pubs.acs.org/IC Article Interplay between β‑Diimino and β‑Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction Navid Jameei Moghaddam, Marcos Gil-Sepulcre, Jia-Wei Wang, Jordi Benet-Buchholz, Carolina Gimbert-Suriñach,* and Antoni Llobet* Cite This: Inorg. Chem. 2022, 61, 16639−16649 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ABSTRACT: Two Ni complexes are reported with κ4-P2N2 β- diimino (BDI) ligands with the general formula [Ni(XBDI)]- (BF4)2, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2- Downloaded via 37.15.237.229 on November 10, 2022 at 15:04:46 (UTC). (diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X in- dicates the substituent in the α-carbon intradiimine position, X = H for 1(BF4)2 and X = Ph for 2(BF4)2. Electrochemical analysis together with UV−vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β- diimino complexes 12+ and 22+ to the negatively charged β- diketiminato (BDK) analogues (1-H)+ and (2-H)+ via deprotona- tion in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1+ and 2+ can also rapidly evolve to the BDK (1-H)+ and (2-H)+, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at Eapp = −1.8 V vs Fc+/0, being the active species the one-electron-reduced derivative 1-H and 2-H. ■ INTRODUCTION β-Diketiminates (often abbreviated as nacnac or BDK) are a small molecules such as carbon dioxide or dinitrogen.14,21 More recently, Meyer and coworkers have published a family of ligands that have been widely used for synthesizing a dinuclear bis(β-diketiminato) complex bridged by a pyrazolate large amount of complexes with different metal centers, all ring, which has been shown to be involved in the across the periodic table.1,2 Since their introduction in stoichiometric proton reduction reaction to produce hydrogen 1968,3−6 BDK ligands have gained lots of interest because of through a proposed metal−metal and metal−ligand coopera- their versatility and tunability as well as their strong binding to tive two-electron reductive process.22,23 In their work, they metal centers. The variation of the ligand’s steric and electronic fully characterized the dihydride species Ni2H2(BDK)2 in properties is possible by changing the substituent on the imine Figure 1 (left) with two Ni(II) centers, prepared from the corresponding bromido-bridged derivative in the presence of N atom and the α-carbons.7,8 In addition, although BDK have KHBEt 3 . The species Ni 2 H 2 (BDK) 2 converts to the been long considered spectator ligands, several reports show corresponding bis(β-diimino) derivative Ni2(BDI)2 containing that they can also act as non-innocent ligands.9−12 There is a two three-coordinated Ni(I) centers in the presence of an acid considerable diversity of bonding modes from pure σ to a source, as demonstrated by a full range of spectroscopic and combination of σ to π donation for β-diketiminato ligands to magnetic techniques. metal complexes, in which most of the coordination modes are In this work, we exploit the ability of BDI/BDK ligands in metal center dependent. The most common is the κ2-N,N′ proton to hydrogen conversion by designing two tetradentate bidentate coordination through the two nitrogen atoms, symmetric κ4-P2N2 diimino ligands containing two aliphatic affording a six-membered metalacyclic ring.1,13 diphenylphosphine arms giving enough flexibility to allow tetra First-row transition metal β-diketiminato complexes have been used for a wide variety of applications including stoichiometric and catalytic transformations such as function- Received: June 22, 2022 alization of alkenes, cross-coupling reactions, and energy- Published: October 5, 2022 related applications.14−20 In addition, late transition metal complexes have been used for synthesizing low coordination metal centers mimicking the active site of metalloproteins for the conversion of organic substrates but also the activation of © 2022 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.inorgchem.2c02150 16639 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article Figure 1. (Left) Stoichiometric H2 evolution from a dihydrido dinickel bis(β-diketiminato) complex reported in the literature.22,23 (Right) β- Diimino ligands containing two phosphine arms designed in this work as precursors to tetradentate β-diketiminato complexes. Scheme 1. Synthesis of Ligands and Complexes Described in This Work Figure 2. ORTEP representation of 12+(a) and 22+(b) at 50% probability level (hydrogens are omitted for further clarification, except for α-carbon hydrogen). Color code: C, dark gray; N, blue; Ni, green; P, purple. coordination with required geometry (Figure 1, right). While π-back bonding ability of phosphine ligands. Two different the BDK/BDI moiety was expected to provide a non-redox- ligands have been designed with different groups (HBDI and active but useful proton relay fragment, the two coordinated PhBDI) in the α-carbon intradiimine position, tuning the phosphine groups would provide extra coordination sites while acidity of the BDI proton to