Comparison of thermodynamic and kinetic aspects of oxidative addition of PhE-EPh (E = S, Se, Te) to Mo(CO)3(PR3)2, W(CO)3(PR3)2, and Mo(N[tBu]Ar) 3 complexes. The role of oxidation state and ancillary ligands in metal complex induced chalcogenyl radical generation

Article abstract of DOI:10.1021/ja063250+

Enthalpies of oxidative addition of PhE-EPh (E = S, Se, Te) to the M(0) complexes M(PⁱPr₃)_2- (CO)₃ (M = Mo, W) to form stable complexes M(^?EPh)(Pⁱr ₃)₂(CO)₃ are reported and compared to analogous data for addition to the Mo(III) complexes Mo(N[^tBu]Ar)₃ (Ar = 3,5-C₆H₃Me₂) to form diamagnetic Mo(IV) phenyl chalcogenide complexes Mo(N[^tBu]Ar)₃(EPh). Reactions are increasingly exothermic based on metal complex, Mo(P ⁱPr₃)₂(CO)₃ < W(P ⁱPr₃)₂(CO)₃ < Mo(N[ ^tBu]Ar)₃, and in terms of chalcogenide, PhTe-TePh < PhSe-SePh < PhS-SPh. These data are used to calculate L₀M-EPh bond strengths, which are used to estimate the energetics of production of a free ^?EPh radical when a dichalcogenide interacts with a specific metal complex. To test these data, reactions of Mo(N[^tBu]Ar) ₃ and Mo(PⁱPr₃)₂(CO)₃ with PhSe-SePh were studied by stopped-flow kinetics. First- and second-order dependence on metal ion concentration was determined for these two complexes, respectively, in keeping with predictions based on thermochemical data. ESR data are reported for the full set of bound chalcogenyl radical complexes (PhE ^?)M(PⁱPr₃)₂(CO)₃; g values increase on going from S to Se, to Te, and from Mo to W. Calculations of electron densities of the SOMO show increasing electron density on the chalcogen atom on going from S to Se to Te. The crystal structure of W( ^?TePh)(PⁱPr₃)₂(CO)₃ is reported.

Full text of DOI:10.1021/ja063250+

Published on Web 07/13/2006
Comparison of Thermodynamic and Kinetic Aspects of
Oxidative Addition of PhE-EPh (E ) S, Se, Te) to
Mo(CO)3(PR3)2, W(CO)3(PR3)2, and Mo(N[tBu]Ar)3 Complexes.
The Role of Oxidation State and Ancillary Ligands in Metal
Complex Induced Chalcogenyl Radical Generation
James E. McDonough, John J. Weir, Kengkaj Sukcharoenphon, Carl D. Hoff,*
Olga P. Kryatova, Elena V. Rybak-Akimova,* Brian L. Scott, Gregory J. Kubas,*
Arjun Mendiratta, and Christopher C. Cummins*
Contribution from the UniVersity of Miami, Coral Gables, Florida 33124, Tufts UniVersity,
Medford, Massachusetts 02155, Massachusetts Institute of Technology, 77 Massachusetts
AVenue, Cambridge, Massachusetts 02139, and Los Alamos National Laboratory, Structural
Inorganic Chemistry Group, Chemistry DiVision, MS J514, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545
Received May 9, 2006; E-mail: c.hoff@miami.edu
Abstract: Enthalpies of oxidative addition of PhE-EPh (E ) S, Se, Te) to the M(0) complexes M(PiPr3)2-
(CO)3 (M ) Mo, W) to form stable complexes M(•EPh)(PiPr3)2(CO)3 are reported and compared to analogous
data for addition to the Mo(III) complexes Mo(N[tBu]Ar)3 (Ar ) 3,5-C6H3Me2) to form diamagnetic Mo(IV)
phenyl chalcogenide complexes Mo(N[tBu]Ar)3(EPh). Reactions are increasingly exothermic based on metal
complex, Mo(PiPr3)2(CO)3 < W(PiPr3)2(CO)3 < Mo(N[tBu]Ar)3, and in terms of chalcogenide, PhTe-TePh
< PhSe-SePh < PhS-SPh. These data are used to calculate LnM-EPh bond strengths, which are used
•
to estimate the energetics of production of a free EPh radical when a dichalcogenide interacts with a
specific metal complex. To test these data, reactions of Mo(N[tBu]Ar)3 and Mo(PiPr3)2(CO)3 with PhSe-
SePh were studied by stopped-flow kinetics. First- and second-order dependence on metal ion concentration
was determined for these two complexes, respectively, in keeping with predictions based on thermochemical
data. ESR data are reported for the full set of bound chalcogenyl radical complexes (PhE•)M(PiPr3)2(CO)3;
g values increase on going from S to Se, to Te, and from Mo to W. Calculations of electron densities of the
SOMO show increasing electron density on the chalcogen atom on going from S to Se to Te. The crystal
structure of W(•TePh)(PiPr3)2(CO)3 is reported.
Scheme 1
Introduction
The thiyl radical, whether free1 (•SR) or bound to a metal
complex2 (LnM(•SR)), has received increasing attention for its
role in biological chemistry.3 The most common sources of thiyl
radicals are (i) oxidation of thiols or thiolates and (ii) radical
cleavage of the sulfur-sulfur bond of disulfides. The latter
process may be important for transition metal complexes,
including metalloenzymes; however it is not clear under what
conditions interaction of disulfides with metal complexes will
lead to radical generation.
Two mechanisms for reaction of a metal complex with a
disulfide are shown in Scheme 1. In the upper reaction manifold,
a single metal center reversibly binds a disulfide and then is
capable of direct splitting of the sulfur-sulfur bond. This
generates a free radical, which in the presence of additional
metal complex would be rapidly trapped. However in an
enzymatic or catalytic process under conditions of low inter-
cepting metal complex concentration, a thiyl radical so generated
could conceivably take part in secondary reactions. The upper
pathway can have a low enthalpy of activation only if the M-SR
bond is stronger than the RS-SR bond. The lower pathway
would be expected to be operative for complexes with weak
(1) Wardman, P. In Sulfur Centered Radicals; Alfassi, Z. B., Ed.; John Wiley:
New York, 1999.
(2) Kimura, S.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt, K. J. Am.
Chem. Soc. 2001, 123, 6025.
(3) (a) Bennati, M.; Robblee, J. H.; Mugnaini, V.; Stubbe, J.; Freed, J. H.;
Borbat, P. J. Am. Chem. Soc. 2005, 127, 15014. (b) Stubbe, J.; Nocera, D.
G.; Yee, C. S.; Chang, C. Y. Chem. ReV. 2003, 103, 2167. (c) Licht, S. S.;
Booker, S.; Stubbe, J. Biochemistry 1999, 38, 1221.
9
10.1021/ja063250+ CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 10295-10303
10295

A R T I C L E S
McDonough et al.
Figure 1. Ball-and-stick representations of Mo(N[tBu]Ar)3 and Mo(PCy3)2(CO)3 highlighting the vacant sites available for approach to an incoming ligand.
individual M-SR bonds. Derived rate laws for these reactions
are complex4 and depend on the relative magnitudes of the
specific rate constants, so that in practice it can be difficult to
distinguish between them on kinetic data alone.
bond. Despite the steric crowding at the metal center in M(CO)3-
(PR3)2(L), ligand substitution reactions are facile. Kinetic
evidence indicates they are even associative in character, i.e., a
concerted process where the agostic C-H “pushes off” as the
incoming group in turn displaces the agostic interaction.8b
Binding of H2 yields a molecular hydrogen complex that exists
in tautomeric equilibrium with a dihydride. Dinitrogen binds
reversibly to form mononuclear M(CO)3(PCy3)2(N2), and in the
case of the sterically less crowded isopropyl phosphine a
dinuclear complex [µ-N2][W(CO)3(PiPr3)2]2 is formed as well.
The crystal structure of this complex has been determined;8c
however splitting of dinitrogen does not occur for these low-
valent complexes.
