Electronic effects in oxo transfer reactions catalysed by salan molybdenum(VI) cis-dioxo complexes

Article abstract of DOI:10.1039/b820754b

A series of molybdenum(vi) cis-dioxo complexes containing tetradentate salan ligands with different para-substitutions on the phenoxy group have been prepared. These complexes catalyse the oxygen atom transfer reaction between dimethylsulfoxide and triphe

Full text of DOI:10.1039/b820754b

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Electronic effects in oxo transfer reactions catalysed by salan molybdenum(VI)
cis-dioxo complexes†
Christopher J. Whiteoak, George J. P. Britovsek,* Vernon C. Gibson* and Andrew J. P. White
Received 21st November 2008, Accepted 9th January 2009
First published as an Advance Article on the web 17th February 2009
DOI: 10.1039/b820754b
A series of molybdenum(VI) cis-dioxo complexes containing tetradentate salan ligands with different
para-substitutions on the phenoxy group have been prepared. These complexes catalyse the oxygen
atom transfer reaction between dimethylsulfoxide and triphenylphosphine. During oxo transfer
catalysis, the complexes are resistant to formation of catalytically inactive oxo-bridged dimeric Mo(V)
complexes. Electronic effects influence the rate of the oxo transfer reaction and the fastest rates are
achieved when the para-phenoxy substituent is an electron withdrawing nitro substituent. Hammett
correlations have shown that the rate-determining step involves nucleophillic attack of the phosphine
on one of the oxo ligands. Electrochemical measurements have shown that all complexes containing
tertiary amine ligands exhibit quasi-reversible behaviour and that the para-substituent has a
considerable effect on the half potentials (E1/2). A linear correlation between the E1/2 values and the
Hammett sp parameter is observed.
dioxo complex with the salan ligand (N,N¢-dimethyl-N,N¢-bis[(2-
hydroxyphenyl)methylene]-1,2-diaminoethane) [WO2(LH)] cataly-
ses oxo-transfer reactions in the conversion of benzoin to benzil
with dimethylsulfoxide (DMSO), which is a relatively reactive oxo
transfer agent.26 Lehtonen and Sillanpa¨a¨ have shown that the
two molybdenum dioxo complexes with ortho/para substituted
salan ligands [MoO2(LMe,Me)] and [MoO2(LtBu,tBu)] catalyse the
oxo transfer reaction between DMSO and triphenylphosphine.25
We were intrigued by these results and decided to investigate
whether the reactivity of these molybdenum catalysts can be
tuned such that, in future, other substrates may be oxidised with
environmentally benign oxidants such as H2O2 or O2.
We report here the synthesis and characterisation of an ex-
tensive series of molybdenum(VI) cis-dioxo complexes containing
tetradentate salan ligands [MoO2(H2LH)] and [MoO2(LX)] and a
systematic study into the effect of the para-phenoxy substituent on
the catalytic oxo transfer properties of these complexes (see Fig. 1).
All complexes have been fully characterised and their catalytic
Introduction
Enzymes containing molybdenum-based active sites for the oxida-
tion of substrates via oxygen atom transfer reactions have been of
interest for some time.1–3 Three families of molybdenum enzymes
have been identified: Sulfite Oxidase (SO), Dimethylsulfoxide
Reductase (DMSOR) and Xanthine Oxidase (XO) and structural
analyses of these enzymes have shown that all contain sulfur
donors in the molybdenum coordination sphere. Understandably,
most studies on model systems for these metallo-enzymes have
used Mo and W complexes containing sulfur based ligands, in
particular dithiolene ligands.3–5 In addition, a number of non-
dithiolene ligands have been investigated, including pyridine
dithiolates,6,7 tris(pyrazolyl)borate ligands,8–13 b-ketiminates14 and
Schiff base ligands of the salen and salan type. The latter
class of ligands were first investigated by Topich and Lyon,
who prepared molybdenum complexes with tridentate Schiff
base ligands and studied the oxo transfer properties of these
complexes with phosphines.15–18 Tetradentate salen and salan
ligands and related ONNO and SNNS type ligands were employed
by Enemark and co-workers19–21 and the use of bulky substituents
was investigated in order to prevent the formation of dimeric
oxo-bridged Mo(V) complexes, which are generally considered
the reason for catalyst deactivation in oxo transfer reactions.22
More recently, Ng23,24 and Lehtonen25 have reported on the use
of tetradentate salan ligands with amine and phenoxy donors.
