E–H Bond Activation of Ammonia and Water by a Geometrically Constrained Phosphorus(III) Compound

The synthesis of a phosphorus(III) compound bearing a N,N-bis(3,5-di-tert-butyl-2-phenoxy)amide ligand is reported. This species has been found to react with ammonia and water, activating the E–H bonds in both substrates by formal oxidative addition to afford the corresponding phosphorus(V) compounds. In the case of water, both O–H bonds can be activated, splitting the molecule into its constituent elements. To our knowledge, this is the first example of a compound based on main group elements that sequentially activates water in this manner.

The controlled activation of polar small molecules,s uch as ammonia and water, is ac hallenging and highly desirable target, as these substrates are abundant and inexpensive feedstocks for the synthesis of value-added chemicals and the generation of renewable energy. [1,2] Theprocesses required to carry out such transformations (for example,o xidative addition and reductive elimination) are most commonly associated with precious-metal catalysts,f or which crustal abundance,e xpense,a nd toxicity are significant issues. Consequently,t he last decade has seen significant advances in the development of main group species that are capable of activating an umber of challenging small-molecule substrates. [3] Such systems include alkyl(amino) carbenes (AACs), [4] low-oxidation-state compounds of the heavier Group 13 and 14 elements, [5,6] and frustrated Lewis pairs (FLPs). [7] These species have been shown to activate substrates including ammonia and/or dihydrogen;s everal such systems have even demonstrated the ability to carry out these processes catalytically. [8] New routes resulting in the activation of NÀHb onds in ammonia are particularly appealing because of the dearth of transition-metal systems capable of effecting such atransformation [9][10][11][12][13][14][15][16][17][18][19][20][21] and the relevance of N À H activation to an umber of potentially important industrial processes. [22] In addition to these high profile examples,r ecent studies have shown that complexes of phosphorus(III) with distorted geometry can cleave arange of polarized EÀHbonds (E=Oor N). Studies by Arduengo et al. and Radosevich and coworkers have shown that the planar T-shaped phosphorus system, A,f acilitates the oxidative addition of ammonia, primary amines,and alcohols ( Figure 1). [23,24] Furthermore, A can be used in conjunction with ammonia-borane to facilitate the catalytic reduction of azobenzene. [25] Related studies have since extended this approach to nonplanar phosphorus triamide (B)a nd diazadiphosphapentalene (C), [26,27] both of which activate E À Hb onds through al igand-assisted mechanism.
Them echanisms by which A-C activate small molecules are strongly dependent on the sterics/electronics of the ligand backbone.F or example,ithas been shown that, following the activation of alcohols by A,p roton migration to the ligand backbone occurs,r egenerating ap hosphorus(III) compound. [23] This inspired us to investigate the utility of the tridentate N,N-bis(3,5-di-tert-butyl-2-phenoxy)amidel igand (denoted [ONO] 3À ), previously studied with transition-metal centers, [28] as asupport for anovel geometrically constrained phosphorus system. It was hypothesized that the aromatic backbone of ONO 3À should preclude proton migration to the ligand following E À Hb ond activation.
Thereaction between H 3 [ONO],PCl 3 ,a nd triethylamine (Scheme 1) leads to the quantitative formation of as ingle product (1). As inglet for the product was detected in the 31 PNMR spectrum at d = 168.6 ppm. [29] This chemical shift falls within the characteristic range exhibited for compounds A, B,a nd C (d = 187.0, 159.8 and 173.2 ppm, respectively). Additionally, 1 HNMR analysis confirmed that the molecule had as ymmetric ligand environment (see the Supporting Information for full experimental details).
1 was isolated as ac ompositionally pure colorless solid, solutions of which turn violet upon exposure to oxygen, presumably because of the redox-active nature of the ONO 3À ligand. Crystals of 1 suitable for single-crystal X-ray diffraction studies were grown from cold pentane and the molecular structure confirmed the formation of the desired phosphorus(III) species ( Figure 2).
