Copper(II) complexes of potentially terdentate N2-[(R)-2-hydroxypropyl]- and N2-[(S)-2-hydroxypropyl]-(S)-phenylalaninamide for chiral recognition: Synthesis of the Ligands and Formation Constants

Copper(II) complexes of the ligands N²-[(R)-2-hydroxypropyl]- and N²-[(S)-2-hydroxypropyl]-(S)-phenylalaninamide performed chiral separation of N-dansyl-protected and unmodified amino acids in HPLC (reversed phase). With the aim of investigating which species are potentially involved in the discrimination mechanism, the two ligands were synthesized and their complexation equilibria with Cu²⁺ studied by potentiometry and spectrophotometry in aqueous solution up to pH 11.7. The formation constants of the species observed, [CuL]²⁺, [CuL₂]²⁺, [CuLH_–1]⁺, [CuL₂H_–1]⁺, [CuL₂H_–2], and [CuL₂H_–3]^?, were quite similar for both compounds and were compared to those of (S)-phenylalaninamide. Most probably, in [CuL₂H_–3]^? the ligands behave as terdentate, with the deprotonated OH group occupying an apical position.     

Chiral Diaminodiamide Copper(II) Complexes for the Enantioselective Recognition of Amino Acids: Synthesis of the Ligands and Formation Constants

(S,S)-N,N′ -Bis(aminoacyl)ethane- and (S,S)-N,N′ -bis(aminoacyl)propanediamines (AA-NN-2 and AA-NN-3, respectively, AA = alanine, phenylalanine, valine) were synthesized as the dihydrochlorides, and their complexes with Cu(II) studied potentiometrically. Since these ligands in the presence of Cu(II) are able to perform chiral resolution of D ,L -dansylamino acids in HPLC (reversed phase), in a certain pH range (6.5–8.5), it is important to know the equilibria existing between ligands and copper in aqueous solution. For AA-NN-2, four species, CuLH³⁺, CuL²⁺, Cu₂L₂H, and CuLH_?2, were detected, whereas for AA-NN-3, only CuLH³⁺, CuL²⁺, and CuLH_?2 were found. The aim is to find out which complexes may be involved in the recognition process.     

Formation and dissociation mechanism of amide complexes. V. Interconversion of dimeric and monomeric Cu2+-complexes with 1,8-diamino-3,6-diaza-2,7-octanedione: Comparison between the reactivity of terminal and internal amide groups

The protonation and deprotonation rates of the coordinated amide groups in the Cu²⁺-complexes of 1,8-diamino-3,6-diaza-2,7-octanedione (DED = L) have been studied by stopped-flow techniques. Starting at low pH from Cu²⁺ and DED the dimeric Cu₂L₂⁴⁺-complex, fully formed within the mixing time of the stopped-flow instrument, reacts in two consecutive steps to yield the final product CuLH_?2. The rate constants of the forward and backward reactions have been determined and are given in Table 1. The intermediate was identified as Cu₂L₂H_?2²⁺ by measuring its VIS.-absorption spectrum. The rate constants for the interconversion of the amide groups from the O- to the N-coordinated form in the Cu²⁺-complexes of DED, 2,10-dioxo-1, 4, 8, 11 tetraazaundecane (DANA) and triglycine (TRIGLY) are compared with each other. It is shown that these rate constants are similar, no matter whether the amide group is terminal or internal as long as the rotation is easily possible as is the case in the dimeric species Cu₂L₂⁴⁺ and Cu₂L₂H_?2²⁺. However, for CuLH_?2 the inter-conversion only takes place after opening of one of the chelate rings in a rapid protonation preequilibrium.     

Metal complexes with macrocyclic ligands. Part XXXVI. Thermodynamic and kinetic studies of bivalent and trivalent metal ions with 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid

NMR, potentiometric, and UV/VIS measurements were run to study the protonation and the In³⁺ and Cu²⁺ stability constants of 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (do3a, L). The protonation of do3a follows the typical scheme with two high and several low log K_H values. Between pH 11 and 13, the protonation mainly occurs at the N-atom, which is not substituted by an acetate side chain. The In³⁺ complex is not appreciably protonated even at low pH values (pH ? 1.7), whereas [CuL] can add up to three protons in acidic solution to give the species [CuLH], [CuLH₂], and [CuLH₃], the stability of which was determined. The formation rates of the Y³⁺, Gd³⁺, Ga³⁺, and In³⁺ complexes with do3a were measured using a pH-stat technique, whereas that of Cu²⁺, being faster, was followed on a stopped-flow spectrophotometer. In all cases, the reaction scheme implies the rapid formation of partially protonated intermediates, which rearrange themselves to the final product in the rate-determining process. ([MLH])_in, an intermediate, in which the metal ion probably is coordinated by two amino acetate groups, proved to be the reactive species for Y³⁺, Gd³⁺, and Ga³⁺. The formation of [Cu(do3a)] was interpreted by postulating that either ([CuLH])_in or ([CuLH])_in, and ([CuLH₂])_in are the reactive complexes. The rates of dissociation of the Y³⁺, Gd³⁺, and Cu²⁺ complexes with do3a were studied spectrophotometrically. For Y³⁺ and Gd³⁺, arsenazo III was used as a scavenger, whereas for Cu²⁺ the absorption associated with d-d* transition was followed. For [Y(do3a)] and [Gd(do3a)], the rate law follows the kinetic expression k_obsd ? k₀ + k₁[H⁺]. The dissociation of [Cu(do3a)] goes through the proton-independent dissociation of [CuLH₃], which is the main species at low pH.     

Chemical relevance of the copper(II)—l-carnosine system in aqueous solution: A thermodynamic and spectrophotometric study

The copper(II)—l-carnosine (L⁻) system has been re-investigated in aqueous solution, at I = 0.1 mol dm⁻¹, different temperatures (5⩽t⩽45°C) and with metal to ligand ratios ranging from 3:1 to 1:3. Both potentiometry and visible spectrophotometry were employed. From an overall consideration of all experiments, [CuLH]²⁺, [CuL]⁺, [CuLH₋₁]°, [Cu₂L₂H₋₂]° and [Cu₂LH₋₁]²⁺ were recognized as the species which provide the best interpretation of experimental data. The complex formation constants, determined at different temperatures, allowed us to obtain reliable values of ΔH° and good estimates of ΔC°_p. From visible spectrophotometric measurements, carried out at different pH and metal to ligand ratios, it was possible to calculate the electronic spectrum of each complex formed in solution. A structure is also proposed for each species, on the basis of thermodynamic and spectral results.     

Binding H2, N2, H-, and BH3 to transition-metal sulfur sites: synthesis and properties of [RuL(PR3)(N2Me2S2)] Complexes (L=eta2-H2, H-, BH3; R=Cy, iPr)

The reactions of [Ru(N₂)(PR₃)(‘N₂Me₂S₂’)] [‘N₂Me₂S₂’=1,2‐ethanediamine‐N,N′‐dimethyl‐N,N′‐bis(2‐benzenethiolate)(2?)] [ 1 a (R=iPr), 1 b (R=Cy)] and [μ‐N₂{Ru(N₂)(PiPr₃)(‘N₂Me₂S₂’)}₂] ( 1 c ) with H₂, NaBH₄, and NBu₄BH₄, intended to reduce the N₂ ligands, led to substitution of N₂ and formation of the new complexes [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PR₃)(‘N₂Me₂S₂’)] [ 3 a (R=iPr), 3 b (R=Cy)], and [Ru(H)(PR₃)(‘N₂Me₂S₂’)]^? [ 4 a (R=iPr), 4 b (R=Cy)]. The BH₃ and hydride complexes 3 a , 3 b , 4 a , and 4 b were obtained subsequently by rational synthesis from 1 a or 1 b and BH₃?THF or LiBEt₃H. The primary step in all reactions probably is the dissociation of N₂ from the N₂ complexes to give coordinatively unsaturated [Ru(PR₃)(‘N₂Me₂S₂’)] fragments that add H₂, BH₄^?, BH₃, or H^?. All complexes were completely characterized by elemental analysis and common spectroscopic methods. The molecular structures of [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PiPr₃)(‘N₂Me₂S₂’)] ( 3 a ), [Li(THF)₂][Ru(H)(PiPr₃)(‘N₂Me₂S₂’)] ([Li(THF)₂]‐ 4 a ), and NBu₄[Ru(H)(PCy₃)(‘N₂Me₂S₂’)] (NBu₄‐ 4 b ) were determined by X‐ray crystal structure analysis. Measurements of the NMR relaxation time T₁ corroborated the η² bonding mode of the H₂ ligands in 2 a (T₁=35 ms) and 2 b (T₁=21 ms). The H,D coupling constants of the analogous HD complexes HD‐ 2 a (¹J(H,D)=26.0 Hz) and HD‐ 2 b (¹J(H,D)=25.9 Hz) enabled calculation of the H? D distances, which agreed with the values found by X‐ray crystal structure analysis ( 2 a : 92 pm (X‐ray) versus 98 pm (calculated), 2 b : 99 versus 98 pm). The BH₃ entities in 3 a and 3 b bind to one thiolate donor of the [Ru(PR₃)(‘N₂Me₂S₂’)] fragment and through a B‐H‐Ru bond to the Ru center. The hydride complex anions 4 a and 4 b are extremely Brønsted basic and are instantanously protonated to give the η²‐H₂ complexes 2 a and 2 b .     

