Facile synthesis of mononuclear early transition-metal complexes of κ³cyclo-tetrametaphosphate ([P₄O₁₂]⁴⁻) and cyclo-trimetaphosphate ([P₃O₉]³⁻)

Abstract

We herein report the preparation of several mononuclear-metaphosphate complexes using simple techniques and mild conditions with yields ranging from 56% to 78%. Treatment of cyclo-tetrametaphosphate ([TBA]₄[P₄O₁₂]·5H₂O, TBA = tetra-n-butylammonium) with various metal sources including (CH₃CN)₃Mo(CO)₃, (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl, MoO₂Cl₂(OSMe₂)₂, and VOF₃, leads to the clean and rapid formation of [TBA]₄[(P₄O₁₂)Mo(CO)₃]·2H₂O, [TBA]₃[(P₄O₁₂)Mo(CO)₂(η³-C₃H₅)], [TBA]₃[(P₄O₁₂)MoO₂Cl] and [TBA]₃[(P₄O₁₂)VOF₂]·Et₂O salts in isolated yields of 69, 56, 68, and 56% respectively. NMR spectroscopy, NMR simulations and single crystal X-ray studies reveal that the [P₄O₁₂]⁴⁻ anion behaves as a tridentate ligand wherein one of the metaphosphate groups is not directly bound to the metal. cyclo-Trimetaphosphate-metal complexes were prepared using a similar procedure i.e., treatment of [PPN]₃[P₃O₉]·H₂O (PPN = bis(triphenylphosphine)iminium) with the metal sources (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl, MoO₂Cl₂(OSMe₂)₂, MoOCl₃, VOF₃, WOCl₄, and WO₂Cl₂(CH₃CN)₂ to produce the corresponding salts, [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)], [PPN]₂[(P₃O₉)MoO₂Cl], [PPN]₂[(P₃O₉)MoOCl₂], [PPN]₂[(P₃O₉)VOF₂]·2CH₂Cl₂, and [PPN]₂[(P₃O₉)WO₂Cl] in isolated yields of 78, 56, 75, 59, and 77% respectively. NMR spectroscopy, NMR simulations and single-crystal X-ray studies indicate that the trianionic ligand [P₃O₉]³⁻ in these complexes also has κ³ connectivity.

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1. Introduction

Bimetallated cyclo-tetrametaphosphate ([P₄O₁₂]⁴⁻) and monometallated cyclo-trimetaphosphate ([P₃O₉]³⁻) moieties have a wide range of applications including use as pigments, catalysts, food additives, and fluorescent materials.^1–6 The cyclo-tetraphosphate anion is of special interest due to its flexible eight-membered ring structure that can produce complexes with different conformations.^1,7–9 Currently, two different strategies are employed for the preparation of bimetallic complexes of cyclo-tetrametaphosphate. One strategy is based on acidification of MCl₂ (M = Co, Fe, Mn, Ni, Cu) by [H₂PO₄]⁻, followed by thermal treatment at high temperatures (≥600 °C), leading to binary oligomeric systems of the general formula M₂P₄O₁₂.^{2–4,10–19}

A second strategy developed by Kamimura et al. comprises stirring a solution containing a salt of the cyclo-tetrametaphosphate ligand with the desired noble transition metal (such as rhodium, iridium, ruthenium, and palladium) at room temperature overnight. The desired product can then be obtained by further extraction and crystallization from dichloromethane and diethyl ether.⁷ They have also described the synthesis of di- and trinuclear μ-oxo titanium(IV) cyclo-tetraphosphate complexes.¹ Even though the preparation of noble and non-noble metal complexes using this ligand has been reported, a strategy for the straightforward preparation of mononuclear complexes of cyclo-tetrametaphosphate using mild conditions and non-noble metals represents a vacancy in the literature. In addition to filling a vacancy, we recognize that metals such as molybdenum and vanadium are useful in oxidation catalysis, such that cyclophosphate complexes of these metals may turn out to be valuable as precatalysts.

The chemistry of complexes incorporating the trianionic cyclo-trimetaphosphate ligand [P₃O₉]³⁻ has also been investigated, specifically in studies of catalytic applications such as vinylidene rearrangement of general internal alkynes via the 1,2-migration of alkyl, aryl, and acyl groups.^5,6 The methodology used for the synthesis of metal-cyclo-trimetaphosphate compounds was first reported by Klemperer and co-workers in 1981, and since then only a handful of publications reporting the preparation of mononuclear cyclo-trimetaphosphate-transition-metal complexes has been published.^5,6,20–29 Additionally, the coordination chemistry of early transition-metal-[P₃O₉] complexes has not been explored extensively. Our group has recently reported the preparation of the first tridentate-cyclo-tetrametaphosphate complex.⁸ Montag et al. reported the preparation of [Na][(P₄O₁₂)Co(TACN)] (TACN = 1,4,7-triazacyclononane) in aqueous media. The structure determination of this salt revealed that the ligand is coordinated to the metal in a κ³ fashion in which one of the phosphate groups is not directly connected to the metal center.

Herein we report the coordination chemistry of mononuclear complexes of cyclo-tetrametaphosphate and cyclo-trimetaphosphate ligands that incorporate non-noble metals (Fig. 1) along with the development of a simple synthetic procedure, using readily accessible precursors and common glove-box techniques.

Fig. 1 General structure of cyclo-tetrametaphosphate (right) and cyclo-trimetaphosphate (left) salts. The letter M represents one of the transition metals used in this work (M = V, W or Mo) and L_n represents the remainder of the metal's coordination sphere.

