Nitric oxide (NO) reactivity studies on mononuclear iron(II) complexes supported by a tetradentate Schiff base ligand

Abstract

Mononuclear high-spin [Fe^II(Gimpy)(Cl)₂], (1) and low-spin [Fe^II(Gimpy)(CN)₂]·3H₂O, (2·3H₂O) complexes, where; Gimpy: (1,2-bis(2-phenyl-2-(pyridin-2-yl)hydrazono)ethane), were synthesized and characterized by various analytical and spectral techniques. The NO reactivity of complex 2·3H₂O via acidified nitrite solution gave rise to a mixture of two different complexes; a ligand nitrated unsymmetrical complex [Fe(Gimpy-NO₂)(CN)₂]·CH₂Cl₂, (3·CH₂Cl₂) via electrophilic route as well as nitrosylation at the metal centre, formation of unstable complex [Fe(Gimpy)(NO)(CN)]²⁺, (4). Formation of unstable complex 4 was monitored via visual colour change from purple to brownish-red and evidence of this complex has been established by means of in situ IR, ESI-MS and UV-visible spectral studies. In addition, the structures of complexes 2·3H₂O and 3·CH₂Cl₂ have been established unambiguously through single crystal X-ray diffraction analysis. Interestingly, after nitration of imine proton twisting of one phenyl ring (near to –NO₂ group) in complex 3 was obtained, leading to an unsymmetrical complex 3 (nitration of one imine proton only is supported by theoretical studies), possibly due to steric hindrance of –NO₂ group and results of electrochemical studies of all complexes (1–3) were also examined.

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Introduction

Among the first row transition series, iron is one of the versatile metals used for various applications¹ due to its flexible range of coordination geometries, oxidation states, spin-states and redox potential.² In the recent years, binding of small molecules such as dioxygen (O₂), carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H₂S) to iron(II) centres has attained considerable interest due to their important role in cellular pathways.³ However, study of NO molecule with transition metal complexes, especially with iron complexes have been of current interest.^3,4 NO, a diatomic molecule has been found to be important in several physiological processes namely, blood pressure regulation, immune, endocrine response, neurotransmission and cell death.⁵ Literature reports projected and formulated NO-based treatments some of which have been validated for clinical applications; in fact sodium nitroprusside (SNP) is used as a drug for NO delivery.³

Interaction of NO with transition metal complexes is not only important for the discovery of novel NO donors but also important for NO scavenging activity.⁶ Quite a lot of reports on iron–nitrosyl have been described with anionic ligands having strong σ-donating property such as carboxamido-N and phenolato-O and were found to be important in coordination and photolability of NO.^6–8 For example Chavez and co-worker reported, [Fe(T1Et₄ⁱPrIP)(OTf)₂] complexes show reversible reactivity with nitric oxide to afford 6-coordinate {FeNO}⁷, S = 3/2 complex with linear Fe–N–O group.⁹ Goldberg and co-worker presented first structural and electronic model of NO-bound cysteine dioxygenase, [Fe(NO)(N₃PyS)]BF₄ which upon photo-irradiation shows reversible release of NO.¹⁰ Interestingly, Mascharak and co-worker reported iron complexes which could deliver NO upon illumination with light and such complexes were important in photodynamic therapy (PDT).^4,7

It is well known that SNP exhibit cyanide poisoning and rhodanese enzymes detoxify the cyanide ion, which converts CN⁻ to SCN⁻.¹¹ However, patients with hepatic problem are not good candidates for SNP therapy as increase in SCN⁻ is not favourable in this condition.¹² Therefore, there is a necessity of substitution of these cyanide groups by proper ligands, the following three reports prompted us to design and synthesize ligand Gimpy and its iron complexes for the synthesis of iron nitrosyl complexes. First, nitric oxide could bind easily to the iron-site in nitrile hydratase.^4,7 Second, ligand PaPy₃ gave rise to Fe(III) complex and reaction of NO gave rise to iron nitrosyl complex having {Fe–NO}⁶ moiety. Third, for NO reactivity, Fe(III) metal centre was reacted with nitric oxide gas to obtain the iron nitrosyl complex.^4,7 In fact facile synthesis of ruthenium nitrosyl complexes were achieved as a part of our on-going research⁸ because ruthenium always provided low-spin complexes with ligands.

