Ab initio study of medium effects on the geometries of the compounds of trigonal-bipyramidal silicon with coordination center ClSiC3O

Journal of Molecular Structure (Theochem) 538 (2001) 59±65

www.elsevier.nl/locate/theochem

Ab initio study of medium effects on the geometries of the compounds of trigonal-bipyramidal silicon with coordination center ClSiC3O V.F. Sidorkin*, E.F. Belogolova, V.A. Pestunovich A.E. Favorsky, Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1, Favorsky Irkutsk 664033, Russian Federation Received 10 May 2000; accepted 16 July 2000

Abstract For a number of (O±Si)chelate and zwitterionic intramolecular complexes with coordination center ClSiC3O, appreciable discrepancies between the experimental solid state geometry and those calculated at the HF/6-31G p level for the gas phase and solution were found. The established regularity of changing the molecular structure of complexes in going from the gas phase to the solution, and the solid state corresponds to that expected from an analysis of the 29Si NMR data and a physical model of the in¯uence of medium on the geometry of hypervalent silicon compounds, developed earlier by the present authors. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Pentacoordination; Medium effect; Ab initio study

1. Introduction A wide range of the chelate (A±C) and zwitterionic (D) complexes of the trigonal-bipyramidal silicon atom (CTBPS) exhibits high sensitivity of the spectral (IR and NMR) characteristics of the coordination center to a change in their aggregation state, the polarity and temperature of the solution [1±7]. In most cases, analysis of the reasons for this phenomenon argues for an essential effect of external factors not on the dissociation constants of CTBPS but on the axial bond lengths and the valence angles of their central

* Corresponding author. Fax: 17-3952-396046. E-mail address: [email protected] (V.F. Sidorkin).

atom [1,2,4±7]. More strong experimental evidences for such unusual structural rearrangement in the CTBPS molecules upon the in¯uence of medium were obtained only for highly symmetric molecules of silatranes (E) [7±9]. According to the X-ray and electron diffraction (ED) data, the transition of 1-methyl- (E1) and 1-¯uorosilatranes (E2) from the solid to the gas phase is accompanied by an unprecedentedly Ê ) elongation of the `coordinate' great (,0.28 A Si±N bond [8,9]. For the majority of other CTBPS, the experimental determination of the gas phase structure and consequently providing a direct information about the possibility of an appreciable medium effect on the geometry of their coordination center are complicated by the low symmetries of the molecules.

0166-1280/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0166-128 0(00)00698-9

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In this situation, the quantum chemical methods acquire decisive signi®cance. However, a reliable quantum chemical prediction of the gas-phase CTBPS structure is not an easy task. Semiempirical methods often lead to wrong results [10]. Ab initio geometry optimizations of real CTBPS were performed only for some silatranes and their analogs [11±14] demonstrating far from the total agreement between the calculated and experimental structures of E1 ±E2. At the same time, quantum chemical (semiempirical [15] and ab initio [10±14,16]) analysis of potential functions of the Si±N bond stretching reveals surprising softness of the XSiO3N coordination center of silatranes and con®rms the possibility of its deformation when the molecules go from the gas phase to the polar media. These results together with successive use of the hypervalency concept [17], perturbation theory and the Onsager reactive ®eld model have allowed a better understanding of the nature of the unusually high sensitivity of the geometrical and spectral characteristics of silatranes to the medium effects [5,7,12,15,16]. The suitability of the developed approach and ideas to other CTBPS is a problem of general interest in the coordination chemistry of a silicon. In this paper, the results of an ab initio study of reality and the extent of the medium-induced structural rearrangement of the ClSiC3O coordination center of (O±Si)chelate and zwitterionic molecules 1±5 have been considered.

