Aromatic nitrogen heterocycles as bridging ligands; a survey

Synthesis and Coordination Chemistry of Doubly-Tridentate Tripodal Pyridazine and Pyrimidine-Derived Ligands: Structural Interplay Between M2L and M2L2 (M = Ni and Pd) Complexes and Magnetic Properties of Iron(II) Complexes

The coordination chemistry of three bridging doubly-tridentate ligands, including the known compound 3,6-bis(di-2-pyridylmethyl)pyridazine (1), which is structurally similar to 1,4-bis(di-2-pyridylmethyl)phthalazine (2), and two pyrimidine-linked compounds 4,6-bis(di-2-pyridylmethyl)pyrimidine (3), and 4,6-bis(di-2-pyridylamino)pyrimidine (4), was investigated with FeII, NiII, and PdII metal salts. Ligands 3 and 4 were synthesized in one-pot reactions from easily obtained starting materials; compound 3 was synthesized from di-2-pyridylmethane and 4,6-diiodopyrimidine in 48% yield, while ligand 4 was prepared by reacting di-2-pyridylamine with 4,6-dichloropyrimidine in 27% yield. During the synthesis of 4, an additional compound, 4-chloro-6-(di-2-pyridylamino)pyrimidine (5), with only one tridentate binding site was obtained in 30% yield. Reactions of 1, 3, and 4 with FeII or NiII salts gave two types of complexes, either discrete M2L or M2L2 assemblies. The PdII complexes obtained were also characterized as discrete M2L complexes. The compounds were characterized by a combination of NMR and IR spectroscopy, microanalysis and X-ray crystallography. Noticeable differences in the structures obtained for NiII coordination complexes with the carbon-linked (3) and nitrogen-linked (4) ligands were observed, whereby the nitrogen linker adopted a trigonal planar geometry and prevented tridentate facial coordination of the octahedral metal centres. The magnetic properties of dinuclear FeII complexes of 1 were examined to see if they showed spin-crossover effects, a feature recently observed by others in other dinuclear helicate complexes, but the complexes remained high-spin at all temperatures between 300 and 2 K.

Torsional potential of 4,4'-bipyridine: ab initio analysis of dispersion and vibrational effects

Ab initio calculations using restricted Hartree-Fock, second-order Møller-Plesset perturbation theory (MP2), density-functional theory (DFT), and coupled-cluster methods have been done to obtain the torsional potential-energy profile of the aza-aromatic molecule 4,4'-bipyridine. The torsional potential is evaluated adiabatically by fixing the normalized sum of the dihedral angles through the C-C inter-ring bond at several values along the torsional path and relaxing the remaining degrees of freedom. Previous discrepancies between MP2 and DFT internal rotation barrier heights are removed, and seen to be mostly due to the underestimation of the dispersion energy in the coplanar conformer by MP2 when using relatively small basis sets. The calculations indicate that the barrier height between the twisted global minimum and the 0 degrees conformer is around 1.5-1.8 kcal mol-1 while that corresponding to the 90 degrees one is about 2.0-2.2 kcal mol-1. This same relative energy ordering of the coplanar and perpendicular conformers was experimentally derived from nuclear magnetic resonance (NMR) measurements of 1H dipolar couplings on 4,4'-bipyridine solutions in a nematic liquid crystal, although the barrier heights are much lower than those estimated from NMR experiments in the gas phase. The DFT infrared spectrum and zero-point vibrational energy corrections to the torsional energy profile have also been calculated, the latter having a small influence on the torsional potential-energy profiles.

Selective thin-layer chromatography of 4-R-1,2,4-triazoles

A convenient and selective TLC of 4-R-1,2,4-triazoles (Rtrz) and nitrogen containing aromatic rings is presented (molecules 1-11). This simple revelation method implies a complexation of heterocycles to Co(II) ions on a TLC plate, followed by a subsequent modification of the reduction potential which enables oxidation of Co(II) by permanganate.

