Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel

Article pubs.acs.org/Organometallics

Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel Complexes in Four Different Oxidation States Blake R. Reed,†,∥ Maryam Yousif,† Richard L. Lord,‡ Meaghan McKinnon,§ Jonathan Rochford,*,§ and Stanislav Groysman*,† †

Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States § Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02125, United States ‡

S Supporting Information *

ABSTRACT: A series of nickel complexes with potentially redox active bis(aldimino)pyridine ligands [NNN] ([NNN] = 1,1′-(pyridine2,6-diyl)bis(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl, mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis(trifluoromethyl)phenyl) were synthesized, and their properties and reactivities were investigated as a function of the overall oxidation state of the system. (Ni[NNN])2+ complexes of ligands featuring bulky electron-rich substituents (1a-Br2 and 1b-Br2, [NNN] = 1,1′(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine) and 1,1′-(pyridine-2,6-diyl)bis(N-mesitylmethanimine), respectively) demonstrated five electrochemical reduction events, the first three of which were quasi-reversible. In contrast, only two quasi-reversible reductions were observed for the less bulky and electron-deficient N-aryl substituents 4-(trifluoromethyl)phenyl and 3,5-bis(trifluoromethyl)phenyl. Chemical reduction of 1a-Br2 and 1b-Br2 with 1 equiv of KC8 or CoCp*2 forms (Ni[NNN])+ complexes of the general formula Ni[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that these complexes feature significant unpaired spin density on the metal, consistent with “nickel(I)” character. This behavior is in contrast with previously reported bis(ketimino)pyridine systems, in which at the (Ni[NNN])+ state the unpaired electron resided exclusively in the ligand. Further reduction forms a series of (Ni[NNN])0 complexes, in which all of the potentially tridentate [NNN] ligands bind via only one iminopyridine unit; the second arm is left unbound in most complexes. Variable temperature NMR spectroscopy demonstrates that bound and unbound arms exchange via a postulated tridentate intermediate. Electrochemical reduction, via three sequential one-electron reductions, of 1a-Br2 and 1b-Br2 in the presence of CO2/H+ forms an active catalyst for H2 evolution at a glassy-carbon electrode surface, again emphasizing the unique redox chemistry of the bulky bis(aldimino)pyridine nickel complexes.

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INTRODUCTION

Wieghardt and co-workers have demonstrated that in neutral M(iminopyridine)2 complexes (bearing related bidentate iminopyridine ligands), earlier metals (chromium, manganese) appear as divalent ions antiferromagnetically coupled to the two ligand radical anions, whereas nickel is best described as nickel(I).9 Rohde and co-workers reported that the reduction of the bis(imino)pyridine nickel(II) dichloride complex Ni[NNN]MeCl2 leads to chloride loss and the formation of Ni[NNN]MeCl (Figure 1), which contains a square-planar nickel(II) center and a [NNN] ligand radical anion.10 Similarly, Gambarotta, Budzelaar, and co-workers have shown that a Ni[NNN]MeMe complex (Figure 1) is best described as a [NNN] radical anion bound to a low-spin nickel(II) center.11

Bis(imino)pyridine ligands are redox-active entities whose ability to accept several electrons can be harnessed in the design of catalytic redox transformations of base and maingroup elements.1,2 Due to their chelating nature and redox properties that enable multiple accessible oxidation states, complexes of bis(imino)pyridine ligands with nearly every transition metal and many main-group elements have been synthesized and their reactivity has been investigated.3−8 For the 3d metals, it has been shown that ligand reduction is generally more accessible than metal reduction.1a,3 Therefore, reduced bis(imino)pyridine complexes generally contain frontier orbitals of primarily ligand character, which is consistent with ligand-based reactivity. However, the precise electronic structure of the metal−ligand ensemble often depends on the specific metal−ligand interaction. Thus, © 2017 American Chemical Society

Received: October 17, 2016 Published: February 1, 2017 582

DOI: 10.1021/acs.organomet.6b00793 Organometallics 2017, 36, 582−593

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Organometallics

mode of CX2 (X = O or S) coordination. Thus, we turned our attention to the tridentate bis(imino)pyridine ligands. In our preliminary communication, we demonstrated that the bis(aldimino)pyridine ligand [NNN]a ([NNN]a = 1,1′-(pyridine2,6-diyl)bis(N-(2,6-diisopropylphenyl)methanimine)) forms the non-square-planar complex Ni[NNN]aX (X = Br, Cl) upon one-electron reduction of the bis(aldimino)pyridine nickel(II) dihalide precursor Ni[NNN]aX2.14 Unlike the previously reported planar bis(ketimino)pyridine complexes Ni[NNN]MeCl and Ni[NNN]MeMe (Figure 1),10,11 for which the spin density was localized on the ligand and Ni was described as low-spin Ni(II), nonplanar Ni[NNN]aX complexes contained significant spin density at the metal, as inferred from X-ray crystallography, EPR measurements, and DFT calculations.14 We have also shown that Ni[NNN]aX demonstrates metal-based reactivity with dioxygen (oxidation),14 whereas Ni[NNN]MeCl underwent ligand oxidation.10 More recently, we demonstrated that the three-electronreduced species (Ni[NNN])− exhibits electrocatalytic reactivity in H+ reduction in the presence of CO2.15 Following these initial reports, we became interested in the coordination chemistry and reactivity of bis(aldimino)pyridine complexes in other accessible oxidation states, with a particular focus on the comparison between the analogous bis(aldimino)pyridine and bis(ketimino)pyridine systems. The present study specifically demonstrates that bis(aldimino)pyridine nickel complexes (Ni[NNN]) can exist in at least four different oxidation states: (Ni[NNN])2+, (Ni[NNN])+, (Ni[NNN])0, and (Ni[NNN])−. The oxidation-state-specific coordination chemistry and reactivity of these complexes are described.

Figure 1. Square-planar Ni[NNN]MeCl and Ni[NNN]MeMe complexes containing nickel(II) centers ligated by one-electron-reduced bis(ketimino)pyridine ligands.10,11

Most of the previous studies had focused on the chemistry of bis(ketimino)pyridine ligands featuring methyl groups in the imino positions. Bis(aldimino)pyridine complexes, on the other hand, have received significantly less attention.12 Although it is feasible that hydrogen substituents in the imino positions would destabilize ligand-based radicals, they might also lead to different electronic structures, coordination chemistry, and chemical reactivity relative to their bis(ketimino)pyridine counterparts. We have been exploring the coordination chemistry and reactivity of mononuclear and dinuclear complexes with iminopyridine ligands in the activation of small molecules, in particular CS2 and CO2.13 We have previously reported that the bidentate iminopyridine ligand [NN] enables side-on coordination of CS2 at square-planar Ni centers, Ni[NN](η2CS2).13b,c However, in spite of the significant elongation of the metal-bound C−S bond, no C−S bond breaking (or C−C bond formation) was observed. We postulated that a related tridentate ligand may force a monodentate, more reactive,

Scheme 1. Coordination Chemistry of (Ni[NNN])n+ Complexes in Three Different Oxidation States (n = 2−0)

