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Coordination networks incorporating the in situ generated ligands [OC(CO2)3]4− and [OCH(CO2)2]3−
Journal of Molecular Structure 796 (2006) 2–8 www.elsevier.com/locate/molstruc
Coordination networks incorporating the in situ generated ligands [OC(CO2)3]4K and [OCH(CO2)2]3K Brendan F. Abrahams, Timothy A. Hudson, Richard Robson * School of Chemistry, University of Melbourne, Victoria 3010, Australia Received 16 January 2006; received in revised form 10 February 2006; accepted 10 February 2006 Available online 8 May 2006
Abstract A coordination polymer of composition [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O separates in crystalline form from basic aqueous reaction mixtures at 40 8C containing Zn(II) together with either dihydroxyfumaric acid or diketosuccinic acid. [Mn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O can be obtained under analogous conditions using diketosuccinic acid but not when dihydroxyfumaric acid is used as the starting material. The ˚ (Zn) and 17.2584(10) A ˚ (Mn)]. The unusual C4 O4K zinc and manganese products are cubic with space group P213 [aZ16.807(8) A 7 component of 4K these compounds is the new oxyanion of carbon, [OC(CO2)3] arising via a metal-promoted benzilic acid-type rearrangement. The C3 HO3K 5 component is the trianion of hydroxymalonic acid generated by the decarboxylation of C4 O4K 7 . The structures of these two compounds consist of cubane-like units inter-linked via C3 HO3K 5 bridges into a 3D network with the intrinsically chiral (10,3)-a topology. In the case of zinc, the 3K crystalline product that separates under somewhat more forcing conditions contains no C4 O4K 7 but only its decarboxylation product C3 HO5 , the composition being [Zn3(C3HO5)2(H2O)6]$2H2O. This compound crystallises in the monoclinic space group P21/c with unit cell dimensions aZ ˚ , bZ5.4564(6) A ˚ , cZ14.9897(16) A ˚ and bZ105.400(2)8. The C3 HO3K 9.8346(11) A 5 unit plays the same triply chelating role seen in the above two compounds, giving rise in this case to a 2D coordination polymer with the (6,3) topology. The sheet structure adopts a strongly undulating character reminiscent of corrugated galvanised sheet. Multiple intra-sheet and inter-sheet hydrogen bonds are present. Basic aqueous solutions containing diketosuccinic acid and zinc(II) under even more forcing conditions (sealed tube, 130 8C) yield the ultimate decomposition product, oxalate, in the crystalline form of the known Zn(C2O4)(H2O)2. q 2006 Elsevier B.V. All rights reserved. Keywords: Coordination polymer; Oxyanion; Rearrangement; Cubane; Hydrogen bonding
1. Introduction It was reported recently that dihydroxyfumaric acid, I, in the presence of certain divalent metal ions in basic aqueous solution at room temperature, undergoes aerial oxidation to diketosuccinic acid, II, followed by benzilic acid-type rearrangement to generate crystalline products containing discrete octametallic molecules of composition ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 (MZMg, Fe, Co and Zn), the C4 O4K 7 component being the alkoxide–tricarboxylate species [OC(CO2)3]4K, a new oxyanion of carbon, III [1]. Since those results were published, Mn and Ni have been added to the list of divalent metal ions giving ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 [2]. The structure of
* Corresponding author. Tel.: C61 3 8344 6469; fax: C61 3 9347 5180. E-mail address: [email protected] (R. Robson).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.02.009
the octametallic molecule is shown in Fig. 1. It contains a cubane-like core consisting of four trigonal prismatic metal centres and four m3 -alkoxo bridges from separate [OC(CO2)3]4K ligands, each of which chelates to three metal K centres in the core. This central ½MII4 ðC4 O7 Þ4 8 unit is ideally arranged to chelate to four other appended metal centres, which have an octahedral coordination environment completed by three facially disposed aqua ligands. The hydroxy-tricarboxylic acid corresponding to [OC(CO2)3]4K, namely HOC(COOH)3 would be expected to be extremely prone to decarboxylation to give HOCH(COOH) 2, hydroxymalonic acid. The [OC(CO2)3]4K unit present in the above crystalline complexes presumably survives without loss of CO2 because, from the moment of its generation, it is stabilised by chelation to metal centres. We report here that, under somewhat more forcing conditions, different crystalline species separate that incorporate the product of decarboxylation, namely the alkoxide– dicarboxylate anion, [OCH(CO2)2]3K (IV) (represented C3 HO3K 5 below) derived from hydroxymalonic acid.
B.F. Abrahams et al. / Journal of Molecular Structure 796 (2006) 2–8
HOOC
HOOC
OH
O–
O CO2–
HO
O
COOH I
CO2–
CO2–
COOH II
III M
M O–
O H
O
O
H O
O
O
O IV
(0.24 mmol). Crystals of ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 ðH3 OÞ$8H2 O, contaminated with finer solid, were obtained upon heating the suspension at 40 8C for 2 days. 2.1.3. Zn3(C3HO5)2(H2O)6]$2H2O from di-sodium diketosuccinic acid To a solution of di-sodium diketosuccinic acid suspended in water (1.00 g, 47.2 mmol) was added a solution of Zn(NO3)2$6H2O (2.10 g, 1.5 mmol). The suspension was then heated for 2 days at 55 8C from which crystals of Zn3(C3HO5)2(H2O)6$2H2O were obtained.
O
O –
–
3
M V
2.1.4. Zn3(C3HO5)2(H2O)6$2H2O from dihydroxyfumaric acid To a solution of partially dissolved dihydroxyfumaric acid (300 mg, 2.03 mmol) in water (100 ml) and tetramethylammonium hydroxide (6.06 mmol) in methanol was added a solution of Zn(NO3)2$6H2O (402 mg, 1.35 mmol) in water (100 ml). The solution was allowed to stand at 40 8C. Initially, a small yield of [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O was obtained but upon standing for 3 weeks colourless crystals of [Zn3(C3HO5)2(H2O)6]$2H2O separated from the solution as the major product. 2.2. X-ray crystallography
Fig. 1. The structure of the ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 molecule. Circles in order of increasing size represent C, O and M, respectively.
