[(Tp(tBu,Me))CrR]: A new class of mononuclear, coordinatively unsaturated chromium(II) alkyls with cis-divacant octahedral structure

Reaction of CrCl2 with Tp(tBu, Me)K yielded [Tp(tBu, Me)Cr(3-tBu,5-MepzH)Cl] (1) and [Tp(tBu, Me)CrCl] (2), while the same reaction in the presence of pyridine gave 1 and [Tp(tBu, Me)Cr(py)Cl] (3). Alkylation of 3 with Grignard reagents produced the chromium(rr) alkyls [Tp(tBu, Me)CrR] (4, R = Et; 5, R = Ph; 6, R = CH2SiMe3), which reversibly added to pyridine to form the five-coordinate adducts [Tp(tBu, Me)Cr(py)R] (7, R = Et; 8, R = Ph; 9, R = CH2SiMe3). 1, 2, 4, and 5 were structurally characterized by X-ray diffraction. The four-coordinate molecules 2, 4, and 5 adopt a highly unusual cis-divacant octahedral coordination geometry, while 1 is the first five-coordinate Tp(tBu, Me)-complex of a first-row transition metal. Despite their coordinative unsaturation, chromium alkyls 4-6 do trot polymerize ethylene, or even react with it. This observation is inconsistent with the catalytic activity commonly ascribed to divalent chromium in heterogeneous polymerization catalysts. Attempts to oxidize 4-6 (e.g., with [Cp2Fe]BPh4) to cationic chromium(III) alkyls failed, yielding [Tp(tBu, Me)Cr(thf)][BPh4] (10) instead.