SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF DIENE AND BENZENE PENTARUTHENIUM CLUSTERS

Reaction of the square-pyramidal cluster [Ru5C(CO)15] 1 with cyclohexa-1.3-diene and Me3NO in CH2Cl2 at ambient temperature affords the novel species [Ru5C(CO)13(mu-eta2:eta2-C6H8)] 2 which has been fully characterized by X-ray diffraction studies [monoclinic, space group P2(1)/n, a = 16.193(3), b = 9.897(2), c = 19.536(4) angstrom, beta = 98.80(3)degrees and Z = 4]. Further reaction of compound 2 with Me3NO in the same solvent results in the dehydrogenation of the C6H8 ligand and the production of two isomers 3a and 3b of the benzene derivative [Ru5C(CO)12(C6H6)]. In agreement with mass spectroscopic and H-1 NMR data, the X-ray diffraction analysis shows that the benzene ligand is bound in a face-capping mode in [Ru5C(CO)12(mu3-eta2:eta2:eta2-C6H6)] 3a and in a terminal mode in [Ru5C(CO)12(eta6-C6H6)] 3b, respectively. Compound 3a is triclinic, space group P1-, a = 9.525(2), b = 14.574(3), c = 9.484(4) angstrom, alpha = 96.29(2), beta = 112.37(3), gamma = 82.33(2)degrees and Z = 2; 3b is monoclinic, space group P2(1)/c, a = 15.26(2), b = 16.675(9), c = 19.016(9) angstrom, beta = 96.04(7)degrees and Z = 8. On heating 3a in hexane a quantitative and irreversible conversion to 3b is observed. Treatment of 3b with carbon monoxide produces the adduct [Ru5C(CO)13(eta6-C6H6)] 4a, which, on the basis of a single crystal X-ray diffraction study, has been shown to contain a 'bridged-butterfly' arrangement of five ruthenium atoms. Compound 4a is monoclinic, space group P2(1)/n, a = 9.792(4), b = 15.718(2), c = 16.446(3) angstrom, beta = 96.37(2)degrees and Z = 4. On standing in CH2Cl2, 4a undergoes loss of carbon monoxide to give the apical isomer [Ru5C(CO)12(C6H6)] 3c. On heating in hexane over a prolonged period 3c undergoes quantitative isomerization to 3b.