Main Group Hydrides Research Articles

Reactions of pentaphenylborole with main group hydrides

The reactivity of the anti-aromatic pentaphenylborole with HBpin (pinacolborane), HGeEt3, and HSnBu3 was investigated. In all cases, the products were 1-bora-cyclopent-3-ene heterocycles resulting from the stereospecific addition of the E–H bond to the borole. Both HGeEt3, and HSnBu3 furnished syn products, while the reaction with pinacolborane yielded the anti-product. The discrepancy between the products is rationalized by previous studies reported on the reactivity of boroles with dihydrogen.

Establishing the Hydride Donor Abilities of Main Group Hydrides

Interest in reductions with main group hydrides has been reinvigorated with the discovery of frustrated Lewis pairs. Computational analysis showed that the borohydride of the commonly used Lewis acid B(C6F5)3 was determined to be 15 kcal/mol less reducing than borohydride ([BH4]−), 22 kcal/mol less reducing than aluminum hydride ([AlH4]−), and 41 kcal/mol less reducing than superhydride ([HBEt3]−). In addition to [HB(C6F5)3]−, a hydride donor ability scale with an estimated error of ∼3 kcal/mol includes 132 main group hydrides with gradually changing reducing capabilities spanning 160 kcal/mol. The scale includes representatives from organosilanes, organogermanes, organostannanes, borohydrides, boranes, aluminum hydrides, NADH analogues, and CH hydride donors. The large variety of reducing agents and the wide span of the scale (ranging from 0.5 to 160 kcal/mol in acetonitrile) make the scale a useful tool for the future design of metal-based or main group reducing agents.

Establishing the Steric Bulk of Main Group Hydrides in Reduction Reactions

AbstractThe steric bulk of reducing agents has been widely employed in the generation of chiral centers and selective reductions of organic compounds, but reports addressing the quantification of the steric bulk surrounding the main group hydride have been virtually non‐existent. To address the limited amount of steric bulk information of main group reducing agents, the cone angles of 84 main group hydrides related to the chemistry of frustrated Lewis pairs are reported. Of the 84 main group hydrides, the steric bulk of two carbon‐based hydrides, four aluminum hydrides, 11 borane‐Lewis base adducts derived from borenium cations, nine organosilanes, two organostannanes, one organogermane, and 55 borohydrides are investigated. In addition to the main group hydride complexes reported, the cone angle of four highly active metal‐hydride complexes, used in the asymmetric reduction of ketones, are determined and compared with chiral main group hydrides. Steric bulk analysis suggests that chiral hydride complexes exhibiting cone angles >165° achieve highly enantioselective reductions (>90 % ee).

Homopolar dihydrogen bonding in main group hydrides: discovery, consequences, and applications.

This perspective describes the recent discovery and investigation of homopolar dihydrogen bonding, and focuses on the identification and characterisation of hydride-hydride interactions in compounds of the main group elements. A highlight of this programme has been an appreciation of the important role played by this interaction in the structural and thermochemical properties of these materials, and in the mechanisms through which they release hydrogen. A fuller understanding of this new class of H∙∙∙H interactions has also allowed us to explore their role in the supramolecular chemistry of hydrogen-rich compounds.

Weakly Coordinated Zinc and Aluminum σ-Complexes of Copper(I)

We report the synthesis and isolation of three new σ-complexes of Cu(I) in which E–H (E = Al, Zn) σ-bonds are coordinated to copper. The addition of the main group hydride to a toluene-solvated Cu(I) complex results in reversible ligand exchange, and the Cu(I) σ-complexes have been crystallized. Experimental and computational data provide a wealth of evidence for weak binding of the E–H bond to Cu(I), which can be ascribed to σ-donation from the E–H bond into the 4s orbital of copper and back-donation from copper into the E–H σ* orbital.

