Reductive elimination of ethane from five-coordinate platinum(IV) alkyl complexes

Article abstract of DOI:10.1021/ic701504z

Five-coordinate platinum(IV) alkyl complexes bearing sterically non-demanding pyridylpyrrolide ligands, (LX)PtMe₃ [LX = 3,5-di-tert-butyl-2-(2-pyridyl)pyrroiide (3a) and 3,5-diphenyl-2-(2-pyridyl) pyrrolide (3b)] have been prepared. An X-ray structure of 3a establishes that it is a five-coordinate Pt^IV complex with a square-pyramidal geometry. Thermolysis of 3a or 3b in C₆D₆ with ethylene results in reductive elimination of ethane (C₂H₆) and methane (CH₄ and CH₃D) and the formation of cyclometalated platinum(II) ethylene complexes 4a or 4b, respectively. Results of kinetic investigations of the reaction of 3b are consistent with a mechanism of direct C-C reductive elimination from the five-coordinate Pt^IV compound. Thermolysis of 3a in C₆D₆ with no ethylene present forms a novel dinuclear complex (5-d₆).

Full text of DOI:10.1021/ic701504z

Inorg. Chem. 2007, 46, 8496−8498
Reductive Elimination of Ethane from Five-Coordinate Platinum(IV) Alkyl
Complexes
Avery T. Luedtke and Karen I. Goldberg*
Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700
Received July 27, 2007
Five-coordinate platinum(IV) alkyl complexes bearing sterically non-
demanding pyridylpyrrolide ligands, (LX)PtMe3 [LX
) 3,5-di-tert-
butyl-2-(2-pyridyl)pyrrolide (3a) and 3,5-diphenyl-2-(2-pyridyl)pyr-
rolide (3b)] have been prepared. An X-ray structure of 3a estab-
lishes that it is a five-coordinate PtIV complex with a square-pyra-
midal geometry. Thermolysis of 3a or 3b in C6D6 with ethylene
results in reductive elimination of ethane (C2H6) and methane (CH4
and CH3D) and the formation of cyclometalated platinum(II) eth-
ylene complexes 4a or 4b, respectively. Results of kinetic
investigations of the reaction of 3b are consistent with a mechanism
t
Figure 1. POV-Ray thermal ellipsoid diagram of 3a (R ) Bu) with a
50% probability. Selected bond lengths (Å) and angles (deg): C1-Pt1,
2.068(5); C2-Pt1, 2.046(5); C3-Pt1, 2.038(5); N1-Pt1, 2.123(4); N2-
Pt1, 2.189(4); C1-Pt1-C3, 86.47(19); N2-Pt1-C1, 104.91(18); N1-Pt1-
N2, 77.40(15).
of direct C−
C reductive elimination from the five-coordinate PtIV
Pt(H)2SiR4]+ (pz′ ) 3,5-dimethylpyrazole; R4 ) Et3, Ph3,
compound. Thermolysis of 3a in C6D6 with no ethylene present
forms a novel dinuclear complex (5-d6).
and Ph2H).5 More recently, the five-coordinate PtIV com-
i
plex (AnIm)PtMe3 (AnIm ) [o-C6H4{N(C6H3 Pr2)}(CHd
i
NC6H3 Pr2)]-) has been crystallographically characterized.6
Coordinatively and electronically unsaturated species are
often proposed as key intermediates in transition-metal-
catalyzed reactions.1 Five-coordinate d6 and three-coordinate
d8 late-transition-metal species have received significant
recent attention in this regard.2 Because of their inherent
reactivity, the isolation, characterization, and studies of such
unsaturated complexes can be challenging. Although pro-
posed for over 30 years as intermediates in reductive
elimination and oxidative addition reactions to form and
cleave C-H, C-C, and C-X bonds at PtIV and PtII centers,
respectively,2c,3 the first isolable examples of five-coordinate
PtIV compounds were only reported in 2001: a series of
A five-coordinate structure has also been proposed for a
(dimethylsilyl)platinum(IV) pydridylindolide complex.7
Notably, each of the known five-coordinate PtIV complexes
contains an anionic bidentate ligand bearing N donor atoms.
Similarly, in this contribution, 3,5-disubstituted 2-(2-pyridyl)-
pyrrolide ligands are used to stabilize unsaturated PtIV.
