Dihydrogen Complexes Research Articles

σ-Alane Complexes of Chromium, Tungsten, and Manganese

Photolytic ligand displacement and salt metathesis routes have been exploited to give access to κ(1) σ-alane complexes featuring Al-H bonds bound to [W(CO)(5)] and [Cp'Mn(CO)(2)] fragments, together with a related κ(2) complex of [Cr(CO)(4)]. Spectroscopic, crystallographic, and quantum chemical studies are consistent with the alane ligands acting predominantly as σ-donors, with the resulting binding energies calculated to be marginally greater than those found for related dihydrogen complexes.

Theoretical study of two states reactivity of methane activation on iron atom and iron dimer

Density functional theory (DFT) calculations have been carried out to explore the catalytic activation of C–H bonds in methane by the iron atom, Fe, and the iron dimer, Fe2. For methane activation on an Fe atom, the calculations suggest that the activation of the first C–H bond is mediated via the triplet excited-state potential energy surface (PES), with initial excitation of Fe to the triplet state being necessary for the reaction to be energetically feasible. Compared with the breaking of the first C–H bond, the cleavage of the second C–H bond is predicted to involve a significantly higher barrier, which could explain experimental observations of the HFeCH3 complex rather than CH2FeH2 in the activation of methane by an Fe atom. For methane activation on an iron dimer, the cleavage of the first C–H bond is quite facile with a barrier only 11.2, 15.8 and 8.4kcal/mol on the septet state energy surface at the B3LYP/6-311+G(2df,2dp), BPW91/6-311+G(2df,2dp) and M06/B3LYP level, respectively. Cleavage of the second C–H bond from HFe2CH3 involves a barrier calculated respectively as 18.0, 10.7 and 12.4kcal/mol at the three levels. The results suggest that the elimination of hydrogen from the dihydrogen complex is a rate-determining step. Overall, our results indicate that the iron dimer Fe2 has a stronger catalytic effect on the activation of methane than the iron atom.

Isolation, observation, and computational modeling of proposed intermediates in catalyticprotonreductions with the hydrogenase mimic Fe2(CO)6S2C6H4

Proposed electrocatalytic proton reduction intermediates of hydrogenase mimics were synthesized, observed, and studied computationally. A new mechanism for H(2) generation appears to involve Fe(2)(CO)(6)(1,2-S(2)C(6)H(4)) (3), the dianions {[1,2-S(2)C(6)H(4)][Fe(CO)(3)(μ-CO)Fe(CO)(2)](2-) (3(2-)), the bridging hydride {[1,2-S(2)C(6)H(4)][Fe(CO)(3)(μ-CO)(μ-H)Fe(CO)(2)]}(-), 3H(-)(bridging), and the terminal hydride 3H(-)(term-stag), {[1,2-S(2)C(6)H(4)][HFe(CO)(3)Fe(CO)(3)]}(-), as intermediates. The dimeric sodium derivative of 3(2-), {[Na(2)(THF)(OEt(2))(3)][3(2-)]}(2) (4) was isolated from reaction of Fe(2)(CO)(6)(1,2-S(2)C(6)H(4)) (3) with excess sodium and was characterized by X-ray crystallography. It possesses a bridging CO and an unsymmetrically bridging dithiolate ligand. Complex 4 reacts with 4 equiv. of triflic or benzoic acid (2 equiv. per Fe center) to generate H(2) and 3 in 75% and 60% yields, respectively. Reaction of 4 with 2 equiv. of benzoic acid generated two hydrides in a 1.7 : 1 ratio (by (1)H NMR spectroscopy). Chemical shift calculations on geometry optimized structures of possible hydride isomers strongly suggest that the main product, 3H(-)(bridging), possesses a bridging hydride ligand, while the minor product is a terminal hydride, 3H(-)(term-stag). Computational studies support a catalytic proton reduction mechanism involving a two-electron reduction of 3 that severs an Fe-S bond to generate a dangling thiolate and an electron rich Fe center. The latter iron center is the initial site of protonation, and this event is followed by protonation at the dangling thiolate to give the thiol thiolate [Fe(2)H(CO)(6)(1,2-SHSC(6)H(4))]. This species then undergoes an intramolecular acid-base reaction to form a dihydrogen complex that loses H(2) and regenerates 3.

