Manganese Chemistry of Anionic Pyrrole-Based Pincer Ligands

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Manganese Chemistry of Anionic Pyrrole-Based Pincer Ligands Ana L. Narro, Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, Texas 78249, United States

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S Supporting Information *

ABSTRACT: The chemistry of the pyrrole-based pincer ligands, RPNP (PNP = anion of 2,5-bis(dialkylphosphinomethyl)pyrrole, R = Cy and t-Bu), with manganese is reported. Metallation of tBuPNP with Mn(II) halide precursors did not afford 1:1 ligand to metal complexes but rather led to the formation of the 2:1 complex, [Mn(κ2-N,P-tBuPNP)2]. Reduction of in situ generated tBu PNP-Mn(II) in the presence of 2,2′-bipyridine generated the apparent, highspin Mn(I) complex, [Mn(bipy)(tBuPNP)], although metric parameters derived from crystallography demonstrated that the compound is best regarded as containing a Mn(II) ion with a bipy radical anion. Reactions of the Mn(I) precursor, [MnBr(CO)5], with RPNP afforded lowspin Mn(I) complexes of the type [Mn(CO)n(RPNP)] (R = Cy, n = 3; R = t-Bu, n = 2). A third equivalent of CO binds reversibly to [Mn(CO)2(tBuPNP)] but is lost readily. Pincer backbone dehydrogenation of [Mn(CO)n(RPNP)] with 1,4benzoquinone produced the related complexes, [Mn(CO)n(RdPNP)] (RdPNP = anion of 2,5-bis(dialkylphosphinomethylene)2,5-dihydropyrrole). [Mn(CO)2(tBudPNP)] was found to undergo protonation by (H{OEt2}2)(BArF4) at the methine position to generate the cationic species, [Mn(CO)2(tBudPNP-H)](BArF4). Reduction of [Mn(CO)2(tBuPNP)] by KC8 produced the rare, molecular Mn(0) species, K[Mn(CO)2(tBuPNP)]. Electron paramagnetic resonance characterization of [Mn(CO)2(tBuPNP)]− is consistent with a metal-based radical. The chemistry of the pincer manganese species is discussed in the context of potential catalysis and compared with more commonly encountered Mn complexes containing neutral pincer ligands.

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INTRODUCTION Discrete, pincer-ligated manganese complexes are promising well-defined catalysts for a host of transformations including hydrogenation and CO2 reduction.1−4 By in large, the catalytic activity of these systems for reductive transformations is reliant upon the availability of ionizable hydrogen atoms (protons) on the pincer ligand.5,6 This requirement likely stems from the difficulty in engaging two-electron redox processes for manganese.7−10 As a result, the lion’s share of catalytic systems to date featuring Mn pincers contain neutral ligands, which are capable of undergoing reversible deprotonation.11−28 By contrast, manganese compounds featuring anionic pincer ligands are much less common, representing only a small fraction of their neutral counterparts. Ozerov and co-workers reported some of the first examples of such compounds employing the amide-based P NP ligand, bis(2(diisopropylphosphino)4-tolyl)amide ( iPr P 2 N). 29,30 The iPr P2N pincer was demonstrated to support both high-spin complexes of Mn(II) and low-spin complexes of Mn(I). More recently, Gade et al. demonstrated the use of well-defined Mn(II) complexes bearing an anionic bis(oxazolinylmethylidene)isoindoline pincer ligand for the enantioselective hydroboration of ketones.31 Notably, this work provided evidence for a reduction mechanism that operates in the absence of metal−ligand cooperativity, highlighting the differences between neutral and anionic pincer ligands. A Mn(I) complex of an anionic phenolate-based POP pincer ligand has also been recently described by Lacy et al.32 This compound was found to be an active catalyst for the © XXXX American Chemical Society

Tishchenko reaction, but as with neutral pincers, reversible deprotonation of the ligand backbone was invoked to explain the catalytic activity. The potential use of anionic pincer ligands in catalysis is intriguing given their robust nature and the disparate reactivity observed for their metal complexes in comparison to neutral versions.33 These considerations paired with the desirable attributes of Mn as an earth-abundant element provide a compelling rationale for investigating these species in more detail. In this contribution, we disclose the chemistry of the pyrrole-based pincer ligands, RPNP (R = t-Bu, Cy; PNP = anion of bis(dialkylphosphinomethyl)pyrrole),34−38 with manganese. The pincer ligand is found to support complexes of Mn in several different oxidation states and coordination geometries including a rare example of a molecular Mn(0) species.

