Metal-bis[poly(pyrazolyl)borate] complexes. Electrochemical, magnetic, and spectroscopic properties and coupled electron-transfer and spin-exchange reactions.

Electrochemical, magnetic, and spectroscopic properties are reported for homoleptic divalent (M = Mn, Fe, Co, Ni, Ru) and trivalent (M = Cr, Mn, Fe, Co) metal-bis[poly(pyrazolyl)borate] complexes, M(pzb)(2), where pzb(-) = hydrotris(pyrazolyl)borate (Tp), hydrotris(3,5-dimethylpyrazolyl)borate (Tp), or tetrakis(pyrazolyl)borate (pzTp). Ligand field strengths in metal-pzb complexes increase as Tp < Tp < pzTp, which reflects the importance of steric rather than electronic effects on spectroscopic properties. However, metal-centered redox potentials become more negative as pzTp < Tp < Tp, which follows the electron-donating ability of the ligands. Co(III)/Co(II) and Mn(III)/Mn(II) electrode reactions are accompanied by a change in metal atom spin-state; i.e., (S = 0) Co(pzb)(2) + e(-) <==> (S = 3/2) [Co(pzb)(2)] and (S = 1) Mn(pzb)(2) + e(-) <==> (S = 5/2) [Mn(pzb)(2)]. Apparent heterogeneous electron-transfer rate constants derived from sweep-rate dependent cyclic voltammetric peak potential separations in 1,2-dichloroethane are small and decrease as pzTp > Tp > Tp for the Co(III)/Co(II) couples. Slow electron transfer is characteristic of coupled electron transfer and spin exchange. M(Tp)(2) redox potentials relative to values for other homoleptic MN(6)(3+/2+) couples change as M varies from Cr to Ni. For early members of the series, M(Tp)(2) potentials nearly equal those of complexes with aliphatic N-donor ligands (e.g., triazacyclononane, sarcophagine). However, M(Tp)(2) potentials approach those of M(bpy)(3) for later members of the series. The variation suggests a change in the nature of the metal-pzb interaction upon crossing the first transition row.