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CHEMICAL PROBLEMS 2020 no. 2 (18) ISSN 2221-8688
145
UDC 54-386[548-735]
FEATURES OF CRYSTAL STRUCTURES OF SYMMETRIC POLYMETHYLFERROCENES
N.Z. Ibrahimova
Acad. M. Nagiyev Institute of Catalysis and Inorganic Chemistry National Academy of Sciences of Azerbaijan H. JavidAve.,113, Baku, AZ1143; e-mail: [email protected]
Received 20.02.2020 Accepted 04.04.2020
Abstract: Features of the crystal structure of the sym. MeFc molecule are compared with features of crystal structures of sym. MeFc (n = 2, 8,10) and nonsymmetrical pentamethylferrocene (Me5C5)FeCsH5. The intra-ring and off-ring valence angles of the 1,2,4-Me3C5H2 ligand in the complex sym. Me6Fc are interpreted by the steric interactions of the vicinal methyl groups with each other, as well as with a single Me-group of the second 1,2,4-Me3C5H2 ligand of the molecule. It found that in spite of the fact that changes in the Fe-(C5 ring center) bond length in a series sym. MenFc (n = 2,6,8,10) correspond to an increase in steric effects between methyl groups of different rings, there is also an electronic effect of methyl groups which does not emanate from superior steric effects in the sym. polymethylferrocenes. (CIF file CCDC No. 1436882).
Keywords: 1,2,4,1',2',4'-hexamethylferrocene, steric effect, electronic effect DOI: 10.32737/2221-8688-2020-2-145-150
Introduction
The feasible applied aspects [1] of ferrocene derivatives and the features of their electronic structure [2, 3] make these compounds one of the most intensively studied chemical systems in the last half a century. Our interest in ferrocene derivatives is related to the possibility of developing reversible electrochemical reference electrodes based on polymethylferrocene and polymethylferricenium cations in non-aqueous medium [4], as well as to the specialty of their crystal and electronic structures [5-8].
One of the factors affecting the energetic characteristics of the electron transfer reaction in a system of polymethylferrocene/ polymethylferricenium cation mentioned in the work on reference electrodes are changes in the inner-sphere reorganization energy in the course
of electron transfer. This composite was directly determined by magnitude of changes in the bond length of the iron atom-center of the cyclopentadienyl ring Fe-(C5 ring center) during the oxidation of the polymethylferrocene molecule. Therefore, first at all, we focused on the influence of steric and electronic effects of the substituents on the bond length Fe-(C5 ring center) in initial sym. Me6Fc molecule.
In the previous article [7], we revealed specific features of the crystal structure of 1,2,4,1 ',2',4'-hexamethylferrocene molecule (sym. Me6Fc). In this article, these structural features are compared with appropriate parameters of other members of a homologous series of symmetric polymethylferrocene and non- symmetric pentamethylferrocene (Me5C5)FeC5H5 [9].
Experimental part
Me6Fc was synthesized in accordance with the procedure described in [10]. 0. 16x0.13x0.11mm of Me6Fc monocrystals were grown in hexane at a temperature of 10°C.
X- ray structural analysis of Me6Fc was carried out on an automatic three-circle diffractometer with a two-coordinate detector -Bruker SMART APEX-II CCD (T = 150 K, ÀMo^x-radiation, X= 0.71073 Ao, graphite
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CHEMICAL PROBLEMS 2020 no. 2 (18)
monochromator, graphite monochromator, 9 and ra-scanning). For the obtained data, calculation of X-ray absorption was made using the SADABS program [11]. The structure was deciphered by direct methods and refined by full-matrix least square method (LSM) referring to F2 with approach to anisotropic approximation for non-hydrogen atoms. The positions of the hydrogen atoms were calculated geometrically and included in the precision
according to the "rider model" with fixed isotropic displacement parameters (Uiso(H) = 1.5Ueq(C) for CH3-groups and Uiso(H) = 1.2Ueq(C) for all other groups). All calculations were performed using the SHELXTL software package [12].
