Sebastian D. Pike

Dehydrogenative Boron Homocoupling of an Amine‐Borane

The dehydrogenative coupling of amine-boranes as catalyzed by transition-metal fragments offers the potential for controlled hydrogen release and the formation of oligomeric and polymeric materials in which head-to-tail coupling yields products with B N bonds that are isoelectronic with technologically pervasive polyolefins. Because of this, the area has received considerable attention recently and there are now a wide range of catalysts available, which operate using inner-sphereor outer-sphere-type mechanisms, that dehydrogenatively couple amine-boranes of the general formula H3B·NRR’H (R, R’= H, alkyl) to give monomeric, cyclic, or polymeric amino-borane materials based on H2B=NRR’. By contrast, the homocoupling of amine-boranes to form welldefined products with B B single bonds has not been reported, although dehydrogenation of H3B·NH3 by [Pd(NCMe)4][BF4] has been reported to form insoluble polymeric materials with B B bonds. This preference for heterocoupling likely stems from the fact that B-H/N-H activation of amine-boranes gives amino-boranes that are well set up for further oligomerization through the formation of dative B N bonds, a process that is also driven thermodynamically by the differences in relative s-bond strengths between B B (70 kcalmol ) and B N (107 kcal mol ) single bonds. Well-defined homocoupling of boranes, as mediated by transition metals, is essentially limited to B B bond formation in polyhedral boranes, for example pentaborane(9) (A), guanidine bases (B), and most recently the homocoupling of HBCat and related derivatives to give the corresponding diboranes (C) (Scheme 1). By contrast, the homocoupling of phosphines or silanes is well established. 14] The homocoupling of boranes requires the B-H activation of two boranes at a metal center; we, and others, have recently reported on B-H activation at group 9 metal centers in both amineand amino-boranes. In particular, H3B·NMe3 undergoes B-H activation at {Rh(PR3)n} + fragments to give bimetallic hydrido-boryl products (n = 1, R3 = Cy3), [17] or in the presence of the alkene tert-butylethene (TBE, n = 2, R3 = iBu2tBu) catalytic hydroboration occurs to afford Me3N·BH2CH2CH2tBu, I. [18] The suggested mechanism for this process involves reversible B-H activation to give a hydrido-boryl complex, alkene insertion, and subsequent reductive elimination of I. Homocoupling of H3B·NMe3 was not observed, possibly because the approach of the second equivalent of H3B·NMe3 to the metal is hindered. However, the Ir pincer system Ir(tBuPOCOPtBu)(H)2 [tBuPOCOPtBu = kP,C,P-1,3-(OPtBu2)2C6H3] catalyzes the dehydropolymerization of H3B·NMeH2, for which polymer growth kinetics suggest a coordination insertion mechanism consistent with the activation of two amine-boranes at the metal center before B N bond formation. Taking clues from this and using a related pincer system based on the {Rh(Xantphos)} fragment, we now report that H3B·NMe3 undergoes stoichiometric homo-dehydrocoupling to form the diborane(4) H4B2·2 NMe3 II (Scheme 1D), a compound previously synthesized from the combination of NMe3 with B3H7L (L = THF, SMe2). [20] Addition of H3B·NMe3 to the precursor [Rh(k 2 P,PXantphos)(NBD)][BAr4] [21] under a H2 atmosphere resulted in rapid hydrogenation of the diene and coordination of the amine-borane to the resulting Rh dihydride to give Scheme 1. Homocoupling to form B B bonds.

Solid-State Synthesis and Characterization of σ-Alkane Complexes, [Rh(L2)(η2,η2-C7H12)][BArF4] (L2 = Bidentate Chelating Phosphine)

The use of solid/gas and single-crystal to single-crystal synthetic routes is reported for the synthesis and characterization of a number of σ-alkane complexes: [Rh(R2P(CH2)nPR2)(η(2),η(2)-C7H12)][BAr(F)4]; R = Cy, n = 2; R = (i)Pr, n = 2,3; Ar = 3,5-C6H3(CF3)2. These norbornane adducts are formed by simple hydrogenation of the corresponding norbornadiene precursor in the solid state. For R = Cy (n = 2), the resulting complex is remarkably stable (months at 298 K), allowing for full characterization using single-crystal X-ray diffraction. The solid-state structure shows no disorder, and the structural metrics can be accurately determined, while the (1)H chemical shifts of the Rh···H-C motif can be determined using solid-state NMR spectroscopy. DFT calculations show that the bonding between the metal fragment and the alkane can be best characterized as a three-center, two-electron interaction, of which σCH → Rh donation is the major component. The other alkane complexes exhibit solid-state (31)P NMR data consistent with their formation, but they are now much less persistent at 298 K and ultimately give the corresponding zwitterions in which [BAr(F)4](-) coordinates and NBA is lost. The solid-state structures, as determined by X-ray crystallography, for all these [BAr(F)4](-) adducts are reported. DFT calculations suggest that the molecular zwitterions within these structures are all significantly more stable than their corresponding σ-alkane cations, suggesting that the solid-state motif has a strong influence on their observed relative stabilities.

