Ruthenium complexes bearing η5-pyrazolato ligands

Full text of DOI:10.1021/ja990109a

4536
J. Am. Chem. Soc. 1999, 121, 4536-4537
Ruthenium Complexes Bearing η5-Pyrazolato
Ligands
Jayani R. Perera, Mary Jane Heeg, H. Bernhard Schlegel, and
Charles H. Winter*
Department of Chemistry, Wayne State UniVersity
Detroit, Michigan 48202
ReceiVed January 12, 1999
Pyrazolato ligands have an extensive coordination chemistry
among the middle to late transition metals and either exhibit η1-
bonding to a single metal or act as bridging ligands between two
metals.1 We and others have recently demonstrated that η2-
pyrazolato ligand coordination is common in the early transition
metals2,3 and is feasible among complexes of the groups 1 and 2
metals.4,5 Among the larger lanthanide and actinide ions, η2-
pyrazolato ligand coordination is most frequently observed6 and
an unusual µ-η2:η2-coordination mode has been recently docu-
mented.7 Conspicuously absent from the literature are complexes
bearing η5-pyrazolato ligands, although the possibility of this
coordination mode has been suggested in several reviews.1 The
lack of complexes bearing η5-pyrazolato ligands is surprising,
since complexes with η5-pyrrolyl ligands are well-known for
metals across the periodic table.8 In the course of exploring
pyrazolato complexes of the early transition metals, we became
interested in investigating the possibility of η5-pyrazolato ligand
coordination. Herein we report the synthesis, structure, and
properties of a series of ruthenium(II) complexes bearing η5-
pyrazolato ligands. This is the first documentation of this
coordination mode in any metal. Molecular orbital calculations
have been carried out on a model ruthenium(II) pyrazolato
complex. The theoretical results demonstrate that η5-pyrazolato
ligands employ orbitals similar to those of η5-cyclopentadienyl
ligands to bond to ruthenium(II) centers, which implies that η5-
pyrazolato ligand coordination should be feasible in many metals.
Figure 1. Perspective view of ((CH3)2C3HN2)(C5(CH3)5)Ru (1) with
thermal ellipsoids at the 50% probability level. Selected bond lengths
(Å) and angles (deg): Ru-C(1) 2.145(3), Ru-C(2) 2.145(3), Ru-C(3)
2.152(3), Ru-C(4) 2.177(3), Ru-C(5) 2.171(3), Ru-C(11) 2.182(3),
Ru-C(12) 2.205(3), Ru-C(13) 2.178(4), Ru-N(1) 2.178(3), Ru-N(2)
2.174(3), Ru-C5(CH3)5(centroid) 1.785(3), Ru-(CH3)2C3HN2(centroid)
1.837(3), C5(CH3)5(centroid)-Ru-(CH3)2C3HN2(centroid) 177.31(13).
Treatment of [(C5(CH3)5)RuCl]49 with 3,5-dimethylpyrazolato-
potassium,4 3,5-diphenylpyrazolato(tetrahydrofuran)potassium,4 or
3,5-di-tert-butylpyrazolatopotassium4 in tetrahydrofuran afforded
(η5-3,5-dimethylpyrazolato)(η5-pentamethylcyclopentadienyl)-
ruthenium(II) (1, 71%), (η5-3,5-di-tert-butylpyrazolato)(η5-pen-
tamethylcyclopentadienyl)ruthenium(II) (2, 72%), or (η5-3,5-
diphenylpyrazolato)(η5-pentamethylcyclopentadienyl)-
ruthenium(II) (3, 71%), as dark green, pale yellow, or dark brown
crystalline solids, respectively (eq 1).10 Complexes 1-3 were
(1) For reviews, see: Cosgriff, J. E.; Deacon, G. B. Angew. Chem., Int.
Ed. 1998, 37, 286. La Monica, G.; Ardizzoia, G. A. Prog. Inorg. Chem. 1997,
46, 151. Sadimenko, A. P.; Basson, S. S. Coord. Chem. ReV. 1996, 147, 247.
Trofimenko, S. Prog. Inorg. Chem. 1986, 34, 115. Trofimenko, S. Chem. ReV.
1972, 72, 497.
