CuII Cluster Research Articles

Tetranuclear CuII Cluster as the Ten Node Building Unit for the Construction of a Metal-Organic Framework for Efficient C2 H2 /CO2 Separation.

Rational design of high nuclear copper cluster-based metal-organic frameworks has not been established yet. Herein, we report a novel MOF (FJU-112) with the ten-connected tetranuclear copper cluster [Cu4 (PO3 )2 (μ2 -H2 O)2 (CO2 )4 ] as the node which was capped by the deprotonated organic ligand of H4 L (3,5-Dicarboxyphenylphosphonic acid). With BPE (1,2-Bis(4-pyridyl)ethane) as the pore partitioner, the pore spaces in the structure of FJU-112 were divided into several smaller cages and smaller windows for efficient gas adsorption and separation. FJU-112 exhibits a high separation performance for the C2 H2 /CO2 separation, which were established by the temperature-dependent sorption isotherms and further confirmed by the lab-scale dynamic breakthrough experiments. The grand canonical Monte Carlo simulations (GCMC) studies show that its high C2 H2 /CO2 separation performance is contributed to the strong π-complexation interactions between the C2 H2 molecules and framework pore surfaces, leading to its more C2 H2 uptakes over CO2 molecules.

Copper(I)-Iodide Clusters as Carriers for Regulating and Visualizing Release of Aroma Molecules.

Achieving the controlled release of functional substances is indispensable in many aspects of life. Especially for the aroma molecules, their effective delivery of flavor and fragrance is challenging. Here, selected pyridines, as highly volatile odorants, were individually coordinated with copper(I) iodide (CuII) via a straightforward one-pot synthesis method, rapidly forming pure or even crystalline CuII cluster-based profragrances at room temperature. The obtained profragrances enabled the stable and high loading of volatile fragrances under ambient conditions and guaranteed their long-lasting release during heating. Furthermore, the intrinsic emission luminescence of these solid-state profragrances decayed along with the aroma release, which can serve as an additional indicator for monitoring the delivery process. This research sets a precedent for using CuII clusters as dual-purpose release agents and greatly expands their potential applications.

A new heterotrimetallic sandwich‐like CuII–LaIII–CuII (3d–4f–3d) cluster as a model anticancer drug in interaction with FS‐DNA and BSA and as a new artificial catalyst for catecholase activity

A new heterotrimetallic sandwich‐like CuII–LaIII–CuII cluster, [(valpn)2Cu2La(NO3)2]NO3(H2O)1.8, (H2valpn = 1,3‐propanediylbis(2‐iminomethylene‐6‐methoxy‐phenol)), was synthesized by the reaction of [Cu(valpn)] with La (NO3)3.6H2O (1:1 M ratio) at room temperature. The resulted cluster was characterized by X‐ray diffraction, UV–Vis and IR spectroscopy, and elemental analysis (CHN). The crystal structure of the cluster revealed that La(III) ion was sandwiched between two [Cu(valpn)] moieties and located in the open compartments of two copper Schiff base complexes and being surrounded by 12 oxygen atoms. This cluster complex, as a model anticancer drug, was investigated in interaction with fish sperm deoxyribonucleic acid (FS‐DNA) using docking study and experimental methods such as fluorescence and UV–Vis spectroscopy, circular dichroism spectroscopy (CD), cyclic voltammetry (CV), viscosity measurements, and agarose gel electrophoresis. These investigations suggested intercalative interaction between the cluster complex and FS‐DNA. Plasma proteins are other targets for the anticancer drugs, so the cluster–bovine serum albumin (BSA) interaction was also investigated. For evaluating the binding, Stern–Volmer constants (KSV) and the number of binding sites (n) were calculated. It was proposed that the electrostatic interaction was the main force in the cluster–BSA binding. Cytotoxic activity was assayed by MTT assay toward LACaP, MCF‐7, and HeLa cancer cell lines. The IC50 values of the cluster for HeLa and LACaP cell lines were lower than for MCF‐7 cell lines. The docking studies were also performed for cluster–BSA and cluster–DNA interactions. The cluster was also investigated as an artificial enzyme in catechol oxidase activity, and kinetics of the oxidation of 3,5‐di‐tert‐butylcatechol catalyzed by the cluster was studied.

Copper(i)-iodide cluster structures as functional and processable platform materials.

