Abstract:
Rational design of title ligands and their transition metal complexes gave the high effective catalysts for hydrogen economy and perspective “stimuli-responsive” luminescent materials. Together with the above novel cyclic aminomehtylphospine ligands have showed a row of unpredicted properties like spontaneous formation of macrocyclic molecules, unique reversible slitting of macrocycles on to the smaller cycles, rapid interconversion of the isomers catalyzed by both acids and transitional metals, bridging behavior of usually chelating ligands and unexpected high influence of handling substituents on N-atoms on to the catalytic and luminescent properties of P-complexes.
Introduction
The exponential growth of publication activity in the field of transitional metal complexes of cyclic aminomethylphosphines has been observed during last decade due to their ability to serve as hydrogenizes enzyme mimetics in the electrochemical hydrogen synthesis and hydrogen oxidation in the fuel cells. A variety of new nickel catalysts were obtained in order to achieve higher catalytic efficiency (high turnover frequency, and low overpotential). Changing nature of the substituent at the phosphorus atom as well as at the nitrogen atom significantly influences not only the on the catalytic efficiency of the complex, but also on the catalytic mechanisms [1]. Along with the above-mentioned features the novel data on unique liability of the title systems as well as unexpected formation of macrocycles via self-assembly processes demonstrate new perspectives of their usage in adaptive coordination chemistry. Herein we introduce a short review on the recent advances of the chemistry of cyclic aminomethylphosphines as ligands for transition metals focused on balance of rational design and novel approaches.
Background
The role of science is widely discussed in modern society. One of the opinions is that chemistry is not a pure science, but the part of chemical industry and is working just for supplying people with modern more effective medical drugs, materials or technologies for their production. And there are no fundamental problems to be solved by chemistry anymore. That opinion has been introduced by Nature’s known provocative writer Philip Ball “it (chemistry) is nothing more than a handy tool – or are there still major chemical questions to crack?” – asked he [2]. On the other hand Nobel prize winner Jean-Marie Lehn believes that chemistry is standing on the new edge moving from the molecular design to the adaptive coordination and supramolecular systems [3]. Indeed during our everyday work even on solving just an applied chemical questions we faced with an unknown previously and unpredicted findings which required the development of our basic chemical knowledge and arising the novel concepts and approaches. The recent development of chemistry of cyclic aminomethylphosphines as a ligand for transition metals is an example of that idea.
Chemistry of cyclic aminomethylphosphines was initiated at the end of the 1970s when two main types of that ligands were obtained, namely 1,5-diaza-3,7-diphosphacyclooctanes [4], [5] (Scheme 1) and phosphatriazaadamantaine [6]. The last compound is a well-known ligand (pta) for producing water-soluble complexes of various transition metals [7], [8], however the chemistry of the first ligands containing two phosphorus and two nitrogen atoms incorporated into the 8-membered cycle and other similar heterocyclic systems with P–CH2N intracyclic fragments are less known and is the theme of the present paper.
Synthesis of first examples of 1,5-diaza-3,7-diphosphacyclooctanes.
The 8-membered aminomethylphosphines were described nearly simultaneously by groups of Prof. B.A. Arbuzov in Russia [4] and Prof. G. Maerkl [5] in Germany (Scheme 1). The structure was unambiguously established in 1982 by X-ray data (Fig. 1) [10].
![Fig. 1: Structure of 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane [9].](/document/doi/10.1515/pac-2016-1022/asset/graphic/j_pac-2016-1022_fig_001.jpg)
Structure of 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane [9].
Since that date the Mannich-type condensations between primary phosphines, formaldehyde, and amines had been regarded as a powerful tool for the synthesis of 1,5-diaza-3,7-diphosphacyclooctanes. Their synthesis is usually performed as a convenient one-pot process. The first step is the formation of bis(hydroxymethyl)organylphosphines from the corresponding primary phosphines and formaldehyde. Bis(hydroxymethyl)organylphosphines are used without additional purification. The second step is an addition of primary amines to the reaction mixture. A very wide variation of the exocyclic substituents both on the nitrogen and phosphorus atoms provides the possibilities to tune both stereoelectronic and physical properties of the cyclic diphosphines. It should be mentioned that this condensation is highly stereoselective giving only isomers with syn-orientation of phosphorus lone electron pairs (P LP of cyclic diphosphines) [9].
According to X-ray analysis data in the solid state all studied 1,5-diaza-3,7-diphosphacyclooctanes have similar chair-chair (“crown”) conformations (Scheme 2). The axial or pseudoaxial substituents on nitrogen atoms and P LP are situated on the same side relative to the heterocyclic plane. Endocyclic nitrogen atoms possess trigonal planar or tetrahedral configuration for N-aryl and N-benzyl substited heterocycles correspondingly [1], [9].
Conformational equilibrium of “chair-chair” and “chair-boat” forms of 1,5-diaza-3,7-diphosphacyclooctanes.
Recently it has been established by dynamic NMR correlation experiments and quantum-chemical calculations that “crown” conformations are predominant among the theoretically possible conformations even in solutions (Figs. 2 and 3). It is noteworthy that the phenyl group at phosphorus does not show any special influence on the equilibrium [11].
![Fig. 2: Main conformations of the 1,5-di-R-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane (R=Me, Ph, tBu) with corresponding relative energies [kcal mol−1].](/document/doi/10.1515/pac-2016-1022/asset/graphic/j_pac-2016-1022_fig_002.jpg)
Main conformations of the 1,5-di-R-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane (R=Me, Ph, tBu) with corresponding relative energies [kcal mol−1].
Schematic representation of the energy profile for CW–CB conformational exchange of 1,5-diaza-3,7-diphosphacyclooctanes.
The number of stereospecific NOEs were the basis for assigning the predominant form to the C2 symmetric CW “crown” form. For the second minor form, the N–CH2–P protons resonate as two AB systems, which means that its structure has lower symmetry, and thus corresponds to the Cs symmetric CB form. This result somehow disagrees with a recent study [12] dealing with the ligand containing tBu-groups on phosphorus atoms: for this compound, two sets of signals of approximately equal intensity were observed in the NMR spectra and were interpreted as a result of the CW and the SD forms being present. According to the calculations, the SD form is essentially disfavored in terms of energy and even if it occurs as a reaction product from kinetic control, it should be converted into the CW or CB forms. The second set of signals in the 1H and 31P NMR spectra may be from either the form with the LP of P directed to a different sides relative to the heterocyclic plane or a macrocycle with more elementary (tBuPCH2NPh)n fragments (n>2). Both of these structures might be products of the reaction and, inherently, they cannot interconvert into the CW/CB forms; this explains the invariability of the NMR spectra. Line-shape analysis (LSA) of the exchange-broadened spectra at 303–183 K lets us estimate the energy barriers for these processes in the range of 10–12 kcal mol−1. So 1,5-diaza-3,7-diphosphacyclooctanes are well-predisposed for the metal chelation [11].