The Simplest Amino‐borane H2B=NH2 Trapped on a Rhodium Dimer: Pre‐Catalysts for Amine–Borane Dehydropolymerization

Abstract The μ‐amino–borane complexes [Rh2(LR)2(μ‐H)(μ‐H2B=NHR′)][BArF 4] (LR=R2P(CH2)3PR2; R=Ph, iPr; R′=H, Me) form by addition of H3B⋅NMeR′H2 to [Rh(LR)(η6‐C6H5F)][BArF 4]. DFT calculations demonstrate that the amino–borane interacts with the Rh centers through strong Rh‐H and Rh‐B interactions. Mechanistic investigations show that these dimers can form by a boronium‐mediated route, and are pre‐catalysts for amine‐borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.

Well‐Defined and Robust Rhodium Catalysts for the Hydroacylation of Terminal and Internal Alkenes

A Rh-catalyst system based on the asymmetric ligand tBu2PCH2P(o-C6H4OMe)2 is reported that allows for the hydroacylation of challenging internal alkenes with β-substituted aldehydes. Mechanistic studies point to the stabilizing role of both excess alkene and the OMe-group.

Effect of the phosphine steric and electronic profile on the Rh-promoted dehydrocoupling of phosphine-boranes.

The electronic and steric effects in the stoichiometric dehydrocoupling of secondary and primary phosphine–boranes H3B·PR2H [R = 3,5-(CF3)2C6H3; p-(CF3)C6H4; p-(OMe)C6H4; adamantyl, Ad] and H3B·PCyH2 to form the metal-bound linear diboraphosphines H3B·PR2BH2·PR2H and H3B·PRHBH2·PRH2, respectively, are reported. Reaction of [Rh(L)(η6-FC6H5)][BArF4] [L = Ph2P(CH2)3PPh2, ArF = 3,5-(CF3)2C6H3] with 2 equiv of H3B·PR2H affords [Rh(L)(H)(σ,η-PR2BH3)(η1-H3B·PR2H)][BArF4]. These complexes undergo dehydrocoupling to give the diboraphosphine complexes [Rh(L)(H)(σ,η2-PR2·BH2PR2·BH3)][BArF4]. With electron-withdrawing groups on the phosphine–borane there is the parallel formation of the products of B–P cleavage, [Rh(L)(PR2H)2][BArF4], while with electron-donating groups no parallel product is formed. For the bulky, electron rich, H3B·P(Ad)2H no dehydrocoupling is observed, but an intermediate Rh(I) σ phosphine–borane complex is formed, [Rh(L){η2-H3B·P(Ad)2H}][BArF4], that undergoes B–P bond cleavage to give [Rh(L){η1-H3B·P(Ad)2H}{P(Ad)2H}][BArF4]. The relative rates of dehydrocoupling of H3B·PR2H (R = aryl) show that increasingly electron-withdrawing substituents result in faster dehydrocoupling, but also suffer from the formation of the parallel product resulting from P–B bond cleavage. H3B·PCyH2 undergoes a similar dehydrocoupling process, and gives a mixture of stereoisomers of the resulting metal-bound diboraphosphine that arise from activation of the prochiral P–H bonds, with one stereoisomer favored. This diastereomeric mixture may also be biased by use of a chiral phosphine ligand. The selectivity and efficiencies of resulting catalytic dehydrocoupling processes are also briefly discussed.

Organometallic synthesis, reactivity and catalysis in the solid state using well-defined single-site species.

Acting as a bridge between the heterogeneous and homogeneous realms, the use of discrete, well-defined, solid-state organometallic complexes for synthesis and catalysis is a remarkably undeveloped field. Here, we present a review of this topic, focusing on describing the key transformations that can be observed at a transition-metal centre, as well as the use of well-defined organometallic complexes in the solid state as catalysts. There is a particular focus upon gas–solid reactivity/catalysis and single-crystal-to-single-crystal transformations.

