Pseudo-tetrahedral Coordination Research Articles

The influence of pseudo-tetrahedral coordination environment of Co2+ ion in polymeric dimethylmalonates on the magnetic anisotropy and slow magnetic relaxation

The interaction of Co(NO3)2·6H2O and MI2CO3 (MI = Rb or Cs) with dimethylmalonic acid (H2Me2mal) leads to the formation of 3D polymeric compounds: {[Co4Rb8(Me2mal)8(H2O)8]·5H2O}n (1) and {[CoCs2(Me2mal)2(H2O)4]·H2O}n, respectively. The structures of 1 and 2 were characterized using X-ray diffraction analysis and IR spectroscopy. Each complex contains CoII atoms arranged in four vertex polyhedra. Both compounds exhibit field-induced slow magnetic relaxation. The dc-magnetic measurements and ab initio calculation results show that 1 has “easy plane” type of magnetization, where the axial parameter D of both crystallographicaly independent Co2+ ions is positive. In contrast, parameter D of 2 is negative, which corresponds to “easy axis” type. For 2 the relaxation process includes the Orbach mechanism, which corresponds to the quantum chemical calculation.

A Four-Coordinate Pr4+ Imidophosphorane Complex.

The imidophosphorane ligand, [NPtBu3]- (tBu=tert-butyl), enables isolation of a pseudo-tetrahedral, tetravalent praseodymium complex, [Pr4+(NPtBu3)4] (1-Pr), which is characterized by a suite of physical characterization methods including single-crystal X-ray diffraction, electron paramagnetic resonance, and L3-edge X-ray near-edge spectroscopies. Variable-temperature direct-current magnetic susceptibility data, supported by multiconfigurational quantum chemical calculations, demonstrate that the electronic structure diverges from the isoelectronic Ce3+ analogue, driven by increased crystal field. The four-coordinate environment around Pr4+ in 1-Pr, which is unparalleled in reported extended solid systems, provides a unique opportunity to study the interplay between crystal field splitting and spin-orbit coupling in a molecular tetravalent lanthanide within a pseudo-tetrahedral coordination geometry.

Front Cover: Static and Dynamic Magnetic Properties of a Co(II)‐Complex with N2O2 Donor Set – A Theoretical and Experimental Study (Eur. J. Inorg. Chem. 10/2023)

The Front Cover shows a Co(II) ion that is actually chelated by two monoanionic sulfur-containing Schiff base ligands. Each ligand contributes with a nitrogen and an oxygen atom to form the illustrated pseudo-tetrahedral coordination. A magnetic field prevents spin transversal by quantum tunnelling at very low temperature, since the spin transversal is connected to a Zeeman energy difference that needs to be compensated by energy from quantized atomic movements. At higher temperatures, the local anisotropy due to the strong orbital momentum mixing to the spin 3/2 of the Co(II) plays a more crucial role, and two-phonon spin-relaxation processes become more and more dominant. More information can be found in the Research Article by B. Schwarz, K. C. Mondal and co-workers.

Kink distortion of the pseudo-S4 axis in pseudotetrahedral [N2O2] bis-chelate cobalt(II) single-ion magnets leads to increased magnetic anisotropy.

Two new mononuclear cobalt(II) complexes with the general formula [Co(L1,2)2] (1 and 2) were synthesized using bidentate Schiff base ligands with NO donor set, namely, 2-(benzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL1) and its methyl substituted derivative 2-(6-methylbenzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL2). X-ray structure analysis reveals a distorted pseudotetrahedral coordination sphere at the cobalt(II) ion, that cannot be described by a simple twisting of the two ligand chelate planes with respect to each other, which would imply a rotation about the pseudo-S4 axis of the complex. Such a pseudo-rotation axis would approximately be colinear with the two vectors defined by the cobalt ion and the two centroids of the chelate ligands, where the angle κ between the two vectors would be 180° in an ideal pseudotetrahedral arrangement. For complexes 1 and 2, the observed distortion can be characterized by a significant bending at the cobalt ion with angles κ of 163.2° and 167.4°, respectively. Magnetic susceptibility and FD-FT THz-EPR measurements together with ab initio calculations reveal an easy-axis type of anisotropy for both complexes 1 and 2, with a spin-reversal barrier of 58.9 and 60.5 cm-1, respectively. For both compounds, frequency-dependent ac susceptibility measurements show an out-of-phase susceptibility under applied static fields of 40 and 100 mT, which can be analyzed in terms of Orbach and Raman processes within the observed temperature range.

