Detection of an Iridium-Dihydrogen Complex: A Proposed Intermediate in Ionic Hydrogenation.

Addition of high pressures of H2 to five-coordinate [(tBu)4(POCOP)Ir(CO)(H)]OTf [(tBu)4(POCOP) = κ3-C6H3-2,6-(OP(tBu)2)2] complexes results in observation of two new iridium-dihydrogen complexes. If the aryl moiety of the POCOP ligand is substituted with an electron withdrawing protonated dimethylamino group at the para position, hydrogen coordination is enhanced. Five-coordinate Ir-H complexes generated by addition of triflic acid to (tBu)4(POCOP)Ir(CO) species show an Ir-H 1H NMR chemical shift dependence on the number of equivalents of acid present. It is proposed that excess triflic acid in solution facilitates triflate dissociation from iridium, resulting in unsaturated five-coordinate Ir-H complexes. The five-coordinate iridium-hydride complexes were found to catalyze H/D exchange between H2 and CD3OD. The existence of the dihydrogen complexes, as well as isotope exchange reactions, provide evidence for proposed ionic hydrogenation intermediates for glycerol deoxygenation.

Oxyfunctionalization with Cp*Ir(III)(NHC)(Me)(Cl) with O2: identification of a rare bimetallic Ir(IV) µ-oxo intermediate.

Methanol formation from [Cp*Ir(III)(NHC)Me(CD2Cl2)](+) occurs quantitatively at room temperature with air (O2) as the oxidant and ethanol as a proton source. A rare example of a diiridium bimetallic complex, [(Cp*Ir(NHC)Me)2(µ-O)][(BAr(F)4)2], 3, was isolated and shown to be an intermediate in this reaction. The electronic absorption spectrum of 3 features a broad observation at ∼660 nm, which is primarily responsible for its blue color. In addition, 3 is diamagnetic and can be characterized by NMR spectroscopy. Complex 3 was also characterized by X-ray crystallography and contains an Ir(IV)-O-Ir(IV) core in which two d(5) Ir(IV) centers are bridged by an oxo ligand. DFT and MCSCF calculations reveal several important features of the electronic structure of 3, most notably, that the µ-oxo bridge facilitates communication between the two Ir centers, and σ/π mixing yields a nonlinear arrangement of the µ-oxo core (Ir-O-Ir ∼ 150°) to facilitate oxygen atom transfer. The formation of 3 results from an Ir oxo/oxyl intermediate that may be described by two competing bonding models, which are close in energy and have formal Ir-O bond orders of 2 but differ markedly in their electronic structures. The radical traps TEMPO and 1,4-cyclohexadiene do not inhibit the formation of 3; however, methanol formation from 3 is inhibited by TEMPO. Isotope labeling studies confirmed the origin of the methyl group in the methanol product is the iridium-methyl bond in the [Cp*Ir(NHC)Me(CD2Cl2)][BAr(F)4] starting material. Isolation of the diiridium-containing product [(Cp*Ir(NHC)Cl)2][(BAr(F)4)2], 4, in high yields at the end of the reaction suggests that the Cp* and NHC ligands remain bound to the iridium and are not significantly degraded under reaction conditions.

Characterization of a palladium dihydrogen complex.

The preparation and isolation of the first palladium dihydrogen complex is described. NMR spectroscopy reveals a very short H-H bond length, but the hydrogen molecule is activated toward heterolytic cleavage. An X-ray crystal structure suggests that proton transfer to the (tBu) PCP (κ(3)-2,6-((t)Bu2PCH2)2C6H3) pincer ligand is possible. The basicity of the ipso-carbon atom of the pincer ligand was investigated in a related complex.

Hydrogenation of carboxylic acids catalyzed by half-sandwich complexes of iridium and rhodium.

A series of half-sandwich Ir and Rh compounds are demonstrated to be competent catalysts for the hydrogenation of carboxylic acids under relatively mild conditions. Of the structurally diverse group of catalysts tested for activity, a Cp*Ir complex supported by an electron-releasing 2,2'-bipyridine ligand was the most active. Higher activity was achieved with employment of Brønsted or Lewis acid promoters. Mechanistic studies suggest a possible reaction pathway involving activated carboxylic acid substrates. The hydrogenation reaction was shown to be general to a variety of aliphatic acids.

