Facile, thermoreversible cycloaddition of small molecules to a ruthenium(II) arene β-diketiminate

Article abstract of DOI:10.1021/om070017r

Formation of a vacant coordination site on a Ru center permits an unusual binding of small molecules such as ethylene, acetylene, and dihydrogen, resulting in the transformation of the chelating β-diketiminate ligand to a β-diimine. These representative species are observed during the catalytic hydrogenation of styrene.

Full text of DOI:10.1021/om070017r

1120
Organometallics 2007, 26, 1120-1122
Facile, Thermoreversible Cycloaddition of Small Molecules to a
Ruthenium(II) Arene â-Diketiminate
Andrew D. Phillips, Ga´bor Laurenczy, Rosario Scopelliti, and Paul J. Dyson*
Institut des Sciences et Inge´nierie Chimiques, Ecole Polytechnique Fe´de´rale de Lasuanne (EPFL),
CH-1015 Lausanne, Switzerland
ReceiVed January 8, 2007
Scheme 1
Summary: Formation of a Vacant coordination site on a Ru
center permits an unusual binding of small molecules such as
ethylene, acetylene, and dihydrogen, resulting in the transfor-
mation of the chelating â-diketiminate ligand to a â-diimine.
These representatiVe species are obserVed during the catalytic
hydrogenation of styrene.
Identification of intermediates during a reaction cycle is
fundamental to the understanding and improvement of basic
catalytic processes. In general, a key step involves the generation
of a coordinatively unsaturated metal center which receives and
activates the incoming substrate.1 Under typical conditions, these
species are normally too reactive to be observed. Hence, in order
to facilitate synthesis and comprehensive characterization, it is
necessary to employ a ligand that can impart high thermody-
namic and kinetic stability through means of strong electron
donation and steric constraints. One class of ligand that satisfies
these requirements are the â-diketiminates (1, abbreviated herein
η2-bound to the metal.6 Although numerous examples of iron
â-diketiminate species are known,2 analogous complexes of the
heavier members of the group 8 triad, Ru and Os, have been
notably absent.2 This is surprising, considering that ruthenium
complexes with smaller unsaturated diazo-coordinating ligands
(i.e., (R)N(CH)nN(R), n ) 1 (anionic amidinates),7 2 (neutral
diazabutadienes)8) have been used for well over two decades,
in addition to tetraazacyclotetradecine species9 (2) and acetyl-
acetone10 chelating compounds. Furthermore, the unusual η3-
(C,N,N′)-bonded ruthenium bisphosphiniminomethanide com-
plex 3 has also been recently reported.11
Our investigations began with the synthesis and characteriza-
tion of complexes based on the highly stable Ru(II)-η6-arene
fragment. Using the lithiated version of 112 in dichloromethane,
combined with [(η6-C6H6)RuCl2]2, afforded clean reactions with
LiCl and (η6-C6H6)RuCl((ArNCMe)2CH) (4a; Ar ) 2,6-
dimethylphenyl) as the only detectable products (Scheme 1).
Removal of the chloride substituent is facilitated by the
stoichiometric addition of Me3SiOTf to 4a, or more conve-
niently, a single-pot reaction is possible using the ruthenium
dimer, 4a, and sodium triflate in equal molar ratios. Compound
4b, [(η6-C6H6)Ru((ArNCMe)2CH)]OTf, is insoluble in Et2O and
hydrocarbon solvents but highly soluble in THF and chlorinated
solvents. In the solid state, 4a demonstrates moderate air
sensitivity, and 4b decomposes over several hours.
as L), which have been utilized for the preparation of a number
of interesting and application-oriented main-group- and transi-
tion-metal-centered compounds.2 In particular, polymerization
catalysts based on 1 in combination with Ti, Zr, Cr, Co, Ni,
and Pd (ML) have been the focus of considerable attention.2
Similar complexes with transition-metal centers are capable of
activating H2,3 N2,4 and O2.5 Examples of alkene and alkyne
coordination by ML species are also known but generally are
(6) (a) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998. (b) Stoian,
S. A.; Yu, Y.; Smith, J. M.; Holland, P. L.; Bominaar, E. L.; Muenck, E.
Inorg. Chem. 2005, 44, 4915. (c) Bai, G.; Wei, P.; Stephan, D. W.
Organometallics 2005, 24, 5901.
(7) Nagashima, H.; Kondo, H.; Hayashida, T.; Yamaguchi, Y.; Gondo,
M.; Masuda, S.; Miyazaki, K.; Matsubara, K.; Kirchner, K. Coord. Chem.