(Nitrito-κ2 O,O')bis-[tris-(4-methyl-phen-yl)phosphane-κP]silver(I).

The mol-ecular structure of the title compound, [Ag(NO2)(C21H21P)2], exhibits a pseudo-tetra-hedral coordination around the central AgI atom. The compound crystallizes with one mol-ecule in the asymmetric unit in the monoclinic space group P21/n with a rather long b axis [33.8752 (2) Å]. Weak C-H⋯O and C-H⋯N inter-actions consolidate the crystal packing. The nitrite-O atoms each occupy a single position in the coordination geometry.

Postsynthetic Treatment of ZIF-67 with 5-Methyltetrazole: Evolution from Pseudo-Td to Pseudo-Oh Symmetry and Collapse of Magnetic Ordering

Reaction of Co(II) nitrate with 2-methylimidazole (2mIm) yields ZIF-67, the structure of which features Co(II) ions in pseudo-tetrahedral coordination geometry. Strong antiferromagnetic interactions between Co(II) ions mediated by the 2mIm ligands lead to antiferromagnetic ordering at 22 K. Postsynthetic treatment of Co(II) ZIF-67 with 5-methyltetrazole (5mT) results in the loss of crystallinity and magnetic order. The local structure of the Co(II) ions was probed by a combination of diffuse-reflectance electronic absorption spectroscopy and Co K-edge X-ray absorption spectroscopy (in the XANES and EXAFS regions). Upon reaction with 5mT, the 4A2(F)-4T1(F) and 4A2(F)-4T1(P) transitions at 1140 and 585 nm, respectively, of the pseudo-tetrahedral Co(II) center in ZIF-67 become less prominent and are replaced by transitions at 990 and 475 nm attributable to the 4T1g(F)-4T2g(F) and 4T1g(F)-4T1g(P) transitions of a pseudo-octahedral Co(II) center, respectively. Furthermore, the 1s-3d pre-edge absorption feature in the Co K-edge XANES spectrum loses intensity during this reaction, and the edge feature becomes more sharp, consistent with a change from pseudo-Td to pseudo-Oh geometry. EXAFS analysis further supports the proposed change in geometry: EXAFS data for ZIF-67 are well fitted to four Co-N scatterers at 1.99 Å, whereas the data for the 5mT-substituted compound are best fitted with 6 Co-N scatterers at 2.14 Å. Our results support the conclusion that a six-coordinate, pseudo-Oh geometry is adopted upon ligand substitution. The increase in coordination number directly increases the Co-N bond distances, which in turn weakens magnetic exchange interactions. No magnetic ordering is found in the 5mT-substituted materials.

Copper(I) Complexes Containing PCP Ligand Catalyzed Hydrogenation of Carbon Dioxide to Formate under Ambient Conditions.

