Comparative kinetics and mechanism of oxygen and sulfur atom transfer reactions mediated by bis(dithiolene) complexes of molybdenum and tungsten

Article abstract of DOI:10.1021/ic040087f

Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [M ^IV(O-p-C₆H

Full text of DOI:10.1021/ic040087f

Inorg. Chem. 2004, 43, 8092−8101
Comparative Kinetics and Mechanism of Oxygen and Sulfur Atom
Transfer Reactions Mediated by Bis(dithiolene) Complexes of
Molybdenum and Tungsten
Jun-Jieh Wang,† Olga P. Kryatova,‡ Elena V. Rybak-Akimova,‡ and R. H. Holm†,*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138, and Department of Chemistry, Tufts UniVersity,
Medford, Massachusetts 02155
Received June 30, 2004
Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined
in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [MIV(O-
-
-
p-C6H4X
′
)(S2C2Me2)2]1
+
Ph3AsQ
f ′ + Ph3As (M ) Mo, W; Q ) O, S) with
[MVIQ(O-p-C6H4X )(S2C2Me2)2]1
variable substituent X
′
have been investigated in acetonitrile in order to determine the relative rates of oxo versus
sulfido transfer at constant structure (square pyramidal) of the atom acceptor and of atom transfer at constant
structure of the atom donor and metal variability of the atom acceptor. All reactions exhibit second-order kinetics
and entropies of activation (
−
25 to
−
45 eu) consistent with an associative transition state. At parity of atom acceptor,
S
O
S
O
k2 (0.25
−
0.75 M-1s-1) > k2 (0.023
−
0.060 M-1s-1) with M
)
Mo and k2 (4.1
−
66.7 M-1s-1) > k2 (1.8
−
9.8
)
W
Mo
Mo
M-1s-1) with M
O) and 16 89 (Q
OMe < H < Br < COMe < CN; increasing electron-withdrawing propensity accelerates reaction rates. The probable
transition state involves significant Ph3AsQ‚‚‚M bond-making (X rate trend) and concomitant As Q bond weakening
(bond energy order As O > As S). Orders of oxo and sulfido donor ability of substrates and complexes are
deduced on the basis of qualitative reactivity properties determined here and elsewhere. This work complements
previous studies of the reaction systems [MIV(O-p-C6H4X )(S2C2Me2)2]1-/XO where the substrates are N-oxides and
)
W. At constant atom donor and X
′
, k2 > k2 with reactivity ratios k2W/k2
)
78−
184 (Q
−
) S). Rate constants refer to 298 K. At constant M and Q, rates increase in the order X′ ) Me
j
′
−
−
−
′
W
Mo
S-oxides and k2 > k2 at constant substrate also applies. The reaction order of substrates is Me3NO > (CH2)4SO
> Ph3AsS > Ph3AsO. This research provides the first quantitative information of metal-mediated sulfido transfer.
Introduction
products [MVIO(OR′)(S2C2R2)2]1- and afford products X )
amines and sulfides. Reactions in tungsten-based systems
in particular are clean and complete. Kinetics data and
mechanistic considerations reduce to second-order reactions
proceeding through an associative transition state with
substantial M‚‚‚OX bond-making character. The leading
aspects of this research are summarized elsewhere.4
Of all metal-mediated atom and group transfer reactions,
oxo transfer is by far the most extensively documented and
throughly investigated. Reaction 1 is merely one example
of a manifold of transformations which proceed by primary
oxo transfer (i.e., reactions in which reactants XO/X are
We have recently developed analogue reaction systems
of the molybdenum and tungsten oxotransferases based on
the minimal oxygen atom transfer reaction 1 with M ) Mo
and W and oxo donors XO ) N-oxides and S-oxides.1-3
The reactant complexes are square pyramidal bis(dithiolenes)
of the general type [MIV(OR′)(S2C2R2)2]1- (R′ ) p-C6H4X′;
R ) Me, Ph), which convert to the distorted octahedral
M
IV + XO f MVIO + X
(1)
* Author to whom correspondence should be addressed. E-mail: holm@
chemistry.harvard.edu.
(2) Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1931-1943.
(3) Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2002, 124, 4312-4320.
(4) Enemark, J. H.; Cooney, J. J. A.; Wang, J.-J.; Holm, R. H. Chem.
ReV. 2004, 104, 1175-1200.
† Harvard University.
‡ Tufts University.
(1) Lim, B. S.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920-1930.
8092 Inorganic Chemistry, Vol. 43, No. 25, 2004
10.1021/ic040087f CCC: $27.50
© 2004 American Chemical Society
Published on Web 11/17/2004

Metal-Mediated Oxo and Sulfido Transfer Reactions
single oxygen atom donors/acceptors, the oxidation state
change Mz/z+2 results from atom transfer only, and the
transferable or transferred atom is oxidic and bound in a
terminal or bridging mode directly to atom M).5 The situation
may be contrasted with primary sulfur atom transfer reactions
with analogous defining features and whose relatively few
examples can be summarized by the minimal reactions 2-6.
The first three are sulfido transfer from substrate resulting
in the formation of MdS units, while the last two are sulfido
transfer to substrate with concomitant metal reduction.
Frequently used sulfur acceptors X are R3P and CN-, and
sulfur donors XS are elemental sulfur, episulfides, RNCS,
RSSSR, or a MdS complex. (The reaction types do not
include MSn species (n g 2), many of which function as
sulfur atom donors.)
has led to reactions 7 and 8 with M ) Mo and W. These are
specific examples of reactions 1 and 3, respectively, and are
set out in detail in Figure 1. The compound Ph3AsS has been
shown to be a sulfur donor to other group 15 compounds in
[MIV(O-p-C6H4X)(S2C2Me2)2]1- + Ph3AsO f
[MVIO(O-p-C6H4X)(S2C2Me2)2]1- + Ph3As (7)
[MIV(O-p-C6H4X)(S2C2Me2)2]1- + Ph3AsS f
[MVIS(O-p-C6H4X)(S2C2Me2)2]1- + Ph3As (8)
reactions for which limited kinetics and thermodynamic data
are available.26-28 It has apparently not been utilized previ-
ously in metal-mediated sulfido transfer reactions. Reactions
7 and 8 provide an opportunity to determine (i) the relative
rates of oxo versus sulfido transfer at constant structure of
the atom acceptor (MoIV or WIV) and nearly constant structure
of the atom donor, and (ii) the relative rates of atom transfer
at constant structure of the atom donor and metal variability
of the atom acceptor. There have been no previous deter-
minations of the kinetics and mechanism of sulfur atom
transfer. Consequently, ii is a matter of interest in its own
right and also because of the existence of polysulfide
reductase. This molybdoenzyme has been classified in the
dimethyl sulfoxide (DMSO) reductase family; hence its
active site is very likely of the bis(dithiolene) type.29
Additionally, we are interested in analogue reaction systems
MII + XS f MIVS + X
(2)
M ) Ti,6 Mo,7,8 W,7-9 Sn10-12
(3)
M
IV + XS f MVIS + X
M ) W13-15
MIVO + XS f MVIO(S) + X
M ) Mo16-18
(4)
(5)
(6)
MVIO(S) + X f MIVO + XS
M ) Mo,18-21 W22
-
MVIS2 + X f MIVS + XS
M ) W22,23
of arsenite oxidase, which converts arsenite (H2AsO3 ) to
arsenate (HAsO42-).30,31 There is very little information on
oxo transfer reactions of arsenic compounds, a matter
redressed to an extent by the reactivity demonstrated here
between Ph3AsO and MoIV and WIV dithiolenes.
