UV–Vis Identification and DFT-Assisted Prediction of Structures of Cu

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UV-Vis Identification and DFT-assisted Prediction of Structures of Cu(II)-Alkyl Chlorocomplexes Oleg Gromov, Ekaterina Zubanova, Elena N. Golubeva, Victor Fedorovich Plyusnin, Georgii Mikhailovich Zhidomirov, and Mikhail Ya. Mel'nikov J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 06 Nov 2012 Downloaded from http://pubs.acs.org on November 6, 2012

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UV-Vis Identification and DFT-assisted Prediction of Structures of Cu(II)-Alkyl Chlorocomplexes. Oleg I. Gromov,*† Ekaterina M. Zubanova,† Elena N. Golubeva,† Victor F. Plyusnin,ab Georgii M. Zhidomirov,†§ Mikhail Ya. Melnikov† †

Chemistry Department, Lomonosov Moscow State University, Leninskiye Gory 1-3, Moscow, Russia a

Insitute of Chemical Kinetics and Combustion SB RAS, 630090 Novosibirsk, Russia b

§

Novosibirsk State University, 630090, Novosibirsk, Russia

Boreskov Institute of Catalysis (BIC) SB RAS, 630090, Novosibirsk, Russia

ABSTRACT

The structures of paramagnetic copper complexes, the products of photolysis of tetrachlorocuprates of quaternary ammonium in frozen solvents, earlier denoted as 1-Cu and 2Cu, were established on the basis of comparison of experimental and theoretical UV-Vis spectra. UV-Vis spectra of photolysis products were registred at 77-116 K. Comparison with the EPR data in this temperature range allowed to assign photolysis products bands in Vis spectrum either to 1-Cu or to 2-Cu. Model structures for 1-Cu and 2-Cu were proposed. TD-DFT calculated

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spectra of model compounds along with CuCl42- anion are in excellent agreement with the experiment. The comparison of UV-Vis and EPR data and results of TD-DFT calculations evidences that 1-Cu and 2-Cu are paramagnetic organochlorocuprates(II) with general formulae Cu(II)Cl2R and Cu(II)Cl3R, respectively, where R is (-C6H12)N+(C6H13)3.

INTRODUCTION Photolysis of tetrachlorocuprates of quaternary ammonium in frozen solvents yields alkyl-type radicals, Cu(I) compounds and two paramagnetic copper complexes, earlier denoted as 1-Cu and 2-Cu1,2, where 1 and 2 are the sequence numbers not related to the structure, oxidation state and other properties of these compounds. They were proposed to include alkyl fragments of counterions or solvent3. The similar compounds with Cu(II)-C(sp3) bond may be important intermediates4

in

radical

reactions

catalyzed

by

copper

complexes5,

such

as

polyhalohydrocarbons addition to alkenes6, living radical polymerization7, cyclization8, etc. Indeed, Cu+ forms organocopper(II) compounds with a wide variety of alkyl-type radicals, such as CH39-11, poly(acrylates) derivatives12,13, or CR1R2OH and CH2CR1R2OH radicals, produced from methanol, ethanol or propan-2-ol14. They have short (10-2 - 10-6 s) lifetimes at ambient conditions, but in a frozen matrix at 77 K organocuprates(II) are stable and could be studied by EPR and UV-Vis spectroscopy. The greatest advantage of EPR technique in application to the system under investigation is that complexes of Cu(I) are EPR silent as they are diamagnetic and do not introduce complications in the spectra interpretation. However, spectra of 1-Cu, 2Cu and alkyl-type radicals strongly overlap that makes it difficult to obtain the individual spectra. On the annealing at 100 K complete conversion of 2-Cu to 1-Cu and alkyl-type radicals vanishing were observed2. As a result the individual EPR-spectrum and Spin-

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Hamiltonian parameters of 1-Cu were obtained15. In our previous paper we have established that compounds of general formula Cu(II)Cl2R, where R is an alkyl-type fragment, have DFT predicted principal g-tensor values similar to experimental ones of 1-Cu16. Though, it was difficult to separate out EPR spectrum of 2-Cu since it was gained in presence of other paramagnetic products of photolysis1. This spectrum previously obtained by multiple subtractions gives rise to some doubts as we are still failing to interpret it2. Thus, the aims of the present work are estimation of the structure of 2-Cu and confirmation of suggestions on 1Cu structure, that may be based on comparison of experimental UV-Vis spectra of 1-Cu and 2Cu, and TD-DFT calculations of model compounds spectra. Fortunately, complexes of copper with radicals have bands in visible region not overlapping with the spectra of Cu(I) complexes3. TD-DFT approach allows to calculate spectra of copper(II) chloride complexes in semi-quantitative agreement with experiment17. Model compounds structures including alkyl radicals of quaternary ammonium cations in copper coordination sphere were proposed earlier16 and can be used here.

