Reaction systems related to dissimilatory nitrate reductase: Nitrate reduction mediated by bis(dithiolene)tungsten complexes

Article abstract of DOI:10.1021/ic040109y

Kinetics of the oxygen atom transfer reactions [M^IV(QC ₆H₂-2,4,6-Prⁱ₃)(S₂C ₂Me₂)₂]^1- + XO → [M ^VIO(QC₆H₂-2,4,6-Prⁱ3)(S ₂C₂Me₂)₂]^1- + X in acetonitrile with substrates XO = NO₃^- and (CH ₂)₄SO have been determined. The reactants are bis(dithiolene) complexes with M = Mo, W and sterically encumbered axial ligands with Q = O, S to stabilize mononuclear square pyramidal structures. The complex [Mo^IV(SC₆H₂-2,4,6-Prⁱ ₃)(S₂C₂Me₂)₂] ^1- is an analogue of the active site of dissimilatory nitrate reductase which in the reduced state contains a molybdenum atom bound by two pyranopterindithiolene ligands and a cysteinate residue. Nitrate reduction was studied with tungsten complexes because of unfavorable stability properties of the molybdenum complexes. Product nitrite was detected by a colorimetric method. All reactions with both substrates are second-order with associative transition states (δS? ? -20 eu). Variation of atoms M and Q, together with data from prior work, allows certain kinetics comparisons to be made. Among them, k₂^W/k₂^Mo = 25 for (Ch ₂)₄SO reduction (Q = S), an expression of the kinetic metal effect. Further, k₂^S/k₂^O = 28 and ?10⁴ for nitrate and (CH₂)₄SO reduction, respectively, effects attributed to relatively more steric congestion in achieving the transition state with hindered phenolate vs thiolate ligands. The effect is more pronounced with the larger substrate. These results demonstrate the feasibility of tungsten-mediated nitrate reduction by direct atom transfer using molecules with both axial thiolate and phenolate ligands. Complexes of the type [M^IV(OR)(S₂C₂Me ₂)₂] are capable of reducing biological N-oxide, S-oxide, and nitrate substrates and thus constitute functional analogue reaction systems of enzymic transformations.

Full text of DOI:10.1021/ic040109y

Inorg. Chem. 2005, 44, 1068−1072
Reaction Systems Related to Dissimilatory Nitrate Reductase: Nitrate
Reduction Mediated by Bis(dithiolene)tungsten Complexes
Jianfeng Jiang and R. H. Holm*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138
Received September 15, 2004
-
Kinetics of the oxygen atom transfer reactions [MIV(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1
+ XO f
[MVIO(QC6H2-2,4,6-
Pri3)(S2C2Me2)2]1
+ X in acetonitrile with substrates XO )
NO3- and (CH2)4SO have been determined. The reactants
-
are bis(dithiolene) complexes with M
)
Mo, W and sterically encumbered axial ligands with Q
)
O, S to stabilize
mononuclear square pyramidal structures. The complex [MoIV(SC6H2-2,4,6-Pri3)(S2C2Me2)2]1 is an analogue of the
active site of dissimilatory nitrate reductase which in the reduced state contains a molybdenum atom bound by two
pyranopterindithiolene ligands and a cysteinate residue. Nitrate reduction was studied with tungsten complexes
because of unfavorable stability properties of the molybdenum complexes. Product nitrite was detected by a
-
colorimetric method. All reactions with both substrates are second-order with associative transition states (
∆
S‡
∼
−
20 eu). Variation of atoms M and Q, together with data from prior work, allows certain kinetics comparisons to
Mo
be made. Among them, k2W/k2
)
25 for (CH2)4SO reduction (Q
) S), an expression of the kinetic metal effect.
