Article pubs.acs.org/Organometallics
Apicophilicity versus Hydrogen Bonding. Intramolecular Coordination and Hydrogen Bonds in N‑[(Hydroxydimethylsilyl)methyl]-N,N′‑propyleneurea and Its Hydrochloride. DFT and FT-IR Study and QTAIM and NBO Analysis Nina N. Chipanina, Nataliya F. Lazareva, Tamara N. Aksamentova, Alexey Yu. Nikonov, and Bagrat A. Shainyan* A. E. Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences, 1 Favorskii Street, Irkutsk 664033, Russia S Supporting Information *
ABSTRACT: Conformers of N-[(hydroxydimethylsilyl)methyl]N,N′-propyleneurea (1) and their hydrochlorides (2) with HCl coordinated to different basic sites have been studied experimentally by FT-IR and theoretically using the density functional theory (DFT) method at the B3LYP/6-311+G(d,p) and M06/6311+G(d,p) levels of theory. The structures of silanols 1 and 2 are determined by the balance of two competing effects: namely, intramolecular CO→Si coordination and intramolecular C O···H−O or intermolecular X···H−Cl hydrogen bonding. The preferred conformation of silanol 1 is that with an equatorial hydroxyl group, in apparent contradiction with the apicophilicity rule. In the crystal, silanol 1 exists as a conformer with a bifurcated bond composed of a weak CO→Si coordinated bond and a substantially more strong CO···H−O hydrogen bond. From the NBO analysis, the energies of the nO → σ*Si−X and nO → σ*H−O orbital interactions responsible for the formation of the coordination and hydrogen bonds, as well as the lengths of these bonds, change in opposite directions. In solution the equilibrium is shifted toward the conformer having only the hydrogen bond and no coordination bond. Its hydrochloride 2 exists in the crystal as a conformer with the axial OH group coordinated to HCl, whereas in solution it appears to be in equilibrium with a conformer having the equatorial OH group, in which a four-centered bifurcated bond is formed by two intramolecular components CO→Si and CO···HO and one intermolecular component CO···HCl. The QTAIM analysis showed the O→Si coordination bonds in the studied compounds to fall in the range from partially covalent and weak donor−acceptor to mainly electrostatic in nature and the hydrogen bonds to vary from weak to medium in energy.
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INTRODUCTION Silanols having one or several Si−OH groups have been actively studied (see, for example, reviews1 and references therein). The interest in these compounds is based on several reasons. First, Si−OH functional groups are widespread in nature as reactive sites on the surface of silicate rocks;2 second, silicic acid (Si(OH)4) and its derivatives are found in low concentration in the aqueous environment;2 finally, silanol groups are involved in the processes of biosilicification in leaving organisms.3 Silanols are widely used in organic synthesis as building blocks and catalysts4 as well as in polymer materials chemistry.1,5 Silanol groups are the key structural unit in zeolites and silicates, which determine their surface and catalytic properties.6 One of the most important features of the silanol groups is their high acidity1a,c,7 coupled with the relatively high basicity.1a,c,7a Note that when the silanol group is involved in hydrogen bonding as an H donor the basicity of its oxygen is strongly enhanced relative to the free silanol group.8 These two © XXXX American Chemical Society
fundamental properties provide the formation of strong intraand intermolecular hydrogen bonds both between the silanol molecules and with a large variety of organic molecules. Intramolecular hydrogen bonds in silanols have been studied by X-ray,9 Raman,10 IR,9d,f,i,11,12 and NMR spectroscopy,9b,g,i as well as quantum chemistry.8,9i,k,11,13 NMR, FT-IR, and theoretical studies of various types of geminal silsesquioxane silanols showed that the formation of intra- and intermolecular hydrogen bonds O−H···O depends on both the medium effects and the steric and electronic effects of the substituents at silicon.9i Aromatic silanediols stabilized by a mesityl group at silicon and having the amide group in a position ortho to the MesSi(OH)2 group are capable both of intermolecular hydrogen bonding (X-ray, NMR) via Si−O−H···O(H)−Si H Received: April 1, 2014
A
dx.doi.org/10.1021/om500349s | Organometallics XXXX, XXX, XXX−XXX
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Scheme 2. Hydrolysis of N[(Chlorodimethylsilyl)methyl]lactams and -carboxamides
bonds and of intramolecular hydrogen bonding with the Si− O−H···OC hydrogen bond closing the seven-membered ring (NMR).9a The methods used provide evidence for the conformational rigidity and binding properties on the basis of the properties of the studied compounds, such as hybridization, sterics, and the H-bonded ring size that affect intra- and intermolecular hydrogen bonding. Among numerous silanols having intramolecular hydrogen bonds, we have found only two (A and B; Scheme 1) with pentacoordinate silicon.
