Organometallics
Communication
(2JPH = 19 Hz) and the carbene resonance, also a triplet, at δC
225.9 (2JPC = 8 Hz) attest to the α-Ru−H elimination
sequence.
From these results, we may begin to discern a pattern in
which reactions of 1 with metal centers will form NHC
complexes if the system is sufficiently electron rich or else result
in a PNP or σ-perimidinyl complex (Scheme 4). We may
ACKNOWLEDGMENTS
■
This work was supported by the Australian Research Council
(DP1093516 and DP110101611). The assistance of Dr.
Anthony C. Willis in the acquisition and interpretation of
crystallographic data is gratefully acknowledged.
REFERENCES
■
(1) (a) Topics in Organometallic Chemistry.; van Koten, G., Milstein,
D., Eds.; Springer: Berlin, Heidelberg, 2013; Vol. 40. (b) The
Chemistry of Pincer Compounds; Morales-Morales, D., Jensen, C. M.,
Eds.; Elsevier: Amsterdam, 2007. (c) Benito-Garagorri, D.; Kirchner,
K. Acc. Chem. Res. 2008, 41, 201. (d) Peris, E.; Crabtree, R. H. Coord.
Chem. Rev. 2004, 248, 2239.
Scheme 4. Mechanistic Conjecture for the Formation of per-
NHC Pincer Complexes via C−H Activation
(2) (a) Topics in Organometallic Chemistry; Glorius, F., Ed.; Springer:
Berlin, Heidelberg, 2007; Vol. 21. (b) Hahn, F. E.; Jahnke, M. C.
Angew. Chem., Int. Ed. 2008, 47, 3122. (c) Marion, N.; Nolan, S. P.
Chem. Soc. Rev. 2008, 37, 1776.
(3) (a) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239.
(b) Pugh, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610.
(4) (a) Fehlhammer, W. P.; Finck, W. J. Organomet. Chem. 1991, 414,
261. (b) Bazinet, P.; Ong, T.-G.; O’Brien, J. S.; Lavoie, N.; Bell, E.;
Yap, G. P. A.; Korobkov, I.; Richeson, D. S. Organometallics 2007, 26,
2885. (c) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc.
̈
2003, 125, 13314. (d) Ozdemir, I.; Alici, B.; Gurbuz, N.; Cetinkaya, E.;
Cetinkaya, B. J. Mol. Catal. A: Chem. 2004, 217, 37. (e) Herrmann, W.
A.; Schuetz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25,
2437. (f) Tu, T.; Malineni, J.; Bao, X.; Dotz, K. H. Adv. Synth. Catal.
̈
therefore surmise that the proligands initially bind via the
phosphine group(s), encouraging one of the amine centers to
interact with the metal, as in complexes 3−5. The proximity of
the central methylene group to the metal center might promote
interaction with one of the C−H bonds, resulting in oxidative
addition to give a σ-perimidinyl hydrido complex such as
[IrHCl(CO){CH(NCH2PR2)2C10H6}]. Subsequent loss of
dihydrogen could then proceed via an addition/elimination
or σ-metathesis pathway depending on the nature of the metal
and coligands. Alternatively, a HL group, where L− is a
nonhydride ligand, may be expelled.
It should be noted that this mechanistic conjecture is based
on isolable ground state geometries, and double C−H
activation may conceivably precede the coordination of the
second phosphine arm or require its dissociation.
In conclusion, while double dehydrogenation of proligand 1a
was observed on reaction with [RuCl2(PPh3)3] to give the per-
NHC complex 2, the analogous reaction of the less electron
donating proligand 1b, as well as reactions of 1a with less
electron rich starting materials, gave the asymmetric PNP
coordinated complexes 3−5, in which no C−H activation had
occurred. These observations have provided further insight into
the likely mechanistic pathway by which proligands 1 form per-
NHC complexes.
2009, 351, 1029. (g) Tsurugi, H.; Fujita, S.; Choi, G.; Yamagata, T.;
Ito, S.; Miyasaka, H.; Mashima, K. Organometallics 2010, 29, 4120.
(h) Verlinden, K.; Ganter, C. J. Organomet. Chem. 2014, 750, 23.
(5) Hill, A. F.; McQueen, C. M. A. Organometallics 2012, 31, 8051.
(6) (a) Prades, A.; Poyatos, M.; Mata, J. A.; Peris, E. Angew. Chem.,
Int. Ed. 2011, 50, 7666. (b) Ho, V. M.; Watson, L. A.; Huffman, J. C.;
Caulton, K. G. New J. Chem. 2003, 27, 1446. (c) Valdes
́
, H.; Poyatos,
M.; Peris, E. Organometallics 2013, 32, 6445.
(7) The formation of 2 was accompanied by a significant proportion
of organometallic side products. NMR data8 also suggested the
formation of a side product in which PPh3 replaces the THF ligand.
This was difficult to separate from complex 2, and analytically pure
samples of 2 could only be obtained by small-scale crystallization.
(8) Characterization data and preparative details are presented in the
Supporting Information.
(9) The reaction of 1b with [RuCl2(PPh3)3] has very recently been
reported to provide a symmetric five-coordinate complex devoid of
direct N−Ru coordination, some NMR data for which correspond to
those for 3.10 The complex was also shown to catalyze the transfer
hydrogenation of acetophenone but under conditions which our
31P{1H} NMR data indicate it is unstable.
(10) Fu, Q.; Zhang, L.; Yi, T.; Zou, M.; Wang, X.; Fu, H.; Li, R.;
Chen, H. Inorg. Chem. Commun. 2013, 38, 28.
(11) Shaw, B. L. J. Am. Chem. Soc. 1975, 97, 3856.
(12) Cambridge Crystallographic Data Centre: ConQuest Version
1.15, 2013 release.
(13) (a) Zeng, J. Y.; Hsieh, M.-H.; Lee, H. M. J. Organomet. Chem.
2005, 690, 5662. (b) Lee, H. M.; Zeng, J. Y.; Hu, C.-H.; Lee, M.-T.
Inorg. Chem. 2004, 43, 6822.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, and CIF files giving crystallographic data for 2
(CCDC 983020), 3 (CCDC 983019), 4 (CCDC 983021), and
5 (CCDC 983022), synthetic procedures, and spectroscopic
and analytical data for the compounds described. This material
■
S
(14) Orpen has suggested demarcating regimes for short (≤ 2.52 Å),
intermediate (2.52−2.95 Å), and long (2.95−3.15 Å) H···Cl−M
hydrogen bonding: Aullon
A.; Orpen, A. G. Chem. Commun. 1998, 653.
́
, G.; Bellamy, D.; Brammer, L.; Burton, E.
AUTHOR INFORMATION
Corresponding Author
■
Notes
The authors declare no competing financial interest.
1912
dx.doi.org/10.1021/om5000985 | Organometallics 2014, 33, 1909−1912