(E = Si, Ge, Sn). A Theoretical Study - American Chemical Society

Sep 26, 2013 - ABSTRACT: Quantum chemical calculations of the reaction profiles for addition of one and two H2 molecules to amido-substituted ditetryl...
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Reaction Pathways for Addition of H2 to Amido-Ditetrylynes R2N− EE−NR2 (E = Si, Ge, Sn). A Theoretical Study Markus Hermann,† Catharina Goedecke,† Cameron Jones,*,‡ and Gernot Frenking*,† †

Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße 1, 35032 Marburg, Germany Department of Chemistry, Monash University, P.O. Box 23, Melbourne, Victoria 3800, Australia

‡

S Supporting Information *

ABSTRACT: Quantum chemical calculations of the reaction profiles for addition of one and two H2 molecules to amido-substituted ditetrylynes have been carried using density functional theory at the BP86/def2TZVPP//BP86/def2-TZVPP level of theory for the model systems L′EEL′ and BP86/def2-TZVPP//BP86/def-SVP for the real compounds. The hydrogenation of the digermyne LGeGeL (L = N(SiMe3)Ar*; Ar* = C6H2Me{C(H)Ph2}2-4,2,6) follows a stepwise reaction course. The addition of the first H2 gives the singly bridged species LGe(μ-H)GeHL, which rearranges with very low activation barriers to the symmetrically hydrogenated compound LHGeGeHL and to the most stable isomer LGeGe(H)2L, which is experimentally observed. The addition of the second H2 proceeds with a higher activation energy under rupture of the Ge−Ge bond, yielding LGeH and LGeH3 as reaction products. Energy calculations which consider dispersion interactions using Grimme’s D3 term suggest that the latter reaction is thermodynamically unfavorable. The second hydrogenation reaction LGeGe(H)2L → L(H)2GeGe(H)2L possesses an even higher activation barrier than the bond-breaking hydrogenation step. Further calculations which consider solvent effects change the theoretically predicted reaction profile very little. The calculations of the model system L′GeGeL′ (L′ = NMe2) give a very similar reaction profile. Calculations of the model disilyne and distannyne homologues L′SiSiL′ and L′SnSnL′ suggest that the reactivity of the amido-substituted ditetrylynes always has the order Si > Ge > Sn. The most stable product of the addition of one H2 to the distannyne L′SnSnL′ is the doubly bridged species L′Sn(μ-H)2SnL′, which has been experimentally observed when bulky groups are employed. Analysis of the H2− L′EEL′ interactions in the transition state for the addition of the first H2 with the EDA-NOCV method reveals that the HOMO− LUMO and LUMO−HOMO interactions have similar magnitudes.

1. INTRODUCTION

The investigations of ditetrylynes experienced an upswing in 2005, when Power et al. reported that H2 can be activated by a m ai n -g ro u p -m e t a l c o m p l e x , A r # G e G e A r # ( A r # = C6H3(C6H3Pri2-2,6)2-2,6), at room temperature and atmospheric pressure.8 The hydrogenation of Ar#GeGeAr# yielded mixtures of Ar#(H)GeGe(H)Ar#, Ar#(H)2GeGe(H)2Ar#, and Ar#GeH3 whose compositions depend on the reaction stoichiometry. The reaction of the analogous distannyne Ar#SnSnAr# with H2 gave a completely different product: i.e., doubly bridged Ar#Sn(μ-H)2SnAr#.9 Using the even more bulky group Ar#+, which has iPr substituents at the 3,5-positions of the central phenyl ring, the hydrogenation of the distannyne Ar#+SnSnAr#+ yields the asymmetrically hydrogenated species Ar#+SnSn(H)2Ar#+.9 This shows that the steric properties of R in ditetrylenes REER may have a profound influence on the reaction course. The reaction mechanism of the addition of dihydrogen to the digermyne Ar#GeGeAr# and the distannyne Ar#SnSnAr# was

