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 Fe2(CO)9 with the ligand precursor 2-chloro-N1,N3-bis­(diisopropylphosphanyl)-N1,N3-diethylbenzene-1,3-diamine (P­(C–Cl)­PNEt-iPr) (1) was investigated. When a suspension of Fe2(CO)9 and 1 in CH3CN was transferred in a sealed microwave glass vial and stirred for 18 h at 110 °C the complex [Fe­(PCPNEt-iPr)­(CO)2Cl] (2) was obtained. In an attempt to prepare the hydride Fe­(II) complex [Fe­(PCPNEt-iPr)­(CO)2H] (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 d7 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·Et2O affords the cationic Fe(0) complex [Fe­(κ3 P,CH,P-P­(CH)­PNEt-iPr)­(CO)­(NO)]­BF4 (6) which features an η2-Caryl-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.