Influence of nitrosyl coordination on the binding mode of quinaldate in selective ruthenium frameworks. Electronic structure and reactivity aspects

Abstract

The nitrosyl complexes, [Ru^II(trpy)(L)(NO⁺)Cl]BF₄, [1]BF₄, and [Ru^II(trpy)(L)(NO⁺)](BF₄)₂, [2](BF₄)₂, (trpy = 2,2′:6′,2′′-terpyridine, L⁻ = deprotonated form of unsymmetrical quinaldic acid) have been synthesized. Single crystal X-ray structures of [1]BF₄ and [2](BF₄)₂ reveal that in the former L⁻ binds to the ruthenium ion selectively in a monodentate fashion through the O⁻ donor whereas the usual bidentate mode of L⁻ (O⁻, N donors) has been retained in [2](BF₄)₂ with the same meridional configuration of trpy being seen in both. The Ru–NO group in [1]BF₄ or [2](BF₄)₂, exhibits almost linear (sp-hybridized form of NO⁺) geometry. The difference in bonding mode of the unsymmetrical quinaldate in [1]BF₄ and [2](BF₄)₂ has been reflected in their corresponding ν(NO)/ν(C=O) frequencies as well as in their NO based two-step reduction processes, {Ru^II–NO⁺} → {Ru^II–NO^•} and {Ru^II–NO^•}→{Ru^II–NO⁻}. The close to bent geometry (sp²-hybridized form of NO^•) of the one-electron reduced 1 or [2]⁺ is been reflected in their DFT optimized structures. The spin density plot of the reduced species reveals that NO is the primary spin-bearing center with slight delocalization onto the metal ion which has been reflected in its radical EPR spectrum. [1]⁺ and [2]²⁺ undergo facile photorelease of NO with significantly different k_NO (s⁻¹) and t_1/2 (s) values which eventually lead to the concomitant formation of the corresponding solvent species. The photoreleased NO^• can be trapped as an Mb–NO adduct. The reduced species 1 selectively reacts with the molecular oxygen (O₂) at pH ∼ 1 to yield the corresponding nitro species, [Ru^II(trpy)(L)(NO₂)Cl]⁻.

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Introduction

The important role of nitric oxide (NO^•) in various biological and physiological processes, such as blood pressure regulation, inflammatory response, and apoptosis has drawn renewed research interest on nitrosyl chemistry in recent years.^1–7 NO^• is generated in biological processes by nitric oxide synthase (NOS) during the oxidation of L-arginine to citrulline which plays important roles in neurotransmission, smooth muscle vasodilation and platelet disaggregation in mammals.⁶ The interaction of NO with the metal ions is important from the broader perspective of inorganic chemistry including in bioinorganic chemistry.^4,5 For example, the reactions of NO^• with oxygenated heme-protein have considerable biological significance.⁵ Though some organic nitrites, nitrates and S-nitrsothiols have medicinal application as vasodilators,² they are not suitable for site specific NO^• delivery processes. However, transition metal–nitrosyl complexes have shown potential application for site-specific NO^•-delivery primarily due to photolabile nature of the metal–nitrosyl bond.^8–10 Among them, ruthenium nitrosyl complexes have been emerged as a promising class of NO-donor due to their reduced activity towards oxygen and stability in water.^8,9,11 Besides that, the non-innocent feature of NO facilitates its accessibility in three different redox states, strongly electrophilic NO⁺, neutral NO^• and anionic NO⁻, in transition metal complexes depending on the electronic environment around the metal center which in turn makes the nitrosyl function as a versatile ligand in co-ordination chemistry.^8,11 Further, ruthenium–nitrosyl complexes have shown considerable applications in pharmaceuticals, catalysis, molecular electronics and photochemical devices.¹²

In this context the present article describes the various aspects of the coordinated nitrosyl function in the newly designed selective molecular frameworks of [Ru^II(trpy)(L)(NO)Cl]ⁿ, [1]ⁿ, and [Ru^II(trpy)(L)(NO)]ⁿ, [2]ⁿ, (trpy = 2,2′:6′,2′′-terpyridine, L⁻ = deprotonated form of unsymmetrical quinaldic acid). Herein we report the synthesis and structural characterization of [1]BF₄ and [2](BF₄)₂. Furthermore, the effect of structural diversity of the co-ligand, L⁻ (monodentate versus bidentate) on the electronic structures of [1]ⁿ and [2]ⁿ with special reference to electrophilicity, redox potential and reactivity of the coordinated nitosyl function have been investigated by experimental studies and DFT calculations.

Results and discussion

Synthesis, characterization and structural aspects

The complexes with Enemark–Feltham notation¹³ of {Ru(NO)}⁶ in [Ru^II(trpy)(L)(NO⁺)Cl](BF₄), [1]BF₄, and [Ru^II(trpy)(L)(NO⁺)](BF₄)₂, [2](BF₄)₂, have been synthesized via the direct reaction of NOBF₄ with the previously structurally characterized precursor complex [Ru^II(trpy)(L)Cl] (A)¹⁴ and the reaction of NOBF₄ with the in situ generated [Ru^II(trpy)(L)(C₂H₅OH)]⁺, respectively, as shown in Scheme 1 (trpy = 2,2′:6′,2′′-terpyridine, L⁻ = deprotonated form of unsymmetrical quinaldic acid, HL).^15,16

Scheme 1 The synthetic outline for [1]BF₄ and [2](BF₄)₂.

