N5-imidazolato complexes of ruthenium

Article abstract of DOI:10.1021/om000810j

The imidazolato complexes [Cp*Ru(u-jj1:ij1Ph2C3HN2)h Cp*Ru(ii5-Ph2C3HN2), and Cp*Ru(r]5MeCsfflSy have been prepared and characterized. The latter complexes represent the first structural documentation of the rf-imidazolato ligand coordination mode to any

Full text of DOI:10.1021/om000810j

Organometallics 2000, 19, 5263-5265
5263
η5-Im id a zola to Com p lexes of Ru th en iu m
J ayani R. Perera, Mary J ane Heeg, and Charles H. Winter*
Department of Chemistry, Wayne State University, Detroit, Michigan 48202
Received September 20, 2000
Summary: The imidazolato complexes [Cp*Ru(µ-η1:η1-
Ch a r t 1
Ph2C3HN2)]3, Cp*Ru(η5-Ph2C3HN2), and Cp*Ru(η5-
Me2C3HN2) have been prepared and characterized. The
latter complexes represent the first structural documen-
tation of the η5-imidazolato ligand coordination mode
to any metal. [Cp*Ru(µ-η1:η1-Ph2C3HN2)]3 converts to
Cp*Ru(η5-Ph2C3HN2) upon heating in tetrahydrofuran,
demonstrating that the η5-imidazolato ligand is thermo-
dynamically preferred over the µ-η1:η1-ligand in the
Cp*Ru(II) system.
imidazolato complex upon heating. Such facile conver-
sion of the µ-η1:η1 ligand to the η5 ligand suggests that
the latter coordination mode may be observed in other
low-valent metal complexes and that such interactions
may be relevant to biological systems.
Transition metal complexes containing imidazolato
ligands are well known.1 Interest in metal complexes
with these ligands has been driven by their structural
resemblance to histidine groups in the metal-binding
sites of certain proteins.2,3 The most common coordina-
tion mode of imidazolato ligands is µ-η1:η1, although η1
coordination has been observed (Chart 1).4,5 Other
coordination modes are presently unknown. Herein we
report the synthesis, structure, and properties of a series
of ruthenium(II) complexes containing µ-η1:η1- and η5-
imidazolato ligands. The latter complexes represent the
first structural documentation of the η5-imidazolato
ligand coordination mode to any metal. A complex
containing a µ-η1:η1-imidazolato ligand has been isolated
and characterized. Unexpectedly, it converts to the η5-
6
Treatment of [(C5(CH3)5)RuCl]4 with 4,5-diphenyl-
imidazolatopotassium in tetrahydrofuran at ambient
temperature for 18 h afforded the trimeric 4,5-diphenyl-
imidazolato complex 1 as a dark red solid after workup
(eq 1).7 The trimeric structure of 1 was established
(1) For reviews, see: Sundberg, R. J .; Martin, R. B. Chem. Rev. 1974,
74, 471. Inoue, M.; Kubo, M. Coord. Chem. Rev. 1976, 21, 1. Steel, P.
J . Coord. Chem. Rev. 1990, 106, 227. J aniak, C.; Kuhn, N. Adv.
Nitrogen Heterocycl. 1996, 2, 179.
(2) For overviews, see: Frau´sto da Silva, J . R. R.; Williams, R. J . P.
The Biological Chemistry of the Elements: The Inorganic Chemistry
of Life; Clarendon Press: New York, 1991. Creighton, T. E. Proteins:
Structures and Molecular Principles; Freeman, W. H.: New York, 1984.
Williams, R. J . P.; Frau´sto da Silva, J . R. R. New Trends in Bioinor-
ganic Chemistry; Academic Press: London, 1978. Pratt, J . M. Inorganic
Chemistry of Vitamin B12; Academic Press: New York, 1972.
(3) For leading references, see: Bertini, I.; Banci, L.; Piccioli, M.
Coord. Chem. Rev. 1990, 100, 67. Valentine, J . S.; de Freitas, D. M. J .