Comparison of the Mechanism of Borane, Silane, and Beryllium Hydride Ring Insertion into N-Heterocyclic Carbene C–N Bonds: A Computational Study

A computational investigation has been carried out on the mechanism and energetics of the experimentally observed insertion/ring expansion of N-heterocyclic carbenes (NHCs) by boranes (H2BNHR, BH3; R = Me, Ph) and beryllium hydrides (BeH2) in comparison with silanes (SiH2R2; R = Me, Ph). The results suggest that the ring insertion mechanisms are similar for boranes, beryllium hydrides, and silanes. The principal mechanism components are (1) hydrogen atom migration to the carbene carbon, (2) C–N bond expansion of the NHC with insertion of the main-group hydride into the ring, and (3) migration of a second hydrogen atom to the carbene carbon. The synthetically important NHC·BH3 adduct is also predicted to be thermodynamically unstable with respect to this transformation but is kinetically stabilized with a high barrier to the first hydrogen atom migration. The BeH2 insertion product provides a rare example of a Be–N π interaction.

Synthesis and Hydride Transfer Reactions of Cobalt and Nickel Hydride Complexes to BX3Compounds

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].

Intercepting low oxidation state main group hydrides with a nucleophilic N-heterocyclic olefin

The N-heterocyclic olefin, IPr=CH(2) (IPr = [(HCNDipp)(2)C], Dipp = 2,6-(i)Pr(2)C(6)H(3)) has been demonstrated to be of sufficient Lewis basicity to stabilize main group hydrides in unusually low oxidation states.

Practical Materials Chemistry Approaches for Tuning Optical and Structural Properties of Group IV Semiconductors and Prototype Photonic Devices

We report new approaches based on rational design and preparation of chemical vapor deposition precursors involving novel main-group hydrides to fabricate new families of Si-based semiconductors and prototype devices that display compositional and structural inheritance, from the parent molecule to the solid end product. This methodology enables materials synthesis at extraordinarily low temperatures that are compatible with complementary metal-oxide-semiconductor (CMOS) processing/selective growth and provides the means for obtaining highly metastable strain states in prototype structures that cannot be obtained by conventional protocols. Some of the materials and devices under development, involving alloys in the Si-Ge-Sn system, open up exciting opportunities in photodetectors and photovoltaics because they grow directly on cheap Si substrates and cover an extended range of the near-infrared spectrum that is not accessible to current photovoltaic and optoelectronic group IV semiconductors.

(Invited) Si-Ge-Sn Technologies: From Molecules to Materials to Prototype Devices

We report new approaches based on rational design and preparation of chemical vapor deposition precursors involving novel main-group hydrides to fabricate new families of Si-based semiconductors and prototype devices that display compositional and structural inheritance, from the parent molecule to the solid end-product. This methodology enables materials synthesis at extraordinarily low temperatures that are compatible with CMOS processing/selective growth, and provides the means for obtaining highly metastable strain states in prototype structures that cannot be obtained by conventional protocols. Some of the materials and devices under development, involving alloys in the Si-Ge-Sn system, open up exciting opportunities in photodetectors and photovoltaics because they grow directly on cheap Si substrates and cover an extended range of the near-infrared spectrum that is not accessible to current photovoltaic and optoelectronic group IV semiconductors.

Gas phase acidities and associated equilibrium isotope effects for selected main group mono- and polyhydrides, carbon acids, and oxyacids: A G4 and W1BD study

Gas phase standard state (298.15 K, 1 atm) enthalpies (Δ acid H° (g)) and free energies (Δ acid G° (g)) of acid dissociation were calculated at the Gaussian-4 (G4) and W1BD levels of theory for a range of perproteated, perdeuterated, pertritiated, and partially isotopically labeled main group mono- and polyhydrides, carbon acids, and oxyacids. Excellent agreement was obtained between the available experimental datasets and the theoretical estimates, with effectively equivalent Δ acid H° (g)/Δ acid G° (g) prediction accuracy for the G4 and W1BD methods on carbon acids and oxyacids. The W1BD level of theory provided Δ acid H° (g)/Δ acid G° (g) errors about one-half those of the G4 method on main group hydrides. G4 and W1BD predicted primary and secondary equilibrium isotope effects (EIEs) on gas phase acidity for deuterium and tritium substitution exhibit periodic trends among the main group hydrides, as well as acid strength and structure–property relationships for some groups of carbon acids and oxyacids. Primary EIEs calculated at the W1BD level for various isotopologues of the H 2, LiH, CH 4, NH 3, SiH 4, SH 2, and ClH main group hydrides using the major isotopes for each non-protic element declined in magnitude with increasing atomic number due to decreasing mass differences between the respective isotopologues.

