Kinetic and mechanistic studies of sulfur transfer from imidomethylrhenium sulfides

Article abstract of DOI:10.1021/ic0255196

The bis(2,6-diisopropylphenylimido)methylrhenium(VII) sulfide dimer, {CH₃Re(NAr)₂}₂(μ-S)₂ (1), reacts with a 1:1 amount of a phosphine or an alkyl isocyanide to yield a dimeric rhenium(VI) species, {CH₃Re(NAr)₂}₂(μ-S) (2), which has been structurally characterized. The two rhenium atoms in 2 are within bonding distance, 280 pm, more than 90 pm shorter than in 1. With excess L, 1 reacts to give a monomeric rhenium(V) complex, CH₃Re(NAr)₂L₂ (3A, L = PZ₃, Z = alkyl, aryl; 3B, L = isocyanide). The rate of formation of 3A is first-order with respect to [1] and second-order with respect to monodentate phosphine concentrations. With bidentate phosphines, however, the order with respect to the phosphine drops to unity. The addition of another (nonoxidizable) coordinating ligand, such as pyridine or one of its derivatives, accelerates the formation of 3A. In the presence of a pyridine ligand the reaction is first-order with respect to phosphine concentration, both monodentate and bidentate. The reactions between phosphines and 2 are slower than those with 1, which excludes [CH₃Re(NAr)₂]₂(μ-S) from being the intermediate in the reactions of 1. To account for that, we have proposed an intervening species that partitions between transformation to 3 with excess L and to 2 otherwise.

Full text of DOI:10.1021/ic0255196

Inorg. Chem. 2002, 41, 4780−4787
Kinetic and Mechanistic Studies of Sulfur Transfer from
Imidomethylrhenium Sulfides
Wei-Dong Wang, Ilia A. Guzei,† and James H. Espenson*
Ames Laboratory and Department of Chemistry, Iowa State UniVersity of Science and Technology,
Ames, Iowa 50011
Received January 29, 2002
The bis(2,6-diisopropylphenylimido)methylrhenium(VII) sulfide dimer, {CH Re(NAr)2}2(µ-S)2 (1), reacts with a 1:1
3
amount of a phosphine or an alkyl isocyanide to yield a dimeric rhenium(VI) species, {CH Re(NAr)2}2(µ-S) (2),
3
which has been structurally characterized. The two rhenium atoms in 2 are within bonding distance, 280 pm, more
than 90 pm shorter than in 1. With excess L, 1 reacts to give a monomeric rhenium(V) complex, CH Re(NAr)2L2
3
(3A, L ) PZ , Z ) alkyl, aryl; 3B, L ) isocyanide). The rate of formation of 3A is first-order with respect to [1]
3
and second-order with respect to monodentate phosphine concentrations. With bidentate phosphines, however,
the order with respect to the phosphine drops to unity. The addition of another (nonoxidizable) coordinating ligand,
such as pyridine or one of its derivatives, accelerates the formation of 3A. In the presence of a pyridine ligand the
reaction is first-order with respect to phosphine concentration, both monodentate and bidentate. The reactions
between phosphines and 2 are slower than those with 1, which excludes [CH Re(NAr)2]2(µ-S) from being the
3
intermediate in the reactions of 1. To account for that, we have proposed an intervening species that partitions
between transformation to 3 with excess L and to 2 otherwise.
Introduction
Compared to the abundant reports on oxo-transfer reac-
tions, relatively few studies have been published on the
analogous sulfur-transfer processes.1-5 Rhenium(VII) and
rhenium(V) complexes are believed to play important roles
in the catalytic desulfurization of thiiranes.6 Here we report
our studies on the sulfur transfer from rhenium(VII) and
rhenium(VI) sulfido complexes to phosphines (PZ3, Z )
alkyl, aryl) and isocyanides, eqs 1 and 2.
Experimental Section
Materials and Instrumentation. Unless stated otherwise, the
phosphines and alkyl isocyanides used for this work were purchased
from Aldrich or Strem, and used as received. Benzene (Aldrich),
toluene-d8 (Aldrich), and benzene-d6 (CIL) were dried with sodium/
benzophenone and stored in a nitrogen-filled glovebox. The
preparation of {MeRe(NAr)2}2(µ-S)2 (1, Ar ) 2,6-diisopropyl-
phenyl) has been reported previously.7 The synthesis of Me2-
PCH2P(O)Me2 was adapted from the reported procedure.8,9 Phos-
phine disulfides Me2P(S)(CH2)nP(S)Me2 (n ) 1 or 2) were prepared
* Author to whom correspondence should be addressed. E-mail:
espenson@ameslab.gov.
† Current address: Department of Chemistry, University of Wisconsin,
Madison, WI 53706.
(1) Woo, L. K. Chem. ReV. 1993, 93, 1125.
(2) Berreau, L. M.; Woo, L. K. J. Am. Chem. Soc. 1995, 117, 1314.
(3) Foley, S. R.; Richeson, D. S.; Darrin, S. Chem. Commun. 2000, 1391.
(4) Young, C. G.; Laughlin, L. J.; Colmanet, S.; Scrofani, S. D. B. Inorg.
Chem. 1996, 35, 5368.
(5) Hall, K. A.; Mayer, J. M. J. Am. Chem. Soc. 1992, 114, 10402.
(6) Jacob, J.; Espenson, J. H. Chem. Commun. 1999, 1003.
(7) Wang, W.-D.; Guzei, I. A.; Espenson, J. H. Inorg. Chem. 2000, 39,
4107.
