Metal ion-biomolecule interactions. III. Versatility of metal ion binding to imidazoles. Synthesis and 1H nmr spectroscopic properties of N- and C-bound mercury(II) complexes

Methylmercury(II) and mercury(II) complexes of imidazole (1), 1-methylimidazole (2), and the 1,3-dimethylimidazolium ion (3) have been prepared in aqueous or ethanolic solution. Elemental analysis and ¹H nmr spectroscopy have been used to characterize the complexes. The MeHg (Me = methyl) binding sites have been identified as N₁, N₃ (1), N₃, C₂ (2), and C₂ (3). Reaction with HgO leads to the formation of Hg-bridged complexes of the type Im-Hg-Im, (Im = imidazole), where bonding occurs through N₁ (1) and C₂ (3); the latter is also formed as a result of symmetrization of the C₂-bound MeHg complex. The formation of the C₂-bound (carbene) complexes is discussed in terms of the increased acidity of the C₂ proton resulting from coordination of an electrophilic species at N₃. Based on electrostatic considerations, there appears to be a “minimum degree of activation” required before C₂ bonding can occur, which explains the lack of this coordination mode in 1. ¹⁹⁹Hg-¹H spin-spin coupling (⁴J) is observed for C-bound mercury, but not for N-bound mercury, which is interpreted in terms of a decreased ligand exchange rate in the former case, due to the greater stability of the Hg-C bond. ²J coupling constants measured in (CD₃)₂SO for a number of MeHg complexes of heterocyclic ligands (including the imidazoles of the present study) correlate well with the ligand pK_a (25°C, aqueous solution), according to ²J = ?3.88 pK_a + 248.5. Results in the present work are discussed in relation to our previous work with nucleosides. The significance of the results to biological systems is considered.     

Gold(I)-NHC complexes of antitumoral diarylimidazoles: structures, cellular uptake routes and anticancer activities

Five new heterocyclic gold carbene complexes were prepared, four chlorido-[1,3-dimethyl-4,5-diarylimidazol-2-ylidene]gold complexes 6a-d and a chlorido-[1,3-dibenzylimidazol-2-ylidene]gold complex 11, and three of them were characterised by X-ray single crystal analyses. They were tested for cytotoxicity against a panel of four human cancer cell lines and non-malignant fibroblasts, for tubulin interaction, and for the pathways of their uptake into 518A2 melanoma cells. All complexes showed cytotoxic activity in the micromolar IC₅₀ range with distinct selectivities for certain cell lines. In stark contrast to related metal-free 1-methyl-4,5-diarylimidazoles, the complexes 6 and 11 did not noticeably inhibit the polymerisation of tubulin to give microtubules. The cellular uptake of complexes 6 occurred mainly via the copper transporter (Ctr1) and the organic cation transporters (OCT-1/2). Complex 11 was accumulated preferentially via the organic cation transporters and by Na⁺/K⁺-dependent endocytosis. The new gold carbene complexes seem to operate by a mechanism different from that of the parent 1-methylimidazolium ligands.     

Synthesis,spectroscopic characterization,electrochemical behavior and computational analysis of mixed diamine ligand gold(III) complexes: antiproliferative and in vitro cytotoxic evaluations against human cancer cell lines

The gold(III) complexes of the type [(DACH)Au(en)]Cl₃, 1,2-Diaminocyclohexane ethylenediamine gold(III) chloride [where 1,2-DACH = cis-, trans-1,2- and S,S-1,2diaminocyclohexane and en = ethylenediamine] have been synthesized and characterized using various analytical and spectroscopic techniques including elemental analysis, UV–Vis and FTIR spectra; and solution as well as solid-state NMR measurements. The solid-state ¹³C NMR shows that 1,2-diaminocyclohexane (1,2-DACH) and ethylenediamine (en) are strongly bound to the gold(III) center via N donor atoms. The stability of the mixed diamine ligand gold(III) was determined by ¹H and ¹³C NMR spectra. Their electrochemical behavior was studied by cyclic voltammetry. The structural details and relative stabilities of the four possible isomers of the complexes were also reported at the B3LYP/LANL2DZ level of theory. The coordination sphere of these complexes around gold(III) center adopts distorted square planar geometry. The computational study also demonstrates that trans- conformations is slightly more stable than the cis-conformations. The antiproliferative effects and cytotoxic properties of the mixed diamine ligand gold(III) complexes were evaluated in vitro on human gastric SGC7901 and prostate PC3 cancer cells using MTT assay. The antiproliferative study of the gold(III) complexes on PC3 and SGC7901 cells indicate that complex 1 is the most effective antiproliferative agent among mixed ligand based gold(III) complexes 1–3. The IC₅₀ data reveal that the in vitro cytotoxicity of complexes 1 and 3 against SGC7901 cancer cells are fairly better than that of cisplatin.     

