Substituent effect on oxygen atom transfer reactivity from oxomolybdenum centers: Synthesis, structure, electrochemistry, and mechanism

Article abstract of DOI:10.1021/ic900579s

Dioxo molybdenum complexes of general formula Tp^*MoO ₂(S-p-RC₆H₄) (1), where Tp^* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and R = OMe, Me, SMe, NHCOMe, H, Cl, CF₃, NO₂, were reacted with trimethyl phosphine (PMe ₃) to convert into complexes of general formula Tp ^*MoO(S-p-RC₆H₄)(OPMe₃) (2) (where R = OMe, Me, SMe, H, Cl, and CF₃). These complexes were isolated and characterized by NMR, IR, UV/vis, and single crystal X-ray crystallography. Electronic and NMR spectra, as well as redox potentials vary as a function of substituent on the thiophenolato ligand. When viewed entirety of the oxygen atom transfer (OAT) reactivity, the reaction of Tp ^*MoO₂(S-p-RC₆H₄) with PMe ₃shows a biphasic behavior, indicating the formation of at least one intermediate. The kinetics of the both steps, that is, the formation of the phosphoryl intermediate and the formation of the solvent coordinated species have been investigated by UV-vis spectroscopy. The first step follows a second order process, first order with respect to both the complex and PMe₃, and the overall second order rate constants at 25 °C range from 98.2 (±0.01) × 10^-2 M^-1 s^-1(for R = OMe) to 223.0 (±0.20) × 10^-2 M^-1 s ^-1 (for R = CF₃); activation parameters were in the ranges & Delta; H^D = 49.3 (±4.1)kJ·mor ^-1(for R = OMe) to 34.0(±7.5) kJ·mol^-1(for R = CF₃), ΔS^D = -154.0(±14.2) J·mol^-1·K^-1(for R = OMe) to -184.3 (±26.1) J·mol^-1 K^-1 (for R = CF ₃), and & Delta; G^D = 95.0 kJ·mol ^-1 (for R = OMe) to 88.7 kJ·mol^-1 (for R = CF ₃). Formation of the acetonitrile complex from the phosphoryl complex follows a first order process with respect to the complex. The first order rate constants at 25 °C range from 3.60 (±0.01) × 10^-4 sec^-1 (for R = OMe) to 6.32 (±0.11) × 10^-4 sec^-1 (for R = CF₃), and the enthalpy of activation and entropy of activation snow variation; ΔH^D = 62.5 (±2.2) to 67.8 (±1.0) kJ·mol^-1, ΔS ^D = -82.5 (±3.3) to -101.3 (±7.5) J·mol^-1·K^-1, but the free energy of activation remains constant Δ G^D ～ 92 (±1) kJ·mol^-1. Large entropies of activation associated with both steps are consistent with associative transition states. The comparable magnitude of the activation energy of the two steps underscores the difficulty in identifying the rate-limiting step in the overall OAT reaction. The first step, however, is more sensitive toward the substituent effects than the second step. Therefore, a change in the substituent can play an important role in deciding the rate-limiting step involved in a two-step OAT reaction.

Full text of DOI:10.1021/ic900579s

Inorg. Chem. 2009, 48, 6303–6313 6303
DOI: 10.1021/ic900579s
Substituent Effect on Oxygen Atom Transfer Reactivity from Oxomolybdenum
Centers: Synthesis, Structure, Electrochemistry, and Mechanism
Partha Basu,* Victor N. Nemykin,† and Raghvendra S. Sengar
Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282.
† Current address: Department of Chemistry, University of Minnesota-Duluth, Duluth, MN
Received March 24, 2009
Dioxo molybdenum complexes of general formula Tp*MoO2(S-p-RC6H4) (1), where Tp* = hydrotris(3,5-dimethyl-1-
pyrazolyl)borate and R = OMe, Me, SMe, NHCOMe, H, Cl, CF3, NO2, were reacted with trimethyl phosphine (PMe3) to
convert into complexes of general formula Tp*MoO(S- p-RC6H4)(OPMe3) (2) (where R = OMe, Me, SMe, H, Cl, and
CF3). These complexes were isolated and characterized by NMR, IR, UV/vis, and single crystal X-ray crystallography.
Electronic and NMR spectra, as well as redox potentials vary as a function of substituent on the thiophenolato ligand.
When viewed entirety of the oxygen atom transfer (OAT) reactivity, the reaction of Tp*MoO2(S-p-RC6H4) with PMe3
shows a biphasic behavior, indicating the formation of at least one intermediate. The kinetics of the both steps, that is,
the formation of the phosphoryl intermediate and the formation of the solvent coordinated species have been
investigated by UV-vis spectroscopy. The first step follows a second order process, first order with respect to both the
complex and PMe3, and the overall second order rate constants at 25 °C range from 98.2 ((0.01) ꢀ 10-2 M-1 s-1
(for R = OMe) to 223.0 ((0.20) ꢀ 10-2 M-1 s-1 (for R = CF3); activation parameters were in the ranges ΔH‡ = 49.3
((4.1) kJ mol-1 (for R = OMe) to 34.0 ((7.5) kJ mol-1 (for R = CF3), ΔS‡ = -154.0 ((14.2) J mol-1 K-1 (for R =
3
3
3
3
OMe) to -184.3 ((26.1) J mol-1 K-1 (for R = CF3), and ΔG‡ = 95.0 kJ mol-1 (for R = OMe) to 88.7 kJ mol-1 (for
3
3
3
R = CF3). Formation of the acetonitrile complex from the phosphoryl complex follows a first order process with respect
to the complex. The first order rate constants at 25 °C range from 3.60 ((0.01) ꢀ 10-4 sec-1 (for R = OMe) to 6.32
((0.11) ꢀ 10-4 sec-1 (for R = CF3), and the enthalpy of activation and entropy of activation show variation; ΔH‡ =
62.5 ((2.2) to 67.8 ((1.0) kJ mol-1, ΔS‡ = -82.5 ((3.3) to -101.3 ((7.5) J mol-1
K
-1, but the free energy of
3
3
3
activation remains constant ΔG‡ ∼ 92 ((1) kJ mol-1. Large entropies of activation associated with both steps are
3
consistent with associative transition states. The comparable magnitude of the activation energy of the two steps
underscores the difficulty in identifying the rate-limiting step in the overall OAT reaction. The first step, however, is more
sensitive toward the substituent effects than the second step. Therefore, a change in the substituent can play an
important role in deciding the rate-limiting step involved in a two-step OAT reaction.
Introduction
reactions. Catalytic OAT reactions are also important in
industrial processes such as the epoxidation of olefin.2-4
The overall OAT reaction is also a defining reaction in
synthetic oxo-molybdenum chemistry.5-11 OAT reactions
are a subset of atom transfer reactions, and as such these
reactions have been reported for other transition metal, for
In nature, pterin-containing mononuclear molybdenum
enzymes are widely distributed through all forms of life.1
These enzymes catalyze key metabolic processes and are
active participants in the global biogeochemical cycles of
elements such as carbon, nitrogen, sulfur, arsenic, and
selenium. In general, enzymes such as sulfite oxidase, di-
methyl sulfoxide reductase, and nitrate reductase catalyze net
oxygen atom transfer (OAT) reactions at the mononuclear
oxomolybdenum active centers.1 Although the details of the
mechanism may vary, non-molybdenum enzymes such as
cytochrome P450 are also thought to proceed via similar
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*To whom correspondence should be addressed. E-mail: basu@duq.edu.
