Pd(II)- and Pt(II)-Assisted P-C Activation/Cyclization Reactions with a Luminescent α-Aminophosphine.

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.