Ligand Design toward Multifunctional Substrate Reductive

Aug 1, 2017 - First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands. Lukas AligMaximilian FritzSven Schneider...
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Ligand Design toward Multifunctional Substrate Reductive Transformations Alexander V. Polezhaev,† Chun-Hsing Chen,† Adam S. Kinne,† Alyssa C. Cabelof,‡ Richard L. Lord,‡ and Kenneth G. Caulton*,† †

Department of Chemistry, Indiana UniversityBloomington, Bloomington, Indiana 47405, United States, Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States

‡

S Supporting Information *

ABSTRACT: The synthesis of bis(N1-phenyl-5-hydroxypyrazol-3-yl)pyridines (“L”) is described, and these are silylated to achieve analogues (“Si2L”) without the variable of the hydroxyl proton mobility. One hydroxyl example is characterized in its bis-pincer iron(II) complex, which shows every OH proton involved in hydrogen bonding. The steric bulk of the silylated N-phenyl-substituted ligands allows the synthesis and characterization of paramagnetic (Si2L)FeCl2 complexes, and one of these is reduced, under CO, to give the diamagnetic (Si2L)Fe(CO)2 species. Structural comparison and density functional theory calculations of the dichloride and dicarbonyl species show that much, but not all, of the reduction occurs at both the ligand pyridine and pyrazole rings, and thus this ligand type is more resistant to reduction than the simpler bis(iminopyridines). The OSiR3 substituent offers a useful diagnostic of reduction at pyrazole via the degree of π-donation to pyrazole by the oxygen lone pairs, and the stereoelectronic features of the NPh moiety are analyzed. The X-ray photoelectron spectroscopy binding energies of both iron and nitrogen are analyzed to show details of the locus of reduction.

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INTRODUCTION Bis(pyrazolyl)pyridines (Pz2Py) are ligands that have attracted a great deal of attention for their modularity, and thus tunability,1 in spin-crossover chemistry,2 always as (Pz2Py)2MII complexes, thus with a sterically saturated metal center. There are a few examples of Pz2Py complexes with 1:1 ligand-to-metal stoichiometry, but all of them include metals in positive oxidation states (FeII, CoII, MnII, and RuII) and high coordination number (6).3−6 We are interested in the catalytic transformations of hard-to-reduce molecules such as N2 and CO2 to valuable products with a single-site catalyst and were attracted to the bis(pyrazolyl)pyridine family of ligands because of their potential to be redox-active and also proton-responsive. Pz2Py contains two redox-active 1,4-diazabutadiene units (in red in Figure 1), connected by the pyridine N atom, thus lowering the energy of electron storage in those units. The Pz2Py isomer should be viewed as a variant of a bis(iminopyridine) ligand but one that is annulated to an aminoolefin unit, which makes it a new electrondonor substituent. This alone places it in a unique 1,1′-(pyridine2,6-diyl)bis(N-arylmethanimine) (PDI) class, by carrying electron-donating substituents versus the usual bulky PDI aryl substituents, whose bulk prevents conjugation of any π effects to the imine. Bis(pyrazoles) should therefore have more reducing power, once reduced. Because there is no precedent of bis(pyrazolyl)pyridines among reduced metal complexes, we worked to explore that topic. We report here a rational design of new bis(pyrazolyl)pyridine-based ligands and the investigation © 2017 American Chemical Society

of their electronic and steric properties in complexes with iron in different oxidation states.

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RESULTS AND DISCUSSION Our preliminary work with H2L iron and cobalt complexes showed that after deprotonation they have a tendency to form polynuclear aggregates.7 Seeking a rational design of bis(pyrazole)pyridine-type ligands for the reductive activation of small molecules, our goals were to retain the electron-rich character of the ligand, increase the steric bulk around the metal center, and keep H+ responsivity. To achieve these goals, we installed phenyl (or other aryl) rings on the pyrazole N atoms and acidic OH groups in the 5 position of the pyrazole ring (Figure 2). Installation of OH groups in the 5 position of the pyrazole rings opened an opportunity to easily turn off the proton responsivity by silylation to avoid H2 formation from 2H+ under reductive conditions. We also envision that R3Si+ groups could be transferred to the substrate (such as CO2) and thus serve as a reduction-resistant analogue of H+. Synthesis of Ligand Precursors. Ligand precursor synthesis used conventional 3-hydroxypyrazole methodology, which involves 1,3-keto ester formation via Claisen condensation and subsequent cyclization of a bis(1,3-keto ester) 2 with phenylhydrazine (Scheme 1).8 The 1H NMR spectrum of H2L in Received: March 26, 2017 Published: August 1, 2017 9505

DOI: 10.1021/acs.inorgchem.7b00785 Inorg. Chem. 2017, 56, 9505−9514

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Figure 1. Bis(pyrazolyl)pyridine and PDI ligand redox properties.

Figure 2. Bis(3-hydroxypyrazolyl)pyridine ligand design (S = substrate).

Figure 3. Mercury view of the non-H atoms of compound 5a showing selected atom labeling. The unlabeled atoms are C. A crystallographic mirror plane relates the two pyrazole arms. Thermal ellipsoids are shown at 50% probability.

dimethyl sulfoxide (DMSO)-d6 shows all seven expected CH signals and indicates 2-fold symmetry, consistent with the equivalent pyrazole arms of the pincer form. Acidic protons appear as a broad singlet at 11.9 ppm. 13C NMR spectra are in agreement with the 2-fold symmetry; the 3-pyrazole carbon signal appears at 86.7 ppm, in agreement with the 3-hydroxypyrazole tautomeric form.9 Silylation of 4 with Me3SiCl or Me2tBuSiCl was performed in tetrahydrofuran (THF) using triethylamine (Et3N) as the base at room temperature (RT) overnight. Subsequent filtration, concentration of a THF solution, and addition of pentane causes precipitation of a white powder in quantitative yield. Silylated ligand precursors show good solubility in most organic solvents including nonpolar (benzene and toluene). 1H NMR spectroscopy shows the disappearance of the OH proton signal and the appearance of the tetramethylsilane (TMS) group proton signals with an intensity of 18H; the 13C NMR spectroscopy, electrospray ionzation mass spectrometry (ESI MS), and molecular structure of compound 5a (Figure 3) confirm the structure of the product. Single crystals of compound 5a were obtained by cooling a concentrated pentane solution to −45 °C. The molecular structure of 5a obtained by X-ray diffraction (XRD; Figure 3) is in agreement with spectroscopic data. The two arms are related by a mirror plane perpendicular to the pyridine ring. The two pyrazole arms are nearly coplanar to the pyridine ring (torsion angle = 7.7°), and the phenyl rings are slightly rotated (torsion angle = 28.3°), showing some conjugation between the aromatic systems. The pyrazole rings are rotated so that the α-N atoms (N2) are trans to the pyridine N atom to avoid N−N lone-pair repulsion. Metalation of Ligand Precursors. The reaction of a suspension of compound 4 in THF with an equimolar slurry of

