Complexation of Ni (ClO4) 2 and Mg (ClO4) 2 with 3-Hydroxyflavone

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Complexation of Ni(Clo) and Mg(ClO) with 3-Hydroxyflavone in Acetonitrile Medium: Conductometric, Spectroscopic and Quantum Chemical Investigation Vira N. Agieienko, and Oleg N. Kalugin J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5080829 • Publication Date (Web): 25 Sep 2014 Downloaded from http://pubs.acs.org on September 30, 2014

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The Journal of Physical Chemistry

Complexation of Ni(ClO4)2 and Mg(ClO4)2 with 3-Hydroxyflavone in Acetonitrile Medium: Conductometric, Spectroscopic and Quantum Chemical Investigation Vira N. Agieienko, Oleg N. Kalugin* Department of Inorganic Chemistry, V. N. Karazin Kharkiv National University, Svoboda sq. 4, Kharkiv, 61022, Ukraine *Corresponding author: Phone/Fax: +38 057 707 55 56. E-mail: [email protected]

The complex formation of Ni(ClO4)2 and Mg(ClO4)2 with of 3-hydroxyflavone (HL, flavonol) in acetonitrile was studied using conductometric and spectroscopic methods. It was found that interaction of nickel cation with HL leads to formation of the doubly charged [Ni(HL)]2+ complex whereas in solutions of magnesium perchlorate the complex with anion [MgClO4(HL)]+ is formed. Using the extended Lee-Wheaton equation the limiting equivalent conductivities of [Ni(HL)]2+ and [MgClO4(HL)]+ and thermodynamic constants of their formation were obtained at 288, 298, 308, 318, and 328 K. Calculated Stoke’s radii indicate weak solvation of the formed complexes and low temperature stability of their solvation shells. Based on the quantum chemical calculations and noncovalent interactions analysis, it is found that in the solvated [Ni(HL)]2+ and [MgClO4(HL)]+ complexes interaction of the Ni2+ and Mg2+ cations with flavonol occurs via the carbonyl group of HL. Complexation with Ni2+ does not change greatly the internal structure of HL: in the [Ni(HL)]2+ 1 ACS Paragon Plus Environment


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complex, flavonol shows intramolecular H-bond between 3-hydroxyl and carbonyl groups. When 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

complex with [MgClO4]+ is formed, OH-group turns out of the plane of chromone moiety that leads to rupture of intramolecular H-bond in the ligand molecule. Moreover, in the [MgClO4(HL)]+ complex, perchlorate anion possesses strong ability to interact with HL forming intracomplex Hbond between hydrogen of 3-hydroxyl group and oxygen of ClO4–. Its strength is more pronounced than in the intramolecular one in both [Ni(HL)]2+ and uncomplexed 3-hydroxyflavone.

Keywords: Magnesium and nickel perchlorates, 3-hydroxyflavone, acetonitrile solutions, conductometry, absorbance and fluorescent spectra, quantum chemical calculations, ionic equilibria, hydrogen bond. 1. Introduction Flavonoids, polyphenol derivatives widely distributed in plants, are of increasing interest due to both health benefits1-4 and specific spectroscopic properties5-6. They possess antioxidant activity2,7, the ability to scavenge active oxygen species8 and act as metal chelators9-10. Due to the ability to possess excited-state intramolecular proton transfer (ESIPT) reaction, which is exceedingly sensitive to temperature11, nature of the solvent environment12-13, trace amounts of water14, and to the presence of cations14-16, flavonoids have been recently concerned as promising substances in development of high-performance fluorescent probes for multi-charged cations17-20. Among salts, perchlorates are frequently used in the composition of flavonol-based solutions21, owing to their good solubility in different solvents22. In the papers published to date, the complexation of flavonoids with perchlorate salts in nonaqueous solutions has been investigated mostly by spectroscopic methods14-16,23. The complexation of magnesium and barium cations with flavonol derivatives in acetonitrile has been studied by absorbance and fluorescence spectroscopy both in ground14 and exited state15. The cation-ligand interactions were evidenced on the basis of both shifts of the existing ligand’s bands and appearance of the new ones. Insignificant changes of the band shapes and the position of maxima in the 2 ACS Paragon Plus Environment

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flavonols spectra with the addition of perchlorate salts authors interpreted as a formation of so-called 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

‘external’ complexes [Kt(HL)]2+ (e.g. Mg2+ and Ba2+ with 3-methoxyflavone, Ba2+ with 3hydroxyflavone) where flavonols bind with cations via carbonyl oxygen. Appearance of the new bands in absorbance and fluorescence spectra (e.g. in case of 3-hydroxyflavone and / or 3-hydroxy4′-dimethylaminoflavone in the presence of Mg(ClO4)2) was interpreted as a formation of the complexes with substitution of hydrogen atom of 3-hydroxyl group by Mg2+ cation15. Disappearance of the ESIPT reaction was also observed in the spectra of 3-hydroxyflavone and 3hydroxy-4′-dimethylaminoflavone with increasing the concentration of Mg(ClO4)2 in solution14. On the addition of Ba(ClO4)2 to acetonitrile solutions of above-mentioned flavonols the changes in their fluorescence spectra were less noticeable. The ability of Mg2+, Ca2+, and Ba2+ cations to bind flavonol molecules in m-nitrobenzyl alcohol matrix has been established by FAB mass spectrometry23. The mass spectrum of flavonol solutions containing magnesium ions revealed formation of [HL·MgClO4]+ along with complexes [MgL]+ and [MgL2]+, where HL and L– are flavonol molecules and deprotonated flavonols, respectively. It was also found that in solutions of Ca(ClO4)2 and Ba(ClO4)2 the cations preferably form complexes with perchlorate anion such as [HL·KtClO4]+ or [KtL·HClO4]+. Proton NMR spectroscopy16 was used to find the binding centers of quercetin and its derivatives complexed by Be2+, Mg2+, Ca2+, Zn2+, Cd2+ in dimethylsulfoxide – d6 solutions. Analysis of the 1H NMR signals originating from protons in non-complexed and complexed flavonols showed the existence of different types of complexes when the cation or position of methyl group in the ligand molecule was varied. Complexes’ stability constants in the ground and exited state in methanol solutions were also obtained using spectrophotometric and spectrofluorometric titrations. The literature survey shows that significant effort was spent on the elucidation of complex formation of flavonols with alkaline-earth cations; however, the answer to the question whether the formed complexes’ composition and structure can be adequately found from the results of spectroscopic experiment has been remained open. Besides, obtained complexation constants are usually calculated without taking into account neither activity coefficients of the ions nor association 3 ACS Paragon Plus Environment


