Thioamide pincer ligands with charge versatility

Article abstract of DOI:10.1021/ic051775h

This paper reports the synthesis and characterization of three complexes, two palladium and one platinum, with 2,6-bis-thioamidophenyl and 2,6-bis-thioamido-pyridine ligands. The ligands show internal charge versatility by losing protons from a phenyl CH (I) or from amide NH's (II and III). The complexes were also examined as Heck catalysts, and the palladacycle, I, was found to be more effective compared to the others. The crystal structures of the complexes are also reported.

Full text of DOI:10.1021/ic051775h

Inorg. Chem. 2006, 45, 964−966
Thioamide Pincer Ligands with Charge Versatility
Rowshan Ara Begum, Douglas Powell, and Kristin Bowman-James*
Department of Chemistry, UniVersity of Kansas, 1251 Wescoe Hall DriVe,
Lawrence, Kansas 66045
Received October 13, 2005
This paper reports the synthesis and characterization of three
complexes, two palladium and one platinum, with 2,6-bis-thioamido-
phenyl and 2,6-bis-thioamido-pyridine ligands. The ligands show
internal charge versatility by losing protons from a phenyl CH (I)
or from amide NH’s (II and III). The complexes were also examined
as Heck catalysts, and the palladacycle, I, was found to be more
effective compared to the others. The crystal structures of the
complexes are also reported.
catalysts for carbon-carbon coupling reactions.8,9 Of these,
sulfur-containing pincers have gained recognition because
of the added stability that the SCS triad imparts to the
catalysts,9 although there is some debate as to whether these
pincers are true or precursor catalysts.10 In terms of sulfur
Lewis base donors, however, thioamides are a relatively
unexplored functional group in ligand design strategies.
Nonetheless, interest in thioamides is gaining momentum,
as is reflected by a number of recent papers reporting on
both anion11 and transition-metal-ion5,6,12,13 chemistry.
Recently, we reported the first example of a ditopic
thioamide-based pincer palladacycle as a catalyst for Heck-
type coupling reactions.12 To our knowledge, that complex
represents one of only a few multitopic SCS pincers.14,15 To
further explore the catalytic utility of thioamides in pincer
frameworks, we synthesized two simple monotopic pincer
ligands, 1 and 2. Ligand 17 and other N,N-dialkylthioben-
zamide ligands4,5 have been synthesized by others. However,
those investigations primarily focused on understanding the
role of sulfur in cyclopalladation reactions4,5 and using the
complexes as precursors to benzothiazoles.7 An interesting
The incorporation of thioamide functional groups into
pincer ligand frameworks has resulted in a class of ligands
with an internal charge versatility that adds a new dimension
to transition-metal coordination chemistry. The chemistry of
thioamides has a lengthy history.1 In addition to their
biological activity, thioamides have been used as therapeutic
agents2 and in transition-metal coordination chemistry.3-7
More recently, pincer ligands, which hold metal ions in
clawlike vices via tridentate coordination, have generated
considerable interest among transition-metal chemists as
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* To whom correspondence should be addressed. E-mail: kbjames@ku.edu.
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(b) Pagani, G.; Pregnolato, M.; Ubiali, D.; Terreni, M.; Piersimoni,
C.; Scaglione, F.; Fraschini, F.; Gasco´n, A. R.; Mun˜oz, J. L. P. J.
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AdV. Synth. Catal. 2005, 347, 161-171. (b) Sommer, W. J.; Yu, K.;
Sears, J. S.; Ji, Y.; Zheng, X.; Davis, R. J.; Sherrill, C. D.; Jones, C.
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964 Inorganic Chemistry, Vol. 45, No. 3, 2006
10.1021/ic051775h CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/07/2006

