Palladium(II) complexes of unsymmetrical CNN pincer ligands

Article abstract of DOI:10.1021/ic800507k

Unsymmetrical 1-(arylimino)-3-(2-hetarylimino)isoindolines have been prepared from 1,3-diiminoisoindoline, an arylamine (aniline, 2-methylaniline, 2-iodoaniline), and a heteroaromatic amine (2-amino-6-methylpyridine, 2-amino-4-methylthiazole) in a stepwise manner by two consecutive condensations. The metalation reactions of these compounds with palladium(II) acetate proceed upon cyclopalladation of the carbocyclic aryl moieties and yield unsymmetrical C,N,N pincer complexes in all cases. X-ray crystallographic analysis were performed on single crystals of hydrogen{acetato[1-phenylimino-3-(6- methylpyridylimino)isoindolinato]palladate(II)} H[(phpi)Pd(OAc)] and pyridine[1-(2-tolylimino)-3-(4-methylthiazolylimino)isoindolinato]palladium(II) [(2-tolti)Pd(py)] by which the coordination mode, the conformation, the protonation site, and the trans influence of the carbon donor were established. For one more C,N,N pincer complex, hydrogen{acetato[1-(2-iodophenylimino)-3-(6- methylpyridylimino)isoindolinato]palladate(II)} H[(2-lphpi)Pd(OAc)], a similar mononuclear coordination mode was confirmed by ¹H NMR spectroscopy, whereas for the product of an oxidative addition reaction of a palladium(0) precursor to the iodoaryl derivative a product with exo coordination was found. First experiments showed the effectivity of one of these complexes as a precatalyst in CC coupling reactions (Heck and Stille coupling).

Full text of DOI:10.1021/ic800507k

Inorg. Chem. 2008, 47, 6404-6412
Palladium(II) Complexes of Unsymmetrical CNN Pincer Ligands
Martin Bro¨ring,* Christian Kleeberg, and Silke Ko¨hler
Fachbereich Chemie, Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse,
35032 Marburg, Germany
Received March 20, 2008
Unsymmetrical 1-(arylimino)-3-(2-hetarylimino)isoindolines have been prepared from 1,3-diiminoisoindoline, an
arylamine (aniline, 2-methylaniline, 2-iodoaniline), and a heteroaromatic amine (2-amino-6-methylpyridine, 2-amino-
4-methylthiazole) in a stepwise manner by two consecutive condensations. The metalation reactions of these
compounds with palladium(II) acetate proceed upon cyclopalladation of the carbocyclic aryl moieties and yield
unsymmetrical C,N,N pincer complexes in all cases. X-ray crystallographic analysis were performed on single
crystals of hydrogen{acetato[1-phenylimino-3-(6-methylpyridylimino)isoindolinato]palladate(II)} H[(phpi)Pd(OAc)] and
pyridine[1-(2-tolylimino)-3-(4-methylthiazolylimino)isoindolinato]palladium(II) [(2-tolti)Pd(py)] by which the coordination
mode, the conformation, the protonation site, and the trans influence of the carbon donor were established. For
one more C,N,N pincer complex, hydrogen{acetato[1-(2-iodophenylimino)-3-(6-methylpyridylimino)isoindo-
1
linato]palladate(II)} H[(2-Iphpi)Pd(OAc)], a similar mononuclear coordination mode was confirmed by H NMR
spectroscopy, whereas for the product of an oxidative addition reaction of a palladium(0) precursor to the iodoaryl
derivative a product with exo coordination was found. First experiments showed the effectivity of one of these
complexes as a precatalyst in CC coupling reactions (Heck and Stille coupling).
Introduction
in studies of bond activation processes (C-H and C-C
activation etc.),4 and reports about their application as
sensoric material, photosensitizer, and in biohybrids have
appeared in the literature.5 Besides these pincer ligands other
ligand systems with similar donor sets have been designed
and investigated, including those with NHC carbene donors,6
Pincer complexes are coordination compounds of metal
ions carrying one meridonal-tridentate ligand.1 Two types
of pincer complexes, both named after the donor atoms of
the chelate ligand, have become of particular importance in
the field, the PCP pincers,2 and the NCN pincers.3 Members
of both types have been demonstrated in the recent past to
be excellent precatalysts for CC coupling reactions, aldol
condensations, and other transformations. In addition, PCP
and NCN pincer complexes have advantageously been used
(4) (a) Frech, C. M.; Shimon, L. J. W.; Milstein, D. Chem.sEur. J. 2007,
13, 7501–7509. (b) Schwartsburd, L.; Pooverenov, E.; Shimon,
L. J. W.; Milstein, D. Organometallics 2007, 26, 2931–2936. (c)
Gagliardo, M.; Chale, P. A.; Brouwer, S.; van Klink, G. P. M.; van
Koten, G. Organometallics 2007, 26, 2219–2227. (d) Gauvin, R. M.;
Rozenberg, H.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D.
Chem.sEur. J. 2007, 13, 1382–1393. (e) Salem, H.; Ben-David, Y.;
Shimon, L. J. W.; Milstein, D. Organometallics 2006, 25, 2292–2300.
(f) Frech, C. M.; Shimon, L. J. W.; Milstein, D. Angew. Chem., Int.
Ed. 2005, 44, 1709–1711. (g) van der Boom, M. E.; Liou, S. Y.;
Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2004,
357, 4015–4023. (h) Naghipour, A.; Sabounchei, S. J.; Morales-
Morales, D.; Canseco-Gonzalez, D.; Jensen, C. M. Polyhedron 2007,
26, 1445–1448. (i) Wang, Z. H.; Eberhard, M. R.; Jensen, C. M.;
Matsukawa, S.; Yamamoto, Y. J. Organomet. Chem. 2003, 681, 189–
195. (j) Morales-Morales, D.; Redon, R.; Yung, W.; Yung, C.; Jensen,
C. M. Chem. Commun. 2000, 1619–1620. (k) Morales-Morales, D.;
Grause, C.; Kasaoka, K.; Redon, R.; Cramer, R. E.; Jensen, C. M.
Inorg. Chim. Acta 2000, 300, 958–963. (l) Zhang, X. W.; Emge, T. J.;
Ghosh, R.; Goldman, A. S. J. Am. Chem. Soc. 2005, 127, 8250–8251.
(m) Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. Organome-
tallics 2004, 23, 1766–1776. (n) Gottker-Schnetmann, I.; White, P. S.;
Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804–1811.
*Towhomcorrespondenceshouldbeaddressed.E-mail:Martin.Broering@
chemie.uni-marburg.de. Fax: (49) 6421 282 5356.
(1) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750–
3781.
(2) (a) Milstein, D. Pure Appl. Chem. 2003, 75, 445–460. (b) van der
Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759–1792. (c)
Sjo¨vall, S.; Wendt, O. F.; Andersson, C. J. Chem. Soc., Dalton Trans.
2002, 1396–1400. (d) Ma, L.; Imbesi, P. M.; Updegraff, J. D., III;
Hunter, A. D.; Protasiewicz, J. D. Inorg. Chem. 2007, 46, 5220–5228.
(3) (a) Gagliardo, M.; Snelders, D. J. M.; Chase, P. A.; Klein Gebbink,
R. J. M.; van Klink, G. P. M.; van Koten, G. Angew. Chem., Int. Ed.
2007, 46, 8558–8573. (b) Slagt, M. Q.; van Zwieten, D. A. P.;
Moerkerk, A. J. C. M.; Klein Gebbink, R. J. M.; van Koten, G. Coord.