The five new copper(I) complexes [Cu2(μ-Cl)2(κ1-PCPt-Bu)] (1), [Cu2(μ-Br)2(κ1-PCPt-Bu)] (2), [Cu2(μ-I)2(κ1-PCPt-Bu)] (3), [Cu2(μ-CN)2(κ1-PCPt-Bu)] (4), and [Cu4(μ3-SCN)4(κ1-PCPt-Bu)2]·CH2Cl2 (5) bearing a 1,3-bis[(di-tert-butylphosphino)methyl]benzene ligand were synthesized and characterized spectroscopically, and the molecular structures of 1, 3, and 5 were determined by single-crystal X-ray diffraction techniques. Structural studies for 1 and 3 revealed their binuclear structures with Cu···Cu separations of 2.609(3) and 2.6359(19) Å, respectively. However, 5 has a tetranuclear cubane structure with an 18-electron configuration at each copper without any metal-metal bonds. The two copper centers in 1 and 3 are bonded to one bridging PCPt-Bu ligand in a κ1-manner and two bridging (pseudo)halido ligands in a μ2-bonding mode to generate a nonplanar Cu2(μ-X)2 framework. The four copper centers in 5 are at the vertices of a tetrahedron. Each copper center has pseudo-tetrahedral coordination provided by two bridging PCPt-Bu ligands in a κ1-manner and the four bridging thiocyanate groups in a μ3-manner. These complexes were used as catalysts for the hydrogenation of CO2 to formate in the presence of DBU as a base to produce valuable energy-rich chemicals, and therefore it is a promising, safe, and simple strategy to conduct reactions under ambient pressure at room temperature. Among all of the five copper(I) complex based catalysts, 3 displayed the best catalytic performance with turnover number (TON) values of 38-8700 in 12-48 h of reaction at 25-80 °C. The outstanding catalytic performance of [Cu2(μ-I)2(κ1-PCPt-Bu)] (3) makes it a potential candidate for realizing the large-scale production of formate by CO2 hydrogenation.

Syntheses, characterization and DFT studies of two new homo-dinuclear zinc(II) complexes based on pyrazole and 3,5-dimethylpyrazole ligands

Two new homo-dinuclear zinc(II) complexes, bis(µ-pyrazolato-) bis[(formato)(pyrazole)zinc(II)], [{Zn(OCHO)(pzH)}2(µ-N2H3C3)2] (1) and bis(µ-3,5-dimethylpyrazolato)bis[(formato)(-3,5-dimethylpyrazole) zinc(II)], [{Zn(dmpzH)(OCHO)}2(µ-N2H7C5)2] (2), were synthesized through the reactions of zinc(II) formate dihydrate, Zn(OCHO)2·2H2O with pyrazole (pzH = C3H4N2) and 3,5-dimethylpyrazole (dmpzH = C5H8N2), respectively, in toluene at room temperature. These dinuclear complexes have been characterized using different analytical techniques; elemental and thermogravimetric analyses, IR and 1H-NMR spectroscopies, single-crystal X-ray diffraction and DFT studies. Compound 1 crystallized in the P space group of the triclinic crystal system with unit cell parameters a = 8.205(3) Å, b = 8.864(5) Å, c = 14.486(7) Å, α = 104.929(14)°, β = 91.214(11)° and γ = 106.921(8)°. Compound 2 crystallized in the P21/c space group of the monoclinic crystal system with cell parameters a = 8.400(3) Å, b = 11.020(4) Å, c = 14.930(5) Å, α = γ = 90° and β = 106.3(10)°. Pseudo-tetrahedral coordination geometries were observed around the zinc(II) centers in both complexes, constructed by one oxygen atom from a formate anion and three nitrogen atoms from one pzH and two pyrazolato in 1 or one dmpzH and two 3,5-dimethylpyrazolato in 2. The bulk of these materials are consolidated by weak and strong N-H···O, C-H···O, C-N···O and C-H···π, N-H···π and C-O···O intra/intermolecular interactions resulting to zigzag networks containing empty voids. DFT studies on both compounds revealed their optimized structures, frontier molecular orbitals, theoretical IR frequencies and the global reactivity descriptors.

TcI(NO)X(Cp)(PPh3)] Complexes (X–= I–, I3–, SCN–, CF3SO3–, or CF3COO–) and Their Reactions

Reactions of [Tc(NO)Cl(Cp)(PPh3)] with a series of monodentate ligands X– (X– = I–, I3–, F3CSO3–, CF3COO–, or SCN–) result in a ready replacement of the chlorido ligand and the formation of complexes of the general composition [Tc(NO)X(Cp)(PPh3)]. Technetium retains its oxidation state “+1” and its pseudotetrahedral coordination environment. [Tc(NO)(SCN)(Cp)(PPh3)] is the first technetium complex with an S-coordinated SCN– ligand. The complexes are stable as solids. In solution, however, a slow isomerization of the thiocyanato compound into the thermodynamically more stable isothiocyanato species is observed, while solutions of [Tc(NO)(I3)(Cp)(PPh3)] undergo an internal oxidation under formation of the technetium(II) complex [Tc(NO)(I)2(Cp)]. All products were characterized by elemental analysis, NMR and IR spectroscopy, and X-ray structure analysis. Particularly, 99Tc NMR spectroscopy proved to be a valuable method for these types of technetium(I) compounds.

