Half-sandwich aminocarbyne complexes of chromium

Abstract

Efficient methods are described for the synthesis of half-sandwich chromium aminocarbyne complexes starting from trans-X(CO) 4CrCN iPr 2 ( 1a: X  Cl; 1b: X  Br). Complexes 1a and 1b are obtained from Cr(CO) 6 in two steps. The first step involves a nucleophilic addition of LiN iPr 2 to Cr(CO) 6 to give the carboxamido complex Li[(CO) 5Cr{C(O)N iPr 2}], followed by reaction with the Lewis acids ClC(O)C(O)Cl and BrC(O)C(O)Br, respectively. Thermal decarbonylation of 1a or 1b with γ-picoline (4-methylpyridine) results in the quantitative formation of X(CO) 2(pic) 2CrCN iPr 2 ( 2a: X  Cl; 2b: X  Br). Complex 2b reacts with NaCp and KTp′ (Tp′= hydridotris(3,5-dimethylpyrazol-1-yl)borate) to afford Cp(CO) 2CrCN iPr 2 ( 3) and Tp′(CO) 2CrCN iPr 2 ( 4), respectively. Similarly, when 2b is treated with KCp★ (Cp★  C 5 Me 5), the half-sandwich aminocarbyne complex Cp★(CO) 2CrCN iPr 2 ( 5) is obtained. Thermal decarbonylation of 1b with tBuNC in refluxing CH 2Cl 2 leads exclusively to the cationic aminocarbyne complex [( tBuNC) 4(CO)CrCN iPr 2] Br ( 6). Treatment of 6 with NaCp and KTp′ results in the formation of Cp(CO)( tBuNC)CrCN iPr 2 ( 7) and Tp′(CO) ( tBuNC)CrCN iPr 2 ( 8), respectively. In both reactions a minor product, [( tBuNC) 5CrCN iPr 2]Br, is formed by a carbonyl substitution reaction of 6 with the released tBuNC. Complex 6 reacts with KCp★ to give a mixture of the half-sandwich aminocarbyne complex Cp★(CO)( tBuNC)CrCN iPr 2 ( 9) and the Cr 0 isocyanide isomers cis-and trans- Cr(CO)(CN iPr)(CN tBu) 4 ( 10a,b), the latter probably originating from an electron-transfer or a proton-abstraction reaction of 6 with KCp★. Formation of 10a/b is avoided when a more electron-rich (less acidic) aminocarbyne complex, such as Br(CO)( tBuNC) 3Cr CN iPr 2 ( 11), is treated with KCp★.