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Complexes of 6-methyl-2-pyridone with the alkaline earth metals magnesium, strontium and barium: Synthesis and structural characterisation
Inorganica Chimica Acta 359 (2006) 3474–3480 www.elsevier.com/locate/ica
Complexes of 6-methyl-2-pyridone with the alkaline earth metals magnesium, strontium and barium: Synthesis and structural characterisation Gary S. Nichol, William Clegg
*
School of Natural Sciences (Chemistry), Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Received 19 October 2005; received in revised form 10 January 2006; accepted 14 January 2006 Available online 24 February 2006 Dedicated to Professor Mike Mingos.
Abstract Reaction in dry methanol of 6-methylpyridine (Hmhp) with M(OR)2 (M = Mg, Sr, Ba) formed three new coordination compounds, with very different crystal structures. Compound [Mg4(mhp)4(MeO)4(MeOH)8] (1) forms discrete clusters containing a rare Mg4O4 cubane, only the tenth reported occurrence of such a motif. Compounds [Sr(mhp)2(Hmhp)] (2) and [Ba(mhp)2(Hmhp)(MeOH)] (3) are both eight-coordinate complexes in polymeric chains. The solvent-free 2 packs as an infinite left-handed helix with elegant sixfold screw symmetry formed as a result of twisting necessary for coordination to Sr2+ by both deprotonated nitrogen atoms. In 3 there is also a molecule of methanol coordinated to Ba2+; there is now no need for a twist to achieve the extra coordination, and so the compound forms instead a straight chain with glide-plane symmetry. 2006 Elsevier B.V. All rights reserved. Keywords: 6-Methyl-2-pyridone; Alkaline-earth metals; Coordination chemistry; Hydrogen bonding; Single-crystal structure determination; Synchrotron radiation
1. Introduction The molecule 2-hydroxypyridine, also known as 2-pyridone, and its family of substituted derivatives exhibit a keto–enol tautomeric equilibrium commonly observed in organic molecules. The equilibrium of this particular family must be one of the most studied transformations in chemistry, with the first observations being made in 1907 [1]. Whilst it is generally held that the ketone (or pyridone) form predominates in such equilibria, the predominant tautomer observed in the case of 2-hydroxypyridines is less well defined. The tautomerisation is influenced strongly by external factors (e.g. the polarity of the solvent and the
*
Corresponding author. E-mail addresses: [email protected] (W. Clegg).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.01.005
pH of the solution) and substituent effects (especially the position of the substituent in the ring and its specific electronic effects). Comprehensive investigations by UV–Vis [2,3], infra-red [4–8], and nuclear magnetic resonance [9,10] spectroscopy, together with gas-phase [11–13], theoretical [11,12,14,15], and crystallographic studies [16–18] have been carried out. These have shown that the pyridone form predominates in polar solvents, whereas in non-polar solvents both tautomers can co-exist. Substituents at the 6-position have the greatest electronic effect on the equilibrium; the electron-withdrawing chloro- and bromo-substituted compounds crystallise as the pyridinol tautomer whereas the unsubstituted molecule crystallises as the pyridone tautomer. Theoretical and spectroscopic research has long predicted that the 6-methyl substituted compound will exist in the solid state as the pyridone tautomer and we were recently able to show that this is indeed true, both for pure
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H3 C
N
OH
H3 C
H N
O
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2. Experimental 2.1. Synthesis
pyridinol
pyridone
Scheme 1. The 2-pyridinol–2-pyridone tautomerism.
6-methyl-2-pyridone [19] and for its pentahydrate [20] (see Scheme 1). 6-Methyl-2-pyridone (Hmhp) has been widely used as a ligand in transition metal coordination chemistry, of which an in-depth review was published in 1995 [21]. A common theme throughout the review is the formation of [M2(mhp)4] dimers by coordination through both the oxygen and nitrogen atoms of the ligand to bridge two metal sites. Such coordination frequently induces bonding between the metal atoms [22] but not always; for example, the [Pd2(mhp)4] dimer contains no formal Pd–Pd bond, ˚ [23]. Other less comdespite the short distance of 2.54 A mon structural motifs include a Cr4O4 cubane [24] and a very unusual polymeric Tc compound [25]. Since 1995 research has continued with the synthesis and characterisation of a variety of mixed-metal complexes of the transition elements with lanthanides or s-block metals. By contrast the volume of published research detailing the coordination of the s-block elements alone by the pyridone family of ligands pales into insignificance when compared to that for the transition elements. There are just two reported s-block metal/pyridone complexes which do not also feature a transition or lanthanide element. These are the simple [K(hp) Æ H2O] [26] and the rather more complicated [(6-LiCH2Py-2-OLi)4(6-CH3Py-2-OLi)2(THF)9] [27] where lithiation of Hmhp removed not only the labile N– H proton but also deprotonated the methyl group of two-thirds of the mhp ligands, giving a mixed anion–dianion lithium cage compound. There are no reports of crystalline complexes of any of the 2-pyridone family of ligands with alkaline earth metals; the only reported occurrence of any alkaline earth complexes with 2-pyridones as ligands are [M(hp)6(L)2], where M = Mg or Ca and L ¼ BF4 , ClO4 and NO3 [28,29], although these complexes have not been crystallographically characterised. To rectify this imbalance we have carried out a series of synthetic and crystallographic investigations on a range of 2-pyridone derivatives with the alkali [30] and alkaline earth metals. We present here the following alkaline earth mhp complexes: [Mg4(mhp)4(MeO)4(MeOH)8] (1) [Sr(mhp)2(Hmhp)] (2) [Ba(mhp)2(Hmhp)(MeOH)] (3) Despite several attempts using a variety of different synthetic routes, we were unable to prepare crystals of any Ca– mhp complex.
