Discovery of the first metallaquinone [11]

Full text of DOI:10.1021/ja001759d

J. Am. Chem. Soc. 2000, 122, 8797-8798
8797
Scheme 1
Discovery of the First Metallaquinone
Nissan Ashkenazi,† Arkadi Vigalok,† Srinivasan Parthiban,†
Yehoshoa Ben-David,† Linda J. W. Shimon,‡
Jan M. L. Martin,*,† and David Milstein*,†
Departments of Organic Chemistry and Chemical SerVices
The Weizmann Institute of Science, 76100 RehoVot, Israel
of 2a and 2b quantitatively produced the dicarbonyl complexes
3a and 3b, respectively.
ReceiVed May 22, 2000
Deprotonation of 3a and 3b with an equimolar amount of KOH
in THF, at 25 °C overnight, or with 1,8-bis(dimethylamino)-
naphthalene (Proton Sponge) or 1,4-Diazabicyclo[2.2.2]octane
(Dabco), upon refluxing in methanol for 4 days, cleanly produced
4. The presence of two strong π-acceptor CO ligands is essential
for the charge transfer from the phenoxide moiety to the metal
center, stabilizing the Ru(0)-metallaquinone formed. For com-
parison, addition of PMe3 to 2a followed by deprotonation results
in decomposition.
Quinoid systems such as quinones, quinone methides, quino-
dimethanes, and quinodiimines have been widely investigated
regarding their chemical, biological, and physical properties.1 Of
special interest are their utility as charge transfer complexes due
to the relatively low energy barrier between the ground and excited
states. Some metal stabilized quinone methides have also been
reported recently.2 Remarkably, no metallaquinone, that is, a
compound in which one of the oxygen atoms of a quinone has
been replaced by a metal, has hitherto been reported. Such a
compound is expected to have a strong dipolar contribution to
the excited state compared to its biradical nature in quinones.
We know of only one example of a stable quinoid compound
which contains a heavier element (phosphorus)3 instead of the
oxygen.
We report here the synthesis and structure of the first stable
metallaquinone. In this compound one of the oxygen atoms of
the p-quinone system is replaced by ruthenium. This compound
interconverts between a quinoid-Ru(0) carbene form in nonpolar
solvents and a Ru(II)-quinolate zwitterionic form in polar solvents.
Ru(0) carbenes are rare.4
Complex 4 was characterized spectroscopically.10 Significantly,
some of its spectral characteristics are solvent-dependent. IR of
a benzene solution shows three significant absorption signals at
1670, 1617, and 1586 cm-1 which are characteristic to CdO and
CdC absorptions of quinoid systems, while these signals are
absent in a methanol solution. The color of the benzene solution
is reddish-orange, whereas the methanol solution is yellow. The
UV-vis spectrum clearly supports this observation as in benzene
there is one absorption signal at λmax ) 450 nm (ꢀ ) 35 400)
while in methanol there is a blue shift of the signal to λmax ) 432
nm (ꢀ ) 22 700). 13C NMR spectra recorded in different solvents
shows even more dramatic changes. In THF-d8 it exhibits signals
at 303.08 ppm (characteristic for the CdRu carbon of carbene
complexes5) and at 187.45 ppm (characteristic for CdO carbon
of quinones). In CD3OD both these signals disappear and two
new signals at 160.64 and 151.55 ppm are observed. In polar
aprotic solvents (i.e. acetone) these signals are observed at 167.04
and 154.95 ppm. These observations led us to conclude that 4
appears in two different forms in different solvents, the quinoid
form 4a is present in less polar solvents such as benzene and
THF and the zwitterionic form 4b in polar solvents such as
methanol and acetone. To the best of our knowledge, this type
of zwitterionic organometallic compound is unprecedented.11
As none of the methods for either the preparation of organic
quinoid structures or of ruthenium(II) carbene systems5 was
suitable for our purpose, we designed a new synthetic pathway
(Scheme 1). Reaction of the new phenolic PCP ligand 3,5-bis-
(di-tert-butylphoshinomethylene)phenol (1),6 with either Ru(O2-
8
CCF3)2CO(PPh3)2‚CH3OH,7 or Ru(DMSO)4Cl2 in the presence
of two equivalents of NEt3, results in a facile cyclometalation
process, similar to the one previously reported,9 forming 2a and
2b, respectively, in high yields. Bubbling CO through solutions
† Department of Organic Chemistry.
‡ Department of Chemical Services.
(1) The Chemistry of the Quinoid Compounds, Parts 1 and 2; Patai, S.,
Ed.; Wiley: London, 1974; The Chemistry of the Quinoid Compounds, Parts
1 and 2; Patai,S., Rappoport, Z., Eds.; Wiley: Chichester, 1988.
