Preparation, Structural Characterization and Ligand Exchange Reactivity of Sterically Hindered 1,3-Bis-(4-methyl-2-thiazolylimino)isoindolinatopalladium(II) Complexes

Isoindoline-derived ligands and applications

During the past decade isoindoline-based ligands became the subject of growing interest due to their modular set-up.

Synthesis, structure and dynamics of NHC-based palladium macrocycles

A series of macrocyclic CNC pincer pro-ligands based on bis(imidazolium)lutidine salts with octa-, deca- and dodecamethylene spacers have been prepared and their coordination chemistry investigated. Using a Ag2O based transmetallation strategy, cationic palladium(II) chloride complexes [PdCl{CNC-(CH2)n}][BAr(F)4] (n = 8, 10, 12; Ar(F) = 3,5-C6H3(CF3)2) were prepared and fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state, by X-ray crystallography. The smaller macrocyclic complexes (n = 8 and 10) exhibit dynamic behaviour in solution, involving ring flipping of the alkyl spacer across the Pd-Cl bond, which was interrogated by variable temperature NMR spectroscopy. In the solid-state, distorted coordination geometries are observed with the spacer skewed to one side of the Pd-Cl bond. In contrast, a static C2 symmetric structure is observed for the dodecamethylene based macrocycle. For comparison, palladium(II) fluoride analogues [PdF{CNC-(CH2)n}][BAr(F)4] (n = 8, 10, 12) were also prepared and their solution and solid-state structures contrasted with those of the chlorides. Notably, these complexes exhibit very low frequency (19)F chemical shifts (ca. -400 ppm) and the presence of C-H···F interactions ((2h)J(FC) coupling observed by (13)C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC-(CH2)8}][BAr(F)4] exceptionally shows C(2v) time averaged symmetry in solution at room temperature (CD2Cl2, 500 MHz) as a consequence of dual fluxional processes of the pincer backbone and alkyl spacer.

Conformational study on sterically hindered bis(arylimino)-isoindolines

Single crystal X-ray crystallographic analyses of three bis(arylimino)isoindoline ligands (bai's) with terminal thiazolyl (bti) and pyridyl groups (bpi) and differing degrees of sterical encumbrance are reported and discussed in the context of conformation, conjugation, and coordination properties. 4-Mebti 2 forms yellow plates from CH 2 Cl 2/pentane at -20 °C in P21/c with Z = 4, 6-Mebpi 3 crystallizes as yellow blocks from CH 2 Cl 2/n-hexane at -20 °C in P21/c with Z = 8 (two independant molecules), and 4-tBubti 4 is obtained from CHCl 3/n-hexane at ambient temperature as yellow plates in P21 with Z = 2. The latter compound forms pseudomerohedrically twinned crystals and was treated accordingly in the structure solution and refinement. The resulting molecular structures provide insight into the stepwise increase of sterical hindrance from 2 over 3 to 4 which results in a rotational displacement of one of the terminal heterocycles from the major plane. The ease of this rotation process proves a low degree of conjugation in bis(arylimino)isoindolines and stands in contrast with the formation of a helically distorted structure for a related, though strongly conjugated, t-butyl terminated tripyrrin 1.