Nickel(II) complexes stabilized by bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine: Synthesis and characterization of complexes stabilized by a hydrogen bonding network

Intramolecular hydrogen bonds in metalloproteins are key in directing reactivity yet these effects have been difficult to achieved in synthetic systems. We have been developing a synthetic system that uses hydrogen-bonding interactions to modulate the secondary coordination around a transition metal ion. This was accomplished with the ligand bis[N-(6-pivalamido-2-pyridylmethyl)]benzylamine (H₂pmb), which contains two carboxyamido units appended from pyridine rings. Several nickel complexes were prepared and structurally characterized. In particular, we found that the appended carboxyamido groups either provide intramolecular H-bond donors or can be converted to bind directly to a metal center. We established that the complex Ni^IIH₂pmb(Cl)₂ can be sequentially deprotonated with potassium tert-butoxide, causing coordination of the carboxyamido oxygen atoms and concomitant loss of the chloro ligands. The chloro ligands were also removed with silver(I) salts in the presence of acetate ions and the complex Ni^IIH₂pmb(κ²-OAc)(κ¹-OAc) was isolated, in which an intramolecular H-bonding network occurs between the H₂pmb ligand and the coordinate acetato ligands.     

Synthesis and anomalous magnetic properties of heterometal dinuclear Fe(II)-Cr(III) complex bridged by 3,5-bis(pyridin-2-yl)-pyrazolate

A Cr(III)-Fe(II) dinuclear complex [(nta)Cr(μ-bpypz)Fe(dpea)]BF₄, where bpypz=3,5-bis(pyridin-2-yl)-pyazolate, nta=nitrilotriacetate and dpea=(2-aminoethyl)-bis(2-pyridylmethyl)amine, has been synthesized and structurally characterized by ESI-MS and ²H NMR spectra. The proposed structure is mostly more planar in the bridging moiety as compared to that of the previously reported [(nta)Cr(μ-bpypz)Fe(picen)]BF₄(N,N^′-bis-(2-pyridylmethyl)ethylenediamine(picen)). The dpea complex is found to be a high spin state, but to exhibit no spin-crossover by magnetic susceptibility and Mössbauer spectra measurements; showing a large antiferromagnetic interaction, which could make the spin transition temperature lower and/or destroy spin-crossover, suggesting complementarity between spin-crossover and magnetic interaction.     

Synthesis and characterization of an N-coordinated amidate copper(II) complex of deprotonated N-(2,6-diisopropylphenyl)-2-(bis-(2-pyridylmethyl))aminoethanamide

The title ligand, N-(2,6-diisopropylphenyl)-2-(bis-(2-pyridylmethyl))aminoethanamide (DIPMAE-H), was prepared by a nucleophilic substitution reaction between N-(2,6-diisopropyl)phenyl-2-bromoethanamide and bis-(2-pyridylmethyl)amine. An analogous ligand (TBPMAE-H) in which the 2,6-diisopropylphenyl group was substituted for a tert-butyl group was also prepared in this manner. Then, [(DIPMAE-H)CuBr]⁺Br⁻ and [(TBPMAE-H)CuBr]⁺Br⁻ were prepared by heating one equivalent of ligand and CuBr₂ in CH₃CN. In both compounds the geometry about the copper center is square pyramidal with distortions due to the geometrical constraints of the ligand. The amide oxygen occupies the axial position, and the three amine nitrogens and the bromide ligand form the basal plane of the square pyramid. Pairs of complexes in the unit cell are associated via weak donation of a lone pair on the bromide ligand of one complex to the copper center of another (Cu?Br distances in the range of 3.3576-3.4022 Å).The title compound, (DIPMAE)CuBr, was prepared by deprotonation of [(DIPMAE-H)CuBr]⁺Br⁻ using NaH. The key feature of (DIPMAE)CuBr is the amidate group η¹- and N-coordinated to the copper center. The compound also exhibits distorted trigonal bipyramidal coordination geometry with the bromide and tertiary amine donors occupying the axial sites and the amidate and pyridyl donors occupying the equatorial positions. The copper atom is displaced from the trigonal plane towards the bromide donor apex due to the geometrical demands of the ligand.     

