Diels-Alder cycloaddition as a new approach toward stable PC(sp 3)P-metalated compounds

Article abstract of DOI:10.1039/b815051f

A straightforward synthetic route toward a new family of dibenzobarrelene-based cyclometalated compounds is described. The Royal Society of Chemistry.

Full text of DOI:10.1039/b815051f

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Diels–Alder cycloaddition as a new approach toward stable
PC(sp3)P-metalated compoundsw
Clarite Azerraf, Alina Shpruhman and Dmitri Gelman*
Received (in Cambridge, UK) 29th August 2008, Accepted 12th November 2008
First published as an Advance Article on the web 11th December 2008
DOI: 10.1039/b815051f
A straightforward synthetic route toward a new family of
dibenzobarrelene-based cyclometalated compounds is described.
on a one-step transformation of readily available and structu-
rally simple anthracene-based 9-C(sp2)-metalated complexes9
into more complex C(sp3)-metalated ones by means of
Diels–Alder cycloaddition as depicted in Scheme 2. Our belief
is that successful application of this strategy to specially
designed transition metal precursors and dienophiles in a
combinatorial manner may result in a diversity-oriented
approach toward the family of C(sp3)-based pincer complexes.
Initially, we tested a reaction between the known and readily
available palladacycle 19a and dimethyl acetylenedicarboxylate
as a dienophile.
Pincer-like compounds are now well studied and recognized as
efficient catalysts in a variety of chemical transformations,
building blocks for construction of advanced materials, as well
as components in supramolecular systems.1 With regard to the
carbometalated pincer complexes, the vast majority of reported
substances possess an sp2-hybridized carbon, while examples of
C(sp3)-based compounds appear in the literature only occasion-
ally.2 This imbalance, arguably, originates from a greater
thermal and conformational stability of the former compared
to the latter. This is especially true of complexes bearing
all-aliphatic ligands; in these cases, the carbometalated com-
pounds often coexist in equilibrium with isomeric olefinic or
carbenic species due to the facile a- or b-hydride elimination.3
On the other hand, available experimental data strongly imply a
very interesting and unique reactivity of C(sp3)-cyclometalated
organometallics due to a stronger donating ability of the sp3
carbon.2,4 It is thus obvious that the development of a facile
synthetic route toward stable compounds of this type will
encourage their utilization in many research areas.
We were glad to realize that the organometallic compound 1
reacts smoothly under thermal conditions resulting in the
formation of the [4 þ 2] adduct 4 in boiling diethylene glycol
diethyl ether (anisole can be used instead) in 71% yield. The
choice of solvents appears to be critical for the successful
transformation. The formation of 4 was only sluggish in less
polar solvents, such as xylene (apparently due to a low solubility
of the transition metal precursors), and less efficient in protic
solvents, such as ethylene glycol monomethyl ether. In the
latter case, we suspect the formation of transesterification
by-products, which complicate the final work-up.
We hypothesized that triptycene-like bidentate ligands
lacking easily abstractable hydrogen atoms represent
a
suitable skeleton for construction of stable C(sp3)-based pincer
complexes (Scheme 1).5
Traditionally, pincer complexes of this type are prepared
either by C–H activation (Scheme 1, right) or by oxidative
insertion of a coordinated transition metal into the C–X bond
of the halogenated spacer (Scheme 1, left). Unfortunately, our
previous experience with triptycene-based ligands reveals that
the central methine hydrogen is insufficiently acidic to be
displaced by a proximate metal center.6,7 By now, we were able
to prepare a single example of very stable but catalytically active
iridium(III) C(sp3)-metalated compound using this strategy,8
while other tested metals (Ni–Pt, Rh and Ru) failed to form
the desired structure.The second route (oxidative addition)
is, apparently, feasible, but synthetically more laborious and
therefore may be applied to a limited number of targets.
Therefore we envisioned a conceptually different synthetic
approach toward the desired class of compounds which relies
Analogous transformations were performed using the known
Ni (2)9a and new Pt (3) C(sp3)-cyclometalated precursors.
The expected products 5 and 6 were isolated in 73 and 81%
yield, respectively.
It is interesting to note that the difference in 31P-NMR
spectra of the precursors versus products is very small in all
cases (1–2 ppm shift). However, the change of hybridization at
1
C-9 and C-10 can be clearly detected by H-NMR measure-
ments thanks to a very characteristic highfield chemical shift of
Institute of Chemistry, The Hebrew University, 91904 Jerusalem,
Israel. E-mail: dgelman@chem.ch.huji.ac.il; Fax: +972-2-6585279;
Tel: +972-2-6584588
w Electronic supplementary information (ESI) available: Detailed
experimental procedures and NMR spectra for all compounds men-
tioned in the text. CCDC 699996–699997. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI:
10.1039/b815051f
Scheme 1 Possible synthetic approaches toward PC(sp3)P-complexes
based on the triptycene spacer.
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reaction is driven to completion by only 0.001 mol% of 4 in 12 h
which corresponds to at least 105 TON.
Scheme 2 Diels–Alder approach toward PC(sp3)P-complexes.
To conclude, we have demonstrated that the unprecedented
