Interrogating the steric outcome during H₂ heterolysis: in-plane steric effects in the regioselective protonation of the PN³P-pincer ligand

Abstract

H₂ heterolysis to generate well-defined nickel hydride-proton complexes was achieved by the 2^nd generation PN³P-pincer nickel platform. The regioselective protonation in the ligand framework was demonstrated for the first time to highlight the importance of in-plane hindrance during the H₂ splitting process.

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Heterolytic splitting of H₂ to generate a proton (H⁺) and hydride (H⁻) has been considered to play a central role in H₂ activation catalyzed by hydrogenase enzymes,¹ which usually either contain a bimetallic core, [FeFe]/[FeNi], or a single Fe center as the active site.^1a,b Investigations of the detailed structure of [FeFe]-hydrogenases have shown that a pendant amine base, acting as a proton relay, serves a vital role in facilitating the formation or heterolysis of H₂.² To mimic the catalysis of hydrogenases (Fig. 1A), a large number of Ni-containing models have been artificially synthesized and studied particularly by DuBois et al. (Fig. 1B).³ These [Ni(P₂^RN₂^R′)₂]²⁺ hydrogenase models can mediate the oxidation and reproduction of H₂. These systems involve either an oxidative addition of H₂ by directly transferring its electrons to the central Ni, or a heterolytic (asymmetric) cleavage of H₂ to produce the hydride-proton intermediate (Fig. 1C).^3f,h Several investigations of heterolytic H₂ cleavage by nickel complexes have supported the asymmetric transition state.^3c,e,4 Heinekey and coworkers reported the first structurally well-defined nickel dihydrogen complex, in which the H₂ molecule cleaves heterolytically in the presence of an external triethylamine as a base (Fig. 2).⁵ Caulton and co-workers showed that the cationic nickel η²-H₂ complex implements H₂ heterolysis facilitated by the internal amine base in the ligand skeleton (Fig. 2).^4a Bullock et al. reported that the oxidation rate of H₂ catalyzed by [Ni(P^Cy₂N^tBu₂)₂](BF₄)₂ increases with the decrease of the base size in the following order: NEt₃ < ^tBuNH₂ < ⁿBuNH₂.^3m While heterolytic H₂ cleavage reactions are highly relevant to enzymatic and organometallic hydrogenation processes, little is known to elucidate the influence of sterics on the protonation step. Herein, we demonstrate the metal–ligand cooperative heterolysis of H₂ by a [(PN³P)Ni]⁺ species with site-selective protonation to feature the importance of the in-plane steric effects (Fig. 2).

Fig. 1 Structures of hydrogenases and nickel mimics. (A) [NiFe] hydrogenase; (B) Ni hydrogenase mimic; (C) proposed transition state for production of H₂ catalyzed by (B).

Fig. 2 Heterolysis of H₂ by cationic pincer nickel complexes.

We have recently reported a new class of nickel compounds supported by a PN³P-pincer ligand containing three nitrogen atoms in the ligand framework through a post-modification strategy (Fig. 3).⁶ Compared with the complexes derived from the conventional 2,6-diaminopyridine based ligands,⁷ these new complexes exhibited unparalleled thermal stability and distinct reactivity.^6,8 In particular, a nickel hydroxide complex (PN³P)NiOH (1) was studied in details with respect to its strong basicity.^8b Although hydrogenolysis of the M–OH bond to produce a M–H compound concomitant with water is a highly valuable reaction,⁹ unfortunately, our Ni–OH complex 1 showed no reaction toward H₂ gas. This is in sharp contrast to other analogous Pd,⁹ Pt¹⁰ or Ru¹¹ complexes, where the H₂ molecule is readily cleaved by the M–OH moiety, presumably originating from the unfavorable formation of a (PN³P)Ni(H₂) intermediate.^4a Caulton and co-workers have reported that a free coordination site for H₂ is important for the heterolysis to proceed in the pincer-type nickel compound.^4a Consistent with their viewpoint, when one equivalent of B(C₆F₅)₃ was introduced to the C₆D₆ solution of 1, the resulting borane adduct (PN³P)Ni(OH)B(C₆F₅)₃ (2) could undergo the hydrogenolysis reaction. This is likely attributed to the property of anionic [(OH)B(C₆F₅)₃]⁻ as a better leaving group than the OH group. However, differently from what was reported by the groups Peters¹² and Limberg,^4b the [(PN³P)Ni(H₂)]⁺ intermediate was not observed.^4a,13 The hydrogenolysis reaction occurred immediately after the H₂ was introduced into 2, as corroborated by the rapid color change from brown to light yellow. The ³¹P{¹H} NMR showed significant downfield chemical shifts, in comparison with those from 2 (δ 103.5 and 103.2 ppm), with two sets of peaks observed at δ 147.5 and 144.9 ppm. The successful heterolysis of H₂ to generate a proton and a hydride was further validated by the ¹H NMR with a doublet of doublet peak at δ −16.20 ppm for the hydride and a signal at δ 9.41 ppm for the proton. The identity of product 3 was confirmed by X-ray crystallography (Fig. 4). Complex 3 consists of an anion [(OH)B(C₆F₅)₃]⁻ and a cation [(PN³HP)Ni(H)]⁺ in which the H⁻and H⁺ derived from the splitting of H₂ bond to the nickel center and one of the nitrogen atoms, respectively (Scheme 1). Although various experiments on H₂ heterolysis catalyzed by nickel compounds have been reported, to our knowledge, no well-defined nickel hydride-proton intermediate was obtained.

