Amine Boranes Dehydrogenation Mediated by an Unsymmetrical

Aug 29, 2018 - The IrIII hydride (tBuPCN)IrHCl (1) containing the tridendate unsymmetrical pincer ligand tBuPCN– {tBuPCN(H) ...
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Article Cite This: Organometallics 2018, 37, 3142−3153

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Amine Boranes Dehydrogenation Mediated by an Unsymmetrical Iridium Pincer Hydride: (PCN) vs (PCP) Improved Catalytic Performance Lapo Luconi,†,# Elena S. Osipova,‡,# Giuliano Giambastiani,*,†,§,∥ Maurizio Peruzzini,† Andrea Rossin,*,† Natalia V. Belkova,*,‡ Oleg A. Filippov,‡ Ekaterina M. Titova,‡,⊥ Alexander A. Pavlov,‡ and Elena S. Shubina*,‡ Downloaded via UNIV OF SUNDERLAND on September 25, 2018 at 14:19:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Istituto di Chimica dei Composti Organometallici − Consiglio Nazionale delle Ricerche (ICCOM - CNR), Via Madonna del Piano 10, 50019, Sesto Fiorentino, Italy ‡ A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences (INEOS RAS), Vavilova Str. 28, 119991 Moscow, Russia § Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES), UMR 7515 CNRS - University of Strasbourg (UdS), 25, rue Becquerel, 67087 Strasbourg Cedex 02, France ∥ Kazan Federal University, Kremlyovskaya Str. 18, 420008 Kazan, Russia ⊥ Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklay St, 117198 Moscow, Russia S Supporting Information *

ABSTRACT: The IrIII hydride (tBuPCN)IrHCl (1) containing the tridendate unsymmetrical pincer ligand tBuPCN− {tBuPCN(H) = 1-[3-[(ditert-butylphosphino)methyl]phenyl]-1H-pyrazole} has been exploited as ammonia borane (NH3BH3, AB) and amine boranes dehydrogenation catalyst in THF solution at ambient temperature. 1 releases one H2 equivalent per AB equivalent, with concomitant cyclic poly(aminoboranes) formation [B-(cyclotriborazanyl)-amine-borane (BCTB) and cyclotriborazane (CTB)] as the final “spent fuel”. 1 has been found to have superior catalytic activity than its symmetrical analogue (tBuPCP)IrHCl, with recorded TOF values of 580 h−1 (AB in THF) and 401 h−1 (DMAB in toluene) at ambient temperature. The reaction has been analyzed experimentally through multinuclear [11B, 31P{1H}, 1H] NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect determination with deuterated AB isotopologues. The hydride/borohydride intermediate (tBuPCN)IrH(η2-BH4) (2) is the catalyst resting state formed during the dehydrogenation process; it is detected by a variable-temperature multinuclear NMR of the reaction course (in the 190−323 K range). A DFT modeling of the reaction mechanism using DMAB as substrate has been performed with the geometry optimization in toluene at the M06 level of theory. The combination of the kinetic and computational data reveals that a simultaneous B−H/N−H activation occurs in the presence of 1, after the preliminary amine borane coordination to the metal center.

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INTRODUCTION Lightweight inorganic hydrides are at the forefront of contemporary chemical hydrogen storage research.1 They are easy-to-handle and thermally stable compounds with relatively high hydrogen content. Among the most studied materials belonging to this class, there are ammonia borane (NH3·BH3, AB, 19.3% H wt.)2 and amine boranes (RNH2·BH3, R2NH· BH3, with R = aliphatic chain or ring). In the past decade, homogeneously catalyzed AB dehydrogenation has become increasingly popular; H2 evolution is mediated by organometallics of the elements from the second and third transition series, like ruthenium,3 rhodium,3d,4 and iridium.5 The exploitation of 4d or 5d metals helps with the stabilization of reaction intermediates, allowing for their isolation and © 2018 American Chemical Society

thorough characterization. The same stabilizing effect of reagents and intermediates is provided by κ3-tridendate pincer ligands,3c,6 with assorted donor atom sets: (PCP), (PNP), (NCN), (SCS), (OCO).7 More recently, unsymmetrical analogues like (PCN)8 have also started to appear in the literature. PCN-type systems are intriguing, because, in (PCN)M(L)n complexes (M = transition metal; L = ancillary ligand), the tridentate ligand contains both hard (N) and sof t (P) donor functions, with a markedly different trans effect. This difference results in the group with the weaker trans effect (N) being more likely to dissociate from the metal center. Received: July 12, 2018 Published: August 29, 2018 3142

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Organometallics

deuterated solvent, while 31P{1H} chemical shifts were referenced to 85% H3PO4 with downfield shift taken as positive. 11B and 11B{1H} were referenced to BF3·OEt2. Synthesis of (tBuPCN)IrHCl (1). [IrCl(COE)2]2 (0.148 g, 0.16 mmol) was added to a solution of tBuPCN(H) (0.100 g, 0.33 mmol) in dry and degassed toluene (7 mL), and the resulting mixture was heated at reflux overnight to obtain a dark-red solution. The solution was allowed to cool to ambient temperature and the solvent was removed in vacuo to give the crude product as a pale red solid. Pentane was added to the residue and the solid was broken up with a spatula. Filtration through a medium-porosity frit afforded a bright red solid, which was washed with several portions of pentane and dried under vacuum (0.150 g, yield 87%). Suitable crystals for X-ray diffraction analysis were obtained through rapid cooling of hot concentrated toluene solutions. NMR spectra in THF-d8 are given in the Supporting Information (Figures S1, S3, and S4).

