Mechanism of DimethylamineâBorane Dehydrogenation Catalyzed by

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The mechanism of dimethylamine borane dehydrogenation catalyzed by iridium(III) PCP-pincer complex Ekaterina M Titova, Elena S Osipova, Alexander A. Pavlov, Oleg Andreevich Filippov, Sergey V Safronov, Elena S. Shubina, and Natalia V. Belkova ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03207 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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The mechanism of dimethylamine borane dehydrogenation catalyzed by iridium(III) PCPpincer complex Ekaterina M. Titova, Elena S. Osipova, Alexander A. Pavlov, Oleg A. Filippov, Sergey V. Safronov, Elena S. Shubina and Natalia V. Belkova* A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia, E-mail: [email protected]

Keywords: iridium pincer complexes, metal hydrides, catalytic dehydrogenation, amineboranes, molecular spectroscopy, DFT calculations, dehydrocoupling mechanism

Abstract

The title complex (tBuPCP)IrH(Cl) (1;

tBu

PCP = κ3-2,6-(CH2PtBu2)2C6H3) appeared to be

moderately active in NHMe2BH3 (DMAB) dehydrogenation allowing the systematic spectroscopic (variable temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate (tBuPCP)IrHCl(η1-BH3NHMe2) complex (3)

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stabilized by NH···Cl hydrogen bond is shown experimentally at the first reaction step. That activates both B-H and Ir-Cl bonds initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)IrH2(η1-BH3NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso-carbon allowing to overcome the steric repulsion between amine borane and tert-butyl groups at phosphorus atoms. Under catalytic conditions (2-5 mol% 1) the hydride complex (tBuPCP)IrH(µ2-H2BH2) (5) was identified that is not a dormant catalytic species, but the steady-state intermediate formed as a result of the B-N bond breaking. DMAB dehydrogenation yields the borazane monomer H2B=NMe2 (detected by 11

B NMR), dimerization of this to give the final product [H2BNMe2]2, and (tBuPCP)IrH4 as the

catalyst resting state. The scenario of the BN bond cleavage in DMAB leading to the by-products of dehydrogenation such as bis(dimethylamino)hydroborane and (tBuPCP)IrH(µ2-H2BH2) (5) is proposed. The results obtained allow suggesting the mechanism of catalytic DMAB dehydrocoupling that could be generalized to other processes.

Introduction Nowadays, hydrogen is globally recognized as a clean energy carrier to be used in environmentally safe high energy-efficient fuel cells that produce water as the only byproduct. Among numerous hydrogen storage materials, B-N compounds bearing covalently bound hydrogen have great potential to achieve the requirements set for on-board hydrogen storage systems.1,2 Amine boranes are also used as precursors to BN-based ceramics and polymeric materials.3 For this reason, the catalytic dehydrocoupling of amine boranes is of broad interest


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despite the difficulties in re-hydrogenation of spent amine borane .4,5 All these transformations can be facilitated by a wide range of transition metal catalysts,6,7 and new systems keep appearing in the literature.8-10 It is recognized that metal-mediated dehydrocoupling reactions of amine-boranes are very complex processes and the detailed mechanism depends on the identity of the metal center and supporting ligands.7,11,12 Over the years there have been numerous reports13,14 on the chemistry of pincer iridium complexes stimulated by a report on the catalytic activity of iridium dihydrides in homogeneous alkane dehydrogenation.15 Based on the isoelectronic structure of amine-boranes and alkanes it was suggested that iridium pincer complexes should catalyze the release of hydrogen chemically stored in amine boranes. Indeed, (tBuPOCOP)IrH2 (0.3 mol % in THF;

tBu

POCOP = κ3-2,6-

(OPtBu2)2C6H3)) efficiently dehydrogenates NH3BH3 (AB) to produce 1 equiv. of H2 in 20 min at room temperature16 and this catalyst remains one of the most active pincer catalysts in the literature.17,18 However, as the reaction proceeds, the catalyst becomes dormant due to the formation of sigma-borane complex, (tBuPOCOP)IrH2(BH3), that should be converted to (tBuPOCOP)IrH4 to restore the catalytic activity.16,19 Several possible reaction pathways going via IrI or IrIII species were analyzed computationally for model (MePOCOP)IrH2 system,20 but the results were not fully consistent with experimental observations for AB dehydrogenation. The IrIII pathway was essentially reproduced in the recent work on the mechanism of H2 release from hydrogenated boron nitride nanotubes (HBNNTs) catalyzed by (MeO-iPrPCP)IrH2 (MeO-iPrPCP = κ3-4-MeO-2,6-(CH2PiPr2)2-C6H2).21 The pincer supported hydrido chloride complexes are typically used only as precursors to corresponding hydrides. Our recent studies on (tBuPCP)IrH(Cl) (1;

tBu

PCP = κ3-2,6-

(CH2PtBu2)2C6H3) showed it can be a convenient model to study the interaction of such pincers


