Insertion of a N-heterocyclic carbene (NHC) into a platinum - Olefin bond

Article abstract of DOI:10.1021/om700447u

The insertion of a N-heterocyclic carbene (NHC) into a coordinated olefin moiety is observed for a Pt-based system. Important intermediates along the reaction pathway have been isolated and fully characterized. Computational work supports a proposed mechanistic pathway involving external attack at the olefin by a NHC ligand rather than an intramolecular migratory insertion route.

Full text of DOI:10.1021/om700447u

3286
Organometallics 2007, 26, 3286-3288
Insertion of a N-Heterocyclic Carbene (NHC) into a
Platinum-Olefin Bond
Serena Fantasia,† Heiko Jacobsen,‡ Luigi Cavallo,§ and Steven P. Nolan*,†
Institute of Chemical Research of Catalonia (ICIQ), AV. Pa¨ısos Catalans, 16, 43007 Tarragona, Spain,
KemKom, Libellenweg 2, 25917 Leck, Germany, and Dipartmento of Chimica, UniVersita´ di Salerno,
Via SalVador Allende, Baronissi (SA) I-84081, Italy
ReceiVed May 7, 2007
Summary: The insertion of a N-heterocyclic carbene (NHC) into
a coordinated olefin moiety is obserVed for a Pt-based system.
Important intermediates along the reaction pathway haVe been
isolated and fully characterized. Computational work supports
a proposed mechanistic pathway inVolVing external attack at
the olefin by a NHC ligand rather than an intramolecular
migratory insertion route.
from Pt centers.8 Other than these important studies, very little
has been reported of the chemistry of NHCs bound to Pt. We
now present some of our findings involving “simple” ligand
substitution reactions of NHCs for coordinated olefin positions.
For a straightforward Pt precursor we selected (1,5-hexadi-
ene)PtCl2 (1), in which the olefinic linkages are known to be
labile.9 We reasoned that this precursor would be an excellent
source of the PtCl2 moiety. Indeed, the olefin can be replaced
by one or two NHCs, depending on the ligand steric hindrance.
Reaction of equimolar amounts of 1 and N,N-bis(2,6-
diisopropylpheny)imidazol-2-ylidene (2; IPr) gives rise to the
substitution of one coordinated double bond by IPr and affords
complex 3 in 80% yield (eq 1).
Since the reported synthesis and X-ray structure determination
of N,N-bis(adamantyl)imidazol-2-ylidene (IAd),1 the use of
N-heterocyclic carbenes (NHCs) in catalysis has experienced a
veritable explosion. These catalytic uses range from their now
well-known role as ligands in ruthenium-based olefin metathesis2
and palladium cross-coupling chemistry3 to their efficient
performance as organocatalysts in transesterification4 and related
transformations.5
As part of a program focusing on the reactivity of NHCs
with late-transition-metal complexes,6 we recently undertook a
study dealing with Pt precursors. The work of Marko´ and co-
workers in this area has clearly demonstrated that Pt-NHC-
containing compounds exhibit excellent activity in the hydro-
silylation of olefins.7 Cavell and co-workers have also reported
unusual reductive elimination of NHC-containing fragments
1H and 13C NMR data show the presence of both a
coordinated double bond (1H, 4.48, 4.12, and 3.79 ppm, JPt-H
) 64-68 Hz; 13C, 88.7 and 67.9 ppm) and an unbound olefin
(1H, 5.55, 4.87, and 4.83 ppm; 13C, 137.5 and 114.9 ppm). An
upfield shift of the IPr carbenic carbon at 144.0 ppm indicates
its coordination. As a platinum-containing by product of reaction
1, the white precipitate 4 was isolated in 6% yield; its 1H NMR
shows the presence of one coordinated double bond at 4.30,
3.70, and 2.81 ppm (JPt-H ) 69-75 Hz). No signal of the free
olefin can be observed, and a multiplet with a very high JPt-H
value of 132 Hz appears at 2.22 ppm. 13C NMR DEPT
correlation data strongly suggest the presence of a Pt-C single
bond.
To unequivocally establish the identity of by product 4, a
single-crystal diffraction study was performed on crystals grown
by slow evaporation of a saturated CHCl3 solution. As shown
by the ball and stick representation depicted in Figure 1, by
product 4 is the result of the formal insertion of IPr into one
coordinated double bond of hexadiene.
* To whom correspondence should be addressed. E-mail: snolan@iciq.es.
† Institute of Chemical Research of Catalonia (ICIQ).
‡ KemKom.
§ Universita´ di Salerno.
(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361-363.
(2) (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann,
W. A. Angew. Chem., Int. Ed. 1998, 37, 2490-2493; 1999, 38, 262
(correction). (b) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J.
Am. Chem. Soc. 1999, 121, 2674-2678. (c) Scholl, M.; Trnka, T. M.;
Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247-2250. (d)
Romero, P. E.; Piers, W. E.; McDonald, R. Angew. Chem., Int. Ed. 2004,
43, 6161-6165.
(3) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309.
(b) Navarro, O.; Kelly, R. A., III; Nolan, S. P. J. Am. Chem. Soc. 2003,
125, 16194-12195. (c) Marion, N.; Navarro, O.; Mei, J.; Scott, N. M.;
Stevens, E. D.; Nolan, S. P. J. Am. Chem. Soc. 2006, 128, 4101-4111. (d)
Navarro, O.; Marion, N.; Mei, J.; Nolan, S. P. Chem. Eur. J. 2006, 12,
5142-5148.
(4) (a) Grasa, G.; Kissling, R. M.; Nolan, S. P. Org. Lett. 2002, 4, 3583-
3586. (b) Nyce, G. W.; Lamboy, J. A.; Connor, E. F.; Waymouth, R. M.;
Hedrick, J. L. Org. Lett. 2002, 4, 3587-3590. (c) Grasa, G. A.; Guveli, T.;
Singh, R.; Nolan, S. P. J. Org. Chem. 2003, 68, 2812-2819.
(5) For an overview on applications of NHC’s in catalysis, see: Nolan,
S. P., Ed. N-Heterocyclic Carbenes in Synthesis; Wiley-VCH: Weinheim,
Germany, 2006.
(6) For example, see: (a) de Fre´mont, P.; Stevens, E. D.; Fructos, M.
R.; D´ıaz-Raquejo, M. M.; Pe´rez, P. J.; Nolan, S. P. Chem. Commun. 2006,
2045-2047. (b) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.;
Nolan, S. P. Angew. Chem., Int. Ed. 2005, 44, 2512-2515. (c) Scott, N.
M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J. Am.
Chem. Soc. 2005, 127, 3516-3526. (d) Dorta, R.; Stevens, E. D.; Scott, N.
M.; Costabile, C.; Cavallo, L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc.
2005, 127, 2485-2495.
When reaction 1 was carried out with excess IPr (1.4 equiv),
the isolated yield of the insertion product 4 increased to 40%.
(7) (a) Marko´, I. E.; Ste´rin, S.; Buisine, O.; Mignani, G.; Branlard, P.;
Tinant, B.; Declercq, J.-P. Science 2002, 298, 204-206. (b) Marko´, I. E.;
Michaud, G.; Berthon-Gelloz, G.; Buisine, O.; Ste´rin, S. AdV. Synth. Catal.
2004, 346, 1429-1434.
(8) (a) McGuinness, D. S.; Cavell, K. J.; Yates, B. F.; Skelton, B. W.;
White, A. H. J. Am. Chem. Soc. 2001, 123, 8317-8328. (b) Bacciu, D.;
Cavell, K. J.; Fallis, I. A.; Ooi, L. Angew. Chem., Int. Ed. 2005, 44, 5282-
5284.
(9) For the synthesis of [Pt(1,5-hexadiene)(Cl)2], see: Jensen, K. Acta
Chem. Scand. 1953, 7, 866-868.
10.1021/om700447u CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/31/2007