Further, k2S/k2
)
28 and
∼
104 for nitrate and (CH2)4SO reduction, respectively, effects attributed to relatively
O
more steric congestion in achieving the transition state with hindered phenolate vs thiolate ligands. The effect is
more pronounced with the larger substrate. These results demonstrate the feasibility of tungsten-mediated nitrate
reduction by direct atom transfer using molecules with both axial thiolate and phenolate ligands. Complexes of the
type [MIV(OR)(S2C2Me2)2] are capable of reducing biological N-oxide, S-oxide, and nitrate substrates and thus
constitute functional analogue reaction systems of enzymic transformations.
Introduction
(and others) in second-order reactions that proceed through
an associative transition state.7,8 Leading aspects of this
research are summarized elsewhere.4
In elaborating bioinorganic aspects of molybdenum and
tungsten chemistry,1-4 we have utilized ene-1,2-dithiolates
as structural and electronic simulators of the pyranopterin-
dithiolene cofactor ligand (S2pd) in Figure 1. This research
has afforded, among other complexes, square pyramidal
desoxo [MIV(OR)(S2C2Me2)2]1- (M ) Mo, W) and distorted
octahedral monooxo [WVIO(OR)(S2C2Me2)2]1-. The molyb-
denum complexes are analogues of the reduced sites of the
DMSOR family under the Hille classification.5,6 Using the
minimal reaction paradigm MIV + XO f MVIO + X,
functional analogue systems have been developed which
reduce the biological substrates XO ) Me3NO and Me2SO
The scope of atom transfer reactions mediated by the
foregoing molybdenum and tungsten complexes with both
biological and abiological substrates is a matter of continuing
interest. Very recently, we have demonstrated related mo-
lybdenum- and tungsten-mediated bimolecular sulfur atom
transfer reactions.9 In addition to DMSOR and trimethyl-
amine N-oxide reductase, the DMSOR family includes
dissimilatory nitrate reductase.5 These enzymes catalyze
reaction 1 and are found in certain bacteria that have the
ability to grow on nitrate and reduce it for respiration. The
structure of NiR from DesulfoVibrio desulfuricans has been
solved at 1.9 Å resolution.10 The oxidized active site is
revealed as [MoVI(OH/OH2)(S‚Cys)(S2pd)2], and the desoxo
(1) Holm, R. H. Coord. Chem. ReV. 1990, 100, 183-221.
(2) Enemark, J. H.; Young, C. G. AdV. Inorg. Chem. 1994, 40, 1-88.
(3) McMaster, J.; Tunney, J. T.; Garner, C. D. Prog. Inorg. Chem. 2004,
52, 539-583.
(4) Enemark, J. H.; Cooney, J. J. A.; Wang, J.-J.; Holm, R. H. Chem.
ReV. 2004, 104, 1175-1200.
(5) Hille, R. Chem. ReV. 1996, 96, 2757-2816.
(6) Abbreviations are given in Chart 1.
(7) Lim, B. S.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920-1930.
(8) Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1931-1943.
(9) Wang, J.-J.; Kryatova, O.; Rybak-Akimova, E. V.; Holm, R. H. Inorg.
Chem. 2004, 43, 8092.
1068 Inorganic Chemistry, Vol. 44, No. 4, 2005
10.1021/ic040109y CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/28/2005

Nitrate Reduction by Bis(dithiolene)tungsten
on the kinetics of nitrate reduction. Although we are unaware
of any tungsten-containing NiR, we utilize tungsten because
of its generally better stability in reducing ligand environ-
ments such as that in bis(dithiolene) complexes. Further, in
this and much of our past work we have attempted to develop
molybdenum and tungsten dithiolene chemistry in parallel,
the principal motivation being the existence of Mo/W
isoenzymes.17
Experimental Section
Preparation of Compounds. All reactions and manipulations
were performed under a pure dinitrogen atmosphere using either
modified Schlenk techniques or an inert atmosphere box. Solvents
were passed through an Innovative Technology solvent purification
system prior to use. Tetramethylenesulfoxide ((CH2)4SO) was
vacuum-distilled before use. (Bu4N)(NO3) was a commercial sample
(Aldrich) and was used as received. (Et4N)(NO2)18 was recrystal-
lized from acetonitrile/ether. The compounds (Et4N)(SC6H2-2,4,6-
Pri3) and (Et4N)(OC6H2-2,4,6-Pri3) were prepared from equimolar
HSC6H2-2,4,6-Pri319 or HOC6H2-2,4,6-Pri314 with 25% Et4NOH in
methanol and were isolated as colorless crystalline solids from
acetonitrile/ether. The compounds (Et4N)[M(SC6H2-2,4,6-Pri3)(S2C2-
Me2)2] (M ) Mo,14 W15) were prepared as described.
