Synthesis,structures, and spectroscopic properties of Hg(II) complexes of bidentate NN and tridentate NNO Schiff-base ligands

Reactions of HgX₂ (X?=?Cl, N₃, NO₃) with (E)-2-methoxy-N-(pyridin-2-ylmethylene)aniline (L¹) and (E)-4-methoxy-N-(pyridin-2-ylmethylene)aniline (L²) in ethanol gave two monomers, [HgL¹(Cl)₂] (1) and [HgL²(NO₃)₂(DMSO)] (5), and three coordination polymers, {[HgL¹(N₃)₂]₂·Hg(N₃)₂}_n (2), [HgL²(Cl)₂]_n (3), and [HgL²(NO₃)₂]_n·nCH₃CN (4). Compounds 1–5 were characterized by elemental analysis, IR, NMR spectroscopy, and single-crystal X-ray diffraction. The common feature of monomeric 1 and 5 is the presence of intra- and intermolecular Hg–O bonds. In the absence of these, polymeric structures arise as a result of azide, chloride, and nitrate bridging in 2, 3, and 4, respectively. Fluorescent properties of 1–5 were also investigated.     

Ni(II) and Cu(II) complexes of new unsymmetrical N2O2 Schiff bases derived from 3-(2-aminobenzylimino)-1-phenylbutan-1-ol: synthesis,structure, antibacterial properties,and electrochemistry

Two new N₂O₂ unsymmetrical Schiff bases, H₂L¹ = 3-[({o-[(E)-(o-hydroxyphenyl)methylideneamino]phenyl}methyl)imino]-1-phenyl-1-buten-1-ol and H₂L² = 3-[({o-[(E)-(2-hydroxy-1-naphthyl)methylideneamino]phenyl}methyl)imino]-1-phenyl-1-buten-1-ol, and their copper(II) and nickel(II) complexes, [CuL¹] (1), [CuL²] (2), [NiL¹] (3), and [NiL²] (4), have been synthesized and characterized by elemental analyses and spectroscopic methods. The crystal structures of these complexes have been determined by X-ray diffraction. The coordination geometry around Cu(II) and Ni(II) centers is described as distorted square planar in all complexes with the CuN₂O₂ coordination more distorted than the Ni ones. The electrochemical studies of these complexes indicate a good correlation between the structural distortion and the redox potentials of the metal centers. The ligand and metal complexes were also screened for their in vitro antibacterial activity.     

Copper(I) and copper(II) complexes with pyrazine-containing pyridylalkylamide ligands N-(pyridin-2-ylmethyl)pyrazine-2-carboxamide and N-(2-(pyridin-2-yl)ethyl)pyrazine-2-carboxamide

Four copper(II) complexes and one copper(I) complex with pyridine-containing pyridylalkylamide ligands N-(pyridin-2-ylmethyl)pyrazine-2-carboxamide (HL^pz) and N-(2-(pyridin-2-yl)ethyl)pyrazine-2-carboxamide (HL^pz?) were synthesized and characterized. The X-ray crystal structures of [Cu₂(L^pz)₂(4,4?-bipy)(OTf)₂] (1, OTf?=?trifluoromethanesulfonate, 4,4?-bipy?=?4,4?-bipyridine) and [Cu(L^pz)(py)₂]OTf·H₂O (2, py?=?pyridine) revealed binuclear and mononuclear molecular species, respectively, while [Cu(L^pz)(μ₂-1,1-N₃)]_n (3), [Cu(L^pz?)(μ₂-1,3-N₃)]_n (4), and [Cu(HL^pz)Cl]_n (5) are coordination polymer 1-D chains in the solid state.     

Dinuclear Schiff-base copper(II) complexes with various bridging groups

Three new centrosymmetric dinuclear copper(II) complexes, [Cu₂Cl₂(L¹)₂] (1), [Cu₂(μ _1,3-NCS)₂(L²)₂] (2), and [Cu₂(μ _1,1-N₃)₂(L³)₂] (3), where L¹, L², and L³ are the deprotonated forms of the Schiff bases 1-[(2-propylaminoethylimino)methyl]naphthalen-2-ol (HL¹), 1-[(3-methylaminopropylimino)methyl]naphthalen-2-ol (HL²), and 2-[(2-isopropylaminoethylimino)methyl]phenol (HL³), respectively, have been prepared and characterized by elemental analysis, IR spectra, and single-crystal X-ray crystallography. Each Cu is coordinated by the three donors of the Schiff bases and by two bridging groups, forming a square-pyramidal geometry.     

