Unusual reactivities of N-heterocyclic carbenes upon coordination to the platinum(ii)-dimethyl moiety

Article abstract of DOI:10.1039/b920482b

Unsaturated NHCs of varying steric bulk undergo a series of unusual oxidative addition and reductive elimination processes upon binding to the Pt(Me)₂ fragment.

Full text of DOI:10.1039/b920482b

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Unusual reactivities of N-heterocyclic carbenes upon coordination
to the platinum(II)–dimethyl moietyw
George C. Fortman,a Natalie M. Scott,a Anthony Linden,b Edwin D. Stevens,c Reto Dorta*b
and Steven P. Nolan*ac
Received (in Cambridge, UK) 1st October 2009, Accepted 22nd December 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b920482b
Unsaturated NHCs of varying steric bulk undergo a series of
unusual oxidative addition and reductive elimination processes
upon binding to the Pt(Me)2 fragment.
N-Heterocyclic carbenes are now well-established and versatile
entities for late-transition metal compounds and have proven
to be often superior ancillary ligands in homogeneous catalytic
processes.1 Among such catalytic transformations, alkane
(and in particular methane) C–H bond activation and
Scheme 1 Unsaturated N-heterocyclic carbenes used with corres-
ponding %Vbur steric parameter values.7
functionalization certainly remains one of the most challenging
tasks.2 While a variety of homogeneous metal compounds
have shown potential for this kind of process, results with
(C–H) and reductive elimination (C–H and C–C) processes
platinum complexes of the Shilov-type are particularly
upon binding to the platinum center.
interesting for the functionalization of methane. Intense
We anticipated a rather straightforward reactivity between
relatively small N-heterocyclic carbene ligands MeIMe
research has gone into trying to understand individual steps
(%Vbur = 25.5), ICy (%Vbur = 26.1), or MeIiPr (%Vbur
=
of the Shilov chemistry with Pt(II)–dimethyl or Pt(IV)–methyl
27.9) and (COD)Pt(Me)2.8 Indeed, mixing (COD)Pt(Me)2
with 2 equivalents of the respective ligand in benzene at
room temperature leads to fast substitution of the COD
moiety and, after appropriate workup, to high yields
of complexes cis-(MeIMe)2Pt(Me)2 (1), cis-(ICy)2Pt(Me)2 (2),
and cis-(MeIiPr)2Pt(Me)2 (3). Since these compounds represented
a new class of NHC–platinum complexes, complete characteri-
zation included X-ray diffraction studies. Ball-and-stick
drawings of cis-1, cis-2 and cis-3 are incorporated in
Scheme 2. Due to the steric crowding of the two cis-configured
NHC ligands in these compounds, deviations from the
expected square-planar geometry are already apparent in these
complexes. The bond angles between the NHCs and the
platinum center increase when going from cis-1 to cis-2 to
cis-3, and lead to a contraction in the CH3–Pt–CH3 angles
(Table S1).w
model compounds surrounded with neutral or anionic
nitrogen-based chelates and phosphine ligands.3 These findings
have subsequently been implemented in an improved catalytic
process for the conversion of methane.4 Despite the obvious
similarity between NHCs and the ligands normally used in this
type of chemistry and despite the fact that interesting
palladium-catalyzed methane reactivity has been disclosed
using a chelating N-heterocyclic carbene ligands,5 efforts to
develop Shilov-type chemistry with platinum–NHC compounds
have been minimal. For instance, reports in the literature
where NHC ligands are part of alkyl- or aryl-containing
platinum species are exceedingly scarce,6 and apparently
simple model compounds such as (NHC)2Pt(Me)2 or
(NHC)2Pt(Me)Cl are unknown at present.
Herein, we report our results on the coordination behavior
of monodentate, unsaturated NHCs of varying steric bulk
(Scheme 1) with (COD)Pt(Me)2,7 and show how the bulkier
ligand members undergo a series of unusual oxidative addition
The increasing distortions in complexes 1–3 already hinted
to possible problems when accommodating sterically more
a EaStCHEM School of Chemistry, University of
St Andrews, St Andrews, KY16 9ST, UK.
E-mail: snolan@st-andrews.ac.uk;
Web: http://chemistry.st-and.ac.uk/staff/spn/group
b University of Zurich, Institute of Organic Chemistry,
Winterthurerstrasse 190, 8057, Zurich, Switzerland.