Iron and cobalt complexes with the ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and derivatives

A series of iron(II)/(III) and cobalt(II)/(III) complexes with the tetradendate tripodal ligands (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp), its methylated derivatives Me₂-uns-penp and Me₄-uns-penp as well as the amide ligand N-acetyl-N,N-bis[(2-pyridyl)methyl]ethylenediamine (acetyl-uns-penp) were synthesized and structurally characterized. They have been investigated in regard to their reactivity towards dioxygen and/or hydrogen peroxide. Complexes of this type seem to have a high potential to be useful in the activation of dioxygen for selective oxidation reactions of organic substrates.     

Coordination modes of N-(2-hydroxyphenyl)methyl-bis-(2-pyridylmethyl)amine with copper (I) and (II) halides

The title ligand, N-(2-hydroxyphenyl)methyl-bis-(2-pyridylmethyl)amine, was prepared via a condensation-reduction synthetic route. The compounds, CuCl(C₁₉H₁₉N₃O) and [CuBr(C₁₉H₁₉N₃O)]⁺Br⁻ · 3H₂O, were readily synthesized from the reaction of CuCl or CuBr₂ and the ligand in acetonitrile. The title copper(I) compound is an O-H ? Cl hydrogen-bonded linear chain of tetrahedrally coordinated copper centers, and the title copper(II) compound exists as two strongly tetragonally distorted dibromide bridged metal cations in a dimer with the phenol hydroxyl groups weakly bound in a trans-fashion to one of the bridging bromides. In the copper(I) complex the phenoxy group acts only as a hydrogen bond donor, whereas in the copper(II) complex it acts both as a ligand and a hydrogen bond donor.     

Syntheses and crystal structures of mononuclear and dinuclear (pentamethylcyclopentadienyl)iridium(III) complexes containing 2-mercaptobenzimidazole

A reaction of [Cp*IrCl₂]₂ {Cp* = η⁵-C₅(CH₃)₅} and 2-mercaptobenzimidazole (H₂bimt) in methanol in a 1:2 molar ratio gave a yellow complex of [Cp*IrCl₂(H₂bimt)]·CH₃OH (1). In compound 1 the H₂bimt acts as a monodentately S-donating (κS) ligand. A similar reaction of [Cp*IrCl₂]₂ and H₂bimt in the presence of NaOCH₃ (molar ratio of 1:2:2) gave an orange product (2) on addition of excess amount of NH₄PF₆. Compound 2 was composed of an unsymmetrical dinuclear complex cation, [(Cp*IrCl)₂(μ-Hbimt)(μ-H₂bimt)]⁺, a PF₆⁻ anion, and water molecules of crystallization. In the complex cation, H₂bimt bridges two Ir^III centers by S atom in the μ-κS:κS mode, while the monodeprotonated Hbimt⁻ ligand bridges via S and N atoms in the μ-κS:κN¹ mode.     

Synthesis, characterization and X-ray crystal structure of [Re(L4)(CO)3]Br · 2CH3OH (L4 = N,N-bis[(2-diphenylphosphino)ethyl]methoxyethylamine): A model compound for novel cationic Tc(I)-tricarbonyl radiotracers useful for heart imaging

This report describes synthesis and evaluation of cationic complexes, [^99mTc(CO)₃(L)]⁺ (L = N-methoxyethyl-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L1), N-[(15-crown-5)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L2) and N-[(18-crown-6)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L3)) as potential radiotracers for heart imaging. Preliminary results from biodistribution studies in female adult BALB-c mice indicated that the cationic ^99mTc(I)-tricarbonyl complex, [^99mTc(CO)₃(L2)]⁺, has a significant localization in the heart at 60 min post-injection. To understand the coordination chemistry of these bisphosphine ligands with the ^99mTc(I)-tricarbonyl core, we prepared [Re(CO)₃(L4)]Br (L4: N,N-bis[(2-diphenylphosphino)ethyl]methoxyethylamine) as a model compound. [Re(CO)₃(L4)]Br has been characterized by elemental analysis, IR, ESI-MS, NMR (¹H, ¹³C, ¹H-¹H COSY, and ¹H-¹³C HMQC) methods, and X-ray crystallography. In solid state, [Re(CO)₃(L4)]⁺ has a distorted octahedron coordination geometry with PNP occupying one facial plane. The chelator backbone adopts a “chair” conformation with phosphine-P atoms at equatorial positions and the amine-N at the apical site. In solution, [Re(CO)₃(L4)]⁺ is able to maintain its cationic nature with no dissociation of carbonyl ligands or any of the three PNP donors.     