During our examination of the reactivity of bis(dithiolene)-
WIV complexes, we observed several instances of sulfur atom
transfer reaction 3 with XS ) (PhCH2S)2S and oxo transfer
reaction 1 with XO ) Ph3AsO.2,15 The latter reagent has
previously been effective in a molybdenum-mediated non-
dithiolene reaction system.24,25 More recent experimentation
Experimental Section
Preparation of Compounds. All reactions and manipulations
were conducted under a pure dinitrogen atmosphere using either
an inert atmosphere box or standard Schlenk techniques. Methanol
was distilled from magnesium; acetonitrile, ether, dichloromethane,
and tetrahydrofuran (THF) were freshly purified using an Innovative
Technology solvent purification system and stored over 4-Å
molecular sieves. Benzene and n-pentane used in column chroma-
tography were of HPLC grade from J. T. Baker. Deuterated solvents
(Cambridge Isotope Laboratories, Inc.) were stored over molecular
sieves under dinitrogen. Commercial samples (Aldrich) of the
compounds Ph3AsO, Ph3SbS, and As2O3 were sublimed and (EtO)3-
As was distilled prior to use. Ph3AsS was obtained by the reaction
of Ph3AsO with (Me3Si)2S in acetonitrile in J90% yield, and NaO-
(5) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449.
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Chem. 1991, 30, 1537-1542.
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1317.
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H. J. Am. Chem. Soc. 1998, 120, 8102-8112.
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(15) Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 1302-1310.
(16) Eagle, A. A.; Laughlin, L. J.; Young, C. G.; Tiekink, E. R. T. J. Am.
Chem. Soc. 1992, 114, 9195-9197.
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Chem. 2001, 40, 4563-4573.
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R. H. J. Am. Chem. Soc. 1993, 115, 2714-2722.
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(26) Baechler, R. D.; Stack, M.; Stevenson, K.; Van Valkenburgh, V.
Phosphorus, Sulfur Silicon Relat. Elem. 1990, 48, 49-52.
(27) Jason, M. E. Inorg. Chem. 1997, 36, 2641-2646.
(28) Capps, K. B.; Wixmerten, B.; Bauer, A.; Hoff, C. D. Inorg. Chem.
1998, 37, 2861-2864.
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Chem. 1996, 35, 5368-5377.
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Soc. 2000, 122, 2946-2947.
(19) Thapper, A.; Donahue, J. P.; Musgrave, K. B.; Willer, M. W.;
Nordlander, E.; Hedman, B.; Hodgson, K. O.; Holm, R. H. Inorg.
Chem. 1999, 38, 4104-4114.
(20) Adam, W.; Bargon, R. M.; Schenk, W. A. J. Am. Chem. Soc. 2003,
125, 3871-3876.
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G. Polyhedron 2004, 23, 385-394.
Inorganic Chemistry, Vol. 43, No. 25, 2004 8093

Wang et al.
Figure 1. Schematic depiction of the atom transfer reactions of bis(dithiolene)M(IV) complexes 1 and 2, including oxo transfer (reactions 7a and 7b) and
sulfido transfer (reactions 8a and 8b). The structures of product oxo (3, 4) and sulfido (6) complexes have been established in previous work. Complex 5
is unstable to internal redox and decays to 8 in solution. Reaction 9 shows the ligand transfer reaction between a MoVIO2 complex and arsenite esters.
p-C6H4X′ was prepared from addition of 1 equiv of NaOMe to
p-X′-C6H4OH (X′ ) H, Me, OMe, Br, COMe, CN) in methanol.
In the following preparations, all volume reduction steps were
performed in vacuo.
1-Methyl-4-arsa-3,5,8-trioxabicyclo[2.2.2]octane. The follow-
ing method is more convenient and leads to higher yields than the
published procedure.32 Arsenic trioxide (9.9 g, 50 mmol) and
2-hydroxymethyl-2-methyl-1,3-propanediol 12.0 g (100 mmol) were
covered with toluene (100 mL) and refluxed for 10 h. A Dean-
Stark trap was used to collect water generated during the reaction.
The solvent was removed; the solid residue was sublimed to give
the product as 8.20 g (85%) of hygroscopic colorless crystals; mp
42 °C, lit.32 41-42°. 1H NMR (CD3CN): δ 0.63 (s, 1), 4.02 (s, 2).
4-Arsa-3,5,7-trioxabicyclo[2.2.1]heptane. The following pro-
cedure is related to a published method.33 A mixture of As2O3 (9.9
g, 50 mmol), glycerol (9.2 g, 100 mmol), and toluene (100 mL)
was refluxed for 8 h, resulting in a colorless, very viscous syrup.
The crude product was sublimed and the sublimate was resublimed,
giving the product as 2.7 g (33%) of extremely hygroscopic crystals;
mp 69 °C, lit.33 66-70 °C. 1H NMR (CD3CN): δ 5.15 (t, 1), 3.88
(m, 2), 3.55 (m, 2).
(Et4N)[As(bdt)2]. A solution of (Et4N)2[MoO2(bdt)2]34 (20 mg,
0.032 mmol; bdt ) benzene-1,2-dithiolate(2-)) in 1 mL of aceto-
nitrile was treated with a solution of (EtO)3As (20 mg, 0.095 mmol)
(33) Wolfrom, M. L.; Holm, M. J. J. Org. Chem. 1961, 26, 273-274.
(34) Ueyama, N.; Oku, H.; Kondo, M.; Okamura, T.; Yoshinaga, N.;
Nakamura, A. Inorg. Chem. 1996, 35, 643-650.
