Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Aromaticity Suppression by Intermolecular Coordination. Optical Spectra and Electronic Structure of Heavy Carbene Analogues with an Amidophenolate Backbone R. R. Aysin,† S. S. Bukalov,† L. A. Leites,*,† A. V. Lalov,‡ K. V. Tsys,§ and A. V. Piskunov§ †
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow 119991, Russia N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 119991, Russia § G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, ul. Tropinina 49, Nizhny Novgorod 603950, Russia
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‡
S Supporting Information *
ABSTRACT: Experimental optical (UV−vis, Raman) and computational magnetic (NICS, GIMIC) criteria were used to study the aromaticity of group 14 N,O-heterocyclic carbenes tBuAPE (AP = amidophenolate, E = Ge, Sn, Pb). The UV−vis and Raman spectra of the monomeric compound with E = Ge were shown to differ significantly from those of dimeric compounds with E = Sn, Pb. The reason for these phenomena is suppression in the dimeric molecules of 10 π-electron aromaticity due to partial filling of the vacant pz orbital of the EII atom caused by intermolecular coordination N→EII. The monomeric forms of all molecules studied are aromatic according to all the criteria used.
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INTRODUCTION Aromaticity is one of the fundamental chemical conceptions, but it is somewhat intuitive; it cannot be directly measured quantitatively but can be only characterized through various molecular properties.1,2 To estimate aromaticity, four conventional criteria are commonly used:2−5 a chemical criterion, that is specific reactivity, as well as energy (RE, ASE, ISE), structural (geometry parameters), and magnetic (NMR, NICS, ACID, GIMIC) criteria. Computational magnetic indices have recently become widespread.6,7 The optical criterion, involving frequencies and intensities in vibrational (Raman, IR) and electronic absorption spectra, is infrequently used, although it can be very informative if properly applied.8−12 As all the aforementioned criteria are based on different physical phenomena, the results of their estimations are by no means obliged to coincide even for organic molecules, even more so for organometallics. However, the very juxtaposition of these results is of much interest. With the aim to study the aromaticity of group 14 organometallics, we first investigated the N-heterocyclic Arduengo carbene and its heavy element analogues (type I, Scheme 1).13−16 The aromaticity of these molecules (cyclic electron delocalization with participation of six π electrons of the ligand and vacant pz orbital of the divalent E atom (E = Si, Ge, Sn)) was demonstrated with the use of various experimental and theoretical methods.16 These included frequency shifts and intensity enhancement in the Raman spectra as well as long-wavelength band positions in the UV−vis spectra (optical criteria), NICS indices © XXXX American Chemical Society
Scheme 1
(magnetic criterion)17 and isomerization stabilization energy (ISE) (energy criterion).18 The same approach was used for the investigation of 10 π-electron aromaticity of benzannulated type II compounds.19,20 X-ray diffraction data for the type II germylene and stannylene with R = Me, [C6H4(MeN)2EII], have shown that the germylene is monomeric while the stannylene is a dimer at the expense of coordination N→Sn bonds, whose energy was estimated as ∼19 kcal/mol. The results of Raman measurements have demonstrated that in THF solution the dimer dissociates to monomers.19 Type II compounds with more voluminous substituents (R = Np), [C6H4(NpN)2EII], were investigated in the whole series EII = C, Si, Ge, Sn, Pb.20 All of these molecules were shown by singlecrystal X-ray diffraction analysis to be monomers and by Raman, UV−vis, NICS, and ISE data to be aromatic. Thus, all of the results obtained for type I and type II molecules confirm their aromaticity and demonstrate a tendency to its increase on going down group 14. Received: June 27, 2019
A
DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
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Computations. All theoretical calculations were performed in the Gaussian25 and ORCA programs26 at the DFT level of theory using the TPSS,27 B3LYP,28 and PBE029 functionals with the Def2-TZVP basis set.30 Implementation of a scalar-relativistic ZORA correction31 had no significant effects on interatomic distances and relative energies (see Table S1 in the Supporting Information). Normalcoordinate analysis and evaluation of Raman intensities were performed using the TPSS functional, and calculations of potential energy distribution (PED) were carried out with use of the NCA99 program.32 Time-dependent (TD) DFT calculations and evaluation of thermodynamic parameters of the dimerization process were done with the PBE0 functional. NICS indices according to the Stanger approach33 were computed at the B3LYP/Def2-TZVP level. Calculations of the magnetic shielding matrix with subsequent application of the GIMIC method6 were done at the B3LYP/Def2TZVP level according to recommendation.6 Visualization of GIMIC results was performed in the Paraview program.34 To integrate IC for each bond, the recommended procedure6 was used.
