Cobalt(III)‐Mediated Permanent and Stable Immobilization of Histidine‐Tagged Proteins on NTA‐Functionalized Surfaces

We present the cobalt(III)‐mediated interaction between polyhistidine (His)‐tagged proteins and nitrilotriacetic acid (NTA)‐modified surfaces as a general approach for a permanent, oriented, and specific protein immobilization. In this approach, we first form the well‐established Co²⁺‐mediated interaction between NTA and His‐tagged proteins and subsequently oxidize the Co²⁺ center in the complex to Co³⁺. Unlike conventionally used Ni²⁺‐ or Co²⁺‐mediated immobilization, the resulting Co³⁺‐mediated immobilization is resistant toward strong ligands, such as imidazole and ethylenediaminetetraacetic acid (EDTA), and washing off over time because of the high thermodynamic and kinetic stability of the Co³⁺ complex. This immobilization method is compatible with a wide variety of surface coatings, including silane self‐assembled monolayers (SAMs) on glass, thiol SAMs on gold surfaces, and supported lipid bilayers. Furthermore, once the cobalt center has been oxidized, it becomes inert toward reducing agents, specific and unspecific interactions, so that it can be used to orthogonally functionalize surfaces with multiple proteins. Overall, the large number of available His‐tagged proteins and materials with NTA groups make the Co³⁺‐mediated interaction an attractive and widely applicable platform for protein immobilization.     

Redox non-innocence of thioether crowns: spectroelectrochemistry and electronic structure of formal nickel(III) complexes of aza-thioether macrocycles

The Ni^II complexes [Ni([9]aneNS₂‐CH₃)₂]²⁺ ([9]aneNS₂‐CH₃=N‐methyl‐1‐aza‐4,7‐dithiacyclononane), [Ni(bis[9]aneNS₂‐C₂H₄)]²⁺ (bis[9]aneNS₂‐C₂H₄=1,2‐bis‐(1‐aza‐4,7‐dithiacyclononylethane) and [Ni([9]aneS₃)₂]²⁺ ([9]aneS₃=1,4,7‐trithiacyclononane) have been prepared and can be electrochemically and chemically oxidized to give the formal Ni^III products, which have been characterized by X‐ray crystallography, UV/Vis and multi‐frequency EPR spectroscopy. The single‐crystal X‐ray structure of [Ni^III([9]aneNS₂‐CH₃)₂](ClO₄)₆?(H₅O₂)₃ reveals an octahedral co‐ordination at the Ni centre, while the crystal structure of [Ni^III(bis[9]aneNS₂‐C₂H₄)](ClO₄)₆?(H₃O)₃? 3H₂O exhibits a more distorted co‐ordination. In the homoleptic analogue, [Ni^III([9]aneS₃)₂](ClO₄)₃, structurally characterized at 30 K, the Ni? S distances [2.249(6), 2.251(5) and 2.437(2) Å] are consistent with a Jahn–Teller distorted octahedral stereochemistry. [Ni([9]aneNS₂‐CH₃)₂](PF₆)₂ shows a one‐electron oxidation process in MeCN (0.2 M NBu₄PF₆, 293 K) at E_1/2=+1.10 V versus Fc⁺/Fc assigned to a formal Ni^III/Ni^II couple. [Ni(bis[9]aneNS₂‐C₂H₄)](PF₆)₂ exhibits a one‐electron oxidation process at E_1/2=+0.98 V and a reduction process at E_1/2=?1.25 V assigned to Ni^II/Ni^III and Ni^II/Ni^I couples, respectively. The multi‐frequency X‐, L‐, S‐, K‐band EPR spectra of the 3+ cations and their 86.2 % ⁶¹Ni‐enriched analogues were simulated. Treatment of the spin Hamiltonian parameters by perturbation theory reveals that the SOMO has 50.6 %, 42.8 % and 37.2 % Ni character in [Ni([9]aneNS₂‐CH₃)₂]³⁺, [Ni(bis[9]aneNS₂‐C₂H₄)]³⁺ and [Ni([9]aneS₃)₂]³⁺, respectively, consistent with DFT calculations, and reflecting delocalisation of charge onto the S‐thioether centres. EPR spectra for [⁶¹Ni([9]aneS₃)₂]³⁺ are consistent with a dynamic Jahn–Teller distortion in this compound.     