All manipulations were carried out under dry N2 using standard Schlenk techniques. Methanol was dried with ele˚ molecular sieves. mental magnesium and stored over 3 A i Mg(OEt)2 and Sr( OPr)2 were purchased from Aldrich, Ba(2-ethylhexoxide) was purchased from Strem Chemicals, and 6-methyl-2-pyridone was purchased from Avocado research chemicals. All were used without further purification and were dried in vacuo prior to use. Elemental analyses were determined by the University of Newcastle upon Tyne Advanced Chemical and Materials Analysis unit. 2.1.1. Synthesis of 1 About 1.1 g (10 mmol, 5 equivalents) of 6-methyl-2-pyridone was placed in a dry Schlenk flask. 10 cm3 of dry methanol was added, forming a colourless solution. In a separate flask was placed 0.25 g (2 mmol, 1 equivalent) of magnesium ethoxide. This was dissolved in 15 cm3 of dry methanol, with gentle heating, and then carefully transferred to the flask containing the 6-methyl-2-hydroxypyridine via a cannula, forming a two-layer mixture. Diffusion at room temperature over three days resulted in the formation of large white crystals suitable for X-ray diffraction. The crystals decompose upon exposure to air; elemental analyses obtained approximately correspond to 1 with the loss of four MeOH molecules per tetramer. (0.25 g, 55%). CHN Calc. C, 47.51; H, 7.48; N, 6.16. Found: C, 46.01; H, 6.64; N, 6.88%. 2.1.2. Synthesis of 2 About 0.218 g (2 mmol, 2 equivalents) of 6-methyl-2pyridone was placed in a dry Schlenk flask. 10 cm3 of dry methanol was added, forming a colourless solution. In a separate flask was placed 0.22 g (1 mmol, 1 equivalent) of strontium iso-propoxide. This was dissolved in 10 cm3 of dry methanol, with gentle heating, and then carefully transferred to the flask containing the 6-methyl-2-hydroxypyridine via a cannula, forming a two-layer mixture. Diffusion at room temperature resulted in the formation of a mass of tiny needle crystals (0.174 g, 42.1%), which were too small for X-ray diffraction using standard equipment, so data were collected using synchrotron radiation. The air-sensitive nature of the sample meant that reliable CHN analyses could not be obtained despite several efforts. 2.1.3. Synthesis of 3 About 0.216 g (2 mmol, 10 equivalents) of 6-methyl-2pyridone was placed in a dry Schlenk flask. 2 cm3 of dry methanol was added, forming a colourless solution. 0.2 cm3 (0.2 mmol, 1 equivalent) of barium 2-ethylhexoxide was added and the flask swirled, forming an orange solution. Small needle-like crystals had grown after approximately 30 min. These were too small for X-ray diffraction. An additional 10 cm3 of dry methanol was
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˚ and Uiso(H) = ride on the parent atom with C–H = 0.95 A 1.2Ueq(C). For 1 data collection was carried out with the crystal at 200 K, as the crystals shattered when flash-cooled to 150 K; there is also disorder of one methyl group, which is discussed below. For 2 the absolute structure parameter of 0.067 (15), and hence the hand of the chiral helical structure, was determined from 1505 Friedel pairs using the method described by Flack [38].
added and the pale orange solution heated to boiling point. Upon cooling slowly to room temperature, a white amorphous precipitate formed. This was removed by filtration, and the pale orange filtrate concentrated. Storage of the solution at 273 K for three weeks resulted in the formation of colourless crystals suitable for X-ray diffraction. (0.067 g, 67.7%). The air-sensitive nature of the sample meant that reliable CHN analyses could not be obtained despite several efforts; results obtained correspond approximately to compound 3 with loss of one of the ligands.
3. Results and discussion
2.2. Crystallography
3.1. Compound 1: [Mg4(mhp)4(MeO)4(MeOH)8]
A summary of the crystallographic experimental parameters for all three compounds is given in Table 1. Data for all three compounds were collected on Bruker SMART 1 K CCD diffractometers using either graphite-monochromated Mo Ka (1 and 3) or silicon[1 1 1]-monochromated synchrotron radiation (2) and recorded using x-scans with narrow frames. SMART and SAINT [31,32] were used for data collection and integration, respectively, with absorption and intensity decay corrections by SADABS [33]. Structure solution was by direct methods using SIR2002 [34] (1) or SHELXTL [35] (2 and 3) with anisotropic refinement for all non-hydrogen atoms by full-matrix least-squares on F2 using SHELXTL. Molecular graphics were produced using DIAMOND 3 [36] and MERCURY 1.4 [37]. Hydrogen atoms were located in a difference Fourier map for all compounds; N–H and O–H hydrogen atoms were freely refined, while methyl hydrogen atoms were constrained to ˚ and Uiso(H) = ride on the parent atom with C–H = 0.98 A 1.5Ueq(C). Aromatic hydrogen atoms were constrained to
Reaction of Hmhp with Mg(OEt)2 in methanol yielded compound 1, shown in Fig. 1. The structure is based around a highly unusual Mg4O4 cubane core, of which there are only nine other examples in the Cambridge Structural Database (CSD) [39]. The asymmetric unit of the compound is actually one half of the cubane, the remainder being symmetry-generated by a crystallographic two-fold rotation axis. What is most striking about this compound is that there are only four anionic mhp ligands rather than the expected eight, even though an excess of Hmhp was used in the reaction. Instead of containing two mhp anions per Mg2+ cation, there is instead just one mhp and one MeO ligand obtained by deprotonation of solvent methanol; this behaviour was totally unexpected. The cubane is constructed from the four Mg2+ centres and the four MeO oxygen atoms, each of which is triply bridging for three Mg2+ centres. Mg–O bond lengths for ˚ the cubane core are in the range 2.0661(15)–2.1130(14) A (Table 2); these are typical for an Mg4O4 cubane and are
Table 1 Crystallographic data for all compounds
Chemical formula Mr Crystal system, space group ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) V (A Z Dx (g cm3) ˚) Radiation type, k (A l (mm1) Temperature (K) Crystal form, colour Crystal size (mm) hmax () Tmin, Tmax No. of measured, independent and observed (F2 > 2r) data No. of refined parameters Rint R[F2 > 2r(F2)], wR(F2), S ˚ 3) Dqmax, Dqmin (e A
1
2
3
[Mg(C6H6NO)(CH3O)(CH3OH)2]4 910.2 monoclinic, C2/c 20.5341 (14) 18.9944 (13) 12.4769 (9) 92.5828 (12) 4861.5 (6) 4 1.244 Mo Ka, 0.71073 0.14 200 plate, white 0.60 · 0.40 · 0.05 28.3 0.910, 0.993 21 534, 5898, 3847
[Sr(C6H6NO)2(C6H7NO)] 413.0 hexagonal, P65 12.651 (3) 12.651 (3) 19.219 (10) 2663.8 (17) 6 1.545 synchrotron, 0.6898 3.06 120 needle, colourless 0.10 · 0.01 · 0.01 24.2 0.750, 0.985 8090, 3118, 2286
[Ba(C6H6NO)2(C6H7NO)(CH3OH)] 494.7 monoclinic, P21/c 11.9906 (8) 21.2107 (13) 8.0731 (5) 94.923 (10) 2045.6 (2) 4 1.606 Mo Ka, 0.71073 1.97 150 plate, colourless 0.30 · 0.10 · 0.05 25.3 0.590, 0.908 14 386, 3702, 3156
298 0.034 0.044, 0.137, 1.02 0.32, 0.30
234 0.122 0.055, 0.123, 0.92 0.71, 0.46
256 0.038 0.029, 0.068, 1.08 1.70, 1.42
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unexceptional for Mg–O bonds generally. Both unique Mg2+ centres have slightly distorted octahedral geometry, the distortions largely being necessary to maintain the integrity of the cubane. One of the methoxy groups is disordered over two positions with refined occupancies of 55% and 45% for the disorder components. Turning to the molecular geometry of the mhp ligands, the C–O bond ˚ , intermediate between lengths are 1.295(2) and 1.299(2) A C@O and C–O exo to an aromatic ring and only slightly ˚ found in the neutral Hmhp mollonger than the 1.270(5) A ecule [19]. There is no coordination through the mhp nitrogen atoms in this structure, the ligands being formally monodentate rather than bidentate; free rotation of the anion about the Mg–O bond is instead prevented by N– H. . .O hydrogen bonds, with O–H. . .O hydrogen bonding securing the coordinated methanol molecules (Fig. 1 and Table 3). There are also two weak C–H. . .O bonds linking two methyl hydrogen atoms to the oxygen atoms of the mhp ligands, but their influence in the structure is minor. By contrast to the unusual molecular geometry, the crystal packing is unexceptional. There are no significant intercluster interactions and the packing consists merely of ripple-shaped stacks of the discrete cubanes along the c-axis. 3.2. Compound 2: [Sr(mhp)2(Hmhp)]
Fig. 1. The molecular structure of 1 (a) with 30% probability displacement ellipsoids, selected atomic labelling and hydrogen bonding indicated by dashed lines. (b) Representation of the Mg4O4 cubane structure. Symmetry operator: a, 2 x, y, 3/2 z.