Non-Heme FeII Diastereomeric Complexes Bearing a Hexadentate Ligand: Unexpected Consequences for the Spin State and Catalytic Oxidation Properties

The reactivity and selectivity of non-heme Fe^II complexes as oxidation catalysts can be substantially modified by alteration of the ligand backbone or introduction of various substituents. In comparison with the hexadentate ligand N,N,N′,N′-tetrakis(pyridin-2-ylmethyl)ethane-1,2-diamine (TPEN), N,N′-bis[1-(pyridin-2-yl)ethyl]-N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (_2MeL₆²) has a methyl group on two of the four picolyl positions. Fe^II complexation by _2MeL₆² yields two diastereomeric complexes with very similar structures, which only differ in the axial/equatorial positions occupied by the methylated pyridyl groups. In solution, these two isomers exhibit different magnetic behaviors. Whereas one isomer exhibits temperature-dependent spin-state conversion between the S=0 and S=2 states, the other is more reluctant towards this spin-state equilibrium and is essentially diamagnetic at room temperature. Their catalytic properties for the oxidation of anisole by H₂O₂ are very different and correlate with their magnetic properties, which reflect their lability/inertness. These different properties most likely depend on the different steric constraints of the methylated pyridyl groups in the two complexes.     

Potentiometric and spectroscopic study of copper(II) complexes with glycyl-glycyl-l-histidyl-l-alanine in aqueous solution

The complex formation between copper(II) and the tetrapeptide glycyl-glycyl-l-histidyl-l-alanine ([H₃L]²⁺) has been studied in aqueous solution at t = 25°C and I = 0.1 mol dm⁻³ by potentiometric, visible spectrophotometric and circular dichroism measurements. All the experimental techniques show that the complex [CuLH₋₂]⁻ is predominant over a wide pH range, while the monodentate [CuLH]²⁺ is formed in the acidic region and a further deprotonated [CuLH₋₃]²⁻ species exists at pH higher than 10. The formation constants of the three above complexes are reported and a structure is proposed on the basis of spectroscopic results. The structure of [CuLH₋₂]⁻ species very probably involves four nitrogen donors in the plane, excluding coordination by the l-alanine residue, while it seems likely that [CuLH₋₃]²⁻ is formed by simple dissociation of the N-1 pyrrole hydrogen.     

An ESR study of the copper(II)–glycyl-l-serine and copper(II)–l-seryl-glycine systems by the simultaneous analysis of multi-component isotropic spectra. Formation constants and coordination modes

The formation constants and the isotropic ESR parameters (g-factors, ⁶³Cu, ⁶⁵Cu, ¹⁴N hyperfine coupling constants and relaxation parameters) of the various species were determined by the simultaneous analysis of a series of spectra, taken in a circulating system at various pH and ligand-to-metal concentration ratio. For both systems the new [CuLH]²⁺ complex was identified in acidic solutions. With the glycyl-l-serine ligand below pH 11.5 the same complexes and coordination modes are formed than with simple dipeptides. The side-chain donor group is bound only over pH 11.5 in the complex [CuLH₋₂(OH)]²⁻, where it is deprotonated and substitutes the carboxylate O in the third equatorial site. For the bis complex [CuLH₋₁(L)]⁻ an isomeric equilibrium was shown, where the difference between the isomers was based on which of the donor atoms of the ‘L’ ligand, the peptide O or the amino N, occupies the fourth equatorial position, and which one is coordinated axially. The l-seryl-glycine ligand forms the same species as simple dipeptides and glycyl-l-serine up to pH 8. The only difference is that the axial binding of the alcoholic OH group fairly stabilizes the bidentate equatorial coordination of the ‘L’ ligand through the amino N and peptide O atoms in the [CuL]⁺ complex as well as in the major isomer of the [CuLH₋₁(L)]⁻ complex. For this system we showed that (1) proton loss and the equatorial coordination of the alcoholic OH group occurs at relatively low pH (over pH 8–9), which results in the [CuL₂H₋₂]²⁻ complex with excess ligand, and also the newly identified species [Cu₂L₂H₋₄]²⁻: (2) this process is in competition with the proton loss of a coordinated water molecule. For both systems, the ESR-inactive species [Cu₂L₂H₋₃]⁻ was also shown.     

