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Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand Kyle E. Rosenkoetter, Michael K. Wojnar, Bronte J. Charette, Joseph W. Ziller, and Alan F. Heyduk* Department of Chemistry, University of California, Irvine, California 92697-2025, United States

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S Supporting Information *

ABSTRACT: Double deprotonation of bis(2-mercapto-4methylphenyl)amine ([SNS]H 3 ) followed by addition to NiCl2(PR3)2 in air-free conditions afforded [SN(H)S]Ni(PR3) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR3)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = 1/2 complex, [SNS]Ni(PCy3) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh3)}2 ({3b}2) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = 1/2 monomers analogous to 3a. Complexes 1−3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol−1, respectively, using the corresponding pKa and E°′ values. Consistent with these BDFE values, TEMPO• reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.

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INTRODUCTION It has long been recognized that the management of proton and electron inventories is a critical factor in the reactions of small-molecule substrates including N2, O2, CO, and CO2.1−4 Such proton-coupled electron-transfer (PCET) reactions, or hydrogen-atom-transfer (HAT) reactions, constitute an important and active area of current research. Detailed thermodynamic and kinetic analyses of HAT reactions have elucidated the key factors that control HAT reactions in both organic and inorganic systems.3 Two types of inorganic HAT coordination complexes have been studied in detail. In the first class, metal hydride complexes, both the proton and electron are transferred in and out of the primary coordination sphere of the metal. The second class comprises coordination complexes in which the metal ion serves as the electron source and a coordinated ligand serves as the proton source. Herein we investigate a third type of coordination complex capable of participating in HAT reactions, where both proton and electron transfer occurs from a noninnocent ligand.5−9 Redox-active ligands are the most commonly discussed type of noninnocent ligand platform.10 Catecholate, which can exist in three different oxidation states when coordinated to a metal ion, is the prototypical redox-active ligand.11,12 In redox-active ligand coordination complexes, ligand-based oxidation state changes often operate in concert with or even in place of more classical metal-based oxidation state changes during redox reactions.13 A second type of noninnocent ligand is one that exerts a secondary steric-type interaction on the metal coordination sphere.14 Second-sphere interaction ligands typically include functionalities that provide hydrogenbonding, dipolar, or van der Waals interactions within the second coordination sphere of the metal ion.15,16 These second-sphere interactions can stabilize reactive species or lower activation barriers. An example of the latter ligand type is © XXXX American Chemical Society

diphosphine ligands with an amine bridgehead that are used in the design of electrocatalysts.17−19 The tertiary amine bridgehead does not coordinate to the metal but instead shuttles protons to and from the metal center during catalysis. A ligand platform that can serve as a reservoir for both protons and electrons can also act as a HAT noninnocent ligand. Herein, we report on the ability of a tridentate [SNS] pincer ligand ([SNS]H3 = bis(2-mercapto-4-methylphenyl)amine)20 to participate in HAT reactions when coordinated to nickel(II). Tridentate pincer ligands have a well-established coordination chemistry with transition-metal ions, and in many cases, ligand-centered HAT chemistry has been observed.21−28 Similarly, there have been several recent reports of redoxnoninnocent ligand platforms that can participate in HAT reactions.5−9 The [SNS] ligand examined here is the sulfur congener of the well-known ligand derived from bis(3,5-di-tertbutyl-2-phenol)amine, which is stable in three oxidation states when coordinated to a metal ion.11,29,30 Structural, spectroscopic, and computational data for the square-planar nickel complexes reported here indicate that the [SNS] ligand platform is redox-noninnocent and capable of electron, proton, and hydrogen-atom transfer reactivity. Redox and pK a measurements are used to benchmark the bond dissociation free energy (BDFE) for the active ligand-bound hydrogen atom, and preliminary HAT reactivity with TEMPO• is reported. Special Issue: Applications of Metal Complexes with LigandCentered Radicals Received: March 13, 2018

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DOI: 10.1021/acs.inorgchem.8b00618 Inorg. Chem. XXXX, XXX, XXX−XXX

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RESULTS Synthesis and Characterization. Square-planar nickel complexes of the bis(2-mercapto-4-methylphenyl)amine ([SNS]H3) proligand were prepared by taking advantage of the different pKa values of the amine and thiol functional groups.31 Under a N2 atmosphere, the reaction of [SNS]H3 with NiCl2(PR3)2 in the presence of 2 equiv of triethylamine led to metalation of the ligand via selective deprotonation of the more acidic thiol groups to afford [SN(H)S]Ni(PR3) complexes (1a, R = Cy; 1b, R = Ph), as shown along the top of Scheme 1. When 3 equiv of the strong base potassium hydride

characterized by a parent-ion peak at 1194 amu, indicative of a dimer with the formula {[SNS]Ni(PPh3)}2 ({3b}2). X-ray diffraction experiments were performed on singlecrystal samples of complexes 1−3, revealing all six complexes to contain square-planar nickel centers. Figure 1 shows the

Scheme 1

was used in place of triethylamine, potassium salts of a nickellate anion were obtained with the formula K[SNS]Ni(PR3) (K[2a], R = Cy; K[2b], R = Ph). Electrospray ionization mass spectrometry (ESI-MS) showed the presence of the appropriate [SN(H)S]Ni(PR3)+ ions for [SN(H)S]Ni(PCy3) (1a) and 1b; in the cases of K[2a] and K[2b], the MS spectra showed the parent ion for the salt as K[SNS]Ni(PR3)+. All four of the above complexes proved to be diamagnetic in solution, showing a single resonance in the 31P{1H} NMR spectrum in the 25−35 ppm range. The 1H NMR spectra of these complexes all showed resonances in the normal diamagnetic range; notably, the spectra for 1a and 1b showed singlet resonances at 7.84 and 8.31 ppm, respectively, attributable to an intact N−H proton. This resonance was not observed in the 1H NMR spectra of K[2a] and K[2b]. Metalation of bis(2-mercapto-4-methylphenyl)amine with nickel(II) salts under aerobic conditions led to the formation of one-electron-oxidized complexes that maintain squareplanar geometry around the nickel center. Solutions of NiCl2(PCy3)2 with bis(2-mercapto-4-methylphenyl)amine and 3 equiv of triethylamine in air changed color from blue to deep green and afforded a blue-green solid upon workup. The ESI-MS spectrum of the product showed a parent-ion peak at 596 amu, indicative of a monomer with the formula [SNS]Ni(PCy3) (3a). Neither the 1H NMR nor the 31P{1H} NMR spectrum of 3a showed prominent resonances, consistent with its characterization as a paramagnetic, S = 1/2 complex. When the same reaction was carried out using NiCl2(PPh3)2 as the nickel source, the isolated product was

