Synthesis, Redox Chemistry, and Electronic Structures of Bis-di(silylamido)silane f-Element Complexes

We report the synthesis and characterization of pseudo-tetrahedral f-element complexes bearing the sterically demanding bidentate di(silylamido)silane ligand, {Me₂Si(NSiiPr₃)₂}^2- (Me₂SiN^†N^†). Two equivalents of the dipotassium salt, [Me₂SiN^†N^†K₂] (1), react with either M^IIII₃(THF)_n (M = Ce, Pr, n = 4; M = Tb, n = 3.5) or [U^IV(BH₄)₄]_n affording trivalent [M^III(Me₂SiN^†N^†)₂{K(toluene)₂}]_n (2M; M = Ce, Pr, Tb) and tetravalent [U^IV(Me₂SiN^†N^†)₂] (3U), respectively. Complex 2Ce was oxidized to tetravalent [Ce^IV(Me₂SiN^†N^†)₂] (3Ce) with Ag^II, but 2Pr and 2Tb were unreactive under these conditions. The electrochemical accessibility of M(IV/III) couples in 2M (M = Ce, Pr, Tb) and 3M (M = Ce, U) was examined by cyclic voltammetry. For 2Ce and 3Ce, a quasi-reversible Ce(IV/III) couple was observed (E_pc = -1.19 V vs [Fc]^+/0), but complexes 2Pr and 2Tb showed anodic waves attributed to ligand oxidation only, which is supported by computational analyses. For complex 3U, U(IV/III), U(V/IV), and U(VI/V) redox couples with E_1/2 values of -2.38, -0.89, and -0.11 V vs [Fc]^+/0, respectively, were found. All complexes have been characterized variously by single-crystal X-ray diffraction, multinuclear NMR, UV-vis-NIR, and IR spectroscopies, SQUID magnetometry, and elemental analyses as appropriate. Electronic structure calculations were employed to rationalize electrochemical results for 2Pr, 2Tb, and 3Ce.