ReV. 2003, 245, 177.
(8) For example: (a) Bailey, P. J.; Mitchell, L. A.; Parsons, S. J. Chem.
Soc. Dalton Trans. 1996, 2839. (b) Zuccaccia, D.; Sabatini, S.; Bellachioma,
G.; Cardaci, G.; Clot, E.; Macchioni, A. Inorg. Chem. 2003, 42, 5465. (c)
tom Dieck, H.; Kollvitz, W.; Kleinwaechter, I. Organometallics 1986, 5,
1449.
(1) Van Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding
the Art; Kluwer Academic: Dordrecht, The Netherlands, 2004.
(2) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
102, 3031 and references cited therein.
(3) (a) Budzelaar, P. H. M.; de Gelder, R.; Gal, A. W. Organometallics
1998, 17, 4121. (b) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem.
Commun. 2004, 764. (c) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am.
Chem. Soc. 2003, 125, 15286. (d) Smith, J. M.; Lachicotte, R. J.; Holland,
P. L. J. Am. Chem. Soc. 2003, 125, 15752. (e) MacAdams, L. A.; Buffone,
G. P.; Incarvito, C. D.; Golen, J. A.; Rheingold, A. L.; Theopold, K. H.
Chem. Commun. 2003, 1164.
(4) (a) Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel,
W. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 10660.
(b) Chai, J.; Hongping, Z.; Stueckl, A. C.; Roesky, H. W.; Magull, J.;
Bencini, A.; Caneschi, A.; Gatteschi, D. J. Am. Chem. Soc. 2005, 127, 9201.
(c) Dai, X.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 4798.
(5) (a) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.;
Lukat-Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland,
P. L. J. Am. Chem. Soc. 2006, 128, 756. (b) Masuda, J. D.; Stephan, D. W.
Can. J. Chem. 2005, 83, 324.
(9) Klose, A.; Solari, E.; Hesschenbrouck, J.; Floriani, C.; Re, N.;
Geremia, S.; Randaccio, L. Organometallics 1999, 18, 360.
(10) Fernandez, R.; Melchart, M.; Habtemariam, A.; Parsons, S.; Sadler,
P. J. Chem. Eur. J. 2004, 10, 5173.
(11) (a) Bibal, C.; Pink, M.; Smurnyy, Y. D.; Tomaszewski, J.; Caulton,
K. G. J. Am. Chem. Soc. 2004, 126, 2312. (b) Smurnyy, Y. D.; Bibal, C.;
Pink, M.; Caulton, K. G. Organometallics 2005, 24, 3849.
(12) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead,
M. M.; Roesky, H. W.; Power, P. P. Dalton Trans. 2001, 3465.
10.1021/om070017r CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/03/2007

Communications
Organometallics, Vol. 26, No. 5, 2007 1121
Figure 1. Ball and stick diagrams of complexes 4b (left), 5a (center), and 5c (right). Solvates and anions have been omitted for clarity.
Scheme 2a
a X ) OTf.
A number of spectroscopic methods were used to characterize
4a and 4b (see the Supporting Information for a complete
description). In particular, the structures (4b is shown in Figure
1; 4a (Figure S1) and metric parameters are given in the
Supporting Information) were determined from X-ray diffraction
data. The structure of 4a features a Ru-Cl bond (2.521(1) Å)
Figure 2. Graphical representation of the computationally modeled
that is within error equal to the longest distance for the Ru η6-
Kohn-Sham orbitals comprising the HOMO (left) and LUMO
arene subset,13 and in 4b, the anion weakly interacts with the
(right) levels in complex 4b.
cation (the nearest H‚‚‚O distance is 2.275 Å). The general
geometric arrangement of 4a and 4b consists of flanking ligand
aryl groups parallel to the metal-arene group, perpendicular
changed color upon the addition of the substrate. Hence,
investigations into prototypical reactions were conducted by
to the plane of the metal-nitrogen heterocycle. In the case of
combining 4b with ethylene or acetylene under atmospheric
4a, the C6H6-Ru fragment is tilted out of the plane Ru-
pressure (Scheme 2). Immediately, solutions of 4b in THF
Ncentroid-C(11) by 154.79°, but not to the extent as in 3, 73.8°.