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between cations and anions of the initial salts. Thus, obtained quantitative characteristics of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

investigated systems can be rather characterized as the apparent complexation constants. The idea to use conductometry for determination of constants of complexation is widely used in physical chemistry. But usually this procedure looks quite simple since (i) the charged ligands are considered, and/or (ii) the constant of complex formation is several order in magnitude exceeds the constant of ion association of corresponding electrolyte, (iii) as a rule the activity coefficients of the charged species are omitted given apparent constants of the investigated equilibria instead of thermodynamic ones. As literature review shows, the conductometry technique has never been used for determination of the constants of complexation with participation of doubly charged cations with neutral ligand taking into account activity coefficients and low values of these constants in nonaqueous media where inter-ion association influences equilibrium concentration of the doubly charged cation. In this paper we have proposed and developed a chemometric approach based on mutual analysis of conductometric data of the given electrolyte of the KtAn2 type, namely Mg(ClO4)2 and Ni(ClO4)2, without and with neutral ligand, 3-hydroxyflavone (Figure 1). Mg(ClO4)2 and Ni(ClO4)2 salts were chosen for the following reason: Mg2+ has a crystallographic ionic radius equal to that of Ni2+ (r = 0.078 nm by Goldschmidt24) but its electronic structure is substantially different. Therefore, one should expect differences in cation – anion, cation – solvent and cation – ligand interactions in the solutions of these salts. As it was shown earlier these cations show the same coordination numbers in acetonitrile25-27 and the primary constants of association KA1 Kt2+ + ClO4– ↔ KtClO4+ ,

KA1,

are quite close to each other, namely KA = 234 and KA = 251 at 298.15 K for Mg(ClO4)2 and Ni(ClO4)2, respectively28. Thus, one can expect that the differences in spectroscopic and conductive properties of the solutions of these salts in the presence of 3-hydroxyflavone will be rather determined by both specific cation – ligand(s) interactions. Thus, it was deemed worthwhile to attempt a joint analysis of conductometric and spectroscopic data and quantum chemical calculations

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to get deeper insight into the complexation behavior of magnesium and nickel perchlorates with HL 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in acetonitrile.

Figure 1. Structure and atomic numbering of 3-hydroxyflavone molecule.

The paper is organized as following: Part 2 gives experimental details concerning chemicals’ preparation, conductometric and spectroscopic measurements, theoretical details of conductometric data treatment. The model of ionic equilibria in the systems under study, discussions of dynamic and thermodynamic characteristics of the complexes are given in Part 3. Part 4 is devoted to the results of quantum chemical calculations and conclusions are outlined in the last section. 2. Experimental methods 2.1. Chemicals. Perchlorates were synthesized starting from corresponding carbonates and perchloric acid (weight fraction 40 %). Magnesium and nickel perchlorates were recrystallized once from water and then twice from acetonitrile, and final products were obtained in form of crystal solvates Mg(ClO4)2·6AN and Ni(ClO4)2·6AN. The precise concentration of Mg(ClO4)2 in stock acetonitrile solution was determined by the standard EDTA titration. The composition of Ni(ClO4)2·6AN was determined gravimetrically by means of dymethylglyoxime in the form of NiDm2. Acetonitrile of chemically pure grade was purified following the standard procedure29. The obtained solvent contained < 50 ppm water according to Karl-Fischer titration; it had specific conductance 1.0·10–7 S cm–1 at 298.15 K. 5 ACS Paragon Plus Environment


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3-hydroxyflavone (99 %) was purchased from Aldrich. The reagent was used after 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

recrystallization from acetonitrile and drying to constant mass. 2.2. Conductometric measurements. The series of 30 solutions was prepared from concentrated solutions gravimetrically by dilution. Resistance was measured using Gwinstek LCR-821 LCR meter at a 1 kHz frequency in three cells made from molybdenum glass with platinated platinum electrodes with accuracy of ca. 0.1 %. The cells were calibrated against the aqueous solutions of KCl by the standard procedure30. Temperature was controlled with accuracy of 0.01 K using water thermostats. The density gradients BNi(ClO4 )2 and BHL for the solutions of HL, Ni(ClO4)2, and HL in the presence of Ni(ClO4)2 needed for the conversion of the concentration m% (mol·(kg of solution)-1) to c (mol·dm-3)

c = d ⋅ m% , d = d 0 + Bsalt ⋅ m% salt + BHL ⋅ m% HL ,

31

(1)

were determined using a vibrating-tube densimeter (VIP-232) at (298.15 ± 0.01) K. Here, d 0 – density of pure solvent. The gradient BMg(ClO4 )2 was calculated from the concentration dependence of density of the Mg(ClO4)2 acetonitrile solutions at 298.15 K33. Bsalts and BHL were considered to be independent of temperature. Experimental equivalent conductivities of investigated ternary systems along with density gradient Bsalts and BHL are listed in Tables S1, S2 of Supporting information. 2.3. Theoretical details. Theoretical equivalent electric conductivity Λ theor is described by eq st st Λ theor eq , j ( сs , j , cL , j , A ) =

1 csst, j

r

∑c

i, j

i =1

⋅ f ( I j , λ10 ,..., λr0 , K1 ,..., K m , R ) ,

(2)

for every j-th experimental point. Here, сsst, j and cLst, j are stoichiometric concentrations of the salt and the ligand, respectively. λ 0 are the limiting equivalent conductivities of all charged particles, K are the formation constants, and R is the distance parameter. The minimization of the sum of the squares k

theor st st of deviations Q = ∑  Λ exp eq , j − Λ eq , j ( cs , j ; cL , j ; A )   ⇒ min was performed by varying the set of fitted 2

j =1

parameters A of the theoretical model describing the concentration dependence of equivalent


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conductivity. We used the extended Lee-Wheaton equation34-36 as a theoretical model for the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

concentration dependence of Λ theor . In accordance with this equation, the equivalent conductivity of eq i-th ion can be written as f ( I , A ) ≡ λeq ,i ( I , A ) = λeq0 ,i 1 + Ei ( I ) + H i ( I )  , s

s

2 3 Ei ( I ) = zi ∑ χ ip ∑ tv0 χ vp  Avp ( t ) ⋅ ( βκ ) + Bvp ( t ) ⋅ ( βκ ) + Cvp ( t ) ⋅ ( βκ )  ,   p =2 v =1

Hi ( I ) = −

zi (κτ ) 2 1 2 5 1 + Vi ( ) ( t ) ⋅ ( βκ ) + Vi ( ) ( t ) ⋅ ( βκ ) + Пi( )t 6 , 2 (1 + t )

(

)

(3)

where tv0 is the limiting transfer number of ν-th ion, t = κ R , τ = eF / ( 299.792458 ⋅ 3πη ) , and η is the viscosity of the solvent. The other terms χ ip , Avp , Bvp , Cvp , Vi ( ) , Vi ( ) , Пi( ) are defined by Lee 1

2

5

and Wheaton in original papers34-35. Some corrections37 were made to the Lee-Wheaton equation before it was used in this work. Calculations of the equilibrium concentrations of all ionic species in solution were performed taking into account ion activity coefficients38-39, the latter were represented by the Debye-Hückel equation. In this work, the flexible polyhedron technique proposed by Nelder and Mead40 was implemented for the minimization of Q functional as a part of the proprietary LWSUM program. Its mathematical foundations were described in38,41-42. Density, dielectric constant, and viscosity of acetonitrile necessary for the calculations were taken from43. 2.4. Spectroscopic Measurements. The UV-vis spectra were recorded on a Hitachi U-3210 spectrophotometer at 293.15 ± 0.1 K. Fluorescence spectra were measured on a Hitachi F-4010 spectrofluorimeter. 3. Experimental results and discussions 3.1. Spectroscopy. Figure 2A represents the absorbance spectrum of 1·10–4 mol dm–3 acetonitrile solution of HL and its spectra in the presence of 2·10–2 mol dm–3 Ni(ClO4)2 and Mg(ClO4)2 solutions.



