Synchrotron X-ray charge density study of coordination polymer Co3(C8H4O4)4(C4H12N)2(C5H11NO)3 at 16 K

The charge density (CD) of coordination polymer Co3(C8H4O4)4(C4H12N)2(C5H11NO)3 (1) has been determined from multipole modeling of structure factors obtained from single-crystal synchrotron X-ray diffraction measurements at 16 K. The crystal structure formally contains a negatively charged framework with cations and neutral molecules in the voids. However, the CD suggests that the framework is close to neutral, and therefore qualitative conclusions based on formal charge counting, e.g., about guest inclusion properties, will be incorrect. There are considerable differences in the charge distributions of the three unique benzenedicarboxylic acid linkers, which are widely used in coordination polymers. This suggests that the electrostatic properties of coordination polymer cavities, and thereby their inclusion properties, are highly tunable. The electron density topology shows that the tetrahedrally coordinated Co atom has an atomic volume which is 15% larger than that of the octahedrally coordinated Co atom. The crystal structure has both ferromagnetic and antiferromagnetic interactions, but no direct metal-metal bonding is evidenced in the CD. The magnetic ordering therefore takes place through superexchange in the oxygen bridges and the aromatic linkers. Bonding analysis of the experimental CD reveals that two oxygen atoms, O(1) and O(11), have significant covalent contributions to the metal-ligand bonding, whereas all other oxygen atoms have closed-shell interactions with the metals. This indicates that these two oxygen atoms are the key mediators of the magnetic ordering.     

A stable silicon(II) monohydride

A stable silicon(II) monohydride is accomplished through a covalent shared interaction of the silylene lone-pair and a sp(3)-hybridized boron atom of the Lewis acidic BH(3). Experimental charge density investigations reveal a central positively charged silicon atom bound to a negatively charged hydrogen atom. The positively charged H-Si-BH(3) moiety is coordinated by the lone-pairs of electrons of the benzamidinate ligand. This coordination is reinforced by a transannular Si1···C1 privileged exchange channel.     

Divalent metal phosphonate coordination polymers constructed from a dipiperidine-based bisphosphonate ligand

The ligand 4,4'-dipiperidine-N,N'-bis(methylenephosphonic acid), H(4)L, has been reacted with divalent metal salts under solvothermal conditions to yield seven new metal phosphonate coordination polymers. The compounds have been characterized by elemental analyses and their structures determined by single-crystal X-ray diffraction. Zn(2)(L)(H(2)O)(2) and Co(2)(L)(H(2)O)(2) have (different) layered structures, while Mn(2)(L)(H(2)O)(3) has a chain motif. In these compounds, the N atoms of the ligand bind to the metal ions. α-Co(2)Cl(2)(H(2)L), formed from CoCl(2)·6H(2)O and H(4)L in ethanol, is also layered but the N atoms of the ligand are protonated. The Co atoms are tetrahedral, coordinated by three phosphonate oxygen atoms and a chloride ion. A polymorph of this compound, β-Co(2)Cl(2)(H(2)L), was obtained from a mixed ionic liquid under microwave irradiation. The primary difference between the polymorphs is the orientation of the phosphonate group relative to the dipiperidine. When reacted hydrothermally with Sn(II)C(2)O(4), H(4)L partially decomposes, producing phosphate ions which are incorporated into the structure of Sn(6)O(2)(H(2)L)(PO(4))(2)·4H(2)O. In this compound, the N atoms of the ligand are protonated, and two oxide anions are incorporated for charge balance. A second phase is obtained from the same reaction, which was determined to be Sn(7)O(L)(3). This compound has a layered structure which contains relatively large voids within the inorganic portion of the layer. These structures are discussed, as well as factors influencing the state of protonation in the final compounds. The choice of solvent and temperature were found to have a significant influence on the type of structure obtained.     