Hence, with this view point, we designed, synthesized and characterized a tetradentate ligand Gimpy, (1,2-bis(2-phenyl-2-(pyridin-2-yl)hydrazono)ethane), (shown in Scheme 1) and their high-spin iron(II) complex. As stated above, NO reactivity depend upon the spin-state of the iron centre,^4,7 we converted high-spin Fe(II) complex to low-spin Fe(II) cyanide complex and their nitric oxide activity was examined. All the complexes were characterized by different physical methods and molecular structures were also determined by X-ray crystallography. Resulted outcome and their reactivity have been reported in this article.

Scheme 1 Synthetic procedure for the complexes 1 and 2.

Results and discussion

Preparation and characterization of iron(II) complexes

The tetradentate Schiff base ligand, Gimpy was synthesized according to the procedure reported earlier by our group.¹³ Synthesis of monomeric high/low-spin iron(II) complexes were carried out in air. Green solid of high-spin [Fe(Gimpy)Cl₂], 1 was prepared using anhydrous FeCl₂ and Gimpy in methanol : dichloromethane mixture. Complex 2 was synthesized by one-step and two-step reaction from complex 1. For one-step reaction, complex 2 was prepared via reaction of equimolar ratio (1 : 1) of Gimpy and FeCl₂ or Fe(ClO₄)₂·xH₂O with two equivalents of sodium cyanide in dichloromethane–methanol–water mixture. For two-step reaction, two equivalents of sodium cyanide were added to complex 1 to yield a purple coloured low-spin Fe(II) complex [Fe(Gimpy)(CN)₂], 2, with removal of two equivalents of sodium chloride. The synthetic procedures of both of these complexes are shown in Scheme 1.

Coordination of imine nitrogen to Fe(II) centre in both the complexes exhibited a characteristic ν_C=N band near to 1575 and 1600 cm⁻¹, for 1 and 2, shifted from 1687 cm⁻¹ in free ligand (Gimpy). A strong ν_CN band at 2093 cm⁻¹ in the IR spectra expressed the presence of the coordinated cyanide in complex 2 (Fig. S1, ESI†).¹⁴ The ESI-MS spectra of 1 and 2 in dichloromethane showed a mass peak at m/z = 485.21 (for 1), 474.11 (for 2), which corresponds to [M⁺ − (Cl)⁻], [M⁺ − (CN)⁻], respectively (Fig. S2, ESI†). Molar conductivities values of 18 and 20 Ω⁻¹ cm² mol⁻¹ for 1 and 2, respectively confirm the non-electrolytic nature of both the complexes, when determined in dimethylformamide at ca. 10⁻³ M at 25 °C.¹⁵ Complex 1, afforded a effective magnetic moment of 4.91 BM, consistent with four unpaired electrons (S = 2) and predicting the stabilization of high-spin d⁶ iron(II) complex. The solution magnetic moment of 1 was also measured by Evans' method and the value is 5.1 BM, which further corroborates a high-spin electronic configuration.¹⁶

Diamagnetic (low-spin) nature of complex 2 was confirmed by ¹³C and ¹H NMR spectroscopy. In comparison to free ligand, which showed imine protons at δ = 7.13–7.15 ppm (Fig. S3 and S4, ESI†), in complex 2 these imine protons move to downfield (δ = 7.64–7.58 ppm) due to reduction of electron density (Fig. S4, ESI†).¹⁷ Similarly, ¹³C NMR spectra of complex 2 also showed a distinct peak at 153.65 ppm of cyanide ion when compare to the ligand, Gimpy (Fig. S3, ESI†). Electronic spectra of complexes 1 and 2 were recorded in dichloromethane and displayed in Fig. 1. Complex 1 display strong absorption maxima at ∼420 nm, which is of metal-to-ligand charge transfer (MLCT) origin, due to Fe(II) “t_2g” orbitals and π* of pyridine orbitals. Similar bands has been noted previously in other iron complexes derived from Schiff base ligands.¹⁸ The shoulder near 426 nm and broad band near 576 nm (for 2) were assigned as MLCT. The broad bands were probably due to the presence of CN⁻ ions.¹⁴

