Stereoselective formation and catalytic activity of hydrido(acylphosphane)(chlorido)( pyrazole)- rhodium( III ) complexes. Experimental and DFT studies †

The reaction of [{RhCl(COD)} 2 ] (COD = 1,5-cyclooctadiene) with L = pyrazole (Hpz), 3(5)-methylpyrazole (Hmpz) or 3,5-dimethylpyrazole (Hdmpz) and PPh 2 ( o -C 6 H 4 CHO) (Rh : L : P = 1 : 2 : 1) gives hydridoacyl complexes [RhHCl{PPh 2 ( o -C 6 H 4 CO)}(L) 2 ] ( 1 ). Stereoselective formation of 1-Hpz and 1-Hmpz with pyra-zoles trans to hydrido and phosphorus and hydrogen bond formation with O -acyl and chlorido occur. 1-Hmpz is a mixture of two linkage isomers in a 9 : 1 ratio, with two 5-methylpyrazole ligands or with one 3-and one 5-methylpyrazole ligand, respectively. Fluxional 1-Hdmpz undergoes metallotropic tautomer-ization and is a mixture of equal amounts of 1a-Hdmpz and 1b-Hdmpz , with hydrido trans to pyrazole or chlorido, respectively. Complexes 1 readily exchange hydrido by chlorido to a ﬀ ord [RhCl 2 {PPh 2 - ( o -C 6 H 4 CO)}(L) 2 ] ( 2-Hpz , 2-Hmpz


Introduction
Acylhydrido transition metal species are well known to be involved in many stoichiometric and catalytic reactions such as hydroacylation 1 or aldehyde decarbonylation reactions, 2 among others. Recently we have reported that hydridoiridaβ-diketones, which contain an acyl and a hydroxycarbene group stabilized by a strong O⋯H⋯O intramolecular hydrogen bond or {(acylphosphane)(diphenylphosphinous acid)}rhodium(III) complexes, which contain a hydrogen bond between the acidic hydrogen atom of the coordinated phosphane and the oxygen atom of a coordinated acyl group, are efficient and robust homogeneous catalysts for the hydrolysis of ammonia-or amine-boranes under air to release hydrogen gas. 3 Pyrazoles, five-membered heterocycles containing two adjacent nitrogen atoms, possess a wide range of useful properties 4 and as ligands to transition metals they have attracted considerable interest. 5 Pyrazoles possess a planar aromatic † Electronic supplementary information (ESI) available: Cartesian coordinates of optimized DFT structures, images generated from those coordinate files, computationally obtained energy values associated with each structure, and crystallographic data file of complexes 2-Hpz, 3a-Hdmpz and 7-Hpz. CCDC 1062495-1062497. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01705j ring with a pyridine-type nitrogen suitable for coordination and an adjacent N-H group that can behave as a hydrogenbond donor. In metal complexes of simple pyrazoles both inter-and intra-molecular hydrogen bonding can be found. Intermolecular forces have been successfully employed for the construction of supramolecular assemblies with various and interesting structures. 6 Intramolecular hydrogen bonds are important in establishing the conformation of the complexes. 7 Halide, hydroxide or peroxo species are known as hydrogenbond acceptors towards metal-coordinated pyrazoles 8 and the presence of strong hydrogen bonds between pyrazole and pyrazolate moieties in late transition metal complexes has also been reported. 9 Therefore, we thought it interesting to prepare acylrhodium derivatives containing pyrazoles, which could form hydrogen bonds involving the oxygen atom of the acyl group and behave as catalysts for the hydrolysis of amineboranes to release hydrogen. Hydrogen is a feasible alternative to fossil fuels but its safe storage and delivery still remains a challenge. 10 Ammonia-borane (AB), a chemical hydrogen storage material with high hydrogen contents (H 3 N-BH 3 , 19.6 wt%), is considered a potential hydrogen source. The homogeneous transition-metal catalysed dehydrogenation of ammonia-or amine-boranes under mild conditions, which usually affords up to one equivalent of hydrogen gas and requires an inert atmosphere, has been the subject of recent intensive research. 11 Hydrolysis reactions can afford up to three equivalents of hydrogen per equivalent of an amineborane adduct. Transition metal heterogeneous systems, 12 which often require an inert atmosphere and include highly efficient rhodium nanoparticles, 13 allow fast H 2 release from these amine-borane adducts at room temperature. The efficient homogeneously catalyzed version of this reaction has been reported more recently using the afore-mentioned hydridoirida-β-diketones or hydrido{(acylphosphane)(diphenylphosphinous acid)}rhodium(III) complexes, 3 and also iridium-PNP complexes, 14 coordinatively unsaturated iridium-carbene derivatives, 15 dicarbonylruthenacyclic compounds 16 or ruthenium-bipyridine-p-cymene complexes. 17 We now report on the preparation of new hydrido(acylphosphane)rhodium(III) complexes containing pyrazole (Hpz), 3(5)methylpyrazole (Hmpz) or 3,5-dimethylpyrazole (Hdmpz). DFT calculations were carried out aimed at identifying the thermodynamically most stable isomer of those complexes that could not be unambiguously characterized otherwise. The catalytic activity of the complexes to promote the hydrolysis of AB or amine-boranes in air to release hydrogen is also tested.