Them olecular structure of 1 differs from the planar geometry exhibited by the analogous 10-P-3 species A (where 10-P-3 denotes a3 -coordinate Pcenter with 10 valence electrons) with pyramidalization at both nitrogen and phosphorus centers providing pseudo C s symmetry,s imilar to B and C.W hilst the O-P-N bond angles (93.21(5)8 8 and 93.36(5)8 8)a re statistically identical, the O-P-O angle is significantly larger because of the constraints of the ligand backbone (109.55(5)8 8). Thes um of the bond angles around the phosphorus center (296.128 8)thus confirms the nonplanar geometry.The PÀNbond length of 1 (1.757(1) ) is the same (within error) as that of B (1.761(1) ) but is significantly longer than that of A (1.703(2) ), suggesting negligible donation of the nitrogen lone pair to the orbitals on phosphorus.T he deviation from planarity also results in as hortening of the PÀOb onds of 1 (1.659(1) a nd 1.652(1) ) relative to A (1.835(2) a nd 1.792(2) ), which now fall within the expected range for P III ÀOb onds. Calculations using density functional theory (DFT) reveal that pyramidal (C s )a nd planar (C 2v )i somers of 1 are within 4kJmol À1 of one another,w ell within the error of the calculations.This suggests that fluxionality between isomers is likely to be present in solution. Thecalculations also reveal an energetically accessible LUMO for the optimized C 2v geometry which is predominantly composed of phosphorus atomic orbital character (54.7 %). This implies that that subsequent reactions involving the activation of small molecules might proceed via an initial nucleophilic pathway, corroborating experimental findings that 1 does not react with non-nucleophilic substrates such as phenylacetylene and phenylsilane (see below).
Thea ctivation of the N À Hb onds of ammonia has long posed as ignificant challenge for transition-metal complexes, on account of the unfavorable coordination/activation equilibrium for this substrate.For this reason examples are scarce, and even fewer constitute classical oxidative additions. [9][10][11][12][13][14][15][16][17][18][19][20][21] Therefore,a dvances in main group mediated activation of ammonia are of great interest. [4, 5, 6c-d,24, 26, 27, 30-32] Ther eaction of 1 with ammonia gas (1 atm) results in the quantitative conversion of 1 to anew product, 2 (Scheme 2). The 31 PNMR spectrum of 2 exhibits ar esonance at d = À46.1 ppm as adoublet of triplets ( 1 J P-H = 819 Hz, 2 J P-H = 11 Hz), consistent with the oxidative addition of as ingle NÀHb ond at the phosphorus center. Furthermore,c omplementary doublets are detected in the 1 HNMR spectrum at d = 8.46 ppm and d = 2.16 ppm integrating to one (PH)and two protons (NH 2 ), respectively.T he proposed structure of 2 was confirmed through single-crystal X-ray diffraction studies ( Figure 3). Although ammonia has been shown to react with both A and B [24,26] in ap rocess that constitutes af ormal oxidative addition, 2 is the first structurally authenticated example of such ac ompound.
Them olecular structure of 2 exhibits at rigonal-bipyramidal structure,inwhich the hydride and two amide moieties adopt equatorial positions.The ONO 3À ligand adopts aplanar conformation, analogous to the mesitylamine activation product of A,[ A(H)(NHMes)]. [24] By contrast, the reaction products of B have been shown to retain af olded ligand geometry. [26] Thea xial disposition of the P À Ob onds of 2 results in their lengthening relative to the analogous bonds in 1,w hilst the increased valencye ffects ac ontraction of the P1ÀN1 bond length (average value of 1.701 ). Substitutional

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Chemie disorder of the amide and hydride substituents over the two equatorial sites precludes the analysis of the relevant bond metrics.
