Potential Tridentate Ligand Research Articles

Transition Metal Compounds of Pyridine‐Amide‐Functionalized Carbene Ligands: Synthesis, Structure, and Electrocatalytic Properties in Proton Reduction

Three pyridyl‐amide substituted (benz)imidazolium salts H2L1Cl, H2L2Cl and H2L3Cl were synthesized and successfully employed as ligand precursors for the syntheses of novel nickel(II) and cobalt(III) complexes. The compounds H2L1Cl and H2L2Cl are precursors to tetradentate ligands and differ in the nature of the N‐heterocyclic carbene (NHC) functionality, being imidazole‐based and benzimidazole‐based, respectively. The ligand precursor H2L3Cl resembles H2L1Cl, but with one of the pyridyl groups replaced with a benzyl group, thus yielding a potential tridentate ligand. The nickel(II) compounds [Ni(L1)]Cl and [Ni(L2)]PF6 were obtained, bearing tetradentate ligands comprising an amidate and two pyridine nitrogen donor atoms and an (NHC) carbon donor. Single crystal X‐ray crystallography revealed that the nickel ions in both compounds are in slightly distorted square‐planar geometries. Reactions of cobalt salts with the ligands H2L1Cl and H2L3Cl resulted in the cobalt(III) compounds [Co(L1)2]Cl and [Co(L3)2]PF6; the cobalt ions in both complexes are in octahedral geometries, bound by two tridentate ligands in a meridional binding mode, with two dangling pyridine and benzyl groups, respectively. The four compounds show electrocatalytic activity in proton reduction in dimethylformamide solutions in presence of acetic acid; their activity is compared using cyclic voltammetry and quantified with gas chromatography.

Crystal structure of 2-[bis(1H-pyrazol-1-yl)meth-yl]pyridine.

The title compound, C12H11N5, was synthesized as a potential tridentate ligand to make catalytic metal complexes. The dihedral angle between the pyrazolyl rings is 67.9 (1)°. The most prominent feature in the crystal packing are C—H⋯N hydrogen-bonding inter­actions that link the mol­ecules into a supra­molecular tape along the b-axis direction.

Synthesis, characterization, and crystal structure determination of palladacycles of para-substituted 2-thiobenzylazobenzenes

Bichelated neutral palladacycles (1–3), [Pd(L)Cl], were synthesized from reaction of the new potential tridentate (C,N,S) ligands, 2-thiobenzylazobenzene (L1), 4′-methyl-2-thiobenzylazobenzene (L2), and 4′-chloro-2-thiobenzylazobenzene (L3) with sodium tetrachloropalladate(II), Na2[PdCl4], in ethanol. The compounds were characterized by elemental analysis, FT-IR, 1H NMR, UV–visible, and thermogravimetric analysis. The crystal structures of L2 and 1–3 were determined by single-crystal X-ray diffraction. In 1–3, the geometry around palladium remains almost square planar, coordinated to carbon, nitrogen, and sulfur of the ligand forming a bichelated cyclopalladate complex. The C–H…Cl type intermolecular hydrogen bonds, weak π…π, C–H…π, and van der Waals interactions are believed to be the stabilizing forces for the crystal packing of these palladacycles.

Synthesis and Characterization of Chiral Gold (I) Phosphine Complexes with New Dithiophosphorus Ligands

The novel chiral-centered amidodithiophosphonates [Fe(η5-C5H5)(η5-C5H4P(= S)(SH)(NHR))] were synthesized by the reaction of [FcP(= S)(μ-S)]2 and chiral amines such as (R)-(+)-1-phenylethyl-amine, (S)-(−)-1-phenylethyl-amine, (R)-(+)-1-phenylpropyl-amine, (S)-(+)-3-methyl-2-butyl-amine, (S)-(+)-1-aminoindane, (-)-cis-myrtanylamine, and (1S,2S)-(+)-2-benzyloxycyclohexyl-amine in toluene. The reaction of chiral-centered amidoferrocenyldithiophosphonates and [(PPh3)AuCl] gave rise to gold (I) complexes in CHCl3 or the CHCl3-toluene mixtures. The ligands 1–7 and the complexes 1a, 3a, and 4a were characterized by elemental analyses, infrared (IR), nuclear magnetic resonance (NMR; 1H‒, 31P‒, 13C‒) spectroscopy, as well as mass spectrometry (MS) measurements. Although amidodithiophosphonates are potential tridentate ligands, they are coordinated to gold only by one sulfur atom. The investigated chiral gold (I) complexes including two different phosphorus ligands are air stable. GRAPHICAL ABSTRACT