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RESULTS AND DISCUSSION Treatment of MnCl2(THF)1.6 with the lithium salt of tBuPNP was found to generate an ill-defined mixture containing, among other species, the protonated pyrrole ligand, H(tBuPNP). Due to the paramagnetism of high-spin Mn(II), 1H NMR analysis yielded little information. Crystallization of the reaction mixture from heptane afforded the 2:1 complex, [Mn(κ2N,P-tBuPNP)2] (1, Figure 1) in low yields. The compound possesses a severely distorted tetrahedral geometry (τ4 = 0.67)39 with bidentate coordination to two tBuPNP ligands. Received: January 29, 2019

A

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the corresponding Mn(II) complex, [MnCl(bipy)(tBuPNP)], by omitting KC8 failed to produce a tractable material. The solid-state structure of 2 is shown in Figure 2. The complex possesses a distorted square-pyramidal geometry with

Figure 2. Thermal ellipsoid (50%) drawing of the solid-state structure of 2. Hydrogen atoms and minor components of the disordered t-Bu group were omitted for clarity. Selected bond distances (Å) and angles (°): Mn(1)-N(1) = 2.115 (2); Mn(1)-N(2) = 2.145 (2); Mn(1)-N(3) = 2.079 (2); Mn(1)-P(1) = 2.7084 (9); Mn(1)-P(2) = 2.6983 (9); P(1)-Mn(1)-P(2) = 145.25 (3); N(1)-Mn(1)-N(2) = 166.25 (9); N(1)-Mn(1)-N(3) = 115.53 (10); N(2)-Mn(1)-N(3) = 77.47 (10).

Figure 1. Thermal ellipsoid (50%) drawing of the solid-state structure of 1. Hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (°): Mn(1)-N(1) = 2.0452 (14); Mn(1)N(2) = 2.0614 (15); Mn(1)-P(2) = 2.6028 (5); Mn(1)-P(3) = 2.5575 (5); N(1)-Mn(1)-N(2) = 117.27 (6); P(2)-Mn(1)-P(3) = 120.702 (18); N(1)-Mn(1)-P(2) = 80.72 (4); N(1)-Mn(1)-P(3) = 144.69 (4); N(2)-Mn(1)-P(2) = 114.02 (5); N(2)-Mn(1)-P(3) = 81.47 (4).

the tBuPNP ligand and one of the nitrogen atoms of the bipyridine unit occupying equatorial positions. The structure is superficially similar to that of [Fe(bipy)(CyPNP)],41 although the bond distances to manganese are longer than those of other characterized κ3-PNP complexes of mid 3d metals, consistent with the high-spin nature of 2.42,43 Given the potential redox non-innocence of the bipyridine ligand, we were also curious to interrogate the metrics about the heterocycle to determine if the electronic structure is better described by a Mn(II)-bipy•− resonance form. The distances within the pyridine rings indicate a substantial alteration favoring a reduced π system (see the Supporting Information).44 Such a picture agrees with that determined by Ozerov and co-workers whose related [Mn(bipy)(iPrP2N)] complex was formulated as a high-spin Mn(II) ion antiferromagnetically coupled to a reduced bipy unit.30 In addition to the metric data, the UV−vis−NIR spectrum of 2 demonstrates several low energy absorptions between 700 and 1100 nm (see the Supporting Information), consistent with a bipy radical anion.45 Given that the majority of catalytically active Mn pincer complexes feature low-spin Mn(I), we next targeted such species by introducing strong-field carbonyl ligands. The treatment of 2 with carbon monoxide did not proceed cleanly to afford low-spin Mn(I)−CO complexes of tBuPNP. Instead, such compounds were accessed by the treatment of [MnBr(CO)5] with the lithium salt of tBuPNP (3, eq 2). Compound 3

