Iron(IV) Alkyl Hydrazido Complexes: Electronic Structure, Fe–C Bond Homolysis, and N–C Bond Forming Migration Reactions

Abstract

High-valent iron-alkyl complexes are rare, as they are typically prone to Fe–C bond homolysis. We show here an unusual way to access formally iron(IV) alkyl complexes through double silylation of iron(I) alkyl dinitrogen complexes to form an NNSi2 group. When the alkyl group is trimethylsilylmethyl, the formally iron(IV) compound is stable at room temperature. Spectroscopically validated computations show that the disilylhydrazido(2–) ligand stabilizes the formal iron(IV) oxidation state through a strongly covalent Fe–N -interaction, in which one -bond fits an "inverted field" description. This means that the two bonding electrons are localized on the metal and not the ligand, and an iron(II) resonance structure is a significant contributor as with the phenyl analogue. However, in contrast to the phenyl analogue which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, and this places one electron in the * orbital and weakens the Fe–N bonding, leading to longer Fe–N bonds. The reactivity of these hydrazido(2–) complexes has an interesting dependence on the specific alkyl group. When the alkyl group is methyl, the formally iron(IV) species undergoes migration of the carbon-based ligand to the NNSi2 group to form a new N–C bond, followed by an intriguing isomerization of the hydrazido ligand. This reactivity is not observed with the bulkier trimethylsilylmethyl complex. When the alkyl group is benzyl, yet another reactivity pathway is evident: the Fe–C bond homolyzes to give a three-coordinate iron(III) complex with a hydrazido(2–) ligand. DFT calculations are used to explain the differences between the behavior with the different alkyl groups. Overall, these formally iron(IV) compounds display a diverse set of reaction pathways associated with the specific alkyl groups.

Dinitrogen Binding and Functionalization from a Low-Coordinate Alkynyliron Complex

“…Though calling the hydrazido(2−) ligand an electrophile might sound unusual given its 2– formal charge, we showed using DFT and CASSCF calculations that the LUMO lies largely on the proximal N atom . This is a characteristic of inverted ligand–field interactions that are found in a number of complexes with formally high oxidation states. , In order to compare the electrophilicity in a quantitative fashion, we performed natural bond orbital (NBO) analysis on the optimized structures of complexes 4 ( S = 1 and 2) and 3 ( S = 2) and the alkyl analogue 3-CH 3 ( S = 2) . The results are shown in Table .…”

“…55,56 In order to compare the electrophilicity in a quantitative fashion, we performed natural bond orbital (NBO) analysis on the optimized structures of complexes 4 (S = 1 and 2) and 3 (S = 2) and the alkyl analogue 3-CH 3 (S = 2). 44 The results are shown in Table 3. Each formally iron(IV) complex has an Fe d-electron count close to 6 e − , consistent with a physical oxidation state of iron(II) for each complex.…”

“…Comparing Iron(IV) Hydrazido Complexes with Other Hydrocarbyl Ligands. The isolation of 3, the isolation of 4 as per the literature, 43 and the isolation of an alkyl analogue in separate work 44 enables us to compare some aspects of the electronic structure of these formally iron(IV) hydrazido(2−) complexes. Detailed analysis will be in a future publication; here, we test several potential explanations for why the alkynyl Table 3.…”

“…In the work described here, we use β-diketiminate supporting ligands, which are strongly binding weak-field ligands that can enforce low coordination numbers through the use of ortho substituents on the flanking aryl groups. , While numerous organometallic iron complexes supported by these ligands have been isolated, there are only a handful of β-diketiminato iron alkynyl complexes (Figure ). − Inspired by recent findings from our group showing that reduced, three-coordinate iron­(I) aryl complexes can bind and functionalize N 2 through a silylation-migration pathway (Scheme ), − we sought to explore the analogous reactivity with alkynyl complexes. Here, we report the synthesis and reactivity of monoalkynyliron complexes supported by a β-diketiminate ligand, including the binding of N 2 at an iron­(I) alkynyl complex and the isolation of a formally iron­(IV) alkynyl species …”