2. Results and discussion

2.1. Synthesis and characterization

[TBA]₄[P₄O₁₂]·5H₂O (1) was prepared from the known salt [Na]₄[P₄O₁₂]·5H₂O³⁰ by dissolving the latter in deionized water and passing the solution through a DOWEX® column which was previously conditioned with [TBA][OH]. The eluent was evaporated to afford a white solid which was dissolved in acetonitrile and then precipitated with diethyl ether to yield the desired compound. In general, salts 2, 3, 4 and 5 were prepared by dissolving [TBA]₄[P₄O₁₂]·5H₂O in dry acetonitrile and adding an appropriate transition-metal reagent (Fig. 2). Typically, the reaction mixture was stirred for one hour, leading to complete and clean conversion into the desired complex with isolated yields ranging from 56 to 69%. The observed less-than-quantitative yields are attributed mainly to losses incurred in the purification process (crystallization).

Fig. 2 Synthesis of molybdenum and vanadium cyclo-trimetaphosphate salts. All reactions were carried out in a glovebox using dry solvents. i = (CH₃CN)₃Mo(CO)₃, ii = (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl, iii = MoO₂Cl₂(OSMe₂)₂, iv = VOF₃.

The salts 7, 8, 9, 10 and 11 were obtained by adding the corresponding metal precursors to a stirring solution of [PPN]₃[P₃O₉]·H₂O in dry acetonitrile, affording complete conversion to the final products (Fig. 3). The compounds [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)] (7) and [PPN]₂[(P₃O₉)MoO₂Cl] (8) spontaneously precipitate ca. two minutes after the addition of the corresponding precursor to the acetonitrile solution of the metaphosphate ligand. The isolated yields of these complexes are very similar to those obtained in the case of the cyclo-tetrametaphosphate complexes, varying from 56 to 78%.

Fig. 3 Synthesis of molybdenum, tungsten, and vanadium cyclo-trimetaphosphate salts. All reactions were carried out in a glovebox using dry solvents. i = (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl, ii = MoO₂Cl₂(OSMe₂)₂, iii = MoOCl₃, iv = VOF₃, v = WOCl₄.

The products were characterized by solid-state FT-IR spectroscopy; stretching frequencies for signature oscillators of cyclo-tetrametaphosphate complexes are summarized in Table S1.† The IR spectrum of complex [TBA]₄[(P₄O₁₂)Mo(CO)₃]·2H₂O (2) features three carbonyl bands at 1874, 1708, and 1702 cm⁻¹ consistent with C₁ symmetry. The compound [TBA]₃[(P₄O₁₂)Mo(CO)₂(η³-C₃H₅)] (3) features two stretching bands at 1898 and 1709 cm⁻¹ for the carbonyl groups which are higher in energy compared to those observed in the case of salt 2, as would be expected as a function of the higher oxidation state of the molybdenum center.^31,32 The salt [TBA]₃[(P₄O₁₂)MoO₂Cl] (4) shows two stretches for the MoO₂ moiety (896 and 880 cm⁻¹), consistent with the presence of a pair of cis oxo ligands.³³ [TBA]₃[(P₄O₁₂)VOF₂]·Et₂O (5) shows only one stretch (912 cm⁻¹) for the V≡O bond. The value observed for this stretch is in the expected range.^34–3751V-NMR shows a broad peak at −594.5 ppm similar to the observations reported by Roesky et al. for the complex Ph₃NVOF₂.³⁴

Complex [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)] (7) has two characteristic C≡O stretches at 1918 and 1812 cm⁻¹ which, as expected, appear at higher energy compared to those of the already reported salt [PPN]₃[(P₃O₉)Mo(CO)₃] (6).³⁸ Complex [PPN]₂[(P₃O₉)MoO₂Cl] evinces two IR bands at 923 and 898 cm⁻¹, characteristic of a molybdenum dioxo moiety, while complex [PPN]₂[(P₃O₉)MoOCl₂] (9) has a single stretch for the Mo=O bond at 932 cm⁻¹. The FT-IR spectrum of compound [PPN]₂[(P₃O₉)VOF₂]·2CH₂Cl₂ (10) manifests one signal at 930 cm⁻¹, characteristic of a V≡O stretch (Table S2†). To the best of our knowledge, there is only one previous report of a fully characterized and stabilized monomeric oxo fluoride vanadium compound, reported by Roesky et al. The complex Ph₃PNVOF₂ was prepared by treating the ligand (Ph₃PNSiMe₃) with the precursor (VOF₃) at low temperature (−73 °C) and stirring for twelve hours.³⁴ The salt [PPN]₂[(P₃O₉)WO₂Cl] (11) exhibits two absorption bands for the WO₂ moiety at 938 and 907 cm⁻¹.

Table 1 summarizes the carbonyl stretches of the compounds reported in this work (i.e.2, 3, 6, and 7) and compounds that contain a tripodal ligand (Kläui, (pyrazolyl)borate and 1,3,5-triazacyclohexane ligands) bearing the same metal–carbonyl moiety. From the values reported, we may conclude that the cyclo-tetrametaphosphate ligand is a better donor than the cyclo-trimetaphosphate ligand and the other tridentate ligands listed in Table 1. This is reflected in decreased ν_C≡O stretching frequencies corresponding to weaker CO bonds.^43,44

Table 1 Comparison of selected IR data between compounds reported in this work and those reported for other tripodal ligands (cm⁻¹). Counter ions and complex charges omitted for simplification

M	MP4O12	MP3O9	M[η^5-CpCo{P(O)(R)2}3]	MBC9H10N6	MC3H6N3Me3
Mo(CO)3	1874	1883	1880^39	1890^40	1900^41
	1708	1723	1738^39	1750^40	1785^41
	1702		1710^39		1750^41
Mo(CO)2C3H5	1898	1928	1921^39	1934^42	nr
	1709	1812	1822^39	1842^42	nr

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In addition to IR spectroscopy, ³¹P-NMR spectroscopy is a helpful tool for determining the connectivity of the metaphosphate ligand with respect to the metal center. All the cyclo-tetrametaphosphate-based complexes reported herein exhibit three signals in their ³¹P-NMR spectra in an intensity ratio of 1 : 2 : 1. This supports that the polyanionic ligand is binding in a tripodal fashion, leading to κ³ connectivity (similar to a previous report from our group).⁸ On the other hand, [TBA]₃[(P₄O₁₂)VOF₂]·Et₂O exhibits only two peaks in a ratio of 1 : 3, possibly due to a fluxional process or to a change to κ² coordination in solution (this could possibly be attributed to the lability of the oxygen trans to the oxo ligand), leading to broad peaks instead of the expected triplets. Variable temperature NMR experiments confirmed the behavior described above. At lower temperatures (Fig. 4) the spectrum for complex 4 shows a new pattern with a spin system AM₂X represented by three triplets in a ratio of 1 : 2 : 1 while at room temperature the spectrum consists of a singlet.

