Introduction to work
The relevance of the work. Complexes of porphyrins with metals in high oxidation states can coordinate bases much more efficiently than M2+ complexes and form mixed coordination compounds in which, in addition to the macrocyclic ligand, the first coordination sphere of the central metal atom contains noncyclic acidoligands and sometimes coordinated molecules. The issues of compatibility of ligands in such complexes are extremely important, since it is in the form of mixed complexes that porphyrins perform their biological functions. In addition, reactions of reversible addition (transfer) of base molecules, characterized by moderately high equilibrium constants, can be successfully used for the separation of mixtures of organic isomers, for quantitative analysis, for the purposes of ecology and medicine. Therefore, studies of the quantitative characteristics and stoichiometry of additional coordination equilibria on metalloporphyrins (MPs) and substitution of simple ligands in them are useful not only from the point of view of theoretical knowledge of the properties of metalloporphyrins as complex compounds, but also for solving the practical problem of searching for receptors and carriers of small molecules or ions. So far, there are practically no systematic studies on complexes of highly charged metal ions.
Goal of the work. This work is devoted to the study of the reactions of mixed porphyrin-containing complexes of highly charged metal cations Zr IV , Hf IV , Mo V and W V with bioactive N-bases: imidazole (Im), pyridine (Py), pyrazine (Pyz), benzimidazole (BzIm), characterization stability and optical properties of molecular complexes, substantiation of stepwise reaction mechanisms.
Scientific novelty. Methods of modified spectrophotometric titration, chemical kinetics, electronic and vibrational absorption and 1 H NMR spectroscopy were used for the first time to obtain thermodynamic characteristics and substantiate the stoichiometric mechanisms of reactions of N-bases with metal porphyrins with a mixed coordination sphere (X) -, O 2-, TPP - tetraphenylporphyrin dianion). It has been established that in the vast majority of cases, the processes of formation of metalloporphyrin-base supramolecules proceed stepwise and include several reversible and slow irreversible elementary reactions of coordination of base molecules and substitution of acid ligands. For each stage of the stepwise reactions, the stoichiometry, equilibrium or rate constants, base orders of slow reactions were determined, and the products were spectrally characterized (UV, visible spectra for intermediate products and UV, visible and IR for final products). Correlation equations have been obtained for the first time, which make it possible to predict the stability of supramolecular complexes with other bases. The equations are used in this work to discuss the detailed mechanism of substitution of OH - in Mo and W complexes by a base molecule. The properties of MR that are promising for the detection, separation, and quantitative analysis of biologically active bases, such as moderately high stability of supramolecular complexes, clear and fast optical response, low sensitivity threshold, and one-second circulation time, are described.
The practical significance of the work. Quantitative results and substantiation of the stoichiometric mechanisms of molecular complex formation reactions are essential for the coordination chemistry of macroheterocyclic ligands. The dissertation work shows that mixed porphyrin-containing complexes exhibit high sensitivity and selectivity with respect to bioactive organic bases, within a few seconds or minutes they give an optical response suitable for the practical detection of reactions with bases - VOCs, components of drugs and food, due to which are recommended for use as components of base sensors in ecology, food industry, medicine and agriculture.
Approbation of work. The results of the work were reported and discussed at:
IX International Conference on Problems of Solvation and Complex Formation in Solutions, Ples, 2004; XII Symposium on Intermolecular Interactions and Conformations of Molecules, Pushchino, 2004; XXV, XXVI and XXIX Scientific Sessions of the Russian Seminar on the Chemistry of Porphyrins and Their Analogues, Ivanovo, 2004 and 2006; VI School-Conference of young scientists of the CIS countries on the chemistry of porphyrins and related compounds, St. Petersburg, 2005; VIII scientific school - conferences on organic chemistry, Kazan, 2005; All-Russian scientific conference "Natural macrocyclic compounds and their synthetic analogues", Syktyvkar, 2007; XVI International Conference on Chemical Thermodynamics in Russia, Suzdal, 2007; XXIII International Chugaev Conference on Coordination Chemistry, Odessa, 2007; International Conference on Porphyrins and Phtalocyanines ISPP-5, 2008; 38th International Conference on Coordination Chemistry, Israel, 2008.
