Phenylpyrazole‐Based Hypervalent Phosphorus Compounds: From Positional Isomerism to Stacking Interactions

The lithium salt of 3,5-dimethyl-1-phenyl-1H-pyrazole (PyrC6H4Li) reacts with (Et2N)2PCl to give the precursor [PyrC6H4P(NEt2)2] (2), which, after reaction with PCl3, affords PyrC6H4PCl2 (3) in 79 % yield. The phosphorus atom in 3 has four-coordinate disphenoidal geometry with an axial arrangement of Cl atoms. The length of the hypervalent N–P bond in this compound [1.775(1) A] is comparable to that of common single bonds. The structure of 3 is better described by canonical formulae containing charges on the N, P, and Cl atoms. The 1H NMR spectrum of 3 in CDCl3 shows five pairs of nonequivalent Me groups, which correspond to the positional isomers that exist in the equilibrium mixture. Compound 3 easily exchanges chlorine with methyl iodide to form the diiodo derivative 4 (PyrC6H4PI2) with nonequivalent P–I bonds. The reduction of 3 with aluminium foil or [GeCl2(diox)] gave cationic diphosphines [(PyrC6H4P)2(μ-Cl)][AlCl4] (8) and [(PyrC6H4P)2(μ-Cl)][GeCl3] (9), respectively, with a chloride bridge between the phosphorus atoms. Compounds 4, 8 and 9 show a disphenoidal arrangement around the hypervalent phosphorus atoms. The dichloro and diiodo derivatives 3 and 4 are destroyed on heating in pyridine to give diphosphines [(PyrC6H4PCl)2] (10) and [(PyrC6H4P)2(μ-I)]+[I] (12), respectively, containing hypervalent (trivalent four-coordinate) phosphorus. The crystal structure of 10 shows multiple C···C, C···Cl, H···H, and C···H short contacts between the adjacent molecules. Compound 3 exchanges chlorine for oxygen atoms on heating in dimethylformamide to give the dioxide PyrC6H4PO2 (13) with an N–PV distance [1.808(1) A] even longer than that in 3.