B-Methylated Amine-Boranes: Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions. Inorganic , (22),

Although the dehydrogenation chemistry of amine-boranes substituted at nitrogen has attracted considerable attention, much less is known about the reactivity of their B-substituted analogs. When the B-methylated amine-borane adducts, RR′NH∙BH 2 Me ( 1a : R = R′ = H; 1b : R = Me, R′ = H; 1c : R = R′ = Me; 1d : R = R′ = i Pr) were heated to 70 °C in solution (THF and toluene), redistribution reactions were observed involving the apparent scrambling of the methyl and hydrogen substituents on boron to afford a mixture of the species RR′NH∙BH 3-x Me x (x = 0 – 3). These reactions were postulated to arise via amine-borane dissociation followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of 1a - 1d with Rh(I), Ir(III) and Ni(0) precatalysts in THF at 20 °C resulted in an array of products, including aminoborane RR′N=BHMe, cyclic diborazane [RR′N-BHMe] 2 , and borazine [RN-BMe] 3 based on analysis by in situ 11 B NMR spectroscopy, with peak assignments further supported by DFT calculations. Significantly, very rapid, metal-free hydrogen transfer between 1a and the monomeric aminoborane, i Pr 2 N=BH 2 , to yield i Pr 2 NH∙BH 3 (together with dehydrogenation products derived from 1a ) was complete within only 10 min. at 20 °C in THF, substantially faster than for the

methyl substituent on boron rather than on nitrogen, the process was more thermodynamically favorable and the activation energy barrier was reduced.

Introduction
Amine-boranes (RR′R′′N·BH3, R, R′, R′′ = H, alkyl or aryl) are isoelectronic with alkanes and have been the recent focus of intense attention as a result of their potential applications in hydrogen storage and transfer, as well as precursors to new inorganic materials. 1 For example, polyaminoboranes, n, which are structurally analogous and isoelectronic to polyolefins, represent an interesting class of polymers accessed from amine-boranes that may possess useful piezoelectric or preceramic properties. 2 Dehydrocoupling of amine-boranes, where the release of hydrogen is accompanied by the formation of new B-N bonds, can be performed thermally, 1c using stoichiometric amounts of a hydrogen acceptor, 1i-l, 3 or much more efficiently with a variety of Rh, 4 Ir, 2d, 5 Ni, 6 Ti, 7 Fe, 6c, 8 Re, 9 Ru 10 precatalysts as well as with other transition metal 11 and main-group species. 12 Under these conditions, ammonia-borane, NH3·BH3, as well as primary, RNH2·BH3, and secondary amine-boranes, RR′NH·BH3 (R, R′ = alkyl, aryl) readily eliminate one equivalent of hydrogen to yield aminoboranes, RR′N=BH2, which can either be stable as a monomer or undergo oligomerization to yield linear RR′NH- x-BH3 or cyclic x borazanes (x = 2 or 3) (Scheme 1). 13 Alternatively, high molecular weight polyaminoboranes, n can be formed if the precursor is ammonia-borane (R = H) or a sterically unhindered primary amine-borane (R = Me, Et, nBu) and the catalyst is selective for dehydropolymerization. 2d, 4e, 5c, 6a, 14 Under most circumstances, elimination of two equivalents of hydrogen occurs with ammonia-borane and primary amineboranes to yield borazines, [RN-BH]3 (Scheme 1). 15