give the corresponding BDK allowing to stabilize metallic low oxidation states, thanks to the derivative. Thus, we describe the synthesis, spectroscopic, and 16640 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article Figure 3. Comparison of the 1H NMR spectra of complex 1(BF4)2 in CD3CN (top) and DMF-d7 (bottom). electrochemical characterization of the nickel complexes Single crystals suitable for X-ray diffraction analysis were derived from HBDI and PhBDI and their conversion to the grown by layering Et2O on top of a saturated solution of corresponding β-diketiminato derivatives in the context of the complexes 1(BF4)2 or 2(BF4)2 in CH3CN while keeping the proton reduction reaction to produce hydrogen. mixture at −30 °C. The ORTEP representations of the cationic part of both complexes, shown in Figure 2, reveal a ■ RESULTS AND DISCUSSION Synthesis and Structural Characterization of Com- distorted square planar geometry with the angles of 94.56° (N1-Ni-N2), 83.06° (N2-Ni-P2), 86.48° (N1-Ni-P1), and 95.85° (P1-Ni-P2) for 12+ and 92.60° (N1-Ni-N2), 86.93° plexes 1(BF4)2 and 2(BF4)2. Symmetrical κ4-P2N2 ligands (N1-Ni- P1), 83.28° (N2-Ni-P2), and 96.64° (P1-Ni-P2) for derived from β-diketiminate are scarce in the literature, despite 22+. The imine C=N bond lengths ranging from 1.279(8) to the well-known capacity of both BDK and phosphine groups to 1.281(1) Å are significantly shorter than the C−N bonds stabilize coordination complexes.24,25 Other examples include linking the phosphine arms, which are in the range of only one phosphine arm connected to the BDK ligand.20,26 1.476(4)−1.495(3) Å. These results are in agreement with HBDI and PhBDI ligands in Figure 1 and Scheme 1 are two localized imine double bonds within a nonplanar ligand core novel tetradentate ligands with two nitrogen and two with the sp3-hybridized α-carbon lying above the plane of the phosphorous donors that differ on the substituent of the α-C backbone, giving rise to a boat conformation.27 position (H for HBDI and Ph for PhBDI). The synthesis of the Compounds 1(BF4)2 and 2(BF4)2 are Ni(II) d8 diamagnetic ligands HBDI and PhBDI was performed through two complexes and could be fully characterized by NMR consecutive condensation steps in oxygen and moisture free spectroscopy (Figure 3 and Figures S1−S26 in the Supporting conditions. Briefly, the first condensation was performed by Information). The 1H NMR spectra in CD3CN show the refluxing the corresponding acetylacetone and 2- characteristic resonances of the α-imino protons of the sp3 (diphenylphosphaneyl)ethylamine in the presence of a carbon at 4.29 ppm integrating 2H for 1(BF4)2 and at 5.50 catalytic amount of acid in toluene, and the second ppm integrating 1H for 2(BF4)2. A single set of resonances per condensation was performed using dimethyl sulfate in a methylene group of the ligand scaffold is indicative of a solvent-free reaction with yields of 30 and 35% for HBDI and symmetric structure of the complexes. This is confirmed by the 31 PhBDI, respectively (Scheme 1). The synthesis of Ni P{H} NMR spectra in CD3CN, which show a unique singlet complexes from the ligands HBDI and PhBDI is straightfor- resonance for the two phosphine ligands at 49.64 and 47.15 ward using Ni(BF4)2·6H2O as metal precursor, obtaining ppm for 1(BF4)2 and 2(BF4)2, respectively. moderate to good reaction yields (36 and 96% for complexes All the resonances corresponding to complex 1(BF4)2 are 1(BF4)2 and 2(BF4)2, respectively). The lower yield obtained significantly shifted when the NMR analysis is performed in for 1(BF4)2 is due to the difficulties in the purification of the deuterated dimethylformamide (DMF-d7) (Figure 3 and compound, which requires a slow crystallization process in a Figures S11 and S23 in the SI). In particular, the α-imino mixture of acetonitrile/diethyl ether at −30 °C, as described in proton appears at 5.31 ppm, accounting for a 1.02 ppm detail in the Experimental Section. downfield shift compared to the resonance in CD3CN. In 16641 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article Figure 4. Zoom of the HSQC spectra of 1(BF4)2 in CD3CN (left) and DMF-d7 (right). Figure 5. UV−vis spectra of 0.125 mM of (a) 1(BF4)2 in CH3CN (black) and DMF (gray) and (b) 2(BF4)2 in CH3CN (red) and DMF (orange). addition, it integrates for one proton instead of two, suggesting The β-diimino compounds 1(BF4)2 and 2(BF4)2 and the β- a deprotonation of the neutral β-diimino ligand to form a diketiminato derivatives (hereafter (1-H)BF4 and (2-H)BF4) negatively charged β-diketiminato derivative (Figure 3). This is were also characterized by UV−vis spectroscopy (Figure 5; see confirmed by the 13C NMR spectrum, where the chemical shift also Figures S38 and S39). Complex 1(BF4)2 in acetonitrile of the C=N imino carbon is upfield shifted from 179.97 ppm in shows an intense absorption below 300 nm and two well- CD3CN to 161.81 ppm in DMF-d7 and the α-carbon is defined absorption bands at 323 nm (ε = 3720 M−1 × cm−1) downfield shifted from 53.71 ppm