Table 3 ˚ , ) Hydrogen bonds for 1 (A
Table 2 ˚) Selected bond distances for 1 (A Mg(1)–O(3) Mg(1)–O(4) Mg(1)–O(7A) Mg(2)–O(2) Mg(2)–O(6) Mg(2)–O(8) O(1)–C(1)
2.0936(17) 2.1016(15) 2.0661(15) 2.0451(16) 2.1093(15) 2.0718(14) 1.295(2)
Symmetry operator a, 2 x, y, 3/2 z.
Reaction of Sr(iOPr)2 with Hmhp yielded needle crystals so small that data were collected at Station 9.8, Daresbury Laboratory SRS, through the EPSRC-funded National Crystallography Service. The asymmetric unit of polymeric 2 is shown in Fig. 2. Although the reaction stoichiometry was maintained at 2:1 Sr(iOPr)2:Hmhp the crystal structure contains a third, neutral, Hmhp ligand coordinated to the Sr2+ centre; there is no coordinated or uncoordinated methanol in the structure. Each mhp and Hmhp ligand uses its O atom as a bridge between two Sr2+ cations, generating a chain polymer. The two deprotonated mhp ligands also coordinate as chelating ligands via their nitrogen atoms, giving a total Sr2+ coordination number of 8. While Sr–N bond distances are in agreement with typical values, there is a spread of Sr–O distances from the ˚ ] to the long Sr–O2 [2.917(6) A ˚ ], a short Sr–O1 [2.417(6) A rather uncommon length for an Sr–O bond. C–O bond lengths vary from the relatively normal (for Hmhp) ˚ to the almost single 1.321(11) A ˚ (Table 4). The 1.274(9) A coordination geometry about Sr is very distorted square
Mg(1)–O(1) Mg(1)–O(7) Mg(1)–O(8) Mg(2)–O(5) Mg(2)–O(7) Mg(2)–O(8A) O(2)–C(7)
2.0618(15) 2.0786(14) 2.1130(14) 2.0985(17) 2.0946(14) 2.0770(14) 1.299(2)
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
O(4)–H(4O). . .N(1) O(6)–H(6O). . .N(2) O(5)–H(5O). . .O(1A) O(3)–H(3O). . .O(2) C(17)–H(17A). . .O(1A) C(17)–H(17B). . .O(2)
0.91(3) 0.96(3) 0.84(3) 0.84(3) 0.98 0.98
1.78(3) 1.70(3) 1.81(3) 1.81(3) 2.59 2.53
2.667(2) 2.645(2) 2.639(2) 2.637(2) 3.228(3) 3.165(3)
165(3) 166(3) 167(3) 171(3) 123 122
Symmetry operator a, 2 x, y, 3/2 z.
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Fig. 2. The asymmetric unit of 2 expanded to show the complete Sr2+ coordination, with 50% probability displacement ellipsoids and hydrogen bonding marked by a dashed line. Symmetry operators: a, x y, x 1, z 1/6; b, y + 1, 1 x + y, z + 1/6.
Table 4 ˚) Selected bond distances for 2 (A Sr–O(1) Sr–O(2) Sr–O(3) Sr–N(1A) O(1)–C(1) O(3)–C(13)
2.417(6) 2.917(6) 2.595(6) 2.731(7) 1.321(11) 1.274(9)
Sr–O(1A) Sr–O(2A) Sr–O(3A) Sr–N(2) O(2)–C(7)
2.555(7) 2.507(6) 2.664(6) 2.637(7) 1.298(11)
Symmetry operator: a, x y, x 1, z 1/6.
antiprismatic, although this is not immediately obvious as the distortions are so gross. The reason for this distortion starts to become clear when one considers the crystal packing of 2. The space group is P65 and, as Fig. 2 shows, the polymeric chain parallel to the c-axis, generated by bridging O atoms of mhp and Hmhp ligands, is helical. The twisting of the ligands necessary for both deprotonated nitrogen atoms to coordinate the Sr2+ centre results in this infinite left-handed helix. When viewed along the c-axis the sixfold screw symmetry of the helix is both evident and elegant. Within the helix there is just one unique hydrogen bond – classical or otherwise – securing in place the neutral Hmhp ligand which is able to coordinate through oxygen only (see Fig. 3 and Table 5). Since the reaction contains no driving force for enantiomeric selectivity, we presume that equal quantities of leftand right-handed helical chains are formed, segregated into enantiomorphic crystals belonging to the pair of space groups P65 and P61. There are no significant inter-helical interactions in this structure; no acceptors exist to accept C–H hydrogen bonding and the distances between the rings
Fig. 3. One of the infinite helices formed in the crystal packing of 2 viewed along the c-axis. The elegant sixfold screw symmetry is obvious from the bonds radiating out from the centre to meet the points of the pseudo Star of David outlined by the dark Sr–O bonds.
