Bifunctional ruthenium(II) hydride complexes with pendant strong lewis acid moieties: Structure, dynamics, and cooperativity

Article abstract of DOI:10.1021/ja3119477

The synthesis of a novel class of bifunctional ruthenium hydride complexes incorporating Lewis acidic BR₂ moieties is reported. Determination of the molecular structures in the solid state and in solution provided evidence for tunable interaction between the two functionalities. Cooperative effects on the reactivity of the complexes were demonstrated including the activation of small Lewis basic molecules by reversible anchoring at the boron center.

Full text of DOI:10.1021/ja3119477

Communication
pubs.acs.org/JACS
Bifunctional Ruthenium(II) Hydride Complexes with Pendant Strong
Lewis Acid Moieties: Structure, Dynamics, and Cooperativity
Thomas G. Ostapowicz, Carina Merkens,† Markus Holscher, Jurgen Klankermayer,* and Walter Leitner
̈
̈
RWTH Aachen University, Institut fur Technische und Makromolekulare Chemie, Worringerweg 1, 52074 Aachen, Germany
̈
S
* Supporting Information
Scheme 1. Bifunctional Ruthenium Hydride Complexes
Bearing Borane-Based Lewis Acid Sites: Possible
Intramolecular Interaction and Substrate (S) Activation
ABSTRACT: The synthesis of a novel class of bifunc-
tional ruthenium hydride complexes incorporating Lewis
acidic BR2 moieties is reported. Determination of the
molecular structures in the solid state and in solution
provided evidence for tunable interaction between the two
functionalities. Cooperative effects on the reactivity of the
complexes were demonstrated including the activation of
small Lewis basic molecules by reversible anchoring at the
boron center.
A series of new ruthenium hydride complexes based on the
[CpRu(H)(CO)(PR3)] framework with pendant boron-based
Lewis acid moieties were synthesized as shown in Scheme 2.
ransition metal complexes containing multiple reactive
Tsites hold great opportunities for the future development
of homogeneous catalysis. In particular, this relates to the
combination of metal hydrides and Lewis or Brønsted acidic
sites to facilitate networks of bond-breaking and -forming
events as required for cooperative small molecule activation.1
Nature uses this concept in various enzymes to facilitate highly
selective multistep transformations under mild conditions.2 In
heterogeneous catalysis, the combination of hydrogenation
active transition metal particles with acidic sites in the support
materials is also a well-established principle, used on a large
scale for example in hydrocarbon reforming.3
Scheme 2. Synthesis of Bifunctional Ru−H/Lewis Acid
Complexes 1, 4, and 5
In sharp contrast, there are only very few examples for late
transition metal hydride complexes containing Lewis acidic
groups within the same molecule and little is known about
possible synergistic effects.4 Hill and Owen reported examples
of late transition metal hydride complexes functionalized with
pendant di- and triaza borane units facilitating hydride transfer
reactions.5 In 1995, Baker, Marder and co-workers isolated a
complex with a Ru−H−B unit by hydroboration of the
cyclometalated [RuH(PMe3)3(η2-CH2PMe2)] complex with H-
9-BBN and already, in 1990, a related Ir-complex.6 Most
recently, a nickel complex with integrated borane function was
shown to enable heterolytic cleavage of dihydrogen.7
In the present study, we focus on bifunctional ruthenium
hydride complexes with BR2 groups as the Lewis acidic sites.
Ruthenium hydride complexes are known to serve as active
intermediates in numerous hydrogen transfer processes. By
systematic variation of the ligand environment at the metal and
the Lewis acidity of the borane moiety, we wanted to explore
(i) whether an interaction between the hydride ligand and the
borane group occurs; (ii) how such an interaction might affect
the reactivity of the Ru−H moiety; and (iii) whether the Lewis
acid could serve as an anchor group to activate small molecules
in the vicinity of the reactive Ru−H function (Scheme 1).
Complex 1 was obtained in 76% yield in a two-step one-pot
reaction starting from commercially available Ru3(CO)12 by
treatment with cyclopentadiene and the phosphane-9-BBN
ligand8 in refluxing n-heptane.9 This method cannot be applied
for the incorporation of the stronger Lewis acidic B(C6F5)2-
group, as the corresponding free phosphine ligand is not
accessible due to B−P Lewis base pair formation. We therefore
synthesized the new diphenylvinylphosphane complexes 2 and
3 as above and then, in a second step, conducted the
hydroboration with Piers’ borane10 at the ligand within the
coordination sphere. Under ultrasound irradiation, the Cp (4)
and Cp* (5) complexes were obtained in good yields of 87%
and 71%, respectively.