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with coordinating solvents and HX donors.22,23 Complex 1 appeared to be moderately active in NHMe2BH3 (DMAB) dehydrogenation allowing the systematic spectroscopic (variable temperature NMR and IR) investigation, under both stoichiometric and catalytic regimes, combined with the DFT calculations that resulted in the detailed mechanistic proposals. Moreover (tBuPCP)IrH(Cl) appears being a convenient, easy-to-handle pre-catalyst that features higher stability than (tBuPCP)IrH4/(tBuPCP)IrH2 catalysts.

Results and discussion Catalytic amine-boranes dehydrogenation. When 1 was added to a toluene solution of DMAB, under an atmosphere of argon, gas evolution was observed. Concomitant with this, the red color of 1 faded to pale yellow suggesting formation of six-coordinate species.22,23 The progress of the reaction was followed by measuring a volume of hydrogen released, typically using an initial DMAB concentration of 0.47 M and a 2 mol % catalyst loading to compare with the results published16,24 for (tBuPOCOP)IrH2. Under these conditions the complete conversion and the release of one equivalent of hydrogen gas can be reached in 24 h at room temperature. This is substantially faster than DMAB dehydrogenation by (tBuPOCOP)IrH2 where less than 1 equiv of H2 was released over 48 h.24 Dehydrogenation of tBuNH2BH3 is slow in toluene (15% conversion in 1 h) probably because of a high degree of its self-association in this non-polar solvent. Use of THF (the solvent used for (tBuPOCOP)IrH2) gives better results (32% conversion in 1 h). The kinetic curves obtained at different catalyst loading (Figure 1) clearly have two regions which slope differs two times (effective rate constants keff´ and keff´´ are equal 1.0·10-4 and 4.9·105 -1

s , respectively). Interestingly, when the hydrogen evolution ends, the reaction mixture restores


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its red color, the recovery of 1 being confirmed by UV-vis (Figure S1) and NMR spectra (vide infra).

Figure 1. Kinetic curves for H2 evolution from DMAB (0.47 M) catalyzed by (tBuPCP)IrH(Cl) (1) in toluene at 300 K. ♦ - 2 mol %, ■ – 5 mol %, ▲ – 10 mol % of 1 as catalyst. The

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B NMR spectra measured at the end of the reaction in toluene-d8 showed formation of

two products: cyclic diborazane [H2BNMe2]2 (C; t, δB 5.4, JBH = 113.6 Hz)25-27 and bis(dimethylamino)hydroborane (Me2N)2BH (D; d, δB 28.9, JBH = 147.6 Hz)28 in the 3:1 ratio (Figure S2). Closer look on the spectra measured after 4 h of the reaction revealed the formation of borazane monomer H2B=NMe2 (A, t, δB 38.2, JBH = 130 Hz)25-27 and cyclic dimethylaminochlorohydroborane dimer (Me2NBHCl)2 (B, d, δB 6.5, JBH = 130.0 Hz)29 as intermediates (Figure 2; Scheme 1). The appearance of the latter could be related to the dissociation of the Ir-Cl bond of the pre-catalyst 1 in the presence of amine-borane. Both species convert to the final cyclic diborazane [H2BNMe2]2 (C) as the reaction progresses while (Me2N)2BH (D) appears to be formed in the beginning and does not undergo further transformations (Scheme 1).


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Figure 2.

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B NMR spectrum (128 MHz) taken after 4h of Me2NHBH3 (c = 0.28 M)

dehydrogenation in the presence of 5 mol % of 1 in toluene-d8 at 290 K. The signal of unidentified species is marked by an asterisk.