Fig. 1 UV-visible spectra of the complexes 1 and 2 (20 μM) in dichloromethane, at room temperature.

Nitric oxide (NO) reactivity

Reactivity of NO in complexes 1 and 2 were investigated. Complexes 1 and 2 were reacted with in situ generated NO derived from acidified NaNO₂ solution.¹⁹ No color change was found on the treatment of dichloromethane solution of complex 1 with acidified NaNO₂ solution. However, treatment of a dichloromethane solution of complex 2 with acidified NaNO₂ solution interestingly gave rise to a mixture of two complexes with two step color changes. We observed a color change from purple to red-brown after five min and ultimately the formation of green color within 15–20 min (Scheme 2). We were unable to isolate and characterized the red-brown complex, as, this red-brown solution convert to green solution instantaneously. However, we have monitored the in situ UV-visible spectral changes for this purple to red conversion, a new band at around 640 nm appear since this red color is unstable, the band around 640 nm is decreased immediately (Fig. S5, ESI†). In situ analysis of this brownish-red solution was also examined via IR, ESI-MS analysis and speculate the possible formation of NO coordinated iron–nitrosyl complex [Fe(Gimpy)(NO)(CN)]²⁺4. The IR spectrum of in situ generated brownish-red complex 4, showed a band near 1917 cm⁻¹ and 2104 cm⁻¹ (Fig. 2B). The peak near 1917 cm⁻¹ indicated the formation of {Fe–NO}⁶ according to Enemark and Feltham notation (Enemark and Feltham have introduced a {M–NO}ⁿ notation to describe the bonding in metal nitrosyls where n represents the sum of metal d and NO π* electrons).¹⁹ The other peak at 2104 cm⁻¹ was due to coordinated cyanide ion. The ESI-MS analysis of in situ red solution showed a peak at ∼507.203 and 504.323 which corresponds to [M⁺ + 3] and [M⁺] (Fig. S6, ESI†). It is important to mention here that the starting complex 2 possesses low-spin Fe(II) center, hence probable formation of {Fe–NO}⁷. As discussed, NO reactivity depends upon the spin-state of metal centre, Mascharak and co-worker reported high-spin Fe(III) complex (Et₄N)[(bpb)Fe(Cl)₂] did not afford any isolable nitrosyl complex, whereas reaction of NO with the low-spin starting complex [(bpb)Fe(Py)₂]ClO₄; gave nitrosyl complexes in different solvent media.⁷ Similarly, in this work NO reactivity of high-spin, complex 1 do not give nitrosyl complex whereas in presence of the cyanide group this reaction works well with acidified NaNO₂via liberation of volatile HCN.²⁰ Further, the green solution on passing through silica column resulted a ligand nitrated unsymmetrical complex [Fe^II(Gimpy-NO₂)(CN)₂], 3 which has been further characterized by UV, IR and single crystal X-ray analysis (vide infra) (Fig. 2A). It has been found in literature that NaNO₂ shows nitration of aromatic compounds in acidic media and the reaction proceeding by NO₂⁺ as an active reagent.²¹ In complex 2, after metallation five membered become highly delocalized and hydrogen atoms behave like aromatic hydrogen. Hence the probable mechanism of this nitration is also supported by NO₂⁺via electrophilic pathway and shown in Scheme 3. Only one imine proton get nitrated in excess of H⁺/H₂O, NaNO₂ and discussed by theoretical calculation (vide infra).