ChemInform Abstract: Phosphine‐Supported Metal—Dihydrogen Complexes: Elongation of H—H Bond to Reversible Release of H2

AbstractReview: 201 refs.

Catalytic Hydrogenation of Carbon Dioxide and Bicarbonates with a Well‐Defined Cobalt Dihydrogen Complex

Catalytic Hydrogenation of Carbon Dioxide and Bicarbonates with a Well‐Defined Cobalt Dihydrogen Complex

Reactions of an Osmium Bis(dihydrogen) Complex under Ethylene: Phosphine Addition to a C–C Double Bond and C–H Bond Activation of Fluoroarenes

The bis(dihydrogen) complex [OsTp(η2-H2)2(PiPr3)]BF4 (1; Tp = hydridotris(pyrazolyl)borate) reacts with ethylene under 2 atm of the gas to afford [OsTp(CH2CH2PiPr3)(η2-CH2═CH2)2]BF4 (2), which promotes the CH bond activation of fluorobenzene and 1,3-difluorobenzene to give OsTp(3-C6H4F)(η2-CH2═CH2)2 (3) and OsTp(3,5-C6H3F2)(η2-CH2═CH2)2 (4), respectively, releasing [EtPiPr3]BF4. The formation of 2 takes place via [OsTp(η2-CH2═CH2)2(PiPr3)]BF4 (5), which has been isolated and characterized by X-ray diffraction analysis.

Theoretical study of hydrogen and dihydrogen bond interaction of B6H10 with the HF molecule

Ab initio calculations were used to analyze the interactions of B6H10 with an HF molecule at the MP2/6-311++g(d,p) and B3LYP/6-311++g(d,p) computational levels. B6H10 could interact with HF through both its terminal (Ht) and bridged (Hb) hydrogens. The interaction of Ht and Hb of B6H10 with HF could result in the formation of Ht···H and Hb···F dihydrogen and hydrogen bond complexes, respectively. The obtained structures have been analyzed with the Atoms in Molecules methodology.

Ruthenium Agostic (Phosphinoaryl)borane Complexes: Multinuclear Solid-State and Solution NMR, X-ray, and DFT Studies

The reactivity of the (o-phosphinophenyl)(amino)borane compound HB(N(i)Pr(2))C(6)H(4)(o-PPh(2)) prepared from Li(C(6)H(4))PPh(2) and HBCl(N(i)Pr(2)) toward the bis(dihydrogen) complex RuH(2)(H(2))(2)(PCy(3))(2) (1) was studied by a combination of DFT, X-ray, and multinuclear NMR techniques including solid-state NMR, a technique rarely employed in organometallic chemistry. The study showed that the complex RuH(2){HB(N(i)Pr(2))C(6)H(4)(o-PPh(2))}(PCy(3))(2) (3), isolated in excellent yield as yellow crystals and characterized by X-ray diffraction, led in solution to PCy(3) dissociation and formation of an unsaturated 16-electron complex RuH(2){HB(N(i)Pr(2))C(6)H(4)(o-PPh(2))}(PCy(3)) (4), with a hydride trans to a vacant site. In both cases, the (phosphinoaryl)(amino)borane acts as a bifunctional ligand through the phosphine moiety and a Ru-H-B interaction, thus featuring an agostic interaction.

Insights into Dehydrogenative Coupling of Alcohols and Amines Catalyzed by a (PNN)–Ru(II) Hydride Complex: Unusual Metal–Ligand Cooperation

