Facile reductive elimination of ethane from strained dimethylpalladium(II) complexes

Full text of DOI:10.1021/ja0056062

J. Am. Chem. Soc. 2001, 123, 4081-4082
4081
Communications to the Editor
systems but never isolated. The only reports of isolated examples,
thus far, are for [(µ-dcpe)Pd]26a,10 and [(µ-dippm)Pd]2.11 We now
report an unusual chelate ring-size-dependent reductive elimina-
tion of ethane from dimethylpalladium(II) complexes bearing
electron-rich chelating bisphosphines to afford the complexes [(µ-
dcpm)Pd]2 and [(µ-dtbpm)Pd]2 under extremely mild conditions.
Recently, Po¨rschke et al. showed that displacement of the tmeda
Facile Reductive Elimination of Ethane from
Strained Dimethylpalladium(II) Complexes
Steven M. Reid, Joel T. Mague, and Mark J. Fink*
Department of Chemistry, Tulane UniVersity
New Orleans, Louisiana 70118
12
ligand in (tmeda)PdMe2 by phosphines affords P2PdMe2 com-
plexes which may undergo facile reductive elimination of ethane
to give zerovalent palladium species.13 We have found that
(tmeda)PdMe2 reacts with bisphosphines Cy2P(CH2)nPCy2 (n )
1-4) in benzene at room temperature to afford dimethyl palladium
complexes 1a-d as air-stable, white, crystalline solids (eq 1).14
An X-ray crystal structure of 1a indicates a highly strained chelate
ring system with the P-Pd-P bond angle being only 73.34(4)°.15
ReceiVed September 13, 2000
Complexes of the type L2Pd(0) are often implicated in textbook
mechanisms as essential intermediates in various palladium-
catalyzed carbon-carbon coupling1 and cross-coupling reactions,2
and have fundamental and practical significance (L ) monoden-
1
tate phosphine P or
/ chelating bisphosphine P-P). Stable
2
complexes P2Pd have been studied for some time,3 but their
chelated counterparts, (P-P)Pd, are known only as reactive
intermediates. The high reactivity of (P-P)Pd has been attributed
to the acute P-Pd-P bond angle that spawns an isolobal
relationship to singlet methylene.4 One fate of these zerovalent
intermediates is their dimerization to complexes in which the
bidentate ligands bridge the two metal centers.5 We have shown
that the stable complex, [(µ-dcpe)Pd]2, dissociates readily and
reversibly in solution to give highly reactive (dcpe)Pd(0) frag-
ments.6 Accordingly, other dimeric complexes of this type could
serve as “bottled” sources of catalytically active (P-P)Pd. For
example, complexes such as [(µ-dippe)Pd]2,7 [(µ-dppm)Pd]2,8 and
Colorless 1a decomposed cleanly in benzene at 20 °C (days)
or more conveniently at 65 °C (18 h) to give ethane (δH ) 0.80)
and the red dipalladium(0) complex [(µ-dcpm)Pd]2, 2 (eq 2). The
[(µ-dtbpm)Pd]2 have been implicated in chemically reacting
9
(1) (a) Hegedus, L. S. In Organometallics in Synthesis; Schlosser, M., Ed.;
John Wiley and Sons, Ltd.: New York, 1994; pp 383-459. (b) Hermann, W.
A. In Applied Homogeneous Catalysis with Organometallic Compounds. A
ComprehensiVe Handbook in Two Volumes; Cornils, B., Hermann, W. A.,
Eds.; VCH Publishers: New York, 1996; Vol. 2, pp 712-732. (c) Farina,
V.; Krishnamurthy, V.; Scott, W. J. Org. React. 1997, 50, 1.
(2) (a) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852. (b) Hartwig, J. F.
Angew. Chem., Int. Ed. Engl. 1998, 37, 2046. (c) Wolfe, J. P.; Wagaw, S.;
Marcoux, J.-F.; Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805.
(3) (a) Otsuka, S.; Yoshida, T.; Matsumoto, M.; Nakatsu, K. J. Am. Chem.