Synthesis and anti-cancer activity of bis-amino-phosphine ligand and its ruthenium(II) complexes

The development of both chemotherapeutic drug resistance as well as adverse side effects suggest that the current chemotherapeutic drugs remain ineffective in treating the various types of cancers. The development of new metallodrugs presenting anti-cancer activity is therefore needed. Ruthenium complexes have gained a great deal of interest due to their promising anti-tumour properties and reduced toxicity in vivo. This study highlighted the effective induction of cell death in a malignant melanoma cell by two novel bis-amino-phosphine ruthenium(II) complexes referred to as GA105 and GA113. The IC₅₀ concentrations were determined for both the complexes, the ligand and cisplatin, for comparison. Both complexes GA105 and GA113 displayed a high anti-cancer selectivity profile as they exhibited low IC₅₀ values of 6.72 µM and 8.76 µM respectively, with low toxicity towards a non-malignant human cell line. The IC₅₀ values obtained for both complexes were lower than that of cisplatin. The new complexes were more effective compared to the free ligand, GA103 (IC₅₀ = >20 µM). Morphological studies on treated cells induced apoptotic features, which with further studies could indicate an intrinsic cell death pathway. Additionally, flow cytometric analysis revealed that the mode of cell death of complex GA113 was apoptosis. The outcomes herein give further insight into the potential use of selected Ru(II) complexes as alternative chemotherapeutic drugs in the future.     

New bio-sensitive and biologically active single crystal of pyrimidine scaffold ligand and its gold and platinum complexes: DFT,antimicrobial, antioxidant,DNA interaction,molecular docking with DNA/BSA and anticancer studies

Novel gold and platinum complexes [AuL₂]·Cl, 1 and [PtL₂]·2Cl, 2 with ligand, 2-methoxy-6-((2-(4-(trifluoromethyl)pyrimidin-2-yl)hydrazono)methyl)phenol (HL) have been synthesized and screened for their antimicrobial, antioxidant, DNA binding and anticancer (in vitro) activities. The single crystal of ligand HL was obtained by slow evaporation technique. The molecular structure of HL was confirmed from single crystal X-ray technique. Density functional theory calculations have been performed to gain insights into the electronic structure of these metal complexes. Antimicrobial result shows that, HL and complexes (1 and 2) have good antimicrobial agents against E. coli (bacteria) and C. albicans (fungi) than others bacterial and fungal strains. Antioxidant assay results suggest that, HL and complexes (1 and 2) possess good radical scavenging activity against diverse free radicals (DPPH, SOD, NO and H₂O₂). The intercalative interactions of HL and complexes (1 and 2) with CT-DNA were confirmed from spectroscopic titrations and viscometric measurements. Furthermore, the interactions of prepared compounds with DNA were confirmed by molecular docking analysis. In order to understand the nature of interactions between these metal complexes and BSA protein results clearly shows that complex 1 binds better than that of complex 2. The antitumor activities of prepared products were tested against single normal and different tumor cell lines by MTT assay. These results reveal that prepared complexes (1 and 2) have significant cytotoxic effect against tumor cell lines.     

