Bis ( σ-B – H ) complexes of copper ( I ) : precursors to a heterogeneous amine – borane dehydrogenation catalyst †

A series of bis(σ-B–H) complexes of copper(I) have been prepared by displacement of arene solvent from a β-diketiminate copper(I) complex by four-coordinate boranes, H3B–L (L = NMe3, lutidine). In the presence of the same copper arene complex, the secondary amine–borane H3B–NMe2H undergoes dehydrogenation. We provide evidence for formation of a heterogengous catalyst from decomposition of the solution species.

Since Hartwig and co-workers reported the isolation and characterisation of [Cp 2 Ti(η 2 -HBcat) 2 ] (HBcat = catecholborane), 1 our understanding of the coordination chemistry of boranes has flourished. 2 Contrasting studies have investigated the interaction of 3-and 4-coordinate boranes with transition metal centres. 3,4 Regardless of the environment at boron and the mode of coordination to the metal, σ-borane complexes have become synonymous with B-H bond activation. These species are invoked as intermediates in the catalytic borylation of C-H bonds, 5 the hydroboration of alkenes and alkynes, 6 and the dehydrogenation of amine-boranes. 7 For example, Shimoi and co-workers studied the coordination of H 3 B-NMe 3 to a series of group 6 carbonyl complexes and demonstrated dehydrogenation of H 3 B-NHR 2 under photochemical conditions. 8 In related studies, Weller, Sabo-Etienne, Aldridge, Manners, Schneider, and others have conducted extensive investigations into the coordination of H 3 B-NR 3 , H 2 BvNR 2 and [H 2 B-NR 2 ] 2 fragments to a series of late transition metals, including ruthenium, rhodium and iridium complexes. 7,9-12 A detailed understanding of not only the electronic structure of a clutch of σ-borane complexes but also the mechanisms of amine-borane dehydrogenation has emerged.
Despite a growing interest in the catalytic applications of the 1 st row transition metals, little is known about σ-complexes of copper. The coordination of σ-bonds to Cu(I) may fore-shadow oxidative addition to Cu(III) and play an unappreciated role in catalysis. In line with these expectations, Bourissou and coworkers have reported the intramolecular coordination of Si-Si and Si-H bonds within carefully designed ligand frameworks to Cu(I). In the case of an Sn-Sn analogue, oxidative addition of the tin-tin bond was observed allowing isolation of the corresponding Cu(III) distannyl complex. 13 Stack, Ribas and co-workers have provided EPR and computational support for an agostic interaction in a Cu(II) metallocycle. 14 Recently we reported the reversible, intermolecular, coordination of Al-H and Zn-H bonds to a two-coordinate copper(I) fragment generated in situ from 1 2 ·toluene (Scheme 1). 15,16 Here we disclose that amine-boranes coordinate reversibly to Cu(I), and demonstrate an effective pre-catalyst for amine-borane dehydrogenation.
The reaction of 1 2 ·toluene 15 with H 3 B-L (L = NMe 3 , lutidine) in a 1 : 2 stoichiometry C 6 D 6 resulted in a minor perturbation of the resonances of 1 and the borane as evidenced by line-broadening and chemical shift changes in both the 1 H and 11 B{ 1 H} NMR spectra. Despite the weak and potentially reversible nature of the interaction, preparative scale reactions allowed the isolation of the corresponding σ-borane complexes 2a-b as yellow crystalline solids in 63-85% yield (Scheme 1).
Infrared data are consistent with the formulation of these species as weakly coordinated boron hydrides and reveal broad B-H stretches (2a, 2423 cm −1 ; 2b, 2403 cm −1 ) in the range expected for terminal boron hydrides with no clear differentiation between the (σ-B-H) and B-H vibrations. 1 H and 11 B NMR spectroscopy show that, upon dissolution in toluene-d 8 , isolated crystalline samples of the Cu(I) σ-complexes establish an equilibrium between 2 + toluene and 1·toluene + amine-borane (Scheme 2). We have previously shown that 1 2 ·toluene forms 1·toluene by solvation, and the latter is the predominant specices in toluene solution. 15 For complex 2b at 298 K in toluene-d 8 the BH 3 unit of the amine-borane is observed as a single resonance at δ = -15.5 ppm in the 11 B{ 1 H} NMR and as a broad quartet at δ = 3.27 ppm in the 1 H NMR. The resonances are assigned to a time-averaged contribution from bound and unbound amineborane; consistent with fast chemical exchange. Upon cooling, decoalescence of a series of resonances assigned to both the amine-borane and the β-diketiminate ligand occurs. At 193 K the slow exchange regime is reached and a mixure of 1·toluene, amine-borane and 2b is observed in solution (ESI, Fig. S1 and S2 †). 20 While at this temperature B-H resonances were apparent as broad signals in the 1 H{ 11 B} NMR between δ = 2.5 and 3. 5

