Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum

Jan 27, 2018 - Synopsis. The electronic structures of all complexes in the three- and four-membered redox series [Cr(MePDP)2]z (z = 1−, 2−, 3−) ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional Theoretical Study Anitha S. Gowda, Jeffrey L. Petersen, and Carsten Milsmann* C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States S Supporting Information *

ABSTRACT: The three- and four-membered redox series [Cr(MePDP)2]z (z = 1−, 2−, 3−) and [Mo(MePDP)2]z (z = 0, 1−, 2−, 3−) were synthesized to study the redox properties of the pincer ligand MePDP2− (H2MePDP = 2,6-bis(5-methyl-3-phenyl1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by X-ray crystallography, UV/vis/NIR spectroscopy, and magnetic susceptibility measurements. Experimental and density functional theory (DFT) studies are consistent with closedshell MePDP2− ligands and +III oxidation states (d3, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(MePDP)2]1− (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo(MePDP)2 which was characterized by X-ray crystallography, UV/ vis/NIR, and magnetic susceptibility measurements. The experimental and computational data indicate metal-centered oxidation to a MoIV complex (d2, S = 1) with two MePDP2− ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of MePDP•3− radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established experimentally by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/vis/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calculations. X-ray crystallographic analyses of the one-electron-reduced species [M(MePDP)2]2− (S = 1, M = Cr, Mo) established structural indicators for pincer reduction and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/vis/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as containing +III metal ions (d3) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, respectively.

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INTRODUCTION Redox-active ligands play an important role in many chemical processes involving reversible electron transfer. Drawing inspiration from metalloproteins incorporating redox-active ligands,1 numerous synthetic applications relying on the interplay of transition metals with ligands that can undergo reversible electron transfer have been developed in recent years.2−7 This concept is particularly successful in enabling multielectron redox processes with first-row transition metals that otherwise prefer single-electron-transfer chemistry and has been utilized in a diverse range of reactions from organometallic catalysis to small molecule activation.8,9 Besides their broad utility in base metal catalysis, redox-active ligands occupy a central role in many energy-related applications.10−12 Coordination compounds that can store electrons in their ligand framework have recently been identified as attractive © XXXX American Chemical Society

materials for the design of nonaqueous redox-flow batteries with large potential windows.13,14 Even more prominently, redox-active ligands are a key components of molecular transition metal photosensitizers.12,15−17 In these photoactive complexes, initial charge separation occurs via photoinduced intramolecular electron transfer between the metal center and the supporting ligand. Among the now well-established, large pool of redox-active ligands, pincer-type ligands containing a central pyridine moiety occupy an outstanding position. While the classic terpyridine framework (terpy) can be found in many electro- and photochemical applications,18−20 more modular pincer systems such as pyridine diimine (PDI), 2,6-bis(imidazol-2-ylidene)Received: November 6, 2017

A

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of [Cr(MePDP)2]1− with Li+ as the Counterion

situ treatment of H2MePDP (2,6-bis(5-methyl-3-phenyl-1Hpyrrol-2-yl)pyridine) with 2 equiv of n-BuLi. Removal of LiCl by filtration followed by recrystallization via vapor diffusion of pentane into a concentrated THF solution of the crude material at −35 °C allowed isolation of [Li(thf)4][Cr(MePDP)2] as a dark green microcrystalline solid. Solutions of the pure compound in THF appear dichroic dark green or red depending on the angle of observation. The 1H NMR spectrum of the complex showed no detectable resonances in the range from −300 to 300 ppm, consistent with the formation of a paramagnetic chromium compound. Solid state magnetic susceptibility measurements confirmed a room temperature magnetic moment of 3.7 μB establishing an S = 3/2 ground state for [Li(thf)4][Cr(MePDP)2] consistent with an octahedral CrIII ion with a d3 electron configuration. While two independent X-ray diffraction data sets established the identity of [Li(thf)4][Cr(MePDP)2], severe disorder of the [Li(thf)4]1+ cation in both cases prevented refinement of a publishable structure model. The synthesis of the corresponding molybdenum complex [Li(thf)4][Mo(MePDP)2] was accomplished by reaction of MoCl3(thf)3 with Li2MePDP in a 1:2 stoichiometry in THF at 70 °C. Recrystallization from THF/pentane at −35 °C provided dark brown crystals suitable for X-ray diffraction, and the molecular structure of [Mo(MePDP)2]1− is shown in Figure 1. A detailed analysis of bond distances and angles will be presented in a separate section of this manuscript (vide infra). The 1H NMR data collected in THF-d8 show seven paramagnetically shifted signals consistent with D2d symmetric [Mo(MePDP)2]1− (Figure S2). In contrast, 1H NMR spectra of the complex recorded in benzene-d6 exhibit 13 broad resonances indicating conversion to a single paramagnetic species of lower symmetry (Figure S2). X-ray diffraction analysis of single crystals obtained from a concentrated benzene solution of [Li(thf)4][Mo(MePDP)2] revealed the identity of the new species as the contact ion pair [Li(thf)2Mo(MePDP)2], in which the lithium cation is coordinated by two of the pyrrole nitrogen atoms of the pincer ligands. The molecular structure is shown in Figure 1 and accounts for the reduced symmetry of the compound observed by NMR spectroscopy in solution. The molybdenum and lithium centers occupy a crystallographic C2 axis rendering the two ligands identical, but coordination of the lithium ion to one pyrrole unit of each ligand removes any additional symmetry element, resulting in a C2 symmetric complex. As for [Li(thf)4][Mo(MePDP)2], important structural parameters will be discussed in a later section of this work (vide infra). Based on the observation of lithium coordination to the ligand, the influence of different alkali metal ions was investigated. Deprotonation of H2MePDP with NaH or KH followed by reaction with MoCl3(thf)3 in a 2:1 ratio at 70 °C resulted in full conversion to a single product. For both reaction conditions, using NaH and KH, the 1H NMR spectrum of the

pyridine (CNC), and 2,6-bis(diphosphinomethyl)pyridine (PNP) that allow straightforward steric manipulation of the coordination environment are now a hallmark of organometallic catalysis.9,21,22 More recently, Caulton and co-workers introduced a new pincer ligand derived from 2,6-bis(1H-pyrrol2-yl)pyridine, H2PDP, containing a central pyridine ring flanked by two pyrrolide units.23,24 By combining the π accepting pyridine ring with two electron-rich, π donating pyrrolide anions, this ligand was proposed to be redox-active under both oxidative and reductive conditions. Initial investigations of the electronic properties of late transition metal complexes with PDP2− ligands showed rich oxidative chemistry which was attributed to ligand-based oxidations.23 However, no ligand-centered reduction events were identified. Using a similar pyridine dipyrrolide ligand in combination with earth-abundant group 4 metals, we recently reported the synthesis of a luminescent bis-ligand zirconium complex, Zr(MePDP)2, and showed that it can be used as a substitute for precious metal photosensitizers in reductive photoredox transformations.25 In addition to these favorable photochemical properties, electrochemical studies of Zr(MePDP)2 and its nonluminescent titanium analogue Ti(MePDP)2 revealed rich redox chemistry with multiple reversible electron-transfer processes at very negative potentials ( 4 × 104 M−1 cm−1) in the UV region of the spectrum which are most likely dominated by π → π* and n → π* transitions within the aromatic systems of the ligands. All compounds are intensely colored in solution due to absorption bands with high molar extinction coefficients (ελ = 0.5 × 104 to 4 × 104 M−1 cm−1) in the visible region of the spectrum. While no electronic transitions above 700 nm are observed for the monoanionic chromium and molybdenum species, neutral Mo(MePDP)2 as well as the dianionic and trianionic compounds exhibit strong absorbance bands (ελ ∼ 1 × 104 M−1 cm−1) between 700 and 1100 nm. Although the intensities of the observed features between 400 and 1100 nm indicate charge transfer transitions, a more detailed analysis cannot be provided at this point due to the complex nature of the spectra. Importantly, the dianionic and trianionic complexes exhibit broad absorption bands between 1400 and 2100 nm. These features are indicative of ligand-to-ligand intervalence charge transfer (LLIVCT) transitions that occur in the presence of ligand radicals and have

