Homoleptic Metal Complexes Research Articles

The First Crystal Structure of an Anti-Geometric Homoleptic Zinc Complex from an Unsymmetric Curcuminoid Ligand

Curcuminoids are widely studied due to their well-recognized therapeutic properties. These molecules are often derivatized with metals, producing their corresponding homoleptic metal complexes. Numerous crystal structures of homoleptic symmetric curcuminoids with physiologically essential metals are known, although the literature lacks reports of homoleptic metal complexes of unsymmetric curcuminoids (or hemi-curcuminoids) as ligands. Three unknowns must be solved when an unsymmetric curcuminoid ligand is reacted with a metal ion: (a) the degree of coordination (MLn); (b) the spatial geometry; and (c) the conformational nature (syn or anti) of the complex. Herein, we report the structure of the anti-isomer of the Zn complex of the hemi-curcuminoid 5-hydroxy-1-(4-methoxyphenyl)hexa-1,4-dien-3-one. While the NMR shows only one set of signals for this homoleptic complex, the unambiguous stereochemistry was established through single-crystal X-ray diffractometry, revealing an anti-hexacoordinated octahedral ML2 structure.

Homoleptic metal complexes derived from hydrazones as ligand; synthesis, cytotoxic activity, photoluminescence properties and ADMET studies

In the present work, N'-(2‑hydroxy-5-nitrobenzylidene)thiophene-2-carbohydrazide (L) and its Cu(II), Ni(II), Co(II), and Fe(II) complexes have been prepared. Their chemical structures were characterized by elemental analysis, molar conductivity, FT-IR, UV–Vis, TGA, LC-MS/MS, 1H and 13C NMR (for ligand). Also, the photoluminescence properties of the free ligand and its metal complexes were investigated in DMF solution. The ligand is emissive at 430–630 nm range and the emission band is almost turned off upon complexation with Cu(II) and Fe(II) ions. Emission spectra of the Ni(II) and Co(II) complexes showed a blue shift compared to the free ligand. Also, in vitro cytotoxic activities of both ligand and its metal complexes against HTB-54 and BEAS-2B cells have been investigated. The [Fe(L)2]⋅2H2O complex (IC50 35.59 μM) showed better cytotoxic activity than cisplatin against HTB-54 cells. Moreover, the ligand and its Fe(II) complex had slight cytotoxic activity against BEAS-2B cell line.

First Gallium and Indium Crystal Structures of Curcuminoid Homoleptic Complexes: All-Different Ligand Stereochemistry and Cytotoxic Potential.

The crystal structure determination of metal complexes of curcuminoids is a relevant topic to assess their unequivocal molecular structure. We report herein the first two X-ray crystal structures of homoleptic metal complexes of a curcuminoid, namely Dimethoxycurcumin (DiMeOC), with gallium and indium. Such successful achievement can be attributed to the suppression of interactions from the phenolic groups, which favor an appropriate molecular setup, rendering Dimethoxycurcumin gallium ((DiMeOC)2-Ga) and Dimethoxycurcumin indium ((DiMeOC)3-In) crystals. Surprisingly, the conformation of ligands in the crystal structures shows differences in each metal complex. Thus, the ligands in the (DiMeOC)2-Ga complex show two different conformers in the two molecules of the asymmetric unit. However, the ligands in the (DiMeOC)3-In complex exhibit three different conformations within the same molecule of the asymmetric unit, constituting the first such case described for an ML3 complex. The cytotoxic activity of the (DiMeOC)2-Ga complex is 4-fold higher than cisplatin against the K562 cell line and has comparable activity towards U251 and PC-3 cell lines. Interestingly, this complex exhibit three times lesser toxicity than cisplatin and even slightly lesser cytotoxicity than curcumin itself.

