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(eta(2)-CH2CHSiMe3)] (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 FeCl2 with two equiv. of IMesMe, one equiv. of CH2=CHSiMe3, and two equiv. of KC8. The characterization data obtained from solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, and Fe-57 Mossbauer spectroscopy, in combination with theoretical calculations suggest an S = 1 ground spin-state for 1 that has enhanced iron-to-alkene it-backdonation as compared to that in the bis(alkene) complex RIPr)Fe(eta(2)-CH2CHSiMe3)(2)] (IPr = 1,3-di-(2'6'-diisopropylphenyl)-2-ylidene). The equimolar reactions of 1 with the internal alkynes PhCCPh, EtCCEt, and PhCCMe yield the iron(0) alkyne complexes [(IMesMe)(2)Fe(eta(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 PhCCMe is found to give a paramagnetic cyclobutadiene iron complex [(IMesMe)(2)Fe(eta(4)-C(Ph)C(CH3)C(Ph)C(CH3))] (5). The molecular structure and Fe-57 Mossbauer spectroscopy data indicate an S = 1 ground spin-state for 2-5. Complex 1 is an effective catalyst for cyclotrimerization reactions of internal alkynes R1C CR2 (R-1 = R-2 = Ph; R-1 = R-2 = Et; R-1 = Me, R-2 = Ph). (C) 2021 Elsevier Ltd. All rights reserved.