Reactions of the Tridentate and Tetradentate Amine Ligands di-(2-picolyl)(N-ethyl)amine and 2,5-Bis-(2-pyridylmethyl)-2,5 diazohexane with Technetium Nitrosyl Complexes

The reaction of the Tc(II) nitrosyl complex (Bu₄N)[Tc(NO)Cl₄] with di-(2-picolyl)(NEt)amine in methanol yields the neutral complex [Tc(NO)Cl(py-N(Et)-py)]. The reaction of the Tc(I) nitrosyl complex [Tc(NO)Cl₂(HOMe)(PPh₃)₂] with this tridentate ligand yields cationic [Tc(NO)Cl(py-N(Et)-py)(PPh₃)]Cl. These two complexes have been structurally characterized. The reaction of [Tc(NO)Cl₂(HOMe)(PPh₃)₂] with the tetradentate ligand 1,4-bis-(2-pyridylmethyl)-1,4-diazobutane yields a mixture of products including cationic [Tc(NO)Cl(py-NH-NH-py)]Cl and cationic [Tc(NO)Cl(PPh₃)(py-NH-NH∼py)]Cl, with a pyridyl terminus left dangling.     

Synthesis, structure and properties of bis[N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamidecopper(II)] perchlorate

In this account we report the synthesis, structure and characterization of a dimeric Cu(II) complex, [Cu₂(PaPy₃H)₂](ClO₄)₄ (bis[N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamidecopper(II)]). In this complex, the coordination of the designed ligand PaPy₃H to the Cu(II) centers is completed by the bis(picolyl)methyl amine portion of one PaPy₃H unit and the oxygen and pyridine nitrogen atoms of the pyridine-2-carboxamide moiety of a second PaPy₃H ligand. The resulting dimeric complex demonstrates a new mode of coordination of the ligand PaPy₃H.     

Synthesis, characterization, X-ray molecular structure and catalase-like activity of a non-heme iron complex: Dichloro[N-propanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III)

In this work we present the synthesis and characterization of the complex dichloro[N-propanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III) [Fe^III(PBMPA)Cl₂]. The ligand LiPBMPA was synthesized through the Michael reaction of BMPA with methylacrylate, followed by alkaline hydrolysis. The complex [Fe^III(PBMPA)Cl₂] has been synthesized by the reaction of the ligand with FeCl₃ · H₂O and was mainly characterized by cyclic voltammetry, conductivimetry, and electronic, infrared and Mössbauer spectroscopies, and by X-ray structural analysis, which showed an iron center coordinated by one carboxylate oxygen in a monodentate way, one tertiary amine, two pyridine groups and two chloride ions. It has been proposed that in water the chloride ligands are shifted by the solvent molecules and the species [Fe^III(PBMPA)(H₂O)₂]Cl₂ is predominant. The catalase-like activity of the complex was tested in water, and it proved to be active in the hydrogen peroxide dismutation. Kinetics studies were conducted following the initial rates method. The reaction is first order in relation to both the complex and the hydrogen peroxide. Based on the presence of a lag phase that depends on the initial complex concentration, we propose that the active species that shows in situ catalase-like activity, is a binuclear complex.     

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.     

N-methyl-bis(2-bromo 6-pyridylmethyl)amine: high yield synthesis, and spectroscopic properties of the dichloroferrous complex with trigonal bipyramidal geometry

The new tridentate title ligand has been synthesized by a very convenient and high yield method. The dichloroferrous derivative was prepared in order to provide spectroscopic bases for solution studies of trigonal bipyramidal derivatives with aminomethylpyridyl containing ligands. Its crystal structure is reported, together with UV-Vis, ¹H NMR and molecular conductivity data. The absence of any MLCT in UV-Vis, significant broadening of the β and β′ resonances in ¹H NMR and neutral electrolytic behavior are reported and taken together, these parameters can now unequivocally serve as criteria to detect any tridentate coordination mode of the tripod in the chemistry of dichloroferrous complexes with tris-(2-pyridylmethyl)amine type ligands.     

