Main Group Hydrides Research Articles

Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent.

In this work, we report the first example of the PCET reactivity for a boron cluster compound, the zwitterionic nido-carboranyl diphosphonium derivative 7-P(H)tBu2-10-P(H)iPr2-nido-C2B10H10. This main-group reagent efficiently transfers two electrons and two protons to quinones to yield hydroquinones and regenerate a neutral closo-carboranyl diphosphine, 1-PtBu2-2-PiPr2-closo-C2B10H10. As we have previously reported the conversion of this closo-carboranyl diphosphine into the zwitterionic nido- derivative upon reaction with main group hydrides, the transformation reported herein represents a complete synthetic cycle for the metal-free reduction of quinones, with the redox-active carboranyl diphosphine scaffold acting as a mediator. The proposed mechanism of this reduction, based on pKa determination, electrochemical studies, and kinetic isotope effect determination, involves the electron transfer from the nido- cluster to the quinone coupled with the delivery of protons.

NHC aluminum chemistry on the rise.

This perspective highlights recent developments of the use of N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (cAACs) in alane and aluminum organyl chemistry. Especially in the last few years this flourishing research field led to some remarkable discoveries including various substitution patterns at the central aluminum atom, different oxidation states, neutral and charged compounds with varying coordination numbers and unique reactivities. Thereby NHCs play a vital role in the stabilization of these otherwise highly reactive compounds, which would not be realizable without the use of this intriguing class of ligands. Nevertheless, main group hydrides and especially NHC ligated alanes also tend to undergo NHC decomposition reactions, which are part of ongoing research and provide important information for NHC research in general.

Accelerating σ-Bond Metathesis at Sn(II) Centers.

Molecular main-group hydride catalysts are attractive as cheap and Earth-abundant alternatives to transition-metal analogues. In the case of the latter, specific steric and electronic tuning of the metal center through ligand choice has enabled the iterative and rational development of superior catalysts. Analogously, a deeper understanding of electronic structure-activity relationships for molecular main-group hydrides should facilitate the development of superior main-group hydride catalysts. Herein, we report a modular Sn-Ni bimetallic system in which we systematically vary the ancillary ligand on Ni, which, in turn, tunes the Sn center. This tuning is probed using Sn L1 XAS as a measure of electron density at the Sn center. We demonstrate that increased electron density at Sn centers accelerates the rate of σ-bond metathesis, and we employ this understanding to develop a highly active Sn-based catalyst for the hydroboration of CO2 using pinacolborane. Additionally, we demonstrate that engineering London dispersion interactions within the secondary coordination sphere of Sn allows for further rate acceleration. These results show that the electronics of main-group catalysts can be controlled without the competing effects of geometry perturbations and that this manifests in substantial reactivity differences.

Donor-acceptor stabilized isomers of composite group 13-14-15 hydrides

Structures and stability of LB·E13H2E14H2E15H2·LA compounds, stabilized by Lewis base (LB) and/or Lewis acid (LA), were computationally explored. Obtained results reveal that the isomers with the middle position of group 14 element are the most stable for all studied combinations of group 13-14-15 elements (B, Al, Ga; Si, Ge, Sn; P, As, Sb). Coordination of LA lowers the energy difference between isomers but increases the activation energy for the isomerization. Stronger LB, such as NHC, also increase the activation energy for the isomerization. Possible synthetic approaches to the main group hydride isomers with the middle position of group 14 element were proposed and computationally examined.

Structures and Stability of Mixed Main Group Hydrides [PBEH6]n (E = C, Si, Ge; n = 1-4) and Donor-Acceptor Stabilization of Monomeric Chain Isomers.

Structures and stability of mixed group 13-14-15 element hydrides PBEH6 (E = C, Si, Ge), their oligomers, and complexes with Lewis acids and Lewis base are computationally studied at the B3LYP-D3/def2-TZVP level of theory. Unsubstituted chain hydrides are unstable and are expected to form cyclic oligomers. Cyclization can be prevented by the donor-acceptor complex formation. For complexes of chain hydrides with Lewis acid in many cases additional reactivity beyond the donor-acceptor complex formation is observed: cyclization and migration of terminal group from the group 13 Lewis acid to the boron or group 14 terminal atoms of the hydride. The most promising compounds for the experimental studies have been identified. Joint stabilization by both W(CO)5 and trimethylamine provides a synergetic effect, allowing stabilization of 13-14-15 chain compounds.

Site-Selective Hydrogenation of Electron-Poor Alkenes and Dienes Enabled by a Rhodium-Catalyzed Hydride Addition/Protonolysis Mechanism.

