Hydrolysis of Element (White) Phosphorus under the Action of Heterometallic Cubane-Type Cluster {Mo3PdS4}

Reaction of heterometallic cubane-type cluster complexes—[Mo3{Pd(dba)}S4Cl3(dbbpy)3]PF6, [Mo3{Pd(tu)}S4Cl3(dbbpy)3]Cl and [Mo3{Pd(dba)}S4(acac)3(py)3]PF6, where dba—dibenzylideneacetone, dbbpy—4,4′-di-tert-butyl-2,2′-bipyridine, tu—thiourea, acac—acetylacetonate, py—pyridine, with white phosphorus (P4) in the presence of water leads to the formation of phosphorous acid H3PO3 as the major product. The crucial role of the Pd atom in the cluster core {Mo3PdS4} has been established in the hydrolytic activation of P4 molecule. The main intermediate of the process, the cluster complex [Mo3{PdP(OH)3}S4Cl3(dbbpy)3]+ with coordinated P(OH)3 molecule and phosphine PH3, have been detected by 31P NMR spectroscopy in the reaction mixture.


Introduction
Both organic and inorganic phosphorus-containing compounds have become widespread agents for various industrial applications. Traditional methods for the preparation of phosphorus compounds involve oxidation and chlorination of the element (white) phosphorus (P 4 ) and use the phosphorus chlorides as phosphorylating agents for the synthesis of various organophosphorus substrates. It should be noted that direct activation and transformation of P 4 is a very harsh and risky process that involves toxic and hazardous reagents and waste, and which negatively impacts on the environment.
The oxidation of P 4 in the presence of H 2 O usually leads to the formation of phosphoric acid H 3 PO 4 , while phosphorous acid H 3 PO 3 is more interesting and important phosphorus precursor which can be used as a phosphorylating agent as it contains a functionally capable P-H bond. Moreover, the current methods for the preparation of H 3 PO 3 leave much to desire and require the use of toxic and hazardous phosphorus trichloride producing a huge amount of corrosive gaseous hydrogen chloride and dangerous phosphorus chloro-derivatives.
We have previously demonstrated that palladium complexes can be efficiently used for the preparation of phosphorous acid H 3 PO 3 directly from P 4 by its mild hydrolysis in the coordination sphere of the metal [1,2]. However, the main limitation of these catalytic systems is the formation of catalytically inactive palladium phosphides and palladium black [3]. It is important to note, that ruthenium-mediated P 4 hydrolysis was thoroughly studied in Florence in the scientific group of M. Peruzzini and P. Stoppioni. It was shown that the hydrolysis of [CpRu(PPh 3 ) 2 (η 1 -P 4 )] (Cp = cyclopentadienyl) complex leads to the formation of phosphine PH 3 and phosphorous acid H 3 PO 3 [4,5]. Moreover, it has been established that the mechanism of this process may involve the formation of binuclear intermediate, when η 2 -P 4 unite is doubly coordinated to two {CpRu(PPh 3 ) 2 } moieties, which is hydrolyzed with the formation of H 3 PO 3 and previously unknown 1-hydroxytriphosphane (PH(OH)PHPH 2 ) as the intermediate of the overall process [6]. Later this process was improved and the kinetic of P 4 hydrolysis was investigated using ruthenium complexes with water-soluble phosphine ligands [7]. There is also a notable example of stabilization of phosphorous acid H 3 PO 3 in its tautomeric form P(OH) 3 on the Ru site [8]. Hence, further development of new methods of white phosphorus hydrolysis is of high interest.
Heterometallic cubane-type cluster complexes with {M 3 PdS 4 } (M = Mo, W) core that were first described by Hidai's group in Japan [9,10], possess a number of attractive properties including unordinary reactivity and catalytic activity of the Pd site [11][12][13][14][15][16][17][18][19][20][21][22]. To cite the most recent example, {Mo 3 PdS 4 } cluster complexes react with fullerene C 60 to form hybrid compounds containing a fullerene molecule coordinated to palladium in the cluster core [23]. The {M 3 PdS 4 } clusters catalyze allylation of aromatics [24][25][26] and nucleophilic addition to triple bonds [10,27]. However, until now there have been no examples of P 4 molecule coordination and activation in the coordination sphere of the {Mo 3 PdS 4 } cluster core, despite reported ability of such clusters to stabilize the unstable species or less-favoured tautomers, such as As(OH) 3 , P(OH) 3 , PhP(OH) 2 , Ph 2 P(OH), HP(OH) 2 through coordination to the palladium site, which indicates high affinity of the latter for pnictogens. [28,29]. From these earlier studies, we have assumed that {Mo 3 PdS 4 } complexes could be involved in the process of P 4 activation and its transformation.