The combination of the copper(i)-iodide entity with organic ligands gives rise to a large variety of CuII polynuclear structures in the form of molecular complexes or extended structures. An appropriate selection of these components allows the preparation of materials showing interesting physicochemical properties and potential applications, mainly focused on organic light-emitting diodes and optical sensors. The most prominent physical feature of these materials is their emission, which can be modulated using the chemical structure and composition. This review article collates the advances in this research field, rationalizing the information into two main blocks as a function of the dimensionality of the structures: molecular complexes and extended networks. We describe the most typical ways of preparation, structures, and properties, with particular attention to the processability of the material as a fundamental aspect of the integration of the materials into real devices. Therefore, we aim to integrate the basic elements of the coordination chemistry of CuII clusters from the materials science perspective to envision this promising research field's potential technological future.

Self-assembled Cu(II) cluster from aerobic oxidation of Cu(I)Br with tris(triazolyl)methanol

Treatment of tris(4-benzyl-1,2,3-triazolyl)methanol (Htbtm) with CuIBr in refluxed CH2Cl2 under aerobic conditions afforded the hexanuclear CuII complexes with the formula Cu6(µ3-O⋯H⋯O-µ3)(µ-tbtm-к3-O,N3,N3′)3(µ-Br)6. The crystal structure reveals two Cu3(µ-Br)3(µ3-O) units linked by three deprotonated tbtm ligands and a proton. Magnetic studies of the hexanuclear CuII cluster show a strong overall antiferromagnetic behavior with a dominant coupling interaction between CuII⋯CuII ions of different Cu3(µBr)3(µ3-O) units.

A potassium-encapsulated Preyssler-based structure modified by binuclear cyclic copper(II) clusters: photocatalytic and electrochemical properties

Through the use of a flexible ligand 1,1′-bis(3,5-dimethyl-1H-pyrazolate)methane (H2bdpm), a K-encapsulated Preyssler-based complex, [Cu4(H2bdpm)4(H2O)10(H6KP5W30O110)]·2(H2bdpm)·14H2O, has been synthesized under hydrothermal conditions. Single crystal X-ray diffraction analysis shows that the complex contains rare K-encapsulated Preyssler anions, which are linked by binuclear cyclic clusters [Cu2(H2bdpm)2]4+ to form a 1D chain. Adjacent chains are further connected by abundant intermolecular forces to construct a 2D supramolecular framework. The photocatalytic and electrochemical properties of the complex have been investigated. (1) The Preyssler-based POM-TMCs are rarely reported. (2) The K-encapsulated Preyssler anion is uncommonly observed in Preyssler-based compound. (3) Preyssler anions are linked by binuclear CuII clusters to form a 1D chain. (4) Compound 1 exhibits photocatalytic and electrochemical properties.

Temperature-controlled formation of Anderson-type compounds and their conversion to [γ-Mo8O26]4−-based variants using pendent ligands

Abstract By tuning the reaction temperature, two Anderson- and two [γ-Mo8O26]4−-based compounds decorated by pendent organic ligands, [CuII 9(bpz)2(pz)2(H2O)24][H2(Cr(OH)5Mo6O19)4]·11H2O (1), [CuII(bpz)2(H2O)2(γ-H4Mo8O26)]·2H2O (2), [CuII 2(tea)2(H2O)6(HCr(OH)6Mo6O18)2]· 6H2O (3) and [AgI(bpz)(H2O)(γ-H4Mo8O26)0.5] (4) (bpz=4-butyl-1H-pyrazole, pz=1H-pyrazole, tea=2-[1,2,4]triazol-4-yl-ethylamine), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectra and elemental analyses. In compound 1, there are two kinds of tri-nuclear CuII clusters induced by bpz and pz ligands, respectively. Four Anderson-type anions are linked by these tri-nuclear clusters to form a “W”-type subunit. In compound 2, the [Cu(bpz)2(H2O)2]2+ subunits connect the γ-Mo8 anions to construct a chain. The remaining two non-coordinated N donors in [Cu(bpz)2(H2O)2]2+ further link two adjacent γ-Mo8 anions through Mo–N bonds. In compound 3, there exists a bi-nuclear CuII cluster [Cu2(tea)2(H2O)6]4+. The discrete bi-nuclear CuII clusters and the CrMo6 anions link each other through abundant hydrogen bonding interactions. In compound 4, the [Ag(bpz)(H2O)]+ subunits connect γ-Mo8 anions to build a zigzag chain. The chains are further fused by other [Ag(bpz)(H2O)]+ cations to form a grid-like layer. There still exist Mo–N bonds in 4. We also have investigated the electrochemical and photocatalytic properties of 1–4.

