Coordination Chemistry of Ru(II) Complexes of an Asymmetric Bipyridine Analogue: Synergistic Effects of Supporting Ligand and Coordination Geometry on Reactivities

The reactivities of transition metal coordination compounds are often controlled by the environment around the coordination sphere. For ruthenium(II) complexes, differences in polypyridyl supporting ligands affect some types of reactivity despite identical coordination geometries. To evaluate the synergistic effects of (i) the supporting ligands, and (ii) the coordination geometry, a series of dicarbonyl–ruthenium(II) complexes that contain both asymmetric and symmetric bidentate polypyridyl ligands were synthesized. Molecular structures of the complexes were determined by X-ray crystallography to distinguish their steric configuration. Structural, computational, and electrochemical analysis revealed some differences between the isomers. Photo- and thermal reactions indicated that the reactivities of the complexes were significantly affected by both their structures and the ligands involved.


Introduction
Polypyridines with multiple covalently bonded pyridine groups exhibit unique photophysical and redox properties [1,2]. Among the polypyridines, bipyridine analogues play an important role in the formation of various transition metal complexes as bidentate ligands with two nitrogen donor atoms [3]. Bipyridine analogues function not only as supporting ligands for stabilizing metal complexes, but also as electron pool sites. For example, in addition to catalysts for the production of useful resources, such as in multi-electron reductions of carbon dioxide and water-gas shift reactions [4][5][6][7][8][9][10][11], bipyridine analogues are also utilized as photosensitizers [12] and phosphorescence materials [13]. A variety of studies that imparted selectivity to various reactions by strictly controlling the coordination sphere have been reported for many ruthenium complexes. Various molecular structures can be readily designed for a ruthenium center because ruthenium can take various oxidation states and can easily interact with the ligand. These structures include examples where differences in the coordination geometry dramatically changed both the electrical and photochemical properties of the complex, and therefore its catalysis for chemical reactions [14][15][16]. Many examples of the relationship between the supporting bipyridine ligands and the reactivity of metal complexes are known. For example, the relationship between the number of heteroatoms involved in the supporting ligand and the reactivity of the complex has been reported in a ruthenium complex containing bipyridine analogues [17].
Recently, we reported the synthesis of ruthenium complexes with 2,2'-bipyridine (bpy) and an analogue, the asymmetric bidentate ligand 2-(2-pyridyl)-1,8-naphthyridine (pynp, Chart 1).  Figure S1), it is reasonable to propose that an intramolecular interaction between the non-coordinating nitrogen atom in the pynp ligand and the adjacent coordinated carbonyl promotes the formation of p-isomers [18]. In addition, only the dp-isomer was selectively isolated in the case of [Ru(pynp)2(CO)2] 2+ ( Figure 2), even though the bis-pynp complex could form three diastereomers (pp, dp, and dd; Chart 3). According to DFT calculations for the three diastereomers, the total energy of the experimentally formed dp-isomer was 3.79 kcal/mol higher than that of the most stable pp-isomer, which appears to contradict the experimental result. We then performed total energy calculations for the two possible intermediates in the coordination of the second pynp ligand to the ruthenium center by assuming that the aqua ligand in the trans position of the coordinated pynp is substituted earlier than the aqua ligand in the cis position (see the structure diagram in Supplementary Materials Figure S1). The results indicate that both intermediates have almost the same energy (ΔE = 0.08 kcal/mol; Figure S2). Therefore, the second pynp ligand may coordinate through a kinetically favorable pathway rather than a thermodynamic one to give the selective dp-isomer.   