Novel coordinatively unsaturated bimetallic complexes, [(η5-C5ME5)Ru(μ2- iPrNC(Me)= NiPr)Ru(η5C5Me5)] +: A bridging amidinate ligand perpendicular to the metal-metal axis effectively stabilizes the highly reactive cationic diruthenium species

Article abstract of DOI:10.1021/ja016899h

Coordinatively unsaturated diruthenium complexes, [(η⁵-C₅Me₅)Ru(μ₂-ⁱPrNC(Me)=NⁱPr)Ru(η⁵-C₅Me₅)]⁺, of which crystallography revealed structures bearing a bridging amidinate ligand perpendicular to the Ru-Ru axis, were synthesized by anion exchange of [(η⁵-C₃Me₅(Ru(μ₂-ⁱPrNC(Me)=NⁱPr)Ru(η⁵-C₅Me₅)]⁺Br- by weakly coordinating anions. Variable-temperature NMR showed rapid motion of the bridging amidinate ligand. The coordinatively unsaturated nature of the cationic complexes provides their high reactivity toward a series of two electron donor ligands. Oxidative addition of molecular hydrogen occurred to give [(η⁵-C₅Me₅)Ru(μ₂-ⁱPrNC(Me)=NⁱPr)(μ-H)Ru(η⁵-C₅Me₅)(H)]⁺, which was isolated and characterized. Copyright

Full text of DOI:10.1021/ja016899h

Published on Web 01/03/2002
Novel Coordinatively Unsaturated Bimetallic Complexes,
[(η5-C5Me5)Ru(µ2-iPrNC(Me)dNiPr)Ru(η5-C5Me5)]+: A Bridging Amidinate
Ligand Perpendicular to the Metal-Metal Axis Effectively Stabilizes the Highly
Reactive Cationic Diruthenium Species
Hideo Kondo, Kouki Matsubara, and Hideo Nagashima*
Institute of AdVanced Material Study, Kyushu UniVersity, and CREST,
Japan Science and Technology Corporation (JST), Kasuga, Fukuoka 816-8580, Japan
Received August 21, 2001
In our previous paper, we reported a diruthenium complex
bearing a bridging amidinate ligand in an unusual bonding mode.1
Bridging amidinates, of which each nitrogen atom is bound to a
different metal center, have been well investigated for their effective
stabilization of a wide variety of dinuclear transition metal
complexes.2,3 In these complexes, the bridging amidinate ligand is
located parallel to the metal-metal bond.3 In sharp contrast, the
Figure 1. The ORTEP drawings of 2d with thermal ellipsoids drawn at
the 50% probability level. The B(C6F5)4 anion is omitted for clarity. θ1, θ2,
and θ3 shown in the figure and the table in the inset are torsion angles
(deg). Representative bond distances (Å) are as follows. 2d: Ru1-Ru2 )
2.9198(3), Ru1-N1 ) 2.097(3), Ru1-N2 ) 2.098(3), Ru2-N1 ) 2.101-
(3), Ru2-N2 ) 2.119(3), Ru2-C1 ) 2.173(3), N1-C1 ) 1.387(4), N2-
C1 ) 1.387(4).
bridging amidinate in our novel complex is perpendicular to the
metal-metal axis, in which one ruthenium center bonds with two
nitrogen atoms, whereas the other ruthenium atom is bound to the
face of the amidinate ligand in the π-allyl-like coordination mode.1
Of keen interest in this new bonding mode is how effectively the
bridging amidinate can stabilize the dinuclear framework. If it is
effective, our novel complex would be a good starting point for
exploration of various organometallic reactions of the dinuclear
systems including catalysis.4,5 In this paper, we wish to report
synthesis of isolable yet highly reactive coordinatively unsaturated
diruthenium complexes, [(η5-C5Me5)Ru(µ2-iPrNC(Me)dNiPr)Ru-
(η5-C5Me5)]+ (2a-d) from [(η5-C5Me5)Ru(µ2-iPrNC(Me)dNiPr)-
Ru(Br)(η5-C5Me5)] (1). Of extreme importance in this outcome is
that the “perpendicularly coordinated” bridging amidinate ligand
in the new cationic compounds strongly binds the two metal centers
even in the reactive coordinatively unsaturated state,6,7 and thereby
addition of two electron donor ligands or bimetallic activation of
molecular hydrogen is actually achieved without decomposing the
dinuclear framework.