Evidence of this dissociative behavior was found by some of us in the palladium derivatives of the pyrazole-based (tBuPCN) ligand {1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole}, with concomitant “rollover” C−H activation on the 5position of the pyrazole ring.9 From a homogeneous catalysis perspective, the ligand hemilability may also provide access to a vacant coordination site at the metal center. It has often been invoked as a critical factor when comparing the catalytic activity of (PCP)M(L)n/(NCN)M(L)n and (PCN)M(L)n analogues; the unsymmetrical complex has consistently been shown to be more active than its symmetric counterparts.6c,10 In a recently published work by some of us, there is evidence that (tBuPCP)IrHCl is an efficient catalyst for dimethylamine borane (DMAB) dehydrogenation.11 The detailed spectroscopic and computational study identified the hexa-coordinate complex (tBuPCP)IrH2(η1-BH3·NHMe2) as the key reaction intermediate. In this complex, the steric repulsion between the amine-borane and tert-butyl groups at the phosphorus atoms is overcome through DMAB coordination trans to the ipso carbon. Compared to its symmetrical tBuPCP− counterpart, tBu PCN− is less sterically demanding (only one −PtBu2 group on the ligand skeleton instead of two) and a weaker σ-donor (pyrazole vs phosphine); these features might provide a more active species for the same reaction. In addition, the coordination flexibility of the pyrazole moiety in tBuPCN− may be a useful feature to get more reactive organometallic catalysts. Following our previous investigations,3a,5b,9,12 in this paper, we report on the preparation and characterization of the novel pyrazole-based pincer iridium(III) hydride (tBuPCN)IrHCl (1) (Scheme 1) to be exploited as homogeneous

1 H NMR (300 MHz, THF-d8, 293 K): δ −33.5 (d, 2JPH = 22.3 Hz, 1H, Ir-H, H19),1.20 (d, 3JPH = 12.8 Hz, 9H, P-C(CH3)3, H12,13,14), 1.39 (d, 3JPH = 12.8 Hz, 9H, P-C(CH3)3, H16,17,18), 3.17 (d, 2JPH = 8.9 Hz, 2H, Ar-CH2-P, H7), 6.61 (m, 1H, CH, H9), 6.77 (m, 1H, CH Ar, H3), 6.97 (m, 1H, CH Ar, H2), 7.09 (m, 1H, CH Ar, H4), 8.00 (m, 1H, CH, H8), 8.29, (m, 1H, CH, H10). 13C{1H} NMR (75 MHz, THF-d8, 293 K): δ 26.6 (m, P-C(CH3)3, C12,13,14,16,17,18), 32.3 (d, 1JPC = 20.4 Hz, P-C(CH3)3, C), 34.7−33.9 (P-C(CH3)3, Ar-CH2-P), 104.0 (C9), 106.3 (C4), 118.8 (C), 118.9 (C), 124.8 (C), 136.0 (C), 141.6 (C), 143.6 (C), 147.7 (C6). 31P{1H} NMR (121 MHz, THF-d8, 293 K): δ 49.7 (s). IR (nujol mull): νIrCl 261 cm−1. Anal. Calcd (%) for C18H27ClIrN2P (530.06): C 40.79, H 5.13, N 5.28; found C 40.65, H 5.07, N 5.13. Synthesis of (tBuPCN)IrH(η2-BH4) (2). A solution of 1 (0.175 g, 0.33 mmol) in THF (7 mL) was added to a suspension of NaBH4 (0.125 g, 3.30 mmol, 10 equiv) in THF (13 mL). The resulting mixture was heated at reflux with vigorous stirring for 3 h. The mixture was allowed to cool down at room temperature, and it was filtered through a Celite pad. The volatiles were removed in vacuo to give a solid residue that was redissolved in toluene and filtered again through a Celite pad. Finally, toluene was removed under vacuum to give 2 as a brown solid (0.110 g, yield 65%). NMR spectra in THF-d8 are given in the Supporting Information (Figures S7−S10).

Scheme 1. Structure of the Pyrazole-Containing Unsymmetrical Pincer Complex (tBuPCN)IrHCl (1)

catalyst in amine boranes dehydrogenation. Its reaction with amine boranes (AB, DMAB) in catalytic and stoichiometric regimes has been followed experimentally [multinuclear 11B, 31 P, 1H NMR and IR spectroscopy at variable temperatures (VT), kinetic rate measurements, and kinetic isotope effect (KIE) studies]. The reaction mechanism using DMAB as substrate (because of its well-defined and soluble byproducts in the experiments) was also analyzed through computational modeling at the DFT//M06//SMD level of theory.

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1 H NMR (300 MHz, THF-d8, 293 K): δ −21.34 (d, 2JPH = 18.6 Hz, 1H, Ir-H, H19), −5.11 (br s, 2H, B-H), 1.33 (d, 3JPH = 13.3 Hz, 9H, P-C(CH3)3, H12,13,14), 1.36 (d, 3JPH = 13.3 Hz, 9H, P-C(CH3)3, H16,17,18), 3.26 (dd, 2JHH = 17.3 Hz, 2JPH = 9.2 Hz, 1H, Ar-CH2-P, H7), 3.47 (dd, 2JHH = 17.3 Hz, 2JPH = 9.2 Hz, 1H, Ar-CH2-P, H7), 5.5 (br s, 2H, B-H), 6.61 (m, 1H, CH, H9), 6.87 (m, 1H, CH Ar, H3), 7.00 (m, 1H, CH Ar, H2), 7.15 (m, 1H, CH Ar, H4), 7.93 (m, 1H, CH, H8), 8.38, (m, 1H, CH, H10). 13C{1H} NMR (75 MHz, THF-d8, 293 K): δ 29.0 (d, 2JPC = 3.1 Hz, P-C(CH3)3, C12,13,14), 29.9 (d, 2JPC = 23.4 Hz, P-C(CH3)3, C12,13,14), 34.7 (d, 1JPC = 23.4 Hz, P-C(CH3)3, C11), 35.4 (d, 1JPC = 23.4 Hz, P-C(CH3)3, C11), 38.0 (d, 1JPC = 36.5 Hz, Ar-CH2P, C7), 106.9 (C-H, Ar), 109.2 (C-H, Ar), 121.6 (C-H, Ar), 122.1 (CH, Ar), 127.6 (C-H, Ar), 139.2 (C-H, Ar), 142.0 (C), 145.9 (C), 147.9 (C6). 31P{1H} NMR (121 MHz, THF-d8, 293 K): δ 58.0 (s); 11 1 B{ H} NMR (96 MHz, C6D6, 293 K): δ 6.99 (br s). IR (KBr): νIrH