Figure S1), it is reasonable to propose that an intramolecular interaction between the non-coordinating nitrogen atom in the pynp ligand and the adjacent coordinated carbonyl promotes the formation of p-isomers [18]. In addition, only the dp-isomer was selectively isolated in the case of [Ru(pynp) 2 (CO) 2 ] 2+ ( Figure 2), even though the bis-pynp complex could form three diastereomers (pp, dp, and dd; Chart 3). According to DFT calculations for the three diastereomers, the total energy of the experimentally formed dp-isomer was 3.79 kcal/mol higher than that of the most stable pp-isomer, which appears to contradict the experimental result. We then performed total energy calculations for the two possible intermediates in the coordination of the second pynp ligand to the ruthenium center by assuming that the aqua ligand in the trans position of the coordinated pynp is substituted earlier than the aqua ligand in the cis position (see the structure diagram in Supplementary Materials Figure S1). The results indicate that both intermediates have almost the same energy (∆E = 0.08 kcal/mol; Figure S2). Therefore, the second pynp ligand may coordinate through a kinetically favorable pathway rather than a thermodynamic one to give the selective dp-isomer. In contrast, initial introduction of the asymmetric pynp ligand leads to the selective formation of p-isomers (Figure 1b Figure S1), it is reasonable to propose that an intramolecular interaction between the non-coordinating nitrogen atom in the pynp ligand and the adjacent coordinated carbonyl promotes the formation of p-isomers [18]. In addition, only the dp-isomer was selectively isolated in the case of [Ru(pynp)2(CO)2] 2+ ( Figure 2), even though the bis-pynp complex could form three diastereomers (pp, dp, and dd; Chart 3). According to DFT calculations for the three diastereomers, the total energy of the experimentally formed dp-isomer was 3.79 kcal/mol higher than that of the most stable pp-isomer, which appears to contradict the experimental result. We then performed total energy calculations for the two possible intermediates in the coordination of the second pynp ligand to the ruthenium center by assuming that the aqua ligand in the trans position of the coordinated pynp is substituted earlier than the aqua ligand in the cis position (see the structure diagram in Supplementary Materials Figure S1). The results indicate that both intermediates have almost the same energy (ΔE = 0.08 kcal/mol; Figure S2). Therefore, the second pynp ligand may coordinate through a kinetically favorable pathway rather than a thermodynamic one to give the selective dp-isomer.  [4,21,22]. In addition, the interatomic distances between the carbonyl carbon and the non-coordinating nitrogen of pynp (2.607(5) to 2.754(14) Å) in the three p-forms are much shorter than the sum of van der Waals radii (3.25 Å) [23]. The shortening suggests that pynp and the adjacent CO ligand interact considerably in these complexes.  (14) 1 Distance between the non-coordinating nitrogen in pynp and the CO carbon atoms. 2 [18]. 3 [20].

Characterization of Complexes
Spectroscopic and electrochemical analyses were performed on the synthesized complexes ( Table 2). Two strong IR bands assignable to νCO were observed around 2100 and 2040 cm −1 in all complexes. These values were similar to those in other dicarbonyl-ruthenium(II) complexes [4,21,22], and no clear differences were observed between the isomers. Given that the complexes contain two carbonyl ligands of the highest order in the spectrochemical series, no obvious absorption was observed in the visible region (Supplementary Materials Figure S3). However, intense polypyridyl-centered π-π* intraligand transitions were observed in the UV region [24].   [4,21,22]. In addition, the interatomic distances between the carbonyl carbon and the non-coordinating nitrogen of pynp (2.607(5) to 2.754(14) Å) in the three p-forms are much shorter than the sum of van der Waals radii (3.25 Å) [23]. The shortening suggests that pynp and the adjacent CO ligand interact considerably in these complexes.  (14) 1 Distance between the non-coordinating nitrogen in pynp and the CO carbon atoms. 2 [18]. 3 [20].