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2009 American Chemical Society
Published on Web 06/01/2009
pubs.acs.org/IC

6304 Inorganic Chemistry, Vol. 48, No. 13, 2009
Basu et al.
example, rhenium, vanadium, and tungsten complexes.12-17
In most cases, OAT reactions involve higher-valent metal
centers, and the reaction proceeds with concomitant reduc-
tion of the donor center by two electrons as shown in
eq 1. OAT reactions involving oxo-molybdenum complexes
the formation of a phosphine oxide-coordinated intermedi-
ate.32,43,44 The phosphoryl intermediate of general formula
[Tp*MoO(OPR3)X], in which Tp*=hydrotris(3,5-dimethyl-
pyrazol-1-yl)borate; X = SPh, OPh, Br, and Cl, and [TpiPr
-
MoO(OPR3)X] (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)
borate; X = Cl, substituted phenolate, or alkyl thiolate)
has been detected, isolated, and characterized.37,38,40,41,45
Mechanistically, the first step involves the formation of the
phosphoryl intermediate, and in the second step the phos-
phine oxide is solvolyzed. In this report we present complexes
with MoVI; S unit which is also present in sulfite oxidase
(SO), where the sulfur donor comes from a cysteine.46 Herein
we report complexes with a Mo-S(thiophenol) unit which
allows systematic perturbation at the thiophenol group
providing a vehicle for a detailed understanding of the
electronic effect on reactivity. Thus, syntheses and spectro-
scopic characterization of Tp*MoO2(S-p-RC6H4) (where
R = OMe, Me, SMe, NHCOMe, H, Cl, CF3, and NO2),
and Tp*MoO(S-p-RC6H4)(OPMe3) (where R = OMe, Me,
SMe, H, Cl, and CF3) (Chart 1) are reported here. In addition,
we report the structures, redox chemistry, and reactivity of both
the dioxo-Mo(VI) and the phosphoryl Mo(IV) complexes. The
complex with an unsubstituted thiophenol has recently been
reported,41 which is used here for completeness.
MOn þ þ XfMðn -2Þ þ þ XO
ð1Þ
containing dithiocarbamate,18,19 ene-dithiolate,20-31 and hy-
drotris(pyrazolyl)borate32-34 as well as other ligands10,35,36
have been reported. While a variety of biological substrates
have been used in investigating such reactions in model
systems, tertiary phosphines (PR3) have become the reagents
of choice because of their high solubility in organic solvents
and the ability to tune their reactivity through substitution
at phosphorus. For a number of years we have been engaged
in delineating the details of OAT reactivity using tertiary
phosphines as model substrates.32,37-41
Traditionally, OAT reactions from MoVIO2 centers to PR3
are thought to proceed via nucleophilic attack by the phos-
phine on an empty ModO π* orbital.5,42 In these reactions,
the existence of a single transition state was supported by
experimental data. More recent theoretical and experimental
investigation indicated that the overall reactions proceed via
Experimental Section
1
Spectroscopy and Electrochemistry. Room temperature H,
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13C, and 31P NMR spectra were recorded using a Bruker ACP-
300 spectrometer at 300.133 MHz, 75.469 MHz, and 121.496
MHz frequencies, respectively. Deuterated solvents were ob-
tained from Cambridge Isotope Laboratory and used as re-
ceived. Infrared spectra were recorded on a Perkin-Elmer FT-IR
1760X spectrometer on NaCl plates or in KBr pellets. Mass
spectra were collected on a Micromass ZMD mass spectrometer
using both negative and positive ionization mode. Acetonitrile
solutions of the samples were injected via a syringe pump with a
flow rate of 0.1-0.2 mL/min. Electronic spectra of complexes
were recorded in Cary 3 and Cary 14 spectrophotometers. Cyclic
voltammograms (CVs) of all the dioxo-MoVI and monooxo-
MoIV complexes were recorded in a Bioanalytical systems (BAS)
model CV-50W using a standard three electrode system con-
sisting of Pt-disk working and reference electrodes and a Pt-wire
auxiliary electrode in dry and degassed acetonitrile, containing
0.1 M Bu4NClO4 as supporting electrolyte. At the end of each
measurement the potentials were internally calibrated with Fc+/
Fc couple, and presented with respect to Fc+/Fc couple at the
same scan rate (100 mV/s). Electrochemical measurements of
monooxo-MoIV phosphoryl complexes were completed in
less than 2 min, and up to 7% of the phosphoryl ligand ex-
changed during this time. The first scan was recorded at a rate of
100 mV/s within 30 s of dissolution of the complex in acetonitrile
during which ∼1% ligand exchanged.
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X-ray Crystallography. Brown crystals of dioxo-MoVI com-
plexes 1a, 1b, 1f, and 1h or bright green crystals of 2a, 2f, and 2g
were obtained by vapor diffusion of hexane into the benzene or
toluene solution of the complex. Crystals of complex 1d were
obtained by slow diffusion of toluene into the acetonitrile
solutions of the complex. X-ray quality single crystals were
mounted on glass fibers and coated with the epoxy resin, the
intensities were recorded on a Rigaku AFC-7R four-circle
X-ray diffractometer using graphite-monochromatized Mo-KR
8604.
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Somogyi, A.; Basu, P. J. Am. Chem. Soc. 2002, 124, 756.
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Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6305
Chart 1
˚
radiation (λ = 0.71069 A) at room temperature using ω-2θ
scan. The structures were solved by Patterson method using
teXsan crystallographic software package;47 then they were
refined using the Crystal for Windows package.48 The solvent
benzene molecules in the phosphoryl complexes (2a and 2f) were
found to be disordered and were modeled as planar with
Kinetics of Solvolysis of Tp*MoO(S-p-RC6H4)(OPMe3). The
solvolysis reactions in acetonitrile were monitored at ∼ 414 nm.
For a typical experiment, ∼2-3 mg of the phosphoryl complex
was dissolved in 1.5 mL of pre-equilibrated acetonitrile in a
cuvette and placed in an isothermal cell holder. Temperature
was equilibrated for 3-4 min before data collection. All mea-
surements were followed for ∼5-6 times of the half-life of the
reaction.
˚
restrained C-C distance as 1.39 ((0.02) A and C-C-C angle
as 120.0 ((0.02)°.
General Procedure of the Synthesis of Dioxo-MoVI Complexes
Tp*MoO2(S-p-RC6H4) R = OMe, Me, SMe, NHCOMe, H, Cl,
CF3, NO2. All reactions were performed anaerobically using
anhydrous solvents. A mixture of suitable para-substituted
benzenethiol and triethylamine in CH2Cl2 was added into a
suspension of Tp*MoO2Cl in CH2Cl2 at the room temperature.
The yellow reaction mixture turned to brown over a period of
1-6 h depending on the benzenethiol used. The progress of
reaction was monitored by thin layer chromatography. After
completion of the reaction, the solvent was evaporated, and the
brown residue was chromatographed on silica gel using toluene
(or a mixture of toluene and acetonitrile) as an eluent.
