No abstract

Solutions of the Lewis acid B(C6F5)3 in 1,4-dioxane are found to effectively catalyze the hydrogenation of a variety of ketones and aldehydes. These reactions, the first to allow entirely metal-free catalytic hydrogenation of carbonyl groups under relatively mild reaction conditions, are found to proceed via a "frustrated Lewis pair" mechanism in which the solvent, a weak Brønsted base yet moderately strong donor, plays a pivotal role.

The catalytic fixation of N by molecular Fe compounds is a rapidly developing field, yet thus far few complexes can effect this transformation, and none are selective for NH production. Herein we report that the simple Fe(0) complex Fe(EtPCHCHPEt)(N) (1) is an efficient catalyst for the selective conversion of N (>25 molecules N fixed) into NH, attendant with the production of ca. one molecule of NH. Notably, the reductant (CoCp*) and acid (PhNHOTf) used are considerably weaker than conventional chemical H and e sources used in previous demonstrations of N turnover by synthetic Fe compounds. These results show that the direct catalytic conversion of N to the hydrazine oxidation state on molecular Fe complexes is viable and that the mechanism of NH formation by such systems may proceed via Fe-NH intermediates.

A new family of electron-deficient tris(aryl)boranes, B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 1-3), has been synthesized, permitting an investigation into the steric and electronic effects resulting from the gradual replacement of C(6)F(5) with C(6)Cl(5) ligands. B(C(6)F(5))(2)(C(6)Cl(5)) (3) is accessed via C(6)Cl(5)BBr(2), itself prepared from donor-free Zn(C(6)Cl(5))(2) and BBr(3). Reaction of C(6)Cl(5)Li with BCl(3) in a Et(2)O/hexane slurry selectively produced B(C(6)Cl(5))(2)Cl, which undergoes B-Cl exchange with CuC(6)F(5) to afford B(C(6)F(5))(C(6)Cl(5))(2) (5). While 3 forms a complex with H(2)O, which can be rapidly removed under vacuum or in the presence of molecular sieves, B(C(6)Cl(5))(3) (6) is completely stable to refluxing toluene/H(2)O for several days. Compounds 3, 5, and 6 have been structurally characterized using single crystal X-ray diffraction and represent the first structure determinations for compounds featuring B-C(6)Cl(5) bonds; each exhibits a trigonal planar geometry about B, despite having different ligand sets. The spectroscopic characterization using (11)B, (19)F, and (13)C NMR indicates that the boron center becomes more electron-deficient as n increases. Optimized structures of B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 0-3) using density functional theory (B3LYP/TZVP) are all fully consistent with the experimental structural data. Computed (11)B shielding constants also replicate the experimental trend almost quantitatively, and the computed natural charges on the boron center increase in the order n = 0 (0.81) < n = 1 (0.89) < n = 2 (1.02) < n = 3 (1.16), supporting the hypothesis that electrophilicity increases concomitantly with substitution of C(6)F(5) for C(6)Cl(5). The direct solution cyclic voltammetry of B(C(6)F(5))(3) has been obtained for the first time and electrochemical measurements upon the entire series B(C(6)F(5))(3-n)(C(6)Cl(5))(n) (n = 0-3) corroborate the spectroscopic data, revealing C(6)Cl(5) to be a more electron-withdrawing group than C(6)F(5), with a ca. +200 mV shift observed in the reduction potential per C(6)F(5) group replaced. Conversely, use of the Guttmann-Beckett and Childs' methods to determine Lewis acidity on B(C(6)F(5))(3), 3, and 5 showed this property to diminish with increasing C(6)Cl(5) content, which is attributed to the steric effects of the bulky C(6)Cl(5) substituents. This conflict is ascribed to the minimal structural reorganization in the radical anions upon reduction during cyclic voltammetric experiments. Reduction of 6 using Na((s)) in THF results in a vivid blue paramagnetic solution of Na(+) [6](•-); the EPR signal of Na(+)[6](•-) is centered at g = 2.002 with a((11)B) 10G. Measurements of the exponential decay of the EPR signal (298 K) reveal [6](•-) to be considerably more stable than its perfluoro analogue.

The synthesis of the bimetallic permethylpentalene complexes Pn*2M2 (M = V, Cr, Mn, Co, Ni; Pn* = C8Me6) has been accomplished, and all of the complexes have been structurally characterized in the solid state by single-crystal X-ray diffraction. Pn*2V2 (1) and Pn*2Mn2 (3) show very short intermetallic distances that are consistent with metal-metal bonding, while the cobalt centers in Pn*2Co2 (4) exhibit differential bonding to each side of the Pn* ligand that is consistent with an eta(5):eta(3) formulation. The Pn* ligands in Pn*2Ni2 (5) are best described as eta(3):eta(3)-bonded to the metal centers. (1)H NMR studies indicate that all of the Pn*2M2 species exhibit D(2h) molecular symmetry in the solution phase; the temperature variation of the chemical shifts for the resonances of Pn*2Cr2 (2) indicates that the molecule has an S = 0 ground state and a thermally populated S = 1 excited state and can be successfully modeled using a Boltzmann distribution (DeltaH(o) = 14.9 kJ mol(-1) and DeltaS(o) = 26.5 J K(-1) mol(-1)). The solid-state molar magnetic susceptibility of 3 obeys the Curie-Weiss law with mu(eff) = 2.78 muB and theta = -1.0 K; the complex is best described as having an S = 1 electronic ground state over the temperature range 4-300 K. Paradoxically, attempts to isolate the "double ferrocene" equivalent, Pn*2Fe2, led only to the isolation of the permethylpentalene dimer Pn*2 (6). Solution electrochemical studies were performed on all of the organometallic compounds; 2-5 exhibit multiple quasi-reversible redox processes. Density functional theory calculations were performed on this series of complexes in order to rationalize the observed structural and spectroscopic data and provide estimates of the M-M bond orders.