In contrast the complex9 Mo(N[R]Ar)3 has three unpaired
electrons in its ground state and shows little tendency to bind
conventional ligands for both steric and electronic reasons. For
the sterically less crowded R ) isopropyl derivative, an agostic
interaction is not present, but rather the potential three-center
agostic C-H bond undergoes oxidative addition to form a
molybdaziridine hydride complex of Mo(V). The latter has been
shown in many cases to act as a “resting state” for the active
Mo(N[R]Ar)3. Binding of dinitrogen to yield a detectable
mononuclear adduct [N2][Mo(N[R]Ar)3] does not occur under
normal conditions, but at low temperature a dinuclear intermedi-
ate [µ-N2][Mo(N[R]Ar)3]2 is formed and subsequently splits
dinitrogen to form 2 mol of the terminal nitrido complex Nt
Mo(N[R]Ar)3.
Despite the importance of oxidative addition of disulfides at
metal centers, relatively few detailed mechanistic studies have
been reported. Earlier work5 has shown that for the 17-electron
•
radical complex Cr(CO)3Cp* reaction with PhS-SPh is first-
•
order in metal radical and generates free SPh, but that due to
its stronger sulfur-sulfur bond, reaction with MeS-SMe obeys
a rate law second-order in metal complex. In the case of the
16-electron fragments M(phen)(CO)3 (M ) Mo, W) concerted
attack by two metal centers was found to occur.6 Bergman and
Aubart7 investigated reaction of a range of aryl disulfides with
both CoCp2 and Cp2Ta(µ-CH2)2CoCp. Strong dependence on
the electrostatic character of the specific disulfide for CoCp2
showed that oxidative addition proceeded mainly by outer-sphere
electron transfer, but that Cp2Ta(µ-CH2)2CoCp reacted with
substantially more covalent character in the transition state.
This paper reports detailed investigation of reactivity of the
phenyl dichalcogenides (PhE-EPh, E ) S, Se, Te) with three
metal complexes M(CO)3(PR3)2 (M ) Mo, W) and Mo(N[R]-
Ar)3. Despite different oxidation states and ancillary ligands,
both of these complexes present a vacant site at the metal for
ligand binding and oxidative addition, as shown in Figure 1.
i
The complexes M(CO)3(PR3)2 (M ) Cr, Mo, W; R ) Pr,
Cy)8 are formally 16-electron complexes with no unpaired
electrons. However, saturation at the metal and achievement of
an 18 valence electron count is attained to some extent since
the vacant site is blocked by a three-center M‚‚‚H-C agostic
Despite chemical differences described above, the phosphine
and amido complexes shown in Figure 1 each present an open
site for ligand binding as well as oxidative addition reactions.
In addition they provide an opportunity to compare and contrast
(4) The derived rate law for the upper manifold is dP/dt ) 2k1k2k4[MLn]2-
[RSSR]/{k-1k-2[MLnSR] + k4[MLn] }. The derived rate law for the lower
manifold is dP/dt ) 2k1k3[MLn]2[RSSR]/{k-1k-3 + k-1k5 + k3k5[MLn]}.
Due to the complexity of these equations, depending on the exact values
of the specific rate constants, as well as reagent concentrations, a range of
behavior could in principle be displayed. However, under most circum-
stances the upper manifold would be expected to reduce to first-order
dependence on metal complex concentration, and the lower manifold to
second-order dependence on metal complex concentration. In the case that
[MLnSR] ) 0 the upper pathway reduces to dP/dt ) 2k1k2[MLn][RSSR]
) kobs[MLn][RSSR] where kobs ) 2k1k2. For the lower pathway for low
relative values of [MLn] the rate law reduces to dP/dt ) 2k1k3[MLn]2[RSSR]/
k-1{ k-3 + k5} ) kobs[MLn]2[RSSR] where kobs ) 2k1k3/k-1{k-3 + k5}.
Due to the rapid nature of the reaction, values of kobs are measured in this
work and frequently could not be resolved into the individual rate constants
shown in this representative scheme.
(8) (a) Wasserman, H. J.; Kubas, G. J.; Ryan, R. H. J. Am. Chem. Soc. 1986,
108, 2294. (b) Zhang, K.; Gonzalez, A. A.; Mukerjee, S. L.; Chou, S.-J.;
Hoff, C. D.; Kubat-Martin, K. A.; Barnhart, D.; Kubas, G. J. J. Am. Chem.
Soc. 1991, 113, 9170. (c) Butts, M. D.; Kubas, G. J.; Luo, X.-L.; Bryan, J.
C. Inorg. Chem. 1997, 36, 3341. (d) Gonzalez, A. A.; Zhang, K.; Nolan,
S. P.; de la Vega, R. L.; Mukerjee, S. L.; Hoff, C. D.; Kubas, G. J.
Organometallics 1988, 7, 2429. (e) Gonzalez, A. A.; Hoff, C. D. Inorg.
Chem. 1989, 28, 4295. (f) Zhang, K.; Gonzalez, A. A.; Hoff, C. D. J. Am.
Chem. Soc. 1989, 111, 3627. (g) Gonzalez, A. A.; Zhang, K.; Mukerjee,
S. L.; Hoff, C. D.; Khalsa, G. R. K.; Kubas, G. J. ACS Symp. Ser. 1990,
428, 133. (h) Kubas, G. J.; Nelson, J. E.; Bryan, J. C.; Eckert, J.;
Wisniewski, L.; Zilm, K. Inorg. Chem. 1994, 33, 2954.
(9) (a) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. (b) Laplaza,
C. E.; Johnson, A. R.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 709.
(c) Laplaza, C. E.; Johnson, M. J. A.; Peters, J.; Odom, A. L.; Kim, E.;
Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623. (d) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685. (e) Tsai,
Y.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C.; Klooster, W. T.;
Koetzle, T. F. J. Am. Chem. Soc. 1999, 121, 10426. Tsai, Y.; Cummins,
C. C. Inorg. Chim. Acta 2003, 345, 63.
(5) Ju, T. D.; Capps, K. B.; Lang, R. L.; Roper, G. C.; Hoff, C. D. Inorg.
Chem. 1997, 36, 614.
(6) Ju, T. D.; Capps, K. B.; Roper, G. C.; Hoff, C. D. Inorg. Chim. Acta
1998, 270, 488.
(7) Aubart, M.; Bergman, R. G. J. Am. Chem. Soc. 1998, 120, 8755.
9
10296 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Chalcogenyl Radical Generation
A R T I C L E S
of the dinuclear complex was added by syringe to 5.4 mg (2.50 ×
10-2) mmol of PhS-SPh. The solution immediately turned a deep blue
color, and gas was evolved. Analysis by FTIR spectroscopy showed
quantitative conversion to product as shown in Supporting Information
Figure S1. Similar equimolar reaction of 3.0 mL of this solution with
PhSe-SePh (7.7 mg) or PhTe-TePh (10.0 mg) resulted in similar rapid
and quantitative formation to give blue-green and green complexes,
respectively. Solutions of these complexes overnight in the glovebox
showed little sign of decomposition for W derivatives, but were less
stable for the Mo derivatives. ESR spectroscopy was done by sealing
toluene solutions in quartz tubes under vacuum. These tubes left in the
freezer were found to be stable for a period of weeks to months for the
W derivatives, but showed some slow decomposition for the Mo
complexes.
Crystals of W(•TePh)(CO)3(PiPr3)2 suitable for structure determina-
tion were prepared by reaction of 0.1 g of W(CO)3(PiPr3)2 and 0.035
g of PhTeTePh in 10 mL of toluene. Upon addition of toluene, the
solution turned the deep blue color characteristic of the radical
W(•TePh)(CO)3(PiPr3)2. Approximately 2 mL of this solution was
filtered into a glass tube under argon, then layered with 8 mL of distilled
heptane. The tube was sealed and placed in a freezer for a period of 2
weeks. During that time slow diffusion of the two solvents occurred
and blue-purple crystals formed. The tube was opened in the glovebox,
and the mother liquor was removed by syringe and replaced with a
small amount of degassed mineral oil. The tube was then sealed again
and stored until mounting for structure determination.
reactivity involving low and intermediate oxidation states at
group 6 metal centers and to probe how oxidation state and
ancillary ligand environments influence the ability of a metal
complex to generate free chalcogenyl radicals in a single step
or bound radicals in a concerted process.
