Synthesis, Structure and Complexation Ability of Novel Metalloreceptors Containing Two Pincer Complexes of Palladium(II)

Abstract

Metalloreceptors 1a and 1b were synthesized by palladation of 42-and 54-membered crown ethers that have two SCSpincer ligand moieties. The X-ray crystallographic analysis reveals that the metalloreceptor 1b has a folded structure in which the two pincer-Pd moieties are stacked in a parallel manner. The metalloreceptor 1b recognizes the aromatic biMetalloreceptors have recently attracted much attention in supramolecular chemistry because the physical and chemical properties of the metal center can be applied in guest binding, sensing, and regulation of molecular functions.[1] In particular, the coordination of various ligands to metals is very useful for the formation of a stable supramolecular system.[2Ϫ6] Combination of the coordinate bonding and other intermolecular interactions provides a strong and selective recognition.Complexes between Pd and pincer ligands are useful as they provide a nice coordination site for host-guest interaction. [7] For example, crown ethers containing a Pd center inside the ring and molecular recognition have been reported.[8Ϫ10] Cooperative binding of the Pd unit and the crown ether moiety to nucleobases such as cytosine and adenine was observed in this system. [11,12] However, a host molecule containing several pincer complexes as a binding site has not been synthesized although this type of host is very interesting as a multi-point receptor.Here we report the synthesis, structure and molecular recognition ability of large macrocyclic hosts 1, which contain two pincer complexes of Pd II . The metallohosts are expected to bind bidentate guests of different sizes in the cavity because the framework of these hosts is flexible so as to cause induced-fit recognition (Scheme 1).Metallohosts 1 containing the two Pd centers were prepared according to Scheme 2. Diols 2a [13] and 2b [14] were treated with p-toluenesulfonyl chloride to afford 3a and 3b in 80% and 63% yield, respectively. Reaction of dithiol 4 [15] with 3a, 3b in the presence of K 2 CO 3 afforded diols 5a, 5b, [a] [16] yielded metallohosts 1a, 1b. All spectroscopic data are consistent with the formula of 1 (see Exp. Sect.). Palladation of 7 caused broadening of the 1 H NMR signals, probably due to the enhanced rigidity of the structure, which restricts the free rotation of the methylene group. On metallation, downfield shifts of the signals for the SCH 2 CH 2 protons (e.g. δ ϭ 2.60 for 7b, δ ϭ 3.32 for 1b, Figure 1) and the benzyl protons is observed. This is elucidated by the decreased electron-density of these protons. ESI-MS for 1b gave signals at m/z ϭ 614. Ϫ anion during the ESI-MS measurements). In addition, the X-ray crystallographic analysis clarified the structure of 1b (Figure 2). The two complex moieties exhibit square-planar geometry at Pd, with three donor atoms provided by the rigid SCS chelate. The PdϪS distances are in

Thioamide Pincer Ligands with Charge Versatility

“…Recently, we reported the first example of a ditopic thioamide-based pincer palladacycle as a catalyst for Heck-type coupling reactions . To our knowledge, that complex represents one of only a few multitopic SCS pincers. , To further explore the catalytic utility of thioamides in pincer frameworks, we synthesized two simple monotopic pincer ligands, 1 and 2 . Ligand 1 7 and other N , N -dialkylthiobenzamide ligands 4,5 have been synthesized by others.…”

15 Noble metals

“…227 New palladacyclic complexes with pyridylphosphane ligands 231 A dinuclear Pd(II) metalloreceptor possessing either a 42 or 54 membered crown ether and two S^C^S pincer groups (H 2 L50) [{Pd(NCCH 3 )} 2 (m-L50)] has been reported, which adopts a folded structure, but recognizes aromatic bidentate guests such as 4,49-bpy. 232 An unusual Pd(II) complex [Pd{g 3 -Ph 2 P(LS)CHPPh 2 (LS)}(PPh 3 )] featuring an S^C^S pincer ligand has been reported in which unlike conventional carbene complexes, coordination occurs through donation from a metal d-orbital to an empty n p -orbital on the carbene ligand. 233 A new tridentate pincer phosphane/N-heterocyclic carbene ligand (L51), [similar to (L23) described earlier], 91 and its palladium complexes were reported.…”