Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Dehydrogenation of Dimethylamine−Borane Catalyzed by HalfSandwich Ir and Rh Complexes: Mechanism and the Role of Cp* Noninnocence Shrinwantu Pal, Shuhei Kusumoto, and Kyoko Nozaki* Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *
ABSTRACT: Half-sandwich Cp*RhIII complexes (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienyl) supported by 2,2′bipyridine or 4,4′-di-tert-butyl-2,2′-bipyridine catalyze dehydrogenation of dimethylamine−borane (Me2NH·BH3) to produce H2 and dimethylamino−borane dimer (Me2NBH2)2 with turnovers of 2200. The IrIII analogues, on the other hand, display dramatically poorer catalytic activity. Mechanistic inferences drawn from stoichiometric reactions and DFT calculations suggest noninnocent involvement of the Cp* moiety as a proton shuttle. by Cp*RhIII complexes27,28 and that of amine−boranes by Cp*supported group 4 complexes29 have been noted. Here we report that half-sandwich Ir and Rh complexes supported by 2,2′-bipyridine and 4,4′-di-tert-butyl-2,2′-bipyridine ligands (i.e., [1-M(THF)](OTf)2 and [1′-M(THF)](OTf)2, respectively, shown in Scheme 1) are active in the catalytic dehydrogenation of dimethylamine−borane (Me2NH· BH3, DMAB), producing dimethylamino−borane dimer (Me2N−BH2)2 and H2 as products of dehydrogenation. Stoichiometric reactions and DFT calculations suggest that
1. INTRODUCTION The use of dihydrogen (H2) as a cleaner alternative to traditional fuels necessitates the development of materials1 that can produce H2 “on demand”. Owing to their low molecular weight and kinetic stability, amine−boranes (R2NH·BH3; R = H, Me, etc.) are attractive candidates.2,3 Although resistant to direct protonolysis of the B−H fragment by the protic N−H fragment,4 amine−boranes can be dehydrogenated to produce amino−boranes (R2NBH2) by employing transition-metal catalysts.5 To this end, group 9 metal complexes6−13 have been employed as some of the most active homogeneous amine− borane dehydrogenation catalysts. For many group 9 catalysts it has been proposed that coordination of the B−H fragment to M14−18 facilitates protonolysis by the N−H fragment, leading to H2 evolution and concomitant formation of an additional B− N bond. Half-sandwich Cp*M (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienyl) complexes of group 9 metals (M = Ir, Rh, etc.) are an important class of organometallic complex with a demonstrated ability to serve as catalysts for both hydrogenation (e.g., of esters19 and carboxylic acids20) and dehydrogenation (e.g., of alcohols21,22 and C−C bonds23) reactions. It is intriguing that despite examples of well-defined σ(B−H)−M interactions in half-sandwich complexes24,25 in the literature, the potential of this conveniently prepared26 important class of complex in amine−borane dehydrogenation has, to the best our knowledge, not previously been reported. To this end, catalytic dehydrogenation of phosphine−boranes © XXXX American Chemical Society
Scheme 1. Dehydrogenation of Dimethylamine−Borane Using Half-Sandwich Complexes of Ir and Rh Supported by Bipyridyl Ligands (R = H, tBu)
Received: December 12, 2017
A
DOI: 10.1021/acs.organomet.7b00889 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics noninnocent involvement of the Cp* fragment enhances activity by acting as a proton shuttle.30
Table 1. Dehydrogenation of DMAB Catalyzed by Cp* Ir and Rh Complexes at 50 °C in THFa
2. RESULTS AND DISCUSSION 2.1. Dehydrogenation of DMAB Catalyzed by Cp* IrIII and RhIII Complexes. Catalyst solutions in THF were prepared by treatment of the corresponding chloride precursors (i.e., [1-M(Cl)]Cl and [1′-M(Cl)]Cl; M = Ir, Rh) with AgOTf in THF (see the Experimental Section). Presumably, in coordinating solvents such as THF, the active catalysts are 18e− solvento complexes, i.e. [1-M(solv)](OTf)2 and [1′M(solv)](OTf)2, with solv = THF coordinated to the MIII center. Indeed, X-ray diffraction of single crystals grown from acetonitrile revealed acetonitrile coordinated to the RhIII center (Figure 1).