2. Experimental 2.1. Syntheses 2.1.1. Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O from dihydroxyfumaric acid To a solution of partially dissolved dihydroxyfumaric acid (100 mg, 0.676 mmol) in water (50 ml) was added tetramethylammonium hydroxide (2.03 mmol) and a solution of Zn(NO3)2$6H2O (134 mg, 0.451 mmol) in water (50 ml). Colourless crystals, contaminated with finer solid, of [Zn 6(C 4O 7)(C 3HO 5) 3(H 2O) 6](H 3O)$8H 2O were obtained upon standing at 40 8C for 7 days. 2.1.2. ½M II6 ðC 4 O7 ÞðC3 HO5 Þ3 ðH 2 OÞ6 ðH 3 OÞ$8H 2 O (MZ Zn or Mn) from di-sodium diketosuccinic acid To a suspension of di-sodium diketosuccinic acid (50 mg, 0.24 mmol) in water (5 ml) was added a solution of MII(NO3)2$xH2O (0.36 mmol) followed by trimethylamine
Crystal data for [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O: ˚ , VZ MrZ1155.60, cubic, space group: P213, aZ16.807(8) A 3 ˚ ˚ 4747(4) A , TZ293 K, lZ1.54180 A , ZZ4, rcalc Z 1.617 g/cm3, mZ4.203 mmK1, F(000)Z2308, no. of measured (and independent) reflections: 1864 (1707), no. of parameters: 190, R1 [IO2s(I)]Z0.0819, wR2 (all data)Z0.2161, max/min ˚ K3. Intensity residual electron density: 0.65 and K0.65 e A data for [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O were collected using an Enraf-Nonius CAD-4 diffractometer fitted with Cu Ka radiation at 293 K. Structure refinements were performed using the SHELXL-97 program [3], which uses a full-matrix least-squares refinement based of F2. Numerical absorption corrections were applied [4]. Peaks of electron density identified in difference maps were consistent with disordered water molecules. These peaks were assigned as oxygen atoms and were refined with partial site occupation factors that were adjusted so as to produce reasonable thermal displacement parameters. At convergence the sum of the of the site occupancies suggested eight water molecules per formula unit. Crystal data for [(Mn)6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2˚, O: MrZ1093.02, cubic, space group: P213, aZ17.2584(10) A ˚ 3, TZ130 K, lZ0.71073 A ˚. VZ5140.5(5) A While a full structure determination was performed for the zinc compound, only a partial structure solution could be obtained in the case of the manganese compound. Despite this, the connectivity of the network is clearly indicated. Crystal data for Zn3(C3HO5)2(H2O)6$2H2O: MrZ574.31, ˚ , bZ5.4564(6) monoclinic, space group: P21/c, aZ9.8346(11) A ˚ , cZ14.9897(16) A ˚ , bZ105.400(2)8, VZ775.49(15) A ˚ 3, A ˚, TZ130 K, lZ0.71073 A ZZ2, rcalcZ2.46 g/cm3, K1 mZ4.705 mm , F(000)Z576, no. of measured (and
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B.F. Abrahams et al. / Journal of Molecular Structure 796 (2006) 2–8
independent) reflections: 4449 (1742), no. of parameters: 160, R1 [IO2s(I)]Z0.0268, wR2 (all data)Z0.0709, max/min residual ˚ K3. Intensity data for electron density: 0.98 and K0.49 e A Zn3(C3HO5)2(H2O)6$2H2O were collected with a Bruker CCD Area Detector diffractometer fitted with Mo Ka radiation and a graphite monochromator. Structure refinements were performed using the SHELXL-97 program [3], which uses a full-matrix leastsquares refinement based of F2. Absorption corrections were performed using the SADABS program [5]. 3. Results and discussion Basic aqueous reaction mixtures containing dihydroxyfumaric acid and zinc(II), held at 40 8C for approximately 3 days, yield colourless crystals that contain C3 HO3K 5 together with some C4 O4K 7 that still survives, the composition being [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O, but after approximately 3 weeks at this temperature the major solid product (especially when more concentrated solutions are used) contains no C4 O4K 7 and has the composition [Zn3(C3HO5)2 (H2O)6]$2H2O. The same [Zn6(C4O7)(C3HO5)3(H2O)6](H3 O)$8H2O product can be obtained from the di-sodium salt of diketosuccinic acid in basic solution with zinc nitrate at 40–50 8C. Crystals of composition [Mn6(C4O7)(C3HO5)3 (H2O)6](H3O)$8H2O can be similarly obtained from the reaction of the di-sodium salt of diketosuccinic acid in basic solution with manganese nitrate at 40–50 8C. However, whilst [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O can be obtained from either dihydroxyfumaric acid or diketosuccinic acid, we have been unable to obtain the Mn analogue from dihydroxyfumaric acid. Similar anomalous behaviour of manganese is seen also in the generation of the ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 series of compounds: although several members of this series can be obtained from either dihydroxyfumaric acid or diketosuccinic acid, we have succeeded in obtaining [Mn8(C4O7)4(H2O)12] only when diketosuccinic acid is the starting material, all our attempts starting with dihydroxyfumaric acid having failed. As represented in V, the C3 HO3K 5 unit is ideally composed so as to chelate three metal centres, and the single crystal X-ray diffraction studies described below reveal that it does indeed play this role, thus acting as a 3-connecting building block in the formation of extended coordination networks. The only reference to a structural study of a metal derivative of C3 HO3K 5 that we have found concerns a discrete, non-polymeric triCu(II) derivative [6]. 3.1. ½M II6 ðC 4 O7 ÞðC 3 HO5 Þ3 ðH 2 OÞ6 ðH 3 OÞ$8H 2 O (MZZn or Mn) The compounds ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 ðH3 OÞ$8H2 O (MZZn or Mn), are cubic with the space group P213 [aZ ˚ (Zn) and 17.2584(10) A ˚ (Mn)]. For all 16.807(8) A the structures reported in this paper, inter-atomic distances and angles, except those mentioned specifically below, are unexceptional and are not tabulated here, but are presented in 4K the Supplementary material. The ligands C3 HO3K 5 and C4 O7 act in concert to generate an anionic 3D network of
Fig. 2. View, slightly displaced from that along the threefold axis, of a cubane unit, in the structure of the ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 K network, showing the three hydroxymalonate trianionic ligands, C3 HO3K 5 , which each provide a m3 alkoxo centre for the cubane. Circles in order of increasing size represent C, O and M, respectively. The fourth oxygen centre of the cubane unit is provided by the single C4 O4K 7 ligand, all atoms of which except its alkoxo atom are omitted here for clarity.
