Thermochemistry and molecular structure of a remarkable agostic interaction in a heterobifunctional ruthenium-boron complex

Article abstract of DOI:10.1021/ja909858a

(Formula Presented) A boron-pendant ruthenium species forms a unique agostic methyl bridge between the boron and ruthenium atoms in the presence of a ligating solvent, acetonitrile. NMR inversion-recovery experiments enabled the activation and equilibrium

Full text of DOI:10.1021/ja909858a

Published on Web 01/20/2010
Thermochemistry and Molecular Structure of a Remarkable Agostic
Interaction in a Heterobifunctional Ruthenium-Boron Complex
Brian L. Conley and Travis J. Williams*
Loker Hydrocarbon Research Institute and Department of Chemistry, UniVersity of Southern California,
Los Angeles, California, 90089-1661
Received November 20, 2009; E-mail: travisw@usc.edu
Hydrocarbon functionalization is an important reaction class in
organic synthesis and has a central role in applications ranging from
1
utilization of hydrocarbon feedstocks to fine-chemical synthesis.
In this area, our group is developing ligand-metal bifunctional
catalysts that manipulate hydrides. Our general approach to C-H
functionalization involves cooperative effects between a transition
metal and a ligand-centered coordination-directing element. This
is analogous to hydrogen-bond-directed oxidation catalysts such
Figure 1. An agostic boron methyl has structural homology to transition
states for coordination-directed C-H activation reactions.
2
as Noyori’s (Ts-DPEN)(cym)RuCl systems and the cyclopenta-
3
4
Scheme 1. Synthesis of Bis(2-pyridyl)borate-Ligated Ruthenium
Complexes
dienone-ligated systems from Shvo and Casey (Figure 1A), except
that a Lewis acid directing group such as boron (Figure 1B) does
not require a protic functionality in the substrate. In this com-
munication, we report the synthesis, X-ray structure, and unique
thermochemistry of an unusually strong C-H agostic methyl bridge
between boron and ruthenium (Figure 1C). This agostic complex
has structural homology to a potential transition state for boron-
directed C-H cleavage (Figure 1B).
Our design of a boron-pendant ruthenium-based oxidation
catalyst requires a ligand that is robust under mild conditions. Along
these lines, we became interested in the dimethylbis(2-pyridyl)borate
ligand(3)asaplatformfromwhichaheterobimetallicboron-transition
5
metal hydride abstraction system could be constructed. Pt(II) and
Pt(IV) complexes of this ligand are known and are stable toward
6
dioxygen at elevated temperatures. A Pt(II) complex of 3 shows
no reactivity between the borate and the metal, but the Pt(IV)
congener participates in slow methyl transfer from boron to platinum
with the intermediacy of an agostic bridge.
Analogous to the Pt(IV) complex, we observe that a boron methyl
on 3 forms an agostic interaction to ruthenium when ligated to
Ru(II). This occurs with expulsion of an acetonitrile ligand from
the metal (Scheme 1). Unlike the analogous platinum system, the
equilibrium between 6 and 7 is very rapid. To our knowledge, this
is the first characterization of an alkyl group bridging boron and
ruthenium in this way. Moreover, formation of the agostic bond
occurs in the presence of acetonitrile, thus defining a very rare
example of ligand displacement from a coordinatively saturated
with an agostic interaction. In an attempt to arrest the agostic
interaction, solutions of 6 + 7 were cooled to -40 °C in acetonitrile-
d
3
2
and -95 °C in dichloromethane-d . The upfield signal remained
a singlet in each case, indicating rapid interconversion among the
three hydrogens of the bridging methyl at these temperatures. A
7
metal by a C-H bond. In fact, hydrolysis of this bridging methyl
affords a µ-OH complex that has reactivity analogous to that of
2
5 °C HMBC spectrum of the equilibrating mixture elucidated the
8
the Shvo catalyst.
carbon chemical shifts of the boron-bound carbon atoms and
The syntheses of agostic complex 7 and precatalyst 1 are outlined
illustrated that the upfield methyl group in 7 was bound both to
in Scheme 1. The route is initiated with the formation of
1
3
5
boron (correlation to the other BMe C: 8 ppm) and ruthenium
chlororuthenium adduct 4 from 3 and 2. Treatment of 4 with
1
3
1
(
correlation to acetonitrile ligands C: 125 ppm). The low JC-H
3
AgOTf in acetonitrile-d in a J. Young NMR tube affects rapid
9
1
values provide further evidence for an agostic interaction. Although
C-H values for the boron methyls of 6 and 7 could not be observed
