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9Клатрохелаты_ МаКрОГ8Т8рОЦМКЛЬ1_Статья
Clathrochelates http://macroheterocycles .¡suet .ru Paper
DOI: 10.6060/mhc2012.120256b
Free-Radical Reaction of the Iron(II) Dichloroclathrochelate with Tetrahydrofuran Radical Derivatives: Synthesis and Structure of the Monotetrahydrofuryl-Containing Cage Complex
Mikhail A. Vershinm,a Alexey B. Burdukov,a@ Natalie V. Pervukhma,a Ilia V. Eltsov,b and Yan Z. Voloshinc
aNikolaev Institute of Inorganic Chemistry SB RAS, 630090 Novosibirsk, Russia Novosibirsk State University, 630090 Novosibirsk, Russia
cA.N. Nesmeyanov Institute of Organoelement Compounds RAS, 110991 Moscow, Russia Corresponding author E-mail: lscc@niic.nsc.ru
Free-radical substitution of the chlorine atoms in the fluoroboron-capped iron(II) dichloroclathrochelate with tetrahydrofuran radical derivatives was studied. The reaction proceeds with high regioselectivity and yields the monofunctionalized macrobicyclic product. The molecular structure of this complex was determined both in solution and in the solid state using multinuclear NMR and the single-crystal X-ray diffraction.
Keywords: Iron complexes, clathrochelates, ligand reactivity, radical substitution, X-ray crystallography, NMR spectroscopy
Introduction
Recently, we have reported the first observation of the free-radical substitution of the ribbed chlorine atoms of the boron-capped tris-dioximate iron(II) dichloroclathrochelate (Scheme 1) via its interaction with 1,4-dioxane radical derivatives. This reaction mainly afforded the mono- and bis-1,4-dioxan-2-yl-substituted iron(II) macrobicyclic complexes.[1] This paper describes a similar reaction of the tetrahydrofuryl radical, as well as the structure of the major monosubstituted tetrahydrofuryl-containing clathrochelate product (both in solution and in the solid state) and its spectral characteristics.
FeBd2(Cl2Gm)(BF)2 FeBd2(Cl(THF)Gm)(BF)2
Scheme 1.
Experimental
The 1H, 11B, 19F and 13C{1H} NMR spectra of the complex obtained were recorded from its CD2Cl2 solution with a Bruker Avance III 500 spectrometer (working frequencies 500.13 (1H), 160.46 (11B), 470.59 (19F), and 125.76 MHz (13C)). The 1H and 13C NMR chemical shifts are given in the 8 scale and referred to residual proton signals of the solvent (5.34 ppm for CHDCl2 and 53.8 ppm for CD2Cl2), the signal assignment in the corresponding spectra and structure determination were carried out on the basis of 2D COSY,
HMBC and HSQC NMR spectra. The nB and 19F NMR chemical shifts are referred to external BF3 • O(C2H5)3 and TFT. XH spin-spin coupling constants in the tetrahydrofuryl fragment and chemical shifts in the XH NMR spectrum were determined by simulation of the spin system with NUMMRIT algorithm and the method of total-lineshape fitting (RMS = 0.1357, R = 0.0067, SpinWorks 3.1.8., Bruker Topspin 2.1).
The clathrochelate precursor FeBd2(Cl2Gm)(BF)2 was obtained as described in [2].
Synthesis
1,8-Bis(2-fluorobora)-2,7,9,14,15,20-hexaoxa-3,6,10,13,16,19-hexaaza-4,5,11,12-tetraphenyl-17-chloro-18-(2-tetrahydrofuryl)bicyclo[6.6.6]eicosa-3,5,10,12,16,18-hexaen(2-) iron(2+), FeBd2(Cl(THF)Gm)(BF)2 (a typical procedure).
