Abstract
In this contribution, we revisit the neglected and forgotten cationic, air-stable, 18-valence electron, heteroleptic sandwich complex (cycloheptatrienyl)(cyclopentadienyl)manganese, which was reported independently by Fischer and by Pauson about 50 years ago. Using advanced high-power LED photochemical synthesis, an expedient rapid access to the parent complex and to functionalized derivatives with alkyl, carboxymethyl, bromo, and amino substituents was developed. A thorough study of these “tromancenium” salts by a range of spectroscopic techniques (1H/13C/55Mn-NMR, IR, UV–vis, HRMS, XRD, XPS, EPR), cyclic voltammetry (CV), and quantum chemical calculations (DFT) shows that these manganese sandwich complexes are unique metallocenes with quite different chemical and physical properties in comparison to those of isoelectronic cobaltocenium salts or (cycloheptatrienyl)(cyclopentadienyl) sandwich complexes of the early transition metals. Electrochemically, all tromancenium ions undergo a chemically partially reversible oxidation and a chemically irreversible reduction at half-wave or peak potentials that respond to the substituents at the Cp deck. As exemplarily shown for the parent tromancenium ion, the product generated during the irreversible reduction process reverts at least partially to the starting material upon reoxidation. Quantum-chemical calculations of the parent tromancenium salt indicate that metal–ligand bonding is distinctly weaker for the cycloheptatrienyl ligand in comparison to that of the cyclopentadienyl ligand. Both the HOMO and the LUMO are metal and cycloheptatrienyl-ligand centered, indicating that chemical reactions will occur either metal-based or at the seven-membered ring, but not on the cyclopentadienyl ligand.
Highlights
Among the huge number of metal sandwich complexes known in organometallic chemistry, the class of heteroleptic(cyclopentadienyl) metal complexes [(Cht)M(Cp)]±n constitutes an interesting family of compounds, mostly restricted to early transition metals, as shown by the work of Green,1 Elschenbroich,2 Tamm,3 and Braunschweig
A general and useful derivatization strategy of cationic sandwich metal complexes is nucleophilic exoaddition of various nucleophiles to provide neutral, functionalized endohydride derivatives that may subsequently be oxidized with tritylium to valuable substituted sandwich metal cations.8a,16 Attempted reactions of tromancenium complex 9 with a range of carbon, nitrogen, or oxygen nucleophiles resulted, only in complete decomposition of the manganese sandwich complex, indicating that tromancenium salts are surprisingly labile in comparison to other cationic metallocenes, due to rather weak bonding of the cycloheptatrienyl ligand
A custom-made high-power LED photoreactor was proven to be highly advantageous in comparison to standard photochemical methodology for the convenient and rapid synthesis of six tromancenium salts
Summary
Among the huge number of metal sandwich complexes known in organometallic chemistry, the class of heteroleptic (cycloheptatrienyl)(cyclopentadienyl) metal complexes [(Cht)M(Cp)]±n constitutes an interesting family of compounds, mostly restricted to early transition metals, as shown by the work of Green, Elschenbroich, Tamm, and Braunschweig. Table 1 summarizes the known representatives commonly called “tro-met-cenes” where “tro” stands for tropylium (C7H7)+, “met” for the metal center, and “cene” for a cyclopentadienide-containing sandwich complex. A general and useful derivatization strategy of cationic sandwich metal complexes (for example cobaltocenium salts) is nucleophilic exoaddition of various nucleophiles to provide neutral, functionalized endohydride derivatives that may subsequently be oxidized with tritylium to valuable substituted sandwich metal cations.8a,16 Attempted reactions of tromancenium complex 9 with a range of carbon, nitrogen, or oxygen nucleophiles resulted, only in complete decomposition of the manganese sandwich complex, indicating that tromancenium salts are surprisingly labile in comparison to other cationic metallocenes, due to rather weak bonding of the cycloheptatrienyl ligand (vide infra) To summarize this synthetic section, high-power LED photochemistry of metal carbonyl complexes is convenient, fast, and superior to standard methodology. Analysis of the Hirshfeld partial charges exemplary calculated with the PBE0 density functional revealed a positively charged metal center (Table S3)
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