Density functional theory calculations were performed to elucidate the mechanism of dehydrogenative coupling of primary alcohols and amines mediated by a PNN-Ru(II) hydride complex (PNN = (2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine)). A plausible reaction pathway was proposed which contains three stages: (1) The alcohol dehydrogenation reaction to generate the aldehyde and H(2); (2) The aldehyde-amine condensation reaction to form the hemiaminal intermediate; (3) The dehydrogenation process of the hemiaminal intermediate to yield the final amide product with the liberation of H(2). The first and third stages occur via a similar pathway: (a) Proton transfer from the substrate to the PNN ligand; (b) Intramolecular rearrangement of the deprotonated substrate to form an anagostic complex; (c) Hydride transfer from the deprotonated substrate to the Ru center to yield the trans-dihydride intermediate and the aldehyde (or amide); (d) Benzylic proton migration from the PNN ligand to the metal center forming a dihydrogen complex and subsequent H(2) liberation to regenerate the catalyst. In all these steps, the metal-ligand cooperation plays an essential role. In proton transfer steps (a) and (d), the metal-ligand cooperation is achieved through the aromatization/dearomatization processes of the PNN ligand. While in steps (b) and (c), their collaboration are demonstrated by the formation of an anagostic interaction between Ru and the C-H bond and two ionic hydrogen bonds supported by the PNN ligand.

Rh{NC5H3-2,6-(CH2PtBu2)2}(PCy3)][BArF4]: A Latent Low-Coordinate Rhodium(I) PNP Pincer Compound

An expedient new route to the cationic [(PNPtBu)Rh(L)]+ metal fragments (PNPtBu = κ3-NC5H3-2,6-(CH2P tBu2)2; L = CO, acetone, ethene, MeCN, N2) by the synthesis of, then substitution of PCy3 in, [(PNPtBu)Rh(PCy3)][BArF4] is reported. This synthetic route also allows for the synthesis of the new dihydrogen complex [(PNPtBu)Rh(H2)][BArF4]. [(PNPR)Rh(PCy3)][BArF4] (R= Ph, Cy, Mes) are also reported, in which the PCy3 ligand is more tightly bound to the metal center.

Water Splitting Promoted by a Ruthenium(II) PNN Complex: An Alternate Pathway through a Dihydrogen Complex for Hydrogen Production

A new mechanism involving the formation of a dihydrogen intermediate is described using the TPSS level of the DFT method for the generation of hydrogen from water catalyzed by the PNN Ru(II) pincer complex 4 (Science2009, 324, 74). The Mulliken charge of the hydride ligand in 4 is highly negative (−1.574) and facilitates a dihydrogen bonding with one of the solvated water molecules to yield 4···H2O. In the next step, water coordinates to the metal center by forcing a conformational change in the orientations of the hydride and CO ligands to form a transient intermediate, 4′(H2O). The activation free energy, Gact for this step is 14.92 kcal/mol. Subsequently, the intermediate liberates the dihydrogen with a Gact of 10.47 kcal/mol. With respect to (4 + H2O) as reference, the effective Gact of the entire mechanism is calculated to be 32.56 kcal/mol. Unlike the previously reported mechanism, the new mechanism operates without the cooperation of the aromatization–dearomatization processes of the pincer ligand and bypasses a highly reversible pathway involving the water-mediated 4 to 1 conversion. Further, a direct pathway for the formation of the cis-dihydroxo intermediate is possible with the new mechanism.

A Mercurial Route to a Cobalt Dihydrogen Complex

Mercury for protection? Mercury protects the highly reactive 14-electron {Co(POCOP)} fragment in a Co–Hg–Co complex, but reaction with H2 gives a rare dihydrogen complex of cobalt, and a second H2 molecule is added at higher pressures and low temperature (see scheme).

Anharmonicity of Weakly Bound M+−H2 Complexes

The anharmonicity of weakly bound complexes is studied using the vibrational self-consistent field (VSCF) approach for a series of metal cation dihydrogen (M(+)-H(2)) complexes. The H-H stretching frequency shifts of M(+)-H(2) (M(+) = Li(+), Na(+), B(+), and Al(+)) complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated H-H stretching frequency of Li(+)-H(2), B(+)-H(2), Na(+)-H(2), and Al(+)-H(2) is red-shifted by 121, 202, 74, and 62 cm(-1), respectively, relative to that of unbound H(2). The calculated red shifts and their trends are in good agreement with the available experimental and previously calculated data. Insight into the observed trends is provided by symmetry adapted perturbation theory (SAPT).