It was of interest to extend these studies to include related benzannulated N,O-heterocyclic carbene analogues of type III (Scheme 1), designated as tBuAPE (AP stands for an amidophenolate ligand), namely germylene (1), stannylene (2), and plumbylene (3); their syntheses and X-ray structures were recently published separately.21−23 X-ray data have borne witness to the monomeric structure of the germylene 1 and dimeric structure of the stannylene 2 and plumbylene 3 due to intermolecular N→EII donor−acceptor interactions (Scheme 2). A peculiar feature of the stannylene 2 (tBuAPSn) crystal22 Scheme 2
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RESULTS AND DISCUSSION Molecular Geometry. Salient structural parameters of 1− 321−23 are juxtaposed in Table 1 along with the results of
appeared to be a secondary Sn···Sn interaction that unifies two dimers into a tetramer. This is evidenced by a conforming shortened interatomic Sn···Sn distance of 3.352 Å. Moreover, topological analysis of the electron density distribution function according to Bader’s AIM theory24 has revealed a (3,−1) bond critical point between these two Sn atoms, which is characterized by a rather high ρ(r) value of 0.043 a.u. pointing to an intermolecular contact. Our aim was to apply to molecules 1−3 the same aromaticity indicators as were used for type II molecules, to compare the data obtained and inspect the influence of the dimerization process on aromaticity.
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EXPERIMENTAL AND COMPUTATIONAL SECTION
Synthesis. All of the compounds studied were synthesized according to the described procedures.21−23 Spectroscopic Measurements. Raman spectra in the region 100−4000 cm−1 were registered using a Jobin-Yvon LabRAM-300 laser Raman spectrometer equipped with a CCD detector and a microscope. The excitation line was the 632.8 nm line of a He/Ne laser, its power not exceeding 2 mW. The solid samples were sealed in capillaries under high vacuum or under an inert atmosphere. UV−vis spectra in the range 200−900 nm were registered with Carl Zeiss M400 and Agilent 8453 spectrophotometers. The samples were either Nujol mulls or solutions in heptane prepared under the Ar atmosphere of an MBraun glovebox.
Figure 1. Electronic absorption spectra of 1−3.
geometry optimization for monomeric and dimeric molecules. Good agreement between the experimental and computed (at PBE0/def2-TZVP level) values is of note. UV−vis spectra. It is well-known that electron delocalization leads to a bathochromic shift of the corresponding
Table 1. Experimental and Calculated (PBE0) Interatomic Distances (Å) for Monomeric and Dimeric Forms of the Molecules tBu APE (E = Ge, Sn, Pb)a E
form
method
E−O
E−N
Ge
monomer dimer monomer monomer dimer dimer monomer dimer dimer
PBE0 PBE0 X-ray21 PBE0 PBE0 X-ray22 PBE0 PBE0 X-ray23
1.819 1.857 1.829 2.099 2.053 2.054 2.116 2.171 2.161
1.865 2.057 1.867 2.065 2.254 2.229 2.164 2.348 2.317
Sn
Pb
N→E 2.149
2.326 2.320 2.425 2.422
C−N
C−O
C1C2
1.392 1.449 1.399 1.389 1.448 1.452 1.384 1.442 1.447
1.346 1.343 1.360 1.337 1.334 1.347 1.330 1.327 1.348
1.411 1.403 1.407 1.421 1.412 1.411 1.427 1.419 1.412
a
These data demonstrate that dimerization due to N→E coordination leads to a significant and regular elongation of the E−N and C−N bonds. B
DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 2. Positions of the Long-Wavelength Absorption Bands (nm) in the UV−vis Spectra of Benzannulated Carbene Analogues
Table 3. Experimental and Computed Data (Frequencies (cm−1), Intensities) and Approximate Assignment of the Most Intense Lines in the Raman Spectrum of Germylene 1
E compound
form
Ge
Sn
Pb
C6H4(MeN)2EII 19
monomer dimer monomer monomer dimer
360
430 294 448 448 306
∼500 ∼530 300
C6H4(NpN)2EII 20 tBu II APE
362 360