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Figure 2. Absorbance (A) and fluorescence (B) spectra of HL, nickel perchlorate and HL with the addition of nickel and magnesium perchlorates in acetonitrile at 293 K. Scheme represents excited state intramolecular proton transfer in 3-hydroxyflavone molecule.

In case of Ni(ClO4)2, no specific changes in the absorbance spectrum of HL occur when the salt is added. One can identify the following peculiarities i) weak intensity decrease due to the dilution of the initial HL solution; ii) the widening of the right arm of the maximum in region of 380 nm, and

iii) the new peak in region of 580 nm. Explanation of the latter observations comes from the absorbance spectrum of Ni(ClO4)2 in acetonitrile which is also shown in Figure 2A. Obviously, the long-wave band with λmax = 580 nm corresponds to the one in the spectrum of solvated Ni2+. It is also clear, that overlapping the short-wave band of the Ni(ClO4)2 spectrum (λmax = 350 nm) with the ligand’s band results in the widening of the latter one in long-wave region. Addition of Mg(ClO4)2 in acetonitrile solution of HL leads to significant changes in the spectral picture. In the absorbance spectrum, we observe the appearance of the new band with λmax = 420 nm in bathochromic region that directly reflects formation of the cation-flavonol complex14.


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The fluorescent spectrum of HL and the ones induced by addition of Ni(ClO4)2 and Mg(ClO4)2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

normalized on the maximum of intensity are presented in Figure 2B. Here, the new peak with λmax = 488 nm generated by the presence of Mg(ClO4)2 is found, whereas adding the Ni(ClO4)2 solution does not change the fluorescence spectrum of the HL solution. Decrease in intensity of the 530 nm band reflecting the existence of ESIPT in HL with increasing the concentration of Mg(ClO4)214 clearly indicates that 3-hydroxyl group plays an important role in the complex formation in the investigated systems. The changes occurred in absorbance and fluorescent spectra of 3-hydroxyflavone in the presence of magnesium and nickel perchlorates clearly display that there can be formed two types of complexes with HL. In the complex with nickel cation, the structure of 3-hydroxyflavone does not differ from that of uncomplexed ligand which is reflected in the invariability of the flavonol’s shape and peak position (see Figure 2) when the salt is added. In HL – Mg(ClO4)2 solutions, the ligand’s structure is greatly affected by the comlexation process resulting in the appearance of the new bands in both absorbance and fluorescence spectra. 3.2. Conductometry. The crucial point of any chemometric analysis of the conductometric data is establishing a set of ion equilibria in the system under study. In the investigated ternary systems, the following equilibria can be written Kt2+ + ClO4– ↔ KtClO4+ ,

KA1,

KtClO4+ + ClO4– ↔ Kt(ClO4)2 , Kt2+ + HL ↔ [Kt(HL)]2+,

KL1,

(I)

KA2,

(II)

(III)

where KA1, KA2 are the first- and second-step association constants, respectively, and KL1 is the constant of complexation. As Mg(ClO4)2 and Ni(ClO4)2 are strong associated electrolytes in acetonitrile (up to 40 % of cations exist as KtClO4+ particles in 5 mmol dm–3 solutions28,33) formation of the complexes with ion associate should be also considered KtClO4+ + HL ↔ [KtClO4(HL)]+,

KL2.

(IV)



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One can assume that flavonol molecule acts with doubly charged cations forming complexes with 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substitution of the hydrogen atom of 3-hydroxyl group, which is in accordance with the abovementioned review14, the next two equations should be also examined Kt2+ + HL ↔ [Kt(L)]+ + H+,

KL3, (V)

KtClO4+ + HL ↔ [KtClO4(L)] + H+,

KL4. (VI)

The full set of fitted parameters A for the investigated systems includes the equivalent limiting conductivities of all charged particles λeq0 (Kt 2+ ) , λ 0 (ClO −4 ) , λ 0 (KtClO +4 ) , λeq0 [Kt(3HL)]2+ ,

λ 0 [KtClO 4 (3HL)]+ , λ 0 [Kt(L)]+ , and λ 0 (H + ) , constants of ion association (KA1, KA2), constants of complex formation (KL1, KL2, KL3, KL4), and the distance parameter R. It is clear that simultaneous optimization of all parameters is an extremely difficult task for reasonable amount of experimental points. On the other hand, some of these parameters can be found from independent conductometric experiment on the Kt(ClO4)2 – acetonitrile systems where only equilibria I and II would take place. The latter systems were successfully studied by us earlier in28 where all the parameters corresponded to reactions I and II were obtained with sufficient precision in temperature range 278.15 – 318.15 K for acetonitrile solutions of Mg(ClO4)2 and Ni(ClO4)2. The main conclusions which can be made from the previous results28 are (i) the second-step association constants KA2 were found to be statistically insignificant in diluted solutions of the investigated electrolytes and, therefore, can be neglected in the systems containing 3-hydroxyflavone in the corresponding range of concentrations; (ii) as the least sensitive parameter (in the set A), distance parameter R was adopted to be equal to the sum of ionic radii plus the diameter of the solvent molecule, R = r (Kt 2 + ) + r (An − ) + d s , in accordance with the Barthel’s chemical model44. Thus, as the addition of an electroneutral ligand does not change the ionic strength of the solutions, such parameters as λeq0 (Kt 2+ ) , λ 0 (ClO −4 ) ,

λ 0 (KtClO +4 ) , K A1 and R

can be taken from the previously found results28 and fixed during

optimizing the experimental conductometric data of the investigated ternary systems. After these transformations, the set of fitted parameters can be reduced as following


