Chemical Science EDGE ARTICLE Ambiphilic boryl groups in a neutral Ni(II) complex: Cite this: Chem. Sci., 2021, 12, 2540 a new activation mode of H2† All publication charges for this article Pablo Rı́os, a Javier Borge, b Francisco Fernández de Córdova,a have been paid for by the Royal Society Giuseppe Sciortino, c Agustı́ Lledós *c and Amor Rodrı́guez *a of Chemistry The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B2Cat2) allows for the synthesis of nickel(II) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H2 is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(II)/Ni(0)/Ni(II) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism Received 2nd November 2020 Accepted 21st December 2020 that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different DOI: 10.1039/d0sc06014c nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing rsc.li/chemical-science the formation of aminoboranes. bonds have proven very effective for the activation of diverse Introduction small molecules.5 Recently, the growing interest in rst-row Since the discovery of pincer ligands in the late 1970s,1 we have transition metal catalysts has also motivated the design of witnessed a tremendous expansion of their use as supporting cooperative platforms based on these metals.6 Among them, ligands in many transition metal complexes that are involved in those systems that bear a boron atom in the ligand architecture, relevant catalytic transformations.2,3 More recently, inspired by whether as a borane or as a boryl functionality, show a different nature, the metal–ligand cooperative processes observed in reactivity pattern, the boron center acting as a Lewis acid, in catalytic reactions mediated by enzymes have been mimicked contrast to that observed when electron-rich moieties such as based on ligand design to develop cooperative catalysts in amides, alkoxides or carbenes are involved (in these situations which both, the metal and the ligand, play a part in the catalytic the Lewis acidic site resides on the metal).7–11 In 2012 Peters and reaction.4 A wide variety of systems having diverse metal–ligand co-workers9 reported the heterolytic cleavage of dihydrogen along with alkene hydrogenation by using a nickel–borane a complex stabilized by a bis(phosphino)borane ligand previously Instituto de Investigaciones Quı́micas, Departamento de Quı́mica Inorgánica, Universidad de Sevilla, Consejo Superior de Investigaciones Cientı́cas, Centro de described by the group of Bourissou (Scheme 1).10 Innovación en Quı́mica Avanzada (ORFEO-CINQA), C/Américo Vespucio 49, 41092 Sevilla, Spain. E-mail: marodriguez@iiq.csic.es b Departamento de Quı́mica Fı́sica y Analı́tica, Centro de Innovación en Quı́mica Avanzada (ORFEO-CINQA), Universidad de Oviedo, C/Julián Claverı́a 8, 33006, Oviedo, Spain c Departament de Quı́mica, Centro de Innovación en Quı́mica Avanzada (ORFEO- CINQA), Universitat Autònoma de Barcelona, Campus UAB, 08193 Cerdanyola del Vallès, Spain. E-mail: agusti@klingon.uab.es † Electronic supplementary information (ESI) available. CCDC 2040006–2040009. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc06014c Scheme 1 Activation of H2 by a nickel–borane complex. 2540 | Chem. Sci., 2021, 12, 2540–2548 © 2021 The Author(s). Published by the Royal Society of Chemistry Edge Article Chemical Science Scheme 2 Activation of H2 by nickel–methyl complex 1. Recently, we have described the activation of dihydrogen mediated by a nickel–methyl complex stabilized by a bis(phos- phino)boryl pincer ligand.11c Mechanistic studies suggest that the H2 bond heterolytic rupture involves a metal–boryl cooper- ation mechanism in which the boryl moiety acts as the elec- Fig. 1 Molecular structure of 3 (H atoms omitted for clarity). Thermal trophilic site accepting a hydride to form, aer loss of methane, ellipsoids are represented at the 50% probability level. Selected bond a Ni(0) s-borane complex that nally evolves to a nickel(II) distances (Å) and angles ( ): Ni(1)–B(4) ¼ 2.015(2); Ni(1)–B(3) ¼ hydride species 2 (Scheme 2).11a 1.942(2); P(1)–Ni(1)–P(2) ¼ 157.74(3); B(3)–Ni(1)–B(4) ¼ 175.0(1). At this point we wondered if