Conformational fluctuation in palladium(II)–methyl aldopentopyranoside complexes

Four methyl d-pentopyranosides (β-Ara, α-Lyx, β-Rib, β-Xyl), as well as Me-β-l-Ara, some of them residing in a well-defined conformation in the solution state (Ara, Xyl) and some showing pronounced chair inversion in solution (Lyx, Rib), form bidentate chelates of the general formula [Pd(chxn)(LH−2)-κO,O′] and [Pd(tmen)(LH−2)-κO,O′], chxn = (R,R)-cyclohexane-1,2-diamine, tmen = N,N,N′,N′-ethane-1,2-diamine and L = glycoside, with PdIIN2-type metal probes. The dynamic behaviour of the free glycosides is maintained in their chelates, the only case where the metal is bonded by a cis-vicinal diol function. Thus, one fluctuating chelate was detected with the lyxopyranoside in the κO2,3 binding mode, and two fluctuating chelates were found for the ribopyranoside (κO2,3 and κO3,4). No fluctuating chelate was found for the arabinopyranoside (the free arabinopyranoside being non-fluctuating as well), or for the xylopyranoside (no cis-vicinal diol function). In addition, syn-diaxial chelation (κO2,4) was observed for the ribopyranoside and the xylopyranoside. The spectroscopic results were supplemented by X-ray analyses.

Conformational fluctuation of a glycoside (methyl β-d-ribopyranoside in the scheme) is maintained on chelation with diamine-palladium(II) probes if, and only if, a cis-vicinal chelate such as the depicted κO2,3-bonded complex is formed. Both in trans-vicinal and syn-diaxial chelates, fluctuation is frozen and the sterically only possible conformer is enriched. The NMR investigation is based on the invention of ‘Pd-tmen’, a new coordinating cellulose solvent.Figure optionsDownload as PowerPoint slide