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 RN᎐ ᎐ CH᎐CH᎐ ᎐ NR = R᎐DAB (R = alkyl, aryl) and the organoplatinum fragments PtMe 2 , PtMe 4 and PtMes 2 (Mes = mesityl) can be reversibly reduced to paramagnetic compounds, formulated as platinum(II) or (IV) species bound by radical anion ligands (R-DAB Ϫ ). 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 1 H(CH) and large 14 N and 195 Pt 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)PtMe 2 ] Ϫ → [(R-DAB)PtMe 4 ] Ϫ could be monitored by EPR spectroscopy.

Influence of the variation of the co-ligands on the electronic transitions and emission properties of [Pt(I)(CH3)3(iPr-DAB)], [Pt(CH3)4(α-diimine)] and [Pt(SnPh3)2(CH3)2(iPr-DAB)]: an experimental and theoretical study

“…1 These investigations included the reactions of these complexes with acids, 2 methyl group transfer reactions and reductive eliminations, 3 and, most importantly for this study, their photoreactivity. [4][5][6][7][8] This photosensitivity had already been mentioned in ref. 1, and was later shown to involve a homolytic Pt-C ax bond splitting reaction from a triplet excited state.…”

The Organometallic Chemistry of Rh‐, Ir‐, Pd‐, and Pt‐Based Radicals: Higher Valent Species

“…The g-value of 2.0060 is clearly influenced by spin-orbit coupling (platinum influence). Both the g value and a 195 Pt coupling suggest that the spin trap has intercepted the organometallic product of the photopromoted homolytic PtÀ ÀC bond splitting reaction (123). Apparently, the spin trap is less effective to capture the methyl radical.…”

“…EPR Data Reported for Cationic [Ir(Cp*)Me(PR 3 )(NO)] .þ Species Obtained by Chemical Reduction of Their Dicationic Precursorsof a metal-centered d orbital in case of the Pt II complexes and for Pt IV it is a PtÀ ÀC ax s orbital with some contribution of a platinum p x orbital(123)(124)(125). Both the Pt II Me 2 and the Pt IV Me 4 complexes show irreversible oxidation waves in their cyclic voltammograms at 0.4 V versus Fc/Fc þ .…”

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

“…Platinum(II) and -(IV) complexes with nitrogen-donor-containing ligands have found interest as chemotherapeutic agents (cytostatica), as catalysts, and as photoemissive 3 or redox-active species . The latter often involve π-accepting chelate ligands of the α-diimine type such as aromatic 2,2‘-bipyridine 3i,4c,f or 1,10-phenanthrolines 4g.…”

“…Platinum(II) and -(IV) complexes with nitrogen-donor-containing ligands have found interest as chemotherapeutic agents (cytostatica), as catalysts, and as photoemissive 3 or redox-active species . The latter often involve π-accepting chelate ligands of the α-diimine type such as aromatic 2,2‘-bipyridine 3i,4c,f or 1,10-phenanthrolines 4g. Detailed photophysical studies, calculations, 3d,4j,k structure determinations, 3c,d,4c,e and (spectro)electrochemical and EPR studies of generated paramagnetic species 4 have been used recently to elucidate the nature of these systems (L)PtX n in ground, reduced, oxidized, or low-lying excited states.…”