Scheme 1. The reaction scheme for DMAB transformation. The

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B NMR spectra showed the appearance of yet another boron-containing species giving

broadened triplet δB -1.1 ppm (Figure S3). Its amount is relatively small and maintained constant after 200 min. Selective decoupling of 1H NMR spectrum revealed its connection to the proton signal at 1.96 ppm and the exchange with 1. However, we were unable to provide better identification. Formation of (poly)boric acid (broad signal at δB 21 ppm)30,31 due to the hydrolysis of the reaction intermediates by water impurities becomes also visible at longer reaction times. Analysis of the time evolution of different

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B signals (Figure S4) shows that DMAB

dehydrogenation leads to the simultaneous accumulation of two major products C and D. This suggests they could be formed through the same intermediate. The corresponding kinetic curves


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(time evolution of mol fraction for different boron-containing species obtained from

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B NMR

data; Figure S5) have two regions resembling those obtained by volumetric measurements of H2 evolution (Figure 1). Second regions have slightly different slopes for two products (Figure S5; k”A = 1.5·10-5 s-1, k”B = 1.2·10-5 s-1) in agreement with a simultaneous accumulation of C and D. Spectroscopic mechanistic studies. When 1 equiv. DMAB was added to a toluene solution of (tBuPCP)IrH(Cl) at low temperature the 1H NMR spectra at 200 K showed a new broad hydride resonance at δH -23.85.32 At 210 K it resolves into a quartet with 2JHH = 13.6 Hz due to the coordination of BH3 group to iridium. Above this temperature, it shifts down-field and transforms into the triplet with 2JPH = 14.5 Hz at 220 K (Figure 3). This signal corresponds to the complex 3 in which DMAB is coordinated to iridium center via BH-group (Scheme 2). The boron hydride group of non-coordinated DMAB appears in 1H NMR spectrum as very broad signal δH(BH3) = 1.5-2.4 ppm (2JBH = 96 Hz) and its methyl resonance δH(Me) appears at 1.89 ppm (2JHH = 5.5 Hz). In complex 3 the resonance of methyl group of coordinated DMAB appears at 2.00 ppm (2JHH = 5.0 Hz) while BH3 group gives two new signals. One of them, δH(BHbr), appears at -1.52 ppm and belongs to µ-BH proton; the other signal δH(BHterm) = 6.29 ppm belongs to the BH protons which remain unbound in complex 3.33 Further support of the complex 3 assignment was provided by the analogous experiment with (tBuPCP)IrH4 (4), which was separately prepared from (tBuPCP)IrH(Cl). No signals of 3 were observed in the 1H NMR spectrum when 1 equiv. DMAB was added at low temperature to tetrahydride 4. At that dehydrogenation of DMAB and H2 evolution was completed with similar rate yielding cyclic diborazane [H2BNMe2]2 (C) and (Me2N)2BH (D) in the 3:1 ratio similar to that obtained with 1 as a catalyst.


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Figure 3. Variable temperature 1H NMR (600 MHz) spectra of (tBuPCP)IrH(Cl) (c = 0.02 M) and its mixture with DMAB (ratio 1:1) in toluene-d8. PtBu2 Ir H

Cl

PtBu2

K1 + Me2NH.BH3

Ir H

P t Bu2

Pt Bu2 Cl H Ir NMe 2 H t H B H P Bu2 H 3

K2

Cl H NMe 2.BH 3

PtBu2 2

1

1/2 [Me2

N.BHCl]

2

PtBu2 Cl Ir

PtBu2 H H Ir H H PtBu2

H

H B PtBu2 H

4

= H NMe 2 H

TS

Scheme 2. The proposed mechanism for the chloride pre-catalyst activation. The hydride resonance of starting (tBuPCP)IrH(Cl) complex (δH -42.67) becomes very broad after the addition of DMAB and almost invisible in the NMR spectrum at 200 K (Figure 3). These changes resemble those observed in the presence of alcohols23 and suggest that the signal belongs to a mixture of complexes 1 and 2; (tBuPCP)IrH(Cl) in the latter being hydrogen bonded


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to DMAB via N-H···Cl bond (Scheme 2). That also causes a strong shift of δH(NH) signal of DMAB (3.2 ppm) to lower field that depends on the temperature (δH(NH) = 4.05 at 200 K in the presence of 1 equiv. 1; Figure S6). The formation of hydrogen bonded complex 2 was also proved by