2. Subjects and methods The choice of the compounds 1±5 as a subject of the present study is not arbitrary. According to X-ray and 29Si NMR spectroscopy data (see Refs. [4,6]), having the same ClSiC3O coordination center (F) are characterized by a wide range

of axial bond strengths (lengths): from weak `coordinate' Si à O and strong `covalent' Cl±Si bonds in (O±Si)chelate 2,2,5-trimethyl-3-(dimethylchlorsilylmethyl)-4-oxozolidinone 1 to strong `covalent' SiO and weak `coordinate' Cl ! Si bonds in zwitterionic 2,4,4,6,6-pentamethyl-6l 5-sila-D 2-dihydro-1,3,4l 4-oxadiazine chloride 5. This allows us to consider the dependence of the extent of deformation of the complex geometry under the effect of medium on their initial stability in the gas phase. Quantum chemical calculations were performed using the Pc Gamess [18] packages of programs. All computations

V.F. Sidorkin et al. / Journal of Molecular Structure (Theochem) 538 (2001) 59±65

61

Table 1 Ê ), valence and dihedral angles (8) and the displacement of Si atom from the equatorial HF 6-31G p//6-31G p values of some bond lengths (d, A Ê ) of molecules 1±5 (the values determined by the X-ray analysis [4,6] are given in parentheses) plane of three C atoms (DSi, A Geometry parameters

d(SiO) d(ClSi) d(SiC2) d(Csp 2N) d(C1N) /C1SiCl /C1SiC2 /NC1Si /SiOCsp 2 /OCsp 2N /C 1SiOCsp 2 /SiOCsp 2N /NC1SiO DSi

Compound 1

2

3

4

5

2.740 (2.448) 2.134 (2.153) 1.878 (1.859) 1.332 (1.327) 1.450 (1.455) 96.6 (93.3) 119.5 (116.2) 118.5 (114.8) 98.7 (103.0) 126.1 (123.7) 0.2 (1.9) 0.8 (2.1) 21.0 (24.9) 0.369 (0.243)

2.348 (2.027) 2.167 (2.259) 1.881 (1.87) 1.379 (1.36) 1.474 (1.46) 93.4 (92.6) 118.0 (121.1) 118.3 (116.6) 112.9 (114.8) 117.8 (117.8) 0.0 0.0 0.0 0.278 (0.160)

2.372 (1.954) 2.176 (2.307) 1.900 (1.862) 1.333 (1.315) 1.458 (1.473) 93.9 (88.0) 118.0 (121.7) 116.9 (109.6) 109.1 (113.2) 120.4 (119.1) 0.9 (5.9) 1.1 (24.7) 22.4 (25.5) 0.271 (0.058)

2.175 (1.879) 2.364 (2.432) 1.883 (1.851) 1.286 (1.283) 1.511 (1.522) 89.8 (82.5) 118.0 (117.7) 121.5 (117.4) 122.6 (122.5) 129.5 (131.3) 27.9 (25.9) 242.1 (240.7) 18.3 (18.2) 0.151 (20.078)

1.966 2.304 1.886 1.290 1.511 85.8 120.6 118.7 125.2 126.6 31.8 245.1 15.3 0.041

were carried out with the full geometry optimization. The local stability of the calculated structures on the potential energy surface was con®rmed by the calculation of the Hessian eigenvalues. 3. Results and discussion In principle, complexes 1±5 could be in equilibrium with the products of their dissociation along the coordinate bond. A detailed study of this problem is beyond the scope of the task to be solved. Nevertheless, experimental results (X-ray structural analysis, IR and 29Si NMR spectroscopy) [4,6] reject the possibility of the dissociation of the CTBPS 1±5 in condensed media. A similar situation evidently takes place in the gas phase also. 1 A number of most informative structural parameters of CTBPS 1±5 calculated at the HF/6-31G p level together with those determined by the X-ray diffraction analysis are presented in Table 1. Its analysis demonstrates nearly a full agreement between the CTBPS conformations observed in crystals and calculated for the gas phase. The differences between the gas and solid phase values of both the bond lengths 1