Oligo(U-terpyridines) and Their Ruthenium(II) Complexes: Synthesis and Structural Properties

A highly efficient domino reaction starting from tetrahydroquinolinone and a series of bisiminium salts provides the corresponding bis(U-terpyridines). These ligands have been treated with [(tpy)RuCl3] to afford novel dinuclear complexes [(tpy)Ru(L)Ru(tpy)]4+. The protocol is also applied for the synthesis of a star-shaped tris(U-terpyridine) and the trinuclear complex [{(tpy)Ru}3(L)]6+. In view of potential applications in the fields of metallopolymers and molecular devices, the electronic spectra, as well as the electrochemical potentials of all the complexes have been obtained. According to these data, no significant intermetal interaction has been observed for the ruthenium complexes presented here.

Metal–metal interactions in dinuclear ruthenium complexes containing bridging 4,5-di(2-pyridyl)imidazolates and related ligands

The dinuclear bis(2,2'-bipyridine)ruthenium complex of 4,5-di(2-pyridyl)imidazolate has been prepared and separated into its (meso and rac) diastereoisomers. The 2-phenyl substituted analogue forms the meso isomer selectively. All three complexes have been characterised by 1H NMR and X-ray crystallography. Electrochemical measurements and spectroelectrochemistry of the mixed-valence states reveal strong metal-metal interactions and IVCT bands that are highly dependent on the electrolyte.

Synthetic Approaches to Polypyridyl Bridging Ligands with Proximal Multidentate Binding Sites

[reactions: see text] A series of 12 bridging ligands was prepared. These ligands include a central linker appended to two 1,8-naphthyrid-2-yl or two 1,10-phenanthrolin-2-yl units. The linkers include pyridazin-3,6-diyl, 1,8-naphthyrid-2,7-diyl, 2,2'-bipyrid-6,6'-diyl, 1,10-phenanthrolin-2,9-diyl, 1,2-di(2'-pyrid-6'-yl)ethyne, and 3,6-di(2'-pyrid-6'-yl)pyridazine. The ligands were synthesized from the diacetyl derivative of the central linker by a Friedländer condensation with either 2-aminonicotinaldehyde or 8-amino-7-quinolinecarbaldehyde. The precursor diacetyl derivatives were, in turn, prepared by pathways involving Stille and Sonogashira couplings. Examination of the electronic absorption spectra of the bridging ligands shows the strongest correlation to be between pairs of ligands having the same central linker. Complexation studies will follow.

Copper(II) Mediated Anion Dependent Formation of Schiff Base Complexes

A tetranuclear mixed ligand copper(II) complex of a pyrazole containing Schiff base and a hydroxyhexahydropyrimidylpyrazole and copper(II) and nickel(II) complexes of the Schiff base having N-donor atoms have been investigated. A 2 equiv amount of 5-methyl-3-formylpyrazole (MPA) and 2 equiv of 1,3-diamino-2-propanol (1,3-DAP) on reaction with 1 equiv of copper(II) nitrate produce an unusual tetranuclear mixed ligand complex [Cu4(L1)2(L2)2(NO3)2] (1), where H2L1 = 1,3-bis(5-methyl-3-formylpyrazolylmethinimino)propane-2-ol and HL2 = 5-methyl-3-(5-hydroxyhexahydro-2-pyrimidyl)pyrazole. In contrast, a similar reaction with nickel(II) nitrate leads to the formation of a hygroscopic intractable material. On the other hand, the reaction involving 2 equiv of MPA and 1 equiv each of 1,3-DAP and various copper(II) salts gives rise to two types of products, viz. [Cu(T3-porphyrinogen)(H2O)]X2 (X = ClO4, NO3, BF4 (2)) (T3-porphyrinogen = 1,6,11,16-tetraza-5,10,15,20-tetrahydroxy-2,7,12,17-tetramethylporphyrinogen) and [Cu(H2L1)X]X x H2O (X = Cl (3), Br (4)). The same reaction carried out with nickel(II) salts also produces two types of compounds [Ni(H2L1)(H2O)2]X2 [X = ClO4 (5), NO3 (6), BF4 (7)] and [Ni(H2L1)X2] x H2O [X = Cl (8), Br (9)]. Among the above species 1, 3, and 5 are crystallographically characterized. In 1, all four copper atoms are in distorted square pyramidal geometry with N4O chromophore around two terminal copper atoms and N5 chromophore around two inner copper atoms. In 3, the copper atom is also in distorted square pyramidal geometry with N4Cl chromophore. The nickel atom in 5 is in a distorted octahedral geometry with N4O2 chromophore, where the metal atom is slightly pulled toward one of the axial coordinated water molecules. Variable-temperature (300 to 2 K) magnetic susceptibility measurements have been carried out for complex 1. The separations between the metal centers, viz., Cu(1)...Cu(2), Cu(2)...Cu(2)A, and Cu(2)A...Cu(1)A are 3.858, 3.89, and 3.858 A, respectively. The overall magnetic behavior is consistent with strong antiferromagnetic interactions between the spin centers. The exchange coupling constants between Cu(1)...Cu(2) and Cu(2)...Cu(2A) centers have turned out to be -305.3 and -400.7 cm(-1), respectively, resulting in a S = 1/2 ground state. The complexes are further characterized by UV-vis, IR, electron paramagnetic resonance, and electrochemical studies.

