Factors influencing linkage isomer preference in palladium(II)-chlorinilate complexes

Abstract

The factors influencing linkage isomer preference in the bonding between palladium(II) and the chloranilate dianion (CA 2−) are addressed through the synthesis and spectroscopic characterization of 16 [Pd(CA)L 2] compounds, where L 2 = 2Cl −, 2CH 3CN, 2H 2O, ox 2−, acac −, 2dmso, 2SCN −, 2F −, 2Br −, bpy, tcnq, 2CN −, CA 2− and cod. The (CCA) linkage isomer containing chloranilate ligated as dicarbanion through CCl carbon atoms is favored in the presence of weak σ-donor ligands towards the soft Pd II center (all L 2 groups except dmso, bpy and CN −), while strong σ-donor invariably give (π-CA) products in which the conventional p-quinonoid form of CA 2− is present. [Pd&(CCA)L 2] compounds are characterized by three infrared characteristics: (1) several strong CO vibrations in the 1620–1720 cm −1 interval but no 1500–1550 cm −1 CO mode; (2) 1–4 medium-intensity CC stretching bands between 1150 and 1220 but no CC feature near 1350 cm −1; and (3) a relatively high-energy CCl vibration, i.e. > 855 cm −1. Distinct C-to_Pd II LMCT transitions in the vicinity of 310 and 230–260 nm indicate that the non-equivalence of PdC bonds first demonstrated by the X-ray crystal structure of K 2[Pd(CCA)Cl 2]·4H 2O is a general feature of the (CCA) series. [Pd(CCA)L 2] complexes are surprisingly susceptible to hydrolysis, affording [Pd(CCA)(H 2O) 2] within several minutes in weakly acidic solution (pH 3.7). A kinetic trans effect linked to the strenghtening of Pd-chloranilate C bonds by the departure of competing σ donor ligands is suggested.