The bond distances about manganese are consistent with highspin Mn(II) and similar to those reported for a related complex prepared by Tondreau and Boncella bearing a pyridine-based pincer ligand.40 Notably, two of the phosphine arms in 1 remain uncoordinated to the metal center resulting in a bidentate chelating mode, which has not been previously observed for this class of ligands. Rational synthesis of 1 was accomplished by the treatment of MnCl2(THF)1.6 with two equivalents of Li(tBuPNP). Attempts to generate Mn(II) complexes with a 1:1 ligand to metal stoichiometry were unsuccessful, even in experiments employing sub-stoichiometric quantities of M(RPNP) (R = tBu, M = Li; R = Cy, M = Na). These results contrast those of Ozerov and co-workers who were able to prepare the manganese(II) “ate” complex, Li[MnCl2(iPrP2N)].30 In the present case, it is not clear whether the differing rigidity or electronic character of the pyrrole-based PNP ligands suppress the formation of a fivecoordinate species akin to Li[MnCl2(iPrP2N)]. A complex of high-spin manganese possessing a 1:1 ligand− metal stoichiometry was generated by reduction of the reaction mixture containing Li(tBuPNP) and MnCl2(THF)1.6 in the presence of 2,2′-bipyridine (2, eq 1). The resulting complex

displayed a solution magnetic moment of 4.9 (2) μB indicating an apparent high-spin Mn(I) ground state. Attempts to prepare B

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Figure 4. Electronic absorption spectra of 3 and 4 in toluene showing the result of CO addition and removal to 3.

binding of a third CO ligand to Mn. In the event that CO is capable of binding to 3, a bleaching of this absorption would be expected. As shown in Figure 4, treatment of 3 with an atmosphere of CO does in fact lead to immediate diminution of the band at 565 nm. Subsequent purging of the solution with N2 restored the absorption profile of 3 establishing a reversible binding for the third CO ligand. Attempts to generate the corresponding dicarbonyl complex of CyPNP via similar purging and/or exposure of 4 to vacuum failed to liberate CO. Therefore, it appears that the greater steric bulk of tBu PNP versus CyPNP is sufficient to discourage the formation of the 18-electron tricarbonyl species. The coordinative unsaturation of 3 coupled with its ability to bind a sixth ligand led us to survey its potential reactivity with molecules relevant to reported manganese-catalyzed processes. The treatment of 3 with H2, CO2, and H2SiPh2 in benzene-d6 solution did not proceed to any new species as judged by NMR spectroscopy, even upon heating to 80 °C. This lack of reactivity is not surprising given that most examples of Mnpincer catalysis feature some degree of metal−ligand cooperation during bond activation. To this end, we proceeded to examine the dehydrogenation of the pyrrole backbone.47,48 Such a modification to the tBuPNP ligand disrupts the aromaticity in the pyrrole ring, potentially creating a site for ligand-based reactivity.49 Accordingly, the reaction of both 3 and 4 with 1,4-benzoquinone (BQ) was found to proceed cleanly to the corresponding dehydrogenated species (3d and 4d, eq 4) with the formation of hydroquinone (HQ). The

Figure 3. Thermal ellipsoid (50%) rendering of 3. Hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (°): Mn(1)-N(1) = 1.9384 (17); Mn(1)-C(23) = 1.770 (2); Mn(1)C(24) = 1.751 (2); Mn(1)-P(1) = 2.3145 (6); Mn(1)-P(2) = 2.3376 (6); P(1)-Mn(1)-P(2) = 161.79 (2); N(1)-Mn(1)-C(23) = 155.37 (9); N(1)-Mn(1)-C(24) = 117.49 (9); C(23)-Mn(1)-C(24) = 87.08 (9).

is best described as square-pyramidal (τ = 0.11) with the CO ligands occupying one axial and one equatorial position. The metal−ligand bond distances are substantially contracted in comparison to 1 and 2, in line with the low-spin nature of 3. A compound analogous to 3 containing low-spin Fe(I) has also been recently reported by Walter and co-workers.46 Analogous treatment of [MnBr(CO)5] with NaCyPNP in dioxane gave rise to a yellow-orange solution in contrast to the purple color observed for 3. Isolation of the reaction product yielded a yellow solid with 1H NMR features comparable to those of 3, in agreement with the presence of low-spin Mn(I). Analysis of the product by IR spectroscopy demonstrated that the compound was a tricarbonyl complex (4, eq 3) with CO

fundamentals appearing at 2010 (w), 1915, and 1893 cm−1, consistent with a meridional isomer. Subsequent crystallographic analysis confirmed the six-coordinate nature of 4 (see below, Figure 5). Bond metrics about Mn were similar to those observed for 3. The dramatic difference in color between 3 and 4 prompted us to examine the nature of the electronic transitions in the two complexes. Figure 4 displays the absorption spectra for both species. Compound 3 exhibits an absorption maximum centered at 565 nm accounting for its vivid purple color, whereas 4 lacks any significant bands in the visible region. Based on these distinctions, we reasoned that the 565 nm transition in 3 would provide a diagnostic hallmark for the

