Complexes of molybdenum(VI) oxide tetrafluoride and molybdenum(VI) dioxide difluoride with neutral N- and O-donor ligands

Article abstract of DOI:10.1016/j.jfluchem.2017.06.015

[MoOF₄(MeCN)], prepared from reaction of MoF₆ with (Me₃Si)₂O in anhydrous MeCN solution, reacts with the neutral O-donor ligands, thf, Ph₃PO, Me₃PO, dmf and dmso, (L) in a 1:1 molar ratio u

Full text of DOI:10.1016/j.jfluchem.2017.06.015

Accepted Manuscript
Title: Complexes of molybdenum(VI) oxide tetrafluoride and
molybdenum(VI) dioxide difluoride with neutral N- and
O-donor ligands
Authors: William Levason, Francesco M. Monzittu, Gillian
Reid, Wenjian Zhang, Eric G. Hope
PII:
S0022-1139(17)30149-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jfluchem.2017.06.015
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27-6-2017
28-6-2017
Please cite this article as: William Levason, Francesco M.Monzittu, Gillian Reid,
Wenjian Zhang, Eric G.Hope, Complexes of molybdenum(VI) oxide tetrafluoride and
molybdenum(VI) dioxide difluoride with neutral N- and O-donor ligands, Journal of
Fluorine Chemistryhttp://dx.doi.org/10.1016/j.jfluchem.2017.06.015
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Complexes of molybdenum(VI) oxide tetrafluoride and molybdenum(VI)
dioxide difluoride with neutral N- and O-donor ligands
William Levason,*1 Francesco M. Monzittu, Gillian Reid and Wenjian Zhang
School of Chemistry, University of Southampton. Southampton SO17 1BJ UK
Eric G. Hope
Department of Chemistry, University of Leicester, Leicester LE1 7RH. UK
*1 Corresponding author e mail wxl@soton.ac.uk
Graphical abstract
Highlights:




First examples of MoOF4 complexes with neutral ligands
X-ray structure of [MoOF4(Ph3PO)]
Comparisons of [MoOF4(L)] and [MoO2F2(L)2] complexes
Comparisons of Mo(VI) and W(VI) oxide fluoride complexes
1

Abstract
[MoOF4(MeCN)], prepared from reaction of MoF6 with (Me3Si)2O in anhydrous MeCN solution,
reacts with the neutral O-donor ligands, thf, Ph3PO, Me3PO, dmf and dmso, (L) in a 1:1 molar ratio
under rigorously anhydrous conditions to form six-coordinate [MoOF4(L)], which have been
characterised by microanalysis, IR, 1H, 19F{1H} and 95Mo NMR spectroscopy. In the presence of
moisture the major products are [MoO2F2(L)2], which can be made directly by reaction of
[MoOF4(L)] with a further equivalent of L and (Me3Si)2O. [MoOF4(MeCN)] and 2,2’-bipyridyl
produce the insoluble [MoOF4(bipy)], which is probably 7-coordinate. Ph3AsO is quantitatively
converted to Ph3AsF2 by [MoOF4(MeCN)], and soft ligands, including Me2S, Me3P and Me3As,
reduce the oxide fluoride on contact. Unstable [MoO2F2(MeCN)2] has also been prepared and the X-ray
structure of [MoO2F2(MeCN)2]MeCN is reported. X-ray crystal structures are reported for
[MoOF4(Ph3PO)], [MoO2F2(Ph3PO)2], [MoO2F2(Me3PO)(H2O)] and [Mo2O4F2(-F)2(Me3PO)2].
Comparisons with the corresponding chemistries of WOF4 and WO2F2 are described.
Keywords: molybdenum; oxide-fluoride; X-ray structures; nitrogen donor ligand; oxygen donor
ligand
1. Introduction
Oxide fluorides of the early d-block metals in their group oxidation state are strong Lewis acids and
form an extensive range of oxido-fluorido anions, often produced in aqueous HF in the presence of
suitable metal cations [1,2]. In contrast, surprisingly few complexes with neutral ligands have been
reported [3], which is due at least in part to the strongly polymerised structures and hence unreactive
nature of many of the oxide fluorides. We recently reported the synthesis of a series of complexes
[WOF4(L)] derived from tungsten(VI) oxide tetrafluoride, made from [WOF4(MeCN)] and neutral N-
or O-donor ligands (L = thf, py, Ph3PO, Me3PO, dmso) in anhydrous MeCN solution [4]. The
[WOF4(MeCN)], which is readily obtained from WF6 and (Me3Si)2O in MeCN, is a convenient
synthon and can be stored for weeks in a glove box, although it decomposes rapidly in air and is
readily hydrolysed by wet solvents. Treatment of [WOF4(L)] with further equivalents of L and
(Me3Si)2O afforded complexes [WO2F2 (L)2] (L = py, Ph3PO, Me3PO, dmso) [4]. Attempts to replace
the nitrile in [WOF4(MeCN)] with soft donor ligands, such as arsines, thioethers or selenoethers,
failed, but two seven-coordinate, pentagonal-bipyramidal complexes, [WOF4(diphosphine)]
(diphosphine = Me2PCH2CH2PMe2 or o-C6H4(PMe2)2), were made from [WOF4(MeCN)] and one
2