(Et4N)[Mo(OC6H2-2,4,6-Pri3)(S2C2Me2)2]. To a suspension of
72 mg (0.19 mmol) of [Mo(CO)2(S2C2Me2)2]20 in 2 mL of
acetonitrile was added a solution of 65 mg (0.19 mmol) (Et4N)-
(OC6H2-2,4,6-Pri3) in 1 mL of acetonitrile. The mixture was stirred
for 15 h to give a red-brown solution which was filtered through
Celite. The solvent was removed. The residue was washed with
ether (3 × 15 mL) and extracted with 1 mL of 1:4 (v/v) acetonitrile/
THF; the extract was covered with 60 mL of ether. A red-brown
crystalline solid appeared upon allowing the mixture to stand for 3
d. The solid was collected, washed with ether (3 × 5 mL), and
dried to afford the product as 69 mg (54%) of a brown micro-
crystalline solid. Absorption spectrum (acetonitrile): λmax (ꢀM) 331
(13000), 415 (4000), 570 (580), 729 (150) nm. 1H NMR (CD3CN,
anion): δ 1.04 (d, 12), 1.14 (d, 6), 2.53 (s, 12), 2.88 (m, 1), 3.24
(m, 2), 6.65 (s, 2).
(Et4N)[W(OC6H2-2,4,6-Pri3)(S2C2Me2)2]. To a suspension of
112 mg (0.24 mmol) of [W(CO)2(S2C2Me2)2]20 in 2 mL of
acetonitrile was added a solution of 82 mg (0.24 mmol) (Et4N)-
(OC6H2-2,4,6-Pri3) in 1 mL of acetonitrile. The mixture became
green-brown immediately with accompanying gas evolution. Sol-
vent was removed after stirring for 2 h. The residue was washed
with ether (3 × 15 mL) and extracted with 2 mL of 1:4 (v/v)
acetonitrile/THF; the extract was covered with 60 mL of ether.
Crystalline solid appeared after allowing the mixture to stand for 1
d. The solid was collected, washed with ether (3 × 5 mL), and
dried to afford the product as 79 mg (44%) of a green-brown
microcrystalline solid. Absorption spectrum (acetonitrile): λmax (ꢀM)
295 (15000), 319 (sh, 11000), 355 (5700), 394 (sh, 2400), 464 (950),
530 (200) nm. 1H NMR (CD3CN, anion): δ 1.00 (d, 12), 1.11 (d,
6), 2.56 (s, 12), 2.80 (m, 1), 3.19 (m, 2), 6.71 (s, 2).
Figure 1. Schematic depictions of the reduced and oxidized sites of
dissimilatory nitrate reductase (DesulfoVibrio desulfuricans) and the
proposed reaction pathway. Also shown is the pyranopterindithiolate cofactor
(R absent or a nucleotide).
site [MoIV(S‚Cys)(S2pd)2] has been proposed to intervene in
catalysis. The situation is set out in Figure 1, where the
NO3- + 2H+ + 2e- h NO2- + H2O
(1)
oxidized site is presented in the deprotonated (oxo) form
accessible in synthetic complexes. The structure of a second
dissimilatory enzyme has been crystallographically estab-
lished. NiR A from Escherichia coli contains a [Mo(O2-
C‚Asp)(S2pd)2] site with an unsymmetrically coordinated
carboxylate group.11 These sites bind two pyranopterindithi-
olene ligands, one of the defining features of the DMSOR
family, but differ in the protein-based ligand between each
other and with other members of the family. In DMSOR
and TMAOR,12,13 the oxidized (deprotonated) site is
[MoVIO(O‚Ser)(S2pd)2], devoid of cysteinate or carboxylate
ligation.
