Gunther Steinfeld

Triazolopyridines as ligands: structural diversity in iron(II), cobalt(II), nickel(II) and copper(II) complexes of 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L10) and spin crossover in [FeII(L10)2(NCS)2]

The coordination chemistry of the ligand 3-(2-pyridyl)-[1,2,4]triazolo[4,3-a]pyridine (L¹⁰) has been investigated and iron(II), cobalt(II), nickel(II) and copper(II) complexes featuring diverse structural motifs have been prepared. In the 2 : 1-type complexes [Co(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (20), [Ni(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (21), [Cu(II)(L¹⁰)₂(MeOH)₂](ClO₄)₂ (22), [Co(II)(L¹⁰)₂(H₂O)₂](ClO₄)₂ (23) and [Cu(II)(L¹⁰)₂(ClO₄)₂] (24) the metal centres are N₄O₂ octahedrally coordinated with two N²,N(pyr) bidentate ligands L¹⁰ in the equatorial positions. In the N₆ octahedral 4 : 1-type complex [Co(II)(L¹⁰)₄](ClO₄)₂·H₂O (25) both axially coordinating N¹ unidentate and equatorially bound N²,N(pyr) bidentate ligands L¹⁰ are observed. The N₆ octahedral 3 : 1-type complex [Fe(II)(L¹⁰)₃](OTf)₂·1.5MeCN·0.13H₂O·0.87MeO(t)Bu (27) features three N²,N(pyr) bidentate ligands L¹⁰ in the mer configuration. The two closely related N₆ octahedral complexes [Fe(II)(L¹⁰)₂(NCS)₂] (29) and [Fe(II)(L¹⁰)₂(dca)₂] (30) have fundamentally different structures. While complex 29 features two equatorially bound N²,N(pyr) bidentate ligands L¹⁰ and axial NCS⁻ co-ligands, complex 30 is a one-dimensional doubly μ1,5-dicyanamido-bridged polymer with N¹ unidentate ligands L¹⁰ in the axial positions. Temperature-dependent magnetic susceptibility measurements of the iron(II) complexes 28 and 29 have shown the 3 : 1-type complex [Fe(II)(L¹⁰)₃](ClO₄)₂·H₂O (28) to be in the low-spin state over the range 300-2 K and the 2 : 1-type complex 29 to be a spin crossover compound with T(1/2) = 269 K whereas the dicyanamido-bridged complex 30 remains in the high-spin state even down to 113 K, according to X-ray diffraction data. A single end-to-end bridging NCS⁻ co-ligand is found in the N₄S square-pyramidal complex [Cu(II)(L¹⁰)(NCS)₂] (31) which shows Curie-Weiss behaviour over the range 300-2 K. A brief review of the coordination chemistry of triazolopyridines is given.

Trapping of a Thiolate→Dibromine Charge‐Transfer Adduct by a Macrocyclic Dinickel Complex and Its Conversion into an Arenesulfenyl Bromide Derivative

Stuck on sulfur: The first transition-metal complexes with S-Br units are surprisingly stable. Solid 3 is stable for at least six months and under vacuum solid 2 does not lose Br(2). The formation of the first structurally characterized transition-metal arenesulfenyl bromide complex 3 occurs with a change of the spin ground state from S = 2 to S = 0.

Reactions at the N2Ni(μ2-SR)2NiN2 Core in Dinuclear Nickel(II) Amine-Thiolate Complexes