Ligands - ions or molecules that are directly associated with the complexing agent and are donors of electron pairs. These electron-rich systems, which have free and mobile electron pairs, can be electron donors, for example: Compounds of p-elements exhibit complexing properties and act as ligands in a complex compound. Ligands can be atoms and molecules
(protein, amino acids, nucleic acids, carbohydrates). The efficiency and strength of the donor-acceptor interaction between a ligand and a complexing agent is determined by their polarizability, i.e., the ability of a particle to transform its electron shells under external influence.
Instability constant:
Knest= 2 /
K mouth \u003d 1 / Knest
Ligand substitution reactions
One of the most important steps in metal complex catalysis, the interaction of the Y substrate with the complex, proceeds via three mechanisms:
a) Replacement of the ligand with a solvent. Usually such a stage is depicted as the dissociation of the complex
The essence of the process in most cases is the replacement of the ligand L by the solvent S, which is then easily replaced by the substrate molecule Y
b) Attachment of a new ligand along a free coordinate with the formation of an associate, followed by dissociation of the substituted ligand
c) Synchronous substitution (type S N 2) without the formation of an intermediate
Ideas about the structure of metalloenzymes and other biocomplex compounds (hemoglobin, cytochromes, cobalamins). Physical and chemical principles of oxygen transport by hemoglobin.
Structural features of metalloenzymes.
Biocomplex compounds vary considerably in stability. The role of the metal in such complexes is highly specific: replacing it even with an element with similar properties leads to a significant or complete loss of physiological activity.
1. B12: contains 4 pyrrole rings, cobalt ion and CN- groups. Promotes the transfer of the H atom to the C atom in exchange for any group, participates in the formation of deoxyribose from ribose.
2. hemoglobin: has a quaternary structure. Four polypeptide chains connected together form an almost regular ball shape, where each chain contacts two chains.
Hemoglobin is a respiratory pigment that gives blood its red color. Hemoglobin is composed of protein and iron porphyrin and carries oxygen from the respiratory organs to body tissues and carbon dioxide from them to the respiratory organs.
Cytochromes- complex proteins (hemoproteins) that carry out stepwise transfer of electrons and / or hydrogen from oxidized organic substances to molecular oxygen in living cells. This produces an energy-rich ATP compound.
Cobalamins- natural biologically active organocobalt compounds. The structural basis of cobalt is a corrin ring, consisting of 4 pyrrole nuclei, in which the nitrogen atoms are bonded to the central cobalt atom.
Physico-chemical principles of oxygen transport by hemoglobin- Atom (Fe (II)) (one of the components of hemoglobin) is able to form 6 coordination bonds. Of these, four are used to fix the Fe (II) atom itself in the heme, the fifth bond is to bind the heme to the protein subunit, and the sixth bond is used to bind the O 2 or CO 2 molecule.
Metal-ligand homeostasis and causes of its violation. Mechanism of toxic action of heavy metals and arsenic based on the theory of hard and soft acids and bases (HMBA). Thermodynamic principles of chelation therapy. Mechanism of cytotoxic action of platinum compounds.
In the body, the formation and destruction of biocomplexes from metal cations and bioligands (porphins, amino acids, proteins, polynucleotides), which include donor atoms of oxygen, nitrogen, and sulfur, continuously occur. The exchange with the environment maintains the concentrations of these substances at a constant level, providing metal ligand homeostasis. Violation of the existing balance leads to a number of pathological phenomena - metal surplus and metal deficiency states. An incomplete list of diseases associated with changes in the metal-ligand balance for only one ion, the copper cation, can be cited as an example. Deficiency of this element in the body causes Menkes syndrome, Morfan syndrome, Wilson-Konovalov disease, cirrhosis of the liver, emphysema, aorto- and arteriopathy, anemia. Excessive intake of the cation can lead to a series of diseases of various organs: rheumatism, bronchial asthma, inflammation of the kidneys and liver, myocardial infarction, etc., called hypercupremia. Professional hypercupreosis is also known - copper fever.
