Sterically Demanding Phosphines with 2,6-Dibenzhydryl-4-methylphenyl Core: Synthesis of Ru, Pd, and Pt Complexes, and Structural and Catalytic Studies.

The synthesis of sterically demanding aminophosphine and phosphinite ligands ArNHPPh (1) and ArOPPh (2), based on 2,6-dibenzhydryl-4-methylphenyl core having bulky benzhydryl groups, and their Ru, Pd and Pt complexes is described. The reactions of 1 and 2 with [Ru(η- p-cymene)Cl] in 2:1 molar ratios produced mononuclear complexes [RuCl{(η- p-cymene)PPh-EAr*}-κ-P] (E = NH (3) and O (4)). Interestingly, complexes 3 and 4, upon refluxing in chlorobenzene, displace the p-cymene ring by one of the phenyl rings of side arms, forming rare η-arene coordinated tethered complexes [RuCl{(PPh-EAr*)-κ-P-η-arene}] (E = NH (5) and O (6)). Treatment of 1 and 2 with [M(COD)Cl] [M = Pd, Pt] in 2:1 ratios afforded mononuclear complexes [MCl{(PPhEAr*)-κ-P}] (E = NH, M= Pd (7), Pt (8); E = O, M= Pd (9), Pt (10)). Similarly, 1:1 reactions of 1 and 2 with [Pd(COD)Cl] produced chloro-bridged dinuclear complexes [PdCl{(PPhEAr*)-κ-P}] (E = NH (11) and O (12)), whereas [Pt(COD)Cl] yielded only the mononuclear complexes 8 and 10. The reactions of 1 and 2 with [Pd(η-CH)Cl] in 2:1 molar ratios produced the mononuclear complexes [PdCl{(η-CH)(PPhEAr*)-κ-P}] (E = NH (13) and O (14)). Many of these complexes have been structurally characterized, which show C-H···π interactions between the methine hydrogen of the benzhydryl and with one of the carbon atoms of the phenyl ring attached to the phosphorus center. The complex [RuCl{(η- p-cymene)PPh-NHAr*}-κ-P] (3) shows very short C-H···π interactions of 2.36 Å; in addition, C-H···M interactions were observed between the methine hydrogen of one of the benzhydryl groups and palladium centers in 11 and 13. The tethered ruthenium complex 5 is found to be an excellent catalyst for the catalytic oxidation of various styrene derivatives.