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A = {λeq0 [Kt(HL)]2+ , λ 0 [KtClO 4 (HL)]+ , λ 0 [Kt(L)]+ , λ 0 [KtClO 4 (L)], λ 0 (H + ), K L1 , K L 2 , K L 3 , K L 4 } . (4) We believe it is unlikely that equilibria III – VI realize in the solutions simultaneously. Thus, in order to clarify the complexation pattern we decided to analyze how the formation of each complex affects conductivity of the investigated ternary systems. To do this, the model experiment was constructed with the following major preconditions: (i) equivalent concentration of Ni(ClO4)2 and Mg(ClO4)2 was equal to 2 mmol dm–3 and did not change during the addition of the ligand in concentration range from 0.1 to 3.0 mmol·dm–3; (ii) the values of λeq0 (Kt 2+ ) , λ 0 (ClO −4 ) ,

λ 0 (KtClO +4 ) , log K A1 , and R were fixed at the level of that given in28 at 298.15 K (corresponding data are listed in Table 1); (iii) we assumed that the limiting equivalent conductivities of complexes were the same as those for the corresponding cations, notably λeq0 [Kt(HL)]2+ = λeq0 (Kt 2+ ) ,

λ 0 [Kt(L)]+ = λeq0 (Kt 2+ ) , and λ 0 [KtClO 4 (HL)]+ = λ 0 (KtClO 4+ ) , the limiting conductance of the λ 0 [KtClO 4 (L)] complex was admitted to be equal to 0; (iv) we assumed that the values of complexation constants for every complex

are equal to 3 logarithm units. The values of all

mentioned parameters are presented in Table 1. Table 1. Parameters of the model conductometric experiment at T = 298.15 Ka

λ0(½Kt2+)

Mg(ClO4)2 – HL – AN 94.2

Ni(ClO4)2 – HL – AN 112.3

λ0(ClO4–)

103.6

103.6

94.0

94.0

88.6

111.0

94.2

112.3

94.2

112.3

88.6

111.0

parameter

0

+

λ (H ) 0

+

λ (KtClO4 ) 0

2+

λ (½[Kt(HL)] ) 0

λ [Kt(F)]

+

0

λ [KtClO4(HL)]

+

log KA

2.37

2.40

KLb

3.00

3.00

log R

8.69

8.69

a

Units: λ , Sm cm mol ; KA, KL, dm mol ; R, Å.

b

Complexation constants have the same value (log KL= 3.00) for each of the reactions (III – VI).

0

2

–1

3

–1



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Figure 3 represents the changes in equivalent electrical conductance in the model acetonitrile 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

solutions of Ni(ClO4)2 and Mg(ClO4)2 when 3-hydroxyflavone is added.

Figure 3. Equivalent conductivities of model acetonitrile solutions of Ni(ClO4)2 and Mg(ClO4)2 (ceq = 2 mmol dm–3) at 298.15 K when the [Kt(HL)]2+, [KtClO4(HL)]+, [Kt(L)]+, and [KtClO4(L)] complexes are formed. Vertical blue dashed line represents a limit where cligand / cKt 2+ ratio is equal to 1.

An inspection of the Figure 3 shows the following features. (1) In case of formation of the [Kt(HL)]2+ complex, the conductance of solution increases (up to 3.2 % for Mg(ClO4)2 and up to 4.1 % for Ni(ClO4)2) and negligibly decreases (ca. 0.1 S cm2 mol–1) when [KtClO4(HL)]+ is formed. (2) As the concentration of HL increases and the [Kt(L)]+ complex is formed, the conductance of the solution dramatically increases (up to 7.7 % for Mg(ClO4)2 and up to 4.7 % for Ni(ClO4)2), approaching the constant values of conductance equal to 167.5 and 178.1 S·cm2·mol–1, respectively. The increase in conductance observed under these conditions is caused by the formation of another conductive particle, solvated proton, with λ 0 (H) + = 94.0 S·cm2·mol–1 at 298.2 ± 0.1 K45. In this case, the ion association between proton and perchlorate anion were also taken into consideration H+ + ClO4– ↔ HClO4, log KH = 2.080945.

(5)

(3) When the neutral complexes [KtClO4(L)] are formed, the slope of the corresponding curves changes drastically in a range where the ratio cligand / cKt 2+ vary from 0 to 1. Equivalent conductivity


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decreases by 43 % and 42 % for Mg(ClO4)2 and Ni(ClO4)2, respectively, from their initial values and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

takes a constant value when both cation and ion associate are fully involved in the complex. Experimental conductometric data on acetonitrile solutions of Mg(ClO4)2 and Ni(ClO4)2 (ceq = 2 mmol dm–3) in the presence of 3-hydroxyflavone are shown in Figure 4. One can be noticed, that no meaningful changes in the values of electrical conductivity are observed when the concentration of HL increases. They do not change the slope when cligand / cKt 2+ = 1 and do not plateau as it mentioned by formation of [Kt(L)]+ and [KtClO4(L)] complexes (see Figure 3).

Figure 4. Experimental equivalent conductivity of acetonitrile solutions of Ni(ClO4)2 and Mg(ClO4)2 (ceq = 2 mmol dm–3) at 298.15 K as a function of HL concentration.

As it is seen from Figure 4, the changes in conductivity caused by addition of the ligand are quite similar to those observed in case of formation the [Kt(HL)]2+ and [KtClO4(HL)]+ complexes. Under these circumstances we can reasonably draw a conclusion that in the investigated Kt(ClO4)2 – HL – AN systems complexation is not accompanied with formation of the [Kt(L)]+ and [KtClO4(L)] complexes and with simultaneous splitting off the hydrogen atom of 3-hydroxyl group of the HL molecule. The above-mentioned conclusion allows one to take into account only equations III and IV resulting in formation the [Kt(HL)]2+ and [KtClO4(HL)]+ complexes, respectively, for adequate description of the complexation in the systems under study. Clearly, the set of fitted parameters 4 reduces converting to the following view 13 ACS Paragon Plus Environment


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A = {λeq0 [Kt(HL)]2+ , λ 0 [KtClO 4 (HL)]+ , K L1 , K L 2 } . (6) reflecting the parameters which result from the complexation processes described by the equilibria III and IV. Then, three models of ionic equilibria were considered. Model A assumes that only [Kt(HL)]2+ complex is formed (Eq. III) in the investigated systems. The corresponding set of parameters is

A = {λeq0 [Kt(HL)]2+ , K L1} .

(7)

In accordance with Model B, the only complex forming in solution is [KtClO4(HL)]+ (Eq. IV). In the case the set of fitted parameters has the following view

A = {λ 0 [KtClO 4 (HL)]+ , K L 2 } .

(8)

Finally, Model C was also considered which assumes that both complexation processes III and IV occur in the solutions. Its set of fitted parameters corresponded to Eq. 6. To make decision concerning the appropriate model, we have analyzed dispersion of approximation, σ =

Q , as a function of complexes’ limiting equivalent conductivities, where k−N

Q is the sum of squares of deviations (see Section 2.3), k – the number of experimental points, N –

number of equations. In order to obtain necessary dependences experimental data was optimized at frozen values of

λeq0 [Kt(HL)]2+ and λ 0 [KtClO 4 (HL)]+ in steps of 10 S cm2 mol–1 (where it was possible) for each proposed model. Obtained dispersions we plotted as functions of

λeq0 [Kt(HL)]2+

and

λ 0 [KtClO 4 (HL)]+ at 298.15 K. The results are shown in Figs. 5 and 6 for the systems containing Ni(ClO4)2 and Mg(ClO4)2, respectively. The choice of the appropriate model was based on the presence of the pronounced minimum on the curves.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Dispersion of approximation, σ, against the values of the limiting equivalent conductivities of [Ni(HL)]2+ and [NiClO4(HL)]+ at 298.15 K in the system Ni(ClO4)2 – HL – AN.

