Can Aromatic Ï-Clouds Complex Divalent Germanium and Tin

Article pubs.acs.org/Organometallics

Can Aromatic π-Clouds Complex Divalent Germanium and Tin Compounds? A DFT Study Lies Broeckaert,† Paul Geerlings,† Aleš Růzǐ čka,§ Rudolph Willem,‡ and Frank De Proft*,† †

Department of General Chemistry (ALGC) and ‡Department of Materials and Chemistry (MACH), High Resolution NMR Centre (HNMR), Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussels, Belgium § University of Pardubice, Faculty of Chemical Technology, Department of General and Inorganic Chemistry, Pardubice 53210, Czech Republic S Supporting Information *

ABSTRACT: The properties of various electron-deficient germylenes and stannylenes are investigated using density functional theory (DFT). The dominant electrophilic character of these divalent group IV compounds is demonstrated by computed DFT-based reactivity descriptors. Next, the interaction of selected model dihalogenated germylenes and stannylenes (GeX2 and SnX2, with X = F, Cl, Br, I) with a series of potential aromatic πdonors is studied; computed classical donor−acceptor σ-interactions with strong Lewis bases serve as a reference. In addition, natural bond orbital analyses were performed in order to study the interactions at the orbital level, consistently indicating that the most important interaction for the π-complexations is the overlap of the formal empty p-orbital on the germanium or the tin atom and the π-orbitals of the aromatic rings. Additional information is obtained from the extent of charge transfer from the π-donors toward the divalent tin and germanium compounds. The existence of a complexation interaction between the π-clouds of the aromatic rings and the divalent compounds is theoretically established. The strength of the π-complexation parallels the trends in electron-donating and electron-withdrawing character of the substituents on the aromatic compounds. Correlations of the total complexation energy with the NBO interaction energy confirm that this πcomplexation is essentially an orbital-controlled interaction. In agreement with experimental data, σ-complexation is found to dominate over π-complexation.

■

INTRODUCTION Germylenes and stannylenes, the heavier divalent group 14 metal analogues of carbenes, have dual properties due to the presence of both a formal empty and therefore electrophilic porbital and a rather inert lone electron pair on their central metal atom. In particular, the category of Ge(II) and Sn(II) dihalides is an important subclass that can be used as precursors for a large series of other metallylenes.1 GeCl2 was first synthesized as an intermolecularly stabilized complex with the Lewis base 1,4-dioxane.2 More recently, the Ge dihalides were obtained via stabilization using a N-heterocyclic carbene.3,4 The Sn(II) dihalides are all stable compounds under ambient conditions.5 Becerra and co-workers observed transient Ge(II) and Sn(II) compounds, GeH2 and Ge(CH3)2, detected in the gas phase by laser flash photolysis.6,7 The highly reactive dimethylstannylene Sn(CH3)2 can likewise be generated as a transient in the gas phase and in hexane solution but decays to a longer lived species through the formation of its dimer, tetramethyldistannene.6,8 Many germylenes are stable in the gas phase or in dilute solutions; they can be monomeric but generally tend to dimerize to digermenes, especially in concentrated solutions and in the solid state.9 The dimerization of metallylenes is generally possible when only small ligands are bound to the metal, disilene, digermene, and distannene dimers being © 2012 American Chemical Society

generated from silylenes, germylenes, and stannylenes, respectively.5,10 The double metal−metal bond containing digermene or distannene compounds result from an intermolecular interaction between the lone pair of one metal atom and the vacant p-orbital of the metal atom of another molecule.11 Stabilization of metallylenes leading to heavy carbene analogues amenable to isolation can be achieved in different ways. The first approach is the introduction of bulky substituents.5,12 When these σ-bonded alkyl- or arylstannylenes contain in addition Lewis bases amenable to coordination expansion of the metal atom by a donor−acceptor interaction, they can even be stable in solution.12 Second, metallylenes containing covalent bonds with atoms such as N, O, and S also display thermodynamic stability, preventing oxidation to their metal(IV) analogues.13−15 Since the first preparation of stable dialkyl and diamido derivatives of Ge(II), Sn(II), and Pb(II) compounds by Lappert and co-workers in the 1970s,16,17 theoretical and experimental interest for these compounds has increased.18 An overview of the coordination chemistry of germanium(II) halides with group 15 and 16 donor ligands (N, Received: September 20, 2010 Published: February 13, 2012 1605

dx.doi.org/10.1021/om100903h | Organometallics 2012, 31, 1605−1617

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complexation reactions of simple metallylenes with THF, methanol, and n-butanol.38 It turned out that this complexation modulates significantly the extent to which the metallylenes undergo dimerization, depending on both substitution patterns on the metal and solvent.38 The germylene- and stannylene-mediated C−H activation of alkanes, alkenes, alkynes, arenes, and ethers, performed for direct functionalization or carbon−carbon bond formation, was investigated by Kavara and co-workers,39−44 using a mixture of the metallylene and phenyl iodide. In their proposed reaction mechanism, in the intermediate formed, the central germanium or tin atom interacts with the lone pair of iodine in a σ-type interaction rather than with the aromatic iodide ring. This observation is a good indication of the relative strength of σand π-interactions, with the former being significantly favored over the latter. Intermolecular π-complexes of dichlorocarbene (CCl2) and anisole derivatives were observed experimentally by UV−vis spectroscopy,45 whereas for nonaromatic σ-complexing ethers such as THF and dioxane, no CCl2 complexation was observed.45 A combined experimental and theoretical DFT study conducted recently by Baines and co-workers emphasized N-heterocyclic carbenes (NHC’s) as excellent electron donors for divalent germanium compounds.4,46,47 The importance of π-interactions in group 14 metallocenes (central metal = Ge, Sn, Pb) is stressed by Lawless and coworkers, who investigated the orbital interactions by DFT calculations.48 An η5-interaction occurs between the central metal (Ge, Sn, or Pb) atom and two parallel (substituted) cyclopentadienyl rings. Very recently, the π-complex between alkenes and the simple germylene GeH2 has been detected experimentally by UV, the data collected being inconsistent with free GeH2 and therefore assigned to its complex with a diene.49 In addition, bismuth−arene π-complexes held together by both group V metal−arene π- and π−π-stacking interactions have also been investigated recently.50 The above overview emphasizes that the complexation behavior of metallylenes is of high relevance to their reactivity in various subsequent reactions, the key to which is the metallylene electrophilicity. The present work focuses mainly on the issue of π-complexation of dihalogermylenes and -stannylenes by π-electron-donating aromatic molecules. In this work, halides are specifically addressed in order to ensure sufficient electrophilicity of the metal in the germylenes and stannylenes so as to increase the likelihood of such π-complexes to be significantly evidenced. Anhydrous GeCl2 and SnCl2 are known to be soluble in aromatic hydrocarbons, at least in the presence of anhydrous AlCl3, while such does not appear to be the case for GeBr2 and GeI2.51 We therefore aimed at finding out to what extent these experimental findings are reflected in theoretically determined π-complexation energies for dihalogermylenes and -stannylenes. In the first part, for the sake of comparison, interaction with a selection of organic molecules bearing a σ-lone pair (Figure 1) is investigated so as to obtain a sound reference to the strength of σ-complexation to dihalogermylenes and -stannylenes. For molecules 1−8 in Figure 1, this complexation deals with the overlap of the formal empty p-orbital of the metal with the lone pair containing orbital of the organic moiety. Subsequently, on the basis thereof, the possibility of πcomplexation of dihalogenated germylenes and stannylenes by