According to preliminary calculations at the HF/6-31G p level complexes 1±3 are more than 5 kcal/mol energetically more favorable as compared with the related tetrahedral structures.

and valence angles not associated with the coordination center are moderate and characteristic of stable donor±acceptor complexes [19]. Only the difference between the lengths of axial bonds, ClSi and especially Ê ). SiO, falls appreciably outside this scope (,0.1 A For each of the compounds 1±5, the gas phase value of the SiO bond length (d(SiO) gas) signi®cantly exceeds the solid phase bond length (d(SiO) solid). In contrast, for the second axial bond, ClSi, the value d(ClSi) gas is markedly smaller than d(ClSi) solid. This is highly symptomatic: according to the model of hypervalency [5,17], increasing (decreasing) the strength of one axial bond at TBP Si atom caused by internal and external factors should lead to the weakening (strengthening) of another. The discrepancy between the solid and gas phase structures of ClSiO fragments is most pronounced by the example of complex 4 (Table 1). According to data of 29Si NMR spectroscopy and X-ray structural analysis [4±6], in the solid phase this compound is in the zwitterionic form characterized by a weak (`coordinate') interaction with the chlorine anion and not with the oxygen atom. It is con®rmed by the displacement of TBP Si atom from the equatorial plane of three C atoms toward the O atom (value of the corresponding displacement, DSi, is negative). On the contrary, as follows from calculations at HF/6-31G p level, in the gas phase this molecule should exist in the

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V.F. Sidorkin et al. / Journal of Molecular Structure (Theochem) 538 (2001) 59±65

Table 2 Ê ) between the Basis set effects on the difference (d gas 2 d solid, A calculated (d gas) and determined by X-ray analysis (d solid) values of some bond lengths of the molecule 2 Bond

SiO ClSi SiC 2 Csp 2O Csp 2N C 1N

Basis STO-3G

3-21G

3-21G p

6-31G

6-31G p

0.131 20.082 0.051 20.012 0.049 0.020

20.054 0.193 0.025 0.004 20.009 0.034

0.040 20.045 0.009 20.005 20.002 0.033

0.126 0.101 0.030 20.005 0.010 0.023

0.321 20.092 0.011 20.041 0.019 0.014

(O±Si)chelate form: Si atom is deviated to the opposite side (DSi . 0, ClSi bond is `covalent', SiO is `coordinate'). The same discrepancy is also characteristic for the complex 5. Calculations predict its (O±Si)chelate structure in the gas phase whereas 29Si NMR spectroscopy indicates its zwitterionic structure in solution [20]. Some contribution to the considered difference (d(SiO/ClSi) gas 2 d(SiO/ClSi) solid) is of course made by errors in the gas phase geometry calculation caused by the basis set incompleteness and a neglect of the electron correlation. Since calculations at a higher level of theory are very expensive, we were unable to perform the complete analysis of these factors. 2 We would like to demonstrate, at an accessible level, their effect on the calculated values of some bond lengths of CTBPS with (O±Si)dimethyl(N,N-diacetylaminomethyl)chlorosilane 2 as an example (see Table 2 and Fig. 1). As follows from Table 2, the choice of a basis set is of great importance for estimating the axial bond lengths of the TBP Si atom. In this respect, our results coincide with those known for the silatranes [7,11,12]. With the use of smaller basis sets than a HF/6-31G p basis, the discrepancy between the calculated d(SiO) gas and d(SiO) solid values decrease considerably. Moreover, the 3-21G p value of the SiO bond length is incorrect, less than the solid state value! (The last result is unexpected since, no matter what the basis set size, the calculated values d(SiN) of E1 and E2 2

In full volume, such analysis was not carried out even for silatranes E1 and E2, though their ab initio calculations were performed using two supercomputers [12].