Fig. 3 Synthesis of 2^nd PN³P-pincer compounds through ligand post-modification strategy.

Fig. 4 Molecular structures of complexes 3⁺, [5a]⁺ and [7b]⁺. Thermal ellipsoids are shown at the 30% probability level; hydrogen atoms except Ni–H and N–H moieties are omitted for clarity. Selected bond lengths [Å] and angles [°]: for 3⁺: Ni(1)–N(1) 1.9015(11), Ni(1)–P(1) 2.1407(4), Ni(1)–P(2) 2.1401(4), Ni(1)–H(1) 1.351, N(2)–H(2) 0.85(1); N(1)–Ni(1)–H(1) 178.16, H(1)–Ni(1)–P(1) 91.33, H(1)–Ni(1)–P(2) 96.83, P(1)–Ni(1)–P(2) 171.776(15). For [5a]⁺: Ni(1)–N(1) 1.9017(7), Ni(1)–P(1) 2.1348(3), Ni(1)–P(2) 2.1300(3), Ni(1)–H(1) 1.374, N(2)–H(2) 0.863; N(1)–Ni(1)–H(1) 178.29, H(1)–Ni(1)–P(1) 95.05, H(1)–Ni(1)–P(2) 92.98, P(1)–Ni(1)–P(2) 171.738(9). For [7b]⁺: Ni(1)–N(1) 1.9022(14), Ni(1)–P(1) 2.1461(5), Ni(1)–P(2) 2.1431(5), Ni(1)–H(1) 1.37(3), N(3)–H(3N) 0.86(3); N(1)–Ni(1)–H(1) 178.3(12), H(1)–Ni(1)–P(1) 94.7(12), H(1)–Ni(1)–P(2) 93.9(12), P(1)–Ni(1)–P(2) 171.382(19).

Scheme 1 Synthesis and H₂ heterolysis of complex 2.

In order to further verify the importance of the leaving ability of ancillary ligand, we prepared a similar complex (4), (PN³P)NiOTf, containing a slightly stronger coordinating group, OTf, as the promoter of H₂ heterolysis. The comparison of the crystal structures of 2 and 4 clearly showed that the length of the Ni–O bond (1.9169(11) Å) in complex 4 is shorter than that (2.0258(15) Å) in complex 2, suggesting that the leaving ability of (OH)B(C₆F₅)₃ moiety is higher than that of the OTf group. Indeed, no reaction occurred for complex 4 in the presence of H₂ at room temperature over 12 h. Only upon heating at 50 °C, a new product, 5a, was slowly formed. These results suggest that the displacement of OTf by H₂ at an elevated temperature to generate [(PN³P)Ni(H₂)]⁺ may play a vital role in promoting the activation of H₂. The ¹H NMR also shows protic and hydride signals at δ 10.16 and −16.19 ppm, respectively, similar to those in complex 3. The X-ray diffraction analysis revealed the structure of 5a to be [(PN³HP)Ni(H)]⁺[OTf]⁻(Fig. 4).

Both NMR and crystal data suggested that the proton from H₂ heterolysis was transferred into one of the side arm nitrogen atoms of the PN³P ligand (Fig. 4). Caulton and co-workers have demonstrated that nitrogen atoms could potentially serve as a base to facilitate H₂ heterolysis.^4a As our ligand framework contains three nitrogen atoms, in principle there are three possible protonated products formed in this system (Fig. 5). 2D NOESY experiments were conducted to determine the structure of the Ni–H cation. The cross peak at 2.89/10.16 ppm was attributed to the correlation of the NH proton and the methylene protons of the two ethyl groups at the same side of the ligand (Fig. S10† for complex 5a). Likewise, the correlation of the NH proton and those of two equivalent tBu groups could also be observed at the position of 1.29/10.16 ppm, further suggesting that the proton was selectively added to one N atom (Ni1_left), consistent with qualitative DFT calculations (Fig. 5). The counterpart Ni1_right with the proton connecting to the N atom on the other side was found to be higher in energy. The relative Gibbs free energy of other structures Ni1_H2, before H₂ splitting, and Ni1_mid were both found to be extremely unfavorable, indicative of the thermodynamic preference of the H₂ cleavage process.

