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

Abstract

The linkage isomerism between (O,O')- and (O,alkene)-chelates has been investigated for the complexes A(2)PtL(2) (A(2) = 2,2-dimethyl-1,3-propanediamine (DMPDA), trans-(+/-)-1,2-diaminocyclohexane (DACH); L(2) = allylmalonate (AM), diallylmalonate (DAM)). The crystal structures of (DMPDA)Pt(AM).2H(2)O (tetragonal P4(2)/m, a = 13.614(3) Å, b = 13.614(3) Å, c = 8.451(4) Å, V = 1566.3(9) Å(3), Z = 4, R = 0.0472) and (DMPDA)Pt(DAM).2H(2)O (monoclinic P2(1)/n, a = 11.021(3) Å, b = 8.996(2) Å, c = 18.765(7) Å, beta = 106.92(3) degrees, V = 1780.0(9) Å(3), 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 ((1)H, (13)C, and (195)Pt) 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 Me(2)SO 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.