Specifically, Ng and co-workers24 have shown that a tungsten
Department of Chemistry Imperial College London, Exhibition Road, South
Kensington, London, UK SW7 2AY. E-mail: g.britovsek@imperial.ac.uk;
Fax: +44 (0)20 75945804; Tel: +44(0)20 75945863
† Electronic supplementary information (ESI) available: further experi-
mental data, cyclic voltammetry studies and Fig. S1. CCDC reference
number 689173. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b820754b
Fig. 1 Molybdenum dioxo salan complexes.
Dalton Trans., 2009, 2337–2344 | 2337
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3
4
potential in oxo transfer reactions between DMSO and PPh3
has been investigated. A detailed Hammett analysis has been
carried out in order to investigate the correlation between the
para-substituent and the reduction potential of these catalysts and
with their catalytic activity.
2H, ArOH), 6.9 (dd, 2H, JHH = 8.1, JHH = 1.8 Hz, ArH), 6.78
(d, 2H, 4JHH = 1.8, ArH), 6.76 (d, 2H, 3JHH = 8.1, ArH), 3.68 (s,
4H, NCH2Ar), 2.68 (s, 4H, N(CH2CH2)N), 2.30 (s, 6H, NCH3),
2.26 (s, 2H, ArCH3). 13C NMR (400 MHz, CDCl3, 298 K): 155.37,
129.30, 129.11, 128.20, 121.32, 115.93 (ArC), 61.84, 54.18 (CH2),
41.77, 20.49 (CH3). MS (ESI): m/z = 329, [M + H]+. Elemental
analysis for C20H28N2O2 (F.W. 328.5): C, 73.14; H, 8.59; N, 8.53%.
Found C, 73.03; H, 8.65; N, 8.48%.
Experimental
All syntheses of complexes were carried out using standard
vacuum line, Schlenk, or cannular techniques or in a conventional
nitrogen-filled glovebox. NMR spectra were collected on a Bruker
DRX-400 spectrometer. Chemical shifts for 1H and 13C NMR are
referenced to the residual protio impurity and to the 13C NMR
signal of the deuterated solvent. 19F and 31P chemical shifts are
reported relative to CFCl3 and H3PO4 (85%), respectively. Mass
spectra were recorded on a VG Autospec spectrometer. Elemental
analysis was performed by the Science Technical Support Unit
at London Metropolitan University. Electrochemical studies were
performed on an AD Instruments MacLab/2e utilising EChem
software equipped with an Ag/AgCl reference, Pt wire counter
and Pt stick working electrodes.
N,N¢-dimethyl-N,N¢-bis[(5-bromo-2-hydroxyphenyl)methylene]-
1,2-diaminoethane (H2LBr)
To a stirred mixture of N,N¢-dimethylethylenediamine (1.00 g,
11.3 mmol) and 4-bromophenol (3.93 g, 22.7 mmol) in 50 mL
of refluxing methanol was added aqueous formaldehyde (3.68 g,
45.4 mmol). The mixture was refluxed overnight and cooled
to room temperature and the white precipitate was filtered off,
washed with two portions of cold methanol (2 ¥ 30 mL) and dried
1
under vacuum to yield a white powder (3.28 g, 63%). H NMR
(400 MHz, CDCl3, 298K): 10.57 (br s, 2H, ArOH), 7.29 (dd, 2H,
4
4
3JHH = 8.6, JHH = 2.4, ArH), 7.09 (d, 2H, JHH = 2.4, ArH),
3
6.74 (d, 2H, JHH = 8.6, ArH), 3.68 (s, 4H, NCH2Ar), 2.66 (s,
4H, N(CH2CH2)N), 2.30 (s, 6H, NCH3). 13C NMR (400 MHz,
CDCl3, 298K): 156.96, 131.74, 131.08, 123.52, 118.11, 110.89
(ArC), 61.27, 53.89 (CH2), 41.65 (CH3). MS (ESI): m/z = 459,
[M + H]+. Elemental analysis for C18H22Br2N2O2 (F.W. 458.2): C,
47.18; H, 4.84; N, 6.11%. Found C, 47.26; H, 4.75; N, 6.02%.
