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]^+/0, 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]^+/0 redox potentials relative to values for other homoleptic MN₆^3+/2+ couples change as M varies from Cr to Ni. For early members of the series, [M(Tp)₂]^+/0 potentials nearly equal those of complexes with aliphatic N-donor ligands (e.g., triazacyclononane, sarcophagine). However, [M(Tp)₂]^+/0 potentials approach those of [M(bpy)₃]^3+/2+ 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.