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Synthesis and crystal structures of the 15-crown-5 solvates of potassium- and rubidium-2-trimethylsilylamidopyridine
Polyhedron 21 (2002) 2451 /2455 www.elsevier.com/locate/poly
Synthesis and crystal structures of the 15-crown-5 solvates of potassium- and rubidium-2-trimethylsilylamidopyridine Stephen T. Liddle, William Clegg * Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Received 15 July 2002; accepted 28 August 2002
Abstract Metathesis of ‘‘[Li(PyNSiMe3)(15C5)]’’ (15C5 /15-crown-5 ether) with potassium tert -butoxide or rubidium 2-ethylhexoxide yields the two new amide complexes [{K(PyNSiMe3)(m-15C5)}2] (3) and [{Rb(m-PyNSiMe3)(15C5)}2] (4). X-ray crystallography reveals 3 to be a ‘slipped’ centrosymmetric dimer, in which the crown ethers exhibit a bridging coordination mode via one crown oxygen from each crown to form a (KO)2 four-membered ring; the amide adopts a terminal chelating mode. In contrast, 4 is a centrosymmetric amido-bridged dimer with a transoid (RbN)2 core having additional bonding via bridging pyridyl groups; the crown ethers cap both ends of the dimer. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Potassium; Rubidium; Amide ligands; 15-Crown-5; Crystal structures
1. Introduction Recent times have witnessed enormous interest in the structure and reactivity of alkali metal derivatives of alkyls, alkoxides, imides and amides [1 /6]. The widespread synthetic utility of these reagents as ligand transfer reagents for the synthesis of transition metal and lanthanide complexes, and for proton abstraction reactions [7], has led to structural investigations of simple systems revealing a wide range of structural trends ranging from monomers, through oligomers, to polymeric species [8]. However, whilst there is an overwhelming amount of data available on lithium amides, reports of heavier alkali metal analogues are still relatively sparse, although they are becoming more common. Since the bonding is primarily ionic in nature, the structures adopted are highly dependent upon the electronic and steric properties of the substituents at the donor centre and the presence of co-ligands such as tetrahydrofuran (THF), other ethers and amines. Given that the coordination of additional donor ligands to
alkali metals generally improves their reactivity by reducing the extent of aggregation, we have become interested in combining alkali metal amides with sizematched or mismatched crown ethers to investigate the structural consequences a multidentate crown ether has for the state of aggregation and the solid state structures adopted. We previously reported [9] the synthesis and crystal structures of the Li, Na, K, Rb and Cs complexes of deprotonated 2-trimethylsilylaminopyridine (TAP), incorporating the crown ether 12-crown-4 (12C4). Of particular pertinence to this work were the homologous potassium and rubidium contact ion pair dimers [{M(mPyNSiMe3)(12C4)}2] [M /K (1) or Rb (2)], which exhibited planar transoid centrosymmetric (MN)2 cores with additional bridging pyridyl groups [9]. In this paper we report the continuation of this on-going work with the synthesis and structural characterisation of the 15crown-5 (15C5) solvates of the potassium and rubidium salts of TAP, which display structural differences dictated by the alkali metal cation.
2. Results and discussion * Corresponding author. Tel.: /44-191-222-6649; fax: /44-191222-6929 E-mail address: [email protected] (W. Clegg).
In our earlier study we reported the characterisation of the compound [Li(PyNSiMe3)(12C4)] as a precursor
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 2 2 5 - 1
S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
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to heavier alkali metal compounds. In this work ‘‘[Li(PyNSiMe3)(15C5)]’’, which could only be isolated as a viscous red oil, was employed. Despite numerous attempts, no crystalline material could be isolated; this implies an ill-defined complex, reflecting the poor host / guest fit of lithium with 15C5. Metathesis of ‘‘[Li(PyNSiMe3)(15C5)]’’ with potassium tert-butoxide or rubidium 2-ethylhexoxide affords 3 and 4, respectively, as red oils, in essentially quantitative yields (Eq. (1)). Recrystallisation of these oils from solutions of methylcyclohexane containing THF yields crystals of 3 and 4 suitable for X-ray crystallographic study. ‘‘[Li(PyNSiMe3 )15C5)]’’MOR 0 1=2[fM(PyNSiMe3 )(15C5)g2 ]LiOR M K; R But M Rb; R CH3 (CH2 )3 CH(CH2 CH3 )CH2 1
13
(1)
The H and C NMR spectra of 3 and 4 exhibit the expected signals in accord with the solid state structures (see below). The 29Si NMR spectra exhibit signals at /19.03 and /18.97 ppm, respectively, reflecting the similarity of potassium and rubidium in terms of Pauling electronegativities, and are commensurate with the soft polarisable nature of higher alkali metals leading to increased shielding at the amido centre. An X-ray crystallographic study reveals that 3 crystallises as a ‘slipped’ dimer with no significant intermolecular interactions (Fig. 1). Selected bond lengths and angles are given in Table 1. The coordination sphere of each potassium ion is dominated by the 15C5, which bridges via one oxygen centre to the other potassium centre, forming a (KO)2 ring that lies on a crystallographic inversion centre. This bridging nature of crown ethers with alkali metals is rare, but some examples have been reported with lithium [10,11], sodium [12] and potassium [13 /15]. The coordination
Fig. 1. The molecular structure of 3, without H atoms.
