Iron Complexes Bearing Imidazolium And/or Phosphine Ligands:Design,Synthesis And Their Catalysis

In this paper, novel ionic Fe （Ⅱ） or Fe（Ⅲ） complexes containing imidazolium cations, imidazolinium cations or benzimidazolium cations （1-15） were synthesized. Among these complexes, complexes 1-8 are ionic Fe （Ⅱ） complexes containing both phosphine ligands and imidazolium cations or imidazolinium cations; complex 9 is an ionic Fe（Ⅱ） complex containing both a N-heterocyclic carbene and an imidazolium cation; complexes 10-15 are ionic Fe（Ⅲ） complexes containing benzimidazolium cations. All of these complexes have been characterized by elemental analysis and X-ray crystallography. In turn, the catalytic behaviors of these complexes have been investigated. The obtained results were shown as followed:1. The reaction of FeX₂ （X= Cl or Br） with 1 equivalent of imidazolium salt or imidazolinium salt produced the intermediate, which was then treated with 1 equivalent of phosphine ligand to produce novel ionic Fe（Ⅱ） complexes 1-8. These complexes are novel ionic Fe（Ⅱ） complexes containing a phosphine ligand and an imidazolium （imidazolinium） cation. Each of complexes 1-7, which are ionic mononuclear Fe（Ⅱ） complexes, contains an imidazolium cation [HIPr]⁺or [HIMes]⁺, or an imidazolinium cation [SHEPr]+（[HIPr]⁺= 1,3-bis（2,6-diisopropylphenyl）imidazolium cation, [HIMes]⁺= 1,3-bis（1,3,5-trimethylphenyl）imidazolium cation, [HSPr]+= 1,3-bis（2,6-diisopropylphenyl）imidazolinium cation） and an Fe（Ⅱ）-based anion [Fe（L）X₃]’ （L= PPh₃ or PCy₃, X= Cl or Br）; Complex 8, which is ionic dinuclear Fe（Ⅱ） complex, contains two imidazolium cations [HIPr]+and an Fe（Ⅱ）-based anion [Fe₂（dppe）Cl₆]-（dppe =1,2-bis（diphenylphosphino）ethane）. The air-stability of these complexes are very close to that of the bis（phosphine） Fe（Ⅱ） complexes, and much better than that of complex 9, which contains both a N-heterocyclic carbene and an imidazolium cation.2. Complexes 1-8 showed high catalytic activities and broad substrate scope in the cross-coupling of alkyl Grignard reagents with aryl phosphates. To our knowledge, this reactin is the first Fe-catalyzed cross-coupling reaction of aryl phosphates with alkyl Grignard reagents. The catalytic activity of these complexes showed clear structure-activity relationships, with complex 4 exhibiting the highest activity. The synergic effects resulting from these complexes were studied. Furthermore we developed a new method for the cross-coupling of phenols and alkyl Grignard reagents through in situ phenol activation mediated by diethyl chlorophosphate.3. As a conventional iron complex, Fe（PPh₃）₂Cl₃ was found to be efficient catalyst for not only intermolecular reductive coupling between unactivated alkyl chlorides and aryl bromides, but also intramolecular reductive cross-coupling in the presence of magnesium turnings. The latter is the first Fe-catalyzed intramolecular eductive cross-coupling reactions, and a range of dihydrobenzofuran and benzopyran can be easily prepared through this reaction. The mechanism of intermolecular cross-coupling has been in detail investigated through four elementary reactions.4. Ionic Fe（Ⅲ） complexes 10-15 containing benzimidazolium cations were synthesized through the reactions of FeX₃ （X= Cl or Br） with benzimidazolium salts in a 1:1 molar ratio. All of the complexes were non-hygroscopic and air-stable, with five of them existing as solids. Each of the molecular structures of complexes 10-15 contains contains an benzimidazolium cation and an Fe（Ⅱ）-based anion [FeX₄]* （X= Cl or Br）.5. Complexes 10-15 showed high catalytic activities and broad substrate scope in the reductive cross-coupling of unactivated alkyl bromides with aryl phosphates in the presence of magnesium turnings. The catalytic activity of complexes 10-15 showed clear structure-activity relationships, with complex 15 exhibiting the highest activity.6. [HIPr][FeBr₄] was found to be an efficient catalyst for the cycloaddition reaction of CO₂ with epoxides under normal pressure, and this reactin is the first Fe-catalyzed cycloaddition reaction of CO₂ with epoxides under atmospheric pressure. The cycloaddition reactions of CO₂ with various monosubstituted terminal epoxides proceeded smoothly in the presence of [HIPr][FeBr₄].7. The synthesis of Fe（Ⅱ）-based indene-NHC complexes have been tested by reactions of （Ind）2Fe with imidazolium salts. There are some unexpected results observed, including: the reaction of （Ind）2Fe with [HIPr]Cl （1,3-bis（2,6-diisopropylphenyl） imidazolium salt） at 25℃ afforded anionic Fe（Ⅱ） complex 9 in 33% yield; the same reaction in refluxing THF provided Fe（IPr）（IPr*）Cl2（16） （IPr= 1,3-bis（2,6-diisopropylphenyl）imidazol-2-ylidene, IPr*= "abnormal" N-heterocyclic carbene）, which is the first biscarbene Fe（Ⅱ） complex containing a "abnormal" N-heterocyclic carbene liagnd, together with a bridged dinuclear monocarbene Fe（II） complex [Fe（IPr）Cl（OCH₂CH₃）]₂（17）; Fe（Mes）₂Cl₂ （18） （IMes= 1,3-dimesitylimidazol-2-ylidene） could be isolated in 34% yield from the reaction of （Ind）2Fe with [HIMes]Cl （1,3-bis（2,4,6-trimethylphenyl）imidazolium salt） at the reflux temperature of THF.