Scheme 2 Visual color change and formation of the complexes 3 (green) and 4 (purple) from complex 2 (red brown).

Fig. 2 (A) UV-visible spectrum of complex 3 (40 μM) in dichloromethane at room temperature, (B) in situ IR spectrum of complex 4.

Scheme 3 Proposed mechanism for ligand nitration and nitrosylation reaction mechanism in presence of acidified NaNO₂.

X-ray structure determination

Molecular structures of complexes 2·3H₂O and 3·CH₂Cl₂ were determined by X-ray crystallography. All the crystallographic parameters (Table S1, ESI†) selected bond distances and bond angles are tabulated in Table 1.

Table 1 Selected bond length (Å) and bond angles (°) for complexes 2·3H₂O and 3·CH₂Cl₂

Complex 2·3H2O		Complex 3·CH2Cl2
Bond distances (Å)
Fe1–N6	1.993(5)	Fe1–N1	1.858(7)
Fe1–N4	1.853(6)	Fe1–N3	2.028(7)
Fe1–N3	1.848(6)	Fe1–N4	1.849(7)
Fe1–N1	2.005(6)	Fe1–N6	1.999(7)
Fe1–C21	1.959(3)	Fe1–C26	1.968(10)
Fe1–C22	1.943(2)	Fe1–C27	1.964(9)
N3–C12	1.294(9)	N4–C3	1.343(11)
N4–C11	1.324(9)	N1–C2	1.294(11)
Bond angle (°)
N3–Fe1–N6	163.1(2)	N4–Fe1–N1	82.5(3)
N1–Fe1–N6	115.67(8)	N6–Fe1–N1	163.2(3)
N6–Fe1–N4	81.4(2)	N3–Fe1–N1	81.0(3)
N4–Fe1–N1	162.8(2)	N3–Fe1–N6	115.8(3)
N3–Fe1–N4	81.64(9)	N3–Fe1–N4	163.2(3)
N3–Fe1–N1	81.2(2)	N4–Fe1–N6	80.7(3)
C22–Fe1–C21	173.21(10)	C26–Fe1–C27	167.0(4)
N4–Fe1–C21	89.0(3)	C27–Fe1–N3	85.8(3)
N3–Fe1–C21	89.5(3)	C27–Fe1–N4	98.3(3)
N1–Fe1–C21	89.1(3)	C27–Fe1–N6	87.3(3)
N6–Fe1–C21	89.3(3)	C27–Fe1–N1	95.0(4)
C22–Fe1–N6	86.8(3)	C26–Fe1–N1	94.7(3)
C22–Fe1–N4	95.9(2)	C26–Fe1–N3	87.2(3)
C22–Fe1–N3	95.8(2)	C26–Fe1–N4	91.6(3)
C22–Fe1–N1	87.6(3)	C26–Fe1–N6	85.9(3)
N7–C21–Fe1	172.5(2)	N7–C25–Fe1	176.6(8)

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Structure of complexes 2·3H₂O and 3·CH₂Cl₂

Crystals of both compounds were obtained in dichloromethane by diethyl-ether diffusion in freezer within a week. The X-ray crystal structures of [Fe(Gimpy)(CN)₂] (2·3H₂O) and [Fe(Gimpy-NO₂)(CN)₂] (3·CH₂Cl₂) are depicted in Fig. 3. Complexes 2·3H₂O and 3·CH₂Cl₂ crystallized in the orthorhombic space group Pn2₁a and triclinic space group P1̄, respectively. In both complexes four nitrogen donors N1, N3, N4, N6 lie in the equatorial plane while two cyanides were situated axially and trans to each other (Fig. 3). The C=N_im distances in complexes 2·3H₂O and 3·CH₂Cl₂ were 1.294(9), 1.324(9) Å and 1.343(11), 1.293(11) Å, respectively.¹⁴

Fig. 3 ORTEP plot of complexes (A) [Fe(Gimpy)(CN)₂], 2·3H₂O (B) [Fe(Gimpy-NO₂)(CN)₂], 3·CH₂Cl₂, hydrogen atoms and solvent were removed for clarity.