Experimental Section
General Considerations. Unless stated otherwise, all operations
were performed in a Vacuum Atmospheres or MBraun drybox under
an atmosphere of purified nitrogen or argon or utilizing standard Schlenk
tube techniques under argon. Toluene and heptane were purified by
distillation under argon from sodium benzophenone ketyl into flame-
dried glassware. Methylene chloride was refluxed under an argon
atmosphere over P2O5 and then distilled. FTIR data were obtained on
a Perkin-Elmer system 2000 spectrometer; ESR data, on a Bruker EMX
spectrometer utilizing X-band radiation in quartz tubes sealed under
vacuum. Stopped-flow kinetic and solution calorimetric data were
obtained using techniques analogous to those described previously8,9,21
and are illustrated with representative procedures below. Dichalco-
genides were obtained from Aldrich Chemical and were recrystallized
from methylene chloride/heptane mixtures by slow evaporation and
cooling.
Preparation of M(•EPh)(CO)3(PiPr3)2 and Crystal Growth of
W(•TePh)(CO)3(PiPr3)2. Reactions of PhSe-SePh and PhTe-TePh
with M(CO)3(PiPr3)2 were performed in a manner strictly analogous to
that reported previously11 for reaction of PhS-SPh. The reactions were
rapid and quantitative, as determined by FTIR spectroscopy. In the
glovebox a solution of (µ2-N2)[W(CO)3(PiPr3)2]2 (0.15 g) was prepared
in a Schlenk tube under Ar in 15 mL of freshly distilled toluene. Three
milliliters of this yellow-orange solution containing 2.5 × 10-2 mmol
Crystal Structure Determination. Due to air sensitivity, a crystal
of W(•TePh)(CO)3(PiPr3)2 was mounted from a pool of mineral oil under
argon gas flow. The crystal was placed on a Bruker P4/CCD
diffractometer and cooled to 203 K using a Bruker LT-2 temperature
device. The instrument was equipped with a sealed, graphite-mono-
chromatized Mo KR X-ray source (λ ) 0.71073 Å). A hemisphere of
data was collected using æ scans, with 30 s frame exposures and 0.3°
frame widths. Data collection and initial indexing and cell refinement
were handled using SMART10a software. Frame integration, including
Lorentz-polarization corrections, and final cell parameter calculations
were carried out using SAINT software.10b The data were corrected
for absorption using the SADABS program.10c Decay of reflection
intensity was monitored via analysis of redundant frames. The structure
was solved using direct methods and difference Fourier techniques,
and two crystallographically independent molecules were identified in
the unit cell. All hydrogen atom positions were idealized and rode on
the atom they were attached to. After all atomic positions were assigned
and refined anisotropically to conversion, four significant residual peaks
(two at 15 e Å-3 and two at 8 e Å-3) remained in the difference map.
These four peaks were refined as two sets of W-Te twin components
based on their distances and locations. The site occupancy factors were
tied to the major W-Te components and refined to approximately a
13% contribution; to obtain convergence, the major and minor
contributions were fixed at 0.87 and 0.13, respectively. The final
refinement included anisotropic temperature factors on all non-hydrogen
atoms, except for the minor Te and W component atoms. Structure
solution, refinement, graphics, and creation of publication materials
were performed using SHELXTL.10d Additional details of data collec-
tion and structure refinement are listed in Table 1. The refinement of
the W(•TePh)(CO)3[PiPr3]2 molecule in a major 87% occupied site and
a minor 13% occupied site is an example of “whole molecule
disorder”.10e
(10) (a) SMART-NT 4; Bruker AXS, Inc.: Madison, WI, 1996. (b) SAINT-NT
5.050; Bruker AXS, Inc.: Madison, WI, 1998. (c) Sheldrick, G. SADABS,
first release; University of Go¨ttingen: Germany. (d) SHELXTL NT Version
5.10; Bruker AXS, Inc.: Madison, WI, 1997. (e) This phenomenon and
the subsequent refinement of the minor contribution are accepted among
crystallographers; see for example: Thomas, C. A.; Zong, K. W.; Abboud,
K. A.; Steel, P. J.; Reynolds, J. R. J. Am. Chem. Soc. 2004, 126, 16440.
Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik, P. J.
J. Am. Chem. Soc. 2004, 126, 16937. John, K. D.; Miskowski, V. M.; Vance,
M. A.; Dallinger, R. F.; Wang, L. C.; Geib, S. J.; Hopkins, M. D. Inorg.
Chem. 1998, 37, 6858.
(11) (a) Lang, R. F.; Ju, T. D.; Kiss, G.; Hoff, C. D.; Bryan, J. C.; Kubas, G.
J. J. Am. Chem. Soc. 1994, 116, 7917. (b) Lang. R. F.; Ju, T. D.; Kiss, G.;
Hoff, C. D.; Bryan, J. C.; Kubas, G. J. Inorg. Chim. Acta 1997, 259, 317.
(c) Data were reported in ref 11a for the enthalpy of reaction W(CO)3-
1
(PiPr3)2 + /2PhS-SPh with ∆H ) -18.9 ( 1.2 kcal/mol; this compares
to data in Table 4 (divided by 2) of ∆H ) -2l.1 ( 0.5 kcal/mol. The
disagreement between these two values is outside experimental error by
0.5 kcal/mol. The derived data rely on the purity of the PhS-SPh. A
possible reason for this slight disagreement is that the current sample was
more pure. Care was taken to ensure that the recrystallized dichalogenides
were not only spectroscopically pure as judged by NMR data but also of
high crystal quality. To compare the three complexes shown in Table 4,
samples from the same batches of dichalcogenides were used for each metal.
(12) Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399.
(13) Armstrong, D. A.; Chipman, D. M. In S-Centered Radicals; Alfassi, ZX.
B., Ed.; John Wiley: New York, 1999.
(14) Landolt-Bernstein. Numerical Data and Functional Relationships in Science
and Technolgy, New Series, Volume 17, Magnetic Properties of Free
Radicals; Fischer, H., Ed.; Springer-Verlag: Berlin, 1988. See also Vol.
9, Hellwege, K. H. 1979, in this series.
(15) Tripathi, G. N. R.; Sun, Q.; Armstrong, D. A.; Chipman, D. M.; Schuler,
R. H. J. Phys. Chem. 1992, 96, 5344.
(16) The isotropic shift for the SPh radical is reported as 2.008 and SePh as
2.007 in ref 14. The isotropic shift of SCH2C6H5 is reported as 2.024 and
SeCH2C6H5 as 2.102. The authors could not find corresponding data for
Te radicals.
Reaction of M(CO)3(PiPr3)2 and PhS-SPh and PhSe-SePh
under H2 Atmosphere. In a typical procedure a solution of 0.12 g of
Mo(CO)3(PiPr3)2 dissolved in 5 mL of freshly distilled toluene was
prepared in a 25 mL Schlenk tube under H2 (99.9995%) at 1.3 atm of
pressure and a temperature of 10 °C. A 1 mL aliquot was removed for
FTIR and showed the presence of Mo(CO)3(PiPr3)2(H2). A solution of
0.05 g of PhS-SPh in 2.5 mL of toluene was then added. The solution
turned deep blue, and an FTIR of the solution showed the near
(17) Springs, J.; Janzen, C. P.; Darensbourg, M. Y.; Calabrese, J. C.; Krusic, P.
J.; Verpeaux, J. N. Amatore, C. J. Am. Chem. Soc. 1990, 112, 5789-5797.
(18) Rieger, P. H. Coord. Chem. ReV. 1994, 135/136, 203; see Table 16, p 249.
