Comproportionation reaction and hindered rotation of coordinated pyridine rings in an acetate-bridged tetraplatinum(II) cluster with pyridine-based ligands in the cluster plane

A series of pyridine-substituted derivatives of octaacetatotetraplatinum(II), [Pt₄(CH₃COO)_8-n(L)_2n]ⁿ⁺ (L = 4-dimethylaminopyridine (dmap), pyridine (py), 4-cyanopyridine (cpy); n = 1 - 4) were prepared, and the tetra- and octasubstituted forms (n = 2 and 4) were isolated. ¹H NMR spectra showed that this type of cluster undergoes a comproportionation reaction. Reactions between clusters in which n = 0 and 2, n = 0 and 4, and n = 2 and 4 afforded Pt₄ clusters with n = 1, 2, and 3, respectively, as a main product in acetonitrile. The dmap-substituted clusters, trans-[Pt₄(CH₃COO)₆(dmap)₄](ClO ₄)₂ · 3CH₃NO₂ (3a(ClO₄)₂·3CH₃NO₂) and [Pt₄(CH₃COO)₄(dmap)₈](ClO ₄)₄·4H₂O (5a(ClO₄)₄·4H₂O), have been structurally characterized. Both 3a and 5a have a square-planar cluster core comprised of four Pt^II ions, and all eight out-of-plane coordination sites are occupied by acetate ligands in a bridging mode. In 5a, all of the in-plane sites are occupied by dmap ligands. In 3a, four dmap ligands occupy the coordination sites at the two mutually opposite edges of the square planar cluster skeleton, giving a trans tetrasubstituted form of [Pt₄(CH₃COO)₈] (1). In octasubstituted 5a, adjacent dmap ligands are so closely arranged that the Pt-N distances (2.20(3), 2.30(3) Å) are longer than those in tetrasubstituted 3a (2.13(1), 2.15(1) Å) and related Pt₄ clusters. Furthermore, rotation of the dmap ligand about the Pt-N bond in 5a was restricted, and the rate constant of the rotation was 4.5s^-1 at 20°C from dynamic NMR study. Cluster [Pt₄(CH₃COO)₅-(dmap)₆]³⁺ (4a) also exhibited similar hindered rotation with the rate constants of 2.0s^-1, 12s^-1 and ≈ 10⁴s^-1 at 20°C depending on the coordination sites of the dmap ligands in 4a.