Amino olefin nickel(I) and nickel(0) complexes as dehydrogenation catalysts for amine boranes

Abstract

A rare paramagnetic organometallic nickel(I) olefin complex can be isolated using the ligand bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine. This complex and related nickel(0) hydride complexes show very high catalytic activity in the dehydrogenation of dimethylamino borane with release of one equivalent of dihydrogen.

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Introduction

Recent years have seen a significant effort to use amine boranes, R_3−xH_xN–BH_yR_y−3, as chemical hydrogen storage materials with relatively high storage capacities (ideally up to 19.6 wt% in the conversion of ammonia borane to boronitride structures).¹ This approach may be questioned as long as boranes and amines are not produced from their natural resources using eco-friendly energy sources and the direct “refuelling” of ‘BN’ compounds remains problematic without pre-digestion of the ‘BN’ material.² The generation of reactive intermediates through dehydrogenations offers interesting perspectives for “waste-free” atom economic synthetic methods. Efforts along these lines also include the development of catalysts free of rare and expensive precious metals which enable fast release of H₂.³

Several mechanisms for the dehydrogenation of amine borane compounds have been proposed⁴ and a variety of metal complexes with, Ti,⁵ Rh,^6–8 Ir,⁹ Ru,¹⁰ Re¹¹ and more recently Mg¹² and Pd,¹³ promote this reaction. Two results are especially noteworthy with respect to this work: Baker et al. reported that N-heterocyclic carbene (NHC) complexes of nickel(0) show a higher catalytic activity compared to Rh(NHC) or Ru(NHC) analogs.¹⁴ Possible reaction mechanisms for the ammonia borane dehydrogenation catalyzed by nickel carbene have been calculated.^15a–c It has been suggested that a dissociated free carbene may participate in the process.^15d Further, ruthenium-amido catalysts have been introduced with the highest reported activity to date in the dehydrogenation of amine boranes.¹⁰

According to Fagnou et al.,^10a the amido group plays the role of a cooperative ligand¹⁶ and the dehydrogenation proceeds according to a Noyori–Morris-related mechanism.¹⁷ Schneider et al. observed and isolated an inactive Ru–N–B–H metallacycle in a side reaction to rationalize the quick release of only 70% of H₂.^10b,c

We discovered that amino olefin ligands, specifically bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine = bistropylidenylamine = trop₂NH,¹⁸ give rise to structurally well-defined rhodium amido complexes like B, which serve as superb transfer hydrogenation catalysts.¹⁹ Furthermore B, which is simply derived through deprotonation of the very stable amine complex [Rh(trop₂NH)(PPh₃)]OTf A is a highly active catalyst in the dehydrogenative coupling (DHC) of primary alcohols to carboxylic derivatives under various reaction conditions.¹⁹ Here we report on novel bistropylidenylamine nickel(I) and nickel(0) complexes which can be applied in the highly efficient catalytic dehydrogenation of Me₂HN–BH₃4.

Results and discussion

The dark green nickel(I) complex 2 was prepared by addition of trop₂NH¹⁷ to nickel(II) trifluoroacetate 1 in the presence of Zn powder. The dark red nickel(0) complex 3 was prepared by further reducing the nickel(I) complex in the presence of PPh₃ (Scheme 1). This reaction can be performed in one step, where [Ni^II(OOCCF₃)₂] 1 is directly converted to 3 when the reduction of 1 with Zn is performed in the presence of trop₂NH and PPh₃. While the reduction of 2 in the presence of PPh₃ proceeds smoothly with Zn powder, a cyclic voltammogram (CV) of complex 2 in THF shows the onset potential for a reduction at rather negative values (−1.99 V vs. Fc⁺/Fc; the peak potential of the irreversible reduction wave is seen at −2.42 V). On the contrary, the CV of complex [Ni(trop₂NH)(PPh₃)] 3 shows a reversible redox wave at −1.28 V for the nickel(I)/nickel(0) couple.

Scheme 1 Synthesis of rhodium(I), nickel(I) and nickel(0) complexes B, 2, and 3.

The molecular structures of 2 and 3 were determined by a single crystal X-ray diffraction study. The results are shown in Fig. 1A and 1B, respectively. Complex 2 is a very rare example of a nickel(I) olefin complex.²⁰ The nickel(I) center is pentacoordinated in a coordination sphere formed through the trop₂NH ligand and the non-symmetrically κ²–binding trifluoroacetate [Ni–O1: 2.176(1) Å; Ni–O2: 2.265(1) Å]. In [Ni(trop₂NH)(PPh₃)] 3 the nickel(0) center resides in a distorted pseudo-tetrahedral coordination sphere. With respect to the Ni(trop₂NH) moiety, the structures of 2 and 3 are rather similar. Not unexpected, the Ni–N bond is slightly longer in the nickel(0) complex 3 (2.1939(2) Å vs. 2.061(2) Å in 2). The Ni–ct1 and Ni–ct2 (ct1 and ct2 represent the centroids of the C4–C5 and C19–C20 double bonds, respectively) distances are marginally shorter and the coordinated C=C_trop bonds (C4=C5, C19=C20) are slightly longer in the nickel(0) complex 3 indicating more metal-to-ligand back donation of the d¹⁰-nickel(0) center into the π-accepting olefinic units. This is also reflected in the NMR data; the ¹³C NMR resonances of the C=C_trop nuclei are observed at rather low frequencies (¹³C NMR δ = 67.9, 69.5 ppm). In comparison, the corresponding resonances in the cationic d⁸-Rh^I complex A are observed at 74.0 ppm and 74.2 ppm in the ¹³C NMR spectra. Note that in A the NH group of the trop₂NH ligand is rather acidic (¹H NMR δ = 5.66 ppm; estimated pK_a DMSO ≈ 17). The nickel(0) complex 3 has a reduced acidity (¹H NMR δ = 1.09 ppm) compared to A. Indeed, adding MeOD to solutions of 3 did not show any evidence for a proton-deuteron exchange and no deprotonation reaction is observed between 3 and KOtBu.