There were not much differences in Fe–N_py and Fe–N_im distance in both the complexes (2·3H₂O and 3·CH₂Cl₂) and these values were consistent with reported data.²² The Fe(II)–CN distances in both complexes (2·3H₂O and 3·CH₂Cl₂) were 1.959(3), 1.943(2) Å and 1.957(9), 1.953(9) Å, respectively and comparable to other low-spin Fe(II) complexes with coordinated cyanide.²² One interesting feature obtained from the comparison of the structures of 2·3H₂O and 3·CH₂Cl₂ were the twisting of phenyl ring in the ligand frame (Fig. S7, ESI†). In 2·3H₂O both the phenyl rings made 153° and 154.15° angles with the mean equatorial plane obtained by all coordinated nitrogen donors from the ligand (Gimpy). However, after nitration of imine proton one of the phenyl rings (near to –NO₂ group) twisted and the angle between equatorial plane and phenyl ring was ∼141°. On the other hand, other phenyl ring shows slight increase of ∼156° with the mean equatorial plane (Fig. S7, ESI†). The phenyl ring which was close to the nitration site got twisted; and we speculate that twisting of one of phenyl ring was possibly due to steric hindrance of –NO₂ group. Significant deviations from ideal octahedral geometry were noted in 2·3H₂O and 3·CH₂Cl₂ due to five-membered chelate rings. Bond angles N3–Fe1–N6 and N3–Fe1–N4 were 163.1(2)° and 81.64(9)°, respectively for 2·3H₂O. However, for 3·CH₂Cl₂ bond angles N4–Fe1–N1 and N3–Fe1–N4 were found to be 82.4(3)° and 163.2(3)°, respectively. Interestingly the bond angle of axial NC–Fe–CN were 173(10)° and 166.9(4)° for 2·3H₂O and 3·CH₂Cl₂, respectively and clearly indicate greater distortion of axial bond in 3·CH₂Cl₂. Packing diagram of complex 2 shows hydrogen bonding interaction of 2.862 Å with oxygen of solvent molecule and N of axial CN molecule (Fig. S8, ESI†).

Electrochemistry

The redox properties of the iron complexes have been studied by cyclic voltammetry. Electrochemical data and representative voltammograms are displayed in Table S2† and Fig. 4. Complex 1 gave rise to an irreversible cyclic voltammogram near −0.26 V vs. Ag/AgCl. On the other hand, complexes 2 and 3 provided irreversible redox couple near 0.78 and 1.17 V vs. Ag/AgCl. It is important to note here that complex 1 is having Fe(II) high-spin iron centre, whereas complexes 2 and 3 contain Fe(II) low-spin metal centre. The presence of electron withdrawing –NO₂ group in the ligand frame of complex 3 was probably responsible for making complex 3 unstable and more difficult to oxidise compared to complex 2.²³

Fig. 4 Cyclic voltammograms of 10⁻³ M solution of complexes 1 (black), 2 (red) and 3 (blue) using working electrode: glassy-carbon, reference electrode: Ag/AgCl; auxiliary electrode: platinum wire, scan rate 0.1 V s⁻¹.