composition ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 K, consisting of interlinked cubane-like units with the connectivity of the (10,3)-a net, a net which is intrinsically chiral [7]. The individual cubane-like units are related to but significantly different from those seen in Fig. 1. As shown in Fig. 2, three of the four oxygen centres within the cubane unit are provided by the alkoxo centres from three separate C3 HO3K 5 ligands, which are related around the threefold axis. The fourth oxygen atom in the cubane is provided by the C4 O4K 7 ligand, all atoms of which except for the alkoxo atom have been omitted from Fig. 2 for clarity. As can be seen in Fig. 2, each of the three C3 HO3K 5 units around the cubane nucleus acts as a bidentate chelating ligand for a metal centre, M(1), external to the cubane. The single C4 O4K 7 ligand per cubane, shown in Fig. 3, which is oriented with its alkoxide C–O located along the threefold axis, plays almost exactly the same role as the four II C4 O4K 7 ligands per cubane in ½M8 ðC4 O7 Þ4 ðH2 OÞ12 represented in Fig. 1. As can be seen in Fig. 3, the C4 O4K 7 ligand provides one alkoxo m3 bridge for the cubane unit and chelates to the 3 equiv. metals centres, M(2) in the figure. The three carboxylate oxygen centres of the C4 O4K 7 unit not directly coordinated to cubane metal centres are ideally arranged, as are those in ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 in Fig. 1, to act as a tridentate donor system to an appended metal centre, M(4) in Fig. 3. The octahedral coordination geometry of M(4) is completed by a facial arrangement of three aqua ligands. The fourth and unique metal centre in the cubane nucleus, M(3) in Fig. 3, is located on the threefold axis. It has an essentially octahedral coordination environment consisting of its three alkoxo oxygen neighbours within the cubane core together with a facial arrangement of three aqua ligands. The three M(2) centres belonging to the cubane nucleus that are related around the threefold axis, have a trigonal prismatic coordination geometry very similar to the
B.F. Abrahams et al. / Journal of Molecular Structure 796 (2006) 2–8
5
Fig. 5. The (10,3)-a network. Circles in order of decreasing size represent the Tris-bidentate coordinated M(1), the centre of the cubane unit and the appended M(4), respectively. Broken lines represent the triple hydrogen bond interactions linking the aqua ligands on M(4) to carboxylato oxygen donors on M(1). Fig. 3. Representation of the cubane unit, the associated C4 O4K 7 ligand and the octahedral coordination environments of M(3) and M(4). Circles in order of increasing size represent C, O and M, respectively. The alkoxo C–O bond of the C4 O4K 7 unit and M(3) and M(4) are all located on the threefold axis.
trigonal prismatic coordination geometry seen for all four cubane metal centres in ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 in Fig. 1. Each M(1) centre is linked to three cubane nuclei by the bidentate C3 HO3K 5 donors they provide, as shown in Fig. 4. Thus, each cubane is linked to three M(1) centres (as can be seen in Fig. 2) and acts as a 3-connecting node, while each M(1) centre in turn is linked to three cubanes (as can be seen in
Fig. 4. The Tris-bidentate chelated environment of M(1), which is linked by bridging C3 HO3K 5 ligands to three cubane units. A threefold axis passes through M(1).
Fig. 4) and acts as a second type of 3-connecting node. An infinite 3D 3-connected network is thereby generated which has the topology of the chiral (10,3)-a net, shown in Fig. 5. Also represented schematically in Fig. 5 are the C4O7M(4)(H2O)3 appendages to the cubane nodes, each of which is directed, across the channels of the (10,3)-a net, towards a particular M(1) node with which it forms a trio of hydrogen bonds, details of which can be seen in Fig. 6. The three aqua
Fig. 6. Details of the cross-channel triple hydrogen bond from the aqua ligands on M(4) to the carboxylato oxygen donors on M(1). Circles in order of increasing size represent C, O and M, respectively.
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ligands on M(4), which act as hydrogen bond donors, form an equilateral triangle almost identical in size to the equilateral triangle formed by the three carboxy oxygen donors around M(1),which act as hydrogen bond acceptors. The contents of the intra-framework spaces, w8H2O per formula unit, are disordered and ill-defined. We propose that the cation providing charge balance for the negatively charged framework is the hydronium ion, H3OC, not identifiable by X-ray diffraction techniques. This assignment is supported by neutron diffraction studies on an anionic yttrium oxalate network, ½YðC2 O4 ÞK 2 n , which showed unambiguously that the counter cation was the hydronium ion closely associated with water molecules by hydrogen bonding [8]. 3.2. Zn3(C3HO5)2(H2O)6]$2H2O Fig. 7. Two views of a [Zn(C3HO5)(H2O)2]K chain. Circles in order of increasing size represent C, O and Zn, respectively. Hydrogen bonds between aqua ligands and bridging alkoxo oxygen centres are indicated by open connections. The views in (a) and (b) are from different angles, both perpendicular to the b axis. The view in (b) highlights the sinusoidal character of the Zn/C3HO5 chain.
After approximately 3 weeks at 40 8C aqueous solutions containing dihydroxyfumaric acid and zinc acetate deposit colourless hexagonal plates of composition [Zn3(C3HO5)2 (H2O)6]$2H2O. The same product can be obtained from aqueous reaction mixtures containing the di-sodium salt of
Fig. 8. Two views of the [Zn3(C3HO5)2(H2O)6] sheet. Circles in order of increasing size represent C, O and Zn, respectively. (a) View perpendicular to the sheet showing the two sorts of hydrogen bonds within the sheet represented as open connections. (b) Edge-on view of the sheet perpendicular to the b axis. The aqua ligands have been omitted in (b) to highlight the pronounced corrugation within the Zn/C3HO5 component. The smallest open circles shown in (b) represent the C–H hydrogen atoms of C3 HO3K 5 .
