Spin exchange between transition metal complexes and nitroxyl radicals in nonaqueous media

Abstract

New complexes of diaza- and tetraaza-containing crown ethers, viz., 1,10-diaza-18-crown-6 (1), 1,4,8,12-tetraazacyclopentadecane (2), 1,4,8,11-tetraazacyclotetradecane (3), and 1,4,8,11-tetraazacyclotetradecane 1,4,8,11-tetrachloride tetraacetic acid tetrahydrate (4), with the divalent copper and nickel ions and the Cl–, Br–, ClO4 –, NO3 –, and AcO– counterions were synthesized. The exchange interactions of these compounds and paramagnetic copper and nickel salts with the TEMPO radical in MeOH—CHCl3 binary mixtures of different compositions were studied. The plots of the linewidths of the hyperfine coupling components of TEMPO vs. concentration of the ions and temperature show that the frequency of diffusion collisions is the rate-limiting step for spin exchange (strong exchange regime). A strong dependence of the exchange rate constant (k ex) on the crown ether and counterion structure was found. The isotropic hyperfine coupling constants (a Cu) and g factors (g i ) were measured for the CuII complexes with the crown ethers. In the case of the crown ether complexes 1—3 with CuCl2, the a Cu constant decreases linearly with an increase in Δg i = g i – 2.0023 in the series 3 < 2 < 1, whereas k ex increases linearly in the same series with a decrease in the contact HFC on the CuII nucleus (K) and a decrease in covalence of bonding. For the complexes of 2 with CuII and different axial ligands (counterions), k ex increases in the series Cl– < ClO4 – ≤ AcO– ≤ Br–; < NO3 –. In the case of the complexes of 2 with NiCl2, k ex increases in the series 1 < 4 < 3 ≈ 2. For the CuII and NiII salts with the Cl–, ClO4 –, and NO3 – anions, the k ex values are almost independent of the anion nature. The correlation of the k ex values with the electron-spin parameters of the complexes is discussed.