The chemistry of low-oxidation-state p-block compounds has been an area of very active research in recent years, which led to the isolation of numerous molecules that exhibit unusual bonding motifs and surprising reactivities.1 A prominent example for the liaison of fundamental and applied research is the isolation and structural characterization of heavy group 14 homologues of alkynes REER (E = Si−Pb) which was achieved from 2000 to 2004 by Power2 (E = Ge−Pb) and Sekiguchi3 (E = Si). While the parent systems HEEH are only stable in a low-temperature matrix, where they possess doubly hydrogen bridged equilibrium geometries4 that were theoretically predicted,5 the substituted molecules REER, which have bulky aryl or silyl groups R, are stable under normal laboratory conditions.2,3,6 The latter systems have a trans-bent geometry of the REER moiety with bending angles between ∼94° (RPbPb)2a and 137°(RSiSi).3a The results of many theoretical analyses of the bonding situation revealed that the E−E bond order in ditetrylynes can range between 3 and 1.7 It was shown that the unusual equilibrium geometries, which strongly deviate from the carbon system and the bond order in REER, can be explained in terms of interactions between the fragments ER in the electronic ground state.7b © XXXX American Chemical Society

Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Received: August 6, 2013

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Figure 1. Calculated reaction profile for the addition of H2 to the model compound L′GeGeL′ (1Ge′) at the BP86/def2-TZVPP level. Bond lengths are given in Å and angles in deg. The dihedral angle θ refers to the NGeGeN fragment.

equilibrium with the hydrido germylene L†(H)Ge:. The homologous distannyne L†SnSnL† could not be prepared, though its formal H2 addition product, L†Sn(μ-H)2SnL†, was synthesized by an alternate route.13 In this paper we report on quantum chemical calculations of the reaction course for dihydrogen addition to 1Ge. We address the question as to why the hydrogenation of LGeGeL gives LGeGe(H2)L as the only isolated reaction product. To this end we calculated the reaction steps for addition of one and two H2 molecules to LGeGeL (1Ge) and to the model compound L′GeGeL′ (1Ge′; L′ = NMe2). We also report on calculated results for the H2 addition to the group 14 homologues distannyne L′SnSnL′ (1Sn′) and disilylene L′SiSiL′ (1Si′), which not yet are experimentally known.

recently investigated in a detailed theoretical study by Wang, Schleyer, and co-workers.10 The calculations suggest that the first step of the H2 addition yields for both systems the singly bridged isomer Ar#E(μ-H)EHAr#, which then may rearrange to other isomers. It was found that the addition of the second H2 molecule has a higher barrier than the first hydrogenation, particularly for the tin system, which explains why the doubly hydrogenated molecule Ar#(H)2SnSn(H)2Ar# was not observed experimentally. The stability and bonding situation of ditetrylenes REER depends strongly on the nature of the substituent R. This has been utilized in a recent study by us,11 where bulky monodentate amido ligands12 were employed for the isolation of the digermyne Ar*(Me3Si)NGeGeN(SiMe3)Ar* (1Ge; Ar* = C6H2Me{C(H)Ph2}2-4,2,6). A theoretical analysis of the bonding situation showed that the Ge−Ge bond in 1, which is significantly longer (2.709 Å) than in Ar#GeGeAr# (2.285 Å), should be classified as a single bond and that the dicoordinated germanium atoms of 1 possess each one electron lone pair.11 The formally vacant p(π) AO of Ge receives electronic charge from the nitrogen lone-pair electrons, which stabilizes the digermyne 1Ge in an unprecedented way. The speculation that the unusual bonding situation in the amido-digermyne might give rise to interesting chemical reactivities was met by the observation that the molecule reacts with dihydrogen in solution and in the solid state at temperatures as low as −10 °C. Surprisingly, the only product that could be isolated is the singly hydrogenated mixed-valence compound LGeGe(H2)L (L = N(SiMe3)Ar*), while the symmetrical product LHGeGeHL was found to be in equilibrium with LGeGe(H2)L in solution and the doubly hydrogenated species LGe(H)2Ge(H)2L was not observed.11 Very recently, we synthesized the related digermyne L†GeGeL† (L† = N(Ar†)(SiPri3), Ar† = C6H2{C(H)Ph2}2Pri2,6,4), which also gives only a singly hydrogenated product.13 Unlike LGeGeL, the molecule is the symmetrically hydrogenated species L†(H)GeGe(H)L†, which in solution is in

