Platinum(IV centres with agostic interactions from either sp2 or sp3 C-H bonds

Article abstract of DOI:10.1039/c0dt01428a

Agostic complexes of platinum(iv) have been isolated and characterised. A Pt(ii) sp³ agostic complex maintains the agostic interaction upon oxidation giving a Pt(iv) sp³ agostic complex; in another Pt(iv) complex the agostic interaction from an sp² C-H bond is sufficient to displace another ligand.

Full text of DOI:10.1039/c0dt01428a

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Platinum(IV) centres with agostic interactions from either sp2 or sp3 C–H
bonds†
Sarah H. Crosby, Robert J. Deeth, Guy J. Clarkson and Jonathan P. Rourke*
Received 20th October 2010, Accepted 2nd December 2010
DOI: 10.1039/c0dt01428a
Agostic complexes of platinum(IV) have been isolated and
characterised. A Pt(II) sp3 agostic complex maintains the
agostic interaction upon oxidation giving a Pt(IV) sp3 agostic
complex; in another Pt(IV) complex the agostic interaction
from an sp2 C–H bond is sufficient to displace another ligand.
the 195Pt satellites on the single sharp resonance for the nine protons
of the Bu group now show a reduced coupling constant – down
t
from 16 Hz in 1 to 5 Hz. Such a change is consistent with oxidation
4
to Pt(IV) and is mirrored in the 19F spectrum where the JF–Pt of
67 Hz reduces to 33 Hz. A platinum chemical shift of -1016 ppm
can be determined for the new complex, again consistent with
a Pt(IV) species. Thus all spectroscopic evidence suggests that the
oxidation proceeded to give a Pt(IV) species that retains the agostic
interaction present in 1, Scheme 1.
Agostic complexes are widely invoked as intermediates in the
activation of C–H bonds in organometallic chemistry,1,2 and are
much sought after as part of the search for the selective and general
transformation of unreactive C–H bonds to other functional
groups.3,4 Many metals have been investigated in the quest for such
transformations: of particular relevance to this paper is the use of
platinum.3,5 Agostic interactions have considerable precedent in
the stabilisation of coordinatively unsaturated metal centres which
then ought to be ideally set up for subsequent C–H activation.
Here we report the synthesis and characterisation of complexes
containing agostic interactions from either sp2 or sp3 hybridised
C–H bonds to platinum(IV) centres. Whilst formally 14e T shaped
platinum(II) complexes containing agostic interactions have long
been known,6–8 with a recent contribution to the area from our own
work,9 agostic interactions within formally 16e square pyramidal
Pt(IV) complexes are very rare, with only one crystallographically
characterised example known.10 The paucity of observed Pt(IV)
agostic complexes is perhaps surprising, given that all substitution
reactions of these 18e octahedral centres are expected to proceed
via dissociative processes11 and therefore formally five coordinate
centres. It is pertinent to note that a number of five coordinate
Pt(IV) complexes have been isolated and characterised;12–22 none
however contain agostic interactions.
Scheme 1
We were able to grow crystals of the new species 2, and the
structure is indeed that of a Pt(IV) species with a d agostic
interaction with the tBu group, Fig. 1.
The structure shown above represents the second reported
example of a crystallographically characterised Pt(IV) agostic
interaction. Whilst we23 have reported solution evidence (backed
up with calculations) for the presence of Pt(IV) agostic species
as reactive intermediates in solution and others have postulated
their existence,24 there has previously been only one definitive
example.10 Our crystal structure of 2 clearly identifies the agostic
interaction as occurring with a single hydrogen, which was directly
˚
During our study into the properties of our9 previously charac-
terised Pt(II) sp3 agostic species 1 we decided to test the resilience of
the agostic interaction, reasoning that a suitably protected metal
centre might maintain an agostic interaction even on oxidation.
Oxidation of the platinum(II) species 1 with the iodine(III) reagent
iodobenzene dichloride proceeds rapidly at room temperature to
located in the difference map: the H–Pt distance is 2.15(3) A
˚
and the corresponding C–Pt distance is 2.721(3) A. This single
interaction is unlike that present in 1 where we identified a
bifurcated dual interaction. It is also pertinent to note that whilst
the H–Pt distances in 1 and 2 are very similar, the C–Pt distance
˚
in 2 is significantly longer than in 1 (2.721(3) versus 2.472(4) A)
1
suggesting the single C–H agostic interaction in 2 is weaker than
the bifurcated one found in 1.
yield cleanly a single species. The room temperature H NMR
spectrum of the new species exhibits the same coupling pattern as
its precursor, with slight changes in chemical shift. Significantly,
1
Variable temperature H NMR studies (on a 500 MHz spec-
trometer) on 2 show the single room temperature◦resonance of
the tBu group noticeably broadening by about -40 C. Although
this broadening had become significant at -95 ◦C (the lowest
temperature we were able to access in solution), this peak was
clearly still a single resonance, indicating the agostic interaction
is rapidly exchanging across all nine hydrogens. A single 13C
Department of Chemistry, University of Warwick, Coventry, UK, CV4 7AL.
