STERIC CROWDING AND REDOX REACTIVITY IN PLATINUM(II) AND PLATINUM(IV)COMPLEXES CONTAINING SUBSTITUTED 1,10-PHENANTHROLINES

The effect of the phenanthroline substituents on the structure and rea ctivity of platinum(II) and platinum(IV) complexes has been investigat ed. The X-ray crystal structures of the compounds [PtI2(4,7-Ph(2)phen) ]. CHCl3 (1dz . CHCl3), [PtI4(4,7-Ph(2)phen)]. CHCl3 (2dz . CHCl3), [P tI2(2,9-Me(2)-4,7-Ph(2)phen)] (1fz), and [PtI4(2,9-Me(2)-4,7-Ph(2)phen )]. I-2 (2fz . I-2) have shown that complexes 1fz and 2fz, containing ortho-substituted phenanthrolines, exhibit a remarkable displacement o f the equatorial iodine atoms from the N-Pt-N' plane (average 0.477(2) and 0.199(2) Angstrom, respectively), a bending of the phenanthroline [angle between outer rings of 19.9(7) and 14.2(7)degrees, respectivel y] and a rotation of the N-C-C'-N' plane with respect to the N-Pt-N' p lane [32.3(10) and 26.5(9)degrees, respectively]. Comparison between t he structures of 1fz and 2fz, both having the phenanthroline with meth yl substituents in the ortho position, indicates that, in the latter c ase, because of the presence of the two axial iodine ligands, the disp lacements of the ligands from the equatorial plane are smaller and fin d a compensation in a narrowing of the I(1)-Pt-I(1') angle (5 degrees) and a lengthening of the Pt-N bonds (0.07 Angstrom). The electrochemi cal behavior of the four-coordinate platinum(II) complexes shows that compounds possessing regular planar geometry have access to the one-el ectron reduced species, whereas those with distorted coordination geom etry are irreversibly reduced by collapsing of the complex geometry. T his is in sharp contrast with the behavior of related nickel complexes for which the pseudo-tetrahedral coordination imposed by bulky 2,9-su bstituents of phenanthroline stabilizes the nickel(I) species. Spectro scopic results allow us to assign a significant Pt(I) character to [1d ](-) monoanions. The electrogenerated, plus one electron, complexes ar e not indefinitely stable and, because of conjugation with the phen li gand, progressively restore the Pt(II) oxidation state by transferring the electron to the peripheral organic ligand. The latter process can involve multiple electron additions in the macroelectrolysis time sca le. The related platinum(IV) complexes [PtX(4)(L)] undergo irreversibl e two-electron reduction accompanied by fast release of the axial liga nds and formation of the corresponding platinum(II) species.