Incorporation of phosphorus atoms into the cyclic back bone lead to the few sequences (1). The mobility of phosphorus lone pairs (P LP) is strictly limited by the cycle (2). Turning of the P LPs to each other in order to form chelate complexes leads to the simultaneous movement of phosphorus atoms to each other (3). The properties of chelate complexes will depend on conformational behavior of heterocyclic ligand on metal matrix (4). Other atoms incorporated into the cycle and their exocyclic substituents are forced to be in close proximity to the central ion of chelate complexes.
These ligands with two phosphine donor centers easily form stable P,P-chelate mono- or bisligand complexes with soft transition metals of different groups (Fig. 4) [1], [9].
Transitional metal complexes of 1,5-diaza-3,7-diphosphacyclooctanes.
Even at the first paper describing X-ray data of the chelate complex of 1,5-diaza-3,7-diphosphacyclooctane was mentioned that obtained metal containing bicyclic structure has a boat-chair conformation and intracyclic N-atom of the boat fragment is situated in close proximity to the metal-center, but does not interacted with it directly [13]. So, the nature of that atom and the substituent at it should influence the reactivity of the central ion. However, only in 2006 in pioneered work of DuBois and co-workers it was demonstrated that intracyclic nitrogen atoms could serve as proton relays in catalysts of hydrogen synthesis via electrolysis based on Ni(II)-complexes making this artificial systems a mimetic of natural hydrogenizes [14].
Rational design of complexes with desired properties based on cyclic aminomethylphosphine ligands
Water-soluble complexes
Reaction of primary phosphines with formaldehyde and primary amines containing hydrophilic substituents namely 3-aminobenzoic, 2-aminoisophathalic acids and salts of 3-aminophenulsulphonic acid, led to the formation of water-soluble heterocyclic bisphosphines [15]. The derivatives of 3-aminobenzoic and 2-aminoisphathalic acids are soluble in the presence of base, the last one is soluble just in water. It should be mentioned that heterocycles demonstrate a high stability in water even on basic conditions. No-evidences of hydrolyses back to hydroxymethylphosphines or cycle interconversions were registered in the NMR spectra. Water solubility is determined by the substituents on phosphorus atoms and is changing from high to the poor in the row of R=Py, o-C6H4OH, Ph, Fc, Mes, Tipp (Fig. 5) [15], [16], [17].
Transitional metal complexes of water-soluble 1,5-diaza-3,7-diphosphacyclooctanes.
The obtained water-soluble ligands readily gives P,P-chelate complexes with various transition metals (Fig. 5) [15], [16]. The formation of traces (less than 3%) of corresponding organometallic bischelate due to the activation of C–H bonds of methyl group in mesityl substituent on P-atom was the only one side process of this systems in water [15].
The unusual water soluble complex of tetraphosphine with core 1,5-diaza-3,7-diphosphacyclooctane ring was formed via on pot synthesis of ligand in presence of iron trichloride or its derivatives (Fig. 5) [18].
It should be mentioned that water-soluble 1,5-diaza-3,7-diphospacyclooctanes and their charged bisligand platinum complexes formed host-guest adducts with calixarens containing four vialogen groups on the upper rim (Fig. 6). The reversible switching of the binding was observed during the cycles of electrochemical reduction-oxidation of vialogen fragment. So, that adduct could be regarded as a kind of molecular devices [19], [20], [21].
The reversible switching of the host-guest adducts of 1,5-diaza-3,7-diphospacyclooctanes and calixarens containing four vialogen groups on the upper rim.
Pd-complexes demonstrate a moderate activity in catalysis of polymerization of ethylene in water, however it is the first example of the catalysis in basic conditions [15].
Chiral complexes
A number of chiral optically active complexes have been obtained on the base of 1,5-diaza-3,7-diphosphacyclooctanes containing chiral R- or S-1-methylbenzyl radicals on nitrogen atoms [22], [23], [24]. Recently a novel approach to the chiral 1,5-diaza-3,7-diphosphacyclooctanes was developed basing on condensation of chiral l-menthylphosphines with formaldehyde and various amines (Scheme 3). However, the steric requirements of l-menthyl fragments prevented the formation of charged bisligand complexes and only complexes with metal to ligand 1 to 1 ratio was formed [25], [26].
CSynthesis of chiral cyclic aminomethylphosphines and their transition metal complexes.
The presence of asymmetric exocyclic substituents makes the whole molecule of the ligand asymmetric. So, at the low temperature protons of the intracyclic methylene groups are registrated as two ABX systems and the conformational behavior of cyclic ligand on the matrix of transition metal was established. It has been shown that position of conformational equilibrium is differing from that of initial heterocyclic phosphines and depends not only on the structure of the ligands but on the nature of the central ions (Fig. 7). So, for the Pt(II) and Pd(II) complexes the chair-boat (CB) conformation is predominant, at the same time for Cu(I) and Mo(0) complexes the predominant conformation is boat-boat similar to the crown conformation of free phosphine [27].
DFT calculated energies of the conformers of 1,5-diaza-3,7-diphosphacycloctane complexes with various metals.
Catalysts for electrochemical hydrogen production and oxidation of hydrogen in fuel cells
As it was mentioned earlier for P-complexes of cyclic aminomethylphosphines, nitrogen atoms incorporated into the cyclic skeleton of the ligand are fixed in close proximity to the metal center and therefore could effectively participate in proton delivery and hydrogen activation processes on the transition metals. Thanks to that the corresponding complexes of no precious transition metals namely nickel(II), cobalt(II) and iron(II) lead themselves as mimetic of hydrogenizes catalyzing electrochemical synthesis of hydrogen [28], [29], [30].