The circulation of heavy metals occurs partially in the form of ions or complexes with amino acids, fatty acids. However, the leading role in the transport of heavy metals belongs to proteins that form a strong bond with them.
They are fixed on cell membranes, block the thiol groups of membrane proteins- 50% of them are protein-enzymes that disrupt the stability of the protein-lipid complexes of the cell membrane and its permeability, causing the release of potassium from the cell and the penetration of sodium and water into it.
A similar effect of these poisons, which are actively fixed on red blood cells, leads to disruption of the integrity of erythrocyte membranes, inhibition of aerobic glycolysis and metabolism processes in them in general, and accumulation of hemolytically active hydrogen peroxide due to inhibition of peroxidase in particular, which leads to the development of one of the characteristic symptoms of poisoning by compounds this group - to hemolysis.
The distribution and deposition of heavy metals and arsenic occur in almost all organs. Of particular interest is the ability of these substances to accumulate in the kidneys, which is explained by the rich content of thiol groups in the renal tissue, the presence of a protein in it - metallobionin, which contains a large number of thiol groups, which contributes to the long-term deposition of poisons. The liver tissue, also rich in thiol groups and containing metallobionin, is also distinguished by a high degree of accumulation of toxic compounds of this group. The term of deposit, for example, of mercury can reach 2 months or more.
The excretion of heavy metals and arsenic occurs in different proportions through the kidneys, liver (with bile), mucous membrane of the stomach and intestines (with feces), sweat and salivary glands, lungs, which is usually accompanied by damage to the excretory apparatus of these organs and manifests itself in the corresponding clinical symptoms.
The lethal dose for soluble mercury compounds is 0.5 g, for calomel 1–2 g, for copper sulphate 10 g, for lead acetate 50 g, for white lead 20 g, for arsenic 0.1–0.2 g.
The concentration of mercury in the blood is more than 10 µg/l (1γ%), in the urine more than 100 µg/l (10γ%), the concentration of copper in the blood is more than 1600 µg/l (160γ%), arsenic is more than 250 µg/l (25γ%) %) in the urine.
Chelation therapy is the removal of toxic particles
from the body, based on their chelation
s-element complexonates.
Drugs used to remove
incorporated in the body of toxic
particles are called detoxifiers.
One of the most important steps in metal complex catalysis, the interaction of the Y substrate with the complex, proceeds via three mechanisms:
a) Replacement of the ligand with a solvent. Usually such a stage is depicted as the dissociation of the complex
The essence of the process in most cases is the replacement of the ligand L by the solvent S, which is then easily replaced by the substrate molecule Y
b) Attachment of a new ligand along a free coordinate with the formation of an associate, followed by dissociation of the substituted ligand
c) Synchronous substitution (type S N 2) without the formation of an intermediate
In the case of Pt(II) complexes, the reaction rate is very often described by the two-way equation
Where k S And k Y are the rate constants of the processes occurring in reactions (5) (with solvent) and (6) with ligand Y. For example,
The last stage of the second route is the sum of three fast elementary stages - cleavage of Cl - , addition of Y and elimination of the H 2 O molecule.
