Isolable yet highly reactive cationic organoruthenium(II) amidinates, [Ru(η6-C6R6)(η-amidinate)] +X-, showing signs of coordinative unsaturation: Isoelectronic complexes of Ru(η5-C5Me5)(η-amidinate)

Article abstract of DOI:10.1246/cl.2001.954

The coordinatively unsaturated ruthenium(II) complexes [Ru(η⁶-C₆R₆)(η-amidinate)] ⁺X^- (R = H, Me, X = TFPB, PF₆), being isoelectronic with Ru(η⁵-C₅Me₅)(η-amidinate), have been isolated and characterized by spectroscopy and crystallography. A weak π-coordination of the amidinate ligands in the solid state was observed by X-ray crystallography. DFT calculations also suggest that such a coordination mode contributes to the stabilization of these complexes. These complexes behave as highly reactive transition metal Lewis acids in the reactions with various two-electron donor ligands.

Full text of DOI:10.1246/cl.2001.954

954
Chemistry Letters 2001
Isolable Yet Highly Reactive Cationic Organoruthenium(II) Amidinates, [Ru(η6-C6R6)(η-amidinate)]+X–,
Showing Signs of Coordinative Unsaturation: Isoelectronic Complexes of Ru(η5-C5Me5)(η-amidinate)
Taizo Hayashida, Yoshitaka Yamaguchi,# Karl Kirchner,†,## and Hideo Nagashima*
Institute of Advanced Material Study, Graduate School of Engineering Sciences, and CREST, Japan Science and Technology
Corporation (JST), Kyushu University, Kasuga, Fukuoka 816-8580.
†Institute of Inorganic Chemistry, Vienna University of Technology, Vienna, Austria
(Received June 11, 2001; CL-010544)
The coordinatively unsaturated ruthenium(II) complexes
[Ru(η6-C6R6)(η-amidinate)]+X– (R = H, Me, X = TFPB, PF6), being
isoelectronic with Ru(η5-C5Me5)(η-amidinate), have been isolated
and characterized by spectroscopy and crystallography. A weak π-
coordination of the amidinate ligands in the solid state was observed
by X-ray crystallography. DFT calculations also suggest that such a
coordination mode contributes to the stabilization of these complex-
es. These complexes behave as highly reactive transition metal
Lewis acids in the reactions with various two-electron donor ligands.
bearing the C6H6 ligand (4a–4c) led to decomposition of the product,
presumably due to facile abstraction of fluorine atoms from the count-
er anion by the highly electrophilic metal center.
Coordinatively unsaturated organometallic complexes have been
recognized as important intermediates in homogenous catalysis, and
their preparation has attracted much attention.1 In recent years, sever-
al stable coordinatively unsaturated ruthenium complexes have been
isolated and their reactivity towards various organic substrates has
been investigated.2 Factors stabilizing these complexes are steric hin-
drance by bulky ligands,2c,d strong σ-donation of N-ligands,2e,f or π-
donation of ligands.2b,g–i We have recently reported the successful
isolation of Ru(η5-C5Me5)(η-amidinate) (1) in which the amidinate
ligand effectively stabilizes the highly reactive metal center.3a
Amidinates usually act as 4-electron σ-donor ligands forming two
strong Ru–N bonds, and 1 is thus formally a coordinatively unsaturat-
ed 16-electron complex. In fact, the complexes 1 instantly react with
various two electron donor ligands or allylic halides.3a,b Although
one explanation why the amidinate ligand stabilizes the 16-electron
ruthenium species is strong σ-donation of the ligand as seen in
Kirchner’s complexes,2e,f possible π-coordination of the amidinate
ligand as an additional stabilizing factor has been suggested from X-
ray structures of 1, and has been evidenced very recently by the isola-
tion of Ru2(η5-C5Me5)2Br(µ2,η-amidinate).3c As an extension of
these studies, we were interested in the synthesis of isoelectronic
cationic complexes, [Ru(η6-C6R6)(η-amidinate)]+, where coordina-
tion of solvents or counter anions is absent. The highly reactive
[Ru(η6-C6R6)(η-amidinate)]+X– [R = H, X = TFPB (3a), or R = Me,
X = TFPB (3d) or PF6 (4d)] (TFPB = [tetrakis{3,5-bis(trifluo-
romethyl)phenyl}borate]) thus obtained is the first example of a
cationic ruthenium-amidinate complex showing signs of coordinative
unsaturation.
