(η5-C5Me5)Ru(amidinate): Highly Reactive Ruthenium Complexes Formally Bearing 16 Valence Electrons Showing Signs of Coordinative Unsaturation

Article abstract of DOI:10.1021/om990872d

Novel coordinatively unsaturated ruthenium complexes, (η⁵-C₅Me₅)Ru(amidinate), were synthesized from [(η⁵-C₅Me₅)Ru(OMe)]₂ and lithium amidinates, which exist as monomers in both

Full text of DOI:10.1021/om990872d

Organometallics 2000, 19, 725-727
725
(η5-C5Me5)Ru (a m id in a te): High ly Rea ctive Ru th en iu m
Com p lexes F or m a lly Bea r in g 16 Va len ce Electr on s
Sh ow in g Sign s of Coor d in a tive Un sa tu r a tion
Yoshitaka Yamaguchi and Hideo Nagashima*
Institute of Advanced Material Study and CREST, J apan Science and Technology Corporation
(J ST), Kyushu University, Kasuga, Fukuoka 816-8580, J apan
Received November 1, 1999
Sch em e 1
Summary: Novel coordinatively unsaturated ruthenium
complexes, (η5-C5Me5)Ru(amidinate), were synthesized
from [(η5-C5Me5)Ru(OMe)]2 and lithium amidinates,
which exist as monomers in both solution and solid
states and are highly reactive toward two-electron
ligands.
Studies on structures and reactivity of coordinatively
unsaturated complexes have received much attention
from organometallic chemists in terms of intermediates
in transition-metal-catalyzed organic reactions. Intro-
duction of sterically bulky ligands is a method to
stabilize compounds having a 16-electron configuration
or fewer,1,2 whereas in complexes having a Ru-X moiety
(X ) OR, SR) electron donation from the ligand is an
alternative method to stabilize the reactive metal
center.3,4 In contrast, [(η5-C5Me5)Ru(TMEDA)]+ was
effectively stabilized by the hard σ-donor character of
TMEDA.5 In this paper, we wish to report novel isolable
yet highly reactive ruthenium complexes, (η5-C5Me5)-
Ru(amidinate). In almost all transition-metal amidi-
nates so far reported,6 the amidinate ligand generally
acts as a bidentate four-electron donor through two
metal-nitrogen bonds. In this context, (η5-C5Me5)Ru-
(amidinate) is expected to have 16 valence electrons and
to be coordinatively unsaturated.
7a
Treatment of [(η5-C5Me5)Ru(OMe)]2 with 2 equiv of
quantitatively by reaction of 4 equiv of 1a with [(η5-C5-
Me5)RuCl]4.7b The amidinate complexes are highly
sensitive to air and moisture in both solution and solid
states, being reactive with various two-electron-donor
ligands. For example, 2a ,b instantly reacted with CO
to give rise to addition of a CO molecule to the
ruthenium atom, as shown in Scheme 1. The structure
of the carbonyl complexes, which follows the 18-electron
rule, was determined by spectral data of 3a ,b; this is
supported by the crystal structure of 3b, as shown in
Figure 2 (left).9 The formation of these coordinatively
saturated CO complexes provides supporting evidence
that the purple solids 2a -c are coordinatively unsatur-
ated (η5-C5Me5)Ru(amidinate).
lithium amidinates 1a -c in THF afforded diamagnetic
purple solids in good yields, of which spectroscopic data
are in accord with (η5-C5Me5)Ru(amidinate) (2a -c)
(Scheme 1).8 The complex 2a was alternatively formed
(1) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.;
Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc. 1996, 118, 10189-
10199.
(2) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J . Chem. Soc., Chem.
Commun. 1988, 278-280.
(3) (a) Caulton, K. G. New J . Chem. 1994, 18, 25-41. (b) J ohnson,
T. J .; Folting, K.; Streib, W. E.; Martin, J . D.; Huffman, J . C.; J ackson,
S. A.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1995, 34, 488-499.
(4) (a) Mashima, K.; Kaneyoshi, H.; Kaneko, S.; Mikami, A.; Tani,
K.; Nakamura, A. Organometallics 1997, 16, 1016-1025. (b) Mashima,
K.; Kaneko, S.; Tani, K.; Kaneyoshi, H.; Nakamura, A. J . Organomet.
Chem. 1997, 545-546, 345-356.