the electrophilic behavior of this boron center might be affected by the nature of the group trans to it and, if possible, selecting the appropriate ligand would positions in metal complexes. In fact, just a few examples of d- transform a Lewis acidic boron center into a nucleophilic one. block compounds containing two mutually trans boryl ligands Therefore, we envisioned that placing a stronger s-donor like have been described.13 To the best of our knowledge, 3 is the a dioxoboryl moiety (which on the other hand has an empty p rst nickel complex of this type. 3 exhibits the Bcat fragment orbital), instead of a methyl group, for the diaminoboryl frag- located almost perpendicular (N(1)–B(3)–B(4)–O(2) ¼ 100.0(3) ) ment may have an inuence on the electronic distribution on to the nickel square plane. The Ni(1)–B(3) bond distance the B–Ni–L axis. (1.942(2) Å) is close in value to that observed in Mindiola's complex (1.9091(18) Å).12 However, the Ni(1)–B(4) bond (2.015(2) Results and discussion Å) is considerably longer as a consequence of the stronger trans- inuence of the diaminoboryl moiety compared to the oxygen- Accordingly, to get access to our target complex, the reaction of containing one.14 1 with bis(catecholato)diboron was performed. We observed the The structure of 3 was analyzed by DFT methods using the instantaneous and clean formation of [(PBP)NiBcat] (3) with PBE0/def2TZVP/def2QZVP level of theory, including Grimme's concomitant formation of 2-methyl-1,3,2-benzodioxaborole as D3 (PBE0-D3) dispersion correction (see ESI† for more infor- the only by-product by 1H and 11B NMR (Scheme 3). It is worth mation and references), exhibiting an excellent agreement mentioning that, up to now, the only nickel boryl complex fully between the experimental and calculated geometries (Table characterized is the diphosphino–amido nickel complex [(PNP) S5†).15 A molecular orbital study was performed on the opti- NiBcat] reported by Mindiola and co-workers.12 mized structure of 3 in order to investigate its electron density Complex 3 displays only one 31P NMR signal (d 117.3 ppm) and to determine whether the perpendicular conformation of according to its symmetry and two new resonances at 49 and the Bcat fragment might be due to the existence of metal (d)– 59 ppm in the 11B NMR spectrum for the two boryl moieties. boryl (p) p-interactions. As displayed in Fig. 2, the HOMO is This species shows a high sensitivity to oxygen and moisture, located on the dxz orbital of nickel and the p aromatic system of although appropriate crystals for X-ray diffraction analysis the PBP ligand (no overlap between Ni(1) and B(3) (PBP) is could be obtained by cooling a concentrated toluene/pentane observed, given the opposite sign of the orbitals), whereas the solution of 3 (Fig. 1). The solid-state structure of 3 reveals LUMO is mainly located on the pz orbital of B(3) (PBP). No a square-planar geometry at nickel with both boryl groups appreciable contribution of the Bcat fragment is observed in adopting a trans conguration due to the constraint exerted by any of them. Interestingly, methyl complex 1 exhibits almost the pincer scaffold. This arrangement is rather uncommon identical frontier orbitals (Fig. 2, le); the main differences are because of the tendency of boryl groups to occupy cis or fac the lack of contribution of B(3) (PBP) to the HOMO of 1 and the presence of electron density on the methyl group. This is in agreement with the electron rearrangement that we previously observed for the hydrogenolysis of the Ni–Me bond in 1 (i.e. the methyl moiety acts as a Lewis base capturing a proton while the boron atom behaves as a Lewis acid, receiving a hydride).11a Additionally, the contribution of B(3) (PBP) to the HOMO in 3 clearly reveals the modication of the electron density along the B–M–L axis compared to 1, as discussed previously. Further Scheme 3 Synthesis of trans-bis(boryl)-nickel complex 3. © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 2540–2548 | 2541 Chemical Science Edge Article Fig. 3 Different bonding scenarios in a MH2BR2 complex. knowledge, 4 represents the rst nickel complex containing