The solution dynamics of adduct formation and electronic communication between ligand and metal core in electron deficient quinoline triosmium clusters

The coordination of ammonia and of aliphatic amines (RNH2(R = Et, n-Bu, s-Bu, t-Bu, cyclohexyl) and Et2NH) to the electron deficient clusters Os-3(CO)(9)(mu(3)-eta(2)-C9H6N)(mu-H) (1) and Os-3(CO)(9)(mu(3)-eta(2)-XC9H5N)(mu- H) (X = 5-NH2, 4; 3-NH2, 5; 6-NH2, 6; 5-Br, 7; 5-CH3, 8) has been studied. The initially formed adducts of the amines with 1 gradually isomerize to give a mixture of two isomers whose ratio varies with the cluster, the solvent and the amine. Studies of the variation of the isomer ratio with temperature yield Delta H degrees and Delta S degrees values for the isomer equilibrium. The primary cause of the change in the isomer ratio is the variation in Delta S degrees which is rationalized as being due to solvation effects. The overall structure of the two isomers is assigned on the basis of NMR measurements to one where the amine occupies an axial position on the cluster and where the hydride and quinoline ring bridge different edges of the cluster. The equilibrium constant for amine complex formation is primarily determined by the steric bulk of the amine. The amine substituted complex 4 shows absolutely no tendency to coordinate amines and this is ascribed to relief of the electron deficiency at the metal core by the mesomeric effects of the para-amino group. The isomeric 5 and 6, however, do undergo coordination of amines at the metal core. The 5-methyl substituted complex 8 does not complex amine either. Compounds 1,4-8 all undergo simple protonation with both coordinating (CF3CO2H) and non-coordinating acids (HBF4) to give cationic dihydrido complexes whose solution structures are elucidated by NMR techniques. The chemistry of these electron deficient clusters is compared with related mu(3)-eta(2) imidoyl complexes and can be understood in terms of the Hammett equation, at least for protonation.