EXPERIMENTAL DETAILS Anhydrous copper(II) chloride was prepared by the azeotropic dehydration of CuCl2⋅2H2O with benzene followed by vacuum pumping at 10-3 Torr. Tetrahexylammonium chloride (C6H13)4N+Cl- from Sigma-Aldrich was used without additional purification. 2-Chlorbutane from Merck was purified by distillation over phosphoric anhydride. 2-Chlorbutane was used as a vitrescent solvent. The solutions of [(C6H13)4N+]2[CuCl4]2- were prepared by dissolution of (C6H13)4N+Cl- and CuCl2 in 2-chlorobutane. The total copper concentration [Cu2+]Σ was 2⋅10-3 mol/dm3, and the ratio [(C6H13)4N+Cl-]:[CuCl2] was 6:1 in order to preclude the formation

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of bi- and polynuclear chlorocuprates18. The purity of the solutions was monitored spectroscopically18. The solution was placed in 2 mm quartz cell equipped with a thermo sensor and was saturated with dry argon to remove dissolved oxygen. The UV-Vis absorption spectra were recorded using HP 8453 spectrophotometer. Stationary irradiation was performed using the light of high-pressure mercury lamp (DRSh-500). Single line of the mercury lamp with λmax=405 nm (24691 cm-1) was isolated by means of a combination of SS-5 and PS-11 filters (T=53% at 405 nm). Photolysis was carried out at 77 K in Dewar’s vessel equipped with flat windows. A series of optical spectra at different temperatures was recorded during spontaneous heating up. For the further analysis all experimental spectra were resolved into Gaussian-shaped absorption bands following Stevenson et. al.19 Fitting parameters were varied simultaneously for all spectra to achieve best overall agreement.

CALCULATION DETAILS Unrestricted DFT calculations of the geometry of CuCl42- dianion with counterions and model Cu (II)-alkyl chlorocomplexes were performed using ORCA program package20 on B(38HF)P8621/def2-TZVPP22 level with RIJCOSX approximation23. Vibrational frequencies were calculated in order to prove the potential energy surface minima achievement. TD-DFT calculations were done using B(38HF)P86 functional with aug-def2-TZVPP24,25 basis set for Cu, Cl and C atom bonded with Cu, and with def2-TZVP basis set for other atoms. All calculations were performed employing COSMO26 solvation model with 2-chlorobutane as the solvent. TD-DFT model absorbance spectra were generated with orca_mapspc utility.

RESULTS AND DISCUSSION

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Stable organic compounds of Cu(II) with Cu-C(sp3) bond are presented only by a single series of copper (II) halide complexes [CuX(tptm)] (X = F, Cl, Br, I; tptm = tris(2-pyridylthio)methyl), studied by X-ray crystallography, EPR, UV-Vis absorption spectroscopy, and DFT calculations27-29. Formation of labile copper complexes with CH3CHOH radicals during photolysis of CuCl42- ethanolic solutions at 77 K was found by Plyusnin et al. 3 The photolysis resulted in appearance of bands with maxima at 22000 cm-1 and 38700 cm-1 in the UV-Vis spectrum. At the same time EPR spectrum of a new copper complex aroused. A complex with the same UV-Vis and EPR spectra was synthesized from Cu(I) ions and independently generated CH3CHOH radicals in frozen ethanolic matrix3. It was suggested that this complex with proposed composition Cu(I)…CH3CHOH˙ has band with maximum at 22000 cm-1. At annealing at 130 K the band with maximum at 22000 cm-1 and EPR spectrum vanished. The band with maximum at 38700 cm-1 was assigned to stable Cu(I) complex as it aroused also during the photolysis at room temperature. Golubeva et al.16 argued that similar complex of Cu with alkyl radical is a chloroorganocopper(II) compound with rather stable Cu(II)-C bond and that scarcity of stable compounds with Cu(II)-C(sp3) bond is mainly due to their high reactivity. As Lobanov2 and Golubeva1 used similar to Plyusnin3 photochemical method and observed similar EPR spectra, UV-Vis spectroscopy and spectroscopic data TD-DFT interpretation are likely to be informative also in the case of 1-Cu and 2-Cu compounds. The chosen computational method (TD-DFT/B(38HF)P86/aug-def2-TZVPP) was tested on CuCl42- anion as it has structure similar to those of the compounds of interest. We registered UV-Vis charge transfer spectrum30,31 of (N(C6H13)4)2[CuCl4] in 2-chlorobutane at 77 K (Fig. 1, black solid line)and found it to be in agreement with the data of Ferguson32, Bird and Day30 for the CuCl42- dianion in different surroundings. TD-DFT calculation of the absorption spectrum was done for a preliminary optimized CuCl42- with two N(CH3)4+

