Reduced N -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)CoCl ₂, 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 (S_Co = 3/2) antiferromagnetically coupled to a bis(imino)pyridine chelate radical anion, [PDI]^? (S_PDI = 1/2). At lower temperatures, the Co(II) ion undergoes a spin transition to the low spin form (S_Co = 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 (S_Co = 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 t_2g cobalt orbital and gives rise to the observed overall quartet ground state.