Table 1 ˚ ) and angles (8) for 3 and 4 Selected bond lengths (A 3a Bond lengths K(1) N(1) K(1) O(1) K(1) O(2A) K(1) O(4) Bond angles N(1) K(1) N(2) N(2) K(1) O(2A) O(2) K(1) O(3) O(4) K(1) O(5) O(2) K(1) O(2A) 4b Bond lengths Rb(1) N(1) Rb(1) N(2) Rb(1) O(1) Rb(1) O(3) Rb(1) O(5)
2.7930(11) 2.8747(11) 2.9510(10) 2.8365(10) 48.87(3) 81.12(3) 61.30(3) 60.16(3) 72.06(3)
3.143(2) 2.979(2) 3.119(2) 3.063(2) 3.177(2)
Bond angles N(1) Rb(1) N(1A) 105.48(5) N(1A) Rb(1) N(2) 92.08(6) N(1) Rb(1) N(2A) 86.52(6) O(1) Rb(1) O(2) 54.57(6) O(4) Rb(1) O(5) 54.64(5) Rb(1) N(1) Rb(1A) 74.52(5)
K(1) N(2) K(1) O(2) K(1) O(3) K(1) O(5)
2.8001(11) 2.8289(10) 2.8658(10) 2.7758(10)
N(1) K(1) O(2A) O(1) K(1) O(2) O(3) K(1) O(4) O(5) K(1) O(1) K(1) O(2) K(1A)
79.37(3) 59.91(3) 59.15(3) 59.67(3) 107.94(3)
Rb(1) N(1A) Rb(1) N(2A) Rb(1) O(2) Rb(1) O(4)
3.036(2) 3.174(2) 2.970(2) 3.019(2)
N(1) Rb(1) N(2) 44.07(6) N(1A) Rb(1) N(2A) 43.45(6) N(2) Rb(1) N(2A) 105.18(5) O(2) Rb(1) O(3) 57.61(6) O(5) Rb(1) O(1) 53.77(5) Rb(1) N(2) Rb(1A) 74.82(5)
a
Symmetry operations for equivalent atoms (A): x , y1, z . Symmetry operations for equivalent atoms (A): x1, y1, z1. b
sphere of each potassium cation is completed by the amide, which is relegated to a terminal chelating role. This bonding mode illustrates the strength of the macrocyclic effect; clearly the potassium cation is not large enough to accommodate bridging amide with a capping 15C5 molecule as with the previously reported dimer 1 [9], so the dimer ‘slips’. The five terminal K/O bond lengths span the narrow ˚ , a smaller range than obrange 2.776(2) /2.875(2) A served in 1, reflecting the better host /guest fit of 15C5 than 12C4 with potassium. This range also compares well with other previously reported examples of K(15C5) complexes [16,17]. Thus, each potassium re˚ from the mean plane of the oxygen atoms sides 1.558 A of the crown, a smaller deviation than in 1. The bridging ˚ is longer, as would be K /O bond length of 2.951(2) A expected for a bridging oxygen centre. The K/N(2) ˚ is somewhat longer than the bond length of 2.880(2) A ˚ . It might be expected K /N(1) bond length of 2.793(2) A that the amido N(2) centre would have the shorter bond length. However, inspection of a space-filling plot reveals that the amido centre is pushed away to relieve steric strain by the bulky trimethylsilyl group. Nevertheless, the observed K/N bond lengths fall within ranges previously reported [9,18]. The ligand bite angle of 48.87(3)8 is substantially smaller than those observed
S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
in the compound [{Na(12C4)2}{Na(PyNSiMe3)2 (THF)}] [9], reflecting the larger ionic radius of potassium compared to sodium (effective ionic radii for coordination number 6 are given as 1.02, 1.38, and ˚ for Na, K , and Rb , respectively [19]). The 1.52 A amides exhibit a hinge angle of 157.88, defined as the dihedral angle between the mean plane of the pyridyl ring and amido N(2), and the KN2 plane; this would be 1808 if the potassium ion lay exactly in the plane of the ligand. This hinge angle results in a deviation from planarity at N(2) [sum of bond angles /356.83(9)8]. The trimethylsilyl groups are staggered with respect to the pyridyl ring to minimise steric strain. The molecular structure of 4 is illustrated in Fig. 2 and selected bond lengths and angles are given in Table 1. The complex crystallises as a contact ion pair dimer constructed around a transoid (RbN)2 ring involving the amido nitrogen centres, with additional coordination by bridging pyridyl groups, and capped on each end by a molecule of 15C5. The molecule is centrosymmetric, the (RbN)2 ring lying on a crystallographic inversion centre. The larger ionic radius of rubidium compared to potassium is demonstrated by the retention of the bridged amide dimer core observed in 1 and 2, instead of ‘slipping’ to the dimer architecture observed in 3. The ˚, Rb /O bond lengths span the range 2.970(2) /3.177(2) A reflecting the increased ionic radius of rubidium compared to potassium and agreeing well with previous examples [20,21]. Consequently the rubidium cation ˚ above the oxygen mean plane of the resides 1.943 A crown, reflecting not only the larger ionic radius of rubidium, but also the more congested coordination sphere of the metal. For the amido ligand, the Rb /N ˚ bond lengths span the ranges 2.979(2) /3.174(2) A ˚ (amide) and 3.036(2) /3.143(2) A (pyridyl). Whilst the lower value for the Rb /N(amide) is in good agreement
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with bond lengths observed in 2, the other three bond lengths are at the higher end of bond lengths associated with tertiary amines coordinated to rubidium. For example, Rb /N bond lengths in the ranges 3.022 / ˚ have been observed in the 3.099 and 3.092 /3.131 A complexes [{Rb(fluorenyl)(PMDETA)}8] [22] and [{RbC(H)(SiMe3)2(PMDETA)}2] [23] (PMDETA / N ,N ,N ?,N ??,N ??-pentamethyldiethylenetriamine), respectively. This is, perhaps, an indication of the crowding in this dimer as a result of increasing the size of the crown. Whereas potassium cannot accommodate the complete coordination environment and gives a ‘slipped’ dimer, rubidium is just large enough, but at the expense of a long Rb /N bond as the macrocyclic effect dominates. The trimethylsilyl groups are again staggered with respect to the pyridyl rings minimising steric strain. The pyridyl rings are tilted from the normal of the Rb(1) Rb(1A) vector. The N(1) pyridyl ring tilts towards Rb(1A) (hinge angles of 143.98 to the plane of N(1)/N(2)/Rb(1) and 134.78 to the plane of N(1)/N(2)/ Rb(1A)). This is a smaller degree of tilt than observed in 1 or 2, reflecting the increased coordination number of rubidium, and hence a diminished requirement of electron density from the p system of the pyridyl ring.
