Synthesis and Reactivity of Iron(II) Acetylide Complexes Relevant to Alkyne Dimerization

Iron(II) phenylacetylide complexes of the pyrrole-based pincer ligands, (R) PNP (R = Cy, Bu-t; PNP = anion of 2,5bis(dialkylphosphinomethyl)pyrrole), have been prepared and characterized. Acetylide compounds of both ligands exist as square planar, intermediate-spin (S = 1) species in solution, but additional Lewis base (pyridine, CO, or bipyridine) coordination is required for the isolation of [Fe(CCPh)((PNP)-P-Cy)]. The iron acetylide complex of (PNP)-P-Cy readily inserts phenylacetylene to produce the enynyl species, [Fe(E-C{Ph}CH{CCPh})((PNP)-P-Cy)], representing an intermediate along the pathway to alkyne dimerization. Catalytic dimerization reactions with the (PNPFe)-P-Cy system lead to a mixture of E- and Z-enyne products, indicating interconversion between the iron E- and Z-enynyl species during turnover. Insertion reactions employing 1,4-diphenylbutadiyne support a pathway for interconversion between enynyl rotamers involving rearrangement of the iron enynyl species. The solid-state structures of several acetylide and enynyl species are reported, and their chemistry is discussed in the context of mechanisms for catalytic alkyne dimerization.

Automated summary

Iron(II) phenylacetylide complexes of pyrrole-based pincer ligands were synthesized and characterized. In solution, the acetylide compounds of both ligands exist as square planar, intermediate-spin species. Interconversion between iron enynyl species was observed during catalytic alkyne dimerization.