P, As, O, S, and Se) has been given by Levason and coworkers.19 Chernov and co-workers20 reported the synthesis and catalytic activity of heteroleptic tin(II) dialkoxides stabilized by intramolecular coordination. Several studies have focused on the synthesis of various stable N-heterocyclic germylenes and stannylenes, the germanium and tin analogues of the Nheterocyclic carbenes.21−23 Roesky and co-workers24,25 described the preparation of stable monomeric tin(II) compounds LSnX, supported by bulky β-diketiminate ligands L, containing terminal methyl, amide, fluoride, and iodide groups (X = Me, N(SiMe3)2, F, I). Our group has focused on the inter- and intramolecular stabilization of silylenes, using conceptual DFT reactivity indices.26−28 Correlations between the reactivity (electrophilicity, nucleophilicity, Lewis acidity, and basicity) and the stability29 of these compounds have been established. The charge-controlled27 interaction between silylenes and various bases appears strongest with the hardest base NH3, classifying these compounds as hard Lewis acids. π-Electron donation into the empty 3p orbital on the central metal atom stabilizes the silylene and decreases its electrophilicity. Two of the most investigated reactions of metallylenes are σbond insertions and π-bond additions.30,31 These reactions proceed in two consecutive steps, the first one being assumed to be initiated by a weak complexation of an electron pair of the organic compound with the empty p-orbital of the metal, followed in the second step by an insertion or addition step involving the lone pair of the metal(II). In these reactions, for the dimethyl-substituted metallylenes, the reactivity of the metal decreases with increasing atomic number (Si(CH3)2 > Ge(CH3)2 > Sn(CH3)2),32 even though, for the addition reaction with 1,3-butadiene, Sn(CH3)2 turns out to be more reactive than Ge(CH3)2. The relative reactivity of M(CH3)2 (M = C, Si, Ge, Sn) for several model insertion and addition reactions was investigated theoretically by Su and co-workers.33−35 These studies reveal that the generally lower reactivity of stannylenes in comparison to that of germylenes is determined by the higher activation barrier associated with the second step of the reaction. In contrast, in the first step, the complexation interaction appears to be stronger in the stannylenes than in the germylenes, suggesting that, for this initial complexation step of the metal atom, tin is more electrophilic than germanium. A number of studies have focused on the nature and properties of this prereactive complex arising from the electrophilic interaction between the empty p-orbital of the metallylene and σ- or π-bonds. Leigh and co-workers32 investigated the interaction of transient germylenes GeR2 with O-donor solvents, such as tetrahydrofuran and methanol. Kira and co-workers36 reported a new synthetic method for cyclic dialkylstannylenes and studied their reversible complexation interaction with tetrahydrofuran (THF), observed by UV spectroscopy. This complexation with THF was not observed for the corresponding germylene.36 Several experimental kinetic and theoretical studies were performed on reactions of diaryl- and dialkylgermylenes and diaryl- and dialkylstannylenes with alcohols and ethers.37 Germylene O−H insertion into simple alcohols38 proceeds through the rapid, reversible formation of a Lewis acid−base complex, followed by a rate-determining proton transfer. Rate and equilibrium constants were determined for several 1606


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The global (absolute) electrophilicity ω was introduced by Parr and co-workers53 and is defined on the basis of the electronic chemical potential μ58 and the chemical hardness η59 of the system ω≡

μ2 (I + A)2 = 2η 8(I − A)

(5)

where

Figure 1. σ lone pair bearing molecules investigated in σ-complexation to dihalogermylenes and -stannylenes: tetrahydrofuran (1), tetrahydrothiophene (2), an N-heterocyclic carbene (3), pyridine (4), nitrobenzene (5), anisole (6), furan (7), and 1H-pyrrole (8).

μ=−

(I + A) 2

(6)

and η=I−A

different aromatic molecules 4−8 and additional compounds,

(7)

with

depicted in Figure 2, is investigated.

I = E(N − 1) − E(N )

(8)

and

A = E(N ) − E(N + 1)

I and A are the vertical ionization energy and electron affinity, respectively, defined in terms of E(N), E(N + 1), and E(N − 1), the energies of the neutral, anionic and cationic species, respectively. The local electrophilicity index, condensed to an atom k, ωk+,60 characterizes the electrophilicity of an atom within the molecule. It can be shown to be the product of the global electrophilicity, ω, and a local index, f k+:

Figure 2. Aromatic Lewis bases investigated in π-complexation to dihalogenated germylenes and stannylenes: (trifluoromethyl)benzene (9), 1,3,5-trifluorobenzene (10), benzene (11), and toluene (12), together with the aromatic molecules from the previous figure.

■

+ ω+ k = ωf k

THEORY AND COMPUTATIONAL DETAILS

ω− A =

⃗

(1)

Figure 3. Schematic representation of the π-complexation interaction between an aromatic electron-donating molecule, with R = H, F, CH3, OCH3, CF3, NO2, and a dihalogenated germylene or stannylene in the singlet ground state (M = Ge, Sn, X = F, Cl, Br, I).

(2)

Next, donor−acceptor complexes of a single Lewis base molecule (3)

and a germylene or stannylene molecule, as depicted in Figure 3 in the

where ρN(r)⃗ and ρN+1(r)⃗ are the electron densities of the neutral species (N electrons), and its anion (N + 1 electrons) at point r⃗ in space. In the condensed-to-atoms formulation, this Fukui function on atom k can be written as56

f k+ = p k (N + 1) − p k (N )

(11)

of a species A with respect This quantity yields the nucleophilicity to a reference electrophile B and thus constitutes, in contrast to the electrophilicity, a relative measure of this property. In accord with previous work,61 the fluorine atom was chosen as a reference (B) in view of its high electrophilicity; for this atom, the nucleophilicity was thus set equal to zero.

Using a finite difference approach toward the computation of the derivative in eq 2, f+(r⃗) can be written as

f + ( r ⃗) = ρN + 1( r ⃗) − ρN ( r ⃗)

2 1 (μA − μB) η 2 (η − η )2 A A B

ωA−

In eq 1, E is the energy of the system and ν(r⃗) the external (i.e., due to the nuclei) potential. The functions f+(r⃗) and f−(r⃗), the right- and lefthand side derivatives of eq 1, corresponding to an increase or decrease of the number of electrons, describe how different regions in a molecule are susceptible to nucleophilic or electrophilic attack. In this work, f+(r⃗) was computed in order to probe the local electrophilic behavior of the electron-deficient Sn and Ge atoms, in their interaction with nucleophilic Lewis bases. ⎛ ∂ρ( r ⃗) ⎞+ ⎟ f + ( r ⃗) = ⎜ ⎝ ∂N ⎠ν( r )

(10)

Finally, the nucleophilicity of the germylenes and stannylenes was probed using the nucleophilicity descriptor ω−, introduced by Jaramillo and co-workers,55 which is defined as

The initial focus on germylene and stannylene compounds is directed toward their electrophilic and nucleophilic character, which is assessed using computed reactivity indices, introduced in the framework of conceptual DFT.52 Relevant physical quantities are the (absolute) electrophilicity index,53 the Fukui function,54 and the nucleophilicity index.55 The Fukui function is a local reactivity index that describes the variation in the electronic density ρ(r⃗) upon changing the number of electrons N in the system:

⎛ ∂ 2E ⎞ ⎛ ∂ρ( r ⃗) ⎞ ⎟⎟ = ⎜ ⎟ f ( r ⃗) = ⎜⎜ ⎝ ∂N δν( r ⃗) ⎠ ⎝ ∂N ⎠ν( r ⃗)

(9)

case of the π-complexation interaction, were considered in this work. The strength of this interaction is quantified by the complexation energy, ΔEcomplexation (eq 12).

