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)P^NEt-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(PCP^NEt-iPr)(CO)₂Cl] (2) was obtained. In an attempt to prepare the hydride Fe(II) complex [Fe(PCP^NEt-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(PCP^NEt-iPr)(CO)₂] (4). Exposure of a benzene solution of 4 to NO gas (1 bar) at room temperature affords the diamagnetic complex [Fe(PCP^NEt-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(κ³P,CH,P-P(CH)P^NEt-iPr)(CO)(NO)]BF₄ (6) which features an η²-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.