Cytotoxic palladium(II) complexes of 8-aminoquinoline derivatives and the interaction with human serum albumin

Palladium(II) complexes are potential antitumor metallodrugs for their chemical resemblance to platinum(II) complexes. Two palladium(II) complexes (1 and 2) in the formula of [PdLⁿCl] [L¹ = N-(tert-butoxycarbonyl)-l-methionine-N′-8-quinolylamide, L² = L-alanine-N′-8-quinolylamide] have been synthesized accordingly. The structures of the complexes were fully characterized by X-ray crystallography. The palladium(II) center in 1 is coordinated by two N atoms and an S atom from L¹ with one chloride anion as the leaving group; while that in 2 is coordinated by three N atoms from L² with one chloride anion as the leaving group. The interaction between complex 1 and human serum albumin (HSA) has been investigated using fluorescence and circular dichroism spectroscopies. The complex seems to react with HSA chiefly through hydrophobic and electrostatic interactions, and it does not alter the α-helical nature of HSA. The cytotoxicity of these complexes has been tested against the human cervical cancer (HeLa), human mammary cancer (MCF-7), and human lung cancer (A-549) cell lines. Complex 1 displays a cytotoxic activity comparable to that of cisplatin, but complex 2 is less active than cisplatin.     

Photoactive trans ammine/amine diazido platinum(IV) complexes

The synthesis and characterisation of eight new octahedral Pt^IV complexes of the type trans,trans,trans-[Pt(N₃)₂(OH)₂(NH₃)(Am)] where Am = methylamine (2), ethylamine (4), thiazole (6), 2-picoline (8), 3-picoline (10), 4-picoline (12), cyclohexylamine (14), and quinoline (16) are reported, including the X-ray crystal structures of complexes 2, 8, and 14 as well as that of two of the precursor Pt^II complexes (trans-[Pt(N₃)₂(NH₃)(methylamine)] (1) and trans-[Pt(N₃)₂(NH₃)(cyclohexylamine)] (13)). Irradiation with UVA light rapidly induces loss in intensity of the azide-to-Pt^IV charge-transfer bands and gives rise to photoreduction of platinum. These complexes have potential for use as photoactivated anticancer agents.     

Mo- and W-N2 and -CO complexes with novel mixed P/N ligands: Structural properties and implications to synthetic nitrogen fixation

Four new ligands containing a pyridine or thiazole group and one or more N-(diphenylphosphinomethyl)amine functions have been prepared and employed for the synthesis of Mo(0) and W(0) carbonyl and dinitrogen complexes. For comparison coordination of the literature-known ligand N,N-bis(diphenylphosphinomethyl)-methylamine (PNP, 1) to such systems has been investigated as well. Two new ligands are N,N-bis(diphenylphosphinomethyl)-2-aminopyridine (pyNP₂, 2) and N,N′-bis(diphenylphosphinomethyl)-2,6-diaminopyridine (PpyP, 3). In a third new ligand, N-diphenylphosphinomethyl-2-aminothiazole (thiazNP, 4), the pyridine group is replaced by thiazol. Finally, the pentadentate ligand N,N,N′,N′-tetrakis(diphenylphosphinomethyl)-2,6-diaminopyridine (pyN₂P₄, 5) has been synthesized. Coordination of ligands 2, 3 and 4 to low-valent metal centers is investigated on the basis of the three molybdenum carbonyl complexes [Mo(CO)₃(NCCH₃)(pyNP₂)] (6), [Mo(CO)₄(PpyP)] (7) and [Mo(CO)₄(thiazNP)] (8), respectively, all of which are structurally characterized. Moreover, employing ligands 1 and 2 the two dinitrogen complexes [W(N₂)₂(dppe)(PNP)] (9) and [Mo(N₂)₂(dppe)(pyNP₂) (10), respectively, are prepared. Both systems are investigated by vibrational and NMR spectroscopy; in addition, complex 10 is structurally characterized.     

