to the addition of water,9 Rh,10 and numerous other ex-
amples.6b,11,12 Several groups have recently reported asym-
metric variants.13
precursor and an effficient metalation strategy.14,20 Pincer
complexes, in general, have shown a great variety of
chemistries.21 A notable recent example is in the area of N-H
bond activation.22
We recently reported a general methodology for the
synthesis of CCC-NHC pincer complexes that exploited the
basicity and electrophilicity of Zr(NMe2)4 to activate three
C-H bonds simultaneously coupled with transmetalation
from Zr to prepare late transition metal CCC-NHC com-
plexes of Rh (Scheme 1, 2b).14,20 We have extended that
methodology to include Ir 2a.
We report the extension of the metalation/transmetalation
methodology for the synthesis of CCC-NHC pincer com-
plexes to Ir14 and the first examples of Rh and Ir NHC pincer
complexes for the catalysis of intramolecular hydroamination/
cyclization of unactivated alkenes. These systems are highly
active catalysts giving near quantitative yields with low
catalyst loadings for secondary amine substrates. They
function in the presence of air and with water as solvent.
The new CCC-NHC pincer catalysts have been prepared
in high yield by a general synthetic methodology and have
been structurally characterized.
Synthesis of NHC pincer complexes has been an area of
intense research activity lately.15 Numerous groups have
contributed to the development of pyridyl bridged,16 xylyl
bridged,17 and 2,6-lutidinyl bridged systems.17a,18 We have
been focused on developing phenylene bridged systems19 and
have developed a methodology for the synthesis of the ligand
Scheme 1. Synthesis of Pincer Complexes 2a and 2b
(7) For recent examples see: Takaya, J.; Hartwig, J. F. J. Am. Chem.
Soc. 2005, 127, 5756-5757. Hoover, J. M.; Petersen, J. R.; Pikul, J. H.;
Johnson, A. R. Organometallics 2004, 23, 4614-4620.
(8) For asymmetric examples see: Dorta, R.; Egli, P.; Zurcher, F.; Togni,
A. J. Am. Chem. Soc. 1997, 119, 10857-10858. Hultzsch, K. C. AdV. Synth.
Catal. 2005, 347, 367-391. Kim, J. Y.; Livinghouse, T. Org. Lett. 2005,
7, 1737-1739.
(9) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127,
1070-1071.
While evaluating several routes to late transition metal
CCC-NHC pincer complexes, it was found that metalation
with Zr followed by transmetalation to Rh or Ir was a high-
yielding process providing Ir 2a in 68% and Rh 2b in 66%
(see Supporting Information). The bis imidazolium salt 1 was
treated in situ with 2.5 equiv of Zr(NMe2)4. It was then stirred
with [IrCl(COD)]2 for 8 h. The resulting iodo-bridged dimer
was isolated in 68% yield. An X-ray quality crystal was
grown by slow evaporation of a CH2Cl2 solution.
A single-crystal X-ray analysis of 2a revealed a structure
that is isomorphous with the Rh analogue (Figure 1). The
iodo-bridges between the Ir centers complete the octahedral
environment. The molecular structure contains a center of
symmetry that relates the two halves of the molecular
structure. Unlike the Rh analogue, no Ir ammine adduct was
noted spectroscopically, but it is anticipated that in solution
this dimer is readily split by coordination to the amine
functinal group of the substrate. Select metric data are
included in Figure 1. Other than the geometric constrains of
the tridentate ligand, the geometry of the complex is within
the normal ranges. Due to the chelating rings the C6-Ir-
C2 angle is only 78°, a significant deviation from the
idealized 90°. Likewise the C2-Ir1-C2′ angle is 156°,
compared to the idealized angle of 180°.
(10) Takemiya, A.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 6042-
6043.
(11) For leading references see: Molander, G. A.; Hasegawa, H.
Heterocycles 2004, 64, 467-474. Field, L. D.; Messerle, B. A.; Vuong, K.
