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The novel organolithium O,C,O-pincer compound
Inorganica Chimica Acta 358 (2005) 2422–2426 www.elsevier.com/locate/ica
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The novel organolithium O,C,O-pincer compound Roman Jambor a,*, Libor Dosta´l a, Ivana Cı´sarˇova´ b, Alesˇ Ru˚zˇicˇka a, Jaroslav Holecˇek a a
Department of General and Inorganic Chemistry, University of Pardubice, na´m. Cˇs. Legiı´ 565, Pardubice CZ-532 10, Czech Republic b Department of Inorganic Chemistry, Faculty of Science, Charles University, Albertov 6, Prague CZ-128 43, Czech Republic Received 3 March 2004; accepted 11 January 2005
Abstract The preparation and characterization of organolithium O,C,O-pincer compound [2,6-(tBuOCH2)2C6H3]-Li (1) is described. The X-ray diffraction techniques revealed the dimeric structure of 1 consisting of two lithium atoms Li(1) and Li(2) and two monoanionic chelating aryl ligands in the solid state, where each lithium atom is coordinated by two oxygen atoms of two different ligands. The NMR spectroscopy and cryoscopy measurements established monomer–dimer equilibrium of 1 in concentrated solutions of non-coordinated solvents, while the diluted solutions of 1 consist of monomer only. 2005 Elsevier B.V. All rights reserved. Keywords: Lithium; O ligands; X-ray diffraction
1. Introduction Organolithium compounds constitute a commonly used and very important class of organometallic compounds frequently used in organic and organometallic synthesis for several decades [1]. The formation of aggregates in solid state [2], in solution [3], and even in gas phase [4] is a well known phenomenon of this class of compounds. It is generally recognized, that the degree of aggregation has a strong influence on the reactivity of the complexes. In order to better understand their reactivity, the structure of this class of compounds has been studied both in the solid state by X-ray crystallography [2a–f] and in solution mainly by NMR spectroscopy [3d–l]. Recently, we reported on the synthesis of O,C,O-chelating ligands, [2,6-(ROCH2)2C6H3]-, where R = Me, *
Corresponding author. Tel.: +420 46 603 7151; fax: +420 46 603 7068. E-mail address: [email protected] (R. Jambor). 0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.01.003
iPr, tBu. These ligands have been used for synthesis and structural studies of hypercoordinated organotin(IV) compounds [5]. The preparation of these compounds was based on the metathetical exchange reaction of organolithium coumpounds (mostly prepared in situ) with appropriate (organo)tin chlorides. For better understanding of the reactivity of the starting organolithium compounds, we isolated one of these derivatives [2,6-(tBuOCH2)2C6H3]Li (1) during the preparation and here we report on the synthesis and the structure of this organolithium compound 1.
2. Experimental All reactions as well as the sample preparations for NMR and cryoscopy were carried out under Schlenk techniques. The solvents were dried by standard methods and distilled prior to use. The 1H, 7Li and 13C NMR spectra were acquired on Bruker Avance500 spectrometer in toluene-d8 at 300 K (in the case of 1H and