(19) Witner, A.; Huttner, G.; Zsolnai, L.; Kronick, P.; Gottlieb, M.; Angew.
Chem., Intl. Ed. Engl. 1984, 23, 975. See also: Lau, P.; Braunwarth, H.;
Huttner, G.; Gunauer, D.; Evertz, K.; Imhof, W.; Emmerich, C.; Zsolnai,
L.; Organometallics 1991, 10, 3861.
(20) Angerhofer, A. A.; Walker, L.; Sukcahroenphon, K.; McDonough, J. E.;
Hoff, C. D. Work in progress.
(21) McDonough, J. E.; Carlson, M. J.; Weir, J. J.; Hoff, C. D.; Kryatova, O.
P.; Rybak-Akimova, E. V.; Cummins, C. C. Inorg. Chem. 2005, 44, 3127.
9
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A R T I C L E S
McDonough et al.
Table 1. Crystal Structure Parameters for W(•TePh)(CO)3(PiPr3)2
complex was established by changing [Mo complex] (0.45, 0.3375,
and 0.225 mM) under pseudo-first-order conditions (4.5 mM Ph2Se2).
The system was tested using conventional UV-vis spectrometry at
room temperature (Figure S2). Regardless of the amount of Ph2Se2
added, the characteristic spectra of green Mo(N[tBu]Ar)3(SePh) were
observed. This product is moderately air-stable, showing little decom-
position 10 min after the cell was opened to air. The same spectral
changes were observed in a stopped-flow cell under any conditions of
concentration/temperature. All of the experiments were performed in
a single-mixing mode of the instrument, with a 1:1 (v/v) mixing ratio.
The reactions were monitored for three to five half-lives. A series of
four to six measurements at each temperature gave an acceptable
standard deviation (within 10%). Data analysis was performed with
IS-2 Rapid Kinetic software from Hi-Tech Scientific, Spectfit/32 Global
Analysis System software from Spectrum Software Associates, or Excel
Solver from Microsoft.
crystal system
space group (No.)
description
cryst size
a, Å
monoclinic
P1h (#2)
black multifaceted block
0.16 × 0.20 × 0.32 mm3
12.154(5)
16.218(7)
16.684(7)
75.872(6)
79.373(8)
89.993(8)
3131(2)
b, Å
c, Å
R, deg
â, deg
γ, deg
volume, Å3
Z
4
D
calcd, g cm-3
1.682
T, K
203
quantitative conversion to Mo(•SPh)(CO)3(PiPr3)2. The H-SPh band,
which is weak and occurs near 2570 cm-1 in toluene, could not be
quantified above baseline noise, and no odor characteristic of thiophenol
was detected. In comparable experiments with W(CO)3(PiPr3)2, thio-
phenol was detected both by its characteristic smell and also by the
ν(H-SPh) present in the FTIR spectrum. The FTIR intensity data
indicated that ∼1/3 mole of HSPh was produced for each mole of
reacting W(CO)3(PiPr3)2 under these conditions. In addition to bands
characteristic of W(•SPh)(CO)3(PiPr3)2 production, extra bands attributed
to uncharacterized products were detected. Reactions using PhSe-SePh
were performed in a strictly analogous manner. The H-SePh stretch
occurs at 2310 cm-1 as determined in our experiments, and no instability
or decomposition of PhSeH was observed in the absence of air. Under
a hydrogen atmosphere, neither Mo(CO)3(PiPr3)2 nor W(CO)3(PiPr3)2
was found to produce H-SePh as detected either by odor or by FTIR.
Reaction of Mo(•SePh)(CO)3(PiPr3)2 and Mo(N[tBu]Ar)3. In the
glovebox a purple solution of 0.255 g of Mo(CO)3(PiPr3)2 in 5 mL of
freshly distilled toluene was prepared in a Schlenk tube. After removing
1 mL of this solution for FTIR, solid PhSe-SePh (0.0580 g) was added
to prepare in situ a bright blue solution of Mo(•SePh)(CO)3(PiPr3)2. To
this solution was added 0.19 g of Mo(N[tBu]Ar)3, and a series of FTIR
spectra were run over a period of 2 h. During this time the FTIR spectral
bands attributed to Mo(•SePh)(CO)3(PiPr3)2 at 1981 and 1877 cm-1
slowly decreased and bands attributed to Mo(CO)3(PiPr3)2 at 1955, 1844,
and 1813 cm-1 grew in.
Stopped-Flow Kinetic Measurements. Toluene solutions of the
reagents were prepared in a Vacuum Atmospheres or a MBraun
glovebox filled with argon and placed in Hamilton gastight syringes.
Time-resolved spectra or single-wavelength kinetic traces were acquired
at temperatures from -80 to +25 °C using a Hi-Tech Scientific
(Salisbury, Wiltshire, UK) SF-43 multimixing CryoStopped-Flow
instrument in a diode array mode or a single-wavelength mode,
respectively. The stopped-flow instrument was equipped with stainless
steel plumbing, a 1.00 cm stainless steel mixing cell with sapphire
windows, and an anaerobic gas-flushing kit. The instrument was
connected to an IBM computer with IS-2 Rapid Kinetic software (Hi-
Tech Scientific). The temperature in the mixing cell was maintained
to (0.1 K, and the mixing time was 2 to 3 ms. The driving syringe
compartment and the cooling bath filled with heptane (Fisher) were
flushed with argon before and during the experiments, using anaerobic
kit flush lines. All flow lines of the SF-43 instrument were extensively
washed with degassed, anhydrous toluene before charging the driving
syringes with reactant solutions.
Calorimetric Measurements. In the glovebox a solution of 0.45 g
of W(CO)3(PiPr3)2 was prepared under argon atmosphere in 6 mL of
toluene. One milliliter of this solution was used for recording an FTIR
spectrum, and the remaining 5 mL was loaded under argon into the
cell of the Setaram-C-80 Calvet microcalorimeter. The solid-containing
compartment of the calorimeter was loaded with 0.0103 g of PhS-
SPh, which had been recently recrystallized from methylene chloride/
heptane and then dried in vacuo. The assembled calorimeter cell was
taken from the glovebox and loaded into the calorimeter. Following
temperature equilibration the reaction was initiated and the thermogram
indicated a rapid reaction, which returned cleanly to baseline with no
thermal signal indicative of secondary reactions occurring. Following
return to baseline, the cell was taken back into the glovebox and a
sample analyzed by FTIR spectroscopy showed the presence of only
starting material, product, and trace W(CO)4(PiPr3)2, which is present
whenever handling W(CO)3(PiPr3)2. The integrated enthalpy of reaction,
-37.06 kcal/mol PhS-SPh, was averaged with four other independent
measurements to give an average value based on solid PhS-SPh of
-36.2 ( 0.6 kcal/mol. Correction for the endothermic enthalpy of
solution of solid PhSSPh (+6.0 ( 0.1 kcal/mol) yields the final value
with all species in toluene solution of -42.2 ( 0.7 kcal/mol.
Results and Discussion
Preparative and Structural Chemistry. Reaction 1 was
found to occur rapidly for E ) Se, Te and M ) Mo, W:
2 M(CO)3(PR3)2 + PhE-EPh f 2 M(•EPh)(CO)3(PR3)2
(1)
This is an extension to the heavier chalcogenides of the
previously studied11 oxidative addition of disulfides to the same
complexes. From a preparative point of view the initial radical-
forming reactions are rapid and quantitative, as determined by
FTIR studies. Spectroscopic data for the complexes are sum-
marized in Supporting Information Figure 1.
Previously we had reported the X-ray structure of the 17-
electron radical complex W(•I)(CO)3(PiPr3)2, but all previous
attempts to obtain X-ray-quality crystals of the thiolate complex
radicals W(•SR)(CO)3(PiPr3)2 failed.11 In the present study, slow
crystallization from toluene/heptane produced crystals of
W(•TePh)(CO)3(PiPr3)2, the structure of which is shown in
Figure 2 and summarized in Table 1.
