Fixation of CO(2) by Zinc(II) Chelates in Alcoholic Medium. X-ray Structures of {[Zn(cyclen)](3)(&mgr;(3)-CO(3))}(ClO(4))(4) and [Zn(cyclen)EtOH](ClO(4))(2)

Nickel(II) Complexes of Tripodal Ligands as Catalysts for Fixation of Atmospheric CO2 as Organic Carbonates

The fixation of atmospheric CO₂ into value-added products is a promising methodology. A series of novel nickel(II) complexes of the type [Ni(L)(CH₃ CN)₂ ](BPh₄ )₂ 1-5, where L=N,N-bis(2-pyridylmethyl)-N', N'-dimethylpropane-1,3-diamine (L1), N,N-dimethyl-N'-(2-(pyridin-2-yl)ethyl)-N'-(pyridin-2-ylmethyl) propane-1,3-diamine (L2), N,N-bis((4-methoxy-3,5-dimethylpyridin-2-ylmethyl)-N',N'-dimethylpropane-1,3-diamine (L3), N-(2-(dimethylamino) benzyl)-N',N'-dimethyl-N-(pyridin-2-ylmethyl) propane-1,3-diamine (L4) and N,N-bis(2-(dimethylamino)benzyl)-N', N'-dimethylpropane-1,3-diamine (L5) have been synthesized and characterized as the catalysts for the conversion of atmospheric CO₂ into organic cyclic carbonates. The single-crystal X-ray structure of 2 was determined and exhibited distorted octahedral coordination geometry with cis-α configuration. The complexes have been used as a catalyst for converting CO₂ and epoxides into five-membered cyclic carbonates under 1 atmospheric (atm) pressure at room temperature in the presence of Bu₄ NBr. The catalyst containing electron-releasing -Me and -OMe groups afforded the maximum yield of cyclic carbonates, 34% (TON, 680) under 1 atm air. It was drastically enhanced to 89% (TON, 1780) under pure CO₂ gas at 1 atm. It is the highest catalytic efficiency known for CO₂ fixation using nickel-based catalysts at room temperature and 1 atm pressure. The electronic and steric factors of the ligands strongly influence the catalytic efficiency. Furthermore, all the catalysts can convert a wide range of epoxides (ten examples) into corresponding cyclic carbonate with excellent selectivity (>99%) under this mild condition.

Utilization of Low-Concentration CO2 with Molecular Catalysts Assisted by CO2-Capturing Ability of Catalysts, Additives, or Reaction Media

Increasing concentration of atmospheric CO₂ is a worldwide concern and continues to trigger various environmental problems. Photo- or electrocatalytic CO₂ reduction (CO₂-Red) using solar energy, i.e., artificial photosynthesis, is a prospective technique owing to its sustainability and the usefulness of the reaction products. Concentrations of CO₂ in exhaust gases from industries are several % to 20%, and that in the atmosphere is about 400 ppm. Although condensation processes of CO₂ require high energy consumption and cost, pure CO₂ has been used in most of the reported studies for photo- and electrocatalytic CO₂-Red because the reaction between CO₂ and the catalyst could be one of the rate-limiting steps. To address these issues and provide a repository of potential techniques for other researchers, this perspective summarizes the catalytic systems reported for the reduction of low-concentration CO₂, which utilize a combination of catalytic CO₂-Red and CO₂-capturing reactions (or CO₂ adsorption). First, we describe CO₂ insertions into M-X bonds of the catalysts, which increase the rate constants and/or equilibrium constants for CO₂ binding on the catalysts, and modifications of the second coordination sphere to stabilize the CO₂-bound catalysts. Furthermore, we discuss the reaction media used for catalytic CO₂-Red that have the unique effect of increasing CO₂ concentrations around the catalysts. These reaction media include typical CO₂-capturing additives, ionic liquids, and metal-organic frameworks.

Aqua(1,4,7,10-tetraazacyclododecane)zinc(II) bis(perchlorate)

The cationic Zn^II part of aqua­(1,4,7,10-tetra­aza­cyclo­dodeca­ne)zinc(II) diperchlorate, [Zn(C₈H₂₀N₄)H₂O](ClO₄)₂, exhibits a slightly distorted square-pyramidal coordination environment with a water mol­ecule in the apical position.