Mechanistic studies of the reductive elimination of ethane
from these new five-coordinate platinum(IV) trimethyl com-
pounds have been carried out, and the results are consistent
with C-C coupling occurring directly from the five-
coordinate complexes. Previous studies of alkyl C-C reduc-
tive elimination from six-coordinate PtIV have consistently
found support for mechanisms involving preliminary ligand
loss to generate steady-state concentrations of five-coordinate
intermediates from which C-C coupling occurs.3 By starting
with a five-coordinate PtIV system, as in the work described
here, a rate constant corresponding directly to the C-C bond-
forming reductive elimination step can be measured.
trimethyl complexes with nacnac ligands, [{(o-R1 -p-
2
R2C6H2)NC(R3)}2CH]PtMe3 (R1-3 ) various combinations
t
i
of H, Me, Bu, and Pr),4 and silyl hydride complexes with
a protonated trispyrazole borate ligand, [κ2-((Hpz′)BHpz′2)-
* To whom correspondence should be addressed. E-mail: goldberg@
chem.washington.edu.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4th ed.; John Wiley & Sons, Inc.: New York, 2005.
(2) (a) Yamashita, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 5344.
(b) Baratta, W.; Stoccoro, S.; Doppiu, A.; Herdtweck, E.; Zucca, A.;
Rigo, P. Angew. Chem., Int. Ed. 2003, 42, 105. (c) Puddephatt, R. J.
Angew. Chem., Int. Ed. 2002, 41, 261. (d) Plutino, M. R.; Scolaro, L.
M.; Romeo, R.; Grassi, A. Inorg. Chem. 2000, 39, 2712. (e)
Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.;
Goldman, A. S. J. Am. Chem. Soc. 2000, 122, 11017.
The reaction of 3,5-di-tert-butyl-2-(2-pyridyl)pyrrole
[DtBPP-H (1a)] or 3,5-diphenyl-2-(2-pyridyl)pyrrole [DPPP-
(4) (a) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 15286. (b) Fekl, U.; Goldberg, K. I. J. Am. Chem. Soc. 2002,
124, 6804. (c) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem.
Soc. 2001, 123, 6423.
(3) For examples, see: (a) Procelewska, J.; Zahl, A.; Liehr, G.; van Eldik,
R.; Smythe, N. A.; Williams, B. S.; Goldberg, K. I. Inorg. Chem.
2005, 44, 7732. (b) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am.
Chem. Soc. 2003, 125, 9442. (c) Williams, B. S.; Goldberg, K. I. J.
Am. Chem. Soc. 2001, 123, 2576.
(5) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am.
Chem. Soc. 2001, 123, 6425.
(6) Kloek, S. M.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 3460.
(7) Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics
2006, 25, 1801.
8496 Inorganic Chemistry, Vol. 46, No. 21, 2007
10.1021/ic701504z CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/18/2007

COMMUNICATION
Scheme 1
Figure 2. Formation of 5-d8 and POV-Ray diagram at a 50% probability.
The H atoms and 1.5 equiv of benzene have been omitted for clarity.
Selected bond lengths (Å) and angles (deg): Pt1-Pt2, 2.7688(6); Pt1-C1,
2.052(9); Pt1-C2 2.003(8); Pt1-C3, 2.067(8); Pt2-C35, 2.061(9); Pt2-
C9, 2.124(9); N1-Pt1-N2, 75.2(3); C9-Pt2-Pt1, 67.3(2).
and alkyl intramolecular C-H bond activation are competi-
tive. The complete deuteration of the cyclometalated Bu
t
H (1b)]8 with KH in tetrahydrofuran affords the green salt
DtBPP-K (2a) or DPPP-K (2b). The reaction of 2a or 2b
with [PtMe3OTf]4 (OTf ) trifluoromethylsulfonate) in
toluene generates the orange five-coordinate complexes
(LX)PtMe3 [LX ) DtBPP (3a) and DPPP (3b); Figure 1].9
An X-ray structure of complex 3a shows a square-pyra-
group would require reversibility of the cyclometalation and
solvent activation. The formation of dinuclear 5-d8 would
then result from trapping of the unsaturated PtII deuterated
cyclometalated species B with a molecule of the five-
coordinate 3a starting compound. If this mechanistic scheme
is viable, it should be possible to trap the PtII cyclometalated
species B with an exogenous ligand.
t
midal geometry (Figure 1). The Bu groups do not provide
intramolecular protection of the open site, and there is no
evidence of intermolecular interactions.9 Thus, steric protec-
tion of the open site is not required for the stability of
unsaturated five-coordinate PtIV complexes.
