Reactivity of Mo-Ot Terminal Bonds toward Substrates Having Simultaneous Proton- And Electron-Donor Properties: A Rudimentary Functional Model for Oxotransferase Molybdenum Enzymes

Article abstract of DOI:10.1021/ic960670z

In order to study the reactivity pattern of M_o-O_t bonds associated with anionic sulfur ligands, precursor complexes MoO₂L-D (H₂L = S-methyl 3-(2-hydroxyphenyl)methylenedithiocarbazate; D = CH₃OH (1), H₂O (2)) were synthesized. Complex 2 crystallizes in the orthorhombic space group P2₁2₁2₁, with a = 6.079(1) ?, b = 11.638(2) ?, c = 17.325(2) ?, V = 1225.7(4) ?³, and Z = 4. In its reaction with PhNHOH, 1 forms a seven-coordinate oxaziridine compound [MoO(η²-ONPh)L-CH₃OH] (3) by oxo-rearrangement (elimination-substitution) without a change in the molybdenum oxidation state. The crystal data for 3 are a = 9.573(3) ?, b = 9.859(2) ?, c = 10.604(3) ?, α = 95.90(2)°, β== 95.81(2)°, γ = 112.12(2)°, V = 911.5(4) ?³, Z = 2, and triclinic space group P1. In contrast to that of the precursor compound 1 (M_o-O_t = 1.700(4) ?), the terminal M_o-O_t distance in 3 (1.668(2) ?) is typical of Mo-O bonds of order 3, which drives the formation of an apparently unstable three-membered metallacycle via spectator oxo stabilization (Rappé, A. K.; Goddard, W. A., III. J. Am. Chem. Soc. 1982, 104, 448). With thioglycolic acid, 1 undergoes an oxo-transfer reaction through a coupled electron-proton transfer mechanism involving two steps, each having first-order dependence on H₂tga concentration. The system offers an interesting reactivity model for the oxo-transfer pathway of oxidoreductase Mo enzymes.

Full text of DOI:10.1021/ic960670z

Inorg. Chem. 1997, 36, 2517-2522
2517
Reactivity of Mo-Ot Terminal Bonds toward Substrates Having Simultaneous Proton- and
Electron-Donor Properties: A Rudimentary Functional Model for Oxotransferase
Molybdenum Enzymes§
Subodh Kanti Dutta,† David B. McConville,‡ Wiley J. Youngs,‡ and Muktimoy Chaudhury*,†
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India, and Department of Chemistry, University of Akron, Akron, Ohio 44325-3601
ReceiVed June 5, 1996X
In order to study the reactivity pattern of Mo-Ot bonds associated with anionic sulfur ligands, precursor complexes
MoO2L‚D (H2L ) S-methyl 3-(2-hydroxyphenyl)methylenedithiocarbazate; D ) CH3OH (1), H2O (2)) were
synthesized. Complex 2 crystallizes in the orthorhombic space group P212121, with a ) 6.079(1) Å, b ) 11.638-
(2) Å, c ) 17.325(2) Å, V ) 1225.7(4) Å3, and Z ) 4. In its reaction with PhNHOH, 1 forms a seven-coordinate
oxaziridine compound [MoO(η2-ONPh)L‚CH3OH] (3) by oxo-rearrangement (elimination-substitution) without
a change in the molybdenum oxidation state. The crystal data for 3 are a ) 9.573(3) Å, b ) 9.859(2) Å, c )
10.604(3) Å, R ) 95.90(2)°, â ) 95.81(2)°, γ ) 112.12(2)°, V ) 911.5(4) Å3, Z ) 2, and triclinic space group
P1h. In contrast to that of the precursor compound 1 (Mo-Ot ) 1.700(4) Å), the terminal Mo-Ot distance in 3
(1.668(2) Å) is typical of Mo-O bonds of order 3, which drives the formation of an apparently unstable three-
membered metallacycle Via spectator oxo stabilization (Rappe´, A. K.; Goddard, W. A., III. J. Am. Chem. Soc.
1982, 104, 448). With thioglycolic acid, 1 undergoes an oxo-transfer reaction through a coupled electron-
proton transfer mechanism involving two steps, each having first-order dependence on H2tga concentration. The
system offers an interesting reactivity model for the oxo-transfer pathway of oxidoreductase Mo enzymes.
MoVIO2L + S h MoIVOL + SO
MoIVOL + H2O h MoVIO2L + 2H+ + 2e
S + H2O h SO + 2H+ + 2e
(1)
(2)
(3)
Introduction
The pterin cofactors of oxotransferase molybdenum enzymes
have similar minimal structures.1,2 X-ray crystallographic
studies reported recently confirmed that these molecules possess
a reduced pyranopterin nucleus with a 6-alkyl side chain carrying
an enedithiolate functionality, chelated to a cis-MoOX (X ) O
or S) center.3,4 Under the constraints of protein microenviron-
ments, reactivity of the Mo-Ot terminal bond(s) of this cis-
MoOX moiety appears to be highly interesting.5 Perhaps a
unique combination of size and electronic structure in molyb-
denum plays a balancing role between the metal ion’s abilities
to abstract an oxo ligand or to lose it to a substrate, thus
to reactivate the enzymes. The net process is an oxygen atom
transfer reaction involving the elements of water (eq 3) with
the molybdenum oxidation state switching between +VI and
+IV Via a short-lived mononuclear EPR-active Mo(V) (S )
1/2) intermediate during the turnover.5a,8,9
This information has prompted several recent reports on
oxomolybdenum complexes of biomimetic ligands.7d,10-25 Nu-
merous reactions that replicate the primary oxygen atom transfer
catalyzing a host of biological oxo-transfer reactions.1,6
A
mechanistic model proposed for these catalytic actions considers
direct transfer of an oxygen atom from the molybdenum center
to the substrates7 (eq 1). This oxo-transfer step is immediately
followed by a coupled electron-proton transfer reaction (eq 2)
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† Indian Association for the Cultivation of Science.
