STRENGTHENING OF N-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-CO HYDROGEN-BONDS UPON INCREASING THE BASICITY OF THE HYDROGEN-BOND ACCEPTOR (CO)

Low temperature crystal structures of (DABCO)H+Co(CO)(4)(-) (1) and (D ABCO)H+Co-(CO)(3)PPh(3)(-) (2) (DABCO = 1,4-diazabicyclooctane) indica te that both salts exhibit N-H ... Co hydrogen bonding. IR and NMR dat a indicate that these hydrogen bonded species persist in nonpolar solv ents such as toluene, but exist as solvent separated ions in more pola r solvents. Replacement of the axial CO ligand by PPh(3) leads to a sh ortening of the N ... Co separation in the solid state from 3.437(3) t o 3.294(6) Hi. This change is accompanied by an increase in the angle between the equatorial carbonyl ligands. Thus, the crystallographic re sults suggest: a strengthening of the N-H ... Co hydrogen bond upon in creasing the basicity of the metal center, the first observation of th is type in the solid state. This assertion is supported by variable-te mperature H-1 and C-13 NMR data in toluene-d(8) solution which, discus sed in the light of ab initio calculations, indicate that the barrier to a fluxional process involving cleavage of the N-H ... Co hydrogen b ond is greater in 2 than in 1. The crystal structures of 1 and 2 have been determined by X-ray diffraction at 135(5) and 123(5) K, respectiv ely [1 monoclinic, P2(1)/n. (No. 14), a = 8.728(2), b = 23.333(5), c = 12.146(2) Angstrom, beta = 95.74(2)degrees, V = 2461.1(9) Angstrom(3) , Z = 8; R(F) = 0.043, R(w)(F) = 0.043, S(F) = 1.21: 2 orthorhombic, P ca2(1) (No. 29), a 16.084(8), b = 8.874(3), c 17.312(3) Angstrom, V = 2471(1) Angstrom(3), Z = 4, R(F) = 0.065, R(w)(F) = 0.060, S(F) = 1.16 ].