Indeed, the thermolysis of 3a and 3b at 85-100 °C in
C6D6 in the presence of ethylene as a trapping ligand results
in C-C reductive elimination to form ethane and methane
(CH4 and CH3D) along with the cyclometalated platinum-
(II) ethylene complexes 4a and 4b (Scheme 2). For reactions
run in C6D6, multiple deuterium incorporation into the
cyclometalated tBu group of 4a was observed, but no deuter-
ium was incorporated into 4b. Yields of 68% for 4a and
88-93% for 4b were determined by 1H NMR spectroscopy.9
The conversion of 3b to 4b at 98 °C in C6D6 was studied
under pseudo-first-order conditions with excess ethylene
(130-720 mM, 9-60 equiv). Clean first-order kinetic
behavior was observed for the disappearance of PtIV in all
kinetic experiments.9 Surprisingly, as shown in Figure 3,
ethylene was found to inhibit the rate of the reaction.
However, the effect is quite small; an increase in [C2H4] by
5.5 times reduced kobs by only 28%.9
In contrast to the square-pyramidal structure observed in
1
the solid state, H NMR spectra of 3a and 3b in C6D6 at
298 K show a single Pt-Me signal indicating that in solution
the three inequivalent Me groups in each complex are
averaged on the NMR time scale. Such high fluxionality in
solution is typical for five-coordinate d6 complexes.4c,10
Complexes 3a and 3b are thermally stable at room temper-
ature. However, when 3a was heated in C6D6 at 75 °C, ethane
and methane (CH4 and CH3D) were formed along with one
major Pt product (5-d8, 57% by 1H NMR). The X-ray struc-
ture of 5-d8, a dinuclear compound, is shown in Figure 2.
Formal electron counting implies two PtIII centers. Consistent
with this assignment, the JPt-H values (77, 72, and 75.5 Hz)
of the Pt-bound Me groups would be unusually high for
PtIV.11 Thermolysis of 3b also produced ethane and methane
(CH4 and CH3D), but no stable Pt product could be identified.
A different way to view the oxidation states in dinuclear
complex 5-d8 would be as the interaction of a three-coordi-
nate cyclometalated PtII species with a molecule of the PtIV
starting complex 3a. While the PtIII dimer senario is more
consistent with the spectral data, the PtII/PtIV view suggests
a mechanism to generate 5-d8 (Scheme 1). Direct reductive
elimination of two Me groups from the five-coordinate 3a
would lead to the three-coordinate PtII intermediate A.12
Intermediate A could undergo oxidative addition of the C6D6
solvent (path a) or cyclometalation (path b) followed by
reductive elimination of methane. The observation of both
CH3D and CH4 (1:1.4) indicates that arene intermolecular
The unforeseen mild inhibition of reductive elimination
by ethylene could be explained if ethylene coordinates to
the open site of 3b and prevents reductive elimination.
(8) Klappa, J. J.; Rich, A. E.; McNeill, K. Org. Lett. 2002, 4, 435.
(9) See the Supporting Information.
(10) (a) Huang, D.; Streib, W. E.; Bollinger, J. C.; Caulton, K. G.; Winter,
R. F.; Scheiring, T. J. Am. Chem. Soc. 1999, 121, 8087. (b) Heyn, R.
H.; Macgregor, S. A.; Nadasdi, T. T.; Ogasawara, M.; Eisenstein, O.;
Caulton, K. G. Inorg. Chim. Acta 1997, 259, 5. (c) Riehi, J.; Jean, Y.;
Eisenstein, O.; Pellssierm, M. Organometallics 1992, 11, 729 and
references cited therein.
(11) (a) Crespo, M.; Puddephatt, R. J. Organometallics 1987, 6, 2548. (b)
Clegg, D. E.; Hall, J. R.; Swile, G. A. J. Organomet. Chem. 1972,
38, 403.
(12) Such unsaturated PtII species may be stabilized by solvent coordination
or an agostic interaction.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8497

COMMUNICATION
Scheme 2
Figure 3. Plot of 1/kobs vs [C2H4] (mM) for the thermolysis of 3b at 98
°C. Ethylene concentrations were evaluated at room temperature.15
bonds of the solvent or the ligand competitively, leading to
the production of both CH3D and CH4 (1:1.3 from 3a and
1:3.8 from 3b). That deuterium from the solvent is incor-
porated into the tBu group of 4a but not the Ph group of 4b
indicates that the cyclometalation step is reversible for 4a
but not for 4b. The higher proportion of CH4 from 3b versus
Olefins are not expected to bind strongly to PtIV because
the back-bonding capacity of these high-valent centers should
be quite small. Indeed, alkene coordination to PtIV has been
reported but is rare.13 Support for the proposal of ethylene
3a
is
con-
1
coordination to 3b is found in H NMR spectra of 3b; the
sistent with faster activation of an aryl versus an alkyl C-H
bond, albeit steric constraints in the intramolecular reaction
may also be important.6 In the presence of ethylene, the three-
coordinate cyclometalated PtII species12 is trapped to produce
4a or 4b. In the absence of ethylene, the five-coordinate PtIV
complex 3a itself serves as the trap to form the dinuclear
species 5-d8. Notably, scrambling of deuterium from C6D6
into an isolated sample of complex 5 is not observed after 1
day at 70 °C. Thus, the H/D scrambling into the cyclo-
Pt-Me signal shifts upfield with increasing ethylene con-
centration and decreasing temperature. More convincing is
1
the H NMR spectrum at 204 K of a toluene-d8 solution of
complex 3b in the presence of ethylene; three inequivalent
Pt-Me signals and a signal for coordinated ethylene (δ 4.31)
are consistent with a platinum(IV) ethylene complex 3b‚
[C2H4] (Figure S4 in the Supporting Information).