(8) Mading, P.; Scheller, D. Z. Anorg. Allg. Chem. 1988, 567, 179.
(9) Robertson, R. A. M.; Poole, A. D.; Payneb, M. J.; Cole-Hamilton, D.
J. Chem. Commun. 2001, 47.
4780 Inorganic Chemistry, Vol. 41, No. 18, 2002
10.1021/ic0255196 CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/16/2002

Sulfur Transfer from Imidomethylrhenium Sulfides
Table 1. Crystallographic Data for {MeRe(NAr)2}2(µ-S) (2, Ar )
by mixing the bidentate phosphines with 2 equiv of sublimed sulfur
in benzene, isolated as white precipitates and analyzed by com-
parison with the literature NMR data.8,10,11 A Bruker DRX-400
spectrometer was used to collect the 1H, 13C, and 31P NMR spectra.
The 1H and 13C chemical shifts were measured relative to residual
2,6-Diisopropylphenyl)
empirical formula C50H74N4Re2S
fw
1135.59
temp
cryst syst
unit cell
173(2) K
triclinic
a ) 10.9556(5) Å unit cell
b ) 13.4586(6) Å
c ) 19.3160(9) Å
2481.5 (2) Å3
1.520 g/cm3
1136
l
0.71073 Å
P1h
space group
R ) 105.780(1)°
1H and 13C resonances in the deuterated solvents C6D5H (δH
)
P
dimensions
dimensions â ) 93.711(1)°
7.16 ppm, δC ) 128.39 ppm) and C6D5CD2H (δ ) 2.09 ppm). 31
γ ) 112.715(1)°
chemical shifts were referenced to 85% H3PO4. Kinetics experi-
ments were carried out under N2 at 298 K by the use of a Shimadzu
UV 3101PC equipped with a thermoelectrical cell holder. Elemental
analyses were performed by Desert Analytics.
Kinetics. The stock solutions of {MeRe(NAr)2}2(µ-S)2 (1) and
{MeRe(NAr)2}2(µ-S) (2) in C6H6 were prepared and stored in a
glovebox. All kinetic runs were performed with at least a 10-fold
excess of phosphine or isocyanide over the rhenium complex. For
more rapid reactions with PMe3, dmpe, and dmpm, kinetic traces
for the formation of CH3Re(NAr)2(PZ3)2 (3A) at 630 nm were
recorded to >4 half-times. Equation 3
vol
Z
m
2
density(calcd)
49.51 cm-1
F000
Ra
0.0231
Rw
0.0484
a
2
-
2
2
a R(wF2) ) ∑[w(Fo
Fc)|.
Fc )]/∑[w(Fo )2]1/2. R ) ∑∆/∑(Fo). ∆ ) |(Fo
-
12H); (13C) δ -31.0 (ReCH3); (31P) δ -23.3 (virtual t, J ) 10.8
Hz), -57.9 (t, J ) 10.8 Hz).
MeRe(NAr)2(CNtBu)2. NMR (C6D6, 280 K): (1H) δ 7.18 (m,
4H), 6.97 (t, J ) 7 Hz, 2H), 3.88 (septet, J ) 6.8 Hz, 4H), 2.91 (s,
3H, ReCH3), 1.47 (d, J ) 6.8 Hz, 24H), 0.74 (s, 18H); (13C) δ
-29.1 (ReCH3).
Abst ) Abs∞ + (Abs0 - Abs∞) × exp(-kψt)
MeRe(NAr)2(CNnBu)2. NMR (C6D6, 283 K): (1H) δ 6.9-7.2
(6H), 3.94 (septet, J ) 6.8 Hz, 4H), 2.96 (s, 3H, ReCH3), 2.66 (t,
J ) 6 Hz, 4H), 0.8 (m, 8H), 0.47 (t, J ) 6 Hz, 6H); (13C) δ -29.0
(ReCH3).
was used to fit the absorbance-time data. For PMe2Ph, the data
were fit by the method of initial rates. The early stages of the
absorbance-time curve at 629 nm (ꢀ3 ) 1.2 × 104 L mol-1 cm-1
)
X-ray Crystallography. A yellow crystal of 2 with approximate
dimensions 0.35 × 0.25 × 0.24 mm3 was selected under oil. The
crystal was mounted to the tip of a glass capillary in a stream of
cold nitrogen at 173 K and centered in the X-ray beam by using a
video camera. The data collection was performed on a Bruker CCD-
1000 diffractometer with Mo KR (λ ) 0.71073 Å) radiation. The
final cell constants were calculated from a set of 5360 strong
reflections from the actual data collection. The data were collected
by using the hemisphere data collection routine. The reciprocal
space was surveyed to the extent of 1.9 hemispheres to a resolution
of 0.80 Å. A total of 22192 data were harvested by collecting three
sets of frames with 0.3° scans in ω with an exposure time 30 s per
frame. These highly redundant datasets were corrected for Lorentz
and polarization effects. The absorption correction was based on
fitting a function to the empirical transmission surface as sampled
by multiple equivalent measurements.13
The systematic absences in the diffraction data were consistent
for the space groups P1 and P1h.14 The E-statistics strongly suggested
the centrosymmetric space group P1h that yielded chemically
reasonable and computationally stable results of refinement. A
successful solution by direct methods provided most non-hydrogen
atoms from the E-map. The remaining non-hydrogen atoms were
located in an alternating series of least-squares cycles and difference
Fourier maps. All non-hydrogen atoms were refined with anisotropic
displacement coefficients. All hydrogen atoms were included in
the structure factor calculation at idealized positions and were
allowed to ride on the neighboring atoms with relative isotropic
displacement coefficients. The final least-squares refinement of 533
parameters against 10037 data resulted in residuals R (based on F2
for I g 2σ) and Rw (based on F2 for all data) of 0.0231 and 0.0484,
respectively. Crystallographic data for 2 are given in Table 1.
were fit to the equation of a straight line; the slope of that line,
Abs′, was converted into the initial rate (in concentration units) by
division by ∆ꢀ3 (1.2 × 104 L mol-1 cm-1. None of the other species
absorb at this wavelength), the difference in molar absorptivity
between reactants and products at wavelength λ: Vi ) Abs′/∆ꢀλ.