In order to increase the activity of the catalysts the steric and electronic characteristics of exocyclic substituents has been varied in wide extend (Scheme 4). The easiest way of modifying of 1,5-diaza-3,7-diphosphacyclooctanes is to change the substituents on nitrogen atoms taking various primary amines into the condensation reaction. This approach allowed to vary the donor ability of key-nitrogen atoms as well as to enhance the proton delivery by additional donor centers or immobilization of the catalysts on the polymeric conductive support [1], [9], [29], [30].
Electrochemical hydrogen synthesis.
The variation of substituents on the phosphorus atoms along with well-known phenyl and benzyl [1], [29], [30], [31] included ethyl and iso-propyl [32], [33], [34], butyl and 2-phenylethyl [35], tert-butyl [12], [36], [37], cyclohexyl [38], [39], [40], [41], [42], l-menthyl [25], ferrocenyl [24], [43] and ferrocenomethyl [17] groups. However, only few examples of aminomethylphosphines with the functionalized substituents on the phosphorus atoms were derived from ortho-phosphinophenol [16].
Introduction of functionalized substituent (o-pyridyl) to the phosphorus atom provides a novel rout for hydrogen delivery to the transition metal or its activation on the central ion. Both abilities has been demonstrated recently by the nickel(II) complexes of 1,5-diR-3,7-di(2-pyridyl)-1,3-diaza-3,7-diphosphacyclooctanes (Scheme 5) [44].
Synthesis of 1,5-diR-3,7-di(2-pyridyl)-1,3-diaza-3,7-diphosphacyclooctanes.
These complexes were the best catalysts working in pure organic solvents (acetonitrile) (Fig. 8) at that time. Turnover frequencies (TOFs) calculated for novel catalysts are much higher than those for similar complexes with phenyl substituents on phosphorus atoms. Namely, TOF of the reaction increases from 100 s−1 (Ph) up to 15200 s−1 (Py) [45].
![Fig. 8: Cyclic voltammograms of 1.5 mM solution of the catalyst (R=CHMePh) in acetonitrile in the presence of various amounts of CF3COOH [46].](/document/doi/10.1515/pac-2016-1022/asset/graphic/j_pac-2016-1022_fig_008.jpg)
Cyclic voltammograms of 1.5 mM solution of the catalyst (R=CHMePh) in acetonitrile in the presence of various amounts of CF3COOH [46].
Main exchange processes in the title biligand complexes of 1,5-diaza-3,7-diphosphacyclooctanes.
Unexpected increase of catalytic activity were demonstrated for phosphinoaminopyridines with bulky substituents on nitrogen atoms. It has been supposed that growth of activity is determined by the distortion of square-planar geometry of central Ni(II) ion to tetrahedral [45].
Indeed complex dynamic NMR investigations and quantum-chemical simulations for the corresponding complexes established that ligands are predominantly in CB-conformation with noticeably distorted geometry of central ions (Figs. 6, 7 and 9) [11].
However, recently rather active nickel catalysts of hydrogen evolution based on 1,5-diaza-3,7-diphosphacyclooctaines with long-chain lipophilic fragments attached to the substituents on nitrogen atoms have been found. The reaction was performed in the protonic ion liquids. The increase of activity had been ascribed to the decreasing of rate of conformational interconversion, which in its turn decreasing the content of “undesired” BB-conformation [46].
So, nickel complexes of 1,5-diaza-3,7-diphosphacyclooctaines could be regarded as the most perspective candidates for the catalysts for electrochemical hydrogen production and hydrogen oxidation in the fuel-cells, which had a potential to replace rather expensive platinum catalysts.
Synthesis of gold(I) complexes of 1,5-diaza-3,7-diphosphacyclooctane with exocyclic chromophoric pyridyl groups.
Design of “stimuli-responsive” luminescent complexes
Complexes of d10-metals seems to be one of the most promising base for design of novel luminescent systems due to the simultaneous participance of metal, ligand and counter ion orbitals in the formation of HOMO and LUMO and therefore in the energies of metal-to-metal, metal-to-ligand and metal-to-halogen charge transfers.
Copper complex occasionally became the first one of diazadiphosphacyclooctane which was studied by the X-ray analysis. It is a monoligand P,P-chelate complex with pyridine as a co-ligand [13]. However, no luminescence was mentioned so far.
Introduction of exocyclic chromophoric pyridyl groups into the 1,5-diaza-3,7-diphosphacyclooctane [45] backbone opens an opportunity for the design of novel “stimuli responsive” luminescent compounds.
Indeed dinuclear gold P,P-complexes are readily formed by simple ligand exchange reaction (Scheme 6). As we expected obtained complexes demonstrate luminescence due to the metal-to-ligand transitions between highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO). It has been shown that (ClAu)2L complexes crystallized from different solvents as pseudo polymorphs with different emission characteristics (Fig. 10) [47]. X-ray data of pseudo-polymorphic forms demonstrates that in all cases heterocyclic bridging ligand possesses “crown” conformation in good accordance with that previously predicted by variable-temperature 1D and 2D NMR and DFT investigations of copper complexes of chiral ligands based on l-menthylphosphine [27].
X-Ray data of monocrystals grown from acetonitrile (a) and DMSO (b) solutions of (ClAu)2L (L=1,5-diaza-3,7-diphosphacyclooctane).
So, both AuCl moieties and both substituents on exocyclic nitrogen atoms are situated on one side of heterocycle forming some kind of intramolecular cavity. The solvent molecules are binded by relatively weak H-pi interactions with aryl groups on the upper rim and by hydrogen X….H bonds with heterocyclic methylene groups on the lower site (Fig. 10).
The formation of supramolecular associates is the reason of found vapochromism (sensitivity to the presence of acetone vapor). Concentration-dependent changes in the emission of complexes in solvent mixtures demonstrates the exchange of weakly bonded solvent molecule (Acetonitrile) by acetone and DMF [48].
The gold complexes could be regarded as the base for luminescent sensors, but they are too expensive for creation of light emitters. In order to made base for copper based emitters [49] we prepared 1,5-diaza-3,7-diphospacyclooctanes with 2(2-pyridyl)ethyl substituents on phosphorus atoms.
Due to the flexible ethylene bridge pyridyl group readily interact with central metal giving chelate polynuclear complexes in the course of interaction with copper iodide.
Recently obtained bi- and hexanuclear copper species (Scheme 7) demonstrate an emission in the wide range of the wave lengths with high quantum yields. DFT calculations show that HOMO is located on metallic fragment whereas LUMO is associated with pyridyl group. Unusual white emission of hexanuclear complex appeared to be a combination of at list two transitions [50].
