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]

The substitution behavior of the organometallic complex cation [Pt{C 6 H 3 (CH 2 NMe 2 ) 2 -2,6}(OH 2 )] + ( 1 ) has been studied with a series of nucleophiles (Cl -, Br -, I -, N 3 -, 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, k obs, for the complex-formation reaction given by k obs = k 1 [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 Δ S ⧧ and Δ V ⧧ . The results are compared to those obtained earlier for the geometrically related neutral zwitterionic complex [Pt{C 6 H 4 (CH 2 NMe 2 )-2}(NC 5 H 4 SO 3 -3)(OH 2 )] ( 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{C 6 H 3 (CH 2 NMe 2 ) 2 -2,6}(OH 2 )] + [OSO 2 CF 3 ] - ( 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 H 2 O molecule (Pt−O = 2.186 (2) Å) trans to the Pt−C aryl bond (Pt−C = 1.903(2) Å); the aggregate results from the involvement of both H atoms of the two H 2 O molecules in asymmetric hydrogen bonding to O atoms of the two triflate anions.