B.F. Abrahams et al. / Journal of Molecular Structure 796 (2006) 2–8
diketosuccinic acid and zinc nitrate after 2 days at 55 8C. The structure of [Zn3(C3HO5)2(H2O)6]$2H2O was studied by single crystal X-ray diffraction. The compound crystallises in the monoclinic space group P21/c with unit cell dimensions of aZ ˚ , bZ5.4564(6) A ˚ , cZ15.9897(16) A ˚ and bZ 9.8346(11) A 105.400(2)8. The hydroxymalonate trianion, C3 HO3K 5 , again plays the triply chelating role seen in V, thus acting as a 3-connecting node and giving rise, in this case, to a 2D coordination polymer with the (6,3) topology. The sheet structure is conveniently envisaged in terms of anionic linear 1D coordination polymers of composition [Zn(C3HO5)(H2O)2]K linked together by coordination to a second type of diaqua Zn2C cation in the way described below. Fig. 7 shows the structure of the [Zn(C3HO5)(H2O)2]K component of the sheet. All zinc centres, Zn(1), in the chain are equivalent, having octahedral coordination geometry consisting of two bidentate chelated C3 HO3K 5 ligands and two cis aqua ligands. The alkoxo oxygen centre of C3 HO3K 5 acts as a bridge between two Zn(1) centres. The backbone of the Zn/(C3HO5) polymeric chain adopts a pronounced sinusoidal arrangement, as can be seen from the angle of view represented in Fig. 7(b). One of the hydrogen atoms on one of the aqua ligands is directed towards the alkoxo oxygen centre of an adjacent C3 HO3K 5 ligand (O/O ˚ ) generating a significant hydrogen separation 2.651(2) A bonding interaction that supports the sinusoidal arrangement, as can be seen in Fig. 7. The [Zn(C3HO5)(H2O)2]K chains are linked together by diaqua zinc centres of a different type, Zn(2), to generate 2D sheets of overall composition [Zn(C3HO5)(H2O)2]2[Zn(H2O)2] or [Zn3(C3HO5)2(H2O)6] as shown in Fig. 8. Zn(2) has an octahedral coordination geometry with trans aqua ligands, the metal being located on a centre of symmetry. Pronounced
7
corrugation is evident within the sheets as can be seen in the edge-on view perpendicular to the b axis in Fig. 8(b). Additional reinforcement within the sheet is provided by hydrogen bonds, which can be seen in Fig. 8(a), from an aqua ligand on Zn(2) to a carboxylato oxygen atom coordinated to ˚ ]. an adjacent Zn(2) [O/O, 2.800(4) A Sheets are stacked in the c direction as shown in Fig. 9. Aqua ligands on Zn(1) centres in one sheet are hydrogen bonded to aqua ligands on Zn(1) centres in the neighbouring ˚ ). The aqua ligands on Zn(1) centres in sheet (O/O, 2.919(5) A one sheet are also hydrogen bonded to aqua ligands on Zn(2) in ˚ ). Lattice water molecules the adjacent sheet (O/O, 2.919(5) A are located between sheets, as shown in Fig. 9; these provide additional sheet-to-sheet binding by forming hydrogen bonds to carboxylato oxygen centres in sheets above and below ˚ and 2.916(5) A ˚ ). The hydrogen atoms on all (O/O, 2.887(5) A the water molecules in the structure (i.e. the two inequivalent cis aqua ligands on Zn(1), the 2 equiv. trans aqua ligands on Zn(2) and the lattice water molecules) are well defined in the structure solution and all of the OH groups act as hydrogen bond donors in significant interactions with other oxygen centres. The combination of this efficient hydrogen bonding with efficient coordinate bonding gives rise to a very tightly knit 3D structure. 3.3. Generation of zinc oxalate When an aqueous solution containing the di-sodium salt of diketosuccinic acid and zinc nitrate is heated in a sealed tube at 130 8C for approximately 18 h large crystals of the known oxalate, Zn(C2O4)(H2O)2 are formed [9].
Fig. 9. Representation of the stacking of the sheets in [Zn3(C3HO5)2(H2O)6]$2H2O. Circles in order of increasing size represent C, O and Zn, respectively. The sheets are bonded together by multiple hydrogen bonds represented here by open connections.
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4. Concluding remarks
Supplementary data
The work reported here extends our knowledge of the remarkable sequence of metal ion-promoted transformations exhibited by dihydroxyfumaric acid. Initial air oxidation is followed by a benzilic acid-type rearrangement to give the new oxyanion of carbon, C4 O4K 7 , as a crucial component of a highly symmetrical octametallic complex with a cubane-like core. Under slightly more forcing conditions the C4 O4K 7 species undergoes partial decarboxylation and the C3 HO3K 5 ion so formed acts in concert with still intact, remnant C4 O4K 7 to generate crystalline coordination polymers in which cubane-like units are linked together with the archetypal 10,3-a topology. Under somewhat more forcing conditions, at least in the case of the Zn2C-promoted reaction, a crystalline coordination polymer 4K of C3 HO3K 5 , containing no C4 O7 , is formed, which contains strongly undulating sheets with multiple intra-sheet and intersheet hydrogen bonds. Under yet more forcing conditions (130 8C, sealed tube) similar reaction mixtures generate zinc oxalate.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2006.02.009. CCDC numbers 292643 and 292644 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK. Fax: C44 1223 336033).
Acknowledgements The authors gratefully acknowledge support from the Australian Research Council.