Zinc(II) and Copper(II) Metal–Organic Frameworks Constructed from a Terphenyl‐4,4′′‐dicarboxylic Acid Derivative: Synthesis, Structure, and Catalytic Application in the Cyanosilylation of Aldehydes

Solvothermal reaction of zinc(II) and copper(II) salts with 3,3′′‐dipropoxy‐[1,1′:4′,1′′‐terphenyl]‐4,4′′‐dicarboxylic acid (H2L) gave rise to the 1D and 2D metal–organic frameworks (MOFs) [Zn(L)(dmf)2]n (1), [Zn2(L)(NO3)2(4,4′‐bipyridine)2]n·n(DMF) (2), [Cu(L)(H2O)2]n·2n(H2O) (3), and [Cu3(OH)2(L)2]n·n(H2O) (4), which were characterized by elemental analysis, FTIR spectroscopy, and X‐ray single‐crystal diffraction. MOFs 1 and 3 have zigzag‐ and linear‐type 1D structures, respectively, whereas 2 and 4 possess rectangular grid‐ and layer‐type 2D structures. In 2 and in 4, a dinuclear ZnII cluster (C2O4Zn2) and a trinuclear µ3‐hydroxo‐bridged CuII cluster [Cu3(OH)2]n, respectively, act as building block units. These frameworks act as heterogeneous catalysts (the Zn‐MOFs are more active than Cu‐MOFs, and 1 is the most efficient) for the cyanosilylation of aldehydes with trimethylsilyl cyanide at room temperature. These MOF‐based heterogeneous catalysts can be easily recovered and reused, at least over a few consecutive cycles, without losing activity.

Two new POM-based compounds containing a linear tri-nuclear copper(II) cluster and an infinite copper(II) chain, respectively

Abstract Using the pendent organic ligand 1-butyl-1H-(1,3,4)triazole (btz), two new polyoxometalate (POM)-based compounds, [Cu3(btz)6(H2O)4(H3P2W18O62)2]·6H2O (1) and [Cu2(btz)(tz)(H2O)2(SiMo12O40)]·5H2O (2) (tz=1H-(1,3,4)triazole), have been synthesized under hydrothermal conditions. In compound 1, three CuII ions are fused by six btz ligands to form a linear tri-nuclear CuII cluster, with water as bridging molecules. Two Wells-Dawson anions offer two terminal O atoms to link two apical CuII atoms of this tri-nuclear cluster. In compound 2, the btz and tz molecules connect CuII ions to construct an infinite CuII chain still with water molecules as bridges. The adjacent chains are linked by one type of POM anions to build a 2D grid-like layer. Considering the long-range coordinative Cu–O bonds, the other types of POM anions connect adjacent layers to form a 3D framework. The electrochemical and photocatalytic properties of compounds 1 and 2 were also studied.

Synthesis, properties and surface self-assembly of a pentanuclear cluster based on the new π-conjugated TTF-triazole ligand

The new π-extended redox-active ligand with both TTF and triazole units, 6-(4,5-bis(propylthio)-1,3-dithiol-2-ylidene)-1H-[1,3]dithiolo[4′,5′:4,5]benzo [1,2-d] [1–3]triazole, has been successfully prepared. Based on the versatile ligand and Cu(tta)2 precursors (tta− = 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-dione), a TTF-based pentanuclear CuII cluster (Cu5(tta)4(TTFN3)6) is synthesized and structurally characterized. Their absorption and electrochemical properties are investigated. Antiferromagnetic couplings are operative between metal ion centers bridged by triazoles in the complex. The self-assembled structure of the cluster complex on a highly oriented pyrolytic graphite (HOPG) surface was observed using scanning tunneling microscopy and density functional theory (DFT) calculations have been performed to provide insight into the formation mechanism. The introduction of the redox-active TTF unit into the cluster complexes with interesting magnetic properties renders them promising candidates for new multifunctional materials.