number of carbonyl ligands in compounds 3d and 4d remained unchanged from those in 3 and 4, respectively, as judged by IR spectroscopy. Despite the increased basicity of the dehydrogenated PNP unit (dPNP), the stretching modes for the CO ligands were found to shift only minimally to a lower energy from their values in 3 and 4. A similar small shift in the νNN vibrational frequency was also observed upon dehydrogenation in the case of [Co(N2)(tBuPNP)].49 C

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Figure 5. Thermal ellipsoid (50%) rendering of the solid-state structures of 4 (left) and 4d (right) One of two crystallographically independent molecules of the asymmetric unit for 4d is displayed. Hydrogen atoms and co-crystallized heptane molecule (4d) were omitted for clarity. Selected bond distances (Å) and angles (°): 4: Mn(1)-N(1) = 2.0326 (17); Mn(1)-C(16) = 1.797 (2); Mn(1)-C(17) = 1.8435 (15); Mn(1)-P(1) = 2.2966 (3); P(1)-Mn(1)-P(1A) = 158.61 (2); N(1)-Mn(1)-C(16) = 180.0; C(17)-Mn(1)-C(17A) = 175.10 (8); C(16)-Mn(1)-C(17) = 87.55 (4). 4d: Mn(1)-N(1) = 2.0057 (14); Mn(1)-C(31) = 1.8423 (19); Mn(1)-C(32) = 1.8321 (18); Mn(1)-C(33) = 1.7902 (18); Mn(1)-P(1) = 2.3486 (5); Mn(1)-P(2) = 2.3359 (5); N(1)-Mn(1)-C(33) = 178.09 (7); P(1)-Mn(1)-P(2) = 158.872 (19); C(31)-Mn(1)-C(32) = 177.58 (8).

assignment of the site of protonation. Nonetheless, the average C−C bond distances about the pyrrole unit were found to be an intermediate between those of 3 and 4d consistent with a disordered methine/methylene site (see the Supporting Information). The challenges encountered in synthesizing divalent manganese complexes of the RPNP ligands (see above) and the observed recalcitrance of 3 toward reaction with simple two-electron substrates, such as H2, led us to question whether any stable metal-based redox events could be accessed. We therefore consulted cyclic voltammetry to determine if the pincer ligand engenders any reversible redox processes in the Mn(I) complexes. Gratifyingly, compound 3 demonstrated a reversible cathode process at −2.14 V (vs Fc/Fc+; see the Supporting Information). An analogous reversible reduction was not identified in the CV of 4. Accordingly, we targeted the reduction of 3 by chemical means in order to prepare a rare example of a mononuclear, molecular Mn(0) complex.52−54 Treatment of 3 with excess KC8 in THF over 18 h led to a color change from purple to green. Isolation and purification of the resulting product afforded a green solid that was identified as the potassium salt of the [Mn(CO)2(tBuPNP)]− anion (6, eq 6). The solid-state structure of the anion of 6 is shown in

The solid-state structure of 4d demonstrates the expected alternation in bond lengths about the pincer backbone, consistent with the depiction in eq 4. The remaining bond metrics about the inner coordination sphere are similar to those in 4 with a slight elongation of the Mn−P distances to accommodate the more rigid CydPNP ligand (Figure 5). The Cy dPNP ligand of 4d is also substantially more planar than the Cy PNP ligand of 4. In similar fashion to 3, solutions of 3d failed to react with H2 indicating that PNP dehydrogenation is insufficient for unlocking cooperative metal−ligand reactivity in these systems. Compound 3d did undergo protonation, however, in the presence of the strong acid, (H{OEt2}2)(BArF4) (eq 5).