E-mail: dorta@oci.uzh.ch
c University of New Orleans, Department of Chemistry,
2000 Lakeshore Dr, New Orleans, LA 70148, USA
w Electronic supplementary information (ESI) available: Table S1
with selected bond lengths and angles of complexes 1–7, experimental
procedures. CCDC 741808–741813 (1–6) and 756172 (7). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b920482b
Scheme 2 Synthesis and ball-and-stick drawings of cis-(MeIMe)2Pt(Me)2
(1), cis-(ICy)2Pt(Me)2 (2), and cis-(MeIiPr)2Pt(Me)2 (3).
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demanding NHC ligands. For the next bulkier ligand in
our series, namely IMes (%Vbur = 34.2), reaction with
(COD)Pt(Me)2 in benzene and subsequent evaporation gave
two compounds in variable ratio depending on the reaction
time. In situ reactions in C6D6 revealed relatively fast displacement
of COD by the two IMes ligands. Based on these studies,
preparative reactions were devised. Workup of the reaction
after 10 minutes gave the first complex in pure form, while
extended reaction times and slight heating led to exclusive
generation of the second compound. This latter complex
was fully characterized and turned out to be the unusual,
cyclometallated derivative cis-(IMes)(IMes0)Pt(Me) (5).w The
X-ray structure of 5 (Scheme 3, below) shows that the two
NHC ligands remain in a cis orientation with the cyclo-
metallated side chain trans to the normal IMes ligand. In spite
of its ready transformation into 5, 1H and 13C NMR
characterization of the first compound, that we presumed to
be cis-(IMes)2Pt(Me)2 (4), was possible. In addition, when a
benzene solution of 4 is layered with diethyl ether, X-ray
quality crystals are swiftly formed and permit the unequivocal
confirmation of its structure (Scheme 3, above).w The cis
orientation of the ligands, together with the steric requirements
of the mesityl wingtips of IMes, leads to a highly distorted
structure in the solid state. In order to accommodate both
IMes ligands, the imidazole rings cannot acquire the usual
coordination mode and tilt away and towards the two
Pt–methyl groups (dihedral angles between the plane of the
imidazole ring and platinum in 4: 159.2(1)1 and 163.0(1)1).
Furthermore, one of the o-methyl groups of IMes approaches
the metal center (3.569(5) A). The overall structure certainly
helps explain the propensity of 4 to undergo subsequent
intramolecular C–H activation of one of the benzylic positions
of the ligand. A straightforward reaction pathway accounting
Scheme 4 Proposed reaction pathway for the formation of 6 with
ball-and-stick drawing of (IPr)2Pt (6).
for the formation of 5 is outlined in Scheme 3 and involves
initial C–H activation of 4 to give a six-coordinate platinum(IV)
complex which subsequently undergoes reductive elimination
of a molecule of methane to give 5. It should be noted that
while intramolecular activation of the benzylic C–H bond of
the aromatic side chain in IMes is known for both Ru and Rh,9
the example here represents the first case of such reactivity with a
group 10 metal.
Activation of the isopropyl groups of the aromatic side
chains in IPr is more difficult than benzylic C–H activation in
IMes.10 We therefore turned our attention to the interaction of
IPr (%Vbur = 35.8) with (COD)Pt(Me)2. Here, reaction of 2
equivalents of IPr with (COD)Pt(Me)2 at room temperature
was less straightforward and led to a mixture of complexes.
Gratifyingly, slight heating of the benzene or toluene solution
(50–80 1C) gave a yellow-orange fluorescent solution from
which a single, bright yellow complex was isolated in high
overall yield after workup. Complete characterization showed
the compound to be the homoleptic 14-electron platinum(0)
complex (IPr)2Pt (6).w The most likely reaction pathway
leading to 6 is shown in Scheme 4 and involves initial
coordination of two IPr ligands. The resulting cis-configured
(IPr)2Pt(Me)2 species would then undergo ready C–C reductive
elimination of ethane because of the steric pressure exerted by
the two IPr ligands on the methyl groups. Such reactivity is
highly unusual and we are not aware of reports on C–C
reductive elimination of simple methyl/alkyl groups from
platinum(II) compounds.11 In the solid state, complex 6 shows
the expected linear two-coordinate arrangement with somewhat
shorter NHC–Pt bond lengths than in the Pt(II) complexes
(Table S1).w
The bulkiest NHC member in our series, namely ItBu
(%Vbur = 37.0), chooses yet another reactivity pathway to
alleviate the severe steric crowding which would arise from a
compound of formula cis-(NHC)2Pt(Me)2. Reaction of 2
equivalents of ItBu with (COD)Pt(Me)2 in diethyl ether leads
to slow precipitation of a white powder that is isolated in good
1
yield after workup. The H NMR spectrum showed a 1 : 1
stoichiometry between the methyl and ItBu ligands, but several
resonances indicated that the complex formed was not cis-
(ItBu)2Pt(Me)2. To elucidate the structure of the compound,
crystals were grown from slow diffusion of diethyl ether
into a concentrated CH2Cl2 solution and were subjected
to an X-ray diffraction study.w The crystal structure of
cis-(nItBu)(aItBu)Pt(Me)2 (7, Scheme 5) shows coordination
of one of the two ItBu moieties via the C4 position of the
central imidazole ring. By repositioning one of the tBu
side chains through abnormal binding of ItBu, the overall
Scheme 3 Probable reaction pathway for the formation of 4 and 5
with ball-and-stick drawings of cis-(IMes)2Pt(Me)2 (4), and of
cyclometallated (IMes)(IMes0)Pt(Me) (5).
