Back-Bonding Signature with High Pressure: Raman Studies on Silver

Aug 3, 2017 - This effect is pronounced when A+ is replaced by Ag+ with difference in coordination ability of latter, resulting in expression of chara...
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Back-Bonding Signature with High Pressure: Raman Studies on Silver Nitroprusside Pallavi P. Ghalsasi,*,†,+ Prasanna S. Ghalsasi,*,‡,+ and D. V. S. Muthu§ †

School of Engineering & Technology, Navrachana University, Vadodara 391410, Gujarat, India Department of Chemistry, Faculty of Science, The M.S. University of Baroda, Vadodara 390001, Gujarat, India § Department of Physics, Indian Institute of Science, Bangalore 560012, India ‡

ABSTRACT: In centrosymmetric molecules, like A n+[M(CN)6]n− (where A is alkali metal cation), normally all stretching vibrations of cyanide (CN−) shift to high frequency in response to nonhydrostatic pressure, whereas, in non-centrosymmetric molecules in which one axial CN ligand is replaced by NO ligand, one observes unusual softening of only equatorial CN stretching modes. This effect is pronounced when A+ is replaced by Ag+ with difference in coordination ability of latter, resulting in expression of characteristic signature of back-bonding. One can correlate this uneven stretching of cyanide to Poisson-like effect, where the axial Fe−N, Fe−C, and C−N stretching modes harden but the equatorial C−N stretching modes soften due to expansion at the equatorial plane. Thus, the present study is focused on results of non-hydrostatic high-pressure Raman measurements on silver nitroprusside up to 11.5 GPa, for not only observing characteristic signature of “back-bonding” interaction, rarely featured in literature, but also for generating reversible flexible structures akin to noncovalent interaction.

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because π-acceptor ligand during back-bonding prefers transposition or remains in a pair opposite to each other around a metal center. Such a centrosymmetric arrangement of ligand actually removes the “elastic or flexible” nature of back-bonding and may restrict expression of its noncovalent interaction ability in design. We are presently involved in exploring this latter aspect of back-donation and its usefulness in generating technologically important property and/or its characteristic signature.17−19 Distance between C−N bond or Fe−C bond in K3[Fe(CN)6] or K4[Fe(CN)6]13 undergo isotropic change when subjected to temperature or pressure. Change of counterion from K+ to Na+ made no difference. One may conclude that centrosymmetric arrangement of ligand around metal center is responsible for isotropic behavior. Interestingly, literature has shown CN bonds have “isotropic effect” even if the centrosymmetric arrangement of −CN− is broken, a case study of Na2[Fe(NO)(CN)5].20 Sodium nitroprusside has been studied for its vasodilator application21 along with related studies focused on understanding physiological role of nitric oxide, NO.22,23 Recently it has been shown that negative thermal expansion ability in hexacyano systems can be observed by replacing Na+ with Ag+ due to presence of argentophilic interaction.24,25 That means Ag+, due to its size and coordinating ability, helped in

INTRODUCTION Back-bonding has remained one of the unique “synergistic” modus operandi between metal cation and π-acceptor ligand, primarily due to σ-donation and later π-acceptance by ligand.1 It is the topological arrangement of metal ions’ d-orbitals and π*-orbitals of ligand that drives such a formation. This not only results in change in bond distance between metal and ligating atoms but is also responsible for unusual conductivity, magnetism, and dynamic properties such as optical behavior,2−5 light-induced reversible metastable states,6,7 and structural stability.8 Most common examples of π-acceptor ligands remain restricted to CO, CN, to some extent NO, and aromatic amines. Traditionally, one of the factors that made back-bonding interesting is due to its “characteristic” signature in Fourier transform infrared (FT-IR) or vibrational spectroscopy.9−11 Unfortunately this technique cannot distinguish between σdonation and π-back-donation, which remains a challenging assignment. But, now with the combined help from extended X-ray absorption spectroscopy (EXAS) study and theoretical calculations, one can predict percent contribution of various bonding electrons.12,13 Back-donation literature leads two important facts: (1) in case of CN as a ligand, σ-donation results in shortening of C− N distance, whereas π-backdonation lengthens it (shortens Fe− C bond). This observation has a periodic trend and therefore has correlation with vibrational frequencies; (2) rarely employed as a tool in designing structures.4,−16 This may be © 2017 American Chemical Society

Received: May 10, 2017 Published: August 3, 2017 9669

DOI: 10.1021/acs.inorgchem.7b01151 Inorg. Chem. 2017, 56, 9669−9675

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Inorganic Chemistry distorting coordination polymeric structures by external parameters such as temperature or pressure. Interestingly, role of cyanide, or back-donation, in these systems was overshadowed due to coordination abilities of Ag+. This prompted us to employ non-hydrostatic high-pressure study on silver nitroprusside, a system with Ag+ ion as well as noncentrosymmetric arrangement of back-bonding ligand for understanding back-donation using Raman spectroscopy. This study will help in renewing interest in back-bonding as a flexible non-covalent interaction for structural effects to generate various physical phenomena such as conductivity, magnetism, and more importantly, thermal expansion,26−28 a topic of our research.