(2) Guzei, I. A.; Baboul, A. G.; Yap, G. P. A.; Rheingold, A. L.; Schlegel,
H. B.; Winter, C. H. J. Am. Chem. Soc. 1997, 119, 3387. Guzei, I. A.; Yap,
G. P. A.; Winter, C. H. Inorg. Chem. 1997, 36, 1738. Guzei, I. A.; Winter, C.
H. Inorg. Chem. 1997, 36, 4415.
characterized by spectral and analytical methods, and the molec-
ular structure of 1 was determined by X-ray crystallography. The
presence of π-bound pyrazolato ligands was strongly suggested
by the positions of the pyrazolato ring hydrogen resonances in
(3) Ro¨ttger, D.; Erker, G.; Grehl, M.; Fro¨lich, R. Organometallics 1994,
13, 3897.
(4) Ye´lamos, C.; Heeg, M. J.; Winter, C. H. Inorg. Chem. 1998, 37, 3892.
(5) Pfeiffer, D.; Heeg, M. J.; Winter, C. H. Angew. Chem., Int. Ed. 1998,
37, 2517.
1
the H NMR spectra (1, δ 4.39; 2, δ 4.72; 3, δ 5.59) and of the
hydrogen-substituted carbon resonances in the 13C{1H} NMR
spectra (1, 79.29 ppm; 2, 71.23 ppm, 3, 72.24 ppm). For
comparison, early transition metal complexes bearing η2-pyra-
zolato ligands show resonances for the pyrazolato ring hydrogen
in the 1H NMR spectra between δ 5.94-6.60 and for the
hydrogen-substituted ring carbon in the 13C{1H} NMR between
106 and 113 ppm.2 Complexes 1-3 exhibit irreversible oxidations
at 0.631, 0.600, and 0.702 V, respectively, by cyclic voltammetry
in acetonitrile.11 These values are slightly more positive than the
analogous value for pentamethylruthenocene (E1/2 ) 0.54 V),12
and indicate that the pyrazolato ligands are less electron-donating
(6) For leading references, see: Cosgriff, J. E.; Deacon, G. B.; Gatehouse,
B. M.; Hemling, H.; Schumann, H. Angew. Chem., Int. Ed. Engl. 1993, 32,
874. Deacon, G. B.; Gatehouse, B. M.; Nickel, S.; Platts, S. M. Aust. J. Chem.
1991, 44, 613. Cosgriff, J. E.; Deacon, G. B.; Gatehouse, B. M. Aust. J. Chem.
1993, 46, 1881. Cosgriff, J. E.; Deacon, G. B.; Gatehouse, B. M.; Hemling,
H.; Schumann, H. Aust. J. Chem. 1994, 47, 1223. Deacon, G. B.; Delbridge,
E. E.; Skelton, B. W.; White, A. H. Eur. J. Inorg. Chem. 1998, 543. Culp, T.
D.; Cederberg, J. G.; Bieg, B.; Kuech, T. F.; Bray, K. L.; Pfeiffer, D.; Winter,
C. H. J. Appl. Phys. 1998, 83, 4918.
(7) Deacon, G. B.; Delbridge, E. E.; Skelton, B. W.; White, A. H. Angew.
Chem., Int. Ed. 1998, 37, 2251.
(8) For selected reports of compounds bearing η5-pyrrolyl ligands, see:
Rakowski DuBois, M.; Parker, K. G.; Ohman, C.; Noll, B. C. Organometallics
1997, 16, 2325. Janiak, C.; Kuhn, N. AdV. Nitrogen Heterocycl. 1996, 2, 179.
Schumann, H.; Rosenthal, E. C. E.; Winterfeld, J.; Kociok-Ko¨hn, G. J.
Organomet. Chem. 1995, 495, C12. Schumann, H.; Winterfeld, J.; Hemling,
H.; Kuhn, N. Chem. Ber. 1993, 126, 2657. Kelly, W. J.; Parthun, W. E.
Organometallics 1992, 11, 4348. Kuhn, N.; Henkel, G.; Stubenrauch, S. J.
Chem. Soc., Chem. Commun. 1992, 760. Kuhn, N.; Ko¨ckerling, M.; Stuben-
brauch, S.; Bla¨ser, D.; Boese, R. J. Chem. Soc., Chem. Commun. 1991, 1368.