Tables of atom coordinates, bond lengths, valence and torsion angles, and anisotropic displacement parameters (ADPs) for sym. Me6Fc were deposited in the Cambridge bank with structural data: CCDC 1861054.
Results and discussion
The geometry of sym. Me6Fc molecule was shown in Fig. 1; and Table 1 shows the lengths of the Fe-(C5 ring center) in a series of
sym. MenFc polymethylferrocenes (n = 2,6,8,10).
Fig. 1. General view of sym. Me6Fc molecule and scheme of numbering the carbon atoms
Table 1. Changes in bond length of Fe-(C5 ring center) in a series of MenFc (n = 2, 6, 8, 10)
MenFc n = 2 n = 6 n = 8 n = 10
Fe-C(MemC5H5_m), À 1.647 (100 K) [13] 1.649 (173 K) [6] 1.649 (150 K) 1.653 (100 K) [14] 1.655 (298 K) [15] 1.6568 (296 K) [15] 1.651 (90 K) [13]
*— values in parenthesis are temperatures at which the X-ray structural analysis of monocrystal was carried out.
As can be seen from Table 1, the Fe-C distance (Me3C5№) (1.649 Â) in the symmetric Me6Fc molecule is situated between respective distances in the sym. Me2Fc (1.647 Â) [13] and sym. MesFc(1.653 Â) [14] and (1.655 Â) [15]. This means that in symmetric methyl homologues of ferrocene with increasing the number of Me-groups, the distance of Fe-(C5 ring center) tends to increase (Table 1), which does not correspond to the electron-donor
properties of Me-substituents.
Most likely, this discrepancy must be related to the steric effect of two substituted MemC5H5-m rings (m - is the number of methyl groups in the ligand) in sym. MenFc molecules. Mutual electrostatic repulsion of the substituted MemC5H5-m rings, preventing the shortening of Fe-(C5 ring center) bond, veiled the electronic properties of Me-groups.
However, during configuring the structure
of nonymmetric polymethylferrocene, in particular, of nonsymmetric
pentamethylferrocene (Me5C5)FeC5H5, due to the absence (or decrease) of the aforementioned steric repulsion, the electron-donor properties of the Me-groups are clearly appeared: the distance between iron and the substituted ring shortens (1.640 A [9] and 1.642 A [16]), but the distance between iron and unsubstituted ring, on the contrary, gets longer (1.653 A [9], 1.658 A [16]), (compared with ferrocene (1.654 A) [17].
The elongation of the Fe-C5H5 bond (as
compared with ferrocene) is most likely due to the antibonding effect of the Fe-C5Me5 bond, on which the electron density is composed of five methyl groups.
In changes in the bond lengths between Fe atom and carbon atoms of the C5-ring of the molecule sym. Me6Fc, the following tendency is observed: the distance from the Fe atom to the ring substituted carbon atoms (Fe-Csubstituted) is longer than distance from this Fe to the unsubstituted carbon atoms (Table 2):
Table 2. Bond length between Fe atom and carbon atoms of the C5-ring of the sym.
Me6Fc molecule
Fe-Csubstituted(C5-ring) A FeCsunsubstituted(C5-ring) A
Fe-C(1) 2.045 Fe-C(2) 2.043
Fe-C(3) 2.051 Fe-C(4) 2.037
Fe-C(5) 2.049
A similar picture is observed in the crystal structures of sym. MesFc [14, 15], Me2Fc [6] and in triple-decker sandwich complexes containing tetramethyl-cyclopentadienyl ring [18]. It is accounted that one of the reasons of elongation of these bonds may be the deviation of the Me- group in the plane of the C5-ring away from the Fe atom [18].
The nature of changes in the intra- and inter-ring valence angles of the 1,2,4-Me3C5H2 ligand allows us to identify these changes as steric effects of the vicinal methyl groups with each other, as well as with the single Me-group of the second 1,2,4-Me3C5H2 ligand bonding with an iron atom.