Solvents and reagents
Acetonitrile, dichloromethane and DMSO were dried by pro-
longed reflux over calcium hydride under a nitrogen atmosphere,
being freshly distilled prior to use. N,N-dimethylformamide
(DMF) and methanol was purchased dry from Sigma-Aldrich
and stored over molecular sieves. All other reagents are com-
mercially available and were used without further purification.
[MoO2(acac)2] was prepared according to a published procedure.27
The ligands H4LH, H2LH and H2LNO2 were prepared according
to previously reported methods.24,28 The synthesis for the ligands
H2LCl, H2LI, H2LOMe and H2LF are analogous to the synthesis of
the bromo derivative given below and are provided in the ESI†. The
synthesis for the complexes [MoO2LMe], [MoO2LBr], [MoO2LI],
[MoO2LNO2], [MoO2LOMe], [MoO2LF] and [MoO2H2LH] have been
provided in the ESI†.
[MoO2LH]
This compound was prepared in a manner similar to that
described previously.25 To
a
solution of H2LH (300 mg,
1.0 mmol) in 20 mL acetonitrile was added bis(2,4-
pentanedionato)dioxomolybdenum(VI) (326 mg, 1.0 mmol). The
reaction mixture was stirred for 6 hours in which time a yellow
precipitate formed. The precipitate was filtered and washed with
methanol (20 mL) and acetonitrile (20 mL) yielding a yellow
powder (303 mg, 71%). 1H NMR (400 MHz, CDCl3, 298K): 7.25
(t, 2H, 3JHH = 7.6, ArH), 7.05 (d, 2H, 3JHH = 7.6, ArH), 6.95–6.80
2
(m, 4H, ArH), 5.10 (d, 2H, JHH = 14.3, ArCH2N), 3.60 (d, 2H,
N,N¢-dimethyl-N,N¢-bis[(5-methyl-2-hydroxyphenyl)methylene]-
2JHH = 14.3, ArCH2N), 3.08 (d, 2H, 2JHH = 9.5, (N(CH2CH2)N)),
1,2-diaminoethane (H2LMe
)
2.74 (s, 6H, NCH3), 2.15 (d, 2H, 2JHH = 9.5, (N(CH2CH2)N)). 13
C
To a stirred solution of 2-hydroxy-5-methyl-benzaldehyde (1.00 g,
7.3 mmol) in 20 mL of methanol was slowly added a solution
of ethylenediamine (220 mg, 3.7 mmol) in 20 mL of methanol.
Upon stirring a yellow precipitate formed and after 30 minutes of
stirring, sodium borohydride (556 mg, 14.7 mmol) was added in
small portions. When the mixture was colourless, it was poured
over 100 mL of water and extracted with dichloromethane to yield
a slightly off white powder (934 mg, 86%). The white powder
(900 mg, 3.0 mmol) was dissolved in 50 mL of tetrahydrofuran.
To this solution was added 10 mL of acetic acid and aqueous
formaldehyde (2.84 g, 35 mmol) and the mixture was stirred
for 1 hour. Sodium borohydride (574 mg, 15 mmol) was added
slowly and the reaction stirred overnight. All volatiles were
removed under vacuum and the residue was hydrolysed with
sodium hydroxide solution. The aqueous phase was extracted with
dichloromethane and the organic phase dried over sodium sulfate.
All volatiles were removed under vacuum yielding a white powder
(854 mg, 87%). 1H NMR (400 MHz, CDCl3, 298 K): 10.48 (br s,
NMR (400 MHz, CDCl3, 298K): 159.55, 129.62, 129.42, 122.09,
120.86, 118.70 (ArC), 64.84, 52.01 (CH2), 48.15 (CH3). MS (ESI):
m/z = 429, [M + H]+. Elemental analysis for C18H22N2O4Mo (F.W.
426.32): C, 50.71; H, 5.20; N, 6.57%. Found C, 50.82; H, 5.29; N,
6.63%.
[MoO2LCl]
This compound was prepared in an analogous manner to that
described above for [MoO2LH], except H2LCl (369 mg, 1.0 mmol)
dissolved in 20 mL of dichloromethane was used in place of H2LH
dissolved in acetonitrile to yield an orange powder (345 mg, 70%).