Fig. 4 VT-³¹P-NMR of (a): [TBA]₃[(P₄O₁₂)MoO₂Cl] and (b): [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)]. The spectra were taken with an increase of 10 °C from +20 °C (top) to −60 °C (bottom) in case of spectra a and from +70 °C (top) to −40 °C (bottom) in case of spectrum b.

Fig. 5 shows the spectrum observed for complex [TBA]₃[(P₄O₁₂)MoO₂Cl] at −60 °C (a) along with the corresponding simulation (b) which was computed using the program gNMR.⁴⁵ The simulation reinforces the proposed four spin system AM₂X in line with the existence of two phosphate moieties which are similar to each other and strongly coupled. These coupled phosphate units have two different phosphate neighbors which do not show any coupling between them. This suggests, for example, a κ³ connectivity of the phosphate to the metal in a fashion similar to that observed by Montag et al.⁸

Fig. 5 ³¹P-NMR of (a): [TBA]₃[(P₄O₁₂)MoO₂Cl] at −60 °C and (b): simulated spectrum using gNMR.⁴⁵

³¹P{¹H}-NMR spectra of compounds 7, 8 and 10 show three signals in a ratio of 4 : 1 : 2 corresponding to [PPN]⁺ and the respective anion. The ³¹P-NMR spectrum of complex 7 was recorded at different temperatures (Fig. 4) showing that at high temperatures it consists of a singlet and at low temperatures it evolves into a triplet and a doublet. This behavior could be related to fluctuation of the ligand around the metal. The ¹H-NMR spectrum of salt 7 shows the presence of four signals corresponding to the hydrogens of the allyl group and [PPN]⁺, wherein the apical and syn protons appear at very similar chemical shifts. The ³¹P{¹H}-NMR spectrum of [(P₃O₉)MoO₂Cl]²⁻ is characteristic of an A₂B spin system indicating a κ³ connectivity of the metaphosphate ligand. The ³¹P{¹H}-NMR spectrum of salts 9 and 11 features only two signals in a 4 : 3 ratio, corresponding to [PPN]⁺ and the cyclo-trimetaphosphate-metal complex respectively.

The simulated spectrum for the complex [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)] at −30 °C is shown in Fig. 6. The simulation demonstrates that the spin system of this molecule is as proposed A₂B giving rise to a doublet coupled to an apparent triplet. This spin system suggests that one out of the three metaphosphate moieties has a unique chemical environment.

Fig. 6 ³¹P-NMR of (a): [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)] at −30 °C and (b): spectrum simulated using gNMR.⁴⁵

2.2. Crystallographic studies

X-ray diffraction quality crystals of [TBA]₃[(P₄O₁₂)Mo(CO)₂(η³-C₃H₅)] (Fig. 7) were grown from a mixture of diethyl ether and dichloromethane. The symmetry of the complex is C₁ and the connectivity of the metaphosphate is κ³. This result is consistent with the conclusions drawn from ³¹P-NMR and FT-IR spectroscopies studies. As the NMR simulations suggested, there are three different phosphorous environments and two of these are not coupled (the dangling metaphosphate and the farthest counterpart). The configuration of the allyl group is endo, and the bond length between the metal and the central carbon is, as expected, shorter than those between the metal and the terminal carbons.⁴⁶ The distances between molybdenum and the oxygen atoms are all similar and only differ by 0.035 Å. The crystal structure of this salt also contains a dichloromethane molecule which is hydrogen bonded to the dangling metaphosphate, partially stabilizing the negative charge on the metaphosphate. The D⋯A distance is 3.065 Å (Table S3†) suggesting that this is a weak hydrogen bond.⁴⁷ This structure suggests that it may be possible to pre-organize a substrate by interaction with the dangling phosphate residue.

Fig. 7 Solid-state molecular structure of anion [(P₄O₁₂)Mo(CO)₂(η³-C₃H₅)]³⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [TBA]⁺ cations and minor disorder are omitted for clarity.

Crystals of [TBA]₃[(P₄O₁₂)MoO₂Cl] were obtained from a mixture of dichloromethane and diethyl ether, and were subjected to study via X-ray crystallography. The conformation of the oxo atoms is cis and the connectivity of the metaphosphate is κ³, which is again in complete agreement with FT-IR and NMR spectroscopy studies (Fig. 8). The values observed for the interatomic distances of compound [TBA]₃[(P₄O₁₂)MoO₂Cl] are very similar to the values for MoO₂Cl₂(OSMe₂)₂.⁴⁹ As was the case for 3, this derivative (complex 4) possesses C₁ symmetry. The crystal structure of this compound also contains a dichloromethane molecule that is hydrogen bonded with one of the oxygen atoms in the metaphosphate ligand. The D⋯A distance is 3.253 Å (Table S3†), indicative of a weak hydrogen bond.⁴⁷ Selected interatomic distances for salts 3 and 4 are summarized in Table 2.

Fig. 8 Solid-state molecular structure of anion [(P₄O₁₂)MoO₂Cl]³⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [TBA]⁺ cations and minor disorder are omitted for clarity.