2. METHODS Geometry optimizations without symmetry constraints were carried out with the Gaussian 09 optimizer14 in conjunction with Turbomole 6.415 energies and gradients at the BP86 level16 of theory using def2TZVPP17 basis sets for the model system and def2-SVP18 for the real system. The tin atom was calculated with a quasi-relativistic effective core potential.19 Stationary points were characterized by calculating the Hessian matrix analytically. Thermal corrections have been taken from these calculations. Improved single-point energies for the real system were calculated at the BP86/def2-TZVPP level using BP86/ def2-SVP optimized geometries. All energies in this paper are ΔG values at 298 K. The effect of dispersion interactions on the calculated energies of the real system has been estimated using Grimme’s D3 term.20 Solvent effects were considered by calculations of the real system with the COSMO model by Klamt.21 The orbital interactions at the transition state between H2 and the substrate have been investigated with the EDA-NOCV method,22 which combines the energy decomposition analysis (EDA)23 with the natural orbitals for chemical valence (NOCV).24 The EDA-NOCV method makes it possible to partition the total orbital interactions into pairwise contributions of the orbital interactions. Details of the method may be found in the literature.25 The EDA-NOCV calculations were carried out at the BP86/TZ2P+ level of theory with the ADF 2010.02b program package. B

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Figure 2. Calculated reaction profile for the addition of H2 to the real compound LGeGeL (1Ge) at the BP86/def2-TZVPP//BP86/def2-SVP level.The values in italics include dispersion interactions which were calculated at the BP86/def2-TZVPP+D3//BP86/def2-SVP level. The values in boldface consider solvent effects which were calculated at the BP86/def2-TZVPP+D3-COSMO//BP86/def2-SVP level. Bond lengths are given in Å and angles in deg. The dihedral angle θ refers to the NGeGeN fragment.

3. RESULTS AND DISCUSSION Figure 1 shows the calculated reaction course for the addition of H2 to the model digermyne L′GeGeL′ (1Ge′), where the bulky substituents L of LGeGeL (1Ge) are replaced by L′ = NMe2. The weakly exergonic initial reaction step (ΔG = −1.0 kcal/mol) leads to the singly bridged molecule L′Ge(μH)GeHL′ (2Ge′) via the transition state TS1−2Ge′, which has an activation barrier of ΔG⧧ = 20.8 kcal/mol. The compound 2Ge′ is only a shallow minimum on the potential energy surface. It rearranges with small barriers to the energetically lower lying 1,2 -hydrogenated isomer L′(H)GeGe(H)L′ (4Ge′) and the mixed-valence form L′GeGe(H)2L′ (3Ge′), which is 2.5 kcal/mol more stable than 4Ge′. The calculations suggest that there is a rapid equilibration among the isomers 3Ge′ ↔ 2Ge′ ↔ 4Ge′, with 3Ge′ being the most stable form.26 This is in agreement with the experimental result of the real system 1Ge, where only the 1,1-isomer LGeGe(H)2L (3Ge) was isolated as a reaction product in the solid state.11 The overall addition of one H2 to 1Ge′, yielding 3Ge′, is exergonic by ΔGR = −7.7 kcal/mol. It is interesting to note that, for the aryl-substituted system Ar#GeGeAr# of Power, the calculations predict that the 1,2-hydrogenated isomer Ar#(H)GeGe(H)Ar# is lower in energy than the mixed-valence isomer Ar#GeGe(H)2Ar#.10 The calculations further suggest that the addition of the second H2 to 1Ge′ proceeds via the reaction cascade 3Ge′ → 2Ge′ → 4Ge′ + H2 → L′(H)2GeGe(H)2L′ (5Ge′), where the last step has a rather high activation barrier of ΔG⧧ = 29.8 kcal/ mol relative to 3Ge′. We could not locate a transition state for the direct H2 addition reaction 3Ge′ + H2 → 5Ge′. Rather, we found a transition state for the reaction 3Ge′ + H2 → L′GeH