Activation parameters were calculated over a temperature range of
283-313 K from the least-squares fit of values of ln(k/T) vs 1/T,
according to eq 4.
k
∆Sq ∆Hq
kB
ln
) ln
+
-
(4)
(T)
(h )
R
RT
Preparation of {MeRe(NAr)2}2(µ-S), 2. To a 50 mL hexane
solution of {CH3Re(NAr)2}2(µ-S)2 (100 mg, 0.0856 mmol) was
added under nitrogen 12.2 µL of PMe2Ph (0.0856 mmol). The
solution was stirred for 16 h, and then the solvent volume was
reduced by vacuum to about 5 mL. The concentrated solution was
cooled to -20 °C, yielding red crystals. NMR (C6D6, 298K): (1H)
δ 1.07 (d, J ) 8.8 Hz, 12H), 1.10 (d, J ) 8.8 Hz, 12H), 1.20 (d,
J ) 8.8 Hz, 24H), 3.41 (s, 3H), 3.45 (s, 3H), 3.69 (septet, J ) 8.8
Hz, 4H), 3.89 (septet, J ) 8.8 Hz, 4H), 7.0 (m, 12H); (13C) δ -4.72
(CH3Re), 2.56 (CH3Re). EI MS: m/z 1135. Anal. Found (Calcd
for C50H74N4Re2S, FW ) 1135.6): C, 52.64 (52.88); H, 6.46 (6.57);
N, 4.85 (4.93); S, 2.76 (2.82).
Characterization of MeRe(NAr)2L2 (3). The structures and
elemental analyses for L ) dmpe and PMe2Ph, as well as
spectroscopic data for 10 analogous compounds, have been reported
previously.12 The corresponding dmpm and RNC complexes,
unreported before, have spectroscopic data very similar to those of
the fully characterized ones (see the Supporting Information).
MeRe(NAr)2(dmpm)2. NMR (C6D6, 298 K): (1H) δ 7.14 (d, J
) 7 Hz, 2H), 7.05 (d, J ) 7 Hz, 2H), 6.92 (t, J ) 7 Hz, 2H), 3.84
(septet, J ) 6.8 Hz, 2H), 3.49 (septet, J ) 6.8 Hz, 4H), 2.83 (t, J
) 4.6 Hz, 3H, ReCH3), 1.68 (m, 4H), 1.38 (br s, 12H), 1.36 (d, J
) 6.8 Hz, 12H), 1.29 (d, J ) 6.8 Hz, 12H), 0.66 (d, J ) 3.6 Hz,
Results
Reactions with Phosphines. When a stoichiometric
amount of PMe2Ph was used to react with 1, the color of
(10) Maeding, P. Z. Chem. 1986, 26, 408.
(11) Karsch, H. H. Chem. Ber. 1982, 115, 818.
(12) Wang, W.-D.; Guzei, I. A.; Espenson, J. H. Organometallics 2001,
20, 148.
(13) Blessing, R. H. Acta Crystallogr., A 1995, 51, 33.
(14) All software and sources of the scattering factors are contained in the
SHELXTL (version 5.1) program library (G. Sheldrick, Bruker
Analytical X-ray Systems, Madison, WI).
Inorganic Chemistry, Vol. 41, No. 18, 2002 4781

Wang et al.
the C6D6 solution slowly changed from green-brown to the
orange-red color characteristic of 2. At completion, only the
free SdPMe2Ph at 31.2 ppm was observed in the 31P NMR
previously, establishing that dmpe acts as a monodentate
ligand.12 Only one of its phosphorus atoms is coordinated
to rhenium, and the two dangling bifunctional phosphines
lie trans to each other. As in the case of the dmpe analogue,
the dmpm product also showed two virtual triplets in the
31P NMR spectrum. The triplet at -23.6 ppm coupled with
the triplet at -59.7 ppm as revealed by the 31P-31P 2D
experiment. The virtual triplet pattern of the 31P resonances
of the dmpm product did not change as the magnetic field
changed from 81 to 202 MHz. For both dmpe and dmpm,
the phosphine products were Me2P(S)(CH2)nPMe2 (n ) 1,
2); the possible disulfide products Me2P(S)(CH2)nP(S)Me2
were not observed. The reaction of 1 with an excess of
Me2P(O)CH2PMe2 yielded a 3A-type Re(V) species and
Me2P(O)CH2P(S)Me2. Detailed spectroscopic data are given
in the Supporting Information.
Certain control experiments were performed to demon-
strate the lack of SsP redistribution reactions between real
or potential PdS products. It was shown that the sulfur
transfer between dppe and Me2P(S)CH2CH2P(S)Me2 or
SdPMe2Ph did not occur under the experimental conditions,
whether potential rhenium catalysts were absent or not.
Compound 3A was also observed when excess dmpe, dmpm,
P(OMe)2Ph, PMe3, or PMe2Ph was added to a solution of 2,
eq 7. However, no further reaction between 2 and an excess
of PMePh2 was observed in 2 days at room temperature. The
reaction of 2 and P(OMe)3 was also very sluggish.