anhydrous FeCl2 in THF yields a lemon-yellow solid and a colorless solution after stirring for 12 h at RT (Scheme 2). The precipitate was filtered and washed with THF. The product is insoluble in most organic solvents except for DMSO, acetonitrile (MeCN), and methanol (MeOH). The 1H NMR spectrum of the resulting product in CD3CN shows seven signals of appropriate intensity for a 2-fold-symmetric pincer ligand (Figure 4a). The range of chemical shifts −8.3 to +58.5 ppm indicates paramagnetism, but all signals including the OH protons are clearly evident; ESI and APCI MS show a positive ion of m/z 845 that is consistent with the formula [(H2L)(HL)Fe]+, indicative of a 2:1 ligand-to-metal stoichiometry. Initially, we expected the formation of a complex with 1:1 M/L stoichiometry but it is well-known for Fe2+ to form 2:1 L/M complexes with planar tridentate ligands (FeCl2 + L = [L2Fe]2+(FeCl4)2−), and polar solvents encourage this transformation.10 The insolubility of compound 4 in nonpolar solvents also influences this result. To solve the solubility problem, the product was treated with 2 equiv of TMSOTf to replace chloride by triflate. In 2 min after the addition of TMSOTf, the yellow slurry turns to a deep-yellow solution. After 2 h of additional stirring, volatiles were removed and the oily residue was triturated with pentane, forming a yellow powder. The 1H NMR spectrum of the product in CD2Cl2 shows the same pattern of signals as that for the starting material 6 (Figure 4b), indicating that the ligand−metal assembly remains untouched and only the counterion was exchanged. The 19F NMR spectrum shows a singlet at −79 ppm, in agreement with the noncoordinated TfO− anion. Mass spectral positive ions at

Scheme 1. Synthesis of Ligand Precursors

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DOI: 10.1021/acs.inorgchem.7b00785 Inorg. Chem. 2017, 56, 9505−9514

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Inorganic Chemistry Scheme 2. Metallation of Ligand Precursor 4

Figure 4. (a) 1H NMR spectrum of compound 6 in CD3CN. (b) 1H NMR spectrum of compound 6(OTf) in CD2Cl2. 1H NMR spectra of compound 6 in CD3OD (c) immediately after dissolution and (d) after 1 h.

Figure 5. Two views of the molecular structure of compound 6 showing selected atom labeling. Thermal ellipsoids are shown at 50% probability. H atoms, triflate counterions, and DCM are omitted for clarity. Selected structural parameters: Fe1−N3 2.116 Å, Fe1−N8 2.110 Å, Fe1−N2 2.178 Å, Fe1−N4 2.204 Å, Fe1−N7 2.243 Å, Fe1−N9 2.183 Å. The figure at the right shows ligand twisting and phenyl conformations.

m/z 845 and 995 due to [(H2L)(HL)Fe]+ and [[(H2L)2Fe](OTf)]+ are in agreement with the 2:1 ligand-to-metal stoichiometry.

The 1H NMR spectrum of [(H2L)2Fe]Cl2 in CD3OD shows the immediate disappearance of the signal at 17.8 ppm due to 9507

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five-coordinate iron structures shows the distances here to be consistent with high-spin FeII. The dihedral angle N2−Cpz− Cpy−N3 is 4.95°; hence, the ligand rings are eclipsed. Electrochemistry. We are interested in reductive catalytic processes, and the character of the reduction (ligand- or metalbased) is a key question for catalysis design. Cyclic voltammetry (CV) was studied at a glassy carbon working electrode in DCM with 0.1 M [NBu4][PF6] at scan rates from 60 to 200 mV s−1. Free ligand precursor 5a shows no oxidative or reductive waves in DCM from +1.5 to −2.7 V versus Fc/Fc+, confirming that Si2L is harder to reduce than PDI13 and consistent with the pyrazole being relatively electron-rich. In contrast, compound 7b shows one reversible oxidation (E1/2 = −0.17 V vs Fc/Fc+) and one irreversible reduction, at Epc of −2.5 V (Figures S1−S3). We next turned to a chemical reductant to reach this potential. Chemical Reduction. 7b was chemically reduced and trapped with CO, forming complex (Si2L)Fe(CO)2 (8). The stirring of 7b with 2 equiv of KC8 in the presence of 1 atm of CO furnished a deep-brown solid identified as 8, based on NMR and IR spectroscopies and single-crystal XRD (Figure 8). At the same time, the reduction of compound 7a under a CO atmosphere does not produce a carbonyl-containing species according to solution IR spectroscopy (Scheme 4). We suggest that the TMS group was attacked during reduction, leading to decomposition. As expected for an S = 0, five-coordinate iron complex, the 1H and 13C NMR spectra of 8 in benzene-d6 or toluene-d8 display the number of resonances in the diamagnetic region expected for a molecule with C2 symmetry. HMQC and HMBC spectra of compounds 5b and 8 allowed the assignment of all of the signals. The pyridine ortho and para carbon signals in the 13C NMR spectrum of compound 8 show significant upfield shifts compared to those of free ligand 5b, which correlates well with results of DFT showing the highest occupied molecular orbital (HOMO) of compound 8 partially located on the pyridine ring (Figures 7 and S4). The redox capability of the ligand 5b can be compared to that of other reduced (PDI)Fe(CO)2 complexes in the literature, including the well-studied pyridine bis(diimine) (PDI)Fe(CO)214,15 and pyridine bis(phosphinomethyl) (PNP)Fe(CO)2.16,17 These complexes serve as standards where reduction occurs partially on the ligand (PDI)Fe(CO)2 versus definitely on the metal (PNP)Fe(CO)2. A comparison of the structural (X-ray), spectroscopic [IR and X-ray photoelectron spectroscopy (XPS)], and electrochemical data can answer the question of the metal oxidation state. In the IR spectrum of 8, two intense carbonyl bands were observed at 1935 and 1868 cm−1, slightly reduced from those reported for (PDI)Fe(CO)217 (1950 and 1894 cm−1) and substantially increased compared to those of (PNP)Fe(CO)216 (1842 and 1794 cm−1). The dramatic shift of the CO bands to higher frequencies compared to authentic iron(0) in (PNP)Fe(CO)2 and their similarity to (PDI)FeII(CO)2 indicates a reduction (or strong back-donation) into the Si2L pincer in 8. Therefore, the degree of pincer reduction in 8 is significantly greater than that in (PNP)Fe(CO)2 and closer to that in (PDI)Fe(CO)2. The slow diffusion of pentane vapor into a concentrated THF solution of 8 at −35 °C produced single crystals suitable for XRD. The solid-state structure is presented in Figure 8. The molecule is rigorously C2-symmetric (axis defined by Fe1 and N3). The overall geometry about the iron in 8 is between squarepyramidal and trigonal-bipyramidal (τ5 = 0.5), in contrast to the distorted square-pyramidal structure observed for (PDI)Fe(CO)2 (τ5 = 0.01) and nearly trigonal-bipyramidal structure for

