Binding of Alkenes to ReS₄⁻

The tetrathioperrhenate anion, ReS 4 - ( 1 ), is shown to form adducts with a variety of alkenes. The alkene adducts form reversibly, and those of norbornene and norbornadiene were isolated. For norbornene, the K eq 's range from 12 000 (CH 2 Cl 2 solution) to 2650 M - 1 (pyridine solution). The following values were determined (MeCN soln): Δ H = −53 kJ mol - 1 and Δ S = −98 J·mol - 1 ·K - 1 . The solid-state structures of adducts derived from norbornene and norbornadiene, Ph 4 P[ReS 2 (S 2 C 7 H 10 )] ( 2 ) and (Ph 4 P) 2 [(ReS 2 ) 2 (S 4 C 7 H 8 )], respectively, reveal tetrahedral Re centers wherein the Re−SR bonds are elongated by 0.1 Å compared to those in 1 . Additionally, the RS−Re−SR angle is contracted to 86.7°, reflecting the constraint of the chelate backbone. Electrochemical measurements show that the reduction of 2 induces dissociation of norbornene from ReS 4 2 - . Ethylene binds to 1 only weakly at room temperature, but this adduct is directly observable by low-temperature UV/vis spectroscopy. The adduct of dimethyl maleate and 1 could only be observed in the neat alkene; 1 does not catalyze the isomerization of dimethyl maleate to dimethyl fumarate.