Scheme 2: Hydrogen transfer between Me2NH•BH3 and iPr2N=BH2.
Although a wide range of N-substituted amine-boranes are known and have been investigated extensively, 4a to date, far fewer examples of B-substituted amine-boranes have been studied. The geometries and dissociation energies of NH3•BHxMe3-x and MexNH3-x•BH3 (x = 0 -3) were theoretically predicted by Boutalib and co-workers whom suggested that the stability of the amine-borane increased upon inclusion of a methyl group at nitrogen with a corresponding decrease when boron possesses methyl substituents. 18 Dixon and co-workers investigated the dehydrogenation energies of the same set of amine-boranes via DFT calculations and reported that having a methyl substituent at boron led to an exothermic dehydrogenation reaction whereas at nitrogen, the process was close to thermoneutral. 19 In addition, our group has previously reported that the B-pentafluorophenyl substituted amine-borane, iPr2NH•BH(C6F5)2 undergoes thermal dehydrogenation at 100 °C to yield the aminoborane iPr2N=B(C6F5)2. 20 We have also synthesized a series of B-thioaryl substituted amine-boranes, including iPr2NH•BH2SR (R = Ph, C6F5) which underwent both thermal and catalytic dehydrogenation to yield B-thioaryl substituted aminoborane, iPr2N=BHSR. 21 Synthesis of two B-methylated amine-boranes, Me3N•BH2Me and Me2NH•BH2Me, has been previously reported by Paul and Roberts,22 and Beachley and Washburn,23 although no further studies of their reactivity were described.
Herein, we report the synthesis of a series of B-methylated amine-boranes (1a: NH3•BH2Me; 1b:  The B-methylated amine-boranes were isolated as either colorless solids (1a, 1b, 1d) or as a liquid (1c) and selected characterization data is given in Table 1. 25 As expected, one signal was observed in the 11 B NMR spectra in the range -8.5 to -15.1 ppm as a triplet ( 1 JBH = 94-96 Hz), for 1a-1d in CDCl3 (Figures S1, S3, S6, S9). The observed chemical shift and coupling pattern was indicative of a four-coordinate boron center with two hydrogen substituents. Comparison of the 11 B NMR chemical shifts of 1a-1d to the analogous amine-boranes; ammonia-borane, NH3•BH3, N-methyl amine-borane, MeNH2•BH3, N-dimethyl amine-borane, Me2NH•BH3 and Ndiisopropyl amine-borane, iPr2NH•BH3, indicated that a downfield shift was observed upon the replacement of a hydrogen for a methyl group at boron (Table 1). 4a The observed 1 H ( Figures S2,   S4, S7, S10) and 13 C ( Figures S5, S8, S11) NMR spectra were unremarkable, but consistent with the assigned structures.  (Figure 1a). 25 In an analogous fashion, crystals of 1b and 1d were grown that were also suitable for X-ray diffraction (Figures 1b and   2). 25  The B-N bond length for 1a was determined to be 1.614(3) Å, which was slightly elongated compared to that of 1b (1.605(2) Å), presumably as the electron donating methyl group at nitrogen strengthened the dative bond from nitrogen to boron (Table 2). On the other hand, the inclusion of two sterically bulky isopropyl groups at nitrogen (in 1d) significantly lengthened the B-N bond to 1.6333(6) Å. Although the increased electron donating ability of the isopropyl groups would be expected to strengthen the dative bond, the steric effect of the bulky alkyl groups dominates, thereby weakening the B-N bond. The B-N bond lengths for 1a, 1b and 1d were found to be significantly longer than for analogous amine-boranes without a methyl group at boron; with reported B-N distances of 1.58(2), 1.5936(13) and 1.5965(13) Å for NH3•BH3, 27 MeNH2•BH3 and Me2NH•BH3, respectively. 28 This was a likely consequence of an increase in electron density at boron induced by the electron donating methyl group, which reduced the strength of the B-N dative bond. Both the boron and nitrogen atoms of 1b were found to exhibit distorted tetrahedral geometries, consistent with approximate sp 3 hybridization, and the C1-N1-B1-C2 chain was found to adopt a gauche conformation with a dihedral angle of 178.27°. Longer-distance, non-covalent intermolecular interactions were observed for 1a between adjacent amine-boranes, with lengths in the range of 2.08-2.34 Å between the hydridic and protic hydrogen atoms on boron and nitrogen, respectively ( Figure 3 and Table 3). These distances were shorter than twice the Van der Waal radius for two hydrogen atoms (2.4 Å). Dihydrogen intermolecular interactions were also detected for amine-boranes 1b and 1d (Figures S61 and   S62 and Table 3). The H-H-N angles were determined to be relatively linear (

