Non-bridging Ligands Research Articles

Introducing non-bridging ligand in metal-organic framework-based electrocatalyst enabling reinforced oxygen evolution in seawater

Using seawater as the replacement of freshwater for electrolysis, with the integration of renewable energy, is deemed as an attractive manner to harvest green hydrogen. However, the complexity of seawater puts forward stricter requirement to the electrocatalyst to alleviate the chlorine electrochemistry and corrosion. Herein, a nanosheet array of NiFe-MOF@Ni2P/Ni(OH)2 is devised by partially substituting terephthalic acid (H2BDC) ligand by ferrocenecarboxylic acid (FcCA). Tailoring the active site into an under-coordinated fashion affords NiFe-MOF@Ni2P/Ni(OH)2 excellent performance towards oxygen evolution reaction (OER), only requiring the overpotentials of 302 mV and 394 mV in alkaline seawater to drive the current densities of 100 and 1000 mA cm−2, respectively. Moreover, the as-obtained electrocatalyst showed robust durability for operating more than 120 h at 500 mA cm−2 under harsh condition (6 M KOH + 1.5 M NaCl, 60 ℃). Density functional theory (DFT) calculations confirmed that tuning the coordination environment of Ni in NiFe-MOF by incorporating the non-bridging FcCA ligands could boost the formation of more active catalytic sites, which can simultaneously enhance the electronic conductivity and accelerate OER kinetics. This work provides beneficial enlightenment of combining MOF-based electrocatalyst with direct electrolysis of seawater.

Spectrophotometric Study of Bridging N-Donor Ligand-Induced Supramolecular Assembly of Conjugated Zn-Trisporphyrin with a Triphenylamine Core.

This article studies the supramolecular assembly behavior of a Zn-trisporphyrin conjugate containing a triphenylamine core (1) with bridging N-donor ligands using the UV-vis spectrophotometric titration method at micromolar concentrations. Our results show that pyridine, a non-bridging ligand, formed a 3:1 open complex with 1. The corresponding binding constant was estimated to be (2.7 ± 0.15) × 1014 M−3. In contrast, bridging ligands, 4,4-bipyridine (BIPY) and 1,3-di(4-pyridyl)propane (DPYP), formed stable 3:2 double-decker complexes with 1 in solution, which collapsed to yield a 3:1 open complex when excess BIPY or DPYP was added. The binding constants for forming BIPY and DPYP double-decker complexes were estimated to be (9.26 ± 0.07) × 1027 M−4 and (3.62 ± 0.16) × 1027 M−4, respectively. The UV-vis titration profiles supported the conclusion that the degradation of the 3:2 double-decker 1∙BIPY complex is less favorable compared to that of 1∙DPYP. Consequently, the formation of the 3:1 1∙DPYP open complex proceeded more readily than that of 1∙BIPY.

Continuous Breathing Rare-Earth MOFs Based on Hexanuclear Clusters with Gas Trapping Properties

Guest responsive porous materials represent an important and fascinating class of multifunctional solids that have attracted considerable attention in recent years. An understanding of how these structures form is essential toward their rational design, which is a prerequisite for the development of tailor-made materials for advanced applications. We herein report a novel series of stable rare-earth (RE) MOFs that show a rare continuous breathing behavior and an unprecedented gas-trapping property. We used an asymmetric 4-c tetratopic carboxylate-based organic ligand that is capable of affording highly crystalline materials upon controlled reaction with RE cations. These MOFs, denoted as RE-thc-MOF-1 (RE: Y3+, Sm3+, Eu3+, Tb3+, Dy3+, Ho3+, and Er3+), feature hexanuclear RE6 clusters that display a highly unusual connectivity and serve as unique 8-c hemi-cuboctahedral secondary building block, resulting in a new (3,3,8)-c thc topology. Extensive single-crystal to single-crystal structural analyses coupled with detailed gas (N2, Ar, Kr, CO2, CH4, and Xe) and vapor (EtOH, CH3CN, C6H6, and C6H14) sorption studies, supported by accurate theoretical calculations, shed light onto the unique swelling behavior. The results reveal a synergistic action involving steric effects, associated with coordinated solvent molecules and 2-fluorobenzoate (2-FBA) nonbridging ligands, as well as cation-framework electrostatic interactions. We were able to probe the individual role of the coordinated solvent molecules and 2-FBA ligands and found that both cooperatively control the gas-breathing and -trapping properties, while 2-FBA controls the vapor adsorption selectivity. These findings provide unique opportunities toward the design and development of tunable RE-based flexible MOFs with tailor-made properties.

