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CHEMICAL PROBLEMS 2023 no. 3 (21) ISSN 2221-8688
203
UDC 54-386[548-735]
HYROGEN BOND IN CRYSTAL STRUCTURE OF SALTS OF SYMMETRIC POLYMETHYLFERRICINIUM CATiONS (SYM.[(CH3)mC5H5-m]2Fe+X", m=3,4,5 or MenFc+X-, n = 6, 8, 10; X- = PF6-, BF4-, Br3 )
N.Z. Ibrahimova1*, S.M. Rustemova2, G.M. Jafarov1, I.U. Lyatifov1
1Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry Ministry of Science and Education of the
Republic of Azerbaijan H.Javid ave., 113, AZ1143 Baku, Azerbaijan
2Azerbaijan Technical University H. Javid Ave. 25, AZ 1073 Baku, Azerbaijan e-mail: [email protected]
Received 12.06.2023 Accepted 09.08.2023
Abstract: The article explains reasons of elongation (0.019 A) of the bond between iron-cyclopentadienyl ring in the 1,2,4,1,2',4-hexamethylferricinium (sym. Ме6Fc+) cation formed due to the one-electron oxidation of iron atom of sym.1,2,4,1,2,4 -hexamethylferrocene (sym. Me6Fc) molecule which is 2-2.5 times smaller (0.04-0.05 A) than that observed in pairs of ferrocene/ferricinium (Fc/Fc+), sym. octamethylferrocene/sym.octamethylferricinium (MeFc/Me8Fc+) and decamethylferro-
cene/decamethylferricinium (Me 10Fc/Me 10Fc+). For this purpose, X-ray structural parameters characterizing the interaction between cation and anion in the sym.Me6Fc+PF6- complex was compared with appropriate parameters in Me8Fc+BF4Me10Fc+Br3 and Me1CFc+PF6-complexes; and it was mooted that there is an F"H type hydrogen bond in this complex taking into account the length of the non-valent F 'H contact (bond) in the sym. Me6Fc+PF6- complex. The lengths of P-F bonds and F-P-F angles of the PF6- anion in the listed 4 sandwich complexes were compared with appropriate parameters in the PF6- anion of the LiPF6 crystal. It has been unequivocally proved that F 'H hydrogen bonds exist in the sym.Me6Fc+PF6- complex. Keywords: sym. 1,2,4,1,2,4 -hexamethylferrocene, sym. 1,2,4,1,2,4 -hexamethylferricinium-hexafluorophosphate, hydrogen bond, Van der Waals radius of the atom DOI: 10.32737/2221-8688-2023-3-203-210
Introduction
The properties of the spatial and electronic structure of sym. polymethylferrocenes and their one-electron oxidation products - sym. polymethylferricinium cations (a) with a
sandwich structure made it possible to use them in various fields such as drugs, catalysts, sensors, inhibitors, nanomaterials, reference electrodes, etc. [1-6].
MenFc ^ MenFc+ + e (a)
Our interest in these complexes is related to the development of an electrochemical reference electrode based on sym. polymethylferrocene/sym. polymethylferricinium (MenFc/ MenFc+, n = 6, 8, 10) redox systems [7-11]. It created a need to comparatively study the crystal structures of the sym. hexamethylferrocene (sym.Ме6Fc)
molecule (b) and sym. hexamethylferricinium-hexafluorophosphate salt (sym. Me6Fc+PF6) (c) in the research carried out in this direction. In the course of the research, we faced a problem of whether there is a hydrogen bond between the cation and the anion in the salt. This article is aimed at studying the presence of F"'H hydrogen bonds in sym. Me6Fc+PF6- salt.
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CHEMICAL PROBLEMS 2023 no. 3 (21)
b)
Fig. 1. Crystal structures of the sym. Me6Fc (b) and sym. Me6Fc+PF6 (c) complexes
Experimental part
Single crystals of sym. Me6Fc+PF6- with 0.170 x 0.140 x 0.120 mm dimensions were grown in acetone at -10° C.
