Group 10 Bis(iminosemiquinone) Complexes ... - ACS Publications

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

Group 10 Bis(iminosemiquinone) Complexes: Measurement of Singlet−Triplet Gaps and Analysis of the Effects of Metal and Geometry on Electronic Structure Kyle M. Conner, Amanda L. Perugini, Miko Malabute, and Seth N. Brown* Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana 46556-5670, United States S Supporting Information *

ABSTRACT: Bis(iminosemiquinone) complexes of divalent group 10 metals have been described as having open-shell singlet ground states characteristic of very strong coupling between the two ligand radicals. By using the nonlinear temperature dependence of the chemical shifts of the 1H NMR spectra, the singlet−triplet gaps in seven of these compounds have been measured, with the nickel compounds having gaps of about 2400 cm−1 and the palladium compounds about 1800 cm−1. Bis(iminosemiquinone)platinum complexes have singlet−triplet gaps too large to measure by this technique (over 2800 cm−1, estimated to be about 3000 cm−1), though bis(3,5-di-tertbutylbenzosemiquinonato)platinum(II) has a measurable singlet−triplet gap of 1850 cm−1. In combination with near-IR absorption data of the neutral, cationic, and anionic bis(iminosemiquinone) complexes, a simplified two-electron, two-orbital bonding model describing these compounds can be fully parametrized based on experimental data. The identity of the central metal principally affects the difference in energy between metal−ligand π nonbonding and metal−ligand π antibonding orbitals, with the strength of the bonding interactions increasing in the order Pd < Ni < Pt. Twisting the ligands out of planarity (by using a 2,2′-biphenylenediyl linker) has a marked effect on the optical spectra of the compounds but not on their singlet−triplet gaps; this indicates that the effect is not due to changes in bonding interactions but rather due to a decrease in the magnitude of the quantum mechanical exchange interactions in the twisted compared to the flat compounds.

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complexes.3 In these compounds, the ability to manipulate the torsion angles in Mo2Cl4(R2PXPR2)2 through judicious choice of diphosphine ligands allowed the strength of the δ interaction to be tuned. Another class of strongly coupled diradicals/weakly bonded species is the neutral, square planar group 10 bis(iminosemiquinone) complexes (Figure 1).4 In these compounds, the two frontier orbitals that collectively must hold two electrons are derived from the redox-active orbitals on the iminosemiquinones, the in-phase combination of pπ orbitals on oxygen and nitrogen that is relatively high in energy due to antibonding interactions with a filled benzene π orbital (Figure 1a).5 In a trans square planar complex, these two orbitals form two symmetry-adapted combinations, of Au and Bg symmetry, that are no longer degenerate. In particular, the Bg combination can interact significantly with one of the (filled) metal dπ orbitals, rendering it π* in character (Figure 1b) and destabilizing it with respect to the Au combination, which is strictly nonbonding with respect to the metal d orbitals (Figure 1c).6 The two frontier orbitals in these compounds are thus metal−ligand nonbonding vs antibonding, as opposed to bonding vs antibonding in the

INTRODUCTION At its heart, chemical bonding revolves around the problem of how to situate two electrons in a bonding and an antibonding orbital.1 If the bonding orbital is much more stable than the antibonding orbital, then the ground state of the system is well described as having both electrons occupying the bonding orbital, with opposite spins in order to obey the Pauli exclusion principle. This is the familiar case of a two-center, two-electron bond. At the opposite extreme is the case where both orbitals have exactly or nearly the same energy. Such species are diradicals,2 where both singlet and triplet states are usually energetically accessible, and the energetics are dominated by Coulomb repulsion and exchange energies rather than features of chemical bonding. In between these two extremes lie species where the extent of covalent bonding is of the same order of magnitude as the exchange and Coulomb interactions. Depending on whether one views the compounds as being derived from bonds or from diradicals, one can describe them as weakly bonded species or as strongly coupled diradicals. They are of interest precisely because they occupy the borderland where neither our intuition about bonding nor our expectations of independent radical centers can be relied upon. A case that has been studied in considerable detail is the δ bond in quadruply bonded dimolybdenum © XXXX American Chemical Society

Received: January 8, 2018

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DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Kohn−Sham orbitals illustrating (a) the ligand redox-active orbital (LUMO of N-phenyliminobenzoquinone), (b) the higherenergy Bg metal−ligand antibonding orbital in (C6H4[NH]O)2Pd, and (c) the lower-energy Au metal−ligand nonbonding orbital in (C6H4[NH]O)2Pd.

Figure 2. State energy diagram for (isq)2M. Subscript 1 refers to the au orbital, and subscript 2 refers to the bg orbital. Energies from ref 2, with the zero of energy set to h1 + h2 + 1/2(J11 + J12).

estimated computationally to be in the range of 3000−4000 cm−1, which is noted to be “too large to be accurately measured experimentally.”12 Here, we describe experimental measurements of the large singlet−triplet gaps in a number of bis(iminosemiquinonato)nickel and -palladium complexes based on the temperature dependence of their 1H NMR chemical shifts.13 When combined with measurements of the optical absorptions of the neutral, cationic, and anionic complexes, this enables a complete experimental parametrization of the two-electron, two-orbital model described above. This allows one to analyze in detail the effects of changing the metal and the ligand on the electronic structure and bonding in the complexes.

metal−metal quadruply bonded compounds or theoretical constructs such as stretched dihydrogen, but the same overall electronic structure still obtains. Group 10 bis(diiminobenzene)7,8 and bis(semiquinone),9,10 compounds are also known and appear to be qualitatively similar to the bis(iminosemiquinone) complexes. In all two-orbital, two-electron species, there are four possible electronic states11 whose energies can be calculated in terms of the one-electron energies of the two orbitals, the exchange energy, and the Coulomb repulsion energies (Figure 2).2 The key qualitative features of this state diagram are: (1) Because the two orbitals are close in energy, the two 1Ag states are strongly mixed by configuration interaction, which lowers the energy of the ground state. It no longer makes sense to talk about a HOMO or a LUMO, since the ground state of the molecule has substantial occupancy of both frontier orbitals. This mixing is the defining characteristic of the “open-shell singlet” ground state of these molecules. (2) The low-lying 1Ag state is close in energy to the triplet 3Bu state in which the two electrons occupy different orbitals. Because the 1Ag is stabilized by differential occupancy of the more bonding orbital, it is almost always the ground state. An exception would be if the Coulomb repulsion terms for the electrons in the same orbitals (J11, J22) significantly exceeded those of the electrons in different orbitals (J12). Group 10 bis(iminosemiquinone) complexes are invariably observed to be diamagnetic, indicating that the (open-shell) singlet ground state lies significantly below the triplet state. This qualitative multiconfigurational description of the bonding in the group 10 bis(iminosemiquinone) complexes is well established. In contrast, quantitative experimental data germane to this electronic structure, apart from the energies of the very intense band due to the 1Ag → 1Bu transition, are difficult to obtain. For example, singlet−triplet gaps for such compounds have been

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RESULTS Preparation of Group 10 Bis(iminosemiquinone) Complexes. In addition to studying known examples of bis(iminosemquinone) complexes of group 10 metals, we wished to have a complete series of analogues with the same ligands (see Scheme 1 for the ligands used in this study). The only ligand for which such a complete series is available in the literature is the parent N-phenyl-3,5-di-tert-butyl-1,2-iminobenzosemiquinone,4,6 which forms only sparingly soluble compounds which are therefore not ideal for spectroscopic (especially NMR) studies. Nickel bis(iminosemiquinone) complexes have typically been prepared by treatment of nickel(II) nitrate or chloride with the appropriate N-aryl-3,5-di-tert-butyl-2-aminophenol in the presence of base and atmospheric oxygen in a polar solvent such as methanol or acetonitrile.4,14−18 In our hands, this approach worked poorly when applied to the highly tert-butyl substituted ligands SnipH2 (N-(3,5-di-tertbutylphenyl)-2-hydroxy-3,5-di-tert-butylaniline) and tBuClipH4 (4,4′-di-tert-butyl-N,N′-bis(3,5-di-tert-butyl-2-hydroxyphenyl)2,2′-diaminobiphenyl), because the ligands are insoluble in B

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

bis(semiquinone) complexes from Ni(CO)4 and the corresponding 1,2-benzoquinones.9,10,22 This sterically encumbered bis(iminoquinone) complex does indeed show a simple oneelectron oxidation to the monocation (Figure 3), though the

Scheme 1. Complexes and Abbreviations Used in This Studya

Figure 3. Cyclic voltammogram of (Diso)2Ni (CH2Cl2, 200 mV s−1, 0.1 M Bu4N[PF6]).

subsequent oxidation event is irreversible. Similar electrochemistry is observed using either Bu4N[PF6] or Bu4N[B(C6H3-3,5(CF3)2)4] as supporting electrolyte. a

The numbering schemes used for descriptions of proton resonances in NMR are shown.

the polar solvents, such as methanol or acetonitrile, required to dissolve the nickel salts. Better results are obtained using nickel(II) 2-ethylhexanoate, which is readily soluble in dichloromethane and could be used to prepare the bis(iminosemiquinone) complexes in the presence of triethylamine and air (eq 1).

