The 4-Electron Cleavage of a N N Double Bond by a Trimetallic TiNi2

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

The 4‑Electron Cleavage of a NN Double Bond by a Trimetallic TiNi2 Complex Peter L. Dunn,†,§ Sudipta Chatterjee,‡,§ Samantha N. MacMillan,‡ Adam J. Pearce,† Kyle M. Lancaster,*,‡ and Ian A. Tonks*,† †

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Department of Chemistry, University of MinnesotaTwin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States ‡ Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: The synthesis and reactivity of a new trimetallic complex Ti(NP)4Ni2 (NP = 2-diphenylphosphinopyrrolide) (3) is reported. Single-crystal X-ray diffraction and X-ray absorption studies point to a unique bonding motif: a d10−d10, Ni0−Ni0 bond stabilized by a proximal d0 TiIV metal center. The coordination chemistry of 3 with a variety of L (L = isocyanide and alkyne) donors has also been explored. In the case of isocyanide coordination, the Ni−Ni bond is broken, while diphenylacetylene binding results in a symmetric butterfly μ2-κ2-alkyne bridge across the Ni−Ni moiety. Finally, complex 3 is capable of the 4-electron cleavage of the NN double bond in benzo[c]cinnoline, the first example of NN bond cleavage by Ni. The resulting product, 7, has been characterized structurally and spectroscopically, and the mechanistic implications are discussed in the context of metal−metal cooperativity.

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INTRODUCTION Multielectron transformations underpin numerous key chemical processes. For example, both biological and industrial nitrogen fixation require efficient, sequential redox events.1 Biological nitrogen fixation, as well as many other important biological redox processes, are catalyzed by protein-supported metal clusters. Such metallo-cofactors afford multiple potential substrate binding sites and provide the possibility of multiple loci participating in redox. This observation inspired a “polynuclear hypothesis”: the idea that multiple metal centers can work in conjunction to facilitate redox reactivity for small molecule activation.2 Indeed, synthetic multimetallic clusters show remarkable redox profiles3−10 as well as reactivity. For example, trinuclear Ru complexes can selectively cleave strong CC double bonds11 as well as perform azobenzene cleavage.12 Similarly, other trinuclear species have been shown to perform multielectron transformations such as N2 cleavage.13 In an attempt to exert better tunability and control over these highly reactive clusters, a number of polydentate ligands have been designed.2,14−18 While there is a rich reactivity © XXXX American Chemical Society

landscape spanned by these multinuclear systems across the periodic table, the reactivity of homogeneous multimetallic Ni systems is largely limited to a few intriguing examples (Figure 1). For example, Agapie synthesized a family of LNiINiI (L = 1,4-bis(2-(diisopropylphosphino)phenyl)benzene) complexes containing Ni−Ni bonds that can undergo cooperative reductive and oxidative C−C bond formation across the Ni− Ni moiety.19−21 Additionally, Johnson has synthesized a pentanuclear Ni cluster ligated by hydrides and phosphines that is able to abstract a carbon atom from ethylene as well as perform ultradeep hydrodesulfurization.22−25 Finally, Uyeda synthesized Ni2 complexes supported by a napthyridinediimine framework which span a wide range of formal oxidation states. These complexes have shown remarkable reactivity stemming from the two Ni centers’ ability to react in a cooperative fashion, resulting in regioselective alkyne trimerization, Pauson−Khand cyclizations, hydrosilylation, carbene transfertype chemistry, and nitrene coupling.26−34 Received: June 18, 2019

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DOI: 10.1021/acs.inorgchem.9b01805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. 50% thermal ellipsoid drawing of 3. Select phenyl groups have been reduced to the ipso carbon and hydrogen atoms omitted for clarity. Bond lengths (Å): Ti1−Ni1, 2.634(1); Ti1−Ni2, 2.289(1); Ni1−Ni2, 2.369(1); Ni1−P1, 2.186(1); Ni1−P3, 2.162(2); Ni1−P4, 2.184(2); Ni2−P2, 2.203(1); Ni2−P3, 2.411(1); Ti1−N1, 2.098(4); Ti1−N2, 1.977(4); Ti1−N3, 2.184(2); Ti1−N4, 2.099(5); Ni2− C28, 2.389(4); Ni2−C29, 2.497(6).

Figure 1. Multinuclear Ni complexes exhibiting a range of cooperative reactions.

phosphines bound to Ni1 and only one to Ni2. The lowcoordinate Ni2 is stabilized by an η2−π interaction with a neighboring phenyl group of a Ni1-bound phosphine (Ni2− C28, 2.389(4) Å; Ni2−C29, 2.497(6) Å). The Ni−Ni distance is short, 2.369(1) Å (FSR = 1.03), while the Ti−Ni distances are asymmetric with one long (Ti1−Ni1, 2.634(1) Å; FSR = 1.07) and one short (Ti1−Ni2, 2.289(1) Å; FSR = 0.93) (FSR = formal shortness ratio;37 (M−M distance)/∑(M covalent radii)). For reference, the bimetallic complex 2 contains a Ti Ni double bond (2.2665(5) Å; FSR = 0.91) and the axially ligated CO species (NP)Ti(NP) 3Ni(CO) (TiNi(CO)) contains no metal−metal bond (2.6136(8) Å; FSR = 1.05).35 Despite the asymmetry in the solid state, 3 appears fluxional in C6D6 solution, showing only two broad 31P NMR signals in a 1:1 ratio. The two different resonances likely arise from two pairs of phosphines, where one set is perpendicular to the Ni−Ni moiety (akin to P3 and P4 in the XRD structure) and the other set is trans across the Ni−Ni bond (similar to P1 and P2). Exchange of P3 and P4 between the two Ni centers may be the cause of the observed fluxionality. Variabletemperature 31P NMR experiments were undertaken to lend insight. Initially, cooling the sample sharpens the more upfield resonance into an apparent triplet (JPP = 48 Hz) at −15 °C before both the downfield and upfield resonances broaden into the baseline at −40 and −85 °C, respectively (Figure S3, Supporting Information (SI)). While there are numerous examples of d10−d10 metal−metal interactions in coinage metals, there are significantly fewer with Ni. Complex 3 is among a limited number of formally Ni0−Ni0 bimetallic complexes in the literature that feature short metal− metal distances.7,38−44 The majority of these complexes are supported by bridging ligands that are strong π-acceptors such as CO, alkynes, nitriles, or isocyanides because there is no thermodynamic driving force for the formation of a metal− metal bond between two d10 metal centers. In 3, Ti could be envisioned as the bridging/supporting ligand. Recently, Zhu et al. reported similar formally Ni0 complexes with U2Ni2 and U2Ni3 cores that contain short metal−metal bonds, albeit with significantly longer Ni−Ni distances (2.58−3.30 Å).45 Given the unusually short Ni−Ni bond distance in 3 (Table 1), as well as the different Ti−Ni bond distances, we were interested

Building on these examples of multinuclear Ni chemistry, we report the synthesis, electronic characterization, and reactivity of a trimetallic TiNi2 complex supported by four bifunctional NP (NP = 2-diphenylphosphinopyrrolide) ligands. This molecule contains a formal d10−d10, Ni0−Ni0 bond stabilized by a proximal d0 TiIV metal center. We explore the simple coordination chemistry of this molecule and demonstrate its ability to perform stoichiometric multielectron transformations in the form of the 4-electron cleavage of the NN bond of benzo[c]cinnoline. This, to the best of our knowledge, is the first report of Ni performing NN bond cleavage.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization of 3. Previously, we found that treatment of the metalloligand Ti(NP)4 (1) with 1 equiv of Ni(COD)2 yields the heterobimetallic TiNi lantern complex 2.35,36 Because dative Ti−P bonds are exceptionally labile in these Ti(NP) systems, we envisioned that it would be possible for 2 to further react with another soft late transition metal through the Ti−P bond. Indeed, treatment with 10 equiv of Ni(COD)2 at 90 °C results in a second metalation event, yielding a new hetero-trimetallic Ti(NP)4Ni2 complex (TiNi2, 3) in modest (32%) yield (Scheme 1). Synthesis of 3 can be accomplished from isolated 2, or directly from exposure of 1 to 10 equiv of Ni(COD)2 at 90 °C. The large excess of Ni(COD)2 is required because it undergoes decomposition at the reaction temperature. The solid-state structure of 3 is presented in Figure 2. Complex 3 is asymmetric in the solid state, with three Scheme 1. Synthesis of TiNi2 (3) from Ti(NP)4 (1)