Communications
Organometallics, Vol. 26, No. 14, 2007 3287
Path a assumes intermolecular attack of a NHC molecule onto
one of the CdC bonds of 1. The alternative paths b assume
displacement of a CdC double bond in 1 by a NHC molecule
to give 3. Complex 3 can undergo intramolecular attack of the
coordinated NHC ligand at the nearby CdC double bond (path
b1) or intermolecular attack of a second NHC molecule at the
coordinated CdC double bond (path b2).
An energy diagram involving the three reaction pathways is
illustrated in Figure 2. Path a thus starts from 1, and through
transition state [1-4]′q, and an energy barrier of ∆Hq ) 18 kJ/
mol, collapses into product 4′, which is 166 kJ/mol more stable
than 1. The alternative pathways start from 3′. The intramo-
lecular path (b1) proceeds through transition state [3-4]′q, with
an energy barrier of ∆Hq ) 167 kJ/mol, and collapses into 4′,
while the intermolecular attack (b2) proceeds through transition
state [3-5]′q, with an energy barrier of ∆Hq ) 32 kJ/mol, and
collapses into 5′, which is 108 kJ/mol lower in energy than 3′.
Dissociation of the NHC ligand from 5′ is an endothermic step,
by 198 kJ/mol, and gives the final complex 4′.
Figure 1. Ball-and-stick representations of 3 and 4. Selected bond
distances (Å) are as follows. 3: Pt-C(1) ) 1.9993(13), Pt-Cl(1)
) 2.3563(4), Pt-Cl(2) ) 2.3043(4), C(32′)-C(33′) ) 1.326(4).
4: Pt-C(29) ) 2.071(5), Pt-Cl(3) ) 2.3075(15), Pt-Cl(6) )
2.470(2).
Scheme 1. Possible Mechanisms for Reaction 1
Figure 2 clearly shows that, according to the calculations,
the preferred reaction pathway is an intermolecular, rather than
an intramolecular, process. Although a NHC ligand binds
strongly to the platinum-diene fragment, the activation energy
for an intramolecular process is 1 order of magnitude larger
than the activation energy for intermolecular processes.
In light of the DFT calculation results, it appears that 3 is
the intermediate leading to final insertion product 4. The
intramolecular process requires 2 equiv of free IPr to achieve
insertion of IPr onto the coordinated double bond, although the
overall processs is catalytic in the second equivalent of the NHC.
This explanation would account for the increased yield observed
when the reaction is performed with excess IPr. It is worth
noting that the energy required to go from 5′ to 4′ is more than
the activation barrier from 5′ to 3′. Thus, an equilibrium between
3′ and 4′ is likely to be present. This might explain the
impossibility of increasing the yield of 4 by increasing the
reaction time. To explore if the behavior of IPr toward
Attempts to increase further the yield by overnight reaction have
thus far failed.
DFT calculations were performed in order to gain insights
into the mechanism of this reaction. Three possible pathways
were considered for the attack of a NHC molecule onto a CdC
double bond of the diene (Scheme 1).
Figure 2. Energy diagram of reaction 1 pathways.