Figure 5. Electronic absorption spectra of [Cr(MePDP)2]z (top; z = 1−, 2−, 3−) and [Mo(MePDP)2]z (bottom; z = 0, 1−, 2−, 3−) recorded in THF solution at room temperature. Artifacts around 1400, 1700, and 1800 nm are due to absorption bands of the solvent.

been observed for several chromium and molybdenum complexes with redox-active ligands.33,35,36 The relatively low intensity (ελ ∼ 1 × 103 M−1 cm−1) for the transitions in [M(MePDP)2]z (z = 2−, 3−) can be attributed to the nearly perpendicular arrangement of the two pincer ligands, which reduces orbital overlap and limits electron transfer between the two ligands. Notably, these NIR LLIVCT bands are absent in the spectra of the neutral and monoanionic complexes. Due to the photoluminescent properties observed for Zr(MePDP)2 upon irradiation with visible light, emission spectra were recorded for the group 6 complexes studied in this work. However, no luminescence was observed for any of the chromium or molybdenum species upon excitation with visible light. Solid State Structures. Molecular structures were determined by high resolution X-ray crystallography at 100 K, and the crystallographic details are summarized in the Supporting Information. Important bond lengths and angles for all compounds are shown in Table 2. With the exception of the contact ion pair [Li(thf)2Mo(MePDP)2] all complexes discussed in this section are best described as well-separated complex ions, [M(MePDP)2]z−, with a distorted octahedral structure in which the two pincer ligands exhibit a nearly perfectly perpendicular arrangement with dihedral angles ranging from 86.97° to 89.98° and Npyridine−M−Npyridine angles of 174.84° to 178.36°. The most significant deviation from an idealized octahedral MN6 coordination environment is F

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Selected Bond Lengths (Å) and Angles (deg) for [M(MePDP)2]z (M = Cr, z = 1−, 2−; M = Mo, z = 0, 1−, 2−) M(1)−N(1) M(1)−N(2) M(1)−N(3) M(1)−N(4) M(1)−N(5) M(1)−N(6) N(2)−C(5) C(5)−C(6) C(6)−C(7) C(7)−C(8) C(8)−C(9) N(2)−C(9) N(5)−C(32) C(32)−C(33) C(33)−C(34) C(34)−C(35) C(35)−C(36) N(5)−C(36) dihedral N(2)−M(1)−N(5)

[Cr(MePDP)2]1−a

[Cr(MePDP)2]2−b

Mo(MePDP)2

[Mo(MePDP)2]1−c

[Li(thf)2 Mo(MePDP)2]

[Mo(MePDP)2]2−d

2.037(2) 1.999(2) 2.048(2) 2.044(2) 2.009(2) 2.042(2) 1.367(3) 1.394(4) 1.388(4) 1.387(4) 1.393(4) 1.356(3) 1.357(3) 1.397(4) 1.387(4) 1.386(4) 1.394(4) 1.360(3) 88.77 177.27(9)

2.035(2) 1.925(2) 2.051(2) 2.059(2) 2.034(2) 2.072(2) 1.394(3) 1.384(3) 1.397(3) 1.401(3) 1.379(3) 1.393(3) 1.357(3) 1.403(3) 1.387(3) 1.386(3) 1.391(3) 1.358(3) 89.98 178.36(7)

2.083(4) 2.076(4) 2.124(4)

2.130(2) 2.116(2) 2.136(2) 2.127(2) 2.122(2) 2.134(2) 1.355(3) 1.393(4) 1.388(4) 1.382(4) 1.393(4) 1.373(3) 1.352(3) 1.389(3) 1.386(4) 1.383(4) 1.392(3) 1.370(3) 88.43 176.56(8)

2.1595(15) 2.1091(14) 2.1271(15)

2.114(2) 2.038(2) 2.124(2) 2.137(2) 2.110(2) 2.154(2) 1.407(3) 1.384(3) 1.393(4) 1.404(4) 1.385(3) 1.395(3) 1.375(3) 1.399(3) 1.390(3) 1.393(3) 1.389(3) 1.370(3) 89.65 177.68(7)

e 1.366(6) 1.383(6) 1.396(6) 1.374(7) 1.373(7) 1.381(6)

e

89.50 176.9(2)

e 1.367(2) 1.393(2) 1.389(3) 1.384(3) 1.397(2) 1.367(2)

e

88.43 175.84(8)

From [K(18-crown-6)(thf)2][Cr(MePDP)2]·THF. bFrom [K(18-crown-6)(MeCN)2][K(18-crown-6)(MeCN)Cr(MePDP)2] ·3MeCN. cFrom [Na(18-crown-6)(thf)2][Mo(MePDP)2]·2THF. dFrom [K(18-crown-6)(MeCN)2][K(18-crown-6)(MeCN)Mo(MePDP)2]·3MeCN. eLigands symmetry equivalent. a

introduced by the geometric constraints of the PDP ligand framework resulting in average Npyrrole−M−Npyrrole angles significantly smaller than 180°. This distortion is more pronounced in the molybdenum species compared to their chromium analogues due to the increased size of the central metal ion and the corresponding longer metal−ligand bonds. The structure of the complex ion [Cr(MePDP)2]1− was independently determined via the structures of the sodium and potassium crown ether salts [Na(18-crown-6)(thf)2][Cr(MePDP)2]·2THF and [K(18-crown-6)(thf)2][Cr(MePDP)2]· THF, respectively. In both structures, the transition metal containing anion is well separated from the alkali metal countercation. Because the geometric parameters of the anions in the two structures are identical within the 3σ limits, the following discussion will focus on the potassium salt, for which a slightly higher quality structure was obtained (Table 2). For the two PDP ligands in [Cr(MePDP)2]1−, the metal−ligand bond lengths as well as the intraligand bond distances and angles are the same within the 3σ criterion. Notably, the structural parameters of the pincer ligands are nearly identical to those determined for the free ligand precursor H2MePDP25 and are consistent with a description as MePDP2− and a resulting electronic structure of [CrIII(MePDP2−)2]1− for the complex ion. The crystallographic data obtained for [K(18-crown-6)(MeCN) 2 ][K(18-crown-6)(MeCN)Cr( Me PDP) 2 ]·3MeCN allow the analysis of the structural features of the dianionic complex [Cr(MePDP)2]2−. As described previously (vide supra), one of the potassium cations is in direct contact with the π system of one of the phenyl substituents in the backbone of one of the PDP ligands with a K(2)−C(23) distance of 3.343(2) Å. Nevertheless, the two pyrrole units of the potassium associated ligand are identical within 3σ, indicating that the phenyl− potassium interaction does not perturb the structure of the pyridine dipyrrolate ring system. In contrast to [Cr(MePDP)2]1−, the two pincer units in [Cr(MePDP)2]2− are not