The evidence of ethylene binding to PtIV and the slight
inhibition of the rate of reductive elimination are consistent
with the mechanism in Scheme 2, wherein an equilibrium
between 3b and 3b‚[C2H4] exists but reductive elimination
occurs only from five-coordinate 3b.14 The rate expression
for the mechanism shown in Scheme 2 is d[PtIV]/dt )
t
metalated Bu group occurs prior to the formation of 5-d8.
In summary, the bidentate anionic (pyridyl)pyrrole ligands
have been used to prepare new five-coordinate platinum-
(IV) alkyl complexes. X-ray characterization shows that the
square-pyramidal geometry is preferred even in the absence
of steric considerations. Kinetic investigations indicate that
C-C coupling occurs directly from the five-coordinate PtIV
complex. Without the preliminary ligand loss as found in
the previous mechanistic studies of C-C reductive elimina-
tion from six-coordinate PtIV,3 kinetic information relating
to the C-C coupling step was accessible and a rate constant
for this elementary step was obtained. The three-coordinate
PtII products of reductive elimination in this system undergo
intra- and intermolecular C-H bond activation and then are
trapped by added ethylene or by PtIV starting material in the
absence of added trapping agent. We are continuing our
investigations of these and related unsaturated model species
to probe the intimate mechanism of the C-C and C-H
coupling and cleavage steps in reductive elimination/oxida-
tive addition at PtIV/PtII.
k
obs[PtIV], where kobs ) k2/(Keq[C2H4] + 1).9 Values of k2 )
6.3(4) × 10-4 s-1 and Keq ) k1/k-1 ) 0.6(2) M-1 can be
calculated from the plot of 1/kobs vs [C2H4] shown in Figure
3.15 The small Keq for ethylene coordination to 3b is con-
sistent with the weak back-bonding expected for PtIV, but
the ethylene binding is significant enough to inhibit the re-
ductive elimination of ethane. Most important in this analysis
is that because the reductive elimination occurs directly from
3b, the k2 value corresponds to the actual C-C bond-forma-
tion step. Previous measurements of the rate constants for
C-C reductive elimination from six-coordinate PtIV have in-
cluded contributions from the preequilibrium ligand loss
step.3
Thus, the proposed reaction sequence that takes place upon
thermolyses of 3a or 3b to form 4a or 4b, respectively, in
the presence of ethylene or to form 5 from 3a in the absence
of ethylene can be related as detailed in Schemes 1 and 2.
Following reductive elimination of ethane from 3a or 3b,
the three-coordinate PtII intermediate A activates the C-H(D)
Acknowledgment. This material is based upon work
supported by the National Science Foundation under Grant
0137394. We thank Dr. W. Kaminsky and J. Benedict for
performing the X-ray structural determinations and Prof. K.
McNeill (University of Minnesota) for the initial samples
of the ligands 1a and 1b.
(13) (a) Howard, W. A.; Bergman, R. G. Polyhedron 1999, 17, 803. (b)
Parsons, E. J.; Larsen, R. D.; Jennings, P. W. J. Am. Chem. Soc. 1985,
107, 1793.
(14) The possibility that reductive elimination of ethane follows after
dissociation of one end of the bidentate N-N ligand cannot be ruled
out. However, the energy of such an unsaturated intermediate (four-
coordinate PtIV) is likely to be very high.
(15) Ethylene concentrations at 100 °C can be estimated, but the values
for k2 [6.5(4) × 10-4 s-1] and Keq [1.4(3) M-1] do not differ
significantly from those calculated using [C2H4] determined by NMR
integration at 25 °C.
Supporting Information Available: Synthetic procedures and
spectral characterization of Pt compounds, experimental details of
kinetic experiments, and X-ray crystallographic data files for 3a
and 5-d8. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC701504Z
8498 Inorganic Chemistry, Vol. 46, No. 21, 2007