Synthesis, structural characterization and electrochemical activity of oxidovanadium(IV/V) complexes of a diprotic ONS chelating ligand

The present work reports the chemistry of a few oxidovanadium(IV) and (V) complexes of the ONS chelating ligand S-benzyl-β-N-(2-hydroxyphenylethylidine) dithiocarbazate (H₂L). Major objective of this work is to arrive at some general conclusions about the influence of binding environment generated by the replacement of an O-donor center by a S-donor point in a ligand (of a similar arrangement of the other O- and N-donor points) on the redox behavior and on the structural features of comparable [VO(OEt)(ONS)] and [VO(OEt)(ONO)] complexes. Synthesis, characterization by various physicochemical techniques (UV-Vis, IR, EPR and elemental analysis), exploration of electrochemical activity of the oxidovanadium(V) complex [V^VO(OEt)L] (1), the mixed ligand complex [V^VO(N-O)L] (3) (where N-O is the mono anion of 8-hydroxyquinoline) and a binuclear complex [V^VO(OEt)L]₂(μ-4,4′-bipy) (2) are reported. Similar studies on of mixed ligand oxidovanadium(IV) complexes of the formula [V^VO(N-N)L] (4,5) (where N-N = 2,2′-bipy and o-phen) are also presented here. The [V^VO(OEt)L] complex is pentacoordinated and distorted square pyramidal, while the [V^IV(N-N)L] complexes are hexacoordinated and octahedral. Structural features of the complex 1 were compared with the corresponding aspects of the previously reported analogous complex [V^VO(OEt)(ONO)] (1′).     

Intramolecular ether oxygen coordination in the zinc complexes with dipicolylamine (DPA)-derived ligands

The ether oxygen coordination to the zinc center in the complexes with dipicolylamine (DPA)-derived ligands, N-(2-methoxyethyl)-N,N-bis(2-pyridylmethyl)amine (L), N-(3-methoxypropyl)-N,N-bis(2-pyridylmethyl)amine (L′), and N-{3-(2-pyridylmethyloxy)propyl}-N,N-bis(2-pyridylmethyl)amine (L^Py) has been discussed. Upon chelation of the oxygen atom, L forms a five-membered chelate ring with respect to the 2-aminoethyl ether moiety whereas L′ forms a six-membered chelate in 3-aminopropyl ether unit. This difference was highlighted by the crystal structures of ZnCl₂ complexes, in which [Zn(L)Cl₂] (1) exhibited ether oxygen coordination but [Zn(L′)Cl₂] (2) had the ether oxygen non-coordinated. The terminal pyridyl group of L^Py facilitates the ether oxygen atom coordination via a metal binding from the basal plane trans to the aliphatic nitrogen.     

Cyanato bridged binuclear nickel(II) and copper(II) complexes with pyridylpyrazole ligand: Synthesis, structure and magnetic properties

Binuclear cyanate bridged nickel(II) complex [Ni(L)(NCO)]₂(PF₆)₂ (1) and copper(II) complex [Cu(L)(NCO)]₂(PF₆)₂ (2), where L is N,N-bis(3,5-dimethylpyrazol-1-ylmethyl)aminomethylpyridine, a tetradentate N₄-coordinated ligand have been synthesized and characterized by physicochemical method. The structures of complexes 1 and 2 have been studied by single crystal X-ray diffraction analysis. The structure analysis reveals that both nickel(II) and copper(II) center are coordinated in distorted octahedral fashion and coordination mode of cyanate ligand is end-to-end (μ-1,3) for complex 1 but it is double end-on (μ-1,1) mode for complex 2. The variable temperature magnetic susceptibility data, measured from 2 to 300 K, show weak antiferromagnetic interaction with J value −6.2(1) cm⁻¹ for complex 1, whereas complex 2 has very weak ferromagnetic interaction with J value +0.5(1) cm⁻¹.     