Structural and solution chemistry, protein binding and antiproliferative profiles of gold(I)/(III) complexes bearing the saccharinato ligand

A series of new gold(I) and gold(III) complexes based on the saccharinate (sac) ligand, namely M[Au(sac)₂] (with M being Na⁺, K⁺ or NH₄⁺), [(PTA)Au(sac)], K[Au(sac)₃Cl] and Na[Au(sac)₄], were synthesized and characterized, and some aspects of their biological profile investigated. Spectrophotometric analysis revealed that these gold compounds, upon dissolution in aqueous media, at physiological pH, manifest a rather favourable balance between stability and reactivity. Their reactions with the model proteins cytochrome c and lysozyme were monitored by mass spectrometry to predict their likely interactions with protein targets. In the case of disaccharinato gold(I) complexes, cytochrome c adducts bearing four coordinated gold(I) ions were preferentially formed in high yield. In contrast, [(PTA)Au(sac)] (PTA = 1,3,5-triaza-7-phosphaadamantane) turned out to be poorly effective, only producing a mono-metalated adduct in very low amount. In turn, the gold(III) saccharinate derivatives were less reactive than their gold(I) analogues: K[Au(sac)₃Cl] and Na[Au(sac)₄] caused moderate protein metalation, again with evidence of formation of tetragold adducts. Finally, the above mentioned gold compounds were challenged against the reference human tumor cell line A2780S and its cisplatin resistant subline A2780R and their respective cytotoxic profiles determined. [(PTA)Au(sac)] turned out to be highly cytotoxic whereas moderate cytotoxicities were observed for the gold(III) complexes and only modest activities for disaccharinato gold(I) complexes. The implications of these results are thoroughly discussed in the light of current knowledge on gold based drugs.     

A novel unsymmetrical quadridentate ligand 1-(2′-aminophenyl)-6-methyl-2,5-diazanona-l,6-diene-8-one and its complexes with copper(II), nickel(II) and palladium(II)

Two routes for the preparation of the title compound (6) have been developed. Reaction of equimolar quantities of 2-aminobenzaldehyde, pentane- 2,4-dione and 1,2-diaminoethane yields the ligand 6 and 2-methyl-3-acetylquinoline as side product. The compound 6 was obtained in high yield in a one- step condensation of 2-aminobenzaldehyde with 1-amino-4-methyl-3-azahept-4-ene-6-one (5). Studies on the condensation of 5 with various 2-aminobenz- aldehyde derivatives revealed that the yield of unsymmetrical ligand is evidently influenced by the acidic properties of the hydrogen atom (of atoms) of the ring substituent in the ortho position to the carbonyl group. Copper(II), nickel(II) and palladium(II) complexes of 6 have been prepared and characterized. Spectroscopic data of nickel and palladium complexes are consistent with their planar structure. A superhyperfine splitting due to nitrogen is observed in the EPR spectrum of the copper complex in spite of the presence of azomethine hydrogen atom.     

A report describing the effect of di-2-pyridyl ligand architecture on the coordination properties with gold(III)

In an effort to develop novel gold-based chemotherapies, gold(III) coordination complexes possessing a series of di-2-pyridyl ligands were targeted as synthetic products. It was found that di-2-pyridyl ligands linked by different groups exhibited varying coordination to gold(III). Di-2-pyridyl sulfide (DPS) exhibited bidentate binding to gold(III), and formed a complex ion with a gold tetrachloride counter ion {[(DPS)AuCl₂]AuCl₄; compound 3}; di-2-pyridyl ether (DPO) formed a neutral monodentate coordination complex with gold(III) {[(DPO)(AuCl₃)]; compound 4}; and attempts to make a gold(III) complex with di-2-pyridyl ketone (DPK) were unsuccessful, as a complex ion possessing the protonated ligand and a gold tetrachloride anion was isolated {[HDPK][AuCl₄]; compound 5}. Compounds 3-5 were structurally characterized using X-ray crystallography, which confirmed the different coordination environments around the gold(III) metal centers.     