Figure 1. ORTEP diagrams of 1a (top), [K(2.2.2-crypt)][2a] (middle), and 3a (bottom). Ellipsoids are shown at 50% probability. A THF solvent molecule has been omitted from the structure of K(2.2.2-crypt)[2a] for clarity.

structures of the family of nickel complexes containing the PCy3 ligand, 1a−3a. The PPh3 complexes 1b and [2b]⊖ are isostructural with the PCy3 complexes 1a and [2a]⊖, respectively (see the Supporting Information), except for an intermolecular hydrogen-bonding interaction between adjacent molecules of 1b that is not present in the structure of 1a. Complex 3b is dimeric, and its structure will be discussed below. Crystals of 1a, 1b, and 3a were obtained from tetrahydrofuran (THF) or acetonitrile (MeCN) solutions of B


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Table 1. Selected Distances (Å) and Angles (deg) for [SN(H)S]Ni(PR3) (1), [K(2.2.2-crypt)]{[SNS]Ni(PR3)} (K(2.2.2crypt)[2]), and [SNS]Ni(PR3) (3) for R = Cy (a) and Ph (b)

Ni−P Ni−N Ni−S(1) Ni−S(2) N−C(1) N−C(7) S(1)−C(2) S(2)−C(8) N−Ni−S(1) N−Ni−S(2) N−Ni−P τ4

[SN(H)S]Ni(PR3) (1a, R = Cy; 1b, R = Ph)

K(2.2.2-crypt){[SNS]Ni(PR3)} (K(2.2.2-crypt)[2a], R = Cy; K(2.2.2crypt)[2b], R = Ph)

{[SNS]Ni(PR3)}n (3a, R = Cy; {3b}2, R = Ph)

2.1983(6); 2.1683(5) 1.9843(16); 1.9652(14) 2.1750(4); 2.1630(5) 2.1865(6); 2.1872(5) 1.471(2); 1.461(2) 1.475(2); 1.469(2) 1.755(4); 1.7559(18) 1.751(2); 1.7714(18) 88.54(11); 89.72(4) 86.94(5); 87.38(4) 169.55(5); 179.53(5) 0.17; 0.08

2.2025(7); 2.1711(6) 1.9085(19); 1.8973(15) 2.1694(7); 2.1688(6) 2.1749(7); 2.1742(6) 1.398(3); 1.395(2) 1.395(3); 1.402(2) 1.754(2); 1.7526(19) 1.750(2); 1.7610(18) 87.61(6); 88.22(5) 88.07(6); 88.33(5) 174.15(6); 175.97(5) 0.08; 0.07

2.2192(6); 2.2090(4) 1.9224(18); 1.9067(12) 2.1665(7); 2.1856(4) 2.1814(7); 2.2117(4) 1.390(3); 1.3936(18) 1.392(3); 1.3797(18) 1.742(2); 1.7466(15) 1.743(2); 1.7436(15) 88.61(6); 87.48(4) 88.18(6); 87.84(4) 179.00(6); 142.81(4) 0.03; 0.32

the respective complex at 25 °C. For K[2a] and K[2b], single crystals were obtained by vapor diffusion of diethyl ether into THF and MeCN solutions, respectively, which also contained 2.2.2-cryptand to sequester the potassium cation. Selected metrical parameters for 1−3 are presented in Table 1. As is evident from Figure 1, complexes 1a−3a all have fourcoordinate, square-planar nickel centers; accordingly, the calculated τ4 value for each complex is close to zero.32 The adjacent five-membered chelate rings, formed upon coordination of the [SNS] pincer ligand to the nickel center, generate acute N−Ni−S angles that readily allow for accommodation of the large PCy3 ligand (cone angle = 170°).33 Bond distances across the PCy3 family of complexes are remarkably similar. The Ni−N bond distance is sensitive to the presence of a proton on the ligand nitrogen with a long Ni−N distance of 1.98 Å in 1a and shorter Ni−N distances of 1.91 and 1.92 Å in [2a]⊖ and 3a, respectively. Similarly, the N−C bond distances within the pincer ligand are sensitive to the presence of the protonated nitrogen with a long distance of 1.47 Å in 1a and shorter N−C distances of 1.40 and 1.39 Å in [2a]⊖ and 3a, respectively. The Ni−P bond distances vary over a narrow 0.02 Å range in 1a−3a and the Ni−S and S−C bond distances show little if any variation across the series of three complexes. In general, the similarity in the structures across the series 1a−3a, including metal−ligand and intraligand bond distances, makes it impossible to make clear-cut metal and ligand oxidation state assignments from the structural data.34 The dimeric formulation of {3b}2, indicated by the mass spectral data, was confirmed in the solid state by single-crystal X-ray diffraction experiments. Figure 2 shows the structure of {3b}2, consisting of two four-coordinate [SNS]Ni(PPh3) fragments stacked on top of one another. The dimer is held together by long (∼2.4 Å) Ni−S interactions between adjacent [SNS]Ni(PPh3) fragments, which stacks the [SNS] ligands on top of one another with centroid-to-centroid distances of 3.45 and 3.63 Å. The dimerization causes a distortion from planarity around each nickel center manifested in an P−Ni−N angle of only 140°, which, in turn, affords a τ4 value of 0.32 (excluding the extra Ni−S interaction). Nickel−ligand bond distances within a monomer unit vary slightly from the PCy3 derivative 3a, with slightly shorter Ni−P and Ni−N distances and slightly longer Ni−S distances. Other key bond distances within the [SNS] ligand are identical between monomeric 3a and dimeric {3b}2.