Scheme 3. Resonance Structures of Protonated Hypervalent Silanols
Scheme 1. Intramolecularly H Bonded Silanols with Pentacoordinate Silicon
new representative of protonated silanols.16 The existence of intramolecular coordination bond CO→Si in molecule 2 was proven by an X-ray study in the crystal and by 29Si NMR spectra in solution. As for other silanol hydrochlorides, compound 2 was synthesized by mild hydrolysis of N[(chlorodimethylsilyl)methyl]-N,N′-propyleneurea with air moisture. The hydrolysis stops at the stage of formation of salt 2, which is formed in close to quantitative yield and is stable enough to be stored in air for a long period. This is surprising, because usually the hydrolysis of chlorosilanes, especially in the absence of HCl scavengers, gives substantial amounts of the products of condensation of silanolsdi- and polysiloxanes. The crystal structure of compound 2 showed that the chlorine atom in the crystal is not bonded directly to the silicon but, rather, is H-bonded to the OH2 moiety, resulting in the formation of a specific crystal structure with the supramolecular layers linked by OH···Cl and NH···Cl hydrogen bonds. It can be assumed that the stability of compound 2 is due to intermolecular hydrogen bonds. Compound 1 is also stable and capable of forming an intramolecular hydrogen bond. Its structure was proved by multinuclear NMR spectroscopy. The 29Si chemical shifts in 1 and 2 are rather similar (2.09 and 4.89 ppm, respectively) and are both shifted upfield in comparison to that of trimethylsilanol (12.57 ppm),17 suggesting the presence of a weak intramolecular CO→Si coordination bond. As was mentioned above, the intramolecular N→Si coordination bond in molecule B is substantially weakened upon the formation of an intramolecular hydrogen bond.9k Except for compound B having a specific structure, there have been no other studies of the effect of intramolecular hydrogen bonding on the pentacoordination of the silicon atom or of silanols having intramolecular coordination and hydrogen bonds in the same coordination motif. The formation of the CO→Si intramolecular coordination bond in molecules 1 and 2 in the conformers with the hydroxyl group in the axial position of the silicon trigonal bipyramid is in compliance with the apicophilicity rule. On the other hand, the structure with the equatorial location of the hydroxyl group could be stabilized by the formation of a CO···H−OSi intramolecular hydrogen bond. Therefore, intramolecular hydrogen bonding with the silanol group in molecules 1 and 2 competes with the a priori preferred apical location of this group. Thus, the question of whether the silicon atom in the solid compound 1 as well as in the isolated molecules of 1 and 2, or in their solutions, is tetra- or pentacoordinate remains open. In addition, the basic sites in these moleculesthe
Oxobis{[(dioxythiobis(4-methyl-6-tert-butyl-o-phenylene))dioxy]hydroxysilane} (A) was the first silanol with pentacoordinate silicon, in which the intramolecular hydrogen bond was proven in the solid state by an X-ray study.9j The siloxane moiety in A is linear with the SiOSi angle equal to 180°. The authors believe that this is due to the presence of two symmetrical SO···H−OSi hydrogen bonds with an O···O distance of 2.899(3) Å. The second recently synthesized compound is 5-(10-benzo[h]quinolyl)-5-hydroxydibenzo[b,f ]silepin (B), in which the nitrogen atom is involved both in hydrogen bonding with the silanol group and in pentacoordination with the silicon atom.9k The most important feature of structure B is the equatorial location of the silanol hydroxyl group, which should be considered to be a violation of the apicophilicity rule. Note that in the Si−F analogue of silanol B the fluorine atom is apical, with the NSiF angle close to linear.9k As a result, the N→Si coordination bond in B is much longer (2.818 Å) (and, hence, weaker) than that in the Si−F analogue (2.341 Å) and the sum of the angles in the equatorial plane in B is smaller (341.55° versus 353.26°).9k N-[(Chlorodimethylsilyl)methyl]lactams, -amides, and -imides are typical and well-studied compounds of pentacoordinate silicon with the coordination unit SiC3OCl (see, for example, reviews14 and references therein). The Si−Cl bond in these compounds is very labile, allowing synthesis of a wide variety of compounds with the coordination unit SiC3OX (X = Hal, OAlk, OAr, OTf, 0.5O(siloxanes)). One of the most interesting and least studied reactions of these compounds is hydrolysis (Scheme 2), which easily occurs even by air moisture in the absence of HCl scavengers and leads to silanol hydrochloridesa new class of compounds of pentacoordinate silicon.15 Pentacoordination of silicon in these compounds was proven by X-ray and multinuclear NMR studies. A theoretical study of the cation [PhC(O)NHCH2SiMe2(OH2)]+ showed that it is stable only in the crystal and should be considered as the waterstabilized silyl cation C rather than the silyloxonium ion D (Scheme 3).15b Recently, we have synthesized N-[(hydroxydimethylsilyl)methyl]-N,N′-propyleneurea (1) and its hydrochloride (2) as a B
dx.doi.org/10.1021/om500349s | Organometallics XXXX, XXX, XXX−XXX
