Intramolecular exchange of coordinated and dangling phosphine groups in pentacarbonyl[(diphenylphosphino)(di-p-tolylphosphino)methane]tungsten(0)

Article abstract of DOI:10.1021/om300092x

Equilibrium constants and rates have been determined for the isomerization of the linkage isomers (OC)₅W[κ¹-PPh ₂CH₂P(p-tolyl)₂] (5) and (OC) ₅W[κ¹-P(p-tolyl)₂CH₂PPh ₂] (6). It is proposed that this intramolecular exchange involves a nucleophilic attack of the pendant phosphine on a cis carbonyl group, followed by ring opening and a 1,2-shift.

Full text of DOI:10.1021/om300092x

Note
pubs.acs.org/Organometallics
Intramolecular Exchange of Coordinated and Dangling
Phosphine Groups in Pentacarbonyl[(diphenylphosphino)-
(di-p-tolylphosphino)methane]tungsten(0)
Richard L. Keiter,*,† Deliang Chen,† Geoffrey A. Holloway,† Ellen A. Keiter,† Yi Zang,† M. Todd Huml,†
Jonathan Filley,† and Douglas E. Brandt‡
‡
†Department of Chemistry and Department of Physics, Eastern Illinois University, Charleston, Illinois 61920, United States
ABSTRACT: Equilibrium constants and rates have been
determined for the isomerization of the linkage isomers
(OC)5W[κ1-PPh2CH2P(p-tolyl)2] (5) and (OC)5W[κ1-P(p-
tolyl)2CH2PPh2] (6). It is proposed that this intramolecular exchange involves a nucleophilic attack of the pendant phosphine on
a cis carbonyl group, followed by ring opening and a 1,2-shift.
revious work has shown that the complexes (OC)5W[κ1-
PPh2CH2CH2P(p-tol)2] (1) and (OC)5W[κ -PPh2CH2-
Scheme 2
1
P
CH(PPh2)2] (3) exchange pendant and coordinated phosphines
in solution, forming linkage isomers that exist in equilibrium. The
rate of isomerization for 3, however, is about 104 times faster than
that for 1 in CDCl3 at 55 °C (Scheme 1).1
Scheme 1
Reaction schemes based on thermodynamic and kinetic results
have been proposed to account for the relative rates (Scheme 2).2
In both part a and part b of Scheme 2, the reactions are initiated by
attack of the pendant phosphine nucleophile on a cis carbonyl carbon,
which leads to the formation of a six-membered ring. Ring opening
allows for a 1,2-shift in both mechanisms, but the shift is unfavorable in
Scheme 2b relative to the formation of a five-membered ring which
can then open to give the new isomer 4 directly.
The exchanging phosphorus atoms in both processes are
separated by two carbon atoms. Complex 4 also has an
uncoordinated phosphine that is separated from the ligated site
by one carbon. Because these two phosphines are symmetry
equivalent, their exchange with the metal cannot be determined
directly by 31P NMR. We have now synthesized two complexes
with nonequivalent exchanging phosphines: (OC)5W[κ1-
RESULTS AND DISCUSSION
■
PPh2CH2P(p-tol)2] (5) and its linkage isomer 6. The Ph2P
and (p-tol)2P groups are isosteric but nonequivalent and have 31P
chemical shifts that differ by 2 ppm, allowing the ratio of the two
isomers in solution to be determined. To further assess the
mechanistic models presented in Scheme 2, we have investigated
the kinetics and thermodynamics of isomerization for these
complexes (Scheme 3).
Syntheses. Two approaches were used to synthesize
Ph2PCH2P(p-tol)2 (7), a ligand not previously reported.
Following the work of Langhans, Ph2PCH2Cl was obtained
Received: February 6, 2012
Published: June 12, 2012
© 2012 American Chemical Society
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Organometallics
Note
Scheme 3
from a two-phase reaction3 and its subsequent reaction with
LiP(p-tol)2 gave 7.
Ph2PH + CH2Cl2 + KOH
→ Ph2PCH2Cl + KCl + H2O
(1)
Ph2PCH2Cl + LiP(p‐tol)2 → 7 + LiCl
Alternatively, 7 was prepared from Ph2PCH2SiMe3 and
(2)
Figure 1. Isomerization of 5 at 40 °C.