Enhancing easy-plane anisotropy in bespoke Ni(II) quantum magnets

We examine the crystal structures and magnetic properties of several S = 1 Ni(II) coordination compounds, molecules and polymers, that include the bridging ligands HF2−, AF62− (A = Ti, Zr) and pyrazine or non-bridging ligands F−, SiF62−, glycine, H2O, 1-vinylimidazole, 4-methylpyrazole and 3-hydroxypyridine. Pseudo-octahedral NiN4F2, NiN4O2 or NiN4OF cores consist of equatorial Ni-N bonds that are equal to or slightly longer than the axial Ni-Lax bonds. By design, the zero-field splitting (D) is large in these systems and, in the presence of substantial exchange interactions (J), can be difficult to discriminate from magnetometry measurements on powder samples. Thus, we relied on pulsed-field magnetization in those cases and employed electron-spin resonance (ESR) to confirm D when J ≪ D. The anisotropy of each compound was found to be easy-plane (D > 0) and range from ≈ 8–25 K. This work reveals a linear correlation between the ratio d(Ni-Lax)/d(Ni-Neq) and D although the ligand spectrochemical properties may play an important role. We assert that this relationship allows us to predict the type of magnetocrystalline anisotropy in tailored Ni(II) quantum magnets.

Synthesis, structure and properties of new heterometallic octanuclear Li2Na2Cu4 and decanuclear Li2Zn8 complexes

A carboxylate containing polydentate ligand in combination with exogeneous succinate (suc) has been used to stabilize heterometallic octanuclear [Li2Na2Cu4(cpdp)2(suc)2(CH3OH)2(H2O)4]Cl2·6H2O (1) and decanuclear Na(H3O)2[Li2Zn8(cpdp)4(suc)2(H2O)4]Cl2Br3·6MeOH·19H2O (2) complexes. The reaction of ligand H3cpdp (H3cpdp = N,N'-bis[2-carboxybenzomethyl]-N,N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol) with stoichiometric amounts of CuCl2/sodium succinate, and ZnCl2/sodium succinate, allowed isolation of complexes 1 and 2, respectively. Analyses of single crystal X-ray structures indicate that complex 1 is capped by two [Cu2(cpdp)]+ molecular building units and two succinate linkers, while complex 2 is overlaid by four [Zn2(cpdp)]+ molecular building units and two succinate linkers. Whereas complex 1 shows monodentate terminal and tridentate μ3:η2:η1:η1 bridging coordination of carboxylate, and μ4:η2:η1:η1:η1 bridging mode of succinate, complex 2 displays monodentate terminal and anti-anti μ2:η1:η1 bidentate bridging coordination of carboxylate, and μ4:η1:η1:η1:η1 bridging mode of succinate. To our knowledge, 1 is the first example of copper/lithium/sodium-based hetero-octanuclear complex with any class of bridging or non-bridging ligand showing three different binding modes of carboxylates. Again, 2 is also the first example of a hetero-decanuclear metallomacrocyclic complex with any class of bridging or non-bridging ligand combining both lithium and zinc. Variable temperature magnetic investigation of 1 discloses sensible antiferromagnetic interactions intermediated among the copper centers. Thermal behavior of 1 and 2 has been examined by thermogravimetric analysis indicating that the complexes are stable up to ∼430 °C.