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absence of any Ni 3d valence vacancies in these complexes, implying the presence of d10 Ni0 centers in both complexes. The overlapping first derivative spectra for complexes 2 and 3 suggest that the charge at Ni (and, hence, the physical oxidation state) remains largely invariant between the two compounds (Figure S6, SI). The pre-edge bands for trimetallic 3 are broadened relative to those of bimetallic 2 (Figure 3a, inset), as would be expected due to the presence of two inequivalent Ni centers in the solid-state structure, shown in Figure 2. The Ti K-edge data are presented in Figure 3b. Considering the +4 oxidation state on Ti for complex 1, complexes 2 and 3 might be presumed to possess more reduced Ti centers as the rising edges appear lower in energy relative to 1. However, differences are predicted in near-edge X-ray scattering given the proximity of a second large Ni scatterer. Moreover, the local geometry about Ti is perturbed. Both effects can confound the relationship between rising edge energy and physical oxidation state.46 Additionally, the electron-rich Ni center(s) could donate electron density to Ti, which would also influence Ti 1s and 4p binding energies (Scheme 2). This

in determining the nature and degree of covalency in these bonds. Table 1. Metal−Metal Distances (Å) and Formal Shortness Ratios (FSRs, in Square Brackets) of Relevant Complexes 35

2

TiNi(CO)35 3 4 5 7

Ti−Ni1

Ti−Ni2

Ni−Ni

2.2665(5) [0.91] 2.6136(8) [1.05] 2.634(1) [1.07] 2.5319(6) [1.02] 2.4749(6) [1.00] 3.065(1) [1.23]

2.289(1) [0.93] 2.4155(6) [0.98] 2.4754(6) [1.00] 2.805(1) [1.13]

2.369(1) [1.03] 2.5276(6) [1.10] 4.0220(5) [1.75] 2.3583(7) [1.026]

X-ray Absorption Spectroscopy. To interrogate the electronic structure and bonding of complex 3, Ni and Ti Kedge X-ray absorption spectra (XAS) were obtained on samples of crystalline 3 pulverized and diluted in BN (Figure 3). To facilitate the interpretation of the data, Ni K-edge XAS

Scheme 2. Hybrid DFT-Calculated Energy Level Diagram (B3LYP, CP(PPP) on Nickel and Titanium; ZORA-def2TZVP(-f) on All Other Atoms) for Titanium K-edge Spectra Showing Progressive Energy Increases in Titanium 1s and 4p upon Addition of Ni0 Centers

latter point is analogous to a situation encountered in the Ti Kedge XAS of TiFe hetero-bimetallic complexes synthesized by Lu and co-workers where Ti−Fe covalency between TiIV and Fe−I/−II centers yielded red-shifted edges.47 Moreover, the Ti K-edges should, in principle, provide some insight into the influence of adding a second Ni center. However, the experimental data alone would seem to imply no effect, as first derivative plots of the Ti K-edge data show that the rising edges for complex 2 and 3 are isoenergetic at 4980.6 eV (Figure S7, SI). Although the origin of the nearly identical Ti K-rising-edge energies of complexes 2 and 3 is initially puzzling, this has been made clear with the help of molecular orbital (MO) calculations (vide infra). Below in Scheme 2, we schematically show that the rising edge energies for complexes 2 and 3 remain roughly the same because successive addition of Ni centers results in an increase of the Ti 1s and 4p energy levels. The values for the different energy levels shown in Scheme 2 have been obtained using theoretical calculations under identical conditions for all of the complexes and discussed in the next section.

Figure 3. Overlaid experimental (a) Ni K-edge and (b) Ti K-edge XAS spectra for hetero-bimetallic complexes 1 (gray), 2 (red), and 3 (black). Insets shown in panels a and b are the zoomed-in pre-edge regions of the experimental Ni and Ti K-edge XAS, respectively.

for hetero-bimetallic complex 2 and Ti K-edge XAS for complexes 1 and 2 were also obtained. The Ni K-edge X-ray absorption near-edge structure (XANES) feature intense and well-resolved pre-edge peaks for both complexes 2 (8332.8 eV) and 3 (8331.7 and 8332.5 eV), attributable to dipole allowed 1s → 4p transitions (Figure 3a). Peak energies were obtained via analysis of second derivative plots of the Ni K-edge XAS (Figure S5, SI). No low-energy pre-edge features, attributable to 1s → 3d excitations, could be resolved. This suggests the C


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Inorganic Chemistry DFT Calculations: MO Analysis of the Bonding in 3. To facilitate interpretation of the XAS data and to help define the bonding in 3, frontier MO diagrams were generated for 1− 3 via single-point DFT calculations using crystallographic coordinates where the phenyl rings of all of the complexes have been truncated to H atoms (Figure 4 and Figures S8−S10, SI).

core and valence) are expected to be raised to higher energy with the consecutive addition of d10 Ni centers to complex 1. As seen in Scheme 2, upon going from complex 2 to complex 3, Ti 1s and 4p are raised in energy to similar degrees (ΔE = 4932.8 vs 4932.7 eV, respectively), in accord with the invariant rising edge inflection points in the Ti K-edge XAS of both complexes. Given the lower binding energy of Ti 3d, the net effect is stabilization of the Ni−Ni-based σ and σ* MOs. Stabilization of these orbitals by Ti 3d admixture results in net σ-bonding in a manner analogous to p/d mixing giving net bonding in d8−d8 dimers.48 Meanwhile, the short Ti−Ni (P1Ni) distance of 2.289(1) Å, such as the Ti−Ni distance (2.2665(5) Å) in complex 2, can be attributed to the additional bonding overlap between Ti and Ni via the π-bonding interaction in MO no. 126. The MO diagram of complex 3 does not reveal any significant bonding interaction between P3Ni and Ti, which is consistent with the long Ti−Ni distance (2.6344(1) Å) observed in the crystal structure. On the basis of the predicted electronic structure, no low-energy Ni 1s → 3d pre-edge transition should be observed, which accords with the experimental data and is supported by the fit of the calculated XAS spectra (vide infra). Time-Dependent DFT Calculations: XAS Assignments. Time-dependent DFT (TDDFT) calculations are now a well-established means to simulate metal K-edge XAS, affording a convenient means to assign spectra.46,47,49,50 Moreover, agreement between calculated and experimental XAS (following energy corrections to account for poor corepotential modeling by DFT) lend support for electronic structure descriptions obtained from DFT. Experimental XAS data for compounds 1−3 are well-reproduced using TDDFT initiated from the aforementioned hybrid DFT single-point calculations, supporting the proposed bonding picture discussed above (Figure 5a and Figures S11 and S12, SI). The calculated Ni K-edge XAS of complex 3, provided in Figure 5a, shows excitations from Ni 1s to mainly Ti 3d-based antibonding orbitals that have significant contributions from Ni 4p (see Table S3). The broad feature in the experimental data for 3 is reproduced by calculations, and it corresponds to two distinct pre-edge peaks at 8331.9 and 8333.1 eV, separated by 1.2 eV, presented in Figure 5a. Deconvolution of the calculated spectrum clearly shows that these two energetically different peaks arise due to the presence of two electronically inequivalent Ni centers in complex 3 (Figure 5b). This is readily attributed to the difference in electron densities on Ni manifested by unique coordination environments about Ni. While the low-energy feature (8331.9 eV) arises due to the transition from P1Ni, the high-energy feature (8333.1 eV) dictates transition from P3Ni. This less intense pre-edge peak at lower energy for P1Ni is due to the transition to a low-lying orbital with less Ni 4p character (4.1% 4p; MO no. 142) compared to P3Ni which has significant contribution from MO no. 144 with 19.5% Ni 4p (see Figure 5c,d and Table S3). Therefore, the presence of more Ni 4p character of the P3Ni center in MO no. 144 enhances the probability of an allowed 1s to 4p transition compared to the less Ni 4p character of the P1Ni center in MO no. 142, ultimately resulting in the increase of the calculated pre-edge peak intensity at higher energy (8331.1 eV), as observed from the fit of the deconvoluted spectra (red trace in Figure 5b). The most intense peak in the calculated spectrum, falling in the experimental rising edge region, corresponds to transitions to higher energy molecular

Figure 4. (a) Abbreviated frontier MO diagram of 3 showing MOs germane to metal−metal bonding. Orbitals and energies were obtained via hybrid DFT using the B3LYP functional. The CP(PPP) basis set was used on Ti and Ni atoms, while the ZORA-def2-TZVP(f) basis set was used on all other atoms. Orbitals are plotted at an isolevel of 0.03 au. Nonbonding (both occupied and unoccupied) Ni and Ti 3d orbitals are shown in the SI (Figure S8). (b) Pictorial MO representation of σ and π bonds between the three metal centers.

