Coordination Chemistry of SCS PdII Pincer Systems

Abstract

We have studied the coordination of substituted pyridines, and phosphorus‐ and sulfur‐containing ligands to an SCS PdII pincer system. These ligands coordinate to PdII (trans to the cyclopalladated aryl group) by quantitative substitution of the labile acetonitrile ligand in complex 1. Competition experiments showed that both electronic and steric effects influence the strength of coordination to the PdII pincer of the substituted pyridines. A quantitative analysis of the substituent effect was achieved by a Hammett correlation. Phosphorus‐containing ligands also coordinate to this SCS PdII motif, as evidenced by NMR spectroscopy and single‐crystal X‐ray diffraction studies. They are much stronger ligands than the pyridines. The coordination strength of the thioureas falls in between those of the pyridines and phosphanes/phosphites. Our results lead therefore to the following order of ligand strength towards PdII in SCS PdII pincers: PR3 > P(OR)3 > N,N′‐disubstituted thiourea > (substituted) pyridines > MeCN.

Noncovalent Synthesis of Water-Soluble SCS PdII Pincer Assemblies

“…The signal of the α-pyridyl protons of 3 is broad and shifted upfield compared to uncoordinated 3, and the broad signal of the CH 2 S protons of T 3ϩ is shifted downfield with respect to [T·3MeCN] 3ϩ (BF 4 Ϫ ) 3 . Both these features are in accordance with previous observations, [67,77] proving the coordination of pyridine ligand 3 to the pincer moieties of T 3ϩ . Changing the deprotecting agent from AgBF 4 to AgNO 3 , thereby introducing hydrophilic nitrate anions instead of hydrophobic tetrafluoroborate anions, resulted in a better water solubility of the trinuclear assembly.…”

“…Coordination of all phosphane ligand to the pincers was evident from both 1 H and 31 P NMR spectroscopy. In the 1 H NMR spectrum of the assembly the doublet at δ ϭ 6.72 ppm indicates the splitting of the Ar Pd protons by 1 H-31 P coupling (as observed before for phosphane coordination to Pd II ), [77] whereas the 31 P NMR spectrum shows a shift from δ ϭ Ϫ20.4 (free 5) to 2.0 ppm upon coordination.…”

“…[77] From this study and work by the group of Fujita, [78] we anticipated that water-solubilizing ligands based on pyridines or phosphanes should be suitable for preparing water-soluble coordination assemblies. A pyridine derivative with a polyhydroxylated tail (3) was synthesized by ringopening of gluconolactone (2) with 4-aminomethylpyridine (1) (Scheme 1).…”

“…In another experiment, the addition of excess pyridine ligand 4 to [M·4] ϩ (BF 4 Ϫ ) in CDCl 3 resulted in two separate signals for free and coordinated ligand, indicating slow ligand exchange on the NMR time scale. The influence of [D 6 ]DMSO on the pyridine exchange rate in CDCl 3 was discussed in a previous article, [77] and the rate increase upon adding [D 6 ]DMSO was rationalized by assuming that DMSO facilitates an associative solvolysis pathway. The 1 H NMR signals for uncoordinated and coordinated pyridine merge into one broad signal upon increasing the percentage of DMSO.…”

“…Having available water-solubilizing ligands 3, 4, and 5, their coordination to a variety of SCS Pd II pincer systems (monopincer M ϩ , [77] dipincer D 2ϩ , [82] tripincer T 3ϩ , [66] and hexapincer H 6ϩ , Scheme 3) was investigated.…”