Chem. Educ. 1985, 62, 990. McMillin, D. R. J . Chem. Educ. 1985, 62,
997. Weser, U.; Schubotz, L. M.; Lengfelder, E. J . Mol. Catal. 1981,
13, 249. Beem, K. M.; Richardson, D. C.; Rajagopalan, K. V. Biochem-
istry 1977, 16, 1930. Fee, J . A.; Briggs, R. G. Biochim. Biophys. Acta
1975, 400, 439. Richardson, J . S.; Thomas, K. A.; Rubin, B. H.;
Richardson, D. C. Proc. Nat. Acad. Sci. U.S.A. 1975, 72, 1349. McCord,
J . M.; Fridovich, I. J . Biol. Chem. 1969, 244, 6049.
(6) Fagan, P. J .; Ward, M. D.; Calabrese, J . C. J . Am. Chem. Soc.
1989, 111, 1698.
(7) A 100 mL Schlenk flask was charged with [Cp*RuCl]4
(0.250 g, 0.229 mmol) and tetrahydrofuran (25 mL). A solution of 4,5-
diphenylimidazalatopotassium was prepared by mixing 4,5-diphenyl-
imidazole (0.202 g, 0.916 mmol) and potassium hydride (0.037 g, 0.916
mmol) in tetrahydrofuran (35 mL). After stirring for 18 h at ambient
temperature, this solution was added by cannula to the ruthenium
reactant. After being stirred for 18 h at ambient temperature the
resultant solution was filtered through a 2 cm pad of Celite on a coarse
glass frit. The volatile components were removed under reduced
pressure to afford 1 as a spectroscopically pure dark red crystalline
solid (0.313 g, 75%). An analytical sample was crystallized from hexane
at -20 °C: mp 177-179 °C; IR (Nujol, cm-1) 3051 (w), 1599 (s), 1501
(s), 1259 (w), 1153 (m), 1070 (m), 1028 (m), 974 (w), 909 (w), 762 (s),
697 (s), 666 (w); 1H NMR (benzene-d6, δ) 7.98 (s, 1 H, Ph2Im C-H),
6.92 (dd, J ) 5 Hz, J ) 2 Hz, 6 H, m- and p-C6H2H2′H), 6.43 (dd, J )
7 Hz, J ) 2 Hz, 4 H, o-C6H2H2′H), 1.56 (s, 15 H, C5(CH3)5); 13C{1H}
NMR (benzene-d6, ppm) 144.12 (s, Ph2Im C-H), 139.31 (s, Ph2Im C),
137.81 (s, ipso C of Ph group), 129.49 (s, ortho C-H of Ph group), 126.02
(s, meta C-H of Ph group), 125.94 (s, para C-H of Ph group), 72.40 (s,
C5(CH3)5), 10.21 (s, C5(CH3)5). Anal. Calcd for C25H26N2Ru: C, 65.91;
H, 5.75; N, 6.15. Found: C, 65.55; H, 5.94; N, 6.16.
(4) Selected examples of µ-η1:η1 complexes: Rettig, S. J .; Storr, A.;
Summers, D. A.; Thompson, R. C.; Trotter, J . J . Am. Chem. Soc. 1997,
119, 8675. Mao, Z.-W.; Chen, M.-Q.; Tan, X.-S.; Liu, J .; Tang, W.-X.
Inorg. Chem. 1995, 34, 2889. Chaudhuri, P.; Karpenstein, I.; Winter,
M.; Lengen, M.; Butzlaff, C.; Bill, E.; Trautwein, A. X.; Flo¨rke, U.;
Haupt, H.-J . Inorg. Chem. 1993, 32, 888. Costes, J .-P.; Dahan, F.;
Laurent, J .-P. Inorg. Chem. 1991, 30, 1887. Lu, Q.; Luo, Q. H.; Dai, A.
B.; Zhou, Z. Y.; Hu, G. Z. J . Chem. Soc., Chem. Commun. 1990, 1429.
Suzuki, M.; Ueda, I.; Kanatomi, H.; Murase, I. Bull. Chem. Soc. J pn.
1983, 56, 3421. Kolks, G.; Frihart, C. R.; Coughlin, P. K.; Lippard, S.
J . Inorg. Chem. 1981, 20, 2933. O’Young, C.-L.; Dewan, J . C.;
Lilienthal, H. R.; Lippard, S. J . J . Am. Chem. Soc. 1978, 100, 7291.
Isied, S. S.; Kuehn, C. G. J . Am. Chem. Soc. 1978, 100, 6754.