The cationic Zn^II part of aqua­(1,4,7,10-tetra­aza­cyclo­dodeca­ne)zinc(II) bis­(perchlorate), [Zn(C₈H₂₀N₄)(H₂O)](ClO₄)₂, exhibits a slightly distorted square-pyramidal coordination environment with a water mol­ecule in the apical position. In the crystal, the macrocyclic ring alternates between two conformations with equal occupancies. Two of the three perchlorate anions are situated about a twofold rotation axis, and one of them shows disorder of the O atoms with occupancies of 0.62 (7) and 0.38 (7). In the crystal, the complexes are connected by inter­molecular hydrogen bonding via the perchlorate anions.

Anion tuning of Zn2+ architectures using a Tris-base salicylic ligand

In this study, a hydroxyl-rich Schiff base ligand, H₄L, and its resulting complexes with ZnCl₂, Zn(CH₃COO)₂ and Zn(ClO₄)₂ were explored.

Ditopic receptors containing urea groups for solvent extraction of Cu(ii) salts

The ditopic receptor L3 [1-(2-((7-(4-(tert-butyl)benzyl)-1,4,7,10-tetraazacyclododecan-1-yl)methyl)phenyl)-3-(3-nitrophenyl)urea] containing a macrocyclic cyclen unit for Cu(ii)-coordination and a urea moiety for anion binding was designed for recognition of metal salts. The X-ray structure of [CuL3(SO₄)] shows that the sulfate anion is involved in cooperative binding via coordination to the metal ion and hydrogen-bonding to the urea unit. This behaviour is similar to that observed for the related receptor L1 [1-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenyl)-3-(3-nitrophenyl)urea], which forms a dimeric [CuL1(μ-SO₄)]₂ structure in the solid state. In contrast, the single crystal X-ray structure of [ZnL3(NO₃)₂] contains a 1 : 2 complex (metal : anion) where one anion coordinates to the metal and the other is hydrogen-bonded to the urea group. Spectrophotometric titrations performed for the [CuL3(OSMe₂)]²⁺ complex indicate that this system is able to bind a wide range of anions with an affinity sequence: MeCO₂^- > Cl^- > H₂PO₄^- > Br^- > NO₂^- > HSO₄^- > NO₃^-. Lipophilic analogues of L1 and L3 extract CuSO₄ and CuCl₂ from water into chloroform with high selectivity over the corresponding Co(ii), Ni(ii) and Zn(ii) salts.

Catalytic fixation of atmospheric carbon dioxide by copper(ii) complexes of bidentate ligands

The copper(ii) complexes of simple bidentate ligands have shown selective fixation and sequestration of atmospheric CO₂. The fixation of CO₂ proceeds via copper(i) species and geometrical interconversions and afforded CO₃²⁻ bound complexes.

Magneto-structural properties of carbonato-bridged copper(ii) complexes: fixation of atmospheric CO2

Weak ferromagnetic [Cu₃(dmMePEA)₃(μ₃-CO₃)(ClO₄)₃]ClO₄ (1) and strong antiferromagnetic [Cu₂(iptren)₂(μ₂-CO₃)](ClO₄)₂·H₂O (2) carbonato-bridged Cu(ii) complexes were synthesized via atmospheric fixation of CO₂ in slightly basic solutions.

A trinuclear copper(II) cryptate and its μ3-CO3 cascade complex: thermodynamics, structural and magnetic properties

The 2,4,6-triethylbenzene-capped hexaamine macrobicycle with pyridyl spacers (pyr) was able to coordinate three copper(II) ions within its cavity. Potentiometric studies performed at 298.2 K in MeOH/H(2)O (50:50 v/v) and at ionic strength 0.10 mol dm(-3) in KNO(3) revealed that trinuclear species predominate in solution from pH 5.0, the hydroxo complexes being the main species, which start forming at unusual very low pH values. The single-crystal X-ray determination of the trinuclear complex showed that the three copper centres have square-planar geometry, arranged in an almost equilateral triangle, and have carbonate bridging the three metal centres. The presence of carbonate resulted from fixation of atmospheric CO(2). The present study represents the first μ(3)-CO(3)-bridged trinuclear copper(II) complex located in the interior of a macrobicyclic cavity. The magnetic data of [Cu(3)(pyr)(μ(3)-CO(3))]·(ClO(4))(4) showed ferromagnetic intramolecular interactions [J=3.80 cm(-1), based on the Hamiltonian H=-J(S(1)S(2)+S(2)S(3)+S(1)S(3))] yielding a spin quartet, S=3/2, ground state. Density functional calculations on the experimental geometry of the trinuclear complex showed that the ferromagnetic nature of the magnetic coupling can be attributed to the syn-anti conformation of the carbonato bridge, and a magneto-structural correlation, based on the different conformations (syn-anti, syn-syn and anti-anti), is presented. The interesting properties observed, namely the lowering of the pK(a) of coordinated water molecules to unusual values and the good fit of the carbonate anion between the copper centres, derive from the special architecture of pyr.

