Diphosphine-induced thiolate-bridge scission of [Re(CO)3(μ,κ2-S,N-thpymS)]2 (thpymS = 1,4,5,6-tetrahydropyrimidine-2-thiolate): Structural and computational studies of configurational isomers of [Re(CO)3(κ2-S,N-thpymS)]2(μ,κ1,κ1-dppe)

Abstract

Reactions of binuclear [Re(CO)3(μ,κ2-S,N-thpymS)]2 (1) with diphosphines have been investigated. At 298 K, dppm reacts to give mononuclear [Re(CO)3(κ1-dppm)(κ2-S,N-thpymS)] (2) through a phosphine-promoted scission of the dithiolate bridges that leaves one of the phosphine moieties free (dangling). Refluxing 2 in toluene leads to CO loss and formation of dinuclear [Re2(CO)4(μ-dppm)(μ,κ2-S,N-thpymS)2] (3) whose rhenium centers are bridged by two thiolate groups and the dppm ligand. Treatment of 1 with dppe at room temperature furnishes [Re(CO)3(κ2-S,N-thpymS)]2(μ,κ1,κ1-dppe) (4) where each phosphine center ligates the respective d6-ML5 rhenium fragment. Complex 4 exists as two distinct configurational isomers (4a and 4b) that have been isolated and the solid-state structures characterized crystallographically. The principal difference in the stereoisomeric products is the orientation of the two [Re(CO)3(κ2-S,N-thpymS)] moieties at the anti-staggered Newman projection involving the P-C-C-P backbone of the dppe ligand. Both stereoisomers retain their identity in solution at ambient temperatures but equilibrate to a 1:1 mixture upon heating at 363 K for 1 h. The reaction of 1 with dppe in toluene at 383 K affords [Re(CO)2(κ1-dppe)2(κ2-S,N-thpymS)] (5) containing two monodentate (dangling) diphosphine ligands. Thus, these seemingly simple reactions afford a range of different products whose composition is highly dependent upon the experimental conditions employed and the nature of the diphosphine backbone. The reaction of 1 with dppe and the process responsible for the equilibration of the two configurational isomers of 4 have been investigated by electronic structure calculations.