absorption band in the UV−vis spectra due to rapprochement of ground- and excited-state levels. That is why it was of interest to measure the electronic absorption spectra of the type III (tBuAPEII) molecules as solids (as Nujol mulls or microcrystalline films) and neutral solutions (heptane) and to compare the results with those for the related type II molecules.19,20 The spectra obtained are presented in Figure 1. For their interpretation, a TD DFT computation was carried out for the first 20 electronic transitions. The results (see Table S3 in the Supporting Information) have shown that the longwavelength transitions belong in all cases to the π−pz type. On examination of Figure 1, a well-marked difference is striking between the spectra of stannylene 2 and plumbylene 3 as solids and in solutions. This difference means that on going to a solution the intermolecular associations are broken up to monomers. This conclusion is supported by a comparison of the absorption band positions (λmax) of all benzannulated molecules studied (Table 2) and by calculated dimerization energy values (Table S2 in the Supporting Information). It is seen that all the monomeric germylenes studied absorb at nearly the same wavelength, ∼360 nm: that is, in this case the particular structure of the chelating ligand only slightly affects the band position. As the λmax values in the solution spectra for stannylene 2 (448 nm) and plumbylene 3 (∼530 nm) are close to those for the monomeric type II stannylene and plumbylene C 6 H 4 ( Np N) 2 E II (448 and ∼500 nm, respectively), dissociation of dimers 2 and 3 in heptane solution is beyond doubt. A sharp blue shift of the longwavelength bands in the spectra of solid compounds 2 and 3 in
exptl
calcd
approx assignt (groups participating in this normal mode)
672 s 822 m 938 vs 1244 vs 1311 s 1413 ms 1565 m
669 (23) 813 (23) 937 (52) 1243 (138) 1298 (190) 1424 (39) 1572 (79)
mainly 5-membered ring mainly tBu νGe−N + νN−C + tBu the whole bicycle the whole bicycle mainly, tBu νCC + δCH of the aryl ring
Figure 3. Comparison of the frequencies and intensities of the Raman lines, corresponding to benzene ring vibrations. The spectra are normalized according to the 2800−3000 cm−1 region (see text).
comparison to solution spectra suggests an essential decrease in aromaticity of these molecules on dimerization, which is in accord with the TD DFT results (see the Supporting Information).
Figure 2. Raman spectra of solid 1−3 and the initial aminophenol. C
DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
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Figure 4. NICS scan curves for the five-membered cycles in model monomeric MeAPE molecules at the GIAO-B3LYP/Def2-TZVP: (■) isotropic NICS; (●) in-plane component; (▲) out-of-plane component.
To be able to compare line intensities between the spectra in Figure 2, the spectra were normalized to equalize the intensity of the regions where the νCH bands of the tBu groups are situated (2900−3000 cm−1). This procedure is well-grounded because the νCH vibrations are well-localized and the C−H bonds are not affected by the EII atom and do not participate in π conjugation. From Figure 2 it is seen that the overall spectral pattern of the monomeric germylene 1 stands in marked contrast to the others in the line positions and their high intensities. Meanwhile, the spectra of the stannylene 2, plumbylene 3, and initial aminophenol are much less intense and have similar patterns. As it is well-known that π delocalization results in enhancement of the Raman lines corresponding to vibrations of conjugated bonds,9−11 then just a comparison of the overall spectral pictures of the germylene 1 with those of stannylene 2 and plumbylene 3 allows one to draw an inference about the aromaticity of monomeric 1 and its substantial weakening in the dimers 2 and 3. This conclusion agrees well with the results of detailed spectral analysis based on normal coordinate calculations (NCA). The results of the latter along with an estimation of Raman intensities allowed us to make band assignments. Experimental and computed data for the most intense Raman lines of molecule 1 are given in Table 3. The results of NCA calculations have shown that almost all vibrations of these complicated molecules are not welllocalized; they are of mixed origin. The vibrations localized mainly in tBu groups manifest themselves as lines of moderate intensity at 822−1413 cm−1. The most intense lines belong to vibrations of the aromatic system. The lines at 672 and 938
Table 4. NICS Values (ppm) at Minima of NICS Curves Computed for the Five-Membered Rings of the Model Me APE Monomers 1′-3′ E at minima of NICSout‑of‑plane (Figure 4) on FiPC-NICS curve (Figure 5)
Ge
Sn
Pb
−5.0 −5.0
−4.7 −4.7
−4.7 −4.6
Figure 5. Dependence of NICSout‑of‑plane on NICSin‑plane curves for the five-membered cycles in model monomeric MeAPE molecules at the GIAO-B3LYP/Def2-TZVP level.