Bis{μ-ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate-κO}bis[aqua(4-{[4-(ethoxycarbonyl)phenyl]diazenyl}phenolato-κO){ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate-κO}potassium

The two potassium cations in the dinuclear molecule of [K2{OC(C14H13ON2)O}2{OC(C14H13ON2)OH}4(H2O)2] are connected through a double bridge involving two centrosymmetrically related ethyl-4-(phenylazophenol)benzoate ligands. Each cation is also bound to a further non-bridging ligand, one ethyl-4-(phenylazophenolate)benzoate anion and a water molecule, leading to a distorted fivefold coordination. The two uncharged ligands are almost planar, whereas in the anionic ligand the aromatic systems display a dihedral angle of 21.14 (11)°. A supramolecular network formed by hydrogen-bonding interactions between phenolate anions, phenol groups and water molecules connects the dimeric species along [001]. Hirshfeld surface calculations revealed the following contributions related to intermolecular interactions: C...H (24.4%), O...H (13.2%) and N...H (7.4%). The azo fragment is disordered over two sets of sites [occupancy ratio 0.824 (15):0.176 (5)].

Dependence of magnetic coupling on ligands at the axial positions of NiII in phenoxido bridged dimers: experimental observations and DFT studies.

A nickel(ii) complex [Ni2LR2(CH3CN)4](ClO4)2·2CH3CN (1) has been synthesized by using a tridentate reduced Schiff base ligand, HLR, 2-[(3-methylamino-propylamino)-methyl]-4-nitrophenol and Ni(ClO4)2·6H2O. Stoichiometrically controlled addition of NH4SCN to the acetonitrile-water solution of 1 produced two other complexes, [Ni2LR2(μ1,1-NCS)(CH3CN)2](ClO4)·CH3CN (2) and [Ni2LR2(NCS)2 (CH3CN)2] (3) by replacing one and two coordinated CH3CN by NCS-, respectively. The dinuclear NiII complexes 1 and 3 are structurally very similar containing a diphenoxido bridge between the NiII ions. Complex 2 also consists of a dinuclear entity, but a μ1,1-isothiocyanato bridge is present in addition to the diphenoxido bridge. The magnetic measurements indicate that antiferromagnetic interactions (J = -33.85 cm-1) for 1, (J = -23.43 cm-1) and 3 and ferromagnetic interaction (J = 5.01 cm-1) for 2 are mediated between the intra-dimer NiII ions. The nature of coupling (ferromagnetic for 2 and antiferromagnetic for 1 and 3) is understandable from the phenoxido bridging angles (91.27° (av. angle) for 2, 102.41° for 1 and 102.42° for 3) but it cannot explain the considerable difference of J values between 1 and 3. Other important factors such as NiO bond distances, the out-of-plane shift of phenyl rings and the hinge distortions are also of no use for a possible explanation. With the help of computational evidence, we have identified for the first time that magnetic coupling could be dramatically modified by the non-bridging axially coordinated ligands: antiferromagnetic coupling becomes considerably stronger for a neutral ligand in comparison to the negatively charged ligand. Moreover, for a negatively charged axially coordinated ligand, antiferromagnetic coupling decreases with increase in crystal field strength.

Structural Features of PbS Nanocube Monolayers upon Treatment with Mono- and Dicarboxylic Acids and Thiols at a Liquid-Air Interface.

This paper describes the ordering of PbS nanocubes (NCs) within free-standing monolayers (suspended on acetonitrile), upon exchanging the native oleate ligands for a series of thiolate and carboxylate ligands at the liquid-air interface. Treatment with either carboxylic acids or thiols effectively decreases the inter-NC separation of nearest-neighbor particles without etching the NC surface. Dicarboxylic acids and dithiols bridge neighboring NCs with an interparticle separation that is consistent with fully extended, bridging ligands. Monocarboxylic acids and monothiols separate NCs by an amount governed by their length, with long-chain ligands showing significant intercalation. (1)H NMR spectroscopy shows carboxylic acids are more effective at replacing the native oleate than are thiols, which we ascribe to the lower pKa values of carboxylic acids. The fast exchange that occurs upon treatment with monocarboxylic acids kinetically traps the clusters of particles in nonclosed packed geometries, so monolayers treated with monocarboxylic acids are, on average, less ordered than those treated with monothiols. Ex situ electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) analyses of deposited films on Si/SiO2 substrates show that NCs exchanged with nonbridging ligands pack more efficiently at long length scales than do NCs exchanged with bridging ligands, due primarily to the creation of defects within the NC lattice in response to the rigidity of the bridging ligand.