Characterization of Complexes
Spectroscopic and electrochemical analyses were performed on the synthesized complexes ( Table 2). Two strong IR bands assignable to νCO were observed around 2100 and 2040 cm −1 in all complexes. These values were similar to those in other dicarbonyl-ruthenium(II) complexes [4,21,22], and no clear differences were observed between the isomers. Given that the complexes contain two carbonyl ligands of the highest order in the spectrochemical series, no obvious absorption was observed in the visible region (Supplementary Materials Figure S3). However, intense polypyridyl-centered π-π* intraligand transitions were observed in the UV region [24]. Although spectroscopic measurements did not show any marked differences between isomers, electrochemistry clearly showed different behavior. The cyclic voltammograms (CV) of all the complexes showed multiple ligand-based reduction waves in the range of −1 to −2 V vs. Fc + /Fc. Since pynp is more easily reduced than bpy or phen, the first reduction peak at ca. −1 V was attributed to the reduction of the pynp ligand [25]. When the cathodic scan was immediately reversed after the first peak potential, the coupled oxidation wave was reversible in the d-isomers (Figure 3a, dotted line); however, those of the corresponding p-isomers were either irreversible or quasi-reversible at the first reduction wave (Figure 3b). Although the first reduction wave of [2p] 2+ in Figure 3b appears to be reversible, the coupled anodic current decreased as the scan rates slowed (Supplementary Materials Figure S4). The basicity of the free nitrogen atom of the 1,8-naphthyridine moiety in pynp increased due to one-electron reduction of pynp, thus making intramolecular nucleophilic attack to the adjacent carbonyl carbons possible, leading to the formation of a metallacyclic compound (Scheme 1) [26]. Due to this structural change, only the p-isomer exhibited irreversible one-electron reduction behavior.   3 Data in parentheses represent oxidation waves caused by the second reduction.
Although spectroscopic measurements did not show any marked differences between isomers, electrochemistry clearly showed different behavior. The cyclic voltammograms (CV) of all the complexes showed multiple ligand-based reduction waves in the range of −1 to −2 V vs. Fc + /Fc. Since pynp is more easily reduced than bpy or phen, the first reduction peak at ca. −1 V was attributed to the reduction of the pynp ligand [25]. When the cathodic scan was immediately reversed after the first peak potential, the coupled oxidation wave was reversible in the d-isomers (Figure 3a, dotted line); however, those of the corresponding p-isomers were either irreversible or quasi-reversible at the first reduction wave (Figure 3b). Although the first reduction wave of [2p] 2+ in Figure 3b appears to be reversible, the coupled anodic current decreased as the scan rates slowed (Supplementary Materials Figure S4). The basicity of the free nitrogen atom of the 1,8-naphthyridine moiety in pynp increased due to one-electron reduction of pynp, thus making intramolecular nucleophilic attack to the adjacent carbonyl carbons possible, leading to the formation of a metallacyclic compound (Scheme 1) [26]. Due to this structural change, only the p-isomer exhibited irreversible one-electron reduction behavior.

Photochemical Reactions
Polypyridylruthenium complexes are generally photoreactive. When cis-[Ru(bpy)2(CO)2] 2+ in acetonitrile is irradiated with light, two carbonyl ligands simultaneously dissociate and the corresponding solvent complex (cis-[Ru(bpy)2(CH3CN)2] 2+ ) is produced [27,28]. However, when pynp is substituted for one of the two bpy ligands, the two carbonyl groups dissociate stepwise due to their electronic non-equivalence [18]. Since no comparisons between the d-and p-isomers were made in the previous report, we compared the photoreactivities of both isomers. As previously reported, there are two reaction steps for both isomers (Scheme 2) [18,29]. In the first step, one carbonyl ligand was dissociated, and, at the same time, the acetonitrile used as the solvent became coordinated (the first step in Scheme 2 and Figure 4a). Steric retention of the complex was supported by structural analysis of the isolated species at this step ( Figure 5a). In the subsequent step, photoisomerization from the d-to p-isomer subsequent slow dissociation of the second CO ligand and solvent coordination occurred (the second step in Scheme 2a and Figure 4b). Reaction analysis and DFT calculations suggest that the formation of photoexcited states of the complex promote such an isomerization to a more stable p-form in similar complexes containing pynp [30,31]. In the bis-pynp complex ([3dp] 2+ ), a similar two-step photoreaction (Scheme 2a) was confirmed from structural analyses of both products (dp-form in Figure 5b and pp-form in Supplementary Materials Figure S5). On the other hand, the final structure of the p-isomers was unchanged from spectroscopic analyses (Scheme 2b).