Fig. 5 Structures of the probable intermediates and protonated products with relative Gibbs free energy data (the energetics in the first and second lines represent the energies in gas phase and in the experimental solvent (benzene) phase, respectively).

The selective protonation of the nitrogen atom may originate from the different electronic and/or steric effects on the nitrogen atoms. To further rationalize the selectivity, an analogous nickel triflate complex (6) containing only two ethyl groups on the PN³P framework was prepared. Interestingly, when complex 6 was used, both N side arms could be protonated to give a mixture of 7a and 7b in a ratio of approx. 1 : 6 (Scheme 3). The major product 7b suggests that the steric effect is likely the dominating factor of the observed selectivity in the protonation step (Fig. 4), since 7a should experience similar electronic effects as 5a. This argument is further supported by very similar atomic charges from NPA of the two nitrogen sites in 4 (q(N2) = −0.616; q(N3) = −0.627). Analyzing the molecular structure of 5a, we notice that the diethyl groups are both out of the plane of the central heterocyclic ring, but the monoethyl group is in the plane, strongly indicating that the selectivity is caused by the in-plane hindrance. To further support this argument, two analogous complexes (8 and 10) were prepared via replacing the Et groups by Me and ⁱPr groups and employed to conduct the heterolytic splitting of H₂. It was found that the selectivity indeed remained the same (Scheme 2). The structures of 9a and 11a were also characterized by NMR and single crystal X-ray diffraction (Fig. 6). The distances between H and Me in 9a (H2–C6: 2.785 Å; H2–C7: 2.892 Å) are longer than those in 9b (H3A–C8: 2.505 Å), in agreement with the observation that protonation of the dimethyl side is more feasible (Fig. 7). When NMR experiments were conducted at an elevated temperature (110 °C), the rations of 5a/5b and 11a/11b changed (Fig. S33 and S35†), indicating that these complexes are interchangeable. These observations suggest that these regioselective products are likely the results of thermodynamic preference driven by the in-plane hindrance between the protonated N–H arm and the H or alkyl group at the sp² carbon of the ring.

Scheme 2 Heterolysis of H₂ by [Ni-OTf] complexes.

Scheme 3 Heterolysis of H₂ by complex 6.

Fig. 6 Molecular structures of complexes [9a]⁺ and [11a]⁺. Thermal ellipsoids are shown at the 30% probability level; hydrogen atoms except Ni–H and N–H moieties are omitted for clarity. Selected bond lengths [Å] and angles [°]: for [9a]⁺: Ni(1)–N(1) 1.9037(11), Ni(1)–P(1) 2.1376(5), Ni(1)–P(2) 2.1379(5), Ni(1)–H(1) 1.28(3), N(2)–H(2) 0.79(2); N(1)–Ni(1)–H(1) 177.645, H(1)–Ni(1)–P(1) 95.299, H(1)–Ni(1)–P(2) 92.700, P(1)–Ni(1)–P(2) 171.994(16). For [11a]⁺: Ni(1)–N(1) 1.908(2), Ni(1)–P(1) 2.1229(7), Ni(1)–P(2) 2.1268(8), Ni(1)–H(1) 1.368, N(2)–H(2) 0.861; N(1)–Ni(1)–H(1) 176.323, H(1)–Ni(1)–P(1) 92.226, H(1)–Ni(1)–P(2) 95.120, P(1)–Ni(1)–P(2) 172.642.

Fig. 7 The distances between H and Me in 9a and 9b.

In summary, we have demonstrated the importance of the leaving ability of coordinated anion for the H₂ heterolysis process in the planar pincer geometry by comparing the effects of the (OH)B(C₆F₅)₃ and the OTf moieties. We also presented a unique model for interrogating the in-plane interactions. Using two different 2^nd generation PN³P-pincer nickel compounds (4 and 6), the H₂ heterolysis and regioselective protonation of the in-plane less hindered nitrogen atoms in the ligand framework was observed. Together with both DFT calculations and control experiments, the significance of the steric factor was confirmed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from King Abdullah University of Science and Technology (KAUST).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed descriptions of the preparation and characterization of compounds 2–11; additional NMR spectra. CCDC 1891945–1891947, 1891951, 1891954, 1909960 and 1909961. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt03003d

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