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Publicly Available Published by De Gruyter March 17, 2017

Hydroxylated phosphines as ligands for chalcogenide clusters: self assembly, transformations and stabilization

  • Maxim N. Sokolov EMAIL logo , Alexander V. Anyushin , Rita Hernandez-Molina , Rosa Llusar and Manuel G. Basallote


This contribution is a documentation of recent advances in the chemistry of chalcogenide polynuclear transition metal complexes coordinated with mono- and di-phosphines functionalized with hydroxo groups. A survey of complexes containing tris(hydroxymethyl)phosphine (THP) is presented. The influence of the alkyl chain in bidentate phosphines, bearing the P–(CH2)x–OH arms, is also analyzed. Finally, isolation and structure elucidation of the complexes with HP(OH)2, P(OH)3, As(OH)3, PhP(OH)2, stabilized by coordination to Ni(0) and Pd(0) centers embedded into chalcogenide clusters, is discussed.

Keyword:: ICPC-21


Phosphine ligands are ubiquitous in modern coordination chemistry. Virtually a limitless number of phosphines bearing almost any imaginable functionality have been made accessible by the organophosphorous chemists. Among this very vast family, the hydroxyphosphines are useful precursors for a variety of important phosphorus-containing compounds including aminomethylphosphines, phosphorus heterocycles and polycycles [1], [2], [3]. The simplest organic hydroxyphosphine is tris(hydroxymethyl)phosphine (THP), reported in 1958 [4]. It is obtained by addition of PH3 to CH2O (Scheme 1) [5], [6].

Scheme 1: Preparation of THP.
Scheme 1:

Preparation of THP.

First complexes with THP were apparently reported in 1973 [7]. In particular, complexes of noble metals with ТНР catalyze hydrogenation of unsaturated aldehydes [8] and hydration of alkynes [9] in biphasic systems. They also catalyze hydroamidation of CO2 into DMF, isomerization of allyl alcohols into ketones [10], and hydrogenation of ketones into alcohols [11]. Their solubility in water enables attachment to a support surface (such as SiO2) through the hydroxylic groups of THP [12]. Moreover, hydroxyphosphines and their complexes may be used for development of modern nanoscale and dendrimeric materials. Air oxidation of THP leads to the formation of the corresponding phosphine oxide, THPO, followed by partial loss of CH2O with the formation of anionic (HOCH2)2PO2, which can be used for construction of coordination polymers [13].

The significance of phosphines in the cluster chemistry comes from the well-known symbiosis between these ligands and transition metal clusters, in particular, chalcogenide clusters. In fact, many chalcogenide clusters are only known as phosphino complexes: more often than not only phosphines are strongly enough coordinating to prevent uncontrollable transformation of a finite cluster core, formed by self-assembly from a metal cation with a sulfide source, such as H2S or (Me3Si)2S, into a bulky metal sulfide [14], [15], [16].

The nature of a phosphine exercises enormous influence on the composition and property of the self-assembled clusters [17]. A significant problem is posed by further functionalization of a particular cluster in order to obtain new compounds with any desired set of functional groups. This can be achieved either by use of a phosphine with a specific functionality at the cluster self-assembly step, or by modification of a coordinated phosphine ligand without affecting the cluster core. In this respect the presence of a P–(CH2)x–OH sequence in a coordinated phosphine ligand is welcome. The hydroxyl group in THP itself is particularly reactive: it can be acylated, silylated, substituted by amino group, halide etc. [18], [19], [20], [21]. Thus, coordination of THP and its analogues not only leads to water soluble sulfide clusters, but can open new ways of cluster functionalization and preparation of dendrimers. This chemistry can be extended to the bidentate phosphines, bearing the P–(CH2)x–OH arms, namely, with x=1, 3 and 4.

Another class of hydroxylated phosphines has OH groups directly attached to P(III) centers. In particular, the unstable P(OH)3 and HP(OH)2 molecules are the simplest water-soluble phosphine ligands and may be of interest as a cheaper alternative to usual water-soluble organic phosphines in two-phase catalytic systems. However, the R2P-OH species are not stable towards isomerization into corresponding hydrophosporyl (R2HP(=O)) tautomers. For the isomerization of P(OH)3 into the common dibasic phosphorous acid log K=10.3(1.5) at 25 °C in aqueous solution [22], and as for HP(OH)2, the estimated ratio [HP(OH)2]/[H2P(O)(OH)] ≤10−12 [23]. The organic hydroxyphosphines RP(OH)2 and R2POH also exist preferentially as the hydrophosphoryl tautomers, except when R is bulky, such as 2,2-dimehtylbicyclo[2,2,2]heptyl-3-methyl [24] or highly electronegative, such as (CF3)2POH [25]. There are, nevertheless, examples of stabilization of P(OH)3 by coordination to a d8 or low-spin d6 metal center, namely, with Ru(II), Pt(II), Mo(0) and Cr(0). Perhaps the first P(OH)3 complex was reported as early as in 1872 as PtCl2·2P(OH)3 by Schützenberger [26]. At that time H3PO3 was regarded as “normal” tribasic acid, and this formulation corresponded to the expected ability of P(OH)3 to form adducts with metal halides. Much later products formulated as [Pt(P(OH)3)4]Cl2 and some polynuclear Rh(III) complexes claimed to contain coordinated P(OH)3 were isolated from reactions of Pt(II) and Rh(III) chloride complexes with H3PO3, but at this stage characterization was insufficient and this chemistry has never been resumed [27], [28]. Another product, a dinuclear K2[Pt2(P2O5H2)4]·2H2O, was isolated in the reaction of K2PtCl4 with H3PO3; it contains four pyrophosphite bridging ligands P2O3(OH)2 (“pop”), derived from condensation of two molecules of P(OH)3 [29]. Carbonyl complexes [M(CO)5(P(OH)3)] were also reported (M=Cr, Mo). The Cr complex and was isolated as a hydrate, [Cr(CO)5(P(OH)3)]·H2O by hydrolysis of [Cr(CO)5(PCl3)] in the presence of Et3N [30]. The synthesis of [Mo(CO)5(P(OH)3)], reported in 2004, was achieved by the one-pot reaction of Mo(CO)6 with HP(O)(OEt)2 and water. The structure of the product was confirmed by X-ray analysis [31], [32]. However, the first clear evidence for isomerization of free H3PO3 upon coordination was obtained in 1980s, by Franco et al., in the reaction with [Ru(NH3)5(H2O)]2+, which produced [Ru(NH3)5(P(OH)3)]2+ [33]. This product was characterized only in solution. The first isolated and structurally characterized Ru(II) complex with P(OH)3 was [CpRu(PPh3)2(P(OH)3)]+ [34], prepared by treatment of [CpRu(PPh3)2Cl] with H3PO3 in the presence of Tl+ or Ag+ as chloride scavengers.