Thermally-Induced Redistribution and Dehydrogenation Reactions of 1a-1d
To determine the thermal stability of the B-methylated amine-boranes, a solution of 1a-1d in either a coordinating (THF) or a weakly coordinating (toluene) solvent was monitored by 11 B NMR spectroscopy at ambient (20 °C) and elevated (70 °C) temperatures over various periods of time. The assignment of the product signals detected by 11 B NMR spectroscopy was further supported by literature chemical shift data (δB,lit), where available, as well as DFT calculations of 11 B NMR chemical shifts (δB,calc) and 1 JBH coupling constants (Table S4). 25

(a) Thermally-Induced Redistribution and Dehydrogenation Reactions of 1a
The thermal stability of 1a at ambient temperature was investigated by 11 B NMR spectroscopy.
No reaction was observed for 1a in a THF solution after 170 h at 20 °C ( Figure S16). However, under analogous conditions in a toluene solution, significant amounts of products derived from the redistribution of the methyl and hydrogen substituents at boron were observed; in addition to unreacted 1a (ca. 40 %), the appearance of singlet, doublet and quartet peaks at -6.1, -9.5 and -  Figure S17).

°C.
As well as the products arising from methyl and hydrogen redistribution at boron for 1a, a broad 11 B NMR peak with no detectable proton coupling was observed at δB,exp 46.3 (s The redistribution reaction of 1a in toluene was also studied at elevated temperature (70 °C).  Figure S18). Similar redistribution products were observed when heating 1a to 70 °C in THF after 24 h ( Figure S19).

(b) Thermally-Induced Redistribution and Dehydrogenation Reactions of 1b-1d.
In contrast to the redistribution of the methyl and hydrogen substituents at boron observed for 1a in toluene at 20 °C, no corresponding reaction was detected for solutions of 1b-1d in either coordinating (THF) or weakly coordinating (toluene) solvents after 170 h by 11 B NMR spectroscopy ( Figures S20-S25). The stability of 1b-1d at elevated temperatures was then investigated; a toluene solution of 1b-1d was heated to 70 °C until quantitative consumption of the amine-borane was detected. As the steric bulk of the alkyl groups increased at nitrogen, the reaction time for complete consumption of the B-methylated amine-borane increased from 48 h (1b) to 170 h (1c) to 500 h (21 days) (1d), as monitored by 11 B NMR spectroscopy.
Similar to the case of 1a, thermolysis of 1b in toluene at 70 °C led to the formation of a complex mixture of redistribution and dehydrogenation products by 11 B NMR spectroscopy, although the process was slower (48 h Figure S25). Analogous redistribution and dehydrogenation products were detected for the thermolysis reaction of 1c ( Figure S26) and for in toluene at 70 °C ( Figure S27). Similar reactivity was also observed for the thermolysis of 1b-1d in the more coordinating solvent, THF, at 70 °C over a period of 170 -340 h (Figures S29-S31).
In contrast to the results for 1a, a significantly faster reaction was observed for 1b with 5 mol%  Figure   S39).
No reaction was observed for the attempted dehydrogenation of 1d with 5 mol% IrH2 (POCOP) in THF at 20 °C after 120 h by 11 B NMR spectroscopy ( Figure S42). In contrast, 1d underwent dehydrogenation to yield the aminoborane, iPr2N=BHMe, as the sole product, using 10 mol% skeletal nickel in THF at 20 °C, as observed by 11 B NMR spectroscopy. Nevertheless, the reaction proceeded significantly slower compared to the reaction of 1d with [Rh(COD)(μ-Cl)]2 as a precatalyst, with 70 % conversion being observed after 210 h (Scheme 8b, Figure S43).

