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

Abstract

Iron(II) phenylacetylide complexes of the pyrrole-based pincer ligands, R PNP (R = Cy, t Bu; PNP = anion of 2,5bis(dialkylphosphinomethyl)pyrrole), have been prepared and characterized. Acetylide compounds of both ligands exist as squareplanar, intermediate-spin (S = 1) species in solution, but additional Lewis base (pyridine, CO, or bipyridine) coordination is required for the isolation of [Fe(CCPh)( Cy PNP)]. The iron acetylide complex of Cy PNP readily inserts phenylacetylene to produce the enynyl species, [Fe(E-C{Ph}CH{CCPh})( Cy PNP)], representing an intermediate along the pathway to alkyne dimerization. Catalytic dimerization reactions with the Cy PNPFe 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.

Homo- and Cross-Coupling of Phenylacetylenes and α-Hydroxyacetylenes Catalyzed by a Square-Planar Rhodium Monohydride

“…Subsequent insertion of the C–C triple bond of another alkyne molecule into the metal-acetylide bond leads to a η 3 -butenynyl intermediate, which then reacts with a new alkyne molecule to give the dimer and regenerate the active acetylide species ( a in Scheme ). In contrast, electron-rich precursors react by homolytic cleavage of the C­(sp)–H bond, to give a hydride–metal–acetylide complex. This species inserts the C–C triple bond of a new alkyne molecule, into the metal-hydride bond, to generate an alkenyl-metal-acetylide intermediate.…”

Z-selective dimerization of terminal alkynes by a (PNNP)FeII complex

“…S21, ESI†) corresponding to the bound phenylacetylide fragments closely matches other Fe II acetylide complexes reported in the literature. 27 Additionally, the zero-field Mössbauer spectrum (4 K) displays a well-resolved quadrupole doublet with an isomer shift ( δ ) of 0.214 mm s −1 and a quadrupole splitting (|Δ E Q |) of 1.971 mm s −1 (Fig. 2b).…”