4
(p-tol)2PCl.
Ph2PCH2SiMe3 + (p‐tol)2PCl → Me3SiCl + 7
(3)
The first approach uses less expensive reactants and mild
reaction conditions and requires only two steps. It works best
when carried out with a large excess of CH2Cl2 (also the solvent)
at temperatures below 10 °C. The second approach leads to a
higher yield (82%) than the first (40%), but a disadvantage of this
reaction is that several steps are required to prepare the starting
materials. Reaction of 7 with (OC)5WNH2Ph gave a mixture of 5
and 6. As 5 is less soluble than 6, it was possible to isolate pure 5
by slow crystallization.
Figure 2. Plot of ln([5] − [5]eq) versus time at 40 °C.
(OC)5WNH2Ph + 7 → 5 + 6 + PhNH2
(4)
Complexes 5 and 6 have identical elemental analyses and
nearly identical IR spectra. To distinguish one isomer from the
other with certainty by 31P NMR, (OC)5W[κ1-P(p-tol)2CH2P(p-
tol)2] (8) and the known (OC)5W[κ1-PPh2CH2PPh2)] were
prepared and their chemical shifts were compared with those of
the new isomers.
Rate constants, half-lives to equilibrium, and equilibrium
constants for the reaction are shown in Table 1. From the slope
Table 1. Rate Data and Equilibrium Constants for 5 ⇌ 6 in
CDCl3
Efforts to obtain 5 selectively from the reaction of
(OC)5WPPh2CH2X (X = Cl (9), Br (10)) with LiP(p-tol)2
were unsuccessful. Presumably, the steric requirements of the
reactants make an SN2 reaction unfavorable. The precursors 9
and 10 were obtained selectively from the reaction of
(OC)5WNH2Ph with PPh2CH2X or from the nonselective
reaction of Li[(OC)5WPPh2] with CH2X2. The latter reaction
also gave (OC)5WPPh2Me,5 possibly forming from a competing
metalation reaction in which (OC)5WPPh2CH2Li is protonated
in the workup.
Thermodynamics and Kinetics. The 5 ⇌ 6 isomerization
was followed at 25, 40, and 55 °C by 31P NMR. Each isomer gives
rise to two first-order doublets resulting from phosphorus−
phosphorus coupling of the nonequivalent phosphorus atoms.
The nonoverlapping signals of the PPh2 and P(p-tol)2 groups
allowed isomer ratios to be obtained via integration. Tungsten−
phosphorus satellites were observed for the coordinated
phosphines. There was no evidence for chelation of 5 or 6 in
any of the isomerization runs, consistent with the previously
reported stability of (OC)5W(κ1-PPh2CH2PPh2).6 In fact, a
CDCl3 solution of 5 and 6 showed no spectroscopic or visual
evidence for degradation over a 2 year period.
ln 2/(k1 + k−1
)
T (K)
k1 (s−1
)
k−1 (s−1
)
(days)
K
328 (2.40 0.12) ×
(1.16 0.12) ×
2.25
15.8
178
2.07
10−6
10−6
313 (3.50 0.02) ×
(1.58 0.02) ×
2.22
2.36
10−7
10−7
298 (3.17 0.06) ×
(1.34 0.06) ×
10−8
10−8
of ln K versus 1/T, ΔH and ΔS for the forward reactions were
found to be −3.35 0.24 kJ/mol and −4.74 0.76 J/(mol K),
respectively. The reaction is thermodynamically favorable (6 is
more stable than 5), but entropy favors the formation of 5.
Application of the Eyring equation allowed determination of
activation parameters. For the forward reaction, ΔH⧧ = 114.7
4.2 kJ/mol and ΔS⧧ = −3.1 14 J/(mol K), and for the reverse
reaction, ΔH⧧ = 118.4 4.1 kJ/mol and ΔS⧧ = 1.9 13 J/(mol K).
The enthalpies of activation are much lower than the estimated
W−P bond strengths for similar complexes7 and are consistent with
a mechanism that includes a significant associative component.
