Facile and Selective Synthetic Approach for Ruthenium Complexes Utilizing a Molecular Sieve Effect in the Supporting Ligand

It is extremely important for synthetic chemists to control the structure of new compounds. We have constructed ruthenium-based mononuclear complexes with the tridentate 2,6-di(1,8-naphthyridin-2-yl)pyridine (dnp) ligand to investigate a new synthetic approach using a specific coordination space. The synthesis of a family of new ruthenium complexes containing both the dnp and triphenylphosphine (PPh 3) ligands, [Ru(dnp)(PPh 3)(X)(L)] −), has been described. All complexes have been spectroscopically characterized in solution, and the nitrile complexes have also been characterized in the solid state through single-crystal X-ray diffraction analysis. Dnp in the present complex system behaves like a " molecular sieve " in ligand replacement reactions. Both experimental data and density functional theory (DFT) calculations suggest that dnp plays a crucial role in the selectivity observed in this study. The results provide useful information toward elucidating this facile and selective synthetic approach to new transition metal complexes.


Introduction
A more accurate regulation of the structure leads to the construction of appropriate reaction systems.For example, some isomeric pairs of [Ru(tpy)(L)Cl] n+ type complexes (tpy = 2,2':6',2''terpyridine, L = asymmetrical pyridyl-based bidentate ligands; quinaldic acid (qu) and 2-(1-methyl-3pyrazolyl)pyridine (pypz-Me)) have been prepared and structurally characterized as precatalysts to investigate the effect of isomeric structural features on the catalytic epoxidation process [1,2].In the complex [Ru(tpy)(qu)(OH 2 )] + , the cis isomer has been established to be an excellent catalyst for the chemoselective epoxidation through limited differences in electronic structural features exist between the isomeric pair [1].In the complex [Ru(tpy)(pypz-Me)(OH 2 )] 2+ , on the other hand, the trans isomer exhibits better activity because it contains a pyridine C-H bond nearly parallel to the Ru-O bond, whereas for the cis isomer this position is occupied by a C-CH 3 group and thus exerts a much stronger steric effect [2].These examples indicate that it is important to clearly distinguish the molecular structures of the compounds.Therefore, much attention has been paid to study both facile and selective synthesis for complex compounds.
In an effort to discover new molecular systems, we have recently reported ruthenium-based mononuclear complexes with the pyridyl-bridged di(1,8-naphthyridin-2-yl) (dnp) ligand [3].The tridentate dnp has two noncoordinating nitrogen atoms and is able to form intramolecular hydrogen bonds between the coordinated water and dnp.In addition, dnp has a narrower coordination space on the equatorial site than the parent ligand, tpy (Chart 1).Therefore, size complementarity of the entering molecule with the coordination space of dnp plays an important role and is apparent in size-selective recognition of entering molecules such as nitriles and halides.We report that dnp in the present complex system behaves like a "molecular sieve" in ligand replacements and it leads to a facile and selective synthesis of a new family of ruthenium complexes.

Synthesis of the Precursor
Dnp was prepared by the Friedländer condensation of two equivalents of 2-aminonicotinaldehyde with 2,6-diacetylpyridine according to procedures that have been reported earlier [4,5].Precursor 1 was synthesized by treating [RuCl 2 (CH 3 CN) 2 (PPh 3 ) 2 ] [6] with dnp in an ethanolic solution (Scheme 1).The analogous complexes containing tpy initially formed chlorido complexes, which can only be hydrated with the assistance of the silver ion to cleave the Ru-Cl bond [7,8].However, the present system spontaneously hydrates under the reaction conditions.Dnp has two additional fused pyridine rings attached to two side-coordinating pyridines of tpy.These rings presumably promote hydration both by electronic repulsion of the chloride leaving group and stabilization of the entering water through hydrogen bonds.
Scheme 1. Preparation of the precursor.
1 was characterized by both 1 H and 31 P{ 1 H} nuclear magnetic resonance (NMR) spectroscopy in acetone-d 6 , and the resulting spectra showed well-resolved signals.A signal observed at 8.23 ppm disappeared on the addition of D 2 O, which could be attributed to the protons of the aqua ligand [9].The 31 P{ 1 H} NMR spectrum of 1 showed a singlet at 27.29 ppm, which indicates two triphenylphosphine (PPh 3 ) ligands situated trans to each other.The crystal structure of 1 by single-crystal X-ray analysis also supports this result in solution [3].