Polymerizations of methyl methacrylate and rac-lactide by zinc(II) precatalyst containing N-substituted 2-iminomethylpyridine and 2-iminomethylquinoline

The reaction of [ZnCl₂] with N-cyclopentyl-1-(quinolin-2-yl)methanimine (L_A), N-cyclohexyl-1-(quinolin-2-yl)methanimine (L_B), N-cyclohexyl-1-(pyridin-2-yl)methanimine (L_C), 2,6-diethyl-N-(pyridin-2-ylmethylene)aniline (L_D), N-cyclopentyl-1-(pyridin-2-yl)methanimine (L_E), and N-phenyl-(pyridin-2-yl)methanimine (L_F) in ethanol produced the bidentate [(NN′)ZnCl₂] complexes, [L_AZnCl₂], [L_BZnCl₂], [L_CZnCl₂], [L_DZnCl₂], [L_EZnCl₂] and [L_FZnCl₂], respectively. The molecular structures revealed that the zinc in [L_nZnCl₂] (L_n = L_A ? L_D) showed a distorted tetrahedral geometry involving two nitrogens of N,N’-bidentate ligands and two chloride ligands. Most of these initiators were effective for polymerization of methyl methacrylate (MMA) and polymerization of rac-lactide (rac-LA). [L_CZnCl₂] (with N-cyclohexyl substituted at imine-pyridine moiety) exhibited the highest catalytic activity for MMA polymerization in the presence of modified methylaluminoxane (MMAO) with an activity of 3.33 × 10⁴ g PMMA/mol·Zn·h at 60 °C, giving moderate syndiotactic poly methyl methacrylate (PMMA) with high molecular weight (9.62 × 10⁵ g/mol). The dimethyl derivatives [L_nZnMe₂] (L_n = L_A ? L_F), generated in situ, polymerized rac-LA with moderate activity and yielded a polylactide (PLA) with good number-average molecular weights and narrower polydispersity indices (PDIs). [L_AZnMe₂] effectively initiates the ring-opening polymerization (ROP) of rac-LA to attain heterotactic PLA (P_r = 0.91).     

Cobalt(II) complexes with pyridine and 5-[(E)-2-(aryl)-1-diazenyl]-quinolin-8-olates: synthesis,electrochemistry and X-ray structural characterization

Abstract

Nine new cobalt(II) compounds, trans-[Co(L^PAQ)₂(Py)₂] (1), trans-[Co(L^PAQ)₂(3-MePy)₂] (2), trans-[Co(L^MeAQ)₂(Py)₂] (3), trans-[Co(L^OMeAQ)₂(Py)₂] (4), trans-[Co(L^OEtAQ)₂(Py)₂]·2(H₂O) (5), trans-[Co(L^CAQ)₂(Py)₂] (6), trans-[Co(L^BAQ)₂(Py)₂] (7), cis-[Co(L^BAQ)₂(3-MePy)₂] (8a) and trans-[Co(L^BAQ)₂(3-MePy)₂]·2(3-MePy) (8b) (primary ligand: L^XAQ?=?substituted 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olate; secondary ligands: Py?=?pyridine, 3-MePy = 3-methylpyridine), have been synthesized and characterized by elemental analysis, IR and UV-vis spectroscopy. Magnetic measurements of the cobalt compounds were performed in solution by ¹H NMR spectroscopy using the Evans’ method while their redox properties were studied by cyclic voltammetry. Single-crystal X-ray diffraction analysis of the compounds revealed their octahedral geometries and trans configuration, except for 8a, which has a cis configuration. Intermolecular noncovalent interactions were detected, π···π interactions in 5, C?–?H···π interactions in 2 and C?–?H···π edge-to-face (T-shaped) arrangements in 3, 4, 6, and 7.     

