Bonding and reactivity in the electronically unsaturated hydrogen-bridged dimer [Ru3(CO)8(μ3-CMe)(μ-H) 2(μ3-H)]2

Article abstract of DOI:10.1021/om2012375

The electronically unsaturated complex [Ru₃(CO) ₈-(μ₃-CMe)(μ-H)₂(μ₃-H)] ₂ (1), viewed as a dimer of the 46-electron fragment Ru ₃(CO)₈(μ₃-CMe)(μ-H)₃, is held together by delocalized bonding involving two triply bridging hydride ligands. Compound 1 exhibits a dynamic activity in solution that equilibrates two of the three types of hydride ligands. Compound 1 reacts with 1,1- bis(diphenylphosphino) methane to form the macrocyclic complex [Ru ₃(CO)₇-(μ₃-CMe)(μ-H)₃] ₂(μ-dppm)₂ (3).

Full text of DOI:10.1021/om2012375

Communication
pubs.acs.org/Organometallics
Bonding and Reactivity in the Electronically Unsaturated
Hydrogen-Bridged Dimer [Ru3(CO)8(μ3-CMe)(μ-H)2(μ3-H)]2
Richard D. Adams,*,† Yuwei Kan,† Qiang Zhang,† Michael B. Hall,*,‡ and Eszter Trufan‡
†Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
‡Department of Chemistry, Texas A & M University, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: The electronically unsaturated complex [Ru3(CO)8-
(μ3-CMe)(μ-H)2(μ3-H)]2 (1), viewed as a dimer of the 46-electron
fragment Ru3(CO)8(μ3-CMe)(μ-H)3, is held together by delocalized
bonding involving two triply bridging hydride ligands. Compound 1
exhibits a dynamic activity in solution that equilibrates two of the three
types of hydride ligands. Compound 1 reacts with 1,1-bis(diphenyl-
phosphino)methane to form the macrocyclic complex [Ru3(CO)7-
(μ3-CMe)(μ-H)3]2(μ-dppm)2 (3).
lectronic unsaturation in chemical compounds can be
E
expressed in a variety of ways. It is readily recognized in
the form of “empty” valence orbitals, such as the one found on
the boron atom in BF3, or in multiple bonds, as found in
alkenes and alkynes.1 In metal complexes it can be found in the
form of empty orbitals at a “vacant” coordination site2 or in
metal−metal multiple bonds.3 In the presence of hydrogen,
unsaturation can be disguised by the formation of delocalized
bonds having hydrogen bridges as found in boranes, such as
B2H6,4 or in polynuclear metal complexes, such as the 46-electron
triosmium complex Os3(CO)10(μ-H)25 (A), the 56-electron
tetrarhenium complex Re4(CO)12(μ-H)46 (B), and higher clusters
such as the five-metal 68-electron complex Pt2Re3(CO)9(PR)3-
(μ-H)6 (C, R = tBu3).7,8
Figure 1. An ORTEP diagram of the molecular structure of
Ru6(CO)16(μ3-CMe)2(μ-H)4(μ3-H)2 (1), showing thermal ellipsoids
at the 30% probability level. Selected interatomic bond distances (Å)
are as follows: Ru(1)−Ru(3) = 2.8544(4), Ru(1)−Ru(2) = 2.8650(3),
Ru(2)−Ru(3) = 2.8840(3), Ru(3)−Ru(3′) = 2.9932(5), Ru(2)−
Ru(3′) = 3.627(1), Ru(1)−C(1) = 2.094(3), Ru(2)−C(1) = 2.097(3),
Ru(3)−C(1) = 2.032(3), Ru(1)−H(1) = 1.78(4), Ru(1)−H(2) =
1.69(4), Ru(2)−H(1) = 1.82(4), Ru(2)−H(3) = 1.81(3), Ru(3)−
H(2) = 1.88(4), Ru(3)−H(3) = 1.85(4).