(2) (a) Rabin, O.; Vigalok, A.; Milstein D. J. Am. Chem. Soc. 1998, 120,
7119. (b) Rabin, O.; Vigalok, A.; Milstein D. Chem. Eur. J. 2000, 6, 454. (c)
Kopach, M. E.; Harman, W. D. J. Am. Chem. Soc. 1994, 116, 6581. (d)
Amouri, H.; Besace, Y.; Le Bras, J.; Vaissermann, J. J. Am. Chem. Soc. 1998,
120, 6171. (e) Amouri, H.; Vaissermann, J.; Rager, M. N.; Grotjahn, D. B.
Organometallics 2000, 19, 1740.
The structure of 4b, which was crystallized from acetone
solution by slow diffusion of diethyl ether, was confirmed by
X-ray crystallography. The unit cell contains a molecule of Dabco
(which was used as a base) per two molecules of 4b. Comparison
of the structures of 4b and its precursor 3a (Figure 1) shows
similar bond lengths. The geometry around the ruthenium atom
in 3a is a distorted octahedron [OC-Ru-CO angle is 92.91-
(14)°], while in 4b it is a distorted square pyramid with the
respective angle of 99.1(3)°.
(3) Sasaki, S.; Murakami, F.; Yoshifuji, M. Angew. Chem., Int. Ed. 1999,
38, 340.
(4) Clark, G. R.; Hoskins, S. V.; Jones, T. C.; Roper, W. R. J. Chem. Soc.,
Chem. Commun. 1983, 719.
(5) (a) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H.; Ziller, J. W. J. Am.
Chem. Soc. 1992, 114, 3974. (b) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W.
J. Am. Chem. Soc. 1993, 115, 9858. (c) Schwab, P.; France, M. B.; Miller, J.
W.; Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2039. (d) Schwab,
P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100. (e)
Belderrain, T. R.; Grubbs, R. H. Organometallics 1997, 16, 4001. (f) Stu¨er,
W.; Wolf, J.; Werner, H.; Schwab, P.; Schulz, M. Angew. Chem., Int. Ed.
1998, 37, 3421. (g) Coalter, J. P., III; Bollinger, J. C.; Huffman, J. C.; Werner-
Zwanziger, U.; Caulton, K. G.; Davidson, E. R.; Ge´rard, H.; Clot, E.;
Eisenstein, O. New J. Chem. 2000, 24, 9.
(10) IR (Film) 1983, 1921, 1659, 1590, 1577, 1030 cm-1. UV-vis-
1
(benzene). λmax) 450 nm (ꢀ ) 35400). H NMR (THF-d8, 250.2 MHz) 6.38
(s, 2H), 3.18 (br.s., 4H), 1.38 (t, 18H, JP-H ) 6.2 Hz), 1.26 (t, 18H, JP-H
)
(6) For general preparation of PCP systems, see: Moulton, C. J.; Shaw,
B. L. J. Chem. Soc., Dalton Trans. 1976, 1020.
6.2 Hz). 13C NMR (THF-d8, 100.6 MHz) 303.08 (t, JP-C ) 5.7 Hz), 205.69
(t, JP-C ) 6.3 Hz) 204.74 (t, JP-C ) 8.3 Hz), 187.45 (s), 148.68 (t, JP-C ) 6.4
Hz), 111.55 (t, JP-C ) 8.9 Hz), 40.34 (t, JP-C ) 11.4 Hz), 36.70 (m), 30.52
(s), 29.85 (s). 31P {1H} (THF-d8, 101.3 MHz) 103.41 (s). The ν(CO) reported
for Ru(CF2)(CO)2(PPh3)2 in ref 4, 1983, 1910 cm-1, are very similar to those
for 4a (see above).
(7) Dobson, A.; Robinson, S. D.; Uttley, M. F. J. Chem. Soc., Dalton Trans.
1975, 370.
(8) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans.
1973, 204.
(9) Van der Boom, M. E.; Kraatz, H.-B.; Hassner, L.; Ben-David, Y.;
Milstein, D. Organometallics 1999, 18, 3873.
(11) For a review on zwitterionic organometalates see: Chauvin, R. Eur.
J. Inorg. Chem. 2000, 577.
10.1021/ja001759d CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/22/2000

8798 J. Am. Chem. Soc., Vol. 122, No. 36, 2000
Communications to the Editor
phase structure is clearly “quinonic” in character, as opposed to
the clearly “zwitterionic” X-ray diffraction structure.
Aside from the C-H and CO ligand stretching frequencies,
the ONIOM(B3LYP/LANL2DZP:HF/LANL1MB) computed IR
spectra of 4a have a number of “fingerprint” features in regions
not masked by the t-Bu vibrations: (a) an intense CparadO stretch
at 1689 cm-1, (b) symmetric and antisymmetric combinations of
the CmetadCpara stretches at 1624 and 1564 cm-1, of which only
the former is intense, and (c) the CipsodRu stretch at 1036 cm-1
.
