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

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

Komi Akatsuka, Ryosuke Abe, Tsugiko Takase and Dai Oyama * Cluster of Science and Engineering, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan; [email protected] (K.A.); [email protected] (R.A.); [email protected] (T.T.) * Correspondence: [email protected]; Tel.: +81-24-548-8199

Received: 29 November 2019; Accepted: 17 December 2019; Published: 19 December 2019

Abstract: 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.

Keywords: bipyridine; phenanthroline; naphthyridine; ruthenium; carbonyl complex; isomerization; crystal structure

1. 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 [4–11], 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 [14–16]. 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, Chart1).

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Molecules 2019, 24, x FOR PEER REVIEW 2 of 16 TheMolecules supporting 2019, 24, x ligands FOR PEER aff REVIEWected the properties of the complexes, and diastereomers with these ligands2 of 16 showed didifferentfferent reactivities reactivities [18 [18].]. As As is the is casethe incase some in ofsome the studiesof the mentionedstudies mentioned above, the above, difference the showed different reactivities [18]. As is the case in some of the studies mentioned above, the differencein reactivities in reactivities based on coordination based on coordination geometry or thegeometry effect of or the the supporting effect of ligandthe supporting on the properties ligand on of difference in reactivities based on coordination geometry or the effect of the supporting ligand on the complexproperties have of the been complex reported have independently, been reported but independently, there are very few but examples there are that very investigated few examples the the properties of the complex have been reported independently, but there are very few examples thatsynergistic investigated effects ofthe both. synergistic In this study, effects we of prepared both. In a seriesthis study, of Ru(II) we complexes prepared (Chart a series2) containing of Ru(II) that investigated the synergistic effects of both. In this study, we prepared a series of2 +Ru(II) complexesthe asymmetric (Chart bidentate 2) containing pynp and the a asymmetric symmetric bidentate bidentate ligand pynp NˆN, and [Ru(pynp)(NˆN)(CO)a symmetric bidentate2] ligand(NˆN =complexesbpy or 1,10-phenanthroline (Chart 2) containing (phen) the asymmetric as a more rigid bidentate ligand, pynp Chart and1), as a asymmetric platform forbidentate investigating ligand N^N, [Ru(pynp)(N^N)(CO)2]2+ (N^N = bpy or 1,10-phenanthroline (phen) as a more rigid ligand, N^N, [Ru(pynp)(N^N)(CO)2]2+ (N^N = bpy or 1,10-phenanthroline (phen) as a more rigid ligand, Chartthese synergistic1), as a platform effects [for19]. investigating In addition tothese characterizing synergistic the effects above [19]. complexes, In addition we performedto characterizing photo- Chart 1), as a platform for investigating these synergistic effects [19]. In addition to characterizing2+ theand above thermal complexes, reactions we on performed the complexes, photo- along and withthermal the reactions bis–pynp on complex the complexes, [Ru(pynp) along2(CO) with2] theas athe reference above complexes, compound. we We performed comprehensively photo- and evaluated thermal reactions the synergistic on the ecomplexes,ffects of the along coordination with the bis–pynp complex [Ru(pynp)2(CO)2]2+ as a reference compound. We comprehensively evaluated the bis–pynp complex [Ru(pynp)2(CO)2]2+ as a reference compound. We comprehensively evaluated the synergisticgeometry and effects supporting of the coordination ligands on the geometry reactivity. and supporting ligands on the reactivity. synergistic effects of the coordination geometry and supporting ligands on the reactivity.

Chart 1. Chemical structures of pynp, bpy, and phen. Chart 1. Chemical structures of pynp, bpy, and phen.

2+ ChartChart 2.2. [Ru(pynp)(NˆN)(CO)[Ru(pynp)(N^N)(CO)22]]2+ complexescomplexes in in this this study study [19]. [19]. Chart 2. [Ru(pynp)(N^N)(CO)2]2+ complexes in this study [19]. 2. Results and Discussion 2. Results and Discussion 2. Results and Discussion 2.1. Diastereoselective Synthesis and Characterization of Ruthenium Complexes with the Asymmetric 2.1.pynp Diastereoselective Ligand Synthesis and Characterization of Ruthenium Complexes with the Asymmetric pynp 2.1. Diastereoselective Synthesis and Characterization of Ruthenium Complexes with the Asymmetric pynp Ligand 2.1.1.Ligand Synthesis and Structure of Complexes 2.1.1. Synthesis and Structure of Complexes 2.1.1.Unlike Synthesis unsubstituted and Structure 2,2’-bipyridine of Complexes and 1,10-phenanthroline, the pynp ligand has no C2 symmetry axis (ChartUnlike1 ),unsubstituted so metal complexes 2,2'-bipyridine containing and pynp(s) 1,10 may-phenanthroline, have several diastereomers.the pynp ligand We has previously no C2 Unlike unsubstituted 2,2'-bipyridine and 1,10-phenanthroline, the pynp ligand has no C2 symmetryreported that axis for (Chart ruthenium 1), so metal carbonyl complexes complexes cont containingaining pynp(s) both may bpy have and pynp,several formation diastereomers. of the symmetry axis (Chart 1), so metal complexes containing pynp(s) may have several diastereomers. Wedesired previously isomers reported can be controlled that for byruthenium the order carb of introductiononyl complexes of the containi two ligandsng both [18 bpy,20]. and Using pynp, this We previously reported that for ruthenium carbonyl complexes containing both bpy and pynp, formationmethod, dicarbonylruthenium of the desired isomers complexes can be controlled containing by both the pynp order and of phenintroduction were successfully of the two prepared. ligands formation of the desired isomers can be controlled by the order of introduction of the two ligands As[18,20]. in the Using previous this method, study, the dicarbonylruthenium distal isomer is produced complexes preferentially containing when both phen pynp is introducedand phen were first [18,20]. Using this method, dicarbonylruthenium complexes containing both pynp and phen were (Figuresuccessfully1a). In prepared. the case ofAsNˆN in the= previousbpy, the productstudy, th obtainede distal isomer by this is method produced is almost preferentially all the d -form,when successfully prepared. As in the previous study, the distal isomer is produced preferentially when whereasphen is introduced in the phen first system (Figure a small 1a). In quantity the case ofof theN^N corresponding = bpy, the productp-form obtained (10–20%) by isthis also method formed. is phen is introduced first (Figure 1a). In the case of N^N = bpy, the product obtained by this method is Thisalmost may all bethe due d-form, to energy whereas diff inerences the phen between system the ad small- and quantityp-isomers. of Densitythe corresponding functional p theory-form almost all the d-form, whereas in the phen system a small quantity of the corresponding p-form (DFT)(10–20%) calculations is also formed. for each This complex may system be due suggest to energy that thedifferencesp-isomers between are more the stable, d- and but thep-isomers. energy (10–20%) is also formed. This may be due to energy differences between the d- and p-isomers. Densitydifference functional between theory the d- and(DFT)p-isomers calculations in the for phen each system complex is smaller system thansuggest that that of the the corresponding p-isomers are Density functional theory (DFT) calculations for each complex system suggest that the p-isomers are morebpy system stable, (NˆN but the= bpy: energy 4.3 kcaldifference/mol, NˆN between= phen: the 1.9d- and kcal /pmol).-isomers The in DFT the calculations phen system indicate is smaller that more stable, but the energy difference between the d- and p-isomers in the phen system is smaller thanthe minor that of formation the corresponding of the p-isomer bpy system in addition (N^N to= bpy: the d 4.3-isomer kcal/mol, is due N^N to the = phen: relatively 1.9 kcal/mol). small energy The than that of the corresponding bpy system (N^N = bpy: 4.3 kcal/mol, N^N = phen: 1.9 kcal/mol). The DFTdifference calculations in the phenindicate system. that the minor formation of the p-isomer in addition to the d-isomer is due DFT calculations indicate that the minor formation of the p-isomer in addition to the d-isomer is due to the relatively small energy difference in the phen system. to the relatively small energy difference in the phen system.