Photochemical Reactions
Polypyridylruthenium complexes are generally photoreactive. When cis-[Ru(bpy) 2 (CO) 2 ] 2+ in acetonitrile is irradiated with light, two carbonyl ligands simultaneously dissociate and the corresponding solvent complex (cis-[Ru(bpy) 2 (CH 3 CN) 2 ] 2+ ) is produced [27,28]. However, when pynp is substituted for one of the two bpy ligands, the two carbonyl groups dissociate stepwise due to their electronic non-equivalence [18]. Since no comparisons between the dand p-isomers were made in the previous report, we compared the photoreactivities of both isomers. As previously reported, there are two reaction steps for both isomers (Scheme 2) [18,29]. In the first step, one carbonyl ligand was dissociated, and, at the same time, the acetonitrile used as the solvent became coordinated (the first step in Scheme 2 and Figure 4a). Steric retention of the complex was supported by structural analysis of the isolated species at this step ( Figure 5a). In the subsequent step, photoisomerization from the dto p-isomer subsequent slow dissociation of the second CO ligand and solvent coordination occurred (the second step in Scheme 2a and Figure 4b). Reaction analysis and DFT calculations suggest that the formation of photoexcited states of the complex promote such an isomerization to a more stable p-form in similar complexes containing pynp [30,31]. In the bis-pynp complex ([3dp] 2+ ), a similar two-step photoreaction (Scheme 2a) was confirmed from structural analyses of both products (dp-form in Figure 5b and pp-form in Supplementary Materials Figure S5). On the other hand, the final structure of the p-isomers was unchanged from spectroscopic analyses (Scheme 2b).  [27,28]. However, when pynp is substituted for one of the two bpy ligands, the two carbonyl groups dissociate stepwise due to their electronic non-equivalence [18]. Since no comparisons between the d-and p-isomers were made in the previous report, we compared the photoreactivities of both isomers. As previously reported, there are two reaction steps for both isomers (Scheme 2) [18,29]. In the first step, one carbonyl ligand was dissociated, and, at the same time, the acetonitrile used as the solvent became coordinated (the first step in Scheme 2 and Figure 4a). Steric retention of the complex was supported by structural analysis of the isolated species at this step ( Figure 5a). In the subsequent step, photoisomerization from the d-to p-isomer subsequent slow dissociation of the second CO ligand and solvent coordination occurred (the second step in Scheme 2a and Figure 4b). Reaction analysis and DFT calculations suggest that the formation of photoexcited states of the complex promote such an isomerization to a more stable p-form in similar complexes containing pynp [30,31]. In the bis-pynp complex ([3dp] 2+ ), a similar two-step photoreaction (Scheme 2a) was confirmed from structural analyses of both products (dp-form in Figure 5b and pp-form in Supplementary Materials Figure S5). On the other hand, the final structure of the p-isomers was unchanged from spectroscopic analyses (Scheme 2b).    From comparisons of the first CO-dissociation rates (Table 3), it was found that the photoreactions of the d-isomers were always faster than those of the corresponding p-isomers (ca. two times in both complexes). This difference was interpreted to be due to their geometries, based on photoreaction behavior seen in similar diastereomers [32]. The pynp ligand has a naphthyridine unit in place of the pyridine unit in bpy (or phen), which is both more delocalized and a π-acceptor. The superior charge acceptor properties of the naphthyridine unit in the d-isomers leads to better labilization of the trans-CO, and thus the d-isomers exhibit faster CO release compared with the corresponding p-isomers, despite having lower extinction coefficients between 300 and 400 nm ( Table 2 and Supplementary Materials Figure S3). We also found that the overall photoreaction of the bpy system proceeded more smoothly than that of the corresponding phen system. This was probably because phen compounds were significantly more stable than their bpy analogues, based on the rigidity of phen [33]. Despite prolonged photoirradiation, incomplete dissociation of the second CO ligand in the phen system consequently prevented isolation of the single disubstituted complex ([Ru(pynp)(phen)(CH3CN)2] 2+ ).   