Mixed-ligand Ru(II) complexes with 2,2′-bipyridine and aryldiazo-β-diketonato auxillary ligands: Synthesis, physico-chemical study and antitumour properties

The complexes of Ru(II)-2,2'-bipyridyl with substituted diazopentane-2,4-diones (L1H-L5H) were synthesized and characterized by elemental analyses, conductance, FAB (fast atom bombardment) mass and spectral (IR, UV/Vis (UV/visible), NMR) studies. Molecular geometry optimization of the complexes was also made. None of the complexes luminesce. However, facilitated oxidation of Ru(II) to Ru(III) was evidenced from their lower reduction potential data. The ligands and their complexes were tested for their antitumour activity against a variety of tumour cell lines. Though activity is found to vary with the type of tumour cell lines used, yet complex 5 with naphtyldiazopentane-2,4-dione as co-ligand was found to be a potential compound as it showed in general significant activity against all cell lines studied.

Ligand design in multimetallic architectures: six lessons learned

Metallosupramolecular chemistry involves the use of combinations of organic ligands and metals for the construction of both discrete and polymeric aggregates. This Account describes some lessons that we have learned about aspects of ligand design in the course of our work in this area. Specifically, we recommend the incorporation of a diverse range of heterocyclic rings and arene cores within the ligands, as well as attention to symmetry considerations, and offer suggestions for the introduction of chirality and flexibility within the ligands and the exploitation of other weak interactions to assist self-assembly processes.

Organometallic Silver(I) Supramolecular Complexes Generated from Multidentate Furan-Containing Symmetric and Unsymmetric Fulvene Ligands and Silver(I) Salts

One new conjugated symmetric fulvene ligand L1 and two new unsymmetric fulvene ligands L2 and L3 were synthesized. Five new supramolecular complexes, namely Ag2(L1)3(SO3CF3)3 (1) (1, monoclinic, P2(1)/c; a = 12.702(3) A, b = 26.118(7) A, c = 13.998(4) A, beta = 96.063(4) degrees, Z = 4), [Ag(L1)]ClO4 (2) (monoclinic, C2/c; a = 17.363(2) A, b = 13.2794(18) A, c = 13.4884(18) A, beta = 100.292(2) degrees, Z = 8), [Ag(L1)(C6H6)SbF6] x 0.5C6H6 x H2O (3) (monoclinic, P2(1)/c; a = 6.8839(11) A, b = 20.242(3) A, c = 18.934(3) A, beta = 91.994(3) degrees, Z = 4), Ag(L2)(SO3CF3) (4) (triclinic, P1; a = 8.629(3) A, b = 10.915(3) A, c = 11.178(3) A, alpha = 100.978(4) degrees, beta = 91.994(3) degrees, gamma = 105.652(4) degrees, Z = 2), and Ag(L3)(H2O)(SO3CF3) (5) (triclinic, P1; a = 8.914(5) A, b = 10.809(6) A, c = 11.283(6) A, alpha = 69.255(8) degrees, beta = 87.163(9) degrees, gamma = 84.993(8) degrees, Z = 2) were obtained through self-assembly based on these three new fulvene ligands in a benzene/toluene mixed-solvent system. Compounds 1-5 have been fully characterized by infrared spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The results indicate that the coordination chemistry of new fulvene ligands is versatile. They can adopt either cis- or trans-conformation to bind soft acid Ag(I) ion through not only the terminal -CN and furan functional groups but also the fulvene carbon atoms into organometallic coordination polymers or discrete complexes. In addition, the luminescent properties of L1-L3 and their Ag(I) complexes were investigated preliminarily in EtOH and solid state.