High-nuclearity silver clusters templated by carbonates generated from atmospheric carbon dioxide fixation

Two high-nuclearity silver clusters have been prepared by reacting AgC(2)Bu(t) with AgBF(4) (or AgOTf) in the presence of TMEDA in air. The reaction takes up atmospheric CO(2) to give CO(3)(2-), which functions as a template for the formation of the cage compounds, whose nuclearities depend on the particular counteranions employed.

Chiral multinuclear macrocyclic polyamine complexes: Synthesis, characterization and their interaction with plasmid DNA

A series of multinuclear macrocyclic polyamine metal (Zn(2+), Cu(2+), Co(2+)) complexes containing chiral dipeptide linkage were synthesized and used as artificial nuclease enzyme model. The interaction between the complexes and plasmid DNA (pUC19) was studied, and the results revealed that these complexes could act as powerful catalysts for the cleavage of plasmid DNA under physiological conditions.

Cadmium(II) and Copper(II) Complexes with Imidazole-Containing Tripodal Polyamine Ligands: pH and Anion Effects on Carbon Dioxide Fixation and Assembling

A new Cd(II) complex [Cd3(L)3(mu3-CO3)](ClO4)4.2CH3CN (1) with two-dimensional (2D) network structure was obtained by reaction of an imidazole-containing tripodal polyamine ligand N1-(2-aminoethyl)-N1-(2-imidazolethyl)-ethane-1,2-diamine (L) with Cd(ClO4)2.6H2O at pH 9.0 in air. The carbonate anions (CO3(2-)) are from the hydration of the atmospheric carbon dioxide, which is the same as in the previously reported Cu(II) complex [Cu3(L)3(mu3-CO3)](ClO4)4.3CH3CN (2). However, the coordination mode of CO3(2-) in 1 is mu3-eta2:eta2:eta2 while the one in 2 is mu3-eta1:eta1:eta1. One-dimensional (1D) chain Cd(II) and Cu(II) complexes [Cd(L)Cl]ClO4.H2O (3) and [Cu(L)(H2O)](ClO4)2 (4) without CO3(2-) were prepared by a similar method as that for 1 and 2 except for the different reaction pH, namely, 3 and 4 were obtained at pH 7 while 1 and 2 were obtained at pH 9. In addition, when Cu(NO3)2 was used to react with L at pH 9, a unique 1D double-stranded helical chain complex [Cu(L)Cl]NO3.1.25H2O (5) was obtained. The results revealed that the reaction pH and the counteranion have great impact on the carbon dioxide absorption and hydration as well as on the assembling and structure of the complexes. The magnetic property of complex 2 was investigated in the temperature range of 1.8-300 K, and weak ferromagnetic coupling among the mu3-eta1:eta1:eta1-CO3(2-) bridged Cu(II) atoms was observed.

Carbon Dioxide Fixation by an Unprecedented Hydroxo Lead−Chromium Carbonyl Complex: Synthesis, Reactivity, and Theoretical Calculations

The novel hydroxo-bridged dimeric lead-chromium carbonyl complex [Et4N]2[{PbCr2(CO)10}2(mu-OH)2] ([Et4N]2[1]) was synthesized from the reaction of PbCl2 and Cr(CO)6 followed by metathesis with [Et4N]Br in a KOH/MeOH solution. The X-ray crystallographic structure shows that dianion 1 consists of two Pb{Cr(CO)5}2 units bridged by two hydroxo fragments in which the Pb atoms are further coordinated with two Cr(CO)5 groups, resulting in a distorted tetrahedral geometry. A CO2 molecule can insert itself into dianion 1 to form two new carbonate complexes, [Et(4)N]2[{PbCr2(CO)10}(CO3)] ([Et4N]2[2]) and [Et4N]2[{PbCr2(CO)10}2(CO3)] ([Et4N]2[3]), depending on the reaction conditions. In addition, complex 2 can be transformed into 3 in CH2Cl2 solution at an elevated temperature. While the carbonate group in dianion 2 is bonded to one Pb atom, which is coordinated with two Cr(CO)5 fragments, the carbonate group in 3 bridges the two Pb centers in a mu-1kappa2OO':2kappa2OO' fashion in which each Pb atom is further bonded to two Cr(CO)5 moieties. Complexes 2 and 3 can be converted back the hydroxo complex 1 under appropriate conditions. All three unprecedented lead-chromium compounds, 1-3, were fully characterized by spectroscopic methods and single-crystal X-ray diffraction analyses. The nature and formation of complexes 1-3 were also examined by molecular orbital calculations using the B3LYP method of the density functional theory.