In case of nickel perchlorate (Figure 5), for both A and B models the minimum on the curves is observed at the values of λeq0 [Ni(HL)]2+ ≈ 90 and λ 0 [NiClO 4 (HL)]+ ≈ 190 S cm2 mol–1. However, we incline to that the complexation process is accompanied with the formation of [Ni(HL)]2+ species. Firstly, the magnitude of the dispersion in the minimum in case of model B is greater than in case of model A, that are 0.82 and 0.77 S cm2 mol–1, respectively. Also, in spite of hypothesized weak solvation of the [NiClO4(HL)]+ complex the value of its limiting conductance in the minimum has extremely high non-physical value, ca. 190 S cm2 mol–1. Thus, the model A implying that Ni2+ forms [Ni(HL)]2+ complex was chosen for final experimental data treatment of the Ni(ClO4)2 – HL – AN system in whole temperature range.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 6. Dispersion of approximation, σ, against the values of the limiting equivalent conductivities of [Mg(HL)]2+ and [MgClO4(HL)]+ (A) and logarithms of complexation constants (log KL and log KL’, B) at 298.15 K in the system Mg(ClO4)2 – HL – acetonitrile. As it can be seen from Figure 6A, existence of a minimum on the curves σ = f (λeq0 [ Mg(HL) ] ) , 2+

σ = f (λ 0 [ MgClO 4 (HL)] ) for the B and C models is observed when Mg(ClO4)2 – HL – AN system +

is considered. The value of dispersion in the minimum point for the model B is greater than that for C one that, however, can be explained by greater amount of the fitted parameters when the model C is used. For this reason, the σ = f (log K L ) functions were also analyzed. The corresponding curves presented in Figure 6B. They show occurrence of the minimum only in the case of model B. Thus, scans of dispersions of approximation against λeq0 and log KL results in the conclusion that the most appropriate model for the description of the complexation pattern in the Mg(ClO4)2 – HL – AN system is the model B. The derived parameters ( λeq0 and log K L ) for the investigated systems in whole temperature range are presented in Table 2. 16 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Limiting equivalent conductivities ( λeq0 , λ 0 ), constants of complexation (log KL), radii of the unsolvated complexes R, and Stock’s radii RSt of [MgClO4(HL)]+ and [Ni(HL)]2+ in acetonitrile at 288.15 – 328.15 Ka T

a

Ni(ClO4)2 – HL, R([Ni(HL)]2+) = 4.50b

Mg(ClO4)2 – HL, R([MgClO4(HL)]+) = 4.84b

λeq0

RSt

log KL1

σ

λ0

RSt

log KL2

σ

288.15

79.9 ± 0.7

4.08

2.02 ± 0.01

0.41

123 ± 4

2.60

1.67 ± 0.01

0.28

298.15

86.0 ± 0.8

4.16

1.93 ± 0.01

0.43

133 ± 4

2.81

1.75 ± 0.01

0.35

308.15

92 ± 1

4.26

1.87 ± 0.01

0.49

137 ± 5

2.84

1.76 ± 0.01

0.47

318.15

93.8 ± 0.1

4.55

1.81 ± 0.01

0.66

140 ± 5

3.00

1.77 ± 0.01

0.58

328.15

102 ± 2

4.63

1.83 ± 0.01

0.71

143 ± 6

3.24

1.76 ± 0.01

0.68

Units: T, K; λ 0 , λeq0 , σ , Sm cm2 mol–1; KL, dm3 mol–1; R, RSt, Å.

b

From the results of quantum chemical calculations in vacuum at the B3LYP/6-31+G(d) level of computation

As it is seen from Table 2, for both systems containing Ni(ClO4)2 and Mg(ClO4)2, the complex formation constants are quite small. For the Ni(ClO4)2 – HL system, logarithms of the complexation constants slightly decrease (0.19 units) with temperature increase, and their values remain nearly constant when the temperature achieves the value of 308.15 K. Thus, this complex is enthalpy stabilized but entropy destabilized. Similar behavior was previously observed for some complexes of d-elements with macrocyclic ligands in non-aqueous solutions, namely pure acetonitrile46-47,

dimethylsulfoxide48, acetonitrile – dimethylformamide49, acetonitrile – dimethylsulfoxide50, dimethylsulfoxide – nitrobenzene48 mixtures. It can be assumed that the decrease in entropy upon complexation is related to a change in the conformational structure of the ligands, from a rather flexible one in the free state to a rigid conformation in the complex. In the case of Mg(ClO4)2 – HL system, we observe a very slight temperature-induced constant increase, namely 0.09 logarithmic units, while the higher the temperature the less the change in the values of log KL2. This behavior indicates that the complex is enthalpy and entropy stabilized. The values of equivalent conductivities of the complexes strongly increase with increasing the temperature which is basically explained by temperature-induced viscosity decrease. In order to estimate the solvation effect on the [Ni(HL)]2+ and [Mg(ClO4)(HL)]+ complexes the Stokes radii for ‘slip’ conditions were calculated using the values of equivalent limiting conductivities as follows 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

RSt = zi eF / 4πηλi0 , (9) where zi – the complex’s charge, F – Faraday constant, η – viscosity of solvent. Obtained Stokes radii are listed in Table 2. The radii of the ‘naked’ complex cations which are necessary for assessment of the thickness of the solvated shells were evaluated from B3LYP/6-31+G(d) calculations by means of Gaussian 03 program package51. Corresponding values are also given in Table 2. As it is clearly seen, the radii of the ‘naked’ complexes are somewhat greater than those obtained from equivalent conductivities of the complexes. This means the [Ni(HL)]2+ and [Mg(ClO4)(HL)]+ species are slightly solvated in acetonitrile. In case of [MgClO4(HL)]+, a difference between Ri and RSt equals 2.00 Å at 298.15 K whereas for [Ni(HL)]2+ this difference is 0.24 Å. Obviously, weak solvation of perchlorate anion and 3-hydroxyflavone in acetonitrile is the main reason of poor solvation of these complexes. Moreover, as [MgClO4(HL)]+ comprises both ClO4– and HL ligands, its Stock’s radius is smaller than that of [Ni(HL)]2+. It is also clear that Stokes radii strongly depend on temperature. They rise by 0.55 Å for [Ni(HL)]2+ and by 0.64 Å for [MgClO4(HL)]+ in the investigated temperature range. This behavior is explained by instability of the complexes’ solvation shells. 4. Quantum chemical calculations Quantum chemical calculations were carried out in order to elucidate the geometry of the investigated complexes with solvation shells being taking into account and to find out how the ligands (HL, ClO −4 , AN) influence each other under the process of complex formation. The following calculations were performed using Gaussian 0351 program package. Geometry optimizations of molecules, solvates, and solvated complexes in vacuum were carried out, without any symmetry constraints, using the 6-31+G(d) basis set. The B3LYP exchange-correlation functional was used for all calculations. Rings and atoms for each structure were numbered in accordance with numbering in Figure 1. Magnesium and nickel were considered as six-coordinated cations in acetonitrile according to the results of IR and electronic spectra of their acetonitrile solutions26-27,52 and molecular dynamics simulations25. Ion associate [MgClO4]+ was presented as a contact ion pair (CIP) which is in line 18 ACS Paragon Plus Environment