(5) (a) Gemel, C.; Mereiter, K.; Schmid, R.; Kirchner, K. Organo-
metallics 1997, 16, 5601-5603. (b) Gemel, C.; Sapunov, V. N.; Mereiter,
K.; Ferencic, M.; Schmid, R.; Kirchner, K. Inorg. Chim. Acta 1999, 286,
114-120.
(6) Review of the amidinate ligands: (a) Barker, J .; Kilner, M. Coord.
Chem. Rev. 1994, 133, 219-300. (b) Edelmann, F. T. Coord. Chem.
Rev. 1994, 137, 403-481. To our knowledge, the only example in which
the amidinate ligand acts as more than a four-electron donor was seen
in a triosmium cluster: Burgess, K.; Holden, H. D.; J ohnson, B. F. G.;
Lewis, J . J . Chem. Soc., Dalton Trans. 1983, 1199-1202.
(7) (a) Koelle, U.; Kossakowski, J . J . Organomet. Chem. 1989, 362,
383-398. (b) Fagan, P. J .; Ward, M. D.; Calabrese, J . C. J . Am. Chem.
Soc. 1989, 111, 1698-1719.
(8) In a typical example, [(η-C5Me5)Ru(OMe)]2 (82 mg, 0.15 mmol)
was treated with Li[MeC(NiPr)2] (46 mg, 0.31 mmol), which was
prepared from MeLi and N,N′-diisopropylcarbodiimide, in THF at -78
°C, and the mixture was warmed to room temperature. After 1 h, the
solvents were removed in vacuo, and the residue was extracted with
pentane. Concentration of the extracts gave 2a as a purple air-sensitive
solid (106 mg, 0.28 mmol, 90% yield). Further purification can be
achieved by sublimation (50 °C, 10-3 mmHg). NMR of 2a in THF-d8:
1H, δ 1.16 (d, J ) 6.4 Hz, 12H, CH(CH3)2), 1.35 (s, 3H, CCH3), 1.71 (s,
15H, C5(CH3)5), 3.06 (sep, J ) 6.4 Hz, 2H, CH(CH3)2); 13C, δ 10.27,
12.99, 25.43, 48.01, 70.00, 168.08. Other data are given in the
Supporting Information.
10.1021/om990872d CCC: $19.00 © 2000 American Chemical Society
Publication on Web 02/08/2000

726 Organometallics, Vol. 19, No. 5, 2000
Communications
monomer in solution.14,15 Thus, three possible struc-
tures, a monomeric structure having a 16-electron
configuration (A), a dimer (B), and a monomer stabilized
by a solvent (C), should be considered as the structure
of (η5-C5Me5)Ru(amidinate) (Figure 1). 1H and 13C NMR
resonances derived from methyl groups in the isopropyl
moiety of 2a are good indicators to determine the
structure in solution. A partial structure of A consisting
of the center of the C5Me5 ligand, the Ru atom, and the
amidinate ligand has C2v symmetry, which makes the
1H and 13C resonances from the methyl group equiva-
lent. In contrast, this partial structure in B and C has
Cs symmetry, leading to the appearance of two inde-
pendent methyl signals. In fact, the CO complex 3a has
a structure analogous to that of C, showing two in-
F igu r e 1.
The (η5-C5Me5)Ru(amidinate) complexes are generally
reactive with other two-electron-donor ligands. Repre-
sentative examples are summarized in Scheme 1.
Treatment of 2a with 2,4,6-Me3C6H2NC or PPh3 af-
forded 4a or 5a , respectively, in quantitative yields. A
mixture of 2a and pyridine afforded an 1H NMR
spectrum assignable to the pyridine complex 6a at -90
°C.10 Reversible and irreversible coordinations were seen
in the reactions with olefins. Treatment of 2a with
TCNE afforded the stable complex 7a . In contrast,
formation of the ethylene complex 8a was evidenced by
spectroscopy when a THF solution of 2a was allowed to
stand under an ethylene atmosphere, but 2a was
regenerated quantitatively when ethylene was removed
from the reaction mixture under reduced pressure.11
The π-acceptor strength of the ligands decreases in the
order CO > RNC > PR3 > pyridine,12 whereas TCNE
> ethylene. The above results indicate that better
π-acceptors are tightly coordinated with (η5-C5Me5)Ru-
(amidinate); in other words, (η5-C5Me5)Ru(amidinate)
species is a good donor.13
1
equivalent methyl signals. H and 13C NMR spectra of
2a showing only a single methyl signal in a noncoordi-
nating solvent, methylcyclohexane-d14, from -110 °C to
room temperature ruled out the existence of the struc-
ture B and the possibility that reversible coordination
of the solvent was crucial for stabilization of the complex
via the structure C. Thus, 2a apparently exists as a
monomeric structure such as A in hydrocarbon solution.
