Diffusion NMR Studies on the Self-Aggregation of Ru-Arene CAP Complexes: Evidence for the Formation of H-Bonded Dicationic Species in Acetonitrile

The self-aggregation of neutral [RuCl2(eta6-p-cymene)(CAP)] (2; p-cymene = 1-methyl-4-(propan-2-yl)benzene) and ionic [RuCl(eta6-p-cymene)(L)(CAP)](PF6) (3, L = CAP; 4, L = MeCN) and [RuCl2(eta6-p-cymene)(CAP-H)](BPh4) (5; CAP-H = monoprotonated CAP) ruthenium(II) arene complexes, bearing the water-soluble phosphine CAP (1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane), in aprotic polar solvents, was investigated by means of PGSE (pulsed field gradient spin-echo) diffusion NMR. In addition, the analogous neutral [RuCl2(eta6-p-cymene)(PTA)] (1) and monoprotonated [RuCl2(eta6-p-cymene)(PTA-H)](BPh4) (6) PTA (1,3,5-triaza-7-phosphaadamantane) complexes were investigated in acetonitrile-d3. Complexes 1-4 do not exhibit any tendency to self-aggregate in acetonitrile-d3 and acetone-d6. In contrast, 5 and 6 monoprotonated complexes undergo a peculiar self-aggregation process, involving almost exclusively the cation, leading to the main formation of dicationic species held together by intermolecular hydrogen bonds. This has been clearly demonstrated by 1H diffusion NMR experiments, which allowed measurement of an average hydrodynamic volume for the cation that was more than double that expected, whereas that of the counterion is only slightly higher than that of the free anion. The trend of the aggregation number (N+) as a function of the concentration of 5 and 6 was fitted using the equation of the equal K indefinite model of association, leading to DeltaG°298 K = -3.1 and -3.0 kcal mol-1, respectively, for the formation of H-bonded dications. The critical role played by the protonated -NH unit in establishing hydrogen bonding was further supported by the detection of 6+2 dinuclear species in acetonitrile-d3 solutions containing equimolar mixtures of 2 and 6. The self-aggregation free energy of 2 (DeltaG°298 K = -3.1 kcal mol-1) and 6+ (DeltaG°298 K = -2.9 kcal mol-1) in such mixed solutions is practically identical with those of 5 and 6 in acetonitrile-d3.