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8094 Inorganic Chemistry, Vol. 43, No. 25, 2004

Metal-Mediated Oxo and Sulfido Transfer Reactions
Table 1. Crystallographic Data for (Et4N)[As(bdt)2]a
in 1 mL of acetonitrile. The light yellow solution was stirred for 3
h and filtered. The filtrate was reduced to one-third of its original
volume. Several volume equivalents of ether were layered onto the
solution, and the mixture was allowed to stand overnight. The
product was collected, washed with ether, and obtained as 4.0 mg
(26%) of colorless crystals. 1H NMR (CD3CN, anion): δ 6.79 (m,
2), 7.14 (q, 2). The compound was identified by an X-ray structure
determination.
formula
fw
crystal system
space group
T, K
C20H28AsNS4
485.59
monoclinic
P21/c
â, deg
106.91(1)
4
2220(3)
1.453
1.912
1.98-22.50
1.277
Z
V, Å3
dcalc, g/cm3
µ, mm-1
θ range, deg
213
a, Å
b, Å
10.462(8)
14.103(10)
15.730(11)
GOF (F2)
c
c, Å
R1 b (wR2
)
5.25 (8.32)
(Et4N)[Mo(O-p-C6H4X′)(S2C2Me2)2]. These compounds were
prepared in an analogous procedure for [Mo(OPh)(S2C2Me2)2]1- 1
with NaO-p-C6H4X′ instead of NaOPh on a 0.16-0.22 mmol scale.
A suspension of NaO-p-C6H4X′ in acetonitrile was added to a
suspension of 1 equiv of [Mo(CO)2(S2C2Me2)2]35 in acetonitrile.
The mixture was stirred for 6-8 h to generate a brown or green-
brown solution. A solution of 1 equiv of Et4NCl in acetonitrile
was added; the mixture was stirred for 10 min and filtered. The
filtrate was reduced to one-third of its original volume, several
volume equivalents of ether were layered onto the solution, and
the mixture was allowed to stand for 2 days. All compounds were
isolated as highly crystalline solids; yields are indicated
X′ ) Me. Brown needle-shaped crystals (56%). 1H NMR
(CD3CN, anion): δ 2.14 (s, 3), 2.57 (s, 12), 6.27 (d, 2), 6.80 (d,
2). Absorption spectrum (acetonitrile) λmax (ꢀM): 272 (sh, 13 400),
344 (12 050), 392 (3600), 473 (1470), 571 (sh, 430), 736 (120)
nm.
a Mo KR radiation (λ ) 0.71073 Å). b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2
2
2
2
) {∑[w(Fo - Fc )]2/∑[w(Fo )2]}1/2
.
OMe: δ 2.31 (s, 12), 3.76 (s, 3), 6.79 (d, 2), 6.91 (d, 2). X′ ) Br:
δ 2.33 (s, 12), 6.90 (d, 2), 7.31 (d, 2). X′ ) COMe: δ 2.34 (s,
12), 2.49 (s, 3), 7.02 (d, 2), 7.86 (d, 2). X′ ) CN: δ 2.35 (s, 12),
7.06 (d, 2), 7.46 (d, 2).
X-ray Structure Determination. The structure of the compound
in Table 1 was determined. Single crystals were obtained by
layering several volume equivalents of ether onto an acetonitrile
solution and allowing the mixture to stand for at least 12 h. A crystal
was mounted on a Siemens (Bruker) SMART CCD-based diffrac-
tometer. Data were collected at 213 K using Mo KR radiation (λ
) 0.71073 D) and ω scans of 0.3°/frame for 30 s, such that 1271
frames were collected for a hemisphere of data. The first 50 frames
were recollected at the end of the data collection to monitor for
decay; no significant decay was detected. Cell parameters were
retrieved using SMART software and refined using SAINT
software. Data reduction was performed with SAINT; absorption
corrections were applied with SADABS. The space group was
assigned from analysis of symmetry and systematic absences using
XPREP. The structure was solved by direct methods and refined
by full-matrix least-squares method against F2 using SHELXL-97.
All non-hydrogen atoms were refined anisotropically; hydrogen
atoms were placed at idealized positions of carbon atoms. Crystal
data and final agreement factors are given in Table 1.37
Kinetics Measurements. Sample preparations and reactions were
performed under strictly anaerobic conditions in acetonitrile or
dimethylformamide (DMF) solutions. The reaction systems (Et4N)-
[M(O-p-C6H4X′)(S2C2Me2)2]/Ph3AsO (M ) Mo, W) and (Et4N)-
[Mo(O-p-C6H4X′)(S2C2Me2)2]/Ph3AsS or Ph3SbS were convention-
ally monitored with a Varian Cary 3 spectrophotometer equipped
with a cell compartment thermostated to (0.5 K. Thermal equi-
librium was reached by placing a quartz cell (1 mm path length)
containing 0.28-mL solutions of the complexes in the cell compart-
ment at least 5 min prior to reaction initiation. Reactions were
initiated by the injection of substrate (in acetonitrile or DMF
solutions) 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:
[MoIV]0 ) 1.76-2.14 mM, [Ph3AsO]0 ) 14.3-29.4 mM; [MoIV]0
) 0.88-1.07 mM, [Ph3AsS]0 ) 2.86-5.88 mM; [WIV]0 ) 1.50-
1.85 mM, [Ph3AsO]0 ) 7.69-25.0 mM. Tight isosbestic points
occurred in each reaction system, demonstrating clean conversions.
For each substrate, reactions were conducted at five temperatures
in the range of 278-333 K. At each temperature, at least five
independent runs with different initial concentrations [Ph3AsO]0
were performed under pseudo-first-order conditions for molybde-
num and tungsten complexes. Plots of ln[(At - A∞)/(A0 - A∞)]
versus time were linear over at least 3 half-lives, and the initial
rate constants kobs were obtained from the data for the first 30 min.
Plots of kobs versus [Ph3AsO]0 were linear and yielded overall
X′ ) OMe. Brown needle-shaped crystals (52%). 1H NMR (CD3-
CN, anion): δ 2.57 (s, 12), 3.61 (s, 3), 6.32 (d, 2), 6.55 (d, 2).
Absorption spectrum (acetonitrile) λmax (ꢀM): 274 (sh, 13 000), 344
(12 000), 389 (4100), 466 (1750), 570 (sh, 490), 737 (210) nm.
1
X′ ) Br. Dark-orange block-shaped crystals (32%). H NMR
(CD3CN, anion): δ 2.59 (s, 12), 6.30 (d, 2), 7.11 (d, 2). Absorption
spectrum (acetonitrile): λmax (ꢀM): 272 (sh, 12 400), 340 (11 050),
396 (3400), 468 (1500), 570 (sh, 465), 730 (150) nm.
X′ ) COMe. Green-brown block-shaped crystals (20%). 1H
NMR (CD3CN, anion): δ 2.42 (s, 3), 2.60 (s, 12), 6.42 (d, 2), 7.64
(d, 2). Absorption spectrum (acetonitrile) λmax (ꢀM): 272 (sh,
13 800), 338 (12 900), 393 (3650), 463 (1250), 571 (sh, 385), 735
(115) nm.
(Et4N)[W(O-p-C6H4COMe)(S2C2Me2)2]. The compound was
prepared in an analogous procedure for (Et4N)[W(O-p-C6H4X′)(S2C2-
Me2)2]1- 3 with use of [W(CO)2(S2C2Me2)2]36 and NaO-p-C6H4-
COMe. The reaction was conducted on a 0.25 mmol scale in THF.