General Procedure of the Synthesis of Monooxo-MoIV Phos-
phoryl Complexes, Tp*MoO(S-p-RC6H4)(OPMe3) R = OMe,
Me, SMe, H, Cl, CF3. All the reaction and workup were
performed under anaerobic conditions using anhydrous sol-
vents. The dioxo-MoVI complex of a particular thiol was
dissolved in dry benzene (or toluene), to which 4-fold excess of
PMe3 was added whereby the color of the mixture changed from
dark brown to green. Partial evaporation of solvent followed by
addition of hexane yielded a green precipitate, which was filtered
and washed with hexane several times yielding the target
compound.
Results
Syntheses. Compounds synthesized in this study are
shown in Chart 1. Dioxo-MoVI complexes, Tp*MoO2(S-
p-RC6H4) (1) were synthesized following literature pro-
cedure49 by reacting Tp*MoO2Cl with para-substituted
thiophenols in the presence of Et3N in CH2Cl2. Reactions
were found to be complete within 2-6 h as evidenced by
thin layer chromatography, and the target compounds
were purified by chromatography on silica columns using
toluene (for R = OMe, Me, H, Cl, and CF3) or a mixture
of toluene and acetonitrile (for R = NHCOMe and NO2)
as the eluent. All dioxo-MoVI complexes were obtained
as brown solids and found to be stable in air in the solid
state.
The OPMe3 coordinated monooxo-MoIV intermediate
complexes Tp*MoO(S-p-RC6H4)(OPMe3) (2) were
synthesized by an incomplete transfer of an oxygen atom
between dioxo complexes and PMe3 in non-coordinating
solvents such as benzene or toluene under ambient con-
ditions. In most cases, the color of the reaction has
changed from brown to green within 30-40 min after
addition of PMe3. The phosphoryl intermediate com-
plexes were purified by precipitation from the reaction
mixture by adding excess hexane. The same reaction with
PMe3 and 1d or 1h did not yield pure phosphoryl com-
plexes in benzene or in toluene.
Kinetic Measurements. Kinetic measurements were per-
formed anaerobically in acetonitrile solutions. Progress of the
reaction was monitored spectrophotometrically at a suitable
wavelength. All kinetic data were analyzed using the Origin 6.1
software packages. The activation parameters were determined
using Arrhenius and Eyring relations using variable tempera-
ture (5-30 °C) kinetic data. For the second order reactions the
rate constants were normalized for concentration.
NMR Spectra. All molybdenum complexes described
in here are diamagnetic and are amenable to NMR
spectroscopy (Supporting Information, Tables S1, S2,
Kinetics of Formation of the Phosphoryl Intermediate,
Tp*MoO(S-p-RC6H4)(OPMe3). Reactions of dioxo-MoVI
complexes with PMe3 followed a second order process, first
order both in complex and in phosphine. The reactions were
followed under pseudo first order conditions with the concen-
tration of the dioxo-MoVI complex 3-5 mM, and an excess of
PMe3. The solutions were equilibrated before mixing, and the
reactions were monitored within 10 s of mixing. The formations
of the intermediate complexes were monitored by single
wavelength assays at an appropriate wavelength between 393
and 398 nm. The wavelengths were selected based on the
location of the isosbestic point for solvation of the phosphoryl
intermediate.
1
and S3). The H NMR spectra revealed Cs symmetry in
1 and C1 symmetry for 2, which is consistent with
the reported Tp*MoO2X and Tp*MoOX(OPMe3) com-
plexes.40,45,50,51 The 31P NMR spectra of 2 in benzene
displayed a resonance at ∼ 62.0 ((1) ppm for the co-
ordinated OPMe3, which was deshielded by about 26 ppm
relative to the free OPMe3 (36.2 ppm). 31P-chemical shifts
were found to be insensitive to the substitution at the
para-position of the thiophenolate ring. Solution of 2 in
(49) Xiao, Z.; Bruck, M. A.; Doyle, C.; Enemark, J. H.; Grittini, C.;
Gable, R. W.; Wedd, A. G.; Young, C. G. Inorg. Chem. 1995, 34, 5950.
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Gable, R. W.; Wedd, A. G.; Young, C. G. Inorg. Chem. 1996, 35, 5752.
(51) Nemykin, V. N.; Basu, P. Dalton Trans. 2004, 1928.
(47) teXsan: Crystal Structure Analysis Package; Molecular Structure
Corporation: The Woodlands, TX, 1985, 1992
(48) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.;
Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.

6306 Inorganic Chemistry, Vol. 48, No. 13, 2009
Basu et al.
acetonitrile exhibited a developing signal for free OPMe3
at 36.2 ppm, indicating dissociation of the coordinated
OPMe3 in the acetonitrile.
probably because of the unfavorable orientation of the S
(pπ) orbitals with respect to the Mo(dxz) atomic orbital,
and similar geometry dependence is known for other
molybdenum complexes with at least one arylthiol li-
gand.54 The energy of this transition does not line-
arly correlate with the Hammett constants on the thio-
phenol ring.
The 1H NMR spectra of Tp*MoO(S-p-OMeC6H4)
(OPMe3) were investigated in four different solvents
(Supporting Information, Table S3), which shows solvent
dependent chemical shifts. In polar solvents, for example,
acetonitrile and acetone, pyrazole methyl protons reso-
nate between 1.8 and 2.6 ppm, while in non-polar aro-
matic solvents, for example, toluene and benzene, they
shift to higher fields (2.20-2.8 ppm). Interestingly, larger
solvent effect was observed for OPMe3 (∼0.7 ppm) and
OMe (∼0.5 ppm) groups but only a small change in the
pyrazole C-H and thiophenol signals. The effect of
temperature has also been studied in acetonitrile from
-40 °C to +35 °C but no significant changes were
observed.
Crystallography. Three-dimensional molecular struc-
tures were determined by single crystal X-ray crystal-
lography (Figures S1 and S2, Supporting Information),
and representative structures are shown in Figure 1. The
crystallographic parameters are listed in Tables 1 and 2,
and the metric parameters are placed in Tables S4-S5
(Supporting Information). The ligands are arranged in a
distorted-octahedral geometry about the metal with a
local Cs geometry in 1 and C1 geometry in 2 at the metal
center. The coordination sphere of the dioxo-complexes is
composed of the facially coordinating tridentate Tp*
ligand, a pair of terminal oxo ligands, and the sulfur of
the thiophenolato ligands. Similarly, in the coordination
sphere of the monooxo-complexes, the three monoden-
tate ligands (dO, S-p-RC6H4, ;OPMe3) are constrained
by the facial stereochemistry of the Tp* ligand.
Infrared Spectra. Tables S1 and S2 (Supporting Infor-
mation) also list selected infrared spectral data. The dioxo
complexes exhibit two strong bands for a cis-MoO2 unit
in 1 at ∼930 cm-1 and 900 cm-1 assigned to the symmetric
and asymmetric vibrational modes, respectively. Other
characteristic bands include a sharp B-H stretch at
2545 cm-1 for 1 which shifted to ∼20 cm-1 toward lower
frequencies in 2. For 2, sharp bands were observed at
∼945 cm-1 and ∼1120 cm-1 due to the MoIV=O vibra-
tion and PdO vibrations, respectively.