MOF functionalization via solvent-assisted ligand incorporation: phosphonates vs carboxylates.

Solvent-assisted ligand incorporation (SALI) is useful for functionalizing the channels of metal-organic framework (MOF) materials such as NU-1000 that offer substitutionally labile zirconium(IV) coordination sites for nonbridging ligands. Each of the 30 or so previous examples relied upon coordination of a carboxylate ligand to achieve incorporation. Here we show that, with appropriate attention to ligand/node stoichiometry, SALI can also be achieved with phosphonate-terminated ligands. Consistent with stronger M(IV) coordination of phosphonates versus carboxylates, this change extends the pH range for retention of incorporated ligands. The difference in coordination strength can be exploited to achieve stepwise incorporation of pairs of ligands-specifically, phosphonates species followed by carboxylate species-without danger of displacement of the first ligand type by the second. Diffuse reflectance infrared Fourier-transform spectroscopy suggests that the phosphonate ligands are connected to the MOF node as RPO2(OH)¯ species in a moiety that leaves a base-accessible -OH moiety on each bound phosphonate.

ChemInform Abstract: Beyond Post‐Synthesis Modification: Evolution of Metal—Organic Frameworks via Building Block Replacement

AbstractReview: 132 refs.

Structure control and sorption properties of porous coordination polymers prepared from M(NO3)2 and 4,4′-bipyridine (M = Co2+, Ni2+)

The inclusion of labile non-bridging ligands in coordination polymers can be used to modify their topology and provide a route to the preparation of materials with vacant metal coordination sites. The reaction of Co(NO3)2 and 4,4′-bipyridine (bpy) in ethanol is known to produce a 2D coordination polymer [Co2(bpy)3(NO3)4]n, 1. The inclusion of non-bridging aqua ligands produces new coordination polymer structures, 1D chains of [Co(bpy)(H2O)2(NO3)2]n, 2, and [Ni(bpy)(H2O)2(NO3)2]n, 3. The structures of the 2 and 3 were determined by single-crystal X-ray diffraction, which showed that they had the same packing motif, layers of parallel chains stacked in an ABAB pattern along the c axis. The crystals had 4Å channels running parallel to the c axis. The disordered guest molecules in the channels of 2 were removed by heating at 90°C, along with the two aqua ligands. This produced a coordination polymer with vacant metal coordination sites that had a high selectivity for the adsorption of water and its coordination to the Co. Aqua ligands also bound to the Co when 1 adsorbed water and, but the process was much slower due to the larger structural changes this required in the 2D framework and the lack of vacant coordination sites.

Beyond post-synthesis modification: evolution of metal-organic frameworks via building block replacement.

Metal-organic frameworks (MOFs) are hybrid porous materials with many potential applications, which intimately depend on the presence of chemical functionality either at the organic linkers and/or at the metal nodes. Functionality that cannot be introduced into MOFs directly via de novo syntheses can be accessed through post-synthesis modification (PSM) on the reactive moieties of the linkers and/or nodes without disrupting the metal-linker bonds. Even more intriguing methods that go beyond PSM are herein termed building block replacement (BBR) which encompasses (i) solvent-assisted linker exchange (SALE), (ii) non-bridging ligand replacement, and (iii) transmetalation. These one-step or tandem BBR processes involve exchanging key structural components of the MOF, which in turn should allow for the evolution of protoMOF structures (i.e., the utilization of a parent MOF as a template) to design MOFs composed of completely new components, presumably via single crystal to single crystal transformations. The influence of building block replacement on the stability and properties of MOFs will be discussed, and some insights into their mechanistic aspects are provided. Future perspectives providing a glimpse into how these techniques can lead to various unexplored areas of MOF chemistry are also presented.