The common approach for synthesis of the {M 3 M'S 4 } clusters involves the reaction of a low-valent metal precursor with a {M 3 S 4 } trinuclear complex in a desired coordination environment [12]. The same synthetic approach has been applied in current work, where [Pd 2 (dba) 3 ] × CHCl 3 was used as the palladium source. The synthetic routes to new cluster complexes 1 and 3 used in this work are depicted in Scheme 1. The cluster complex 2 has been obtained according to a previously published procedure [36].
In the case of 3, the IR spectrum is complicated by a significant overlap between bands from acac and py ligands. The band at 1603 cm −1 (ν(C=C + C-N)) relates to the bonded pyridine, and the two characteristic bands (ν(C=C + C=O)) at 1578 and 1523 cm −1 are associated with the acetylacetonate ligand. The two intensive bands at 838 and 556 cm −1 are explained by the presence of PF6 − group. The highly intensive band at 1627 cm −1 reveals the presence of coordinated dba [37].
The 1 H NMR spectra of both 1 and 3 in CDCl3 demonstrate a complicated pattern due to the presence of overlapping signals in the 7-9 ppm area,that originate from the protons of various aromatic rings present indbbpy, py, and dba ligands. There are also signals related to alkene fragments in dba (δ ~ 7 ppm). The characteristic signals generated by protons of the tert-butyl group (δ 1.40-1.45 ppm) in dbbpy ligand are detected in the case of 1. The spectrum of 3 contains peaks associated with CH3-(δ 1.85 ppm) and CH-groups The de-coordination of the pyridine ligands is expectable under ionization conditions and was also observed for the trinuclear {Mo3S4} precursor [38]. All the peaks have been assigned both from m/z and characteristic isotope patterns. Scheme 1. Synthesis of cubane-type cluster complexes 1 and 3.
In the case of 3, the IR spectrum is complicated by a significant overlap between bands from acac and py ligands. The band at 1603 cm −1 (ν(C=C + C-N)) relates to the bonded pyridine, and the two characteristic bands (ν(C=C + C=O)) at 1578 and 1523 cm −1 are associated with the acetylacetonate ligand. The two intensive bands at 838 and 556 cm −1 are explained by the presence of PF 6 − group. The highly intensive band at 1627 cm −1 reveals the presence of coordinated dba [37].
The 1 H NMR spectra of both 1 and 3 in CDCl 3 demonstrate a complicated pattern due to the presence of overlapping signals in the 7-9 ppm area, that originate from the protons of various aromatic rings present in dbbpy, py, and dba ligands. There are also signals related to alkene fragments in dba (δ~7 ppm). The characteristic signals generated by protons of the tert-butyl group (δ 1. 40 3 ] + ) are detected in the spectrum of 3. The de-coordination of the pyridine ligands is expectable under ionization conditions and was also observed for the trinuclear {Mo 3 S 4 } precursor [38]. All the peaks have been assigned both from m/z and characteristic isotope patterns.

Interaction with P 4
The reactivity of heterometallic cluster complexes [Mo 3 {Pd(dba)}S 4 Cl 3 (dbbpy) 3 ]PF 6 (1), [Mo 3 {Pd(tu)}S 4 Cl 3 (dbbpy) 3 ]Cl (2) and [Mo 3 {Pd(dba)}S 4 (acac) 3 (py) 3 ]PF 6 (3) towards P 4 was investigated both in the absence and in the presence of water. According to 31 P NMR spectra, addition of the equimolar amount of P 4 to the solutions of these complexes in DMF, THF and CH 2 Cl 2 does not lead to transformation of P 4 molecule, and no new signals from phosphorus-containing species were detected.