Synthesis of Complexes
In dnp, the coordination space of the equatorial position is narrower than that of the similar tpy.By using this steric feature, it is possible to impart selectivity when replacing the labile aqua ligand with another one.The aqua ligand of 1 was replaced with the rodlike molecule CH 3 CN at room temperature to give the corresponding 2 (acetonitrile complex) (Figure 1a).In addition, 3 (benzonitrile (PhCN)-bound complex) formed in the reaction with PhCN, though PhCN contains a bulky phenyl group.It was confirmed by both X-ray structural and various spectroscopic analyses that the aqua ligand of 1 can be replaced by nitriles (see below).When a substitution reaction was performed using the linear thiocyanate anion (SCN − ) to investigate the effect of electronic repulsion between dnp and incoming ligands, corresponding 4 (SCN-coordinated complex) was obtained.Based on 1 H NMR and Electrospray ionization mass spectrometry (ESI-MS) analyses, the SCN ligand in 4 was observed to be coplanar with dnp.It is well-known that SCN − is an ambidentate ligand.Although we could not identify the coordination atom from the infrared spectrum data (ν N≡C = 2134 cm −1 ) of 4, SCN − would likely coordinate with its smaller (relative to sulfur) nitrogen atom in 4 because SCN − was coordinated selectively to the ruthenium center at the nitrogen atom in a similar complex containing dnp [10].
On the other hand, the aqua ligand of 1 did not exchange with the nitrite ion (NO 2

−
), which is another ambidentate ligand; instead, the axial ligand (PPh 3 ) underwent substitution to produce 5 (Figure 1b).Two characteristic bands in the fingerprint region (1438 and 1304 cm −1 ) from the IR (infrared) spectrum of 5 suggest that 5 is a nitrogen atom-coordinated nitro species [11,12].In addition to NO 2 − , 1 reacted with halide ions Cl − and Br − to give the axial ligand-substituted 6 (chlorido complex) and 7 (bromido complex), respectively.It was confirmed by both 1 H NMR and mass spectral analyses that complexes 5-7 each have one PPh 3 ligand and retain the aqua ligand.Reaction of the acetonitrile-substituted complex with chloride (i.e., 2 and Cl − ) and the chlorido complex with acetonitrile (i.e., 6 and CH 3 CN) both lead to 8 (double-substituted complex) (Figure 1c).To summarize, the synthetic reactions using 1 are classified into two groups based on the combination of size selectivity of dnp and complementarity of size and shape of the incorporated molecule.The ligand having both linear molecular shape and smaller coordination atoms (the second period elements) incorporates into the coordination space of dnp to replace the coordinated water molecule (Figure 1a).On the other hand, nonlinear molecules or the third or later period elements are unable to approach the coordination space of dnp.As a result, an axial ligand, rather than the aqua ligand, is involved in the substitution reaction (Figure 1b).Thus, the supporting dnp acts as a so-called "molecular sieve."

Electronic Absorption Spectra
The electronic absorption spectral data for the series complexes are summarized in Table 1.The complexes exhibit intense high-energy d→π* metal-to-ligand charge transfer (MLCT) and π→π* intraligand transitions in the UV region.The weak UV absorptions observed in the 310-360 nm range have been assigned to unresolved ligand-field d→d transitions on the ruthenium(II) center.In addition, the stronger visible transitions in the 370-600 nm range have been assigned to low-energy MLCT transitions [13].Dnp may be considered as a dipyrido-fused analogue of tpy, which is both more delocalized and more electronegative [14].As a result, dnp is a much better charge acceptor.This difference is evidenced by the absorption maxima of 8 and the corresponding terpyridine analogue ([Ru(tpy)(PPh 3 )(CH 3 CN)Cl] + ), where the latter (tpy-complex) MLCT band is at 466 nm [15], and 8 absorbs at 563 nm.It is worth noting that the more planar system gives rise to a better delocalization and consequently exhibits lower energy absorption.
Figure 2 shows the visible absorption spectra of [Ru(dnp)(PPh 3 )(OH 2 )X] n+ -type aqua complexes (X = PPh 3 : 1, NO 2 : 5, Cl: 6, and Br: 7) in acetone solution.The MLCT bands of anion-bound complexes (5, 6, and 7) are red-shifted with regard to the corresponding 1 (phosphine complex) because of the relative destabilization of dπ(Ru) levels caused by the anionic ligands.The relative shifts in the MLCT transitions to lower energies (X = Br − < Cl − < NO 2 − < PPh 3 ) follow the spectrochemical series.Ligand variation has altered the MLCT band energies, suggesting that substitution of these ligands moves the dπ orbital energies of the ruthenium(II) metal center.

