Influence of Cyclopentadienyl Ring‐Tilt on Electron‐Transfer Reactions: Redox‐Induced Reactivity of Strained [2] and [3]Ruthenocenophanes

Abstract

In contrast to ruthenocene [Ru(η(5) -C5 H5 )2 ] and dimethylruthenocene [Ru(η(5) -C5 H4 Me)2 ] (7), chemical oxidation of highly strained, ring-tilted [2]ruthenocenophane [Ru(η(5) -C5 H4 )2 (CH2 )2 ] (5) and slightly strained [3]ruthenocenophane [Ru(η(5) -C5 H4 )2 (CH2 )3 ] (6) with cationic oxidants containing the non-coordinating [B(C6 F5 )4 ](-) anion was found to afford stable and isolable metalmetal bonded dicationic dimer salts [Ru(η(5) -C5 H4 )2 (CH2 )2 ]2 [B(C6 F5 )4 ]2 (8) and [Ru(η(5) -C5 H4 )2 (CH2 )3 ]2 [B(C6 F5 )4 ]2 (17), respectively. Cyclic voltammetry and DFT studies indicated that the oxidation potential, propensity for dimerization, and strength of the resulting RuRu bond is strongly dependent on the degree of tilt present in 5 and 6 and thereby degree of exposure of the Ru center. Cleavage of the RuRu bond in 8 was achieved through reaction with the radical source [(CH3 )2 NC(S)SSC(S)N(CH3 )2 ] (thiram), affording unusual dimer [(CH3 )2 NCS2 Ru(η(5) -C5 H4 )(η(3) -C5 H4 )C2 H4 ]2 [B(C6 F5 )4 ]2 (9) through a haptotropic η(5) -η(3) ring-slippage followed by an apparent [2+2] cyclodimerization of the cyclopentadienyl ligand. Analogs of possible intermediates in the reaction pathway [C6 H5 ERu(η(5) -C5 H4 )2 C2 H4 ][B(C6 F5 )4 ] [E=S (15) or Se (16)] were synthesized through reaction of 8 with C6 H5 EEC6 H5 (E=S or Se).

Recent developments with strained metallocenophanes

“…Thus, thermolysis of rac-14 in the presence of an excess of MgCp 2 afforded acyclic, ferrocene-derivative 15 (Scheme 4), via cleavage of both the CeC bond derived from the dicarba bridge, and the FeeCp bond [20]. Interestingly, both ring-opened analogue 16 and the linear PFE 17 also undergo homolytic CeC cleavage of the dicarba linker under thermolysis conditions to afford species 15 and 18 respectively, the latter through complete depolymerisation of polymer 17 (Scheme 3).…”

“…Thermolysis of rac-14, 16 and 17 at 300 C[20].Scheme 5. Oxidation of dicarba[2]ruthenocenophane 19 to afford dimer dication 20.…”

Synthesis and Characterization of a Family of Air-Stable Ferrocene- and Ruthenocene-Containing Primary, Secondary, and Tertiary Phosphines

“…18 Depending on the conditions, for example when an ethane bridge is introduced between the cyclopentadienyl substituents in dicarba [2]ruthenocenophanes, oxidation leads to formation of isolable dimers linked through a Ru-Ru bond. 14,19 Scheme 2. Summary of the known chemical oxidation behavior of ruthenocene in coordinative/non-coordinative media.…”

A Phosphine-Based Heterotrimetallic (M = Fe, Ru, W) Homopolymer

“…21,[23][24] The electrochemical properties of ruthenocene are far more complex than those of ferrocene, due to the extremely Lewis acidic character of ruthenocenium that often leads to the formation of acid/base adducts and/or disproportionation and dimerization reactions. 23,[28][29] This behavior is responsible for the irreversible character of the ruthenocene oxidation wave, and also effects the shape and symmetry of the ferrocene wave mentioned above. Upon coordination to tungsten, the electrochemical properties of 4•W(CO) 5 changed dramatically, revealing independent electrochemical features associated with each of the transition metals present.…”

“…The current response of the wave centered at −150 mV was less than that expected for a one-electron process, ruling out the possibility that it results from phosphine oxidation. Furthermore, the adsorption behavior observed has been previously explored in detail for closely related phosphines. ,, The electrochemical properties of ruthenocene are far more complex than those of ferrocene, due to the extremely Lewis acidic character of ruthenocenium that often leads to the formation of acid/base adducts and/or disproportionation and dimerization reactions. ,, This behavior is responsible for the irreversible character of the ruthenocene oxidation wave, and also affects the shape and symmetry of the ferrocene wave mentioned above. Upon coordination to tungsten, the electrochemical properties of 4·W­(CO) 5 changed dramatically, revealing independent electrochemical features associated with each of the transition metals present.…”