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

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 yz,2A2⟩ (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 z2,2A1⟩ 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 (Delta E < 25 kJmol(-1)) following substrate coordination. DFT suggests the main geometrical difference between the LS and HS systems is the severe puckering of the N2O2 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.

Automated summary

This study investigates the ground state changes of (R,R-')-N,N-'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino Co(II) complex upon coordination of different pyridyl substrates. The results show a tentative dependency on the pKa-H of the substrates for the spin distribution in the resulting adducts.