1
spectrum. The H NMR spectrum showed two singlets at
3.45 and 3.41 ppm with the same intensity; the corresponding
13C chemical shifts were at 2.56 and -4.72 ppm, respectively.
These resonances were assigned to the two inequiValent
methyl groups bound to the rhenium atoms of 2. There were
also two sets of septets from the methine protons of the
1
arylimido groups, which a H-1H 2D-COSY experiment
showed were coupled to three doublets of the methyl protons
of isopropyl moieties at ∼1 ppm. Other phosphines, such as
1
PMe3, dmpe, and dmpm, also led to identical H and 13C
NMR spectra for rhenium product 2. On the basis of the
1
intensities of the H signals and the lack of indication of
phosphine binding, the rhenium product of the reactions
between 1 and an equimolar amount of phosphine (with an
excess, further reaction ensued) was formulated as {MeRe-
(NAr)2}2(µ-S) (2), eq 5.
{MeRe(NAr)2}2(µ-S)2 + PZ3 f
{MeRe(NAr)2}2(µ-S) + SdPZ3 (5)
The presence of a single sulfur atom in 2 is consistent with
the elemental analysis and the EI mass spectrum. The
inequivalent methyl groups as shown in the NMR spectra
suggest that the two rhenium centers are found in different
chemical environments. This has been confirmed by the
X-ray diffraction studies, as presented subsequently. Unex-
pectedly, the chemical shifts of the two rhenium methyl
protons approached one another as the temperature decreased.
For example, the chemical shift difference in C7D8 was 0.082
ppm at 323 K, but dropped to 0.027 ppm at 283 K. The two
singlets merged into a single resonance at 263 K. When the
temperature was decreased further, two singlets reappeared.
The variable-temperature NMR data suggest that the two
methyl groups remain inequivalent through the above-
mentioned temperature range and the Re-Re bond is likely
intact.
Compound 2 and phosphine sulfide were also formed with
P(OMe)3, P(OMe)2Ph, P(OMe)Ph2, PMePh2, and P(iPr)3.
With these phosphines, but not the ones given before, 2
persists long enough to be observed eVen in the presence of
excess phosphine. No reaction between 1 and an excess of
PPh3 or PCy3 was observed in 2 days at room temperature.
When PMe2Ph was added in g10-fold excess to a C6D6
solution of 1, the color of the solution changed to the intense
green characteristic of 3A. Compound 2 was not observed;
however, 1H and 31P NMR spectra indicated that the
structurally characterized compound MeRe(NAr)2(PMe2Ph)2
was produced.12 Similar products MeRe(NAr)2(PZ3)2 (3A)
were also observed when excesses of PMe3, dmpe, and dmpm
were used, eq 6.
excess PZ3
{MeRe(NAr)2}2(µ-S)
8
MeRe(NAr)2(PZ3)2 + SdPZ3 (7)
Reactions with Isocyanides. Like phosphines, alkyl
n
t
isocyanides RNC (R ) Bu and Bu) react with 1 equiv of
1 to yield RNCS and 2, both of which were identified by
1
their H and 13C NMR resonances. When excess RNC was
used to react with 1, or more RNC was added to a solution
of 2, the compound MeRe(NAr)2(CNR)2, 3B, was recognized
from its spectroscopic similarities to 3A. Specifically, the
electronic spectrum of the deep blue-green colored MeRe-
(NAr)2(CNtBu)2 also showed an intense band at 609 nm (ꢀ3B
) 1.5 × 104 L mol-1 cm-1), paralleling the 580-630 nm
band of 3A. For the tBuNC product, the sharp singlet at 2.91
1
ppm in the H NMR spectrum with a corresponding 13C
chemical shift at -29.1 ppm assigned to the methyl group
bound to rhenium was characteristic of CH3Re(NAr)2(L)2
(3) complexes. At 280 K, the resonances for the free and
coordinated tBuNC ligand at 0.85 and 0.74 ppm, respectively,
were noticeably broader than that for tBuNCS at 0.77 ppm.
The NAr groups exhibited a septet and a doublet at 3.88
and 1.47 ppm with the same coupling constant of 6.8 Hz.
The integrations for the rhenium methyl, coordinated tBuNC,
and NAr moieties were consistent with the formula of MeRe-
1
(NAr)2(CNBut)2. The H NMR resonance of the Re-CH3
group shifted from 2.91 ppm in C6D6 at 280 K to 2.79 ppm
at 320 K.
excess PZ3
{MeRe(NAr)2}2(µ-S)2
8
Addition of PMe2Ph or dmpe to the solution of MeRe-
(NAr)2(CNBut)2 yielded 3A and free isocyanide. Ligand
substitution was also observed with P(OMe)3. Qualitatively,
MeRe(NAr)2(PZ3)2 + SdPZ3 (6)
The molecular structure of the dmpe product 3A was reported
4782 Inorganic Chemistry, Vol. 41, No. 18, 2002

Sulfur Transfer from Imidomethylrhenium Sulfides
Figure 2. Second-order dependences of the pseudo-first-order rate constant
(PMe3) and the rhenium-normalized initial rate (PMe2Ph) for reactions of
1 on the concentrations of these monodentate phosphines in benzene at
298 K.
in 1. It should be noted that the two rhenium centers are not
chemically equivalent even though the sulfido bridge is
symmetric. The methyl group on one rhenium is pointing
away from the other rhenium, while the other methyl group
is pointing toward the neighboring rhenium. The different
environments in which the two methyl groups reside may
result in the peculiar H chemical shift changes in the
variable-temperature studies. Selected bond distances and
angles are given in Table 2.