Soc. 1976, 98, 5850. (b) Otsuka, S. J. Organomet. Chem. 1980, 200, 191. (c)
Urata, H.; Suzuki, H.; Moro-oka, Y.; Ikawa, T. J. Organomet. Chem. 1989,
364, 235. (d) Paul, F.; Patt, J.; Hartwig, J. F. J. Am. Chem. Soc. 1994, 116,
5969. (e) Paul, F.; Patt, J.; Hartwig, J. F. Organometallics 1995, 14, 3030. (f)
Kuran, W.; Musco, A. Inorg. Chim. Acta 1975, 12, 187.
(4) Hofmann, P. In Organometallics in Organic Synthesis; de Meijere, A.,
tom Dieck, H., Eds.; Springer-Verlag: Berlin, 1987; pp 1-35.
(5) (a) Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. ReV. 1988,
86, 191. (b) Puddephatt, R. J. Chem. Soc. ReV. 1983, 12, 99. (c) Anderson,
G. K. AdV. Organomet. Chem. 1993, 35, 1. (d) Mague, J. T. J. Cluster Sci.
1995, 6, 217.
formation of 2 likely proceeds through the dimerization of (dcpm)-
Pd(0) generated by the reductive elimination of ethane from 1a.
The related analogue [(µ-dtbpm)Pd]2, 3, was obtained as bright
orange crystals from the direct reaction between (tmeda)PdMe2
and dtbpm in hot benzene (eq 3). The intermediate (µ-dtbpm)-
(6) (a) Pan, Y.; Mague, J. T.; Fink, M. J. J. Am. Chem. Soc. 1993, 115,
3842. (b) Abbreviations used in this study: dcpm ) bis(dicyclohexylphos-
phino)methane; dcpe ) 1,2-bis(dicyclohexylphosphino)ethane; dcpp ) 1,3-
bis(dicyclohexylphosphino)propane; dcpb ) 1,4-bis(dicyclohexylphosphino)-
butane; dippm
) bis(diisopropylphosphino)methane; dippe ) 1,2-bis-
PdMe2 could not be isolated, presumably because rapid elimina-
tion of ethane occurs under the reaction conditions necessary for
its formation.16 The air-sensitive complexes 2 and 3 each gave
(diisopropylphosphino)ethane; dppm ) bis(diphenylphosphino)methane; dppe
) 1, 2-(bisdiphenylphosphino)ethane; dppp ) 1,3-bis(diphenylphosphino)-
propane; dmpm ) bis(dimethylphosphino)methane; dmpe ) 1,2-bis(dimeth-
ylphosphino)ethane; dtbpm ) bis(di-tert-butylphosphino)methane; tmeda )
N,N,N′,N′-tetramethylethylenediamine.
(10) We recently found an efficient and improved route to [(µ-dcpe)Pd]2
and [(µ-dippe)Pd]2; results will be published elsewhere.
(11) Dohring, A.; Goddard, R.; Hopp, G.; Jolly, P. W.; Kokel, N.; Kruger,
C. Inorg. Chim. Acta 1994, 222, 179.
(12) (a) de Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van
Koten, G. Organometallics 1989, 8, 2907. (b) de Graaf, W.; Boersma, J.;
Grove, D.; Spek, A. L.; van Koten, G. Recl. TraV. Chim. Pays-Bas 1988,
107, 299.
(13) Krause, J.; Cestaric, G.; Haack, K.-J.; Seevogel, K.; Storm, W.;
Po¨rschke, K.-R. J. Am. Chem. Soc. 1999, 121, 9807.
(14) The structural assignment of the compounds 1a-d are entirely
consistent with spectroscopic data. The mononuclear nature of these species
is established by a combination of mass spectral techniques including FAB,
electrospray, and chemical ionization.
(7) (a) Fryzuk, M. D.; Clentsmith, G. K. B.; Rettig, S. J. Organometallics
1996, 15, 2083. (b) Trebbe, R.; Goddard, R.; Rufinska, A.; Seevogel, K.;
Po¨rschke, K.-R. Organometallics 1999, 18, 2466. (c) Schager, F.; Bonrath,
W.; Po¨rschke, K.-R.; Kessler, M.; Kru¨ger, C.; Seevogel, K. Organometallics
1997, 16, 4276. (d) Schager, F.; Haack, K. J.; Mynott, R.; Rufinska, A.;
Po¨rschke, K.-R. Organometallics 1998, 17, 807.
(8) (a) Gauthron, I.; Mugnier, Y.; Hierso, K.; Harvey, P. D. New J. Chem.