Pd(II) complexes containing N-alkyl-3-pyridine-5-trifluoromethyl pyrazole ligands: Synthesis, NMR studies and X-ray crystal structures

Palladium [PdCl₂(L)] complexes with N-alkylpyridylpyrazole derived ligands [2-(5-trifluoromethyl-1H-pyrazol-3-yl)pyridine (L¹), 2-(1-ethyl-5-trifluoromethyl-1H-pyrazol-3-yl)pyridine (L²), 2-(1-octyl-5-trifluoromethyl-1H-pyrazol-3-yl)pyridine (L³), and 2-(3-pyridin-2-yl-5-trifluoromethyl-pyrazol-1-yl)ethanol (L⁴) were synthesised. The crystal and molecular structures of [PdCl₂(L)] (L = L², L³, L⁴) were resolved by X-ray diffraction, and consist of monomeric cis-[PdCl₂(L)] molecules. The palladium centre has a typical square-planar geometry, with a slight tetrahedral distortion. The tetra-coordinate metal atom is bonded to one pyridinic nitrogen, one pyrazolic nitrogen and two chlorine ligands in cis disposition. Reaction of L (L², L⁴) with [Pd(CH₃CN)₄](BF₄)₂, in the ratio 1M:2L, gave complexes [Pd(L)]₂(BF₄)₂. Treatment of [PdCl₂(L)] (L = L², L⁴) with NaBF₄ and pyridine (py) and treatment of the same complexes with AgBF₄ and triphenylphosphine (PPh₃) yielded [Pd(L)(py)₂](BF₄)₂ and [Pd(L)(PPh₃)₂](BF₄)₂ complexes, respectively. Finally, reaction of [PdCl₂(L⁴)] with 1 equiv of AgBF₄ yields [PdCl(L⁴)](BF₄).     

(Pyrazolylmethyl)pyridine platinum(II) and gold(III) complexes: Synthesis, structures and evaluation as anticancer agents

Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1), 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2), 2-(3,5-di-tert-butylpyrazol-1-ylmethyl)pyridine (L3) and 2-(3-p-tolylpyrazol-1-ylmethyl)pyridine (L4) with K₂[PtCl₄] in a mixture of ethanol and water formed the dichloro platinum complexes [PtCl₂(L1)] (1), [PtCl₂(L2)] (2), [PtCl₂(L3)] (3) and [PtCl₂(L4)] (4). Complex 1, [PtCl₂(L1)], could also be prepared in a mixture of acetone and water. Performing the reactions of L2 and L3 in a mixture of acetone and water, however, led to C-H activation of acetone under mild conditions to form the neutral acetonyl complexes [Pt(CH₂COCH₃)Cl(L2)] (2a) and [Pt(CH₂COCH₃)Cl(L3)] (3a). The same ligands reacted with HAuCl₄ · 4H₂O in a mixture of ethanol and water to form the gold salts [AuCl₂(L1)][AuCl₄] (5) [AuCl₂(L2)][Cl] (6) [AuCl₂(L3)][Cl] (7) and [AuCl₂(L4)][AuCl₄] (8); however, with the pyrazolyl unit in the para position of the pyridinyl ring in 4-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L5), 4-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L6) neutral gold complexes [AuCl₃(L5)] (9) and [AuCl₂(L6)] (10) were formed; signifying the role the position of the pyrazolyl group plays in product formation in the gold reactions. X-ray crystallographic structural determination of L6, 2, 33a, 8 and 10 were very important in confirming the structures of these compounds; particularly for 3a and 8 where the presence of the acetonyl group confirmed C-H activation and for 8 where the counter ion is . Cytotoxicity studies of L2, L4 and complexes 1-10 against HeLa cells showed the Au complexes were much less active than the Pt complexes.     

Syntheses, structures and bioactivities of silver(I) complexes with dithioether ligands that contain a di- or triethylene glycol chain and different terminal groups

A series of flexible dithioethyl ligands that contain ethyleneoxy segments were designed and synthesized, including bis(2-(pyridin-2-ylthio)ethyl)ether (L¹), 1,2-bis(2-(pyridin-2-ylthio)ethoxy)ethane (L²), bis(2-(benzothiazol-2-ylthio)ethyl)ether (L³) and 1,2-bis(2-(benzothiazol-2-ylthio)ethoxy)ethane (L⁴). Reactions of these ligands with AgNO₃ led to the formation of four new supramolecular coordination complexes, [Ag₂L¹(NO₃)₂]₂ (1), [Ag₂L²(NO₃)₂] (2), [AgL³(NO₃)]_∞ (3) and [AgL⁴(NO₃)] (4) in which the length of the (CH₂CH₂O)_n spacers and the terminal groups of ligands cause subtle geometrical differences. Studies of the inhibitory effect to the growth of Phaeodactylum tricornutum show that all four complexes are active and the compound 4 has the highest inhibitory activity.     