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Notes and references
1 (a) N-Heterocyclic Carbenes in Synthesis, ed. S. P. Nolan,
Wiley-VCH, Weinheim, 2006; (b) N-Heterocyclic Carbenes
in Transition Metal Catalysis, ed. F. Glorius, Topics in Organo-
metallic Chemistry, Springer, Berlin, 2007, vol. 21; (c) S. Dı
Gonzalez, N. Marion and S. P. Nolan, Chem. Rev., 2009, 109,
3612.
´
ez-
´
2 (a) Activation and Catalytic Reactions of Saturated Hydrocarbons in
the Presence of Metal Complexes, ed. A. E. Shilov and
G. B. Shul’pin, Springer, Berlin, 2000; (b) Activation and Functio-
nalization of C–H Bonds, ed. K. I. Goldberg and A. S. Goldman,
Oxford University Press, Oxford, 2004.
3 For a recent review, see: M. Lersch and M. Tilset, Chem. Rev.,
2005, 105, 2471.
4 R. A. Periana, D. J. Taube, S. Gamble, H. Taube, T. Satoh and
H. Fujii, Science, 1998, 280, 560.
5 M. Muehlhofer, T. Strassner and W. A. Herrmann, Angew. Chem.,
Int. Ed., 2002, 41, 1745.
Scheme 5 Possible reaction pathway leading to the formation of 7
with ball-and-stick drawing of cis-(nItBu)(aItBu)Pt(Me)2 (7). C(7)–Pt
and C(11)–Pt distances: 3.278(8) and 3.193(8) A.
6 (a) E. M. Prokopchuk and R. J. Puddephatt, Organometallics,
2003, 22, 563; (b) J. S. Owen, J. A. Labinger and J. E. Bercaw,
J. Am. Chem. Soc., 2004, 126, 8247; (c) R. Lindner, C. Wagner and
D. Steinborn, J. Am. Chem. Soc., 2009, 131, 8861.
crowding is considerably reduced and the X-ray structural
data permit the quantification of the steric pressure around the
Pt center. This can be gauged by measuring the %Vbur values
for both normal (37.0) and abnormal (29.4) ItBu ligands
present in 7. As the numbers clearly indicate, an abnormal
binding mode significantly relieves the bulk around the Pt
center, rendering the sterics of this coordination mode more
similar to the overall %Vbur values of less sterically demanding,
normally bound complexes 1–3.
7 For calculation of %Vbur, the web-based SambVca application
was used: https://www.molnac.unisa.it/OMtools/sambvca.php.
See also: A. Poater, B. Cosenza, A. Correa, S. Giudice,
F. Ragone, V. Scarano and L. Cavallo, Eur. J. Inorg. Chem.,
2009, 1759.
8 For complexation of corresponding monophosphines and thermo-
chemical data: C. M. Haar, S. P. Nolan, W. J. Marshall,
K. G. Moloy, A. Prock and W. P. Giering, Organometallics,
1999, 18, 474.