Synthesis of bi- and teranuclear coppper(I) complexes.
So, the application of cyclic aminomethyl phosphine is one of the growing and very promising areas.
Unpredicted findings in the chemistry of cyclic aminomethylphosphines
Unexpected effective self-assembly of cage macrocycles
During the last 15 years chemistry of cyclic aminomethylphoshines brings us a few surprising phenomenon forcing us to develop a new concepts and approaches.
At the very begining of new age it was found that reaction of primary phenylphosphine, formaldehyde and primary diamines lead to the formation of cage macrocyclic tetraphosphines (Schemes 8 and 9) instead of expected polymeric species [51], [52]. The high yield reaction was stereoselective giving cyclophane formed by four phenylene and two cis-diazadiphosphacyclooctaine fragments. All four phosphorus atoms have the same configuration with phosphorus LP situated on one side of aminomethylphosphine ring and looking just into the molecular cavity of cyclophane [52].
The supposed key intermediates of the macrocyclization process.
Covalent self-assembly of cage macrocyclic tetrakisphosphines.
In order to explain reaction efficiency the concept of covalent self-assembly was engaged [53], [54]. In contrast to known strategies of macrocyclic design – template synthesis and reactions in high dilution conditions – relatively slow totally reversible reactions at normal concentration of reagents and without any templating agents are the base for covalent self-assembly. A number of intermediates, by-product and “wrong” products (Scheme 8) formed in the reaction mixtures should be able to dissociate and re-associate in a “right” manner giving rise to the most thermodynamically favorable macrocylic structures [52], [55].
In short time it became obvious that the phenomenon of cyclophane self-assembly is more general [53]. A wide variety of primary phosphines namely: aliphatic – benzyl [52], bulky – 2,4,6-trimethyl and 2,4,6- tri-i-propylphenyl [52], [56], chiral – l-menthyl [57], functional – o-pyridyle and 2-(o-pyridyl)ethylphosphines [58] were involved into the macrocycles formation. Moreover diamines with two [51], [52], [55], [58], [59], [60], three [56], [58], [61] and even four [57] phenylene fragment in the spacer were used to give cage macrocycles with extremely large intramolecular cavities (Scheme 9).
In all cases the reactions was stereoselective and LP of all four phosphorus atoms are looking into the cavity. The free volumes of intramolecular cavities are changed in the range from dozen to few hundreds cubic angstroms.
For example, 38-membered cyclophane incorporate benzene molecule in the cavity (Fig. 11) [56]. Small molecules could reversibly enter and leave the cavity due to the found processes of conformational equilibrium of “twisting” and cylindrical forms (Fig. 12) of cage macrocycles [62].
Structure of 38-membered cage macrocycle and sace-filling model (view along the a axis) showing encapsulation of a benzene molecule in the macrocyclic cavity.
Schematically presentation of the main conformers of 38-membered cyclophanes.
The same process is responsible for the formation of binuclear platinum complexes (Scheme 10) with out of cavity disposition of metal atoms [57].
Synthesis of binuclear platinum(II) complexes of cage macrocycles.
The availability of the macrocyclic compounds is one of the key points for their use in the field of supramolecular chemistry and nanomaterials [54]. So, cage macrocyclic tetracisphosphines available via covalent self-assembly with a unique shape, distinct architecture, and set of functional groups should start to inspire the imagination of the chemists to the wide and effective search for novel molecular materials and devices.
Effective self-assembly of macrocyclic corands and lability of exocyclic -PCH2N- backbond
“Smart” ligands
All macrocycles described above were synthesized with the use of the covalent self-assembly approach on the basis of diamines with spatially divided amine groups as the structure-forming building blocks. However, in the course of the systematic studies in the field of the synthesis of heterocyclic diphosphines by the condensations of secondary diphosphines with formaldehyde and primary amines we unexpectedly met another example of the covalent self-assembly of flexible macrocyclic tetraphosphines.
Attempts to synthesize cyclic bisphosphines with one aliphatic chain between phosphorus atoms via reaction of secondary diphosphines, formaldehyde and primary amines lead to the desired compounds only in the case of rather short chains – bis(arylphosphino)methane [63], -ethane [64], [65], [66] and in one case –propane [67] (Scheme 11).
Synthesis of cyclic aminomethylposphines by condensation of secondary bisphosphines, formaldehyde and primary amines.
The interaction of bis(arylphosphino)methanes and –ethanes with the wide set of primary amines in the presence of formaldehyde led to expected six- [63] and seven-membered [64], [65], [66] heterocycles (Scheme 11) which were obtained in good yields.
The reactions of 1,3-bis(arylphosphino)propanes with formaldehyde and 5-aminoisophtalic acid proceeded similarly giving corresponding water-soluble 1-aza-3,7-diphosphacyclooctanes (Scheme 11) [67].
It has been shown that rac- and meso-isomers of 7-membered 1-aza-3,6-diphosphacycloheptanes are interconverted much faster than it could be expected for trivial inversion on phosphorus atoms. The kinetic study demonstrates that the reaction has a second order at list on the few stages [65]. The formation of 14-membered intermediate has been supposed.
Indeed interaction of 1,2-bis(hydroxymethyl(aryl)phosphino)ethane with primary alkyl amines led to the formation of 14-membered macrocyclic tetraphosphine (Scheme 12) [68]. The formation of only one racemic RRRR/SSSS-isomer instead of mixture of five possible isomers is the evidence of self-assembly process.
Stereoselective synthesis of 14-, 16-, 18- and 20-membered macrocycles.
For α,ω-bis(arylphosphino)alkanes (propane, butane and pentane) the formation of corresponding 16- [69], [70], [71], [72], [73], 18- [74] and 20-membered [74] macrocyclic tetraphosphines become a predominant direction. Alternating SSSS/RRRR or RSSR diastereomers were formed in the row of the 14-, 16-, 18- and 20-membered macrocyclic aminomethylphosphines (Scheme 12) despite the fact that the starting material, α,ω-bis(arylphosphino)propanes, was used as equimolar diastereomeric mixtures of meso and rac isomers [74].