31

P NMR. Whereas the resonance of complex 1 appears at δP 67.5, and that of

complex 3 at δP 45.7, the 31P resonance observed upon DMAB addition at 200 K appears shifted to 60.8 ppm and becomes broad at 220 K. Upon warming the sample to 290 K this phosphorous signal shifts to 66.9 ppm in line with the equilibrium shift observed in the 1H NMR spectra. Though the formation of N-H···Ir hydrogen bond instead of N-H···Cl interaction could be an option for the d6 metal, it does not occur in this system. As we have shown, the metal atom in 1 behaves as a Lewis acid and the chloride ligand is a preferred basic center.23 The integration of full proton spectra reveals that complex 2 co-exists with 3 in 1.5:1 ratio at 200 K. This ratio changes to 11:1 at 240 K (Figure 4). These data allow estimation of thermodynamic parameters for 2 ↔ 3 equilibrium, yielding ∆H° = -4.5 kcal/mol, ∆S° = -23.4 cal/(mol·K). At 240 K the signal δH -9.17 (2JPH = 9.8 Hz) of the well-known tetrahydridocomplex (tBuPCP)IrH4 (4)15,34 appears in the 1H NMR spectrum. At 270 K the resonance of complex 3 completely disappears but the resonance of complex 2 grows in intensity in agreement with the low entropy of 3. At 290 K the resonance of molecular hydrogen δ(H2) = 4.5 ppm appears in 1H NMR spectrum while the amount of tetrahydride increases, the ratio 4 to 2+1 becoming 1:4 (Figure 4).


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1.0

0.8

0.6

1+2 3 4

0.4

0.2

0.0 180

200

220

240

260

280

300

T, K

Figure 4. The temperature dependencies of 2, 3 and 4 molar fractions during the reaction of (tBuPCP)IrH(Cl) (c = 0.02 M) with DMAB (ratio 1:1). The IR spectra measured for equimolar DMAB: (tBuPCP)IrH(Cl) mixture in toluene show the appearance of new BH bands at 190 K (Figure 5). These two bands νBH = 2490 and 2087 cm-1 correspond to the stretching vibrations of the terminal and bridging BH groups, respectively. The same picture was observed for the equimolar mixture of (tBuPCP)IrH(Cl) with tBuNH2BH3 (νBHbr = 2108 сm-1 and νBHterm = 2465 сm-1; Figure S7). These observations confirm the formation of new complex 3 with amine-borane molecule coordinated to the metal center. Warming the reaction mixture results in the intensity decrease of νBH bands of 3 and the simultaneous increase of the band belonging to DMAB dehydrogenation product (νBH = 2426 cm-1). Besides that the band of IrH stretching vibrations of tetrahydrido iridium complex 4 (νIrH = 2070 cm-1)35 becomes visible at room temperature. Moreover, the far-IR spectra show the graduate disappearance of the IrCl stretching vibration band of complex 1 (νIrCl = 276 cm-1) upon addition of DMAB at ambient temperature. Interestingly, Me3NBH3 does not react with iridium complex 1, suggesting the NH


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functionality that might be involved in hydrogen bonding with chloride ligand is important for the reaction.

Figure 5. Variable temperature IR spectra (νBH region) for equimolar DMAB: (tBuPCP)IrH(Cl) mixture. Toluene, 190 – 290 K, c = 0.025 M. The 1H NMR monitoring at ambient temperature shows the hydride resonance of complex 1 (δH -42.67) becomes broader and appears down-field upon DMAB addition (∆δH = 1.35 ppm) but progressively shifts toward its original position as the reaction proceeds (Figure 6). This down-field shift could be explained by the equilibrium between the starting hydride 1, hydrogen bonded 2 and 3. The latter two species are present in small amounts at ambient temperatures (Figure 4) and are not directly observed. However, their fast exchange with 1 on the NMR time scale under these conditions shifts the signal to lower field. As DMAB is consumed in the reaction course, the equilibrium (Scheme 2) shifts to the left and the signal drifts toward its position in the non-bound 1. The presence of such exchange and involvement of DMAB in the equilibrium with 1 is further supported by measurements of 1H{11B} spectra. The non-selective decoupling causes splitting of the hydride signal into two peaks at -41.48 and -41.67 ppm, whereas selective decoupling of δB -12.8 signal (DMAB) leaves only the lower frequency singlet δH -41.48 of increased intensity (Figure 7).


