Structure of Coordination Complexes: The Synergy between NMR Spectroscopy and Computational Chemistry

Abstract

Illustrative examples of how NMR spectroscopy and computational chemistry data can be used in synergy to gain information on structure, coordination mode, bonding, symmetry and isomeric distribution of transition metal complexes, is presented. Isomer distribution and the most stable structures in a series of Ti(β-diketonato)₂Cl₂ and Ti(β-diketonato)₂(biphen) complexes as determined by density functional theory (DFT) methods and the application of the Boltzmann equation, are in agreement with crystal structures and variable temperature NMR results. Secondly, the DFT determined coordination mode of the 4-amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole, (bpt-NH₂) which has the appropriate chemical geometry to behave as anionic or neutral bidentate chelating group to form a 5- or 6-membered complex, is shown to be in agreement with ¹H NMR shifts for [Rh(bpy)₂(bpt-NH)]²⁺, [Rh(phen)₂(bpt-NH)]²⁺, [Rh(bpt-NH)(cod)] and [Ir(bpt-NH)(cod)] (cod = 1,5-cyclooctadiene, phen = 1,10-phenanthroline, bpy = 2,2’-bipyridine). The oxidative addition of CH₃I to [Rh(β-diketonato)(CO)(PPh₃)] complexes consist of three reaction steps and involves isomers of two different RhIII-alkyl and two different RhIII-acyl species. For this reaction experimental ¹H NMR techniques complement the stereochemistry of reaction intermediates and products as calculated by density functional theory. NMR properties, in agreement with computational results, proved to be useful to access the nature of the κ³ to κ² distortion in coinage metal-ethylene complexes supported by tris(pyrazolyl)borates. The last example showed that NMR, X-ray crystal and computational results showed C₂ symmetry for a series of metal(II) complexes coordinated to a 16-membered pentaaza macrocycle.

Keywords: DFT, computational, organometallic, NMR spectroscopy