Transition Metal Chemistry

, Volume 35, Issue 1, pp 111–115

Molybdenum(VI) imido complexes: formation of [Mo(NPh)(S2CNR2)3]X and [Mo(NPh)(S2CNR2)2(μ-MoO4)]2 (R = Me, Et) from the reaction of [MoO2(S2CNR2)2] and PhNCO under aerobic conditions

Authors

    • Department of ChemistryUniversity College London
  • Idris Richards
    • Department of ChemistryUniversity College London
Article

DOI: 10.1007/s11243-009-9302-0

Cite this article as:
Hogarth, G. & Richards, I. Transition Met Chem (2010) 35: 111. doi:10.1007/s11243-009-9302-0

Abstract

Products from the thermolysis of PhNCO and [MoO2(S2CNR2)2] (R = Me, Et) are highly dependent upon the reaction conditions. When carried out in air, the major products are cations, [Mo(NPh)(S2CNR2)3]+, as shown by a crystal structure of [Mo(NPh)(S2CNEt2)3]2[Mo6O19]. Under rigorously anaerobic conditions, reaction of two equivalents of PhNCO with [MoO2(S2CNR2)2] affords [Mo(NPh)2(S2CNR2)2] as the major product. However, chloroform solutions of the bis(imido) complexes hydrolyze in air to afford [Mo(NPh)(S2CNR2)2(μ-MoO4)]2, in which molybdate groups bridge between molybdenum(VI) imido-bis(dithiocarbamate) centers. These results are placed in context of our earlier studies of these reactions that lead to the formation of oxo-disulfide [MoS2(NPh)(S2CNR2)2] and dimeric molybdenum(V) [MoO(μ-NPh)(S2CNR2)]2 complexes, thus allowing a full picture of these transformations to be established.

Graphical Abstract

Heating PhNCO with [MoO2(S2CNR2)2] in air gives [Mo(NPh)(S2CNR2)3]X (R = Me, Et), but when the reaction is carried out under anaerobic conditions, [Mo(NPh)2(S2CNR2)2] initially results, chloroform solutions of which hydrolyze in air to afford [Mo(NPh)(S2CNR2)2(μ-MoO4)]2.
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Introduction

Over the past twenty years, transition metal imido chemistry has been intensively investigated, and many synthetic routes have been developed [1]. One of these, the replacement of a metal-bound oxo-ligand upon reaction with an organic isocyanate is potentially attractive, since a wide range of isocyanates is commercially available, transition metal oxides are relatively common, and the by-product is CO2 [26].
$$ {\text{M}}{=}{\text{O}} + {\text{RNCO}} \to {\text{M}}{=}{\text{NR}} + {\text{CO}}_{2} $$
In earlier work, we attempted to utilize this approach toward the synthesis of molybdenum(VI) imido complexes upon thermolysis of the dioxo complexes, [MoO2(S2CNR2)2], with aryl isocyanates [711]. The outcome of these reactions was found to be highly dependent upon the nature of the aryl group, but when bulky 2,6-disubstituted aryl isocyanates were used, the target bis(imido) complexes, [Mo(NAr)2(S2CNR2)2], were produced in reasonable yields [11, 12]. In contrast with phenyl isocyanate, a somewhat surprising result was noted, products being the molybdenum(VI) imido-disulfide complexes, [MoS2(NPh)(S2CNR2)2] together with dimeric molybdenum(V) complexes, [MoO(μ-NPh)(S2CNR2)]2 [7, 8]. In subsequent work [12], we showed that while the bis(arylimido) complexes were stable to further heating, the intermediate oxo-imido complexes, [MoO(NAr)(S2CNR2)2], were not; suggesting that it was further reactions of these that resulted in the observed products (Fig. 1).
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Fig. 1

Reaction of [MoO2(S2CNR2)2] with arylisocyanates under anaerobic conditions

Recently, we sought to repeat these preparations in order to further explore the reactivity of the dimeric molybdenum(V) complexes, [MoO(μ-NPh)(S2CNR2)]2 [13]. However, oxygen and water were not rigorously excluded from the reaction solvent (toluene), and this led to the formation of a quite different product, namely [Mo(NPh)(S2CNR2)3][X]. The formation of the latter has led us to further explore these reactions in order to get a full picture of all observed transformations. These studies are reported herein and include the isolation of the molybdate-bridged complexes [Mo(NPh)(S2CNR2)2(μ-MoO4)]2 and the crystallographic characterization of [Mo(NPh)(S2CNEt2)3]2[Mo6O19].

