Solid-state high-resolution NMR studies on spin fluctuation associated with valence tautomerism of metal–benzoquinone system: Cobalt complex in the stable and meta-stable phases

Equilibrium between low-spin [CoIII(SQ)(Cat)(N–N)] and high-spin [CoII(SQ)2(N–N)] redox isomers, where SQ is semiquinonate (charge: −1, spin: 1/2), Cat is catecholate (charge: −2, spin: 0) and N–N is chelating nitrogen donor ligand, respectively, is a representative valence tautomeric phenomenon. To elucidate independently the spin state of the cobalt ion and that of benzoquinone-derived ligands in the solid state, we measured 13C MAS NMR spectrum of 3,5-di-t-butyl-1,2-benzoquinone and 2H MAS NMR spectrum of deuterated 2,2′-bipyridine for [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2′-bipyridine)] · x(C6H5CH3) and its deuterated analogue in a temperature range of 200–350 K. Irreversible change of an effective magnetic moment μeff of a virgin sample was observed around 370 K due to a partial loss of crystal solvent and a change of crystal structure, whereas the sample annealed at 390 K showed a crystal structure different from the reported one and a reversible change of μeff, which is ascribed to equilibrium between Co(III)-form (S = 1/2) and Co(II)-form (S = 3/2). Based on the shifts and the number of NMR peaks for the annealed sample, we concluded that (1) interconversion between redox isomers occurs faster than NMR time scale (>104 s−1) in the solid state, (2) intraconversion between SQ and Cat in Co(III)-form also occurs much faster than 5 × 104 s−1 even at 198 K and (3) electron spins on SQ ligands in Co(II)-form are quenched probably due to a strong antiferromagnetic coupling between the two SQ ligands. The enthalpy and the entropy of the interconversion were estimated to be 17 kJ/mol and 54 J/(K mol), respectively. For the virgin metastable phase, SQ and Cat were clearly distinguished by 13C MAS NMR spectrum. The solid-state high-resolution NMR spectrum is useful to detect independently the change of spin states of benzoquinone-derived radical and metal ion.

Spin fluctuation associated with valence tautomrtic interconversion was characterized for [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2′-bipyridine)] · x(C6H5CH3) in its stable and meta-stable phases by solid-state high-resolution 2H and 13C NMR spectrum. Change of the spin state of the organic radical ligands and of the cobalt ion was detected independently by 13C MAS NMR spectrum of catecholate and semiquinoate ligand, and by 2H MAS NMR spectrum of deuterated bpy-d18 ligand, respectively.Figure optionsDownload as PowerPoint slide