Silylation of Iron-Bound Carbon Monoxide Affords a Terminal Fe Carbyne

A series of monocarbonyl iron complexes in the formal oxidation states 0, +1, and +2 are accessible when supported by a tetradentate tris(phosphino)silyl ligand (SiPiPr3 = [Si(o-C6H4PiPr2)3]−). X-ray diffraction (XRD) studies of these carbonyl complexes establish little geometrical change about the iron center as a function of oxidation state. It is possible to functionalize the terminal CO ligand of the most reduced carbonyl adduct by addition of SiMe3+ to afford a well-defined iron carbyne species, (SiPiPr3)FeCOSiMe3. Single-crystal XRD data of this iron carbyne derivative reveal an unusually short FeCOSiMe3 bond distance (1.671(2) Å) and a substantially elongated C−O distance (1.278(3) Å), consistent with Fe−C carbyne character. The overall trigonal bipyramidal geometry of (SiPiPr3)FeCOSiMe3 compares well with that of the corresponding carbonyls, (SiPiPr3)Fe(CO)−, (SiPiPr3)Fe(CO), and (SiPiPr3)Fe(CO)+. Details regarding the electronic structure of the carbyne complex have been explored via the collection of comparative Mössbauer data for all of the complexes featured and also via DFT calculations. In sum, these data point to a strongly π-accepting Fischer-type carbyne ligand that confers stability to a low-valent iron(0) rather than high-valent iron(IV) center.