Characterization of the complexes 3a, 3d, and 4d was carried out
by spectroscopic methods and elemental analyses, which was support-
ed by the X-ray structure determination of 4d. The ORTEP drawing
of 4d is shown in Figure 1.5 This molecular structure indicates that the
complex 4d is monomeric. Neither the counter anion nor the solvent
was coordinated.6,7 In other words, the complexes 3a, 3d, and 4d exist
as an ion pair. The cationic ruthenium center should be coordinatively
unsaturated, if the formal electron count (amidinate = 4-electron
donor) is adopted. Of importance is the distance between the rutheni-
um atom and the central carbon of the amidinate, 2.431(5) Å, which
indicates existence of the bonding interaction; a similar Ru–C distance
of coordinatively saturated 2d is 2.570(10) Å. The shorter Ru–C (cen-
ter of the amidinate) distance actually provided the folded structure
shown in Figure 1, in which a plane consisting of the ruthenium atom
and two nitrogen atoms makes an angle of 31.4° with a plane of the
amidinate N–C–N moiety. This folded structure of 4d is similar to
that seen in the isoelectronic complexes 1, suggesting that possible π-
coordination of the amidinate ligand could mitigate the coordinatively
unsaturated nature of [Ru(η6-arene)(η-amidinate)]+.
Variable temperature NMR studies of [Ru(η6-arene)(η-amidi-
nate)]+ in CD2Cl2, in which the diastereotopic N-iPr groups of the
amidinate ligand is a convenient probe to observe stereochemical
change of the complexes, e.g. from a C2v-symmetric structure
shown in Scheme 1 to the folded structure in Figure 1, did not
afford evidence of π-interaction of the amidinate ligand. Thus, 1H
resonances due to methyl protons of the iPr groups in the amidinate
of 3b, 3d, and 4d appeared around δ 1.0–1.5 ppm as a single dou-
blet in the temperature range from –90 °C to room temperature,
showing that the complex has a C2v symmetric structure. These
results indicate that even if there exists the π-interaction of the
amidinate in the complexes 3 and 4 in solution, it is weak,
reversible, and unable to detect on the NMR time scale. The solu-
tion dynamics also suggesst that possible coordination of the count-
er anion or solvent cannot be ruled out in solution. However, that is
reversible and cannot be detected by the NMR technique.
As shown in Scheme 1, halogeno-precursors 2a–2d were
obtained by the reaction of [Ru(η6-C6R6)Cl2]2 with Li(amidinate) in
THF. These complexes were characterized by spectroscopic methods
and X-ray structure determination.3d Anion exchange of 2 with
NaTFPB in C6H5F or AgPF6 in CH2Cl2 gave the TFPB complexes 3
or PF6 complexes 4. The complexes 3a, 3d, and 4d having bulky
substituents on the amidinate or the arene ligand were isolable in
good yields (62–93%) as air and moisture sensitive blue solids.4 Less
stable 3b and 3c were characterized by NMR, but attempted isolation
of them has so far been unsuccessful. Preparation of PF6 homologues
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
955
amidinate)]+ was evidenced by its high reactivity towards various
donor ligands. These new results are interesting in comparison with
the chemistry of isoelectronic complexes, and further investigation on
reactivity of [Ru(η6-C6R6)(η- amidinate)]+ is currently in progress.
This work was partially supported by Grant-in-Aid for
Scientific Research (10450343, 13540374, and 1302090).
Dedicated to Prof. Hideki Sakurai on the occasion of his 70th
birthday.
References and Notes
#
Present address: Department of Materials Chemistry, Faculty of
Engineering, Yokohama National University, Yokohama, Kanagawa 240-
8501, Japan.
