Neutral and Cationic Paramagnetic Amino-amidinate Iron(II) Complexes: ¹⁹F NMR Evidence for Interactions with Weakly Coordinating Anions

Two alkylamino-benzamidinates, [RNC(Ph)NCH2CH2NMe2]− (ArL, R = 2,6-iPr2C6H3; SiL, R = SiMe3), have been employed as ligands for Fe(II). The ligands adopt bridging coordination modes in their diiron(II) dichloro complexes [(μ-RL)FeCl]2 (1). Their structure and reactivity are strongly dependent on the nature of R. With CO, [(μ-ArL)FeCl]2 (1a) forms the monomeric, 18-electron dicarbonyl compound [κ3-C(O)(2,6-iPr2C6H3)NC(Ph)NCH2CH2NMe2]FeCl(CO)2 (3), in which CO has inserted into the Fe−N(amidinate) bond, whereas [(μ-SiL)FeCl]2 (1b) is unreactive under similar conditions. Alkylation of 1a was unsuccessful, leading to ligand redistribution and isolation of (κ2-ArL)2Fe (2), in which the ligands adopt a κ2-amido-amino coordination mode, but 1b reacts with KCH2Ph to give the 14-electron diiron dibenzyl derivative [(μ-SiL)FeCH2Ph]2 (4). Benzyl abstraction by B(C6F5)3, [Ph3C][B(C6F5)4], or [PhNHMe2][B(C6F5)4] affords the paramagnetic monocation [(μ-SiL)2Fe2CH2Ph]+ (5+) partnered with [PhCH2B(C6F5)3]− or [B(C6F5)4]−, respectively. Interactions of these weakly coordinating anions with the paramagnetic Fe(II) centers in solution are apparent from significantly shifted and broadened resonances in the 19F NMR spectra.