Activation of Hydrogen by Palladium(0): Formation of the Mononuclear Dihydride Complex trans‐[Pd(H)2(IPr)(PCy3)]

An even split : In sharp contrast with the general behavior of Pd⁰ complexes, [Pd(IPr)(PCy₃)] is able to activate the H? H bond. The resulting trans‐[Pd(H)₂(IPr)(PCy₃)] is the first isolated mononuclear dihydride palladium compound. Its formation is supported by multinuclear NMR spectroscopy, density functional calculations, and X‐ray diffraction studies. The stability and reactivity of this new species are examined.

     

Temperature- and solvent-dependent binding of dihydrogen in iridium pincer complexes

Mixtures of deuterium labeled complexes (p-XPOCOP)IrH2-xDx (1-6-d0-2) {POCOP = [C6H2-1,3-[OP(tBu)2]2] X = MeO (1), Me (2), H (3), F (4), C6F5 (5), and ArF = 3,5-(CF3)2-C6H3 (6)} have been generated by reaction of (p-XPOCOP)IrH2 complexes with HD gas in benzene followed by removal of the solvent under high vacuum. Spectroscopic analysis employing 1H and 2D NMR reveals significant temperature and solvent dependent isotopic shifts and HD coupling constants. Complexes 1-6-d1 in toluene and pentane between 296 and 213 K exhibit coupling constants JHD of 3.8-9.0 Hz, suggesting the presence of an elongated H2 ligand, which is confirmed by T1(min) measurements of complexes 1, 3, and 6 in toluene-d8. In contrast, complex 6-d1 exhibits JHD = 0 Hz in CH2Cl2 or CDCl2F whereas isotopic shifts up to -4.05 ppm have been observed by lowering the temperature from 233 to 133 K in CDCl2F. The large and temperature-dependent isotope effects are attributed to nonstatistical occupation of two different hydride environments. The experimental observations are interpreted in terms of a two component model involving rapid equilibration of solvated Ir(III) dihydride and Ir(I) dihydrogen structures.     

Active-site models for iron hydrogenases: reduction chemistry of dinuclear iron complexes

Reduction of Fe2(mu-S2C3H6)(CO)6 (1) in tetrahydrofuran with 1 equiv of decamethylcobaltocene (Cp*2Co) affords a tetranuclear dianion 2. The IR spectra of samples of 2 in solution and in the solid state exhibit a band at 1736 cm(-1), suggestive of the presence of a bridging carbonyl (CO) ligand. X-ray crystallography confirms that the structure of 2 consists of two Fe2 units bridged by a propanedithiolate moiety formulated as [Fe2(mu-S2C3H6)(CO)5(SCH2CH2CH2-mu-S)Fe2(mu-CO)(CO)6](2-). One of the Fe2 units has a bridging CO ligand and six terminal CO ligands. The second subunit exhibits a bridging propanedithiolate moiety. One CO ligand has been replaced by a terminal thiolate ligand, replicating the basic architecture of Fe-only hydrogenases. The reduction reaction can be reversed by treatment of 2 with 2 equiv of [Cp2Fe][PF6], reforming complex 1 in near-quantitative yield. Complex 2 can also be oxidized by acids such as p-toluenesulfonic acid, regenerating complex 1 and forming H2.     

Dihydrogen complexes of electrophilic metal centers: observation of Cr(CO)5(H2), W(CO)5(H2) and [Re(CO)5(H2)]+

Photolysis of dichloromethane solutions of M(CO)6 (M = Cr, W) at low temperature in the presence of hydrogen gas affords W(CO)5(H2) (1) and Cr(CO)5(H2) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T1 values for the hydride resonances and a large HD coupling of 35.3 Hz (W) and 35.8 Hz (Cr) in the HD derivatives. A cationic analogue, [Re(CO)5(H2)]+ (3), was prepared by reaction of Re(CO)5Cl with [Et3Si][B(C6F5)4] in fluorobenzene under hydrogen. Complex 3-d1 exhibits JHD = 33.9 Hz. Complex 3 is strongly acidic, with complete deprotonation by diethyl ether; complexes 1 and 2 are moderately acidic. Deprotonation of 1 is complete in the presence of one equivalent of triethylamine.     

