A delicate balance between sp2 and sp3 C-H bond activation: A Pt(II) complex with a dual agostic interaction

Article abstract of DOI:10.1021/ja905046n

(Chemical Equation Presented) 2-tert-Butyl-6-(4-fluorophenyl)pyridine reacts with K₂PtCl₄ via the activation of an sp ² C-H bond to give a cyclometalated complex that contains a bifurcated agostic interaction. Rearrangemen

Full text of DOI:10.1021/ja905046n

Published on Web 09/18/2009
A Delicate Balance between sp2 and sp3 C-H Bond Activation: A Pt(II)
Complex with a Dual Agostic Interaction
Sarah H. Crosby, Guy J. Clarkson, and Jonathan P. Rourke*
Department of Chemistry, UniVersity of Warwick, CoVentry CV4 7AL, U.K.
Received June 19, 2009; E-mail: j.rourke@warwick.ac.uk
The selective and general transformation of unreactive C-H bonds
to other functional groups is of tremendous worldwide interest and
significance;1,2 many metals have been utilized to activate C-H bonds,
with platinum1,3 complexes showing particular promise and palladium4
and iridium5,6 complexes being utilized catalytically. Cyclometalation
reactions,7 such as those of phenylpyridines,8 are seen as analogues
of intermolecular C-H activations, and such complexes have uses in
their own right.9 Agostic complexes are widely invoked as intermedi-
ates in the activation of C-H bonds in organometallic chemistry10,11
and have considerable precedent in the stabilization of coordinatively
unsaturated metal centers; of particular relevance to the work reported
herein are the formally 14e T shaped Pt(II) complexes.12-14 Such
electrophilic 14e platinum centers ought to be ideally set up for
subsequent C-H activation, where it is generally recognized that a
preference exists for the activation of sp2 over sp3 C-H bonds.3 Here
we report the synthesis and characterization of an sp2 cyclometalated
complex containing an agostic interaction that exchanges the sp2 ring
for an sp3 cyclometalated ring, before eventually activating both C-H
bonds to give a doubly cyclometalated species.
Definitive confirmation of the structure came from an X-ray
structure (Figure 1), where the notionally T-shaped geometry of
the complex is clear. Interestingly, agostic interactions are present
from two of the hydrogens on one of the methyl groups (Pt-H
distances of 2.15(3) and 2.14(4) Å and a Pt-C distance of 2.472(4)
Å), with these hydrogens being directly located in the X-ray
diffraction study. This type of bifurcated agostic interaction has
only rarely been observed with late transition metals,20-22 and never
1
before with platinum. A variable temperature H NMR study was
undertaken to try and establish whether this agostic interaction could
be frozen out sufficiently for separate resonances to be observed
for any of the hydrogens of the tBu, but even at -95 °C in CD2Cl2
t
only one sharp peak was observed for the Bu group, indicating a
rapid exchange of the agostic interaction across all nine hydrogens.
Other features of note in the solid-state structure of 2 include an
infinite Pt-Pt stacking interaction of 3.5734(1) Å with a Pt-Pt-Pt
angle of 156° along the chain.
Direct reaction of potassium tetrachloroplatinate with 2-tert-butyl-
6-(4-fluorophenyl)pyridine, 1, in ethanoic acid slowly, but cleanly,
activates one of the C-H bonds of the fluorinated phenyl ring (with
the elimination of 1 equiv of HCl and 2 equiv of KCl) to give a
1
cyclometalated product, as evidenced by the H NMR coupling
pattern on the phenyl ring and by the presence of platinum satellites
on the only 19F NMR resonance. Normally one would expect the
product of this type of reaction either to complete a square-planar
coordination geometry with an additional equivalent of pyridine
or to become a chloride bridged dimer.15-17 In the past we have
established that larger ligands such as 2-ethyl-6-(4-fluorophenyl)py-
ridine are too large to complex to the cyclometalated product,
leaving the chloride bridged dimer as the expected product.18
However, we designed the ligand 1 with the express intention of
preventing, through unfavorable steric interactions, even the forma-
tion of chloride bridged dimers. Additional platinum satellites of
16 Hz are present on the singlet (relative integral 9) at 1.67 ppm in