Bis(iminosemiquinonato)palladium(II) complexes are typically prepared analogously to their nickel congeners, from a palladium(II) halide precursor and the appropriate aminophenol in the presence of base and air.4,14,16,19,23 In the preparation of (tBuClip)Pd, however, it is more convenient to use palladium acetate as the metal source, and under these conditions, 2 equiv of metal is needed, even in the presence of air, with one serving to metalate the ligand and the second serving as oxidant (eq 3). The palladium black that deposits is removed by filtration through silica gel.

Almost no bis(iminosemiquinonato)nickel(II) compounds reported to date show a reversible one-electron oxidation in their cyclic voltammograms, in contrast to the heavier congeners. An exception is the 2,2-dimethyl-1,3-propanediyl-bridged complex (Clap)Ni (Scheme 1).14 We hypothesized that the usual observation of an irreversible one-electron oxidation or of a two-electron oxidation is driven by association of solvent or counterion with the nickel center in the initially formed monocation. Ligand association on oxidation has been documented for a complex with pendant thioethers,17 and a six-coordinate complex with bound perchlorates has been isolated.19 In order to inhibit axial coordination, we targeted the 2,6-diisopropylphenyl-substituted complex (Diso)2Ni (Scheme 1). In this case, the preparation of the corresponding aminophenol has not been described,20 but the compound is readily prepared using the nickel(0) precursor Ni(cod)2 and the appropriate iminoquinone21 (eq 2). This procedure mirrors the preparation of nickel

The bidentate aminophenol SnipH2 does not undergo oxidation under analogous conditions, instead forming the bis(aminophenolate) complex (SnipH)2Pd (eq 4). In the solid state, the complex exists as dimers held together by hydrogen bonding between the amine hydrogens and the phenolate oxygens (Figure 4). This motif has been seen previously in the structure of the chelated (ClapH2)Pd;14 the present structure demonstrates that dimer formation is not limited to cis complexes or to alkylamines. Judging from the low solubility of the compound and the downfield shift of the NH proton in C

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Structural and Spectroscopic Characterization of (isq)2M Complexes. All of the newly prepared complexes except (tBuClip)Pd have been structurally characterized in the solid state by X-ray crystallography (Table S1). Two representative structures, the bis(bidentate) complex (Diso)2Ni and the tetradentate complex (tBuClip)Pt, are illustrated in Figure 5; the other structures are similar to these and to previously characterized analogues, and are shown in the SI. Metrical data for the newly characterized compounds are in close agreement with known analogues (Table 1), and the periodic trends in bond distances (short Ni−ligand bonds, with Pd−L and Pt−L bond distances nearly identical) are as expected. Of more interest are the intraligand bond distances (Table S2), which are related to the degree of ligand oxidation; this relationship can be quantitated by a “metrical oxidation state” (MOS).25 In all cases, the MOS values are reasonably close to −1, supporting a view of these ligands as semiquinone-like. However, there are subtle, but systematic, variations in the values. For example, in the planar complexes, the MOS values for the nickel and palladium are quite close (the nickel complexes perhaps tending to have more negative MOS values, but this is on the edge of statistical significance), but the two platinum complexes have MOS values about 0.1 units more negative. Such variations have been ascribed to variations in π bonding between complexes, as delocalization of the electrons into or out of the ligand redox-active orbital affects the bond distances in the same way as outright reduction or oxidation.25,26 Here, an increasingly negative value of the MOS might be interpreted in terms of greater back-donation of the metal into the iminosemiquinone π orbital. Another systematic pattern in the MOS values is that the twisted 2,2′-biphenyl-bridged complexes consistently show ∼0.1 unit more negative MOS values than their planar analogues. This effect does not appear to be a result of the cis geometry of these compounds, as the MOS values of the other known cis compounds, (Clap)Ni (−1.11(4)),14 (Clap)Pd (−1.04(4)),14 and (pFlip)Pd (−1.07(7)),23 are not unusually negative. Spectroscopic and electrochemical characteristics of the compounds are also similar to those of previously reported analogues (Table 2). The most diagnostic feature of the compounds is their extremely intense, relatively narrow LLCT transitions in the near-IR (Figure S2). The wavelength of this band is relatively insensitive to the ligand structure, except that the 2,2′-biphenylene bridge causes a red shift of ∼1000 cm−1 compared to the planar compounds.16 All of the Pd and Pt compounds show two reversible reductions and two reversible oxidations in their cyclic voltammograms, while the unhindered nickel compounds show two reversible one-electron reductions but a single two-electron oxidation (Figure S3). As discussed above, (Diso)2Ni does show a reversible one-electron oxidation (Figure 3). All of these redox processes have been previously assigned as ligand-centered.4,6 The compounds are diamagnetic and show 1H and 13C NMR signals in the normal diamagnetic region (see below for more detailed discussion of the 1H NMR spectra). In particular, the

Figure 4. Thermal ellipsoid plot of {(SnipH)2Pd}2 (50% ellipsoids). Hydrogen atoms (except for NH) and methyl carbons are omitted for clarity.

the 1H NMR (δ 8.57 in CDCl3), the dimeric structure is likely retained in solution, which may explain the compound’s resistance to ligand oxidation. The oxidized complex (Snip)2Pd can be prepared by carrying out the metalation in the presence of diethyl azodicarboxylate (eq 5), which presumably oxidizes the monomeric (SnipH)2Pd before dimerization takes place. Treatment of isolated {(SnipH)2Pd}2 with diethyl azodicarboxylate also results in formation of (Snip)2Pd, but much more slowly (weeks at room temperature).

Only one platinum bis(iminosemiquinone), prepared from PtCl2 in acetonitrile, has been reported previously.6 We were unable to prepare bis(iminosemiquinones) from tBuClipH4 and dichloroplatinum complexes such as PtCl2(SEt2)2 or (cod)PtCl2. A common route to metalate salenH2 ligands with platinum(II) uses K2PtCl4 in the presence of sodium acetate in DMF at elevated temperatures.24 Under these conditions, in the presence of air to oxidize the ligands, the platinum bis(iminosemiquinones) can be prepared in moderate yield (eq 6). D

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Thermal ellipsoid plots (50% ellipsoids) of (a) (Diso)2Ni and (b) (tBuClip)Pt·2C6H6·2DMSO. Hydrogen atoms, lattice solvent, and the minor component of the disordered N-aryl group in (Diso)2Ni are omitted for clarity.

Table 1. Selected Metrical Data from Group 10 Bis(iminosemiquinonate) Complexes

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DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Our analysis of the data from the bis(iminosemiquinonato) complexes differs from the prior approaches in two minor respects, both necessitated by the rather large singlet−triplet gaps displayed by these compounds. First, the large singlet− triplet gaps mean that very little compound is in the triplet state, which gives rise to modest paramagnetic contributions to the chemical shift. Particuarly at low temperature, this means that the temperature dependence of the chemical shift is dominated by the small, linear changes characteristic of typical diamagnetic chemical shifts (Figure 6b). In order to satisfactorily fit the data, a parameter accounting for this linear temperature dependence must be included in the model. Second, since, even at the highest temperatures, very little triplet is present, the nonlinear portions of the δ vs T curves do not approach a saturating value, as has been occasionally observed in other systems. The relatively generic appearance of the observed portion of the curve means that, mathematically, the singlet−triplet gap is highly correlated with the triplet hyperfine coupling constants (A values) of the observed hydrogens. In other words, decreasing the singlet−triplet gap produces much the same effect as increasing the A values of all the hydrogens. Global optima for the parameters could be obtained for all compounds from these fits (Table S3), but the strong mathematical correlation led to physically unreasonable variations of A between similar compounds. To address this correlation, the chemical shift data were fit subject to the constraint that the A value of the H-5, the proton para to nitrogen in the iminosemiquinone ring, which has the largest A value for most compounds, was set to the value calculated by DFT (B3LYP) for the triplet state. Unlike the singlet ground state of these compounds, the triplet is not close in energy to any other triplet states and so is not subject to significant configuration interaction and thus should be well described by DFT. DFT generally does a good job of predicting 1 H hyperfine constants in organic radicals,33 and the suitability of the method for these compounds is substantiated by the reasonable agreement between the calculated A values and the A values obtained from the fits to the NMR data (Table 3). Constraining the value of AH‑5 usually has a small effect on the calculated value of ΔES‑T, with most values agreeing within the experimental uncertainties, though in two cases the values change by up to 400 cm−1 (Table S3). The overall model does