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1a to be more stable energetically, while 1b has lower free energy and, hence, is more stable chemically. Conformer 1c having two apical oxygen atoms is the next most stable, showing that the effect of H bonding in 1a,b is more important than the energy gain due to the apicophilicity rule. Note that the M06 method gives a ΔG value almost twice as low as the ΔE value. Interestingly, conformer 1c, being substantially more stable than the least stable conformer 1d, has a longer O···Si distance, suggesting that the effect of electronegativity of the equatorial oxygen atom in 1d increasing the positive charge on silicon is more important for the O→Si−X coordination than the apical location of such a poor leaving group as hydroxyl in 1c. The degree of pentacoordination of the silicon atom was shown to increase in the order X = Me ≪ 0.5O, AlkO < PhO, F < PhCOO < MeCOO < C6F5O < PhSO2O < MeSO2O < Cl < CF3COO ≪ Br ≪ TfO < I.14d This proves that nucleofugality of X plays the determining role and electronegativity is of less importance. Although 1c is ∼3 kcal/mol less stable in comparison to 1a,b, the O→Si coordination bond in 1c is 0.2 Å shorter than in 1a, and the bond angle COSi increases to 102° as compared to 98 and 87° in 1a,b. Earlier, the analysis of a large set of experimental and theoretical data on compounds with an intramolecular CO→Si bond has shown that the optimal value of the COSi angle in five-membered heterocycles closed by this bond is 116°,23 and in the crystal of compound 2 it is equal to 115°.16 The Si−Cax bond in 1d is 0.03 Å longer than the Si−Ceq bonds or the Si−C bonds with a tetrahedral silicon atom in 1e. Since the reasons for existence of conformers 1a,b as separate minima on the potential energy surface, although proved by both DFT methods (Table 1), are not evident, we have analyzed their interconversion by scanning the O···Si distance with all other parameters being optimized. The curve obtained by the B3LYP method (Figure 2a) shows the absence of a noticeable barrier between 1a,b, while the M06 method gives two clearly separated minima (Figure 2b). Their existence is the result of competing between the CO→Si coordination and intramolecular CO···H−O H bonding. To analyze the nature of the intramolecular interactions in conformers 1a−e and in the reference molecules 3, 4, and 6, a QTAIM analysis24 was performed (Table 2), which was successfully used earlier to study bifurcated hydrogen bonds in complexes with Lewis acids and bases as well as coordination bonds in SiF3-substituted derivatives of imides.21,25 Bond critical points (BCPs) in the molecules were found and their topological properties determined: the electron density ρ(rc), the Laplacian of electron density ∇2ρ(rc), and the total energy densities H(rc). The energies of interaction (E) were calculated by the equation18c,d,26
carbonyl oxygen, the amide nitrogen, and the silanol oxygen atomscan participate in the formation of intermolecular hydrogen bonds. The goal of the present study is to examine the role of intra- and intermolecular hydrogen bonds in the stabilization of conformational isomers of N[(hydroxydimethylsilyl)methyl]-N,N′-propyleneurea (1) and N-[(hydroxydimethylsilyl)methyl]-N,N′-propyleneurea hydrochloride (2) and the effect of these bonds on the intramolecular CO→Si coordination interaction (Scheme 4). For this, DFT calculations together with QTAIM and NBO analysis, along with FT-IR spectroscopy, were employed. For comparison, compounds 3−6 were used as reference structures. Scheme 4. Studied Silanols and Related and Model Molecules
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RESULTS AND DISCUSSION Theoretical Calculations: Geometry, Energies, QTAIM, NBO Analysis, Vibrational Modes. Conformational Isomers of N-[(Hydroxydimethylsilyl)methyl]-N,N′-propyleneurea (1). The six conformers 1a−e (Figure 1) were located as minima on the potential energy surface of molecule 1. Conformers 1a,b have the CO···H−O intramolecular hydrogen bond closing the seven-membered ring and are very close in energy (Table 1). The O···Si distance and the length of the O−H···OC hydrogen bond in conformers 1a,b vary in opposite directions (Figure 1). This is typical for three-centered bifurcated H bonds formed by an oxygen atom and is a demonstration of cooperative effects18 leading to strengthening of one component (O−H···OC) at the expense of the other (CO→Si). This allows to conclude that in 1a,b we have a rare case of a bifurcated bond with one component represented by a hydrogen bond and the other one by a coordination bond, both being intramolecular. The carbonyl oxygen of urea and its derivatives is known to form bifurcated hydrogen bonds and to stabilize specific conformations19 as well as to result in regioselective H-bonded packing via self-sorting by recognition between N−H···O(S) and O−H···O intermolecular hydrogen bonds.20 The CO···H bond angles in 1a,b are equal to 140 and 125°, respectively, which favors the formation of a hydrogen bond because it is much larger than the critical value of 72−80° found for the five- and six-membered rings closed by H bonds.21 The O···Si distances are 2.908 (1a) and 3.298 Å (1b), that is, less than the sum of the van der Waals (vdW) radii (3.62 Å),22 suggesting, at least for 1a, CO→Si coordination in the structure with the electronegative substituent in the equatorial plane of the silicon trigonal bipyramid. From the analysis of Table 1, the following conclusions can be drawn. The most stable conformers are 1a,b, stabilized by the intramolecular hydrogen bond. Both DFT methods show
E = 1/2Vc
Vc = 1/4∇2 ρ(rc) − 2Gc
where Gc is the local kinetic electron energy density. Although this equation is an approximation,18d it is widely used for estimation of the energy of interaction in the QTAIM analysis. BCPs between the oxygen and silicon atoms were found in conformers 1c,d having no intramolecular hydrogen bonds. The BCPs of the O→Si bond are characterized by low values of ρ(rc) (∼0.02 au) and H(rc) (−0.0005 and 0.0006 au) and by low positive values of ∇2ρ(rc) (0.06 and 0.05 au), typical for weak donor−acceptor interactions, with energies (E) of 4.2 and 4.3 kcal/mol. The O→Si bond in 1c with H(rc) < 0 can be considered as partially covalent. No BCPs corresponding to the C
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Figure 1. Molecular structure of the conformers of silanol 1 and N-[(chlorodimethylsilyl)methyl]-N′-(trimethylsilyl)-N,N′-propyleneurea (6).