1998, 237. (b) Young, S. J.; Kellenberger, B.; Reibenspies, J. H.; Himmel, S.
E.; Manning, M.; Anderson, O. P.; Stille, J. K. J. Am. Chem. Soc. 1988, 110,
5744.
(9) (a) Hofmann, P.; Heiss, H.; Neiteler, P.; Mu¨ller, G.; Lachmann, J.
Angew. Chem., Int. Ed. Engl. 1990, 29, 880. (b) Hofmann, P.; Heiss, H.;
Neiteler, P.; Mu¨ller, G.; Lachman, J. Angew. Chem. 1990, 102, 935.
10.1021/ja0056062 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/05/2001

4082 J. Am. Chem. Soc., Vol. 123, No. 17, 2001
Communications to the Editor
Scheme 1. Reactions with Other Bisphosphines
dissociation of one phosphine “arm”,22 a step that is much more
favored for the dcpm ligand in strained 1a than those ligands in
the unstrained analogues 1b-d.
The reaction between equimolar amounts of dppm and (tmeda)-
PdMe2 in benzene afforded a very sparingly soluble white solid
whose mass spectrum indicated the formation of (dppm)PdMe2.
Attempts to purify and characterize this complex, however, were
prevented by the extremely rapid reductive elimination of ethane
in polar and nonpolar solvents to give the well-known (µ-
dppm)3Pd223 (Scheme 1).24 Although it is also likely that divalent
(dppm)Pd is formed initially, the detailed mechanism for the
formation of (µ-dppm)3Pd2 is uncertain at this time since we were
unable to detect any intermediates by NMR.
The addition of less sterically demanding dmpm to (tmeda)-
PdMe2 unexpectedly furnished dimeric [(dmpm)PdMe2]2, 4, in
high yield. An X-ray crystal structure of 4 showed that it is
comprised of two square-planar cis-PdMe2 units bridged by two
dmpm ligands in a “twist saddle” conformation, a structural motif
observed previously for related platinum(II) complexes.5a,25 The
eight-member ring system of 4 is thermally stable and does not
eliminate ethane in solution.
We have demonstrated an unusual dependence of the reductive
elimination of ethane from dimethylpalladium complexes bearing
bisphosphine ligands on the size of the chelate ring. Reductive
elimination of ethane from highly strained dimethyl palladium
complexes containing chelating bisphospinomethane ligands may
be a practical and highly efficient route to certain stable d10-d10
dipalladium species, [(µ-(P-P)Pd]2. We are currently exploring
the scope of these elimination reactions and the potential of these
unique dinuclear complexes to serve as “bottled” sources of highly
reactive 14 electron (P-P)Pd fragments.
Figure 1. ORTEP drawing of 2 with thermal ellipsoids drawn at 30%
probability except for the cyclohexyl carbons which are shown as arbitrary
spheres for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)-
Pd(2), 2.8582(6); Pd(1)-P(1), 2.268(1); Pd(1)-P(2), 2.259(1); Pd(2)-
P(3), 2.281(1); Pd(2)-P(4), 2.262(1); P(1)-Pd(1)-P(2), 178.56(5); P(3)-
Pd(2)-P(4), 178.68(5); P(1)-C(1)-P(4), 112.7(3); P(2)-C(26)-P(3),
114.7(2); P(1)-Pd(1)-Pd(2)-P(3), 148.50(5); P(2)-Pd(1)-Pd(2)-P(4),
148.59(5).
rise to sharp singlets in 31P{H} NMR spectra at δP 28.59 and
62.87, respectively, and exhibit molecular ions in the correspond-
ing high-resolution mass spectra. The X-ray crystal structure of
2 shows that the two dcpm ligands that bridge the dipalladium
core are highly twisted relative to each other (P1-P4-P3-P2
≈ 45°) (Figure 1), an arrangement that minimizes interactions
between the cyclohexyl groups.17 The Pd-Pd interatomic distance
of 2.8582(6) Å is slightly longer than that found for the related
complex [(µ-dcpe)Pd]2 (Pd-Pd ) 2.7611(5) Å),6a but consider-
ably longer than formal single bonds in Pd(I)-Pd(I) complexes.18
The weak Pd-Pd interaction has been ascribed to d-p mixing
in the σ bonding orbital.19 Accordingly, the long wavelength dσ*
f pσ electronic absorption20 for 2 (λmax ) 480 nm) is somewhat
lower in energy than that for [(µ-dcpe)Pd]2 (λmax ) 456 nm)
consistent with the reduced orbital overlap associated with a longer
Pd-Pd distance.