To the authors’ knowledge, the structure in Figure 2 is the
first reported for a W(I) chalcogenyl radical complex. The
structure is essentially distorted octahedral and resembles that
previously reported11for W(•I)(CO)3(PiPr3)2. The W-Te-C(22)
angle of 108.23° does not appear to show unusual distortion. It
is worth noting that the P-W-P angle of 166.82° is bent down
These techniques are illustrated for study of the kinetics of reaction
of Mo(N[tBu]Ar)3 and PhSe-SePh. The reaction was studied by rapid
scanning spectrophotometry from -40 to +25 °C under second-order
(0.3 mM solutions) and under pseudo-first-order conditions (0.45 mM
for Mo complex and 4.5 mM for Ph2Se2). The rate dependence on [Ph2-
Se2] was studied at -40 °C using [Ph2Se2] ) 0.45 mM before mixing
and varying [Ph2Se2] from a 1- to 20-fold excess, and the reaction order
in Ph2Se2 was established by method of initial rates. The order in Mo
9
10298 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Chalcogenyl Radical Generation
A R T I C L E S
Table 2. Isotropic g Values for Complexes at Room Temperature
in Toluene Solution.
metal
S
Se
Te
Te−Naphth
Mo
W
2.0372
2.0787
2.0626
2.1003
2.1076
2.1378
2.1075
2.1389
Thiyl radicals are known to have broad, poorly resolved ESR
spectra13 with g values ranging from 2.0 to 2.25. Data14 indicate
that for •SPh the g value is near 2.008, and for •SePh near 2.007.
This could be interpreted as indicating some delocalization of
•
the electron into the arene ring for the EC6H5 radicals since
the g value is close to the typical value near 2.003 commonly
observed for organic radicals. Theoretical calculations15 indicate
•
that for SPh the SOMO is a predominantly sulfur-based S pπ
Figure 2. Thermal ellipsoid plot of W(•TePh)(CO)3(PiPr3)2. Selected bond
distances (Å) and angles (deg): W(1)-Te(1) ) 2.7535(12), W(1)-P(1) )
2.5524(15), W(1)-P(2) ) 2.5600(14), W(1)-C(1) ) 1.994(5), W(1)-C(2)
) 1.956(5), W(1)-C(3) ) 2.019(4), C(2)-W(1)-Te(1) ) 177.90(14),
P(1)-W(1)-P(2) ) 166.87(4), C(1)-W(1)-C(3) ) 172.31(18). Relevant
structural parameters are collected in Table 1; full crystal data are available
as Supporting Information.
orbital involved to only a minor extent with arene orbitals. The
•
•
isotropic g values for SCH2Ph (2.024) and SeCH2Ph (2.102)
are higher and may reflect even higher localization on the
chalcogen. Data for •TePh radicals16 could not be found by the
authors.
A complete set of transition metal M-E-Ph radicals
comparable to these for comparison could also not be found by
the authors. The metastable radicals (•SR)(Cr(CO)5)2 formed
on low-temperature oxidation of anionic [RS][Cr(CO)5]2- have
been studied,17 and ESR data for these radicals were found to
be consistent with formulation as organometallic analogues of
sulfuranyl radicals R3S•. Isotropic g values were near 2.025 for
a range of complexes, and the unpaired electron was believed
to be in a predominantly 3pz orbital of the trigonal sulfur. Data
for several Mn(II) piano stool complexes CpMn(CO)2(X) have
been collected.18 Of relevance to this work, Huttner and co-
workers19 have reported isotropic g values of 2.03 for Cp*-
(CO)2Mn(•StBu) and 2.07 for Cp*(CO)2Mn(•SePh). As discussed
in the following section, calculations indicate that the SOMO
is predominantly a mixture of chalcogen and metal-based
orbitals. The nearly identical nature of the ESR data for phenyl
and naphthyl derivatives would support little influence of the
aromatic ring. Data in Table 2 show increasing g values on going
from S to Se to Te. That would be in keeping with data discussed
above for the E-CH2C6H5 derivatives. In addition it should be
noted that there is an increase in g value on going from Mo to
W for all three chalcogens. This could be due to changes in the
SOMO or the metal contribution to the g value; however more
detailed analysis of this point is beyond the scope of this paper.
Work in progress on low-temperature ESR and ENDOR on
these complexes may yield additional insight.20
Calculation of the SOMO for M(•EPh)(CO)3(PH3)2 for M
) Mo, W; E ) S, Te. To address theoretically the question of
the odd-electron orbital in complexes of formula M(•EPh)(CO)3-
(PR3)2 (E ) S, Se, Te; M ) Mo, W), the structure of the
representative model system Mo(•SPh)(CO)3(PH3)2 was opti-
mized. The multipole-derived spin density (MDC-q) on the
atoms was inspected and its SOMO visualized. From the output
of the Amsterdam Density Functional (ADF) calculation, only
a few atoms had significant spin density (Table 3). These results
indicate that the unpaired electron is located primarily on the
Mo and S atoms with some spin density on the carbonyl trans
to the SPh ligand. The values above are quite consistent with
the appearance of the SOMO, shown in Figure 4. The role of
the chalcogenide was investigated by doing a similar geometry
optimization and analysis for E ) Te. The complex Mo(•TePh)-
Figure 3. Normalized plots of ESR spectra (room temperature, toluene
solution) for M(•EPh)(CO)3(PiPr3)2 radicals: Mo (upper spectra) and W
(lower spectra) for S (green), Se (blue), and Te (red). Spectra for TeNaphth
(Naphth ) naphthyl, not shown for clarity) are virtually identical to those
of TePh in terms of position and shape.
away from the W-TePh bond. The Te-W-C(2) bond angle
of 177.59° is close to linear.
ESR Studies of M(•EPh)(CO)3(PiPr3)2. The stable free
radical complexes prepared here were all characterized by
solution phase room-temperature ESR spectroscopy. Recent
work12 has pointed out the difficulty and importance of knowing
the location of the unpaired electron in such radical complexes.
Two limiting views are (i) a metal-based radical and (ii) a
complexed thiyl or chalcogenyl radical. In addition, the organic
group can potentially serve as a site of delocalization. The room-
temperature ESR spectra of these radical complexes are shown
in Figure 3, and measured isotropic g values are summarized
in Table 2.
9
J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006 10299

A R T I C L E S
McDonough et al.
Table 3. Calculated Spin Densities for Mo(•SPh)(CO)3(PH3)2
Table 4. Enthalpies of Oxidative Addition and Derived M-EPh
Bond Strength Estimates
MDC-q spin density
b
b
c
Mo(PiPr3)2(CO)3
W(PiPr3)2(CO)3
Mo(N[tBu]Ar)3
Mo
S
C
O
P
P
-0.361
-0.356
-0.110
-0.070
-0.038
-0.012
E
E
−
Ea
∆Hrxn
BDE
∆Hrxn
BDE
∆Hrxn
BDE
S
Se
Te
46
41
33
-30.4
-27.8
-21.4
(38)
(34)
(27)
-42.2d
-37.0
-34.0
(44)
(39)
(33)
-63.2
-62.8
-48.0
(55)
(52)
(41)
a All data in kcal/mol. Data for the PhE-EPh bond strength as a function
of E are taken from ref 21. b Enthalpies of oxidative addition in toluene
solution at 25 °C and derived M-EPh bond strengths in kcal/mol. Error
limits on calorimetric measurements are typically (1 kcal/mol. Derived
absolute bond strengths in toluene solution are considered accurate to (3
kcal/mol. c Data for PhE-Mo(N[tBu]Ar)3 used to calculate bond strengths
here have been reported previously.22 d Value repeated and slightly revised
from earlier data.11
The data in Table 4 allow comparison of the bond strengths
to Mo(PiPr3)2(CO)3, W(PiPr3)2(CO)3, and Mo(N[tBu]Ar)3. For
the M(PiPr3)2(CO)3 complexes, oxidative addition to W is about
12 kcal/mol more exothermic than for Mo for all three
chalcogenides. As a direct result, the W-EPh bond is ∼6 kcal/
mol stronger than the Mo-EPh bond in this system. Considering
that the bonding radii of Mo and W are typically the same, this
indicates the increased coordinating power of W in the
complexes M(PR3)2(CO)3. Whether or not this difference may
be extrapolated to W(N[tBu]Ar)3 compared to Mo(N[tBu]Ar)3
is at present unknown.