Tridentate and Tetradentate Iminophosphorane-Based Ruthenium Complexes in Catalytic Transfer Hydrogenation of Ketones

Tetradentate iminophosphorane complexes [RuH(PNNP)][BArF4] (5) and [RuHCl(PNNP)] (6) (PNNP = PPh2CH2PPh2NCH2CH2NPPh2CH2PPh2), prepared by in situ deprotonation of the aminophosphonium ligands in the presence of the dihydrogen complex RuH(H2)(Cl)(PCy3)2, are active precatalysts for the catalytic transfer hydrogenation of ketones at 80 °C under basic conditions in HOiPr. Complex 6 was characterized by X-ray diffraction. With a predominant focus on acetophenone, a comparative study with [RuH(H2NNPP)(PCy3)][BArF4] (3) incorporating a tridentate ligand (H2NNPP = H2N(C6H4)NPPh2CH2PPh2) does not indicate an NH effect. It is postulated that the pentacoordinated configuration around Ru with a hydride trans to a vacant site is essential in the catalyst resting state; however, the stability of this structural arrangement is disadvantageous for high activities. Further mechanistic studies, including labeling experiments, indicate facile protonation of these systems and, in addition, structural variations as a function of protonating agent (HOiPr, HX, H2O). The hydrido(chloro) complex RuH(Cl)(HNP)(NP) (7) (NP = Ph2PCH2PPh2NCH2tBu) was also isolated and characterized by X-ray diffraction, and a hydrogen bond between the chlorine and the NH group of the pendant ligand was ascertained by DFT calculations.

Immobilized contrast-enhanced MRI: Gadolinium-based long-term MR contrast enhancement of the vein graft vessel wall

An implantable MR contrast agent that can be covalently immobilized on tissue during surgery has been developed. The rationale is that a durable increase in tissue contrast using an implantable contrast agent can enhance postsurgical tissue differentiation using MRI. For small-vessel (e.g., vein graft) MRI, the direct benefit of such permanent “labeling” of the vessel wall by modification of its relaxation properties is to achieve more efficient imaging. This efficiency can be realized as either increased contrast leading to more accurate delineation of vessel wall and lesion tissue boundaries, or, faster imaging without penalizing contrast-to-noise ratio, or a combination thereof. We demonstrate, for the first time, stable long-term MRI enhancement using such an exogenous contrast mechanism based on immobilizing a modified diethylenetriaminepentaacetic acid gadolinium3+ dihydrogen complex on a human vein using a covalent amide bond. Signal enhancement due to the covalently immobilized contrast agent is demonstrated for excised human vein specimens imaged at 3 T, and its long-term stability is demonstrated during a 4-month incubation period. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc.

ChemInform Abstract: First‐Row Transition Metal Catalyzed Reduction of Carbonyl Functionalities: A Mechanistic Perspective

AbstractReview: 170 refs.

Preparation of a Dihydrogen Complex of Cobalt

Ammonia borane (AB) is an attractive candidate for the chemical storage of hydrogen. Recently, our research group reported that the complex [(pocop)IrH2] (pocop= k -C6H31,3-[OP(tBu)2]2) has facilitated the rapid release of H2 from AB under mild conditions. While this result is promising, iridium is too expensive for widespread application. Our efforts to extend the catalytic chemistry of [(pocop)IrH2] to Co have led to several intriguing complexes. Direct analogues of the Ir catalyst were obtained, thereby demonstrating the unusual ligation of H2 on a Co center. Herein we report the first cobalt–dihydrogen complex, which was characterized by NMR spectroscopy and studied by means of theoretical calculations. The metalation of pocop–H with Co, achieved with a method similar to procedures developed for Ir or Ni, did not proceed in satisfactory yield. An alternative approach that makes use of an iodinated pocop ligand was pursued, as described for a similar synthesis. Complex 1 was prepared in good yield by activation of the ligand with nBuLi and the addition of CoI2·THF (Scheme 1).