in CD3CN to 102.16 ppm and 348 nm (ε = 4528 M−1 × cm−1) with a shoulder at 414 in DMF-d7. The assignment of the α-carbon is confirmed by nm (ε = 460.72 M−1 × cm−1) (black trace in Figure 5a). These the clear crossed signals in the HSQC spectra (see Figure 4 bands are attributed to intraligand π−π* transitions. Finally, a and Figures S5 and S15 in the SI). This impressive shift can very weak absorption band at 512 nm (ε = 196.56 M−1 × only be explained by the changes in hybridization and formal cm−1) appears that is assigned to the d−d transition of the low- spin square planar Ni(II) complex.29 charge of the carbon atoms going from a β-diimino to a β- As discussed above, dissolving 1(BF4)2 in DMF affords the diketiminate ligand. A similar phenomenon is observed for β-diketiminato derivative, which shows a very similar UV−vis complex 2(BF4)2 where the resonance corresponding to the α- spectrum (gray trace in Figure 5a) with exactly the same bands CH proton disappears (Figure S25). Likewise, the C=N but less intense at 323 nm (ε = 3050 M−1 × cm−1), a shoulder carbon shifts from 181.52 ppm in CD3CN to 144.81 ppm in at 414 nm (ε = 356.56 M−1 × cm−1), and 512 nm (ε = 134.24 DMF-d7 and the α-carbon from 67.97 ppm in CD3CN to M−1 × cm−1). In contrast, the band at 348 nm (ε = 4578 M−1 112.96 ppm in DMF-d7 (Figure S26). 2D-NMR experiments of × cm−1) is slightly more intense (Figure 5a, inset). The both complexes in DMF-d7 rule out the possible α-imino differences in the absorption spectra of 2(BF4)2 and (2-H)BF4 proton exchange with the solvent and support the deprotona- are more pronounced, although the band positions are also tion of the β-diimino ligand (Figure S27). Indeed, the acidity very similar (Figure 5b, red and orange traces, respectively). of the α-CH proton is rather high, and it is increased once The spectrum in CH3CN shows bands at 284 nm (ε = 13770 coordinated to the metal center, so that even very weak bases M−1 × cm−1), a shoulder at 313 nm (ε = 6499 M−1 × cm−1), such as DMF induce the deprotonation of the ligand.2,28 and two other bands at 365 nm (ε = 1507 M−1 × cm−1) and 16642 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article Figure 6. (a) Cyclic voltammetry experiments of 0.5 mM solution of 12+ (black) and 22+ (red) in CH3CN and a scan rate of 100 mV/s. (b) Comparison of voltammograms of 12+ in CH3CN at different scan rates. Table 1. Redox Potential Data for Complexes 12+ and 22+ in CH3CN and DMFb solvent complex E1pa E1pc E11/2 ΔE1 (mV) E2pa E2pc E21/2 ΔE2 (mV) a CH3CN 12+ −1.14 −1.20 −1.17 60 −1.73 −1.79 −1.76 66 22+ −1.08 −1.14 −1.11 59 −1.74 −1.83 −1.78 90 DMF 12+ −1.72 −1.8 −1.76 80 22+ −1.72 −1.8 −1.76 80 a b +/0 Peak separation potential at high scan rates (υ = 0.3 V/s). All potentials are reported in V versus Fc . 528 nm (ε = 112.72 M−1 × cm−1). In comparison, 2(BF4)2 in Thus, the first redox event from a formal Ni(II) to Ni(I) is DMF shows bands at 270 nm partially overlapped with the influenced by the electron-withdrawing nature of the phenyl solvent subtraction noise, at 325 nm (ε = 2526 M−1 × cm−1), ring in the α-position of the diimino complex. The reversibility 365 nm (ε = 2832 M−1 × cm−1), and 521 nm (ε = 193.6 M−1 of this first redox wave at slow scan rates of ≤0.2 V/s is fully × cm−1). reversible for 22+ as opposed to 12+, suggesting that the Electrochemical Characterization. The redox properties chemical reaction following the first reduction is less favored of complexes 12+ and 22+ were investigated by cyclic for 22+. voltammetry (CV) in anhydrous acetonitrile or dimethylfor- Cyclic voltammetry of the β-diketiminato derivatives (1-H)+ mamide containing 0.1 M [(n-Bu)4N](PF6)(TBAPF6) as a and (2-H)+ in DMF shows only one quasi-reversible peak supporting electrolyte in a three-electrode one-compartment appearing at the same potential for both complexes with (2- cell consisting of a glassy carbon working electrode, Pt as a H)+ showing lower reversibility (E1/2 = −1.76 V vs Fc+/0, ΔE = counter electrode, and Ag/AgNO3 solution in acetonitrile 80 mV; Figures 7 and 8a and Table 1; see also Figures S30 and (0.01 M Ag/AgNO3, 0.1 M TBAPF6) as a reference electrode, S31 in the SI). Compound (2-H)+ was also characterized by under nitrogen atmosphere. Ferrocene was used as the internal performing a CV of 22+ in acetonitrile in the presence of reference, which was added at the end of each experiment. triethylamine, obtaining the same electrochemical profile Cyclic voltammetry of 12+ in acetonitrile solution shows two (Figure S32). Spectroelectrochemical experiments in an reduction waves. The first one is irreversible with Epc = −1.20 V vs Fc+/0, and the second one is quasi-reversible with E1/2 = −1.76 V vs Fc+/0 (ΔE = 66 mV) (Figure 6a, black trace; see also Table 1 and Figure S33). At higher scan rates (υ ≥ 0.3 V/ s), the first reduction peak becomes reversible (E1/2 = −1.17 V vs Fc+/0, ΔE = 60 mV; Figure 6b), indicating that a chemical reaction follows the electrochemical step.30 A plot of redox peak current varies linearly with the square root of the scan rate (ipc, ipa vs υ1/2) for both redox events, which is expected from homogeneous diffusion-controlled electrochemical events (Figure S28 in the SI). The electrochemical behavior of complex 22+ is similar to that of 12+ showing a first redox wave at E1/2 = −1.11 V vs Fc+/0 (ΔE = 59 mV) and a second one at E1/2 = −1.78 V vs Fc+/0 (ΔE = 90 mV), accounting for an anodic shift of 67 mV for the former and only 20 mV cathodic shift for the latter Figure 7. Cyclic voltammogram of 12+ in CH3CN (bottom) and (Figure 6a, red trace; see also Figures S29 and S33 in the SI). DMF (top). 