Uridine Binding by Zn(II) Macrocyclic Complexes: Diversion of RNA Cleavage Catalysts

Zn(II) complexes of 1-oxa-4,7,10-triazacyclododecane (12[ane]N3O), 1,5,9-triazacyclododecane (12[ane]N3), and 1-hydroxyethyl-1,4,7-triazacyclononane (9[ane]N3OH) promote cleavage of the RNA analogue, 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) at pH 8.0, I=0.10 M (NaCl), 25 degrees C with second-order rate constants of 8.9x10(-3), 9.0x10(-3), and 3.3x10(-3) M-1 s-1, respectively. Cleavage of HpPNP by these catalysts is inhibited by uridine with inhibition constants (Ki) of 1.2, 0.46, and 45 mM, respectively, under these conditions. Binding constants derived from these inhibition constants are 2-200-fold larger than those for binding of related Zn(II) complexes to phosphate diesters under similar conditions, suggesting that uridine sequences in RNA will inhibit Zn(II)-catalyzed cleavage by competing with phosphate diester binding sites. Further studies are carried out that utilize pH-potentiometric titrations to monitor uridine binding to five Zn(II) macrocyclic complexes in aqueous solution at 25 degrees C, I=0.10 M (NaCl). The data are consistent with binding of the Zn(II) complexes to the N3-deprotonated form of uridine to give log KU.-values of 5.29, 4.57, 4.56, 3.47, and 2.65 for the Zn(II) complexes of 12[ane]N3, 12[ane]N4, 12[ane]N3O, 15[ane]N3O2, and 9[ane]N3OH, respectively (12[ane]N4=1,4,7,10-tetraazacyclododecane, 15[ane]N3O2=1,4-dioxa-7,10,13-triazacyclopentadecane). For the five Zn(II) complexes studied, there is a linear relationship between uridine anion binding constants and hydroxide binding constants.

New Polynuclear Manganese Clusters from the Use of the Hydrophobic Carboxylate Ligand 2,2-Dimethylbutyrate