Chart 1
Scheme 1
Treatment of 1 with either AgBF4, AgPF6, AgSbF6, or NaB-
(C6F5)4 in CH2Cl2 for 1 h afforded the corresponding [(η5-C5-
Me5)Ru(µ2-iPrNC(Me)dNiPr)Ru(η5-C5Me5)]+(X)- (2a-d) in 75-
99% yield as shown in Scheme 1. The crystal structures of the PF6
(2a)8 and B(C6F5)4 (2d)8 derivatives clearly demonstrate that the
counteranions are outside of the coordination sphere. No coordinat-
ing solvent was visible. Thus, these compounds are coordinatively
unsaturated in the solid states. Representing this, the ORTEP
drawing of 2d is shown in Figure 1. The compounds have a Cs-
symmetric structure with a mirror plane including centers of two
C5Me5 ligands, Ru1, Ru2, and C1 and its adjacent methyl carbon.
The Ru1-Ru2 distance in each complex is in the range of 2.91 (
0.01 Å, indicating the existence of a Ru-Ru single bond. Each of
the two Ru atoms is bonded to both of two nitrogen atoms, and
there is a relatively long Ru2-C1 bond (2.17-2.19 Å). This
coordination mode of the bridging amidinate ligand is similar to 1
having a Ru-Br bond; however, the longer Ru2-C1 bond distance
(1: Ru2-C1 ) 2.104(4) Å)1 and larger θ3 angle indicate weaker
π-coordination of the bridging amidinate ligand.
whereas four methyl moieties of the isopropyl substituent of the
bridging amidinate group were magnetically equivalent to give a
sharp doublet in the temperature range from -80 to 35 °C. This is
in sharp contrast to the fact that the Cs-symmetric structure of 1,
which is supported by crystallography, provided two singlets and
two doublets due to the C5Me5 and methyl moieties of the isopropyl
groups in the µ2-amidinate group. A singlet for C5Me5, as well as
one doublet for the isopropyl group in the NMR down to -80 °C,
indicates rapid motion (time averaged C2V structure) of the amidinate
ligand in 2 in solution. In other words, replacement of the
coordinating Br atom in 1 by other counteranions resulted in
formation of cationic species shown in A of Scheme 1, which are
interconverted with B in CD2Cl2. This interconversion is very rapid
in the NMR time scale to provide the spectra suggesting formally
C2V-symmetric structures.9
Isolation of coordinatively unsaturated diruthenium complexes
stimulated us to explore reactivity of these compounds with various
substrates. The complex 2d instantly reacted with CO (1 atm) or
isonitriles in CH2Cl2 to give the corresponding adducts quantita-
tively. Striking evidence showing the coordinatively unsaturated
nature of these complexes is successful oxidative addition of H2.
Variable-temperature 1H NMR spectra of 2a-d in CD2Cl2
showed that two C5Me5 groups were visible as a sharp singlet,
9
534 VOL. 124, NO. 4, 2002 J. AM. CHEM. SOC.
10.1021/ja016899h CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This work was partially supported by the
Japan Society for the Promotion of Science (Grants-in-Aid for
Scientific Research 10450343, 13540374, 13029090).
Supporting Information Available: Text giving experimental
details and analytical data on new complexes and tables of X-ray
structural information, including data collection parameters, positional
and thermal parameters, and bond lengths and angles for complexes
2a, 2d, 2e, and 5d (PDF). A crystallographic file in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. The ORTEP drawing of 5d with the thermal ellipsoid drawn at
the 50% probability level. The B(C6F5)4 anion is omitted for clarity.
Representative bond distances (Å) are as follows. Ru1-Ru2 ) 2.7741(4),
Ru1-N1 ) 2.105(3), Ru1-N2 ) 2.098(3), Ru1-H1 ) 1.59(5), Ru1-H2
) 1.86(5), Ru2-H2 ) 1.68(5), Ru2-N1 ) 2.135(3), Ru2-N2 ) 2.144-
(3), Ru2-C1 ) 2.113(3), N1-C1 ) 1.380(4), N2-C1 ) 1.384(4).