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Table 1. Observed Raman-Mode Frequencies at Ambient Conditions Raman frequency (cm−1)

IR frequency (cm−1)

323 432

480 (ω1) 648 (ω2) 652 (ω3) 1940 2138 2163 (ω4) 2177 (ω5) 2188 (ω6)

EXPERIMENTAL SECTION

Silver nitroprusside was synthesized by mixing aqueous solutions of sodium nitroprusside, dihydrate, Na2[Fe(CN)5NO]·2H2O (298 mg in 10 mL H2O), and AgNO3 (340 mg in 10 mL of H2O). The mixture was stirred at room temperature for ∼10 min until a peach color precipitate is formed. The precipitate is then washed with distilled water, filtered, and dried at room temperature for 1 d. The resultant compound was obtained in the form of powder. The sample is characterized by ambient Raman and FTIR spectroscopy. Raman measurements were performed on a custom-built microRaman setup consisting of Jobin Yvon iHR550 single monochromator and 532 nm laser as an excitation source. High pressure was generated using a Mao Bell-type diamond anvil cell (DAC) (High Pressure Diamond Optics Ltd., USA). Sample was filled inside a 75 μm thick pre-indented stainless steel gasket without any pressure transmission medium. Pressure calibration was done using ruby fluorescence technique.29 FTIR transmission spectra were recorded using Hyperion microscope, by keeping silver nitroprusside powder on ZnSe disks.

assignment A2

424

E

434 467 492 613 658 1939

E A1 E A1 E A1 E B1 A1 A1

2174

vibration Fe−C−N Equatorial bending Fe−C Equatorial stretching Fe−C−N bending Fe−C axial stretching Fe−C−N bending Fe−N stretching Fe−N−O bending N−O stretching C−N equatorial C−N equatorial C−N equatorial C−N axial

N−O bending modes around 650 cm−1 (ω2, ω3), equatorial C−N stretching modes (ω4, ω5) at 2163 and 2177 cm−1, respectively, and axial C−N stretching modes (ω6) at 2188 cm−1 could be traced as a function of pressure. Figure 2 shows Raman spectra in the region of 300−750 cm−1 as a function of pressure. All the modes became broad and weak in intensity as the pressure was increased. The axial Fe−C stretching mode (ω1) broadened much more as compared to the Fe−N stretching and Fe−N−O bending modes (ω2, ω3). The comparative plot showing the variation of full width at half-maximum (fwhm) of (ω1, ω2, ω3) with pressure is given in Figure 3b. On the other hand, many significant changes are observed in the line shapes of Fe−N stretching (ω2) and Fe−N−O bending mode (ω3) with increasing pressure. It was observed that at ambient pressure or the lowest pressure, ω3 is very small in intensity and is seen just as a weak shoulder of ω2. However, with increase in pressure, the relative intensity of ω2 with respect to ω3 reduces, and the intensity of ω3 modes becomes enhanced. This indicates that, with pressure, Fe−N−O bending modes contribute to the Raman cross-section more as compared to the Fe−N stretching vibrations. The variation of the relative intensities I(ω2)/I(ω3) as a function of pressure is shown in Figure 3a. At this stage let us compare our results with the highpressure effects observed in case of sodium nitroprusside,20 which reported that the intensity of the Fe−N−O bending mode becomes equal to that of Fe−N stretching mode ∼0.8 GPa, and the intensity switchover takes place above 0.8 GPa making the Fe−N−O bending mode stronger than Fe−N stretching mode. In case of silver nitroprusside, the intensities of these two modes become equal at ∼8−9 GPa. We conjecture that, because of the changes in Fe−N−O bending motion with pressure, the axial Fe−C stretching mode also gets strongly affected making it broader. Figure 3c shows linear fit to the Raman frequencies of all three modes. ω2 and ω3 indicate blue shift with very small pressure derivatives, with ω1 having the least pressure derivative among them. Figure 4 shows Raman spectra of the CN stretching modes as a function of pressure. In this frequency range too, the overall effect of pressure was to broaden the Raman modes. Of the three CN stretching modes ω4, ω5, and ω6, the ω5 mode becomes stronger with pressure, and just ∼0.8 GPa, its intensity is almost the same as that of ω6. Further increase in pressure makes ω5 stronger, and a broad peak develops. Figure 5a,b