‡ University of Akron.
§ Dedicated to the memory of Professor Nobumasa Kitajima.
X Abstract published in AdVance ACS Abstracts, April 15, 1997.
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S0020-1669(96)00670-2 CCC: $14.00 © 1997 American Chemical Society

2518 Inorganic Chemistry, Vol. 36, No. 12, 1997
Dutta et al.
acetylacetone), S-methyl dithiocarbazate,39 â-phenylhydroxylamine,40
and H2L41 were prepared as previously described. Carbon disulfide,
hydrazine hydrate (both from E. Merck), methyl iodide (Loba),
acetylacetone (Fluka), and salicylaldehyde (BDH) were freshly distilled
before use. All other chemicals were obtained commercially and used
as received.
step, both forward and reverse (eq 1), have been achieved with
many of these complexes using nitrate, sulfite, benzoin, and a
variety of phosphines, amine N-oxides, and sulfoxides as
substrates. In contrast, examples of model complexes that
regenerate cis-MoO22+ species Via a correlated electron-proton
transfer step (eq 2) have been few.26,27
Syntheses. [MoO2L‚CH3OH], 1. A methanolic solution (30 mL)
of [MoO2(acac)2] (0.65 g, 2 mmol) was added to a suspension of the
ligand (H2L) in an equimolar amount (0.45 g) in the same solvent (30
mL). The mixture was stirred for 2 h, resulting in a clear orange
solution. The volume of the solution was concentrated to ca. 10 mL,
after which orange crystals appeared during slow cooling in the air.
The solid was filtered off, washed with diethyl ether, and finally dried
under vacuum. The product was recrystallized from methanol.
Yield: 0.69 g (90%). Anal. Calcd for MoC10H12N2O4S2: C, 31.25;
H, 3.12; N, 7.29; Mo, 25.0. Found: C, 31.4; H, 3.2; N, 7.3; Mo, 24.8.
As part of a program relating to oxo-transfer reactivity of
Mo-Ot terminal bonds toward various electron- and/or proton-
donor reagents, we have reported the isolation and characteriza-
tion of a series of mononuclear molybdenum complexes in
biologically relevant oxidation states using derivatives of
dithiocarboxylic acids as (S,S)- donor ligands.28-31 Herein we
report an extension of this study with a tridentate ligand,
S-methyl 3-(2-hydroxyphenyl)methylenedithiocarbazate (H2L),
having (ONS) donor sites. cis-MoO2 complexes of tridentate
ligands have a distinct advantage as models for oxo-transfer
due to the presence of a labile or vacant coordination site for
potential binding of substrates.32,33 The work presented here
includes synthesis of the six-coordinated complexes cis-
[MoO2L‚D] (D: CH3OH, 1; H2O, 2) and the crystal structure
of 2. The reactivity of the terminal Mo-Ot bond toward
substrates having simultaneous proton- and electron-donor
properties, thioglycolic acid (H2tga) and â-phenylhydroxylamine
has been investigated.34,35 Two different kinds of reactivity are
observed, oxo-transfer with H2tga and oxo-rearrangement
(elimination-substitution) with PhNHOH. In the latter case,
the product has been identified by X-ray crystallographic
analysis. The kinetics of the oxo-transfer reaction with H2tga
have been investigated in detail.
IR (KBr pellet), cm-1: ν(C N) 1590 s; ν(C-O/phenolate) 1540 s;
‚ ‚
ν(ModO) 930 s, 900 s; ν(C-S) 642 m. UV-vis (CH3OH) [λmax/nm
(ꢀ/M-1 cm-1)]: 405 (3070), 344 (26 400), 317 (31 200).
[MoO2L‚H2O], 2. This compound was prepared by following the
same procedure as described for 1 using wet ethanol as solvent. It
was recrystallized from absolute ethanol. Yield: 88%. Anal. Calcd
for MoC9H10N2O4S2: C, 29.19; H, 2.70; N, 7.56; Mo, 25.95. Found:
C, 29.1; H, 2.6; N, 7.5; Mo, 25.5. IR (KBr pellet), cm-1: ν(OH) 3320
‚ ‚
b; ν(C N) 1590 s; ν(C-O/phenolate) 1545 s; ν(ModO) 940 s, 910 s;
ν(C-S) 642 m. UV-vis (CH3OH) [λmax/nm (ꢀ/M-1 cm-1)]: 402
(3000), 340 (25 300), 310 (32 000).
Diffraction-quality crystals were grown from absolute ethanol by
slow evaporation.
[MoO(η2-ONPh)L‚CH3OH], 3. To a stirred suspension of 1 (0.55
g, 1.45 mmol) in methanol (60 mL) was added a solid sample of
â-phenylhydroxylamine (0.17 g, 1.55 mmol) in portions. A darker clear
solution was obtained which was reduced to ca. 20 mL in volume on
a rotary evaporator. On standing at room temperature, the solution
yielded shining brown crystals. These were collected by filtration,
washed with Et2O, and dried under vacuum. Yield: 0.55 g (80%).