Chem. ReV. 2004, 248, 2275–2282. (c) Slagt, M. Q.; Rodriguez, G.;
Grutters, M. M. P.; Klein Gebbink, R. J. M.; Klopper, W.; Jenneskens,
L. W.; Lutz, M.; Spek, A. L.; van Koten, G. Chem.sEur. J. 2004,
10, 1331–1344. (d) Ma, L.; Imbesi, P. M.; Updegraff, J. B., III; Hunter,
A. D.; Protasiewicz, J. D. Inorg. Chem. 2007, 46, 5220–5228.
6404 Inorganic Chemistry, Vol. 47, No. 14, 2008
10.1021/ic800507k CCC: $40.75  2008 American Chemical Society
Published on Web 06/05/2008

Palladium(II) Complexes of Unsymmetrical CNN Pincer Ligands
S donors,7 or others.8 However, reports concerning the
preparation and use of unsymmetric donor pincer ligands
are rare.9
Scheme 1. Formation of Cyclopalladated (bpi*)Pd Complexes 3 and 4
New examples of CNN pincer complexes have recently
been found with the sterically congested 1,3-bis(6-meth-
ylpyridylimino)isoindoline ligands 6-Me-bpi 110 and 4,6-
Me2-bpi 211 (Scheme 1). In the course of reactions of 1 and
2 with [Pd(cod)Cl2] and palladium(II)acetate, respectively,
the spontaneous, strain triggered rotation of one of the
6-methylpyridyl rings, followed by cyclopalladation in the
respective 3-positions of these rings, has been observed.
The anionic [(6-Me-bpi*)Pd(X)]- species (X ) Cl, OAc;
the asterisk indicates a C-H activated, dianionic ligand) that
are initially formed in this process then bind to a proton or,
with dimerization, to a second equiv of palladium(II) acetate,
and have been isolated as neutral CNN pincer 3 and 4,
respectively (Scheme 1). X-ray crystallographic work per-
formed on these complexes provided detailed insight into a
strong trans influence of the carbon donor.
(5) (a) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000,
406, 970–974. (b) Albrecht, M.; Lutz, M.; Schreurs, A. M. M.; Lutz,
E. T. H.; Spek, A. L.; van Koten, G. J. Chem. Soc., Dalton Trans.
2000, 3797–3804. (c) Sutter, J.-P.; Beley, M.; Collin, J.-P.; Veldman,
N.; Spek, A. L.; Sauvage, J.-P.; van Koten, G. Mol. Cryst. Liq. Cryst.
1994, 253, 215–222. (d) Gagliardo, M.; Rizzo, F.; Lutz, M.; Spek,
A. L.; van Klink, G. P. M.; Merbach, A. E.; De Cola, L.; van Koten,
G. Eur. J. Inorg. Chem. 2007, 2853–2861. (e) Wadman, S. H.; Kroon,
J. M.; Bakker, K.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van
Koten, G. Chem. Commun. 2007, 1907–1909. (f) Batema, G. D.; van
de Westelaken, K. T. L.; Guerra, J.; Lutz, M.; Spek, A. L.; van Walree,
C. A.; van Koten, G. Eur. J. Inorg. Chem. 2007, 1422–1435. (g)
Kruithof, C. A.; Casado, M. A.; Guillena, G.; Egmond, M. R.; van
der Kerk-van Hoof, A.; Heck, A. J. R.; Klein Gebbink, R. J. M.; van
Koten, G. Chem.sEur. J. 2005, 11, 6869–6877. (h) Consorti, C. S.;
Ebeling, G.; Rodembusch, F.; Stefani, V.; Livotto, P. R.; Rominger,
F.; Quina, F. H.; Yihwa, C.; Dupont, J. Inorg. Chem. 2004, 43, 530–
536.
It can be expected that the trans influence of the carbon
donor of CNN pincer complexes like 3 and 4 increases the
lability of the bound terminal heterocycle (i.e., the pyridyl
moiety in these cases). This increase may gain additional
support by a well-positioned steric constraint so that the
terminal heterocycle of sterically constrained (bai*)-based
CNN pincer complexes becomes hemilabile (bai* ) C-H
activated dianion of a bis(arylimino)isoindoline). This hemi-
lability leaves the option of two reactive sites at the palladium
ion in the cis position and thus the promise for catalytic
activity in CC coupling reactions.8 Unsymmetric 1-(arylimino)-
3-(hetarylimino)isoindolines should be ideal candidates for
the selective preparation of such CNN pincer complexes.
However, literature mentions members of this class of ligands
only as components in complex mixtures.12 We have now
studied preparation strategies for sterically hindered 1-(arylimi-
no)-3-(hetarylimino)isoindolines and cyclopalladation reac-
tions13 of the products and report here the successful
syntheses and first applications of such pincer compounds.
(6) (a) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. Chem. Commun.
2001, 210–211. (b) Loch, J. A.; Crabtree, R. H. Pure Appl. Chem.
2001, 73, 119–128. (c) Moser, M.; Wucher, B.; Kunz, D.; Rominger,
F. Organometallics 2007, 26, 1024–1030. (d) Lee, H. M.; Zeng, J. Y.;
Hu, C.-H.; Lee, M.-T. Inorg. Chem. 2004, 43, 6822–6829.
(7) (a) Meijer, M. D.; Mulder, B.; van Klink, G. P. M.; van Koten, G.
Inorg. Chim. Acta 2003, 352, 247–252. (b) Arliguie, T.; Doux, M.;
Mezailles, N.; Thuery, P.; Le Floch, P.; Ephritikhine, M. Inorg. Chem.
2006, 45, 9907–9913.
(8) (a) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Milstein, D. Dalton
Trans. 2007, 107–113. (b) Feller, M.; Ben-Ari, E.; Gupta, T.; Shimon,
L. J. W.; Leitus, G.; Diskin-Posner, Y.; Weiner, L.; Milstein, D. Inorg.
Chem. 2007, 46, 10479–10490.
Experimental Section
(9) (a) Poverenov, E.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.;
Ben-David, Y.; Milstein, D. Organometallics 2005, 24, 1082–1090.
(b) Bianchini, C.; Lenoble, G.; Oberhauser, W.; Parisel, S.; Zanobini,
F. Eur. J. Inorg. Chem. 2005, 4794–4800. (c) Zucca, A.; Cinellu,
M. A.; Pinna, M. V.; Stoccoro, S.; Minghetti, G.; Manassero, M.;
Sansoni, M. Organometallics 2000, 19, 4295–4304. (d) Seoccoro, S.;
Chelucci, G.; Cinellu, M. A.; Zucca, A.; Minghetti, G. J. Organomet.
Chem. 1993, 450, C15–C16. (e) Seoccoro, S.; Soro, B.; Minghetti,
G.; Zucca, A.; Cinellu, M. A. J. Organomet. Chem. 2003, 679, 1–9.
(f) Fang, X.; Scott, B. L.; Watkin, J. G.; Kubas, G. J. Organometallics
2000, 19, 4193–4195. (g) Kuzenetov, V. F.; Lough, A. J.; Gusev, D. G.
Inorg. Chim. Acta 2006, 359, 2806–2811. (h) Eberhard, M. R.;
Matsukawa, S.; Yamamoto, Y.; Jensen, C. M. J. Organomet. Chem.
2003, 687, 185–189. (i) Alsters, P. L.; Engel, P. F.; Hogerheide, M. P.;
Copijn, M.; Spek, A. L.; van Koten, G. Organometallics 1993, 12,
1831–1844. (j) Aye, K. T.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.;
Scott, J. D.; Watson, A. A. Organometallics 1989, 8, 1518–1522. (k)
Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J. D.