In planar square complexes of transition metals, a trans effect is observed, formulated by I.I. Chernyaev - the effect of LT on the rate of substitution of a ligand that is in a trans position to the LT ligand. For Pt(II) complexes, the trans effect increases in the series of ligands:
H2O~NH3 The presence of the kinetic trans effect and thermodynamic trans effect explains the possibility of synthesizing inert isomeric complexes Pt(NH 3) 2 Cl 2: Reactions of electrophilic substitution (S E) of hydrogen by a metal in the coordination sphere of the metal and their reverse processes SH - H 2 O, ROH, RNH 2, RSH, ArH, RCCH. Even H 2 and CH 4 molecules are involved in reactions of this type Insertion reactions L by bond M-X In the case of X=R (an organometallic complex), metal-coordinated molecules are also introduced at the M-R bond (L–CO, RNC, C 2 H 2 , C 2 H 4 , N 2 , CO 2 , O 2 , etc.). Insertion reactions are the result of an intramolecular attack by nucleophile X on a - or -coordinated molecule. Reverse reactions - - and -elimination reactions Oxidative addition and reductive elimination reactions M 2 (C 2 H 2) M 2 4+ (C 2 H 2) 4– Apparently, in these reactions there is always a preliminary coordination of the attached molecule, but this is not always possible to fix. Therefore, the presence of a free site in the coordination sphere or a site associated with the solvent, which is easily replaced by the substrate, is an important factor affecting the reactivity of metal complexes. For example, bis--allyl complexes of Ni are good precursors of catalytically active species, since due to the easy reductive elimination of bis-allyl, a complex with a solvent, the so-called. bare nickel. The role of free seats is illustrated by the following example: Reactions of nucleophilic and electrophilic addition to - and -metal complexes As intermediates of catalytic reactions, there are both classical organometallic compounds having M-C, M=C and MC bonds, as well as non-classical compounds in which the organic ligand is coordinated according to 2 , 3 , 4 , 5 and 6 -type, or is an element of electron-deficient structures - bridging CH 3 and C 6 H 6 groups, non-classical carbides (Rh 6 C (CO) 16, C (AuL) 5 +, C (AuL) 6 2+, etc.). Among the specific mechanisms for classical -organometallic compounds, we note several mechanisms. Thus, 5 mechanisms of electrophilic substitution of a metal atom at the M-C bond have been established. electrophilic substitution with nucleophilic assistance AdEAddition-elimination AdE(C) Attachment to C atom in sp 2 hybridization AdE(M) Oxidative addition to metal Nucleophilic substitution at the carbon atom in the reactions of demetallation of organometallic compounds occurs as a redox process: It is possible that an oxidizing agent may be involved in this step. CuCl 2 , p-benzoquinone, NO 3 - and other compounds can serve as such an oxidizing agent. Here are two more elementary stages characteristic of RMX: M-C bond hydrogenolysis and homolysis of the M-C bond An important rule relating to all reactions of complex and organometallic compounds and related to the principle of least motion is Tolman's 16-18 electron shell rule (Section 2). The main substitution reaction in aqueous solutions - the exchange of water molecules (22) - was studied for a large number of metal ions (Fig. 34). The exchange of water molecules in the coordination sphere of a metal ion with the bulk of water molecules present as a solvent proceeds very quickly for most metals, and therefore the rate of such a reaction was studied mainly by the relaxation method. The method consists in disturbing the equilibrium of the system, for example, by a sharp increase in temperature. Under new conditions (higher temperature), the system will no longer be in equilibrium. Then measure the rate of equilibrium. If it is possible to change the temperature of the solution within 10 -8 sec, then it is possible to measure the rate of a reaction that requires a time interval greater than 10 -8 sec. It is also possible to measure the rate of substitution of coordinated water molecules in various metal ions by ligands SO 2-4, S 2 O 3 2- , EDTA, etc. (26). The rate of such a reaction depends on the concentration of the hydrated metal ion and does not depend on the concentration of the incoming ligand, which makes it possible to use the first-order equation (27) to describe the velocity of these systems. In many cases, the rate of reaction (27) for a given metal ion does not depend on the nature of the incoming ligand (L), be it H 2 O molecules or SO 4 2- , S 2 O 3 2- , or EDTA ions. This observation, and the fact that the rate equation for this process does not include the concentration of the incoming ligand, suggests that these reactions proceed by a mechanism in which the slow step is to break the bond between the metal ion and water. The resulting compound is then likely to rapidly coordinate nearby ligands. In sec. 4 of this chapter, it was indicated that more highly charged hydrated metal ions, such as Al 3+ and Sc 3+ , exchange water molecules more slowly than M 2+ and M + ions; this gives grounds