With the availability by synthesis of the reactant complexes
[MIV(SR)(S2C2Me2)2]1- (M ) Mo,7,14 W15) and their oxida-
tion products [WVIO(SR)(S2C2Me2)2]1-,15 an examination of
nitrate reduction by oxygen atom transfer in analogue
reaction systems with thiolate ligation becomes feasible. In
addition, the reactant complexes [MIV(OR)(S2C2Me2)2]1-
(M ) Mo,14 W16) and [WVIO(OR)(S2C2Me2)2]1- 8 admit
examination of the effect of sulfur vs oxygen axial ligation
(10) Dias, J. M.; Than, M. E.; Humm, A.; Huber, R.; Bourenkov, G. P.;
Bartunik, H. D.; Bursakov, S.; Calvete, J.; Caldeira, J.; Carneiro, C.;
Moura, J. J. G.; Moura, I.; Roma˜o, M. J. Structure 1999, 7, 65-79.
(11) Bertero, M. G.; Rothery, R. A.; Palak, M.; Hou, C.; Lim, D.; Blasco,
F.; Weiner, J. H.; Strynadka, N. C. J. Nature Struct. Biol. 2003, 10,
681-687.
(Et4N)[WO(OC6H2-2,4,6-Pri3)(S2C2Me2)2]. To a solution of 49
mg (0.062 mmol) of (Et4N)[W(OC6H2-2,4,6-Pri3)(S2C2Me2)2] in 1
(12) Li, H.-K.; Temple, C.; Rajagopalan, K. V.; Schindelin, H. J. Am. Chem.
Soc. 2000, 122, 7673-7680.
(17) Garner, C. D.; Stewart, L. J. Met. Ions Biol. Syst. 2002, 39, 699-726.
(18) Hyde, M. R.; Garner, C. D. J. Chem. Soc., Dalton Trans. 1975, 1186-
1191.
(19) Blower, P. J.; Dilworth, J. R.; Hutchinson, J. P.; Zubieta, J. A. J. Chem.
Soc., Dalton Trans. 1985, 1533-1541.
(20) Fomitchev, D. V.; Lim, B. S.; Holm, R. H. Inorg. Chem. 2001, 40,
645-654.
(13) Czjzek, M.; Dos Santos, J.-P.; Pommier, J.; Me´jean, V.; Haser, R. J.
Mol. Biol. 1998, 284, 435-447.
(14) Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-
273.
(15) Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 1302-1310.
(16) Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275-1281.
Inorganic Chemistry, Vol. 44, No. 4, 2005 1069

Jiang and Holm
Chart 1. Designation of Complexes and Abbreviations
Determination of Nitrite. Nitrite generated in the system was
measured quantitatively by a colorimetric method.21 After comple-
tion of an oxo transfer reaction, the reaction mixture was reduced
to dryness and the residue was extracted with 100 mL of water.
An aqueous solution of sulfanilamide and N-(1-naphthyl)ethylene-
diamine dihydrochloride was added to the extract to give a purple
solution. The absorbance at 550 nm was measured and the
concentration was acquired by comparison with a standard curve
measured with NaNO2. Nitrite generated in the various systems
corresponded to 85-95% of the theoretical conversion for a
complete reaction.
Other Physical Measurements. All measurements were made
under anaerobic conditions. NMR spectra were recorded on Varian
Mercury 300 or 400 spectrometers. Absorption spectra were
determined with a Varian Cary 3 or Varian Cary 50 spectropho-
tometer at 298 K.
Results and Discussion
mL of acetonitrile was added a solution of 22 mg (0.066 mmol) of
Ph3AsO in 1 mL of THF. The green-brown solution became dark
brown within 2 min; the reaction mixture was stirred for 15 min.
