Iron coordination with bis-phosphinite PONOP and bis-phosphite PONOP pincer ligands: toward the development of iron alkylidenes

Includes bibliographical references (pages 53-55)A series of bis-phosphinite and bis-phosphite PONOP iron complexes were prepared and characterized by NMR and IR. Bis-phosphinite PONOP iron dichlorides [(RPONOP)FeCl2 (RPONOP=2,6-(R2PO)2(C5H3N); R=iPr, tBu)] were prepared through complexation of the free ligands with FeCl2 and their solid-state structures were determined by X-ray diffraction. Bis-phosphite PONOP iron complexes [(OEtPONOP)Fe(PMe3)2 and (CatPONOP)Fe(PMe3)2; (Cat=catechol)] were synthesized through complexation of the free ligands to Fe(PMe3)4. Carbonyl complexes of both bis-phosphinite and bis-phosphite PONOP were prepared and characterized by IR. The monocarbonyl (iPrPONOP)Fe(CO)Cl2 was accessed through exposure of (iPrPONOP)FeCl2 to an atmosphere of CO and the solid-state structure was determined by X-ray diffraction. The CO stretching frequency was observed at 1969 cm-1. Dicarbonyl complexes (iPrPONOP)Fe(CO)2 and (OEtPONOP)Fe(CO)2 were accessed through reduction of the corresponding chloride complexes with sodium amalgam under a CO atmosphere. Carbonyl stretching frequencies for (iPrPONOP)Fe(CO)2 and (OEtPONOPFe)(CO)2 were observed at 1824 and 1876 cm-1, and at 1871 and 1927 cm-1 respectively. The bis-phosphite PONOP complexes exhibit a more oxidized metal center than the bis-phosphinite PONOP complexes, as would be expected for the stronger ??-acceptor character of these ligands. The electronic properties of the bis-phosphinite PONOP and bis-phosphite PONOP iron complexes are intermediate between previously reported PNP and PDI structures, with the PONOP ligands exhibiting stronger electron donating ability than PDI ligands, but promoting a more oxidized metal center than found in analogous PNP iron complexes