a H2Bcat moiety.19 The structure of 4 shows a k2 coordination Fig. 2 Frontier orbitals of complexes 1 (left) and 3 (right). Isodensity mode for the H2Bcat group with two different B–H bond value: 0.06 e au3. Most of the hydrogen atoms and some tert-butyl distances (1.190(2) and 1.267(3) Å). These distances are groups have been omitted for clarity. considerably shorter than those reported for niobocene Cp2Nb [BH2(O2C6H4)] (1.62(5) and 1.69(5) Å), whose structure resem- bles bonding scenario II (Fig. 3).19a The Ni(1)–B(3) distance analysis of 3 revealed the presence of energetically lower (1.966(2) Å) is signicantly shorter than the Ni(1)–B(4) distance molecular orbitals with the right symmetry for p-backbonding, (2.145(3) Å) which is, in fact, longer than the sum of the covalent namely HOMO-6 and HOMO-7, although the coefficients of the radii of nickel and boron (2.09 Å).20 The Ni–H bond distances p orbitals of the boron atoms in these molecular orbitals are (1.614(3) and 1.750(2) Å) are comparable to those found in considerably small (Fig. S58†).16 similar species previously reported.17 Additional support for In order to explore the effect of this electronic picture on the classifying 4 as a dihydroborate comes from QTAIM analysis on cooperative bond activation of dihydrogen, we exposed a solu- its optimized structure.21 Indeed, bond critical points (BCPs) tion of 3 in C6D6 to an atmosphere of H2 (4 bar) at room were observed for Ni(1)–H(1), Ni(1)–H(2), B(4)–H(1) and B(4)– temperature. We observed an immediate change of colour from H(2), but not between Ni(1) and B(4). Moreover, a ring critical yellow to pale orange upon diffusion of dihydrogen into the point (RCP) was also detected inside the kite-shaped cycle solution of 3. Analysis by 1H, 31P{1H} and 11B{1H} NMR spec- formed by the bond paths among Ni(1), H(1), H(2) and B(4) troscopy conrmed the full conversion of 3 into a new species 4 (Fig. S59†). These results are in good agreement with some (Scheme 4). previously reported complexes of osmium containing H2Bcat This new complex is characterized by a signal at 107.1 ppm fragments.19f in the 31P{1H} NMR spectrum. The 11B{1H} NMR spectrum At this point the question that arises is whether the mech- shows two resonances; one is shied upeld (18 ppm) anism of H2 activation is analogous to that operating when 1 is compared to that observed for 3 (59 ppm), as expected for employed (i.e. the diaminoboryl group acting as an electrophile) a tetra-coordinated boron center, while the other one appears at or, in contrast, the presence of a dioxoboryl group trans to it the typical chemical shi of a diaminoboryl group (41 ppm). affects its electronic behaviour to allow a different H–H bond The 1H NMR spectrum of 4 is very similar to that of 3, yet there activation mode. Accordingly, the mechanism for the formation is a singlet that integrates to two protons at d ¼ 1.49 ppm. These of 4 was modelled by means of DFT calculations.22 NMR data suggest the formation of a dihydro–borate complex When considering the potential mechanisms that could lead (I, Fig. 3).17 The 2H NMR spectrum of a sample prepared by to the activation of the H2 molecule, several possibilities were reacting 3 with deuterium shows a single broad resonance at the same chemical shi. Moreover, addition of a 1 : 1 mixture of H2 and D2 (2 bar) to a solution of 3 leads to the formation of HD, demonstrating the reversibility of the reaction (see ESI†).18 Fortunately, crystallization of 4 from C6D6/pentane at 20  C afforded suitable crystals for X-ray diffraction analysis which conrmed the proposed structure by NMR (Fig. 4). Among the nickel dihydro–borate complexes reported, only a few have been characterized by X-ray diffraction analysis and, to our Fig. 4 Molecular structure of 4 (H atoms omitted for clarity expect for the H2B unit). Thermal ellipsoids are represented at the 50% probability level. Selected bond distances (Å) and angles ( ): Ni(1)–B(4) ¼ 2.145(3); Ni(1)–B(3) ¼ 1.966(2); B(4)–H(1) ¼ 1.190(2); B(4)–H(2) ¼ 1.267(3); Ni(1)– Scheme 4 Dihydrogen activation by 3. H(1) ¼ 1.614(3); Ni(1)–H(2) ¼ 1.750(2). 