Oxido- versus imido-transfer reactions in oxido-imido molybdenum(VI) complexes: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.poly.2015.02.015

The reaction of the oxido-imido molybdenum(VI) compounds [Mo(O)(Nmes)(S₂CNR₂)₂] (mes = 2,4,6-C₆H₂Me₃, R₂ = ⁱPr₂, 1a; C₄H₄, 1b) with an excess of PMe₃ was investigated. [Mo(Nmes)(S₂CNR₂)₂(PMe₃)] (R₂ = ⁱPr₂, 2a; C₄H₄, 2b) complexes and the corresponding phosphane oxide OPMe₃ were exclusively detected as reaction products, according to an oxygen atom transfer (OAT) reaction. No evidence of the possible imido transfer reaction was observed. In order to explain the selective OAT reaction in this system, DFT calculations were carried out with the model compound [Mo(O)(N-2,6-Me₂C₆H₃)(S₂CNMe₂)₂], 1c, and PMe₃ as reactant. The two possible oxido transfer and imido transfer pathways were considered and the nucleophilic attack of the phosphane to the multiple bonded atom was the associative intermolecular processes modelled. The oxido transfer is thermodynamic and kinetically favoured with respect the imido one in agreement with the experimental results.

Full text of DOI:10.1016/j.poly.2015.02.015

Accepted Manuscript
Oxido- versus imido- transfer reactions in oxido-imido molybdenum(VI) com-
plexes: a combined experimental and theoretical study
Antonio Pastor, Francisco Montilla, Agustín Galindo
PII:
S0277-5387(15)00098-4
DOI:
http://dx.doi.org/10.1016/j.poly.2015.02.015
POLY 11201
Reference:
Polyhedron
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Accepted Date:
23 December 2014
12 February 2015
Please cite this article as: A. Pastor, F. Montilla, A. Galindo, Oxido- versus imido- transfer reactions in oxido-imido
molybdenum(VI) complexes: a combined experimental and theoretical study, Polyhedron (2015), doi: http://
dx.doi.org/10.1016/j.poly.2015.02.015
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REVISED
Oxido- versus imido- transfer reactions in oxido-imido molybdenum(VI)
complexes: a combined experimental and theoretical study
*
Antonio Pastor, Francisco Montilla, and Agustín Galindo
Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo
1203, 41071 Sevilla, Spain. E-mail: galindo@us.es
Keywords: molybdenum(VI), oxido, imido, oxigen atom transfer, imido-transfer, DFT
____________________
* Corresponding author.
E-mail address: galindo@us.es (A. Galindo).

ABSTRACT
The reaction of the oxido-imido molybdenum(VI) compounds [Mo(O)(Nmes)(S2CNR2)2] (mes =
i
2,4,6-C6H2Me3, R2 = Pr2, 1a; C4H4, 1b) with an excess of PMe3 was investigated.
i
[Mo(Nmes)(S2CNR2)2(PMe3)] (R2 = Pr2, 2a; C4H4, 2b) complexes and the corresponding
phosphane oxide OPMe3 were exclusively detected as reaction products, according to an oxygen
atom transfer (OAT) reaction. No evidence of the possible imido transfer reaction was observed.
In order to explain the selective OAT reaction in this system, DFT calculations were carried out
with the model compound [Mo(O)(N-2,6-Me2C6H3)(S2CNMe2)2], 1c, and PMe3 as reactant. The
two possible oxido transfer and imido transfer pathways were considered and the nucleophilic
attack of the phosphane to the multiple bonded atom was the associative intermolecular processes
modelled. The oxido transfer is thermodynamic and kinetically favoured with respect the imido
one in agreement with the experimental results.
1. Introduction
A large number of compounds containing the oxido or the imido ligands are now known
for the majority of the transition metals.1 Both groups permit stabilization of high formal
oxidation states and, in particular, for molybdenum the area of dioxido2 and bis(imido)3
complexes are very extensive. This situation contrasts with the relatively low number of mixed
oxido-alkylimido or oxido-arylimido derivatives of this metal4,5,6,7,8 and the relatively scarce data
about their reactivity.4c,k,5b,7b,8 Molybdenum catalysed oxygen atom transfer (OAT) reactions are
of extraordinary interest in many processes at several levels: academic, industrial and biological.9
For this reason, several research groups designed model molybdenum complexes and studied
their OAT chemistry.10,11,12 In marked contrast, the related imido transfer reaction is much less
2