Complex FeBd2(Cl2Gm)(BF)2 (0.100 g, 0.13 mmol) was dissolved in THF (20 ml), and 5.5 M solution of tert-butylhydroperoxide in decane (0.7 ml) was added. The reaction mixture was refluxed for 2 h and left overnight. Then an additional portion of the tert-butylhydroperoxide solution (0.5 ml) was added, and the reaction mixture was refluxed for 2 h; the reaction course was monitored by TLC (SiO2 foil, eluent: chloroform - hexane 2:1 mixture). Then the reaction mixture was rotary evaporated to dryness and the solid residue was extracted with chloroform. The extract was purified by column chromatography (column: 1x20 cm, SiO2, eluent: chloroform). The head elute, containing mainly the clathrochelate precursor, was thrown out, and the second elute was collected. This elute was evaporated to dryness and recrystallized from dichloromethane - heptane 1:2 mixture. Yield: 0.053 g (50%). Calculated, %: C 52.1, H 3.5, N 10.7. C34H27N6O7ClB2F2Fe. Found, %: C 52.1, H 3.7, N 10.5. *H NMR (CD2Cl2) 8 ppm: 2.12 (m, 1H, CH2), 2.23 (m, 2H, C4 H2 C3 H2), 2.43 (m, 1H, C3 H2), 4.01 (td, 1H, C5 H O), 4.19 (td, 1H, C5 H2O), 5.66 (t, 1H, C2 HO), 7.38 (m, 20H, Ph). 13C(XH} NMR (CD2Cl2) 8 ppm: 27.30 (s, C4H2), 30.88 (s, C3H2), 70.22 (s, C5H2), 73.65 (s, C2H), 128.4 (s, 2-C (Ph)), 129.4 (s, 1-C (Ph)), 130.7 (s, 4-C (Ph)), 130.9 (s, 3-C (Ph)), 132.28 (s, Cl C=N), 157.38, 157.53, (two s, PhC=N), 157.71 (s, (THF)C=N).
5, ppm
CH2 C4H C4H2
Table 1. Proton spin coupling parameters for the tetrahydrofuryl fragment
J, Hz
C5H_CH_CH_
C2H 0.02 -0.07 7.41
C5H -8.01 -0.21
CH2 0.16
CH1 CH2
C4H1 C4H2
Proton spin coupling parameters for the tetrahydrofuryl fragment are listed in Table 1. "B NMR 5 ppm: 3.56 (d, JUb 19F = 16 Hz), 3.62 (d, JiiB 19F = 15 Hz). 19F NMR 5 ppm: -169.37 (q),-169.50 (q). IR v cm-1: 692, 925, 1063, 1108, 1169 v(N-O), 1215m v(B-O) + v(B-F), 1360, 1444 5(C-H), 1547m v(ClC=N), 1578w v(PhC=N), 2876w, 3059w v(C-H).
X-ray crystallography
Single crystals of the complex FeBd(Cl(THF)Gm) (BF)2-2.2CH2Cl2 suitable for the X-ray crystallography were grown by slow evaporation of its saturated solution in dichloromethane-heptane 1:2 mixture at room temperature. The crystal system of C3630H3160B2Cl5 60F2FeN6O7 (M = 977.87) is orthorhombic at 293 K: a = 15.73350(10), b = 16.6740(9), c = 17.0570(9) A, V = 4474.6(4) A3, space group P212121, Z = 4, dcalc = 1.452 g cm-3. The intensities of 19891 reflections were measured with a Bruker Nonius X8Apex equipped with a 4K CCD detector using graphite monochromated Mo-Ka radiation (X = 0.71073 A, 20 < 55°). 4685 independent reflections (R(int) = 0.0466) were used for the solution and refinement of the structure. The semiempirical absorption correction was applied using intensities of equivalent reflections (SADABS).[3] The structure was solved by the direct method[4] and refined by full-matrix least squares against F2. Non-hydrogen atoms were found on difference Fourier maps and refined with anisotropic displacement parameters (except for solvate dichloromethane molecules). The positions of hydrogen atoms were calculated and included in the refinement in isotropic approximation by the riding model. The solvate dichloromethane molecules were refined with restrained C - Cl bond lengths. The final convergence factors were R1(F) = 0.1004 and wR2 = 0.3068 for 4130 reflections with I > 2a(I). Goodness-of-fit (F2) = 1.537 for all reflections included in the last stage of refinement. All calculations were made using the SHELXTL-97program package.[5] CCDC reference number is 866037.