Table 2 Selected interatomic distances (Å) of salts 3 and 4

Bond	3	4
3: [TBA]3[(P4O12)Mo(CO)2(η^3-C3H5)], 4: [TBA]3[(P4O12)MoO2Cl].
Mo–O1	2.181(4)	2.0764(4)
Mo–O2	2.216(4)	2.163(4)
Mo–O3	2.200(5)	2.172(4)
Mo–CO	1.979(11)–2.038(13)
Mo–Cl		2.396(2)
Mo–Om		1.691(4)–1.794(8)

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The decreased Mo–OP distance in 4versus that of 3 correlates with the increased oxidation state in the former. Even though there was no crystal structure reported for the starting material (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl, the crystal structure of (CH₃CN)₂Mo(CO)₂(η³-2-methyl-allyl)Cl was reported by Baker et al. The data from the solid-state structure of this complex show that the values of the interatomic distances of Mo=O and Mo–Cl are very similar to those observed in our complexes.⁵⁰

Crystal structures were obtained for all the cyclo-trimetaphosphate salts reported herein. In general, all the complexes have C_s symmetry. Crystals of [PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)] were grown by vapor diffusion of diethyl ether into a dilute dichloromethane solution (Fig. 9). The solid-state structure of [(P₃O₉)Mo(CO)₂(η³-C₃H₅)]²⁻ exhibits κ³ connectivity (consistent with the ³¹P{¹H}-NMR spectrum) wherein all of the Mo–OP bonds differ by 0.085 Å. As suggested by the NMR simulations, one of the Mo–O distances is shorter than the other two (Mo–O₁) leading to a slightly different phosphorus environment. The Mo–CO interatomic distances are similar to those reported for [(P₃O₉)Mo(CO)₃]³⁻.³⁸ The configuration of the allyl group in the solid state is endo, and the bond length between the metal and the central carbon is shorter than the bond between the metal and the external carbons by 0.14 Å.

Fig. 9 Solid-state molecular structure of anion [(P₃O₉)Mo(CO)₂(η³-C₃H₅)]²⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [PPN]⁺ cations are omitted for clarity.

Crystals of [PPN]₂[(P₃O₉)MoO₂Cl] and [PPN]₂[(P₃O₉)WO₂Cl] were grown by vapor diffusion of diethyl ether into a saturated acetonitrile solution affording colorless crystals (Fig. 10 and 11 respectively). The crystal structure of the compound [PPN]₂[(P₃O₉)WO₂Cl] shows that the configuration of the two oxo atoms is cis which is consistent with the result obtained by IR spectroscopy. Crystals of [PPN]₂[(P₃O₉)MoOCl₂] were grown from a mixture of dichloromethane–toluene (2 : 1) affording green crystals (Fig. 12).

Fig. 10 Solid-state molecular structure of anion [(P₃O₉)MoO₂Cl]²⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [PPN]⁺ cations and minor disorder are omitted for clarity.

Fig. 11 Solid-state molecular structure of the anion [(P₃O₉)WO₂Cl]²⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [PPN]⁺ cations and solvent are omitted for clarity.

Fig. 12 Solid-state molecular structure of anion [(P₃O₉)MoOCl₂]²⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [PPN]⁺ cations and minor disorder are omitted for clarity.

Crystals of [PPN]₂[(P₃O₉)VOF₂]·2CH₂Cl₂ were grown from a mixture of dichloromethane and diethyl ether (Fig. 13). The V≡O distance is 1.6242(19) Å indicating a bond order of three.⁵¹ In the solid-state structure of salt 10, two solvent molecules are present. One dichloromethane solvent molecule engages in a weak hydrogen bond (D⋯A distance 3.317 Å) to one of the fluoride ions attached to the vanadium center (Table S3†).⁴⁷

Fig. 13 Solid-state molecular structure of the anion [(P₃O₉)VOF₂]²⁻ rendered using ORTEP⁴⁸ with ellipsoids at the 50% probability level; the [PPN]⁺ cation and solvent are omitted for clarity.

Selected interatomic distances for all the crystal structures of cyclo-trimetaphosphate complexes are summarized in Table 3. In general, the higher the oxidation state of the metal, the shorter the Mo–OP bond. The M–O_m bond (O_m = terminal oxo atom) becomes shorter as the oxidation state of the metal increases. The bond length of V≡O is shorter than the other M–O_m bonds, lending credence to its interpretation as a triple bond.⁵¹

Table 3 Selected interatomic distances (Å) of salts 6, 7, 8, 9, 10, and 11

Bond	6	7	8	9	10	11
6: [PPN]3[(P3O9)Mo(CO)3]; 7: [PPN]2[(P3O9)Mo(CO)2(η^3-C3H5)]; 8: [PPN]2[(P3O9)MoO2Cl]; 9: [PPN]2[(P3O9)MoOCl2]; 10: [PPN]2[(P3O9)VOF2]·2CH2Cl2; 11: [PPN]2[(P3O9)WO2Cl]; X = CO, Cl, F; Om = terminal oxo atom.
M–O1	2.278(3)	2.1467(11)	2.1966(14)	2.204(7)	2.1725(16)	2.1904(11)
M–O2	2.272(3)	2.2204(13)	2.1938(14)	2.068(7)	2.0395(17)	2.0453(17)
M–O3	2.300(3)	2.2325(14)	2.0587(15)	2.083(8)	1.9692(16)	2.1790(18)
M–X	1.940(4)–1.961(2)	1.942(2)–1.945(2)	2.3628(11)	2.352(10)–2.360(18)	2.347(18)–2.35(4)	2.3562(6)
						2.3562(6)
M–Om			1.687(4)–1.694(2)	1.73(5)	1.6242(19)	1.7152(13)–1.711(3)

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3. Conclusions

In summary, the facile synthesis of a series of early transition-metal-cyclophosphate complexes (bearing labile ligands) is reported. The complexes (2–5 and 7–11) were isolated in good yield and were fully characterized by means of IR and NMR spectroscopy. The obtained crystal structures and NMR spectroscopy data for these complexes showed that both cyclo-triphosphate and cyclo-tetraphosphate behave as tridentate ligands connected in κ³ mode: in the case of [P₄O₁₂]⁴⁻, one of the metaphosphate units is uncoordinated.