hydrogen/deuterium (H/D) exchange of acidic OH protons and the slow (in 1 h) disappearance of the signal at 54.8 ppm associated with H/D exchange at the 4 position of pyrazole rings due to reversible 5-hydroxypyrazole/5-pyrazolone tautomerization (Figure 4c,d). ESI MS(+) of the deuterated product shows that up to eight protons per cation were exchanged to deuterium. [(H2L)2Fe](OTf)2 was synthesized by an alternative route by stirring 2 equiv of H2L with Fe(OTf)2 in THF for 2 h. All spectral data for this product are identical with those obtained from [(H2L)2Fe]Cl2 using TMSOTf (Scheme 2). Needlelike yellow crystals of [(H2L)2Fe](OTf)2 were grown from THF by the slow diffusion of pentane vapors at −45 °C. The molecular structure of [(H2L)2Fe](OTf)2 (Figure 5) shows two independent cations [(H2L)2Fe]2+, both having distorted octahedral geometry around iron. Both pincer ligands are not perfectly planar; all four planes formed by pyrazole rings deviate from the corresponding pyridine planes at different angles (3.5°, 17.0°, 14.0°, and 22.1°). The phenyl ring planes are significantly rotated at different angles to the plane of the corresponding pyrazoles (40.6°, 53.1°, 40.2°, and 54.3°). At the same time, the planes of the phenyl ring of one pincer ligand are nearly parallel (16.4°, 15.3°, 5.4°, and 15.5°) to the planes of the pyridine ring of the second pincer ligand with interplane distances of 3.65−3.93 Å that are in the range of strong π−π interaction.11 Iron to pyridine nitrogen bond distances (Fe−Npy) are in the range of 2.11−2.14 Å, and iron to pyrazole nitrogen distances (Fe−Npz) are in the range of 2.18−2.24 Å, all in good agreement with the previously reported dicationic high-spin iron(II) bis(pyrazolyl)pyridines.1 Three of the four triflate anions are disordered, but all of them show strong hydrogen-bonding interactions with pyrazole OH groups (O−O ∼ 2.6 Å). Metalation of silylated ligands 5a and 5b proceeds smoothly in typical organic solvents. A colorless solution of 5a in dichloromethane (DCM), after the addition of solid FeCl2, changes color to pink in minutes, and after 4 h, the suspension becomes homogeneous, indicating complete reaction (Scheme 3). After Scheme 3. Metalation of Silylated Ligand Precursors 5a (R = Me) and 5b (R = tBu) (“Si2LFeCl2”)

the concentration and addition of pentane, a yellow solid precipitates from the solution. The 1H NMR spectrum of the precipitate (Si2L)FeCl2 (7b; from SiMe2tBu) in CD2Cl2 shows eight sharp resonances in the range of 80 ppm, indicating paramagnetism with relative intensities of 1:2:2:2:4:4:12:18, a characteristic pattern for C2 symmetry. The slow diffusion of pentane vapors into a DCM solution of the product affords needlelike yellow crystals suitable for XRD analysis. The molecular structure of compound SiLFeCl2 (7a) confirms the desired 1:1 ligand-to-metal stoichiometry (Figure 6). The molecule has crystallographic C2 symmetry, in full agreement with conclusions from the 1H NMR spectrum. The Fe−Npy distances are 0.08 Å shorter than the Fe−Npz distances. The three rings of the pincer ligand do not deviate significantly from coplanarity, and the five-coordinate structure has a τ5 parameter of 0.34,12 where a trigonal-bipyramidal structure has τ = 1.0 and a square pyramid τ = 0.0. A comparison to the other 9508

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Figure 6. Molecular structure of compound 7a showing selected atom labeling. The unlabeled atoms are C. Thermal ellipsoids are shown at 50% probability. H atoms are omitted for clarity. A crystallographic C2 axis is defined by Fe and N3. Selected structural parameters: Fe1−N3 2.110 Å, Fe1−N2 2.293 Å, Fe1−Cl1 2.298 Å, C1−O1 1.333 Å; N2−Fe1−N3 74.30°, N2−Fe1−N2* 148.61°, N3−Fe1−Cl1 115.98°, Cl1−Fe−Cl1 128.03°.

(PNP)Fe(CO)2 (τ5 = 0.77). In 8, the two carbonyl ligands and the pyridine N atom define the equatorial plane with a bond angle C−Fe−N of 125.77(18)°. Complex 8 has a more contracted C−Fe−C angle [108.5(4)°] compared to (PNP)Fe(CO)2 (119.9°) and a wider angle than that in (PDI)Fe(CO)2 (97.0°), consistent with a reduced π-back-bonding to the pincer ligand compared to (PNP)Fe(CO)2 and increased compared to (PDI)Fe(CO)2. The Fe−C and C−O structural parameters in 8 lie between the reduced Fe center in (PNP)Fe(CO)2 and the oxidized iron in (PDI)Fe(CO)2. In (PNP)Fe(CO)2 and (PDI)Fe(CO)2, the Fe−C bond distances of 1.7325(9) and 1.7809(19)/1.7823(19) Å are observed along with the C−O bond lengths of 1.1734(11) and 1.147(2) Å, respectively. The corresponding distances in 8, 1.765(6) and 1.162(6) Å, are intermediate. Pincer-based reduction is associated with significant changes in the intrapincer bond lengths compared to nonreduced forms. Significant shortness of the central bond in two diazabutadiene fragments of the PDI ligand together with elongation of the C−N imino bond and in the pyridine ring is always observed in PDI complexes with formally low-valent metals. In 8, the same tendency is observed. The C−C bonds between the pyrazole and pyridine rings (Figure 9) are 0.04 Å shorter for 8 than those in the parent

Scheme 4. Reduction of Compound 7b under a CO Atmosphere

Figure 7. Difference in the 13C NMR chemical shifts of compounds 8 and 5b and the calculated HOMO of compound 8.