Raman Spectra. Raman spectra registered for solid 1−3 as well as for the initial aminophenol tBuAPH2 are presented in Figure 2.
Figure 6. NICS-scan curves for the five-membered cycles in Sn and Pb dimers. D
DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
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Figure 7. Induced current surfaces for monomers HAPE (1″−3″) at isovalue 0.025 a.u.
Table 5. IC Strength Values (nA/T) for Model HAPE Molecules monomer
energy of the Sn···Sn intermolecular interaction in the tetramer 2 is much less22 than that of an ordinary Sn−Sn covalent bond (∼60 kcal/mol), there is no escape from mentioning this spectral peculiarity. NICS. Negative values of the NICS (nucleus independed chemical shift) index are generally taken as aromaticity indicators.17 However, Stanger33 has shown that simple NICS estimations, especially given small values of this index, are not sufficient for such a judgment. That is why Stanger et al. proposed a modified method (NICS-scan) which consists of separating isotropic NICS values into in-plane and out-of-plane contributions and then scanning them over the distance of ghost Bq atom from the molecular plane. Torres-Vega et al.37 proposed to use the NICSout‑of‑plane/NICSin‑plane dependences, to consider the value of “free of in-plane component (FiPC) NICS” and to examine the shape of the corresponding curves. Thus, the authors33,37 make inferences about aromaticity and antiaromaticity on the basis of the positions and shapes of the curves obtained. For polycyclic compounds, NICS indices are computed for each cycle separately.17,33 We applied NICS-scan methods to model monomeric molecules with an Me substituent at the nitrogen atom, MeAPE. The results obtained for the sixmembered cycles involved in type III molecules are typical for aryls and are of no interest, in contrast to those for the fivemembered [C2NOE] cycles that are presented in Figure 4 and Table 4. All of the NICS values obtained are negative and close to analogous values for [C2N2E] cycles in type II molecules.20 The shape of the curves and their mutual alignment (Figure 5) are characteristic of π-aromatic systems. In particular, NICS out-of-plane curves have minima at ∼1 Å distance from the molecular plane. Thus, these data also confirm the aromaticity of the type III monomers. However, analogous computations for the five-membered cycles in the dimeric molecules 2′ and 3′ furnished an opposite result (Figure 6). Here the curve shapes and their relative positions point to the lack of aromaticity. These curve patterns correspond to typical conjugated but not aromatic systems.33 GIMIC. To solve the problem raised, we have also invoked a recently elaborated6 computational method, GIMIC (gaugeincluding magnetically induced current), which seems to be a powerful magnetic aromaticity criterion. It is based on the calculation of magnetically induced current (IC) density and has shown itself as a useful tool for aromaticity studies, applicable to all types of molecules and allowing one to compare different but related molecular systems. It is very important that this approach gives the possibility to visualize conjugated domains by graphical representation of isosurfaces for signed modulus of the IC density (separately for diamagnetic and paramagnetic contributions) and to carry out numeric evaluation of the ring-current strength by integrating the current-density flow along the defined bonds.
dimer
bond
E = Ge (1″)
E = Sn (2″)
E = Pb (3″)
E = Sn (2″)
E = Pb (3″)
E−N E−O C−N C−O C···C(1)a C···C(2) C···C(3) C···C(4) C···C(5) C···C(6)
7.1 7.1 6.2 6.5 5.1 12.0 12.2 12.2 12.1 9.4
7.1 7.0 5.9 6.4 5.0 11.8 11.9 11.9 11.9 8.4
7.3 7.1 6.0 6.5 4.6 11.6 11.8 11.7 11.7 8.0
1.1 1.1 1.6 1.6 7.7 9.9 10.1 10.1 10.2 7.9
1.6 1.6 1.4 1.4 7.8 9.7 9.9 10.0 10.0 8.0
a
Numeration of the C···C bonds in the aryl ring:
Figure 8. IC surface for dimericHAPSn (2″) molecule.