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with the results of dielectric relaxation spectroscopy, IR spectroscopy33 and conductometric 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

investigation28 of Mg(ClO4)2 in acetonitrile. In the present paper, solvent shared associates [Kt – AN – ClO4]+ were not examined. The optimized structure of the solvates [Ni(AN)6]2+ (A) and [Mg(AN)5ClO4]+ (B) are presented in Figure 7.

Figure 7. Structures of the [Ni(AN)6]2+ (A) and [MgClO4(AN)5]+ (B) solvates along with NCI surfaces. The s = 0.5 au isosurface is colored according to a blue-green-red scheme over the range – 0.03 < sign(λ2)ρ < 0.02 au.



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Perchlorate anion and 3-hydroxyflavone were considered as non-solvated ligands which is in line 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with the results of IR spectra of perchlorates salts33, diffusion coefficients of flavonoids53 and the conclusions based on flavonoids’ solubility in series of solvents54. The complexes’ composition corresponded to the results of conductometric data processing presented above (Table. 2). As far as HL is considered as a monodentate ligand25, the complexes are the products of substitution of one solvent molecule in the solvation shells of [MgClO4(AN)5]+ and [Ni(AN)6]2+, shown in Figure 7. Thus, the solvate-complexes with 3-hydroxyflavone have the following composition: [MgClO4(AN)4(HL)]+ and [Ni(AN)5(HL)]2+. Optimized structures, corresponding to the minimum energies of formation, are shown in Figure 8.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Optimized structures of 3-hydroxyflavone (A), [Ni(AN)5(HL)]2+ (B) and [MgClO4(AN)4(HL)]+ (C) complexes obtained from quantum chemical calculations at B3LYP/631+G(d) level along with NCI surfaces. The s = 0.5 au isosurface is colored according to a bluegreen-red scheme over the range – 0.03 < sign(λ2) ρ < 0.02 au.



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To elucidate the nature of interactions within the complexes the noncovalent interactions (NCIs) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

55-56

were estimated. The NCI index enables to study the domains of the electronic density associated

with weak interactions that exhibit both low electron density and low reduced density gradient(s):

s=

∇ρ 1 , (10) 1/3 2(3π ) ρ 4/3

where ρ is the electron density. By multiplying the density by the sign of the second eigenvalue of the density Hessian (λ2), one can distinguish the strength and the attractive or repulsive nature of the interactions55-56. The NCIs in solvates, 3-hydroxyflavone, and complexes are displayed as isosurfaces in Figs. 7 and 8, respectively. Here, blue indicates strong attraction, green indicates weak interaction, and red indicates strong repulsion. Spherical rings observed in case of [Ni(AN)6]2+ and [Ni(HL)(AN)5]2+ between nickel cation and nitrogen atom of AN and carbonyl oxygen of HL demonstrate formation of coordination Ni – N and Ni – O4 bonds. This type of isosurface has been previously found by NCI analysis of some d-elements in composition of complexes, namely between Cu(I) and Cl, P, S57, between Fe and benzene in ferrocenes58-59, between Au atoms in gold clusters and between Au and H, N, O in complexes of guanine with gold clusters60. Quantum chemical calculations at B3LYP/6-31+G(d) level lead to an almost planar structure of 3hydroxyflavone molecule in vacuum. The molecular structures of HL calculated at B3LYP/6-31G*61 and B3LYP/6-31G(d,p)62 levels have been also published earlier and consisted in completely flat molecule too. Main structural characteristics (bonds, distances, angles and dihedrals) as well as their experimental values obtained from the results of X-Ray data collection of single crystal63 are listed in Table 3. Comparison of the experimentally observed and calculated values shows a good agreement between each other. Thus, one can conclude that chosen level of computation is sufficient to describe adequately the structural characteristics of the HL molecule. Table 3 also summarizes main geometric parameters of 3-hydroxyflavone in composition of the [Ni(AN)5(HL)]2+ and [MgClO4(AN)4(HL)]+ complexes.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Main structural parameters of 3-hydroxyflavone (I) and the [MgClO4(AN)4(HL)]+ (II) and [Ni(AN)5(HL)]2+ (III) complexes, obtained at B3LYP/6-31+G(d) level of computationa and experimental structural characteristics of HL (the result of X-Ray investigation of single crystal) bonds O1C2 C2 C3 C3 C4 C4C10 C10C5 C5 C6 C6 C7 C7 C8 C8 C9 C9C10 C9O1 C2C1’ C1’C2’ C2’C3’ C3’C4’ C4’C5’ C5’C6’ C6’C1’ C4O4 C3O3 O3H3 Distances O4···O3 O4···H3 C4···H3 O3···Kt O4···Kt H3···Kt O3···Cl с O4···Cl с H3···Cl с H3···O1’с

I b

1.369 1.363 1.457 1.446 1.387 1.366 1.372 1.385 1.401 1.392 1.377 1.474 1.389 1.364 1.400 1.377 1.397 1.394 1.232 1.357 0.96 2.677 2.20

this paper 1.375 1.371 1.460 1.457 1.410 1.385 1.410 1.388 1.402 1.404 1.361 1.469 1.411 1.395 1.397 1.398 1.393 1.411 1.244 1.356 0.986 2.616 1.985 2.225

II

III

angles

1.360 1.385 1.443 1.454 1.412 1.384 1.411 1.387 1.401 1.403 1.359 1.470 1.410 1.394 1.398 1.399 1.392 1.411 1.261 1.359 0.990

1.357 1.388 1.431 1.443 1.414 1.384 1.412 1.386 1.401 1.408 1.358 1.461 1.413 1.393 1.398 1.400 1.391 1.414 1.280 1.361 0.981

