Iron(II) High-Spin and Low-Spin Complexes from Pentaisopropylcyclopentadienyliron(II) Bis(trimethylsilyl)amide

Two new complexes, [(C5Me5)FeBr(tmeda)] (tmeda = tetramethylethylenediamine) and [{C-5(CHMe2)(5)} FeN(SiMe3)(2)], have been synthesized and characterized as high-spin iron(II) complexes with typical distances between the iron atom and the center of gravity of the C-5 ring (Fe-Cp-cent) exceeding 190 pm and displaying paramagnetic behavior in solution. Both are analogues of known complexes, namely, the Jonas complex [(C5Me5)FeCl(tmeda)] and the Siemeling "pogo-stick" complex [(C5Me5)FeN(SiMe3)(2)]. The silylamide [{C-5(CHMe2)(5)}FeN(SiMe3)(2)] was treated with water to yield the bridged hydroxo dimer [{C-5(CHMe2)(5)}Fe(mu-OH)](2) with high-spin iron(II) configuration. With 2,5-dimethylpyrrole the azaferrocene derivative [{C-5(CHMe2)(5)}Fe(NC4H2Me2-2,5)] was obtained, and with diphenylphosphine a few crystals of the diphosphine hydride [{C-5(CHMe2)(5)}FeH(HPPh2)(2)] as well as a sample of its dehydrogenative P-P coupling product [{C-5(CHMe2)(5)}FeH(Ph2P-PPh2)](n) (n = 1 or 2) have been obtained, which are both diamagnetic 18-valence-electron complexes. Two-coordinated [{C-5(CHMe2)(5)}-FeN(SiMe3)(2)] features sizeable negative magnetic anisotropy and slow magnetic relaxation with an effective energy barrier for spin reversal of 113 cm(-1) under an applied field of 3000 Oe.