From the activation of tetraphosphorus to the chemistry of diphosphorus and beyond

Abstract

The niobium-phosphorus triple bond in [P=-Nb(N[Np]Ar)3]- (Np = CH2tBu; Ar = 3,5-Me2C6H3) has produced the first case of P4 activation by a metal-ligand multiple bond. Treatment of P4 with the sodium salt of the niobium phosphide complex in weakly-coordinating solvents led to the formation of the C3-symmetric cyclo-P3 anion, while in THF, it led to the formation of the cyclo-P5 anion [(Ar[Np]N)([eta]4-P5)Nb(N[Np]Ar)2]-. The latter represents a rare example of a substituted pentaphospha-cyclopentadienyl ligand and may be interpreted as the product of trapping an intermediate h5-P5 structure through the migration of one anilide ligand. A search for methods of activating P4 that avoid tedious metal-mediated steps led to the discovery of an incredibly simple procedure involving only commercial reagents. Irradiation of solutions containing P4 and readily available 1,3-dienes produced bicyclic organic diphosphanes in an atom-economical, one-step protocol. Use of 2,3-dimethylbutadiene allowed the isolation of the bicyclic diphosphane P2(C6H10)2 in gram-quantities, but other dienes such as 1,3-butadiene, isoprene, 1,3-pentadiene, and 1,3-cyclohexadiene also provided evidence for incorporation of P2 units via double Diels-Alder reactions. Theoretical investigations provided support for the formation of P2 molecules from photo-excited P4. Investigations into the physical and chemical characteristics of P2(C6H10)2 uncovered an unprecedented stability towards cleavage of the P-P bond relative to other diphosphanes. P2(C6H10)2 exhibits a flexible, yet robust bicyclic framework containing lone pairs disposed at an angle of ca. 45°, and proved to be ideally suited to form multiple bridges between two metal centers. Dinuclear complexes containing tetrahedral, zero-valent group 10 metals bridged by three diphosphane ligands were investigated in detail. These contain D3h-symmetric {M2P6} barrelene cages with metal-metal distances of 4 Å , and exhibited substitution reactions where the cages remain intact. Alternatively, diphosphane P2(C6H10)2 allowed for unprecedented selectivity towards functionalization of a single phosphorus lone pair. Additional functionalization proceeds at a significantly slower rate, thus enabling the selective isolation of various phosphoranes (EP2(C6H10)2 and E2P2(C6H10)2; E = O, S, N-R). Metalation reactions with the bulky diiminodiphosphorane ligand (MesN)2P2(C6H10)2 allowed for multiple metal complexes, showing that such ligands provide an attractive pre-organized binding pocket for transition metals, as well as post-transition metals.