Figure 2. ORTEP diagram of {3b}2. Ellipsoids are shown at 50%. Hydrogen atoms and MeCN solvent molecules are omitted for clarity.

Spectroscopic Characterization of Oxidized [SNS] Nickel Complexes. Complexes 3a and {3b}2 are one-electron oxidation products of the anions [2a]⊖ and [2b]⊖. Given that [2a]⊖ and [2a]⊖ are both closed-shell, diamagnetic, nickel(II) complexes with a potentially redox-active [SNS] ligand, the nature of the oxidation in 3a and {3b}2 is of interest. As expected for a one-electron oxidation product, 3a is a paramagnetic complex that is NMR-silent. Figure 3 shows the X-band electron paramagnetic resonance (EPR) spectrum of 3a in a THF solution at both 10 and 298 K (inset). The room-temperature spectrum is an isotropic signal centered at g = 2, consistent with the type of spectra observed for ligandlocalized, S = 1/2 spin systems.35 Upon cooling to 10 K, the spectrum becomes rhombic, indicative of a nickel contribution to the singly occupied molecular orbital (SOMO). The lowtemperature spectrum was fit with g values of 1.99, 2.01, and 2.03 (see the Supporting Information). The magnetic behavior of dimeric {3b}2 is more complex than that of monomeric 3a. When dissolved in acetonitrile-d3 (CD3CN), {3b}2 is NMR-active, affording slightly broadened 1 H and 31P NMR signatures in the normal diamagnetic region of the spectrum. Consistent with the NMR results, MeCN solutions of {3b}2 are EPR-silent. Both the NMR and EPR results in MeCN agree with the dimeric formulation of {3b}2 observed by ESI-MS and single-crystal X-ray diffraction, where C


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Figure 3. X-band EPR spectrum of 3a dissolved in THF at 10 and 298 K (inset).

antiferromagnetic coupling between an unpaired electron on each monomer would lead to diamagnetic behavior. In a benzene solution, however, the behavior of {3b}2 is the opposite. Solutions of {3b}2 in C6D6 are NMR-silent. The EPR spectrum of {3b}2 dissolved in benzene shows an isotopic, S = 1 /2 signal at 298 K, which upon cooling became rhombic with g values at 1.95, 2.00, and 2.02 (see the Supporting Information). These results are analogous to those for monomeric 3a, indicating that the dimeric formulation for {3b}2 is dynamic in solution. Polar solvents favor the dimeric [SNS]Ni(PPh3), and nonpolar solvents favor dissociation to monomeric [SNS]Ni(PPh3). Density Functional Theory (DFT) Computations. Nickel complexes 1b−3b were examined using DFT computations to help elucidate their electronic structures. Gas-phase DFT computations were carried out at the TPSS/ def2-TZVP level of theory.36−38 In particular, the locus of the unpaired electron in monomeric 3b (and by analogy 3a) was of interest. For 1b and [2b]⊖, the solid-state structures were used as the starting points for the geometry optimizations. Complexes 1b and [2b]⊖ were well-described by a singlet closed-shell Kohn−Sham DFT solution. Metal−heteroatom bond distances fell within 0.02 Å of the solid-state values, whereas intraligand C−S, C−S, and C−C bond distances all fell within 0.004 Å of the solid-state values. For anion [2b]⊖, the highest occupied molecular orbital (HOMO) is mainly [SNS] π* in character with a smaller nickel contributions of 20% (see the Supporting Information). The lowest unoccupied molecular orbital (LUMO), as expected for a square-planar nickel(II) complex, is nominally M−L σ* with the overall symmetry of the metal dx2−y2 orbital (see the Supporting Information). Neutral 1b, with the protonated [SN(H)S]2− ligand, has the analogous M−L σ* orbital as the LUMO, but thanks to protonation of the pincer ligand, the HOMO is an M−L π* orbital involving the nickel dxz orbital (see the Supporting Information). In 1b, the metal contribution to this M−L π* HOMO is 44%. Open-shell [SNS]Ni(PPh3) (3b) was modeled as an S = 1/2 monomer using a spin-unrestricted DFT computational scheme. To build a model for monomeric 3b, the geometry optimization calculation was carried out by starting with half of the solid-state dimer, {3b}2. After geometry optimization, metal−heteroatom and intraligand C−S, C−S, and C−C bond distances for this 3b model were consistent with those obtained by X-ray diffraction for 3a. Figure 4a shows a

Figure 4. (a) Qualitative molecular orbital diagram and Kohn−Sham molecular orbitals and (b) total spin-density plot for monomeric 3b.

qualitative molecular orbital diagram with Kohn−Sham molecular orbitals for 3b, correlated to the frontier molecular orbitals of [2b]⊖. Given that 3b is the one-electron oxidation product of anion [2b]⊖, it was expected that an electron would be removed from the HOMO of this anion. Accordingly, the lowest-energy, unoccupied β-orbital component correlates well with the HOMO of [2b]⊖ and is best described as an [SNS] π* orbital with small contributions from the nickel center (13%) and phosphine ligand (2%). The filled α component of this [SNS] π* orbital is lower in energy, falling below both the α and β components of the M−L π* orbital that is the HOMO−1 in [2b]⊖, as shown in Figure 4a. The total spindensity plot, shown in Figure 4b, is consistent with these orbital assignments, with 91% of the total spin density residing on the [SNS] ligand. Electrochemistry. To better elucidate the redox behavior of the [SNS]Ni platform, voltammetric experiments were carried out. The electrochemical response of complexes 2 and 3 were well-behaved in a MeCN solution containing 0.1 M [Bu4N][PF6] as the supporting electrolyte (see the Supporting Information). All potentials were referenced to the [Cp2Fe]+/0 redox couple using an internal standard. The cyclic voltammogram of [SNS]Ni(PCy3)⊖ ([2a]⊖) showed a reversible (ipa/ipc ≅ 1; ΔE ≅ 70 mV) one-electron redox process at −0.64 V versus [Cp2Fe]+/0 (Figure 5, top), corresponding to oxidation of the complex to 3a. An irreversible reduction of [2a]⊖ also was observed at −2.74 V versus [Cp2Fe]+/0. When 3a was analyzed by cyclic voltammetry, the same two redox processes were observed with the expected shift in the open-circuit potential (see the Supporting Information). For the PPh3 derivatives [2b]⊖ and {3b}2, analogous one-electron redox processes were observed, shifted only slightly to −0.61 and −2.54 V versus [Cp2Fe]+/0 (see the Supporting Information). In the case of [2b]⊖ and {3b}2, the process at −0.61 V is reversible (ipa/ipc ≅ 1; ΔE ≅ 78 mV), suggesting that the event D


