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.