Chelate and pincer carbene complexes of rhodium and platinum derived from hexaphenylcarbodiphosphorane, Ph3P=C=PPH3

Article abstract of DOI:10.1021/ja0502831

The reaction of [(cod)RhCl]₂ with Ph₃P=C=PPh₃ (1) gave the bidentate Rh(I) carbene complex, (cod)Rh[η²-C{P(C₆H₄)Ph₂}{PPh₃}] (2), in which one of the Ph groups in 1 underwent orthometalation to form the chelate. Displacement of cod by 2 equiv of PMe₃ transformed 2, via a second orthometalation event, into the Rh(III) C,C,C pincer carbene complex, HRh(PMe₃)₂[η³-C{P(C₆H₄)Ph₂}₂] (3). The reaction of [Me₂Pt(SMe₂)]₂ with 1 led directly to the analogous C,C,C pincer carbene complex of Pt(II), (Me₂S)Pt[η³-C{P(C₆H₄)Ph₂}₂] (4). DFT calculations on a model form of 3 suggest a net single σ-bonding interaction between Rh and an sp²-hybridized carbene center, with a HOMO that is predominantly carbene p_z in character. Copyright

Full text of DOI:10.1021/ja0502831

Published on Web 03/22/2005
Chelate and Pincer Carbene Complexes of Rhodium and Platinum Derived
from Hexaphenylcarbodiphosphorane, Ph3PdCdPPh3
Kazuyuki Kubo,† Nathan D. Jones,‡ Michael J. Ferguson,‡ Robert McDonald,‡ and
Ronald G. Cavell*,‡
Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, Higashi-Hiroshima, 739-8526 Japan,
and Department of Chemistry, UniVersity of Alberta, Edmonton, AB, Canada T6G 2G2
Received January 16, 2005; E-mail: ron.cavell@ualberta.ca
Chart 1
Cyclometalated “pincer” complexes (Chart 1, A) have become
important in the last 30 years,1 both in the variety of known
compounds and in the range of their catalytic applications.2 Reaction
types now include diverse transformations: alkane dehydrogena-
tion,2a,b activation of small molecules (e.g., CO22c and N22d), C-X
bond formation2e and activation2f,g (X ) C, N, O), polymerization
of alkynes2h and alkenes,2i,j and transfer hydrogenation catalysis.2k
Applications as sensors1a,2l and “molecular switches”2m have also
emerged.
A few cyclometalated pincer N-heterocyclic carbenes (NHCs)
Scheme 1. Synthesis of 2
(Chart 1, B; E ) C) have recently been reported by Crabtree3 and
Danopoulos.4 Related C,N,C (NHC) pincers (Chart 1, B; E ) N)
have been used as catalysts in alkene oligomerization and
polymerization,2i,j Heck,3a-c,5 Sonogashira,6 transfer hydrogenation,3e,4b
and oxidative cleavage3e reactions.
In extending our studies of phosphorus-stabilized pincer carbene
complexes of the late metals,7,8 we have investigated the chemistry
of the carbodiphosphoranes, R3PdCdPR3 (R3P+-C2--+PR3).
Although a formal carbene resonance form (six valence electrons
on C) cannot be drawn for these compounds, we feel that they have
strong (and underappreciated) parallels9,10 with Bertrand- and
Arduengo-type carbenes in that they are stable, neutral, two-electron
σ-donors, with bent structures in the solid state, and have ylidic
resonance forms that place eight valence electrons on the “carbenic”
carbon. While the NHCs are recognized as “push-push” carbenes11
and the Bertrand-type (phosphanyl-silyl) carbenes as “push-
pull”,11 carbodiphosphoranes may be conceptualized as a “pull-
pull” variety considering that the positively charged phosphonium
substituents withdraw electron density from the central, formally
indicative of sp2 hybridization. The Rh-C(1) bond (2.165(2) Å) is
longer than those reported for Rh-NHC complexes (2.00-2.10
Å)13 and can be regarded as a single bond. The C(1)-P(1) and
C(1)-P(2) distances are indistinguishable despite the asymmetry
of the ligand. Complex 2 can be formulated as a 16-electron Rh(I)
complex wherein the central C atom acts as a net neutral, 2-electron,
σ-donor. The bidentate carbene ligand in 2 is reminiscent of our
C,N-bidentate bis(phosphoranimine) carbene complex of Pt (see
Supporting Information, Figure S1).7
Reaction of 2 with 2 equiv of PMe3 gave the Rh(III) C,C,C pincer
carbene complex, HRh(PMe3)2[η3-C{Ph2P(C6H4)}2] (3), in 85%
yield (Scheme 2) by replacement of the cod ligand and a second
phenyl orthometalation event on the free PPh3 end of the ligand.