Coordination arrays — Synthesis and characterization of tetranuclear complexes of grid-type

A series of tetranuclear metal complexes of grid-type consisting of four bis-tridentate ligands and four divalent transition metal ions were synthesized and characterized. The 1H NMR spectra of diamagnetic complexes containing Zn(II), Cd(II), Fe(II), and Ru(II) was correlated to the radius of the metal ion. The UV–vis and electrochemical results indicated that the bridging ligand π* orbital and the dπ metal orbital are stabilized by complexation of more than one metal ion. Furthermore, the Co(II) and Fe(II) grids exhibit metal–metal interaction mediated by the bis-tridentate ligands as indicated by electrochemical and spectroscopic methods. These results provide guidelines for the design of larger grids bearing several metal centres in a square arrangement, which also represent potential components of molecular electronic devices.Key words: complexes with nitrogen ligands, octahedral metal ions, self-assembly, supramolecular chemistry.

Nitrogen heterocycles as building blocks for new metallosupramolecular architectures

The syntheses of representative examples of five classes of new heterocyclic ligands are described. These include N,N'-chelating bis-heterocycles, binucleating ligands, cyclometallated compounds, chiral ligands and a family of polyheteroaryl-linked arenes.

Synthesis, crystal structure, magnetic properties, and theoretical studies of [(Cu(mepirizole)Br)2(mu-OH)(mu-pz)] (mepirizole=4-methoxy-2-(5-methoxy-3-methyl-1H-pyrazol-1-yl)-6-methylpyrimidine; pz=pyrazolate), a novel mu-pyrazolato-mu-hydroxo-dibridged copper(II) complex

A novel mu-pyrazolato-mu-hydroxo-dibridged copper(II) complex has been synthesized and structurally characterized: [(Cu(mepirizole)Br)2(mu-OH)(mu-pz)] (mepirizole=4-methoxy-2-(5-methoxy-3-methyl-1H-pyrazol-1-yl)-6-methylpyrimidine; pz=pyrazolate). The title compound crystallizes in the monoclinic system, space group P2(1)/c, with a=15.618(2) A, b=15.369(3) A, c=16.071(3) A, and beta=112.250(1) degrees. The structure is built up of dinuclear [(Cu(mepirizole)Br)2(mu-OH)(mu-pz)] units with five-coordinated copper(II) ions (CuBrN3O chromophores) linked by mu2-OH and mu2-pyrazolato bridges that are well separated from each others. The intramolecular copper-copper distance is 3.378(3) A. Magnetic susceptibility data show that the copper atoms are strongly antiferromagnetically coupled with J=-770 cm(-1). The obtained triplet-singlet energy gap is compared with those reported for a series of related dimers. The strong antiferromagnetic coupling arising from the complementarity of the hydroxo and pyrazolato bridges has been discussed on the basis of DFT calculations.

Chiral Heterocyclic Ligands. X. Synthesis and Metal Complexes of Hindered and Chiral 2,2′-Bipyrimidines

The first examples of 2,2′-bipyrimidine ligands incorporating bulky or chiral substituents have been prepared by nickel(0)-mediated homo-coupling reactions of chloropyrimidines. The 4,4′-di-(tert-butyl) derivative 2 and the chiral derivative 5, with 4,5-fused bornane groups, have been shown to form mononuclear metal complexes in which they coordinate through the less hindered nitrogen donors.

