RADICAL SUPEREXCHANGE IN SEMIQUINONE COMPLEXES CONTAINING DIAMAGNETIC METAL-IONS - 3,6-DI-TERT-BUTYL-1,2-SEMIQUINONATE COMPLEXES OF ZINC(II), COBALT(III), GALLIUM(III), AND ALUMINUM(III)

The tris(3,6-di-tert-butylsemiquinonato)gallium(III), -aluminum(III), and -cobalt(III) complexes have been studied structurally, spectrally, and magnetically. Complexes of the three diamagnetic metals have octahedral structures of D3 symmetry (Ga(3,6-DBSQ)3, monoclinic, C2/c, a = 22.069(6) angstrom, b = 19.365(5) angstrom, c = 10.163(3) angstrom, beta = 94.90(2)degrees, V = 4327(2) angstrom3, Z = 4; Co(3,6-DBSQ)3, monoclinic, C2/c, a = 21.799(6) angstrom, b = 19.376(8) angstrom, c = 10.167(3) angstrom, beta = 94.99(3)degrees, V = 4278(3) angstrom3, Z = 4). EPR spectra recorded on Ga(3,6-DBSQ)3 and Al(3,6-DBSQ)3 in a toluene glass at 77 K show signals at g values of 2, 4, and 6, indicative of a S = 3/2 ground state. Variable-temperature magnetic measurements show an increase in magnetic moment below 70 K from 3.0 mu(B), the value expected for three noninteracting S = 1/2 radicals, toward the 3.8 mu(B) value for a S = 3/2 ferromagnetically coupled ground state. This behavior has been fit to a theoretical model with J values of 6.2 and 8.6 cm-1 for the Ga and Al complexes, respectively. EPR spectra recorded on Co(3,6-DBSQ)3 show evidence for a S = 1/2 ground state at temperatures below 77 K and magnetic moment decreases at low temperature consistent with an antiferromagnetically coupled doublet ground state (J = -39.1 cm-1). The zinc(II) complex Zn(tmeda)(3,6-DBSQ)2 has been prepared and fully characterized (Zn(tmeda)(3,6-DBSQ)2, monoclinic, P2(1)/c, a = 17.157(5) angstrom, b = 11.563(3) angstrom, c = 19.864(4) angstrom, beta = 113.26(2)degrees, V = 3620(1) angstrom3, Z = 4). The monomeric complex shows an EPR spectrum at 77 K with a zero-field-split DELTAM(S) = 1 transition and a half-field DELTAM(S) = 2 transition from a S = 1 spin state. Magnetic measurements indicate that the complex has a diamagnetic singlet ground state (J = -33.7 cm-1) and that EPR activity results from thermal population of the triplet state that lies 67.4 cm-1 above the singlet. The difference in the exchange interaction between radical ligands bonded to the Al and Ga metal ions relative to Co and Zn appears related to d-orbital participation in metal-SQ pi bonding. The mixed-charge ligand complex (Ga(tmeda)(3,6-DBSQ)(3,6-DBCat) has been prepared to study the possibility of interligand electron transfer. The complex shows no band that may be assigned as an interligand intervalence-transfer transition, but the EPR spectrum of the complex recorded in solution at 210 K shows a pattern of hyperfine coupling that is consistent with time-averaged quinone ligands.