A bimetallic Ru2Pt complex containing a trigonal-planar μ3-carbido ligand: Formation, structure, and reactivity relevant to the Fischer-Tropsch process

Article abstract of DOI:10.1021/ja907387w

(Figure Presented) The bimetallic Ru₂Pt complex [(Cp*Ru)₂(μ₂-NHPh)(μ₂-H) (μ₃-C)PtMe(PMe₃)₂][OTf] (3; Cp* = E⁵-C₅Me₅) containing a planar three-co

Full text of DOI:10.1021/ja907387w

Published on Web 11/30/2009
A Bimetallic Ru2Pt Complex Containing a Trigonal-Planar µ3-Carbido Ligand:
Formation, Structure, and Reactivity Relevant to the Fischer-Tropsch Process
Shin Takemoto,* Hidenobu Morita, Kenji Karitani, Hideki Fujiwara, and Hiroyuki Matsuzaka*
Department of Chemistry, Graduate School of Science, Osaka Prefecture UniVersity, Gakuen-cho 1-1,
Naka-ku, Sakai, Osaka 599-8531, Japan
Received September 1, 2009; E-mail: takemoto@c.s.osakafu-u.ac.jp; matuzaka@c.s.osakafu-u.ac.jp
1
In contrast to the common occurrence of trigonal sp2-hybridized
carbon atoms in covalent substances, transition-metal complexes
that contain a planar three-coordinate carbon as a ligand are
extremely rare.1,2 This essentially unexplored class of complexes
should provide an interesting analogy to metal carbene3 and
carbyne4 complexes [i.e., LnMdCR2 and LnM(µ2-CR)MLn, respec-
tively] and serve as useful models of transition-metal surface carbide
species, as exemplified by other complexes of low-coordinate
carbides.1,2,5-10 Herein we report the synthesis and structure of a
bimetallic Ru2Pt complex that contains a trigonal-planar µ3-carbido
ligand generated by a double C-H bond activation of a cluster-
bound µ2-methylene ligand.9,10 The resulting µ3-carbido ligand
exhibited a reactivity relevant to the Fischer-Tropsch process,11-14
producing a µ2-ethylidene ligand via coupling with nearby methyl
and hydride moieties.
and 8 Hz, JPtC ) 858 Hz). This 13C chemical shift is similar to
those reported for the µ3-C ligand in [KCMo(NRAr)3]2 [δ 482.8;
R ) CMe(CD3)2, Ar ) 3,5-C6H3Me2]1 and the µ2-CH ligand in
[Cp2Fe2(CO)3(µ2-CH)][PF6] (δ 490.2).4a A signal due to the Pt-Me
carbon was observed at δ 3.7 (2JPC ) 76 and 8 Hz, JPtC ) 557
1
1
Hz). In the H NMR spectrum of 3, resonances attributable to the
Ru-bound hydride and the amido N-H proton were observed at δ
-18.7 (s) and 6.17 (br s), respectively.
Treatment of the cluster [(Cp*Ru)2(µ2-CH2)(µ3-NPh)Pt(PMe3)2]
(1; Cp* ) η5-C5Me5)15 with MeOTf in Et2O afforded the cationic
methyl derivative 2 in 85% yield (Scheme 1). Single-crystal X-ray
diffraction (XRD) analysis of 2 revealed the terminal coordination
of the methyl ligand to a Ru center. Although complex 1 showed
no signs of reaction when heated in toluene at 110 °C for 24 h,
complex 2 underwent a facile thermal isomerization reaction to
produce the carbido complex [(Cp*Ru)2(µ2-NHPh)(µ2-H)(µ3-
C)PtMe(PMe3)2][OTf] (3) when heated for 3 days at 40 °C in a
toluene suspension (Scheme 1). Repeating the reaction using 13C-
enriched 2, [(Cp*Ru)2CH3(µ2-13CH2)(µ3-NPh)Pt(PMe3)2][OTf] (2-
13CH2), resulted in exclusive enrichment at the µ3-carbido carbon
in 3, demonstrating that the µ3-carbido ligand in 3 arises from the
µ2-CH2 ligand in 2. Complex 3 was isolated in 93% yield as a red
Figure 1. ORTEP drawing of the cationic part of 3 (50% probability level).
H atoms have been omitted. Selected bond lengths (Å) and angles (deg):
Ru(1)-C(1), 1.943(7); Ru(2)-C(1), 1.959(7); Pt(1)-C(1), 1.993(7);
Pt(1)-C(2), 2.111(6); Ru(1)-N(1), 2.086(5); Ru(2)-N(1), 2.081(5);
Pt(1)-P(1), 2.3094(17); Pt(1)-P(2), 2.3171(18); Ru(1)-C(1)-Ru(2),
77.6(3); Ru(1)-C(1)-Pt(1), 143.6(4); Ru(2)-C(1)-Pt(1), 137.9(4).
Scheme 1
Figure 2. Highest occupied MOs involved in σ and π bonding of the µ3-
carbido ligand in [(CpRu)2(µ2-H)(µ2-NH2)(µ3-C)PtH(PMe3)2]+.
A thermal ellipsoid plot of the cationic part of 3 is shown in
Figure 1. The carbon atom C(1) has trigonal-planar geometry (sum
of the bond angles ) 359.1°) and acts as a link between the Ru2
and Pt fragments to form a Y-shaped Ru2(µ3-C)Pt core. The
Ru-C(1) distances in 3 [1.943(7) and 1.959(7) Å] are shorter than
the Ru-C single-bond lengths observed for the µ2-methylene ligand
in 2 [2.034(4) and 2.043(5) Å] and are analogous to the Ru-C
distances observed for the µ2-ethylidyne ligand in [Cp*Ru(µ2-
CMe)(µ2-NPh)RuCp*][OTf] [1.929(7) and 1.914(7) Å].16 The
Pt-C(1) distance in 3 [1.993(7) Å] is similar to that reported for
[(IMes)Pt(dmso)Cl2] [1.981(8) Å; IMes ) 1,3-dimesitylimidazo-
lidine-2-ylidene],17 in which the N-heterocyclic carbene acts as a
σ-donor ligand. Other structural features include the pyramidal
geometry of N(1), indicating its formulation as a µ2-amido nitrogen,
crystalline solid and characterized by elemental analysis, multi-
nuclear NMR spectroscopy (1H, 13C{1H}, and 31P{1H}), and single-
crystal XRD. The 13C{1H} NMR spectrum of 3 showed a downfield
resonance assignable to the carbido carbon at δ 464.6 (2JPC ) 110
9
18026 J. AM. CHEM. SOC. 2009, 131, 18026–18027
10.1021/ja907387w  2009 American Chemical Society