Pb(II) and Bi(III) complexes involving a Schiff base ligand: syntheses,structures, physical,and antioxidant properties

Two new complexes, {[PbL(NO₃)₂H₂O]H₂O}_n (1) and [BiL₂(NO₃)₂]NO₃ (2), based on (E)-3-chloro-6-[2-(pyrazin-2-ylmethylene)hydrazinyl]pyridazine (L) were synthesized and characterized by IR spectra, single-crystal X-ray diffraction, and elemental analyzes. X-ray single crystal diffraction experiments of 1 and 2 display that extensive hydrogen bonds and π…π stacking interactions construct the 1-D infinite chain {[PbL(NO₃)₂H₂O]H₂O}_n and [BiL₂(NO₃)₂]NO₃ into two 3-D supramolecular frameworks. Interestingly, pure phase PbO nano-particles were synthesized by thermolysis of 1 and characterized by scanning electron microscopy and X-ray powder diffraction analyzes. Furthermore, antioxidant activities of L, 1, and 2 were also studied.     

Coordination of di- and triimine ligands at ruthenium(II) and ruthenium(III) centers: structural,electrochemical and radical scavenging studies

Herein, we explore the coordination of di- and triimine chelators at ruthenium(II) and ruthenium(III) centers. The reactions of 2,6-bis-((4-tetrahydropyranimino)methyl)pyridine (thppy), N¹,N²-bis((3-chromone)methylene)benzene-1,2-diamine (chb), and tris-((1H-pyrrol-2-ylmethylene)ethane)amine (H₃pym) with trans-[Ru^IICl₂(PPh₃)₃] afforded the diamagnetic ruthenium(II) complex cis-[RuCl₂(thppy)(PPh₃)] (1) and the paramagnetic complexes [mer-Ru₂(μ-chb)Cl₆(PPh₃)₂] (2), and [Ru(pym)] (3), respectively. The complexes were characterized by IR, NMR, and UV–vis spectroscopy and molar conductivity measurements. The structures were confirmed by single crystal X-ray diffraction studies. The redox properties of the metal complexes were probed via cyclic- and squarewave voltammetry. Finally, the radical scavenging capabilities of the metal complexes towards the NO and 2,2-di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH) radicals were investigated     

Synthesis,crystal structure,and antibacterial activity of mononuclear nickel(II) and cobalt(III) Schiff-base complexes

Four new mononuclear complexes, [Ni(L¹)(NCS)₂] (1), [Ni(L²)(NCS)₂] (2), [Co(L¹)(N₃)₂]ClO₄ (3), and [Co(L²)(N₃)₂]ClO₄ (4), where L¹ and L² are N,N′-bis[(pyridin-2-yl)methylidene]butane-1,4-diamine and N,N′-bis[(pyridin-2-yl)benzylidene]butane-1,4-diamine, respectively, have been prepared. The syntheses have been achieved by reaction of the respective metal perchlorate with the tetradentate Schiff bases, L¹ and L², in presence of thiocyanate (for 1 and 2) or azide (for 3 and 4). The complexes have been characterized by microanalytical, spectroscopic, single crystal X-ray diffraction and other physicochemical studies. Structural studies reveal that 1–4 are distorted octahedral geometries. The antibacterial activity of all the complexes and their constituent Schiff bases have been tested against Gram-positive and Gram-negative bacteria.     

Syntheses,characterization, and crystal structures of ruthenium(II)/(III) complexes with tridentate salicylaldiminato ligands

Treatment of [Ru(PPh₃)₃Cl₂] with one equivalent of tridentate Schiff base 2-[(2-dimethylamino-ethylimino)-methyl]-phenol (HL) in the presence of triethylamine afforded a ruthenium(III) complex [RuCl₃(κ²-N,N-NH₂CH₂CH₂NMe₂)(PPh₃)] as a result of decomposition of HL. Interaction of HL and one equivalent of [RuHCl(CO)(PPh₃)₃], [Ru(CO)₂Cl₂] or [Ru(tht)₄Cl₂] (tht = tetrahydrothiophene) under different conditions led to isolation of the corresponding ruthenium(II) complexes [RuCl(κ³-N,N,O-L)(CO)(PPh₃)] (2), [RuCl(κ³-N,N,O-L)(CO)₂] (3), and a ruthenium(III) complex [RuCl₂(κ³-N,N,O-L)(tht)] (4), respectively. Molecular structures of 1·CH₂Cl₂, 2·CH₂Cl₂, 3 and 4 have been determined by single-crystal X-ray diffraction.     

