Communication pubs.acs.org/IC
Molecular Structures of N,N′‑Dimethylbenzimidazoline-2-germylene and -stannylene in Solution and in Solid State by Means of Optical (Raman and UV−vis) Spectroscopy and Quantum Chemistry Methods R. R. Aysin,† L. A. Leites,*,† S. S. Bukalov,† A. V. Zabula,‡ and R. West*,‡ †
Scientific and Technical Center on Raman Spectroscopy, A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilova street, Moscow 119991, Russia ‡ Organosilicon Research Center, University of Wisconsin, Madison, Wisconsin 53706, United States S Supporting Information *
Crystals of germylene 1 suitable for an X-ray diffraction study were obtained by sublimation in vacuo. Germylene 1 crystallizes at room temperature in the tetragonal P42/mbc space group of symmetry. The molecular structure for 1 is given in Figure 1, and
ABSTRACT: X-ray data obtained for germylene 1 evidence its monomeric structure, unlike that of stannylene 2, which had been shown previously to form a coordination dimer. Raman spectra of solid and liquid 1 are identical, whereas the Raman spectra of solid 2 and its solution 2a differ significantly. The spectrum of 2 is complicated and contains the lines corresponding to N → Sn coordination bonds forming a dimer. The spectrum of 2a is simpler and close to that of monomeric 1, thus pointing to the rupture of the dimer in solution. The UV− vis spectrum of solid 2 exhibits a band corresponding to a transition involving the N → Sn coordination bonds. Quantum theory of atoms in molecules data estimate the energy of this bond as ∼19 kcal/mol. The aromaticity of 1 and 2 with their 10 π-electron systems including divalent Ge or Sn atoms is confirmed by negative nucleusindependent chemical shift values.
Figure 1. X-ray structure of 1 (left) and its crystal packing fragment (right).
crystallographic data are given in Table S1 (see the Supporting Information, SI). Germylene 1 shows weak intermolecular Ge1··· N1* interactions [3.591(5) Å] with the N atoms of two adjacent germylene units, thus making the intramolecular Ge1−N1 and Ge1−N2 distances slightly different [1.854(5) and 1.869(5) Å]. These interactions result in the formation of chains in the crystal of 1. Related, but much stronger, E···N contacts were observed for dimeric stannylene 2 [2.361(2) Å].3 Selected experimental bond lengths (Å) for 1 and 2 and their calculated values for the monomers are given in Table 1. Raman spectra of 1 and 2 were investigated, and their assignment was based on the results of normal coordinate analysis (NCA) at the density functional theory (DFT) PBE level (see Tables S2 and S3 in the SI) for isolated molecules 1 and 2 as well as for the dimeric 2. The spectra of 1 were recorded for a solid sample and for a liquid formed as a result of its sublimation; they appeared to be identical, and their frequencies and intensities were very close to those calculated by NCA for a monomer. The spectra of liquid 1, solid 2, and solution 2a in THF-d8 are juxtaposed in Figure 2. The simple pattern of the spectrum of 2a resembles that of monomeric germylene 1. The most intense lines in both spectra (at 632 cm−1 for 1 and 618 cm−1 for 2a) correspond to a totally symmetric mode of a complex origin with appreciable contribution from the νE−N coordinates. The E−N bond stretch also participates in the complex normal modes at 474, 338, and 256 cm−1 (for 1) and 451, 287, and 243 cm−1 (for
B
enzannulated systems with divalent group 14 elements bound to the 1,2-diaminophenylene moiety play a special role among N-heterocyclic carbenes (NHCs) and their analogues because of their stability, accessibility, and catalytic activity.1 Some of these compounds had been known before the synthesis of the first NHC. In most cases, the kinetic stability of benzannulated NHCs and their analogues was raised by virtue of bulky substituents (t-Bu, Np, Mes, etc.). Benzannulated derivatives bearing sterically nondemanding substituents have also been prepared. The synthesis of benzannulated germylene 1 with N,N′-Me substituents was reported in 1989 by Meller et al.2 The corresponding tin analogue 2 was synthesized and structurally characterized more recently.3 It was shown that 2 exists as a bimolecular aggregate, where two stannylene units are glued together via strong intermolecular coordination N → Sn (2.36 Å). The 119Sn NMR study for 2 suggests a N → Sn bond rapture in a tetrahydrofuran (THF) solution and formation of, presumably, 2·2THF units due to coordination of THF molecules to the electron-deficient stannylene center, while in a benzene solution, the dimer breaks up into two monomers. Herein, we report the aggregation pattern and electronic structure for 1 and 2 in both the solid and liquid states by means of vibrational and electronic spectra and quantum chemistry calculations. © XXXX American Chemical Society
Received: March 6, 2016
A
DOI: 10.1021/acs.inorgchem.6b00572 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry Table 1. Selected Experimental Bond Lengths (Å) for 1 and 2 and Their Calculated Values for the Monomers germylene 1 N1−E N2−E C1−C2 C1−C6 C2−C3 C3−C4 C5−C6 C4−C5 N1−C2 N2−C1 N1−C7 N2−C8
stannylene 2
exptl
calcd PBE0
exptl dimer3
calcd PBE0 monomer
1.854(5) 1.869(5) 1.428 1.398 1.397 1.374 1.362 1.393 1.380 1.384 1.466 1.465
1.868
2.092 2.189 1.425
2.060
1.418 1.396
1.418 1.401 1.391
1.393 1.377 1.445
1.378 1.424 1.451 1.478
Figure 3. UV−vis spectra of solid 1 and 2.