equivalent of the diphosphine in anhydrous Et2O solution. Six-coordinate [WOF4(PMe3)] was also
isolated [4,5].
Here we report an investigation of some chemistry in the analogous molybdenum system. Two oxide
fluorides of molybdenum(VI) are known. The first, MoOF4, is a white powder made by fluorination
of MoO3, or by reaction of molybdenum metal with O2/F2 mixtures [2]. The solid structure contains
polymeric chains with two cis fluoride bridges and hence a distorted octahedral molybdenum centre
[6]. In the gas phase it is monomeric with a square-pyramidal geometry [7,8]. There are a variety of
reported routes to MoO2F2 involving fluorination of MoO3 or MoO2Cl2 [1], although a recent study
[9] cast doubt on a number of these. The structure has also proved elusive, but powder X-ray
diffraction data suggest it is based upon triangular Mo3(-F)3 units with axial bridging oxide groups
linking the layers, and with disordered terminal Mo-F and Mo=O groups [9]. The only literature
report of a neutral organic ligand complex of MoOF4, is of [MoOF4(Ph3PO)], identified in situ by its
19F NMR spectrum, but not isolated [10]. There are also adducts of MoOF4 with XeF2 and KrF2
[11,12]. In contrast, a number of complexes of the form [MoO2F2L2] (L = Ph3PO, Me3PO, Ph2MePO,
dmso; L2 = 2,2’-bipyridyl, 1,10-phenanthroline) have been prepared, mostly by addition of ethanolic
solutions of the appropriate ligand to a solution of “molybdic acid” (MoO3nH2O) in aqueous HF, or,
in some cases, from the corresponding [MoO2Cl2(L)2] and Me3SnF [13-18]. The [MoO2F2(thf)2] was
made by decomposition of [MoF4(NCl)] in thf [19], and [MoO2F2(dmf)2] by dissolution of MoO2F2 in
dmf [9]. The spectroscopic characterisation of these complexes is patchy, but single crystal X-ray
structures are available for a few examples [9,14,16,17,19].
Here we report on the synthesis and characterisation of the first series of neutral N- or O-donor
complexes of MoOF4 and some structural and spectroscopic data on complexes of MoO2F2 for
comparison.
2. Results and Discussion
2.1 Synthons
The route developed to prepare MoOF4 complexes is based upon [MoOF4(MeCN)] which is
readily obtained by reaction of MoF6 with (Me3Si)2O (HMDSO), in a 1 : 1 molar ratio in anhydrous
MeCN. The [MoOF4(MeCN)] is isolated as a very moisture sensitive white powder, which can be
easily scaled to 2-3 g quantities and stored in a sealed container in the freezer for several weeks. The
solid slowly darkens, becoming grey and then blue in a glove box at ambient temperatures. It is very
moisture sensitive in solution in MeCN, turning blue on hydrolysis, and although it also dissolves
easily in anhydrous CH2Cl2, the solution slowly decomposes, again turning blue, and the 19F{1H}
NMR spectrum shows some CH2FCl formed [20]. The 19F{1H} NMR spectrum of [MoOF4(MeCN)]
in MeCN shows a singlet at δ = +146.5 with weak six-line pattern satellites due to coupling with 95Mo
3

with 1JMoF = 64 Hz (95Mo, I = 5/2, 15.9%, Q = 0.12 x 10-28 m2). The other molybdenum isotope with a
nuclear spin, 97Mo (I = 5/2, 9.5%), has a large Q = 1.1 x 10-28m2 and coupling to it is not observed.
The chemical shifts observed in CH2Cl2 solution are only slightly different (Section 3), but due to the
slow reaction with the chlorocarbon solvent, MeCN was preferred for most NMR studies in this work.
The 95Mo NMR spectrum is a binomial quintet at δ(95Mo) = −287.3. The value is shifted significantly
to lower frequency than those observed for [MoO2F2L2] complexes (Table 1) which have δ(95Mo)
between around ─40 to ─170 and 1JMoF > 100 Hz. The IR spectrum (Nujol mull) shows ν(CN) =
1
1
2324, ν(C–C) + δ(CH3) = 2293 cm , both shifted to high frequency from free MeCN (2292, 2253 cm ),
1
1
a very strong, broad feature at 1019 cm assigned as ν( MoO) and broad peak at 652 cm due to
ν(MoF).
2.2 MoOF4 complexes
The reaction of [MoOF4(MeCN)] with Ph3PO in a 1:1 molar ratio in rigorously dry MeCN, produces
[MoOF4(Ph3PO)] as a white powder. If an excess of Ph3PO is used, also under anhydrous conditions,
after several days the 19F{1H} and 31P{1H} NMR spectra show that [MoOF4(Ph3PO)] is still the only
major product, although a small amount of Ph3PF2 [21] has formed, indicating some slow O/F
exchange. If “wet” (i.e. commercial laboratory grade) MeCN is used as solvent for the
[MoOF4(MeCN)] + Ph3PO synthesis, [MoO2F2(Ph3PO)2] is isolated upon concentration of the solvent.
This is identified by its characteristic NMR and IR spectra (Section 2.3). The supernatant solution
shows [HF2], [Ph3POH]+ [15, 22] and a number of other unidentified species. The 19F{1H} NMR
spectrum of [MoOF4(Ph3PO)] is a singlet at δ = +139 with weak 95Mo satellites (1JMoF = 65 Hz) and
the 95Mo NMR spectrum is a quintet at δ = −270.0 (Figure 1) , which clearly distinguish this complex
from [MoO2F2(Ph3PO)2] (δ(19F{1H}) = −51.1, δ (95Mo) = −114 (t), 1JMoF = 120 Hz) (Table 1).
The sensitivity in solution made growing crystals of these complexes for X-ray analysis challenging.
However, after numerous attempts, crystals of [MoOF4(Ph3PO)] were obtained via slow evaporation
from a concentrated solution of the complex in MeCN. The structure (Figure 2) shows the presence of
a six-coordinate molybdenum centre with the phosphine oxide trans to Mo=O. The molybdenum lies
slightly out of the F4 plane towards the oxido-group.
The crystals were isomorphous with those of [WOF4(Ph3PO)] [4], and the bond lengths and angles are
very similar between the molybdenum and tungsten analogues, as expected.
The complex [MoOF4(Me3PO)] was made similarly from [MoOF4(MeCN)] and Me3PO in anhydrous
MeCN. It is also very moisture sensitive in solution, and [Me3POH]+ [23] and [MoO2F2(Me3PO)2]
[15] were identified among the hydrolysis products by multinuclear NMR studies. In contrast, the
reaction of [MoOF4(MeCN)] with Ph3AsO in dry MeCN results in immediate and quantitative
formation of Ph3AsF2 [24], with no evidence for a molybdenum complex formed.
4

