Controlled, reversible conversion of a ketene ligand to carbene and CO ligands on a single metal center [7]

Full text of DOI:10.1021/ja000243r

5222
J. Am. Chem. Soc. 2000, 122, 5222-5223
be recoupled upon addition of chloride. This opens the way to
studying the fundamental interconversion illustrated by eq 1 in
both directions on the same metal fragment MLn.
Controlled, Reversible Conversion of a Ketene
Ligand to Carbene and CO Ligands on a Single
Metal Center
Our strategy for studying ketene cleavage was to prepare a
ketene complex with a ligand which could be easily removed
under mild conditions. We hoped that removal of such a ligand
cis to the ketene would lower the formal electron count and result
in breaking the ketene ligand into carbene and CO ligands cis to
each other. Our experience with ketene complexes of trans-ClM-
(PR3)2 (M ) Ir13-15 or Rh16) fragments has thus far involved
monodentate, bulky phosphines PR3, in which two phosphines
appear mutually trans to each other, leaving the two remaining
coordination sites trans, not cis as desired. Therefore, the use of
a chelating diphosphine was explored. Combination of the
phosphine (t-Bu)2PCH2P(t-Bu)2 (dtbpm) with the iridium(I) dimer
[(µ-Cl)Ir(cyclooctene)]2 leads to a very air-sensitive cyclooctene-
free species, formulated as halide-bridged dimer 1.17 When this
Douglas B. Grotjahn,*,† Galina A. Bikzhanova,†
Laura S. B. Collins,† Thomas Concolino,‡
Kin-Chung Lam,‡ and Arnold L. Rheingold‡
Department of Chemistry, San Diego State UniVersity
5500 Campanile DriVe, San Diego, California 92182-1030
Department of Chemistry and Biochemistry
UniVersity of Delaware, Newark, Delaware 19716
ReceiVed January 24, 2000
The formation of ketenes from metal-coordinated carbenes and
CO is implicated as a key step in several useful synthetic reactions.
Carbene-CO complexes (eq 1) can be used as sources of ketene
intermediates in benzannulation reactions,1 ketene-imine or
ketene-alkene cycloadditions,2 or electrophilic substitution reac-
tions.3 In a few cases, ketene ligands have actually been observed
(1)
on single metal centers after coupling of carbene and CO
ligands.4-6 In contrast, the reverse reaction, cleavage of a ketene
ligand to carbene and CO ligands, has never been seen on a single
metal center, until this work. The interaction of ketenes with some
transition metal complexes has given carbene complexes, but these
reactions have led to dinuclear7,8 or trinuclear9 carbene complexes
featuring a bridging carbene ligand, which is not related to ketene-
forming reactions used in organic synthesis. Among mononuclear
systems, a few ketene complexes have been shown to decompose
to metal-CO complexes and alkenes (probable carbene dimer-
ization products).10,11 Starting with an iron-ketene complex,
incorporation of external 13CO into the bound ketene ligand has
been seen, presumably by way of an undetected carbene-CO
intermediate.12 However, from these reactions a carbene-CO
complex has not yet been isolated. Here, we report such an
occurrence for the first time. Removal of a chloride ligand from
a 16-electron Ir-ketene complex breaks the ketene CdC bond,
giving an Ir complex with carbene and CO ligands, characterized
by X-ray diffraction. Moreover, the carbene and CO ligands can
ligand exchange reaction is performed in the presence of
diphenylketene, ketene complex 2 is formed in 61% yield. The
η2-(C,C) binding of the ketene ligand in 2 was suggested by the
strong IR absorption at 1728 cm-1 and the 13C NMR shifts of
the ketene carbons [195.50 (dd, J ) 6.2, 3.5 Hz, OdCdC) and
23.6 (d, J ) 47.5 Hz, OdCdC)].13,16,18 Furthermore, the fact that
all four ortho protons appeared as a single resonance was
consistent with an η2-(C,C) complex, whereas an η2-(C,O) isomer
(not shown) would have inequivalent phenyl groups. We note
that the same phosphine and same ketene on Ni(0) gave an η2-
(C,O) complex.19 The phosphorus nuclei in 2 are inequivalent
(-8.39 and -22.39 ppm, two doublets, J ) 30.3 Hz). An X-ray
diffraction study verified the proposed structure (Figure 1),20 only
the fifth structure of a ketene in η2-(C,C) binding mode.21-24
Complex 2 cannot be described as having square planar geometry.
† San Diego State University.
‡ University of Delaware.
(13) Grotjahn, D. B.; Lo, H. C. Organometallics 1995, 14, 5463-5465.
(14) Grotjahn, D. B.; Lo, H. C. J. Am. Chem. Soc. 1996, 118, 2097-2098.
(15) Lo, H. C.; Grotjahn, D. B. J. Am. Chem. Soc. 1997, 119, 2958-2959.
(16) Grotjahn, D. B.; Bikzhanova, G. A.; Hubbard, J. L. Organometallics
1999, 18, 5614-5619.
(1) Do¨tz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587-608.
