New Insight into the Nature of Bonding in the Dimers of Lappert's

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A new insight into the nature of bonding in the dimers of Lappert’s stannylene and its Ge analogs. A quantum mechanical study Robert Sedlak, Olga A. Stasyuk, Celia Fonseca Guerra, Jan #ezá#, Ales Ruzicka, and Pavel Hobza J. Chem. Theory Comput., Just Accepted Manuscript • DOI: 10.1021/acs.jctc.6b00065 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016

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Journal of Chemical Theory and Computation

A new insight into the nature of bonding in the dimers of Lappert’s stannylene and its Ge analogs. A quantum mechanical study Robert Sedlak,[a,b] Olga A. Stasyuk,[a] Célia Fonseca Guerra,[c] Jan Řezáč,[a] Aleš Růžička*[d] and Pavel Hobza*[a,b] Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,

a

166 10 Prague 6, Czech Republic Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry,

b

Palacký University, 771 46 Olomouc, Czech Republic Department of Theoretical Chemistry and Amsterdam Center for Multiscale Modeling, VU

c

Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University

d

of Pardubice, Studentská 573, CZ532 10, Pardubice, Czech Republic

stannylene • dimer • dative bond • dispersion interaction • electrostatic potential • energy decomposition analysis

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The strength and nature of the connection in Lappert’s stannylene dimer ({Sn[CH(SiMe3)2]2}2) and its smaller analogs, simplified stannylenes, as well as similar Ge complexes were studied by means of DFTD3 calculations, energy decomposition analysis (EDA), electrostatic potential (ESP) and natural population analysis. The transbent structure of the investigated molecules was rationalized by means of EDA, ESP and molecular orbital (MO) analyses. The different ESPs for the monomers studied are a result of different hybridization of the Sn (Ge) atoms. The comparably strong stabilization in the largest and the smallest systems with a dramatically different substituent size is explained by the different nature of the binding between monomers. For all complexes, it has been found that the total attractive interaction is mostly provided by the electrostatic component (> 50 %), followed by orbital interaction and dispersion. In the largest molecule (Lappert’s stannylene), the dispersion interaction plays a more significant role in stabilization and its magnitude is comparable to that of orbital interaction; on the other hand in the smallest molecule (SnH2), where bulky substituents are replaced by H only, the dispersion energy is less important and the EE bond is more of a chargetransfer character, caused by donoracceptor orbital interactions. The charge transfer in Ge dimers is greater than in the Sn ones due to shorter distances between monomers, which cause better overlaps. The easier dimerization of Lappert’s stannylene as compared to Kira’s ({Sn[(Me3Si)2CHCH2CH2CH(SiMe3)2κ2C,C']}) stannylene is explained by the different orientation of their substituents – asymmetry promotes dimerization.

Introduction


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Lappert’s seminal discovery of lowvalent and lowcoordinate Group 14 coordination and organometallic compounds14 opened an unprecedented source of maingroup metal chemistry areas, inspired or influenced by that particular species, has been opened. In the last twenty years, the phenomena described have included, for example, the lowcoordinate bulky terphenyl liganddecorated organometallics511 and analogs of acetylenes, highly silylated compounds,12,13 various cyclic or acyclic metal amides,1417 lowvalent metal complexes containing heterocyclic or carbene ligands,1823,24,25 the use of lowvalent maingroup metal species as ligands for transitionmetal complexes2631 or various areas of the activation of small molecules.511,1417,32,33 In agreement with Lappert’s original observations, the possibility of the (co)existence of both singlet and triplet states of tetrylenes (lowvalent Group 14 metal (E) complexes – R2E(14))2631,34 and even the aggregation of such molecules to the dimers – ditetrenes (R2E(14)=E(14)R2)34 (Scheme 1) by some authors described as heavier congeners of olefins have been observed many times both experimentally 2631 and theoretically.35,36 It is believed that the sterical protection of the metal center is the reason for the existence of the monomeric tetrylene,37 and slightly controversially also for the excitedstate stabilization (a triplet state).34 According to several studies, the stabilization of the monomeric species R 2E(14) over the dimeric ones (R2E(14)=E(14)R2) could be achieved not only by the steric properties of the metal substituents (even by cyclic silylated hydrocarbons), 37,38,39 but also by the introduction of a heteroatom to the ligand skeleton, 28 using πelectronrich cyclic chelating ligands,25 the employment of an adjacent donor atom in both intramolecular and intermolecular fashion 4042 and the interaction of the metal centre with a small atom of such a ligand as fluorine 2631 or hydrogen (an agostic interaction).4349 On the other hand, the lowvalent tin compounds of Marschner &


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Müller50 and Wesemann51,52 reveal higher steric protection by silyl groups and polyaromates, respectively, but behave as endocyclic distannenes. Scheme 1.