2D Compound Constructed by Cu3 Units with Mixed Azido and Tetrazole Double Bridges: Synthesis, Structure, and Theoretical Study on Magnetic Properties

AbstractThe two‐dimensional (2D) layer CuII compound [Cu3(L)2(N3)4] (1) [L = 2‐amino‐3‐(5‐tetrazole)‐methyate‐N‐pyridine] was synthesized by in‐situ hydrothermal reaction of CuCl2·2H2O, NaN3, and 3‐(5‐tetrazole)‐methyate‐N‐pyridine. The central Cu1 and Cu2 atoms are located in five‐coordinate and six‐coordinate arrangements, respectively. Three CuII ions are linked by mixed double EO (end‐on)‐azido‐tetrazole bridges to give trinuclear CuII clusters, which are further extended by EE (end‐to‐end) azido bridges to form 2D metal‐organic layers. The magnetic exchange interactions in complex 1 were investigated by DFT calculations, and the calculated exchange interaction (J = –849 cm–1) revealed that the double EO‐azido‐tetrazole bridges transmit antiferromagnetic coupling between CuII ions.

Interaction of the Trinuclear Triangular Secondary Building Unit [Cu3(μ3-OH)(μ-pz)3]2+ with 4,4′-Bipyridine. Structural Characterizations of New Coordination Polymers and Hexanuclear CuII Clusters. 2°

By reacting 4,4′-bipyridine (bpy) with selected trinuclear triangular CuII complexes, [Cu3(μ3-OH)(μ-pz)3(RCOO)2(LL′)] [pz = pyrazolate anion; R = CH3, CH3CH2, CH2═CH, CH2═C(CH3); L, L′ = Hpz, H2O, MeOH] in MeOH, the substitution of monotopic ligands by ditopic bpy was observed. Depending on the stoichiometric reaction ratios, different compounds were isolated and structurally characterized. One- and two-dimensional coordination polymers (CPs), as well as two hexanuclear CuII clusters were identified. One of the hexanuclear clusters self-assembles into a supramolecular three-dimensional structure, and its crystal packing shows the presence of two intersecting channels, one of which is almost completely occupied by guest bpy, while in the second one guest water molecules are present. This compound also shows a reversible, thermally induced, single-crystal-to-single-crystal transition.

Influence of N-donor sites in 5-(x-pyridyl)-1H-tetrazole ligands (x = 2, 4) on assembly of polyoxometalate-based compounds modified by multinuclear metal clusters and infinite chains

Through tuning the N-donor site of the pyridyl group in 5-(x-pyridyl)-1H-tetrazole ligands (x = 2, 4), four new Keggin-based compounds, [Cu7(2-ptz)8(OH)2(H2O)2(HPMoVI10MoV2O40)]·4H2O (1), [Cu5(2-ptz)6(HPMoVI10MoV2O40)(H2O)4]·4H2O (2), [Ag6(4-ptz)4(H2SiMo12O40)] (3) and [Ag5(4-ptz)4(H2PMo12O40)]·5H2O (4) (2-ptzH = 5-(2-pyridyl)-1H-tetrazole, 4-ptzH = 5-(4-pyridyl)-1H-tetrazole), were successfully synthesized under hydrothermal conditions and structurally characterized. Compound 1 possesses two kinds of CuII subunits: penta-nuclear clusters and mono-nuclear subunits, which connect with each other to form a one dimensional (1D) stair-like metal–organic chain. The adjacent 1D stair-like chains are further linked by tetradentate Keggin anions to construct a 2D layer. In compound 2, the tetra-nuclear CuII clusters and mono-nuclear CuII subunits construct a 2D metal–organic layer, which is further linked by bidentate Keggin anions to build a 3D framework. In compound 3, the 4-ptz ligand links AgI ions with tetrazole groups to form 1D infinite Ag–ptz chains, which are extended by the pyridyl group of 4-ptz and the tetrazole groups of other 4-ptz molecules to construct a 2D layer. The adjacent 2D layers are further linked by 6-connected Keggin anions to build a 3D framework. In compound 4, the 4-ptz ligands are linked by AgI ions to form 1D infinite Ag–ptz belts. The Keggin anions are linked by AgI ions to construct another infinite inorganic chain, in which each Keggin anion provides six oxygen atoms to coordinate with the AgI ions from adjacent Ag–ptz belts to construct a 2D layer. The roles of the ptz ligands with different N-donor sites in the construction of multinuclear clusters and 1D chains are discussed. The electrochemical and photocatalytic properties of the title compounds were investigated as well.