X-ray diffraction study of sym. Me6Fc+PF6- was carried out on a Bruker SMART APEX-II CCD three-circle automated diffractometer with an array detector (T = 150 K, MoXa-radiation, X = 0.71073 Â, graphite monochromator, 9- and ra-scan modes). The absorption corrections were applied by the SADABS program [12]. The structure was solved by the direct methods and refined by
full-matrix least-squares method on F2 in the anisotropic approximation for nonhydrogen atoms. The hydrogen atom positions were calculated geometrically and included in the refinement in accord to the riding model with fixed isotropic parameters (Tiso(H) = 1.5Fequiv(C) for CH3 groups and &iso(H) = 1.2Fgquiv(C) for all other groups). All calculations were performed using the SHELXTL software package [13]. The main experimental details and the unit cell parameters are summarized in Table 1.
Table 1. Crystallographic data on X-ray experiment and structure refinement details for sym.
Me6Fc+PF6-_
Parameter Value Parameter Value
M, g/mol 415.15 Crystal size, mm 0.170 x 0.140 x 0.120
Temperature, K 150(2) 0 range, deg. 3.305 - 25.686
X, A 0.71073 Ranges of indices h, k, l -8<h<5, -18<k<16, -20<l<20
System Monoclinic
Space group C2/c
Unit cell parameters: a, A b, A c, A 6.7313(10) 15.331(2) 16.7169(19) Total number of reflections 5247
Number of unique reflections (flint) 1622 [Rint = 0.0490]
GOOF for F2 1.135
P, deg. 90.097(5) fl-factor (I > 2o(I) R1 = 0.0623, wR2 = 0.1629
V, A3 1725.1(4)
Z 4 fl-factor (all ref lections) R1 = 0.0718, wR2 = 0.1670
p(calcd.), g/cm3 1.598
Apmax/Apmin, e A3 0.968/-0.750
p, mm-1 1.022
F(000) 852
The conditions of growing single crystals of sym. Me6Fc complex and the conducted experiment and obtained structural data by the method of X-ray structural analysis are reflected in our previous works [7, 8].
At the same time, supplementary materials on the crystal structures of sym. Me8Fc+BF4-and Me10Fc+PF6- complexes were also analyzed in order to confirm unambiguously the obtained results on these structures.
Results and discussion
The parameters of the crystal structures of sym. hexamethylferrocene (sym. Me6Fc) molecule and sym. hexamethylferricinium-hexafluorophosphate salt (sym. Me6Fc+PF6) obtained by us [7, 8] were analyzed comparatively with the appropriate structural parameters of the salts of octa- and decamethylferricinium cations ([MenFc+X-, n = 8, 10; X- = PF6-, BF4-, Br3-] known from references to investigate the presence of hydrogen bonds in the sym. Me6Fc+PF6-complex.
Results of the analysis are indicative that the elongation (Al) of the bond between the
iron-cyclopentadienyl ring (Fe-A) (A is the center of the cyclopentadienyl ring) in the ferricinium (Fc+), octamethylferricinium (sym. Me8Fc+) and decamethylferricinium (Me10Fc+) cations formed as a result of one-electron oxidation of iron atom in ferrocene (Fc), octamethylferrocene (sym. Me8Fc) and decamethylferrocene (Me10Fc) molecules is in the order of 0.04-0.05 A, whereas this difference (elongation of Fe-A bonding, Al) in the our studied sym.Me6Fc and sym. Me6Fc+PF6- complexes is 0.019 A, namely, is 2-2.5 times less [8].
Al = l(Fe-A)Me6Fc+ - l(Fe-A)Me6Fc = 0.019 A Fig. 2. Elongation (Al) of Fe-A bond during one-electron oxidation of sym. Me6Fc molecule (The H
atoms were not shown for the sake of simplicity)
This indicates that the ring in the sym. Me6Fc+ cation formed as a result of oxidation cannot move far from the iron atom due to some reason. It cannot be explained by the fact that the Fe-A bond in the Me6Fc+ cation is stronger than in the Me10Fc+ cation, because the number of electron-donating methyl groups in the Me10Fc+ cation is more. In other words, theoretically, the Fe-A bond in the Me6Fc+ cation should be weaker than that in the Me10Fc+ cation.