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C NMR at the range of 300–170 K). Appropriate chemical shifts were calibrated on residual peak of toluene: 1H, dH = 2.09 ppm; 13C, dC = 20.39 ppm. The lithium chemical shifts are given relative to external 0.1 M LiBr in THF solution (d = 0.00 ppm). 2,6-bis(tert-butoxymethyl)phenyl was prepared analogously to the literature [6]. 2.1. Preparation of 2,6-bis(tert-butoxymethyl) phenyllithium (1) To a stirred hexane (20 mL) solution of 2,6(tBuOCH2)C6H4 (1.0 g, 4.0 mmol) was added n-butyllithium (2.5 mL of 1.6 M solution in pentane, 4.0 mmol) via syringe at 78 C, after which the solution was allowed to warm up to R.T. and was stirred for additional 5 h. The remaining solution was concentrated to 5 mL and compound 1 was isolated by crystallization and filtration at 78 C. Yield: 0.9 g (89%). Anal. Calc. for C16H25LiO2 (256.32): C, 74.98; H, 9.83. Found: C, 74.72; H 9.55%. 1H NMR (toluene-d8, 0.1 M solution of 1): d (ppm) 1.2 (s, 18H, CH3), 4.2 (s, 4H, CH2), 7.3 (d, 2H, H(3,5)), 7.5 (t, 1H, H(4)). 7Li NMR (toluened8): d (ppm) 2.9. 13C NMR (toluene-d8): d (ppm) (determined by conventional 2D experiments) 27.0 (CH3), 63.8 (CH2), 71.2 (C(CH3)3), 124.4 (C(3,5)), 124.7 (C(4)), 126.1 (C(1)), 140.4 (C(2,6)). 1 H NMR (toluene-d8, 0.5 M solution of 1): d (ppm) 1.0 (s, 18H, CH3), 4.6 (s, 4H, CH2, broad signal at 300 K, AX pattern (4.81, 4.47) at 220 K, 2J(Ha, Hb) = 9.46 Hz), 6.9–7.2 (complex pattern, 3 H(Ar)). 7Li NMR (toluene-d8): d (ppm) 0.9. 13C NMR (toluened8): d (ppm) 27.3, 27.4, 27.5 (CH3), 70.1 (CH2), 74.3 (C(CH3)3), 122.0 (C(3,5)), 124.1 (C(4)), 149.5 (C(2,6)), 181.9 (C(1), seven line pattern with 1:2:3:4:3:2:1 intensity ratio, 1J(7Li, 13C) = 20.5 Hz). 2.2. X-ray data and crystal structure determination Crystals suitable for X-ray analysis were obtained from hexane solution of 1 at 30 C. C32H50Li2O4, M = 512.6, monoclinic, P21/c (No. 14), a = 29.5550(4) ˚ , b = 23.0230(6) A ˚ , c = 18.6100(6) A ˚ , b = 97.8970 A ˚ 3, Z = 16, Dx = 1.086 Mg m3. (12), V = 12543.0(5) A A colorless air-sensitive crystal of dimensions 0.5 · 0.31 · 0.25 mm was mounted into Lindenmann glass capillary and measured at Nonius KappaCCD diffractometer by monochromatized Mo Ka radiation ˚ ) at 150(2) K. An absorption was ne(k = 0.71073 A glected (l = 0.068 mm1); from very poorly diffracting crystal a total of 60 543 diffractions were measured in the range h = 33 to 33, k = 26 to 26, and l = 21 to 21 (hmax = 24.2), from which 19 245 were unique (Rint = 0.081) and 10 605 observed according to the I > 2r(I) criterion. Cell parameters from 110 850 reflections (h = 1–24.1). The structure was solved by direct
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methods (SIR92, Altomare, 1994) and refined by fullmatrix least squares based on F2 (SHELXL 97). The hydrogen atoms were fixed into idealized positions (riding model) and assigned temperature factors either Hiso(H) = 1.2Ueq(pivot atom) or Hiso(H) = 1.5Ueq(pivot atom) for methyl moiety. The refinement converged (D/rmax = 0.001) to R = 0.077 for observed reflections and wR = 0.218, GOF = 1.009 for 1417 parameters and all 19 245 reflections. The final difference map displayed no peaks of chemical significance (Dqmax = 0.448, ˚ 3). Some atoms of tert-butyl moiety Dqmin = 0.471 e A have required large displacement parameters during refinement, however, the quality of diffraction data did not allow to describe this potential disorder in the other way.
3. Results and discussions Two reaction paths have been developed for the synthesis of 1. The first one is based on the lithium-halogen exchange reaction [6]. Recently, the second procedure, similar to that used for analogous organolithium N,C,N-pincer complex [7], has been applied for the synthesis of 1, as well (Eq. (1)) (direct aromatic lithiation of 2,6-(tBuOCH2)2C6H4 in hexane at 78 C).