Solvent was removed and the residue was rinsed with ether (3 ×
3 mL). The residue was extracted with 1 mL of acetonitrile. Ether
(50 mL) was added to the extract, and the mixture was maintained
at -40 °C for 2 d. The solid that separated was collected, washed
with ether (3 × 3 mL), and dried. The product was obtained as 25
mg (50%) of a dark brown microcrystalline solid. Absorption
spectrum (acetonitrile): λmax (ꢀM) 353 (sh, 6000), 401 (sh, 3300),
Reaction Systems. Bis(dithiolene)molybdenum and -tung-
sten complexes with axial phenolate ligands, exemplified by
1 and 4, have been highly effective in mediating clean and
complete oxo and sulfido transfer reactions.7-9 However, the
corresponding complexes with benzenethiolate or relatively
small thiolate or selenolate ligands in general have resisted
isolation. Instead, binuclear species such as [MoIV2(µ2-SePh)2-
(S2C2Ph2)4]2- 14 or mononuclear [WV(QPh)2(S2C2Me2)2]1- (Q
) S, Se)22 were obtained. However, the use of the sterically
demanding substituents R ) 2-adamantyl7 or 2,4,6-triiso-
propylphenyl14,15 has allowed isolation of the desired com-
plexes [MIV(SR)(S2C2Me2)2]1-. No attempt has been made
to determine the smallest R group that stabilizes complexes
of this formulation. To augment complexes of the hindered
phenolate type, 2 and 5 were prepared in this work. The
ability of sterically crowded complexes to sustain oxo transfer
is demonstrated by reaction 2 (M ) W) with XO/X ) Ph3-
AsO/Ph3As, in which 5 (Q ) O) is converted to 7 (50%)
and 6 (Q ) S) to 8 (68%)15 in the indicated isolated yields.
The complexes [MoIV(OR)(S2C2Me2)2]1- support oxo transfer
with N-oxides and S-oxides, but in no case has the reaction
product been sufficiently stable for isolation. As shown in a
detailed study of the reactions of phenolate complex 1,
species of the type [MoVIO(OR)(S2C2Me2)2]1- undergo an
internal redox reaction to afford the products [MoVO(S2-
C2Me2)2]1- and RO•, which is quenched to form ROH.7
1
533 (1700), 667 (1300) nm. H NMR (CD3CN anion): δ 1.10 (d,
12), 1.18 (d, 6), 2.20 (s, 12), 6.89 (s, 2), 2.84 (m, 2), 3.80 (m, 1).
In the following sections, complexes are numerically designated
according to Chart 1.
Kinetics Measurements. Procedures and data analysis were
performed as in previous kinetics studies.7 Sample preparations and
reactions were carried out under strictly anaerobic conditions in
acetonitrile solutions. Acetonitrile (Burdick & Jackson) was stored
over 4-Å molecular sieves for 2 d prior to use. The reaction systems
were conventionally monitored with a Varian Cary 3 spectropho-
tometer equipped with a cell compartment thermostated to (0.5
K. Thermal equilibrium was reached by placing a quartz cell (1
mm path length) containing a 0.25 mL solution of the substrate in
the cell compartment at least 5 min prior to reaction initiation.
Reactions were initiated by the injection of complexes using a
gastight syringe (100 µL) through the rubber septum cap of the
quartz cell followed by rapid shaking of the solution mixtures. The
following initial concentrations were used: [6]0 ) 0.20-0.60 mM,
[(Bu4N)(NO3)]0 ) 4.0-60 mM; [6]0 ) 0.30-0.60 mM, [(CH2)4-
SO]0 ) 20-800 mM; [5]0 ) 0.30-0.60 mM, [(Bu4N)(NO3)]0 )
90-800 mM; [5]0 ) 0.30-0.60 mM, [(CH2)4SO]0 ) 1.9-7.9 M;
[3]0 ) 0.30-0.60 mM, [(CH2)4SO]0 ) 0.16-4.7 M. Tight isosbestic
points were observed in each reaction system, indicating no
significant accumulation of an intermediate. For each substrate,
reactions were conducted at five different temperatures in the range
of 288-338 K. At each temperature, at least five independent runs
with different initial substrate concentrations were performed under
pseudo-first-order conditions for these complexes. Plots of ln[(At
- A∞)/(A0 - A∞)] vs time were linear over 3-5 half-lives and the
initial rate constants kobs were obtained from the data for the first
30 min. Plots of kobs vs [substrate]0 were linear and yielded overall
second-order rate constants k2 at each temperature. Activation
parameters were determined from linear plots of the Eyring equation
k ) (kBT/h)[exp(∆S‡/R - ∆H‡/RT)]. Standard deviations of rate
constants were estimated by using linear least-squares error analysis
with uniform weighting of data points.
