Titration Calorimetry System for Measurement of Formation Enthalpies of Organometal Complexes

点击翻译按钮获取中文摘要

With the aim of calorimetric measurement for the formation of organo-transition metal complexes, we prepared a new vessel for titration calorimetry. The volume of initial test solution is reduced to 4 ml with a new vessel (2 cmφ x 4 cmh). Heat sensor is a couple of thermistors, which can detect temperature changes by 10^<-5> degree. In the present work, we studied complexation of soft-metal soft-ligand Ag^+-EPh_3 (E = P, As, Sb) systems in acetonitrile, which is a stronger π acceptor but weaker σ donor than dimethyl sulfoxide (DMSO). The formation constant of [Ag(EPh_3)]^+ reduced in the order P > As > Sb, and the mono-, bis-, tris- and tetrakis-PPh_3 complexes are formed in AN. The result is compared with that in pyridine (Py), which is a stronger π acceptor and σ donor than DMSO. The soft Ag^+ ion thus shows a specific dπ-π interaction with AN and Py but not with DMSO. The [Ag(PPh_3)_4]^+ is found in AN but not in DMSO and Py. Enthalpies of transfer of individual [Ag(PPh_3)_n]^+ (n = 0-3) species from DMSO to AN show that the value is close to zero for n=0 and become more positive for the complex with increasing n, indicating that the specific dπ-π Ag^+-AN interaction is weakened by bound PPh_3 ligand. Similarly, enthalpies of transfer of individual [Ag(PPh_3)_n]^+ (n = 0-3) species from DMSO to Py show that the value is negative and large for n=0 and become less negative for the complex with increasing n, and the value is slightly negative for [Ag(PPh_3)_3]^+. This indicates that the specific dπ-π Ag^+-Py interaction is almost disappeared by bound PPh_3 ligand. These facts leads to the conclusion that weakened dπ-π interaction and weak σ donating ability of AN is essential for the formation of tetrahedral [Ag(PPh_3)_4]^+ complex.