An Intramolecular N-H…(#mu#-H)Re_2 Dihydrogen Bond and a Novel #mu#_3-#eta#~2 Coordination Mode of the Pyrazolate Anion on a Triangular luster Face

摘要

The quantitative addition of pyrazole (Hpz) to the 44 valence-electron, triangular cluster anion [Re_3(#mu#_3-H)-(#mu#-H_3)_3(CO)_9]~- gives the novel unsaturated anion [Re_3(#mu#-H)_4(CO)_9(Hpz)]~- (1, 46 valence electrons), which contains a pyrazole molecule that is terminally coordinated on a cluster vertex. Solid-state X-ray and IR analyses reveal a rather weak hydrogen-bonding interaction between the NH proton and one of the hydrides bridging the opposite triangular cluster edge (#DELTA#Hdeg = -3.1 kcal mol~(-1) from the Iogansen equation). Both IR and NMR data indicate that such a proton-hydride interaction is maintained in the major conformer present in CD_2Cl_2, but also provide evidence of h presence of minor conformers of 1 in which the NH proton is involved in an intermolecular hydrogen bond with the solvent. The #mu#-H…HN bond length evaluated in solution through the T_1 minimum value (2.07 A) and that determined in the solid state by X-ray diffraction (2.05 A) are in good agreement. NMR experiments show that, in acetone, intermolecular N-H…solvent interactions replace the intramolecular dihydrogen bond. At room temperature in CH_2Cl_2, the pyrazole ligand in 1 is labile and 1 slowly "disproportionates" to [Re_3(#mu#_3-H)-(#mu#-H)_3(CO)_9]~- and [Re_3(#mu#-H)_3(CO)_9]-#mu#-#eta#~2-pz)(Hpz)]~-, with H_2 evolution. Slow H_2 evolution also leads to he formation of the anion [Re_3(#mu#-H)_3(CO)_9(pz)]~- (5), in which the pyrazolate anion adopts a novel #mu#_3-#eta#~2-coordination mode, as revealed by a single-crystal X-ray analysis. The analysis of the bond lengths indicates that the pyrazolate anion in 5 acts as a six-electron donor, with loss of the aromaticity. The formation of 5 from 1 is much faster in solvents with a high dielectric constant such as acetone or DMF. Anion 5 as also obtained from the reaction of pyrazole with [Re_3(#mu#-H)_3(CO)_9(#mu#_3-CH_3)]~- through the intermediate formation of two isomeric addition derivative and following CH_4 evolution.