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well-resolved paramagnetically shifted 1H NMR spectra that span approximately a 70−80 ppm range (from −10 to +70 ppm). We assign the lowest-field peak at 69.1 ppm in the spectrum of 1b-Br2 to the β-H of the pyridine. Bryliakov and co-workers have reported 1H NMR spectra of the related bis(ketimino)pyridine nickel(II) dichloride complexes, in which the peak around 70 ppm was also assigned to a pyridine β-H.17 Pyridine β protons of other complexes all appear in a similar region (68.9 ppm for previously reported 1a-Br2, 66.4 ppm for 1a-Cl2, 61.9 ppm for 1c-Br2, 60.7 ppm for 1d-Br2). The pyridine γ-H resonance is likely observed at 11.4 ppm for 1aBr2;14 similar chemical shift values are observed for other complexes (12.5 ppm for 1b-Br2, 10.8 ppm for 1c-Br2, and 11.0 ppm for 1d-Br2). The NMR spectra of complexes 1a-Br2−1e-Br2 clearly demonstrate their paramagnetic nature. We have previously reported solution magnetic moments for compounds 1a-Br2 and 1a-Cl2 that were measured in a coordinating solvent, CD3CN. These magnetic moments were consistent with two unpaired electrons (μeff = 3.0 μB). In our present investigation, we have also measured solution magnetic moments in the noncoordinating solvent CD2Cl2. The observed values were also consistent with two unpaired electrons, as expected for high-spin nickel(II) complexes. Cyclic voltammograms (CV) of complexes 1b-Br2−1e-Br2 are given in Figure 2 and are compared with the previously published CV of complexes 1a-Br2 and 1a-Cl2. All electrochemical data are summarized in Table 1. Complex 1a-Br2 demonstrates five reduction events at −0.68, −1.22, −2.27, −2.56, and −2.72 V vs FeCp2+/0, the first three of which appear to be quasi-reversible. Complexes 1a-Cl 2 and 1b-Br 2

RESULTS AND DISCUSSION (Ni[NNN])2+ Oxidation State: Synthesis, Characterization, and Reactivity of Ni(II) Dihalides. We have previously reported the synthesis, spectroscopic and structural characterization of complexes 1a-Br2 and 1a-Cl2 with [NNN]a (Scheme 1). In this work, we sought to prepare additional bis(aldimino)pyridine ligands [NNN]b−e (Scheme 1 and 2) and Scheme 2. Synthesis of complexes 1d-Br2 and 1e-Br2

their Ni(II) complexes, in order to evaluate the steric and electronic effects of the N substituents on the coordination chemistry, properties, and reactivity of bis(aldimino)pyridine nickel complexes. [NNN]a features bulky electron-rich 2,6diisopropylphenyl substituents, while [NNN]b has slightly less bulky mesityl groups. [NNN]c has electron-donating, nonbulky 4-methoxyphenyl substituents, while [NNN]d and [NNN]e b o t h f e a t u r e n o n b u l k y , el e c t r o n - w i t h d r a w i n g 4(trifluoromethyl)phenyl ([NNN]d) and 3,5-bis(trifluoromethyl)phenyl ([NNN]e) groups. [NNN] ligands with electron-rich substituents (mesityl, [NNN]b; p-methoxyphenyl, [NNN]c) were synthesized by condensation of dicarboxypyridine with the corresponding anilines, as described in the Experimental Section; syntheses of [NNN]b and [NNN]c have been previously reported.16 We have previously shown that Cl and Br complexes (of [NNN]a) lead to a very similar coordination chemistry in both “Ni(II)” and “Ni(I)” states;14 thus, the present work focuses mostly on bromide complexes. Treatment of THF solutions of NiBr2(dme) (dme = dimethoxyethane) with 1 equiv of [NNN]a−c leads to the formation of the pentacoordinate Ni(II) complexes Ni[NNN]a−cBr2 (1a-Br2−1c-Br2), which were isolated as orange crystalline solids in good to excellent yields. For the electron-deficient N-aryl substituents (p(trifluoromethyl)phenyl, [NNN]d; m-bis(trifluoromethyl)phenyl, [NNN]e), we were not able to isolate the free ligand in good yield due to the more difficult condensation of the electron-poor anilines. Instead, the corresponding Ni(II) complexes were prepared in a one-pot reaction, in which mixing of dicarboxypyridine with 2 equiv of the respective anilines was followed by addition of the nickel(II) bromide dimethoxyethane complex. It is possible that these products form in a template synthesis, in which aniline condensation occurs in the coordination sphere of Ni(II). The resulting Ni(II) complexes 1d-Br2 and 1e-Br2 were obtained as acetonitrile adducts by recrystallization of the crude reaction products from CH3CN/ether. The complexes were isolated in good (75%) to excellent (96%) yields. The complexes [NNN]a−eNiBr2 were characterized by 1H NMR spectroscopy, cyclic voltammetry, mass spectrometry, solution magnetic susceptibility measurements, and elemental analysis; selected complexes were also characterized by X-ray crystallography. Compounds 1a-Br2−1e-Br2 all demonstrate

Figure 2. CV of complexes 1a-Cl2−1e-Br2 at 1 mM concentration recorded at a glassy-carbon electrode in 0.1 M Bu4NPF6 acetonitrile supporting electrolyte under 1 atm of Ar at a scan rate of 100 mV/s. 584


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Supporting Information), consistent with a diamagnetic compound in solution. It is likely that the diamagnetic nature of [1a-Cl](PF6) results from the square-planar geometry at the Ni(II) d8 center.18 These experiments indicate that squarepyramidal complexes 1a-X2−1e-Br2 retain their structure in solution and confirm that halide loss only occurs upon chemical or electrochemical reduction. We note that bis(ketimino)pyridine nickel(II) methyl cations were also reported to demonstrate a diamagnetic NMR spectrum due to their square-planar geometry.19 We have previously reported the structures of compounds 1a-Br2 and 1a-Cl2, obtained as orange crystals by vapor diffusion of ether into THF or CH2Cl2 solutions (for the structure of 1a-Br2, see Figure 3, top). We have also crystallized

Table 1. Electrochemical Data for All Ni[NNN]X2 Complexes Recorded in 0.1 M Bu4NPF6 Acetonitrile Electrolyte at a Glassy-Carbon Working Electrode Reported vs the Nonaqueous Ferrocenium/Ferrocene (FeCp2+/0) Pseudo Referencea E vs FeCp2+/0 (V) 1a-Cl2 1a-Br2 1b-Br2 1c-Br2 1d-Br2 1e-Br2

−0.86 −0.68 −0.75 −0.89 −0.82 −0.86

−1.23 −1.22 −1.29 −1.36 −1.28b −1.33

−2.28 −2.27 −2.31b −2.08b −1.90b −2.05b

−2.61b −2.56b −2.44b −2.34b

−2.73b −2.72b −2.55b

−2.30b

a

Conditions: 1 mM sample concentration; 3 mm diameter glassycarbon working electrode; Pt-wire counter electrode; Ag/AgPF6 nonaqueous reference electrode; 0.1 V s−1 scan rate. bIrreversible; Epc reported.

demonstrate similar behavior. In contrast, four reductions are observed for complex 1c-Br2 and only three for 1d-Br2 in the solvent-accessible range. For all complexes, the first two events take place at similar potentials, implying that the reductions are not strongly dependent on the steric or electronic nature of the N-aryl substituents. The accessibility of the first two reduction potentials implies that bis(aldimino)pyridine nickel systems can be isolated in at least three different oxidation states. It is likely that the first two (quasi-reversible) events are bis(imino)pyridine ligand based (at least on a CV time scale).14 We note that in a related study on bis(imino)pyridine complexes with main-group elements (Zn and Mg), Berben and co-workers found that the first reductions take place at similar potentials but were irreversible due to chloride loss.5b Intriguingly, in the present case the first electrochemical reduction is quasireversible; however, chemical reduction of complexes 1a-Br2 (or 1a-Cl2) and 1b-Br2 leads to the isolation of monohalide complexes (2a-Br/Cl and 2b-Br; vide infra). Scan-ratedependent studies for 1a-Cl2 and 1a-Br2 show evidence of an electrochemical−chemical (EC) mechanism at slow scan rates consistent with the assignment of a ligand-based reduction followed by halide dissociation.15 The latter study also demonstrates that the second reduction potential of 1a-Cl2 and 1a-Br2 also follows an EC mechanism: i.e., following oneelectron reduction and generation of 2a-Cl and 2a-Br at the electrode surface subsequent ligand reduction occurs, resulting in another halide loss and formation of the neutral species in each case, perhaps structurally analogous to dimer 4a (vide infra). Given their electrochemical quasi-reversibility, facile first reduction, and the monohalide structures of the monoreduced products (vide infra), we were curious whether the tetracoordinate {Ni[NNN]RX}+ cationic complexes could be isolated. To test for that, we decided to abstract Cl− from 1aCl2 and then analyze the products using 1H NMR spectroscopy. Treatment of 1a-Cl2 with TlPF6 results in the precipitation of TlCl and formation of a yellow solution, from which the cationic compound [1a-Cl](PF6) was isolated. The 1H NMR spectrum of [1a-Cl](PF6) is drastically different from the spectrum of 1a-Cl2. The spectrum of 1a-Cl2 demonstrates six resonances spread over an ∼70 ppm spectral window, consistent with a paramagnetic compound in solution.14 In contrast, [1a-Cl]+ contains all of the expected resonances within a 1−9 ppm spectral window (Figure S9 in the