The new complexes 1, 4, and 5 are yellow crystalline solids,
and the molecular structures obtained from single crystal X-ray
are depicted in Figure 1. The hydride ligands could be fully
Received: December 13, 2012
Published: January 30, 2013
© 2013 American Chemical Society
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Communication
In solution, the B−H interaction is strongly dependent on
the Lewis acidity of the BR2 group: At room temperature, NMR
spectroscopy shows evidence of only a very weak boron−
hydride interaction for complex 1 bearing the 9-BBN moiety.
The 11B resonance is found in the typical region of three-
coordinated boron at 65.7 ppm. The hydride signal appears as
2
two sharp doublets at −13.88 and −13.90 ppm (both JHP
=
26.8 Hz) in the ratio 4:1 (Figure 2b, Table 2). This can be
1
Figure 2. Hydride region of the H NMR spectra of complexes 1 (a)
and 4 (b) at 25 °C (lower line) and −50 °C (upper line). Solvents were
C6D6 or CD2Cl2.
Table 2. Selected 1H NMR Data of the Neutral Complexes 1,
4, and 5 and the Cationic Complexes 6, 7, and 8
a
b
b
b
+
+
δ(H) /
T1(H) /
δ(H2 ) /
T1(H2 ) /
compl.
ppm
ms
compl.
ppm
ms
c
1
4
5
−13.8
−15.4
−13.9
b
443
205
230
6
7
−7.2
−7.2
−6.9
15
25
8
808
a
c
Rt in C6D6. −50 °C in CD2Cl2. Two doublets (4:1).
1
attributed to an isotope-induced chemical shift Δ10/11B(1H)
arising from a very weak interaction with 10/11B in analogy to
n
other Δ10/11B(X) shifts where the equilibrium lies on the side
of the threefold coordination.12 The observation of the isotopic
shift reflects the long relaxation time T1 in the typical range for
terminal Ru−H groups.13 By lowering the temperature to −50
°C this signal shifts to −16.2 ppm appearing as a broad singlet
(h1/2 = 47 Hz) indicative of formation of an internal Lewis base
adduct (Figure 2a).
In contrast, complexes 4 and 5 bearing the strong Lewis
acidic B(C6F5)2 group show broad hydride signals already at
room temperature appearing at −15.4 and −13.9 ppm,
respectively (Figure 2a, Table 2). With T1 values of about
200 ms the relaxation times of the hydride signals of 4 and 5 are
considerably shortened, reflecting the interaction with the
quadrupole nucleus 11B, whose signals appear at −4.2 and −4.1
ppm in the typical region of four-coordinated boron.
Furthermore, the 19F NMR spectra of 4 and 5 exhibit two
sets of three individual signals, resulting from the diastereotopic
C6F5 substituents next to the chiral-at-metal CpRu(CO)(PR3)
H unit.14 Raising the temperature to 70 °C for 4 and to 100 °C
for 5 results in a coalescence of the 19F NMR signals indicating
a dynamic process that can be associated with the weakening of
the B−H interaction (see SI for figure).
Figure 1. Molecular structures of complexes 1 (top), 4 (middle), and 5
(bottom) as obtained from single crystal X-ray diffraction. Hydrogen
substituents except H1 were omitted for clarity. Thermal ellipsoids are
at the 50% probability level.
refined on the basis of the corresponding electron density,
providing a reliable determination of their positions (see
Supporting Information (SI)). In the solid state, all three
compounds reveal a cyclic conformation with the Ru···B and
B−H distances indicative of a Lewis acid/Lewis base interaction
similar to that for comparable structures.6b,11 Both distances are
shorter by about 10 pm for 4 and 5 as compared to 1, reflecting
the stronger Lewis acidity of the B(C6F5)2-group (Table 1).
The Ru···B distance in 5 is also slightly longer than that in 4.
This may occur from increased steric repulsion between the
Cp* and the B(C6F5)2 groups, as the B−H distances remain
largely identical in both complexes.