16643 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article Figure 8. (a) Cyclic voltammogram of 22+ in CH3CN (bottom) and DMF (top). The red arrow indicates the applied potential in the bulk electrolysis (BE) experiment (Eapp = −1.3 V vs Fc+/0). (b) UV−vis spectra of 22+ after bulk BE at Eapp = −1.3 V vs Fc+/0 (red) compared to the UV−vis of (2-H)+ in DMF (orange). See Figure S35 for the details in the BE experiment. Scheme 2. Proposed Electrochemical and Chemical Events during Reduction of 12+ and 22+ in CH3CN (top) and DMF (bottom) Showing the Interconversion between β-Diimino and β-Diketiminato Complexes optically transparent thin-layer electrochemical (OTTLE) cell The peak position of the unique redox event observed for show isosbestic points for the conversion of (1-H)+ and (2- (1-H)+ and (2-H)+ exactly matches that of the second redox H)+ to the neutral 1-H and 2-H derivatives, respectively event observed for 12+ and 22+ (compare black and gray in (Figures S34 and S35 in the SI). The process is reversible, as Figure 7 and red and orange in Figure 8a; see also Table 1). demonstrated by the full recovery of the initial species in the These results suggest a putative conversion of the mono- reverse scan. Thus, the low reversibility of the redox process in reduced β-diimino complexes 1+ and 2+ to the β-diketiminato DMF is associated to a slow electrochemical process, which is complexes (1-H)+ and (2-H)+ within the time scale of the enhanced in the case of the α-phenyl-substituted derivative (2- experiment. This hypothesis is supported by spectroscopic H)+, suggesting that geometrical rearrangement is involved in analysis of the one-electron-reduced species derived from 22+. the process and is hindered by the bulky phenyl group. The low potential of the reduction of (1-H)+ and (2-H)+ This was achieved by performing a bulk electrolysis (BE) (E1/2 = −1.76 V) compared to the first reduction observed for experiment at −1.3 V vs Fc+/0 of a solution of the β-diimino the parent complexes 12+ (E1/2 = −1.17 V) and 22+ (E1/2 = complex 22+ in acetonitrile (Figure 8b and Figure S37 in the −1.11 V) accounts for a cathodic shift of 580 and 650 mV, SI). UV−vis analysis of the resulting mixture matches the respectively, and is in agreement with the negatively charged profile of the spectrum of the β-diketiminato (2-H)+ (Figure nature of the β-diketiminato versus the neutral β-diimino 8b). An analogous experiment using compound 12+ leads to ligands. decomposition of the complex during the long time of the bulk 16644 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article Figure 9. (a) Cyclic voltammetry experiments of a 0.5 mM solution of 22+ in DMF (0.1 M TBAPF6) after consecutive addition of p-TsOH·H2O at a scan rate of 0.1 V/s; WE, GC (Φ = 1 mm); CE, Pt; REF electrode, Ag/AgNO3 (Ag wire in a 0.01 M solution of AgNO3 and 0.1 M TBAPF6 in acetonitrile). (b) Bulk electrolysis experiments at Eapp = −1.8 V vs Fc+/0 in the presence of 60 eq. p-TsOH·H2O in the presence (red) and in the absence (gray) of a 0.5 mM solution of 22+ in DMF (0.1 M TBAPF6); WE, GC rod (S = 3.1 cm2); CE, Pt mesh; REF electrode, Ag/AgNO3 (Ag wire in a 0.01 M solution of AgNO3 and 0.1 M TBAPF6 in acetonitrile), Vcathode = Vanode = 5 mL. electrolysis experiment, highlighting the lower stability of this metal and metal−β-diketiminate ligand cooperative stoichio- complex compared to the α-phenyl-substituted analogue 22+. metric hydrogen evolution by a bimolecular dihydrido nickel The conversion of 1+ and 2+ into (1-H)+ and (2-H)+, complex in the presence of a proton source (Figure 1).22,23 respectively, implies a formal loss of a hydrogen atom, which Further, the dihydride derivative was stable enough to be seems unlikely from the product resulting from the one- isolated and characterized but only in the presence of an acid electron reduction (Scheme 2, top). Instead, we propose a source, and thanks to the cooperative protonation of the BDK/ conversion of the mono-reduced formally Ni(I) complexes to a BDI ligand, the complex was able to evolve H2 gas. In contrast, Ni(III) hydride after intramolecular proton transfer from the in the case of complexes 12+ and 22+ reported here, the putative acidic α-diimino ligand to form the hydride derivative hydrido species 1′+ and 2′+ are too reactive to be isolated, and (Ni(III)-H, 1′+). Such a transformation of low-valent Ni they rapidly transform into (1-H)+ and (2-H)+, respectively. complexes is ubiquitous in molecular hydrogen evolution Electrochemical Proton Reduction. The electrochemical catalysis, but it usually takes place in an intermolecular fashion behavior of 12+ and 22+ in the presence of acid was performed from external acid sources or through an intramolecular proton in CH3CN and DMF using p-toluene sulfonic acid (p-TsOH) transfer from pending protonated amino groups.31 For (pKa = 8.6 in CH3CN).35 In acetonitrile solution, the addition complexes 12+ and 22+, the process is favored by the relatively of acid results in a slight increase in the current of the first high acidity of the diimino ligand and close proximity of the reduction wave for both complexes 12+ and 22+ and a loss of two reactive sites.33 The resulting Ni(III) hydride could then reversibility for both (Figures S40 and S41 in the SI). Upon be involved in a homolytic bimolecular Ni-H cleavage releasing further addition, the peak intensity continues to increase but a molecule of H2 or to a hydrogen atom abstraction by a quickly reaches a plateau after adding approximately 2−3 solvent molecule via metal hydride hydrogen atom transfer equivalents of acid. A similar phenomenon is observed when (MHAT) pathways, producing in both cases the β- [DMFH](CF3SO3) is used as acid (pKa = 6.8 in CH3CN; diketiminato derivatives (1-H)+ and (2-H)+ (Scheme 2, top Figure S42 in the SI). The early saturation of the wave right).32−34 The latter show the same electroreduction feature in CH3CN and in DMF at E1/2 = −1.76 V vs Fc+/Fc (Scheme intensity suggests an irreversible reaction promoted by the 2, bottom). The transformation of diimino to diketiminate presence of acid but with poor or null hydrogen evolution upon reduction and hydrogen atom loss, either by H2 catalytic nature. Indeed, bulk electrolysis experiments at Eapp = evolution or by a MHAT process, evidences the non-innocent −1.2 V vs Fc+/0 in the presence of 20 equivalents of p-TsOH· nature of this type of ligands that can get involved in H2O did not generate any hydrogen gas, as confirmed by cooperative metal−ligand reactivity. A bulk electrolysis experi- analyzing the headspace of the cathodic compartment of the ment of a solution of 22+ at the first one-electron reduction electrochemical cell using GC-TCD. Thus, we propose that wave (Eapp = −1.2 V vs Fc+/Fc) in the absence of any proton under these conditions, the one-electron-reduced species 1+ source followed by analysis of the headspace of the cathodic and 2+ or their Ni(III)-hydride derivatives 1′+ and 2′+ compartment by gas chromatography coupled to thermal protonate to form the intermediate species (1 + H)2+ and (2 conductivity detector (GC-TCD) indicates the evolution of + H)2+ (Scheme S1 in the Supporting Information). Such hydrogen gas, while no hydrogen gas was detected in an species could lead to the electroreduction of one or two of the analogous blank experiment performed in the absence of the imino ligands to form the amino counterparts (formal complex (Figure S36 in the SI). These results not only confirm hydrogenation of the C=N group). Such transformations that the hydrogen atom loss occurs via hydrogen gas have been proposed and even confirmed for related nickel and production but also indicate a putative involvement of a cobalt complexes containing imino ligands.36−38 Since the Ni(III) hydride as a key intermediate for hydrogen production. irreversible conversion of complexes 12+ and 22+ prevented the In this context, a recent report has demonstrated both metal− hydrogen evolution reaction, we did not further investigate this 16645 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article process but focused on the proton reduction ability of 12+ and decomposition might be related to deposited species on the 22+ in DMF at lower potential. electrode since a broad reduction wave in the range of E = Figure 9a shows the cyclic voltammetry profile after −1.1 to −1.7 V vs Fc+/0 is observed in both cases (12+ and 22+) successive additions of p-TsOH in a solution of 22+ in DMF. and that might be responsible for the slight increase of current The reversible wave at −1.8 V associated with the one-electron in this range of potentials (Figure S46 in the SI). Importantly, reduction of (1-H)+ and (2-H)+ becomes irreversible and no significant increase in the current is observed at the bulk increases in intensity with increasing concentration of acid. In electrolysis potential (Eapp = −1.8 V vs Fc+/0) for the rinsed addition, an increase in the current is also observed at E < −1.8 electrode when compared to a blank electrode, supporting the V, which cannot be attributed solely to the blank (see Figure homogeneous molecular nature of the catalytic process. S43 in the SI). A characteristic reversible wave at E = −1.1 V is The fact that 12+ and 22+ are able to trigger hydrogen also observed, attributed to the redox signature of the first evolution in the presence of protons in DMF at Eapp = −1.8 V reduction of 22+, which results from the