HP(OH)2 was even more elusive. Prior to our work there were only indirect indications that HP(OH)2 might be involved in some catalytic cycles. Thus, Pd-catalyzed formation of HP(O)(OR)(OH) from NaH2PO2 and ROH (with H2 as by-product) is believed to proceed via HP(OH)2 [35], and cross-coupling of H2PO2 with ArX with the formation of ArP(H)(O)(OH) is catalyzed by [Pd(PPh3)4], presumably via {Ar–Pd–PH(OH)2} intermediates [36]. More recently, [СрRu(PPh3)2(HP(OH)2)]+ was obtained by treatment of [CpRu(PPh3)2Cl] with H3PO2 in the presence of Tl+ or Ag+ as chloride scavengers and structurally characterized [34]. It is worth mentioning that elusive phosphine oxide (H3PO) was first obtained in solution by electrochemical methods starting from white phosphorus and trapped as a ligand in half-sandwich CpRu(II) complexes following tautomerization to phosphinous acid, H2P(OH) [37].

Stabilization of Ph2POH by coordination was reported for [Au(Ph2POH)Cl], [Pt(Ph2POH)2Cl2], [Au(Ph2POH)GeCl3], [Au(Ph2POH)2]+ [38], obtainable either directly from diphenylphosphine oxide or by hydrolysis of coordinated Ph2PCl molecule. Mono and dinuclear complexes of PdCl2 with tBu2POH were prepared from [(COD)PdCl2] and tBu2P(O)H. These structures were confirmed by X-ray analysis, and the complexes were successfully used in cross-coupling reactions [39]. Pt(II) complexes with Me2POH and Ph2POH efficiently catalyze nitrile hydrolysis under neutral conditions [40]. Complex [(tBuPhP(OH))2Pd2Cl4], prepared from [(COD)PdCl2] and enantiopure tBuPhP(O)H, catalyzes asymmetric allylic substitution [41]. Oxydation of Ph2PH in the coordination sphere of Rh(III) also gives complexes with Ph2POH and μ-Ph2PO without breaking the Rh–P bond [42].

We have found that Ni(0) and Pd(0) centers, incorporated into cuboidal {M3Q4}-based chalcogenide clusters (M=Mo, W; Q=S, Se), revert the tautomerization and stabilize the R2P–OH tautomers by direct coordination. This property seems to be general and has allowed isolation and structure elucidation of the complexes with HP(OH)2, P(OH)3, PhP(OH)2. Elusive As(OH)3 has also been pinned down in this way.

Assessment of coordination properties of THP

It is well known that the coordination ability of the phosphines depends on the nature of the substituent groups at the P atom: electron-accepting groups increase, and electron-donating groups decrease this property. Allen and collaborators have suggested use of the 1JP−Se coupling constants from the selenides (R3PSe) as a probe for the electronic effects of the substituents and thus the donor properties of the phosphines: electron-acceptor groups will increase this parameter [43]. For this purpose, we have prepared THPSe by reactions of THP with KSeCN and Ph3PSe (reaction of [P(CH2OH)4]Cl with Se or KSeCN also produces THPSe). THPSe has following NMR characteristics (in CD3OH): δ 31P 37.2, δ 13C 58.4, δ 1H 4.22, δ 77Se −508.5 ppm, 1JC−H 147.5, 1JP−Se 678.0 Hz. The latter value is close to the value reported for Me3PSe (684 Hz) [44]. This closeness means that introduction of the hydroxylic substituents (i.e. transformation of –CH3 groups into –CH2OH) does not significantly alter inherently strong basicity of the aliphatic phosphines, and THP can be regarded as strong donor, au pair with PMe3, and much stronger that PPh3 (1JP−Se 732 Hz). Experimental confirmation of the strong donating ability of THP is our observation of its complexing with Cd2+ in 1:3 M:L stoichiometry: with other phosphines the number of coordinated PR3 ligands does not exceed two [13]. Consequently, THP must be fully suitable for efficient coordination of the cluster core in a nascent chalcogenide cluster to prevent its incontrollable growth into binary metal sulfides, if the strategy of cluster formation by metal cation sulfidation in the presence of a phosphine, put forward by the Cecconi and Fenske groups, is followed. Indeed, this strategy proved successful with transition metals from 8 to 10 groups, as discussed below.