Density functional theory (DFT) calculations

Geometry optimization for ligand, Gimpy, complexes 2 and 3 have been carried out at DFT level (Fig. S9, ESI†). The coordinates of DFT-calculated compounds were shown in Table S3 and S4, ESI.† The geometrical parameters as bond lengths in gas phase were calculated using Gaussian 09 package.²⁴ Comparison of the data shows good agreement between experimental and calculated bond parameters and listed in Table S5, ESI.† TD-DFT calculations were carried out to compare calculated and experimental structural data for representative complexes (2 and 3) by using B3LYP/LAN2DZ level with Gaussian 09.²⁴ Contour plots of molecular orbitals of complexes 2 and 3 were generated using Gauss view 5.0 and shown in Fig. S10, ESI.† Electronic transition data extracted from the TD-DFT calculations their spectra and significant transitions along with their orbital contribution are given in Table S6 and Fig. S11, ESI.† Transitions below 400 nm are ligand based with oscillator strengths (f) of around 0.11–0.22 for both the complexes 2 and 3. Interestingly, theoretical data further support nitration of only one imine hydrogen rather both. According to mechanism (Scheme 4), NO₂⁺, the attacking species, prefers to go to molecule with high electron density on ligand or metal centre. If we see, highest occupied molecular orbital (HOMO) of complex 2 showed more electron density on both the imine protons and after nitration in complex 3, imine-1 showing the percentage contribution of 27% while imine-2 only 3%, due to nitration decreases the electrophilicity of imine-2 that's why only single nitration occur over the imine bonds (Tables S7 and S8, ESI†). Similar observation was reported by Darensbourg and co-worker where methylation of coordinated thiolates in zinc complex even though both thiolates in the coordination sphere are chemically equivalent, only one gets methylated because methylation decreases the nucleophilicity of the remaining un-methylated thiolate.²⁵ Highest occupied molecular orbital (HOMO) of ligand Gimpy also shows equal electron density on both imine protons (frontier orbitals diagram for ligand Gimpy, and their percentage contribution are shown in Fig. S12 and Table S9, ESI,† respectively).

Scheme 4 Frontier orbitals diagram for the HOMO of complexes 2 and 3, showing only single nitration of imine protons.

Further, at HOMO of complex 3, no electron density was found on imine proton and support only single nitration of imine proton.

Experimental

Synthetic procedures

Synthesis of 1. A batch of (127 mg, 1 mM) anhydrous FeCl₂ in 2 mL of methanol was added dropwise to a batch of Gimpy (392 mg, 1 mM) in 30 mL dichloromethane. The colour of solution was changed to green from yellow and further stirred for 3–4 h. The solution was kept at room temperature for over-night; green semi-crystalline material was obtained at the bottom of the beaker on next day. This solid was filtered and washed with ether and dried in vacuum. Shiny green crystals were obtained by ether diffusion to dichloromethane solution in freezer. Yield: 195 mg, (38%). Selected IR data (KBr, ν_max/cm⁻¹): 1580, ν_C=Nimine, 1566. UV-visible [CH₂Cl₂, λ_max/nm (ε/M⁻¹ cm⁻¹)]: 241 (14 150), 363 (21 450), 417 (6200). Λ_M/Ω⁻¹ cm² mol⁻¹ (in DMF): 18 (neutral). μ_eff (294 K) = 4.90 BM. Anal. calcd for C₂₄H₂₀N₆Cl₂Fe: C, 55.52; H, 3.88; N, 16.19, found: C, 55.29; H, 3.99; N, 16.91.