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counterions in 2-chlorobutane environment. Positions of absorption bands in the model spectrum scaled by 0.91 factor (Fig. 1, green dash line) agree well with the experiment.

Figure 1. UV-Vis spectrum of tetrahexylammonia tetrachlorocuprate ((N(C6H13)4)2[CuCl4]) at 77 K in frozen 2-chlorbutane (black solid), simulation with Gaussian-shaped bands (red dash) and TD-DFT calculated spectrum of CuCl42- dianion with two N(CH3)4+ counterions in 2chlorobutane as the environment (green dash, spectrum wavenumbers scaled by 0.91 factor).

Reasonable agreement of calculated spectrum with experiment allows using TD-DFT approach to simulate spectra of the model chloroorganocuprates.

Tetrachlorocuprates Photolysis Photolysis of tetrahexylammonia tetrachlorocuprate frozen solution in 2-chlorobutane at 77 K resulted in consumption of CuCl42- accompanied by decreasing of intensities of its absorption in UV-Vis region (Fig. 2). Simultaneously, appearance of new absorption bands with maxima near 22000 cm-1 and 37700 cm-1 and increasing of the absorbance in the UV region were observed.

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According to EPR data of Lobanov2, Golubeva1, Plyusnin3 generation of alkyl-type radicals was observed during CuCl42- photolysis. Alkyl-type radicals are products of oxidation of ethanol3 or tetrahexylammonia alkyl chains1,2 by Cu(II) complexes, thus Cu(I) compounds are likely to be found in the system. Mononuclear Cu(I) chlorocomplexes have characteristic charge transfer to solvent (CTTS) spectra in the UV region19,33. Cu(I)Cl32- in neutral water solution has intensive band with maximum at 36500 cm-1 whereas Cu(I)Cl2- may have only a weak shoulder at this region and the first well resolved band in its spectrum has maximum at 43500 cm-1. Though, CTTS spectra are dependent on surrounding34. Therefore, absorption in the UV region can be explained by generation of at least Cu(I)Cl32- species. Plyusnin et. al.

3

found that copper complexes, supposed to be complexes of copper(I) with

organic radicals, have the absorption with maximum at approximately 22000 cm-1. Photolysis of tetrahexylammonia tetrachlorocuprate in 2-chlorobutane also yields a new absorption near 22000 cm-1. Cu(I) chloride complexes have no bands in this area19,33. The new absorption in 20000-26000 cm-1 region has clearly complex structure that could be explained by superposition of 1-Cu, 2-Cu, and CuCl42- bands. In order to eliminate 2-Cu, photolysed sample was annealed as in15.

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Figure

2.

UV-Vis

spectra

of

photolysed

tetrahexylammonia

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tetrachlorocuprate

((N(C6H13)4)2[CuCl4]) in 2-chlorobutane at 77 K during the photolysis. Consecutively, minutes: 0 (black), 6 (green), 12 (brown), 22 (blue), 38 (purple), 60 (cyan) and 90 (red).

On heating of the photolysed sample increasing of intensity of the absorption in visible region (20000-26000 cm-1) was observed, while absorbance maximum position shifts by approximately 1000 cm-1 (Fig. 3). The shape of spectra at 15000-20000 cm-1 also was changed and its intensity increased.