Synthesis, characterization and structure of a new zinc(II) complex containing the hexadentate N,N′,N,N′-bis[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)]-ethylenediamine ligand: Generation of phenoxyl radical species

This work summarizes the results of our studies on the structural, spectral and redox properties of a mononuclear zinc(II) complex with the new H₂L ligand (H₂L = N,N′,N,N′-bis[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)]-ethylene diamine). The crystal structure of the complex [Zn^II(HL)] · ClO₄ (1) was determined by X-ray crystallographic analysis. The structure of this complex consists of a discrete mononuclear cation [Zn^II(HL)]⁺, in a strongly distorted geometry with a slight tendency toward a distorted square pyramidal geometry, as reflected by the structural index parameter τ of 0.44. The zinc(II) cation is coordinated to one oxygen and four nitrogen atoms: the pyridine nitrogen atoms (N22 and N32), tertiary amine nitrogen atoms (N1 and N4) and phenolate oxygen atom (O10). ¹H and ¹³C NMR spectral data show a rigid solution structure for 1 in agreement with X-ray structure. Potentiometric studies of complex 1 were also performed and revealed three titratable protons which are attributed to the protonation/deprotonation of two phenol groups (p[K]_a1 = 4.04 and p[K]_a3 = 11.34) and dissociation of a metal-bound water molecule (p[K]_a2 = 7.8). The phenolate groups in complex 1 are suitably protected by bulky substituents (tert-butyl) in the ortho- and para-positions, which through electrochemical oxidation generate a one-electron oxidized phenoxyl species in solution. This radical species was characterized by UV-Vis, EPR and electrochemical studies. The Zn(II)-phenoxyl radical species is of bioinorganic relevance, since its spectroscopic, redox and reactivity properties can be used to establish the role of phenoxyl radicals in biological and catalytical systems.     

Studies on galactose oxidase active site model complexes: effects of ring substituents on Cu(II)-phenoxyl radical formation

Copper(II) complexes of new N₃O- and N₂O₂-donor tripodal ligands bearing one or two o-substituted phenol moieties have been synthesized as models for the galactose oxidase active site. The complexes of 2-[N-(1-methyl-2′-imidazolylmethyl)-N-(6″-methyl-2″-pyridylmethyl)-aminomethyl)]-4-methyl-6-methylthiophenol (MeSL), [Cu(MeSL)Cl], and N-(6-methyl-2-pyridylmethyl)-N,N-bis(2′-hydroxy-3′,5′-di-tert-butylbenzyl)amine (t-buL2mepy), [Cu(t-buL2mepy)(H₂O)], have been revealed by X-ray structural analysis to have a square-pyramidal structure with one and two phenolate oxygens in the basal plane, respectively. [Cu(MeSL)Cl] was converted into a Cu(II)-o-methylthiophenoxyl radical species by electrochemical or Ce(IV) oxidation. An o-methoxyphenoxyl radical in a similar complex was considerably more stable than the 2,4-di(tert-butyl)phenoxyl radical. While t-buL2mepy reacted with Cu(ClO₄)₂ to give [Cu(t-buL2mepy)(H₂O)] without disproportionation, an N2O2-donor ligand containing an o-methoxyphenol, a 2,4-di(tert-butyl)phenol, and an N-methylimidazole moiety gave a phenoxyl radical complex exhibiting the characteristic absorption peak at 478 nm as a reddish powder by the reaction with Cu(ClO₄)₂ as a result of spontaneous disproportionation. It exhibited a quasi-reversible redox wave at E_1/2=0.34 V (vs. Ag/AgCl) in CH₃CN, which is lower than the potentials of the copper complexes of various N₃O-donor ligands, and oxidized ethanol to acetaldehyde with a low turnover number.     