EXPERIMENTAL SECTION

General Considerations. All reactions were performed using standard Schlenk procedures under a dry nitrogen or argon atmosphere. The tBuPCN(H) ligand,9 [IrCl(COE)2]2,13 NH3BD3, ND3BH3, and ND3BD314 were prepared according to the published procedures. Commercial reagents (ammonia-borane, dimethylamineborane, sodium borohydride) were purchased from Aldrich and used as received, without further purification. Tetrahydrofuran (THF) and toluene were purified by standard distillation techniques. THF-d8 (Aldrich) was stored over 4 Å molecular sieves and degassed by three freeze−pump−thaw cycles before use. NMR spectra were recorded on Bruker AVANCE 600, 400, and 300 FT-NMR spectrometers at different temperatures (200−320 K). 1H and 13C{1H} chemical shifts are reported in parts per million (ppm) downfield of tetramethylsilane (TMS) and were calibrated against the residual resonance of the 3143

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Organometallics 1924 cm−1; νBH 2442/2397/2291/2218 cm−1. Anal. Calcd (%) for C18H31BIrN2P (509.45): C 41.59, H 5.62, N 5.39; found C 41.65, H 5.77, N 5.35. NMR Spectroscopic Data for (tBuPCN)IrH(Cl)(NH3BH3) (3; THF-d8, 293 K). 1H NMR (400 MHz): δ −23.65 (br s, 1H, Ir-H), 1.35 (d, 3JPH = 12.8 Hz, 9H, P-C(CH3)3), 1.3 (d, 3JPH = 13.1 Hz, 9H, P-C(CH3)3), 3.09 (dd, 2JPH = 9.6 Hz, 1H, Ar-CH2-P), 3.19 (dd, 2JPH = 8.6 Hz, 1H, Ar-CH2-P), 6.59 (m, 1H, CH), 6.69 (t, 1H, CH Ar), 6.88 (d, 1H, CH Ar), 7.04 (d, 1H, CH Ar), 7.85 (m, 1H, CH), 8.30, (m, 1H, CH). 31P{1H} NMR (121 MHz): δ 55.3 ppm. NMR Spectroscopic Data for (tBuPCN)IrH(Cl)(Me2NHBH3) (4; THF-d8, 230 K). 1H NMR (600 MHz): δ −23.84 (br s, 1H, Ir-H), 1.37 (d, 3JPH = 12.47 Hz, 18H, P-C(CH3)3), 3.13 (dd, 2JPH = 9.9 Hz, 1H, Ar-CH2-P), 3.19 (dd, 2JPH = 8.4 Hz, 1H, Ar-CH2-P), 6.68 (m, 1H, CH), 6.74 (t, 1H, CH Ar), 6.92 (d, 1H, CH Ar), 7.11 (d, 1H, CH Ar), 7.96 (m, 1H, CH), 8.45, (m, 1H, CH). 31P{1H} NMR (121 MHz): δ 55.0 ppm. NMR Monitoring of the Reactions of 1 with Amine Boranes. A quartz 5 mm NMR tube was loaded with 14 mg of 1 (molecular mass = 530 μ, 0.026 mmol) under an inert atmosphere, and then 0.3 mL of dry and degassed tetrahydrofuran-d8 was transferred into the tube via cannula, under nitrogen. The solution obtained was first used to record the 1H and 31P{1H} NMR spectra of the catalysts. The reactions with amine boranes were monitored as follows: 1 equiv of the amine borane of choice was dissolved in 0.2 mL of THF-d8 into a separate Schlenk under nitrogen, and the resulting solution was syringed into the NMR tube containing the hydride sample, at low temperature. The temperature was slowly raised to 298 K within the NMR spectrometer probe head, and new sets of multinuclear 1H, 31P, and 11B NMR data were collected at each temperature. The total experiment time is 7 h. Finally, to complete the NMR spectra data set, a check of the reaction mixture was performed 48 h after the end of the VT experiment at ambient temperature. Kinetic Measurements. The H2 production during the reaction of AB with 1 in THF was monitored through the Man on the Moon X102 kit (see Figure S11 for details). In a typical experiment, 5.3 mg of 1 (0.01 mmol) was placed under an inert atmosphere in a two-neck round-bottom 40 mL flask connected to a switchable three-way valve through a Torion screw. The solid was dissolved in 1 mL of THF; then, a solution of AB (50 equiv) in THF (2 mL) was added by syringe. The as-obtained mixture was then kept at T = 298 K under stirring; then the valve was switched to the pressure transducer which is connected via wireless to the software recording the kinetic profile online for a total time of 24 h. The resulting kinetic data were corrected by the vapor pressure of pure THF at the same temperature recorded on a blank solution; the pressure increase caused by THF was used to calculate the correct number of released H2 equivalents. The calculation was made assuming ideality for hydrogen gas, applying the PV = nRT ideal gas law. The insoluble residue obtained after 24 h was washed twice with degassed pentane and dried under vacuum before recording the IR spectrum (KBr). Alternatively, the reaction progress was followed by measuring the volume of hydrogen released. In a typical experiment, DMAB (0.01 g, 0.28 mmol) was dissolved in toluene (0.6 mL) in a 10 mL round-bottom flask and the flask was closed with a tight-fitting rubber septum. The required amount (2−10 mol %) of 1 in toluene was transferred via syringe to the stirred amine borane solution. Timing was started when the catalyst was injected. The hydrogen gas was collected in a water-filled, upturned buret through a Teflon tube. The volume of hydrogen gas collected was recorded periodically (every minute during the first halfhour) until the reaction was complete. Variable-Temperature Infrared (VT-IR) Measurements. IR spectra were recorded at different temperatures (190−293 K) using a home-modified cryostat (Carl Zeiss Jena) with a Nicolet 6700 spectrometer using 0.1−0.2 cm CaF2 cells for the mid-IR region and polyethylene cells (d = 0.1 cm) for the far-IR region. The accuracy of the experimental temperature was ±0.5 °C. The cryostat modification allows for transfer of the reagents (premixed at either low or room temperature) under an inert atmosphere directly into the cells.