[1]BF₄ and [2](BF₄)₂ exhibit satisfactory microanalytical and mass spectral data (Fig. S1†) and show 1 : 1 and 1 : 2 molar conductivities in acetonitrile solution, respectively (see Experimental Section).

The formation of [1]BF₄ and [2](BF₄)₂ have been authenticated by their single crystal X-ray structures (Fig. 1 and Table 1). The five-membered chelate ring of L⁻ (through the O1⁻ and N1 donors of L⁻) in the precursor A has been retained in [2](BF₄)₂ along with the usual meridional configuration of trpy.¹⁴ However, the direct nitrosylation of A (Scheme 1) surprisingly leads to the concomitant transformation of the bidentate (O⁻, N donors) mode of L⁻ to the monodentate (O⁻ donor) in [1]BF₄ retaining the same meridional configuration of trpy.

Fig. 1 ORTEP diagrams of (a) [1]BF₄·5H₂O and (b) [2](BF₄)₂. Thermal ellipsoids are drawn at 50% probability. The solvents of crystallization, counter anions and hydrogen atoms are omitted for clarity.

Table 1 Selected crystallographic parameters

	[1]BF4·5H2O	[2](BF4)2
Empirical formula	C25H17B1Cl1F4N5O8Ru	C25H17B2F8N5O3Ru
Mr	738.77	710.13
Crystal system	Triclinic	Triclinic
Space group	P1̄	P1̄
a/Å	8.5160(3)	8.4478(2)
b/Å	10.1858(4)	8.9532(2)
c/Å	16.7736(6)	17.6431(4)
α (°)	92.631(3)	94.079(2)
β (°)	102.713(3)	91.367(2)
γ (°)	93.147(3)	98.405(2)
V/Å^3	1414.66(9)	1315.96(5)
Z	2	2
μ/mm^−1	0.733	0.693
T/K	150(2) K	120(2)
D c/g cm^−3	1.734	1.792
F(000)	736	704
2θ range (°)	6.54 to 50	6.58 to 50
data/restraints/parameters	4952/0/434	4622/0/397
R 1, wR2 [I > 2σ]	0.0698, 0.2092	0.0396, 0.1046
R 1, wR2 (all data)	0.0795, 0.2136	0.0435, 0.1067
GOF	1.136	1.061
largest diff. peak, hole/e Å^−3	2.399 and −0.755	1.307 and −0.889

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The bond distances and bond angles in [1]BF₄ and [2](BF₄)₂ (Table 2) are in good agreement with the reported data of analogous complexes.^8,9,11 The geometrical constraint due to the meridional mode of trpy has been reflected in the appreciably smaller trans angles involving the trpy ligand, N2–Ru–N4 of 156.5(3)° and 159.35(12)° in [1]BF₄ and [2](BF₄)₂, respectively. The central Ru–N3(trpy) bond lengths of 2.015(6) Å in [1]BF₄ and 1.981(3) Å in [2](BF₄)₂ are significantly shorter than the corresponding distances involving the terminal pyridine rings of trpy, Ru–N2(trpy), 2.083(6) Å and 2.075(3) Å, and Ru–N4(trpy), 2.079(6) Å and 2.079(3) Å, respectively. The central Ru–N3(trpy) distance in [1]BF₄ is 0.034 Å longer than that in [2](BF₄)₂ due to the effect of the strongly π-accepting NO⁺trans to the Ru–N3(trpy). The Ru–O⁻(monodentate L⁻) bond distance in [1]BF₄ (2.044(5) Å) is appreciably longer relative to Ru–O⁻(chelated L⁻) in [2](BF₄)₂ (1.990(2) Å) due to the effect of oppositely directed σ/π-donor Cl⁻versus the strongly π-accepting NO⁺trans to the Ru–O⁻(L⁻) bond. The impact of varying coordination situations in the complexes has been reflected in their trans angles: O1–Ru–Cl, 171.85(15)° and N3–Ru–N5, 175.6(3)° in [1]BF₄versus O1–Ru–N5, 176.43(12)° and N(3)–Ru–N(1), 163.12(12)° in [2](BF₄)₂. The almost linear mode of Ru–N5–O3 (NO), 173.0(8)° in [1]BF₄ and 175.9(3)° [2](BF₄)₂ as well as the triple bond feature of N5≡O3 (NO) (1.141(9) Å in [1]BF₄ and 1.130(4) Å in [2](BF₄)₂) establish the sp-hybridized state of the nitrogen atom of the NO⁺ group as expected from the coordinated π-accepting nitrosonium ion. However, the presence of the π-accepting trpy ligand (N3) trans to Ru–N5–O3 in [1]BF₄ makes the Ru–N–O angle (173.0(8)°) relatively more tilted compared to that in [2](BF₄)₂ (175.9(3)°) where the Ru–N–O group is trans to the σ-donating O⁻ of the chelated L⁻. The pendant quinaldate ring in [1]BF₄ is slightly tilted (about 96°) with respect to the equatorial plane. The Ru–O1–C1 angle of 118.9(5)° implies that the bound oxygen (O1) is close to the sp²-hybridized state which in turn introduces a resonating feature of the carboxylate group (O1–C1, 1.276(9) Å and C1–O2, 1.242(10) Å) in [1]BF₄ (Table 2).