Fig. 2. Intra- and inter-ring valence angles of 1,2,4-Me3C5№ ligand in sym. Me6Fc molecule
Fig. 2 shows that the steric effect of vicinal Me-groups (nearest H ... H contacts in the crystal are 2.377 A) leads to a slight increase in the angles ZC1C5C8 (126.2°) and ZC5C1C6 (126.8°) from the ideal value (126°).
Differences in the values of these angles (126.2° and 126.8°), as well as of the adjacent ZC4C5C8 (126.3°) and ZC2C1C6 (125.4°) angles indicate that either the steric effect of the H8A atom (Me at C5) with the H4 atom (hydrogen at C4) is stronger than the steric effect of the H6C atom (Me at C1) with the H2 atom (hydrogen at C2), or these differences are due to the interaction of vicinal Me-groups with a single Me-group located in the second C5-ring
of the molecule. (The capital letters A, B, and C are used to distinguish the hydrogen atoms of the mentioned methyl group. Hydrogen A is located almost perpendicular to the plane of the C5-ring and away from the iron atom). However, the distances H8A (Me at C5) ... H4 and H6C (Me at C1) ... H2 are greater than the sum of the van der Waals radii of the hydrogen atom. Therefore, the observed difference between two pairs of adjacent angles is most likely explained by a stronger steric effect between H6C and H7B atoms (2.436 A), which opening the ZC5C1C6 (126.8°) angle, leads to a decrease in the ZC2C1C6 (125.4°) angle.
Fig. 3. H...H unbonded contacts between vicinal Me-groups of the 1,2,4-Me3C5H2 ligand, as well as between vicinal Me-groups and the single Me group of different 1,2,4-Me3C5H2 ligands of the Me6Fc molecule
The steric effect between vicinal Me-groups within C5-ring, as well as their interaction with a single Me-group of the second C5-ring, in turn, led the decrease of intra-ring angles ZC4C5C1 (107.8°), ZC2C1C5 (107.5°) and ZC2C3C4 (106.5°), as compared with their ideal value (108°).
Since, the C5-ring of the Me3C5H2 ligand has a flat structure; a slight decrease in the angles (ZC4C5C1 and ZC2C1C5) is also redounded on intra-ring angles at the unsubstituted carbon atoms of the ring -ZC5C4C3 and ZC1C2C3: the latter increased (108.7 and 109.4°) relative to the ideal value (108°).
The unbounded lengths between the Me-groups of different C5-rings (Fig. 3) also explain differences between deviating angles of Me-groups from the plane of the C5-ring. So, from two vicinal Me-groups (C6 and C8), the group (C6) that has the smallest unbounded
contact with a single Me-group of the other C5 ring (2.436 A between the H6C and H7B atoms) deviates more from the plane of the C5-ring (1.64°) than the Me group (C8), which has a slightly elongated unbounded contact (2.492 A between the H8B and H7C atoms) (0.87°). In turn, a single Me- group (C7), spatially interacting with both vicinal Me-groups (C6 and C8) of the other C5-ring, deviates even more from the plane of the C5-ring (2.49°) than each of the vicinal Me-groups.
Thus, the features of the crystal structure of the sym-Me6Fc molecule determined in limit of the Me3C5H2 ligand can be interpreted by steric effects between vicinal methyl groups, and also between latter and Me-groups located in another Me3C5H2 ligand. The tendency of change the bond length of Fe-(C5 ring center) in the series of sym. MenFc (n = 2,6,8,10) also corresponds to an increase in steric effect between methyl groups of different rings.
However, a comparison of these results with respect to bond lengths of Fe-(C5H5 ring center) and Fe- (C5Me5 ring center) in the nonsymmetric pentamethylferrocene indicates that a change in the bond lengths of the Fe-(C5
ring center) in sym. polymethylferrocenes also occurs under the influence of electron-donor properties of methyl groups, which do not appear in symmetric polymethylferrocenes due to the predominance of steric effects.