COORDINATION OF CU2± BY MONO-AMINOALKYLOXAMIDES IN AQUEOUS SOLUTION

Abstract

Complex formation equilibria of Cu^2± complexes of N-(2-aminoethyl)(oxamide, N-3-aminopropyl)oxamide, 1,8-diamino-3,6-diazaoctane-7,8-dione and 1,10-diamino-4,8-diazaoctane-9,10-dione in aqueous solution at 25°C ± 0.1°C and I = 0.1 mol dm^?3 (KNO₃) have been studied using potentiometric and spectrometric titrimetry. Mixed ligand titrations using 2,2′-bipyridyl as the second ligand have been added in order to obtain unambiguous results. The Cu^2± complexes of the monoalkyl substituted oxamides studied can be classified into three groups: (1) CuLH₁ and CuLH₂ complexes; these complexes have a single deprotonated oxamide group in a trans configuration; (2) a CuLH_?3 complex; this complex has a doubly deprotonated oxamide group in a cis configuration; (3) Cu₂LH_?2, Cu₃L₂H_?4 and Cu₃L₂H_?5 complexs; these polynuclear complexes have the doubly deprotonated oxamide group in a trans configuration. Deprotonation of the primary amide group in the Cu₂LH_?2 complex of these ligands occurs before pH = 5. This unprecedented deprotonation of a primary amide group under these conditions is due to the cooperation of both strong and optimally positioned coordinating groups. The concept of amide oxygen anchoring is introduced.     

A Comparative Study of Stabilities and Coordination Modes of β-Alaninephosphonic Acid in Copper(II) Heteroligand Complexes with Ethylenediamine, Diethylenetriamine or N,N,N′,N′,N″-Pentamethyldiethylene Triamine in Aqueous Solution

Solution equilibrium studies on Cu²⁺?CL₁?CL₂ ternary systems have been performed by pH-potentiometry, UV?CVis spectrophotometry and EPR methods {where L₁ corresponds to a polyamine such as ethylenediamine (en), diethylenetriamine (dien), N,N,N??,N??,N??-pentamethyldiethylenetriamine (Me₅dien)} and L₂ denotes 2-aminoethylphosphonic acid (??-alaninephosphonic acid)}. The results suggest the formation of heteroligand complexes with [Cu(L₁)(??-Ala(P))] stoichiometry in all of the studied systems. Additionally, in the system with en, [Cu(en)(??-Ala(P))H_?1]^? is formed in basic solutions. Our spectroscopic results indicate tetragonal geometry for the [Cu(en)(??-Ala(P))] species, a geometry slightly deviated from square pyramidal for the [Cu(dien)(??-Ala(P))] complex, and somewhat stronger geometry distortion was present for the [Cu(Me₅dien)(??-Ala(P))] complex. The coordination modes in these heteroligand complexes are discussed.     

Synthesis,crystal structure,thermal decomposition,and XPS studies of homo and heterotrinuclear Cu(II)–Cu(II)–Cu(II) and Cu(II)–Ni(II)–Cu(II) complexes obtained from salpn type ligands