Thef acile activation of ammonia by 1 encouraged us to explore its reactivity towards other more challenging substrates.Remarkably,itwas discovered that 1 undergoes rapid addition of water at the phosphorus center to form 3,with the reaction proceeding quantitatively in the presence of asingle equivalent of substrate (Scheme 3). The 31 PNMR spectrum of the reaction mixture shows abroad doublet resonance at d = À36.9 ppm ( 1 J P-H = 881 Hz), which collapses to asinglet upon proton decoupling.T he signal for the corresponding hydride is detected as ad oublet in the 1 HNMR spectrum at d = 8.22 ppm whilst abroad resonance at d = 3.87 ppm is evidence of the hydroxyl proton. Furthermore,F TIR spectral analysis of the product reveals ab road band at 3425 cm À1 ,c onsistent with an OÀHb ond stretch. Crystals of 3 were grown from ac oncentrated toluene solution and single-crystal X-ray diffraction analysis confirmed the oxidative addition of water over the phosphorus center ( Figure 4). This observation comes in stark contrast to the reactivity of A,w hich is reported to hydrolyze to the corresponding free ligand and phosphoric acid. [23] Theo xidative activation of water is of immense current interest, representing ac rucial step in metal-catalyzed water splitting.D espite this fact, there are few transition-metal complexes that are able to effect this transformation. [2] Although there are several reports outlining similar reactivity at main group centers, [33][34][35][36][37][38][39][40][41] there are currently only two examples that are able to do so without the use of forcing conditions,n on-equimolar loadings of water, or secondary activating substrates. [37,41] Ther oom-temperature reaction of 3 with one equivalent of deuterium oxide results in exchange of the hydroxyl proton. Interestingly,when the reaction is carried out at 70 8 8C, exchange of the phosphorus-bound hydride is also observed, providing a1:1 isotopic distribution. Therelative ease of H/D exchange is in agreement with aprevious report by Pçrschke et al. [35] In addition to the previously observed doublet, attributable to the phosphorus center of 3,t he 31 PNMR spectrum of the reaction mixture also shows at riplet at d = À37.7 ppm ( 1 J P-D = 133 Hz). Acomplementary doublet is also Figure 3. Molecular structure of one the two independentmolecules of 2 in the asymmetric unit (thermal ellipsoids set at 50 %probability;all hydrogen atoms, with the exception of those bonded to phosphorus and nitrogen, are omitted for clarity). [29] Despite the substitutional disorder between the hydride (H1) and amide (N2H 2 )moieties we were able to locate the proton positions in the difference Fourier map, however the bond lengths needed to be restrained.S elected bond lengths [] and angles [8 8]ofone of the independent molecules within the crystal lattice:P 1 ÀN1 1.700(2), P1ÀO1 1.718(2), P1ÀO2 1.710(2); O1-P1-O21 76.67 (11),N 1-P1-O18 8.64 (11), N1-P1-O28 8.36 (11).  . .

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Communications observed in the 2 DNMR spectrum at d = 8.75 ppm). The magnitude of the coupling constant is consistent with the difference in gyromagnetic ratio of the hydrogen and deuterium nuclei (g H /g D = 6.5).
Thelability of the hydroxyl proton of 3 encouraged us to investigate the equimolar reaction of this product with as econd equivalent of 1 (Scheme 3). At room temperature, the reaction proceeds slowly,however,after 12 hat708 8C, the 31 PNMR spectrum of the reaction mixture indicates that 3 has been fully converted to an ew product, 4,w hich exhibits as econd-order multiplet resonance at d = À44.0 ppm (m, 1 J P-H = 913 Hz, 2 J P-P = À30 Hz, 3 J P-H = 1Hz), which collapses to as inglet on proton decoupling.Asimilar multiplet is also observed in the corresponding 1 HNMR spectrum at d = 8.43 ppm. Single-crystal X-ray diffraction studies revealed that the product 4 featured an oxygen-bridged dimeric structure ( Figure 5), arising from oxidative addition of the remaining O À Hb ond over the phosphorus(III) center of 1, thereby completely splitting water into its constituent elements.Similarly,reaction of 1 with substoichiometric amounts of H 2 Oalso leads to the formation of 4.Subsequent reaction of 4 with water regenerates the monomeric precursor 3.T o our knowledge,s tepwise oxidative addition of water by am ain group system has not previously