Synthesis, structural characterization, and chemistry of a monomeric cationic iridium carbyne complex [17]

Full text of DOI:10.1021/ja982641o

11008
J. Am. Chem. Soc. 1998, 120, 11008-11009
Synthesis, Structural Characterization, and
Chemistry of a Monomeric Cationic Iridium
Carbyne Complex
Hans F. Luecke and Robert G. Bergman*
Department of Chemistry, UniVersity of California
Berkeley, California, and the Chemical Sciences DiVision
Lawrence Berkeley National Laboratory,
Berkeley, California 94720
ReceiVed July 27, 1998
The synthesis and characterization of compounds containing
multiple bonds between carbon and transition metals has played
an important role in fundamental organometallic chemistry, in
advancing our understanding of a variety of industrially important
processes and in the development of novel synthetic transforma-
tions.1-8 While a number of terminal and bridging carbyne
complexes have been prepared, terminal carbyne complexes
containing a group 9 metal are very rare.9-11 We report here the
generation, isolation, structure determination, and preliminary
chemistry of a stable terminal iridium carbyne complex,
[(Cp′)(PMe3)IrtCPh]+BArf- (1) (Cp′ ) η5-C5Me4Et; BArf )
B[C5H3(3,5-CF3)2]4).
Figure 1. ORTEP diagram of the cationic portion of 1.
Scheme 1
The synthetic route to 1 employed is outlined in Scheme 1.
These reactions proceed cleanly with the analogous η5-C5Me5
(Cp*) complexes, but because of their increased crystallinity, only
the η5-C5Me4Et(Cp′) complexes are presented here. This synthetic
approach involves O-methylation of iridium acyl complex 2, to
afford the cationic hydrido Fischer carbene complex 3 in 89%
yield. Deprotonation of 3 using LiN(TMS)2 in THF at -40 °C
affords the neutral Fischer carbene complex 4 in 62% yield. The
addition of 1 equiv of trimethylsilyl triflate to 4 in pentane at
-40 °C results in gradual formation of carbyne salt 5 as an off-
white precipitate. Upon evacuation, the solid and solution darken
to a gray-green color. Analysis of the volatile products indicates
that trimethylsilyl methyl ether is the only byproduct of this
reaction. We presume that this reaction proceeds by direct
silylation of the oxygen atom to form a silyloxonium complex,
which subsequently evolves the silyl ether byproduct. When more
conventional abstraction agents such as boranes12 are employed,
a mixture of products is obtained. Analysis of these product
mixtures by NMR spectroscopy indicates that the cationic carbyne
is present in small quantities. Crude 5, despite being clean by
NMR, cannot be recrystallized. Therefore, the triflate counterion
was metathesized using NaBArf to afford 1 in 84% yield. The
1H NMR spectrum of 1 in CD2Cl2 exhibits a single trimeth-
ylphosphine resonance at δ 1.65 ppm (d), two ring methyl
resonances at δ 2.22 and 2.25 ppm, ring ethyl resonances at δ
1.18 (t) and 2.49 ppm (q), and three phenyl resonances at δ 7.45
(t), 7.83 (d), and 7.89 ppm (t), as well as resonances corresponding
to the BArf anion. The 31P{1H} NMR spectrum exhibits a singlet
at δ -49.7 ppm. The 13C{1H} NMR spectrum displays a broad
singlet resonance at δ 296.9 ppm, which is within the range of
chemical shifts observed for other transition metal carbyne
complexes.2,12
A single crystal was grown by slow vapor diffusion of Et2O
into a CH2Cl2 solution of 1 at -40 °C. An X-ray diffraction
study was carried out on this sample and the structure solved by
direct methods and refined using least-squares methods. An
ORTEP diagram of the cationic portion of 1 is shown in Figure
1. The Ir-C(15) bond distance is 1.734(6) Å, the Ir-C(15)-
C(16) angle is 175.7(4)°, and the iridium, phosphorus, and carbon
atoms are coplanar. Additionally, there are no unusual contacts
between the cation and the anion. A full description of the data
collection and positional parameters is presented in the Supporting
Information.