Synthesis, crystal structure and catalytic activity in reductive amination of di-chlorido-(η6-p-cymene)(2'-di-cyclo-hexyl-phosphanyl-2,6-di-meth-oxy-biphen-yl-κP)ruthenium(II).

The title compound, [RuCl2(C10H14)(C26H35O2P)] (I), crystallizes in the monoclinic space group P21/c with two crystallographically independent mol-ecules (A and B) in the asymmetric unit. The geometries of both mol-ecules are very similar and distinguished only by the twist angles of the two benzene rings in the phosphine substituents [89.54 (14) and 78.36 (14)° for mol-ecules A and B, respectively]. The Ru atoms have classical pseudo-tetra-hedral piano-stool coordination environments. The conformation of each mol-ecule is stabilized by intra-molecular C-H⋯O and C-H⋯Cl hydrogen bonds and C-H⋯π inter-actions. The two mol-ecules are linked by a C-H⋯Cl hydrogen bond. In the crystal, the mol-ecules are further linked by C-H⋯ π inter-actions, forming -A-B-A-B- chains propagating along the a-axis direction. Complex I is an active catalyst for reductive amination reaction. The catalytic activity of this complex can be explained by the lability of the p-cymene ligand, which can be replaced by two-electron ligands such as CO or amine.

Synthesis, Characterization and DFT Studies of Two Zinc(II) Complexes Based on 2-Isopropylimidazole

Two novel coordination compounds, [Zn(L)2(OOCH)2] (1) and [Zn(L)3(OCHO)](OCHO)]·H2O (2) (where L = 2-isopropylimidazole, C6H10N2) have been prepared by reaction of 2-isopropylimidazole with zinc(II) formate at room temperature using toluene as solvent. These compounds were characterized by elemental and thermal analyses, IR, 1HNMR and 13CNMR spectroscopies, single crystal X-ray diffraction and DFT studies. The Zn centers in 1 and 2 adopt pseudo-tetrahedral coordination geometries. Compound 1 crystallizes in the monoclinic system P2/c space group whereas compound 2 crystallizes in the P-1 space group of the triclinic crystal system. Several types of hydrogen intra-/intermolecular interactions are observed in these materials and extend into a two-dimensional leaf like network in 1 and a two-dimensional lattice of rectilinear pillars in 2. Compounds 1 and 2 were also optimized and their frontier molecular orbitals, global reactivity descriptors, molecular electrostatic potential, natural bond orbitals were investigated using density functional theory (DFT). In fact the induced structural differences from complex 1 to complex 2 led to the reduction of the frontier molecular orbital energy gap by 1.338 eV and a decrease of the chemical hardness by 0.669 eV.

Analytical and structural investigation via infrared spectroscopy and density functional methods of cuprous complexes of the antioxidant tripeptide glutathione (GSH). Synthesis and characterization of a novel CuI-GSH compound

