Variable coordination modes and catalytic dehydrogenation of B -phenyl amine – boranes †

The chemistry of N -substituted amine – boranes and their reactivity towards transition metal centres is well established but the chemistry of B -substituted amine – boranes is not. Here we present the coordination chemistry of H 2 PhB·NMe 3 towards a range of Rh( I ) fragments with di ﬀ erent P – Rh – P ligand bite angles, {Rh(P i Pr 3 ) 2 } + , {Rh(P i Bu 3 ) 2 } + , {Rh( i Pr 2 P(CH 2 ) 3 P i Pr 2 )} + , {Rh(Ph 2 P(CH 2 ) n PPh 2 )} + ( n = 3, 5), as characterised by NMR spectroscopy and single-crystal X-ray di ﬀ raction. This reveals a di ﬀ erence in the coordination mode of the amine – borane, with large bite angle fragments favouring η 2 -coordination through a sigma-interaction with BH 2 , whereas fragments with small bite angles favour η 6 -coordination through the aryl group of the amine – borane. The catalytic dehydrocoupling of H 2 PhB·NMe 2 H is also explored, with the aminoborane HPhB v NMe 2 found to be the sole dehydrogenation product. Stoichiometric reactivity with H 2 PhB·NMe 2 H again showed small bite angle fragments to prefer η 6 -aryl coordination, while the larger bite angle {Rh(P i Pr 3 ) 2 } + gave rapid dehydrogenation to form a mixture of the Rh( III ) dihydride [Rh(P i Pr 3 ) 2 (H) 2 ( η 2 -H 2 PhB·NMe 2 H)][BAr F4 ] and the low coordinate aminoboryl complex [Rh(P i Pr 3 ) 2 (H)-(BPhNMe 2 )][BAr F4 ]. These results suggest that precatalysts which η 6 -bind arenes strongly should be avoided for the dehydrocoupling of amine – boranes bearing aryl substituents.


Introduction
Amine-boranes, defined by the simplest example H 3 B·NH 3 , have been the subject of significant interest and research effort in the past decade with regard to their potential as molecular hydrogen storage materials (i.e. dehydrogenation) 1 and as precursors to polyaminoboranes (i.e. dehydrocoupling). 2 Much of this research has focussed on developing homo-and heterogeneous catalytic methodologies for dehydrogenation/ dehydrocoupling that allow for control of kinetics and final product distributions. 3 N-Alkyl substituted amine-boranes, particularly those bearing methyl groups 4 (although aryl substituents are also known 5 ) have received the bulk of attention because of their thermal stability (N-alkyl especially as N-aryl undergo spontaneous dehydrocoupling 5b ) relative ease of synthesis, high weight% H, and as precursors to polyaminoboranes. 4 The coordination chemistry, and subsequent reactivity, of such species is also well developed, often operating through 3 centre-2 electron (3c-2e) sigma M⋯H-B interactions. 3,4,6 Developments in B-substituted analogues have, surprisingly, lagged behind; perhaps due to their more challenging synthesis, 7 and potential instability due to weaker B-N bonds. 7b,8 The reactivity and coordination chemistry of B-alkyl (or heteroalkyl) substituted amine-boranes, particularly with respect to dehydrocoupling, has only recently attracted significant attention. Manners and co-workers reported the synthesis of B-substituted amine-boranes containing relatively exotic substituents [e.g. C 6 F 5 or SR, Scheme 1(i)] 9 that undergo dehydrogenation to form the corresponding aminoboranes. Liu and co-workers have developed a range of cyclic amineboranes [selected examples shown in Scheme 1(ii)], 10 that can be dehydrocoupled by transition metal catalysts to form discrete, well-characterised products. 11 In some cases intermedi-ate sigma-complexes can be isolated, e.g. A Scheme 2. 11f Liu and Manners have independently described the dehydrogenation of the B-methyl amine-boranes MeH 2 B·NMe x H 3−x [x = 0, 1, 2; Scheme 1(iii)] by catalytic and non-catalytic (thermal) routes. 7 Reports of B-aryl amine-boranes are scarce. H 2 PhB·NMe 3 (I) 12 has been shown to form sigma-complexes with suitable group 6 and 7 metal fragments (e.g. B, Scheme 2). 13 H 2 PhB·NMe 2 H (II) is known 14 but its coordination chemistry or dehydrocoupling has not been reported. The B-phenyl amine-borane H 2 PhB·NH 3 can be dehydrocoupled using [Pd(NCMe) 4 ][BF 4 ] 2 to form a material tentatively identified as [PhBNH x ] n , but insolubility prevented further characterisation. 15 We report here a detailed study into the coordination chemistry of B-aryl substituted amine-boranes (I) and (II) with {Rh(L 2 )} + fragments (L 2 = (PR 3 ) 2 or chelating diphosphine) in which the steric and electronic (bite angle, β 16 ) demands of the phosphine ligands are varied. Unlike B-alkyl (or N-alkyl) substituted amine-boranes, B-aryl analogues offer two potential binding motifs: either through the aryl (e.g. η 6 ) or 3c-2e Rh⋯H-B interactions (e.g. η 2 ), Scheme 3. The relative strength of amine-borane sigma binding with increasing bite angle has been commented upon before in [Rh(L 2 )(η 2 -H 3 B·NMe 3