Anal. Calcd for MoC16H17N3O4S2: C, 40.42; H, 3.57; N, 8.84; Mo,
20.21. Found: C, 40.3; H, 3.6; N, 8.9; Mo, 20.2. IR (KBr pellet),
During the progress of this work, Bhattacharyya et al.36
2+
reported the synthesis and characterization of similar cis-MoO2
complexes of H2L and related ligands.
Experimental Section
All operations were carried out under purified dinitrogen. Reagent
grade solvents, dried and distilled by standard methods,37 were used in
all cases unless otherwise mentioned. [MoO2(acac)2]38 (Hacac )
cm-1: ν(C N) 1590 s; ν(C-O/ phenolate) 1540 s; ν(ModO) 975 s;
‚ ‚
ν(C-S) 640 m. UV-vis (CH3OH) [λmax/nm (ꢀ/M-1 cm-1): 433 (sh),
362 (10 900), 322 (18 200).
Diffraction-quality crystals were obtained from methanol by slow
evaporation.
(18) Oku, H.; Ueyama, N.; Kondo, M.; Nakamura, A. Inorg. Chem. 1994,
33, 209.
(19) Roberts, S. A.; Young, C. G.; Cleland, W. E., Jr.; Ortega, R. B.;
Enemark, J. H. Inorg. Chem. 1988, 27, 3044.
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108, 6992.
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Soc. 1992, 114, 6934.
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Yamanouchi, K.; Carducci, M. D.; Enemark, J. H. Inorg. Chem. 1990,
29, 3650.
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Trans. 1990, 387.
(25) Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1985, 107, 917.
(26) Xiao, Z.; Youngs, C. G.; Enemark, J. H.; Wedd, A. G. J. Am. Chem.
Soc. 1992, 114, 9194.
(27) Young, C. G.; Wedd, A. G. In Encyclopedia of Inorganic Chemistry;
King, R. B., Ed.; Wiley: New York, 1994; p 2330 and references
therein.
(28) Dutta, S. K.; Kumar, S. B.; Bhattacharyya, S.; Chaudhury, M. J. Chem.
Soc., Dalton Trans. 1994, 97.
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2169.
(30) Chaudhury, M. Inorg. Chem. 1985, 24, 3011.
(31) Chaudhury, M. Inorg. Chem. 1984, 23, 4434.
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(33) Craig, J. A.; Harlan, E. W.; Snyder, B. S.; Whitener, M. A.; Holm, R.
H. Inorg. Chem. 1989, 28, 2082.
[MoOL], 4. Method 1. To a stirred suspension of 1 (0.19 g, 0.5
mmol) in CH3OH (30 mL) was added a solution of thioglycolic acid
(H2tga) (0.1 g, ca. 1 mmol) in the same solvent (5 mL). Stirring was
continued for 2 h at room temperature, during which the color of the
reaction mixture changed to dark-brown from its initial orange shade.
It was filtered and the filtrate volume reduced to ca. 15 mL by rotary
evaporation. The dark microcrystalline compound obtained at this stage
was collected by filtration, washed with Et2O, and dried under vacuum.
Recrystallization was not possible due to the low solubility of this
compound in common organic solvents. Yield: 0.11 g (65%).
Method 2. A mixture of 1 (0.27 g, 0.7 mmol) and PPh3 (0.37 g,
1.4 mmol) in methanol (40 mL) was refluxed for 2 h, during which a
dark, shining microcrystalline product appeared. It was collected by
filtration, washed with Et2O, and dried under vacuum. Yield: 0.18 g
(76%). Anal. Calcd for MoC9H8N2O2S2: C, 32.14; H, 2.38; N, 8.33;
Mo, 28.57. Found: C, 31.8; H, 2.3; N, 8.1; Mo, 28.2. IR (KBr pellet),
cm-1: ν(C N) 1590 s; ν(C-O/phenolate) 1530 s; ν(ModO) 962 s;
‚ ‚
ν(C-S) 630 m. UV-vis (DMF) [λmax/nm (ꢀ/M-1 cm-1)]: 680 (sh),
471 (5600), 402 (sh), 365 (sh), 309 (11 800).
Physical Measurements. Details of IR and UV-visible measure-
ments have been described elsewhere.42 Elemental analyses for C, H,
and N were performed in this laboratory (IACS) with a Perkin-Elmer
(34) Martin, J. F.; Spence, J. T. J. Phys. Chem. 1970, 74, 3589.
(35) Thakuria, B. M.; Gupta, Y. K. J. Chem. Soc., Dalton Trans. 1975,
77.
(36) Bhattacharjee, S.; Bhattacharyya, R. J. Chem. Soc., Dalton Trans. 1993,
1151.
(37) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: Oxford, England, 1980.
(38) Chen, G. J.-J.; McDonald, J. W.; Newton, W. E. Inorg. Chem. 1976,
15, 2612.
(39) Bohr, G.; Sehleitzer, G. Z. Anorg. Allg. Chem. 1955, 280, 176.
(40) Vogel, A. I. A Text-Book of Practical Organic Chemistry, 3rd ed.;
Longman: London, 1973; p 629.
(41) Dutta, S. K.; Tiekink, E. R. T.; Chaudhury, M. Polyhedron, in press.

Reactivity of Mo-Ot Terminal Bonds
Inorganic Chemistry, Vol. 36, No. 12, 1997 2519
Table 1. Summary of Crystallographic Data
experiment was repeated at least three times, and the solutions were
maintained at a constant temperature ((0.1 °C) with a Haake F3
thermostated bath. Absorption changes at 425 nm during the course
of reaction were monitored by conventional spectrophotometry using
a Shimadzu UV-2100 spectrophotometer. All computation work for
the analyses of kinetic data were performed on an IBM PC 486
computer using a locally developed least-squares program.