3
1H NMR (400 MHz, CDCl3, 298K): 7.19 (dd, 2H, JHH = 8.7,
4
4JHH = 2.5, ArH), 7.04 (d, 2H, JHH = 2.5, ArH), 6.80 (d, 2H,
2
3JHH = 8.7, ArH), 5.02 (d, 2H, JHH = 14.5, ArCH2N), 3.55 (d,
2
2
2H, JHH
=
14.5, ArCH2N), 3.03 (d, 2H, JHH
=
9.7,
2
(N(CH2CH2)N)), 2.71 (s, 6H, NCH3), 2.19 (d, 2H, JHH = 9.7,
(N(CH2CH2)N)). 13C NMR (400 MHz, CDCl3, 298K): 158.17,
2338 | Dalton Trans., 2009, 2337–2344
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129.34, 129.20, 125.41, 123.42, 120.05 (ArC), 64.22, 52.07 (CH2),
48.08 (CH3). MS (ESI): m/z = 497, [M + H]+. Elemental analysis
for C18H20Cl2N2O4Mo (F.W. 495.2): C, 43.66; H, 4.07; N, 5.66%.
Found C, 43.70; H, 4.13; N, 5.73%.
Procedure B. (used to determine the rate equation for
[MoO2LNO2]): Triphenylphosphine (2.5 mmol) was placed in an
ampoule and oxidised in 5 mL dimethylsulfoxide in the presence
of an amount of catalyst and aliquots removed from the bulk at
30, 60, 90, 120 and 150 minutes. These samples were mixed with
25% v/v of CDCl3 and analysed by 31P NMR spectroscopy.
X-Ray crystallography
Crystals of [MoO2LCl] suitable for X-ray diffraction were grown
from chloroform by slow evaporation of the solvent. Crystal data
for [MoO2LCl]: C18H20Cl2MoN2O4, M = 495.20, monoclinic, I2/a
Cyclic voltammetry
The complex [MoO2LX] (20 mg, ca. 0.05 mmol) was dissolved in
50 mL of a 0.2 M solution of NBu4PF6 in DMF. Part of this solu-
tion was transferred to an electrochemical cell. The data for cyclic
voltammograms (3 scans) at scan rates of 50, 100 and 200 mV s-1
were collected. Individual scans can be found in the ESI†.
˚
(no. 15), a = 14.45109(12), b = 7.45243(6), c = 18.62581(17) A,
◦
3
˚
b = 98.5896(8) , V = 1983.42(12) A , Z = 4 [C2 symmetry],
Dc = 1.658 g cm-3, m(Cu–Ka) = 8.122 mm-1, T = 173 K, yellow
plates, Oxford Diffraction Xcalibur PX Ultra diffractometer; 1894
independent measured reflections (Rint = 0.0214), F2 refinement,
R1(obs) = 0.021, wR2(all) = 0.060, 1757 independent observed
absorption-corrected reflections [|Fo| > 4s(|Fo|), 2qmax = 143◦],
123 parameters. The structure was solved and refined using the
SHELXTL (PC version 5.1, Bruker AXS, Madison, WI, 1997) and
SHELX-97 (G. Sheldrick, Institut Anorg. Chemie, Tammannstr.
4, D37077 Go¨ttingen, Germany, 1998) software packages. CCDC
689173. Further details may be found in the ESI.†
Results and discussion
Ligand and complex synthesis
The unsubstituted salan ligands H4LH and H2LH were prepared
as previously described.28 Condensation of ethylenediamine with
salicylaldehyde followed by a reduction with sodium borohydride
to yield H4LH and subsequent methylation of the amine to yield
H2LH is shown in eqn (1). It was also possible to synthesise H2LMe
by this method, but not the other ligands.
Oxidation reactions
Ortho/para-disubstituted salan ligands are generally prepared
by a Mannich condensation reaction.29 Kerton and co-workers
have reported the synthesis of a salan ligand with no ortho-
substituents (H2LMe) via a Mannich condensation reaction,30 but
this method failed in our hands for this particular ligand. However,
the Mannich condensation protocol was successfully employed for
the synthesis of H2LF, H2LI, H2LBr, H2LCl and H2LOMe (eqn (2)). It
was not possible to synthesise H2LNO2 by any of the routes shown in
eqn (1) and (2). Instead, it was necessary to react 2-(chloromethyl)-
4-nitrophenol with N,N¢-dimethylethylenediamine (eqn (3)) as
described previously by Ng.24
Two different experimental procedures were used for the oxidation
of PPh3:
Procedure A. (used for the Hammett analysis): To the catalyst
[MoO2Lx] (0.025 mmol) and 0.25 mmol (10 equivalents) of
PPh3 was added 5 mL of DMSO at room temperature and the
mixture was stirred vigorously for 2 minutes to ensure complete
dissolution. An aliquot of the reaction solution was placed in
the NMR Spectrometer, which had been pre-heated to 130 ◦C. A
spectrum was obtained every 3 minutes for 6 hours (apart from
[MoO2LNO2] where, due to the fast rate of the reaction, a spectrum
was obtained every minute). A representative example is shown in
Fig. 2.