The crystal structure of 2b provides additional evidence
that these complexes exist as monomers without coor-
dination of the solvents, though there is an unprec-
edented difference in the crystal structures from the
structure A: the center of the Cp* ring, the Ru atom,
and the two nitrogen atoms lie on the same plane,
whereas a plane consisting of the Ru atom and two
nitrogen atoms makes an angle of 48.9(4)° with a plane
of the amidinate N-C-N moiety.16 The ORTEP draw-
ing is illustrated in Figure 2 (right).
In summary, we have achieved the first successful
isolation of (η5-C5Me5)Ru(amidinate) complexes, which
exist as monomers in both solution and solid states and
are highly reactive with two-electron-donor ligands such
as CO and olefins. To our knowledge, this is the first
case where amidinate ligands6 play an important role
in effectively stabilizing a coordinatively unsaturated
metal center.17 Further investigation on the structures
and reactivity of (η5-C5Me5)Ru(amidinate) is in progress.
The next question is the structure of (η5-C5Me5)Ru-
(amidinate) complexes, which showed signs of coordi-
native unsaturation as described above. A related
compound, (η5-C5Me5)Ru(acetylacetonate), formally bear-
ing 16 valence electrons and reactive with several
2-electron donors was reported by Koelle and co-work-
ers, who revealed that this complex existed as a dimer
in the solid state and was in equilibrium with a
(9) In a typical experimental procedure, 2b (82 mg, 0.18 mmol) was
treated with CO (1 atm) at -78 °C. The mixture was warmed to room
temperature. After 30 min, the solvents were removed in vacuo, and
the resulting yellow solid was recrystallized from hexane at -20 °C to
give 3b as yellow crystals (45 mg, 0.091 mmol, 45% yield). 1H NMR in
THF-d8: δ 0.90 (s, 18H), 1.77 (s, 15H), 7.17-7.31 (m, 5H). IR (KBr):
1888 cm-1. Other data are summarized in the Supporting Information.
Crystal data for 3b: monoclinic, space group P21/n (No. 14), a ) 18.958-
(3) Å, b ) 15.336(2) Å, c ) 8.775(2) Å, â ) 90.296(13)°, V ) 2551.4(8)
Å3, Z ) 4; R1 ) 0.0520, wR2 ) 0.1640 (I > 2σ(I)); R1 ) 0.0993, wR2
) 0.2819 (all data).
(14) Koelle, U.; Kossakowski, J .; Raabe, G. Angew. Chem., Int. Ed.
Engl. 1990, 29, 773-774. The dimeric structure of (η5-C5Me5)Ru(acac)
was reported by reinterpretation of the published crystallographic
data: Smith, M. E.; Hollander, F. J .; Andersen, R. A. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1294.
(15) (a) Koelle, U.; Rietmann, C.; Raabe, G. Organometallics 1997,
16, 3273-3281. (b) Koelle, U. Chem. Rev. 1998, 98, 1313-1334.
Recently Koelle briefly mentioned that a Cp*RuL (L ) nitrogen-
containing conjugated ligand) may be a monomeric 16e complex (p 1331
in ref 15b).
(16) Crystal data for 2b: orthorhombic, space group Pnma (No. 62),
a ) 19.171(3) Å, b ) 15.245(3) Å, c ) 8.163(3) Å, V ) 2385.7(9) Å3; Z
) 4, R1 ) 0.0347, wR2 ) 0.0920 (I > 2σ(I)); R1 ) 0.0596, wR2 ) 0.1074
(all data). The molecular structure of 2c was monomeric and solvent-
free. The angle θ is 29.7(2)°. See the Supporting Information.