Organometallics 1988, 7, 1363–1367.
All reagents were purchased from commercial sources and used
as received. Solvents were dried prior to use by standard procedures
and stored under Argon in the dark. Reactions were performed in
Schlenk equipment under an atmosphere of purified argon. NMR
spectra were obtained on a Bruker DRX 400, DRX 300, or ARX
200 spectrometer. Chemical shifts (δ) are given in ppm relative to
residual protio solvent resonances (1H spectra), deuterio solvent
(13C), or phosphoric acid (31P). Mass spectra were recorded on a
VB Tribid or a Varian CH7 (EI, 70 eV), an IonSpec Ultima (ESI),
or a QStarPulsar i (ESI and APCI). m/z values are given for the
most abundant isotopes only. UV-vis data was collected on a
Shimadzu UV-1601 PC spectrophotometer. GC measurements were
(12) (a) Spiessens, L. I.; Anteunis, M. J. O. Bull. Soc. Chim. Belg. 1984,
93, 459–477. (b) Spiessens, L. I.; Anteunis, M. J. O. Bull. Soc. Chim.
Belg. 1988, 97, 431–451. (c) Sato, R.; Nakayama, M.; Yuzawa, Y.;
Goto, T.; Saito, M. Chem. Lett. 1985, 1887–1902.
(10) (a) Dietrich, B. L.; Egbert, J.; Morris, A. M.; Wicholas, M.; Anderson,
O. P.; Miller, S. M. Inorg. Chem. 2005, 44, 6476–6781. (b) Bro¨ring,
M.; Kleeberg, C.; Co´nsul Tejero, E. Eur. J. Inorg. Chem. 2007, 3208–
3216.
(13) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. ReV. 2005, 105, 2527–
2571.
(11) Bro¨ring, M.; Kleeberg, C. Dalton Trans. 2007, 1101–1103.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6405

Bro¨ring et al.
undertaken on a Agilent 6850 Network GC using helium as carrier
gas, a Agilent HP-1 column, and a FID for quantitative detection.
123.8, 125.9, 129.6, 130.1, 130.9, 131.2, 133.7, 134.7, 146.7, 149.3,
149.5, 153.1, 170.9; MS (EI, 70 eV): m/z ) 332 (M+•); HRMS
(EI): Calcd for C19H16N4S: 332.1096; found: 332.1089.
Preparation of 1-Arylimino-3-iminoisoindolines 9-11: General
Procedure. 1,3-Diiminoisoindoline 5 (0.72 g, 4.95 mmol) and the
aniline derivative 6, 7, or 8 (1 equiv) are suspended in ethanol (10
mL) and heated to reflux for 6 (9) or 18 h (10, 11), respectively.
The solvent is then removed in vacuo, and the residue is subjected
to silica chromatography. In a first fraction, unreacted aniline
derivative is eluted with dichloromethane. After changing the
solvent to ethyl acetate the product is obtained in the second,
yellowish fraction.
1-Phenylimino-3-iminoisoindoline (9). Obtained (227 mg, 21%)
as a yellow solid; 1H NMR (300 MHz, CD2Cl2): δ ) 4.61 (s br, 2
H, NH2), 7.00-7.03 (m, 2 H, 2-CHPh), 7.15-7.21 (m, 1 H, 4-CHPh),
7.35-7.41 (m, 2 H, 3-CHPh), 7.48-7.52 (m, 1 H, R-CH), 7.55-7.66
(m, 2 H, ꢀ′-CH, ꢀ-CH), 7.90-7.96 (m, 1 H, R′-CH); 13C NMR
(75 MHz, CD2Cl2): δ ) 120.2, 121.9, 122.4, 123.5, 129.0, 130.9,
131.9, 132.5, 133.2, 137.4, 150.9, 157.1; MS (EI, 70 eV): m/z )
221 (M+•); HRMS (EI): Calcd for C14H11N3: 221.0953; found:
221.0955.
1-(6-Methylpyridylimino)-3-(1-iodophenylimino)isoindoline (16). Ob-
tained (175 mg, 40%) after chromatography on silica with pentane/
ethyl acetate (5:1) as a yellow solid; 1H NMR (200 MHz, CDCl3):
δ ) 2.18 (s, 3 H, Me), 6.84-6.95 (m, 2 H, 5-CHPh, 5-CHPy),
7.15-7.26 (m, 2 H, 3-CHPh, 3-CHPy), 6.44 (td, J ) 7.9 Hz, J )
1.5 Hz, 1 H, 4-CHPh), 7.61 (t, J ) 7.7, 1 H, 4-CHPy), 7.67-7.74
(m, 2 H, ꢀ-CH, ꢀ′-CH), 7.98 (dd, J ) 6.6 Hz, J ) 1.3 Hz, 1 H,
6-CHPh), 8.02-8.21 (m, 2 H, R-CH, R′-CH), 12.31 (br.s, 1 H, NH);
13C NMR (50 MHz, CDCl3): δ ) 24.3, 92.5, 119.8, 120.5, 120.8,
122.7, 122.9, 125.8, 129.5, 132.0, 133.9, 136.7, 138.6, 139.7, 151.0,
152.5, 153.9, 156.1, 160.1; UV-vis (CH2Cl2): λmax ) 231, 273,
349 nm; MS (EI, 70 eV): m/z ) 438 (M+•); HRMS (EI): Calcd for
C20H15IN4: 438.0341; found: 438.0349.
Preparation of Palladium(II) Complexes of 1-Hetarylimino-3-
aryliminoisoindolines 17-19: General procedure. 1-Hetarylimino-
3-aryliminoisoindoline 14, 15, or 16 (1 equiv, 0.4 mmol) and
Pd(OAc)2 (1 equiv, 0.4 mmol) are suspended in methanol (8 mL)
at ambient temperature and stirred for 16 h. After removal of all
volatiles the residue is purified as indicated below.
1-(2-Tolylimino)-3-iminoisoindoline (10). Obtained (218 mg,
1
19%) as a yellow solid; H NMR (300 MHz, CD2Cl2): δ ) 2.11
(s, 3 H, CH3), 6.86-6.90 (m, 1 H, CHPh), 7.13-7.30 (m, 3 H,
CHPh), 7.37-7.40 (m, 1 H, R-CH), 7.54-7.66 (m, 2 H, ꢀ-CH),
7.95-7.99 (m, 1 H, R-CH), no signal for NH protons observed;
13C NMR (75 MHz, CD2Cl2): δ ) 17.7, 119.8, 121.2, 122.4, 122.9,
126.3, 129.5, 130.2, 130.6, 131.7, 133.8, 137.8, 151.0, 160.1, 168.2;
MS (EI, 70 eV): m/z ) 235 (M+•); HRMS (EI): Calcd for C15H13N3:
235.1109; found: 235.1116.