For the first time it was demonstrated that the stereochemical result of the reaction depends on the even or odd number of the methylene groups between the two chiral phosphorus atoms in the initial α,ω-bisphosphines. Thus, the configuration at phosphorus in the compounds studied seems to obey a rule: if the two chiral phosphorus centers in the macrocycle are linked by an odd number of methylene groups, the RPSPSPRP stereoisomer is adopted. If the phosphorus atoms in the macrocycle are linked by an aliphatic chain consisting of an even number of methylene groups, the SPSPSPSP/RPRPRPRP isomer is formed (Fig. 13) [74].
Structure and main parameters of 14-, 16-, 18- and 20-membered aminomethylphosphine corands.
The lability of the P-CH2-N-fragment is a key property causing the self-assembly and stereoconversion processes of the cyclic aminomethylphosphines. It has been shown that the 14-membered macrocycles are not stable in the solution demonstrating unique splitting onto the two molecules of 7-membered 1-aza-3,6-diphosphacyclohexane [68]. After ca. 14 days the system changed significantly. 1H and 31P spectra (Fig. 14) corresponded to the mixture of RR/SS and RS-isomers of 7-membered 1-aza-3,6-diphosphacyclohexane as well as initial 14-membered corand with integral intensities as ca. 75: 22: 3. The quantity of macrocycles in the equilibrium mixture is only 2–6%. Unexpectedly, in spite of prevalence of 7-membered heterocycles (73–83%), only crystals of macrocycles were obtained in nearly quantitative yields after the slow evaporation of solvent from equilibrium mixtures. These results indicate that the splitting processes are reversible [68].
1H (a) and 31P (b) NMR spectra of 14 membered corand (2a) and formed 7-memberd cycles (2b and 2c) (correspondingly colored) in C6D6 at T=303 K after ca. 14 days. (c) Schematic representation of the macrocycle and structures of the 7-membered isomers. The signals on the spectra are colored, respectively.
The NMR monitoring of the reaction showed that two processes take place: the splitting of the macrocycle onto RR/SS- and RS-isomers of 7-membered and their intercoversion into each other.
It has became obvious that all this phenomena – covalent self-assembly, macrocycle splitting and stereoisomers interconversion is the sequence of lability of covalent bonds of P–CH2–N fragment. So, the reactivity of that kind of ligands should be differ from that of common bisphosphines.
Indeed slow addition of transitional metal derivatives (in manner characteristic for classical organic reactions) to the solution of ligands described above lead to the ligand adoptation in order to form the most stable complexes. For example, rac-isomers of 1-aza-3,7-diphosphacycloheptanes with phosphorus LP situated on different sides of the heterocycle react with platinum(II) derivatives giving rise of chelate complexes with meso-isomer of corresponding ligands (Scheme 13) [75].
Synthesis of chelate Pt(II) complexes from rac-isomer of 1-aza-3,7-diphosphacycloheptane.
The slow isomerization of rac to meso isomer can be exploited for the exclusive formation of P,P-chelate complexes by slow addition of metal complex to a mixture of diastereomers.
Moreover, copper(I) complexes based on macrocyclic tetraphosphine corands exhibit a high structural diversity.
14-Membered corands form very stable cationic P4-coordinated copper complexes with retention of phosphorus atom configurations, whereas their higher homologues 16-, 18- and 20-membered macrocycles under the same conditions give neutral binuclear complexes simalteniously transforming into the novel RRSS-isomers (Fig. 15). Synthesis and structure of copper complexes are described in the other paper of the present issue [76].
Copper(I) complexes of 14-, 16-, 18- and 20-membered macrocyclic aminomethylphosphines.
So, cyclic and macrocyclic aminomethylphosphines could be regarded as “smart” ligands changing their stereochemical structure in the course of complex formation according to the metal demands and as promising candidates for ligands in future adaptive coordination chemistry of transitional metals. The lability of the P–CH2–N-fragment should be taken into account for further design of transition metal complexes and catalytic active systems on their basis, e.g. catalysts for electrochemical hydrogen transformations.
Conclusion
Cyclic aminomethylphosphine ligands are powerful tools for design of transition metal complexes with predicted properties, however their ability for reversible transformations should be also taking into account. The future of the chemistry is a permanent balancing between fundamental findings and rational design.
Article note:
A collection of invited papers based on presentations at the 21st International Conference on Phosphorous Chemistry (ICPC-21) held in Kazan, Russia, 5–10 June 2016.
Acknowledgments
This work was supported by Russian Science Foundation grant No. 15-13-30031.
References
[1] A. A. Karasik, A. S. Balueva, E. I. Musina, O. G. Sinyashin. Mendeleev Commun.23, 237 (2013).10.1016/j.mencom.2013.09.001Search in Google Scholar
[2] Ph. Ball. Nature. 442, 500 (2006).10.1038/442500aSearch in Google Scholar
[3] J.-M. Lehn. Angew.Chem. Int. Ed.54, 3276 (2015).10.1002/anie.201409399Search in Google Scholar
[4] B. A. Arbuzov, O. A. Erastov, G. N. Nikonov. Izv. AN SSSR Ser. Khim.12, 2771 (1979).Search in Google Scholar
[5] V. Märkl, G. Jin, Ch. Schoerner. Tetrahedron Lett.21, 1409 (1980).10.1016/S0040-4039(00)92732-1Search in Google Scholar
[6] D. J. Daigle, A. B. Pepperman Jr., S. L. Vail. J. Heterocycl. Chem.11, 407 (1974).10.1002/jhet.5570110326Search in Google Scholar
[7] A. D. Phillips, L. Gonsalvi, A. Romerosa. Coord. Chem. Rev.248, 955 (2004).10.1016/j.ccr.2004.03.010Search in Google Scholar
[8] J. Bravo, S. Bolaño, L. Gonsalvi, M. Peruzzini. Coord.Chem. Rev.254, 555 (2010).10.1016/j.ccr.2009.08.006Search in Google Scholar
[9] E. Musina, A. Karasik, O. Sinyashin, G. Nikonov. Book Advances in Heterocyclic Chemistry. Scriven E.F.V., Ramsden C.A. ed., Elsevier, 117, 83 (2015).10.1016/bs.aihch.2015.10.001Search in Google Scholar
[10] B. A. Arbuzov, O. A. Erastov, G. N. Nikonov, I. A. Litvinov, D. S. Yufit, Yu. T. Struchkov. Dokl. Akad. Nauk SSSR257, 127 (1981).Search in Google Scholar
[11] Sh. Latypov, A. Strelnik, A. Balueva, Yu. Spiridonova, A. Karasik, O. Sinyashin. Eur. J. Inorg. Chem.7, 1068 (2016).10.1002/ejic.201501331Search in Google Scholar
[12] E. S. Wiedner, J. Y. Yang, W. G. Dougherty, W. S. Kassel, R. M. Bullock, M. Rakowski DuBois, D. L. DuBois. Organometallics29, 5390 (2010).10.1021/om100395rSearch in Google Scholar
[13] A. A. Karasik, G. N. Nikonov, A. S. Dokuchaev, I. A. Litvinov. Russ. J. Coord. Chem. 20, 300 (1994).Search in Google Scholar
[14] D. Wilson, R. Newell, M. McNevin, J. Muckerman, M. Rakowski DuBois, D. DuBois. J. Am. Chem. Soc. 128, 358 (2006).10.1021/ja056442ySearch in Google Scholar PubMed
[15] A. A. Karasik, R. N. Naumov, A. S. Balueva, Yu. S. Spiridonova, O. N. Golodkov, H. V. Novikova, G. P. Belov, S. A. Katsyuba, E. E. Vandyukova, P. Lönnecke, E. Hey-Hawkins, O. G. Sinyashin. Heteroat. Chem.17, 499 (2006).10.1002/hc.20272Search in Google Scholar
[16] A. A. Karasik, I. O. Georgiev, E. I. Musina, J. Heinicke, O.G. Polyhedron20, 3321 (2001).10.1016/S0277-5387(01)00950-0Search in Google Scholar
[17] A. A. Karasik, R. N. Naumov, R. Sommer, O. G. Sinyashin, E. Hey-Hawkins. Polyhedron21, 2251 (2002).10.1016/S0277-5387(02)01168-3Search in Google Scholar
[18] A. D. Burrows, D. Dodds, A. S. Kirk, J. P. Lowe, M. F. Mahon, J. E. Warren, M. K. Whittlesey. Dalton Trans.5, 570 (2007).10.1039/B614726GSearch in Google Scholar PubMed
[19] G. R. Nasybullina, V. V. Yanilkin, N. V. Nastapova, A. Yu. Ziganshina, D. E. Korshin, Yu. S. Spiridonova, A. A. Karasik, A. I. Konovalov. Russ. Chem. Bull.61, 2295 (2012).10.1007/s11172-012-0324-ySearch in Google Scholar
[20] V. V. Yanilkin, G. R. Nasybullina, N. V. Nastapova, A. Y. Ziganshina, D. E. Korshin, Y. S. Spiridonova, M. Gruner, W. Habicher, A. A. Karasik, A. I. Konovalov. Electrochimica Acta111, 466 (2013).10.1016/j.electacta.2013.07.039Search in Google Scholar
[21] A. Y. Ziganshina, G. R. Nasybullina, V. V. Yanilkin, N. V. Nastapova, D. E. Korshin, Yu. S. Spiridonova, R. R. Kashapov, M. Gruner, W. D. Habicher, A. A. Karasik, A. I. Konovalov. Russ. J. Electrochem.50, 142 (2014).10.1134/S1023193513040150Search in Google Scholar
[22] A. A. Karasik, R. N. Naumov, O. G. Sinyashin, G. P. Belov, H. V. Novikova, P. Lönnecke, E. Hey-Hawkins. Dalton Trans. 11, 2209 (2003).10.1039/B300754ESearch in Google Scholar
[23] E. V. Novikova, A. A. Karasik, E. Hey-Hawkins, G. P. Belov. Russ. J. Coord. Chem.31, 260 (2005).10.1007/s11173-005-0087-1Search in Google Scholar
[24] A. A. Karasik, Y. S. Spiridonova, D. G. Yakhvarov, O. G. Sinyashin, P. Lönnecke, R. Sommer, E. Hey-Hawkins. Mendeleev Commun.15, 89 (2005).10.1070/MC2005v015n03ABEH002119Search in Google Scholar
[25] S. Ignatieva, A. Balueva, A. Karasik, Sh. Latypov, A. Nikonova, O. Naumova, P. Lönnecke, E. Hey-Hawkins, O. Sinyashin. Inorg. Chem.49, 5407 (2010).10.1021/ic902564nSearch in Google Scholar PubMed
[26] G. R. Nasybullina, V. V. Yanilkin, A. Yu. Ziganshina, V. I. Morozov, E. D. Sultanova, D. E. Korshin, Yu. S. Spiridonova, A. S. Balueva, A. A. Karasik, A. I. Konovalov. Russ. Chem. Bull.62, 2158 (2013).10.1007/s11172-013-0315-7Search in Google Scholar
[27] Sh. Latypov, A. Strelnik, S. Ignatieva, E. Hey-Hawkins, A. Balueva, A. Karasik, O. Sinyashin. J. Phys. Chem. A. 116, 3182 (2012).10.1021/jp209281cSearch in Google Scholar PubMed