O→Si interaction were found in conformers 1a,b having H bonds, which may be (though not necessarilysee NBO Analysis) indicative of electrostatic character of this interaction. The energies of the hydrogen bonds obtained from QTAIM analysis can be defined as weak for 1a (5.6 kcal/mol) and medium for 1b (7.4 kcal/mol). No BCP was found between the oxygen and silicon atoms in molecule 3, having a trimethylsilyl group. However, the O→Si distance in 3 (2.961 Å) is much less than the sum of the vdW radii; the interaction between the oxygen and silicon atoms is also electrostatic in nature, leading to a slight elongation by 0.01 Å of the axial C−Si bond relative to the equatorial bonds. A large contribution of electrostatic interaction in the formation of the intramolecular O→Si bond
was noted in a theoretical study of a wide series of pentacoordinate silicon compounds (see ref 23 and references therein). The relationship between geometrical parameters of the silicon trigonal bipyramid in the studied molecules is also consistent with electrostatic interactions which play, if not a predominant, at least an important role. Variation of the stretching frequencies of the CO and OH groups in conformers 1a−e with respect to the reference compounds 3−5 provides an independent criterion of the degree of involvement of these groups in various intramolecular interactions. The calculated value of 1758 cm−1 of ν(CO) for the “free” carbonyl group in urea 4 decreases to 1718 cm−1 in 3 and 1e, in which it is not involved in coordination with the D
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and 2e-C, complexes with HCl coordinated to the nitrogen atom of the NCH2SiOH moiety in conformers 1a,e (Figure 3, Table 3). An X-ray study of compound 216 proved the structure of conformer 2c (2c-A), in which the silanol OH group is in the axial position, being coordinated to HCl. The different structure of compound 1, with the silanol OH group in the equatorial position, and its hydrochloride 2, having it in the axial position, is rationalized by the increase of nucleofugality of H2O+ with respect to OH as a leaving group, bearing in mind that it is the nucleofugality of X that plays a decisive role in pentacoordination of Si−X-containing molecules. The distance between the silicon and the carbonyl oxygen of 1.865 Å allows us to characterize the O→Si coordination bond as weak.62 The optimized O→Si distance in the isolated complex 2c-A (2.459 Å) is shortened by 0.253 Å with respect to the starting silanol 1c, while the axial Si−OH bond is elongated by 0.048 Å. Correspondingly, the calculated ν(CO) and ν(OH) frequencies in 2c-A become ∼20 cm−1 lower than in 1c. Nevertheless, from the QTAIM analysis, the O→Si bond still is a weak donor−acceptor bond, although its energy E is increased to 6.61 kcal/mol. Coordination of HCl to the equatorial OH group in complex 2d-A with an O→Si bond energy E of 6.14 kcal/mol has a smaller effect on the surroundings of the silicon atom. The O→Si and Si−OH bonds are respectively shortened and elongated by 0.168 and 0.025 Å with respect to 2d, although the ν(CO) and ν(OH) frequencies in 1d are as sensitive to coordination with HCl as in 1c, being reduced also by ∼20 cm−1. In complex 2a-A, coordination of HCl to the OH group involved in the intramolecular H bond results in its shortening by 0.1 Å with the simultaneous elongation of the equatorial O−Si bond by 0.045 Å relative to 1a, while the O→Si distance remains intact and its energy increases to 7.09 kcal/mol. The interaction of HCl with the carbonyl oxygen in conformer 2a-B results in the formation of a four-centered bifurcated bond and in a negative cooperative effect, as witnessed by elongation of the two intramolecular components: O→Si by 0.036 Å and OH···OC by 0.104 Å. Finally, coordination of HCl to the nitrogen atom of the NCH2SiOH moiety in conformer 2a-C causes elongation of the intramolecular H bond by 0.057 Å and shortening of the O→Si bond by 0.049 Å, thus demonstrating a competitive effect of strengthening of the coordination interaction and weakening of the hydrogen bonding. A power dependence between the energies and the lengths of the intramolecular Si→O coordination and O−H···OC hydrogen bonds for a series of hydrochlorides and silanols is plotted in Figure 4. An analysis of the plot shows that the most stable complexes are those with HCl coordinated to the carbonyl oxygen atom. Its protonation (EH···OC = 152.18 kcal/ mol) occurs in complex 2e-B, in which the oxygen atom is not involved in any intramolecular interaction. The lowest values, EH···N ≈ 7 kcal/mol, correspond to coordination of HCl to nitrogen in complexes 2a-C, 2e-C, and 3·HCl-C; they are practically independent of the structure of the base. The weakest coordination of HCl with the hydroxyl oxygen is observed in the complex with trimethylsilanol 5·HCl-A (EH···OH = 9.94 kcal/mol) and in complex 2e-A (EH···OH = 10.64 kcal/ mol), lacking intramolecular bonds. The value of EH···OH is increased to 12.05 kcal/mol in complex 2a-A, where the OH group is involved in the O−H···OC bond, which enhances the basicity of the oxygen atom. The highest basicity of the hydroxyl oxygen atom is observed for the axial OH group in
Table 1. B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) (in Italics) Relative Energies (ΔE, kcal/mol) and Free Energies (ΔG, kcal/mol) of Conformers 1a−e molecule
ΔE
ΔE(ZPE)a
ΔG
1a
0 0 0.45 0.46 2.92 2.91 7.69 4.31
0 0 0.38 0.74 2.59 2.41 6.98 3.58
0.30 0.02 0 0 2.70 1.58 6.42 2.85
1b 1c 1d 1e a
ZPE corrected.
Figure 2. B3LYP/6-311+G(d,p) (a) and M06/6-311+G(d,p) (b) energy profiles for scanning the O···Si distance in conformers 1a,b.
