Reduced <i>N</i>-Alkyl Substituted Bis(imino)pyridine Cobalt Complexes: Molecular and Electronic Structures for Compounds Varying by Three Oxidation States

The stepwise 1−3 electron reduction of the N-alkyl substituted bis(imino)pyridine cobalt dichloride complexes, (RAPDI)CoCl2, was studied where RAPDI = 2,6-(RNCMe)2C5H3N, R = C6H11 (Cy), CHMe2 (iPr). One electron reduction with either zinc metal or NaBEt3H furnished the bis(imino)pyridine cobalt monochloride compounds, (RAPDI)CoCl. X-ray diffraction on the (iPrAPDI)CoCl derivative established a distortion from square planar geometry where the chloride ligand is lifted out of the idealized cobalt-chelate plane. Superconducting Quantum Interference Device (SQUID) magnetometry on both compounds established spin crossover behavior with an S = 1 state being predominant at room temperature. Computational studies, in combination with experimental results, establish that the triplet spin isomer arises from a high spin Co(II) center (SCo = 3/2) antiferromagnetically coupled to a bis(imino)pyridine chelate radical anion, [PDI]− (SPDI = 1/2). At lower temperatures, the Co(II) ion undergoes a spin transition to the low spin form (SCo = 1/2) and antiferromagnetic coupling gives rise to the observed diamagnetic ground state. Replacing the chloride ligand with a methyl group, namely (RAPDI)CoCH3, also yielded distorted compounds, albeit less pronounced, that are diamagnetic at room temperature. Two electron reduction of the (RAPDI)CoCl2 derivatives with excess 0.5% sodium amalgam or 2 equiv of NaBEt3H furnished the bis(chelate)cobalt complexes, (RAPDI)2Co, while three electron reduction with 3 equiv of sodium naphthalenide yielded the cobalt dinitrogen anions, [Na(solv)3][(RAPDI)CoN2] (solv = THF, Et2O). Both bis(chelate) compounds were crystallographically characterized and determined to have S = 3/2 ground states by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. Computational studies, in combination with metrical parameters determined from X-ray diffraction, establish a high spin (SCo = 3/2) cobalt(II) center with two bis(imino)pyridine chelate radical anions. Antiferromagnetic coupling between the two chelate centered radicals is mediated by a doubly occupied t2g cobalt orbital and gives rise to the observed overall quartet ground state.