A copper(I) compound of reduced glutathione (Glu,S;Cys,R)-(GSH) was isolated from the reaction of CuICl and zwitterionic H3GSH(+−) in acetonitrile:water medium. The analytical data are in agreement with the formula CuI(H2GSH)(H2O)2. The characterization was carried out via infrared spectroscopy and density functional (DFT) method computations, that allowed to fully optimize structures for the free ligands and for several complex molecules of the type [CuI(S,O,O-H2GSH)(OH2)] having a pseudo-tetrahedral coordination arrangement and the Gly residue deprotonated at carboxylic group. The molecules were fully optimized at DFT-B3LYP/6-31G∗∗ (BS1) and in some cases at B3LYP/6-311++G∗∗ (BS2) and the solvent treatment (water, via PCM method) was taken into account (BS1W). The comparative analyses between experimental and computed infrared spectra confirmed that the free ligand was reduced glutathione in the zwitterionic form at the Glu residue. The analysis showed also that the metal compound consisted of complex molecules of the type [CuI(S,O,O-H2GSH(Gly))str(H2O)] where the ligand had a stretched (str) arrangement assisted by a network of hydrogen bonds. The presence of a globular (glo) structure for the complex molecule that brought the Glu-COOH and Glu-NH2 groups towards the Cys-SH and the Gly-COO− donors, respectively, could not be ruled out.

The silver(I) complex [HB{3-(CF3),5-(CH3)Pz}3]AgNCCH3 supported by a partially fluorinated scorpionate ligand.

Tris(pyrazolyl)borates are used extensively in metal coordination chemistry and belong to a class of ligands generally referred to as scorpionates. The steric and electronic properties of these ligands can be modified quite easily by varying the substituents on the 3-, 4-, and 5-positions of the pyrazolyl moieties on the B atom. Fluorinated tris(pyrazolyl)borates are useful in the stabilization of rare silver(I) complexes. The silver(I) adduct (acetonitrile-κN){tris[5-methyl-3-(trifluoromethyl)pyrazol-1-yl-κN2]hydroborato}silver(I), [Ag(C15H13BF9N6)(CH3CN)] or [HB{3-(CF3),5-(CH3)Pz}3]AgNCCH3, was obtained by treating [HB{3-(CF3),5-(CH3)Pz}3]Na with CF3SO3Ag in the presence of acetonitrile, and was isolated in 85% yield. Single-crystal X-ray diffraction analysis reveals that the AgI center has a pseudo-tetrahedral all-nitrogen coordination sphere, and is supported by a tris(pyrazolyl)borate ligand that binds to the AgI center in a κ3-fashion.

Nanoscaled zero valent iron/graphene composite as an efficient adsorbent for Co(II) removal from aqueous solution

A magnetic graphene, i.e., nanoscaled zero valent iron/graphene (0FG) composite, was prepared, characterized and applied for the removal of Co(II) from aqueous solution. The magnetic graphene (0FG) was synthesized through reduction of graphene oxide (GO) and ferrous ions by potassium borohydride. The kinetics and isotherms of Co(II) adsorption onto 0FG were investigated. The mechanism for Co(II) removal was proposed based on the Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and the X-ray absorption fine structure (XAFS) analysis. The results showed that pseudo second-order models and the Freundlich isotherm model fitted well with the data obtained. The adsorption capacity of 0FG was calculated from the Langmuir isotherm, which was 65.58, 101.60 and 134.27mg/g at 10, 20 and 30°C, respectively. Thermodynamic parameters suggested that the adsorption process was endothermic and spontaneous. Co2+ was stabilized by γ-FeOOH/γ-Fe2O3/Fe3O4 on the surface of graphene sheets, forming CoFe2O4-like nanocrystals. The coordination numbers and interatomic distances indicated that Co2+ mainly occupied the octahedral site, while pseudo-tetrahedral coordination may occur by dehydroxylation of Co(O,OH)6. Magnetic graphene is a potential adsorbent for Co2+ removal.

Cobalt(II)-Based Single-Ion Magnets with Distorted Pseudotetrahedral [N2O2] Coordination: Experimental and Theoretical Investigations.