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1+2+3 1 + DMAB 110min 1 + DMAB 50min 1 + DMAB 40min 1 + DMAB 20min 1

-8.5

-9.0 -9.5 Chemical Shift (ppm)

-35

-40 -45 Chemical Shift (ppm)

-50

-55

Figure 6. 1H NMR monitoring of the reaction (tBuPCP)IrH(Cl) (1, с = 0.023 М) with DMAB (ratio 1:1) in toluene-d8 at 290 К.

-41.0

-41.5 Chemical Shift (ppm)

-42.0

Figure 7. 1H NMR spectra of equimolar (tBuPCP)IrH(Cl)/DMAB mixture measured ca. 50 min after mixing. Green spectrum – 1H; blue – 1H{11B}; red – selectively decoupled

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B signal

at -12.8 ppm. с = 0.023 М, toluene-d8, 290 К. Under catalytic conditions (5 mol% 1) the 1H NMR spectra show the formation of hydride complex 5, which was identified as (tBuPCP)IrH(µ2-H2BH2) by the comparison to the recently described analogues33,36 and confirmed by independent synthesis via the treatment of 1 with NaBH4 (Figure S8). Its signals (δH -5.37 (br s, 1H, µ-BH); -7.33 (br s, 1H, µ-BH); −20.19 (t, 1H, H, 2JPH = 10.6 Hz), δB 12.2 (br s)) become visible after 1 hour of the reaction till the complete consumption of DMAB. Such spectral pattern is in agreement with the presence of two non-


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equivalent µ-BH hydrides as confirmed by DFT calculations (vide infra). We believe, that in contrast to (tBuPOCOP)IrH2(BH3)

16,19

, complex 5 is not a dormant catalytic species, but the

steady-state intermediate formed as a result of B-N bond breaking. DFT calculations. The geometry optimizations were performed at the DFT/M0637 level without any ligand simplification. Optimizations of all reported species were performed both in the gas phase and toluene; data for the latter are reported if not mentioned otherwise. Several local minima were found for DMAB coordination to 1 featuring both NH···Cl and BH···Ir interactions (Figure S9). Among those the η1-BH coordination trans to ipso-carbon is less favourable (formation enthalpy calculated in toluene ∆Hsolv = -1.9 kcal/mol) than trans to hydride (∆Hsolv = -4.4 kcal/mol), but NH···Cl bonded complex 2 is even more stable (∆Hsolv = 10.0 kcal/mol). However, the most stable structure possesses both interactions (complex 3, Figure 8). In this complex the simultaneous amine-borane coordination to iridium and strong hydrogen bonding to chloride, N-H···Cl-Ir, result in strong activation of the Ir-Cl bond. This bond becomes almost dissociated with Ir-Cl distance 2.604 Å in the gas phase (2.629 Å in toluene) being well above the sum of the covalent radii 2.4 Å (1.02 and 1.41 Å for Cl and Ir, respectively38) and much longer than in the starting 1 [d(Ir–Cl) =2.425 Å (X-ray),39 2.469 Å (calc)] and its complexes with N-bases [2.542 Å (X-ray)22, 2.530-2.576 Å (calc)23]. Complex 3 has similar stability as hydrogen bonded complex 2 in toluene, ∆Hsolv = -10.0 kcal/mol, but due to the lower entropy it is observed only at low temperatures (Figure 4). The chloride detachment from 3 requires only 7.4 kcal/mol yielding complex 3’ (Figure 8; ∆Hsolv = -2.6 kcal/mol relative to reactants). Both complexes 3 and 3’ are identified as local minima on PES. The TS for their interconversion was estimated as a result of stepwise Ir-Cl bond elongation being ~0.8 kcal/mol above 3’. Formation of iridium tetrahydride 4eq (Figure S10) and cyclic (Me2NBHCl)2 –


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suggested intermediates of the (PCP)IrHCl reaction with DMAB (Scheme 3) – is calculated as almost ergoneutral (-0.2 kcal/mol in ∆Esolv scale and -3.5 kcal/mol in ∆Hsolv scale), confirming the possibility of their conversion back to the starting hydrido chloride 1 in the end of the catalytic cycle. Thus, the DFT calculations show the low energy cost of (tBuPCP)IrH(Cl) activation and entry to the catalytic cycle.