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FigureFigure 1. Diastereoselective1. Diastereoselective synthesis synthesis ofof [1[1]]22++ and [ [22]]2+2.+ .(a) (a) synthetic synthetic route route for forthe the d-isomersd-isomers and and Figure 1. Diastereoselective synthesis of [1]2+ and [2]2+. (a) synthetic route for the d-isomers and molecularmolecular structure structure of of [2 d[2]d2+]2+;; (b) (b) syntheticsynthetic route for for the the pp-isomers-isomers and and molecular molecular structure structure of [ of2p []2+2p. ]2+. molecular structure of [2d]2+; (b) synthetic route for the p-isomers and molecular structure of [2p]2+. Counteranions,Counteranions, H atoms,H atoms, and and solvent solvent molecules molecules are are omitted omitted for for clarity. clarity. The The asymmetric asymmetric unitunit ofof thethe [2d] crystalCounteranions,[2d] containscrystal contains twoH atoms, chemically two andchemically solvent identical identical molecules complex comp are pairs,lex omitted pairs, of which of forwhich onlyclarity. only one Theone is displayed. isasymmetric displayed. unit of the [2d] crystal contains two chemically identical complex pairs, of which only one is displayed. In contrast,In contrast, initial initial introduction introduction of of the the asymmetric asymmetric pynp pynp ligandligand leadsleads to the selective formation formation of p-isomersofIn p -isomerscontrast, (Figure (Figureinitial1b). Considering introduction 1b). Considering the of molecularthe theasymmetr molecular structureic pynp structure of anligand isolated of leads an and isolatedto analyzedthe selective and intermediateanalyzed formation of intermediatep-isomers (Figure cis(solv)-[Ru(pynp)(CO) 1b). Considering2+ 2(solv) the2]2+ ;molecular Supplementary structure Materials of Figuan reisolated S1), it isand reasonable analyzed cis(solv)-[Ru(pynp)(CO)2(solv)2] ; Supplementary Materials Figure S1), it is reasonable to propose thatintermediateto an propose intramolecular cis that(solv)-[Ru(pynp)(CO) an intramolecular interaction between interaction2(solv)2 the]2+ ;between non-coordinatingSupplementary the non-coordinating Materials nitrogen Figu atom nitrogenre S1), in the itatom is pynp reasonable in the ligand andto proposepynp the adjacentligand that and an coordinated intramolecularthe adjacent carbonyl coordinate interaction promotesd carbonyl between the promotes formation the non-coordinating the of formationp-isomers of [ 18nitrogen p].-isomers In addition, atom [18]. inIn only the addition, only the dp-isomer was selectively isolated in the case of [Ru(pynp)2(CO)2]2+ (Figure 2), thepynpdp -isomerligand and was the selectively adjacent isolatedcoordinate ind the carbonyl case of promotes [Ru(pynp) the(CO) formation]2+ (Figure of p2-isomers), even though[18]. In even though the bis-pynp complex could form three diastereomers2 2(pp, dp, and dd; Chart 3). theaddition, bis-pynp only complex the dp-isomer could formwas threeselectively diastereomers isolated in (pp the, dp case, and ofdd [Ru(pynp); Chart3).2(CO) According2]2+ (Figure to DFT 2), According to DFT calculations for the three diastereomers, the total energy of the experimentally even though the bis-pynp complex could form three diastereomers (pp, dp, and dd; dp-Chart 3). calculationsformed dp- forisomer the was three 3.79 diastereomers, kcal/mol higher the than total that energy of the most of the stable experimentally pp-isomer, which formed appearsisomer to According to DFT calculations for the three diastereomers, the total energy of the experimentally wascontradict 3.79 kcal the/mol experimental higher than result. that We of thethen most perfor stablemed totalpp-isomer, energy calculations which appears for the to two contradict possible the formed dp-isomer was 3.79 kcal/mol higher than that of the most stable pp-isomer, which appears to experimentalintermediates result. in the We coordination then performed of thetotal second energy pynp calculations ligand to the for ruthenium the two possiblecenter by intermediates assuming contradict the experimental result. We then performed total energy calculations for the two possible in thethat coordination the aqua ligand of in the the second trans position pynp ligand of the coordinated to the ruthenium pynp is centersubstituted by assuming earlier than that the theaqua aqua ligandintermediatesligand in thein thetrans in cistheposition position coordination of (see the the coordinated of structure the second pynpdiagram pynp is substituted in ligand Supplementary to earlierthe ruthenium than Materials the aquacenter Figure ligand by S1). assuming in The the cis positionthatresults the aqua (see indicate theligand structure that in both the diagramintermediatestrans position in Supplementary have of the almost coordinated the Materials same pynp energy Figure is (substitutedΔE S1).= 0.08 The kcal/mol; earlier results Figurethan indicate the S2). aqua that bothligandTherefore, intermediates in the thecis secondposition have almostpynp (see ligand thethe samestructure may energycoordinate diagram (∆E =through0.08 in Supplementary kcal a kinetically/mol; Figure favorable Materials S2). Therefore, pathway Figure the rather S1). second The pynpresultsthan ligand indicate a thermodynamic may that coordinate both oneintermediates to through give the a selective kineticallyhave almost dp-isomer. favorable the same pathway energy (Δ ratherE = 0.08 than kcal/mol; a thermodynamic Figure S2). oneTherefore, to give the the second selective pynpdp-isomer. ligand may coordinate through a kinetically favorable pathway rather than a thermodynamic one to give the selective dp-isomer.

Chart 3. Possible diastereomers in [Ru(pynp)2(CO)2]2+.

2+ ChartChart 3. 3.Possible Possible diastereomersdiastereomers inin [Ru(pynp)[Ru(pynp)22(CO)(CO)22]]2+. .

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2+ 2+ Figure 2. Molecular structure of dp-[Ru(pynp)2(CO)2] ([3dp] ) with atom labels and displacement Figure 2. Molecular structure of dp-[Ru(pynp)2(CO)2]2+ ([3dp]2+) with atom labels and displacement ellipsoids for non-H atoms drawn at the 50% probability level. Counteranions and H atoms are omitted ellipsoids for non-H atoms drawn at the 50% probability level. Counteranions and H atoms are for clarity. omitted for clarity. 2+ Table1 summarizes the structural parameters of a series of [Ru(pynp)(NˆN)(CO) 2] complexes. The data around the ruthenium center and the {Ru–CO}2+ moieties are within the typical range of Table 1 summarizes the structural parameters of a series of [Ru(pynp)(N^N)(CO)2]2+ complexes. dicarbonylruthenium(II) complexes that have polypyridines as supporting ligands [4,21,22]. In addition, The data around the ruthenium center and the {Ru–CO}2+ moieties are within the typical range of the interatomic distances between the carbonyl carbon and the non-coordinating nitrogen of pynp dicarbonylruthenium(II) complexes that have polypyridines as supporting ligands [4,21,22]. In (2.607(5) to 2.754(14) Å) in the three p-forms are much shorter than the sum of van der Waals radii addition, the interatomic distances between the carbonyl carbon and the non-coordinating nitrogen (3.25 Å) [23]. The shortening suggests that pynp and the adjacent CO ligand interact considerably in 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 these complexes. radii (3.25 Å) [23]. The shortening suggests that pynp and the adjacent CO ligand interact considerably in these complexes. Table 1. Selected bond and interatomic distances (Å) and angles (◦) for dicarbonyl complexes.

Table 1.Parameter Selected bond and[1d ]interatomic2+ 2 [1 pdistances]2+ 3 (Å)[2 andd]2+ angles (°)[2 forp]2 +dicarbonyl[3dp complexes.]2+

ParameterRu–C 1.905 [1d]2+ (3) 2 1.925[1p]2+ (11) 3 [2 1.903d]2+(9) [2 1.917p]2+ (4)[3dp 1.919]2+ (10) 1.900 (3) 1.910 (10) 1.888 (9) 1.911 (4) 1.886 (8) Ru–C 1.905 (3) 1.925 (11) 1.903 (9) 1.917 (4) 1.919 (10) 1.909 (9) 1.900 (3) 1.910 (10) 1.8881.895 (9) (10) 1.911 (4) 1.886 (8) C–O 1.134 (4) 1.123 (13)1.909 1.145 (9) (11) 1.133 (5) 1.132 (10) 1.124 (4) 1.119 (12)1.895 1.123 (10) (11) 1.122 (5) 1.120 (11) C–O 1.134 (4) 1.123 (13) 1.1451.133 (11) (11) 1.133 (5) 1.132 (10) 1.121 (11) 1.124 (4) 1.119 (12) 1.123 (11) 1.122 (5) 1.120 (11) Ru–C–O 176.9 (3) 177.6 (8) 177.0 (8) 176.0 (4) 175.7 (7) 176.6 (3) 174.3 (9)1.133 175.5 (11) (8) 171.6 (4) 175.6 (8) 1.121177.4 (11) (8) Ru–C–O 176.9 (3) 177.6 (8) 177.0177.1 (8) (8) 176.0 (4) 175.7 (7) 1 C ... N 176.6- (3) 174.3 2.671 (14)(9) 175.5 - (8) 171.6 2.607 (4) (5) 175.6 2.754 (8) (14) 1 Distance between the non-coordinating nitrogen in177.4 pynp and(8) the CO carbon atoms. 2 [18]. 3 [20]. 177.1 (8) 2.1.2. CharacterizationC…N of 1 Complexes - 2.671 (14) - 2.607 (5) 2.754 (14) Spectroscopic1 Distance between and electrochemical the non-coordinating analyses nitrogen were in performed pynp and onthethe CO synthesized carbon atoms. complexes 2 [18]. 3 [20]. (Table 2). 1 Two strong IR bands assignable to νCO were observed around 2100 and 2040 cm− in all complexes. These2.1.2. Characterization values were similar of Complexes to those in other dicarbonyl-ruthenium(II) complexes [4,21,22], and no clear diﬀerencesSpectroscopic were observed and electrochemical between the isomers. analyses Given were that performed the complexes on containthe synthesized two carbonyl complexes ligands (Tableof the highest2). Two order strong in IR the bands spectrochemical assignable series,to νCO no were obvious observed absorption around was 2100 observed and 2040 in thecm− visible1 in all complexes.region (Supplementary These values Materials were Figuresimilar S3). to However,those in intenseother polypyridyl-centereddicarbonyl-ruthenium(II)π–π* intraligandcomplexes [4,21,22],transitions and were no observed clear differenc in thees UV were region observed [24]. 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].

