Ansa [1]Trochrocenophanes and Their Related Unstrained 1,1‘-Disubstituted Counterparts:  Synthesis and Electronic Structure

The heteroleptic sandwich complex [Cr(η⁵-C₅H₅)(η⁷-C₇H₇)] (trochrocene) was prepared by subsequent treatment of CrCl₃ with NaCp and Mg in the presence of cycloheptatriene in yields of 40%. Selective dimetalation employing ^tBuLi/tmeda (N, N, N‘, N‘-tetramethylethylenediamine) afforded the highly reactive species [Cr(η⁵-C₅H₄Li)(η⁷-C₇H₆Li)]·tmeda. An X-ray crystal-structure determination of its thf solvate revealed a symmetrical, dimeric composition in the solid state, that is, a formula of [Cr(η⁵-C₅H₄Li)(η⁷-C₇H₆Li)]₂·(thf)₈, where the C₅H₄ moieties of both units are connected by two bridging lithium atoms. Addition of different element dihalides to the dilithio precursor facilitated the isolation of ansa complexes with boron and germanium in the bridging position. Structural characterization by X-ray diffraction studies on [Cr(η⁵-C₅H₄)-BN(SiMe₃)₂-(η⁷-C₇H₆)] and [Cr(η⁵-C₅H₄)-GeMe₂-(η⁷-C₇H₆)] emphasized the strained character with tilt angles of 23.87(13)° and 15.07(17)°, respectively. In contrast, the isolation of the appropriate [1]stannatrochrocenophane failed because of the thermal lability of the resulting product. However, the corresponding 1,1‘-disubstitued derivatives [Cr(η⁵-C₅H₄R)(η⁷-C₇H₆R)] (R = B(Cl)NⁱPr₂, SiMe₃, GeMe₃, SnMe₃) were obtained by reverse addition of the dilithio precursor to an excess of the element (di)halide. The unstrained nature was proven by a crystal structure analysis of the 1,1‘-diborylated species. The electronic structure of these substituted trochrocene derivatives, as well as of the [2]bora and [n]sila congeners (n = 1, 2), was investigated by means of UV−vis spectroscopy and DFT methods. As a consequence of the strong electronic influence of the B−N π-system on the LUMOs, the UV−vis studies revealed a complementary correlation of the lowest energy band maxima as a function of molecular distortion for the boron containing species on the one hand, and the boron-free compounds on the other hand. These trends were reproduced fairly well by time dependent DFT calculations.