Evaporation of a solution of [MoOF4(MeCN)] in anhydrous thf resulted in a cream solid that turned
dark blue overnight. The instability prevented a microanalysis being obtained, but the spectroscopic
properties of the freshly prepared complex, viz 19F{1H} NMR (CD2Cl2, 293 K) +143.1 (s, 1JMo-F = 67
95
1
Hz), Mo NMR −271.6 (quintet), IR (cm ) 1097 (s, thf), 1016 (vs, MoO), 872 (s, thf), 675 (vs, MoF),
identified it as [MoOF4(thf)]. This instability makes [MoOF4(thf)] less suitable as a synthon than the
nitrile complex. The complexes [MoOF4(L)] (L = dmf, dmso) (Scheme 1) were obtained similarly to
the phosphine oxides and have the expected spectroscopic features (Table 1). Both decompose,
turning into blue powders, overnight at ambient temperatures.
Repeated attempts to isolate a pure complex from the [MoOF4(MeCN)]-pyridine system were
unsuccessful. In contrast, addition of an MeCN solution of 2,2’-bipyridyl to [MoOF4(MeCN)] in
MeCN, results in immediate precipitation of a cream solid, identified by microanalysis as
[MoOF4(bipy)]. This complex is completely insoluble in solvents such as MeCN or CH2Cl2,
precluding solution spectroscopic measurements or the growth of crystals. However, the IR spectrum,
1
1
which shows ν(MoO) at 959 cm and ν(MoF) at 622, 595, 532 and 476 cm , suggests that like
[WOF4(bipy)] [25], [WOF4(py)2] [26,27] and [WOF4(diphosphine)] [5], it is seven-coordinate,
probably pentagonal bipyramidal with axial O/F. The filtrate from this reaction shows some
[MoO2F2(bipy)], but attempts to convert a suspension of [MoOF4(bipy)] in MeCN to [MoO2F2(bipy)]
by stirring with HMDSO failed, with the [MoOF4(bipy)] being recovered, presumably due to its
insolubility in the reaction mixture. [MoOF4(bipy)] also appears to be stable in air, which contrasts
with the sensitivity to moisture of the [MoOF4(L)] complexes described above.
Although [WOF4(PMe3)] and [WOF4(diphosphine)] (diphosphine = Me2PCH2CH2PMe2 or o-
C6H4(PMe2)2) are formed from the reaction of the phosphines with [WOF4(MeCN)] in Et2O solution
[5], the same ligands immediately produced dark tarry precipitates on contact with [MoOF4(MeCN)]
in Et2O. Similarly, combination of AsMe3, SMe2 or SeMe2 with [MoOF4(MeCN)] in Et2O or MeCN
solution immediately produced deep red or brown solutions and dark precipitates, indicating reduction
of the molybdenum.
2.3 MoO2F2 complexes
As described in the Introduction, [MoO2F2(L)2] complexes are prepared readily from MoO3⋅nH2O
and the appropriate ligand in aqueous HF/EtOH or by Cl/F exchange from the corresponding
[MoO2Cl2(L)2]. In view of the much greater difficulties of routinely handling MoF6, syntheses based
upon this would be much less convenient, and the work described in this section aims to explore the
relationship between corresponding [MoOF4(L)] and [MoO2F2(L)2], rather than devise alternative
syntheses for the latter. Spectroscopic and structural data for several [MoO2F2(L)2] complexes
obtained during this study are also presented.
5

The reaction of MoF6 with ≥ 2 equivalents of HMDSO in anhydrous MeCN, or reaction of
[MoOF4(MeCN)] with HMDSO in MeCN was carried out numerous times under a variety of
conditions on a preparative scale and also as in situ NMR studies; all gave basically the same results.
The products were white solids which turned blue over time in the solid state and more rapidly if left
in contact with the MeCN solution. The blue colour also develops slowly from the dry solids in
contact with the glass vessels, and 19F{1H} NMR spectra identified some [SiF5]− present, indicating
the borosilicate glass is slowly attacked (cf. [9]). The MeCN filtrates from the preparations deposited
insoluble blue solids over time. The in situ solution NMR data showed a molybdenum species with
δ(19F{1H}) = −33.7 (s) and δ(95Mo) = −162.8 (t,1JMo-F = 115 Hz), which were consistent with the
formation of [MoO2F2(MeCN)2]. However, all attempts to isolate a pure solid sample were unsuccessful;
work-up of the solutions generating white/blue solids that were only partially soluble in MeCN.
Microanalyses of these solids showed the C,H,N content to be variable, but typically about 1/3 of the
values expected for [MoO2F2(MeCN)2]. The IR spectra of these solids showed weak nitrile bands at
1
1
2312, 2285 cm and very broad absorptions at ~ 1020890 and ~ 670450 cm . Superimposed upon the
1
1
latter were sharperfeatures at 959, 938 cm (MoO) and 567 cm (MoF). Serendipitously, one attempt to
grow crystals of [MoOF4(MeCN)] by evaporation of an MeCN solution in the dry box, gave a few
clear crystals, which were shown by X-ray structure analysis to be [MoO2F2(MeCN)2]⋅MeCN. The
structure (Figure 3) shows a distorted octahedral molecule with a cis-MoO2 unit and with the nitriles trans
to Mo=O. The bond lengths are unexceptional, and the wide OMoO angle (104.1(2)°) and the axial
FMoF unit bent away from the MoO2, are typical of other complexes of MoO2F2 [9,14,15]. These
results show that [MoO2F2(MeCN)2] is formed in the reactions, but is unstable, losing MeCN (even in
MeCN solution) and depositing what is probably largely MoO2F2 polymer. Solids isolated by evaporation
of the solution are mainly a mixture of MoO2F2 polymer [9,28] and [MoO2F2(MeCN)2] (we cannot rule
out the presence of other insoluble Mo/O/F species). Extraction of the [MoO2F2(MeCN)2] into MeCN or
CH2Cl2 results in partial decomposition with generation of more MoO2F2. The very broad IR bands below
1
~1050 cm are attributed to the polymer, while the sharper superimposed features correspond to
[MoO2F2(MeCN)2]. Attempts to prepare [NbOF3(MeCN)2] [29 ] or [WO2F2(MeCN)2] [4] from the parent
fluoride and HMDSO in MeCN solution, failed, with only the nitrile-free polymeric oxide/fluoride
isolated. The present case of MoO2F2 shows that the MeCN adduct can be formed, but that it is unstable
with respect to irreversible loss of MeCN.
The reaction of [MoOF4(L)] with a further equivalent of L (L = Ph3PO, Me3PO, dmso) and HMDSO in
anhydrous MeCN solution produced [MoO2F2(L)2] (Scheme 1), which were identified by comparison of
their key spectroscopic features (Table 1) with literature data. The complexes [MoO2F2(L)2] (L = Ph3PO,
Me3PO, dmso) were also isolated from reactions of [MoOF4(L)] with the appropriate ligands in “wet”
solvents (i.e. as supplied laboratory grade). Since [MoO2F2(L)2] were originally prepared by combining
6