Upon protonation of the Ru−H unit, the boron hydride
interaction was cleaved for all three complexes as indicated by
11B signals in the three-coordinate region. Treatment of 1 and 4
with HBF4 in ether resulted in the formation of the cationic
dihydrogen complexes 6 and 7, whereas complex 5 gave the
dihydride 8 as can be seen by comparison of the T1 values
(Scheme 3, Table 2).15 This is fully consistent with the
corresponding reactivity of the nonfunctionalized Cp(*)Ru−H
Table 1. Structural Parameters of Complexes 1, 4, and 5
complex d(BH)/pm d(RuH)/pm d(Ru···B)/pm d(CO)/pm
1
4
5
152(6)
143(6)
141(3)
159(5)
160(6)
168(2)
301.9(6)
289.5(6)
292.2(3)
115.3(7)
115.3(6)
114.9(3)
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Journal of the American Chemical Society
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Ru−H for deuterium was observed for this species. For
complex 1, however, H/D exchange was not complete even
after two weeks under identical conditions. This is most
consistent with the formation of species 11, where the binding
at the strong Lewis acid site B(C6F5)2 enhances the acidity of
the MeOH molecule in close proximity to the hydridic Ru−H
moiety, whose signal appears at −11.9 ppm in the adduct. This
arrangement holds great potential for catalytic heterolytic
reduction of polar groups via an outer sphere mechanism, in
the presence of appropriate hydrogen donors.16
Scheme 3. Protonation of Complexes 1, 4, and 5 To Give
Cationic Dihydrogen Complexes 6 and 7 or Dihydride 8
In summary we reported the synthesis, structure, and
reactivity of a novel class of bifunctional ruthenium hydride/
borane complexes. The results provide evidence for synergistic
interactions between the hydridic and Lewis acid moieties:
Whereas the principal bonding situation at the Ru−H unit is
mainly controlled by the ancillary ligands (here Cp and Cp*),
additional control factors are provided by the incorporation of a
sufficiently strong Lewis acidic site such as the B(C6F5)2
function. This includes modulation of the reactivity of the
M−H group as well as coordination and activation of small
molecules in close proximity to a hydride ligand. Thus, further
investigations toward the application of this new class of
bifunctional complexes in catalysis appear highly promising and
are currently under investigation in our group.
units, indicating that the Lewis acid group has no interaction
with the cationic complexes, as expected.
The different strengths of the interactions in the neutral
monohydrides are, however, clearly reflected by their reactivity
toward chlorinated hydrocarbons (Scheme 4). The hydride
Scheme 4. Reactivity towards Chlorinated Hydrocarbons
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
ligand of the 9-BBN complex 1 is exchanged for chloride to
quantitatively yield 9 within 2 h at 40 °C in CD2Cl2 solution.9
In contrast, no H/Cl exchange was observed with complexes 4
and 5 even after 24 h at 40 °C. Thus, the boron hydride
interaction appears to shield the Ru−H group preventing the
substitution process.
AUTHOR INFORMATION
Corresponding Author
jklankermayer@itmc.rwth-aachen.de
■
Finally, the behavior of complexes 1 and 4 toward external
Lewis bases was investigated. In the case of complex 1, no
changes in the 11B NMR data associated with adduct formation
at the weakly Lewis acidic 9-BBN group could be observed in
the presence of acetonitrile, tetrahydrofurane, or pyrrolidine. In
contrast, complex 4 with the strong Lewis acidic B(C6F5)2
moiety was found to bind these molecules under opening of the
Ru−H interaction. The 11B NMR signals at −3 to −6 ppm
clearly indicate the four-coordinate boron. The sharp hydride
Present Address
†RWTH Aachen University, Institut fur Anorganische Chemie,
̈
Landoltweg 1, 52056 Aachen, Germany.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a Kekule
■
1
́
grant (FCI) to Th.G.O.
resonances at −11.9 to −12.0 ppm in the H NMR with a
2
and the DFG (International research training group SeleCa).
Additional support was received from Cluster of Excellence
“Tailor-Made Fuels from Biomass” (TMFB), which is funded
by the Excellence Initiative of the German federal and state
governments to promote science and research at German
universities.
doublet splitting of JHP = 32 Hz together with the appearance
of only one set of signals in the 19F NMR rule out an internal
interaction with the hydride ligand, confirming the binding of
the external Lewis base. The adduct formation was fully
reversible and the parent complex 4 was reformed upon
removing the Lewis bases in vacuum.
Most significantly, the potential for cooperative activation of
external Lewis bases was revealed by the reactivity of complexes
1 and 4 toward methanol (Scheme 5). Again, the NMR data
showed reversible binding of MeOH only for complex 4. In
deuterated methanol, a fast exchange within seconds of the
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