While the latter may seem surprising at first, we note that ωe of
the RuC diatomic is known20 to be 1030 cm-1. (c) is the first
assignment of the absorption band of a Ru-carbene bond.
The experimental IR spectrum of neat 4a has an intense band
at 1030 cm-1 which would correspond to (c), and a triplet of
intense bands at 1577, 1590, and 1659 cm-1 which are most easily
explained as (a) and (b). We note that the CdO and symmetric
Figure 1. Elipsoid plots of complexes 4b (left) and 3a (right) Hydrogen
atoms were omitted for clarity.12
High-level calculations have revealed some unique features
regarding the structure and energetic properties of this system.
All calculations were carried out using the Gaussian 98 program
system.13 B3LYP14/LANL2DZ15 geometry optimizations and
harmonic frequency calculations were carried out on a model
compound for the full system 4a (in which all tertiary butyl
substituents were replaced by hydrogen atoms), while the full
system was also studied by means of the ONIOM16 (B3LYP/
LANL2DZ:HF/LANL1MB15) approach. In both cases, three
stationary points were found that are very similar in all respects
relevant to this discussion: we will focus on the Cs structure which
is marginally more stable than the others.
Clearly quinonic geometries were found, even when a “zwit-
terionic” starting geometry was used deliberately. For instance,
the Cipso-Cortho, Cortho-Cmeta, and Cmeta-Cpara bond lengths in 4a
are 1.463, 1.368, and 1.463 Å, respectively, at the ONIOM(B3LYP/
LANL2DZP:17HF/LANL1MB) level, while the CparadO bond
distance is 1.239 Å, much closer to the distances at that level of
theory for quinone than for hydroquinone. The computed Wiberg
bond order18 for CparadO is 1.56 at the B3LYP/LANL2DZ level,
and CparadO and CipsodRu bonds are clearly present in the natural
bond orbital (NBO)19 analysis. Summarizing, the computed gas-
C
orthodCmeta bands exhibit appreciable coupling in the computed
spectrum, and therefore (a) and (b) are best considered together;
we also note that at the level of theory concerned, reproduction
of computed IR intensities will be semiquantitative at best: this
should be kept in mind when comparing the computed intensities
with the observed peak areas.
Computational limitations precluded treatment of the electronic
spectrum of 4a; however, since the essential structural and
vibrational features found in 4a are present in the model
compound as well, we have carried out time-dependent density
functional theory21 calculations of its five lowest-lying electronic
transitions. Only one band (514 nm) with a high oscillator strength
is present in the visible range; it corresponds to an excitation from
the RudC π HOMO into the ring π* LUMO. Experimentally,
indeed one peak (452 nm in C6H6, 432 nm in MeOH) is seen in
the visible range.
By means of a constrained ONIOM (B3LYP/LANL2DZ:HF/
LANL1MB) optimization on the model compound, we find that
distortion to the zwitterionic structure (not a local minimum on
the potential surface) requires about 13 kcal/mol.
In summary, we have prepared and characterized the first
metallaquinone, which can be easily transformed into its zwitter-
ionic form. This transformation is reversible. We are currently
investigating the utility of this novel compound.
(12) Relevant distances (Å) and angles (deg): 4b: Ru(1)-C(11) 2.121-
(6); O(1)-C(14) 1.373(8); C(11)-C(12) 1.396(9); C(11)-C(16) 1.397(9);
C(12)-C(13) 1.386(9); C(13)-C(14) 1.372(9); C(14)-C(15) 1.393(10);
C(15)-C(16) 1.410(9); P(2)-Ru(1)-P(3) 157.46 (6); C(2)-Ru(1)-C(3) 99.1-
(3). 3a: Ru(1)-C(11) 2.117(3); O(1)-C(14) 1.370(4); C(11)-C(12) 1.410-
(4); C(11)-C(16) 1.398(4); C(12)-C(13) 1.387(5); C(13)-C(14) 1.380(5);
C(14)-C(15) 1.392(4); C(15)-C(16) 1.392(5); P(2)-Ru(1)-P(3) 157.50-
(3); C(5)-Ru(1)-C(6) 92.91(14).
(13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
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G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B.
B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.;
Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A.
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Acknowledgment. D.M. is the Israel Matz Professor of Organic
Chemistry, and J.M., the Incumbent of the Helen and Milton A.
Kimmelman Career Development Chair. This research was supported by
the Minerva Foundation, Munich, Germany, by the U.S.-Israel Binational
Science Foundation, Jerusalem, Israel, and by the Tashtiyot program of
the Ministry of Science (Israel).
Supporting Information Available: Text describing the synthesis
and characterization of compounds 1-4 and tables of crystal data and
structure refinement, atomic coordinates, bond lengths and angles,
anisotropic displacement parameters, and hydrogen atom coordinates for
3a and 4b and Cartesian coordinates of all computed compounds (PDF).
This material is available free of charge via the Internet at http::/pubs.acs.org.
JA001759D
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