Our expectation that 5 would isomerize more slowly than 3
was realized. This expectation was based on the assumption that
isomerizations of 5 and 1 follow analogous mechanistic pathways
in which the rate-determining step is a 1,2-shift (Scheme 2). Both
5 and 1 are missing a second uncoordinated phosphine arm,
which precludes the faster pathway available to 3.
The plot in Figure 1, which depicts the isomerization of 5 at
40 °C, shows a decrease in the concentration of 5 with time and a
corresponding increase in the concentration of 6 until
equilibrium is reached.
A plot of ln([5] − [5]eq) versus time (Figure 2) gave a
straight line, as expected for first-order kinetics, and from its slope,
−(k1 + k−1), and K both rate constants were determined.
However, the isomerization of 5 at 328 K is 100 times faster
than that of 1. To account for this difference, a more detailed look
at the mechanism is required (Schemes 4 and 2a). Specifically,
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Note
consideration of the solution conformations of 5 and 1 is
instructive.
increasing the probability of a 1,2-shift. Thus, k for the
isomerization of 5 is larger than for 1. This difference could
also be viewed as resulting from the greater rotational degrees of
freedom in 1, which would decrease the probability of collision
between its pendant phosphine and an equatorial carbonyl.
That a pendant group of a coordinated ligand in group 6 metal
carbonyl complexes may influence substitution reaction rates is
not a new idea. Examples involving the distal oxygen and sulfur
atoms of acetate, carboxylate, and thiolate ligands have been
reported.11
Scheme 4
The difference in the W−P bond dissociation energies in 1 and
5 may make a small contribution to the faster isomerization
rate observed for 5. The pKa's of Ph2PCH2PPh2 (dppm) and
Ph2PCH2CH2PPh2 (dppe) are 3.86 and 3.81, respectively,
showing that dppe is slightly more basic (toward protons) than
is dppm.12 While this does not tell us anything about the π
capacities of the two ligands, it does suggest that there is likely to
be too little difference in W−P bond strengths to account for the
large difference in rates.
The crystal structure of (OC)5W(κ1-PPh2CH2PPh2), which is
isosteric with 5, has been determined.8 It reveals that the pendant
phosphine points toward the W(CO)4 equatorial plane of the
molecule with a phosphorus−carbonyl carbon separation of
3.42 Å (Scheme 5), which is about the sum of the van der Waals
CONCLUDING REMARKS
■
For many years it was not recognized that complexes of group 6
pentacarbonyls with monocoordinated polyphosphines undergo
exchange of the coordinated and uncoordinated phosphine
moieties. It was thought that such complexes could only serve as
ligands toward Lewis acids or undergo chelation if heated or
photolyzed. The discovery that 3 undergoes phosphine exchange
much more quickly than expected led us to investigate reactions
of this type in more detail.7c,13 We have found that exchange is
slowed when the coordinated and pendant phosphines are
separated by one or two carbon atoms if a third phosphine arm is
missing. Thus, 1 and 5 exhibit much slower phosphine exchange
than does 3. For one-armed diphosphine complexes, reactions
proceed by nucleophilic attack on the carbon of M−CO, which
leads to phosphine dissociation followed by a 1,2-shift. For the
two-armed 3, the 1,2-shift is avoided by formation of a new ring
followed by dissociation of the phosphine from the M−CO
group. Complex 5 isomerizes more quickly than 1 primarily
because, in the predominant conformational arrangement in 5
(but not 1), the pendant phosphine lies toward the carbonyl
substrate, thereby facilitating nucleophilic attack. In summary,
complexes 1, 3, and 5 all undergo phosphorus exchange more
quickly than chelation at modest temperatures and the exchange
rates vary considerably (k = 10−4−10−8 s−1 at 55 °C), depending
on the nature of the phosphine.
Scheme 5
radii (3.5 Å).9 Spectroscopic evidence previously reported
suggests that this conformation largely persists in solution.1b,2
The pendant arm is not localized on a particular carbonyl group
but rather can be viewed as moving about the equatorial carbonyl
plane. The phosphorus lone pair of the pendant phosphine is
thus poised for nucleophilic attack on the nearby carbonyl
carbon, thereby creating a five-membered ring and increasing the
electron density at the tungsten atom. This leads to a weakening
of the W−P bond and increases the probability of dissociation,
which in turn allows a 1,2-shift.