silicon atom or the hydroxyl group; this is due to the donor effect of the SiMe3 group in 3 or the SiMe2OH group in 1e. However, the same value of 1718 cm−1 was calculated for ν(CO) in conformer 1d in spite of it having the shortest O→ Si distance (Figure 1). Therefore, electrostatic interaction between the carbonyl oxygen and the silicon atom, while affecting the geometrical parameters of the coordination unit, has a small, if any, effect on the calculated ν(CO) frequencies. In conformer 1c, having the axial OH group at silicon and a reduced O→Si contact, the value of ν(CO) is lowered to 1709 cm−1. In the most stable conformers 1a,b, having the silanol OH group equatorial, the decrease of the ν(CO) frequencies to 1706 cm−1 for 1a and to 1698 cm−1 for 1b is completely due to their involvement in intramolecular hydrogen bonding with the carbonyl group and is parallel to their strength (Table 2). The same is true for the stretching vibrations of the hydroxyl group. The ν(OH) band at 3902 cm−1 in trimethylsilanol 5 and conformer 1e remains practically intact in conformers 1c,d but sharply drops to 3796 and 3654 cm−1 in conformers 1a,b, respectively. The lowest values of ν(OH) and ν(CO) in conformer 1b having the strongest hydrogen bond were also obtained from M06 calculations. N-[(Hydroxydimethylsilyl)methyl]-N,N′-propyleneurea Hydrochloride (2). The basic sites in molecule 1 (SiOH, CO, NH) can participate in intermolecular hydrogen bonding with HCl to form various silanol hydrochlorides 2. We have studied theoretically the following conformers: 2a-A and 2c-A−2e-A, complexes with HCl coordinated to the silanol oxygen in conformers 1a,c−e; 2a-B and 2e-B, complexes with HCl coordinated to the carbonyl oxygen in conformers 1a,e; 2a-C E
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Table 2. Calculated Frequencies ν(CO) and ν(OH) (cm−1), Bond Lengths (l, Å), BCP Properties (ρ(rc), ∇2ρ(rc), H(rc); au), and Energies (E, kcal/mol) of Intra- and Intermolecular Hydrogen Bonds and Intramolecular Coordination Bonds CO→Si in Conformers 1a−e and in Complexes with HCl (2a-A−2a-C, 2c-A, 2d-A, 2e-A−2e-C, 4-B, and 4-C)a molecule 1a
ν(CO)
ν(OH)
bond
l
ρ(rc)
∇2ρ(rc)
H(rc)
E
1706 1756
3796 3883
CO···H−O
2.040 1.970 2.058 2.908 2.873 2.826 1.896 1.950 3.298 3.324 2.712 2.590 2.630 2.662 2.333 2.028 1.941 1.976 2.909 2.846 1.712 2.144 2.944 1.601 2.097 2.859 1.920 2.459 2.442 1.646 1.729 2.494 1.738 1.753 1.079 2.181 1.937 1.544 1.921
0.022
0.084
0.0016
5.63
0.028
0.102
−0.0009
7.43
0.019
0.057
−0.0005
4.20
0.021 0.035
0.050 0.049
0.0006 0.0085
4.33 9.17
0.026
0.102
−0.0015
7.09
0.042 0.018
0.132 0.068
0.0027 −0.0014
12.05 4.50
0.059 0.020
0.136 0.077
0.0116 −0.0016
17.93 5.03
0.034 0.028
0.080 0.052
0.0021 0.0041
7.63 6.61
0.051
0.138
0.0070
15.25
0.027 0.040 0.039 0.242 0.028 0.034 0.068 0.034
0.050 0.124 0.124 −1.327 0.073 0.076 0.135 0.081
0.0035 0.0021 0.0015 0.4084 0.0005 0.0020 0.018 0.002
6.14 11.10 10.64 152.18 6.03 7.26 21.62 7.44
CO→Si
1b
1698 1744
3654 3762
CO···H−O CO···Si
1c
1709 1755
3904 3984
CO→Si
1d 6
1721 1624
3896
CO→Si CO→Si
2a-A
1701 1750
3720 3821
CO···H−O CO→Si
2a-B
1644
3842
2a-C
1724
3829
2c-A
1692 1737
3882 3968
H−O···H−Cl CO···H−O CO→Si CO···H−Cl CO···H−O CO→Si N···H−Cl CO→Si H−O···H−Cl
2d-A
1704
3869
2e-A 2e-B
1716 1641
3879 3895
2e-C 4-B 4-C
1747 1655 1773
3898
CO→Si H−O···H−Cl H−O···H−Cl CO···H−Cl N−H···Cl−H N···H−Cl CO···H−Cl N···H−Cl
a
See Figure 3. Legend for values: Roman type, B3LYP/6-311+G(d,p); boldface type, SCRF/B3LYP/6-311+G(d,p) calculations with DMSO as a solvent (ε 46.7); italic type, M06/6-311+G(d,p).
a wide series of compounds.28 Calculations of the isolated molecule 6 give an O→Si distance equal to 2.333 Å, which is ∼0.4 Å larger than the distances measured experimentally by Xray diffraction (1.905 and 1.923 Å).16 The energy of the O→Si coordination bond in the gas phase is 9.17 kcal/mol (Table 2), and according to the QTAIM analysis this bond is a weak donor−acceptor. In polar DMSO, the O→Si distance is reduced to 2.028 Å, exceeding the experimental value by as little as ∼0.1 Å. The O→Si coordination bond in molecule 1c is also strengthened in solution, being shortened from 2.712 Å in the gas phase to 2.590 Å in DMSO. A higher sensitivity of the O→Si bond in molecule 6 in comparison to that in molecule 1 to the polarity of the medium is due to a greater degree of pentacoordination at silicon in the former, which in turn results from the axial location of chlorine, as a good leaving group, in contrast to 1, having the more sluggish leaving group OH in the
complex 2c-A, in which the value of EH···OH reaches 15.25 kcal/ mol due to charge transfer via pentacoordination in the C O→Si−OH moiety. Intramolecular hydrogen bonds in the conformers of molecules 1 and 2 also fall on the curve in Figure 4, lying on the lowest part of the plot with an energy of 4−7 kcal/mol. The calculated O→Si distance in the isolated complex 2c-A is 0.6 Å longer than that measured experimentally by X-ray diffraction. Assuming that this discrepancy may be due to the difference between the gas and condensed states, we performed calculations of compounds 1 and 2 and the model compound 6 using the self-consistent reactive field (SCRF) and DMSO as solvent (ε = 46.7).27 The effect of polarity of the solvent on the strength of the O→Si bond and the frequencies of the stretching vibrations of the axial bonds in the silicon trigonal bipyramid was studied both theoretically and experimentally on F
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Figure 3. Molecular structure of the conformers of hydrochloride 2.