1.396 1.389
additional distinct band at 294 nm is observed. For spectra interpretation, a time-dependent DFT calculation of the first 10 transitions for monomeric 1 and monomeric and dimeric 2, as well as visualization of the boundary molecular orbitals at the PBE0 level, was carried out. The molecular orbital plots are given in Figure S1 in the SI. The results show that the low-energy bands of 1 and 2 (360 and 430 nm, respectively) correspond to the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) transition and the next one to the HOMO[−1]−LUMO one (Table 2), with all of the
1.457
2a), respectively. The normal modes with frequencies higher than 1100 cm−1, involving only the 1,2-N2C6H4 fragment, are of a heavily mixed origin, but they still feel the influence of E, slightly decreasing from Ge to Sn. As can be seen from Figure 2, the spectra of 2 and 2a differ significantly. The spectrum of the solid sample is more complicated; the lines involving the Sn−N bond stretches are notably shifted compared to those of 2a (608 → 618, 434 → 451, and 228 → 243 cm−1), and some other lines are split (see, e.g., the region of 1400−1500 cm−1). “Extra” features are observed in 2 that are absent in the solution spectrum; e.g., in the low-frequency region (100−200 cm−1), lines are present that correspond (according to the NCA results) to normal modes involving the N → Sn coordination bonds. Thus, it is evident from the Raman spectra that in the THF solution the dimer of 2 breaks up completely. To check the proposal, previously made in the literature,3 about possible coordination of monomeric 2a in the THF solution with the solvent molecules, we carried out energetic parameters and NCA calculations for a model adduct 2·THF. The results show that coordination of the stannylene molecule to THF is weak (the O → Sn distance is 2.70 Å; see also Table S5 in the SI) and does not appreciably change the frequencies of monomeric 2a. For both solids 1 and 2, the UV−vis absorption spectra were registered (Figure 3). The spectrum of 1 contains three absorption bands (360, 248, and 224 nm), while in the spectrum of 2, along with analogous bands at 430, 250, and 214 nm, an
Table 2. UV−Vis Absorption Band Positions and Their Assignment
1 (Ge) 2 (Sn)
λexp, nm
λcacl, nm
360 248 430 250
340 261 463 256
294
312
transition HOMO−LUMO HOMO[−1]-LUMO monomer HOMO−LUMO monomer HOMO[−1]− LUMO dimer HOMO[−1]− LUMO[+2]
type π−π* π−π* π−π* π−π* π−(N → Sn)*
transitions mentioned being of the π−π* type. The results also show that the absorption band of solid 2 at 294 nm, specific only to a dimer, corresponds to a transition from the HOMO[−1] to LUMO[+2] level; the latter includes the N → Sn coordination bond orbital. The results of quantum theory of atoms in molecules (QTAIM) analysis4 of dimer 2 based on the X-ray geometry3 are presented in Table S4 in the SI and Figure 4.
Figure 2. Raman spectra of melted 1, solid 2, and solution 2a in THF-d8. Asterisks indicate lines of THF-d8. B
DOI: 10.1021/acs.inorgchem.6b00572 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
In conclusion, our data evidenced that the solid-state germylene 1 is a monomer unlike stannylene 2, which is dimeric because of strong Sn ← N intermolecular coordination. Raman and NMR3 data show that in solution dissociation of 2 takes place, which is in accordance with the results of DFT calculations. These results also underline the usefulness of Raman spectroscopy for the investigation of aggregation patterns in both solution and solid state.
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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.inorgchem.6b00572. Experimental details, computational details, Tables S1− S5, and Figures S1−S3 (PDF) X-ray crystallographic data in CIF format for 1 (CIF) xyz coordinates (ZIP)
Figure 4. QTAIM molecular graph of dimer 2.