protonation of (2-H)+ suggests that at this low potential, the decomposition pathways after a few additions of acid. This phenomenon is very clear at observed at lower potentials and attributed to the reduction of 9 equivalents of p-TsOH (Figure 9a, inset). These results show the imine motifs are avoided. Thus, the reduction of the key how subtle changes in acid concentration and acid strength can hydride species 1′+ and 2′+ in Scheme 2 is proposed as the displace the equilibrium between the β-diimino and β- driving force of the hydrogen evolution reaction through the diketiminato complexes (Scheme 3).39−41 A very similar result formal Ni(II)-hydride species 1′ and 2′ (see also Scheme S2 in is obtained using compound 12+, as shown in Figure S44 in the the Supporting Information). Alternatively, the formal Ni(I) SI. diimino derivatives 1+ and 2+ could also be reduced to formal Ni(0) complexes that could lead to hydrogen, most likely Scheme 3. Equilibria between β-Diketiminato and β- through a Ni(I)-hydride complex. However, the latter route Diimino Complexes (1-H)+/12+ and (2-H)+/22+, Highly seems less likely taking into account the possible conversion of Dependent on Acid Concentration and Acid Strength the involved β-diimino species to the β-diketiminato derivatives by fast intramolecular proton to hydride transfer. ■ CONCLUSIONS The spectroscopic and electrochemical characterization of the two novel nickel κ4-P2N2 β-diimino (BDI) complexes 12+ and 22+ shows the interconversion of the BDI ligand to a β- diketiminato (BDK) ligand upon reduction and hydrogen atom loss. Two pathways are possible for this transformation, both starting from a metal-based reduction of the initial Ni(II) in 12+ and 22+ to form a formal Ni(I) species that rapidly evolves to the Ni(III)-hydride derivatives 1′+ and 2′+ upon intramolecular deprotonation of the α-carbon acidic proton in the BDI ligand. At this point, 1′+ and 2′+ can be involved either To assess the proton reduction ability of 12+ and 22+ in in hydrogen evolution through bimolecular reductive elimi- DMF, a bulk electrolysis experiment at Eapp = −1.8 V vs Fc+/0 nation or in the metal hydride-induced hydrogen atom transfer in the presence of 60 equivalents of p-TsOH was performed. (MHAT) reaction to the acetonitrile solvent. Both routes After 20 min, the headspace of the electrochemical cell was would lead to the formation of the BDK complexes (1-H)+ and analyzed by GC-TCD, confirming the formation of hydrogen (2-H)+. A bulk electrolysis experiment of 22+ in the absence of gas. The accumulated charge and accumulated hydrogen gas in acid at the first reduction wave potential leads to hydrogen gas the bulk electrolysis experiments are significantly higher than formation detected in the headspace of the cathodic compart- those observed for analogous experiments performed in the ment, proving that the bimolecular reductive elimination is the absence of complexes, confirming the role of 12+ and 22+ in preferred pathway. promoting the hydrogen evolution reaction (Figure 9b and Hydrogen detection experiments in the presence of acid Figures S44b and S47 in the SI). Taking into account the pKa sources rule out a hydrogen evolution reaction in the first of p-TsOH in DMF, an approximate overpotential of 0.78 V is reduction wave of 12+ and 22+ (Eapp = −1.2 V vs Fc+/0), calculated for the HER by 12+ and 22+ (see the Supporting presumably due to the electroreduction of the imino ligands Information).42,43 Complex 22+ shows a fairly stable current that evolve to the amino derivatives in the presence of protons over the whole 20 min period at about −0.60 mA (−0.19 mA/ and blocking the reductive elimination route toward H2. In cm2), and complex 12+ shows a lower current intensity (−0.36 contrast, hydrogen evolution is demonstrated in the second mA, −0.11 mA/cm2) that starts to decrease slowly after 8 min, reduction wave where the decomposition route is partially suggesting a higher stability for the former. Cyclic voltammetry overcome by further reduction of intermediates 1′+ and 2′+ at experiments performed immediately after the bulk electrolysis low potentials (Eapp = −1.8 V vs Fc+/0). Importantly, the experiments show the characteristic redox feature of complex conversion of (1-H)+ and (2-H)+ to 12+ and 22+, respectively, 22+ at E = −1.1 V (after 1.8 turnover numbers), while complex in the presence of acid is clearly demonstrated by the recovery 12+ is more difficult to identify due to the irreversible nature of of the first reduction wave of 22+ in DMF. Overall, these results the first reduction, which shows to be very broad and pointing highlight the easy interconversion between BDI and BDK to partial decomposition (Figure S45 in the SI). Faradaic ligands, depending on subtle changes in the acid strength of efficiencies of 33 and 35% were obtained for 12+ and 22+, species in the media and their concentration and manifest their respectively. A rinse test of the working electrode performed non-innocent character as well as their potential role as proton after the bulk electrolysis experiment suggests that part of the relays in catalytic processes. 