Monitoring the Substrate‐Induced Spin‐State Distribution in a Cobalt(II)‐Salen Complex by EPR and DFT

Abstract Ground state changes of (R,R ’ )‐N,N ’ ‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexane‐diamino Co(II), following coordination of various pyridyl substrate has been examined by CW EPR, pulsed relaxation measurements and DFT. The solution‐based Co(II) complex possesses a low spin (LS) state (with g ‐values of 1.96, 1.895, 3.14). Upon coordination of the pyridyl substrate, the resulting bound adduct reveals a distribution of LS ‘base‐on’ species, possessing a electronic ground state (with g ‐values of 2.008, 2.2145, 2.46) and a high spin (HS) species (with g eff = 4.6). DFT indicated that the energy gap between the LS and HS state is dramatically lowered (ΔE<25 kJ mol −1 ) following substrate coordination. DFT suggests the main geometrical difference between the LS and HS systems is the severe puckering of the N 2 O 2 ligand backbone. The results revealed a tentative dependency on the pKa−H of the substrates for the spin distribution where, in most cases, the higher pKa−H substrate values favoured the HS species.