The influence of coordination geometry and valency on the K-edge absorption near edge spectra of selected chromium compounds

X-ray absorption spectra at the chromium K-edge are reported for a number of selected chromium compounds of known chemical structure. The spectra were obtained with use of synchrotron radiation available at the ectron tretcher ccelerator ELSA in Bonn. The compounds studied include the tetrahedrally coordinated compounds Ca₂Ge_0.8Cr_0.2O₄, Ba₂Ge_0.1Cr_0.9O₄, Sr₂CrO₄, Ca₂(PO₄)_x(CrO₄)_1−xCl (x=0.25,0.5), Ca₅(CrO₄)₃Cl, CrO₃, the octahedrally coordinated compounds Cr(II)-acetate, CrCl₃, CrF₃, Cr₂O₃, KCr(SO₄)₂ · 12H₂O, CrO₂ and cubic coordinated metallic chromium. In these compounds chromium exhibits a wide range of formal oxidation states (0 to VI). The absorption features in the near edge region are shown to be characteristic of the spatial environment of the absorbing atom. The occurrence of a single pre-edge line easily allows one to distinguish between tetrahedral and octahedral coordination geometry, whereas the energy position of the absorption edge is found to be very sensitive to the valency of the excited chromium atom. Calculations of the ionisation potential of Cr in different oxidation states using the non-relativistic Hartree–Fock method (Froese–Fischer) confirm that the ionisation limit shifts to higher energy with increasing Cr valency. More detailed information on the electronic structure of the different compounds is gained by real-space full multiple scattering calculations using the FEFF8 code.     

The coordination of uranyl in water: a combined quantum chemical and molecular simulation study

The coordination environment of uranyl in water has been studied using a combined quantum mechanical and molecular dynamics approach. Multiconfigurational wave function calculations have been performed to generate pair potentials between uranyl and water. The quantum chemically determined energies have been used to fit parameters in a polarizable force field with an added charge transfer term. Molecular dynamics simulations have been performed for the uranyl ion and up to 400 water molecules. The results show a uranyl ion with five water molecules coordinated in the equatorial plane. The U-O(H(2)O) distance is 2.40 A, which is close to the experimental estimates. A second coordination shell starts at about 4.7 A from the uranium atom. No hydrogen bonding is found between the uranyl oxygens and water. Exchange of waters between the first and second solvation shell is found to occur through a path intermediate between association and interchange. This is the first fully ab initio determination of the solvation of the uranyl ion in water.     

Basicity of uranyl oxo ligands upon coordination of alkoxides

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide ligands. Imposition of steric congestion at the metal center prevents oxo-alkoxide ligand exchange in the reactions using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3. Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions, and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group. These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4, single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of aggregation of compounds 2-4 in various solvents. Vibrational spectroscopic measurements of 2-4 imply that, in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl U=O bonds are weakened. Crystal data are as follows. 1: monoclinic space group C2/m, a = 10.6192(8) A, b = 18.36(1) A, c = 10.6151(8) A, beta = 109.637(1) degrees, V = 1949.1(3) A3, Z = 2, dcalc = 1.297 g cm-3. Refinement of 2065 reflections gave R1 = 0.045. 2: monoclinic space group P2(1)/c, a = 6.1796(4) A, b = 15.669(1) A, c = 16.169(1) A, beta = 95.380(1) degrees, V = 1558.7(2) A3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections gave R1 = 0.036. 4: tetragonal space group I4, a = 17.8570(6) A, b = 17.8570(6) A, c = 11.4489(6) A, V = 3650.7(3) A3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.     

Assembly of coordination nanostructures via ligand derivatization of oxide surfaces

A scheme is presented for the construction of coordination nanostructures on oxide surfaces (glass, Si/SiO2, quartz), based on application of epoxy-terminated monolayers as anchors for covalent grafting of ligands. Two ligands bearing amine groups were reacted with epoxysilane monolayers on oxide surfaces, providing ligand-terminated substrates. The ligands employed were (i) a pyridine moiety, used for subsequent binding of cobalt tetraphenylporphine (CoTPP), and (ii) deferoxamine (DFX), which contains hydroxamic acid moieties, used for subsequent construction of various Zr4+-based coordination layers. The results suggest that a dense ligand layer was obtained in both cases, allowing the formation of coordination overlayers on the oxide surfaces. The growth of coordinated layers was similar to analogous overlayers assembled on Au substrates, indicating that high ligand coverage is achieved by the epoxy-amine surface reaction. Epoxy-based functionalization of oxide substrates is a mild and efficient method for preparing high-quality coordination overlayers. Moreover, the method makes use of commercially available silane and amine reactants, providing the basis for wide application.     