References [1] B.F. Abrahams, T.A. Hudson, R. Robson, J. Am. Chem. Soc. 126 (2004) 8624. [2] T.A. Hudson, unpublished results. [3] G.M. Sheldrick, Programs for Crystal Structure Analysis, University of Go¨ttingen, Germany, 1998. [4] G.M. Sheldrick, SHELX-76, A Program for Crystal Structure Determination, Cambridge, United Kingdom, 1976. [5] SADABS: Area-detector Absorption Correction; Siemens Industrial Automation: Madison, WI, 1996. [6] A.V. Ablov, G.A. Popovich, G.I. Dimitrova, G.A. Kiosse, I.F. Bourshteyn, T.I. Malinovskii, B.M. Shchedrin, Dokl. Akad. Nauk SSSR (Russ.) (Proc. Natl. Acad. Sci. USSR) 229 (1976) 611. [7] A.F. Wells, Three Dimensional Nets and Polyhedra, Wiley–Interscience, New York, 1977, p. 35. [8] G.D. Brunton, C.K. Johnson, J. Chem. Phys. 62 (1975) 3797. [9] G. Giester, Zeitschrift fuer Kristallographie 212 (1997) 720–723.
Coordination networks incorporating the in situ generated ligands [OC(CO2)3]4K and [OCH(CO2)2]3K Brendan F. Abrahams, Timothy A. Hudson, Richard Robson * School of Chemistry, University of Melbourne, Victoria 3010, Australia Received 16 January 2006; received in revised form 10 February 2006; accepted 10 February 2006 Available online 8 May 2006
Abstract A coordination polymer of composition [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O separates in crystalline form from basic aqueous reaction mixtures at 40 8C containing Zn(II) together with either dihydroxyfumaric acid or diketosuccinic acid. [Mn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O can be obtained under analogous conditions using diketosuccinic acid but not when dihydroxyfumaric acid is used as the starting material. The ˚ (Zn) and 17.2584(10) A ˚ (Mn)]. The unusual C4 O4K zinc and manganese products are cubic with space group P213 [aZ16.807(8) A 7 component of 4K these compounds is the new oxyanion of carbon, [OC(CO2)3] arising via a metal-promoted benzilic acid-type rearrangement. The C3 HO3K 5 component is the trianion of hydroxymalonic acid generated by the decarboxylation of C4 O4K 7 . The structures of these two compounds consist of cubane-like units inter-linked via C3 HO3K 5 bridges into a 3D network with the intrinsically chiral (10,3)-a topology. In the case of zinc, the 3K crystalline product that separates under somewhat more forcing conditions contains no C4 O4K 7 but only its decarboxylation product C3 HO5 , the composition being [Zn3(C3HO5)2(H2O)6]$2H2O. This compound crystallises in the monoclinic space group P21/c with unit cell dimensions aZ ˚ , bZ5.4564(6) A ˚ , cZ14.9897(16) A ˚ and bZ105.400(2)8. The C3 HO3K 9.8346(11) A 5 unit plays the same triply chelating role seen in the above two compounds, giving rise in this case to a 2D coordination polymer with the (6,3) topology. The sheet structure adopts a strongly undulating character reminiscent of corrugated galvanised sheet. Multiple intra-sheet and inter-sheet hydrogen bonds are present. Basic aqueous solutions containing diketosuccinic acid and zinc(II) under even more forcing conditions (sealed tube, 130 8C) yield the ultimate decomposition product, oxalate, in the crystalline form of the known Zn(C2O4)(H2O)2. q 2006 Elsevier B.V. All rights reserved. Keywords: Coordination polymer; Oxyanion; Rearrangement; Cubane; Hydrogen bonding
1. Introduction It was reported recently that dihydroxyfumaric acid, I, in the presence of certain divalent metal ions in basic aqueous solution at room temperature, undergoes aerial oxidation to diketosuccinic acid, II, followed by benzilic acid-type rearrangement to generate crystalline products containing discrete octametallic molecules of composition ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 (MZMg, Fe, Co and Zn), the C4 O4K 7 component being the alkoxide–tricarboxylate species [OC(CO2)3]4K, a new oxyanion of carbon, III [1]. Since those results were published, Mn and Ni have been added to the list of divalent metal ions giving ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 [2]. The structure of
* Corresponding author. Tel.: C61 3 8344 6469; fax: C61 3 9347 5180. E-mail address: [email protected] (R. Robson).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.02.009
the octametallic molecule is shown in Fig. 1. It contains a cubane-like core consisting of four trigonal prismatic metal centres and four m3 -alkoxo bridges from separate [OC(CO2)3]4K ligands, each of which chelates to three metal K centres in the core. This central ½MII4 ðC4 O7 Þ4 8 unit is ideally arranged to chelate to four other appended metal centres, which have an octahedral coordination environment completed by three facially disposed aqua ligands. The hydroxy-tricarboxylic acid corresponding to [OC(CO2)3]4K, namely HOC(COOH)3 would be expected to be extremely prone to decarboxylation to give HOCH(COOH) 2, hydroxymalonic acid. The [OC(CO2)3]4K unit present in the above crystalline complexes presumably survives without loss of CO2 because, from the moment of its generation, it is stabilised by chelation to metal centres. We report here that, under somewhat more forcing conditions, different crystalline species separate that incorporate the product of decarboxylation, namely the alkoxide– dicarboxylate anion, [OCH(CO2)2]3K (IV) (represented C3 HO3K 5 below) derived from hydroxymalonic acid.
B.F. Abrahams et al. / Journal of Molecular Structure 796 (2006) 2–8
HOOC
HOOC
OH
O–
O CO2–
HO
O
COOH I
CO2–
CO2–
COOH II
III M
M O–
O H
O
O
H O
O
O
O IV
(0.24 mmol). Crystals of ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 ðH3 OÞ$8H2 O, contaminated with finer solid, were obtained upon heating the suspension at 40 8C for 2 days. 2.1.3. Zn3(C3HO5)2(H2O)6]$2H2O from di-sodium diketosuccinic acid To a solution of di-sodium diketosuccinic acid suspended in water (1.00 g, 47.2 mmol) was added a solution of Zn(NO3)2$6H2O (2.10 g, 1.5 mmol). The suspension was then heated for 2 days at 55 8C from which crystals of Zn3(C3HO5)2(H2O)6$2H2O were obtained.
