Three-coordinate Bis( N -heterocyclic carbene)iron(0) complexes with alkene and alkyne ligation: Synthesis and characterization

The synthesis and characterization of the three-coordinate iron(0) complex [(IMesMe) 2 Fe( η 2 -CH 2 CHSiMe 3 )] ( 1, IMesMe = 1-(2′4′6′-trimethylphenyl)-3-methyl-2-ylidene) as well as its reactions with alkynes have been investigated. Complex 1 was prepared in 76% yield from the reaction of FeCl 2 with two equiv. of IMesMe, one equiv. of CH 2 CHSiMe 3, and two equiv. of KC 8. The characterization data obtained from solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, and 57 Fe Mössbauer spectroscopy, in combination with theoretical calculations suggest an S  = 1 ground spin-state for 1 that has enhanced iron-to-alkene π-backdonation as compared to that in the bis(alkene) complex [(IPr)Fe( η 2 -CH 2 CHSiMe 3 ) 2 ] (IPr = 1,3-di-(2′6′-diisopropylphenyl)-2-ylidene). The equimolar reactions of 1 with the internal alkynes PhC CPh, EtC CEt, and PhC CMe yield the iron(0) alkyne complexes [(IMesMe) 2 Fe( η 2 -RCCR′)] (R = R′ = Ph, 2 ; R = R′ = Et, 3 ; R = Ph, R′ = Me, 4 ). In addition, the reaction of 1 with five equiv. of PhC CMe is found to give a paramagnetic cyclobutadiene iron complex [(IMesMe) 2 Fe( η 4 -C(Ph)C(CH 3 )C(Ph)C(CH 3 ))] ( 5 ). The molecular structure and 57 Fe Mössbauer spectroscopy data indicate an S  = 1 ground spin-state for 2 – 5. Complex 1 is an effective catalyst for cyclotrimerization reactions of internal alkynes R 1 C CR 2 (R 1  = R 2  = Ph; R 1  = R 2  = Et; R 1  = Me, R 2  = Ph).