From comparisons of the first CO-dissociation rates (Table 3), it was found that the photoreactions of the d-isomers were always faster than those of the corresponding p-isomers (ca. two times in both complexes). This difference was interpreted to be due to their geometries, based on photoreaction behavior seen in similar diastereomers [32]. The pynp ligand has a naphthyridine unit in place of the pyridine unit in bpy (or phen), which is both more delocalized and a π-acceptor. The superior charge acceptor properties of the naphthyridine unit in the d-isomers leads to better labilization of the trans-CO, and thus the d-isomers exhibit faster CO release compared with the corresponding p-isomers, despite having lower extinction coefficients between 300 and 400 nm ( Table 2 and Supplementary Materials Figure S3). We also found that the overall photoreaction of the bpy system proceeded more smoothly than that of the corresponding phen system. This was probably because phen compounds were significantly more stable than their bpy analogues, based on the rigidity of phen [33]. Despite prolonged photoirradiation, incomplete dissociation of the second CO ligand in the phen system consequently prevented isolation of the single disubstituted complex ([Ru(pynp)(phen)(CH3CN)2] 2+ ). From comparisons of the first CO-dissociation rates (Table 3), it was found that the photoreactions of the d-isomers were always faster than those of the corresponding p-isomers (ca. two times in both complexes). This difference was interpreted to be due to their geometries, based on photoreaction behavior seen in similar diastereomers [32]. The pynp ligand has a naphthyridine unit in place of the pyridine unit in bpy (or phen), which is both more delocalized and a π-acceptor. The superior charge acceptor properties of the naphthyridine unit in the d-isomers leads to better labilization of the trans-CO, and thus the d-isomers exhibit faster CO release compared with the corresponding p-isomers, despite having lower extinction coefficients between 300 and 400 nm ( Table 2 and Supplementary Materials Figure S3). We also found that the overall photoreaction of the bpy system proceeded more smoothly than that of the corresponding phen system. This was probably because phen compounds were significantly more stable than their bpy analogues, based on the rigidity of phen [33]. Despite prolonged photoirradiation, incomplete dissociation of the second CO ligand in the phen system consequently prevented isolation of the single disubstituted complex ([Ru(pynp)(phen)(CH 3 CN) 2 ] 2+ ).

Thermochemical Reactions
As shown in Scheme 3, in thermal reactions one carbonyl ligand of a dicarbonylruthenium(II) complex undergoes nucleophilic attack from solvent molecules [34,35]. Thus, when the d-isomers of the dicarbonyl complexes, which are expected to be more reactive (see Section 2.2.1.), were heated in water/acetonitrile or alcohol (methanol or ethanol)/acetonitrile mixtures, one of the coordinated CO moieties underwent nucleophilic attack from the solvent to the CO ligand at the trans position of pynp. As expected, molecular structures of the isolated complexes showed the formation of the hydroxycarbonyl (-C(O)OH in Figure 6a) and the methoxy-or ethoxycarbonyl (-C(O)OC 2 H 5 in Figure 6b and Supplementary Materials Figure S6) complexes. Given that the pynp ligand is more π-acidic than bpy or phen, the CO carbon in the trans position of pynp has a more positive charge.

