Structural, morphological and optical studies of zinc and cadmium sulfide thin films with different contents of phosphorus.

Zinc complexes of neutral bidentate ligands that contain an electron-poor diacylated cyclic guanidine are reported, along with their reactivity in solvent-free polymerization of rac-lactide. The diacyl groups significantly decrease the electron-donating ability of the ligand compared to the corresponding nonacylated derivative. This manifested itself through restricted rotation about the formal C−N exo double bond, with the Gibbs free-energy rotational barrier (ΔG ‡) ranging from 47.4 to 59.6 kJ mol −1 . Restricted rotation was also observed for the isopropyl substituent syn to the N exo aryl ring with ΔG ‡ values of 47.5−51.9 kJ mol −1 . Coordination of the ligands to zinc caused further π-electron delocalization from the endocyclic to the exocyclic nitrogen, thereby decreasing the Wiberg bond index of the C−N exo bond from 1.67−1.70 to 1.48−1.50 and eliminating all restricted rotations at room temperature. All three complexes were active in the solvent-free polymerization of rac-lactide with the second-order polymerization rate constant for the anthranilate-based complex of 7.30 × 10 −3 M −1 s −1 . The resulting polymers had a slight heterotactic bias (P r = 0.60) with molecular weights lower than anticipated, possibly due to transesterification, as suggested by mass spectrometry. The mass spectrum of the polymer did not show evidence of the ligand dissociating from the metal during the initiation step and its incorporation into the polymer structure. The acylation of the cyclic guanidine thus imparts advantages to the ligand, with the second building block offering additional opportunities to further improve the catalytic performance of these zinc complexes.