Scheme 3. Structural Changes upon One-Electron Reduction of [CrIII(MePDP2−)2]1− to [CrIII(MePDP•3−)(MePDP2−)]2−

identical but show several structural differences, which are summarized in Scheme 3. While the geometric parameters of one PDP unit are experimentally indistinguishable from those observed in the monoanion and H2MePDP, the second pincer ligand exhibits substantial alterations to the central pyridine ring. Most significantly, the Npyridine−Cortho bonds are considerably elongated by 3−4 pm while the Cr−Npyridine bond length is contracted by 7 pm. In addition, the pyridine Cortho−Cmeta and Cmeta−Cpara bond distances of the central heterocycle in the second pincer ligand show signs of contraction and elongation, respectively. However, these changes are less pronounced and lie within the 3σ range. The observed structural changes are consistent with ligand-centered reduction of one of the PDP ligands upon one-electron reduction of [Cr III ( Me PDP 2− ) 2 ] 1− to [Cr III ( Me PDP 2− )(MePDP•3−)]2−, in which the additional electron is localized on only one of the two pincer ligands (Scheme 3). Based on these structural data, the most reliable reporter for ligand reduction is the elongation of the C−N bonds of the pyridine ring, which suggests that the additional electron is predominantly localized on this heterocycle. Similar geometric G

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

structure of the dianionic molybdenum complex is best described as [MoIII(MePDP2−)(MePDP•3−)]2−. Computational Studies. Broken-symmetry (BS) density functional theory (DFT) calculations were performed to obtain further insight into the electronic structure of the three- and four-membered redox series for [Cr(MePDP)2]z (z = 3−, 2−, 1−) and [Mo(MePDP)2]z (z = 3−, 2−, 1−, 0), respectively. Unless noted otherwise, all ground state geometries were optimized at the B3LYP level of DFT starting from the coordinates obtained by crystallographic analysis of the monoanionic complexes without any truncations to the molecules and the optimized geometries are in good agreement with the experimental structures from X-ray crystallography (Table S5−S7). Calculated Electronic Structure of [Cr(MePDP)2]1− and [Mo(MePDP)2]1−. Based on their experimentally determined S = 3/2 ground state, the monoanionic complexes [Cr(MePDP)2]1− and [Mo(MePDP)2]1− were computed as quartets using spin-unrestricted methods (UKS4). All attempts to converge on broken-symmetry solutions failed and yielded the same UKS4 state. The electronic structures of both complexes are best described as containing a MIII ion (d3) coordinated by two closed-shell dianionic MePDP2− ligands, [MIII(MePDP2−)2]1−. Inspections of the molecular orbital manifolds for both monoanions (Figure S8 and Figure S9) show that three unpaired electrons occupy the t2g orbitals of idealized octahedral symmetry, which are split into an e set (dxz, dyz) and a b2 orbital (dxy) according to the D2d symmetric ligand field. Two empty orbitals of b1 (dx2−y2) and a1 (dz2) symmetry can be found at higher energy. Further illustration is provided by the spin density distributions (Figure 6) obtained via Mulliken population analysis, which are consistent with three unpaired electrons on each metal center. Despite the qualitatively similar electronic structures, the spin density distributions reveal important differences between [Cr(MePDP)2]1− and [Mo(MePDP)2]1−. For chromium, a spin density value slightly larger than three is observed due to spin polarization of the Cr−N σ bonds. In the presence of three unpaired electrons in the dxz, dyz, and dxy orbitals of CrIII, interactions between nitrogen lone pairs and the unfilled dx2−y2 and dz2 orbitals of the metal lead to an increase in spin density on the metal and small, negative spin density values on the nitrogen donors. For molybdenum on the other hand, the spin density value on the metal is lower than three. While the presence of σ bond spin polarization similar to the chromium case is reflected in negative spin density on the nitrogen donors, additional π interactions between the metal SOMOs and the conjugated π systems of the ligands result in an overall decrease in spin density on the molybdenum center and positive spin density values throughout the PDP backbone. This increase in metal−ligand π interactions for Mo is the result of better overlap of the larger 4d orbitals with the π system of the ligand compared to the Cr 3d orbitals. Calculated Electronic Structure of [Mo(MePDP)2]0. Computations for [Mo(MePDP)2]0 were performed assuming a triplet ground state based on the available magnetic data. Similar to the monoanionic complex, no stable broken-symmetry solutions were found and all approaches converged to the same UKS3 electronic structure. The Mulliken spin density plot shown in Figure 6 indicates two unpaired electrons on the metal center consistent with a central MoIV ion (d2) and an electronic structure of [MoIV(MePDP2−)2]0. The increase in metal oxidation state compared to [MoIII(MePDP2−)2]1− is also

