Pyridine(diimine) Chelate Hydrogenation in a Molybdenum Nitrido

Apr 5, 2019 - ... the rhodium hydride precatalyst (η5-C5Me5)(py-Ph)Rh(H) ([Rh–H]; py-Ph = 2-phenylpyridine) resulted ... Wang, Loose, Chirik, and K...
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Pyridine(diimine) Chelate Hydrogenation in a Molybdenum Nitrido Ethylene Complex Mat́ e ́ J. Bezdek and Paul J. Chirik* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States

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S Supporting Information *

ABSTRACT: Addition of H2 gas to the aryl-substituted pyridine(diimine) molybdenum nitride (iPrPDI)Mo(N)(C2H4) ([1-(N)(η2-C2H4)]; iPrPDI = 2,6-(2,6-iPr2-C6H3N CMe)2C5H3N) in the presence of the rhodium hydride precatalyst (η5-C5Me5)(py-Ph)Rh(H) ([Rh−H]; py-Ph = 2phenylpyridine) resulted in partial hydrogenation of the central pyridine of the iPrPDI chelate to yield (iPrTHPDI)Mo(N)(C2H4) ([2-(N)(η2-C2H4)]; iPrTHPDI = 2,6-(2,6iPr2-C6H3NCMe)2C5H6N). The product, [2-(N)(η2-C2H4)], was structurally and spectroscopically characterized, and its electronic structure was examined by density functional theory (DFT). The stepwise addition of H atoms from [Rh−H] to [1(N)(η2-C2H4)] was computed to be thermodynamically viable by DFT.

I

Scheme 1. Selected PDI Ligand Modifications: (a) Imine CAlkylation;18 (b) Pyridine N-Alkylation;19 (c) Double Imine C-Alkylation;20 (d) H-Atom Migration;21 (e) Deprotonation−Dimerization;22b,c (f) Alkylation− Dimerization;18,22a,23 (g) Oxidation;11a (h) Metal−Ligand Cooperation;9d (i) the Pathway Reported in This Work

nitially prepared as modular terpyridine mimics, pyridine(diimines) (PDIs) have emerged as a privileged ligand class in homogeneous catalysis, particularly with Earth-abundant transition metals.1 Following independent reports from Brookhart,2 Bennett,3 and Gibson4 describing high-activity PDI iron(II) and cobalt(II) dihalide catalyst/methylaluminoxane (MAO) activator combinations for ethylene polymerization,5 PDI ligands have since been applied in the cycloaddition and hydrofunctionalization of olefins,6 carbonyl hydrosilylation,7 and C−H functionalization8 as well as smallmolecule activation including N2,9 NH3,10 O2,11 and CO2.12 The rich reaction chemistry observed with transition-metal, main-group,13 and f-block14 elements bearing PDI ligands can often be traced to their chemical and electronic versatility, owing to their relative ease of synthesis15 and the accessibility of low-lying π* orbitals of a2 and b2 symmetry.16 As a consequence, PDI ligands can exhibit both redox activity and noninnocence by either engaging in reversible 1e− redox chemistry with a metal center or serving as traditional π-acids with a high degree of covalency, respectively.17 PDI ligands are also chemically noninnocent and can undergo alkylation,18−20 H atom migration,21 oxidation,11a deprotonation, and, consequently, dimerization chemistry upon treatment with nucleophiles and bases (Scheme 1a−g).18,22,23 While ligand modification chemistry arising from electrophilic sites and acidic C−H bonds can be detrimental to the catalytic performance of PDI complexes, these features can also be leveraged to achieve productive reactivity such as the activation of inert small molecules by metal−ligand cooperation (Scheme 1h).9d Therefore, identifying new ligand modification pathways continues to be of interest both in the context of mechanistic investigations and for the development of next-generation catalysts featuring PDI ligands. Molybdenum complexes supported by PDI ligands exhibit ligand modification chemistry as well as unusual electronic © XXXX American Chemical Society

structures. For example, addition of NH3 to the arylsubstituted PDI dinitrogen complex [{(iPrBPDI)Mo(N2)}2(μ 2 ,η 1 ,η 1 -N 2 )] ( i P r BPDI = 2,6-(2,6-iPr 2 -C 6 H 3 N CPh)2C5H3N) resulted in imine NimineCimine bond cleavage and conversion to a terminal aryl imide, where ammonia Received: February 13, 2019

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DOI: 10.1021/acs.organomet.9b00095 Organometallics XXXX, XXX, XXX−XXX