Crystal Data Collection. Single crystal X-ray data for 1 were collected at low temperature (T = 100 K) on an Oxford Diffraction XcaliburPX diffractometer equipped with a CCD area detector using a Cu Kα radiation (λ = 1.5418 Å). The program used for the data collection was CrysAlis CCD 1.171.15 Data reduction was carried out with the program CrysAlis RED 1.171,16 and the absorption correction was applied with the program ABSPACK 1.17. Direct methods implemented in Sir9717 were used to solve the structures, and the refinements were performed by full-matrix least-squares against F2 implemented in SHELXL-2014.18 The hydride ligand H(100) was located on the residual electron density maps during the refinement process; it was refined isotropically with a thermal factor equal to 1.5 Ueq(Ir). All the non-hydrogen atoms were refined anisotropically while the hydrogen atoms [apart from H(100)] were fixed in calculated positions and refined isotropically with the thermal factor depending on the one of the atom to which they are bound. The high residual density found at the end of the refinement (alerts “A” in the checkcif) is due to bad crystal quality. No solvent-accessible voids were found in the crystal lattice with the PLATON SQUEEZE software.19 The geometrical calculations were performed by PARST97,20 and molecular plots were produced by the program ORTEP3.21 CCDC-1838328 contains the supplementary crystallographic data for this paper. Computational Details. Calculations were performed on the real structure of 1 with the Gaussian 0922 package at the DFT/M0623 level. Effective core potentials (ECP) and associated SDD basis set24 supplemented with f-polarization functions (SDD(f))25 were applied to the Ir atom. The C atoms of aromatic rings and P atoms of the pincer, the H and Cl ligands and the BH and NH groups of DMAB were described with the 6-31++G(d,p) basis set,26 while all the other atoms were described with a 6-31G basis set.26d The structures of the reactants and complexes were fully optimized with this basis set without any symmetry restrictions in toluene (ε = 2.3741), which was introduced within the SMD solvation model.27 The full geometry optimization was followed by the thermochemistry calculations. The nature of all the stationary points on the potential energy surface was confirmed by vibrational analysis.28 No scaling factors were applied to the calculated frequencies. The transition state structures showed only one negative eigenvalue in their diagonalized force constant matrices, and their associated eigenvectors were confirmed to correspond to the motion along the reaction coordinate under consideration using the Intrinsic Reaction Coordinate (IRC) method.29

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RESULTS AND DISCUSSION Hydrides Synthesis and Characterization. The direct reaction of the IrI dimer [(COE)IrCl]2 (COE = ciscyclooctene) with the asymmetric pincer ligand tBuPCN(H) {1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole}9 in toluene at reflux led to the isolation of the new iridium(III) hydride (tBuPCN)IrHCl (1), which is the product of oxidative addition of the ipso C−H bond of the pincer phenyl ring to iridium(I) (Scheme 2). Complex 1 has been isolated in the form of an air-sensitive bright red powder; it has been characterized both in solution (multinuclear 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, Figures S1−S5) and in the solid state (single-crystal X-ray diffraction analysis). In THF-d8, the hydride signal in the 1H NMR spectrum of 1 is a doublet falling at δH = −33.5 ppm Scheme 2. Synthesis of 1

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Organometallics (2JH‑P = 22.3 Hz, Figure S1), while it is broad and observed at higher field (δH = −39.6 ppm) in C6D6 (Figure S2a). The hydride signal disappears at 210 K in toluene-d8, and several signals get visible around −30 ppm (Figure S2b). All the other signals are rather broad at this temperature. These could mean a fluxionality and interaction with the solvent. The same effect is also observed on the 31P{1H} NMR spectra, presumably because of THF coordination to iridium that causes a significant variation of δ value and peak shape, from δP = 49.7 ppm (sharp, THF-d8, Figure S4) to δP = 57.5 ppm (broad, C6D6, Figure S5). The UV−vis measurements of 1 in toluene show a very strong band of the pyrazolate substituent at λmax = 340 nm and three wide overlapping MLCT bands11 with λmax = 427, 480, and 512 nm. The latter bands disappears in THF, because of its coordination to the metal center (Figure S6). As shown before for (tBuPCP)IrH(Cl), the formation of sixcoordinate complexes with bases causes a color fading and the disappearance of absorption bands in the electronic spectra.11,30 1 crystallizes in the P21/c monoclinic space group, with four molecules per unit cell. The coordination geometry around the metal center is square pyramidal (Figure 1), with the hydride ligand occupying the apical position; the