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Table 2.2. Spectral andand electrochemicalelectrochemical datadata forfor dicarbonyldicarbonyl complexes.complexes. Technique [1d]2+ [1p]2+ [2d]2+ [2p]2+ [3dp]2+ Technique [1d]2+ [1p]2+ [2d]2+ [2p]2+ [3dp]2+ IR (νCO/cm–1) 2097 2097 2099 2093 2094 IR (νCO/cm 1) 2097 2097 2099 2093 2094 − 2044 2045 2045 2044 2040 2044 2045 2045 2044 2040 max UV–visUV–vis (λmax (/λnm)/nm) 339 339 (1.78) (1.78) 343 343 (2.90) (2.90) 342 342 (1.64) (1.64) 349 349 (2.05) (2.05) 336 (2.93) 336 (2.93) (ε/ 10(ε4/×10M 14 cmM−1 1cm) −1) 314 314 (2.22) (2.22) 315 315 (2.63) (2.63) 273 273 (3.19) (3.19) 272 272 (3.79) (3.79) × − − CV (CVE/V ( vs.E/V Fc vs.+/Fc) Fc+/Fc) 1.09−1.091 1 −1.071.071 1 −1.121.12 11 −1.091.09 1 1 −1.11 1.111 1 − 1 − 1 − 1 − 1 − 1 1.45 1 1.48 1 1.591 1.491 1.291 − −1.45 −−1.48 −−1.59 −1.49− −1.29− 0.98 2 1.05 2 1.03 2 1.03 2 − −0.98 2 −−1.05 2 −1.03− 2 −1.03− 2 ( 0.78) 3 ( 0.87) 3 ( 0.79) 3 − (−0.78) 3 (−−0.87) 3 (−0.79)− 3 1 2 3 Epc. Epa. Data in parentheses represent oxidation waves caused by the second reduction. 1 Epc. 2 Epa. 3 Data in parentheses represent oxidation waves caused by the second reduction.

AlthoughAlthough spectroscopic measurements did not showshow any marked didifferencesfferences between isomers, electrochemistryelectrochemistry clearly clearly showed showed diff erentdifferent behavior. behavi Theor. cyclicThe voltammogramscyclic voltammograms (CV) of all(CV) the complexesof all the showedcomplexes multiple showed ligand-based multiple ligand-based reduction wavesreduction in thewaves range in the of range1 to of2 V−1 vs.to − Fc2 V+/ vs.Fc. Fc Since+/Fc. pynpSince − − ispynp more is easilymore easily reduced reduced than bpythan or bpy phen, or phen, the first the reduction first reduction peak peak at ca. at ca.1 V − was1 V was attributed attributed to the to − reductionthe reduction of the of pynpthe pynp ligand ligand [25]. [25]. When When the cathodicthe cathodic scan scan was immediatelywas immediately reversed reversed after after the first the peakfirst peak potential, potential, the coupled the coupled oxidation oxidation wave wave was reversiblewas reversible in the ind -isomersthe d-isomers (Figure (Figure3a, dotted 3a, dotted line); however,line); however, those ofthose the correspondingof the correspondingp-isomers p-isomers were either were irreversible either irreversible or quasi-reversible or quasi-reversible at the first at reductionthe first reduction wave (Figure wave3 (Figureb). Although 3b). Although the first the reduction first reduction wave of wave [ 2p]2 of+ in[2p Figure]2+ in Figure3b appears 3b appears to be reversible,to be reversible, the coupled the coupled anodic currentanodic decreasedcurrent decreased as the scan as ratesthe scan slowed rates (Supplementary slowed (Supplementary Materials FigureMaterials S4). Figure The basicity S4). The of basicity the free of nitrogen the free atom nitrogen of the atom 1,8-naphthyridine of the 1,8-naphthyridine moiety in pynpmoiety increased in pynp dueincreased to one-electron due to one-electron reduction of reduction pynp, thus of makingpynp, th intramolecularus making intramolecular nucleophilic nucleophilic attack to the attack adjacent to carbonylthe adjacent carbons carbonyl possible, carbons leading possible, to the formation leading ofto athe metallacyclic formation compound of a metallacyclic (Scheme1 )[compound26]. Due to(Scheme this structural 1) [26]. Due change, to this only structural the p-isomer change, exhibited only the irreversible p-isomer one-electron exhibited irreversible reduction one-electron behavior. reduction behavior.

2+ 2+ 1 Figure 3. Cyclic voltammograms of (a)[2d] and (b)[2p] in CH3CN (v = 0.1 V s− , c = 1.0 mM). Figure 3. Cyclic voltammograms of (a) [2d]2+ and (b) [2p]2+ in CH3CN (v = 0.1 V s−1, c = 1.0 mM).

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Scheme 1. Reversible metallacyclization induced by one-electron transfer in [Ru(pynp)(CO)2(PPh3)2]2+ [26]. 2+ SchemeScheme 1. Reversible1. Reversible metallacyclization metallacyclization induced by one-electron induced transferby inone-electron [Ru(pynp)(CO) 2transfer(PPh3)2 ] in[26 ]. 2.2. Reactivities[Ru(pynp)(CO) of Complexes2(PPh3)2]2+ [26]. 2.2. Reactivities of Complexes 2.2. Reactivities of Complexes 2.2.1. Photochemical Photochemical Reactions Reactions

Polypyridylruthenium complexes are generally photoreactive. When cis-[Ru(bpy)2(CO)2]2+ in2+ 2.2.1.Polypyridylruthenium Photochemical Reactions complexes are generally photoreactive. When cis-[Ru(bpy)2(CO)2] acetonitrilein acetonitrile is isirradiated irradiated with with light, light, two two carbonyl carbonyl ligands ligands simultaneously simultaneously dissociate dissociate and and the Polypyridylruthenium complexes are generally photoreactive. When cis-[Ru(bpy)2(CO)2]2+ in corresponding solvent complex (cis-[Ru(bpy)2(CH3CN)22+]2+) is produced [27,28]. However, when corresponding solvent complex (cis-[Ru(bpy)2(CH3CN)2] ) is produced [27,28]. However, when pynp pynpacetonitrile is substituted is irradiated for one withof the light, two bpytwo ligands, carbonyl the ligandstwo carbonyl simultaneously groups dissociate dissociate stepwise and duethe is substituted for one of the two bpy ligands, the two carbonyl2+ groups dissociate stepwise due to their tocorresponding their electronic solvent non-equivalence complex (cis [18].-[Ru(bpy) Since 2no(CH comparisons3CN)2] ) is betweenproduced the [27,28]. d- and However, p-isomers whenwere pynpelectronic is substituted non-equivalence for one [18of ].the Since two nobpy comparisons ligands, the between two carbonyl the d- andgroupsp-isomers dissociate were stepwise made in due the madeprevious in the report, previous we compared report, we the compared photoreactivities the photoreactivities of both isomers. of Asboth previously isomers. reported,As previously there reported,to their electronic there are non-equivalence two reaction steps [18]. for Since both no is comparisonsomers (Scheme between 2) [18,29]. the d -In and the p- firstisomers step, were one madeare two in reaction the previous steps forreport, both we isomers compared (Scheme the2 )[photoreactivities18,29]. In the first of step,both oneisomers. carbonyl As ligandpreviously was carbonyldissociated, ligand and, was at the dissociated, same time, and, the at acetonitrile the same usedtime, asthe the acetonitrile solvent became used as coordinated the solvent (thebecame first coordinatedreported, there (the are first two step reaction in Scheme steps 2 and for Figure both is4a).omers Steric (Scheme retention 2) of[18,29]. the complex In the wasfirst supported step, one carbonylstep in Scheme ligand2 andwas Figuredissociated,4a). Steric and, retention at the same of the time, complex the acetonitrile was supported used byas the structural solvent analysis became byof thestructural isolated analysis species at of this the step isolated (Figure species5a). In theat subsequentthis step (Fig step,ure photoisomerization 5a). In the subsequent from thestep,d- photoisomerizationcoordinated (the first from step thein Scheme d- to p-isomer 2 and Figure subsequent 4a). Steric slow retention dissociation of the of complex the second was COsupported ligand byto p-isomerstructural subsequent analysis slowof the dissociation isolated species of the secondat this CO step ligand (Figure and solvent5a). In coordinationthe subsequent occurred step, and(the solvent second stepcoordination in Scheme occurred2a and Figure(the second4b). Reaction step in Scheme analysis 2a and and DFT Figure calculations 4b). Reaction suggest analysis that andphotoisomerization DFT calculations from suggest the thatd- to the p-isomer formation subsequent of photoexcited slow dissociation states of the of complexthe second promote CO ligand such andthe formationsolvent coordination of photoexcited occurred states (the of thesecond complex step promotein Scheme such 2a and an isomerization Figure 4b). Reaction to a more analysis stable anp-form isomerization in similar complexesto a more containingstable p-form pynp in [similar30,31]. Incomplexes the bis-pynp containing complex pynp ([3dp [30,31].]2+), a similarIn the and DFT calculations suggest2+ that the formation of photoexcited states of the complex promote such bis-pynptwo-step photoreactioncomplex ([3dp (Scheme] ), a 2similara) was confirmedtwo-step photoreactio from structuraln (Scheme analyses 2a) of both was products confirmed ( dp -formfrom structuralan isomerization analyses to of a both more products stable p(dp-form-form in in similar Figure complexes5b and pp-form containing in Supplementary pynp [30,31]. Materials In the bis-pynpin Figure5 bcomplex and pp -form([3dp in]2+), Supplementary a similar two-step Materials photoreactio Figure S5).n On(Scheme the other 2a) hand, was theconfirmed final structure from Figureof the p S5).-isomers On the was other unchanged hand, the from final spectroscopic structure of the analyses p-isomers (Scheme was unchanged2b). from spectroscopic analysesstructural (Scheme analyses 2b). 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).