(5) Selected examples of η1 complexes: Mandon, D.; Ott-Woelfel, F.;
Fischer, J .; Weiss, J .; Bill, E.; Trautwein, A. X. Inorg. Chem. 1990, 29,
2442. Quinn, R.; Strouse, C. E.; Valentine, J . S. Inorg. Chem. 1983,
22, 3934. Storm, C. B.; Freeman, C. M.; Butcher, R. J .; Turner, A. H.;
Rowan, N. S.; J ohnson, F. O.; Sinn, E. Inorg. Chem. 1983, 22, 678.
10.1021/om000810j CCC: $19.00 © 2000 American Chemical Society
Publication on Web 11/14/2000

5264 Organometallics, Vol. 19, No. 25, 2000
Communications
by X-ray crystallographic analysis.8 In addition, the
imidazolate ring C-H unit of 1 resonated at δ 7.98 in
1
the H NMR and at 144.12 ppm in the 13C{1H} NMR
spectrum. These shifts, relative to the values observed
for 4,5-diphenylimidazole (1H NMR δ 8.02; 13C{1H}
NMR 148.69 ppm), do not suggest η5-coordination of the
imidazolato ligand, but are consistent with the observed
µ-η1:η1 coordination mode. Refluxing of 1 for 18 h in
tetrahydrofuran or treatment of [(C5(CH3)5)RuCl]4 with
4,5-diphenylpyrazolatopotassium in refluxing tetrahydro-
furan for 18 h afforded (η5-4,5-diphenylimidazolato)(η5-
pentamethylcyclopentadienyl)ruthenium (2) as a brown
crystalline solid after workup.9 Adoption of the η5 coor-
dination mode in 2 was suggested by the upfield shifts
of the imidazolato ring CH unit (1H NMR δ 6.45;
13C{1H} NMR 109.35 ppm), relative to 1. As described
below, the X-ray crystal structure of 2 confirmed the
presence of the η5-imidazolato ligand. Treatment of [(C5-
(CH3)5)RuCl]4 with 2,4-dimethylimidazolatopotassium
in tetrahydrofuran at ambient temperature afforded low
(<50%) yields of (η5-2,4-dimethylimidazolato)(η5-penta-
methylcyclopentadienyl)ruthenium (3), apparently due
to the low solubility of the potassium reagent.10 There
was no evidence for a µ-η1:η1 complex similar to 1 in
the crude reaction mixture, apparently due to steric
congestion caused by the methyl group in the 2-position
of the imidazolato ligand. However, conducting the
reaction in refluxing tetrahydrofuran for 2 h afforded 3
in 73% yield as a light brown solid after workup. The
presence of a η5-2,4-dimethylimidazolato ligand in 3 was
F igu r e 1. Perspective view of ((C6H5)2C3HN2)(C5(CH3)5)-
Ru (2) with thermal ellipsoids at the 50% probability level.
Selected bond lengths (Å) and angles (deg): Ru-C(1)
2.164(4), Ru-C(2) 2.186(3), Ru-C(3) 2.209(3), Ru-N(1)
2.239(3), Ru-N(2) 2.221(3), Ru-C(16) 2.169(4), Ru-C(17)
2.160(4), Ru-C(18) 2.152(4), Ru-C(19) 2.151(4), Ru-C(20)
2.174(4), Ru-C5(CH3)5(centroid) 1.794(3), Ru-(C6H5)2C3-
HN2(centroid) 1.861(3), C5(CH3)5(centroid)-Ru-(C6H5)2C3-
HN2(centroid) 178.1(1).
suggested by the upfield shifts of the imidazolato ring
CH unit (1H NMR δ 5.42; 13C{1H} NMR 96.15 ppm),
relative to 1, and was confirmed by an X-ray crystal
structure determination.8
Figure 1 shows a perspective view of 2, along with
selected bond lengths and angles.8 Complex 2 exists in
a pseudo-metallocene structure, with the five-membered
rings being twisted 12.8(1)° from the eclipsed conforma-
tion. The ruthenium-carbon bond lengths associated
with the pentamethylcyclopentadienyl ligand range
from 2.151 to 2.174 Å. Within the imidazolato ligand,
the ruthenium-carbon bond lengths range between
2.164 and 2.209 Å, while the ruthenium-nitrogen bond
lengths are 2.221(3) and 2.239(3) Å. Accordingly, the
ruthenium-carbon bond lengths to the pentamethyl-
cyclopentadienyl ligand are shorter than the related
values for the imidazolato ligand. The differential
bonding of ruthenium to the two π-bonded ligands
is further illustrated by the ruthenium-pentamethyl-
cyclopentadienyl (centroid) and ruthenium-imidazolato
(centroid) distances of 1.794(3) and 1.861(3) Å, respec-
tively. The two π-bonded ligands in 2 are essentially co-
planar, with a pentamethylcyclopentadienyl (centroid)-
ruthenium-imidazolato (centroid) angle of 178.1(1)°.