In contrast to related carbyne complexes,9-11 the cationic
benzylidyne complex 1 is exceptionally thermally stable in dry
degassed CH2Cl2 and THF, decomposing to unknown products
only at temperatures above 105 °C. The thermal stability of 1
has allowed us to explore its reactivity. The carbyne complex
reacts readily with pyridine N-oxide to form cationic phenyl
carbonyl complex 6 in 94% yield. We presume that this reaction
proceeds by the mechanism outlined in Scheme 2. The initial
step is oxygen atom transfer to the iridium-carbon triple bond
of 1 to form an intermediate complex which can be represented
by either an oxairidacyclopropene (7) or an η2-acyl (8) resonance
structure.13 The cationic acyl complex 8 has been proposed as
(1) Kim, H. P.; Angelici, R. J. AdV. Organomet. Chem. 1987, 27, 51.
(2) Mayr, A.; Hoffmeister, H. AdV. Organomet. Chem. 1991, 32, 227.
(3) Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schrock, R. R.; Schubert, U.;
Weiss, K. Carbyne Complexes; VCH: Weinheim, 1988.
(4) Schrock, R. R. Acc. Chem. Res. 1986, 19, 342.
(5) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; 2nd ed.; University
Science Books: Mill Valley, CA, 1987.
(6) Muetterties, E. L.; Stein, J. Chem. ReV. 1979, 79, 479.
(7) Herrmann, W. A. In Applied Homogeneous Catalysis with Organome-
tallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim,
1996; Vol. 2.
(8) Wender, I. Fuel Process. Technol. 1996, 48, 189.
(9) Ho¨hn, A.; Werner, H. Angew. Chem., Int. Ed. Engl. 1986, 25, 737.
(10) Ho¨hn, A.; Werner, H. J. Organomet. Chem. 1990, 382, 255.
(11) Rappert, T.; Nu¨rnberg, O.; Mahr, N.; Wolf, J.; Werner, H. Organo-
metallics 1992, 11, 4156.
(13) Related atom-transfer reactions have been observed with osmium
carbyne complexes, in which the metallacyclopropene complexes are isolable
products: Gallop, M. A.; Roper, W. R. AdV. Organomet. Chem. 1986, 25,
121.
(12) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John
Wiley and Sons Inc.: New York, 1988.
10.1021/ja982641o CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/09/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 42, 1998 11009
Scheme 2
complex displays a singlet resonance at δ 25.5 ppm, which is in
the range typical of phosphonium ylide complexes.16,17 The
19F{1H} NMR spectrum displays normal resonances for the BArf-
and OTf- moieties. Formation of 12 is consistent with initial
generation of benzylidene complexes 13, which is unstable with
respect to phosphine migration.18 Due to the instability of this
compound in solution, it has been characterized by derivatization
with PPNCl to form Cp′Ir(CH(Ph)PMe3)Cl2 (14), which has been
generated independently by the addition of trimethylbenzyli-
denephosphorane to [Cp′IrCl2]2.
To obtain additional information about the protonation of
carbyne complex 1, we monitored its reaction with HOTf at -78
1
°C in CD2Cl2 by low-temperature NMR spectroscopy. The H
NMR spectrum at -30 °C displays a broad singlet at δ 18.74
ppm, broad aryl resonances, and broadened resonances for the
Cp′ and PMe3 ligands. The 31P{1H} NMR spectrum exhibits a
singlet resonance at δ -45.1 ppm, indicative of an iridium-bound
trimethylphosphine ligand.19 These data are consistent with the
benzylidene structure 13 shown in Scheme 4. When DOTf is
used, the resonance at δ 18.74 ppm is absent, which further
supports the assigned structure. Benzylidene complex 13 is stable
at temperatures below -20 °C, but at higher temperatures,
rearrangement to 12 is observed. Addition of the stronger acids
HBF4 and H(Et2O)BArf to 1 at -78 °C results in immediate
decomposition. This indicates that the presence of the triflate
anion is important for the stabilization of 13, most likely by
coordination to the electrophilic iridium center. Addition of
NaBArf to 12 also results in decomposition. Interestingly, the
addition of styrene to 13 at -30 °C results in the formation of
1,2-diphenylcyclopropane (eq 1), isolated in 95% yield as a 2:1
Scheme 3
Scheme 4
ratio of trans and cis isomers.