[MIV(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- + XO f
[MVIO(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- + X (2)
The kinetics of reaction 2, which is set out schematically
in Figure 2, have been established. The square pyramidal
stereochemistry of the reactants is based on the structures
of 1,7 3,14 4,16 6,15 and [MoIV(OC6H3-2,6-Pri2)(S2C2Me2)2]1-.14
The metal atom is situated 0.78-0.83 Å (M-OR) or 0.70-
0.73 Å (M-SR) above the equatorial S4 plane toward the
axial ligand. Other parameters are M-OR ) 1.86-1.90 Å,
(21) Basset, J.; Denney, R. C.; Jeffery, G. H.; Mendham, J. In Vogel’s
Textbook of QuantitatiVe Chemical Analysis; Longman Scientific and
Technical: 1989; pp 702 ff.
(22) Sung, K.-M.; Holm, R. H. Inorg. Chem. 2001, 40, 4518-4525.
1070 Inorganic Chemistry, Vol. 44, No. 4, 2005

Nitrate Reduction by Bis(dithiolene)tungsten
Figure 2. Upper: schematic representation of oxygen atom transfer reactions of MoIV (2, 3) and WIV (5, 6) complexes with sterically hindered phenolate
or thiolate axial ligands containing the 2,4,6-triisopropylphenyl substituent. The MoVIO product complexes are too unstable to be isolated whereas the WVI
O
complexes (7, 8) are isolable. Lower: associative reaction pathway for nitrate reduction.
Table 1. Kinetics Parameters for the Oxygen Atom Transfer Reaction
Sytems [MIV(QR)(S2C2Me2)2]/XO in Acetonitrile
a
M
QR
XO
k2 (M-1 s-1
)
∆H‡ (kcal/mol) ∆S‡ (eu)
Mob OPh
(CH2)4SOd 1.5(2) × 10-4
(CH2)4SOd 9.0(3) × 10-4
10.1(4)
16.8(6)
11.6(4)
18.6(7)
15.8(4)
14.6(9)
12.6(9)
-39(1)
-16(2)
-33(1)
-22(2)
-16(1)
-16(3)
-20(3)
Mo SC6H2Pri3 (CH2)4SOe 1.4(1) × 10-3
Wc OPh
W
W
W
W
OC6H2Pri3 (CH2)4SOg 2.3(1) × 10-6
- f
OC6H2Pri3 NO3
6.1(1) × 10-3
SC6H2Pri3 (CH2)4SOe,g 3.5(1) × 10-2
- f
SC6H2Pri3 NO3
1.7(1) × 10-1
a Measured values at 298 K. b Reference 7. c Reference 8. d k2W/k2
)
Mo
6.0. e k2W/k2 ) 25. f k2S/k2 ) 28. g k2S/k2 ) 104.
Mo
O
O
M-SR ) 2.32-2.34 Å, M-O-C ) 135,147° (1, 4), 167°,14
and M-S-C ) 102-104° (3, 6). In the phenolate series,
the presence of Pri groups on ring increase the Mo-O-C
angle by 32°; bond distances are not significantly affected
by the larger axial ligands. The distorted octahedral product
configuration is demonstrated by the structures of [WVIO-
(OPh)(S2C2Me2)2]1- 8 and 8.15 These structures are metrically
very similar except for the 0.44 Å difference in W-QR bond
lengths.