A novel two‐dimensional nickel(II) coordination polymer with 1,4‐bis(1,2,4‐triazol‐1‐ylmethyl)benzene and azide ligands

The coordination geometry of the Ni^II atom in the title complex, poly[diazidobis[μ‐1,4‐bis(1,2,4‐triazol‐1‐ylmethyl)benzene‐κ²N⁴:N^4′]nickel(II)], [Ni(N₃)₂(C₁₂H₁₂N₆)₂]_n, is a distorted octahedron, in which the Ni^II atom lies on an inversion centre and is coordinated by four N atoms from the triazole rings of two symmetry‐related pairs of 1,4‐bis(1,2,4‐triazol‐1‐ylmethyl)benzene (bbtz) ligands and two N atoms from two symmetry‐related monodentate azide ligands. The Ni^II atoms are bridged by four bbtz ligands to form a two‐dimensional (4,4)‐network.     

Effect of Heavy‐Atom (Br) at the Phenyl Rings of Schiff‐Base Ligands on the NIR Luminescence of their Bimetallic Zn‐Nd Complexes

Two heterobimetallic Zn‐Nd phenylene‐bridged Schiff‐base ligands complexes [ZnNd L¹ (Py)(NO₃)₃] ( 1 ) and [Zn L² Nd(Py)(NO₃)₃]·MeCN ( 2 ) (Py = pyridine, H₂L¹ = N,N′‐bis‐ (3‐methoxy‐salicylidene)phenylene‐1,2‐diamine, H₂L² = N,N′‐bis‐5‐bromo‐3‐methoxy‐salicylidene)phenylene‐1,2‐diamine) were obtained. Both 1 and 2 were structurally characterized by X‐ray crystallography, and their near‐infrared (NIR) luminescent properties were determined. For the two complexes, the occupation of pyridine at the axial position of 3d Zn²⁺ ions could effectively prevent luminescent quenching arising from OH‐, NH‐ or CH oscillators of the solvates around the 4f Nd³⁺ ions, and the heavy‐atom (Br) effect of the Schiff‐base ligands on their NIR luminescent properties is also discussed.     

A novel octanuclear nickel(II)–molybdenum(VI) heterometallic cluster based on the salicylhydroxamate ligand

Salicylhydroxamic acid (H₃shi) is known for its strong coordination ability and multiple coordination modes, and can easily coordinate to metal cations to form compounds with five‐ or six‐membered rings, as well as mono‐, di‐ and multinuclear compounds with interesting structures having potential applications in organic chemistry, coordination chemistry, and the materials and biological sciences. A novel octanuclear nickel(II)–molybdenum(VI) heterometallic cluster based on the salicylhydroxamate ligand, namely di‐μ₃‐acetato‐di‐μ₂‐acetato‐di‐μ₃‐hydroxido‐di‐μ₃‐oxido‐tetraoxidooctakis(pyridine‐κN)bis(μ₅‐salicylhydroxamato)hexanickel(II)dimolybdenum(VI) monohydrate, [Mo₂Ni₆(C₇H₄NO₃)₂(C₂H₃O₂)₄O₅(OH)₂(C₅H₅N)₈]·H₂O, (I), was synthesized by the reaction of sodium molybdate, nickel acetate and salicylhydroxamic acid in a dimethylformamide/pyridine/methanol solution at room temperature. The salicylhydroxamate(3−) (shi³⁻), acetate and oxide ligands adopt complicated coordination modes and link six Ni^II and two Mo^VI cations into the octanuclear heterometallic cluster. All of the metal cations exhibit octahedral coordination geometries and are connected to each other through the sharing of corners, edges or planes. The heterometallic clusters are further connected to form two‐dimensional supramolecular layers through weak C—H…O hydrogen bonds. Studies of the magnetic properties of the title compound reveal antiferromagnetic interactions between the Ni^II cations.     