2
3
formula
fw
space group
a, Å
C9H10N2O4S2Mo
370.25
P212121 (No. 19 )
6.079(1)
C16H17N3O4S2Mo
475.38
P1h (No. 2)
9.573(3)
9.859(2)
10.604(3)
95.90(2)
95.81(2)
112.12(2)
911.5(4)
2
b, Å
c, Å
11.638(2)
17.325(2)
Results and Discussion
R, deg
â, deg
γ, deg
V, Å3
Synthesis. The orange complex [MoVIO2L‚CH3OH], 1, is
obtained by the reaction of [MoO2(acac)2] with a stoichiometric
amount of H2L in methanol at room temperature. When wet
ethyl alcohol is used as solvent, the corresponding product is
[MoVIO2L‚H2O], 2. Both these complexes are soluble in
common organic solvents. In the solid state, 1 loses the
coordinated methanol slowly with simultaneous disappearance
of its shining nature. The product is probably an oxo-bridged
oligomer,46 revertible to its original mononuclear form when
recombined with dry methanol.
1225.7(4)
4
Z
T, K
140
0.710 73
2.006
14.17
171
140
λ(Mo KR), Å
0.710 73
1.728
F
calcd, g cm-3
µ, cm-1
9.76
236
no. of params refined
R1a (wR2b)
0.0143 (0.0371)
0.0257 (0.0641)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2.
2
w ) 1/[σ2(Fo ) + (dP)2 + eP], where P ) (Fo2 + 2Fc2)/3, d ) 0.0279
Reaction of 1 with â-phenylhydroxylamine in methanol (eq
4) involves an oxo-rearrangement process. The Mo(VI) product
and e ) 0.0000 for 2, and d ) 0.0326 and e )1.0823 for 3.
2400 elemental analyzer. Molybdenum content was estimated gravi-
metrically as the quinolin-8-olate.
X-ray Crystallography. The X-ray data collection on crystals of
2 and 3, both orange blocks of dimensions (0.40 × 0.30 × 0.18) and
(0.36 × 0.22 × 0.18 mm), respectively, was performed on a Syntex
P21 diffractometer equipped with graphite-monochromated Mo KR (λ
) 0.710 73 Å) radiation. Automatic centering and least-squares routines
were carried out on several reflections between 20 and 30° in 2θ (31
for complex 2 and 26 for complex 3) to obtain the cell dimensions.
The intensity data were collected at 140 K using the ω-scan method in
the range 4.2 e 2θ e 46.36° for 2; the corresponding range for 3 was
3.9 e 2θ e 47.82°. For compound 2, a total of 1444 (3409 for 3)
reflections were scanned, and of these, 1322 (2833 for 3) independent
reflections were used for structure solution and refinement. Three
standard reflections measured every 97 reflections showed no significant
fluctuation in intensity. The data were corrected for Lorentz and
polarization effects. A semiempirical absorption correction (transmis-
sion factors were in the ranges 0.624-0.731 for 2 and 0.762-0.846
for 3) from ψ scans was applied.
The structure of 2 was solved using the automated Patterson routine
in the SHELXTL PLUS package,43 while the structure of 3 was solved
by the direct methods program SIR 92.44 Both structures were refined
using SHELXL-9345 Via full-matrix least-squares calculations on F2.
All non-hydrogen atoms were refined with anisotropic thermal param-
eters, and hydrogen atoms were placed in calculated positions and
refined using a riding model. Refinement to convergence of 171
parameters against all 1322 unique data yielded wR2 ) 3.71%, GOF
) 1.054 (R1 ) 1.43% for I > 2.0σ(I)) for complex 2. For complex 3,
236 parameters were refined using all 2833 unique data to give wR2
) 6.41%, GOF ) 1.038 (R1 ) 2.57% for I > 2.0σ(I)). Conventional
R factors, R1, are based on F while weighted R factors, wR2, are based
on F2. Relevant crystallographic data and the details of final refinement
processes are provided in Table 1.
3, which is stable at room temperature, has two interesting
features. It contains a three-membered molybdoxaziridine47
(MoON) ring. The remaining Mo-Ot bond (spectator) appears
to be converted from a double to a triple bond, as suggested by
X-ray crystallographic analysis (Vide infra). Thus the seemingly
innocent spectator oxo group may be intimately involved in
stabilization of the three-membered metallacycle. This experi-
mental observation is in complete agreement with the predictions
of Rappe´ and Goddard from ab initio theoretical calculations.48
Compound 3 is soluble in methanol and CH2Cl2.
Treatment of 1 with H2tga (1:2 mole ratio) generates the
monooxo Mo(IV) complex [MoOL], 4, and dtga, the oxidized
form of thioglycolic acid34 by an oxo-abstraction reaction, shown
by eq 5. The kinetics and mechanism of this reaction have been
[MoVIO2L‚CH3OH] + 2H2tga f
[MoIVOL] + dtga + H2O + CH3OH (5)
examined in detail (Vide infra). Compound 4, once isolated,
fails to react with Lewis bases such as pyridine, picoline, or
imidazole, suggesting a polymeric structure, perhaps binuclear.49
This dark-brown Mo(IV) compound can also be prepared by
an alternative route (method 2) using PPh3 as the oxo abstractant.
It is sparingly soluble in common organic solvents except in
DMF in which it has only limited stability, thus eluding all
attempts at recrystallization.