O
O –
–
3
M V
2.1.4. Zn3(C3HO5)2(H2O)6$2H2O from dihydroxyfumaric acid To a solution of partially dissolved dihydroxyfumaric acid (300 mg, 2.03 mmol) in water (100 ml) and tetramethylammonium hydroxide (6.06 mmol) in methanol was added a solution of Zn(NO3)2$6H2O (402 mg, 1.35 mmol) in water (100 ml). The solution was allowed to stand at 40 8C. Initially, a small yield of [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O was obtained but upon standing for 3 weeks colourless crystals of [Zn3(C3HO5)2(H2O)6]$2H2O separated from the solution as the major product. 2.2. X-ray crystallography
Fig. 1. The structure of the ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 molecule. Circles in order of increasing size represent C, O and M, respectively.
2. Experimental 2.1. Syntheses 2.1.1. Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O from dihydroxyfumaric acid To a solution of partially dissolved dihydroxyfumaric acid (100 mg, 0.676 mmol) in water (50 ml) was added tetramethylammonium hydroxide (2.03 mmol) and a solution of Zn(NO3)2$6H2O (134 mg, 0.451 mmol) in water (50 ml). Colourless crystals, contaminated with finer solid, of [Zn 6(C 4O 7)(C 3HO 5) 3(H 2O) 6](H 3O)$8H 2O were obtained upon standing at 40 8C for 7 days. 2.1.2. ½M II6 ðC 4 O7 ÞðC3 HO5 Þ3 ðH 2 OÞ6 ðH 3 OÞ$8H 2 O (MZ Zn or Mn) from di-sodium diketosuccinic acid To a suspension of di-sodium diketosuccinic acid (50 mg, 0.24 mmol) in water (5 ml) was added a solution of MII(NO3)2$xH2O (0.36 mmol) followed by trimethylamine
Crystal data for [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O: ˚ , VZ MrZ1155.60, cubic, space group: P213, aZ16.807(8) A 3 ˚ ˚ 4747(4) A , TZ293 K, lZ1.54180 A , ZZ4, rcalc Z 1.617 g/cm3, mZ4.203 mmK1, F(000)Z2308, no. of measured (and independent) reflections: 1864 (1707), no. of parameters: 190, R1 [IO2s(I)]Z0.0819, wR2 (all data)Z0.2161, max/min ˚ K3. Intensity residual electron density: 0.65 and K0.65 e A data for [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O were collected using an Enraf-Nonius CAD-4 diffractometer fitted with Cu Ka radiation at 293 K. Structure refinements were performed using the SHELXL-97 program [3], which uses a full-matrix least-squares refinement based of F2. Numerical absorption corrections were applied [4]. Peaks of electron density identified in difference maps were consistent with disordered water molecules. These peaks were assigned as oxygen atoms and were refined with partial site occupation factors that were adjusted so as to produce reasonable thermal displacement parameters. At convergence the sum of the of the site occupancies suggested eight water molecules per formula unit. Crystal data for [(Mn)6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2˚, O: MrZ1093.02, cubic, space group: P213, aZ17.2584(10) A ˚ 3, TZ130 K, lZ0.71073 A ˚. VZ5140.5(5) A While a full structure determination was performed for the zinc compound, only a partial structure solution could be obtained in the case of the manganese compound. Despite this, the connectivity of the network is clearly indicated. Crystal data for Zn3(C3HO5)2(H2O)6$2H2O: MrZ574.31, ˚ , bZ5.4564(6) monoclinic, space group: P21/c, aZ9.8346(11) A ˚ , cZ14.9897(16) A ˚ , bZ105.400(2)8, VZ775.49(15) A ˚ 3, A ˚, TZ130 K, lZ0.71073 A ZZ2, rcalcZ2.46 g/cm3, K1 mZ4.705 mm , F(000)Z576, no. of measured (and
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independent) reflections: 4449 (1742), no. of parameters: 160, R1 [IO2s(I)]Z0.0268, wR2 (all data)Z0.0709, max/min residual ˚ K3. Intensity data for electron density: 0.98 and K0.49 e A Zn3(C3HO5)2(H2O)6$2H2O were collected with a Bruker CCD Area Detector diffractometer fitted with Mo Ka radiation and a graphite monochromator. Structure refinements were performed using the SHELXL-97 program [3], which uses a full-matrix leastsquares refinement based of F2. Absorption corrections were performed using the SADABS program [5]. 3. Results and discussion Basic aqueous reaction mixtures containing dihydroxyfumaric acid and zinc(II), held at 40 8C for approximately 3 days, yield colourless crystals that contain C3 HO3K 5 together with some C4 O4K 7 that still survives, the composition being [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O, but after approximately 3 weeks at this temperature the major solid product (especially when more concentrated solutions are used) contains no C4 O4K 7 and has the composition [Zn3(C3HO5)2 (H2O)6]$2H2O. The same [Zn6(C4O7)(C3HO5)3(H2O)6](H3 O)$8H2O product can be obtained from the di-sodium salt of diketosuccinic acid in basic solution with zinc nitrate at 40–50 8C. Crystals of composition [Mn6(C4O7)(C3HO5)3 (H2O)6](H3O)$8H2O can be similarly obtained from the reaction of the di-sodium salt of diketosuccinic acid in basic solution with manganese nitrate at 40–50 8C. However, whilst [Zn6(C4O7)(C3HO5)3(H2O)6](H3O)$8H2O can be obtained from either dihydroxyfumaric acid or diketosuccinic acid, we have been unable to obtain the Mn analogue from dihydroxyfumaric acid. Similar anomalous behaviour of manganese is seen also in the generation of the ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 series of compounds: although several members of this series can be obtained from either dihydroxyfumaric acid or diketosuccinic acid, we have succeeded in obtaining [Mn8(C4O7)4(H2O)12] only when diketosuccinic acid is the starting material, all our attempts starting with dihydroxyfumaric acid having failed. As represented in V, the C3 HO3K 5 unit is ideally composed so as to chelate three metal centres, and the single crystal X-ray diffraction studies described below reveal that it does indeed play this role, thus acting as a 3-connecting building block in the formation of extended coordination networks. The only reference to a structural study of a metal derivative of C3 HO3K 5 that we have found concerns a discrete, non-polymeric triCu(II) derivative [6]. 3.1. ½M II6 ðC 4 O7 ÞðC 3 HO5 Þ3 ðH 2 OÞ6 ðH 3 OÞ$8H 2 O (MZZn or Mn) The compounds ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 ðH3 OÞ$8H2 O (MZZn or Mn), are cubic with the space group P213 [aZ ˚ (Zn) and 17.2584(10) A ˚ (Mn)]. For all 16.807(8) A the structures reported in this paper, inter-atomic distances and angles, except those mentioned specifically below, are unexceptional and are not tabulated here, but are presented in 4K the Supplementary material. The ligands C3 HO3K 5 and C4 O7 act in concert to generate an anionic 3D network of
Fig. 2. View, slightly displaced from that along the threefold axis, of a cubane unit, in the structure of the ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 K network, showing the three hydroxymalonate trianionic ligands, C3 HO3K 5 , which each provide a m3 alkoxo centre for the cubane. Circles in order of increasing size represent C, O and M, respectively. The fourth oxygen centre of the cubane unit is provided by the single C4 O4K 7 ligand, all atoms of which except its alkoxo atom are omitted here for clarity.