Figure 3. UV/visible spectra for the specified reaction (R ) C6H2-2,4,6-
Pri3) in acetonitrile at room temperature; band maxima and an isosbestic
point are indicated. The system contained [6]0 ) 0.22 mM and [(Bu4N)-
(NO3)]0 ) 26.2 mM; spectra were recorded every 50 s.
that the product in these systems was not oxo complex 7 or
8. A limited kinetics study indicated that the rate constants
of these reactions were ca. 5-10 times larger than that for
nitrate reduction (see below). Consequently, the reactions 2
were performed with, usually, 50 equiv or more of nitrate.
Under these pseudo-first-order conditions, well-behaved
kinetics were observed.
Substrate Reduction Kinetics. Addition of ca. 100 equiv
of nitrate to an acetonitrile solution of 6 resulted in a gradual
color change from red-brown to purple. Spectral changes are
shown in Figure 3. The intense features at 294 and 345 nm
decrease with time; the final spectrum with λmax ) 556 and
693 nm is identical to that of complex 8 measured separately.
Note the tight isosbestic point at 377 nm. In this and other
nitrate reduction reactions, 85-95% of nitrite expected from
stoichiometric conversion was detected by a colorimetric
method. Reduction of XO ) NO3- and (CH2)4SO by 5 and
6 were found to produce tight isosbestic points and final
spectra which are those of 7 and 8, respectively, and to obey
the second-order rate law 3, a consistent feature of all
substrate transformations mediated by bis(dithiolene)molyb-
denum and -tungsten complexes.7-9 The parameters collected
Initial observations revealed that the molybdenum com-
plexes 2 and 3 are inadmissible for examination of nitrate
reduction. Phenolate complex 2 showed an extremely slug-
gish reaction with excess nitrate in acetonitrile. Reaction
systems containing thiolate complex 3 and excess nitrate in
both acetonitrile and DMF solutions were not well-behaved.
The complex underwent decomposition to [Mo(S2C2Me2)3]1-,20
identified spectrophotometrically, at rates much faster than
oxo transfer. However, an intrinsic ability of 3 to support
oxo transfer was demonstrated by the clean reduction of
(CH2)4SO to (CH2)4S; kinetics data are contained in Table
1. With the finding that complexes of the native metal of
dissimilatory NiR are unsuitable, attention was directed to
the tungsten-based systems in reaction 2 with 5 or 6 and
-
-
XO/X ) NO3 /NO2 . To provide controls and a comparative
rate basis, reduction of the well-studied substrate X ) (CH2)4-
SO,7,8 the least hindered S-oxide, was examined in parallel
with nitrate reduction. In reaction systems involving 5 and
6 and 5-10 equiv of nitrite, it was observed that a rapid
color change from red-brown to purple ensued. Equimolar
quantities of the complexes and nitrite gave a slower reaction;
the 1H NMR and UV/visible spectra of the reaction showed
-d[MIV]/dt ) k2[MIV][XO]
(3)
Inorganic Chemistry, Vol. 44, No. 4, 2005 1071

Jiang and Holm
in Table 1 allow several kinetics comparisons and other
features to be examined.
state possibly arising from less specific solvation owing to
the larger axial substituents. For the reactions studied here,
∆H‡ is 68-78% of ∆G‡ and ∆H‡O > ∆H‡S at constant
substrate, although the effect is not large (3-4 kcal/mol).
We interpret this to mean that rate differences arise primarily
from energy required to bind substrate in the more congested
coordination environment afforded by an axial W-OR than
W-SR ligand, a matter supported by the W-QR bond
distances and W-Q-C bond angles specified above. In this
picture, the parameters of the initial square pyramidal
configuration, especially the W-QR bond lengths and
W-Q-C bond angles (see above), are the dominant factors
as the ground-state structure evolves to the transition state
geometry by binding substrate cis to the QR ligand, as
observed in the product stereochemistry. The much smaller
discrimination factor for nitrate reduction by 5 and 6 in (iii)
is proposed to arise because of the small planar structure of
the substrate, whose binding is less sensitive to steric
influences. This factor is ca. 103 larger for (CH2)4SO which,
while not a sterically crowded molecule, is evidently large
enough to differentiate among different steric environments.
