Synthesis, structure, and properties of low-spin manganese(III)-poly(pyrazolyl)borate complexes

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)(2)SbF6, where pzb(-) = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)(2) compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (mu(eff) = 2.9-3.8 mu(B)). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) angstrom in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d(4) center. N-Mn-N bite angles of slightly less than 90(o) within the facially coordinated pzb(-) ligands produce a small trigonal distortion and effective D-3d symmetry in 1 and 2. These angles increase to 90.0(4)(o) in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)(2)(+). Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)(2)(+) complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (k(s,h)) for reduction of t(2g)(4) Mn(pzb)(2)(+) to t(2g)(3)e(g)(2) Mn(pzb)(2) (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t(2g)(4) Mn(pzb)(2)(+) to t(2g)(3) Mn(pzb)(2)(2+). k(s,h) values decrease as Tp* > pzTp > Tp for the Mn(pzb)(2)(+/0) electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)(2)(+/0) and Co(pzb)(2)(+/0) couples.