References
1. Astruc D. Why is ferrocene so exceptional?. Eur. J. Inorg. Chem., 2017, vol. 1, pp. 6-29.
2. Yang Y., Lei Y. Theoretical investigations of ferrocene/ferrocenium solvation in imidazolium-based room-temperature ionic liquids. Phys. Chem. Chem. Phys., 2013, vol. 15, pp. 2669-2683.
3. Guiling Z., Hui Z., Miao S., Yanhong L., Xiaohong P., Xiaoyang Y., Bo L., Zesheng L. Substitution Effect on the Geometry and Electronic Structure of the Ferrocene. J. Comput. Chem. 2007, vol. 28, pp. 22602274.
4. Geiger W.E. Complementary Use of Electrochemistry and Synthetic Redox Chemistry in the Oxidation of Decamethylferrocene: An Integrated Advanced Laboratory Experiment. Journal of Chemical Education. 2018, vol. 95, pp. 1648-1653.
5. Andrew J. A., Christoph R. J., Matthias B. Probing the Electronic Structure of Substituted Ferrocenes with High-Resolution XANES Spectroscopy. Chem. Eur. J. 2012, vol. 18, no.23, pp. 7021-7025.
6. Lousada, C. M.; Pinto, S. S.; Canongia Lopes, J. N.; Minas da Piedade, F. M.; Diogo, H. P.; Minas da Piedade, M. E. Experimental and Molecular Dynamics Simulation Study of the Sublimation and Vaporization Energetics of Iron Metalocenes. Crystal Structures of Fe(n5-C5H4CH3)2 and Fe[(n5-(C5H5)(n5-C5H4CHO)]. J. Phys. Chem. A. 2008, vol. 112, pp.2977-2987.
7. Ibrahimova N.Z., Jafarov G.M., Tagiyev D.B., Lyatifov I.U. Crystal Structure of 1,2,4,1',2',4'-Hexamethylferrocene. Russian Journal of Coordination Chemistry. 2020, vol. 46, no. 1, pp. 53-57.
8. Ibrahimova N.Z., Mammadov I.G., Jafarov G.M., Salimov R.M., Lyatifov I.U. 1H NMR investigation of diamagnetic 1,2,4,1',2',4'-
hexamethylferrocene, paramagnetic
1,2,4,1',2',4'-hexamethylferricinium hexafluorophosphate and electron exchange between them. Chemical Problems. 2017, no. 1, pp. 51-58.
9. Zanin I.E., Antipin M.Yu., Struchkov Yu.T., Kudinov A.R., Rybinskaya M.I. The molecular and crystal structure of pentamethylferrocene (q5-(C5H5)Fe(q5-(C5Me5) in the range 153-293 K. Analysis of thermal motion in a crystal by X-ray diffraction data. Metallorganic Chemistry. 1992, vol. 5, no.3, pp. 579-588.
10. Ibrahimova N.Z., Mammadov I.G., Jafarov G.M., Salimov R.M., Lyatifov I.U. Poly-methylferrocenes and relevant polymethylferricinium cations. News of Baku University. 2016, no. 1, pp. 27-32.
11. Sheldrick G.M. SADABS v. 2.03, Bruker/Siemens Area Detector Absorption Correction Program, Bruker AXS, Inc., Madison, WI, 2003.
12. Sheldrick G.M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, C71, pp. 3.
13. Makal A.M., Plazuk D., Zakrzewski J., Misterkiewicz B. and Wozniak K. Experimental Charge Density Analysis of Symmetrically Substituted Ferrocene Derivatives. Inorg.Chem. 2010, vol. 49, pp. 4046-4059.
14. Sutton L. E., Ed. "Tables of Interatomic Distances and Configuration in Molecules and Ions." Supplement 1956-1959, Chemical Society: London, 1965.
15. Struchkov Yu.T., Andrianov V.G., Salnikova T.N., Lyatifov I.R., Materikova R.B. Crystal and molecular structures of two polymethylferrocenes: sym-octamethylferrocene and decamethylferrocene. J. Organomet. Chem. 1978, vol. 145, pp. 213-223.