(6Ge′) + L′GeH3 (7Ge′), where the Ge−Ge bond breaks during the reaction (Figure 1). The calculated activation barrier for the H2 addition to 3Ge′ with concomitant breaking of the Ge−Ge bond is only slightly higher in energy (ΔG⧧ = 32.0 kcal/mol) than the stepwise hydrogenation of 3Ge′, yielding 5Ge′ (ΔG⧧ = 29.8 kcal/mol). Therefore, we calculated the hydrogenation reaction of the real system 1Ge. Figure 2 shows that the reaction profile for the hydrogenation of the real digermyne 1Ge is not very different from the H2 addition to the model system 1Ge′ (Figure 1). The first hydrogenation of 1Ge has a slightly lower barrier for the ratedetermining entrance step TS1-2Ge (ΔG⧧ = 20.4 kcal/mol) than for the model compound, and the overall reaction to the most stable mixed-valent product 3Ge is slightly less exergonic (ΔGR = −7.5 kcal/mol) than the formation of 3Ge′, but the differences are very small. A qualitative difference between the hydrogenations of the real system and the model system is found for the addition of the second H2. Figure 2 shows that the energy barrier for the reaction cascade 3Ge → 2Ge → 4Ge + H2 → L(H)2GeGe(H)2L (5Ge) is now higher (ΔG⧧ = 29.3 kcal/mol) than the activation energy for hydrogenation with concomitant Ge−Ge bond rupture: 3Ge + H2 → LGeH (6Ge) + LGeH3 (7Ge) (ΔG⧧ = 25.0 kcal/mol). The calculated reaction profile for the hydrogenation of the real germanium system 1Ge is in accord with the experimental finding that the addition of one H2 gives the mixed-valence compound 3Ge at low temperatures. A product of the second hydrogenation was not observed experimentally.11 1H NMR measurements of the reaction product at low temperature showed a spectrum with one hydride resonance and two sets of amide resonances, which agrees with the formation of the C

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fully hydrogenated molecule Ar#(H)2GeGe(H)2Ar# (ΔG⧧ = 19.3 kcal/mol) or the fragments Ge(H)3Ar# + Ge(H)Ar# (ΔG⧧ = 19.6 kcal/mol) are very similar. This explains why the products of both reactions are experimentally observed.8 The activation barrier for the second hydrogenation of the energetically lower lying symmetrical isomer Ar#(H)GeGe(H)Ar# is much higher than for hydrogenation of Ar#GeGe(H)2Ar#. This order is also found for the hydrogenation of L(H)GeGe(H)L and LGeGe(H)2L. However, the calculations of the latter system gave only one transition state for the hydrogenation of LGeGe(H)2L, which leads under rupture of the Ge−Ge bond to the fragments Ge(H)3L + Ge(H)L. Unlike the hydrogenation of Ar#GeGe(H)2Ar#, the latter process is endergonic; thus, it is not observed. The second relevant difference between the two systems is that we could not locate a transition state for the hydrogenation of LGeGe(H)2L yielding L(H)2GeGe(H)2L. The calculations show that the theoretically predicted reaction course for the first hydrogenation of the model system 1Ge′ is very similar to the real system. The only difference which is relevant is the second hydrogenation step. The calculated reaction energies and activation barriers for the most important step of the hydrogenation of germanium model system and the real system are given in Table 1.