Constrained geometry optimizations of H atoms’ positions were carried out using the BP86 functional on these truncated structures (see Experimental Section for further details). These calculations employed the B3LYP hybrid density functional and the scalar relativistically recontracted ZORA-def2-TZVP(f) basis set on all atoms except for Ti and Ni where CP(PPP) basis set was used in all calculations. Lö wdin orbital coefficients for the metal-based orbitals are provided in Tables S1−S3, SI. The MO diagram for 3 indicates the presence of two metal−metal σ bonds (one between P3Ni and P1Ni and the other between P1Ni and Ti) and one π bond (between P1Ni and Ti). Table S3 shows the contribution of ca. 10% Ti 3d character in the Ni−Ni bonding MO (MO no. 123; Figure 4). A dπ−dπ interaction is operative only between P1Ni and Ti in MO no. 126 (Figure 4) despite having notable Ni 3d character on P3Ni (ca. 15.4%). The remaining occupied Nibased valence MOs are nonbonding with respect to either of the Ni centers, except for MO no. 135 which shows σ* interaction between the two Ni centers (Figure 4 and Figure S8, SI). Both Ni−Ni σ and σ* orbitals mix with Ti 3d. As a result of this addition of electron density to Ti, Ti orbital energies (both D


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Inorganic Chemistry

Figure 5. (a) Comparison of the experimental (solid line) and TDDFT-calculated (dotted line) Ni K-edge spectra of complex 3. Sticks under calculated spectrum represent transitions from Ni 1s orbitals. (b) Deconvoluted Ni K-edge XAS spectra for the two different Ni centers in 3. (c, d) Transitions involved in the pre-edge region for complex 3. (e) Acceptor MOs during transitions are shown in this panel with Löwdin orbital composition.

orbitals dominated by ligand character (MO nos. 146 and 147). Initial Reactivity Studies. Compound 3 was treated with several L-donor ligands (Scheme 3). Reaction of 3 with Scheme 3. Reaction of 3 with L-Donor Ligands

Figure 6. 50% thermal ellipsoid drawings of 4 with side-on (left) and front (right) views. Phenyl groups on P have been reduced to the ipso carbon and hydrogen atoms omitted for clarity. Bond lengths (Å): Ti1−Ni1, 2.5317(6); Ti1−Ni2, 2.4155(6); Ni1−Ni2, 2.5276(6); Ti1−N1, 2.058(2); Ti1−N2, 2.051(2); Ti1−N3, 2.088(2); Ti1−N4, 2.076(2); Ni1−P2, 2.2530(8); Ni1−P4, 2.2449(8); Ni2−P1, 2.2793(9); Ni2−P3, 2.2418(8); Ni1−C65, 1.971(3); Ni1−C72, 1.946(3); Ni2−C65, 1.953(3); Ni2−C72, 1.987(3); C65−C72, 1.326(4).

In contrast, reaction of 3 with 2 equiv of CNtBu rearranges the trimetallic core, yielding Ti((μ2-NP)2Ni(CNtBu))2 (5) (Scheme 3). The solid-state structure of 5 is presented in Figure 7. The trimetallic core has been split by coordination of one isocyanide to each Ni, resulting in complete cleavage of the Ni−Ni bond as shown by the ca. 4 Å Ni−Ni distance. The Ni centers are equivalent by NMR and XRD analysis and are each ligated by two phosphines and a bridging pyrrolide (Ti1− N3, 2.104(2) and Ti1−N4, 2.103(2) Å; Ni1−N3, 2.070(2) and Ni2−N4, 2.061(2) Å). This results in a pseudo-trigonal bipyramidal Ni that is capped by an isocyanide. The Ti−Ni distances are 2.4749(6) and 2.4754(6) Å (FSR = 1.005), consistent with a single bond. We next treated 3 with various nitrene donors in order to probe its ability to perform multielectron transformations. Reaction of 3 with adamantyl azide resulted in decomposition,

PhCCPh at 90 °C resulted in formation of the bridging alkyne complex, Ti(NP)4Ni2(PhCCPh) (4). The solid-state structure of 4 is presented in Figure 6. Complex 4 maintains a triangular trimetallic core while binding the alkyne perpendicular to the Ni−Ni bond, resulting in two equivalent Ni centers. The C−C bond length of the alkyne increases from 1.194(4) to 1.328(4) Å and shows deviation from planarity due to back-bonding from the two adjacent Ni0 centers, as has previously been observed in other Ni2 alkyne-bridged systems.51,52 The Ni−Ni bond length increases to 2.5273(6) Å in complex 4 from 2.369(1) Å in complex 3. The Ti−Ni distances are more symmetric than the parent structure with values of 2.5319(6) and 2.4152(6) Å (FSR = 1.023 and 0.977). E


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Inorganic Chemistry

Figure 7. 50% thermal ellipsoid drawing of 5. Phenyl groups on P have been reduced to the ipso carbon and hydrogen atoms omitted for clarity. Bond lengths (Å): Ti1−Ni1, 2.4749(6); Ti1−Ni2, 2.4754(6); Ni1−P4, 2.1880(9); Ni1−P2, 2.2176(9); Ni2−P1, 2.2155(9); Ni2−P3, 2.1832(8); Ti1−N1, 2.074(2); Ti1−N2, 2.075(2); Ti1−N3, 2.103(2); Ti1−N4, 2.104(2); Ni1−N3, 2.070(2); Ni2−N4, 2.061(2).

Figure 8. 50% thermal ellipsoid drawing of 7. Phenyl groups on P have been reduced to the ipso carbon and hydrogen atoms omitted for clarity. Bond lengths (Å): Ti1−Ni1, 3.065(1); Ti1−Ni2, 2.805(1); Ni1−Ni2, 2.3583(7); Ti−N1, 2.145(4); Ti1−N2, 2.018(3); Ti1−N3, 2.113(4); Ti1−N5, 2.085(4); Ti1−N6, 1.915(3); Ni1−P3, 2.166(1); Ni1−N4, 1.874(4); Ni1−N6, 1.837(3); Ni2−P2, 2.226(1); Ni2−P4, 2.235(1); Ni2−N6, 1.920(4); P1−N5, 1.631(1).

as evidenced by crystallographic characterization of an oxidized phosphinimine complex of Ni, Ni(NP(Ph)2NAd)2 (6) (Figure S20, SI). While there was no reaction of 3 with azobenzene, reaction of 3 with benzo[c]cinnoline resulted in the isolation of complex 7 (Scheme 4). The solid-state structure of 7 is

downfield due to phosphine oxidation. On the basis of P−P coupling constants, we tentatively assign the major species to be similar to the crystal structure (where only the phosphines coordinated to Ni2 are coupling), while the minor species, which has 3 coupled P resonances, is likely an isomer where all 3 phosphines are bound to the same Ni. Heating to 100 °C and cooling to −100 °C revealed no interchange between the two species by 31P NMR spectroscopy. N−N bond cleavage reactions have received significant attention due to their intermediacy in N 2 reduction catalysis.53−58 Molecular examples of N−N triple- and double-bond scission reactions are most often reported at early transition metal centers, where the metals are significantly more reducing and formation of strong metal−ligand π bonds acts as a thermodynamic driving force.59−71 Additionally, these metals can occupy a large range of formal oxidation states that can provide the requisite electrons to facilitate these types of transformations at a single metal center. There are significantly fewer reports of these reactions involving late transition metals, and those that do typically involve multiple centers to provide the necessary electrons.72−75 To our knowledge, there are no reports of NN cleavage reactions featuring nickel. We propose that formation of 7 likely proceeds first through binding of benzo[c]cinnoline across the Ni−Ni edge of 3 (Scheme 4) to form a μ−κ1:κ1-bridged azo species such as A. Although DFT calculations indicate that the LUMO of 3 is Tibased (Figure 4), all reactions of π-acceptor ligands with 3 have resulted in binding at Ni. Furthermore, Holland et al. has observed a similar bridged bimetallic benzo[c]cinnoline species upon reaction with low-valent diketiminate Fe complexes that have been shown to fully break other NN bonds.72 NN cleavage could then occur either through (1) complete NN cleavage across the two Ni atoms to form two P2NiII bridging imidos, in analogy to Holland’s Fe azobenzene NN cleavage;73 or (2) Ti-assisted cleavage,68 wherein the formation of a strong Ti−N bond assists in the reduction. In either case, rearrangement to a structure similar to the facially capped μ3-imido intermediate B seems likely en route to the formation of 7. From B, phosphine oxidation by the remaining Ni2 μ2-imido and subsequent NP ligand rearrangement would then lead to 7.