distortions in the central pyridine ring have been reported for the related 2,6-bis(imidazol-2-ylidene)pyridine (CNC) ligand.42 Structural parameters for [Mo(MePDP)2]1− were obtained from studies of [Li(thf)4][Mo(MePDP)2]·3THF, [Na(18crown-6)(thf)2][Mo(MePDP)2]·2THF, and [K(18-crown-6)(thf)2][Mo(MePDP)2]·3THF. According to these data, the nature of the cation has no significant influence on the structure of the [Mo(MePDP)2]1− anion and all bond lengths and angles between the three structures are identical within 3σ. The highest quality structure was obtained for [Na(18-crown6)(thf)2][Mo(MePDP)2]·2THF and will be used as the reference for the discussion of the metrical parameters of [Mo( Me PDP) 2 ] 1− . Like its chromium analogue, [Mo(MePDP)2]1− exhibits a symmetric structure in which the two PDP ligands are identical within experimental error. The short Npyridine−Cortho bond lengths in both PDP ligands are consistent with dianionic pincer ligands and an electronic structure [MoIII(MePDP2−)2]1−. A comparison of the structural parameters of [Mo(MePDP)2]1− with those observed for the contact ion pair [Li(thf)2Mo(MePDP)2] reveals that association of the lithium cation exerts only minor changes to the overall structure of the [Mo(MePDP)2] moiety. Despite its bridging position between two adjacent pyrrole units, the lithium cation does not significantly affect the dihedral angle between the two ligand planes, which remain close to perpendicular at 88.43°. The majority of the intraligand bond distances remain unperturbed, indicating dianionic PDP ligands. The only exception is a slight elongation of the N(1)−C(1) bond distance due to the direct Li−N(1) interaction. A second effect of lithium association with the pyrrole nitrogen is an elongation of the Mo−N(1) bond distance from 2.132(2) Å in [Mo(MePDP)2]1− to 2.160(2) Å in [(thf)2LiMo(MePDP)2], which is consistent with reduced π donation of the N(1) lone pair. Overall, this structural comparison suggests that the electronic structure of the [MoIII(MePDP2−)2]1− core is independent of lithium cation binding. The crystallographic data for the neutral complex Mo(MePDP)2 shows only minor changes to the overall structure upon one-electron oxidation of [Mo(MePDP)2]1−. The most significant change can be observed in the shorter metal−ligand distances for Mo(MePDP)2 (Table 2). The Mo(MePDP)2 molecule lies on a crystallographic C2 axis, rendering the two PDP ligands equivalent, and the intraligand bond distances are consistent with dianionic pincer ligands. Overall, the crystallographic analysis is consistent with an electronic structure description as MoIV(MePDP2−)2 and a metal-centered redox event for the MoIV(MePDP2−)2/[MoIII(MePDP2−)2]1− couple, supporting the interpretation of the electrochemical data. Structural data for [Mo(MePDP)2]2− could be obtained from crystallographic characterization of [K(18-crown-6)(MeCN)2][K(18-crown-6)(MeCN)Mo(MePDP)2]·3MeCN. Similar to the chromium analogue, a short contact between one of the potassium cations and a phenyl group of one PDP ligand (K(2)−C(23) = 3.395 Å) was identified but seems to have no influence on the metrical parameters of the ligand. The [Mo(MePDP)2]2− moiety exhibits two distinct PDP ligands with significantly elongated Npyridine−Cortho bonds within the central pyridine ring and a short Mo−Npyridine bond for one of the ligands indicating ligand reduction to MePDP•3− (Table 2). The corresponding bonds in the second PDP unit are consistent with a dianionic pincer. According to this data, the electronic H

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Two electronic structures were considered for [Cr(MePDP)2]2− based on the experimentally determined S = 1 ground state: A simple spin-unrestricted triplet (UKS3) corresponding to a low spin d4 [CrII(MePDP2−)2]2− description and a BS(3,1) state representing [Cr III ( Me PDP 2− )(MePDP•3−)]2− containing a trianionic radical ligand (SL = 1/ 2) antiferromagnetically coupled to a d3 metal center (SCr = 3/ 2). Both approaches resulted in identical electronic structures consistent with a BS(3,1) solution. Spontaneous symmetry breaking for the simple UKS3 approach highlights the favorability of the broken-symmetry state. The Mulliken population analysis (Figure 7) reveals a spin density of close to three on the metal center, in agreement with an oxidation state assignment as CrIII, which is further supported by the identification of three singly occupied and two empty metal orbitals. A single unpaired electron of opposite spin is found on the ligand framework. A qualitative molecular orbital (MO) diagram for the dianionic chromium complex is shown in Figure 7. The spatial overlap of S = 0.62 between the ligandcentered SOMO and one of the Cr-centered SOMOs indicates strong antiferromagnetic coupling and is typical for open-shell systems containing ligand-centered radicals. Despite the symmetric input geometry, the two MePDP ligands in [Cr(MePDP)2]2− are not equivalent in the optimized structure, which indicates localization of the radical on one of the pincer ligands. This result allows the direct comparison between the DFT predicted structural parameters for a closed-shell Me PDP2− and a reduced MePDP•3− radical ligand. Because the majority of the spin density in MePDP•3− is localized on the pyridine ring of the pincer ligand, changes in the pyridine bond distances should be the most sensitive geometric reporters for the oxidation state of the ligand. In fact, based on the calculated geometry the only significant change is predicted for the Cortho−N bond length, which increases from 1.360 Å in Me PDP2− to 1.385 Å in MePDP•3−. This computational result is consistent with the crystallographic data that show elongated Npyridine−Cortho bond distances in the central pyridine ring for only one of the ligands in [CrIII(MePDP2−)(MePDP•3−)]2− compared to the ligand precursor H 2 M e PDP and [CrIII(MePDP2−)2]1−. The strong preference for an electronic structure with a localized radical ligand observed both experimentally and in silico is most likely due to the orthogonal orientation of the π systems of the two ligands in combination with the low overlap between metal and ligand SOMOs. Based on the presence of a ligand radical in [Cr(MePDP)2]2−, several electronic structures were evaluated for [Cr( M e PDP) 2 ] 3 − . Further ligand reduction leading to [CrIII(MePDP•3−)2]3− was probed via a BS(3,2) approach, metal reduction corresponding to [Cr I I ( Me PDP 2− )(MePDP•3−)]3− with a low spin d4 metal center was investigated by a BS(2,1) approach, and reductively induced intramolecular electron transfer yielding [CrI(MePDP2−)2]3− containing a low spin d5 ion was taken into account via a simple UKS2 calculation. All of these computational approaches represent doublet ground states consistent with the experimental data. Similar to the dianionic complex, all calculations converged to a single electronic structure representing a BS(3,2) solution. The Mulliken spin density plot and the qualitative MO diagram shown in Figure 8 establish [CrIII(MePDP•3−)2]3− as the preferred electronic structure. In agreement with the presence of two ligand radicals, the computed Npyridine−Cortho bond lengths are identical in both ligands at 1.389 Å resulting in a D2d symmetric structure with equivalent ligands.

Figure 6. Spin density distributions obtained via Mulliken population analysis for [Cr(MePDP)2]1− (top), [Mo(MePDP)2]1− (middle), and [Mo(MePDP)2]0 (bottom).

reflected in the shortening of the metal−ligand bond distances, which is observed experimentally and in the calculated structures. Consistent with the electrochemical data, the [MoIV(MePDP2−)2]0/[MoIII(MePDP2−)2]1− couple represents a predominantly metal-centered redox event and the electronic structures of the compounds differ only in the occupation of the dxy orbital (b2, Figure S10). Calculated Electronic Structure of [Cr(MePDP)2]2− and [Cr(MePDP)2]3−. While the calculations for the monoanionic complexes and the neutral molybdenum compound yielded straightforward electronic structure descriptions in agreement with simple Werner-type coordination compounds, calculations for dianionic [Cr(MePDP)2]2− and trianionic [Cr(MePDP)2]3− revealed more complicated electronic structures. The possible electronic structure descriptions are summarized in Scheme 4. I

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 4. Possible Electronic Structures for the Reduced Species [M(MePDP)2]2− and [M(MePDP)2]3− (M = Cr, Mo)

Figure 8. Qualitative molecular orbital diagram (top) and Mulliken spin density plot (bottom) for [Cr(MePDP)2]3−.

configuration (UKS3) and a broken-symmetry solution (BS(3,1)). As for chromium, both approaches converged to the same configuration representing a BS(3,1) state. However, a closer inspection of the optimized structural parameters, molecular orbital manifold, and spin density distribution of [Mo(MePDP)2]2− revealed significant differences from the chromium congener. The Mulliken population analysis (Figure 9) exhibits a spin density of 2.21 on molybdenum, much lower

Figure 7. Qualitative molecular orbital diagram (top) and Mulliken spin density plot (bottom) for [Cr(MePDP)2]2−.

