Chemistry of the diaminochalcogenophosphinic chloride–aluminium trichloride system: preparation and crystal structures of new chalcogenophosphonium cations

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Abstract
A comprehensive investigation of the reactions between diaminothiophosphinic chlorides, (R2N)2P(S)Cl (R = Me, Et, or Pri), and aluminium trichloride confirms the potential for at least two modes of reactivity. Typical Lewis acid–base complexes have been observed at room temperature in solution by 31P and 27Al n.m.r. spectroscopy. However, in the solid state, novel dimeric heterocyclic diphosphonium systems have been isolated for the Me2N and Et2N derivatives (crystal data for [{(Et2N)2PS}2][AlCl4]2: monoclinic, space group P21/n, a= 10.598(2), b= 8.976(2), c= 19.370(4)Å, β= 98.65(2)°, Z= 2, R= 0.052). In contrast, the Pri 2N derivative maintains the covalent Lewis acid–base adduct structure in the solid state [crystal data for (Pri 2N)2P(Cl)S·AlCl3: monoclinic, space group P21/c, a= 12.705(5), b= 9.504(3), c= 18.016(6)Å, β= 92.85(3)°, Z= 4, R= 0.066]. The diaminoselenophosphinic chlorides show no evidence of adduct formation in solution; however, identical heterocyclic diphosphonium salts have been isolated in the solid state for the Me2N and Et2N derivatives (crystal data for [{(Et2N)2PSe}2][AlCl4]2: triclinic, space group P, a= 10.635(7), b= 12.335(8), c= 15.159(9)Å, α= 95.94(8), β= 93.46(7), γ= 110.99(9)°, Z= 2, R= 0.066). The new heterocycles represent examples of heterocyclic thiophosphonium (and selenophosphonium) cations, and are structurally related to known neutral isovalent phosphetanes. In solution, the thiophosphonium salts dissociate and reform the Lewis acid–base adducts, while the selenium analogues adopt an equilibrium involving only ionic species. The delicate energetic balance between ionic and covalent structures is further demonstrated for the sulphur systems by the promotion of the ionic structures in solutions containing an excess of AlCl3. However, the solution species react with CH2Cl2 by means of an electrophilic attack at the sulphur centre.

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