Previous work by Nishibayashi and co-workers with [Fe(N2)(tBuPNRP)] (R = H, Me) has established that the protonation of PNP can occur at both the C2 and C3 positions of the pyrrole ring.50,51 In the case of 3d, however, dehydrogenation of the pincer backbone imparts an enamine-like character to the methine bridges enhancing their Brønsted basicity. Consequently, the reaction of 3d with (H{OEt2}2)(BArF4) was found to produce a new species, 5, which displayed 1H NMR features consistent with protonation at the methine carbon atom (see the Supporting Information). In line with the cationic nature of 5, the νCO modes were found to shift to a higher energy (1942 and 1871 cm−1) compared to those in 3 and 3d. The solid-state structure of 5 was obtained (Figure 6), but a crystallographic disorder about the methine and methylene positions of the ligand prohibited the definitive

Figure 7. Somewhat surprisingly, 6 was not found to retain solvent molecules upon crystallization. Instead, the potassium cations coordinate to the π face of the pyrrolide rings with additional interactions involving the carbonyl ligands giving rise to a coordination polymer (see the Supporting Information). We previously observed a similar polymer for a related Fe(0) complex of CyPNP; however, in that instance, D

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Figure 6. Thermal ellipsoid (50%) drawing of the solid-state structure of 5. Hydrogen atoms and minor components of the disordered CF3 groups were omitted for clarity. Selected bond distances (Å) and angles (°): Mn(1)-N(1) = 1.9384 (17); Mn(1)-P(1) = 2.3145 (6); Mn(1)-P(2) = 2.3376 (6); Mn(1)-C(23) = 1.770 (2); Mn(1)-C(24) = 1.751 (2); C(1)-N(1) = C(1)-C(5) = 1.490 (3); C(4)-C(14) = 1.499 (3); C(5)-P(1) = 1.852 (2); C(14)-P(2) = 1.859 (2); P(1)-Mn(1)-P(2) = 161.79 (2); N(1)-Mn(1)-C(24) = 117.49 (9); C(23)-Mn(1)-C(24) = 87.08 (9).

is displayed in Figure 8. The signal, centered at g = 2.005, displays a modest anisotropy with an average line spacing of 17

Figure 7. Thermal ellipsoid (50%) drawing of the solid-state structure of 6. Potassium cations and hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (°): Mn(1)-N(1) = 2.0092 (19); Mn(1)-C(23) = 1.759 (2); Mn(1)-C(24) = 1.765 (2); Mn(1)P(1) = 2.2882 (7); Mn(1)-P(2) = 2.2930 (7); P(1)-Mn(1)-P(2) = 152.25 (3); N(1)-Mn(1)-C(23) = 161.57 (10); C(23)-Mn(1)-C(24) = 93.54 (11).

Figure 8. X-band EPR spectrum of 6 in THF at room temperature.

G. Both these parameters are comparable to those observed by Figueroa et al. for a five-coordinate Mn(0) species bearing terphenyl isocyanide ligands, further supporting the description of 6 as a low-spin complex of zero-valent manganese.53 Preliminary investigation of the reactivity of compound 6 with simple radical species demonstrated a facile re-oxidation to Mn(I). For example, the treatment of 6 with an atmosphere of NO(g) led to an immediate color change from dark green to purple. NMR and IR analyses of the reaction mixture identified compound 3 as the only discernable Mn product. The apparent robust nature of the MnI/0 couple may therefore bode well for potential electrocatalytic applications of this system.52,55,56

the potassium counterions were found to bind solvent molecules.42 The bond metrics about Mn in 6 are similar to those in 3 with a maximal deviation (0.07 Å) observed for the Mn−N contact, consistent with the retention of the low-spin state. Intra-ring bond distances for the pyrrolide moiety of 6 do not evince any evidence of ligand-based reduction suggesting that the complex is best described as a bona fide Mn(0) species. Moreover, the νCO signatures for 6 were found to shift substantially to lower frequency (1817 and 1722 cm−1) consistent with a metal-based reduction. To provide further proof for the formulation of 6 as a Mn(0) complex, we turned to electron paramagnetic resonance (EPR) spectroscopy. The solution spectrum of 6 at room temperature E