formation of 5 [ H δ(B-Me) ) +0.09, +0.04 ppm]; solvent then
displaces cymene in minutes at elevated temperature to give an
equilibrating mixture of 6 and 7 (1:1 at 90 °C), which can be
1
J
directly (see the Supporting Information), each was measured in a
1
13
1
hydrolyzed to give 1 [ H δ(B-Me) ) +0.29 ppm].
C-coupled HSQC experiment:
J
C-H(agostic) ) 100 Hz;
1
1JC-H(free) ) 107 Hz in 7. For comparison, 6 has JC-H (free) )
109 Hz. Further comparison of the spectral details for 6 and 7 is
presented in the Supporting Information. The nearest examples of
1
The spectral data for 7 feature H resonances at +0.18 and -5.13
ppm (∆δ ) 5.3 ppm). While the downfield shift is consistent with
bonding only at boron, the peak at -5.13 ppm is more consistent
1
764 9 J. AM. CHEM. SOC. 2010, 132, 1764–1765
10.1021/ja909858a  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Catalytic Reactivity of 1
diffraction methods. Cleavage of the bridge has kinetic order with
respect to [acetonitrile], indicating that bridge dissociation is either
a rapid pre-equilibrium or concerted displacement. Thus, this is a
very rare situation in which an agostic interaction is in equilibrium
with a tightly binding ligand. A hydrolyzed form of this ligand-metal
bifunctional complex is a catalyst for transfer dehydrogenation of
alcohols, although the mechanism of this reaction has not been
established. We are currently investigating the mechanism of this
oxidation and optimizing the heterobifunctional motif for general
hydride manipulation reactions.
Figure 2. ORTEP diagrams of 1 (left) and 7 (right). Selected hydrogens
and the counterions have been omitted for clarity. Ellipsoids are drawn at
1
1
the 50% level. For comparison sake, the bond lengths (Å) for the agostic
IV
BMe-M interactions in 7 and [(3)Pt Me
3
] are Ru1-H1, 1.72; B1-C1,
1
.66; Ru1-C1, 2.53; Ru1-B1, 2.89, and Pt1-H1, 2.02; B1-C1, 1.68;
Pt1-C1, 2.76; Pt1-B1, 3.08, respectively.
6
a metal-bound alkyl borate in the literature include a Pt(IV) case
and a bis(pyrazolyl)diethylborate-ligated Mo(II) complex.
1
0
Acknowledgment. We thank the University of Southern
California, Loker Hydrocarbon Research Institute, the Hydrocarbon
Research Foundation, the Anton Burg Foundation, and the ACS
Petroleum Research Fund (47987-G1) for research support and the
NSF (DBI-0821671) and NIH (S10-RR25432) for NMR spectrom-
eters. We are grateful to G. K. S. Prakash, T. Flood, A. Kershaw,
S. Lynch, and C. Cares for valuable discussions and to T. Stewart
and R. Haiges for assistance with X-ray crystallography. We thank
Denver Guess for assistance with synthetic scale-up.
Importantly, the NMR data for 7 show that the lifetimes of 6
and 7 are on the order of 1 s in solution, and [7]/[6] decreases at
lower temperature. Nonetheless, low-temperature vapor diffusion
enabled crystallization of 7 in dichloromethane at low acetonitrile
concentration. Remarkably, these crystals enabled collection of an
X-ray structure that clearly defines the atomic connectivity in 7
(
Figure 2). A direct comparison of the X-ray structures of 1 and 7
illustrates the positions of the dipyridylborate ligand with an oxygen
or agostic bridge (Figure 2). The agostic C-H in 7 was positioned
using the electron difference map. The Ru1-C1 bond distance of
Supporting Information Available: Experimental procedures,
kinetics data, graphical spectra, and crystallographic data (CIF) for
compounds 1, 4, and 7. This material is available free of charge via
the Internet at http://pubs.acs.org.
2
.53 Å in 7 is in contrast to the bond distance of 2.10 Å for Ru1-O1
in 1.
The thermochemistry for the equilibration of 6 and 7 was
measured by NMR spectroscopy. This equilibrium has a linear van’t
Hoff plot from 20 to 80 °C with ∆H ) 5.5(2) kcal/mol and ∆S )
References
2
0.1(5) eu. The conversion of 7 to 6 was conveniently studied by
NMR magnetization transfer: at 85 °C, bridge cleavage has k-1,obs
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(
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q
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(
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alcohol and 40 mol % t-BuOK in acetone resulted in >95% yield
of the corresponding ketone. The mechanism of this reaction is
unknown.
In summary, we have reported here the first example of an agostic
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(
1
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(
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JA909858A
J. AM. CHEM. SOC. 9 VOL. 132, NO. 6, 2010 1765