PERIODIC TRENDS IN CHARGE-DISTRIBUTION FOR TRANSITION-METAL COMPLEXES CONTAINING CATECHOLATE AND SEMIQUINONE LIGANDS - METAL-MEDIATED SPIN COUPLING IN THE BIS(3,5-DI-TERT-BUTYL-1,2-SEMIQUINONE) COMPLEXES OF PALLADIUM(II) AND PLATINUM(II), M(DBSQ)2 (M = PD, PT)

Reactions carried out with M2(DBA)3 (M = Pd, Pt; DBA = dibenzylideneacetone) and 3,5-di-tert-butyl-1,2-benzoquinone gave as major products the M(DBSQ)2 complexes. Crystallographic characterization on both complexes showed that they are monomeric, with square planar geometries for the metals and metrical values that reflect the semiquinone character of the ligands and the divalent charges of the metal ions. Both complexes appear diamagnetic in magnetic susceptibility measurements. However, paramagnetic broadening and shifting of tert-butyl H-1-NMR resonances of Pd(DBSQ)2 shows the effect of residual, low-level paramagnetism. Resonances of Pt(DBSQ)2 are sharp indicating stronger radical-radical exchange through the Pt ion and complete diamagnetism for the complex. Electrochemical characterization shows that both complexes undergo two reversible reductions and one irreversible oxidation. A minor product obtained from the synthesis of Pd(DBSQ)2 was characterized structurally and found to be Pd2[Pd(DBSQ)2]2. The complex molecule consists of two planar cis-Pd(DBSQ)2 units bridged by two Pd atoms pi-bonded to allylic regions of the DBSQ ligands. A minor product was also obtained from the synthesis of Pt(DBSQ)2. This complex is paramagnetic and EPR active, giving a spectrum with hyperfine coupling that may be described as a single DBSQ ligand coupled to a diamagnetic Pt atom. Otherwise, the form of this product remains unclear. The localized charge distribution of Pd(DBSQ)2 and Pt(DBSQ)2 is quite different from the delocalized electronic structures of the diimine and 1,2-dithiolene analogs studied earlier. Differences in magnetic coupling illustrate the metal dependence of the SQ-SQ exchange interaction through the two metals. The complexes illustrate the shift in charge distribution for transition metal quinone complexes across the second and third transition series of the periodic table.