3. Conclusion The new potassium and rubidium amides are readily prepared by a metathesis protocol in excellent yields. The structures adopted in the solid state are cation dictated; in 3 a slipped dimer is formed by bridging crowns, giving a (KO)2 ring with terminal chelating amides, whereas for 4 a dimer with a (RbN)2 ring is formed, with additional coordination by bridging pyridyl groups and capping crowns, as a consequence of the larger size of rubidium compared to potassium.
4. Experimental 4.1. General comments
Fig. 2. The molecular structure of 4, without H atoms.
All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry nitrogen. Methylcyclohexane and THF were distilled from sodium /benzophenone ketyl radical under an atmosphere of dry nitrogen and stored over activated 4A molecular sieves. 15-Crown-5 ether was dried by, and stored over activated 4A molecular sieves. 2-Trimethylsilylaminopyridine was prepared as described previously [9]. Butyllithium was purchased from Aldrich as a 2.5 M solution in hexanes. Potassium tert -butoxide was purchased from Aldrich and was baked at 100 8C (10 2 mmHg) and stored under an atmosphere of dry nitrogen prior to use. Rubidium-2-ethylhexoxide was prepared by
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S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
a literature method [24]. Deuteriated solvents were distilled from a potassium mirror, deoxygenated by three freeze /pump/thaw cycles, and stored over activated 4A molecular sieves. The 1H and 13C NMR spectra were recorded on a Bruker AC200 spectrometer and 29Si spectra on a Bruker WM300 operating at 200.1, 50.3 and 59.6 MHz, respectively, and chemical shifts are quoted in ppm relative to tetramethylsilane. Elemental analyses were obtained by Elemental Microanalysis Ltd., Okehampton, UK.
4.2. Preparation of [{K(PyNSiMe3)(m-15C5)}2] (3) Bun Li (1.28 ml, 3.19 mmol) was added dropwise to a solution of 2-trimethylsilylaminopyridine (0.53 g, 3.19 mmol) and 15-crown-5 (0.63 ml, 3.19 mmol) in THF (40 ml) and stirred for 1 h to afford a yellow solution. Dropwise addition of this solution to But OK (0.36 g, 3.19 mmol) gave a dark orange solution. Removal of volatiles in vacuo yielded a viscous red oil; recrystallisation from methylcyclohexane/THF solution at 5 8C gave crystals of 3 suitable for X-ray crystallography (0.98 g, 72.3%). Microanalysis for 3: C, 50.68; H, 8.21; N, 6.31. C36H66K2N4O10Si2 requires C, 50.91; H, 7.83; N, 6.60%. Spectroscopic data for 3: dH ([2H]8 THF) 0.10 (18H, s, SiMe3), 3.67 (40H, s, 15C5), 5.68 (2H, t, b-H-Py), 6.05 (2H, d, b?-H-Py), 6.78 (2H, t, g-H-Py) and 7.68 (2H, d, a-H-Py). dC ([2H]8 THF) 2.79 (SiMe3), 69.64 (15C5), 103.01 (b-C-Py), 115.27 (b?-C-Py), 134.66 (g-C-Py), 149.53 (a-C-Py) and 170.47 (a?-C-Py). dSi ([2H]8 THF) /19.03 (s, SiMe3).