Table 5 ˚ , ) Hydrogen bond for 2 (A D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
N(3)–H(3N). . .O(2A)
0.89(13)
2.00(13)
2.873(10)
165(12)
Symmetry operator: a, x y, x 1, z 1/6.
are too great to permit aromatic stacking interactions. It is thus not surprising that the compound crystallised as tiny needles. 3.3. Compound 3: [Ba(mhp)2(Hmhp)(MeOH)] Chemically, the only difference between 2 and 3, apart from the change from Sr to Ba, is the addition of one coordinated methanol molecule in compound 3. Structurally, however, the difference is rather more profound. The repeat unit of 3 is shown in Fig. 4. It can be readily seen that, although an additional molecule (of solvent) is coordinated to the metal centre, the overall coordination number does not change, since one of the deprotonated nitrogen atoms is now acting as a hydrogen bond acceptor instead of being coordinated to the metal ion, as it was in 2. ˚ to As in 2 C–O bonds range from the normal 1.265(4) A ˚ the long 1.305(5) A. Ba–O and Ba–N bond lengths are unexceptional, although the Ba coordination geometry is distorted such that it is not possible to describe it as anything approaching a regular eight-coordinate polyhedral shape (Table 6).
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Fig. 4. The expanded asymmetric unit of 3 with 50% probability displacement ellipsoids and with hydrogen bonding indicated by dashed lines. Symmetry operators: a x, 1/2 y, z 1/2; b x, 1/2 y, z + 1/2.
Table 6 ˚) Selected bond distances for 3 (A Ba–O(1) Ba–O(2) Ba–O(3) Ba–O(4) O(1)–C(1) O(3)–C(13)
2.765(3) 2.849(3) 2.811(3) 2.734(3) 1.287(5) 1.305(5)
Ba–O(1A) Ba–O(2A) Ba–O(3A) Ba–N(3) O(2)–C(7)
2.702(3) 2.843(2) 2.740(2) 2.876(3) 1.265(4)
Symmetry operator: a, x, 1/2 y, z 1/2.
If the differences in the asymmetric units of 2 and 3 reveal little about the structural consequence of the coordinated methanol molecule, then the crystal packing certainly does. In 2 we noted a polymeric left-handed helix caused by the twisting necessary for the coordination of both deprotonated nitrogen atoms. In 3 such twisting is not necessary, as only one nitrogen site is coordinating. The second acts as a hydrogen bond acceptor for the methanol OH hydrogen atom (Table 7). Consequently the polymer has no twist and it forms instead a regular straight chain along the c-axis, with an alternating zig-zag arrangement of Ba2+ centres. Each pair of adjacent Ba2+ ions is bridged by three oxygen atoms and the chain has overall twofold glide symmetry only, and not the sixfold screw symmetry observed in 2 (Fig. 5). As with 2 there are no C–H hydrogen bonds or aromatic stacking interactions connecting the chains together. Table 7 ˚ , ) Hydrogen bonds for 3 (A
Fig. 5. The polymeric chain in 3 viewed along the c-axis, with glide-plane symmetry.
4. Conclusions Reaction in dry methanol of Hmhp with M(OR)2 (M = Mg, Sr, Ba) formed three new coordination compounds with very different crystal structures. In 1 methanol was observed to be deprotonated to provide charge-balancing methoxy in the formation of a rare Mg4O4 cubane, despite an excess of Hmhp in the reaction mixture. Additional solvent coordination satisfied the Mg2+ coordination, which has octahedral geometry with typical Mg–O bond lengths. The larger coordination capacity of Sr2+ (2) and Ba2+ (3) cannot be satisfied with deprotonated ligands and solvent alone. In 2 we see the inclusion of a neutral Hmhp ligand in the metal coordination, but no solvent. The resulting eight-coordinate polymeric compound has an infinite left-handed helix structure with elegant six-fold screw symmetry formed as a result of the twisting necessary for coordination to Sr by the deprotonated nitrogen atoms of two mhp ligands. In 3 there is also a molecule of methanol coordinated. An additional O–H. . .N hydrogen bond prevents one of the nitrogen atoms coordinating to Ba, so the coordination number in 3 is still eight. However, there is now no need for a twist to achieve the extra coordination by nitrogen, so the polymeric compound forms a straight chain. Acknowledgements
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
<(DHA)
N(2)–H(2N). . .O(3A) O(4)–H(4O). . .N(1)
0.79(5) 0.88(6)
1.98(5) 1.78(6)
2.738(4) 2.645(4)
159(5) 169(5)
Symmetry operator: a, x, 1/2 y, z 1/2.
We thank Dr. Ross Harrington, Luca Russo and Zhanhui Yuan for data collection and processing of 2 at Station 9.8, SRS Daresbury Laboratory as part of the EPSRC-funded
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National Crystallography Service. We thank the CCLRC for synchrotron beam-time and the EPSRC for equipment and studentship funding. Appendix A. Supplementary material Crystallographic data for 1–3 have been deposited with the Cambridge Crystallographic Data Centre, deposition Nos. 286918–286920, 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). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2006.01.005. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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P. Beak, J.B. Covington, J.M. White, J. Org. Chem. 45 (1980) 1347. P. Beak, J.B. Covington, J. Am. Chem. Soc. 100 (1978) 3961. A. Kvick, Acta Crystallogr. Sect B. 32 (1976) 220. A. Kvick, I. Olovsson, Ark. Kem. 30 (1969) 71. B.R. Penfold, Acta Crystallogr. 6 (1953) 591. G.S. Nichol, W. Clegg, Acta Crystallogr. C61 (2005) o383. W. Clegg, G.S. Nichol, Acta Crystallogr. E60 (2004) 1433. J.M. Rawson, R.E.P. Winpenny, Coord. Chem. Rev. 139 (1995) 313. F.A. Cotton, R.A. Walton, Multiple Bonds Between Metal Atoms, Oxford University Press, 1993. W. Clegg, C.D. Garner, M.H. Al-Samman, Inorg. Chem. 21 (1982) 1897. L. Akhter, W. Clegg, D. Collison, C.D. Garner, Inorg. Chem. 24 (1985) 1725. F.A. Cotton, P.E. Fanwick, L.D. Gage, J. Am. Chem. Soc. 108 (1980) 1570. A.J. Blake, P.E.Y. Milne, R.E.P. Winpenny, Acta Crystallogr. Sect. C. 47 (1991) 2461. S.T. Liddle, W. Clegg, Chem. Commun. (2001) 1584. J. Reedijk, Rec. Trav. Chim. 83 (1969) 1139. J. Reedijk, J.A. Smit, Rec. Trav. Chim. 91 (1972) 681. D.M. Tooke, Studies in the structural chemistry of complexes between 2-pyridones and alkali metals, Ph.D. Thesis, University of Newcastle upon Tyne, Newcastle upon Tyne, 2004. Bruker, SAINT, Bruker AXS, Madison, WI, USA, 2001. Bruker, SMART, Bruker AXS, Madison, WI, USA, 2001. G.M. Sheldrick, SADABS, University of Go¨ttingen, Germany, 2003. M.C. Burla, M. Camalli, B. Carrozzini, G.L. Cascavano, C. Giacovazzo, G. Polidori, R. Spagna, J. Appl. Crystallogr. 36 (2003) 1103. G.M. Sheldrick, SHELXTL Version 6, Bruker AXS Inc., Madison, WI, USA, 2001. K. Brandenburg, H. Putz, DIAMOND-3, University of Bonn, Germany, 2004. I.J. Bruno, J.C. Cole, P.R. Edgington, M. Kessler, C.F. Macrae, P. McCabe, J. Pearson, R. Taylor, Acta Crystallogr. B 58 (2002) 389. H.D. Flack, Acta Crystallogr. A39 (1983) 876. F.H. Allen, Acta Crystallogr. B58 (2002) 380.