Thermochemical Reactions
As shown in Scheme 3, in thermal reactions one carbonyl ligand of a dicarbonylruthenium(II) complex undergoes nucleophilic attack from solvent molecules [34,35]. Thus, when the d-isomers of the dicarbonyl complexes, which are expected to be more reactive (see Section 2.2.1.), were heated in water/acetonitrile or alcohol (methanol or ethanol)/acetonitrile mixtures, one of the coordinated CO moieties underwent nucleophilic attack from the solvent to the CO ligand at the trans position of pynp. As expected, molecular structures of the isolated complexes showed the formation of the hydroxycarbonyl (-C(O)OH in Figure 6a) Figure S6) complexes. Given that the pynp ligand is more π-acidic than bpy or phen, the CO carbon in the trans position of pynp has a more positive charge.

Thermochemical Reactions
As shown in Scheme 3, in thermal reactions one carbonyl ligand of a dicarbonylruthenium(II) complex undergoes nucleophilic attack from solvent molecules [34,35]. Thus, when the d-isomers of the dicarbonyl complexes, which are expected to be more reactive (see Section 2.2.1.), were heated in water/acetonitrile or alcohol (methanol or ethanol)/acetonitrile mixtures, one of the coordinated CO moieties underwent nucleophilic attack from the solvent to the CO ligand at the trans position of pynp. As expected, molecular structures of the isolated complexes showed the formation of the hydroxycarbonyl (-C(O)OH in Figure 6a) and the methoxy-or ethoxycarbonyl (-C(O)OC2H5 in Figures 6b and Supplementary Materials Figure S6) complexes. Given that the pynp ligand is more π-acidic than bpy or phen, the CO carbon in the trans position of pynp has a more positive charge.  We next investigated other thermochemical reactions using monocarbonyl complexes that do not undergo nucleophilic attack on the coordinated carbonyl. When the monocarbonyl complex (d-[Ru(pynp)(phen)(CO)(CH3CN)] 2+ ), which was produced by the photoreaction described in Section 2.2.1, was heated in acetone, it isomerized from the d-to the p-isomer (Figure 7). Notably, this thermal isomerization reaction was accelerated more than 70 times when water was added to the solution. A similar isomerization was observed in d-[Ru(pynp)(bpy)(CO) (CH3CN)] 2+ . In contrast, such thermal isomerization could not be confirmed for other monocarbonyl complexes with anionic ligands (-COR − ; R − = OH, OCH3, or OC2H5). These results strongly suggest that the thermal isomerization reaction involves a donor-acceptor interaction between the solvent and the complex. In ruthenium(II) complexes, terminal CO ligands that interact with the donor solvent tend to exhibit νCO IR frequencies over 2000 cm −1 [36,37]. In this study, the CO stretching frequency of the We next investigated other thermochemical reactions using monocarbonyl complexes that do not undergo nucleophilic attack on the coordinated carbonyl. When the monocarbonyl complex (d-[Ru(pynp)(phen)(CO)(CH 3 CN)] 2+ ), which was produced by the photoreaction described in Section 2.2.1, was heated in acetone, it isomerized from the dto the p-isomer (Figure 7). Notably, this thermal isomerization reaction was accelerated more than 70 times when water was added to the solution. A similar isomerization was observed in d-[Ru(pynp)(bpy)(CO)(CH 3 CN)] 2+ . In contrast, such thermal isomerization could not be confirmed for other monocarbonyl complexes with anionic ligands (-COR − ; R − = OH, OCH 3 , or OC 2 H 5 ). These results strongly suggest that the thermal isomerization reaction involves a donor-acceptor interaction between the solvent and the complex. In ruthenium(II) complexes, terminal CO ligands that interact with the donor solvent tend to exhibit νCO IR frequencies over 2000 cm −1 [36,37]. In this study, the CO stretching frequency of the thermally isomerized acetonitrile complexes exceeded 2000 cm −1 (2008 and 2009 cm −1 ), while those of complexes containing an anionic ligand were 1950-1960 cm −1 . It would therefore be expected that weak interactions between the coordinated carbonyl and the solvent (acetone or water) could induce thermal isomerization. That is, the interaction between the coordinated CO and the solvent significantly changes the electronic state of the ruthenium center, resulting in lowering the activation energy for isomerization of the d-isomer to the thermally more stable p-isomer [38].