2542 | Chem. Sci., 2021, 12, 2540–2548 © 2021 The Author(s). Published by the Royal Society of Chemistry Edge Article Chemical Science taken into account: (a) decoordination of one of the phosphine boron atom of the PBP ligand) and a Lewis acid center (the Ni ligands in order to create a vacant position where H2 could bind atom).25a In agreement with this picture, natural charge analysis the metal atom; (b) cooperativity of the PBP pincer ligand, in shows incipient charge separation between the two H atoms: a similar fashion to that reported previously9,11 (Schemes 1 and whereas the one close to the PBP ligand possesses a charge of 2), and (c) metathesis processes.23 Decoordination of one of the +0.023, the one next to Bcat has a charge of 0.052. Moreover, phosphine groups turned out to be too energy demanding (the both boron atoms also differ in their natural charges (B(PBP) ¼ resulting product was 38.3 kcal mol1 higher than complex 3), +0.587 vs. B(cat) ¼ +0.826). However, in contrast to what and exploratory calculations involving metathesis pathways happens in usual ligand-assisted H–H heterolytic splitting, the evolved towards participation of the PBP ligand. Thus, the hydride does not remain bonded to the metal; instead it ends up mechanism in which the pincer ligand assists the activation of (Int1) connected to a more acidic center, namely the boron atom the H2 was computed, as displayed in Fig. 5 (energy prole) and of the Bcat ligand. The metal is just assisting the heterolytic 6 (optimized structures of intermediates and transition states). H–H scission between the two boron centers, which play Starting from 3, the approach of H2 to the pincer complex gives different roles during H2 activation (see below). the corresponding adduct 3$H2, which is 12.1 kcal mol1 above Upon cleavage, each boryl moiety receives a H atom, giving the origin. The increased energy and the small degree of elon- the corresponding Ni(0) bis s-BH complex Int1, with a relative gation of the H–H bond (0.803 Å in 3$H2 vs. 0.744 Å in the Gibbs energy of 3.8 kcal mol1. This species seems to contain optimized free H2 molecule, see Fig. 6) reect the weakness of two s-BH interactions (average values: Ni–H z 1.62 Å; B–H z the binding. This is not surprising, given the absence of 1.29 Å; B–H–Ni z 85 ).25b Interestingly, the Ni–B(cat) distance a coordination vacant site in the complex, its neutral character (1.979 Å) is slightly shorter than that of Ni–B(PBP) (2.037 Å). The and the electron conguration on the metal.24 In 3$H2 the explanation for this shortening might be found in the small dihydrogen ligand is slightly leaned towards the boron atom of degree of pyramidalization of the boron atom of the B(cat) unit the PBP ligand (H–B(PBP) ¼ 2.387 Å, H–B(cat) ¼ 2.407 Å), (sum of angles around B(cat) ¼ 343.9 vs. 351.7 in B(PBP)), already anticipating the course of the reaction. The perpendic- suggesting a stronger interaction with the metal center. ular orientation of the Bcat ligand precludes any interaction However, QTAIM analysis only evinces BCPs for the B–H and with H2 at this point. Although the formation of the dihydrogen Ni–H bonds, suggesting h1 interactions (Fig. S60†). Aer Int1 complex seems to be unfavourable, H–H cleavage can take place catecholborane rotates in a similar way to that described by easily from 3$H2 through TS1 (19.3 kcal mol1). TS1 reveals Hartwig et al.,26 placing both H atoms in a trans arrangement in substantial elongation of the H2 molecule (1.244 Å), where one species Int10 falling at 8.9 kcal mol1 (Fig. 6). The most appre- of the H atoms is located close to the B atom of the PBP ligand ciable consequences of this rotation are the slight contraction (H–B(PBP) ¼ 1.632 Å). In contrast, the other H atom is farther of the Ni–H(Bcat) bond to 1.567 Å and the B–Ni–B angle from away from Bcat (H–B(cat) ¼ 2.211 Å). Both hydrogen atoms 179.1 to 133.0 . Closing this angle renders the complex with an remain close to Ni and almost equidistant (H–Ni ¼ 1.495 and appropriate geometry for H transfer from PBP to Bcat: the cat- 1.503 Å). This geometrical arrangement of TS1 ts with an echolboryl fragment is orienting the