Metal Ions Drive Thermodynamics and Photochemistry of the Bis(styryl) Macrocyclic Tweezer

UV/Vis and NMR spectroscopy were used for the structural elucidation and thermodynamic and photochemical studies of the metal‐coordinated crown‐containing macrocyclic tweezer (E,E)‐ 1 . The bis(styryl) tweezer (E,E)‐ 1 formed two types of complexes with magnesium(II): a 1:1 intramolecular asymmetric sandwich complex [(E,E)‐ 1 ]?Mg²⁺ and a 1:2 complex [(E,E)‐ 1 ]?(Mg²⁺)₂. In the former case, there is direct cation intramolecular exchange (0.299 s^?1, ΔG^≠=69.4 kJ mol^?1) between two parts of the bis(styryl) tweezer (E,E)‐ 1 . Addition of barium(II) to the bis(styryl) tweezer (E,E)‐ 1 led to an intramolecular centrosymmetric sandwich 1:1 complex [(E,E)‐ 1 ]?Ba²⁺. Irradiation of [(E,E)‐ 1 ]?Ba²⁺ afforded reversible intramolecular [2π+2π] photocyclization with excellent stereoselectivity and quantitative yield. In contrast, irradiation of [(E,E)‐ 1 ]?(Mg²⁺)₂ resulted in reversible stepwise E,Z‐isomerization.     

Rational Construction of Polymeric Cadmium(II) Benzimidazole for Transition Metal Ion Exchange

A 1D double‐helical coordination polymer {[Cd(pbbm)₂]₂(ClO₄)₄(H₂O)₂}_n ( 1 ) was successfully constructed by the reaction of Cd(ClO₄)₂ · 6H₂O with 1,1′‐(1,5‐pentanediyl)bis‐1H‐benzimidazole (pbbm). Interestingly, polymer 1 exhibits highly selective capacity for the ionic exchange of Zn²⁺ and Cu²⁺ over Co²⁺ and Ni²⁺ ions in the crystalline solid state when the crystals of 1 are immersed in the aqueous solutions of the perchlorate salts of Cu²⁺, Zn²⁺, Co²⁺, and Ni²⁺ ions, respectively, which indicates that central Cd^II ion exchange might be considered as being dominated by the coordination ability of metal ions to free functional groups, ionic radii of exchanged metal ions, and the solution concentration of adsorbed metal salts. The parent material‐ and ion‐exchange‐induced products are identified by FT‐IR spectroscopy, PXRD patterns as well as SEM and EDS measurements. In addition, the thermal stability of 1 was also investigated.     

Bis(1,5‐di­aza­cyclo­octane‐N,N′)nickel(II) dibromide

The crystal structure of the title complex, [Ni(C₆H₁₄N₂)₂]Br₂, consists of discrete [Ni(C₆H₁₄N₂)₂]²⁺ cations and bromide counter‐anions. The Ni^II ion is at the center of symmetry and is four‐coordinated by four nitro­gen donors of the mesocyclic ligand 1,5‐di­aza­cyclo­octane (DACO) [Ni—N 1.935 (2)–1.937 (2) Å]. The coordination geometry of Ni^II can be considered as square planar and both DACO ligands take the boat–chair conformation. The bromide anions are hydrogen bonded with the nitro­gen donors of the ligands to form a macrocycle‐like ring system.     

Reversible Immobilization of Peptides: Surface Modification and In Situ Detection by Attenuated Total Reflection FTIR Spectroscopy

A generic method is described for the reversible immobilization of polyhistidine‐bearing polypeptides and proteins on attenuated total reflecting (ATR) sensor surfaces for the detection of biomolecular interactions by FTIR spectroscopy. Nitrilotriacetic acid (NTA) groups are covalently attached to self‐assembled monolayers of either thioalkanes on gold films or mercaptosilanes on silicon dioxide films deposited on germanium internal reflection elements. Complex formation between Ni²⁺ ions and NTA groups activates the ATR sensor surface for the selective binding of polyhistidine sequences. This approach not only allows a stable and reversible immobilization of histidine‐tagged peptides (His–peptides) but also simultaneously allows the direct in situ quantification of surface‐adsorbed molecules from their specific FTIR spectral bands. The surface concentrations of both NTA and His–peptide on silanized surfaces were determined to be 1.1 and 0.4 molecules nm^?2, respectively, which means that the surface is densely covered. A comparison of experimental FTIR spectra with simulated spectra reveals a surface‐enhancement effect of one order of magnitude for the gold surfaces. With the presented sensor surfaces, new ways are opened up to investigate, in situ and with high sensitivity and reproducibility, protein–ligand, protein–protein, protein–DNA interactions, and DNA hybridization by ATR–FTIR spectroscopy.     