yielded pale yellow precipitates (5a and 5b), which are highly
The effect of chloride removal and formation of a coordinatively
1
soluble in CH2Cl2. H COSY spectroscopy revealed that the
unsaturated metal center is readily apparent in the Ru-N bond
ethylene and acetylene had inserted between the Ru and central
distances (4b, 1.997(5), 1.994(5) Å; 4a, 2.099(2) Å). The
R-position carbon of the â-diketiminate ligand. Additionally,
N-Ru-N bite angle (4a, 86.7(1)°; 4b, 88.5(2)°) is also
the frequency of the R-proton is significantly shifted to a higher
significantly less than that observed for the series 2 of tetraazo
field than in 4b. Crystals suitable for X-ray diffraction analyses
species (average 97.4°).12 The arene centroid to Ru distance
were obtained in both cases (Figure 1). The structures reveal a
three-legged piano-stool configuration with a Ru σ-bonded CH2
or CH group forming alkanyl (5a) and alkenyl (5b) complexes.
Additionally, the entire â-diketiminate ligand has undergone
(4a, 1.688 Å; 4b, 1.670 Å) is typical of species with five- or
four-membered diaza-coordinating ligands. In contrast to arene-
ruthenium chelated amidinates or bisphosphiniminomethanide
species, where the R-carbon forms a direct Ru-C bond ([η6-
modification, including a shortening of the Câ-N bonds and,
C6Me6Ru(N(iPr))2CPh]PF6, 2.431(5) Å;14 3, 2.224(3) Å),11a in
more significantly, the Ru and R-carbon centers being folded
complexes 4a and 4b this feature is absent, the separation
out of the original metal-ligand plane, thus breaking the
distance being significantly greater (3.384 and 3.386 Å,
electronic π-delocalization along the N-C3-N chain. The most
respectively). Another interesting aspect upon removal of the
chloride from 4a is the change in color. Normally, unsaturated
16-electron ruthenium complexes are dark red or purple;
however, 4b is dark orange. This difference can be attributed
to a strong metal-to-ligand charge transfer, as indicated by a
broad adsorption at 434 nm (with ꢀ ) 5455) in the UV-visible
striking difference between 5a and 5b is the C-C distance of
the inserted substrate, which is primarily manifested as a shorter
Ru-C distance in the alkenyl complex: 5b, 2.065(2) Å; 5a,
2.159(3) Å. Surprisingly, both types of cycloaddition reactions
are reversible, with 5b quantitatively reverting back to 4b at
room temperature, in solution, over a period of 1 day under a
spectrum of 4b dissolved in dichloromethane. Similar types of
N2 atmosphere. Similarly, 5a loses ethylene, but only at elevated
MLCT interactions have been observed for arene-Ru-diaz-
temperatures (>50 °C). Other examples of transition-metal
â-diketiminate species that undergo ligand insertion reactions
at the R-carbon position are known (e.g., FeCl(ArNCMe)2CH)
abutadiene complexes.8c
During evaluations of 4b as a catalyst for styrene hydrogena-
tion (vide infra), it was observed that the solutions rapidly
withdiazoacetate15 andmethyltransferwithinPt(CH3)5(ArNCMe)2-
CH; Ar ) 2,6-iPr2C6H3),16 but none of the above are reported
(13) Statistical data obtained from the CCDC (version 5.28, August
2006).
(14) Hayashida, T.; Yamaguchi, Y.; Kirchner, K.; Nagashima, H. Chem.
Lett. 2001, 954.
(15) Gregory, E. A.; Lachicotte, R. J.; Holland, P. L. Organometallics
2005, 24, 1803.

1122 Organometallics, Vol. 26, No. 5, 2007
Communications
to be reversible. A product analogous to 5b resulting from
ethylene insertion into an Al â-diketiminate has been character-
ized in solution, but the reaction is only reversible following
the addition of a strong nucleophile.17
ruthenium (+0.05), creates a 1,4-metal-based dipole within the
complex. Moreover, both the HOMO and LUMO are mainly
comprised of contributions from the dyz orbital of Ru and the
py orbital associated with the R-carbon of the chelating â-diket-
amine (Figure 2). Both results suggest that substrate activation
by complex 4b involves a concerted mechanism, whereby the
HOMO of the complex interacts with the LUMO of the substrate
(the π*-MO of the olefin or the σ*-MO of H2) and simulta-
neously the LUMO of the complex combines with the substrate
HOMO. A number of other metal â-diketiminate complexes
are known to react with H2, including those containing Rh,3a
Ir,3b and Pt,3c yielding terminal hydrides, while Fe3d and Cr3e
form dimeric species with bridging hydrides. However, none
of these systems involve the interaction and reversible trans-
formation of the â-diketiminate ligand.