Scheme 4. Synthesis of 7 and Hypothesized Mechanism of Formation

presented in Figure 8. Complex 7 contains the benzo[c]cinnoline moiety; however, the NN bond has been completely cleaved resulting in the formation of a phospinimide (P1−N5, 1.631(3) Å) and a μ3-imido that facially caps all three metal centers (Ti1−N6, 1.915(3) Å; Ni1−N6, 1.837(3) Å; Ni2−N6, 1.920(4) Å). Additionally, one NP ligand has completely migrated from titanium to chelate across the nickel centers. While the triangular metal−metal core is maintained, the Ti−Ni distances have elongated significantly (Ti1−Ni1, 3.065(1) Å; Ti1−N2, 2.805(1) Å) and a short nickel−nickel contact is maintained (Ni1−Ni2, 2.3583(7) Å). At room temperature, the 31P NMR spectrum of 7 consists of only a single resonance with a number of underlying broad features. Cooling the sample reveals two sets of 1:1:1:1 resonances in an approximately 2:1 ratio, consistent with two species each containing 4 inequivalent phosphines. Importantly, each species contains a singlet shifted significantly F


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(solvent) molecule. An additional B alert is present in the structure of 5, likely due to a pseudo-symmetry element. Details regarding refined data and cell parameters are available in Tables S4 and S5.

Unfortunately, we have been unable to isolate/observe the presumed dinickel diimido or Ti imido/Ni imido intermediates. However, we favor these cooperative imido pathways for several reasons: (1) both low-valent Ti and Fe have been shown to cleave the NN bond of azobenzene via cooperative bimetallic mechanisms, while bisphosphine mononickel azo complexes have been isolated but have no reported cleavage reactivity;76 (2) control experiments with complex 2 have shown no reaction with benzo[c]cinnoline at room temperature, indicating both nickel centers are necessary for reactivity; (3) bisphosphine mononickel imidos are known;77,78 and (4) metal-imidos across the periodic table are known to oxidize phosphine ligands.79−81 Due to the lack of useful in situ spectral analysis and the complications of ligand rearrangement, the exact order of operations for NN cleavage remains uncleara more robust ligand system will be needed to fully understand the details of this reaction.

X-RAY ABSORPTION SPECTROSCOPY

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COMPUTATIONAL DETAILS

Sample Preparation and Measurements. Solid samples for Xray spectroscopic analysis were prepared in an inert-atmosphere glovebox. For Ni K-edge XAS measurements, the solid samples were finely ground using an agate mortar and pestle with boron nitride (BN) into a homogeneous mixture comprising 5% (w/w) Ni. These mixtures were pressed into 1 mm Al spacers and sealed with 38 μm Kapton tape. Samples for Ti K-edge XAS were prepared by grinding neat solids to a fine powder and spreading them to vanishing thickness onto 38 μm low-S Mylar tape. XAS data were obtained at the Stanford Synchrotron Radiation Lightsource (SSRL) at beamline 7-3 (Ni) and 4-3 (Ti) under ring conditions of 3 GeV and 500 mA. For Ni, a Si(220) double-crystal monochromator was used for energy selection, while, for Ti, a Si(111) double-crystal monochromator was used. For Ni, a Rh-coated mirror (set to an energy cutoff of 9 keV) was used for harmonic rejection. Energy calibrations were performed by assigning the first inflection points of Ni and Ti foil spectra to 8331.6 and 4966 eV, respectively. Ni K-edge data were obtained with samples maintained at 10 K in an Oxford liquid He flow cryostat. Ti K-edge data were obtained on samples held at ambient temperature within a He atmosphere. Ni K-edge data were obtained in transmission mode using Ar-filled ionization chambers placed before and after the sample. Ti K-edge XAS data were obtained using fluorescence detection measured with a PIPS detector. Data Analysis. Data were collected from 8010 to 8730 eV for Ni and from 4734 to 5360 eV for Ti. Multiple scans were measured and averaged with the SIXPACK88 software package. No spectral changes due to photodamage were observed after multiple scans for these complexes. Data were normalized to postedge jumps of 1.0 (Ti, 4985 eV; Fe, 8350 eV) in SIXPACK by applying a Gaussian normalization for the pre-edge and a quadratic normalization for the postedge to produce the final spectra. The final processed spectra were plotted using Igor Pro 6.37.

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CONCLUSIONS In summary, we report the synthesis of a trimetallic TiNi2 complex supported by phosphinopyrrolide ligands. Electronic structure calculations supported by XAS suggest the presence of two metal−metal bonding interactions: a σ Ni−Ni bonding interaction that manifests via stabilization of the filled Ni d10− d10 σ/σ* manifold by Ti admixture and a Ni−Ti π bond. Remarkably, reaction of the TiNi2 complex with benzo[c]cinnoline results in 4-electron cleavage of the NN double bond, the first reported example of such a cleavage with Ni. This double-bond cleavage may occur through a diimido intermediate or via the assistance of Ti. The synthesis and reactivity of this species should spur further studies using metalloligands to support multinuclear fragments in order to discover and promote unique multielectron reactivity.

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EXPERIMENTAL SECTION

General Considerations and Instrumentation. All air- and moisture-sensitive compounds were manipulated in a glovebox under a nitrogen atmosphere. Solvents for air- and moisture-sensitive reactions were vacuum transferred from sodium benzophenone ketyl (benzene-d6 and toluene-d8) or predried on a Pure Process Technology solvent purification system (hexanes, THF, and benzene). Diphenylacetylene and tert-butyl isocyanide were purchased from Aldrich and used without further purification. Ti(NP)436 and Ni(COD)282 were prepared according to literature procedure. 1H and 31P spectra were recorded on a Bruker Avance III 400 MHz spectrometer. Chemical shifts are reported with respect to residual protiosolvent impurity for 1H (s, 7.16 ppm for C6D5H), 13C (t, 128.06 ppm for C6D5H), and an PPh3 external standard for 31P{1H} (s, −6 ppm for C6D6). X-ray Crystal Data: General Procedure. Crystals were removed quickly from a scintillation vial to a microscope slide coated with oil. Samples were selected and mounted on the tip of a 0.1 mm diameter glass capillary. Data collection was carried out on a Bruker APEX II CCD diffractometer with a 0.71073 Å Mo Kα source. The data intensity was corrected for absorption and decay (SADABS).83 Final cell constants were obtained from least-squares fits from all reflections. Crystal structure solution was done through intrinsic phasing (SHELXT-2014/5),84 which provided most non-hydrogen atoms. Full matrix least-squares/difference Fourier cycles were performed (using SHELXL-2016/6 and GUI ShelXle)85,86 to locate the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. A disordered benzene molecule was removed from the unit cells of 4 and 7 using Platon SQUEEZE.87 A B level alert is present in 6 due to short C−C bonds in a benzene

