PROTONATION OF DIENE COMPLEXES OF RHODIUM, IRIDIUM, RUTHENIUM, AND OSMIUM - A FINE BALANCE BETWEEN TERMINAL AND AGOSTIC HYDRIDES

The structures of the cationic hydrido complexes formed on addition of HPF6 to (eta(5)-pentamethylcyclopentadienyl)- and (eta(6)-arene)metal complexes containing various 1,3-dienes or 1,5-cyclooctadiene have been investigated by IR and NMR (1-H, C-13) spectroscopy. The rhodium complexes [RhH(eta-C5Me5)(diene)]+ (diene = 1,3-cyclohexadiene (1), 2,3-dimethylbutadiene (3)) are highly fluxional eta(3)-enyl complexes with a M-H-C interaction (agostic hydrides), as shown by their H-1 and C-13 NMR spectra at -100-degrees-C and by deuteration experiments. As with other compounds of this class, two reversible processes can be observed by variable-temperature NMR spectroscopy: (1) M-H bond cleavage to give a 16e eta(3)-enyl complex, which leads to exchange of the endo C-H bonds of 1 and exchange of the agostic methyl hydrogen atoms of 3; (2) C-H bond cleavage to give a diene metal hydride, which, in combination with process 1, averages separately the endo and exo protons of 1 and the five dienyl protons of 3. The free energy of activation DELTA-G double-ended dagger for process 2 is slightly larger than for process 1, the estimated values being about 9.0 and 7.5 kcal/mol in the case of 3. The complexes [IrH(eta-C5Me5)(diene)]+ (diene = 1,3-cyclohexadiene (2), 2,3-dimethylbutadiene (4)) and [OsH(eta-arene)(diene)]+ (arene = C6H6, diene = 1,3-cyclohexadiene (5), 2,3-dimethylbutadiene (7); arene = 1,3,5-C6H3Me3, diene = 1,3-cyclohexadiene (6), 2,3-dimethylbutadiene (8)) are terminal hydrides in which the hydride ligand migrates between metal and diene reversibly and rapidly on the NMR time scale above room temperature (DELTA-G double-ended dagger congruent-to 12 kcal/mol for 2 and 4). The coupled C-13 NMR spectrum of [RuH(eta-C6H6)(C6H8)]+ (9) at -100-degrees-C suggests that this compound contains an agostic hydride similar to 1 and 3, but the H-1 and C-13 NMR spectra above -100-degrees-C resemble those expected for a highly fluxional terminal hydrido diene complex, the free energy of activation DELTA-G double-ended dagger for reversible Ru-H bond cleavage being 8.8 kcal/mol. In contrast to the rhodium complexes and most other agostic hydrides formed from protonation of diene complexes, therefore, DELTA-G double-ended dagger for C-H bonds cleavage (process 2) in 9 is less than that for M-H bond cleavage (process 1) and is probably about 5-6 kcal/mol. The compounds [RuH(eta-C6Me6)(C6Me6)]+ (10) and [RuH(eta-arene)(1,3-diene)]+ (diene = 2,3-dimethylbutadiene, arene = C6H6 (11), C6H3Me3 (12), C6Me6 (13); arene = C6Me6, diene = isoprene (14), 2-methyl-1,3-pentadiene (15), 3-methyl-1,3-pentadiene (16)) are also agostic, but in most cases limiting spectra cannot be obtained, even for process 1, at -100-degrees-C. Protonation of M(eta-arene)(1,5-COD) gives terminal hydrido diene complexes [MH(eta-arene)(1,5-COD)]+ (M = Ru, arene = C6H6, 1,3,5-C6H3Me3, C6Me6; M = Os, arene = C6H6, C6H3Me3). The compound obtained from Ru(eta-C6Me6)(1,5-COD) and DPF6 incorporates deuterium at the methylene carbon atoms of the coordinated diene, which implies that [RuH(eta-C6Me6)(1,5-COD)]+ is in equilibrium with eta(1), eta(2)-cyclooctenyl and possibly agostic eta(3)-cyclooctenyl species. All the protonated diene complexes except [OsH(eta-arene)(1,5-COD)]+ react with 2e-donor ligands (L) to give nonfluxional 18e complexes of the type [M(eta-C5Me5)(eta(3)-enyl)(L)]+ (M = Rh, Ir; L = t-BuNC) and [M'(eta-arene)(eta(3)-enyl)(L)]+ [M' = Ru, Os; L = CO, t-BuNC, P(OMe)3 (not all possible combinations)].