Results and discussion
The reaction of [{RhCl(COD)} 2 ] (COD = 1,5-cyclooctadiene) with PPh 2 (o-C 6 H 4 CHO) (Rh/P = 1 : 1) in the presence of pyrazole (Rh/ Hpz = 1 : 2) leads to the displacement of COD and the formation of the hydridoacyl complex [RhHCl{PPh 2 (o-C 6 H 4 CO)}(Hpz) 2 ] (1-Hpz), shown in Scheme 1i, in line with the well-known ability of the aldehyde-phosphane ligand to promote the chelate-assisted oxidative addition of the aldehyde to late transition metal complexes. 2e The NMR spectra indicate the presence of only one species in solution. The 31 P{ 1 H} NMR spectrum contains only one doublet due to coupling with rhodium ( J (Rh,P) = 166 Hz) at 79.0 ppm, the characteristic low field due to the five-member ring effect. 18 The 1 H NMR spectrum shows two sets of pyrazole signals at 8.47, 7.64 and 6.35 ppm or 7.00, 6.95 and 5.85 ppm, respectively, due to two non-equivalent ligands. These resonances are sharp and different for protons 3 and 5 of each pyrazole ring, indicating the absence of metallotropic tautomerization and both pyrazoles being firmly bonded to rhodium. 8,19 In the low field region, the N-H resonances, also sharp, appear at 14.02 and 11.75 ppm and suggest the presence of hydrogen bonding to oxygen and chlorido. Adscription of the former to N-H⋯O bonding between the acyl group and the pyrazole trans to phosphorus appears likely. 3d A doublet of doublet in the high field region, due to a hydrido bonded to rhodium ( J (Rh,H) = 21.0 Hz) and cis to phosphorus ( J (P,H) = 15.4 Hz) is observed. The chemical shift, at −15.79 ppm, is consistent with a hydrido trans to an electronegative atom. Due to the equivalent trans influence exerted by chlorido and N-donor ligands, 20 it is difficult to determine the relative cis/trans disposition of the chlorido and the hydrido ligands. Also, the identification of the partners involved in the generation of intramolecular hydrogen bonds to the donor group of the pyrazole may be difficult due to the dependency of the hydrogen bond strength on its geometry, as well as on the nature of the substituent in the trans position to the acceptor ligand. 21 DFT calculations (see below) indicate the structure depicted in Scheme 1i, with hydrido cis to chlorido and trans to pyrazole, to be the most stable for 1-Hpz.
The reaction with 3(5)-methylpyrazole affords a mixture of two isomers shown in Scheme 1ii, 1a-Hmpz : 1a′-Hmpz = 9 : 1. The spectroscopic features, similar to those observed for 1-Hpz, indicate the absence of metallotropic tautomerization. In this case we believe that isomers 1a-Hmpz and 1a′-Hmpz are linkage isomers. 1H-Pyrazoles exist in solution as mixtures of two tautomeric forms whereas in the solid state, with very few exceptions, only one tautomer is observed. 4 When coordinated to transition metals, 3(5)-(R)pyrazoles (R = alkyl or aryl) usually appear in the 5-(R)pyrazole coordination mode, with the R group in the most distant position from the coordinating N-atom, in order to avoid steric congestion. 22 The tautomerism of 3(5)-methylpyrazole, involving the proton transfer between nitrogen sites has been thoroughly studied. 23 This ligand is unique and a few examples of complexes that contain both coordination modes, 5-methylpyrazole and 3-methylpyrazole, on the same metal atom have been reported. 6a,24 Due to steric constraints, in our case the most abundant isomer, 1a-Hmpz, is expected to contain two less demanding 5-methylpyrazole ligands and the minor isomer, 1a′-Hmpz, could contain at least one 3-methylpyrazole ligand. DFT calculations (vide infra) confirm this assumption and show that isomer 1a′-Hmpz, containing both coordination modes 5-and 3-methylpyrazole, must be the minor linkage isomer.