Self-assembly of the transition metal sulfide clusters in the presence of THP

Reaction of FeCl2·4H2O and P(CH2OH)3 with H2S in methanol produces [Fe6S8(P(CH2OH)3)6]2+ (Fig. 1) as the only product according to 31P NMR [45]. UV-Vis and ESR spectra (Fig. 2) of [Fe6S8(P(CH2OH)3)6]Cl2 contain three signals (geff=6.5, 4.2 и 2.0) in agreement with the values reported for structurally characterized [Fe6S8(PEt3)6](BPh4)2 (geff=7.0, 4.9 и 2.0) [46]. Intercalation of [Fe6S8(P(CH2OH)3)6]Cl2 into MoS2 [47], [48] gives MoS2([Fe6S8(P(CH2OH)3)6]Cl2)0.06. This composition is typical for incorporation of hexanuclear clusters of such type [49], [50]. Magnetic susceptibility for MoS2([Fe6S8(P(CH2OH)3)6]Cl2)0.06 estimated in the range 80–300 K (9 kOe) yielded, for six Fe atoms, μeff=11.8(1) μB and S=5.42(4) (i.e. μeff=4.83(5) μB, S=1.97(3) per Fe atom). This means that low-spin: high-spin Fe3+ ions ratio is 4:2. Thus the effective magnetic moment for [Fe6S8(P(CH2OH)3)6]Cl2 cluster is higher than for other reported {Fe6S8P6}2+ clusters (five low-spin and one high-spin Fe3+) [51], [52], [53]. The coordination with P(CH2OH)3 unexpectedly increases the paramagnetism of the cluster core.

Fig. 1: View of the {Fe6S8}2+ cluster core with coordinated THP (only P atoms are shown).
Fig. 1:

View of the {Fe6S8}2+ cluster core with coordinated THP (only P atoms are shown).

Fig. 2: Absorption (left, r.t.) and ESR (right, 77 K) spectra of [Fe6S8(P(CH2OH)3)6] in MeOH solution.
Fig. 2:

Absorption (left, r.t.) and ESR (right, 77 K) spectra of [Fe6S8(P(CH2OH)3)6] in MeOH solution.

No Ru analogues of [Fe6S8(P(CH2OH)3)6]2+ could be obtained. Instead, reaction of [Ru(PPh3)3Cl2] with THP in ethanol leads to [Ru2(μCl)3(P(CH2OH)3)6]Cl as single detectable product, according to 31P NMR. The same complex was obtained from RuCl3 and THP in poor yield (9%). Crystal structure of [Ru2(μCl)3(P(CH2OH)3)6]Cl contains binuclear cations [Ru2(μCl)3(P(CH2OH)3)6]+ and Cl anions. The binuclear complex has {P3Ru(μ-Cl)3RuP3}+ core built of two confacial octahedra with non-bonding Ru…Ru distance of 3.350 Å. Attempts at sulfidation did not give identifiable products [54].

Reaction of CoCl2·6H2O and P(CH2OH)3 with H2S in ethanol gives [Co6S8(P(CH2OH)3)6] as the only product (31P NMR: δ=28.6 ppm) [55]. This can be isolated only as a dark-brown oil of the composition [Co6S8(P(CH2OH)3)6]·5H2O. Acylation of [Co6S8(P(CH2OH)3)6] with excess of (CH3CO)2O or (C2H5CO)2O gives acylated products [Co6S8(P(CH2OC(O)CH3)3)6]·H2O and [Co6S8(P(CH2OC(O)CH2CH3)3)6] in moderate yields; it is known that direct acylation of free P(CH2OH)3 produces P(CH2OC(O)CH3)3 [21]. The crystal structure of [Co6S8(P(CH2OC(O)CH3)3)6]·H2O contains hexanuclear clusters [Co6S8(P(CH2OC(O)CH3)3)6] (Fig. 3) with nearly regular octahedral {Co63-S)8} cores with short Co…Co distances (2.78–2.82 Å), typical for all known Co6S8 clusters with alkyl phosphines. The Co–P distances in [Co6S8(P(CH2OC(O)CH3)3)6]·H2O is 2.105 Å, being somewhat shorter than in [Co6S8(PEt3)6](BPh4) [56]) and [Co6S8(PPh3)6] (ca. 2.16 Å) [57]). The shortening might be due to the increase in π-acceptor properties of the phosphines ligands on going from alkyl and aryl phosphines to P(CH2OC(O)CH3)3 with electronegative acetate substituent in the alkyl radical. High-resolution 31P, 1H and 13C NMR spectra and corresponding JP−X values show that [Co6S8(THP)6] and [Co6S8(P(CH2OOCCH2CH3)3)6] possess similar geometry. Cyclic voltammetry shows reversible redox process with Ea and Ec at 0.16 and −0.05 V, and an irreversible anodic wave at + 0.52 V (vs. Ag/AgCl couple) [55].

Fig. 3: View of [Co6S8(P(CH2O(O)CCH3)3)6] in [Co6S8(P(CH2OC(O)CH3)3)6]·H2O. The hydrogen atoms and two phosphine ligands are omitted for clarity.
Fig. 3:

View of [Co6S8(P(CH2O(O)CCH3)3)6] in [Co6S8(P(CH2OC(O)CH3)3)6]·H2O. The hydrogen atoms and two phosphine ligands are omitted for clarity.