Complexes 2 and 3 exhibit irreversible oxidations at
0.750 and 0.736 V, respectively, by cyclic voltammetry
in acetonitrile.11 These values are slightly more positive
than the analogous values for pentamethylruthenocene
(E1/2 ) 0.54 V) and the ruthenium pyrazolato complexes
(C5(CH3)5)(3,5-R2pz)Ru (R ) CH3 (0.631 V), tBu (0.600
V), Ph (0.702 V))12 and indicate that the imidazolato
ligands are less electron donating than a cyclopenta-
dienyl ligand and are even slightly less donating than
isomeric pyrazolato ligands.
(8) Crystal data for 2: crystals grown from hexane at -20 °C,
C25H26N2Ru, triclinic, group P1h, a ) 7.5054(6) Å, b ) 9.7217(7) Å, c )
15.8937(11) Å, V ) 1048.31(13) Å3, Z ) 2, T ) 295(2) K, Dcalcd ) 1.443
g cm-3, R(F) ) 4.54% for 4648 observed reflections (2.68° e 2θ e
56.58°). All non-hydrogen atoms in 2 were refined with anisotropic
displacement parameters. Full data for the X-ray crystal structure
determination of 2 are included in the Supporting Information. The
X-ray crystal structures of 1 and 3 were also determined. The poor
crystal quality of 1 prohibits a detailed structure report; however, the
identity of the compound was unambiguously determined. Complex 3
exhibits a sandwich structure similar to that of 2. Complete details
will be reported in a later full paper.
(9) A 100 mL Schlenk flask was charged with [Cp*RuCl]4 (0.250 g,
0.229 mmol) and tetrahydrofuran (35 mL). To this mixture was added
a solution of 4,5-diphenylimidazalatopotassium (prepared by mixing
4,5-diphenylimidazole (0.202 g, 0.916 mmol) and potassium hydride
(0.037 g, 0.916 mmol) in tetrahydrofuran (35 mL)) The combined
solutions were refluxed for 18 h. Upon cooling, the brown solution was
filtered through a 2 cm pad of Celite on a coarse glass frit. The volatile
components were removed under reduced pressure to afford 2 as a
brown powder (0.287 g, 69%). Brown crystals were grown by crystal-
lization from hexane at -20 °C: mp 151-153 °C; IR (Nujol, cm-1) 3033
(w), 1600 (m), 1502 (m), 1422 (m), 1220 (m), 1189 (w), 1114 (w), 1067
(m), 1025 (m), 948 (m), 903 (w), 761 (s), 695 (s), 670 (m), 657 (s); 1H
NMR (benzene-d6, δ 7.82 (dd, J ) 8.0 Hz, J ) 1.5 Hz, 4 H, o-C6H2H2′H),
7.12 (tt, J ) 7.5 Hz, J ) 1.5 Hz, 4 H, m-C6H2H2′H), 7.04 (tt, J ) 7.5
Hz, J ) 1.7 Hz, 2 H, p-C6H2H2′H), 6.45 (s, 1 H, Ph2Im C-H), 1.53 (s, 15
H, C5(CH3)5); 13C{1H} NMR (benzene-d6, ppm) 134.08 (s, ipso C of Ph
group), 129.25 (s, ortho C-H of Ph group), 128.13 (s, meta C-H of Ph
group), 127.61 (s, para C-H of Ph group), 110.12(s, Ph2Im C), 109.35
(s, Ph2Im C-H), 85.26 (s, C5(CH3)5), 10.53 (s, C5(CH3)5). Anal. Calcd
for C25H26N2Ru: C, 65.91; H, 5.75; N, 6.15. Found: C, 65.50; H, 5.90;
N, 6.39.