Ruthenium(II) Acetate Catalyst for Direct Functionalisation of sp2‐CH Bonds with Aryl Chlorides and Access to Tris‐ Heterocyclic Molecules

AbstractThe in situ generated (p‐cymene)ruthenium diacetate [Ru(OAc)2(p‐cymene)] catalyst 2, prepared from the (p‐cymene)ruthenium dichloride dimer {[RuCl2(p‐cymene)]2, 1} and potassium acetate (KOAc), acts as an excellent catalyst for ortho CH bond functionalisation of 2‐pyridylbenzene with unactivated aryl chlorides in the presence of potassium carbonate (K2CO3). Quantitative diarylation can be reached in 1 h at 120 °C. The diarylation of 2‐pyridylbenzene with 2‐halopyridines and 2‐ and 3‐halothiophenes was performed with 1 in the presence of KOAc or K2CO3 under more drastic conditions to generate the potential tridentate ligands, tris[1,2,3‐(2‐pyridyl)]benzenes and bis(2,6‐thiophenyl)2‐pyridylbenzenes.

2-[(2-Hydroxy-2,2-diphenylethyl)(methyl)amino]-N,N-dimethylethanaminium bromide

The title compound, C19H27N2O+·Br−, is the hydro­bromide of the trapping product of lithia­ted N,N,N′,N′-tetramethylethylenediamine (TMEDA) with benzophenone. Thereby, the N atom of the NMe2 group is selectively protonated and the respective trapping product represents a potential tridentate ligand with one O and two N donor atoms. The H atoms at N (H2N) and O (H1O) are involved in hydrogen bonds with the Br−. The mol­ecular structure shows all donor atoms to be arranged on one side of the mol­ecule, thus indicating a potential threefold coordination of a Lewis acid.

Synthesis and structural peculiarities of gallium Complexes with novel paullone derivatives

Abstract 9-Bromo-7,12-dihydroindolo[3,2-d][1]benzazepin-6-ylhydrazine was reacted with 2-acetylpyridine to give a Schiff base as a potential tridentate ligand. The reaction of this ligand with gallium chloride afforded complexes of 1:1 and 2:1 stoichiometry. The results of X-ray diffraction studies of the ligand and both gallium complexes are reported and compared with the data for a related gallium complex with a Schiff base obtained from 9-bromo-7,12-dihydroindolo[3,2-d][1]benzazepin-6-ylhydrazine and 2-hydroxybenzaldehyde.

Benzo[b]-1,10-phenanthrolines. V. Synthesis and Properties of 3,3’-Polymethylene-2-(pyrid-2’-yl)benzo[b]-1,10-phenanthrolines

A series of 3,3'-polymethylene-2-(pyrid-2'-yl)benzo[b]-1,10-phenanthrolines were prepared by the Friedlander reactions of 4-aminoacridine-3-carbaldehyde with pyrido[b]cycloalkanones as potential tridentate ligands. Dehydrogenation of dimethylene-bridged system afforded the corresponding fully aromatized system. Flexibility of annulated bridges is highly dependent on the length of the carbon chain, where the tetramethylene-bridge is rigid enough to differentiate eight bridge protons in 1 H NMR time scale at rt.

Spectrum–structure correlation for visible absorption spectra of copper(II) complexes showing axial co-ordination in aqueous solution

We attempted to draw some general considerations regarding the ability of the visible absorption spectra to provide information on axial co-ordination around copper(II) ion in aqueous solution. A series of visible spectra was recorded on the copper(II)–2,2′-bipyridine and copper(II)– l-aspartic acid systems. 2,2′-Bipyridine is known for its ability to co-ordinate copper(II) in axial position(s) as well, while l-aspartic acid is a potential tridentate ligand whose ability to chelate in axial position has been here investigated. Combined pH-metric and visible absorption spectrophotometric measurements were performed: visible absorption spectra were recorded during alkalimetric titrations as a function of pH. Further computer-assisted treatment allowed obtaining absorption spectra of single complexes. A series of axially co-ordinated copper(II) complexes found in the literature was also considered. Structural assignment of planar complexes is based on an empirical equation previously published by our research group, which allows an estimation of the ligand field strength of the equatorial donor groups involved. This equation can now help to strengthen the discussion and to assess a criterion to establish the axial co-ordination mode of copper(II) ion. The location of a donor group along the z-axis of the copper(II) distorted octahedron causes a red shift in the absorption spectrum. A quantitative estimation of the red shift per donor group was performed and ranges of red shifts for single donor groups were identified: we calculated a mean value of 36 nm, while extreme values are 19 and 58 nm.