composition ½MII6 ðC4 O7 ÞðC3 HO5 Þ3 ðH2 OÞ6 K, consisting of interlinked cubane-like units with the connectivity of the (10,3)-a net, a net which is intrinsically chiral [7]. The individual cubane-like units are related to but significantly different from those seen in Fig. 1. As shown in Fig. 2, three of the four oxygen centres within the cubane unit are provided by the alkoxo centres from three separate C3 HO3K 5 ligands, which are related around the threefold axis. The fourth oxygen atom in the cubane is provided by the C4 O4K 7 ligand, all atoms of which except for the alkoxo atom have been omitted from Fig. 2 for clarity. As can be seen in Fig. 2, each of the three C3 HO3K 5 units around the cubane nucleus acts as a bidentate chelating ligand for a metal centre, M(1), external to the cubane. The single C4 O4K 7 ligand per cubane, shown in Fig. 3, which is oriented with its alkoxide C–O located along the threefold axis, plays almost exactly the same role as the four II C4 O4K 7 ligands per cubane in ½M8 ðC4 O7 Þ4 ðH2 OÞ12 represented in Fig. 1. As can be seen in Fig. 3, the C4 O4K 7 ligand provides one alkoxo m3 bridge for the cubane unit and chelates to the 3 equiv. metals centres, M(2) in the figure. The three carboxylate oxygen centres of the C4 O4K 7 unit not directly coordinated to cubane metal centres are ideally arranged, as are those in ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 in Fig. 1, to act as a tridentate donor system to an appended metal centre, M(4) in Fig. 3. The octahedral coordination geometry of M(4) is completed by a facial arrangement of three aqua ligands. The fourth and unique metal centre in the cubane nucleus, M(3) in Fig. 3, is located on the threefold axis. It has an essentially octahedral coordination environment consisting of its three alkoxo oxygen neighbours within the cubane core together with a facial arrangement of three aqua ligands. The three M(2) centres belonging to the cubane nucleus that are related around the threefold axis, have a trigonal prismatic coordination geometry very similar to the
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5
Fig. 5. The (10,3)-a network. Circles in order of decreasing size represent the Tris-bidentate coordinated M(1), the centre of the cubane unit and the appended M(4), respectively. Broken lines represent the triple hydrogen bond interactions linking the aqua ligands on M(4) to carboxylato oxygen donors on M(1). Fig. 3. Representation of the cubane unit, the associated C4 O4K 7 ligand and the octahedral coordination environments of M(3) and M(4). Circles in order of increasing size represent C, O and M, respectively. The alkoxo C–O bond of the C4 O4K 7 unit and M(3) and M(4) are all located on the threefold axis.
trigonal prismatic coordination geometry seen for all four cubane metal centres in ½MII8 ðC4 O7 Þ4 ðH2 OÞ12 in Fig. 1. Each M(1) centre is linked to three cubane nuclei by the bidentate C3 HO3K 5 donors they provide, as shown in Fig. 4. Thus, each cubane is linked to three M(1) centres (as can be seen in Fig. 2) and acts as a 3-connecting node, while each M(1) centre in turn is linked to three cubanes (as can be seen in
Fig. 4. The Tris-bidentate chelated environment of M(1), which is linked by bridging C3 HO3K 5 ligands to three cubane units. A threefold axis passes through M(1).
Fig. 4) and acts as a second type of 3-connecting node. An infinite 3D 3-connected network is thereby generated which has the topology of the chiral (10,3)-a net, shown in Fig. 5. Also represented schematically in Fig. 5 are the C4O7M(4)(H2O)3 appendages to the cubane nodes, each of which is directed, across the channels of the (10,3)-a net, towards a particular M(1) node with which it forms a trio of hydrogen bonds, details of which can be seen in Fig. 6. The three aqua
Fig. 6. Details of the cross-channel triple hydrogen bond from the aqua ligands on M(4) to the carboxylato oxygen donors on M(1). Circles in order of increasing size represent C, O and M, respectively.
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ligands on M(4), which act as hydrogen bond donors, form an equilateral triangle almost identical in size to the equilateral triangle formed by the three carboxy oxygen donors around M(1),which act as hydrogen bond acceptors. The contents of the intra-framework spaces, w8H2O per formula unit, are disordered and ill-defined. We propose that the cation providing charge balance for the negatively charged framework is the hydronium ion, H3OC, not identifiable by X-ray diffraction techniques. This assignment is supported by neutron diffraction studies on an anionic yttrium oxalate network, ½YðC2 O4 ÞK 2 n , which showed unambiguously that the counter cation was the hydronium ion closely associated with water molecules by hydrogen bonding [8]. 3.2. Zn3(C3HO5)2(H2O)6]$2H2O Fig. 7. Two views of a [Zn(C3HO5)(H2O)2]K chain. Circles in order of increasing size represent C, O and Zn, respectively. Hydrogen bonds between aqua ligands and bridging alkoxo oxygen centres are indicated by open connections. The views in (a) and (b) are from different angles, both perpendicular to the b axis. The view in (b) highlights the sinusoidal character of the Zn/C3HO5 chain.