Reversible photoswitching of the coordination numbers of silicon in organosilicon compounds bearing a 2-(phenylazo)phenyl group

Several organosilicon compounds bearing a 2-(phenylazo)phenyl group were synthesized from the corresponding chlorosilanes and 2-lithioazobenzene prepared by halogen-lithium transmetalation of 2-iodoazobenzene. Their structures were determined by (1)H, (13)C, (19)F, and (29)Si NMR spectra, UV-vis spectra, and X-ray crystallographic analyses. In the UV-vis spectra, silyl groups caused red shifts of both the n-pi and pi-pi transitions of the azo group compared with the transitions of the unsubstituted azobenzene. The E-isomers of the fluorosilanes showed an intramolecular interaction between a nitrogen atom of the azo group and the silicon atom, leading their intermediate structures between a distorted trigonal bipyramidal structure and a tetrahedral structure around the silicon atoms, which were revealed by the X-ray crystallographic analyses and the NMR spectra. On the other hand, silanes without fluorine atoms showed tetrahedral structures in the absence of such an interaction. The photoirradiation of the E-isomers of the fluorosilanes afforded reversibly the corresponding Z-isomers in good yields. The silicon atoms of the Z-isomers were found to be tetracoordinate in the absence of Si-N interactions by the (29)Si NMR spectra. The coordination numbers of the silicon atom of the fluorosilanes were reversibly switched between four and five by photoirradiation. These properties were compared to those of a tetrafluoro[2-(phenylazo)phenyl]silicate.     

Isomers in the chemistry of iron coordination compounds

The coordination chemistry of iron covers a wide field, as shown by a survey covering the crystallographic and structural data of almost one thousand and three hundred coordination complexes. About 6.7% of these complexes exist as isomers and are summarized in this review. Included are distortion (96.6%) and cis — trans (3.4%) isomers. These are discussed in terms of the coordination about the iron atom, bond length and interbond angles. Distortion isomers, differing only by degree of distortion in Fe-L, Fe-L-Fe and L-Fe-L parameters, are the most common. Iron is found in the oxidation states zero, +2 and +3 of which +3 is most common. The stereochemistry around iron centers are tetrahedral, five — coordinated (mostly trigonal — bipyramid) and six — coordinated. The most common ligands have O and N donor sites.     

Salts of the Lewis-acidic dianion [Hg(closo-1-CB11F11)2]2-: coordination of acetonitrile and water

The coordination of acetonitrile and water to the Hg atom in [Hg(closo-1-CB(11)F(11))(2)](2-) (1) reveals the Lewis acidity of the Hg(II) center, which is unprecedented, since 1 is a dianion. Both coordination compounds were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, and differential scanning calorimetry (DSC). In contrast, the Hg atom in [PhHg(closo-1-CB(11)F(11))](-) (2) does not coordinate to CH(3)CN and H(2)O, although it has only a single negative charge.     

Syntheses and structures of alkaline earth metal bis(diphenylamides)

Various preparative procedures are employed in order to synthesize alkaline earth metal bis(diphenylamides) such as (i) metalation of HNPh2 with the alkaline earth metal M, (ii) metalation of HNPh2 with MPh2, (iii) metathesis reaction of MI2 with KNPh2, (iv) metalation of HNPh2 with PhMI in THF, and (v) metathesis reaction of PhMI with KNPh2 followed by a dismutation reaction yielding MPh2 and M(NPh2)2. The magnesium compounds [(diox)MgPh2]infinity (1) and (thf)2Mg(NPh2)2 (2) show tetracoordinate metal atoms, whereas in (dme)2Ca(NPh2)2 (3), (thf)4Sr(NPh2)2 (4), and (thf)4Ba(NPh2)2 (5) the metals are 6-fold coordinated. Additional agostic interactions between an ipso-carbon of one of the phenyl groups of the amide ligand and the alkaline earth metal atom lead to unsymmetric coordination of the NPh2 anions with two strongly different M-N-C angles in 3-5.     