Synthesis, structure, and reactivity of a nickel dihydrogen complex.

Hydrogen activation by nickel: A tBuPCP pincer ligand facilitates formation of cationic Ni(II) dihydrogen and terminal dinitrogen complexes. The compounds have been characterized by X-ray crystallography and NMR spectroscopy. Addition of base promotes heterolytic cleavage of H(2) to form the corresponding neutral hydride complex.

Dihydrogen complexes of iridium and rhodium.

A series of iridium and rhodium pincer complexes have been synthesized and characterized: [(POCOP)Ir(H)(H(2))] [BAr(f)(4)] (1-H(3)), (POCOP)Rh(H(2)) (2-H(2)), [(PONOP)Ir(C(2)H(4))] [BAr(f)(4)] (3-C(2)H(4)), [(PONOP)Ir(H)(2))] [BAr(f)(4)] (3-H(2)), [(PONOP)Rh(C(2)H(4))] [BAr(f)(4)] (4-C(2)H(4)) and [(PONOP)Rh(H(2))] [BAr(f)(4)] (4-H(2)) (POCOP = κ(3)-C(6)H(3)-2,6-[OP(tBu)(2)](2); PONOP = 2,6-(tBu(2)PO)(2)C(5)H(3)N; BAr(f)(4) = tetrakis(3,5-trifluoromethylphenyl)borate). The nature of the dihydrogen-metal interaction was probed using NMR spectroscopic studies. Complexes 1-H(3), 2-H(2), and 4-H(2) retain the H-H bond and are classified as η(2)-dihydrogen adducts. In contrast, complex 3-H(2) is best described as a classical dihydride system. The presence of bound dihydrogen was determined using both T(1) and (1)J(HD) coupling values: T(1) = 14 ms, (1)J(HD) = 33 Hz for the dihydrogen ligand in 1-H(3), T(1)(min) = 23 ms, (1)J(HD) = 32 Hz for 2-H(2), T(1)(min) = 873 ms for 3-H(2), T(1)(min) = 33 ms, (1)J(HD) = 30.1 Hz for 4-H(2).

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.

A carbonyl-rich bridging hydride complex relevant to the Fe-Fe hydrogenase active site.

Protonation of Fe(2)(µ-S(2)C(3)H(6))(CO)(6) with an acid derived from [SiEt(3)][B(C(6)F(5))(4)] and HCl affords a hydride-bridged dimer.

Dihydrogen/dihydride or tetrahydride? An experimental and computational investigation of pincer iridium polyhydrides.

The iridium pincer complexes (PCP)IrH(4) (1; PCP = [kappa(3)-1,3-(CH(2)P(t)Bu(2))(2)C(6)H(3)]) and (POCOP)IrH(4) (2; POCOP = [kappa(3)-1,3-(OP(t)Bu(2))(2)C(6)H(3)]) have proven to be effective catalyst precursors for dehydrogenation of alkanes. The complex (POCOP)IrH(2) has also been applied successfully as a catalyst for release of H(2) from ammonia borane. Investigation of the "tetrahydride" forms of these complexes by solution NMR methods suggests their formulation as dihydrogen/dihydride species. This is in contrast to the solid state structure of 1, determined by neutron diffraction (at 100 K), which indicates a compressed tetrahydride structure with only weak H-H interactions. Complex 1 (C(24)H(47)IrP(2)) crystallizes in the space group P4(2), tetragonal, (Z = 2) with a = 11.7006 (19) A, c = 9.7008(27) A, and V = 1328.1(5) A(3). Electronic structure calculations on 1 and 2 indicate that the global minima on the potential energy surfaces in the gas phase are tetrahydride structures; however, the dihydrogen/dihydride forms are only slightly higher in energy (1-3 kcal/mol). A dihydrogen/dihydride species is calculated to be the global minimum for 2 when in solution. The barriers to interconversion between the tetrahydride and dihydrogen/dihydride species are almost negligible.