Hydrogen{acetato-[1-(6-methylpyridylimino)-3-phenyliminoisoin-
dolinato]palladate(II)} H[(phpi)Pd(OAc)] (17). Obtained from 14 (19
mg, 10%) after fractionating recrystallization from n-hexane/
chloroform (1:1) as a bright red solid; 1H NMR (400 MHz,
[D6]DMSO): δ ) 1.90 (s, 3 H, OAc), 2.93 (s, 3 H, -CH3),
6.95-6.99 (m, 1 H, CHPh), 7.14-7.19 (m, 2 H, CHPy, CHPh),
7.29-7.34 (m, 2 H, CHPy, CHPh), 7.58-7.62 (m, 2 H, ꢀ,ꢀ′-CH),
7.66 (d, J ) 7.2 Hz, 1 H, CHPh), 7.87 (t, J ) 7.7 Hz, 1 H, 4-CHPy),
7.92-7.99 (m, 2 H, R′-CH), 11.93 (br.s, 1 H, NH); 13C NMR
(HMQC, 400 MHz, [D6]DMSO): δ ) 20.4, 26.0, 120.5, 120.8,
121.5, 123.5, 125.0, 128.6, 129.6, 131.0, 132.8, 138.6 (no quar-
1-(1-Iodophenylimino)-3-iminoisoindoline (11). Obtained (610
1
mg, 36%) as a yellow solid; H NMR (300 MHz, CDCl3): δ )
5.75 (br.s, 2 H, NH2), 6.92 (dt, J ) 7.8 Hz, J ) 1.3 Hz, 1 H, CHPh),
7.01 (dd, J ) 7.9 Hz, J ) 1.6 Hz, 1 H, CHPh), 7.33-7.40 (m, 2 H,
R-CH, CHPh), 7.53-7.68 (m, 2 H, ꢀ′-CH, ꢀ-CH), 7.91 (dd, J )
7.9 Hz, J ) 1.3 Hz, 1 H, CHPh), 8.02 (d, 1 H, R′-CH); 13C NMR
(75 MHz, CDCl3): δ ) 121.7, 122.6, 123.2, 124.3, 128.4, 130.7,
131.7, 132.2, 132.8, 136.6, 138.6, 152.6; MS (EI, 70 eV): m/z )
347 (M+•); HRMS (EI): Calcd for C14H11IN3: 346.9919; found:
346.9914.
ternary carbon centers were observed); UV-vis (CH2Cl2): λmax
)
232, 337, 448 nm; MS (APCI): m/z ) 416 ([M-HOAc]-); HRMS
(APCI): Calcd for C20H14N4Pd: 416.0248; found: 416.0248. CHN
determinations did not result in reproducible data due to varying
amounts of cocrystallized 17′.
Pyridin-[1-(4-methylthiazolylimino)-3-(2-tolylimino)isoindolinato]-
palladium(II) [(2-tolti)Pd(py)] (18). Obtained from cyclopalladated
15 by recrystallization from pyridine/n-hexane as a reddish-brown
solid (91%); 1H NMR (400 MHz, CD2Cl2/C5D5N): δ ) 1.09 (s, 3
H, Me), 2.79 (s, 3H, Me), 5.81 (d, 1 H, J ) 7.8 Hz, CHPh), 6.40 (s,
1 H, CHTh), 6.60 (t, 1 H, J ) 7.5 Hz, CHPh), 7.06 (d, 1 H, J ) 7.0
Hz, CHPh), 7.35-7.41 (m, 1 H, ꢀ-CH), 7.40-7.50 (m, 1 H, ꢀ-CH),
7.97 (d, 1 H, J ) 7.5 Hz, R-CH), 8.11 (d, 1 H, J ) 7.3 H, R-CH);
13C NMR (100 MHz, CD2Cl2/C5D5N): δ ) 17.0, 19.7, 20.8, 110.1,
121.1, 121.4, 124.0, 126.6, 128.9, 130.0, 133.1, 135.7, 138.6, 139.0,
141.6, 146.3, 149.0, 155.8, 171.4, 172.8; UV-vis (CH2Cl2): λmax
) 237, 440 nm; MS (ESI, CH3CN): m/z ) 478 ([M - py + CH3CN
+ H]+); HRMS (ESI): Calcd for C21H18N5PdS ([M - py + CH3CN
+ H]+): 478.0312; found: 478.0319; CHN analysis: Calcd for
C24H19N5PdS C 55.87, H 3.71, N 13.57; found C 55.74, H 3.55, N
13.27.
Preparation of 1-Hetarylimino-3-aryliminoisoindolines 14-16:
General Procedure. 1-Arylimino-3-iminoisoindoline 9, 10, or 11 (1
equiv, 1.0 mmol) and heterocycle 12 or 13 (1 equiv, 1.0 mmol)
are suspended in n-butanol (10 mL) and heated to reflux for 7 h.
After removal of all volatiles, the residue is purified by column
chromatography as indicated below.
1-(6-Methylpyridylimino)-3-phenyliminoisoindoline (14). Obtained
(194 mg, 88%) after chromatography on silica with pentane/ethyl
acetate (3:1) as a yellow solid; 1H NMR (300 MHz, CD2Cl2): δ )
2.16 (s, 3 H, Me), 6.85 (d, J ) 7.3 Hz, 1 H, 5-CHPy), 7.15-7.24
(m, 4 H, 4 × CHPh), 7.41-7.48 (m, 2 H, 3-CHPh), 7.56-7.63 (m,
1 H, 4-CHPy), 7.64-7.78 (m, 2 H, ꢀ-CH, ꢀ′-CH), 7.95-8.05 (m,
2 H, R-CH, R′-CH), 12.28 (br.s, 1 H, NH); 13C NMR (75 MHz,
CD2Cl2): δ ) 120.2, 121.9, 122.4, 123.5, 129.0, 130.9, 131.9, 132.5,
133.2, 137.4, 150.9, 157.1; MS (EI, 70 eV): m/z ) 221 (M+•);
HRMS (EI): Calcd for C14H11N3: 221.0953; found: 221.0955.