The analogous µ-O complex {MeRe(NAr)2}2(µ-O) has
previously been structurally characterized.12 As expected, the
Re-Re distance in the µ-O case is shorter than that in 2,
268.4 vs 279.7 pm, respectively. Also resulting from the size
difference of oxygen and sulfur, the O-Re-Re angle is
about 6° less than the S-Re-Re angle. The remarkable
difference between the two compounds is the arrangement
of the two methyl groups. The oxo compound adopts a
syn,syn arrangement. On the other hand, the sulfido complex
exists in a syn,anti arrangement with Cs symmetry. 1H NMR
studies in the range 243-323 K showed no indication of a
syn,anti to syn,syn conversion for the µ-S compound. Even
at 263 K, when the two methyl proton resonances merged
into one, there were still two resonances for the methine
protons of the isopropyl groups, in contrast to one methyl
resonance and one methine resonance observed for the µ-O
complex. The reversibility of the variable-temperature spectra
also excludes the possibility of a process involving kinetic
and thermodynamic isomers.
Figure 1. An ORTEP view of the molecular structure of {MeRe(NAr)2}2-
(µ-S) (2, Ar ) 2,6-diisopropylphenyl) with thermal ellipsoids at the 30%
probability level.
Table 2. Selected Bond Lengths (pm) and Bond Angles (deg) for
{MeRe(NAr)2}2(µ-S) (2, Ar ) 2,6-Diisopropylphenyl)
1
Re(1)-Re(2)
Re(1)-N(1)
Re(1)-N(2)
Re(1)-C(1)
Re(1)-S
279.74(2)
173.8(3)
174.9(3)
214.4(3)
229.65(9)
Re(2)-S
228.93(9)
176.2(3)
176.1(3)
213.9(4)
Re(2)-N(3)
Re(2)-N(4)
Re(2)-C(26)
Re(1)-S-Re(2)
S-Re(1)-Re(2)
N(1)-Re(1)-N(2)
75.18(3)
52.29(2)
N(1)-Re(1)-C(1)
N(1) -Re(1) -S
99.47(13)
107.00(9)
129.83(13)
121.35(13) N(3) -Re(2) -N(4)
C(1)-Re(1)-Re(2) 75.79(10)
Re(1)-N(1)-C(14) 170.4(2)
C(26)-Re(2)-Re(1) 134.93(11)
Re(2)-N(3)-C(39)
Re(2)-N(4)-C(27)
175.8(4)
176.4(3)
Re(1)-N(2)-C(2)
169.0(2)
t
the binding abilities of BuNC and P(OMe)3 toward Re(V)
are of the same order of magnitude. Unlike most of the
phosphine analogues, only one set of isopropyl groups in
MeRe(NAr)2(CNtBu)2 was observed even in C7D8 at 243 K.
We suggest that their equivalence arises from fast rotation
along the (Ar)C-N bond as a result of the isocyanide ligand
being less sterically demanding than phosphine.
Neither nBuNC nor tBuNC reacts with MeRe(NAr)2O. For
example, addition of 10 mM MeRe(NAr)2O and 10 mM
tBuNC led only to the formation of equilibrated amounts of
MeRe(NAr)3 and MeRe(NAr)O2 after 10 days at room
t
temperature in C6D6. The formation of BuNCO was not
observed.
Kinetics. The initial rate method was used to study the
reaction between 1 and PMe2Ph. The formation of MeRe-
(NAr)2(PMe2Ph)2 was monitored at 629 nm (ꢀ3 ) 1.2 × 104
L mol-1 cm-1), with [1] and [PMe2Ph] varied in the ranges
0.1-0.5 and 7-50 mM, respectively. The initial rate was
first-order with respect to [1], but second-order with respect
to [PMe2Ph], Figure 2 and eq 8. The slope of the plot of
Vi/[1] against [PMe2Ph]2 yielded a third-order rate constant,
km ) 3.3(1) L2 mol-2 s-1. Full kinetic profiles were collected
for reactions with 32 and 42 mM PMe2Ph. The traces fitted
eq 3 exactly, and k ) 3.4 L2 mol-2 s-1 was obtained. The
Structure of 2. The molecular structure of {MeRe-
(NAr)2}2(µ-S) (2) is given in Figure 1. Unlike the asymmetric
bissulfido bridges in {MeRe(NAr)2}2(µ-S)2 (1),7 the mono-
sulfido bridge in 2 is symmetric between the two rhenium
centers with the Re-S bond distance at 229.6 pm. Therefore,
the rhenium atoms in 2 may better be viewed as Re(VI)
instead of one Re(V) and one Re(VII), such that the unpaired
electron on each rhenium interacts to form a Re-Re single
bond. The distance between the two rhenium atoms in 2 is
279.7 pm, which is much shorter than the 371.9 pm distance
Inorganic Chemistry, Vol. 41, No. 18, 2002 4783

Wang et al.
Figure 3. Pseudo-first-order rate constants in benzene at 298 K for the
reactions of {MeRe(NAr)2}2(µ-S)2 (1, Ar ) 2,6-diisopropylphenyl) against
the concentrations of bifunctional phosphines showing that these reactions
are first-order with respect to [dmpe] and [dmpm].
Figure 4. kψ against [PZ3] for the reactions between {MeRe(NAr)2}2(µ-
S) (2, Ar ) 2,6-diisopropylphenyl) and phosphines; both monodentate and
bidentate phosphines show first-order dependences.