Figure 8. Toluene optimized geometries of complexes 3 (left) and 3’ (right). Principal bond distances in Å. Hydrogen atoms of PCP ligand are removed, tBu substituents at phosphorus and Me groups of DMAB are shown as a wireframe.

Scheme 3. Suggested catalytic cycles.


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The key intermediate of the catalytic cycle for DMAB dehydrogenation (cycle a, Scheme 3) is the hexacoordinate dihydrido complex (tBuPCP)IrH2(DMAB) (6, Figure 9) which structure is similar to that of (tBuPCP)IrHCl(DMAB) complex 3 (Figure 8). That is the only structure we were able to optimize for (tBuPCP)IrH2(DMAB) due to the repulsion between the methyl groups of DMAB and the bulky tertiary butyl groups of the phosphine moiety. Expectedly, the formation of 6 from the five-coordinate dihydride is energetically favorable (∆Hsolv = -6.9 kcal/mol). However, H2 dissociation from (tBuPCP)IrH4 (4eq) costs about the same, making the overall substitution reaction slightly endothermic in toluene (∆Hsolv = 0.8 kcal/mol) in contrast to the exothermic exchange in the gas phase (∆Hgas = -2.1 kcal/mol) (Figure 10). The coordination of DMAB activates its BH and NH bonds by 0.032 and 0.007 Å, respectively. In the same time, Ir-H bond involved in the dihydrogen bonding with the nitrogen-bound proton becomes 0.049 Å longer than free Ir-H in 6 (Figure 9). Although the DMAB arrangement in 6 differs from that of ammonia-borane in its adducts with model catalysts (MeO-iPrPCP)IrH2 21 and (MePOCOP)IrH2 20 as well as (tBuPOCOP)IrH2,21 in all cases the simultaneous η1-BH coordination and N-H···H-Ir dihydrogen bonding determine the six-membered configuration of the subsequent transition state. The same complex 6 could precede the B-N bond dissociation (cycle b, Scheme 3) yielding (tBuPCP)IrH(µ2-H2BH2) complex 5 having two non-equivalent µ-BH hydrides (Figure S11). This process could be promoted by a nucleophile which could be even adventitious water40 or BH ligand of DMAB.41 Me2NH released at this step is probably trapped by Me2N=BH2 to form product D ((Me2N)2BH) detected in the 11B NMR spectra (Figure 2). Product D formation could be formalized as indicated on Scheme 4 with total reaction energy being ∆Hsolv = +2.0 kcal/mol and ∆Gsolv -11.5 kcal/mol. Hydrolysis of coordinated BH3 in 5 would give (poly)boric acid and


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should proceed much easier than hydrolysis of DMAB or BH4– itself due to the BH activation in the metal coordination sphere.42-44

Scheme 4. Formation of (tBuPCP)IrH(µ2-H2BH2) and (Me2N)2BH via the B-N bond dissociation. Amine-borane dehydrogenation in 6 goes via concerted proton-hydride transfer. However, the normal coordinate analysis shows that the proton movement makes the biggest impact to the imaginary frequency νi = 607 cm-1: 67% impact of H(N), 29% of H(Ir) and 2% of H(B) movement. The transition state has a product like geometry with broken N-H bond and completed formation of H-H ligand (Figure 9). Proton transfer from N-H to a catalyst is usually considered as a rate limiting step of amine-borane dehydrogenation. In the present case of (tBuPCP)IrH2(DMAB) system, it occurs simultaneously with B-H bond breaking. The reaction barrier ∆H≠solv = 15.4 kcal/mol relative to starting (tBuPCP)IrH4/DMAB is comparable to the literature data for (MePOCOP)IrH2 and(MeO-iPrPCP)IrH2.20,21 The calculated TS free energy, ∆G≠solv = 21.0 kcal/mol relative to (tBuPCP)IrH4/DMAB, is in agreement with the experimental values ∆G≠ ≈ 23 kcal/mol obtained from the reaction rate constants k (see above). Then H2 ligand in tetrahydride complex 4eq is replaced by next amine-borane molecule closing the catalytic cycle.


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νi = -607 cm-1 Figure 9. Geometries of complex 6 (left) and TS for DMAB dehydrogenation (right) optimized in toluene. Principal bond distances in Å. Hydrogen atoms of PCP ligand are removed, tBu substituents at phosphorus and Me groups of DMAB are shown as a wireframe.