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RESULTS AND DISCUSSION Silver nitroprusside crystallizes in monoclinic unit cell in the Pc space group30,31 with two formula units per unit cell. According to the group theoretical calculations, for the [Fe(CN)5NO]2− anion with C4v symmetry, there are 33 fundamental internal vibrations,32−34 and the irreducible representation can be written as Γ = 8A1 (IR,R) + 1A2 (inactive) + 4B1 (R) + 2B2 (R) + 9E (IR,R). Figure 1 shows ambient Raman and IR spectra, respectively. The observed Raman and IR mode frequencies at ambient conditions outside DAC along with their assignments are given in Table 1. However, inside DAC, only axial Fe−C stretching mode at 480 cm−1 (ω1), Fe−N stretching, and Fe−

Figure 1. Ambient Raman and FTIR spectra of AgNP. 9670

DOI: 10.1021/acs.inorgchem.7b01151 Inorg. Chem. 2017, 56, 9669−9675

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Figure 2. Raman Spectra of AgNP at high pressures in 300−750 cm−1 range.

Figure 3. Variation of (a) relative intensity (b) fwhm (c) Raman-mode frequency with pressure.

shows relative intensities I(ω5)/I(ω6) and fwhm of ω4, ω5, and ω6 as a function of pressure. Figure 5c shows frequencies of ω4, ω5, and ω6 with pressure. ω4 and ω5 show appreciable red shift with pressure with ω6 with small blue shift with pressure. The overall effect of non-hydrostatic pressure was found to make the material more and more disordered. The measurements were performed up to 11.5 GPa, beyond which the

Raman spectra were not discernible. From Figures 2−5, clearly all the Raman modes broaden with pressure. The frequencies of Fe−C (axial) and CN (axial) stretching modes do not shift appreciably. On the other hand, the frequencies of Fe−N stretch and δ(Fe−N−O) bending blue shift, and that of CN (equatorial) stretching red shifts. The pressure dependence of frequencies can be explained on the basis of back-donation of 9671

DOI: 10.1021/acs.inorgchem.7b01151 Inorg. Chem. 2017, 56, 9669−9675

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Figure 4. Raman Spectra of AgNP at high pressures in 2000−2400 cm−1 range.

Figure 5. Variation of (a) relative intensity, (b) fwhm, and (c) Raman-mode frequency with pressure.

electrons to π*(NO) antibonding orbital.35 Electronic structure of the nitroprusside ion gets dominated by the short Fe−N distance as is common in most of the complexes of transition metal ions with nitric oxides. According to the above scheme, the degenerate 6e(dxz; dyz) orbital is composed of mainly metal d character and contains four electrons. The highest occupied molecular orbital (HOMO) is made of 2b2(dxy) orbital and has

mainly metal d character occupied with two electrons. The lowest unoccupied molecular orbital (LUMO) is made of the antibonding 7e(π*NO) ligand orbital. There are higher levels at 3b1(dx2−dy2) and 5a1(dz2). According to Mulliken population analysis, the 2b2 orbital is composed of 84.5% dxy(Fe), 13.89% π(CN), and 1.58% π*(CN). The charge transfer from 2b2(dxy) 9672

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Figure 6. Comparison of Raman spectra of AgNP for starting sample with that of decompressed from 11.5 GPa (a) 300−750 cm−1 range and (b) 2100−2300 cm−1 range.

nitroprusside molecules and Agb+ makes comparatively weaker coordination bonds with three N atoms of three axial CN bonds of different silver nitroprusside molecules. The expansion of equatorial CN bonds against nonchanging axial CN bond can be understood in terms of this silver coordination. On the one hand, equatorial CN bridges, due to presence of dicoordinated Aga+ ion, bend N−Ag−N bond needed to accommodate elongation along equatorial CN bonds. On the other hand, axial Agb+ cation coordination being with three nitrogen atoms, though weak, does not encourage the expansion of axial CN bond and just increases disorder by slight reduction in axial CN bond length. At a higher pressure of ∼11.5 GPa, the disorder is large enough to cause the material to amorphize. The Raman spectra of the original sample and that of decompressed sample are shown in Figure 6a,b. It can be seen that on decompression from 11.5 GPa, the spectra were not completely reversible, but the features were reversible partially.