Kinetics Measurements. All manipulations associated with the
kinetic measurements were performed under a dinitrogen atmosphere.
Approximately 10-4 M solutions of 1 in dry methanol were employed.
For each molybdenum concentration, experiments were carried out with
five or more different H2tga concentrations and the kinetics followed
for at least 3 half-lives. Pseudo-first-order conditions were used
throughout by maintaining the H2tga concentration between 20- and
60-fold molar excess over the molybdenum concentration. Each
The free ligand H2L exhibits a ν(CdS) vibration at 1040
cm-1, as well as ν(N-H) at ca. 2960 and ν(O-H) at 3100 cm-1
.
All of these suggest that the Schiff base ligand is in its thione
form.36 The disappearance of these bands and appearance of a
new band at 645-630 cm-1 due to the ν(C-S) stretch indicate
a tridentate ONS mode of coordination from the thioenol36 form
in the complexes. Compounds 1 and 2 both show two strong
(42) Bhattacharyya, S.; Kumar, S. B.; Dutta, S. K.; Tiekink, E. R. T.;
Chaudhury, M. Inorg. Chem. 1996, 35, 1967.
(43) SHELXTL PLUS; Siemens Analytical X-ray Instruments, Inc.: Madi-
son, WI, 1994.
(46) Rajan, O. A.; Chakravorty, A. Inorg. Chem. 1981, 20, 660.
(47) Liebeskind, L. S.; Sharpless, K. B.; Wilson, R. D.; Ibers, J. A. J. Am.
Chem. Soc. 1978, 100, 7061.
(44) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(45) Sheldrick, G. M. SHELXL-93: Program for the Refinement of Crystal
Structures. University of Go¨ttingen, Germany, 1993.
(48) Rappe´, A. K.; Goddard, W. A., III. J. Am. Chem. Soc. 1982, 104,
448.
(49) Caradonna, J. P.; Harlan, E. W.; Holm, R. H. J. Am. Chem. Soc. 1986,
108, 7856.

2520 Inorganic Chemistry, Vol. 36, No. 12, 1997
Dutta et al.
Table 3. Selected Bond Distances and Angles for 3
Distances (Å)
Mo-O(4)
Mo-O(2)
Mo-N(3)
Mo-O(1)
Mo-N(1)
Mo-O(3)
Mo-S(1)
1.965(2)
2.006(2)
2.091(2)
1.668(2)
2.180(2)
2.321(2)
2.417(1)
S(1)-C(8)
S(2)-C(8)
O(2)-C(1)
O(4)-N(3)
N(1)-C(7)
N(1)-N(2)
N(3)-C(11)
1.739(3)
1.743(3)
1.330(4)
1.386(3)
1.298(4)
1.417(3)
1.442(4)
Angles (deg)
O(1)-Mo-O(4)
O(1)-Mo-O(2)
O(4)-Mo-O(2)
O(1)-Mo-N(3)
O(4)-Mo-N(3)
O(2)-Mo-N(3)
O(1)-Mo-N(1)
O(4)-Mo-N(1)
O(2)-Mo-N(1)
N(3)-Mo-N(1)
O(1)-Mo-O(3)
O(4)-Mo-O(3)
O(2)-Mo-O(3)
N(3)-Mo-O(3)
N(1)-Mo-O(3)
O(1)-Mo-S(1)
100.5(1)
102.4(1)
78.07(9)
103.3(1)
39.81(9)
115.71(9)
92.12(9)
159.75(9)
83.81(9)
151.0(1)
174.24(9)
83.80(8)
74.57(9)
82.42(9)
82.72(8)
97.66(8)
O(4)-Mo-S(1)
O(2)-Mo-S(1)
N(3)-Mo-S(1)
N(1)-Mo-S(1)
O(3)-Mo-S(1)
C(8)-S(1)-Mo
C(1)-O(2)-Mo
C(10)-O(3)-Mo
N(3)-O(4)-Mo
C(7)-N(1)-Mo
N(2)-N(1)-Mo
O(4)-N(3)-C(11)
O(4)-N(3)-Mo
C(11)-N(3)-Mo
C(12)-C(11)-N(3)
C(16)-C(11)-N(3)
115.89(7)
153.01(7)
76.24(7)
77.53(7)
83.79(6)
98.7(1)
132.1(2)
130.5(2)
75.0(1)
125.3(2)
123.1(2)
112.0(2)
65.2(1)
117.2(2)
123.7(3)
116.3(3)
Figure 1. Molecular structure and crystallographic numbering scheme
for the complex [MoO2L‚H2O], 2.
Mo-O(4) angle of 169.69(9)°. Thus the MoO2 moiety has a
cis configuration with conventional50 average values for the
Mo-O(2)/O(3) distance (1.70 Å) and O(2)-Mo-O(3) bond
angle (105.9(1)°), which is close to the energy-minimum value
estimated from MO theory.51 The Mo-O(4) bond trans to the
terminal oxygen atom O(2) has been significantly lengthened
(2.308(2) Å) compared to the Mo-O(1) distance (1.928(2) Å)
occupying a cis position, indicating that the H2O molecule is
weakly coordinated to the metal center.
Figure 2. Molecular structure and crystallographic numbering scheme
for the complex [MoO(ONPh)L‚CH3OH], 3.