Table 2 Selected bond distances and bond angles in [1]BF₄ and [2](BF₄)₂

Bond distance (Å)/Bond angle (°)	[1]^+		[2]^2+
	X-Ray	DFT	X-Ray	DFT
Ru–N1	—	—	2.116(3)	2.19
Ru–N2	2.083(6)	2.11	2.075(3)	2.14
Ru–N3	2.015(6)	2.03	1.981(3)	2.02
Ru–N4	2.079(6)	2.11	2.079(3)	2.13
Ru–N5	1.754(7)	1.79	1.758(3)	1.80
Ru–O1	2.044(5)	2.05	1.990(2)	1.97
Ru–Cl	2.361(2)	2.44	—	—
O(1)–C1	1.276(9)	1.30	1.317(4)	1.34
O(2)–C1	1.242(10)	1.24	1.202(5)	1.21
N(5)–O3	1.141(9)	1.14	1.130(4)	1.15
N1–Ru–N3	—	—	163.12(12)	160.82
N2–Ru–N4	156.5(3)	156.32	159.35(12)	158.22
O1–Ru–N5	98.3(3)	97.80	176.43(12)	175.03
N3–Ru–N5	175.6(3)	175.95	94.83(13)	94.98
Cl–Ru–O1	171.85(15)	171.59	—	—
N5–Ru–N2	100.1(3)	100.80	96.49(13)	90.86
N5–Ru–N3	175.6(3)	175.95	94.83(13)	94.98
N5–Ru–N4	103.4(3)	102.88	91.29(13)	95.83
N5–Ru–N1	—	—	101.82(12)	104.18
N1–Ru–N4	—	—	96.52(11)	99.17
N3–Ru–O1	85.8(2)	86.11	84.26(11)	81.61
Ru–N5–O3	173.0(8)	170.25	175.9(3)	174.02
Ru-1-C1	118.9(5)	117.21	116.6(2)	119.15

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The DFT calculated bond parameters (Table 2) based on the optimized structures of [1]⁺ and [2]²⁺ (Fig. S2†) are in general agreement with the X-ray data.

The corresponding one-electron reduced species with {Ru(NO)}⁷ configuration in [Ru^II(trpy)(L)(NO^•)Cl] (1) and [Ru^II(trpy)(L)(NO^•)]⁺ ([2]⁺) can be generated electrochemically.^8,11 The DFT calculated structural parameters of [1]⁺/1 and [2]²⁺/[2]⁺ (Fig. 2 and Table S1†) reveal similar structural differences between the two sets of complexes based on the electronic environment around the {Ru–NO} groups. The σ-donating ability of the carboxylate oxygen (O1) of L⁻ makes the Ru–N5–O3 angle relatively more bent in [2]⁺ (139.6°) than in 1 (141.85°) where the Ru–NO group is situated trans to the π-accepting trpy (N3) ligand (Fig. 3). The difference in calculated Ru–N5–O3 angles between [2]²⁺ and [2]⁺ of ∼35° is reasonably larger than that between [1]⁺ and 1 (∼29°) implying greater electron density on N(5) in [2]⁺ as has also been revealed by the NBO studies (Table 3). The double bond feature of N5–O3 (NO), 1.176 Å and 1.18 Å in 1 and [2]⁺, respectively, suggests the sp² character of the nitrogen atom. The lengthening of the Ru–N5 bond upon NO based reduction, 0.14 Å for [1]⁺/1 and 0.12 Å for [2]²⁺/[2]⁺ due to increasing electron–electron repulsion upon addition of an electron is in agreement with the established concept of labilization of the Ru–NO bond on reduction.^11h

Fig. 2 The DFT optimized geometry of (a) 1 and (b) [2]⁺. The hydrogen atoms are omitted for clarity.

Fig. 3 Schematic representation of 1 and [2]⁺.

Table 3 The detailed NBO results of {Ru(NO)}ⁿ, (n = 6,7)

	Total atomic charge				Natural charge
	Ru	N(5)	O(3)	Cl	Ru	N(5)	O(3)	Cl
[1]^+	0.733	0.135	−0.068	−0.226	0.737	0.383	−0.065	−0.491
[1]	0.617	0.026	−0.182	−0.349	0.635	0.184	−0.194	−0.576
[2]^2+	1.013	0.054	−0.070	—	0.802	0.317	−0.072	—
[2]^+	0.840	−0.022	−0.189	—	0.687	0.139	−0.196	—

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Spectral and redox aspects

The ¹H NMR spectra of [1]⁺ and [2]²⁺ in (CD₃)₂SO exhibit the calculated number of seventeen partially overlapping aromatic proton resonances in each case (Fig. S3†). The extent of overlap of the proton signals is appreciably larger in [2]²⁺ as compared to [1]⁺. The maximum downfield shifted signals in [2]²⁺ and [1]⁺ appear at 10.2 ppm and 9.2 ppm, respectively. This can be attributed to the proton trans to the quinoline nitrogen N(1) of the chelated L⁻ opposite to the central pyridine ring of the π-acceptor trpy in [2]²⁺ whereas the same atom in the monodentate quinoline ring in [1]⁺ exists away from the coordination sphere.