4.3. Preparation of [{Rb(m-PyNSiMe3)(15C5)}2] (4) Bun Li (1.35 ml, 3.37 mmol) was added dropwise to a solution of 2-trimethylsilylaminopyridine (0.56 g, 3.37 mmol) and 15-crown-5 (0.67 ml, 3.37 mmol) in THF (40 ml) and stirred for 1 h to afford a yellow solution. Addition of this solution to rubidium-2-ethylhexoxide (0.72 g, 3.37 mmol) gave a dark orange solution. Removal of volatiles in vacuo yielded a viscous red oil; recrystallisation from a methylcyclohexane/THF solution at 5 8C gave crystals of 4 suitable for X-ray crystallography (1.15 g, 72.4%). Microanalysis for 4: C, 46.25; H, 7.40; N, 5.65. C36H66N4O10Rb2Si2 requires C, 45.90; H, 7.06; N, 5.95%. Spectroscopic data for 4: dH ([2H]8 THF) 0.10 (18H, s, SiMe3), 3.62 (40H, s, 15C5), 5.72 (2H, t, b-H-Py), 6.04 (2H, d, b?-H-Py), 6.79 (2H, t, g-H-Py) and 7.73 (2H, d, a-H-Py). dC ([2H]8 THF) 2.77 (SiMe3), 69.90 (15C5), 103.11 (b-C-Py), 115.60 (b?-CPy), 134.75 (g-C-Py), 149.65 (a-C-Py) and 170.16 (a?-CPy). dSi ([2H]8 THF) /18.97 (s, SiMe3).
4.4. X-ray crystallography Crystal data for complexes 3 and 4 are listed in Table 2. Crystals were examined on a Bruker AXS SMART CCD area detector with graphite-monochromated Mo ˚ ) at 160 K with an Oxford Ka radiation (l /0.70173 A Cryostream low-temperature device. Cell parameters were refined from positions of all strong reflections in each data set. Intensities were corrected semi-empirically for absorption, based on symmetry-equivalent and repeated reflections. The structures were solved by direct methods and refined on F2 values for all unique data. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in geometrically calculated positions and constrained with a riding model; U (H) was set at 1.2 (1.5 for methyl groups) times Ueq for the parent atom. Programs were Bruker AXS SMART (control) and SAINT (integration) [25], and SHELXTL for structure solution, refinement and molecular graphics [26]. Table 2 Crystal data and structure refinement parameters for 3 and 4 Compound
3
4
Formula Formula weight Temperature (K) Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Absorption coefficient (mm 1) Crystal size (mm) umax (8) Reflections measured Unique reflections Reflections with F2 2s (F2) Transmission factors Rint Number of parameters R a [F2 2s (F2)] Rw b (all data) Goodness-of-fit c (S ) Largest difference peak and ˚ 3) hole (e A
C36H66K2N4O10Si2 849.3 160 monoclinic P 21/c
C36H66N4O10Rb2Si2 942.1 160 triclinic P 1¯
12.7950(6) 9.4632(4) 18.5977(8)
2249.41(17) 2 0.32
10.6890(7) 11.2629(7) 11.7875(8) 103.220(2) 112.535(2) 108.048(2) 1144.57(13) 1 2.24
0.76 0.48 0.36 28.6 18968 5377 4410
0.80 0.80 0.38 28.6 9981 5226 3813
0.754 /0.881 0.0185 247 0.0318 0.0922 1.084 0.65 and 0.26
0.176 /0.361 0.0396 248 0.0358 0.0840 0.851 1.06 and 0.86
a
92.661(2)
Conventional R a½½Fo½½Fc½½/a½Fo½ for ‘observed’ reflections having F2 2s (F2). b Rw [aw (F/2o/F/2c )/2/aw (F/2o )/2]1/2 for all data. c S [Sw (F/2o/F/2c )/2/(no. of unique reflectionsno. of parameters)]1/2.
S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
5. Supplementary information Crystallographic data for the structural analysis of compounds 3 and 4 have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 179632 and 179631 for compounds 3 and 4, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).
Acknowledgements We thank the EPSRC for partial funding of diffractometer equipment.
References [1] W.N. Setzer, P. von, R. Schleyer, Adv. Organomet. Chem. 24 (1985) 353. [2] C. Schade, P. von, R. Schleyer, Adv. Organomet. Chem. 27 (1987) 169. [3] K. Gregory, P. von, R. Schleyer, R. Snaith, Adv. Inorg. Chem. 37 (1991) 47. [4] R.E. Mulvey, Chem. Soc. Rev. 20 (1991) 167. [5] M.A. Beswick, D.S. Wright, in: E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive Organometallic Chemistry II, Pergamon Press, Oxford, 1995, p. 1. [6] J.D. Smith, Adv. Organomet. Chem. 43 (1999) 267.