solutions of L in EtOH and MoO3⋅nH2O in aqueous HF, their formation here from hydrolysis is
unsurprising.
Crystals of [MoO2F2(Ph3PO)2] were obtained by slow evaporation from a CH2Cl2 solution and the
structure is shown in Figure 4 [30].
Although they are not isomorphous, the structure of [MoO2F2(Ph3PO)2] is very similar to those of
[WO2F2(Ph3PO)2] [4] and [MoO2F2(Ph2MePO)2] [14], the key features being a cis-O=M=O unit with
a wide angle (103.8(4) °) and a near linear FMF (158.7(2) °) bent away from the MO2 group. The
bond lengths are unexceptional.
A few crystals each of three complexes containing Me3PO were isolated from the filtrates from the
preparations of [MoOF4(Me3PO)]. The first shown in Figure 5 was identified by the X-ray structure
solution as [MoO2F2(Me3PO)(H2O)] which has trans FMoF and cis-MoO2 groups. The Mo=O
distances are surprisingly disparate, Mo1−O2trans-Me3PO = 1.7433(19) Å and Mo1−O1trans-H2O = 1.684(2)
Å, but still clearly double bonds.
The second crystal contained the dimer [Mo2O4F2(-F)2(Me3PO)2] with the structure shown in (Figure
2

6), which is related to that of the known anion [Mo2O4F4(-F)2] [31,32] with the Me3PO replacing
two terminal fluorides.
The third crystal was [Me3POH]2[Mo2O4F4(-F)2] (see ESI) which contains the known dimeric
Mo(VI) anion. The protonated phosphine oxide, [Me3POH]+ was observed in the 31P{1H} NMR
spectra of hydrolysed [MoOF4(Me3PO)] solutions.
3. Experimental
Infrared spectra were recorded as Nujol mulls between CsI plates using a Perkin-Elmer Spectrum 100
1
spectrometer over the range 4000200 cm . 1H, 19F{1H}, 31P{1H} and 95Mo NMR spectra were recorded
using a Bruker AV–II 400 spectrometer and are referenced to the protio resonance of the solvent, external
CFCl3 , 85% H3PO4 and [MoO4]− in H2O/D2O at pH 11 respectively. Microanalyses were undertaken by
London Metropolitan University. Solvents were dried prior to use: THF and Et2O by distillation from
sodium benzophenone ketyl, MeCN and CH2Cl2 from CaH2. MoF6 was obtained from Fluorochem.
Ligands were obtained commercially (Sigma Aldrich) and dried over molecular sieves (dmf), CaH2
(dmso), by sublimation in vacuo (Me3PO) or by melting in vacuo (bipy, Ph3PO) and all reactions were
carried out under dinitrogen in rigorously dry solvents, using Schlenk and glove box techniques.
7

CAUTION: MoF6 is highly corrosive and HF is generated by hydrolysis of the complexes, both of which
can cause serious skin burns on contact. Appropriate safety precautions should be taken.
3.1 [MoOF4(MeCN)]
A mixture of MeCN (50 mL) and HMDSO (1.6 g, 12.0 mmol) were frozen in liquid nitrogen, and then
MoF6 (2.5 g, 12.0 mmol) condensed in under vacuum. The mixture was allowed to warm slowly to room
temperature turning yellow-brown and then colourless on melting. It was stirred for 5 h, giving a clear,
colourless solution. The solvent was removed in vacuo to give a white powder. Yield: 2.5 g, 94%. Anal.
Required for C2H3F4MoNO (229.0): C, 10.49; H, 1.32; N, 6.12%. Found: C, 10.50; H, 1.35; N, 6.00%. 1H
NMR (CD2Cl2, 293 K): 2.30 (s); (CD3CN, 293 K): 2.10 (s). 19F{1H} NMR (CD2Cl2, 293 K): +148.2 (s,
1JMo-F = 67 Hz); (CD3CN, 293 K): +146.5 (s, 1JMo-F = 64 Hz). 95Mo NMR (CD3CN, 293 K): −287.3
1
(quintet 1JMoF = 64 Hz). IR (Nujol/cm ): 2324 (m, CN), 2293 (ν(C–C) + δ(CH3)), 1019 (s, MoO), 652
(br, MoF).
3.2 [MoOF4(thf)]
[MoOF4(MeCN)] (0.23 g, 1 mmol) was added to dry thf (10 mL) to produce a clear cream solution. After
1 h. the solvent was removed in vacuo to leave a sticky cream solid which darkened becoming dark blue
overnight. The complex is very moisture sensitive turning deep blue on exposure to air, and slowly gives
a blue precipitate in chlorocarbon solvents. 1H NMR (CD2Cl2, 293 K): 1.92 (s, [H]), 3.86 (s, [H]).
19F{1H} NMR (CD2Cl2, 293 K): +143.1 (s, 1JMo-F = 67 Hz). 95Mo NMR (CD2Cl2 293 K): −271.6 (quintet,
1
1JMoF = 65 Hz). IR (Nujol/cm ): 1097 (s, thf), 1016 (vs, MoO), 872 (s, thf), 675 (vs, MoF).
3.3 [MoOF4(Ph3PO)]
A solution of Ph3PO (0.28 g, 1 mmol) in MeCN was added to a solution of [MoOF4(MeCN)] (0.23 g, 1
mmol) in MeCN giving a clear slightly yellow solution. The reaction mixture was left stirring for 1 h. The
solvent was then reduced approximatively to 5 mL when a white solid precipitated out of solution. The
solid was filtered and dried in vacuo. Yield: 0.24 g, 52%. Anal. Required for C18H15F4MoO2 (466.2): C,
46.37; H, 3.24%. Found: C, 46.50; H, 3.34%. 1H NMR (CD3CN, 293 K): 7.857.54 (m). 19F{1H} NMR
(CD3CN, 293 K): +139.1 (s, 1JMo-F = 67 Hz). 31P{1H} NMR (CD3CN, 293 K): +42.8 (s). 95Mo NMR
1
(CD3CN, 293 K): −270.0 (quintet, 1JMoF = 65 Hz). IR (Nujol/cm ): 1085 (s, P=O), 990 (s, MoO), 640 (s,
MoF).
3.4 [MoOF4(Me3PO)]
A solution of Me3PO (0.097 g, 1 mmol) in CH3CN was added to a solution of [MoOF4(MeCN)] (0.23 g,
1 mmol) in MeCN giving a clear, slightly yellow solution. The reaction mixture was left stirring for 1 h.
The solvent was then reduced approximatively to 2 mL, when a white solid precipitated out of solution.
8

