η2 and η4 olefin complexation by S, S-[Ru II(Me 2edda)]: a structural change to accommodate bidentate dienes

Abstract

[Ru II(Me 2edda)(H 2O) 2] ( 1), Me 2edda 2− = N, N′-dimethylethylenediaminediacetate, exhibits a sterically-controlled molecular recognition in forming η 2 and η 4 olefin complexes. 1 exists with an N 2O 2 in-plane set of chelate donors and axial H 2O ligands. The two CH 3 functionalities of Me 2edda 2− are poised above and below the N 2O 2 plane of the glycinato rings. Studies herein of the 2,2′-bipyridine complex, [Ru II(Me 2edda)(bpy)], with bidentate bpy chelation as established via 1H NMR and electrochemical methods show 1 to be ligated in the S,S configuration with the glycinato rings in-plane as a cis-O form. 1 is sterically discriminating in forming η 2 complexes with smaller olefins (ethylene, 2-propene, cis-2-butene, methyl vinyl ketone and 3-cyclohexene-1-methanol), but rejects larger decorated ring structures and branched olefins (1,2-dimethyluracil, cyclohexene-1-one 2-methyl-2-propene). η 2 complexes of 1 have characteristic Ru II/III DPP waves near 0.55 V which vary slightly with olefin structure. Potentially bidendate dienes (1,3-butadiene, 1,3-cyclohexadiene and 2,5-norbornadiene (nbd) form η 4 complexes as shown by Ru II/III waves between 0.94 and 1.30 V, indicate of a highly stabilized Ru II center by π-backboning. An η 2⇌ η 4 ‘equilibrium’ with apparent K = 22 at 25 °C is observed for nbd coordinated to 1. (The η 2 and η 4 distribution may be a kinetic one and not a thermodynamic one). To allow formation of the cis η 4 complexes, 1 must undergo a shift of one or both glycinato donors from the N 2O 2 plane into the axial site away from the dimethyl functionalities. η 4 chelation by 1,3-butadiene has been confirmed by 1H NMR spectral assignments of two [Ru II(Me 2edda)] isomers, one in the axial rans-O glycinato configuration, e.g. 1,3-butadiene is bidentate in the original N 2O 2 plane and a second unsymmetrical glycinato arrangement with in-plane and axial glycinato as well as in-plane and axial η 4-1,3-butadiene coordination. [Ru II(hedta)(H 2O)] − ( 2), hedta 3− = N-hydrpxyethylenediaminetriacetate, is less discriminating for olefin structures, forming η 2 complexes with all eleven olefins and dienes mentioned for studies with 1. However, 2 does not undergo displacement of a carboxylate donor by the second olefin unit of a diene [Ru II(hedta)(diene)] − complexes possess a pendant non-coordinated olefin and on η 2-bound olefin in the complex, indicated by a normal Ru II(pac)(olefin)Ru II/III wave near 0.55 V.