been reported. However,similar reactivity has been postulated by Driess and co-workers who isolated amixed-valence disiloxane from the reaction between water and asilylene,the synthesis of which was proposed to proceed via an unisolable [LSi(OH)(H)] intermediate. [36] Tr ansition-metal complexes of silyenes are also know to react in such amanner. [42] Additionally,Bercaw and Hillhouse have reported the related Group 4c omplexes [{Cp* 2 M(H)} 2 m-O] (M = Zr and Hf;C p* = pentamethylcyclopentadienyl), formed via s-bond metathesis pathways. [10] It is worth noting at this stage that 2 does not react further with 1 to afford the related imide-bridged species. Themolecular structures of 3 and 4 are both significantly distorted from at rigonal-bipyramidal geometry about the phosphorus center with the geometry of the latter much closer to square-based pyramidal (t = 0.52 and 0.17-0.33, respectively). In both cases,t he N-P-H bond angles are relatively large and the N-P-O (OH/m-O) angles small, due to ag reater degree of negative hyperconjugation into the PÀO (OH/m-O) s*orbital than into the respective PÀN s*orbital of 2.T he chelating ligands of both species are shown to adopt aplanar conformation and in the case of 3 (Figure 4), this is coincident with ac rystallographic mirror plane that disorders the hydride and hydroxide substituents over two positions.T he dimeric species 4 exhibits af olded structure in which the ligands of each phosphorus center diverge,b ringing the two hydrides closer together in space,w hilst as light torsion around the P À O (m-O) bonds reduce the proximity of the bulky tert-butyl groups ( Figure 5). TheP 1 À O3 bond of 3 (1.774 (3) ) is significantly longer than both ligand P À O bonds and appreciably longer than the PÀO(Ph) bond in the phenol activation product of B,[B(H)(OPh)] (1.657 (2) ). [26] It is also slightly larger than the sum of the respective covalent radii but well within the sum of the Va nder Waals radii. [43,44] In contrast, the P À O (m-O) bonds of 4 are much shorter (1.601(2)-1.631(2) ), consistent with ag reater electrostatic interaction between the bridging oxo group and the two phosphorus centers.T he ligand PÀOand PÀNbonds of both species are comparable to those of 2.
Interestingly,w hen as olid sample of 3 is heated under adynamic vacuum at 100 8 8Cf or 36 h, the 31 PNMR spectrum indicates the formation of 4 after redissolving the solid. This suggests that water activation may be reversible (that is, 1 is formed which subsequently reacts with 3 to form 4). An alternative mechanism involving ac ondensation reaction between two molecules of 3 is also possible.T op robe this further, we decided to investigate the behavior of 2 under related conditions where the possibility of the dimeric imidebridged system is not possible.Although there is no evidence for reversibility of the oxidative addition product detected in solution, heating solid samples of 2 at 100 8 8Cunder adynamic vacuum does regenerate small amounts of 1.T hese observations contrast with those made for [A(H)(NH 2 )],w hich is shown to sublime without decomposition at 40 8 8C(1mmHg). Ther elated primary amine and alcohol activation products have,h owever, been shown to be reversible under forcing conditions. [24] To conclude,w eh ave shown that the novel phosphorus-(III) species 1 is capable of activating the polar EÀHbonds of Figure 5. Molecular structure of one the two independentmolecules of 4 in the asymmetric unit (thermal ellipsoids set at 50 %probability;all hydrogen atoms, with the exception of those bonded to phosphorus, are omitted for clarity). [29] Selected bond lengths for one of the two independentmolecules in the asymmetric unit [] and angles

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Chemie water and ammonia. In the case of water, both of the OÀH bonds can be activated sequentially,atransformation which, while postulated in the chemical literature for both transitionmetal and main group compounds,h as never permitted for the isolation of both the hydride/hydroxide and oxo-bridged products in am ain group system. Studies are currently ongoing in our research groups with the goal of converting these interesting stoichiometric reactions into catalytically viable processes capable of transforming abundant and inexpensive small molecules such as H 2 Oa nd NH 3 into value-added chemicals.