(4)

ΔEcomplexation = E(complex) − E(metallylene)

with pk(N + 1) and pk(N) the electronic populations on atom k in the N + 1 and N electron systems, respectively, as determined by natural population analysis (NPA).57

− E(isolated Lewis base) 1607

(12)


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Table 1. Global Electrophilicities ω and Nucleophilicities ω− of a Series of Germylene and Stannylene Compoundsa

Detailed insight into these interactions was subsequently obtained by a natural bond orbital (NBO) analysis of the Lewis acid−base complexes.57,62 All calculations were performed using the Gaussian 0363 and Gaussian 0964 programs. All complexes were fully optimized at the DFT level, using the B3LYP functional,65 and the aug-cc-pVTZ basis set on all atoms.66 For tin and iodine atoms, a large-core (46-electron) SDB (Stuttgart−Dresden−Bonn) relativistic effective core potential (RECP)67 was combined with this Dunning type of basis set. This basis set was also used for calculating the (local) electrophilicities and nucleophilicities of some germylenes and stannylenes. All structures were confirmed to be minima on the potential energy surface by performing frequency calculations at the same level of theory. Complexation energies were computed with eq 12, applying the Counterpoise correction for the basis set superposition error (BSSE).68 The difference between BSSE corrected and uncorrected complexation energy values was found to be small, varying between 0.01 and 0.6 kcal/mol. This is not completely unexpected, in view of the extended basis set used for the calculation of the complexation energies. For the most polarizable halostannylene investigated in this work, SnBr2, for which it can be expected that the influence of dispersion will be the largest, we have also investigated in detail the effect of dispersion on the π-complexation energies using dispersion corrected DFT (DFT-D) calculations according to the scheme proposed by Grimme and co-workers.69 The geometries of the different π-complexes were optimized at the B3LYP-D level using the same basis set as in the other calculations. The complexation energies were computed and a correction for the basis set superposition error was applied. The results are given in the Supporting Information. A good linear correlation exists between the dispersion corrected and uncorrected energies; both the regression line slope and the correlation coefficient are close to 1. The intercept is 7.2 kcal mol−1, indicating that, in the case of the most polarizable stannylene, dispersion increases the interaction energy of the complexes with 7.2 kcal mol−1. The only outlier in the regression is encountered for the complex with toluene, where a slight change in the geometry of the complex is observed. It can however be concluded that the trends in the complexation energies remain unaffected to a large degree and that the conclusion that aromatic π-clouds can complex germylenes and stannylenes is strengthened. NBO analyses of the metallylenes and their optimized complexes were performed at the same level of theory with the Gaussian NBO version 3.1 of the NBO program (natural atomic orbital and natural bond orbital analysis), developed by Weinhold and co-workers.70

molecule

ω

ω−

molecule

ω

ω−

GeI2 GeBr2 GeCl2 GeF2 GeH2 Ge(CH3)2 Ge(OCH3)2

46.5 44.9 43.6 41.1 39.3 29.0 27.6

4.9 4.6 4.4 3.8 5.3 7.2 6.9

SnBr2 SnCl2 SnF2 SnH2 Sn(CH3)2 Sn(OCH3)2

45.2 44.4 42.0 37.9 30.0 29.7

4.7 4.5 4.1 5.8 7.2 6.8

All values are in kcal mol−1. SnI2 is not included, because no stable geometry could be obtained for this compound (see text). a

referring to the fluorine atom, while electrophilicities are absolute values, the germylene and stannylene compounds can nevertheless be classified as being predominantly electrophilic, an issue addressed further below in the NBO analysis. These results are in line with a previous finding of Walsh, emphasizing that many reactions are initiated with these compounds acting as electrophiles.7 The electrophilicity values are in good agreement with the general trends in electron-withdrawing and -donating character of the substituents on germanium and tin; halogen substituents increase the global electrophilicity of the compounds and electron-donating substituents (methyl and methoxy groups) decrease ω values.27,29 In order to gain more insight into the contribution of each atom to the global electrophilicity, its local counterpart, the local electrophilicity ωk+, was calculated using f+(r)⃗ condensed to f k+ (eq 10). Results are given in Table 2. Also, lower Table 2. Local Electrophilicities ωk+ of the Metallylenesa molecule GeI2 GeBr2 GeCl2

■

GeF2

RESULTS AND DISCUSSION 1. Electrophilicity and Nucleophilicity of Germylenes and Stannylenes. In this first part, the electrophilicity and nucleophilicity of different germylenes and stannylenes is probed. It can be anticipated that these compounds have both electrophilic and nucleophilic character, due to the simultaneous presence of a formal empty p-orbital and a lone pair on the central metal atom. In order to quantify both properties, the global electrophilicity ω and nucleophilicity ω− of simple germylenes and stannylenes was computed using eqs 5 and 11. The results are presented in Table 1. As can be seen from Table 1, electrophilicity and nucleophilicity values are quite similar for identically substituted germylenes and stannylenes; in most cases, the stannylenes are both slightly more electrophilic and nucleophilic than the corresponding germylenes. This electrophilicity sequence is in good agreement with the electronegativity trends predicted for EXn type functional groups, E being a group IV atom.71 Although a comparison of the values of the electrophilicity and nucleophilicity is less straightforward, due to the fact that the nucleophilicity values are expressed as relative quantities

GeH2 Ge(CH3)2

Ge(OCH3)2

atom k

ωk+

Ge I Ge Br Ge Cl Ge F Ge H Ge C H* Ge O C H*

30.0 8.3 32.0 6.5 34.0 4.8 37.7 1.7 41.0 −0.8 24.0 −0.9 1.1 22.8 0.9 −0.2 0.6

molecule

SnBr2 SnCl2 SnF2 SnH2 Sn(CH3)2

Sn(OCH3)2

atom k

ωk+

Sn Br Sn Cl Sn F Sn H Sn C H* Sn O C H*

35.6 4.8 37.3 3.6 39.7 1.1 39.9 −1.0 26.1 −0.9 1.0 25.7 0.4 −0.2 0.6

a

For hydrogen atoms marked with asterisks, the average value of the local electrophilicities of the nonequivalent methyl protons is given in the table. All values are in au kcal mol−1.

electrophilicities in most cases correspond to higher nucleophilicity values. In previous work, it was shown that a decreasing electrophilicity corresponds to a higher stability of the metallylenes (larger singlet−triplet gap).27 Table 2 confirms that the germanium and tin atoms are the main electrophilic centers in the molecules, since their contribution to the global electrophilicity originates from the local ωk+ value of the metal atom. On average, the metal atoms 1608