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phenolate anions [SNS]Ni(PR3)⊖ ([2a]⊖ and [2b]⊖) are related by the removal of a proton from the [SN(H)S]2− ligand, it was of interest to examine the acid−base properties of these complexes. The addition of the strong base 1,8diazabicyclo[5.4.0]undec-7-ene [DBU; pK a (MeCN) = 24.34]39 to an MeCN solution of 1a resulted in an immediate color change from orange to blue, signaling formation of the [2a]⊖ anion. Similarly, the addition of DBU to the PPh3 derivative 1b resulted in the quantitative formation of [2b]⊖. The protonation reactions also proceeded smoothly, with the addition of 4-cyanoanilinium tetrafluoroborate [pKa(MeCN) = 7.0]40 to solutions containing either [2a]⊖ or [2b]⊖ resulting in the quantitative formation of 1a or 1b, respectively. Complexes 1a, 1b, and [2a]⊖ and [2b]⊖ show very different spectroscopic features in the UV−vis region of the electromagnetic spectrum, allowing for the ready determination of [SN(H)S]2− pKa values by spectrophotometric titration. Accordingly, the portionwise addition of 2,4,6-trimethylpyridine [pKa(MeCN) = 14.98]39 to an MeCN solution of 1a resulted in the growth of an intense charge-transfer band at 368 nm, characteristic of the nickelate anion of [2a]⊖. From the titration data, a pKa of 17.4 ± 0.5 was determined for complex 1a, which contains the PCy3 ancillary ligand. Analogous titration of 1b formed [2b]⊖ and afforded a pKa of 15.9 ± 0.5 for the derivative with the PPh3 ancillary ligand. HAT Reactivity. From a thermodynamic perspective, aminophenolate complexes 1a and 1b are related to oxidized complexes 3a and 3b by HAT. To test for a viable HAT pathway relating these complexes, reactions with both TEMPO• and TEMPO-H (BDFE = 66.5 kcal mol−1 for TEMPO-H)3 were investigated. Aminophenolate complexes 1a and 1b rapidly reacted with 1 equiv of TEMPO•, resulting in the formation of 3a and 3b, respectively, along with TEMPO-H, in quantitative spectroscopic yields. Conversely, when 3a or 3b were treated with 1 equiv of TEMPO-H, no forward reaction was observed with complete recovery of the starting materials after several hours at room temperature.

Figure 5. Cyclic voltammograms for [2a]⊖ (top) and 1a (bottom) in MeCN containing 0.1 M [Bu4N][PF6] using a glassy carbon working electrode. Potentials are referenced to the [Cp2Fe]+/0 couple.

involves the monomeric form of 3b rather than the dimeric form observed in the solid-state structure. The reversibility of this reduction, along with its similarity to the analogous reduction of 3a, further suggests that the monomer−dimer equilibrium for 3b and {3b}2 is fast on the electrochemical time scale. In the cases of the protonated derivatives 1a and 1b, an irreversible reduction leads to the observation of redox features consistent with the formation of complexes [2a]⊖ and [2b]⊖, respectively. Upon an initial scanning in the cathodic direction between −0.6 and −2.0 V versus [Cp2Fe]+/0, samples of 1a showed an irreversible reduction at −1.76 V (Figure 5, bottom). When the scan window was widened to include potentials from −0.3 to −2.8 V, additional, reversible, oneelectron processes were observed at −2.74 and −0.64 V versus [Cp2Fe]+/0 after the initial irreversible reduction. These redox processes correspond to the reduction and oxidation processes, respectively, of the [2a]⊖ anion, suggesting that the initial, irreversible reduction of 1a is followed by the loss of 1/2 equiv of H2 in an EC-type mechanism (Scheme 2). Analogous

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DISCUSSION Redox Behavior. The [SNS] ligand platform derived from bis(2-mercapto-4-methylphenyl)amine is a direct structural analogue of the well-known [ONO] pincer ligand derived from bis(3,5-di-tert-butyl-2-phenol)amine.29 When coordinated to a metal ion, the [ONO] ligand is stable in three different oxidation states: the trianionic catecholate form, the dianionic semiquinonate radical form, and the monoanionic quinonate.11,30 A key question to answer about the [SNS] platform is the degree of redox activity present in the ligand that contains sulfur donors in place of the oxygen atoms. While dithiolate ligands were some of the first noninnocent ligand platforms to be studied, the substitution of sulfur for oxygen in catecholate-type ligands typically results in a decrease in the redox activity because sulfur does not form strong π bonds with carbon as readily as oxygen does. The synthesis of anionic [SNS]Ni(PR3)⊖ ([2a]⊖ and [2b]⊖) and the observation of a reversible, one-electron oxidation in the cyclic voltammogram of each anion provided an opportunity to examine the redox activity of the [SNS] ligand platform. For both PCy3 and PPh3 derivatives, these anionic nickel complexes contained diamagnetic, squareplanar, nickel(II) centers. The [SNS] ligand in [2a]⊖ and [2b]⊖ is consistent with a trianionic pincer-type ligand, as has been previously observed when the ligand is coordinated to