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[7] M.F. Lappert, P.P. Power, A.R. Sanger, R.C. Srivastava, Metal and Metalloid Amides, Horwood-Wiley, Chichester, 1980, p. 24. [8] A.M. Sapse, P. von R. Schleyer (Eds.), Lithium Chemistry. A Theoretical and Experimental Overview, Wiley Interscience, New York, 1995. [9] S.T. Liddle, W. Clegg, J. Chem. Soc., Dalton Trans. (2001) 402. [10] A. Dietrich, B. Neumuller, K. Dehnicke, Z. Anorg. Allg. Chem. 625 (1999) 619. [11] M.P. Murchie, J.W. Bovenkamp, A. Rodrigue, K.A. Watson, S. Fortier, Can. J. Chem. 66 (1988) 2515. [12] K.A. Watson, S. Fortier, M.P. Murchie, J.W. Bovenkamp, Can. J. Chem. 69 (1991) 687. [13] S. Chadwick, K.R. Senge, Chem. Eur. J. 4 (1998) 1768. [14] M.E. Fraser, S. Fortier, A. Rodrigue, J.W. Bovenkamp, Can. J. Chem. 64 (1986) 816. [15] G. Krauter, F. Weller, K. Dehnicke, Z. Naturforsch. 44 (1989) 444. [16] T.E. Waldman, B. Waltermire, A.L. Rheingold, R.D. Ernst, Organometallics 12 (1993) 4161. [17] B. Borgsen, F. Weller, K. Dehnicke, Z. Anorg. Allg. Chem. 596 (1991) 55. [18] P.C. Andrews, D.R. Baker, W. Clegg, R.E. Mulvey, P.A. O’Neil, Polyhedron 10 (1991) 1839. [19] R.D. Shannon, Acta Crystallogr., Sect. A 32 (1976) 751. [20] D.L. Ward, S.B. Dawes, O. Fussa, J.L. Dye, Am. Cryst. Assoc., Abstr. Papers (Winter) 13 (1985) 25. [21] N.S. Fender, S.S. Finegan, D. Miller, M. Mitchell, I.A. Kahwa, F.R. Fronczek, Inorg. Chem. 33 (1994) 4002. [22] D. Hoffmann, F. Hampel, P. von, R. Schleyer, J. Organomet. Chem. 456 (1993) 13. [23] P.B. Hitchcock, M.F. Lappert, W.P. Leung, L. Diansheng, T. Shun, J. Chem. Soc., Chem. Commun. (1993) 1386. [24] M.H. Chisholm, S.R. Drake, A.A. Naiini, W.E. Streib, Polyhedron 10 (1991) 337. [25] SMART and SAINT software for CCD diffractometers, Bruker AXS Inc., Madison, WI, USA, 1997. [26] G.M. Sheldrick, SHELXTL user manual, version 5.1, Bruker AXS Inc., Madison, WI, USA, 1997.
Synthesis and crystal structures of the 15-crown-5 solvates of potassium- and rubidium-2-trimethylsilylamidopyridine Stephen T. Liddle, William Clegg * Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Received 15 July 2002; accepted 28 August 2002
Abstract Metathesis of ‘‘[Li(PyNSiMe3)(15C5)]’’ (15C5 /15-crown-5 ether) with potassium tert -butoxide or rubidium 2-ethylhexoxide yields the two new amide complexes [{K(PyNSiMe3)(m-15C5)}2] (3) and [{Rb(m-PyNSiMe3)(15C5)}2] (4). X-ray crystallography reveals 3 to be a ‘slipped’ centrosymmetric dimer, in which the crown ethers exhibit a bridging coordination mode via one crown oxygen from each crown to form a (KO)2 four-membered ring; the amide adopts a terminal chelating mode. In contrast, 4 is a centrosymmetric amido-bridged dimer with a transoid (RbN)2 core having additional bonding via bridging pyridyl groups; the crown ethers cap both ends of the dimer. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Potassium; Rubidium; Amide ligands; 15-Crown-5; Crystal structures
1. Introduction Recent times have witnessed enormous interest in the structure and reactivity of alkali metal derivatives of alkyls, alkoxides, imides and amides [1 /6]. The widespread synthetic utility of these reagents as ligand transfer reagents for the synthesis of transition metal and lanthanide complexes, and for proton abstraction reactions [7], has led to structural investigations of simple systems revealing a wide range of structural trends ranging from monomers, through oligomers, to polymeric species [8]. However, whilst there is an overwhelming amount of data available on lithium amides, reports of heavier alkali metal analogues are still relatively sparse, although they are becoming more common. Since the bonding is primarily ionic in nature, the structures adopted are highly dependent upon the electronic and steric properties of the substituents at the donor centre and the presence of co-ligands such as tetrahydrofuran (THF), other ethers and amines. Given that the coordination of additional donor ligands to
alkali metals generally improves their reactivity by reducing the extent of aggregation, we have become interested in combining alkali metal amides with sizematched or mismatched crown ethers to investigate the structural consequences a multidentate crown ether has for the state of aggregation and the solid state structures adopted. We previously reported [9] the synthesis and crystal structures of the Li, Na, K, Rb and Cs complexes of deprotonated 2-trimethylsilylaminopyridine (TAP), incorporating the crown ether 12-crown-4 (12C4). Of particular pertinence to this work were the homologous potassium and rubidium contact ion pair dimers [{M(mPyNSiMe3)(12C4)}2] [M /K (1) or Rb (2)], which exhibited planar transoid centrosymmetric (MN)2 cores with additional bridging pyridyl groups [9]. In this paper we report the continuation of this on-going work with the synthesis and structural characterisation of the 15crown-5 (15C5) solvates of the potassium and rubidium salts of TAP, which display structural differences dictated by the alkali metal cation.
2. Results and discussion * Corresponding author. Tel.: /44-191-222-6649; fax: /44-191222-6929 E-mail address: [email protected] (W. Clegg).