The transition metal catalyzed hydrogenation of alkenes is a well-developed technology used on lab scale as well as on large scales in the chemical industry. Site- and chemoselective mono-hydrogenations of polarized conjugated dienes remain challenging. Instead, stoichiometric main-group hydrides are used rather than H2 . As part of an effort to develop a scalable route to prepare geranylacetone, we discovered that Rh(CO)2 acac/xantphos based catalysts enable the selective mono-hydrogenation of electron-poor 1,3-dienes, enones, and other polyunsaturated substrates. D-labeling and DFT studies support a mechanism where a nucleophilic RhI -hydride selectively adds to electron-poor alkenes and the resulting Rh-enolate undergoes subsequent inner-sphere protonation by alcohol solvent. The finding that (Ln )Rh(H)(CO) type catalysts can enable selective mono-hydrogenation of electron-poor 1,3-dienes provides a valuable tool in the design of related chemoselective hydrogenation processes.

Metal-Free Bond Activation by Carboranyl Diphosphines.

We report metal-free bond activation by the carboranyl diphosphine 1-PtBu2-2-PiPr2-C2B10H10. This main group element system contains basic binding sites and possesses the ability to cycle through two-electron redox states. The reported reactions with selected main group hydrides and alcohols occur via the formal oxidation of the phosphine groups and concomitant reduction of the boron cage. These transformations, which are driven by the cooperation between the electron-donating exohedral substituents and the electron-accepting cluster, differ from those of "regular" phosphines and are reminiscent of oxidative addition to transition metal centers, thus representing a new approach to metal-free bond activation.

Hydride Affinities for Main-Group Hydride Reductants: Assessment of Density Functionals and Trends in Reactivities.

In the present study, we have examined hydride affinities relevant to a range of group 13 and group 14 reductants. We use the high-level W1X-G0, G4(MP2)-XK, and DSD-PBEP86 methods to obtain the RHA42 set of accurate reductant hydride affinities. Assessment of DFT methods with the RHA42 set shows that all functionals that we have examined are fairly accurate. Overall, we find ωB97X-V to be the most accurate. The MN12-SX screened-exchange functional and the nonhybrid B97-D3BJ method also perform well, and they may provide a lower-cost means for obtaining hydride affinities. The trend in the hydride affinities suggests an increased reducing power when one moves down the periodic table, e.g., with TlH3 being a stronger reductant than BH3. We also find that group 13 hydrides are stronger reductants than the group 13 analogues. In general, substitution of a hydrogen, e.g., BH2+ → BHMe+, and the formation of dimer, e.g., BH2+ → B2H5+, also lead to stronger reductants. A notable observation is the small hydride affinities for silyl cations, which are indicative of the potential of silanes as strong reducing agents. In particular, poly(methylhydrosiloxane) (PMHS) cations are associated with especially small hydride affinities owing to the presence of intramolecular oxygen atoms that can stabilize the cation center. We have further found the germanium analogues of the silanes to be more reactive, and they may further widen the scope of main-group hydride reducing agents.

Silyl Anion Initiated Hydroboration of Aldehydes and Ketones.

Hydroboration is an emerging method for mild and selective reduction of carbonyl compounds. Typically, transition‐metal or reactive main‐group hydride catalysts are used in conjunction with a mild reductant such as pinacolborane. The reactivity of the main‐group catalysts is a consequence of the nucleophilicity of their hydride ligands. Silicon hydrides are significantly less reactive and are therefore not efficient hydroboration catalysts. Here, a readily prepared silyl anion is reported to be an effective initiator for the reduction of aldehydes and ketones requiring mild conditions, low catalyst loadings and with a good substrate scope. The silyl anion it is shown to activate HBpin to generate a reactive borohydride in situ which reacts with aldehydes and ketones to afford the hydroboration product.

Theoretical Investigation of Main-Group Element Hydride Insertion into Phosphorus-Heterocyclic Carbenes (PHCs)

Initial reports of ring expansion reactions (RER) of N-heterocyclic carbenes (NHCs) with main-group element hydrides have led to several synthetic and theoretical investigations, which include reports of insertion by Be, B, Al, Si, and Zn hydrides. The RERs generally lead to insertion of the heteroatom into the endocyclic C–N bond with formation of an expanded heterocyclic ring. Following the recent isolation of a P-heterocyclic carbene (PHC), here we report results from a computational study (RI-SCS-MP2/def2-TZVP//M06–2X/def2-TZVP) of RERs with a series of PHCs for the ring-insertion of silicon (SiH4, SiH2Ph2) and boron (BH3, BH2NMe2) hydrides. In order to explore the roles of both electronic and steric effects on PHCs and their reactivity, a series of P-substituent PHCR (R=H, Me, Ph, and bulky Ar groups) were investigated. Bulky R groups serve to maximise ring planarity and the σ-donating capability of the PHC. For RER, the PHC analogues exhibit facile initial hydride transfer from the main-group hydrides to the carbene carbon, with barriers that are substantially lower than with NHCs. However, the full ring insertion mechanisms for PHCs are, in general, kinetically unfavourable due to a large barrier associated with the ring-expansion step. While bulky P-substituents maximise heterocycle planarity towards that of NHCs, the RER reactivity with bulky PHCs does not reflect that of an NHC.