SYNTHESIS AND COORDINATION CHEMISTRY OF N-(DIPHENYLPHOSPHINO) DIALLYLAMINE

N-(diphenylphosphino)diallylamine (1) is a potential tridentate ligand containing both phosphorus and olefinic donor sites. Its preparation is straightforward and involves the addition of chlorodip...

Syntheses and crystal structures of the potential tridentate ligand formed from condensation of ferrocenoylacetone and S-benzyldithiocarbazate and its bivalent metal complexes

A new potential tridentate ligand ( H 2 L) was synthesized from condensation of ferrocenoylacetone (C 5H 5FeC 5H 4C(O)CH 2C(O)CH 3) with S-benzyldithiocarbazate in refluxing benzene. Reaction of the ligand ( H 2 L) with a bivalent metal salt (M(OAc) 2 · xH 2O, M=Cu, Ni, Mn, Hg, Zn; SnCl 2 · 2H 2O) afforded the corresponding bivalent metal complexes M( HL) 2 ( I, M=Cu; II, M=Ni; III, M=Mn; IV, M=Hg; V, M=Zn; VI, M=Sn). All the new compounds have been characterized by elemental analyses, 1H NMR, IR and UV spectroscopies. A single-crystal X-ray analysis indicates that the ligand exists as a single iminone and not a tautomeric mixture of the iminone and enaminone. Crystallography of complex VI shows that the ligand acts as monoanionic tridentate coordinated to the tin atom via the carbonyl oxygen, deprotonated enamine nitrogen and thione sulfur atoms and the coordination geometry of the central metal is a distorted octahedron.

Synthesis of a new potential tridentate anthracene ligand bearing deprotectable methoxymethoxy (OMOM) group at 1,8-positions: attempt to synthesize anionic hypervalent carbon compounds.

A novel potential tridentate ligand bearing deprotectable coordinating atoms, 1,8-bis(methoxymethoxy)-9-bromoanthracene (15), was synthesized. The key steps are as follows: 1) stepwise mono-oxygenation from 1,8-dibromo-9-methoxyanthracene by use of electrophilic oxaziridine and gaseous dioxygen, and 2) selective reduction of the methoxy group by LDBB (lithium di-tert-butylbiphenylide) followed by treatment with BrCF(2)CF(2)Br. The corresponding 1,8-bis(methoxymethoxy)-9-lithio-anthracene (14), which should be a useful versatile trianion equivalent, could be generated by treatment of the bromide with one equivalent of nBuLi. The lithioanthracene reacted with hexafluoroacetone to give the deprotected ether 17 together with the adduct alcohol 16. The ether could easily be deprotected to give 1,2-dihydro-1,1-bis(trifluoromethyl)-2-oxa-9-hydroxyanthrylene (8), which was deprotonated with KH in the presence of [18]crown-6 to give the corresponding anion (9-K([18]crown-6)). The X-ray structure and NMR spectra of 9-K([18]crown-6) showed that it has an unsymmetrical structure probably due to the interaction between the oxygen atom of the phenoxide and the potassium cation surrounded by the crown ether.

Synthesis of a novel potential tridentate anthracene ligand, 1,8-bis(dimethylamino)-9-bromoanthracene, and its application as chelate ligand for synthesis of the corresponding boron and palladium compounds.

A novel potential tridentate ligand, 1,8-bis(dimethylamino)-9-bromoanthracene, was synthesized. The key steps are as follows: 1) dimethylamination of 1,8-dibromo-9-methoxyanthracene by a modified Buchwald's method to afford 1,8-bis(dimethylamino)-9-methoxyanthracene, and 2) reduction of the methoxy group by LDBB (lithium di-tert-butylbiphenylide) followed by treatment with BrCF2CF2Br. The corresponding 1,8-bis(dimethylamino)-9-lithioanthracene, which should be a useful versatile tridentate ligand, could be generated by treatment of the bromide with one equivalent of nBuLi. The lithioanthracene reacted with B-chloroborane derivatives to give three 9-boryl derivatives. Although we recently reported that the crystal structure of 1,8-dimethoxy-9-B-catecholateborylanthracene was a symmetrical compound with the almost identical two O-B distances (2.379(2) and 2.441(2) A), the newly prepared 1,8-bis(dimethylamino)-9-borylanthracene derivatives clearly have unsymmetrical structures with coordination of only one NMe2 group toward the central boron atom. However, the energy difference between the unsymmetrical and symmeterical structures was found to be very small based on 1H NMR measurements, in which symmetrical anthracene patterns in the aromatic region (two kinds of doublets and a triplet) and a sharp singlet signal of the two NMe2 groups were observed even at -80 degrees C. 1,8-Bis(dimethylamino)-9-bromoanthracene itself can be a versatile ligand for transition metal compounds. In fact, direct palladation of the bromide took place by the reaction with [Pd2(dba)3].CHCl3 in THF to give the 9-palladated product. X-ray crystallographic analysis of the Pd compound showed that the square planar palladium atom was coordinated in a symmetrical fashion by both NMe2 groups (Pd-N bonds are 2.138(5) and 2.146(5) A).