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processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with CrysAlisPro63 and SCALE3 ABSPACK,64 respectively. The structure was solved with the SHELXT structure solution program65 within Olex266 using direct methods and refined (on F2) with the SHELXL package67 using fullmatrix, least-squares techniques. All nonhydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atom positions were determined by geometry and refined by a riding model. Crystallographic structure and refinement parameters appear in the Supporting Information. [Mn(κ2-N,P-tBuPNP)2] (1). A scintillation vial was charged with 0.018 g (0.077 mmol) of MnCl2(THF)1.6, 0.060 g (0.15 mmol) of Li(tBuPNP), and 5 mL of THF. The solution was allowed to stir overnight at room temperature, after which time the solvent was removed under reduced pressure. The dark yellow residue was extracted into toluene and filtered through a pad of Celite, and the filtrate was evaporated to dryness. The resulting yellow solid was washed with pentane and isolated on a frit, yielding 0.038 g (60%) of crude material. Attempted crystallization of the crude solid from THF/pentane resulted in the precipitation of small amounts of a colorless material, which was discarded. The remaining mother liquor was decanted and evaporated to dryness. Dissolution of the resulting residue in heptane afforded crystals of 1 suitable for X-ray diffraction after standing at room temperature for 24 h. Anal. Calcd for C44H84MnN2P4: C, 64.45; H, 10.33; N, 3.42. Found: C, 64.88; H, 11.10; N, 3.18. The slightly elevated values found for C and H are consistent with the retention of residual heptane from purification. [Mn(bipy)(tBuPNP)] (2). A flask was charged with 0.150 g (0.581 mmol) of MnCl2(THF)1.6, 0.226 g (0.581 mmol) of Li(tBuPNP), 0.114 g (0.668 mmol) of 2,2′-bipyridine, and 5 mL of THF. To the dark brown solution, 0.086 g (0.58 mmol) of KC8 was added. The resulting mixture was allowed to stir at room temperature for 18 h. All volatiles were removed in vacuo leaving a dark brown residue that was extracted into toluene. The toluene extract was filtered through a pad of Celite and evaporated to dryness. The resulting brown solid was washed with pentane and isolated by filtration to afford 0.183 g (48%). Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at −30 °C. μeff = 4.9 (2) μB. Anal. Calcd for C32H50MnN3P2: C, 64.74; H, 8.48; N, 7.08. Found: C, 64.84; H, 8.70; N, 6.70. [Mn(CO)2(tBuPNP)] (3). A flask was charged with 0.181 g (0.657 mmol) of [MnBr(CO)5], 0.257 g (0.659 mmol) of Li(tBuPNP), and 20 mL of THF. The resulting dark orange mixture was allowed to stir for 18 h at 65 °C, during which time it acquired a deep purple color. All volatiles were removed in vacuo, and the remaining purple residue was extracted into toluene and filtered through a pad of Celite. The solution was evaporated to dryness, and the resulting purple solid was washed with pentane and isolated by filtration to afford 0.240 g (74%). Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at −30 °C. 1H NMR (300 MHz, δ): 6.55 (s, 2 pyr-CH), 3.02 (m, 4 CH2), 1.14 (m, 36 t-Bu); 31P NMR (δ): 116.34. IR (ν C O ): 1904, 1833 cm − 1 . Anal. Calcd for C24H45MnNO2P2: C, 58.41; H, 8.58; N, 2.84. Found: C, 59.27; H, 8.81; N, 2.62. The slightly elevated values found for C and H are consistent with the retention of residual heptane from purification. [Mn(CO)3(CyPNP)] (4). A flask was charged with 0.200 g (0.728 mmol) of [MnBr(CO)5], 0.371 g (0.728 mmol) of Na(CyPNP), and 20 mL of 1,4-dioxane. The light orange mixture was allowed to stir overnight at 100 °C, during which time it became yellow in color. All volatiles were removed in vacuo, and the remaining yellow residue was extracted into toluene. The toluene extract was filtered through a pad of Celite and evaporated to dryness. The resulting yellow solid was washed with pentane and isolated by filtration to afford 0.237 g (56%). Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at −30 °C. 1H NMR (300 MHz, δ): 6.53 (s, 2 pyr-CH), 3.04 (m, 2 CH2), 2.03 (m, 8 Cy), 1.82 (app d, 4 Cy), 1.64 (app br s, 8 Cy), 1.53 (m, 4 Cy), 1.42 (app t, 4 Cy), 1.25 (m, 4 Cy), 1.11 (m, 12 Cy); 31P NMR (δ): 89.44. IR (νCO): 2010 (w), 1915, 1893 cm−1. Anal. Calcd for C33H50MnNO3P2: C, 63.35; H,

CONCLUSIONS In this contribution, we have detailed the chemistry of a series of pyrrole-based PNP ligands with manganese. The compounds reported herein join a small list of known Mn complexes bearing anionic pincer ligands. Whereas the coordination of RPNP to Mn(II) complex was accomplished only in the presence of bipyridine, the ligand was found to support several Mn(I) species. Unlike anionic pincer complexes of Mn(I) derived from neutral precursors, compounds 3 and 4 did not demonstrate reactivity toward molecular hydrogen, consistent with the inability of RPNP to participate in ligand-based proton transfer events. Despite this lack of reactivity, both compounds readily underwent dehydrogenation in the presence of benzoquinone to afford variants containing a modified pincer backbone (3d and 4d). In the case of 3d, treatment with HBArF4 produced a novel cationic species demonstrating protonation at the methine arm. Finally, the reduction of 3 with KC8 was found to generate 6, a rare example of a stable molecular Mn(0) complex. Spectroscopic and solid-state data for 6 were consistent with a 17-electron metal-based radical.

