Pd(II)- and Pt(II)-Assisted P–C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine
Mikhail Yu. Afonin, Pavel A. Martynenko, Dmitry S. Kolybalov, Radmir M. Khisamov, Sergey N. Konchenko, Taisiya S. Sukhikh
Abstract
There is unceasing interest toward transformations of phosphine derivatives, which are facilitated by transition metals. We report a facile Pd(II)- and Pt(II)-assisted P–C bond cleavage in a luminescent 2-phenylbenzothiazole-based α-methylaminophosphine ( PCN, 1 ). Specifically, reactions between 1 and [M(COD)Cl 2 ] (M = Pd, Pt; COD = cycloocta-1,5-diene) in different solvents (methylene chloride, acetonitrile, pyridine, toluene) resulted in the formation of PPh 2 –, captured either as a bridging ligand in binuclear complexes with a {M 2 (PPh 2 ) 2 } moiety or as an adduct to COD in [Pt 2 (PPh 2 COD) 2 Cl 2 ]. The heterocyclic part transforms to annulated c-CN + species with a 1,2-dihydroquinazoline cycle formed. In the presence of pyridine as a base, annulated form c-CN + destabilizes and undergoes reverse cyclization transforming to deprotonated CN form. Quantum-chemical density functional theory (DFT) calculations predict that a crucial step in the reactions involves proton transfer from the N atom of the amino group of PCN to a neighboring molecule. A combination of high photophysical sensitivity of c-CN + toward its immediate environment and rich structural capabilities in assembling ( c-CN ) 2 2+ pairs in different crystal packings in a family of phases with the general formula ( c-CN ) 2 [M 2 (PPh 2 ) 2 Cl 4 ] allows one to fine-tune the luminescence properties of the latter. The results were rationalized as a variation of π–π intercationic spacings, which tunes the degree of excited-state charge transfer between c-CN + cations. As a result, compounds with relatively short interplanar π–π-separation between the cations show a stronger charge-transfer-mediated bathochromic shift.