ð1Þ The X-ray diffraction analysis (XRD) showed that the unit cell of 1 consists of four independent molecules. Their appropriate parameters are given in Table 1. Each of the molecules has a dimeric structure consisting of two lithium atoms Li(1) and Li(2) and two monoanionic chelating aryl ligands (see Fig. 1 illustrating one of the molecules A). This dimer made up of a Li2C2 four-membered ring is by far the most common solid-state aggregate in dual side arm coordination [8]. The coordination geometry of the lithium atoms is tetrahedral, each lithium atom is coordinated by two oxygen atoms of two different ligands and two bridging aryl C(1) atoms. The C(1) atom of each ligand is bonded to both lithium atoms with an acute angle Li(1)–C(1)–Li(2) (range 64.3(3)–66.4(3)), indicating 2-electron-3-center bridging aryl anion. The structure of 1 shows distortions in the lithium-bonded aryl ring [9]. The range of C(2)–C(1)–C(6) angles (113.3(3)–114.8(3)) is less than 120 and these distortions are also seen in the lengthening of the C–C bonds containing C(1) (range of bond length of C(1)–C(2,6) is ˚ , whereas the range of length of the 1.404(6)–1.421(5) A
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Table 1 ˚ ) and bond angles () of four symmetrically independent molecules of 1 (A–D) Selected bond distances (A ˚ , ) Bond lengths, angles (A A B C C(1a)–Li(1) C(1a)–Li(2) C(1b)–Li(1) C(1b)–Li(2) Li(1)–Li(2) Li(1)–O(1a) Li(1)–O(1b) Li(2)–O(2a) Li(2)–O(2b) Li(1)–C(1a)–Li(2) Li(1)–C(1b)–Li(2) C(1a)–Li(1)–O(1a) C(1a)–Li(2)–O(2a)
2.219(7) 2.214(8) 2.171(8) 2.167(7) 2.360(9) 1.946(7) 1.984(6) 1.913(8) 1.972(7) 64.3(3) 65.9(3) 86.5(3) 87.1(3)
Fig. 1. A view of one of the molecules in the structure of 1(A), the others are differing mainly in the orientation of tBu moiety. Displacement ellipsoids are drawn on 30% probability level; hydrogen atoms are omitted for clarity.
˚ ). The coordinaother C–C bonds is 1.367(5)–1.400(5) A tion of the O-donor substituents has formed two five-membered rings (the range of bite angles C(1)– Li(1)–O(1) is 86.3(3)–87.1(3)). The structure and structural parameters of 1 [bond distances Li–C, Li–Li, and bite angle C(1)–Li(1)–O] are very similar to those of other dimeric lithium aryls containing N,C,N-chelating ligand, where two fivemembered rings are formed through the Li–N coordination too [8a–e]. In contrast with the dimeric structure of 1, similar organolithium compounds containing two oxygen donor atoms (2,6-di-tert-butoxyphenyl)lithium [10] and (2,6-
2.157(7) 2.190(8) 2.238(7) 2.180(8) 2.357(10) 1.974(7) 1.960(7) 1.960(8) 1.957(8) 65.7(3) 64.5(3) 86.7(3) 86.1(3)
D
2.203(8) 2.235(7) 2.207(8) 2.177(7) 2.396(10) 1.982(8) 1.999(8) 1.939(6) 2.004(6) 65.4(3) 66.3(3) 86.3(3) 86.3(2)
2.238(8) 2.191(8) 2.148(8) 2.198(7) 2.379(9) 1.955(7) 1.997(7) 1.983(6) 2.014(6) 65.0(3) 66.4(3) 86.8(3) 86.5(2)
dimethoxyphenyl)lithium [11], where the four-membered ring is formed by Li–O coordination, have been shown to be the trimer and the tetramer in the solid state. In the trimeric structure of (2,6-di-tert-butoxyphenyl)lithium, the favored trigonal bonding geometry around the ether oxygen was distorted to pyramidal for some of the oxygen atoms resulting to Li–O(trigonal) and Li–O(pyramidal) coordination. The values of the Li–O bond distances ˚ ) are comparable to those in 1 (range 1.913(8)–2.004(6)A ˚ ), whereas the Li– of Li–O(trigonal) (average 1.992 A ˚ ) are O(pyramidal) bond distances (average 2.076 A somewhat longer in (2,6-di-tert-butoxyphenyl)lithium. The tetrameric structure of (2,6-dimethoxyphenyl)lithium consists of two interacting dimeric units. The different values of Li–O bond distances of (2,6-dimeth˚ ) and 1 have been oxyphenyl)lithium (average 2.024 A established as well. These comparisons indicate that stronger Li–O interaction in 1 results in the stabilization of dimeric structure. NMR studies, to determine the aggregation state of 1, showed the concentration dependence of NMR spectra of 1 in [D8]toluene (for selected NMR parameters, see Table 2). In 1H, 13C and 7Li NMR spectra, the only one set of sharp signals has been observed in diluted 0.1 M [D8]toluene solution of 1. Both 1H and 13C NMR spectra are
Table 2 The NMR parameters of 1 in [D8]toluene Chemical shifts (ppm) d(1H(CH2)) d(1H(CH3)) d(13C(1)) d(7Li) a
Concentration 0.1 M
0.5 M
4.2 1.2 126.1 0.9
4.2/4.6a 1.2/1.0 126.1/181.9b 0.9/2.9
Broad signal at 300 K, AX pattern at 220 K (d(1H(CH2)) at 4.81 and 4.47 ppm, 2JH, H = 9.46 Hz). b Seven line pattern at 300 K (1J(7Li, 13C) = 20.5 Hz).
