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(NSi<i>i</i>Pr₃)₂}^2- (Me₂SiN^†N^†). Two equivalents of the dipotassium salt, [Me₂SiN^†N^†K₂] (<b>1</b>), react with either M^IIII₃(THF)_<i>n</i> (M = Ce, Pr, <i>n</i> = 4; M = Tb, <i>n</i> = 3.5) or [U^IV(BH₄)₄]_<i>n</i> affording trivalent [M^III(Me₂SiN^†N^†)₂{K(toluene)₂}]_<i>n</i> (<b>2M</b>; M = Ce, Pr, Tb) and tetravalent [U^IV(Me₂SiN^†N^†)₂] (<b>3U</b>), respectively. Complex <b>2Ce</b> was oxidized to tetravalent [Ce^IV(Me₂SiN^†N^†)₂] (<b>3Ce</b>) with Ag^II, but <b>2Pr</b> and <b>2Tb</b> were unreactive under these conditions. The electrochemical accessibility of M(IV/III) couples in <b>2M</b> (M = Ce, Pr, Tb) and <b>3M</b> (M = Ce, U) was examined by cyclic voltammetry. For <b>2Ce</b> and <b>3Ce</b>, a quasi-reversible Ce(IV/III) couple was observed (<i>E</i>_pc = -1.19 V vs [Fc]^+/0), but complexes <b>2Pr</b> and <b>2Tb</b> showed anodic waves attributed to ligand oxidation only, which is supported by computational analyses. For complex <b>3U</b>, U(IV/III), U(V/IV), and U(VI/V) redox couples with <i>E</i>_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 <b>2Pr</b>, <b>2Tb</b>, and <b>3Ce</b>.