X-ray Spectroscopy and Molecular Orbital Analysis. Density functional theory (DFT) calculations were performed to facilitate interpretation of XAS data and to interrogate the bonding within the molecules discussed. All electronic structure and spectroscopic calculations were performed using version 3.03 of the ORCA computational chemistry package.89,90 Geometry optimizations were carried out starting from crystal structure coordinates and were performed using the BP8691,92 functional, the zero-order regular approximation for relativistic effects (ZORA) as implemented by van Wüllen93−95 and scalar relativistically recontracted Ahlrich’s def2TZVP(-f)(def2-TZVP(-f)-ZORA)96−98 basis set. Solvation was modeled using the conductor-like screening model (COSMO) using a dielectric of 9.08 (CH2Cl2).99 Geometry optimizations were carried out for all of the complexes in which bulky phenyl rings were truncated to H atoms and only H atom positions of the complexes were optimized (Tables S6−S8, SI). Ni and Ti K-edge XAS spectra were calculated using time-dependent density functional theory (TDDFT) calculations with before-mentioned geometry optimized coordinates utilizing B3LYP functional,100 the CP(PPP)101 basis set on Ni and Ti using an integration grid accuracy of 7, and the def2TZVP(-f)-ZORA basis set on all other atoms. Calculations with hybrid functionals used the RIJCOSX algorithm to speed the calculation of Hartree−Fock exchange.102 Errors in core-potential energetics were evaluated by plotting the calculated XAS peak energies against experimental peak energies. These fits were used to shift the calculated spectra for comparison to experiment. All molecular orbital images were generated using UCSF Chimera package.103 Of note, the use of B3LYP and the CP(PPP) basis sets have previously been shown to satisfactorily reproduce Ni and Ti Kedge XAS data.47,49 G


Inorganic Chemistry

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Synthesis of Ti((2-Ph2P)C4H3N)4Ni2, Ti(NP)4Ni2 (3). Solid Ti(NP)4 (1) (540 mg, 0.515 mmol, 1 equiv) was added to a 20 mL scintillation vial and dissolved in 10 mL of benzene. Freshly crystallized Ni(COD)2 (1.42 g, 5.15 mmol, 10 equiv) was then added as a solid. The vial was sealed with a Teflon-coated cap and heated at 90 °C overnight. The dark solution was then allowed to cool and filtered over Celite (Caution! The reduced Ni black f umes upon contact with air). Note: The reaction can be monitored by 31P{1H} NMR. If mixtures of 2 and 3 are obtained, the material should be re-subjected to initial reaction conditions as 2 and 3 are inseparable. After filtration, volatiles were removed to leave a black, oily residue, which was taken up in THF and layered with pentane. The solution was cooled to −35 °C and allowed to crystallize overnight. The resulting black crystals were washed thoroughly with pentane and dried to give 192 mg (32% yield) of 3. Alternatively, minimal benzene can be added to the oily residue followed by pentane to precipitate 3 as a solid. 1H NMR (400 MHz, C6D6; ppm): δ 6.45−6.52 (4H, br), 6.60− 6.93 (32H, br) 6.93−7.13 (16H, br). 13C{1H} (100 MHz, C6D6, ppm): δ 117.0−119.0 (br), 132.0−135.0 (br), 110.8−111.6 (br). 31 1 P{ H} NMR (121 MHz, C6D6; ppm): δ −5.0 to −8.0 (br, 2P), −15.0 to −18.0 (br, 2P). Elemental analysis from crystalline material was repeatedly unsuccessful due to low carbon percentages. Synthesis of Ti(NP)4Ni2(PhCCPh) (4). Solid Ti(NP)4Ni2 (3) (30 mg, 0.026 mmol, 1 equiv) was dissolved in 3 mL of benzene, and diphenylacetylene (4.6 mg, 0.026 mmol, 1 equiv) was added. The solution was warmed to 90 °C for approximately 1 h. Reaction progress can be monitored via 31P NMR analysis. The solution was then layered with pentane and allowed to crystallize overnight. The resulting dark crystals were washed with pentane and dried to give crystalline 4 quantitatively. Isolated 4 is extremely insoluble, which precludes detailed spectroscopic analysis of the crystalline material; however in situ NMR analysis of the reaction mixture allows for 1H and 31P NMR characterization of 4. 1H NMR (400 MHz, C6D6; ppm): δ 5.79 (br, 4H), 6.33 (br, 8H), 6.44 (br, m, 4H), 6.94−6.51 (br, m, 33H). 31P{1H} NMR (121 MHz, C6D6; ppm): δ −1.10. Elem. Anal. Calcd (C78H62N4P4TiNi2): C, 69.68; H, 4.65; N, 4.17. Found: C, 69.60; H, 5.02; N, 3.89. Synthesis of Ti((NP)2Ni(CN-tBu))2 (5). Solid Ti(NP)4Ni2 (3) (30 mg, 0.026 mmol, 1 equiv) was dissolved in 3 mL of benzene, and tert-butyl isocyanide was added (4.3 mg, 0.052 mmol, 2 equiv) and stirred for 1 h. The solution was layered with pentane and allowed to crystallize overnight. The resulting crystals were washed with pentane and dried to give crystalline 5 (quantitative by 31P NMR analysis). 1H NMR (400 MHz, C6D6; ppm): δ 1.01−1.05 (s, 18H), 6.31 (m, 2H), 6.39−6.41 (m, 6H), 6.56−6.62 (m, 9H), 6.66−6.71 (m, 3H), 6.77− 6.81 (m, 2H), 6.86−6.93 (m, 5H), 6.98−7.05 (m, 7H), 7.07−7.13 (m, 5H), 7.44−7.52 (m, 6H), 7.54−7.58 (s, 2H), 7.80−7.88 (m, 4H). 31 1 P{ H} NMR (121 MHz, C6D6; ppm): δ −6.62 (d, Jpp = 11.7 Hz, 4P). Satisfactory elemental analysis could not be obtained due isocyanide loss under prolonged vacuum. Synthesis of Ti(NP)4Ni2(benzo[c]cinnoline) (7). Solid Ti(NP)4Ni2 (3) (62 mg, 0.053 mmol, 1 equiv) was dissolved in 3 mL of benzene and benzo[c]cinnoline (9.6 mg, 0.053 mmol, 1 equiv) was added. The solution was stirred for 4 h and then layered with pentane and allowed to crystallize overnight. The resulting black crystals were washed to pentane and dried to give 46 mg (64% yield) of 7. 1H NMR (400 MHz, C6D6; ppm): δ Species A and B, 5.20 (m, 1H), 5.63 (m, 1H), 5.70 (m, 1H), 6.00 (m, 1H), 6.03 (m, 1H), 6.06 (m, 2H), 6.13−6.50 (br m, 24H), 6.50−6.58 (m, 2H), 6.60- 7.15 (broad m, aryl), 7.19−7.45 (m, 8H), 7.57−7.67 (m, 5H), 7.80−7.84 (br, 1H), 7.89−7.98 (m, 5H), 8.32−8.38 (m, 3H), 9.64 (m, 1H). 31P{1H} NMR (121 MHz, C7D8, −70 °C; ppm): δ Species A, −23.5 (s, 1P), −2.65(d, Jpp = 23.4 Hz, 1P), 1.99 (d, Jpp = 23.4 Hz, 1P), 35.75 (s, 1P); species B, −9.96 (d, Jpp = 155.8 Hz, 1P), 2.66 (d, Jpp = 49.7 Hz, 1P), 6.01 (dd, Jpp = 155.8, 49.7 Hz, 1P), 30.7 (s, 1P). Elem. Anal. Calcd (C76H60N6P4TiNi2): C, 67.79; H, 4.49; N, 6.24. Found: C, 67.81; H, 4.51; N, 5.97.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01805. Full experimental procedures, characterization data, and spectra (PDF) Accession Codes

CCDC 1841829−1841833 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 Authors

*(I.A.T.) E-mail: [email protected]. *(K.M.L.) E-mail: [email protected]. ORCID

Sudipta Chatterjee: 0000-0003-4977-3840 Samantha N. MacMillan: 0000-0001-6516-1823 Kyle M. Lancaster: 0000-0001-7296-128X Ian A. Tonks: 0000-0001-8451-8875 Author Contributions §

P.L.D. and S.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the Inorganometallic Catalyst Design Center, an EFRC funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (Grant DE-SC0012702 to I.A.T.) and the National Science Foundation (NSF) (Grant CHE1454455 to K.M.L.). I.A.T. and K.M.L. gratefully acknowledge the Alfred P. Sloan Foundation for support. P.L.D. acknowledges a UMN Doctoral Dissertation Fellowship for funding. Equipment for the University of Minnesota Chemistry Department NMR facility was supported through a grant from the National Institutes of Health (S10OD011952) with matching funds from the University of Minnesota. XAS data were obtained at SSRL, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the Department of Energy’s Office of Biological and Environmental Research, and by NIH/NIGMS (including Grant P41GM103393).