Magnetic interactions in oxide-bridged dichromium(III) complexes. Computational determination of the importance of non-bridging ligands

The magnetic susceptibility of the dinuclear chromium(III) complex[(CH3CN)5CrOCr(NCCH3)5](BF4)4·2 CH3CN has been measured and analyzed. With a fitted value of the triplet energy J=650cm−1, the antiferromagnetic coupling is the strongest hitherto determined for an unsupported linear oxide-bridged dinuclear Cr(III) system. The applicability of DFT for computational prediction of exchange in strongly coupled chromium(III) dimers was examined and an optimal and accurate modeling approach was devised. By such modeling it was shown possible to reproduce experimental exchange coupling constants with small relative errors typically of less than 10% ranging from the strongest coupled systems to systems with moderately strong couplings. A significant influence (>20%) of the chemical nature of the peripheral, non-bridging ligands on the exchange coupling was found and rationalized.

Structural diversity in manganese, iron and cobalt complexes of the ditopic 1,2-bis(2,2′-bipyridyl-6-yl)ethyne ligand and observation of epoxidation and catalase activity of manganese compounds

A ditopic 1,2-bis(2,2'-bipyridyl-6-yl)ethyne ligand, L, has been synthesized for the first time by consecutive Suzuki and Sonogashira coupling reactions either in a one- or two-step synthesis. Coordination of L with some first-row transition metals, Fe, Mn and Co showed a very rich structural diversity that can be obtained with this ligand. Reaction of L with Mn(II)(OAc)(2) yielded a dimanganese(II) complex, [Mn(2)L(mu-OAc)(3)]PF(6), (1) where the two somewhat inequivalent trigonal-bipyramidal Mn atoms separated by 3.381 A are bridged by L and three acetate moieties. A similar reaction of L with Mn(III)(OAc)(3) yielded a very different dimanganese complex [Mn(2)L'(OH)(OAc)(2)(DMF)(2)]PF(6) x DMF (2) where L' is a E-1,2-bis(2,2'-bipyridyl-6-yl)ethene fragment that was formed in situ. The L' ligand bridges between the two Mn centers, despite its trans configuration, which leads to a very strained ethene bridging moiety. The Mn atoms are also bridged by two acetate ligands and a hydroxy group that bridges between the Mn atoms and the ethene fragment; DMF completes the octahedral coordination around each Mn atom which are separated by 3.351 A. A comproportionation reaction of L with Mn(II)(OAc)(2) and n-Bu(4)NMnO(4) yielded a tetramanganese compound, [Mn(4)(mu(3)-O)(2)(OAc)(4)(H(2)O)(2)L(2)](PF(6))(2) x 2 CH(3)CN (3). Compound 3 has a dimer of dimers structure of the tetranuclear Mn core that consists of binuclear [Mn(2)O(OAc)(2)L](+) fragment and a PF(6) anion. BVS calculations indicate that 3 is a mixed-valent 2Mn(II) plus 2Mn(III) compound where two [Mn(II)(2)O(OAc)(2)L](+) fragments are held together by Mn(III)-O inter-fragment linkers which have a distorted octahedral geometry. The Mn atoms in the [Mn(2)O(OAc)(2)L](+) fragments have a capped square-pyramid configuration where an aqua ligand is capped on one of the faces. Although the aqua ligand is well within a bonding distance to a carbon atom of the proximal ethyne bridge, there does not appear to be an oxygen-carbon bond formation, rather the ligand is constrained in this position, as deduced by the observation that the bond lengths and angles of the ligand are essentially the same as those for the free ligand, L. Reaction of L with perchlorate or triflate salts of Fe(II), Mn(II) and Co(II) in dry acetonitrile yielded binuclear triple helicate structures (2:3 metal to L ratios) [Fe(2)L(3)](CF(3)SO(3))(4) x CH(3)CN (4), [Mn(2)L(3)](ClO(4))(4) x 1.7 CH(3)CN x 1.65 EtOEt (5) and [Co(2)L(3)](ClO(4))(4) x 2 CH(3)CN x 2 EtOEt (6) where each M(II) center with a slightly distorted octahedral geometry is bridged by three of the ditopic ligands. The M-M distances varied; 5.961 A (Mn), 6.233 A (Co) 6.331 A (Fe). Reaction of L with Co(ClO(4))(2) x 6 H(2)O in wet acetonitrile yielded a dicobalto(III) compound, [Co(2)L'(3)(O)(2)](ClO(4))(2) x H(2)O (7), with two types of L' fragments; one bridging between the two Co centers and two non-bridging ligands, each bonded to a Co atom via one bipyridyl group where the other is non-bonding. The octahedral coordination sphere around each Co atom is completed by the formation of a cobalt-carbon bond from the two carbon atoms of the ethene moiety of the bridging ligand and by a hydroxy moiety that is also bonded to the ethene group of the non-bridging ligand. Reaction of L with Co(ClO(4))(2) x 6 H(2)O in dry acetonitrile in the presence of Et(3)N yielded the tetracobalto(II) complex {[Co(2)L(4)(OH)(4)](ClO(4))(4)}(2) (8) with a unique twisted square configuration of cobalt ions with Co-Co distances of 3.938 to 4.131 A. In addition to the L bridging ligand the Co atoms are linked by hydroxy moieties. Some preliminary catalytic studies showed that the Mn compounds 1 and 2 were active (high yield within 3 min) for alkene epoxidation with peracetic acid and hydrogen peroxide dismutation (catalase activity).

