Antimicrobial resistance is an ever-increasing problem throughout the world and has already reached severe proportions. Bacteria can develop ways to render traditional antibiotics ineffective, ...raising a crucial need to find new antimicrobials with novel mode of action. We demonstrate here a novel class of pyrazine functionalized Ag(I) and Au(I)-NHC complexes as antibacterial agents against human pathogens that are resistant to several antibiotics. Complete synthetic and structural studies of Au(I) and Ag(I) complexes of 2-(1-methylimidazolium) pyrimidinechloride (L-1), 2,6-bis(1-methylimidazol)pyrazinechloride (L-2) and 2,6-bis(1-methyl imidazol) pyrazinehexa-fluorophosphate (L-3) are reported herein. Chloro2,6-bis(1-methyl imidazol)pyrazinegold(I), 2b and chloro 2,6-bis(1-methyl imidazol)pyrazinesilver(I), 2a complexes are found to have more potent antimicrobial activity than other synthesized compounds and several conventionally used antibiotics. Complexes 2b and 2a also inhibit the biofilm formation by Gram-positive bacteria, Streptococcus mutans and Gram-negative bacteria, Escherichia coli, causing drastic damage to the bacterial cell wall and increasing membrane permeability. Complexes 2b and 2a strongly binds to both Lys and Dap-Type peptidoglycan layers, which may be the reason for damage to the bacterial cell wall. Theoretical studies of all the complexes reveal that 2b and 2a are more reactive than other complexes, and this may be the cause of differences in antibacterial activity. These findings will pave the way towards developing a new class of antibiotics against different groups of conventional antibiotic-resistant bacteria.
Two series of Zn(II)-azido complexes of diimine (
N
C
C
N
) and azoimine (
N
N
C
N
) are described. The ligands belong to α/β-naphthyl-(2-pyridylmethylene)amine (diimine) and ...1-alkyl-2-(naphthyl-α/β-azo)imidazoles (azoimine). The complexes are structurally characterized. The diimine system has end-on azido bridged ligands and the azoimine system gives terminal azido bridged complexes. They exhibit high intense ligand centered emissions.
Two new series of Zn(II) complexes of α/β-naphthyl-(2-pyridylmethylene)amine and 1-alkyl-2-(naphthyl-α/β-azo)imidazoles with
N
3
-
as a counter ion are described in this article. The single crystal X-ray structure of Zn(β-naphthyl-(2-pyridylmethylene)amine)(N
3)
2
2 has revealed an end-on azido bridged dimer while Zn(1-ethyl-2-(naphthyl-β-azo)imidazole)
2(N
3)
2 is a monomer. The complexes are spectroscopically characterized. Examination of luminescence properties of the complexes suggests that the Schiff bases are emissive and the naphthylazo compounds are non-emissive.
1-Alkyl-2-(naphthyl-(α/β)-azo)imidazole α-NaiR (
2)/β-NaiR (
3); R=Me (
a), Et (
b), CH
2Ph (
c) are N,N′-bidentate chelating ligands. The structure of 1-ethyl-2-(naphthyl-α-azo)imidazolium ...hexaflurophosphate has been confirmed by single crystal X-ray structure study and shows supramolecular geometry through non-covalent interactions (C–H⋯F, C–H⋯π, π⋯π). Mercury(II) complexes of the formula Hg(NaiR)Cl
2 (
4/5) have been synthesised and characterised by spectral (IR, UV–Vis and
1H and
13C NMR) studies. α-NaiR has been used to synthesise organomercury derivatives, Hg(α-NaiR–H)Cl (
6) and the site of mercuration has been supported by charge density calculation using a semi-empirical PM3 technique.
A series of coordinated and cyclometallated mercury(II) compounds 0f 1-alkyl-2-(naphthyl-(α/β)-azo)imidazoles are described. The X-ray structure of 1-ethyl-2-(naphthyl-α-azo)imidazolium hexaflurophosphate shows supramolecular geometry through noncovalent interactions. The C(9)–H activation by Hg(OAc)
+ has been supported by charge density calculation using PM3 single point Hamiltonian.
Title compounds have been synthesized by Ag-assisted route to remove Cl from Ru(bpy)
2Cl
2 followed by chelation with NaiR (1-alkyl-2-(naphthyl-α/β-azo) imidazole). The structure has been established ...by X-ray crystallography. Redox property gives Ru(III)/Ru(II) couple and ligand reductions.
Heteroleptic tris-chelates, Ru(bpy)
2(α/β-NaiR)(ClO
4)
2 (bpy
=
2,2′-bipyridine; α/β-NaiR
=
1-alkyl-2-(naphthyl-α-azo)imidazole, α-NaiR and 1-alkyl-2-(naphthyl-β-azo)imidazole, β-NaiR) have been synthesized either by reacting Ru(bpy)
2Cl
2 and α/β-NaiR in presence of 2 equivalent of AgNO
3 or from Ru(bpy)
2Cl
2
+
Ag(α/β-NaiR)
2(ClO
4) reaction under refluxing condition in MeCN. The single crystal X-ray diffraction study in one case, and by other spectroscopic studies has supported the structure of the complexes. All the complexes show metal-to-ligand charge transfer (MLCT) transitions in the visible region. Cyclic voltammetric studies exhibit high potential Ru(III)/Ru(II) (1.2–1.3
V vs. SCE) and three successive ligand reductions at negative to SCE in acetonitrile solution.
