Quantum Mechanical Exchange Coupling in Polyhydride and Dihydrogen Complexes

Exploring the Tl 2 H 2 potential energy surface: A comparative analysis with group 13 systems and experiment

Thallium chemistry is experiencing unprecedented importance. Therefore, it is valuable to characterize some of the simplest thallium compounds. Stationary points along the singlet and triplet Tl  2 H  2 potential energy surface have been characterized. Stationary point geometries were optimized with the CCSD(T)/aug-cc-pwCVQZ-PP method. Harmonic vibrational frequencies were computed at the same level of theory while anharmonic vibrational frequencies were computed at the CCSD(T)/aug-cc-pwCVTZ-PP level of theory. Final energetics were obtained with the CCSDT(Q) method. Basis sets up to augmented quintuple-zeta cardinality (aug-cc-pwCV5Z-PP) were employed to obtain energetics in order to extrapolate to the complete basis set limits using the focal point approach. Zero-point vibrational energy corrections were appended to the extrapolated energies in order to determine relative energies at 0 K. It was found that the planar dibridged isomer lies lowest in energy while the linear structure lies highest in energy. The results were compared to other group 13 M  2 H  2 (M = B, Al, Ga, In, and Tl) theoretical studies and some interesting variations are found. With respect to experiment, incompatibilities exist.

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

Pairwise H2/D2 Exchange and H2 Substitution at a Bimetallic Dinickel(II) Complex Featuring Two Terminal Hydrides

A compartmental ligand scaffold HL with two β-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi₂(H)₂], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi₂(μ-Br)] (1) with MHBEt₃ (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K·2 with dibenzo(18-crown-6) led to the separated ion pair [LNi₂(H)₂][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D₂ was investigated by a suite of ¹H and ²H NMR experiments, revealing an unusual pairwise H₂/D₂ exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of ∼62 kJ/mol, to give H₂ and an antiferromagnetically coupled [LNi^I₂]^- species. This was confirmed by SQUID monitoring during H₂ release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na⁺ or K⁺) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)₂] → H₂ transition derived from 2D ¹H EXSY spectra confirmed first-order dependence of H₂ release on M·2 concentration and a strong effect of the alkali metal cation M⁺. Treating [LNi₂(D)₂]^- with phenylacetylene led to D₂ and dinickel(II) complex 3^- with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNi^II₂(H)₂]^- having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H₂ upon substrate binding is reminiscent of the proposed N₂ binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.

Elongated Dihydrogen versus Compressed Dihydride in Osmium Complexes

Small modifications on the co-ligands of complexes containing two coordinated hydrogen atoms can determine the elongated dihydrogen versus compressed dihydride nature of these species and therefore their chemical behavior. 2,6-diphenylpyridine favors the formation of the osmium(IV) cation [OsH₂ (C₆ H₄ pyPh)(PiPr₃ )₂ ]⁺ , whereas 2-phenoxy-6-phenylpyridine, which contains an oxygen atom between the heterocycle and one of the phenyl groups, stabilizes the osmium(II) elongated dihydrogen species [Os(C₆ H₄ pyOPh)(η² -H₂ )(PiPr₃ )₂ ]⁺ . In contrast to the latter, the former shows a marked tendency to undergo reductive elimination of the heterocycle.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

NHC carbene supported half-sandwich hydridosilyl complexes of ruthenium: the impact of supporting ligands on Si⋯H interligand interactions

Electron donating NHC carbene ligand IPr exerts stronger RuH⋯Si interactions in complexes Cp(IPr)RuH₂(SiR₃) than in related ⁱPr₃P complexes Cp(ⁱPr₃P)RuH₂(SiR₃).

Heterolytic cleavage of H2 by bifunctional manganese(i) complexes: impact of ligand dynamics, electrophilicity, and base positioning

A balance of metal electrophilicity and ligand steric influences is required for facile, reversible H–H heterolytic cleavage in Mn complexes with pendant amines.

Dihydrogen bonding in complex (PP3)RuH(η(1)-BH4) featuring two proton-accepting hydride sites: experimental and theoretical studies

Combining variable-temperature infrared and NMR spectroscopic studies with quantum-chemical calculations (density functional theory (DFT) and natural bond orbital) allowed us to address the problem of competition between MH (M = transition metal) and BH hydrogens as proton-accepting sites in dihydrogen bond (DHB) and to unravel the mechanism of proton transfer to complex (PP3)RuH(η(1)-BH4) (1, PP3 = κ(4)-P(CH2CH2PPh2)3). Interaction of complex 1 with CH3OH, fluorinated alcohols of variable acid strength [CH2FCH2OH, CF3CH2OH, (CF3)2CHOH (HFIP), (CF3)3COH], and CF3COOH leads to the medium-strength DHB complexes involving BH bonds (3-5 kcal/mol), whereas DHB complexes with RuH were not observed experimentally. The two proton-transfer pathways were considered in DFT/M06 calculations. The first one goes via more favorable bifurcate complexes to BHterm and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP3)RuHeq(H2)](+)[OR](-). The second pathway starts from the less-favorable complex with RuH ligand but shows a lower activation barrier (23.5 kcal/mol for HFIP) and eventually leads to the final product via the isomerization of intermediate [(PP3)RuHax(H2)](+)[OR](-). The B-Hbr bond breaking is the common key step of all pathways investigated.