Molecular Structures
Single-crystal X-ray analyses of the nitrile complexes (2, 3, and 8) have been conducted to understand the geometry of series complexes in detail.The molecular structure of 1 has been reported elsewhere [3].Selected parameters are summarized in Table 2, and the ORTEP diagrams for these cations are shown in Figures 3-5.All of the Ru-N distances of dnp are in the range 2.118(2)-2.167(6)Å with the exception of the central Ru-N bond, which is shorter (1.9660( 14)-2.007(7)Å), as expected (Table 2).The phosphine ligands are in axial positions with respect to the tridentate dnp.The Ru-P bond distances of bis-phosphine complexes (2.4306(7) Å for 2, 2.430(2) and 2.436(2) Å for 3) are longer than that of the 2.3272(7) Å distance in 8, which has a trans chlorido ligand.However, these distances are in the range previously observed in Ru(II)-phosphine complexes [15][16][17][18].The nitrile units (N≡CR) are essentially linear and the Ru-N, N-C, and C-C bond distances are similar to typical values in other Ru(II)-N≡CR complexes [15].The phenyl ring of benzonitrile in 3 is nearly coplanar with dnp; the dihedral angle between the central pyridyl ring and the phenyl ring is 13.0(4)°.This geometry is attributed to steric hindrance between the phenyl rings of PPh 3 and PhCN.In addition, the bond distance of Ru-Cl (2.4412(7) Å) in 8 is within the typical range for Ru(II)-chlorido complexes bearing tridentate pyridyl ligands [15].A remarkable structural feature is that the interatomic distances between noncoordinating nitrogen atoms of dnp are short.As described above, the complex system contains dnp that has two naphthyridine moieties on both sides of pyridine.The additional naphthyridine moieties could cause significant steric hindrance for Cl − binding in 1 (or 2).Thus, the presence of dnp seems to influence the selective coordination to the equatorial position.To examine this steric effect, density functional theory (DFT) calculations have been performed on 8 (and its geometrical isomer 8′).The DFT-optimized structure for 8 is similar to its experimental structure.For example, for 8, the calculated Ru-N and N-C bond distances and the angle of N-C-C in the acetonitrile ligand are 2.09 Å, 1.15 Å, and 179.5°, respectively, which are comparable to the experimental values (Table 2).Figure 6 depicts the optimized structures of 8 and 8′ and demonstrates greater steric hindrance at the equatorial ligand plane of 8′ than 8.There is a large calculated energy difference between 8 and 8′. 8 prefers trans-P, Cl geometry, which is 14.1 kcal/mol lower than that of 8′.As shown in Figure 6, this geometrical preference is due to the narrower coordination space generated by the naphthyridine moieties in 8, which prefers a linear CH 3 CN coordination.This calculated result is strongly in accord with the fact that 8′ does not experimentally form.
Elemental analysis data were obtained on a Perkin Elmer 2400II series CHN analyzer. 1H and 31 P{ 1 H} NMR spectra were recorded on a JEOL JMN-AL300 spectrometer (25 °C) operating at 1 H and 31 P frequencies of 300 and 121 MHz, respectively.ESI-MS data were obtained on a Bruker Daltonics microTOF.Electronic absorption spectra were obtained in 1-cm quartz cuvettes on a JASCO V-570 UV/VIS/NIR spectrophotometer.IR spectra were obtained using the KBr pellet method with a JASCO FT-IR 4100 spectrometer.DFT calculations were performed on 8 and 8′.The ground-state, gas-phase structures of these complexes were optimized at the DFT level (B3LYP) [19,20].The DGDZVP basis set was used for Ru atoms [21,22].The 6-31G(d) basis set was employed for H, C, N, P, and Cl atoms [23,24].A vibrational analysis was carried out to confirm the optimized geometry is a true minimum with no imaginary frequency.All of the calculations were performed using the Gaussian 09W program package [25].

X-ray Crystallographic Analyses
Single crystals of the perchlorate salt of 2 and 8•2H 2 O were obtained by diffusion of diethyl ether into an acetonitrile solution of each of the complexes.Single crystals of 3 were obtained by diffusion of diethyl ether into a benzonitrile solution of the complex.An orange crystal of 2 with dimensions 0.30 × 0.30 × 0.15 mm, an orange crystal of 3 with dimensions 0.20 × 0.10 × 0.05 mm, and a deep violet crystal of 8•2H 2 O with dimensions 0.30 × 0.25 × 0.25 mm were mounted on a glass fiber.Measurements were made on a Rigaku RAXIS-RAPID diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71075 Å).Data were collected to a maximum 2θ value of 55°.All calculations were conducted using the CrystalStructure crystallographic software package [27] except for refinement, which was performed using SHELXL97 [28].The structures were solved by direct methods [29,30] and were expanded using Fourier techniques.Multi-scan absorption corrections were applied [31].The non-hydrogen atoms were refined anisotropically, and hydrogen atoms were included as raiding atoms.Crystallographic parameters of 2, 3, and 8•2H 2 O are summarized in Table 3

Conclusions
We have described the synthesis and X-ray crystallographic characterization of a new ruthenium complex system containing the highly functional dnp ligand.Computational analysis suggests that the incorporation of dnp not only stabilizes the complex but also enables this system to be synthesized with improved stereospecificity.The results provide useful information to elucidate this facile and selective synthetic approach to new transition metal complexes.For example, regulation of linkage isomerization in ruthenium complexes involving SCN − could contribute to materials science in the development of photosensitizers for dye-sensitized solar cells (DSCs).

Figure 3 .
Figure 3. Molecular structure of the cation in complex 2. Counteranions and hydrogen atoms are omitted for clarity.

Figure 4 .
Figure 4. Molecular structure of the cation in complex 3. Counteranions and hydrogen atoms are omitted for clarity.

Figure 5 .
Figure 5. Molecular structure of the cation in complex 8.The counteranion, solvent molecules, and hydrogen atoms are omitted for clarity.

Figure 6 .
Figure 6.Space-filling models of the lowest energy structures of geometrical isomers (8 and 8′) with the electronic energy difference.
aThe central pyridyl ring of dnp vs. the phenyl ring of benzonitrile.