isomer 3Ge. The NMR signals broaden at higher temperature, which suggests a rapid interconversion with the energetically higher lying isomer 4Ge. However, the calculated barrier for the first hydrogenation step (ΔG⧧ = 20.4 kcal/mol) is only 4.6 kcal/mol lower than the barrier for the second hydrogenation with associated Ge−Ge bond rupture (ΔG⧧ = 25.0 kcal/mol). The difference is too small to explain the rapid addition of one H2, while the second hydrogenation does not take place. Therefore, we carried out further calculations of the real system which consider dispersion interactions using Grimme’s D3 term and solvent effects using the COSMO approach. The calculated data are also shown in Figure 2. The inclusion of dispersion interactions at the BP86/def2TZVPP+D3//BP86/def2-SVP level which are given in italics changes the activation barriers only slightly. The two most important barriers for the first hydrogenation (ΔG⧧ = 18.4 kcal/mol) and second hydrogenation with Ge−Ge bond rupture (ΔG⧧ = 24.9 kcal/mol) become slightly smaller, but the difference of ΔΔG⧧ = 6.5 kcal/mol still remains rather small. A dramatic change is found, however, for the reaction energy of the second step. The calculations which include dispersion forces suggest that the reaction 3Ge + H2 → 6Ge + 7Ge is an endergonic process by 4.8 kcal/mol. This means that the second reaction step does not take place, because it is thermodynamically unfavorable. This is an interesting result which is related to a recent report about stable ethanes R3C− CR3 that carry bulky substituents. While hexaphenylethane (R = C6H5) is not a stable molecule but dissociates to Gomberg’s radical C(C6H5)3, more bulky aryl groups Ar* which carry two tert-butyl substituents at each phenyl ring strongly stabilize the compound (Ar*)3C−C(Ar*)3, which melts at 214 °C.27 The present results indicate that the second hydrogenation with concomitant Ge−Ge bond breaking is prevented by dispersion interactions which stabilize 3Ge. The calculations which consider solvent effects using the COSMO model give results which do not change the above conclusion. The calculated data at the BP86/def2-TZVPP +D3+COSMO//BP86/def2-SVP level which are in boldface in Figure 2 change the activation barriers and reaction energies by only ∼1 kcal/mol. Finally, we also carried out broken-symmetry calculations at the BP86/def2-SVP level in order to see if the single-reference calculations are reliable for the transition states, which have some radical character. The activation barriers changed by less than 0.1 kcal/mol. It is instructive to compare the calculated reaction profile for the hydrogenation of the amido-digermyne LGeGeL which is shown in Figure 2 with the theoretical results for the arylsubstituted digermyne Ar#Ge(μ-H)2GeAr# that were reported by Zhao et al.10 The calculated reaction courses exhibit some similarities but also some distinctive differences. The first hydrogenation step leads in both cases to the singly bridged isomer RGe(μ-H)GeHR, where R denotes a general substituent. The activation barriers ΔG⧧ of the exergonic reactions are not very different (18.4 kcal/mol for R = Ar#, 20.4 kcal/mol for R = L). The singly bridged species RGe(μ-H)GeHR rearranges in both cases with low barriers to classical isomers. As noted above, the symmetrical form R(H)GeGe(H)R is slightly lower in energy than the mixed-valence isomer RGeGe(H)2R when R = Ar#, while the reverse order is calculated for R = L. The most important difference between the two systems concerns the second hydrogenation step. Zhao et al.10 found that the activation barriers for hydrogenation of the mixed-valence isomer Ar#GeGe(H)2Ar# which yield the

Table 1. Calculated Activation Energies ΔG⧧ and Reaction Energies ΔGR at the BP86/def2-TZVPP Level (kcal/mol) of the Most Important Steps of the Hydrogenation of L′EEL′ and LGeGeL 1E′ + H2 → 2E′ ΔG⧧ ΔGR 3E′ + H2 → → → 5E′ ΔG⧧ ΔGR 3E′ + H2 → 6E′ + 7E′ ΔG⧧ ΔGR

E = Si

E = Gea

E = Sn

15.2 −14.0

20.8 (20.4; 18.4; 19.0) −1.0 (−3.3; -3.7; -3.0)

23.0 5.8

19.2 −24.3

29.8 (29.3; 30.2; 29.8) −4.1 (−6.4; -8.0; 7.5)

41.3b 17.1b

19.3 −8.0

32.0 (25.0; 24.9; 25.6) 1.3 (−13.2; 4.8; 2.7)

34.8 15.3

a

The values in Roman text in parentheses refer to the real system LGeGeL which were calculated at the BP86/def2-TZVPP//BP86/ def2-SVP level. The data in italics include dispersion interactions at the BP86/def2-TZVPP+D3//BP86/def2-SVP level. The values in boldface also consider solvent effects at the BP86/def2-TZVPP+D3COSMO//BP86/def2-SVP level. bValues for the reaction of 8Sn′.