Synthesis of 2·3H₂O.
Method A. A batch of (89 mg, 0.7 mM) FeCl₂ (or 0.8 mM of Fe(ClO₄)₂·xH₂O) in 10 mL of methanol was added dropwise to stirred solution of (274 mg, 0.7 mM) Gimpy in 30 mL dichloromethane. To above mixture, a batch of (68 mg, 1.4 mM) sodium cyanide in methanol : water (5 : 1) was added after 1/2 h. The reaction mixture was further stirred for 5 h, the colour of solution was changed to purple from green during this period. After 5 h of stirring solvent was evaporated, purple-brown solid was obtained which was washed with excess of water. This solid was again dissolved in dichloromethane filtered and solvent was evaporated. Purple colour block shaped crystals were obtained by ether diffusion in dichloromethane–methanol system at −10 °C. Yield: 210 mg, (60%). Selected IR data (KBr, ν_max/cm⁻¹): 2093, ν_CN, 1598, ν_C=Nimine, 1564, 1484, 1439. UV-visible [CH₂Cl₂, λ_max/nm (ε/M⁻¹ cm⁻¹)]: 237 (21 150), 297 (22 450), 368 (19 300), 426 (3115), 576 (1450). Λ_M/Ω⁻¹ cm² mol⁻¹ (in DMF): 20 (neutral). ¹H NMR (CDCl₃, δ/ppm, 500 MHz): 8.51–8.50 (d, J = 5.5 Hz, 2H), 7.64–7.58 (m, 6H), 7.52–7.46 (m, 6H), 7.06–7.03 (m, 2H), 6.84 (s, 2H), 6.08–6.06 (d, J = 8.5 Hz, 2H). ¹³C NMR (CDCl₃, δ/ppm, 500 MHz): 158.52, 153.65, 150.02, 139.23, 135.96, 134.40, 131.36, 131.08, 129.36, 119.90, 106.94. Anal. calcd for C₂₆H₂₆N₈O₃Fe: C, 56.33; H, 4.73; N, 20.21, found: C, 56.29; H, 4.71; N, 21.09.
Method B. A batch of sodium cyanide (59 mg, 1.2 mM) in 5 mL H₂O : methanol (2 : 3) was added to stirred solution of complex 1 (250 mg, 0.5 mM) in 15 mL of dichloromethane. The green solution of complex 1 was converted to purple after 1 h and the reaction mixture was further stirred for 5 h. Purple colour reaction mixture was filtered and evaporated. The residue was taken in dichloromethane and filtered. Shiny purple crystals were collected from ether layering of this filtrate at −10 °C. Yield: 110 mg, (41%).

Synthesis of 3·CH₂Cl₂. 5 mL of 0.01 M HCl was added to stirred solution of complex 2 (15 mg, 0.03 mM) in 10 mL dichloromethane. To this stirred solution 50 mg of solid NaNO₂ was added, the purple colour of solution was changed to brownish-green within 30 min. After 30 min of stirring, the brownish-green layer was separated using separating funnel. This brownish-green solution was run through an alumina column and clear green solution was eluted in dichloromethane. The green solution was concentrated and layered with diethyl-ether and kept for crystallization in freezer. Green coloured crystals were appeared after seven days which was filtered and dried in vacuum. Yield: 5 mg, (27%). Selected IR data (KBr, ν_max/cm⁻¹): 2110, ν_CN, 1635, ν_C=N. UV-visible [CH₂Cl₂, λ_max/nm (ε/M⁻¹ cm⁻¹)]: 232 (18 430), 276 (10 820), 357 (6520), 454 (4175), 640 (930). Λ_M/Ω⁻¹ cm² mol⁻¹ (in DMF): 10 (neutral). Anal. calcd for C₂₇H₂₁N₉O₂Cl₂Fe: C, 51.45; H, 3.36; N, 20.01, found: C, 51.01; H, 3.51; N, 19.60.

Synthesis of complex 4. A drop of 0.01 M, HCl was poured to the dichloromethane (5 mL) solution of complex 2 (15 mg, 0.03 mM). Slowly, a pinch of NaNO₂ was added to the aqueous layer. Within a minute, the colour of solution changed from purple to red and finally green. This red colour was not stable therefore in situ characterization was done by UV-visible, IR and ESI-MS studies. IR data (NaCl, ν_max/cm⁻¹): 2104, ν_CN, 1917, ν_NO. ESI-MS (DCM): 507.203, [M⁺ + 3] and 504.323, [M⁺].