The metric parameters of dioxo-MoVI complexes were
found to be similar to those observed for 1e.41 The ModO
bond distances lie in the range from 1.670(11)-
1.710(2) A, in the region of a typical oxo-MoVI bond. The
˚
Electronic Spectra. UV-visible spectra of 1 and 2 were
recorded in methylene chloride and in acetonitrile, re-
spectively, and the data are tabulated in Tables S1 and S2
(Supporting Information). For 1, a band around 415 nm
(ε>1000 M-1cm-1) due to the S f Mo charge transfer
band was observed. A hypsochromic (or blue) shift from
1a (424 nm) to 1h (404 nm) was observed because of
the substituent present at the para-position of the thio-
phenolato ligand. An intense transition around 260 nm
(ε>13000 M-1cm-1) can be assigned as intraligand
excitation. In 1h this band moved to 335 nm probably
because of the electron withdrawing NO2 substituent. The
energy of this low-energy band in 1 at ∼500 nm varies
linearly with the Hammett constants (σp) of the substi-
tuents (σp: OMe, -0.27; Me, -0.17; SMe, 0.0; NHCOMe,
0.0; H, 0.0; Cl, +0.23; CF3, +0.54; NO2, +0.78)52
following eq 2. The linear relation (R2 = 0.88) indicates
that the electronic contribution from the thiophenol ring
Mo-S bond distances, as expected, range from 2.408(4)-
2.419(2) A. There is no systematic variation in the
˚
Mo-S bond distance as a function of the para-
substituents on the thiophenolato ligand. The S-C(41)
˚
bond distances (1.767(17)-1.806(14) A) are also consis-
tent with the reported values for thiophenol and disul-
fides.55 A strong trans influence from the oxo-ligands
lengthened the Mo-N bond (i.e., Mo-N(11) and Mo-
˚
N(21): 2.278(9)-2.349(6) A) compared to the Mo-N(31)
˚
bond (2.156(6)-2.183(11) A). The O(1)dModO(2) bond
angles (103.2(2)-104.4(5)°) are higher than the octahe-
dral bond angle (90°) as are the OdMo-S bond
angles presumably because of repulsions between the
π-electrons of the donor atoms. It is interesting to note
that O(1)dMo-S and O(2)dMo-S bond angles are
slightly different, which makes the two terminal oxo-
groups inequivalent. The inequivalency of the two term-
inal oxo-groups in a dioxo-Mo center was explained
on the basis of the anisotropic covalent interac-
tion of thiophenolato ligand.56 The covalent interaction
is modulated by the OdMo-S-C(41) dihedral and
the OdMo-S bond angles (Supporting Information,
Table S4). The two dihedral angles O(1)dMo-S-C(41)
and O(2)dMo-S-C(41) are distinctly different which
corroborates the inequivalency of the two oxo groups.
In these structures the position of the Tp* ligand is
constant, but the orientation of the thiophenol ring along
the S-C bond is found to be different. This change in the
orientation is understood with the help of Mo-S-
C(41)-C(42/46) torsion angle and Mo-S-C(41) bond
angles. Different torsion angles in compounds with
Eðcm-1Þ ¼ 18836 ð ( 113Þ þ 1428 ð ( 269Þ ꢀ σp ð2Þ
makes significant contribution to the transition energy.
The Mo(IV) phosphoryl complexes (2a-2g) exhibit a
low energy low intensity d-d transition at ∼760 nm which
is consistent with other similar phosphoryl complexes.45
This band has been tentatively assigned as due to
the dxyfdxz,yz transition. A band around ∼410 nm
can be assigned as the S(pπ) f Mo(dxz) charge transfer
transition with possible contributions from dxy
f
dx2-y2 excitation expected for this energy region.45,53
Interestingly, this band (ε ∼ 360-680 M-1cm-1) exhibits
lower intensity than a typical charge transfer transition,
(54) McNaughton, R. L.; Mondal, S.; Nemykin, V. N.; Basu, P.; Kirk, M.
L. Inorg. Chem. 2005, 44, 8216.
(55) Sengar, R. S.; Nemykin, V. N.; Basu, P. New J. Chem. 2003, 27, 1115.
(56) Helton, M. E.; Pacheco, A.; McMaster, J.; Enemark, J. H.; Kirk, M.
L. J. Inorg. Biochem. 2000, 80, 227.
(52) Hansch, K.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(53) Nemykin, V. N.; Basu, P. Inorg. Chem. 2003, 42, 4046.

Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6307
Figure 1. ORTEP diagrams of 1a and 2a shown at 40% probability levels. Hydrogen atoms have been omitted for clarity. These two structures are
shown as representative examples, complete ORTEP structures are placed in the Supporting Information.
Table 1. Crystallographic Data for Tp*MoO2(S-p-RC6H4) (1)
1a
1b
1d
1f
1h
formula
C
22H29B1Mo1N6O3S1
C22H29B1Mo1N6O2S1
548.30
monoclinic
P21/c
18.772(13)
8.136(6)
18.494(13)
C30H38B1Mo1N7O3S1
683.49
triclinic
C24H29B1Mo1N6O2S1Cl1
607.80
triclinic
P1
10.378 (4)
13.601 (4)
10.352 (2)
107.346 (18)
97.29 (3)
89.52 (3)
1382.8 (7)
2
C21H26B1Mo1N7O4S1
579.30
monoclinic
P21/c
11.898(2)
formula mass
crystal system
space group
564.33
triclinic
P1
P1
˚
a(A)
10.993(4)
12.151(4)
9.892(2)
96.21(2)
104.05(2)
88.57(3)
1274.2(7)
2
1.471
3362
0.067
0.114
12.4168(17)
13.7887(18)
10.5214(15)
96.732(11)
113.826(10)
81.333(11)
1625.9(4)
2
1.396
6782
0.033
0.065
˚
b(A)
16.203(3)
12.887(3)
˚
c(A)
R(deg)
β(deg)
γ(deg)
117.723(9)
91.55(3)
3
˚
V(A )
2500.0(3)
4
1.457
1659
0.053
0.218
2483.5(8)
4
1.549
2409
0.114
0.345
Z
dcalc (mg/m3)
data collected
R(F2)
1.460
2639
0.086
0.143
wR(F2)
Table 2. Crystallographic Data for Tp*MoO(S-p-RC6H4)(OPMe3) (2)
2a
2f
2g
formula
C
31H44B1Mo1N6O3S1P1
C30H41B1Mo1N6O2S1P1Cl1
C32H43B1Mo1N6O2S1P1F3
formula mass
crystal system
space group
718.52
monoclinic
P21/c
11.991 (2)
10.675 (2)
28.378(6)
94.47(3)
3621.5(13)
4
1.318
4264
0.075
0.155
722.94
monoclinic
P21/c
18.150(4)
11.194(2)
18.367(4)
106.14(3)
3584.6(13)
4
1.340
4728
0.054
0.118
770.52
monoclinic
C2/c
28.649(8)
11.032(6)
26.998(7)
118.046(18)
7531(5)
8
1.359
5992
0.067
0.165
˚
a(A)
˚
b(A)
˚
c(A)
β(deg)
3
V(A )
˚
Z
dcalc (mg/m3)
data collected
R(F2)
wR(F2)
different substituents indicate rotational freedom of the
phenyl ring along the C(41)-S bond. The Mo-S-C(41)
angle also changed from 101.3(2)° in 1a to 110.7(6)° in 1h.
Both factors would dictate the orientation of the S π
orbitals with respect to the molybdenum orbitals and
therefore could influence the Mo-S covalency.
