A flame retardant-hardener for epoxy resins: Synthesis, structural, and DFT studies of the [Cu(H2NC2H4NH2)2(H2O)Cl]Cl complex

The aqua-bis(ethylenediamine)-chloro-copper(II) chloride complex, [Cu(en)2(H2O)(Cl)]Cl (1), was synthesized by direct interaction of CuCl2×2H2O with pepa (pepa is a polyethylenepolyamine containing ethylenediamine (en)). The crystalline complex 1 was characterized by IR spectra and structurally studied. Crystals of this complex consist of the [Cu(en)2(H2O)(Cl)]+ discrete cations, whose Cu2+ ion is chelated by two en molecules. The Cu(II) coordination polyhedron has the shape of an elongated square bipyramid, in which four nitrogen atoms of the –NH2 groups of en molecules form the base of this bipyramid, and the oxygen atom of the water molecule and Cl− are in its apical positions. Three-dimensional framework 1 is formed due to hydrogen bonds O–H…Cl and N–H…Cl. The Cu(II)–(H2NC2H4NH2) chelation was studied by DFT using the 6-31G* basis set. The calculated electron-stereochemical parameters are in good agreement with the ability of complex 1 to simultaneously be a flame retardant and a curing agent of epoxy resins.


Introduction
Recently, chelate complexes [1] of many d-metals with polyamines [2] due to their thermal stabilization [3][4][5][6][7][8] caused by the chelating effect are widely used as flame retardants and hardeners of epoxy resins [9][10][11][12][13][14][15].Among the great number of polydentate ligands, ethylenediamine (en) is known to be used as an efficient curing agent to produce epoxy-polymer composites.In turn, high-performance materials based on epoxy resins are one of the most demanded classes of polymers used in industry today, from simple two-component adhesives to high-tech applications [16].However, the epoxypolymer composites are combustible materials, which prevents their wider use.Nevertheless, their combustibility can be significantly reduced if polyamine complexes of transition metals are used as a modifier for the production of epoxyamine composites.In this regard, the chelate complex of incombustible copper(II) chloride with en is of particular interest [17].To better understand how chelate complexes containing a polyamine ligand bonded to a central metal atom can affect the efficiency of epoxy-amine polymerization and increase the flame retardant properties of copper(II)-containing epoxy-amine composites, reliable information is needed on the crystal structure of chelate complexes.Unfortunately, the data on the crystal structure of the chelate complex of copper(II) chloride with ethylenediamine in [17] turned out to be inaccurate.
This study reports the interaction of polyethylenepolyamine (pepa) with copper(II) chloride to form the crystalline complex [Cu(en) 2 (H 2 O)(Cl)]Cl (1) (en is one of the components of pepa), the crystal structure of which was determined more precisely, and its electron-stereochemical parameters were calculated by DFT.

Experimental part 2.1. Synthesis
The crystal complex [Cu(en) 2 (H 2 O)(Cl)]Cl (1) was obtained by direct interaction of copper(II) chloride with pepa (pepa = H 2 N[-C 2 H 4 NH-] n H containing en = NH 2 C 2 H 4 NH 2 ).Crystalline CuCl 2 ×2H 2 O (1.7 g, 0.01 mol) was placed in a porcelain mortar, an excess of pepa was added, and the resulting mixture was triturated until a homogeneous liquid suspension of dark blue color appeared.The contents were left for several days at room temperature until crystalline phase 1 appeared according to the reaction (1).

Abstract:
The aqua-bis(ethylenediamine)-chloro-copper(II) chloride complex, [Cu(en) 2 (H 2 O)(Cl)]Cl (1), was synthesized by direct interaction of CuCl 2 ×2H 2 O with pepa (pepa is a polyethylenepolyamine containing ethylenediamine (en)).The crystalline complex 1 was characterized by IR spectra and structurally studied.Crystals of this complex consist of the [Cu(en) 2 (H 2 O)(Cl)] + discrete cations, whose Cu 2+ ion is chelated by two en molecules.The Cu(II) coordination polyhedron has the shape of an elongated square bipyramid, in which four nitrogen atoms of the -NH 2 groups of en molecules form the base of this bipyramid, and the oxygen atom of the water molecule and Cl -are in its apical positions.Three-dimensional framework 1 is formed due to hydrogen bonds O-H…Cl and N-H…Cl.The Cu(II)-(H 2 NC 2 H 4 NH 2 ) chelation was studied by DFT using the 6-31G* basis set.The calculated electron-stereochemical parameters are in good agreement with the ability of complex 1 to simultaneously be a flame retardant and a curing agent of epoxy resins.