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EXPERIMENTAL SECTION

General Comments. Manipulations of air- and moisture-sensitive materials were performed under an atmosphere of purified nitrogen gas using standard Schlenk techniques or in a Vacuum Atmospheres glovebox. Tetrahydrofuran, diethyl ether, pentane, and toluene were purified by sparging with argon and passing through two columns packed with 4 Å molecular sieves (all solvents) and alumina (THF and ether). Benzene-d6 was dried over sodium ketyl and vacuumdistilled prior to use. 1H NMR, 13C NMR, and 31P NMR spectra were recorded in benzene-d6 on Varian spectrometers operating at 300 or 500 MHz (1H) and referenced to the residual 1H resonance of the solvent or the deuterium lock frequency (31P). FT-IR spectra were recorded in benzene-d6 solution with a ThermoNicolet iS 10 spectrophotometer running the OMNIC software; an air-tight liquid transmission cell (Specac OMNI) with KBr windows (path length = 0.05 mm) was used for all measurements. UV−vis spectra were recorded on a Cary 60 (UV−vis) or Cary 1000 (UV−vis−NIR) spectrophotometer in air-tight Teflon-capped quartz cells. Solution EPR spectra were recorded at the X-Band on a Bruker EMX EPR spectrometer in THF at 293 K. Data were recorded in 4 mm o.d. quartz EPR tubes capped with a tight-fitting rubber septum. Cyclic voltammetry was performed in THF at 23 °C on a CH Instruments 620D electrochemical workstation. A three-electrode setup was employed comprising a 2 mm diameter glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl quasireference electrode. Triply recrystallized Bu4NPF6 was used as the supporting electrolyte. All electrochemical data were referenced to the ferrocene/ferrocenium couple at 0.00 V. Solution magnetic susceptibility measurements were determined by the Evans method without a solvent correction using reported diamagnetic corrections.57 Elemental analyses were performed by the CENTC facility at the University of Rochester. In each case, recrystallized material was used for the combustion analysis. Materials. MnCl2(THF)1.6,58,59 Li(tBuPNP),51 Na(CyPNP),42 HBArF4,60 and potassium graphite (KC8)61 were prepared according to published procedures or slight modifications thereof. [MnBr(CO)5], bipyridine, and 1,4-benzoquinone were purchased from commercial suppliers and used as-received. Crystallography. Crystals suitable for X-ray diffraction were mounted, using Paratone oil, onto a nylon loop. Data were collected at 98 (2) K using a Rigaku AFC12/Saturn 724 CCD fitted with MoKα radiation (λ = 0.71075 Å). Low-temperature data collection was accomplished with a nitrogen cold stream maintained by an XStream low-temperature apparatus. Data collection and unit cell refinement were performed using CrystalClear software.62 Data F

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Organometallics Accession Codes