The MOs of [1]⁺ and [2]²⁺ (Tables S2 and S4†) predict that in both cases the HOMOs are primarily composed of L⁻ based orbitals. The LUMO and LUMO + 1 are however dominated by NO based orbitals which in effect make them susceptible towards reduction (Tables S2 and S4†). Accordingly, [1]⁺ and [2]²⁺ exhibit two successive reductions, E°₂₉₈,V (ΔE_p, mV) at 0.31(70), −0.34(80) and 0.05(60), −0.64(90) in CH₃CN versus SCE corresponding to the {Ru^II–NO⁺} → {Ru^II–NO^•} (I) and {Ru^II–NO^•} → {Ru^II–NO⁻} (II) couples, respectively (Fig. 4).^8,11 The selective trans orientation of two π-accepting groups, trpy and NO⁺ in [1]⁺ makes the reduction processes relatively easier than that in [2]²⁺ where the NO⁺ is trans to the electron rich O⁻ donor of L⁻. This has also been reflected in the greater natural charge on N(5) (NO) in [1]⁺ than [2]²⁺ (Table 3). The significant contribution of trpy based orbitals in higher UMOs results in trpy based quasi-reversible reductions at further negative potentials, E°₂₉₈,V (ΔE_p, mV) of −0.84(100), −1.65(110) and −0.7(110), −1.15(110) for [1]⁺ and [2]²⁺, respectively.^{8a–c,11a,h–m,14}

Fig. 4 Cyclic voltammograms of (a) [1]⁺ and (b) [2]²⁺ in CH₃CN/0.1 M [Et₄N](ClO₄) versus SCE, scan rate:100 mV s⁻¹.

[1]⁺ and [2]²⁺ exhibit moderately intense transitions in the near-UV region, 326 nm and 348 nm, respectively, followed by several intense higher energy intraligand transitions in the UV region in CH₃CN (Fig. 5).^8,11 The bands in the near-UV region are assigned on the basis of the TD-DFT calculations on the optimized structures of [1]⁺ and [2]²⁺ as the Ru^II(dπ)/L(π) → NO⁺(π*) transition (Tables S5 and S6†). The slight difference in electronic spectra in the complexes can be attributed to the mixing of chloride ion orbitals in the HOMOs of [1]⁺ and the denticity of L⁻. Upon one-electron reduction to [1] and [2]⁺ the near UV region bands shift to the lower energy region at 556 nm and 493 nm, respectively, which are assigned to the Ru^II(dπ)/NO^•(π) → π*(trpy) and Ru^II(dπ)/NO^•(π) → L(π*) transitions, respectively, based on the TD-DFT calculations.

Fig. 5 Electronic spectra in CH₃CN of (a) [1]⁺ (black), 1 (red) and (b) [2]²⁺ (black), [2]⁺ (red).

The ν(C=O) frequency of the coordinated L⁻ in the precursor [(Ru^II(trpy)(L)(Cl)] (A) at 1635 cm⁻¹.¹⁴ has been appreciably shifted to 1667 cm⁻¹ and 1696 cm⁻¹ in [1]BF₄ and [2](BF₄)₂, respectively (Fig. S4†) due to their different electronic environments. The characteristic vibrations of the BF₄⁻ counter anion appear near 1600 cm⁻¹ and 1080 cm⁻¹. The reasonably high ν(NO) frequencies of [1]BF₄ and [2](BF₄)₂ of 1895 cm⁻¹ (DFT: 1906 cm⁻¹) and 1926 cm⁻¹ (DFT: 1939 cm⁻¹), respectively, imply their moderately electrophilic character. Upon one-electron reduction to 1 and [2]⁺, the NO bands at 1895 cm⁻¹ and 1926 cm⁻¹ disappear. However, the ν(NO^•) frequency of the reduced state in the expected SWIR region of 1600–1700 cm⁻¹ did not resolve properly due to the presence of other characteristic vibrations of the C=O of coordinated L⁻, BF₄⁻ counter anion as well as aryl ring vibrations.

The EPR spectrum of the representative one-electron reduced paramagnetic [2]⁺ in frozen CH₃CN/0.1 M Bu₄N(PF₆) solution yields g-components at 2.012 (g₁), 1.988 (g₂) and 1.869 (g₃) (Fig. S5†) with a g-anisotropy (g₁ − g₃) of 0.143, which is about twice a large as that observed for the related {Fe(NO)}⁷ species due to ζ(Ru) ≈ 2ζ(Fe) (ζ = spin–orbit coupling constant).^10b,11h However, the value of <g> = [1/3(g₁² + g₂² + g₃²)]^1/2 = 1.957 implies that the spin is primarily localized on the N(5) of NO.^17,18 The overall negative shift of the g value (Δg = 45.0 ppt) with respect to the free electron value of 2.0023 arises due to the admixture of higher excited states with nonzero angular momentum (eqn (1)).¹⁸ The radical EPR feature of [2]⁺ (Fig. S5†) has been reflected in the significant bending of Ru–N(5)–O(3) angle (139.69°) and lengthening of the N(5)–O(3) bond (1.181 Å).

ξ: spin orbit coupling constant

L: angular momentum operator

E ₀: energy of the SOMO

The SOMO of [2]⁺ indicates σ-donation from the NO(π*) orbital to the Ru(d_z²) center which reveals {Ru^II–NO^•} as the predominant oxidation state formulation along with considerable mixing of the {Ru^I–NO⁺} state due to the presence of 23% metal contribution. The presence of ∼10% spin on the metal in spin density analysis also reveals that spin is mainly concentrated on NO (Fig. 6). Moreover, σ-interaction between Ru and NO results in spin density at the axial position (Fig. 6) which leads to a significant hyperfine splitting in the EPR spectrum due to the N(5) of NO.