Scheme 3. Synthesis of 2

11

B{1H}, and 31P{1H} NMR spectroscopy at ambient temperature (Figures S7−S10). The hydride signals in the 1H NMR spectrum of 2 are all well-resolved; they fall at δH = −21.3 ppm (terminal hydride), δH = −5.1 ppm (bridging borohydrides), and δH = 5.5 ppm (terminal borohydrides). The η2coordination of the borohydride ligand to iridium is supported by the presence of two 1H NMR signals featured by the same (2:1) integration with respect to the classical hydride. In addition, the analogous (tBuPOCOP)Ir(H)(BH4) species reported by Heinekey and Goldberg in 2006 and characterized via single-crystal neutron diffraction shows the same coordination mode.5d Unfortunately, numerous attempts did not lead to the obtainment of high quality crystals of 2 suitable for XRD analysis. Consequently, the hydride ligands around the iridium atom could not be located from the residual electron density maps.33 The position of the boron atom with respect to the iridium center though [d(Ir−B) = 2.25 Å; α(C− Ir−B) = 147°] is very similar to that found in (tBuPOCOP)Ir(H)(BH4) [d(Ir−B) = 2.18 Å; α(C−Ir−B) = 159°].5d This supports the hypothesis of an identical η2-coordination mode of the borohydride group in the two complexes. Reaction of 1 with Amine Boranes: Determination of the Kinetic Parameters. The catalytic activity of 1 in AB dehydrogenation was analyzed through volumetric measurements of the H2 equivalents produced at ambient temperature in THF. Using the Man on the Moon X102 device (Figure S11), the reaction was monitored by measuring the pressure variation vs time in a closed reaction vessel. Kinetic laws and parameters were also inferred from this set of data. Initial rate experiments performed at different AB concentrations and also at variable catalyst concentration (Figures S12 and S13) showed that the overall reaction is first-order in both [AB] and [1]: −d[AB]/dt = kobs [AB] = k[1][AB]. In the presence of 2 mol % of 1, 1 equiv of hydrogen is released from NH3·BH3 in 10 h at room temperature. The reaction rate constant k is 0.018 M−1 s−1, and the TOF value calculated from the first collected points (1 min) reaches 580 h−1. Raising the temperature up to 313 K causes significant reaction acceleration (Figure 2), with a calculated rate constant of 0.119 M−1 s−1 and a TOF of 1380 h−1. Quantitative conversion of ammonia borane is achieved within 1.5 h. In order to confirm the reaction mechanism, the reaction with deuterated amine-boranes (NH3BD3, ND3BH3, ND3BD3) was also investigated. Kinetic Isotope Effect (KIE) showed that a normal rate decrease with deuteration is observed for Ndeuteration [ND3BH3; kH/kD = 2.9] and B-deuteration [NH3BD3; kH/kD = 1.6]. In the fully deuterated compound [ND3BD3, kH/kD = 4.4], the effect is approximately equal to the product of the ND and BD values [KIE(ND3BD3) = KIE(NH3BD3)·KIE(ND3BH3) = 4.6]. This result suggests that both B−H and N−H activation take place during the rate-determining step. The 11B NMR spectrum recorded during the reaction course (Figure 3) shows two increasing signals of the dehydrogenation products: B-(cyclotriborazanyl)-amine-borane (BCTB; δB = −7.0, −13.2,

Figure 1. ORTEP diagram (ellipsoids at 50% probability) of the structure of 1. Hydrogen atoms on the ligand omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ir(1)−H(100) 1.54(18), Ir(1)−N(1) 2.070(12), Ir(1)−C(1) 1.988(14), Ir(1)−Cl(1) 2.447(4), Ir(1)−P(1) 2.256(4); C(1)−Ir(1)−H(100) 87(7), Cl(1)−Ir(1)−H(100) 98(7), N(1)−Ir(1)−Cl(1) 92.2(4), P(1)− Ir(1)−Cl(1) 105.34(13), C(1)−Ir(1)−Cl(1) 170.9(5), N(1)−Ir(1)− P(1) 161.9(4).

pincer donor atoms and the chloride ligand are forming the pyramid basal plane. The hydride and chloride ligands are cis to each other, as in the analogous symmetrical complex (tBuPCP)IrHCl (tBuPCP = 2,6-C6H3(CH2PtBu2)2).31 The main bond lengths fall in the same range as those observed for similar hydrides in the Cambridge Structure Database [mean d(Ir−H) value = 1.46 Å; mean d(Ir−Cl) = 2.48 Å; mean d(Ir− C) = 2.03 Å].7c,9,30−32 Extensive C−H···Cl intermolecular short contacts involving the tert-butyl and the methylene groups on the pincer are present in the solid state. See Table S1 for the crystallographic parameters of 1. Starting from 1, a chloride ligand replacement by tetrahydroborate is achieved through prolonged reflux in THF solution with an excess NaBH4 (10 equiv), to afford the hydride/borohydride complex (tBuPCN)IrH(η2-BH4) (2, Scheme 3) as an air-sensitive brown powder. It has been characterized in solution through multinuclear 1H, 13C{1H}, 3145

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equivalent per monomer is produced at the end of the catalysis. When 2 mol % of 1 is added to a Me2NH·BH3 (DMAB) solution in toluene at T = 298 K, the reaction goes faster (TOF = 401 h−1) than the same reaction with (tBuPCP)IrHCl (TOF = 80 h−1).11 The dehydrogenation of the mono-alkyl amine-borane tBuNH2·BH3 is very slow in toluene, probably because of the high degree of its selfassociation in apolar solvents. The use of THF gives again better results, but tBuNH2·BH3 dehydrogenation is still slower than that of DMAB in toluene (Table 1). Table 1. Summary of the Catalytic Performance of 1 in the Dehydrogenation of Different Amine Boranes (T = 298 K) substrate

Figure 2. Kinetic curves of AB dehydrogenation in THF at different temperatures (2 mol % of 1).

DMAB

BuNH2·BH3

t

NH3BH3

solvent

method

C6H5F toluene toluene THF toluene THF THF THF

IRb vol.c vol.c MoMd vol.c vol.c MoMd MoMd

mol % 1 TOFa 10 10 2 3.5

58 96 401 665

2 2 3.5

212 580 358

time (h)/ conversion (%) 3/60 1/66 1.5/62 0.5/100 very slow 4/65 20/100 1/100

TOFs are determined for the first reaction minute. bIR monitoring of νBH decrease. cVolumetric measurements. dMan on the Moon. a