Table 3. Summary of Rate Constantsa for the Reactions of Phosphines
with {MeRe(NAr)2}2(µ-S)2 (1) and {MeRe(NAr)2}2(µ-S) (2)
1
2
L
km
kb
k2
PMe3
PMe2Ph
Me2P(O)CH2PMe2
dmpe
dmpm
184(7)
3.3(1)
2.15(4)
0.195(3)
0.017(1)
0.080(9)
0.108(2)
0.070(3)
b
10.3(2)a
32.5(10)a
tert-butyl isocyanide
0.082(2)
a Units: km, L2 mol-2 s-1; kb and k2, L mol-1 s-1
.
b Not determined.
reaction of 1 with PMe3 was faster than that with PMe2Ph,
and kinetic traces for the entire time course were collected.
The plot of kψ against [PMe3]2, Figure 2, gave k ) 184(7)
L2 mol-2 s-1. As with other monodentate phosphines, the
reaction of 1 with Me2P(O)CH2PMe2 studied by the use of
the initial rate method was directly proportional to [Me2P(O)-
CH2PMe2]2. The reactions of 1 with the bifunctional phos-
phines dmpe and dmpm show first-order dependences on
their concentrations, Figure 3 and eq 9. The two rate
equations are given in eqs 8 and 9, and the rate constants km
and kb are summarized in Table 3.
Figure 5. Effects of [Py] on the reaction between 1 and PMe2Ph in C6H6
at 298 K. The intercept is a function of k1[PMe2Ph]2; see the text.
Conditions: [1] ) 0.045 mM; open circles, [PMe2Ph] ) 26 mM; squares,
[PMe2Ph] ) 13 mM; solid circles, [PMe2Ph] ) 7.8 mM. The inset shows
that the slopes are first-order with respect to [PMe2Ph].
-d[1]/dt ) km[1][PZ3]2 (monodentate phosphine) (8)
bound to Re in 1 did not change upon addition of pyridine,
indicating a weak interaction. However, pyridine did show
an effect on the reaction of 1 with PMe2Ph. When [PMe2Ph]
was kept constant at 7.8, 13, or 26 mM, the observed first-
order rate constant for the formation of 3 was linearly
proportional to [Py] in the range 9-68 mM, Figure 5. The
slopes of the straight lines were a linear function of
[PMe2Ph]; see the Figure 5 inset. The above results and those
in the absence of pyridine could be described by eq 11.
-d[1]/dt ) kb[1][Ph2P(CH2)nPPh2] (bidentate phosphine)
(9)
The reaction between 2 and a large excess of PMe3 fit
single-exponential kinetics exactly. The pseudo-first-order
plot, Figure 4, clearly showed that the reaction with 2 was
first-order with respect to [PMe3]. The same rate law holds
for PMe2Ph, dmpe, and dmpm, as shown in Figure 4. The
second-order rate constants from least-squares fitting are
given in Table 3. The rate equation is
kψ ) km[PMe2Ph]2 + km′[PMe2Ph][Py]
(11)
-d[2]/dt ) k2[2][any phosphine]
(10)
The unmistakable intercept in Figure 5 arises from the first
term of eq 11. The computer program Scientist was used to
Phosphine Reactions in the Presence of Pyridine. Unlike
the case of CH3ReO3,15 the chemical shift of the CH3 protons
(15) Wang, W.-D.; Espenson, J. H. J. Am. Chem. Soc. 1998, 120, 11335.
4784 Inorganic Chemistry, Vol. 41, No. 18, 2002

Sulfur Transfer from Imidomethylrhenium Sulfides
Discussion
Kinetics and Mechanisms. For monodentate phosphines
PZ3 (PMe3 or PMe2Ph), the rate of formation of 3A from 1,
eq 1, is second-order with respect to [PZ3], but first-order
with respect to [PZ3] for reaction 2 when 2 affords 3. Though
one cannot directly compare the rate constants with different
units, under the experimental conditions the formation of 3A
from 1 is clearly faster than that from 2, on the basis of the
rates at which the changes in the UV/vis spectra occur. For
bidentate phosphines PP (dmpe or dmpm), the formation of
3A is first-order with respect to [PP] regardless of whether
the reaction starts with 1 or 2. The rate constants for the
reactions of dmpe and dmpm with 1 are at least 100 times
larger than those with 2. Again, it seems unlikely that
compound 2 is the intermediate toward the formation of 3A
for the reactions of 1 with excess phosphines. To account
for the kinetic observations, we propose the mechanism
presented in Scheme 1. The phosphorus atoms of the
bidentate ligands interact with the rhenium and sulfur atoms
of 1 to form a five-membered ring for dmpm (shown in the
scheme as Ia) and an analogous six-membered ring inter-
mediate for dmpe. Note that one P donor atom attacks the
µ-S group of 1 and the other P donor is coordinated to Re.
The finding that Py acts as a stand-in for the second
monodentate phosphine allows us to argue that the second
P donor does not attack the second µ-S group. The possi-
bility that the two phosphorus atoms interact with the two
sulfur atoms was additionally ruled out on the basis that
phosphine disulfides Me2P(S)(CH2)nP(S)Me2 are absent; only
Me2P(S)(CH2)nPMe2 was obtained, combined with our
experimental evidence showing the absence of PdS redis-
tribution.
Figure 6. Agreement between kinetic data fitted to the rate law in eq 11
versus experimental kinetic data.