Table 2. Spectroscopic and Electrochemical Characterization of M(isq)2 (CH2Cl2) compound (Snip)2Ni (Diso)2Ni (tBuClip)Ni (Snip)2Pd (tBuClip)Pd (Snip)2Pt (tBuClip)Pt

λmax, nm (ε, L mol−1 cm−1) 877 897 979 865 963 796 861

(2.52 (1.76 (2.36 (1.42 (1.85 (6.44 (5.21

× × × × × × ×

4

10 ) 104) 104) 104) 104) 104) 104)

E°, V vs Cp2Fe+/Cp2Fe δ 13C, CO −1.91, −1.20, 0.03 −1.22, 0.11 −1.59, −0.99, 0.11 −1.67, −1.19, 0.00, 0.42 −1.43, −0.95, 0.11, 0.55 −1.91, −1.24, 0.14, 0.68 −1.65, −1.03, 0.24, 0.78

172.3 171.3 170.2 175.8 174.1 177.8 174.4

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C chemical shift of the aromatic carbon bonded to oxygen has been suggested as a useful marker for the ligand oxidation state.23 The values shown by these complexes range from 170 to 178 ppm, downfield of the 150−155 ppm typical of amidophenoxides27,28 and upfield of the 185−187 ppm found for iminoquinones.29 Determination of Singlet−Triplet Gaps by VariableTemperature 1H NMR Spectroscopy. 1H NMR spectra of the bis(iminosemiquinone) complexes are invariably observed in the normal diamagnetic ranges and show equivalence of the two ligands on the metal center. However, a few of the complexes show some broadened resonances at room temperature (see, e.g., the spectra of (tBuClip)Pd in Figure 6a). Furthermore, the peaks often undergo significant and nonlinear changes in chemical shift as a function of temperature (Figure 6b). These features are characteristic of compounds with diamagnetic, singlet ground states that are in rapid chemical exchange with lowlying, thermally populated triplet excited states. Analysis of the temperature dependence of chemical shifts in such cases was pioneered by Cotton and co-workers13 and the method subsequently independently reinvented by Limberg30 and by Autschbach and Keister31 and used by a variety of others.32 Of particular relevance to the present work, Autschbach and Keister analyzed the chemical shift data published by Pierpont on the most downfield tert-butyl resonance of the palladium bis(benzosemiquinone) (DTBSQ )2Pd from −85 to 100 °C in toluene-d89 and determined a singlet−triplet gap for this compound of 869(17) cm−1. We measured the chemical shifts for this compound in CD2Cl2 (−109 to 33 °C) and find a value identical within the experimental uncertainty (888(17) cm−1).

Figure 6. (a) 1H NMR spectra of (tBuClip)Pd (aromatic region) in CDCl2CDCl2. (b) Temperature dependence of chemical shifts of (tBuClip)Pd (CDCl2CDCl2). The solid lines are fit to eq 7′. F

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Descriptions of Singlet−Triplet Equilibria in M(isq)2 and Related Compounds ΔES‑T, cm−1

compound

T range (K)

(DTBSQ )2Pd

164−306

888(18)

AH‑5, MHz DFT −4.7

AH‑3, MHz Expt, DFT 0.58(5), 0.33

(DTBSQ )2Pt (Snip)2Ni

224−413 254−442

1850(30) 2500(300)

−4.5 −6.2

0.09(4), 0.33 −1.3(2), −3.4

(Diso)2Ni

254−434

2210(130)

−6.2

−2.41(12), −3.4

(Clap)Ni (tBuClip)Ni

254−430 297−414

2370(170) 2400(400)

−5.7 −4.9

−1.7(5), −2.9 −2.0(3), −2.0

(Snip)2Pd

236−413

1860(110)

−5.7

−2.06(11), −3.1

(pFlip)Pd

214−431

1610(50)

−5.5

−1.96(7), −2.7

(tBuClip)Pd

224−413

1940(50)

−4.4

−1.31(7), −1.7

other A, MHz Expt, DFT 4-tBu: 0.62(5), 0.52 6-tBu: 0.082(10), 0.04 2′,6′: 0.6(2), −1.8 4′: 0.4(2), −1.6 3′,5′: 0.49(11), 1.3 4′: −0.40(11), −1.6 CH2: 16(2), 17.5 3′: −3.0(3), −2.4 5′: −2.3(3), −2.3 6′: −0.3(3), 1.0 2′,6′: −1.19(11), −2.0 4′: −0.87(10), −2.0 2′,6′: −1.16(7), −1.1 3′,5′: 0.98(6), 1.0 3′: −2.41(9), −2.7 5′: −1.98(8), −2.5 6′: 0.33(7), 1.1

and cobaltocene, respectively, in dichloromethane solution (Figure 8 and Figure S2). In all cases, the cyclic voltammograms of the compounds indicated that these two reagents would have a redox potential sufficient to ensure complete conversion to the singly oxidized or reduced state, but too small to allow further oxidation or reduction. In most cases, this was confirmed by observation that the optical spectra stopped changing after addition of 1.0 equiv of oxidant or reductant. However, some of the measured potentials of the 0/−1 couples are close to that of Cp2Co+/0, and significant curvature was observed in the titration plots, consistent with incomplete reduction of the metal complexes by stoichiometric cobaltocene. We attribute this difference to the easier charge separation in CH2Cl2 containing 0.1 M electrolyte, in which the cyclic voltammograms were measured. Consistent with this, normal titration curves were observed for these compounds when the titrations were carried out in the presence of 0.1 M Bu4N[PF6]. The radical cation and anion of ([2-CF3C6H4]NC6H2tBu2O)2Pd have been previously generated and characterized as being square planar, four-coordinate species containing ligand-centered radicals.19 As the spectroscopic features of [(Snip)2Pd]+ and [(Snip)2Pd]− are virtually identical to those reported, and those of the other Pd and Pt species are very similar, there can be little doubt that these species are all analogous. The nickel complexes are potentially more ambiguous. For example, the isoelectronic bis(diiminobenzosemiquinonato)nickel complexes are sometimes observed to be square planar, but sometimes they are found to be tetrahedral, with markedly different spectroscopic properties and electronic structures.35 In particular, the tetrahedral species lack the intense near-IR transitions observed in all oxidation states of the planar compounds. Reduced, monoanionic [([2-CF3C6H4]NC6H2tBu2O)2Ni]− has been characterized in the solid state as square planar, with a delocalized ligand-centered radical.15 The similarity of its optical spectrum, featuring near-IR peaks at 1340 nm (ε = 1.0 × 104 L mol−1 cm−1) and 885 nm (ε = 3 × 103 L mol−1 cm−1), to the radical anions of nickel reported here substantiates that they retain their planar structure in solution. No four-coordinate [L2Ni]+ monocations have been characterized structurally, but the similarity of the spectra to the corresponding palladium complexes, in particular their intense long-wavelength bands, again is consistent with a

a very good job of modeling the chemical shifts as a function of temperature for all compounds (Figure S4). While each of the palladium and nickel bis(iminosemiquinone) complexes showed noticeable curvature in their δ vs T plots, the platinum complexes (Snip)2Pt and (tBuClip)Pt did not (up to 140 °C), indicating that their singlet−triplet gaps are too large to measure by this technique (>2800 cm−1). Likewise, the singlet−triplet gap in the bis(diiminobenzosemiquinonato) palladium complex (PhNC6H4NH)2Pd7 also proved to be too high to measure. We were able to determine the singlet−triplet gap of one platinum complex, the bis(benzosemiquinonato) complex (DTBSQ )2Pt9 (Figure S4j), with a singlet−triplet gap of 1850(30) cm−1. Optical Spectroscopy of Metal Bis(iminosemiquinone) Radical Cations and Anions. Additional information about the electronic structure of the M(isq)2 compounds is potentially provided by the one-electron oxidized and one-electron reduced forms of the compounds, which instantiate one-electron/ two-orbital and three-electron/two-orbital systems (Figure 7).

Figure 7. Schematic optical transitions in (a) M(isq)2+ and (b) M(isq)2−.