In our earlier study we reported the characterisation of the compound [Li(PyNSiMe3)(12C4)] as a precursor
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 2 2 5 - 1
S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
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to heavier alkali metal compounds. In this work ‘‘[Li(PyNSiMe3)(15C5)]’’, which could only be isolated as a viscous red oil, was employed. Despite numerous attempts, no crystalline material could be isolated; this implies an ill-defined complex, reflecting the poor host / guest fit of lithium with 15C5. Metathesis of ‘‘[Li(PyNSiMe3)(15C5)]’’ with potassium tert-butoxide or rubidium 2-ethylhexoxide affords 3 and 4, respectively, as red oils, in essentially quantitative yields (Eq. (1)). Recrystallisation of these oils from solutions of methylcyclohexane containing THF yields crystals of 3 and 4 suitable for X-ray crystallographic study. ‘‘[Li(PyNSiMe3 )15C5)]’’MOR 0 1=2[fM(PyNSiMe3 )(15C5)g2 ]LiOR M K; R But M Rb; R CH3 (CH2 )3 CH(CH2 CH3 )CH2 1
13
(1)
The H and C NMR spectra of 3 and 4 exhibit the expected signals in accord with the solid state structures (see below). The 29Si NMR spectra exhibit signals at /19.03 and /18.97 ppm, respectively, reflecting the similarity of potassium and rubidium in terms of Pauling electronegativities, and are commensurate with the soft polarisable nature of higher alkali metals leading to increased shielding at the amido centre. An X-ray crystallographic study reveals that 3 crystallises as a ‘slipped’ dimer with no significant intermolecular interactions (Fig. 1). Selected bond lengths and angles are given in Table 1. The coordination sphere of each potassium ion is dominated by the 15C5, which bridges via one oxygen centre to the other potassium centre, forming a (KO)2 ring that lies on a crystallographic inversion centre. This bridging nature of crown ethers with alkali metals is rare, but some examples have been reported with lithium [10,11], sodium [12] and potassium [13 /15]. The coordination
Fig. 1. The molecular structure of 3, without H atoms.
Table 1 ˚ ) and angles (8) for 3 and 4 Selected bond lengths (A 3a Bond lengths K(1) N(1) K(1) O(1) K(1) O(2A) K(1) O(4) Bond angles N(1) K(1) N(2) N(2) K(1) O(2A) O(2) K(1) O(3) O(4) K(1) O(5) O(2) K(1) O(2A) 4b Bond lengths Rb(1) N(1) Rb(1) N(2) Rb(1) O(1) Rb(1) O(3) Rb(1) O(5)
2.7930(11) 2.8747(11) 2.9510(10) 2.8365(10) 48.87(3) 81.12(3) 61.30(3) 60.16(3) 72.06(3)
3.143(2) 2.979(2) 3.119(2) 3.063(2) 3.177(2)
Bond angles N(1) Rb(1) N(1A) 105.48(5) N(1A) Rb(1) N(2) 92.08(6) N(1) Rb(1) N(2A) 86.52(6) O(1) Rb(1) O(2) 54.57(6) O(4) Rb(1) O(5) 54.64(5) Rb(1) N(1) Rb(1A) 74.52(5)
K(1) N(2) K(1) O(2) K(1) O(3) K(1) O(5)
2.8001(11) 2.8289(10) 2.8658(10) 2.7758(10)
N(1) K(1) O(2A) O(1) K(1) O(2) O(3) K(1) O(4) O(5) K(1) O(1) K(1) O(2) K(1A)
79.37(3) 59.91(3) 59.15(3) 59.67(3) 107.94(3)
Rb(1) N(1A) Rb(1) N(2A) Rb(1) O(2) Rb(1) O(4)
3.036(2) 3.174(2) 2.970(2) 3.019(2)
N(1) Rb(1) N(2) 44.07(6) N(1A) Rb(1) N(2A) 43.45(6) N(2) Rb(1) N(2A) 105.18(5) O(2) Rb(1) O(3) 57.61(6) O(5) Rb(1) O(1) 53.77(5) Rb(1) N(2) Rb(1A) 74.82(5)
a
Symmetry operations for equivalent atoms (A): x , y1, z . Symmetry operations for equivalent atoms (A): x1, y1, z1. b
sphere of each potassium cation is completed by the amide, which is relegated to a terminal chelating role. This bonding mode illustrates the strength of the macrocyclic effect; clearly the potassium cation is not large enough to accommodate bridging amide with a capping 15C5 molecule as with the previously reported dimer 1 [9], so the dimer ‘slips’. The five terminal K/O bond lengths span the narrow ˚ , a smaller range than obrange 2.776(2) /2.875(2) A served in 1, reflecting the better host /guest fit of 15C5 than 12C4 with potassium. This range also compares well with other previously reported examples of K(15C5) complexes [16,17]. Thus, each potassium re˚ from the mean plane of the oxygen atoms sides 1.558 A of the crown, a smaller deviation than in 1. The bridging ˚ is longer, as would be K /O bond length of 2.951(2) A expected for a bridging oxygen centre. The K/N(2) ˚ is somewhat longer than the bond length of 2.880(2) A ˚ . It might be expected K /N(1) bond length of 2.793(2) A that the amido N(2) centre would have the shorter bond length. However, inspection of a space-filling plot reveals that the amido centre is pushed away to relieve steric strain by the bulky trimethylsilyl group. Nevertheless, the observed K/N bond lengths fall within ranges previously reported [9,18]. The ligand bite angle of 48.87(3)8 is substantially smaller than those observed
S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