)][BAr F
4 ] complexes, with larger L-Rh-L bite angles favouring tighter Rh⋯H 2 B interactions (as measured by NMR spectroscopy). 17 Conversely, larger bite angles in the simple arene complexes [Rh(L 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] result in weaker Rh⋯arene interactions, as measured by collision-induced dissociation in Electrospray Mass Spectrometry (ESI-MS) and solution equilibrium measurements. 18 These trends presumably reflect the optimisation of bonding between the d 8 -Rh(I)-{ML 2 } fragment and either the B-H sigma donating orbitals 13 or the π-arene orbitals, 19 as modified by the L-Rh-L angle. 20 This can be interpreted by the energy of the C 2v -{ML 2 } + LUMO that is of π-symmetry (b 1 ) becoming lower in energy with increasing bite angle, 17a,21 thus finding a worse match with the arene HOMO and a better one with the relatively low lying B-H σ-orbitals. In this contribution we demonstrate empirically that with B-aryl amine-boranes the L-Rh-L bite-angle dictates which mode of binding is observed (i.e. η 6 or η 2 ), present equilibrium thermochemical data on the relative binding strengths of each motif when the two binding modes are finely balanced, and show that dehydrocoupling of H 2 PhB·NMe 2 H forms an unusual example of a B-substituted acyclic aminoborane which undergoes subsequent B-H activation to form a B-substituted amino-boryl complex.

Results and discussion
Synthesis of precursors H 2 PhB·NMe 3 (I) 12 and H 2 PhB·NMe 2 H (II) 14 have been reported, and their original syntheses comes from the reaction of diboranes (PhBH 2 ) 2 with NMe 3 or NMe 2 H respectively. An alternative, expedient, synthesis of (I) and (II) is based on the methods of Hawthorne, 22 Shimoi, 13 95 Hz. In contrast to N-aryl amine boranes, such as H 3 B·NPhH 2 , 5b compounds (I) and (II) were found be stable towards thermal dehydrocoupling or B-N bond cleavage, remaining unchanged on heating (C 6 H 5 F, 80°C, 12 h).

Coordination chemistry of H 2 PhB·NMe 3
Reaction of a stoichiometric amount of (I) with [Rh(L 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] [L 2 = (P i Pr 3 ) 2 , 24 (P i Bu 3 ) 2 , 25 i Pr 2 P(CH 2 ) 3 P i Pr 2 , 18b Ph 2 P(CH 2 ) 3 PPh 2 and Ph 2 P(CH 2 ) 5 PPh 2 17b ] in 1,2-difluorobenzene solvent resulted in displacement of the fluorobenzene ligand and formation of new complexes in solution as determined by NMR spectroscopy. These fragments were chosen to probe changes in phosphine bite-angle, while keeping the electronic contribution from the phosphine substituent as constant as possible. For example P i Bu 3 and P i Pr 3 have different cone angles of 143°& 160°respectively but similar electronic properties; 26 L-Rh-L bite angles can be varied in Ph 2 P(CH 2 ) n PPh 2 (n = 3 or 5); and mono-dentate versus chelating coordination modes can be probed with P i Pr 3 and i Pr 2 P-(CH 2 ) 3 P i Pr 2 . These fragments have also been used to form well-defined sigma amine-borane complexes with, for  F 4 ] (1) in which the amine-borane binds through two Rh⋯H-B 3c-2e interactions (Scheme 4). In the 11 B{ 1 H} NMR spectrum a single broad peak is observed at δ 34.8, with a characteristic downfield shift (35.6 ppm) of the borane resonance compared to free ligand (δ −0.8) that signals η 2 Rh⋯H-B binding. 27,28 In the 1 H NMR spectrum the BH 2 resonance is observed at δ −6.36 (2 H relative integral), an upfield shift of 8.73 ppm compared to free ligand. The aryl protons [δ 7.37, 3H; δ 7.25, 2H] are not significantly shifted from free ligand. 13 In the 31 P{ 1 H} NMR spectrum a doublet is observed at δ 69.6 [J (PRh) = 176 Hz], shifted 14.1 ppm downfield from the starting material. 24 In the solid state the complex crystallises with two cations (and two anions) in the asymmetric unit. An overlay of the independent cations (ESI) did not reveal any significant difference in amine-borane geometry but the P i Pr 3 ligands vary slightly in position and conformation [e.g. P(1)-Rh(1)-P(2) 106. 19(3) Å, P(3)-Rh(2)-P(4) 101.86(3)°] which we attribute to crystal packing effects due to a rather flat potential energy surface as only one set of resonances could be observed in the 1 H, 11 B and 31 P{ 1 H} NMR spectra. This observation of different ligand conformation/bite angles of two independent molecules in the asymmetric unit has been noted in amineborane complexes of H 3 B·NMe 2 BH 2 ·NMe 2 H and [Me 2 NBH 2 ] 2 with the {Rh(P i Bu 3 ) 2 } + fragment. 27 17a perhaps as a result of the extra steric demand caused by B-substitution in 1. High quality X-ray diffraction data allowed the hydrogen atoms of the BH 2 unit to be located in the difference map and refined freely, confirming the η 2 -coordination mode.