The solid was filtered and dried in vacuo. Yield: 0.087 g, 31%. Anal. Required for C3H9F4MoO2 (280.0):
C, 12.87; H, 3.24%. Found: C, 12.85; H, 3.27%. 1H NMR (CD3CN, 293 K): 1.88 (d, 1JP-H = 15 Hz).
19F{1H} NMR (CD3CN, 293 K): +135.9 (s, 1JMo-F = 62 Hz).31P{1H} NMR (CD3CN, 293 K): +64.2. 95Mo
1
NMR (CD3CN, 293 K): −257.0 (quintet, 1JMo-F = 66 Hz). IR (Nujol/cm ): 1079 (s, P=O), 970 (s, MoO),
606 (s, MoF).
3.5 [MoOF4(dmf)]
A solution of dmf (0.083 g, 1 mmol) in CH3CN was added to a solution of [MoOF4(MeCN)] (0.23 g, 1
mmol) in MeCN giving a clear solution. The reaction mixture was left stirring for 1 h. The solvent was
then removed in vacuo giving a slightly yellow oil. Hexane (2 mL) was added causing the formation of a
white solid which was filtered off and dried in vacuo (0.075 g, 29 %). Anal. – complex significantly
decomposed in ~ 12 h, turning blue; this precluded microanalytical measurements. 1H NMR (CD3CN,
293 K): 8.07 (s), 3.09 (s), 2.97 (s). 19F{1H} NMR (CD3CN, 293 K): +138.5 (s, 1JMo-F = 67 Hz).95Mo NMR
1
(CD3CN, 293K) −266.3 (quintet, 1JMo-F = 66 Hz). IR (Nujol/cm ): 1007 (s, MoO), 643 (s, MoF).
3.6 [MoOF4(dmso)]
A solution of dmso (0.089 g, 1 mmol) in CH3CN was added to a solution of [MoOF4(MeCN)] (0.21 g, 1
mmol) in CH3CN giving a clear, slightly yellow solution. The reaction mixture was left stirring for 1 h.
The solvent was then removed in vacuo giving a slightly yellow waxy solid. Anal. – complex
significantly decomposed in ~ 24 h, turning blue; this precluded microanalytical measurements. 1H NMR
(CD3CN, 293 K): 2.75 (s). 19F{1H} NMR (CD3CN, 293 K): +139.1 (s, 1JMo-F = 65 Hz). 95Mo NMR
1
(CD3CN, 293 K): −251.5 (quintet, 1JMo-F = 64 Hz). IR (Nujol/cm ): 1030 (s, SO), 1007 (s, MoO), 653 (s,
MoF).
3.7 [MoOF4(bipy)]
A solution of bipy (0.157 g, 1 mmol) in CH3CN was added to a solution of [MoOF4(MeCN)] (0.23 g, 1
mmol) in CH3CN causing, after few minutes, the precipitation of a pale yellow solid. The solid was
filtered off and dried in vacuo (0.220 g, 65 %). Anal. Required for C10H8F4MoN2O2 (344.1): C, 34.90; H,
2.34; N, 8.14. Found: C, 35.03; H, 2.32; N, 8.24 %. IR (Nujol/cm-1) 959 (s,MoO), 622, 595,532 and 476
(MoF).
3.8 [MoO2F2(MeCN)2]
A mixture of MeCN (15 mL) and HMDSO (0.80 g, 4.5 mmol) were frozen in liquid nitrogen, and then
MoF6 (0.42 g, 2 mmol) condensed in under vacuum. The mixture was allowed to warm slowly to room
temperature and stirred for 5 h, giving a colourless solution with some white solid. The solvent was
removed in vacuo to give a very pale blue-white powder. The product was only partially soluble in dry
9