developed and the examples of molybdenum mediated imido transfer reactions are not
abundant.13,14
A common model OAT reaction is the oxido-transfer to phosphanes and our research
group investigated the oxido-transfer, and also the imido-transfer reaction, to PMe3 by using
dioxido and bis(imido) molybdenum(VI) compounds, respectively.15,16 The ligand exchange
reaction of dioxido and bis(imido) derivatives gave a straightforward synthetic route for the
preparation of mixed oxido-imido complexes.5b,17 This fact allowed the possibility of
investigating the transfer of one or both groups to phosphane reagents. In fact, the reaction of
phosphanes with oxido-p-tolylimido and oxido-N-tosylimido molybdenum(VI) complexes was
early studied by Maatta18 and Holm,19 respectively. More recently, other research groups became
interested in this reaction and, for example, oxido-tert-butylimido compounds with an
unsymmetrical Schiff base ligand were analysed by Heinze8 and oxido-tert-butylimido
compounds with β-ketiminato or η2-pyrazolato ligands were studied by Mösch-Zanetti.7 Herein,
as a continuation of our own work in the area of imido-molybdenum compounds,20,21 we describe
a combined experimental and theoretical study of the reactivity of oxido-imido complexes of
molybdenum(VI) versus PMe3 substrate.
2. Experimental section
2.1. General
All preparations and other operations were carried out under dry oxygen-free nitrogen
atmosphere following conventional Schlenk techniques. Solvents were dried and degassed before
1
13
use. Infrared spectra were recorded on Perkin-Elmer Model 883 spectrophotometer. H, C and
31
31P NMR spectra were run on Bruker AMX-300 and Bruker AMX-500 spectrometers. P shifts
were measured with respect to external 85 % H3PO4. 13C NMR spectra were referenced using the
13C resonance of the solvent as an internal standard but are reported with respect to SiMe4.
Compounds [Mo(Nmes)(O)(S2CNR2)2], 1a,b, were prepared according to the literature
procedure.21
3

2.2.
Reaction
of
[Mo(Nmes)(O)(S2CNiPr2)2]
with
PMe3:
Formation
of
[Mo(Nmes)(S2CNiPr2)2(PMe3)] (2a)
To a solution of [Mo(Nmes)(O)(S2CNiPr2)2], 1a, (0.06 g, 0.1 mmol) in THF (10 ml) was
added an excess of PMe3 (0.5 ml, 1 M solution in toluene). The colour of the mixture changed
from red to violet. Within 10 minutes, the solution turned brown and the reaction was stirred
overnight. Then, the volatiles were removed in vacuum. The resulting residue was a mixture of
OPMe3 and compound [Mo(Nmes)(S2CNiPr2)2(PMe3)], 2a, which was characterized by NMR.
Yield: quantitative (NMR), 86 % (isolated). Data for 2a: 1H NMR (500 MHz, C6D6): δ 6.58 (s, 2,
CH, Ph), 2.49 (s, 6, o-CH3), 1.97 (s, 3, p-CH3), 1.42 (d, 7.5 Hz, 9, PMe3), 1.2 (very br, 12, CH3).
The CH signals of iPr groups were not discerned due to a dynamic process similar to that reported
13
for complex 1a.21 31P{1H} NMR (C6D6): δ 6.84 (s). C{1H} NMR (125 MHz, C6D6): δ 220.4,
199.4 (s, S2C), 154.9 (s, ipso-C), 134.9, 132.2 (s, p- and o-C), 128.5 (s, m-C), 50.9 (br, CH), 49.9
(br, CH), 21.0 (s, p-CH3), 19.8 (s, o-CH3), 19.5 (br, CH3), 16.5 (d, JCP = 13 Hz, PMe3).
2.3.
Reaction
of
[Mo(Nmes)(O)(S2CNC4H4)2]
with
PMe3:
Formation
of
[Mo(Nmes)(S2CNC4H4)2(PMe3)] (2b)
The reaction of complex [Mo(Nmes)(O)(S2CNC4H4)2], 1b, with an excess of PMe3 was
carried out following a similar procedure. The resulting residue was again a mixture of OPMe3
and compound [Mo(Nmes)(S2CNC4H4)2(PMe3)], 2b. Yield: quantitative (NMR), 81 % (isolated).
Compound 2b was identified by comparison of its NMR data with those previously reported.20
1H-NMR (300 MHz, C6D5CD3): δ 7.63, 7.48 (pseudo t, JHH = 2.3 Hz, 2H, NC4H4), 6.34 (s, 2H,
CH, Ph), 6.04, 5.93 (pseudo t, JHH = 2.3 Hz, 2H, NC4H4), 1.95 (s, 6H, o-CH3), 1.86 (s, 3H, p-
31
13
CH3), 1.07 (d, JHP = 9.2 Hz, 9H, PMe3). P{1H}-NMR (C6D5CD3): δ -8.10 (s). C{1H}-NMR
(75 MHz, C6D5CD3): δ 220.4, 206.7 (s, S2C), 153.3 (s, ipso-C), 136.6, 135.4 (s, p-C and o-C),
128.5 (s, m-C), 121.1, 118.4, 114.3, 109.1 (s, NC4H4), 20.8 (s, p-CH3), 19.3 (s, o-CH3), 14.8 (d,
JCP = 28 Hz, PMe3).
4