Table 2. Selected Bond Distances and Angles for 2
Distances (Å)
Mo-O(3)
Mo-O(2)
Mo-O(1)
Mo-N(1)
Mo-O(4)
Mo-S(1)
1.697(2)
1.702(2)
1.928(2)
2.275(3)
2.308(2)
2.4403(9)
S(1)-C(8)
S(2)-C(8)
O(1)-C(1)
N(1)-C(7)
N(1)-N(2)
N(2)-C(8)
1.740(3)
1.739(3)
1.344(4)
1.286(4)
1.406(4)
1.291(4)
The molecular structure of 3 confirms seven-coordination with
an η2-disposition of the ONPh group around molybdenum. The
MoO4N2S core defines a distorted pentagonal-bipyramidal
geometry with the planar ONS donor ligand displaying mer
coordination along with the O(4) and N(3) atoms of the ONPh
group (Figure 2). Such a coordination geometry is a common
feature for seven-coordinate monooxomolybdenum(VI) com-
plexes.47,52-54 In this description, the terminal oxo ligand O(1)
occupies one of the axial positions, the other being occupied
by an oxygen atom, O(3), of a coordinated methanol molecule.
N(3), O(4), O(2), N(1), and S(1) define the pentagonal plane
about the Mo atom and lie -0.031(2), 0.105(2), -0.106 (1),
0.085(1), and -0.053(1) Å, respectively, out of the weighted
least-squares plane through them. The Mo atom is 0.323(1) Å
above this plane in the direction of the O(1) atom. Replacement
of a strongly π-donating terminal oxo ligand in 2 by the η2-
ONPh moiety drastically reduces the bond length between Mo
and the spectator oxygen O(1) in 3 from 1.700(4) to 1.668(2)
Å. The latter is typical of Mo-O bonds of order 3.55 The Mo-
O(3) axial bond (2.321(2) Å) is much longer, indicating the
strong trans-labilizing influence of the Mo-O(1) terminal bond.
The Mo-N(1) distance (2.180(2) Å) in 3, on the other hand, is
shortened compared to the corresponding distance (2.275(3) Å)
in the parent compound 2, another effect of this oxo-rearrange-
Angles (deg)
O(3)-Mo-O(2)
O(3)-Mo-O(1)
O(2)-Mo-O(1)
O(3)-Mo-N(1)
O(2)-Mo-N(1)
O(1)-Mo-N(1)
O(3)-Mo-O(4)
O(2)-Mo-O(4)
O(1)-Mo-O(4)
N(1)-Mo-O(4)
105.9(1)
104.04(9)
99.3(1)
O(3)-Mo-S(1)
O(2)-Mo-S(1)
O(1)-Mo-S(1)
N(1)-Mo-S(1)
O(4)-Mo-S(1)
C(8)-S(1)-Mo
C(1)-O(1)-Mo
C(7)-N(1)-Mo
N(2)-N(1)-Mo
N(2)-C(8)-S(1)
91.12(7)
97.01(8)
153.65(7)
75.86(7)
80.79(6)
100.5(1)
133.6(2)
123.6(2)
122.8(2)
126.7(2)
158.81(9)
92.4(1)
82.88(9)
84.27(9)
169.69(9)
79.46(8)
77.26(9)
bands at ca. 930 and 900 cm-1, assigned to symmetric and
antisymmetric vibrations, respectively, of the cis-MoO2 core.
As expected, compounds 3 and 4 each show only one such
terminal ν(ModO) absorption at 975 and 962 cm-1, respec-
tively.
Description of the Crystal Structures. The molecular
structures and the atom-numbering schemes for compounds 2
and 3 are shown in Figures 1 and 2, respectively. Selected bond
lengths and angles are given in Tables 2 and 3. Complex 2
has a distorted octahedral geometry (Figure 1) with one nitrogen,
one sulfur, and four oxygen atoms. Three atoms, O(1), N(1),
and S(1) (from the tridentate ligand), and one terminal oxo atom,
O(3), occupy the meridional plane and lie -0.092(1). -0.028-
(1), -0.065(1), and -0.071(1) Å, respectively, out of the least-
squares plane through them; the Mo atom lies 0.256(1) Å out
of this plane in the direction of the O(2) atom, the other terminal
oxygen. Both O(2) and O(4), the latter from the coordinated
water molecule, occupy the axial positions and form an O(2)-
(50) Ueyama, N.; Oku, H.; Kondo, M.; Okamura, T.; Yoshinaga, N.;
Nakamura, A. Inorg. Chem. 1995, 35, 643.
(51) Tatsumi, K.; Hoffmann, R. Inorg. Chem. 1980, 19, 2656.
(52) Dirand, J.; Ricard, L.; Weiss, R. J. Chem. Soc., Dalton Trans. 1976,
278.
(53) Schlemper, E. O.; Schrauzer, G. N.; Hughes, L. A. Polyhedron 1984,
3, 377.
(54) Bristow, S.; Enemark, J. H.; Garner, C. D.; Minelli, M.; Morris, G.
A.; Ortega, R. B. Inorg. Chem. 1985, 24, 4070.
(55) Cotton, F. A.; Wing, R. M. Inorg. Chem. 1965, 4, 867.

Reactivity of Mo-Ot Terminal Bonds
Inorganic Chemistry, Vol. 36, No. 12, 1997 2521
Figure 3. Spectral changes in the reaction of [MoO2L‚CH3OH], 1 (4.0
× 10-4 M), with H2tga (2.8 × 10-2 M) in methanol at 10 °C. Spectra
were recorded every 2 min over a period of 14 min.
ment reaction. The equatorial plane of this molecule and that
of the phenyl ring of the ONPh group are not coplanar, as
indicated by the Mo-O(4)-N(3)-C(11) torsion angle of
110.8°.