The crystal structure of a representative monooxo-
MoIV phosphoryl complex is shown in Figure 1 and all
structures can be found in Figure S2, Supporting Infor-
mation. The metric parameters are listed in Table S5
(Supporting Information). The ModO bond distance
˚
(1.665(4)-1.686(5) A) was found to be typical for such
complexes.37,38,40,41,45,57 In most cases, the ModO dis-
tance is, however, slightly shorter than that in corres-
ponding dioxo-MoVI complexes. The Mo-O bond
˚
lengths (2.153(6)-2.175(5) A) are similar to those ob-
served in other OdMo-OPR3 complexes.37,38,40,41,45,57
(57) Doonan, C. J.; Millar, A. J.; Nielsen, D. J.; Young, C. G. Inorg.
Chem. 2005, 44, 4506.

6308 Inorganic Chemistry, Vol. 48, No. 13, 2009
Basu et al.
The O(2)dP bond lengths (1.484(4)-1.519(4) A) are
sufficiently smaller than the O-P single bond found in
P4O10 (1.6 A), indicating the involvement of multiple
Table 3. Redox Potentials of 1 and 2 in MeCN at Room Temperaturea
˚
complexes
E1/2(V)
ΔEp(mV)
40
˚
bonding. Interestingly, O(1)-Mo-O(2) bond angles
(98.6(2)-99.8(3)°) are reduced from their dioxo-Mo
counterparts and are closer to a typical octahedral value
indicating a reduction in the π-electron interactions be-
tween two oxygen ligands.
1a
1b
1c
1d
1e
1f
-1.170
-1.130
-1.120
-1.150
-1.110
-1.070
-1.030
126
88
68
79
120
65
134
Compared to the dioxo complexes, there is no notice-
able change in the Mo-S bond lengths, although the
1g
2a
2b
2c
2e
2f
-0.550
-0.540
-0.510
-0.530
-0.490
-0.460
81
68
68
69
66
78
˚
S-C(41) bond length is found to be slightly (ca. 0.01 A)
longer. The Mo-N(n1) (n = 1-3) distances are consis-
tent with the expected values based on the trans influence
of the coligands, namely, dO > S-p-RC6H4 > OPMe3.
The O(2)-Mo-N(n1) angles approached to the ideal
octahedral arrangement (∼90°) for n = 2 and 3 but for
n = 1, the values are still obtuse (by ∼ 10-15°). The O-
(1)-Mo-S angle shows a small increase (∼3°), however,
O(2)-Mo-S shows a large decrease of about 15°
compared to the dioxo complexes. Unlike the parent
dioxo-complexes, superimposition of the structures of
the phosphoryl complexes exhibits little variation.
Electrochemistry. The electrochemical behaviors of the
dioxo-MoVI complexes and the monooxo-MoIV phos-
phoryl complexes were investigated by cyclic voltamme-
try, the redox potentials are summarized in Table 3, and
representative voltammograms are shown in Figure 2.
With the exception of 1h, all dioxo-complexes exhibited a
well-defined one-electron redox couple due to Mo(VI/V)
reduction similar to those reported earlier.49,58 The redox
couple in complex 1h was irreversible, the precise reason
for the irreversibility is currently not clear, although we
assume the redox noninnocent nature of the p-NO2
substituent is playing a role.
2g
a Potentials are expressed with respect to Fc+/Fc couple.
solution.32,41 Consistent with this, freshly prepared acet-
onitrile solution of pure 2a slowly changes from green to
cyan color, which has been monitored by optical spectro-
scopy (Figure 3b). At 22 °C, two distinct isosbestic points
(at 398 nm and at 670 nm) were observed in this case,
indicating a clean transformation at this temperature.
The brown color acetonitrile solution of the dioxo-
complexes changed to green when reacted with PMe3 and
finally to cyan. Figure 3a shows the time dependent
optical spectra of acetonitrile solution of 1a when reacted
with PMe3 at 10 °C. In this case, no distinct isosbestic
point was observed. The two step process is evident as the
LMCT transition at 414 nm decreases in two steps-first
a quick decrease (∼3-5 min depending upon the amount
of PMe3 and the temperature), followed by a slower
(∼40-60 min) process. At the same time a low energy
band due to the d-d transition at ∼750 nm grew at a
comparable rate to that of the decay of the 414 nm band.
As mentioned, the optical spectra of the acetonitrile
solution of 2a exhibit distinct isosbestic points at 398
and 670 nm; however, no such isosbestic point was
observed for the reaction of 1a with PMe3 (inset of
Figure 3a). Assuming no other reactions contributing
significantly to the overall spectral profile, any spectral
change at 398 nm in the reaction of 1a with PMe3
represents the formation of 2a. Thus, single wavelength
assay were conducted at 398 nm for the formation of the
phosphoryl complex 2a. In the case of decomposition
of 2a, maximum change in absorbance was observed at
414 nm and therefore assays were performed at that
wavelength. The activation parameters were computed
from temperature dependent rate constants (Table 4).
The reaction of 1a with PMe3 was followed at ∼398 nm
under pseudo first order condition from which the second
order rate constants were calculated (Supporting Informa-
tion, Table S6). Similarly reactions of PMe3 with other
dioxo-MoVI complexes were monitored at an appropriate
wavelength (Supporting Information, Table S7), and
the activation parameters were determined from tempera-
ture dependent rate constants (Table 4). Both steps of the
reaction of 1 with PMe3 could be followed spectrophoto-
metrically as time constants of the two steps are different.
The phosphoryl complexes also exhibit a reversible
one-electron oxidation couple due to the MoV/MoIV
couple. The linear dependence of the peak currents with
the square root of the scan rate and the ratio of the
cathodic-to-anodic peak current close to unity indicate
a diffusion controlled process. The peak-to-peak separa-
tion values are found to be close to what is expected for a
typical one electron redox process. The reversible nature
of the voltammograms indicates that the compounds
are electrochemically stable. Displacement of phosphine
oxide by a solvent (MeCN in this case) generate com-
pound 3 which exhibits a more positive reduction poten-
tial (Figure 2). This spontaneous transformation has been
discussed in detail for compound 2e.41 At room tempera-
ture the rate of transformation of 2e to 3e was determined
from electrochemistry to be 5.6 ꢀ 10-4 sec-1, which is
consistent with that determined by optical spectroscopy.
The reduction potentials of the phosphoryl complexes as
well as the parent dioxo-MoVI complexes follow a linear
relation with the Hammett constant of the para-substi-
tuent at the thiophenol (Figure 2, right panel). The effect
of substituents on the reduction potential is discussed
later.
Kinetics. We have reported that phosphoryl complexes
undergo solvolysis reaction in coordinating solvents such
as acetonitrile, as evidenced by a change in color of the
Discussion
A series of new dioxo-Mo(VI) complexes coordinated by a
substituted thiophenol have been synthesized and characterized.
(58) Millar, A. J.; Doonan, C. J.; Laughlin, L. J.; Tiekink, E. R. T.;
Young, C. G. Inorg. Chim. Acta 2002, 337, 393.

Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6309
Figure 2. Cyclic voltammograms of a representative dioxo-MoVI complex, Tp*MoO2(S-p-CF3C6H4), (1g) (a, left panel, top), and a representative
phosphoryl-MoIV complex, Tp*MoO (S-p-CF3C6H4)(OPMe3), (2g) (b, left panel, bottom), in MeCN at a scan rate of 100 mV/s. An extra redox response at
around 100 mV found in 2g is due to the formation of solvent coordinated complex; note that this trace was recorded after 15 minutes of dissolution of 2g.