Recent developments in the chemistry of low valent Group 14 hydrides

This Perspective discusses the synthesis and reactivity of low valent Group 14 hydrides of germanium and tin. The use of hindered terphenyl ligands has facilitated the isolation of a number of unusual new main group hydride structural types (each predicted by computational work), and has culminated in the use of these species as precursors to new cluster archetypes.

Isomeric Forms of Heavier Main Group Hydrides: Experimental and Theoretical Studies of the [Sn(Ar)H]2 (Ar = Terphenyl) System

A series of symmetric divalent Sn(II) hydrides of the general form [(4-X-Ar')Sn(mu-H)]2 (4-X-Ar' = C6H2-4-X-2,6-(C6H3-2,6-iPr2)2; X = H, MeO, tBu, and SiMe3; 2, 6, 10, and 14), along with the more hindered asymmetric tin hydride (3,5-iPr2-Ar*)SnSn(H)2(3,5-iPr2-Ar*) (16) (3,5-iPr2-Ar* = 3,5-iPr2-C6H-2,6-(C6H2-2,4,6-iPr3)2), have been isolated and characterized. They were prepared either by direct reduction of the corresponding aryltin(II) chloride precursors, ArSnCl, with LiBH4 or iBu2AlH (DIBAL), or via a transmetallation reaction between an aryltin(II) amide, ArSnNMe2, and BH3.THF. Compounds 2, 6, 10, and 14 were obtained as orange solids and have centrosymmetric dimeric structures in the solid state with long Sn...Sn separations of 3.05 to 3.13 A. The more hindered tin(II) hydride 16 crystallized as a deep-blue solid with an unusual, formally mixed-valent structure wherein a long Sn-Sn bond is present [Sn-Sn = 2.9157(10) A] and two hydrogen atoms are bound to one of the tin atoms. The Sn-H hydrogen atoms in 16 could not be located by X-ray crystallography, but complementary Mössbauer studies established the presence of divalent and tetravalent tin centers in 16. Spectroscopic studies (IR, UV-vis, and NMR) show that, in solution, compounds 2, 6, 10, and 14 are predominantly dimeric with Sn-H-Sn bridges. In contrast, the more hindered hydrides 16 and previously reported (Ar*SnH)2 (17) (Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2) adopt primarily the unsymmetric structure ArSnSn(H)2Ar in solution. Detailed theoretical calculations have been performed which include calculated UV-vis and IR spectra of various possible isomers of the reported hydrides and relevant model species. These showed that increased steric hindrance favors the asymmetric form ArSnSn(H)2Ar relative to the centrosymmetric isomer [ArSn(mu-H)]2 as a result of the widening of the interligand angles at tin, which lowers steric repulsion between the terphenyl ligands.

Diverse World of Unconventional Hydrogen Bonds

AbstractFor Abstract see ChemInform Abstract in Full Text.

Diverse World of Unconventional Hydrogen Bonds

This Account presents our view of unconventional intermolecular hydrogen bonds (HBs) for organometallic complexes and transition-metal or main-group hydrides. Over the past decade, low-temperature spectroscopic (IR, UV, and NMR) studies combined with theoretical calculations have disclosed the static and dynamic features of different HBs. Their guiding role in the proton-transfer processes was determined, as well as the energetic characteristics of HB intermediates and the activation barriers. Nevertheless, there is still much to explore in terms of the prediction of HB properties and control of protonation/deprotonation processes.

Molecular dialane and other binary hydrides.

The dialane molecule (Al2H6, see structure) is highly unstable in the condensed phase, and hence it was more than half a century after the discovery of aluminum hydride that it was identified in a hydrogen matrix. Apart from such fundamental findings, the rapidly developing chemistry of the binary main-group hydrides is also aimed at synthetic applications.

Heats of hydrogenation of compounds featuring main group elements and with the potential for multiply bonding.