Figure 4. Kinetics plot for the reaction of [MoO2L‚CH3OH] (4.22 ×
10-4 M) with H2tga (1.95 × 10-2 M) at 10 °C, illustrating the biphasic
nature of the reaction. Consecutive observed rate constants kIobs (inset)
Electronic Spectra. The electronic spectra of 1 and 2 in
methanol are relatively simple, each featuring a single ligand-
to-metal S(π) f Mo(dπ) charge transfer band in the near-UV
region at 405 (3070) and 402 nm (3000 M-1 cm-1), respectively.
The corresponding band for the oxaziridine molecule 3 appears
as a shoulder at 433 nm. For the Mo(IV) compound 4, the
spectrum recorded in degassed DMF appears to be a little more
complicated by the presence of multiple absorptions.7d,19,56
Following the ligand-field classification, it would be reasonable
to assign the low-energy shoulder at 680 nm to a d-d transition
(ꢀ 138 M-1 cm-1), while the remaining two high-intensity bands
at 471 nm (5600 M-1 cm-1) and 402 nm (sh) are probably due
to metal-ligand charge transfers. All the reported complexes
have two UV bands appearing near or below 360 nm due to
ligand internal transitions.
and kII were obtained from the slopes.
obs
6 and 7 give the rate law dependencies for the two stages, where
kIobs ) k1[H2tga ] + k-1
kIIobs ) k2[H2tga ] + k-2
(6)
(7)
k1, k2 are the rates of forward reactions and k-1, k-2, their
backward contributions. The rate constants are calculated from
the slopes and intercepts of the lines shown in Figure 5: k1 )
(72.08 ( 4.98) × 10-3 M-1 s-1, k2 ) (39.00 ( 3.60) × 10-3
M-1 s-1, k-1 ) (1.11 ( 0.09) × 10-3 s-1, and k-2 ) (0.56 (
0.07) × 10-3 s-1 at 10 °C. Corresponding values at 25 °C are
(218.36 ( 8.94) × 10-3 M-1 s-1, (129.84 ( 6.94) × 10-3 M-1
Kinetics Studies: Oxygen Atom Transfer from [MoO2L‚-
CH3OH]. The kinetics of reaction 5 were investigated in dry
methanol under pseudo-first-order conditions by conventional
spectrophotometry. At 25 °C, the reaction was followed to ca.
3 half-lives. The absorption band at 405 nm in 1 decays with
time at the expense of a new band at 452 nm. The same
reaction, when followed at 10 °C up to 1 half-life, generates a
tight isosbestic point at 382 nm with the peak position at 425
nm (Figure 3), consistent with the involvement of only two
components under these conditions (Vide infra). Pseudo-first-
order plots of log(A∞ - At) against time (the symbols have their
usual significance) indicate biphasic kinetics and are reminiscent
of what is expected for a consecutive reaction scheme.57 As
shown in Figure 4, these plots can be resolved into two straight
lines according to the method of Weyh and Hamm.58 The
straight-line segment corresponding to the longer time span
when extrapolated to zero time, gives an intercept (Ae) that
accounts for the contribution of the second step relative to the
earlier one. The observed rate constant for the second-step
reaction (kIIobs) is obtained from the slope of the extrapolated
line, and that of the first step (kIobs) is determined by least-
squares analysis of the line obtained from the logarithmic
variation of corrected absorbances [(A∞ - At) - Ae] versus time
(Figure 4, inset).
s-1, (2.76 ( 0.15) × 10-3 s-1, and (1.75 ( 0.11) × 10-3 s-1
,
respectively. The observed values at 25 °C are approximately
3 times higher than those at 10 °C.
The overall reaction thus proceeds in two consecutive steps.
Equations 8-10 provide a consistent interpretation. The first
step is a substitution reaction involving replacement of the
[LMoVIO2‚CH3OH ] + H2tga yk1z
k-1
[ LMoVIO2‚(Htga) ]- + CH3OH + H+ (8)
[LMoVIO2‚(Htga) ]- + H2tga yk2z
k-2
[LMoIVO] + OH- + dtga (9)
fast
2[LMoIVO] 8 [LMoIVO ]2
(10)
coordinated methanol by a thioglycolate anion to generate a
dioxomolybdenum(VI) species [LMoVIO2‚(Htga)]- with sulfur
from Htga- being the likely donor atom. The onset of this
replacement process is clearly observable (Figure 3) in the form
of an isosbestic point at 10 °C up to ca. 1 half-life of the reaction
beyond which the second step of the consecutive process
becomes operative.
In the next step (eq 9), a second H2tga molecule generates
the Mo(IV) product [LMoIVO] by an oxo-transfer reaction
coupled to proton-electron transfers. The one-step two-electron
Mo(VI)/Mo(IV) reduction is balanced electronically by the
oxidation of 2 equiv of thiol to form the disulfide product (dtga).
This crucial oxo-transfer step is particularly operative at 25 °C
These observed rates (kIobs and kIIobs) have first-order depen-
dence on substrate concentration [H2tga] when present in large
excess (Figure 5; see also Supporting Information). Equations
(56) Laughlin, L. J.; Young, C. G. Inorg. Chem. 1996, 35, 1050.
(57) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition
Metal Complexes, 2nd ed.; VCH: New York, 1991.
(58) Weyh, J. A.; Hamm, R. E. Inorg. Chem. 1969, 8, 2298.