However, addition of water to the DMF solutions containing complexes 1 or 2 and P 4 has led to the appearance of the signals associated with inorganic oxo-acids H 3 PO 3 (δ 2.1 ppm) and H 3 PO 4 (δ 0.9 ppm) with integral ratio of 3.4:1.0 (for 1) and 7.5:1.0 (for 2). Additionally, new signals around 110 ppm were detected. These signals correspond to the formation of [Mo 3 {PdP(OH) 3 }S 4 Cl 3 (dbbpy) 3 ] + (Figure 1), in which the Pd atoms bears the tautomeric form of phosphorous acid H 3 PO 3 (P(OH) 3 ) formed by the hydrolysis of white phosphorus. These results nicely fit with the previously published data [28,29], where the signals around 115 ppm in 31  DMF, THF and CH2Cl2 does not lead to transformation of P4 molecule,and nonew signalsfrom phosphorus-containing species were detected.
However, addition of water to the DMF solutions containing complexes 1 or 2 and P4 has led to the appearance of the signals associated with inorganic oxo-acids H3PO3 (δ 2.1 ppm) and H3PO4 (δ 0.9 ppm) with integral ratio of 3.4:1.0 (for 1) and 7.5:1.0 (for 2). Additionally, new signals around 110 ppm were detected. These signals correspond to the formation of [Mo3{PdP(OH)3}S4Cl3(dbbpy)3] + (Figure 1), in which the Pd atoms bears the tautomeric form of phosphorous acid H3PO3 (P(OH)3) formed by the hydrolysis of white phosphorus. These results nicely fit with the previously published data [28,29], where the signals around 115 ppm in 31 P NMR spectra were attributed to the complexes [Mo3{PdP(OH)3}S4(H2O)9-xClx] (4-x)+ obtained by the reaction of [Mo3{PdCl}S4(H2O)9] 3+ with PCl3 or H3PO3 in 4M HCl. The total conversion of white phosphorus (by 31 P NMR spectroscopy) in these reactions was 96.7% for complex 1 and 68.0% for complex 2. In order to boost the activity of clusters 1 and 2, we attempted modification of their ligand surrounding by the substitution of the chloride-ions from the first and the second coordination spheres with weakly coordinated ions, and in this way to increase the electrophilic properties of Pd. Indeed, addition of TlNO3 as a halide scavenger increased the reactivity of the cluster towards P4. As a result, increased intensity of the signals related to H3PO3 and H3PO4, and the decreased intensity of the P4signal were observed in 31 P NMR spectra. In the case of complex 1, full conversion of P4 was accomplished with a 72.0% yield of H3PO3. Complex 2 gave 74.4% conversion of P4 and 53.1% yield of H3PO3.
The solvent influence on the reactivity of P4 and its hydrolysis was also investigated, using the complex 1 as the benchmark. Addition of an excess of H2O to the reaction mixture containing 1 and P4 in CH2Cl2 allowed for detection of the signals associated with [Mo3{PdP(OH)3}S4Cl3(dbbpy)3] + (δ 110 ppm), H3PO3 (δ 4.7 ppm) and H3PO4 (δ 1.1 ppm). Adding TlNO3 increased the signal intensities, and the observed molar ratio of H3PO3:H3PO4 was 3:2. The signal at δ-243.6 ppm related to the formation of PH3 was also observed, as minor peak. The conversion of P4 was 43.1% with only 8.6% yield of H3PO3. In order to boost the activity of clusters 1 and 2, we attempted modification of their ligand surrounding by the substitution of the chloride-ions from the first and the second coordination spheres with weakly coordinated ions, and in this way to increase the electrophilic properties of Pd. Indeed, addition of TlNO 3 as a halide scavenger increased the reactivity of the cluster towards P 4 . As a result, increased intensity of the signals related to H 3 PO 3 and H 3 PO 4 , and the decreased intensity of the P 4 signal were observed in 31 P NMR spectra. In the case of complex 1, full conversion of P 4 was accomplished with a 72.0% yield of H 3 PO 3 . Complex 2 gave 74.4% conversion of P 4 and 53.1% yield of H 3 PO 3 .
The solvent influence on the reactivity of P 4 and its hydrolysis was also investigated, using the complex 1 as the benchmark. Addition of an excess of H 2 O to the reaction mixture containing 1 and P 4 in CH 2 Cl 2 allowed for detection of the signals associated with [Mo 3 {PdP(OH) 3 }S 4 Cl 3 (dbbpy) 3 ] + (δ 110 ppm), H 3 PO 3 (δ 4.7 ppm) and H 3 PO 4 (δ 1.1 ppm). Adding TlNO 3 increased the signal intensities, and the observed molar ratio of H 3 PO 3 :H 3 PO 4 was 3:2. The signal at δ −243.6 ppm related to the formation of PH 3 was also observed, as minor peak. The conversion of P 4 was 43.1% with only 8.6% yield of H 3 PO 3 .