Synthesis and structures of complexes with axially chiral isoquinolinyl-naphtholate ligands

The synthesis of axially chiral ligands 1-(3',6'-di-t-butyl-2'-hydroxy-1'-naphthyl)-3-R-isoquinoline (R = H, Pr(i), Bu(t)) (LR-H) is described. Ligands with unsubstituted isoquinolinyl rings tend to give 1:2 metal complexes. The syntheses and crystal structures of Li2(LH)2(THF)2 (9), (LH)2Ti(OPr(i))2 (12), Zn(LH)2 (13) and [Mg(LH)2]2 (14) are reported. Complex formation is highly stereoselective; the ligands in 1:2 complexes have the same stereochemistry (i.e. R,R and S,S but not R,S), whereas in the binuclear magnesium compound 14 the bridging and non-bridging ligands LH have opposite stereochemistry. The reaction of LH-H with Pd(acac)2 afforded the N,O chelate Pd(acac)(LH) (10), whereas towards K2PtCl4 the same ligand acts as an N-donor only, to give trans-PtCl2(LH-H)2 (11) in which the OH groups are hydrogen-bonded to one of the two chloride ligands. The more bulky ligand with a t-butyl substituent in the 3-position of the isoquinolinyl ring reacts with zinc and magnesium bis(amides) to give the mixed-ligand species (LBu)ZnN(SiMe3)2 and (LBu)MgN(SiMe3)2, respectively, which catalyse the ring-opening polymerisation of epsilon-caprolactone (CL) and rac-lactide (LA).

Analyzing the catalytic role of Asp97 in the methionine aminopeptidase from Escherichia coli