Figure 3. (top) Structure of 1a-Br2, with 50% probability ellipsoids. (bottom) Structure of 1d-Br2, with 50% probability ellipsoids. H atoms and cocrystallized solvent were omitted for clarity. Selected distances (Å) and angles (deg) for 1a-Br2: Ni−Br1 2.4303(3), Ni−Br2 2.3530(3), Ni−N1 1.972(2), Ni−N2 2.147(2); N1−Ni1−Br2 155.50(5), N1−Ni1−Br1 90.26(5). Selected distances (Å) and angles (deg) for 1d-Br2: Ni−Br1 2.5639(5), Ni−Br2 2.5493(5), N−N1 1.976(2), Ni−N2 2.155(3), Ni−N3 2.132(3), Ni−N4 2.020(3); N1− Ni−N4 177.6(1), Br2−Ni−Br1 177.92(2).

compound 1d-Br2 from CH3CN/ether and solved its solidstate structure (Figure 3, bottom). While compounds 1a-Br2 and 1a-Cl2 have a distorted-square-pyramidal geometry, compound 1d-Br2 is octahedral, with acetonitrile serving as an additional ligand to the nickel center. The structure of 1dBr2 demonstrates a trans geometry of the bromide ligands, with NCCH3 binding trans to the pyridine donor of [NNN]. (Ni[NNN])+ Oxidation State: Synthesis and Reactivity of Ni[NNN]X Complexes. We have previously reported that the one-electron reduction of Ni(II) complexes 1a-Cl2 and 1aBr2 led to the formation of complexes 2a-Cl and 2a-Br 585


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structure of 2a-Br.14 In the structure of 2b-Br, the Npy−Ni−Br angles range from the nearly linear 178.2(1)° to the distorted 170.4(1) and 165.4(1)° (Figure 5). Significantly, these values correlate well with the deviation of the N-aryl substituent from its perpendicular (to the NNN plane) arrangement (measured by the Cα−N−Cβ−Cγ dihedral angle; Figure 4), as was previously predicted by DFT calculations for the 2a-Br system. Thus, for the nearly linear Npy−Ni1−Br1 angle of 178.2(1)°, a nearly perpendicular dihedral angle of 87(1)° is observed (conformer 2b-Br-I), for the slightly distorted angle of 170.4(1), the N-mesityl group slightly bends from the perfectly perpendicular arrangement (72(1)°, 2b-Br-II), and finally for the Npy−Ni3−Br3 angle of 165.4(1)°, further distortion from the perpendicular values is found, as manifested by the value of 67(2)° (2b-Br-III). We repeated the constrained optimizations varying the Npy− Ni−Br angle for 2b-Br, similar to those calculated for 2a-Br and its ketimine analogue10 in our previous report,14 and the results are shown in Figure 6. These plots have been fit to a

(Scheme 1).14 The structures of complexes 2a-Cl and 2a-Br demonstrated nonplanar geometries; high-field EPR spectroscopy and DFT calculations suggested a substantial amount of spin density on the metal. This effect was attributed to the decreased steric impact of the H-imino substituents (vs Me substituents), which allows for the rotation of the bulky N-aryl groups. This rotation leads to deviation of the Npy−Ni−X angles from linearity, which in turn allows migration of the spin density to the metal. DFT calculations proposed that this process demands less energy for the aldimino groups than for the ketimino groups, requiring, for example, only about 2 kcal/ mol for the Npy−Ni−X angle of 150°. Significantly, this hypothesis was confirmed by an observed range of Npy−Ni−X angles; compound 2a-Br crystallized as two different polymorphs, featuring Npy−Ni−Br angles of 161.4(1) and 156.5(1)°. A schematic drawing of 2a-Br, along with the discussed structural parameters, is given in Figure 4.

Figure 4. Depiction of the Npy−Ni−Br and dihedral Cα−N−Cβ−Cγ angles discussed in the paper. R = 2,6-diisopropylphenyl (2a-Br), mesityl (2b-Br). Figure 6. DFT calculations demonstrating a correlation between the Cim−Nim−CAr−CAr dihedral angle deviation from perpendicular and the deviation of the Npy−Ni−Br angle from linearity. Fit lines: (i) y = −0.0128x2 + 0.9968x, R2 = 0.98 (black); (ii) y = −0.0105x2 + 1.0747x, R2 = 0.99 (red); (iii) y = −0.0119x2 + 1.1433x, R2 = 1.00 (blue).

Next, we investigated whether this effect was limited to a relatively bulky N-aryl group, 2,6-diisopropylphenyl. Thus, we interrogated the one-electron reduction of other Ni(II) complexes, 1b-Br2−1e-Br2. The one-electron reduction of 1b-Br2 formed a purple solution. Recrystallization of the crude product from cold ether led to the formation of dark brown 2bBr in 60% yield. Reduction of 1c-Br2−1e-Br2 led to the isolation of paramagnetic brown solids that did not produce Xray-quality crystals. Complex 2b-Br was characterized by solution magnetometry, IR spectroscopy, and X-ray crystallography. The measured magnetic moment of 2b-Br was 1.71 μB, consistent with one unpaired electron. The crystal structure of 2b-Br (obtained from cold ether) demonstrates three different conformers differing primarily in the Npy−Ni−Br angle (Figure 5). These findings agree with our previous observation of two different conformers (differing in Npy−Ni−Br angle) in the crystal

second-order polynomial with an intercept set to 0 (i.e., no dihedral deviation when the Npy−N−Br angle is linear). This assumption is validated by the good fits, and one can conclude that the dihedral deviation from perpendicular becomes increasingly difficult as the Npy−N−Br angle changes from 180°. There is a notable difference between the ketimine and aldimine backbones; the greater steric demand of the ketimine backbone causes a plateau in the dihedral at an earlier value (with deviation from linear by between 20 and 30°), and to a smaller torsional value (∼20°), in comparison to the species with the aldimine backbone. Our calculations predict a minimal difference, within the errors of the method, for 2a-Br vs 2b-Br,

Figure 5. X-ray structures (40% probability ellipsoids) of the three conformers (left, 2b-Br-I; center, 2b-Br-II; right, 2b-Br-III) observed in the asymmetric unit of 2b-Br. 586