Ruthenium(II) carbonyl chloride complexes containing pyridine-functionalised bidentate N-heterocyclic carbenes: Synthesis, structures, and impact of the carbene ligands on catalytic activities

A series of new ruthenium(II) carbonyl chloride complexes with pyridine-functionalised N-heterocyclic carbenes [Ru(Py-NHC)(CO)₂Cl₂], [Py-NHC = 3-methyl-1-(2-pyridyl)imidazol-2-ylidene, 1 (1a and 1b); 3-methyl-1-(2-picoyl)imidazol-2-ylidene, 2 (2a and 2b); 3-methyl-1-(2-pyridyl)benzimidazolin-2-ylidene, 3 (3b); 3-methyl-1-(2-picoyl)benzimidazolin-2-ylidene, 4 (4a and 4b); 1-methyl-4-(2-pyridyl)-1,2,4-triazoline-5-ylidene, 5 (5a and 5b)] have been prepared by transmetallation from the corresponding silver carbene complexes and characterized by NMR, IR spectroscopy and elemental analysis. In these complexes with bidentate Py-NHC ligands, one CO ligand is trans to the Py ligand. In 1a, 2a, 4a, and 5a, the NHC ligand is trans to the other CO ligand, thus leaving the two Cl⁻ ligands trans to each other. In 1b, 2b, 3b, 4b, and 5b, the NHC ligands are trans to one Cl⁻ ligand, and the two Cl⁻ ligands are cis to each other. The structures for 1b, 2b, 3b and 4b have been determined by single-crystal X-ray diffraction. These complexes are efficient catalysts in the transfer hydrogenation of acetophenone and their catalytic activities are found to be influenced by electronic effect of the N-heterocyclic carbene ligands.     

Tumor cytotoxicity of 5,6-dimethyl-1,10-phenanthroline and its corresponding gold(III) complex

A gold(III) complex possessing 5,6-dimethyl-1,10-phenanthroline (5,6DMP) was synthesized and fully characterized using standard spectroscopic techniques, as well as X-ray crystallography and elemental analysis. The complex [(5,6DMP)AuCl₂][BF₄] (2) was found to possess a distorted square planar geometry about the gold(III) center, commonplace for d⁸ Au(III) cations possessing sterically un-hindered polypyridyl ligands. Compound 2 was evaluated for its potential use as an anticancer therapeutic. It was determined that the complex is stable in phosphate buffer over a 24-hour period, thought it does undergo rapid reduction in the presence of equimolar amounts of reduced glutathione (GSH) and ascorbic acid. The DNA binding and in vitro tumor cytotoxicity of the title compound 2 were also determined. It was found to undergo weak and reversible binding to calf thymus DNA, and was more cytotoxic towards a panel of human cancer cell lines than the commonly used chemotherapy agent cisplatin. Cytotoxicity experiments with the free 5,6DMP ligand indicate that the ligand has IC₅₀ values that are slightly lower than those observed for the gold complex (2), and coupled with the fact that the ligand appears to be released from the gold(III) metal center in reducing environments, this suggests the ligand itself may play an important role in the antitumor activity of the parent gold complex.     

Palladium(II) complexes of quinolinylaminophosphonates: synthesis, structural characterization, antitumor and antimicrobial activity

Three types of palladium(II) halide complexes of quinolinylaminophosphonates have been synthesized and studied. Diethyl and dibutyl [α-anilino-(quinolin-2-ylmethyl)]phosphonates (L1, L2) act as N,N-chelate ligands through the quinoline and aniline nitrogens giving complexes cis-[Pd(L1/L2)X₂] (X═Cl, Br) (1-4). Their 3-substituted analogues [α-anilino-(quinolin-3-ylmethyl)]phosphonates (L3, L4) form dihalidopalladium complexes trans-[Pd(L3/L4)₂X₂] (5-8), with trans N-bonded ligand molecules only through the quinoline nitrogen. Dialkyl [α-(quinolin-3-ylamino)-N-benzyl]phosphonates (L5, L6) give tetrahalidodipalladium complexes [Pd₂(L5/L6)₃X₄] (9-12), containing one bridging and two terminal ligand molecules. The bridging molecule is bonded to the both palladium atoms, one through the quinoline and the other through the aminoquinoline nitrogen, whereas terminal ligand molecules are coordinated each only to one palladium via the quinoline nitrogen. Each palladium ion is also bonded to two halide ions in a trans square-planar fashion. The new complexes were identified and characterized by elemental analyses and by IR, UV-visible, ¹H, ¹³C and ³¹P nuclear magnetic resonance and ESI-mass spectroscopic studies. The crystal structures of complexes 1-4 and 6 were determined by X-ray structure analysis. The antitumor activity of complexes in vitro was investigated on several human tumor cell lines and the highest activity with cell growth inhibitory effects in the low micromolar range was observed for dipalladium complexes 11 and 12 derived from dibutyl ester L6. The antimicrobial properties in vitro of ligands and their complexes were studied using a wide spectrum of bacterial and fungal strains. No specific activity was noted. Only ligands L3 and L4 and tetrahalidodipalladium complexes 9 and 11 show poor activities against some Gram positive bacteria.     