Complex 3 represents a distinctly new class of pincer complexes
(Chart 1, C) and is the first example of a pincer carbene complex
derived from 1. The second orthometalation step is probably
facilitated by (i) a strong σ-donor contribution from PMe3 and (ii)
displacement of cod to alleviate the requirement for the mutual cis
orientation of ancillary ligands. Such C,C,C pincer systems are rare;
C
2- atom to give it a distinct carbenic character. Here, we describe
new members of the small class of C,C,C pincer complexes and
introduce a unique category of pincer carbenes (Chart 1, C) which,
unlike B, incorporates only a single carbene donor and two, as
opposed to one, cyclometalated phenyl rings. Rh(III) and Pt(II)
prototypes are presented, with the Rh case also yielding a Rh(I)
C,C bidentate carbene intermediate.
Treatment of [RhCl(cod)]2 (cod ) 1,5-cyclooctadiene) with
Ph3PdCdPPh3 (1)12 gave (cod)Rh[η2-C{P(C6H4)Ph2}{PPh3}] (2)
in 78% yield and the phosphonium salt, [HC(PPh3)2][Cl] (Scheme
1). Complex 2 is air sensitive but is thermally stable under an inert
atmosphere to at least 60 °C. We propose an initial bridge splitting
coordination of 1 to Rh followed by phenyl orthometalation at one
end of the ligand to form a putative hexacoordinated Rh(III)-
hydride intermediate which is dehydrohalogenated by free 1.
Structural characterization of 2 (Figure 1) shows a Rh(I) atom
in a characteristic distorted square-planar coordination geometry.
The C(1) center is trigonal planar (sum of angles ) 360.0°),
Figure 1. ORTEP illustration of the molecular structure of 2 (20%
ellipsoids) showing only cod H atoms and only the ipso carbon atoms of
the phenyl rings, except for those in the orthometalated ring. Selected bond
lengths (Å) and angles (°): Rh-C(1) 2.165(2), Rh-C(12) 2.072(2), C(1)-
P(1) 1.692(2), C(1)-P(2) 1.693(2), P(1)-C(1)-P(2) 124.50(13).
† Hiroshima University.
‡ University of Alberta.
9
5314
J. AM. CHEM. SOC. 2005, 127, 5314-5315
10.1021/ja0502831 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Synthesis of 3
contribution on Rh (Figure S2). This picture is similar to that given
by Le Floch and co-workers for the HOMO of the S,C,S pincer
carbene complex, (Ph3P)Pd[C{Ph2PdS}2].16 Calculated Mulliken
charges for 3′ are Rh +0.26, Ccarbene -0.87, P +0.40 (including H
atoms), and Cphenyl +0.17. Detailed calculations are in progress as
are reactivity studies of these new pincer carbene complexes.
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada and ACS-PRF (35314-AC3)
for financial support. K.K. thanks the Japan Ministry of Education,
Culture, Sports, Science and Technology for an Overseas Visiting
Scholarship. N.D.J. thanks the Killam Trust for a postdoctoral
fellowship.
Supporting Information Available: Synthetic and characterization
data for 2, 3, and 4, crystallographic data in CIF for 2‚2.5C6H6 and 3,
calculation details, and selected MO representations for 3′. This material
is available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750-
3781. (b) van der Boom, M. E.; Milstein, D. E. Chem. ReV. 2003, 103,
1759-1792. (c) Slagt, M. Q.; van Zwieten, D. A. P.; Moerkerk, A. J. C.