Metal compounds for the treatment of parasitic diseases

The cysteine proteases of the trypanosomatid parasitic protozoa have been validated as targets for chemotherapy of Chagas’ disease and leishmaniasis. Metal complexes of gold, platinum, iridium, palladium, rhodium and osmium have been reported to have activity against a variety of trypanosomatids, but the molecular target of these compounds has not been defined. The activity of gold(III) and palladium(II) cyclometallated complexes, and oxorhenium(V) complexes against mammalian and parasitic cysteine proteases was investigated. All gold(III) complexes (1-6) inhibited cathepsin B with IC₅₀ values in the range of 0.2-1.4 μM. Of the six palladium compounds, aceto[2,6-bis[(butylthio-κS)methyl]phenyl-κC]-, (SP-4-3)-palladium(II) (11) was the most potent inhibitor of cathepsin B with an IC₅₀ of 0.4 μM. A clear structure-activity relationship was observed with the oxorhenium(V) complexes with chloro[2,2′-(thio-κS)bis[ethanethiolato-κS)]] oxorhenium(V) (16) being the most potent inhibitor of cathepsin B with an IC₅₀ of 0.009 μM. Six complexes were further tested against the parasite cysteine proteases, cruzain from T. cruzi, and cpB from L. major; the most potent inhibitors were the two rhenium complexes (2(1H)-pyridinethionato-κS²)[2,6-bis[(mercapto-κS)methyl]pyridine-κN¹] oxorhenium(V) (15) and chloro[2,2′-(thio-κS)bis[ethanethiolato-κS)]] oxorhenium(V) (16). The compounds were also evaluated in assays for parasite growth. Two oxorhenium(V) compounds ((p-methoxyphenylthiolato-S)[2,6-bis[(mercapto-κS)methyl]pyridine-κN¹] oxorhenium(V) (14) and (methanethiolato)[2,2′-(thio-κS)bis[ethanethiolato-κS)]] oxorhenium (V) (18)) and the palladium compound 11 inhibited T. cruzi intracellular growth, and compound 11 inhibited promastigote growth in three Leishmania species. In conclusion this preliminary data indicates that metal complexes targeted at parasite cysteine proteases show promise for the treatment of both Chagas’ disease and leishmaniasis.     

New dinuclear arene ruthenium complexes with P,S- or P,O-chelating ligands

Two equivalents of 2-diphenylphosphinobenzoic acid react with 1,2-ethanedithiol and 1,8-diaminonaphthalene under peptidic coupling conditions to give the new ligands 1,2-bis-S-[2^′-(diphenylphosphino)benzoyl]dithioethane (dppte) (1) and 1,2-bis-N-[2^′-(diphenylphosphino)benzoyl]diaminonaphthalene (dppan) (2), respectively. 1 and 2 have been characterised by mass spectrometry, elemental analysis, NMR, IR spectroscopy, and by single-crystal X-ray structure analysis. 2 is easily oxidised by air to give the monophosphine oxide derivatives (3). Single-crystal X-ray structure analysis of 3 shows an intramolecular hydrogen bond between an amido and the phosphoryl oxygen atom. Compounds 1 and 2 react with [RuCl₂(η⁶-p-cymene)]₂ to give the dinuclear complexes [RuCl(η⁶-p-cymene)(dppte)RuCl(η⁶-p-cymene)]²⁺ (4) and [RuCl(η⁶-p-cymene)(dppan)RuCl(η⁶-p-cymene)]²⁺ (5). As determined by single-crystal X-ray structure analysis, 4 and 5 adopt different coordination modes to the ruthenium atoms. In 4 the symmetric dppte ligand is P,S coordinated to the ruthenium atom, whereas in 5 the dppan ligand prefers a P,O coordination mode.     

