Indium(III)/2-benzoylpyridine chemistry: interesting indium(III) bromide-assisted transformations of the ligand

Reactions of 2-benzoylpyridine, (py)(ph)CO, with InX₃ (X = Cl, Br) in EtOH at room temperature have been studied. The InCl₃/(py)(ph)CO system has provided access to complex [InCl₃{(py)(ph)CO}(EtOH)]·{(py)(ph)CO} (1) and the byproduct {(pyH)(ph)CO}Cl (2). The reaction of InBr₃ with (py)(ph)CO has led to a mixture of (L)[InBr₄{(py)(ph)CO}] (3) and [In₂Br₄{(py)(ph)CH(O)}₂(EtOH)₂] (4), where L⁺ is the 9-oxo-indolo[1,2-a]pyridinium cation and (py)(ph)CH(O)^- is the anion of (pyridin-2-yl)methanol. Based on solubility and crystallisation time differences between the two components of the mixture, complex 4 was isolated in pure form, i.e. free from 3. The formations of the counterion L⁺ and the coordinated (py)(ph)CH(O)^- anion represent clearly InBr₃-promoted/assisted transformations. Reaction mechanisms have been proposed for the formation of 2, 3 and 4. Complex 4 could also be isolated by the reaction of InBr₃ and pre-formed (py)(ph)CH(OH) in EtOH. The solid-state structures of 1, 3 and 4 were determined by single-crystal X-ray crystallography, while the identity of the salt 2 was confirmed by microanalyses and a variety of spectroscopic techniques, including ESI-MS spectra. In the indium(III) complexes, the metal ions are 6-coordinate with a distorted octahedral geometry. The halogeno groups (Cl^-, Br^-) in the three complexes are terminal. The (py)(ph)CO molecule behaves as a N,O-bidentate (1.11) ligand in 1 and 3. A terminal EtOH ligand completes the coordination sphere of In^III in 1. The alkoxo oxygen atoms of the two 2.21 (py)(ph)CH(O)^- ligands doubly bridge the In^III centers in 4 creating a {InIII2(μ-OR)₂}⁴⁺ core; a nitrogen atom of one reduced organic ligand, two bromo ions and one terminal EtOH molecule complete the 6-coordination at each metal centre. Complexes 1, 3 and 4 were characterised by IR and Raman spectroscopies, and the data were discussed in terms of their known solid-state structures. Molar conductivity data and ¹H NMR spectra were used in an attempt to probe the behaviour of the complexes in DMSO. The to-date observed metal ion-assisted/promoted transformations of (py)(ph)CO are also discussed.