NBO Analysis. To assess the degree of the donor−acceptor interaction of the lone electron pairs (LP) of the carbonyl oxygen and the Si−O, Si−C, and H−O bonds of the studied molecules, an NBO analysis was performed. The second-order perturbation energies E(2) corresponding to the most important donor → acceptor charge transfer orbital interactions, nO → σ*Si−O, nO → σ*Si−C, and nO → σ*H−O, were determined to characterize the strength of the coordination and hydrogen bonds (Table 4).
equatorial position. In molecule 1a, both components of the bifurcated bondthe coordination bond and the H bond become stronger with an increase of polarity of the medium being shortened by 0.04 (O→Si) and 0.07 Å (O−H···OC). Distinct from that, geometry optimization of complex 2c-A in DMSO as a solvent results in strengthening of the O→Si coordination to the covalent O−Si bond with simultaneous elimination of a water molecule and formation of cation 1c+ (Scheme 5). G
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the nO → σ*Si−Cax and nO → σ*Si−Oax interactions and the length of the coordination bond O→Si (Figure 5). The fact that the point for conformer 1a lies on the same curve is an additional argument for the existence of a coordination interaction in 1a. Note that only points corresponding to a weak O→Si interaction fall on the curve. A large deviation of point 6 from the curve in Figure 5 is due to the aforementioned weakening of the nO → σ*Si−X interaction for strong pentacoordination in the systems with good leaving groups X. Along with axial bonds, the equatorial Si−O and Si−C bonds contribute to the interaction with the lone pairs of the carbonyl oxygen, although the corresponding E(2) values are much smaller. In molecule 1a, there is a CO···H−O hydrogen bond, the O→Si bond has electrostatic nature, and the energy of the nO → σ*Si‑Cax interaction is only 1.85 kcal/mol and is accompanied by weak interactions with the equatorial bonds nO → σ*Si−Oeq (1.27 kcal/mol) and nO → σ*Si−Ceq (0.54 kcal/ mol). In hydrochloride 2a-C, coordination of HCl to the nitrogen atom must decrease the donor ability of the carbonyl oxygen. Actually, the nO → σ*H−O interaction responsible for the H bonding becomes ∼2 kcal/mol weaker and the nO → σ*Si−Cax responsible for the pentacoordination becomes ∼0.6 kcal/mol stronger relative to 1a (Table 4). This is an indication of the formation of a bifurcated bond with the components with oppositely varying properties. Conformer 1b, having an energy of the nO → σ*H−O interaction of 7.03 kcal/mol, has no nO→σ*Si−Cax interaction and, hence, no even electrostatic coordination, which is consistent with the O···Si distance being almost equal to the sum of the vdW radii. Note that there is a qualitative correlation of the calculated degree of pentacoordination of the silicon atom (ηα)29 with the E(2) values determined by the NBO analysis (Table 4), as well as with the Si→O bond energies from the QTAIM analysis. The largest degree of pentacoordination is observed in molecules 6 (62%) and 2c-A (52%), having electronegative groups in axial positions. Conformers 1a,c,d and hydrochlorides 2a-C and 2d-A with a low degree of pentacoordination (ηα < 40%), according to the suggested classification,30 can be assigned to pseudochelate structures. The minimal value of ηα, equal to 3%, in conformer 1b proves the tetrahedral arrangement of the silicon atom in it. FT-IR Spectra. In the FT-IR spectrum of crystalline hydrochloride 2 (2c-A), having an intramolecular coordination O→Si bon,d16 the O−H+ stretching vibrations appear as a wide weak band with a maximum at 2210 cm−1 (Figure S1, Supporting Information). Three weak bands at 3232, 3195, and 3165 cm−1 are overtones, and there are compound tones of the intense doublet band of mixed vibrations ν(CO), δ(NH) (1625 cm−1) and ν(CN), δ(CH2) (1589 cm−1). These weak bands are overlapped with an intense wide band in the range 3400−3200 cm−1 caused by the NH and OH stretching vibrations of hydrochloride 2 participating in the formation of its self-associates having NH···Cl and OH···Cl intermolecular hydrogen bonds observed in the X-ray diffraction pattern.16 The spectrum of hydrochloride 2 in methylene chloride is quite different (Figure S2, Supporting Information). First, weak bands appear at 3685 cm−1 (ν(OH)) and 3445 cm−1 (ν(NH)), belonging to stretching vibrations of free OH groups in silanols7a,31 and NH in cyclic ureas,32 respectively. Their intensity increases with dilution, suggesting an increased amount of molecules formed by dissociation of hydrochloride 2 and its self-associates (Figure S3, Supporting Information). The ν(O−H+) band in solution is more intense than that in the
Table 3. B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) (in Italics) Relative Energies and Free Energies (ΔE, ΔE(ZPE), ΔG, kcal/mol) for Hydrochlorides 2a-A−2a-C, 2c-A, 2d-A, and 2e-A−2e-Ca molecule
ΔE
ΔE(ZPE)
ΔG
2a-A
0 0 0.204 5.857 2.560 1.840 6.794 5.949 0.545 11.704
0.003 0 0 5.956 2.223 2.032 6.489 5.146 1.307 11.152
0 0 0.484 7.465 2.258 2.930 7.073 4.370 2.180 11.941
2a-B 2a-C 2c-A 2d-A 2e-A 2e-B 2e-C a
See Figure 3.
Figure 4. CO···HO (boldface Roman type) and CO···HCl, H− O···HCl, and N···HCl (boldface italics) bond energies as a function of their length (Å).
