Crispin Lichtenberg

Low-valent iron(i) amido olefin complexes as promotors for dehydrogenation reactions.

Fe(I) compounds including hydrogenases show remarkable properties and reactivities. Several iron(I) complexes have been established in stoichiometric reactions as model compounds for N2 or CO2 activation. The development of well-defined iron(I) complexes for catalytic transformations remains a challenge. The few examples include cross-coupling reactions, hydrogenations of terminal olefins, and azide functionalizations. Here the syntheses and properties of bimetallic complexes [MFe(I) (trop2 dae)(solv)] (M=Na, solv=3 thf; M=Li, solv=2 Et2 O; trop=5H-dibenzo[a,d]cyclo-hepten-5-yl, dae=(N-CH2 -CH2 -N) with a d(7) Fe low-spin valence-electron configuration are reported. Both compounds promote the dehydrogenation of N,N-dimethylaminoborane, and the former is a precatalyst for the dehydrogenative alcoholysis of silanes. No indications for heterogeneous catalyses were found. High activities and complete conversions were observed particularly with [NaFe(I) (trop2 dae)(thf)3 ].

A Low-Valent Iron Imido Heterocubane Cluster: Reversible Electron Transfer and Catalysis of Selective C-C Couplings

Enzymes and cofactors with iron-sulfur heterocubane core structures, [Fe4 S4 ], are often found in nature as electron transfer reagents in fundamental catalytic transformations. An artificial heterocubane with a [Fe4 N4 ] core is reported that can reversibly store up to four electrons at very negative potentials. The neutral [Fe4 N4 ] and the singly reduced low-valent [Fe4 N4 ](-) heterocubanes were isolated and fully characterized. The low-valent species bears one unpaired electron, which is localized predominantly at one iron center in the electronic ground state but fluctuates with increasing temperatures. The electrons stored or released by the [Fe4 N4 ]/[Fe4 N4 ](-) redox couple can be used in reductive or oxidative CC couplings and even allow catalytic one-pot reactions, which show a remarkably enhanced selectivity in the presence of the [Fe4 N4 ] heterocubanes.

Reactivity of an All‐Ferrous Iron–Nitrogen Heterocubane under Reductive and Oxidative Conditions

The reactivity of the all-ferrous FeN heterocubane [Fe4 (Ntrop)4 ] (1) with i) Brønsted acids, ii) σ-donors, iii) σ-donors/π-acceptors, and iv) one-electron oxidants has been investigated (trop = 5H-dibenzo[a,d]cyclo-hepten-5-yl). 1 showed self-re-assembling after reactions with i) and proved surprisingly inert in reactions with ii) and iii), with the exception of CO. Reductive and oxidative cluster degradation was observed in reactions with CO and TEMPO, respectively. These reactions yielded new cluster compounds, namely a trinuclear bis(μ3 -imido) 48 electron complex in the former case and a tetranuclear all ferric μ-oxo-μ-imido species in the latter case. Characterization techniques include NMR and in situ IR spectroscopy, single crystal X-ray analysis, Mössbauer spectroscopy, cyclic voltammetry, magnetic susceptibility measurements, and DFT calculations.