After the addition of 1 equiv of Et4NCl, the reaction mixture was
stirred for 10 min. The dark green-brown residue after solvent
removal was dissolved in 1 mL of acetonitrile, and the solution
was filtered. Ether (15 mL) was layered onto the filtrate, causing
the separation of dark green-brown block-shaped crystals over 1
day, which were dried to afford the product (40%). 1H NMR
(CD3CN, anion): δ 2.44 (s, 3), 2.67 (s, 12), 6.50 (d, 2), 7.67 (d,
2). Absorption spectrum (acetonitrile): λmax (ꢀM) 282 (sh, 14 500),
297 (15 400), 323 (sh, 8900), 404 (sh, 2500), 485 (sh, 900), 654
(600) nm.
(Et4N)[WS(O-p-C6H4X′)(S2C2Me2)2]. These compounds were
formed in situ by the immediate and complete reaction of [W(O-
p-C6H4X′)(S2C2Me2)2]1- (7.1-7.6 mmol) with 2.0 equiv of Ph3AsS
1
in CD3CN; reactions were monitored by H NMR spectroscopy.
Product solutions are dark red-brown; methyl chemical shifts (δ
2.27-2.35) overlap that of [WS(OPh)(S2C2Me2)2]1- (δ 2.29).14 X′
) Me: δ 2.27 (s, 3), 2.32 (s, 12), 6.86 (d, 2), 7.03 (d, 2). X′ )
(35) Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263-
273.
(36) Goddard, C. A.; Holm, R. H. Inorg. Chem. 1999, 38, 5389-5398.
(37) See paragraph at the end of this article for Supporting Information
available.
Inorganic Chemistry, Vol. 43, No. 25, 2004 8095

Wang et al.
Chart 1. Designation of Complexes
second-order rate constants k2 at each temperature. In case of mixed-
second-order conditions for the sulfido transfer reactions to
molybdenum complexes, at each temperature at least five inde-
pendent runs with different initial concentrations [Ph3AsS]0 were
performed. The second-order rate constants k2 (M-1 s-1) were
obtained from the plots of {ln([Ph3AsS]t/[MoIV]t) - ln([Ph3AsS]0/
[MoIV]0)}/([Ph3AsS]0 - [MoIV]0) versus time, in which the values
of [MoIV]t and [Ph3AsS]t were calculated from the equations of
[MoIV]t ) [MoIV]0 - (At - A0)/(A∞- A0)[MoIV]0 and [Ph3AsS]t )
[Ph3AsS]0 - (At - A0)/(A∞ - A0)[MoIV]0, respectively. Rate
constants k2 were averaged from the values which were obtained
from multiple independent runs.38
Because of faster reaction rates, the systems (Et4N)[W(O-p-
C6H4X′)(S2C2Me2)2]/Ph3AsS were investigated using a Hi-Tech
Scientific (Salisbury, Wilshire, UK) SF-43 Multi-Mixing Cryo-
Stopped-Flow instrument in a diode array mode. The instrument
was equipped with stainless steel plumbing, a stainless steel mixing
chamber with sapphire windows, and an anaerobic gas-flushing kit.
The temperature in the mixing cell (1 cm) was maintained to (0.1
K, and the mixing time was 2-3 ms. Experiments were performed
in a single-mixing mode of the instrument. Reactions were studied
under second-order conditions within the 0.25-0.75 mM concentra-
tion range of complex and substrate (after mixing) at 293-323 K
(283-323 K for X′ ) CN). A series of four to six shots at each
temperature gave standard deviations within 10%. Data analysis
was performed with IS-2 Rapid Kinetic (Hi-Tech) and Spectfit/32
Global Analysis System (Spectrum Software) software with the
relationship At ) A∞ - (A∞ - A0)/(1 + k2C), where C is the
concentration of reagents under equimolar conditions. Activation
parameters were determined from plots of the Eyring equation k )
(kBT/h)[exp(∆Sq/R - ∆qH/RT)]. Standard deviations were estimated
using linear least-squares error analysis with uniform weighting of
the data points.38
Figure 2. Structure of [As(bdt)2]1- as the Et4N+ salt showing 50%
probability ellipsoids, the atom labeling scheme, and selected bond distances
(Å) and angles (deg).
including (EtO)3As and two cyclic esters (cf Experimental
Section). Equimolar reaction systems of the esters and
[MoO2(bdt)2]2-, intended as an analogue of the (possibly
protonated) MoVIO2 active site of the oxidized enzyme,31,41
in acetonitrile for ca. 8 h resulted in ligand substitution rather
than oxo transfer and 70-80% conversion to [As(bdt)2]1-.
When conducted on a preparative scale with (EtO)3As, the
Et4N+ salt was obtained in (nonoptimized) 26% yield. The
overall process is described by reaction 9 (Figure 1) in which
the MoVI product is formulated by analogy to [Mo2O4(OCH2-
CH2OMe)6]2-.42 (Et4N)[As(bdt)2] is isomorphous with the
corresponding antimony compound,43 and the anions are
isostructural. The structure and metric features of [As(bdt)2]1-
are shown in Figure 2. The anion has a distorted trigonal
bipyramidal stereochemistry with atoms S2 and S4 in axial
positions, the stereochemically active lone pair in an equato-
rial position, and an AsS4 unit with C2V symmetry. The angles
Sax-As-Sax ) 166.35(4)° and Seq-As-Seq ) 106.48(3)°
Other Physical Measurements. All measurements were made
under anaerobic conditions. NMR spectra were recorded on Bruker
AM-500N and Varian Mercury 400 spectrometers. Infrared spectra
were otained in KBr pellets and measured with a Nicolet Nexus
470 FT-IR spectrometer. Absorption spectra were determined with
a Varian Cary 50 Bio spectrophotometer at 298 K.
with mean bond lengths As-Sax ) 2.51 Å and As-Seq
)
2.26 Å. Analogous results were obtained with [WO2(bdt)2]2-.
This unexpected ligand transfer reaction implies a require-
ment for oxoarsenic substrates in AsIII/AsV conversions by
In the sections that follow, complexes are designated as in Chart
1.
(39) Lim, B. S.; Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2000, 122,
7410-7411.
Results and Discussion
(40) Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275-1281.
(41) Conrads, T.; Hemann, C.; George, G. N.; Pickering, I. J.; Prince, R.
C.; Hille, R. J. Am. Chem. Soc. 2002, 124, 11276-11277.
(42) Turova, N. Y.; Kessler, V. G.; Kucheiko, S. I. Polyhedron 1991, 22,
2617-2628.
Reactions with Arsenite Esters. In this work, the first
experiments with arsenic substrates involved AsIII esters,
(38) Clifford, A. A. MultiVariate Error Analysis; Halsted Press: New York,
(43) Wegener, J.; Kirschbaum, K.; Giolando, D. M. J. Chem. Soc., Dalton
Trans. 1994, 1213-1218.
1973.