ppm, we have been unable to resolve terminal B-H and (σ-B-H) resonances. We suggest that even at 193 K fast exchange between the terminal B-H and (σ-B-H) units within 2b occurs.
The observation of a weak and reversible coordination of the B-H bond to Cu(I) parallels that reported for analogous Al-H and Zn-H σ-complexes. 15 This fluxional process was observed for not only 2b (vide supra) but also 2a. VT NMR on toluene-d 8 samples of 2a across the 193 to 353 K range allowed quantification of the equilibrium depicted in Scheme 2. Van't Hoff analysis gave ΔH rxn = -1.40(4) kcal mol −1 , ΔS rxn = -5.87(2) cal K −1 mol −1 and ΔG rxn = +0.17(3) kcal mol −1 . The data suggest that binding of H 3 B-NMe 3 to 1·toluene is slightly endergonic.
In order to gain a deeper understanding of the strength and nature of the bonding within the bis(σ-B-H) complexes, a series of DFT calculations were undertaken. A minimum on the potential energy surface with a bis(σ-B-H) coordination mode was obtained for the series of complexes presented in Scheme 2 Reversible σ-complex formation with copper(I). Scheme 1. All attempts to optimise mono(σ-B-H) geometries led to this structure. The calculated B-H bond lengths are significantly longer than those determined in 2b by X-ray crystallography and range from 1.20-1.23 Å. Furthermore, across a choice of functionals, and in contrast to the X-ray data, the (σ-B-H) lengths were determined to be only slightly longer than the terminal B-H bond in these calculations (Δ = 0.03 Å). Based on the known difficulty in assigning the position of the hydrogen atoms in X-ray diffraction experiments, the calculated B-H bond lengths represent a more realistic description of the ground-state structure.
NBO calculations suggest only a minor perturbation of borane within the coordination complexes 2a and 2b. The Wiberg Bond Indices (WBIs) for the (σ-B-H) bonds are similar to that of the terminal B-H. Furthermore, both the Cu-H and Cu-B WBIs are low, suggestive of a weak interaction (Fig. 2). Second order perturbation analysis allows a quantification of the donor-acceptor interactions, donation of electrons from each of the two B-H σ-bonds occurs to the 4s orbital of copper (2a, 21.7 + 22.3 kcal mol −1 : 2b, 15.6 + 16.8 kcal mol −1 ), significant back-donation from Cu(I) to the B-H σ*-orbitals is not recorded for either 2a or 2b.
These data were further underscored by a quantum theory atoms-in-molecules (QTAIM) calculation on 2a which revealed bond critical points (BCPs) between the Cu/B and H atoms, but not between Cu and B. These data show a bending of the (σ-B-H) bond critical paths toward Cu and are consistent with two 3-centre,2-electron interactions (Fig. 2). In line with the NBO analysis, the QTAIM data for coordinated (ρ bcp = 0.154; Further modification of the amine-borane to a substrate that contained both hydridic and acidic protons resulted in facile dehydrogenation and boron-nitrogen bond formation. While reaction of H 3 B-NHMe 2 with 1 2 ·toluene resulted in the generation of the corresponding σ-complex, compound 2c was short-lived and only observed in situ. All attempts to isolate this latter species resulted in dehydrogenation of H 3 B-NHMe 2 (Scheme 3).
In line with these expectations, 1 2 ·toluene catalysed the dehydrogenation of H 3 B-NHMe 2 in 5 mol% loading at 80°C in C 6 D 6 solution (Scheme 2). Notably 2c was observed as an intermediate in solution by 1 H NMR spectroscopy. In this case, at 298 K the 1 J 11B-1H coupling can be resolved and the 1 J 11B-1H for the equilibrium mixture of 1·toluene, 2c and H 3 B-NMe 2 H (90.4 Hz) is slightly smaller than that of the independent amine-borane (96.4 Hz). Monitoring catalytic reactions by 1 H and 11 B NMR spectroscopy revealed the formation of known products [H 2 B-NMe 2 ] 2 (3a) and HB(NMe 2 ) 2 (3b). The reaction proceeded with concomitant formation of a Cu(0) mirror on the interior of the reaction vessel. Following a catalytic run, re-exposure of the Cu(0) mirror to the reaction conditions reestablished amine-borane dehydrogenation. An Hg(0) drop experiment resulted in a significant inhibition of catalysis. In this instance, data are consistent with 2c acting as a homogeneous precursor to a heterogeneous species. 21 These data contrast those found by Philips and co-workers for the dehydrogenation of ammonia-borane catalysed by a ruthenium analogue of 1·benzene and by Bertrand and co-workers using a CAAC-stabilised copper borohydride complex. 22,23

Conclusions
In summary, we have reported the first examples of isolable and crystallographically characterised σ-borane complexes of Cu(I). While in the solid-state amine-boranes coordinate via an η 2 :η 2 -mode, in solution displacement of this ligand by arene solvent is both fast and reversibile. Inclusion of both hydridic and protic hydrogen atoms on the ligand leads to a decomposition of the coordination compound and production of a heterogeneous copper catalyst that is capable of the dehydrogenation of an amine-borane. We are grateful to the Royal Society for provision of a research fellowship (MRC) and the EPSRC for project funding, including a prize research fellowship (AEN). We are grateful to Pete Haycock for his assistance with multinuclear and VT NMR experiments.