Herein we describe a new form of this hydrogen-bridged
unsaturation which is located in the linkage between two
triruthenium carbonyl clusters in the hexaruthenium carbonyl
complex [Ru3(CO)8(μ3-CMe)(μ-H)2(μ3-H)]2 (1).
Compound 1 was obtained by a silica gel induced decar-
bonylation of the 48-electron complex Ru3(CO)9(μ3-CMe)-
(μ-H)3 (2)9,10 and was characterized by a low-temperature
single-crystal X-ray diffraction analysis. An ORTEP diagram of
the molecular structure of compound 1 is shown in Figure 1.
The compound can be viewed as a centrosymmetrically
coupled dimer of two 46-electron triruthenium fragments
“Ru3(CO)8(μ3-CMe)(μ-H)3”, each formed by the loss of one
CO ligand from a molecule of 2. The two Ru3 clusters
are linked by a long, hydrogen-bridged metal−metal bond:
Received: December 13, 2011
Published: December 21, 2011
© 2011 American Chemical Society
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Ru(3)−Ru(3′) = 2.9932(5) Å. The Ru−Ru bonds within the
Ru3 triangles are shorter, Ru(1)−Ru(3) = 2.8544(4) Å, Ru(1)−
Ru(2) = 2.8650(3) Å, and Ru(2)−Ru(3) = 2.8840(3) Å, and
are similar to those found in 2, 2.841(6) and 2.844(6) Å.11 As
in 2, each Ru−Ru bond within the Ru3 triangles contains a
bridging hydrido ligand (located and refined in the structural
analysis), but two of these, H(3) and H(3′), serve as triply
bridging ligands by extending to the ruthenium atom, Ru(3), in
the neighboring Ru3 triangle. The Ru(2)···Ru(3′) distance
between the two Ru3 clusters, 3.627(1) Å, is too long for a
direct bonding interaction.
Compound 1 contains a total of 92 valence electrons. A six-
metal cluster with seven metal−metal bonds should have 94
electrons, (6 × 18) − (7 × 2), if all of the metal atoms formally
have an 18-electron configuration.12 In order to obtain a clearer
picture of bonding in 1, several DFT calculations have been
performed: first, a geometry optimization starting with the
structure as found in the solid state, followed by a vibrational
frequency calculation to confirm the stationary point as a
minimum, and then a fragment analysis to help explain the
bonding (details are given in the Supporting Information). To
better understand the stability of the dimer, 1, DFT calculations
were also performed on the two Ru3(CO)8(μ3-CMe)(μ-H)3
monomer units of 1, and the proposed intermediate that
contains the direct Ru(3)−Ru(3′) bond, but without triply
bridging H(3) or H(3′) ligands. The details of the optimized
structures for these three species are shown in Figure 2. Our
the geometry that they display in the optimized dimer struc-
ture, rather than at their independently optimized geometry
as described above. These calculations show that the total
(intrinsic) electronic bond energy of −33.40 kcal/mol is
comprised mostly of orbital interactions (−31.58 kcal/mol) and
a small contribution (−1.82 kcal/mol) from the sum of the
electrostatic (attraction in this case) and Pauli repulsions. The
source of this orbital stabilization is mixing of the LUMO of
each monomer fragment with several occupied fragment
orbitals, specifically the HOMO-2, HOMO-8, and HOMO-
11, of the other fragment. The key features of this mixing are
described below, and related orbital contour drawings are
shown in Figure 3. The most significant pair of interactions
Figure 3. Orbital contour diagrams at the isosurface value of 0.03 for
the fragment orbitals and 0.02 for the dimer orbitals showing (a)
HOMO-11 of fragment A (the electron-donor orbital) in relation to
the LUMO of fragment B (the electron-acceptor orbital), (b) HOMO-
27, the major MO making a net contribution to the bonding between
the two fragments in the dimer 1, (c) HOMO-8 of fragment A (the
electron-donor orbital) in relation to the LUMO of fragment B (the
electron-acceptor orbital), (d) HOMO-18, a MO making a net
contribution to the bonding between the two fragments in the dimer
1, (e) the HOMO-2 of fragment A (the electron-donor orbital) in
relation to the LUMO of fragment B (the electron-acceptor orbital),
and (f) HOMO-5, the second most important MO making a net con-
tribution to the bonding between the two fragments in the dimer 1.