Results and discussion

Heating a toluene solution of [MoO2(S2CNMe2)2] and PhNCO for 20 h in air resulted, after chromatography, in the isolation of relatively small amounts of [MoS2(NPh)(S2CNMe2)2] and [MoO(μ-NPh)(S2CNMe2)]2 [7], while the major product under these conditions was [Mo(NPh)(S2CNMe2)3][X], being eluted with 1% methanol in dichloromethane and isolated as a yellow solid. Characterization of the cation was relatively straightforward, a molecular ion being observed in the FAB mass spectrum at m/z 548. In the 1H NMR spectrum, along with the aromatic resonances, four methyl resonances were observed in the expected 2:2:1:1 ratio, consistent with a pentagonal-bipyramidal coordination environment in which one sulfur and the imido group occupy the two axial sites. The precise nature of the anion [X] was still unclear at this stage, and it may be a mixture of chloride and hydroxide.

Heating PhNCO and [MoO2(S2CNEt2)2] in air resulted in a similar outcome, the major yellow band eluting with 10% methanol in dichloromethane. While the 1H NMR spectrum clearly showed the expected cation, again no signals were observed for the anion. The crude product was dissolved in DMSO and slow evaporation led to the formation of a small amount of yellow crystalline solid characterized by elemental analysis and X-ray crystallography as [Mo(NPh)(S2CNEt2)3]2[Mo6O10] (Fig. 2). The cation adopts the expected distorted pentagonal-bipyramidal coordination environment. The axial imido ligand is characterized by a short molybdenum-nitrogen distance of 1.727(5) Å, and approximately linear Mo–N–C vector of 170.2(4)o. Also as expected, the axial molybdenum–sulfur interaction is somewhat elongated at 2.585(2) Å, when compared to those in the equatorial plane [Mo-S(av) 2.499 Å], a result of the trans-influence exerted by the phenylimido ligand. The molybdate anion lies between pairs of cations, and no significant intermolecular contacts are observed. This structure is analogous to those recently reported by Minelli et al. [14] for [Mo(N-2,6-Me2C6H3)(S2CNEt2)3]2[Mo6O10] and [Mo(N-2,4,6-Me3C6H2)(S2CNEt2)3]2[Mo6O10], both being prepared from the reaction of [MoOX2(S2CNEt2)2] (X = Cl, Br) with the corresponding aniline in the presence of two equivalents of triethylamine. A difference between them and [Mo(NPh)(S2CNEt2)3]2[Mo6O10] is the relative insolubility of the latter in all simple organic solvents, which has precluded NMR characterization.
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Fig. 2

Molecular structure of the cation in [Mo(NPh)(S2CNEt2)3]2[Mo6O10] (anion excluded) with selected bond lengths (Å) and angles (o); Mo(1)-S(1) 2.4931(17), Mo(1)-S(2) 2.5160(16), Mo(1)-S(3) 2.4856(16), Mo(1)-S(4) 2.5126(16), Mo(1)-S(5) 2.4889(16), Mo(1)-S(6) 2.5848(16), Mo(1)-N(4) 1.727(5), Mo(1)-N(4)-C(20) 170.2(4), S(6)-Mo(1)-N(4) 174.43(16), S(1)-Mo(1)-S(2) 68.27(5), S(3)-Mo(1)-S(4) 68.45(5), S(5)-Mo(1)-S(6) 69.84(5)

The precise mode of formation of imido cations, [Mo(NPh)(S2CNR2)3]+, is unclear. It is known that addition of acids to [MoO2(S2CNR2)2] results in formation of [MoO(S2CNR2)3]+, which in turn reacts with PhNCO to give [Mo(NPh)(S2CNR2)3]+ [15]. Alternatively, oxo-substitution may occur first to generate [MoO(NPh)(S2CNR2)2], which is then susceptible to oxo-displacement by further dithiocarbamate. In order to probe the latter, we initially attempted to prepare [MoO(NPh)(S2CNMe2)2] via thermolysis of [MoO2(S2CNMe2)2] with one equivalent of PhNCO under rigorously anaerobic conditions; however, a complex mixture ensued. In contrast, when we heated [MoO2(S2CNMe2)2] and 2.5 equivalents of PhNCO in dry toluene for 1 h, the major product (ca. 80%) was the bis(imido) complex [Mo(NPh)2(S2CNMe2)2], as shown by comparison of spectroscopic data with that of an authentic sample prepared upon addition of two equivalents of NaS2CNMe2 to [Mo(NPh)2Cl2(dme)] [13]. Other minor products that could be identified by 1H NMR spectroscopy were the expected imido-disulfide and molybdenum(V) complexes, [MoS2(NPh)(S2CNMe2)2] and [MoO(μ-NPh)(S2CNMe2)]2, respectively.

