Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity

Abstract

The possibility to alter properties of metal complexes without significant steric changes is a useful tool to tailor the reactivity of the complexes. Herein we present the synthesis of iron complexes with the tripodal phosphane ligands Triphos and Triphos(Si) and report on their different coordination properties. Whereas reaction of Triphos(Si) and FeX2 (X = Cl, Br) exclusively afforded (Triphos(Si))FeX2 with a κ(2)-coordinated ligand, the homologous C-derived Fe complexes show rapid conversion in solution to afford [(Triphos)Fe(CH3CN)3][Fe2Cl6] or [(Triphos)Fe(CH3CN)3][FeBr4], respectively. The structural conversion was found to be temperature- and solvent-dependent and was accompanied by a linear change of the overall magnetization. The different ligand influence was shown to have a significant effect on the ability of (Triphos(Si))FeCl2 and (Triphos)FeCl2 to perform the Sonogashira cross-coupling reaction of 4-iodotoluene and phenyl acetylene as well as the hydrosilylation of acetophenone. The results presented herein show the different coordination properties of two structurally homologous tripodal ligands and demonstrate the importance of geometrically controlled ligand field splitting on the stability and reactivity of metal complexes. The C/Si exchange therefore provides a simple and straightforward tool to manipulate properties and reactivity of metal complexes.

Carbon/Silicon Exchange at the Apex of Diphos‐ and Triphos‐Derived Ligands – More Than Just a Substitute?

“…While this exchange does not commonly influence either the acceptor/donor strength of the donating atoms or the sterics at the metal center directly, the structural requirements of silicon enforce geometrically controlled ligand field splitting. Furthermore, while the C‐derived complexes favor low‐spin states, the Si counterparts preferentially exist in high‐spin configurations , . Such changes are typically observed for Triphos and its Si‐derived counterpart Triphos Si for metals that have high‐spin and low‐spin states with only low energy barriers between the two states .…”

“…In contrast, the analogous silicon complex was solely reported as [κ 2 ‐PhSi(CH 2 PPh 2 ) 3 CoCl 2 ] ( 10b ) . In addition, high‐spin iron(II) complex [κ 2 ‐MeC(CH 2 PPh 2 ) 3 FeCl 2 ] ( 12a ) can also be reversibly interconverted to low‐spin [κ 3 ‐MeC(CH 2 PPh 2 ) 3 Fe(MeCN) 3 ] 2+ ( 13 ) in acetonitrile (Figure ) . The analogous silicon complex, [κ 2 ‐MeSi(CH 2 PPh 2 ) 3 FeCl 2 ] ( 12b ), did not undergo a similar interconversion process.…”

“…Exchange of the carbon for silicon at the apex of phosphine ligands can therefore be used as a tool to modulate the properties of the complexes, which can result in analogous complexes that are biased for different catalytic reactions. For example, while [κ 2 ‐MeC(CH 2 PPh 2 ) 3 FeCl 2 ] ( 12a ) is able to catalyze Sonogashira cross‐coupling reactions, the silicon‐containing complex [κ 2 ‐MeSi(CH 2 PPh 2 ) 3 FeCl 2 ] ( 12b ) is inactive for this reaction . In a similar manner, 12b catalyzes the hydrosilylation of acetophenone with Et 3 SiH with catalyst loadings of 5 mol‐% at –78 °C.…”

“…Solvent-controlled rearrangements of tripodal Co and Fe complexes. [25][26][27] The examples shown clearly indicate that the choice of conditions is crucial for complex stability and needs to be considered in order to prevent unforeseen products. On the other hand, one can utilize the differences in reactivity to synthesize a desired complex.…”

Towards Iron-Catalyzed Sonogashira Cross-Coupling Reactions

“…Recently, we reported on Ni‐catalyzed Sonogashira cross‐coupling reactions utilizing Triphos Si ‐ and Triphos‐ligands, showing that the reactivity of a catalytic system can be easily tuned by different geometric constraints induced by a C/Si exchange in the ligand backbone . We could later show that, contrary to Triphos Si FeCl 2 , TriphosFeCl 2 allowed for stoichiometric C−C bond coupling of aryl‐iodides and phenylacetylene under Sonogashira cross‐coupling conditions . In continuation of our efforts to establish an iron‐based Sonogashira cross‐coupling reaction, we herein report on the use of FeCl 2 (bdmd) ( 3 ), in combination with a Cu(I) salt, for this type of reaction (Scheme and Figure ).…”