Right panels of a and b show the linear dependence of redox potentials with σp.
The synthetic route leading to these compounds is already
established, and the successful syntheses of the molecules
described in here underscore the robustness of the methodol-
ogy.41,49 The ModO vibrations (∼923 and ∼893 cm-1) do
not exhibit a large variation; however, the lower energy band
Structural Changes. The metric parameters of the
dioxo-MoVI and phosphoryl intermediate complexes are
similar to those reported earlier, which includes the
ModO, Mo-N, and Mo-S bond lengths.37,38,40,41,45
The metal centers in both dioxo-MoVI and monooxo-
MoIV complexes are distorted octahedral with strong
trans-effect from the ModO bond, and in the phosphoryl
complexes, the PdO bond exhibits a double bond char-
acter. Figure 4 shows the superimposition of the structure
of crystallographically characterized dioxo-complexes
which highlights different orientations of the thiophenol
ring. Compared to the dioxo-MoVI complexes, in general
there is an increase in the Mo-S-C angle in the phos-
phoryl complexes, as well as a change in the orientation of
the thiophenol ring. A significant structural reorientation
of the thiophenol ring in Tp*MoO(SC6H5)(OPMe3) com-
pared to Tp*MoO2(SC6H5) has been observed recently.41
A very similar structural reorganization was observed in
the reaction pairs Tp*MoO2(S-p-OMeC6H4)/ Tp*MoO
(S-p-OMeC6H4)(OPMe3) and Tp*MoO2(S-p-ClC6H4)/
Tp*MoO(S-p-ClC6H4)(OPMe3) (Figure 5). These struc-
tural changes are observable with a small tertiary phos-
phine (PMe3) but was not observed with a larger
phosphine, that is, PEt3 in a similar system, where a
phenol takes the position of the thiophenol.57 While the
origin of this structural reorganization is yet to be under-
stood clearly, superimposition of the structures of
near 500 nm in their optical spectra differ by 37 nm (1420 cm-1
)
from electron donating OMe substituent in 1a to electron
withdrawing NO2 group in 1h. The protons of the thiophenol
ring are appropriately shifted upfield (for electron donating
substituent) or downfield (for electron withdrawing substi-
tuent) in their 1H NMR spectra; however, little or no change
was observed in the chemical shift of the pyrazole ring. This
indicates that the remote substitution at the thiophenol ring
does not influence the electronic environment of the Tp*.
Molybdenum(IV) phosphoryl intermediate complexes
were synthesized by incomplete oxo-transfer reaction with
PMe3 as an oxo-abstractor. However, compounds 1d and 1h
are sparingly soluble in benzene or toluene, and the reactions
with PMe3 did not yield the target phosphoryl complex;
instead red colored solutions resulted that are reminiscent of
μ-oxo-dimeric compounds. Like 1, no or little change in the
ModO vibration (∼947 cm-1) or PdO (∼1114 cm-1) has
been observed. The 31P NMR was found to be invariant,
while the low energy band near ∼778 nm in the optical
spectra showed a modest variation (by ∼15 nm, 287 cm-1).
Thus, the substituent effect on the equatorial thiophenol
ring is more prominent in the dioxo-MoVI complexes.

6310 Inorganic Chemistry, Vol. 48, No. 13, 2009
Basu et al.
offers more structural flexibility, aromatic thiols provides
more electron conjugation.
Substituent Effects on the Redox Potentials. In an
octahedral oxo-MoV center, the redox orbital is the
half-filled dxy orbital; reduction to oxo-MoIV would
involve further addition of an electron into this orbital,
and oxidation would involve loss of an electron from this
orbital. A number of factors influence the redox potency
of oxo-MoV centers such as the chelate ring size and
torsion angle, as well as the electronic properties of
remote substituents.62-65 In contrast to the MoV com-
plexes, the redox properties of MoIV or MoVI centers have
received less attention, in part because of the lack of
reversible redox couples in dioxo-MoVI complexes.
Dioxo-MoVI complexes of pyrazolyl borate such as 1,
however, exhibit reversible MoVI/V reduction couple.49
The phosphoryl complexes (2) also exhibit a reversible
one-electron MoIV/V oxidative couple. The MoVI/V re-
duction potential in 1 appears at a more negative poten-
tial than the MoV/IV couple in 2. The MoVI/V reducion
potential span from -1166 mV in 1a to -1031 mV in 1g, a
difference of 135 mV, whereas the Mo(V/IV) potential
span from -549 mV in 2a to -463 mV in 2g, a difference
of 86 mV. The reduction potentials for both couples vary
as a function of substituents at the para position of the
thiophenolato ring. In 1, electron withdrawing groups
make the complex easier to reduce and in 2, electron-
withdrawing groups make the oxidation process difficult.
The sensitivity of reduction potentials is understood from
the Hammett correlation. The reduction potentials vary
linearly (Figure 2) with the Hammett constants (σp) of
the substituents at the para position of the thiophenol
ring (eqs 3 and 4 are for 1 and 2, respectively):
E1=2ðmVÞ ¼ 158 ð ( 23Þꢀ
σp -1118 ð ( 6Þ ðR2 ¼ 0:91Þ
ð3Þ
Figure 3. UV-visible spectral changes in MeCN. (a) During the oxida-
tion of PMe3 by 1a at 10 °C, [1a]:[PMe3] = 1:20 ([1a] = 0.35 mM); time
interval between each scan = 1.4 min. The inset shows the absence of an
isosbestic point. (b) Decomposition of 2a at 22 °C, [2a] = 2.53 mM, time
interval between each scan = 5.5 min.
E1=2ðmVÞ ¼ 109 ð ( 13Þꢀ
σp -518 ð ( 4Þ ðR2 ¼ 0:95Þ
ð4Þ
The slope of the lines indicates the sensitivity of the
process on the substituent. Clearly, the dioxo-MoVI com-
plexes are more sensitive to the substituents compared to
the oxo-MoIV phosphoryl complexes, which indicates
stronger π interaction of the sulfur orbitals with the Mo
d-orbitals. This effect is also manifested in the smaller
Mo-S-C angle. Interestingly, the redox potentials in
oxo-MoV centers in Tp*MoOCl(O-p-RC6H4) and
Tp*MoO(O-p-RC6H4)2 complexes are more sensitive to
the substitutents.62 While these complexes are composi-
tionally and structurally different, a metal ligand distance
(d(Mo-S) > d(Mo-O)) may lead to a higher sensitivity.
Mechanism of OAT Reaction. The first step in the OAT
reaction from a dioxo-MoVI center to a tertiary phos-
phine involves nucleophilic attack on the ModO
Tp*MoO2(S-p-OMeC6H4)/Tp*MoO(S-p-OMeC6H4)
(OPMe3) pair indicates that a twist of the thiophenol ring
away from the point of attack by the phosphine, rotating
the thiophenolato ring clockwise along the Mo
B
3 3 3
vector. Such a conformational change would influence
the metal-sulfur interaction and covalency as proposed
in Cu-S(cysteine) and Mo-S(cysteine) interactions in
native systems.59,60 The large variations in the structural
features in the thiophenolato ligand show the involve-
ment of the first coordination sphere as well as the second
coordination sphere in the OAT reaction, which has been
proposed to play an important role in the redox and OAT
processes of analogue and natural oxotransferase sys-
tems.60,61 While the aliphatic thiol, for example, cysteine,
(62) Graff, J. N.; McElhaney, A. E.; Basu, P.; Gruhn, N. E.; Chang, C.-S.