Group 14 hydrides with low valent elements for activation of small molecules

Transition metal compounds are well known as activators of small molecules, and they serve as efficient catalysts for a variety of homogeneous and heterogeneous transformations. In contrast, there is a general feeling that main group compounds cannot act as efficient catalysts because of their inability to activate small molecules. Traditionally, the activation of small molecules is considered one of the key steps during a catalytic cycle with transition metals. As a consequence, researchers have long neglected the full range of possibilities in harnessing main group elements for the design of efficient catalysts. Recent developments, however, have made it possible to synthesize main group compounds with low-valent elements capable of activating small molecules. In particular, the judicious use of sterically appropriate ligands has been successful in preparing and stabilizing a variety of Group 14 hydrides with low-valent elements. In this Account, we discuss recent advances in the synthesis of Group 14 hydrides with low-valent elements and assess their potential as small-molecule activators. Group 14, which comprises the nonmetal C, the semimetals Si and Ge, and the metals Sn and Pb, was for years a source of hydrides with the Group 14 element almost exclusively in tetravalent form. Synthetic difficulties and the low stability of Group 14 hydrides in lower oxidation states were difficult to overcome. But in 2000, a divalent Sn(II) hydride was prepared as a stable compound through the incorporation of sterically encumbered aromatic ligands. More recently, the stabilization of GeH(2) and SnH(2) complexes using an N-heterocyclic carbene (NHC) as a donor and BH(3) or a metal carbonyl complex as an acceptor was reported. A similar strategy was also employed to synthesize the Si(II) hydride. This class of hydrides may be considered coordinatively saturated, with the lone pair of electrons on the Group 14 elements taking part in coordination. We discuss the large-scale synthesis of hydrides of the form LMH (where M is Ge or Sn, L is CH(N(Ar)(CMe))(2), and Ar is 2,6-iPr(2)C(6)H(3)), which has made it possible to test their reactivity in the activation of small molecules. Unlike the tetravalent Group 14 hydrides, the Ge(II) and Sn(II) hydrides were found to be able to activate a number of small molecules in the absence of any externally added catalyst. For example, the Ge(II) hydride and Sn(II) hydride can activate CO(2), and the reaction results in the formation of Ge(II) and Sn(II) esters of formic acid. This product represents a prototype of a new class of compounds of Group 14 elements. Moreover, we examined the activation of carbonyl compounds, alkynes, diazo and azo compounds, azides, and compounds containing the C═N bond. These Group 14 hydrides with low-valent elements are shown to be able to activate a number of important small molecules with C≡C, C═O, N═N, and C═N bonds. The activation of small molecules is an important step forward in the realization of main group catalyst development. Although it is not yet customary to assay the potential of newly synthesized main group compounds for small-molecule activation, our results offer good reason to do so.

Aromatic diosmatricyclic nitrogen-containing compounds

Aromatic diosmatricyclic nitrogen-containing compounds are prepared from Os(VI) complex OsH6(PiPr3) by double 1,3-C-H bond activation of aromatic six-membered cycles with imino substituents meta disposed.

Hydrocarbon-Soluble Calcium Hydride: A “Worker-Bee” in Calcium Chemistry

The reactivity of the hydrocarbon-soluble calcium hydride complex [{CaH(dipp-nacnac)(thf)}(2)] (1; dipp-nacnac=CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)) with a large variety of substrates has been investigated. Addition of 1 to C=O and C=N functionalities gave easy access to calcium alkoxide and amide complexes. Similarly, reduction of the C[triple chemical bond]N bond in a cyanide or an isocyanide resulted in the first calcium aldimide complexes [Ca{N=C(H)R}(dipp-nacnac)] and [Ca{C(H)=NR}(dipp-nacnac)], respectively. Complexation of 1 with borane or alane Lewis acids gave the borates and alanates as contact ion pairs. In reaction with epoxides, nucleophilic ring-opening is observed as the major reaction. The high reactivity of hydrocarbon-soluble 1 with most functional groups contrasts strongly with that of insoluble CaH(2), which is essentially inert and is used as a common drying agent. Crystal structures of the following products are presented: [{Ca{OC(H)Ph(2)}(dipp-nacnac)}(2)], [{Ca{N=C(H)Ph}(dipp-nacnac)}(2)], [{Ca{C(H)=NC(Me)(2)CH(2)C(Me)(3)}(dipp-nacnac)}(2)], [{Ca{C(H)=NCy}(dipp-nacnac)}(2)], [Ca(dipp-nacnac)(thf)](+)[H(2)BC(8)H(14)](-) and [{Ca(OCy)(dipp-nacnac)}(2)]. The generally smooth and clean conversions of 1 with a variety of substrates and the stability of most intermediates against ligand exchange make 1 a valuable key precursor in the syntheses of a wide variety of beta-diketiminate calcium complexes.