9 (a) J. Huang, E. D. Stevens and S. P. Nolan, Organometallics,
2000, 19, 1194; (b) M. J. Chilvers, R. F. R. Jazzar, M. F. Mahon
and M. K. Whittlesey, Adv. Synth. Catal., 2003, 345, 1111;
A closer inspection of the X-ray structure of 7 also indicates
that the tBu groups of the normally bound ItBu ligand
approach the metal considerably and both C(7)–Pt and
C(11)–Pt distances are within the range of weak agostic
(c) D. Giunta, M. Holscher, C. W. Lehmann, R. Mynott,
¨
C. Wirtz and W. Leitner, Adv. Synth. Catal., 2003, 345, 1139;
(d) K. Abdur-Rashid, T. Fedorkiw, A. J. Lough and R. H. Morris,
Organometallics, 2004, 23, 86.
interactions (3.278(8)
A and 3.193(8) A; closest H–Pt
distances; 2.45(7) A and 2.61(7) A).12 Both these X-ray data
as well as the fact that ItBu is sterically more demanding
than IMes and IPr support the mechanistic proposal for its
generation (Scheme 5). Accordingly, the pathway to 7 would
proceed via intermolecular C–H activation of the alkenyl C–H
bond of the imidazole fragment by a transient (ItBu)Pt(Me)2
complex to give an unsaturated Pt(IV) species. Subsequent
formal C–H reductive elimination via hydrogen migration
from the platinum center to the C2 position of the imidazole
moiety would generate the observed complex 7. Formation
of monometallic complex 7 is very unusual because the
abnormal NHC binding mode is directly generated from the
free monodentate carbene ligand.13
10 For the only known example (with Ir), see: (a) C. Y. Tang,
W. Smith, D. Vidovic, A. L. Thompson, A. B. Chaplin and
S. Aldridge, Organometallics, 2009, 28, 3059; activation of the
methyl group of the o-isopropylphenyl fragment on a nacnac-type
ligand is known for platinum, see: (b) U. Fekl and K. I. Goldberg,
J. Am. Chem. Soc., 2002, 124, 6804.
11 For thermolytic C–C reductive elimination from strained platina-
cyclobutanes: (a) R. Di Cosimo and G. M. Whitesides, J. Am.
Chem. Soc., 1982, 104, 3601. For thermolytic reductive C–C
elimination from diarylplatinum(II) compounds: (b) S. E. Himmel
and G. B. Young, Organometallics, 1988, 7, 2440;
(c) R. K. Merwin, R. C. Schnabel, J. D. Koola and
D. M. Roddick, Organometallics, 1992, 11, 2972.
12 (a) A. Albinati, C. G. Anklin, F. Ganazzoli, H. Ruegg and
P. S. Pregosin, Inorg. Chem., 1987, 26, 503; (b) A. Albinati,
C. Arz and P.S. Pregosin, Inorg. Chem., 1987, 26, 508.
13 Examples of abnormal N-heterocyclic carbenes generated directly
from free NHCs are rare and either involve multidentate NHC
chelates or metal clusters, see: (a) X. Hu, I. Castro-Rodriguez and
K. Meyer, Organometallics, 2003, 22, 3016; (b) A. A. Danopoulos,
N. Tsoureas, J. A. Wright and M. E. Light, Organometallics,
2004, 23, 166; (c) C. E. Ellul, M. F. Mahon, O. Saker and
M. K. Whittlesey, Angew. Chem., Int. Ed., 2007, 46, 6343;
(d) M. R. Crittall, C. E. Ellul, M. F. Mahon, O. Saker and
M. K. Whittlesey, Dalton Trans., 2008, 4209. For a review on
abnormal/remote NHCs, see: (e) O. Schuster, L. Yang,
H. G. Raubenheimer and M. Albrecht, Chem. Rev., 2009, 109,
3445. For a very low-yielding case (6%), see ref. 10a.
In conclusion, the present results indicate that N-hetero-
cyclic carbene ligands can be used to generate a remarkable
range of high-yielding, unusual products when reacted with
platinum–dimethyl precursors. All of the NHCs studied bind
readily to (COD)Pt(Me)2.14 With the bulkier members of the
NHCs tested, a series of unprecedented transformations have
been uncovered. Studies aimed at exploiting the reactivities
found here are ongoing in our laboratories.
NMS wishes to thank the American Australian Association
for a Sir Keith Murdoch Fellowship. RD thanks the
Alfred Werner Foundation for an Assistant Professorship.
SPN thanks the ERC for an Advanced Researcher Grant
(FUNCAT).
14 Qualitatively, this fact can be appreciated through the simple
observation that IMes, IPr and ItBu interact readily with the
platinum precursor, while similarly bulky P(tBu)3 does not bind
to (COD)Pt(Me)2 (see ref. 8a). Preliminary quantitative data via
reaction calorimetry support this view.
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This journal is The Royal Society of Chemistry 2010
1052 | Chem. Commun., 2010, 46, 1050–1052