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Organometallics demonstrating that the aldimine’s structural flexibility allows electronic flexibility between Ni(I)/L(0) and Ni(II)/L(1−), whereas the ketimine remains in the Ni(II)/L(1−) state. Both species agree within 1−2° for the dihedral for each Npy−N−Br angle and show a still-rising value in the dihedral of ∼27° for the largest forced angle deformation probed. Thus, packing effects within the crystal structure, which are completely absent from our model, may be important for explaining the larger variation in the crystallographically observed Npy−Ni−Br angles, and arm dihedrals, in 2b-Br in comparison to 2a-Br. We have also investigated the reactivity of 2a-Br. We have previously reported that the reaction of 2a-Br with O2 leads to the formation of 1a-Br2, free ligand, and an insoluble nickel salt, possibly nickel oxide.14 On treatment with CS2, a purple solution of complex 2a-Br undergoes a color change to brown. Crystallization of the reaction products from hexanes affords bright yellow crystals confirmed to be the free ligand by 1H NMR and XRD. Our repeated attempts to determine the nature of the metalated products were not successful. No reaction with CO2 was observed, both with 1 equiv of CO2 and under 1 atm of CO2. The reaction of 2a-Br with PhSSPh leads to the formation of (Ni(SPh)2)11 (see the Supporting Information for the structure of (Ni(SPh)2)11), along with the free ligand (observed by 1H NMR spectroscopy). While the formation and structure of (Ni(SPh)2)11 was previously reported,20 it was synthesized via a salt metathesis reaction of NiCl2 with NaSPh, while in the present case it is obtained via a one-electron oxidation with a “PhS•” synthon. In summary, most of the observed reactions of 2a-Br involve demetalation of Ni[NNN]Br to afford a free ligand along with various Ni(II) salts. (Ni[NNN])0 Oxidation State. Our initial strategy to access (Ni[NNN])0 complexes relied on the use of the “Ni(0)” precursor Ni(COD)2. Treatment of Ni(COD)2 with 1 equiv of [NNN]a at room temperature resulted in the formation of a mixture of Ni[NNN]a(COD) (3a-COD), Ni([NNN]a)2 (3a), and Ni(COD)2. Running the reaction at −30 °C led to a similar mixture of products. Extraction and recrystallization of this mixture from hexane led reproducibly to a mixture of crystals of Ni[NNN]a(COD) (3a-COD) and Ni([NNN]a)2 (3a). Both types of crystals are dark (green-black) and feature similar morphology. A 1H NMR spectrum of the mixture demonstrated the presence of resonances attributable to both 3a and 3a-COD, as well as Ni(COD)2 (Figure S12 in the Supporting Information). Treatment of Ni(COD)2 with 2 equiv of [NNN]a led to the formation of Ni([NNN]a)2 (3a), isolated as dark blue-green crystals by recrystallization from ether. X-ray crystal structures of 3a-COD and 3a (Figures 7 and 8) demonstrate that [NNN]a is bound to Ni in both cases via only one iminopyridine unit; the second imine is unbound. The bidentate coordination mode of [NNN]a is reminiscent of the coordination mode of the bidentate iminopyridine ligands in the reduced Ni[NN] complexes.9,13 Specific bond lengths are given in Table 2. A 1H NMR spectrum of 3a indicated that in solution the complex also features bidentate binding modes of bis(imino)pyridines, similar to the case for the solid-state structure. The imino protons appear to be particularly sensitive to the iminopyridine environment. Two sets of the iminopyridine resonances are observed, consistent with bound and unbound forms. The imino protons of the bound iminopyridine resonate at 10.03 ppm, which is close to the values previously found for the iminopyridine-Ni-COD and Ni(iminopyridine)2 com-

Figure 7. X-ray structure of complex 3a-COD, with 50% probability ellipsoids.

Figure 8. X-ray structure of complex 3a, with 50% probability ellipsoids.

plexes.13 For the unbound imine, the imino protons resonate at 9.19 ppm, which is close to the value (8.50 ppm) observed for the free ligand. We postulated that the “bidentate” binding mode of COD prevents tridentate coordination of bis(imino)pyridine and therefore decided to replace COD with diphenylacetylene (DPA). Addition of DPA to a mixture of Ni(COD)2 and [NNN]a resulted in the formation of 3a-DPA (isolated in 33% yield). X-ray crystallography again indicates that 3a-DPA contains an κ2-bound bis(imino)pyridine with one coordinated and one free iminopyridine chelate (Figure 9). The structure demonstrates significant elongation of the CC triple bond of acetylene, consistent with its activation.21 The experimental data above indicate that our complexes do not exhibit the anticipated tridentate coordination of [NNN]a at the (Ni[NNN])0 state. To determine whether it is the steric effect (of bulky 2,6-diisopropylphenyl groups) that is responsible for the bidentate ligation of [NNN]a, we evaluated the reactivity of [NNN]b with Ni(COD)2. Treatment of Ni(COD)2 with [NNN]b (1 equiv) led to the formation of 0.5 equiv of Ni([NNN]b)2 (3b) along with unreacted Ni(COD)2. Pure 3b can be obtained by treating Ni(COD)2 with 2 equiv of [NNN]b, followed by recrystallization. Both the solid-state (Figure 10) and the solution structure of 3b (see below) are 587


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Organometallics Table 2. Selected Bond Lengths for 1a-Br2, 3a, 3a-COD, and 3b CNima 1a-Br2 3a(1)d 3a(2)d 3a-COD 3b 3a-DPA

1.274(3) 1.326(8) 1.320(7) 1.316(4) 1.332(4) 1.294(3)

c

C−Ca 1.475(3) 1.412(9) 1.413(8) 1.415(4) 1.415(5) 1.442(3)

c

CNpya

CNpyb

c

c

1.335(3) 1.365(8) 1.377(7) 1.379(4) 1.381(4) 1.367(3)

1.335(3) 1.390(8) 1.387(8) 1.381(4) 1.370(4) 1.361(3)

C−Cb 1.473(3) 1.476(9) 1.482(9) 1.460(4) 1.468(5) 1.465(3)

CNimb c

1.271(3)c 1.278(8) 1.282(8) 1.279(4) 1.270(4) 1.273(3)

a Bound iminopyridine. bUnbound iminopyridine. cIn 1a-Br2, both iminopyridines are bound. dDue to the non-C2-symmetric structure of 3a, two different sets of bond lengths are observed.

respective bond lengths of complexes 3a, 3a-COD, 3b, and 3aDPA, in comparison to the nickel(II) complex 1a-Br2. 1a-Br2 shows typical CN and C−C bond lengths corresponding to a neutral ligand: CN imino bonds are 1.274(3) and 1.271(3) Å, and the Cimino−Cpyridine bonds are 1.475(3) and 1.473(3) Å. The unbound imine arm of the (Ni[NNN])0 complexes shows bond lengths comparable to bond lengths of a neutral ligand. For instance, CNimino distances of 1.270(4) and 1.279(4) Å and Cimino−Cpyridine distances of 1.468(5) and 1.460(4) Å (C− C) are observed for the free imino arms of 3b and 3a-COD, respectively. In contrast, bound imine shows significantly longer CN bonds and shortened C−C bonds. CNimino bond distances of 1.332(4) and 1.316(4) Å and Cimino−Cpyridine bond distances of 1.415(5) and 1.415(4) Å (C−C) are observed for the coordinated imino arms of 3b and 3a-COD, respectively. A 1H NMR spectrum of 3a demonstrates two sharp sets of signals at room temperature, one attributable to the coordinated iminopyridine and the other to the “dangling” side. In contrast, a room-temperature proton spectrum of 3b shows broad peaks, which may indicate a dynamic process that exchanges the bound site for the unbound site via a presumed tridentate intermediate. To shed light on this transformation, we conducted a VT NMR study in deuterated toluene (C7D8). Figure 11 shows the aromatic regions of the proton spectra taken at different temperatures; full spectra are shown in the Supporting Information. The room-temperature spectrum (295 K) shows two sharp resonances, a triplet at 7.74 ppm, attributed to the protons in the γ position of the pyridine, Hc, and a singlet around 6.62 ppm, attributed to the four aryl protons of mesityl groups. Cooling the sample to 233 K slows the dynamic process, allowing the peaks of the bound and unbound imine arms to be resolved. Specifically, the imino proton of the bound arm (Ha) appears as a sharp singlet at 9.43 ppm, and the imino proton of the unbound arm (Hb) resonates at 8.93 ppm. Pyridine β protons (3′- and 5′-positions, He and Hd) are observed as two sharp doublets, at 9.10 and 6.40 ppm. Finally, the protons of the bound N-Mes appear as two singlets, while the protons of the unbound N-Mes appear as a single sharp singlet. The overall number of the peaks is consistent with the C2 symmetry observed in the solid state. Heating the sample to higher temperatures (i.e., 348 K) equilibrates the imino and N-Mes protons that appear as a sharp singlet each (relative intensities 2 and 4). The equilibration likely proceeds through a coordination of the “unbound” arm to the metal, followed by the detachment of the “bound” arm. A ΔG⧧ value of approximately 14 kcal mol−1 was obtained from the coalescence temperature of the imine peaks.22 As the direct reaction of [NNN] ligands with Ni(COD)2 did not allow the isolation of non-bis(homoleptic) complexes, we turned to the investigaion of the reduction of (Ni[NNN])2+ complexes as precursors to (Ni[NNN])0. Treatment of 1a-Br2

Figure 9. X-ray structure of complex 3a-DPA, with 50% probability ellipsoids. Two independent molecules are observed in the asymmetric unit displaying similar structures.