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than that of the chlorides and the bromides. With regard to the particular case of SnH2, the numerical outcome is considered questionable because it lies far outside the value range found for the other compounds. Closer examination of the NBO of this compound reveals that the lowest lying antibonding lone pair orbital does not correspond to a formally empty p-orbital. The reason that this happens with this particular compound is unclear, especially in view of the finding for the Ge analogue, which displays a normal value in view of the other compounds. For the alkyl-substituted metallylene, it is observed that the occupation of the formal empty p-orbital is very low; the charge Q on the metal atom is also highly positive and slightly higher than in the case of the bromides but lower than in the case of the alkoxides. The orbital energy of the formally empty porbital is the highest of all compounds considered. As a result, in a charge-controlled complexation reaction, the alkylsubstituted germylene and stannylene will be less electrophilic than the alkoxides but slightly more electrophilic than the bromides. When the complexation occurs with a soft donor atom, i.e. under orbital control, the electrophilicity will be low. It can be concluded that the electrophilic behavior of the metallylenes is basically the result of the interplay of three effects: the positive charge Q as well as the local electrophilicity on the metal atom and the relative energy of the formally empty p-orbital on the metal with respect to lone electron pairs in the Lewis base. 2. σ-Complexation of Halogermylenes and Stannylenes. We investigated σ-complexes of the electrophilic germanium(II) and tin(II) halides, GeX2 and SnX2 (X = F, Cl, Br, I), with various Lewis bases. This classical σ-type interaction with Lewis bases is included in order to provide reference values for comparison with the aromatic π-complexations, which will be reported on subsequently in the paper. The Lewis bases (Figure 1) investigated are tetrahydrofuran (THF (1)), tetrahydrothiophene (THT (2)), a widely used stable N-heterocyclic carbene (3, from the family of imidazol-2ylidenes), H2O, and H2S. They are complexing the metal atom through the lone pair of the C, O, S, or N donor atoms in a “classical” Lewis acid−base interaction based on σ-donation. Pyridine (4), anisole (6), furan (7). and nitrobenzene (5), which can also potentially be involved in π-complexation, can also form σ-complexes through the lone pairs of either the N or the O atom which are not involved in π-delocalization. In the case of SnI2, no results are provided because no stable complexes were found, due to systematic convergence failure in the iterative self-consistent field (SCF) optimization procedure. First, all GeX2 and SnX2 molecules are considered in a classical Lewis acid−base interaction with THF, often used as a coordinating Lewis base in experimental works.23,31,32,36,38 The complexation strength and NBO interaction energies are presented in Table 4. The positive NBO interaction energies represent stabilizing interactions in which higher energy values correspond to stronger orbital interactions. From Table 4, it is clear that the strongest complexation occurs with the stannylenes, the complexation energy systematically increasing (becoming more negative) from SnBr2 to SnF2. ΔENBO, probing the energy change associated with the orbital overlap between the formal empty p-orbital on the metal and the lone pair of the oxygen in THF (1), is the highest for the germylenes and increases from F to Br (I); this is in agreement with the energy of the formal empty p-orbital (see Table 3). The highest value for ΔENBO occurs for the most negative orbital energies, enabling a priori a more favorable

of the stannylenes have higher local electrophilicities than those of the germylenes. The higher (local) electrophilicity of the tin compounds in comparison to the germanium values are in agreement with previous findings.71 The most highly electrophilic species are the halogenated germylenes and stannylenes, and these can thus be expected to combine with electron-rich species in a complexation interaction. Interestingly, whereas the global electrophilicity of the metallylene increases along the series EF2 < ECl2 < EBr2 ( ECl2 > EBr2 (>EI2). This can be understood more thoroughly by considering the atomic charges on the Sn and Ge atoms, together with the population of the formal empty porbital on these atoms and the energy of this orbital; these properties, obtained using the natural bond orbitals, are given in Table 3. As can be seen, the formal empty p-orbital becomes Table 3. Occupancies and Energies of the Formal Empty pOrbitals of Various Metallylenes, Together with the Atomic Charge Q on the Metal Atoma

a

molecule

p occupancy

p energy

Q

GeF2 GeCl2 GeBr2 GeI2 GeH2 Ge(CH3)2 Ge(OCH3)2 SnF2 SnCl2 SnBr2 SnH2 Sn(CH3)2 Sn(OCH3)2

0.123 0.220 0.260 0.320 0.000 0.046 0.245 0.083 0.163 0.194 0.000 0.028 0.199

−0.082 −0.131 −0.142 −0.151 −0.109 −0.048 −0.080 −0.072 −0.125 −0.132 +4.137 −0.040 −0.074

+1.409 +0.953 +0.790 +0.543 +0.566 +0.928 +1.236 +1.581 +1.170 +1.019 +0.776 +1.084 +1.417

All values are in au.

more populated when going from F to Br (I); this is in agreement with the more efficient overlap of the more diffuse lone pair p-type orbitals of the heavier halogens with the formal empty p-orbital on Sn. As a result, this orbital is the least populated in the fluorine compounds, resulting in the highest positive atomic charge and highest local electrophilicity of the metal atom in these fluorinated molecules; it should also be noted that increasing the population of the formal empty porbitals lowers their energy, thus facilitating the interaction with lower lying lone-pair orbitals of Lewis bases. The reverse trend for the global electrophilicity (cf. eq 5) of these compounds can be traced back to the decreasing hardness of the metallylenes upon increasing the atomic number of the halogens. For the alkoxides, the occupation of the formal empty porbital is comparable with that of the bromide in the case of both metals; the charge on the metal atom is highly positive, the charge being higher only in the fluorides. Also, the orbital energy of the formally empty p-orbital is comparable to that of the fluoride. It can thus be concluded that in a chargecontrolled complexation reaction (i.e. the complexation of the metallylene with a hard donor atom), the alkoxy-substituted germylene and stannylene are only slightly less electrophilic than the corresponding fluorides but more electrophilic than the chlorides and bromides. When the complexation occurs with a soft donor atom, i.e. under orbital-control, the electrophilicity is comparable to that of the fluorides, but less 1609


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Table 4. Complexation Energies (kcal mol−1), NBO Interaction Energies (kcal mol−1), and the Amount of Charge Transfer, ΔQ (au), for the Interaction of THF (1) with Several GeX2 and SnX2 Germylenes and Stannylenes (X = F, Cl, Br, I) Lewis acid

ΔEcomplexation

ΔENBO

ΔQ

GeI2 GeBr2 GeCl2 GeF2 SnBr2 SnCl2 SnF2

−13.5 −14.2 −14.5 −14.6 −14.3 −15.0 −16.4

48.1 46.4 45.5 36.6 35.0 34.0 20.7

0.107 0.105 0.104 0.088 0.087 0.086 0.051

interaction with the lone pair orbital on O and a larger charge transfer in the complex. As can be seen from Table 4, the total complexation energies with THF do not follow the trend in the NBO interaction energies but parallel the trends in the atomic charge on the metal atom (Table 3). Thus, this interaction is essentially charge controlled, which is not surprising since the complexing atom in the Lewis base, oxygen, is a hard atom, thus favoring hard−hard or electrostatically controlled interactions. Table 4 shows that the most stable complexes are encountered for SnF2. Accordingly, in Table 5, the σ-type

Figure 4. Optimized σ-complexes of SnF2 and GeF2 with THF (a and b) and an N-heterocyclic carbene (c and d).