The research on the length of non-valent contacts (bonds) in the crystal structure of the sym. Me6Fc+PF6 complex showed that the
reason for the slight elongation of the Fe-A bond in this complex may be due to the FH hydrogen bond that can form between the cation and the anion. It is known that, according to the mechanism of intermolecular hydrogen bond formation, this type of bond is usually formed between one F, O, sometimes Cl, N, Br atom particle and another H atom (H+5) particle with partial positive charge. Therefore, at the first glance, the hydrogen bonding through the H atoms bonded to the sp3-hybridized C atom in methyl homologues of sandwich complexes formed through weakly polarized chemical bonds was not very convincing. However, the
analysis of literature data in this aspect shows that it has been proved both by theoretical calculations [14] and experimentally [15] that F atoms of PF6- and BF4- anions are able to form hydrogen bonds with H atoms bonded with sp2-and sp3-hybridized C atoms. Taking this into account, the structural parameters of sandwich complexes (sym. Me6Fc+PF6-, sym. Me8Fc+BF4-, Me10Fc+PF6-, Me10Fc+Br3-) were comparatively and carefully investigated in respect of the aspect whether there are hydrogen bonds or not.
This study showed that the F...H hydrogen bond was observed in polymethylferricinium salts for the first time in the structure of the Me8Fc+BF4- complex [15]. However, since the purpose of the research topic discussed in the article [15] is dedicated to another problem, the authors haven't studied the effect of the F"H type hydrogen bond on the structure of the sym.Me8Fc+cation, for example, the "staggered" conformation (d) of the cyclopentadienyl ligands relative to each other and the length of the Fe-A bond.
e)
Fig. 3. Staggered (d) and eclipsed (e) conformation of ferrocene [16]
The study also showed that unlike the sym. Me8Fc+BF4- complex, the article on the structure of the completely methylated Me10Fc+PF6- complex of the ferricinium cation [17] does not provide any specific information about the F"H type-hydrogen bond. Therefore, we obtained the supplementary material for this article and analyzed the appropriate structural data in order to completely determine whether there are F" H hydrogen bonds in the Me10Fc+PF6- complex [17] or not.
The analysis showed that the lengths of the P1-F1, P1-F2 and P1-F3 bonds are 1.6085 A, 1.603 A, and 1.601 A, respectively [17] in the PF6- anion of the Me10Fc+PF6- complex with a "staggered" conformation. The lengths of these bonds are practically the same as the length of the P-F bond (l.603 (1) A) [18] observed in the anion of the pure LiPF6 crystal (that is, in the state of absence of F" H type-
hydrogen bond). From this, we come to the conclusion that the F atoms of the PF6- anion in the Me10Fc+PF6 complex do not form F"H type-hydrogen bonds in the crystal state, and the fact that the cyclopentadienyl rings of the Me10Fc+ cation are in the "staggered" conformation is related to the crystal forces in the crystal lattice. Because, if there were F"H hydrogen bonds in the Me10Fc+PF6- complex, the length of P-F bond would be significantly different from 1.603 (1) A.
It ought to be noted that according to the authors [19], "there is no also interconnecting bond between tribromide Br3- anion and Me10Fc+ cation" in the Me10Fc+Br3- complex. Therefore, in our opinion, the "eclipsed" conformation (e) observed in the Me10Fc+Br3-complex was formed by the crystal packing effect of the crystal, just like the conformation (d) of the Me10Fc+PF6- complex.
Fig. 4. Scheme of F' H hydrogen bonds in the sym. Me6Fc+PF6 complex
Unlike the 3 complexes discussed above (sym. Me8Fc+BF4-, MewFc+PF6-, Me10Fc+Br3-), the structure of sym.Me6Fc+PF6- complex clearly reflects the sym. F"'H type-hydrogen bond. Thus, the sum of the Van der Waals radii of the fluorine and hydrogen atoms is 2.67 Â [20], while the lengths of FL..H(8)B(C8)* and F2'H4(C4)* non-valent bonds are 2.053 Â and 2.527 Â, which is significantly smaller than 2.67 Â. The fact that the lengths of the hydrogen bonds are smaller than the sum of the Van der Waals radii (less than 2.67 Â) indicates the presence of F" H hydrogen bonds in the sym.Me6Fc+PF6- complex.