Rhodium behaves differently. Interaction of RhCl3·3H2O with P(CH2OH)3 in methanol gives a mononuclear complex (31P: δ 34.5 ppm). Bubbling H2S through this solution gives a dark-red solution and a complicated pattern in 31P NMR spectrum (doublet of triplets and triplet of doublets in 1:2 ration, Fig. 4, 1JP−Rh=122.7 Hz, 2JP−P=14.3 Hz for 12.0 ppm signal, 1JP−Rh=81.9 Hz, 2JP−P=15.8 Hz for 40.4 ppm signal). This pattern closely resembles that reported for [Rh33S)22S)(μ2Cl)2(PEt3)6](PF6), obtained by Cecconi et al. in 3% yield [58], and thus corresponds to the formation of [Rh33S)22S)(μ2Cl)2(THP)6]+ (Fig. 5). According to 31P NMR data, this cluster is the only product. In the crystal structure of [Rh33S)22S)(μ2Cl)2(THP)6]Cl the cluster cation has Rh…Rh distances of 3.26 Å, indicating no metal-metal bonding. The Rh–P and Rh–(μ2S,Cl) bonds are 2.32 Å and 2.42 Å, respectively. The Rh-(μ3S) bond distance is 2.41 Å.

Fig. 4: 31P NMR spectrum of [Rh3(μ3S)2(μ2S)(μ2Cl)2(P(CH2OH)3)6]Cl solution in methanol.
Fig. 4:

31P NMR spectrum of [Rh33S)22S)(μ2Cl)2(P(CH2OH)3)6]Cl solution in methanol.

Fig. 5: Structure of the [Rh3(μ3S)2(μ2S)(μ2Cl)2(THP)6]+ cation.
Fig. 5:

Structure of the [Rh33S)22S)(μ2Cl)2(THP)6]+ cation.

Reaction of NiCl2 with two equivalents of THP in ethanol gives dark-orange solution of [Ni(THP)2Cl2] (31P NMR: −6.2 ppm). Passing H2S through this solution gives a dark-red solution (λmax 347 nm; reported [16] for [Ni3S2(PEt3)6]2+ 360 nm), with the 31P NMR signal shifted to + 14.0 ppm, corresponding to the formation of [Ni3S2(P(CH2OH)3)6]2+ [59]. This cluster undergoes further evolution: in 3 days the signal of [P(CH2OH)4]+ at + 24.0 ppm appears, together with two other signals of equal intensity at + 14.5 and at + 42.3 ppm, attributable to the {P,P-PCH2OP} and {P,P-PCH2OP} atoms in the bidentate ligand (HOCH2)2PCH2OP(CH2OH)2, corresponding to the formation of [Ni3S2((HOCH2)2PCH2OP(CH2OH)2)3]2+. Formation of [P(CH2OH)4]+ can be explained by disproportionation 2P(CH2OH)3 + H+=[P(CH2OH)4]+ + HP(CH2OH)2, which simultaneously furnishes the secondary phosphine necessary for condensation. Oxidation of HP(CH2OH)2 into HOP(CH2OH)2, followed by condensation with remaining THP, produces (HOCH2)2PCH2OP(CH2OH)2 – an interesting analogue of (HOCH2)2PCH2CH2P(CH2OH)2 (dhmpe). This condensation was reported for monomeric Pt, Pd [5] and Rh [60] complexes. Reaction of [MCl2(P(CH2OH)3)2] (M=Pd, Pt) with P(CH2OH)3 in excess produced [M((CH2OH)2PCH2OP(CH2OH)2)2]2+, but attempts to isolate free {P,P-PCH2OP} ligand failed because of its rapid decomposition into P(CH2OH)3 and HP(O)(CH2OH)2 [6].

It must be born in mind that the presence of large amounts of OH-groups around the cluster core makes these complexes highly hydrophilic and their crystallization is often a formidable task. Our attempts to isolate [Ni3S2((HOCH2)2PCH2OP(CH2OH)2)3]2+ with PF6, BF4 or BPh4 ions, commonly used for such purpose, failed, but with [Mo6Cl14]2−, which can be regarded as a larger equivalent of PF6, we succeeded in growing single crystals of [Ni3S2{(CH2OH)2PCH2OP(CH2OH)2}3][Mo6Cl14]·0.8H2O. The crystal structure contains trinuclear cations [Ni3S2({P,P-PCH2OP})3]2+ (Fig. 6, (HOCH2)2PCH2OP(CH2OH)2={P,P-PCH2OP}, cluster anions [Mo6Cl14]2− and solvent water molecules which form hydrogen bonds with OH groups of the {P,P-PCH2OP} ligands. Each Ni atom has square planar environment and is coordinated with two sulfur atoms and two phosphorus atoms of the THP with the following geometry: Ni…Ni 2.784(1), Ni-S 2.205(1), Ni–P 2.115(2) Å [59].

Fig. 6: View of [Ni3S2{(HOCH2)2PCH2OP(CH2OH)2}3]2+. Hydrogen atoms and OH groups of the ligand are omitted for clarity.
Fig. 6:

View of [Ni3S2{(HOCH2)2PCH2OP(CH2OH)2}3]2+. Hydrogen atoms and OH groups of the ligand are omitted for clarity.

Reaction of [Pt{P(CH2OH)3}2Cl2] [6] with H2S gives [Pt3S2{P(CH2OH)3}6]Cl2 as single product [61]. This water soluble colorless chloride salt is stable in solutions for at least 1 year, and crystallizes from concentrated solutions. In the presence of NH4PF6 [Pt3S2(P(CH2OH)3)6](PF6)(OH)·H2O was obtained [62]. The crystals of both salts contain trinuclear cations [Pt3S2{P(CH2OH)3}6]2+ (Fig. 7) with central {Pt33-S)2}2+ core featuring non-bonding Pt…Pt distances (3.15–3.20 Å). Each Pt atom has square planar environment from two capping sulfides and two phosphorous atoms of the P(CH2OH)3 ligands. The Pt–S and Pt–P bond lengths are 2.36 и 2.26, respectively. This geometry is very close to that observed in [Pt3S2(PMe2Ph)6](BEt4)2 [63] In cyclic voltammogramm single reversible one-electron redox couple appears at −0.63 V, followed by irreversible reduction at −1.59 V. The irreversibility of the second step may be related to the change of coordination geometry on going from square planar Pt(II) in [Pt3S2{P(CH2OH)3}6]2+ to tetrahedral Pt(0) in the product of two-electron reduction. A closely related complex, [Pt3S2(dppe)3]2+, exhibits a single reduction peak at −2.10 V. Presumably in this case reduction proceeds in single two-electron step.