The complexes were prepared by reaction of the ligand with
[MoO2(acac)2] in methanol and a solvent system in which
the ligand dissolves, typically acetonitrile or a mixture of
Fig. 2 A series of 31P NMR spectra for the reaction between PPh3 and DMSO at 130 ◦C, catalysed by [MoO2LCl] according to procedure A.
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(1)
(2)
(3)
dichloromethane and acetonitrile. This gave yellow or orange
complexes in moderate to good yields, which did not require any
further purification. All ligands and subsequent complexes have
1
been characterised by H, 13C and where applicable 19F NMR
spectroscopy as well as mass spectrometry and elemental analysis.
In the case of [MoO2LCl], crystals suitable for X-ray analysis were
obtained from chloroform by slow evaporation of the solvent.
Fig. 3 shows the molecular structure of [MoO2LCl] along with
selected bond lengths and angles in Table 1. The complex has
crystallographic C2 symmetry about an axis which passes through
the molybdenum and bisects the C9–C9¢ bond and the ligand has
adopted a cis-a (R*,R*) topology, whereby R* refers to the relative
configuration at the amine nitrogen atoms. 1H NMR analysis
and in particular the 19F NMR spectrum of the related complex
[MoO2LF], which contains only a single resonance at -121 ppm,
indicate that this is also the only geometry observed in solution.
Bond lengths and angles are unremarkable and comparable
to similar Mo(VI) dioxo complexes containing salan type
ligands.21,22,25,31
Fig. 3 The molecular structure of [MoO2LCl].
Triphenylphosphine oxidation studies
The ability of molybdenum(VI) cis-dioxo complexes to catalyse
oxo-transfer reactions has been investigated using PPh3 as the
substrate and DMSO as the oxygen transfer agent (for details
see Experimental). The decrease of the concentration of PPh3
was monitored over time (see Fig. 2) for all Mo(VI) complexes
and from the data shown in Fig. 4, where ln[PPh3]/[PPh3]0 is
plotted against time, it can be seen that all reactions are first
order with respect to the triphenylphosphine concentration. An
important outcome of this study is that the rate of oxidation of
PPh3 appears to be directly related to the electron-withdrawing
ability of the para phenoxy substituent on the molybdenum
catalyst.
◦
Cl
˚
Table 1 Selected bond lengths (A) and angles ( ) for [MoO2L ]
Mo–O
Mo–N(8)
1.7035(15)
2.3945(19)
Mo–O(1)
1.9381(14)
O–Mo–O(1)
O–Mo–O¢
O–Mo–N(8¢)
O(1)–Mo–O(1¢)
N(8)–Mo–N(8¢)
96.54(6)
108.17(11)
88.24(7)
155.86(9)
75.39(10)
O–Mo–N(8)
163.55(8)
O–Mo–O(1¢)
O(1)–Mo–N(8)
O(1)–Mo–N(8¢)
97.56(7)
79.69(6)
81.26(6)
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Fig. 4 Variation in [PPh3] versus time in the catalysed oxo transfer reaction between DMSO and PPh3. Conditions: [PPh3]0 = 50 mM (10 equiv.);
[catalyst] = 5 mM (1 equiv) in DMSO at 130 ◦C.