M.; Gebbink, R. J. M. K.; van Koten, G. Coord. Chem. ReV. 2004, 248,
2275-2282.
Figure 2. ORTEP representation of the molecular structure of 3 (20%
ellipsoids). Only the hydride H atom is shown. All but the ipso C atoms of
the phenyl rings are omitted except for those on the orthometalated rings.
Selected bond lengths (Å) and angles (°): Rh-C(1) 2.202(3), C(1)-P(1)
1.672(3), C(1)-P(2) 1.675(3), P(1)-C(1)-P(2) 138.32(18).
(2) (a) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am.
Chem. Soc. 1999, 121, 4086-4087. (b) Jensen, C. Chem. Commun. 1999,
2443-2449. (c) Lee, D. W.; Jensen, C. M.; Morales-Morales, D.
Organometallics 2003, 22, 4744-4749. (d) Vigalok, A.; Ben-David, Y.;
Milstein, D. Organometallics 1996, 15, 1839-1844. (e) Beletskaya, I.
P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009-3066 and references
therein. (f) Liou, S.-Y.; van der Boom, M. E.; Milstein, D. Chem. Commun.
1998, 687-688. (g) Gandelman, M.; Milstein, D. Chem. Commun. 2000,
1603-1604. (h) Yao, J.; Wong, W. T.; Jia, G. J. Organomet. Chem. 2000,
598, 228-234. (i) McGuinness, D. S.; Gibson, V. C.; Steed, J. W.
Organometallics 2004, 23, 6288-6292. (j) McGuinness, D. S.; Gibson,
V. C.; Wass, D. F.; Steed, J. W. J. Am. Chem. Soc. 2003, 125, 12716-
12717. (k) Dani, P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten,
G. Angew. Chem., Int. Ed. 2000, 39, 743-745. (l) Albrecht, M.; Gossage,
R. A.; Lutz, M.; Spek, A. L.; van Koten, G. Chem.sEur. J. 2000, 6,
1431-1445. (m) Steenwinkel, P.; Grove, D. M.; Veldman, N.; Spek, A.
L.; van Koten, G. Organometallics 1998, 17, 5647-5655.
Scheme 3. Synthesis of 4
we know of only two other reported types: a Pt complex of ours8
(Figure S1) and three substitutional variants of a group of Pd-
NHC complexes of type B (E ) C) from the groups of Crabtree3a
and Danopoulos.4c
The Rh center in 3 has a distorted octahedral geometry (Figure
2). The Rh-C(1) bond (2.202(3) Å) is slightly longer than that in
2. The sum of the angles around C(1) is 359.83°, indicative once
again of sp2 hybridization. We formulate 3 as an 18-electron Rh-
(III) species in which the central carbon atom, again, acts as a
neutral, 2-electron, σ-donor.
Although the C(1)-P bonds in 2 and 3 are in the range previously
reported for LnM-C(PPh3)2 complexes (M ) Re, Ni, Cu, Au;
1.66-1.78 Å),14 those in 2 (av 1.693(2) Å) are slightly longer than
those in 3 (av 1.674(3) Å), indicating lower C(1)-P bond orders
in 2. The stronger Rh-C(1) bond in 2 may act to reduce the
negative charge on C(1) and, consequently, the π-donation from
C(1) to P. The P(1)-C(1)-P(2) angle (138.32(18)°) in 3 is the
greatest throughout the range of complexes of 1 (123.1-136.0°),14
including 2 (124.50(13)°).
Reaction of 1 with [Me2Pt(SMe2)]2 gave an analogous C,C,C
pincer carbene complex of Pt(II) (4) directly via double orthometa-
lation with elimination of 2 equiv of CH4 (Scheme 3). No inter-
mediate species were observed. The complex is stable in the same
fashion as 2. Full details are given in the Supporting Information.