2.774 2.587 2.578 3.862 2.014 3.429 3.815 3.774 2.841 1.770

2.621 2.011 2.260 4.065 2.071 3.245

O 1 C2 C3 C2 C3 C4 C3C4C10 C4C10C9 C9C10C5 C10C5C6 C5 C6 C7 C6 C7 C8 C7 C8 C9 C8C9C10 C10C9O1 C9 O 1 C2 O1C2C1’ C2C1’C2’ C1’C2’C3’ C2’C3’C4’ C3’C4’C5’ C4’C5’C6’ C5’C6’C1’ C6’C1’C2’ C3 C4 O 4 C4O4H3 O4H3O3 C4 C3 O 3 C3O3H3 C3O3Kt C4O4Kt O3H3Kt O3H3O1’ с H3O1’Cl с C3C2C1’C2’ C4C3O3H3 C4C3O3Kt

I b

119.7 122.8 116.0 117.9 121.2 120.8 120.1 118.7 121.2 123.4 120.2 111.1 122.7 121.0 121.3 118.5 120.0 121.3 117.8 119.8 120.8 82.5 110.0 109.0

5.5

this paper 119.1 122.4 116.0 118.4 118.9 120.2 119.9 120.8 118.8 121.2 122.0 122.2 112.4 121.7 120.3 120.7 119.4 120.4 120.7 118.5 118.6 119.5 83.7 114.2 104.0

–0.1 0.6

II

III

120.1 121.0 116.6 118.8 118.4 120.3 120.2 120.7 118.6 121.8 121.2 122.2 112.1 122.0 120.2 120.6 119.7 120.2 120.6 118.8 121.9 75.5 90.3 119.2 109.9 87.9 162.3 108.7 164.2 120.1 25.2 48.2 –5.7

118.1 122.1 117.4 117.7 118.3 120.3 120.3 120.7 118.6 121.8 121.3 123.2 113.2 121.8 120.1 120.5 119.8 120.2 120.5 118.8 117.5 83.6 118.3 116.1 104.4 83.1 144.1 142.3

–15.5 –4.4 15.8

a

Units: bond length are in Å, valence and dihedral angles are in °.

b

Original atom numbering from ref 63 were converted in accordance with IUPAC nomenclature.

с

Oxygen and chlorine atoms of perchlorate anion in the [MgClO4(AN)4(HL)]+ complex.

The data given in Table 3 show that complexation of [MgClO4(AN)5]+ and [Ni(AN)6]2+ with HL does not lead to significant changes in geometry of the A and B rings of the HL molecule. Visible changes are observed only for bonds and angles connected to carbonyl group and C ring as a 23 ACS Paragon Plus Environment


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consequence. The shortening of the O1C2, C3C4 and C4C10 bonds is accompanied with an increase of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C2C3 and C4O4 bonds. However, neither C3O3 nor O3H3 bond lengths are changed when complexes are formed. In [Ni(AN)5(HL)]2+ shown in Figure 8B, the dihedral angle ∠C3C2C1’O2’ between phenyl ring and chromone part of HL is equal to – 15.5 °. The NCI analysis indicates that the HL’s conformation in the complex with Ni2+ is additionally stabilized by van der Waals forces between O1 and O3 atoms of the ring C and hydrogen atoms of the B ring (shown as green regions on the isosurfaces). One can clearly see from Figure 8A that these interactions favor molecular planarity of uncomplexed 3hydroxyflavone as well. In the [MgClO4(AN)(HL)]+ (Figure 8C), dihedral ∠C3C2C1’O2’ equals 25.2 °. NCIs between the fragments of B and C rings of HL also promote molecule to stay comparative planar in this solvatecomplex. Thus, 3-hydroxyflavone retains extensive conjugation between the heterocyclic ring and exocyclic phenyl ring being complexed by both Ni2+ and MgClO4+. This only realizes when the magnitude of ∠C3C2C1’O2’ is approximately within the range 0 < θ < 30 °. In the complexes, distances Kt – O3 equal 3.862 and 4.073 Å for Kt = Mg and Ni, respectively, and are ca. 2 Å greater than corresponding Kt – O4 ones. These data point that interaction of the cations with flavonol occurs via the carbonyl group of HL. It is additionally confirmed by the analysis of NCIs: strong attractive interactions are observed between carbonyl oxygen O4 and Kt2+ whereas O3 does not interact with Mg2+ and Ni2+ cations directly. The most pronounced interactions hydroxyl oxygen possesses with one of the AN molecules of the cations’ solvation shells. In the [Ni(AN)5(HL)]2+ complex (Figure 8B), HL possesses intramolecular H-bond between carbonyl and hydroxyl group of the molecule. The attractive region’s intensity on NCIs’ isosurfaces is comparable to that of the uncomplexed 3-hydroxyflavone (Figure 8A). This agrees with fluorescence spectrum of 3HF in the presence of Ni(ClO4)2 in acetonitrile. Persistence of the longwavelength phototautomer band clearly indicates retention of the ESIPT effect in the studied system. This can occur only if the intramolecular H-bond in the 3HF molecule in the first coordination shell of Ni2+ cation is preserved. 24 ACS Paragon Plus Environment

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Opposed to them, intramolecular H-bond of HL in [MgClO4(AN)4(HL)]+ (Figure 8C) undergoes 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

significant changes. Distances O4···H3 and C4···H3 increase by 0.602 and 0.353 Å, respectively, and are accompanied with an increase of the dihedral angle ∠ C4C3O3H3 of 47.6 °. Thus, intramolecular H-bond between 3-hydroxyl group and carbonyl oxygen is broken. However, such geometric characteristics as r(H3···Cl) = 2.841, r(H3···O1’) = 1.770 Å, and ∠ O3H3O1’ = 164.2 ° clearly indicate formation of intracomplex hydrogen bond between oxygen atom of perchlorate anion and H3 atom of HL. It is clear that in this configuration 3-hydroxyflavone cannot exhibit the ESIPT due to geometric hindrance. Indeed, as it was mentioned earlier (see Section 3.1) addition of Mg(ClO4)2 to acetonitrile solutions of HL results in complete disappearance of the tautomer fluorescence band. Analysis of NCIs shows that the intensity of H-bonding between perchlorate anion and 3-hydroxyl group is much stronger than that between carbonyl and hydroxyl groups in uncomplexed HL and in composition

of

[Ni(AN)5(HL)]2+.