Fig. 6 Mulliken atomic spin density plot for [2]⁺ (N(5): 0.514, O(3): 0.329, Ru: 0.096).

Electronic features: theoretical insights

Molecular orbital analysis reveals that the LUMOs of [1]⁺ and [2]²⁺ originate from the interaction of the π*-orbital of NO⁺ with the metal's t_2g(d_xy) and e_g(d_z²) orbitals, respectively, (Tables S2 and S4†).^{10b,11c–h,17} This is attributed to stronger back-bonding in [1]⁺ between the metal t_2g(dπ) orbital and the π*-orbital of trpy in the filled MOs. The presence of 10–25% metal contribution in the LUMOs lowers the energy of the metal t_2g(dπ) orbitals and thus the HOMOs are mainly composed of the orbitals of the co-ligands (trpy, L or Cl).^10b The significant metal contribution (∼58%) in [2]²⁺ has been predicted in the HOMO − 5 state whereas around 40% metal contribution in [1]⁺ has been detected in HOMO − 11 which also provides evidence for the lesser extent of back-bonding in [2]²⁺. The presence of a σ/π-donating chloride ion plays a crucial role for the stronger back-bonding in [1]⁺. This is reflected in a greater natural charge on the metal ion in [2]²⁺ than [1]⁺ (Table 3). The observed more linearity of the {Ru–N(5)–O(3)} bond in [2]²⁺ (175.9(3)°) as compared to [1]⁺ (171.0(8)°) suggests relatively larger degeneracy of the π*-orbitals (LUMO and LUMO + 1) in [2]²⁺.

The addition of one-electron to the π*-orbital of {RuNO}⁶ in [1]⁺ or [2]²⁺ leads to the formation of a {RuNO}⁷ species, 1 or [2]⁺ and hence lifts the said degeneracy of the π*-orbitals (LUMO and LUMO + 1) (Tables S3–S4†).^10b,17a Such a splitting can be viewed as a Jahn–Teller splitting due to the lowering of symmetry and the spin–orbit interaction which is reflected in the bending mode of Ru–NO with Ru–N(5)–O(3) angles of 141.85° and 139.69 in 1 and [2]⁺, respectively, as revealed in their DFT optimized structures. The SOMO of 1 or [2]⁺ is primarily composed of the 2p(π)-atomic orbitals of N5 and O3 (π*(NO^•)) along with a lesser extent of 4d_xy(Ru^II) orbital of 1 and dz²(Ru^II) orbital of [2]⁺. The appreciable extent of metal contribution in the SOMO suggests {Ru^II–NO^•} as the predominant oxidation state along with partial mixing of the {Ru^I–NO⁺} state.^11c–i,l The SOMO of 1 suggests the presence of a 4d_xy(Ru^II) → pπ*(NO^•) back-bonding interaction while the SOMO of [2]⁺ reveals the σ-charge donation from the low-lying singly-occupied pπ*(NO^•) to the metal e_g-orbitals. This can be attributed to the different coordination environment of the NO function in the complexes: NO is trans to the π-accepting trpy ligand in the equatorial plane in [1]⁺ whereas in [2]²⁺ the NO is trans to the σ-donating O⁻ of L⁻ in an axial orientation.

The approximate bent geometry of {Ru–NO}⁷ in 1 and [2]⁺ introduces the possibility of a pair of conformational isomers based on the dihedral angle (θ) of L_trans–Ru–N5–O3, with the idealized eclipsed and staggered angles of θ = 0° and θ = 45°, respectively.^11b–d,h The geometry optimization at the G03/(U)B3LYP level reveals that the eclipsed conformation is more stable by 0.01 eV in [2]⁺. However, neither eclipsed nor staggered conformation is found to be perfectly suitable for 1 where the dihedral angle (θ) of L_trans(N3)–Ru–N5–O3 is 28.3° (Fig. S6†). Such a pseudo-staggered (θ = 28.3°) conformation in 1 is more stable by 0.05 eV and 0.12 eV with respect to the eclipsed (θ = 0°) and staggered (θ = 45°) forms, respectively. Another theoretically possible staggered conformation with the dihedral angle of θ = −45° is destabilized under the same level of DFT presumably due to the repulsive interaction between O3 and O2. The stereoelectronic effect of the chloride and O2 from bulky L in 1 collectively prevent it exhibiting either the eclipsed (θ = 0°) or staggered (θ = 45°) conformation, leading to an energetically favored pseudo-staggered (θ = 28.3°) conformation.