Forcing the P-Rh-P bite angle to be significantly smaller, while keeping the electronic contribution of the P-substituents the same, is achieved by use of the chelating phosphine complex [Rh( i Pr 2 P(CH 2 ) 3 P i Pr 2 )(η 6 -C 6 H 5 F)][BAr F 4 ]. Reaction of this with a stoichiometric amount of amine-borane (I) resulted in the formation of an orange solution, rather than the purple one observed for 1. X-ray diffraction quality crystals were obtained from a 1,2-difluorobenzene/pentane recrystallisation, from which a single crystal X-ray diffraction study demonstrated η 6 -binding of the arene, rather than Rh⋯H-B bonding: [Rh( i Pr 2 P(CH 2 ) 3 P i Pr 2 )(η 6 -PhH 2 B·NMe 3 )][BAr F 4 ] (2) (Scheme 4B). The P-Rh-P bite angle [94.04(4)°] is significantly smaller than in η 2 -bound complex 1 [e.g. 101.86(3)°]. Consistent with this different binding mode, that does not involve the borane fragment, in the 11 B{ 1 H} NMR spectrum a single resonance is observed at δ −2.1 that is now only slightly shifted from free amine-borane (δ −0.8). In the 31 P{ 1 H} NMR spectrum a single species is observed [δ 45.0; J (RhP) = 195 Hz] a chemical shift that is barely changed when compared with [Rh( i Pr 2 P-(CH 2 ) 3 P i Pr 2 )(η 6 -C 6 H 5 F)][BAr F 4 ]. 18b No resonance was observed in the high-field region of the 1 H NMR spectrum that would signal Rh⋯H 2 B interactions, but peaks at δ 6.93 [relative integral 1 H] and δ 6.31 [4 H, a 2 + 2 coincidence] demonstrate η 6 -binding through the phenyl moiety of (I). 31 Thus a change in the bite angle from 101.83(3)°in (1) to 94.04(4)°in (2) is also reflected in a change in the coordination mode from η 2 to η 6 . This preference comes into fine balance when the monodentate phosphine P i Bu 3 is used, that has a cone angle of 143°a nd thus might be expected to have a smaller P-Rh-P bite angle than (1). 17a Reaction of (I) with the precursor complex [Rh(P i Bu 3 ) 2 (η 6 -C 6 H 5 F)][BAr F 4 ] again led to formation of a purple solution. However, more complicated NMR data were observed than for either (1) or (2) which suggested the presence of two species in solution. In the 11 B{ 1 H} NMR spectrum (CD 2 Cl 2 ) two peaks are observed at δ 29.5 and −2.1 in a ratio of 10 : 11 respectively. The peak observed at δ 29.5 suggested the formation of a sigma complex with a η 2 -Rh⋯H 2 B interaction, being shifted 30.3 ppm downfield compared to I, cf. complex (1). The higher field signal at δ −2.1 is only shifted 1.3 ppm upfield compared with free ligand suggesting an alternative coordination mode for the amine-borane, more like (2). In the 31 P{ 1 H} NMR spectrum two resonances are observed in the same ratio as measured in the 11 (2). These data indicate both η 2 -Rh⋯H 2 B and η 6 -aryl bound complexes are present in solution. The 1 H NMR spectrum is consistent with this description. In the high field region a broad resonance is observed at δ −5.06 (Rh⋯H 2 B) which integrates to 1.1 H relative to the [BAr F 4 ]signals, and 2 singlets are observed at δ 2.76 and 2.50 corresponding to NMe 3 protons in the different coordination modes of the amine-borane. In addition, resonances can be observed upfield of the aryl region indicative of η 6 -aryl coordination. These two complexes are formulated as [Rh(P i Bu 3 A variable temperature NMR spectroscopy study was carried out to determine if exchange between these isomers was occurring in solution. At 298 K in 1,2-difluorobenzene, the concentrations of η 2 -3 and η 6 -3 were found to be approximately equal. Lowering the temperature to 240 K resulted in a relative increase in η 6 -3 while at higher temperature (330 K) (η 2 -3) was favoured, demonstrating the two isomers to be in dynamic equilibrium. The equilibrium constant at each temperature was calculated from integration of the 31 P{ 1 H} NMR spectra; and the resulting Van't Hoff plot (Scheme 5B) allowed for determination of the thermodynamic parameters for this exchange: ΔH°= −11.3 ± 0.5 kJ mol −1 ; ΔS°= −39.1 ± 1.7 J K −1 mol −1 ; ΔG(298 K) = 0.4 ± 0.4 kJ mol −1 . Thus binding between the two modes is