[28] Ed. by R. M. Bullock. Catalysis Without Precious Metals, Wiley-VCH, Weinheim, (2010).10.1002/9783527631582Search in Google Scholar
[29] M. Rakowski DuBois, D. DuBois. Chem. Soc. Rev. 38, 62 (2009).10.1039/B801197BSearch in Google Scholar PubMed
[30] R. Bullock, M. Helm. Acc. Chem. Res.48, 2017 (2015).10.1021/acs.accounts.5b00069Search in Google Scholar PubMed
[31] V. V. Khrizanforova, Yu. G. Budnikova, I. D. Strelnik, E. I. Musina, M. I. Valitov, M. K. Kadirov, A. A. Karasik, O. G. Sinyashin. Russ. Chem. Bull.62, 1003 (2013).10.1007/s11172-013-0131-0Search in Google Scholar
[32] M. D. Doud, K. A. Grice, A. M. Lilio, C. S. Seu, C. P. Kubiak. Organometallics31, 779 (2012).10.1021/om201168hSearch in Google Scholar
[33] S. Wiese, U. J. Kilgore, D. L. DuBois, R. M. Bullock. ACS Catal. 2, 720 (2012).10.1021/cs300019hSearch in Google Scholar
[34] P. Das, R. M. Stolley, E. F. van der Eide, M. L. Helm. Eur. J. Inorg. Chem.27, 4611 (2014).10.1002/ejic.201402250Search in Google Scholar
[35] U. J. Kilgore, M. P. Stewart, M. L. Helm, W. G. Dougherty, W. S. Kassel, M. Rakowski DuBois, D. L. DuBois, R. M. Bullock. Inorg. Chem.50, 10908 (2011).10.1021/ic201461aSearch in Google Scholar PubMed
[36] C. J. Weiss, E. S. Wiedner, J. Roberts, A. M. Appel. Chem. Commun.51, 6172 (2015).10.1039/C5CC01107HSearch in Google Scholar PubMed
[37] T. Liu, X. Wang, Ch. Hoffmann, D. L. DuBois, Bullock, R. M. Angew. Chem. Int. Ed.53, 5300 (2014).10.1002/anie.201402090Search in Google Scholar PubMed
[38] B. R. Galan, J. Schoeffel, J. C. Linehan, C. Seu, A. M. Appel, J. A. S. Roberts, M. L. Helm, U. J. Kilgore, J. Y. Yang, D. L. DuBois, C. P. Kubiak. J. Am. Chem. Soc.133, 12767 (2011).10.1021/ja204489eSearch in Google Scholar PubMed
[39] J. Y. Yang, Sh. Chen, W. G. Dougherty, W. S. Kassel, R. M. Bullock, D. L. DuBois, S. Raugei, R. Rousseau, M. Dupuis, M. Rakowski DuBois. Chem. Commun.46, 8618 (2010).10.1039/c0cc03246hSearch in Google Scholar PubMed
[40] K. Weber, T. Weyhermueller, E. Bill, O. Erdem, W. Lubitz. Inorg. Chem. 54, 6928 (2015).10.1021/acs.inorgchem.5b00911Search in Google Scholar PubMed
[41] S. Lense, A. Dutta, J. A. S. Roberts, W. Shaw. Chem. Commun.50, 792 (2014).10.1039/C3CC46829ASearch in Google Scholar
[42] A. Dutta, J. A. S. Roberts, W. Shaw. Angew. Chem. Int. Ed.53, 6487 (2014).10.1002/anie.201402304Search in Google Scholar PubMed
[43] L.-C. Song, H. Tan, F.-X. Luo, Y.-X.Wang, Zh. Ma, Zh. Niu. Organometallics33, 5246 (2014).10.1021/om500571nSearch in Google Scholar
[44] E. Musina, V. Khrizanforova, I. Strelnik, M. Valitov, Yu. Spiridonova, D. Krivolapov, I. Litvinov, M. Kadirov, P. Lönnecke, E. Hey-Hawkins, Yu. Budnikova, A. Karasik, O. Sinyashin. Chem. Eur. J.20, 3169 (2014).10.1002/chem.201304234Search in Google Scholar PubMed
[45] V. V. Khrizanforova, E. I. Musina, M. N. Khrizanforov, T. P. Gerasimova, S. A. Katsyuba, Yu. S. Spiridonova, D. R. Islamov, O. N. Kataeva, A. A. Karasik, O. G. Sinyashin, Yu. H. Budnikova. Organometallic Chem.789–790, 14 (2015).10.1016/j.jorganchem.2015.04.044Search in Google Scholar
[46] J. Hou, M. Fang, A. Cardenas, W. Shaw, M. Helm, R. Bullock, J. Roberts, M. O’Hagan. Energy Environ. Sci.7, 4013 (2014).10.1039/C4EE01899KSearch in Google Scholar
[47] I. D. Strelnik, V. V. Gurzhiy, V. V. Sizov, E. I. Musina, A. A. Karasik, S. P. Tunik, E. V. Grachova. Cryst. Eng. Comm. 18, 7629 (2016).10.1039/C6CE01272HSearch in Google Scholar
[48] N. Shamsutdinova, I. Strelnik, E. Musina, T. Gerasimova, S. Katsyuba, V. Babaev, D. Krivolapov, I. Litvinov, A. Mustafina, A. Karasik, O. Sinyashin. New J. Chem.40, 9853 (2016).10.1039/C6NJ02277DSearch in Google Scholar
[49] E. I. Musina, A. V. Shamsieva, I. D. Strelnik, T. P. Gerasimova, D. B. Krivolapov, I. E. Kolesnikov, E. V. Grachova, S. P. Tunik, C. Bannwarth, S. Grimme, S. A. Katsyuba, A. A. Karasik, O. G. Sinyashin. Dalton Transactions45, 2250 (2016).10.1039/C5DT03346BSearch in Google Scholar
[50] A. A. Karasik, E. I. Musina, I. D. Strelnik, A. S. Balueva, Yu. H. Budnikova, O. G. Sinyashin. Phosphorus Sulfur Silicon Relat. Elem.190, 729 (2015).10.1080/10426507.2014.989431Search in Google Scholar
[51] R. M. Kuznetsov, A. S. Balueva, I. A. Litvinov, A. T. Gubaidullin, G. N. Nikonov, A. A. Karasik, O. G. Sinyashin. Russ. Chem. Bull.51, 151 (2002).10.1023/A:1015086419302Search in Google Scholar
[52] A. S. Balueva, R. M. Kuznetsov, S. N. Ignat’eva, A. A. Karasik, A. T. Gubaydullin, I. A. Litvinov, O. G. Sinyashin, P. Lönnecke, E. Hey-Hawkins. Dalton Trans. 3, 442 (2004).10.1039/B311592ESearch in Google Scholar
[53] A. A. Karasik, A. S. Balueva, O. G. Sinyashin. C. R. Chimie13, 1151 (2010).10.1016/j.crci.2010.04.006Search in Google Scholar