Photo-lability of the {Ru–NO} bond

The facile photo-labilization of the {Ru^II–NO^•} bond maintaining the integrity of the remaining part of the molecule is believed to be significant particularly from the perspective of the biochemically desired target oriented NO^• delivery process.^8,9 In this context both the nitrosyl complexes, [Ru^II(trpy)(L)(NO⁺)Cl]⁺, [1]⁺, and [Ru^II(trpy)(L)(NO⁺)]²⁺, [2]²⁺, are found to undergo the facile photocleavage of the {Ru–NO} bond under the exposure of light in CH₃CN as evidenced by their spectroscopic signatures (Fig. 7).¹⁹ The photolabilization of the {Ru–NO} bond is accompanied by the formation of the corresponding solvent bound ruthenium(II)-photoadducts, [Ru^II(trpy)(L)(Cl)(CH₃CN)] and [Ru^II(trpy)(L))(CH₃CN)]⁺, respectively, and the transformation proceeds through several isosbestic points (Fig. 7). The formation of an Mb–NO adduct on passing the liberated “NO” through the aqueous solution of reduced Mb under deoxygenated conditions has been evidenced by its characteristic absorption band at λ_max = 420 nm (Fig. S7†). The estimated first-order rate constant (k/s⁻¹) and t_1/2/s values of the photocleavage process are 2.6 × 10⁻¹, 2.66 and 2.9 × 10⁻², 23.8 for [1]⁺ and [2]²⁺, respectively. The ten-fold faster photolability of the {Ru^II–NO^•} bond in [1]⁺ as compared to [2]²⁺ can be rationalized based on their structural differences including the trans influence of co-ligands. The cleavage of the {Ru^II–NO⁺} bond via the photo-irradiation process is known to proceed through the formation of the intermediate excited S = 1 state in {Ru^III–NO^•}^* as shown below.^8f

Fig. 7 Time evolution of the electronic spectra of (a) [1]⁺, concentration: 0.57 × 10⁻⁴ M in CH₃CN, time intervals: 2 s, and (b) [2]²⁺, concentration: 0.21 × 10⁻⁵ M in CH₃CN, time intervals: 5 s under the exposure of light (Xe lamp, 350 W). Insets show the absorbance versus time plots at (a) 326 nm and (b) 482 nm corresponding to the solvent species in each case.

[Ru^II–NO⁺] + hν → [Ru^III–NO^•]^* → [Ru^II–solvent] + NO^•

The formation of a solvated Ru(II) species instead of Ru(III)–solvate as a photoproduct has been attributed to the absence of the otherwise expected EPR signal of Ru(III) as has also been established earlier.⁸ The electronic effect of π-accepting ligands certainly facilitates the formation of {Ru^II–CH₃CN} in the ground state of the photo-product, as has also been established recently by us.^8g It should be noted that the selective photocleavage of the M–NO bond, maintaining the integrity of the rest of the molecule is biologically significant.⁹ The competition between the pπ-orbitals of NO and N3(trpy) for the same dπ-orbital of the metal ion (observed in HOMO − 4) makes the {Ru–NO} bond in [1]⁺ more photolabile. On the other hand, the greater extent of dπ(Ru) → pπ(NO) back-bonding as well as trans-influence of the σ-donating carboxylate group (O1 of L⁻) are the likely factors for the relatively slower photo-dissociation process in [2]²⁺.

Reactivity of {Ru–NO} towards molecular oxygen: dioxygenase activity

The reaction of NO with the oxygenated heme-proteins has immense biological importance.⁵ Though nitrogen monoxide plays crucial roles in various physiological processes, overproduction of NO can also lead to several toxicological processes, such as cell death and DNA damage, primarily due to the formation of highly reactive peroxynitrite (⁻OON=O).²⁰ Nitric oxide dioxygenase (NODs) catalyzes the reaction of NO with dioxygen to yield the environmentally and physiologically benign nitrate anion (NO₃⁻) in mammals.²¹ Highly reactive peroxynitrite (⁻OON=O or oxoperoxonitrate) is known to be the powerful oxidant/nitrating agent in nitric oxide biochemistry and has an influential role in stress injury.²¹ The selective conversion of peroxynitrite (⁻OON=O) to biologically benign nitrate takes place in oxygenated heme-proteins to control the overproduction of NO^•22,23but in synthetic model systems, peroxynitrite usually transforms to nitrite.^24a–b,d

Although the reduced [2]⁺ remains inactive towards molecular oxygen (O₂), 1 (in situ generated via the addition of N₂H₄ in the CH₃CN solution of [1]⁺ or [2]²⁺) is found to activate molecular oxygen at 298 K. Upon bubbling dioxygen (O₂) in an acetonitrile solution of 1 at pH ∼ 1, the intensity of the peaks at 525 nm and 376 nm corresponding to 1 periodically decreases with the concomitant growth of a new peak at 476 nm (Fig. S8†). The transformation proceeds through several isosbestic points at 501 nm, 399 nm and 355 nm, implying a clean process with the selective involvement of two species, [Ru^II(trpy)(L)(NO^•)Cl] (1) and [Ru^II(trpy)(L)(NO₂)Cl]⁻ ([1a]⁻). The formation of [1a]⁻ is evidenced from its ESI-MS(+) peak at 554.88 corresponding to {([1a]–Cl) + H⁺} (calcd. mass: 554.03) (Scheme 2 and Fig. S9†). The transformation of 1 to [1a]⁻ is likely to occur via the intermediate transient peroxynitrite species {Ru–(OON=O)} ([1a′]⁻). Consequently, the oxygenation reaction of 1 in the presence of tyrosine-mimic 2,4-di-tert-butylphenol at pH ∼ 1 results in the nitrated product, 2,4-di-tert-butyl-6-nitrophenol (NO₂–DTBP) and oxidative coupling product, 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butyl-1,1′-biphenyl as depicted in Scheme 2.²⁴

Scheme 2 Dioxygenase activity of 1 at pH ∼ 1.

Conclusions

The present article highlights the following points:

- The unprecedented hemilabile feature of the chelated quinaldate ligand (L⁻) in the presence of NO⁺ in the molecular framework of [1]⁺ comprising of selective co-ligands, π-acepting trpy and σ/π-donating chloride.

