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Linkage Isomerism Dependent on Solvent and Temperature. Synthesis and Structural Properties of Diamineplatinum(II) Complexes of Allyl- and Diallylmalonate Ligands

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journal contribution
posted on 09.01.1999, 00:00 by Young-A Lee, Young Keun Chung, Youn Soo Sohn
The linkage isomerism between (O,O‘)- and (O,alkene)-chelates has been investigated for the complexes A2PtL2 (A2 = 2,2-dimethyl-1,3-propanediamine (DMPDA), trans-(±)-1,2-diaminocyclohexane (DACH); L2 = allylmalonate (AM), diallylmalonate (DAM)). The crystal structures of (DMPDA)Pt(AM)·2H2O (tetragonal P42/m, a = 13.614(3) Å, b = 13.614(3) Å, c = 8.451(4) Å, V = 1566.3(9) Å3, Z = 4, R = 0.0472) and (DMPDA)Pt(DAM)·2H2O (monoclinic P21/n, a = 11.021(3) Å, b = 8.996(2) Å, c = 18.765(7) Å, β = 106.92(3) °, V = 1780.0(9) Å3, Z = 4, R = 0.0531) have been solved. Each platinum atom adopts a typical square planar arrangement with two nitrogen atoms in cis positions. However, surprisingly, the AM anionic ligand is coordinated to the platinum atom via (O,O‘)-chelation mode through its two carboxylate groups with the alkene group uncoordinated in the solid state, breaking the hard/soft rule. The tetradentate DAM ligand is chelated to the platinum atom through one carboxylate and one alkene group resulting in (O,alkene)-chelation mode with another uncoordinated carboxylate and alkene group. Multinuclear (1H, 13C, and 195Pt) NMR studies clearly disclose that the linkage isomerism depends on the solvents employed. Both allyl- and diallylmalonate ligands are chelated exclusively to the platinum(II) atom via (O,O‘)-mode in dimethylformamide or Me2SO solution whereas only (O,alkene)-chelation mode is observed in an aqueous solution. At room temperature, the complexes both of the AM and DAM ligands exist in methanol as a mixture of (O,O‘)- and (O,alkene)-modes. Furthermore, interconversion between the two isomers occurs reversibly depending on temperature:  the (O,alkene)-chelate is predominant at low temperatures while the (O,O‘)-chelate is favorable at elevated temperatures.