entry cat. 1 2 3 4 5 6 7
[1-Ir(THF)] (OTf)2 [1′-Ir(THF)] (OTf)2 [1-Rh(THF)] (OTf)2 [1′Rh(THF)] (OTf)2 [1′Rh(THF)] (OTf)2 [1′Rh(THF)] (OTf)2 AgOTf
concn of DMAB (M)
loading (mol %)
time (h)
TONb
TOF (h−1)c
4.0
0.1
10
20
2
4.0
0.1
10
28
3
4.0
0.1
10
244
24
4.0
0.1
10
516
52
8.0
0.025
24
1153
48
8.0
0.025
48
2240
47
24
0
0
8.0
a
Reactions performed in screw-cap NMR tubes attached to a prepurged N2 manifold equipped with a bubbler. bTONs (defined as moles of DMAB consumed/mole of catalyst) are an average of two experiments calculated from 11B NMR spectroscopy. cTOFs were rounded to the nearest integer.
At 50 °C, using 4.0 M THF solutions of DMAB with 0.1 mol % loading, the Ir complexes still demonstrated poor catalytic activity (Table 1, entries 1 and 2). In sharp contrast, the Rh complexes [1-Rh(THF)](OTf)2 and [1′-Rh(THF)](OTf)2 were found to be appreciably active under identical conditions, furnishing TONs of 244 and 516 in 10 h corresponding to TOFs (h−1) of 24 and 52 (entries 3 and 4). Some of the evolved gas was collected in a gas bag, analyzed by GC-BID, and confirmed to be H2 by comparison with an authentic sample. To justify the use of integral intensities derived from 11 B NMR spectroscopy as a convenient tool for the evaluation of conversion of DMAB to dimethylamino−borane, an independent experiment was conducted in THF-d8 with benzene as internal standard; TONs calculated from 1H NMR and 11B NMR spectroscopy were found to be in excellent agreement with each other (see Figure S10 in the Supporting Information). Using 8.0 M DMAB solutions and a lower catalyst loading of 0.025 mol % of [1′-Rh(THF)](OTf)2, TONs of 1153 and 2245 were observed in 24 and 48 h (Table 1, entries 5 and 6, respectively). As a control reaction, as well as to rule out the involvement of Ag-containing species in the catalysis, a reaction
Figure 1. X-ray structure of a representative solvento complex [1′Rh(solv)](OTf)2 (solv = acetonitrile). Triflate counterions are omitted.
First, the activities of the complexes [1-Ir(THF)](OTf)2 and [1-Rh(THF)](OTf)2 were roughly estimated at 25 °C. For Ir, when 0.1 mol % of [1-Ir(THF)](OTf)2 was treated with DMAB in THF, a bright yellow solution was observed which persisted for at least 24 h. According to 11B NMR spectroscopy, no conversion of DMAB (−14.4 ppm, q, 1JBH = 97 Hz) was observed. On the other hand, for Rh, a purple solution (see Mechanistic Studies) formed immediately upon addition of DMAB to 0.1 mol % of [1-Rh(THF)](OTf)2 in THF at room temperature, which over a few hours progressed to a wine red coloration. After the mixture was stirred for 24 h at 25 °C, a small amount of the cyclic dimer of dimethylamino−borane, i.e. (Me2N−BH2)2, appeared as a triplet (1JBH = 114 Hz) at 4.6 ppm in the 11B NMR spectrum (see Figure 2 for a related analysis of the 11B NMR resonances). Subsequently, dehydrogenation of DMAB was examined at 50 °C. Representative results are summarized in Table 1.
Figure 2. Stacked 11B NMR spectra showing evolution of (Me2N−BH2)2 as the major B-containing product from dehydrogenation of DMAB. Minor amounts of unidentified byproducts were observed (see the Supporting Information for these signals). B
DOI: 10.1021/acs.organomet.7b00889 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
after which progressive suppression in reaction rate was observed, ultimately leading to saturation at ∼10 h. On the basis of the available headspace in the NMR tube, the H2 pressure at this point was calculated to be approximately 5 atm. The reaction could not be reversed, and under 4 MPa of H2, catalyst decomposition into a fine black precipitate and sequentially hydrogenated Cp* ligand was observed (see Figure S12 in the Supporting Information). 2.2. Mechanistic Studies. Starting from the THF-bound complex, i.e. [1-Rh(THF)]2+ as shown in Scheme 2, we
was performed using AgOTf (entry 7) and no conversion was observed. To evaluate the time course and the temperature dependence of catalyst activity, the evolution of (Me2N−BH2)2 from DMAB catalyzed by 0.1 mol % of [1′-Rh(THF)](OTf)2 in the range of 50−70 °C was continuously monitored by 11B NMR spectroscopy in 1,2-dimethoxyethane (DME) every 30 min over a period of 10 h. DME was chosen instead of THF to allow experiments at ≥60 °C. A representative stacked plot of the 11B NMR spectra shown in Figure 2 shows formation of (Me2N−BH2)2 as the major B-containing species over 10 at 1 h intervals. Minor amounts of unidentified byproducts were observed at 1.3, 20, and 27 ppm (see Figure S11 in the Supporting Information). A trace amount of Me2NBH2, appearing as a triplet at 37 ppm (1JBH = 130 Hz), was also observed. Average TOFs of 45, 50, and 62 h−1 (corresponding to TONs of 453, 504, and 622) were observed at 50, 60, and 70 °C, respectively, indicating a nominal dependence of the overall TOFs on temperature. The time course plots shown in Figure 3a demonstrate sustained catalyst activity under the reaction
Scheme 2. Mechanisms of DMAB Dehydrogenation Catalyzed by Half-Sandwich Ir and Rh Complexesa
a
Triflate counterions have been omitted for clarity.