- The electronic aspects of the coordinated nitrosyl function in [1]BF₄ and [2](BF₄)₂ differ appreciably based on their specific structural features as evidenced by their ν(NO) frequencies and E°(Ru^II–NO⁺/Ru^II–NO^•) potentials.

- The built in electronic structural differences in [1]⁺ and [2]²⁺ have further been reflected in the rate of light induced {Ru^II–NO⁺} bond cleavage.

- Reduced 1 has been selectively transformed to the corresponding {Ru^II–NO₂} species through contact with molecular oxygen, although the corresponding [2]⁺ failed to exhibit any such activity with O₂.

Experimental section

Materials

The precursor complexes Ru(trpy)Cl₃^25a and [Ru^II(trpy)(L)(Cl)] ¹⁴ (trpy = 2,2′:6′,2′′-terpyridine) were prepared according to literature procedures. The ligand quinaldic acid (HL) and other reagents and chemicals were obtained from Aldrich and used as received. For spectroscopic and electrochemical studies HPLC-grade solvents were used.

Physical measurements

¹H NMR spectra were recorded in (CD₃)₂SO on a 400 MHz Bruker spectrometer. Chemical shift data are quoted as δ in ppm and as s, d, dd, t, q and m representing singlet, doublet, doublet of doublet, triplet, quartet, sextet and multiplet peaks, respectively. IR and UV-vis spectra were recorded using Thermo Nicolet 320 and Perkin Elmer Lambda 950 spectrophotometers, respectively. ESI-mass spectra were recorded using micromass Q-TOF. Cyclic voltammetric studies were carried out using a PAR model 273A electrochemistry system. Platinum wire working and auxiliary electrodes and an aqueous SCE were used in a three-electrode configuration. The supporting electrolyte was 0.1 mol dm⁻³ [NEt₄](ClO₄), and the solute concentration was ∼10⁻³ mol dm⁻³. The half-wave potential E°₂₉₈ was set equal to 0.5(E_pa + E_pc), where E_pa and E_pc are the anodic and cathodic cyclic voltammetric peak potentials, respectively. Elemental analyses were carried out on Perkin-Elmer 240C elemental analyzer. The 2,4-di-tert-butyl phenol (DTBP) reactions were monitored by gas chromatographic technique with a FID detector (Shimadzu GC-2014 gas chromatograph) as well as GCMS (Hewlett-Packard GCD-HP1800A). The EPR measurement was made in a two-electrode capillary tube with a X-band Bruker system ESP300.^25b

Synthesis of [Ru^II(trpy)(L)(NO⁺)Cl]BF₄ ([1]BF₄). 50 mg of [Ru^II(trpy)(L)(Cl)] (A, 0.09 mmol) and 4 equivalent of NOBF₄ (42 mg, 0.36 mmol) were taken in 20 cm³ dichloromethane. The mixture was stirred initially at 0 °C for 30 min followed by at 298 K for another 2 h. During the course of the reaction the initially blue solution of [Ru^II(trpy)(L)(Cl)] (A) changed to red and a dark yellow precipitate was formed. The precipitate was allowed to settle inside the refrigerator for 1 h and then filtered off under reduced pressure and washed thoroughly with dichloromethane and diethyl ether and dried in vacuo. The solid mass was recrystallized from 1 : 1 acetonitrile–benzene solution. Yield: 45 mg (75%). Anal. calcd. for C₂₅H₁₇N₅ClO₃BF₄Ru (Mol. wt.: 659.01): C, 45.52; H, 2.60; N, 10.62. Found: C, 45.35; H, 2.58; N, 10.53%. Molar conductivity (Λ_M [Ω⁻¹ cm² M⁻¹], CH₃CN): 105. ESI(+)-MS (m/z, CH₃CN): 572.05 ([1]⁺), 543.04 ([1–NO + H]⁺), 537.08 ([1–NO–Cl]⁺). ¹H NMR (400 MHz, (CD₃)₂SO): δ (ppm) (J/Hz): 9.25 (1H, d, 8.4), 9.09 (2H, d, 8.4), 9.02 (2H, d, 8.0), 8.97 (1H, d, 7.2), 8.70 (1H, d, 8.8), 8.67 (3H, m), 8.34 (2H, m), 8.15 (1H, t, 7.84, 7.20), 8.02 (2H, d, 7.3), 7.79 (2H, m). IR (KBr) ν(BF₄⁻): 1083, 1601; ν(NO⁺):1895; ν(C=O): 1667 cm⁻¹. λ/nm (CH₃CN) (ε/dm³ mol⁻¹ cm⁻¹): 455(sh), 349(10 380), 326(15 120), 315(14 120), 283(16 540).