Intermetallic coinage metal-catalyzed functionalization of alkanes with ethyl diazoacetate: Gold as a ligand

The complexes [Au₂M₂(C₆F₅)₄(NCMe)₂]_n (M = Cu, 1; M = Ag, 2) have been tested as catalysts for the functionalization of alkanes by the carbene insertion methodology, using ethyl diazoacetate as the carbene source. Moderate to high conversions have been obtained. The observed selectivities seem to favor the proposal that the active metal for catalysis is the Cu/Ag center, the Au(C₆F₅)₂ unit acting as a spectator ligand in both cases.     

New gold carbene complexes as candidate anticancer agents

Three structurally related gold(I) carbene complexes with bulky hydrophobic ligands i.e. 1–3 were investigated in solution for further consideration as candidate anticancer agents. Cytotoxic assays were subsequently conducted on bone marrow-derived preosteoclast cell line of human origin (FLG 29.1) and human colon cancer cells (HCT-116). A far greater cytotoxic activity was measured for compound 1 against HCT-116 cells compared to 2 and 3; conversely, all compounds were highly and similarly active against FLG 29.1 cells. Results obtained for the reaction of complexes 1 and 2 with RNase A documented the occurrence of a weak interaction with this model protein and the formation of a tiny amount of the corresponding adduct. Moreover, a certain reactivity of the complex 2 was also detected toward GSH. The general implications of the obtained results are discussed.     

Norfloxacin Zn(II)-based complexes: acid base ionization constant determination, DNA and albumin binding properties and the biological effect against Trypanosoma cruzi

Zn(II) complexes with norfloxacin (NOR) in the absence or in the presence of 1,10-phenanthroline (phen) were obtained and characterized. In both complexes, the ligand NOR was coordinated through a keto and a carboxyl oxygen. Tetrahedral and octahedral geometries were proposed for [ZnCl₂(NOR)]·H₂O (1) and [ZnCl₂(NOR)(phen)]·2H₂O (2), respectively. Since the biological activity of the chemicals depends on the pH value, pH titrations of the Zn(II) complexes were performed. UV spectroscopic studies of the interaction of the complexes with calf-thymus DNA (CT DNA) have suggested that they can bind to CT DNA with moderate affinity in an intercalative mode. The interactions between the Zn(II) complexes and bovine serum albumin (BSA) were investigated by steady-state and time-resolved fluorescence spectroscopy at pH 7.4. The experimental data showed static quenching of BSA fluorescence, indicating that both complexes bind to BSA. A modified Stern–Volmer plot for the quenching by complex 2 demonstrated preferential binding near one of the two tryptophan residues of BSA. The binding constants obtained (K _b ) showed that BSA had a two orders of magnitude higher affinity for complex 2 than for 1. The results also showed that the affinity of both complexes for BSA was much higher than for DNA. This preferential interaction with protein sites could be important to their biological mechanisms of action. The analysis in vitro of the Zn(II) complexes and corresponding ligand were assayed against Trypanosoma cruzi, the causative agent of Chagas disease and the data showed that complex 2 was the most active against bloodstream trypomastigotes.     