The nature of binding in Lappert’s stannylenes and their Ge and Pb analogs has very recently been investigated in Ref. 53. The authors have concluded that stabilization in these systems is exclusively caused by London dispersion energy based on the B3W91D3 method. They have deduced the role of dispersion from the values of D3 dispersion energies. The respective dispersion energy is, however, strongly underestimated. This is attributable to the fact that the original Becke–Johnsson damping function is not applicable at such short X–X (X= Sn, Ge, Pb) distances as in the present systems. When an appropriate damping function is applied (see below), the dispersion energy reaches much higher values (even by one order of magnitude) and has no relation to the original values. Consequently, the conclusions of Ref. 53 should be taken with caution. This report offers a novel theoretical explanation of the association behavior in the dimers of Lappert’s stannylene and its Sn and Ge analogs based on a careful analysis of the monomer’s electrostatic potential and noncovalent interactions acting in the dimer using the


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KohnSham MO theory. Furthermore, Lappert’s first stannylene has been compared with its analogs, Kira’s stannylene and some germanium compounds. Results and Discussion Stabilization in Lappert’s stannylenes possessing the transbent form (cf. Figure 1) was first believed to originate in the double electrondonor – electronacceptor dative interaction,2631 between the inplane oriented lone pair of sp character on the Sn of the first subsystem and the vacant perpendicular pz orbital on the Sn of the second subsystem. The reference plane is defined by the tin atom and the two carbon atoms covalently bonded to the tin atom. This transbent model of distannylene was first suggested by Trinquier and Malrieu as a part of the GCTM model5457 and comprehensively discussed in Ref. 58. On the other hand, the discussion about the character of this connection revolves around the terms of dative bond, double bond, slipped bond,59 double dative donor–acceptor bond and others. The dative bond resembles a standard covalent bond where, however, the shared electron pair originates from one subsystem only. The prototype dative bond exists in the H 3N→BH3 system, where the arrow implies that electrons are provided by one subsystem (NH 3 moiety).60,61 The bond length in the abovementioned dative bond is 1.694 Å, which is greater than that in the standard covalent bond between N and B (e.g. the NB bond in borazine has length 1.44 Å), but it is still much shorter than that in most of the noncovalent complexes (e.g. the length of a hydrogen bond in water is ~ 2 Å). Figure 1. A schematic depiction of a double dative bond in an analog of Lappert’s stannylene (Me2Sn)2; color legend: vacant pz orbitals = blue ellipsoids; occupied lone pairs = red ellipsoids.


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The vacant pz orbital has perpendicular orientation, whereas the occupied lone pair has inplane orientation. The reference plane is defined by the tin atom and the two carbon atoms covalently bonded to the tin atom.

The transbent structure of dimers of Lappert’s stannylenes with the same hybridization of the Sn (or similar) atom exists in systems with bulky as well as small (e.g. only H) substituents. The bulky substituents protect the electronic environment of the Sn atoms, which makes these systems kinetically much more stable. It has been shown that bulky substituents can dramatically influence the bonding character and the length between Sn atoms in stannylenes.62 Several questions, however, arise. Do bulky substituents play only this role or do they also participate in a different type of stabilization, which has not been discussed yet? Is the dative bond similar when the substituents are dramatically different? Is the ability of Lappert’s stannylenes to create transbent dimers (contrary to Kira’s stannylenes) connected with the symmetry of the system considered and its substituents? Figure 2. A–H: The structures of Lappert’s stannylene dimers and their analogs; the structure of Kira’s stannylene; color legend: magenta = Sn, blue = Si, green = C and white = H.


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In the present study, we have tried to answer these questions on the basis of the monomer’s electrostatic potential and noncovalent interactions in the dimers and thus provide a new interpretation of stability for different Lappert’s stannylenes and their Ge analogs. The systems investigated All of the Sncontaining structures studied are visualized in Figure 2. First, the crystal structure (1a) was fully relaxed (1b). Then we removed the extended aliphatic chain on Sn and instead of CH(Si(CH3)3)2, we considered the CH(SiH3)2 substituent (2a). Here all the bond angles and dihedral angles were kept frozen from 1a, while in 2b, the systems were fully relaxed. Further reduction of substituents (from CH(SiH 3)2 to CH3) leads to systems 3a and 3b, respectively. Finally, the replacement of CH3 by H provided systems 4a and 4b; in the former one, all the bond angles as well as dihedral angles were the same as in 1a, molecules 3a and 4a, whereas full relaxation was allowed for 3b and 4b. Computations The abovementioned binary complexes were subjected to partial or complete geometry optimization at the B97D2/def2QZVP level of theory. 63,64,65,66 The default convergence criteria of the Gaussian0967 program package were adopted (maximum force

New Insight into the Nature of Bonding in the Dimers of Lappert's

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