Figure 10. Computed (DFT/M06) free energy profile (∆Gsolv) for the proposed catalytic cycle of DMAB dehydrogenation catalyzed by [(tBuPCP)Ir] in toluene.


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Conclusions Our results show the possibility of direct use of hydrido-chloride iridium pincers as precatalysts of amine-borane dehydrogenation. In the present case of the

tBu

PCP supported system,

the chloride substitution at the activation step proceeds even faster (via lower barrier) than the catalytic cycle itself. The latter involves (tBuPCP)IrH4 as the catalyst resting state as was found for ammonia borane dehydrogenation catalyzed by (tBuPOCOP)IrH2.16 Very recent publication reports on catalytic activity of [3,5-R2(POCOP)IrHX] complexes (3,5-R2(POCOP) = κ3-C5HR22,6-(OPtBu2)2 with R = t-Bu, COOMe; X = Cl, H) in dehydrogenation of hydrazine borane.45 In agreement with our findings, there is a minor difference in the performance of iridium(III) dihydrido and hydrido-chloride species.45 Both PCP and POCOP complexes are thoroughly studied, though the data are disseminated in the literature. Despite some structural difference (the metal center of (tBu4POCOP)Ir is much less sterically hindered than that of (tBu4PCP)Ir) the electronic differences are fairly subtle and complex.13,46 The corresponding hydrides possess very similar NMR spectral characteristics (Table S2) whereas their relative catalytic activity varies depending on the process.47,48 In amineborane dehydrogenation the two systems (PCP and POCOP) apparently operate through the same resting states. Being less effective than POCOP-based complexes, (tBuPCP)IrHCl allows the detailed experimental mechanistic investigation and spectroscopic characterization of the reaction intermediates. Formation of the hexacoordinate (tBuPCP)IrHCl(η1-BH3NHMe2) complex (3) stabilized by NH···Cl hydrogen bond activates not only the B-H but also the Ir-Cl bond initiating the pre-catalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by our DFT calculations for the key intermediate of the catalytic cycle, (tBuPCP)IrH2(η1-BH3NHMe2) complex (6). In these complexes DMAB is coordinated trans to


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the ipso-carbon in contrast to cis-coordination considered in previous theoretical works.20,21 We believe that namely this geometry (preserved in the TS) allows overcoming the steric repulsion between amine-borane and tert-butyl groups at phosphorus atoms. The same complex 6 can serve as a precursor of the BN bond cleavage in DMAB that occurs in the presence of nucleophiles as e.g. adventitious water or BH ligand of amine-borane itself. (tBuPCP)IrH(µ2H2BH2) (5) formed as the result of the BN bond cleavage is not a dormant catalytic species, as its POCOP analogue, but an intermediate of the complementary catalytic cycle yielding bis(dimethylamino)hydroborane. The overall mechanism of catalytic dehydrocoupling of DMAB suggested herein on the basis of thorough spectroscopic and DFT studies should be operative in other processes catalyzed by pincer iridium complexes. Furthermore, these observations can be generalized for application to many of the transition-metal and main-group systems that catalyze the amine-boranes dehydrocoupling.

Experimental section All reactions were carried out under an argon atmosphere using standard Schlenk techniques. Solvents were carefully dried by conventional methods and distilled under argon before use. {2,6-bis[(di-tert-buthylphosphino)methyl]phenyl}chlorohydridoiridium ((tBuPCP)IrH(Cl), 1) was prepared from 1,3-bis[(di-tert-buthylphosphino)methyl]benzene and [Ir(COE)2Cl]2 in analogy with the procedure reported for the p-methoxy derivative (MeO-PCP)IrH(Cl).49 (tBuPCP)IrH4 (4) was prepared from (tBuPCP)IrH(Cl) as described in ref.

15 1

. H,

31

P{1H} and

11

B NMR spectra

were recorded with a Bruker Avance 600, Bruker Avance 300 and Bruker Avance 400 FT-NMR spectrometers at different temperatures (200-293 K). The resonances were calibrated relative to the residual solvent peaks (1H) or referred to external 85% H3PO4 (31P). 11B NMR spectra were


