Preparation, Structure, and Reactivity of Nonstabilized Organoiron Compounds. Implications for Iron-Catalyzed Cross Coupling Reactions

Abstract

A series of unprecedented organoiron complexes of the formal oxidation states -2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo beta-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me 4Fe)(MeLi)][Li(OEt 2)] 2 ( 3) and [Ph 4Fe][Li(Et 2O) 2][Li(1,4-dioxane)] ( 5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to beta-hydride elimination reduce FeX n ( n = 2, 3) to clusters of the formal composition [Fe(MgX) 2] n . The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C 2H 4) 4][Li(tmeda)] 2 ( 8), [Fe(cod) 2][Li(dme)] 2 ( 9), [CpFe(C 2H 4) 2][Li(tmeda)] ( 7), [CpFe(cod)][Li(dme)] ( 11), or [Cp*Fe(C 2H 4) 2][Li(tmeda)] ( 14). Such electron-rich complexes, which are distinguished by short intermetallic Fe-Li bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such C-C bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(-2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively.

Iron Pincer Complexes as Catalysts and Intermediates in Alkyl–Aryl Kumada Coupling Reactions

“…Fürstner and co-workers showed that Fe-"ate" complexes, in which the formal oxidation states of Fe range from -2 to +2, might be catalytically active; 8,15 treatment of FeCl 3 with a large excess of aryl Grignard reagents could lead to Fe(0)-"ate" species. 8 Further, they showed that a formal Fe(-2) complex,…”

Iron(III) Half‐Sandwich Complexes of the Two‐Legged Piano‐Stool [CpFe(aryl)Cl] Type from the Corresponding Aryliron(II) Precursors

“…[11] The Fe III -C bonds in N-heterocyclic carbene complexes are even longer, for example, the Fe-CH 3 Scheme 3. Palladium(II) chloride did not rearrange the mesityl ligand to form a heterodinuclear species but resulted in the oxidized aryliron complex 5. distances are 2.060(11) Å in the 17-valence-electron iron(III) complex [Cp*Fe(η 3 -C 3 H 5 )(CH 3 )] [12] and 2.090(2) Å in the 1,3-diisopropylimidazol-2-ylidene complex of iron(III) chloride. [13] The tert-butyl substituent of C-4 is located directly above the mesityl moiety and bent out of the plane of the cyclopentadienyl ring by ca.…”

Reactions of [FeL2(CH3CN)2](PF6)2 (L = N-pyrimid-2-ylimidazol-ylidene) with N-, P-, O-, and S-donors and its catalytic activity

“…The catalytic efficiencies of iron salts in Kumada cross coupling reactions of Grignard reagents with aryl or alkyl halides have been demonstrated previously [13,[44][45][46][47]. However, investigations of Kumada coupling catalyzed by a welldefined iron-NHC complex are rare.…”