When the reaction is performed using 3,5-dimethylpyrazole, a different behaviour is observed. The 1 H and 31 P{ 1 H} NMR spectra at 298 K indicate the presence of almost equal amounts of the two species 1a-Hdmpz and 1b-Hdmpz in solution. The 31 P{ 1 H} NMR spectrum contains two doublets in the 77-78 ppm range and the 1 H NMR spectrum shows two doublets of doublets in a very narrow range, between −15.5 and −16.5 ppm, due to the corresponding hydridos. We believe that both isomers differ in the group trans to hydrido as shown in Scheme 1iii, pyrazole in 1a-Hdmpz or chlorido in 1b-Hdmpz. DFT calculations (see below) confirm the similar stability of these complexes, 1a-Hdmpz and 1b-Hdmpz. Furthermore, in the 1 H NMR spectrum, the resonances due to the pyrazole groups are extremely broad, suggesting that both isomers undergo metallotropic tautomerization and proton transfer in addition to metal exchange between nitrogen sites. 6b,22a,b,25 The metallotropic tautomerization is much less frequent than that observed for the free ligands. On lowering the temperature, the signals due to the pyrazole become sharper, so that by 213 K the expected resonances due to four non-equivalent groups, two per isomer, with eight non-equivalent methyl groups, can be observed. These results show that at low temperatures the metallotropic tautomerization, which can be attributed to steric congestion, becomes inhibited. The resonances due to the phosphanes and the hydridos remain unaltered in the 298-213 K range.
Complexes 1 are stable at room temperature in the solid state but show low stability in solution, therefore single crystals for a crystallographic study could not be obtained. As mentioned earlier, theoretical calculations were performed to determine the particular disposition of the ligands around the central metal and to describe the intramolecular hydrogen bonds established in the obtained compounds. On the basis of its reported accuracy, 26 the M062X level DFT calculations were carried out in order to identify the most stable isomers and rotamers, involving different hydrogen bonding options, of complexes 1. Comparison of PCM corrected relative ΔG values for all possible isomers and rotamers of compounds 1 at the M062X level was shown to be an appropriate tool to identify the thermodynamically most stable isomer/rotamer in each case. The absolute Gibbs free energies of the most stable structures (Tables 1 and SI-1, ESI †) were used in each case to estimate the relative stability (ΔG) of the rotamers/isomers. Up to 12 alternative isomers/rotamers with chlorido cis to phosphorus were considered in the case of 1-Hpz-type structures (Table SI-1 †). The isomer with chlorido trans to phosphorus and both Hpz ligands cis to each other was found to be 12.51 kcal mol −1 less stable than compound 1-Hpz (data not shown). In view of this significant thermodynamic destabilization, the theoretical study of additional trans (chlorido/phosphorus) isomers was found unnecessary. As observed in Table 1, the cis-H,Cl isomer 1-Hpz was identified as the most stable and unique structure, since the energetically closest structure, a trans-H,Cl isomer, turned out to be 1.86 kcal mol −1 less stable (Table SI-1 †), a significant energy difference, which rules out the possibility of the experimental observation of both species in the NMR spectra. This is in agreement with the experimental data. Compound 1-Hpz corresponds therefore to the cis-H,Cl isomer shown in Fig. SI-1a, † where the pyrazoles trans and cis to phosphorus would establish hydrogen bonds with the acceptors O-acyl and chlorido, respectively. Any rotamer/isomer where the hydrido acts as a hydrogen bond acceptor was shown to be consistently unstable with respect to 1-Hpz (see Fig. SI-2 †). With respect to 1-Hmpz-type structures, the inclusion of all plausible tautomeric species led to the analyses of 48 structures, which, again, cover all plausible combinations of intramolecular hydrogen bonds (Table SI-1 †). As shown in Table 1, complexes 1a-Hmpz and 1a′-Hmpz were identified as the