2.4. Computational details
The electronic structure and geometries of the model compounds were computed using
density functional theory at the B3LYP level.22 The Mo atom was described with the LANL2DZ
basis set23 while the 6-31G(d,p) basis set was used for all the remaining atoms. The optimized
geometries of all the compounds were characterized as energy minima either by a nonexistence of
imaginary frequencies (NImag = 0) or by very low vibrational frequencies (< 20 cm-1) in the
diagonalisation of the analytically computed Hessian (vibrational frequencies calculations). These
very low vibrational frequencies that do not lead to energy minimum optimisation have been
reported in other cases, and it seems to be associated to numerical errors in the DFT integration
grid. They could be eliminated by much more expensive calculation with a better grid.24
Transition states were located using the quadratic synchronous transit (QST2) approach25 and
frequency calculations were performed in order to check the stationary states (NImag = 1). The
DFT calculations were performed using the Gaussian 03 suite of programs.26 Cartesian
coordinates of all optimized compounds are collected in the Supporting Information.
3. Results and discussion
3.1. Reaction of oxido-imido molybdenum(VI) complexes with PMe3
The dithiocarbamate derivatives [Mo(O)(Nmes)(S2CNR2)2] (R2 = iPr2, 1a; C4H4,1b) were
selected to investigate the possible competitive transfer of oxido and imido groups to tertiary
phosphane PMe3. Addition of an excess of PMe3 to solutions of these complexes leads to
formation of the compounds [Mo(Nmes)(S2CNR2)2(PMe3)] (R2 = iPr2, 2a; C4H4, 2b) (Scheme 1).
Both reactions were monitored by 31P{1H} NMR. The spectra showed signals due to the
coordinated PMe3 (δ 6.8, 2a; -8.3, 2b), one resonance due to the O=PMe3 (around 30.8 ppm) and
no evidence of the possible imido transfer reaction was observed. The structures shown in the
Scheme 1 are easily deduced from their NMR data that reveal the presence of two non-equivalent
dithiocarbamate ligands since, for instance, two singlets for the two S2C groups are observed in
5

