The Role of Weak Interactions in Strong Intermolecular M··· Cl

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The Role of Weak Interactions in Strong Intermolecular M···Cl Complexes of Coinage Metal Pyrazolates: Spectroscopic and DFT Study Aleksei A. Titov,†,‡ Ekaterina A. Guseva,† Oleg A. Filippov,† Galina M. Babakhina,† Ivan A. Godovikov,† Natalia V. Belkova,† Lina M. Epstein, and Elena S. Shubina*,† †

A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova str. 28, 119991 Moscow, Russia People’s Friendship University of Russia, Miklukho-Maklay str. 6, 117198 Moscow, Russia

‡

S Supporting Information *

ABSTRACT: The nondestructive reversible complexation of the macrocyclic group 11 metal pyrazolates {[3,5-(CF3)2Pz]M}3 (M = Cu(I), Ag(I)) to the halogen atom X = Cl, Br of η3allyliron tricarbonyl halides (η3-2-R-C3H4)Fe(CO)3X is revealed by the variable-temperature spectroscopic (IR, NMR) study combined with density functional theory calculations. The composition of all complexes at room temperature is determined as 1:1. In the case of the [AgL]3 macrocycle, complexes 1:2 are observed at low temperature ( (η3-2-Ph−C3H4)Fe(CO)3Cl in complexes with [AgL]3. The ΔH° values for complexation with [CuL]3 determined for (η3-2-R-C3H4)Fe(CO)3Br demonstrate even greater decrease of ΔH° on going from Me to Ph. Replacement of the halogen atom (Cl with Br) also reduces the complex strength (11−12%). Such sequences suggest the steric effect being the main factor affecting ΔH° and ΔS° values. The spectral measurements for yet another iron halide CpFe(CO)2Cl (7)were performed in CH2Cl2 due to low solubility of 7 in hexane. However, the former solvent substantially decreases complex formation constants19 as was again observed for 7 (Table 3). Still, the increase of Kf with cooling (>40 times) was appreciably greater than in the case of

Figure 5. IR spectra in the ν(CO) range of 2 (c = 0.0009 mol/L) at 230 K (black) and in the presence of 0.5 equiv of [AgL]3 at 290 K (green), 270 K (red), 230 K (blue); d = 2.2 mm, hexane.

method allowed us to assign the new additional band ν(CO)′in compl to the complex of 1:2 composition (two molecules of halide per one macrocycle molecule), for example, 2b′ (Figure 6, bottom). Formation Constants. The value of Kf was found to be rather high at room temperature. Thus, Kf290 is 235 L/mol for the complex of (η3-2-Me-C3H4)Fe(CO)3Cl with [CuL]3 (3a) and 1650 L/mol for the complex with [AgL]3 (3b), and the values grow approximately an order of magnitude when cooled 7033

DOI: 10.1021/acs.jpca.6b06579 J. Phys. Chem. A 2016, 120, 7030−7036

Article

The Journal of Physical Chemistry A Table 3. Thermodynamic Characteristicsa and Kf of the Complexes [{3,5-(CF3)2Pz}M]3 with Iron Halogenides R 3a 5a 6a 7a 2b 3b 4b 5b 7b a

X

Me Cl Me Br Ph Br CpFe(CO)2Clb H Cl Me Cl Ph Cl Me Br Cp(Fe(CO)2Clb

ΔH°, kcal/mol −5.4 −4.9 −3.6 −3.4 −7.7 −5.8 −4.9 −5.1 −5.5

± ± ± ± ± ± ± ± ±

0.1 0.2 0.4 0.2 0.1 0.1 0.2 0.1 0.3

ΔS°, cal/(mol K) −7.7 −6.4 −0.9 −3.0 −11.7 −5.0 −1.6 −1.4 −6.0

± ± ± ± ± ± ± ± ±

0.2 0.7 0.5 0.7 0.4 0.1 0.4 0.5 1.0

Kf290·× 10−3, L/mol

Kf250·× 10−3, L/mol

0.2 0.2 1.2 0.08 1.7 1.9 1.9 3.7 0.7

1.1 0.7 2.1 0.2 13.3 9.6 7.4 14.5 2.9

ΔH° and ΔS° determined in 290−230 K range. bMeasurements in CH2Cl2

slightly preferred for exo isomer over endo one (the complexation energies being ΔHgas = −20.9 and −21.8 kcal/ mol for endo and exo isomers of 3, respectively; ΔGgas = 9.5 and 9.8 kcal/mol). This results in the change of the relative energies of isomers. ΔΔH and ΔΔG calculated as

allyl complexes. Thus, its temperature dependence gives the formation enthalpy values comparable to those obtained for allyl complexes in hexane. Unfortunately, we failed to obtain crystals suitable for the Xray analysis. This was due to rather low solubility of both iron halides and their complexes with macrocycles. When low concentrations and consecutive long-time growth techniques were utilized the destruction of complexes was observed, even in argon atmosphere. In the absence of the structural data, we were enforced to characterize the complexes by the DFT. The allyl halogenides (η3-C3H5)Fe(CO)3X could exist as endo and exo isomers (Scheme 3, left), which cannot be