The syntheses, structures, and magnetic properties are reported of [Mn12O12(O2CPe(t))16(MeOH)4] (4), [Mn6O2(O2CH2)(O2CPe(t))11(HO2CPe(t))2(O2CMe)] (5), [Mn9O6(OH)(CO3)(O2CPe(t))12(H2O)2] (6), and [Mn4O2(O2CPe(t))6(bpy)2] (7, bpy = 2,2'-bipyridine), where Pe(t) = tert-pentyl (Pe(t)CO2H = 2,2-dimethylbutyric acid). These complexes were all prepared from reactions of [Mn12O12(O2CPe(t))16(H2O)4] (3) in CH2Cl2. Complex 4 x 2MeCN crystallizes in the triclinic space group P1 and contains a central [Mn(IV)4O4] cubane core that is surrounded by a nonplanar ring of eight alternating Mn(III) and eight mu3-O(2-) ions. This is only the third Mn12 complex in which the four bound water molecules have been replaced by other ligands, in this case MeOH. Complex 5 x (1/2)CH2Cl2 crystallizes in the monoclinic space group P2(1)/c and contains two [Mn3(mu3-O)]7+ units linked at two of their apexes by two Pe(t)CO2(-) ligands and one mu4-CH2O2(2-) bridge. The complex is a new structural type in Mn chemistry, and also contains only the third example of a gem-diolate unit bridging four metal ions. Complex 6 x H2O x Pe(t)CO2H crystallizes in the orthorhombic space group Cmc2(1) and possesses a [Mn(III)9(mu3-O)6(mu-OH)(mu3-CO3)]12+ core. The molecule contains a mu3-CO3(2-) ion, the first example in a discrete Mn complex. Complex 7 x 2H2O crystallizes in the monoclinic space group P2(1)/c and contains a known [Mn(III)2Mn(II)2(mu3-O)2]6+ core that can be considered as two edge-sharing, triangular [Mn3O] units. Additionally, the synthesis and magnetic properties of a new enneanuclear cluster of formula [Mn9O7(O2CCH2Bu(t))13(THF)2] (8, THF = tetrahydrofuran) are reported. The molecule was obtained by the reaction of [Mn12O12(O2CCH2Bu(t))16(H2O)4] (2) with THF. Complexes 2 and 4 display quasireversible redox couples when examined by cyclic voltammetry in CH2Cl2; oxidations are observed at -0.07 V (2) and -0.21 V (4) vs ferrocene. The magnetic properties of complexes 4-8 have been studied by direct current (DC) and alternating current (AC) magnetic susceptibility techniques. The ground-state spin of 4 was established by magnetization measurements in the 1.80-4.00 K and 0.5-7 T ranges. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating a full powder average and including only axial anisotropy, gave S = 10, g = 2.0(1), and D = -0.39(10) cm(-1). The complex exhibits two frequency-dependent out-of-phase AC susceptibility signals (chi(M)'') indicative of slow magnetization relaxation. An Arrhenius plot obtained from chi(M)'' vs T data gave an effective energy barrier to relaxation (U(eff)) of 62 and 35 K for the slower and faster relaxing species, respectively. These studies suggest that complex 4 is a single-molecule magnet (SMM). DC susceptibility studies on complexes 5-8 display overall antiferromagnetic behavior and indicate ground-state spin values of S < or = 2. AC susceptibility studies at < 10 K confirm these small values and indicate the population of low-lying excited states even at these low temperatures. This supports the small ground-state spin values to be due to spin frustration effects.

Lewis Acid Properties of Zinc(II) in Its Cyclen Complex. The Structure of [Zn(Cyclen)(SC(NH2)2](ClO4)2 and the Bonding of Thiourea to Metal Ions. Some Implications for Zinc Metalloenzymes

The structure of the complex [Zn(cyclen)Tu](NO(3))(2) (1) is reported (cyclen = 1,4,7,10-tetraazacyclododecane; Tu = thiourea): orthorhombic, space group P2(1)2(1)2(1), a = 11.4170(11) A, b = 12.1995(11) A, c = 12.5299(12) A, Z = 4, R = 0.0504. The coordination of the cyclen is the same as that found for other similar Zn(II) complexes, with square pyramidal coordination around the Zn(II) and mean Zn-N bond lengths of 2.16 A. The coordinated Tu occupies the axial coordination site, with Zn-S = 2.31 A. The Zn-S-C-N torsion angle, involving the coordinated Tu, of 75.4 degrees is unusually large, because such torsion angles involving coordinated Tu are normally closer to 0 degrees. The bonding between Zn and S is discussed in terms of overlap with the p orbitals on S, which favors the eclipsed (Zn-S-C-N torsion = 0 degrees) mode of coordination of Tu. The energies of eclipsed and staggered modes (Zn-S-C-N = 90 degrees) of coordination of Tu to metal ions are examined by means of ab initio calculations, using the STO-3G basis set. It is concluded that the rather low formation constant for the Tu complex with Zn(II)/cyclen reported in this work was due to steric effects in 1, which prevent the adoption of the lower energy eclipsed conformation. These steric effects, because of clashes that would occur between Tu in the eclipsed conformation and the cyclen ring, cause the coordination of Tu with a higher energy conformation, with Zn-S-C-N = 75.4 degrees. The latter approaches the high energy staggered conformation that has Zn-S-C-N = 90 degrees. log K(1) values for Cl(-), Br(-), I(-), and CN(-) are reported and shown to be consistent with the binding site on the Zn(II) in the Zn(II)/cyclen complex being softer in the hard and soft acids and bases (HSAB, Pearson 1997) sense than the Zn(II) aqua ion, but not as soft as Zn(II) in triaza macrocycles that promote tetrahedral coordination. The change in HSAB character from intermediate in the Zn(II) aqua ion to softer in the cyclen complex, and softer still in tridentate N-donor ligands in model complexes, and in the Zn(II) active site of carbonic anhydrase as representative of Zn(II) metalloenzymes in general, is discussed in terms of the role of such effects in the functioning of metalloenzymes.