Syntheses, Structures, and Bioactivities Evaluation of some Transition Metal Complexes with 4,4'‐Diacetylcurcumin

Stoichiometric reactions of 4,4'‐diacetylcurcumin (HL) with series of transition metal ions, namely Fe3+, Co2+, Ni2+ and Zn2+, in methanol result in the corresponding homoleptic metal complexes. All the obtained complexes were characterized by elemental analysis, high resolution mass spectrometry, IR spectroscopy, magnetic moment and single‐crystal X‐ray diffraction. Structural analyses are unprecedentedly performed for the FeIII, CoII, and NiII complexes and reveal octahedral mononuclear complexes with the compositions [Fe(L)3] and [M(L)2(MeOH)2] (M = Co2+, Ni2+, Zn2+) for trivalent and divalent metal ions, respectively. In all complexes, the deprotonated ligands serve as monoanionic and bidentate ligands with (O,O)‐chelating β‐diketonate moieties. The free ligand HL exhibits considerable antiproliferative effects for the human MCF‐7 breast and HepG2 liver cancer cells with IC50 values of 20.91 ± 2.16 μg·mL–1 and 12.85 ± 1.85 μg·mL–1, respectively. The CoII and ZnII complexes with IC50 values in the range of 14.53–20.80 μg·mL–1 for MCF‐7 breast and 8.48–10.68 μg·mL–1 for HepG2 liver cancer cells show stronger antiproliferative effects than HL, the FeIII and NiII complexes cause weaker reductions of the growth of the two tested cancer cell lines.

Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8 ]4+ (M=MoIV , WIV ) from Superacids.

Octacyanometalates K4[Mo(CN)8] and K4[W(CN)8] are completely protonated in superacidic mixtures of anhydrous hydrogen fluoride and antimony pentafluoride. The resulting hydrogen isocyanide complexes [Mo(CNH)8]4+ [SbF6]− 4 and [W(CNH)8]4+ [SbF6]− 4 are the first examples of eight‐coordinate homoleptic metal complexes containing hydrogen isocyanide (CNH) ligands. The complexes were crystallographically characterized, revealing hydrogen‐bonded networks with short N⋅⋅⋅H⋅⋅⋅F contacts. Low‐temperature NMR measurements in HF confirmed rapid proton exchange even at −40 °C. Upon protonation, ν(C≡N) increases of about 50 cm−1 which is in agreement with DFT calculations.

The SALSAC approach: comparing the reactivity of solvent-dispersed nanoparticles with nanoparticulate surfaces.

We demonstrate that the ‘surface-as-ligand, surface-as-complex’ (SALSAC) approach that we have established for annealed nanoparticulate TiO2 surfaces can be successfully applied to nanoparticles (NPs) dispersed in solution. Commercial TiO2 NPs have been activated by initial treatment with aqueous HNO3 followed by dispersion in water and heating under microwave conditions. We have functionalized the activated NPs with anchoring ligands 1–4; 1–3 contain one or two phosphonic acid anchoring groups and 4 has two carboxylic acid anchors; ligands 1, 2 and 4 contain 6,6′-dimethyl-2,2′-bipyridine (Me2bpy) metal binding domains and 3 contains a 2,2′:6′,2′′-terpyridine (tpy) unit. Ligand functionalization of the activated NPs has been validated using infrared and 1H NMR spectroscopies, and thermogravimetric analysis. NPs functionalized with 1, 2 and 4 react with [Cu(MeCN)4][PF6] and those with 3 react with FeCl2·4H2O; metal binding has been investigated using solid-state absorption spectroscopy and scanning electron microscopy (SEM). Competitive binding of ligands 1–4 to TiO2 NPs has been investigated and shows preferential binding of phosphonic acid over carboxylic acid anchors. For the phosphonic acids, the binding orders are 3 > 1 > 2 which is rationalized in terms of relative pKa values (phosphonic acid and [HMe2bpy]+ or [Htpy]+) and the number of anchoring groups in the ligands. Ligand exchange between ligand-functionalized NPs and homoleptic metal complexes gives NPs functionalized with heteroleptic copper(i) or iron(ii) complexes.