8096 Inorganic Chemistry, Vol. 43, No. 25, 2004

Metal-Mediated Oxo and Sulfido Transfer Reactions
Table 2. Kinetics Data for the Reaction Systems [MIV(O-p-C6H4X)(S2C2Me2)2]1-/Ph3AsQ (M ) Mo, W; Q ) O, S) in Acetonitrile Solutions
k2 (M-1s-1) (298 K)
∆Hq (kcal/mol)
M ) Mo M ) W
∆Sq (eu)
W/Mo reactivity
ratiob
complex
substrate
M ) Mo
M ) W
M ) Mo
M ) W
[M(O-p-C6H4Me)(S2C2Me2)2]1-
Ph3AsO
Ph3AsS
Ph3AsO
Ph3AsS
Ph3AsO
Ph3AsS
Ph3SbS
Ph3AsO
Ph3AsS
Ph3AsO
Ph3AsS
Ph3AsO
Ph3AsS
2.3(1) × 10-2
2.5(2) × 10-1
2.4(2) × 10-2
2.6(1) × 10-1
2.8(1) × 10-2
2.9(1) × 10-1
5.5(1) × 10-2
3.7(1) × 10-2
5.2(2) × 10-1
6.0(3) × 10-2
7.5(2) × 10-1
a
1.8(2)
4.1(3)
1.9(2)
4.8(3)
3.1(1)
6.7(1)
1.4(1)
6.8(2)
22.7(3)
9.8(1)
66.7(4)
25.2(3)
228(4)
78
16
79
18
111
23
25
184
44
163
89
7.3(2)
4.9(1)
7.3(3)
4.9(2)
7.2(3)
4.8(3)
6.0(2)
9.8(3)
6.9(5)
9.8(3)
6.7(5)
9.3(3)
6.6(5)
8.8(2)
-42(1)
-45(2)
-40(3)
-45(2)
-39(2)
-44(4)
-44(3)
-26(2)
-32(4)
-25(1)
-33(4)
-26(1)
-32(3)
-28(2)
[M(O-p-C6H4OMe)(S2C2Me2)2]1-
[M(OPh)(S2C2Me2)2]1-
[M(O-p-C6H4Br)(S2C2Me2)2]1-
[M(O-p-C6H4COMe)(S2C2Me2)2]1-
[M(O-p-C6H4CN)(S2C2Me2)2]1-
6.3(2)
4.6(1)
9.1(3)
6.4(5)
8.8(2)
6.1(5)
-39(3)
-44(3)
-25(2)
-29(4)
-25(2)
-27(4)
a
Mo
a Mo complex could not be prepared. b k2W/k2
.
oxo transfer, a matter supported by the reactivity of Ph3AsO
in the systems which follow.
Reaction Systems for Oxo and Sulfido Transfer. This
investigation centers on reactions 7 and 8, which are
implemented by the systems 1 or 2/Ph3AsO and 1 or
2/Ph3AsS in acetonitrile. Complexes 1a1 and 2a40 have
isostructural, practically isometric square pyramidal structures
with indistinguishable mean M-S distances (2.32 Å),
dihedral angles between chelate rings (126-127°), and
displacements of the metal atom from the S4 plane toward
the axial phenoxide ligand (0.78-0.79 Å). Axial bond
distances (Mo-O 1.898(5) Å, W-O 1.861(3) Å) differ by
ca. 0.03 Å. The substrates are simple tetrahedral molecules
with nearly identical bond angles and bond lengths As-S
(2.079(4), 2.095(5) Å)44,45 and As-O (1.65, 1.641(3) Å)46
that differ by 0.44-0.45 Å, including p-chloro derivatives.47
The gas-phase As-O bond dissociation energy of Ph3AsO
calculated from other thermodynamic data is 103 kcal/mol,
essentially the same as that for MoOCl4 (101 kcal/mol).48
Hoff and co-workers28 have estimated from gas-phase and
solution enthalpies that the As-S bond energy in Ph3AsS is
70 kcal/mol. The two substrates are, therefore, stereochemi-
cally congruous but differ in bond length, bond strength, and
presumably, basicity (vide infra).
The reaction systems are suitable to address the determina-
tions (i) and (ii) specified above. Rate constants at 298 K
were measured for five MoIV (1a-1e) and six WIV (2a-2f)
complexes, each with both Ph3AsO and Ph3AsS as substrates,
in acetonitrile solutions. Complexes differ in terms of the
para-substituent (X′ ) H, Me, OMe, Br, COMe, CN) of the
axial phenolate ligand. Activation parameters were deter-
mined for four MoIV and five WIV systems with both
substrates. Kinetic parameters are collected in Table 2.
Oxo Transfer. Reactions 7a (M ) Mo) and 7b (M ) W)
are depicted schematically in Figure 1. Spectral changes over
Figure 3. UV/visible spectra for the reaction systems [M(OPh)(S2C2-
Me2)2]1-/Ph3AsO (M ) Mo, W) in acetonitrile at 298 K. The systems intially
contained 1.88 mM MoIV and 25.0 mM Ph3AsO (upper), and 1.50 mM
WIV and 1.88 mM Ph3AsO (lower). In these and other spectra, arrows
indicate changes in absorbance with time; peak maxima and isosbestic points
are indicated.
the course of typical reactions are shown in Figure 3. For
the MoIV system 1a/Ph3AsO, equimolar components at the
millimolar concentration level resulted in no appreciable
reaction over ca. 8 h, after which decomposition occurred.
The use of J5 equiv of substrate resulted in nearly complete
reaction in 2 h. In this system, a tight isosbestic point is
observed at 523 nm. The bands at 576 and 830 nm increase
in intensity with time and arise from MoV complex 7. The
reaction proceeds by relatively slow atom transfer to form
(44) Boorman, P. M.; Codding, P. W.; Kerr, K. A. J. Chem. Soc., Dalton
Trans. 1979, 1482-1485.
(45) Narayana, S. V. L.; Shrivastava, H. N. Acta Crystallogr. 1981, B37,
1186-1189.
(46) Shao, M.; Jin, X.; Tang, Q.; Huang, Y. Tetrahedron Lett. 1982, 23,
5343-5346.
(47) Belsky, V. K.; Zavodnik, V. E. J. Organomet. Chem. 1984, 265, 159-
165.
(48) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571-589.
Inorganic Chemistry, Vol. 43, No. 25, 2004 8097

Wang et al.