In this study, a mononuclear CuL complex was prepared by the use of bis-N,N′-(salicylidene)-1, 3-propanediamine (LH₂) and Cu²⁺ ion. NiCl₂ and NiBr₂ salt were treated with this complex in dioxanewater medium and two new complexes [(CuL)₂NiCl₂(H₂O)₂] and [(CuL)₂NiBr₂(H₂O)₂)] with Cu(II)–Ni(II)–Cu(II) nucleus structure were obtained. In addition to this bis-N,N′-(2-hydroxybenzyl)-1,3-diaminopropane (L^HH₂) was prepared by the reduction of LH₂ with NaBH₄ in MeOH medium. The treatment of this reduced complex with Cu²⁺ ion resulted a complex [(CuL^H)₂CuCl₂] with a structure of Cu(II)–Cu(II)–Cu(II). The complexes prepared were characterized by the use of elemental analysis, IR spectroscopy, thermogravimetric and X-ray diffraction methods. The crystal structures of [(CuL)₂NiBr₂(H₂O)₂] (СIF file CCDC 1448402) and [(CuL^H)₂CuCl₂] (СIF file CCDC 1448401) complexes were elucidated. It was found that halogen ions are coordinated to terminal Cu²⁺ ions which are in a distorted square pyramid coordination sphere. It was determined that the central Cu(II), which joins terminal square pyramidal Cu(II), was coordinated only by the phenolic oxygens of the ligand while the central Ni(II) was coordinated by two phenolic oxygens of the organic ligand and two water molecules. These complexes were investigated by XPS and it was found that the terminal and central Cu²⁺ ions were different in Cu(II)–Cu(II)–Cu(II) complex. Also, the thermal degradation of the CuLH complex unit was observed to exothermic in contrast to the expectations.

     

Syntheses,crystal structures and solvatochromic properties of dinuclear oxalato-bridged copper(II) complexes

Three dinuclear copper(II) complexes, [Cu₂(L¹)₂(μ-ox)](ClO₄)₂?2(CH₃CN), [Cu₂(L²)₂(μ-ox)](ClO₄)₂?H₂O, and [Cu₂(L³)₂(μ-ox)](ClO₄)₂ where ox = oxalato; L = N,N-dimethyl,N′-benzylethane-1,2-diamine, L¹, N,N-diethyl,N′-benzylethane-1,2-diamine, L², N,N-diisoprophyl,N′-benzylethane-1,2-diamine, L³, were prepared and characterized by elemental analyses, spectral (IR, UV–Vis) data and molar conductance measurements. The crystal structures of [Cu₂(L¹)₂(μ-ox)](ClO₄)₂?2(CH₃CN) and [Cu₂(L³)₂(μ-ox)](ClO₄)₂ have been determined by single-crystal X-ray analysis. Solvatochromic behaviors were investigated in various solvents, showing positive solvatochromism. The effect of steric hindrance around the copper ion imposed by N-alkyl groups of the diamine chelates on the solvatochromism property of the complexes is discussed. Solvatochromism was also studied with different solvent parameter models using stepwise multiple linear regression method.     

Ten‐vertex rhodadithiaborane ­chemistry: [8‐{I(CH2)5}‐3‐(η5‐C5Me5)‐arachno‐3,7,8‐RhS2B8H9]

Neutral 8‐(5‐iodo‐n‐pentyl)‐3‐(η⁵‐penta­methyl­cyclo­pentadi­enyl)‐arachno‐3‐rhoda‐7,8‐di­thia­undecaborane, [Rh(C₅H₁₉B₈­IS₂)­(C₁₀H₁₅)], obtained from the [arachno‐7,8‐S₂B₉H₁₀]^? anion by treatment with I(CH₂)₅I followed by [Rh(C₅Me₅)Cl₂]₂ and N,N,N′,N′‐tetra­methyl‐1,8‐di­amino­naphthalene, has the 11‐vertex cluster geometry of [arachno‐7,8‐S₂B₉H₁₀]^?, but with an {Rh(C₅Me₅)} unit in the 3‐position instead of a {BH} unit, and with a –(CH₂)₅I chain attached exo to an S atom.     

Designing a heterotrinuclear CuII—NiII—CuII complex from a mononuclear CuII Schiff base precursor with dicyanamide as a coligand: synthesis,crystal structure,thermal and photoluminescence properties