cm−1 correspond to normal modes localized mainly in the fivemembered cycle. The lines in the region 1200−1320 cm−1 correspond to normal modes involving the whole bicycle. The line in the region ∼1600 cm−1 corresponding to a normal mode localized in the benzene ring deserves special attention. Parameters of this line, its frequency and intensity, are often treated as indicators of conjugation.11,12 A significant frequency decrease and intensity enhancement of this line on going from the spectra of 2 and 3 to the spectrum of 1 (presented in detail in Figure 3) also reinforce the conclusion made above. An interesting feature of the Raman spectrum of solid stannylene 2 is the presence of very intense lines in the lowfrequency region, at 126 and 170 cm−1 (Figure 2). It is pertinent to remember here that the stretching vibration νSn−Sn of a “full-value” covalent Sn−Sn bond due to the heavy Sn atom and its high polarizability manifest themselves just as very intense low-frequency Raman lines (192 cm−1 in the spectrum of distannane Me3Sn−SnMe335 and 110−150 cm−1 in the spectra of polymeric polystannanes [R2Sn]n36). Although the E
DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
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Organometallics We have already applied this approach to type I and II molecules.38 The results have demonstrated the applicability of the GIMIC method to the carbene analogues containing a group 14 atom in the cycle. The data obtained by this method for model monomeric type III molecules with an H substituent at nitrogen atom, HAPEs 1″−3″, are presented in Figure 7 and Table 5. It is seen that the outer violet surfaces for 1″−3″, corresponding to diamagnetic IC outside the ring, are large and continuous; they spread out all the bonds of the bicycle. The violet surfaces dominate over the cyan surfaces of paramagnetic IC inside the ring. Such isosurfaces are typical of aromatic systems. In contrast, analogous plots for dimeric molecules exhibit another IC pattern; an example for a dimer of the HAPSn (2″) molecule is presented in Figure 8. It is evident that the surfaces of diamagnetic and paramagnetic IC are commensurate and exhibit discontinuities. As was said above, the GIMIC method allows numerical characterization of magnetically induced ring current strength by integration.6 The results of numerical net IC strength evaluation for all the bonds present in monomeric and dimeric type III molecules are given in Table 5. The IC strength values for the five-membered cycle in monomeric 1″−3″ molecules lie in the interval 6−7 nA/T and are similar for the germylene, stannylene, and plumbylene. However, these values differ markedly from those in dimeric molecules 2″ and 3″, which are much lower (1−2 nA/T), evidently due to the lack of aromaticity. For the C···C bonds, their averaged IC strength values, ∼10 nA/T, are typical for aryl rings.6 The only exception is the reduced value for the C··· C(1) bond, which is explained by compensation of local ring currents in two adjacent cycles of the bicyclic system.38 The IC strength values given in Table 5 for the monomeric type III molecules are similar to those obtained for the type II molecules.38
A. V. Piskunov: 0000-0002-8200-9433 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge partial financial support from the Ministry of Science and Higher Education of Russia. Synthesis of compounds 1−3 was performed in the framework of Russian Science Foundation project # 17-03-01428.
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CONCLUSIONS Thus, all the experimental and theoretical approaches used point to 10 π-electron aromaticity of the monomeric molecules 1−3, as opposed to the dimers 2 and 3, where aromaticity of the five-membered cycles is suppressed due to formation of the intermolecular coordination bonds N→EII. The degree of aromaticity of [C2NOE] cycles in the monomers 1−3 is close to that of [C2N2E] cycles in type II compounds.
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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.organomet.9b00434.
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REFERENCES
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Additional description of quantum-chemistry calculations and their results (PDF) Optimized Cartesian coordinates (XYZ)
AUTHOR INFORMATION
Corresponding Author
*L.A.L.: e-mail,
[email protected]; tel, +74991359262. ORCID
R. R. Aysin: 0000-0003-1402-9878 S. S. Bukalov: 0000-0003-2342-7093 L. A. Leites: 0000-0001-9686-2876 F
DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.organomet.9b00434 Organometallics XXXX, XXX, XXX−XXX