Oxidative addition to Mo(N[tBu]Ar)3 is consistently more
exothermic than to Mo(PiPr3)2(CO)3. The experimentally ob-
served enthalpies of oxidative addition are nearly twice as
exothermic and the Mo-EPh bond is some 14-18 kcal/mol
stronger. It should be kept in mind that M(PiPr3)2(CO)3 has a
three-center C-H agostic bond blocking its vacant site with a
bond strength on the order of 10 kcal/mol and does not possess
a truly vacant site.8 This would reduce both the exothermicity
of the oxidative addition reactions and the defined measure of
the bond strength in the complexes M(PiPr3)2(CO)3 if no agostic
interaction were present. Such an accounting might be of interest
for interpreting theoretical results, but for practical utility in
terms of reaction mechanisms, and in the formal definition of
bond strength, it is the reactions of M(PiPr3)2(CO)3 with its
agostic bond present that are observed.
Figure 4. Calculated probability density contour for SOMO of Mo(•SPh)-
(CO)3(PH3)2.
(CO3)(PH3)2 was found to have a SOMO very similar in
appearance to its E ) S congener. The odd electron in the TePh
case resides more on Te than on Mo, with MDC-q spin densities
as follows: Te, -0.44; Mo, -0.39. Additionally there is less
calculated spin density on atoms other than Te and Mo than
there was for the SPh case.
Calculation of the SOMO also yields important insight into
the chemical bonding in M(•SPh)(PR3)2(CO)3. Because EPh is
a π-donor and the t2g orbital set is filled at the metal for these
systems, when SPh binds as an anion in the complex
M(SPh)(PR3)2(CO)3-, there is no net π bond (4e repulsion). In
the neutral radical a single electron is removed from the Mo-S
π* orbital for a π bond order of one-half. This may have
consequences in the net bond strength in the radical. For
example it was observed earlier that enhanced stability of the
M(•SPh)(PR3)2(CO)3 radical may explain the fact that thiophenol
bound to M(PR3)2(CO)3 undergoes facile H atom transfer11
Thermochemistry of Oxidative Addition of PhE-EPh.
Solution calorimetric data on the thermochemistry of oxidative
addition of dichalcogenides as shown in eq 1 for the full range
of S, Se, and Te derivatives of Mo(PiPr3)2(CO)3 and W(PiPr3)2-
(CO)3 are collected in Table 4. Data for enthalpies of oxidative
addition are used to generate bond strength estimates based on
recently generated values for PhE-EPh bond strengths.21 Data
for PhS-SPh and W(PiPr3)2(CO)3 were reported earlier, but are
repeated here.11c Also shown in Table 4 are data reported
earlier22 for Mo(N[tBu]Ar)3.
A direct outcome of the data in Table 4 is the ability to
calculate the net energy change for direct radical cleavage
reactions such as that shown in eq 2:
LnM + PhE-EPh f LnM-EPh + •EPh
(2)
calcd
∆Hrxn, kcal/mol
Mo(CO)3(PiPr3)2
W(CO)3(PiPr3)2
Mo(N[tBu]Ar)3
E ) S (+8) Se (+7) Te (+6)
E ) S (+2) Se (+2) Te (0)
E ) S (-12) Se (-11) Te (-9)
Reaction 2 represents what could be a potential first step in
•
oxidative addition in which a free EPh radical would be
generated. Beneath the equation are shown the calculated
enthalpies of this radical cleavage step obtained using data from
Table 4. It is clear that for Mo(N[tBu]Ar)3 this reaction is
exothermic for all three dichalcogenides and that for Mo(PiPr3)2-
(CO)3 it is endothermic. For the complex W(PiPr3)2(CO)3,
radical cleavage is calculated to be essentially thermoneutral.
(22) Mendiratta, A.; Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova, E. V.;
McDonough, J. E.; Hoff, C. D. Inorg. Chem. 2003, 42, 8621.
9
10300 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Chalcogenyl Radical Generation
A R T I C L E S
Figure 5. Time-resolved (100 s scan, 10 s intervals) spectral changes and
kinetic trace (at 580 nm) obtained upon mixing Mo(N[tBu]Ar)3 (0.45 mM)
with Ph2Se2 (4.5 mM) in a stopped-flow cell at -40 °C.
Figure 6. Plot of initial rates of appearance of [PhSe]Mo(N[tBu]Ar)3 vs
[Ph2Se2] at -40 °C. The initial concentration of Mo(N[tBu]Ar)3 was 0.225
mM (before mixing). A least-squares fit of the data gives an intercept of
0.0003 (∼3%) (R2 ) 0.998).
These results have implications for kinetic studies discussed
below. If the energetics are favorable, simple radical cleavage
as shown in eq 2 can prevail. A second mechanism would
involve coordination of PhE-EPh to two metal centers prior
to cleaving the chalcogen-chalcogen bond. There is precedent
for that type of intermediate in that stable complexes have been
prepared as shown in eq 3:
2W(CO)5(THF) + PhTe-TePh f
[PhTe-TePh][W(CO)5]2 (3)
Here the low reductive power of the W(CO)5 group enables
stable complexes with an intact Te-Te bond to be prepared
and structurally characterized.23
Figure 7. Plot of the temperature dependence of the second-order rate
constant for the formation of (PhSe)Mo(N[tBu]Ar)3 upon mixing Mo(N[tBu]-
Ar)3 (0.45 mM) with Ph2Se2 (4.5 mM) in toluene. The fit is to the linear
form of the Eyring equation, giving activation parameters of ∆Hq ) 5.0 (
0.5 kcal/mol-1, ∆Sq ) -32 ( 4 eu (R2 ) 0.999).
Rate and Mechanism of Reaction of Mo(N[tBu]Ar)3 with
Ph2Se2. This reaction was studied under both pseudo-first-order
(large excess PhSe-SePh) and second-order conditions [Mo-
1
(N[tBu]Ar)3] ) /2[PhSe-SePh]. The kinetic behavior at all
It is of interest to compare the rate of oxidative addition to
the rate of ligand binding in this system. The binding of
adamantyl isonitrile showed24 ∆Hq ) 5.5 kcal/mol and ∆Sq )
-15 cal/mol deg:
conditions studied was in agreement with first-order dependence
on both reactants, and the rate law d[P]/dt ) kobs[Mo(N[tBu]-
Ar)3][PhSe-SePh] was strictly obeyed from -40 to +25 °C.
Representative spectroscopic data and calculated and observed
kinetic data under first-order conditions are shown in Figure 5.
Mo(N[tBu]Ar)3 + AdNC f Mo(N[tBu]Ar)3(CNAd) (4)
Concentration experiments using 1:1 to 1:20 ratios of the
reagents were performed to determine initial rates of the reaction.
A plot of initial rate versus [Se] is a straight line, with intercept
close to zero, as shown in Figure 6, and indicates first-order
dependence on [PhSe-SePh].
Varying the concentration of Mo complex (0.45 to 0.225 mM)
under pseudo-first-order conditions ([Ph2Se2] ) 4.5 mM) at 25
°C gave the same k1obs ) 0.48 s-1 (k2 ) 213 ( 7 s-1 M-1).
These data suggest that reaction is first-order in Mo complex
as well and follows the rate law V ) kobs[Mo(N[tBu]Ar)3][Ph2-
Se2]. An Eyring plot of the derived second-order rate constants
is shown in Figure 7. All of the data are consistent with reaction
of Mo(N[tBu]Ar)3 with PhSe-SePh by a mechanism similar to
that shown in the upper portion of Scheme 1. Due to the rapid
nature of oxidative addition, resolution of kobs into individual
rate constants such as those shown in Scheme 1 was not
possible.4
The enthalpies of activation for both the ligand binding of AdNC
and the oxidative addition of PhSe-SePh are similar within
experimental error. It is primarily the entropy of activation that
is different for the two processes, and this implies a more
ordered transition state for the oxidative addition reaction.
