Iron PCP Pincer Complexes in Three Oxidation States: Reversible Ligand Protonation To Afford an Fe(0) Complex with an Agostic C-H Arene Bond

In the current investigation, the reaction of Fe-2(CO)(9) with the ligand precursor 2-chloro-N-1,N-3-bis-( diisopropylphosphanyl)-N-1,N-3-diethylb enzene-1,3-diamine (P(C-Cl)P-NEt-ipr) (1) was investigated. When a suspension of Fe-2(CO)(9) and 1 in CH3CN was transferred in a sealed microwave glass vial and stirred for 18 h at 110 degrees C the complex [Fe(PCPNEt-iPr)(CO)(2)Cl] (2) was obtained. In an attempt to prepare the hydride Fe(II) complex [Fe(PCPNEt-iPr)(CO)(2)H] (3), 2 was reacted with 1 equiv of Li[HBEt3] in THF. Instead of ligand substitution, this complex underwent a one electron reduction which led to the formation of the low-spin d(7) Fe(I) complex [Fe(PCPNEt-iPr)(CO)(2)] (4). Exposure of a benzene solution of 4 to NO gas (1 bar) at room temperature affords the diamagnetic complex [Fe(PCPNEt-iPr)(CO)(NO)] (5). This is the first iron PCP nitrosyl complex. Protonation of 5 with HBF4 center dot Et2O affords the cationic Fe(0) complex [Fe((KP)-P-3,CH,P-P(CH)P-NEt-iPr)(CO)(NO)BF4 (6) which features an eta(2)-C-aryl-H agostic bond. Even with relatively weak bases such as NEt3 the agostic C-H bond can be deprotonated with reformation of the starting material 5. Therefore, protonation of 5 is completely reversible.