## Visiting Professor of Institute of Advanced Material Study, Kyushu
University. (Jan.–Apr., 2000)
The existence of a weak π-coordination of the amidinate ligand
is also evident from DFT calculations (B3LYP level) with the model
species [Ru(η6-C6H6){η-(ΝΗ)2CH}]+.8 The optimized structure
reveals a “folded” amidinate ligand with a bond distance between the
ruthenium atom and the central carbon of the amidinate ligand of 2.40
Å. The inversion of the folded Cs symmetric structure proceeds
through a planar C2v-symmetric transition state with a very small acti-
vation barrier of 3.3 kcal / mol which is easily overcome under exper-
imental conditions in line with the above NMR experiments.
1
2
For a review; R. Poli, Chem. Rev., 96, 2135 (1996).
a) K. G. Caulton, New. J. Chem., 18, 25 (1994) and references sited there-
in. b) T. J. Johnson, K. Folting, W. E. Streib, J. D. Martin, J. C. Huffman,
S. A. Jackson, O. Eisenstein, and K. G. Caulton, Inorg. Chem., 34, 488
(1995). c) M. Ogasawara, S. A. Macgregor, W. E. Streib, K. Folting, O.
Eisenstein, and K. G. Caulton, J. Am. Chem. Soc., 118, 10189 (1996). d)
D. Huang, W. E. Streib, J. C. Bollinger, K. G. Caulton, R. F. Winter, and
T. Scheiring, J. Am. Chem. Soc., 121, 8087 (1999). e) C. Gemel, K.
Mereiter, R. Schmid, and K. Kirchner, Organometallics, 16, 5601 (1997).
f) C. Gemel, J. C. Huffman, K. G. Caulton, K. Mauthner, and K.
Kirchner, J. Organomet. Chem., 594, 342 (2000). g) B. K. Campion, R.
H. Heyn, and T. D. Tilley, J. Chem. Soc., Chem. Commun., 1988, 278. h)
K. Mashima, H. Kaneyoshi, S. Kaneko, A. Mikami, K. Tani, and A.
Nakamura, Organometallics, 16, 1016 (1997). i) K.-J. Haack, S.
Hashiguchi, A. Fujii, T. Ikariya, and R. Noyori, Angew. Chem., Int. Ed.
Engl., 36, 285 (1997).
Similar to the complexes 1 the [Ru(η6-C6H6)(η-amidinate)]+
species are highly reactive towards various two electron donor lig-
ands. For instance, 3a instantly reacted with PPh3, pyridine, tBuNC,
CO, and ethylene in CH2Cl2 to give the corresponding adducts
[Ru(η6-C6H6)(η-amidinate)(L)]+TFPB– (5–9) in high yields.
(Scheme 2) However, the electronic character of the complexes 3 is
very different from their isoelectronic complexes 1. The CO stretch-
ing frequency of 8 [νCO; 2050 cm–1] is the highest among those of
previously reported half-sandwich ruthenium(II) carbonyl
complexes.9 This is in sharp contrast to the low νCO of [Ru(η5-
C5Me5){η-(NtBu)2CPh}(CO)] (1888 cm–1).3a Furthermore, rotation
of the ethylene ligand about the axis including the ruthenium atom
3
4
a) Y. Yamaguchi and H. Nagashima, Organometallics, 19, 725 (2000). b)
H. Kondo, Y. Yamaguchi, and H. Nagashima, Chem. Commun., 2000,
1075. c) H. Kondo, Y. Yamaguchi, and H. Nagashima, J. Am. Chem.
Soc., 123, 500 (2001). d) H. Hayashida, Y. Yamaguchi, and H.
Nagashima, unpublished results.
In a typical example, 2a (157 mg, 0.35 mmol) and NaTFPB (311 mg,
0.35 mmol) were dissolved in fluorobenzene (ca. 3 mL) at –40 oC. The
reaction mixture was allowed to warm to room temperature with stirring
for 30 min. The color of the solution changed from reddish brown to dark
blue. Insoluble sodium salts were removed by filtration. The resulting
dark blue solution was concentrated to ca. 1 mL in vacuo, and dry pentane
was added. At –35 °C, highly air sensitive blue solids of 3a were precipi-
tated (396 mg, 0.31 mmol, 90%). Anal. Calcd for C53H41N2BF24Ru: C,
49.98; H, 3.24; N, 2.20%. Found: C, 50.50; H, 3.59; N, 2.20%. 1H NMR
of 3a in CD2Cl2: δ 1.22 (s, 18H; C(CH3)3), 6.13 (s, 6H; C6H6), 7.01 (dt, J
= 7.1, 1.8 Hz, 2H; C6H5), 7.38 (t, J = 7.9 Hz, 2H; C6H5), 7.47 (tt, J = 7.4,
1.1 Hz, 1H; C6H5), 7.56 (s, 4H; (CF3)2C6H3), 7.73 (t, J = 2.4 Hz, 8H;
(CF3)2C6H3).