Note that the orientation has the bridging Ru atoms upper right and
lower left.
Figure 2. Schematic representations of the monomer, the intermediate
with the Ru(3)−Ru(3′) bond, and the dimer, showing bond distances
of the optimized structures and the relative Gibbs free energies of the
systems.
occurs when the LUMO of one of the monomer fragments
accepts electron density from the HOMO-11 of the other
fragment, as shown in Figure 3a for one of the pairs. This
mixing leads to significant bonding in the HOMO-27 of the
dimer 1, as shown in Figure 3b. The HOMO-8 of each
fragment has a similar, but somewhat weaker, interaction with
the LUMO of the other fragment, as shown in Figure 3c. This
second interaction enhances the bonding in the HOMO-18 of
the dimer 1, shown in Figure 3d. Finally, the LUMO of each
fragment also mixes with the HOMO-2 of the other fragment
(Figure 3e), in a way that enhances the bonding from the
HOMO-5 of 1, shown in Figure 3f.
The unsaturation of the monomer fragment is concentrated
in the LUMO, which is the orbital that a two-electron-donor
ligand such as a carbonyl or a phosphine would use to make an
additional bond. In the dimer 1, the unsaturation is resolved by
electron donation from the HOMO-11, HOMO-8, and
analysis shows that the combination of the two Ru3(CO)8-
(μ3-CMe)(μ-H)3 units to form the intermediate with only the
Ru(3)−Ru(3′) bond lowers the Gibbs free energy (ΔG°) of the
system by 2.73 kcal/mol and creates a metal−metal bond at
2.91 Å. The rearrangement of this structure into the observed
dimer with contributions from the triply bridging H(3) and
H(3′) lowers the ΔG° of the system to 8.96 kcal/mol below
that of the two monomers and lengthens the Ru(3)−Ru(3′)
bond to 3.06 Å. These results indicate that the Ru(3)−Ru(3′),
H(3)−Ru(3′), and H(3′)−Ru(3) interactions all contribute to
the bonding in 1.
The fragment analysis describes the nature of the bonding
between two monomers (labeled A and B) that are frozen in
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HOMO-2 orbitals of the other fragment. Thus, the dimer has
both triply bridging H's and direct Ru(3)−Ru(3′) bonding.
Variable-temperature 1H NMR spectra of the hydride ligands
in 1 show that the compound is dynamically active in solution.
At low temperature (−90 °C), the spectrum exhibits three
resonances at−16.51 (br, 2H), −16.96 (s, 2H) and −20.68
ppm (br, 2H) for each of the three types of bridging hydride
ligands, which is in accord with the structure found in the solid
state. However as the temperature is raised, the two broad
resonances at −16.51 and −20.68 ppm broaden further and
merge into a single peak, which is sharp at −18.30 ppm at room
temperature (see Figure S2 in the Supporting Information).
These changes can be explained by a rocking rearrangement
motion in which the environments of the triply bridging
hydride ligands H(3) are exchanged in a pairwise fashion with
the pair of edge-bridging hydride ligands H(2), but they are not
exchanged with the other bridging hydride ligands H(1), as
shown in Scheme 1 (CO labels are not shown in Scheme 1).