Investigation of main group promoted carbon dioxide reduction

The reduction of carbon dioxide (CO2) is of interest to the chemical industry, as many synthetic materials can be derived from CO2. To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO2 to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride's ability to insert CO2 into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO2 insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO2 adduct of tBu3P and B(C6F5)3 resulted in converting the sequestered CO2 molecule to CO. Reduction of the CO2 adduct of tBu3P and B(C6F5)3 with both electrons and protons resulted in only proton reduction.

Heterolytic bond activation at gold: evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage.

The coordinatively unsaturated gold(iii) chelate complex [(C^N-CH)Au(C6F5)]+ (1 +) reacts with main group hydrides H-BPin and H-SiEt3 in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C6H3Bu t )-6-(C6H4Bu t )pyridine anion; Pin = OCMe2CMe2O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuC6F5}2(μ-H)]+ (2 +) and (C^N-CH)AuH(C6F5) (5), together with spectroscopically detected [Et3Si-OEt2]+. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature (-20 °C). H2 activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt2)2]+, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C6F5H. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt2 with Hantzsch ester is essentially instantaneous and quantitative at -30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors towards gold.

Syntheses and Structures of Ruthenium Complexes Containing a Ru-H-Tl Three-Center-Two-Electron Bond.

Treatment of CpRuH(PP) (PP=dppm, dppe) with TlPF6 produced [CpRu(H)(Tl)(PP)]PF6 . X-ray diffraction and computational studies suggest that the complexes contain a Ru-H-Tl 3c-2e bond and can be viewed as the first σ-complexes of period 6 main-group hydrides [CpRu{η2 -(H-Tl)}(PP)]PF6 or [Tl{η2 -H-RuCp(PP)}]PF6 . The complexes can be stored as a solid at room temperature for days without appreciable decomposition, but are unstable in solution and evolved to the trimetallic complexes [{CpRu(PP)}2 (μ-Tl)]PF6 .

Bioinspired Catalytic Generation of Main-group Electrophiles by Cooperative Bond Activation.

Catalytic processes involving cooperativity have seen tremendous progress in recent years and impressive new synthetic methodologies have been developed. Inspired by the cooperative heterolytic H₂ splitting in [NiFe] hydrogenases, Ohki and Tatsumi designed cationic ruthenium thiolate complexes with a tethered sulfur ligand. Over the last decade, we have demonstrated the facile activation of main-group hydrides such as hydrosilanes, hydroboranes, DIBAL-H, and hydrostannanes by the Ru-S bond in Ohki-Tatsumi complexes. This account illustrates these E-H bond activations and highlights selected catalytic applications, particularly dehydrocouplings, of the generated main-group electrophiles.

Bimolecular Reaction Dynamics in the Phenyl-Silane System: Exploring the Prototype of a Radical Substitution Mechanism.

We present a combined experimental and theoretical investigation of the bimolecular gas-phase reaction of the phenyl radical (C6H5) with silane (SiH4) under single collision conditions to investigate the chemical dynamics of forming phenylsilane (C6H5SiH3) via a bimolecular radical substitution mechanism at a tetracoordinated silicon atom. Verified by electronic structure and quasiclassical trajectory calculations, the replacement of a single carbon atom in methane by silicon lowers the barrier to substitution, thus defying conventional wisdom that tetracoordinated hydrides undergo preferentially hydrogen abstraction. This reaction mechanism provides fundamental insights into the hitherto unexplored gas-phase chemical dynamics of radical substitution reactions of mononuclear main group hydrides under single collision conditions and highlights the distinct reactivity of silicon compared to its isovalent carbon. This mechanism might be also involved in the synthesis of cyanosilane (SiH3CN) and methylsilane (CH3SiH3) probed in the circumstellar envelope of the carbon star IRC+10216.

A Viewpoint on Chemical Reductions of Carbon–Oxygen Bonds in Renewable Feedstocks Including CO2 and Biomass

This Viewpoint presents a perspective on recent successes, challenges and research opportunities in the catalytic reduction of renewable carbon feedstocks, such as CO2 and biomass. Operating a shift from petrochemistry to utilize sustainable carbon resources imposes the development of efficient reduction methods, able to promote the conversion of C-O bonds to C-H bonds in an energy and atom efficient manner. Current reductive processes based on the utilization of main group hydrides, H2, formic acid and Hantzsch esters are compared to highlight their potential and limitations. Catalysis is key in present and future developments of sustainable reductive transformations and the design of efficient catalytic systems is examined to overcome the challenges facing each class of reductant.