1-Alkyl-2-(naphthyl-(α/β)-azo)imidazoles (α/β-NaiR; R = Me, Et and CH
2
Ph) react with (NH
4
)
2
OsCl
6
and complexes OsCl
2
(NaiR)
2
are isolated in two isomeric forms: blue-violet (3, 4) and ...red-violet (5, 6). The ligand is a bidentate N (N(imidazole)), N' (N(azo)) donor type. With reference to the pairs of Cl, Cl; N, N and N', N' atoms the blue-violet and red-violet isomers are assigned to cis-trans-cis (ctc) and cis-cis-cis (ccc) configurations respectively. IR spectra of the complexes show two v(Os-Cl) bands and support the cis-OsCl
2
configuration.
1
H NMR spectra also support the ctc and ccc-configuration. The structure of the blue-violet complex OsCl
2
(α-NaiEt)
2
has been determined by X-ray crystallography and the ctc configuration has been confirmed. The structure shows an unusually long N=N bond length, 1.331(4) Å, which is elongated by 0.07 Å compared to the free ligand value.
1-Alkyl-2-(naphthyl-α-azo)imidazoles (NaiR,
2) (C
10H
7NNC
3H
2N
21R; R=Me (
a), Et (
b), PhCH
2 (
c)) have been reacted with Na
2PdCl
4 in MeOH or MeCN solutions of Pd(MeCN)
2Cl
2 to synthesise ...Pd(NaiR)Cl
2 (
3). The reaction of Pd(OAc)
2 in boiling benzene with NaiR followed by the addition of LiCl has resulted in the synthesis of the cyclopalladated complex Pd(NaiRH)Cl (
4). The ligand, NaiR, acts as a
N,
N′-bidentate chelator while NaiRH acts as a tridentate
N,
N′,
C-cyclometallating ligand. The infrared spectra of
3 exhibit two PdCl stretches correspond to a
cis-PdCl
2 geometry, and a single
ν(PdCl) band in
4 suggests one PdCl bond. Cyclopalladation is supported by a single crystal X-ray crystal structural study of Pd(NaiEtH)Cl (
4b) and the metallation takes place at C(8′)-position. The solution of Pd(NaiR)Cl
2 (
3) is also irreversibly transformed into Pd(NaiRH)Cl (
4) when the pH is adjusted to 4.5–6.0 by NaOAc or other bases (NaOMe, NaOH, LiOH, Li
2CO
3 etc.). At higher pH values (8–10) the reaction shows the chelative hydroxylation at the C(2′)-site to synthesise Pd(NaiRO)Cl (
5). The structure of the hydroxylated blue product is also supported by a single crystal X-ray crystal structure of Pd(NaiEtO)Cl (
5b). The reaction of Pd(NaiR)Cl
2 in MeCN with dilute sodium hydroxide in air, or aqueous silver nitrate under boiling conditions, or its treatment with Tollen's reagent in MeCN solution under ambient conditions has also yielded the hydroxylated product. All the compounds have been characterised by elemental analyses, IR, UV–vis and
1H-NMR data. The solution spectral behaviour has been interpreted by EHMO calculations.
In search of a new cyclometallating reagent containing heterocyclic system, we have designed 1-alkyl-2-(naphthyl-α-azo)imidazoles. The cycloplladation is carried out at pH 4.5–6.0 and the metallation takes place at the C(8′)-position. Thus the ligand behaves as a NN′C system. At higher pH values (8–10) chelatative hydroxylation is observed at the C(2′)-site. Cyclopalladation vis-à-vis hydroxylation at two different sites of the naphthyl ring in dichloro-1-ethyl-2-(naphthyl-α-azo)imidazole palladium (II) is supported by the X-ray crystal structure as in Figs. 1 and 2, respectively.
Cadmium(II) complexes of
N-(2-pyridyl)methyliden-α (or β)-aminonaphthalene (α- or β-NaiPy) are described. They have been spectroscopically characterized and structural confirmation has been carried ...out by an X-ray diffraction study in one case. The complexes are highly fluorescent. Naphthylazoimidazole complexes are non-fluorescent.
Cadmium(II) complexes of
N-(2-pyridyl)methyliden-α (or β)-aminonaphthalene (α or β-NaiPy) and 1-alkyl-2-(naphthyl-α (or β)-azo)imidazoles (α- or β-NaiR) using Cl
−,
ClO
4
-
and
NO
3
-
as anions are spectroscopically characterized. Molar conductance data in nitromethane show that the chloro and nitrite compounds are non-conducting, while the perchlorate complexes are 1:2 electrolytes. The single crystal X-ray structure of Cd(N-{(2-pyridyl)methyliden}-β-aminonaphthalene)
2(ONO
2)
2 has been determined. The luminescence properties of Cd(II) complexes of α/β-NaiPy are studied and they exhibit ligand centered π–π
∗ emissions. Naphthylazoimidazoles and their cadmium complexes are non-fluorescent.