16646 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article ■ EXPERIMENTAL SECTION All organic reagents and metal precursors were purchased from imino)-3-phenylpent-2-en-2-amine (1.4 g, 4.36 mmol) and dimethyl sulfate (0.4 mL, 4.36 mmol) were stirred for 30 min. During this time, fume appeared, and the mixture became dense. The reaction mixture Sigma-Aldrich and used without further purification, unless otherwise was maintained overnight without stirring, producing a very dense stated. Anhydrous solvents were taken from a solvent purification red-orange oily mixture. A solution of 2-diphenylphosphinoethyl- system (SPS). amine (1 g, 4.36 mmol) in 2 mL of dry and degassed methanol was Synthesis of 4-((2-(Diphenylphosphaneyl)ethyl)amino)- added. 20 min after the addition, a white dense solid appeared, which pent-3-en-2-one. A round-bottom flask equipped with a Dean− was diluted in 2 mL of methanol to make the reaction mixture easier Stark apparatus was charged with a catalytic amount of p-toluene to stir. After 3 h, a solution of sodium methoxide (0.23 g, 4.36 mmol) sulfonic acid (0.04 g) and the setup subjected to vacuum−nitrogen in 10 mL of dry and degassed methanol was added and immediately a cycles to provide an inert atmosphere. A solution of acetyl acetone white precipitate appeared. Stirring continued for another 3 h. The (0.43 g, 4.36 mmol) in 20 mL of dry and degassed toluene was added resulting white solid was filtered off, washed with 100 mL of MeOH, by cannula, and the mixture was stirred under nitrogen atmosphere at and dried under vacuum (900 mg, 1.53 mmol, 35%). 1H NMR (500 RT. After 1 h of stirring, 2-diphenylphosphinoethylamine (1 g, 4.36 MHz, C6D6) δ 13.27 (s, 1H), 7.60−7.50 (m, 7H), 7.29−7.19 (m, mmol) in 20 mL of dry and degassed toluene was added, and the 8H), 7.19−7.05 (m, 10H), 3.52 (dt, J = 9.6, 7.4 Hz, 4H), 2.63 (dd, J = reaction mixture was warmed up to reflux under nitrogen atmosphere. 8.2, 6.6 Hz, 4H), 1.57 (s, 6H). 13C NMR (126 MHz, C6D6) δ 160.2, After 24 h, the reaction was cooled down to room temperature, and 144.4, 139.9, 139.8, 133.4, 133.3 (d, J = 5.5 Hz), 126.2, 44.3, 44.1, the volatiles were removed by reduced pressure, resulting in a yellow 31.6 (d, J = 10.6 Hz), 17.8. 31P{H} NMR (202 MHz, C6D6) δ oily compound, identified as the desired compound by NMR −17.19. Anal. Calcd: C, 78.24; H, 6.73; N, 4.68. Found: C, 78.18; H, spectroscopy (1.35 g, 4.36 mmol, 100%). 1H NMR (500 MHz, 6.66; N, 4.53; ESI-Mass: calcd for m/z: [C31H40N2P] = 598.3. Found C6D6) δ 11.36 (s, 1H), 7.38−7.29 (m, 4H), 7.16−7.08 (m, 6H), 4.92 [M + H]+m/z: 599.29. (s, 1H), 3.00−2.91 (m, 2H), 2.13 (s, 3H), 2.05−1.95 (m, 2H), 1.32 Synthesis of 1(BF4)2. In a round-bottom flask equipped with a (s, 3H). 13C NMR (126 MHz, C6D6) δ 194.6, 161.5, 138.6−138.5 reflux condenser, Nickel(II) tetrafluoroborate hexahydrate (60 mg, (m), 133.2, 95.8, 40.2, 29.1, 18.3. 31P{H} NMR (202 MHz, C6D6) δ 0.177 mmol) was dissolved in 30 mL of anhydrous and degassed −18.22. acetonitrile affording a blue solution. Ligand HBDI (100 mg, 0.177 Synthesis of (2,4)-N-(2-(Diphenylphosphaneyl)ethyl- (diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine, HBDI. mmol) was dissolved in 15 mL of DCM inside the glovebox, and the Inside the glovebox, 4-((2-(diphenylphosphaneyl)ethyl)amino)pent- resulting yellow solution was transferred to the dissolved nickel salt 3-en-2-one (1 g, 3.21 mmol) and dimethyl sulfate (0.4 mL, 4.4 mmol) solution by a cannula outside the glovebox. Upon addition, the color were stirred for 30 min. During this time, fume appeared and the of the reaction changed from blue to red. The reaction mixture was reaction mixture became dense. The reaction mixture was maintained stirred at 65 °C under nitrogen overnight. The volatiles were removed overnight without stirring. Then, 2-diphenylphosphinoethylamine (1 under reduced pressure, leaving a red oily compound, which was g, 4.36 mmol) in 2 mL of dry and degassed methanol was added and dissolved in the minimum amount of acetonitrile. To this acetonitrile stirred, forming a viscous red oil. After 3 h, sodium methoxide (0.17 g solution, Et2O was added, and the color of the solution became 3.21 mmol) dissolved in 10 mL of dry and degassed methanol was cloudy. It was placed at −30 °C, causing green crystals to appear that added and immediately after the mixture became cloudy and a white were filtered and washed with Et2O (40 mg, 0.0637 mmol, 36%). precipitate started to appear. The reaction stirring continued for Crystals suitable for single-crystal X-ray diffraction analysis were another 3 h. The white precipitate was filtered, washed with 100 mL grown by slow diffusion of Et2O to a saturated acetonitrile solution. 1 of MeOH, and dried under vacuum (500 mg, 0.96 mmol, 30%). 1H H NMR (500 MHz, CD3CN) δ 7.55 (t, J = 7.5 Hz, 3H), 7.47−7.43 NMR (500 MHz, C6D6) δ 7.59−7.49 (m, 8H), 7.16−7.07 (m, 12H), (m, 9H), 7.35 (t, J = 7.6 Hz, 8H), 4.29 (s, 2H), 3.66 (s, 4H), 2.74− 4.63 (s, 1H), 3.49−3.37 (m, 4H), 2.59−2.52 (m, 4H), 1.66 (s, 6H). 