Dilithiation of Bis(benzene)molybdenum and Subsequent Isolation of a Molybdenum-Containing Paracyclophane

The homoleptic sandwich complex bis(benzene)molybdenum, [Mo(η6-C6H6)2], was successfully
dilithiated by employing an excess of BuLi in the presence of N,N,N‘,N‘-tetramethylethylenediamine (up to
6 equiv each) at slightly elevated temperatures furnishing the highly reactive, ring metalated species [Mo(η6-C6H5Li)2]·tmeda in high yields. Alternatively, this compound was synthesized upon prolonged sonication
with 5 equiv of tBuLi/tmeda without heating. An X-ray crystal structure determination revealed a symmetrical,
dimeric composition in the solid state, i.e., a formula of [Mo(η6-C6H5Li)2]2·(thf)6, where the six-membered
rings are connected by two pairs of bridging lithium atoms. The synthesis of an elusive ansa-bridged complex
failed in the case of a [1]bora and a [1]sila bridge due to the thermal lability of the resulting compounds.
Instead, reverse addition of the dilithio precursor to an excess of the appropriate element dihalide facilitated
the isolation of several unstrained, 1,1‘-disubstituted derivatives, namely, [Mo{η6-C6H5(BN(SiMe3)2X)}2] (X
= Cl, Br) and [Mo{η6-C6H5(SiiPr2Cl)}2], respectively. However, the incorporation of a less congesting [2]sila bridge was accomplished. In addition to the formation of [Mo{(η6-C6H5)2Si2Me4}], a molybdenum-containing paracylophane complex was isolated and characterized by means of crystal structure analysis.
The ancillary formation of 1 equiv of bis(benzene)molybdenum strongly suggests that this species is
generated by deprotonation of the ansa-bridged complex by the dilithiated precursor and subsequent reaction
with a second equivalent of the disilane.