Diels–Alder reaction of organometallic dienes with organic
dienophiles is indeed possible and appears to be a straight-
forward synthetic route toward dibenzobarrelene-based
C(sp3)-metalated pincer complexes. More detailed studies on
the new family of compounds will follow.
the H-10 signal (from ca. 8.3 ppm in 1–3 to ca. 5.8 ppm in 4–6)
that takes place over the course of the reaction. Other NMR
data also match the proposed structure (see supporting
informationw).
Despite the unambiguity in identification of the new com-
pounds, the single crystals of 4 and 5 were subjected to X-ray
analysis10 (ORTEP pictures are given in Fig. 1). As expected,
unlike in the square planar 1, the palladium center in C(sp3)-
metalated 4 is strongly distorted from planarity toward a
butterfly-like environment. For example, the observed
P(1)–Pd–P(2), C(1)–Pd–Cl(1) angles and P(1)–P(2) intramole-
cular distance between the two phosphorus atoms are
150.43(4)1, 171.86(11)1 and 4.427 A, respectively. As expected,
the Pd–Cl bond in 4 is slightly longer than the corresponding
bond in the previously reported 1 due to a stronger trans
influence exerted by the C(sp3) ligand.9a Very similar structural
features were observed for the nickel analog 5.
We thank the Israel Science Foundation (Grant No. 866/06)
for financial support. We also thank Dr Shmuel Cohen for
solving X-ray structures.
Notes and references
1 (a) C. Crocker, R. J. Errington, W. S. McDonald, K. J. Odell,
B. L. Shaw and R. J. Goodfellow, J. Chem. Soc., Chem. Commun.,
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N. Veldman, A. L. Spek, J.-P. Sauvage and G. van Koten, Angew.
Chem., Int. Ed. Engl., 1994, 33, 1282; (c) M. Albrecht, M. Lutz,
A. L. Spek and G. van Koten, Nature, 2000, 406, 970; (d) M. E. van
der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759;
(e) K. B. Renkema, Y. V. Kissin and A. S. Goldman, J. Am. Chem.
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Chem. Res., 2008, 41, 201; (g) W. Leis, H. A. Mayer and
W. C. Kaska, Coord. Chem. Rev., 2008, 252, 1787; (h) The Chemistry
of Pincer Compounds, ed. D. Morales-Morales and C. M. Jensen,
Elsevier, Amsterdam, 2007.
2 (a) G. R. Clark, T. R. Greene and W. R. Roper, J. Organomet.
Chem., 1985, 293, C25; (b) M. McLoughlin, H. A. Mayer, R. Flesher
and W. C. Kaska, Organometallics, 1994, 13, 3816; (c) W. Lesueur,
E. Solari, C. Floriani, A. Chiesi-Villa and C. Rizzoli, Inorg. Chem.,
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J. Chem. Soc., Dalton Trans., 2002, 1396; (e) M. Kanzelberger,
X. W. Zhang, T. J. Emge, A. S. Goldman, J. Zhao, C. Incarvito
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978.
Thermogravimetric tests showed that the thermal stability
of the new compounds is exceptional and in some cases
exceeds the stability of C(sp2)-metalacycles. For example, the
first weight loss detected for 4 takes place at 370 1C and this is
120 1C higher than for the parent 1. 5 and 6 also demonstrated
decomposition points far over 280 1C.
Since very often an extreme stability may indicate chemical
inertness, it was important for us to demonstrate at early stages
of the study that this is not the case for the new family of
compounds. We tested 4 as a promoter in the well-explored
Mizoroki–Heck reaction which typically serves as a model reac-
tion for evaluating the catalytic activity of pincer complexes.11
We were pleased to find that despite the exceptional thermal
stability the new complexes are catalytically active. The following
3 (a) A. Vigalok and D. Milstein, Organometallics, 2000, 19, 2061;
(b) D. G. Gusev and A. J. Lough, Organometallics, 2002, 21, 5091;
(c) V. F. Kuznetsov, K. Abdur-Rashid, A. J. Lough and
D. G. Gusev, J. Am. Chem. Soc., 2006, 128, 14388.
4 M. Ohff, A. Ohff, M. E. van der Boom and D. Milstein, J. Am.
Chem. Soc., 1997, 119, 11687.
5 It is noteworthy that bridgehead carbon atoms of bicyclo[2.2.2]octane
have no effect on the charge transfer rates in donor–bridge–acceptor
systems which highlights the compounds as attractive candidates
for construction of conducting materials: R. H. Goldsmith,
J. Vura-Weis, A. M. Scott, S. Borkar, A. Sen, M. A. Ratner and
M. R. Wasielewski, J. Am. Chem. Soc., 2008, 130, 7659.
6 O. Grossman, C. Azerraf and D. Gelman, Organometallics, 2006,
25, 375.
7 The lack of reactivity of transition metals toward carbometalation
is not very surprising considering the low acidity of the methine
hydrogen: L. H. Schwartz, J. Org. Chem., 1968, 33, 3977.
8 C. Azerraf and D. Gelman, Chem.–Eur. J., 2008, 14, 10364.
9 (a) M. W. Haenel, D. Jakubik, C. Krueger and P. Betz, Chem. Ber.,
1991, 124, 333; (b) M. W. Haenel, S. Oevers, K. Angermund,
W. C. Kaska, H.-J. Fan and M. B. Hall, Angew. Chem., Int. Ed.,
2001, 40, 3596; (c) M. Yamashita, K. Kamura, Y. Yamamoto and
K.-Y. Akiba, Chem.–Eur. J., 2002, 8, 2976; (d) N. Solin,
J. Kjellgren and K. J. Szabo, J. Am. Chem. Soc., 2004, 126, 7026.
Fig. 1 ORTEP drawing (50% probability ellipsoids) of the structures
4 and 5. Hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond lengths (A) and angles (1) for 4: Pd1–C1
(2.057(3)), Pd1–Cl1 (2.3694(9)), Pd1–P1 (2.2908(9)), Pd1–P2
(2.2874(9)), Cl1–Pd1–C1 (173.14(10)), P1–Pd1–P2 (148.53(3)),
C1–Pd1–P1 (96.96(3)). Selected bond lengths (A) and angles (1) for
5: Ni1–C1 (1.961(4)), Ni1–Cl1 (2.2069(12)), Ni1–P1 (2.1872(12)),
Ni1–P2 (2.1977(12)), Cl1–Ni1–C1 (165.57(12)), P1–Ni1–P2
(146.64(5)), C1–Ni1–P1 (86.60(12)).
10 Crystallographic data for 4: C44H33ClO4P2Pd,
M
¼
829.49,
orthorhombic, a ¼ 8.3159(5), b ¼ 11.2104(7), c ¼ 38.813(2) A,
ꢀc
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Chem. Commun., 2009, 466–468 | 467