Preparation of a Dihydrogen Complex of Cobalt

H2 & Co.: Ein neuer Cobalt(II)-Pinzettenkomplex kann in Abwesenheit weiterer Liganden unter Bildung der hoch reaktiven Quecksilber-verbrückten Dicobaltspezies 1 reduziert werden. Nach der Behandlung mit H2 bei tiefer Temperatur gelingt es, den ungewöhnlichen Cobalt-Diwasserstoffkomplex 2 zu beobachten. Co blau, P gelb, O rot, Hg dunkelgrau. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Diverse Roles of Hydrogen in Rhenium Carbonyl Chemistry: Hydrides, Dihydrogen Complexes, and a Formyl Derivative

Rhenium carbonyl hydride chemistry dates back to the 1959 synthesis of HRe(CO)₅ by Hieber and Braun. The binuclear H₂Re₂(CO)₈ was subsequently synthesized as a stable compound with a central Re₂(μ-H)₂ unit analogous to the B₂(μ-H)₂ unit in diborane. The complete series of HRe(CO)(n) (n = 5, 4, 3) and H₂Re₂(CO)(n) (n = 9, 8, 7, 6) derivatives have now been investigated by density functional theory. In contrast to the corresponding manganese derivatives, all of the triplet rhenium structures are found to lie at relatively high energies compared with the corresponding singlet structures consistent with the higher ligand field splitting of rhenium relative to manganese. The lowest energy HRe(CO)₅ structure is the expected octahedral structure. Low-energy structures for HRe(CO)(n) (n = 4, 3) are singlet structures derived from the octahedral HRe(CO)₅ structure by removal of one or two carbonyl groups. For H₂Re₂(CO)₉ a structure HRe₂(CO)₉(μ-H), with one terminal and one bridging hydrogen atom, lies within 3 kcal/mol of the structure Re₂(CO)₉(η²-H₂), similar to that of Re₂(CO)₁₀. For H₂Re₂(CO)(n) (n = 8, 7, 6) the only low-energy structures are doubly bridged singlet Re₂(μ-H)₂(CO)(n) structures. Higher energy dihydrogen complex structures are also found.

Hydride Ion Transfer from Ruthenium(II) Complexes in Water: Kinetics and Mechanism

Reactions of hydride complexes of ruthenium(II) with hydride acceptors have been examined for Ru(terpy)(bpy)H(+), Ru(terpy)(dmb)H(+), and Ru(η(6)-C(6)Me(6))(bpy)(H)(+) in aqueous media at 25 °C (terpy = 2,2';6',2''-terpyridine, bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine). The acceptors include CO(2), CO, CH(2)O, and H(3)O(+). CO reacts with Ru(terpy)(dmb)H(+) with a rate constant of 1.2 (0.2) × 10(1) M(-1) s(-1), but for Ru(η(6)-C(6)Me(6))(bpy)(H)(+), the reaction was very slow, k ≤ 0.1 M(-1) s(-1). Ru(terpy)(bpy)H(+) and Ru(η(6)-C(6)Me(6))(bpy)(H)(+) react with CH(2)O with rate constants of (6 ± 4) × 10(6) and 1.1 × 10(3) M(-1) s(-1), respectively. The reaction of Ru(η(6)-C(6)Me(6))(bpy)(H)(+) with acid exhibits straightforward, second-order kinetics, with the rate proportional to [Ru(η(6)-C(6)Me(6))(bpy)(H)(+)] and [H(3)O(+)] and k = 2.2 × 10(1) M(-1) s(-1) (μ = 0.1 M, Na(2)SO(4) medium). However, for the case of Ru(terpy)(bpy)H(+), the protonation step is very rapid, and only the formation of the product Ru(terpy)(bpy)(H(2)O)(2+) (presumably via a dihydrogen or dihydride complex) is observed with a k(obs) of ca. 4 s(-1). The hydricities of HCO(2)(-), HCO(-), and H(3)CO(-) in water are estimated as +1.48, -0.76, and +1.57 eV/molecule (+34, -17.5, +36 kcal/mol), respectively. Theoretical studies of the reactions with CO(2) reveal a "product-like" transition state with short C-H and long M-H distances. (Reactant) Ru-H stretched 0.68 Å; (product) C-H stretched only 0.04 Å. The role of water solvent was explored by including one, two, or three water molecules in the calculation.