4. Experimental

General

Unless stated otherwise, all the operations were performed in a Vacuum Atmospheres drybox under an atmosphere of purified nitrogen. Mo(CO)₆ was purchased from Strem and used without further purification. Tetrabutylammonium hydroxide 1 M and DOWEX® 50 WX4-200 ion-exchange resin were purchased from Sigma-Aldrich. [Na]₄[P₄O₁₂]·5H₂O,³⁰ [PPN]₃[P₃O₉]·H₂O,⁵² (CH₃CN)₃Mo(CO)₃,⁵³ (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl,⁵⁴ MoO₂Cl₂(OSMe₂)₂,⁴⁹ MoOCl₃,⁵⁵ WO₂Cl₂(CH₃CN)₂,⁵⁵ WOCl₄ ⁵⁵ and [PPN]₃[(P₃O₉)Mo(CO)₃] (6)³⁸ were prepared according to published procedures. Aqueous solutions were prepared using reagent grade deionized water (r ≥ 18 MΩ cm; Rica Chemical Company, USA). Diethyl ether, acetonitrile, dichloromethane and tetrahydrofuran were dried and deoxygenated by the method of Grubbs using a system built by SG Water USA, LLC and stored over 4 Å molecular sieves. 4 Å molecular sieves were dried under reduced pressure at a temperature above 200 °C over the course of one week. ¹H, ¹³C{¹H} and ³¹P{¹H}-NMR spectra were recorded on Varian Mercury-300 or Bruker AVANCE-400 spectrometers. NMR solvents were obtained from Cambridge Isotope Laboratories. ¹H and ¹³C chemical shifts are reported with respect to internal solvent resonances (DMSO-d₆, δ: 2.50 and 39.52 ppm respectively). ³¹P-NMR chemical shifts are reported with respect to an external reference (85% H₃PO₄, δ: 0.0 ppm). ⁵¹V-NMR chemical shifts are reported with respect to an external reference (VOCl₃, δ: 0.0 ppm). J values are given in Hz. Infrared spectra were recorded on a Bruker TENSOR37 FT-IR spectrometer. X-ray data collections were carried out on a Siemens Platform three-circle goniometer with a CCD detector using Mo-Kα radiation, λ = 0.71073 Å. An Agilent Technologies 5975C Mass Selective Detector operating in electron impact ionization mode was used to collect mass spectrometric data. Mass Spectrometry was performed on a Micromass Q-TOF ESI spectrometer in anionic mode. Samples were prepared in a glovebox and placed into a 1.5 mL GC vial with a septum to prevent exposure to moisture and oxygen and immediately analyzed by ESI mass spectrometry. All samples were prepared at a concentration of 0.1 mg mL⁻¹ in acetonitrile. Prior to injection of the samples, the capillary tubing was flushed with 3 mL of dry acetonitrile (dried over 4 Å molecular sieves for 3 days). The anions of interest were observed to be sensitive to instrument voltages. Optimum instrument settings were obtained with a capillary voltage of 3200 V, a MCP detector voltage of 2500 V, source temperature of 100 °C and a desolvation gas temperature of 150 °C. EPR measurements were performed using a Bruker EMX EPR spectrometer, an ER 4199HS cavity and a Gunn diode microwave source producing X-band (8–10 GHz) radiation.

[TBA]₄[P₄O₁₂]·5H₂O (1). Dowex® (18.0 g) was stirred with a solution of 0.5 M tetrabutylammonium hydroxide (120 mL) for 12 hours. The Dowex was loaded into a column and washed with distilled water (40 mL). An aqueous solution of [Na]₄[P₄O₁₂]·5H₂O (4.0 g in 200 mL) was loaded into the column. The first 40 mL of eluent was discarded and the remainder was combined with effluent generated by washing the column with 40 mL of water after the elution was finished. The eluent was evaporated to dryness under vacuum at 40–50 °C yielding a white powder. The powder was washed with diethyl ether (3 × 20 mL) and further dried in a Schlenk line at 50 °C overnight. The solid was extracted with acetonitrile (20 mL) and the insoluble solids were filtered out and washed with acetonitrile (5 mL). The filtrate was concentrated down to 5 mL. A large excess of diethyl ether (250 mL) was added leading to the formation of a white solid that was collected by filtration, washed with diethyl ether (3 × 10 mL) and dried in vacuo in the Schlenk line overnight at 50 °C (6.0 g, 57% yield). Elem. Anal. Found: C, 55.68; H, 11.24; N, 4.13%. Calc. for C₆₄H₁₅₄N₄O₁₇P₄: C, 55.87; H, 11.28; N, 4.07%. δ_H (400 MHz; DMSO-d₆) 0.95 (t, ³J_HH = 7.2, CH₃–CH₂, 48H), 1.34 (m, CH₂–CH₂, 32H), 1.59 (m, CH₂–CH₂, 32H), 3.21 (t, ³J_HH = 7.2, CH₂,–N, 32H) ppm. δ_C (100 MHz; DMSO-d₆) 57.2 (CH₂, 16C), 23.2 (CH₂, 16C), 19.1 (CH₂, 16C), 13.4 (CH₃, 16C) ppm. δ_P (121.5 MHz; CH₃CN) −23.2 ppm (s, 4P).

