Syntheses and structures of bridge and chelate isomers of tetraplatinum(II) cluster complex with diethyldithiophosphate ion (Et₂dtp) at in-plane sites, [Pt₄(CH₃COO)₄(Et₂dtp)₄], and their mutual isomerization via cluster core rotation

A reaction of [Pt₄(CH₃COO)₃] (1) with an excess of diethyldithiophosphate (Et₂dtpH) in CH₃CN affords a tetrasubstituted derivative, [Pt₄(CH₃COO)₄(Et₂dtp)₄], which is a mixture of two isomers. X-ray analyses of the isolated two isomers revealed that Et₂dtp ligands in both isomers are coordinated in the pane of the Pt₄ cluster core, and that coordination mode of Et₂dtp is the chelate type in one isomer (2) whereas it is the bridge type in the other (3). ¹H NMR studies showed that both 2 and 3 isomerize slowly in solution to give the same equilibrium mixture. The rate constants for the isomerization in CDCl₃ at 40 °C were k₁ = 1.2 x 10^-4 s^-1 (for the reaction from 2 to 3) and k₂ = 1.5 x 10^-4 s^-1 (for the reaction from 3 to 2), respectively. Both 2 and 3 undergo the isomerization also in acetonitrile with similar reaction rate, but the mechanism is different from that in chloroform. Activation entropy for the reaction starting from 2 to 3 was -9 ± 9 J nol^-1 K^-1 in chloroform and -47 ± 11 J mol^-1 K^-1 in acetonitrile. The small |ΔS(+)| value for the reaction n chloroform suggests that the isomerization proceeds without bond cleavage through an intermediate where three sulfur atoms from two Et₂dtp's are coordinated to each platinum. That is, the isomerization in chloroform is caused by rotation of the Pt₄ cluster core within an array of eight S atoms arranged circularly in a single plane. On the other hand, the isomerization in acetonitrile proceeds via a Pt-S bond cleaved intermediate possibly by the solvent assisted mechanism. All the kinetic parameters for the reactions in CDCl₃ and CD3Cl₃ are reported.