Fig. 7: Crystal structure of cation [Pt3S2(P(CH2OH)3)6]2+.
Fig. 7:

Crystal structure of cation [Pt3S2(P(CH2OH)3)6]2+.

Our attempts to obtain the Pd analogue failed. Reaction of K2[PdCl4] with P(CH2OH)3, generated in situ from [P(CH2OH)4]Cl and KOH, gave orange needles of K[Pd(P(CH2OH)3)Cl3] in 40% yield [64].

Coordination of hydroxyalkyldiphosphines to the M3S44+ unit

The M3S44+ (M=Mo, W) cluster core is a robust entity, offering unique possibilities of cluster modification from which valuable properties result. Coordination of diphosphine ligands such as in [M3S4Y3(diphosphine)3]+ (Y=halogen, hydrogen) enhances the cluster stability, and proper functionalization of the ligand with hydroxo group leads to water-soluble clusters that are stable within a wide pH range. These water-soluble complexes are excellent candidates as catalysts under biphasic conditions or as a co-catalyst to be immobilized in solid surfaces. Small ligand variations can have significant effects on the solubility, reactivity, and aqueous speciation of the resulting molecular clusters. We have studied coordination of hydroxyalkyldiphosphines (HO(CH2)xPC2H4P(CH2)xOH)2 with x=1 (dhmpe), x=3 (dhprpe) and x=4 (dhbupe).

The cluster complex with hydroxydiphosphine [Mo3S4Cl3(dhmpe)3]Cl [65] was prepared by reaction of dhmpe in methanol and (Bu4N)2[Mo3S7Cl6] in acetonitrile, in the presence of 0.1 M aqueous HCl, in order to suppress ligand deprotonation. Solutions of [Mo3S4Cl3(dhmpe)3]Cl in aqueous HCl are green, while water solutions are brown. This difference has been interpreted as the result of partial substitution of Cl with a deprotonated hydroxylic group of dhmpe at one or two of the metal centers, Fig. 8. Accordingly, ESI-mass-spectra contain signals of [Mo3S4L(L-H)2(H2O)]2+ and [Mo3S4L(L-H)2]2+ as major species in aquous solutions (Fig. 8), resulting from the ability of a hydroxyl group of one or two dhmpe (L) to coordinate to the cluster {Mo3S4} core as tridentate P,P,O ligand (L–H). Acidification of the solutions of [Mo3S4L(L–H)2(H2O)]2+ in the presence of X (X=Cl, NCS) yields [Mo3S4X3L3]+, while with KOH [Mo3S4L3(OH)3]+ is the product.

Fig. 8: From left to write structures of dhmpe, [Mo3S4L(L–H)2(H2O)]2+, [Mo3S4L(L–H)2]2+, [Mo3S4X3L3]+, and [Mo3S4X3L3]+ (X=Cl, SCN, OH) are shown (L=bidentate P-coordinated dhmpe, L–H=tridentate P– and O-coordinated deprotonated dhmpe).
Fig. 8:

From left to write structures of dhmpe, [Mo3S4L(L–H)2(H2O)]2+, [Mo3S4L(L–H)2]2+, [Mo3S4X3L3]+, and [Mo3S4X3L3]+ (X=Cl, SCN, OH) are shown (L=bidentate P-coordinated dhmpe, L–H=tridentate P– and O-coordinated deprotonated dhmpe).

The complex with dhprpe (dhprpe=(HOCH2CH2CH2)2PC2H4P(CH2CH2CH2OH)2) [Mo3S4Cl3(dhprpe)3]Cl was prepared from [Mo3S4Cl4(PPh3)3(H2O)2] and dhprpe in acetonitrile. Upon increasing the length of the hydroxyalkyldiphosphine alkyl chain from methyl to propyl, water solubility increases by a factor of one hundred, and the product becomes highly hygroscopic and difficult to crystallize. Crystal structure was determined for [Mo3S4Cl3(dhprpe)3]2[Mo6Cl14]·3H2O [66], which was obtained from acidified solutions after addition of [Mo6Cl14]2−. From basic solutions crystals of [Mo3S4(dhprpe-H)3]PF6·5H2O with tridentate deprotonated dhprpe-H (P,P,O) ligand were obtained (Fig. 9). Kinetics of acid-base equilibrium between [Mo3S4Cl3L3]+ and [Mo3S4(L–H)3]+ (Fig. 9) was investigated by 31P{1H} NMR, ESI-MS and stop-flow techniques.

Fig. 9: Equilibrium between [Mo3S4Cl3L3]+ and [Mo3S4(L–H)3]+ (L=bidentate Pcoordinated dhprpe, L–H=tridentate P– and O-coordinated deprotonated dhprpe).
Fig. 9:

Equilibrium between [Mo3S4Cl3L3]+ and [Mo3S4(L–H)3]+ (L=bidentate Pcoordinated dhprpe, L–H=tridentate P– and O-coordinated deprotonated dhprpe).

