Formation of a Phosphorus−Phosphorus Bond by Successive One-Electron Reductions of a Two-Phosphinines-Containing Macrocycle:  Crystal Structures, EPR, and DFT Investigations

Abstract

Chemical and electrochemical reductions of the macrocycle 1 lead to the formation of a radical monoanion anion [1](*)(-) whose structure has been studied by EPR in liquid and frozen solutions. In accord with experimental (31)P hyperfine tensors, DFT calculations indicate that, in this species, the unpaired electron is mainly localized in a bonding sigma P-P orbital. Clearly, a one-electron bond (2.763 A) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 A apart (crystal structure). A subsequent reduction of this radical anion gives rise to the dianion [1](2)(-) which could be crystallized by using, in the presence of cryptand, Na naphthalenide as a reductant agent. As shown by the crystal structure, in [1](2)(-), the two phosphinine moieties adopt a phosphacyclohexadienyl structure and are linked by a P-P bond whose length (2.305(2) A) is only slightly longer than a usual P-P bond. When the phosphinine moieties are not incorporated in a macrocycle, no formation of any one-electron P-P bond is observed: thus, one-electron reduction of 3 with Na naphthalenide leads to the EPR spectrum of the ion pair [3](*)(-) Na(+); however, at high concentration, these ion pairs dimerize, and, as shown by the crystal structure of [(3)(2)](2)(-)[(Na(THF)(2))(2)](2+) a P-P bond is formed (2.286(2) A) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon-sodium-phosphorus bridges.

Aromatic λ3 heterocycles XIV. Fosfinine-ether macrocyclesheterocycles XIV. Fosfinine-ether macrocycles

“…Phenyl substituents of phosphinine units are situated in a plane roughly perpendicular to the phosphinine rings, in order to minimize steric repulsion with methyl groups. Cavity dimensions in PE1 and PE2 are close to those of PE3 obtained by X-ray crystallography that suggest possible "host-guest" metal complexes [32].…”

“…In Table 1 bond legths of phosphabenzene, Si-O and P-P distances in comparison with the reported X-ray 66 geometries [32] are presented. Valence angles of phosphabenzene units and exocyclic angles of P-C-Si and C-Si-O are listed in Table 2 together with experimental data for PE3.…”

“…σ-Coordinative character of phosphinine is comparable with that of phosphine while π-donor character is similar to benzene [25]. Phosphorus is better π-acceptor than carbon, LUMO level of phosphinine with dominant π character and large coefficient at phosphorus confer good π back donation ability, this characteristic was used for the stabilization of the reduced metallic species [28,29,[30][31][32]. Phosphinineether macrocycle bridged with dimethylsiloxi moieties manifest very interesting behavior towards one electron reduction, which allows the formation of one electron bond between the phosphorus atoms due to the spatial arrangement of the macrocycle that provides a close contact [33].…”

Confirmed by X‐ray Crystallography: The B⋅B One‐Electron σ Bond

“…[3] Owing to their intrinsically high reactivity, however, most molecules containing one-electron bonds are highly elusive and, in striking contrast to the large body of theoretical knowledge accumulated on such systems in the past decades, the paucity of experimentally characterized examples is remarkable: [4] With two exceptions, [5] experimental evidence has only been gathered for highly sensitive substances by means of elaborate techniques, such as matrix isolation, g-irradiation, H-atom abstraction, or one-electron reduction at low temperatures. [6][7][8][9][10][11][12][13][14][15][16] Specifically, radical anions of the type [R 3 B·BR 3 ]C À ([I]C À , Scheme 1; R = H, OMe) were detected. [13][14][15][16] Moreover, during electrochemical studies on the reduction of Ph 3 B, Mills and DuPont found indications of B·B adduct formation between [Ph 3 B]C À and unreduced Ph 3 B.…”

Physical Chemistry at the University of Geneva

“…macrocycle 2). [14] ii) The development of instrumentation permitting the electrochemical oxidation or reduction of substances in situ in the EPR cell at variable temperature, an example of a spectrum being shown in Fig. 8.…”