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Organometallics served as the irreversible source of hydrogen (Scheme 1h).9d More recently, our group reported the PDI molybdenum nitride [(iPrPDI)Mo(N)(C2H4)] ([1-(N)(η2-C2H4)]; iPrPDI = 2,6-(2,6-iPr2-C6H3NCMe)2C5H3N), which can be prepared from coordinated NH3 and exhibits a ligand-centered singly occupied molecular orbital (SOMO).10 While pyridine(diimine)-based radicals are commonplace with first-row transition metals, PDIs more frequently adopt a closed-shell, dianionic form with second- and third-row metals,24,25 highlighting the unusual electronic structure of [1-(N)(η2C2H4)]. Here we describe an extension of these studies and report a distinct PDI modification pathway involving partial hydrogenation of the pyridine ring (Scheme 1i). The molecular and electronic structures of the molybdenum complex bearing the hydrogenated chelate were determined. As part of our laboratory’s ongoing efforts to explore the proton-coupled electron transfer (PCET)26 reactivity of molybdenum complexes supported by potentially redox active ligands,10,27 the reactivity of [1-(N)(η2-C2H4)] with the known hydrogen atom donor [(η5-C5Me5)(py-Ph)Rh(H)] ([Rh−H]; py-Ph = 2-phenylpyridine)28 was examined. Addition of 3 equiv of the rhodium hydride [Rh−H] to [1(N)(η2-C2H4)] in THF solution resulted in a color change from orange-red to green over the course of 15 min at room temperature. Monitoring the reaction by 1H NMR spectroscopy revealed the formation of a diamagnetic product identified as [( iPr THPDI)Mo(N)(C 2 H 4 )] ([2-(N)(η 2 C2H4)]; iPrTHPDI = 2,6-(2,6-iPr2-C6H3NCMe)2C5H6N), the product of three H-atom additions from [Rh−H] to the iPr PDI chelate in [1-(N)(η2-C2H4)] (Figure 1). The benzened6 1H NMR spectrum of blue (λmax = 625 nm) [2-(N)(η2-

C2H4)] exhibits the number of resonances consistent with Cs molecular symmetry, as indicated by four distinct iPr methyl resonances at 1.43, 1.19, 1.01, and 0.98 ppm. The partial hydrogenation of the iPrPDI chelate was identified by the lack of diagnostic downfield C−H resonances assignable to the 3-, 4-, and 5-pyridine positions along with the concomitant appearance of signals in the aliphatic region (2.01−1.21 ppm). Also consistent with Cs molecular symmetry was the observation of a doublet of doublets in the 13C NMR spectrum of [2-(N)(η2-C2H4)] at 51.71 ppm assigned to the ethylene carbons (1JC−H = 156.7, 149.2 Hz). Analysis of [2-(15N)(η2C2H4)], prepared from hydrogenation of [1-(15N)(η2-C2H4)], by 15N{1H} NMR spectroscopy revealed a singlet at +1040 ppm, consistent with the presence of a terminal molybdenum nitride.29 The solid-state structure of [2-(N)(η2-C2H4)] was determined by single-crystal X-ray diffraction and confirmed coordination of ethylene and nitride ligands (d(CC) = 1.420(3) Å; d(MoN) = 1.644(2) Å) as well as the partial hydrogenation of the central pyridine of the iPrPDI chelate. Ligand reduction was also evidenced by structural distortion with the lowering of the 4-pyridine carbon below the chelate plane by 0.654 Å (Figure 1b), a geometry expected from the generation of three sp3 carbon centers in [2-(N)(η2-C2H4)]. Remarkably, reduction of the ethylene and imine ligands was not observed despite ample precedent for these functionalities to undergo Rh-catalyzed hydrogenation.30 The unusual chelate modification in [2-(N)(η2-C2H4)] prompted determination of the electronic structure of the complex and assignment of the redox state of the new ligand. While the diamagnetism of [2-(N)(η2-C2H4)] limits the number of experimental observables for an electronic structure determination, DFT computations were nevertheless carried out to investigate the oxidation state of the molybdenum and the corresponding redox state of the iPrTHPDI ligand. Calculations were conducted at the B3LYP level of theory and used to construct a qualitative molecular orbital diagram for [2-(N)(η2-C2H4)]. As shown in Figure 2, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of [2-(N)(η2-C2H4)] are iPrTHPDI ligand based π* orbitals of a′′ and a′ symmetry, respectively, supporting a closed-shell, anionic description for iPr THPDI.31 HOMO-7 and HOMO-8 exhibit molybdenum dyz

Figure 1. (a) Partial hydrogenation of the pyridine(diimine) chelate in [1-(N)(η2-C2H4)] by stoichiometric or catalytic reaction with [Rh−H]. (b) Solid-state structure of [2-(N)(η2-C2H4)] with ellipsoids at 30% probability. Hydrogen atoms (except those connected to C4, C5, and C6) have been omitted for clarity. Selected bond distances (Å): Mo1−N1 2.215(2), Mo1−N2 2.171(2), Mo1− N3 2.214(2), Mo1−N4 1.644(2), C34−C35 1.420(3), N1−C2 1.326(2), C2−C3 1.423(2), C3−C4 1.501(3), C4−C5 1.524(3), C5−C6 1.526(3), C6−C7 1.497(3), C7−C8 1.419(3), N3−C8 1.323(2).