Mild, green copper/4-dimethylaminopyridine catalysed aerobic oxidation of alcohols mediated by nitroxyl radicals in water

A novel copper/4-dimethylaminopyridine catalyst system has been found to catalyse the aerobic oxidation of a variety of primary and secondary alcohols in water at room temperature, in the presence of a TEMPO or ABNO nitroxyl radical. A tetranuclear CuII cluster was revealed to be a plausible reactive intermediate.

Assembly and Catalytic Properties of Cobalt(II) and Copper(II) Coordination Polymers Based on Bis(imidazole) and Carboxylic Acid Mixed Ligands

AbstractTwo CoII/CuII coordination polymers, [Co(bim)(ip)]n (1) and [Cu3(nip)2(OH)(bim)(H2O)]n (2) [bim = bis(imidazol‐1‐yl)methane, H2ip = isophthalic acid and H2nip = 5‐nitroisophthalic acid], were synthesized by reactions of metal acetate with H2ip or H2nip in the presence of bim ligand under hydrothermal conditions. The polymers were characterized by elemental analyses, IR spectroscopy, TGA, XRPD, and single‐crystal X‐ray diffraction. Compound 1 shows a 2D (4, 4) layer structure with CoII ions at the nodes, which is further extended in to a 3D supramolecular network through C–H···π interactions. Compound 2 manifests a four‐connected {Cu5(OH)2}8+ pentameric kernel, which is bridged by bim ligand molecules and two kinds of nip ligands with different coordination modes. This complex also displays a 2D (4, 4) network, which is constructed from pentanuclear CuII clusters as secondary building unit (SBU). In addition, the catalytic properties of the title compounds for the degradation of congo red were investigated.

Oxacalix[4]arene-supported di-, tetra- and undecanuclear copper(ii) clusters

Oxacalix[4]arenes containing either one or two oxa-bridges afford di and undecanuclear Cu(II) clusters respectively upon reaction with cupric nitrate under facile conditions. Variation in reaction conditions results in formation of a tetranuclear Cu(II) cluster with mono-oxacalix[4]arene, representing a structural expansion of the dinuclear assembly.

Two pH‐Dependent 1D Copper(II) Coordination Polymers based on a Bis‐triazole‐bis‐amide Ligand: Syntheses, Structures, and Properties

AbstractTwo bis‐triazole‐bis‐amide‐based copper(II) coordination polymers, [Cu4(μ3‐OH)2(dtb)(DNBA)6(H2O)2]·2H2O (1) and Cu(dtb)(DNBA)2 (2) [dtb = N,N′‐bis(4H‐1, 2, 4‐triazole)butanamide and HDNBA = 3, 5‐dinitrobenzoic acid], were synthesized and structurally characterized. Polymer 1 shows a one‐dimensional (1D) chain based on tetranuclear CuII cluster and the dtb ligands, whereas polymer 2 exhibits a 1D chain constructed from CuII ions and the dtb ligands. The pH values have a crucial influence on the formation of 1 and 2. In addition, the electrochemical and luminescent properties of 1 and 2 as well as magnetic properties of 1 were investigated.

New Coordination Polymers and Porous Supramolecular Metal Organic Network Based on the Trinuclear Triangular Secondary Building Unit [Cu3(μ3-OH)(μ-pz)3]2+ and 4,4′-Bypiridine. 1°

The trinuclear triangular CuII complex [Cu3(μ3-OH)(μ-pz)3(HCOO)2(Hpz)2] (Hpz = pyrazole) reacts with 4,4′-bipyridine (bpy) yielding a two-dimensional (2D) waved sheets, two three-dimensional (3D) coordination polymers (CPs), as well as a hexanuclear CuII cluster, depending on the reagent ratios and reaction conditions. Single crystal X-ray diffraction (XRD) determinations point out that, while CPs crystal structures are not porous, the hexanuclear CuII clusters are packed in the solid state generating a stable porous 3D supramolecular network, where two kinds of perpendicular, hydrophobic channels (ca. 4.83 × 5.86 Å2 and 4.99 × 4.79 Å2, corresponding to the 24.7% of the total crystal volume) are present. In the “as-synthesized” compound, channels of one kind are empty, while the other ones are occupied by guest bpy molecules which can be removed by soaking the crystals in suitable solvents (benzene, toluene, c-hexane) maintaining intact the crystal skeleton. Moreover, two of the above complexes act as catalysts (or catalyst precursors) in the peroxidative oxidation of cyclohexane.