Acknowledgment. We thank Dow Corning Corporation for financial
support of this work and Aldrich Chemical Co. for generous gifts of PdCl2
and phosphines. We also thank Sarah Shealy and David Bostwick of
Georgia Institute of Technology for their assistance with mass spectrom-
etry.
Surprisingly, complexes 1b-d do not eliminate ethane under
the experimental conditions found for 1a. Facile reductive
elimination of ethane from 1a is likely facilitated via the formation
of a three-coordinate “T-shaped” intermediate21 following initial
(15) This is a comparatively low value among some other complexes of
Group 10 metals that also bear “short-bite” bisphosphinomethane ligands: (a)
Reference 9. (b) Hofmann, P.; Heiss, H.; Mueller, G. Z. Naturforsch., B:
Chem. Sci. 1987, 42, 395. (c) Hofmann, P.; Perez-Moya, L. A.; Krause, M.
E.; Kumberger, O.; Mueller, G. Z. Naturforsch., B: Chem. Sci. 1990, 45,
897. (d) Barkley, J.; Ellis, M.; Higgins, S. J.; McCart, M. K. Organometallics
1998, 17, 1725. (e) Steffen, W. L.; Palenik, G. J. Inorg. Chem. 1976, 15,
2432.
Supporting Information Available: Full experimental details and
compound characterizations; tables of crystal data, structure solution and
refinement, atomic coordinates, bond lengths and angles, and anisotropic
thermal parameters for 1a, 2, and 4 (PDF). Three X-ray crystallographic
files (CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) No reaction occurs at ∼20 °C, but upon heating a singlet at δP ) 9.8
and a PdMe2 multiplet at δH ) 0.9, likely arising from the intermediacy of
JA0056062
1
(dtbpm)PdMe2, appear along with 31P and H resonances for 3.
(17) The synthesis and X-ray crystal structure of [(µ-dcpm)Pt]2 were
mentioned but details have not yet appeared; see: Hofmann, P. In Organo-
silicon Chemistry: From Molecules to Materials; Auner, N., Weis, J., Eds.;
VCH: New York, 1994; pp 231-250.
(22) Mechanistic studies on the decomposition of P2PdMe2 complexes: (a)
Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem. Soc.
Jpn. 1981, 54, 1857. (b) Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102,
4933. (c) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc.
Jpn. 1981, 54, 1868.
(23) (a) Stern, E. W.; Maples, P. K. J. Catal. 1972, 27, 120. (b) Hunt, C.
T.; Balch, A. L. Inorg. Chem. 1981, 20, 2267. (c) Kirss, R. U.; Eisenberg, R.
Inorg. Chem. 1989, 28, 3372.
(24) Facile reductive elimination of ethane also thwarted attempts to
synthesize (dppm)NiMe2 whereas (dppe)NiMe2 and (dppp)NiMe2 are stable:
Kohara, T.; Yamamoto, T.; Yamamoto, A. J. Organomet. Chem. 1980, 192,
265.
(25) Puddephatt, R. J.; Thomson, M. A. J. Chem. Soc., Chem. Commun.
1981, 805.
(18) Kullberg, M. L.; Lemke, F. R.; Powell, D. R.; Kubiak, C. P. Inorg.
Chem. 1985, 24, 3589 and references therein.
(19) (a) Sakaki, S.; Ogawa, M.; Musashi, Y. J. Phys. Chem. 1995, 99,
17134. (b) Dedieu, A.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 2074.
(20) (a) Harvey, P. D.; Gray, H. B. J. Am. Chem. Soc. 1988, 110, 2145.
(b) Harvey, P. D. J. Cluster Sci. 1993, 4, 377.
(21) Direct reductive elimination from tetracoordinate palladium would
likely be prohibitive due to the generation of high-energy nonlinear (P-P)Pd
intermediates: (a) References 4 and 15b. (b) Dierkes, P.; van Leeuwen, P.
W. N. M. J. Chem. Soc., Dalton Trans. 1999, 1519.