Regioselective Nucleophilic Additions to Diiron Carbonyl Complexes Containing a Bridging Aminocarbyne Ligand: A Synthetic, Crystallographic and DFT Study

Abstract

Diiron μ-aminocarbyne compounds, 1a-e, are prepared in two steps from Fe 2 Cp 2 (CO) 4 , negating the need for difficult purification procedures of intermediate species; they are efficiently isolated by alumina chromatography. Minor amounts of μ-aminocarbyne aryl isocyanide compounds, 2a-c, are obtained as side products. The structures of the cations in 1a,c,e are calculated using DFT; the carbyne carbon is generally predicted to be the thermodynamic site of hydride addition, in agreement with a previous experimental finding concerning 1a. Accordingly, the reaction of 1e with NaBH 4 affords a bridging aminocarbene complex, 4, in 85 % yield. Otherwise, the reaction of 1c with NaBH 4 yields the aminocarbyne-cyclopentadiene derivative 3 (70 %), presumably as a consequence of the [a] Scheme 1. Regioselective additions of nucleophiles to the diiron aminocarbyne complex 1a. Results and DiscussionThe commercial compound [Fe 2 Cp 2 (CO) 4 ] was reacted with the appropriate isocyanide, in a ca. 3:2 molar ratio, in acetonitrile solution. [16] The reactions with alkyl isocyanides were conducted under reflux conditions, whereas the reactions with aryl isocyanides proceeded at room temperature. The resulting mixtures were dried under vacuum and the residues were dissolved in dichloromethane and then treated with methyl triflate, thus affording the μ-aminocarbyne complexes 1a-e (Scheme 2). The difficult isolation of the monoisocyanide intermediates (see the Introduction) was unnecessary. The final products 1a-e were efficiently purified by alumina chromatography and were then isolated as microcrystalline, air-stable compounds in 65-92 % yields. The synthesis of 1c-e was accompanied by the side formation of minor products derived from di-isocyanide species, 2a-c. Compounds 2a-c were recovered by the chromatography in 3-12 % yields, although 2a was formerly reported as being Scheme 2. Synthesis of diiron μ-aminocarbyne complexes.Eur. J. Inorg. Chem. 2018, 960-971 www.eurjic.org

Anticancer Potential of Diiron Vinyliminium Complexes

“…The vinyliminium complexes [ 2 – 19 ]CF 3 SO 3 were prepared from Fe 2 Cp 2 (CO) 4 following a four‐step procedure (Scheme ). The aminocarbyne precursors [ 1 a – c ]CF 3 SO 3 were obtained by carbon monoxide/isocyanide substitution and subsequent alkylation in near quantitative yields (Scheme , steps 1–2) . One carbonyl ligand is subsequently replaced by a more labile acetonitrile molecule, which allows the insertion of the alkyne into the iron−carbyne bond, forming the vinyliminium ligand (Scheme , steps 3–4).…”

Constructing Organometallic Architectures from Aminoalkylidyne Diiron Complexes

“…Diiron cationic complexes with one aminoalkylidyne ligand, 1 , have been known since a long time, and are available from [Fe 2 Cp 2 (CO) 4 ] by a straightforward two step procedure (Scheme ), consisting in carbon monoxide/isocyanide substitution followed by alkylation of the coordinated isocyanide . Analogous bis‐aminoalkylidyne species were also reported in the past .…”

“…These reactions are generally selective across three possible sites, i.e. the Cp ring, the CO ligand and the aminocarbyne carbon,, depending on the nature of the nucleophile (Scheme ). A bulky R substituent on the nitrogen atom may have a significant impact on the reactivity of the complex: for instance, the reaction of 1 (R = Xyl = 2,6‐dimethylphenyl) with cyanide salts results in direct CO replacement (Scheme e) rather than attack to the carbyne (Scheme b) …”

“…The C–N bond length in the X‐ray structures of 1 ranges from 1.280(8) to 1.297(4) Å, accounting for a partial iminium character due to competition of the nitrogen lone pair for the empty π orbital on the carbyne carbon. Solution IR spectroscopy in the ν(CO) region is a practical tool to easily monitor the course of the transformations of 1 and also all the succeeding compounds.…”

“…On the other hand, NMR spectroscopy is a diagnostic technique for the recognition of the aminoalkylidyne group, since the carbyne nucleus typically resonates at down fields (e.g. at δ = 315.5 ppm in [D 6 ]DMSO for R = Me, and at δ = 327.8 ppm in CDCl 3 for R = Xyl = 2,6‐dimethylphenyl).…”