Preliminary DFT electronic structure calculations15 using the
model complex, HRh(PH3)2[η3-C{H2P(C6H4)}2] (3′), revealed one
net bonding MO of σ symmetry with respect to the Rh-Ccarbene
axis, which is best described as a bonding overlap between an sp2-
hybridized orbital on C and a d-orbital on Rh. The HOMO is
predominantly a Ccarbene pz-orbital with a small antibonding d-orbital
(3) (a) Gru¨ndemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.; Crabtree, R.
H. Organometallics 2001, 20, 5485-5488. (b) Peris, E.; Loch, J. A.; Mata,
J.; Crabtree, R. H. Chem. Commun. 2001, 201-202. (c) Loch, J. A.;
Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.; Crabtree, R. H.
Organometallics 2002, 21, 700-706. (d) Crabtree, R. H. Pure Appl. Chem.
2003, 75, 435-443. (e) Poyatos, M.; Mata, J. A.; Falomir, E.; Crabtree,
R. H.; Peris, E. Organometallics 2003, 22, 1110-1114. (f) Peris, E.;
Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239-2246.
(4) (a) Tulloch, A. A. D.; Danopoulos, A. A.; Tizzard, G. J.; Coles, S. J.;
Hursthouse, M. B.; Hay-Motherwell, R. S.; Motherwell, W. B. Chem.
Commun. 2001, 1270-1271. (b) Danopoulos, A. A.; Winston, S.;
Motherwell, W. B. Chem. Commun. 2002, 1376-1377. (c) Danopoulos,
A. A.; Tulloch, A. A. D.; Winston, S.; Eastham, G.; Hursthouse, M. B. J.
Chem. Soc., Dalton. Trans. 2003, 1009-1015. (d) Danopoulos, A. A.;
Tsoureas, N.; Wright, J. A.; Light, M. E. Organometallics 2004, 23, 166-
168. (e) Danopoulos, A. A.; Wright, J. A.; Motherwell, W. B.; Ellwood,
S. Organometallics 2004, 23, 4807-4810.
(5) Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247-2273.
(6) Mas-Marza, E.; Segarra, A. M.; Claver, C.; Peris, E.; Fernandez, E.
Tetrahedron Lett. 2003, 44, 6595-6599.
(7) Jones, N. D.; Lin, G.; Gossage, R. A.; McDonald, R.; Cavell, R. G.
Organometallics 2003, 22, 2832-2841 (erratum 5378).
(8) Lin, G.; Jones, N. D.; Gossage, R. A.; McDonald, R.; Cavell, R. G. Angew.
Chem., Int. Ed. 2003, 42, 4054-4057.
(9) Schmidbaur, H.; Hasslberger, G.; Deschler, U.; Schubert, U.; Kappenstein,
C.; Frank, A. Angew. Chem., Int. Ed. Engl. 1979, 18, 408-409.
(10) Bruce, A. E.; Gamble, A. S.; Tonker, T. L.; Templeton, J. L. Organo-
metallics 1987, 6, 1350-1352.
(11) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39-91.
(12) Ramirez, F.; Desai, N. B.; Hansen, B.; McKelvie, N. J. Am. Chem. Soc.
1961, 83, 3539-3540.
(13) Despagnet, E.; Miqueu, K.; Gornitzka, H.; Dyer, P. W.; Bourissou, D.;
Bertrand, G. J. Am. Chem. Soc. 2002, 124, 11834-11835.
(14) Vicente, J.; Singhal, A. R.; Jones, P. G. Organometallics 2002, 21, 5887-
5900 and references therein.
(15) B3LYP/LANL2DZ. Frisch, M. et al. Gaussian 98, 2002. For full citation,
see the Supporting Information.
(16) Cantat, T.; Me´zailles, N.; Ricard, L.; Jean, Y.; Le Floch, P. Angew. Chem.,
Int. Ed. 2004, 43, 6382-6385.
JA0502831
9
J. AM. CHEM. SOC. VOL. 127, NO. 15, 2005 5315