Table 2. Reference of bond lengths and bond angles (Z) of sym. Me6Fc+PF6 , Me10Fc+PF6 _and Li PF6 coordination compounds in PF6 anion_
Coordination compounds PF6-
P-F1 P-F2 P-F3 ZF1PF2 ZF1PF3 ZF3PF2
Me6Fc+PF6- 1.570 Â 1.590 Â 1.576 Â 89.7° 91.5° 89.4°
Me10Fc+PF6- 1.6085 Â 1.603 Â 1.601 Â 90° 90° 90°
Li+PF6- 1.603 Â 1.603 Â 1.603 Â 90° 90° 90°
At the same time, the presence of F"'H type hydrogen bond is also confirmed by the values of the P-F bond lengths and F-P-F angles of the PF6anion in the sym.Me6Fc+PF6-complex. Thus, the length of the P1-F1, P1-F2 and P1-F3 bonds in PF6- anion are 1.570 A, 1.590 A, and 1.576 A, respectively; in other words, when F" H hydrogen bonds are formed in the investigated sandwich complexes, the P-F bonds in the PF6- anion should get shorter and significantly differ from the value (1.603 (1) A) observed in the PF6- anion of the LiPF6 crystal [18] (table 2).
So, the comparative study of the crystal structure of the sym. Me6Fc+PF6- complex unambiguously proves that there is an F" H type hydrogen bond in the sym.Me6Fc+PF6- complex.
It should be noted that the hydrogen bonds shown in the sym. Me6Fc+PF6- complex are oriented in space in such a way that, on the one
hand, they prevent the elongation of the Fe-A bond (see Fig. 4, hydrogen bonds F2...H and F1...H), and on the other hand, lead to the arrangement of two Me3C5H2 rings relative to each other in the sym. Me6Fc+ cation, as shown in Fig. 5 ("staggered" configuration).
Fig. 5. "Staggered" configuration of C5-rings in the sym. Me6Fc+ cation
Conclusion
The comparative analysis of crystal structures of sym. Me6Fc+PF6-, sym. Me8Fc+BF4-, Me1oFc+Br3 and Me^Fc+PF6-complexes shows that slight charging of the sym. Me6Fc+ cation with Me groups as compared to Me8Fc+ and Me10Fc+ cations gives the PF6- anion the opportunity to get very close to an iron atom and, as a result, form F1...H(8)B(C8) and F2...H4(C4) type hydrogen bonds with H atoms of the Me groups and ring. Since F1...H(8)B(C8) and
F2...H4(C4) hydrogen bonds are formed between different rings of the same cation, these bonds do not allow the rings to move away from each other (i.e. from the iron atom). As a result, during oxidation of the neutral sym. Me6Fc molecule to the sym.Me6Fc+ cation, the elongation of the metal-ring bond (0.019 A) is 2-2.5 times smaller than that observed in other MenFc/MenFc+ (n = 0, 8, 10) redox couples (from 0.04-0.05 A).
References
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ВОДОРОДНАЯ СВЯЗЬ В КРИСТАЛЛИЧЕСКОЙ СТРУКТУРЕ СОЛЕЙ СИМ. КАТИОНОВ ПОЛИМЕТИЛФЕРРИЦИНИЯ [сим.(СНз)тС5Н5-т]2Ее+Х", m=3,4,5 или
MenFs+X", n = 6, 8, 10; Х- = PFe", BF4", Вгз")
1 2 11 Н.З. Ибрагимова , С.М. Рустамова , Г.М. Джафаров , И.У. Лятифов
1 Институт катализа и неорганической химии Министерства науки и образования Азербайджана 2Азербайджанский Технический Университет e-mail: nigar-ibrahimova93@mail. ru
Аннотация: В статье интерпретированы причины 2-2.5-кратного уменьшения разности длины связи железо-циклопентадиенильное кольцо при одноэлектронном окислении молекулы сим. 1,2,4,1' ,2 ' ,4 ' -гексаметилферроцена (сим. Ме^) в катион сим. 1,2,4,1 ',2 ',4 ' -гексаметилферрициния (сим. Ме^+;Д1=0,019А) по сравнению с этой же разностью в парах ферроцен-феррициний (Fs-Fs+), сим. октаметилферроцен-сим. октаметилферрициний (сим. Ме^-сим. Me8Fs ) и декаметилферроцен-декаметилферрициний (МеloFs-МеloFs+) (Д1=0,04-0,05А). Для этого были сравнены структурные параметры, характеризующие взаимодействие катиона и аниона комплекса сим. Ме^+PF^ с соответствующими параметрами комплексов сим. Ме8Fs+BF4-, Ме1^Вг3 и Ме10Fs+PF6- и учитывая длины невалентных F' H контактов (связей) высказана идея о наличии водородных связей типа F' H в
комплексе сим. Ме6Fs+PF6-. Сравнение длин Р-Р связей и величин углов F-P-F аниона РР6- в 4-х вышеперечисленных сэндвичевых комплексах (сим. Ме6Fs+PF6-, сим. Ме8Fs+BF4-, Ме1^^г3 и Ме10Fs+PF6)- с соответствующими параметрами аниона РР6- в кристалле LiРF6 позволило однозначно доказать существование водородной связи типа Р.. Н в комплексе Ме6Fs+PF6- .