R. Jambor et al. / Inorganica Chimica Acta 358 (2005) 2422–2426
Fig. 2. Proposed monomer–dimer equilibrium and their NMR parameters [ppm].
13
C(1H)
temperature independent and there is no broadening of sharp signals at 200 K and the C(1) signal remains unsplit even at this temperature. However, the second set of signals (in addition to previous ones) appeared after concentration of the sample (0.5 M solution of 1 in [D8]toluene) in all NMR spectra (see Section 2). In 13C NMR spectrum (obtained after the concentration), the second signal for C(1) was detected at 181.9 ppm as a seven line pattern with 1:2:3:4:3:2:1 intensity at 295 K, indicating a coupling with two equivalently bonded lithium atoms. The 1H NMR spectrum is temperature dependent and broad signal of benzylic protons at 300 K (from the second set) can be seen as an AX pattern at 220 K (see Table 2). On the basis of these findings, the second set of signals, observed in the concentrated sample, can be assigned to the dimeric structure of 1 in concentrated toluene solution. The structure is probably similar to that found in the solid state (see Fig. 2(b), C2 symmetry of 1 and therefore the CH2 protons are diastereotopic). The decoalescence of CH2 protons in 1H NMR is probably caused by Li–O exchange (DGà of the fluxional processes calculated from the Eyring equation 1 is 41.6 kJ mol1) in the dimeric structure of 1. Following the characterization of dimeric structure of 1 in concentrated solution (mentioned above), the monomeric form of 1 seems to exist in the diluted toluene solution (Fig. 2(a)). The C(1) signal is unsplit probably due to intermolecular exchange, which is still rapid enough to cancel out 13 C–7Li coupling [11,12]. The cryoscopy measurement proved our hypotheses [13], and the determined relative molecular weight of 0.3 M benzene solution of 1 is 331 g mol1 (molecular weight of proposed monomer is 256 and 512 g mol1 for dimer) and established 71% monomer–29% dimer equilibrium. This ratio agrees well with the fact determined by 1H NMR (66% monomer–33% dimer equilibrium in 0.5 M [D8]toluene solution of 1) and it is p Eyring equation: DGà = RTc ln[2ph(Dm)/kTc 3] with DGà = free energy of activation (J), Tc = coalescence temperature (K), and Dm = chemical shift difference (Hz); the other symbols have their usual meanings. 1
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interesting to see that within experimental error, the ratio of 66:33 found for monomer–dimer at ambient temperature does not change over the temperature range from 170 to 300 K. What else, the determined relative molecular weight of 0.1 M benzene solution of 1 is 265 g mol1. To the best of our knowledge, similar monomer–dimer equilibrium was not detected in [(2,6-dialkylaminomethyl)phenyl]lithiums nor in (2,6-dialkoxyphenyl)lithiums in non-coordinated solvents, while this process has been observed in THF or Et2O solutions of organolithium compounds previously [8a–e,11,14].
Acknowledgments The authors thank the Grant Agency of the Czech Republic (Grant Nos. 203/03/P128 and 203/04/0223) and The Ministry of Education of the Czech Republic (FR 330211-L.D., LN00A028 project) for financial support.
Appendix A. Supplementary data Details of the crystal X-ray structure analysis, full tables of atomic positions, bond lengths, angles, anisotropic thermal parameters, and table of the structure factors are available from the authors on request. CCDC-215858 also contains the supplementary crystallographic data for this paper. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EY, UK (Fax: +44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2005.01.003.