13
the C{1H} NMR (at 220.4 and 199.4 ppm for 2a). The formulation of the product 2b was also
confirmed by comparison of its NMR data with that of the same sample, reported previously by
us, and obtained by treatment of [Mo(Nmes)Cl2(PMe3)3]with two equivalents of KS2CNC4H4.20
Conversely to that observed by Holm in a related system,19 in these reactions there was not signal
of imido transfer and formation of mesN=PMe3. Transfer of the oxido ligand to phosphane and
reduction from molybdenum(VI) to molybdenum(IV) with coordination of PMe3 takes place.
This behaviour is similar to that observed by Heinze8 and Mösch-Zanetti7a,c in related oxido-
imido molybdenum(VI) complexes.
Scheme 1
3.2 Theoretical study of the reaction of model complex [Mo(O)(N-2,6-Me2C6H3)(S2CNMe2)2]
with PMe3: oxido versus imido transfer
After our experimental studies about the oxido-transfer reaction to PMe3, we decided to
investigate, using DFT at the B3LYP level,27 the reaction of model complex [Mo(O)(N-2,6-
Me2C6H3)(S2CNMe2)2], 1c, with PMe3, in order to give justification to our experimental
observations. Although the nature of the N-substituents of dithiocarbamate ligand has a subtle
effect on the OAT,28 the modellization of dithiocarbamate with methyl substituents is adequate.
As a proof of that, the calculated thermodynamic of the reaction of [Mo(O)2(S2CNMe2)2]
complex with PMe3 (∆H = -31.7 kcal·mol-1) compares well with the measured thermochemical
data for [Mo(O)2(S2CNEt2)2] + PPh3 (∆H = -29.0 2.5 kcal·mol-1).9e The atom transfer reactions
6

are associative intermolecular processes in which the nucleophilic attack proceeds by the
approaching of the phosphane (or the substrate, in general) to the atom multiple bonded to the
metal. For instance, previous theoretical studies, reported for the oxido transfer reaction in the
system [Mo(O)2(NH3)2(SH)2] + PMe3, showed that the first step is the nucleophilic attack of
PMe3 on a π* Mo=O orbital perpendicular to the [Mo(O)2] unit and at a Mo-O-P angle of ca.
130º.29 This affords a transition state with a weakened Mo-O bond and the phosphane bounded to
one oxygen atom. Intermediates of this process, complexes [TpiPrMo(O)(OPh)(OPEt3)],30
[Tp*Mo(O)Cl(OPMe2R)] (R = Me, Ph)31 and [Tp*Mo(O)(SPh)(OPMe3)],32 were isolated and
X-ray characterized showing Mo-O-P angles within the 130-144º range. The oxido transfer
reaction in our system was modelled similarly and the reaction profile, as relative Gibbs free
energy, the calculated structures of the transition state and intermediates are shown in Fig. 1. The
energy reference was taken as the energy of the separate reagents ([Mo(O)(N-2,6-
Me2C6H3)(S2CNMe2)2] + PMe3). The optimised key intermediates and transition state structures
appear in Fig. 2.
Fig. 1 Reaction profile for the oxido transfer reaction to PMe3 of [Mo(O)(N-2,6-
Me2C6H3)(S2CNMe2)2] complex and the following ligand substitution (relative Gibbs free energy
in kcal·mol-1).
7

