Synthesis and property of pentakis(μ-acetato-O,O′) (linear-hexamine)tetraplatinum(II), [Pt₄(μ-acetato-O,O′)₅(L)]³⁺; correlation between rate of in-plane acetate ligand exchange and catalytic activity for hydration of acetonitrile

Three new hexadentate polyamine ligands (L), their acetate bridged tetranuclear platinum complexes, [Pt4(μ-CH₃COO)₅(L)]³⁺ (L³ = 1,5,8,11,14,18-hexaazaoctadecane (3), L⁵ = 1,11-bis(2-pyridylmethyl)-1,4,8,11-tetraazaundecane (5), L⁶ = N,N′-bis(((2-pyridylmethyl)amino)ethyl)piperazine (6), and the same type of Pt₄ complex with L⁴ = 1,10-bis(2-pyridylmethyl)-1,4,7,10-tetraazadecane) (4) were prepared. Single crystal X-ray analysis of 6(ClO₄)₃·2C₄H₆O ₃-CHCl₃ shows that the hexamine ligand with the sequence of the bridge-chelate-bridge-chelate-bridge mode and one acetate occupy coordination sites in the plane of the cluster. Crystal data: triclinic space group P 1̄, a = 14.001(4), b = 19.029(3), c = 12.562(3) Å, α = 100.23(2), β = 113.9(2), γ = 77.60(2)°, V = 2972(1) ų, Z = 2. Ligand exchange rates of the in-plane acetate ligand in these complexes were determined by means of ¹H NMR in CD₃CN at 25°C: k = 1.2 × 10^-2 for 3, 1.4 × 10^-4 for 4, 8.7 × 10^-6 for 5, 1.2 × 10^-6 mol^-1 kg s^-1 for 6. All the Pt₄ complexes show, to some extent, catalytic activity for hydration of acetonitrile to give acetamide: turnover number per hour under experimental conditions was 0.51 for 3, 0.46 for 4, 0.09 for 5, and 0.09 for 6, respectively. It was found, for the series of compounds including [Pt₄(μ-CH₃COO)₈] and [Pt₄(μ-CH₃COO)₆(3,2,3-tet)], that a qualitatively positive correlation holds between the ligand exchange rate and the catalytic activity, that is, the faster the ligand exchange, the higher the catalytic activity in general. This fact indicates that the catalytic activity is associated with the lability of coordination sites trans to the Pt-Pt bond.