Complexes of 6-methyl-2-pyridone with the alkaline earth metals magnesium, strontium and barium: Synthesis and structural characterisation Gary S. Nichol, William Clegg
*
School of Natural Sciences (Chemistry), Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Received 19 October 2005; received in revised form 10 January 2006; accepted 14 January 2006 Available online 24 February 2006 Dedicated to Professor Mike Mingos.
Abstract Reaction in dry methanol of 6-methylpyridine (Hmhp) with M(OR)2 (M = Mg, Sr, Ba) formed three new coordination compounds, with very different crystal structures. Compound [Mg4(mhp)4(MeO)4(MeOH)8] (1) forms discrete clusters containing a rare Mg4O4 cubane, only the tenth reported occurrence of such a motif. Compounds [Sr(mhp)2(Hmhp)] (2) and [Ba(mhp)2(Hmhp)(MeOH)] (3) are both eight-coordinate complexes in polymeric chains. The solvent-free 2 packs as an infinite left-handed helix with elegant sixfold screw symmetry formed as a result of twisting necessary for coordination to Sr2+ by both deprotonated nitrogen atoms. In 3 there is also a molecule of methanol coordinated to Ba2+; there is now no need for a twist to achieve the extra coordination, and so the compound forms instead a straight chain with glide-plane symmetry. 2006 Elsevier B.V. All rights reserved. Keywords: 6-Methyl-2-pyridone; Alkaline-earth metals; Coordination chemistry; Hydrogen bonding; Single-crystal structure determination; Synchrotron radiation
1. Introduction The molecule 2-hydroxypyridine, also known as 2-pyridone, and its family of substituted derivatives exhibit a keto–enol tautomeric equilibrium commonly observed in organic molecules. The equilibrium of this particular family must be one of the most studied transformations in chemistry, with the first observations being made in 1907 [1]. Whilst it is generally held that the ketone (or pyridone) form predominates in such equilibria, the predominant tautomer observed in the case of 2-hydroxypyridines is less well defined. The tautomerisation is influenced strongly by external factors (e.g. the polarity of the solvent and the
*
Corresponding author. E-mail addresses: [email protected] (W. Clegg).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.01.005
pH of the solution) and substituent effects (especially the position of the substituent in the ring and its specific electronic effects). Comprehensive investigations by UV–Vis [2,3], infra-red [4–8], and nuclear magnetic resonance [9,10] spectroscopy, together with gas-phase [11–13], theoretical [11,12,14,15], and crystallographic studies [16–18] have been carried out. These have shown that the pyridone form predominates in polar solvents, whereas in non-polar solvents both tautomers can co-exist. Substituents at the 6-position have the greatest electronic effect on the equilibrium; the electron-withdrawing chloro- and bromo-substituted compounds crystallise as the pyridinol tautomer whereas the unsubstituted molecule crystallises as the pyridone tautomer. Theoretical and spectroscopic research has long predicted that the 6-methyl substituted compound will exist in the solid state as the pyridone tautomer and we were recently able to show that this is indeed true, both for pure
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H3 C
N
OH
H3 C
H N
O
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2. Experimental 2.1. Synthesis
pyridinol
pyridone
Scheme 1. The 2-pyridinol–2-pyridone tautomerism.
6-methyl-2-pyridone [19] and for its pentahydrate [20] (see Scheme 1). 6-Methyl-2-pyridone (Hmhp) has been widely used as a ligand in transition metal coordination chemistry, of which an in-depth review was published in 1995 [21]. A common theme throughout the review is the formation of [M2(mhp)4] dimers by coordination through both the oxygen and nitrogen atoms of the ligand to bridge two metal sites. Such coordination frequently induces bonding between the metal atoms [22] but not always; for example, the [Pd2(mhp)4] dimer contains no formal Pd–Pd bond, ˚ [23]. Other less comdespite the short distance of 2.54 A mon structural motifs include a Cr4O4 cubane [24] and a very unusual polymeric Tc compound [25]. Since 1995 research has continued with the synthesis and characterisation of a variety of mixed-metal complexes of the transition elements with lanthanides or s-block metals. By contrast the volume of published research detailing the coordination of the s-block elements alone by the pyridone family of ligands pales into insignificance when compared to that for the transition elements. There are just two reported s-block metal/pyridone complexes which do not also feature a transition or lanthanide element. These are the simple [K(hp) Æ H2O] [26] and the rather more complicated [(6-LiCH2Py-2-OLi)4(6-CH3Py-2-OLi)2(THF)9] [27] where lithiation of Hmhp removed not only the labile N– H proton but also deprotonated the methyl group of two-thirds of the mhp ligands, giving a mixed anion–dianion lithium cage compound. There are no reports of crystalline complexes of any of the 2-pyridone family of ligands with alkaline earth metals; the only reported occurrence of any alkaline earth complexes with 2-pyridones as ligands are [M(hp)6(L)2], where M = Mg or Ca and L ¼ BF4 , ClO4 and NO3 [28,29], although these complexes have not been crystallographically characterised. To rectify this imbalance we have carried out a series of synthetic and crystallographic investigations on a range of 2-pyridone derivatives with the alkali [30] and alkaline earth metals. We present here the following alkaline earth mhp complexes: [Mg4(mhp)4(MeO)4(MeOH)8] (1) [Sr(mhp)2(Hmhp)] (2) [Ba(mhp)2(Hmhp)(MeOH)] (3) Despite several attempts using a variety of different synthetic routes, we were unable to prepare crystals of any Ca– mhp complex.
