Conformational fluctuation in palladium(II)-methyl aldopentopyranoside complexes

摘要

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)N_2-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.