Reactions of organo-rhodium complexes with multidentate N,N and N,S-heterocycles and exchange studies by NMR

Dihalobridged binuclear complexes Rh(diolefin)(μ-X)]₂ {diolefin = 1,5-cyclooctadiene (cod), X = Cl or Br; diolefin = norbornadiene (nbd), X = Cl}, undergo halide bridge cleavage reactions with multidentate N,N-heterocycles 1,3,5-tris(benzimidazolyl)benzene (L¹H₃), 1,3,5-tris(N-methylbenzimidazolyl)benzene (L²H₃) and N,S-heterocycle 1,3,5-tris(benzothiazolyl)benzene (L³H₃) to yield trinuclear heterocycle bridged complexes {RhX(cod)}₃(μ-LH₃)] and {RhCl(nbd)}₃(μ-LH₃)] (LH₃ = L¹H₃, L²H₃, L³H₃). ¹H NMR exchange measurements have shown resonances for olefinic protons 1″, 2″, 5″ and 6″ of cod at different chemical shifts, perhaps due to restricted Rh–N bond rotation. The olefinic and aliphatic protons would undergo exchange with each other and also with intermediate species. The exchange mechanism may be visualized to involve Rh–N bond breaking, rotation of the cod ligand of the T-shaped (three-coordinate) intermediate species followed by recomplexation. An alternate mechanism may be Rh–cod bond breaking at olefin positions 5″ and 6″, isomerisation of the T-complex such that 5″/6″ moves trans to X coupled with rotation of the heterocycle about the Rh–N bond (made easier by the reduced coordination number of the intermediate), followed by recoordination of 1″/2″ trans to N, followed by recomplexation. NMR signals from the intermediate species in one dimensional ¹H, ¹³C and 2D NMR spectra have supported the exchange of protons.