Single crystal structure determination
Diffraction data for the single crystal of complex 1 were collected on an ENRAF NONIUS CAD-4T diffractometer with Mo K α radiation.Crystal data, data collection, and structure refinement details are summarized in Table 1.The crystal structure was solved and then refined by least squares method on F 2 using the WinCSD, Version 4.19 software package [18].The final refinement was done by ShelXL software [19] with the following graphical user interface of OLEX 2 [20].H atoms of -NH 2 groups were derived from difference Fourier maps and refined with U iso (H) = 1.2 U eq (N).Water H atoms were also derived from difference Fourier maps and refined isotropically with O-H fixed distance.The other H atoms were refined in ideal positions (riding model), with C-H = 0.97 (methylene) and with U iso (H) = 1.2U eq (C).The position and thermal parameters of atoms for complex 1 is given in Table S1, Suppl.Info.Selected bond lengths and angles for complex 1 are presented in Table 2. Figures of the crystal structure 1 were drawn using DIAMOND 3.1 software.

Testing for flammability
The temperatures of ignition (T ign ) and self-ignition (T self-ign ) for en and [Cu(en) 2 (H 2 O)(Cl)]Cl were measured using the TF apparatus for temperature tests according to all-Union State Standard 12.1.044-89[21], described in detail in Data S2, Suppl.Info.There were three measurements for each type of specimens; the resulting values were averaged.The values of T ign and T self-ign for en were 45(1) °C and 380(1) °C, respectively, while the crystalline complex 1 did not ignite or self-ignite even when the temperature in a reaction chamber reached 450(1) ℃ or 600(1) ℃, respectively.

DFT study
Quantum-chemical modeling of chelating processes in the pepa -CuCl 2 •H 2 O system was carried out by the DFT method.The restricted formalism of B3LYP method with a 6-31G* orbital basis set was performed using the HyperChem program version 8.0.6.The [Cu(H 2 O) 2 Cl 2 ] and [Cu(en) 2 (H 2 O)(Cl)]Cl discrete clusters, as well as a free molecule of en, were constructed using the crystallographic data of CuCl 2 •2H 2 O [22] and 1.The charge density distribution on atoms was calculated with no geometrical optimization of the [Cu(H 2 O) 2 Cl 2 ], and [Cu(en) 2 (H 2 O)(Cl)]Cl structural fragments, as well as the en molecule.The calculations were carried out under the assumption that the clusters and the ligand molecule are in vacuum as isolated particles.

Structural features on Cu(II)-(en) binding in a κ 2 mode
The ability of the amino-group to easily coordinate with copper(II) salts by donor-acceptor type, as well as the terminal arrangement of the amino-groups in the en molecule make this bidentate ligand an applicable chelating agent to form the [Cu(H 2 NC 2 H 4 NH 2 ) 2 (H 2 O)(Cl)]Cl complex.Figure 1 shows the structural unit of complex 1, in which the [Cu(en) 2 (H 2 O) (Cl)] + discrete complex cation is formed, containing Cu 2+ ions chelated by two bidentate molecules of en.The Cu(II)-(en) bonding in a κ 2 mode, in turn, causes the deformation of the initially quadrilateral coordination core of Cu(II) generated by four N atoms of two en (see Table 2).
Nevertheless, the Cu 2+ ion in complex 1 is hexa-coordinated.In addition to four nitrogen atoms, the coordination core of Cu(II) also includes an oxygen atom of the H 2 O molecule and one Cl -ion.Thus, the coordination environment of Cu(II) is an elongated square bipyramid (Figure 2), in which the N(1), N(2), N(3), and N(4) atoms form the basis of this bipyramid, while the O and Cl(1) atoms occupy two opposite apical positions at distances of 2.659(4) and 2.831(3) Å for Cu-O and Cu-Cl(1), respectively (the Jahn-Teller effect [23]).It should be noted here that in the crystal structure of the complex under consideration, studied by the X-ray photographic method in 1967 [17], the same Cu-O distance is 1.9(2) Å due to the incorrect determination of the positional parameters for the oxygen atom.Another chlorine atom, Cl(2), is an outer-sphere anion.Thus, this outer-sphere Cl -anion is incorporated into the crystal framework 1, crosslinking the [Cu(en) 2 (H 2 O)(Cl)] + complex cations by hydrogen bonds N-H…Cl and O-H…Cl [24] ((N)H(4A)...Cl( 2) and (O)H(1F)...Cl(2) distances are 2.55(9) and 2.22(3) Å, respectively), which additionally stabilizes the crystal structure of complex 1.
The obtained structural data for complex 1 are in a good agreement with the observed shift in the vibration frequencies of N-H bonds in the IR spectra upon coordination with the metal atom.In particular, the CuCl 2 -(NH 2 C 2 H 4 NH 2 ) chelation is reflected in the vibration frequency of the N-H bonds, which corresponds to the values of Dn N-H = 20 cm -1 and Dd N-H = 34 cm -1 for stretching and bending of -NH 2 groups, respectively.