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REFERENCES

(1) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Molecular Catalysts for Multielectron Redox Reactions of Small Molecules: The “Cofacial Metallodiporphyrin” Approach. Angew. Chem., Int. Ed. Engl. 1994, 33 (15−16), 1537−1554. (2) Powers, T. M.; Betley, T. A. Testing the Polynuclear Hypothesis: Multielectron Reduction of Small Molecules by Triiron Reaction Sites. J. Am. Chem. Soc. 2013, 135 (33), 12289−12296. (3) Barden, B. A.; Culcu, G.; Krogman, J. P.; Bezpalko, M. W.; Hatzis, G. P.; Dickie, D. A.; Foxman, B. M.; Thomas, C. M. Assessing the Metal−Metal Interactions in a Series of Heterobimetallic Nb/M H


Article

Inorganic Chemistry Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties. Inorg. Chem. 2019, 58 (1), 821−833. (4) Clouston, L. J.; Siedschlag, R. B.; Rudd, P. A.; Planas, N.; Hu, S.; Miller, A. D.; Gagliardi, L.; Lu, C. C. Systematic Variation of Metal− Metal Bond Order in Metal−Chromium Complexes. J. Am. Chem. Soc. 2013, 135 (35), 13142−13148. (5) Baumann, J. A.; Salmon, D. J.; Wilson, S. T.; Meyer, T. J.; Hatfield, W. E. Electronic structure and redox properties of the clusters [Ru3O(CH3CO2)6L3]n+. Inorg. Chem. 1978, 17 (12), 3342− 3350. (6) Horak, K. T.; Velian, A.; Day, M. W.; Agapie, T. Arene noninnocence in dinuclear complexes of Fe, Co, and Ni supported by a para-terphenyl diphosphine. Chem. Commun. 2014, 50 (34), 4427− 4429. (7) Horak, K. T.; Lin, S.; Rittle, J.; Agapie, T. Heterometallic Effects in Trinuclear Complexes Supported by p-Terphenyl Diphosphine Ligands. Organometallics 2015, 34 (18), 4429−4432. (8) Hernández Sánchez, R.; Zheng, S.-L.; Betley, T. A. Ligand Field Strength Mediates Electron Delocalization in Octahedral [(HL)2Fe6(L′)m]n+ Clusters. J. Am. Chem. Soc. 2015, 137 (34), 11126−11143. (9) Li, F.; Meyer, R. L.; Carpenter, S. H.; VanGelder, L. E.; Nichols, A. W.; Machan, C. W.; Neidig, M. L.; Matson, E. M. Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate−alkoxide cluster. Chem. Sci. 2018, 9, 6379−6389. (10) VanGelder, L. E.; Brennessel, W. W.; Matson, E. M. Tuning the redox profiles of polyoxovanadate-alkoxide clusters via heterometal installation: toward designer redox Reagents. Dalton Trans. 2018, 47, 3698−3704. (11) Hu, S.; Shima, T.; Hou, Z. Carbon−carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride. Nature 2014, 512, 413. (12) Nakajima, Y.; Suzuki, H. Nitrogen−Nitrogen Double Bond Cleavage of Azobenzene by a Triruthenium Pentahydrido Complex, (Cp′Ru)3(μ3-H)2(μ-H)3 (Cp′ = η5-C5Me5), and Catalytic Hydrogenation of Azobenzene and 1,2-Diphenylhydrazine. Organometallics 2005, 24 (8), 1860−1866. (13) Shima, T.; Hu, S.; Luo, G.; Kang, X.; Luo, Y.; Hou, Z. Dinitrogen Cleavage and Hydrogenation by a Trinuclear Titanium Polyhydride Complex. Science 2013, 340 (6140), 1549−1552. (14) Murray, L. J.; Weare, W. W.; Shearer, J.; Mitchell, A. D.; Abboud, K. A. Isolation of a (Dinitrogen)Tricopper(I) Complex. J. Am. Chem. Soc. 2014, 136 (39), 13502−13505. (15) Lionetti, D.; de Ruiter, G.; Agapie, T. A trans-Hyponitrite Intermediate in the Reductive Coupling and Deoxygenation of Nitric Oxide by a Tricopper−Lewis Acid Complex. J. Am. Chem. Soc. 2016, 138 (15), 5008−5011. (16) Kanady, J. S.; Tsui, E. Y.; Day, M. W.; Agapie, T. A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II. Science 2011, 333 (6043), 733−736. (17) Tsui, E. Y.; Tran, R.; Yano, J.; Agapie, T. Redox-inactive metals modulate the reduction potential in heterometallic manganese−oxido clusters. Nat. Chem. 2013, 5, 293. (18) Powers, T. M.; Fout, A. R.; Zheng, S.-L.; Betley, T. A. Oxidative Group Transfer to a Triiron Complex to Form a Nucleophilic μ3Nitride, [Fe3(μ3-N)]−. J. Am. Chem. Soc. 2011, 133 (10), 3336−3338. (19) Velian, A.; Lin, S.; Miller, A. J. M.; Day, M. W.; Agapie, T. Synthesis and C−C Coupling Reactivity of a Dinuclear NiI−NiI Complex Supported by a Terphenyl Diphosphine. J. Am. Chem. Soc. 2010, 132 (18), 6296−6297. (20) Lin, S.; Day, M. W.; Agapie, T. Nickel Hydrides Supported by a Non-Innocent Diphosphine Arene Pincer: Mechanistic Studies of Nickel−Arene H-Migration and Partial Arene Hydrogenation. J. Am. Chem. Soc. 2011, 133 (11), 3828−3831. (21) Chao, S. T.; Lara, N. C.; Lin, S.; Day, M. W.; Agapie, T. Reversible Halide-Modulated Nickel−Nickel Bond Cleavage: Metal− Metal Bonds as Design Elements for Molecular Devices. Angew. Chem., Int. Ed. 2011, 50 (33), 7529−7532.

(22) Shoshani, M. M.; Johnson, S. A. Facile Deep and Ultradeep Hydrodesulfurization by the [(iPr3P)Ni]5H6 Cluster Compared to Mononuclear Ni Sources. Inorg. Chem. 2015, 54 (24), 11977−11985. (23) Shoshani, M. M.; Johnson, S. A. Cooperative carbon-atom abstraction from alkenes in the core of a pentanuclear nickel cluster. Nat. Chem. 2017, 9, 1282. (24) Shoshani, M. M.; Semeniuchenko, V.; Johnson, S. A. Dismantling of Vinyl Ethers by Pentanuclear [(iPr3P)Ni]5H6: Facile Cooperative C−O, C−C and C−H Activation Pathways. Chem. - Eur. J. 2018, 24 (53), 14282−14289. (25) Beck, R.; Shoshani, M.; Johnson, S. A. Catalytic Hydrogen/ Deuterium Exchange of Unactivated Carbon−Hydrogen Bonds by a Pentanuclear Electron-Deficient Nickel Hydride Cluster. Angew. Chem., Int. Ed. 2012, 51 (47), 11753−11756. (26) Zhou, Y.-Y.; Uyeda, C. Catalytic reductive [4 + 1]cycloadditions of vinylidenes and dienes. Science 2019, 363 (6429), 857−862. (27) Powers, I. G.; Andjaba, J. M.; Luo, X.; Mei, J.; Uyeda, C. Catalytic Azoarene Synthesis from Aryl Azides Enabled by a Dinuclear Ni Complex. J. Am. Chem. Soc. 2018, 140 (11), 4110− 4118. (28) Zhou, Y.-Y.; Hartline, D. R.; Steiman, T. J.; Fanwick, P. E.; Uyeda, C. Dinuclear Nickel Complexes in Five States of Oxidation Using a Redox-Active Ligand. Inorg. Chem. 2014, 53 (21), 11770− 11777. (29) Steiman, T. J.; Uyeda, C. Reversible Substrate Activation and Catalysis at an Intact Metal−Metal Bond Using a Redox-Active Supporting Ligand. J. Am. Chem. Soc. 2015, 137 (18), 6104−6110. (30) Zhou, Y.-Y.; Uyeda, C. Reductive Cyclopropanations Catalyzed by Dinuclear Nickel Complexes. Angew. Chem. 2016, 128 (9), 3223− 3227. (31) Hartline, D. R.; Zeller, M.; Uyeda, C. Well-Defined Models for the Elusive Dinuclear Intermediates of the Pauson−Khand Reaction. Angew. Chem., Int. Ed. 2016, 55 (20), 6084−6087. (32) Behlen, M. J.; Zhou, Y.-Y.; Steiman, T. J.; Pal, S.; Hartline, D. R.; Zeller, M.; Uyeda, C. Dinuclear oxidative addition reactions using an isostructural series of Ni2, Co2, and Fe2 complexes. Dalton Trans. 2017, 46 (17), 5493−5497. (33) Pal, S.; Zhou, Y.-Y.; Uyeda, C. Catalytic Reductive Vinylidene Transfer Reactions. J. Am. Chem. Soc. 2017, 139 (34), 11686−11689. (34) Pal, S.; Uyeda, C. Evaluating the Effect of Catalyst Nuclearity in Ni-Catalyzed Alkyne Cyclotrimerizations. J. Am. Chem. Soc. 2015, 137 (25), 8042−8045. (35) Dunn, P. L.; Carlson, R. K.; Gagliardi, L.; Tonks, I. A. Structure and bonding of group 4-nickel heterobimetallics supported by 2(diphenylphosphino)pyrrolide ligands. Dalton Trans. 2016, 45 (24), 9892−9901. (36) Dunn, P. L.; Reath, A. H.; Clouston, L. J.; Young, V. G.; Tonks, I. A. Homo- and heteroleptic group 4 2-(diphenylphosphino)pyrrolide complexes: Synthesis, coordination chemistry and solution state dynamics. Polyhedron 2014, 84, 111−119. (37) Pauling, L. Atomic Radii and Interatomic Distances in Metals. J. Am. Chem. Soc. 1947, 69 (3), 542−553. (38) DeLaet, D. L.; Fanwick, P. E.; Kubiak, C. P. Binuclear complexes of nickel(0): comparison of a bridging methyl isocyanide and a bridging (methylamino)carbyne ligand. Organometallics 1986, 5 (9), 1807−1811. (39) Simón-Manso, E.; Kubiak, C. P. Dinuclear Nickel Complexes as Catalysts for Electrochemical Reduction of Carbon Dioxide. Organometallics 2005, 24 (1), 96−102. (40) Lee, C. H.; Laitar, D. S.; Mueller, P.; Sadighi, J. P. Generation of a Doubly Bridging CO2 Ligand and Deoxygenation of CO2 by an (NHC)Ni(0) Complex. J. Am. Chem. Soc. 2007, 129 (45), 13802− 13803. (41) Suseno, S.; Horak, K. T.; Day, M. W.; Agapie, T. Trinuclear Nickel Complexes with Metal−Arene Interactions Supported by Trisand Bis(phosphinoaryl)benzene Frameworks. Organometallics 2013, 32 (23), 6883−6886. I


