A. Inorg. Chem. 1986, 25, 2674-2676. (10) (a) Carreck, P. W.; Charalambous, J.; Kensett, M. J.; McPartlin, M.; Sims, R. Inorg. Nucl. Chem. Lett. 1974,lO. 749-751. (b) Charalam- bous, J.; Kensett, M. J.; Jenkins, J. M. Inorg. Chim. Acra 1976, zy 16, (1 1) Charalambous, J.; Maple, P.; Nassef, N. A,; Taylor, F. B. Inorg. Chim. Acta 1978, 26, 107-1 11. 213-217. 0020-1669/88/1327-3292$01.50/0 0 1988 American Chemical Society