Figure 9. Difference in the pincer bond distances between the dichloro and dicarbonyl adducts of (PDI)Fe (right) and (Si2L)Fe (left).

(Si2L)FeCl2 7a, and the C−Npy and C−Npz bonds are 0.04 and 0.03 Å longer, respectively. Elongation of the C−Npy bond is bigger than that of C−Npz in 8, in agreement with the pyridine being easier to reduce than pyrazole. In 8, the C−O(Si) bond length is 0.04 Å longer than that in (Si2L)FeCl2 and very close to the C−O bond length in the free (Si2L) ligand. This shows reduced R3SiO π donation to the pyrazole ring when the complex is reduced. The electrophilic

Figure 8. Mercury drawing of complex 8. Thermal ellipsoids are shown at 50% probability, and H atoms are omitted for clarity. Selected structural parameters: Fe1−N2 1.947(3) Å, Fe1−N3 1.886(5) Å, Fe1− C19 1.765(6) Å, Si1−O1 1.675(3) Å, O1−C1 1.340(5) Å, O2−C19 1.162(6) Å; N2#1−Fe1−C19 101.16(17)°, C19#1−Fe1−C19 108.5(4)°, N2−Fe1−C19 92.54(18)°, N3−Fe1−C19 125.77(18)°, Si1−O1−C1 124.8(3)°, O2−C19−Fe1 175.6(4)°. 9509

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Inorganic Chemistry Fe2+ in (Si2L)FeCl2 encourages oxygen-lone-pair donation to the aromatic system of the pyrazole ring and thus a shorter C−O bond. After reduction, a partial negative charge located on the aromatic system of the pyrazole rings decreases oxygen π donation and elongation of the C−O bond back to the length in the free ligand. The torsion angle between the pyrazole and phenyl rings is also changed from 38° in 7b to 72° (28° in the free ligand 5a) because of repulsion with the carbonyl ligands in 8, and this indicates that steric protection of the metal center is achieved as planned. XPS. We also employed XPS for characterization of the electron density at the metal center.18−24 With respect to determination of the metal oxidation states using XPS, it is independently reported21,25 that, irrespective of the metal, a formal oxidationstate change of 1 is accompanied by a change in the electron binding energy (BE) of 0.8−1.2 eV. XPS data for a series of [pincer]Fe(L)2 complexes (L = Hal, CO) were recently reported by Milstein et al.23,24 The BE of 709.7 eV for 7b with Fe 2p3/2 is in good agreement with those previously reported. Both the Fe 2p3/2 and Fe 2p1/2 peaks have broad shake-up satellites at ∼4 eV higher BEs that are also characteristic of FeII. For 8, we observe a BE value of 708.7 eV (Figures S5−S8). The literature values for low-valent iron complexes (PDI)Fe(CO)2 (708.4 eV) and (PNP)Fe(CO)2 (708.1) are in a good agreement with our observation for 8. Chirik et al. specifically avoid a single-oxidation-state assignment of (PDI)Fe(CO)2 (708.4 eV),14 and Neidig et al.24 considered (PNP)Fe(CO)2 (708.1) to be an Fe0 example with a significant amount of backdonation to the ligand. We believe that absolute values of the BE obtained from XPS are not reliable to distinguish FeI from Fe0, but the absence of shake-up satellites and significantly lower BEs strongly supports a statement that Fe is in a lower oxidation state than 2+. We measured the high-resolution XPS spectra of all elements present for compounds 5b, 7b, and 8 to follow changes in the BEs of heteroatoms in the ligand. N 1s spectra of the free ligand 5b show two well-resolved peaks at 399.1 and 401.1 eV in a 3:2 ratio. We assigned the lower BE peak to pyridine and pyrazole imine N atoms and the highest to pyrazole amine N atoms based on the literature data.26,27 After coordination with FeCl2, the same pattern was observed, with atoms directly coordinated to Fe shifted to a 0.5 eV higher BE of 399.6 eV, while the amine N atoms shifted only by 0.2 eV up to 401.3 eV. Analysis of the N 1s BE of compound 8 was complicated by the presence of minor oxidized species. A more complicated signal was observed because of the presence of a low BE component of 397.7 eV due to significant ligand reduction (see the Supporting Information). The O 1s peak in compounds 5b and 7b is almost at the same position of 532.5 eV, which agrees with the values known for siloxanes.28,29 The O 1s signal in compound 8 shows two major components: 532.9 eV due to O atoms from the Si2L ligand and 531.22 eV due to carbonyl O atoms.27,28,30 A minor peak at lower BEs (529.8 eV) is due to an oxidized impurity, which agrees with the literature values for iron(II) oxides.27,31 DFT Evaluation of the Electronic Structure. DFT calculations at the B3LYP/6-31G(d) level of theory (see the Supporting Information for full details) elucidate the electronic structures of 7a and 8 where ligand L is 5a, including N phenyl and SiMe3. We have studied in detail the influence of both phenyl and OSiMe3 rotational conformations and found that these have no chemically significant impact on the results that follow (see the Supporting Information for details). We seek to learn whether the LFeCl2 species contains FeII(L0), FeIII(L1−), or

FeIV(L2−). Among the possible singlet, triplet, quintet, and septet spin states for LFeCl2, the quintet was found to be more stable by at least ∼26 kcal/mol, and thus we focused on that spin state for further analysis. For comparison, (PNP)FeCl2 also shows the quintet to be the lowest-energy configuration.24 A spin-density plot of the quintet LFeCl2 (Figure 10) showed that all unpaired

Figure 10. Spin-density isosurface plot (0.002 au) of S = 2 LFeCl2. The Mulliken spin densities for the individual atoms are as follows: Fe, 3.78; Cl, 0.07; Npy, 0.01; Npz, 0.01.

electrons reside on the metal center. This rules out the possibility that the metal center is FeIII because this would yield spin density on the pincer. This leaves either high-spin FeII or FeIV; a corresponding orbital analysis (Figure 11) supports high-spin FeII

Figure 11. Corresponding orbitals of the quintet LFeCl2 with an isosurface value of 0.05 au.

showing four unpaired electrons on the metal center and one doubly occupied d orbital. There is negligible pincer ligand character in these orbitals. In order to further support an oxidation assignment of FeII and L0, we investigated the structural parameters within the ligand by simulating redox chemistry incorporating a redox-inert metal, zinc. Whether the electron goes to the pyridine or pyrazole ligand, it will occupy a π* orbital, elongating the intraring bonds. The Cpz−Cpy bonds are the best diagnostic for the amount of communication between the different heterocycles (Figures S8−S10). LFeCl2 has bond lengths that are most similar to the neutral ZnII species, providing additional evidence that its ligand is neutral and that the Fe center is a high-spin (S = 2) FeII. 9510

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suggest FeI and L(−1), but the orbital analysis suggests strong π-back-donation versus electron transfer in this system.