1-(4-Methylthiazolylimino)-3-(2-tolylimino)isoindoline (15). Ob-
tained (120 mg, 36%) after chromatography on silica with pentane/
ethyl acetate (6:1) as a yellow solid; 1H NMR (300 MHz, CD2Cl2):
δ ) 2.15 (s, 3 H, Me), 2.26 (s, 3 H, Me), 6.60 (s, 1H, CHTh),
7.01-7.07 (m, 1 H, CHPh), 7.09-7.18 (m, 1 H, CHPh), 7.25-7.33
(m, 2 H, CHPh), 7.61-7.72 (m, 2 H, ꢀ-CH), 7.94-7.96 (m, 1 H,
R-CH), 8.05-8.08 (m, 1 H, R-CH), 11.65 (br.s, 1 H, NH); 13C
NMR (75 MHz, CD2Cl2): δ ) 16.2, 16.8, 110.1, 118.9, 121.8,
Hydrogen{acetato-[1-(6-methylpyridylimino)-3-(1-iodophenylimi-
no)isoindolinato]palladate(II)} H[(2-Iphpi)Pd(OAc)] (19). Obtained
from 16 (99 mg, 41%) after extraction in toluene, evaporation of
the solvent and recrystallization from dichloromethane/n-hexane
1
as a bright red solid; H NMR (400 MHz, [D6]DMSO): δ ) 1.88
(s, 3 H, OAc), 2.73 (s, 3 H, -CH3), 6.69 (t, J ) 7.6 Hz, 1 H,
4-CHPh), 7.21 (d, J ) 7.3 Hz, 1 H, 3-CHPy), 7.36 (d, J ) 7.8 Hz,
1 H, 5-CHPy), 7.60-7.67 (m, 2 H, ꢀ′-CH, ꢀ-CH), 7.67-7.71 (m,
1 H, CHPh), 7.80 (d, J ) 7.3 Hz, 1 H, CHPh), 7.90 (t, J ) 7.6 Hz,
6406 Inorganic Chemistry, Vol. 47, No. 14, 2008

Palladium(II) Complexes of Unsymmetrical CNN Pincer Ligands
Table 1. Crystallographic Data for the Complexes 17, 18, and 20
compound
17
18
20
formula
molecular mass
a/Å
b/Å
c/Å
C
22.925H18.925Cl2.77N4O2Pd1.02
C24H19N5Pd S
515.90
12.737(4)
12.633(7)
17.184(6)
90
130.34(2)
90
4
C57H47Cl2IN4P2Pd
1154.13
589.34
23.2418(17)
11.5637(11)
17.9680(12)
90
104.537(5)
90
12.0384(13)
13.2267(15)
15.940(2)
83.055(14)
86.798(14)
82.664(13)
2
R/°
ꢀ/°
γ/°
Z
8
d/g·cm-3
cryst syst
space group
diffractometer
radiation
monochromator
cryst size/mm
temperature/K
data collection
Θ range/°
h range
1.675
monoclinic
C2/c
IPDS 1
Mo KR
graphite
0.36 × 0.24 × 0.14
193(2)
1.626
monoclinic
P21/c
IPDS 1
Mo KR
graphite
0.26 × 0.08 × 0.07
193(2)
1.535
triclinic
j
P1
IPDS 1
Mo KR
graphite
0.24 × 0.23 × 0.07
193(2)
σ scan
σ scan
σ scan
1.98-25.87
-28 e h e 28
-14 e k e 14
-21 e l e 21
22 437
2.24-25.99
-15 e h e 15
-15 e k e 15
-21 e l e 20
16 023
1.91-26.07
-14 e h e 14
-16 e k e 16
-19 e l e 19
24 990
k range
l range
measured reflns
indep. reflnsa
abs. coefficient
struct. solutionb
final R1 value
final wR2 value
3370
1.151
direct
0.0434
2192
1.002
direct
0.0281
7374
1.204
Patterson
0.0303
0.0758
0.1053
0.0544
a I < 2σ(I); b All structures were solved using SHELXS, Program for Crystal Structure Determination14 and refined with SHELXL, Program for Crystal
Structure Refinement.15
1 H, 4-CHPy), 7.94 (d, J ) 7.1 Hz, 1 H, R′-CH), 7.99 (d, J ) 7.1
Hz, 1 H, R-CH), 11.8 (br.s, 1 H, NH); 13C NMR (100 MHz,
CD2Cl2): δ ) 21.0, 25.8, 104.4, 120.8, 120.9, 121.0, 121.6, 124.6,
129.5, 129.9, 130.7, 133.5, 135.5, 137.5, 138.9, 154.9, 158.8, 179.4,
201.9; UV-vis (CH2Cl2): λmax ) 236, 334, 443 nm; MS (ESI):
m/z ) 543 ([M-OAc]+); HRMS (ESI): Calcd for C20H14IN4Pd
([M-OAc]+): 542.92980; found: 542.92168; CHN analysis: Calcd
for C22H17IN4PdO2 × CH2Cl2 C 43.84, H 2.84, N 9.30; found C
43.94, H 3.11, N 9.05.
Synthesis of 20. H(1-Iphpi) 16 (30 mg, 0.07 mmol) and Pd(PPh3)4
(79 mg, 0.07 mmol) are dissolved in absolute THF (2 mL) in the
dark and are stirred at ambient temperature for 16 h. The product
crystallizes directly from the reaction mixture and can be filtered
off as a bright yellow material.
Yield: 47 mg, 64%; 1H NMR (300 MHz, CD2Cl2): δ ) 2.41 (s,
3 H, CH3), 6.34-6.38 (m, 2 H, CHPh), 6.58 (t, J ) 7.6 Hz, 1 H,
CHPh), 6.92 (d, J ) 7.3 Hz, 1 H, CHPy), 7.10-7.18 (m, 21 H, CHPh,
CHPPh3), 7.48-7.64 (m, 11 H, CHPy, CHPPh3), 7.75 (t, J ) 6.6 Hz,
1 H, CHPy), 7.77-7.97 (m, 2 H, ꢀ,ꢀ′-CH), 8.01 (d, J ) 7.6 Hz, 1
H, R′-CH), 8.44 (d, J ) 7.6 Hz, 1 H, R-CH), 11.2 (s, 1 H, NH);
13C NMR (75 MHz, CD2Cl2): δ ) 24.5, 119.1, 119.8, 120.3, 122.3,
124.1, 127.4 - 127.5 (m, PPh3), 130.0, 131.1, 131.6, 132.5, 132.8,
133.1, 134.9-135.1 (m, PPh3), 135.6, 136.0, 136.8, 138.4, 146.9,
150.0, 153.8, 156.2, 160.2, 205.5; 31P NMR (81 MHz, CD2Cl2): δ
) 25.0; UV-vis (CH2Cl2): λmax ) 224, 342 nm; MS (ESI): m/z )
416 ([20 - I - 2 PPh3]+); several attempted combustion analyses
gave C values deviating from theory by g20% for unclear reasons.
X-ray Analyses. X-ray quality crystals of 17/17′ and 20 were
grown by slow evaporation from chloroform or dichloromethane
solutions, respectively, at ambient temperature. A crystal of 18 grew
from a pyridine solution layered with n-hexane. Crystallographic
data for all three compounds is given in Table 1.
Results and Discussions
Preparation of Unsymmetrical H(bai) Species. Whereas
several protocols for the preparation of symmetrical H(bai)
species (bai ) anion of a bis(arylimino)isoindoline) are
known,16 unsymmetrical H(bai) ligands have only been
prepared as mixtures so far.12 On the base of known
preparations of symmetrical H(bai)s, four different two-step
strategies can be proposed for the synthesis of unsymmetric
H(bai)s with one terminal phenyl and one terminal hetaryl
functionality. The introduction of the first arylimino group
can be attempted by condensation of either the aniline or
the hetarylamine with either phthalodinitrile or 1,3-diimi-
noisoindoline, followed by the introduction of the respective
other terminal aromatic unit in a second condensation step.
To effectively perform the first step, that is, the preparation
of 1-(arylimino)-3-iminoisoindolines, in an acceptable yield
and purity the possible double condensation to the sym-
metrical H(bai) has to be controlled. This can be done if the
starting material is more reactive than the product of
monocondensation, which is usually the case for 1,3-
diiminoisoindoline 5.12a,16a Two issues deserve consideration
to choose the appropriate amino component for the first
condensation. On the one hand, the monocondensation
product should be as unreactive as possible, and on the other
hand the possible scrambling during the second condensation
(14) Sheldrick, G. M. SHELXS-97, A Program for Automatic Solution
of Crystal Structure; Universita¨t Go¨ttingen: Go¨ttingen, Germany,
1997.
(15) G. M. Sheldrick SHELXL-97, A Program for Crystal Structure
Refinement; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(16) (a) Elvidge, J. A.; Linstead, R. P. J. Chem. Soc. 1952, 5000–5007.
(b) Siegl, W. O. J. Org. Chem. 1977, 42, 1872–1878. (c) Siegl, W. O.
J. Heterocyclic Chem. 1981, 18, 1613–1618.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6407

Bro¨ring et al.
Scheme 2. Two-Step Preparation of Unsymmetric H(bai) 14-16
Scheme 3. Cyclometalation of H(phpi) 14 with Palladium(II) Acetate
shift dramatically to 12.28, 11.65, and 12.31 ppm after
condensation with the hetarylamine, indicating the presence
of the imino form with an intramolecular NH · · ·N bridge
for the new unsymmetric H(bai)s 14, 15, and 16, respectively.