The synthesis and magnetic properties of cobalt(II) complexes with sterically demanding Schiff-base ligands are reported. The compounds [Co(L(Br))2] (1) and [Co(L(Ph))2]·CH2Cl2 (2·CH2Cl2) are obtained by the reaction of cobalt(II) acetate with the ligands HL(Br) and HL(Ph) in a dichloromethane/methanol mixture. 1 and 2 crystallize in the space groups P21212 and P1̅, respectively. X-ray diffraction studies revealed mononuclear constitution of both complexes. For 1, relatively short intermolecular Co-Co distances of 569 pm are observed. In compound 2, a hydrogen-bonded dichloromethane molecule is present, leading to a solvent aggregate with remarkable thermal stability for which desolvation is taking place between 150 and 210 °C. Magnetic measurements were performed to determine the zero-field-splitting (ZFS) parameter D for both complexes. Frequency-dependent susceptibility measurements revealed slow magnetic relaxation behavior with spin-reversal barriers of 36 cm(-1) for 1 and 43 cm(-1) for 2 at an applied external field of 400 Oe. This observation is related to an increasing distortion of the pseudotetrahedral coordination geometry for complex 2. These distortions can be decomposed in two major contributions. One is the elongation effect described by the parameter ϵT, which is the ratio of the averaged obtuse and acute bond angles. The other effect is related to a twisting distortion of the chelate coordination planes at the cobalt center. A comparison with literature examples reveals that the elongation effect seems to govern the overall magnetic behavior in pseudotetrahedral complexes with two bidentate chelate ligands. Ab initio calculations for complexes 1 and 2 using the CASPT2 method show strong splitting of the excited (4)T2 term, which explains the observed strong ZFS. Spin-orbit calculations with the RASSI-SO method confirm the single-molecule-magnet behavior because only small transversal elements are found for the lowest Kramers doublet for both complexes.

Valence Tautomerism in Main‐Group Complexes? Computational Modeling of Si, Ge, Sn, and Pb Bischelates with o‐Iminoquinone Ligands

AbstractThe potential valence tautomeric (VT) properties of bischelate complexes of group 14 elements with 2,6‐di‐tert‐butyl‐4‐[(3,5‐di‐tert‐butyl‐2‐hydroxyphenyl)imino]cyclohexa‐2,5‐dienone (I) and 4,6‐di‐tert‐butyl‐N‐(R)‐o‐aminophenol (II, R = Ph and tBu) were studied computationally. According to DFT B3LYP/6‐311++G(d,p)/SDD calculations, the silicon complexes will be present as SiIV isomers with pseudotetrahedral coordination sites. Geometry optimizations of both lead bischelates I and II result in tetragonal‐pyramidal PbII structures. In the germanium complex I, the energy difference (5.2 kcal mol–1) between the electromeric forms allows the expectation of valence tautomerism, but the large energy barrier of the spin‐forbidden reaction (20.8 kcal mol–1) will complicate the occurrence of a thermally initiated redox process. Tin complex II (R = tBu) is characterized by a narrow energy gap between the tetragonal‐pyramidal and pseudotetrahedral electromeric forms (7.0 kcal mol–1) and a low energy barrier (11.7 kcal mol–1) for their interconversion and, therefore, may be considered as a promising candidate for the observation of thermally driven VT rearrangements.

Influence of the Homopolar Dihydrogen Bonding C-H⋅⋅⋅H-C on Coordination Geometry: Experimental and Theoretical Studies.

The reaction of the N-thiophosphorylated thiourea (HOCH2 )(Me)2 CNHC(S)NHP(S)(OiPr)2 (HL), deprotonated by the thiophosphorylamide group, with NiCl2 leads to green needles of the pseudotetrahedral complex [Ni(L-1,5-S,S')2 ]⋅0.5 (n-C6 H14 ) or pale green blocks of the trans square-planar complex trans-[Ni(L-1,5-S,S')2 ]. The former complex is stabilized by homopolar dihydrogen C-H⋅⋅⋅H-C interactions formed by n-hexane solvent molecules with the [Ni(L-1,5-S,S')2 ] unit. Furthermore, the dispersion-dominated C-H⋅⋅⋅ H-C interactions are, together with other noncovalent interactions (C-H⋅⋅⋅N, C-H⋅⋅⋅Ni, C-H⋅⋅⋅S), responsible for pseudotetrahedral coordination around the Ni(II) center in [Ni(L-1,5-S,S')2 ]⋅0.5 (n-C6 H14 ).