Full Structural Characterization of Homoleptic Complexes of Diacetylcurcumin with Mg, Zn, Cu, and Mn: Cisplatin-level Cytotoxicity in Vitro with Minimal Acute Toxicity in Vivo.

At the present time, scientists place a great deal of effort worldwide trying to improve the therapeutic potential of metal complexes of curcumin and curcuminoids. Herein, the synthesis of four homoleptic metal complexes with diacetylcurcumin (DAC), using a ligand designed to prevent the interaction of phenolic groups, rendering metal complexes through the β-diketone functionality, is reported. Due to their physiological relevance, we used bivalent magnesium, zinc, copper, and manganese for complexation with DAC. The resulting products were characterized by ultraviolet-visible (UV-Vis), fluorescence spectroscopy, infrared spectroscopy (IR), liquid and solid-state nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), magnetic moment, mass spectrometry (MS), single crystal, and powder X-ray diffraction (SCXRD and PXRD). Crystallization was achieved in dimethylsulfoxide (DMSO) or N,N-dimethylformamide (DMF) as triclinic systems with space group P-1, showing the metal bound to the β-diketone function, while the 1H-NMR confirmed the preference of the enolic form of the ligand. Single crystal data demonstrated a 1:2 metal:ligand ratio. The inhibition of lipid peroxidation was evaluated using the thiobarbituric acid reactive substance assay (TBARS). All four metal complexes (Mg, Zn, Cu, and Mn) exhibited good antioxidant effect (IC50 = 2.03 ± 0.27, 1.58 ± 0.07, 1.58 ± 0.15 and 1.24 ± 0.10 μM respectively) compared with butylated hydroxytoluene (BHT) and α-tocopherol. The cytotoxic activity in human cancer cell lines against colon adenocarcinoma (HCT-15), mammary adenocarcinoma (MCF-7), and lung adenocarcinoma (SKLU-1) was found comparable ((DAC)2Mg), or ca. 2-fold higher ((DAC)2Zn) than cisplatin. The acute toxicity assays indicate class 5 toxicity, according to the Organization for Economic Co-operation and Development (OECD) guidelines at doses of 3 g/kg for all complexes. No mortality or changes in the behavior of animals in any of the treated groups was observed. A therapeutic potential can be envisaged from the relevant cytotoxic activity upon human cancer cell lines in vitro and the undetected in vivo acute toxicity of these compounds.

A New Family of Homoleptic Copper Complexes of Curcuminoids: Synthesis, Characterization and Biological Properties.

We report herein the synthesis and crystal structures of five new homoleptic copper complexes of curcuminoids. The scarcity of reports of homoleptic complex structures of curcuminoids is attributed to the lack of crystallinity of such derivatives, and therefore, their characterization by single crystal X-ray diffraction is rare. The ligand design suppressing the phenolic interaction by esterification or etherification has afforded a significant increase in the number of known crystal structures of homoleptic metal complexes of curcuminoids revealing more favorable crystallinity. The crystal structures of the present new copper complexes show four-fold coordination with a square planar geometry. Two polymorphs were found for DiBncOC-Cu when crystallized from DMF. The characterization of these new complexes was carried out using infrared radiation (IR), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and single crystal X-ray diffraction (SCXRD) and the antioxidant and cytotoxic activity of the obtained complexes was evaluated.

Inside Back Cover: Noninnocent β‐Diiminate Ligands: Redox Activity of a Bis(alkylimidazole)methane Ligand in Cobalt and Zinc Complexes (Chem. Eur. J. 45/2017)

The first example of a redox non-innocent β-diketiminate-type ligand, bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane anion, BMIMPh2−, is presented. In homoleptic metal complexes, these ligands are susceptible toward oxidation, covering three different oxidation states. The redox behavior of complexes featuring BMIMPh2− or related ligands are expected be a versatile tool in contemporary research within the context of chemical catalysis or activation of small molecules. More information can be found in the Communication by R. J. M. Klein Gebbink et al. on page 10732. (Background illustration: Invisible_studio – Freepik.com; Scientific figures: P. Ghosh).