Synthesis of [Ru^II(trpy)(L)(NO⁺)](BF₄)₂ ([2](BF₄)₂). A mixture of [Ru^II(trpy)(L)(Cl)] (50 mg, 0.09 mmol) and AgNO₃ (153 mg, 0.9 mmol) were taken in 25 cm³ of ethanol and heated to reflux with constant stirring for 1 h. The solution was cooled to room-temperature and filtered through Celite to remove the white precipitate of AgCl. Nitrosonium tetrafluoroborate (NOBF₄, 16 mg, 0.14 mmol) was then added to the above filtrate and the resulting solution was stirred for 6 h. Partial removal of the solvent under reduced pressure resulted in dark solid which was collected through filtration. The solid mass thus obtained was washed several times with diethyl ether and dried in vacuo. The product was recrystallized from 1 : 1 benzene–acetonitrile. Yield: 26 mg (55%). Anal. calcd. for C₂₅H₁₇N₅O₃B₂F₈Ru (Mol. wt. 711.04): C, 42.19; H, 2.41; N, 9.85. Found: C, 42.25; H, 2.45; N, 9.89%. Molar conductivity (Λ_M [Ω⁻¹ cm² M⁻¹], CH₃CN): 190. ESI(+)-MS (m/z, CH₃CN): 624.31 ([2](BF₄)₂–BF₄]⁺). ¹H NMR (400 MHz, (CD₃)₂SO): δ (ppm) (J/Hz): 10.25 (1H, d, 8.5), 9.18 (2H, m), 8.92 (5H, m), 8.49 (2H, m), 8.28 (1H, d, 8.4), 7.94 (3H, m), 7.75 (1H, m), 7.61 (2H, m). IR (KBr), ν(BF₄⁻): 1083, 1599; ν(NO⁺):1927; ν(C=O): 1696 cm⁻¹. λ/nm (CH₃CN) (ε/dm³ mol⁻¹ cm⁻¹): 346(20 920), 325(20 360), 289(24 380), 279(26 770).

Trapping of photoreleased “NO” by myoglobin

3.0 cm³ acetonitrile solution of the nitroso species ([1]⁺ or [2]²⁺) was initially taken in a quartz cuvette of optical path length of 1 cm. The cuvette was sealed with a rubber septum and the solution was deoxygenated by purging nitrogen gas. The photolyis was carried out for 10 min using a Xe 350 W lamp. The photoreleased free “NO” was allowed to pass through the reduced myoglobin solution in water using a cannula and the UV-vis. spectrum was recorded.

Dioxygenase activity

The conversion process of [Ru^II(trpy)(L)(NO^•)Cl] (1) → [Ru^II(trpy)(L)(NO₂)Cl]⁻ ([1a]⁻) in acetonitrile at pH ∼ 1 and in presence of bubbling O₂ was monitored by following the increase in absorbance of the new band at 476 nm corresponding to λ_max of the nitro species [1a]⁻ till the changes of intensity completely levelled off. The above experiment was then repeated in presence of 2,4-di-tert-butylphenol (DTBP) at 298 K and the formation of corresponding products, 2,4-di-tert-butyl-6-nitrophenol (NO₂-DTBP) and oxidative coupling product, 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butyl-1,1′-biphenyl were confirmed by GC and GCMS.

Crystallography

Single crystals of [1]BF₄ and [2](BF₄)₂ were grown by slow evaporation of their 1 : 1 acetonitrile–benzene solutions. Single crystal X-ray diffraction data were collected using an OXFORD XCALIBUR-S CCD single crystal X-ray diffractometer at 150 K. The structures were solved and refined by full-matrix least-squares techniques on F² using the SHELX-97 program.²⁶ All data were corrected for Lorentz polarization and absorption effects, and the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the refinement process as per the riding model. The complex [1]BF₄ was crystallized with five water molecules. CCDC-838835 and CCDC-838836 contain the supplementary crystallographic data for [1]⁺ and [2]²⁺, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

Computational details

Full geometry optimizations were carried out at the (R)B3LYP and (U)B3LYP levels^27,28 using density functional theory method with Gaussian 03 (revision C.02).²⁹ All elements except ruthenium were assigned the 6-31G(d) basis set during geometry optimizations and 6-31G(d,p) during single point and property calculations. The LanL2DZ basis set with effective core potential was employed for the ruthenium atom.^30,31 Vertical electronic excitations based on (R)/(U)-B3LYP optimized geometries were computed for the time-dependent density functional theory (TD-DFT)³² in acetonitrile using the Polarizable continuum model (PCM) of Tomasi and co-workers,³³ specifically, the conductor like PCM (CPCM) in conjugation with the united atom topological model (UAO radii, implemented in Gaussian 03) was applied.^33–35 GaussSum was used to calculate the fractional contributions of various groups to each molecular orbital.³⁶ No symmetry constraints were imposed during structural optimizations, and the nature of the optimized structures and energy minima were defined by subsequent frequency calculations. Natural bond orbital analyses were performed using the NBO 3.1 module of Gaussian 03 on optimized geometry.³⁷ All the calculated structures were visualized with ChemCraft.³⁸

Acknowledgements

The financial support received from Department of Science and Technology (DST), New Delhi and Council of Scientific and Industrial Research (CSIR), New Delhi (fellowship to A.D.C.) and University Grant Commission (UGC) (fellowship to P.D.), New Delhi is gratefully acknowledged. X-Ray and GCMS studies were carried out at the National Single-Crystal X-ray Diffraction Facility and Sophisticated Analytical Instrumentation Facilities (SAIF), IIT Bombay, respectively. Computational facilities from the Department of Chemistry, IIT Bombay are gratefully acknowledged. We thank Mr. Thomas Scherer, Institut für Anorganische Chemie, Universität Stuttgart for EPR measurement.

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Footnote

† Electronic supplementary information (ESI) available: Characterization details of the complexes, crystallographic material and DFT results (Table S1–S7 and Fig. S1–S9). CCDC reference numbers 838835 and 838836. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra00953f

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