3288 Organometallics, Vol. 26, No. 14, 2007
Communications
coordinated olefin is general for other NHC ligands, the reaction
between 1 and a 2-fold excess of ItBu (6) was performed (eq
2).
insertion behavior of a NHC in ruthenium-acetylide systems,10a
but to our knowledge this is the first system where the insertion
intermediate and product have been isolated and fully character-
ized. This insertion also appears to be catalytic in the second
equivalent of NHC employed. This behavior has dire conse-
quences for Pt-based catalysis, as deactivation in this manner
would presumably lead to inactive species. Judicious selections
of Pt precursor and NHC become key issues in inhibiting this
reactivity manifold. The exact driving force and factors influ-
encing this insertion behavior are presently being further
explored in our laboratories.
The ItBu insertion product 7 was obtained in 55% yield. 1H
and 13C NMR spectra show the same features as for 4. A single-
crystal diffraction study unambiguously confirms the atom
connectivity.
The noninnocent behavior of coordinated NHC with metal
centers or adjacent ligands has been documented,10 as have
examples where a TM-NHC bond (TM ) transition metal)
displays lability.11 Cavell has proposed a similar insertion
behavior with Pd(0) and Ni(0).12 Kirchner has reported the
Acknowledgment. The ICIQ Foundation is gratefully ac-
knowledged for financial support, as are Eli Lilly, Degussa, and
Umicore for gifts of materials. S.P.N. is an ICREA Research
Professor.
Supporting Information Available: Text and figures giving
experimental details, a detailed discussion of the computational
treatment, NMR, mass spectra and CIF files giving crystal data for
complexes 3, 4, and 7. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data have also
been deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K., and can be obtained on request, free of charge, by quoting
the publication citation and the deposit numbers 639902 (3), 639901
(4), and 639903 (7).
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Drewitt, M. J.; Baird, M. C. Organometallics 2002, 21, 2958-2963.
OM700447U
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