MeCN in which it gave a bluish solution which darkened over time. Solution data on the soluble
component (see text): 1H NMR (CD3CN, 293 K): 1.97 (s). 19F{1H} NMR (CD3CN, 293 K): −33.7 (s).
1
95Mo NMR (CD3CN, 293 K): −162.8 (t, 1JMo-F = 115Hz). IR (Nujol/cm ): 2312 (m, CN), 2285 (m, ν(C–
C) + δ(CH3)), 959(m), 938 (m) MoO), 567 (br, MoF). There are also very broad absorptions at ~
1
1020890 and ~ 670450 cm .
3.9 [MoO2F2(Ph3PO)2]
A solution of Ph3PO (0.28 g, 1 mmol) in laboratory grade (i.e. wet) MeCN (20 mL) was added to a
solution of [MoOF4(MeCN)] (0.23 g, 1 mmol) in MeCN (10 mL) giving a clear slightly yellow solution.
The reaction mixture was left stirring for 1 h. The solvent was then reduced approximately to 5 mL, when
a cream solid precipitated out of solution. The solid was filtered and dried in vacuo. Yield: 0.24 g, 35%
(based upon OPPh3). Anal. Required for C36H30F2MoP2O4 (722.5): C, 59.85; H, 4.19; Found: C, 59.50; H,
4.17%. 1H NMR (CD2Cl2, 293 K): 7.687.40 (m). 19F{1H} NMR (CD3CN, 293 K): −49.0 (s). 31P{1H}
1
NMR (CD2Cl2, 293 K): +40.2 (s). IR (Nujol/cm ): 1143(s), 1085(s) (P=O), 949(m), 916 (m) (MoO),
587 (s, MoF).
3.10 [MoO2F2(dmso)2]
Dmso (0.08 g, 1 mmol) was added to a solution of [MoOF4(MeCN)] (0.23 g, 1 mmol) in laboratory
grade MeCN (20 mL) to produce a clear solution. After 2 h the solution was concentrated in vacuo to ~
3 mL, resulting in precipitation of a white powder, which was filtered off and dried in vacuo. Anal.
Required for C4H12F2MoO4S2 (332.0): C, 14.91; H, 3.75%. Found: C, 14.71; H, 3.81%. 1H NMR
(CD3CN, 293 K): 2.75 (s). 19F{1H} NMR (CD3CN, 293 K): ─50.8 (s). 95Mo (CD3CN, 293 K): ─92.7 (t,
1
1JMo-F = 112 Hz). IR (Nujol/cm ): 1002(s), 994 sh (S=O) 928(m), 897(m) (MoO), 583(m), 554 (w,br,
MoF). The complex was also made from [MoOF4(MeCN)], dmso, and HMDSO in a 1:2:2 molar ratio,
and was spectroscopically identical.
3.11 X-ray experimental
Crystals of the complexes were grown from MeCN solutions of the complexes as described above.
Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG)
Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å)
rotating anode generator with VHF Varimax optics (70 micron focus) with the crystal held at 100 K.
Structure solution and refinement were performed using SHELX(S/L)97, SHELX-2013 or SHELX-
2014/7 [33]. H atoms bonded to C were placed in calculated positions using the default C−H distance,
and refined using the riding model. Details of the crystallographic parameters are given in Table 2.
CCDC reference numbers in cif format are [MoO2F2(MeCN)2]: CCDC 1542410;
[Me3POH]2[Mo2O4F4(μ-F)2]: CCDC 1542411; [MoO2F2(Me3PO)(H2O)]: CCDC 1542412;
10

[Mo2O4F2(μ-F)2(Me3PO)2]: CCDC 1542413; [MoOF4(Ph3PO)]: CCDC 1542414; [MoO2F2(Ph3PO)2]:
CCDC 1542415.
4. Conclusions
The first series of neutral ligand (N- or O-donor) complexes of MoOF4, specifically [MoOF4(L)] (L =
MeCN, thf, dmf, dmso, Ph3PO and Me3PO) have been prepared and characterised spectroscopically
(IR and multinuclear NMR), and the X-ray structure of [MoOF4(Ph3PO)] determined. All are
extremely readily hydrolysed in both the solid state and solution, and the corresponding [MoO2F2(L)2]
were identified as the major molybdenum containing hydrolysis products. The reaction of the
[MoOF4(L)] with L and HMDSO produces the [MoO2F2(L)2] analogues, although the latter are more
easily made in other ways. In contrast, the [MoOF4(bipy)] is insoluble in common solvents and
appears stable in air as a solid and does not react with HMDSO in MeCN, presumably due to its
insolubility. Full spectroscopic data are reported for the [MoO2F2(L)2] complexes, along with X-ray
structures for [MoO2F2(Ph3PO)2], [MoO2F2(Me3PO)(H2O)] and [Mo2O4F2(-F)2(Me3PO)2 ]. In general
there is a close similarity between the corresponding complexes of MoOF4 and WOF4 as might be
expected, but the former are very significantly less stable both as solids and in solution. In contrast,
corresponding MoO2F2 and WO2F2 complexes are both robust, presumably due to the increased
stabilisation of the metal centre by the second π-donor oxide ligand. The instantaneous reduction of
[MoOF4(MeCN)] by soft donor ligands, contrasting with the isolation of WOF4-phosphine complexes
[5], is a reflection of the significantly greater oxidising power of analogous 4d versus 5d metal
centres.
Acknowledgements
We thank EPSRC for support (EP/K039466, EP/M023664, and EP/N035437/1). FMM thanks the
EPSRC and GE Healthcare for a Case studentship.
11

References
[1] M. Gerken, H. P. A. Mercier, G. J. Schrobilgen, in Advanced Inorganic Fluorides, T. Nakajima, B.
Zemva, A. Tressaud (eds), Elsevier, Amsterdam, 2000, Chap. 5.
[2] J. H. Holloway, D. Laycock, Adv. Inorg. Chem. Radiochem. 28 (1984) 73-99.
[3] S. L. Benjamin, W. Levason, G. Reid, Chem. Soc. Rev. 42 (2013) 1460-1499.
[4] W. Levason, G. Reid, W. Zhang, J. Fluorine Chem. 194 (2016) 50-57.
[5] J. W. Emsley, W. Levason, G. Reid, W. Zhang, J. Fluorine Chem. 197 (2017) 74-79.
[6] A. J. Edwards, B. R. Steventon, J. Chem. Soc. (A) (1968) 2503-2510.
[7] K. Iijima, Bull. Chem. Soc. Japan 50 (1977) 373-375.
[8] W. Levason, R. Narayamaswamy, J. S. Ogden, A. J. Rest, J. W. Turff, J. Chem. Soc., Dalton Trans.
(1981) 2501-2507.
[9] H. Shorafa, H. Ficicioglu, F. Tamadon, F. Girgsdies, K. Seppelt, Inorg. Chem. 49 (2010) 4263-4267.
[10] M. E. Ignatov, E. G. Il’in, B. V. Golovanov, V. D. Butski, Yu. A. Buslaev, Dokl. Akad. Nauk
SSSR 277 (1984) 375-378.
[11] J.H. Holloway, G. J. Schrobilgen, Inorg. Chem. 19 (1980) 2632-2640.
[12] J. H. Holloway, G. J. Schrobilgen, Inorg. Chem. 20 (1981) 3363-3368.
[13] R. Kergoat, J. E. Guerchais, Bull. Soc. Chim. Fr. (1970) 2932-2937.
[14] G. Wang, G. Chen, R. L. Luck, Z. Wang, Z. Mu, D. G. Evans, X. Duan, Inorg. Chim. Acta 357
(2004) 3223-3229.
[15] M. F. Davis, W. Levason, R. Ratnani, G. Reid, T. Rose, M. Webster, Eur. J. Inorg. Chem. (2007)
306-313.
[16] W. Wang, Y. Zhang, X. Jin, X. Du, Acta Crystallogr. Sect. E. E65 (2009) m1094.
[17] I. Sens, H. Stenger, U. Müller, K. Dehnicke, Z. Anorg. Allg. Chem. 610 (1992) 117-120.
[18] R. J. Butcher, B. R. Penfold, E. Sinn, J. Chem. Soc. Dalton Trans. (1979) 668-675.
[19] M. Rhiel, S. Wocadlo, W. Massa, K. Dehnicke, Z. Anorg. Allg. Chem. 622 (1996) 1195-1199.
[20] J. H. Holloway, E. G. Hope, P. J. Towson, R. L. Powell, J. Fluorine Chem. 76 (1996) 105-107.
12