Molecules 2019, 24, x FOR PEER REVIEW 9 of 16 thermally isomerized acetonitrile complexes exceeded 2000 cm −1 (2008 and 2009 cm −1 ), while those of complexes containing an anionic ligand were 1950-1960 cm −1 . It would therefore be expected that weak interactions between the coordinated carbonyl and the solvent (acetone or water) could induce thermal isomerization. That is, the interaction between the coordinated CO and the solvent significantly changes the electronic state of the ruthenium center, resulting in lowering the activation energy for isomerization of the d-isomer to the thermally more stable p-isomer [38].

General Remarks
All chemicals were purchased from commercial sources and used without further purification unless otherwise stated. All solvents used for the syntheses were anhydrous. Acetonitrile for electrochemical measurements was purified by passing through solvent purification columns (Glass Contour, Laguna, CA, USA). Then 2-(2-pyridyl)-1, 8
IR spectra were obtained using a JASCO FT-IR 4100 spectrometer (Tokyo, Japan). Electrospray ionization mass spectrometry (ESI-MS) data were obtained using a Bruker Daltonics micrOTOF spectrometer. Electronic spectra were obtained on a JASCO V-560 spectrophotometer. The 1 H and 13 C{ 1 H}-NMR spectra were acquired on a JEOL JMN-AL300 spectrometer (Tokyo, Japan) operating at 1 H and 13 C frequencies of 300 and 75.5 MHz, respectively. Elemental analysis data were obtained on a PerkinElmer 2400II series CHN analyzer (Yokohama, Japan). Electrochemical measurements were performed on an electrochemical analyzer (ALS/CHI model 660E, Tokyo, Japan) with a solution of the complex in acetonitrile (1 mM) and n-Bu4NClO4 (0.1 M) as a supporting electrolyte in a cell consisting of a glassy carbon working electrode (ϕ = 1.6 mm), a Pt wire counter electrode, and Ag/AgNO3 (0.01 M) as the reference electrode. All potentials are reported in volts versus the ferrocenium/ferrocene couple (Fc + /Fc) under Ar at 25 °C. DFT calculations were performed using the quantum computation software Gaussian 09 [41]. The geometries of the Ru complexes were fully optimized using a restricted DFT method employing the B3LYP function [42,43], with a 6-31G(d) basis set for the light elements (H, C, N, and O) [44,45] and a LanL2DZ basis set [46] for the Ru atom. The solvent effect of acetonitrile was evaluated using an implicit solvent model and a polarizable continuum model. The vibrational analyses were performed at the same calculation level employed for geometry optimization.

General Remarks
All chemicals were purchased from commercial sources and used without further purification unless otherwise stated. All solvents used for the syntheses were anhydrous. Acetonitrile for electrochemical measurements was purified by passing through solvent purification columns (Glass  [18,39,40].