major and minor species (9 : 1), respectively, as concluded from the significant energy difference among the two (ΔG = 1.35 kcal mol −1 ). As observed in Fig Experimental NMR data for 1-Hdmpz-type compounds revealed the existence of almost equimolar amounts of two species. In agreement with this, DFT results pointed out that two or even three species may exist simultaneously in solution. A slightly more abundant species (60%) was identified to consist of two rotamers of the trans-H,Cl isomer, 1b-Hdmpz and 1b′-Hdmpz . We thus observe that the difference in energy between trans-H,Cl and cis-H,Cl isomers is much smaller for the Hdmpz ligand than for the Hpz or Hmpz ligands, and allows the formation of both isomers in the Hdmpz case. In an attempt to identify the driving force that provokes the lower stability difference between isomers, steric clashes between the methyl groups present as substituents on the Hdmpz ligand (cis to phosphorus and also cis to acyl) with the neighbouring O-acyl were encountered. This steric effect would destabilize the cis-H,Cl geometry reducing the energy gap with the trans-H,Cl geometry, containing a Hdmpz ligand trans to acyl.
In a chloroform solution complexes 1 readily exchange the hydrido with chlorido to afford complexes [RhCl 2 {PPh 2 -(o-C 6 H 4 CO)}(L) 2 ] (L = Hpz, 2-Hpz; Hmpz, 2-Hmpz; Hdmpz, 2-Hdmpz). Such a reaction has several precedents. 27 As shown in Scheme 2, in all the three cases single species are obtained, which are static in solution on the NMR time scale at room temperature. An X-ray diffraction study on complex 2-Hpz (see below) indicates cis chloridos and the existence of N-H⋯Cl hydrogen bonds.
It is remarkable that upon exchange of hydrido by chlorido, the 31 P NMR signal moves significantly towards a higher field and appears in the 57-59 ppm range. A reduction in the coupling constant, J (Rh,P) = 136 Hz, is also observed. Their 13 C{ 1 H} NMR spectra show the resonance due to the acyl group in the low field, 222-228 ppm range, as a doublet of doublet due to Table 1 Ranked PCM corrected relative free energy values (ΔG, kcal mol −1 ) calculated at the M062X/6-311++G(2d,2p)//B3LYP/6-31G (d,p)& LANL2DZ level of theory, for the most stable isomers/rotamers of compounds 1-Hpz, coupling with rhodium ( J (Rh,C) = 28 Hz) and with a cis phosphorus atom ( J (P,C) = 3 Hz).
Complexes 1 react with triphenylphosphane that displaces the pyrazole ligand trans to phosphorus to give complexes [RhHCl{PPh 2 (o-C 6 H 4 CO)}(PPh 3 )L] (L = Hpz, 3-Hpz; Hmpz, 3-Hmpz; Hdmpz, 3a-Hdmpz and 3b-Hdmpz) as shown in Scheme 3i. The reaction is stereoselective for Hpz and Hmpz affording only one isomer, which, in accord with the DFT studies performed for complexes 1, we assume to be the cis-H, Cl species, while Hdmpz affords an almost equimolar mixture of two isomers that, as shown by the NMR spectra, can only differ in the group trans to the hydrido, pyrazole in 3a-Hdmpz or chlorido in 3b-Hdmpz.
The 31 P{ 1 H} NMR spectra show two doublets of doublets in the 65-62 and the 44-38 ppm range, due to the chelating acylphosphane and PPh 3 respectively. The J (P,P) coupling constants, in the 376-379 Hz range, indicate two mutually trans phosphorus atoms. In the 13 C{ 1 H} NMR spectra, the resonance due to the acyl group appears as a doublet ( J (Rh,C) in the range 28- 35 Hz) in the expected low field (232-235 ppm range), 28 and the resonances due to the 3 and 5 carbon atoms of the pyrazole rings are different, indicating the absence of tautomerization in all cases. In the 1 H NMR spectra, the hydrido appears as a doublet of doublet of doublet due to coupling to rhodium and to two cis phosphorus atoms, in the −14.0 to −14.5 ppm range, in a slightly lower field than in complexes 1. Sharp singlets in the 11.0-11.8 ppm range are also observed, due to the pyrazole being involved in N-H⋯Cl hydrogen bonding, and the resonances for protons or methyl groups on the 3 and 5 carbon atoms of the pyrazole rings are different. An X-ray diffraction study of 3a-Hdmpz confirms the structure depicted in Scheme 3i.