2+ Scheme 2. Photoreactions of [Ru(pynp)(NˆN)(CO)2] [29]. (a) The two-step reaction of d-isomers 2 2+ Scheme2+ 2. Photoreactions2+ 2+ of [Ru(pynp)(N^N)(CO) ] [29]. (a) The two-step reaction of d-isomers2+ ([1d]2+ ,[2d]2+ , and [3dp2+] ) with isomerization; (b) The two-step reaction of p-isomers ([1p] 2+ and ([1d]2+, [2d] , and [3dp] ) with isomerization; (b) The two-step reaction of p-isomers ([1p] and [2p] ) without isomerization. The structures of these species2+ were conﬁrmed by X-ray crystallographic [Scheme2p]2+) without 2. Photoreactions isomerization. of [Ru(pynp)(N^N)(CO) The structures of2] these[29]. (aspecies) The two-step were confirmedreaction of byd-isomers X-ray analyses2+ (Figure2+ 5, Supplementary2+ Materials Figure S5, and reference [18]). 2+ crystallographic([1d] , [2d] , and analyses [3dp] (Figure) with isomerization;5, Supplementa (ryb) MaterialsThe two-step Figure reaction S5, and of reference p-isomers [18]). ([1 p] and [2p]2+) without isomerization. The structures of these species were confirmed by X-ray crystallographic analyses (Figure 5, Supplementary Materials Figure S5, and reference [18]).

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2+ Figure 4. Changes in the absorption spectra of [2+d] in CH3CN upon photoirradiation (λ = 300–400 Figure 4. Changes in thethe absorptionabsorption spectraspectra ofof [[22dd]]2+ in CH3CNCN upon photoirradiation ( λλ = 300–400 nm). nm). (a) 1st reaction step (t < 900 s); (b) 2nd reaction step3 (t > 900 s). Inset: First-order plot based on (nm).a) 1st (a reaction) 1st reaction step step (t < (900t < 900 s); (s);b) ( 2ndb) 2nd reaction reaction step step (t >(t 900> 900 s). s). Inset: Inset: First-orderFirst-order plot based based on on the the absorbance values at 475 nm. absorbancethe absorbance values values at 475at 475 nm. nm.

Figure 5. Figure 5.Molecular Molecular structures structures ofof monosubstitutedmonosubstituted complexes complexes with with atom atom labels labels and and displacement displacement Figure 5. Molecular structures of monosubstituted complexes with atom labels and displacement ellipsoidsellipsoids for for non-H non-H atoms atoms drawn drawn atat thethe 50%50% probabilityprobability level. level. Counteranions, Counteranions, H H atoms, atoms, and and solvent solvent ellipsoids for non-H atoms drawn at the 50% probability level. Counteranions,2+ H atoms, and solvent molecules are omitted for clarity. (a) d-[Ru(pynp)(phen)(CO)(CH CN)] ;(b) dp-[Ru(pynp)2+ (CO) molecules are omitted for clarity. (a) d-[Ru(pynp)(phen)(CO)(CH3 3CN)] ; 2(b) molecules 2+ are omitted for clarity. (a) d-[Ru(pynp)(phen)(CO)(CH3CN)]2+; (b) (CHdp3-[Ru(pynp)CN)] . 2(CO)(CH3CN)]2+. dp-[Ru(pynp)2(CO)(CH3CN)]2+. FromFrom comparisons comparisons of theof firstthe CO-dissociationfirst CO-dissociation rates (Tablerates3 ),(Table it was 3), found it was that thefound photoreactions that the From comparisons of the first CO-dissociation rates (Table 3), it was found that the ofphotoreactions the d-isomers wereof the always d-isomers faster were than always those faster of the than corresponding those of thep -isomerscorresponding (ca. two p-isomers times in (ca. both photoreactions of the d-isomers were always faster than those of the corresponding p-isomers (ca. complexes).two times in This both di complexes).fference was This interpreted difference towas be interpreted due to their to be geometries, due to their based geometries, on photoreaction based on two times in both complexes). This difference was interpreted to be due to their geometries, based on behaviorphotoreaction seen in behavior similar seen diastereomers in similar diastereomers [32]. The pynp [32]. ligand The pynp has aligand naphthyridine has a naphthyridine unit in place unit of photoreaction behavior seen in similar diastereomers [32]. The pynp ligand has a naphthyridine unit thein pyridine place of the unit pyridine in bpy (orunit phen), in bpy which(or phen), is both which more is both delocalized more delocalized and a π -acceptor.and a π-acceptor. The superior The in place of the pyridine unit in bpy (or phen), which is both more delocalized and a π-acceptor. The chargesuperior acceptor charge properties acceptor of properties the naphthyridine of the naphthyridine unit in the d-isomers unit in leadsthe d-isomers to better labilizationleads to better of the superior charge acceptor properties of the naphthyridine unit in the d-isomers leads to better translabilization-CO, and of thus the the transd-isomers-CO, and exhibit thus fasterthe d-isomers CO release exhibit compared faster withCO release the corresponding compared withp-isomers, the 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 despitecorresponding having p lower-isomers, extinction despite coehavingfficients lower between extinction 300 coefficients and 400 nm between (Table2 300and and Supplementary 400 nm (Table 2 and Supplementary Materials Figure S3). We also found that the overall photoreaction of Materials(Table 2 and Figure Supplementary S3). We also Materials found that Figure the overall S3). We photoreaction also found that of the bpyoverall system photoreaction proceeded of more the bpy system proceeded more smoothly than that of the corresponding phen system. This was smoothlythe bpy system than that proceeded of the corresponding more smoothly phen than system.that of the This corresponding was probably phen because system. phen This compounds was probably because phen compounds were significantly more stable than their bpy analogues, based wereprobably significantly because phen more compounds stable than were their significantly bpy analogues, more based stable on than the their rigidity bpy ofanalogues, phen [33 based]. Despite on the rigidity of phen [33]. Despite prolonged photoirradiation, incomplete dissociation of the prolongedon the rigidity photoirradiation, of phen [33]. incompleteDespite prolonged dissociation photoirradiation, of the second incomplete CO ligand dissociation in the phen of the system second CO ligand in the phen system consequently prevented isolation of the single disubstituted2 + consequentlysecond CO ligand prevented in the isolationphen system of the consequently single disubstituted prevented complex isolation ([Ru(pynp)(phen)(CH of the single disubstituted3CN)2 ] ). complex ([Ru(pynp)(phen)(CH3CN)2]2+). complex ([Ru(pynp)(phen)(CH3CN)2]2+). Table 3. Rates of the first photoreaction step (298 K) of the dicarbonyl complexes.

Rate [1d]2+ [1p]2+ [2d]2+ [2p]2+ [3dp]2+ 1 3 3 3 3 3 kobs/sec− 12 10− 6.5 10− 2.4 10− 1.3 10− 7.6 10− × × × × ×

Molecules 2019, 24, x FOR PEER REVIEW 8 of 16

Table 3. Rates of the first photoreaction step (298 K) of the dicarbonyl complexes.