All manipulations were carried out under dry N2 using standard Schlenk techniques. Methanol was dried with ele˚ molecular sieves. mental magnesium and stored over 3 A i Mg(OEt)2 and Sr( OPr)2 were purchased from Aldrich, Ba(2-ethylhexoxide) was purchased from Strem Chemicals, and 6-methyl-2-pyridone was purchased from Avocado research chemicals. All were used without further purification and were dried in vacuo prior to use. Elemental analyses were determined by the University of Newcastle upon Tyne Advanced Chemical and Materials Analysis unit. 2.1.1. Synthesis of 1 About 1.1 g (10 mmol, 5 equivalents) of 6-methyl-2-pyridone was placed in a dry Schlenk flask. 10 cm3 of dry methanol was added, forming a colourless solution. In a separate flask was placed 0.25 g (2 mmol, 1 equivalent) of magnesium ethoxide. This was dissolved in 15 cm3 of dry methanol, with gentle heating, and then carefully transferred to the flask containing the 6-methyl-2-hydroxypyridine via a cannula, forming a two-layer mixture. Diffusion at room temperature over three days resulted in the formation of large white crystals suitable for X-ray diffraction. The crystals decompose upon exposure to air; elemental analyses obtained approximately correspond to 1 with the loss of four MeOH molecules per tetramer. (0.25 g, 55%). CHN Calc. C, 47.51; H, 7.48; N, 6.16. Found: C, 46.01; H, 6.64; N, 6.88%. 2.1.2. Synthesis of 2 About 0.218 g (2 mmol, 2 equivalents) of 6-methyl-2pyridone was placed in a dry Schlenk flask. 10 cm3 of dry methanol was added, forming a colourless solution. In a separate flask was placed 0.22 g (1 mmol, 1 equivalent) of strontium iso-propoxide. This was dissolved in 10 cm3 of dry methanol, with gentle heating, and then carefully transferred to the flask containing the 6-methyl-2-hydroxypyridine via a cannula, forming a two-layer mixture. Diffusion at room temperature resulted in the formation of a mass of tiny needle crystals (0.174 g, 42.1%), which were too small for X-ray diffraction using standard equipment, so data were collected using synchrotron radiation. The air-sensitive nature of the sample meant that reliable CHN analyses could not be obtained despite several efforts. 2.1.3. Synthesis of 3 About 0.216 g (2 mmol, 10 equivalents) of 6-methyl-2pyridone was placed in a dry Schlenk flask. 2 cm3 of dry methanol was added, forming a colourless solution. 0.2 cm3 (0.2 mmol, 1 equivalent) of barium 2-ethylhexoxide was added and the flask swirled, forming an orange solution. Small needle-like crystals had grown after approximately 30 min. These were too small for X-ray diffraction. An additional 10 cm3 of dry methanol was
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˚ and Uiso(H) = ride on the parent atom with C–H = 0.95 A 1.2Ueq(C). For 1 data collection was carried out with the crystal at 200 K, as the crystals shattered when flash-cooled to 150 K; there is also disorder of one methyl group, which is discussed below. For 2 the absolute structure parameter of 0.067 (15), and hence the hand of the chiral helical structure, was determined from 1505 Friedel pairs using the method described by Flack [38].
added and the pale orange solution heated to boiling point. Upon cooling slowly to room temperature, a white amorphous precipitate formed. This was removed by filtration, and the pale orange filtrate concentrated. Storage of the solution at 273 K for three weeks resulted in the formation of colourless crystals suitable for X-ray diffraction. (0.067 g, 67.7%). The air-sensitive nature of the sample meant that reliable CHN analyses could not be obtained despite several efforts; results obtained correspond approximately to compound 3 with loss of one of the ligands.
3. Results and discussion
2.2. Crystallography
3.1. Compound 1: [Mg4(mhp)4(MeO)4(MeOH)8]
A summary of the crystallographic experimental parameters for all three compounds is given in Table 1. Data for all three compounds were collected on Bruker SMART 1 K CCD diffractometers using either graphite-monochromated Mo Ka (1 and 3) or silicon[1 1 1]-monochromated synchrotron radiation (2) and recorded using x-scans with narrow frames. SMART and SAINT [31,32] were used for data collection and integration, respectively, with absorption and intensity decay corrections by SADABS [33]. Structure solution was by direct methods using SIR2002 [34] (1) or SHELXTL [35] (2 and 3) with anisotropic refinement for all non-hydrogen atoms by full-matrix least-squares on F2 using SHELXTL. Molecular graphics were produced using DIAMOND 3 [36] and MERCURY 1.4 [37]. Hydrogen atoms were located in a difference Fourier map for all compounds; N–H and O–H hydrogen atoms were freely refined, while methyl hydrogen atoms were constrained to ˚ and Uiso(H) = ride on the parent atom with C–H = 0.98 A 1.5Ueq(C). Aromatic hydrogen atoms were constrained to
Reaction of Hmhp with Mg(OEt)2 in methanol yielded compound 1, shown in Fig. 1. The structure is based around a highly unusual Mg4O4 cubane core, of which there are only nine other examples in the Cambridge Structural Database (CSD) [39]. The asymmetric unit of the compound is actually one half of the cubane, the remainder being symmetry-generated by a crystallographic two-fold rotation axis. What is most striking about this compound is that there are only four anionic mhp ligands rather than the expected eight, even though an excess of Hmhp was used in the reaction. Instead of containing two mhp anions per Mg2+ cation, there is instead just one mhp and one MeO ligand obtained by deprotonation of solvent methanol; this behaviour was totally unexpected. The cubane is constructed from the four Mg2+ centres and the four MeO oxygen atoms, each of which is triply bridging for three Mg2+ centres. Mg–O bond lengths for ˚ the cubane core are in the range 2.0661(15)–2.1130(14) A (Table 2); these are typical for an Mg4O4 cubane and are