The molecular graph of dimeric 2 exhibits all of the expected critical points (CPs) and bond paths, including those for N → Sn coordination bonds. Their topological parameters, given in Table S4 in the SI, show that the latter interactions are of the intermediate type;4 the energy of each coordination bond, estimated according to the Espinoza phenomenological correlation,5 is 19 kcal/mol.6 Asymmetry of the molecule due to formation of the coordination dimer leads to different Sn−N, C−N, and C−C distances (Table 1) and thus to their different bond CP parameters (Table S4 in the SI). Energy characteristics of the coordination dimer formation of 1 and 2 were calculated as energy differences between the monomers and dimers and are presented in Table 3. It is seen
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*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.R.A. acknowledges a grant of the President of Russia for Young Scientists (Grant MK-5227.2015.3).
Table 3. Energetic Characteristics of Coordination Dimer Formation E 1 (Ge)
2 (Sn)
monomer dimer Δ, kcal/mol monomer dimer Δ, kcal/mol
E°, au
H°, au
G°, au
−2496.56167 −4993.11361 +6.1 −634.16857 −1268.34612 −5.6
−2496.55052 −4993.09038 +6.7 −634.15723 −1268.32327 −5.5
−2496.59742 −4993.16503 +18.7 −634.20506 −1268.39632 +8.6
Table 4. NICS Values (ppm) at the GIAO-B3LYP Level 1 (Ge) 2a (Sn) a
FiPC-NICSa
−5.40 −5.14
−5.37 −5.13
REFERENCES
(1) Zabula, A. V.; Hahn, F. E. Eur. J. Inorg. Chem. 2008, 2008, 5165− 5179. (2) Meller, A.; Pfeiffer, J.; Noltemeyer, M. Z. Anorg. Allg. Chem. 1989, 572, 145−150. (3) Hahn, F. E.; Wittenbecher, L.; Le Van, D.; Zabula, A. V. Inorg. Chem. 2007, 46, 7662−7667. (4) Bader, R. F. W. Atoms in Molecules. A Quantum Theory; Clarendon Press: Oxford, U.K., 1990. (5) (a) Espinosa, E.; Molins, E.; Lecomte, C. Chem. Phys. Lett. 1998, 285, 170−173. (b) Espinosa, E.; Alkorta, I.; Rozas, I.; Elguero, J.; Molins, E. Chem. Phys. Lett. 2001, 336, 457−461. (6) It is notable that the intermolecular coordination energy (ICE) value, estimated according to ref 5, is an “intrinsic” coordination bond energy; it does not include molecular relaxation (in our case, the latter involves N-atom pyramidalization and other molecular rearrangements). That is why the ICE value differs from the thermodynamic parameters presented in Table 3. (7) (a) Lee, V. Y.; Sekiguchi, A. Organometallic Compounds of LowCoordinate Si, Ge, Sn and Pb: From Phantom Species to Stable Compounds; John Wiley & Sons, Ltd: Chichester, U.K., 2010. (b) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem. Rev. 2009, 109, 3479. (c) Herndon, J. W. Coord. Chem. Rev. 2000, 206, 237. (8) (a) Stanger, A. J. Org. Chem. 2006, 71, 883−893. (b) Stanger, A. J. Org. Chem. 2010, 75, 2281−2288. (c) Gershoni-Poranne, R.; Stanger, A. Chem. - Eur. J. 2014, 20, 5673−5688. (9) Torres-Vega, J. J.; Vasquez-Espinal, A.; Caballero, J.; Valenzuela, M. L.; Alvarez-Thon, L.; Osorio, E.; Tiznado, W. Inorg. Chem. 2014, 53, 3579−3585.
that 1 does not want to exist as a dimer (positive value of the enthalpy of dimer formation). The data for 2 show that this enthalpy is negative, while the entropy factor leads to a positive value of the free Gibbs energy, thus favoring dimer dissociation in solutions [which is actually confirmed by the Raman (2a) and NMR3 spectra]. The tendency of stannylenes to form coordination dimers, unlike germylenes, is typical and was observed earlier for many cases.7 To evaluate the aromaticity of 1 and 2a, the nucleusindependent chemical shift (NICS) values for their fivemembered heterocycles were calculated according to the Stanger8 and Torres-Vega et al.9 refinements. The negative NICS values (Table 4) and the shapes of the curves obtained (see Figures S2 and S3 in the SI) confirm the aromaticity of the compounds studied with their delocalized 10 π-electron systems.
min NICSout‑of‑plane
AUTHOR INFORMATION
Corresponding Authors
“Free of in-plane component” NICS according to ref 9. C
DOI: 10.1021/acs.inorgchem.6b00572 Inorg. Chem. XXXX, XXX, XXX−XXX
