The propensity for phosphorus to form catenated compounds is evidenced by the extensive arrays of structurally characterized polyphosphines [1, 2] and homopolyatomic anions reported. [3] In contrast, comprehensively characterized polyphosphorus cations are limited to phosphinophosphonium 1, [4][5][6][7][8][9] phosphinodiphosphonium 2, [10] diphosphiranodiphosphonium 3, [11] and phosphidodiphosphonium 4 [12,13] ions (Scheme 1). Nevertheless, recent and unique examples of cations 5, [7,14] 6, [15] 7, [8,14,16] and 8 [17] illustrate the potential for diversification and highlight catena-polyphosphorus cations as an underexplored avenue in phosphorus chemistry. In this context, we have exploited facile reactions of polyphosphines (di, tetra, and penta species) to prepare a series of new organosubstituted diphosphinophosphonium 9 and cyclotetraphosphinophosphonium cations 10.
The 31 P NMR spectra for reaction mixtures of tetramethyldiphosphane or tetraphenyldiphosphane with Me 2 PCl or Ph 2 PCl in the presence of Me 3 SiOSO 2 CF 3 (TMSOTf) [18] show rapid, quantitative formation of the corresponding organo-substituted catena-diphosphanophosphonium cations 9 a, 9 b, 9 c, and 9 f (Scheme 2). Derivatives of 9 can be envisaged as diphosphine ligands on phosphenium Lewis acceptors (analogous to 1, thus representing complexes of R 3 P on R 0 2 P + ), [7] and are structural isomers of 4. [12] Cation 9 f is a rearrangement product of 9 d or the product of Me 2 P + insertion into the PΓP bond of Ph 2 PΓPPh 2 . The formation of derivative 9 e was not observed. The preferred formation of 9 f over 9 d and 9 c over 9 e is likely to be a result of the steric interactions between the substituents and the relative donor (PMe 2 versus PPh 2 )/acceptor (PMe 2 + versus PPh 2 + ) properties of the PR 2 units.
An unusual eclipsed/staggered (C s ) conformation is observed for the cation of 9 a-OTf (OTf = trifluoromethanesulfonate) in the solid state (Figure 1). Retention of this nonsymmetric arrangement in solution is evidenced by the slight nonequivalence (Dd < 0.1 ppm, DJ = 11-35 Hz) of the terminal phosphorus centers in the 31 P NMR spectra of 9 a, 9 b, and 9 f at 193 K (Table 1; Figure 2 shows the 31 P NMR spectrum of 9 b-OTf as an example). The 31 P NMR spectra of all derivatives of 9 at RT show broad, poorly defined triplets and doublets, thus indicating dynamic behavior that may enable rearrangement of 9 d to 9 f by dissociation to Ph 2 PΓ PMe 2 and Ph 2 P + (Scheme 2). Scheme 1. Previously characterized polyphosphorus cations.