atom and the (expectedly) soft metal centers. This is also in line with the fact that a Sn−S covalent bond is stronger than a Sn−O bond.73a However, experimentally, S coordination to tin is much weaker than the corresponding O coordination.23 This is reflected, at least in part, by the fact that the total complexation energies of THT and THF differ only slightly, in spite of the much higher ΔENBO value of the former than of the latter. When complexation of H2S is compared with that of H2O, it turns out that the complexation energy is much stronger in the latter than in the former with, in contrast with THT and THF, NBO interaction energy contributions of comparable magnitude. This confirms indirectly a major contribution of electrostatic complexation character in H2O, since in H2S the lower NBO interaction energy in comparison to that of THT also increases the complexation energy difference between oxygen and sulfur compounds. This additionally shows that electrostatic contribution to the complexation is less determinant for the sulfur than for the oxygen ligands, in line with oxygen being harder than sulfur. This strongly suggests that the significantly stronger Sn−S covalent bond in THT can be assigned to its higher NBO interaction energy in comparison to that of THF, H2O and H2S. The somewhat surprising higher NBO interaction energy of H2O in comparison to that of THF is not reflected in a higher total complexation energy for H2O in comparison to that for THF, which again provides indirect support for a strong electrostatic interaction in the oxygen compounds. Pyridine, anisole, and nitrobenzene can complex the electrophiles through either a nitrogen or an oxygen lone pair or the aromatic π-cloud. However, it has to be mentioned that pyridine and nitrobenzene are deactivated toward electrophilic aromatic attack in comparison to benzene. The π-interaction energies with aromatic compounds activated for electrophilic aromatic substitution, such as anisole (6), are expected to be larger. Anisole (6) can form a complex with germylenes or stannylenes, either by σ-interaction, through the donor O atom (Table 5), or by π-interaction, through the aromatic π-electron cloud (Tables 6 and 7, see below). The strong complex between the stannylene and NH3 suggests that N is a stronger σ-donor than O, as predicted by the calculations of Olah and

Table 5. σ-Interaction of Furan (7), Pyrrole (8), H2S, Nitrobenzene (5), Anisole (6), H2O, THT (2), THF (1), Pyridine (4), NH3 and NH-Carbene (3) with SnF2 Lewis base

ΔEcomplexation

ΔENBO

ΔQ

furan (7) pyrrole (8) H2S nitrobenzene (5) anisole (6) H2O THT (2) THF (1) pyridine (4) NH3 NH-carbene (3)

−6.5 −7.8 −8.0 −10.8 −13.2 −14.7 −16.0 −16.4 −19.3 −19.4 −32.2

10.7 9.3 21.5 21.1 21.6 24.3 40.2 20.7 41.7 43.0 107.2

0.033 0.048 0.117 0.037 0.053 0.072 0.162 0.051 0.100 0.115 0.211

interaction energies of SnF2 with a series of other Lewis bases, often used in experimental work,23,47 are given. Two representative optimized complexes of SnF2 and GeF2 are also shown in Figure 4. Again, since the trends in the total complexation energy values do not correspond to the trends in NBO interaction energy values, it can be deduced that these σ-complexations are largely electrostatic in nature: i.e., they can be considered as interactions of a formally negatively charged carbon, chalcogen, or nitrogen atom with the positively charged metal atom. This is again in agreement with the fact that the complexing atoms in the Lewis bases are essentially hard atoms. Accordingly, a good example is the interaction with THF (1), with a hard basic center, which strongly contrasts with THT (2), containing a soft basic center, favoring orbital interactions as supported by the much higher ΔENBO. Thus, the NBO overlap energy and the corresponding amount of charge transfer are larger for THT than for THF complexes (Table 5), in good agreement with Pearson’s hard and soft acids and bases principle,72 predicting a more favorable interaction between the soft sulfur 1610


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chemical shifts, since benzene has no potential σ-donor while THF has no potential π-donor. Thus, 119Sn NMR chemical shifts not only demonstrate the divalent character of the tin atoms but also have the power to reveal additional σ-coordination by O- and N-containing donors. Finally, Lee and co-workers78 studied the electrochemical properties of dihalostannylenes and dihalogermylenes and their complexes with σ-donating Lewis bases, such as 1,4-dioxane, pyridine, and PPh3. Reduction and oxidation potentials were measured in CH3CN (acetonitrile). Quantum chemical calculations showed that the complexation destabilizes both the HOMO and the LUMO of the carbene analogues, in agreement with our results presented in Table 8 below. In the electrochemical study of Egorov and co-workers79 an additional complex was formed between EX2 and some transition-metal (Cr, Mo, W) pentacarbonyls, leading to a (CO)5M EX2·(base) complex. The voltammetric study provided experimental equilibrium constants of the complex formation of dichlorogermylene GeCl2 with Lewis bases (PPh3, AsPh3). The equilibrium constants are on the order of 103−104. All of these experimental results confirm the existence of our calculated σ-type interactions. 3. π-Complexation of Halogermylenes and Stannylenes. The equilibrium geometries obtained for a series of Lewis acid−base π-complexes are given in the Supporting Information. Overall, the results reveal that the plane of the germylene or stannylene and the plane of the aromatic molecule are parallel, positioning the formal empty p-orbital of the metal above a ring carbon at distances ranging from 3 to 3.5 Å. These distances are well below the sum of the van der Waals radii of the metal and carbon atoms. The calculated intermolecular distances are in good agreement with experimental results on N-heterocyclic germylenes and stannylenes and their adducts.22,23 The presence of weak intermolecular [η4-(C6H4)···M] and [η2-(C5H3N)···M] interactions between the germanium or tin atom and the carbon atoms of aromatic (benzene) rings confirms the existence of weak π-interactions.22,23 Experimental contact distances for π-interactions with Sn(II) are available for aggregates of two divalent Sn(II) compounds with aromatic ligands. Thus, parallel oriented stannylene molecules dimerize through the formation of intermolecular [η6-(C6H4)···Sn]2 interactions with intermolecular Sn···C6(centroid) contact distances of 3.23 Å, in excellent agreement with our results. Several analogous complexes with such π-complexations display interaction distances ranging from 3.2 to 4 Å.22,23 The work of Mansell and co-workers22 and Zabula and Hahn23 on the synthesis of N-heterocyclic germylenes and stannylenes and their dimers confirms the existence of weak intermolecular π-interactions. Experimental contact distances ranging typically from 2.5 to 3.5 Å also agree with our calculations. The results of our calculations do not reveal such η6-type interactions between the investigated benzene derivatives and the metallylenes. Some representative examples of interaction between SnF2 and GeF2 with anisole, benzene, and nitrobenzene are in given in Figure 5. These particular three compounds are highlighted here because anisole contains a mesomeric electron-releasing substituent, nitrobenzene contains a mesomeric electron-withdrawing substituent, and benzene, without any substituent, can be used as a reference compound. In order to make comparisons possible between the

