LOW-SPIN TRIS(QUINONE OXIMATES) OF MANGANESE(II,III) - SYNTHESIS, ISOMERISM, AND EQUILIBRIA

The manganese(III) tris chelates Mn(RQ)3 (1, R = Cl, Br, Me, (t)Bu) synthesized from 1,2-quinone oximes (HRQ) and Mn(MeCO2)3-2H20 are low spin (S = 1, 2.9-3.1 mu(B) at 298 K) both in the solid state and in solution. Their mer geometry is proven by H-1 NMR spectra in CDC13 Solution. The manganate(II) anions, Mn(RQ)3-, have been electrogenerated in solution from Mn(RQ)3 and are uniformly low spin (S = 1/2, 1.8-2.0 mu(B) at 298 K). The salt [Et4N][Mn(CIQ)3] (1.77 mu(B) at 298 K) has been isolated and structurally characterized: chemical formula C26H29N406-Cl3Mn; crystal system triclinic; space group P1BAR; a = 12.205 (6) angstrom, b = 13.645 (9) angstrom, c = 18.076 (10) angstrom; alpha = 91.04 (5)-degrees, beta = 94.07 (4)-degrees, gamma = 96.06 (4)-degrees; V = 2985 (3) angstrom3; Z = 4; R = 8.80%, R(w) = 8.88%. The lattice consists of an ordered arrangement of equal proportions of mer and fac isomers. The average Mn-O and Mn-N distances are 1.97 (1) and 1.93 (2) angstrom, respectively. Isomer-specific nonbonded Cl ... Cl interactions are important in crystal packing. Variable-temperature voltammetry of Mn(RQ)3- has revealed the presence of isomeric equilibria in solution-the equilibrium constant K(II) (=[mer]/[fac]) lying in the range 2-3 at 260 K in dichloromethane. On the other hand, the clectrochemistry of Mn(RQ3) isconsistent with the virtually exclusive presence of the mer isomer. The equilibrium constant K(III)(=[mer]/[fac]) for Mn(RQ)3 has been estimated to be approximately 10(4) at 260 K in dichloromethane. The unstable fac-Mn(RQ)3 isomer can be observed by stereoretentive voltammetry of the fac-mer equilibrium mixture of Mn(RQ)3-. It spontaneously isomerizes with a rate constant of approximately 0.03 s-1 (260 K, dichloromethane). The manganese(III)-manganese(II) formal potentials are isomer dependent, and for a given R, E-degrees(fac) is 170-180 mV more positive than E-degrees(mer). The inequalities K(III) > K(II) and E-degrees(fac) > E-degrees(mer) are thermodynamically interrelated, and some of the factors controlling them are examined.