EPR study of electron transfer and group transfer in organoplatinum(II) and (IV) compounds†

Abstract

Chelate complexes between the 1,4-diazabuta-1,3-diene ligands RNCH–CHNR = R–DAB (R = alkyl, aryl) and the organoplatinum fragments PtMe₂, PtMe₄ and PtMes₂ (Mes = mesityl) can be reversibly reduced to paramagnetic compounds, formulated as platinum(II) or (IV) species bound by radical anion ligands (R-DAB^˙–). EPR studies in fluid and frozen solution support this assignment; however, the metal contribution to the singly occupied MO is higher for the paramagnetic Pt^II species than for the Pt^IV systems. Comparison with related radical complexes of the main group and transition element series reveals that even the organoplatinum(IV) compounds exhibit a relatively high degree of ligand-to-metal spin transfer as evident from small ¹H(CH) and large ¹⁴N and ¹⁹⁵Pt EPR hyperfine coupling constants. The tetramethylplatinum(IV) complexes are photoreactive; a platinum-containing primary dissociation product from platinum–carbon σ bond homolysis has been detected by EPR spectroscopy using Bu^t-NO as a spin trap reagent during irradiation. Group transfer reactivity has also been noted for radical anions where the conversion [(R-DAB)PtMe₂]^˙– → [(R-DAB)PtMe₄]^˙– could be monitored by EPR spectroscopy.