Carbene complexes. Part VII. Chloromethyleneammonium chlorides. Electron-rich carbenoids, as precursors to secondary carbene metal complexes; crystal and molecular structure of trichloro(dimethylaminomethylene)bis(triethylphosphine)rhodium(III)

Abstract

The reaction of dialkyl(chloromethylene)ammonium chlorides (R₂NCHCl)⁺Cl^–, (R = Me, Et, Prⁱ, or Buⁿ), with Rh^I substrates [RhCl(PPh₃)₃], [RhCl(CO)PPh₃]₂, or [RhCl(CO)₂]₂ affords Rh^III secondary carbene complexes, [L′RhCl₃CHNR₂]_n[L′= CO, n= 2; L′=(PPh₃)CO, n= 1; L′=(PEt₃)₂ or (PPh₃)₂, n= 1], via a three-fragment oxidative-addition reaction. The secondary carbene Rh^III complexes are characterised by a high degree of CN double-bonding, greater than in tertiary carbene complexes or carboxylic acid dialkylamides. The C_carb–H¹H n.m.r. resonance is at low field (τca. –1), ν(C_carb–H) at ca. 3050 cm^–1, and ν(C_carb[graphic omitted]N) at ca. 1600 cm^–1; coupling of C_carb–H resonances are identified: ²J(¹⁰³Rh–¹H), ³J(³¹P–¹H), and cis- and trans-⁴J(C¹H₃N–¹H). A single-crystal X-ray analysis of [RhCl₃(PEt₃)₂CHNMe₂] has been carried out to R= 0·050; the complex has an approximately octahedral Rh environment with mutually trans-phosphine ligands. Bond distances (Å) are Rh–C_carb= 1·961(11), C_carb–N = 1·289(14), C_carb–H = 1·14(4), Rh–P = 2·368(3) and 2·372(3), Rh–Cl (mutually trans)= 2·357(4) and 2·364(3), Rh–Cl (trans-carbene)= 2·445(4). The high trans-influence of the secondary carbene ligand is discussed in terms of X-ray, i.r., and n.m.r. data, and it appears to have the highest trans-influence of a neutral ligand of Rh^III.