Q.; Turner, P. Organometallics 2005, 24, 4241-4250. Lai, R. Y.; Surekha,
K.; Hayashi, A.; Ozawa, F.; Liu, Y. H.; Peng, S. M.; Liu, S. T.
Organometallics 2007, 26, 1062-1068. Field, L. D.; Messerle, B. A.;
Vuong, K. Q.; Turner, P.; Failes, T. Organometallics 2007, 26, 2058-
2069. Burling, S.; Field, L. D.; Messerle, B. A.; Rumble, S. L. Organo-
metallics 2007, 26, 4335-4343.
(12) For reviews see: Bytschkov, I.; Doye, S. Eur. J. Org. Chem 2003,
935-946. Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673-686.
Molander, G. A.; Romero, J. A. C. Chem. ReV. 2002, 102, 2161-2185.
Muller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675-703.
(13) Martinez, P. H.; Hultzsch, K. C.; Hampel, F. Chem. Commun. 2006,
2221-2223. Horrillo-Martinez, P.; Hultzsch, K. C.; Gil, A.; Branchadell,
V. Eur. J. Org. Chem. 2007, 3311-3325. Gribkov, D. V.; Hultzsch, K. C.;
Hampel, F. J. Am. Chem. Soc. 2006, 128, 3748-3759. Watson, D. A.; Chiu,
M.; Bergman, R. G. Organometallics 2006, 25, 4731-4733. Wood, M. C.;
Leitch, D. C.; Yeung, C. S.; Kozak, J. A.; Schafer, L. L. Angew. Chem.,
Int. Ed. 2007, 46, 354-358. Kim, H.; Kim, Y. K.; Shim, J. H.; Kim, M.;
Han, M. J.; Livinghouse, T.; Lee, P. H. AdV. Synth. Catal. 2006, 348, 2609-
2618.
(14) Rubio, R. J.; Andavan, G. T. S.; Bauer, E. B.; Hollis, T. K.; Cho,
J.; Tham, F. S.; Donnadieu, B. J. Organomet. Chem. 2005, 690, 5353-
5364.
(15) For a recent review see: Peris, E.; Crabtree, R. H. Coord. Chem.
ReV. 2004, 248, 2239-2246.
(16) For a leading reference see: Wright, J. A.; Danopoulos, A. A.;
Motherwell, W. B.; Carroll, R. J.; Ellwood, S.; Sassmannshausen, J. Eur.
J. Inorg. Chem. 2006, 4857-4865.
(17) (a) Grundemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.;
Crabtree, R. H. Organometallics 2001, 20, 5485-5488. (b) Nielsen, D. J.;
Cavell, K. J.; Skelton, B. W.; White, A. H. Inorg. Chim. Acta 2002, 327,
116-125. (c) Danopoulos, A. A.; Tulloch, A. A. D.; Winston, S.; Eastham,
G.; Hursthouse, M. B. Dalton Trans. 2003, 1009-1015. (d) Magill, A. M.;
McGuinness, D. S.; Cavell, K. J.; Britovsek, G. J. P.; Gibson, V. C.; White,
A. J. P.; Williams, D. J.; White, A. H.; Skelton, B. W. J. Organomet. Chem.
2001, 617, 546-560.
(18) Tulloch, A. A. D.; Danopoulos, A. A.; Tizzard, G. J.; Coles, S. J.;
Hursthouse, M. B.; Hay-Motherwell, R. S.; Motherwell, W. B. Chem.
Commun. 2001, 1270-1271.
(19) Vargas, V. C.; Rubio, R. J.; Hollis, T. K.; Salcido, M. E. Org. Lett.
2003, 5, 4847-4849.
Our initial evaluation of these complexes as catalysts for
the intramolecular hydroamination/cyclization focused on
(20) Andavan, G. T. S.; Bauer, E. B.; Letko, C. S.; Hollis, T. K.; Tham,
F. S. J. Organomet. Chem. 2005, 690, 5938-5947.
(21) For recent reviews see: The Chemistry of Pincer Compounds;
Morales-Morales, D.; Jensen, C., Eds.; Elsevier: New York, 2007.
(22) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao,
J.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644-13645
and references therein.
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