J.; Enemark, J. H. Inorg. Chem. 2002, 41, 2642.
(63) Chang, C. S. J.; Pecci, T. J.; Carducci, M. D.; Enemark, J. H. Inorg.
(59) Gamelin, D. R.; Williams, K. R.; LaCroix, L. B.; Houser, R. P.;
Tolman, W. B.; Mulder, T. C.; de Vries, S.; Hedman, B.; Hodgson, K. O.;
Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 613.
Chem. 1993, 32, 4106.
(60) Peariso, K.; Helton, M. E.; Duesler, E. N.; Shadle, S. E.; Kirk, M. L.
Inorg. Chem. 2007, 46, 1259.
(61) Kirk, M. L.; Peariso, K. Polyhedron 2004, 23, 499.
(64) Chang, C. S. J.; Enemark, J. H. Inorg. Chem. 1991, 30, 683.
(65) Chang, C. S. J.; Collison, D.; Mabbs, F. E.; Enemark, J. H. Inorg.
Chem. 1990, 29, 2261.

Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6311
Table 4. Activation Parameters for the Formation of 2 and 3
starting complex
λabsa (nm)
Ea‡ (kJ mol-1
)
ln A
ΔH‡ (kJ mol-1
)
ΔS‡ (J mol-1 K-1
)
ΔG‡297 kJ mol-1
3
3
3
Tp*MoO2(S-p-RC6H4) + OPMe3 f Tp*MoO(S-p-RC6H4)(OPMe3)
1a
1b
1e
1f
398
395
393
396
398
51.7((4.1)
49.7((8.7)
41.2((1.3)
48.3((1.0)
36.4((7.5)
11.9((1.7)
12.0((3.6)
9.0((0.6)
12.1((0.4)
8.3((3.2)
49.3((4.1)
47.3((8.7)
38.8((1.3)
45.9((1.0)
34.0((7.5)
-154.0((14.2)
-153.3((30.1)
-178.4((4.7)
-152.6((3.6)
-184.3((26.1)
95.0
92.8
91.8
91.2
88.7
1g
Tp*MoO(S-p-RC6H4)(OPMe3) + MeCN f Tp*MoO(S-p-RC6H4)(MeCN) + OPMe3
2a
2b
2c
2e
2f
414
414
414
412
418
424
64.9((2.2)
69.5((2.5)
68.3((1.0)
70.4((1.0)
67.6((0.1)
65.0((3.3)
18.3((1.0)
20.2((1.0)
19.8((0.4)
20.6((0.4)
19.6((0.1)
18.8((1.3)
62.5((2.2)
66.9((2.5)
65.8((1.0)
67.8((1.0)
65.1((0.2)
62.6((3.3)
-101.3((7.5)
-85.7((8.3)
-88.3((8.8)
-82.5((3.3)
-90.0((0.5)
-96.8((11.0)
92.7
92.4
92.0
92.3
91.8
91.4
2g
a Wavelength at which the assay was conducted.
Figure 5. Superimposition of the crystal structures (tube diagrams) of
Tp*MoVIO2(S-p-ClC6H4)/Tp*MoIVO(S-p-ClC6H4)(OPMe3) in left and
Tp*MoVIO2(S-p-OMeC6H4)/Tp*MoIVO(S-p-OMeC6H4)(OPMe3). Note
the rotation of the thiophenol ring away from the coordinated phosphine
oxide.
Considering the substrate being the same (PMe3), the
difference in the enthalpy of activation likely to reflect a
difference in the nucleophilicity as a function of substi-
tuent. Overall, the free energy of activation (ΔG‡) showed
a small change of ∼6.3 kJ mol-1 depending on the
3
substituent at the para-position of the thiophenolato
ligand.
The substituent effect on the rate of formation of
the phosphoryl intermediate complex 2 showed a linear
Figure 4. Superimposition of the crystal structures (line diagrams) of
Tp*MoO2(S-p-RC6H4).
logðkIR=kIHÞ ¼ Fσp ¼ 0:93 ð ( 0:29Þσp
ð5Þ
π*-orbital by the phosphorus lone pair, with simulta-
neous nucleophilic attack on the P-C σ* orbital by the
terminal oxo-group. This results in a hypervalent char-
acter of the phosphorus at the transition state and ulti-
mately two-electron reduced phosphoryl intermediate
complex.32 The next step of reaction is a solvolysis reaction
where the phosphine oxide is replaced by a solvent molecule.
Variable temperature kinetic measurements allowed deter-
mining the activation parameters for both steps (Table 4).
For the first step, a large negative values of the entropy
dependence of rate constants on the Hammett cons-
tant of the substituent, eq 5. The normalized rate
constant (kIR/kIH; where kI represents the rate of
the formation of phosphoryl intermediates with substi-
R
tuent R and kI represents the same reaction with
H
unsubstituted thiophenol) increases with electron-
withdrawing groups while it decreases with electron-
releasing groups. The linearity (with R2 = 0.85) indi-
cates that reaction with different substituents follow
a similar mechanism. The slope of the plot (F) is 0.93
which suggest a strong dependence of the reaction rate.66,67
of activation (ΔS‡: -152.6 to -184.3 J mol-1 K-1
)
suggest an associative transition state. A relatively small
3
3
activation enthalpy (ΔH‡: 34.0 to 49.3 kJ mol-1) is
3
associated with this process similar to that observed for
other alkyl phosphine (e.g., PEt3) suggests the formation
of a strong PdO bond at the expense of a ModO bond.32
(66) Arias, J.; Newlands, C. R.; Abu-Omar, M. M. Inorg. Chem. 2001, 40,
2185.
(67) Seymore, S. B.; Brown, S. N. Inorg. Chem. 2000, 39, 325.

6312 Inorganic Chemistry, Vol. 48, No. 13, 2009
Basu et al.
Figure 6. Schematic representation of the OAT reaction of 1 showing the effect of remote substitution.
This observation is consistent with the nucleophilic nature
of the phosphine attack where electron-withdrawing
groups on dioxo-MoVI center would decrease the electron
density of the oxo-group and therefore making it more
susceptible for the nucleophilic attack by the phosphine.
Under the mechanistic proposal, the binding and for-
mation of the phosphine oxide is concomitant with the
electron transfer to the metal center. While there is no
reliable way to determine the redox potential of this
activated complex accurately, the parent compound ex-
hibits a reversible one-electron redox couple, which pro-
vides the ground state reduction potential. This ground
state reduction potential is indicative of the intrinsic
reducibility of the metal center and thus can be corre-
lated with the rate of the reaction. Thus, a plot of the log
(kIR/kIH) (where R = OMe, H, Cl, and CF3) versus
difference in the reduction potential (ER - EH) provides
a linear relation (R2 = 0.93) indicating a similar process is
operating in all cases (eq 6). Under the reaction condi-
tions, the linearity then suggests that the reduction of the
the first step, they indicate an associative transition state.
