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(II)N₂-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.