Scheme 2

electrochemical behavior is observed for 1b, with the irreversible reduction occurring at −1.60 V versus [Cp2Fe]+/0. To test for the generation of hydrogen gas upon reduction of 1a and 1b, an MeCN solution of 1b was treated with 1 equiv of decamethylcobaltocene (Cp*2Co). Analysis of the reaction headspace by thermal conductivity gas chromatography revealed the formation of 0.45 equiv of hydrogen gas (90% yield), whereas UV−vis spectroscopy of the resulting solution confirmed the quantitative formation of [2b]⊖. pKa Determinations. Given that the aminophenolate complexes [SN(H)S]Ni(PR3) (1a and 1b) and the amidoE


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Scheme 4.3,49 For the PCy3 derivative, 1a, a pKa of 17.4 ± 0.5 and an E°′(0/−) of −0.64 V versus [Cp2Fe]+/0 afforded an

tungsten or molybdenum.20,41−43 When metalation of the [SNS] ligand platform was carried out in air, oxidized versions of the nickel complexes were obtained. In the case of the PCy3 derivative, 3a, the product was a square-planar, S = 1/2 nickel complex. In the case of the PPh3 derivative, the complex dimerizes in the solid state as {3b}2 but appears to establish a rapid monomer−dimer equilibrium based on solution EPR spectroscopic and electrochemical data. This dimerization behavior is reminiscent of the behavior of some copper(II) thiolate complexes with ligands that favor planar geometries.44−46 At 298 K, the X-band EPR spectra of 3a and 3b show the type of single-derivative signal at g = 2.00 that is diagnostic for ligand-centered radicals in oxidized, semiquinonate ligands.11,35 Upon cooling of the sample to 10 K, the signal broadens to a rhombic pattern that is still centered at gave = 2.01. This rhombic character and the slight deviation of the g values from 2.00 are suggestive of nickel d orbital participation in the SOMO.35 Consistent with this suggestion, the low-temperature EPR spectra of 3a and 3b both include hyperfine coupling to the 31P of the phosphine ligand, indicating that the SOMO is at least partially delocalized onto the nickel ion. Authentic nickel(III) complexes typically show rhombic spectra with more pronounced deviations of gave from the free electron.47 For example, the nickel(III) complex derived from the N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide pincer ligand displays a rhombic EPR spectrum at 77 K with g values of 2.00, 2.23, and 2.32.27 By way of comparison, oxidation of [PNP]NiCl ([PNP]− = N[2P(CHMe2)2-4-methylphenyl]2) by one electron generates an S = 1/2 complex with a slightly rhombic EPR at 292 K (g = 2.01, 2.02, and 2.04), which was assigned as a nickel(II) species with the hole residing mostly on the [PNP] ligand.48 To further elucidate the electronic structures of 3a and 3b, DFT calculations were carried out on monomeric 3b. These computations support the contention that the unpaired electron and the hole in 3b are localized on the [SNS] ligand with only a small contribution from the nickel ion. The presence of an unpaired electron on the [SNS] ligand of 3a and monomeric 3b constitutes evidence for one-electron ligand-based redox activity. As such, the conversion of [2]⊖ to 3 can be written as a primarily ligand-based oxidation, with the [SNS] ligand adopting a trianionic, catecholate oxidation state in [2]⊖ (denoted as [SNScat]3−) and a dianionic, open-shell, semiquinonate oxidation state in 3 (denoted as [SNSsq•]2− in Scheme 3).

Scheme 4

N−H BDFE of 63.9 ± 0.8 kcal mol−1. In the case of the PPh3 derivative 1b, a slightly more acidic pKa of 15.9 ± 0.5 and an E°′(0/−) of −0.61 V afforded an N−H BDFE of 62.4 ± 0.8 kcal mol−1. Similar N−H BDFE values were recently reported for bidentate aminophenolate ligands coordinated to palladium and platinum.50 The calculated N−H BDFE values for 1a and 1b are similar, owing to the minor electronic impact that results from the difference between ancillary PCy3 and PPh3 ligands, respectively. Assuming that the difference in the steric profiles does not impact the N−H bond strength, the difference between these two ancillary ligands lies in their donor ability. While PCy3 is a marginally stronger donor than PPh3 (2056 cm−1 vs 2069 cm−1),33 this difference in donor ability has contradictory effects on the calculated BDFE. While the stronger donor PCy3 ligand pushes more electron density toward the [SN(H)S]Ni fragment, this donation manifests a higher pKa but a more negative E°′(0/−). As a result, the N− H BDFE values for 1a and 1b are within experimental error of one another. It is worth noting that more dramatic changes to the ancillary ligand may result in a larger spread in the BDFE values. The calculated N−H BDFE values for 1a and 1b are consistent with the preliminary HAT reactivity studies. The addition of a stoichiometric quantity of TEMPO• to a solution of 1a or 1b resulted in a rapid reaction to produce TEMPO-H and 3a or 3b, respectively. Conversely, the reverse reactions using 3a and 3b with stoichiometric TEMPO-H did not produce any observable 1a and 3b. These results are consistent with a BDFE for TEMPO-H of 66.5 kcal mol−1 in MeCN.3 The BDFE values measured for 1a and 1b fall at the low end of the range typically observed for transition-metal HAT or PCET systems, especially for those where the hydrogen atom or proton binds to a nitrogen or oxygen atom of a coordinated ligand.3 Given that the trends in the ligand-based redox potentials are fairly well established, it stands to reason that a great deal of control over the BDFE, and hence the thermodynamics of HAT, can be realized through judicious choice of the redox-active ligand platform in a given metal complex.