Another possible factor is the intrinsic binding propensity
of a W-SR vs W-OR site in the absence of steric effects.
We find this artificial situation difficult to assess and leave
the matter open. Depiction of the pathway of nitrate reduction
with axial thiolate ligation is included as reaction 4 in Figure
2.
(i) Steric Effects. The rate constant ratio k2(4)/k2(5) ≈
400 signifies a moderate rate retardation in the reaction of
unhindered vs hindered WIV phenolate complexes with the
same substrate. This factor is likely to be substrate-dependent
(not tested).
(ii) Kinetic Metal Effect. The ratios k2W(4)/k2Mo(1) ) 68
and k2W(6)/k2Mo(2) ) 25 are another manifestation of oxo
transfer from substrate to metal being faster with tungsten
than molybdenum. The rate discrimination for these reac-
tions, which implicate MIV f MVI oxidation, is not large,
ratios typically being j50. We have encountered no excep-
tions to this behavior, which we take as a periodic property.
Rate constant data and discussion of this behavior are
available elsewhere.4,8,23 Isoelectronic molybdenum and
tungsten complexes with identical ligation are isostructural
and nearly isodimensional and, at least with bis(dithiolenes),
follow the same rate law at constant substrate. Consequently,
a practical utility of the kinetic metal effect is definition of
reaction pathways of molybdenum systems of insufficient
robustness with use of analogous, more stable tungsten
systems which can deliver all necessary information except
exact rate constants. Note, however, that for substrate
reductions these values are likely to agree within 2 orders
of magnitude. The present study of nitrate reduction offers
an application of the kinetic metal effect, which is also
manifested by molybdenum and tungsten isoenzymes.4
Last, there are prior examples of molybdenum- and
tungsten-mediated nitrate reduction systems.24,25 At least one
of them was designed to simulate biological nitrate reduc-
tion,25 but was executed before the structure of any molyb-
doenzyme site was known. Nonetheless, the present inves-
tigation together with the previous system demonstrates that
nitrate reduction by primary (direct) oxo transfer24 is a
feasible reaction pathway. Oxygen isotope labeling experi-
ments, possibly designed like those which demonstrated oxo
transfer in DMSOR,26 are required to demonstrate primary
atom transfer from substrate to molybdenum in NiR.
S
(iii) Sulfur vs Oxygen Axial Ligands. The ratio k2 (6)/
O
k2 (5) ) 28 reveals that nitrate is reduced faster at a W-SR
vs a W-OR site at constant R ) C6H2-2,4,6-Pri3 and
dithiolene ligands. A much greater discrimination factor of
∼104 is found with the same complexes and (CH2)4SO as
substrate. The first result implies that an enzymic site with
W-S‚Cys binding is a more effective catalyst than a
W-O‚Ser site in the absence of protein environmental
effects. The kinetic metal effect suggests that the statement
can be extrapolated to molybdenum sites. However, this
matter cannot be tested at present, as there are no reports of
nitrate reduction by other members of the DMSOR family
nor of N-oxide or S-oxide reduction by NiR.
Acknowledgment. This work was supported by NSF
Grant CHE 0237419. We thank Dr. S. C. Lee for useful
discussions.
(iv) Reaction Pathway. For all reactions in Table 1, ∆S‡
is negative, consistent with an associative transition state.
Values are less negative than for systems based on 1 and 4
with the same substrate, suggesting a less-ordered transition
IC040109Y
(24) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449, and references therein.
(25) Craig, J. A.; Holm, R. H. J. Am. Chem. Soc. 1989, 111, 2111-2115.
(26) Schultz, B. E.; Holm, R. H.; Hille, R. J. Am. Chem. Soc. 1995, 117,
827-828.
(23) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998, 37,
1602-1608.
1072 Inorganic Chemistry, Vol. 44, No. 4, 2005