pz orbital of boron in a way heterolytic cleavage of the H–H bond between a basic site (the that can receive one H atom. Indeed, the H atom from the Fig. 5 Gibbs energy profile in toluene for the reaction of 3 with H2. Relative Gibbs energies at 298 K and 1 M in kcal mol1. Some of the phosphine groups (PtBu2) have been abbreviated as P for clarity. © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 2540–2548 | 2543 Chemical Science Edge Article Fig. 6 Optimized structures of intermediates and transition states in the Gibbs energy profile for the reaction of 3 with H2. Selected distances in Å. H atoms bonded to carbon atoms have been omitted for clarity. diaminoborane can bounce to the boron atom of Bcat through reaction observed for methyl derivative 1, a localized orbital TS2, giving 4 as the nal and most thermodynamically stable analysis was performed. This method involves the trans- species of the prole (Fig. 6). The energy required for this formation of Kohn–Sham orbitals into maximally localized transition state is considerably small (4.7 kcal mol1 with orbitals (LMOs) whose centroids are computed for the selected respect to Int10 ), presumably because of the preorganized structures.28 In the case of methyl complex 1 and boryl species 3, geometry.27 In TS2 the hydrogen atom that is being transferred a direct comparison of both species gives little information, as is almost halfway between both boron centres (H–B(PBP) ¼ shown in Fig. S50;† nonetheless, studying the electron rear- 1.703 Å, H–B(Bcat) ¼ 1.735 Å) and remains close to the Ni atom rangement along the reaction paths provides valuable infor- (H–Ni ¼ 1.515 Å). TS2 can be considered as the transition state mation, since the rupture and formation of chemical bonds can for the hydride transfer between the two boron atoms, assisted be studied visually, similar to an arrow-pushing scheme. Thus, by the nickel atom (see Fig. S51, S52 and S65†). analysis of the centroids involved in the H–H cleavage in TS1 The importance of both boryl groups in hydrogen activation revealed that the boron atom from the PBP ligand acts as was further evidenced when the boryl group of the PBP ligand a Lewis base, capturing a formally protic hydrogen, whereas the was replaced by a carbene fragment (similar to a benzimidazol- catecholboryl moiety receives the attack from the hydride-like H 2-ylidene moiety), giving a transition state similar to TS1 with fragment (Fig. 7a and b top). This is exactly the opposite result a much higher Gibbs energy (34.3 kcal mol1, Fig. S54†). In this compared to that observed for complex 1, where the boryl cationic TS1 a heterolytic H–H rupture also takes place, but the fragment acts as a Lewis acid accepting a hydride-like unit and Bcat boron receives the formally protic hydrogen (H–B(Bcat) ¼ the methyl ligand behaves as a Lewis base, capturing a formally 1.452 Å) and the hydride is kept bonded to the nickel centre (Ni– protic hydrogen (Fig. 7b, bottom).11 This manifests the ambi- H ¼ 1.500 Å). TS1 with the carbene ligand is a late transition philic character of the boryl ligand and how it can be tuned by state (H–H ¼ 1.416 Å vs. 1.244 Å with the PBP ligand). In this way modifying the ligand trans to it. the energy barrier for the H–H rupture is much higher and in Studying the centroids in TS2 reveals how nickel assists the agreement with the Hammond postulate the resulting inter- transfer of a hydride from the diaminoborane to the boron atom mediate is not stabilized (just 0.1 kcal mol1 below TS1), giving of HBcat, forming the H2Bcat– fragment. At the same time, an a highly endergonic prole. electron pair of Ni moves away from the metal to forge a Ni–B To analyze the inuence of the pincer ligand in the reaction, bond with the boron atom of the PBP ligand, reconstituting the we have computed the energy prole on an unconstrained pincer scaffold in a process where the oxidation state of the (PMe3)2(B(NH)2R)Ni(Bcat) complex (R ¼ C6H4). A similar metal increases from Ni(0) to Ni(II) (red arrow, Fig. S51†). This mechanism can operate for the H2 activation (see Fig. S55†). mechanism for H2 activation is novel compared to