E-mail: j.rourke@warwick.ac.uk; Fax: 44 2476 524112; Tel: 44 2476 523263
† Electronic supplementary information (ESI) available: Full experimental
details. CCDC reference number 790276. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0dt01428a
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that of 4 with two exceptions: there are no signals corresponding
2
to a coordinated DMSO and the JH–Pt on the CH2 group has
increased from 72 to 87 Hz. Complex 5 has a single 195Pt resonance
at -851 ppm and, in the absence of any other ligating species, we
propose a structure with an agostic interaction from the phenyl
ring completing the coordination sphere of the platinum centre,
Scheme 2.
Whilst platinum satellites are not obvious on the phenyl ring
proton ortho to the pyridine (meta to the F) in 5, a 1H-195Pt NMR
correlation experiment shows a clear interaction – presumably
the satellites are lost in sides of the peaks. Satellites at 7 Hz can
be seen on the corresponding carbon resonance providing strong
evidence for an agostic interaction.25,26 As with complex 2, the 1JC–H
coupling constant for the appropriate C–H bond in 5 changes little
from the values observed in the uncoordinated ligand (162 Hz).
Fig. 1 Molecular structure of 2; thermal ellipsoids at 50% probability
level. Selected bond lengths (A) and angles (deg): Pt(1)–H(16A) 2.16(3);
˚
1
The H spectrum of 5 is sharp at room temperature, but begins
Pt(1)–C(16) 2.721(3); C(16)–H(16A) 1.04(3); C(16)–H(16B) 1.03(3);
C(16)–H(16C) 0.93(3); Pt(1)–C(12) 1.996(3); Pt(1)–N(1) 2.029(2);
Pt(1)–Cl(1) 2.2984(6); Pt(1)–Cl(3) 2.3021(7); Pt(1)–Cl(2) 2.3161(6);
C(13)–C(16) 1.533(4); C(12)–Pt(1)–N(1) 83.21(10); C(12)–Pt(1)–Cl(1)
97.41(8); N(1)–Pt(1)–Cl(1) 179.30(7); C(12)–Pt(1)–H(16A) 164.0(9);
N(1)–Pt(1)–H(16A) 87.3(8); Cl(1)–Pt(1)–H(16A) 92.2(8); Cl(3)–
Pt(1)–H(16A) 76.7(9); Cl(2)–Pt(1)–H(16A) 102.1(9); C(2)–N(1)–C(6)
121.7(2).
to broaden upon cooling until at around -60 ◦C (on a 500 MHz
spectrometer) two separate signals of relative intensity one are seen
for the CH2 group (a single broad signal is still seen for the CH3
groups) and at least six signals are seen in the aromatic region. Even
on further cooling to -95 ◦C the 1H signals in the aromatic region
remained too broad to be fully resolved, as were the 13C signals.
It is possible to extract an experimental barrier to equilibration of
the resonances of 9.5 1 kcal mol-1 from this data.
resonance for the methyl groups was also observed at both room
and low temperature. One further parameter we were able to
Quantum chemical calculations were used to determine a likely
minimum energy conformation of 5: this was found to be one in
which the phenyl ring is inclined ~50◦ with respect to the pyridine
ring (0◦ = coplanar, 90◦ = perpendicular), and this brings one of the
1
determine, the JC–H coupling constant of the methyl groups of
t
the Bu, showed an insignificant change between uncoordinated
ligand (128 Hz) and agostic species 2 (127 Hz), indicating little
back-bonding from the Pt(IV) centre.
˚
hydrogens into close (~2.65 A) proximity to the platinum centre.
Such a conformation would account for the low temperature
NMR spectra as it not only renders the four protons of the
phenyl ring different, it also makes the two CH2 protons, and
the two methyl groups different. Rotational profiles about the
phenyl-pyridine bond were calculated using both B3LYP and MP2
methods (Fig. 2): a definite but small barrier to rotation through
the perpendicular arrangement of the phenyl and pyridine rings
is predicted (~1.5 kcal mol-1), which at least qualitatively explains
the experimental results, Fig. 2.
In parallel to our studies on oxidation of the agostic species 1, we
also looked at the oxidation of the sp3 cyclometalated species 3.
Oxidation with iodobenzene dichloride results in the rapid and
clean formation of a new species 4 at -40 ◦C. Spectroscopic
evidence points clearly to the formulation depicted in Scheme
1
2 below: the coupling patterns of the signals in the H NMR
spectrum remain similar to those observed in the starting material
3, a single signal is observed in the 19F spectrum, and a platinum
resonance is seen at -1568 ppm. The presence of a single 1H
resonance for the six protons of the DMSO, and likewise a single
resonance for the CH2 group (both with 195Pt satellites) confirms
the DMSO has not moved to a position cis to both the N and the
CH2; we can rule out its unlikely movement to the crowded position
trans to the CH2 group on the basis of NOE measurements: clear
NOE enhancements are seen between the signals of the DMSO
and the CH2 group (and vice versa) with none seen between the
aromatic signals and the DMSO.