Kuhn, N.; Kuhn, A.; Lampe, E.-M. Chem. Ber. 1991, 124, 997. Kuhn, N.;
Horn, E.-M.; Boese, R.; Augart, N. Angew. Chem., Int. Ed. Engl. 1989, 28,
342. Kuhn, N.; Horn, E.-M.; Boese, R.; Augart, N. Angew. Chem., Int. Ed.
Engl. 1988, 27, 1368. Kuhn, N.; Horn, E.-M.; Zauder, E.; Bla¨ser, D.; Boese,
R. Angew. Chem., Int. Ed. Engl. 1988, 27, 579. Efraty, A.; Jubran, N. Inorg.
Chim. Acta 1980, 44, L191.
(9) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc. 1989,
111, 1698.
(10) Preparative procedures, spectral data, and analytical data for 1-3 are
contained in the Supporting Information.
(11) The cyclic voltammetry experiments were conducted with a BAS-
100 electrochemical analyzer using a glassy carbon working electrode and a
Ag/AgCl reference electrode. The solvent was acetonitrile containing 0.1 M
tetrabutylammonium hexafluorophosphate. The sweep rate was 100 mV/s. The
reported potential values are for irreversible oxidations. The oxidation
potentials are relative to an internal ferrocene standard (E° ) 0.31 V): Gagne,
R. R.; Koval, C. H.; Lisensky, G. C. Inorg. Chem. 1980, 19, 2854.
10.1021/ja990109a CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/27/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 18, 1999 4537
than a cyclopentadienyl ligand. The X-ray crystal structure of 1
was determined.13 Figure 1 shows a perspective view of the
molecule, along with selected bond lengths and angles. Complex
1 exists in a pseudo-metallocene structure, with the five-membered
rings being twisted 24.3(2)° from the eclipsed conformation. The
ruthenium-carbon bond lengths associated with the pentamethyl-
cyclopentadienyl ligand range between 2.145 and 2.177 Å. Within
the pyrazolato ligand, the ruthenium-carbon bond lengths range
between 2.182 and 2.205 Å, while the ruthenium-nitrogen bond
lengths are 2.174(3) and 2.177(3) Å. Accordingly, the ruthenium-
carbon bond lengths to the pentamethylcyclopentadienyl ligand
are shorter than the related values for the pyrazolato ligand. The
differential bonding of ruthenium to the two π-bonded ligands is
further illustrated by the ruthenium-pentamethylcyclopentadienyl
(centroid) and ruthenium-pyrazolato (centroid) distances of
1.785(3) and 1.837(3) Å, respectively. The two π-bonded ligands
in 1 are essentially coplanar, with a pentamethylcyclopentadienyl
(centroid)-ruthenium-pyrazolato (centroid) angle of 177.31(13)°.
The dihedral angles between the planes of the five-membered
rings and the methyl substituents average 1.5(3)° for the pen-
tamethylcyclopentadienyl ligand and 2.6(4)° for the 3,5-dimeth-
ylpyrazolato ligand.
To understand the bonding between the pyrazolato ligands and
ruthenium(II), ab initio calculations were carried out at the
B3LYP/LANL2DZ level of theory on ruthenocene (4) and the
model complex (η5-pyrazolato)(η5-cyclopentadienyl)ruthenium-
(II) (5).14 The calculations on 5 show the cyclopentadienyl and
Figure 2. Two of the occupied orbitals of 5, emphasizing (a) an orbital
involved in π-bonding to the ruthenium atom, and (b) the orbital
containing the symmetric combination of the nitrogen lone pairs.
further investigation of complexes bearing π-pyrazolato ligands.
The successful syntheses of 1-3 probably originates from kinetic
stabilization due to the bulky (C5(CH3)5)Ru(II) fragment and the
substituents in the 3- and 5-positions of the pyrazolato ligands.
Interestingly, we have been unable to prepare (η5-pyrazolato)-
(η5-pentamethylcyclopentadienyl)ruthenium(II) as a pure com-
pound, supporting the idea that subsitution at the 3- and 5-
positions is crucial to the stability to 1-3. In this vein, bulky
groups in the 2- and 5-positions of pyrrolyl ligands have been
shown to confer kinetic stability to complexes bearing η5-pyrrolyl
ligands.8 The calculations demonstrate that the overlap between
the pyrazolato p-orbitals and the ruthenium d-orbitals is not as
extensive as in ruthenocene. Consistent with this prediction, the
oxidation potentials of 1-3 are 0.09-0.16 V more positive than
that of pentamethylruthenocene and the molecular structure of 1
shows longer bond lengths to the pyrazolato ligand atoms than
to the pentamethylcyclopentadienyl ligand carbons. On the basis
of our results, it is likely that η5-pyrazolato ligand coor-
dination will be feasible in many metal complexes, particularly
low-valent mid to late transition metal complexes. Furthermore,
anions of related nitrogen-rich five-membered heterocycles should
also be capable of π-coordination to many metal centers. Unique
characteristics of the η5-pyrazolato ligand include its electron-
withdrawing character relative to that of the cyclopentadienyl
ligand, as well as the presence of basic lone pairs on the nitrogen
atoms. Such characteristics should lead to a rich and varied
reaction chemistry in complexes bearing η5-pyrazolato ligands.16,17
We are continuing to explore the chemistry of 1-3 and related
complexes. These studies will be reported in due course.