Synthesis and Crystal Structures of Lithium,Sodium, and ­Potassium Derivatives of 2‐(Methylthio)aniline and 2‐(Phenylthio)aniline

2‐(Methylthio)aniline (H₂L¹) and 2‐(phenylthio)aniline (H₂L²) were treated with n‐butyllithium to yield the corresponding anilides [LiHL¹] and [LiHL²]. Recrystallization from diethyl ether and THF afforded the solvates [LiHL¹(Et₂O)] and [LiHL²(THF)₂]. The X‐ray crystal structure determination revealed dimeric molecules which exhibit a centrosymmetric Li₂N₂ ring. In the case of [LiHL¹(Et₂O)] the SMe group is involved in Li coordination and in the case of [LiHL²(THF)₂] the SPh group is part of an intramolecular N–H ··· S hydrogen bridge. The sodium anilides [NaHL¹(DME)] and [NaHL²(DME)] were obtained from the reaction of H₂L¹ and H₂L² with sodium amide in DME as solvent. Like in the case of the lithium amides the sodium derivatives [NaHL¹(DME)] and [NaHL²(DME)] display centrosymmetric Na₂N₂ cores. The coordination sphere of the sodium atoms is completed by DME molecules, which act as chelating ligands. In the case of [NaHL¹(DME)] the DME molecules enable additionally a linkage of the dimeric subunits to give a chain structure. The potassium derivatives [KNHL¹] and [KNHL²(DME)] were obtained from H₂L¹ and H₂L² and potassium hydride in DME as solvent. [KNHL¹] displays a distinct structure based on [(KNHL¹)₂] dimers, which are linked by additional [KNHL¹] units to give a 3D coordination polymer with {4.8.16(3)} topology. [KNHL²(DME)] forms dimers linked by bridging DME molecules to give a chain‐like coordination polymer.     

Synthesis,crystal structures,and biological activity of oxovanadium(V) complexes with similar tridentate hydrazone ligands

Two new oxovanadium(V) complexes, [VOL¹(OEt)(EtOH)] (1) and [VOL²(OMe)(MeOH)] (2), were prepared by reaction of [VO(acac)₂] (where acac?=?acetylacetonate) with N′-(3-bromo-2-hydroxybenzylidene)-4-methylbenzohydrazide (H₂L¹) in ethanol and N′-(3-bromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide (H₂L²) in methanol, respectively. Crystal and molecular structures of the complexes were determined by elemental analysis, infrared spectra, and single-crystal X-ray diffraction. The V ions have octahedral coordination. Thermal stability and the inhibition of urease of the complexes were studied.     

Synthesis,crystal structures,spectroscopic studies and antibacterial properties of a series of mononuclear cobalt(III) Schiff base complexes

New complexes of cobalt(III) with the tridentate and tetradentate Schiff base ligands: 3-methoxy-2-{(Z)[(2-hydroxyphenyl)imino]methyl}phenol (H₂L¹), 4-[(2-hydroxyphenyl)imino]-2-pentanone (H₂L²); and 2-((E)-1-(2-((E)-1-(2-hydroxy-4,5-dimethylphenyl)ethylideneamino)ethylimino)ethyl)-4,5 dimethylphenol (H₂L³), namely [Co^III(L¹)(N-MeIm)₃]PF₆ (1), [Co^III(L¹)(py)₃]ClO₄ (2), [Co(L¹)(py)₃][Co(L¹)₂] (3) and [Co^III(L²)(N-MeIm)₃]PF₆ (4) and [Co(L³)(N-MeIm)₂]PF₆ (5), were synthesized and characterized by physico-chemical and spectroscopic methods. The crystal structures of the complexes were determined by X-ray crystallography. In each of these complexes, the cobalt(III) centre has a slightly distorted octahedral environment, utilizing all available coordination centres of the ligands. The complexes were also screened for in vitro antibacterial activities against four human pathogenic bacteria, and their minimum inhibitory concentrations indicated good antibacterial activities.     