We calculated the hydrogenation of the analogous silicon and tin model compounds 1Si′ and 1Sn′ in order to find out about the trend of their reactivities. Figure 3 shows the calculated reaction profile for hydrogenation of the model disilyne L′SiSiL′ (1Si′). The qualitative features of the theoretically predicted reaction course of the silicon system are similar to the results for the germanium system (Figure 1). The first step of the hydrogenation of 1Si′ has a lower barrier (ΔG⧧ = 15.2 kcal/mol) and the formation of the most stable isomer 3Si′ is more exergonic (ΔGR = −24.0 kcal/mol) than for germanium. The calculations predict that the disilyne 1Si′ is kinetically and thermodynamically more reactive than the digermyne 1Ge′. In addition, the second step of the hydrogenation reaction of 1Si′ is calculated to be more facile than for the germanium compound. The activation barrier for the second hydrogenation with retention of the Si−Si bond, 3Si′ → 2Si′ → 4Si′ + H2 → L′(H)2SiSi(H)2L′ (5Si′), has an D

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Figure 3. Calculated reaction profile for the addition of H2 to the model compound L′SiSiL′ (1Si′) at the BP86/def2-TZVPP level. Bond lengths are given in Å and angles in deg. The dihedral angle θ refers to the NSiSiN fragment.

Figure 4. Calculated reaction profile for the addition of H2 to the model compound L′SnSnL′ (1Sn′) at the BP86/def2-TZVPP level. Bond lengths are given in Å and angles in deg. The dihedral angle θ refers to the NSnSnN fragment.

activation energy of ΔG⧧ = 19.2 kcal/mol. The second hydrogenation reaction with concomitant Si−Si bond rupture, 3Si′ + H2 → L′SiH (6Si′) + L′SiH3 (7Si′), has nearly the same barrier of ΔG⧧ = 19.3 kcal/mol. It is interesting to note that the calculated activation barriers do not conform with the thermodynamic stabilities of the reaction products. The molecule L′(H)2SiSi(H)2L′ (5Si′) is 16.3 kcal/mol lower in

energy than the isomers L′SiH (6Si′) + L′SiH3 (7Si′) (Figure 3). The calculated activation barriers suggest that the products 5Si′, 6Si′, and 7Si′ should be found in nearly identical amounts. The calculated results of the model system 1Si′ let us predict that the real silicon compound LSiSiL (1Si) reacts more easily with dihydrogen than the germanium complex LGeGeL (1Ge). The reaction product of the addition of one H2 to 1Si should E

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Figure 5. EDA-NOCV analysis of the transition state TS1−2Ge′: (a) qualitative sketch of the orbital interactions between H2 and Me2N−GeGe− NMe2 (1Ge′); (b) frontier orbitals of the reacting species H2 and 1Ge′; (c) EDA-NOCV stabilization energies between the two most important pairs of interacting orbitals in the transition state TS1-2Ge′. The deformation densities Δρ′ and Δρ″ give the charge flow from the yellow to the green areas.