Synthesis and Structural Determination of New Potential Chiral Tridentate Ligands Derivated from 1-(R)-(+)-Camphor

New alcohols 3d, 3e and 3f were efficiently prepared in enantiopure form (2 steps, 50–58%), from available 1-(R)-(+)-camphor 1. The absolute configurations were assigned by NMR and relative X-ray crystallographic analysis.

Asymmetric allylic alkylation catalyzed by palladium complexes with new chiral ligands

A potential chiral tridentate ligand based on 2,6-disubstituted pyridine has been prepared and its palladium complexes have been used in asymmetric allylic alkylation. Up to 75 % ee is achieved in the reaction between 1,3-diphenyl-2-propenyl acetate and dimethyl malonate. A control experiment with a similar ligand without pyridine suggests that the bidentate diphosphine coordination is responsible for the asymmetric reaction with this potential tridentate ligand.

N-(2-Hydroxyphenyl)salicylaldimine

The title compound, 2-(salicylideneamino)phenol, C 13 -H 11 NO 2 , a potential tridentate ligand, has two crystallographically independent molecules in the asymmetric unit, one of which is disordered. Intramolecular hydrogen bonds occur between the pairs of atoms 0(16) and N(8) [2.625 (7) A], and O(1) and 0(16) [3.571(7) A], the H atoms essentially being bonded to the O atoms.

Neuartige basische Liganden für die homogenkatalytische Homologisierung von Methanol zu Ethanol, II [1]. Synthese und komplexchemisches Verhalten potentiell zwei- und dreizähniger P–N- und P–O-haltiger Chelatliganden / Novel Basic Ligands for the Homogeneous Catalytic Homologation of Methanol to Ethanol, II [1]. Synthesis and Complex Chemical Behaviour of Potential Bi- and Tridentate P–N and P–O Containing Chelate Ligands

Tridentate P-N and P-O ligands of the type RP(XC5H4N)2 (1a, b, a′, b′) [X = O, NH: R = CH3 (a, a′), t-Bu (b, b′)] and PhP(CH2C4H7O)2 (1c″) are obtained by reaction of RPCl2 with 2-hydroxy- and 2-aminopyridine and Li2PPh with tetrahydrofurfuryil chloride, respectively. In a similar way the potential bidentate ligand Ph2PCH2C4H7O (2c″) is formed by the action of LiPPh2 on ClCH2C4H7O. Potential tridentate ligands as RP(C4H3O)2 (3a‴-c‴) [R = CH3 (a‴), t-Bu (b‴), Ph (c‴)| with two furyl-phosphorus bonds can be isolated from RPCl2 and furyllithium. The complex chemical behaviour towards Mo(CO)6 or carbonyl derivatives of chromium was elucidated for 2c‴ and 3a‴- c‴, resulting in the formation of (OC)5MoPPh2CH2C4H7O (4c″) and trans-(OC)4Cr[RP(C4H3O)2]2 (5a‴-c‴), respectively. In the case of 5a‴ an X-ray investigation proved 3a‴-c‴ to be only monodentate P-ligands 5a‴ crystallizes in the monoclinic space group P21/c with Z = 2. 1c″ is the most versatile species within this series and functions probably as a tri-, bi- and monodentate P-O ligand. With 1c″ and 3c″ some homologation experiments of methanol at 200 °C and 304 bar synthesis gas pressure were carried out