Table 3.Rate Rates of the [1 dfirst]2+ photoreaction[1p]2+ step[2 (298d]2+ K) of the[2p dicarbonyl]2+ [3dp complexes.]2+ −1 −3 −3 −3 −3 −3 Molecules 2020, 25, 27 kobsRate/sec 12 [1 ×d 10]2+ 6.5[1 ×p 10]2+ 2.4[2 ×d 10]2+ 1.3[2 ×p 10]2+ 7.6[3 dp× 10]2+ 8 of 16 kobs/sec−1 12 × 10−3 6.5 × 10−3 2.4 × 10−3 1.3 × 10−3 7.6 × 10−3 2.2.2. Thermochemical Reactions 2.2.2. Thermochemical Reactions 2.2.2.As Thermochemical shown in Scheme Reactions 3, in thermal reactions one carbonyl ligand of a dicarbonylruthenium(II) As shown in Scheme3, in thermal reactions one carbonyl ligand of a dicarbonylruthenium(II) complexAs shownundergoes in Scheme nucleophilic 3, in thermal attack from reactions solvent on emolecules carbonyl [34,35].ligand ofThus, a dicarbonylruthenium(II) when the d-isomers of thecomplex dicarbonyl undergoes complexes, nucleophilic which attackare expected from solvent to be more molecules reactive [34 (see,35]. Section Thus, when2.2.1.), the wered-isomers heated in of thecomplex dicarbonyl undergoes complexes, nucleophilic which attack are expected from solvent to be moremolecules reactive [34,35]. (see Thus, Section when 2.2.1 the.), wered-isomers heated of water/acetonitrilethe dicarbonyl complexes, or alcohol which (methanol are expected or ethanol) to be/acetonitrile more reactive mixtures, (see Section one of 2.2.1.), the coordinated were heated CO in moietiesin water /acetonitrileunderwent nucleophilic or alcohol (methanol attack from or ethanol)the solvent/acetonitrile to the CO mixtures, ligand at one the of trans the coordinated position of COwater/acetonitrile moieties underwent or alcohol nucleophilic (methanol attack or ethanol) from the/acetonitrile solvent to mixtures, the CO ligandone of atthe the coordinatedtrans position CO pynp. As expected, molecular structures of the isolated complexes showed the formation of the ofmoieties pynp. underwent As expected, nucleophilic molecular structuresattack from of the the solvent isolatedto complexes the CO ligand showed at the formationtrans position of the of hydroxycarbonylpynp. As expected, (–C(O)OH molecular in structuresFigure 6a) of an thed the isolated methoxy- complexes or ethoxycarbonyl showed the formation(–C(O)OC 2ofH 5 thein Figureshydroxycarbonyl 6b and Supplementary (–C(O)OH in Materials Figure6a) Figure and the S6) methoxy-complexes. or Given ethoxycarbonyl that the pynp (–C(O)OC ligand is2 Hmore5 in hydroxycarbonyl (–C(O)OH in Figure 6a) and the methoxy- or ethoxycarbonyl (–C(O)OC2H5 in πFigure-acidic6b than and bpy Supplementary or phen, the MaterialsCO carbon Figure in the S6)trans complexes. position of Given pynp thathas a the more pynp positive ligand charge. is more πFigures-acidic 6b than and bpy Supplementary or phen, the CO Materials carbon inFigure the trans S6) complexes.position of pynpGiven has that a the more pynp positive ligand charge. is more π-acidic than bpy or phen, the CO carbon in the trans position of pynp has a more positive charge.

Scheme 3. Nucleophilic attack of ROH on the coordinated CO in dicarbonyl-ruthenium(II) complexes. Scheme 3.3.Nucleophilic Nucleophilic attack attack of ROHof ROH on the on coordinated the coordi COnated in dicarbonyl-ruthenium(II) CO in dicarbonyl-ruthenium(II) complexes. complexes.

Figure 6. Molecular structures with atom labels and displacement ellipsoids for non-H atoms drawn at the 50% probability level. Counteranions, Counteranions, H H atom atoms,s, and and solvent solvent molecules molecules are are omitted omitted for for clarity. clarity. Figure 6. Molecular structures with atom labels and displacement ellipsoids for non-H atoms drawn + + (a) d-[Ru(pynp)(bpy)(CO)(C(O)OH)]+;;( (bb)) dd-[Ru(pynp)(bpy)(CO)(C(O)OC-[Ru(pynp)(bpy)(CO)(C(O)OC2HH5)])]+. . at the 50% probability level. Counteranions, H atoms, and solvent molecules2 5 are omitted for clarity.

(a) d-[Ru(pynp)(bpy)(CO)(C(O)OH)]+; (b) d-[Ru(pynp)(bpy)(CO)(C(O)OC2H5)]+. We next investigatedinvestigated other thermochemical reactions using monocarbonyl complexes that do not undergo nucleophilic attack on the coordinatedcoordinated carbonyl. When the monocarbonyl complex We next investigated other thermochemical2+ reactions using monocarbonyl complexes that do (d-[Ru(pynp)(phen)(CO)(CH CN)]2+ ), which was produced by the photoreaction described in (notd-[Ru(pynp)(phen)(CO)(CH undergo nucleophilic attack33CN)] on), whichthe coordi was natedproduced carbonyl. by the Whenphotoreaction the monocarbonyl described in complex Section 2.2.1,Section was 2.2.1 heated, was heatedin acetone, in acetone, it isomerized it isomerized from the from d- theto thed- to p-isomer the p-isomer (Figure (Figure 7). Notably,7). Notably, this (d-[Ru(pynp)(phen)(CO)(CH3CN)]2+), which was produced by the photoreaction described in Section thermalthis thermal isomerization isomerization reaction reaction was was accelerated accelerated more more than than 70 70 times times when when water water was was added added to to the 2.2.1, was heated in acetone, it isomerized from the d- to the p-isomer (Figure2+ 7). Notably, this solution. A similar isomerization was observed in d-[Ru(pynp)(bpy)(CO)(CH CN)] . In2+ contrast, such solution.thermal isomerizationA similar isomerization reaction was was accelerated observed more in d-[Ru(pynp)(bpy)(CO)(CH than 70 times when3 water3CN)] was .added In contrast, to the suchthermal thermal isomerization isomerization could could not be not conﬁrmed be confirme for otherd for monocarbonyl other monocarbonyl complexes complexes with anionic with anionic ligands solution. A similar isomerization was observed in d-[Ru(pynp)(bpy)(CO)(CH3CN)]2+. In contrast, (–COR ;R = OH,− − OCH , or OC H ). These results strongly suggest that the thermal isomerization ligandssuch thermal− (–COR− isomerization; R = OH,3 couldOCH2 3not, 5or be OC confirme2H5). Thesed for otherresults monocarbonyl strongly suggest complexes that withthe thermal anionic isomerizationreaction involves reaction a donor-acceptor involves a donor-acceptor interaction between interaction the solvent between and the complex.solvent and In ruthenium(II)the complex. ligands (–COR−; R− = OH, OCH3, or OC2H5). These results strongly suggest that the thermal Incomplexes, ruthenium(II) terminal complexes, CO ligands terminal that interact CO ligands with thethat donor interact solvent with tend the donor to exhibit solventνCO tend IR frequencies to exhibit isomerization reaction1 involves a donor-acceptor interaction between the solvent and the complex. νoverCO 2000IR frequencies cm− [36,37 over]. In 2000 this cm study,−1 [36,37]. the CO In stretching this study, frequency the CO ofstretching the thermally frequency isomerized of the In ruthenium(II) complexes, terminal CO1 ligands that interact1 with the donor solvent tend to exhibit acetonitrile complexes exceeded 2000− cm1 − (2008 and 2009 cm− ), while those of complexes containing νCO IR frequencies over 2000 cm 1 [36,37]. In this study, the CO stretching frequency of the an anionic ligand were 1950–1960 cm− . 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 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 complexesMolecules 2020 containing, 25, 27 an anionic ligand were 1950–1960 cm−1. It would therefore be expected9 that of 16 weak interactions between the coordinated carbonyl and the solvent (acetone or water) could induce thermalof the ruthenium isomerization. center, resultingThat is, inthe lowering interaction the activationbetween energythe coordinated for isomerization CO and of thethed -isomersolvent significantlyto the thermally changes more the stable electronicp-isomer state [38 of]. the ruthenium center, resulting in lowering the activation energy for isomerization of the d-isomer to the thermally more stable p-isomer [38].