Scheme 1. Dynamic Rearrangement of 1 in Solution
Figure 4. ORTEP diagram of the molecular structure of
[Ru3(CO)7(μ3-CMe)(μ-H)3]2(μ-dppm)2 (3), showing thermal ellip-
soids at the 20% probability level. Selected interatomic bond distances
(Å) are as follows: Ru(1)−Ru(3) = 2.8533(14), Ru(1)−Ru(2) =
2.8710(13), Ru(2)−Ru(3) = 2.8445(15), Ru(1)−C(4) = 2.071(11),
Ru(2)−C(4) = 2.054(13), Ru(3)−C(4) = 2.072(15), Ru(1)−P(1) =
2.436(3), Ru(2)−P(2) = 2.451(3), Ru(1)−H(1) = 1.67(10), Ru(1)−
H(2) = 1.38(14), Ru(2)−H(1) = 1.70(10), Ru(2)−H(3) = 1.33(15),
Ru(3)−H(3) = 1.95(16), Ru(3)−H(2) = 2.04(13).
Alternatively, the molecule could simply dissociate into two
Ru3(CO)8(μ3-CMe)(μ-H)3 fragments and then recombine in
such a way that H(2) and H(3) have been interchanged. Line-
shape analyses have provided the following activation
parameters: ΔH⧧ = 8.7(5) kcal/mol, ΔS⧧ = 2.5 cal/K. The
small value of ΔS⧧ would suggest a nondissociative mechanism.
Electronically unsaturated compounds generally exhibit
higher reactivity than saturated compounds. Such is also the
case with 1. When treated with CO at room temperature,
compound 1 was rapidly converted back to 2, quantitatively.
When treated with 1,1-bis(diphenyphosphino)methane
(dppm), compound 1 was converted to the new compound
[Ru3(CO)7(μ3-CMe)(μ-H)3]2(μ-dppm)2 (3) in 25% yield at
room temperature within 5 min. Compound 3 was
characterized structurally by a single crystal X-ray diffraction
analysis, and an ORTEP diagram of its molecular structure is
shown in Figure 4.
Scheme 2. Formation of 3 from 1
Scheme 3. Formation of 4 from 2
Compound 3 is a centrosymmetrical dimer linked by two
bridging dppm ligands; each phosphorus atom of the dppm is
coordinated to a different Ru3 cluster. The hydrogen-bridged
link between the two triruthenium clusters of 1 was completely
ruptured and a 10-membered macrocycle which contains two
dppm-substituted triruthenium clusters was created in its place
(see Scheme 2). The Ru−Ru bonding within each Ru3 triangle,
Ru(1)−Ru(3) = 2.8533(14) Å, Ru(1)−Ru(2) = 2.8710(13) Å,
and Ru(2)−Ru(3) = 2.8445(15) Å, is similar to that in 2.11
Notably, there was no formation of compound 3 in the
reaction between compound 2 and dppm at room temperature
in 2 h, although after 20 h, one is able to isolate instead the
chelate complex Ru3(CO)7(μ3-CMe)(μ-H)3(μ-dppm) (4) in
31% yield (see Scheme 3 and Figure S3 in the Supporting
Information).
cluster complexes. One such species is the hexaruthenium
complex Ru6(CO)14[μ4-η2-OCH2CHNC(Me)OC(Ph)]2-
(μ3-H)2, which is held together in part by two bridging hydride
ligands and two polydentate bridging oxazoline ligands.13
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, tables, figures, and CIF files giving experimental details,
characterization data, and crystal data for the structural analyses.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Other molecules with unsaturation such as 1 may exist as
intermediates or even as isolable compounds in decarbonyl-
ation reactions involving hydride containing metal carbonyl
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AUTHOR INFORMATION
Corresponding Author
*E-mail: adams@chem.sc.edu (R.D.A.); hall@science.tamu.edu
(M.B.H.).
■
ACKNOWLEDGMENTS
■
This research was supported by the National Science Foundation
(Grant No. CHE-1111496, R.D.A.). The authors from Texas
A&M University gratefully acknowledge the National Science
Foundation (Grant Nos. CHE-0541587 and CHE-0910552) and
The Welch Foundation (Grant No. A-0648) for support.
ABBREVIATIONS
■
LUMO, lowest unoccupied molecular orbital; HOMO, highest
occupied molecular orbital; ORTEP, Oak Ridge thermal
ellipsoid plot; DFT, density functional theory
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