Fig. 2 The energy profile of 5 as a function of rotation about the
phenyl-pyridine bond.
Scheme 2
We thus have identified two clear examples of Pt(IV) metal
complexes containing agostic interactions: in 2 the agostic inter-
action comes from an sp3 hybridised C–H bond, whereas in 5 the
interaction is from an sp2 hybridised C–H bond. In both cases,
our evidence points to minimal back-bonding from the electron
poor Pt(IV) centre suggesting these interactions might be relatively
Whilst we were able to clearly identify the new complex 4, we
were unable to isolate it: at about 0 ◦C 4 begins to transform
cleanly to another new species 5. The new complex 5 is sufficiently
stable to be isolated and manipulated at room temperature in
1
air. At room temperature the H NMR spectrum of 5 resembles
1228 | Dalton Trans., 2011, 40, 1227–1229
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weak. However, in the case of complex 5, Scheme 2, the agostic
interaction, together with an expected release of steric strain is
sufficiently strong to displace a DMSO ligand.27
10 E. Khaskin, P. Y. Zavalij and A. N. Vedernikov, Angew. Chem., Int. Ed.,
2007, 46, 6309–6312.
11 P. V. Bernhardt, C. Gallego and M. Martinez, Organometallics, 2000,
19, 4862–4869.
We thank Warwick University for a WPRS award to SHC and
support from Advantage West Midlands (AWM) (part funded by
the European Regional Development Fund) for the purchase of
an XRD system, and a high resolution mass spectrometer.
12 U. Fekl, W. Kaminsky and K. I. Goldberg, J. Am. Chem. Soc., 2001,
123, 6423–6424.
13 U. Fekl and K. I. Goldberg, J. Am. Chem. Soc., 2002, 124, 6804–
6805.
14 U. Fekl, W. Kaminsky and K. I. Goldberg, J. Am. Chem. Soc., 2003,
125, 15286–15287.
15 D. Karshtedt, J. L. McBee, A. T. Bell and T. D. Tilley, Organometallics,
2006, 25, 1801–1811.
Notes and references
16 S. M. Kloek and K. I. Goldberg, J. Am. Chem. Soc., 2007, 129,
1 M. Brookhart, M. L. H. Green and G. Parkin, Proc. Natl. Acad. Sci.
U. S. A., 2007, 104, 6908–6914.
2 E. Clot and O. Eisenstein, “Agostic interactions from a computational
perspective: One name, many interpretations”, In Principles and
Applications of Density in Inorganic Chemistry II, 2004; Vol. 113,
pp 1–36.
3460.
17 A. T. Luedtke and K. I. Goldberg, Inorg. Chem., 2007, 46, 8496–8498.
18 S. Reinartz, P. S. White, M. Brookhart and J. L. Templeton, J. Am.
Chem. Soc., 2001, 123, 6425–6426.
19 P. Sangtrirutnugul and T. D. Tilley, Organometallics, 2008, 27, 2223–
2230.
20 N. M. West, P. S. White, J. L. Templeton and J. F. Nixon,
Organometallics, 2009, 28, 1425–1434.
3 J. A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507–514.
4 R. G. Bergman, Nature, 2007, 446, 391–393.
5 A. S. Goldman and K. I. Goldberg, In Activation and Functionalization
of C–H Bonds, K. I. Goldberg and A. S. Goldman, ed. ACS,
Washington DC, 2004; Vol. ACS Symposium Series vol 885.
6 N. Carr, B. J. Dunne, A. G. Orpen and J. L. Spencer, J. Chem. Soc.,
Chem. Commun., 1988, 926–928.
7 N. Carr, L. Mole, A. G. Orpen and J. L. Spencer, J. Chem. Soc., Dalton
Trans., 1992, 2653–2662.
8 M. J. Ingleson, M. F. Mahon and A. S. Weller, Chem. Commun., 2004,
2398–2399.
9 S. H. Crosby, G. J. Clarkson and J. P. Rourke, J. Am. Chem. Soc., 2009,
131, 14142–14143.
21 S. B. Zhao, G. Wu and S. Wang, Organometallics, 2008, 27, 1030–1033.
22 J. L. McBee and T. D. Tilley, Organometallics, 2009, 28, 3947–3952.
23 S. H. Crosby, G. J. Clarkson, R. J. Deeth and J. P. Rourke,
Organometallics, 2010, 29, 1966–1976.
24 A. Baber, C. Fan, D. W. Norman, A. G. Orpen, P. G. Pringle and R. L.
Wingad, Organometallics, 2008, 27, 5906–5910.
25 W. H. Bernskoetter, C. K. Schauer, K. I. Goldberg and M. Brookhart,
Science, 2009, 326, 553–556.
26 A. B. Chaplin and A. S. Weller, Organometallics, 2010, 29, 2332–2342.
27 B. L. Conley and T. J. Williams, J. Am. Chem. Soc., 2010, 132, 1764–
1765.
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