Electron preconditions for chelate complex formation
It is not less interesting to be considered the electron density distribution in the Cu(II) coordination core of complex 1. Changes in the electronic parameters of the coordinated en molecule in comparison with its uncoordinated state are the result of chelation occurring in the pepa -CuCl 2 •2H 2 O system.The DFT study showed that in the Cu(II) coordination core, the electron density of N atoms is effectively shifted to the metal atom due to the chelating effect.The charge (±d, ē) on nitrogen atoms of -NH 2 groups in an uncoordinated en molecule are -0.371, and -0.364 ē and the d value on the copper atom in CuCl 2 •2H 2 O is +0.239 ē (Figures 3a and 3b).The electron density on N atoms of -NH 2 groups of en molecule noticeably decreases, owing to the formation of the [Cu(en) 2 (H 2 O)Cl]Cl chelate complex (the d values are -0.175,-0.171, -0.183, and -0.210 ē for coordinated nitrogen atoms).On the contrary, the electron density on the central copper atom is increased (in complex 1, the d value for Cu atom is -0.176 ē) (Figure 3c).

Flame retardant-hardener properties of [Cu(en) 2 (H 2 O)Cl]Cl
It can be expected that the observed electron density redistribution in the Cu(II) coordination core will have a positive effect on the epoxy-curing by the [Cu(en) 2 (H 2 O)Cl]Cl complex.Apparently, the Cu(II)-(en) chelating is able to cause polarization of N-H bonds, which results in an increase in the electrophilic ability of H atoms in amino groups.As a result, the positive charge on the H atoms in the coordinated amino-groups increases in comparison with the uncoordinated amino-groups (in complex 1, the d values for H atoms of -NH 2 groups are ranged from +0.163 to +0.236 ē) (see Figures 3a  and 3c).All this facilitates the electrophilic addition of the H atom to the O atom of the oxirane ring as much as possible and, at the same time, enhances the ability of the N atoms to nucleophilic attack on the C atoms of epoxy groups (Figure 4).Thus, the analysis of charges on atoms of complex 1 clearly shows that en coordinated to Cu (II) is a more effective curing agent for epoxy resins than uncoordinated en.
On the other hand, the formation of a chelate complex is accompanied by the efficient binding of a combustible organic amine with a noncombustible inorganic salt.This interaction largely determines the thermal stability of the resulting chelate complex, which is able to act as a fire retardant hardener of epoxy resins in the epoxy-amine polymerization.Flammability tests have shown that en in the free state ignites at 45 ℃.However, en being in the chelate complex 1 does not ignite at all.In other words, en as a combustible substance, after binding with CuCl 2 and the formation of the [Cu(en) 2 (H 2 O)Cl]Cl complex, turns into a practically noncombustible substance.This fact can be explained by additional chemical bonds that form between en and CuCl 2 .DFT calculations of the energies of chemical bonds in a square-bipyramidal environment of Cu(II), carried out for the [Cu(en) 2 (H 2 O)Cl]Cl chelate complex, showed that the sum of the energies of four Cu-N bonds, one Cu-O bond, and one Cu-Cl bond is 338.1 kJ•mol -1 .To break these bonds, it is necessary to use up a significant part of the thermal energy coming from the ignition source.In addition, for the ignition of en, a gas mixture of en and air must form above its surface.In this mixture, the saturated vapor concentration of en must exceed the lower limit of the concentration of flame propagation.However, even at temperature above 450 ℃, the ignition of the [Cu(en) 2 (H 2 O) Cl]Cl complex was not observed.It should be noted that the previously studied chelate complex ethylenediamine-N,N'diethylenetriamine-N,N' ,N''-copper(II) hexafluorosilicate, [Cu(en)(dien)]SiF 6 [6] containing en as a curing agent for epoxy resins and CuSiF 6 as a flame retardant exhibits the properties of a flame retardant-hardener similar to complex 1. Thermal analysis data performed for the [Cu(en)(dien)]SiF 6 complex and its components (pepa and CuSiF 6 ) showed that, in contrast to pepa, for which the total weight loss is observed in the temperature range from 20 ℃ to 170 ℃, the [Cu(en)(dien)]SiF 6 complex completely decomposes at a temperature of many times higher than 544 ℃.This fact is explained by the ability of polyamine molecules to be firmly held in the complex by means of Cu(II)-N bonds.This chelation also makes it difficult for the polyamines to evaporate and ignite further.It is obvious that the main reason for the flame retardant influence of copper(II) salts on nitrogen-containing hydrocarbons is the complexing processes occurring in the polyamine-inorganic copper(II) salt system.