[TBA]₄[(P₄O₁₂)Mo(CO)₃]·2H₂O (2). Mo(CO)₃(MeCN)₃ (0.047 g, 0.15 mmol) was added to a solution of [TBA]₄[P₄O₁₂]·5H₂O (0.20 g, 0.16 mmol) in acetonitrile (ca. 2 mL). The solution was stirred for one hour, during which it became dark yellow. The solution was concentrated to 0.5 mL and added to excess diethyl ether (ca. 20 mL), yielding a yellow oil. The solvent was then decanted and the remaining oil was treated with pentane and then dried under vacuum, affording an orange solid (0.15 g, 69% yield). Elem. Anal. Found: C, 53.84; H, 10.37; N, 4.02% Calc. for C₆₇H₁₄₈MoN₄O₁₇P₄: C, 53.58; H, 9.93; N, 3.73%. FT-IR (ATR): ν_max/cm⁻¹ 1702vs (CO), 1708vs (CO) and 1874vs (CO) cm⁻¹. FT-IR (in acetonitrile) ν_max/cm⁻¹ 1702vs (CO), 1708vs (CO) and 1874vs (CO). δ_H (400 MHz; DMSO-d₆) 0.95 (t, ³J_HH = 7.2, CH₃–CH₂, 48H), 1.34 (m, CH₂–CH₂, 32H), 1.57 (m, CH₂–CH₂ = 7.3, 32H), 3.17 (t, ³J_HH = 7.2, CH₂–N, 32H) ppm. δ_C (100 MHz; DMSO-d₆) 231.5 (s, CO, 3C), 57.4 (CH₂, 16C), 23.0 (CH₂, 16C), 19.1 (CH₂, 16C), 13.4 (CH₃, 16C) ppm. δ_P (121.5 MHz; CH₃CN) −16.6 (b, 1P), −19.6 (b, 2P), −25.6 (b, 1P) ppm.

[TBA]₃[(P₄O₁₂)Mo(CO)₂(η³-C₃H₅)] (3). [TBA]₄[P₄O₁₂]·5H₂O (0.50 g, 0.36 mmol) was added to a suspension of (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl (0.12 g, 0.39 mmol) in acetonitrile (ca. 2 mL). The suspension became homogeneous after addition of the tetrametaphosphate salt. The solution was stirred for 30 min whereupon the solvent was evaporated yielding a yellow solid. The solid was dissolved in dichloromethane (ca. 0.5 mL) and diethyl ether was added dropwise up to saturation. The solution was allowed to stand overnight, yielding a yellow crystalline solid which was collected by filtration and washed with tetrahydrofuran (3 × 1 mL) (0.25 g, 56%); Elem. Anal. Found: C, 51.49; H, 8.90; N, 3.36%. Calc. for C₅₃H₁₁₃MoN₃O₁₄P₄: C, 51.49; H, 9.21; N, 3.40%. FT-IR (ATR): ν_max/cm⁻¹ 1789vs (CO) and 1898vs (CO). FT-IR (in acetonitrile) ν_max/cm⁻¹ 1807s (CO) and 1914vs (CO). δ_H (400 MHz; DMSO-d₆) 0.66 (d, H^anti, ³J_HH = 9.2, 2H), 0.95 (t, ³J_HH = 7.2, CH₃–CH₂, 36H), 1.34 (m, CH₂–CH₂, 24H), 1.57 (m, CH₂–CH₂, 24H), 3.17 (t, ³J_HH = 7.2, CH₂–N, 24H), 3.17 (b, H^syn, 2H), 3.92 (b, H^apical, 1H) ppm. δ_C (100 MHz; DMSO-d₆) 231.5 (s, CO, 2C), 63.7 (s, C^allyl, 1C), 61.0 (s, C^allyl, 2C), 57.4 (CH₂, 12C), 23.0 (CH₂, 12C), 19.1 (CH₂, 12C), 13.4 (CH₃, 12C) ppm. δ_P (162 MHz; CH₃CN) −17.3 (t, ²J_PP = 27.0, 1P), −21.6 (t, ²J_PP = 27.0, 2P), −25.7 (t, ²J_PP = 27.0, 1P) ppm. ESI-MS (m/z, CH₂Cl₂): 995.3415 ([TBA]₂M⁻).

[TBA]₃[(P₄O₁₂)MoO₂Cl] (4). MoO₂Cl₂(OSMe₂)₂ (0.052 g, 0.15 mmol) was added to a solution of [TBA]₄[P₄O₁₂]·5H₂O (0.20 g, 0.15 mmol) in acetonitrile (ca. 2 mL). The solution was stirred for one hour and the solvent was removed in vacuo, affording an oil. The oil was dissolved in dichloromethane (ca. 0.5 mL) and diethyl ether was added dropwise up to saturation. The solution was allowed to stand overnight providing light yellow crystals which were collected by filtration and washed with tetrahydrofuran (3 × 1 mL) (0.12 g, 68%). Elem. Anal. Found: C, 47.54; H, 8.83; N, 3.44%. Calc. for C₄₈H₁₀₈ClMoN₃O₁₄P₄: C, 47.78; H, 9.02; N, 3.48%. FT-IR (ATR): ν_max/cm⁻¹ 880m (M=O) and 896m (M=O). δ_H (400 MHz; DMSO-d₆) 0.87 (t, ³J_HH = 7.2, CH₃–CH₂, 36H), 1.27 (m, CH₂–CH₂, 24H), 1.52 (m, CH₂–CH₂, 24H), 3.14 (t, ³J_HH = 7.2, CH₂–N, 24H) ppm. δ_C (100 MHz; DMSO-d₆) 57.2 (CH₂, 12C), 23.2 (CH₂, 12C), 19.1 (CH₃, 12C), 13.4 (CH₃, 12C) ppm. δ_P (162 MHz; CH₃CN) −21.2 (b, 4P) ppm.