The 11B NMR monitoring of DMAB dehydrogenation (2 mol % 1) at 298 K in THF-d8 (Figure S15) allows identifying the final reaction products: the cyclic dimer (H2BNMe2)2 (t, δB 3.5, JBH = 113.5 Hz) and bis(dimethylamine) borane [(Me2N)2BH] (d, δB 26.6, JBH = 149.0 Hz), the latter being a product of B−N bond cleavage (Scheme 5). In addition, several intermediates were observed in 11B spectra at the earlier reaction stage: the “inorganic isobutene analogue” − aminoborane NMe2BH2 (t, δB 35.9, JBH = 125.4 Hz) and the linear diborazane BH3NMe2-BH2NHMe2 (t, δB 1.5, JBH = 113.5 Hz). The presence of monomeric aminoborane in the spectrum suggests its “off-metal” dimerization yielding the cyclic dimer. On the other hand, the formation of linear diborazane BH3NMe2-BH2NHMe2 is the result of an “on-metal” coupling, following the mechanistic scheme proposed by Weller and coworkers on the IrIII hydride [Ir(PCy3)2H2]+.36 Both mechanistic options were considered in the DFT calculations on this system (see DFT analysis section). The product of the IrCl precatalyst activation, dimethylaminochlorohydroborane dimer (Me2NBHCl)2 (t, δB 4.1, JBH = 127.7 Hz), is formed within the first 5 min after reagents mixing and its amount is constant during the reaction. The 31P NMR spectrum measured at the end of the catalysis (48 h) showed at least partial restoration of the IrCl species. The IR spectra of the 1/DMAB mixture measured at room temperature for 10 mol % of 1 in aromatic hydrocarbons (toluene, C6H5F) show the simultaneous decrease of the νBH (2368 cm−1) and νNH (3284 cm−1) bands of the initial amineborane and the growth of the band of the dehydrogenation product (2434 cm−1; Figure 4). The rate constant obtained from these spectral data k(BH) = 0.0065 M−1 s−1 is in good agreement with that obtained from volumetric measurements under the same conditions (k = 0.0068 M−1 s−1). These values correspond to the activation free energy ΔG⧧298 K of 20.1 kcal· mol−1.

Figure 3. 11B NMR spectrum (128 MHz) recorded during NH3BH3 dehydrogenation by 1 (5 mol %) in THF-d8.

and −23.7 ppm) and cyclotriborazane (CTB, δB = −13.2 ppm, Scheme 4),34 together with another broad signal at δB = 7.1 Scheme 4. B-Containing Byproducts from the Reaction of 1 with AB

ppm assigned to 2. The identity of these poorly soluble boroncontaining products was also confirmed by IR spectroscopy (Figure S14);35 their chemical nature is in agreement with the only one H2 equivalent per monomer released from AB under these conditions. The hydrido chloride 1 is an efficient dehydrogenation catalyst also for primary and secondary amine-boranes (Me2NH·BH3, tBuNH2·BH3). As in the AB case, one H2 3146

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Organometallics Scheme 5. B-Containing Products from the Reaction of 1 with DMAB

Figure 4. Time evolution of IR spectra of DMAB (c = 0.1 M) in the presence of 10 mol % of 1 at room temperature in C6H5F (A) and corresponding kinetic curves (B).

Reactions of 1 with Amine Boranes: Spectroscopic Mechanistic Study. To get insight into the reaction mechanism, the interaction of complex 1 with amine boranes was monitored through variable-temperature (VT) multinuclear NMR and IR spectroscopy. When 1 equiv of NH3· BH3 was added to (tBuPCN)IrHCl in THF-d8, the 1H NMR spectra (T = 273 K) showed new broad hydride signals at δH = −23.4 ppm (IrH) and δH = −1.12 ppm (3H, BH3) (Figure 5).

Figure 6. Variable-temperature 31P NMR spectra (400 MHz) of (tBuPCN)IrHCl and its mixture with NH3BH3 (1:1 ratio) in THF-d8. Dotted blue line: evidence of (tBuPCN)IrH(BH4) 2 as reaction intermediate.

In the presence of AB at 293 K (40 min after the reagents mixing), two new hydride doublets appear in the 1H NMR spectrum at δH = −21.3 and −21.9 ppm (in a 2:1 ratio) with corresponding doublets at δP = 58.2 and 58.0 ppm (JHP = 17.8 Hz) in the phosphorus spectrum. This set of δH and δP signals was assigned to two positional isomers of the hydrido borohydride complex (tBuPCN)IrH(η2-BH4) (2), with two μBH hydrides (see the Experimental Section). The minor isomer is short-living and disappears at 303 K. Complex 2 is the sole iridium hydride species observed during the reaction with BH3NH3 under catalytic conditions (5 mol % 1 at 298 K). In the case of the equimolar 1/DMAB mixture, the resonances of complex 2 appeared in 1H and 31P NMR spectra at 303 K. This behavior suggests higher stability of the precursor complex 4 (vide inf ra) in agreement with a higher B−N bond energy in DMAB.37 Interestingly, when 5 equiv of DMAB in THF-d8 was added to 1 at room temperature and the mixture was subsequently cooled to 220 K, the δIrH

Figure 5. Variable-temperature 1H NMR spectra (400 MHz, hydride region) of (tBuPCN)IrHCl and its mixture with NH3BH3 (1:1 ratio) in THF-d8. Dotted blue line: evidence of (tBuPCN)IrH(BH4) 2 as reaction intermediate.

Under these conditions, the initial resonance of 1 in the 31P NMR spectrum (δP = 49.8 ppm) is replaced by a broad signal at δP = 55.3 ppm (Figure 6). These proton and phosphorus resonances correspond to complex 3 (Scheme 6). They become even broader upon increasing the temperature to 323 K and shift toward the position of the initial hydrido chloride resonances due to the equilibrium with the initial complex 1. The same signals (δIrH = −25.7, δP = 54.2 ppm) were observed in the presence of 1 equiv of DMAB (at 273−323 K) assigned to complex 4 (Scheme 6). 3147

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Organometallics Scheme 6. Interaction of Amine Boranes with 1

Figure 7. 1H NMR spectrum (hydride region, 600 MHz) of 1 in the presence of 1 equiv of DMAB in THF-d8 at 210 K.