approximately thermoneutral. The negative enthalpy indicates η 6 -binding of the aryl group is stronger than the η 2 -binding through BH 2 but this is moderated by the associated negative entropy, which is likely to be the result of loss of free rotation of the phenyl group upon η 6 -binding. The negative entropy also means that η 2 Rh⋯H-B binding will become increasingly favoured at higher temperature. A similar entropy change (ΔS°= −16.3 ± 3.3 J K −1 mol −1 ) upon loss of phenyl group free rotation was observed in the epimerisation of 2-phenyl-c-4,c-6-dimethyl-1,3-dioxane; 32 while for the anion exchange equilibrium between [1-closo-CB 11 H 6 Br 6 ] − and [BAr F 4 ] − η 6 -binding through an aryl group of [BAr F 4 ] − was also shown to be enthalpically favoured but entropically disfavoured (ΔS°= −87.6 ± 0.8 J K −1 mol −1 ). 33 Layering a 1,2-difluorobenzene solution of this mixture with pentane at −30°C led to formation of purple crystals and an orange oil. Isolation of a crystal suitable for X-ray diffraction by mechanical separation allowed the solid-state structure of the purple material to be determined (Scheme 5C). As for complex (1), two cations are present in the asymmetric unit; an overlay of these independent structures (ESI) did not reveal significant differences in amine-borane binding and orientation, although some conformational differences and a difference in ligand bite angle was observed for the P i Bu 3 ligands. The structure shows a close interaction between the rhodium centre and the BH 2 moiety in [Rh(P i Bu 3 with Rh⋯B distances of 2.153(6) and 2.172(6) Å consistent with η 2 -binding. Although the hydrogen atoms could not be located in the difference map and were placed in calculated positions, the metrical data are consistent with this description as well as the NMR data. The two P-Rh-P bite angles measured for each independent molecule, 95.14(5) and 98.84(5)°, are smaller than for (1), but larger than for (2), consistent with the equilibrium observed in solution.
Bulk mechanical separation of the crystals from the oil for further analysis was not possible, but we propose the orange oil to be the η 6 -phenyl bound amine-borane complex [Rh(P i Pr 3 ) 2 (η 6 -PhH 2 B·NMe 3  that showed the same NMR spectra as a freshly prepared sample. To extend this study into the effect of bite angle [Rh(Ph 2 P-(CH 2 ) 5 PPh 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] was used as a starting material, which has a flexible chelating phosphine with a 5-carbon backbone. This ligand has been shown to be able to access to a wide range of bite angles, and values of 93.98(4) to 117.3(1)°h ave been determined crystallographically. 16b,34 Reaction of a stoichiometric amount of this starting material with (I) resulted in an orange solution with NMR data characteristic of an η 6 -aryl bound. Crystallisation from layering a dichloromethane solution with pentane allowed a single crystal X-ray diffraction study to be carried out and confirmed the η 6coordination mode in [Rh(Ph 2 P(CH 2 ) 5 PPh 2 )(η 6 -PhH 2 B·NMe 3 )] [BAr F 4 ] (4) (Fig. 1). The phosphine ligand bite angle was found to be only 92.21(4)°, the smallest observed crystallographically for this ligand but consistent with the observed binding mode. By contrast the corresponding H 3 B·NMe 3 complex is η 2 -bound, [Rh(Ph 2 P(CH 2 ) 5 PPh 2 )(η 2 -H 3 B·NMe 3 )][BAr F 4 ], and shows a P-Rh-P bite angle of 98.18 (3)°. This shows that the observed bite angle for a flexible ligand such as Ph 2 P(CH 2 ) 5 PPh 2 is very dependent on the ancillary ligands. The analogous complex formed with the smaller bite angle aryl diphosphine, [Rh(Ph 2 P(CH 2 ) 3 PPh 2 )(η 6 -PhH 2 B·NMe 3 (15)°, ESI] also shows a η 6 -coordination mode. The data for the ligation of (I) is summarised in Fig. 2; in which a plot of Rh⋯B distance against bite angle shows that larger bite angles give η 2 -complexes, smaller bite angles result in η 6 -complexes, with a crossover point at approximately 95°. The Rh⋯B distance in the η 2 -binding mode appears to be rather insensitive to bite angle.