preceding However, in the unconstrained complex the isomer with the two reports according to recent classications (Fig. 8)7b due to the boryl ligands mutually cis is 8.7 kcal mol1 more stable than the presence of two boryl ligands with different Lewis acid/base trans boryl isomer. The pincer ligand is forcing the system to behaviour. In fact, some previous methodologies for acti- adopt the trans disposition of the boryls required for the reac- vating dihydrogen with nickel involve cationic species with tion to take place. a vacant site to form the H2 complex followed by deprotona- In order to investigate the electron rearrangement that takes tion,24,29 or the use of electron rich, Ni(0) derivatives able to place upon exposure of 3 to H2 and compare it to the analogous weaken the H–H bond via back-donation to the s* orbital of H2 2544 | Chem. Sci., 2021, 12, 2540–2548 © 2021 The Author(s). Published by the Royal Society of Chemistry Edge Article Chemical Science Fig. 7 Electron rearrangement that takes place upon H2 activation in Scheme 5 Reactivity of 3 with amines and ammonia. complex 3 (a) and comparison with complex 1 (b).11 This reactivity towards amines might be consistent with in combination with Lewis acidic ligands. As a comparison, 3 9,30 a highly Lewis acidic boron center at the Bcat moiety that is a neutral Ni(II) four-coordinate complex with no vacant enables the initial coordination of the amine followed by acti- position in its coordination sphere, and the aforementioned vation of the N–H bond whether by a cooperative mechanism data suggest that the presence of the two boryl ligands is involving the second boron center or by a likely metathesis responsible for the successful H2 activation. pathway. Encouraged by these results we sought to explore if the Given the reactivity observed with 3 towards dihydrogen and borylation of NH3 can also be accomplished by 3. Activation of the conclusions extracted from DFT calculations we were keen to ammonia is quite challenging due to the tendency of this study the electrophilic behavior of the dioxoboryl group. Thus, we molecule to form stable Werner-type complexes that do not explored the reactivity of 3 towards different nucleophilic species. undergo further N–H activation as a consequence of the high First, we performed the reaction towards amines in order to learn strength of the N–H bond.32,33 Different approaches to overcome whether the coordination to the electrophilic boron center takes these difficulties encompass the use of electron rich transition place (Scheme 5). Thus, treatment of 3 with secondary amines, metal complexes to favor N–H cleavage by oxidative addition,34 such as diethylamine and pyrrolidine, allows the instantaneous main group systems,35 bimetallic species,36 transition metal formation in both cases of a new species that was identied as systems with non-innocent ligands that operate cooperatively,37 the nickel hydride complex (2, see Scheme 5), and the corre- metal coordination to induce N–H bond-weakening38 or acti- sponding aminoborane products 5 and 6.31 These species were vation of ammonia through different concerted mechanisms.39 characterized by 1H, 11B and 13C NMR spectroscopy and in the The reaction of ammonia with 3 yielded nickel–hydride case of 6 also by X-ray diffraction analysis. 5 shows only one 11B complex 2,40 similarly to that observed with amines, and the chemical shi at 25.9 ppm in solution, characteristic of a three- double borylation product of ammonia (HNBcat2). This species coordinated boron atom. In the case of 6, which is a dimer in was characterized by comparison of its NMR spectra with those the solid state, the 11B NMR spectrum shows two different reso- of a sample independently synthesized by dehydrogenative nances, one at 25 ppm and another one at 9 ppm, which is the borylation of NH3 and HBCat using a platinum catalyst (see ESI† expected chemical shi for a tetra-coordinated boron, revealing for details).31 It is important to mention that no reaction was a monomer–dimer equilibrium in solution for this species (see observed when a solution of either 2 or nickel methyl complex 