Aspects of the chemistry of some highly crowded aromatic ligands

Abstract

A series of ortho-substituted arylchlorophosphoranes has been prepared. The structures of these compounds have been studied by the use of (^31)Cl n.q.r. and solid-state (^31)P n.m.r. . It has been shown that ortho substitution by groups such as CH(_3) or CF(_3) in phosphoranes of the type P(Ar)Cl4 prevents complexation with Lewis bases, and this is also found to be the case for the derived phosphoniumions [P(Ar)Cl(_3)](^+). The bis(aryl)chlorophosphoranes with these bulky aryl groups (where it is possible for them to be formed) are found to have ionic phosphonium salt structures, regardless of the electronic properties of the aryl group. In some cases, where the aryl groups are too bulky and too electronegative, P-C bond cleavage takes place on chlorination of the bis(aryl)chlorophosphine, and the most bulky group is lost. The phosphoranes PRCl(_4) (where R = CH(_2)Cl or CHCl(_2)) were also prepared. For R = CH(_2)Cl, the solid phosphorane was found to have the ionic, [PRCl(_3)] [PRCl(_5)], structure. This is the first example of an organo-substituted chlorophosphorane to be shown to have the "phosphorus pentachloride structure” in the solid state. By contrast, P(CHCl(_2))Cl(_4) was shown by solid-state (^31)P n.m.r. to be a molecular, 5-coordinate species. The generation of phosphoranides of the type [P(Ar)(CN)(_2)X](^-), with ortho- substituted aryl substituents, has also been investigated. From (^31)P n.m.r. it has been shown that the 2-(methyl)phenyl group can be incorporated into such systems. Various aryl- and aryloxy-1,3,2-dioxaphospholanes were prepared, with the aim of generating metaphosphate species via ethylene elimination. Data from mass spectrometry and from flow pyrolysis suggest that transient metaphosphates of the type ArPO(_2) can be formed in this manner. Results would also indicate that where Ar = 2,6-(CF(_3))(_2)C(_6)H(_3) the respective metaphosphate undergoes a rearrangement via insertion into the ortho-CF(_3) group. It has been shown that the symmetrical diphosphene ArP=PAr (where Ar - 2,4,6-(CF(_3))(_3)C(_6)H(_2)) can be formed by magnesium couphng of the aryldichloro- phosphine. However, attempts to form asymmetrical diphosphines and other low- coordinate phosphorus species by DBU coupling of P(2,6-(CF(_3))(_2)C(_6)H(_3))H(_2) with the respective chloro species were unsuccessful, due to preferred formation of Ar(H)P- P(H)Ar or ArP=PAr. The 2,4,6-(CF(_3))(_3)C(_6)H(_2) group was successfully incorporated into various dia- zophosphine systems. Of particular note was the characterization of the first ever P(III)-P(III) stabilized nit rile imine. By varying the aryl groups on phosphorus in such systems (C(_6)H(_5), C(_6)F(_5) or 2,4,6-(CF(_3))(_3)C(_6)H(_2)) it was possible to obtain further insights into the requirements for the stabilization of nitrile imines, and various trapping reaction were also performed.