Both of these systems therefore lack the exchange interaction and configuration interaction of the two-electron system, but are sensitive to the bonding differences between the one-electron orbitals and (in the case of the radical anion) to the difference in electron−electron repulsion (Coulomb integrals) in the two orbitals. This kind of comparison between different oxidation states has been used to elucidate the δ bonding in metal−metal quadruple bonds.34 The radical cation and radical anion complexes were generated in solution by titration with acetylferricinium hexafluorophosphate G

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Titration of (tBuClip)Pt in CH2Cl2 with (a) acetylferricinium hexafluorophosphate, to generate [(tBuClip)Pt]+, and (b) cobaltocene, to generate [(tBuClip)Pt]−. Initial spectra are in black, final spectra in blue.

as (J22 − J11) and (J11 − J12). (There is a third independent value needed to describe the three Coulomb repulsions, but this affects only the absolute energies of the states, not their relative energies.) This bonding modelessentially a CASSCF(2,2) pictureis of course simplified, and neglects, for example, configurations involving other metal- or ligand-centered orbitals, as well as dynamic correlation effects. Nevertheless, for the reasons set out above, it seems in this system to capture the essential bonding features and should thus be suitable for at least a semiquantitative description of the bonding. It seemed to us that such a simplified bonding model offered the opportunity to elucidate trends in the electronic structure of these compounds that was experimentally rather than computationally derived, and that allowed differences between compounds to be mapped onto a limited set of parameters that would be subject to straightforward physical interpetation. One initial difficulty in realizing this program is that only one energy difference between states is readily measured experimentally, the 1Ag → 1Bu transition that is observed as a narrow, intense band in the near-infrared. For many of the compounds, we have been able to supplement this with measurements of the thermal excitation of the 1Ag → 3Bu transition based on its effect on the 1H NMR chemical shifts of the compounds as a function of temperature. This exactly parallels the methodology used to elucidate the δ bonding in metal−metal quadruply bonded compounds. In the analysis of δ bonding, these two observables sufficed to determine K and ΔW, since all the Coulomb terms were assumed to be equal, based on the small overlap between the atomic d orbitals that combined to form the δ and δ* orbitals.34 This assumption seemed likely to be untenable for the group 10 bis(iminosemiquinone) complexes, since the different amount of metal participation in the ϕ1 (au) and ϕ2 (bg) molecular orbitals would be expected to give rise to differences in J11 compared to J22 (or J12). To gain further information about the Coulomb (and bonding) interactions, we therefore turned to analysis of the radical cations and radical anions, as had also been done in the study of δ bonding.34 To use data from these species in conjunction with data from the neutral compounds relies on the assumption that the bonding and the Coulomb integrals do not change appreciably as a function of the oxidation state of the complex. While this is surely not exactly correct, the great extent of delocalization in these complexes, as well as the

planar, and not tetrahedral, structure for the monocations. Note that [(Clap)Ni]+ and [(Clap)Ni]− have been generated previously in spectroelectrochemistry experiments,14 and their optical spectra (previously reported only with λ < 1100 nm) agree fully with those obtained here by redox titration.

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DISCUSSION Parametrization of the Simplified Bonding Model for M(isq)2. The electronic structure of square planar bis(iminosemiquinonate) complexes of group 10 metals, and isoelectronic analogues, has been established by a thorough series of previous theoretical and experimental studies.4,6,12 The key frontier orbitals are the two linear combinations of the ligandbased π redox-active orbitals (Figure 1a), which are relatively well separated in energy from the other ligand-based orbitals. They are significantly higher in energy than the occupied metalcentered orbitals, and significantly lower in energy than the empty metal-centered orbitals. Because of this energetic isolation, the frontier orbitals are largely ligand-localized, with unambiguous oxidation states of +2 for the metal and −1 for each of the ligands. Nevertheless, there is enough interaction with the metal orbitals that the antibonding interaction between one of the filled metal dπ orbitals and the Bg ligand π combination (Figure 1b) raises it appreciably in energy relative to the strictly nonbonding Au combination (Figure 1c). This interaction is most simply considered as the antibonding component of a π-back-bonding interaction toward the electron-deficient iminosemiquinone. In a typical back-bonding interaction, the ligand acceptor orbital is formally empty. In these compounds, the acceptor orbital is not entirely empty because the energetic difference between the Bg and Au orbitals is small enough that the ground state is not well described by the single (au)2 configuration. Instead, there is a substantial amount of mixing with the (bg)2 configuration, and this multiconfigurational ground state is an open-shell singlet with substantial diradical character. The relative energies of the four possible states for species with two electrons in two orbitals, exemplified by (stretched) dihydrogen, can be enumerated explicitly in terms of four parameters: the difference in energy between the two (one-electron) orbitals, ΔW; the quantum mechanical exchange energy, K12; and two measures of the relative Coulomb repulsion energies for electrons among the three orbitals, which we take for convenience H

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 4. Summary of Optical Transitions and Singlet−Triplet Energy Gaps in Bis(iminosemiquinone) Complexes, with Derived Electronic Parametersa

a

Values in italics are calculated by assuming J11 − J12 to be 100 cm−1.

The sign of the difference indicates that there is more electron− electron repulsion in the bg metal−ligand antibonding orbital than in the au metal−ligand nonbonding (strictly ligand-centered) orbital. This indicates a significant buildup of electron density on the metal atom in the bg orbital; if only a small amount of electron density were accumulated on the metal, then the orbital would be more delocalized than the au orbital and would have a smaller Coulomb repulsion term. (3) The difference in Coulomb energies J11 − J12 is small, typically about 100 cm−1 (it must always be positive since the repulsion of two electrons in different orbitals can never exceed that of two electrons in the same orbital). Since the au orbital is so highly delocalized to begin with (significant, and similar, electron density on 12 atoms; see Figure 1), the amount that is lent from each ligand atom to the central metal in the bg orbital is relatively small. In four of the nine bis(iminosemiquinone) complexes analyzed, only three observations can be made. For the nickel compounds (Snip)2Ni and (tBuClip)Ni, the radical cations are unstable. For the two platinum compounds, the singlet−triplet gaps are too high to be measured. However, if we estimate J11 − J12 as ∼100 cm−1 for these compounds, we can reduce the number of unknown parameters to three and allow their determination. This estimate should introduce only small errors into these parameters (and the associated missing observables), based on the small size and relative constancy of the values observed in the five compounds where this parameter can be calculated directly. (Any error in J11 − J12 translates to an error similar in magnitude in the derived parmeters and in the calculated singlet−triplet gaps.) The derived values (Table 4, italicized entries) are physically reasonable. For example, the singlet−triplet gaps estimated in this way for the platinum compounds (Snip)2Pt and (tBuClip)Pt (∼3000 cm−1), are indeed outside of the range accessible from the NMR measurements (>2800 cm−1). The increase of the Pt singlet−triplet gaps by about 1200 cm−1 compared to those observed for the Pd analogues is similar to the experimentally measured difference of 960 cm−1 in the bis(semiquinone) complexes (DTBSQ )2M (890 cm−1 for Pd and 1850 cm−1 for Pt). Electronic Structure Trends in M(isq)2: Effects of Metal, of Ligand Twisting, and of Ligand Geometry. The energies of the optical transitions in the neutral bis(iminosemiquinone) compounds increase in the order Pd ≲ Ni < Pt. This nonmonotonic periodic trend is also usually (though not invariably) observed in group 10 complexes of the type (catecholate) M(diimine), where intense LLCT transitions are observed at

covalency of the metal−ligand bonds and the localization of the redox events in orbitals with weak metal−ligand interactions, ensures that structural changes between oxidation states are small and thus that changes in the underlying electronic structure are small as well. For example, reduction of ([2-CF3C6H4]NC6H2t Bu2O)2Ni by one electron causes changes in the metal−ligand bond lengths of less than 0.01 Å,15 while the corresponding palladium complex shows similar changes on reduction and only slightly larger changes on one-electron oxidation (0.01 Å elongation of Pd−O bonds and 0.025 Å elongation of Pd−N bonds).19 This geometric invariance contrasts with the situation in early metal27,36 or actinide37 complexes, where the more ionic bonding leads to significant elongations of the metal−nitrogen bonds on ligand oxidation. In all cases where the cations and anions can be prepared, they exhibit intense, relatively narrow absorptions in the nearIR which can be identified as transitions between the two principally ligand-centered orbitals. (With one or three electrons, there are only two possible low-energy states (Figure 7), a 2Au and a 2Bg state.) Since the radical cations are one-electron systems, neither electron−electron repulsion nor exchange interactions are operative, and the observed optical transition energies directly correspond to the difference in energy between the two orbitals, ΔW. The transition in the three-electron system depends on both this energy and the difference in electron−electron repulsion between the two electrons in the nonbonding au orbital (J11) and between the two electrons in the antibonding bg orbital (J22). Thus, for the systems where the radical cations and anions can be examined and where a singlet−triplet gap can be measured in the neutral, the four spectroscopic observables allow calculation of all four parameters needed to describe the relative energies of the states (Table 4). A number of general observations about the values of these parameters can be made. (1) The bonding parameter ΔW and the exchange integral K12 are both large and are similar in magnitude for all the compounds (on the order of 5000 cm−1). If ΔW were much larger than K12, then the compounds would be well described as closed-shell singlets; if ΔW were much smaller than K12, then they would be weakly coupled diradicals. The similarity of the two terms is in agreement with the established description of the compounds as strongly coupled diradicals. (2) The difference in Coulomb energies between the two orbitals, J22 − J11, is much smaller but still significant (on the order of 1000 cm−1), and is always positive. This follows from the significantly higher-energy absorptions found in the radical anions compared to the radical cations. I