Figure 8. VT-IR spectra (νBH region) (A) and their time evolution at 298 K (B) for the equimolar mixture of DMAB and (tBuPCN)IrHCl, toluene, c = 0.01 M.

resonance of complex 2 resolved into the doublet of doublets with JHP = 18.1 Hz and JHH = 11.0 Hz (Figure S16). The NMR observations are very similar for AB and DMAB, suggesting that the same underlying mechanism is operative in the two cases. More intriguing results were obtained at low temperatures. When DMAB was added to the solution of 1 in THF-d8 at 210 K, the hydride signal (δH = −33.5) of the starting 1 shifts to −32.63 ppm due to the N-H···Cl-Ir hydrogen bonding in complex 4′ (Scheme 6) as observed for the (tBuPCP)IrH(Cl) analogue.11 Simultaneously, two new hydride signals appear at −23.86 (broad doublet) and −25.55 (doublet, JHP = 20.91 Hz) ppm with two corresponding signals in the BH-region (δ −6.5 and −2.2 ppm, respectively), suggesting the formation of sixcoordinate species. We propose that the two pairs of signals observed for 1 in the presence of DMAB belong to two isomers of complexes 4a (major, broad) and 4b (minor, sharp) where DMAB is coordinated trans to the hydride and trans to the ipso-carbon, respectively (Figure 7). Previously, the formation of only two out of three possible isomers has been shown for complexes of (tBuPCP)IrH(Cl) with pyridines.30

The base coordination in the apical position (opposite to the hydride ligand in the original square pyramidal complex) gave signals in a lower field broadened due to the exchange, whereas equatorial coordination (trans to the benzene ring) gave sharp, well-resolved resonances in the higher field.30 Interestingly, the 11 B decoupling causes the intensity increase only for the major Ir-H resonance at δH = −23.86 of 4a (Figure S17) that can be explained by a trans effect. Warming the solution leads to the disappearance of the hydride signal of 4b and broadening of the resonance 4a which shifts to higher field. Above 270 K, the latter signal remains the only resonance observed in the highfield region. We assign this resonance to the equilibrium mixture of complexes 4, 4′, and 1 (Scheme 6), which rapidly exchange in the NMR time-scale. Similar experiment in toluene-d8 showed the same hydride signals of 4a and 4b at δH = −23.08 (sharp doublet, JHP = 18.34 Hz) and −25.45 ppm (doublet, JHP = 20.54 Hz). The ratio of these isomers is always constant (8:1) and does not depend on the solvent used. Moreover, the temperature increase up to 273 K does not affect the ratio and the shape of these signals (Figure S18), unlike observed in THF-d8 solution. This indicates higher 3148

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Organometallics stability of complex 4 in toluene and correlates with the IR data under similar conditions (vide infra). Variable-temperature IR spectroscopic measurements revealed that the addition of 1 to Me3N·BH3 (in an equimolar ratio) in toluene does not lead to any changes in the spectra either at low or at room temperature. On the other hand, in the case of DMAB, VT-IR spectra show the appearance of new BH bands corresponding to the stretching vibrations of terminal (2450 cm−1) and bridging (2013 cm−1) BH groups (Figure 8A). These observations prove that DMAB coordinates to the metal center through its BH groups and evidence a vital role of the NH functionality for the formation of the adduct 4. As observed in the interaction of 1 with fluorinated alcohols of variable acidic strength (Figure S19), the chloride ligand is the proton accepting site in hydrogen bonding with alcohols. Consequently, it should be the preferential site of interaction with the N−H bond of DMAB. In agreement with NMR observations, complex 4 is stable up to room temperature (Figure 8B), in contrast to the similar (tBuPCP)IrHCl···DMAB complex that dissociates above 220 K.11 DFT Analysis of DMAB Dehydrogenation. The DFT calculations of the reaction mechanism were performed at the DFT//M06 level of theory taking the real system for the structure optimization in toluene, which was introduced within the SMD model. This theoretical investigation confirms DMAB coordination to (tBuPCN)IrH(Cl) in both the axial (η1-BH trans to hydride, 4a; ΔG298 K = −2.2 kcal/mol) and equatorial position (η1-BH trans to ipso-carbon, 4b; ΔG298 K = −1.9 kcal/mol) (Figure S20). Such a small free energy difference for two isomers is in agreement with their experimental observation in an 8:1 ratio (Figure 7). As well as in the case of (tBuPCP)IrHCl,11 the precatalyst activation initiated with DMAB coordination (Ir−Cl bond elongates by 0.166 Å in 4b) could proceed via Ir−Cl bond dissociation, yielding (tBuPCN)IrH4 and the cyclic dimer [BHClNMe2]2. This reaction is nearly ergoneutral, with ΔH° = −0.5 kcal/mol. In contrast to the analogous (PCP) system, the (tBuPCN)IrH4 complex (5) is found to be less stable than the dihydride (tBuPCN)IrH2 (Figures S21−S23) and the six-coordinated (tBuPCN)IrH2(DMAB) adduct (6). This explains the lack of experimental evidence of di- or tetra-hydrido species even during the dehydrogenation reaction.38 Geometry optimization of complexes 6 gave two isomeric structures of close energy, 6a being 1.9 kcal/mol below 6b in ΔH° scale (2.7 kcal/mol in ΔG298 K, Figure S24), analogues to 4a and 4b. This is in contrast with the case of (tBuPCP)IrH2(DMAB), for which only one equatorial isomer was found.11 The existence of two isomers offers two alternative dehydrogenation pathways (Scheme 7). The equatorial DMAB coordination in 6b precedes the concerted protonhydride transfer (TSNH/BH, cycle i, Scheme 7). Afterward, the NMe2BH2 molecule is eliminated from the metal coordination sphere with concomitant formation of the tetrahydride species 5. Finally, molecular hydrogen is replaced by another substrate molecule. The second reaction pathway implies the axial coordination of DMAB (formation of 6a), followed by the stepwise proton (TSNH) and hydride (TSBH) transfer that also produces a dihydrido complex and NMe2BH2 (cycle ii, Scheme 7). Thus, the (PCN) ligand asymmetry and lower steric hindrance in comparison with the symmetric (PCP) and (POCOP) analogues opens a new reaction pathway. In this case, the sequential proton and hydride transfer also allows for the “on-metal” amine-borane dimerization36 route, yielding