MoVIO complex 3 followed by a rapid internal redox reaction
to produce 7 and the phenoxyl radical, which is quenched
to phenol by hydrogen atom abstraction. The final spectrum
is that of 7 (blue-violet) measured separately. The situation
is closely analogous to the reactions of 1a with S-oxides
and has been analyzed by the same procedure, including
spectrophotometric demonstration of the conversion 3 f 7
and demonstration of the formation of phenol.1 Despite
numerous attempts, we have never been able to isolate
pure samples of complex 3 and related species such as
[MoVIO2(S2C2R2)2]2- (R ) Ph, CO2Me). The requirement
of a more electronegative dithiolene ligand, as in [MoO-
(OSiR3)(bdt)2]1-,49 [MoO2(bdt)2]2-,34 and [MoO2(S2C2-
(CN)2)2]2-,50 allowing isolation and structure determination,
makes evident the susceptibility of monooxo and dioxo MoVI
dithiolenes to internal reduction in an electron-rich environ-
ment. The related WVIO complexes 4 are sufficiently stable
for isolation;2,14 consequently, tungsten-mediated oxo-transfer
reactions are free of the complication of a follow-up redox
process.
The WIV system 2a/Ph3AsO with equimolar components
proceeded to near-completion within 3 h and develops a clean
isosbestic point at 335 nm and bands at 408, 514, and 637
nm that increase in intensity as the reaction progresses. The
final spectrum is that of the WVIO complex 4a, which is
readily distinguished from that of [WVO(S2C2Me2)2]1-.
Reactions 7 follow the second-order rate law eq 10 (Q )
O). Kinetics parameters are collected in Table 2. Values for
4a are in excellent agreement with an earlier determination.2
For some six complexes, rate constants k2 define the ranges
Figure 4. UV/visible spectra for the reaction systems [M(OPh)(S2C2-
Me2)2]1-/Ph3AsS (M ) Mo, W) in acetonitrile at 298 K. The systems intially
contained 0.94 mM MoIV and 5.00 mM Ph3AsS (upper), and 0.75 mM WIV
and 0.75 mM Ph3AsS (lower).
-d[MIV]
) k2[MIV][Ph3AsQ]
(10)
dt
Prior to the kinetics runs, the complexes 6 were generated
in situ by reaction 8b. The formation of WVIS complexes
was assured by comparison of methyl chemical shifts with
that of 6a, which has been prepared and whose X-ray
structure has been determined.14 The equimolar WIV reaction
system 2a/Ph3AsS reacts immediately and generates a tight
isosbestic point at 351 nm and bands at 400, 493, and 603
nm that intensify as the reaction proceeds. The final spectrum
is that of WVIS complex 6a. The reactions 8 are described
by rate law 10 (Q ) S). The faster reaction rates necessitated
stopped-flow measurements, illustrated in Figure 5 for the
system 2c/Ph3AsS. Rate constants k2 are 0.25-0.75 M-1s-1
for molybdenum and 4.1-66.7 M-1s-1 for tungsten with
reactivity ratios 16-89. Eyring plots for the reactions of
complexes 1c and 2c, shown in Figure 5, display the strict
linear behavior found in these and other systems. As for
molybdenum-mediated sulfido transfer, activation entropies
accord with an associative transition state. These are the first
kinetics results for metal-mediated sulfido transfer reactions.
Reactivity Ratios. The ratios in Table 2 demonstrate that
0.023-0.060 M-1 s-1 for molybdenum and 1.8-9.8 M-1
Mo
s-1 for tungsten, with the reactivity ratios k2W/k2 ) 78-
184. Activation parameters were evaluated for eight systems.
Values of ∆Sq are large and negative for both molybdenum-
and tungsten-mediated reactions, consistent with an associa-
tive transition state.
Sulfido Transfer. Reactions 8a and 8b are described in
Figure 1. The spectrophotometric time courses of two typical
reactions are presented in Figure 4. The MoIV system 1a/
Ph3AsS displays a well-defined isosbestic point at 368 nm
and bands at 459, 577, and 731 nm that increase in intensity
with time. The final spectrum is that of disulfido-bridged
complex 8, previously isolated in low yield from the
heterogeneous reaction of [Mo(CO)2(S2C2Me2)2] and Na2S
in acetonitrile and structurally characterized.35 Sulfur atom
transfer occurs, presumably generating MoVIS complex 5a
which has not been directly detected, followed by internal
reduction. This and other complexes 5 are too unstable to
isolate, another manifestation of the reducibility of MoVI
complexes containing electron-rich dithiolene ligands. This
complication is absent in the analogous tungsten system.
W
Mo
in every case k2 > k2 and that for a given complex the
ratio is always larger for Ph3AsO than Ph3AsS. The results
provide one more proof that at parity of structure and ligation
oxo transfer from substrate to metal (MIV f MVIO) is faster
with tungsten and that to substrate from metal (MVIO f MIV)
is faster with molybdenum. Prior examples of this result have
(49) Donahue, J. P.; Goldsmith, C. R.; Nadiminti, U.; Holm, R. H. J. Am.
Chem. Soc. 1998, 120, 12869-12881.
(50) Das, S. K.; Chaudhury, P. K.; Biswas, D.; Sarkar, S. J. Am. Chem.
Soc. 1994, 116, 9061-9070.
8098 Inorganic Chemistry, Vol. 43, No. 25, 2004

Metal-Mediated Oxo and Sulfido Transfer Reactions
thermodynamic scale have reaction enthalpies in the range
∆H ) -43 to -74 kcal/mol. The related WIV complexes
are, of course, strong oxo acceptors. Reactions 11-13 are
classified as intermetal oxo transfer processes.5
[W(OPh)(S2C2Me2)2]1- + [MoO2(bdt)2]2-
[WO(OPh)(S2C2Me2)2]1- + [MoO(bdt)2]2-
f
(11)
(12)
[W(OPh)(S2C2Me2)2]1- + [WO2(bdt)2]2-
f
[WO(OPh)(S2C2Me2)2]1- + [WO(bdt)2]2-
[WO(bdt)2]2- + [MoO2(bdt)2]2-
[WO2(bdt)2]2- + [MoO(bdt)2]2- (13)
f
Figure 5. Stopped-flow time-resolved UV/visible spectra of the reaction
system [W(O-p-C6H4COMe)(S2C2Me2)2]1-/Ph3AsS (both at 25 mM) in
acetonitrile at 298 K. Spectra were recorded at 5-s intervals. Inset: kinetics
trace of the reaction at 500 nm (dots) superimposed with a second-order fit
(k2 ) 66.7 M-1 s-1).
[MoO2(bdt)2]2- + Ph3As f
[MoO(bdt)2]2- + Ph3AsO (14)
been observed for both synthetic and enzymic systems.2,4,51
The present results provide the first demonstration that the
same regularity applies to sulfido transfer. Both sets of results
reveal a kinetic metal effect on atom transfer.
Me3NO > (CH2)4SO > [MoO2(bdt)2]2- > {Ph3AsO >
[WO2(bdt)2]1-} > [WO(OPh)(S2C2Me2)2]1- > Ph3PO >
Ph2MePO (15)
Atom Donor/Acceptor Series. We have introduced a
thermodynamic series for oxo transfer based on the enthalpies
A thermodynamic scale for sulfur atom transfer has not
yet been devised. Reaction 16 with excess phosphine
proceeds rapidly and nearly completely, showing that 6a is
a better sulfur atom donor than Ph3PS. With this and other
information,26,28 the short sulfido donor series 17 is formu-
lated.