Structures of Cu(II)-(histidine) 2 in solution

We have examined the structure of Cu(II)-(histidine) 2 in solution using optical, electron paramagnetic resonance (EPR) and electron spin-echo (ESE) spectroscopies. Histidine is a potential tridentate ligand with three groups capable of binding Cu(II): 1) carboxyl oxygen 2) imidazole nitrogen and 3) amino nitrogen. Histidine is involved in the coordination of metal ions in a number of copper proteins, including superoxide dismutase, ceruloplasmin, ascorbate oxidase, galactose oxidase, etc. In addition, histidine has been implicated in the in vivo transport of copper between albumin and cells. Previously the structure of Cu(II)-(histidine) 2 was determined using proton NMR and optical spectroscopies, and X-ray crystallography. Two structures were suggested. In the first, the Cu(II) is coordinated by two imidazole and by two amino nitrogen atoms. In the second structure the Cu(II) is ligated to a single imidazole nitrogen, and an amino nitrogen from one of the histidines and an amino nitrogen and a carboxyl oxygen from the second histidine. Computer aided analysis of the optical pH titration of Cu(II)-(histidine) 2, at 25 °C, shows two transitions, with pK's of 3.6 and 5.5. Both titrations have n= 1. An EPR pH titration at 77 K, shows 2 transitions with pK's of 2.8 and 4.4 and n values of 2 and 1, respectively. The lower pK represents the complexation of Cu(II)-aquo (g ∥ = 2.417, A ∥ = 14.1 mK) by histidine to form an intermediate complex (g ∥ = 2.306, A ∥ = 18.6 mK). The second pK represents the conversion of this intermediate to the form of the complex present at the physiological pH (g∥ = 2.242, A∥ = 18.8 mK). The changes in g∥ suggest the binding of an additional equatorial nitrogen when the intermediate is converted to the neutral pH form. In a separate study, we used ESE spectroscopy to determine the number of imidazole nitrogen atoms equatorially coordinated Cu(II) in the bis histidine complex. The observed periodicities in the ESE decay envelope are due to interactions of the unpaired electron with the remote, protonated 14N of bound imidazole. Directly coordinated 14N is not observed in these experiments. Fourier transformation of the modulation pattern for the complex prepared at pH 7.6 shows frequencies at 0.7, 1.5, and 4.0 MHz, which are characteristic of Cu(II) coordinated by imidazole. The depth of the modulation pattern has been shown to be a product function of the number of interacting nuclei and their distance. Thus if two imidazoles are coordinated to Cu(II), the depth of modulation is the square of that seen for a single coordinated imidazole provided that the CuN bond lengths are the same. Using Cu(II)diethylenetriamineimidazole, and Cu(II)oxalate(imidazole) 2 as a single, and double imidazole models, we can quantitate the number of imidazoles bound to Cu(II) in Cu(II)-(histidine) 2. The depth of modulation for Cu(II)(histidine) 2 is comparable to that observed for Cu(II)diethylenetriamineimidazole. Therefore, at physiological pH, a single imidazole is equatorially coordinated to the metal ion. At pH 3.4, where the predominant species are Cu(II)-aquo and the low pH intermediate, the modulation pattern characteristic of coordinated imidazole is still observed. Thus imidazole remains bound to Cu(II) here as well.

Structure of Mn 2+ and Cu 2+ complexes with l-methionine, [formula omitted], l-threonine and l-serine

A comparison of the stability constants determined by several authors for complexes between metal ion, Mn 2+ through Zn 2+, and potential tridentate amino acids, methionine, S-methylcysteine, threonine or serine, with those of the bidentate ligands, alanine or α-amino- n-butyric acid, showed that no significant increase in stability for the complexes with the potential tridentate ligands occurred. This does not necessarily suggest that an interaction between metal ions and the thioether or hydroxy groups, which are possible binding sites in these amino acids, is nonexistent. By means of nuclear magnetic resonance spectroscopy, Mn 2+ and Cu 2+ complexes with the previously mentioned amino acids were investigated. Strong evidence has suggested that the sulfur atom of S- methyl- l-cysteine is involved in complex formation with Cu 2+, while such interaction is either small or nonexistent with Mn 2+. Neither metal ion interacted significantly with the sulfur of l-methionine. The hydroxy group of l-threonine was involved with the binding to Mn 2+, while no interaction seemd to occur with Cu 2+; this behavior was probably due to the different preferred coordination numbers of these metal ions. The proton signals of l-serine resulted in a complex pattern; therefore, no detailed conclusions could be drawn for this ligand, although it was assumed that the structures of the complexes were similar to those for l-threonine. Structures for the investigated complexes are suggested, and the significance of weak interactions in metal ion-enzyme complexes is discussed.