Reaction of Mo(CO)3(PiPr3)2 with Ph2Se2. In contrast to
the relatively simple picture that emerged for reaction of Mo-
(N[tBu]Ar)3 and PhSe-SePh, reaction of Mo(PiPr3)2(CO)3
yielded surprising results. Despite being less thermodynamically
favored than Mo(N[tBu]Ar)3 with respect to oxidative addition
(see eq 2), Mo(PiPr3)2(CO)3 was observed to react more rapidly
under roughly comparable conditions. All experiments were
done in toluene under excess diselenide in the temperature range
T ) -80 to 15 °C, [Mo(PiPr3)2(CO)3] ) 0.18 to 0.45 mM, [Ph2-
Se2] ) 2.6 mM. Due to the rapid nature of the reaction, it could
only be followed using a single wavelength (700 nm). Attempts
(23) Pasynskii, A. A.; Torubaer, Y. V.; Eremenko, I. L.; Vegini, D.; Nefedov,
S. E.; Dobrokhotova, A. V.; Yanovskii, A. I.; Struchkov, Y. T. Russ. J.
Inorg. Chem. 1996, 41, 1901.
(24) Stepehens, F. H.; Figueroa, J. S.; Cummins, C. C.; Kryatova, O. P.; Kryatov,
S. V.; Rybak-Akimova, E. V.; McDonough, J. E.; Hoff, C. D. Organo-
metallics 2004, 23, 3126.
9
J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006 10301

A R T I C L E S
McDonough et al.
to use lower concentrations of the reagents failed due to
decomposition of the very air-sensitive Mo(PiPr3)2(CO)3 in
highly dilute solution. All measurements were done at λ ) 700
nm, where no photochemistry was expected.
does not bind quantitatively here), for which we extrapolate to
298 K a value of 3.3 × 105 M-1 s-1. Thus Mo(CO)3(PCy3)2
would be expected to bind ligands roughly 6 times faster at
room temperature compared to Mo(N[tBu]Ar)3. However, rates
of ligand addition reported by us earlier were not determined
for Mo(CO)3(PiPr3)2. To compare Mo(PiPr3)2(CO)3 to Mo-
(PCy3)2(CO)3, and hence ultimately to Mo(N[tBu]Ar)3, qualita-
tive competition experiments were performed with regard to
binding of AdNC, as shown in eq 6:
In the temperature interval -80 to -40 °C ([Mo] ) 0.45
mM, [Ph2Se2] ) 2.6 mM) attempts to fit the kinetic traces by
plotting ln[Mo] versus time yielded curved plots (Figure S3),
whereas those for plots of 1/[A∞ - A ] (Figure S4) were linear
and leave no doubt that at low temperature the reaction is
second-order in metal complex. A complete fit to a second-
order dependence on Mo(PiPr3)2(CO)3 in the -40 to -80 °C
temperature range is shown in Figure S5. At temperatures above
-40 °C, deviation from second-order dependence in [Mo-
(PiPr3)2(CO)3] does not yield such a good fit, indicating a
crossover to mixed first-order and second-order dependence on
Mo(PiPr3)2(CO)3.
Activation parameters for the -40 to -80 °C temperature
range in which second-order dependence on metal concentration
is followed are derived from Figure S6 as ∆Hq ) 4.4 kcal/mol
and ∆Sq ) -28 cal/mol deg. Surprisingly, under the mM
concentration conditions used, the reaction of Mo(PiPr3)2(CO)3,
which is second-order in [Mo(PiPr3)2(CO)3], is found to be
slightly faster than Mo(N[tBu]Ar)3, which follows a rate law
that is first-order in metal complex. Activation parameters, which
overlap within experimental error, are slightly more favorable
for reaction of PhSe-SePh with Mo(PiPr3)2(CO)3 compared to
Mo(N[tBu]Ar)3.
5Mo(PiPr3)2(CO)3 + 5Mo(PCy3)2(CO)3 + AdNC f
Products (6)
Addition of a limited amount of AdNC to excess, equal amounts
of both complexes showed that reaction of Mo(PiPr3)2(CO)3 was
much faster (roughly estimated as 10-100 times more product)
than Mo(PCy3)2(CO)3. On the basis of these observations, the
rate of ligand binding to Mo(PiPr3)2(CO)3 is estimated to be 2
to 3 orders of magnitude greater than to Mo(N[tBu]Ar)3.
It was also of interest to compare the rate of ligand binding
to that of oxidative addition for Mo(PiPr3)2(CO)3. In a related
competition experiment, to a vigorously stirred solution of
AdNC and PhSeSePh was slowly added a small amount of Mo-
(PiPr3)2(CO)3:
5AdNC + 2.5PhSe-SePh + Mo(PiPr3)2(CO)3 f
In addition to the second-order dependence on metal deter-
mined in the rate law, additional support of concerted cleavage
is provided by the enthalpy of activation. The fact that it was
found to be lower (4.4 kcal/mol) than the estimate for radical
cleavage in eq 2 (7 kcal/mol) also supports concerted cleavage.
The entropy of activation (ca. -28 cal/mol K) appears to be
not as highly negative as might be expected of a ternary
transition state [Mo(PiPr3)2(CO)3](PhSe-SePh)[Mo(PiPr3)2-
(CO)3], which is presumed to be obtained by displacement of
the weak agostic bonds by relatively weak dative bonds to
selenium. Displacement of the agostic bonds at the metal centers,
however, may result in some increased flexibility of the bound
isopropyl phosphine ligands and partially compensate for this.
Comparison of Rates of Ligand Binding to Mo(N[tBu]-
Ar)3 and Mo(CO)3(PiPr3)2. The more rapid nature of reaction
of Mo(CO)3(PiPr3)2 prompted additional qualitative studies. It
is clear that the rate of binding of PhSe-SePh plays an integral
role in the rate of oxidative addition. Comparison of the rates
of ligand binding to the tricyclohexylphosphine derivative Mo-
(CO)3(PCy3)2 versus that to Mo(N[tBu]Ar)3 can be made based
on published literature data. It is known that the complexes
M(CO)3(PR3)2 (M ) Cr, Mo,W) despite having the vacant site
blocked by a three-center C-H agostic bond all undergo rapid
ligand addition. Kinetic studies performed earlier, however, were
reported only for binding of ligands to the cyclohexyl phosphine
substituted derivatives and not the isopropyl derivatives studied
here. The binding of pyridine shown in eq 5 was determined
using previous methodology8d-g to have a second-order rate
constant of 2.0 × 106 M-1 s-1 at 298 K:
Products (7)
Due to the fact that PhSe-SePh can bind at either of the two
Se centers, only 2.5 molar equiv of the oxidant was used. At
-70 °C, both Mo(PiPr3)2(CO)3(AdNC) and Mo(PiPr3)2(CO)3-
(•SePh) were formed in roughly equimolar amounts. This
indicates that ligand binding and oxidative addition are competi-
tive reactions25 even at low T for Mo(PiPr3)2(CO)3.
These observations add support to a mechanism in which
rapid ligand exchange occurs for Mo(PiPr3)2(CO)3 resulting in
ultimate formation of Mo(PiPr3)2(CO)3(PhSe-SePh)Mo(PiPr3)2-
(CO)3, which then undergoes radical cleavage with little barrier.
The more rapid nature of reaction of the Mo(0) system studied
is attributed to its much greater rate of ligand exchange despite
weaker reductive power. Such a mechanism would correspond
to the lower pathway in Scheme 1 and would not generate free
•SePh radicals. Formation of Mo(PiPr3)2(CO)3(PhSe-SePh)
shown as a reversible equilibrium process in Scheme 1 is logical
based on the rapid ligand exchange reactions known to occur
for Mo(PR3)2(CO)3(L) complexes8 and the weak donor ability
of neutral organoselenium compounds.