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Figure

3.

UV-Vis

spectra

of

photolysed

tetrahexylammonia

tetrachlorocuprate

((N(C6H13)4)2[CuCl4]) in 2-chlorobutane during the heating. Consecutively: at 77 K (black), 95 K (purple), 100 K (green), 103 K (blue), 105 K (red).

The absorption in 15000-26000 cm-1 region can be resolved into five Gaussian-shaped bands: CuCl42- bands with maxima at 22513 cm-1 and 24378 cm-1 and three bands with maxima at 18862 cm-1, 23696 cm-1 and 22066 cm-1. During the heating of the sample to 108 K intensities of the bands with maxima at 18862 cm-1 and 23696 cm-1 were simultaneously increased more than three times, intensity of the band with maximum at 22066 cm-1 after a slight increase dropped and vanished (Fig. 4). It was found1,2 from EPR data that content of 1-Cu had a maximum at heating, while 2-Cu vanished already at 100 K. Hence, the bands with maxima at 18862 cm-1 and 23696 cm-1 were assigned to 1-Cu, the band with maximum at 22066 cm-1 was assigned to 2-Cu (Table 1).

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Figure 4. Normalized intensities of the bands with maxima at 18862 cm-1 and 23696 cm-1 (black) and 22066 cm-1 (red) during the heating of photolysis products of tetrahexylammonia tetrachlorocuprate ((N(C6H13)4)2[CuCl4]) in 2-chlorobutane.

Table 1. Experimental Gaussian-shape bands positions, their assignment and TD-DFT calculated transitions of N+(CH3)4CuCl42-, I and II model structures* N+(CH3)4CuCl42-

Gausian band position, cm-1

Assignment

22513

CuCl42-

24378

CuCl42-

29145

33810

CuCl42-

CuCl42-

41260

CuCl42-

18862

1-Cu

23696

1-Cu

22066

2-Cu

cm-1

osc strength

23934

0.000163

25030

0.016805

25143

0.016625

29105

0.004199

33183

0.007758

33356

0.037807

33952

0.041694

33863

0.013267

40994

0.061797

I

II

cm-1

osc strength

18862

0.021997

18982

0.000979

23646

0.079154

cm-1

osc strength

22008

0.100654

* All calculated values are scaled by 0.91 factor

TD-DFT calculation of the model structures spectra

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According to our previous paper16 1-Cu has the structure similar to one presented at Fig 5 (I). TD-DFT calculated spectrum of the model structure I (Fig. 6 black dash) being scaled by 0.91 factor matches the experimental spectrum of 1-Cu (Fig. 6 black solid). The same scaling factor was used in the case of CuCl42- calculated spectrum. This agreement confirms our previous conclusion on the structure of 1-Cu16 based on comparison of experimental and calculated EPR parameters of 1-Cu. 2-Cu is known to form during the photolysis of ((N(C6H13)4)2[CuCl4]) at 77 K and to transform into 1-Cu on annealing1,15, hence it may have structure intermediate between CuCl42- and 1-Cu. Thus, it is suggested to contain three chlorine atoms instead of two chlorine atoms in 1-Cu and alkyl-type fragment in copper coordination sphere. The model structure for 2-Cu is presented at Fig. 5 (II). Indeed, TD-DFT calculated spectrum of II being scaled by 0.91 factor (Fig. 6 red dash) is in agreement with the experimental spectrum of 2-Cu (Fig. 6 red solid). Therefore, 1-Cu and 2-Cu are concluded to have Cu(II)Cl2R and Cu(II)Cl3R general formulae, respectively, and the structures similar to model compounds I and II.

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Figure 5. Model structures corresponding to 1-Cu (I) and 2-Cu(II).

Figure 6. Gaussian-shaped bands of 1-Cu (black solid) and TD-DFT calculated spectra of model structure I (black dash, spectrum wavenumbers scaled by 0.91 factor) and Gaussian-shaped band of 2-Cu (red solid) and TD-DFT calculated spectra of model structure II (red dash, spectrum wavenumbers scaled by 0.91 factor).