The mixture was then taken up in wet chloroform and exposed to air, with the aim of preparing [MoO(NPh)(S2CNMe2)2] as a result of imido hydrolysis. Indeed, over a period of hours, a yellow precipitate was deposited from the solution. This was collected, washed and dried to afford [Mo(NPh)(S2CNMe2)2(μ-MoO4)]2 (Fig. 3). It was characterized on the basis of elemental analyses and IR spectra, being too insoluble to obtain meaningful NMR or mass spectra. In the IR spectrum, two strong bands are observed at 929 and 910 cm−1 being associated with the cis terminal oxo groups of the molybdate-bridges. A similar experiment with [MoO2(S2CNEt2)2] also deposited a yellow solid that showed strong bands in the IR spectrum at 952 and 918 cm−1. While these absorptions are characteristic of a cis-dioxo-containing material, unfortunately we were unable to obtain satisfactory elemental analysis for this material, probably due to the inclusion of chloroform and water.
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Fig. 3

Proposed mode of formation of [Mo(NPh)2(S2CNMe2)2] and subsequent hydrolysis

Interestingly, Maatta and Wentworth have previously made similar observations, describing an insoluble yellow solid of formula [Mo2O4(NPh)(S2CNEt2)2] being deposited from the prolonged reflux of [MoO(S2CNEt2)2] and PhN3, which showed strong IR bands at 946 and 910 cm−1 [16, 17]. At that time, the precise formulation was unclear, but later we reported the 2,6-diisopropyl substituted derivative, [Mo(N-2,6-iPr2C6H3)(S2CNEt2)2(μ-MoO4)]2, prepared upon protonation of [Mo(N-2,6-iPr2C6H3)2(S2CNEt2)2] with HBF4 and crystallization from a dichloromethane-methanol mixture [18]. While we characterized the latter (which was completely insoluble in all organic solvents) crystallographically, at the time, it was not clear how it had resulted. Now, it seems that hydrolysis of a nitrogen-containing ligand had occurred under the conditions of crystallization, presumably to afford [MoO(N-2,6-iPr2C6H3)(S2CNEt2)2], which in turn decomposes to give the observed cyclic tetramolybdenum complex. Hence, it appears that the hydrolysis of oxo-imido complexes [MoO(NAr)(S2CNR2)2] is a general process, which can be represented by Eq. 1:
$$ 4\left[ {{\text{MoO}}\left( {\text{NAr}} \right)\left( {{\text{S}}_{2} {\text{CNR}}_{2} } \right)_{2} } \right] + 2{\text{H}}_{2} {\text{O}} + {\text{O}}_{2} \to \left[ {{\text{Mo}}\left( {\text{NAr}} \right)\left( {{\text{S}}_{2} {\text{CNR}}_{2} } \right)_{2} \left( {\mu{-}{\text{MoO}}_{4} } \right)} \right]_{2} + 2\left( {{\text{R}}_{2} {\text{NCS}}_{2} } \right)_{2} + 2{\text{ArNH}}_{2} $$
(1)
Thus, the transformation involves both the hydrolysis of some of the imido ligands and also oxidation of some of the dithiocarbamate ligands to the thiuram disulfides. In support of this, we monitored both the decomposition of a pure sample of [Mo(NPh)2(S2CNMe2)2] and that generated from the PhNCO reaction in wet CDCl3 by 1H NMR spectroscopy. In both cases after about 1 day, a yellow precipitate was deposited, and aniline and tetramethylthiuram disulfide were clearly observed, concomitant with the decay of signals associated with [Mo(NPh)2(S2CNMe2)2]. Other resonances, including that of [MoO2(S2CNMe2)2], were also observed in both cases, so it is clear that the transformation is not clean. Notably, upon decomposition of the pure sample of [Mo(NPh)2(S2CNMe2)2], peaks associated with [Mo(NPh)(S2CNMe2)3]+ were absent, suggesting that formation of these cations does not proceed via oxo-imido intermediates, and supporting the proposal that oxo-substitution by a dithiocarbamate is the primary process in the aerobic reaction of [MoO2(S2CNR2)2] with PhNCO (Fig. 4).
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Fig. 4

Proposed mode of formation of [Mo(NPh)(S2CNR2)3]+

Further, we have noted that thermolysis of a toluene solution of [Mo(NPh)(S2CNMe2)3][X] does not result in formation of either [MoS2(NPh)(S2CNMe2)2] or [MoO(μ-NPh)(S2CNMe2)]2, ruling it out as a possible intermediate in the formation of these complexes from [MoO2(S2CNMe2)2] and PhNCO.