Calculated Electronic Structure of [Mo(MePDP)2]2− and [Mo(MePDP)2]3−. Two computational models were tested for [Mo(Me PDP) 2 ] 2− to discern between a simple triplet J

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

calculations using COSMO has previously been reported by Wieghardt and co-workers.43 Despite the favorable changes in the computed geometry, the spatial overlap between the magnetic orbitals (S = 0.86) remains high even with inclusion of the dielectric medium, and only minor changes to the spin density on molybdenum (2.29) were observed. Given the high orbital overlap value, it is questionable whether a brokensymmetry description including MoIII and a ligand radical should be applied to [Mo(MePDP)2]2− or whether its electronic structure is better described as MoII with strong π backdonation to the pyridine. A similarly ambiguous result was obtained for the trianionic species [Mo(MePDP)2]3−. All tested computational approaches (UKS2, BS(2,1), and BS(3,2)) converged to the same solution consistent with a BS(3,2) state. However, the low spin density value for the molybdenum center at 2.11 is again indicative of strong interactions between the molybdenum 4d orbitals and the π system of the ligand and is reflected in the large spatial overlap of 0.79 between the magnetic orbitals of the molybdenum center and each of the ligand SOMOs (Figure 10).

Figure 9. Top: Magnetic orbitals of [Mo(MePDP)2]2− obtained from a BS(3,1) calculation including COSMO(MeCN). Middle: Mulliken spin density plot for [Mo(MePDP)2]2− obtained from a BS(3,1) calculation including COSMO(MeCN). Bottom: Mulliken spin density plot for [Mo(MePDP)2]2− obtained from a BS(3,1) calculation without modeling of a dielectric medium.

than expected for an octahedral d3 configuration coupled to a ligand radical. Consistent with this result, inspection of the molecular orbital manifold reveals a high spatial overlap value of S = 0.91 between the magnetic orbitals of the ligand and molybdenum. Furthermore, the spin density on the ligand manifold is distributed evenly across both pincer ligands, rendering them equivalent. This is reflected in identical bond distances for the two ligands and is inconsistent with the experimental structure. To probe the effect of a dielectric medium on the computed geometry and electronic structure, the BS(3,1) calculation was repeated including the conductorlike screening model (COSMO) using the two experimentally relevant solvents THF (dielectric constant ε = 7.25) and MeCN (ε = 36.6). Both calculations yielded identical results and provided structures with clearly distinguishable ligands reflected most clearly in the C−N bond distances of the central pyridine (1.367 Å vs 1.394 Å). Consistent with the structural parameters, the spin population analysis shows localization of negative spin density on only one of the two pincer ligands. This effect of radical localization in broken-symmetry

Figure 10. Qualitative molecular orbital diagram (top) and Mulliken spin density plot (bottom) for [Mo(MePDP)2]3−.

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SUMMARY AND CONCLUDING REMARKS The two redox series [Cr(MePDP)2]z (z = 1−, 2−, 3−) and [Mo(MePDP)2]z (z = 0, 1−, 2−, 3−) have been investigated and the electronic structures of all members have been firmly established via a combination of experimental and computational techniques (Scheme 5). The monoanionic complexes [CrIII(MePDP2−)2]1− and [MoIII(MePDP2−)2]1− exhibit a distorted octahedral structure with D2d symmetry and are best described as classical Werner-type coordination compounds with closed-shell dianionic pincer ligands and metals in a +III oxidation state (d3 electron configuration) giving rise to a quartet ground state. Solvent dependent contact ion pair K

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 5. Summary of Redox Events in the Series [Cr(MePDP)2]z (z = 1−, 2−, 3−) and [Mo(MePDP)2]z (z = 0, 1−, 2−, 3−)

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formation with alkali metal cations was established via 1H NMR spectroscopy for [Mo(MePDP)2]1−. However, structural characterization of [(thf)2LiMoIII(MePDP2−)2] revealed only minor perturbations to the geometric and electronic structure of the [MoIII(MePDP2−)2]1− moiety. For molybdenum, one-electron oxidation is reversible and purely metal-centered, yielding the neutral Werner-type complex MoIV(MePDP2−)2 (d2, S = 1). No evidence for reversible ligand oxidation was found for either chromium or molybdenum bis(pyridine dipyrrolate) complexes. In contrast, one- and two-electron reduction of [CrIII(MePDP2−)2]1− is predominantly ligand-centered resulting in complexes best described as [CrIII(MePDP2−)(MePDP•3−)]2− and [CrIII(MePDP•3−)2]3−. In agreement with the experimentally determined triplet and doublet ground states for the dianionic and trianionic complexes, respectively, strong antiferromagnetic coupling between the d3 metal center (SCr = 3/2) and the ligand π radical(s) is supported by brokensymmetry DFT calculations. The presence of LLIVCT bands in the NIR region of the electronic absorption spectra of the reduced complexes provides experimental evidence for the proposed electronic structures. Changes in the intraligand bond distances upon reduction were tracked via X-ray diffraction studies of [Cr III ( Me PDP 2− ) 2 ] 1− and [Cr III ( Me PDP 2− )(MePDP•3−)]2−, which established the Npyridine−Cortho bond lengths as the most reliable reporters for the oxidation state of the ligand. Consistent with the structural data, DFT calculations predict a localized electronic structure for [CrIII(MePDP2−)(MePDP•3−)]2−, in which the majority of the unpaired electron is localized on the central pyridine ring of a single MePDP•3− ligand. Delocalization of the ligand radical over both ligands is most likely hindered by the near perpendicular arrangement of the two pincer ligands. Experimental data from electrochemical studies, electronic absorption spectroscopy (UV/vis/NIR), and X-ray crystallography for the one- and two-electron reduction products of [MoIII(MePDP2−)2]1− indicate similar electronic structures best described as [Mo I I I ( M e PDP 2 − )( M e PDP • 3 − )] 2 − and [MoIII(MePDP•3−)2]3−. However, broken-symmetry DFT calculations suggest a significant increase in metal−ligand π interactions for the second row transition metal molybdenum. This effect is reflected most clearly in the high spatial overlap values for the magnetic orbitals. These results highlight the trend toward more covalent metal−ligand interactions for second row transition metals compared to their first row congeners due to the increased size of the metal d orbitals.

EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were carried out using standard high vacuum line, Schlenk, or cannula techniques or in an MBraun inert atmosphere drybox containing an atmosphere of purified nitrogen. Solvents for air- and moisture-sensitive manipulations were dried and deoxygenated using a Glass Contour Solvent Purification System and stored over 4 Å molecular sieves. Deuterated solvents for NMR spectroscopy were distilled from sodium metal (C6D6), sodium benzophenone (THF), or CaH2 (CD2Cl2 and CDCl3) and stored over 4 Å molecular sieves. Tetrabutylammonium hexafluorophosphate for electrochemical experiments was ground into a fine powder and dried under vacuum at 150 °C for 48 h to remove any trace amounts of water. The ligand precursor H2MePDP,25 CrCl3(thf)3,44 and MoCl3(thf)345 were prepared according to literature procedures. Elemental Analysis. Despite repeated attempts for each compound, the collection of satisfactory elemental analysis data proved to be problematic throughout this study. Considerable effort was expended to optimize the analysis conditions through sample preparation (crystalline material vs powder samples), addition of combustion aids, and analysis by three different analytical testing laboratories for selected compounds. While the values obtained for hydrogen and nitrogen were generally accurate and reproducible within experimental error, the values for carbon were consistently low by 0.5−10% across all samples and varied dramatically even between samples from the same batch of compound. We propose that these unreliable carbon data are due to the formation of metal carbides during combustion analysis, which are known to be exceedingly stable for chromium and molybdenum.46 We also note that we did not observe any indication of impurities in any of the diverse experimental techniques employed in this study. For full transparency, we report the most accurate elemental analysis data for each compound in the following experimental section. Preparation of [Li(thf)4][Cr(MePDP)2]. At room temperature, nBuLi (0.97 mL (1.6 M in hexanes), 1.56 mmol) was added slowly to a 20 mL vial charged with a solution of H2MePDP (0.300 g, 0.77 mmol) in 8 mL of THF. The resulting luminescent dark orange solution was stirred for 30 min and subsequently added to a 20 mL vial containing a suspension of CrCl3(thf)3 (0.144 g, 0.38 mmol) in THF (4 mL). The reaction mixture was stirred at room temperature for 20 h, upon which the color of the solution turned from dark orange to dichroic dark green/red. All volatiles were removed in vacuum, and the obtained solid was redissolved in toluene and the solution filtered to remove LiCl. Removal of toluene in vacuum followed by recrystallization of the obtained solid from THF (8 mL) and pentane (8 mL) at −35 °C yielded dark green, needle shaped crystals identified as [Li(thf)4][Cr(MePDP)2], which were collected via filtration, washed with pentane, and dried in vacuum. Yield: 0.26 g (61%). Anal. Calcd for C70H74CrLiN6NaO4: C, 74.91; H, 6.65; N, 7.49. Found: C, 74.33; H, 6.39; N, 7.92. Preparation of [Na(18-crown-6)(thf)2][Cr(MePDP)2]. Step 1. At room temperature, solid NaH (0.063 g, 2.63 mmol) was added to a 20 mL vial containing a solution of H2MePDP (0.50 g, 1.28 mmol) in L

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry THF (10 mL). The yellow suspension was stirred for 16 h, furnishing a luminescent dark orange solution, which was subsequently added to a 50 mL round-bottom flask charged with a suspension of CrCl3(thf)3 (0.241 g, 0.64 mmol) in THF (15 mL). A dichroic, dark green/red solution was obtained after stirring for 16 h, and all volatiles were removed in vacuum. The remaining solid was dissolved in a 3:1 mixture of THF and toluene and the resulting solution filtered to remove NaCl. The filtrate was collected and solvents were removed in vacuum. Recrystallization from THF (30 mL) and pentane (30 mL) at −35 °C furnished a dark green, microcrystalline material that was used without further purification in the next step. Yield: 0.530 g (73% based on [Na(thf)4][Cr(MePDP)2]). Step 2. A sample of the dark green material obtained in step 1 (0.110 g, 0.10 mmol based on [Na(thf)4][Cr(MePDP)2]) was dissolved in THF (5 mL) and treated with a solution of 18-crown-6 (0.028 g, 0.11 mmol) in THF (2 mL). After stirring at room temperature for 1 h, all volatiles were removed in vacuum and the remaining dark green solid was washed with ether and recrystallized using THF (2 mL) and pentane (2 mL) at −35 °C, yielding dark green crystals identified as [Na(18-crown-6)(thf)2][Cr(MePDP)2]. Yield: 0.12 g (98%). Anal. Calcd for C74H82CrN6NaO8: C, 70.64; H, 6.57; N, 6.68. Found: C, 70.14; H, 6.20; N, 6.78. Preparation of [K(18-crown-6)(thf)2][Cr(MePDP)2]. Crystals identified as [K(18-crown-6)(thf)2][Cr(MePDP)2]·THF were prepared via a similar two step procedure outlined for the sodium analogue [Na(18-crown-6)(thf)2][Cr(MePDP)2] using KH instead of NaH. Anal. Calcd for C74H82CrN6KO8: C, 69.73; H, 6.48; N, 6.59. Found: C, 67.58; H, 6.00; N, 6.78. Preparation of [Li(thf)2Mo(MePDP)2]. At room temperature, nBuLi (0.32 mL (1.6 M in hexanes), 0.51 mmol) was added slowly to a 20 mL vial charged with a solution of H2MePDP (0.100 g, 0.26 mmol) in 3 mL of THF. The resulting luminescent dark orange solution was stirred for 30 min and subsequently added to a thick-walled glass vessel containing a suspension of MoCl3(thf)3 (0.054 g, 0.13 mmol) in THF (3 mL). The reaction mixture was heated to 70 °C for 15 h, furnishing a green-brown solution. All volatiles were removed in vacuum, the obtained solid was redissolved in toluene, and the solution was filtered to remove LiCl. Removal of toluene in vacuum followed by recrystallization of the obtained solid from benzene (2 mL) at room temperature yielded dark brown crystals identified as [Li(thf)2Mo(MePDP)2]·C6H6. Yield: 0.095 g (73%). 1H NMR (400 MHz, C6D6; δ, ppm): 137.81, 82.56, 78.23, 59.01, 22.38, 14.72, 11.89, −0.58, −1.32, −5.64, −6.82, −145.23. Preparation of [Li(thf)4][Mo(MePDP)2]. The synthesis of [Li(thf)4][Mo(MePDP)2] followed the same procedure outlined for [(thf)2LiMo(MePDP)2]. The final recrystallization after removal of LiCl was conducted using THF (4 mL) and pentane (4 mL) at −35 °C, yielding dark brown crystals identified as [Li(thf)4][Mo(MePDP)2]. Yield: 0.080 g (53%). 1H NMR (400 MHz, THF-d8; δ, ppm): 118.77 (12H), 68.86 (4H), 24.21 (4H), 13.76 (8H), −4.81 (4H), −5.69 (8H), −153.10 (2H). Anal. Calcd for C70H74LiMoN6O4: C, 72.09; H, 6.40; N, 7.21. Found: C, 70.31; H, 5.51; N, 7.91. Preparation of [Na(18-crown-6)(thf)2][Mo(MePDP)2]. Step 1. At room temperature, solid NaH (0.063 g, 2.63 mmol) was added to a 20 mL vial containing a solution of H2MePDP (0.50 g, 1.28 mmol) in THF (10 mL). The yellow suspension was stirred for 16 h, furnishing a luminescent dark orange solution, which was subsequently added to a thick-walled glass vessel charged with a suspension of MoCl3(thf)3 (0.269 g, 0.64 mmol) in THF (10 mL). The reaction mixture was heated to 70 °C for 16 h. The resulting brown solution was cooled to room temperature, and the volatiles were removed in vacuum. The solid residue was dissolved in 1:2 THF/toluene and filtered. After removal of solvents, recrystallization from THF (30 mL) and pentane (20 mL) at −35 °C yielded needle shaped brown crystals, which were used directly for the next step. Yield: 0.550 g (73%). 1H NMR (400 MHz, C6D6; δ, ppm): 137.00, 91.02, 76.25, 59.22, 24.90, 14.48, 13.00, 12.50, −2.35, −2.79, −5.31, −6.48, −148.04. Step 2. A THF solution (8 mL) of the material obtained in step 1 (0.200 g, 0.169 mmol) was treated with 18-crown-6 (0.050 g, 0.189 mmol) and stirred for 30 min at room temperature. The solvent was