[54] A. A. Karasik, O. G. Sinyashin. Catalysis by metal complexes. V. 37. Phosphorus Compounds: Advanced Tools in Catalysis and Material Sciences (Gonsalvi L., Peruzzini M., Eds). Dordrecht: Springer Netherlands, ch. 12, 375 (2011).10.1007/978-90-481-3817-3_12Search in Google Scholar
[55] S. N. Ignat’eva, A. S. Balueva, A. A. Karasik, D. V. Kulikov, A. V. Kozlov, Sh. K. Latypov, P. Lönnecke, E. Hey-Hawkins, O. G. Sinyashin. Russian Chem. Bull. Int. Ed.56, 1828 (2007).10.1007/s11172-007-0284-9Search in Google Scholar
[56] A. A. Karasik, D. V. Kulikov, A. S. Balueva, S. N. Ignat’eva, O. N. Kataeva, P. Lönnecke, A. V. Kozlov, Sh. K. Latypov, E. Hey-Hawkins, O. G. Sinyashin. Dalton Trans.3, 490 (2009).10.1039/B812508BSearch in Google Scholar PubMed
[57] Yu. Nikolaeva, A. Balueva, S. Ignatieva, E. Musina, A. Karasik. Izv. Akad. Nauk, Ser.Khim.5, 1319 (2016).10.1007/s11172-016-1455-3Search in Google Scholar
[58] Yu. Nikolaeva, A. Balueva, E. Musina, A. A. Karasik, O. G. Sinyashin Macroheterocycles8, 402 (2015).10.6060/mhc150976bSearch in Google Scholar
[59] A. S. Balueva, D. V. Kulikov, R. M. Kuznetsov, A. T. Gubaidullin, L. Ricard, A. A. Karasik, O. G. Sinyashin. J. Incl. Phen. Macrocyclic Comp.60, 321 (2008).10.1007/s10847-007-9381-5Search in Google Scholar
[60] A. A. Karasik, D. V. Kulikov, R. M. Kuznetsov, A. S. Balueva, S. N. Ignat’eva, O. N. Kataeva, P. Lönnecke, O. G. Sharapov, Sh. K. Latypov, E. Hey-Hawkins, O. G. Sinyashin. Macroheterocycles4, 324 (2011).10.6060/mhc2011.4.08Search in Google Scholar
[61] D. V. Kulikov, A. A. Karasik, A. S. Balueva, O. N. Kataeva, I. A. Livinov, E. Hey-Hawkins, O. G. Sinyashin. Mendeleev Commun.17, 195 (2007).10.1016/j.mencom.2007.06.001Search in Google Scholar
[62] S. K. Latypov, A. V. Kozlov, E. Hey-Hawkins, A. S. Balueva, A. A. Karasik, O. G. Sinyashin. J. Phys. Chem. A114, 2588 (2010).10.1021/jp908052fSearch in Google Scholar PubMed
[63] A. A. Karasik, S. V. Bobrov, A. I. Ahmetzianov, R. N. Naumov, G. N. Nikonov, O. G. Sinyashin. Russ. J. Coord. Chem. 24, 5305 (1998).Search in Google Scholar
[64] A. A. Karasik, A. S. Balueva, E. I. Moussina, R. N. Naumov, A. B. Dobrynin, D. B. Krivolapov, I. A. Litvinov, O. G. Sinyashin. Heteroat. Chem.19, 125 (2008).10.1002/hc.20397Search in Google Scholar
[65] E. I. Musina, A. A. Karasik, A. S. Balueva, I. D. Strelnik, T. I. Fesenko, A. B. Dobrynin, T. P. Gerasimova, S. A. Katsyuba, O. N. Kataeva, P. Lönnecke, E. Hey-Hawkins, O. G. Sinyashin. Eur. J. Inorg. Chem.11, 1857 (2012).10.1002/ejic.201101337Search in Google Scholar
[66] T. I. Fesenko, I. D. Strelnik, E. I. Musina, A. A. Karasik, O. G. Sinyashin. Russ. Chem. Bull.61, 1776 (2012).10.1007/s11172-012-0244-xSearch in Google Scholar
[67] A. A. Karasik, R. N. Naumov, Yu. S. Spiridonova, O. G. Sinyashin, P. Lönnecke, E. Hey-Hawkins. Z. Anorg. Allg. Chem.633, 205 (2007).10.1002/zaac.200600227Search in Google Scholar
[68] E. I. Musina, T. I. Fesenko, I. D. Strelnik, F. M. Polyancev, Sh. K. Latypov, P. Lönnecke, E. Hey-Hawkins, A. A. Karasik, O. G. Sinyashin. Dalton Trans.44, 13565 (2015).10.1039/C5DT01910ASearch in Google Scholar PubMed
[69] R. N. Naumov, A. A. Karasik, O. G. Sinyashin, P. Lönnecke, E. Hey-Hawkins. Dalton Trans.33, 357 (2004).10.1039/B313487CSearch in Google Scholar
[70] R. N. Naumov, A. A. Karasik, K. B. Kanunnikov, A. V. Kozlov, Sh. K. Latypov, K. V. Domasevich, E. Hey-Hawkins, O. G. Sinyashin. Mendeleev Commun.18, 80 (2008).10.1016/j.mencom.2008.03.008Search in Google Scholar
[71] R. N. Naumov, A. V. Kozlov, K. B. Kanunnikov, S. Gomez-Ruiz, E. Hey-Hawkins, Sh. K. Latypov, A. A. Karasik, O. G. Sinyashin. Tetrahedron Letters51, 1034 (2010).10.1016/j.tetlet.2009.12.056Search in Google Scholar
[72] A. A. Karasik, R. N. Naumov, K. B. Kanunnikov, D. B. Krivolapov, I. A. Litvinov, P. Lönnecke, A. S. Balueva, E. I. Musina, E. Hey-Hawkins, O. G. Sinyashin. Macroheterocycles7, 181 (2014).10.6060/mhc140507bSearch in Google Scholar
[73] E. I. Musina, A. V. Shamsieva, D. B. Krivolapov, L. I. Musin, A. A. Karasik Macroheterocycles9, 46 (2016).10.6060/mhc151195mSearch in Google Scholar
[74] R. N. Naumov, E. I. Musina, K. B. Kanunnikov, T. I. Fesenko, D. B. Krivolapov, I. A. Litvinov, P. Lönnecke, E. Hey-Hawkins, A. A. Karasik, O. G. Sinyashin. Dalton Trans.43, 12784 (2014).10.1039/C4DT01619JSearch in Google Scholar
[75] E. I. Musina, T. I. Wittmann, I. D. Strelnik, O. E. Naumova, A. A. Karasik, D. B. Krivolapov, D. R. Islamov, O. N. Kataeva, O. G. Sinyashin, P. Lönnecke, E. Hey-Hawkins Polyhedron100, 344 (2015).10.1016/j.poly.2015.08.033Search in Google Scholar
[76] E. I. Musina, T. I. Wittmann, A. B. Dobrynin, P. Lönnecke, E. Hey-Hawkins, A. A. Karasik, O. G. Sinyashin. PAC current issue (2016).Search in Google Scholar
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