In conclusion, we have now fully elucidated the reaction pathways involved when PhNCO is reacted with molybdenum(VI) dioxo-complexes [MoO2(S2CNR2)2]. These studies show that the precise reaction conditions play a major role in the products generated and in order to prepare bis(imido) complexes, water and oxygen should be rigorously removed.

Experimental section

General

All reactions were carried out under a nitrogen atmosphere in dried degassed solvents unless otherwise stated. [MoO2(S2CNR2)2] (R = Me, Et) was prepared by literature methods [19]. NMR spectra were run on a Bruker AMX400 spectrometer and referenced internally to the residual solvent peak (1H) or externally to P(OMe)3 (31P). Infrared spectra were run on a Nicolet 205 FT-IR spectrometer as KBr disks. Fast atom bombardment mass spectra were recorded on a VG ZAB-SE high-resolution mass spectrometer and elemental analyses were performed in house. Chromatography was carried out on deactivated alumina support. The crude mixtures were dissolved in a small amount (ca. 5 ml) of dichloromethane, absorbed onto alumina (ca. 10 g) and dried. This was then added to the top of the wet chromatography column.

Reaction of [MoO2(S2CNMe2)2] with PhNCO in air

[MoO2(S2CNMe2)2] (1.00 g, 2.72 mmol) was dissolved in ca. 50 ml of toluene, PhNCO (0.40 ml, 3.70 mmol) was added, and the mixture was refluxed in air for 20 h resulting in formation a dark red-brown solution. The solvent was removed under reduced pressure to give a dark brown solid. This was chromatographed on alumina. Elution with dichloromethane-petrol (2:3) gave a purple band, which afforded [Mo(NPh)S2(S2CNMe2)2] (0.19 g, 15%). Elution with dichloromethane-petrol (9:1) gave a yellow band, which afforded [MoO(μ-NPh)(S2CNMe2)]2 (0.02 g, 2%). Elution with dichloromethane-methanol (99:1) gave a yellow band, which afforded [Mo(NPh)(S2CNMe2)3][X] (0.57 g) as a yellow solid. 1H NMR(CDCl3) δ 7.38–7.32 (m, 5H, Ph), 3.47 (s, 6H, Me), 3.41 (s, 3H, Me), 3.41 (s, 6H, Me), 3.27 (s, 3H, Me); IR (KBr) ν/cm−1 1651w, 1556m, 1473m, 1442w, 1398s, 1388w, 1527m, 1149w, 1120w, 1072w, 1052w, 1023w, 955m, 928s, 799s, 763m, 726w, 695m, 666w, 570w, 523w, 441w, 421w.

Reaction of [MoO2(S2CNEt2)2] with PhNCO in air

This was carried out as above using [MoO2(S2CNEt2)2] (1.08 g, 2.55 mmol) and PhNCO (0.35 ml, 3.19 mmol). Elution with dichloromethane-petrol (2:3) gave purple [MoS2(NPh)(S2CNEt2)2] (0.11 g, 8%) and with dichloromethane-petrol (9:1) gave orange-yellow [MoO(μ-NPh)(S2CNEt2)]2 (0.07 g, 8%). Elution with dichloromethane-methanol (9:1) gave an orange band, which afforded [Mo(NPh)(S2CNEt2)3][X] as yellow-green solid (0.27 g). 1H NMR (CDCl3) δ 7.36–7.33 (m, 5H, Ph), 3.91–3.77 (m, 10H, CH2), 3.68 (q, J 7.2, 2H, CH2), 1.35 (t, J 7.2, 6H, Me), 1.32 (t, J 7.2, 3H, Me), 1.31 (t, J 7.2, 6H, Me), 1.19 (t, J 7.2, 3H, Me). This solid was recrystallized from DSMO, and upon slow evaporation a yellow crystalline solid was obtained identified as [Mo(NPh)(S2CNEt2)3]2[Mo6O19] (0.69 g, 7.1%). A small yellow block crystal was used for X-ray diffraction data collection. IR (KBr) ν/cm−1 1650w, 1636w, 1559w, 1525m, 1505w, 1438w, 1394w, 1356w, 1275w, 1205w, 1154w, 1066w, 953vs, 918s, 847s, 798m, 757w, 716m, 661w, 556w, 525w, 475w, 439w; Elemental analysis calc. for C42H70N8O19S12Mo8 (found), %C 23.53 (23.89), %H 3.27 (3.28), %N 5.23 (5.01).