Si-E (E = N, O, F) bonding in a hexacoordinated silicon complex: new facts from experimental and theoretical charge density studies

The concept of hypervalency in molecules, which hold more than eight valence electrons at the central atom, still is a topic of constant debate. There is general interest in silicon compounds with more than four substituents at the central silicon atom. The dispute, whether this silicon is hypervalent or highly coordinated, is enlightened by the first experimental charge density determination and subsequent topological analysis of three different highly polar Si-E (E = N, O, F) bonds in a hexacoordinated compound. The experiment reveals predominantly ionic bonding and much less covalent contribution than commonly anticipated. For comparison gas-phase ab initio calculations were performed on this compound. The results of the theoretical calculations underline the findings of the experiment.     

Cyclophosphazenes as nodal ligands in coordination polymers

Herein, we show that cyclotriphosphazenes carrying organo amino side chains, (RNH)6P3N3 [R = n-propyl (1), cyclohexyl (2), benzyl (3)], and (C4H8N)6P3N3 (4) produce supramolecular coordination compounds in conjunction with silver salts by formation of linear N-Ag-N connections via nitrogen centers of the phosphazene ring. Crystalline materials were obtained by layering methanol solutions containing phosphazene ligands with methanol solutions of AgClO4 and AgNO3. The donor ability of the anion and the steric demand of the lipophilic ligand sphere R control the topology of the coordination network: (1)2(AgClO4)3 forms a graphite-type (6,3) network. All three N(ring) atoms of the phosphazene ligand coordinate to silver ions, which, in return, linearly bridge two phosphazene ligands. The phosphazene-Ag(I) arrangement in 1(AgNO3)2 exists of zigzag chains featuring one bridging silver ion and one terminally coordinated silver ion per ligand molecule. The terminally located Ag(I) ions of neighboring chains are bridged by nitrate ions, resulting in a 2D network. Both 2(AgClO4) and 4(AgClO4) contain only one bridging silver ion per phosphazene ligand, which leaves one N(ring) site vacant and gives 1D zigzag chain arrangements. The crystal structures of 3(AgClO4)2 and 3(AgNO3)2 resemble that of 1(AgNO3)2, but show additional Ag-pi(aryl) interactions between the terminally arranged silver ions and benzyl groups. Treatment of 3 with a methanol solution containing both AgNO3 and AgClO4 leads to the heteroanion derivative 3(AgNO3)(AgClO4). Phosphazene ligands 1-3 have the ability to undergo hydrogen bonding to anions via the six NH groups, and the coordination polymers containing these ligands feature dense networks of NH...O bonds.     

DFT study of electronic structure of saccharin,thiosaccharin, and their respective ions: Effects of metal coordination on thiosaccharinate electronic structure

A rationalization of the conspicuous different abilities of saccharin and thiosaccharin to form coordination compounds with the first series of transition metal elements and to interact with heavy metal cations is sought. Their electronic molecular structures as well as those of their respective ions are compared performing natural bond orbitals (NBO) analyses of the four species. Upon deprotonation, the negative charge at the N atom in saccharin is almost constant while it decreases in thiosaccharin and the negative charge at the monocoordinated chalcogen atom is notably more increased in the latter than in the former. Apparently, the negative charge reorganization makes difficult the coordination of thiosaccharin with the first series of transition metal elements and favors its experimentally observed interaction with heavy metal cations such as cadmium, lead, mercury, silver, and thallium. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

A novel two‐dimensional zinc(II) coordination polymer with 6‐mercaptonicotinic acid

The novel title Zn^II coordination polymer, poly[bis(μ‐6‐thioxo‐1,6‐dihydropyridine‐3‐carboxylato‐κ²S:O)zinc(II)], [Zn(C₆H₄NO₂S)₂]_n, consists of two crystallographically independent zinc centers and two 6‐mercaptonicotinate (Hmna⁻) ligands. Each Zn^II atom is four‐coordinated and lies at the center of a distorted tetrahedral ZnS₂O₂ coordination polyhedron, bridged by four Hmna⁻ ligands to form a two‐dimensional (4,4)‐network. Each Hmna⁻ ion acts as a bridging bidentate ligand, coordinating to two Zn^II atoms through the S atom and a carboxyl O atom. The metal centers reside on twofold rotation axes. The coordination mode of the S atoms and N—H...O hydrogen‐bonding interactions between the protonated N atoms and the uncoordinated carboxyl O atoms give the extended structure a wavelike form.     