A discrete dinickel complex with a central N2NiII(μ2-SR)2NiIIN2 core has been synthesized and investigated in the context of ligand binding and oxidation state changes. Reaction of the hexadentate amine-thiolate ligand N,N′-bis-[2-thio-3-aminomethyl-5-tert-butylbenzyl]propane-1,3-diamine (8) with Ni(ClO4)2 · 6 H2O in methanol affords [NiII28][ClO4]2 · 3 CH3OH (9), the structure of which has been determined by X-ray crystallography. Complex 9 contains a central N2Ni(μ2-SR)2NiN2 core with two approximately planar cis-N2S2Ni coordination polyhedra bridged at the thiolate sulfur atoms. The cis-N2S2Ni units differ in that one nickel atom forms a six-membered chelate ring with the two secondary amine nitrogen atoms of 8 whereas the other nickel atom is bound to the remaining primary nitrogen atoms. Complex 9 binds a variety of substrates. Binding of anions to the N2Ni(μ2-SR)2NiN2 core in 9 occurs selectively at the Ni atom bound to the secondary nitrogen atoms because of a slightly weaker ligand-field strength. This is demonstrated by X-ray structure determination of the isothiocyanate complex [NiII28(NCS)2] · MeOH (10) formed by the reaction of 9 with 2 equiv. of KSCN in methanol. Complex 10 is the first example of a complex with adjacent octahedral cis-N4S2Ni and planar cis-N2S2Ni sites. The overall dinuclear structure of the parent complex 9 is retained in 10, except for trans-axially bound isothiocyanate ions at Ni(1) and an as yet unexplained inversion of configuration at both secondary amine nitrogen atoms. In DMF solution both complexes undergo two successive reductions at potentials of –0.95 V and –1.53 V vs SCE, assigned to the formation of mixed-valent [NiINiII8]1+ and [NiI28]0 species, respectively. The similar electrochemical properties of 9 and 10 suggest that the trans-axially bound isothiocyanate ions in 10 are replaced by DMF molecules in DMF solution. Upon reduction, these solvent molecules decoordinate to produce an unsolvated [NiINiII8]1+ species with two planar N2S2Ni units.

Preparation and characterization of mononuclear Co, Ni, and Zn complexes of dinucleating macrocyclic hexaaza-dithiophenolate ligands and their open-chain derivatives

The preparation and characterization of mononuclear complexes of the dinucleating 24-membered hexazadithiophenolate macrocycles H2L2 and H2L3 and their open-chain N3S2 analogues H2L4 and H2L5 are reported. The highly crystalline compounds [Ni(L4)] (4), [Ni(L5)] (5), [Co(L5)] (6), [NiH2(L2)]2+ (7), [ZnH2(L2)]2+ (8), and [NiH2(L3)]2+ (9) could be readily prepared by stoichiometric complexation reactions of the hydrochlorides of the free ligands with the corresponding metal(II) dichlorides and NEt3 in methanolic solution. All complexes were characterized by X-ray crystallography. Monometallic complexes 4-6 of the pentadentate ligands H2L4 and H2L5 feature distorted square pyramidal MN3S2 structures (tau = 0.01 to 0.44). Similar coordination geometries are observed for the macrocyclic complexes 7-9 of the octadentate ligands H2L2 and H2L3. The two hydrogen atoms in 7-9 are attached to the noncoordinating benzylic amine functions and are hydrogen bonded to the metal-bound thiophenolate functions. A comparison of the structures of 4-9 reveals that the macrocycles L2 and L3 have a rather flexible ligand backbone that do not confer unusual coordination geometries on the metal ions. We also report on the ability of the monometallic complexes 7 and 8 to serve as starting materials for the preparation of dinuclear complexes.

Novel Synthesis of Macrocyclic Amine‐Thiophenolate Ligands: X‐ray Crystal Structure of a Ni4 Complex of an N8S4 Ligand

A novel route to macrocyclic amine-thiophenolate ligands is described. The new, air-stable thiophenolate precursor 1,2-bis(4-tert-butyl-2,6-diformyl-phenylsulfanyl)ethane (4) is readily condensed with two equivalents of 1,2-ethanediamine or 1,3-propanediamine under medium to high dilution conditions to give 2 × 4 condensation products. The smaller 1 × 2 macrocyclic compounds are not produced under these conditions. Subsequent reduction with NaBH4 (reduction of imine groups) and Na/NH3 (reductive cleavage of aryl-alkylsulfides) provides the 36- and 40-membered amine-thiophenolate ligands H46a and H46b. The macrocyclic compounds are versatile ligands for the preparation of polynuclear transition metal complexes. With divalent nickel H46a forms the di- and tetranuclear complexes [Ni2(6a)] (7) and [Ni4(6a)][ClO4]4 (8). Reaction of 8 with four equivalents of NH4SCN yields the novel isothiocyanate complex [Ni4II(6a)(NCS)4]·10MeCN (9). The structure consists of well-separated molecules of the tetranuclear complex [NiII4(6a)(NCS)4] (Ci symmetry). Two symmetry-related binuclear [N2Ni(μ2-SR)2NiN4] fragments composed of thiolate-bridged distorted planar {N2S2Ni}- and distorted cis-octahedral {(SCN)2N2S2Ni} units reside within the cavity of the macrocycle. The intramolecular distance between the two binuclear units is 6.144(1) A.