in the compound [{Na(12C4)2}{Na(PyNSiMe3)2 (THF)}] [9], reflecting the larger ionic radius of potassium compared to sodium (effective ionic radii for coordination number 6 are given as 1.02, 1.38, and ˚ for Na, K , and Rb , respectively [19]). The 1.52 A amides exhibit a hinge angle of 157.88, defined as the dihedral angle between the mean plane of the pyridyl ring and amido N(2), and the KN2 plane; this would be 1808 if the potassium ion lay exactly in the plane of the ligand. This hinge angle results in a deviation from planarity at N(2) [sum of bond angles /356.83(9)8]. The trimethylsilyl groups are staggered with respect to the pyridyl ring to minimise steric strain. The molecular structure of 4 is illustrated in Fig. 2 and selected bond lengths and angles are given in Table 1. The complex crystallises as a contact ion pair dimer constructed around a transoid (RbN)2 ring involving the amido nitrogen centres, with additional coordination by bridging pyridyl groups, and capped on each end by a molecule of 15C5. The molecule is centrosymmetric, the (RbN)2 ring lying on a crystallographic inversion centre. The larger ionic radius of rubidium compared to potassium is demonstrated by the retention of the bridged amide dimer core observed in 1 and 2, instead of ‘slipping’ to the dimer architecture observed in 3. The ˚, Rb /O bond lengths span the range 2.970(2) /3.177(2) A reflecting the increased ionic radius of rubidium compared to potassium and agreeing well with previous examples [20,21]. Consequently the rubidium cation ˚ above the oxygen mean plane of the resides 1.943 A crown, reflecting not only the larger ionic radius of rubidium, but also the more congested coordination sphere of the metal. For the amido ligand, the Rb /N ˚ bond lengths span the ranges 2.979(2) /3.174(2) A ˚ (amide) and 3.036(2) /3.143(2) A (pyridyl). Whilst the lower value for the Rb /N(amide) is in good agreement
2453
with bond lengths observed in 2, the other three bond lengths are at the higher end of bond lengths associated with tertiary amines coordinated to rubidium. For example, Rb /N bond lengths in the ranges 3.022 / ˚ have been observed in the 3.099 and 3.092 /3.131 A complexes [{Rb(fluorenyl)(PMDETA)}8] [22] and [{RbC(H)(SiMe3)2(PMDETA)}2] [23] (PMDETA / N ,N ,N ?,N ??,N ??-pentamethyldiethylenetriamine), respectively. This is, perhaps, an indication of the crowding in this dimer as a result of increasing the size of the crown. Whereas potassium cannot accommodate the complete coordination environment and gives a ‘slipped’ dimer, rubidium is just large enough, but at the expense of a long Rb /N bond as the macrocyclic effect dominates. The trimethylsilyl groups are again staggered with respect to the pyridyl rings minimising steric strain. The pyridyl rings are tilted from the normal of the Rb(1) Rb(1A) vector. The N(1) pyridyl ring tilts towards Rb(1A) (hinge angles of 143.98 to the plane of N(1)/N(2)/Rb(1) and 134.78 to the plane of N(1)/N(2)/ Rb(1A)). This is a smaller degree of tilt than observed in 1 or 2, reflecting the increased coordination number of rubidium, and hence a diminished requirement of electron density from the p system of the pyridyl ring.
3. Conclusion The new potassium and rubidium amides are readily prepared by a metathesis protocol in excellent yields. The structures adopted in the solid state are cation dictated; in 3 a slipped dimer is formed by bridging crowns, giving a (KO)2 ring with terminal chelating amides, whereas for 4 a dimer with a (RbN)2 ring is formed, with additional coordination by bridging pyridyl groups and capping crowns, as a consequence of the larger size of rubidium compared to potassium.
4. Experimental 4.1. General comments
Fig. 2. The molecular structure of 4, without H atoms.
All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry nitrogen. Methylcyclohexane and THF were distilled from sodium /benzophenone ketyl radical under an atmosphere of dry nitrogen and stored over activated 4A molecular sieves. 15-Crown-5 ether was dried by, and stored over activated 4A molecular sieves. 2-Trimethylsilylaminopyridine was prepared as described previously [9]. Butyllithium was purchased from Aldrich as a 2.5 M solution in hexanes. Potassium tert -butoxide was purchased from Aldrich and was baked at 100 8C (10 2 mmHg) and stored under an atmosphere of dry nitrogen prior to use. Rubidium-2-ethylhexoxide was prepared by
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S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
a literature method [24]. Deuteriated solvents were distilled from a potassium mirror, deoxygenated by three freeze /pump/thaw cycles, and stored over activated 4A molecular sieves. The 1H and 13C NMR spectra were recorded on a Bruker AC200 spectrometer and 29Si spectra on a Bruker WM300 operating at 200.1, 50.3 and 59.6 MHz, respectively, and chemical shifts are quoted in ppm relative to tetramethylsilane. Elemental analyses were obtained by Elemental Microanalysis Ltd., Okehampton, UK.