Figure 2. Molecular orbital illustrations and populations for [2(N)(η2-C2H4)] obtained from a spin-restricted DFT calculation at the B3LYP level of theory. The z axis is defined as the MoN vector. iPr THPDI symmetry designations are from the Cs point group. L = ligands. B

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Organometallics and dxz character engaged in π-bonding with nitrogen py and px orbitals, respectively, consistent with a nitrido MoN triple bond. Finally, HOMO-1 exhibits significant ethylene π* character, consistent with the elongated ethylene (CC) bond length of 1.420(3) Å observed in the solid-state structure of [2-(N)(η2-C2H4)]. This value is statistically indistinguishable from the ethylene (CC) bond length of 1.425(6) Å reported for the cationic molybdenum ethylene complex [(PhTpy)(PPh2Me)2Mo(C2H4)][BArF]24 (PhTpy = 4′-Ph2,2′,6′,2″-terpyridine, ArF24 = [C6H3-3,5-(CF3)2]4) that exhibits EPR spectroscopic features consistent with a Mo(III) metallacyclopropane.27d Furthermore, the CH coupling constants (1JC−H = 156.7, 149.2 Hz) observed for coordinated ethylene in the 13C NMR spectrum of [2-(N)(η2-C2H4)] are in the range reported for methylene carbons of ethyl (147.3 Hz)27d and “alkylidene-like” η2-vinyl (160 Hz)32 ligands of molybdenum. Therefore, the electronic structure of the complex is likely best described as a Mo(VI) metallacyclopropane nitride supported by a singly reduced iPrTHPDI ligand. In this electronic structure description, (iPrTHPDI)− is isoeletronic with anionic dipyridylazaallyl ligands which exhibit a rich coordination chemistry with a host of transition metals.33 Localized orbital bonding analysis (LOBA)34 was performed by DFT and also supports a 6+ oxidation state assignment at the molybdenum in [2-(N)(η2-C2H4)]. Because the complex [2-(N)(η2-C2H4)] was the only product observed by 1H NMR spectroscopy from the reaction of [1-(N)(η2-C2H4)] with [Rh−H], the fate of the Rh complex was investigated by EPR spectroscopy. A single isotropic signal at g = 2.070 was observed at the completion of the reaction in toluene solution at room temperature, consistent with the formation of the paramagnetic Rh(II) complex [(η5-C5Me5)(py-Ph)Rh] ([Rh]), the product of Hatom loss from [Rh−H].35 Because [Rh] is known to react with H2 gas to regenerate the rhodium hydride starting material, [Rh−H] was next employed in catalytic amounts using H2 gas as the terminal reductant. Indeed, conducting the reaction using 5 mol % of [Rh−H] under 4 atm of H2 in THF generated [2-(N)(η2-C2H4)] in quantitative NMR yield after 72 h at room temperature.36 To investigate whether related molybdenum iPrPDI complexes exhibit similar reactivity, [(iPrPDI)MoCl3], [(iPrPDI)Mo(η6-C6H6)], and [(iPrPDI)Mo(CO)4] were treated with 25 mol % of [Rh−H] and stirred under 4 atm of H2. However, in each case, no reaction was observed for up to 6 days at room temperature, and intractable product mixtures were obtained upon heating to 60 °C for 18 h. These results suggest that [1-(N)(η2-C2H4)] is uniquely suited for well-defined hydrogenation chemistry with [Rh−H] and is likely due to the stability of the molybdenum−nitrido− ethylene core. Consequently, reactivity takes place at the iPr PDI ligand, resulting in a Mo(VI) oxidation state in the final product. Given that the coordinatively saturated, 18e− complex [Rh− H] is an effective H-atom donor that is known to react by PCET,35,37 DFT computations were conducted in order to probe the thermodynamics of a plausible C−H bond forming PCET pathway leading to the formation of [2-(N)(η2-C2H4)]. As presented in Figure 3 and Table 1, sequential addition of H atoms from [Rh−H] was computed at both the B3LYP and PW6B95 levels of theory to be approximately thermoneutral or exergonic for plausible PCET pathways, forming the intermediates [3-H1], [4-H1], and [3,4-H2] with C−H bond dissociation free energies (BDFEs) in the range of 48−60 kcal

Figure 3. Plausible H-atom addition pathways in [1-(N)(η2-C2H4)].