Synthesis and Coordination Chemistry of Novel Binucleating Macrocyclic Ligands with Amine-thioether and Amine-thiophenolate Donor Functions

Abstract The ability of the aromatic tetraaldehyde l,2-bis(4-tert-butyl-2,6-diformylphenylsulfanyl)-ethane (1) to function as a precursor in the preparation of binucleating hexaamine-dithiolate ligands has been investigated. Reductive amination of compound 1 with bis(aminoethyl)amine under medium-dilution conditions affords the macrobicyclic hexaamine-dithioether compound L1. Deprotection of the [1+2] condensation product gives the corresponding 24-membered hexa-amine-dithiophenol ligand H2L2. The formulation of L1 as a macrobicyclic amine-thioether was confirmed by an X-ray crystal structure determination of the tetranuclear nickel(II) complex of L1, [{(L1)Ni2Cl2}2(μ-Cl)3](BPh4) (2b). The formulation of the doubly deprotonated form (L2)2- of H2L2 as a 24-membered amine-thiophenolate ligand was confirmed by an X-ray crystal structure determination of the dinuclear cobalt(III) complex, [(L2)CoIII(μ-OH)](ClO4)2 · Cl (3). The preparation and the crystal structures of the new compounds are described.

The effect of N-methylation on the chemical reactivity of binuclear Ni amine-thiophenolate complexes

Macrocyclic amine thiophenolate ligands are shown to form face-sharing bioctahedral nickel complexes with a central N3Ni(μ-SR)2(μ-Cl)NiN3 core structure. The bridging halide ion can be readily replaced when all six nitrogen atoms are tertiary amine donors.

Preparation and Characterization of Mononuclear Ni Complexes of Tetradentate Amine-thioether and Amine-thiolate Ligands

A short route for the preparation of tetradentate amine-thioether and amine-thiolate ligands derived from thiosalen is reported. The ligating properties of several of the synthesized ligands towards Ni(II) has been examined. The diamine-dithiophenolate ligands (L6)2− [H2L6 = N,N′-dimethyl-N,N;-di(2- mercaptobenzyl)-ethane-1,2-diamine] and (L7)2− [H2L7 = N,N′-di(2-mercaptobenzyl)-piperazine] support the formation of four-coordinate NiIIN2S2 complexes [NiII(L6)] (10) and [NiII(L7)] (11). By contrast, the amine-thioethers 2 [N′,N″-bis(2-(tert-butylthio)benzyl)ethane-1,2-diamine], L2 [8,11- diaza-5,13-dibenzo-1,4-dithia-cyclotetradecane] and its N-methylated derivative L2,Me were found to produce the six-coordinate Ni(II) complexes [NiIICl2(2)2] (9), [NiII2(μ-Cl)2(L2)2][ClO4]2 (12), [NiII(NCS)2(L2)] (13), and [NiIICl2(L2,Me)] (14). The results of IR, NMR and UV/vis spectroscopy and the crystal structures of complexes 9 - 13 are reported.

Characterisation of a triply thiolate-bridged Ni–Fe amine–thiolate complex: insights into the electronic structure of the active site of [NiFe] hydrogenase

The synthesis and structural characterisation of a new Ni–Fe amine–thiolate complex are described; its chemical properties are related to the active site of [NiFe] hydrogenase.

Rapid structure determination of microcrystalline molecular compounds using electron diffraction

Abstract Chemists of all fields currently publish about 50 000 crystal structures per year, the vast majority of which are X‐ray structures. We determined two molecular structures by employing electron rather than X‐ray diffraction. For this purpose, an EIGER hybrid pixel detector was fitted to a transmission electron microscope, yielding an electron diffractometer. The structure of a new methylene blue derivative was determined at 0.9 Å resolution from a crystal smaller than 1×2 μm2. Several thousand active pharmaceutical ingredients (APIs) are only available as submicrocrystalline powders. To illustrate the potential of electron crystallography for the pharmaceutical industry, we also determined the structure of an API from its pill. We demonstrate that electron crystallography complements X‐ray crystallography and is the technique of choice for all unsolved cases in which submicrometer‐sized crystals were the limiting factor.