An active site aspartate residue, Asp97, in the methionine aminopeptidase (MetAPs) from Escherichia coli (EcMetAP-I) was mutated to alanine, glutamate, and asparagine. Asp97 is the lone carboxylate residue bound to the crystallographically determined second metal-binding site in EcMetAP-I. These mutant EcMetAP-I enzymes have been kinetically and spectroscopically characterized. Inductively coupled plasma-atomic emission spectroscopy analysis revealed that 1.0 +/- 0.1 equivalents of cobalt were associated with each of the Asp97-mutated EcMetAP-Is. The effect on activity after altering Asp97 to alanine, glutamate or asparagine is, in general, due to a approximately 9000-fold decrease in k(ca) towards Met-Gly-Met-Met as compared to the wild-type enzyme. The Co(II) d-d spectra for wild-type, D97E and D97A EcMetAP-I exhibited very little difference in form, in each case, between the monocobalt(II) and dicobalt(II) EcMetAP-I, and only a doubling of intensity was observed upon addition of a second Co(II) ion. In contrast, the electronic absorption spectra of [Co_(D97N EcMetAP-I)] and [CoCo(D97N EcMetAP-I)] were distinct, as were the EPR spectra. On the basis of the observed molar absorptivities, the Co(II) ions binding to the D97E, D97A and D97N EcMetAP-I active sites are pentacoordinate. Combination of these data suggests that mutating the only nonbridging ligand in the second divalent metal-binding site in MetAPs to an alanine, which effectively removes the ability of the enzyme to form a dinuclear site, provides a MetAP enzyme that retains catalytic activity, albeit at extremely low levels. Although mononuclear MetAPs are active, the physiologically relevant form of the enzyme is probably dinuclear, given that the majority of the data reported to date are consistent with weak cooperative binding.

Synthesis, X-ray structures, and magnetic properties of heterometal dinuclear Cr(III)–M(II) complexes

New 3,5-bis(2-pyridyl)pyrazolato (bpypz) bridged heterometal dinuclear complexes [(nta)Cr(μ-bpypz)M II(picen)] + (M = Mn(II), Ni(II)) and [(acac) 2Cr(μ-bpypz)Ni II(picen)] 2+ (nta = nitrilotriacetate, picen = N, N′-bis(2-pyridylmethyl)ethylenediamine, acac = acetylacetate) were synthesized and characterized by the X-ray analysis, ESI-MS and the magnetic measurements, and/or 2H NMR spectra. The molecular structures were compared from a viewpoint of the conformation of the picen depending on M II ionic radii or different modes of hydrogen bonds. The picen in [(nta)Cr(μ-bpypz)Mn II(picen)]BF 4 takes an abnormal conformation with intramolecular bifurcated three-center hydrogen bonds between two carboxylate oxygens of nta and an amine proton of the picen as found for the previously reported corresponding Fe(II) complex [K. Ni-iya, A. Fuyuhiro, T. Yagi, S. Nasu, K. Kuzushita, S. Morimoto, S. Kaizaki, Bull. Chem. Soc. Jpn. 74 (2001) 1891]. On the other hand, for both Ni(II)-nta and Ni(II)-acac complexes, the picen takes a normal conformation with only a two-center hydrogen bond between non-bridging ligands. The magneto-structural relation is discussed for the Cr(III)–Ni(II) complexes in connection with the orthogonality or orbital overlap arising from the difference in distortion around Cr(III) moiety.

Two Nearly Isoformular Isomeric Dinuclear Platinum(II) Complexes with Planar and Butterfly [Pt2(μ2-S)2] Core Structures

Two dinuclear PtII complexes bearing fluorinated phosphine and thiolate ligands were synthesized from mononuclear starting materials, and characterized. Complex [(PPh2(C6F5))(SC6F5)Pt(μ2-SC6F5)2Pt(SC6F5)(PPh2(C6F5))] has phospine ligands arranged trans and presents a planar [Pt2(μ2-S)2] core structure, imposed by symmetry. In contrast, the almost isoformular complex [(PPh(C6F5)2)(SC6F5)Pt(μ2-SC6F5)2Pt(SC6F5)(PPh(C6F5)2)] revealed to be a butterfly molecule with phosphine ligands in a cis configuration. In both complexes, two pentafluorobenzenethiolate ligands bridge metal centers in an anti conformation. Both compounds have been X-ray characterized. Trans complex: a = 11.8576(13) A, b = 12.3237(14) A, c = 13.4644(16) A, α = 90.528(9)°, β = 113.866(9)°, γ = 107.698(9)°, $$ P\,\bar 1 $$ ; cis complex: a = 24.368(2) A, b = 15.043(2) A, c = 37.596(7) A, β = 93.69(1)°, C2/c. Two Nearly Isoformular Isomeric Dinuclear Platinum(II) Complexes with Planar and Butterfly [Pt 2 (μ 2 -S ) 2 ] Core Structures Sylvain Bernes, Luis Villanueva & Hugo Torrens Two dinuclear PtII complexes bearing fluorinated phosphine and thiolate ligands were X-ray characterized, [(PPh2(C6F5))(SC6F5)Pt(μ2-SC6F5)2Pt(SC6F5)(PPh2(C6F5))] and [(PPh(C6F5)2)(SC6F5)Pt(μ2-SC6F5)2Pt(SC6F5)(PPh(C6F5)2)]. The [Pt2(μ2-S)2] core is constrained by symmetry to be planar in the former complex, affording a trans arrangement for non-bridging ligands, whereas the latter is a butterfly molecule with a cis configuration.