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Note
The novel organolithium O,C,O-pincer compound Roman Jambor a,*, Libor Dosta´l a, Ivana Cı´sarˇova´ b, Alesˇ Ru˚zˇicˇka a, Jaroslav Holecˇek a a
Department of General and Inorganic Chemistry, University of Pardubice, na´m. Cˇs. Legiı´ 565, Pardubice CZ-532 10, Czech Republic b Department of Inorganic Chemistry, Faculty of Science, Charles University, Albertov 6, Prague CZ-128 43, Czech Republic Received 3 March 2004; accepted 11 January 2005
Abstract The preparation and characterization of organolithium O,C,O-pincer compound [2,6-(tBuOCH2)2C6H3]-Li (1) is described. The X-ray diffraction techniques revealed the dimeric structure of 1 consisting of two lithium atoms Li(1) and Li(2) and two monoanionic chelating aryl ligands in the solid state, where each lithium atom is coordinated by two oxygen atoms of two different ligands. The NMR spectroscopy and cryoscopy measurements established monomer–dimer equilibrium of 1 in concentrated solutions of non-coordinated solvents, while the diluted solutions of 1 consist of monomer only. 2005 Elsevier B.V. All rights reserved. Keywords: Lithium; O ligands; X-ray diffraction
1. Introduction Organolithium compounds constitute a commonly used and very important class of organometallic compounds frequently used in organic and organometallic synthesis for several decades [1]. The formation of aggregates in solid state [2], in solution [3], and even in gas phase [4] is a well known phenomenon of this class of compounds. It is generally recognized, that the degree of aggregation has a strong influence on the reactivity of the complexes. In order to better understand their reactivity, the structure of this class of compounds has been studied both in the solid state by X-ray crystallography [2a–f] and in solution mainly by NMR spectroscopy [3d–l]. Recently, we reported on the synthesis of O,C,O-chelating ligands, [2,6-(ROCH2)2C6H3]-, where R = Me, *
Corresponding author. Tel.: +420 46 603 7151; fax: +420 46 603 7068. E-mail address: [email protected] (R. Jambor). 0020-1693/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.01.003
iPr, tBu. These ligands have been used for synthesis and structural studies of hypercoordinated organotin(IV) compounds [5]. The preparation of these compounds was based on the metathetical exchange reaction of organolithium coumpounds (mostly prepared in situ) with appropriate (organo)tin chlorides. For better understanding of the reactivity of the starting organolithium compounds, we isolated one of these derivatives [2,6-(tBuOCH2)2C6H3]Li (1) during the preparation and here we report on the synthesis and the structure of this organolithium compound 1.
2. Experimental All reactions as well as the sample preparations for NMR and cryoscopy were carried out under Schlenk techniques. The solvents were dried by standard methods and distilled prior to use. The 1H, 7Li and 13C NMR spectra were acquired on Bruker Avance500 spectrometer in toluene-d8 at 300 K (in the case of 1H and