For the species LFe(CO)2, we considered electronic configurations Fe0, FeI(L−), and FeII(L2−) with possible spin states of singlet, triplet, quintet, and septet. Calculations optimized the singlet as the lowest-energy species, with all other spin states at least ∼7 kcal/mol higher in energy, in agreement with the findings on (PNP)Fe(CO)2.24 Key bond lengths are shown in Table S3. Compared to Table S4, several bond lengths for LFe(CO)2 are intermediate between those of LZnCl2 and [LZnCl2]−. For example, the computed N2pyr−N1pyr bond length of 1.375 Å in the LFe(CO)2 complex falls between the 1.331 and 1.395 Å bond lengths from the L0 and L− zinc complexes, respectively. N1pz−Cpz and Cpz−Cpy also have intermediate bond lengths between the L0 and L− complexes, which brought our focus to the Fe0 or FeI(L−) possibilities. Visualization of the frontier orbitals showed only a single empty d orbital with electron density localized on the metal, which provides strong evidence of a Fe0 d8 center (Figure 12).

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CONCLUSIONS The chemistry described here shows great potential for this pincer ligand type in electron-rich complex chemistry, and the use of siloxy substituents adds a new tuning feature. The installation of steric bulk at the pyrazole β-N atoms is effective in flanking the substrate binding site trans to the pyridine N atom, and that site thus has the potential for manipulation of the bound substrate, as well as avoiding bis-pincer complexes ML2, which lack substrate binding sites. Previous publications with related pincer ligands have tended to focus on whether there is full electron transfer to the pincer from an electron-rich metal, and increasingly the conclusion is that the intraligand bond-length changes, XPS BEs, and Mössbauer parameters are better explained by back-donation. Nonsinglet species are more revealing of their electronic character because electron paramagnetic resonance yields g-value anisotropy and ligand hyperfine parameters that help to characterize the atomic composition of the singly occupied molecular orbital, all unavailable for distinguishing a closed-shell singlet from an open-shell singlet. The extreme of back-donation is, of course, full integral electron transfer from metal to ligand within a molecule. We choose to analyze the trends here among redoxactive pincer ligands in terms of a continuous variable, π basicity of the metal, rather than limit ourselves to integer changes of the oxidation state. A continuous variable is clearly a more flexible representation of the subtle variations explored here because it includes full electron transfer as one extreme. With that condition, 8 here is best considered to be a product of a very good energy match of the metal d-orbital energies to the π* orbitals of the pincer ligand 5b and thus exhibits back-donation to the CO π* orbitals competing with π* of the pyridine ring. The siloxy donor substituents clearly respond to changing metal electron richness, from FeCl2 to Fe(CO)2 partners, judging by the bond distances. As more electron density accumulates on the pyrazoles, the C atom adjacent to the siloxy groups is more electron-rich and thus the O lone pairs should donate less (Tables S5 and S6). The immediate result of this should be increasing the C−O bond length and decreasing the Si−O length. In keeping with crystallographic data, DFT calculations show a similar change within the pincer ligand structure and show dif ferently delocalized frontier orbitals between these two metal partners. There is no evidence that an open-shell singlet state, with full electron transfer away from Fe, is present. DFT structural and orbital contour representations of a control analogue with a redox-inert metal, zinc, show both structural trends of full electron transfer to pincer and confirm that backdonation and full electron transfer have a similar effect on the bond lengths. Furthermore, they show that pyridine is first reduced but that f urther reduction shows delocalization of the electron density into the siloxy-substituted pyrazole. Finally, even with significant back-donation into the Si2L pincer ligand, there is no evidence that CO ligands are weakly held at Fe [i.e., no loss of CO in vacuum, as is true of (PDI)Fe(N2)2].15

Figure 12. Frontier orbitals of singlet LFe(CO)2 plotted at an isosurface value of 0.05 au.

In contrast to LFeCl2, both HOMO and HOMO−1 show some π delocalization onto the pincer π system, which we attribute to back-donation onto the pyridine ring rather than complete electron transfer. This will slightly elongate the intraring bond lengths and contract the bridging carbon bonds, which accounts for the intermediate 0 versus −1 ligand bond lengths that were computed for LFe(CO)2. To put these results in context, (PDI)Fe(CO)213 has both Fe0 and FeII character, based on Mössbauer and structural data, respectively. The NMR and crystal structure data for (PNP)Fe(CO)224 suggest a FeI metal center; however, Mössbauer data and DFT/time-dependent DFT calculations showed that the ligand is more likely acting as a π-acid rather than an anionic ligand. The authors attributed the conflicting experimental data to the fact that the crystal structure only reflects the ligand properties directly, and therefore bond-length data alone are insufficient to determine the metal oxidation state. The HOMO of (L)Fe(CO)2 is computed to have 55% character on the Fe(CO)2 fragment and 45% character on the (L) ligand. This is less ligand character than was computed for the HOMO of (PDI)Fe(CO)2 with 68% ligand character, but we cannot make comparisons to (PNP)Fe(CO)2 because such calculations were not performed. Taken literally, these values

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Materials and Methods. All manipulations were carried out under an atmosphere of ultrahigh-purity nitrogen using standard Schlenk techniques or in a glovebox under argon. Solvents were purchased from commercial sources, purified using an Innovative Technology SPS-400 PureSolv solvent system or by distilling from conventional 9511

