Group 5 <i>ansa</i>-Metallocenes: Structural and Dynamic Properties of Tetrahydroborate Complexes

The new compounds [Nb{(η-C5H4)X(η-C5H4)}Cl2] (X = CEt2 1a, C(C5H10) 1b, C2Me4 1c), [Nb{(η-C5H3tBu)C2Me4(η-C5H3tBu)}Cl2] 1d, [Nb{(η-C5H4)X(η-C5H4)}(η2-BH4)] (X = CEt2 2a*, C(C5H10) 2b, C2Me4 2c*, SiMe2 2e*), [Nb{(η-C5H3tBu)C2Me4(η-C5H3tBu)}(η2-BH4)] (2d*), [Nb{(η-C5H4)X(η-C5H4)}(η2-BD4)] (X = C2Me4 3c, CMe2 4), and [V{(η-C5H4)C2Me4(η-C5H4)}(η2-BH4)] (5*) have been prepared. The asterisk indicates the crystal structure has been determined. The hydrogen scrambling processes in the tetrahydroborate complexes 2a−e, 3c, 4, 5, and [Nb{(η-C5H4)CMe2(η-C5H4)}(η2-BH4)] have been studied. The free energy barrier ΔG⧧ to bridge-terminal hydrogen exchange is considerably reduced when the bridging unit imposes significant structural changes in the metallocene

Studies of <i>ansa</i>-bis(cyclopentadienyl)tungsten derivatives

The new compounds [W{(η-C5H4)X(η-C5H4)}Cl2] 1, [W{(η-C5H4)X(η-C5H4)}Me2] 2, [W{(η-C5H4)SiMe2(η-C5H4)}(SPh)2] 3, [W{(η-C5H4)X(η-C5H4)}H(Cl)] 4, [W{(η-C5H4)X(η-C5H4)}H2] 5 and [W{(η-C5H4)X(η-C5H4)}H3]+Y– 6, where X = SiMe2, SiEt2 or CMe2CMe2, and Y = BF4 or Cl, have been prepared. The crystal structures of [W{(η-C5H4)X(η-C5H4)}Me2] (X = SiMe2 or CMe2CMe2) have been determined

Synthesis, structure and density functional study of the <i>ansa</i>-rhenocene complex [Re{(η-C₅H₄)CMe₂(η-C₅H₄)}Cl]

The ansa-rhenocene compound [Re{(η-C5H4)CMe2(η-C5H4)}Cl] 1 has been prepared by reaction of ReCl5 with [K2(C5H4)CMe2(C5H4)]. X-Ray crystallography shows the molecular structure to be unsymmetrical, with the angle defined by the Re-bridgehead carbon vector and the chlorine atom found to be 170.4°. A density functional study compares the electronic structure of [Re{(η-C5H4)CH2(η-C5H4)}Cl] with [Re{(η-C5H5)2Cl] and shows that for the ansa-bridged species, the chlorine binds less effectively in the central position. The calculated structure of [Re{(η-C5H4)CH2(η-C5H4)}Cl] is also found to be unsymmetrical

Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes

A new air-stable catalyst for the oxidative dehydrogenation of benzylic alcohols under ambient conditions has been developed. The synthesis and characterization of this compound and the related monomeric and dimeric V(IV)- and V(V)-pinF (pinF = perfluoropinacolate) complexes are reported herein. Monomeric V(IV) complex (Me4N)2[V(O)(pinF)2] (1) and dimeric (μ-O)2-bridged V(V) complex (Me4N)2[V2(O)2(μ-O)2(pinF)2] (3a) are prepared in water under ambient conditions. Monomeric V(V) complex (Me4N)[V(O)(pinF)2] (2) may be generated via chemical oxidation of 1 under an inert atmosphere, but dimerizes to 3a upon exposure to air. Complexes 1 and 2 display a perfectly reversible VIV/V couple at 20 mV (vs Ag/AgNO3), whereas a quasi-reversible VIV/V couple at -865 mV is found for 3a. Stoichiometric reactions of 3a with both fluorenol and TEMPOH result in the formation of (Me4N)2[V2(O)2(μ-OH)2(pinF)2] (4a), which contains two V(IV) centers that display antiferromagnetic coupling. In order to structurally characterize the dinuclear anion of 4a, {K(18C6)}+ countercations were employed, which formed stabilizing K···O interactions between the counterion and each terminal oxo moiety and H-bonding between the oxygen atoms of the crown ether and μ-OH bridges of the dimer, resulting in {K(18C6)}2[V2(O)2(μ-OH)2(pinF)2] (4b). The formal storage of H2 in 4a is reversible and proton-coupled electron transfer (PCET) from crystals of 4a regenerates 3a upon exposure to air over the course of several days. Furthermore, the reaction of 3a (2%) under ambient conditions with excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in the selective formation of fluorenone (82% conversion), cinnamaldehyde (40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol dehydrogenation (OAD) chemistry known for VOx surfaces and demonstrating, in air, the thermodynamically challenging selective oxidation of alcohols to aldehydes/ketones