[21] R. Appel, A. Gilak, Chem. Ber. 107 (1974) 2169-217.
[22] P. Mehta, M. Zeldin, Inorg. Chim. Acta 22 (1977) L33-34.
[23] F. Seel, H.-J. Bassler, Z. Anorg. Allg. Chem. 423 (1976) 67-74.
[24] I. Ruppert, V. Bastian, Angew. Chem. Int. Ed. Engl. 17 (1978) 214-215.
[25] L. Arnaudet, R. Bougon, B. Ban, P. Charpin, J. Isabey, M. Lance, M. Nierlich, J. Vigner, Can. J.
Chem. 68 (1990) 507-512.
[26] L. Arnaudet, R. Bougon, B. Ban, P. Charpin, J. Isabey, M. Lance, M. Nierlich, J. Vigner, Inorg.
Chem. 28 (1998) 257-262.
[27] L. Arnaudet, R. Bougon, B. Ban, J. Fluorine. Chem. 74 (1995) 223-225.
[28] M. J. Atherton, J. H. Holloway, J. Chem. Soc., Chem. Comm. (1979) 254-255.
[29] W. Levason, G. Reid, J. Trayer, W. Zhang, Dalton Trans. 43 (2014) 3649-3659.
[30] The unit cell of [MoO2F2(Ph3PO)2], described as monoclinic, was reported in Ref 18 and the
CCDC (CCDC code FXPOMO), but no space group or structural information were provided. It is not
the same cell or space group as reported here.
[31] G. Veryasov, D. Morozov, E. Goreshnik, A. Jesih, J. Fluorine Chem. 156 (2013) 240-245.
[32] T. Kanatani, K. Matsumoto, R. Hagiwara, Eur. J. Inorg. Chem. (2010) 1049-1055.
[33] G. M. Sheldrick, Acta Crystallogr. Sect. A 64 (2008) 112–122.
13

(a)
(b)
Figure 1(a). 19F{1H} and (b) 95Mo NMR spectra of [MoOF4(Ph3PO)] at 295 K.
Figure 2. View of the structure of [MoOF4(Ph3PO)] showing the atom numbering scheme. Ellipsoids
are drawn at the 50% probability level and H-atoms omitted for clarity. Selected bond lengths (Å)
and angles (°): Mo1−F4 = 1.8490(15), Mo1−F3 = 1.8704(14), Mo1−F2 = 1.8540(15), Mo1−F1 =
1.8676(14), Mo1−O1 = 1.6643(18), Mo1−O2 = 2.1533(16), P1−O2 = 1.5171(17), F4−Mo1−F2 =
90.68(7), F4−Mo1−F1= 87.58(7), F2−Mo1−F3 = 90.66(7), F1−Mo1−F3 = 88.11(7), O1−Mo1−F1 =
95.91(8), O1−Mo1−F4 = 97.97(8), O1−Mo1−F3 = 95.47(8), O1−Mo1−F2 = 96.77(8), F3−Mo1−O2 =
83.46(6), F2−Mo1−O2 = 83.02(6), F1−Mo1−O2 = 84.31(6), F4−Mo1−O2 = 83.11(6).
14

Figure 3. View of the structure of [MoO2F2(MeCN)2].MeCN showing the atom numbering scheme.
Ellipsoids are drawn at the 50% probability level and H-atoms omitted for clarity. The disordered
MeCN solvate molecule is also omitted. Selected bond lengths (Å) and angles (°): Mo1−O1 =
1.695(3), Mo1−F1 = 1.898(4), Mo1−F2 = 1.901(4), Mo1−N1 = 2.350(4), O1−Mo1−O1i = 104.1(2),
O1−Mo1−F1 = 98.44(13), O1−Mo1−F2 = 98.47(13), F1−Mo1−F2 = 152.34(16), O1−Mo1−N1 =
89.51(14), F1−Mo1−N1 = 79.85(12), F2−Mo1−N1 = 78.57(12), O1−Mo1−N1 = 89.51(13),
N1−Mo1−N1i = 76.86(17).
Figure 4 View of the structure of [MoO2F2(Ph3PO)2] showing the atom numbering scheme.
Ellipsoids are drawn at the 50% probability level and H-atoms omitted for clarity. Selected bond
lengths (Å) and angles (°): Mo1−F1 = 1.893(5), Mo1−O2 = 1.689(6,), Mo1−O1 = 2.230(5),
F1−Mo1−F1i = 158.7(2), F1−Mo1−O2 = 95.5(2), F1−Mo1−O2i = 97.6(2), O2−Mo1−O2i = 103.8(4),
O1−Mo1−F1 = 80.72(17), O1−Mo1−F1i = 82.48(19), O1−Mo1−O2 = 90.4(2), O1−Mo1−O1i =
75.5(2).
15