co-workers.27 This stabilizing interaction with oxygen and nitrogen donors is often found in stannylene compounds and complexes.15,22 The strongest complex is formed between SnF2 and the N-heterocyclic carbene 3. This result is in good agreement with the study of Baines and co-workers,4,46,47 which reports a strong coordination between several GeX2 compounds and N-heterocyclic carbenes. The Nheterocyclic carbene (NHC)−GeX2 complexes serve as precursors for the noncoordinated germylenes GeX2, because the complexes are easier to isolate and to handle. The introduction of the Lewis base reduces the reactivity of the germylenes; NHC complexes are more nucleophilic and less electrophilic than noncoordinated germylenes, in good agreement with our calculations (Tables 1 and 2) Baines and coworkers46,47 used quantum chemical models to prove that the substituents on germanium strongly affect the formation energy of the NHC−germylene complex. This is of importance, since NHC−GeX2 complexes undergo consecutively to the complexation a cycloaddition reaction with dimethylbutadiene. The latter reaction being thermodynamically unfavorable for NHC−GeX2 in the cases X = F, Cl emphasizes once more the high stability of the complexes also found in our work. Finally, these complexes are also reactive toward methyl iodide, CH3I, GeX2 inserting into the C−I bond to give rise to tetravalent germanium compounds. Mansell and co-workers and Zabula and Hahn21−23 reported intra- and intermolecular M···N and M···O distances (M = Ge, Sn) of some N-heterocyclic germylenes and stannylenes and their dimers, ranging from 2.2 to 3.2 Å. These bond distances are a good indication for weak σ-interactions. For comparison, the (experimental) coordinative Sn−O distance in an SnCl2·(1,4-dioxane) complex is 2.527 Å.21 In the work of Hahn and co-workers on the synthesis of benzimidazolin-2ylstannylenes, they reported a strong intermolecular Sn···N distance of 2.361 Å and intramolecular Sn···O distances of 3.060 and 3.120 Å. Levason and co-workers74 give an overview of the polymeric chain (solid-state) structures of GeCl2, GeBr2, and GeI2 coordinated by O donors, such as THF and dioxane. Intermolecular Ge···O distances around 2.3−2.4 Å are reported. These findings are in good agreement with our results: the calculated Sn···O distance in our optimized SnCl2 THF complex amounts to 2.403 Å. For the corresponding germylene complex this calculated intermolecular Ge···O distance reduces to 2.205 Å. Khrustalev and co-workers75 reported the synthesis of stable heteroleptic germanium(II) and tin(II) compounds with intramolecular N- and O-coordination. The inter- and intramolecular Sn···N and Sn···O interactions, in stannylenes with (N and O) donor atoms in their ligands, lead to three- and four-coordinated tin atoms. In solution, the sterically unprotected tin atoms also coordinate easily with (solvent) donor molecules, such as THF. Higher coordination of the tin atom in stannylenes is known to result in upfield shifts of their 119Sn chemical shifts.21−23,76,77 Such changes in δ(119Sn) values, Δδ, at least qualitatively reflect the strength of inter- and intramolecular interactions between the tin atoms and potential donors.76,77 Hahn and co-workers21 observed such upfield shifts for various benzimidazolin-2-ylstannylenes in THF-d8 in comparison to the signals for benzene-d6. Thus, Δδ varies from 6 to 115 ppm, depending on the structure of the stannylenes and the nature of the donor ligands in their side chains. Even though π-complexation can potentially occur with benzene, the latter is nevertheless an appropriate reference solvent for assessing pure σ-donation of THF using 119Sn 1611


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taking the benzene (11) values from Tables 6 and 7 as a reference, nitro- (5), trifluoro- (10), and trifluoromethylbenzene (9) have lower complexation energies and NBO interaction energies, due to the electron-withdrawing substituents on the aromatic ring. Nitrobenzene (5) is deactivated toward an electrophilic attack; in principle, the two meta positions are less disfavored for interaction with an electrophile. The germylene and stannylene compounds appear centered above the intermediate para position, to be seen as an average position between the two neighboring meta locations. This complexation mode is also found for pyridine (4), where the germanium or tin atom is positioned above the carbon atom in the para position of the ring. In anisole (6), the methoxy substituent is expected to activate the para position of the aromatic ring toward interaction with an electrophile,80,81 our calculations confirming this expectation, one of the strongest complexation energies being found for this compound. A slight distortion of the MF2 moiety away from the methoxy substituent, resulting in a kind of trans arrangement, is noteworthy, because this is not at all observed in substituted benzenes with a C2 axis. In most metal dihalides, not unexpectedly, the strongest πcomplexation interaction occurs with pyrrole (8) and anisole (6), which are also known to be activating compounds toward electrophilic aromatic substitution. As expected, the deactivated nitrobenzene (5), trifluoromethylbenzene (9), and trifluorobenzene (10) molecules display also the lowest complexation energies, even though these interactions remain stabilizing in most complexes, as well as low NBO interaction energy values. For a given stannylene complex, the complexation is only very slightly stronger than for the corresponding germylene complex. The charge transfer ΔQ associated with the complexation interaction, derived in this work from our NBO analyses, is also given in Tables 6 and 7. In all cases, the aromatic molecule is slightly positively charged, and consequently the stannylene or germylene bears a small negative charge, in agreement with the high electrophilicity of the metallylenes. The NBO analysis confirms that the main NBO interaction is an electron donation from the electron-rich π-donors toward the electrophilic germanium or tin center. Moreover, the NBO analysis of the complexes always indicates that the main orbital interaction between the two compounds is the overlap of the formally empty p-NBO orbital on the Ge or Sn atom with a molecular πtype NBO in the aromatic ring. In all cases, the “back-donation”

Figure 5. Optimized π-complexes of GeF2 and SnF2 with nitrobenzene (a and b), benzene (c and d), and anisole (e and f).

σ-complexation with SnF2 described above, we focus in Figure 5 on the π-complexation involving both metallylene fluorides interacting with these three aromatic compounds. Figure 5 reveals essentially π-complexation of the η1 type, since, when starting from an initial geometry positioning the metallylene above the aromatic ring center, the minimum energy structure is systematically one positioning the metal atom above a single ring carbon. There is a general trend toward a preferred position above the aromatic ring carbon atom most remote from the substituent, whatever the nature of the latter. This essentially also holds for all complexes reported in Tables 6 and 7, which also reveal the π-complexation energy to be significantly lower than the σ-complexation energy (compare with Table 5). Also, there is a clear increasing trend in this πcomplexation energy from electron-withdrawing substituted aromatic compounds up to electron-releasing ones. Thus,

Table 6. Total π-Complexation Energies (kcal mol−1) of GeF2, GeCl2, GeBr2, and GeI2 in Interaction with Aromatic Molecules 4−12, together with ΔENBO, the Corresponding NBO (Second-Order Perturbation) Interaction Energies (kcal mol−1) of the Complexesa GeF2

GeCl2

GeBr2

GeI2

Lewis base

ΔEcomplexation

ΔENBO

ΔQ

ΔEcomplexation

ΔENBO

ΔQ

ΔEcomplexation

ΔENBO

ΔQ

ΔEcomplexation

ΔENBO

ΔQ

1,3,5-trifluorobenzene (10) nitrobenzene (5) pyridine (4) trifluoromethylbenzene (9) benzene (11) furan (7) toluene (12) anisole (6) pyrrole (8)

−0.2 −1.3 −1.8 −1.9 −3.3 −2.8 −4.0 −3.7 −5.7

1.0 2.5 2.7 3.6 5.7 10.2 7.0 6.5 13.3

0.002 0.007 0.011 0.014 0.029 0.051 0.030 0.038 0.074

0.1 −0.7 −1.2 −1.3 −2.8 −3.3 −3.8 −4.2 −6.1

0.3 0.9 0.6 1.0 9.8 15.5 11.4 14.2 20.6

0.006 0.014 0.019 0.026 0.049 0.079 0.055 0.080 0.116

0.0 −0.5 −1.0 −1.1 −2.2 −3.2 −3.3 −4.0 −6.0

3.9 3.8 5.1 6.3 10.8 16.7 13.1 16.9 22.1

0.017 0.014 0.021 0.027 0.053 0.086 0.056 0.088 0.126

0.0 −0.2 −0.8 −0.8 −2.2 −2.9 −2.7 −4.0 −5.9

5.3 4.2 5.3

0.000 0.011 0.018 0.025 0.058 0.094 0.071 0.099 0.135

a

11.6 17.8 13.4 17.9 23.3

Also listed is the amount of charge transfer ΔQ (au) from the aromatic π-donor molecules toward GeX2. 1612