2+ Figure 7. Thermal isomerizationisomerization of ofd -[Ru(pynp)(NˆN)(CO)(CHd-[Ru(pynp)(N^N)(CO)(CH3CN)]3CN)]2+and and the the molecular molecular structure structure of ofthe the isomerized isomerizedp-form p-form (counteranions, (counteranions, H atoms,H atoms, and and solvent solvent molecules molecules are are omitted omitted for for clarity). clarity). 3. Materials and Methods 3. Materials and Methods 3.1. General Remarks 3.1. General Remarks All chemicals were purchased from commercial sources and used without further purification 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 unless otherwise stated. All solvents used for the syntheses were anhydrous. Acetonitrile for electrochemical measurements was purified by passing through solvent purification columns (Glass electrochemical measurements was purified by passing through solvent purification columns (Glass Contour, Laguna, CA, USA). Then 2-(2-pyridyl)-1,8-naphthyridine (pynp), [Ru(CO)2Cl2]n, trans-(Cl)- Contour, Laguna, CA, USA). Then 2-(2-pyridyl)-1,8-naphthyridine (pynp), [Ru(CO)2Cl2]n, [Ru(bpy)(CO)2Cl2], trans-(Cl)-[Ru(phen)(CO)2Cl2], cis(OH2)-[Ru(bpy)(CO)2(OH2)2](CF3SO3)2, cis-(OH2)- trans-(Cl)-[Ru(bpy)(CO)2Cl2], trans-(Cl)-[Ru(phen)(CO)2Cl2], [Ru(phen)(CO)2(OH2)2](CF3SO3)2, and cis-(OH2)-[Ru(pynp)(CO)2(OH2)2](CF3SO3)2 were prepared cis(OH2)-[Ru(bpy)(CO)2(OH2)2](CF3SO3)2, cis-(OH2)-[Ru(phen)(CO)2(OH2)2](CF3SO3)2, and according to previously reported procedures [18,39,40]. cis-(OH2)-[Ru(pynp)(CO)2(OH2)2](CF3SO3)2 were prepared according to previously reported IR spectra were obtained using a JASCO FT–IR 4100 spectrometer (Tokyo, Japan). Electrospray procedures [18,39,40]. ionization mass spectrometry (ESI–MS) data were obtained using a Bruker Daltonics micrOTOF IR spectra were obtained using a JASCO FT–IR 4100 spectrometer (Tokyo, Japan). Electrospray spectrometer. Electronic spectra were obtained on a JASCO V-560 spectrophotometer. The 1H and ionization mass spectrometry (ESI–MS) data were obtained using a Bruker Daltonics micrOTOF 13C{1H}-NMR spectra were acquired on a JEOL JMN-AL300 spectrometer (Tokyo, Japan) operating at spectrometer. Electronic spectra were obtained on a JASCO V-560 spectrophotometer. The 1H and 1H and 13C frequencies of 300 and 75.5 MHz, respectively. Elemental analysis data were obtained on a 13C{1H}-NMR spectra were acquired on a JEOL JMN-AL300 spectrometer (Tokyo, Japan) operating PerkinElmer 2400II series CHN analyzer (Yokohama, Japan). Electrochemical measurements were at 1H and 13C frequencies of 300 and 75.5 MHz, respectively. Elemental analysis data were obtained performed on an electrochemical analyzer (ALS/CHI model 660E, Tokyo, Japan) with a solution of the on a PerkinElmer 2400II series CHN analyzer (Yokohama, Japan). Electrochemical measurements complex in acetonitrile (1 mM) and n-Bu4NClO4 (0.1 M) as a supporting electrolyte in a cell consisting were performed on an electrochemical analyzer (ALS/CHI model 660E, Tokyo, Japan) with a of a glassy carbon working electrode (φ = 1.6 mm), a Pt wire counter electrode, and Ag/AgNO3 (0.01 M) solution of the complex in acetonitrile (1 mM) and n-Bu4NClO4 (0.1 M) as a supporting electrolyte in as the reference electrode. All potentials are reported in volts versus the ferrocenium/ferrocene couple a cell+ consisting of a glassy carbon working electrode (ϕ = 1.6 mm), a Pt wire counter electrode, and (Fc /Fc) under Ar at 25 ◦C. DFT calculations were performed using the quantum computation software Ag/AgNO3 (0.01 M) as the reference electrode. All potentials are reported in volts versus the Gaussian 09 [41]. The geometries of the Ru complexes were fully optimized using a restricted DFT ferrocenium/ferrocene couple (Fc+/Fc) under Ar at 25 °C. DFT calculations were performed using the method employing the B3LYP function [42,43], with a 6-31G(d) basis set for the light elements (H, C, quantum computation software Gaussian 09 [41]. The geometries of the Ru complexes were fully N, and O) [44,45] and a LanL2DZ basis set [46] for the Ru atom. The solvent effect of acetonitrile optimized using a restricted DFT method employing the B3LYP function [42,43], with a 6-31G(d) was evaluated using an implicit solvent model and a polarizable continuum model. The vibrational basis set for the light elements (H, C, N, and O) [44,45] and a LanL2DZ basis set [46] for the Ru atom. analyses were performed at the same calculation level employed for geometry optimization. The solvent effect of acetonitrile was evaluated using an implicit solvent model and a polarizable continuum3.2. Synthesis model. of Complexes The vibrational analyses were performed at the same calculation level employed for geometry optimization. 3.2.1. Synthesis of the Distal Isomers ([1d](PF6)2 and [2d](PF6)2) 3.2. Synthesis of Complexes [Ru(bpy)(CO)2(OH2)2](CF3SO3)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 3.2.1. Synthesis of the Distal Isomers ([1d](PF6)2 and [2d](PF6)2) to room temperature. The reaction mixture was condensed to 3 mL under reduced pressure. A light orange[Ru(bpy)(CO) precipitate2 formed(OH2)2](CF on3 additionSO3)2 (53 ofmg, a saturated0.082 mmol) aqueous and pynp solution (20 mg, of KPF 0.0986 to mmol) the mixture. were added The productto methanol was (10 collected mL). The by mixture filtration, was washed refluxed with with cold stirring water andfor 1.5 diethyl h. The ether, reaction and vessel dried was in vacuo cooled to obtain the product (56 mg, 84%). Anal. Calcd for [1d](PF ) :C H N O F P Ru: C, 37.05; H, 2.11; N, 6 2 25 17 5 2 12 2 Molecules 2020, 25, 27 10 of 16

2+ 2+ 8.64. Found: C, 37.40; H, 2.03; N, 8.51. ESI–MS (CH3CN): m/z 260.5 ([M] ), 246.5 ([M–CO] ). IR (KBr): 1 1 2097, 2044 cm− (νCO). H-NMR (acetone-d6): δ 9.76–9.71 (m, 2H), 9.25 (d, J = 8.4 Hz, 1H), 9.09–8.99 (m, 2H), 8.77–8.71 (m, 2H), 8.64 (dd, J = 6.6, 1.8 Hz, 2H), 8.57 (td, J = 6.3, 1.5 Hz, 1H), 8.49 (dd, J = 1.8, 1.5 Hz, 1H), 8.28–8.25 (m, 2H), 8.17–8.13 (m, 1H), 7.76–7.69 (m, 2H), 7.66–7.63 (m, 1H). 13C{1H}-NMR (acetone-d6): δ 192.08 and 191.49 (CO), 158.89, 158.73, 157.02, 156.71, 156.61, 156.16, 150.19, 145.33, 143.16, 142.76, 141.93, 139.61, 131.27, 130.70, 129.41, 129.07, 128.95, 127.20, 126.18, 125.92, 125.49, 125.39, 122.23. A similar reaction between [Ru(phen)(CO)2(OH2)2](CF3SO3)2 (60 mg, 0.089 mmol) and pynp (23 mg, 0.109 mmol) gave a mixture of both [2d](PF6)2 and [2p](PF6)2. Yield: 63 mg (84%). The crude product was recrystallized from a mixture of acetonitrile, acetone, and diethyl ether. Anal. Calcd for [2d](PF6)2:C27H17N5O2F12P2Ru CH3CN 0.5C2H6CO: C, 40.50; H, 2.56; N, 9.29. Found: C, 40.38; H, · · 2+ 2+ 1 2.29; N, 9.14. ESI–MS (CH3CN): m/z 272.6 ([M] ), 258.6 ([M–CO] ). IR (KBr): 2099, 2045 cm− (νCO). 1 H-NMR (acetone-d6): δ 10.16 (dd, J = 5.1, 1.2 Hz, 1H), 9.80 (dd, J = 5.8, 0.9 Hz, 1H), 9.28 (dd, J = 7.6, 0.9 Hz, 1H), 9.18 (dd, J = 8.5, 1.2 Hz, 1H), 8.99 (s, 2H), 8.87 (dd, J = 8.3, 1.2 Hz, 1H), 8.80 (td, J = 8.1, 1.5 Hz, 1H), 8.52–8.47 (m, 2H), 8.39–8.23 (m, 4H), 8.08 (dd, J = 5.4, 1.5 Hz, 1H), 7.95 (dd, J = 8.1, 5.4 Hz, 13 1 1H), 7.60 (dd, J = 8.4, 4.5 Hz, 1H). C{ H}-NMR (acetone-d6): δ 192.33 and 191.47 (CO), 160.51, 159.49, 158.91, 157.29, 156.00, 155.95, 151.31, 147.34, 147.26, 145.17, 143.21, 141.68, 140.94, 139.41, 132.07, 132.06, 131.24, 129.16, 129.10, 128.46, 127.63, 127.59, 125.94, 125.83, 122.27.