to assume that bond breaking plays an important role in the stage that determines the rate of the entire process. The conclusions obtained in these studies are not conclusive, but they give reason to believe that S N 1 processes are important in substitution reactions of hydrated metal ions. Probably the most studied complex compounds are cobalt(III) ammines. Their stability, ease of preparation, and slow reactions with them make them especially suitable for kinetic studies. Since the studies of these complexes were carried out exclusively in aqueous solutions, it is first necessary to consider the reactions of these complexes with solvent molecules - water. It was found that, in general, ammonia or amine molecules coordinated by the Co(III) ion are so slowly replaced by water molecules that substitution of ligands other than amines is usually considered. The rate of reactions of type (28) was studied and found to be of the first order with respect to the cobalt complex (X is one of many possible anions). Since in aqueous solutions the concentration of H 2 O is always approximately 55.5 M, then it is impossible to determine the effect of changing the concentration of water molecules on the reaction rate. The rate equations (29) and (30) for an aqueous solution are experimentally indistinguishable, since k is simply equal to k" = k". Therefore, it is impossible to tell from the reaction rate equation whether H 2 O will participate in the step that determines the rate of the process. The answer to the question whether this reaction proceeds according to the S N 2 mechanism with the replacement of the X ion by the H 2 O molecule or according to the S N 1 mechanism, which first involves dissociation followed by the addition of the H 2 O molecule, must be obtained using other experimental data. This problem can be solved by two types of experiments. Hydrolysis rate (substitution of one Cl ion per water molecule) trance- + about 10 3 times the rate of hydrolysis 2+ . An increase in the charge of the complex leads to strengthening of the metal-ligand bonds, and, consequently, to inhibition of the breaking of these bonds. The attraction of incoming ligands and the facilitation of the substitution reaction should also be taken into account. Since a decrease in the rate was found as the charge of the complex increased, in this case a dissociative process (S N 1) seems more likely. Another way of proof is based on the study of the hydrolysis of a series of complexes similar to trance- + . In these complexes, the ethylenediamine molecule is replaced by similar diamines, in which the hydrogen atoms at the carbon atom are replaced by CH 3 groups. Complexes containing substituted diamines react faster than the ethylenediamine complex. Replacing hydrogen atoms with CH 3 groups increases the volume of the ligand, which makes it difficult for another ligand to attack the metal atom. These steric hindrances slow down the reaction by the S N 2 mechanism. The presence of bulky ligands near the metal atom promotes the dissociative process, since the removal of one of the ligands reduces their accumulation at the metal atom. The observed increase in the rate of hydrolysis of complexes with bulky ligands is good evidence that the reaction proceeds according to the S N 1 mechanism. So, as a result of numerous studies of Co(II) acidoamine complexes, it turned out that the replacement of acid groups by water molecules is a dissociative process in nature. The cobalt atom-ligand bond lengthens to a certain critical value before water molecules begin to enter the complex. In complexes with a charge of 2+ and higher, the breaking of the cobalt-ligand bond is very difficult, and the entry of water molecules begins to play a more important role. It was found that the replacement of the acido group (X -) in the cobalt(III) complex with a group other than the H 2 O molecule (31) first proceeds through its substitution by the molecule solvent - water, followed by its replacement with a new group Y (32). Thus, in many reactions with cobalt(III) complexes, the rate of reaction (31) is equal to the rate of hydrolysis (28). Only the hydroxyl ion differs from other reagents in terms of reactivity with Co(III) amines. It reacts very quickly with amminic complexes of cobalt(III) (about 10 6 times faster than water) according to the type of reaction basic hydrolysis (33). This reaction was found to be first order with respect to the substituting ligand OH - (34). The overall second order of the reaction and the unusually fast progression of the reaction suggest that the OH ion is an exceptionally effective nucleophilic reagent with respect to Co(III) complexes and that the reaction proceeds via the S N 2 mechanism through the formation of an intermediate. However, this property of OH - can also be explained by another mechanism [equations (35), (36)]. In reaction (35), complex 2+ behaves like an acid (according to Bronsted), giving complex + , which is amido-(containing)-compound - a base corresponding to an acid 2+. Then the reaction proceeds according to the mechanism S N 1 (36) with the formation of a