Insufficient quality of the diffraction data did not provide the unequivocal localization of the oxygen atom in the tetrahydrofuryl substituent. Use of the model of the merohedral twinning did not improve the refinement and afforded the statistically senseless Flack parameter of -0.0001(0.73). Therefore, no twinning model was used in the final refinement stage.
Results and Discussion
The free-radical substitution in the dichloroclathrochelate precursor FeBd2(Cl2Gm)(BF)2 with tetrahydrofuran radicals can be initiated by different radical initiators both in an inert atmosphere (Table 2) and in air; a typical synthetic procedure is given in the Experimental section. The monotetrahydrofuryl substituted clathrochelate FeBd2(Cl(THF)Gm)(BF)2 was obtained in a good yield by the reflux of the macrobicyclic precursor in tetrahydrofuran in
8.68 -0.34 -0.24 5.66
0.04 6.56 8.05 4.19
0.40 4.60 7.76 4.02
-12.41 3.97 7.84 2.43
8.19 9.02 2.22
-12.24 2.25
2.11
the presence of fert-butylhydroperoxide as a radical initiator. The benzoyl peroxide and 1,1'-azo-bis(cyclohexane-1-carbnitrile) (VAZO catalyst 88) also can be used as the radical initiators (Table 2). Moreover, the reaction studied proceeds in air, and this suggests that the dichloroclathrochelate precursor FeBd2(Cl2Gm)(BF)2 successfully competes as a radical scavenger with air oxygen for the tetrahydrofuryl radicals formed. The results of the experiments in an inert atmosphere with substoichiometric amounts of radical initiators showed that this reaction has a chain character, and its average propagation length is approximately 2.5.
Table 2. The yields of the monotetrahydrofuryl-containing macrobicycle FeBd2(Cl(THF)Gm)(BF)2 and the average propagation length in the reactions of the dichloroclathrochelate precursor FeBd2(Cl2Gm)(BF)2 with tetrahydrofuran initiated by different radical initiators ([initiator] = 10 mol %, reflux in argon atmosphere for 3 h)
Initiator Yield, % Unreacted precursor, % Average propagation length
tert-BuOOH 29 65 1.5
BzOOBz 39 60 2
(CN)C6HNNC6H4(CN) 58 40 2.5
The molecular structure of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2 in solution was studied by the one- and two-dimensional 1H and 13C NMR spectroscopies (Figures 1-5, the atom labeling is shown in Scheme 1). As it can be seen from the 1H and COSY NMR spectra, this macrobicyclic molecule contains two independent spin subsystems: the first one is formed by the aromatic protons with the total integral intensity of 20H (multiplet at 5H = 7.38 ppm), whereas the second one with the total intensity of 7H is formed by the spin-coupled 1H atoms. A set of the signals with the total intensity of 4H in the range 2.0 - 2.5 ppm is characteristic of the aliphatic protons of the alkyl chain bounded to an electron-withdrawing group (atom). The HSQC spectrum clearly demonstrated that these four protons were coupled with two carbon atoms at 27.30 and 30.88 ppm. Thus, the hydrogen atoms at these aliphatic groups of FeBd2(Cl(THF)Gm)(BF)2 are chemically non-equivalent and have the different chemical shifts, and this may be explained by the presence of a chiral center nearby. As it can be seen from the HSQC spectrum, two signals with the total integral intensity of 2H in the range 4.0 - 4.2 ppm belong to the hydrogen atoms attached to the same carbon
atom. These 5H values are characteristic of the fragments that are directly attached to an ether oxygen atom. The signal in the 'H NMR spectrum at 5.66 ppm correlates with the carbon atom at 73.65 ppm that is not bound to other hydrogen atoms. These 5H and 5C values are characteristic of a CH moiety attached to a strong electron-withdrawing group (atom), and this is the C2H fragment of the ribbed tetrahydrofuryl substituent at the macrobicyclic framework. This conclusion is supported by analogous spectral pattern observed earlier for the 1,4-dioxanyl-substituted iron(II) clathrochelate[1] and the - 13C HMBC spectrum used for the study of the longrange - 13C interactions (vide infra).