removed in vacuum, and excess 18-crown-6 was removed from the solid residue by washing with diethyl ether. Dark brown crystals identified as [Na(18-crown-6)(thf)2][Mo(MePDP)2] were obtained after recrystallization from THF and pentane at −35 °C. Yield: 0.210 g (96%). 1H NMR (400 MHz, THF-d8; δ, ppm): 117.46 (12H), 68.71 (4H), 24.95 (4H), 13.74 (8H), 3.46 (24H), −4.87 (4H), −5.71 (8H), −152.01 (2H). Anal. Calcd for C74H82MoN6NaO8: C, 68.24; H, 6.35; N, 6.45. Found: C, 65.33; H, 6.01; N, 6.42. Preparation of [K(18-crown-6)(thf)2][Mo(MePDP)2]. Crystals identified as [K(18-crown-6)(thf)2][Mo(MePDP)2]·2THF were prepared via a similar two step procedure outlined for the sodium analogue [Na(18-crown-6)(thf)2][Mo(MePDP)2] using KH instead of NaH. Anal. Calcd for C74H82MoN6KO8: C, 67.41; H, 6.27; N, 6.37. Found: C, 65.71; H, 5.76; N, 6.47. Preparation of Mo(MePDP)2. A solution of I2 (13 mg, 0.051 mmol) in THF (2 mL) was added to a 20 mL vial containing a solution of [Na(18-crown-6)(thf)2][Mo(MePDP)2] (100 mg, 0.085 mmol) in THF (4 mL). An immediate color change from dark brown to red-brown was observed, and the reaction mixture was stirred for an additional 3.5 h at room temperature. The solvent was removed in vacuum, and the solid residue was washed several times with diethyl ether. Recrystallization from CH2Cl2 and diethyl ether furnished dark brown crystalline material identified as Mo(MePDP)2. Single crystals suitable for X-ray crystallography were obtained from a concentrated solution of the product in benzene at room temperature. Yield: 0.060 g (81%). 1H NMR (400 MHz, THF-d8; δ, ppm): 75.18 (12H), 70.45 (4H), 63.35 (4H), 11.05 (8H), 1.54 (4H), −0.52 (8H), −89.60 (2H). Anal. Calcd for C54H42MoN6·0.5CH2Cl2: C, 71.67; H, 4.75; N, 9.20. Found: C, 72.24; H, 4.56; N, 8.98. Preparation of [Na(18-crown-6)(thf)2]2[Cr(MePDP)2]. Step 1. A sodium naphthalenide solution was prepared by stirring naphthalene (0.007 g, 0.054 mmol) with excess sodium metal in THF (3 mL) for 1 h. The resulting dark green solution was added dropwise to a 20 mL vial containing a dark brown THF solution (2 mL) of [Na(thf)4][Cr(MePDP)2] (0.050 g, 0.044 mmol). An immediate color change to red-brown was observed, and the solution was stirred for 4 h. The solvents were removed in vacuum, and the solid residue was washed several times with pentane to remove naphthalene. Further purification via addition of pentane to a concentrated THF solution of the crude material yielded a dark red-brown powder. Yield: 0.058 g (91%) based on [Na(thf)4]2[Cr(MePDP)2]. Step 2. A THF solution (4 mL) of the material obtained in step 1 (0.050 g, 0.034 mmol) was treated with 18-crown-6 (0.020 g, 0.076 mmol), resulting in the immediate formation of a red-brown precipitate. The suspension was stirred for 30 min at room temperature, and the solid was isolated by filtration. Excess 18crown-6 was removed from the product by washing with diethyl ether. Yield: 0.044 g (76%). Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Preparation of [K(18-crown-6)(thf)2]2[Cr(MePDP)2]. The complex [K(18-crown-6)(thf)2]2[Cr(MePDP)2] was prepared via a similar two step procedure outlined for the sodium analogue [Na(18-crown6)(thf)2]2[Cr(MePDP)2] using KC8 instead of sodium naphthalenide. The byproduct graphite was removed by filtration before addition of 18-crown-6. Recrystallization via slow diffusion of pentane into a concentrated acetonitrile solution of the product at −35 °C yielded crystals identified as [K(18-crown-6)(MeCN)2][K(18-crown-6)(MeCN)Cr(MePDP)2]·3MeCN by X-ray diffraction. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Preparation of [Na(18-crown-6)(thf)2]2[Mo(MePDP)2]. The complex [Na(18-crown-6)(thf)2]2[Mo(MePDP)2] was prepared via a similar two step procedure outlined for the chromium analogue [Na(18-crown-6)(thf)2]2[Cr(MePDP)2]. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Preparation of [K(18-crown-6)(thf)2]2[Mo(MePDP)2]. The complex [K(18-crown-6)(thf)2]2[Mo(MePDP)2] was prepared via a similar two step procedure outlined for the sodium analogue [Na(18-crownM

