Formation of a phosphorus-phosphorus bond by successive one-electron reductions of a two-phosphinines-containing macrocycle:: Crystal structures, EPR, and DFT investigations

Chemical and electrochemical reductions of the macrocycle I 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 P-31 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 Angstrom) was formed between two phosphorus atoms which, in the neutral molecule, were 3.256 Angstrom 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) Angstrom) 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) Angstrom) between two phosphinine rings which adopt a boat-type conformation, the whole edifice being stabilized by two carbon-sodium-phosphorus bridges.