Synthesis and bioactivity of novel methyl 6-deoxy-6-(N'-alkyl/aryl-N'-benzothiazol-2-yl)guanidino-alpha-D-glucopyranosides

A series of new methyl 6-deoxy-6-[N′-alkyl/aryl-N″-(benzothiazol-2-yl)]guanidino-α-d-glucopyranosides were obtained from the reaction of an alkyl/aryl amine in the presence of HgCl₂ and sugar-thiourea derivatives, followed by the removal of protecting groups. The sugar-thiourea derivatives were obtained from the treatment of 2-aminobenzothiazole derivatives with methyl 2,3,4-tri-O-acetyl-6-deoxy-6-isothiocyanato-α-d-glucopyranoside in dry pyridine. Some of the synthesized guanidines displayed anti-influenza activity.     

Synthesis and characterization of three mononuclear Fe(III) complexes containing bipodal and tripodal ligands: X-ray molecular structure of the dichloro[N-propanamide-N,N-bis-(2-pyridylmethyl)amine]iron(III) perchlorate

In this work, we present the synthesis and characterization of three mononuclear iron(III) complexes: dichloro[N-propanamide-N,N-bis-(2-pyridylmethyl)amine]iron(III) perchlorate (1), trichloro[N-methylpropanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III) (2) and trichloro[bis-(2-pyridylmethyl)amine]iron(III) (3). The complexes were characterized by cyclic voltammetry, conductivimetry, elemental analyses, and by electronic, infrared and Mössbauer spectroscopies. Complex 1 was also characterized by X-ray structural analysis, which showed an iron center coordinated to one amide, one tertiary amine, two pyridine groups and two chloride ions. While for 1 the X-ray molecular structure and the infrared spectrum confirm the coordination of the amide group by the oxygen atom, the infrared spectrum of 2 indicates that the ester group present in the ligand is not coordinated, resulting in a N₃Cl₃ donor set, similar to the one present in 3. However, in 3 there is a secondary amine while in 2 a tertiary amine exists. These structural differences result in distinguishable variations in the Lewis acidity of the iron center, which could be evaluated by the analysis of the redox potential of the complexes, as well as by Mössbauer parameters. Thus, the Lewis acidity decreases in the following order: 1 > 2 > 3. It is important to notice that 1 has the amide group coordinated to the iron center, a feature present in metalloenzymes as lipoxygenase and isopenicillin N synthase, and in a small number of mononuclear iron(III) complexes.     

Synthesis and characterization of cobalt(II)-salicylate complexes derived from N4-donor ligands: Stabilization of a hexameric water cluster in the lattice host of a cobalt(III)-salicylate complex

The syntheses and structural characterization of four cobalt(II)-salicylate complexes, [(TPA)Co^II(HSA)](ClO₄) (1), [(isoBPMEN)Co^II(HSA)](BPh₄) (2), [(TPzA)Co^II(HSA)](ClO₄) (3) and [(6Me₃TPA)Co^II(HSA)](BPh₄) (4) [TPA = tris(2-pyridylmethyl)amine, isoBPMEN = N¹,N¹-dimethyl-N²,N²-bis(2-pyridylmethyl)ethane-1,2-diamine, TPzA = tris((3,5-dimethyl-1H-pyrazole-1-yl)methyl)amine and 6Me₃TPA = tris(6-methyl-2-pyridylmethyl)amine] are described. While 2, 3 and 4 are unreactive towards dioxygen, 1 reacts slowly with molecular oxygen to a cobalt(III)-salicylate complex, [(TPA)Co^III(SA)](ClO₄) (1a). Two different crystalline forms, 1a and 1a·4H₂O were isolated depending upon the condition of oxidation and crystallization. The solid-state structures of cobalt(III)-salicylate unit in both 1a and 1a·4H₂O show a six-coordinate distorted octahedral coordination geometry at the cobalt(III) center ligated by the tetradentate ligand (TPA) where the dianionic salicylate (SA) binds in a bidentate fashion through one carboxylate and one phenolate oxygen. The hydrated form 1a·4H₂O reveals a hexameric water cluster formation in the inorganic lattice host. The complex cation and the perchlorate counterion are involved in stabilizing the (H₂O)₆ cluster in a rare ‘pentamer planar+1’ conformation. A one-dimensional water tape consisting of edge-shared water hexamers is observed. The water tape represents a subunit of ice structure.     