8.06; N, 2.24. Found: C, 62.34; H, 8.00; N, 1.98. Repeated combustion analyses returned low values for carbon. [Mn(CO)2(tBudPNP)] (3d). A scintillation vial was charged with 0.080 g (0.16 mmol) of [Mn(CO)2(tBuPNP)], 0.044 g (0.40 mmol) of 1,4-benzoquinone, and 5 mL of THF. The dark purple mixture was allowed to stir for 18 h at room temperature, during which time it became dark green in color. All volatiles were removed in vacuo, and the remaining green residue was extracted into toluene and filtered through a pad of Celite. The filtrate was evaporated to dryness to afford 0.060 g (75%) of a green solid. Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at −30 °C. 1 H NMR (500 MHz, δ): 6.70 (app t, 2 pyr-CH), 4.57 (m, 2 CHP), 1.28 (m, 36 t-Bu); 31P NMR (δ): 116.73. IR (νCO): 1900, 1828 cm−1. Anal. Calcd for C24H40MnNO2P2: C, 58.65; H, 8.20; N, 2.85. Found: C, 58.79; H, 8.50; N, 2.79. [Mn(CO)3(CydPNP)] (4d). A flask was charged with 0.109 g (0.174 mmol) of [Mn(CO)3(CyPNP)], 0.047 g (0.43 mmol) of 1,4benzoquinone, and 5 mL of THF. The yellow mixture was allowed to stir at room temperature for 4 h, during which time it took on a bright orange color. All volatiles were removed in vacuo, and the remaining orange residue was extracted into toluene and filtered through a pad of Celite. The filtrate was evaporated to dryness, and the resulting dark orange solid was washed with copious amounts of pentane and collected by filtration to afford 0.075 g (69%). Crystals suitable for X-ray diffraction were grown from a saturated heptane solution at −30 °C. 1H NMR (500 MHz, δ): 6.71 (app t, 2 pyr-CH), 4.51 (m, 2 CHP), 2.20 (m, 8 Cy), 2.09 (app d, 4 Cy), 1.76 (app d, 4 Cy), 1.69 (m, 8 Cy), 1.59 (app d, 4 Cy), 1.50 (app q, 4 Cy), 1.28 (app q, 4 Cy), 1.18 (m, 8 Cy); 31P NMR (δ): 89.27. IR (νCO): 2011 (w), 1916, 1893 cm−1. Anal. Calcd for C33H48MnNO3P2·1 2 C7H16: C, 65.07; H, 8.38; N, 2.08. Found: C, 64.47; H, 8.23; N, 2.01. [Mn(CO)2(tBudPNP-H)](BArF4) (5). A flask was charged with 0.0367 g (0.075 mmol) of 3d, 0.0907 g (0.090 mmol) of HBArF4, and 5 mL of THF. The mixture was allowed to stir at room temperature overnight, during which time it went from green to a dark blue color. All volatiles were removed in vacuo. The remaining residue was washed with benzene affording 0.0929 g of a blue solid (91%). Crystals suitable for X-ray diffraction were grown from a saturated diethyl ether solution at −30 °C. 1H NMR (500 MHz, δ): 8.16 (br s, 8 o-BArF4), 7.62 (s, 4 p-BArF4), 7.30 (m, 1 CH), 7.29 (app s, 1  CHP), 6.84 (s, 1 CH), 2.95 (d, J2HP = 8.0 Hz, 2 CH2P), 0.83 (d, J3HP = 13.5 Hz, 18 t-Bu), 0.76 (d, J3HP = 14.5 Hz, 18 t-Bu); 31P NMR (δ): 122.08 (d, J2PP = 66 Hz), 120.46 (d, J2PP = 66 Hz). IR (νCO): 1942, 1871 cm−1. Anal. Calcd for C56H53BF24MnNO2P2: C, 49.61; H, 3.94; N, 1.03. Found: C, 49.14; H, 3.54; N, 0.58. K[Mn(CO)2(tBuPNP)] (6). A scintillation vial was charged with 0.080 g (0.16 mmol) of [Mn(CO)2(tBuPNP)] and 5 mL of THF. To the deep purple solution was added 0.054 g (0.40 mmol) of KC8. The mixture was allowed to stir at room temperature overnight, during which time it became green with an apparent dark precipitate. The mixture was filtered through a pad of Celite to give a bright green solution, which was subsequently evaporated to dryness to give 0.054 g (62%) of a green solid. Crystals suitable for X-ray diffraction were grown by vapor diffusion of pentane into a concentrated THF solution at room temperature. IR (νCO): 1817, 1722 cm−1. Anal. Calcd for C24H42KMnNO2P2: C, 54.13; H, 7.95; N, 2.63. Found: C, 54.32; H, 8.15; N, 2.52.

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CCDC 1894210−1894215 and 1904189 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

Zachary J. Tonzetich: 0000-0001-7010-8007 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Welch Foundation (AX-1772) for financial support of this work. NMR facilities at UTSA are supported by a grant from the National Science Foundation (CHE-1625963). Mr. Ian Davis is acknowledged for assistance in acquiring EPR data and Mr. Vance Thompson is thanked for fruitful discussions.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00058. NMR, IR, and UV−vis spectra, cyclic voltammograms, additional structural diagrams, and tables of crystallographic data and refinement parameters (PDF) G

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