Attempts to Trap •EPh Radicals with M(CO)3(PR3)2(H2).
Chemical evidence for generation of thiyl radicals was generated
earlier by us in the case of the oxidative addition of PhS-SPh
to W(CO)3(PCy3)2. Under an atmosphere of H2, thiophenol was
produced, and the mechanism shown in eqs 8-11 was pro-
posed.11
(25) Additional, more quantitative studies of ligand binding for these complexes
are planned. The fact that ligand binding of AdNC and oxidative addition
of PhSeSePh by Mo(CO)3(PiPr3)2 occur at similar net rates is somewhat
surprising. Since oxidative addition presumably incorporates ligand binding
as a first step, it is clear that PhSe-SePh must bind equal to or faster than
AdNC. The authors could not find data comparing rates of binding at
crowded metal centers of Se versus C donor ligands in the literature.
Mo(CO)3(PCy3)2 + py f Mo(CO)3(PCy3)2(py) (5)
This rate of ligand binding can be compared to the rate of
binding24 of adamantyl isocyanide to Mo(N[tBu]Ar)3 (pyridine
9
10302 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006

Chalcogenyl Radical Generation
A R T I C L E S
transfer in eq 12. In keeping with thermochemical predictions,
FTIR study of reaction 12 showed it to occur over a time period
of 1 or 2 h under millimolar concentration conditions at room
temperature. The slow nature of this thermodynamically favored
group transfer is attributed primarily to steric repulsive forces
in the presumed (PiPr3)2(CO)3Mo(µ-•SePh)Mo(N[tBu]Ar)3 tran-
sition state.
W(CO)3(PCy3)2(H2) f W(CO)3(PCy3)2 + H2
W(CO)3(PCy3)2 + PhS-SPh f
W(•SPh)(CO)3(PCy3)2 + •SPh (9)
•SPh + W(CO)3(PCy3)2(H2) f
W(CO)3(PCy3)2(H•) + HSPh (10)
W(CO)3(PCy3)2(H•) + PhS-SPh f
(8)
Conclusions
Preparation of a full set of M(•EPh)(PiPr3)2(CO)3 complexes
is reported as well as investigation of their kinetic, thermody-
namic, and spectroscopic properties. These reactions are com-
pared to those for Mo(N[tBu]Ar)3(EPh) complexes in terms of
bond strength and rate of reaction. Thermochemical factors play
an important role in determining whether direct cleavage of the
PhE-EPh bond to radicals is uphill for Mo(PiPr3)2(CO)3,
approximately thermoneutral for W(PiPr3)2(CO)3, or significantly
exothermic for Mo(N[tBu]Ar)3.
The mechanism of oxidative addition of PhSe-SePh to Mo-
(PiPr3)2(CO)3 is second-order in metal complex at low temper-
ature, in keeping with a concerted cleavage of PhSe-SePh by
coordination of two metals in the transition state. In contrast,
first-order (in metal) cleavage of PhSe-SePh is observed for
Mo(N[tBu]Ar)3. This is in keeping with the greater Mo-EPh
bond strength observed for Mo(N[tBu]Ar)3. The most surprising
result of this work is that, despite second-order dependence on
metal complex, oxidative addition is more rapid for Mo(PiPr3)2-
(CO)3. This is attributed to a much faster rate of ligand binding
to Mo(PiPr3)2(CO)3 compared to Mo(N[tBu]Ar)3, albeit via what
is proposed to be a concerted reaction not involving generation
of free •EPh radicals. In view of the importance of •EPh radicals
(either free or trapped at a metal complex), additional studies
of these and related reactions by thermodynamic, kinetic, and
theoretical methods are in progress.
W(•SPh)(CO)3(PCy3)2 + HSPh (11)
In the current work, we have investigated the same reaction
qualitatively for W(CO)3(PiPr3)2 and Mo(CO)3(PiPr3)2, where
oxidative addition with PhS-SPh and PhSe-SePh was per-
formed under an atmosphere of H2. At room temperature and a
total pressure of 2 atm of H2, the Mo complex was found to
produce no more than trace PhSH (less than 5%) and the W
complex gave ∼30% of PhSH, presumably by a mechanism
similar to that shown in eqs 8-11. It was thus observed that
reactions in keeping with radical cleavage are observed as a
greater percentage of the reaction channel for W compared to
Mo. This is in keeping with thermochemical data in eq 2, which
show a less endothermic nature to direct cleavage to radicals
for the W compared to Mo systems.
Additional investigations involved analogous reactions of
PhSe-SePh with Mo(PiPr3)2(CO)3 and W(PiPr3)2(CO)3 under
H2. For the Mo system, there was no observation of formation
of PhSeH, but the radical Mo(•SePh)(PiPr3)2(CO)3 was formed
in high yield. Investigation of oxidative addition of PhSe-SePh
with the W complex under H2 did not produce any free PhSeH.
However, the complex W(•SePh)(PiPr3)2(CO)3 was not formed
quantitatively, and additional uncharacterized organometallic
complexes were formed. This implies that if the PhSe• radical
was generated as shown in eq 9, then it did not take part in
reactions leading to free PhSeH but may have undergone some
additional unknown reaction.
Acknowledgment. The authors thank the National Science
Foundation for funding via grant CRC 0209977. G.J.K. is
grateful to the Department of Energy, Office of Basic Energy
Sciences, Chemical Sciences Division, for funding. Special
thanks to Dr. Joshua Telser, Roosevelt College, for helpful
discussions regarding ESR data.
In summary, only for the combination of PhS-SPh and
W(PiPr3)2(CO)3(H2) was significant PhEH found when oxidation
was performed under an atmosphere of H2. This observation
•
implies that SPh was generated in the reaction and abstracted
H from W(PiPr3)2(CO)3(H2). The lower (trace or less) yield of
PhSH for corresponding reactions of Mo(PiPr3)2(CO)3(H2) is
in keeping with a lower percentage of oxidative addition
proceeding through a free radical mechanism. These observa-
tions are also in keeping with the thermochemical data in eq 2.
Reaction of Mo(•SePh)(PiPr3)2(CO)3 and Mo(N[tBu]Ar)3.
Thermochemical data in Table 4 indicate that reaction 12 is
exothermic by approximately 10 kcal/mol:
Supporting Information Available: Figure S1 showing FTIR
data for Mo(•EPh)(PiPr3)2(CO)3 complexes; Figure S2 showing
UV-vis study of formation of Mo(N[tBu]Ar)3(SePh); Figure
S3 showing first-order kinetic plot for reaction of Ph2Se2 and
Mo(PiPr3)2(CO)3 at -80 °C; Figure S4 showing second-
orderkinetic plot for reaction of Ph2Se2 and Mo(PiPr3)2(CO)3
at -80 °C; Figure S5 showing complete best fit data from -40
to -80 °C for Mo(CO)3(PiPr3)2 + Ph2Se2 to second-order
dependence on metal complex concentration; Figure S6 showing
Eyring plot for reaction of Mo(PiPr3)2(CO)3 and Ph2Se2; tables
of crystal data, 24 pages, including crystal data and structure
refinement, atomic coordinates and equivalent isotropic dis-
placement parameters, bond lengths and angles, anisotropic
displacement parameters, hydrogen coordinates, isotropic dis-
placement parameters, as well as full data in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Mo(•SePh )(CO)3(PiPr3)2 + Mo(N[tBu]Ar)3 f
Mo(CO)3(PiPr3)2 + Mo(N[tBu]Ar)3(SePh) (12)
Furthermore, the Mo-SePh bond strength in the radical complex
(34 kcal/mol) is actually weaker than the PhSe-SePh bond (41
kcal/mol), as discussed above. The fact that oxidative addition
of PhSe-SePh to both Mo(PiPr3)2(CO)3 and Mo(N[tBu]Ar)3
occurs rapidly enough that it must be followed by stopped-flow
•
kinetics prompted qualitative investigation of the SePh group
JA063250+
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