In summary, we have synthesized and crystallographically
characterized a terminal iridium carbyne complex. Complex 1
reacts readily with protic acids and amine N-oxides; both processes
probably occur by reaction at the R-carbon atom. This is
supported by the fact that protonation of 1 at low temperature
forms a highly electrophilic alkylidene complex which is capable
of mediating cyclopropanation of styrene. We are currently
examining the scope and mechanism of the cyclopropanation
reaction and further studying the reactivity of this novel carbyne
complex.
an intermediate in the C-H activation of benzaldehyde by (η5-
C5Me5)(PMe3)Ir(Me)(OTf); it rapidly rearranges to form the
analogous cationic phenyl carbonyl complex by migratory dein-
sertion.14
Upon exposure of 1 to dry degassed CH3OH at 25 °C, the
hydrido Fischer carbene complex 3 is regenerated cleanly and
isolated in 95% yield. Two mechanistic extremes for the addition
of CH3OH to 1 are shown in Scheme 3. Pathway a involves
initial nucleophilic attack of the oxygen atom at the carbyne
carbon of 1 to form the oxonium complex 9, which subsequently
rearranges by a proton shift to form the observed product 3.
Pathway b involves initial proton transfer, followed by attack of
methoxide at Ir to form benzylidene complex 10, which rearranges
to a methoxy benzyl complex 11 and subsequently forms 3 via
R-hydrogen migration. Heating a CD2Cl2 solution of 3 in the
presence of 4-Å molecular sieves at 95 °C for extended periods
of time does not afford 1. We therefore assume that this
equilibrium greatly favors carbene complex 3.15
Acknowledgment. This research was supported by the Director,
Office of Energy Research, Office of Basic Energy Sciences, Chemical
Sciences Division, of the U.S. Department of Energy under Contract DE-
AC03-76SF00098. The structural study of 1 was carried out by Dr. F. J.
Hollander, director of the U.C. Berkeley X-ray Diffraction Facility
(CHEXRAY). This paper is dedicated to Professor Helmut Werner, on
the occasion of his 65th birthday, for his important contributions to our
understanding of the chemistry of metal-carbon multiple bonds.
Supporting Information Available: Spectroscopic and analytical data
for all new complexes and, additionally, X-ray diffraction data (ORTEP
diagrams, crystal and data collection parameters, positional parameters,
and intramolecular distances and angles) for 1 (14 pages, print/PDF).
See any current masthead page for ordering information and Web access
instructions.
We have also examined the reactivity of 1 with stronger protic
acids. The addition of HOTf to 1 in CH2Cl2 at 25 °C results in
rapid formation of ylide complex 12 (Scheme 4), which can be
isolated in 85% crude yield. The 31P{1H} NMR spectrum of this
JA982641O
(16) Fryzuk, M. D.; Gao, X.; Joshi, K.; MacNeil, P. A.; Massey, R. L. J.
Am. Chem. Soc. 1993, 115, 10581.
(17) Luecke, H. F.; Arndtsen, B. A.; Burger, P.; Bergman, R. G. J. Am.
Chem. Soc. 1996, 118, 2517.
(14) Alaimo, P. J.; Arndtsen, B. A.; Bergman, R. G. J. Am. Chem. Soc.
1997, 119, 5269.
(15) For a recent example of formation of an osmium carbyne complex by
methanol extrusion from a Fischer carbene intermediate, see: Spera, M. L.;
Chen, H.; Moody, M. W.; Hill, M. M.; Harman, W. D. J. Am. Chem. Soc.
1997, 119, 12772.
(18) For a recent example of an Ir(III) alkylidene complex that is unstable
with respect to hydride migration, see: Alias, F. M.; Poveda, M. L.; Sellin,
M.; Carmona, E. J. Am. Chem. Soc. 1998, 120, 5816.
(19) The resonances in the 13C{1H} NMR spectrum of 13 are broad;
therefore, we have not been able to assign a resonance to the R-carbon.