reactions which possess higher barriers than hydrogenation of the germanium compound 1Ge′ and the silicon compound 1Si′. The calculated data indicate that the reactivities of the amido-substituted ditetrylynes always exhibit the order Si > Ge > Sn. The activation barriers for the first and second hydrogenation of L′EEL′ become higher for the heavier atoms E. This could be related to the strengths of the E−H bonds, which are weaker for the heavier atoms. The calculated reaction energies ΔGR which are shown in Table 1 indicate that all hydrogenation reactions become thermodynamically less favorable when E becomes larger.29 The calculations predict that the addition of the first H2 to L′EEL′ has for all atoms E a lower activation barrier than the addition of the second H2. We analyzed the nature of the H2− L′EEL′ interactions with the EDA-NOCV method in order to find out which orbitals are involved in the addition reaction of the first step. The geometry of the transition state TS1-2Ge′ indicates that H2 attacks one atom E rather than the E−E moiety but that the second atom E is involved in the reaction, which leads to the singly bridged species L′E(μ-H)EHL′ (2E′). The top part of Figure 5 schematically shows for the germanium system the structure of the interacting species. Figure 5a displays qualitatively the frontier orbitals of H2 and L′GeGeL′. The HOMO and LUMO of the two molecules at the equilibrium geometries are shown in Figure 5b. The crucial point on the PES is the transition state where the two species undergo significant distortion relative to the starting geometry. Figure 5c shows the deformation densities which give the charge flows (yellow → green) that are associated with the most important orbital interactions of H2 and L′GeGeL′ in the transition state TS1-2Ge′. It becomes obvious that the strongest orbital interaction (ΔEorb′ = −37.0 kcal/mol) comes from charge donation of H2 to the germanium atom at the right-hand side. The second strongest term (ΔEorb″ = −35.1 kcal/mol) exhibits a charge flow in the reverse direction

be the mixed-valence compound LSiSi(H)2L, while the addition of the second H2 should take place in a kinetically controlled reaction with rupture of the Si−Si bond, yielding LSiH + LSiH3. Figure 4 shows the calculated reaction profile for hydrogenation of the model compound distannyne L′SnSnL′ (1Sn′). The overall reaction course is similar to the hydrogenation of the analogous silicon and germanium compounds. However, the symmetrically hydrogenated first product L′(H)SnSn(H)L′ (4Sn′) is now slightly 0.7 kcal/mol lower in energy then the mixed-valence isomer L′SnSn(H)2L′ (3Sn′). More important is the occurrence of the doubly bridged species L′Sn(μ-H)2SnL′ (8Sn′), which is the most stable isomer of the singly hydrogenated distannyne 1Sn′. The calculations predict that 8Sn′ is 4.3 kcal/mol more stable than 4Sn′ (Figure 4). This is in agreement with our recent experimental finding of the formal L†SnSnL† hydrogenation product, L†Sn(μ-H)2SnL†.13 The calculations suggest that the analogous germanium and silicon systems L′E(μ-H)2EL′ are higher in energy than the classical isomers L′EE(H)2L′ and L′(H)EE(H)L′.28 Therefore, we did not consider them in the above discussion. The formation of the doubly bridged distannyne 8Sn′ may easily take place via rupture of the Sn−Sn bond of 4Sn′ and recombination of the fragments in the reaction 4Sn′ → 2 6Sn′ → 8Sn′ (Figure 4). The latter reactions should take place with small activation barriers. The overall reaction of the model compound 1Sn′ → → 8Sn′ is slightly endergonic by 0.7 kcal/mol, but the use of bulky substituents will cause the reaction to become thermodynamically feasible. Not unexpectedly, the hydrogenation of the tin compound is thermodynamically and kinetically less favored than the reactions of the lighter homologues. This becomes obvious when the calculated data in Table 1 for the different metals are compared with each other. The calculations suggest that the additions of one and two H2 molecules to 1Sn′ are endergonic F

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and is mainly associated with charge accumulation between the bridging hydrogen atoms and the germanium atom on the lefthand side. Thus, HOMO−LUMO and LUMO−HOMO interactions between the two fragments have nearly the same strength. Other pairwise orbital interactions are much weaker ( Ge > Sn. Unlike the disilynes and digermynes, the addition of one H2 to L′SnSnL′ leads to the doubly bridged L′Sn(μ-H)2SnL′ as the most stable reaction product. Analysis of the H2−L′EEL′ interactions in the transition state for the addition of the first H2 with the EDANOCV method reveal that the HOMO−LUMO and LUMO− HOMO interactions have similar magnitudes.