Design of a structural database for homoleptic transition metal complexes

The interest and possible contents of a structural database for homoleptic metal complexes (HCD) is analyzed with the help of a set of some 1400 crystal and gas-phase structural data sets. Some examples of stereochemical and bonding trends that can be obtained from such a data collection are given. Attention is paid also to pseudohomoleptic complexes. These are defined as compounds which are formally homoleptic, in the sense that all the ligands around a metal are chemically equivalent, but in one case two very different bonding situations are found, notably due to Jahn–Teller distortions. Another case of pseudohomoleptic complexes correspond to those molecules that are described as homoleptic but that show short intra- or intermolecular contacts to extra donor groups, making it effectively heteroleptic.

Structures and bonding of homoleptic bis(2,3-dihydro-1,3-diborole) complexes of nickel and platinum

Homoleptic metal complexes of the boron heterocycle 2,3-dihydro-1,3-diborole {(R1C)2(R2B)2R3(H)C} 1 are described. X-Ray crystal structure determinations of two nickel and platinum derivatives are presented. In the nickel complex [Ni(1d)2] 6d (R1 = R2 = R3 = Et) the essentially coplanar heterocycles attain a pentahapto coordination mode with a gauche orientation with respect to one another. An 18 VE count is attained. In contrast, in the 14 VE platinum complex [Pt(1a)2] 4a (R1 = R2 = Et, R3 = Me) the ligands are strongly folded and adopt a tetrahapto coordination. The molecule is centrosymmetric in the crystalline state. DFT MO calculations are presented to establish the relative stabilities of these coordination modes for nickel and platinum, respectively.

Water-soluble bis(4′-[2,2,2-tris(hydroxymethyl)ethoxy]-2,2′:6′,2″-terpyridine)metal complexes

A series of homoleptic metal complexes containing the pentaerythritol-functionalized ligand 1 (4′-(2,2,2-tris(hydroxymethyl)ethoxy)-2,2′:6′,2″-terpyridine) has been synthesized. In MeOH and under microwave conditions, the 2,2,2-tris(hydroxymethyl)ethoxy substituent is cleaved and replaced by a methoxy group originating from the solvent. This has been confirmed from the single crystal structure of [Zn(2)2][PF6]2 (2=4′-methoxy-2,2′:6′,2″-terpyridine). The complexes [Zn(1)2][OAc]2, [Fe(1)2]Cl2 and [Co(1)2][PF6]2 were therefore prepared under mild conditions; [Co(1)2][PF6]2 readily oxidizes to [Co(1)2][PF6]3; the conversion of the paramagnetic [Co(1)2]2+ to diamagnetic [Co(1)2]3+ is readily monitored by 1H NMR spectroscopy. The homoleptic complexes are hygroscopic and are highly soluble in water; the most soluble is [Zn(1)2][OAc]2 (71mmoldm−3). Structural data for 1, [Zn(1)2][PF6]2, 4{[Zn(1)2][PF6]2}·3MeCN, [Zn(1)(OAc-O)2]·2MeOH and 2{[Co(1)2][PF6]3}·1.5MeCN·1.5H2O confirm that dominant packing forces involve hydrogen bonds between pendant 2,2,2-tris(hydroxymethyl)ethoxy units.

New homoleptic metal complexes of Schiff bases derived from 2,4-di-p-tolyl-3-azabicyclo[3.3.1]nonan-9-one