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Table 7. Total π-Complexation Energies (kcal mol−1) of SnF2, SnCl2, and SnBr2 in Interaction with Aromatic Molecules 4−12, Together with ΔENBO, the Corresponding NBO (Second-Order Perturbation) Interaction Energies (kcal mol−1) of the Complexesa SnF2

a

SnCl2

SnBr2

Lewis base

ΔEcomplexation

ΔENBO

ΔQ

ΔEcomplexation

ΔENBO

ΔQ

ΔEcomplexation

ΔENBO

ΔQ

1,3,5-trifluorobenzene (10) nitrobenzene (5) pyridine (4) trifluoromethylbenzene (9) benzene (11) furan (7) toluene (12) anisole (6) pyrrole (8)

−0.8 −2.1 −2.8 −2.8 −4.4 −4.3 −4.8 −5.0 −7.4

2.5 2.7 3.1 3.7 5.7 9.4 5.3 5.4 8.0

0.014 0.011 0.015 0.018 0.022 0.053 0.035 0.039 0.065

−0.4 −1.3 −2.1 −2.1 −3.8 −4.8 −4.4 −5.2 −8.0

2.4 3.6 4.9 5.6 9.3 14.7 10.2 12.6 20.1

0.017 0.016 0.022 0.027 0.033 0.076 0.056 0.061 0.107

−0.2 −1.0 −1.8 −1.8 −3.4 −3.7 −4.4 −4.9 −7.8

2.6 3.0 4.8 5.7 6.5 13.4 7.4 13.3 21.4

0.016 0.013 0.020 0.027 0.042 0.073 0.037 0.073 0.111

Also listed is the amount of charge transfer ΔQ (au) from the aromatic π-donor molecules toward SnX2.

interaction from the lone pair of Ge or Sn toward the antibonding π* orbitals of the aromatic molecules is negligible in comparison to the electron donation from the π-cloud toward the electrophilic Ge or Sn atom. The latter interaction energies are small and differ by one order of magnitude from the energies for the interaction between filled π-orbitals of the aromatics with the “empty” p-orbital of the metal atom. This observation holds for all complexes investigated and again confirms the dominant electrophilic character of these germylenes and stannylenes, as stated earlier (Table 1). The charge transfer from the lone pair orbital on the metal atom toward the π*-type NBOs in the aromatic ring system, representing the antibonding molecular orbitals between two adjacent carbon atoms, is negligible. For the complexes of pyridine, the complexation energies, NBO interaction energies, and amount of charge transfer (Tables 5−7) are one order of magnitude larger for the direct orbital overlap of the σ-lone pair orbital of the nitrogen atom with the empty p-orbital on germanium and tin, in comparison to the interaction in the π-complex. Similar results were obtained for anisole and nitrobenzene. These results confirm that the σ-interaction with the lone pair of a Lewis base is considerably stronger than the interaction with a π-cloud. Plots of the total complexation energy of all complexes from Tables 6 and 7 against the specific NBO interaction energy (see the Supporting Information), reveal medium to good linear correlations: R2 = 0.79 for GeF2 complexes, R2 = 0.90 for GeCl2, 0.95 for GeBr2, 0.94 for GeI2, 0.60 for SnF2, 0.96 for SnCl2 and 0.85 for SnBr2 complexes. This indicates that while, as shown above, σ-interactions turned out to be rather charge-controlled, the π-interactions are now essentially orbital-controlled, as supported by substantial charge transfer contributions. The correlation between the complexation energies and the NBO interaction energies is worse in the case of aromatic compounds with electron-withdrawing substituents; also, the complexation energies of the difluorides only poorly correlate with the NBO interaction energies. Both of these observations can be explained using a schematic orbital interaction diagram (Figure 6). A similar analysis was performed recently82 for the interaction of hard and soft electrophiles with different monosubstituted benzenes. The energy of the formally empty p-orbital on the metal atom increases (i.e., becomes less negative) from the bromides to the fluorides; the π-orbital energy of the aromatic compounds, which is always lower than

Figure 6. Schematic orbital interaction diagram of the interaction between the lowest unoccupied NBO orbitals of the Lewis acids and the highest occupied NBO orbital of some Lewis bases. The numbers in the figure correspond to the aromatic Lewis bases depicted in Figure 1: anisole (6), benzene (11), and nitrobenzene (5).

that of the formally empty p-orbital of the metal atom, also becomes less negative when switching from electron-withdrawing to electron-releasing substituents. Indeed, the highest ionization energies (proportional approximately to the negative of the energy of the highest occupied molecular orbital) are found for aromatic compounds with electron-withdrawing substituents, the lowest ones occurring for those with releasing substituents. The magnitude of the NBO interaction energies is inversely proportional to the difference in the orbital energy of the HOMO of the aromatic π-donor and the LUMO of the metallylene. From this analysis, it is clear that, for a given metallylene, the NBO interaction energy will be the highest with electronreleasing groups and the lowest with electron-withdrawing groups. For a given π-donor, the NBO interaction energy decreases in the order Br > Cl > F. Finally, for two sample metallylenes, GeBr2 and SnF2, we have investigated the effect of the complexation on the NBO properties of the electrophile. Given that no data could be obtained for one of the two metal diodides (SnI2), these metallylenes were selected because SnF2 represents the electrophile with highest charge-controlled interaction while GeBr2 displays the highest orbital-controlled interaction. GeBr2 is also suitable because of the high amount of experimental data4,46,47 available. Their NBO properties are given in Table 8. The results are given for four selected σ-complexes of high relevance for widely available experimental data,4,23,32,38,46,47 while for the three pairs of π-complexes the same aromatic compounds as for Figure 5, nitrobenzene, benzene, and anisole, were considered. Those of noncomplexed SnF2 and GeBr2, 1613


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Table 8. NBO Properties (Atomic Charge Q, Occupancy and Energy of the Formal Empty p-Orbital on the Metal) for Selected σ- and π-Complexes of SnF2 and GeBr2a SnF2

a

GeBr2

Lewis base

p occupancy

p energy

Q

p occupancy

p energy

Q

furan (7) H2O THF (1) NH-carbene (3) nitrobenzene (5) benzene (11) anisole (6)

0.123 0.109 0.117 0.244 0.091 0.101 0.105 0.083

−0.054 −0.038 −0.004 +0.066 −0.065 −0.046 −0.041 −0.072

+1.527 +1.537 +1.540 +1.369 +0.011 +1.560 +0.039 +1.581

0.296 0.228 0.232 0.401 0.266 0.276 0.285 0.260

−0.121 −0.098 −0.068 −0.019 −0.142 −0.124 −0.113 −0.142

+0.761 +0.840 +0.828 +0.303 +0.014 +0.791 +0.088 +0.790

The values of the noncomplexed molecules are also given. All values are in au.