Metalation of New H(bai) Species. The treatment of
phenylimino-6-methylpyridyliminoisoindoline [H(phpi)] 14
with 1 equiv of palladium(II)acetate results in the formation
of the desired cyclopalladated complex H[(phpi)Pd(OAc)]
17, accompanied by large amounts of many unassigned
byproduct (Scheme 3). 17 can be purified by a tedious
fractionating crystallization protocol with loss of most of the
material and was obtained in 10% yield only. The protonation
of an exocyclic nitrogen site appears to occur as a conse-
quence of charge balance and had been observed before in
palladium(II) and cadmium(II) chelates of bai ligands.10a,17
1H NMR and H,H COSY spectra confirm the 2-position
of the phenyl ring as the site of CH activation by the
observation of four adjacent CH groups with multiplet signals
at 7.66, 7.32, 7.16, and 6.97 ppm. In addition, the multiplet
signals for a second ABCD spin system at the isoindoline
subunit and those for the three CH groups of the pyridyl
moiety are present in the aromatic region of the spectrum.
In the aliphatic region, two singlets are observed for the
acetato ligand (1.90 ppm) and for the pyridyl methyl group,
which is markedly shifted by about 0.8 ppm with respect to
the free ligand 14 to 2.93 ppm. A broad absorption at 11.9
ppm indicates the presence of an acidic N-bonded proton.
A well-resolved 13C NMR spectrum of 17 could not be
recorded because of the insufficient solubility of the complex
in common solvents. Resonance data for all nonquarternary
carbon atoms, however, was obtained indirectly from the CH
HMQC spectrum of the compound.
A single crystal suitable for an X-ray diffraction study was
obtained from a chloroform solution of unpurified 17 and
was investigated crystallographically. 17 crystallizes in the
monoclinic system, space group C2/c, with Z ) 8. Two
crystallographically identical molecules are found in the
asymmetric unit as chloroform solvates of a doubly hydrogen-
bonded dimer. About two percent of the lattice sites of each
of these dimers are statistically occupied by a tetranuclear
dimer of 17′. Because of the small amount of this side
product only the sites for the additional palladium atoms Pd2
can be found, whereas no changes for the light atom sites
are apparent. The molecular structures of the dimers of 17
and of 17′ are presented in Figures 1 and 2, respectively.
Table 1 gives crystallographic details. Selected bond lengths
and angles are summarized in Table 2.
step is required to be kept to a minimum. Because of the
well-known influence of the electronic properties of the
arylimino moiety and its ability to act as hydrogen bond
acceptor on the tautomeric amino-imino equilibrium12a,b,17
of substituted 1,3-diiminoisoindolines, the first point clearly
favors the more electron-rich aniline derivative as a precursor
for the first condensation step. The second criterium also
favors the initial introduction of the more electron-rich
arylamine because this moiety is supposed to act sluggishly
as a leaving group for the undesired ipso substitution. These
general considerations about the optimum synthetic scheme
were confirmed by the successful syntheses of 14, 15, and
16 (Scheme 2).
9 has been described before18 so that only supplementary
spectroscopic data are reported for this compound in the
Experimental section. For 10, 11 and 14-16 the composition
was proven by high resolution mass spectra. The NMR
spectra of the new 1,3-diiminoisoindolines are composed
from the subspectra of the different aromatic moieties. The
unsymmetric molecular structures, however, lead to very
1
congested aromatic regions in the H NMR spectra, which
require 2D methods for interpretation. Characteristic and
easily detectable are the singlets for the different methyl
groups that appear in the 1H NMR spectra at 2.11 (10), 2.16
(14), 2.15/2.26 (15) and 2.18 ppm (16). These signals are
well suited to serve as spectroscopic probes for reaction
control during the metalation experiments. The broad signals
at 4.61 and 5.75 ppm for nitrogen bound protons of 9 and
11, respectively, indicate that the less-reactive amino tau-
tomers prevail for the monocondensation products (for 10,
no NH signal could be detected). The NH-derived signals
(17) Anderson, O. P.; Cour, A. I.; Berg, A.; Garrett, A. D.; Wicholas, M.
Inorg. Chem. 2003, 42, 4513–4515.
The results from the X-ray crystallographic structure
determination of 17 confirms the connectivities derived from
the NMR study. The palladium(II) ion is bound in a distorted
(18) Clark, P. F.; Elvidge, J. A.; Linstead, R. P. J. Chem. Soc. 1953, 3593–
3601.
6408 Inorganic Chemistry, Vol. 47, No. 14, 2008

Palladium(II) Complexes of Unsymmetrical CNN Pincer Ligands
Scheme 4. Preparation of the Iodophenyl Derivative 19 and of the Exo
Complex 20
Figure 1. Doubly H-bonded dimer of 17. Ellipsoids are set at the 50%
probability level. Hydrogen atoms and solvent molecules removed for clarity.
the conformation of the bai* ligand of 17 is also distorted
from planarity mainly by the displacement of the py-
ridylimino moiety.
The protonated (bai*)PdOAc units dimerize via two
identical H bridges N4-H4a · · · O2′ and N4′-H4a′ · · · O2,
with NH and OH distances of 0.81 and 2.07 Å, respectively,
and with an almost linear NHO angle of 175°. The presence
of the hydrogen bridges in the solid state allows the
localization of the acidic protons at the N4, N4′ position.
The (bai*)PdOAc fragments of 17 are situated in an anti-
periplanar fashion atop of each other in a distance of 3.172
Å. In solution, however, this dimerization does not seem to
occur to a detectable amount because no special high-field
shift for those protons above the aromatic ring current of
the other half-have been found for any investigated concen-
tration between 10-4 and 10-6 mol/L.
A second palladium complex, the tetranuclear dimer 17′,
is present in the crystal and occupies statistically about 2%
of the sites of the dimeric units of 17. Figure 2 shows the
putative molecular structure of 17′ on the basis of the
crystallographic data. From a chemical point of view, the
assumed structure also appears reasonable due to the similar-
ity with that of the known tetranuclear dimer 4. In the
structure of the main molecule 17, the space segment needed
to host a fourth, neutral ligand on the external Pd2 atom of
the minor 17′ is occupied by a molecule of chloroform. The
identity of the fourth ligand could not be unraveled.
Obviously, 17′ derives from a double cyclopalladation
taking place on the same aryl moiety of a bai ligand, and
the question arises, how the second metalation step occurs.
17 might be attacked by palladium(II) directly at the
peripheral N4 position and subsequently undergoes a σ
metathesis under formation of a four-membered ring. Al-
ternatively palladium(II) may attack at the hemilabile N1
site of the pyridylimino moiety and cleave the original
Figure 2. Assumed molecular structure of the tetranuclear dimer 17′.
Localization of Pd2/Pd2′ and of the solvated chloroform molecules in the
main structure (hydrogen atoms omitted, ellipsoids set at 50% probability).
Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees) for
17 and 18a
compound
17/17′
18
Pd1-C20
1.980(5)
2.163(4)
1.972(3)
2.065(3)
1.903(15)
2.087(16)
1.509(14)
170.16(14)
90.67(15)
91.04(14)
89.41(13)
90.96(12)
167.76(12)
72.6(5)
2.013(4)
2.147(3)
1.972(3)
2.049(3)
Pd1-N1
Pd1-N3
Pd1-O1/N5
Pd2-C16
Pd2-N4
Pd2-O2′
C20-Pd1-N1
C20-Pd1-N3
C20-Pd1-O1/N5
N1-Pd1-N3
N1-Pd1-O1/N5
N3-Pd1-O1/N5
C16-Pd2-N4
C16-Pd2-O2′
N4-Pd2-O2′
N4-H4a
176.72(14)
89.16(13)
91.24(14)
87.57(12)
92.04(12)
176.72(14)
166.9(11)
105.0(8)
0.81(4)
2.07(5)
175(4)
O2′· · ·H4a
N4-H4a-O2′
a Unified numbering scheme used for 17/17′ and for 18 to indicate
equivalent positions.
square-planar fashion to two nitrogen (N1,N3), one carbon
(C20) and one oxygen donor (O1) in distances of 2.163(4),
1.972(3), 1.980(5), and 2.065(3) Å, respectively. The trans
influence of the carbon donor C20 onto the juxtaposed
Pd1-N1 bond is clearly visible and of similar strength as
reported for 3 (Pd-N 2.152 Å)10a and 4 (Pd-N 2.174 Å).10b
For comparison, the Pd-N bonds to the terminal pyridins
in NNN bonded (bpi)PdX species are typically in the range
of 2.027-2.090 Å.10b,11,19 The steric interaction between the
pyridine bound methyl group and O1 induces a distortion
of the coordination sphere toward a tetrahedron. In addition,
(19) (a) Meder, M.; Galka, C. H.; Gade, L. H. Monatsh. Chem. 2005, 136,
1693–1706. (b) Siggelkow, B. A.; Gade, L. H. Z. Anorg. Allg. Chem.
2005, 631, 2575–2584. (c) Meder, M. B.; Haller, I.; Gade, L. H. Dalton
Trans. 2005, 1403–1415. (d) Siggelkow, B. A.; Meder, M. B.; Galka,
C. H.; Gade, L. H. Eur. J. Inorg. Chem. 2004, 3424–3435. (e) Bro¨ring,
M.; Kleeberg, C. Inorg. Chim. Acta 2007, 360, 2905–2910.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6409

Bro¨ring et al.
seen occasionally for tripyrrin complexes of palladium(II).20
18 is obviously racemic in the solid state. In solution,
however, the helix seems to invert rapidly so that the process
can not be visualized by proton NMR techniques.
In analogy to 18, the complex carrying the iodo-substituted
ligand 2-iodophenyl-4-methylthiazolylisoindoline [H2-Iphpi)]
H(2-Iphpi)Pd(OAc) 19 forms upon treatment of 16 with
palladium acetate and can easily be isolated in a pure form
in 41% yield. Deiodination and formation of 17 was not
observed during the transformation. The composition
C22H17IN4O2Pd was confirmed by CHN analysis and mass
spectra. The site of CH activation can be demonstrated by
1H NMR and H,H COSY spectroscopy. Besides the ABCD
system of the isoindoline unit, two different ABC spin
systems are detected in the H,H COSY at 6.69, 7.69, and
7.80 ppm, and at 7.21, 7.36, and 7.90 ppm for the iodophenyl
and the methylpyridyl ring, respectively. As before, two
singlets for the methyl groups are present at 1.88 and 2.73
ppm for the acetate and the methylpyridine unit, with the
latter one displaying the expected low-field shift, and the
acidic proton produces a broad signal at 11.8 ppm.
A complex analogous to 17 but with an anionic iodido
ligand should be available simply by treatment of H(2-Iphpi)
16 with zerovalent palladium and oxidative addition of the
carbon-iodine bond. If equimolar amounts of 16 and
Pd(PPh3)4 are mixed in THF solution, however, the new
species 20 with an exobonded palladium ion forms instead
(scheme 4).
Figure 3. Molecular structure of 18. Ellipsoids are set at the 50% probability
level. Hydrogen atoms were removed for clarity. The numbering scheme
is adapted to that of 17/17′ for better comparison.
Pd-N3 bond. This would leave one palladium ion bonded
to C16 and the other palladium ion bonded to N1,N3. With
respect to the strain of the four-membered ring, the second
scenario appears more probable. All attempts to prepare 17′
selectively failed for the formation of complex product
mixtures. The lesson learned from these experiments is that
there is a general but unselective reactivity of the mono-
nuclear 17 with excess palladium(II) acetate.
To reduce the number of side products and to ease the
workup and purification of the desired CNN bonded com-
plexes, the reactive 2-position of the phenylimino subunit
was substituted in further experiments with a methyl or an
iodo substituent. The treatment of 2-tolyl-4-methylthiazolyl-
isoindoline [H(2-tolti)] 15 with palladium acetate indeed
yields the desired complex H(2-tolti)Pd(OAc), which was
recrystallized from pyridine/n-hexane to give [(2-tolti)Pd(py)]
18 as the only isolated product in 80% yield. The assumed
constitution of 18 was proven by elemental analysis and mass
spectra.
The presence of two equivalent phosphine ligands and thus
of a trans-configured palladium(II) complex is apparent from
31P NMR spectroscopy, which shows only one singlet at
1
25.02 ppm. The aromatic region of the H NMR spectrum
18 was characterized spectroscopically by 1D and 2D
NMR techniques in a pyridine-d5/dichloromethane-d2 mix-
ture. Because of the reduced number of CH groups in 18-
of 20 is governed by the strong absorptions of the PPh3
phenyl groups at 7.14 and 7.56 ppm. Therefore only the
singlet for the methyl group at 2.41 ppm and the broad signal
at 11.2 ppm for the acidic NH proton are of diagnostic value.
The connectivities within 20 were established by a single
crystal X-ray diffraction study. 20 crystallizes in the triclinic
1
d5, the aromatic region of the H NMR spectrum is less
congested and more easily interpreted than in the case of
17. The three aromatic subunits of the 2-tolti ligand display
one singlet at 6.40 ppm for the thiazole, an ABC spin system
at 5.81, 6.60, and 7.06 ppm for the 2-tolyl moiety, and the
typical ABCD system of the central isoindoline unit at 7.38,
7.45, 7.97, and 8.11 ppm. In addition, singlets for methyl
groups at the 2-tolyl group and the thiazole unit are present
at 1.09 and 2.79 ppm, with the latter one showing the typical
low-field shift of the palladated ligand already observed for
17.
A single-crystal X-ray diffraction study could be under-
taken on a small red crystal of 18 (Tables 1 and 2) and
revealed an unexpected helical twist of the 2-tolti ligand.
Figure 3 illustrates the solid-state molecular structure of 18.
As expected, the palladium ion is coordinated to N1, N3,
N5, and C20 in distances similar to those found for 17/17′.
The PdN3C coordination unit is, however, almost planar, and
the strain induced by the presence of the pyridine ligand is
mainly compensated for by an elongation of the Pd-C bond
to 2.013 Å and by a helical distortion of the bai* ligand.
This distortion mode is new for (bai)Pd species but has been
j
system, space group P1, with Z ) 2 and one molecule of
disordered dichloromethane per formula unit. The molecular
structure and selected parameters of 20 are given in Figure
4. For crystallographic details, see Table 1.
The molecular structure of 20 shows a trans configured
palladium(II) complex bound only to the C20 atom of the
bai′ ligand. As expected, the palladium atom is coordinated
in a distorted square-planar geometry, and the bond lengths
within the PdIP2C coordination unit are in agreement with
values found for many similar compounds.21 It is interesting
to note that 20 is not transformed into a CNN chelate complex
(20) (a) Bro¨ring, M.; Brandt, C. D. Chem. Commun. 2001, 499–500. (b)
Bro¨ring, M.; Brandt, C. D. J. Chem. Soc., Dalton Trans. 2002, 1391–
1395. (c) Bro¨ring, M.; Brandt, C. D.; Prikhodovski, S. J. Porphyrins
Phthalocyanines 2003, 7, 17–24. (d) Bro¨ring, M.; Brandt, C. D.;
Stellwag, S. Chem. Commun. 2003, 2344–2345. (e) Bro¨ring, M.;
Prikhodovski, S.; Brandt, C. D.; Co´nsul Tejero, E. Z. Anorg. Allg.