Catalytic dehydrogenation of H 2 PhB·NMe 2 H (II)
{Rh(P 2 )} + fragments have been shown to catalyse the dehydrocoupling of secondary and primary amine-boranes; 17,27 and the ligand bite angle has been shown to affect the rate of de-hydrocoupling of H 3 B·NMe 2 H in particular. 17 Although empirically it is found that the smaller bite angles promoted in larger turnover frequencies, the precise factors behind these differences are not yet fully delineated and likely involve a combination of relative accessibility of Rh(I)/Rh(III) oxidation states/ease of H 2 loss/relative barriers to BH and NH activation all as modified by the bite angle. 3,35 We therefore sought to probe the effect of the ligands on the dehydrocoupling of secondary amine-borane (II) by comparing two electronically similar precatalysts but with very different P-Rh-P angles: [Rh-(P i  Reaction of 5 mol% of these two precatalysts with (II) in 1,2-difluorobenzene (25°C, closed system) resulted in very slow 3 consumption of (II) in both cases: less than 15% conversion in 5 hours (TOF less than 0.6 h −1 ). Although slow, dehydrogenation is also not fast with other amine-boranes using these systems. 11f,17a The major 11 B-containing product displayed a single resonance at δ 39.4 which split into a doublet [J (BH) = 123 Hz] in the 11 B NMR spectrum. This was assigned as aminoborane HPhBvNMe 2 (6) from its characteristic 11 11f Heating to 77°C in a sealed NMR tube resulted in the complete consumption of (II) in less than 1 hour for both catalysts. The product (>95% by 11 B NMR spectroscopy) of dehydrocoupling was again found to be free aminoborane HPhBvNMe 2 from the in situ NMR spectrum. Unfortunately, due to its apparent instability and similar volatility to the 1,2-difluorobenzene solvent, separation and isolation of pure (6) was not possible due to decomposition upon vacuum distillation. Nevertheless NMR data are unambiguous,  and as we show in situ generated (6) can also be used for onward reactivity. The formation of (6) is in contrast with the metal catalysed dehydrocoupling of H 2 MeB·NMe 2 H which forms cyclic B-dimethyl-N-tetramethyldiborazane, 7b [Me 2 NBHMe] 2 , as well the aminoborane, HMeBvNMe 2 . The B-phenyl group in (6) inhibits any appreciable dimerisation to the corresponding diborazane.
[Rh(Ph 2 P(CH 2 ) 3 PPh 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] has been shown to be an excellent catalyst for dehydrocoupling of H 3 B·NMe 2 H (0.2 mol%, open system, TOF ∼ 1250 h −1 ). 17b However this was also a slow catalyst for dehydrocoupling of (II) at room temperature, with full conversion to (6) only observed after 23 hours at 5 mol% catalyst loading (25°C, TOF ∼ 1 h −1 ). In order to probe the causes of the slow dehydrogenation of (II) with this catalyst, stoichiometric studies were performed.

Stoichiometric reactivity of H 2 PhB·NMe 2 H (II)
The presence of the phenyl group which provides a competitive (η 6 ) site for amine-borane binding at the metal centre is a possible cause of the slow dehydrogenation of (II), as B-H activation at the metal centre requires the formation of a precursor sigma complex. 2b,3 Preferential η 6 -coordination through the aryl ring makes this less likely.