Article

Inorganic Chemistry

Imides, Nitrides, and N,N-Dimethylamine. J. Am. Chem. Soc. 2005, 127 (2), 500−501. (61) Milsmann, C.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J. Azo NN Bond Cleavage with a Redox-Active Vanadium Compound Involving Metal−Ligand Cooperativity. Angew. Chem., Int. Ed. 2012, 51 (22), 5386−5390. (62) Kaleta, K.; Arndt, P.; Beweries, T.; Spannenberg, A.; Theilmann, O.; Rosenthal, U. Reactions of Group 4 Metallocene Alkyne Complexes with Azobenzene: Formation of Diazametallacyclopropanes and NN Bond Activation. Organometallics 2010, 29 (11), 2604−2609. (63) Aubart, M. A.; Bergman, R. G. Tantalum-Mediated Cleavage of an NN Bond in an Organic Diazene (Azoarene) to Produce an Imidometal (MNR) Complex: An η2-Diazene Complex Is Not an Intermediate. Organometallics 1999, 18 (5), 811−813. (64) Lockwood, M. A.; Fanwick, P. E.; Eisenstein, O.; Rothwell, I. P. Mechanistic Studies of the Facile Four-Electron Reduction of Azobenzene at a Single Tungsten Metal Center. J. Am. Chem. Soc. 1996, 118 (11), 2762−2763. (65) Monillas, W. H.; Yap, G. P. A.; MacAdams, L. A.; Theopold, K. H. Binding and Activation of Small Molecules by Three-Coordinate Cr(I). J. Am. Chem. Soc. 2007, 129 (26), 8090−8091. (66) Kilgore, U. J.; Yang, X.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Activation of Atmospheric Nitrogen and Azobenzene NN Bond Cleavage by a Transient Nb(III) Complex. Inorg. Chem. 2006, 45 (26), 10712−10721. (67) Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Formation of a terminal aryl-imido compound of titanium by cleavage of the nitrogen nitrogen double bond in benzo[c]cinnoline. Inorg. Chem. 1991, 30 (5), 1143−1144. (68) Canich, J. A. M.; Albert Cotton, F.; Duraj, S. A.; Roth, W. J. The preparation of Ta 2 Cl 6 (PhN) 2 (Me 2 S) 2 by reaction of Ta2Cl6(Me2S)3 with PhNNPh: Crystal structure of the product. Polyhedron 1986, 5 (3), 895−898. (69) Cotton, F. A.; Duraj, S. A.; Roth, W. J. A new double bond metathesis reaction: conversion of an niobium:niobium and an nitrogen:nitrogen bond into two niobium:nitrogen bonds. J. Am. Chem. Soc. 1984, 106 (17), 4749−4751. (70) Ikeda, H.; Nishi, K.; Tsurugi, H.; Mashima, K. Metathesis cleavage of an NN bond in benzo[c]cinnolines and azobenzenes by triply-bonded ditungsten complexes. Chem. Commun. 2018, 54 (30), 3709−3711. (71) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Nitrogen−nitrogen multiple bond cleavage and reduction in diphenyldiazomethane and azobenzene by a titanium (III) complex. J. Chem. Soc., Chem. Commun. 1982, No. 17, 1015−1017. (72) Bellows, S. M.; Arnet, N. A.; Gurubasavaraj, P. M.; Brennessel, W. W.; Bill, E.; Cundari, T. R.; Holland, P. L. The Mechanism of N− N Double Bond Cleavage by an Iron(II) Hydride Complex. J. Am. Chem. Soc. 2016, 138 (37), 12112−12123. (73) Verma, A. K.; Lee, S. C. Reductive Cleavage of the N−N Bond: Synthesis of Imidoiron(III) Cubanes. J. Am. Chem. Soc. 1999, 121 (46), 10838−10839. (74) Pick, F. S.; Leznoff, D. B.; Fryzuk, M. D. Redox behaviour of ([fc(NPiPr2)2]Fe)2, formation of an iron−iron bond and cleavage of azobenzene. Dalton Trans. 2018, 47 (32), 10925−10931. (75) Kiernicki, J. J.; Zeller, M.; Szymczak, N. K. Hydrazine Capture and N−N Bond Cleavage at Iron Enabled by Flexible Appended Lewis Acids. J. Am. Chem. Soc. 2017, 139 (50), 18194−18197. (76) Ittel, S. D.; Ibers, J. A. The preparation and structure of bis(trip-tolylphosphine)(azobenzene)nickel(0), Ni[P(C6H4CH3)3]2[(C6H5NNC6H5]. J. Organomet. Chem. 1973, 57 (2), 389−402. (77) Mindiola, D. J.; Hillhouse, G. L. Terminal Amido and Imido Complexes of Three-Coordinate Nickel. J. Am. Chem. Soc. 2001, 123 (19), 4623−4624. (78) Waterman, R.; Hillhouse, G. L. η2-Organoazide Complexes of Nickel and Their Conversion to Terminal Imido Complexes via