IR spectra were obtained using a JASCO FT-IR 4100 spectrometer (Tokyo, Japan). Electrospray ionization mass spectrometry (ESI-MS) data were obtained using a Bruker Daltonics micrOTOF spectrometer. Electronic spectra were obtained on a JASCO V-560 spectrophotometer. The 1 H and 13 C{ 1 H}-NMR spectra were acquired on a JEOL JMN-AL300 spectrometer (Tokyo, Japan) operating at 1 H and 13 C frequencies of 300 and 75.5 MHz, respectively. Elemental analysis data were obtained on a PerkinElmer 2400II series CHN analyzer (Yokohama, Japan). Electrochemical measurements were performed on an electrochemical analyzer (ALS/CHI model 660E, Tokyo, Japan) with a solution of the complex in acetonitrile (1 mM) and n-Bu 4 NClO 4 (0.1 M) as a supporting electrolyte in a cell consisting of a glassy carbon working electrode (φ = 1.6 mm), a Pt wire counter electrode, and Ag/AgNO 3 (0.01 M) as the reference electrode. All potentials are reported in volts versus the ferrocenium/ferrocene couple (Fc + /Fc) under Ar at 25 • C. DFT calculations were performed using the quantum computation software Gaussian 09 [41]. The geometries of the Ru complexes were fully optimized using a restricted DFT method employing the B3LYP function [42,43], with a 6-31G(d) basis set for the light elements (H, C, N, and O) [44,45] and a LanL2DZ basis set [46] for the Ru atom. The solvent effect of acetonitrile was evaluated using an implicit solvent model and a polarizable continuum model. The vibrational analyses were performed at the same calculation level employed for geometry optimization. [Ru(bpy)(CO) 2 (OH 2 ) 2 ](CF 3 SO 3 ) 2 (53 mg, 0.082 mmol) and pynp (20 mg, 0.098 mmol) were added to methanol (10 mL). The mixture was refluxed with stirring for 1.5 h. The reaction vessel was cooled to room temperature. The reaction mixture was condensed to 3 mL under reduced pressure. A light orange precipitate formed on addition of a saturated aqueous solution of KPF 6 to the mixture. The product was collected by filtration, washed with cold water and diethyl ether, and dried in vacuo to obtain the product (56 mg, 84%

Photochemical Reactions
Photochemical reactions of the complexes were conducted in the degassed CH 3 CN. The solution was irradiated with UV-visible light (λ = 300-400 nm) through a cutoff filter using an Asahi Spectra MAX-302 (Tokyo, Japan) with a xenon lamp (0.1 mW cm −2 ). In a typical reaction, an acetonitrile solution of [1d](PF 6 ) 2 (0.10 mM) was placed in a quartz flask with a stopper and irradiated with a xenon lamp for 30 min. The reaction mixture was condensed to 1 mL under reduced pressure. Addition of diethyl ether to the solution resulted in the formation of a precipitate of the mono-or disubstituted complexes in moderate yields (65-75%). In a typical reaction, d-[Ru(pynp)(phen)(CO)(CH 3 CN)](PF 6 ) 2 (4 mg, 0.005 mmol) was dissolved in acetone (20 mL) and refluxed until the solution became dark red (~72 h). The solution was condensed using a rotary evaporator, and addition of diethyl ether resulted in the formation of a precipitate. After cooling overnight, the isomerized product was collected by filtration, washed with diethyl ether, and then dried in vacuo. The yield was 3 mg (75%). A change of the solvent (acetone/water, 1:20 v/v) brought about a considerable reduction in reaction time (1 h). Similar reactions using d-[Ru(pynp)(bpy)(CO)(CH 3 CN)](PF 6 ) 2 or dp-[Ru(pynp) 2 (CO)(CH 3 CN)](PF 6 ) 2 gave the corresponding por pp-isomer, respectively. These products were characterized by spectroscopic and X-ray crystallographic analyses.  Tables 4-6 and Supplementary Materials Table S1. CCDC-1966877-1966887 contains the supplementary crystallographic data for this paper.

Conclusions
This study successfully formed non-equivalent CO-ligand environments by lowering the molecular symmetry of the well-known benchmark complex, [Ru(bpy) 2 (CO) 2 ] 2+ . In addition to the diastereoselective synthesis of the desired complexes, the relationship between reaction selectivity and coordination geometry was demonstrated using redox properties. We found that the monoacetonitrile complexes undergo thermal isomerization in the d-isomers, whereas the diacetonitrile complexes undergo photoisomerization to give the corresponding p-form. This study will conduct further research on synthetic chemistry, stereochemistry, and structure-reactivity relationships in metal complexes.