Chem. 2007, 633, 458–464.
(21) (a) Spee, M. P. R.; Ader, B.; Steenwinkel, P.; Kooijman, H.; Spek,
A. L.; van Koten, G. J. Organomet. Chem. 2000, 598, 24–27. (b) van
der Voort, E.; van der Sluis, P.; Spek, A. L. Acta. Cryst. C 1987, 43,
461–463. (c) Hartley, F. R. Organomet. ReV. A 1970, 6, 119–137.
6410 Inorganic Chemistry, Vol. 47, No. 14, 2008

Palladium(II) Complexes of Unsymmetrical CNN Pincer Ligands
Table 3. Results from the Catalytic CC Coupling Study
catalyst
substrate
yield (%)
TON
TOF (min-1
)
Heck reaction
Pd(OAc)2
21
21
22
23
24
25
73
96
33
1
98
78
297
385
133
4
392
315
0.27
0.36
0.12
0.003
0.36
0.29
18
18
18
18
18
Stille reaction
Pd(OAc)2
21
21
22
24
25
23
40
39
35
20
89
154
153
135
78
0.09
0.16
0.15
0.14
0.08
Figure 4. Molecular structure of the exo complex 20. Selected bond lengths
(Angstroms) and angles (Degrees): Pd-I 2.6926(4), Pd-P1 2.3345(7),
Pd-P2 2.3255(7), Pd-C20 2.021(3); I-Pd-P1 91.06(2), I-Pd-P2
90.96(2), I-Pd-C20 172.92(8), P1-Pd-P2 1.73.77(3), P1-Pd-C20
87.73(7), P2-Pd-C20 90.98(7). Ellipsoids are set at the 50% probability
level. Hydrogen atoms and solvent molecules removed for clarity.
18
18
18
18
Chart 1. Overview of Palladium(II) Chelates in This Work 17-20
Scheme 5. CC Coupling Reactions Catalyzed by (2-tolti)Pd(py) 18
increase in steric repulsion from 21 over 24 to 25 has only
a minor effect on the outcome of the reaction. The activity
of 18 as a precatalyst is higher compared to palladium
acetate, but does not reach the performance of some other
palladium complexes that have been studied earlier.2c,23,24
A similar picture is provided by the attempts to catalyze Stille
coupling reactions. In this case, however, the influence of
steric bulk is much more pronounced than for the Heck
reaction. In all cases, the reaction mixtures turned dark upon
heating, and the addition of mercury stopped the transforma-
tions immediately, indicating that the active species is
elemental palladium throughout.25
even under prolonged heating. The unstrained complex with
the strong Pd-P bonds is obviously favored over the strained
chelate species with the weak Pd-N bonds and does not
convert.
CC Coupling Catalyzes with 18. Pincer complexes of
palladium(II) are of general interest in the context of catalytic
CC coupling reactions.1,22 We have attempted to use the
easily accessible 18 as a precatalyst for Heck- and Stille-
type coupling reactions as depicted in Scheme 5. The
catalytic performance was studied as a function of electronic
and steric factors of the substrate. For comparison, palla-
dium(II) acetate was used as standard. The results are given
in Table 3.
Conclusions
In the Heck reaction, the reactivity decreases as expected
in the order X ) I > X ) Br > X ) Cl,23 whereas the
In summary, we have presented a strategy for the prepara-
tion of unsymmetric CNN pincer complexes by a stepwise
condensation to bis(arylimino)isoindolines and successive
cyclopalladation. Several new species from this class could
be prepared, characterized, and visualized by X-ray crystal-
lographic analyses. The simple phenylimino substituted bai
ligand H(phpi) reacts with palladium(II) acetate upon cy-
clopalladation to yield the expected complex [(phpi)Pd(OAc)]
but tends to side reactions. As a product from one such side
(22) (a) Nilsson, P.; Wendt, O. F. J. Organomet. Chem. 2005, 690, 4197–
4202. (b) Hossain, M. A.; Lucarini, S.; Powell, D.; Bowman-James,
K. Inorg. Chem. 2004, 43, 7275–7277.
(23) (a) Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H. J. Am.
Chem. Soc. 2006, 128, 6829–6836. (b) Amatore, C.; Jutand, A.;
Mensah, L.; Meyer, G.; Fiaud, J.-C.; Legros, J.-Y. Eur. J. Org. Chem.
2006, 1185–1192. (c) Kozuch, S.; Shaik, S. J. Am. Chem. Soc. 2006,
128, 3355–3365. (d) Nilsson, P.; Puxty, G.; Wendt, O. F. Organo-
metallics 2006, 25, 1285–1292. (e) Kozuch, S.; Shaik, S.; Jutand, A.;
Amatore, C. Chem.sEur. J. 2004, 10, 3072–3080. (f) Kozuch, S.;
Amatore, C.; Jutand, A.; Shaik, S. Organometallics 2005, 24, 2319–
2330. (g) Gooꢀen, L. J.; Koley, D.; Hermann, H.; Thiel, W. Chem.
Comm. 2004, 2141–2143. (h) Christmann, U.; Vilar, R. Angew. Chem.,
Int. Ed. 2005, 44, 366–374. (i) Espinet, P.; Echavarren, A. M. Angew.
Chem., Int. Ed. 2004, 43, 4704–4734. (j) Amatore, C.; Jutand, A. Acc.
Chem. Res. 2000, 33, 314–321.
(24) Yoon, M. S.; Ryu, D.; Kim, J.; Ahn, K. H. Organometallics 2006,
25, 2409–2411.
(25) (a) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855–859.
(b) Olsson, D.; Nilsson, P.; Masnaouy, M. E.; Wendt, O. F. Dalton
Trans. 2005, 1924–1929.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6411

Bro¨ring et al.
reaction, a dimetalated product could be detected crystallo-
graphically. If the coordination site for a second palladium
atom, that is, the 2-position of the phenyl group, is substituted
by methyl or even by iodine, the respective ligands H(2-
tolti) and H(2-Iphti) react cleanly and produce the desired
mononuclear products in acceptable yields. In this system,
the iodophenyl group is not attacked by palladium(II) acetate
but can easily be used for oxidative addition to the zerovalent
palladium precursor [Pd(PPh3)4] to yield an exo complex.
The performance of one of the new compounds as precata-
lysts in CC coupling reactions has been proven for Heck-
and Stille-type reactions with a number of substrates differing
in sterics and electronics, and it could be shown that the
active catalyst is elemental palladium rather than the complex
itself. These first results open new possibilities for a ligand
design based on bai-type molecules and show promise for
further studies.
Acknowledgment. We gratefully acknowledge financial
support by the Deutsche Forschungsgemeinschaft (DFG).
Supporting Information Available: 2D-NMR spectra (COSY,
NOESY) for 17-20, and crystallographic information files (CIF)
for 17, 18, and 20. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC800507K
6412 Inorganic Chemistry, Vol. 47, No. 14, 2008