(17) The reason the amidinate ligands can effectively stabilize the
coordinatively unsaturated metal center requires further investigation;
the amidinate ligands are not bulky enough to protect the reactive
metal center and are not hard σ-donors because of N-CdN conjuga-
tion. Weak coordination of π-electrons on the amidinate ligand may
contribute to the stabilization. The folded structures of 2b and 2c
provided Ru-C (center of the amidinate ligand) distances of 2.336(5)
(2b) and 2.489(2) Å (2c), which are apparently shorter than the
corresponding distances in the 18-electron complex 3b (2.596(6) Å). If
the conjugate π-electrons in the amidinate ligand act as a π-donor to
mitigate the coordinative unsaturation of 16-electron (η-C5Me5)Ru-
(amidinate) species, this additional stabilizing effect would be respon-
sible for the shorter Ru-C bond distances, though it is so weak as to
be undetectable by spectroscopy in solution. Although the coordination
ability of π-electrons on the amidinate ligand to transition metals was
proposed in a review, clear evidence to suggest it is rare.6a
(10) At -90 °C in THF-d8, the 1H NMR spectrum of 6a showed two
independent methyl signals derived from the methyl groups of the
isopropyl group of the amidinate ligand. Signals assignable to the
coordinated pyridine were visible at δ 7.20 (t), 7.64 (t), and 8.67 (d)
(the corresponding signals from uncoordinated pyridine under the same
conditions were δ 7.36, 7.77, and 8.58, respectively). The only signals
of uncoordinated pyridine and 2a were seen at room temperature. At
-70 to -20 °C, broadening of the signals was observed. We consider
that these spectral features are derived from reversible coordination
of pyridine to 2a , and the details are under study.
(11) 1H NMR resonances of the coordinated ethylene were seen at
δ 2.15 (d, J ) 11.2 Hz) and 2.65 (d, J ) 11.2 Hz) at -100 °C in THF-
d8. At room temperature, the peak from the coordinated ethylene was
visible as a broad singlet at δ 2.51, due to the rapid rotation of the
ethylene ligand.
(12) Elschenbroich, Ch.; Salzer, A. Organometallics; VCH: Wein-
heim, Germany, 1992; p 230.
(13) This is supported by a typically low CO stretching frequency
of 3a or 3b (1901 cm-1 for 3a and 1888 cm-1 for 3b, cf. (η5-C5Me5)Ru-
(acac)(CO)14 1915 cm-1, (η5-C5Me5)Ru(PCy3)(CO)Cl2 1908 cm-1, (η5-C5-
Me5)Ru(PiPr2Ph)(CO)X (X ) I, Br, Cl, OR, NHPh)3b 1903-1930 cm-1
)
due to strong back-donation from the (η5-C5Me5)Ru(amidinate) frag-
ment to the CO ligand.

Communications
Organometallics, Vol. 19, No. 5, 2000 727
F igu r e 2. ORTEP drawings of the 18-electron complex 3b (left) and the “16-electron complex” 2b (right) with thermal
ellipsoids drawn at the 50% probability level. Four atoms, Ru1, N1, N2, and C20, lie on the same plane in 3b. In contrast,
2b has the “folded” structure, as described in the text. Representative bond distances (Å) and angles (deg) are as follows.
3b: Ru1-N1 ) 2.133(5), Ru1-N2 ) 2.135(5), Ru1-C20 ) 2.596(6), Ru1-C11 ) 1.828(7), N1-C20 ) 1.334(8), N2-C20
) 1.319(8); N1-Ru1-N2 ) 61.1(2), N1-Ru1-C11 ) 96.1(3), N2-Ru1-C11 ) 93.9(3), N1-C20-N2 ) 109.8(5). 2b: Ru1-
N1(N1#) ) 2.073(3), Ru1-C7 ) 2.336(5), N1-C7 ) 1.347(4); N1-Ru1-N1# ) 64.4(2), N1-C7-N1# ) 110.1(4).
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
Ack n ow led gm en t. We are grateful to Prof. H.
mental details and analytical data on new complexes and
tables of X-ray structural data, including data collection
parameters, positional and thermal parameters, and bond
Suzuki, Dr. M. Oshima, and Mr. Y. Ohki (Tokyo Tech)
and Dr. Kouki Matsubara (Kyushu University) for help
with the X-ray analyses. Part of this work was finan-
cially supported by the J apan Society for the Promotion
of Science (Grant-in-Aid for Scientific Research
10450343).
lengths and angles for complexes 2b,c, 3b, and 7a . This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM990872D