High-Spin Iron(II) Alkynyl Complexes with N-Heterocyclic Carbene Ligation: Synthesis, Characterization, and Reactivity Study

Abstract

High-spin iron(II) alkynyl complexes [(IPr 2 Me 2 ) 2 -Fe(CCBu t ) 2 ] (1) and [(IPr 2 Me 2 ) 2 Fe(CCR)(NHMes)] (R = Bu t 2, SiMe 3 3) bearing a monodentate N-heterocyclic carbene ligand IPr 2 Me 2 (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) have been prepared by salt metathesis and/or amine elimination methods and characterized by various spectroscopic methods. Complex 1 reacts with PMe 3 (4 equiv) and Bu t NC (4 equiv) to form trans-[(PMe 3 ) 4 Fe(CCBu t ) 2 ] (4) and [η 3 -{(Bu t CC)-(Bu t )CC(IPr 2 Me 2 )C(NBu t )}Fe(NCBu t ) 3 ] (5), respectively. In contrast, the reactions of 1 with 4-Pr i -C 6 H 4 NCO and Pr i NCNPr i lead to the formation of the zwitterionic salts 4-Pr i -C 6 H 4 NC(O)(IPr 2 Me 2 ) and (Pr i N) 2 C(IPr 2 Me 2 ), respectively. The interaction of 1 with I 2 gives Bu t CCCCBu t and (IPr 2 Me 2 ) 2 FeI 2 . The C(sp)−C(sp 3 ) cross-coupling products n-C 8 H 17 CCBu t and c-C 6 H 11 CCBu t are formed in high yields when 1 is treated with the corresponding alkyl halides n-C 8 H 17 X and c-C 6 H 11 X (X = Br, Cl). The formation of the ring-opening product 7,7-dimethyloct-1-en-5-yne in the reaction of 1 with cyclopropylmethyl bromide supports the radical character of the cross-coupling reaction.

Intermediates and Reactivity in Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl Halides

“…48 In contrast, the neutral bis-alkynylated iron(II)-NHC complex (IPr 2 Me 2 ) 2 Fe(CC- t Bu) 2 reported by Deng and co-workers is characterized by a distorted tetrahedral geometry at iron and is high spin ( S = 2). 54 …”

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

“…103 Deng and co-workers also described a stoichiometric alkynylalkyl coupling system mediated by the Fe II (C≡CtBu)2(IPr2Me2) complex (Table 1, Entry 25). 104 Bedford and co-workers reported the Suzuki-Miyaura cross-coupling of arylboronates with N-pyrrole amide-based aryl chlorides mediated by a NHCstabilised iron complex. A radical mechanism was proposed due to inhibition of the coupling process in the presence of radical scavenger (TEMPO, Table 1, entry 26).…”

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…promote M–NHC dissociation via ligand displacement. 25 , 138 , 218 – 220 Catalytic poisons that capture metals can shift the equilibrium and thus facilitate the M/NHC complexes decomposition. 221 Parameters of the equilibrium and the rates of ligand exchange obviously depend on the relative energies of dissociation of M–NHC and M–L′ bonds ( Scheme 10 ), concentrations, solvent, temperature and the presence of ancillary species.…”

“…To illustrate this point, an interesting case of Fe II /NHC complex 46 reacting with Bu t NC to give compound 48 can be mentioned. 138 This reaction may be considered as a simple ligand displacement with subsequent transformations of the released species. At the first stage of the reaction, NHC is displaced by Bu t NC ligand.…”