Addition of a slight excess of (II) (1.2 equiv.) to [Rh( i Pr 2 P-(CH 2 ) 3 P i Pr 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] results in the formation of an η 6 -bound complex [Rh( i Pr 2 P(CH 2 ) 3 P i Pr 2 )(η 6 -PhH 2 B·NMe 2 H)] [BAr F 4 ] (7) alongside a small amount of dehydrogenation product (6). Complex (7) was characterised by NMR spectroscopy and single crystal X-ray diffraction (Fig. 3). The solidstate structure reveals a ligand bite angle of 94.27(4)°which is very similar to that in (2) [94.04(4)°] which also displayed an η 6 -coordination mode. In addition to the expected resonances in the NMR spectra an N-H resonance is observed in the 1 H NMR spectrum at δ 3.55. A similar complex is formed on reaction of [Rh(Ph 2 P(CH 2 ) 3 PPh 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] with (II), as characterised by NMR spectroscopy: [Rh(Ph 2 P(CH 2 ) 3 PPh 2 ) (η 6 -PhH 2 B·NMe 2 H)][BAr F 4 ] (8). When three equivalents of (II) were combined with [Rh( i Pr 2 P(CH 2 ) 3 P i Pr 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] slow dehydrocoupling (hours) to form aminoborane (6) occurs, with η 6 -bound (7) observed at the end of catalysis. That η 6 -coordination of ligand (II) in (7) and (8) is preferred to η 2 -binding suggests that this competitive binding mode contributes to the slow dehydrogenation rate under catalytic conditions for these chelating systems. However, that dehydrogenation does occur catalytically, albeit slowly, indicates that if an inner sphere mechanism is operating, access to the η 2 -coordination mode through BH 2 is possible, but the equilibrium lies heavily in favour of η 6 -coordination. Complexes (7) and (8) do not dehydrogenate to any significant degree in the absence of exogenous amine-borane, and we, and others, have previously commented upon the role of B-H⋯H-N interactions in lowering barrier to dehydrocoupling. 37 Given the η 6 binding mode we cannot discount an outer-sphere mechanism in which π-coordination 38 of the metal activates the amine-borane to alternative dehydrogenation pathways. However, the N-H resonance does not change significantly on coordination [δ 3.55 versus δ 3.52] suggesting only a minimal perturbation to this bond.
By using a metal fragment which can adopt a large ligand bite angle, {Rh(P i Pr 3 ) 2 } + , in which η 2 -coordination is favoured (i.e. complex 1) this effect of competitive aryl binding can potentially be avoided. Upon mixing equal amounts of [Rh(P i Pr 3 ) 2 (η 6 -C 6 H 5 F)][BAr F 4 ] and (II) in 1,2-F 2 C 6 H 4 solvent a blue solution was immediately formed which rapidly decolourised (less than 5 min) to yield a very pale yellow solution. This blue colour likely results from the sigma-complex [Rh(P i  We propose the doublet at δ 69.3 is therefore likely to result from (9). In the 11 B{ 1 H} NMR spectrum no signals for free (II) or (6) were observed. There was no further change after 24 hours. Recrystallisation at −26°C resulted in the formation Scheme 6 Catalytic dehydrogenation of amine-borane (II) to form aminoborane (6)   of two distinct crystalline products: colourless block-and pale yellow plate-type crystals that could be separated mechanically. Although relatively poor crystal quality compounded with significant disorder of the phosphine alkyl groups in both complexes prevented collection of high-quality data in single crystal X-ray diffraction experiments, the data were sufficient to identify the products as [Rh(P i Pr 3 Fig. 4. The solid-state structure of (10) contains two independent cations (and two anions) in the asymmetric unit with broadly similar metric parameters, but disorder is observed in the i Pr groups of the phosphine ligand. The N-H and B-H hydrogen atoms were placed in calculated positions, the Rh-H hydride ligands could not be reliably placed and so were omitted, although their presence was confirmed by NMR spectroscopy and ESI-MS of pure, isolated material, vide infra. The structure, and NMR data, of (10) are similar to the closely related complex [Rh(P i Pr 3 17a with the P i Pr 3 ligands in trans orientation, cis Rh-H functionality and Rh-B distances of 2.274(9) and 2.333(8) Å respectively]. The 31 P{ 1 H} NMR spectrum (CD 2 Cl 2 ) of isolated (10) confirmed this species as one of the products observed in the mixture, δ 64.7 [d, J (RhP) = 108 Hz]. Presumably the amine-borane in (10) is undergoing a fluxional process that makes the phosphine ligands equivalent at room temperatures, similar to that observed in related complexes 39 in which an η 2 to η 1 change in coordination is accompanied by a rotation around the Rh⋯H-B bond. In the 1 H NMR spectrum a broad resonance at δ −2.08 is assigned to the Rh⋯H 2 B interaction, and a sharper one at δ −19.05 (integral 2 H) assigned to Rh-H. The 11 B{ 1 H} NMR spectrum shows a signal at δ 8.0 assigned to the amineborane. ESI-MS confirmed the formulation of the cation to be [Rh(P i Pr 3 ) 2 (H) 2 (η 2 -H 2 PhB·NMe 2 H)] + (m/z = 560.32 (found), 560.32 (calculated)). Complex (10) forms by sequential B-H/ N-H activation at a Rh(I) centre, to form a Rh(III) dihydride, which then coordinates another equivalent of (II) to liberate free (6). Such activation processes are well established. 3,27 The second product isolated from the reaction mixture, [Rh(P i Pr 3 (11), is more unusual. The solid-state structure shows a complex in which the phosphine ligands are arranged in a trans orientation, and a molecule of aminoborane (6) has undergone overall oxidative addition of the B-H bond at the {Rh(P i Pr 3 ) 2 } + fragment to form a terminal hydride (located in the final difference map) and a direct Rh-B bond, i.e. an amino-boryl species.
