Synergic Monodeprotonation of Bis(benzene)chromium by Using Mixed Alkali Metal-Magnesium Amide Bases and Structural Characterization of the Heterotrimetallic Products

摘要

Developed by Elschenbroich in the late 1960s, the reaction of bis(benzene)chromium with TMEDA-activated nBuLi still represents today the standard method for metalating the sandwich complex (TMEDA is N,N,N',N'-tetramethylethylenediamine). Metalation plays a pivotal part in the development of bis(benzene)chromium chemistry, which celebrates its 50th anniversary in 2005, as in transforming the parent neutral compound to a more reactive carbanionic form, it opens up a gateway to a plethora of selectively functionalized derivatives. The original lithiation method, which employs a 5:1 molar excess of base, produces mainly the heteroannular dilithio derivative "[Cr(C_6H_5Li)_2]" in yields of about 50 % at best, as determined indirectly by mass spectrometric studies of D_2O-quenched reaction mixtures. Lithiation of one benzene ring strongly activates the molecule towards a second lithiation (on the other ring), hence the yield of the monolitho derivative "[Cr(C_6H_5Li)(C_6H_6)]" is always much poorer (typically 5-20 %). A lack of crystallographic evidence means that neither lithio reagent has been structurally authenticated, though an analogy with the TMEDA adduct of heteroannular dilithioferrocene [{Fe(C_5H_4)_2}_3Li_6·(TMEDA)_2] suggests both could be molecularly complex. Following our recent report that ferrocene can be regioselectively tetrametalated by the synergic action of mixed sodium/magnesium tris(diisopropylamide), manifested in the sixteen-membered inverse crown complex [{Fe(C_5H_3)_2}Na_4Mg_4(iPr_2N)_8], we posed the question how would such synergic base mixtures behave towards bis(benzene)chromium?. The surprising and contrasting answers to this question when the π-arene complex is subjected to synergic amide mixtures based on either 1,1,1,3,3,3-hexamethyldisilazide (HMDS) or 2,2,6,6-tetramethylpiperidide (TMP) provides the basis for this Communication. Moreover, all of the metal products synthesized in this study have been successfully characterized crystallographically, thus belatedly revealing the first crystal structures of polar metal complexes of bis(benzene)chromium.