Silver(I) complexes of 1-alkyl-2-(naphthyl-(α/β)-azo)imidazole are described. The single crystal X-ray diffraction study of bis-1-ethyl-2-(naphthyl-α-azo)imidazolesilver(I) perchlorate confirms the ...structure of the complexes. The complexes show intramolecular and intermolecular H-bonding leading to a 1D helical framework.
Reaction of AgNO
3 and 1-alkyl-2-(naphthyl-(α/β)-azo)imidazole (α-/β-NaiR) in MeOH followed by the addition of NaClO
4 has isolated Ag(α-/β-NaiR)
2(ClO
4) (
3/
4). The complexes are characterized by microanalytical and spectral data. Single crystal X-ray diffraction study of bis-1-ethyl-2-(naphthyl-α-azo)imidazolesilver(I) perchlorate confirms the structure of the complex. Argentophilicity of imidazole-N and steric crowding by naphthyl groups have forced a {2+2} distorted linear geometry. The structure shows intramolecular and intermolecular H-bonding in a 1D helical framework along with face-to-face π–π interactions.
Ruthenium(II) and ruthenium(0) carbonyl complexes of pyridyl-azo-imidazoles are described. The ligand is an unsymmetric biheterocyclic azo system having an azoimine function. The structure of ...Ru(CO)-ligand is confirmed by a single crystal X-ray diffraction study in one case along with other spectral data. The Ru(II) complexes exist in two isomeric forms. The redox properties of the complexes are examined by cyclic voltammetry.
The reaction of 2-(3′-pyridylazo)imidazole (3′-PyaiH) or its 1-alkyl derivative, 3′-PyaiR, with Ru(CO)
3(PPh
3)
2 has synthesized air stable, moisture insensitive, diamagnetic Ru(0) complexes, Ru(CO)(3′-PyaiR)(PPh
3)
2. The ligands serve as an unsymmetric N,N′-chelating agent (N(imidazole) and N(azo) are abbreviated as N and N′, respectively). The X-ray structure determination of Ru(CO)(3′-PyaiH)(PPh
3)
2 shows a square pyramidal geometry. The square plane is made up of Ru, 2P, C(O), N(1), and with N(3) at the apex. Other spectroscopic studies (IR, UV–Vis, NMR) support the stereochemistry. Electrochemistry shows three consecutive anodic peaks (
E
pa) and suggest irreversible redox responses of Ru(I)/Ru(0), Ru(II)/Ru(I), Ru(III)/Ru(II) and azo reductions. Oxidation by Cl
2 of Ru(CO)(3′-PyaiR)(PPh
3)
2 has isolated the Ru(II)-complex as the perchlorate salt, Ru(CO)(Cl)(3′-PyaiR)(PPh
3)
2(ClO
4). A solution of RuCl
3 and 3′-PyaiR in ethanol has isolated two isomers of the composition Ru(3′-PyaiR)
2Cl
2. They have been characterized by spectral and electrochemical data.
Pyridine-2-carboxaldehyde reacts with α/β-naphthylamine to give α/β-naphthyl-(2-pyridylmethylene)amine α-L (1), β-L (2). L belongs to the unsymmetric diimine (--N=C--C=N--) family which can form ...five-membered chelate rings with metal ions. {donor centers are abbreviated as NN(Py) and N^sup '^N(nap)} Ag(L)^sub 2^^sup +^ complexes were prepared and characterized by spectroscopic data. The reaction between L and RuCl^sub 3^ in boiling EtOH yielded green and blue-green compounds of composition RuCl^sub 2^(L)^sub 2^. I.r., u.v.-vis. and ^sup 1^H-n.m.r. data determined the stereochemistry of the complexes as trans-cis-cis (green) and cis-trans-cis (blue-green) according to the sequence of the coordination pair of Cl, N N(Py) and N' N(nap). Upon treatment of Ag(L)^sub 2^^sup +^ with Ru(bpy)^sub 2^Cl^sub 2^ in alcoholic suspension the ternary complexes, Ru(bpy)^sub 2^(L)(ClO^sub 4^)^sub 2^, were isolated and characterized by spectroscopic data. Ru(bpy)(L)^sub 2^(ClO^sub 4^)^sub 2^ complexes were synthesized similarly from ctc-Ru(L)^sub 2^Cl^sub 2^ and 2,2'-bipyridine (bpy) in the presence of AgNO^sub 3^ and NaClO^sub 4^. These complexes show well-defined m.l.c.t transitions in the visible region. The sterochemistry of the complexes was established by ^sup 1^H-n.m.r. data. Cyclic voltammetry shows a high potential Ru^sup III^/Ru^sup II^ couple and follows the order: Ru(bpy)(L)^sub 2^^sup 2+^ > Ru(bpy)^sub 2^(L)^sup 2+^ > Ru(β-L)^sub 2^Cl^sub 2^ > Ru(α-L)^sub 2^Cl^sub 2^.PUBLICATION ABSTRACT