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C NMR at the range of 300–170 K). Appropriate chemical shifts were calibrated on residual peak of toluene: 1H, dH = 2.09 ppm; 13C, dC = 20.39 ppm. The lithium chemical shifts are given relative to external 0.1 M LiBr in THF solution (d = 0.00 ppm). 2,6-bis(tert-butoxymethyl)phenyl was prepared analogously to the literature [6]. 2.1. Preparation of 2,6-bis(tert-butoxymethyl) phenyllithium (1) To a stirred hexane (20 mL) solution of 2,6(tBuOCH2)C6H4 (1.0 g, 4.0 mmol) was added n-butyllithium (2.5 mL of 1.6 M solution in pentane, 4.0 mmol) via syringe at 78 C, after which the solution was allowed to warm up to R.T. and was stirred for additional 5 h. The remaining solution was concentrated to 5 mL and compound 1 was isolated by crystallization and filtration at 78 C. Yield: 0.9 g (89%). Anal. Calc. for C16H25LiO2 (256.32): C, 74.98; H, 9.83. Found: C, 74.72; H 9.55%. 1H NMR (toluene-d8, 0.1 M solution of 1): d (ppm) 1.2 (s, 18H, CH3), 4.2 (s, 4H, CH2), 7.3 (d, 2H, H(3,5)), 7.5 (t, 1H, H(4)). 7Li NMR (toluened8): d (ppm) 2.9. 13C NMR (toluene-d8): d (ppm) (determined by conventional 2D experiments) 27.0 (CH3), 63.8 (CH2), 71.2 (C(CH3)3), 124.4 (C(3,5)), 124.7 (C(4)), 126.1 (C(1)), 140.4 (C(2,6)). 1 H NMR (toluene-d8, 0.5 M solution of 1): d (ppm) 1.0 (s, 18H, CH3), 4.6 (s, 4H, CH2, broad signal at 300 K, AX pattern (4.81, 4.47) at 220 K, 2J(Ha, Hb) = 9.46 Hz), 6.9–7.2 (complex pattern, 3 H(Ar)). 7Li NMR (toluene-d8): d (ppm) 0.9. 13C NMR (toluened8): d (ppm) 27.3, 27.4, 27.5 (CH3), 70.1 (CH2), 74.3 (C(CH3)3), 122.0 (C(3,5)), 124.1 (C(4)), 149.5 (C(2,6)), 181.9 (C(1), seven line pattern with 1:2:3:4:3:2:1 intensity ratio, 1J(7Li, 13C) = 20.5 Hz). 2.2. X-ray data and crystal structure determination Crystals suitable for X-ray analysis were obtained from hexane solution of 1 at 30 C. C32H50Li2O4, M = 512.6, monoclinic, P21/c (No. 14), a = 29.5550(4) ˚ , b = 23.0230(6) A ˚ , c = 18.6100(6) A ˚ , b = 97.8970 A ˚ 3, Z = 16, Dx = 1.086 Mg m3. (12), V = 12543.0(5) A A colorless air-sensitive crystal of dimensions 0.5 · 0.31 · 0.25 mm was mounted into Lindenmann glass capillary and measured at Nonius KappaCCD diffractometer by monochromatized Mo Ka radiation ˚ ) at 150(2) K. An absorption was ne(k = 0.71073 A glected (l = 0.068 mm1); from very poorly diffracting crystal a total of 60 543 diffractions were measured in the range h = 33 to 33, k = 26 to 26, and l = 21 to 21 (hmax = 24.2), from which 19 245 were unique (Rint = 0.081) and 10 605 observed according to the I > 2r(I) criterion. Cell parameters from 110 850 reflections (h = 1–24.1). The structure was solved by direct
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methods (SIR92, Altomare, 1994) and refined by fullmatrix least squares based on F2 (SHELXL 97). The hydrogen atoms were fixed into idealized positions (riding model) and assigned temperature factors either Hiso(H) = 1.2Ueq(pivot atom) or Hiso(H) = 1.5Ueq(pivot atom) for methyl moiety. The refinement converged (D/rmax = 0.001) to R = 0.077 for observed reflections and wR = 0.218, GOF = 1.009 for 1417 parameters and all 19 245 reflections. The final difference map displayed no peaks of chemical significance (Dqmax = 0.448, ˚ 3). Some atoms of tert-butyl moiety Dqmin = 0.471 e A have required large displacement parameters during refinement, however, the quality of diffraction data did not allow to describe this potential disorder in the other way.
3. Results and discussions Two reaction paths have been developed for the synthesis of 1. The first one is based on the lithium-halogen exchange reaction [6]. Recently, the second procedure, similar to that used for analogous organolithium N,C,N-pincer complex [7], has been applied for the synthesis of 1, as well (Eq. (1)) (direct aromatic lithiation of 2,6-(tBuOCH2)2C6H4 in hexane at 78 C).