Poly[[bis{μ2‐N2,N6‐bis[(pyridin‐3‐yl)methyl]pyridine‐2,6‐dicarboxamide}dichloridonickel(II)] N,N‐dimethylformamide tetrasolvate]

In the title compound, {[NiCl₂(C₁₉H₁₇N₅O₂)₂]·4C₃H₇NO}_n, the Ni^II atom is located on an inversion centre and is in a six‐coordinated octahedral geometry, formed by four pyridine N atoms from four N²,N⁶‐bis[(pyridin‐3‐yl)methyl]pyridine‐2,6‐dicarboxamide (BPDA) ligands occupying the equatorial plane and two chloride anions at the axial sites. The bidentate bridging BPDA ligands link the Ni^II atoms into a two‐dimensional corrugated grid‐like flexible layer with a (4,4)‐connected topology, which consists of left‐ and right‐handed helical chains sharing the common Ni^II atoms. Investigation of the thermal stability shows that the network is stable up to 573 K.     

A New Domain of Reactivity for High‐Valent Dinuclear [M(μ‐O)2M′] Complexes in Oxidation Reactions

The strikingly different reactivity of a series of homo‐ and heterodinuclear [(M^III)(μ‐O)₂(M^III)′]²⁺ (M=Ni; M′=Fe, Co, Ni and M=M′=Co) complexes with β‐diketiminate ligands in electrophilic and nucleophilic oxidation reactions is reported, and can be correlated to the spectroscopic features of the [(M^III)(μ‐O)₂(M^III)′]²⁺ core. In particular, the unprecedented nucleophilic reactivity of the symmetric [Ni^III(μ‐O)₂Ni^III]²⁺ complex and the decay of the asymmetric [Ni^III(μ‐O)₂Co^III]²⁺ core through aromatic hydroxylation reactions represent a new domain for high‐valent bis(μ‐oxido)dimetal reactivity.     

Three transition‐metal complexes with the macrocyclic ligand meso‐5,7,7,12,14,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane (L): [Cu(ClO4)2(L)], [Zn(NO3)2(L)] and [CuCl(L)(H2O)]Cl

The three transition‐metal complexes, (meso‐5,7,7,12,14,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane‐κ⁴N)bis(perchlorato‐κO)copper(II), [Cu(ClO₄)₂(C₁₈H₄₀N₄)], (I), (meso‐5,7,7,12,14,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane‐κ⁴N)bis(nitrato‐κO)zinc(II), [Zn(NO₃)₂(C₁₈H₄₀N₄)], (II), and aquachlorido(meso‐5,7,7,12,14,14‐hexamethyl‐1,4,8,11‐tetraazacyclotetradecane‐κ⁴N)copper(II) chloride, [CuCl(C₁₈H₄₀N₄)(H₂O)]Cl, (III), are described. The molecules display a very similarly distorted 4+2 octahedral environment for the cation [located at an inversion centre in (I) and (II)], defined by the macrocycle N₄ group in the equatorial sites and two further ligands in trans‐axial positions [two O–ClO₃ ligands in (I), two O–NO₂ ligands in (II) and one chloride and one aqua ligand in (III)]. The most significant difference in molecular shape resides in these axial ligands, the effect of which on the intra‐ and intermolecular hydrogen bonding is discussed. In the case of (I), all strong hydrogen‐bond donors are saturated in intramolecular interactions, while weak intermolecular C—H...O contacts result in a three‐dimensional network. In (II) and (III), instead, there are N—H and O—H donors left over for intermolecular interactions, giving rise to the formation of strongly linked but weakly interacting chains.     