Hammett parameters allow the quantification of the effect
of electron-donating and electron-withdrawing substituents on
the outcome of a reaction and this can give important insight
into the mechanism of the reaction.32 For example, Holm and
Sung have applied Hammett parameters in a related mechanistic
study of the oxo transfer process catalysed by bis(dithiolene)
tungsten complexes.33 Previous studies on oxidation reactions with
molybdenum(VI) cis-dioxo complexes as catalysts featured salan
ligands with both ortho- and para-substituted phenoxy groups.23–25
This double substitution precludes the use Hammett parameters,
as both substituents will affect the catalytic activity and the ortho
substituent will have an additional steric effect. In order to gain
further insight into the mechanism of oxygen transfer catalysed
by salan molybdenum dioxo complexes we have applied Hammett
analysis to the para-substituted complexes studied here.
with the previously proposed mechanism involving nucleophilic
attack by the phosphine at an oxo ligand.10,34,35 In several related
molybdenum-based systems, this nucleophilic attack has been
identified as the rate determining step in the oxygen atom transfer
process.8,36 From the Hammett analysis shown in Fig. 5, it can be
concluded that this is also the case here, whereby the nucleophilic
attack results in a charge build-up at the molybdenum centre.
Electron-withdrawing substituents will lower the energy of the
transition state by dissipating the negative charge, which results in
faster reaction rates.
Catalyst decompostion
Previous studies have shown that during oxygen atom transfer
catalysis, a potential deactivation pathway for the molybdenum
catalysts is the comproportionation reaction between Mo(VI)
dioxo and Mo(IV) oxo complexes to give oxo-bridged dimeric
Mo(V) complexes (eqn (4)).22,37–39 Once formed, these dimeric
complexes are rather stable and can no longer participate in further
catalytic turnover. If the order of reaction with respect to catalyst
The kinetic data obtained in Fig. 4 have been used to construct
a Hammett diagram, as shown in Fig. 5. The straight line
(R2 = 0.986) and the gradient (r value) of 2.2 indicate the
accumulation of negative charge during the rate-determining
step in the catalytic cycle. These observations are consistent
Fig. 5 Relative rate constants log kX/kH versus the Hammett sp parameter for the oxo transfer reaction between DMSO and PPh3, catalysed by salan
Mo complexes.
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Table 2 Half potentials for Mo(VI)/Mo(V) redox couples in salan molyb-
substantially deviates from the expected first order, it is plausible
that the deviation is caused by the formation of inactive oxo-
bridged Mo(V) complexes. In order to investigate this potential
catalyst deactivation pathway, the conversion of PPh3 to Ph3PO
was monitored using different concentrations of the most active
catalyst [MoO2LNO2].
Using the integrated rate law (as shown in Fig. 4) it was possible
to assign the rate equation as first order with respect to PPh3
concentration. Using eqn (5)–(7) and the relationship between
lnkapp and ln[catalyst], as shown in Fig. 6, it can be seen that the
rate order with respect to catalyst concentration is approximately
unity (x = 1.0128) and hence formation of inactive dimeric oxo-
bridged Mo(V) complexes either does not occur or the formation
is fast and reversible. The rate constant for the oxygen transfer
reaction between Me2SO and PPh3 by the most active catalyst
denum dioxo complexes
Complex
E1/2/V vs. Ag/AgCl
[MoO2(H2LH)]
Irreversible
[MoO2LOMe
]
-1.271
[MoO2LMe
[MoO2LH]
[MoO2LF]
]
-1.255
-1.206
-1.177
[MoO2LCl]
[MoO2LBr]
[MoO2LI]
[MoO2LNO2
-1.111
-1.112
-1.097
]
-0.803, -1.304 (Mo(V)/Mo(IV))
reduction from Mo(VI) to Mo(V) complexes (eqn (9)). These values
indicate that these salan dioxomolybdenum complexes are slightly
easier to reduce than the related bis(phenoxy)diamine complexes
reported by Ng.23 The cyclic voltammogram of [MoO2LNO2],
shown in Fig. 7, displays an additional quasi-reversible electron
transfer couple. This peak is assigned to a further reduction
of Mo(V) to Mo(IV) (eqn (10)). The electron-withdrawing para-
nitro substituents lower the reduction potential to such an extent
that further reduction to Mo(IV) becomes accessible. This second
reduction is not observed with any of the other complexes, in
which case reduction of the DMF solvent takes place before any
further reduction to Mo(IV) can occur. Another exception is com-
plex [MoO2(H2LH)], which displays irreversible electrochemical
reduction behaviour. Similar observations were made previously
by Spence and co-workers22 and this was attributed to ligand
oxidation (eqn (11)).