ð1Þ The X-ray diffraction analysis (XRD) showed that the unit cell of 1 consists of four independent molecules. Their appropriate parameters are given in Table 1. Each of the molecules has a dimeric structure consisting of two lithium atoms Li(1) and Li(2) and two monoanionic chelating aryl ligands (see Fig. 1 illustrating one of the molecules A). This dimer made up of a Li2C2 four-membered ring is by far the most common solid-state aggregate in dual side arm coordination [8]. The coordination geometry of the lithium atoms is tetrahedral, each lithium atom is coordinated by two oxygen atoms of two different ligands and two bridging aryl C(1) atoms. The C(1) atom of each ligand is bonded to both lithium atoms with an acute angle Li(1)–C(1)–Li(2) (range 64.3(3)–66.4(3)), indicating 2-electron-3-center bridging aryl anion. The structure of 1 shows distortions in the lithium-bonded aryl ring [9]. The range of C(2)–C(1)–C(6) angles (113.3(3)–114.8(3)) is less than 120 and these distortions are also seen in the lengthening of the C–C bonds containing C(1) (range of bond length of C(1)–C(2,6) is ˚ , whereas the range of length of the 1.404(6)–1.421(5) A
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Table 1 ˚ ) and bond angles () of four symmetrically independent molecules of 1 (A–D) Selected bond distances (A ˚ , ) Bond lengths, angles (A A B C C(1a)–Li(1) C(1a)–Li(2) C(1b)–Li(1) C(1b)–Li(2) Li(1)–Li(2) Li(1)–O(1a) Li(1)–O(1b) Li(2)–O(2a) Li(2)–O(2b) Li(1)–C(1a)–Li(2) Li(1)–C(1b)–Li(2) C(1a)–Li(1)–O(1a) C(1a)–Li(2)–O(2a)
2.219(7) 2.214(8) 2.171(8) 2.167(7) 2.360(9) 1.946(7) 1.984(6) 1.913(8) 1.972(7) 64.3(3) 65.9(3) 86.5(3) 87.1(3)
Fig. 1. A view of one of the molecules in the structure of 1(A), the others are differing mainly in the orientation of tBu moiety. Displacement ellipsoids are drawn on 30% probability level; hydrogen atoms are omitted for clarity.
˚ ). The coordinaother C–C bonds is 1.367(5)–1.400(5) A tion of the O-donor substituents has formed two five-membered rings (the range of bite angles C(1)– Li(1)–O(1) is 86.3(3)–87.1(3)). The structure and structural parameters of 1 [bond distances Li–C, Li–Li, and bite angle C(1)–Li(1)–O] are very similar to those of other dimeric lithium aryls containing N,C,N-chelating ligand, where two fivemembered rings are formed through the Li–N coordination too [8a–e]. In contrast with the dimeric structure of 1, similar organolithium compounds containing two oxygen donor atoms (2,6-di-tert-butoxyphenyl)lithium [10] and (2,6-
2.157(7) 2.190(8) 2.238(7) 2.180(8) 2.357(10) 1.974(7) 1.960(7) 1.960(8) 1.957(8) 65.7(3) 64.5(3) 86.7(3) 86.1(3)
D
2.203(8) 2.235(7) 2.207(8) 2.177(7) 2.396(10) 1.982(8) 1.999(8) 1.939(6) 2.004(6) 65.4(3) 66.3(3) 86.3(3) 86.3(2)
2.238(8) 2.191(8) 2.148(8) 2.198(7) 2.379(9) 1.955(7) 1.997(7) 1.983(6) 2.014(6) 65.0(3) 66.4(3) 86.8(3) 86.5(2)
dimethoxyphenyl)lithium [11], where the four-membered ring is formed by Li–O coordination, have been shown to be the trimer and the tetramer in the solid state. In the trimeric structure of (2,6-di-tert-butoxyphenyl)lithium, the favored trigonal bonding geometry around the ether oxygen was distorted to pyramidal for some of the oxygen atoms resulting to Li–O(trigonal) and Li–O(pyramidal) coordination. The values of the Li–O bond distances ˚ ) are comparable to those in 1 (range 1.913(8)–2.004(6)A ˚ ), whereas the Li– of Li–O(trigonal) (average 1.992 A ˚ ) are O(pyramidal) bond distances (average 2.076 A somewhat longer in (2,6-di-tert-butoxyphenyl)lithium. The tetrameric structure of (2,6-dimethoxyphenyl)lithium consists of two interacting dimeric units. The different values of Li–O bond distances of (2,6-dimeth˚ ) and 1 have been oxyphenyl)lithium (average 2.024 A established as well. These comparisons indicate that stronger Li–O interaction in 1 results in the stabilization of dimeric structure. NMR studies, to determine the aggregation state of 1, showed the concentration dependence of NMR spectra of 1 in [D8]toluene (for selected NMR parameters, see Table 2). In 1H, 13C and 7Li NMR spectra, the only one set of sharp signals has been observed in diluted 0.1 M [D8]toluene solution of 1. Both 1H and 13C NMR spectra are
Table 2 The NMR parameters of 1 in [D8]toluene Chemical shifts (ppm) d(1H(CH2)) d(1H(CH3)) d(13C(1)) d(7Li) a
Concentration 0.1 M
0.5 M
4.2 1.2 126.1 0.9
4.2/4.6a 1.2/1.0 126.1/181.9b 0.9/2.9
Broad signal at 300 K, AX pattern at 220 K (d(1H(CH2)) at 4.81 and 4.47 ppm, 2JH, H = 9.46 Hz). b Seven line pattern at 300 K (1J(7Li, 13C) = 20.5 Hz).