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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ligands. However, inspection of the parameters of the electronic structure in Table 4 shows a clear correlation in the values of J22 − J11 with geometry. The cis complexes ((Clap)Ni, (pFlip)Pd, and (tBuClip)M) typically show smaller values of this parameter (∼1000 cm−1), while the trans complexes ((Diso)2Ni, (Snip)2M) typically show larger values (∼2000 cm−1). It is plausible to expect the geometry to slightly affect the magnitude of the Coulomb repulsion terms. Due to the asymmetry of the ligand-based redox-active orbital (more density on N than O), the cis isomers will have slightly larger interligand repulsions than will the trans isomers. This effect will be more pronounced in ϕ1 than in ϕ2, because of the significant metal contribution to ϕ2, so the net effect is that the cis isomers will have slightly greater values of J11 than the trans isomers, and so slightly smaller values of J22 − J11.

long wavelength, although data on complete series with all three group 10 metals and the same ligands are difficult to find. In comparisons between two metals, in (Cl4C6O2) M(Me2C2[NC6H3-2,6-Me2]2), the Ni complex (λmax = 660 nm) absorbs at slightly higher energy than the Pd complex (λmax = 690 nm);38 in (DTBCat)M(tBu2bpy), λmax(Pt) (592 nm)39 < λmax(Ni) (620 nm);40 and in (DTBCat)M(2-phenylazopyridine), λmax(Pt) (970 nm) < λmax(Pd) (1378 nm).41 In addition to the optical transitions of the neutral compounds, the radical anions and cations also show the same trend in energy of the optical transitions (Table 4). This trend has also been observed in the low-energy optical transitions of the singly oxidized [(salen)M]+ complexes, shown to have ligand-centered radicals, where ν̅max Pd (4100 cm−1) < Ni (4700 cm−1) < Pt (5450 cm−1).42 (Since the HOMO of the salicyldeneamine anion, like acetylacetonate,43 is analogous to the HOMO of the amidophenolate ion, these cations are isoelectronic to [M(isq)2]−.) The singlet−triplet gap energies in M(isq)2 also increase in the same order, albeit with a greater distinction between nickel and palladium, Pd < Ni < Pt. Both the bonding interaction ΔW and the exchange integral K12 affect these values, but inspection of Table 4 clearly establishes that the differences among the metal complexes are predominantly due to the differences in the bonding interactions ΔW, which vary in the order Pd (∼5000 cm−1) < Ni (∼5500 cm−1) < Pt (∼6500 cm−1). The fact that platinum has the greatest metal−ligand π interaction is not surprising, as third-row transition typically form the strongest covalent bonds.44 The reason for the ordering of nickel and palladium is less obvious. In the earlier study of [(salen)M]+, the difference was attributed to a lower valence orbital energy for palladium, as assessed by the elements’ second ionization energies (the first ionization energies are not useful since the neutral atoms have different ground state electron configurations).42 A lower orbital energy would result in a weaker interaction of the metal with the higher-lying ligand redox-active orbital, resulting in a less antibonding interaction in the bg orbital and a smaller value of ΔW. The effect of twisting the ligand out of planarity, in the 2,2′-biphenylene-bridged compounds, is apparent in the ∼1000 cm−1 red shift of their near-IR optical absorptions. This twisting effect has been observed in other contexts as well. For example, the bulky 3,5-di-tert-butylphenyl substituents in the palladium bis(diiminobenzosemiquinone) complex (C6H4[NC6H3tBu2]2)2Pd force the complex to twist, with a 20.0° angle between the Pd−N−C−C−N chelate planes,35 and its near-IR absorption is red-shifted by 800 cm−1 compared to that shown by planar (PhNC6H4NH)2Pd.7 The perfluorophenyl-substituted compound (C6H4[NC6F5]2)2Pd shows a similar twist (22.4°) and a nearly identical red-shifted near-IR band.45 Remarkably, the qualitative electronic structure analysis (Table 4) indicates that the differences in optical spectra due to twisting are not principally due to differences in bonding but rather to a decreased value of the exchange integral K12 in the twisted compounds. Experimentally, the signature of this is that, while the 1Ag → 1Bu transition energies decrease upon twisting, there is not much change in the singlet−triplet gap between the (tBuClip)M compounds and flat analogues such as (Snip)2M. Since the difference in these two energies is the difference in energy between the 1Bu and 3Bu states, which is just 2K12, this indicates a decrease in the value of K12 upon twisting. In contrast to the effect of twisting on the spectroscopic observables, there is no similar obvious correlation of the observables with the geometric arrangement (cis vs trans) of the

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CONCLUSIONS Variable-temperature NMR methods extend the measurable range of singlet−triplet gaps to about 2800 cm−1, which allows one to determine the singlet−triplet gaps of very strongly coupled systems such as group 10 bis(iminosemiquinone) complexes. In combination with measurements of the optical spectra of the neutral, cationic, and anionic complexes, one can fully parametrize a description of the bonding of these compounds using a previously established general formulation applicable to any two-electron, two-orbital system. In particular, this dissection allows the calculation of both bonding terms, namely, the difference in (one-electron) energy ΔW between the metal− ligand nonbonding and metal−ligand antibonding orbitals, as well as the exchange integral K12 that is centrally important to the behavior of diradicals. These bis(iminosemiquinone) complexes have been characterized by much prior experimental and theoretical work as “strongly coupled diradicals”. The present work quantitatively affirms this description by showing that both K12 (the “diradical” part of the description) and ΔW (the “strongly coupled” part of the description) are large and of similar magnitude (both ∼5000 cm−1). In considering differences among the compounds, the trends in the energies of the near-IR optical transitions, echoed in the singlet−triplet gaps, follow the nonmonotonic periodic trend of Pd < Ni < Pt. The electronic structure model indicates that this difference is due to differences in metal−ligand π bonding, with Pd showing the weakest bonding and Pt the strongest. More surprisingly, the differences apparent upon twisting the structures out of planarity are seen to be due not to differences in bonding, but rather to attenuation of the exchange integral upon twisting.

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EXPERIMENTAL SECTION

Unless otherwise noted, procedures were carried out without precautions to exclude air or moisture. NMR spectra were measured on Bruker Avance DPX 400 or 500 spectrometers. Chemical shifts for 1 H and 13C{1H} spectra are reported in ppm downfield of TMS, with spectra referenced using the known chemical shifts of the solvent residuals. Infrared spectra were measured on a Jasco-6300 FT-IR spectrometer as powders on ATR plates. UV−visible−NIR spectra were recorded in 1 cm quartz cells on a Jasco V-670 spectrophotometer. Elemental analyses were performed by M-H-W Laboratories (Phoenix, AZ). The aminophenol ligands 4,4′-di-tert-butyl-N,N′-bis(3,5-di-tert-butyl-2hydroxyphenyl)-2,2′-diaminobiphenyl (tBuClipH4)28 and N-(3,5-di-tertbutylphenyl)-2-hydroxy-3,5-di-tert-butylaniline (SnipH2)46 were prepared by condensing the appropriate aniline with 3,5-di-tert-butylcatechol in hexane as described in the literature. N-(2,6-Diisopropylphenyl)-3,5di-tert-butyl-o-quinoneimine (Diso) was prepared by acid-catalyzed condensation of 2,6-diisopropylaniline with 3,5-di-tert-butyl-o-benzoquinone.21 J