Synthesis and characterization of ruthenium(II) complexes based on diphenyl-2-pyridylphosphine and their applications in transfer hydrogenation of ketones

Synthesis and characterization of the ruthenium complexes [RuH(CO)Cl(κ¹-P-PPh₂Py)₂(PPh₃)] (1) and [Ru(CO)Cl₂(κ¹-P-PPh₂Py)(κ²-P-N-PPh₂Py)] (2) containing diphenyl-2-pyridylphosphine (PPh₂Py) are described. Spectral and structural data suggested linkage of the PPh₂Py in κ¹-P bonding mode in 1 and both the κ¹-P and κ²-P-N bonding modes in 2. The complex 1 reacted with N,N-donor bases viz., ethylenediamine (en), N,N′-dimethyl-(ethylenediamine) (dimen), 1,3-diaminopropane (diap), 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) and di-2-pyridylaminomethylbenzene (dpa) to afford cationic complexes of formulation [RuH(CO)(κ¹-P-PPh₂Py)₂(N-N)]⁺ (3-8) [N-N = en, 3; dimen, 4; diap, 5; bipy, 6; phen, 7; and dpa, 8], which have been isolated as their tetrafluoroborate salts. The complexes under investigation have been characterized by elemental analyses, spectroscopic and electrochemical studies. Molecular structures of 2, 3, 6, and 8 have been determined by single crystal X-ray diffraction analyses. Further, the complexes 1-8 act as effective precursor catalyst in transfer hydrogenation of acetophenone/ketones in basic 2-propanol.     

Synthesis and structures of dimeric iron(III)-Oxo and -imido complexes containing intramolecular hydrogen bonds

Hydrogen bonding networks proximal to metal centers are emerging as a viable means for controlling secondary coordination spheres. This has led to the regulation of reactivity and isolation of complexes with new structural motifs. We have used the tridenate ligand bis[(N′-tert-butylureido)-N-ethyl]-N-methylaminato ([H₂1]²⁻) that contains two hydrogen bond donors to examine the oxidation of the Fe^II-acetate complex, [Fe^IIH₂1(η²-OAc)]⁻ with dioxygen, amine N-oxides, and xylyl azide. A complex with Fe^III-O-Fe^III core results from the oxidation with dioxygen and amine N-oxides, in which the oxo ligand is involved in hydrogen bonding to the [H₂1]²⁻ ligand. A distinctly different hydrogen bonding network was found in Fe^III dimer isolated from the reaction with the xylyl azide: a rare Fe^III-N(R)-Fe^III core was observed that does not have hydrogen bonds to the bridging nitrogen atom. The intramolecular H-bond networks within these dimers appear to adjust to the presence of the bridging species and rearrange to its size and electron density.     

Syntheses, structures, and properties of Co(III) complexes derived from polypyridine ligands containing one carboxamido nitrogen donor

Four cobalt(III) complexes containing the polypyridine pentadentate ligands N,N-bis(2-pyridylmethyl)amine-N′-ethyl-2-pyridine-2-carboxamide (PaPy₃H), N,N-bis(2-pyridylmethyl)amine-N′-[1-(2-pyridylethyl)acetamide (MePcPy₃H), and N,N-bis(2-pyridylmethyl)amine-N′-(2-pyridylmethyl)acetamide (PcPy₃H), have been synthesized. All three ligands bind the Co(III) center in the same fashion with the exception of loss of conjugation between the carboxamide moiety and the pyridine ring in the latter two. The structures of [(PaPy₃)Co(OH)][(PaPy₃)Co(H₂O)](ClO₄)₃ · 3H₂O (1), [(PaPy₃)Co(NO₂)](ClO₄) · 2MeCN (2), [(MePcPy₃)Co(MeCN)](ClO₄)₂ · 0.5MeCN (3), and [(PcPy₃)Co(Cl)](ClO₄) · 2MeCN (4) have been determined. These ligands with strong-field carboxamido N donor stabilize the +3 oxidation state of the Co center as demonstrated by the facile oxidation of the corresponding Co(II) complexes (prepared in situ) by H₂O₂, [Fe(Cp)₂](BF₄), or nitric oxide (NO). The Co-N_amido bond distances of 1-4 lie in the narrow range of 1.853-1.898 Å. ¹H NMR spectra of these complexes confirm the low-spin d⁶ ground states of the metal centers.