2522 Inorganic Chemistry, Vol. 36, No. 12, 1997
Dutta et al.
which is reluctant to undergo a comproportionation reaction with
available Mo(VI) starting material. In fact, we have been
successful in isolating the oxomolybdenum(IV) compound 4
from the cis-dioxo Mo(VI) precursor 1 by oxo abstraction in
the presence of either H2tga or PPh3. In addition to its
characteristic electronic spectral properties,6a,19 4 in an anaerobic
medium is extremely inert to addition or substitution type
reactions, which suggests a ligand-bridged structure (Vide supra)
for this compound with sufficient geometrical constraints to form
a µ-oxo Mo(V) dimer. Bhattacharyya et al. in a recent report36
identified 4 as a Mo(IV) compound with a polymeric structure.
Holm et al.,6a following the structure determination of an
analogous zinc(II) complex,63 proposed a similar thiolato-
bridged dimeric structure as a viable alternative for the molyb-
denum(IV) complex [MoO(L-NS2)(DMF)] (L-NS2 ) 2,6-
bis(2,2-diphenyl-2-sulfidoethyl)pyridinate(2-)), which is known
to hinder µ-oxo Mo(V) dimer formation.25
Figure 5. Dependence of observed rate constants kIobs (b) and kII
(4) on [H2tga] at 25 °C.
obs
or higher temperature. At 10 °C, however, it occurs after the
first half-life of the reaction, as previously mentioned. Finally,
the monomeric Mo(IV) product is rapidly converted to a stable
condensed form, perhaps binuclear (eq 10).
Concluding Remarks
Oxo-transfer from cis-dioxomolybdenum(VI) compounds of
sulfur ligands to oxomolybdenum(IV) products by arenethiols
is known.49,59 Comparatively less acidic aliphatic thiols, on the
other hand, are capable of such reduction only in the presence
of extraneous protons,60 which presumably promote the rate of
the oxo-transfer process through protonation of a terminal oxo
ligand. Although H2tga contains a much less acidic (pK2 )
9.78) aliphatic thiol group, it is the relatively stronger carboxylic
acid component (pK1 ) 3.58)61 that seems to serve as the source
of protons needed to increase the electrophilicity of the
molybdenum center, thus boosting the reduction process.
Martin and Spence34 studied the oxidation of H2tga by the
Mo(VI) ion in aqueous media in a restricted pH range and found
the reaction to proceed in two steps. Mo(VI) is reduced first
to a µ-oxo dimeric Mo(V)-tga complex and then to a stable
Mo(IV) product of unknown composition. Under the present
set of experimental conditions, the reaction of 1 with H2tga
appears to proceed through a Mo(VI) intermediate to an
oxomolybdenum(IV) product. It is now accepted, almost as a
general feature, that oxo-transfer reactions such as (1) are
accompanied by the formation of a µ-oxo Mo(V) dimer, either
in an irreversibe reaction or in an equilibrium process unless
impeded by steric or geometrical restraints.6,62 In the absence
of a ligand with sufficient steric bulk, our present system
presents an interesting example involving a Mo(IV) product
Two substrates PhNHOH and H2tga, both known to function
simultaneously as proton and electron donors,34,35 have been
used to study the reactivity pattern of Mo-Ot bonds associated
with an anionic sulfur ligand. With PhNHOH, the product of
the reaction with 1 is a seven-coordinate oxaziridine Mo(VI)
compound 3, formed by the rearrangement of one terminal oxo
ligand. The Mo-Ot distance involving the remaining (spectator)
oxo ligand O(1) in 3 is drastically reduced to 1.668(2) Å, typical
of Mo-O bonds of order 3, and can be explained as being due
to the one σ and two π bonds which the spectator oxo ligand is
allowed to form on removal of the other oxygen atom from the
cis-MoO2 moiety.55,64 This observation is a classic example of
spectator oxo stabilization of a strained metallacycle as predicted
by Rappe´ and Goddard.48 With H2tga, on the other hand, an
oxygen atom transfer reaction proceeds through coupled electron-
proton transfer steps that successfully model the turnover
reaction of oxidoreductase Mo enzymes.
Acknowledgment. We wish to thank Professor P. Banerjee
for helpful discussions. Financial assistance received from the
Council of Scientific and Industrial Research, New Delhi, is
gratefully acknowledged.
Supporting Information Available: Tables of atomic coordinates,
anisotropic thermal parameters, bond distances and angles, least-squares
planes, torsional angles, and kinetics data (kobs vs [H2tga]) (17 pages).
Ordering information is given on any current masthead page. Structure
factor amplitudes are available from the authors on request.
(59) McDonald, J. W.; Corbin, J. L.; Newton, W. E. Inorg. Chem. 1976,
15, 2056.
(60) Llopis, E.; Domenech, A.; Ramirez, T. A.; Cervilla, A.; Palanca, P.;
Picher, T.; Sanz, V. Inorg. Chim. Acta 1991, 189, 29.
(61) Stability Constants of Metal Ion Complexes; Sillen, L. G., Martell, A.
E., Eds.; Special Publication No. 17; The Chemical Society: London,
1964; p 376.
IC960670Z
(62) (a) Subramanian, P.; Spence, J. T.; Ortega, R.; Enemark, J. H. Inorg.
Chem. 1984, 23, 2564. (b) Spence, J. T.; Minelli, M.; Kroneck, P. J.
Am. Chem. Soc. 1980, 102, 4538.
(63) Kaptein, B.; Wang-Griffin, L.; Barf, G.; Kellogg, R. M. J. Chem. Soc.,
Chem. Commun. 1987, 1457.
(64) Schro¨der, F. A. Acta Crystallogr. 1975, B31, 2294.