Cobalt(II) Terpyridin-4 '-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material

Spin-crossover (SCO) was studied in [Co(L)(2)](CF3SO3)(2), where L stands for diamagnetic 2,2':6',2 ''-terpyridine (tpy) and its paramagnetic derivative, 4-{4-tert-butyl(N-oxy)aminophenyl}-substituted tpy (tpyphNO). The X-ray crystallographic analysis clarified the Co-N bond length change (d) in high- and low-temperature structures; Delta d(central) = 0.12 and Delta d(distal) = 0.05 angstrom between 90 and 400 K for L = tpy and Delta d(central) = 0.11 and Delta d(distal) = 0.06 angstrom between 90 and 300 K for L = tpyphNO. The low- and high-temperature structures can be assigned to approximate low- and high-spin states, respectively. The magnetic susceptibility measurements revealed that the chi T-m value of [Co(tpyphNO)(2)](CF3SO3)(2) had a bias from that of [Co(tpy)(2)](CF3SO3)(2) by the contribution of the two radical spins. The tpy compound showed a gradual SCO around 260 K and on cooling the chi T-m value displayed a plateau down to 2 K. On the other hand, the tpyphNO compound showed a relatively abrupt SCO at ca. 140 K together with a second decrease of the chi T-m value on further cooling below ca. 20 K. From the second decrease, Co-nitroxide exchange coupling was characterized as antiferromagnetic with 2J(Co-rad)/k(B) = -3.00(6) K in the spin-Hamiltonian H = -2J(Co-rad)(S-Co center dot S-rad1 + S-Co center dot S-rad2). The magnetic moment apparently switches double-stepwise as 1 mu(B)reversible arrow 3 mu(B)reversible arrow 5 mu(B)reversible arrow by temperature stimulas.