R. Jambor et al. / Inorganica Chimica Acta 358 (2005) 2422–2426
Fig. 2. Proposed monomer–dimer equilibrium and their NMR parameters [ppm].
13
C(1H)
temperature independent and there is no broadening of sharp signals at 200 K and the C(1) signal remains unsplit even at this temperature. However, the second set of signals (in addition to previous ones) appeared after concentration of the sample (0.5 M solution of 1 in [D8]toluene) in all NMR spectra (see Section 2). In 13C NMR spectrum (obtained after the concentration), the second signal for C(1) was detected at 181.9 ppm as a seven line pattern with 1:2:3:4:3:2:1 intensity at 295 K, indicating a coupling with two equivalently bonded lithium atoms. The 1H NMR spectrum is temperature dependent and broad signal of benzylic protons at 300 K (from the second set) can be seen as an AX pattern at 220 K (see Table 2). On the basis of these findings, the second set of signals, observed in the concentrated sample, can be assigned to the dimeric structure of 1 in concentrated toluene solution. The structure is probably similar to that found in the solid state (see Fig. 2(b), C2 symmetry of 1 and therefore the CH2 protons are diastereotopic). The decoalescence of CH2 protons in 1H NMR is probably caused by Li–O exchange (DGà of the fluxional processes calculated from the Eyring equation 1 is 41.6 kJ mol1) in the dimeric structure of 1. Following the characterization of dimeric structure of 1 in concentrated solution (mentioned above), the monomeric form of 1 seems to exist in the diluted toluene solution (Fig. 2(a)). The C(1) signal is unsplit probably due to intermolecular exchange, which is still rapid enough to cancel out 13 C–7Li coupling [11,12]. The cryoscopy measurement proved our hypotheses [13], and the determined relative molecular weight of 0.3 M benzene solution of 1 is 331 g mol1 (molecular weight of proposed monomer is 256 and 512 g mol1 for dimer) and established 71% monomer–29% dimer equilibrium. This ratio agrees well with the fact determined by 1H NMR (66% monomer–33% dimer equilibrium in 0.5 M [D8]toluene solution of 1) and it is p Eyring equation: DGà = RTc ln[2ph(Dm)/kTc 3] with DGà = free energy of activation (J), Tc = coalescence temperature (K), and Dm = chemical shift difference (Hz); the other symbols have their usual meanings. 1
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interesting to see that within experimental error, the ratio of 66:33 found for monomer–dimer at ambient temperature does not change over the temperature range from 170 to 300 K. What else, the determined relative molecular weight of 0.1 M benzene solution of 1 is 265 g mol1. To the best of our knowledge, similar monomer–dimer equilibrium was not detected in [(2,6-dialkylaminomethyl)phenyl]lithiums nor in (2,6-dialkoxyphenyl)lithiums in non-coordinated solvents, while this process has been observed in THF or Et2O solutions of organolithium compounds previously [8a–e,11,14].
Acknowledgments The authors thank the Grant Agency of the Czech Republic (Grant Nos. 203/03/P128 and 203/04/0223) and The Ministry of Education of the Czech Republic (FR 330211-L.D., LN00A028 project) for financial support.
Appendix A. Supplementary data Details of the crystal X-ray structure analysis, full tables of atomic positions, bond lengths, angles, anisotropic thermal parameters, and table of the structure factors are available from the authors on request. CCDC-215858 also contains the supplementary crystallographic data for this paper. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EY, UK (Fax: +44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2005.01.003.
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