presented above in Table 3, are repeated in Table 8 for comparison. For both metallylenes, the complexation only slightly reduces the charge on the metal atom, except for the NH-carbene (3). This charge reduction is accompanied by a slight increase of the population in the formal empty p-orbital, except again for compound 3. This general trend is in agreement with the small charge transfers listed in Tables 6 and 7. In the case of H2O and THF complexation to GeBr2, however, quite surprisingly, the population of the formally empty p-orbital decreases, accompanied by an increase of the positive charge on the metal atom. Also the energy of the formal p-orbital increases and, in the case of the NH-carbene complex, it becomes even positive. From these observations, in view of the values for the noncomplexed metallylenes, it can be concluded that the complexation slightly reduces their electrophilicity. The largest reduction occurs with the NH-carbene, in agreement with experimental findings.4,46,47 Experimentally, 119Sn NMR data on weak donor−acceptor interactions based on π-complexation are rather scarce. Data are available, however, for strong π-complexation of the type observed in group 14 metallocenes.48,73b In stannocenes, the tin atom has a formal coordination number of 10 and the highest 119 Sn nuclear shielding is observed (−2199 ppm for (C5H5)2Sn and −2171 ppm for (C5H4CH3)2Sn).76,77 The tin atom can also interact with its aromatic ligands in an η4- or η2-type interaction.83 The donor−acceptor weak π-complexes of our study reveal mainly η1-type interactions for nonmetallocene complexations. 4. Case Study of GeH2, Ge(CH3)2, SnH2, and Sn(CH3)2. Since the donor−acceptor interactions investigated above, constituting the initial step of a number of O−H insertions on dimethylmetallylenes, are very well documented by experimental data,6,7 we also investigated such interactions of GeH2, Ge(CH3)2, SnH2, and Sn(CH3)2 with methanol, THF, and benzene (Table 9). The hydrides were also investigated here because an extensive perspective by Becerra and Walsh7 was devoted to mechanistic themes of intermediate complexes involving GeH2 and SnH2, together with the silicon analogue. In addition, we believe that the hydrides are relevant as reference compounds for our studies on the halides discussed above and the dimethyl compounds of the present section. Thus, O−H insertion of (dimethyl) metallylenes into methanol has been shown to proceed after such a complexation, through a rate-determining proton transfer.6,7,34,84 In such a two-step process, an electrophilic interaction precedes a nucleophilic one. From kinetic studies, the activation barrier

Table 9. Complexation Energies, NBO Interaction Energies, and (Local) Electrophilicities of the Complexes of EH2 and E(CH3)2 (E = Ge, Sn) with CH3OH, THF (1), and Benzene (11)a Lewis base CH3OH

THF (1)

Benzene (11)

a

ΔEcomplexation ΔENBO ω ω+ ΔEcomplexation ΔENBO ω ω+ ΔEcomplexation ΔENBO ω ω+

GeH2

Ge(CH3)2

SnH2

Sn(CH3)2

−14.7 48.8 23.8 8.4 −16.6 53.7 21.5 3.4 −5.3 15.3 28.5 19.1

−8.8 39.2 20.1 1.4 −10.0 41.6 18.9 0.2 −0.8 4.8 24.3 15.8

−12.8 35.4 24.0 13.4 −14.3 38.2 21.6 7.7 −4.4 10.9 28.0 22.3

−8.8 28.4 20.7 5.5 −10.0 30.9 18.8 2.6 −1.5 4.6 24.2 16.9

All values are in kcal mol−1.

associated with the second step was demonstrated to be higher for the stannylene than for the germylene.6,7,84 Calculations performed by Su33−35 on such an insertion reaction with the same substrate show the same trend of higher activation barriers for tin compounds, in comparison to the corresponding germylenes. The tin precursor complexes formed in the first complexation step are more stabilized than the corresponding equally substituted germanium complexes, but the calculated transition state for the second insertion (proton transfer) step lies higher for the tin than for the germanium compound, which, accordingly, is the ratedetermining step of the reaction. The proton transfer step determines the overall reaction rate, and the higher barrier corresponds to a lower reactivity of the stannylenes. Su’s theoretical work on the CH3OH insertion reaction35 indeed established that the energy difference between the transition state and the precursor complex amounts to 22 kcal mol−1 for the dimethylgermanium compound and 34 kcal mol−1 for the corresponding tin compound (calculated at the CCSD(T) level of theory). At the DFT/B3LYP level these values become 23.8 and 30.0 kcal mol−1, respectively. The main conclusion from this theoretical work is that the reaction barrier for insertion is higher for Sn(CH3)2, due to the more stabilized tin complexes and the higher (local) electrophilicity of Sn. The calculated σ-complexes presented in Table 9 focus on the complexation energy of the first step of this reaction mechanism. At first glance there is no significant difference in the global electrophilicity values ω of the complexes; however, 1614


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the local electrophilicity ω+ is higher in all Sn complexes than in the Ge complexes. Also, the metal atom is significantly more electrophilic in the dihydrides than in dimethyl metallylenes. This higher local ω+ was also observed for the free stannylenes (see Table 2). As expected, the complexation with σ- and πdonors reduces the (local) electrophilicity of the central Ge and Sn atoms. For comparison, Su’s35 calculations for Ge(CH3)2 and Sn(CH3)2 provided complexation energies with methanol of −7.7 and −10.2 kcal mol−1, respectively, which compare fairly well in order of magnitude with the value of 8.8 kcal mol−1 that in our calculations turns out to be identical for both compounds.

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CONCLUSIONS

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ASSOCIATED CONTENT

REFERENCES

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Computations show that π-electron clouds can complex electrophilic germylenes and stannylenes, resulting in stable acid−base complexes, albeit only weakly. Conceptual DFT reactivity indices confirm that the electrophilicity of the germylenes and stannylenes prevails over their nucleophilic power. Strong σ-complexes, with (among others) pyridine, THF, and water, provide reference interaction values for the significantly weaker π-complexation interaction. For substituted benzenes, the strength of the complexation increases from the electron-withdrawing to the electron-donating character of the substituent(s) on the aromatic core. In the group of aromatic heterocycles, the highest complexation strength is observed for pyrrole complexes. Linear correlations between the total complexation energy and the NBO interaction energies were observed, suggesting that this interaction is orbital-controlled. Charge transfer trends fairly follow trends in complexation strength. As far as reactivity in insertion reactions is concerned, our work clearly shows that the stronger complexation of the stannylenes in comparison to that of germylenes is one but not the only factor determining their overall rate. The higher stability of the latter with respect to the former indeed increases the difference between the activation barrier of the second ratedetermining step and the complex energy. The activation barrier height of the second rate-determining step is, however, even more strongly tuned by the nature of the donor, as evidenced by the comparison of our complexation data with experimental kinetic data on the overall reaction.

S Supporting Information *

Tables and figures giving complex geometries and energies. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

ACKNOWLEDGMENTS

L.B., F.D.P., and P.G. wish to acknowledge the Research Foundation Flanders (FWO) and the Vrije Universiteit Brussel (VUB) for continuous support to the research group. R.W. is indebted to the Research Council of the VUB for financial support (grant GOA31). A.R. wishes to acknowledge the Czech Science Foundation for financial support (Grant No. 104/09/ 0829). We thank the reviewers for their constructive comments and suggestions. 1615


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