Reaction enthalpies are calculated for the hydrogenation reactions of main group hydrides with the potential for multiple bonding, and thus the unsaturated character of these species is determined. In addition to the global minimum structures, which leave in some cases no hope for even a single E-E bond (E=Group 13, 14, or 15 element), calculations are also performed for geometries with maximum potential for multiple bonding. The trends down the groups and the periods are established. Interpretations have to take several factors into account. These factors sometimes work hand in hand but also against each other. We also include in our survey the species [HGaGaH]2- as a free anion and Na2[HGaGaH] as well as their hydrogenation products [H2GaGaH2]2- and Na2[H2GaGaH2]2-. The results show that the presence of the Na+ ions has a significant impact on their chemistry, and thus suggests that they are involved to a large extent in the bonding. Our results indicate that the compounds should be described as cluster compounds.

Absolute Rate Expressions for Hydrogen Atom Abstraction from Molybdenum Hydrides by Carbon-Centered Radicals

A new family of basis rate expressions for hydrogen atom abstraction by primary, secondary, and tertiary alkyl radicals in dodecane and benzyl radical in benzene from the molybdenum hydride Cp*Mo(CO)3H and for reactions of a primary alkyl radical with CpMo(CO)3H in dodecane are reported (Cp* = η5-pentamethylcyclopentadienyl, Cp = η5-cyclopentadienyl). Rate expressions for reaction of primary, secondary, and tertiary radical clocks with Cp*Mo(CO)3H were as follow: for hex-5-enyl, log(k/M-1 s-1) = (9.27 ± 0.13) − (1.36 ± 0.22)/θ, θ = 2.303RT kcal/mol; for hept-6-en-2-yl, log(k/M-1 s-1) = (9.12 ± 0.42) − (1.91 ± 0.74)/θ; and for 2-methylhept-6-en-2-yl, log(k/M-1 s-1) = (9.36 ± 0.18) − (3.19 ± 0.30)/θ (errors are 2σ). Hydrogen atom abstraction from CpMo(CO)3H by hex-5-enyl is described by log(k/M-1 s-1) = (9.53 ± 0.34) − (1.24 ± 0.62)/θ. Relative rate constants for 1°:2°:3° alkyl radicals were found to be 26:7:1 at 298 K. Benzyl radical was found to react 1.4 times faster than tertiary alkyl radical. The much higher selectivities for Cp*Mo(CO)3H than those observed for main group hydrides (Bu3SnH, PhSeH, PhSH) with alkyl radicals, together with the very fast benzyl hydrogen-transfer rate, suggest the relative unimportance of simple enthalpic effects and the dominance of steric effects for the early transition-state hydrogen transfers. Hydrogen abstraction from Cp*Mo(CO)3H by benzyl radicals is described by log(k/M-1 s-1) = (8.89 ± 0.22) − (2.31 ± 0.33)/θ.

Separability of spin–orbit and correlation energies for the sixth-row main group hydride ground states

The spin–orbit energy contributions to the ground state potential energy curves for the main group hydrides, TIH through AtH are estimated by differencing multireference, single promotion, configuration interaction (MRS-CI) energies with and without the spin–orbit operator. The spin–orbit contributions are then summed into the energies determined at the λ−s MRSD-CI level (both single and double promotions). The agreement between the resultant curves and those obtained using intermediate coupling MRSD-CI is within 1.2 kcal/mol over a range of internuclear separations. This suggests that, contrary to previous arguments, spin–orbit coupling and correlation energies are very nearly separable for the main group hydride ground states. Furthermore, the computational effort expended by this separate evaluation is up to 12 times less than that for a comparable intermediate coupling CI. The analysis of some properties of these hydrides indicates that bond length shifts due to spin–orbit coupling are small (0.03 Å) while harmonic vibrational frequencies decrease by up to 9%. Dissociation energies are predicted to change considerably in the presence of the operator in agreement with previous findings.

Helical, Polymeric Chains in Crystalline Gallaborane [GaBH6]n at 110 K

Distorted tetrahedral GaH4 and tetrahedral BH4 moieties are the building blocks of the helical, polymeric chains formed by gallaborane at 110 K (schematically shown on the right). There are two distinct types of BH4 groups: one is involved in Ga(μ-H)2B coordination and the other in unusual monohydrogen bridging of two adjacent GaH2 units. The structure invites comparison with those of other main group hydrides.