1 ESI†). Additionally, when 3 is reacted with deuterated diethyl- was treated with ammonia (even at 70  C), highlighting the role amine (Et2ND) the formation of the nickel-deuteride (2-D) of the dioxoboryl group in this process. complex is conrmed by 2H NMR spectroscopy (see ESI†). Finally, further study of the electrophilic behavior of Bcat on 3 was performed by analysis of its reactivity towards carbon dioxide. Unfortunately, no reaction was observed with CO2, even aer prolonged heating at 70  C. This lack of reactivity might be an indication of the insufficient electrophilic character of this boron center to react with the oxygen atoms of CO2 but also of the low nucleophilicity of the dioxoboryl moiety that is not basic enough to attack the electrophilic carbon of carbon dioxide.41 Conclusions In conclusion, we have prepared a bis(phosphino)boryl (PBP) nickel complex that accommodates a second boryl group (Bcat) trans to the boron atom of the pincer ligand. This nickel complex is able to reversibly activate dihydrogen in a mecha- nism that involves the cooperation between the metal and both boryl moieties through a concerted ve-center process. We have shown that the electronic behavior of the diaminoboryl group Fig. 8 Different types of H2 activation. on the PBP ligand can be changed by modifying the ligand trans © 2021 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2021, 12, 2540–2548 | 2545 Chemical Science Edge Article to it (boryl instead of methyl) and we provide valuable infor- 6 J. I. van der Vlugt, Eur. J. Inorg. Chem., 2012, 363–375. mation on the key role of the second boryl group in the reactivity 7 (a) Y. Li, C. Hou, J. Jiang, Z. Zhang, C. Zhao, A. J. Page and of this compound. We have found that aer the splitting of the Z. Ke, ACS Catal., 2016, 6, 1655–1662; (b) G. R. Owen, dihydrogen molecule, the boron atom from the PBP ligand Chem. Commun., 2016, 52, 10712–10726; (c) G. Bouhadir behaves as a nucleophile, accepting a formally protic hydrogen, and D. Bourissou, Chem. Soc. Rev., 2016, 45, 1065–1079; (d) whereas the hydride-like fragment ends up bonded to the cat- Y. Li, J. Zhang, S. Shu, Y. Shao, Y. Liu and Z. Ke, Chinese. J. echolboryl moiety that acts as an electrophile. Additionally, the Org. Chem., 2017, 37, 2187–2202; (e) Q. Zhao, Lewis acidic character of the Bcat group was further corrobo- R. D. Dewhurst, H. Braunschweig and X. Chen, Angew. rated by reactivity towards Lewis bases such as amines and Chem., Int. Ed., 2019, 58, 3268–3278; (f) Y. Li, J. Liu, ammonia. X. Huang, L.-B. Qu, C. Zhao, R. Langer and Z. Ke, Chem.– This work discloses an unprecedented mechanism that Eur. J., 2019, 25, 13785–13798; (g) C. Hou, Y. Li and Z. Ke, shows the impact of the boryl–metal–boryl arrangement on the Inorg. Chim. Acta, 2020, 511, 119808. facile and reversible activation of the H–H bond, which could be 8 (a) N. Tsoureas, Y.-Y. Kuo, M. F Haddow and G. R. Owen, useful for the design of catalytic systems that may be able to Chem. Commun., 2011, 47, 484; (b) W.-C. Shih and perform other non-polar bond activations. O. V. Ozerov, J. Am. Chem. Soc., 2017, 139, 17297–17300; (c) W.-C. Shih and O. V. Ozerov, Organometallics, 2017, 36, Conflicts of interest 228–233; (d) Y. Cao, W.-C. Shih and O. V. Ozerov, Organometallics, 2019, 38, 4076–4081; (e) Y. Cao, There are no conicts to declare. W.-C. Shih, N. S. Bhuvanesh and O. V. Ozerov, Chem. Sci., 2020, 11, 10998–11002. Acknowledgements 9 (a) W. H. Harman and J. C. Peters, J. Am. Chem. Soc., 2012, 134, 5080–5082; (b) See also for a DFT analysis: G. Zeng Financial support (FEDER contribution) from the MINECO and S. Sakaki, Inorg. Chem., 2013, 52, 2844–2853; (c) (Projects CTQ2016-76267-P, CTQ2017-87889-P, PID2019- W. H. Harman, T.-P. Lin and J. C. Peters, Angew. Chem., 109312GB-IOO and RED2018-102387-T), the CSIC (AEPP- Int. Ed., 2014, 53, 1081–1086, for related work see also: (d) CTQ2016-76267-P) and the use of computational facilities of R. C. Cammarota and C. C. Lu, J. Am. Chem. Soc., 2015, the Supercomputing Center of Galicia (CESGA) is gratefully 137, 12486–12489; (e) R. C. 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