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving calculated coordinates (Å) and energies (hartree) of the molecules. This material is available free of charge via the Internet at http://pubs.acs.org. G

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Organometallics

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(10) Zhao, L. L.; Huang, F.; Lu, G.; Wang, Z.-X.; Schleyer, P.v.R. J. Am. Chem. Soc. 2012, 134, 8856. (11) Li, J.; Schenk, C.; Goedecke, C.; Frenking, G.; Jones, C. J. Am. Chem. Soc. 2011, 133, 18622. (12) Li, J.; Stasch, A.; Schenk, C.; Jones, C. Dalton Trans. 2011, 40, 10448. (13) Hadlington, T. J.; Hermann, M.; Li, J.; Frenking, G.; Jones, C. Angew. Chem., Int. Ed. 2013, 52, 10199. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K, N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian, Inc., Wallingford, CT, 2009. (15) TURBOMOLE V6.4 2012, a development of the University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007, TURBOMOLE GmbH, since 2007; available from www.turbomole. com. (16) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (17) (a) Weigend, F.; Häser, M.; Patzelt, H.; Ahlrichs, R. Chem. Phys. Lett. 1998, 294, 143. (b) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. (18) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (19) Metz, B.; Stoll, H.; Dolg, M. J. Chem. Phys. 2000, 113, 2563. (20) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. (21) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799. (22) Mitoraj, M. P.; Michalak, A.; Ziegler, T. J. Chem. Theory Comput. 2009, 5, 962. (23) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1. (24) Mitoraj, M. P.; Michalak, A. J. Mol. Model. 2007, 13, 347. (25) Mitoraj, M. P.; Michalak, A.; Ziegler, T. Organometallics 2009, 28, 3727. (26) There is yet another isomer L′Ge(μ-H)2GeL′ where the germanium atoms are doubly bridged by the hydrogen atoms. Such an isomer was also found by Zhao et al.10 for the system Ar#Ge(μH)2GeAr#. The doubly bridged species L′Ge(μ-H)2GeL′ is energetically higher lying (ΔG = −1.6 kcal/mol) than 3Ge′ and 4Ge′; therefore, it is not shown in Figure 1. The calculations of Ar#Ge(μH)2GeAr# which are reported by Zhao et al.10 show that this isomer does not play a role in the reaction course. (27) Grimme, S.; Schreiner, P. R. Angew. Chem., Int. Ed. 2011, 50, 12639. (28) The germanium species L′Ge(μ-H)2GeL′ is 6.1 kcal/mol higher in energy than 3Ge′, while the silicon isomer L′Si(μ-H)2SiL′ is 15.7 kcal/mol higher in energy than 3Si′. (29) The reactivity of the ditetrylynes REER toward hydrogenation appears also to depend on the nature of the substituent R. Zhao et al. report in ref 10 that the first hydrogenation of Ar#SnSnAr# which yields Ar#Sn(μ-H)SnHAr# has an activation barrier of ΔG⧧ = 14.4 kcal/mol that is lower than the first hydrogenation of Ar#GeGeAr#, which has ΔG⧧ = 18.4 kcal/mol. (30) The attentive reader will notice that the strength of the attractive pairwise orbital interactions increases from Si (−68.3 kcal/ mol) to Ge (−72.9 kcal/mol) and Sn (−76.7 kcal/mol), which appears to contradict the increase of the activation barrier. Please note that we give only the attractive orbital term of the H2−L′EEL′ interactions.

The total interactions comprise the electrostatic interactions and the Pauli repulsion. The latter term is the reason the energy of the transition state is higher than that of the educt. (31) (a) Fukui, K. Acc. Chem. Res. 1971, 4, 57. (b) Fukui, K. Theory of Orientation and Stereoselection; Springer Verlag: Berlin, 1975.

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dx.doi.org/10.1021/om4007888 | Organometallics XXXX, XXX, XXX−XXX