Coordination diversity in tin compounds with bis(benzoxazole)phenol as a polydentate ligand: Synthesis and crystal structure studies

Abstract

The reaction of 2,6-bis(benzoxazolyl)-4-tert-butylphenol (HL) with [ⁿBu_xSnCl_4?x] (x?=?0, 1) in 1:1 stoichiometry yielded the tin coordination complexes [(HL)SnⁿBu_xCl_4?x] [x?=?0 (1); x?=?1 (2)]. Deprotonation of HL was performed using reagents having groups with high basicity such as ⁿBuLi or [Sn{N(SiMe₃)₂}₂]. These basic reagents prompted the coordination of the ligand in its anionic form, yielding the complexes [(thf)₂Li(L)SnCl₄] (3) and [(L)Sn{N(SiMe₃)₂}] (4), respectively. The molecular structure of HL displayed an intramolecular hydrogen bond OH—N and a planar arrangement of the bis(benzoxazolyl)phenolic system. In the molecular structures of both complexes containing HL an intramolecular hydrogen bond of NH—O type was also present. The coordination of the ligand in either neutral or anionic form is described by a κO,κN chelate mode toward Sn. All complexes displayed bis(benzoxazolyl)phenolic moieties close to planar; the least planar system was observed in 4 that was also studied by DFT methods.     

Novel platinum(II) and (IV) complexes of naphthaldimine schiff bases

Naphthaldimines containing N₂O₂ donor centers react with platinum(II) and (IV) chlorides to give two types of complexes depending on the valence of the platinum ion. For [Pt(II)], the ligand is neutral, [(H₂L¹)PtCl₂]·3H₂O (1) and [(H₂L³)₂Pt₂Cl₄]·5H₂O (3), or monobasic [(HL²)₂Pt₂Cl₂]·2H₂O (2) and [(HL⁴)₂Pt]·2H₂O (4). These complexes are all diamagnetic having square-planar geometry. For [Pt(IV)], the ligand is dibasic, [(L¹)Pt₂Cl₄(OH)₂]·2H₂O (5), [(L²)Pt₃Cl₁₀]·3H₂O (6), [(L³)Pt₂Cl₄(OH)₂]·C₂H₅OH (7) and [(L⁴)Pt₂Cl₆]·H₂O (8). The Pt(IV) complexes are diamagnetic and exhibit octahedral configuration around the platinum ion. The complexes were characterized by elemental analysis, UV-Vis and IR spectra, electrical conductivity and thermal analyses (DTA and TGA). The molar conductances in DMF solutions indicate that the complexes are non-ionic. The complexes were tested for their catalytic activities towards cathodic reduction of oxygen.     

Mercury(II) halide adducts of a dicarboxylate-like ligand: Crystal structures of polymeric mercury(II) complexes with 1,4-diazoniobicyclo[2.2.2]octane-1,4-diacetate

Three mercury(II) complexes containing double-betaine and halide ligands, [(HgCl₂)₂(L¹)] _n (1), [(HgBr₂)₂(L¹)(H₂O)₂] _n (2), and [2HgCl₂(HL¹)·Hg₂Cl₆] _n (3) [L¹=^–O₂CCH₂N⁺(CH₂CH₂)₃-N⁺H₂CO₂ ^–], have been prepared and shown to have polymeric structures by single-crystal X-ray analysis. Complex (1) exhibits an infinite zigzag chain in which each mercury(II) atom is coordinated by pairs of carboxylate oxygen atoms and chloride ligands in a distorted tetrahedral geometry. In complex (2), the mercury(II) atom is in an unusual square-planar coordination geometry, and weak mercury-ligand and hydrogen bonding extend the structural skeletons into a three-dimensional network. Complex (3) consists primarily of an assembly of HgCl₂(HL¹) moieties and [Hg₂Cl₆]^– anions, and the mercury(II) atoms are in planar T-shaped and distorted tetrahedral coordination environments, respectively. The resulting three-dimensional network is based on the cross-linkage of nearly planar, wide ribbons running in thea direction.     