Scheme 5. Elimination of Water Molecule and Chloride Ion from 2c-A Leading to Cation 1c+
The largest values (12.81 and 8.87 kcal/mol) correspond to the nO → σ*Si−O and nO → σ*Si−C interactions with the axial Si−O and Si−C bonds in conformers 2c-A and 2d-A, in which HCl is coordinated to the axial or equatorial silanol hydroxyl group. These values exceed the energy of the nO → σ*Si−Cl interaction in molecule 6 (7.04 kcal/mol), which, according to the QTAIM analysis, has the strongest O→Si bond. This apparent contradiction is due to different meanings of the energy of orbital interaction leading to the formation of the coordination bond and the binding energy of this bond estimated from the BCP properties. Indeed, the stronger the O→Si coordination bond, the weaker is the Si−X covalent bond with the second axial ligand and hence the higher the σ*Si−X orbital and the smaller the nO → σ*Si−X interaction. A power dependence similar to that mentioned earlier between EH···X and lH···X in Figure 4 is observed between the energies of H
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Table 4. B3LYP/6-311+G** Calculated Intramolecular Bond Lengths (l, Å), Second-Order Perturbation Energies (E(2), kcal/ mol), and Degrees of Pentacoordination of the Silicon Atom (ηα, %) in Conformers of Compounds 1 and 2 and in N[(Chlorodimethylsilyl)methyl]-N′-(trimethylsilyl)-N,N′-propyleneurea (6) E(2) molecule 1a 1b 1c 1d 3 6 2a-C 2c-A 2d-A a
bond
l
CO···H−O CO→Si CO···H−O CO···Si CO→Si CO→Si CO→Si CO→Si CO···H−O CO→Si CO→Si CO→Si
2.040 2.908 1.896 3.298 2.712 2.662 2.961 2.333 2.097 2.859 2.459 2.494
nO → σ*Si‑Cax
nO → σ*Si‑Oax
1.85
nO → σ*Si‑Oeq/nO → σ*Si‑Ceq
nO → σ*H−O
ηα, %
2.80
15
7.03
3
0.87
35 21 18 62 18
1.27/0.54
5.02 5.11 2.18 7.04a 2.46
−/1.29, 0.73, 1.00 2.27/2.33, 1.30 −/1.46, 1.27, 1.36
12.81 8.87
1.99/0.75, 2.46 −/4.78, 2.90, 3.46 5.19/4.35, 3.02
52 34
nO → σ*Si‑Clax.
centered bifurcated bond with a very strong (18 kcal/mol) intermolecular component and two weakened intramolecular components. According to calculations, this results in lowering the frequency of the ν(CO) band by ∼50 cm−1 with respect to complex 2c-A and corresponds to a low-frequency shift of the maximum of the corresponding band in the spectrum by 48 cm−1. In the FT-IR spectrum of solid compound 1 (Figure S4, Supporting Information), as in the spectrum of its hydrochloride 2, three weak high-frequency bands in the range 3400−3000 cm−1 are two overtones (3300, 3065 cm−1) and a compound tone (3216 cm−1) of the two intense bands of mixed vibrations ν(CO), δ(NH) (1648 cm−1) and ν(CN), δ(CH2) (1540 cm−1), and these are overlapped with a wide band of stretching vibrations of associated NH groups. This band appears as a low-frequency asymmetry on the band at 3216 cm−1 and leads to an increase of intensity of all three highfrequency bands. As a result, in the spectra of dilute solutions of 1 in CH2Cl2 (Figure S5, Supporting Information), where only the monomeric molecules are present, their intensity decreases and the vibration band belonging to free NH groups appears with a maximum at 3450 cm−1. The maximum of the band of mixed vibrations ν(CO), δ(NH) in the spectrum of solid 1 at 1648 cm−1 appears 23 cm−1 higher than in the spectrum of solid hydrochloride 2. In contrast, in moderately dilute solutions of compound 1, its frequency (1645 cm−1) is 14 cm−1 lower than in the spectrum of solution of hydrochloride 2 (1659 cm−1). The observed low-frequency shift is indicative of the participation of the carbonyl oxygen of compound 1 in the formation of intramolecular bonds in the absence of intermolecular bonds. From this point of view, most probable is the presence of conformer 1b in solution, which contains a O−H···OC intramolecular bond stronger than that in 1a. The calculated low-frequency shift of its ν(O−H) band with respect to trimethylsilanol 5 is 250 cm−1. The experimental confirmation is, first of all, the absence of the band of vibrations of free O−H groups in the spectra of solutions in the presence of the band of vibrations of free N−H groups. Vibrations of the O−H group involved in the intramolecular H bond appear as a weak band at 3232 cm−1, whose intensity is independent of the degree of dilution. The stretching vibration bands in the spectra of compound 1 in the methylene chloride solutions at 1645, 1526, and 1513 cm−1 are shifted relative to the bands in the
Figure 5. Dependence of the energy of orbital interactions nO → σ*Si‑Cax and nO → σ*Si‑Oax on the O→Si distance (Å).
