Selective Preparation of the [3,5‐tBu₂‐1,2,4‐C₂P₃]⁻ Ion and Synthesis and Structure of the Cationic Species nido‐[3,5‐tBu₂‐1,2,4‐C₂P₃]⁺, Isoelectronic with [C₅R₅]⁺

Abstract

Selective Preparation of the [3, There has been significant progress in the chemistry of transition-metal complexes carrying the h 5 -3,5-tBu 2 -1,2,4-C 2 P 3 ligand.[1] Although the isolobal relationship PMCH and diagonal relationship P/C suggest that complexes carrying this ligand should behave like the corresponding h 5 -cyclopentadienyl-substituted analogues, recent work has shown that the phosphorus-containing systems exhibit subtle differences in both bonding and reactivity. [2][3][4] This situation suggests that further investigations could be rewarding. In a new approach to this chemistry we focused on the reactivity of the interesting tricyclic C 2 P 3 compound 1 (Scheme 1), first reported by Regitz et al.[5] First, we used this species as a convenient starting point for a new selective synthetic approach to the aromatic ion [3,5-tBu 2 -1,2,4-C 2 P 3 ] À (3). Second, and again reflecting on the consequences of replacing CH groups by isolobal P atoms, we explored the possibility of utilizing 1 as a precursor to cations [C 2 tBu 2 P 3 ]+ . The published synthetic pathways to the anion 3, [6][7][8] which derive from earlier work by Becker et al., [9,10] involve reaction of PCtBu with alkali metals (M). However, this procedure is unselective, and to obtain pure 3 it is necessary to repeatedly recrystallize the moisture-sensitive alkali-metal salts to remove the second product of these reactions, the [2,4,5-tBu 3 -1,3-C 3 P 2 ]À ion. Therefore, we recognized that if the full potential of this area was to be realized, it was important to devise a more efficient and selective pathway to 3. We reasoned that if the tricyclic compound 1, which is readily formed in good to excellent yield by treating the readily accessible zirconium complex 2 with PCl 3 , [5] was treated with an alkali metal in THF, the anionic species A should be generated (Scheme 1). We then hypothesized that such a species should undergo consecutive ring-opening reactions (A!B!3) and thus selectively provide the desired anion.Reaction (RT, 2 h, THF) of 1 with Li or Na metal leads to quantitative formation of the lithium or sodium salt of 3, which confirms our ring-opening hypotheses. Both Li·3 and Na·3 react quantitatively with Ph 3 SnCl to give the known compound 3,5-tBu 2 -1-SnPh 3 -1,2,4-C 2 P 3 (4), which has been shown to be a valuable reagent for the syntheses of complexes of the h 5 -C 2 P 3 ligand. [7] We believed that 1 might react with Lewis acids to provide an entry point to the cationic [C 2 tBu 2 P 3 ] + manifold, related to the isoelectronic pentamethylcyclopentadienyl cation, a species that has recently attracted considerable attention. [11][12][13][14][15] In particular, in view of the isolobal relationship PMCH, and Scheme 1. a) PCl 3 ; b) M, THF, ÀMCl; c) Ph 3 SnCl, ÀMCl (* = CtBu; .. = lone pair; M = Li, Na.

Theoretical Prediction for Synthetic Realization: Pyramidal Systems ClE[E′4R4] (E = B–Ga, E′ = C–Ge, R = SiMe3, SiMetBu2): A DFT Study

“…Among the great variety of polyhedral cluster compounds, pyramidanes are of particular importance due to their unusual structures and bonding situations, especially nonclassical interaction mode between their apexes and bases [1][2][3][4]. However, despite such evident interest, pyramidanes eluded their synthetic realization until very recently and became synthetically accessible only several years ago [5][6][7][8][9][10][11][12][13]. The closest approximation to the most challenging but still hypothetical all-carbon pyramidanes C[C 4 R 4 ] represents derivatives in which apical carbon is replaced with the main group element.…”

Monocyclic Di- and Triphosphinophosphonium Cations: New Foundational Frameworks forcatena-Phosphorus Chemistry

“…As catena -phosphorus anions (phosphinophosphide ions) are composed of phosphine ( I ) and phosphide ( II ) units, catena -phosphorus cations (phosphinophosphonium ions) are envisaged as a combination of phosphine ( I ) and phosphonium ( III ) units. Despite the ubiquitous nature of phosphonium salts, relatively few derivatives of catena -phosphorus cations have been reported, − yet the similar geometrical arrangement and isolobal nature of PH 4 + and CH 4 highlight a greater potential for parallels to catena -carbon chemistry (organic chemistry).

…”

A Selective Synthesis of the 1,3,4-Triphospholide Anion

“…Until recently, the apparent parallels between the ligating properties of the η 5 -C 5 H 5 and η 5 -2,5-(C t Bu) 2 -1,3,4-P 3 ligands have been interpreted in terms of the diagonal relationship between C and P and the isolobal relationship between CR and P. , However, recent experimental and theoretical studies have suggested that there is a delicate balance between the η 5 -2,5-(C t Bu) 2 -1,3,4-P 3 ligand behaving as a surrogate η 5 -C 5 H 5 ligand and as a ligand where the order of the frontier orbitals of the η 5 -P 3 C 2 system is reversed, with interesting consequences for bonding and reactivity. − It can be argued that in order to capitalize on this interesting behavior it is important to develop more efficient and selective pathways to the 2,5-(C t Bu) 2 -1,3,4-P 3 anion, for which the most practical reported synthesis is from the reaction of the phosphaalkyne t BuC⋮P and alkali metals in a nonselective reaction requiring repeated recrystallization of moisture-sensitive alkali metal salts in order to separate the 1,3,4-triphospholide anion (42% yield) from the 1,3-diphospholide anion (37% yield) . In a new approach, we have focused on the reduction of the tricyclic ClP 3 (C t Bu) 2 compound 1a (Scheme ) and have found that reaction with either Li or Na metal in thf leads to the selective and quantitative formation of the lithium and sodium salts of the aromatic anion [ 2 ] - , which corresponds to an overall yield from t BuC⋮P of 53%. In this paper we show that (i) our new synthetic pathway can be expanded to provide quantitative and regioselective access to both K and Mg salts of the triphospholide anion and (ii) the mechanistic insights gained from these studies lead to a new and “economic” synthesis of the [2,5-(C t Bu) 2 -1,3,4-P 3 ] - anion from the phosphalkyne t BuC⋮P.…”