[TBA]₃[(P₄O₁₂)VOF₂]·Et₂O (5). VOF₃ (0.021 g, 0.17 mmol) was added to a solution of [TBA]₄[P₄O₁₂]·5H₂O (0.20 g, 0.15 mmol) in acetonitrile (ca. 2 mL). The solution was stirred for one hour and the solvent was then removed in vacuo. The remaining solid was dissolved in dichloromethane, and diethyl ether was added dropwise up to saturation. The solution was allowed to stand overnight, affording light yellow crystals which were collected by filtration and washed with tetrahydrofuran (3 × 1 mL) (0.10 g, 56%). Elem. Anal. Found: C, 51.21; H, 9.77; N, 4.09%. Calc. for C₅₂H₁₁₈F₂N₃O₁₄P₄V: C, 51.10; H, 9.73; N, 3.44%. FT-IR (ATR) ν_max/cm⁻¹ 912m (V≡O). δ_H (400 MHz; DMSO-d₆) 0.88 (t, ³J_HH = 7.2, CH₃–CH₂, 36H), 1.28 (m, CH₂–CH₂, 24H), 1.53 (m, CH₂–CH₂, 24H), 3.15 (t, ³J_HH = 7.1, CH₂–N, 24H) ppm. δ_C (100 MHz; DMSO-d₆) 57.4 (CH₂, 12C), 23.0 (CH₂, 12C), 19.1 (CH₂, 12C), 13.4 (CH₃, 12C) ppm. δ_V (104 MHz; DMSO-d₆) −594.5 ppm.

[PPN]₂[(P₃O₉)Mo(CO)₂(η³-C₃H₅)] (7). [PPN]₃[P₃O₉]·H₂O (0.48 g, 0.26 mmol) was added to a suspension of (CH₃CN)₂Mo(CO)₂(η³-C₃H₅)Cl (0.07 g, 0.26 mmol) in acetonitrile (ca. 2 mL). Upon addition of the trimetaphosphate salt, the suspension became homogeneous. After stirring for ca. 30 seconds yellow solid precipitated. The slurry was stirred for additional 30 minutes in order to ensure completion of the reaction. The solid was collected by filtration and washed with tetrahydrofuran (3 × 1 mL) (0.25 g, 78% yield). Elem. Anal. Found: C, 61.95; H, 4.45; N, 2.20%. Calc. for C₇₇H₆₅MoN₂O₁₁P₇: C, 61.36; H, 4.35; N, 1.86%. FT-IR (ATR): ν_max/cm⁻¹ 1812vs (CO) and 1918s (CO). FT-IR (in acetonitrile) ν_max/cm⁻¹ 1818vs (CO) and 1923vs (CO) cm⁻¹. δ_H (400 MHz; DMSO-d₆) 0.81 (d, ²J_HH = 9.2, H^anti, 2H), 3.15 (m, H^syn and H^apical, 3H), 7.57 (m, H^Ar, 48H), 7.72 (m, H^Ar, 12H) ppm. δ_C (100 MHz; DMSO-d₆) 230.0 (s, CO, 2C), 133.7 (m, para, 12C), 132.0 (m, ortho, 24C), 129.6 (m, meta, 24C), 126.3 (dd, ¹J_PC = 107.1, ³J_PC = 1.5, ipso, 12C), 73.7 (s, C^allyl, 1C), 59.9 (s, C^allyl, 2C) ppm. δ_P (162 MHz; CH₃CN) +22.3 (s, 4P), −12.9 (s, 1P), −13.8 (s, 2P) ppm. ESI-MS (m/z), CH₂Cl₂: 969.9943 (M − PPN)⁻, 215.9049 (M)²⁻.

[PPN]₂[(P₃O₉)MoO₂Cl] (8). [PPN]₃[P₃O₉]·H₂O (1.6 g, 0.86 mmol) was added to a solution of MoO₂Cl₂(OSMe₂)₂ (0.31 g, 0.84 mmol) in acetonitrile (ca. 4 mL). The solution was stirred for 2 hours, during which the formation of white precipitate was observed. The precipitate was collected by filtration, washed with tetrahydrofuran (3 × 1 mL) and dried in vacuo (0.68 g, 56% yield). Elem. Anal. Found: C, 59.18; H, 4.12; N, 2.07%. Calc for C₇₂H₆₀ClMoN₂O₁₁P₇: C, 58.53; H, 4.09; N, 1.90%. FT-IR (ATR): ν_max/cm⁻¹ 898m (M=O) and 923m (M=O). δ_H (400 MHz; DMSO-d₆) 7.57 (m, H^Ar, 48H), 7.72 (m, H^Ar, 12H) ppm. δ_C (100 MHz; DMSO-d₆) 133.7 (m, para, 12C), 132.3 (m, ortho, 24C), 129.9 (m, meta, 24C), 126.33 (dd, ¹J_PC = 107.1, ³J_PC = 1.5, ipso, 12C). δ_P (121.5 MHz; CH₃CN) +22.3 (s, 4P), −18.2 (t, ²J_PP = 18.1, 1P), −19.0 (d, ²J_PP = 18.1, 2P) ppm. ESI-MS (m/z, CH₂Cl₂): 939.9281 (M − PPN)⁻, 200.8703 (M)²⁻.