Synthesis of new palladium(II) compounds with several bidentate nitrogen-donor ligands: Structural analyses by H and C{H} NMR spectroscopy and crystal structures

The reaction of [PdCl₂(CH₃CN)₂] with N-alkylaminopyrazole (NN′) ligands, 1-[2-(ethylamino)ethyl]-3,5-dimethylpyrazole (deae), 1-[2-(ⁱpropylamino)ethyl]-3,5-dimethylpyrazole (deai), and 1-[2-(^tbutylamino)ethyl]-3,5-dimethylpyrazole (deat), affords a series of square planar Pd(II) complexes [PdCl₂(NN′)] (NN′ = deae (1), deai (2) and deat (3)). The solid-state structures of complexes 1 and 3 were determined by single crystal X-ray diffraction studies. The NN′ ligands are coordinated through the N_pz and N_amine atoms to the metal atom, which completes its coordination with two chlorine atoms in a cis disposition. These palladium(II) compounds were characterised by elemental analyses, conductivity measurements, IR, ¹H and ¹³C{¹H} NMR spectroscopies. The NMR studies of the complexes prove the rigid conformation of the ligands when they are complexed.     

Synthesis, characterization, solid-state photo-luminescence and anti-tumor activity of zinc(II) 4′-phenyl-terpyridine compounds

Reactions between 4′-phenyl-terpyridine (L) and several Zn(II) salts (sulfate, nitrate, chloride or acetate) led to the formation of the complexes [Zn₂(μ-O₂SO₂)₂L₂(CH₃OH)₂] (1), [Zn(NO₃)L(H₂O)]NO₃ (2), [Zn(Cl)₂L] (3) and [Zn(CH₃COO)₂L] (4) which were characterized by IR, ¹H NMR and fluorescence spectroscopies, elemental analysis and single crystal X-ray diffraction. In the dinuclear molecule of 1 the Zn atom is hexacoordinated, with a N₃O₃ coordination environment and forms an octagonal ZnOSOZnOSO metallacycle. In the remaining structures, the metal atom is envisaged as possessing highly distorted N₃X₂ (X = O or Cl) square pyramid coordination geometries. The structure of 3 presents two different packing patterns which lead to distinct π-π stackings. In both structures 2 and 4, strong intermolecular hydrogen bonds were identified. The complexes exhibit promising in vitro tumor-inhibiting activities, which are higher than that of cisplatin, against the following human tumor cell lines: promyelocyticfina leukaemia (HL-60), hepatocellular carcinoma (Bel-7402), gastric carcinoma (BGC-823) and nasopharyngeal carcinoma (KB).     

Synthesis, crystal structures and, antibacterial and antiproliferative activities in vitro of palladium(II) complexes of triphenylphosphine and thioamides

Palladium(II) complexes with triphenylphosphine (PPh₃) and thioamides of the general formulae, [Pd(L)₂(PPh₃)₂]Cl₂ and [Pd(L)₂(PPh₃)₂] have been prepared and characterized by elemental analysis, IR and NMR (¹H, ¹³C and ³¹P) methods, and two of them (trans-[Pd(PPh₃)₂(Dmtu)₂]Cl₂·(H₂O)(CH₃OH)_0.5 (1) and trans-[Pd(PPh₃)₂(Mpy)₂] (2)) by X-ray crystallography; where L = thiourea (Tu), methylthiourea (Metu), N,N′-dimethylthiourea (Dmtu), tetramethylthiourea (Tmtu), 2-mercaptopyridine (Mpy), 2-mercaptopyrimidine (Mpm) and thionicotinamide (Tna). The spectral data of the complexes are consistent with the sulfur coordination of thioamides to palladium(II). The crystal structures of the complexes show that (1) has ionic character consisting of [Pd(PPh₃)₂(Dmtu)₂]⁺² cations and uncoordinated Cl⁻ ions, while (2) is a neutral complex with Mpy behaving as anionic thiolate ligand. The coordination environment around palladium in (2) is nearly regular square-planar, while in (1) the trans angles show significant distortions from 180°. The complexes were screened for antibacterial effects, brine shrimps lethality bioassay and antitumor activity. These complexes showed significant activities in most of the cases against the tested bacteria as compared to that of a standard drug. Their antitumor activity against prostate cancer cells (PC3) is comparable with doxorubicin, together with no cytotoxic effects in brine shrimps lethality bioassay study.     