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 Pd(0) complexes, [Pd(IPr)(PCy(3))] is able to activate the H-H bond. The resulting trans-[Pd(H)(2)(IPr)(PCy(3))] 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.

Sigma-borane complexes of iridium: synthesis and structural characterization.

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.

Iridium-catalyzed dehydrogenation of substituted amine boranes: kinetics, thermodynamics, and implications for hydrogen storage.

Dehydrogenation of amine boranes is catalyzed efficiently by the iridium pincer complex (kappa (3)-1,3-(OP ( t )Bu 2) 2C 6H 3)Ir(H) 2 ( 1). With CH 3NH 2BH 3 (MeAB) and with AB/MeAB mixtures (AB = NH 3BH 3), the rapid release of 1 equiv of H 2 is observed to yield soluble oligomeric products at rates similar to those previously reported for the dehydrogenation of AB catalyzed by 1. Delta H for the dehydrogenation of AB, MeAB, and AB/MeAB mixtures has been determined by calorimetry. The experimental heats of reaction are compared to results from computational studies.

H2 addition to (Me4PCP)Ir(CO): studies of the isomerization mechanism.

Reduced steric demand of the Me4PCP pincer ligand (PCP = κ3-C6H4-1,3-[CH2PR2]2, R = Me), allows for a more open metal center. This is evident through structure and reactivity comparisons between (Me4PCP)Ir derivatives and other (R4PCP)Ir complexes (R = tBu, iPr, CF3). In particular, isomerization from cis-(R4PCP)Ir(H)2(CO) to trans-(R4PCP)Ir(H)2(CO) is more facile when R = Me than when R = iPr. Deuterium incorporation in the hydride ligands from solvent C6D6 was observed during this isomerization when R = Me. This deuterium exchange has not been observed for other analogous R4PCP iridium complexes. A kinetic study of the cis/trans isomerization combined with computational studies suggests that the cis/trans isomerization proceeds through a migratory-insertion pathway involving a formyl intermediate.

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

The dicationic complexes [Os(H(2))(PR(3))(2)(bpy)(CO)](2+) [PR(3) = PPh(3), PMePh(2) (2a,b)], [Os(H(2))(PPh(3))(2)(phen)(CO)](2+) (2c), and [Ru(H(2))(PPh(3))(2)(bpy)(CO)](2+) (4) (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) have been prepared by the protonation of the corresponding monocationic hydrides using an excess of trifluoromethanesulfonic acid. The presence of a bound dihydrogen ligand is indicated by short T(1) minimum values consistent with H-H distances of 0.92-1.04 Å. For the partially deuterated derivatives, J(HD) values of 25.1-31.0 Hz were observed. The dicationic complexes are strong acids, 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. The complex [Os(H(2))(PPh(3))(2)(bpy)(CO)](2+) (2a) has been shown to be very stable in solution at room temperature. In contrast, the ruthenium analogue, [Ru(H(2))(PPh(3))(2)(bpy)(CO)](2+) (4), decomposes in solution at room temperature but is relatively stable at temperatures less than 245 K.

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 Å. 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 Å, 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 Å. 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.

Photochemical generation of dihydrogen complexes of chromium and tungsten.

Photolysis of solutions of M(CO)(6) (M = Cr, W) at low temperature in the presence of hydrogen gas affords Cr(CO)(5)(H(2)) (1) and W(CO)(5)(H(2)) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T(1) values for the hydride resonances and the observation of a large HD coupling in the HD derivatives. Irradiation of a phosphine-substituted derivative (PMe(3))Cr(CO)(5) in the presence of hydrogen gas gave similar results. Thus cis-(PMe(3))Cr(CO)(4)(H(2)) (3) and trans-(PMe(3))Cr(CO)(4)(H(2)) (4) were prepared and characterized by (1)H and (31)P NMR spectroscopy. When the photolysis reactions were carried out in methylene chloride, solvent binding competitive with hydrogen binding was observed. This was not observed in less coordinating solvents such as alkanes. Subsequent displacement of solvent by H(2) leads to the dihydrogen complexes. Complexes 1 and 2 are moderately acidic, with deprotonation effected by mild bases.