Ключевые слова: 1,2,4,1',2',4'- сим. гексаметилферроцен, гексафторфосфат-сим. 1,2,4,1',2',4'-гексаметилферрициния, водородная связь, ван-дер-Ваальсовый радиус атома
SiMMETRÍK QURULU§LU POLiMETiLFERRÍSiNiUM KATiONLARIN DUZLARININ (SiM. [(СНэ)тС5Н5-т]2Ре+Х", т=3,4,5 уэ уа МепЕ8+Х", п = 6, 8, 10); X- = РЕв", ВЕ4", Вг3) KRÍSTAL QURULU§UNDA HiDROGEN RABiTЭSi
N.Z. ibrahimova1, 8.М. Rustэmoya2, О.М. сэгэгоу1, 1.и. ьэШоу1
1Kataliz vэ Qeyri-йzvi Ктуа ^ИШи
2Azэrbaycan Texniki e-mail: [email protected]
Хи^э: Mэqalэdэ sim. 1,2,4,1 ',2',4'-heksametilferrosen (sim. Ме6Fs) molekulunda dэmir atomunun birelektronlu oksidlэ§mэsi nэticэsindэ эmэlэ gэlэn sim. 1,2,4,1',2',4'-heksametilferrisinium (sim. Ме6Fs+) kationunda dэmir-tsiklopentadienil halqasl arasmdakl rabitэnin uzanmasmm (0.019 А) ferrosen-ferrisinium (Fs-Fs+), sim. oktametilferrosen-sim. oktametilferrisinium (sim. Ме8Fs-sim. MeFs+) vэ dekametilferrosen-dekametilferrisinium (Меl0Fs-Меl0Fs+) сш!эг^э mй§aЫdэ olunandan (0.04-0.05 А) 2-2.5 dэfэ о1тазтт sэbэbi izah olunmu§dur. Випип й?йп sim. Ме6Fs+PF6- kompleksindэ kationla anion arasmda qar§lhqh tэsiri xarakterizэ edэn rentgenqurulu§ parametrlэr sim. Ме8Fs+BF4-, Меl0Fs+Br3 vэ Меl0Fs+PF6-komplekslэrindэ mйvafiq parametrlэrlэ mйqayisэ edilmi§ уэ sim. Ме6Fs+PF6- kompleksindэ qeyri-valent F' Н kontaktm (rabitэnin) uzunlugunu nэzэrэ alaraq bu kompleksdэ F"'H tipli hidrogen rabitэsinin mбvcudlugu haqqmda 6ыг irэli sйrulmй§dйr. Sadalanan 4 sendvi5 kompleksin tэrkibindэki PF6- anionun Р-F rabitэlэrinin uzunluqlaп уэ F-P-F bucaqlaп LiРР6 kristalmm РР6- anionundaki uygun parametlэrlэ mйqayisэ edilэrэk sim. Ме6Fs+PF6- kompleksindэ F"'H hidrogen rabitэsinin oldugu birmэnall sйbut edilmi§dir.
А^аг sozlэr: sim. 1,2,4,1',2',4'-heksametilferrosen, sim. 1,2,4,1',2',4'-heksametilferrismшm-heksafluoгfosfat, hidrogen rabitэsi, atomun van-der-Vaals radiusu
