Isomerism of amides coordinated to platinum. X-ray crystal structure of dichloro-bisacetamide-platinum(II)

Addition of water to coordinated nitriles in the platinum complexes trans-[PtCl2(N=C-R)(2)] (R=Me, Bu(t), Ph) takes place readily under basic conditions. The Z isomer (corresponding to the trans addition of water to the C=N triple bond) is the only isomer present in chlorinated solvents (CHCl3 or CH2Cl2), however in polar solvents (acetone, DMSO or methanol) a partial isomerization to the E isomer is observed for R=Me. Such an isomerization is not observed in the case of R=Bu(t) or Ph where an increase of the steric bulk of the R group further destabilizes the E isomer (Pt and R groups cia to one another with respect to the C=N double bond) over the Z isomer (Pt and R groups trans to one another with respect to the C=N double bond). Crystals of trans-[PtCl2[HN=C(OH)Me](2)] belong to the triclinic system, space group <P(1)over bar> (No.2), a=4.7892(3), b=5.6329(9), c=8.9695(8) Angstrom, alpha=74.005(10), beta=84.379(6), gamma=83.004(9)degrees, V=230.36(4) Angstrom(3), Z=1. The Pt atom, located at the inversion center, is linked to two chloride ions, trans to each other, and to two nitrogen atoms from the amide ligands. The amides have the Z configuration and the H(O) atom has a short contact to platinum (Pt ... H, 2.56(2) Angstrom; sum of the van der Waals radii, 2.9-3.2 Angstrom). Such a Pt ... H interaction is closely related to hydrogen bonding and stabilizes the Z isomer. Hence the E isomer gains stability only in oxygenated solvents which are capable of competing with the metal for H bonding interaction with the acetamide hydroxyl group (acetone, DMSO or methanol).