3.2.2. Synthesis of the Proximal Isomers ([1p](PF6)2 and [2p](PF6)2)

Using a protocol similar to that described for the synthesis of [1d](PF6)2, the reaction between [Ru(pynp)(CO)2(OH2)2](CF3SO3)2 (106 mg, 0.161 mmol) and bpy (31 mg, 0.197 mmol) or phen (39 mg, 0.197 mmol) gave [1p](PF6)2 or [2p](PF6)2 in 81% and 86% yield, respectively. Anal. Calcd for [1p](PF6)2: C25H17N5O2F12P2Ru: C, 37.05; H, 2.11; N, 8.64. Found: C, 37.22; H, 2.01; N, 8.60. ESI–MS (CH3CN): 2+ 2+ 1 1 m/z 260.5 ([M] ), 246.5 ([M–CO] ). IR (KBr): 2097, 2045 cm− (νCO). H-NMR (acetone-d6): δ 9.64 (d, J = 5.1 Hz, 1H), 9.49 (d, J = 3.9 Hz, 1H), 9.36 (d, J = 8.4 Hz, 1H), 9.17–9.11 (m, 2H), 9.02–8.99 (m, 2H), 8.86 (d, J = 8.1 Hz, 1H), 8.69 (t, J = 3.8 Hz, 1H), 8.50 (t, J = 4.7 Hz, 1H), 8.35 (t, J = 4.8 Hz, 1H), 8.22–8.16 13 1 (m, 2H), 8.02 (d, J = 6.3 Hz, 1H), 7.84–7.77 (m, 2H), 7.50 (t, J = 7.4 Hz, 1H). C{ H}-NMR (acetone-d6): δ 193.12 and 192.20 (CO), 161.36, 158.14, 157.00, 156.88, 156.71, 155.91, 153.77, 151.45, 151.15, 145.42, 143.02, 142.94, 142.87, 140.57, 130.70, 130.65, 129.66, 128.19, 127.20, 126.92, 126.76, 126.03, 122.99. Anal. Calcd for [2p](PF6)2:C27H17N5O2F12P2Ru 0.5CH3CN 0.5C2H6CO: C, 40.08; H, 2.45; N, 8.72. Found: · · 2+ 2+ C, 39.90; H, 2.17; N, 8.68. ESI–MS (CH3CN): m/z 272.6 ([M] ), 258.6 ([M–CO] ). IR (KBr): 2093, 1 1 2044 cm− (νCO). H-NMR (acetone-d6): δ 10.04 (dd, J = 5.1, 1.2 Hz, 1H), 9.48 (dd, J = 3.9, 1.8 Hz, 1H), 9.40 (d, J = 8.4 Hz, 1H), 9.30 (dd, J = 8.4, 1.2 Hz, 1H), 9.18 (d, J = 8.7 Hz, 1H), 9.10 (d, J = 7.8 Hz, 1H), 9.03 (dd, J = 7.8, 1.8 Hz, 1H), 8.96 (dd, J = 8.4, 0.9 Hz, 1H), 8.56–8.38 (m, 4H), 8.23–8.17 (m, 2H), 7.88 (d, 13 1 J = 4.8 Hz, 1H), 7.82 (dd, J = 9.0, 5.4 Hz, 1H), 7.60 (dd, J = 7.5, 1.2 Hz, 1H). C{ H}-NMR (acetone-d6): δ 192.08 (CO), 161.61, 158.85, 157.05, 156.69, 153.96, 152.34, 151.63, 147.09, 146.35, 145.47, 142.88, 142.05, 141.81, 140.59, 132.99, 132.47, 130.39, 129.46, 129.38, 128.88, 128.13, 127.84, 127.19, 126.77, 123.01.

3.2.3. Synthesis of [3dp](PF6)2

A similar reaction between [Ru(pynp)(CO)2(OH2)2](CF3SO3)2 (58 mg, 0.083 mmol) and pynp (20 mg, 0.098 mmol) in methanol (20 mL) gave [3dp](PF6)2. Yield: 67 mg (94%). Anal. Calcd for [3dp](PF6)2:C28H18N6O2F12P2Ru H2O: C, 38.23; H, 2.29; N, 9.56. Found: C, 37.95; H, 1.94; N, 9.32. 2·+ 2+ 1 1 ESI–MS (CH3CN): m/z 286.1 ([M] ), 272.1 ([M–CO] ). IR (KBr): 2094, 2040 cm− (νCO). H-NMR (acetone-d6): δ 9.85 (d, J = 4.8 Hz, 1H), 9.49 (dd, J = 2.2, 1.8 Hz, 1H), 9.28 (d, J = 8.4 Hz, 2H), 9.06–9.01 (m, 3H), 8.85–8.82 (m, 2H), 8.78 (t, J = 1.5 Hz, 1H), 8.53 (dd, J = 6.6, 1.8 Hz, 1H), 8.39–8.16 (m, 3H), 7.88 (d, J = 4.8 Hz, 1H), 7.75 (t, J = 1.5 Hz, 1H), 7.66 (dd, J = 2.2, 1.8 Hz, 1H), 7.54–7.49 (m, 1H). 13C{1H}-NMR (acetone-d6): δ 192.58 and 192.27 (CO), 160.76, 160.25, 158.70, 157.76, 157.19, 155.92, 155.86, 151.01, 145.17, 144.22, 142.97, 142.70, 140.27, 139.57, 139.51, 131.42, 130.27, 129.16, 127.64, 127.58, 126.57, 126.11, 125.68, 125.63, 122.23, 122.03. Molecules 2020, 25, 27 11 of 16

3.3. Photochemical Reactions

Photochemical reactions of the complexes were conducted in the degassed CH3CN. The solution was irradiated with UV–visible light (λ = 300–400 nm) through a cutoff filter using an Asahi Spectra 2 MAX-302 (Tokyo, Japan) with a xenon lamp (0.1 mW cm− ). In a typical reaction, an acetonitrile solution of [1d](PF6)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%).

2+ d-[Ru(pynp)(bpy)(CO)(CH3CN)](PF6)2: ESI–MS (CH3CN): m/z 246.5 ([M–CH3CN] ). IR (KBr): 1 1 1 2009 cm− (νCO). Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 250 (31,200), 312 (18,500), 341 2+ (19,400), 480 (950). p-[Ru(pynp)(bpy)(CH3CN)2](PF6)2: ESI–MS (CH3CN): m/z 232.5 ([M–(CH3CN)2] ). 1 1 Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 239 (26,400), 288 (33,600), 407 (4400), 473 (6500). 2+ d-[Ru(pynp)(phen)(CO)(CH3CN)](PF6)2: ESI–MS (CH3CN): m/z 258.5 ([M–CH3CN] ). IR (KBr): 2008 1 1 1 cm− (νCO). Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 268 (33,400), 341 (16,700), 440 (1600). 2+ 2+ dp-[Ru(pynp)2(CO)(CH3CN)](PF6)2: ESI–MS (CH3CN): m/z 272.0 ([M–CH3CN] ), 292.5 ([M] ). IR 1 1 1 (KBr): 2007 cm− (νCO). Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 241 (40,200), 340 (32,000), 2+ 420sh (2600). pp-[Ru(pynp)2(CH3CN)2](PF6)2: ESI–MS (CH3CN): m/z 258.0 ([M–(CH3CN)2] ), 278.5 2+ 1 1 ([M–CH3CN] ). Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 315 (40,900), 496 (6400). Kinetic measurements for the photoreaction were followed at an appropriate wavelength for each complex (476 nm for [1d]2+, 478 nm for [1p]2+, 475 nm for [2d]2+ and [2p]2+, and 497 nm for [3dp]2+), and the logarithm of the complex concentration versus time plots were generated for the ﬁrst step of the reaction in each measurement.

3.4. Thermochemical Reactions

3.4.1. Reaction of the Coordinated CO in [1d]2+ and [2d]2+ with Solvents

The [1d](PF6)2 (50 mg, 0.0617 mmol) was dissolved in acetonitrile (5 mL) and a suitable cosolvent (water, ethanol, or methanol) (30 mL) and reﬂuxed for 24 h. The volume was reduced to 1 mL using a rotary evaporator. Addition of diethyl ether to the solution resulted in precipitation of the reaction products ([Ru(pynp)(bpy)(CO)(C(O)R)]PF6,R = OH, OC2H5, or OCH3) in moderate to high + yields (50%–82%). [Ru(pynp)(bpy)(CO)(C(O)OH)]PF6: ESI–MS (CH3CN): m/z 538.1 ([M] ). IR (KBr): 1 + 1957 cm− (νCO). [Ru(pynp)(bpy)(CO)(C(O)OC2H5)]PF6: ESI–MS (CH3CN): m/z 566.1 ([M] ). IR (KBr): 1 + 1954 cm− (νCO). [Ru(pynp)(bpy)(CO)(C(O)OCH3)]PF6: ESI–MS (CH3CN): m/z 552.1 ([M] ). IR (KBr): 1 1954 cm− (νCO).

[Ru(pynp)(phen)(CO)(C(O)R)](PF6) complexes were also isolated in 50 to 75% yields using a similar procedure starting from [2d](PF6)2. [Ru(pynp)(phen)(CO)(C(O)OH)]PF6: ESI–MS (CH3CN): m/z 562.0 + 1 ([M] ). IR (KBr): 1960 cm− (νCO). [Ru(pynp)(phen)(CO)(C(O)OC2H5)]PF6: ESI–MS (CH3CN): m/z + 1 590.2 ([M] ). IR (KBr): 1961 cm− (νCO). [Ru(pynp)(phen)(CO)(C(O)OCH3)]PF6: ESI–MS (CH3CN): + 1 m/z 576.1 ([M] ). IR (KBr): 1960 cm− (νCO).

3.4.2. Thermal-induced Isomerization Using Monocarbonyl Complexes 2+ 2+ d-[Ru(pynp)(NˆN)(CO)(CH3CN)] (NˆN = bpy or phen) and dp-[Ru(pynp)2(CO)(CH3CN)]

In a typical reaction, d-[Ru(pynp)(phen)(CO)(CH3CN)](PF6)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)(CH3CN)](PF6)2 or dp-[Ru(pynp)2(CO)(CH3CN)](PF6)2 gave the Molecules 2020, 25, 27 12 of 16 corresponding p- or pp-isomer, respectively. These products were characterized by spectroscopic and X-ray crystallographic analyses.