Group 9 amino-boryl species have been isolated previously, coming from B-H activation, e.g.  (15) Å] suggests aminoborane character is retained, and the B-C aryl bond distance of 1.536 (14) Å is consistent with a single bond. The angles around boron sum to 360°, demonstrating sp 2 character, although the Rh-B(1)-C(1) angle of 98.3(7)°is much smaller than might be expected for such hybridisation. Overall these metrics point to an aminoboryl species, rather than an alternative borylene structure. 42 There appears to be a vacant site that sits cis to Rh-H and Rh-B. There are no close 25,43 Rh⋯C interactions from the i Pr ligands that would point to an agostic interaction [shortest Rh⋯C 3.227 Å]. There is, however, a relatively short Rh⋯C distance to the ipso-phenyl carbon atom [2.634(8) Å] that, when combined with the compressed Rh-B-C angle, suggests a Rh⋯C(ipso) interaction. The next closest distance to the phenyl group is 3.042(9) Å (ortho carbon), longer than would be expected for a η 2 -arene type interaction. The geometry is reminiscent of the η 2 -benzyl complexes that interact via methylene and ipso carbon atoms. 44 Including the Rh⋯C(ipso) interaction complex (11) can be described as a 16-electron Rh(III) species, and (11) is also related to the 16-electron Rh(P i Pr 3 ) 2 (Bcat)(H)Cl that comes from oxidative addition of HBcat to a Rh(I) precursor (cat = catechol). 45 Interestingly, in the system when a chelating phosphine is used an η 6 -complex is isolated, Rh( i Pr 2 PCH 2 CH 2 P i Pr 2 ){(η 6 -cat)Bcat}, paralleling the observations described herein.  In the 31 P{ 1 H} NMR spectrum of (11) a doublet is observed [δ 48.8, J (RhP) = 120 Hz]. The chemical shift of δ 50.1 for the aminoborane observed in the 11 B{ 1 H} NMR spectrum places the complex as a boryl, rather than a borylene 42  A plausible mechanism for the formation of complex (11) invokes dehydrogenation of (II) to form (10) and aminoborane (6), followed by much faster reaction of the latter with residual [Rh(P i Pr 3 ) 2 (η 6 -C 6 H 5 F)][BAr F 4 ] to form (11), overall in equal ratio to (10). When three equivalents of (II) were combined with [Rh(P i Pr 3 ) 2 (η 6 -C 6 H 5 F)][BAr F 4 ] the rapid (15 minutes) formation of only (10) was observed followed by the slow dehydrocoupling (hours) to form aminoborane (6), during which time (10) is observed as a resting state (Scheme 8). From this reaction mixture (10) could be isolated pure in good yield (57%) by layering with pentane and storage at −25°C. A solution of pure (10) did not show any changes, suggesting (11) does not form from (10). Using aminoborane (6) generated catalytically (Scheme 6) reaction (overall oxidative addition) with [Rh(P i Pr 3 ) 2 (η 6 -C 6 H 5 F)][BAr F 4 ] is very rapid (on time of mixing) to form (11). In contrast there is no reaction of (6) with the Rh(I) sigma-complex (1) over 24 h, showing that aminoborane (6) will not displace amine-borane (I) under these conditions. A proposed mechanism is summarised in Scheme 9.