Organotin meclofenamic complexes: Synthesis, crystal structures and antiproliferative activity of the first complexes of meclofenamic acid - Novel anti-tuberculosis agents

The complexes [Me₂(Meclo)SnOSn(Meclo)Me₂]₂ (2) and [Ph₃Sn(Meclo)] (3) where HMeclo is meclofenamic acid, N-(2,6-dichloro-m-tolylanthranilic acid)], have been prepared and structurally characterized by means of vibrational, ¹H and ¹³C NMR spectroscopies. The crystal structure of complexes (2) and (3) have been determined by X-ray crystallography. Three distannoxane rings are present to the dimeric tetraorganodistannoxane of planar ladder arrangement of (2). The structure is centro symmetric and features a central rhombus Sn₂O₂ unit two additional tin atoms linked at the oxygen atoms. Five- and six-coordinated tin centers are present in the dimer distannoxane. X-ray analysis of (3) revealed a penta-coordinated structure containing Ph₃Sn coordinated to the chelated carboxylato group. The polar imino hydrogen atom participates in intra-molecular hydrogen bonds. Complexes (2) and (3) are self-assembled via π → π, C-H-π, stacking interactions and intra-molecular hydrogen bonds. Meclofenamic acid and [Ph₃Sn(Meclo)] have been evaluated for antiproliferative activity in vitro against three human cancer cell lines: MCF-7 (human breast cancer cell line), T24 (bladder cancer cell line), A-549 (non-small cell lung carcinoma) and a mouse L-929 (a fibroblast-like cell line cloned from strain L). The [Ph₃Sn(Meclo)] complex exhibited high cytotoxic activity against all the cancer cell lines. Meclofenamic and [Ph₃Sn(Meclo)] were tested for anti-mycobacterial activity against Mycobacterium tuberculosis H37Rv. The [Ph₃Sn(Meclo)] complex was found to be a promising anti-mycobacterial lead compound, displaying high activity against M. tuberculosis H37Rv.     

Bis(O2S2 macrocycle) complexes of palladium(II)

Two isomeric dibenzo-O₂S₂ macrocycles L¹ and L² have been synthesised and their coordination chemistry towards palladium(II) has been investigated. Two-step approaches via reactions of 1:1-type complexes, [cis-Cl₂LPd] (1a: L = L¹, 1b: L = L²), with different O₂S₂ macrocycle systems (L¹ and L²) have led to the isolation of the following bis(O₂S₂ macrocycle) palladium(II) complexes in the solid state: [Pd(L¹)₂](ClO₄)₂ (2a) and a mixture of [Pd(L¹)₂](ClO₄)₂ (2a) + [Pd(L²)₂](ClO₄)₂ (2b).     

Synthesis of metal complexes of peptide-phosphinites: Metal complexes in asymmetric Heck reaction

The synthesis of seven peptide-derived phosphinites, N-Boc-Phe-Tyr(OPPh₂)-OMe (4), N-Boc-Phe-Tyr(OPEt₂)-OMe (5), N-Boc-Phe-Tyr(OPCy₂)-OMe (6), N-Boc-Phe-Ser(OPPh₂)-OMe (7), N-Boc-Phe-Ser(OP^tBu₂)-OMe (8), N-Boc-Phe-Thr(OPPh₂)-OMe (9), N-Boc-Phe-Thr(OP^tBu₂)-OMe (10) is reported. These ligands are readily coordinated to Pd(II) and Pt(II) centers giving the corresponding complexes of the type ML₂Cl₂ (11-20). The palladium complexes, [N-Boc-Phe-Tyr(OPPh₂)-OMe]₂PdCl₂ (16), [N-Boc-Phe-Tyr(OPEt₂)-OMe]₂PdCl₂ (17), [N-Boc-Phe-Tyr(OPCy₂)-OMe]₂PdCl₂ (18), [N-Boc-Phe-Ser(OPPh₂)-OMe]₂PdCl₂ (19) and [N-Boc-Phe-Thr(OPPh₂)-OMe]₂PdCl₂ (20) catalyze the asymmetric phenylation of 2,3-dihydrofuran in moderate to high yields with high ee’s. The steric and electronic influences of the ligand substituents in driving the catalytic process are also discussed.     

