The controlled transfer of multiple equivalents of protons and electrons is fundamental to many important chemical reactions. While H 2 is a potent reducing agent, transition metal catalysts are often required to overcome kinetic barriers to activating H 2 . Transition metals often mediate reductive transformations via hydride intermediates arising from oxidative addition of H 2 which then perform insertion reactivity. 1 While this primary sphere H 2 transfer is well-established, especially with second-or third-row metals, such two electron processes can be much more challenging with first-row metals.

One strategy to improve the reactivity of first row complexes in these transformations is to engage the secondary coordination sphere. Nature uses this approach, frequently relying on proton/electron transfer from the protein scaffold or cofactors to supply reducing equivalents to transition metal active sites. 2 The elegance of these systems has inspired synthetic chemists to discover new molecular systems which can mimic this reactivity. Incredible advances have been made in designing ancillary ligand scaffolds that can mediate electron transfer, 3 hydrogen bonding, 4 or proton shuttling. 5 Nevertheless, supporting ligand systems which can store both protons and electrons are still uncommon. 6 Recently several well-defined systems that can reversibly store H-atom equivalents have been reported. 7 Systems that can store full H 2 equivalents in a supporting ligand backbone are still rare. 8 In order to explore this relatively scarce area we have been investigating reversible ligand-based H 2 transfer using dihydrazonopyrrole (DHP) complexes of Ni. 9 The 2,5-pyrrole pincer scaffold is attractive for this reactivity because 2e À reduction/ oxidation of the conjugated system coupled with protonation/ deprotonation of the pincer arms can reversibly transfer H 2 without any redox changes at the metal center (Scheme 1). We have previously demonstrated that this scaffold can support the storage of both protons and electrons, but this reactivity had been limited to storage of an electron or H-atom equivalent, not storage of a full H 2 equivalent. 9a,b We now report that Ni complexes of the previously reported tBu,Tol DHP ligand ( tBu,Tol DHP = 2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) can support secondary sphere storage of H 2 . Furthermore, this reactivity is reversible and enables hydrogenation catalysis. Kinetic and computational analysis indicates that ligand hydrogenation proceeds in a process that is first-order in [Ni] and involves H 2 association followed by H-H scission.

Deprotonation of the previously reported tBu,Tol DHPÁ2HCl with 3 eq. of n-BuLi followed by dropwise addition to (dme)NiCl 2 (dme = 1,2-dimethoxyethane) in THF provides ( tBu,Tol DHPH 2 )NiCl (1) as a red crystalline solid in 56% yield (Scheme 2). The The reactivity of 1 was investigated to determine whether the ligand-stored H 2 equivalent could be transferred to substrates. However, 1 shows little to no reactivity with substrates including air, olefins, and carbonyls. We hypothesized that a comparatively strongly coordinating Cl À ligand might inhibit reactivity by occupying a potential site of substrate coordination and therefore abstracted this ligand. Complex 1 reacts cleanly with AgOTf to give the corresponding triflate complex ( tBu,Tol DHPH 2 )NiOTf (2) (Scheme 1). SXRD analysis shows a structure very similar to that of 1 with the triflate bound in the fourth coordination site (Fig. 1, bottom). Hydrogen bonding interactions to the triflate ligand are also clear, with one interaction to O2 of moderate strength and two weaker interactions to O1 based on OÁ Á ÁH distances of B1.9 and 2.4 Å respectively.

We then turned to see if this ligand substitution enabled H 2 transfer reactivity. Hydrogen transfer was tested by stirring 2 with benzoquinone at room temperature which resulted in slow formation of hydroquinone and the previously reported dehydrogenated complex ( tBu,Tol DHP)NiOTf (3) as indicated by 1 H NMR spectroscopy (Scheme 1, see ESI †). 9c This reactivity demonstrates an unusual example where an H 2 equivalent stored on a ligand backbone can be transferred to a substrate. Examples of H 2 transfer between a supporting ligand and a substrate are rare. 8 In order to test catalytic H 2 transfer, we then investigated whether 2 could be regenerated from 3 with H 2 gas. Encouragingly, 1 H NMR analysis of this reaction indicates that complex 2 is formed as the major product when 3 is reacted with one atmosphere of H 2 with mild heating (see ESI †). Given this result, we then placed 3 and excess benzoquinone under an atmosphere of H 2 to determine whether catalytic hydrogenation was feasible. Monitoring by 1 H NMR spectroscopy at room temperature shows conversion of 3 to 2 with concomitant conversion of two equivalents of benzoquinone to hydroquinone indicating this process is catalytic (see ESI †). This reactivity provides important proof of Scheme 2 Synthesis of 1 and 2 and interconversion of 2 and 3 with H 2 and benzoquinone. concept for ligand-based H 2 transfer and shows the viability of the DHP scaffold for reversible H 2 donation.

