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 PtMe 2, PtMe 4, 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 π* orbitals of the R-DAB ligands, either from metal d orbitals (Pt II ) or from metal−carbon σ bond combinations (Pt IV ). These spectral assignments are supported by DFT calculations on model complexes between HN CHCH NH and PtMe 2 or PtMe 4 . The calculations also reproduce the structural results for the complex between CyN CHCH NCy and PtMe 4, 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 PtMe 2 and PtMe 4 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(IV) complexes are photoreactive and undergo metal-to-ligand methyl transfer reactions in agreement with the structurally confirmed weaker bonding to the axial methyl groups.