Experimental and Theoretical Investigations of the Redox Behavior of the Heterodichalcogenido Ligands [(EPiPr2)(TePiPr2)N]− (E = S, Se): Cyclic Cations and Acyclic Dichalcogenide Dimers

Abstract

The two-electron oxidation of the lithium salts of the heterodichalcogenidoimidodiphosphinate anions [(EPiPr2)(TePiPr2)N]− (1a, E = S; 1b, E = Se) with iodine yields cyclic cations [(EPiPr2)(TePiPr2)N]+ as their iodide salts [(SPiPr2)(TePiPr2)N]I (2a) and [(SePiPr2)(TePiPr2)N]I (2b). The five-membered rings in 2a and 2b both display an elongated E−Te bond as a consequence of an interaction between tellurium and the iodide anion. One-electron reduction of 2a and 2b with cobaltocene produces the neutral dimers (EPiPr2NPiPr2Te−)2 (3a, E = S; 3b, E = Se), which are connected exclusively through a Te−Te bond. Two-electron reduction of 2a and 2b with 2 equiv of cobaltocene regenerates the corresponding dichalcogenidoimidodiphosphinate anions as ion-separated cobaltocenium salts Cp2Co[(EPiPr2)(TePiPr2)N] (4a, E = S; 4b, E = Se). The ditellurido analogue Cp2Co[(TePiPr2)2N] (4c) has been prepared in the same manner for comparison. Density functional theory calculations reveal that the preferential interaction of the iodide anion with tellurium is determined by the polarization of the lowest unoccupied molecular orbital [σ*(E−Te)] of the cations in 2a and 2b toward tellurium and that the formation of the dimers 3a and 3b with a central Te−Te linkage is energetically more favorable than the structural isomers with either E−Te or E−E bonds. Compounds 2a, 2b, 3a, 3b, 4a, 4b, and 4c have been characterized in solution by multinuclear NMR spectroscopy and in the solid state by X-ray crystallography.