The reaction of complexes 1 with diphenylphosphane oxide affords complexes [RhHCl{PPh 2 (o-C 6 H 4 CO)} {(PPh 2 O)H} L] (L = Hpz,Hmpz,Hdmpz, shown in Scheme 3ii. As in the reaction with PPh 3 , 1-Hpz affords a single isomer 4-Hpz. 1-Hmpz gives two linkage isomers, 4a-Hmpz : 4a′- Hmpz = 9 : 1, as in the starting material. At variance with the reaction with PPh 3 , 1-Hdmpz, which consists of an almost equimolar mixture of two isomers, affords now a single isomer 4-Hdmpz. The spectroscopic features of complexes 4 are similar to those reported for complexes 3 (see Experimental). In the low field region of the 1 H NMR spectra, a sharp singlet is observed, in the 10.7-11.2 ppm range due to N-H⋯Cl. Another sharp singlet is observed in the 12.0-12.7 ppm range that can be attributed to the presence of a hydrogen bond between the oxygen atom of the diphenylphosphinous acid and the oxygen atom of the coordinated acyl group. 3d,29 This O-H⋯O hydrogen bond appears to contribute to the stereoselectivity of this reaction to afford a single isomer also in the Hdmpz case. DFT calculations (see below) show this isomer to be the cis-H,Cl species.
The same theoretical study as before was carried out to identify the particular isomers/rotamers obtained for 3-Hdmpz-, 4-Hdmpz-and 5-Hdmpz-type complexes. Phosphorus atoms were considered to be in the trans position to each other in all cases since NMR data for the mentioned complexes were unambiguous in this respect. Structures with intramolecular hydrogen bonds established between Hdmpz and any acceptor atom other than chlorido were not considered in these cases (Table SI-2 †), since our DFT studies on complexes 1 have shown that pyrazole ligands cis to phosphorus form hydrogen bonds exclusively with chlorido. DFT results ( Table 2) for 3-Hdmpz-type complexes revealed that two species with a moderate energy difference (0.60 kcal mol −1 ) may exist in a 3 : 7 ratio, namely, the cis-H,Cl 3a-Hdmpz and the trans-H,Cl 3b-Hdmpz isomers (Fig. SI-3a and b, † respectively), the trans-H,Cl isomer being the most abundant species. The latter result is similar to that previously described for 1-Hdmpz and agrees with the experimental NMR data, which show the presence of two different species in solution. In the case of 4-Hdmpz-type complexes, and in agreement with the NMR data, the cis-H,Cl isomer 4-Hdmpz was predicted to be the only species in solution ( Fig. SI-3c †). The energy difference (2.99 kcal mol −1 ) with the energetically closest structure ruled out any other possibility (Table SI-2 †). As depicted in Fig. SI-3c, † the acidic hydrogen atom belonging to the diphenylphosphinous acid would establish a hydrogen bond with the O-acyl moiety. In this case, a combination of a lesser steric hindrance of PPh 2 OH versus PPh 3 along with fixation of the O-acyl atom by O-H⋯O bond formation, relieves the steric clashes between pyrazole methyl substituents and the neighbouring O-acyl, thus stabilizing the cis-H,Cl conformation. Theoretical calculations on 5-Hdmpz-type complexes revealed that the 5a-Hdmpz cis-H,Cl isomer is 0.82 kcal mol −1 more stable that the trans-H,Cl isomer 5b-Hdmpz (Table 2) and that both isomers may exist in an 8 : 2 ratio. The latter results are in agreement with the corresponding NMR data, which contain signals due to two different species in solution. As observed in Fig. SI-3d and SI-3e † a quantitatively significant N-H⋯Cl intramolecular bond is established, while a second hydrogen bond, which may be quantitatively much less meaningful, is also observed between the hydrogen atom of the aldehyde moiety and either the hydrido (5a-Hdmpz) or the O-acyl group (5b-Hdmpz). A visual inspection of 5a-Hdmpz and 5b-Hdmpz suggests a suboptimal spatial disposition of the acyl-phosphane ligand in the trans-H,Cl isomer, derived from its attempt to establish a H-bond with the H atom of the aldehyde moiety of the PPh 2 (o-C 6 H 4 CHO) ligand, which could rationalize the observed isomer ratios.