Copper(I)-1,1,1-tris(diphenylphosphinomethyl)ethane complexes with different coordination modes tuned by auxiliary ligands and their spectroscopic properties

Three mono-, bi- and tetranuclear copper(I) complexes, [Cu(phen)(triphos-O)]BF4 (1) (phen = 1,10-phenanthroline, triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane), [Cu2(bipy)(triphos)2](BF4)2 (2) (bipy = 4,4′-bipyridine), and [Cu4(MeOC^N^N)4(triphos)2(bipy)](BF4)4 (3) (MeOC^N^N = 6-(4-methoxyphenyl)-2,2′-bipyridine), have been synthesized and characterized by NMR spectroscopy, electrospray ionization, and matrix-assisted laser desorption ionization time-of-flight mass spectrometries, elemental analysis, and X-ray crystal analysis. The crystal structure investigation revealed the copper ions of the complexes have pseudo-tetrahedral coordination geometry. The electronic absorption spectra of 1, 2, and 3 contain low-energy bands at 350–500 and 400–650 nm, which are assigned to d(Cu) → π*(phen or bipy) and a mixture of d(Cu) → π*(MeOC^N^N) and d(Cu) → π*(bipy) transitions, respectively. Complex 2 displays a strong, long-lived solid-state emission with a maximum at 555 nm and lifetime of 13.6 μs at room temperature. Photoinduced electron-transfer properties of 2 and 3 involving nanosecond time-resolved absorption spectroscopy and electron spin resonance techniques were studied.

Synthesis, characterization and structure of nickel and copper compounds containing ligands derived from keto-enehydrazines and their catalytic application for aerobic oxidation of alcohols.

Ligand precursors HL(R,Ph) (R = Me, Ph) were synthesised by condensation of acetylacetone and the corresponding N,N-substituted hydrazines and were characterised spectroscopically and structurally. Both in the solid state and in solution they behave as (Z)-keto-enehydrazines and this was confirmed by DFT calculations which showed that this form was the most stable of their possible tautomers. The reaction of HL(R,Ph) compounds with copper acetate and nickel acetate in EtOH afforded the corresponding complexes [M(L(R,Ph))2] (M = Cu, Ni; R = Me, Ph). The methyl-substituted derivatives were structurally characterised by X-ray methods. A four-coordinate environment around the metal centre, where the two L(Me,Ph) ligands act as bidentate N,O-chelators and lie in a pseudo-trans conformation, was found for both compounds. The dihedral angle between the two six-membered metallacycles M(L(Me,Ph)) was 0° for nickel, a typical square planar coordination, meanwhile it was 23° for copper, a square planar slightly distorted to pseudotetrahedral coordination. Copper complexes [Cu(L(R,Ph))2] were tested as catalysts, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of 4-nitrobenzyl alcohol as the model reaction. An almost complete conversion to the corresponding aldehyde was observed after 1 h at 60 °C and 1 bar of dioxygen, in toluene as the solvent. Importantly, air at atmospheric pressure was also observed to be appropriate for the oxidation, although longer reaction times were required. After the optimization of the reaction conditions, the study was extended to other alcohol substrates and good catalytic activity was found for benzylic-type alcohols, while low yield was found for 1-octanol.

Single-Molecule-Magnet Behavior in Mononuclear Homoleptic Tetrahedral Uranium(III) Complexes

The magnetic properties of the two uranium coordination compounds, [K(18c6)][U(OSi(O(t)Bu)3)4] and [K(18c6)][U(N(SiMe3)2)4], both presenting the U(III) ion in similar pseudotetrahedral coordination environments but with different O- or N-donor ligands, have been measured. The static magnetic susceptibility measurements and density functional theory studies suggest the presence of different ligand fields in the two compounds. Alternating-current susceptibility studies conducted at frequencies ranging from 95 to 9995 Hz and at temperatures in the 1.7-10 K range revealed for both compounds slow magnetic relaxation already at zero static magnetic field with similar energy barriers U ∼24 K.