Synthesis and characterization of the cobalt(III) complexes of two pendant-arm cross-bridged cyclams

The cobalt(III) complexes of 4,11-diacetato-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), [Co(1)]PF₆, and 4,11-diacetamido-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (2), [Co(2)][PF₆]₃, have been synthesized and characterized. The crystal structure of [Co(1)]PF₆ consists of an octahedral cobalt(III) cation coordinated to all four ligand nitrogen donors in the macrobicycle’s cavity, as well as to the deprotonated carboxylate oxygen atoms of both pendant arms. Analytical and spectroscopic data indicates that the ligand in [Co(2)][PF₆]₃ is not deprotonated, suggesting coordination through the amide carbonyl oxygens. Study of the electronic spectra of these novel complexes and comparison with data from related cobalt(III) complexes characterizes the ligands as strong field with Δ₀=24,040 and Δ₀=24,250 cm⁻¹ for 1 and 2, respectively. Cyclic voltammograms were obtained for both complexes with large variations observed due to the differences in ligand charge and coordination.     

A convenient synthesis of phosphine-functionalized N-heterocyclic carbene ligand precursors, structural characterization of their palladium complexes and catalytic application in Suzuki coupling reaction

A series of imidazolium chlorides as ligand precursors, L · HCl (L = (1-R)-(3-diphenylphosphanylethyl)-imidazol-2-ylidene; R = aryl, benzyl, naphthylmethyl), for the phosphine-functionalized N-heterocyclic carbene (NHC), L, were prepared by a convenient synthetic procedure of reacting 1,2-dichloroethane with appropriate N-substituted imidazoles to give (β-chloroethyl)imidazolium chlorides, which were subsequently reacted with HPPh₂ producing L · HCl in good yield. Palladium complexes of L, PdLCl₂ (4), were prepared by a one pot reaction of PdCl₂, sodium acetate, and L · HCl in DMSO. Complexes 4b (R = naphthylmethyl) and 4e (R = m-methoxybenzyl) were characterized by X-ray crystallography. Catalytic studies have shown that the palladium complexes are efficient in Suzuki coupling reactions of aryl bromides with phenylboronic acid.     

Comparison of structural features of three new cis-dioxomolybdenum(VI) complexes with 2-hydroxy-1-naphthaldehyde-S-methylisothiosemicarbazone: Possible role of intermolecular interactions on the geometry of the cis-MoO2 unit

Three closely related [MoO₂(L)(ML)] complexes, where L is the 2-hydroxy-1-naphthaldehyde-S-methylisothiosemicarbazone ligand, and ML is EtOH (I), Py (II) and DMSO (III), were synthesized, characterized by NMR and IR spectra, and their X-ray crystal structures were determined. The crystal structure properties of these three closely related complexes were compared. Two cis-Mo-O bond lengths were almost the same in the crystal structure of complexes II and III, while in complex I a significant difference between the two cis-Mo-O bond lengths was observed. At the same time, the geometry of L ligand in complex I is different, compared to II and III. DFT calculations on the isolated molecule I, as well as geometrical analysis of the complexes indicate that intramolecular interactions are not responsible for these structural differences. On the other hand, the pattern of intermolecular contacts in the crystal structure of I differs from those observed in II and III. Analyses indicate that differences in cis-Mo-O bond lengths and in the geometry of ligand L could be related to intermolecular interactions. These results suggest the possibility that in enzymes oxotransferases or in their model systems, the Mo-O bond length could be designed by the interactions of chelate ligands with the surroundings.     

Homoleptic carbene complexes: Part IX. Bis(imidazolin-2-ylidene-1-yl)borate complexes of palladium(II), platinum(II) and gold(I)