Materials and methods

Anhydrous FeCl₂, Fe(ClO₄)₂·xH₂O and NaNO₂ were purchased from Sigma Aldrich, Steinheim, Germany and NaCN was purchased from S. D. Fine-Chem Ltd. (India). Solvent used for spectroscopic studies were HPLC grade and purified by standard procedure before use. Ligand, Gimpy was prepared according to reported procedures.¹³ Elemental analyses were carried out microanalytically at Elementar Vario EL III. The infrared spectra were recorded with Thermo Nikolet Nexus FT-IR spectrometer after preparing KBr pellets with complexes. Electronic absorption spectra were recorded with an Evolution 600, Thermo Scientific UV-visible spectrophotometer. ESI-MS spectra were measured on a Thermo Scientific Exactive. Molar conductivities were determined in DMF at 10⁻³ M at 25 °C with a Systronics 304 conductometer. ¹H and ¹³C NMR were recorded on Bruker AVANCE, 500.13 MHz spectrometer in the deuteriated solvents. Magnetic susceptibilities were measured at 296 K with Vibrating Sample Magnetometer model 155, using nickel as a standard. Diamagnetic corrections were carried out with Pascal's increments.²⁶ Cyclic voltammetry measurements were carried out using a CHI-600C electroanalyzer in solvents like dichloromethane and acetonitrile. A conventional three-electrode arrangement consisting of platinum wire as auxiliary electrode, glassy-carbon as working electrode and Ag(s)/AgCl electrode as reference electrode, was used. These measurements were performed in the presence of 0.1 M tetrabutyl ammonium perchlorate (TBAP) as the supporting electrolyte, using complexes concentration 10⁻³ M. The ferrocene/ferrocenium couple was found at E_1/2 = +0.42 (72) V vs. Ag/AgCl under the same experimental conditions. All experiments were performed at room temperature and solutions were thoroughly degassed with nitrogen prior to beginning the experiments and during the measurements nitrogen atmosphere was maintained.

Crystallography. The X-ray data collection and processing for complexes 2·3H₂O and 3·CH₂Cl₂ were performed on Bruker Kappa Apex-II CCD diffractometer by using graphite monochromated Mo-Kα radiation (λ = 0.71070 Å) at 296 K. Crystal structure were solved by direct methods. Structure solution, refinement and data output were carried out with the SHELXTL program.²⁷ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in geometrically calculated positions and refined using a riding model. Image was created with the DIAMOND program.²⁸

Computational details. The DFT calculation for complexes 2 and 3 were carried out using Gaussian 09 program package.²⁴ The Becke's three parameters hybrid exchange functional with the Lee–Yang–Parr (LYP) non-local correlation functional was used throughout the computational study. A LANL2DZ basis set was used in the calculation. The Gauss View-5 program was used for pictorial representation of frontier molecular orbitals. Time dependent density functional theory (TD-DFT) calculations were also employed on the optimized geometries to evaluate the electronic transitions.

Conclusions

In conclusion, a new family of mononuclear high-spin [Fe^II(Gimpy)(Cl)₂], (1) and low-spin [Fe^II(Gimpy)(CN)₂], (2·3H₂O) complexes have been synthesized and characterized by various analytical, spectral and electrochemical techniques. Low-spin nature of complex 2 was further supported by ¹H and ¹³C NMR spectroscopy. Complex 1 is unable to show reactivity towards NO, as NO reactivity dependent on the spin-state of iron centre. However, complex 2 show NO reactivity to give a mixture of two different complexes; a ligand nitrated unsymmetrical complex 3via electrophilic pathways and an unstable nitrosylated metal complex 4. The formation of unstable complex evidence of by in situ IR and ESI-MS studies. Additionally, molecular structure of complexes 2·3H₂O and 3·CH₂Cl₂ were determined by X-ray crystallography. Excitingly, after nitration of imine proton twisting of one phenyl ring (near to –NO₂ group) in complex 3 was obtained, leading to a unsymmetrical complex 3, possibly due to steric hindrance of –NO₂ group. Theoretical studies further supported the nitration of one imine proton only. Isolation of unstable nitrosyl complex is under progress.

Acknowledgements

K. G. is thankful to CSIR Project of India for financial assistance (No. 01/(2720)/13/EMR-II) dated 17/4/2013. Nidhi Tyagi is thankful CSIR and Ovender Singh is thankful to MHRD for financial assistance.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S12 and Tables S1–S9 showing the characterization of the compounds and their studies under different conditions. CCDC 1489735 (complex 2) and 1489736 (complex 3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra21659e

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