After approximately 3 weeks at 40 8C aqueous solutions containing dihydroxyfumaric acid and zinc acetate deposit colourless hexagonal plates of composition [Zn3(C3HO5)2 (H2O)6]$2H2O. The same product can be obtained from aqueous reaction mixtures containing the di-sodium salt of
Fig. 8. Two views of the [Zn3(C3HO5)2(H2O)6] sheet. Circles in order of increasing size represent C, O and Zn, respectively. (a) View perpendicular to the sheet showing the two sorts of hydrogen bonds within the sheet represented as open connections. (b) Edge-on view of the sheet perpendicular to the b axis. The aqua ligands have been omitted in (b) to highlight the pronounced corrugation within the Zn/C3HO5 component. The smallest open circles shown in (b) represent the C–H hydrogen atoms of C3 HO3K 5 .
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diketosuccinic acid and zinc nitrate after 2 days at 55 8C. The structure of [Zn3(C3HO5)2(H2O)6]$2H2O was studied by single crystal X-ray diffraction. The compound crystallises in the monoclinic space group P21/c with unit cell dimensions of aZ ˚ , bZ5.4564(6) A ˚ , cZ15.9897(16) A ˚ and bZ 9.8346(11) A 105.400(2)8. The hydroxymalonate trianion, C3 HO3K 5 , again plays the triply chelating role seen in V, thus acting as a 3-connecting node and giving rise, in this case, to a 2D coordination polymer with the (6,3) topology. The sheet structure is conveniently envisaged in terms of anionic linear 1D coordination polymers of composition [Zn(C3HO5)(H2O)2]K linked together by coordination to a second type of diaqua Zn2C cation in the way described below. Fig. 7 shows the structure of the [Zn(C3HO5)(H2O)2]K component of the sheet. All zinc centres, Zn(1), in the chain are equivalent, having octahedral coordination geometry consisting of two bidentate chelated C3 HO3K 5 ligands and two cis aqua ligands. The alkoxo oxygen centre of C3 HO3K 5 acts as a bridge between two Zn(1) centres. The backbone of the Zn/(C3HO5) polymeric chain adopts a pronounced sinusoidal arrangement, as can be seen from the angle of view represented in Fig. 7(b). One of the hydrogen atoms on one of the aqua ligands is directed towards the alkoxo oxygen centre of an adjacent C3 HO3K 5 ligand (O/O ˚ ) generating a significant hydrogen separation 2.651(2) A bonding interaction that supports the sinusoidal arrangement, as can be seen in Fig. 7. The [Zn(C3HO5)(H2O)2]K chains are linked together by diaqua zinc centres of a different type, Zn(2), to generate 2D sheets of overall composition [Zn(C3HO5)(H2O)2]2[Zn(H2O)2] or [Zn3(C3HO5)2(H2O)6] as shown in Fig. 8. Zn(2) has an octahedral coordination geometry with trans aqua ligands, the metal being located on a centre of symmetry. Pronounced
7
corrugation is evident within the sheets as can be seen in the edge-on view perpendicular to the b axis in Fig. 8(b). Additional reinforcement within the sheet is provided by hydrogen bonds, which can be seen in Fig. 8(a), from an aqua ligand on Zn(2) to a carboxylato oxygen atom coordinated to ˚ ]. an adjacent Zn(2) [O/O, 2.800(4) A Sheets are stacked in the c direction as shown in Fig. 9. Aqua ligands on Zn(1) centres in one sheet are hydrogen bonded to aqua ligands on Zn(1) centres in the neighbouring ˚ ). The aqua ligands on Zn(1) centres in sheet (O/O, 2.919(5) A one sheet are also hydrogen bonded to aqua ligands on Zn(2) in ˚ ). Lattice water molecules the adjacent sheet (O/O, 2.919(5) A are located between sheets, as shown in Fig. 9; these provide additional sheet-to-sheet binding by forming hydrogen bonds to carboxylato oxygen centres in sheets above and below ˚ and 2.916(5) A ˚ ). The hydrogen atoms on all (O/O, 2.887(5) A the water molecules in the structure (i.e. the two inequivalent cis aqua ligands on Zn(1), the 2 equiv. trans aqua ligands on Zn(2) and the lattice water molecules) are well defined in the structure solution and all of the OH groups act as hydrogen bond donors in significant interactions with other oxygen centres. The combination of this efficient hydrogen bonding with efficient coordinate bonding gives rise to a very tightly knit 3D structure. 3.3. Generation of zinc oxalate When an aqueous solution containing the di-sodium salt of diketosuccinic acid and zinc nitrate is heated in a sealed tube at 130 8C for approximately 18 h large crystals of the known oxalate, Zn(C2O4)(H2O)2 are formed [9].
Fig. 9. Representation of the stacking of the sheets in [Zn3(C3HO5)2(H2O)6]$2H2O. Circles in order of increasing size represent C, O and Zn, respectively. The sheets are bonded together by multiple hydrogen bonds represented here by open connections.
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4. Concluding remarks
Supplementary data
The work reported here extends our knowledge of the remarkable sequence of metal ion-promoted transformations exhibited by dihydroxyfumaric acid. Initial air oxidation is followed by a benzilic acid-type rearrangement to give the new oxyanion of carbon, C4 O4K 7 , as a crucial component of a highly symmetrical octametallic complex with a cubane-like core. Under slightly more forcing conditions the C4 O4K 7 species undergoes partial decarboxylation and the C3 HO3K 5 ion so formed acts in concert with still intact, remnant C4 O4K 7 to generate crystalline coordination polymers in which cubane-like units are linked together with the archetypal 10,3-a topology. Under somewhat more forcing conditions, at least in the case of the Zn2C-promoted reaction, a crystalline coordination polymer 4K of C3 HO3K 5 , containing no C4 O7 , is formed, which contains strongly undulating sheets with multiple intra-sheet and intersheet hydrogen bonds. Under yet more forcing conditions (130 8C, sealed tube) similar reaction mixtures generate zinc oxalate.
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2006.02.009. CCDC numbers 292643 and 292644 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK. Fax: C44 1223 336033).
Acknowledgements The authors gratefully acknowledge support from the Australian Research Council.
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