Hydrogen-bonded networks through second-sphere coordination

The reaction of 4, 7-phenanthroline (1) with aqueous transitionmetal complexes [Mn(H2O)6][NO3]2, [Co(H2O)6][NO3]2, [Ni(H2O)6[NO3]2, [Mn(H2O)6][ClO4]2, and [Co(H2O)6][ClO4]2 does not produce coordination complexes between these metal cations and the N-donor ligand as expected. Instead, supramolecular hydrogenbonded networks are formed between the nitrogen donor atoms of 4, 7-phenanthroline and the OH groups of coordinated water molecules: M-O-H...N interactions. This motif of second-sphere coordination for 1 can be exploited as a tool for crystal engineering. As a demonstration of the generality of this new interaction as a supramolecular building block, five X-ray crystal structures are reported that utilise this hydrogen bonding scheme; [Co(H2O)4(NO3)2].(1)2 (2a), [Co(MeCN)2(H2O)4][ClO4]2.(1)2 (2b), [Ni(H2O)4(NO3)2].(1)2 (3a), [Mn(H2O)4(NO3)2].(1)2 (4a), and [Mn(H2O)6][ClO4]2.(1)(4).4H2O (4b). Each network involves complete saturation of the hydrogen-bond donor sets between the aqua complex and 1 using primarily M-O-H...N(1) and M-O-H...O(anion), interactions. Thermogravimteric analysis shows these materials to have stabililities similar to coordination polymers involving metal-ligand bonds; this demonstrates that second-sphere hydrogen bonding has potential for the construction of polymeric metal-containing materials.     

Preparation of stable and metastable coordination compounds: insight into the structural, thermodynamic, and kinetic aspects of the formation of coordination polymers

The reaction of ZnI2 and pyrimidine in acetonitrile results in the formation of the 1:2 compound ZnI2(pyrimidine)2 (1), which consists of discrete tetrahedral building blocks. Slow heating of 1 at 1 degrees C/min leads to its transformation into the ligand-deficient intermediate 1:1 compound ZnI2(pyrimidine) (3), which upon further heating decomposes into the most ligand-deficient 2:1 compound (ZnI2)2(pyrimidine) (4). In contrast, the 2:3 compound (ZnI2)2(pyrimidine)3 (2) is formed as an intermediate by decomposing 1 using a faster heating rate of 8 degrees C/min. Compound 2 consists of oligomeric units in which each ZnI2 unit is coordinated by two iodine atoms and one bridging and one terminal pyrimidine ligand. The crystal structure of compound 3 is built up of ZnI2 units, which are connected by the ligands into chains. For the thermal transformation of 1 into 3 via 2 as the intermediate, a smooth reaction pathway is found in the crystal structure, for which only small translational and rotational changes are needed. The metastable solvated compound (ZnI2)(pyrimidine)(acetonitrile)0.25 (5) consisting of (ZnI2)4(pyrimidine)4 rings is obtained by quenching the reaction of ZnI2 and pyrimidine in acetonitrile using an antisolvent. On heating, 5 decomposes into a new polymorphic 1:1 compound 6, which consists of (ZnI2)(pyrimidine) chains. On further heating, 6 transforms into a third polymorphic 1:1 compound 7, which consists of (ZnI2)3(pyrimidine)3 rings, and finally into the 1:1 compound 3. Solvent-mediated conversion experiments reveal that compounds 1-4 are thermodynamically stable, whereas compounds 5-7 are metastable. Time-dependent crystallization experiments unambiguously show that compound 7 is formed by kinetic control and transforms within minutes into compound 6, which finally transforms into 3. Compound 3 represents the thermodynamically most stable 1:1 modification, whereas compounds 6 and 7 are metastable. The different compounds obtained by thermal decomposition and by crystallization from solution represent a snapshot of the species in solution and thus provide insight into the formation of coordination compounds.     