Synthesis, Characterization, and Reactivity of Dicationic Dihydrogen Complexes of Osmium

The dicationic Os(II) complex [Os(bpy)(PPh(3))(2)(CO)(H(2))](2+) has been prepared as the triflate salt. The presence of a bound dihydrogen ligand is indicated by a short T(1) minimum value consistent with an H-H distance of 1.05 ?. In the partially deuterated derivative J(HD) = 25.5 Hz was observed. By comparison to other structurally characterized complexes, the observed H-D coupling is most consistent with a H-H distance greater than 1 ?, which requires that the bound H(2) ligand be in the slow rotation regime. The dicationic complex is a strong acid, indicating that the bound H(2) is substantially activated toward heterolytic cleavage. The H(2) ligand is tightly bound to the metal center, and does not undergo exchange with D(2) over the course of several weeks at room temperature. A related dicationic Os(II) complex, [Os(bpy)(2)(CO)(H(2))](2+), has also been prepared. A short T(1) minimum value and a J(HD) value of 29.0 Hz in the partially deuterated derivative is most consistent with a H-H distance of 0.99 ?. The bound H(2) ligand of this complex is significantly less activated toward heterolytic cleavage and is stable in solution for less than a day at room temperature.     

KMgF3:Ni2+晶体的占位、局部结构、吸收光谱和顺磁g因子的统一解释

利用完全对角化方法和强场耦合方案,采用半自洽场(semi-SCF)自由Ni2+的d轨道模型和Ni2+-6X-(x=F,Cl,Br,I)络合物的μ-κ-α模型研究,建立了含有过渡族金属离子的晶体的局域结构与吸收光谱和顺磁g因子之间的定量关系,对KMgF3:Ni2+晶体的占位、局域结构、吸收光谱和顺磁g因子作出了统一解释,预测了KMgF3:Ni2+晶体的光谱精细结构.所得理论计算结果与实验值符合得很好.     

Efficient catalysis of ammonia borane dehydrogenation

In the presence of an iridium pincer complex, dehydrogenation of ammonia borane (H3NBH3) occurs rapidly at room temperature in tetrahydrofuran to generate 1.0 equivalent of H2 and [NH2BH2]5. A metal borohydride complex has been isolated as a dormant form of the catalyst which can be reactivated by reaction with H2.     

An oxidized active site model for the FeFe hydrogenase: reduction with hydrogen gas

Models for the oxidized form of the FeFe hydrogenase active site have been prepared. These cationic complexes contain two iron atoms, carbonyl ligands, a propanedithiolate bridge, and one other bridging group. Reduction of these complexes with hydrogen gas is demonstrated.     

Structure and dynamics of a dihydrogen/hydride ansa molybdenocene complex

In contrast to [Cp(2)MoH(3)](+), which is a thermally stable trihydride complex, the ansa-bridged analogue [(eta-C(5)H(4))(2)CMe(2)MoH(H(2))](+) (1) is a thermally labile dihydrogen/hydride complex. Partial deuteration of the hydride ligands allows observation of J(H)(-)(D) = 11.9 Hz in 1-d(1) and 9.9 Hz in 1-d(2) (245 K), indicative of a dihydrogen/hydride structure. There is a slight preference for deuterium to concentrate in the dihydrogen ligand. A rapid dynamic process interchanges the hydride and dihydrogen moieties in complex 1. Low temperature (1)H NMR spectra of 1 give a single hydride resonance, which broadens at very low temperature due to rapid dipole-dipole relaxation (T(1) = 23 ms (750 MHz, 175 K) for the hydride resonance in 1). Low temperature (1)H NMR spectra of 1-d(2) allow the observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation with DeltaG(150) = 7.4 kcal/mol, but H atom exchange is still rapid at all accessible temperatures (down to 130 K). Density functional calculations confirm the dihydrogen/hydride structure as the ground state for the molecule and give estimates for the energy of two hydrogen exchange processes in good agreement with experiment. The presence of the C ansa bridge is shown to decrease the ability of the metallocene fragment to donate to the hydrogens, thus stabilizing the (eta(2)-H(2)) unit and modulating the barrier to H(2) rotation.