Experimental and Computational Investigations of Tautomerism and Fluxionality in PCP- and PNP-Bridged Heavy Chalcogenides

The reaction of H2C(PCl2)2 with four equivalents of iPrMgCl produces H2C(PiPr2)2, which was treated with tellurium in boiling toluene, or selenium in toluene at room temperature, to give the monochalcogenides EPiPr2CH2PiPr2 (E = Te, 4a; E = Se, 4b) in high yields. X‐ray structural determinations show that 4a and 4b exist as the CH2 tautomers in the solid state with E–P–C–P dihedral angles of 56.1(2)° and 56.7(1)°, respectively. DFT calculations were carried out for the isolectronic series EPR2CH2PR2 and EPR2NHPR2 (E = Se, Te; R = Me, iPr, tBu, Ph) and for their non‐chalcogenated precursors in order to elucidate the factors that determine the preference for PH tautomers in some PNP‐bridged systems. Compounds 4a and 4b were also characterized by multinuclear (1H, 13C, 31P, 77Se, 125Te) NMR spectroscopy. In solution, 4a exhibits fluxional behavior, which has been investigated by variable‐temperature and variable‐concentration multinuclear NMR spectroscopy. The observed behavior is consistent with an intermolecular tellurium transfer with an activation energy of 21.9 ± 3.2 kJ mol–1; consideration of selenium exchange in 4b indicates a much higher energetic barrier. DFT calculations provide insights into the pathway for the chalcogen exchange process in 4a (ΔE = 20.4 kJ mol–1). The outcome of reactions of 4a with selenium and nBuLi reflects the lability of the P‐Te functionality. The monochalcogenides EPiPr2CH2PiPr2 (E = Se, Te) exist as CH2 tautomers in the solid state; the contrast in this finding with that of the preferential formation of PH tautomers by PNP‐bridged analogues is addressed through DFT calculations. In solution, the PCP‐bridged tellurium derivative undergoes rapid intermolecular tellurium exchange with an activation energy of 21.9 ± 3.2 kJ mol–1.