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 (, 1). Specifically, reactions between 1 and [M(COD)Cl₂] (M = Pd, Pt; COD = cycloocta-1,5-diene) in different solvents (methylene chloride, acetonitrile, pyridine, toluene) resulted in the formation of PPh₂^-, captured either as a bridging ligand in binuclear complexes with a {M₂(PPh₂)₂} moiety or as an adduct to COD in [Pt₂(PPh₂COD)₂Cl₂]. The heterocyclic part transforms to annulated ⁺ species with a 1,2-dihydroquinazoline cycle formed. In the presence of pyridine as a base, annulated form ⁺ destabilizes and undergoes reverse cyclization transforming to deprotonated 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 to a neighboring molecule. A combination of high photophysical sensitivity of ⁺ toward its immediate environment and rich structural capabilities in assembling ()₂²⁺ pairs in different crystal packings in a family of phases with the general formula ()₂[M₂(PPh₂)₂Cl₄] 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 ⁺ cations. As a result, compounds with relatively short interplanar π-π-separation between the cations show a stronger charge-transfer-mediated bathochromic shift.