Synthesis, characterization, and electrochemistry of a series of iron(II) complexes containing self-assembled 1,5-Diaza-3,7-diphosphabicyclo 3.3.1 nonane Ligands

The reaction between PPh(CH2OH)(2), iron(II) sulfate, ammonium sulfate, and formaldehyde in aqueous solution gives the iron(II) complex [Fe(kappa(2)-O2SO2)L-2] (1), where L is the bidentate phosphine ligand 3,7-diphenyl-1,5-diaza-3, 7-diphosphabicyclo[3.3.1]nonane. During the course of the reaction, the ligand L self-assembles on the metal center. The reaction between PPh(CH2OH)(2), iron(II) chloride, ammonium chloride, and formaldehyde under similar conditions gives cis[FeCl2L2] (cis-2). The complex cis-2 is converted into trans-2 in Et2O, whereas in water it is converted into cis-[Fe(OH2)(2)L-2](2+), though both of these interconversions are reversible. The chloro ligands in cis-2 are readily displaced by reaction with thiocyanate, azide, and carbonate to give cis- and trans-[Fe(NCS)(2)L-2] (cis- and trans-3), cis- and trans-[Fe(N-3)(2)L-2] (cis- and trans-4), and [Fe(kappa(2)-O2CO)L-2] (5), respectively. The complex cis-2 reacts with CO in water to give trans-[FeCl(CO)L-2]Cl (trans-6), whereas trans-2 reacts with CO in diethyl ether to give cis-[FeCl(CO)L-2]Cl (cis-6), though cis-6 isomerizes in water to form trans-6. The reaction of cis-2 with sodium borohydride gives the hydride chloride complex trans-[FeCl(H)L-2] (7). Electrochemical studies have been undertaken on complexes 1, cis-2, and 7. These reveal reversible oxidations for cis-2 and 7, with the latter giving rise to an unusual 17-electron iron(III) hydride chloride complex. Crystal structures have been obtained for 1, trans-2, trans-3, 5, and 7.