References
(1) Kondo, H.; Yamaguchi, Y.; Nagashima, H. J. Am. Chem. Soc. 2001, 123,
500-501.
(2) (a) For reviews of amidinate, see: Barker, J.; Kilner, M. Coord. Chem.
ReV. 1994, 133, 219-300. Edelmann, F. T. Coord. Chem. ReV. 1994,
137, 403-481. (b) For organoruthenium amidinates, see: Yamaguchi,
Y.; Nagashima, H. Organometallics 2000, 19, 725-727.
Scheme 2
(3) For a review on dinuclear organotransition metal amidinates, see: Cotton,
F. A. Inorg. Chem. 1998, 37, 5710-5720.
(4) Recent reviews on metal cluster, see: (a) Su¨ss-Fink, G.; Meister, G. AdV.
Organomet. Chem. 1993, 35, 41-133. (b) Catalysis by Di- and Polynulear
Metal Cluster Complexes; Adams, R. D., Cotton, F. A., Eds.; VCH: New
York, 1998. (c) Metal Clusters in Chemistry; Braunstein, P., Oro, L. A.,
Raithby, P. R., Eds.; VCH: New York, 1999.
(5) For recent progress on reactive diruthenium compounds, see: (a) Haines,
R. J. ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 7,
pp 625-681. (b) Ko¨elle, U. Chem. ReV. 1998, 98, 1313-1334. (c) Suzuki,
H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Fukushima, M.; Tanaka, M.;
Moro-oka, Y. Organometallics 1994, 13, 1129-1146. (d) Iwasa, T.;
Shimada, H.; Takami, A.; Matsuzaka, H.; Ishii, Y.; Hidai, M. Inorg. Chem.
1999, 38, 2851-2859. (e) Braun, T.; Laubender, M.; Gevert, O.; Werner,
H. Chem. Ber./Recl. 1997, 130, 559-563.
(6) Since the “perpendicularly” coordinated amidinate ligand is a 7 electron
donor to the neutral diruthenium moiety, the total electron count [sum of
two Cp* (5e × 2), Br (1e), µ2-amidinate (7), and Ru-Ru (2e)] of 1 is 36.
In contrast, that of 2a-2d is 34, if the ruthenium-ruthenium bond is
single. A vacant site for the coordination of the ligand or oxidative addition
of H2 is on the Ru(1) atom.
(7) Recent progress on coordinatively unsaturated mononuclear complexes:
(a) Poli, R. Chem. ReV. 1996, 96, 2135-2204. (b) Caulton, K. G. New J.
Chem. 1994, 18, 25-41. (c) Tenorio, M. J.; Mereiter, K.; Puerta, H. C.;
Valerga, P. J. Am. Chem. Soc. 2000, 122, 11230-11231. (d) Gemel, C.;
Huffman, J. C.; Caulton, K. G.; Mauthner, K.; Kirchner, K. J. Organomet.
Chem. 2000, 594, 342-353.
(8) Crystal data for 2a: monoclinic space group C2/c, a ) 31.5792(15) Å, b
) 15.1007(5) Å, c ) 14.4272(7) Å, â ) 110.9710(10)°, V ) 6424.2(5)
Å3, Z ) 8, R1 ) 0.0401, wR2 ) 0.0956 (I > 2σ(I)), R1 ) 0.0579, wR2 )
0.1046 (all data). 2d: monoclinic space group P21/c, a ) 14.1770(5) Å,
b ) 15.2535(4) Å, c ) 23.5483(8) Å, â ) 92.7110(10)°, V ) 5086.6(3)
Å3, Z ) 4, R1 ) 0.0377, wR2 ) 0.1070 (I > 2σ(I)), R1 ) 0.0481, wR2 )
0.1221 (all data). Analytical as well as spectroscopic data of 2a-d are
listed in the Supporting Information.
In a typical example, treatment of 2d with H2 (1 atm) in CH2Cl2
resulted in rapid color change of the solution from purple to yellow.