C O M M U N I C A T I O N S
and the folding of the Ru(1)-N(1)-Ru(2)-C(1) ring, which
suggests the existence of a µ2-hydride ligand between the Ru atoms
[dihedral angle between the Ru-N(1)-C(1) planes ) 153.3°].
A density functional theory (DFT) calculation was conducted
be viewed as involving C-H and C-C bond-forming reductive
eliminations involving C, H, and Me ligands. The fact that the
reaction is induced by coordination of CO may suggest a role for
CO adsorption in facilitating the coupling of surface-bound C, H,
and CHx species in the Fischer-Tropsch hydrocarbon synthesis.11-14
In summary, a planar three-coordinate carbido ligand has been
generated on a bimetallic Ru2Pt cluster via double C-H activation
of a bridging methylene ligand. XRD and DFT studies indicated
the existence of delocalized Ru-C π bonds and an N-heterocyclic
carbene-like Pt-C bond in the µ3-CRu2Pt moiety. A reactivity study
revealed CO-promoted coupling of the carbido ligand with a hydride
and methyl ligands to form an ethylidene moiety.
on
a simplified model of 3, [(CpRu)2(µ2-H)(µ2-NH2)(µ3-
C)PtH(PMe3)2]+. The four highest occupied molecular orbitals
(HOMOs) are predominantly metal-centered and contribute little
to metal-ligand bonding. The next HOMOs (Figure 2) represent
bonding between the carbido ligand and the metal centers. HOMO-4
and HOMO-5 represent Ru-C σ and π bonds, respectively, in the
Ru(µ3-C)Ru moiety, showing the multiple-bond nature of the
Ru-carbido bonds. Both of these orbitals have repulsive π
interactions with respect to the Pt-carbido bond. Thus, the
Pt-carbido bond is made chiefly by σ interactions, as represented
by HOMO-6. Recently, an analogy between the ruthenium terminal
carbido complex [(Cy3P)2Cl2RuC] and carbon monoxide has been
proposed,8g and an adduct of this complex with a {PdCl2(SMe2)}
fragment has been isolated.8b The diruthenium carbido fragment
{(Cp*Ru)2(µ2-H)(µ2-NHPh)(µ3-C)} in 3 seems to be analogous to
π-donor-stabilized carbenes,18 since the carbido carbon atom is sp2-
hybridized, has π bonds with the adjacent Ru centers, and serves
as a two-electron σ-donor to the 14-electron {PtMe(PMe3)2}+
fragment.
Acknowledgment. We acknowledge support of this work from
the Ministry of Education, Culture, Sports, Science and Technology,
Japan (Grants 20750050, 21550064, 19027050) and Toyota Motor
Corporation.
Supporting Information Available: Experimental procedures,
characterization data, computational details, and CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
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2
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.
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