Some Substituent Effects in the Formation of Binuclear Metalladienes and 1,2-Dimetallacycloheptadienones From Reactions Between (η-C₅H₅)₂Rh₂(μ-CO)(CF₃C₂CF₃) and Alkynes

Abstract

The reaction between (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃)(1) and alkynes, RC=CR′, can proceed by two alternative pathways. One gives the complexes (η- C₅H₅)₂Rh₂{C₄(CF₃)₂RR′CO} (2) in which a pentadienone unit bridges the Rh - Rh bond. The other produces the binuclear metalladiene complexes (η-C₅H₅)₂Rh₂{C₄(CF₃)₂RR?} (3) plus the dicarbonyl complex (η-C₅H₅)₂Rh₂(CO)₂(CF₃C₂CF₃) (4). The formation of (2) is strongly favoured with the dialkylacetylenes BuC≡CBu and MeC =CR (R = Et, Prⁱ, Bu^t). With the unsymmetrical acetylenes, two isomers of (2) are isolated. The tendency to form (3) + (4) increases when there are electron-withdrawing substituents such as CN on the alkyne, and becomes dominant when the alkyne is polar (e.g. MeC≡CR with R = Ph, CF₃ or CO₂Me). The reactions with MeC≡CR are highly regioselective, with only one of the two possible regioisomers being isolated in each case. This has the methyl group in the 3-position of the metalladiene ring. Unexpected products formed in the reactions with MeC≡Cet and MeC≡CPrⁱ have been characterized by X-ray crystallography. One is a 1,2- dimetallacycloheptadienone complex, (η-C₅H₅)₂Rh₂{C₄(CF₃)₂Me(CMe₂OH)CO}. The compound crystallizes with 16 molecules in the orthorhombic space group Fdd2 in a unit cell of dimensions a 31.728(15), b 27.923(14), c 9.275(5) Ǻ. The structure was solved by heavy atom methods and refined to R 0.037 based on 2644 observed reflections. The CMe₂(OH) group is adjacent to the carbonyl in the bridging group. The other is a binuclear metalladiene complex (η-C₅H₅)₂Rh₂{C₄(CF₃)₂Me( COMe )}. It crystallizes with eight molecules in the orthorhombic space group P bca in a unit cell of dimensions a 17.349(8), b 15.819(8), c 13.720(7) Ǻ. The structure was solved by heavy atom methods and refined to R 0.053 based on 3045 observed reflections above background. The acyl group is adjacent to the metal in the metalladiene ring. These two complexes are formed from (1) and impurities present in the alkynes.