A Tetrapodal Penta‐Carbene Ligand and Iron(III) Complex

Abstract

We report the first tetrapodal pentadentate ligand composed of five N‐heterocyclic carbene donors. The proto‐ligand [CC4H5Me](OTf)3 (3) is formed in good yields from commercially available reagents. Upon removal of 5 proton equivalents from 3 with bulky non‐nucleophilic bases a dianionic penta‐carbene framework is provided in good yields as a dilithium complex CC4MeLi2 (4). Addition of FeCl3 to a solution of 4 formed in situ provides CC4MeFeCl (5) in moderate yield. Solution‐state magnetism measurements of 5 are consistent with a S=1/2 Fe(III) center. The related diamagnetic Fe(II) compound CC4MeFe (6) can be formed through reduction of 5 using KC8 though poor solubility characteristics have hampered its formation on a preparative scale and full characterization.

An Atypically Bent Isocyanide at Iron in a Tetrapodal Penta‐Carbene Framework

“…Me Fe (1) (Figure S1). [18] We attribute the 1 H signals at 7.95, 7.42, and 7.33 ppm to the protons of the phenyl rings while those at 7.06, 6.60, and 6.39 ppm are attributed to the backbone protons of the four arms and central imidazole-2ylidene ring. We attribute the resonance at 4.09 ppm to the protons of the four N-methyl groups.…”

“…We previously reported the formation of compound 1 from reaction of the ferric species in THF with methyllithium or by stirring over KC 8 at À 78 °C. [18] Subsequent attempts to provide 1 on a preparatory scale have been stymied by solubility issues. We judged that improvements to the reduction of the ferric complex could be made by using a more soluble reducing agent.…”

“…[15][16][17] We designed the dianionic tetrapodal penta-carbene [ Ph CC 4 Me ] 2À aiming to provide a strongly donating framework for application in the activation of multiply-bonded small molecules like N 2 . [18] Improvements in N 2 reduction will lead to energy savings [19] thereby reducing fossil hydrocarbon consumption while also enabling society to transition to alternative energy schemes that do not rely upon carbon as an energy carrier. [20] In this report using a combination of synthetic, spectroscopic, and computational methods we evaluate the donor ability of the square-capping [ Ph CC 4 Me ] 2À framework through the preparation and characterization of the iron(II) isocyanide complex Ph CC 4 Me FeÀ CN t Bu (2) from reaction of Ph CC 4 Me Fe (1) with tert-butylisocyanide (Scheme 1).…”

A S=3/2 Cr(III) Complex In A Square‐Capping NHC Framework

“…A rational as to why a salt metathesis reaction between 1 and the aforementioned organolithium reagents does not occur can found in our XRD and computational results. Comparison of the XRD structures of 1 to the ferric analog shows the Cr atom of 1 resides nearly 0.09 Å further above the mean plane defined by the four equatorial NHC donors than does the Fe atom in the related complex Ph CC 4 Me FeCl [28] . While the elongation of the C (ax) ‐M bond (Cr1‐C1 2.029(2) Å vs. Fe1‐C1 1.944(3) Å) can be anticipated to lower the trans influence of the axial NHC donor, the subtle geometric change is likely too little to improve orbital overlap between Cr and an incoming σ‐donor like the PhC 2 − , H 3 C − or t BuN(H) − moiety to result in the formation of strong bonds.…”

“…In an extension of our previous work describing the preparation of a square‐capping NHC framework and coordination chemistry of an Fe III complex [28] we report herein the preparation and characterization of a Cr III complex as an entry‐point to the study of the coordination chemistry of Cr in this strongly donating framework. Besides finding metallurgical applications, Cr is used in industrial scale chemical transformations including the Philips chromium catalyst and chromium oxide‐based dehydrogenation catalyst.…”