Complex (11) reacts with 2 equivalents of amine-borane (II) to give (10) and (6) on time of mixing. The current data do not discriminate between two possible mechanisms for this transformation. A sigma-bond metathesis/β-elimination of (11) with (II) to eliminate (6), or a reversible reductive elimination of (6) to give a {Rh(P i Pr 3 ) 2 } + fragment which undergoes reaction with (II) as described (Scheme 8). Whatever the mechanism, complex (10) and aminoborane (6) are the ultimate products when excess amine-borane is present, consistent with their observation during catalysis. Pure complex (10) was found to be a slow catalyst for the dehydrogenation of (II) to form (6). Slow amine-borane dehydrogenation when catalysts sit in a Rh(III) dihydride resting state has been noted previously. 35b

Conclusions
We have shown here that the bite angle in {Rh(P 2 )} + type fragments can have a significant effect in determining whether Rh⋯H 2 B η 2 -sigma amine-borane complexes or Rh⋯arene η 6 complexes are formed with B-substituted amine-boranes. Wider bite angles (i.e. monodentate phosphines) tend to favour η 2 coordination modes, and relatively rapid B-H/N-H activation with a secondary B-substituted amine-borane to afford a Rh(III) dihydride complex and a B-substituted aminoborane. With constrained, chelating, phosphines η 6 complexes can be isolated instead in which the amine-borane moiety is intact and dehydrogenation is slow. This difference in stoichiometric reactivity balances out the reported large differences in catalytic dehydrocoupling rate of {Rh(PR 3 ) 2 (H) 2 } + fragment (slow) versus {Rh(chelating phosphine)} + (fast) with H 3 B·NMe 2 H, which does not bear aryl substituents. Thus, although {Rh(P i Pr 3 ) 2 (H) 2 } + dehydrocouples H 2 PhB·NMe 2 H slowly, the η 6 coordination mode observed with {Rh( i Pr 2 P-(CH 2 ) 3 P i Pr 2 )} + means that B-H (and subsequent N-H) activation by an inner sphere coordination/activation mechanism are also slowed so that now both fragments operate at a similar rate. Such observations are potentially important in the design of systems that dehydropolymerise arene-substituted amine-boranes (i.e. BN polystyrene analogues) as rapid dehydrogenation to form putative aminoborane intermediates that then can undergo B-N bond forming process are likely central to any successful catalyst system. We thus suggest that systems that form strong adducts with arene π-systems are less likely to be good candidates for such transformations.

General experimental details
All manipulations, unless otherwise stated, were performed under an atmosphere of argon, using standard Schlenk and glove-box techniques. Glassware was oven dried at 130°C overnight and flamed under vacuum prior to use. Dichloromethane, diethyl ether and pentane were dried using a Grubbs type solvent purification system (MBraun SPS-800) and degassed by successive freeze-pump-thaw cycles. 46 CD 2 Cl 2 and 1,2-F 2 C 6 H 4 were distilled under vacuum from CaH 2 and stored over 3 Å molecular sieves, 1,2-F 2 C 6 H 4 was stirred over alumina for two hours prior to drying. NMR spectra were recorded on a Bruker AVD 500 MHz spectrometer at room temperature unless otherwise stated. For NMR spectra measured in situ in 1,2-F 2 C 6 H 4 , the spectrometer was prelocked and pre-shimmed using a C 6 D 6 (0.1 mL) and 1,2-F 2 C 6 H 4 (0.3 mL) sample and 1 H NMR spectra were referenced to the centre of the downfield solvent multiplet (δ 7.07). 31 P and 11 B NMR spectra were referenced against 85% H 3 PO 4 (external) and Et 2 O·BF 3 (external) respectively. Chemical shifts are quoted in ppm and coupling constants in Hz. ESI-MS were recorded on a Bruker MicrOTOF instrument. In all ESI-MS spectra there was a good fit to both the principal molecular ion and the overall isotopic distribution. Microanalyses were performed by Stephen Boyer at the London Metropolitan University.
Metal precursor compounds [Rh(P i Pr 3 (20 mL). The mixture (a suspension of white solid) was stirred vigorously for 2 hours and evolution of hydrogen was observed. The mixture was evaporated to dryness in vacuo and pentane (100 ml) added and the mixture stirred vigorously. The solution was transferred by filter cannula to another Schlenk and the remaining white solid washed with pentane (2 × 10 mL). The combined fractions were evaporated to dryness in vacuo to yield the amine-borane PhH 2 B·NMe 3 (309 mg, 58%) or PhH 2 B·NMe 2 H (340 mg, 64%) as white solids which were stored in the glove box.
Both compounds have been reported previously although prepared by slightly different synthetic routes. The synthesis of PhH 2 B·NMe 3 has been reported several times 12,13,22,47 and NMR spectroscopy data have been reported in C 6 D 6 . 13 Our data for PhH 2 B·NMe 3 matched that previously reported. The synthesis of PhH 2 B·NMe 2 H has been reported by a different route 14 but no NMR data was given and so is reported below for the first time.