<b>Non-bridging ligand effects on the kinetics of reduction of chloro- and azido-pentaamminecobalt(III) by some polypyridyl complexes of ruthenium(II)</b>

Pentaamminecobalt(III) complexes, [Co(NH 3 ) 5 X] 2+ (X = Cl - , N 3 - ), are reduced by [Ru(bipy) 3 ] 2+ and [Ru(terpy)(bipy)Cl] + in aqueous media at a constant ionic strength of 0.5 M (HCl/LiCl). At 308 K, the second order rate constants (M -1 s -1 ) are as follows: 17.9 for the reduction of the azidocobalt(III) complex by [Ru(bipy) 3 ] 2+ , and 1.41 and 2.63 for the [Ru(terpy)(bipy)Cl] + reduction of the azido- and chlorocobalt(III) complexes, respectively. Activation enthalpies (ΔH # ) and entropies (ΔS # ) were determined from temperature dependence measurements with the following results: ΔH # = 72.1 kJ mol -1 and ΔS # = 13.3 J mol -1 K -1 for the [Ru(bipy) 3 ] 2+ reduction of the azidocobalt(III) complex, while for the reduction of the cobalt(III) complexes by [Ru(terpy)(bipy)Cl] + , ΔH # (N 3- ) = 20.3 kJ mol -1 , ΔH # (Cl - ) = 40.6 kJ mol -1 , ΔS # (N 3 - ) = -177 J mol -1 K -1 , and ΔS # (Cl - ) = -106 J mol -1 K -1 . The relative rates of electron transfer in the different reactions and the influence of π-acceptor ligands on the ruthenium(II) reduction of the cobalt(III) complexes are discussed. KEY WORDS : Non-bridging ligands, Pentaamminecobalt(III) complexes, Azidocobalt(III) complex, Chlorocobalt(III) complex, Kinetics of reduction of cobalt(III) complexes, Ruthenium(II) polypyridyl complexes Bull. Chem. Soc. Ethiop. 2006 , 20(2), 227-234.

From Parquets to Bricks: A Series of Lanthanide Coordination Polymers with Bis(diphenylphosphino)ethane Dioxide

A series of new layered coordination polymers consisting of Ln[Ph2P(O)-CH2CH2-P(O)Ph2]1.5 monomeric units, where Ln = Pr(NO3)3, Nd(NO3)3, NdCl3, Sm(NO3)3, GdCl3, Dy(NO3)3, were synthesized and structurally characterized. The majority of prepared compounds display approximately rectangular building blocks. However, the adopted architecture strongly depends on additional nonbridging ligands in the coordination sphere of the cation, as well as on the reaction conditions and the employed solvent. Accordingly, the solvothermally prepared polymers exhibit either parquet-floor or brick-wall architecture, whereas the polymers from gel-diffusion reactions resemble honeycomb networks. The solvothermal reaction of the smallest lanthanide cation Lu3+ with Ph2P(O)-CH2CH2-P(O)Ph2 led to a molecular compound instead of a polymer.