In the 13C NMR spectra for the complexes of this study, long-
range phosphorus−carbon coupling is observed between the
pendant phosphine and the cis carbonyls of 5 (4JPC = 3.2 Hz) and
6 (4JPC = 2.9 Hz), as reported for (OC)5W(κ1-PPh2CH2PPh2)
(4JPC = 3.0 Hz), but it is not observed for 1. No coupling is
observed to the trans carbonyl for any of the complexes. The
long-range coupling is thought to have a significant “through-
space” contribution and is consistent with the close approach of
the short phosphine arm to the equatorial carbonyl plane.2,8
Unlike isomers 5 and 6, the most favorable conformation for 1
is one in which the pendant phosphine is oriented away from
the carbonyl plane (Scheme 6).10 As a consequence, it is less
EXPERIMENTAL SECTION
■
All reactions were carried out under a nitrogen atmosphere.
Tetrahydrofuran (THF) was dried with sodium metal in the presence
of benzophenone and was freshly distilled under N2 before use. All other
solvents were used without further purification. The starting materials
P(p-tol)3,14 LiP(p-tol)2,15 (p-tol)2 PCl,16 Ph2 PCH2 Cl,3
Ph2PCH2SiMe3,4 (OC)5WNH2Ph,17 (p-tol)2PCH2P(p-tol)2,18 and
Li[W(CO)5PPh2]19 were prepared by literature methods. Phospho-
rus-31 NMR spectra (referenced to 85% phosphoric acid) and carbon-
13 NMR spectra (referenced to TMS) of CDCl3 solutions were
recorded with a GE QE-300 NMR spectrometer; all spectra were
proton-decoupled. Chemical shifts are quoted in ppm and coupling
constants in Hz. Infrared spectra of CHCl3 solutions were recorded with
a Nicolet 20 DXB FT-IR spectrometer, and results are reported as cm−1.
Elemental analyses were performed at the University of Illinois
Microanalytical Laboratory in Urbana, IL.
Scheme 6
probable that the dangling phosphine in 1 will interact with the
carbonyl group to form a ring and, therefore, less likely that there
will be a 1,2-shift. In other words, the equilibria for the forward
reactions are shifted further to the right for 5 than for 1, thereby
(Diphenylphosphino)(di-p-tolylphosphino)methane,
Ph2PCH2P(p-tol)2 (7). (a) To a freshly prepared solution of
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Note
Ph2PCH2Cl (9.2 mmol in 60 mL of THF) was slowly added a red
solution of LiP(p-tol)2 (12.5 mmol in 25 mL of THF). The red color
disappeared rapidly at first and then more slowly until at 19 mL the red
color persisted. The solution was stirred for 20 h and the THF removed
by vacuum. Treatment of the resulting oil with ethanol gave a white solid
(1.50 g, 40%).
ACKNOWLEDGMENTS
■
We thank the Camille and Henry Dreyfus Foundation for a
Scholar Fellow grant (SF-98-004) and the National Science
Foundation (CHE-9708342) for support of this work.
REFERENCES
■
(b) A solution prepared from Ph2PCH2SiMe3 (5.5 mmol) and (p-
tol)2PCl (4.9 mmol) was slowly heated to 150 °C over 4 h. The volatile
Me3SiCl was removed by vacuum and the remaining solid crystallized
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from ethanol (1.9 g, 82%). 31P NMR: AB quartet, δ −22.1, −23.8; 2JPP
126.7.
