Copper(II) and Cobalt(II) Complexes with Derivatives of Salen and Tetrahydrosalen: An Electron Spin Resonance, Magnetic Susceptibility, and Quantum Chemical Study

Abstract

2,3-diamino-2,3-dimethylbutane; H2[I&]L1 = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-2,3-diamino2,3-dimethylbutane; H2L2 = N,~-bis(3-tert-butyl-5-chlorosalicylidene)-2,3-di~no-2,3-dimethylbutane; H2[H4]L2 = N,~-bis(2-hydroxy-3-tert-butyl-5-chlorobenzyl)-2,3-diamino-2,3-dimethylbutane). The ESR spectra of Cu(I1) complexes in frozen (100 K) toluene solution exhibit a well-resolved perpendicular part. In addition to hyperfine structure, superhyperfine lines are also seen. The superhyperfine structure in the perpendicular region for both CuL' and CuL2 complexes could be well accounted for by the interaction of two equivalent protons along with the two nitrogen nuclei. The protons here belong to the carbon atoms adjacent to the nitrogen nuclei. In the presence of pyridine (5% v/v) there is a considerable shift in both gll and gL values. The higher gll values compared with those of the parent complexes are consistent with the square pyramidal geometry implying axial (py) coordination. An almost negligible effect of an electronwithdrawing substituent (X5 = C1) on spin Hamiltonian parameters was observed. Due to the aggregation of molecules, no resolved spectrum could be obtained from frozen toluene solutions of both CoL' and CoL2 complexes. The addition of an axial pyridine leads to a better resolution of the spectra. In the presence of dioxygen and pyridine (5% v/v) the frozen (100 K) toluene solution of both CoL' and CoL2 exhibits rhombic symmetry with well-resolved hyperfine structures in all three directions. The shape of the spectra and spin Hamiltonian parameters indicate the interaction of the square pyramidal cobalt core with dioxygen. The interaction of the complexes CoL(py) with molecular oxygen leads to a spin-spin pairing process which results in a partial ligand to metal charge transfer and a large spin density on the oxygen moiety. CoL complexes are low-spin d7 systems with peff = 2.49,~~ for CoL'. The copper complexes CuL and Cu[&]L are magnetically normal = 1.81,~~ for CuL2). The calculated spin densities show that the unpaired electron is localized on the molecular orbital of b2 symmetry which is almost the dv orbital of the central atom. Only negligible spin density appears at the pyridine nitrogen atom, which is in agreement with the ESR measurements. The [CoL'] system exhibits its unpaired electron at the molecular orbital of a1 symmetry which is the net d,z metal orbital. The IND0/2 method yields the description of the dioxygen adduct which matches well with the generally accepted MO model. The QR-INDO/I failed in the prediction of the spin pairing process. It prefers either Co(t). * -02(?4) or Co(4). -02(tt) types of interaction. This may be due to an improper balance of the resonance and exchange contributions to the magnetic coupling.