five-coordination intermediate compound, which then reacts with solvent molecules, which leads to the final reaction product (37). This reaction mechanism is consistent with the second-order reaction rate and corresponds to the S N 1 mechanism. Since the reaction in the rate-determining step involves a base conjugated to the initial acid complex, this mechanism is given the designation S N 1CB. It is very difficult to determine which of these mechanisms best explains experimental observations. However, there is strong evidence supporting the S N 1CB hypothesis. The best arguments in favor of this mechanism are as follows: octahedral Co(III) complexes generally react according to the dissociative S N 1 mechanism, and there are no convincing arguments why the OH ion should cause the S N 2 process. It has been established that the hydroxyl ion is a weak nucleophilic reagent in reactions with Pt(II), and therefore its unusual reactivity with Co(III) seems unreasonable. Reactions with cobalt(III) compounds in non-aqueous media provide excellent evidence for the formation of five-coordination intermediates provided for by the S N 1 CB mechanism. The final proof is the fact that in the absence of N - H bonds in the Co(III) complex, it slowly reacts with OH - ions. This, of course, gives grounds to believe that the acid-base properties of the complex are more important for the reaction rate than the nucleophilic properties of OH. This reaction of the basic hydrolysis of ammine complexes of Co (III) is an illustration of the fact that kinetic data can often be interpreted in more than one way, and In order to exclude this or that possible mechanism, it is necessary to carry out a rather subtle experiment. At present, substitution reactions of a large number of octahedral compounds have been studied. If we consider their reaction mechanisms, then the dissociative process is most often encountered. This result is not unexpected since the six ligands leave little space around the central atom for other groups to attach to it. Only a few examples are known where the occurrence of a seven-coordination intermediate has been proven or the effect of an incorporating ligand has been detected. Therefore, the S N 2 mechanism cannot be completely rejected as a possible pathway for substitution reactions in octahedral complexes. Reactions of substitution, addition or elimination of ligands, as a result of which the coordination sphere of the metal changes. In a broad sense, substitution reactions are understood as the processes of substitution of some ligands in the coordination sphere of the metal by others. Dissociative (D) mechanism. The two-stage process in the limiting case proceeds through an intermediate with a smaller coordination number: ML6<->+L; + Y --» ML5Y Associative (A) mechanism. The two-stage process is characterized by the formation of an intermediate with a large coordination number: ML6 + Y = ; = ML5Y + L Reciprocal exchange mechanism (I). Most of the exchange reactions proceed according to this mechanism. The process is single-stage and is not accompanied by the formation of an intermediate. In the transition state, the reagent and the leaving group are bound to the reaction center, enter its nearest coordination sphere, and during the reaction one group is displaced by another, the exchange of two ligands: ML6 + Y = = ML5Y+L internal mechanism. This mechanism characterizes the process of ligand substitution at the molecular level. 2. Features of the properties of lanthanides (Ln) associated with the effect of lanthanide compression. Ln 3+ compounds: oxides, hydroxides, salts. Other oxidation states. Examples of reducing properties of Sm 2+ , Eu 2+ and oxidizing properties of Ce 4+ , Pr 4+ . The monotonic decrease in atomic and ionic radii as one moves along the 4f-element series is called lanthanide contraction. I. It leads to the fact that the atomic radii of the 5d-transition elements of the fourth (hafnium) and fifth (tantalum) groups following the lanthanides turn out to be practically equal to the radii of their electronic counterparts from the fifth period: zirconium and niobium, respectively, and the chemistry of heavy 4d- and 5d-metals has a lot in common. Another consequence of f-compression is the closeness of the ionic radius of yttrium to the radii of the heavy f-elements: dysprosium, holmium, and erbium. All rare earth elements form stable oxides in the +3 oxidation state. They are refractory crystalline powders that slowly absorb carbon dioxide and water vapor. Oxides of most elements are obtained by calcining hydroxides, carbonates, nitrates, oxalates in air at a temperature of 800-1000 °C. Form oxides M2O3 and hydroxides M(OH)3 Only scandium hydroxide is amphoteric Oxides and hydroxides readily dissolve in acids Sc2O3 + 6HNO3 = 2Sc(NO3)3 + 3H2O Y(OH)3 + 3HCl = YCl3 + 3H2O Only scandium compounds hydrolyze in aqueous solution. Cl3 ⇔ Cl2 + HCl All halides are known in the +3 oxidation state. All are hardboilers. Fluorides are poorly soluble in water. Y(NO3)3 + 3NaF = YF3↓+ 3NaNO3
Reactions of coordinated ligands
Reactions of organometallic compounds