Table 1. DFT-Computed C−H BDFEs for Sequential HAtom Addition in [1-(N)(η2-C2H4)] complex

pyridine position

[3-H1]

3

[4-H1]

4

[3,4-H2]

3 4

[3,5-H2]

3/5

[2-(N)(η2-C2H4)]

3/5 4

a

level of theory

C−H BDFEa

B3LYP PW6B95 B3LYP PW6B95 B3LYP PW6B95 B3LYP PW6B95 B3LYP PW6B95 B3LYP PW6B95 B3LYP PW6B95

50 52 48 50 60 59 58 57 32 32 69 70 94 96

Values computed in kcal mol−1 in the gas phase.

mol−1 (BDFERh−H = 52 kcal mol−1).38 In contrast, consecutive formation of C−H bonds at the 3- and 5-pyridine positions was computed to be endergonic by ∼20 kcal mol−1 leading to intermediate [3,5-H2] (BDFEC−H = 32 kcal mol−1) and is likely disfavored (Figure 3, dashed arrow). These computational results suggest that PCET from [Rh−H] to the pyridine(diimine) chelate in [1-(N)(η2-C2H4)] is thermodynamically feasible for each H-atom addition step. However, it is important to note that the initial site of H-atom addition to [1(N)(η2-C2H4)] is unknown, as reaction intermediates were not observed during the course of the hydrogenation. Therefore, the structures depicted in Figure 3 can, in principle, be accessed by intramolecular hydrogen atom transfer in analogy to intramolecular alkyl group migrations observed in PDI ligands.39 In addition, the possibility that a PCET pathway is circumvented altogether by pyridine (C−C) insertion into the rhodium−hydrogen bond cannot be rigorously excluded. In summary, partial hydrogenation of a pyridine(diimine) chelate was demonstrated in a molybdenum nitride and the thermodynamics of a plausible C−H bond forming PCET pathway were examined. The transformation described herein provides unique insight into the hydrogenation chemistry of a molybdenum nitride bearing a redox-active ligand.

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Organometallics

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Experimental details, characterization data including NMR spectra of complexes, and computational methods and results (PDF) Computed coordinates for [1-(N)(η2-C2H4)], [3-H1], [4-H1], [3,4-H2], [3,5-H2], and [2-(N)(η2-C2H4)] (XYZ) Accession Codes

CCDC 1894932 contains 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 for P.J.C.: [email protected]. ORCID

Máté J. Bezdek: 0000-0001-7860-2894 Paul J. Chirik: 0000-0001-8473-2898 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0006498. M.J.B. thanks the Natural Sciences and Engineering Research Council of Canada for a predoctoral fellowship (PGS-D) and Princeton University for a Porter Ogden Jacobus Honorific Fellowship. We thank Dr. István Pelczer (Princeton) for assistance with the acquisition of 15 N NMR data.

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REFERENCES

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DOI: 10.1021/acs.organomet.9b00095 Organometallics XXXX, XXX, XXX−XXX