Table 4. Activation Parameters for Reactions between dmpm and
Sulfidorhenium Compounds
∆Sq/J mol-1 K-1
∆Hq/kJ mol-1
{MeRe(NAr)2}2(µ-S)2 (1)
{MeRe(NAr)2}2(µ-S) (2)
175(4)
133(7)
13(1)
40(2)
carry out the multivariate data analysis. The values obtained
are km ) 3.3(1) L2 mol-2 s-1 (which agrees with the entry
in Table 3) and km′ ) 3.4(2) L2 mol-2 s-1. The plot of the
calculated values of kψ obtained from the best fit to eq 11
against the experimental ones is shown in Figure 6. The
accelerating effect was also observed in the presence of
4-methoxypyridine. It is worth noting that the observed rate
constant for the reaction of 1 with 1.5 mM dmpm in the
presence of 24 mM Py is the same as the value in the absence
of Py. On the other hand, 4-(dimethylamino)pyridine ranging
from 15 to 150 mM did speed up the reaction with dmpm.
Product 3A is stable toward these pyridines, but unstable
toward 2-dimethylaminoaniline.
An intermediate containing Re(V)-Re(VII) (Ib) is sub-
sequently formed. The remaining steps, ligand substitution
and terminal sulfur transfer, are thought to be faster. The
kinetic and spectroscopic data are in accord with this. For
monodentate phosphines or a bidentate phosphine with one
of the binding sites blocked by oxidation of one phosphine
to its oxide, a second phosphine is needed to form the Ia-
like intermediate. This proposal accounts for the second-
order dependence on [Me2P(O)CH2PMe2]. Neither Ia nor Ib
was detected; however, a Re(V)-Re(VII) intermediate,
Me(NAr)2Re‚ORe(NAr)2Me, was observed for the reaction
of MeRe(NAr)2O with phosphines.12 When phosphine is
limiting in concentration or when a less nucleophilic phos-
phine is used, a competition reaction that leads to the
formation of 2 occurs.
As expected, the formation of 3B from the reaction of 1
with tBuNC also took place with a second-order dependence
on [tBuNC]. The observed rate constants were 1.9 × 10-3
and 2.7 × 10-4 s-1 when the concentrations of the excess
reagent of tBuNC were 0.15 and 0.058 mol L-1, respectively.
Thus, the third-order rate constant is 8.2(2) × 10-2 L2 mol-2
s-1.
When sulfur transfer was carried out in the presence of
pyridine, further rate acceleration was observed. Because
there is no observable interaction between 1 and pyridine
was observed, it is likely that the phosphine attack at the
sulfur atom is the initial step. The pyridine is believed to
coordinate to rhenium. The modified reaction scheme is
presented in Scheme 2.
Temperature Profiles. The temperature dependence of
the reactions of 1 and 2 with dmpm was studied in a
temperature range of 283-313 K. Activation parameters
were calculated from the least-squares fit of values of ln(k/
T) to 1/T, according to eq 4. The resulting activation
q
q
enthalpies for dppm were ∆Hb ) 13(1) kJ mol-1 and ∆H2
) 40(2) kJ mol-1 for the reaction with 1 and 2, respectively.
Both reactions proceed with large negative activation entro-
pies, Table 4.
Though it is known that sulfur transfer between phos-
phorus centers occurs at elevated temperatures, it does not
happen during these reactions. Tests showed that S re-
Inorganic Chemistry, Vol. 41, No. 18, 2002 4785

Wang et al.
Scheme 1
Scheme 2
Scheme 3
distribution between Me2PCH2CH2PMe2 and Me2P(S)CH2-
CH2P(S)Me2 did not occur under the experimental conditions.
This finding makes the point that the disulfide is not an initial
reaction product that is later converted to the observed
monosulfide.
dimethylphenyl)amino ligand.20,21 Williams and Schrock have
isolated a number of three-coordinate nearly trigonal planar
Re(V) anions and structurally characterized the bis(tri-
phenylphosphoranylidene)ammonium (PPN+) salt of Re-
-
(NAr)3 . It is interesting to note that the nucleophilic
Re(NAr)3- can be alkylated and protonated to yield MeRe-
(NAr)3 and HRe(NAr)3, respectively. For mercuric ion, X-ray
studies showed the crystal structure with a molecular unit
of (ArN)3Re-Hg-Re(NAr)3.22 As in the reactions with 1,
no intermediatesneither the Re(V) dimer nor the three-
coordinate Re(V) monomerswas observed. Pyridine ac-
celeration was observed only in the reactions of 1, not 2,
possibly because the Re-Re distance in 2 is about 90 pm
shorter than in 1, leading to additional steric crowding. Attack
of PZ3 at the µ-S group of 2 is being proposed, because (a)
the product is phosphine sulfide and (b) pyridine does not
convert 2 to MeRe(NAr)2Py2, analogous to 3.
A proposed mechanism involving a dimeric intermediate
for the reactions of 2 with phosphines is given in Scheme 3.
The sulfur-transfer step is believed to be rate controlling.
Attempts to prepare the CH3(NAr)2RedRe(NAr)2CH3 inter-
mediate by using a limiting amount of phosphine failed. An
alternative to this has the reaction occurring via two three-
coordinate Re(V) intermediates. Cummins and others have
prepared a number of three-coordinate compounds with
sterically hindered amino ligands.16-18 When the substituents
were not sufficiently large, a dimer was obtained. For
example, the dimer Mo2(NMe2)6 was isolated,19 but a
monomer, Mo[N(R′)(Ar′)]3, formed with the tert-butyl(3,5-
Comparison of Oxo- and Sulfur-Transfer Structures.