The X-ray structures of 2-Hpz (dichloromethane adduct), 3a-Hdmpz (chloroform adduct) and 7-Hpz have been determined. Selected bond distances and angles are listed in Table 3. Fig. 1-3 show molecular drawings for these complexes together with the atomic labelling scheme used. All the three complexes show a slightly distorted octahedral arrangement, with two positions occupied by the phosphorus and carbon atoms of an acyl-phosphane ligand and a chlorido ligand trans to the acyl carbon atom. 2-Hpz also contains two mutually cis pyrazole ligands that are trans to a second chlorido and to phosphorus. Compounds 3a-Hdmpz or 7-Hpz contain a single pyrazole ligand and hydrido and triphenylphosphane or a second acyl-phosphane ligand, respectively. In the latter compounds the pyrazole ligand lies trans to hydrido or to an acyl carbon atom, respectively, thus allowing the phosphorus atoms to adopt a trans arrangement. The Rh1-N1 distances: 2.046(2), 2.205(2) and 2.221(6) in 2-Hpz, 3a-Hdmpz and 7-Hpz respectively and the Rh1-N3 distance of 2.136(2) Å in 2-Hpz reflect the trans influence order of the ligands: chlorido <  phosphane < hydrido ≈ acyl. 31 In all the three structures the pyrazole moieties and the chlorido ligands are mutually cis and the formation of N-H⋯Cl hydrogen bonds is observed.
The pyrazole protons involved in these hydrogen bonds could be clearly located as Fourier peaks in a difference map. The N⋯Cl distances, between 3.017 Å and 3.067 Å, for all the three complexes are as expected for moderately strong hydrogen bonds, however the N-H⋯Cl angles, between 128(1) and 140(1)°, are in the lower range of the corresponding values. 7,32 Nevertheless, the importance of the hydrogen bonding can also be assessed by the dihedral angle between two planes, that of the pyrazole ring (including the N-H bond) and the plane defined by the coordinated N atom of pyrazole, chlorido, and their trans atoms to the metal and the metal, which should be close to 0°. 8 In 2-Hpz these angles are very small, 4.8 and 6.6°f or the planes involving the N4 and N2 atoms, respectively, indicating that the N-H vector is directed in order to maximize the intramolecular hydrogen bonding. For 3a-Hdmpz and 7-Hpz the values of these dihedral angles are 9.8 and 13.7°, respectively. In 2-Hpz the planes of the heterocycles are almost perpendicular (θ = 86.8°), and this feature may be related to the presence of the hydrogen bonds. Also in 2-Hpz the chlorido trans to acyl forms another hydrogen bond, N2-H2N⋯Cl1″, weaker, with the pyrazole trans to chlorido in a neighbouring molecule (N2-Cl1″ = 3.241(2) Å). This intermolecular interaction leads to the formation of dimers. Complexes 1 and 4, containing N-H⋯O or O-H⋯O hydrogen bonds respectively involving the acyl groups, were tried as catalysts for the hydrolysis of AB, TBAB (tert-butylamineborane), and DMAB (dimethylamine-borane) in THF/H 2 O = 1/1 mixtures, at 298 K, in the presence of air. According to eqn (1),   up to three equivalents of hydrogen per equivalent of an adduct may be released.
With an initial AB, TBAB or DMAB concentration of 0.46 M and a 0.5 mol% catalyst loading of complexes 1, the hydrolysis and the release of 75-90% of the maximum hydrogen content that could be produced, was observed within 25 min ( Table 4). The nature of the product gas was verified as H 2 by NMR of the catalytic hydrolysis of AB in a THF-d 8 /D 2 O mixture, which shows the gradual disappearance of the quartet centred at δ = 1.37 ppm ( 1 J B-H = 92 Hz) due to AB and the formation of HD at δ = 4.52 (t) ppm ( 1 J D-H = 43 Hz). Blank tests under the same experimental conditions, but in the absence of a catalyst, show that the hydrolysis of AB, TBAB or DMAB affords a maximum amount of only 10% of the hydrogen content after 4 min Fig. SI-4 †).
