Reactions of new organoplatinum(II) and -(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 chelate ligands RN=CHCH=NR (R-DAB; R = alkyl, aryl) and the organoplatinum fragments PtMe2, PtMe4, and PtMes(2) (Mes = mesityl) were prepared and characterized with respect to their electronic structures. All compounds are distinguished by low-energy charge-transfer transitions to low-lying pi(*) orbitals of the R-DAB ligands, either from metal d orbitals (Pt-II) or from metal-carbon a bond combinations (Pt-IV). These spectral 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 groups. The distinct solvatochromism of the long-wavelength transitions is described, as are the UV/vis spectroelectrochemical results for reversible reduction to Pt-IV(R-DAB(.-)) or Pt-II(R-DAB(.-)) species (no evidence for a Pt-I state). In contrast, the oxidation is electrochemically irreversible except for the dimesitylplatinum compounds. The electrochemical potentials of corresponding PtMe2 and PtMe4 compounds are very similar, demonstrating that the binding of two additional methyl carbanions compensates for the effect of the higher metal oxidation state. While the organoplatinum(II) species are emissive, the tetramethylplatinum(TV) complexes are photoreactive and undergo metal-to-ligand methyl transfer reactions-in agreement with the structurally confirmed weaker bonding to the axial methyl groups.