Ethyl-, n-propyl- and [(trimethylsilyl)methyl]niobium (4e)-alkyne complexes Tp*Nb(Cl)(CH2R)(PhC⋮CR‘) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; R = Me, R‘ = Me (2a), Et (2b), n-Pr (2c); R = Et, R‘ = Me (3a), Et (3b); R = SiMe3, R‘ = Me (4a)) exhibit α-agostic structures in solution (1H and 13C NMR) and in the solid state (X-ray structure at 173 K for 4a). The phenylpropyne complexes 2a, 3a, and 4a undergo a thermally induced rearrangement which yields niobium methyl complexes Tp*Nb(Cl)(Me)(PhC⋮CEt), Tp*Nb(Cl)(Me)(PhC⋮C−n-Pr), and Tp*Nb(Cl)(Me)(PhC⋮CCH2SiMe3), respectively. This reaction follows a first-order rate law. The rate decreases with increased steric bulk of the alkyl group in the starting complex. For 2a, a full kinetic analysis yields a high enthalpic barrier and an entropy of activation close to zero. Heating a toluene solution of Tp*Nb(Cl)(n-Pr)(PhC⋮CEt) (3b) establishes a 1:1 equilibrium between 3b itself and Tp*Nb(Cl)(Et)(PhC⋮C−n-Pr) (2c). Heating a solution of Tp*Nb(Cl)(Et)(PhC⋮CEt) (2b) and PhC⋮CMe yields the η1-alkenyl (4e)-phenylpropyne complex Tp*Nb(Cl)(η1-CPhCEt2)(PhC⋮CMe) (6), demonstrating that alkyl migration is a key step of the rearrangement. Under pseudo-first-order conditions, this reaction does not depend on the phenylpropyne concentration. Potential 2e-donor ligands fail to act as efficient traps. Accordingly, a successful trapping experiment involves dinitrogen extrusion from N3P(N−i-Pr2)2 in the presence of 2b which yields an unprecedented (phosphinoimido)niobium complex:  Tp*Nb(Cl)(η1-CPhCEt2)[N-P(N−i-Pr2)2] (7). The crystal structure of 7 confirms the η1-alkenyl formulation and reveals a formal Nb−N triple bond, a linear Nb−N−P link, and a pyramidal P. Deoxygenation of propene oxide and desulfurization of ethylene sulfide gives, respectively, the oxo and sulfido η1-alkenyl complexes Tp*Nb(Cl)(η1-CPhCEt2)(X) (X = O (8), S (9)). The observed thermal rearrangements are unique examples of reversible migratory insertion/β-alkyl elimination in transition metal alkyl alkyne complexes. In the absence of efficient trapping agent, the unsaturated η2-alkenyl intermediate that is first generated reversibly ring opens and ring closes. This realizes the intramolecular C−C bond activation which leads to the observed alkyl switch. Kinetic and trapping data indicate that the migratory insertion of the alkyl group on to the alkyne is the rate-determining step of the whole process.

The trifunctional ligands (R)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (2R), (S)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (2S), and (S)-(phenyl(2-anisyl)phosphino)(2-pyridyl)methane (3) have been synthesized, as well as the corresponding RuCl2(PPh3)(L) complexes. The complexes RuCl2(PPh3)(2R) (5) and RuCl2(PPh3)(2S) (6) were isolated as mixtures of two isomers, 5a and 5b and 6a and 6b, respectively. In each of these isomers, the ligands 2 are η3-(P,N,O) bound. They differ by the position of the triphenylphosphine, which is either in a trans position relative to the pyridyl ring (5a or 6a) or in a trans position relative to the ether function (5b or 6b). Variable temperature NMR experiments have shown that the hemilabile character in solution of the ligands 2 is through their pyridyl arm in the isomers a or their ether arm in b. The complexes 6b and RuCl2(PPh3)(3) (9) were characterized by X-ray diffraction. The complexes 5, 6, and 9 are very active catalysts for the transfer hydrogenation of acetophenone by propan-2-ol in basic media, the higher activity being observed for 9 (turnovers frequency 48 900 h-1). The enantioselectivity is modest and dependent on the reactions conditions. The best result has been observed for 5, with an ee of 60%.

The bonding properties of the optically active chiral-at-P polydentate ligands C5H3N(2-R‘)(6-CH2PPhR) (R = Me, R‘ = H, Me; R = o-anisyl, R‘ = H, Me) and the achiral ligand C5H3N(2-Me)(6-CH2PPh2) toward (COD)Rh+ are reported. The results show that steric hindrance of the 2-position of the pyridyl ring induces a labile character of the Rh−N bond. Moreover, for R = o-anisyl and R‘ = Me, a dynamic competition for bonding between the N- and O-donating centers is observed. Comparison of the solid state structures of complexes [Rh(COD)(C5H4N(2-CH2PPhMe)][BF4] and [Rh(COD)(C5H3N(2-Me)(6-CH2PPh2)][BF4] shows a significant bond lengthening of the Rh−N bond in the latter complex, consistent with its fluxional behavior observed in solution.