2+ p-[Ru(pynp)(bpy)(CO)(CH3CN)](PF6)2: ESI–MS (CH3CN): m/z 246.5 ([M–CH3CN] ). IR (KBr): 1 1 1 1992 cm− (νCO). Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 251 (31,300), 312 (19,600), 343 (21,400), 383 (4700), 490 (1200). p-[Ru(pynp)(phen)(CO)(CH3CN)](PF6)2: ESI–MS (CH3CN): m/z 258.5 2+ 1 1 1 ([M–CH3CN] ). IR (KBr): 1994 cm− (νCO). Electronic spectrum (CH3CN): λmax/nm (ε/M− cm− ) 268 (34,500), 332sh (11,000), 521 (2200). pp-[Ru(pynp)2(CO)(CH3CN)](PF6)2: ESI–MS (CH3CN): m/z 272.0 2+ 2+ 1 ([M–(CH3CN)] ), 292.5 ([M] ). IR (KBr): 1994 cm− (νCO). Electronic spectrum (CH3CN): λmax/nm 1 1 (ε/M− cm− ) 245 (31,200), 269 (26,900), 343 (28,800), 423sh (3000), 499 (1000).

3.5. X-ray Crystallographic Analyses Single crystals of [2d] (PF ) (CF SO ) H O, [2p](PF ) CH CN, [3dp](CF SO ) , and d-[Ru(pynp) 2 6 3 3 3 · 2 6 2· 3 3 3 2 (phen)(CO)(CH3CN)](PF6)2 were obtained by vapor diffusion of diethyl ether into an acetonitrile/acetone solution of the complex. p-[Ru(pynp)(phen)(CO)(CH CN)](PF ) CH OH crystals 3 6 2· 3 were obtained by vapor diffusion of diethyl ether into an acetonitrile/acetone/methanol solution of the complex. [Ru(pynp)(CO) (OH )(CH CN)](CF SO ) C H OH crystals were obtained by vapor 2 2 3 3 3 2· 2 5 diffusion of diethyl ether into an acetonitrile/acetone/ethanol solution of the complex. Single crystals of dp-[Ru(pynp) (CO)(CH CN)](PF ) (CH ) CO and [Ru(pynp)(bpy)(CO)(C(O)OH)]PF (CH ) CO 2 3 6 2· 3 2 6· 3 2 were obtained by vapor diffusion of diethyl ether into an acetone solution of the complex. Single crystals of pp-[Ru(pynp)2(CH3CN)2](PF6)2 were obtained by vapor diffusion of diethyl ether into an acetonitrile solution of the complex. Single crystals of d-[Ru(pynp)(bpy)(CO)(C(O)OC H )]PF (CH ) CO 2 5 6· 3 2 and d-[Ru(pynp)(phen)(CO)(C(O)OC2H5)]CF3SO3 were obtained by layering diethyl ether over an acetone/ethanol solution of the complex. Crystal structure determination and refinement data for the complexes are given in Figures1,2 and5–7, Supplementary Materials Figures S1, S5 and S6, and Tables4–6 and Supplementary Materials Table S1. CCDC-1966877–1966887 contains the supplementary crystallographic data for this paper.

Table 4. Crystallographic data for dicarbonyl complexes.

Parameter [2d] [2p] [3dp]

Chemical formula C55H34F21N10O8P3Ru2S C29H20F12N6O2P2Ru C30H18F6N6O8RuS2 Formula weight 1689.02 875.51 869.69 Temperature (K) 93 93 93 Crystal system monoclinic triclinic triclinic Space group P21/c P-1 P-1 a (Å) 13.2730(3) 8.57310(10) 9.68729(18) b (Å) 22.5773(6) 12.7558(3) 13.0592(3) c (Å) 20.2047(5) 15.8850(4) 13.6489(3) α (◦) 90 82.856(6) 83.1931(7) β (◦) 93.7571(8) 75.919(6) 81.5465(7) γ (◦) 90 74.843(6) 72.0749(7) V (Å3) 6041.7(3) 1622.90(8) 1620.10(6) Z 4 2 2 Calcd density (g/cm3) 1.857 1.792 1.783 µ (Mo Kα) (mm–1) 0.744 0.691 0.709 No. unique reflns 62,024 16,768 16,909 No. obsd reflns 13,818 7331 7304 Refinement method Full-matrix least-squares on F2 Parameters 901 470 478 R (I > 2σ(I)) 1 0.0941 0.0552 0.1119 wR (all data) 2 0.2536 0.1483 0.2633 S 1.155 1.077 1.189 1 2 2 2 2 2 2 1/2 R = Σ(||Fo| |Fc||)/Σ|Fo|; wR = {Σw(Fo Fc ) /Σw(Fo ) } . − − Molecules 2020, 25, 27 13 of 16

2+ Table 5. Crystallographic data for [Ru(pynp)(NˆN)(CO)(CH3CN)] (NˆN = phen or pynp).

Parameter d-(phen) p-(phen) dp-(pynp)

Chemical formula C28H20F12N6OP2Ru C29H20F12N6O2P2Ru C32H27F12N7O2P2Ru Formula weight 847.50 875.51 932.61 Temperature (K) 93 93 93 Crystal system orthorhombic triclinic monoclinic Space group Pna21 P-1 P21/n a (Å) 13.8734(9) 11.9822(4) 10.067(4) b (Å) 24.7954(15) 12.5252(3) 32.588(12) c (Å) 9.0869(5) 13.2975(4) 11.329(5) α (◦) 90 84.1114(11) 90 β (◦) 90 87.5838(8) 109.861(5) γ (◦) 90 63.326(2) 90 V (Å3) 3125.9(3) 1773.87(9) 3496(2) Z 4 2 4 Calcd density (g/cm3) 1.801 1.639 1.772 1 µ (Mo Kα) (mm− ) 0.712 0.632 0.648 No. unique reflns 30,815 18,631 35,054 No. obsd reflns 7100 8064 7963 Refinement method Full-matrix least-squares on F2 Parameters 452 436 508 R (I > 2σ(I)) 1 0.0506 0.0832 0.0745 wR (all data) 2 0.1237 0.2503 0.1623 S 1.028 1.049 1.113 1 2 2 2 2 2 2 1/2 R = Σ(||Fo| |Fc||)/Σ|Fo|; wR = {Σw(Fo Fc ) /Σw(Fo ) } . − − + Table 6. Crystallographic data for [Ru(pynp)(bpy)(CO)(COR)] (R = OH or OC2H5).

Parameter R = OH R = OC2H5

Chemical formula C28H24F6N5O4PRu C30H28F6N5O4PRu Formula weight 740.56 768.62 Temperature (K) 93 93 Crystal system triclinic monoclinic Space group P-1 P21/n a (Å) 8.35369(15) 17.2474(5) b (Å) 12.9208(3) 8.1493(2) c (Å) 13.3343(3) 22.1009(6) α (◦) 88.9616(7) 90 β (◦) 79.0700(7) 102.3249(8) γ (◦) 88.2204(7) 90 V (Å3) 1412.36(5) 3034.77(14) Z 2 4 Calcd density (g/cm3) 1.741 1.682 1 µ (Mo Kα) (mm− ) 0.697 0.652 No. unique reflns 12,245 30,159 No. obsd reflns 4934 6960 Refinement method Full-matrix least-squares on F2 Parameters 406 463 R (I > 2σ(I)) 1 0.0583 0.0475 wR (all data) 2 0.1331 0.1132 S 1.219 1.067 1 2 2 2 2 2 2 1/2 R = Σ(||Fo| |Fc||)/Σ|Fo|; wR = {Σw(Fo Fc ) /Σw(Fo ) } . − − Molecules 2020, 25, 27 14 of 16

4. Conclusions This study successfully formed non-equivalent CO-ligand environments by lowering the 2+ molecular symmetry of the well-known benchmark complex, [Ru(bpy)2(CO)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.

Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/25/1/27/s1, 2+ Figure S1: Molecular structure of [Ru(pynp)(CO)2(OH2)(CH3CN)] , Figure S2: Optimized structures of the monosubstituted precursor of [3]2+ with the electronic energy difference, Figure S3: Electronic spectra of [1d]2+,[1p]2+,[2d]2+,[2d]2+, and [3dp]2+, Figure S4: Cyclic voltammograms of [2p]2+ at various scan rates, 2+ Figure S5: Molecular structure of [Ru(pynp)2(CH3CN)2] , Figure S6: Molecular structure of [Ru(pynp)(phen)(CO) + 2+ 2+ (C(O)OC2H5)] , Table S1: Crystallographic data for [Ru(pynp)(CO)2(OH2)(CH3CN)] , [Ru(pynp)2(CH3CN)2] , + and [Ru(pynp)(phen)(CO)(C(O)OC2H5)] . Author Contributions: Syntheses and characterization of compounds, K.A. and R.A.; investigation and data analysis, K.A. and R.A.; crystallography and computational analysis, T.T.; writing—original draft preparation, K.A.; writing—review and editing, D.O.; supervision and funding acquisition, D.O. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by JSPS KAKENHI, grant number JP17K05799. Acknowledgments: We thank Ryota Kimura at Fukushima University for his experimental assistance at an early stage of the project. We would like to thank Editage (www.editage.com) for English language editing. Conflicts of Interest: The authors declare no conflict of interest.

References and Notes

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Sample Availability: Samples of the compounds[1d]2+,[1p]2+,[2d]2+,[2p]2+, and [3dp]2+ are available from the authors.

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