Ni–Fe and Pd–Fe Interactions in Nickel(II) and Palladium(II) Complexes of a Ferrocene-Bridged Bis(imidazolin-2-imine) Ligand

The
bis­(imidazolin-2-imine) ligand N,N′-bis­(1,3-diisopropyl-4,5-dimethyl­imidazolin-2-ylidene)-1,1′-ferrocene­diamine,
fc­(NIm)2 (1) was prepared. Its reaction with
[NiCl2(dme)] (dme = 1,2-dimethoxyethane) or [PdCl2(MeCN)2] afforded the tetrahedral, paramagnetic complex
[(1-κ2N,N′)­NiCl2] (6a) or the diamagnetic,
square-planar complex [(1-κ2N,N′)­PdCl2] (6b), respectively. For the latter, slow rearrangement to the ionic
complex [(1-κFe,κ2N,N′)­PdCl]­Cl, [7]­Cl, was observed,
which was followed by 1H NMR and UV/vis spectroscopy. Treatment
of [7]Cl with NaBF4 afforded [7]­BF4; the palladium atoms in both cations adopt square-planar
environments with short Fe–Pd bonds (ca. 2.65 Å). In addition,
a series of dicationic complexes of the type [(1-κFe,κ2N,N′)­ML]­(BF4)2 (8a: M = Ni, L = MeCN; 8b: M = Pd, L = MeCN; 9a: M = Ni, L = PMe3; 9b: M = Pd, L = PMe3) was prepared from 6a (M = Ni) or [7]­BF4 by chloride abstraction
with NaBF4 or AgBF4 in the presence of acetonitrile
or trimethylphosphine, respectively. In the presence of triphenylphosphine,
the palladium­(II) complex [(1-κFe,κ2N,N′)­Pd­(PPh3)]­(BF4)2 (10) was isolated. Iron–nickel
and iron–palladium bonding in these complexes was studied experimentally
by NMR, UV/vis, and Mössbauer spectroscopy and by cyclic voltammetry.
Detailed DFT calculations were carried out for the cations [(1-κFe,κ2N,N′)­M­(MeCN)]2+ in the 8a/8b couple, with Bader’s atoms in molecules theory revealing
the presence of noncovalent, closed-shell metal–metal interactions.
Potential energy surface scans with successive elongation of the Fe–M
bonds allow an estimation of the iron–metal bond dissociation
energies (BDE) as BDE­(Fe–Ni) = 11.3 kcal mol–1 and BDE­(Fe–Pd) = 24.3 kcal mol–1.