Scheme 3

HAT Reactivity. Complexes 1−3 are related by electronand proton-transfer and HAT reactions. Given the conclusion that the redox change associated with the oxidation of 2 to 3 is localized primarily on the [SNS] ligand platform, then the HAT reaction that relates complexes 1 and 3 also must be localized primarily on the ligand platform. Hess’ law allows for determination of the N−H BDFE associated with the HAT reaction relating 1 and [SNSsq•]Ni(PR3) (3), as outlined in

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out using standard vacuum-line, Schlenk-line, and glovebox techniques F


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Inorganic Chemistry unless otherwise noted.51 Hydrocarbon and ethereal solvents were sparged with argon before being deoxygenated and dried by serial passage through Q5 and activated alumina columns. To test for effective oxygen and water removal, aliquots of each solvent were treated with a few drops of a purple solution of sodium benzophenone ketyl radical in tetrahydrofuran (THF). Halogenated solvents were sparged with argon and dried by passage through two activated alumina columns. Acetonitrile-d3 (CD3CN) was dried over calcium hydride and distilled under reduced pressure prior to use. Triphenylphosphine (Sigma) and tricyclohexylphosphine (20 wt % in toluene, Sigma) were all used as received. The ligand precursor bis(2-mercapto-4-methylphenyl)amine20 and the nickel starting materials NiCl2(PR3)2 (R = Cy, Ph)52 were prepared as previously described in the literature. Ferrocene and decamethylferrocene (Acros) were purified by sublimation, and tetrabutylammonium hexafluorophosphate (Acros) was recrystallized from ethanol three times and dried under vacuum.53 Spectroscopic Methods. Elemental analyses were conducted on a PerkinElmer 2400 Series II CHNS elemental analyzer. NMR spectra were collected on a Bruker Avance 600 MHz spectrometer in dry, degassed CD3CN. 1H NMR spectra were referenced to tetramethylsilane using the residual proteo impurities of the solvent (1.94 ppm for CD3CN).54 All 31P{1H} NMR spectra were referenced to phosphoric acid using an external standard (85% H3PO4). Chemical shifts are reported using the standard δ notation in parts per million. Electronic absorption spectra were recorded with a Jasco V-670 absorption spectrometer or a Cary 60 UV−vis spectrometer equipped with fiber-optic cables. UV−vis spectra were recorded for samples dissolved in dry, degassed acetonitrile (MeCN) and contained in 10 mm quartz cells. Electrospray ionization mass spectrometry (ESI-MS) data were collected on samples dissolved in dry, degassed MeCN or THF using a Waters LCT Premier mass spectrometer. Electron paramagnetic resonance (EPR) spectra were collected on a Bruker EMX X-band spectrometer equipped with an ER041XG microwave bridge. Electrochemical Methods. Electrochemical data were collected on a Gamry Series G 300 potentiostat/galvanostat/ZRA (Gamry Instruments, Warminster, PA) using a standard three-electrode configuration comprising a 3.0 mm glassy carbon working electrode, a platinum wire auxiliary electrode, and a silver wire pseudoreference electrode. Electrochemical experiments were performed at 25 °C in a glovebox under an atmosphere of N2. Electrochemical samples were 1.0 mM analyte solutions in MeCN containing 0.1 M [Bu4N][PF6] as the supporting electrolyte. All potentials were referenced to the [Cp2Fe]+/0 couple using ferrocene or decamethylferrocene (E°′ = −0.59 V) as an internal standard.55 pKa Determinations. The pKa values for 1a and 1b were determined by spectrophotometric titration following published methods.56 In a typical experiment, an MeCN stock solution of the [SN(H)S]Ni(L) complex (L = PPh3, 1.67 mM; L = PCy3, 1.42 mM) was prepared inside a nitrogen-filled glovebox. Aliquots (200 μL) were removed from the stock solution and diluted with 3.0 mL of MeCN to give concentrations between 60 and 70 μM and a total solution volume of 3.20 mL. After an initial spectrum was obtained, the solution was titrated with 2,4,6-collidine (2.5−10 μL per addition) using a volumetric syringe. Spectral changes after each addition were recorded with a Cary 60 UV−vis spectrometer equipped with fiberoptic cables. Gas Chromatography. To quantify hydrogen production, headspace analyses were performed using an Agilent 7890B gas chromatograph system equipped with a thermal conductivity detector (GC-TCD). The GC-TCD system was equipped with a 30 m HPMolesieve column (catalog no. 19095P-MS6) with a diameter of 0.530 mm and a film thickness of 25 μm. The method for hydrogen separation consisted of a 4 min run with an inlet temperature of 200 °C (splitless), a detector temperature of 220 °C, a 40 °C isotherm, and a flow rate of 4.7 mL min−1 for the nitrogen carrier gas. The retention time of molecular hydrogen under these conditions was 2.26 min. Hydrogen was quantified for the method using a calibration curve obtained from six known hydrogen concentrations.