Fig. 1 A) ORTEP plot of 2 at 50% ellipsoid probability (hydrogen atoms (apart from N1–H) and solvent (THF) molecule are omitted for clarity). Selected bond lengths [Å] and angles [°]: Ni1–N1 2.061(2), Ni1–O1 2.176(1), Ni1–O2 2.265(1), Ni1–ct1 1.993, Ni–ct2 2.001, C4–C5 1.388(3), C19–C20 1.392(3), O1–C31 1.249(2), O2–C31 1.249(2); N1–Ni1–O1 104.91(6), N1–Ni1–O2 164.94(5), O1–Ni1–O2 60.05(5), ct1–Ni–ct2 133.67, ct1–Ni–O1 112.33, ct2–Ni1–O1 109.41. B) ORTEP plot of 3 at 50% ellipsoid probability hydrogen atoms (apart from N1–H) are omitted for clarity. Selected bond lengths [Å] and angles [°]: Ni1–N1 2.194(2), Ni1–P1 2.1636(5), Ni1–ct1 1.925, Ni–ct2 1.925, C4–C5 1.409(3), C19–C20 1.411(3); N1–Ni1–P1 119.18(4), ct1–Ni–ct2 132.00, ct1–Ni–P1 108.14, ct2–Ni1–P1 111.54.

Magnetic susceptibility measurements (SQUID, T = 2–300 K) of [Ni^I(trop₂NH)(OOCCF₃)] 2 gave an effective magnetic moment of μ_eff = 1.81 μ_B in accordance with the existence of one unpaired electron.

Inspection of the EPR spectrum of 2 in a frozen 2-Me-THF solution clearly reveals besides the major species 2a (∼90% of the total spectral intensity), the presence of a second minor species 2b (∼5%), and a detailed line shape analysis suggests the presence of even a third minor species, 2c (∼5%) (Fig. 2).²¹

Fig. 2 Top: Experimental and simulated X-band EPR spectrum of [Ni^I(trop₂NH)(OOCCF₃)] in frozen 2-methyl THF. Experimental conditions: 120 K, microwave frequency 9.500720 GHz, microwave power 2.012 mW, modulation amplitude 1.00 Gauss. Bottom: Individual components 2a (90%), 2b (4%) and 2c (6%) contributing to the EPR spectrum. (The simulation was obtained using the parameters in Table 3 of the ESI†).

The major species 2a must correspond to [Ni^I(trop₂NH)(κ²-OOCCF₃)], as the DFT calculated parameters of the optimized geometry of 2 correspond well to the experimental parameters of 2a (for details concerning the EPR spectra and DFT calculations see the ESI†).

Although trop₂NH may be regarded as a redox non-innocent ligand when coordinated to late transition metals,^16,18 it behaves “innocently” in 2. The EPR and DFT data clearly reveal metal-centered spin density (ρ_DFT^Ni > 80%).

The nickel complexes 2 and 3 and the rhodium amide B were investigated for the catalytic dehydrogenation of Me₂HN–BH₃4 in THF at room temperature under an inert atmosphere. The amidoborane salt K[Me₂NBH₃] 5 was used as a co-catalyst, added either in substance²² or prepared in situ from Me₂HN–BH₃4 and tBuOK, (Scheme 2). The nickel(I) complex [Ni(trop₂NH)(OOCCF₃)] 2 is an especially active catalyst and even with 0.3 mol% and 1 mol% of co-catalyst, one equivalent of H₂ is released in less than one minute from 1 M solutions of substrate 4 in THF (Fig. 3).²³ To our knowledge this is the highest rate for the dehydrogenation of 4 mediated by any non-noble metal catalyst under mild conditions reported to date. The nickel(0) phosphane complex [Ni(trop₂NH)(PPh₃)] 3 is significantly slower and no conversion is observed when the catalysis is performed in the presence of an excess of PPh₃.²⁴

Scheme 2 Catalytic dehydrogenation of Me₂HN–BH₃4 mediated by complexes 2 and 3 and catalytic amounts of K[Me₂NBH₃] 5 or tBuOK.

Fig. 3 Volumetric measurement of H₂ release from Me₂HN–BH₃4 mediated by complexes 2 (■ 1 mol%; ▲ 0.3 mol%), 3 (■ 1 mol%), and B (♦ 0.2 mol%).