Figure 10. X-ray structure of complex 3b, with 50% probability ellipsoids. The molecule exhibits crystallographic C2 symmetry with only half the molecule occupying an asymmetric unit. The cocrystallized solvent molecule (hexane) and H atoms are omitted for clarity.

consistent with the bidentate coordination of bis(imino)pyridine. We conclude that the bidentate coordination of [NNN] to the reduced nickel does not depend on the sterics of the ligand but is likely determined by the favorable electronic structure of the Ni([NN])2 fragment, which allows for the delocalization of the reducing equivalents into both iminopyridines.13a,c Comparative examination of the structures of 3a, 3a-COD, 3b, and 3a-DPA demonstrates that there is a significant difference in the corresponding C−C and CN bond distances between bound and unbound iminopyridines, signifying a monoreduced state of nickel-bound iminopyridines vs neutral unbound iminopyridines.9,13 Table 2 gives the 588


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Organometallics

Figure 11. VT NMR spectra of compound 3b (in toluene-d8). Only the aromatic region (6.3−9.5 ppm) is presented; full spectra are given in the Supporting Information. Signals marked with an asterisk belong to the toluene solvent.

with 2 equiv of KC8 produced compound 4a (Scheme 1), which was isolated as black crystals by recrystallization from hexane. Thus, chemical reduction of (Ni[NNN]a)2+ leads to a different product than the direct reaction of Ni(0) with the ligand precursor. In contrast, two-electron reduction of 1b produced again bis(homoleptic) 3b. Treatment of complexes 1c-Br2, 1d-Br2, and 1e-Br2 with 2 equiv of KC8 led to the formation of dark solutions from which no isolable products could be obtained. Compound 4a is a dimer of “Ni[NNN]” units, in which each bis(imino)pyridine ligand binds via an iminopyridine chelate on one side to one nickel and via an η2 imino bond on the other side to another nickel. The schematic representation of the structure of 4a is given in Figure 12A; the ORTEP plot of 4a is given in Figure S2 in the Supporting Information. A similar dimeric structure with the bis(ketimino)pyridine ligand was reported by Budzelaar, Gambarotta, and co-workers (Figure 12B).11 The chief difference between the previously reported structure and the present structure is that the bis(ketimino)pyridine ligand undergoes chemical modification via dimerization of imino CH2 groups, whereas bis(aldimino)pyridine [NNN]a appears unmodified in the structure of 4a. The overall coordination around nickel centers in 4a is similar to that in the Budzelaar/Gambarotta structure and features distorted-squarepyramidal metal geometry: the dihedral angles between Ni1N3C1/Ni2C2N6 and Ni1N1N2/N2N4N5 planes are 157 and 161°. The side-on-bound imino groups demonstrate

Figure 12. (A) Structure of 4a, obtained by the reduction of 1a-Br2 with 2 equiv of KC8. (B) Structure of the bis(ketimino)pyridine “analogue” of 4a featuring a C−C bond between deprotonated iminomethyl groups.11 Structural differences between the two complexes are highlighted.

significant elongation of CN bonds, 1.37(2) and 1.41(2) Å, consistent with previous reports of nickel-bound η2 imino groups.23 The 1H NMR spectrum of demonstrated effective C2 symmetry in solution, presenting one signal for the imino protons at 8.59 ppm and the signal for the “nickel aziridine” proton at 5.41 ppm. Following the synthesis of compound 4a, we investigated its reactivity with CS2 and CO2. The reactivity with CS2 was conducted in toluene-d8. The addition of 1 equiv of CS2 to a black solution of 4a produced a blue solution and a black 589


Organometallics precipitate. The NMR of the soluble phase demonstrated the presence of the free ligand; we were unable to determine the nature of the black precipitate. No reaction with CO2 was observed upon treatment of the toluene solution of 4a with a stoichiometric amount of CO2 or stirring of 4a under a CO2 atmosphere for 1 h. (Ni[NNN])− Oxidation State: Investigation of the Reactivity of Ni[NNN]a−e Species under CV Conditions. Ni[NNN]+ and Ni[NNN]0 failed to react with CO2. However, CV experiments presented in Figure 2 demonstrate that Ni[NNN] complexes, at least in the case of the electron-rich bis(aldimino)pyridines, are capable of reaching lower oxidation states. Along these lines, we have recently reported that threeelectron reduction, via three sequential one-electron reductions, of Ni[NNN]aX2 leads to the formation of a hydrogen evolution catalyst that is activated in the presence of CO2.15 Herein, we decided to investigate whether other complexes would be also capable of similar reactivity in the (Ni[NNN])− state. To answer this question, we conducted electrocatalysis experiments under 1 atm of CO2. These results clearly indicated that 1b-Br2, featuring comparably bulky and electron-rich mesityl groups, also demonstrates electrocatalytic activity. A turnover frequency of 21 s−1 (icat/ip = 5.2 at v = 100 mV s−1) was exhibited by 1bBr2, which is a slight improvement over that observed by 1aBr2 at 2.9 s−1. In contrast, no activity was observed for the complexes 1c-Br2, 1d-Br2, and 1e-Br2. The CV trace of 1b-Br2 recorded under 1 atm of CO2 is plotted in Figure 13 and is

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SUMMARY AND CONCLUSIONS

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EXPERIMENTAL SECTION

This work has focused on the comparative investigation of the oxidation-state-dependent coordination chemistry and reactivity of bis(aldimino)pyridine nickel complexes with the related bis(ketimino)pyridine nickel complexes previously reported by others. Electrochemistry demonstrated that at least four different oxidation states were accessible in bis(aldimino)pyridine [Ni(NNN)] systems. Complexes in three different oxidation states, [Ni(NNN)]2+, [Ni(NNN)]+, and [Ni(NNN)]0, were isolated and characterized. While we were not able to isolate [Ni(NNN)]− species, they displayed electrocatalytic CO2/H+ reactivity. Relatively insignificant differences were observed between bis(aldimino)pyridine and bis(ketimino)pyridine nickel complexes in the [Ni(NNN)]2+ state. Upon attainment of the [Ni(NNN)]+ state, however, the coordination chemistry of bis(aldimino)pyridines and bis(ketimino)pyrides diverges significantly. Previous studies have demonstrated that, in bis(ketimino)pyridine complexes, the [Ni(NNN)]+ state features a ligand radical anion ligated to the high-spin nickel(II) ion. In contrast, in the bis(aldimino)pyridine system, the [Ni(NNN)]+ state contains a significant unpaired spin density at the metal center, as demonstrated by X-ray crystallography, EPR spectroscopy, and DFT calculations. Furthermore, the reactivity observed at the [Ni(NNN)]+ state was primarily metal based for the bis(aldimino)pyridine ligands, as opposed to being ligand based for the bis(ketimino)pyridine ligands. Rather surprisingly, the potentially tridentate bis(aldimino)pyridine ligand was found to bind in a bidentate fashion at the [Ni(NNN)]0 state, with one imino arm left unbound, under all reaction conditions attempted. Again, this finding contradicts previous findings for the bis(ketimino)pyridine nickel complexes, in which the [NNN] ligand was found to coordinate nickel in a tridentate fashion. Overall, our study demonstrates that bis(aldimino)pyridines are less potent as “redox-active” ligands. As a result, they bind transition metals significantly more weakly and undergo facile demetalation. On the other hand, these ligands are less prone to the decomposition pathways which originate in the radical character of the imino positions and lead instead to nickelbased chemistry with small molecules that may be beneficial for catalyst development. Our future work will focus on attempts to isolate [Ni(NNN)]− species, and we will interrogate their stoichiometric reactivity.