The in situ synthesis of the monoanionic chelating dicarbene ligand bis(imidazolin-2-ylidene-1-yl)borate (BIS^R, R=Me (2a), Et (3a), ⁱPr (4a)) from potassium bis(imidazol-1-yl)dihydridoborate (1) via dialkylation with alkyl iodide and deprotonation with LiⁿBu is described. Treatment of PdI₂ and PtCl₂ with THF solutions of BIS^R (1:2) leads to the first neutral homoleptic tetracarbene complexes of these metals 5–9, which are highly soluble in organic media. According to the X-ray structure analyses of 5, 6 and 9, the bischelates adopt centrosymmetric trans double-boat conformations exhibiting the usual stereochemical features. Neutral heteroleptic dicarbene–phosphine complexes of palladium and platinum of the general formula [M(BIS^R)(I)(PEt₃)] (14–19) result in good yields from the reaction of BIS^R with [M(μ-Cl)Cl(PEt₃)]₂ (2:1). From X-ray structure determinations of 15, 16 and 18 two findings should be emphasized: the heavy out-of-plane bending of the boat-shaped chelate ring (which is the cause of chirality of these complexes), and the existence of two rather different M to carbene carbon bond lengths (indicating the stronger trans influence of PEt₃ as compared to I⁻). With Au(Cl)(PPh₃), instead of forming small chelate rings, the bidentate ligands BIS^R switch to a μ₂-η¹:η¹-bridging ligand type of function giving rise to the compounds 10–12. An X-ray study of 11 reveals the structure of a 12-membered dimetallacycle in a twisted boat-like conformation with a trans-annular Au⋯Au separation of 3.3610(7) Å, i.e. only a weak interaction. In addition, two by-products — the three-coordinate iodobis(triphenylphosphine)gold(I) (13) and trans-diiodobis(triethylphosphine)palladium(II) (20) — have been structurally characterized.     

Synthesis and spectroscopic investigations of four-coordinate nickel complexes supported by a strongly donating scorpionate ligand

Two four-coordinate nickel complexes, HB(^tBuIm)₃NiBr and HB(^tBuIm)₃NiNO, were prepared by reaction of a bulky tris(carbene)borate ligand with NiBr₂(PPh₃)₂ and NiBr(NO)(PPh₃)₂, respectively, and structurally and spectroscopically characterized. In addition to standard techniques, high-frequency and -field electron paramagnetic resonance (HFEPR) was employed to understand the spin triplet (S = 1) ground state of the bromo complex. HFEPR, combined with electronic absorption spectroscopy allows comparison of this novel complex with other paramagnetic four-coordinate Ni(II) species. The tris(carbene)borate ligand is a stronger σ-donor than corresponding tris(pyrazolyl)borates (traditional “scorpionate” ligands). The tris(carbene)borate ligand may also act as a π-acceptor, in contrast to tris(pyrazolyl)borates, which show relatively little π-bonding interactions. The influence of tris(carbene)borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO}¹⁰ derivatives. HFEPR spectra of HB(^tBuIm)₃NiBr exhibit hyperfine coupling from Br, which indicates the strong electronic interaction between Ni(II) and this halide ligand, consistent with studies on tris(pyrazolyl)borate Ni(II) complexes.     

Effect of metal coordination and intra-molecular H-bond on the acidity of phenolic proton in a set of structurally characterized octahedral Ni(II) complexes of l-histidine derivative

Four different mononuclear octahedral Ni(II) complexes with protonated and deprotonated form of the same ligand have been synthesized by controlling reaction conditions and structurally characterized. The complexes are [Ni(HL^l-his)(benzoate)(MeOH)] (1), [Ni(HL^l-his)(SCN)(MeOH)] (2), [Ni(HL^l-his)₂] (3) and [Ni(L^l-his)(imidazole)₂] (4) where H₂L^l-his is (S)-2-(2-hydroxybenzylamino)-3-(1H-imidazol-4-yl)-propionic acid. The ligand behaves as a monobasic tetradentate ligand in 1 and 2, monobasic tridentate ligand in 3 and dibasic tetradentate ligand in 4. Ni(II) coordinated phenolic proton of the ligand in the complexes 1-2 shows strong intra-molecular H-bonding with benzoate in 1 and lattice water in 2, whereas 3 shows intermolecular H-bonding between uncoordinated phenols with neighbouring carboxylate. The pH titration of the complexes revealed that metal coordination and H-bond in complexes 1 and 2 considerably lowers the acidity of ligand phenol (pK_a 6.8 and 7.0 respectively) compared to phenol (pK_a 10). The complex 4 does not show any proton loss due to the absence of phenolic proton. All the complexes show extensive H-bonded network in the crystals including narrow (7.8 × 5.2 Å) water filled one dimensional channel in 2.