Sugar-alcohol complexes of palladium(II): on the variable rigidity of open-chain carbohydrate ligands

The C4, C5, and C6 sugar alcohols erythritol (Eryt), D-threitol (D-Thre), D-arabitol (D-Arab), ribitol (Ribt), xylitol (Xylt), dulcitol (Dulc), and D-mannitol (D-Mann) form chelate complexes upon dissolution in Pd-en, an aqueous solution of [Pd(II)(en)(OH)2]. Stability rules are derived from the proportion of a respective species in the solution equilibrium. Crystal-structure analysis supports the NMR spectroscopic results for a series of binuclear compounds that contain the sugar alcohols as tetraanionic polyolato ligands: [Pd2(en)2(ErytH(-4))] x 10 H2O, [Pd2(en)2(D-Arab1,2;3,4H(-4))] x 7 H2O, [Pd2(en)2(Xylt1,2;3,4H(-4))] x 4 H2O, [Pd2(en)2(D-Mann1,2;3,4H(-4))] x 5 H2O, and [Pd2(en)2(Dulc2,3;4,5H(-4))] x 6 H2O. In the case of the pentitols and hexitols, the metalated tetraanions are stabilized by intramolecular hydrogen bonds. The hydrogen bonds uniformly connect an alkoxide acceptor to the hydroxy donor group located at the delta carbon atom. As a consequence of hydrogen bonding, the open-chain carbohydrate ligands become rigid. Crystal-structure analysis provides information on the configurational requirements for rigidity. According to these rules, the hydrogen-bond-supported Dulc2,3;4,5H(-4) tetraanion provides a geometrically persistent ligating pattern. Intramolecular hydrogen bonding seems to be the most-competitive variable to metalation of a polyol. [Pd2(tm-2,1:3,2-tet)(OH)3]OH (tm-2,1:3,2-tet = 1,3-bis(2'-dimethylaminoethyl)hexahydropyrimidine) is a metallizing agent that can force full metalation even in a case as intractable as that of dulcitol. Accordingly, [Pd4(tm-2,1:3,2-tet)2-(DulcH(-6))]Cl2 x 16 H2O contains the fully deprotonated hexitol as the ligand.