level reduces the charge density in the 2b2 orbital, which is 14% π(CN) character. The HOMO−LUMO excitation is possible with the bluegreen light, which initiates charge transfer from the 2b2(dxy) orbital to the antibonding 7e (π*NO) orbital. Bond order is the basis of bond strength. Since bond order = (no. of electrons in bonding orbital) − (no. of electrons in antibonding orbital) × 1/2, when the antibonding orbital receives electrons, the corresponding bond weakens. This could explain large red shift in NO stretching mode frequency as observed in high-pressure infrared spectra of iron(II)−mesotetraphenyl porphyrinate.36 The softening of NO bond is due to increase in its length. The Fe−N−O bond is nonlinear in the latter compound. However, in case of sodium nitroprusside,20 in which the Fe−N−O bond is linear, the NO bond frequency was shown to blue shift as a function of pressure. In silver nitroprusside, we could not trace NO mode frequency. However, because of the linear Fe−N−O bond, we can expect blue shift of NO bond frequency in silver nitroprusside as well. In addition, Fe−N−O angle depends on the population of the π*(NO) orbital,37,38 which is again dependent on the charge transfer. The change in Fe−N−O angle then can cause blue shift in δ(Fe−N−O) bending mode frequency as observed in silver nitroprusside, consistent with the study on sodium nitroprusside.20 The bending of Fe−N−O bond angle also leads to splitting of 7e(π*NO) orbital into two orbitals π1*(NO) and π2*(NO) originating possibly from lowering of the symmetry, spin−orbit coupling, or Jahn−Teller effects. Consequently the Fe−N bond shortens and shows blue shift, whereas we observed negative pressure dependence of the two equatorial CN stretching frequencies with the axial CN stretching frequency almost unchanged. Since the HOMO has 15% contribution from CN, the back-donation should reduce bond order of CN. The selective softening of equatorial CN bonds over the axial ones indicates that somehow the backdonation just affects the Fe−N−O stretch and two of the equatorial CN stretches, leaving the axial Fe−C and axial CN modes unaffected, somewhat like Poisson effect as downward compression along Fe−N−O axis causes expansion for equatorial CN bonds. According to the structure reported by Rodriguez-Hernandez,31 silver nitroprusside has atypical coordination in which silver ion has two types of possible coordinations, namely, Aga+ and Agb+. The Aga+ makes strong linear coordination with two N atoms of two equatorial CN bonds of different silver

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CONCLUSION Under non-hydrostatic pressure environment, silver nitroprusside shows that the material undergoes partially reversible amorphization above 11.5 GPa. The results indicate strong coupling between the Fe−N, Fe−N−O, and equatorial CN stretching vibrations. The Poisson effect-like behavior as observed along contraction in Fe−N bond and expansion along equatorial CN plane can be understood as a result of pressure-induced effective back-donation from Fe and NO due to diverse coordination ability of silver ions. These results are different than the pressure effects on sodium nitroprusside or Na-hexacyanides, where normally all the Raman modes, metalC, and metal-N, do show appreciable blue shift, in spite of noncentrosymmetric arrangement of π-back-bonding ligands in sodium nitroprusside, but they had no role of Ag+ ion. Thus, present work shows that back-donation needs to be effectively employed and recognized as a noncovalent interaction for generating novel structures with properties.14−16

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (P.P.G.) *E-mail: [email protected]. (P.S.G.) 9673

DOI: 10.1021/acs.inorgchem.7b01151 Inorg. Chem. 2017, 56, 9669−9675

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Pallavi P. Ghalsasi: 0000-0002-4589-5608 Author Contributions +

Equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.P.G. would like to acknowledge Prof. A. K. Sood for usage of his research laboratory, instruments, and encouragement. P.P.G. is thankful for financial support from Summer Research Fellowship Programme hosted by Academies of Sciences, DAEBRNS (2012/37P/15/BRNS/806) and DST-WOS(A) (SR/ WOS-A/PS-34/2011(G)). P.P.G. also acknowledges Sophisticated Analytical Instruments Facility at Indian Institute of Technology, Bombay, for measurements of ambient FTIR spectra, Centre for Nanoscience and Engineering, Indian Institute of Science Bangalore for measurements of ambient Raman spectrum. P.S.G. acknowledges grant from UGC-DAE Consortium for Scientific Research at Indore, India (CRS-IC-/ CRS-86/2012-15/593).

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