Ê ) those determined by signi®cantly exceed (.0.3 A the X-ray diffraction [7,11±14].) The use of a minimal, STO-3G, basis leads to results close to the ones received with the 6-31G basis. The latter, however, does not qualitatively describe the reliably established [4±7] and substantiated [5,17] inverse correlation between the lengths of the TBP Si atom axial bonds. At last, observed by the X-ray diffraction shortening of d(SiO) on going from compound 2 to 3 is not reproduced at the 6-31G p level (see Table 1). Available data do not allow predicting with con®dence the character of changing the difference d(SiO) gas 2 d(SiO) solid for CTBPS 1±5 on additional basis set improvements in Hartree±Fock calculations. However, there are reasons to assume that the difference between the geometrical parameters of the coordination center ClSiC3O calculated with the 6-31G p basis and with 6-31 1 G(2d,f) should not be dramatic. Indeed, this change of the basis set size for atoms of coordination center XSiO3N of the molecule E2 has Ê ) decrease of the resulted in just a slight (by 0.04 A calculated value d(SiN) [12]. A signi®cant effect of the electron correlation on the calculated values of the lengths of axial bonds, d(SiO) gas and d(ClSi) gas, expected for molecules 1±5 is readily illustrated by Fig. 1. Taking into account the MP2 correlation, the corresponding correction leads Ê decrease in the SiO bond length calcuto a ,0.1 A lated for complex 2 at the HF/6-31G p level. Evidently,

Fig. 1. The relative energy of the molecule 2 computed at the HF 631G p//6-31G p and MP2 6-31G p//6-31G p level as a function of Si±O distance (the value of the total energy in minimum is set to zero).

V.F. Sidorkin et al. / Journal of Molecular Structure (Theochem) 538 (2001) 59±65 Table 3 Effect of the choice of the Onsager radius (a) value on the HF 631G p-SCRF values of the lengths of axial bonds and the displacement of Si atom from the equatorial plane of three C atoms of the complexes 1±5 in DMSO solution Ê) Molecule a(A 5.69 a 5.05 b 4.24 c 5.07 a 5.07 b 3.92 c 5.61 a 4.69 b 3.98 c 5.27 a 4.85 b 4.99 a 4.76 b

1 2 3 4 5

Ê ) d(ClSi) solvent (A Ê ) DSisolvent (A Ê) d(SiO) solvent (A 2.685 2.650 2.027 2.249 2.249 1.973 2.106 1.964

2.144 2.151 2.349 2.194 2.194 2.352 2.258 2.368

0.351 0.339 0.202 0.236 0.236 0.071 0.150 0.054

2.027 1.990 1.865 1.835

2.422 2.445 2.441 2.517

0.068 0.044 20.059 20.113

a

Taken equal to the van der Waals radius of the molecules. Calculated quantum chemically using a procedure described in Ref. [22]. c Determined from the density of the crystals. b

the value of the correlation determined through the shift of potential function minimum should be viewed with some caution. However, a similar consideration of the electron correlation effect on the calculated Ê ) resulted Si à N bond length in silatrane E2 (,0.24 A in a good agreement with the d(SiN) value obtained by ED analysis [13,17]. In view of these facts, a possible elongation of the SiO bond in compound 2 in the gas phase with respect to that in the solid phase Ê (Table 1). may be estimated as ,0.2 A As follows from the above, the question of the gas phase geometry of 1±5 is still open. The reason for this is evidently a rather high requirement on the quality of ab initio calculation caused by the hypervalent, three-center four-electron (3c±4e), character of bonding in the axial ClSiO fragment of such complexes [5]. Nevertheless, there is no serious reason to have doubts about the decisive contribution of the medium effect on the considerable discrepancy between the values of the lengths of both the ClSi and SiO bonds in 1±5 calculated at the HF/6-31G p level and found by X-ray analysis. Thus, Fig. 1 shows a very symptomatic softness of the SiO bond in complex 2. When the SiO bond is compressed by Ê of the equilibrium position, the energy of this 0.2 A