Conclusions
The chelate complex [Cu(en) 2 (H 2 O)Cl]Cl (1), which can be successfully used as a flame retardant-hardener of epoxy resins, was synthesized by direct interaction of CuCl 2 •2H 2 O with pepa.Crystalline complex 1 consists of discrete complex cations [Cu(en) 2 (H 2 O)Cl] + , whose Cu 2+ ions are chelated by two en ligands.The complex cation is an elongated square bipyramid, the ligands of which are two bidentate en molecules, a water molecule, and a chloride ion.Hydrogen bonds N-H…Cl and O-H…Cl additionally stabilize crystal structure 1. DFT analysis of electron structure 1 showed that en coordinated with Cu(II) is a more effective epoxy curing agent than en in the free state.The Cu(II)-en chelation is able to affect the flammability of the crystalline complex 1.As a result, the combustible en, after bonding with copper(II) chloride, turns into a practically noncombustible substance.All this allows crystals 1 to be used as an effective flame retardanthardener for epoxy resins.The method is carried out in the temperature range from 25 °C to 600 °C and is not used to test metal powders.

Apparatus
Scheme of TF apparatus for determination of the ignition and self-ignition temperatures is shown in Figure 8.The TT apparatus, which is a vertical electric furnace with two coaxially located cylinders made of quartz glass.One of the cylinders with an inner diameter of (80 ± 3) mm and a height of 240 mm is a reaction chamber; the second cylinder of the same height has an inner diameter of (101 ± 3) mm.Spiral electric heaters with a total power of at least 2 kW are wound on the cylinders, which makes it possible to create a working zone temperature of 600 °C in no more than 40 minutes.4.7.1.2.A container with a diameter of (45 ± 1) mm, a height of (18.0 ± 0.4) mm, made of steel mesh or steel sheet with a thickness of no more than 0.5 mm, is used to place a sample of the test substance (material).The container is placed in a holder ring with a diameter of (49.0 ± 0.6) mm.The holder is a metal tube made of heat-resistant steel with a ring welded at the bottom to accommodate the container.4.7.1.3.A gas burner for ignition of a sample, which is a tube with an inner diameter of (6 ± 1) mm, is made of heat-resistant steel, sealed from below and has a hole on the side surface with a diameter of (0.8 ± 0.1) mm at a distance of (5.0 ± 0.5) mm from the sealed end.4.7.1.4.A laminator made of sheet heat-resistant steel in the shape of a cone with an upper diameter of no more than 78 mm, a lower diameter of no more than 11 mm and a length of (500 ± 5) mm serves to supply a natural air flow into the reaction chamber.4.7.1.5.Thermoelectric converters with a diameter of thermoelectrodes no more than 0.5 mm.Thermoelectric converter 9 is used to measure the temperature of the sample and is located in such a way as to ensure contact with the bottom and wall of the container (see Figure 8).Thermoelectric converter 10 serves to control and regulate the furnace temperature and is located inside the reaction chamber at a distance of (140 ± 5) mm from the upper edge of the chamber and (5 ± 1) mm from the chamber wall.The measurement error of the instruments regulating and recording the temperature should not exceed 0.5%.By regulating the supply of gas and air to the burner, the flame of the gas burner is formed in the form of a wedge with a length of (10 ± 2) mm.4.7.3.3.The holder with the container is removed from the chamber.The sample is placed in the container for a time not exceeding 15 s and it is introduced into the reaction chamber.The electric drive of the gas burner is switched on to the specified operating mode.The frequency of approaching the