2.68 (m, 4H), 2.27 (s, 6H). 13C NMR (126 MHz, CD3CN) δ 180.0, 13 C NMR (126 MHz, C6D6) δ 160.1, 139.9, 139.8, 133.4, 133.39, 133.9, 133.2, 129.8117.6, 53.7, 52.7, 23.4. 31P{H} NMR (202 MHz, 95.4, 44.0, 43.8, 31.6 (d, J = 10.7 Hz), 19.41. 31P{H} NMR (202 CD3CN) δ 49.64. 1 MHz, C6D6) δ −17.26. Anal. Calcd: C, 75.84; H, 6.94; N, 5.36. H NMR (500 MHz, DMF-d7) δ 7.63 (ddd, J = 8.2, 6.7, 3.1 Hz, Found: C, 75.04; H, 6.62; N, 5.26; ESI-Mass: calcd for [C33H36N2P2] 8H), 7.55 (t, J = 7.5 Hz, 4H), 7.36 (t, J = 7.6 Hz, 8H), 5.31 (s, 1H), = 522.2. Found: [M + H]+m/z: 523.2. 3.45 (ddt, J = 19.6, 13.3, 6.5 Hz, 4H), 2.88 (p, J = 6.4 Hz, 4H), 2.22 Synthesis of 4-((2-(Diphenylphosphaneyl)ethyl)amino)-3- (s, 6H). 13C NMR (126 MHz, DMF) δ 135.1 (t, J = 5.2 Hz), 133.2, phenylpent-3-en-2-one. A round-bottom flask equipped with a 130.1, 128.3−126.9 (m), 102.2, 52.0, 33.8, 24.6. 31P{H} NMR (202 Dean−Stark apparatus containing a catalytic amount of p-toluene MHz, DMF(d7)) δ 57.64. Anal. Calcd: [C33H36N2NiP2](BF4)2 + sulfonic acid (0.04 g) and 3-phenyl-2,4-pentanedione (0.76 g, 4.36 Et2O: C, 52.55; H, 5.17; N, 3.51. Found: [C33H36N2NiP2](BF4)2 + mmol) was subjected to vacuum−nitrogen cycles to provide an inert Et2O: C, 52.30; H, 4.60; N, 3.50; ESI-Mass: calcd for m/z: atmosphere. Dry and degassed toluene (20 mL) was added by a [C33H36N2NiP2]2+ = 580.2. Found: [M − H]+m/z: 579.16. cannula, and the mixture was stirred under nitrogen atmosphere at Synthesis of 2(BF4)2. In a round-bottom flask equipped with a RT. After 1 h, a solution of 3-diphenylphosphinoethylamine (1 g, 4.36 reflux condenser, Nickel(II) tetrafluoroborate hexahydrate (56 mg, mmol) in 20 mL of dry and degassed toluene was added, and the 0.167 mmol) was dissolved in 30 mL of anhydrous and degassed reaction mixture was heated up to reflux under nitrogen. After 24 h, acetonitrile under nitrogen affording a blue solution. Ligand PhBDI refluxing of the reaction was stopped, and the volatiles were removed (100 mg, 0.167 mmol) was dissolved in 15 mL of DCM inside the under reduced pressure. The resulting yellow oil was identified as the glovebox, and the resulting yellow solution was transferred to the desired compound (1.2 g, 3.48 mmol, 80%). 1H NMR (500 MHz, nickel salt solution by a cannula outside the glovebox. Upon the C6D6) δ 12.63 (s, 1H), 7.41−7.33 (m, 3H), 7.27−7.20 (m, 3H), addition of the ligand, the color of the reaction changed from blue to 7.20−7.06 (m, 9H), 3.12−3.03 (m, 2H), 2.21−2.11 (m, 2H), 2.08 (s, red. The reaction mixture was heated to 65 °C under nitrogen 3H), 1.30 (s, 3H). 13C NMR (126 MHz, C6D6) δ 194.2, 160.6, 142.0, overnight. Removing volatiles under reduced pressure afforded a red 141.8, 138.9, 138.7, 138.5, 133.2, 133.0, 132.7, 132.6 (d, J = 2.6 Hz), oily compound, which was dissolved in the minimum amount of 131.6, 131.5−131.2 (m), 131.0, 130.9, 129.1, 128.8 (dd, J = 13.6, 6.7 acetonitrile. To this acetonitrile solution, Et2O was added, which Hz), 126.9, 126.6, 109.9, 40.8, 40.6, 37.3, 37.1, 37.0, 32.3, 30.2, 29.9, caused the precipitation of a yellow solid that was filtered and washed 29.8, 29.5, 29.2, 29.1, 23.1, 22.8, 21.2, 16.4, 16.2, 14.4, 1.4. 31P{H} with Et2O (80 mg, 0.16 mmol, 96%). Crystals suitable for single- NMR (202 MHz, C6D6) δ −18.04. crystal X-ray diffraction analysis were grown by slow diffusion of Et2O Synthesis of (2,4)-N-(2-(Diphenylphosphaneyl)ethyl)-4-((2- to an acetonitrile solution of 2(OTf)2. 1H NMR (500 MHz, CD3CN) (diphenylphosphaneyl)ethyl)imino)-3-phenylpent-2-en-2- δ 7.76−7.70 (m, 2H), 7.66−7.54 (m, 4H), 7.52−7.26 (m, 16H), amine, PhBDI. Inside the glovebox, (2,4)-N-(2- 7.26−7.19 (m, 3H), 5.50 (s, 1H), 3.75 (ddt, J = 29.1, 14.8, 8.0 Hz, (diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)- 5H), 2.97 (dp, J = 12.6, 6.9 Hz, 2H), 2.62 (dt, J = 13.9, 7.0 Hz, 3H), 16647 https://doi.org/10.1021/acs.inorgchem.2c02150 Inorg. Chem. 2022, 61, 16639−16649 Inorganic Chemistry pubs.acs.org/IC Article 2.31 (d, J = 0.9 Hz, 7H). 13C NMR (126 MHz, CD3CN) δ 181.52, Notes 133.6−133.5 (m), 133.4, 133.1, 130.6, 130.2, 129.7, 128.9, 117.6, The authors declare no competing financial interest. 68.0, 53.0, 38.3, 28,3, 23.2. 31P{H} NMR (202 MHz, CD3CN) δ 47.15. 1 H NMR (500 MHz, DMF-d7) δ 8.04 (s, 2H), 7.70−7.33 (m, 10H), 7.22−7.14 (m, 1H), 3.72 (s, 14H), 3.56 (dt, J = 29.9, 6.2 Hz, ■ ACKNOWLEDGMENTS Financial support from Ministerio de Ciencia e Innovación, 1H), 3.56 (s, 1H), 2.97−2.89 (m, 2H), 2.80−2.74 (m, 3H), 1.95 (s, FEDER, and AGAUR through grants PID2019-111617RB-I00, 2H). 13C NMR (101 MHz, DMF) δ 144.8, 134.1, 132.2 ,130.1− SO-CEX2019-000925-S, and 2017-SGR-1631 is gratefully 128.5, 128.3−125.6, 113.0, 52.1, 34.26, 23.3. 31P{H} NMR (162 acknowledged. N.J.-M. thanks AGAUR for a PhD grant, MHz, DMF-d7) δ 57.87. Anal. Calcd: [C39H40N2NiP2](BF4)2 + Et2O: 2018FI_B0108, G43619550. C, 56.21; H, 5.18; N, 3.37. 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