the 1H NMR spectrum which comes from the tBu group. Platinum
satellites are also visible on the 13C resonance of the corresponding
carbon atom.19 It is apparent that this coupling is too large to have
been transmitted through the bonding to the platinum via the
pyridine structure. In the absence of any other potentially ligating
species we tentatively assigned an agostic structure 2 to this product
(eq 1). Further support for a formally 14e species comes from the
high resolution mass spectrum which shows the presence of ions
of mass (M+H)+ and (M+Na)+.
Figure 1. Molecular structure of 2; thermal ellipsoids at 50% probability
level. Selected bond lengths (Å) and angles (deg): Pt(1)-C(16) 2.472(4),
Pt(1)-H(16A) 2.15(3), Pt(1)-H(16B) 2.14(4), C(16)-H(16A) 1.00(4),
C(16)-H(16B) 1.08(4), C(16)-H(16C) 0.92(4), Pt(1)-C(12) 1.951(4),
Pt(1)-N(1) 1.997(3), Pt(1)-Cl(1), 2.3086(8), C(12)-Pt(1)-N(1) 83.0(1),
C(12)-Pt(1)-Cl(1) 97.9(1), N(1)-Pt(1)-Cl(1) 178.67(8), C(12)-Pt(1)-C(16)
160.2(1), N(1)-Pt(1)-C(16) 77.3(1).
When an excess of dmso (∼5 equiv) was added to a deuterated
chloroform solution of agostic 2, a clean, albeit slow (∼3 days for
completion at room temperature), reaction occurred to give a
different cyclometalated product. Now it is apparent that one of
t
1
the C-H bonds of the Bu group has been activated (in the H
NMR spectrum, there is a signal of relative integral 2 at 2.43 ppm
with Pt satellites at 59 Hz, together with a singlet of relative integral
6 at 1.61 ppm), that there is a sulfur coordinated dmso ligand (1H
NMR signal of relative integral 6 at 3.37 ppm with Pt satellites at
27 Hz), and that the previously cyclometalated phenyl ring is now
only connected to the platinum via the pyridine (two sets of protons
each of relative integral 2, and no platinum coupling to the 19F
signal) (eq 2).
9
14142 J. AM. CHEM. SOC. 2009, 131, 14142–14143
10.1021/ja905046n CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
the addition of water to be crucial in assisting the second
cyclometalation step (DFT calculations suggested the hydration and
ionization of the liberated HCl was the driving force for the
reaction).16
In summary, reactions of pyridine 1 allow the study of the
competitive activation of both sp2 and sp3 C-H bonds and provide
additional insight into this mechanistically rich area of Pt(II)
chemistry.1
High resolution mass spectra confirm the formulation as depicted,
and NOE measurements strongly suggest that the dmso is coordi-
nated trans to the pyridine.19 Presumably there is insufficient space
around the central platinum for an additional ligand to coordinate
directly to 2, and only by swapping the site of cyclometalation can
the weak agostic interactions be replaced by a strong dmso-Pt
bond. The observations that no deuterium is incorporated in the
product 3 when the reaction is carried out in deuterated solvent
and that reaction of a 50/50 mixture of 2 and a deuterated version
of 2 results in only fully protiated 3 and deuterated 3 with no
scrambling of isotopes19 strongly suggest an intramolecular trans-
cyclometalation reaction. We can thus see that the balance between
the two possible cyclometalations of the pyridine 1 is rather fine
and depends on the availability of additional ligating species. In
an attempt to gauge the relative ease of cyclometalation of the two
sites, the reaction to synthesize 2 was undertaken in CH3COOD.
After a reaction time of 5 days (80 °C) we see extensive deuteration
Acknowledgment. We thank University of Warwick for a
WPRS award to S.H.C. and support from Advantage West Midlands
(AWM) (part funded by the European Regional Development Fund)
for the purchase of an XRD system.
Supporting Information Available: Full experimental procedures
for the synthesis of all complexes, together with all spectroscopic data,
the details of the X-ray crystallographic study, the data for the analysis
of deuteration of 2, and the CIF file for 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
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We have previously synthesized doubly cyclometalated species
where both cyclometalated rings are aryl, where again we found
JA905046N
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