The first step is the nucleophilic attack of PMe3 to a molecular orbital π* Mo=O, roughly
perpendicular to the moiety [Mo(O)(N-2,6-Me2C6H3)]. The resulting transition state TS(1c-3c)
shows an elongated Mo=O bond (1.863 versus 1.718 Å in 1c) and the phosphane approaching to
this oxygen atom (1.929 Å) (see Fig. S1 in supplementary data). Similar structural parameters
were found in the calculations of the transition state for the [TpiPrMo(O)2(OPh)] + PMe3 system.33
The asymmetry of the O-P-C angles (155.6, 95.2 and 94.2º) is evident in TS(1c-3c) in agreement
with other related calculations.34 The barrier for the nucleophilic attack of PMe3 is 37.0
kcal·mol-1. This value is somewhat higher than the ∆G#
experimentally observed for the
298
formation of [TpiPrMo(O)(OPh)(OPR3)]35 or [Tp*Mo(O)Cl(OPR3)]36 intermediates in
acetonitrile. The transition state TS(1c-3c) goes forward to the intermediate 3c in which the Mo-
O bond reaches 2.278 Å and the phosphane is definitively bonded to the oxygen atom (1.525 Å).
In the intermediate 3c the OPMe3 ligand is now produced, as occurred in complex
[Tp*Mo(O)Cl(OPMe3)],31 with similar Mo-OPMe3 and O-P distances. In particular, the Mo-O-P
angle of 128.7º is analogous to the complex [TpiPrMo(O)(OPh)(OPEt3)] (experimental,30b 130.9º;
calculated,37 128.5º). The formation of the final product is produced by substitution of the OPMe3
ligand by PMe3 in a stepwise process: dissociation of OPMe3 ligand with formation of the
intermediate [Mo(N-2,6-Me2C6H3)(S2CNMe2)2], 4c and, then, PMe3 coordination affording the
product [Mo(N-2,6-Me2C6H3)(S2CNMe2)2(PMe3)], 2c, (Mo-P bond distance of 2.494 Å). Both,
4c and 2c, are Mo(IV) imido compounds in which the Mo-N bond distance is reduced (1.733 and
1.760 Å for 4c and 2c, respectively) in comparison with that found in 1c for the same distance
(1.780 Å). This is in agreement with the bond order increase produced by the involvement in 4c
and 2c of an additional metal d-π orbital initially shared in 1c with the oxido ligand. The overall
oxido transfer reaction is exergonic (-11.4 kcal·mol-1).
8

Fig. 2 Structures of the intermediates and the transition state in the oxido transfer reaction to
PMe3 of [Mo(O)(N-2,6-Me2C6H3)(S2CNMe2)2] complex.
Concerning the imido transfer pathway, the reaction profile and the calculated structures
of the transition state and intermediates are shown in Fig. 3 and 4, respectively. The relative
Gibbs free energy reference was taken as the energy of the separate reagents. Again the first step
is the nucleophilic attack of PMe3 to a molecular orbital π* Mo=N-2,6-Me2C6H3. The transition
state TS(1c-5c) shows an elongated Mo=N bond (1.906 versus 1.780 Å in 1c) and the phosphane
approaching to the nitrogen atom (2.244 Å) (see Fig. S2 in supplementary data). In this case, the
nucleophilic attack of PMe3 shows a barrier of 39.4 kcal·mol-1. From this transition state is
formed the intermediate 5c that displayed the Me3P=N-2,6-Me2C6H3 ligand N-coordinated to the
metal (2.229 Å). The dissociation of this ligand is exergonic affording the intermediate
[Mo(O)(S2CNMe2)2], 6c. The geometrical parameters of this intermediate are comparable to
those found in structurally known [Mo(O)(S2CNR2)2] complexes.38 Finally, the last part of the
profile is the PMe3 coordination that yields the product [Mo(O)(S2CNMe2)2(PMe3)], 7c, with a
9

Mo-P bond distance of 2.515 Å. The PMe3 association, from 6c to 7c, is an endergonic step by
10.2 kcal·mol-1 (7.6 kcal·mol-1 for the related 4c to 2c transformation) and this theoretical result
actually corroborates a related previous experimental observation. In fact, complex
[Mo(O)(S2CNEt2)2(PMe3)], reported by some of us,39 readily loses PMe3 in solution to give
[Mo(O)(S2CNEt2)2] and the synthesis of the phosphane adduct requires the presence of an excess
of PMe3. The Mo(IV) oxido compounds, 6c and 7c, show Mo-O bond distances (1.680 and 1.696
Å for 6c and 7c, respectively) that are lower than that found in 1c for the same distance (1.718 Å).
As stated before for the imido-molybdenum(IV) complexes, this fact agrees with the bond order
increase produced by the involvement in 6c and 7c of an additional metal d-π orbital, initially
shared in 1c with the imido ligand. The overall imido transfer reaction is slightly endergonic (1.2
kcal·mol-1).
Fig. 3 Reaction profile for the imido transfer reaction to PMe3 of [Mo(O)(N-2,6-
Me2C6H3)(S2CNMe2)2] complex and the following ligand substitution (relative Gibbs free energy
in kcal·mol-1).
10