Aliphatic and aromatic C-H activation of benzo[h]quinolines by Rh(I). Unique precursor dependent formation of mono-, di- and trinuclear complexes

Treatment of 7,8-benzo[h]quinoline (bhq-H, 1) and 10-methyl benzo[h]quinoline (bhq-Me, 3) with [Rh(C₂H₄)₂(THF)₂][BF₄] resulted in double C-H activation of aliphatic and aromatic C-H bonds, yielding the Rh(III) complexes 4 and 5, respectively. The structures of 4 and 5 were revealed by X-ray diffraction. The reaction of 1 with two other slightly different rhodium precursors, [Rh(olefin)_n(THF)₂][BF₄] (COE (n = 2), COD (n = 1)), led to completely different products, a dinuclear complex 7 and a trinuclear complex 6, respectively, which were characterized by X-ray diffraction. Complex 6 exhibits a rare linear Rh-Rh-Rh structure. Utilizing excess of 1 with [Rh(COD)(THF)₂][BF₄] led to the formation of a new product 8 with no C-H bond activation taking place. Additional C-H activation products of 1, cationic and neutral, in the presence of PⁱPr₃ (9a, 9b and 10) are also presented.     

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.     

Bioinspired FeIIICdII and FeIIIHgII complexes: synthesis, characterization and promiscuous catalytic activity evaluation

In this work we report on the synthesis, crystal structure, and physicochemical characterization of the novel dinuclear [Fe^IIICd^II(L)(μ-OAc)₂]ClO₄·0.5H₂O (1) complex containing the unsymmetrical ligand H₂L = 2-bis[{(2-pyridyl-methyl)-aminomethyl}-6-{(2-hydroxy-benzyl)-(2-pyridyl-methyl)}-aminomethyl]-4-methylphenol. Also, with this ligand, the tetranuclear [Fe₂^IIIHg₂^II(L)₂(OH)₂](ClO₄)₂·2CH₃OH (2) and [Fe^IIIHg^II(L)(μ-CO₃)Fe^IIIHg^II(L)](ClO₄)₂·H₂O (3) complexes were synthesized and fully characterized. It is demonstrated that the precursor [Fe^III₂Hg^II₂(L)₂(OH)₂](ClO₄)₂·2CH₃OH (2) can be converted to (3) by the fixation of atmospheric CO₂ since the crystal structure of the tetranuclear organometallic complex [Fe^IIIHg^II(L)(μ-CO₃)Fe^IIIHg^II(L)](ClO₄)₂·H₂O (3) with an unprecedented {Fe^III(μ-O_phenoxo)₂(μ-CO₃)Fe^III} core was obtained through X-ray crystallography. In the reaction 2 → 3 a nucleophilic attack of a Fe^III-bound hydroxo group on the CO₂ molecule is proposed. In addition, it is also demonstrated that complex (3) can regenerate complex (2) in aqueous/MeOH/NaOH solution. Magnetochemical studies reveal that the Fe^III centers in 3 are antiferromagnetically coupled (J = − 7.2 cm^− 1) and that the Fe^III-OR-Fe^III angle has no noticeable influence in the exchange coupling. Phosphatase-like activity studies in the hydrolysis of the model substrate bis(2,4-dinitrophenyl) phosphate (2,4-bdnpp) by 1 and 2 show Michaelis-Menten behavior with 1 being ~ 2.5 times more active than 2. In combination with k_H/k_D isotope effects, the kinetic studies suggest a mechanism in which a terminal Fe^III-bound hydroxide is the hydrolysis-initiating nucleophilic catalyst for 1 and 2. Based on the crystal structures of 1 and 3, it is assumed that the relatively long Fe^III…Hg^II distance could be responsible for the lower catalytic effectiveness of 2.