Building a bridge between coordination compounds and clusters: bonding analysis of the icosahedral molecules [M(ER)12] (M = Cr, Mo, W; E = Zn, Cd, Hg)

The bonding situation of the icosahedral compounds [M(EH)(12)] (M = Cr, Mo, W; E = Zn, Cd, Hg), which are model systems for the isolated species [Mo(ZnCp*)(3)(ZnMe)(9)] possessing the coordination number 12 at the central atom M, have been analyzed with a variety of charge and energy decomposition methods (AIM, EDA-NOCV, WBI, MO). The results give a coherent picture of the electronic structure and the nature of the interatomic interactions. The compounds [M(EH)(12)] are transition metal complexes that possess 12 M-EH radial bond paths (AIM) that can be described as 6 three-center two-electron bonds (MO). The radial M-EH bonds come from the electron sharing interactions mainly between the singly occupied valence s and d AOs of the central atom M and the singly occupied EH valence orbitals (MO, EDA-NOCV). The orbital interactions provide ~42% of the total attraction, while the electrostatic attraction contributes ~58% to the metal-ligand bonding (EDA-NOCV). There is a weak peripheral E-E bonding in [M(EH)(12)] that explains the unusually high coordination number (MO). The peripheral bonding leads for some compounds [M(EH)(12)] to the emergence of E-E bond paths, while in others it does not (AIM). The relative strength of the radial and peripheral bonding in [Al(13)](-) and [Pt@Pb(12)](2-) is clearly different from the situation in [M(EH)(12)], which supports the assignments of the former species as cluster compounds or inclusion compounds (MO, WBI). The bonding situation in [WAu(12)] is similar to that in [M(EH)(12)].     

CO coordination at XNi4 clusters with impurities X = H, C, O. A density functional study

We report a computational investigation of CO adsorption on small nickel clusters that contain single impurity atoms H, C, or O. At bare Ni 4 and clusters with H or O impurity, the most stable coordination of the probe molecule is on top of a Ni atom which interacts with the impurity. The CNi 4 cluster is an exception where 3-fold coordination of CO was determined to be more stable than that on top, however, by 4 kJ/mol only. Our results suggest that the heteroatoms X (X = H, C, O) affect only weakly the reactivity of the cluster with respect to CO; the binding energy of CO in the most stable complexes (CO)XNi 4 increases at most by 10% compared to the value for bare Ni 4, 194 kJ/mol. The impurity induces a small decrease of the CO infrared frequency shift for on-top coordinated CO, compared to Ni 4, because of partial oxidation of the metal moiety. A notable difference is predicted for clusters that contain a C impurity because of the different preferred coordination mode which results in a strong CO frequency red shift of approximately 300 cm (-1). The calculated characteristic CO frequency shifts may be helpful in identifying experimentally clusters with impurity atoms.     

Hybrid organic-inorganic framework structures: influence of cation size on metal-oxygen-metal connectivity in the alkaline earth thiazolothiazoledicarboxylates

We report the synthesis of four organic-inorganic frameworks of alkaline earth cations with the organic ligand 2,5-thiazolo[5,4-d]thiazoledicarboxylate (C6N2S2O4(2-), Thz(2-)). Structures with remarkably different connectivities result when Mg(2+), Ca(2+), Sr(2+), and Ba(2+) react with Thz(2-). Mg(Thz)(H2O)4 (I) forms a 1-D coordination polymer in which one carboxylate oxygen on each terminus of the ligand connects individual MgO6 octahedra from their axial positions, while the remaining equatorial sites are coordinated by water molecules. Ca2(Thz)2(H2O)8 (II) forms a 1-D coordination polymer in which dimeric clusters with 7-fold Ca coordination are connected via the ligand in a linear fashion, with a second, uncoordinated Thz(2-) providing charge balance. Sr(Thz)(H2O)3 (III) has 1-D infinite inorganic connectivity built from edge-sharing SrO7N polyhedra having one carboxylate oxygen and one water molecule acting as M-O-M bridges. Ba2(Thz)2(H2O)7 (IV) has 2-D inorganic connectivity based upon face- and edge-sharing BaO9N polyhedra. One carboxylate oxygen and all water molecules act as bridges between each Ba(2+) and its three neighbors. We shall discuss the manner in which the increasing coordination requirements of the cations (MgO6 < CaO7 < SrO7N < BaO9N) lead to an increase in inorganic connectivity through the series.