COMMUNICATION
Scheme 1. Synthesis of Thioamide-Based Metal Complexes
Figure 1. ORTEP plots for (A) I, (B) II, and (C) III. Thermal ellipsoids
are drawn at 50% probability.
Table 1. Selected Bond Lengths (Å) for I-III
complex
I
II
IIIA
IIIB
M1-X
1.958(2)
2.2927(6)
2.2725(6)
2.388(8)
1.707(2)
1.704(2)
1.319(3)
1.325(3)
1.444(3)
1.442(3)
1.993(16)
2.2951(6)
2.2775(6)
2.3122(5)
1.694(2)
1.752(2)
1.323(3)
1.282(3)
1.432(3)
1.413(3)
1.996(13)
2.284(5)
2.295(5)
2.212(4)
1.743(17)
1.738(16)
1.31(2)
1.31(2)
1.41(2)
1.38(2)
1.985(13)
2.284(5)
2.285(5)
2.210(1)
1.738(18)
1.775(16)
1.31(2)
1.28(2)
1.43(2)
1.42(2)
M1-S1
M1-S2
M1-Y
S1-C7
S2-C15
C7-N8
C15-N16
N8-C9
N16-C17
pyrrolidine-containing thioamide pincer complex with plati-
num that exhibits photoluminescent properties has also been
reported.13 However, 2 has not to our knowledge been
previously reported. The SNS pincer ligand was synthesized
in order to examine the structural aspects of different donor
groups on pincer formation. Herein are reported palladium
and platinum complexes of a new SNS pincer (2) and
structural and catalytic comparisons with the palladium
complex of the SCS analogue (1; Scheme 1).
The precursor amide ligands have both previously been
synthesized.16,17 Both 1 and 2 were synthesized from the
corresponding diamides in good yields using Lawesson’s
reagent.18 (The previous synthesis of 1 was by P2S5.7) The
three complexes were obtained by reacting either 1 or 2 in
dimethyl sulfoxide (DMSO) with a stoichiometric amount
of K2PdCl4 or MCl2(PhCN)2 (M ) Pt and Pd) in CH2Cl2
(Scheme 1). Crystals suitable for X-ray crystallography were
obtained from DMSO for all three complexes. The two
palladium complexes, I and II, crystallized as yellow and
red prisms, respectively, while the platinum complex, III,
was isolated as deep-red crystals.
All three complexes crystallize with the pincer ligand
occupying three coordination sites (Figure 1). In I and II,
chloride serves as the fourth ligand, while in III, an S-bonded
DMSO molecule sits at the remaining site. The negative
charge on ligand 1 derives from a deprotonated phenyl
hydrogen (Scheme 1, A). Ligand 2 becomes either mono-
or dinegatively charged as a result of the loss of either one
or two of the thioamide NH’s, resulting in the existence of
two possible resonance structures: either the charge remains
on the nitrogen, which would result in an N-bonded ligand,
or the charge resonates to the sulfur with the formation of a
CdN imine bond (Scheme 1, B/D and C/E, respectively).
In all three cases, however, S-bonded complexes result: in
I, SCS coordination and, in both II and III, SNS coordina-
tion, with the latter being understood in terms of HSAB
binding preferences.19
The palladacycle I (Figure 1, A) crystallizes with two
molecules of DMSO hydrogen-bonded with the amide NH’s.
Bond lengths and angles (Table 1) are comparable to those
previously reported for the dipalladium macrocyclic com-
plex12 and are indicative of NsCdS bonding. The second
palladium complex, II (Figure 1, B), is similar in overall
geometry to I but is isolated as the monoimine obtained from
the resonance structure after deprotonation of one thioamide
NH (Scheme 1, C), and contains only a single DMSO of
crystallization. A shorter C15-N16 bond and longer S2-
C15 bond compared with C7-N8 and S1-C7, respectively,
confirm this resonance form. The remaining NH forms a
hydrogen bond with the DMSO molecule.
The platinum complex, III, crystallizes with two crystal-
lographically independent platinum units that form a dimer
pair with a shared DMSO of crystallization (Figure 2). The
two planar (to within 0.04 Å) PtNS3 coordination groups
are within 1.6° of being parallel, with their metals directly
above one another at a Pt-Pt distance of 3.702(1) Å. The
coordination planes for the two square-planar arrangements
are rotated by ∼122° about the Pt-Pt vector. The next
shortest Pt-Pt separation in the crystal is 5.487(1) Å. In the
absence of a coordinated chloride, two negative charges are
(16) Kavallieratos, K.; Gala, S. R.; Austin, D. J.; Crabtree, R. H. J. Am.
Chem. Soc. 1997, 119, 2325-2326.
(17) (a) Ray, M.; Ghosh, D.; Shirin, Z.; Mukharjee, R. Inorg. Chem. 1997,
36, 3568-3572. (b) Patra, A. K.; Mukharjee, R. Inorg. Chem. 1999,
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2000, 19, 1423-1428.
(18) Scheibye, S.; Pedersen, B. S.; Lawesson, S.-O. Bull. Soc. Chim. Belg.
1978, 87, 229-238.
(19) Pearson, R. G. SurV. Prog. Chem. 1969, 5, 1-52.
Inorganic Chemistry, Vol. 45, No. 3, 2006 965