16. Bildstein B., Hradsky A., Kopacka H., Malleier R., Ongania K.H. Functionalized pentamethylferrocenes:Synthesis, structure, and electrochemistry. J. Organomet. Chem. 1997, vol. 540, pp.127-145.
17. Freyberg D.P., Robbins J.L., Raymond K.N. and Smart J.C. Crystal and molecular structures of decamethylmanganocene and decamethylferrocene. Static Jahn-Teller
distortion in a metallocene. J. Amer. Chem. Soc. 1979, vol. 101, pp. 892-897. 18. Schwartz A.J., Pike R.D., Herber R.H. and Watson E.J. Syntheses, Structures, and Mossbauer Effect Spectroscopy of TripleDecker Complexes Incorporating Nonamethylferrocene. Organometallics. 2016, vol. 35, no 1, pp. 62-69.
SiMMETRiKPOLiMETiLFERROSENLdRINKRiSTAL QURULUgLARININ
XÜSUSiYYdTLdRi
N.Z. ibrahimova
AMEA-nm akad. M.Nagiyev adina Kataliz va Qeyri-üzvi Kimya înstitutu AZ1143, Baki, H.Cavid pr., 113; e-mail: [email protected]
Sim. MeeFc molekulunun kristal quruluçunun xüsusiyyatlari sim. polimetilferrosenlarin MenFc (n = 2,8,10) va qeyri-sim. pentametilferrosenin (Me5C5)FeC5H5 kristal quruluçlarinin xüsusiyyatlari ila müqayisa edilir. Sim. MeeFc kompleksinda 1,2,4-Me3C2H5 ligandinin daxili va xarici valent hucaqlari molekulda visinal metil qruplarinin ham hir-hiri ila, ham da ikinci 1,2,4-Me3CsH2 ligandinda yerla§mi§ tak Me qrupu ila faza qarçiliqli tasiri ila izah olunur. Göstarilir ki, sim. MenFc (n = 2,6,8,10) sirasinda metil qruplarinin faza qarçiliqli tasiri onlarin elektron qarçiliqli tasirindan güclü oldugu ügün Fe-C(MemCsHs-m) rahitasinin uzunlugunda mü§ahida olunan dayiçiklik metil qruplarinin halqalar arasindaki faza qarçiliqli tasirinin artmasi ila alaqadardir.(CIF file CCDC No. 1436882).
Äçar sözlw. 1,2,4,1',2',4'-heksametilferrosen, faza qarçiliqli tasir, elektron qarçiliqli tasir
ОСОБЕННОСТИ КРИСТАЛЛИЧЕСКИХ СТРУКТУР СИМЕТРИЧНЫХ
ПОЛИМЕТИЛФЕРРОЦЕНОВ
Н.З. Ибрагимова
Институт катализа и неорганической химии им. акад. М.Нагиева Национальной АН Азербайджана AZ1143 Баку, пр. Г. Джавида, 113; e-mail: nigar-ibrahimova93@mail. ru
Особенности кристаллической структуры молекулы сим. МеЕе сравниваются с особенностями кристаллических структур сим. МепЕс (n=2,8,10) и несим. пентаметилферроцена (Ме5С5)ЕеС5Н5. Внутри- и внекольцевые валентные углы 1,2,4-МезС5Н2 лиганда в комплексе сим. МеЕс интерпретированы стерическими взаимодействиями вицинальных метильных групп как между собой, так и с одиночной Ме группой второго 1,2,4-МезС5Н2 лиганда молекулы. Показано, что, хотя изменение длины связи Ее-С(МетС5Н5-т) в ряду сим. МепЕс (n=2,6,8,10) и соответствует росту стерического взаимодействия между метильными группами разных колец, однако, в симметричных полиметилферроценах имеет место и электронное влияние метильных групп, которое не проявляется из-за преобладания стерических взаимодействий. (CIF file CCDC No. 1436882). Ключевые слова: 1,2,4,1',2',4'-гексаметилферроцен, стерическое взаимодействие, электронное взаимодействие
