Synthesis and Conformational Features of Luminescent Phosphoguanidine with a Phenylbenzothiazole Substituent

We report a three-stage scheme (1) Pbt–NH₂ + CS₂ → (Pbt–NH)₂C=S (1) (Pbt = 4-(1′,3′-benzothiazole-2′-yl)phenyl), (2) 1 + PPh₃ + I₂ → Pbt–N=C=N–Pbt (2), (3) 2 + Ph₂PH → (Pbt–N)(Pbt–NH)CPPh₂ (3) for the synthesis of a novel luminescent phosphoguanidine 3 with an unprecedentedly high yield (90%) at the last catalyst-free stage. It is demonstrated by the density functional theory (DFT) method that high reactivity of 2, leading to such an yield, is explained by a high electrostatic potential at the central carbon atom. For 3, two polymorphs 3α, 3β and a solvatomorph 3γ·THF are prepared. The structures of 2, 3α, 3β, and 3γ·THF are determined by single-crystal XRD. The tendency of crystals of different phenylbenzothiazole derivatives to form different conformations is explained by the computational (DFT) data indicating that the energy change of the molecule of 3 considered as a function of the torsion angle between phenyl and benzothiazole fragments does not exceed 2 kJ/mol in the –15…30° range. Photophysical properties of 3β and 3γ·THF phases are studied. It is shown that these compounds exhibit photoluminescence with an emission maximum at 510 nm.