From the reaction mixture, the corresponding oxidative adduct 5d
was isolated quantitatively. NMR spectra suggest the Cs-symmetric
structure of 5d, showing inequivalent C5Me5 groups and two sets
of signals due to the isopropyl groups of the bridging amidinate
ligand. Two Ru-H peaks were seen as sharp singlets at δ -5.60
and -7.77 ppm. The T1 value of these two Ru-H signals at 293 K
was 4.060 (δ -5.60 ppm) and 2.854 s (δ -7.77 ppm), suggesting
that these are not nonclassical hydrides.10 The IR spectrum of 5d
showed a νRu-H absorption at 1956 cm-1. These assignments were
2
supported by H NMR spectra of 5d-d2, which was synthesized
from 2d with D2, showing Ru-D signals at δ -5.44 and -7.72
ppm. The IR spectrum of 5d-d2 resulted in the disappearance of
the absorption at 1956 cm-1 by the isotopic shift. The Cs-symmetric
diruthenium dihydride structure was proved by crystallographic
analysis of 5d,11 in which arrangement of the C5Me5 groups and
the bridging amidinate are similar to those seen in 2d. Of particular
interest is that one hydride is located at the terminal position,
whereas the other bridges the Ru-Ru bond. The terminal hydride
is in the trans position to the bridging hydride as shown in Figure
2. The existence of the bridging hydride caused the relatively short
Ru-Ru bond (2.7741(4) Å), which is 0.15 Å shorter than that of
2d.
In summary, we have accomplished the preparation of isolable
yet highly reactive “cationic coordinatively unsaturated” diruthe-
nium compounds bearing a bridging amidinate ligand and weakly
coordinating counteranions,12 which is a rare example of dinuclear
coordinatively unsaturated compounds.13 The µ2-amidinate ligand
in the unusual coordination mode compared with other dinuclear
transition amidinates plays an essential role in stabilizing the co-
ordinatively unsaturated bimetallic centers, though further investiga-
tion is required to clarify how the perpendicular µ2-amidinate ligand
stabilizes the bimetallic moiety in the coordinatively unsaturated
state. We believe that these results are an important clue to the
development of new reactions and catalysis of diruthenium com-
pounds involving activation of various substrates by coordinatively
unsaturated organodiruthenium species, and further investigation
on the reactivity of these new compounds is actively in progress.
(9) Although a possibility that reversible coordination of the solvent is involved
in the interconversion cannot be completely excluded, it was confirmed
that there was no sign of coordination of the solvent in the NMR spectra
at -80 °C.
(10) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126-
4133.
(11) Crystal data for 5d: monoclinic space group P21/c, a ) 14.1154(3) Å, b
) 15.3888(3) Å, c ) 23.4856(7) Å, â ) 93.4660(9)°, V ) 5092.2(2) Å3,
Z ) 4, R1 ) 0.0417, wR2 ) 0.1248 (I > 2σ(I)), R1 ) 0.0481, wR2
0.1316 (all data).
)
(12) It is known that a triflate anion can be coordinated to the transition metal.14
The crystal structure of the OTf complex, 2e, which was available by
treatment of 1 with AgOTf, showed that the Ru-OTf distance is 2.360
(2) Å, indicating a weak Ru-O bonding interaction, in sharp contrast to
the fact that the Ru-counteranion distances are over 3.9 Å in 2a or 2d.
The Ru-C1 distance in 2e is 2.126 (3) Å, and θ1, θ2, and θ3 are 106.7,
153.1, and 100.2°, respectively; these values are between those of 1 and
2a-d. The Ru-O bonding interaction is only seen in the solid states,
and easily cleaved in solution leading to facile “swing” of the bridging
amidinate ligand (VT studies). Furthermore, reaction of 2e with CO,
isonitriles, or H2 proceeded in a fashion similar to that of 2d to give the
corresponding CO (3e), 2,4,6-Me3C6H2NC (4e), and dihydride (5e)
complexes in 87, 95, and 97% yield. The results are summarized in the
Supporting Information.
(13) Only one example reported: Matsuzaka, H.; Qu¨, J.-P.; Ogino, T.; Nishio,
M.; Nishibayashi, Y.; Ishii, Y.; Uemura, S.; Hidai, M. J. Chem. Soc.,
Dalton Trans. 1996, 4307-4312.
(14) For an example of the Ru-η1-OTf complex, see: Ontko, A. C.; Houlis, J.
F.; Schnabel, R. C.; Roddick, D. M. Organometallics 1998, 17, 5467-
5476 and references therein.
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