1
or free energies of the reaction X + /2O2 f XO, allowing
an ordering of molecules X/XO on the basis of their affinity
for acceptance or donation of an oxygen atom.48 In the
absence of thermodynamic data, certain partial series may
be constructed on the basis of qualitative observations of
reactivity. In the systems following, reactions were performed
at ambient temperature. For example, reactions 11 (1:1) and
12 (5:1), with the indicated MVIO2/WIV molar ratios and
involving previously reported M ) MoVI,IV 34,49 and
WVI,IV 13,52 complexes, proceed to at least 80% completion
in acetonitrile in ca. 1 h. The reactions were monitored by
1H NMR, which distinguishes reactants and products,
especially 2a (δMe 2.61) and 4a (δMe 2.19). In these reactions,
the [MO2(bdt)2]2- complexes are stronger oxo donors than
the WVIO dithiolene product. No reaction occurs between
[MO(bdt)2]2- or 2a and Ph3PO. Further, reaction 13 occurs
to ca. 70% completion within 1 h, confirming the expected
result that MoVIO2 is a stronger donor than WVIO2 in the bdt
series. Reaction 14 proceeds completely to the right with
excess Ph3As; therefore, [MoO2(bdt)2]2- is a stronger oxo
donor than Ph3AsO. The system [WO(bdt)2]2-/excess Ph3AsO
was not well-behaved. No reaction is observed under the
same conditions with [WO2(bdt)2]2-, nor with any WIV or
MIVO complex and Ph3PO. On the basis of observations in
this and previous work,1,2,26,34 oxo donor series 15 is
formulated, in which only isolable complexes are included
and the bracketed entries cannot be ordered with information
currently available. As might be expected, the oxo-WVI
complexes are relatively weak oxo donors and on the
[WS(OPh)(S2C2Me2)2]1- + Ph3P f
[W(OPh)(S2C2Me2)2]1- + Ph3PS (16)
Ph3SbS > Ph3AsS > Ph2MeAsS >
[WS(OPh)(S2C2Me2)2]1- > Ph3PS > Ph2MePS (17)
Reaction Pathway. To address the question of the
pathway of oxo transfer mediated by bis(dithiolene) molyb-
denum and tungsten complexes, the reactions 18 were
examined in some detail.2,3 In common with reactions 7 and
8, these are second-order with large negative activation
entropies and associative transition states. Axial (X′) and in-
plane (Y) ligand substituents were independently varied,
affording linear free energy relationships including ∆Gq
versus σp. The most important finding was that electron-
withdrawing groups (EWGs) accelerate rates and electron-
donating groups (EDGs) decrease rates, relative to the system
with X′ ) Y ) H, in reactions with constant substrate. It
was concluded that the probable pathway proceeds through
an early transition state with substantial XO‚‚‚W bond-
making character. This description is thought to apply to the
[WIV(OC6H4-p-X′)(S2C2(C6H4-p-Y)2)2]1- + (CH2)4SO f
(51) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998, 37,
[WVIO(OC6H4-p-X′)(S2C2(C6H4-p-Y)2)2]1- + (CH2)4S
1602-1608.
(52) Ueyama, N.; Oku, H.; Nakamura, A. J. Am. Chem. Soc. 1992, 114,
7310-7311.
(18)
Inorganic Chemistry, Vol. 43, No. 25, 2004 8099

Wang et al.
Figure 7. Linear free-energy relationships between the activation free
energies and substituent constants for reactions 7 and 8. Correlation
coefficients and slopes (kcal/mol): MoIV/Ph3AsO, R ) 0.934, -2.61; MoIV
/
Ph3AsS, R ) 0.894, -0.94; WIV/Ph3AsO, R ) 0.972, -1.26; WIV/Ph3AsS,
R ) 0.994, -2.71.
plots, we have used relatively recent substituent constants
for -CH3 (-0.14) and -OCH3 (-0.12)53 that are nearly the
same. The negative slopes convey that EWGs groups
accelerate rates in the four reaction sets [MX′/O] and [MX′/
S]. The effect is best developed in the set [WX′/S], which
has the largest slope and five reaction components.
Figure 6. Eyring plots for two sulfido-transfer systems in acetonitrile based
on [M(O-p-C6H4OMe)(S2C2Me2)2]1- with M ) MoIV (upper) and WIV
(lower).
corresponding molybdenum systems which also display
second-order kinetics and an associative pathway.
Concerning feature i, the same trend in parameters has
been observed in the systems 1a, 2a/(CH2)4SO, for which
∆Sq ) -39 (Mo) and -33 (W) eu.1,2 In this sense, the
behavior of the two oxo donor substrates is consistent. The
main contributors to ∆Sq are rearrangement of the MIV
complex from square pyramidal to a distorted octahedral
configuration like that of 2a or 6a and concomitant substrate
binding and solvation changes. The structures of molybde-
num complexes 3 and 5, while unavailable, surely will be
isostructural with the corresponding tungsten complexes. The
smaller values of ∆SqW may reflect a less solvated transition
state, perhaps with more tightly bound substrate.
Feature ii points to a differential ∆Hq component to rate
differences in the systems [MX′/O] versus [MX′/S], which
could reflect strengths of M‚‚‚QAsPh3 bond-making or As-Q
bond-weakening in the transition state. As one measure of
basicity, we note that Ph3AsO is readily protonated and that
As-O bond lengths in various (Ph3AsOH)+ salts (1.69-
1.73 Å)54-58 exceed that in Ph3AsO by ca. 0.04-0.08 Å.
We are unaware of structural information for protonated
Ph3AsS. Attachment of either substrate to an electrophilic
center, as in a transition state, is predicted to weaken the
Q-As bond. The only available measure of relative basicity
An analogous but less extensive approach to the reaction
pathway was applied to reactions 7 and 8 utilizing MoIV
complexes 1a-1c and 1e and WIV complexes 2a-2c, 2e,
and 2f. The data in Table 2 may be considered in terms of
the reaction sets [MIVX′/Q] in which the metal M ) Mo or
W, para substituent X′ of the axial phenolate, and substrate
containing Q ) O or S are independently varied and other
components of the set are held constant. Activation param-
eters reveal that |T∆Sq| > ∆Hq except for the set [WIVX′/
O]; these values are apportioned such that overall ∆Hq is
ca. 30-60% of ∆Gq. Consequently, neither enthalpy nor
entropy factors predominate. Certain features of the activation
parameters emerge across the reaction sets.
(i) Variable M, constant Q and X′: ∆Hq > ∆Hq
,
Mo
W
|T∆Sq|Mo > |T∆Sq|W. The faster rates with tungsten at
constant substrate, expressed by the reactivity ratios 78-
184 (Q ) O) and 16-89 (Q ) S), are due mainly to smaller
(less negative) activation entropies which tend to compensate
the 2-3 kcal/mol higher activation enthalpies.