The comparison of the profiles shown in Figs. 1 and 3 clearly demonstrates that the oxido
transfer is kinetically (lower energy barrier for the transition state TS(1c-3c) with respect to
TS(1c-5c)) and thermodynamically favoured (overall exergonic reaction for the formation of 2c, -
11.4 kcal·mol-1, versus overall endergonic reaction for the formation of 7c, 1.2 kcal·mol-1) with
respect the imido transfer, in agreement with the experimental results. Another point of interest is
the specific contrast of the oxido and the imido transfer steps, namely the transformation of 1c
into 3c or 5c, respectively. For the oxido transfer the process for the formation of 3c is exergonic,
-3.5 kcal·mol-1, whereas the imido transfer step that affords 5c is significantly endergonic, 12.1
kcal·mol-1. This may suggest an insufficient lifetime of the intermediate 5c, which may prevent
further chemical evolution to the final product 7c. In fact, the reverse reaction, from 5c to 1c,
would be exergonic by -12.1 kcal·mol-1 with a lower energy barrier of 27.3 kcal·mol-1.
Fig. 4 Structures of the intermediates and the transition state in the imido transfer reaction to
PMe3 of [Mo(O)(N-2,6-Me2C6H3)(S2CNMe2)2] complex.
11

In order to elucidate if steric hindrance has importance in the imido transfer step, the
profile shown in Fig. 3 was calculated again for the [Mo(O)(NPh)(S2CNMe2)2] model complex,
1d, with the phenyl imido substituent. For the imido transfer process, the results established a
diminution of the energy barrier for the new transition state TS(1d-3d) of 1.4 kcal·mol-1 (see Fig.
S3 in the supplementary data and Table S2), which confirmed the weight of steric features in the
transfer reaction. Obviously, the most important difference between the isoelectronic oxido and
imido functionalities is the diverse steric requirements. The phosphane entering group could have
steric conflicts with the aryl substituent of the imido ligand and, consequently, the imido transfer
reaction would be more impeded than the analogous oxido transfer from a steric point of view.
4. Conclusions
In summary, the reaction of [Mo(O)(Nmes)(S2CNR2)2] complexes with PMe3 occurs with
a selective oxygen atom transfer reaction with formation of [Mo(Nmes)(S2CNR2)2(PMe3)]
compounds and the corresponding phospane oxide OPMe3. The oxido transfer is thermodynamic
and kinetically favoured with respect the imido transfer according to DFT calculations carried out
with the model compound [Mo(O)(N-2,6-Me2C6H3)(S2CNMe2)2] and PMe3 as reactant. In this
system in which both functionalities are simultaneously coordinated to the molybdenum center,
the nucleophilic attack of PMe3 to the multiple bonded atom is favoured for the oxido group with
respect the imido ligand. The significance of the crowding of the metal surroundings in such a
process is demonstrated by the calculations carried out with the model compound
[Mo(O)(NPh)(S2CNMe2)2] in which the imido ligand is less encumbered.
Acknowledgements
Financial support (FEDER contribution) from the Spanish Ministerio de Ciencia e
Innovación (Project CTQ2010-15515) and the Junta de Andalucía (Proyecto de Excelencia FQM-
12

7079) is gratefully acknowledged. We thank to the Centro de Servicios de Informática y Redes de
Comunicaciones (CSIRC), Universidad de Granada, for providing the computing time.
Appendix A. Supplementary data
Major component of the imaginary frequencies found in the transition states, coordinates and
structures of the optimised compounds. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/...
13

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17

Oxido- versus imido- transfer reactions in oxido-imido molybdenum(VI)
complexes: a combined experimental and theoretical study
*
Antonio Pastor, Francisco Montilla, and Agustín Galindo
Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo
1203, 41071 Sevilla, Spain. E-mail: galindo@us.es
GRAPHICAL ABSTRACT
Selective oxido transfer reaction to PMe3 from oxido-imido molybdenum(VI) compounds
[Mo(O)(N-2,4,6-C6H2Me3)(S2CNR2)2] was observed without evidence of imido transfer.
The oxido transfer was calculated to be thermodynamic and kinetically favoured with
respect the imido one in agreement with the experimental results.
18