The cluster complexes with dhbupe were prepared for both Mo and W clusters [67]. Highly hygroscopic samples [Mo3S4Cl3(dhbupe)3]Cl and [W3S4Br3(dhbupe)3]Br were obtained by reacting [M3S4X4(PPh3)(H2O)2] (M=Mo, X=Cl or M=W, X=Br) with the ligand in acetonitrile, and crystal structure was determined for (H3O)4[Mo3S4Cl3(dhbupe)3]2[Mo6Cl4]3 [66]. In the basic pH range the halide ligands are substituted by hydroxo groups to afford the corresponding [Mo3S4(OH)3(dhbupe)3]+ and [W3S4(OH)3(dhbupe)3]+ complexes. Unlike dhmpe and dhprpe, dhbupe is not able to form complexes in the (P,P,O) tridentate mode due to unfavorable chelate effect (a seven-membered ring would be expected).

Thus, reactivity, water solubility, and speciation of the cluster complexes with hydroxyalkyldiphosphines (HO(CH2)xPC2H4P(CH2)xOH)2 are strongly affected by the length of the alkyl chain between P atom and OH group. Acidic solutions containing coordinating X anions, i.e. Cl, in all cases contain the complex with X coordinated to each metal center and the hydroxyalkyldiphosphine acting as a P,P-bidentate ligand. However, diverging behavior patterns that strongly depend on the value of x are observed under neutral and basic conditions. In all cases the phosphorus atoms remain coordinated but the third coordination site at each metal center, i.e. that occupied by X in acid media, can be occupied by water, OH or a deprotonated hydroxyalkyl phosphine group. When x=4, the tris(hydroxo) complex is formed in basic solutions, and kinetic studies suggest a mechanism with two parallel reaction pathways involving water and OH attacks resulting in the formal substitution of X by hydroxo ligands. The reverse reaction of the hydroxo clusters with HX acids occurs with protonation of the OH ligands followed by substitution of coordinated water by X. In contrast, when x=2 or 3, the chemistry in solution is mainly determined by the formation of species with coordinated deprotonated hydroxyalkyl groups. Detailed mechanistic studies reveal that the opening of the chelate ring to form the complexes with coordinated X occurs through two parallel attacks by H+ and X with close rates. The reverse reaction involves chloride substitution and chelate ring closure, and also occurs through two parallel paths, in this case involving attacks by H2O and OH. An additional observation that illustrates the subtle factors affecting the reactivity of these species is that when x=3, the interconversion between the complexes containing X and deprotonated hydroxyalkyl groups occurs in a single kinetic step, thus showing that the reactions proceed at the three metal centers with statistically controlled kinetics. In contrast, when x=1, separate kinetic steps are observed at the three metal centers, showing significant deviations from the statistical predictions.

Heterometallic Mo3M′Q4 clusters and the problem of stabilization of PIII-OH species

The incomplete cuboidal clusters {M3Q4L9} (M=Mo, W; Q=S, Se) can incorporate more than 20 chalcophilic transition and post-transition metals in low oxidation states (Fig. 10). For group 10, heteroatoms in zero oxidation states can be used as the source of M’, including bulk Ni sheets and Pd black, in situ generated Pd(0) and Ni(0), or Pd(dba)2/Pd2(dba)3 (dba is dibenzylideneacetone) [68]. These clusters offer unique possibility to study chemistry at the Ni(0) and Pd(0) centers in a sulfide (or selenide) environment. The most intriguing aspect of the reactivity of [M3M′Q4(H2O)10]4+ aqua complexes is their high affinity towards the hydrophosphoryl compounds R2P(O)H (R=OH, Ph, H). The clusters induce their isomerization into the R2P–OH species, and the P atom uses the lone pair released after isomerization for coordination at Ni or Pd. In this way phosphorous and hypophosphorous acids H3PO3, H3PO2, and their phenyl-substituted derivatives Ph2P(O)H and Ph(OH)P(O)(H) isomerize into P(OH)3, HP(OH)2, PhP(OH)2, and Ph2POH. The high affinity of the Ni and Pd sites in the clusters for P-donors constitutes main driving force for these reactions. The 31P NMR spectra provide unambiguous evidence for isomerization of the ligands. The P–H bond cleavage results in the disappearance of the 1JP−H doublet in the case of H3PO3 and Ph(OH)P(O)(H) (formation of P(OH)3 and PhP(OH)2), or in the transformation of the triplet into the doublet (only one P–H bond remains in HP(OH)2). The coordination of HP(OH)2 was detected with 31P NMR, by appearance of the characteristic 1JP−H double pattern, for the following complexes listed in Table 1.

Fig. 10: Structure of dhbupe and equilibrium between [M3S4X3L3]+ and of [M3S4(OH)3L3]+ are shown (L=bidentate Pcoordinated dhprpe).
Fig. 10:

Structure of dhbupe and equilibrium between [M3S4X3L3]+ and of [M3S4(OH)3L3]+ are shown (L=bidentate Pcoordinated dhprpe).

Table 1:

Chemical shifts and constants of spin-spin interaction for [M′M3(HP(OH)2Q4(H2O)9]3+ (M=Mo, W; M′=Ni, Pd).

Clusterδ (ppm)1JP–H

These values are in excellent agreement with the value reported for [CpRu(PPh3)2(HP(OH)2)]+ (406 Hz) . All these complexes are stable under Ar in 2–4 M HCl, except for [Mo3Pd(HP(OH)2)S4(H2O)9]4+, which in 1 h quantitatively gives the P(OH)3 complex [Mo3Pd(P(OH)3)S4(H2O)9]4+. According to X-ray data, in {[W3Ni(HP(OH)2)Q4(H2O)9]Cl4(cuc)}·11H2O coordinated HP(OH)2 has the folowing geometry: Ni–P 2.128(3), P–O 1.588(11) – 1.612(12) Å.