[PPN]₂[(P₃O₉)MoOCl₂] (9). To a thawing stirring solution of [PPN]₃[P₃O₉]·H₂O (0.50 g, 0.27 mmol) in acetonitrile (ca. 4 mL) was added dropwise a thawing acetonitrile solution (ca. 4 mL) of MoOCl₃ (0.058 g, 0.27 mmol). The color changed after 15 min from brown to yellow and then to a green-yellow color. The reaction mixture was stirred overnight, during which time the color turned into green and a solid precipitate was observed. After this time, the solvent was removed in vacuo and the residue was dissolved in dichloromethane (ca. 4 mL) and the product was precipitated using diethyl ether (ca. 8 mL). The residue was separated from the clear solution by decantation. Dissolution, precipitation, and decantation were repeated three times to get rid of PPNCl coproduct. The residue was then dried under vacuum and then crystallized overnight using a 2 : 1 CH₂Cl₂–toluene mixture to provide the desired product as green crystals which were collected by filtration and washed with a mixture 1 : 3 acetonitrile–diethyl ether (3 × 2 mL) (0.30 g, 75%). Elem. Anal. Found: C, 57.81; H, 4.20; N, 1.75%. Calc for C₇₂H₆₀Cl₂MoN₂O₁₀P₇: C, 57.77; H, 4.04; N, 1.87%. FT-IR (ATR) ν_max/cm⁻¹ 932m (M=O). δ_H (300 MHz; DMSO-d₆) 7.56 (m, H^Ar, 48H), 7.71 (m, H^Ar, 12H) ppm. δ_C (100 MHz; DMSO-d₆): 133.6 (m, para, 12C), 131.8 (m, ortho, 24C), 129.7 (m, meta, 24C), 126.8 (dd, ¹J_PC = 107.1, ³J_PC = 1.5, ipso, 6C) ppm. δ_P (121.5 MHz, CH₃CN): +21.54 (s, 4P) ppm. EPR (CH₂Cl₂, g-value): 1.934. ESI-MS (m/z, CH₂Cl₂): 421.72 (MH)⁻, 385.74 (M − Cl)⁻ and 341.75 (M − PO₃)⁻.

[PPN]₂[(P₃O₉)VOF₂]·2CH₂Cl₂ (10). VOF₃ (0.022 g, 0.16 mmol) was added to a solution of [PPN]₃[P₃O₉]·H₂O (0.30 g, 0.16 mmol) in acetonitrile (ca. 2 mL). The solution was allowed to stir for one hour and the volatile materials were removed in vacuo. The remaining yellow oil was dissolved in dichloromethane, and diethyl ether was added up to saturation. The solution was allowed to stand overnight, affording light yellow crystals which were collected by filtration and washed with tetrahydrofuran (3 × 1 mL) (0.15 g, 59%). Elem. Anal. Found: C, 55.16; H, 4.65; N, 1.52%. Calc. for C₇₄H₆₄Cl₄F₂N₂O₁₀P₇V: C, 55.94; H, 4.06; N, 1.76%. FT-IR (ATR): ν_max/cm⁻¹ 930m (V≡O). δ_H (400 MHz; DMSO-d₆) 7.59 (m, H^Ar, 60H) ppm. δ_C (100 MHz; DMSO-d₆): 133.6 (m, para, 12C), 131.9 (m, ortho, 24C), 129.5 (m, meta, 24C), 126.8 (dd, ¹J_PC = 107.1, ³J_PC = 1.5, ipso, 6C) ppm. δ_P (121.5 MHz; CH₃CN) +22.16 (s, 4P), −18.19 (s, 1P), −22.15 (s, 2P). δ_V (104 MHz; DMSO-d₆): −593.4 ppm.

[PPN]₂[(P₃O₉)WO₂Cl] (11), method 1. A thawing acetonitrile solution (ca. 4 mL) of [PPN]₃[P₃O₉]·H₂O (5.0 g, 0.27 mmol) was added dropwise to a stirring, thawing acetonitrile suspension (4 mL) of WOCl₄ (0.091 g, 0.27 mmol). The suspension was allowed to stir for 2 hours during which time the reaction mixture became colorless and homogeneous. After this time, the solvent was removed under vacuum to give a white residue. The obtained white residue was dissolved in acetonitrile (ca. 4 mL) and precipitated using diethyl ether (ca. 8 mL). The mother liquor was decanted. Dissolution and precipitation were repeated three times, in order to get rid of PPNCl coproduct. The white residue was recrystallized by slow diffusion of diethyl ether into an acetonitrile, or acetonitrile–dichloromethane (1 : 1), solution of the residue to give colorless crystals which were collected by filtration and washed with a mixture 1 : 3 acetonitrile–diethyl ether (3 × 2 mL) (0.32 g, 77%). Elem. Anal. Found: C, 55.38; H, 3.90; N, 1.75%. Calc. for C₇₂H₆₀ClN₂O₁₁P₇W: C, 55.24; H, 3.86; N, 1.79%. FT-IR (ATR) ν_max/cm⁻¹ 907m (W=O), 938m (W=O). δ_H (300 MHz; DMSO-d₆) 7.56 (m, H^Ar, 48H), 7.71 (m, H^Ar, 12H) ppm. δ_C (100 MHz; DMSO-d₆) 133.6 (m, para, 12C), 131.8 (m, ortho, 24C), 129.7 (m, meta, 24C), 126.8 (dd, ¹J_PC = 107.1, ³J_PC = 1.5, ipso, 6C) ppm. δ_P (121.5 MHz; CH₃CN) +22.20 (s, 4P), −17.79 (m, 3P) ppm. ESI-MS (m/z, CH₂Cl₂) 488.80 (MH)⁻, 452.81 (M − Cl)⁻, 408.83 (M − PO₃)⁻, 243.89 (M)²⁻.

Method 2. A thawing solution of [PPN]₃[P₃O₉]·H₂O (0.50 g, 0.27 mmol) in acetonitrile (ca. 4 mL) was added dropwise to a stirring, thawing solution of WO₂Cl₂(MeCN)₂ (0.099 g, 0.27 mmol) in acetonitrile (ca. 4 mL). The solution was allowed to stir for two hours whereupon the solvent was removed in vacuo. The obtained white residue was dissolved in acetonitrile (ca. 4 mL) and precipitated using diethyl ether (ca. 8 mL). The mother liquor was decanted. Dissolution and precipitation were repeated three times, in order to get rid of PPNCl coproduct. White crystals (0.30 g, 72%) suitable for X-ray studies were obtained by crystallization of the obtained white residue by slow diffusion of diethyl ether into acetonitrile solution. Spectroscopic data for the complex synthesized using this method were identical to those obtained according method 1.

Acknowledgements

The authors thank Eni SpA under the Eni-MIT Alliance Solar Frontiers Program for financial support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 898619–898621, 898623, 898624, 928307 and 928308. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52526k

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