Formation of polynuclear palladium complexes with the benzimidazole-2-thiolate anion

The reactions of palladium(II) salts with 2-mercaptobenzimidazole (HL) and its 5,6-difluorinated derivative (HL^F) were investigated. In the presence of hydrochloric acid, PdCl₂ and K₂PdCl₄ react with HL and HL^F in the ethanol—water and acetonitrile—water systems to form the mono-nuclear dicationic complexes [Pd(HL)₄]Cl₂ (1) and [Pd(L^F)₄]Cl₂ (2). In the absence of HCl, the reactions afford the tetranuclear complex Pd₄[(L)₂(μ₃-S,N-(L))₂(μS,N-(L))₄] (3). The reaction of triethylamine with an ethanolic solution of 3 leads to degradation of 3 and the formation of the lantern-type dinuclear complex Pd₂[(μ₂-(L)₄] (4), in which the palladium atoms are in the nonequivalent coordination environment, PdN₄ and PdS₄. The reaction of K₂PdCl₄ with HL or HL^F in the THF—water or acetonitrile—water systems (for the reaction with HL^F) in the presence of Et₃N produces the lantern-type dinuclear complexes Pd₂[(μS,N′-(L³))₄] and Pd₂[(μ-S,N′-(L^F))₄] (5), in which the metal atoms are in the equivalent coordination environment (cis-PdN₂S₂). Dedicated to Academician G. A. Tolstikov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 45–52, January, 2008.     

Syntheses and Structures of Nitridorhenium(V) and Nitridotechnetium(V) Complexes with N,N‐[(Dialkylamino)(thiocarbonyl)]‐N′‐(2‐hydroxyphenyl)benzamidines

[MNCl₂(PPh₃)₂] complexes (M = Re, Tc) react with N‐[(dialkylamino)(thiocarbonyl)]‐N′‐(2‐hydroxyphenyl)benzamidines (H₂L¹) with formation of neutral, five‐coordinate nitrido complexes of the composition [MN(L¹)(PPh₃)]. The products have distorted square‐pyramidal coordination spheres with each a tridentate, double‐deprotonated benzamidine and a PPh₃ ligand in their basal planes.     

Supramolecular architectures with ladder and lamellar topologies using metal-ligand coordination units via C–H···Cl and O–H···Cl hydrogen bonding

New Mn^II/Cu^II/Zn^II complexes [(L¹)MnCl₂] (1), [(L²)CuCl₂]·0.5H₂O (2) and [(L²)ZnCl(H₂O)][ClO₄] (3), containing (2-pyridyl)alkylamine ligands, N-methyl-N,N-bis(2-pyridylmethyl)amine (L¹) and methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine (L²), have been prepared and characterized, including X-ray crystallography. The most striking feature of the structures of these complexes is the formation of molecular ladder and lamellar topology through the crystal packing arrangement, determined by both strong O–H···Cl and weak (however, multiple) C–H···Cl hydrogen-bonding interactions, to maintain the neutral/cationic metal-ligand coordination units linked to each other. In 3, additional secondary interactions are observed involving coordinated solvent and the counter-ion. The results presented here demonstrate that (i) the choice of organic ligands to provide flexibility and inherent potential to participate in hydrogen-bonding interactions, (ii) the coordination geometry preferences of metal ions, (iii) the number of metal-bound chloride ion and (iv) the presence of solvent/counter-anion have a great influence on supramolecular network topology.     

Iridium (III) and rhodium (III) triazoles by 1,3-dipolar cycloadditons to a coordinated azide in iridium (III) and rhodium (III) compounds

The reaction of [(η⁵-C₅Me₅)M(μCl)Cl]₂ with the ligand (L^∩L) in the presence of sodium methoxide yielded compounds of general formula [(η⁵-C₅Me₅)M(L^∩L)Cl] (1–10) (where M = Ir or Rh and L^∩L = N^∩O or O^∩O chelate ligands). Azido complexes of formulation [(η⁵-C₅Me₅)M(L^∩L)N₃] (11–20) have been prepared by the reaction of [(η⁵-C₅Me₅)M(μN₃)(X)]₂ (X = Cl or N₃) with the corresponding ligands or by the direct reaction of [(η⁵-C₅Me₅)M(L^∩L)Cl] with NaN₃. These azido complexes [(η⁵-C₅Me₅)M(L^∩L)N₃] undergo 1,3-dipolar cycloaddition reaction with substituted alkynes in CH₂Cl₂ and for the first time in ethanol at room temperature to yield iridium (III) and rhodium (III) triazoles (21–28). The compounds were characterized on the basis of spectroscopic data, and the molecular structures of 2 and 26 have been established by single crystal X-ray diffraction.