presumed that formal transfer of hydride from the B center of DMAB to the MIII center could occur, especially since welldefined examples of σ(B−H)−M interactions24,25,31 have been reported for similar complexes in the literature. Such a transfer (step A) would result in the formation of stoichiometric amounts of the monocationic M−hydride intermediate [2-M]+ and Me2NH−BH2(THF)+ cation.32,33 To liberate H2 and form amino−borane, i.e. Me2NBH2 or (Me2N−BH2)2, as dehydrogenated products, two mechanisms, namely, a direct protonolysis pathway (step A−B, highlighted in gray) or an alternate Cp* noninnocence mediated pathway (step A−C−D−F−G), were hypothesized. Since B to MIII hydride transfer would also generate stoichiometric amounts of an acidic [Me2NH−BH2(THF)]+ cation,34 the evolution of H2 via the direct protonolysis pathway involves straightforward protonolysis of the M−H fragment. Alternatively, starting from [2-M]+, the reaction may also proceed via a sequence of H shuttling mediated by Cp* noninnocence. Recently, the noninnocent behavior of the Cp* fragment in similar Rh−H complexes,35−39 as opposed to its apparent innocence in related Ir−H complexes, has been the subject of renewed attention.
Figure 3. Comparison of time courses of DMAB dehydrogenation catalyzed by 0.1 mol % of [1′-Rh(THF)](OTf)2 in dimethoxyethane: (a) at 50 °C (blue circles), 60 °C (green diamonds), 70 °C (orange squares), and 70 °C under 1 atm of H2 (red triangles); (b) at 50 °C with H2 allowed to escape (green circles) and in a sealed high-pressure tube (orange squares).
conditions employed in our study. Neither an induction period nor significant deceleration of the reaction was observed. It is notable that allowing the escape of H2 from the reaction system was essential for the effective conversion of DMAB to (Me2N− BH2)2. When the reaction was performed at 70 °C under 1 atm of H2, in 10 h a TON of 554 was observed (as opposed to 622 when the reaction was performed under argon). In order to investigate the possibility of completely shutting down the reaction by allowing the buildup of H2, we performed an otherwise identical reaction in a sealed high-pressure NMR tube. Figure 3b shows a comparison of the time courses observed at 50 °C. The reactions behaved similarly up to 1 h, C
DOI: 10.1021/acs.organomet.7b00889 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 4. Characteristic 1H NMR resonances observed after reaction of [1-M(THF)](OTf)2: M = Ir (a, at 25 °C) and M = Rh (b, at −50 °C) with DMAB in THF-d8. See the Supporting Information for detailed assignments and 1H−13C HSQC and 1H−1H COSY spectra.
