REACTIONS OF NEW ORGANOPLATINUM(II) AND ORGANOPLATINUM(IV) COMPLEXES OF 1,4-DIAZA-1,3-BUTADIENES WITH LIGHT AND ELECTRONS - EMISSION VS PHOTOCHEMISTRY AND THE ELECTRONIC-STRUCTURES OF GROUND, REDUCED, OXIDIZED, AND LOW-LYING CHARGE-TRANSFER EXCITED-STATES

Complexes between the 1,4-disubstituted 1,4-diaza-1,3-butadiene chelat e ligands RN=CHCH=NR (R-DAB; R = alkyl, aryl) and the organoplatinum f ragments PtMe2, PtMe4, and PtMes(2) (Mes = mesityl) were prepared and characterized with respect to their electronic structures. All compoun ds are distinguished by low-energy charge-transfer transitions to low- lying pi() orbitals of the R-DAB ligands, either from metal d orbital s (Pt-II) or from metal-carbon a bond combinations (Pt-IV). These spec tral assignments are supported by DFT calculations on model complexes between HN=CHCH=NH and PtMe2 or PtMe4. The calculations also reproduce the structural results for the complex between CyN=CHCH=NCy and PtMe4 , which exhibits significantly longer Pt-C bonds to the axial methyl g roups. The distinct solvatochromism of the long-wavelength transitions is described, as are the UV/vis spectroelectrochemical results for re versible reduction to Pt-IV(R-DAB(.-)) or Pt-II(R-DAB(.-)) species (no evidence for a Pt-I state). In contrast, the oxidation is electrochem ically irreversible except for the dimesitylplatinum compounds. The el ectrochemical potentials of corresponding PtMe2 and PtMe4 compounds ar e very similar, demonstrating that the binding of two additional methy l carbanions compensates for the effect of the higher metal oxidation state. While the organoplatinum(II) species are emissive, the tetramet hylplatinum(TV) complexes are photoreactive and undergo metal-to-ligan d methyl transfer reactions-in agreement with the structurally confirm ed weaker bonding to the axial methyl groups.