This,

apparently,

is

an

additional

factor

of

the

[MgClO4(AN)4(HL)]+ complex stabilization. We are also confident that this observation is the most prominent explanation of significant changes in the spectral pattern of 3-hydroxyflavone acetonitrile solutions in the presence of Mg(ClO4)2. Insignificant changes are observed in the structure of solvation shells when complexes are formed: i) distances Kt···N increase by ca. 0.02 Å for [MgClO4(AN)4(HL)]+; they do not change when [Ni(AN)5(HL)]2+ is formed; ii) deviations in the position of solvated AN molecules are also insignificant but more remarkable for [MgClO4(AN)4(HL)]+; iii) the effect of HL on the position of perchlorate anion is almost negligible; iv) in [MgClO4(AN)4(HL)]+, van der Waals forces between AN molecules and MgClO4+ slightly weaken compared to solvated associate, whereas no significant changes in attractive interactions are observed when [Ni(AN)6]2+ and [Ni(AN)5(HL)]2+ are concerned; v) the attractive interactions between cations and interacting sites of all the ligands of the first coordination shells ( ClO −4 , HL, AN) are of the same intensity. Analysis of the above-mentioned data results in the conclusion that the listed neutral particles (AN, HL) act as almost equivalent ligands with respect to the Ni2+ and MgClO4+ cations with the same mechanism of complex formation through lone-electron pairs. This ‘equivalency’ reflects in 25 ACS Paragon Plus Environment


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quite low values of constants of complexation which are 1.93 and 1.75 logarithms units at 298.15 K 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for the [Ni(AN)5(HL)]2+ and [MgClO4(AN)4(HL)]+ complexes, respectively. As an additional confirmation of the latter conclusion, the complexes’ formation energies as a difference between the formation energies of the products (Ep) and the reagents (Er) were estimated n

m

p =1

r =1

∆E = ∑ E p − ∑ Er , (10) where n and m are a number of the product and reagent species. Table 4 collects corresponding energies along with the reactions of complex formation themselves. One can notice that the [MgClO4(AN)4(HL)]+ complex is more energetically stable than the [Ni(AN)5(HL)]2+ one with corresponding values of energies – 24.27 and – 12.16 kJ mol–1. Table 4. Energies of the reactions of complexation in vacuum at the B3LYP/6-31+G(d) level of computation equilibriaa

reaction of complex formation

III

[Ni(AN)6]2+ + HL → [Ni(AN)5(HL)]2+ + AN

IV a

+

∆E / kJ mol–1

+

[Mg(AN)5ClO4] + HL → [MgClO4(AN)4(HL)] + AN

– 12.16 – 24.27

Correspond to the reactions given in item 3.2 taking into account solvated AN molecules

However, for both III and IV reactions, substitution of the acetonitrile molecule in the solvation shell of the Ni2+ and MgClO4+ cations by the HL molecule is not accompanied with significant energetic effect. This observation points out the fact that intensity of the cation – HL interactions is approximately the same as the cation – acetonitrile ones.

5. Conclusions In this paper the results of joint spectroscopic, conductometric and quantum chemical investigations of nickel and magnesium perchlorates with 3-hydroxyflavone in acetonitrile are presented. It was shown that in the absorbance and fluorescent spectra of the acetonitrile solutions of HL in the presence of Mg(ClO4)2 the new bands appear in comparison with acetonitrile solution of pure flavonol whereas the addition of Ni(ClO4)2 does not change greatly the spectral pattern of 3hydroxyflavone. Explanation of these differences was found by thorough analysis of conductometric 26 ACS Paragon Plus Environment

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data. It was established that complexation of the above-mentioned cations is not accompanied with 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substitution of the hydrogen atom of 3-hydroxyl group of the HL molecule. By means of analysis of dispersion of approximation as a function of limiting equivalent conductivities of the [Kt(HL)]2+ and [KtClO4(HL)]+ complexes, it was shown that complexation of Mg(ClO4)2 with HL results in formation of the complex with associate cation [MgClO4]+, while in solutions of Ni(ClO4)2 the complex with Ni2+ is formed. Using the extended Lee-Wheaton equation the limiting equivalent electrical conductivities of [Ni(HL)]2+ and [MgClO4(HL)]+ and thermodynamic constants of their formation were obtained. From the values of λeq0 , Stokes radii of the complexes were calculated. Their temperature dependences indicate both extremely weak solvation of these complex cations and low thermal stability of their solvation shells. A very slight temperature-induced formation constant dependence can be related to a little change in the conformational structure of HL when the complexes are formed. Quantum chemical calculations of HL, [Ni(AN)6]2+, [MgClO4(AN)5]+, [Ni(AN)5(HL)]2+, and [MgClO4(AN)4(HL)]+ were performed to elucidate the structure details in the structure of the formed complexes. Analysis of the geometric characteristics and noncovalent interactions shows that 3hydroxyflavone acts as a monodentate ligand with respect to the Ni2+ and MgClO4+ cations interacting via the carbonyl oxygen of 3-hydroxyflavone. Noncovalent interactions calculated from the promolecular densities of the particles indicate that formation of [Ni(AN)5(HL)]2+ is not accompanied with breaking the intramolecular H-bond in HL. Attractive interactions between hydrogen atom of 3-hydroxyl and oxygen atom of carbonyl groups in the complex and in uncomplexed flavonol are of the same intensity. Opposed to them, formation of [MgClO4(AN)4(HL)]+ results in breaking the intramolecular hydrogen bond in HL molecule whereas the intracomplex H-bond between the hydrogen atom of 3hydroxyl group and perchlorate anion is formed. Moreover, analysis of NCIs clearly shows that strength of this intracomplex H-bond is significantly stronger than that in uncomplexed ligand. This apparently is an additional factor of stabilization of the [MgClO4(AN)4(HL)]+ complex. This result is 27 ACS Paragon Plus Environment


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in line with absorbance and fluorescent spectra of acetonitrile solutions of 3-hydroxyflavone which 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

undergo significant changes in the presence of Mg(ClO4)2. Analysis of Kt2+ – AN and Kt2+ – O4(HL) NCIs along with the results of formation energies of the complexes leads to a conclusion that both AN and HL act as almost equivalent ligands with respect to Ni2+ and MgClO4+ cations. This deduction agrees with quite small values of complexation constants which are 1.93 and 1.75 logarithm units at 298.15 K for [Ni(AN)5(HL)]2+ and [MgClO4(AN)4(HL)]+ complexes, respectively.

Acknowledgements The authors gratefully acknowledge Prof. A. Doroshenko and Dr. A. Roshal (Research Institute of Chemistry, V. N. Karazin Kharkiv National University) for fruitful discussions and the help in carrying out the spectroscopic experiment. SSI “Institute for Single Crystals” and Institute for Scintillation Materials of National Academy of Science of Ukraine are acknowledged for computational facilities of joint computational cluster. Dr. Bogdan Marekha ( LASIR laboratory of Lille 1 University) is acknowledged for assistance and discussions in the NCI analysis. The present study was supported by Ministry of Education and Science of Ukraine (Grant No. 0111U010518 and grant No. 0111U006845).

Supporting Information Available Conductance of ternary systems Mg(ClO4)2 – 3-hydroxyflavone – acetonitrile and Ni(ClO4)2 – 3hydroxyflavone – acetonitrile at 288.15, 298.15, 308.15, 318.15, and 328.15 K. This information is available free of charge via the Internet at http://pubs.acs.org.

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Complexation of Ni (ClO4) 2 and Mg (ClO4) 2 with 3-Hydroxyflavone

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