Dimetallic complexes of acyclic pyridine-armed ligands derived from 3,6-diformylpyridazine

A bis(pyridine-armed) acyclic Schiff base ligand L1 has been synthesised from 3,6-diformylpyridazine and two equivalents of 2-(2-aminoethyl)pyridine. Reduction of this ligand using NaBH(4) resulted in the formation of the amine analogue L2. Complexes of the form [M(2)L1(mu-X)]Y(2)ClO(4)[where: M = Cu(II), X = OH(-) and Y = ClO(4)(-) 1, Cl(-) 2, Br(-) 3 or I(-) 4; M = Co(II), X = OH(-) and Y = ClO(4)(-) 5; M = Ni(II), X = SCN(-) 6 or X = N(3)(-) 7 and Y = ClO(4)(-)], and [Cu(2)L2(mu-OH)](ClO(4))(3) 8 were prepared and characterised. The complexes 1 and 5-7 have been characterised by single-crystal X-ray diffraction. The acyclic L1 ligand provides three nitrogen donor atoms per metal centre, including a pyridazine bridge between the metal centres, and the anion X also bridges the two metal centres. As required, coordinating solvent molecules or additional anions make up the remainder of the coordination sphere. The two copper centres of 1 are very strongly antiferromagnetically coupled (2J=-1146 cm(-1))via the pyridazine and hydroxide ion bridges, whereas the competing antiferromagnetic pyridazine bridging pathway and ferromagnetic 1,1-bridging azide pathway resulted in the observation of weak antiferromagnetic exchange in the dinickel(II) complex 7 (2J=-14 cm(-1)). Electrochemical examination of L1, L2 and complexes 1 and 5-8 revealed multiple redox processes. These have been tentatively assigned to a mixture of metal centred and ligand centred redox processes on the basis of cyclic voltammetry and coulometry results and comparisons with literature examples.

A novel series of complexones with bis- or biazole structure as mixed ligands of paramagnetic contrast agents for MRI

We describe the syntheses, physicochemical properties and biological evaluation of a novel series of complexones containing bis- or biazoles moieties and two iminodiacetic acid units as novel ligands for paramagnetic lanthanides. The complexones were prepared by reaction of the corresponding 1,1'-bishaloethylbi- or bispyrazoles with methyl iminodiacetate and subsequent NaOH hydrolysis. 1,1'-Bisbromoethyl precursors were obtained by direct alkylation with an excess of 1,2-dibromoethane, or by heating the corresponding alcohol in HCl. Sigmoidal binding isotherms and MO calculations supported as most stable structures in solution, those containing two Gd(III) atoms bound per molecule of complexone with half saturation values S(0.5) (M(-1), 22 degrees C, pH 7.2) in the range 6.5 10(-6)<S(0.5)<36.1 10(-6). Relaxivity properties [r(1), r(2), s(-1) mM(-1) Gd(III)] determined at 1.5 Tesla gave values (12.0<r(1)<17.7, 12.2<r(2)<20), improving significantly the relaxivities of reference compounds such as Gd(III)EDTA (5.2, 5.6) or Gd(III)DTPA (4.30, 4.30). These improvements involve mainly increased hydration and slower rotational motions. In vitro toxicity experiments are reported.

Friedländer approach for the incorporation of 6-bromoquinoline into novel chelating ligands

Nitration of 3-bromobenzaldehyde followed by sodium dithionite reduction provides 5-bromo-2-aminobenzaldehyde, which undergoes the Friedländer condensation with a variety of enolizable ketones to afford bidentate and tridentate 6-bromoquinoline derivatives. These species may be dimerized with Ni(0) to form biquinolines or treated under Sonogashira conditions to afford 6-alkynyl derivatives. Examination of optical properties indicate an unusually high emission quantum yield for 6,6'-biquinolines. [structure: see text]