ChemInform Abstract: Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

AbstractReview: 362 refs.

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.

Modified synthesis of mixed-ligand dinuclear Ru–Ir, Ru–Rh, and Ru–Ru polyhydride-bridged complexes, CpsRuH3ML (Cps = C5Me5 (Cp*), C5tBu3H2 (Cp‡); M = Rh, Ir, Ru; L = C5(CH3)5, C6H6, p-CH3C6H4CH(CH3)2)

Hetero-bimetallic polyhydride clusters exhibit a regio- and chemoselective activation of a substrate. The rational synthetic method for hetero-bimetallic polyhydride complexes using readily available halide complexes is reported. The reaction of hetero-bimetallic Ru–Ir trichloride complex, Cp*Ru(μ-Cl)3IrCp* (5a), in 2-propanol in the presence of a base afforded Cp*Ru(μ-H)3IrCp* (6a) by sequential salt metathesis and β-hydrogen elimination. CpsRu(μ-H)3ML [Cps = Cp*, Cp‡; ML = IrCp*, RhCp*, Ru(p-cymene), Ru(benzene)] were also selectively synthesized by reacting a mixture of CpsRuCl/n and [LM(μ-Cl)Cl]2 via the formation of CpsRu(μ-Cl)3ML. The IrCp*, RhCp*, and Ru(arene) complexes, Cp‡Ru(μ-H)3IrCp* (6b), Cp‡Ru(μ-H)3RhCp* (8), and Cp‡Ru(μ-H)3Ru(arene) (10), were newly synthesized by this method. The reaction mechanism was discussed based on the hetero-bimetallic chloro-hydride intermediates. Absence of main group hydride reagents was responsible for maintaining the hetero-bimetallic structure during the introduction of the hydride ligand, which lead to selective formation of dinuclear mixed-metal trihydride-bridged complexes.

Electrochemical Characterization of Novel Organoborohydride Compounds

Sodium borohydride (NaBH4) is a solid white powder that finds wide application as a reducing agent in organic and inorganic synthesis [1]. This compound possesses relatively mild reducing abilities, but certain applications call for reducing powers greater than NaBH4, while others require the reducing powers to be attenuated. Organic chemists have successfully tuned the reducing power of NaBH4by attaching organic electron donating or withdrawing groups to the boron center, which can also drive regio- and stereoselectivity. These organoborohydrides have received much recent attention for their role in frustrated Lewis pair systems [2], as well as in hydrogen fuel carriers. Computational methods have established the tuning power of different organic substituents on the boron center, but experimentally quantifying reducing power remains difficult [3]. The present study seeks to quantify reducing power using electrochemical methods and elucidate the relationship between organic tuning group and reducing ability. In this work, benzylborohydride (C7H7BH3Na) was prepared by reduction of potassium benzyltrifluoroborate with a metal hydride. The compound structure was further characterized by multinuclear NMR and FTIR. Whereas, the electrochemical characterization was carried out using a standard three-electrode setup, by cyclic voltammetry (CV) in static conditions and by linear scan voltammetry (LSV) using a rotating ring-disk electrode (RRDE) system. Platinum and gold disk macroelectrodes and microelectrodes were used to study the electrooxidation of these compounds. The organoborohydrides were tested in DMSO + 0.03 M C7H7BH3Na at temperatures ranging from 25 to 65 ºC. The results were directly compared with NaBH4solutions in the same experimental conditions. The main electrochemical parameters were calculated, including number of exchanged electrons, charge transfer coefficient, reaction rate constant, and diffusion coefficient for the selected range of temperatures. Moreover, the activation energy for the benzylborohydride oxidation reaction was calculated assuming Arrhenius behavior. The formation of intermediate compounds was checked by RRDE studies, elucidating the possible steps involved in the oxidation mechanism. Several other organoborohydride compounds are planned to be studied and that will allow finding a direct relationship between organic substituents and the reducing power of the borohydride center. 1. H.O. House, Modern synthesis reactions, Benjamin, W.A., Menlo Park, CA, 1972. 2. J. Paradies, Metal-free hydrogenation of unsaturated hydrocarbons employing molecular hydrogen, Angew. Chem. Int. Ed. 53 (2014) 3552. 3. Z.M Heiden, A.P. Lathem, Establishing the hydride donor abilities of main group hydrides, Organometallics 34 (2015) 1818.