4.2. Preparation of [{K(PyNSiMe3)(m-15C5)}2] (3) Bun Li (1.28 ml, 3.19 mmol) was added dropwise to a solution of 2-trimethylsilylaminopyridine (0.53 g, 3.19 mmol) and 15-crown-5 (0.63 ml, 3.19 mmol) in THF (40 ml) and stirred for 1 h to afford a yellow solution. Dropwise addition of this solution to But OK (0.36 g, 3.19 mmol) gave a dark orange solution. Removal of volatiles in vacuo yielded a viscous red oil; recrystallisation from methylcyclohexane/THF solution at 5 8C gave crystals of 3 suitable for X-ray crystallography (0.98 g, 72.3%). Microanalysis for 3: C, 50.68; H, 8.21; N, 6.31. C36H66K2N4O10Si2 requires C, 50.91; H, 7.83; N, 6.60%. Spectroscopic data for 3: dH ([2H]8 THF) 0.10 (18H, s, SiMe3), 3.67 (40H, s, 15C5), 5.68 (2H, t, b-H-Py), 6.05 (2H, d, b?-H-Py), 6.78 (2H, t, g-H-Py) and 7.68 (2H, d, a-H-Py). dC ([2H]8 THF) 2.79 (SiMe3), 69.64 (15C5), 103.01 (b-C-Py), 115.27 (b?-C-Py), 134.66 (g-C-Py), 149.53 (a-C-Py) and 170.47 (a?-C-Py). dSi ([2H]8 THF) /19.03 (s, SiMe3).
4.3. Preparation of [{Rb(m-PyNSiMe3)(15C5)}2] (4) Bun Li (1.35 ml, 3.37 mmol) was added dropwise to a solution of 2-trimethylsilylaminopyridine (0.56 g, 3.37 mmol) and 15-crown-5 (0.67 ml, 3.37 mmol) in THF (40 ml) and stirred for 1 h to afford a yellow solution. Addition of this solution to rubidium-2-ethylhexoxide (0.72 g, 3.37 mmol) gave a dark orange solution. Removal of volatiles in vacuo yielded a viscous red oil; recrystallisation from a methylcyclohexane/THF solution at 5 8C gave crystals of 4 suitable for X-ray crystallography (1.15 g, 72.4%). Microanalysis for 4: C, 46.25; H, 7.40; N, 5.65. C36H66N4O10Rb2Si2 requires C, 45.90; H, 7.06; N, 5.95%. Spectroscopic data for 4: dH ([2H]8 THF) 0.10 (18H, s, SiMe3), 3.62 (40H, s, 15C5), 5.72 (2H, t, b-H-Py), 6.04 (2H, d, b?-H-Py), 6.79 (2H, t, g-H-Py) and 7.73 (2H, d, a-H-Py). dC ([2H]8 THF) 2.77 (SiMe3), 69.90 (15C5), 103.11 (b-C-Py), 115.60 (b?-CPy), 134.75 (g-C-Py), 149.65 (a-C-Py) and 170.16 (a?-CPy). dSi ([2H]8 THF) /18.97 (s, SiMe3).
4.4. X-ray crystallography Crystal data for complexes 3 and 4 are listed in Table 2. Crystals were examined on a Bruker AXS SMART CCD area detector with graphite-monochromated Mo ˚ ) at 160 K with an Oxford Ka radiation (l /0.70173 A Cryostream low-temperature device. Cell parameters were refined from positions of all strong reflections in each data set. Intensities were corrected semi-empirically for absorption, based on symmetry-equivalent and repeated reflections. The structures were solved by direct methods and refined on F2 values for all unique data. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in geometrically calculated positions and constrained with a riding model; U (H) was set at 1.2 (1.5 for methyl groups) times Ueq for the parent atom. Programs were Bruker AXS SMART (control) and SAINT (integration) [25], and SHELXTL for structure solution, refinement and molecular graphics [26]. Table 2 Crystal data and structure refinement parameters for 3 and 4 Compound
3
4
Formula Formula weight Temperature (K) Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z Absorption coefficient (mm 1) Crystal size (mm) umax (8) Reflections measured Unique reflections Reflections with F2 2s (F2) Transmission factors Rint Number of parameters R a [F2 2s (F2)] Rw b (all data) Goodness-of-fit c (S ) Largest difference peak and ˚ 3) hole (e A
C36H66K2N4O10Si2 849.3 160 monoclinic P 21/c
C36H66N4O10Rb2Si2 942.1 160 triclinic P 1¯
12.7950(6) 9.4632(4) 18.5977(8)
2249.41(17) 2 0.32
10.6890(7) 11.2629(7) 11.7875(8) 103.220(2) 112.535(2) 108.048(2) 1144.57(13) 1 2.24
0.76 0.48 0.36 28.6 18968 5377 4410
0.80 0.80 0.38 28.6 9981 5226 3813
0.754 /0.881 0.0185 247 0.0318 0.0922 1.084 0.65 and 0.26
0.176 /0.361 0.0396 248 0.0358 0.0840 0.851 1.06 and 0.86
a
92.661(2)
Conventional R a½½Fo½½Fc½½/a½Fo½ for ‘observed’ reflections having F2 2s (F2). b Rw [aw (F/2o/F/2c )/2/aw (F/2o )/2]1/2 for all data. c S [Sw (F/2o/F/2c )/2/(no. of unique reflectionsno. of parameters)]1/2.
S.T. Liddle, W. Clegg / Polyhedron 21 (2002) 2451 /2455
5. Supplementary information Crystallographic data for the structural analysis of compounds 3 and 4 have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 179632 and 179631 for compounds 3 and 4, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).
Acknowledgements We thank the EPSRC for partial funding of diffractometer equipment.
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