Thus, we propose the transition state is a seven-coordi-
nated oxo-MoIV center where both outgoing OPMe3 and
incoming MeCN associated with the metal center. It is
instructive to point out that the mechanism is different
from that found in our previous study with phenolato
complexes where small positive entropy of activation
indicated a dissociative interchange mechanism.32 How-
ever, a seven-coordinate transition state was computa-
tionally located at a higher energy. We attribute this
difference in the mechanism to the difference in the metal
ligand bond length (d(Mo-S > d(Mo-O)) and a smaller
substrate. Furthermore, in the previous study we used a
more bulky pyrazolyl borate ligand (TpiPr) whereas the
present investigation used a smaller pyrazolyl borate
(Tp*) ligand, which can also contribute to the mechanistic
difference.
The major contributing factor to the free energy of
activation is the enthalpy of activation, which varies by
4.3 kJ/mol. But the free energy of activation (ΔG‡) is found
to be very small (1.3 kJ/mol), and almost invariant to the
substituents. The change in the enthalpy is compensated by
the entropic contribution which is reminiscent of the en-
tropy compensation effect.68 It is interesting to note that in
the second step of the reaction, the entropic term (TΔS‡)
contributes only ∼30% to the ΔG‡, whereas in the first step
of the reaction the ΔG‡ has a much larger contribution
(48%-62%) from the entropic term, even though both
steps pass through an associative transition state.
logðkIR=kIHÞ ¼ 0:139 ð ( 0:022ÞðER -EHÞ þ
0:007 ð ( 0:007Þ
ð6Þ
metal center is concomitant with the formation of the phos-
phoryl intermediate. Interestingly, this finding is inherently
different from that reported for the tungsten dithiolene
complexes where lack of a correlation with the redox
potential with the reaction rate, in part, led to the proposal
of a two step model for the overall reaction.25 In the tungsten
system, the metal center acts as an oxo-acceptor while in
the present case the metal center acts as an oxo-donor.
The next step in the reaction sequence is the coordina-
tion of solvent molecule replacing the coordinated phos-
phine oxide. In coordinating solvent such as acetonitrile,
the reaction proceeds as a first order process depending
only on the complex. For this step, a fairly large negative
entropy of activation (ΔS‡, -82.5 to -101.3 J/mol.K)
was observed. While the values are smaller than those in
The effect of the substituents on the reaction was
further understood through the Hammett relation. The
log(kIIR/kIIH) (where kIIR represents the rate of solvolysis
reaction with substituent R and kIIH represents the same
reaction with unsubstituted thiophenol) follows a linear
relation with the Hammett constant (eq 7).
logðkIIR=kIIHÞ ¼ Fσp ¼ 0:28 ð ( 0:04Þσp
ð7Þ
(68) Leung, D. H.; Bergman, R. G.; Raymond, K. N. J. Am. Chem. Soc.
2008, 130, 2798.

Article
Inorganic Chemistry, Vol. 48, No. 13, 2009 6313
A linear correlation (R2 = 0.96) indicates that all com-
plexes follow a similar mechanism with electron with-
drawing group increasing the reaction rate; the use of
electrophilic substitution constants (σp+) resulted in a
poor correlation. The slope of the line, F ∼ 0.28 indicates
negative charge build up, although modestly, and more
importantly suggests a nucleophilic ligand substitution
reaction.
The free energy of activation of the two steps are very
similar (Table 4); ΔG‡297 ∼ 91((4) kJ mol-1). This two-
step reaction profile can be depicted pictorially in Fig-
ure 6, which shows two transition states T1 and T2 and
one intermediate. The first transition state T1 primarily
involves a partial bond formation between phosphine and
one of the oxo ligand in [MoO2] unit. The second transi-
tion state T2 involves a partial solvent coordination to
give a seven coordinated environment, resulting in dis-
sociation of the phosphine oxide and formation of a
monooxo-MoIV solvent coordinated product.
Figure 7. Ratio of rate constants at 20 °C as a function of remote
substituent.
oxygen donor (Me2SO) to a desoxo-WIV analogue com-
plex [W(OPh)(S2C2Me2)2].23 The proposed intermediate
complex and nature of the transition states were described
from chemical principles and kinetic studies. In our
proposed profile, the intermediate complex involved in
the OAT reaction has been isolated, thoroughly charac-
terized, and in one case (R = H) the solvent coordinated
product has also been structurally characterized.41 The
nature of the transition states have been defined by
variable temperature kinetic measurements.
This study shows that electron withdrawing groups
(EWGs) and electron donating groups (EDGs) have
opposite effects in the electron transfer step, that is, the
first step of the reaction: the EDGs increase the ΔG‡ value
that decrease the rate of the reaction. In this case, the first
step becomes rate limiting. Similarly, EWGs decrease the
ΔG‡ value and increase the rate of reaction for the first
step which makes the second step rate determining.
Because the remote substituents affect the two steps of
the reaction differently, it is possible to alter the rate
limiting step in the overall reaction even with a single
remote substituent. Figure 7 shows the ratio of the rate
constant (kI/kII) at 20 °C which clearly indicates the
limiting step. For R = Me, both steps are equally
balanced, and with an electron donating group the second
step becomes rate limiting, while for electron withdrawing
groups the first step becomes rate limiting. In the present
case, we have used only one phosphine, PMe3, and it is
important to note that the basicity of the substrate also
influences the reaction such that the reaction rate is
accelerated with more basic phosphines.69 It is provoca-
tive to suggest that in biological systems hydrogen bond-
ing at the active site can play an important role in poising
the electron donation to the metal center. In the case of
pterin-containing molybdenum enzymes, another possi-
bility is the distortion of the cofactor either at the
dithiolene plane or at pyran ring modulating the electron
redistribution. In such cases, the rate limiting step of the
overall process can be altered. Thus, the redox potential
to be modulated by other means such as the hydrogen
bonding which is known to modulate the redox properties
of dioxo-MoVI centers.70
Conclusion
This study elucidates the structural and spectroscopic
properties of several dioxo-MoVI complexes coordinated by
a single thiophenol ligand, as a function of substituents at the
para position that are systematically changed. Many of the
complexes have similar molecular structures thus exhibiting
small structural change among the dioxo-MoVI complexes.
Reaction of the dioxo-MoVI complexes with PMe3 led to the
formation of monooxo-MoIV phosphoryl complexes which
were also characterized structurally, and spectroscopically.
The kinetics studies depict that the two-step OAT reaction
involves associated transition states in both of the steps.
Comparison of the rates from both steps shows that the first
step is more sensitive toward the substituent effects on the
thiophenolato ligand. A similar property has also been
extended to the redox potentials of dioxo and intermediate
complexes, where redox potentials of dioxo complexes are
more sensitive toward substituent effects than the phosphoryl
intermediate complexes. Thus, by changing the electronic
environment around the metal center one can affect the
activation barrier of the first step relative to that of the
second step.
Acknowledgment. The authors are grateful to the Na-
tional Institutes of Health (GM06155502) for financial
support.
A two step reaction profile similar to the one described
in here has been proposed for the OAT reaction from an
Supporting Information Available: Crystallographic data in
CIF format. Tables of characterization data, metric parameters,
and variable temperature rate constants, and ORTEP figures.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(69) Kail, B. W. Ph.D. dissertation, Duquesne University, Pittsburgh,
2006.
(70) Sengar, R. S.; Miller, J. J.; Basu, P. Dalton Trans. 2008, 2569.