A series of 2-phenoxypyridyl and 2-phenoxyjmino ligands, H2LR,R' [2,2'-(5,5'-(1,2-phenylenebis(ethyne-2,1-diyl))bis-(pyridine-5,2-diyl))d iphenol, where R = H, Me, or t-Bu, and R'=H or Ph] and H2BIPSMe,Ph [(3,3'-(1E,1'E)-(3,3'-sulfonylbis(3,1-phenylene)bis(azan-1-yl-1-ylidene) )bis(methan-1-yl-1-ylidene)bis(5-methylbiphenyl-2-ol)], were synthesized as platforms for nonheme diiron(II) protein model complexes, UV-vis spectrophotometric studies and preparative-scale reactions of L-R,L-R' or BIPSMe,Ph, where L-R,L-R' and BIPSMe,Ph are the deprotonated forms of H2LR,R' and H2BIPS(Me,Ph), respectively, with iron(II) revealed that the presence of sterically protective o-phenol substituents is necessary to obtain discrete dinuclear spcies. The reaction of L-Me,L-Ph with iron(II) in tetrahydrofuran (THF) afforded the doubly bridged compound [Fe-2(L-Me,L-Ph)(2)(THF)(3)] (1), which was characterized in the solid state by X-ray crystallography. A large internal cavity in this complex facilitates its rapid reaction with dioxygen, even at -50 degrees C, to produce the thermodynamically stable [Fe-2(mu-O)(L-Me,L-Ph)(2)] (2) species O-18(2) instead of O-16(2) with 1 led to a shift in the Fe-O-Fe vibrational frequency from 833 to 798 cm(-1), confirmig the presence of the (mu-oxo)diiron(III) core and molecular oxygen as the source of the bridging oxo group. The L-Me,L-Ph ligand is robust toward oxidative decomposition and does not display any reversible redox activity.