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referenced against external BF3·OEt2 standard (δ = 0 ppm). The samples for NMR were prepared with c = 0.02 M. IR spectra were recorded at different temperatures (190-293 K) with a Nicolet 6700 spectrometer using CaF2 cells for high frequency region and CsI cells (d = 0.1 cm) for low frequency region. (tBuPCP)IrH(Cl)(Me2NHBH3), 3, toluene-d8, 220 K: 1H NMR (600 MHz):δ 6.29 (br s, 2H, BH2); 3.44 (d(AB), 2H, CH2, 2JPH = 15.0 Hz); 2.85 (m, 2H, CH2); 2.00 (d, 6H, CH3NH, 2JHH = 5.0 Hz); 1.39 (br s, 18H, CH3); 1.12 (br s, 18H, CH3);-1.52 (br s, 1H, µ-BH); −23.80 (t, 1H, H, 2

JPH = 14.5Hz). 31P{1H} NMR (121 MHz): δ 45.7. (tBuPCP)IrH(µ2-H2BH2), 5, toluene-d8, 290 K: 1H NMR (400 MHz):δ 7.53 (br s, 2H, BH2);

6.95 (m, 3H, Ph); 3.2 (dt(AB), 2H, CH2, 2JPH = 16.4 Hz, 2JHH = 3.2 Hz); 2.99 (dt(AB), 2H, CH2, 2

JPH = 16.6 Hz, 2JHH = 3.7 Hz); 1.31 (t, 18H, CH3, 2JHH = 6.4 Hz); 1.17 (t, 18H, CH3, 2JHH = 6.4

Hz);-5.37 (br s, 1H, µ-BH); -7.33 (br s, 1H, µ-BH); −20.19 (t, 1H, H, 2JPH = 10.6 Hz). 11B (128 MHz) δ 11.8 . 31P{1H} NMR (162 MHz): δ 61.0. Quantification of Hydrogen 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 stoppered 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 burette through a Teflon tube. The volume of hydrogen gas collected was recorded periodically (every minute during the first half-hour) until the reaction was complete. Computational details Calculations were performed with the Gaussian0950 package at the DFT/M0637 level without any ligand simplification. Effective core potentials (ECP) and its associated SDD basis set51-54


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supplemented with f-polarization functions (SDD(f))55 were applied to the Ir atom. In the tBuPCP ligand carbon atoms of benzene ring and P atoms were associated with 6-31++G(d,p) basis set; H and Cl ligands, as well as BH and NH groups of DMAB were described with 6-31++G(d,p) basis set,56-59 while the rest of atoms (methyls of DMAB molecule and tBu groups of PCP) was described with a 6-31G basis set.56 The structures of the reactants, complexes with acids and bases, were fully optimized with this basis set without any symmetry restrictions. The nature of all of the stationary points on the potential energy surface was confirmed by vibrational analysis.60 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.61,62 Solvent (toluene, ε = 2.3741) was introduced within the SMD solvation model

63

; full

geometry optimization of certain reactants, intermediates, and transition states was performed at this level followed by the thermochemistry calculations. Acknowledgements This work was financially supported by the Russian Science Foundation (grant № 14-1300801). Supporting Information Additional figures, discussion on the relative stability of (tBuPCP)IrH4 isomers, atomic coordinates (XYZ tables) and absolute energies (in Hartrees) of all optimized stationary points


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Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 09, Revision B.1; Gaussian, Inc.: Wallingford CT, 2009. (51) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theoret. Chim. Acta 1990, 77, 123. (52) Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.; Schwerdtfeger, P.; Pitzer, R. M. Mol. Phys. 1993, 78, 1211. (53) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535 (54) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P. J. Chem. Phys. 1996, 105, 1052. (55) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.; Höllwarth, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem Phys Lett 1993, 208, 111. (56) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (57) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; DeFrees, D. J.; Pople, J. A.; Gordon, M. S. J. Chem. Phys. 1982, 77, 3654. (58) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. R. J Comput Chem 1983, 4, 294. (59) Hariharan, P. C.; Pople, J. A. Theoret. Chim. Acta 1973, 28, 213. (60) Fritsch, J.; Zundel, G. J. Phys. Chem. 1981, 85, 556. (61) Fukui, K. Acc Chem Res 1981, 14, 363. (62) Hratchian, H. P.; Schlegel, H. B. In Theory and Applications of Computational Chemistry; Frenking, G., Kim, K. S., Scuseria, G. E., Eds.; Elsevier: Amsterdam, 2005, p 195. (63) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378.


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SYNOPSIS


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Mechanism of DimethylamineâBorane Dehydrogenation Catalyzed by

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