In case of THF as the solvent, the addition of H 2 O to the reaction mixture containing 1 and P 4 yielded the signal associated with [Mo 3 {PdP(OH) 3 }S 4 Cl 3 (dbbpy) 3 ] + with δ +113.8 ppm in 31 P NMR spectrum. It is worth noting that there were no signals of any phosphoruscontaining acids in 31 P NMR spectra in this case. However, the activation of the complex with TlNO 3 caused both the signal growth and the appearance of new signals related to H 3 PO 3 (δ 3.3 ppm), H 3 PO 4 (δ 1.1 ppm), and PH 3 (δ −244.4 ppm). The integrated intensity ratio H 3 PO 3 :H 3 PO 4 was 8:3, and the observed conversion of P 4 was 85.5% with 24.4% yield of H 3 PO 3 .
The summary of the results obtained in the reaction of {Mo 3 PdS 4 } complexes with P 4 is presented in Table 1. It should be noted, that the presence of the Pd site in the cluster moiety to realise the hydrolysis of P 4 molecule is mandatory, as this reaction does not proceed with Pd-free trinuclear cluster complexes [Mo 3 S 4 Cl 3 (dbbpy) 3 ]Cl and [Mo 3 S 4 Cl 3 (dbbpy) 3 ]PF 6 . This fact confirms that the transformation and followed hydrolysis of P 4 requires the presence of Pd center. Moreover, the use of cluster core {Mo 3 PdS 4 } for hydrolysis of white phosphorus tetrahedron and its transformation into phosphorous acid (H 3 PO 3 ) allows to avoid the formation of the insoluble and inactive Pd-black that is very important for the further use of these {Mo 3 PdS 4 } clusters as catalysts for the hydrolysis of white phosphorus.

Conclusions
Based on the experimental data, we can conclude that heterometallic cubane-type clusters [Mo 3 {Pd(dba)}S 4 Cl 3 (dbbpy) 3 ]PF 6 (1), [Mo 3 {Pd(tu)}S 4 Cl 3 (dbbpy) 3 ]Cl (2) and [Mo 3 {Pd (dba)}S 4 (acac) 3 (py) 3 ]PF 6 (3) efficiently promote the hydrolysis of P 4 molecule leading to the formation of H 3 PO 3 as the major product. The complexes 1 and 2 bearing dbbpy ligand demonstrate higher activity in comparison with complex 3 containing acac ligand. Moreover, removal chloride anions from the coordination sphere of the cluster core with TlNO 3 increases both the activity of the cluster complexes in P 4 activation process and the yield of H 3 PO 3 . The use of the cluster with embedded Pd atom allows to avoid the Pd black formation which occurs when non-cluster Pd complexes are used for white phosphorus hydrolysis process. Thus, this work opens up prospects for studying the potential of heterometallic cubane-type clusters as catalysts for the selective conversion of white phosphorus to phosphorous acid. Further studies are in progress.

Experimental Section
CAUTION: White phosphorus and phosphine mentioned in this communication are hazardous compounds. White phosphorus needs to be stored under water in a wellventilated dark place. White phosphorus is highly toxic and burns spontaneously when exposed to air. In an emergency, white phosphorus can be treated with aqueous copper(II) sulfate solution or sand. On contact with skin, white phosphorus gives highly painful, badly healing burns. In case of skin burns, washing with diluted aqueous solutions of KMnO 4 or CuSO 4 is advised. The continuous inhaling of white phosphorus vapors results in disease of the bone tissue, loss of teeth, and necrosis of parts of the jaw. An aqueous copper(II) sulfate solution (2%) can be used as an immediate antidote for poisoning. All reactions and handling of phosphine and white phosphorus must be carried out under an inert atmosphere in a well-ventilated hood.
All experiments related to the synthesis of the complexes, preparation of the solutions, solvents, and the manipulations with white phosphorus and all chemical reagents were performed under nitrogen atmosphere using standard Schlenk-line techniques.