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The complexes [Fe(C 5 H 4 C 6 H 4 -4-NC 6 H 2 -3,5- t Bu 2 -2-O) 2 ]Pd ([pFlip]Pd),23 [(CH3)2C(CH2NC6H2-3,5-tBu2-2-O)2]Ni ((Clap)Ni),14 and (3,5-tBu2C6H2O2)M ((DTBSQ)2M, M = Pd, Pt)9 were synthesized according to literature methods. (tBuClip)Ni. In a 25 mL Erlenmeyer flask, 0.5014 g of tBuClipH4 (0.711 mmol) and 0.4196 g of nickel(II) 2-ethylhexanoate (Strem, 78% in 2-ethylhexanoic acid, 0.948 mmol) were dissolved in 5 mL of dichloromethane. Triethylamine (500 μL) was added to the solution, which was stirred for 48 h in the air. The resulting dark brown solution was layered with 20 mL of acetonitrile to induce precipitation. After 1.5 h, the layers were mixed and the precipitate was collected via vacuum filtration. Additional acetonitrile (30 mL) was then added to the filtrate, and the precipitate was combined with the first crop of solid. The product was washed with 2 × 3 mL of acetonitrile and airdried for 15 min to give 0.3889 g of (tBuClip)Ni as dark brown crystals (72%). 1H NMR (CDCl3): δ 1.13, 1.35, 1.56 (s, 18H each, tBu), 6.78 (d, 1.7 Hz, 2H, H-3), 7.16 (d, 1.5 Hz, 2H, H-5), 7.39 (dd, 8.2, 1.7 Hz, 2H, H-5′), 7.62 (d, 1.6 Hz, 2H, H-3′), 7.69 (d, 8.2 Hz, 2H, H-6′). 13 C{1H} NMR (CDCl3): δ 29.90, 30.97, 31.43 (C(CH3)3); 35.05, 35.07, 35.29 (C(CH3)3); 115.04, 122.79, 123.16, 124.41, 127.41, 131.65, 141.58, 144.70, 146.39, 150.61, 152.08, 170.16 (CO). IR (cm−1): 2951 (m), 2903 (m), 2866 (m), 1599 (w), 1536 (m), 1523 (m), 1484 (m), 1461 (m), 1437 (m), 1384 (m), 1362 (s), 1315 (m), 1284 (m), 1269 (s), 1246 (m), 1228 (s), 1201 (s), 1177 (s), 1140 (s), 1107 (m), 1022 (m), 1002 (m), 961 (w), 910 (m), 863 (m), 822 (m), 768 (m), 736 (w), 715 (w), 676 (m), 647 (m), 623 (m). Anal. Calcd for C48H64N2NiO2: C, 75.88; H, 8.49; N, 3.69. Found: C, 75.38; H, 8.36; N, 3.68. (tBuClip)Pd. In a 25 mL Erlenmeyer flask, 0.5013 g of tBuClipH4 (0.711 mmol) and 0.3516 g of palladium(II) acetate (1.56 mmol) were dissolved in 15 mL of dichloromethane, and stirred for 2.5 d in the air. The resulting dark green solution was passed through a silica plug using dichloromethane as the eluent to remove the Pd(0) that was formed. The eluate was collected in a 100 mL round-bottom flask and the solvent was removed using a rotary evaporator. The dark green solid was slurried in 10 mL of acetonitrile, collected via vacuum filtration on a glass frit, and washed with 2 × 5 mL of acetonitrile. The product was air-dried for 20 min, yielding 0.4775 g of (tBuClip)Pd as dark green crystals (83%). 1H NMR (CDCl3): δ 1.19, 1.32, 1.53 (s, 18H each, tBu), 6.81 (d, 1.7 Hz, 2H, H-3), 7.02 (br s, 2H, H-5), 7.29 (dd, 8.2, 1.4 Hz, 2H, H-5′), 7.55 (d, 8.3 Hz, 2H, H-6′), 7.59 (d, 1.2 Hz, 2H, H-3′). 13C{1H} NMR (CDCl3): δ 29.90, 31.27, 31.40 (C(CH3)3); 34.91, 35.22, 35.48 (C(CH3)3); 115.73, 122.44, 123.28, 125.69, 128.11, 132.84, 141.35, 144.77, 145.29, 151.03, 152.40, 174.06 (CO). IR (cm−1): 2949 (m), 2902 (w), 2865 (w), 1526 (w), 1482 (m), 1462 (w), 1431 (w), 1384 (m), 1361 (m), 1313 (w), 1268 (m), 1245 (m), 1228 (m), 1201 (m), 1177 (m), 1147 (m), 1110 (m), 1023 (m), 1001 (w), 909 (w), 863 (m), 822 (m), 766 (w). Anal. Calcd for C48H64N2O2Pd: C, 71.40; H, 7.99; N, 3.47. Found: C, 71.44; H, 7.94; N, 3.34. (tBuClip)Pt. In a 50 mL round-bottom flask, 0.5025 g of tBuClipH4 (0.713 mmol), 0.3400 g of potassium tetrachloroplatinate (0.819 mmol), and 0.1502 g of sodium acetate (1.83 mmol) were dissolved in 25 mL of dimethylformamide and the flask was placed in a 60 °C oil bath. The reaction mixture was stirred in the air for 6 h. The resulting dark green solution was transferred to a 500 mL separatory funnel containing 250 mL of distilled water and extracted with 5 × 50 mL of hexanes. The combined organic layers were washed with 5 × 100 mL of water, then evaporated to dryness via rotary evaporation. The residue was dissolved in a minimum volume of 2% acetone in hexanes and purified by column chromatography on silica gel, eluting with the same solvent mixture. The first-eluting dark green fractions were collected and concentrated to dryness. The resulting residue was suspended in 5 mL of acetonitrile, collected via vacuum filtration on a fritted funnel, and washed with 3 × 5 mL of acetonitrile. The product was air-dried for 1 h; yield 0.2145 g (34%). 1H NMR (CDCl3): δ 1.23, 1.32, 1.62 (s, 18H each, tBu), 6.99 (d, 1.8 Hz, 2H, H-3), 7.16 (d, 1.8 Hz, 2H, H-5), 7.33 (dd, 8.3, 1.8 Hz, 2H, H-5′), 7.63 (d, 1.8 Hz, 2H, H-3′), 7.76 (d, 8.3 Hz, 2H, H-6′). 13C{1H} NMR (CDCl3): δ 29.98, 31.31, 31.32 (C(CH3)3); 34.96, 35.18, 35.47 (C(CH3)3); 115.38, 122.54, 122.98,