pyrazolato ligands to be η5-coordinated and coplanar (centroid-
Ru-centroid angle 178.8°), like the results of the structural
analysis for 1. This particular level of theory overestimates the
ruthenium-carbon and ruthenium-nitrogen bond lengths by ca.
0.1 Å. However, the difference in the bond lengths between the
pyrazolato and cyclopentadienyl centroids (0.08 Å in 5) is similar
to the value observed in 1 (0.052 Å). In 4, the carbon p-orbitals
bond equally strongly to ruthenium. However, in 5 the pyrazolato
nitrogen p-orbitals do not participate as strongly as the carbon
p-orbitals in the bonding to ruthenium (Figure 2a). Although the
lower lying, symmetric combination of the nitrogen lone pairs is
fairly distinct (Figure 2b), the higher lying, antisymmetric
combination mixes with a number of metal and ligand orbitals
of the proper symmetry.
The experimental and theoretical results establish that η5-
coordination of pyrazolato ligands is easily achieved. A previous
attempt to prepare pyrazolato analogues of ferrocene did not yield
isolable compounds.15 This report has probably discouraged
Acknowledgment. We are grateful to the National Science Foundation
(Grant no. CHE-9807269) for support of this research and to Prabhoda
Wijetunge for help with the cyclic voltammetry experiments.
(12) Gassman, P. G.; Winter, C. H. J. Am. Chem. Soc. 1988, 110, 6131.
(13) Crystal data for 1: crystals grown from hexane at -20 °C, C15H22N2-
Ru, orthorhombic, group P212121, a ) 10.7505(6) Å, b ) 11.1008(7) Å, c )
12.4859(8) Å, V ) 1490.1(2) Å3, Z ) 4, T ) 295(2) K, Dcalcd ) 1.477 g
cm-3, R(F) ) 3.00% for 3479 observed reflections (4.92° e 2Θ e 56.60°).
All non-hydrogen atoms in 1 were refined with anisotropic displacement
parameters. Full data for the X-ray crystal structure determination of 1 are
enclosed in the Supporting Information. The X-ray crystal structure of 2 was
also determined and reveals a sandwich structure similar to that of 1. Complete
details will be reported in a later full paper.
Supporting Information Available: Synthetic procedures, analytical
and spectroscopic data for 1-3, and tables of positional parameters for
4 and 5 (PDF) and X-ray crystallographic file for 1, in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
JA990109A
(14) Computed with Gaussian 98 (Frisch, M. J.; et al., Gaussian, Inc.,
Pittsburgh, PA, 1998). The LANL2DZ basis set and pseudopotentials (Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270, 284, 299) were used with
the B3LYP (Becke, A. D. Phys. ReV. A 1988, 38, 3098; Lee, C.; Yang W.;
Parr, R. D. Phys. ReV. B 1988, 37, 785; Becke, A. D. J. Chem. Phys. 1993,
98, 5648) hybrid density functional method. Geometries were fully optimized
using redundant internal coordinates (Peng, C.; Ayala, P. Y.; Schlegel, H. B.;
Frisch, M. J. J. Comput. Chem. 1996, 17, 49.
(15) Seel, F.; Sperber, V. J. Organomet. Chem. 1968, 14, 405.
(16) Recent work from the Fu group has demonstrated that azametallocenes
can function as nucleophilic catalysts in a variety of organic transformations.
For leading references, see: Garrett, C. H.; Fu, G. C. J. Am. Chem. Soc. 1998,
120, 7479.
(17) Initial reactivity studies have demonstrated that 1 can be mercurated
and forms an adduct with boron trifluoride. Reactivity studies of 1-3 are
ongoing.