Scheme 7. Proposed Catalytic Cycles for DMAB Dehydrogenation Involving [(tBuPCN)Ir] Species

linear diborazane BH3NMe2-BH2NHMe2 which is observed as a kinetic dehydrocoupling product (Scheme 5). The free energy calculations (ΔG298 K) show very close values of the activation energy (TSNH/BH and TSNH) for the two processes depicted in cycles i and ii (Figure 9); thus, both options are

Figure 9. Computed (DFT/M06) free energy profile (ΔG298 K, in kcal/mol) for equatorial (red) and axial (blue) pathways of DMAB dehydrogenation by [(tBuPCN)IrH2]. The profile for similar equatorial pathway of DMAB dehydrogenation by [(tBuPCP)IrH2] (black) computed at the same level of theory11 is given for comparison.

reasonable. This is in full agreement with the experimental detection of aminoborane (Me2NBH2) and linear diborazane (BH3NMe2-BH2NHMe2), corresponding to the kinetic products of “equatorial” (cycle i) and “axial” (cycle ii) dehydrogenation cycles. In the latter, the energy of TSBH should be very low. A potential energy surface scan using the B−H bond length as a reaction coordinate shows an energy maximum at ca. 0.3 kcal/mol from the initial geometry that we were not able to converge to TSBH. Interestingly, the activation energy of the “equatorial” cycle i is substantially lower than that found for (tBuPCP)IrH2 (TSNH/BH). This is in line with the higher [(tBuPCN)Ir] vs [(tBuPCP)Ir] catalytic activity recorded experimentally. 3149

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Complexes 6 could also precede the B−N bond dissociation, yielding (tBuPCN)IrH(η2-BH4) (Figure S25) and dimethylamine. The latter could trap aminoborane (Me2NBH2), producing (NMe2)2BH and another H2 molecule (Scheme 8).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00488. Table S1: crystal data and structure refinement for 1. Figures S1−S25: 1H, 13C{1H}, 31P{1H}, 11B{1H} NMR spectra, UV−vis spectra, Man on the Moon device, AB dehydrogenation kinetics, FTIR spectrum, IR spectra, optimized geometries, energy profile (PDF) Cartesian coordinates (XYZ)

Scheme 8. Mechanism of B−N Bond Dissociationa

Accession Codes

CCDC 1838328 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 Authors

*E-mail: *E-mail: *E-mail: *E-mail:

a

Nu = nucleophile (THF, H2O traces, amine).

The overall reaction of complex 2 formation is 6 + DMAB → 2 + (NMe2)2BH + 2H2, and it is almost ergoneutral (1.0 kcal/ mol in the ΔH scale). That is in agreement with the experimental observation of (NMe2)2BH and tetrahydroborate complex 2 as the catalyst resting state that can be converted back into the active (tBuPCN)IrH2(DMAB) intermediate.

[email protected] (A.R.). [email protected] (G.G.). [email protected] (N.V.B.). [email protected] (E.S.S.).

ORCID

Giuliano Giambastiani: 0000-0002-0315-3286 Andrea Rossin: 0000-0002-1283-2803 Natalia V. Belkova: 0000-0001-9346-8208 Oleg A. Filippov: 0000-0002-7963-2806 Elena S. Shubina: 0000-0001-8057-3703

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CONCLUSIONS The presence of the less sterically demanding and weaker σdonor pyrazolate arm in the (PCN) pincer ligand 1-[3-[(di-tertbutylphosphino)methyl]phenyl]-1H-pyrazole described in this work enhances the catalytic activity of the corresponding iridium complexes in amine boranes dehydrogenation in comparison to the symmetrical (PCP) analogues. The activity of (tBuPCN)IrH(Cl) (1) as dehydrogenation catalyst is higher than that of (tBuPCP)IrH(Cl)11 but lower than that of (tBuPOCOP)IrH2.5d The lower steric hindrance allows for the formation of both possible isomers of six-coordinate iridium amine-borane complexes (tBuPCN)IrH(Cl)(HBH2NHR2) (in contrast to the tBuPCP analogue) as revealed by variable-temperature multinuclear 11B, 31P{1H}, 1H NMR spectroscopy, IR spectroscopy, and DFT calculations. Changes in the Lewis acidity of the central iridium atom as a consequence of the (PCP) → (PCN) ligand modification lead to the increased stability of this intermediate, which is observed up to room temperature. On the other hand, the same modification in the ligand structure leads to lower stability of the tetrahydride (tBuPCN)IrH4, which was not observed experimentally in contrast with (tBuPCP)IrH4 and (tBuPOCOP)IrH4. The combination of the kinetic and computational data shows that a simultaneous B−H/N−H activation occurs in the presence of 1, after a preliminary amine borane coordination to the metal center. The formation of two six-coordinate iridium amine-borane isomers opens two different dehydrogenation pathways: a concerted or a stepwise proton-hydride transfer. Both processes have very close values of activation Gibbs energy, with kinetic barriers (ΔG#) considerably lower than those reported for (tBuPCP)IrH2.

Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The bilateral project CNR (Italy) − RFBR (Russian Federation) 2018-2020 (RFBR grant No. 18-53-7818) and the Russian Foundation for Basic Research (RFBR, grants No. 17-03-01128, 16-03-00324) are acknowledged for financial support. E.M.T. thanks the RUDN University Program 5-100. G.G. would also like to thank the Italian MIUR through the PRIN 2015 Project SMARTNESS (2015K7FZLH) “Solar driven chemistry: new materials for photo- and electrocatalysis” for financial help. G.G. also thanks the TRAINER project (Catalysts for Transition to Renewable Energy Future) of the “Make our Planet Great Again” program (ref. ANR-17MPGA-0017) for support. This work is also performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.

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REFERENCES

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