The coordination of Ph2POH and PhP(OH)2 was detected for {Mo3PdQ4} (X=S, Se), {W3PdS4} and {Mo3NiS4} clusters.

Typical order of reactivity is Mo<W; Ni<Pd, S≈Se, which correlates with the relative stability of the M(0) oxidation state in the clusters, and Ph2P(O)H>Ph(OH)P(O)H >> H3PO3 [68]. The nickel cluster [Mo3(NiCl)S4(H2O)9]3+ is the least reactive and only prolonged heating afforded the complexes with HP(OH)2, P(OH)3, Ph2P(OH), and PhP(OH)2 [69]. The {W3Ni} and {W3Pd} clusters give much more stable complexes with the hydroxophosphines, which can be purified with cation-exchange chromatography [70]. Titration of [W3NiS4(H2O)10]4+, [W3NiSe4(H2O)10]4+, [W3PdS4(H2O)10]4+ and [W3PdSe4(H2O)10]4+ with H3PO2, H3PO3, and As(OH)3 allowed spectrophotometric determination of the stability constants, listed in Table 2 [71], [72].

Table 2:

Formation constants for the reaction of [M3(PdCl)S4(H2O)9]3+ (M=Mo, W) with HP(OH)2, P(OH)3, and As(OH)3.

ClusterLigand (medium)K (mol–1)
[W3(PdCl)S4(H2O)9]3+HP(OH)2 (2 M HCl)2900
[W3(PdCl)S4(H2O)9]3+As(OH)3 (2 M HCl)3300
[W3(PdCl)S4(H2O)9]3+HP(OH)2 (2 M HCl)43
[W3(PdCl)S4(H2O)9]3+As(OH)3 (2 M HCl)183
[W3(PdCl)S4(H2O)9]3+P(OH)3 (4 M HCl)20
[W3(PdCl)Se4(H2O)9]3+P(OH)3 (4 M HCl)3
[W3(NiCl)S4(H2O)9]3+HP(OH)2 (2 M HCl)22
[W3(NiCl)Se4(H2O)9]3+HP(OH)2 (2 M HCl)273
[W3(NiCl)S4(H2O)9]3+As(OH)3 (2 M HCl)36
[W3(NiCl)Se4(H2O)9]3+As(OH)3 (2 M HCl)1054

Kinetic studies of the reaction between [Mo3PdS4(H2O)10]4+ and H3PO2 indicate that this reaction occurs with biphasic kinetics. The first step corresponds to the formation of a Pd–O coordinated intermediate, with tet-H3PO2 (e.g. the hydrophosphoryl tautomer) acting as the ligand. The second step corresponds to the isomerization into pyr-HP(OH)2. The rate constants for both steps show first order dependence on CH3PO2, indicating the participation of another H3PO2 molecule in each kinetic step. The second order rate constants derived for both steps are k1=(12.5 ± 0.3)×10−2 M−1s−1 and k2=(2.6 ± 0.1)×10−2 M−1s−1. For a better understanding of the mechanism DFT calculations were carried out and they indicate a mechanism for tautomerization of the H3PO2 molecule coordinated in the first step, that involves an 1,2 H-shift catalyzed by the second H3PO2 molecule, which is made possible by its capability for accepting a proton from a P–H bond [73].

Coordination of H3PO3 to [Mo3(PdCl)Q4(H2O)9]3+ (Q=S, Se) is also biphasic. The first, very rapid, step corresponds to coordination of the tet-HP(O)(OH)2 through oxygen (Scheme 2), and the second step, which is slow and reversible, corresponds to isomerization into pyr-P(OH)3 complex with the following parameters: kf (1.18 ± 0.05)·10−4 M−1s−1, kb (2.3 ± 0.25)·10−5 s−1 (Q=S); kf (3.0 ± 0.1)·10−3 M−1s−1, kb (4.5 ± 0.2)·10−3 s−1 (Q=Se) [69]. A significant difference with respect to H3PO2 is that the rate of the second resolved kinetic step in the tautomerization of H3PO3 does not depend on the H3PO3 concentration but on the concentration of the Hpts acid used for the control of the ionic strength. Nevertheless, a mechanism quite similar to that previously discussed for H3PO2 has been proposed, in which the tautomerization process involves initial protonation of the intermediate with the tet-H3PO3 with an external proton, i.e. the H-shift does not take place through the deprotonation–protonation of the O-coordinated intermediate but through its protonation–deprotonation, the initial attack being carried out by H3O+.

Scheme 2: Tautomeric forms of phosphorous acid.
Scheme 2:

Tautomeric forms of phosphorous acid.

The kinetics of the reaction of [W3PdSe4(H2O)10]4+ with H3PO2 similarly involves two steps, the first one corresponding to the substitution at Pd to form an intermediate with O-coordinated tetrahedral H3PO2, and the second step to the isomerization of the coordinated H3PO2. However, in this case both rate constants are independent on CH3PO2, k1obs=(6.8±0.8)×10−2 s−1 and k2obs=(1.3±0.3)×10−3 s−1 [72]. Nevertheless, these results can be interpreted within the mechanism discussed above. In the first resolved kinetic step the substitution occurs with saturation kinetics with respect to the H3PO2 concentration, and tautomerization in the second resolved step occurs with a mechanism similar to that previously commented, but with the rate-determining step displaced from protonation of the O-coordinated intermediate to isomerization of the O-coordinated deprotonated intermediate with pyramidal H2PO2 into the more stable P-coordinated form.

