Comparative Rates of Ligand Substitution Reactions of Pt−C-Bonded Complexes in Aqueous Solution and the X-ray Crystal Structure of [Pt{C₆H₃(CH₂NMe₂)₂-2,6}(OH₂)][OSO₂CF₃]

Abstract

The substitution behavior of the organometallic complex cation [Pt{C6H3(CH2NMe2)2-2,6}(OH2)]+ (1) has been studied with a series of nucleophiles (Cl-, Br-, I-, N3-, NCS-, thiourea, N,N‘-dimethylthiourea, N,N,N‘N‘-tetramethylthiourea) as a function of nucleophile concentration ([Nu]), pH, temperature, and pressure. Complex 1 affords pseudo-first-order rate constants, kobs, for the complex-formation reaction given by kobs = k1[Nu] + k-1; the k-1 term is due to the reverse aquation reaction and is insignificant for the stronger, S-donor nucleophiles. The mechanism for the substitution of the coordinated water molecule is associative, as demonstrated by the large negative values of ΔS⧧ and ΔV⧧. The results are compared to those obtained earlier for the geometrically related neutral zwitterionic complex [Pt{C6H4(CH2NMe2)-2}(NC5H4SO3-3)(OH2)] (2). Both cyclometalated complexes 1 and 2 have very high substitution reactivities that can be ascribed to labilization which arises from the σ-bonded carbon atom of the ortho-metalated aryl ligand in combination with back-bonding to the in-plane aryl ligand that increases the electrophilic character of the Pt(II) center. In complex 1 the σ-donicity of the second chelating dimethylamine group and/or the increased chelation effect of the tridentate ligand lead to a significantly increased lability of the coordinated water molecule, compared to complex 2. The X-ray structure of the ionic complex [Pt{C6H3(CH2NMe2)2-2,6}(OH2)]+[OSO2CF3]- (1a) is described. The crystal structure comprises an aggregate of two complex cations and two triflate anions. The structure contains two fully equivalent square-planar coordinated Pt(II) centers with an O-ligated neutral H2O molecule (Pt−O = 2.186 (2) Å) trans to the Pt−Caryl bond (Pt−C = 1.903(2) Å); the aggregate results from the involvement of both H atoms of the two H2O molecules in asymmetric hydrogen bonding to O atoms of the two triflate anions.