Computational Studies of the Electronic Structures and Ring Contraction Reactions of Pn (n=4-6) Ring in Double and Triple Decker Complexes, and an Orbital Engineering Approach to Stabilise Borozene (B6H6)

Abstract

The well established chemistry of ferrocene1-3 and different sandwich scaffolds4,5 with organic rings prompted us to explore the electronic structures and reactivity of sandwich complexes involving inorganic rings, i.e., cyclo-Pn and cyclo-B6H6. Cyclopolyphosphorus rings (cyclo-Pn), which are isolobal to organic rings (cyclo-(CH)n), are well-explored as a ligand in sandwich complexes. Scheer and coworkers have demonstrated the reactivity of cyclo­-Pn with different nucleophiles.6,7 An anionic nucleophile not only contracts the ring but also leads to products with retention and expansion of the cyclo-Pn ring, suggesting high reactivity but low selectivity.6 Conversely, neutral nucleophiles like N-heterocyclic carbene (NHC) only induce ring contraction.7 The discrepancy in the ring contraction with variation in the ring size of Pn (n=4,5) in double-decker complexes, [CpMPn] (M=Co, n=4, and M=Fe, n=5) is intriguing.7 We have correlated this discrepancy with the inherent ring strain in cyclophosphanes ((PH)n); a strain-free P5 shows reluctance towards contraction to strained P4, whereas comparable strain energies between P4 and P3 enable facile contraction.8 This trend in strain energies ((PH)5<(PH)4≈(PH)3) aligns with that in cycloalkanes ((CH2)n), though the magnitude of strain is significantly higher in (CH2)n than (PH)n. A similar reaction in triple-decker complexes with Pn as a middle deck, [CpMPnMCp] (M=Mo/V, n=6), shows the discrepancy with variation in the metal.7 We have correlated this variation with valence electron count (VEC), which directly impacts the shape of the middle deck.9 Recently, Scheer et al. have shown a facile contraction of strain-free P5 in a double-decker complex, with the subsequent attack of nucleophiles and electrophiles.10 Is it possible to achieve this with one reagent? Indeed, iodine can act as an electrophile, and its anionic polyiodide form can act as a nucleophile. Scheer and coworkers have shown the contraction of [CpFeP5] with iodine, leading to a unique nortricylane derivative product.11 We have investigated mechanistically this fascinating chemistry of I2 as a reagent.12 While cyclo-Pn has been extensively studied as a ligand, cyclo-BnHn, especially cyclo-B6H6, remains underexplored. We have shown its stabilisation in planar or near planar form in the sandwich scaffold using transition metal13 and group 13 main-group elements.14 An orbital engineering strategy is adopted to determine the perfect match among the various group 13 elements for capping the B6H6 ring in hexagonal bipyramidal geometry.14