Table 1 Crystallographic data for all compounds
Chemical formula Mr Crystal system, space group ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) V (A Z Dx (g cm3) ˚) Radiation type, k (A l (mm1) Temperature (K) Crystal form, colour Crystal size (mm) hmax () Tmin, Tmax No. of measured, independent and observed (F2 > 2r) data No. of refined parameters Rint R[F2 > 2r(F2)], wR(F2), S ˚ 3) Dqmax, Dqmin (e A
1
2
3
[Mg(C6H6NO)(CH3O)(CH3OH)2]4 910.2 monoclinic, C2/c 20.5341 (14) 18.9944 (13) 12.4769 (9) 92.5828 (12) 4861.5 (6) 4 1.244 Mo Ka, 0.71073 0.14 200 plate, white 0.60 · 0.40 · 0.05 28.3 0.910, 0.993 21 534, 5898, 3847
[Sr(C6H6NO)2(C6H7NO)] 413.0 hexagonal, P65 12.651 (3) 12.651 (3) 19.219 (10) 2663.8 (17) 6 1.545 synchrotron, 0.6898 3.06 120 needle, colourless 0.10 · 0.01 · 0.01 24.2 0.750, 0.985 8090, 3118, 2286
[Ba(C6H6NO)2(C6H7NO)(CH3OH)] 494.7 monoclinic, P21/c 11.9906 (8) 21.2107 (13) 8.0731 (5) 94.923 (10) 2045.6 (2) 4 1.606 Mo Ka, 0.71073 1.97 150 plate, colourless 0.30 · 0.10 · 0.05 25.3 0.590, 0.908 14 386, 3702, 3156
298 0.034 0.044, 0.137, 1.02 0.32, 0.30
234 0.122 0.055, 0.123, 0.92 0.71, 0.46
256 0.038 0.029, 0.068, 1.08 1.70, 1.42
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unexceptional for Mg–O bonds generally. Both unique Mg2+ centres have slightly distorted octahedral geometry, the distortions largely being necessary to maintain the integrity of the cubane. One of the methoxy groups is disordered over two positions with refined occupancies of 55% and 45% for the disorder components. Turning to the molecular geometry of the mhp ligands, the C–O bond ˚ , intermediate between lengths are 1.295(2) and 1.299(2) A C@O and C–O exo to an aromatic ring and only slightly ˚ found in the neutral Hmhp mollonger than the 1.270(5) A ecule [19]. There is no coordination through the mhp nitrogen atoms in this structure, the ligands being formally monodentate rather than bidentate; free rotation of the anion about the Mg–O bond is instead prevented by N– H. . .O hydrogen bonds, with O–H. . .O hydrogen bonding securing the coordinated methanol molecules (Fig. 1 and Table 3). There are also two weak C–H. . .O bonds linking two methyl hydrogen atoms to the oxygen atoms of the mhp ligands, but their influence in the structure is minor. By contrast to the unusual molecular geometry, the crystal packing is unexceptional. There are no significant intercluster interactions and the packing consists merely of ripple-shaped stacks of the discrete cubanes along the c-axis. 3.2. Compound 2: [Sr(mhp)2(Hmhp)]
Fig. 1. The molecular structure of 1 (a) with 30% probability displacement ellipsoids, selected atomic labelling and hydrogen bonding indicated by dashed lines. (b) Representation of the Mg4O4 cubane structure. Symmetry operator: a, 2 x, y, 3/2 z.
Table 3 ˚ , ) Hydrogen bonds for 1 (A
Table 2 ˚) Selected bond distances for 1 (A Mg(1)–O(3) Mg(1)–O(4) Mg(1)–O(7A) Mg(2)–O(2) Mg(2)–O(6) Mg(2)–O(8) O(1)–C(1)
2.0936(17) 2.1016(15) 2.0661(15) 2.0451(16) 2.1093(15) 2.0718(14) 1.295(2)
Symmetry operator a, 2 x, y, 3/2 z.
Reaction of Sr(iOPr)2 with Hmhp yielded needle crystals so small that data were collected at Station 9.8, Daresbury Laboratory SRS, through the EPSRC-funded National Crystallography Service. The asymmetric unit of polymeric 2 is shown in Fig. 2. Although the reaction stoichiometry was maintained at 2:1 Sr(iOPr)2:Hmhp the crystal structure contains a third, neutral, Hmhp ligand coordinated to the Sr2+ centre; there is no coordinated or uncoordinated methanol in the structure. Each mhp and Hmhp ligand uses its O atom as a bridge between two Sr2+ cations, generating a chain polymer. The two deprotonated mhp ligands also coordinate as chelating ligands via their nitrogen atoms, giving a total Sr2+ coordination number of 8. While Sr–N bond distances are in agreement with typical values, there is a spread of Sr–O distances from the ˚ ] to the long Sr–O2 [2.917(6) A ˚ ], a short Sr–O1 [2.417(6) A rather uncommon length for an Sr–O bond. C–O bond lengths vary from the relatively normal (for Hmhp) ˚ to the almost single 1.321(11) A ˚ (Table 4). The 1.274(9) A coordination geometry about Sr is very distorted square
Mg(1)–O(1) Mg(1)–O(7) Mg(1)–O(8) Mg(2)–O(5) Mg(2)–O(7) Mg(2)–O(8A) O(2)–C(7)
2.0618(15) 2.0786(14) 2.1130(14) 2.0985(17) 2.0946(14) 2.0770(14) 1.299(2)
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
O(4)–H(4O). . .N(1) O(6)–H(6O). . .N(2) O(5)–H(5O). . .O(1A) O(3)–H(3O). . .O(2) C(17)–H(17A). . .O(1A) C(17)–H(17B). . .O(2)
0.91(3) 0.96(3) 0.84(3) 0.84(3) 0.98 0.98
1.78(3) 1.70(3) 1.81(3) 1.81(3) 2.59 2.53
2.667(2) 2.645(2) 2.639(2) 2.637(2) 3.228(3) 3.165(3)
165(3) 166(3) 167(3) 171(3) 123 122
Symmetry operator a, 2 x, y, 3/2 z.
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Fig. 2. The asymmetric unit of 2 expanded to show the complete Sr2+ coordination, with 50% probability displacement ellipsoids and hydrogen bonding marked by a dashed line. Symmetry operators: a, x y, x 1, z 1/6; b, y + 1, 1 x + y, z + 1/6.