Three different bonding modes of cyano groups in the coordination polymer [Cu(en)2(H2O)][Cu(en)2Ni2Cu2(CN)10]·2H2O (en is 1,2‐diaminoethane)

Partial reduction of the Cu^II ions in the aqueous system Cu^II–en–[Ni(CN)₄]^2? (1/1/1) (en is 1,2‐di­amino­ethane) yields a novel heterobimetallic mixed‐valence compound, poly­[[aqua­bis(1,2‐di­amino­ethane)copper(II)] [hexa‐μ‐cyano‐tetra­cyano­bis(1,2‐di­amino­ethane)­tricopper(I,II)­dinickel(II)] dihydrate], [Cu(C₂H₈N₂)₂(H₂O)][Ni₂Cu₃(CN)₁₀(C₂H₈N₂)₂]·2H₂O or [Cu(en)₂(H₂O)][Cu(en)₂Ni₂Cu₂(CN)₁₀]·2H₂O. The structure is formed by a negatively charged two‐dimensional array of the cyano complex [Cu(en)₂Ni₂Cu₂(CN)₁₀]_n^2n?, [Cu(en)₂(H₂O)]²⁺ complex cations and water mol­ecules of crystallization. These last are involved in a complicated hydrogen‐bonding system. The cyano groups act as terminal, μ₂‐bridging or μ₃‐bridging ligands.     

Synthesis of silica nanospheres with Ni2+-iminodiacetic acid for protein separation

Silica (SiO₂) nanospheres (NSs) with immobilized metal ligands have been prepared for the affinity separation of proteins. First, SiO₂ NSs were prepared by controlled hydrolysis of tetraethoxysilane in a basic aqueous-ethanol solution. Then through reaction of iminodiacetic acid (IDA) with 3-glycidoxypropyltrimethoxysilane and immobilization of them onto the surfaces of above SiO₂ NSs, novel affinity adsorbents with IDA chelating groups were obtained. After chelating Ni²⁺ ions, the SiO₂–IDA–Ni²⁺ NSs were applied to separate his-tagged proteins directly from the mixture of lysed cells. The SiO₂–IDA–Ni²⁺ NSs present negligible nonspecific protein adsorption and high protein binding ability (28.3 mg/g).     

Bis(trimethylsilyl)diazene revisited. Density functional theory (DFT) calculations of nitrogen NMR parameters of some azo‐compounds

Calculations of nitrogen NMR parameters [chemical shifts δN and indirect nuclear spin–spin coupling constants J(N,N), J(N,¹³C), J(²⁹Si,N)] of noncyclic azo‐compounds R¹ NN R² (R¹, R² = H, Me, Ph, SiH₃, SiMe₃) and cyclic azo‐compounds [NNCH₂, NN(CH₂)₃ NN(CH₂)₂SiH₂, and NN(SiH₂CH₂SiH₂)] by density functional theory (DFT) methods [B3LYP/6‐311+G(d,p) level of theory] provide data in reasonable agreement with experimental values. The influence of cis‐ and trans‐geometry is reflected by the calculations, and amino‐nitrenes are also included for comparison. The spin–spin coupling constants are analyzed with respect to contact (Fermi contact term, FC) and non‐ contact contributions (paramagnetic and diamagnetic spin‐orbital terms, PSO and DSO, and spin‐dipole term, SD). Bis(trimethylsilyl)diazene 6a can be generated by an alternative method, using the reaction of bis(trimethylsilyl)sulfur diimide with bis‐ (trimethylsilyl)amino‐trimethylsilylimino‐phosphane. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:84–91, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20075     

Synthesis,Structure and Magnetic Properties of a Novel Nickel(II) Radical Heterospin Complex with Salicylaldoxime

A novel heterospin complex containing both Ni^II and nitroxide radical ligands: [Ni(salox)₂(NIT4Py)₂] ( 1 ) (salox = salicylaldoxime, NIT4Py = 2‐(4′‐pyridyl)‐4,4,5,5‐ tetramethyl‐imidazoline‐1‐oxyl‐3‐oxide) has been synthesized and structurally characterized. The structure consists of neutral Ni(salox)₂(NIT4Py)₂ moieties bridged by intermolecular hydrogen bonds, forming a one‐dimensional chain structure. Magnetic measurements show intramolecular antiferromagnetic interactions between NIT4Py and Ni²⁺ ion.     