Attempted Dehydropolymerization of 1a and 1b with Skeletal Nickel
We have previously prepared high molecular weight poly(N-methyl aminoborane), [  Figure S44). After allowing the reaction to continue for 24 h, Next, we explored the analogous reaction of 1b with 100 mol% skeletal nickel in THF at 20 °C.
However, each set of 11 B and 11 B{ 1 H} NMR spectra also showed the presence of several other peaks, of which some were not successfully assigned. Nonetheless, one very minor product, generally detected at a level corresponding to ca.  Figure S50). Consistent with these results, the interception of NH2=BHMe by cyclohexene almost completely prevents the formation of the oligomer, [NH2- As with the case of 1a as a hydrogen donor, a stoichiometric amount of iPr2N=BH2 (in THF) was added to solid 1b at 20 °C. The reaction was monitored by 11 B NMR spectroscopy with 90 % hydrogenation of iPr2N=BH2 to iPr2NH•BH3 detected after 10 min. and subsequent formation of MeNH=BHMe, [MeNH-BHMe]2 and (MeNH)2BMe (Scheme 11a, Figure S51). No further reaction was detected by 11 B NMR spectroscopy after 1 h ( Figure S52).

Scheme 11: Hydrogen transfer of (a) 1b, (b) 1c and (c) 1d with iPr2N=BH2 in THF at 20 °C.
Similar to the case of the hydrogen transfer reaction between 1a and iPr2N=BH2, trapping of the  Figure S53).
Slower hydrogen transfer was detected between 1c and iPr2N=BH2, with 45 % hydrogenation after 10 min. in THF at 20 °C by 11 B NMR spectroscopy. As well as the formation of iPr2NH•BH3, the dehydrogenated product Me2N=BHMe was also detected (Scheme 11b, Figure   S54). After 1 h, further hydrogenation (72 %) was observed as well as the appearance of the cyclic diborazane, [Me2N-BHMe]2 ( Figure S55). In the case of 1c, the presence of cyclohexene resulted in no new products corresponding to the trapped aminoborane, Me2N=BMeCy, being observed for the reaction of 1c with one equivalent of iPr2N=BH2 and two equivalents of cyclohexene in THF at 20 °C after 1 h by 11 B NMR spectroscopy ( Figure S56).
However, after 24 h, a small amount of hydrogen transfer was detected, as shown by 11 B NMR spectroscopy with hydrogenation of iPr2N=BH2 to iPr2NH•BH3 and concomitant dehydrogenation of 1d to iPr2N=BHMe (Scheme 11c, Figure S58). The reaction progressed until an apparent equilibrium was established 1j after 56 days with 95 % hydrogenation of iPr2N=BH2 ( Figure S59).
The redistribution of substituents at boron in three-coordinate boranes is well-known and the mechanism has been proposed to occur via a diborane intermediate. 38   and toluene (exp = -15.7) suggests that such an effect, if it exists, has only a small effect on the B-N bond strength. The B-methylated amine-boranes 1a-1d were also calculated to be slightly more stable as a solution in THF than in toluene, by 8.0 -13.8 kJmol -1 (Table S5). 25 As a more polar solvent, THF probably provides a solution environment in which the amine-borane with polar N-H and B-H bonds is more stable, thereby providing a possible explanation why redistribution was not observed for 1a in THF at 20 °C.

Scheme 13: Structure of postulated THF stabilized B-methyl amine-borane (1a•THF).
Redistribution of methyl and hydrogen substituents in the B-methylated amine-boranes 1b-1d was not detected in toluene at 20 °C after 170 h, but scrambling was observed at 70 °C, with a rate for complete consumption of the amine-borane in the order 1b > 1c > 1d (see section 2.2b).
The electron donating alkyl groups at nitrogen would be expected to strengthen the dative bond between the nitrogen and boron, resulting in an increased resistance to dissociation. This was supported by DFT calculations for 1b and 1c, where the bond dissociation energies in toluene increased from +124.9 kJmol -1 for 1a to +142.9 and +150.5 kJmol -1 for 1b and 1c, respectively (Table 4). However, the bond dissociation energy for 1d (+128.7 kJmol -1 ) was calculated to be close to that for 1a. This was not unexpected as the B-N bond length for 1d determined by X-ray diffraction was elongated compared to 1a and 1b (Table 2), where the steric effect of the isopropyl groups on nitrogen appeared to have a greater influence on the bond distance than their electron donating characteristic. The much lower reactivity of 1d towards redistribution despite the apparently weaker B-N bond was likely a consequence of the unfavorable formation of the redistribution products, iPr2NH•BHMe2 and iPr2NH•BMe3, on steric grounds. Indeed, neither species were detected on thermolysis at 70 °C after 500 h. Instead, the exclusive formation of their dehydrogenated derivatives iPr2N=BMe2 and iPr2N=BHMe was evidenced, together with iPr2NH•BH3 ( Figure S27). This suggests that in the case of 1d, the reaction was driven by the subsequent dehydrogenation of the initially formed, sterically disfavored redistribution products and the very slow amine-borane consumption rate was a likely consequence.  1d 500 +128.7