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The spectra in the UV region In the UV region the spectra have only one distinct maximum, belonging supposedly to Cu(I)Cl32-19. Besides this band, Cu(I)Cl32- and Cu(I)Cl2-, an expected product of Cu(I)Cl32transformation, have intensive absorption in UV region19,33. A change of environment during annealing is supposed to effect Cu(I) chloride complexes CTTS spectrum. Thus, decomposition into individual bands in the UV region is more arguable. TD-DFT calculations indicate that 1-Cu also has bands with maxima at 35766 cm-1 and 37072 cm-1 and 2-Cu has a band with maximum at 36050 cm-1.Nevertheless, UV spectra were passably resolved into two CuCl42- bands, three Gaussian components of Cu(I)Cl32- spectrum, three Gaussian components of Cu(I)Cl2- spectrum and three components of 1-Cu and 2-Cu spectra (Fig. 7).

Figure 7. UV-Vis spectrum of tetrahexylammonia tetrachlorocuprate ((N(C6H13)4)2[CuCl4]) in 2chlorobutane at 77 K after 90 minutes photolysis: model spectrum (red dash), model spectrum of CuCl42- (olive), model spectrum of 1-Cu (green), model spectrum of 2-Cu (blue), model spectrum of CuCl32- (purple).

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On the basis of qualitative analysis of the UV spectra after the photolysis completion the main Cu(I) containing product is Cu(I)Cl32- (Fig. 7). On heating the absorption of Cu(I)Cl32- decreases in parallel with increase of absorption of Cu(I)Cl2- likely because of conversion of less stable19 Cu(I)Cl32- into Cu(I)Cl2-.

CONCLUSIONS The comparison of UV-Vis and EPR1,2,15,16 data, as well as results of TD-DFT calculations evidences that two paramagnetic compounds produced under photolysis of tetrahexylammonia tetrachlorocuprate

solutions

earlier

denoted

as

1-Cu

and

2-Cu

appear

to

be

organochlorocuprates(II). The scheme by Lobanov et. al.2 can now be completed with the more precise formulae (Scheme 1): 1-Cu has general formula Cu(II)Cl2R, where R is (-C6H12)N(C6H13)3+, and structure similar to model compound I (Fig. 6, I), 2-Cu has general formula Cu(II)Cl3R and structure similar to model compound II (Fig. 6, II). In the primary photochemical act an oxidation of ammonia alkyl chain to alkyl-type radical and Cu(II)Cl42reduction to Cu(I)Cl32- occurs with removal of HCl molecule1,1 . Cu(II)Cl3R assumedly forms in a photochemical reaction or as a result of Cu(I)Cl32- interaction with alkyl radical. In the next step another Cl- is detached from copper coordination sphere in Cu(II)Cl3R resulting in Cu(II)Cl2R appearance.

Scheme 1. Interconversions of products of tetrahexylammonia tetrachlorocuprate photolysis. P – purely organic products.

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On annealing Cu(I)Cl32- may transform into Cu(I)Cl2- or interact with alkyl radical resulting in Cu(II)Cl3R or Cu(II)Cl2R formation. Cu(II)Cl3R completely transforms into Cu(II)Cl2R, which yields Cu(I)Cl2- and some diamagnetic organic products (P) in reaction with another Cu(II)Cl2R specie or alkyl-type radicals on further annealing.

AUTHOR INFORMATION Corresponding Author *E-mail: (O.I.G.) [email protected].

ACKNOWLEDGMENT

The research is partially supported by RFBR Grants 10-03-00603a and 12-03-31130

REFERENCES

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29. Santo, R.; Miyamoto, R.; Tanaka, R.; Nishioka, T.; Sato, K.; Toyota, K.; Obata, M.; Yano, S.; Kinoshita, I.; Ichimura, A.; Takui, T. Angew. Chem. Int. Ed. 2006, 45, 7611 –7614. 30 Bird, B. D.; Day, P. J. Chem. Phys. 1968, 49, 392. 31 Ehara, M.; Piecuch, P.; Lutz, J. J.; Gour, J. R. Chem. Phys. 2012, 399, 94–110. 32 Ferguson, J. J. Chem. Phys. 1964, 40, 3406. 33 Davis, D. D.; Stevenson, K. L.; Davis, C. R. J. Am. Chem. Soc. 1978, 100, 5344-5349. 34 Blandamer, M. J.; Fox, M. F. Chem. Rev. 1970, 70, 59–93.

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