complex complex free free ΔΔE = (Eexo − Eendo ) − (Eexo − Eendo )

for complex 3a take values −0.9 and −0.3 kcal/mol, respectively. This effect is much more pronounced for Phsubstituted bases 4 and 6, for which formation of exo-complex with Cu macrocycle is favored by ΔΔG = −3.7 and −1.9 kcal/ mol, respectively. This effect could be explained by the ability of Ph substituent to form multiple weak van der Waals interactions or CH···π hydrogen bonds with macrocycle, when η3-allyl ligand is in the appropriate position (exo). Both endo and exo isomers of (η3-2-R-C3H4)Fe(CO)3X could be identified by NMR spectroscopy. The 1H NMR spectra of free 5 possess four proton signals in the 3.31−4.41 ppm range (Figure 8). Each of these signals corresponds to the pair of symmetrical protons of two equivalent CH2 groups in the endo and exo isomers of 5. Earlier it was suggested for the series of (η3-C3H5)Fe(CO)3X compounds that major form corresponds to the endo isomer.36 Thus, we assign dominating “inner pair of resonances” δ 4.31 and 3.51 to endo-5. Upon addition of 1 equiv of 1a or 1b all four signals undergo highfield shifts (up to −0.19 ppm) and remarkably change their intensity. This pattern implies that both endo and exo isomers of 5 form complexes with the macrocycle. High-field shifts of allyl CH2 protons suggest the absence of direct [ML]3-(η3-2-RC3H4) interaction in the complex formed. On the basis of the long-range HMBC inverse H−C correlation (presence of cross peak between 1H(CH2) and 13C(CO) signals that corresponds to 3JCH close to 10 Hz) we suggest that in the major form of complex 5b (“outer” signals at 4.34 and 3.15 ppm) the iron carbonyl adopts the exo configuration. The intensity redistribution observed for the two pairs of resonances suggests the shift of the endo/exo equilibrium toward the exo complex. Thereby as predicted by computations the preferred complexation of the exo isomer of 5 with 1a and 1b shifts the endo/exo ratio from 6:1 in free 5 to 1:1.4 and 1:7 for the complexes 5b and 5a, respectively. For 5b this change corresponds to ca. −1.3 kcal/mol change of the free energy value ΔΔG, which is in qualitative agreement with the quantum chemistry data.

Scheme 3. Endo and Exo Isomers of (η3-2-RC3H4)Fe(CO)3X (left) and Their Interaction with Macrocycles

discriminated by IR spectroscopy. NMR studies proved the dominance of endo isomers over the exo ones in solution.36 The endo configuration was also reported for all the X-ray characterized examples.29,30 Our DFT analysis of possible complexes for [CuL] 3 macrocycle with η3-allyliron tricarbonyl halides 3−6 reveals that both exo and endo isomers form complexes via Hal···Cu interactions (Figure 7). One of the metal atoms of 1a forms a short bond (2.5−2.8 Å) with halide ligand, while other two (for exo isomer) or one (for endo isomer) metal atoms have much longer contact with halogen atom of 3−6. This structural motifs closely resemble that found in the crystal structure of [AgL]3 complex with hydride ligand.22 Obviously, halide ligand is more accessible in the endo isomers of 2-substituted η3-allyliron halides 3−6. Surprisingly it appeared that formation of the complex with macrocycle is

4. CONCLUSIONS The IR spectroscopic studies evidence for the first time the nondistructive complexation of the Cu(I) and Ag(I) pyrazolates to the halogen ligand of organometallic complexes (η3-2-R-C3H4)Fe(CO)3X and CpFe(CO)2Cl in solution and in 7034

DOI: 10.1021/acs.jpca.6b06579 J. Phys. Chem. A 2016, 120, 7030−7036

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The Journal of Physical Chemistry A

Figure 7. Optimized structures of [CuL]3 complexes with endo- (left) and exo- (right) isomers of 3.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7-499-135-1871. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (Project No. 15-03-08017). Figure 8. 1H NMR spectra of 5 (blue) and in the presence of 1 equiv of 1b (red). Integration for complex 5b is given.

the solid state. The composition of all complexes is 1:1 at ambient temperature. Complexes with two iron halide molecules per one macrocycle are found at low temperatures and at high concentration of iron halide in the case of Ag pyrazolate. The thermodynamic characteristics of the complexes (formation constants and enthalpies in solution) are higher for [AgL]3 than for [CuL]3; their values decrease due to the steric effects of the substituents in the allyl moiety and with the growth of the halogen atom size. Surprisingly, complexation leads to the shift of the endo/exo equilibrium of the parent (η32-R-C3H4)Fe(CO)3X in favor of the exo isomer due to ability of bulky substituents (Me and Ph) to form multiple weak secondary interactions with the macrocycle. This is the first illustration of macrocycle’s ability to segregate different isomers of close energy.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b06579. Table of frequencies, illustrated molecular structures, NMR spectra, details of the DFT calculations, complete ref 27 (PDF) 7035

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