COPPER(II) AND COBALT(II) COMPLEXES WITH DERIVATIVES OF SALEN AND TETRAHYDROSALEN - AN ELECTRON-SPIN-RESONANCE, MAGNETIC-SUSCEPTIBILITY, AND QUANTUM-CHEMICAL STUDY

The salen complexes ML (M(II) = Cu and Co) and the corresponding tetra hydrosalen complexes M[H-4]L (M(II) = Cu) were investigated by ESR spe ctroscopy, by magnetic susceptibility, and by quantum chemical study ( L(2-) and [H-4]L(2-) are anions of the following: (H(2)L(1) = ethylsal icylidene)-2,3-diamino-2,3-dimethylbutane; H-2[H-4]L(1), N,N'-bis(2-hy dtoxy-3-tert-butyl-5-methylbenzyl) 2,3-dimethylbutane; H(2)L(2) = N,N' -bis(3-tert-butyl-5-chlorosalicylidene) H-2 [H-4]L(2) = N,N'-bis(2-hyd roxy-3-tert-butyl-5-chlorobenzyl)-2, 3-diamino-2, 3-dimethylbutane). T he ESR spectra of Cu(II) complexes in frozen (100 K) toluene solution exhibit a well-resolved perpendicular part. In addition to hyperfine s tructure, superhyperfine fines are also seen. The superhyperfine struc ture in the perpendicular region for both CuL(1) and CuL(2) complexes could be well accounted for by the interaction of two equivalent proto ns along with the two nitrogen nuclei. The protons here belong to the carbon atoms adjacent to the nitrogen nuclei. In the presence of pyrid ine (5% v/v) there is a considerable shift in both g(parallel to) and g perpendicular to values. The higher g(parallel to) values compared w ith those of the parent complexes are consistent with the square pyram idal geometry implying axial (py) coordination. An almost negligible e ffect of an electron withdrawing substituent (X(5) = Cl) On spin Hamil tonian parameters was observed. Due to the aggregation of molecules, n o resolved spectrum could be obtained from frozen toluene solutions of both CoL(1) and CoL(2) complexes. The addition of an axial pyridine l eads to a better resolution of the spectra. In the presence of dioxyge n and pyridine (5% v/v) the frozen (100 K) toluene solution of both Co L(1) and CoL(2) 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 pyramid al cobalt core with dioxygen. The interaction of the complexes CoL(py) with molecular oxygen leads to a spin-spin pairing process which resu lts in a partial ligand to metal charge transfer and a large spin dens ity on the oxygen moiety. CoL complexes are low-spin d(7) systems with , mu(eff) = 2.49 mu(B) for CoL(1). The copper complexes Cut and Cu[H-4 ]L are magnetically normal (mu(eff) 1.81 mu(B) for CuL(2)). The calcul ated spin densities show that the unpaired electron is localized on th e molecular orbital of b(2) symmetry which is almost the d(xy), orbita l of the central atom. Only negligible spin density appears at the pyr idine nitrogen atom, which is in agreement with the ESR measurements. The [CoL(1)] system exhibits its unpaired electron at the molecular or bital of al symmetry which is the net d(z)2 metal orbital. The INDO/2 method yields the description of the dioxygen adduct which matches wel l with the generally accepted MO model. The QR-INDO/1 failed in the pr ediction of the spin pairing process. It prefers either Co(up arrow... O-2(up down arrow) or Co(down arrow)) ...O-2(up arrow up arrow) types of interaction. This may be due to an improper balance of the resonanc e and exchange contributions to the magnetic coupling.