The half-potential values E1/2 for the Mo(VI)/Mo(V) redox
couple have been plotted against the Hammett sp values for
the para substituents as shown in Fig. 8. A linear relationship
between the E1/2 values and the Hammett parameter is observed,
which indicates that the substituents in the para phenoxy position
directly affect the electron density at the molybdenum centre. A
linear relationship has been previously observed for molybdenum
dioxo complexes containing tridentate Schiff base ligands.18
The correlation between the E1/2 values and the catalytic oxo
transfer activity indicates that the variation in catalytic activity
is directly related to electronic effects at the metal centre. Steric
effects do not appear to be involved in this series of para-phenoxy
[MoO2LNO2] at 130 ◦C was determined as k0 = 7.13 x 10-2
L
mol-1 s-1. The rate equation shown in eqn (8) is the same as
the one determined previously for the reaction of EtPPh2 with
dioxomolybdenum(VI) complexes containing tridentate Schiff base
ligands,16 which confirms that the rate determining step is indeed
the initial reaction of the phosphine and the dioxo complex.
(4)
rate = kapp[PPh3]
(5)
(6)
(7)
(8)
kapp = k0 [MoO2LNO2
]
x
lnkapp = lnk0 + xln[MoO2LNO2
]
rate = k0[PPh3][MoO2LNO2
]
Cyclic voltammetry studies
The electrochemical properties of the salan molybdenum dioxo
complexes [MoO2LX] used in the oxo transfer experiments,
together with the complex which contains a secondary amine
ligand [MoO2(H2LH)], have been investigated in DMF. The cyclic
voltammograms exhibit a single quasi-reversible peak for most
complexes, with E1/2 values ranging from -1.097 V for [MoO2LI]
to -1.271 V for [MoO2LOMe] (see Table 2), which correspond to a
Fig. 6 The relation ship between lnkapp and ln[catalyst] for the oxo transfer reaction between DMSO and PPh3, catalysed by salan Mo complexes.
2342 | Dalton Trans., 2009, 2337–2344 This journal is The Royal Society of Chemistry 2009
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Fig. 7 Cyclic voltammogram of [MoO2LNO2]. Conditions: room temperature in DMF, [complex] = 1 mM; [NBu4PF6] = 0.2 M; scan speed 100 mV s-1;
working electrode, Pt stick; auxilliary electrode, Pt wire; reference electrode, Ag/AgCl.
Fig. 8 Relationship between E1/2 values for the Mo(VI)/Mo(V) redox couple and the Hammett values sp. Conditions: room temperature, DMF,
[complex] = 1 mM, [NBu4PF6] = 0.2 M; scan speed 100 mV s-1; working electrode, Pt stick; auxiliary electrode, Pt wire; reference electrode, Ag/AgCl.
substituted catalysts, but they will become important with ortho-
phenoxy substitution, which will be the subject of future studies.
Interestingly, a similar relationship has been previously observed
in alkene epoxidation catalysed by related salen manganese
complexes.40
in the catalytic cycle. These observations are consistent with the
mechanism shown in Scheme 1, which was first proposed by Holm
and co-workers.41 Computational studies have complemented
these kinetic studies and oxo atom transfer processes are currently
believed to go through two transition states, one involving the
formation of a phosphine oxide intermediate and one involving
a substitution reaction, which leads to product release.10 The
first reaction involves nucleophilic attack by the phosphine at
-
-
e
MoO2 LX ææÆ MoO2 LX
È
˘
È
˘
(9)
(10)
(11)
(
)
(
)
Î
˚
Î
˚
=
-
2-
the oxygen atom, specifically attack of P on the Mo O p*
-
e
MoO2 LX
ææÆ MoO2 LX
È
˘
È
˘
Î
˚
Î
˚
orbital, to give a phosphine oxide intermediate.8 Intermediates
of this type have been isolated recently for complexes of the type
[LMo(IV)O(OPR3)X], where L = hydrotris(3-isopropylpyrazolyl-
1-yl)borate (TpiPr) and X = Cl-, phenolates, thiolates),8,9,41,42 and
where L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*) and
X = Cl-.10,43 The next step involves a substitution at the octahedral
d2 Mo(IV) intermediate which, based on previous theoretical
studies,34 was described as an associative or associative interchange
(Ia) process in the case of displacement of OPR3 by H2O. However,
(
)
(
)
-
-
e
MoO2 H LH ææÆ MoO2 LH
+ H2O
È
˘
È
˘
(
)
(
)
2
Î
˚
Î
˚
Mechanistic implications
The positive slope in the Hammett analysis in Fig. 5 has shown that
in the catalytic oxygen transfer reaction between DMSO and PPh3,
a negative charge must build up during the rate-determining step
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 2337–2344 | 2343
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4 J. McMaster, J. M. Tunney and C. D. Garner, Prog. Inorg. Chem., 2003,
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5 C. G. Young, in ‘Biomimetic Oxidations Catalyzed by Transition Metal
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6 J. M. Berg and R. H. Holm, J. Am. Chem. Soc., 1985, 107, 917.