General Considerations. All air-sensitive compounds were made in a nitrogen-filled glovebox. [NNN]a, [NNN]b, 1a-Cl2, 1a-Br2, 2a-Cl, and 2a-Br were prepared as previously described.14,16 NiBr2(DME), NiCl2(DME), Ni(COD)2, TlPF6, CoCp*2, KC8, and all anilines were purchased from Aldrich and used as received. 2,6-Pyridinedicarboxaldehyde was purchased from TCI Chemicals and used as received. All solvents were purchased from Fisher and were of HPLC grade. Solvents were purified using an MBRAUN purification system and stored over 3 Å molecular sieves. NMR spectra and magnetic susceptibility measurements were recorded at the Lumigen Instrument Center (Wayne State University) on a Varian 400 MHz NMR spectrometer at room temperature in C6D6, C7D8, C4D8O, or CD2Cl2. Elemental analysis was conducted by Midwest Microlab, LLC. Our attempts to obtain satisfactory elemental analysis for 2b-Br and 3a were not successful due to the sensitivity of these compounds. Electrochemical measurements were recorded on a CH620E instrument (see the Supporting Information for further details). X-ray Crystallographic Details. The structures of 1d-Br2, 2b-Br, 3a-COD, 3a, 3b, 3a-DPA, and 4a were determined by X-ray

Figure 13. Electrocatalytic reactivity of bis(aldimino)pyridine nickel complexes 1a-Cl2, 1a-Br2, and 1b-Br2 recorded under 1 atm of argon and 1 atm of CO2.

compared with the CV traces of 1a-Cl2 and 1a-Br2. We conclude that bulky N substituents are necessary for the electrocatalytic reactivity of bis(aldimino)pyridine nickel complexes. Consistent with our previous bulk electrolysis studies on 1a-Cl2 and 1a-Br2, the mesityl system 1b-Br2 also displays a high selectivity for H2 evolution even under 1 atm of CO2. 590


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Organometallics crystallography. The structures of 1a-Br2, 1a-Cl2, 2a-Cl, and 2a-Br were previously reported.14 A Bruker APEXII/Kappa three-circle goniometer platform diffractometer with an APEX-2 detector was used for data collection. A graphic monochromator was employed for wavelength selection of the Mo Kα radiation (λ = 0.71073 Å). The data were processed, and the structure was solved using the APEX-2 software supplied by Bruker-AXS. The structure was refined by standard difference Fourier techniques with SHELXL.24 Hydrogen atoms were placed in calculated positions using a standard riding model and refined isotropically; all other atoms were refined anisotropically. The structure of 1d-Br2 contained three CH3CN molecules in the asymmetric unit, in addition to one acetonitrile coordinated to the metal. The structure of 2b contained two THF molecules (one disordered over two conformations) and one ether molecule in each asymmetric unit. The structure of 3a contained one ether molecule per symmetric unit. The structure of 3a-DPA contained two ether molecules per asymmetric unit; one ether was fully refined, while the other was found to be disordered over multiple (at least three) conformations and therefore was not further refined. The structure of 4a contained one hexane molecule per asymmetric unit that was found to be disordered over multiple conformations and therefore was also not refined further. We also note that the structure of 4a was of overall low quality and is mostly used to demonstrate connectivity. Structure-specific data collection and refinement information is given in Table S1 in the Supporting Information. Synthetic Procedures. [NNN]c. To a solution of 2,6-pyridinedicarboxaldehyde (500 mg, 3.70 mmol) in 5 mL of toluene was added 2 drops of glacial acetic acid, followed by p-anisidine (911 mg, 7.40 mmol) in 3 mL of toluene. The solution immediately turned cloudy yellow. After 24 h, the solution was filtered, and the resulting solid was washed with toluene. The solid was collected and dried, giving the product as a yellow solid (1.00 g, 2.4 mmol, 66%). 1H NMR (400 MHz, C6D6): δ 8.86 (s, 2H, imino-H), 8.31 (d, J = 8 Hz, 2H, py-β-H), 7.30 (d, J = 8 Hz, 4H), 7.17 (t, J = 8 Hz, 1H, py-γ-H), 6.75 (d, J = 8 Hz, 4H), 3.27 (s, 6H) ppm. Anal. Calcd for C21H19N3O2: C, 73.03; H, 5.54; N, 12.17. Found: C, 73.04; H, 5.56; N, 12.18. 1b-Br2. To a solution of nickel(II) bromide dimethoxyethane complex (42 mg, 0.135 mmol) in 2 mL of THF was added a solution of [NNN]b (50 mg, 0.135 mmol) in 1 mL of THF. After 1 h, volatiles were removed in vacuo, and the resulting brown solid was washed with 5 mL of ether. The solid was dissolved in a minimal amount of THF, covered with 15 mL of ether, and allowed to stand for 24 h at −30 °C. The solid was collected, washed with ether, and dried, giving the product as a reddish brown crystalline solid (75 mg, 0.128 mmol, 95%). 1H NMR (400 MHz, CD2Cl2): δ 69.10 (2H, Δν1/2 = 92 Hz, pyβ-H), 16.04 (6H, Δν1/2 = 12 Hz), 14.13 (4H, Δν1/2 = 16 Hz), 12.52 (1H, Δν1/2 = 40 Hz, py-γ-H), 12.09 (12H, Δν1/2 = 88 Hz), 1.59 (2H, Δν1/2 = 20 Hz) ppm. μeff (Evans, 400 MHz, CD2Cl2, 298 K): 2.48 μB. Anal. Calcd for C25H27Br2N3Ni: C, 51.07; H, 4.63; N, 7.15. Found: C, 50.69; H, 4.63; N, 6.85. ESI-MS: calcd for [C25H27BrN3Ni]+, 506.0742; found, 506.0714. 1c-Br2. To a solution of nickel(II) bromide dimethoxyethane complex (89 mg, 0.290 mmol) in 2 mL of THF was added a solution of [NNN]c (100 mg, 0.290 mmol) in 1 mL of THF. After 1 h, volatiles were removed in vacuo, and the resulting brown solid was washed with 5 mL of ether. The solid was dissolved in a minimal amount of THF, covered with 15 mL of ether, and allowed to stand for 24 h at −30 °C. The solid was collected, washed with ether, and dried, giving the product as a reddish brown crystalline solid (149 mg, 0.264 mmol, 91%). 1H NMR (400 MHz, CD2Cl2): δ 61.93 (2H, Δν1/2 = 140 Hz, py-β-H), 10.77 (1H, Δν1/2 = 36 Hz, py-γ-H), −6.75 6H, Δν1/2 = 8 Hz), −7.00 (4H, Δν1/2 = 16 Hz), −10.13 (2H, Δν1/2 = 8 Hz), −10.50 (4H, Δν1/2 = 24 Hz) ppm. μeff (Evans, 400 MHz, CD2Cl2, 298 K): 2.50 μB. Anal. Calcd for C21H19Br2N3NiO2·CH3CN: C, 45.66; H, 3.67; N, 9.26; Found: C, 45.19; H, 3.52; N, 9.10. ESI-MS: calcd for [C27H19N3NiO2·3CH3CN]2+, 263.0814; found, 263.0454. 1d-Br2. To a solution of 2,6-pyridinedicarboxaldehyde (50 mg, 0.370 mmol) in 5 mL of THF was added 2 drops of glacial acetic acid, followed by 4-trifluoromethylaniline (0.093 mL, 0.740 mmol). After the reaction mixture was stirred for 1 h, nickel(II) bromide