(10) In a manner similar to the preparation of 2, [Cp*RuCl]4 (0.500
g, 0.459 mmol), 2,4-dimethylimidazole (0.177 g, 1.839 mmol), and
potassium hydride (0.074 g, 1.839 mmol) in tetrahydrofuran (70 mL)
were combined to afford 3 as a brown powder (0.445 g, 73%). Crystals
suitable for single-crystal X-ray diffraction were grown by sublimation
at 70 °C (0.1 mmHg): mp 83-84 °C; IR (Nujol, cm-1) 3054 (w), 1599
(s), 1545 (w), 1500 (s), 1260 (w), 1154 (m), 1070 (w), 1029 (m), 991 (w),
975 (m), 909 (w), 978 (w), 762 (s), 714 (m), 696 (s), 667 (m), 590 (w);
1H NMR (benzene-d6, δ) 5.42 (s, 1 H, Me2Im C-H), 2.36 (s, 3 H, Me2Im
CH3), 1.91 (s, 3 H, Me2Im CH3), 1.68 (s, 15 H, C5(CH3)5); 13C{1H} NMR
(benzene-d6, ppm) 121.53 (s, Me2Im C-CH3), 109.36 (s, Me2Im C-CH3),
96.15 (s, Me2Im ring C-H)), 84.31 (s, C5(CH3)5), 15.08 (s, Me2Im CH3),
12.37 (s, Me2Im CH3), 10.96 (s, C5(CH3)5). Anal. Calcd for C15H22N2-
Ru: C, 54.36; H, 6.69; N, 8.45. Found: C, 53.93; H, 6.73; N, 8.18.
The major finding described herein is documentation
of η5-imidazolato ligand coordination to ruthenium in
2 and 3. This is the first example of this bonding mode
(11) The cyclic voltammetry experiments were conducted in aceto-
nitrile containing 0.1 M tetrabutylammonium hexafluorophosphate
under the conditions previously described in ref 12.
(12) Perera, J . R.; Heeg, M. J .; Schlegel, H. B.; Winter, C. H. J . Am.
Chem. Soc. 1999, 121, 4536.

Communications
Organometallics, Vol. 19, No. 25, 2000 5265
to any metal.13 The previous lack of complexes with η5-
imidazolato ligands is particularly surprising, since 1
converts quantitatively to 2 upon heating, indicating
that the η5-imidazolato ligand is thermodynamically
preferred over the µ-η1:η1-imidazolato coordination mode.
The facile conversion of 1 to 2 argues that η5-imidazolato
ligand coordination may be possible in other low-valent
metal complexes. Additionally, similar µ-η1:η1- to η5-
imidazolato isomerizations may be thermodynamically
favorable in at least some of the many known mid to
late transition metal complexes containing µ-η1:η1-
imidazolato ligands.1,4 This raises the intriguing pos-
sibility that η5 coordination of histidine-derived ligands
can occur in metalloenzymes and that such ligation may
play a role in catalytic activity.2,3 In addition to the
above issues, Fu and co-workers have recently demon-
strated that azaferrocenes and azaruthenocenes can
function as nucleophilic catalysts in a variety of organic
transformations.14 Presumably the lone pairs of elec-
trons on the nitrogen atoms of 2 and 3 retain substantial
basicity, and complexes containing η5-imidazolato ligands
may serve as similar nucleophilic catalysts.
Ack n ow led gm en t. We are grateful to the National
Science Foundation (Grant No. CHE-9807269) for sup-
port of this research.
Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-
dures, analytical and spectroscopic data for 1-3, tables of
crystal data, structure solution and refinement, atomic coor-
dinates, bond lengths and angles, and anisotropic thermal
parameters for 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM000810J
(13) For unsuccessful attempts to prepare η5-imidazolato complexes
of iron, see: Seel, F.; Sperber, V. Angew. Chem., Int. Ed. Engl. 1968,
7, 70. Seel, F.; Sperber, V. J . Organomet. Chem. 1968, 14, 405.
(14) For leading references, see: Ruble, J . C.; Fu, G. C. J . Org. Chem.
1996, 61, 7230. Garrett, C. H.; Fu, G. C. J . Am. Chem. Soc. 1998, 120,
7479.