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U ¼ 3618.4(4) A3, T ¼ 173 K, space group P212121, Z ¼ 4, 40 630
reflections collected, unique 7909 (Rint ¼ 0.0515), final R indices
[I 4 2s(I)] R1 ¼ 0.0376, wR2 ¼ 0.0818. Crystal data for 5:
11 (a) S. Sjoevall, O. F. Wendt and C. Andersson, J. Chem. Soc.,
Dalton Trans., 2002, 1396; (b) K. Inamoto, J.-i. Kuroda,
K. Hiroya, Y. Noda, M. Watanabe and T. Sakamoto, Organo-
metallics, 2006, 25, 3095; (c) E. M. Schuster, M. Botoshansky and
M. Gandelman, Angew. Chem., Int. Ed., 2008, 47, 4555; (d) B. M. J.
M. Suijkerbuijk, S. D. Herreras Martinez, G. van Koten and R. J.
M. Klein Gebbink, Organometallics, 2008, 27, 534;
(e) M. Broering, C. Kleeberg and S. Koehler, Inorg. Chem.,
2008, 47, 6404.
C48H41ClNiO5P2,
M
¼
853.91, triclinic,
a
¼
10.870(2),
b
¼
14.914(2), c ¼ 17.181(3) A, U ¼ 2538.0(7) A3, T ¼ 173 K, space group
ꢀ
P1, Z ¼ 2, 26 275 reflections collected, unique 9920 (Rint ¼ 0.0676),
final R indices [I 4 2s(I)] R1 ¼ 0.0802, wR2 ¼ 0.2254. CCDC 699997
and CCDC 699996 contain the supplementary crystallographic data
for this paper (4 and 5, respectively).
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This journal is The Royal Society of Chemistry 2009
468 | Chem. Commun., 2009, 466–468