63 3

complex changes by only 1.0±1.2 kcal/mol. This indicates that crystal packing forces can produce an essential shortening of the SiO bond of molecule 2 in the solid state. The reality of the change in molecules 1±5 geometry on going from the gas phase to the polar solution can be demonstrated quantitatively within the Onsager reaction ®eld model (SCRF) [21]. In the framework of this model, the energy of interaction, ES, between the molecule (with a dipole moment m and radius of a spherical cavity a) and the polar solvent (with the dielectric constant e ) is determined by the following expression: ES ˆ

22…e 2 1† 23 2 ´a ´m …2e 1 1†

The values of the axial bond lengths of the TBP silicon and the displacement of Si atom from the plane of equatorial substituents in molecules 1±5 in DMSO solution …e ˆ 45† calculated at the HF/6-31G p level using the SCRF model are presented in Table 3. For these compounds, in contrast to the silatranes [12], the use of values a equal to their crystal radius leads to improbable results. For example, the predicted SiO bond length in molecules 1 and 2 in DMSO is found to be less than in the solid phase (Tables 1 and 3). The reasonable geometry of these complexes in the polar solution is obtained only when a is equal to their van der Waals radius or calculated from the molar volume computed by the quantum mechanical procedure described in Ref. [22]. In these cases, a sequence of changing the calculated gas phase, solution phase and the experimentally obtained solid phase values of axial bonds lengths for each of the studied molecules looks as follows (Tables 1 and 3): d…SiO†gas . d…SiO†solvent . d…SiO†solid ; d…ClSi†gas , d…ClSi†solvent , d…ClSi†solid : This is consistent with a physically substantiated, experimentally con®rmed by the example of the silatranes, concept on the in¯uence of medium on the CTBPS geometry [5,7,12,15]. Indeed, within the Onsager solvation model the structural rearrangement 3 The silatranes E exibit an analogous ¯at character of the potential function of the SiN bond deformation too [7,11,12,16].

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the initial explanation [1±7,15] for the nature of change in the CTBPS spectral characteristics upon varying the external factors. Acknowledgements The ®nancial support from the Russian Foundation for Basic Research and INTAS is gratefully acknowledged (Grants No.: INTAS-RFBR 95-070, RFBR 9903-33032).

Fig. 2. Dependence of the dipole moment of the molecule 2 on the SiO and ClSi bond lengths calculated at the HF 6-31G p//6-31G p level.

of such molecules induced by the polar medium should be directed towards increasing their dipole moment. For molecules 1±5 this is accompanied by the shortening of the endocyclic SiO bond and the elongation of the exocyclic ClSi …2m=2d…SiO† , 0 and 2m=2d…ClSi† . 0; see Fig. 2). In agreement with the 29Si NMR spectroscopy data [4,6], the calculations suggest a (O±Si)chelate (Cl± Si à O) structure for compound 4 and a zwitterionic (Cl ! Si±O) structure for the complex 5 in DMSO solution. It should be borne in mind that the former is inherent in these compounds in the gas phase and the latter in their crystals. This is determined by the signs of the corresponding DSi values (Tables 1 and 3). At a ®rst glance, it seems that among the series of compounds 1±5 the extent of SiO bond deformation by the polar solvent should increase in line with its weakening (lengthening) in the gas phase: 5 , 4 , 3 , 2 , 1: However the calculations predict minimal and nearly equal shortening of this bond in the complexes with the strongest (5) and the weakest (1) SiO interaction in the solution (Tables 1 and 3). In a qualitative sense, it agrees with the 29Si NMR spectroscopy data [4,6] and is explained by the dependence of a difference (d(SiO) solvent 2 d(SiO) gas) not only on the initial (gas phase) strength of `coordinate' bond, but also on other characteristics of the complexes [5,15]. Thus, the results obtained unambiguously reveal the signi®cant sensitivity of the ClSiC3O coordination center geometry of the complexes 1±5 to the effect of polar medium. Thereby they con®rm the validity of

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