Catalytic Dehydrocoupling Reactions of 1a-1d
The reactivity of the B-methylated amine-boranes 1a-1d with respect to catalytic dehydrocoupling/dehydrogenation varied significantly (see section 2.3). As previously noted, DFT calculations highlight that the inclusion of a methyl group at boron energetically favors dehydrogenation. 19 Favorable kinetics were also apparent, with shorter reaction times being observed for B-methylated amine-borane dehydrocoupling/dehydrogenation, compared to analogous amine-boranes without a methyl group at boron.

Attempted Dehydropolymerization of 1a and 1b with Skeletal Nickel
Our group have previously reported the reaction of MeNH2•BH3 with a stoichiometric amount of skeletal nickel, whereby after 2 h in THF at 20 °C, high molecular weight poly(N-methyl aminoborane) was isolated with 60 % yield. 6a With the aim of isolating the first high molecular weight polyaminoborane with non-hydrogen substituents at boron, the amine-boranes 1a and 1b were therefore also treated with a stoichiometric amount of skeletal nickel in THF at 20 °C. In the case of 1a, partial consumption of the amine-borane after 1 h was detected, together with the presence of the tentatively assigned oligomer/polymer, x, observed as a broad peak between -7.8 and -9.1 ppm by 11 B NMR spectroscopy. However, after 24 h, both the tentatively assigned oligomer/polymer and the B-methylated amine-borane were no longer detectable and the final products were borazine, [NH-BMe]3, and bis(amino)borane (NH2)2BMe. A similar observation was noted for the reaction of 1b with 100 mol% skeletal nickel in THF at 20 °C. The methyl group at boron presumably increases the hydridic nature of the hydrogen cosubstituents at boron, favoring further dehydrogenation of oligo/poly(B-methyl aminoborane) to borazine.
Thus the oligomer/polymer was only observed as an intermediate before undergoing further dehydrocoupling to the more thermodynamically favorable borazine.