reported that addition of a strong acid such as DMF-H+ (DMF = N,N-dimethylformamide) to [2-Rh]+ led to immediate H2 evolution, while weaker acids such as Et3NH+ did not.36 2.2.1.3. Step C (Cp*−H Reductive Elimination). In contrast to the clean formation of [2-Ir](OTf) from [1-Ir](OTf)2, treatment of [1-Rh](OTf)2 in THF-d8 with 10 equiv of DMAB afforded a purple solution containing two species, attributable to [2-Rh](OTf) and [3-Rh](OTf),35,36,45 as shown in Scheme 2. A doublet at −9.27 ppm (1JHRh = 21 Hz) in the 1H NMR spectrum (labeled as Ha in Figure 4(b)) was assigned to the Rh−H fragment of [2-Rh](OTf). A singlet corresponding to the Cp* fragment (labeled as Hb) of [2-Rh](OTf) was also observed. Interestingly, a characteristic doublet at 0.65 ppm (3JHH = 5.8 Hz) corresponding to the CH3(CH) fragment of the η4-Cp*H moiety of [3-Rh](OTf) (labeled as Hc) was observed (see Figure S17 in the Supporting Information for details). Consistent with the assigned structure of [3-Rh](OTf), Hc exhibited a vicinal coupling to the CH3(CH) fragment (labeled Hd) in the 1H−1H COSY NMR spectrum (see Figure S18 in the Supporting Information). Singlets corresponding to the CH3 fragments (labeled as He and Hf) of [3-Rh](OTf) could also be assigned on the basis of previous reports35 and 1H−13C HSQC NMR spectroscopy (see Figure S19 in the Supporting Information). It is interesting that while Miller35 reported complete conversion of the related [2-Rh(Cl)] complex to form [3Rh(Cl)] at room temperature, in our case, at 25 °C, the complexes [2-Rh](OTf) and [3-Rh](OTf) were found to be in a 1:1 equilibrium according to 1H NMR spectroscopy (see Figure S20 in the Supporting Information). We presumed that coordination of chloride was responsible for offering some energetic advantage to the RhI center, leading to clean formation of [3-Rh(Cl)]. We rationalized that, in the presence of a noncoordinating anion such as triflate, the observation of an ∼1:1 mixture of [2-Rh](OTf) and [3-Rh](OTf) might be justifiable. Indeed, immediately upon treatment of [3-Rh(Cl)]35 with NaOTf in THF-d8, formation of an ∼1:1 mixture of [2-Rh](OTf) and [3-Rh](OTf) was identified by the observation of a broad resonance at −9.3 ppm (Rh−H of [2Rh](OTf)) and a quartet at 2.8 ppm (3JHH = 5.8 Hz; CH3(CH) fragment of the η4-Cp*H moiety of [3-Rh](OTf)) in ∼1:1 integral intensity (see Figure S21 in the Supporting
Is the higher activity in the catalytic dehydrogenation of DMAB observed for the Rh complexes simply a function of the differences in the hydricities of otherwise similar Rh−H and Ir−H complexes,40−42 or is it a consequence of Cp* noninnocence? Experimentally, we observed noninnocent behavior of Cp* in the Rh complexes under catalytic conditions. Computationally, we show that noninnocent involvement of Cp* can mediate H shuttling, effectively lowering the reaction barrier required for M−H protonolysis. For the Ir complexes no apparent advantage is offered by Cp* involvement. 2.2.1. Experimental Studies. 2.2.1.1. Step A (B to MIII Hydride Transfer). Reaction of [1-Ir(THF)](OTf)2 with 10 equiv of DMAB in THF-d8 at 25 °C resulted in the immediate conversion to the IrIII−H complex [2-Ir](OTf). The formation of [2-Ir](OTf) was confirmed by comparing the unremarkable but characteristic singlet resonances corresponding to the Ir−H (δH −11.5 ppm) and Ir−Cp* (δH 1.9 ppm) fragments observed in the 1H NMR spectrum (see Figure 4a) to that reported previously43 (see Figure S13 in the Supporting Information for details). A broad triplet (THF-d8, δB 3.5 ppm, 1JBH = 115 Hz) observed in the 11B NMR spectrum (see Figure S14 in the Supporting Information) of the reaction mixture was assigned to the solvent-bound boron-containing product of this reaction, [Me2NH−BH2(THF-d8)](OTf). Consistent with the assignment of this 11B NMR resonance to the solvated [Me2NH− BH2(THF)]+ cation, as opposed to the reported related zwitterionic Me2NH−BH2−OTf (CD2Cl2, δB −0.4 ppm, 1JBH = 121 Hz),34 we observed a shift of the 11B NMR resonance from 0.18 ppm (see Figure S15 in the Supporting Information) to 4.01 ppm (see Figure S16 in the Supporting Information) upon addition of a drop of THF to a CD2Cl2 solution of Me2NH−BH2−OTf prepared freshly, as reported by Manners.34 A similar cationic ether-bound [Me2NH−BH2(OEt2)](BArF) species was reported to also exhibit a similar δB value of 3.7 ppm.34 2.2.1.2. Step B (H2 Evolution via Direct Protonolysis of MIII−H). Although Me2NH−BH2−OTf is acidic enough to protonate amines or pyridines,34 Fukuzumi’s reports are worth noting, wherein the Ir−H fragment of [2-Ir]+ could only be protonolyzed at pH