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(d, 4H, JH,H = 8.1 Hz, o-PhCH), 7.89 (t, 1H, JH,H = 7.4 Hz, p-pyr-CH), 7.94 (d, 2H, JH,H = 7.4 Hz, m-pyr-CH), 11.88 (s, 2H, OH, D2O-exchangeable). 13C NMR (CDCl3, 101 MHz): δ 154.25, 151.89, 150.91, 139.24, 137.62, 129.43, 126.46, 121.82, 118.44, 86.69. MS: m/z 396 (M + 1). Compound 5a. In a glovebox, compound 4 (0.395 g, 1 mmol) was dispersed in 10 mL of dry THF and 0.5 mL of dry Et3N was added, followed by the dropwise addition of Me3SiCl (0.5 mL). After stirring for 12 h at RT, the white precipitate of Et3N/HCl was filtered. The reminaing mother liquor was concentrated to 1 mL, and excess pentane was added. The solution was stored for 12 h at −35 °C, after which colorless blocks very sensitive to water (reforming 4) and suitable for X-ray analysis were collected by filtration. Yield: 0.410 g (76%). 1H NMR (CDCl3, 400 MHz): δ 0.26 (s, 18H, SiCH3), 6.34 (s, 2H, pyrazCH), 7.23 (t, 2H, JH,H = 8.8 Hz, p-PhCH), 7.39 (t, 4H, JH,H = 8.9 Hz, m-PhCH), 7.73 (d, 4H, JH,H = 7.3 Hz, o-PhCH), 7.75 (t, 1H, p-pyr-CH), 7.95 (d, 2H, m-pyr-CH), 13C NMR (CDCl3, 101 MHz): δ 150.78, 150.57, 149.90, 149.11, 137.56, 135.49, 127.42, 125.05, 121.28, 117.29, 87.83, −1.57. MS (ESI): m/z 540.20 (M + H). Compound 5b. Compound 4 (0.395 g, 1 mmol) was dispersed in 10 mL of dry THF and 0.5 mL of dry Et3N was added, followed by the dropwise addition of TBDMSCl (0.33 g, 2.2 equiv). After stirring for 12 h at RT, the white precipitate of Et3N/HCl was filtered, the mother liquor was concentrated to 1 mL, an excess of pentane was added, and the solution was stored for 12 h at −35 °C. A white powder was collected by filtration. Yield: 0.586 g (94%). 1H NMR (CDCl3, 400 MHz): δ 8.35 (d, J = 7.8 Hz, 2H), 7.89 (d, J = 7.8 Hz, 4H), 7.35 (t, J = 7.8 Hz, 1H), 7.17 (t, J = 7.4 Hz, 4H), 7.01 (t, 2H), 6.88 (s, 2H), 0.84 (s, 18H), 0.06 (s, 6H). 13 C NMR (CDCl3, 101 MHz): δ 152.92, 152.23, 151.00, 139.70, 136.96, 128.84, 126.57, 123.49, 118.94, 90.09, 25.62, 18.21, −5.01. MS (ESI): m/z 624.31 (M + H), 510.22 (M − tBuMe2Si + 2H). [(L)2Fe]Cl2 (6Cl). In a scintillation vial, FeCl2 (0.013 g, 0.1 mmol) and compound 4 (0.079 g, 0.2 mmol) were mixed in THF. After 12 h, the reaction mixture turned from a milky-white slurry to a lemon-yellow slurry. The solvent was decanted, and the reminaing residue was washed with THF and dried under vacuum, giving a quantitative yield of a yellow powder, insoluble in DCM, THF, and MeCN and soluble in DMSO and MeOH. IR (KBr): 3146, 1596, 1563 cm−1. 1H NMR (CD3CN, 400 MHz): δ 58.47 (s, 2H), 54.77 (s, 2H, D2O-exchangeable), 26.47 (s, 1H), 17.82 (s, 2H, D2O-exchangeable), 11.46 (s, 2H), 8.74 (s, 4H), 3.29 (s, 2H), −8.29 (s, 4H). MS (APCI): m/z 889 [(M − H)+], 887 [(M − 3H)−]; after exchange with CD3OD, m/z 897 [(M − D)+]. [(L)2Fe]OTf2 (6OTf). Method A. In a scintillation vial, FeOTf2 (0.035 g, 0.1 mmol) and compound 4 (0.079 g, 0.2 mmol) were mixed in DCM. After 12 h, the reaction mixture turned from a milkywhite slurry to a deep-yellow solution. The solvent was partially removed, and pentane was added. A yellow solid precipitated, the mother liquor decanted, and the residue dried under vacuum. Yield: 95% (0.108 g). Method B. In a scintillation vial, compound 5 (0.089 g, 0.1 mmol) and TMSOTf (0.04 mL, 0.22 mmol) were mixed in DCM. After 12 h, the reaction mixture turned from a yellow slurry to a deepyellow solution. The solvent was partially removed, and pentane was added. A yellow solid precipitated, the mother liquor decanted, and the residue dried under vacuum. Yield: 0.10 g (92%). IR (KBr): 3186, 1597, 1564 cm−1. 1H NMR (CD3CN, 400 MHz): δ 58.47 (s, 2H), 54.77 (s, 2H, D2O-exchangeable), 26.47 (s, 1H), 17.82 (s, 2H, D2Oexchangeable), 11.46 (s, 2H), 8.74 (s, 4H), 3.29 (s, 2H), −8.29 (s, 4H). 18F NMR (CD3CN, 376 MHz): δ −78.42. MS (APCI): m/z 889 [(M − H)+], 887 [(M − 3H)−]; after exchange with CD3OD, m/z 897 [(M − D)+]. SiLFeCl2 (7a). In a scintillation vial, FeCl2 (0.013 g, 0.1 mmol) and compound 5a (0.052 g, 0.1 mmol) were mixed in DCM. After 12 h, the reaction mixture turned from a milky-white slurry to a pink solution. The solvent was removed under vacuum, and a light-pink solid was dried, recrystallized from DCM/pentane at −45 °C, and recovered in a quantitative yield. 1H NMR (CD2Cl2, 400 MHz): δ 49.81 (s, 2H), 38.69 (s, 2H), 6.29 (s, 2H), 5.81 (s, 4H), 3.70 (bs, 4H), 1.40 (s, 18H), −27.78 (s, 1H). MS (APCI): m/z 557.05 [(M − TMSCl)+].