Compound 2 adopts the anti,syn arrangement, whereas its
µ-O counterpart 4 was characterized as syn,syn.12 A long-
lived intermediate with the same molecular formula was
1
detected as preceding 4.12 Its H and 13C NMR spectra
consisted of resonances from two ReMe groups, two ArCHMe2
septets, and four ArCHMe2 doublets. On that basis, structure
5 was suggested. A reviewer of this paper has added another
(16) Cummins, C. C. Chem. Commun. 1998, 1777.
(17) Alyea, E. C.; Basi, J. S.; Bradley, D. C.; Chisholm, M. H. Chem.
Commun. 1968, 495.
(18) Alyea, E. C.; Bradley, D. C.; Copperthwaite, R. G. J. Chem. Soc.,
Dalton Trans. 1972, 1580.
(19) Johnson, M. J. A.; Lee, P. M.; Odom, A. L.; Davis, W. M.; Cummins,
C. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 87.
(20) Eller, P. G.; Bradley, D. C.; Hursthouse, M. B.; Meek, D. W. Coord.
Chem. ReV. 1977, 24, 1.
(21) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685.
(22) Williams, D. S.; Schrock, R. R. Organometallics 1993, 12, 1148.
4786 Inorganic Chemistry, Vol. 41, No. 18, 2002

Sulfur Transfer from Imidomethylrhenium Sulfides
plausible formulation for the intermediate, suggesting that
it may be 6, the anti,syn analogue of 2.
the monomeric form in toluene solution at room tempera-
ture;24 however, we believe that the sulfur analogue {MeRe-
(NAr)2S}2 (1) remains as a dimer in solution.7 The kinetic
data for the reactions of monodentate phosphines with 1,
with a second-order dependence on [PZ3], are clearly
different from those with MeRe(NAr)2O, which show a first-
order dependence on [PZ3]. The second-order dependence
on phosphines for the reactions with 1 supports our conclu-
sion that 1 is a dimer in solution. For dmpe and dmpm, and
even for PMe3 and PMe2Ph (different units for the rate
constants), sulfur transfer is much faster than oxo transfer.
Unlike the oxo-transfer situation, the π-acidity of phosphines
seems not to be the main factor in sulfur transfer. The
reaction of MeRe(NAr)2O with P(OMe)2Ph is over 3 × 105
times faster than the reaction with PMe2Ph. This remarkable
effect of the alkoxyl group in phosphines on the oxo-transfer
reaction was not observed in the analogous sulfur reaction,
possibly resulting from the divergence of bridging sulfur
versus terminal oxo. The basicity and steric bulkiness of the
phosphines are more important than π-acidity in the sulfur-
transfer reaction.
Another interesting point to make is that only 1 reacts with
isocyanides to yield MeRe(NAr)2(RNC)2 and RNCS. The
reaction of MeRe(NAr)2O with isocyanides does not occur
under the same conditions. The bond dissociation energies
for RNCdO and RNCdS differ by about 60 kcal mol-1,
similar to the difference between R3PdO and R3PdS.25,26
Thus, a rhenium-sulfur bond, being weaker than a rhenium-
oxygen bond, is likely to give rise to the different reactivities
of isocyanides toward oxo- and thio-transfer reactions.
If 6 is indeed the precursor to 4, then the ring-opened 5
might well intervene, as in 6 f 5 f 4, under the presumption
that Re-Re bond opening in 6 is rate controlling and that
internal rotation in 5 is relatively rapid. The rate of
conversion of the intermediate to 2 has a t1/2 of roughly 1
day, so ∆Gq is about 100 kJ mol-1. Relatively few com-
parisons are possible. Thermal reactions of metal-metal
dimers have been observed and estimates made for the BDE
of the M-M bonds, such as Re2(CO)10 (180 kJ mol-1),23
and the very labile cases of {C5H5Cr(CO)3}2 (6.2 kJ mol-1)
and {C5Me5Cr(CO)3}2 (19 kJ mol-1).
Comparison of Oxo- and Sulfur-Transfer Reactions.
Both {MeRe(NAr)2O}224 and {MeRe(NAr)2S}27 are dimeric
in the solid state. They react with phosphines to give
MeRe(NAr)2(PZ3)2 and the corresponding phosphine oxides
and sulfides, respectively. The reactivity of PMenPh3-n
toward {MeRe(NAr)2O}2 and {MeRe(NAr)2S}2 increases as
n changes from 1 to 3. Also, steric effects were found to
play an important role in both oxo- and sulfur-transfer
reactions. Despite the similarities between {MeRe(NAr)2O}2
and {MeRe(NAr)2S}2, certain differences regarding these two
species and the corresponding atom-transfer reactions are
worth noting. The oxo compound exists predominantly in
Acknowledgment. This research was supported by a
grant from the National Science Foundation. Some experi-
ments were conducted with the use of the facilities of the
Ames Laboratory. We are grateful to Dr. Cole-Hamilton at
the University of St. Andrews for providing the detailed
synthetic procedure for Me2P(O)CH2PMe2.
Supporting Information Available: X-ray crystallographic
tables and NMR data. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC0255196
(23) Chernova, V. I.; Sheiman, M. S.; Rabinovich, I. B.; Syrkin, V. G. Tr.
Khim. Khim. Tekhnol. 1973, 43-44.
(24) Herrmann, W. A.; Ding, H.; Ku¨hn, F. E.; Scherer, W. Organometallics
1998, 17, 2751.
(25) Benson, S. W. Chem. ReV. 1978, 78, 23.
(26) Gilheany, D. G. In The Chemistry of Organophosphorus Compounds;
Hartley, F. R., Ed.; Wiley: New York, 1992; Vol. 2, p 1.
Inorganic Chemistry, Vol. 41, No. 18, 2002 4787