O
S
(ii) Variable Q, constant M and X′: ∆Hq > ∆Hq
,
M
M
S
|T∆Sq|M > |T∆Sq|MO. Smaller activation enthalpies are
mainly responsible for faster rates with Ph3AsS than those
with Ph3AsO. The constant order of activation entropies
(although small differences) may reflect a large extent of
solvation of the more polar Ph3AsO in achieving the
transition state.
(53) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry; Kluwer
Academic/Plenum Publishers: New York, 2000; Part A, p 208.
(54) Ferguson, G.; Macaulay, E. W. Chem. Commun. 1968, 1288-1290.
(55) March, F. C.; Ferguson, G. J. Chem. Soc., Dalton Trans. 1975, 1381-
1384.
(iii) Variable X′, constant M and Q: ∆Hq and |T∆Sq| are
essentially constant contributors to ∆Gq within each set; for
example, in [WX′/O] and [WX′/S] ∆Hq is 55-57% and 41-
43%, respectively. The effects are manifested as the linear
free energy relationships ∆Gq versus σp in Figure 7. In these
(56) Jones, P. G.; Olbrich, A.; Schwarzmann, E. Acta Crystallogr. 1988,
C44, 2201-2202.
(57) Massabni, A. C.; Nascimento, O. R.; Halvorson, K.; Willett, R. D.
Inorg. Chem. 1992, 31, 1779-1784.
(58) Batt, R. V.; Bullivant, D. P.; Elkington, K. E.; Hill, S. E.; Hilton, J.;
Houghton, T. J.; Hovell, M.; Wallwork, S. C. Polyhedron 1998, 17,
2173-2178.
8100 Inorganic Chemistry, Vol. 43, No. 25, 2004

Metal-Mediated Oxo and Sulfido Transfer Reactions
Figure 8. Simple depiction of the proposed reaction pathway for the atom transfer reactions of tungsten complexes 2 with Ph3AsQ to produce oxo complexes
4 and sulfido complexes 6. The associative transition state is proposed to involve Q‚‚‚W bond-making and Q-As bond weakening. Systems based on the
MoIV complexes 1 are expected to have an analogous pathway.
in solution indicates that Ph3AsO (pKa 1.25) is a stronger
proton base than Ph3AsS (pKa -2.91) in acetic anhydride.59
Feature iii parallels the behavior of the reactions 18. We
suggest a pathway, depicted in Figure 8, which implicates a
transition state with certain probable characteristics. Its
structure is expected to resemble the distorted cis-octahedral
configurations of oxo and sulfido reaction products 4a2 and
6a,14 respectively. Appreciable M‚‚‚QAsPh3 bond making
is consistent with the linear free energy behavior in Figure
7. Here, EWGs would make the MIV center relatively more
electrophilic, which should promote substrate binding and
therewith lower ∆Gq for formation of an associative transition
state. Some degree of Q-As bond weakening reflects the
ca. 30 kcal/mol lower bond energy of the sulfide than that
of the oxide and in this interpretation is a contributor to the
faster reaction rates of the former substrate. The bond
weakening feature was not detected in reaction 18, where
the substrate is constant. Bond strengths may contribute to
the ca. 106 times larger rate constant for the reaction of
molybdenum complex 1a with Me3NO (D ) 61 kcal/mol)1,60
than that of 1a with (CH2)4SO (D ) 86-87 kcal/mol).48,61
More detailed discussions of the factors which underlie the
proposed reaction pathway together with depictions of
reaction coordinates are presented elsewhere.2,3
(2) Sulfido transfer to a MoIV or WIV center is always faster
than oxo transfer at parity of ligand by a nearly constant
S
O
factor: k2 /k2 ) 10-14.
(3) Oxo or sulfido transfer to a WIV center is always faster
W
than to a MoIV center at parity of ligand and substrate: k2
k2 ) 78-184 (oxo), 16-89 (sulfido).
/
Mo
(4) Increasing electron-withdrawing capacity of group X′
in molybdenum or tungsten complexes [MIV(O-p-C6H4X′)-
(S2C2Me2)2]1- increases the rate of sulfido or oxo transfer.
(5) The proposed transition state involves a significant
component of bond making between substrate and WIV (or
MoIV) center and concomitant As-O or As-S bond weak-
ening. A common reaction pathway having the characteristics
in 1 appears operative in oxo and sulfido transfer reactions
of bis(dithiolene)MoIV and -WIV complexes.1-3
In assessing the findings of this investigation, it should
be borne in mind that comparative rate effects are small and
that the main conclusions are based on trends rather than on
quantitative differences. In this regard, note the results for
Ph3SbS (Table 2), a known sulfur atom donor26,27 with a
stereochemistry strictly analogous to Ph3AsQ.62 With the
MoIV and WIV, this substrate reacts more slowly by a factor
of 5 than does Ph3AsS, whose bond energy of 70 kcal/mol
is only 3 kcal/mol more than that estimated for Ph3SbS.28
We are uncertain as to the cause of this small but unexpected
rate reversal.
Summary. The following are the principal results and
conclusions of this investigation, whose primary purpose has
been determination of the relative kinetics of oxygen and
sulfur atom transfer in the form of reactions 7 and 8 in
acetonitrile with the homologous substrates Ph3AsO and
Ph3AsS. Rate constants refer to 298 K.
Acknowledgment. This research was supported by NSF
grants CHE 0237419 at Harvard University and 0111202 at
Tufts University. We thank Dr. J. Jiang for a crystal structure
determination and Dr. S. C. Lee for useful discussions.
(1) All oxo and sulfido transfer reactions follow second-
order kinetics with associative transition states connoted by
large negative activation entropies: ∆Sq ) -39 to -45
Supporting Information Available: X-ray crystallographic files
in CIF format for the structure determination of the compound in
Table 1. This material is available free of charge via the Internet
at http://pubs.acs.org.
Mo
eu, ∆Sq ) -25 to -33 eu.
W
(59) Gel’fond, A. S.; Galyametdinov, Yu. G.; Khalikova, N. A.;
Chernokal’skii, B. D. Zh. Obshch. Khim. 1976, 46, 2077-2082.
(60) Acree, W. E., Jr.; Tucker, S. A.; Ribeiro da Silva, M. D. M. C.; Matos,
M. A. R.; Gonc¸alves, J. M.; Ribeiro da Silva; Pilcher, G. J. Chem.
Thermodyn. 2000, 27, 391-398.
(61) Jenks, W. S.; Matsunaga, N.; Gordon, M. J. Org. Chem. 1996, 61,
1275-1283.
IC040087F
(62) Pebler, J.; Weller, F.; Dehnicke, K. Z. Anorg. Allg.Chem. 1982, 492,
139-147.
Inorganic Chemistry, Vol. 43, No. 25, 2004 8101