Synthesis of [Mo(NPh)(S2CNMe2)2(μ-MoO4)]2

[MoO2(S2CNMe2)2] (1.00 g, 2.72 mmol) was dissolved in ca. 50 ml of dry toluene, PhNCO (0.79 ml, 6.40 mmol) was added, and the mixture was refluxed under nitrogen for 14 h, after which the solvent was removed under reduced pressure. Spectroscopic data showed the major product (>80%) to be [Mo(NPh)2(S2CNMe2)2] [13]. 1H NMR(CDCl3) δ 7.25 (t, J 7.4, 4H, Ph), 7.18 (d, J 7.2, 4H, Ph), 7.02 (t, J 7.4, 2H, Ph), 3.43 (s, 12H, Me). Small amounts of [MoS2(NPh)(S2CNMe2)2] and [MoO(μ-NPh)(S2CNMe2)]2 were also present. This mixture was dissolved in wet chloroform (ca. 100 ml) and exposed to air. After a few hours, a yellow precipitate formed which was collected, washed with copious quantities of chloroform and ether, and dried. The product that was completely insoluble in all common organic solvents was identified as [Mo(NPh)(S2CNMe2)2(μ-MoO4)]2 (0.360 g, 45%). IR (KBr) ν/cm−1 1660w, 1558vs, 1475m, 1444m, 1401s, 1281w, 1248m, 1157m, 1051w, 1033w, 929s, 910s, 865w, 845w, 804m, 759m, 748w, 715w, 694w, 663m, 572w, 558w; Elemental analysis calc. for C24H34N6O8S8Mo4 (found), %C 23.53 (23.89), %H 3.27 (3.28), %N 5.23 (5.01).

Synthesis of [Mo(NPh)(S2CNEt2)2(μ-MoO4)]2

[MoO2(S2CNEt2)2] (1.08 g, 2.73 mmol) was dissolved in ca 50 ml of dry toluene, and PhNCO (0.52 ml, 4.20 mmol) was added, and the mixture was refluxed under nitrogen for 14 h. Removal of volatiles gave a dark brown oily solid. This was dissolved in wet chloroform (ca. 80 ml) and exposed to air. After a few hours, a yellow precipitate started to form. The solution was left standing overnight, and the resulting solid was washed with copious quantities of chloroform and ether and dried. The product was identified as [Mo(NPh)(S2CNEt2)2(μ-MoO4)]2 (0.189 g, 22%). IR (KBr) ν/cm−1 1525s, 1502s, 1475w, 1434m, 1379w, 1352w, 1275m, 1205m, 1149m, 1095m, 1066m, 952s, 918s, 850w, 779m, 760w, 753w, 723w, 696w, 650m, 526m, 501m.

X-ray data collection and solution

A single crystal of [Mo(NPh)(S2CNEt2)3]2[Mo6O10] was mounted on a glass fiber, and all geometric and intensity data were taken from this sample using a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 ± 2 K. Data reduction was carried out with SAINT + [20], and absorption correction applied using the program SADABS [21]. Structures were solved by direct methods [22] and developed [23] by using alternating cycles of least-squares refinement and difference-Fourier synthesis. All non-hydrogen atoms were refined anisotropically. Hydrogens were generally placed in calculated positions (riding model). Structure solution used SHELXTL PLUS V6.10 program package [24].

Crystallographic data for [Mo(NPh)(S2CNEt2)3]2[Mo6O10]; yellow block, dimensions 0.12 × 0.11 × 0.08 mm, triclinic, space group P\( \bar{1} \), a = 11.7184(14), b = 11.7375(14), c = 14.0324(17) Å, α = 92.601(2), β = 109.933(2), γ = 94.978(2)o, V = 1,801.8(4) Å3, Z = 1, F(000) = 1,058, dcalc = 1.975 g cm−3, μ. = 1.759 mm−1, Tmax/Tmin = 0.872/0.817. 15,386 reflections were collected, 8,155 were unique [R(int) = 0.0313] of which 5,990 were observed [I > 2.0σ(I)]. At final convergence, R1 = 0.0580, wR2 = 0.1047 [I > 2.0σ(I)] and R1 = 0.0878, wR2 = 0.1141 (all data), for 398 parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 756070.

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