Nickel as a Lewis Base in a T‐Shaped Nickel(0) Germylene Complex Incorporating a Flexible Bis(NHC) Ligand

Flexible, chelating bis(NHC) ligand 2 , able to accommodate both cis‐ and trans‐coordination modes, was used to synthesize ( 2 )Ni(η²‐cod), 3 . In reaction with GeCl₂, it produced ( 2 )NiGeCl₂, 4 , featuring a T‐shaped Ni⁰ and a pyramidal Ge center. Complex 4 could also be prepared from [( 2 )GeCl]Cl, 5 , and Ni(cod)₂, in a reaction that formally involved Ni–Ge transmetalation, followed by coordination of the extruded GeCl₂ moiety to Ni. A computational analysis showed that 4 possesses considerable multiconfigurational character and the Ni→Ge bond is formed through σ‐donation from the Ni 4s, 4p, and 3d orbitals to Ge. (NHC)₂Ni(cod) complexes 9 and 10 , as well as (NHC)₂GeCl₂ derivative 11 , incorporating ligands that cannot accommodate a wide bite angle, failed to produce isolable Ni–Ge complexes. The isolation of ( 2 )Ni(η²‐Py), 12 , provides further evidence for the reluctance of the ( 2 )Ni⁰ fragment to act as a σ‐Lewis acid.     

A comparative study of the packing of two polymorphs of the nickel(II) pincer complex [2,6‐bis(di‐tert‐butylphosphinoyl)‐4‐(3,5‐dinitrobenzoyloxy)phenyl‐κ3P,C1,P′]chloridonickel(II)

Pincer complexes can act as catalysts in organic transformations and have potential applications in materials, medicine and biology. They exhibit robust structures and high thermal stability attributed to the tridentate coordination of the pincer ligands and the strong σ metal–carbon bond. Nickel derivatives of these ligands have shown high catalytic activities in cross‐coupling reactions and other industrially relevant transformations. This work reports the crystal structures of two polymorphs of the title Ni^II POCOP pincer complex, [Ni(C₂₉H₄₁N₂O₈P₂)Cl] or [NiCl{C₆H₂‐4‐[OCOC₆H₄‐3,5‐(NO₂)₂]‐2,6‐(OP^tBu₂)₂}]. Both pincer structures exhibit the Ni^II atom in a distorted square‐planar coordination geometry with the POCOP pincer ligand coordinated in a typical tridentate manner via the two P atoms and one arene C atom via a C—Ni σ bond, giving rise to two five‐membered chelate rings. The coordination sphere of the Ni^II centre is completed by a chloride ligand. The asymmetric units of both polymorphs consist of one molecule of the pincer complex. In the first polymorph, the arene rings are nearly coplanar, with a dihedral angle between the mean planes of 27.9 (1)°, while in the second polymorph, this angle is 82.64 (1)°, which shows that the arene rings are almost perpendicular to one another. The supramolecular structure is directed by the presence of weak C—H…O=X (X = C or N) interactions, forming two‐ and three‐dimensional chain arrangements.     

First‐row transition metal–pyridine (py)–sulfate [(py)xM](SO4) complexes (M = Ni,Cu and Zn): crystal field theory in action

The crystal structures of three first‐row transition metal–pyridine–sulfate complexes, namely catena‐poly[[tetrakis(pyridine‐κN)nickel(II)]‐μ‐sulfato‐κ²O:O′], [Ni(SO₄)(C₅H₅N)₄]_n, (1), di‐μ‐sulfato‐κ⁴O:O‐bis[tris(pyridine‐κN)copper(II)], [Cu₂(SO₄)₂(C₅H₅N)₆], (2), and catena‐poly[[tetrakis(pyridine‐κN)zinc(II)]‐μ‐sulfato‐κ²O:O′‐[bis(pyridine‐κN)zinc(II)]‐μ‐sulfato‐κ²O:O′], [Zn₂(SO₄)₂(C₅H₅N)₆]_n, (3), are reported. Ni compound (1) displays a polymeric crystal structure, with infinite chains of Ni^II atoms adopting an octahedral N₄O₂ coordination environment that involves four pyridine ligands and two bridging sulfate ligands. Cu compound (2) features a dimeric molecular structure, with the Cu^II atoms possessing square‐pyramidal N₃O₂ coordination environments that contain three pyridine ligands and two bridging sulfate ligands. Zn compound (3) exhibits a polymeric crystal structure of infinite chains, with two alternating zinc coordination environments, i.e. octahedral N₄O₂ coordination involving four pyridine ligands and two bridging sulfate ligands, and tetrahedral N₂O₂ coordination containing two pyridine ligands and two bridging sulfate ligands. The observed coordination environments are consistent with those predicted by crystal field theory.