(42) Bochmann, M.; Hawkins, I.; Yellowlees, L. J.; Hursthouse, M. B.; Short, R. L. Synthesis and electrochemical oxidation of tetranuclear nickel carbonyl phosphine clusters. The structure of Ni4(CO)6(PMe3)4. Polyhedron 1989, 8 (10), 1351−1355. (43) Osborn, J. A.; Stanley, G. G.; Bird, P. H. A nickel(0) dimer/ monomer system with facile and reversible carbon monoxide uptake: Ni2(CO)x(dppm)2 (x = 3, 4) and Ni(CO)3(η1-dppm). J. Am. Chem. Soc. 1988, 110 (7), 2117−2122. (44) Tsui, E. Y.; Agapie, T. Carbon dioxide cleavage by a Ni2 complex supported by a binucleating bis(N-heterocyclic carbene) framework. Polyhedron 2014, 84, 103−110. (45) Feng, G.; Zhang, M.; Shao, D.; Wang, X.; Wang, S.; Maron, L.; Zhu, C. Transition-metal-bridged bimetallic clusters with multiple uranium−metal bonds. Nat. Chem. 2019, 11 (3), 248−253. (46) MacMillan, S. N.; Lancaster, K. M. X-ray Spectroscopic Interrogation of Transition-Metal-Mediated Homogeneous Catalysis: Primer and Case Studies. ACS Catal. 2017, 7 (3), 1776−1791. (47) Moore, J. T.; Chatterjee, S.; Tarrago, M.; Clouston, L. J.; Sproules, S.; Bill, E.; Bernales, V.; Gagliardi, L.; Ye, S.; Lancaster, K. M.; Lu, C. C. Enhanced Fe-centered Redox Flexibility in Fe-Ti Heterobimetallic Complexes. Inorg. Chem. 2019, 58, 6199−6214. (48) Mann, K. R.; Gray, H. B. Further Studies of the Spectroscopic Properties and Photochemistry of Binuclear Rhodium(I) Isocyanide Complexes. Adv. Chem. Ser. 1979, 173, 225−235. (49) Agnew, D. W.; DiMucci, I. M.; Arroyave, A.; Gembicky, M.; Moore, C. E.; MacMillan, S. N.; Rheingold, A. L.; Lancaster, K. M.; Figueroa, J. S. Crystalline Coordination Networks of Zero-Valent Metal Centers: Formation of a 3-Dimensional Ni(0) Framework with m-Terphenyl Diisocyanides. J. Am. Chem. Soc. 2017, 139 (48), 17257−17260. (50) Chandrasekaran, P.; Stieber, S. C. E.; Collins, T. J.; Que, L., Jr.; Neese, F.; DeBeer, S. Prediction of high-valent iron K-edge absorption spectra by time-dependent Density Functional Theory. Dalton Trans. 2011, 40 (42), 11070−11079. (51) Barrios-Francisco, R.; Benítez-Páez, T.; Flores-Alamo, M.; Arévalo, A.; García, J. J. Nickel(0) Complexes with Fluorinated Alkyne Ligands and their Reactivity towards Semihydrogenation and Hydrodefluorination with Water. Chem. - Asian J. 2011, 6 (3), 842− 849. (52) Hollingsworth, R. L.; Bheemaraju, A.; Lenca, N.; Lord, R. L.; Groysman, S. Divergent reactivity of a new dinuclear xanthenebridged bis(iminopyridine) di-nickel complex with alkynes. Dalton Trans. 2017, 46 (17), 5605−5616. (53) Siedschlag, R. B.; Bernales, V.; Vogiatzis, K. D.; Planas, N.; Clouston, L. J.; Bill, E.; Gagliardi, L.; Lu, C. C. Catalytic Silylation of Dinitrogen with a Dicobalt Complex. J. Am. Chem. Soc. 2015, 137 (14), 4638−4641. (54) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic conversion of nitrogen to ammonia by an iron model complex. Nature 2013, 501, 84. (55) Lee, Y.; Mankad, N. P.; Peters, J. C. Triggering N2 uptake via redox-induced expulsion of coordinated NH3 and N2 silylation at trigonal bipyramidal iron. Nat. Chem. 2010, 2, 558. (56) Shaver, M. P.; Fryzuk, M. D. Activation of Molecular Nitrogen: Coordination, Cleavage and Functionalization of N2 Mediated By Metal Complexes. Adv. Synth. Catal. 2003, 345 (9−10), 1061−1076. (57) Roux, Y.; Duboc, C.; Gennari, M. Molecular Catalysts for N2 Reduction: State of the Art, Mechanism, and Challenges. ChemPhysChem 2017, 18 (19), 2606−2617. (58) Nishibayashi, Y. Recent Progress in Transition-Metal-Catalyzed Reduction of Molecular Dinitrogen under Ambient Reaction Conditions. Inorg. Chem. 2015, 54 (19), 9234−9247. (59) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data1. J. Am. Chem. Soc. 1996, 118 (36), 8623− 8638. (60) Shaver, M. P.; Fryzuk, M. D. Cleavage of Hydrazine and 1,1Dimethylhydrazine by Dinuclear Tantalum Hydrides: Formation of J


Article

Inorganic Chemistry Dinitrogen Extrusion. J. Am. Chem. Soc. 2008, 130 (38), 12628− 12629. (79) Wu, B.; Gramigna, K. M.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Heterobimetallic Ti/Co Complexes That Promote Catalytic N−N Bond Cleavage. Inorg. Chem. 2015, 54 (22), 10909− 10917. (80) Jenkins, D. M.; Betley, T. A.; Peters, J. C. Oxidative Group Transfer to Co(I) Affords a Terminal Co(III) Imido Complex. J. Am. Chem. Soc. 2002, 124 (38), 11238−11239. (81) Morello, L.; Yu, P.; Carmichael, C. D.; Patrick, B. O.; Fryzuk, M. D. Formation of Phosphorus−Nitrogen Bonds by Reduction of a Titanium Phosphine Complex under Molecular Nitrogen. J. Am. Chem. Soc. 2005, 127 (37), 12796−12797. (82) Schunn, R. A.; Ittel, S. D.; Cushing, M. A.; Baker, R.; Gilbert, R. J.; Madden, D. P. Bis(1,5-Cyclooctadiene)Nickel(0). Inorg. Synth. 2007, 28, 94−98. (83) Blessing, R. An empirical correction for absorption anisotropy. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51 (1), 33−38. (84) Sheldrick, G. SHELXT - Integrated space-group and crystalstructure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71 (1), 3−8. (85) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (86) Hübschle, C. B.; Sheldrick, G. M.; Dittrich, B. ShelXle: a Qt graphical user interface for SHELXL. J. Appl. Crystallogr. 2011, 44, 1281−1284. (87) Spek, A. Structure validation in chemical crystallography. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, 65 (2), 148−155. (88) Webb, S. SIXPACK; Stanford Synchrotron Lightsource, Stanford Linear Accelerator Center, Stanford University, Stanford, CA, USA, 2002. (89) Neese, F. ORCAAn ab initio, Density Functional and Semiempirical Electronic Structure Package, 3.03; Universität Bonn, Bonn, Germany, 2014. (90) Neese, F. The ORCA program system. Wiley Interdisciplinary Reviews: Computational Molecular Science 2012, 2 (1), 73−78. (91) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33 (12), 8822−8824. (92) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38 (6), 3098−3100. (93) van Wüllen, C. Molecular density functional calculations in the regular relativistic approximation: Method, application to coinage metal diatomics, hydrides, fluorides and chlorides, and comparison with first-order relativistic calculations. J. Chem. Phys. 1998, 109 (2), 392−399. (94) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. Relativistic total energy using regular approximations. J. Chem. Phys. 1994, 101 (11), 9783−9792. (95) Lenthe, E. v.; Baerends, E. J.; Snijders, J. G. Relativistic regular two-component Hamiltonians. J. Chem. Phys. 1993, 99 (6), 4597− 4610. (96) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr. J. Chem. Phys. 1994, 100 (8), 5829−5835. (97) Schäfer, A.; Horn, H.; Ahlrichs, R. Fully optimized contracted Gaussian basis sets for atoms Li to Kr. J. Chem. Phys. 1992, 97 (4), 2571−2577. (98) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7 (18), 3297−3305. (99) Klamt, A.; Schüürmann, G. COSMO: a new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient. J. Chem. Soc., Perkin Trans. 2 1993, No. 5, 799−805. (100) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular

Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98 (45), 11623−11627. (101) Neese, F. Prediction and interpretation of the 57Fe isomer shift in Mössbauer spectra by density functional theory. Inorg. Chim. Acta 2002, 337, 181−192. (102) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient, approximate and parallel Hartree−Fock and hybrid DFT calculations. A ‘chain-of-spheres’ algorithm for the Hartree−Fock exchange. Chem. Phys. 2009, 356 (1), 98−109. (103) Pettersen, E. F.; Goddard, T. D.; Huang, C. C.; Couch, G. S.; Greenblatt, D. M.; Meng, E. C.; Ferrin, T. E. UCSF ChimeraA visualization system for exploratory research and analysis. J. Comput. Chem. 2004, 25 (13), 1605−1612.

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