Our preliminary evaluation of styrene hydrogenation with 4a
and 4b (1:1000 catalyst to substrate ratio) in THF at 80 °C and
40 bar of H2 reveals turnover frequencies of 1000 and 838 mol
mol-1 h-1, respectively. ESI-MS of nanoelectrosprayed aliquots
from the catalytic reactions reveal for both systems21 the
identical dominant species [(C6H5Et)Ru(ArNCMe)2CH)]+ at m/z
515, while for 4b, a styrene-coordinated analogue of 5b is
present (m/z 617), and for 4a several dimeric chloro-bridged
species, i.e., [(Ru(ArNCMe)2CH))2Cln]+, are observed. Related
halogen-bridged Ru complexes with phosphine ligands have
been characterized and shown to be efficient hydrogenation
catalysts.22,23
In the interest of identifying possible intermediates formed
during the catalytic hydrogenation process, the reaction between
4b and H2 was also examined. Under 100 bar of H2 pressure,
solution NMR revealed instantaneous formation of a hydride
species (δ(1H) -5.10 ppm, t1 ) 1.392 s), which is also formed,
1
albeit more slowly, under 5 bar of pressure. In general, the H
and 13C NMR spectra of 5c resemble those of 5a and 5b,
including a low-field-shifted diastereotopic R-CH2 group.
Product 5c can be isolated on a larger scale in an autoclave.
However, the isolated highly reactive solid readily reverts back
to 4b in the absence of a H2 atmosphere at room temperature,
but its lifetime is prolonged if kept below -25 °C. Nevertheless,
a solid-state ATR-IR spectrum was obtained, revealing a
diagnostic Ru-H stretch at 1883 cm-1. Crystals were obtained
for X-ray diffraction studies by exploiting the insoluble nature
of 5c in THF. The structure is analogous with those of 5a and
5b, featuring an identical folding pattern of the â-diimine
ligand.18 Hence, this reaction represents a rare example of
reversible heterolytic cleavage of dihydrogen by a nucleophilic
carbon center, whereas for the majority of cases, H2 cleavage
is facilitated by electronegative elements such as N, O, S, and
Cl.19 A DFT-optimized model of 4b20 reveals that the R-carbon
center has the greatest (-0.48) negative charge within the Ru-
N-C3-N ring. This, combined with the positively charged
In conclusion, we have demonstrated that facile generation
of a vacant coordination site in arene-Ru-â-diketiminate
complexes readily activates H2, alkenes, and alkynes, resulting
in thermally reversible cycloaddition with ligand participation.
A future report will examine the hydrogenation ability of a series
of arene-Ru- and arene-Os-â-diketiminate complexes with
different ligand substitution patterns.
(16) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001,
123, 6423.
(17) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.
1998, 120, 9384.
(18) For other R-substituted H,H′-â-diimine complexes see: (a) Feldman,
J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.; Calabrese, J. C.; Arthur,
S. D. Organometallics 1997, 16, 1514. (b) Landolsi, K.; Richard, P.;
Bouachir, F. J. Organomet. Chem. 2005, 690, 513. (c) Landolsi, K.; Rzaigui,
M.; Bouachir, F. Tetrahedron Lett. 2002, 43, 9463. (d) Cope-Eatough,
E. K.; Mair, F. S.; Pritchard, R. G.; Warren, J. E.; Woods, R. J. Polyhedron
2003, 22, 1447.
Acknowledgment. This research was supported by a grant
from the European Marie Curie Action (A.D.P., CARCAS
Project Contract no. MEIF-CT-2005-025287) and the EPFL.
We wish to thank Drs. Euro Solari and Christian G. Hartinger
for their technical assistance.
(19) Peruzzini, M.; Poli, R. Recent AdVances in Hydride Chemistry;
Elsevier: Amsterdam, 2001.
(20) Calculated at the B3LYP level of theory with C2V symmetry
constraints. Further details are provided in the Supporting Information.
(21) Daguenet, C.; Scopelliti, R.; Dyson. P. J. Organometallics 2004,
23, 4849.
(22) Joshi, A. M.; MacFarlane, K. S.; James, B. R. J. Organomet. Chem.
1995, 488, 161.
Supporting Information Available: Text, tables, figures, and
CIF files giving experimental procedures with characterization data
and crystallographic details for 4a,b and 5a-c and computational
details. This material is available free of charge via the Internet at
http://pubs.acs.org.
(23) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer
Academic: Dordrecht, The Netherlands, 2001.
OM070017R