dimethoxyethane complex (114 mg, 0.370 mmol) in 2 mL of THF was added. The reaction mixture was stirred at room temperature. After 24 h, volatiles were removed in vacuo, and the resulting reddish brown solid was washed with 5 mL of ether. The solid was dissolved in a minimal amount of CH3CN, covered with 15 mL of ether, and allowed to stand for 24 h at −10 °C. The solid was collected, washed with ether, and dried, giving the product as a reddish brown crystalline solid (227 mg, 0.355 mmol, 96%). 1H NMR (400 MHz, CD2Cl2): δ 60.67 (2H, Δν1/2 = 136 Hz, py-β-H), 10.96 (1H, Δν1/2 = 40 Hz, py-γH), −6.76 (2H, Δν1/2 = 20 Hz), −10.20 (4H, Δν1/2 = 92 Hz) ppm. 19 F NMR (400 MHz, CD2Cl2): δ −50.53 ppm. μeff (Evans, 400 MHz, CD2Cl2, 298 K): 2.11 μB. Anal. Calcd for C21H13Br2F6N3Ni·3H2O: C, 36.35; H, 2.76; N, 6.06. Found: C, 35.95; H, 2.27; N, 5.90. ESI-MS: calcd for [C21H13F6N3Ni·3CH3CN]2+, 301.0582; found, 301.0779. 1e-Br2. To a solution of 2,6-pyridinedicarboxaldehyde (50 mg, 0.370 mmol) in 5 mL of THF was added 2 drops of glacial acetic acid, followed by 3,5-bis(trifluoromethyl)aniline (0.066 mL, 0.740 mmol). After the reaction mixture was stirred for 1 h, nickel(II) bromide dimethoxyethane complex (114 mg, 0.370 mmol) in 2 mL of THF was added. The reaction mixture was stirred at room temperature. After 24 h, volatiles were removed in vacuo, and the resulting reddish brown solid was washed with 5 mL of ether. The solid was dissolved in a minimal amount of CH3CN, covered with 15 mL of ether, and allowed to stand for 24 h at −10 °C. The solid was collected, washed with ether, and dried, giving the product as a reddish brown crystalline solid (215 mg, 0.277 mmol, 75%). 1H NMR (400 MHz, CD2Cl2): δ 71.80 (2H, Δν1/2 = 132 Hz, py-β-H), 23.07 (1H, Δν1/2 = 52 Hz), 7.02 (2H, Δν1/2 = 36 Hz), 3.69 (2H, Δν1/2 = 80 Hz), 1.94 (4H, Δν1/2 = 104 Hz) ppm. 19F NMR (400 MHz, CD2Cl2): δ −63.64, −65.19 ppm. μeff (Evans, 400 MHz, CD2Cl2, 298 K): 2.88 μB. Anal. Calcd for C23H11Br2F12N3Ni: C, 35.61; H, 1.43; N, 5.42. Found: C, 34.91; H, 1.64; N, 5.16. ESI-MS: calcd for [C23H11F12N3Ni·3CH3CN]2+, 369.0456; found, 369.0189. [1a-Cl](PF6). To a suspension of thallium hexafluorophosphate (TlPF6; 18 mg, 0.051 mmol) in CH2Cl2 (1 mL) was added an orange solution of Ni[NNN]Cl2 (30 mg, 0.051 mmol) in CH2Cl2 (2 mL). The reaction mixture turned red. The reaction mixture was stirred at ambient temperature for 1 h and filtered, and the solvent was evaporated. The resulting orange solid was dissolved in CH2Cl2 (ca. 2 mL) covered in hexanes (ca. 10 mL). The solvent was decanted, and the solid was dried to give the product in 74% yield (26 mg, 0.038 mmol). Anal. Calcd for C31H39ClF6N3NiP: C, 53.75; H, 5.67; N, 6.07. Found: C, 53.13; H, 5.40; N, 5.95. 1H NMR (400 MHz, CD2Cl2): δ 8.53 (t, J = 8 Hz, 1H, py-γ-H), 8.28 (br s, 2H), 7.94 (br s, 2H), 7.32 (br s, 2H, N-(Ar-p-H)), 7.18, (br s, 4H, N-(Ar-m-H)), 3.44 (br s, 4H, Ar−CH(CH3)2), 1.42 (br s, 12H, Ar−CH(CH3)2), 1.25 (br s, 12H, Ar−CH(CH3)2). 19F NMR (400 MHz, CD2Cl2): δ −72.22 ppm. 2b-Br. To an orange solution of 1b-Br2 (142 mg, 0.241 mmol) in THF (3 mL) was added a suspension of KC8 (33 mg, 0.241 mmol) in THF (1 mL). The reaction mixture turned dark purple. The reaction mixture was stirred at ambient temperature for 1 h, filtered, and evaporated. The resulting brown solid was washed with hexanes and ether and dissolved in THF (ca. 1 mL). The solution was covered with 15 mL of hexanes and allowed to stand for 24 h at −30 °C. The solid was collected, washed with hexanes, and dried, giving the product as a brown crystalline solid (73 mg, 0.143 mmol, 60%). 2b-Br is highly airand moisture-sensitive. μeff (Evans, 400 MHz, C6D6, 298 K): 1.71 μB. Absorption spectrum (THF): λmax (εM) 335 (943), 531 (1935), 703 (1010), 939 (1060) nm. X-ray-quality crystals for the X-ray data collection were obtained by vapor diffusion of ether into a saturated THF solution. 3a. To a yellow solution of Ni(COD)2 (15 mg, 0.055 mmol) in THF (2 mL) was added a yellow solution of [NNN]a (50 mg, 0.110 mmol) in THF (3 mL). The reaction mixture slowly turned bluegreen. The reaction mixture was stirred at ambient temperature for 24 h, filtered, and evaporated. The resulting blue-green solid was dissolved in ether (ca. 2 mL) and allowed to stand for 24 h at −30 °C, which led to the formation of dark green crystals. The crystals were dried to give the product in 56% yield (30 mg, 0.031 mmol). 3a is air and moisture sensitive. 1H NMR (400 MHz, C6D6): δ 10.03 (s, 591