COMMUNICATION
a trace amount of the trans isomer after 18 h. Clearly, I, the
palladacycle, performed the most efficiently of the two
palladium complexes. In the presence of a large excess of
elemental mercury ([Hg]:[Pd] ) 300:1), catalysis is slowed
but still occurs; however, catalyst poisoning is still under
investigation. These reactions can also be compared with the
ditopic palladacyclic complex, which shows similar catalytic
activity, based on the mole percent of Pd.12
In summary, these simple thioamide-based ligands provide
opportunities for charge versatility as well as pincer ligands
with the potential as catalysts for carbon-carbon coupling
reactions. In all three cases, SXS binding (X ) CH or N) is
observed despite the presence of potential amide nitrogen
donors, with HSAB issues regulating the preferred resonance
form of the ligand.19 The increased acidity of the thioamide
NH bond as opposed to amides could assist in the facile
deprotonation of the amide hydrogens in the absence of
added base.20 While it is under debate whether SCS palla-
dacycles are catalysts or precatalysts, these new thioamide-
based pincer ligands provide promising new coordination
compounds for further chemical and catalytic exploration.
Figure 2. ORTEP plot for the complex III showing the dimer pair with
one DMSO molecule (A) parallel to the metal centers and (B) through the
metal centers. Thermal ellipsoids are drawn on 50% probability.
Table 2. Heck Coupling of 4-Iodotoluene with Styrene Using
Complexes I-III as Catalystsa
entry catalyst (mol %) reaction (h) yield (%) (trans/gem) TONb
1
2
3
4
5
6
I (1 × 10-3
I (1 × 10-3
I (0.2)
II (0.2)
II (0.2)
)
)
18
48
0.25
4
16
18
56/3.5
64/6
89/9
26/1.3
53/5.6
c/0
59500
70000
490
137
293
III (0.2)
Acknowledgment. The authors thank the National Sci-
ence Foundation under Grant CHE-0316623 for support of
this work and under Grant CHE-0079282 for purchase of
the X-ray diffractometer. Helpful discussions with Dr. Victor
Day of the University of Kansas X-ray Crystallography
Laboratory are also greatly appreciated.
a In dimethylacetamide at 165 °C. b TON ) (moles of product)/(moles
of catalyst). c Trace.
required for a neutral complex and 2 has lost both thioamide
NH’s, resulting in a diimine ligand with two negatively
charged sulfurs (Scheme 1, E, and Figure 1, C). In both units
A and B, the two CdN distances are about the same and
reasonable for a double bond, given the rather large standard
deviations. The C-S bonds are clearly elongated, as was
anticipated for single bonds.
Supporting Information Available: Detailed synthetic, crystal-
1
lographic, and catalytic procedures and time-dependent H NMR
(PDF) and one crystallographic file for the three structures in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
The results of coupling reactions between 4-iodotoluene
and styrene in the presence of I-III in air are presented in
Table 2. As might have been anticipated, catalysis with the
platinum complex III was exceedingly slow, yielding only
IC051775H
(20) Lee, H.-J.; Choi, Y.-S.; Lee, K.-B.; Park, J.; Yoon, C.-J. J. Phys. Chem.
A 2002, 106, 7010-7017.
966 Inorganic Chemistry, Vol. 45, No. 3, 2006