drying agents, and degassed by the freeze−pump−thaw method twice prior to use. Glassware was oven-dried at 150 °C overnight and flamedried prior to use. THF, including THF-d8, was stored over activated 4 Å molecular sieves or sodium metal pieces. NMR spectra were recorded in various deuterated solvents at 25 °C on a Varian Inova-400 spectrometer (1H: 400.11 MHz). Proton chemical shifts are reported in ppm versus solvent protic impurity but referenced finally to SiMe4. MS analyses were performed on an Agilent 6130 MSD (Agilent Technologies, Santa Clara, CA) quadrupole mass spectrometer equipped with a multimode (ESI and APCI) source. All starting materials were obtained from commercial sources and used as received without further purification. CV used glassy carbon as the working electrode, Pt as the counter electrode, and Ag/AgCl wire as the reference electrode. 0.1 M TBAPF6 was employed as a supporting electrolyte, in THF or DCM solvents. All cyclic voltammograms are referenced to internal Fc/Fc+ as the standard, added at the end of the study of the experimental sample. An attempt to perform voltammetry of compounds 7a and 7b in THF in order to have a more negative potential limit than DCM was unsuccessful. The solubility of both was poor, and after the addition of an electrolyte, a color change was observed and multiple peaks were detected by CV, indicating several species in solution. It is likely that the polar solvent and electrolyte encourage a rearrangement of [Si2L]FeCl2 according to [Si2L]FeCl2 + 2Bu4NPF6 + nTHF = [(Si2L)2Fe]2+ + (THF)nFe2+ + 2Cl− + 2PF6−. Compound 2. To a mixture of 2,6-pyridinedicarboxylic ethyl ester (1; 2.23 g, 10 mmol) and ethyl acetate (2.44 mL, 25 mmol), in dry THF (25 mL), was added sodium hydride (1.8 g, 50% in mineral oil). The reaction mixture was refluxed for 4 h and then allowed to cool. The volatiles were evaporated, and the residual solid was washed with diethyl ether (Et2O; 2 × 30 mL), filtered off, and dried. The crude product was dispersed in water (30 mL), and the resulting alkaline solution was treated with 1 M hydrochloric acid until it became slightly acidic (pH 5). The precipitated solid product was filtered off, washed with water, dried, and finally recrystallized from Et2O/hexane. If oil formed during acidification, it was extracted with DCM, and the organic phase was separated, dried over MgSO4, and then evaporated. Residual oil was recrystallized, and a yellowish solid formed in 2−3 days at −20 °C. Yield: 78% (2.4 g). IR (KBr): 1635, 1690 cm−1. 1H NMR (CDCl3, 400 MHz; a mixture of the diketo and keto enol forms): δ 1.21−1.33 (t + t + t, 6H, CH2CH3), 4.15−4.28 (q + q, 4H, CH2CH3), 4.14 (s, CH2 of the diketo form), 6.36 (s, CH of the keto enol form), 7.35 (t, p-CH of the diketo form), 8.03 (t, p-CH of the keto enol form), 8.08 (d, m-CH of the diketo form), 8.24 (d, m-CH of the keto enol form), 12.40 (s, OH of the keto enol form). 13C NMR (CDCl3, 101 MHz): δ 193.81, 193.28, 173.10, 168.00, 167.66, 167.45, 151.37, 150.31, 138.63, 138.26, 125.68, 124.45, 123.21, 89.87, 61.31, 61.25, 60.65, 45.18, 44.78, 14.21, 14.02, 13.97. MS (ESI): m/z 308.11 (M + H). Compound 3. A mixture of compound 2 (0.31 g, 1 mmol) and phenylhydrazine (0.30 g, 3 mmol), in ethanol (10 mL), was refluxed for 6 h. The yellow solid formed was filtered off, washed with Et2O, and dried to afford 3. Yield: 90% (0.44 g). 1H NMR (CDCl3, 400 MHz): δ 1.25 (t, 6H, JH,H = 6.8 Hz, CH2CH3), 4.18 (q, 4H, JH,H = 6.8 Hz, CH2CH3), 4.23 (s, 4H, CH2), 6.93 (t, 2H, JH,H = 7.6 Hz, p-PhCH), 7.23 (d, 4H, JH,H = 7.6 Hz, o-PhCH), 7.41 (t, 4H, JH,H = 7.6 Hz, m-PhCH), 7.69 (t, 1H, JH,H = 8.2 Hz, p-PyCH), 8.06 (d, 2H, JH,H = 8.2 Hz, m-PhCH), 8.91 (s, 2H, NH, D2O-exchangeable), 13C NMR (CDCl3, 101 MHz): δ 170.06, 153.61, 144.75, 137.86, 136.38, 129.26, 120.93, 118.59, 113.58, 61.54, 31.97, 14.11. MS (ESI): m/z 488 (M + H). Compound 4. Compound 3 (0.487 g, 1 mmol) was dissolved in 10 mL of acetic acid. The solution immediately turned deep red, and after reflux for 4 h, a light-yellow precipitate was formed. The reaction mixture was cooled and mixed with 50 mL of crushed ice. A white precipitate was filtered, washed with water, and dried under vacuum. For purification, the yellowish solid was dissolved in a minimum amount of hot DMF and CHCl3 was added to this hot solution until a solid first appeared. Then the mixture was stored for 12 h at −20 °C. A white precipitate was filtered, washed with 2 × 10 mL of dry and cold THF, and dried in high vacuum. Yield: 83%. IR (KBr): 3186, 1597, 1564 cm−1. 1 H NMR (CDCl3, 400 MHz): δ 6.23 (s, 2H, pyraz-CH), 7.33 (t, 2H, JH,H = 7.6 Hz, p-PhCH), 7.53 (t, 4H, JH,H = 8.1 Hz, m-PhCH), 7.86 9512

DOI: 10.1021/acs.inorgchem.7b00785 Inorg. Chem. 2017, 56, 9505−9514

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Bu

SiLFeCl2 (7b). In a scintillation vial, FeCl2 (0.013 g, 0.1 mmol) and compound 5b (0.063 g, 0.1 mmol) were mixed in DCM. After 12 h, the reaction mixture turned from a milky-white slurry to a pink solution. The solvent was removed under vacuum, and a light-pink solid was dried, recrystallized from DCM/pentane at −45 °C, and recovered in a quantitative yield. 1H NMR (CD2Cl2, 400 MHz): δ 49.55 (s, 2H), 38.95 (s, 2H), 6.21 (s, 2H), 5.81 (s, 4H), 4.12 (s, 4H), 1.84 (s, 18H), 1.25 (s, 12H), −28.04 (s, 1H). IR (KBr): 1597, 1564 cm−1. MS (APCI): m/z 599.15 [(M − TBDMSCl)+].