Crystallographic Methods. X-ray diffraction data were collected at low temperature on a single crystal covered in Paratone and mounted on a glass fiber. Data were acquired using a Bruker SMART APEX II diffractometer equipped with a CCD detector using Mo Kα radiation (λ = 0.71073 Å), which was wavelength-selected with a single-crystal graphite monochromator. The SMART program package was used for determination of the unit-cell parameters and for data collection. The raw frame data were processed using SAINT57 and SADABS58 to yield the reflection data file. Subsequent calculations were carried out with SHELXTL.59 The structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques. Analytical scattering factors for neutral atoms were used throughout the analyses.60 Hydrogen atoms were generated in calculated positions and refined using a riding model. ORTEP diagrams were generated using ORTEP-3 for Windows.61 Theoretical Calculations. All calculations were performed with TURBOMOLE 7.262,63 using the TPSS meta-generalized-gradientapproximation (meta-GGA) functional.36 For computational efficiency, initial geometry optimizations were performed using a moderate split-valence plus polarization basis set (def2-SVP)37 and then refined using a triple-ζ valence plus polarization basis set (def2TZVP).38 Atomic coordinates from the solid-state structure of 3a were used as the starting point for the geometry optimization of [SNS]Ni(PCy3); no molecular symmetry was imposed. For monomeric 3b, the solid-state structure of dimeric {3b}2 was split in half and used as the starting point for the geometry optimization of [SNS]Ni(PPh3). Molecular geometries and orbital energies were evaluated self-consistently to tight convergence criteria (energy converged to 0.1 μHartree; maximum norm of the Cartesian gradient ≤10−4 au). Molecular orbital and spin-density surface images were rendered using Visual Molecular Dynamics (VMD 1.9.3) software.64 [SN(H)S]Ni(PCy3) (1a). Under a N2 atmosphere, a 20 mL scintillation vial was charged with NiCl2(PCy3)2 (673 mg, 0.977 mmol, 1.00 equiv), toluene (7 mL), and a magnetic stir bar. To the stirring suspension was added [SNS]H3 (256 mg, 0.980 mmol, 1.00 equiv) dissolved in 2 mL of toluene. The addition of triethylamine (275 μL, 1.97 mmol, 2.02 equiv) caused an immediate color change to dark orange. The mixture was stirred for 30 min at ambient temperature, filtered through a glass frit, and concentrated to approximately 5 mL under reduced pressure. The addition of 100 mL of pentane did not induce precipitation; however, when the total solution volume was reduced to approximately 20 mL, an orange microcrystalline solid formed. This solid was collected by filtration, washed with cold pentane, and dried under reduced pressure to afford the product in 51% yield (256 mg). X-ray-quality crystals of the product were obtained from an MeCN solution of the complex at 25 °C. Anal. Calcd for C32H46NNiPS2: C, 64.28; H, 7.96; N, 2.13. Found: C, 64.22; H, 7.75; N, 2.34. 1H NMR (500 MHz, CD3CN): δ 7.83 (br s, 1H, −NH), 6.99 (m, 2H, aryl-H), 6.85 (m, 2H, aryl-H), 6.57 (m, 2H, aryl-H), 2.19 (m, 6H, −CH3), 2.05 (m, 6H, −Cy), 1.74 (m, 15H, −Cy), 1.28 (m, 9H, −Cy). 31P{1H} NMR (162 MHz, CD3CN): δ 33.9 (s). UV−vis [MeCN; λmax, nm (ε, M−1 cm−1)]: 244 (27000), 270 (22500), 374 (2200). ESI+-MS (THF): m/z 598 ([M]+). [SN(H)S]Ni(PPh3) (1b). Under a N2 atmosphere, a 20 mL scintillation vial was charged with NiCl2(PPh3)2 (655 mg, 1.00 mmol, 1.00 equiv), toluene (7 mL), and a magnetic stir bar. To the stirring suspension was added [SNS]H3 (261 mg, 1.00 mmol, 1.00 equiv) dissolved in 2 mL of toluene. An immediate color change was observed from dark blue to maroon. After 5 min of stirring, triethylamine (280 μL, 2.00 mmol, 2.00 equiv) was added, which resulted in the immediate precipitation of an orange solid. The solid was collected on a glass filter frit and washed with toluene (2 × 20 mL) and pentane (3 × 20 mL). The orange solid was collected and dried under vacuum (423 mg, 73%). X-ray-quality crystals were obtained from an MeCN solution of the complex at 25 °C. Anal. Calcd for C32H28NNiPS2: C, 66.23; H, 4.86; N, 2.41. Found: C, 65.37; H, 4.78; N, 2.33. 1H NMR (500 MHz, CD3CN): δ 8.31 (br s, 1H, −NH), 7.78 (m, 5H, aryl-H), 7.52 (m, 2H, aryl-H), 7.42 (m, 6H, aryl-H), 7.20 (m, 6H, aryl-H), 6.89 (m, 4H, aryl-H), 6.62 (m, 2H, G