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 6)(thf)2]2[Cr(MePDP)2] using KC8 instead of sodium naphthalenide. The byproduct graphite was removed by filtration before addition of 18-crown-6. Recrystallization via slow diffusion of pentane into a concentrated acetonitrile solution of the product at −35 °C yielded crystals identified as [K(18-crown-6)(MeCN)2][K(18-crown-6)(MeCN)Mo(MePDP)2]·3MeCN by X-ray diffraction. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Preparation of [Na(18-crown-6)(thf)2]3[Cr(MePDP)2]. Step 1. A sodium naphthalenide solution was freshly prepared by stirring naphthalene (0.013 g, 0.101 mmol) with excess sodium metal in THF (3 mL) for 1 h. The resulting dark green solution was added dropwise to a 20 mL vial containing a dark brown THF solution (2 mL) of [Na(thf)4][Cr(MePDP)2] (0.050 g, 0.044 mmol). An immediate color change to a dark ink-blue was observed, and the solution was stirred for 4 h. All volatiles were removed in vacuum, and the dark solid residue was washed several times with pentane to remove naphthalene. Further purification via addition of pentane to a concentrated THF solution of the crude material yielded a blackpurple powder. Yield: 0.070 g (91%) based on [Na(thf)4]3[Cr(MePDP)2]. Step 2. A THF solution (4 mL) of the material obtained in step 1 (0.050 g, 0.028 mmol) was treated with 18-crown-6 (0.024 g, 0.091 mmol), resulting in the immediate formation of a black precipitate. The suspension was stirred for 30 min at room temperature, and the solid was isolated by filtration. Excess 18-crown-6 was removed from the product by washing with diethyl ether. Yield: 0.045 g (75%). Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Preparation of [K(18-crown-6)(thf)2]3[Cr(MePDP)2]. The complex [K(18-crown-6)(thf)2]3[Cr(MePDP)2] was prepared via a similar two step procedure outlined for the sodium analogue [Na(18-crown6)(thf)2]3[Cr(MePDP)2] using KC8 instead of sodium naphthalenide. The byproduct graphite was removed by filtration before addition of 18-crown-6. Preparation of [Na(18-crown-6)(thf)2]3[Mo(MePDP)2]. The complex [Na(18-crown-6)(thf)2]3[Mo(MePDP)2] was prepared via a similar two step procedure outlined for the outlined for the chromium analogue [Na(18-crown-6)(thf)2]3[Cr(MePDP)2]. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Preparation of [K(18-crown-6)(thf)2]3[Mo(MePDP)2]. The complex [K(18-crown-6)(thf)2]3[Mo(MePDP)2] was prepared via a similar two step procedure outlined for the sodium analogue [Na(18-crown6)(thf)2]3[Cr(MePDP)2] using KC8 instead of sodium naphthalenide. The byproduct graphite was removed by filtration before addition of 18-crown-6. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful due to the high air and moisture sensitivity of the compound. Physical Measurements. Room temperature magnetic susceptibility measurements were performed with a Johnson Matthey Mark 1 instrument that was calibrated with HgCo(SCN)4. Cyclic voltammetry measurements were conducted under nitrogen atmosphere inside an MBraun drybox using a Gamry Interface 1000 electrochemical workstation in a single compartment cell using 1 mM sample solutions in THF with 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte. A three electrode setup was employed with a glassy carbon electrode as working electrode, a platinum sheet as the counter electrode, and a silver wire as a quasi-reference electrode. Ferrocene was added as an internal standard after completion of the measurements, and all potentials are referenced versus the Fc+/Fc couple. Electronic spectra (200−1100 nm) were recorded using a Shimadzu UV-1800 spectrophotometer in gas-tight quartz cuvettes with a 10 mm path length fitted with J-Young valves. Near-IR spectra (900−2200 nm) were recorded using a Thermo Scientific Nexus 870 FT-IR spectrometer in gas-tight quartz cuvettes with a 10 mm path length fitted with J-Young valves. Emission spectra were obtained in 10 mm path length gas-tight quartz cuvettes with JYoung valves using a Shimadzu RF-5301 PC spectrofluorophotometer.

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H and 13C {1H} NMR spectra were recorded on an Agilent 400 MHz spectrometer or a Varian INOVA 600 MHz spectrometer. All chemical shifts are reported relative to SiMe4 using 1H (residual) chemical shifts of the solvent as a secondary standard. All NMR spectra were processed using the MNova 10.0 software. Continuous wave EPR spectra were recorded on an X-band Bruker EMXPlus spectrometer equipped with an EMX standard resonator and a Bruker PremiumX microwave bridge. The spectra were simulated using EasySpin for MATLAB.44 X-ray Crystallography. Single crystals suitable for X-ray diffraction were coated with polyisobutylene oil (Sigma-Aldrich) in a drybox, transferred to a nylon loop, and then quickly transferred to the goniometer head of a Bruker AXS D8 Venture fixed-chi X-ray diffractometer equipped with a Triumph monochromator, a Mo Kα radiation source (λ = 0.71073 Å), and a PHOTON 100 CMOS detector. The samples were cooled to 100 K with an Oxford Cryostream 700 system and optically aligned. The APEX2 software program (version 2014.1-1)47 was used for diffractometer control, preliminary frame scans, indexing, orientation matrix calculations, least-squares refinement of cell parameters, and the data collection. Three sets of 12 frames each were collected using the omega scan method with a 10 s exposure time. Integration of these frames followed by reflection indexing and least-squares refinement produced a crystal orientation matrix for the crystal lattice that was used for the structural analysis. The data collection strategy was optimized for completeness and redundancy using the Bruker COSMO software suite. The space group was identified, and the data were processed using the Bruker SAINT+ program and corrected for absorption using SADABS. The structures were solved using direct methods (SHELXS) completed by subsequent Fourier synthesis and refined by full-matrix least-squares procedures using the programs provided by SHELXL2014.48 For several structures, heavily disordered solvent molecules were treated as diffuse electron density contributions with the aid of the SQUEEZE routine in the program PLATON.49,50 Further collection and refinement detail can be found in the Supporting Information. Calculations. All DFT calculations were performed with the ORCA program package.51 Geometry optimizations of the complexes and single-point calculations on the optimized geometries were carried out at the B3LYP level of DFT.52−54 The all-electron Gaussian basis sets were those developed by the Ahlrichs group.55−57 Triple-ζ quality basis sets def2-TZVP with one set of polarization functions on the metal and on the atoms directly coordinated to the metal center were used. For the carbon and hydrogen atoms, slightly smaller polarized split-valence def2-SVP basis sets were used that were of double-ζ quality in the valence region and contained a polarizing set of d functions on the non-hydrogen atoms. Auxiliary basis sets to expand the electron density in the resolution-of-the-identity (RIJCOSX)58−60 approach were chosen to match the orbital basis.61−63 The conductorlike screening model (COSMO) was applied to model solvent effects.64 Throughout this paper we describe our computational results by using the broken-symmetry (BS) approach by Ginsberg65 and Noodleman.66 Because several broken-symmetry solutions to the spinunrestricted Kohn−Sham equations may be obtained, the general notation BS(m,n) has been adopted, where m (n) denotes the number of spin-up (spin-down) electrons at the two interacting fragments. All molecular orbital and spin density plots were generated using the program Gabedit.67

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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.7b02809. H NMR spectroscopic data for [Mo(MePDP)2]z (z = 0, 1−), crystallographic collection and refinement details, and computational details and input file examples (PDF) 1

N

DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

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CCDC 1585647−1585654 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

Carsten Milsmann: 0000-0002-9249-5199 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Alan Bristow (Department of Physics, WVU) for assistance with NIR spectroscopic data collection and Nadia Leonard (Chirik group, Department of Chemistry, Princeton University) for assistance with EPR data collection. West Virginia University is acknowledged for financial support. This work used X-ray crystallography (CHE-1336071) and NMR (CHE-1228336) equipment funded by the National Science Foundation. The WVU High Performance Computing facilities are funded by the National Science Foundation EPSCoR Research Infrastructure Improvement Cooperative Agreement #1003907, the state of West Virginia (WVEPSCoR via the Higher Education Policy Commission), the WVU Research Corporation, and faculty investments.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02809 Inorg. Chem. XXXX, XXX, XXX−XXX