Simple phosphinate ligands access zinc clusters identified in the synthesis of zinc oxide nanoparticles

The bottom-up synthesis of ligand-stabilized functional nanoparticles from molecular precursors is widely applied but is difficult to study mechanistically. Here we use 31P NMR spectroscopy to follow the trajectory of phosphinate ligands during the synthesis of a range of ligated zinc oxo clusters, containing 4, 6 and 11 zinc atoms. Using an organometallic route, the clusters interconvert rapidly and self-assemble in solution based on thermodynamic equilibria rather than nucleation kinetics. These clusters are also identified in situ during the synthesis of phosphinate-capped zinc oxide nanoparticles. Unexpectedly, the ligand is sequestered to a stable Zn11 cluster during the majority of the synthesis and only becomes coordinated to the nanoparticle surface, in the final step. In addition to a versatile and accessible route to (optionally doped) zinc clusters, the findings provide an understanding of the role of well-defined molecular precursors during the synthesis of small (2–4 nm) nanoparticles.

C–Cl activation of the weakly coordinating anion [B(3,5-Cl2C6H3)4]− at a Rh(I) centre in solution and the solid-state

Addition of H2 to [Rh((i)Bu2PCH2CH2P(i)Bu2)(NBD)][BAr(Cl)4] (NBD = norbornadiene, Ar(Cl) = 3,5-Cl2C6H3) in the solid-state results in the rapid formation of zwitterionic [Rh((i)Bu2PCH2CH2P(i)Bu2){(η(6)-C6H3Cl2)BAr(Cl)3}] by a gas/solid reaction. This undergoes slow C-Cl bond cleavage in the solid-state to ultimately afford the dimeric Rh(III) complex [RhCl((i)Bu2PCH2CH2P(i)Bu2){C6H3Cl(BAr(Cl)3)}]2. This reactivity is mirrored in solution (CH2Cl2). Kinetic data for the C-Cl activation in both the solid-state and solution are reported.

Reversible Redox Cycling of Well-Defined, Ultrasmall Cu/Cu2O Nanoparticles

Exceptionally small and well-defined copper (Cu) and cuprite (Cu2O) nanoparticles (NPs) are synthesized by the reaction of mesitylcopper(I) with either H2 or air, respectively. In the presence of substoichiometric quantities of ligands, namely, stearic or di(octyl)phosphinic acid (0.1-0.2 equiv vs Cu), ultrasmall nanoparticles are prepared with diameters as low as ∼2 nm, soluble in a range of solvents. The solutions of Cu NPs undergo quantitative oxidation, on exposure to air, to form Cu2O NPs. The Cu2O NPs can be reduced back to Cu(0) NPs using accessible temperatures and low pressures of hydrogen (135 °C, 3 bar H2). This striking reversible redox cycling of the discrete, solubilized Cu/Cu(I) colloids was successfully repeated over 10 cycles, representing 19 separate reactions. The ligands influence the evolution of both composition and size of the nanoparticles, during synthesis and redox cycling, as explored in detail using vacuum-transfer aberration-corrected transmission electron microscopy, X-ray photoelectron spectroscopy, and visible spectroscopy.

Colloidal Cu/ZnO catalysts for the hydrogenation of carbon dioxide to methanol: investigating catalyst preparation and ligand effects

The production of methanol from CO2 hydrogenation is a promising potential route to a renewable liquid fuel and renewable energy vector. Herein, three distinct routes to make colloidal catalysts based on mixtures of Cu(0) and ZnO nanoparticles (NPs) and using low-temperature organometallic procedures are reported. The colloids are surface coordinated by a phosphinate ligand: dioctylphosphinate ([DOPA]−), which delivers a high solubility in organic solvents. Further, the synthetic routes allow fine control of the ZnO:Cu and ligand loadings. The catalysts are prepared by mixing small NPs (2 nm) of either Cu(0) or air-stable Cu2O NPs with ZnO NPs (3 nm), or by the synthesis of Cu(0) in presence of ZnO NPs (ZnO: 2 nm, Cu: 6 nm). The resulting colloidal catalysts are applied in the liquid phase hydrogenation of CO2 to methanol (210 °C, 50 bar, 3 : 1 molar ratio of CO2 : H2). The catalysts typically exhibit 3 times higher rates when compared to a heterogeneous Cu–ZnO–Al2O3 commercial catalyst (21 vs. 7 mmolMeOH gCuZnO−1 h−1). The characterisation of the post-catalysis colloids show clear Cu/ZnO interfaces (HR-TEM), which are formed under reducing conditions, as well as differences in the Cu(0) NP size (from 3 to 7 nm) and nanoscale restructuring of the catalysts. The combination of characterisation and catalytic results indicate that the activity is mostly dictated by the Cu(0) particle size and ligand loading. Smaller Cu(0) NPs exhibited lower turnover frequency (TOF) values, whereas higher ligand loadings ([DOPA]−:(Cu + Zn) of 0.2–1.1) lead to smaller Cu(0) NPs and reduce the formation of Cu/ZnO interfaces. UV-vis spectroscopy reveals that the Cu(0) NPs are more stable to oxidation under air after catalysis than beforehand, potentially due to migration of ZnO onto the Cu surface whilst under catalytic conditions.