(2) Hegedus, L. S. Tetrahedron 1997, 53, 4105-4128.
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1462-1466.
(4) Herrmann, W. A.; Planck, J. Angew. Chem., Int. Ed. Engl. 1978, 17,
525-526.
(17) (a) Meier, C. Diplomarbeit, Technische Universita¨t Mu¨nchen, 1989.
An X-ray crystal structure17b and thorough study of the chemistry of the dimer
[ClRh(dtbpm)]2 and the fragment ClRh(dtbpm)17c have been reported by
Hofmann’s group. (b) Hofmann, P.; Meier, C.; Hiller, W.; Heckel, M.; Riede,
J.; Schmidt, M. U. J. Organomet. Chem. 1995, 490, 51-70. (c) Hofmann, P.;
Meier, C.; Englert, U.; Schmidt, M. U. Chem. Ber. 1992, 125, 353-365.
(18) Geoffroy, G. L.; Bassler, S. L. AdV. Organomet. Chem. 1988, 28,
1-83.
(5) Bodnar, T. W.; Cutler, A. R. J. Am. Chem. Soc. 1983, 105, 5926-
5928.
(6) Schwab, P.; Mahr, N.; Wolf, J.; Werner, H. Angew. Chem., Int. Ed.
Engl. 1993, 32, 1480-1482.
(7) Hong, P.; Nishii, N.; Sonogashira, K.; Hagihara, N. J. Chem. Soc., Chem.
Commun. 1972, 993.
(8) Yamamoto, T.; Garber, A. R.; Wilkinson, J. R.; Boss, C. B.; Streib,
W. E.; Todd, L. J. J. Chem. Soc., Chem. Commun. 1974, 354-356.
(9) Arce, A. J.; Deeming, A. J. J. Chem. Soc., Chem. Commun. 1982, 364-
365. See also Morrison, E. D.; Steinmetz, G. R.; Geoffroy, G. L.; Fultz, W.
C.; Rheingold, A. L. J. Am. Chem. Soc. 1984, 106, 4783-4789.
(10) Miyashita, A.; Shitara, H.; Nohira, H. Organometallics 1985, 4, 1463-
1464.
(19) Hofmann, P.; Perez-Moya, L. A.; Steigelmann, O.; Riede, J. Orga-
nometallics 1992, 11, 1167-1176.
(20) Orthorhombic, Pbca, orange plate, 0.40 mm × 0.20 mm × 0.10 mm,
a ) 11.5550(2) Å, b ) 20.3396(2) Å, c ) 27.4672(2) Å, V ) 6455.45(11)
Å3, Z ) 8, T ) 173 K, Dcalc ) 1.495 g cm-3, R1 ) 3.09%, wR2 ) 11.26%
for 7270 observed independent reflections, GOF ) 1.014.
(21) Redhouse, A. D.; Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1976,
15, 615-616.
(11) Miyashita, A.; Sugai, R.; Yamamoto, J. J. Organomet. Chem. 1992,
428, 239-247. η2-(C,O)-(CH2dCdO)Ni(PCy3)2 is known: Wright, C. A.;
Thorn, M.; McGill, J. W.; Sutterer, A.; Hinze, S. M.; Prince, R. B.; Gong, J.
K. J. Am. Chem. Soc. 1996, 118, 10305-10306.
(12) Bkouche-Waksman, I.; Ricci, J., J. S.; Koetzle, T. F.; Weichmann, J.;
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10.1021/ja000243r CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/13/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 21, 2000 5223
Figure 1. ORTEP diagrams of 2. Thermal ellipsoids at 30% probability. Hydrogens have been omitted for clarity. Key bond distances (Å) and angles:
C(1)-C(2), 1.488(7); C(1)-O(1), 1.222(6); Ir-C(1), 1.986(5); Ir-C(2), 2.245(4); Ir-P(1), 2.3176(12); Ir-P(2), 2.3833(11); Ir-Cl(1), 2.3941(12);
P(1)-Ir-P(2), 73.59(4); P(1)-Ir-Cl(1), 152.16(4); P(2)-Ir-Cl(1), 91.78(4). Figure 1b emphasizes the distortion from square planar geometry. Distances
of coordinated atoms from to the plane defined by P(1), Ir, and P(2): center of the C(1)-C(2) bond, 1.206(5) Å; C(1), 1.694(5) Å; C(2), 0.716(5) Å;
Cl(1), -0.976(2) Å (+ and - values signify points on opposite sides of the plane).
