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

Abstract

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-pyridyl-methyl)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 · 2C4H6O3 · 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 P̄1, 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) Å3, Z = 2. Ligand excharge 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 or 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 quantitatively 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.