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 Pd^IIN₂-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 κO^2,3 binding mode, and two fluctuating chelates were found for the ribopyranoside (κO^2,3 and κO^3,4). 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 (κO^2,4) was observed for the ribopyranoside and the xylopyranoside. The spectroscopic results were supplemented by X-ray analyses.