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Hall and P. Basu, Chem.–Eur. J., 2006, 12, 7501.
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G. Young, Chem.–Eur. J., 2005, 11, 3255.
10 V. N. Nemykin and P. Basu, Inorg. Chem., 2005, 44, 7494.
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Scheme 1
recent theoretical studies on the displacement of OPR3 by CH3CN,
suggest a dissociative or dissociative-interchange (Id) mechanism
for this reaction.8 More studies are clearly needed to unravel all
aspects of the mechanism of this intriguing oxo transfer process.
In summary, we have prepared a series of para-substituted salan
ligands and their corresponding Mo(VI) dioxo complexes and we
have evaluated their ability to catalyse the oxo transfer reaction
between triphenylphosphine and dimethylsulfoxide. The relative
rate constants have been plotted against Hammett sp parameters
and it has been shown that the rate-determining step involves a
nucleophilic attack of the lone pair of the phosphorus atom at
the oxo ligand. We have also shown that, at least for the most
active para-nitro substituted catalyst, the formation of inactive
oxo-bridged dimeric Mo(V) complexes does not appear to occur.
Electrochemical studies have shown that E1/2 and Hammett sp
parameters can be correlated for these complexes and that all
the complexes exhibit quasi-reversible redox behaviour. These
studies have shown that electron withdrawing substituents can be
used to increase the catalytic activity of salan molybdenum dioxo
catalysts and in future studies we will apply these findings for the
development of catalysts for the oxidation of more challenging
substrates such as alkenes with cheaper and more environmentally
friendly oxidants such as O2 or H2O2.
23 Y.-L. Wong, J.-F. Ma, W.-F. Law, Y. Yan, W.-T. Wong, Z.-Y. Zhang, T.
C. W. Mak and D. K. P. Ng, Eur. J. Inorg. Chem., 1999, 313.
24 Y.-L. Wong, Y. Yan, E. S. H. Chan, Q. Yang, T. C. W. Mak and D. K.
P. Ng, J. Chem. Soc., Dalton Trans., 1998, 3057.
25 A. Lehtonen and R. Sillanpa¨a¨, Polyhedron, 2005, 24, 257.
26 R. H. Holm and J. P. Donahue, Polyhedron, 1993, 12, 571.
27 B. Soptrajanov, A. Nikolovski and I. Petrov, Spectrochim. Acta, 1968,
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28 J. Balsells, P. J. Carroll and P. J. Walsh, Inorg. Chem., 2001, 40,
5568.
29 E. Y. Tshuva, N. Gendeziuk and M. Kol, Tetrahedron Lett., 2001, 42,
6405.
30 F. M. Kerton, A. C. Whitwood and C. E. Willans, Dalton Trans., 2004,
2237.
31 H. Elias, F. Stock and C. Ro¨hr, Acta Crystallogr., 1997, C53, 862.
32 C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165.
33 K.-M. Sung and R. H. Holm, J. Am. Chem. Soc., 2002, 124, 4312.
34 M. A. Pietsch and M. B. Hall, Inorg. Chem., 1996, 35, 1273.
35 S. B. Seymore and S. N. Brown, Inorg. Chem., 2000, 39, 325.
36 K. Heinze and A. Fisher, Eur. J. Inorg. Chem., 2007, 1020.
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Holm, Inorg. Chem., 1989, 28, 2082.
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Acknowledgements
We gratefully acknowledge financial support from the Department
of Chemistry, Imperial College, London. We are grateful to Peter
Haycock and Richard Sheppard for their assistance in kinetic
NMR measurements.
40 M. Palucki, N. S. Finney, P. J. Pospisil, M. L. Guler, T. Ishida and E.
N. Jacobsen, J. Am. Chem. Soc., 1998, 120, 948.
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2344 | Dalton Trans., 2009, 2337–2344
This journal is
The Royal Society of Chemistry 2009
©