The narrow P(1)-Ir-P(2) angle [73.59(4)°] enforced by the one-
carbon bridge of the diphosphine ligand is to be expected.17b,25
However, the profound deviation from square-planar geometry
is shown by Figure 1b. The ketene CdC bond [as defined by the
C(1)-C(2)-O(1) plane] is almost orthogonal (82.9°) to the
P-Ir-P plane. In 2, the C-C bond of the ketene is the longest
reported among η2-(C,C)-bound ketenes.21-24
When an orange solution of 2 in CD2Cl2 or CH2Cl2 is treated
with 1 equiv of AgPF6 to remove the chloride ligand, darkening
of the solution color begins immediately. Spectral data indicate
that within 2 h, formation of the deep red-brown complex 3 is
complete. Filtration and use of benzene allows crystallization of
3 in 53% yield. A pure sample of 3 in solution is stable at room
temperature for several days. The ligand set in 3 is clear from
the spectral data. A CO ligand in 3 is indicated by IR (νCO 1999
cm-1, very strong absorption) and 13C NMR [181.63 ppm (d,
J ) 78.6 Hz)] data. Most importantly, the presence of a carbene
ligand is secured by observation of a signal far downfield at 326.1
Figure 2. ORTEP diagram of 3. Thermal ellipsoids at 30% probability.
ppm (dd, J ) 2.0, 60.9 Hz). The effect of a deshielding carbene
ligand on the ipso carbons of the phenyl rings is shown by their
downfield chemical shift (δ 158.87 ppm), compared to 144.31
ppm for the same carbons in 2.26 In distinct contrast to what is
seen in 2, the crystal structure of 3 (Figure 2)27 reveals a somewhat
distorted square-planar geometry: the central Ir and four atoms
directly bound to it are within 0.084 Å of the plane defined by
those five atoms. The IrdCPh2 distance of 1.996(8) Å can be
compared to a shorter MdC distance of 1.868(9) Å in an Ir(I)d
CH2 complex28 and a similar distance of 1.997(5) Å in an Ir(I)-
dialkylcarbene complex,29 and to the MdCPh2 distances of
1.863(4) Å in trans-(Cl)Rh(CPh2)[Sb(i-Pr)3]2,6 and 2.13-2.15-
(2) Å in (CO)5W(CPh2).30 The large diphenylcarbene ligand
experiences more steric repulsion with the nearby P(t-Bu)2 groups
than does the CO ligand [angle P(1)-Ir-C(1) ) 101.1(2)°, angle
P(2)-Ir-C(14) ) 93.9(3)°].
Hydrogens and PF6- ion have been omitted for clarity. Key bond distances
(Å) and angles (deg): Ir-C(1), 1.996(8); Ir-C(14), 1.859(9); C(14)-
O(1), 1.133(11); Ir-P(1), 2.417(2); Ir-P(2), 2.391(2); P(1)-Ir-P(2),
72.92(7); P(1)-Ir-C(1), 101.1(2); P(2)-Ir-C(14), 93.9(3); C(1)-Ir-
C(14), 91.8(3).
Significantly, reaction of 3 with chloride ion regenerates 2,
with the intact ketene ligand. Addition of a solution of the salt
[Ph3PdNdPPh3]Cl in CD2Cl2 to a deep red solution of 3 in CD2-
Cl2 at room temperature led to loss of the carbene ligand, but
when the addition was performed at -50 °C, with subsequent
warming to room temperature within 0.5 h, a brownish-orange
solution containing both ketene complex 2 (64%) and CO complex
4 (31%) was formed. Conditions for the regeneration of 2 from
3 have not yet been optimized, but the results clearly show that
we haVe found a system for studying both ketene cleaVage and
formation on a single metal center for the first time. The extension
of our strategy to other ketenes and metal fragments, so as to
study the influence of steric and electronic factors on the
fundamental interconversion identified in eq 1 is under active
investigation in our laboratory, and will be reported in due course.
(24) Bleuel, E.; Laubender, M.; Weberndo¨rfer, B.; Werner, H. Angew.
Chem., Int. Ed. 1999, 38, 156-159.
(25) Hofmann, P.; Meier, C.; Englert, U.; Schmidt, M. U. Chem. Ber. 1992,
125, 353-365.
(26) The resonances described so far are sharp in spectra obtained at 30
°C, but the proton resonances for the t-Bu groups and the phosphorus nuclei
are broadened unless spectra are obtained at - 20 to 0 °C.
(27) Monoclinic, P21/c, brown block, 0.25 mm × 0.20 mm × 0.10 mm,
a ) 9.3207(5) Å, b ) 28.2934(17) Å, c ) 12.8302(7) Å, â ) 98.579(2)°,
V ) 3345.6(6) Å3, Z ) 4, T ) 173 K, Dcalc ) 1.659 g cm-3, R1 ) 4.37%,
wR2 ) 9.80% for 4920 observed independent reflections, GOF ) 1.086.
(28) Fryzuk, M. D.; MacNeil, P.; Rettig, S. J. J. Am. Chem. Soc. 1985,
107, 6708-6710.
Acknowledgment. Acknowledgment is made to the donors of the
Petroleum Research Fund, administered by the ACS, for partial support
of this research.
Supporting Information Available: Complete details of the X-ray
diffraction study on 2 and 3 and NMR spectra (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(29) Crocker, C.; Empsall, H. D.; Errington, R. J.; Hyde, E. M.; McDonald,
W. S.; Markham, R.; Norton, M. C.; Shaw, B. L.; Weeks, B. J. Chem. Soc.,
Dalton Trans. 1982, 1217-1224.
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Chem. Soc. 1977, 99, 2127-2134.
JA000243R