Novel Platinum(IV)−Carbohydrate Complexes:  Metal Ion Coordination Behavior of Monosaccharides in Organic Solvents

New platinum(IV)-coordinated carbohydrate complexes [PtMe₃L]BF₄ (14E−21E, 22D−24D) have been prepared from the reaction of [PtMe₃(Me₂CO)₃]BF₄ (1) with a wide range of isopropylidene-protected carbohydrates L‘ in acetone. These complexation reactions can be accompanied by a platinum-promoted cleavage of an isopropylidene group (14E, L‘ = 1,2;5,6-di-O-isopropylidene-α-d-glucofuranose (1A), L = 1,2-O-isopropylidene-α-d-glucofuranose; 15E, L‘ = 3-O-acetyl-1,2;5,6-di-O-isopropylidene-α-d-glucofuranose (2A), L = 3-O-acetyl-1,2-O-isopropylidene-α-d-glucofuranose; 16E, L‘ = 3-O-(methylsulfonyl)-1,2;5,6-di-O-isopropylidene-α-d-glucofuranose (3A), L = 3-O-(methylsulfonyl)-1,2-O-isopropylidene-α-d-glucofuranose; 17E, L‘ = 1,2;5,6-di-O-isopropylidene-α-d-allofuranose (4A), L = 1,2-O-isopropylidene-α-d-allofuranose; 18E, L‘ = 2,3;4,6-di-O-isopropylidene-2-keto-l-gulonic acid (5A), L = 2,3-O-isopropylidene-2-keto-l-gulonic acid; 19E, L‘ = 2,3;4,6-di-O-isopropylidene-α-l-sorbofuranose (6A), L = 2,3-O-isopropylidene-α-l-sorbofuranose). In some cases, complexation proceeds without the loss of an isopropylidene group (14E, L = L‘ = 1,2-O-isopropylidene-α-d-glucofuranose (10B); 15E, L = L‘ = 3-O-acetyl-1,2-O-isopropylidene-α-d-glucofuranose (11B); 20E, L = L‘ = 1,2-O-isopropylidene-α-d-xylofuranose (12B), 21E, L = L‘ = 5,6-O-isopropylidene-d-gulono-γ-lactone (13B)). Furthermore, complexes 22D−24D with diprotected carbohydrate ligands undergo a platinum-promoted cleavage of an isopropylidene group in wet methylene chloride to give 19E, cleavage of the carbohydrate ligand L (7A) to give [PtMe₃(H₂O)₃]BF₄, and substitution of the isopropylidene ligand by an aqua ligand to give [PtMe₃L(H₂O)]BF₄ (24D‘, L = 2,3;4,5-di-O-isopropylidene-β-d-fructopyranose), respectively. Complex 14E reacted in acetone within 1 week in a platinum-promoted addition of an isopropylidene protecting group, yielding [PtMe₃L‘]BF₄ (25D, L‘ = 1,2;5,6-di-O-isopropylidene-α-d-glucofuranose). The results show that carbohydrates can act as tridentate neutral ligands with a wide variety of donor sites:  3 × OH (14E, 17E−19E), 2 × OH + O_COMe (15E), 2 × OH + O_SO2Me (16E), 2 × OH + O_ring (20E), 2 × OH + O_acetal (21E), 1 × OH + O_ring + O_acetal (22D - 25D). Bidentate ligation (OH + O_ring) is observed in 24D‘. Complexes 14E singly ¹³C-labeled at each of the saccharide carbons were prepared, and an analysis of the J couplings involving the labeled carbons permitted an assignment of the solution conformation of the carbohydrate ligand. X-ray structure analysis of 19E (monoclinic, P2₁, a = 8.607(3) Å, b = 9.955(4) Å, c = 11.415(4) Å, Z = 2) and 24D‘ (orthorhombic, P2₁2₁2₁, a = 9.851(2) Å, b = 11.141(4) Å, c = 20.418(6) Å, Z = 4) exhibits the same coordination in the solid state as in solution, namely, via the three hydroxyl groups, yielding a strained-cyclic system (19E) and a five-membered 1,3,2-dioxaplatina ring (24D‘).