Abstract
The Rh(I) complex [Rh(CO)(PPh 3 )(mnt)] - (mnt = maleonitriledithiolate) reacts with alkyl halides R-X (X = I, R = Me, Et, n-Pr, n-Bu, n-C 10 H 21 ; X = Br, R = Bz) to form a series of stable Rh(III) acyl anions. The complexes have been characterized by IR and 1 H NMR spectroscopy, and the structure of a representative member of the series (R = Et) has been determined by single-crystal x-ray diffraction. In all of the complexes (except for R = Me) the α-methylene protons of the acyl ligand are magnetically nonequivalent indicating that the acyl group is bonded to a chiral center which is preserved on the NMR time scale. The structure determination of (AsPh 4 ) [RhI(COEt)(PPh 3 )(mnt)] shows the complex to crystallize in space group P2 1 /a (C 2h 5 ) in a cell of dimensions a = 18.466 (6), b = 13.608 (5), c = 19.088 (8) Å, β = 106.39 (1)° with four molecules per unit cell (ρ exptl = 1-54 (2), ρ calcd = 1.558 g/cm 3 ). The Rh(III) acyl anion adopts a square pyramidal structure with the acyl group occupying the apical position at a distance of 2.006 (14) Å from the Rh(III) ion. In the base of the square pyramid the Rh-S distances are unequal, reflecting the differing influence of the trans ligands (2.323 (3) Å trans to phosphine vs. 2.269 (3) Å trans to iodide). The structure determination thus affirms the diastereotopic nature of the α-methylene protons in the static structure. Addition of Ag + to solutions of the Rh(III) acyl anions results in the removal of iodide and the formation of neutral acyl species in which the α-methylene protons appear magnetically equivalent. Solvent exchange is presumed to ocur rapidly in these systems since neutral five-coordinate complexes can be isolated in the presence of coordinating solvents. Heating solutions of the neutral acyl species in THF or CH 2 Cl 2 results in the migration of the R group (R = Me, Et, Bz) from the acyl carbon to one of the sulfur donor atoms of the dithiolene ligand to give S-alkylated Rh(I) complexes of the formula [Rh(CO)(PPh 3 )(R-mnt)]. The reaction of [Rh(CO)(PPh 3 )(mnt)] - with OEt 3 + yields a mixture of the S-ethyl complex and the neutral propanoyl species which upon warming in THF converts entirely to the Rh(CO)(PPh 3 )(Et-mnt) complex. The dicarbonyl complex [Rh(CO) 2 (mnt)] - is considerably less reactive than its monosubstituted triphenylphosphine derivative.