124.35, 127.90, 133.46, 140.14, 142.99, 145.77, 151.00, 151.02, 174.36 (CO). IR (cm−1): 2952 (m), 2904 (w), 2866 (w), 1600 (w), 1528 (w), 1485 (w), 1460 (w), 1385 (w), 1361 (m), 1267 (w), 1227 (m), 1198 (m), 1175 (s), 1147 (s), 1111 (w), 1024 (w), 1000 (w), 911 (w), 863 (w), 822 (w), 765 (w), 628 (m). Anal. Calcd for C48H64N2O2Pt: C, 64.33; H, 7.20; N, 3.13. Found: C, 64.26; H, 7.15; N, 3.10. (Snip)2Ni. In a 25 mL Erlenmeyer flask, 0.2869 g of SnipH2 (0.700 mmol) and 0.1617 g of nickel(II) 2-ethylhexanoate (78% in 2-ethylhexanoic acid, 0.365 mmol) were dissolved in 5 mL of dichloromethane. After adding 280 μL of triethylamine, the solution was stirred for 3 d in the air. The resulting dark green solution was layered with 20 mL of acetonitrile to induce precipitation. After 2 h, the layers were mixed and the precipitate was collected via vacuum filtration, washed with 5 × 5 mL of acetonitrile, and air-dried for 1 h to give 0.1768 g of (Snip)2Ni as dark green crystals (58%). 1H NMR (CD2Cl2): δ 1.03 (s, 18H, tBu), 1.07 (s, 18H, tBu), 1.39 (s, 36H, tBu), 6.37 (s, 2H, H-3), 6.90 (s, 2H, H-5), 7.46 (d, 1.3 Hz, 4H, H-2′,6′), 7.51 (t, 1.3 Hz, 2H, H-4′). 13C{1H} NMR (CD2Cl2): δ 29.87, 30.87, 31.95 (C(CH3)3); 34.81, 35.16, 35.36 (C(CH3)3); 113.67, 121.19, 121.81, 124.38, 140.84, 145.77, 148.77, 151.09, 155.80, 172.32 (CO). IR (cm−1): 3077 (w), 2962 (m), 2867 (m), 1696 (w), 1596 (w), 1581 (m), 1533 (w), 1457 (m), 1419 (w), 1393 (w), 1381 (w), 1361 (m), 1325 (m), 1288 (m), 1248 (m), 1227 (m), 1198 (m), 1175 (m), 1124 (w), 1108 (m), 1043 (m), 1024 (m), 1009 (m), 971 (w), 910 (m), 899 (w), 878 (w), 854 (m), 824 (w), 771 (m), 734 (w), 700 (m), 668 (m). Anal. Calcd for C56H82N2NiO2: C, 76.96; H, 9.46; N, 3.21. Found: C, 77.00; H, 9.73; N, 3.26. {(SnipH)2Pd}2. In the drybox, 0.3202 g of SnipH2 (0.782 mmol) and 0.1771 g of palladium(II) acetate (0.789 mmol) were added to a 50 mL round-bottom flask and dissolved in 6 mL of dichloromethane. The reaction mixture was stirred for 2 d under nitrogen. The flask was opened to the air and the solvent was removed on a rotary evaporator. The brown residue was suspended in 3 mL of acetonitrile, collected via vacuum filtration on a fritted funnel, and washed with 4 × 3 mL of acetonitrile. The product was then allowed to air-dry for 15 min to give 0.3189 g of {(SnipH)2Pd}2 as a brown solid (89%). 1H NMR (CDCl3): δ 0.89, 1.01, 1.03, 1.16 (s, 36H each, tBu), 6.10 (d, 1.9 Hz, 4H, H-3), 6.19 (t, 1.6 Hz, 4H, NArH), 6.87 (d, 2.0 Hz, 4H, H-5), 7.01 (t, 1.4 Hz, 4H, NArH), 7.41 (t, 1.6 Hz, 4H, NArH), 8.57 (s, 4H, NH). 13 C{1H} NMR (CDCl3): δ 29.59, 31.47, 31.65, 32.00 (C(CH3)3); 33.94, 34.69, 34,78, 35.21 (C(CH3)3); 117.24, 118.61, 118.87, 121.60, 122.54, 136.56, 136.83, 138.63, 147.59, 150.44, 151.61, 164.45 (CO). IR (cm−1): 3066 (w, br, νNH), 2953 (m), 2903 (w), 2868 (w), 1594 (w), 1468 (m), 1440 (m), 1410 (m), 1362 (m), 1293 (m), 1251 (s), 1201 (w), 1159 (w), 1119 (w), 1070 (w), 972 (m), 873 (m), 829 (m), 769 (w), 714 (m), 610 (m). Anal. Calcd for C56H84N2O2Pd: C, 72.82; H, 9.17; N, 3.03. Found: C, 73.09; H, 8.97; N, 2.76. (Snip)2Pd. In a 25 mL Erlenmeyer flask, SnipH2 (0.3962 g, 0.967 mmol), palladium(II) acetate (0.1343 g, 0.598 mmol), and diethyl azodicarboxylate (155 μL, 0.988 mmol) were dissolved in 5 mL of dichloromethane. The solution was stirred for 8 h, turning from red to dark blue. The solution was filtered through a plug of silica gel, eluting with dichloromethane. The eluent was evaporated to dryness on a rotary evaporator and the residue was slurried in 3 mL of acetonitrile. The product was collected on a glass frit, washed with 3 × 5 mL of acetonitrile, and air-dried 30 min to give 0.3684 g of (Snip)2Pd as dark blue crystals (83%). 1H NMR (CD2Cl2): δ 1.06 (s, 18H, tBu), 1.09 (s, 18H, tBu), 1.34 (s, 36H, tBu), 6.33 (br s, 2H, H-3), 6.70 (br s, 2H, H-5), 7.23 (d, 1.5 Hz, 4H, H-2′,6′), 7.36 (t, 1.5 Hz, 2H, H-4′). 13 C{1H} NMR (CD2Cl2): δ 29.58, 30.93, 31.72 (C(CH3)3); 34.70, 34.80, 35.10 (C(CH3)3); 113.38, 120.85, 121.05, 125.62, 139.90, 145.19, 146.91, 150.73, 155.83, 175.82 (CO). IR (cm−1): 2954 (m), 2902 (w), 2867 (w), 1582 (w), 1532 (w), 1517 (w), 1477 (w), 1431 (w), 1382 (w), 1359 (m), 1325 (m), 1248 (s), 1199 (m), 1177 (m), 1110 (m), 1043 (m), 1025 (m), 1008 (m), 973 (w), 908 (m), 876 (m), 857 (m), 822 (w), 770 (m), 706 (m), 672 (s). Anal. Calcd for C56H82N2O2Pd: C, 72.98; H, 8.97; N, 3.04. Found: C, 72.85; H, 9.17; N, 2.84. (Snip)2Pt. Into a 50 mL round-bottom flask were placed 0.2083 g of SnipH2 (0.508 mmol), 0.1175 g of K2PtCl4 (0.283 mmol), 0.0646 g of NaOAc (0.787 mmol), and 10 mL of dimethylformamide. The flask K

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry was placed in a 65 °C oil bath and the reaction mixture was stirred in the air for 6 h at this temperature. The resulting indigo solution was chilled 1 h in a −40 °C freezer, suction filtered, and the solid was washed with 4 × 4 mL of acetonitrile. The product was purified further by dissolving in dichloromethane and filtering through a silica plug using dichloromethane as the eluent. The eluate was evaporated to dryness; the dark blue solid was suspended in 3 mL of acetonitrile and isolated by suction filtration. After washing with 2 × 3 mL of acetonitrile and air-drying 1 h, the yield of (Snip)2Pt was 0.1069 g (42%). 1H NMR (CD2Cl2): δ 1.12 (s, 18H, tBu), 1.16 (s, 18H, tBu), 1.37 (s, 36H, tBu), 6.39 (d, 1.6 Hz, 2H, H-3), 6.89 (d, 1.6 Hz, 2H, H-5), 7.36 (d, 1.4 Hz, 4H, H-2′,6′), 7.49 (t, 1.4 Hz, 2H, H-4′). 13C{1H} NMR (CD2Cl2): δ 29.91, 31.03, 31.87 (C(CH3)3); 35.11, 35.24, 35.46 (C(CH3)3); 114.38, 121.74, 121.93, 125.72, 138.28, 143.54, 147.94, 151.53, 153.36, 177.77 (CO). IR (cm−1): 3074 (w), 2960 (m), 2904 (w), 2868 (w), 1599 (w), 1581 (w), 1539 (m), 1476 (m), 1457 (w), 1419 (w), 1393 (w), 1383 (w), 1359 (m), 1321 (m), 1283 (w), 1245 (m), 1227 (m), 1195 (m), 1174 (m), 1112 (m), 1045 (m), 1024 (w), 1011 (m), 982 (w), 912 (w), 899 (w), 876 (w), 856 (m), 844, 828 (m), 770 (m), 734 (w), 709 (m), 668 (m). Anal. Calcd for C56H82N2O2Pt: C, 66.57; H, 8.18; N, 2.77. Found: C, 66.65; H, 8.25; N, 2.70. (Diso)2Ni. In the drybox, 0.1960 g of Diso (0.516 mmol) and 0.0777 g of bis(1,5-cyclooctadiene)nickel(0) (Strem, 0.282 mmol) were dissolved in 5 mL of dry tetrahydrofuran. The solution was stirred for 2 h under nitrogen. The resulting dark green solution was exposed to air and the solvent was evaporated off using a rotary evaporator. The residue was dissolved in dichloromethane and eluted through a silica plug using CH2Cl2. The green eluate was evaporated to dryness to yield a dark green solid that was slurried in 3 mL of acetonitrile, collected by vacuum filtration on a fritted funnel, washed with 3 × 3 mL of CH3CN, and air-dried 30 min to give 0.1492 g of (Diso)2Ni (74%). 1 H NMR (CDCl3): δ 0.99 (s, 18H, tBu), 1.04 (s, 18H, tBu), 1.05 (d, 6.8 Hz, 12H, CH(CH3)(CH′3)), 1.34 (d, 6.8 Hz, 12H, CH(CH3)(CH′3)), 3.31 (sept, 6.8 Hz, 4H, CH(CH3)2), 6.13 (d, 1.1 Hz, 2H, H-3), 6.86 (sl br s, 2H, H-5), 7.22 (d, 7.7 Hz, 4H, H-3′,5′), 7.38 (t, 7.7 Hz, 2H, H-4′). 13C{1H} NMR (CDCl3): δ 24.14, 24.65 (CH(CH3)2); 29.11 (CH(CH3)2); 29.97, 30.93 (C(CH3)3); 34.52, 34.85 (C(CH3)3); 113.39, 123.14, 124.57, 126.89, 140.30, 143.76, 144.03, 145.25, 156.39, 171.31 (CO). IR (cm−1): 2961 (m), 2927 (w), 2904 (w), 2867 (w), 1533 (w), 1521 (w), 1462 (w), 1445 (w), 1380 (w), 1359 (m), 1318 (w), 1284 (w), 1255 (w), 1237 (w), 1199 (m), 1168 (m), 1106 (m), 1021 (m), 1001 (w), 921 (w), 857 (w), 826 (w), 799 (w), 772 (w), 742 (m), 687 (w), 669 (m). Anal. Calcd for C52H74N2NiO2: C, 76.37; H, 9.12; N, 3.43. Found: C, 76.46; H, 8.94; N, 3.34. Cyclic Voltammetry. Cyclic voltammograms were run using a Metrohm Autolab PGSTAT128N potentiostat. The working and counter electrodes were glassy carbon, and potentials were controlled relative to a silver/silver chloride pseudo-reference electrode. The electrodes were connected to the potentiostat through electrical conduits in the drybox face plate. Samples were 1 mM in analyte dissolved in CH2Cl2, with 0.1 M Bu4NPF6 as the electrolyte. In the case of (Snip)2Ni, voltammograms were also run using Bu4N[B(C6H3-3,5(CF3)2)4] as electrolyte.47 Potentials were referenced to ferrocene/ ferrocenium at 0 V,48 with the reference potential established by spiking the test solution with a small amount of decamethylferrocene (E° = −0.565 V vs Cp2Fe+/Cp2Fe).49 Scans were started in the oxidative direction from an initial potential of approximately −0.5 V. Determination of Singlet−Triplet Gaps by Variable-Temperature NMR. NMR spectra were measured on a Bruker 400 MHz or a Varian 500 MHz spectrometer. Samples were dissolved in the air in C2D2Cl4 or 1,2-C6D4Cl2 (Cambridge Isotope Laboratories, used as received) in Teflon-valved J. Young NMR tubes; the solutions were subsequently degassed on the vacuum line and backfilled with nitrogen. Spectra were typically measured every 10 °C and were referenced relative to the solvent residual signals (δ 5.90 for C2HDCl4 and δ 7.19 for the downfield residual in C6D4Cl2). The temperature-dependent behavior of the chemical shifts was modeled as described in the literature,13 except that, due to the large singlet−triplet gaps exhibited by these compounds, satisfactory fitting,

