Bent and Linear Uranium(IV) Metallocenes with Terminal and Bridging Cyanide Ligands

Abstract

Treatment of Cp*2UI2 with KCN in thf led to the formation of Cp*2U(CN)2 (2), which further reacted with NR4CN to give [Cp*2U(CN)3][NR4] (R = Et, 3; R = nBu, 3‘) and [Cp*2U(CN)5][NR4]3 (R = Et, 4; R = nBu, 4‘). While the triscyanide 3‘ adopts the familiar bent sandwich configuration, the pentacyanide 4 is, after the [Cp*2U(NCMe)5]2+ cation, the second example of a linear metallocene resulting from complete saturation of the equatorial girdle. Compound 3‘ was also obtained by oxidation of the trivalent compound [Cp*2U(CN)3][NnBu4]2; the rapid and reversible electron transfer between the U(III) and U(IV) complexes was revealed by 1H NMR spectroscopy. The NMR spectra also revealed that 4 is partially dissociated in thf into 3, providing the first example of an equilibrating couple of bent and linear metallocenes [K = 4.24(4) × 10-5 at 25 °C, ΔH = 199(6) kJ mol-1, and ΔS = 586(20) J mol-1 K-1]. The trinuclear compound [Cp*2UCl2(μ-CN)]2Mg(thf)4 (1) and the 2D polymeric complex [Cp*2U(dmf)3(μ-NC)2(AgI)2]n (5), which were obtained during initial attempts on the synthesis of 2−4 and uranium(V) derivatives, exhibit a bent and linear sandwich structure, respectively.