HALF-SANDWICH AMINOCARBYNE COMPLEXES OF CHROMIUM

Efficient methods are described for the synthesis of half-sandwich chr omium aminocarbyne complexes starting from transX(C0)(4)Cr=(CNPr2)-Pr- i (1a: X = Cl; 1b: X = Br). Complexes 1a and 1b are obtained from Cr(C O)(6) in two steps. The first step involves a nucleophilic addition of (LiNPr2)-Pr-i to Cr(CO)(6) to give the carboxamido complex Li[(CO)(5) Cr{C(O)(NPr2)-Pr-i}], followed by reaction with the Lewis acids ClC(O) C(O)Cl and BrC(O)C(O)Br, respectively. Thermal decarbonylation of 1a o r 1b with gamma-picoline (4-methyl-pyridine) results in the quantitati ve formation of X(CO)(2)(pic)(2)Cr=(CNPr2)-Pr-i (2a: X = Cl; 2b: X = B r). Complex 2b reacts with NaCp and KTp' (Tp' =hydridotris(3,5-dimethy lpyrazol-1-yl)borate) to afford Cp(CO)(2)Cr=(CNPr2)-Pr-i (3) and Tp'(C O)(2)Cr=(CNPr2)-Pr-i (4), respectively. Similarly, when 2b is treated with KCp (Cp* = C(5)Me(5)), the half-sandwich aminocarbyne complex Cp (CO)(2)Cr=-(CNPr2)-Pr-i (5) is obtained. Thermal decarbonylation of 1 b with (t)BuNC in refluxing CH2Cl2 leads exclusively to the cationic a minocarbyne complex [((t)BuNC)(4)(CO)Cr=(CNPr2)-Pr-i]Br (6). Treatment of 6 with NaCp and KTp' results in the formation of Cp(Co)((t)BuNC)Cr =(CNPr2)-Pr-i (7) and Tp'(CO)((t)BuNC)Cr=(CNPr2)-Pr-i (8), respectivel y. In both reactions a minor product, [((t)BuNC)(5)Cr=(CNPr2)-Pr-i]Br, is formed by a carbonyl substitution reaction of 6 with the released (t)BuNC. Complex 6 reacts with KCp to give a mixture of the half-sand wich aminocarbyne complex Cp(CO)((t)BuNC)Cr=(CNPr2)-Pr-i (9) and the Cr-0 isocyanide isomers cis-and trans-Cr(CO)((CNPr)-Pr-i)(CN(t)Bu)(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 av oided when a more electron-rich (less acidic) aminocarbyne complex, su ch as Br(CO)((t)BuNC)(3)Cr=(CNPr2)-Pr-i (11), is treated with KCp.