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(27) (a) Bezdek, M. J.; Guo, S.; Chirik, P. J. Coordination-induced weakening of ammonia, water, and hydrazine X−H bonds in a molybdenum complex. Science 2016, 354, 730−733. (b) Bezdek, M. J.; Pappas, I.; Chirik, P. J. Determining and Understanding N−H Bond Strengths in Synthetic Nitrogen Fixation Cycles. Top. Organomet. Chem. 2017, 60, 1−21. (c) Bezdek, M. J.; Chirik, P. J. Interconversion of Molybdenum Imido and Amido Complexes by Proton−Coupled Electron Transfer. Angew. Chem., Int. Ed. 2018, 57, 2224−2228. (d) Bezdek, M. J.; Chirik, P. J. Proton−Coupled Electron Transfer to a Molybdenum Ethylene Complex Yields a β-Agostic Ethyl: Structure, Dynamics and Mechanism. J. Am. Chem. Soc. 2018, 140, 13817−13826. (28) Hu, Y.; Li, L.; Shaw, A. P.; Norton, J. R.; Sattler, W.; Rong, Y. Synthesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodium(III) Hydrides. Organometallics 2012, 31, 5058−5064. (29) For examples of reported 15N NMR chemical shifts for terminal molybdenum nitrido complexes, see: (a) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J. Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data. J. Am. Chem. Soc. 1996, 118, 8623−8638. (b) Hebden, T. J.; Schrock, R. R.; Takase, M. K.; Muller, P. Cleavage of dinitrogen to yield a (tBuPOCOP)molybdenum(IV) nitride. Chem. Commun. 2012, 48, 1851−1853. (c) Bhattacharya, P.; Heiden, Z. M.; Wiedner, E. S.; Raugei, S.; Piro, N. S.; Kassel, W. S.; Bullock, R. M.; Mock, M. T. Ammonia Oxidation by Abstraction of Three Hydrogen Atoms from a Mo−NH3 Complex. J. Am. Chem. Soc. 2017, 139, 2916−2919. (30) Andersson, P. G.; Munslow, I. J. Modern Reduction Methods; Wiley: New York, 2008. (31) Disregarding potential redox activity, addition of 3 equiv of hydrogen atoms to the iPrPDI ligand in [1-(N)(η2-C2H4)] would generate a chelate bearing an unpaired electron in its neutral form in [2-(N)(η2-C2H4)]. See the Supporting Information for details. (32) Byrnes, M. J.; Dai, X.; Schrock, R. R.; Hock, A. S.; Müller, P. Some Organometallic Chemistry of Molybdenum Complexes that Contain the [HIPTN3N]3‑ Triamidoamine Ligand, {[3,5-(2,4,6-iPr3C6H2)2C6H3NCH2CH2]3N}3‑. Organometallics 2005, 24, 4437− 4450. (33) For selected examples, see: (a) Westerhausen, M.; Kneifel, A. N. Unexpected formation of zinc bis[1,3-di(2-pyridyl)-2-azapropenide] during the thermolysis of methylzinc bis(2-pyridylmethyl)amide. Inorg. Chem. Commun. 2004, 7, 763−766. (b) Frazier, B. A.; Wolczanski, P. T.; Lobkovsky, E. B.; Cundari, T. R. Unusual Electronic Features and Reactivity of the Dipyridylazaallyl Ligand: Characterizations of (smif)2M [M = Fe, Co, Co+, Ni; smif = {(2py)CH}2N] and [(TMS)2NFe]2(smif)2. J. Am. Chem. Soc. 2009, 131, 3428−3429. (c) Frazier, B. A.; Wolczanski, P. T.; Lobkovsky, E. B. Aryl-Containing Chelates and Amine Debenzylation to Afford 1,3-Di2-pyridyl-2-azaallyl (smif): Structures of {κ-C,N,N py 2 -(2pyridylmethyl)2N(CH2(4-tBu-phenyl-2-yl))}FeBr and (smif)CrN(TMS)2. Inorg. Chem. 2009, 48, 11576−11585. (d) Frazier, B. A.; Wolczanski, P. T.; Keresztes, I.; DeBeer, S.; Lobkovsky, E. B.; Pierpont, A. W.; Cundari, T. R. Synthetic Approaches to (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) Reveal Redox Non-Innocence and C-C Bond-Formation. Inorg. Chem. 2012, 51, 8177−8186. (34) Thom, A. J. W.; Sundstrom, E. J.; Head-Gordon, M. LOBA: A localized orbital bonding analysis to calculate oxidation states, with application to a model water oxidation catalyst. Phys. Chem. Chem. Phys. 2009, 11, 11297−11304. (35) Hu, Y.; Norton, J. R. Kinetics and Thermodynamics of H−/ H•/H+ Transfer from a Rhodium(III) Hydride. J. Am. Chem. Soc. 2014, 136, 5938−5948. (36) Control experiments established no reaction between [1-(N) (η2-C2H4)] and H2 in the absence of [Rh−H] under these reaction conditions. (37) Pappas, I.; Chirik, P. J. Ammonia Synthesis by Hydrogenolysis of Titanium-Nitrogen Bonds Using Proton Coupled Electron Transfer. J. Am. Chem. Soc. 2015, 137, 3498−3501. E

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Organometallics (38) Pappas, I.; Chirik, P. J. Catalytic Proton Coupled Electron Transfer from Metal Hydrides to Titanocene Amides, Hydrazides and Imides: Determination of Thermodynamic Parameters Relevant to Nitrogen Fixation. J. Am. Chem. Soc. 2016, 138, 13379−13389. (39) Budzelaar, P. H. M. Radical Chemistry of Iminepyridine Ligands. Eur. J. Inorg. Chem. 2012, 2012, 530−534.

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DOI: 10.1021/acs.organomet.9b00095 Organometallics XXXX, XXX, XXX−XXX