The 13C NMR spectrum of the clathrochelate obtained contains the above-mentioned four carbon signals of the tetrahydrofuryl fragment as well as the signals of the phenyl substituents in the two benzildioximate chelate fragments. These signals were assigned using both the 1H - 13C HSQC spectrum and the data for its clathrochelate analogs described earlier.[67] Four signals of the azomethine carbons of the phenyl-containing a-dioximate fragments are observed in the low-field region, whereas the signal of the chlorine-containing donor oxime group in the 13C NMR spectra usually appears in the range 130 - 135 ppm;[17-9] in the case of the synthesized clathrochelate it is observed at 132.28 ppm. As
Figure 1. 'H NMR spectrum of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2.
O > ul Vfi CO CÏ '.Ü -
V
159.0 157.5 ppcn
CIC=N
3-C
2-C
4-C
1-C
132 131 130 129
C2H ^H
1
CH I 1 CU
L-jl.
16U 1SS ISO IdS 140 13S 130 12S 120 IIS 110 10S 100 9S 90 SS 00 ?S 70 6b £0 SS SO Jb 40 3S 30 2S ppm
Figure 2.13C{'H} NMR spectrum of the clathrochelate FeBd2(Cl(THF)Gm)(BF)r MaKpœemepo^mu /Macroheterocycles 2012 5(1) 11-16
C4h2
^Tl2
—^ c5h2
^^ C'h1
UL
-i5tT 1/-1 % 2
6.0 5.5 5.0 4.5 4.0 3.5
Figure 3. COSY NMR spectrum of the clathrochelate FeBd2(Cl(THF)Gm)(BF)
C4H1
m
3.0 2.5 2.0 ppm
C2H
ClC=N
PhC=N _ (THF)C=N
c'h1 c5h2
JUL
C4h*
C3h2
c'h1 c4h2
_A_AX_ ppm
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
Figure 4. Щ - 13C HSQC NMR spectrum of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2.
Jtt 2
c h
4тт2
C h
c h
2 . 0
c h
C h
2 . 5
3 . 0
3 . 5
4 . 0
4 . 5
5 . 0
5 . 5
c h
5 . 0
c h
C4H2
C3H,
30
40
50
60
C3H2
70
80
100
110
120
130
140
150
Си1
Ch C4h2 JLÀ*_ ppm
PhC=N (THF)C=N
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5
Figure 5. 'H - 13C HMBC NMR spectrum of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2.
it can be seen from the HMBC spectrum, the signals with 8C of approximately 157 ppm belong to the azomethine carbon atoms of both benzildioximate chelate moieties (at 157.38 and 157.53 ppm) and the tetrahydrofuryl-substituted donor oxime group (at 157.71 ppm). A large number of the signals in this 13C{ 1H} NMR spectrum assigned to the phenyl and azomethine carbon atoms indicates an absence of the symmetry plane passing through the middles of the chelate C-C bonds in the a-dioximate ribbed fragments of the macrobicyclic molecule and the encapsulated iron(II) ion. The 11B and 19F NMR spectra also clearly showed the non-equivalence of the capping O3BF fragments of this molecule.
A valuable information about the molecular structure of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2 was obtained from its 1H - 13C HMBC spectrum. As it was mentioned above, the signals at 5.66 ppm in 1H NMR and at 73.65 ppm in 13C NMR spectra, as well as the corresponding doublet in the HMBC spectrum, belong to the C2H group of the ribbed tetrahydrofuryl substituent that is inherently bound to the functionalized a-dioximate chelate fragment. The hydrogen atom of this group has long-range interactions with the carbon atom of the tetrahydrofuryl-containing donor oxime group (the cross-peak at 157.71 ppm) and that of the Cl - C=N one (the cross-peak at 132.28 ppm); the cross-peak at 30.88 ppm was assigned to its interaction with the methylene unit C3H2. The latter is coupled both with the other methylene units of the tetrahydrofuryl substituent and with the azomethine carbon atom (SC = 157.71 ppm).
