Inorganic Chemistry Chart 2. E (left) and Z (right) Isomers of 5c Article Chart 3. Coordination of L3 [7b−L3 (left) and 4b′−L3 (right) Tautomers] L5) were reacted with the ruthenium(II) dimer in a 2:1 molar ratio in ethanol under reflux to give [RuCl(η6-arene)(L)]Cl (1c and 3c−5c) in quantitative yield. In the case of L2, the reaction carried out under similar conditions resulted in the formation of [RuCl(η6-arene)(L2−H)] (2c−HCl), which was further con- verted into 2c by acidification with HCl. ESI-MS spectra of 1c−5c in MeOH show peaks correspond- ing to ions [M − Cl]+ and [M − HCl − H]−, confirming their structures. Additional peaks resulting from the loss of the chlorido ligand along with concomitant deprotonation of the organic ligands, namely, [M − HCl − Cl]+ and [M − 2HCl − H]−, are observed with peaks attributed to [M − HCl + Na]+ ions. NMR spectra of 1c−4c and 2c−HCl show one set of signals, whereas complex 5c was found to undergo E/Z isomerization at the exocyclic amidine bond (C6′N13′) in solution (Chart 2). Analogous behavior was documented recently for [MCl(η6- p-cymene)(L5)]Cl (where M = Ru, Os).21 The full assignment of proton, nitrogen, and carbon resonances for [RuCl(μ- Cl)(η6-arene)]2 and 1c−5c is given in Tables S1−S6 in the Supporting Information. The E/Z isomerization of 5c is solvent-dependent; the relative intensities of the two signal sets for 5c in DMSO-d6 change from 1:0.6 immediately after dissolution to 1:2.4 at equilibrium after 48 h. According to the 1H,1H ROESY NMR plot, the predominant signal set at equilibrium belongs to the Z isomer, which shows H14′,H5′ cross-peaks (Figure S7 in the Supporting Information). In MeOH-d4, the E/Z equilibrium for 5c is reached faster than that in DMSO-d6 and the relative abundance of E and Z isomers changes from 1:0.5 to 1:0.36 in 3 h (1:0.33 after 24 h). The dominant E isomer was identified due to the 1H,1H ROESY NMR couplings of H5′ with arene protons (H1d−H5d), as well as H7′ with H14′. The Z isomer shows cross-peaks of H7′ with arene protons (H1d−H5d). It should be noted that only one NH (H5′) signal, originating from the E isomer, is present in the 1H NMR spectrum after dissolution in MeOH-d4. This dissapears gradually (the intensity decreases by a factor of 10.8 after 3 h, 65 after 9 h to zero after 14 h). Dissociation of the complexes may be excluded because the chemical shifts of both signal sets differed from that of metal- free L5 (Table S5 in the Supporting Information). Moreover, these two sets were not affected by excess chloride ions in MeOH-d4, providing evidence against solvolysis of the Ru−Cl bond. Thus, at equilibrium the Z isomer dominates in DMSO- d6, whereas the E isomer dominates in MeOH-d4, in line with reported data for [MCl(η6-p-cymene)(L5)]Cl.21 The coordination of the ligands (L1−L5) to the ruthenium- (II) arene moiety results in significant changes to the resonances of both the ligands and η6-arene. For instance, significant upfield shifts were observed for the resonances of the η6-phenyl fragment protons H1d, H5d, H2d, and H4d of 4- formylphenoxyacetyl-η6-benzylamide in 4c and 5c (E isomer) compared to those in [RuCl(μ-Cl)(η6-arene)]2, whereas they are shifted downfield in 1c−3c, 2c−HCl, and 5c (Z isomer); the resonance of the H3d proton in all complexes is shifted downfield. The number of signals in the 1H NMR spectra of 1c−3c and 2c−HCl is in agreement with their C1 symmetry, and five- membered chelate cycle formation via the nitrogens N2a and N3b is evident. The 1H,1H ROESY NMR coupling of H1b (14.03 ppm) with the CH2 group (H10b at 4.87 ppm) in 3c indicates stabilization of the 7b−L3 tautomer (Chart 3). The same coordination mode was reported for [MCl(η6-p-cymene)- (L3)]Cl (where M = Ru, Os).25 Upon coordination of L1−L3 to the ruthenium(II) arene moiety, significant shifts were observed for the resonances of the benzimidazole ring protons: H1b [by 1.71 (L1), 0.7 (L2 in 2c−HCl), 1.23 (L2 in 2c), 0.78 (L3) ppm] and H4b [by 0.29 ppm in 3c; the H4b and H7b proton resonances in L1 and L2 (at 7.54 and 7.75−7.77 ppm) were not assigned; the proton H4b gives a peak at 8.1, 8.06, and 8.01 ppm for 1c, 2c, and 2c−HCl, 12674 dx.doi.org/10.1021/ic201801e | Inorg. Chem. 2011, 50, 12669−12679