gas burner to the sample at a distance of (10 ± 1) mm from its surface should be (10 ± 2) s.Observe the sample in the working chamber using a mirror.4.7.3.4.If the sample ignites at the test temperature, then the test is stopped, the burner is stopped in the "outside the oven" position, the container with the sample is removed from the chamber.Record the flash point in the report and the next test is carried out with a new sample at a lower temperature (for example, 50 °C less).If the sample does not ignite within 20 minutes or smoke emission completely stops before this time, then the test is stopped and the refusal is noted in the protocol.The test temperature is taken as the readings of the thermoelectric converter, which measures the sample temperature.
4.7.3.5.Using the method of successive approximations, using new samples and changing the test temperature, determine the minimum temperature of the sample at which, during the holding time in the furnace for no more than 20 minutes, the sample will ignite and burn for more than 5 s after removing the burner, and at a temperature of 10 °C less ignition is absent.

Figure 1 .
Figure 1.Crystallographically independent fragment of the crystal structure 1 with numbered atoms.Thermal ellipsoids are displayed at the 50% probability level for nonhydrogen atoms.

Figure 2 .
Figure 2. XZ plane projection of the crystal structure 1.The coordination polyhedron of Cu(II) is highlighted.

A
flame retardant-hardener for epoxy resins: Synthesis, structural and DFT studies of the [Cu(H2NC2H4NH2)2(H2O)Cl]Cl complex Borys MYKHALICHKO, Helen LAVRENYUK and Oleg MYKHALICHKO S1.Position parameters of atoms and their thermal parameters for the [Cu(H2NC2H4NH2)2(H2O)Cl]Cl complex (1) S2.Experimental determination details of the ignition and self-ignition temperatures of solids and materials.

Figure 8 .
Figure 8. TF apparatus: 1 -glass cylinders; 2 -spiral electric heaters; 3 -heatinsulating material; 4 -steel screen; 5 -sample holder; 6 -container; 7 -gas burner; 8 -burner electric drive; 9, 10 -thermoelectric converters; 11 -laminator.4.7.1.1.The TT apparatus, which is a vertical electric furnace with two coaxially located cylinders made of quartz glass.One of the cylinders with an inner diameter of (80 ± 3) mm and a height of 240 mm is a reaction chamber; the second cylinder of the same height has an inner diameter of (101 ± 3) mm.Spiral electric heaters with a total power of at least 2 kW are wound on the cylinders, which makes it possible to create a working zone temperature of 600 °C in no more than 40 minutes.

4. 7 . 4 .
Evaluation of the results 4.7.4.1.The arithmetic mean of two temperatures differing by no more than 10 °C is taken as the ignition temperature of the test substance (material), at one of which three samples are ignited, and at the other, three failures.The resulting temperature value is rounded to the nearest 5 °C.4.7.4.2.The convergence of the method at a confidence level of 95% should not exceed 7 °C.4.7.4.3.The reproducibility of the method at a confidence level of 95% should not exceed 20 °C.4.7.4.4.Test conditions and results are recorded in the report.

Table 1 .
Crystal data and experimental details for the single crystal of complex 1.

Table S1 .
Position parameters of atoms and their thermal parameters for complex 1 Templates for determining the distance from the bottom edge of the torch to the surface of the sample and for centering the container inside the chamber.Depending on the volume of the sample, it is determined using templates and the position of the container inside the chamber and the distance between the gas burner and the surface of the sample are recorded.