(2) Olguín, J.; Brooker, S. Spin crossover active iron(II) complexes of selected pyrazole-pyridine/pyrazine ligands. Coord. Chem. Rev. 2011, 255, 203−240. (3) Umehara, K.; Kuwata, S.; Ikariya, T. N−N Bond Cleavage of Hydrazines with a Multiproton-Responsive Pincer-Type Iron Complex. J. Am. Chem. Soc. 2013, 135, 6754−6757. (4) Yoshinari, A.; Tazawa, A.; Kuwata, S.; Ikariya, T. Synthesis, Structures, and Reactivities of Pincer-Type Ruthenium Complexes Bearing Two Proton-Responsive Pyrazole Arms. Chem. - Asian J. 2012, 7, 1417−1425. (5) Umehara, K.; Kuwata, S.; Ikariya, T. Synthesis, structures, and reactivities of iron, cobalt, and manganese complexes bearing a pincer ligand with two protic pyrazole arms. Inorg. Chim. Acta 2014, 413, 136− 142. (6) Wang, L.; Yang, Q.; Chen, H.; Li, R.-X. A novel cationic dinuclear ruthenium complex: Synthesis, characterization and catalytic activity in the transfer hydrogenation of ketones. Inorg. Chem. Commun. 2011, 14, 1884−1888. (7) Cook, B. J.; Chen, C.-H.; Pink, M.; Lord, R. L.; Caulton, K. G. Coordination and electronic characteristics of a nitrogen heterocycle pincer ligand. Inorg. Chim. Acta 2016, 451, 82−91. (8) Belmar, J.; Ortiz, L.; Quezada, J.; Parra, M.; Jiménez, C. A. Synthesis and Characterization of Phenylene-bis-pyrazolones and Nitrosation Derivatives. J. Heterocyclic Chem. 2014, 51, E98−E103. (9) Tarabová, D.; Šoralová, S.; Breza, M.; Fronc, M.; Holzer, W.; Milata, V. Use of activated enol ethers in the synthesis of pyrazoles: reactions with hydrazine and a study of pyrazole tautomerism. Beilstein J. Org. Chem. 2014, 10, 752−760. (10) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E. NHeterocyclic Pincer Dicarbene Complexes of Iron(II): C-2 and C-5 Metalated Carbenes on the Same Metal Center. Organometallics 2004, 23, 166−168. (11) Wheeler, S. E. Understanding Substituent Effects in Noncovalent Interactions Involving Aromatic Rings. Acc. Chem. Res. 2013, 46, 1029− 1038. (12) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2[prime or minute]-yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349−1356. (13) Darmon, J. M.; Turner, Z. R.; Lobkovsky, E.; Chirik, P. J. Electronic Effects in 4-Substituted Bis(imino)pyridines and the Corresponding Reduced Iron Compounds. Organometallics 2012, 31, 2275−2285. (14) Tondreau, A. M.; Milsmann, C.; Lobkovsky, E.; Chirik, P. J. Oxidation and Reduction of Bis(imino)pyridine Iron Dicarbonyl Complexes. Inorg. Chem. 2011, 50, 9888−9895. (15) Bart, S. C.; Chłopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. Electronic Structure of Bis(imino)pyridine Iron Dichloride, Monochloride, and Neutral Ligand Complexes: A Combined Structural, Spectroscopic, and Computational Study. J. Am. Chem. Soc. 2006, 128, 13901−13912. (16) Pelczar, E. M.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. Unusual Structural and Spectroscopic Features of Some PNP-Pincer Complexes of Iron. Organometallics 2008, 27, 5759−5767. (17) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Bis(diisopropylphosphino)pyridine Iron Dicarbonyl, Dihydride, and Silyl Hydride Complexes. Inorg. Chem. 2006, 45, 7252−7260. (18) Brant, P.; Feltham, R. D. An x-ray photoelectron spectral study of iron and cobalt nitrosyl complexes of o-phenylenebis(dimethylarsine). Inorg. Chem. 1980, 19, 2673−2676. (19) Brant, P.; Feltham, R. D. X-ray photoelectron spectra of iron complexes: Correlation of iron 2p satellite intensity with complex spin state. J. Electron Spectrosc. Relat. Phenom. 1983, 32, 205−221. (20) Enemark, J. H.; Feltham, R. D. Principles of structure, bonding, and reactivity for metal nitrosyl complexes. Coord. Chem. Rev. 1974, 13, 339−406.

t

Bu SiLFe(CO)2 (8). In a 15 mL Schlenk flask, compound 7b (0.075 g, 0.1 mmol) was dispersed in 5 mL of THF and a flask was charged with 1 atm of CO gas after three pump−thaw operations. The reaction mixture was cooled to −78 °C, 2 equiv of KC8 was quickly added in one batch, and the vial was resealed. After 6 h of stirring, the mixture was warmed to RT and filtered from a graphite powder, volatiles were removed under vacuum, and a deep-brown residue was dried and recrystallized from Et2O/pentane at −45 °C. IR (THF): 1935, 1868 cm−1. 1 H NMR (C6D6 500 MHz): δ 7.69 (d, J = 7.1 Hz, 2H), 7.47 (d, J = 7.4 Hz, 4H), 7.32 (t, J = 7.1 Hz, 1H), 7.20 (m, overlapping with residual solvent signal), 7.01 (t, J = 8.2 Hz, 2H), 6.22 (s, 2H), 0.65 (s, 18H), −0.08 (s, 12H). 13C NMR (C6D6 126 MHz): δ 214.93, 151.53, 149.06, 139.64, 137.51, 129.67, 129.65, 129.19, 115.30, 85.19, 25.18, 17.88, −5.15. Unfortunately, compound 8 always formed in a maximum 50% yield, together with the free Si2L ligand. The solubility of both 5b and 8 were very similar in most organic solvents, but several recrystallizations from a Et2O/heptane/pentane mixture allowed one to obtain a crystalline material with low yield but high purity.

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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.7b00785. Listings of synthetic procedures, NMR, XPS, DFT, and X-ray diffractometry details (PDF) Accession Codes

CCDC 1549938−1549941 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

Richard L. Lord: 0000-0001-6692-0369 Kenneth G. Caulton: 0000-0003-3599-1038 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Indiana University’s Office of the Vice President for Research and the Office of the Vice Provost for Research through the Faculty Research Support Program and the National Science Foundation, Chemical Synthesis Program (SYN), by Grant CHE-1362127.

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REFERENCES

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