New bidentate Schiff-base ligands 2-(2,4-di-p-tolyl-3-azabicyclo[3.3.1]nonan-9-ylidene)hydrazinecarbothioamide HL1 and 2-(2,4-di-p-tolyl-3-azabicyclo[3.3.1]nonan-9-ylidene)hydrazinecarboxamide HL2 were synthesized from the condensation of 2,4-di-p-tolyl-3-azabicyclo[3.3.1]nonan-9-one with thiosemicarbazide and semicarbazide, respectively. Homoleptic complexes of these ligands, of general formula K[Cr(L n )2Cl2], K2[Mn(L n )2Cl2], K2[Fe(L1)2Cl2] and [M(L n )2] (where M = Co(II), Ni(II) Cu(II), Zn(II), Cd(II), and Hg(II) ions; n = 1 or 2) are reported. The mode of bonding and overall geometry of the complexes were determined through IR, UV-Vis, NMR and mass spectral studies, magnetic moment measurements, elemental analysis, metal content, and conductance. These studies revealed octahedral geometry for Cr(III), Mn(II), and Fe(II) complexes, square planar for Cu(II), Co(II), and Ni(II) complexes and tetrahedral for Zn(II), Cd(II), and Hg(II) complexes.

Stereochemistry and Spin State in Four-Coordinate Transition Metal Compounds

A systematic DFT computational study of the stereochemistry associated with each spin state for first transition series four-coordinate d(n) (n = 0-10) homoleptic metal complexes is presented. The stereochemistry of [MMe4](x-) complexes in the 21 spin configurations analyzed can be predicted from the d orbital occupation in the ideal tetrahedral geometry, grouped in three families with tetrahedral, square planar, or intermediate structures that can be described in some cases as sawhorses. The effect of the following factors on the spin state and stereochemical preferences has also been studied: (a) substitution of the sigma-donor methyl ligands by pi-donor chlorides, (b) a high (+4) oxidation state of the metal, and (c) substitution of the metal atom by a second transition series one. Through those factors, low-spin tetrahedral structures can be achieved, as summarized by a magic cube.

Verdazyl radicals as oligopyridine mimics: structures and magnetic properties of M(II) complexes of 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (M = Mn, Ni, Cu, Zn).

The verdazyl radical 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (3) was prepared, and its homoleptic metal complexes M(3)(2)(2+).2X(-) (5, M = Mn(II); 6, M = Ni(II); 7, M = Cu(II); 8, M = Zn(II); X = ClO(4), PF(6)) were characterized by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Relevant crystallographic parameters are as follows: 5, monoclinic space group Pna2(1), a = 18.755(4) A, b = 11.154(3) A, c = 16.594(4) A, alpha = 90.00 degrees, beta = 90.00 degrees, gamma = 90.00 degrees, V = 3471.4(13) A(3), and Z = 4; 7, triclinic space group Ponedblac;, a = 9.4638(18) A, b = 9.8442(19) A, c = 18.769(4) A, alpha = 103.746(3) degrees, beta = 92.925(3) degrees, gamma = 94.869(3) degrees, V = 1687.8(6) A(3), and Z = 2; 8, triclinic space group Ponedblac;, a = 9.4858(14) A, b = 9.7919(14) A, c = 18.889(3) A, alpha = 104.196(3) degrees, beta = 92.855(3) degrees, gamma = 94.216(3) degrees, V = 1692.1(4) A(3), and Z = 2. In all cases, the two verdazyl-based ligands bind almost perpendicular to each other in meridional positions, yielding pseudooctahedral metal complexes whose general structural features are strongly reminiscent of metal bis(terpyridine) complexes. The intramolecular metal-verdazyl magnetic exchange coupling is strongly ferromagnetic in 6 (J(Ni-vd) = +240 cm(-1)), and strongly antiferromagnetic in 5 (J(Mn-vd) = -93 cm(-1)). Complex 7 exhibits weak ferromagnetic coupling (J(Cu-vd) = -4.5 cm(-1)). Intramolecular radical-radical coupling in the zinc complex 8 was found to be weakly antiferromagnetic (J(vd-vd) = -8 cm(-1)). Intramolecular radical-radical exchange was generally weak in the four metal complexes, ranging from -10 cm(-1) (for 5) to +2 cm(-1) (for 7). The low-temperature magnetic behavior of 7 and 8 is complex, possibly arising from a combination of intra- and intermolecular interactions.

Unusual metal alkyls

Abstract