The formation of the complexes was confirmed by X-ray analysis of supramolecular adducts with macrocyclic cavitand cucurbit[6]uril ((C36H36N24O12), CUC[6]). The adducts with cucurbit[6]uril were purposefully used to crystallize the complexes out of dilute solutions by the formation of complementary hydrogen bonds between water molecules, coordinated to the cluster, and the carbonyl groups of the cavitand portals. Crystal structures were determined for {[Mo3PdP(OH)3S4Cl3(H2O)6]2·CUC[6]}Cl2·nH2O (Pd-P 2.26 Å) (Fig. 11), [Mo3Ni(P(OH)3)S4(H2O)8Cl)]Cl3·CUC[6]·14H2O [W3Pd(P(OH)3)S4(H2O)8Cl)]Cl3∙CUC[6]·12.5H2O, {[W3Pd(PhP(OH)2)S4(H2O)7Cl2]Cl2}2·CUC[6]·9.5H2O (Pd–P 2.247(5) Å), [W3(Ni(HP(OH)2))Q4(H2O)9]Cl4·CUC[6]·11H2O (Q=S, Se) [70], [71], [72], [73], [74], [75], [76], [77].

Fig. 11: View of [Mo3(PdE(OH)3)S4Cl3(H2O)6]+ cluster complex.
Fig. 11:

View of [Mo3(PdE(OH)3)S4Cl3(H2O)6]+ cluster complex.

The bond lengths and angles in coordinated P(OH)3 can be compared with the values calculated for free P(OH)3 [78]. The P–O bonds become shorter upon coordination (1.57–1.59 Å in the (Mo3NiS4}, 1.56 Å in {Mo3PdS4}, and 1.54–1.60 Å in the {W3PdS4} complex, instead of 1.64–1.67 Å in free P(OH)3. The O–H bonds hardly change, but there is also a tendency towards less distorted tetrahedral geometry in coordinated P(OH)3 (the OPO angles are 99–105, 100–105, 100–105, and 94–105°, respectively).

It is known that free As(OH)3 is present in solutions of As2O3 in water, but cannot be isolated. However, coordination to the Ni or Pd site allowed us to isolate first complexes with As(OH)3 as As-donor ligand. The reaction of [W3NiSe4(H2O)10]4+ with As(OH)3 is very fast, and even [Mo3(NiCl)S4(H2O)9]3+, despite being the least reactive of all the series, quantitatively goes into [Mo3(NiAs(OH)3)S4(H2O)9]4+, while for H3PO2 and H3PO3 the electronic spectra showed the presence of much unreacted [Mo3NiS4(H2O)10]4+. X-ray structures were determined for {[Mo3PdAs(OH)3S4Cl3(H2O)6]2}·CUC[6]}Cl2·nH2O (Pd–As 2.368(3) Å, As–OH 1.765(16) Å) and for [W3(NiAs(OH)3)S4(H2O)9]Cl4·CUC[6]·13H2O (Ni–As 2.25(4) Å, As–OH 1.72–1.74 Å) [72], [75]. The apparent ease of formation and the greater stability of the As(OH)3 complexes (contrary to the well-known sequence P>As>Sb >> Bi) is explained by the contribution from the highly unfavorable values of the equilibrium constants of the reactions H2P(O)(OH) → HP(OH)2 and HP(O)(OH)2 → P(OH)3 [71], [72], [74].

The geometry of the PhP(OH)2 ligand was determined for the first time. The P–O bonds are 1.59–1.60 Å, P–C 1.760(21) Å, the two O–P–C angles are 98.1 and 102.2°, the O–P–O angle – 105.3°. Both P–O and P–C distances are considerably (by 0.05–0.07 Å) shorter than in free, non-coordinated PhP(OR)2 molecules [33]. The Pd–P bond is 2.256(5) Å, that is, virtually the same as in the complex with P(OH)3. The S–Pd–P angles are 114–120°. The phenyl ring plane is almost orthogonal to the Mo3 plane in the cluster. Another example of an RP(OH)2 ligand [34] is P(OMe)(OH)2 found in [Ru(P(OMe)(OH)2)(dppe)2]2+.


The symbiosis between the chalcogenide polynuclear transition metal complexes and phosphines functionalized with hydroxo groups turned out not only possible but rather fruitful. Apart from the obvious utilitarian goal (making clusters water soluble and amenable to further functionalization), this combination has allowed to reveal unexpected reactivity patterns, such as condensation of the phosphine ligands promoted by the cluster formation; pH-dependent change in the coordination mode of phosphines; and, perhaps most exciting, stabilization of such elusive species as HP(OH)2, P(OH)3, As(OH)3 by direct coordination to cluster core. However, we feel strongly that synthetic potential of this chemistry is far from exhausted. For example, the clusters discussed in this review, are, topologically speaking, excellent platforms for making dendrimers by easy functionalization of the appended OH groups. From the point of view of fundamental chemistry, nothing is known about the reactivity of coordinated P(OH)3, HP(OH)2 and its analogues. Last, but not the least, catalytic activity of these clusters is waiting to be tested.

Article note:

A collection of invited papers based on presentations at the 21st International Conference on Phosphorous Chemistry (ICPC-21) held in Kazan, Russia, 5–10 June 2016.


The financial support of the Spanish Ministerio de Economía y Competitividad (Grant CTQ2015-65207-P), Universitat Jaume I (Research project P1.1B2013-19) and Generalitat Valenciana (PrometeoII/2014/022) is gratefully acknowledged. The work has been financially supported financial support by the Spanish Ministerio de Economía y Competitividad and FEDER funds of the E.U. (grants CTQ2015-65707-C2-1-P and CTQ2015-65707-C2-2-P). RHM acknowledges CTQ2015-65707-C2-2-P for funding.


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Published Online: 2017-3-17
Published in Print: 2017-3-1

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