Schiff bases are considered `versatile ligands' in coordination chemistry. The design of polynuclear complexes has become of interest due to their facile preparations and varied synthetic, structural and magnetic properties. The reaction of the `ligand complex' [CuL] {H₂L is 2,2′‐[propane‐1,3‐diylbis(nitrilomethanylylidene)]diphenol} with Ni(OAc)₂·4H₂O (OAc is acetate) in the presence of dicyanamide (dca) leads to the formation of bis(dicyanamido‐1κN¹)bis(dimethyl sulfoxide)‐2κO,3κO‐bis{μ‐2,2′‐[propane‐1,3‐diylbis(nitrilomethanylylidene)]diphenolato}‐1:2κ⁶O,O′:O,N,N′,O′;1:3κ⁶O,O′:O,N,N′,O′‐dicopper(II)nickel(II), [Cu₂Ni(C₁₇H₁₆N₂O₂)₂(C₂N₃)₂(C₂H₆OS)₂]. The complex shows strong absorption bands in the frequency region 2155–2269 cm⁻¹, which clearly proves the presence of terminal bonding dca groups. A single‐crystal X‐ray study revealed that two [CuL] units coordinate to an Ni^II atom through the phenolate O atoms, with double phenolate bridges between Cu^II and Ni^II atoms. Two terminal dca groups complete the distorted octahedral geometry around the central Ni^II atom. According to differential thermal analysis–thermogravimetric analysis (DTA–TGA), the title complex is stable up to 423 K and thermal decomposition starts with the release of two coordinated dimethyl sulfoxide molecules. Free H₂L exhibits photoluminescence properties originating from intraligand (π–π*) transitions and fluorescence quenching is observed on complexation of H₂L with Cu^II.     

Molecular Ruby under Pressure

The intensely luminescent chromium(III) complexes [Cr(ddpd)₂]³⁺ and [Cr(H₂tpda)₂]³⁺ show surprising pressure‐induced red shifts of up to ?15 cm^?1 kbar^?1 for their sharp spin‐flip emission bands (ddpd=N,N′‐dimethyl‐N,N′‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine; H₂tpda=2,6‐bis(2‐pyridylamino)pyridine). These shifts surpass that of the established standard, ruby Al₂O₃:Cr³⁺, by a factor of 20. Beyond the common application in the crystalline state, the very high quantum yield of [Cr(ddpd)₂]³⁺ enables optical pressure sensing in aqueous and methanolic solution. These unique features of the molecular rubies [Cr(ddpd)₂]³⁺ and [Cr(H₂tpda)₂]³⁺ pave the way for highly sensitive optical pressure determination and unprecedented molecule‐based pressure sensing with a single type of emitter.     

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.     

Formation and Dissociation of Tetrahedral Nickel(II) and Cobalt(II) Complexes of ‘Dithioimidodiphosphato’ Ligands,Measurement of Kinetically Determined Stability Constants,and Kinetic Investigations of the Rapid Formation of Tetrahedral Bis(ligand)copper(II) Complexes and Their Rates of Reduction to Trinuclear Copper(I) Species

The stopped‐flow technique was used to measure the rates of formation and dissociation of tetrahedral [ML₂] complexes (M²⁺=Ni²⁺ or Co²⁺) of four bidentate S₂‐donor ‘dithioimidodiphosphato’ ligands L^? (HL=[R¹R²P(?S)]NH[P(?S)R³R⁴], R¹ to R⁴=alkyl) at 25.0° in MeOH/H₂O 95 : 5 (v/v) solution and in the presence of either MOPS (=3‐(morpholin‐4‐yl)propane‐1‐sulfonic acid) or 2,6‐lutidine (=2,6‐dimethylpyridine) buffers. The kinetically determined equilibrium formation constants for [ML]⁺ ions (M=Ni or Co) are 10^?5 K=0.50±0.01 or 1.64±0.07 l mol^?1 for L=L³ (R¹=R²=Me(CH₂)₂CH(Me), R³=R⁴=Me₂CH), 1.27±0.02 or 7.93±0.09 l mol^?1 for L=L⁷ (R¹ to R⁴=Me₂CHCH₂), 0.88±0.04 or 3.84±0.13 l mol^?1 for L=L⁸ (R¹ to R⁴=Me₂CH), and in case of Ni²⁺ 1.88±0.04 l mol^?1 for L=L⁶ (R¹=R³=Bu, R²=R⁴=^tBu) (see Table 3; for L³ and L⁶–L⁸, see Table 1). Whereas the tetrahedral Ni²⁺ complexes dissociate more slowly than the analogous Co²⁺ species, in all cases, the Co²⁺ complexes are more stable than those of Ni²⁺ due to their larger formation rate constants (Table 3). Reactions of Cu²⁺ with eight ligands HL (R¹ to R⁴=alkyl, alkoxy, aryl, and aryloxy) show that formation of intensely colored tetrahedral [Cu^IIL₂] species is too fast be measured with the available stopped‐flow apparatus (t_1/2<2 ms), but the subsequent rates of reduction of [Cu^IIL₂] to give trinuclear products [Cu^I₃L₃] are measurable. An X‐ray analysis establishes the structure of one of the [Cu₃L₃] complexes, where R¹=R²=Me₂CHO and R³=R⁴=2‐(tert‐butyl)phenyl (L=L⁵), and a multiwavelength stopped‐flow kinetic experiment establishes the spectrum of a tetrahedral [Cu^IIL₂] species prior to the reduction reactions. The redox reactions proceed at 25.0° with first‐order rate constants in the range 0.285 s^?1 (R¹ to R⁴=PhO; L=L¹¹) to 2.58?10^?4 s^?1 (R¹ to R⁴=Me₂CHCH₂; L=L⁷) (Table 4).     

Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H₄]L, and N,N′-dimethylated tetrahydrosalen type complexes, Cu[H₂Me₂]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H₂salphen=N,N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H₄]L and Cu[H₂Me₂]L are air-stable at ambient temperature, except for the complex Cu(^tBu, Me)[H₄]salphen [H₂(^tBu, Me)[H₄]salphen=N,N′-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu(^tBu, Me)[H₄]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH₂–NH–→–C=N–) to form Cu(^tBu, Me)[H₂]salphen and finally, in the presence of base, Cu(^tBu, Me)salphen. X-ray structure analysis of Cu(^tBu, Me)[H₂Me₂]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN₂O₂ coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H₄]L and Cu[H₂Me₂]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin, ¹⁴N donor nuclei and to a distant interaction with two equivalent protons [A^Cu(iso)≈253 MHz, A^N(iso)≈43 MHz, A^N(iso)≈20 MHz]. These protons are attached to the carbon atoms adjacent to the ¹⁴N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [A^N(iso)≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the C=N bond hydrogenation, the coordination polyhedra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu(^tBu, Me)[H₄]salphen and Cu(^tBu, Me)[H₂Me₂]salphen complexes [A^Cu(iso)≈215 MHz].     

Synthesis,electrochemical and e.p.r. spectral studies of binuclear copper(II) complexes with new pentadentate L-proline-based binucleating ligands

The synthesis, reduction, optical and e.p.r. spectral properties of a series of new binuclear copper(II) complexes, containing bridging moieties (OH^–, MeCO₂ ^–, NO₂ ^–, and N₃ ^–), with new proline-based binuclear pentadentate Mannich base ligands is described. The ligands are: 2,6-bis[(prolin-1-yl)methyl]4-bromophenol [H₃L¹], 2,6-bis[(prolin-1-yl)methyl]4-t-butylphenol [H₃L²] and 2,6-bis[(prolin-1-yl)methyl]4-methoxyphenol [H₃L³]. The exogenous bridging complexes thus prepared were hydroxo: [Cu₂L¹(OH)(H₂O)₂] · H₂O (1a), [Cu₂L²(OH)(H₂O)₂] · H₂O (1b), [Cu₂L³(OH)(H₂O)₂] · H₂O (1c), acetato [Cu₂L¹(OAc)] · H₂O (2a), [Cu₂L²(OAc)] · H₂O (2b), [Cu₂L³(OAc)] · H₂O (2c), nitrito [Cu₂L¹(NO₂)(H₂O)₂] · H₂O (3a), [Cu₂L²(NO₂)(H₂O)₂] · H₂O (3b), [Cu₂L³(NO₂)(H₂O)₂] · H₂O (3c) and azido [Cu₂L¹(N₃)(H₂O)₂] · H₂O (4a), [Cu₂L²(N₃)(H₂O)₂] · H₂O (4b) and [Cu₂L³(N₃)(H₂O)₂] · H₂O (4c). The complexes were characterized by elemental analysis and by spectroscopy. They exhibit resolved copper hyperfine e.p.r. spectra at room temperature, indicating the presence of weak antiferromagnetic coupling between the copper atoms. The strength of the antiferromagnetic coupling lies in the order: NO₂ N₃ OH OAc. Cyclic voltammetry revealed the presence of two redox couples Cu^IICu^II Cu^IICu^I Cu^ICu^I. The conproportionality constant K _con for the mixed valent Cu^IICu^I species for all the complexes have been determined electrochemically.