Neutral and cationic paramagnetic amino-amidinate Iron(II) complexes: (19)F NMR evidence for interactions with weakly coordinating anions

Two alkylamino-benzamidinates, [RNC(Ph)NCH(2)CH(2)NMe(2)](-)((Ar)L, R = 2,6-iPr(2)C(6)H(3); (si)L, R = SiMe(3)), have been employed as ligands for Fe(II). The ligands adopt bridging coordination modes in their diiron(II) dichloro complexes [(mu-(R)L)FeCl](2) (1). Their structure and reactivity are strongly dependent on the nature of R. With CO, [(mu-(Ar)L)FeCl](2) (1a) forms the monomeric, 18-electron dicarbonyl compound [K(3)-C(O)(2,6iPr(2)C(6)H(3))NC(Ph)NCH(2)CH(2)NMe(2)]FeCl(CO)(2) (3), in which CO has inserted into the Fe-N(amidinate) bond, whereas [(mu-(si)L)FeCl](2) (1b) is unreactive under similar conditions. Alkylation of la was unsuccessful, leading to ligand redistribution and isolation Of (K(2)-(Ar)L)(2)Fe (2), in which the ligands adopt a K(2)-amido-amino coordination mode, but 1b reacts with KCH(2)Ph to give the 14-electron diiron dibenzyl derivative [(mu-(si)L)FeCH(2)Ph](2) (4). Benzyl abstraction by B(C(6)F(5))(3), [Ph(3)C][B(C(6)F(5))(4)], Or [PhNliMe(2)][B(C(6)F(5))(4)] affords the paramagnetic monocation [(mu-(si)L)(2)Fe(2)CH(2)Ph](+) (5(+)) partnered with [PhCH(2)B(C(6)F(5))3](-) or,[B(C(6)F(5))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 (19)F NMR spectra.