o
and center of the ethylene in 9 was not frozen even at –100 C in
CD2Cl2, while in Ru(η5-C5Me5){η-(NtBu)2CPh}(η2-CH2=CH2) no
dynamic behavior of ethylene was observed in THF-d8 at –60 °C.3a
The lower rotational barrier of ethylene in 9 than that in Ru(η5-
C5Me5)(η-amidinate)(η2-CH2=CH2) indicates a weaker π-back dona-
tion of the ruthenium to the ethylene ligand. In sharp contrast to the
reversible coordination to 1 having π-donor property, pyridine was
irreversibly bound to the cationic metal center of 3a. These data
show that [Ru(η6-C6H6)(η-amidinate)]+ is a typical transition metal
Lewis acid, whereas in sharp contrast, the isoelectronic Ru(η5-
C5Me5)(η-amidinate) is electron rich and a good π-donor.
5
6
Crystal data for 4d: C20H35N2F6PRu; MW = 559.54, triclinic, a =
11.2702(17), b = 11.638(2), c = 10.0284(14) Å, α = 114.848(8), β =
–
91.205(9), γ = 84.317(8)°, U = 1187.4(3) Å3, T = 223 K, space group, P1
(No. 2), Z = 2, µ(Mo Kα) = 0.783 mm–1, 5086 reflections measured, 5086
unique (Rint = 0.000), 4192 observed (>2σ), final residuals R1 = 0.0567,
wR2 = 0.1452 [I > 2σ(I)]; R1 = 0.0759, wR2 = 0.1584 (all data).
A possibility that the PF6 anion may interact with the ruthenium center in
the crystal structure of 4d is ruled out; even the shortest distance of Ru–F
bond, which is 3.59 Å, is much longer than that of the complex, Ru(CO)-
(NO)(PtBu2Me)(BF4) (2.2984(18) Å)7a or Ru2(O2CMe)4(Py)(BF4) (2.368
(7) Å), 7b the counter anion of which coordinates to the metal center.
a) M. Ogasawara, D. Huang, W. E. Streib, J. C. Huffman, N. Gallego-
Planas, F. Maseras, O. Eisenstein, and K. G. Caulton, J. Am. Chem. Soc.,
119, 8642 (1997). b) F. A. Cotton, J. Lu, and A. Yokochi, Inorg. Chim.
Acta, 275-276, 447 (1998).
In summary, we have achieved the successful isolation and struc-
ture elucidation of novel coordinatively unsaturated cationic ruthenium
complexes [Ru(η6-C6R6)(η-amidinate)]+X-, which are isoelectronic
species to Ru(η5-C5Me5)(η-amidinate). The π-coordination of the
amidinate ligand as an additional stabilizing factor was indicated by X-
ray structure of 4d, and supported by the DFT calculations. The coor-
dinatively unsaturated nature of the Lewis acidic [Ru(η6-C6R6)(η-
7
8
All calculations were performed using the Gaussian 98 software package
on the Silicon Graphics Power Challenge of the Vienna University of
Technology. The geometry and energy of the model complex and the
transition state were optimized at the B3LYP level with the
Stuttgart/Dresden ECP (SDD) basis set to describe the electrons of the
ruthenium atom. For all other atoms the 6-31G** basis set was
employed.
9
[Ru(η5-C5H5)(tmeda)(CO)]+TFPB–,2f 1961 cm–1; Ru(η5-C5Me5)-
(PCy3)(CO)Cl, 2g 1908 cm–1; Ru(η5-C5Me5)(PiPr2Ph)(CO)X (X = I, Br,
OR, NHPh),2b 1903–1930 cm–1; Ru(η6-C6Me6)(S2C6H4)(CO), 2h 1965
cm–1.