As final products, the cyclic 1,3-diaza-diboretane 6 (¹¹B NMR δ = 5.10 ppm, t, ¹J_BH = 112 Hz) is quantitatively obtained and traces of the monomer Me₂N=BH₂ (¹¹B NMR δ = 37.2 ppm, ¹J_BH = 121 Hz) are detected. In the slower reaction with 3 as the catalyst, the linear N–B–N–B product 7 is the kinetically controlled product which is converted in a slower follow-up reaction to 6.²⁵

Given the significance of paramagnetic nickel complexes in hydrogenase enzymes,²⁶ it is tempting to assume that the nickel(I) species and radical pathways are involved in the H₂ release. However, presently we cannot exclude that nickel(0) complexes are involved in the catalytic cycle based on the following observations from reactions performed under non-catalytic conditions: (i) [Ni^I(trop₂NH)(OOCCF₃)] 2 is cleanly reduced to [Ni⁰(trop₂NH)(PPh₃)] 3 with one equivalent of K[Me₂HN–BH₃] 5 in the presence of one equivalent PPh₃. In this reaction, the monomer Me₂N=BH₂ is clearly detected which dimerizes over time to the 1,3-diaza-diboretane 6. (ii) The reaction between the nickel(I) complex 2 and a slight excess of amido borane 5 results in a brown solution, showing similar appearance to the catalytic reaction, in which a bridging nickel hydride complex 8 (¹H NMR δ = −8.12 ppm) is observed beside amine borane 4 and the heterocycle 6. The hydride species could not be isolated but was characterized by NMR and we propose the C₂ symmetric structure shown in Scheme 3 (see the ESI† for details). (iii) If K[Me₂NBH₃] is formed in situ from 4 with an excess of KOtBu, the reaction leads to an additional nickel hydride 9 (¹H NMR δ = −4.59 ppm) which after a short time becomes the main product. So far we have not been able to isolate this species but on the basis of the ¹H and ¹³C NMR data we have assigned the structure 9.²⁷ (iv) Finally, the mixture containing both hydrides 8 and 9 also catalyzes comparably fast hydrogen release from amine borane 4.

Scheme 3 Different reaction pathways of [Ni^I(trop₂NH)(OOCCF₃)] 2 to Ni⁰ complexes 3, 8, and 9 in the presence of amine borane 4 and amido borane 5.

Conclusions

In summary, we were able to isolate and fully characterize a very rare example of a nickel(I) olefin complex. This species serves as a precursor to remarkably active non-noble metal catalysts for the dehydrogenation of amine boranes.²⁸ Again, olefinic binding sites proved suitable as steering ligands in a catalytic reaction while in reactions with transition metal complexes from the fourth period they commonly serve only as place holders for “vacant” coordination sites.

Acknowledgements

This work was supported by the joint SNF-DFG project “Unconventional Approaches to the Activation of Dihydrogen” (2OPA21-126711/1) and the ETH Zürich.

Notes and references

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21. The minor species 2b and 2c are perhaps penta-coordinated solvent adducts and/or species with a κ¹-OOCCF₃ coordination mode instead of the κ²-OOCCF₃ coordination mode. The isotropic g-value (2.190) measured at 298 K (featureless, relatively broad isotropic signal) corresponds roughly with the weighted averaged anisotropic g-values of the species 2a, 2b and 2c, suggesting that the rapidly frozen solution spectrum reflects approximately the kinetically trapped equilibrium distribution of these same species in solution.
22. K[Me₂NBH₃] 5 was prepared by treatment of Me₂HN–BH₃4 with 1.0 equiv of KH in THF at room temperature, according to the method adapted from: R. O. Hutchins, K. Learn, F. El-Telbany and Y. P. Stercho, J. Org. Chem., 1984, 49, 2438 . Control experiments with only K[Me₂NBH₃] or KOtBu showed no H₂ evolution. The base/catalyst ratio is crucial for the performance of the catalytic reaction. Reducing the amount of base with respect to the catalyst loading proved to have a detrimental effect in conversion of Me₂HN–BH₃4.
23. The catalytic reaction can be re-loaded with five charges of amine borane without loss of activity. No color change of the solution or precipitation of nickel metal was observed. Only after addition of the sixth charge of substrate, the release of H₂ slowed down but went to completion (release of 1 equivalent of H₂). The catalytic reaction performed in the presence of elemental mercury did not show any reduction of the activity of the catalytic system. This finding may suggest no participation of colloidal heterogeneous nickel metal catalysts.
24. This observation indicates that PPh₃ dissociation from 3 is necessary and an unsaturated Ni complex like [Ni⁰(trop₂NH)] is the catalytically active species which may coordinate Me₂HNBH₃ (4) as substrate.
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28. Preliminary results show that the parent amino borane, (NH₃BH₃), is dehydrogenated with equal efficiency under loss of one equivalent of H₂. Further results using related substrates are also currently under investigation.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC reference numbers 767256–767257. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0sc00483a

‡ Dedicated to Prof. Dr José Barluenga on the occasion of his 70th birthday

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