The catalytic activity for complexes 1-Hpz, 1-Hmpz and 1-Hdmpz was similar. As shown in Fig. 4, the amount of hydrogen evolved from all the three substrates was analogous. When using complexes 4 a similar hydrogen release requires at least 50 min reaction times (Table 4). After completion of the catalytic reactions, the 11 B NMR spectra of the remaining solutions contain only singlets in the 16-20 ppm range, which indicates the transformation of the borane adducts into borate species. 13a,b In the presence of Hg similar results were obtained, confirming the homogeneous nature of the catalytic reaction. 13c Fig. 4 shows the kinetic profiles when using 1-Hmpz as a catalyst. The kinetic profiles obtained in the hydrolysis of AB and TBAB can be considered to follow a first order reaction rate model with respect to the [substrate]. When the experimental points are fitted to a first order law by non-linear least-squares regression, the values of the rate constants k obs = (1.89 ± 0.06) × 10 −3 and (1.98 ± 0.10) × 10 −3 s −1 can be obtained respectively for AB and TBAB. The kinetic profile of the hydrolysis of DMAB does not show a defined kinetic model. The activity shown by complexes 1 is comparable to that observed for acylrhodium complexes containing pyridine and diphenylphosphinous acid that hydrolyse AB and TBAB at 298 K with rate constants k obs = (1.54 ± 0.07) × 10 −3 and (0.95 ± 0.05) × 10 −3 s −1 , respectively. 3d

Conclusions
The stereoselective formation of hydrido(acylphosphane)-(chlorido)( pyrazole)rhodium(III) complexes, with chlorido trans to acyl and hydrido trans to pyrazole and stabilized by the formation of hydrogen bonds, is accomplished. The formation of N-H⋯Cl and N-H⋯O hydrogen bonding occurs and the former is preferred to the latter. Steric hindrance lowers the stereoselectivity of the reaction by lowering the energy difference between isomers. In close agreement with the experimental NMR data, DFT calculations were able to predict the isomer/rotamer proportions and the particular intramolecular hydrogen bonds established in each case. The bis( pyrazole) complexes are efficient homogeneous catalysts for the hydrolysis of ammonia-or amine-borane adducts in air to produce hydrogen.

DFT calculations
Quantum chemical calculations were carried out with the Gaussian 09 series of programs. 37 Full geometry optimizations of each species were performed in the gas phase by employing the hybrid density functional B3LYP 38 with the 6-31G (d,p) basis set for nonmetal atoms together with the LANL2DZ 39 for the metal. When available, X-ray structures were used as starting geometries. Energy values were then refined by singlepoint calculations on the B3LYP/LANL2DZ geometries at the M062X/6-311++G(2d,2p) level of theory and solvent (benzene) contributions to free energy were considered by means of polarizable continuum model 40 (PCM) calculations. The nature of the stationary points was verified by analytical computations of harmonic vibrational frequencies at the M062X/ 6-311++G(2d,2p) level of theory. A pressure of 1 atm and a temperature of 298.15 K were assumed in the calculations. The theoretical ratio of the isomers/rotamers present in solution was predicted assuming a Boltzmann distribution. 41 Dehydrogenation of RR′R″N-BH 3 with complexes 1 or 4 A typical dehydrogenation experiment is described here for the hydrolysis of H 3 N-BH 3 using 2.70 ml of THF/H 2 O = 1 : 1 mixtures and 0.5 mol% catalyst loading: a solution of 38.8 mg (1.25 mmol) of H 3 N-BH 3 in H 2 O was prepared in a round bottom 40 ml flask fitted with a gas outlet and with a side arm sealed with a tight-fitting septum cap. The flask was connected via the gas outlet to a water-filled gas burette. A solution of 0.006 mmol of the corresponding catalyst in dry THF was then syringed through the septum, magnetic stirring was connected and timing started. Gas evolution began immediately and the amount of gas evolved was determined periodically by measuring the displacement of water in the burette. Volumes were measured at atmospheric pressure and 298 K.