Table 4 ˚) Selected bond distances for 2 (A Sr–O(1) Sr–O(2) Sr–O(3) Sr–N(1A) O(1)–C(1) O(3)–C(13)
2.417(6) 2.917(6) 2.595(6) 2.731(7) 1.321(11) 1.274(9)
Sr–O(1A) Sr–O(2A) Sr–O(3A) Sr–N(2) O(2)–C(7)
2.555(7) 2.507(6) 2.664(6) 2.637(7) 1.298(11)
Symmetry operator: a, x y, x 1, z 1/6.
antiprismatic, although this is not immediately obvious as the distortions are so gross. The reason for this distortion starts to become clear when one considers the crystal packing of 2. The space group is P65 and, as Fig. 2 shows, the polymeric chain parallel to the c-axis, generated by bridging O atoms of mhp and Hmhp ligands, is helical. The twisting of the ligands necessary for both deprotonated nitrogen atoms to coordinate the Sr2+ centre results in this infinite left-handed helix. When viewed along the c-axis the sixfold screw symmetry of the helix is both evident and elegant. Within the helix there is just one unique hydrogen bond – classical or otherwise – securing in place the neutral Hmhp ligand which is able to coordinate through oxygen only (see Fig. 3 and Table 5). Since the reaction contains no driving force for enantiomeric selectivity, we presume that equal quantities of leftand right-handed helical chains are formed, segregated into enantiomorphic crystals belonging to the pair of space groups P65 and P61. There are no significant inter-helical interactions in this structure; no acceptors exist to accept C–H hydrogen bonding and the distances between the rings
Fig. 3. One of the infinite helices formed in the crystal packing of 2 viewed along the c-axis. The elegant sixfold screw symmetry is obvious from the bonds radiating out from the centre to meet the points of the pseudo Star of David outlined by the dark Sr–O bonds.
Table 5 ˚ , ) Hydrogen bond for 2 (A D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
\(DHA)
N(3)–H(3N). . .O(2A)
0.89(13)
2.00(13)
2.873(10)
165(12)
Symmetry operator: a, x y, x 1, z 1/6.
are too great to permit aromatic stacking interactions. It is thus not surprising that the compound crystallised as tiny needles. 3.3. Compound 3: [Ba(mhp)2(Hmhp)(MeOH)] Chemically, the only difference between 2 and 3, apart from the change from Sr to Ba, is the addition of one coordinated methanol molecule in compound 3. Structurally, however, the difference is rather more profound. The repeat unit of 3 is shown in Fig. 4. It can be readily seen that, although an additional molecule (of solvent) is coordinated to the metal centre, the overall coordination number does not change, since one of the deprotonated nitrogen atoms is now acting as a hydrogen bond acceptor instead of being coordinated to the metal ion, as it was in 2. ˚ to As in 2 C–O bonds range from the normal 1.265(4) A ˚ the long 1.305(5) A. Ba–O and Ba–N bond lengths are unexceptional, although the Ba coordination geometry is distorted such that it is not possible to describe it as anything approaching a regular eight-coordinate polyhedral shape (Table 6).
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Fig. 4. The expanded asymmetric unit of 3 with 50% probability displacement ellipsoids and with hydrogen bonding indicated by dashed lines. Symmetry operators: a x, 1/2 y, z 1/2; b x, 1/2 y, z + 1/2.
Table 6 ˚) Selected bond distances for 3 (A Ba–O(1) Ba–O(2) Ba–O(3) Ba–O(4) O(1)–C(1) O(3)–C(13)
2.765(3) 2.849(3) 2.811(3) 2.734(3) 1.287(5) 1.305(5)
Ba–O(1A) Ba–O(2A) Ba–O(3A) Ba–N(3) O(2)–C(7)
2.702(3) 2.843(2) 2.740(2) 2.876(3) 1.265(4)
Symmetry operator: a, x, 1/2 y, z 1/2.
If the differences in the asymmetric units of 2 and 3 reveal little about the structural consequence of the coordinated methanol molecule, then the crystal packing certainly does. In 2 we noted a polymeric left-handed helix caused by the twisting necessary for the coordination of both deprotonated nitrogen atoms. In 3 such twisting is not necessary, as only one nitrogen site is coordinating. The second acts as a hydrogen bond acceptor for the methanol OH hydrogen atom (Table 7). Consequently the polymer has no twist and it forms instead a regular straight chain along the c-axis, with an alternating zig-zag arrangement of Ba2+ centres. Each pair of adjacent Ba2+ ions is bridged by three oxygen atoms and the chain has overall twofold glide symmetry only, and not the sixfold screw symmetry observed in 2 (Fig. 5). As with 2 there are no C–H hydrogen bonds or aromatic stacking interactions connecting the chains together. Table 7 ˚ , ) Hydrogen bonds for 3 (A
Fig. 5. The polymeric chain in 3 viewed along the c-axis, with glide-plane symmetry.
4. Conclusions Reaction in dry methanol of Hmhp with M(OR)2 (M = Mg, Sr, Ba) formed three new coordination compounds with very different crystal structures. In 1 methanol was observed to be deprotonated to provide charge-balancing methoxy in the formation of a rare Mg4O4 cubane, despite an excess of Hmhp in the reaction mixture. Additional solvent coordination satisfied the Mg2+ coordination, which has octahedral geometry with typical Mg–O bond lengths. The larger coordination capacity of Sr2+ (2) and Ba2+ (3) cannot be satisfied with deprotonated ligands and solvent alone. In 2 we see the inclusion of a neutral Hmhp ligand in the metal coordination, but no solvent. The resulting eight-coordinate polymeric compound has an infinite left-handed helix structure with elegant six-fold screw symmetry formed as a result of the twisting necessary for coordination to Sr by the deprotonated nitrogen atoms of two mhp ligands. In 3 there is also a molecule of methanol coordinated. An additional O–H. . .N hydrogen bond prevents one of the nitrogen atoms coordinating to Ba, so the coordination number in 3 is still eight. However, there is now no need for a twist to achieve the extra coordination by nitrogen, so the polymeric compound forms a straight chain. Acknowledgements
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
<(DHA)
N(2)–H(2N). . .O(3A) O(4)–H(4O). . .N(1)
0.79(5) 0.88(6)
1.98(5) 1.78(6)
2.738(4) 2.645(4)
159(5) 169(5)
Symmetry operator: a, x, 1/2 y, z 1/2.
We thank Dr. Ross Harrington, Luca Russo and Zhanhui Yuan for data collection and processing of 2 at Station 9.8, SRS Daresbury Laboratory as part of the EPSRC-funded
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National Crystallography Service. We thank the CCLRC for synchrotron beam-time and the EPSRC for equipment and studentship funding. Appendix A. Supplementary material Crystallographic data for 1–3 have been deposited with the Cambridge Crystallographic Data Centre, deposition Nos. 286918–286920, 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). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2006.01.005. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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