Application of NOE and PGSE NMR methodologies to investigate non-covalent intimate inorganic adducts in solution

NOE and PGSE NMR experiments provide crucial information for the structural characterization of non-covalent intimate adducts in solution. The possible presence and the favorite relative orientation of the interacting units can be deduced from NOE results, while the size of the non-covalent adducts can be estimated through PGSE measurements. The complementarity of the two methodologies has been successfully used to investigate transition metal complex ion pairs and, to a lesser extent, intermolecular adducts. The main results concerning the solution structures of non-covalent inorganic adducts are reported and compared with those in the solid state and those from theoretical calculations

NMR techniques for investigating the supramolecular structure of coordination compounds in solution

The text explores important spectroscopic approaches. It first describes intermolecular nuclear Overhauser effect (NOE) NMR experiments and diffusion experiments, offering examples that demonstrate theoretical aspects of the methodolog

Diffusion and NOE NMR studies on the interactions of neutral amino-acidate arene ruthenium(II) supramolecular aggregates with ions and ion pairs

The interaction between [RuCl(AA)(cymene)] supramolecular aggregates (1, AA = alpha-amino-acidate = alpha-aminoisobutyrate; 2, AA = N,N-dimethyl-Gly; 3, AA = Ala; 4, AA = Pro; cymene = 4-isopropyltoluene) and ionic species derived from NBu4PF6 and KPF6 is investigated through diffusion NMR measurements and F-19,H-1-hetero-nuclear Overhauser effect spectroscopy experiments in CDCl3 and CD2Cl2. Aggregates containing the -NH2 functionality (1 and 3) interact strongly with NBu4PF6 as demonstrated by the observation of intense nuclear Overhauser effects between the fluorine atoms of PF6- and the protons of [RuCl(AA)(cymene)]. Unexpectedly, diffusion NMR measurements indicate that the average size of the aggregates increases when a small amount of NBu4PF6 is added (C-salt/C-Ru < 0.1) in CD2Cl2. At higher concentration levels of NBu4PF6 or in CDCl3, NBu4PF6 exerts a destructive effect that reduces the average size of the aggregates. [RuCl(AA)(cymene)](n) aggregates with NR-H (4) and NR2 (2) functionalities are little affected by the addition of NBu4PF6. KPF6 also interacts with [RuCl(AA)(cymene)](n) aggregates as demonstrated by the fact that it becomes noticeably soluble in CDCl3 and CD2Cl2. Diffusion H-1-NMR experiments show that the addition of KPF6 does not markedly alter the average size of [RuCI(AA)(cymene)] supramolecular aggregates. Interestingly, the average size of PF6--containing supramolecular aggregates is, in some cases, slightly higher than that of the ones that do not contain PF6-. This was deduced by independent measurements of the hydrodynamic volume of the anion and of the ruthenium complexes by diffusion F-19-and H-1-NMR experiments, respectively. Copyright (C) 2008 John Wiley & Sons, Ltd

Determining accurate molecular sizes in solution through NMR diffusion spectroscopy

This tutorial review deals with the methodological procedures that can be used to obtain accurate molecular sizes in solution from diffusion NMR spectroscopy. The critical aspects associated with the estimation of the size of molecules from the measured translational self-diffusion coefficient, using the Stokes - Einstein equation, are highlighted and discussed. From a theoretical point of view, it is shown how to take into account the size of the solute with respect to that of the solvent and its non-spherical shape using the appropriate correction factors in the frictional coefficient. From a practical point of view, the advantages of introducing an internal standard in the sample are presented. Initially, non-aggregating systems are considered in an attempt to clarify what hydrodynamic dimensions mean. Successively, aggregating systems are addressed showing how it is possible to understand the aggregation level and derive the thermodynamic parameters for some illustrative aggregation processes

Aggregation tendency and reactivity toward AgX of cationic half-sandwich ruthenium(II) complexes bearing neutral N,O-ligands

The aggregation tendency of complexes [Ru(eta(6)-cymene)(N,O)Cl]X[N,O = 2-benzoylpyridine (2-bzpy), 1, and 2-acetylpyridine (2-acpy), 2, X- = BPh4- or PF6-] has been studied by means of PGSE NMR experiments. It was found that complexes with PF6- as counterion are mainly present in CD2Cl2 as ion pairs at low concentration, as a mixture of ion triples and free anions at medium concentration and as ion quadruples at elevated concentration. F-19, H-1-HOESY NMR experiments revealed that in ion triples and ion quadruples two cationic Ru-units pair up. Consistently, in the solid-state structure of 1PF(6), determined through X-ray single-crystal investigation, two cationic Ru-units are held together by an intermolecular pi-pi stacking interaction between the pyridyl rings. Complexes having BPh4- as counterion are only present in solution as even aggregates, namely ion pairs at low concentration and ion quadruples at elevated concentration. In such a case a counteranion bridges two cationic Ru-units as observed in the solid-state structure of 1BPh(4). The reactivity of complexes 1 - 2 toward AgX salts has been investigated in different solvents. Bicationic [Ru(eta(6)-cymene)(N,O)(MeCN)]X-2 (N, O = 2-bzpy, 3, and 2-acpy, 4) and [Ru(MeCN)(4)(N, O)]X-2 (N, O = 2-bzpy, 5, and 2-acpy, 6) complexes were obtained by the reaction of 1 and 2 with AgX in the presence of three equivalents of acetonitrile or in acetonitrile, respectively. The reaction of 1 with AgPF6 in acetone afforded complex [Ru(eta(6)-cymene)(N, O, O)]PF6 (7, where N, O, O = 4-alcoxide-4-phenyl-4-(pyridin-2-yl) butan-2-one) from the C - C coupling of a deprotonated methyl group of the coordinated acetone and the C=O moiety of 2-bzpyn ligand