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Inorganic Chemistry aryl-H), 2.33 (s, 6H, CH3). 31P{1H} NMR (162 MHz, CD3CN): δ 29.0 (s). UV−vis [MeCN; λmax, nm (ε, M−1 cm−1)]: 260 (23700). ESI+-MS (THF): m/z 579 ([M]+). K(2.2.2-crypt){[SNS]Ni(PCy3)} (2a). Under a N2 atmosphere, a 20 mL scintillation vial was charged with KH (43 mg, 1.1 mmol, 3.2 equiv), THF (3 mL), and a magnetic stir bar. A solution of [SNS]H3 (89 mg, 0.34 mmol, 1.0 equiv) in THF (3 mL) was added, resulting in the liberation of gas. Once the gas evolution had ceased, solid NiCl2(PCy3)2 (236 mg, 0.343 mmol, 1.00 equiv) had an immediate color change to dark greenish blue. The mixture was stirred for 30 min and filtered through a fritted glass filter. The filtrate was concentrated to approximately 1 mL, and 100 mL of pentane was added to force the precipitation of a baby-blue solid. The solid was collected by filtration, washed with pentane (3 × 20 mL), and dried under vacuum to afford 161 mg of product (74%). X-ray-quality crystals were obtained by diffusion of diethyl ether (Et2O) into a THF solution of the complex and 2.2.2-cryptand at 25 °C. Anal. Calcd for C50H81N3NiPO6S2K (with 2,2,2-cryptand): C, 59.28; H, 8.06; N, 4.15. Found: C, 58.93; H, 8.19; N, 3.80. 1H NMR (600 MHz, CD3CN): δ 7.27 (s, 2H, aryl-H), 6.87 (d, J = 1.60 Hz, 2H, aryl-H), 6.36 (m, 2H, aryl-H), 2.16 (s, 6H, −CH3), 2.06 (m, 6H, −Cy), 1.79 (m, 15H, −Cy), 1.75 (m, 3H, −CH (PCy3)), 1.28 (d, J = 7.78 Hz, 9H, −Cy). 31P{1H} NMR (162 MHz, CD3CN): δ 33.0 (s). UV−vis [MeCN; λmax, nm (ε, M−1 cm−1)]: 236 (33600), 276 (31000), 368 (10700). ESI+-MS (THF): m/z 635 ([M + K]+). ESI−-MS (THF): m/z 596 ([M]−). K(2.2.2-crypt){[SNS]Ni(PPh3)} (2b). Complex 2b was prepared using a procedure analogous to that used for the preparation of 2a. Using 255 mg of [SNS]H3 (0.977 mmol, 1.00 equiv), 117 mg of KH (2.93 mmol, 3.00 equiv), and 638 mg of NiCl2(PPh3)2 afforded 400 mg of product as a microcrystalline purple solid (83% yield). X-rayquality crystals were obtained by the diffusion of Et2O into an MeCN solution of the complex and 2,2,2-cryptand at 25 °C. Anal. Calcd for C32H27NNiPS2K: C, 62.15; H, 4.40; N, 2.26. Found: C, 60.16; H, 4.35; N, 2.17. 1H NMR (600 MHz, CD3CN): δ 7.82 (m, 5H, aryl-H), 7.43 (d, J = 7.10 Hz, 2H, aryl-H), 7.39 (t, J = 7.21 Hz, 10H, aryl-H), 6.80 (br s, 2H, aryl-H), 6.43 (d, J = 8.24 Hz, 2H, aryl-H), 2.14 (br s, 6H, −CH3). 31P{1H} NMR (162 MHz, CD3CN): δ 36.0 (s). UV−vis [MeCN; λmax, nm (ε, M−1 cm−1)]: 294 (29200), 364 (12300). ESI+MS (MeCN): m/z 617 ([M + K]+). [SNS]Ni(PCy3) (3a). In air, a 250 mL round-bottomed flask was charged with NiCl2(PCy3)2 (553 mg, 0.802 mmol, 1.00 equiv) in toluene (50 mL) to give a magenta suspension. The proligand [SNS]H3 (208 mg, 0.797 mmol, 0.984 equiv) dissolved in 3 mL of toluene was added to the suspension but did not result in an immediate color change. Triethylamine (500 μL, 3.60 mmol, 3.70 equiv) was added by syringe, resulting in an immediate color change to a dark-blue/purple solution with the concomitant formation of a precipitate. The reaction mixture was stirred for 2 h, and the volatiles were removed under reduced pressure. The solid residue was suspended in THF (50 mL) and stirred for 30 min. The suspension was filtered to remove Et3NHCl, and the solvent was removed from the filtrate. The dark-greenish-blue solid was dissolved in 5 mL of THF, and excess pentane was added to induce the precipitation of a dark-blue solid in 36% yield (215 mg). X-ray-quality crystals of the product were obtained from a concentrated analyte solution in MeCN at 25 °C. Anal. Calcd for C32H45NNiPS2: C, 64.33; H, 7.49; N, 2.34. Found: C, 64.29; H, 7.59; N, 2.80. UV−vis [MeCN; λmax, nm (ε, M−1 cm−1)]: 271 (16700), 300 (14200), 376 (6100), 677 (1900). ESI+MS (THF): m/z 596 ([M]+). {[SNS]Ni(PPh3)}2 ({3b}2). Following the same procedure as that used for the preparation of 3a, 1.25 g of NiCl2(PPh3)2 (1.91 mmol, 1.00 equiv) was suspended in toluene (70 mL) in a 250 mL roundbottomed flask equipped with a magnetic stir bar. To the suspension was added 504 mg of [SNS]H3 1.93 mmol, 1.01 equiv) and 800 μL of triethylamine (5.80 mmol, 3.00 equiv) in 75 mL of toluene, which afforded 626 mg of product (56% yield) as a blue-green solid. X-rayquality crystals were obtained from a concentrated analyte solution in MeCN at 25 °C. Anal. Calcd for C64H54N2Ni2P2S4: C, 66.34; H, 4.70; N, 2.42. Found: C, 65.97; H, 5.36; N, 2.34. 1H NMR (600 MHz,

CD3CN): δ 7.66 (m, 2H, aryl-H), 7.64 (m, 2H, aryl-H), 7.62 (m, 4H, aryl-H), 7.60 (m, 4H, aryl-H), 7.51 (m, 15H, PPh3), 7.44 (m, 15H, PPh3) (note: −CH3 resonances for the [SNS] ligand were not distinguishable because they overlapped with proteo-MeCN in CD3CN). 31P{1H} NMR (162 MHz, CD3CN): δ 26.0 (s). UV−vis [MeCN; λmax, nm (ε, M−1 cm−1)]: 282 (46800), 378 (sh, 13700), 661 (8200). ESI+-MS (THF): m/z 1194 ([M]+). One-Electron Reduction of 1b. A 50 mL round-bottomed flask was charged with a magnetic stir bar, complex 1b (3.4 mg, 5.8 μmol), and decamethylcobaltocene (Cp*2Co; 2.5 mg, 7.6 μmol). The roundbottomed flask was sealed with a ribbed rubber septum inside a nitrogen-filled glovebox. To the vessel was added by syringe 15 mL of dry, degassed MeCN. The initial bright-orange solution rapidly turned to emerald green. The mixture was stirred for 10 min before a sample of the headspace was removed using a gastight syringe. Analysis of this gas by GC-TCD confirmed the formation of 2.7 μmol of hydrogen (0.45 equiv, 90% yield). Analysis of the reaction solution by UV−vis spectrometry revealed the quantitative formation of [CoCp*2]{[SNS]Ni(PPh3)}. Reactions of 1a and 1b with TEMPO•. In a typical experiment, a 20 mL scintillation vial was charged with 1 equiv of the nickel aminophenolate complex 1a or 1b and 1 equiv of TEMPO• in 15 mL of MeCN. The initial orange color of the solution rapidly turned to dark blue. After 1 h, the reaction volatiles were removed under reduced pressure, and the product was washed with pentane. Analysis of the solid residue by UV−vis and NMR or EPR spectroscopy revealed the formation of the corresponding oxidized nickel complex 3a or 3b. Reactions of 3a and 3b with TEMPO-H. In a typical experiment, a 20 mL scintillation vial was charged with 1 equiv of the nickel complex 3a or 3b and 1 equiv of TEMPO-H. The resulting dark-blue solution was stirred for 1 h with no observed color change. After removal of the volatiles under reduced pressure and washing of the solid residue with pentane, spectroscopic analysis (UV−vis and EPR or NMR) showed only the presence of an unreacted nickel starting material with no evidence for the formation of 1a or 1b, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00618. X-ray diffraction data, NMR and EPR spectra, and spectrophotometric pKa titrations (PDF) Accession Codes

CCDC 1831023−1831028 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alan F. Heyduk: 0000-0003-2183-7099 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Science Foundation (Grant CHE-1464832). The authors thank Prof. Andy Borovik for the use of his EPR spectrometer and Claudia Ramirez for assistance with fitting EPR spectra. H


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Inorganic Chemistry

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