Structure and Properties of a Novel Square-Pyramidal Cluster:  Heptahydridopentadecacarbonylpentarhenate Dianion

Abstract

H 4 Re 3 (CO) 9 -][Re(CO) 3 (DMF) 3 + ] (1) in chloroform solution containing DMF undergoes cluster transformation, yielding [H 7 Re 5 (CO) 15 2-][Re(CO) 3 (DMF) 3 + ] (2) and [H 5 Re 4 (CO) 12 -]-[Re(CO) 3 (DMF) 3 + ] (3). The anionic cluster in 2 is the first example of a 74-e square-pyramidal penta-transition-metal cluster without a non-hydride interstitial atom. The seven hydrides in the anion of 2 bridge all edges of the square-pyramidal pentarhenium cluster except one edge in the base. These hydrides are fluxional with ∆H q) 15.0((0.4) kcal mol -1 and ∆S q ) 3.8((1.6) eu. The yields of 2 and 3 depend on the concentration of DMF. When [DMF] > 1 M, 2 and 3 are not formed. At [DMF] around 0.15 M, the formation of 2 is favored, while at concentration around 0.03 M, 3 is favored. The rate law for the formation of 2 is that rateThe proposed mechanisms for the formation of 2 and 3 are presented. 2 also undergoes cluster transformation in dichloromethane solution, and the pathways are dependent on the concentrations of DMF and 2. At very low concentration (2.1 × 10 -5 M) of 2, it transforms to [H 7 Re 6 (CO) 18 -][Re(CO) 3 (DMF) 3 + ]. In the presence of DMF (1 × 10 -3 ), 2 (2.2 × 10 -5 M) transforms to 1. However, at moderate concentration (4.3 × 10 -3 M) of 2, it transforms to a mixture of 1 and 3. The mechanisms for these transformations are proposed.