=
Pentacarbonyl[(diphenylphosphino)(di-p-tolylphosphino)-
methane]tungsten(0), (OC)5W[κ1-PPh2CH2P(p-tol)2] (5) and
(OC)5W[κ1-P(p-tol)2CH2PPh2] (6). To a solution of (OC)5WNH2Ph
(0.83 g, 2.2 mmol in 30 mL of CH2Cl2) was added Ph2PCH2P(p-tol)2
(0.90 g, 2.2 mmol in 30 mL of CH2Cl2). The solution was stirred for
24 h, and the solvent was removed by vacuum. The resulting solid was
dissolved in 10 mL of a 1:1 solution of CH2Cl2 and CH3OH and placed in a
freezer at −15 °C, where a white solid precipitated consisting of 5 and 6 (1.2
g, 82%). IR: νCO 2070 (m), 1980 (w), 1939 (s). The overlapping signals for
the two isomers were slightly broadened and were not resolved. 31P NMR:
(5) McFarlane, H. C. E.; McFarlane, W.; Rycroft, D. S. J. Chem. Soc.,
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5, δ 10.1, δ
−26.1, 2JPP = 104.9, JWP = 245.0; 6, δ −23.9, δ
PPh2
P(tol)2
PPh2
P(tol)2
8.0, 2JPP = 104.0, JWP = 244.4. 13C NMR (CO): 5, δcis 205.6 (dd, 2JPC = 7.1,
4JPC = 2.9), δtrans 208.0 (d, 2JPC = 21.7); 6, δcis 205.7 (dd, 2JPC = 7.0, 4JPC
=
(7) (a) Mukerjee, S. L.; Lang, R. F.; Ju., T.; Kiss, G.; Hoff, C. D.; Nolan,
3.2), δtrans 208.2 (d, 2JPC = 21.9). Pure white crystals of 5 were obtained by
dissolving the isomer mixture in CH2Cl2, adding a layer of CH3OH, and
refrigerating for 3 weeks. Mp: 168−170 °C dec. IR: νCO 2071 (m), 1981
(w), 1940 (s). Anal. Calcd for C32H26O5P2W: C, 52.20; H, 3.60; P, 8.41.
Found: C, 51.99; H, 3.44; P, 8.80.
Pentacarbonyl[bis(di-p-tolylphosphino)methane]tungsten-
(0), (OC)5W[κ1-P(p-tol)2CH2P(p-tol)2] (8). This compound was
prepared in 58.8% yield from (OC)5WNH2Ph and (p-tol)2PCH2P(p-
tol)2 by the same method used for 5 and 6. IR: νCO 2070 (m), 1979 (w),
1938 (s). 31P NMR: δPW 7.89, δP −25.7, 2JPP = 102.8, JWP = 243.4. Anal.
Calcd for C34H30O5P2W: C, 53.42; H, 3.96; P, 8.10. Found: C, 53.19; H,
3.80; P, 7.65.
[(Chloromethyl)diphenylphosphine]pentacarbonyltungsten(0),
(OC)5WPPh2CH2Cl (9). (a) This compound was obtained from the
reaction of (OC)5WNH2Ph (1.0 g, 2.4 mmol) with Ph2PCH2Cl (0.56 g,
2.4 mmol) in toluene (25 mL). Addition of an equal volume of methanol
followed by refrigeration led to 0.64 g of crystalline product (45%). IR:
νCO 2074 (m), 1984 (w), 1942(s). 31P NMR: δP 20.7, JWP = 246.3. Anal.
Calcd for C18H12O5PWCl: C, 38.71; H, 2.17; P, 5.55. Found: C, 38.30;
H, 1.70; P, 5.40.
(b) 9 was also formed from the reaction of Li[(OC)5WPPh2] with
CH2Cl2. Similarly, (OC)5WPPh2CH2Br (10) was obtained. IR: νCO
2073 (m), 1982 (w), 1943 (s). 31P NMR: δP 19.4, JWP = 245.8. However,
in the preparation of both the chloride and bromide complexes by this
method, (OC)5WPPh2Me5 was formed as a side product (25%). 31P
NMR: δP −3.27, JWP = 237.0.
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Reaction Rates and Equilibrium Constants. Into an NMR tube
was placed 30 mg of pure 5 or a mixture of 5 and 6, dissolved in 0.5 mL of
CDCl3. The tube was vacuum-sealed and placed into a constant-
temperature bath. 31P NMR spectra were recorded periodically for each
of three temperatures: 25, 40, and 55 °C. The NMR probe was brought
to the appropriate temperature before each spectrum was collected. The
ratio of the two isomers was determined by integration of the
phosphorus signals.1a Equilibrium was assumed to have been reached
when the ratio no longer changed with time.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rlkeiter@eiu.edu.
Notes
The authors declare no competing financial interest.
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