Single‐Strand Molecular Wheels and Coordination Polymers in Copper(II) Benzoate Chemistry by the Employment of α‐Benzoin Oxime and Azides: Synthesis, Structures, and Magnetic Characterization

AbstractThe use of α‐benzoin oxime (bzoxH2) in copper(II) benzoate chemistry, in the absence or presence of ancillary azido ligands, is reported. The reaction of Cu(O2CPh)2·2H2O with one equivalent of bzoxH2 in N,N‐dimethylformamide (DMF) affords the decanuclear complex [Cu10(bzox)10(DMF)4] (1) in good yield. Dissolution of 1 in CH2Cl2 leads to the subsequent isolation of the solvent‐free complex [Cu10(bzox)10] (2) in moderate yields. Complexes 1 and 2 are isostructural and possess a loop or single‐strand molecular wheel topology. The bzox2– dianions behave as η1:η1:η2:μ3 ligands, which give rise to an overall [Cu10(μ‐ONR)10(μ‐OR′)10] core. Both 1 and 2 stack to form nanotubular columns with beautiful supramolecular architectures. The reaction of Cu(O2CPh)2·2H2O with bzoxH2 and NaN3 in a 1:1:1 molar ratio in MeOH gives the bzoxH2‐free complex [Cu(N3)(O2CPh)(MeOH)]n (3), which is a 1D chain. The CuII atoms in 3 are linked by a single, end‐on N3– group, a syn,syn‐η1:η1:μ PhCO2– ion, and an oxygen atom from the bridging MeOH ligand. The 1D chains are hydrogen bonded into 2D sheets through Nazide···H(OMeOH) interactions. Variable‐temperature, solid‐state direct‐current magnetic studies were carried out on 1–3. The data for 1 and 2 indicate very strong antiferromagnetic exchange interactions and a S = 0 ground state, which is expected for even‐membered loop arrays of CuII atoms. In contrast, 3 exhibits ferromagnetic exchange interactions; the data were fitted to the appropriate equation derived from the Hamiltonian H = –JΣ(Si·Si+1), which includes a zJ′ interchain interaction term. The best‐fit parameters were J = +49.6(4) cm–1, g = 2.067(3), and zJ′ = 2.3(1) K. The combined results demonstrate the ligating flexibility of both the bzoxH2 and azido groups and their usefulness in the synthesis of polynuclear CuII clusters and coordination polymers.

An “S”-shaped pentanuclear CuII cluster derived from the metal-assisted hydrolysis of pyCOpyCOpy: structural, magnetic and spectroscopic studies

Reaction of [Cu2(O2CMe)4(H2O)2] with 2,6-di-(2-pyridylcarbonyl)-pyridine (pyCOpyCOpy or dpcp) in MeCN-H2O 10:1, led to the pentanuclear copper(II) complex [Cu5(O2CMe)6{pyC(O)(OH)pyC(O)(OH)py}2] () which crystallizes in the triclinic P1 space group. The copper(II) atoms are arranged in an "S"-shaped configuration, and are bridged by the doubly deprotonated bis(gem-diol) form of the ligand, pyC(O)(OH)pyC(O)(OH)py2-. Magnetic susceptibility data indicate the interplay of both ferro- and antiferromagnetic intramolecular interactions stabilizing an S=3/2 ground state. Fitting of the data according to a next-nearest-neighbour model {H=-[J1(S1S2+S1'S2')+J2(S2S3+S3'S2')+J3(S1S3+S3'S1')+J4(S2S2')]} yields exchange coupling constants J1=+39.7 cm(-1), J2=-15.9 cm(-1), J3=-8.3 cm(-1) and J4=+4.3 cm(-1), leading to an S=3/2 ground state. X-Band EPR spectroscopy indicates a zero-field splitting of the ground state with |D3/2|=0.38 cm(-1).