We were interested in understanding more about the mechanistic details of addition of H 2 to the ligand backbone and therefore we performed kinetic analyses on the interconversion of 3 and 2. Monitoring the reaction of 3 with H 2 by UV-visible spectroscopy shows conversion of 3 to 2 with kinetics consistent with a first-order reaction in [Ni] under pseudo first-order conditions (Fig. 2). Comparison of the product peak intensities at 355 nm with intensities from isolated 2 indicates a high yield for this conversion (480%), consistent with 1 H NMR analysis (see ESI †), but some small amount of an intermediate or byproduct with an absorbance around 550 nm is also formed. We have thus far been unable to obtain further information on this species. The first order dependence on [Ni] for the consumption of 3 is consistent with the hypothesis of a single metal complex reacting to store H 2 across the ligand framework as opposed to a bimolecular reaction, as has been observed in the homolytic activation of O-H bonds with a related Ph,Tol DHP complex. 9b An Eyring analysis shows a DH ‡ of 13.9(4) kcal mol À1 and DS ‡ of À18( 5) cal (mol K) À1 , which gives an overall DG ‡ at 298 K of B21 kcal mol À1 (Fig. 2). The comparatively large and negative DS ‡ suggests that association of H 2 is at least rate-contributing. We also performed the same analysis with D 2 to determine the deuterium kinetic isotope effect (KIE) for this hydrogenation reaction. Comparison of the rates under H 2 versus D 2 shows an inverse deuterium KIE of 0.8 (1). If scission of the H-H bond was the sole rate contributing step a normal primary KIE would be expected. In contrast, the observed inverse KIE for this hydrogenation reaction could potentially arise from an equilibrium isotope effect (EIE) in an H 2 binding pre-equilibrium. 11 For this reason, as well as the convolution of the energetics of the H 2 cleavage steps by an H 2 association step, we have examined this reaction in more detail through additional calculations.

We have examined the thermodynamics of H 2 addition to the ligand framework by performing density functional theory (DFT) calculations. We initially attempted calculations on OTf À -bound species but observed dissociation of the anion along the reaction coordinate. Experimental evidence for OTf À dissociation is equivocal (see ESI †), but the increased reactivity on moving from 1 to 2 supports that anion dissociation may be required for reactivity. To simplify our computational analysis, we have instead examined the energetics and geometries of H 2 cleavage along a singlet manifold starting from a putative cationic intermediate [( tBu,Tol DHP)Ni] + (Fig. 3).

The first optimized transition state is an H 2 splitting step across the Ni-N bond to form the intermediate hydride species [( tBu,Tol DHPH)NiH] + . This transition state is 28.4 kcal mol À1 higher in energy than [( tBu,Tol DHP)Ni] + + H 2 . The second transition state has a similar barrier of 25.9 kcal mol À1 versus [( tBu,Tol DHP)Ni] + + H 2 and results in the favorable (À6.5 kcal mol À1 from the reactants) formation of the hydrogenated product. Examining the Mulliken charge densities of the H-atoms along the reaction coordinate suggests that TS1 is best described as a proton transfer (see ESI †). While the charges in TS2 are much more covalent, the balanced reaction and Mulliken charges suggests this is step is best considered as a hydride transfer. Overall, this analysis suggests that H 2 binding and scission should be accessible with reasonably good agreement between theoretical and experimental energetics.

These calculations also enable us to test the origin of the inverse experimental KIE. Examination of the isotope dependence of the first transition state (TS1) suggests that a primary KIE would be expected (see ESI †), in contrast with experimental observations. Conversely, if the isotope dependence of free H 2 /D 2 and LNiH 2 + /LNiD 2 + are considered, an inverse isotope dependence is predicted for reversible H 2 binding, consistent with experiment (see ESI †). These observations suggest that the origin of the observed isotope dependence likely arises from an H 2 association EIE. 11 Similar inverse isotope effects have recently been observed in paramagnetic transition metal systems as well as across bimetallic frustrated Lewis pairs. 12,13 The studies presented here show an unusual example of metal-ligand cooperativity enabled hydrogenation reactivity where the ligand can store a full H 2 equivalent. The catalytic hydrogenation of benzoquinone provides an important proof of concept for this area. Kinetic data show that the bifunctional splitting of H 2 proceeds in a process that is first-order in [Ni], but that proceeds with an inverse deuterium isotope dependence. Calculations suggest that H 2 scission is energetically accessible and that the source of the observed inverse deuterium isotope dependence is an EIE arising from H 2 association. While catalytic hydrogenations of other substrates have been less successful thus far, the enhanced reactivity in moving from 1 to 2 (Cl À to OTf À ) suggests that the primary metal coordination sphere still plays an important role in this primarily ligand-centered reactivity and offers a potential avenue to expand hydrogenation reactivity to other substrates.