Unprecedented triphosphinine iron interactions: Intramolecular electron transfer, reactivity round a corner, and a low-activated ring element exchange reaction

2,4,6-Tri-tert-butyl-1,3,5-triphosphinine (7) was combined with 1,5-COD or 2,4-di-tert-butyl-1,3-diphosphete to form iron π-complexes. The ligands of highly reactive [(2,4-di-tert-butyl-1,3-diphosphete)(2,4,6-tri-tert-butyl-1,3,5-triphosphinine)Fe] (8) are formed by cyclooligomerization of tert-butylphosphaalkyne (3) under the influence of the iron atom. 8 rearranges spontaneously to yield penta-tert-butyl-pentaphosphaferrocene (9) as the isolable product. An intramolecular electron transfer product [(1,5-COD)(η6-1,3,5-triphosphacyclohexa-2,5-dine-1,4-diyl)Fe(2+)] (12) is obtained with 1,5-COD. Addition of [(CO)5Cr(THF)] initiates an interligand hydrogen transfer to form [(η5-trihydropentalenyl)Fe(μ,1-3-η-4,5,6-trihydro-1,3,5-triphosphinine)Cr(CO)5] (13) Extensive DFT calculations of isolated and reactive species, and several possible intermediates agree with the experimental observations and revealed for the first time a possible reaction sequence, which allows a low-activated exchange of ring elements between the ligands of sandwich complex 8 to form 9. The process is based on the specific combination of the metal and its heterocyclic π-ligands. Both, singlet and triplet spin states play an important role in the process.