Solvent and Temperature Dependence of 59Co NMR Chemical Shifts of Tris(acetylacetonato)cobaIt(III) and Tris(dipivaloylmethanato)cobalt(III)

Abstract

Cobalt-59 NMR chemical shifts of Co(acac)3, and Co(dpm)3 (acac− = acetylacetonate ion and dpm− = dipivaloylmethanate ion) in 14 organic solvents, C6H14, C6H6, CH2Cl2, CHCl3, CCl4, CH3CN, CH3OH, C2H5OH, CH3CH(OH)CH3, (C2H5)2O, (CH3)2CO, (CH3)2SO, (CH3)2NCHO and C6H5NO2, were measured at five temperatures ranging from 289 to 329 K. The observed chemical shift (δobs) was linearly correlated to the maximum absorption wavelength in the visible spectra (λmax) corresponding to the d-d electronic transition energy between the ground 1A1g and excited 1T1g states. The δobs-λmax relation was explained by the ligand field theory. The temperature coefficients of δobs, of each complex showed a negative correlation with δobs. The δobs, of Co(acac)3 decreased with the increasing electrophilic ability of the solvent (Mayer's acceptor number), whereas no tendency was observed in the case of Co(dpm)3.