Figure 5 View of the structure of [MoO2F2(Me3PO)(H2O)] showing atom numbering scheme.
Ellipsoids are drawn at the 50% probability level and H-atoms omitted for clarity. Selected bond
lengths (Å) and angles (°): Mo1−F2 = 1.9042(15), Mo1−F1=1.9067(15), Mo1−O4 = 2.2881(19),
Mo1−O3 = 2.1475(19), Mo1−O2 = 1.7433(19), Mo1−O1= 1.684(2), F2−Mo1−F1 = 155.92(7), F2−
Mo1−O4 = 79.66(7), F2−Mo1−O3 = 80.40(7), F1−Mo1−O4 = 79.47(7), F1−Mo1−O3 = 83.80(7),
O3−Mo1−O4 = 78.54(7), O2−Mo1−F2 = 94.57(8), O2−Mo1−F1 = 95.97(8), O2−Mo1−O4 =
86.36(9), O1−Mo1−F2 = 100.42(8), O1−Mo1−F1 = 98.48(8), O1−Mo1−O3 = 93.15(9), O1−Mo1−O2
= 102.0(1) .
Figure 6 View of the structure of [Mo2O4F2(-F)2(Me3PO)2] showing atom numbering scheme.
Ellipsoids are drawn at the 50% probability level and H-atoms omitted for clarity. Selected bond
lengths (Å) and angles (°): Mo1−F1 = 1.898(4), Mo1−F2 = 2.210(4), Mo1−F2i = 2.105(4), Mo1−O1=
2.028(4), Mo1−O2 = 1.696(5), Mo1−O3 =1.685(5), Mo1− F2−Mo1i = 111.49(16), F1−Mo1−F2 =
83.99(15), F1−Mo1−F2i = 83.36(15), F2−Mo1−F2i = 68.51(16), O1−Mo1−F2 = 81.55(16),
16

O1−Mo1−F2i = 79.53(16), O2−Mo1−F1 = 98.4(2), O2−Mo1−F2 = 88.55(19), O2−Mo1−O1 =
93.9(2), O3−Mo1−F1 = 98.2(2), O3−Mo1−F2 = 98.4(2), O3−Mo1−O1 = 93.0(2), O3−Mo1−O2
=104.2(2).
Scheme 1
17

Table 1 Selected spectroscopic data
complex
δ(19F{1H}) a
δ(95Mo) a
(1JMoF /Hz)
−287.3 (q)
ν (Mo=O) cm-1 b
ν (MoF) cm-1 b
(1JMoF /Hz)
[MoOF4(MeCN)]
[MoOF4(thf)]
+146.5 (s, 64)
1019
1016
990
652
+143.1 (s, 67)c −271.6 (q)c
+139.1 (s, 65) −270.0 (q)
+135.9 (s, 62) −257.0 (q)
+ 139.1 (s, 65) −251.5 (q)
+138.5 (s, 67) ─266.3 (q)
675
[MoOF4(Ph3PO)]
[MoOF4(Me3PO)]
[MoOF4(dmso)]
[MoOF4(dmf)]
[MoOF4(bipy)]
640
990
639
1007
1007
959
653
643
Insol.
Insol.
622, 595, 532, 476
[MoO2F2(MeCN)2]
[MoO2F2(Ph3PO)2]
−33.7 (s)
−51.8 (s).
−162.8 (t, 115) 959, 930
567
−114 (t, 120)c
−95 (t, 115)c
−92.7 (t, 112)
−58.1 (t, 109)
949, 916
955, 906 d
928, 897
947, 918e
587
[MoO2F2(Me3PO)2] −57.2 (s)c
566 d
[MoO2F2(dmso)2]
[MoO2F2(bipy)]
−50.8 (s)
−52.8 (s)
583, 554
585, 554e
a. in CD3CN, 293 K. b. Nujol mull. c. CD2Cl2 solution, 295 K. d. data from Ref. 14. e. data from
Refs. 13 and 17.
18

Table 2 X-ray crystallographic data a
Compound
[MoO2F2(Me3PO)(H2O)]
[Mo2O4F4(Me3PO)2] [MoO2F2(Ph3PO)2]
Formula
C3H11F2MoO4P
276.03
monoclinic
P21/c (14)
7.3572(2)
7.1853(3)
16.8798(8)
90
C6H18F4Mo2O6P2
516.02
monoclinic
P21/n (14)
8.3279(5)
10.4904(5)
9.1604(5)
90
C36H30F2MoO4P2
722.48
orthorhombic
Fdd2 (43)
18.4570(10)
33.049(3)
9.9908(10)
90
M
Crystal system
Space group (no.)
a /Å
b /Å
c /Å
α /
91.036(3)
90
96.681(5)
90
90
 /
90
 /
U /Å3
892.18(6)
4
794.84(8)
2
6094.3(9)
8
Z
(Mo-Kα) /mm–1
F(000)
1.652
1.837
0.589
544
504
2944
Total number reflns
Rint
7463
5870
6531
0.023
0.049
0.071
Unique reflns
1748
1564
2892
No. of params,
restraints
111, 0
94, 0
204, 1
b
R1, wR2 [I > 2σ(I)]
0.024, 0.054
0.032, 0.058
0.043, 0.135
0.046, 0.136
0.055, 0.109
0.082, 0.136
R1, wR2 (all data)
a Common items: T = 293 K; wavelength (Mo-Kα) = 0.71073 Å; θ(max) = 27.5°; b R1 = Σ||Fo|-|Fc||/Σ|Fo|;
wR2=[Σw(Fo2-Fc2)2/ΣwFo4]1/2
19

Table 2 cont.
Compound
[MoO2F2(MeCN)2]·MeCN [MoOF4(Ph3PO)2]
[Me3POH]2[Mo2O4F4(-F)2]
C3H10F3MoO3P'
278.02
Formula
M
C6H9F2MoN3O2
289.10
C18H15F4MoO2P
466.21
monoclinic
P21/c (14)
10.2899(3)
8.4748(2)
20.5446(6)
90
Crystal system
orthorhombic
triclinic
Space group (no.) Pnma (62)
P-1 (2)
a /Å
9.1511(10)
12.5300(10)
9.0718(10)
90
7.3172(6)
7.3172(6)
8.2791(7)
77.370(7)
67.015(8)
86.101(7)
427.45(7)
2
b /Å
c /Å
α /
90
94.754(3)
90
 /
90
 /
U /Å3
1040.20(18)
4
1785.42(9)
4
Z
(Mo-Kα) /mm–1
F(000)
1.270
568
0.873
1.732
928
272
Total no. reflns
Rint
8057
14889
6271
0.026
1072
0.031
0.024
Unique reflns
3517
1675
No. of params,
restraints
81, 2
235, 0
104, 0
R1, wR2 [I >
2σ(I)]
0.0350, 0.0966
0.027, 0.061
0.034, 0.064
0.024, 0.066
0.025, 0.066
b
R1, wR2 (all data) 0.0367, 0.0978
20