especially in the low-temperature regime, required accounting for the temperature dependence of the diamagnetic chemical shift, which was fit as linearly dependent on temperature (eq 7). (Because of its small singlet−triplet gap and consequently large temperature dependence, the temperature dependence of the diamagnetic chemical shifts in (DTBSQ)2Pd was neglected.)

δobsd = δdia,0°C + α(T − 273.15 K) +

2gβA (3 + eΔE / RT )−1 (γNuc/2π )kT

(7)

δobsd = δdia,0°C + α(T − 273.15) A + (63150) (3 + eΔE /0.001987T )−1 T

(7′)

1 H NMR chemical shifts were thus fit to eq 7′, with δ in units of ppm, T in K, A in MHz, and ΔE in kcal mol−1. For the bis(iminosemiquinone) complexes, all aromatic resonances were fit, as was the methylene resonance in (Clap)Ni. In (DTBSQ)2Pd, the H-5 resonance was too broad to be observed, but the observed aromatic resonance and both tert-butyl resonances were modeled; for (DTBSQ)2Pt, both aromatic resonances were modeled. Unweighted nonlinear leastsquares fitting was carried out, optimizing the diamagnetic chemical shift at 0 °C δdia,0°C, the temperature-dependence of the diamagnetic shift α, and the hyperfine coupling constant A for each resonance, as well as a single global value of ΔE for all resonances, using the Levenberg−Marquardt algorithm as implemented in the Solver routine of Microsoft Excel.50 Global optima for these least-squares fits could be obtained in all cases (Table S3). However, because of the large singlet−triplet gaps of the iminosemiquinones, the accessible temperature ranges show only moderately curved chemical shift vs temperature plots, and consequently, the fitted values of ΔE and A are highly correlated (lowering ΔE has approximately the same effect on the calculated values as raising all the A values proportionately). In order to prevent this correlation from introducing nonphysical A values (and correspondingly inaccurate ΔE values), the A value of one resonance (H-5, located para to nitrogen in the iminosemiquinone ring, usually the proton with the A value that was largest in magnitude) was fixed at the value calculated by DFT for the triplet state of the molecule (except for (DTBSQ)2Pd, where this resonance is too broad to be observed). All other A values, as well as the ΔE value, were then optimized to give the values in Table 3. Uncertainties in the fitted parameters were estimated using established methods.51 Because fixing AH‑5 artificially reduces the apparent statistical uncertainty of the fit, the stated uncertainties are those derived from the unconstrained fits. Redox Titrations. In the drybox, in a 20 mL scintillation vial, 0.010 g of the analyte was dissolved in 10 mL of dichloromethane. This stock solution (100 μL, or 40 μL for Pt compounds due to their higher extinction coefficients) was added to a cuvette containing 2 mL of dichloromethane and sealed with a septum cap. A stock solution of acetylferrocenium hexafluorophosphate52 or cobaltocene was prepared by adding 0.005 g of the redox agent to a 20 mL scintillation vial and dissolving in 10 mL of dichloromethane. An aliquot of this solution was transferred to a 2 mL vial and sealed with a septum cap to be brought to the UV−vis−NIR spectrophotometer. For the nickel complexes, reduction was incomplete in dichloromethane, and in these instances, the titrations were carried out in 100 mM [Bu4N]PF6 in dichloromethane rather than neat dichloromethane. Absorption maxima were unaffected by the presence or absence of the electrolyte. The spectrophotometer was blanked against neat dichloromethane, and an initial spectrum of the neutral analyte was taken. Incremental additions of the redox agent solution were then syringed into the cuvette with an airtight Hamilton syringe, and spectra were measured after each addition. The redox titration was deemed complete once no changes to the optical spectra were observed for five consecutive additions of redox agent. X-ray Crystallography. Crystals were grown by liquid−liquid diffusion (for (SnipH)2Pd·0.07CHCl3 and (Snip)2Ni·CHCl3, CH3CN

L

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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into CHCl3; for (Snip)2Pd·C6H6, CH3CN into C6H6; for (Diso)2Ni, CH3CN into CDCl3; for (tBuClip)Ni, CH3CN into CH2Cl2; and for (tBuClip)Pt·2C6H6·2DMSO, DMSO into C6H6). Crystals were placed in Paratone oil before being transferred to the cold N2 stream of the diffractometer (T = 120 K). Data were reduced and corrected for absorption using the program SADABS. After structure solution using direct or Patterson methods, non-hydrogen atoms not apparent from the initial solutions were found on difference Fourier maps, and all heavy atoms were refined anisotropically. Disordered tert-butyl groups (C87 in (SnipH)2Pd·0.07CHCl3, C37 in (tBuClip)Ni) were treated by constraining opposite methyl groups in the two orientations to have the same thermal parameters and allowing the occupancy factors of the two orientations to refine. The lattice THF in (Snip)2Pt·THF was disordered about the inversion center, and was modeled as being in two half-occupied orientations related by the inversion center. In (Diso)2Ni, the diisopropylphenyl group was found in two locations, and modeled with corresponding atoms in the two components constrained to have the same thermal parameters and allowing the occupancy to refine. Hydrogen atoms were typically found on difference maps and refined isotropically, with the exceptions of the hydrogens on disordered groups. The exceptions to this general treatment were (SnipH)2Pd· 0.07CHCl3, in which only the hydrogens bonded to N were found on difference maps and refined, and (Diso)2Ni and (tBuClip)Pt·2C6H6· 2DMSO, where all hydrogens were placed in calculated positions. Calculations used SHELXTL (Bruker AXS),53 with scattering factors and anomalous dispersion terms taken from the literature.54 Further details about the structure are given in Table S1. Computational Methods. Computationally, all structures were stripped down to replace all tert-butyl groups with hydrogen atoms, except for (DTBSQ)2Pd and (DTBSQ)2Pt, where the tert-butyl groups were included. Geometry optimizations were performed on all compounds separately as triplet gas-phase molecules using an SDD basis set for the metal atom and a 6-31G* basis set for all other atoms and a B3LYP functional using the Gaussian09 suite of programs.55 Geometries were optimized by minimizing the energies of the compounds, and optimized geometries were confirmed to be minima by the lack of imaginary frequencies in the vibrational analysis. Plots of calculated Kohn−Sham orbitals were generated using Gaussview (v. 5.0.8) with an isovalue of 0.04.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (CHE-1465104). We gratefully acknowledge the assistance of Dr. Allen G. Oliver with the X-ray crystallography.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00062. X-ray summary data, cyclic voltammograms, redox titrations, plots of chemical shift vs temperature data, comparison of constrained and unconstrained fits of the VT NMR data, and Cartesian coordinates and energies of calculated structures (PDF) Accession Codes

CCDC 1814601−1814607 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Seth N. Brown: 0000-0001-8414-2396 Notes

The authors declare no competing financial interest. M

DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00062 Inorg. Chem. XXXX, XXX, XXX−XXX