Article

Organometallics 2H, imino-H), 9.19 (s, 2H, imino-H), 9.08 (d, J = 7 Hz, 2H, py-β-H), 7.67 (t, J = 6 Hz, 2H, py-γ-H), 7.03 (m, 12H, N-Ar), 6.97 (t, J = 7 Hz, 2H, py-γ-H), 6.41 (d, J = 7 Hz, 2H, py-β-H), 5.11 (sept, J = 7 Hz, 2H, Ar-CH(CH3)2), 3.07 (sept, J = 7 Hz, 4H, Ar-CH(CH3)2), 2.22 (sept, J = 7 Hz, 2H, Ar-CH(CH3)2), 1.20 (d, J = 7 Hz, 12H, Ar-CH(CH3)2), 1.17 (d, J = 7 Hz, 12H, Ar-CH(CH3)2), 0.98 (d, J = 7 Hz, 6H, ArCH(CH3)2), 0.93 (d, J = 7 Hz, 6H, Ar−CH(CH3)2), 0.90 (d, J = 7 Hz, 6H, Ar−CH(CH3)2), 0.51 (d, J = 7 Hz, 6H, Ar-CH(CH3)2). X-rayquality crystals for the X-ray data collection were obtained by crystallization from ether at −30 °C. 3a/3a-COD. To a yellow solution of Ni(COD)2 (15 mg, 0.055 mmol) in THF (2 mL) was added a yellow solution of [NNN]a (25 mg, 0.055 mmol) in THF (3 mL). The reaction mixture slowly turned blue. The reaction mixture was stirred at ambient temperature for 1 h, filtered, and evaporated. The resulting blue solid was dissolved in hexanes (ca. 2 mL) and allowed to stand for 24 h at −30 °C, which led to the formation of dark green-black crystals. Characterization of the crystals by XRD and 1H NMR demonstrated the presence of both 3a and 3a-COD. 1H NMR (400 MHz, toluene-d8): δ 11.65 (2H, 3aCOD, imino-H), 10.00 (2H, 3a), 9.13 (2H, 3a), 9.03 (2H, 3a), 8.68 (2H, 3a-COD), 8.63 (2H, 3a-COD), 8.47 (2H, 3a-COD), 8.25 (2H, 3a-COD), 7.67 (2H, 3a), 7.45 (2H, 3a-COD), 7.26 (4H, 3a-COD), 6.77 (2H, 3a-COD), 6.45 (2H, 3a), 5.53 (Ni(COD)2), 5.06 (2H, 3a), 3.88 (2H, 3a-COD), 3.49 (2H, 3a-COD), 3.26 (2H), 3.08 (4H, 3a), 2.71 (2H, 3a-COD), 2.40 (2H, 3a-COD), 2.20 (Ni(COD)2), 1.30 (br s, 24H, 3a/3a-COD), 1.14 (br s, 48H, 3a/3a-COD), 0.93 (m, 12H, 3a/3a-COD), 0.44 (br s, 6H, 3a). Some signals belonging to 3a (7.03, 12 H; 6.97, 2H; 2.22, 2H), and possibly some signals belonging to 3aCOD are obscured by toluene resonances. X-ray-quality crystals for the X-ray data collection were obtained by crystallization from hexanes at −30 °C. 3b. To a purple solution of 2b-Br (126 mg, 0.248 mmol) in THF (3 mL) was added a suspension of KC8 (34 mg, 0.248 mmol) in THF (1 mL). The reaction mixture turned black. The reaction mixture was stirred at ambient temperature for 1 h, filtered, and evaporated. The resulting black solid was dissolved in ether (ca. 2 mL) and allowed to stand for 24 h at −30 °C, which led to the formation of black crystals. The crystals were dried to give the product in 95% yield (94 mg, 0.118 mmol). 3b is air- and moisture-sensitive. Anal. Calcd for C50H54N6Ni: C, 75.28; H, 6.82; N, 10.54. Found: C, 74.88; H, 7.06; N, 10.35. 1H NMR (500 MHz, toluene-d8, 348 K): δ 9.35 (s, 4H, imino-H), 7.76 (t, J = 7 Hz, 2H, py-γ-H), 6.68 (s, 8H, N-C6Me3H2), 2.16 (s, 12H, Ar-pCH3), 2.01 (s, 24H, Ar-o-CH3), py-β-H signals are not observed at this temperature. 1H NMR (500 MHz, toluene-d8, 233 K): δ 9.43 (s, 2H, imino-H), 9.10 (d, J = 8 Hz, 2H, py-β-H), 8.93 (s, 2H, imino-H), 7.73 (t, J = 8 Hz, 2H, py-γ-H), 6.78 (s, 2H, N-Mes), 6.70 (s, 4H, N-Mes), 6.64 (s, 2H, N-Mes), 6.40 (d, J = 8 Hz, 2H, py-β-H), 2.43 (s, 6H), 2.31 (s, 6H), 2.24 (s, 12H), 2.08 (s, 6H), 1.43 (s, 6H). X-ray-quality crystals for the X-ray data collection were obtained by crystallization from hexanes at −30 °C. Synthesis of 3b via Ni(COD)2. To a yellow solution of Ni(COD)2 (37 mg, 0.135 mmol) in THF (2 mL) was added a yellow solution of [NNN]b (107 mg, 0.289 mmol) in THF (2 mL). The reaction mixture turned dark blue. The reaction mixture was stirred at ambient temperature for 24 h, filtered, and evaporated. The resulting black solid was dissolved in ether (ca. 2 mL) and allowed to stand for 24 h at −30 °C, which led to the formation of black crystals. The crystals were dried to give the product in 55% yield (59 mg, 0.074 mmol). 1H NMR spectrum of the product matched the spectrum of 3b obtained via the reduction of 2b. 3a-DPA. To a yellow solution of Ni(COD)2 (18 mg, 0.066 mmol) in THF (2 mL) was added a yellow solution of [NNN]a (30 mg, 0.066 mmol) in THF (1 mL). Over the course of 5 min, the reaction mixture turned blue. After the color change, a colorless solution of diphenylacetylene (12 mg, 0.066 mmol) in THF (1 mL) was added. The reaction mixture turned blue-green. The reaction mixture was stirred for 1 h, and the solvent was evaporated. The solid was dissolved in hexanes and allowed to stand for 3 days at −30 °C, which led to the formation of green-black crystals. The crystals were dried to give the product in 33% yield (15 mg, 0.022 mmol). 3a-DPA is air- and

moisture-sensitive. Anal. Calcd for C45H49N3Ni: C, 78.26; H, 7.15; N, 6.08. Found: C, 78.08; H, 7.20; N, 5.50. X-ray-quality crystals for the X-ray data collection were obtained by crystallization from ether at −30 °C. The compound demonstrates broad peaks in its 1H NMR spectrum at room temperature (Figure S14 in the Supporting Information) possibly due to the dynamic behavior in solution similar to that of 3b. 4a. To a purple solution of 2a-Br (96 mg, 0.162 mmol) in THF (3 mL) was added a suspension of KC8 (22 mg, 0.162 mmol) in THF (1 mL). The reaction mixture turned black. The reaction mixture was stirred at ambient temperature for 1 h, filtered, and evaporated. The resulting black solid was dissolved in hexanes (ca. 2 mL) and allowed to stand for 24 h at −30 °C, which led to the formation of black crystals. The crystals were dried to give the product ((Ni[NNN])2, 4a) in 60% yield (50 mg, 0.049 mmol). 4a is air- and moisturesensitive. Anal. Calcd for C31H39N3Ni: C, 72.67; H, 7.67; N, 8.20. Found: C, 72.85; H, 7.98; N, 7.95. 1H NMR (400 MHz, THF-d8): δ 8.59 (s, 2H, imino-H), 7.78 (d, J = 8 Hz, 2H, py-β-H), 7.54 (t, J = 7 Hz, 2H, py-γ-H), 7.26 (m, 4H), 7.16 (d, J = 7 Hz, 2H), 7.12 (t, J = 7 Hz, 2H), 6.76 (m, 6H), 5.41 (s, 2H, Ni-C(N)H), 4.35 (m, 6H), 3.09 (m, 2H, Ar-CH(CH3)), 1.46 (d, J = 6 Hz, 6H, Ar-CH(CH3)), 1.23 (d, J = 6 Hz, 6H, Ar−CH(CH3)), 1.00 (m, 12H, Ar-CH(CH3)), 0.96 (d, J = 7 Hz, 6H, Ar-H(CH3)), 0.90 (m, 18 H, Ar-CH(CH3)). X-ray-quality crystals for the X-ray data collection were obtained by crystallization from hexanes at −30 °C.

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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.organomet.6b00793. Crystallographic information for all compounds (CIF) Cartesian coordinates for the calculated structures (XYZ) Crystallographic information for all compounds, ORTEP diagrams of the structures of [Ni(SPh)2]11 and 4a, NMR spectra, mass spectra, and DFT calculation details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.R.: [email protected]. *E-mail for S.G.: [email protected]. ORCID

Richard L. Lord: 0000-0001-6692-0369 Jonathan Rochford: 0000-0003-2397-9162 Stanislav Groysman: 0000-0003-3578-7985 Present Address ∥

Olivet College, 320 S. Main St., Olivet, MI 49076, USA.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.G. thanks Wayne State University for initially supporting this project and the National Science Foundation for current support under grant number CHE-1349048. J.R. thanks the National Science Foundation for support under grant number CHE-1301132. Computational resources were provided by NSF-MRI award #CHE-1039925 through the Midwest Undergraduate Computational Chemistry Consortium.

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REFERENCES

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Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel

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