solid-state spectrum and appears in the range 2000−2400 cm−1 with a maximum at ∼2200 cm−1. Its intensity decreases in the spectra of diluted solutions, indicative of further dissociation of hydrochloride 2. A weak ν(OH) band at 3600 cm−1 in the spectra in solution corresponds to the formation of a small amount of conformer 1c, having an intramolecular O→Si bond. According to calculations, the ν(O−H) band in 1c is 103 cm−1 lower than that in trimethylsilanol 5 (Table 2). Three bands in the range 3140−3290 cm−1 have the same nature as in the spectrum in the solid state, and are shifted in accordance with the shift of the ν(CO), δ(NH) and ν(CN), δ(CH2) bands in solution. Thus, the intense band of mixed vibrations ν(CO), δ(NH) is shifted to high frequency to 1659 cm−1, relative to 1625 cm−1 in the solid-state spectrum, while the intense band of ν(CN), δ(CH2) vibrations at 1589 cm−1 becomes a doublet with maxima at 1611 and 1595 cm−1. With dilution, the peak intensity of its high-frequency component increases, being followed by a sharp drop of intensity of the band at 1659 cm−1 belonging to the ν(CO) vibrations of hydrochloride 2c-A. This allows us to conclude that in a dilute solution of hydrochloride 2 the dynamic equilibrium is shifted toward the formation of complex 2a-B, which is 2.2 kcal/mol more stable than complex 2c-A. In complex 2a-B the molecule of HCl is coordinated to the carbonyl oxygen atom, which forms a fourI
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solid-state spectrum and have overtones at 3289 and 3064 cm−1 (the latter band is clearly seen in CCl4 (Figure S6, Supporting Information) but is masked by the solvent in CH2Cl2), as well as the compound tone at 3160 cm−1. The low-frequency shift of the ν(O−H) band of 450 cm−1 with respect to the band of the free O−H groups of organosilanols exceeds the calculated value for conformer 1b. However, it is in good agreement with Δν(OH) of 440 cm−1 typical for the FT-IR spectra of intermolecular H complexes of triphenylsilanol with tetramethylene sulfoxide and DMF.33 The calculated value of Δν(OH) for complexes of silanols 1 and 5 with DMF does not exceed 300 cm−1 and is also lower than that measured experimentally. At the same time, even for interaction of silanols and silanediols with ethers, the Δν(OH) value is higher than 300 cm−1.9i,33,34 Conformer 1b, with the strongest intramolecular H bond, is formed also in the solutions of compound 1 in carbon tetrachloride and cyclohexane (Figure S7, Supporting Information). The ν(CO) band in these solutions is shifted to high frequencies by 12 and 16 cm−1, respectively, relative to the solution in CH2Cl2, being sensitive to the polarity of the solvent. Vibration bands of the free OH groups in these spectra are lacking, and a wide weak band of vibrations of the OH group involved in the intramolecular O− H···OC bond has a maximum at 3227 cm−1. In the solid state, most probably, compound 1 exists in the form 1a, since the value of ν(CO) in its spectrum is 3 cm−1 higher than that in the spectrum of its solution in CH2Cl2. This corresponds to the formation of an H bond, which is weaker than that in 1b, in the presence of a weak O→Si bond.
spectroscopy data, compound 1 in the solid state exists as conformer 1a, while in solution the equilibrium is shifted to conformer 1b, having a stronger H bond in the lack of CO→ Si coordination. Its hydrochloride 2, according to an X-ray diffraction study and FT-IR spectroscopy, in the solid state has an intramolecular CO→Si bond and intermolecular N−H··· Cl and O−H···Cl bonds. In solution, a dynamic equilibrium exists between the product of dissociation 2c and conformer 2a-B, in which the oxygen atom forms a four-centered bifurcated bond with intramolecular components CO→Si and CO···H−O and intermolecular component CO··· HCl.
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
General Considerations. Compounds 1 and 2 have been synthesized by the described procedure.16 The IR spectra of solid compounds and of their solutions in CH2Cl2, CCl4, and C6H12 were taken on ATR/FT-IR and FT-IR Varian 3100 spectrometers. Computational Details. Calculations were performed by the B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) methods as implemented in the Gaussian0335 and Gaussian0936 program packages. All calculated structures correspond to minima on the potential energy surface (PES), as proved by positive eigenvalues of the corresponding Hessian matrices. The DFT energies were calculated with the ZPE correction. The NBO analysis37 as implemented in the Gaussian03 package was performed using the 6-311+G** basis set on the previously DFT optimized structures. The QTAIM analysis was performed by the use of the AIM2000 program (version 2.0)38 with the wave function taken from the MP2/6-311++G** single-point calculations on the previously DFT optimized structures. The degree of pentacoordination of the silicon atom was calculated by the formula29 ηa = (((109.5 − 1/3)Σn3 = 1θn)/(109.5 − 90)) × 100%, where θ denotes the angle between axial and equatorial bonds at silicon.
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CONCLUSIONS DFT analysis at the B3LYP/6-311+G(d,p) and M06/6311+G(d,p) levels of theory of the conformers of N[(hydroxydimethylsilyl)methyl]-N,N′-propylen urea 1a−e and their hydrochlorides 2a-A−2a-C, 2c-A, 2d-A, and 2e-A−2e-C showed that their structure and stability are governed by an interplay between the CO→Si coordination, apical preference and nucleofugality of the electronegative group, and intraand intermolecular hydrogen bonding. The most stable are conformers 1b, having the intramolecular H bond CO···H− O and 1a being a rare case of an intramolecular bifurcated bond with one component represented by an H bond and the other component by an Si→O coordination bond. Conformer 1c, with the OH group in the axial position at silicon, is ∼3 kcal/ mol less stable than 1a,b proving that the axial preference of the electronegative group prevails over intramolecular H bonding only for good leaving groups. A power dependence is found between the O→Si distance and the energy of orbital interactions nO → σ*Si‑Cax and nO → σ*Si‑Oax. A QTAIM analysis of bond critical points showed the O→Si coordination bonds in the studied molecules to fall in the range from partially covalent to electrostatic and hydrogen bonds varying from weak to medium. Coordination of HCl to the OH group oxygen at the axial position at silicon in conformer 2c-A strengthens the CO→Si bond. Competitive and cooperative effects of bifurcated bonds are manifested in various ways. Thus, coordination of HCl to hydroxyl oxygen in 2a-A leads to strengthening of the H bond and has no effect on the CO→ Si coordination. Coordination of HCl to nitrogen in the NCH2SiOH moiety in 2a-C leads to strengthening of the C O→Si bond and weakening of the H bond, while coordination of HCl to the carbonyl group in 2a-B results in weakening of both the coordination and hydrogen bonds. From FT-IR
S Supporting Information *
Figures giving FT-IR spectra of compounds 1 and 2 in the solid state and in solution, tables giving Cartesian coordinates of the calculated structures, and an xyz file containing the computed Cartesian coordinates of all of the molecules reported in this study (this file may be opened as a text file to read the coordinates or opened directly by a molecular modeling program such as Mercury (version 3.3 or later, http://www. ccdc.cam.ac.uk/pages/Home.aspx) for visualization and analysis). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for B.A.S.:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/om500349s | Organometallics XXXX, XXX, XXX−XXX
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dx.doi.org/10.1021/om500349s | Organometallics XXXX, XXX, XXX−XXX