Hydrogen Transfer Reactions of 1a-1d with iPr2N=BH2
Hydrogen transfer was detected between B-methylated amine-boranes 1a-1d and iPr2N=BH2 in THF at 20 °C, although the reaction progressed dramatically slower in the case of 1d. To compare the hydrogen donating ability of 1a-1c to amine-boranes without a methyl group at boron, the percentage hydrogenation of iPr2N=BH2 to iPr2NH•BH3 was determined after 10 min. and 1 h by relative integration in the 11 B NMR spectra (Table 5). 1j, 45 After 10 min., at least 90 % hydrogenation of iPr2N=BH2 to iPr2NH•BH3 was detected using 1a and 1b as hydrogen donors.
For amine-boranes without a methyl group at boron, the percentage hydrogenation decreased significantly from 89 % (for NH3•BH3) to 4 % (for Me2NH•BH3) after 10 min.. In comparison, analogous amine-boranes with a methyl group at boron, larger values and a less substantial decrease were observed from 100 % (for 1a) to 45 % (for 1c).  20 °C. 1i, 1j This suggests that although the sterically encumbered nature of 1d results in a slow reaction, the favourable thermodynamics enables the reaction to reach near completion.
The increased hydrogen donating ability observed for 1a-1d towards iPr2N=BH2 is presumably a consequence of the presence of the electron donating methyl group at boron. The enhanced hydridic character of the hydrogen substituents at boron would be expected to lead to faster dehydrogenation reactions. The hydrogen transfer from 1a-1d also appears to be thermodynamically more favourable, which is in agreement with the previous work by Dixon and co-workers where dehydrogenation was calculated to be more exergonic as the number of methyl substituents at boron increased. 19 To further probe the mechanism of hydrogen transfer, DFT studies were conducted for 1a-1c with iPr2N=BH2 ( Figure 5, S65 and S66). 25 In each case, a concerted mechanism with a transition state of a six-membered ring was determined, indicating that both hydrogen atoms were transferred in the same step. An analogous transition state was identified for the hydrogen transfer reaction of Me2NH•BH3 with iPr2N=BH2. 1j In our previously investigated reaction of Me2NH•BH3 with iPr2N=BH2, the activation energy associated with the formation of the six-membered cyclic transition state was found to be +86.9 kJmol -1 , 46 which was higher than that calculated for 1a-1c (+64.7 to +76.0 kJmol -1 ) ( Table 5).
Hydrogen transfer from Me2NH•BH3 to iPr2N=BH2 was reported to be slightly endergonic (+9.1 kJmol -1 ), in contrast to the Gibbs free energies determined for 1a-1c, which were exergonic (-14.7 to -23.6 kJmol -1 ). Prior and subsequent to the formation of the cyclic transition state, encounter complexes were located, which were attributed to the presence of the cyclic hydrogen bonding. Although these complexes were determined to be slightly endergonic compared to initial and final products, the activation energy was effectively reduced, further favoring the hydrogen transfer reaction. This shows that both kinetically and thermodynamically, Bmethylated amine-boranes 1a-1c have an improved ability to function as hydrogen donors towards the aminoborane iPr2N=BH2 than Me2NH•BH3.
The hydrogen transfer reaction of 1a with iPr2N=BH2 resulted in the detection of very small quantities of the unexpected product, iPr2N=BHMe, with substituents apparently derived from each of the reactants. In addition, the borazine [NH-BH]3, with no methyl groups at boron, was also formed as another surprising product. The mechanism of formation for these species is unclear, 47 and is the subject of ongoing studies. Rapid dehydrocoupling of B-methylated amine-boranes with Rh(I), Ir(III) and Ni(0) precatalysts was detected to yield a variety of products including aminoboranes, cyclic diborazanes and borazines, based on in situ 11 B NMR spectroscopy with peak assignments supported by DFT calculations. In comparison to the well-studied catalytic dehydrocoupling of N-methylated amine-boranes, faster dehydrocoupling/dehydrogenation reactions were observed for the B-methylated analogs. This is suggested to be a consequence of the increased hydridic nature of the hydrogen atoms at boron induced by the electron donating methyl group.

Summary
Attempts to synthesize high molecular weight polyaminoboranes with a methyl substituent at boron were made via catalytic dehydropolymerization of 1a and 1b. However, any oligomeric or polymeric B-methylated species that formed under the reaction conditions appeared to readily undergo further dehydrogenation to yield mainly the B-methylated borazine, [RN-BMe]3 (R = H, Me). We are continuing our efforts in this area with the aim to increase the yield of poly(Bmethyl aminoborane) and other analogous materials under conditions where further dehydrocoupling is absent.
Very rapid hydrogenation of iPr2N=BH2 to iPr2NH•BH3 was observed for the B-methylated amine-borane, 1a, with complete hydrogen transfer observed after 10 min. in THF at 20 °C, with very good hydrogenation rates also observed for 1b and 1c. The hydrogen donating ability of Bmethylated amine-boranes was significantly increased compared to amine-boranes without a methyl group at boron. DFT calculations revealed that the pathway for hydrogen transfer occurred via a cyclic six-membered transition state. In addition, the reaction was determined to be exergonic with a lower activation energy barrier than for our previous model using Me2NH•BH3 as the hydrogen donor. These results show that both kinetically and thermodynamically, B-methylated amine-boranes 1a-1c have an improved hydrogen donating ability towards the aminoborane iPr2N=BH2 than the previously investigated amine-borane, Me2NH•BH3. Our ongoing studies focus on transfer hydrogenations involving B-methylated amine-boranes with unsaturated organic substrates.