Thus, these 1D and 2D NMR spectra confirmed the bonding of the functionalizing tetrahydrofuryl substituent to
the clathrochelate framework through the C2 atom. Hence, the reaction studied has a high regioselectivity. This result may be explained by the stabilization of the tetrahydrofur-2-yl radical by the adjacent heteroatom.[10]
The molecular structure of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2 in the solid state was obtained from a single-crystal X-ray diffraction experiment. In the crystal of FeBd2(Cl(THF)Gm)(BF)2 • 2.2 CH2Cl2, the solvate dichloromethane molecules partially occupy three positions. The macrobicyclic molecule FeBd2(Cl(THF)Gm) (BF)2 has a chiral center at the bridging C2 atom, and the studied complex crystallizes in a chiral space group P212121. Therefore, its single crystal should contain only one of the two possible enantiomers. However, a poor quality of the best-available single crystal did not allow making unequivocal conclusions.
The clathrochelate molecule FeBd2(Cl(THF)Gm) (BF)2 (Figure 6) has a distorted trigonal-prismatic geometry characteristic of the boron-capped tris-dioximate iron(II) clathrochelates:[9] the average distortion angle ф of the FeN6-coordination polyhedron is equal to 26.6° (ф = 0° for an ideal trigonal prism; ф = 60° for a trigonal antiprism), and the height h of this polyhedron is approximately 2.34 Â. The Fe-N distances and the bite angles a (half of the chelate angles N-Fe-N) also have the typical values (1.871(10) -1.933(12) Â and 39.4(3) - 40.4(4)°, respectively).
The functionalizing tetrahydrofuryl substituent has an envelope conformation: its three carbon atoms and one oxygen atom (the oxygen position proved by NMR) are located in one plane (their deviations from the mean plane do
40
50
60
70
110
120
130
Макрогетероциклы /Macroheterocycles 2012 5(1) 11-16
15
Figure 6. General view of the clathrochelate FeBd2(Cl(THF)Gm)(BF)2. Hydrogen atoms are omitted for clarity.
not exceed 0.03 Â), whereas the bridging C2 atom deviates from this plane by 0.48 Â.
Conclusions
The free-radical substitution of the iron(II) dichloroclathrochelate precursor with radical tetrahydrofuran derivatives afforded the monofunctionalized tetrahydrofuryl-containing macrobicyclic complex, this substitution occuring via the C2 atom of the tetrahydrofuryl moiety. The reaction studied has a high regioselectivity; the total yield of the clathrochelate complexes isolated after this reaction (the sum of the precursor FeBd2(Cl2Gm)(BF)2 and the only resulting macrobicycle FeBd2(Cl(THF)Gm)(BF)2) is approximately 95-98% (100% corresponds to the initial amount of the
dichloroclathrochelate precursor). This result was explained by the stabilization of the tetrahydrofur-2-yl radical by the adjacent oxygen atom.
Acknowledgement. This study was supported by RFBR grants No 10-03-00403, 11-03-90458, 12-03-00961, 12-0390706 and 12-03-90431. The authors are indebted to Dr. N. Kuratieva (NIIC SB RAS) for obtaining the single-crystal X-ray diffraction data. I.V.E. is grateful to Carl Zeiss AG for financial support.
References
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3. Bruker AXS Inc., APEX2 (Version 1.08), SAINT (Version 7.03), and SADABS (Version 2.11). Bruker Advanced X-ray Solutions, Madison, Wisconsin, USA, 2004.
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5. Sheldrick G.M. Acta Cryst. 2008, A64, 112-122.
6. Vershinin M.A., Burdukov A.B., Boguslavskii E.G., Pervukhina N.V., Kuratieva N.V., Eltsov I.V., Reznikov V.A., Varzatskii O.A., Voloshin Y.Z., Bubnov Y.N. Inorg. Chim. Acta 2011, 366, 91-97.
7. Burdukov A.B., Vershinin M.A., Pervukhina N.V., Kozlova S.G., Eltsov I.V., Voloshin Y.Z. Russ. Chem. Bull. 2011, 24552460.
8. Voloshin Y.Z., Kostromina N.A., Krämer R. Clathrochelates: Synthesis, Structure and Properties. Amsterdam: Elsevier, 2002. 420 p.
9. Burdukov A.B., Vershinin M.A., Eltsov I.V., Pervukhina N.V., Voloshin Y.Z. Inorg. Chem. Comm. 2009, 12, 919-922.
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Received 17.02.2012 Accepted 30.03.2012
