Investigation of Thiaborane closo–nido Conversion Pathways

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Investigation of Thiaborane closo−nido Conversion Pathways Promoted by N‑Heterocyclic Carbenes Jan Vrań a,† Josef Holub,‡ Zdeňka Růzǐ cǩ ova,́ † Jindrǐ ch Fanfrlík,*,§ Drahomír Hnyk,*,‡ and Aleš Růzǐ cǩ a*,†

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Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic ‡ Institute of Inorganic Chemistry, Czech Academy of Sciences, 250 68 Ř ež, Czech Republic § Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo náměstí 542/2, 166 10 Praha 6, Czech Republic S Supporting Information *

ABSTRACT: The 12-X-closo-SB11H10 (X = H or I) thiaboranes react with one or two molar equivalents of various N-heterocyclic carbenes (NHCs) to give the deprotonated 12-vertex species of [12-X-SB11H9· NHC]−[NHC-H]+composition as kinetic products. The use of one molar equivalent of a sterically more hindered NHC reactant leads to the formation of 12-X-SB11H10·NHC adducts with a heavily distorted cage and the nido electron count. Further reaction of 12-I-SB11H10· NHC to deboronated 12-X-SB10H9·NHC proceeds in acetone to complete the closo−nido reaction pathway under the thermodynamic control. The structures of all compounds have been investigated by NMR spectroscopy and diffraction techniques. The results are supported by theoretical methods.

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INTRODUCTION The reactivity of boron clusters has been extensively studied in the recent decades.1 In contrast to organic chemistry, the corresponding reaction mechanisms can be very complex since there are very small energy differences between many intermediates and transition states. On that basis, reactions of boron hydrides can involve many competing pathways. However, much progress has been made so far in the understanding of the reaction mechanisms of boron hydrides and the carboranes derived from them.1 One of the reaction schemes in this area of chemistry comprises the dismantling of a cluster, specifically the removal of one or more vertices.2 This can be done through cluster deboronation or deprotonation, which can be demonstrated on reactions of icosahedral 12-vertex closo-1,2-C2B10H12, known as o-carborane, with various Lewis bases.3 It can also happen that selected Lewis bases simply open the closo arrangement without removing a BH unit. Indeed, HNP(NMe2)3 opens closo-1,2-C2B10H12 to the isolated and structurally characterized adduct C2B10H12·HNP(NMe2)3 (Scheme 1A),3a whose molecular shape has been treated as if it were derived from the still elusive 13-vertex closo docosahedron with a 5-coordinate vertex removedcloso-[B13H13]2− (for its modeled structure, see the SI).4 The final product of that closo−nido conversion is [H2NP(NMe2)3]+[nido-7,8-C2B9H12]− (Scheme 1B). N-Heterocyclic carbenes (NHCs) as neutral bases have also been made to react with various boranes5 and o-carborane.6 Indeed, © XXXX American Chemical Society

the deboronation of the latter by NHC substituted by tPe groups finally resulted in nido-7,8-C2B9H12−. Interestingly, the extremely air- and moisture-sensitive two-cage [C4B20H23]− (Scheme 1C) intermediate was identified during such a deboronation process.6a The exclusive product of dismantling in the same reaction with less sterically hindered NHCs, substituted by Me or Et groups, was nido-carborane with the exoskeletal BH group coordinated by two NHCs (Scheme 1D). When the sterically demanding NHC with Dip (2,6iPr2C6H3) groups was used in the same reaction, only a 1:1 adduct with a coordinated exoskeletal BH group was isolated.6b In addition, the structure of the hypothetical deprotonation product of o-carborane by NHC substituted by tPe (tertpentyl-, 1,1-dimethylpropyl-) group upon the formation of the 1,3-di-tert-pentylimidazolium closo-7,8-C2B10H11− ion pair was optimized by theoretical methods.6a In this respect, nido-1,2C 2 B 10 H 12 2− was computationally shown to be a key intermediate in polyhedral expansion reactions from 12- to 13-vertex carboranes and metallacarboranes.7 Moreover, 13vertex closo-1,2-(CH2)3-1,2-C2B11H118 reacts with the NHCs to provide ion pairs (Scheme 2A) or zwitterionic adducts (Scheme 2B) of 13-vertex nido-carboranes as kinetic and thermodynamic products, respectively.9 Received: October 27, 2018

A

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

borane into account. An isoelectronic and isostructural thiaborane in relation to closo-1,2-C2B10H12 is closo-1-SB11H11 (1a), which has been structurally characterized10 and whose charge distribution has been experimentally confirmed.11 This molecule has recently received attention because the sulfur atom in 1a has highly positive σ-holes,12 which could accept an external electron donor. This seems to be the most significant difference between 1a and closo-1,2-C2B10H12, in which the midpoint of the C−C vector also bears a positive charge,13 but the occurrence of the σ-holes is precluded by the presence of the hydrogen atoms bound to both carbon atoms. According to the above-mentioned structural conclusions,14 thiaboranes could interact via a chalcogen bond with several bases and coordinating solvents. These attempts to react mostly nido-thiaboranes with several nitrogen and phosphorus bases have already been described. In fact, only the products of the deprotonation or substitution of parent thiaboranes have been isolated and structurally characterized.15 To obtain a deeper insight into the reaction mechanisms of thiaboranes, we report here reactions with the subsequent exploration of the corresponding reaction pathway of icosahedral thiaboranes with NHCs.

Scheme 1. Known Products of Heteroborane Reactions with NHCs

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RESULTS AND DISCUSSION First, we tried to mix and cocrystallize closo-1-SB11H11 (1a) and 12-I-closo-1-SB11H10 (1b) 12-vertex thiaboranes with DMSO, HMPA, triethylamine, DABCO, triphenylphosphine, and triphenylstibane under an argon atmosphere to explore the interaction ability of these parent compounds. Unfortunately, as observed by 1H, 13C, and 11B NMR spectroscopy techniques, no interaction between thiaboranes and the mentioned bases takes place in inert deuterated solvents. None of the attempts to cocrystallize these species to achieve structural information using sc-XRD methods was successful (Scheme 3). On the basis of these facts, we decided to increase the s-electron density of the base using various NHCs known to be very good stabilization ligands for highly reactive maingroup metal fragments.16 To study the ability of 1a and 1b to achieve their reactions via chalcogen bonding, we opted for employing the less sterically demanding NHC (1,3-diisopropylimidazol-2-ylideneNHCiPr) with them in the 2:1 molar ratio in relation to the carbene in dry diethyl ether (Scheme 3). [As is obvious, most of the compounds reported in this paper are formally derived from the hypothetical closo-B13H132−. Taking into account the different numbering of atoms of diverse types of reported boron clusters, we have decided to use a simplified numbering of atoms in the figures, experimental section, and discussion. For all compounds except for 6a, the sulfur atom always has number 1 and the boron atom substituted by the carbene ligand number 2. The remaining atoms are numbered according to the IUPAC rules.] Immediately after the addition of the carbene, the final products 2a and 2b were formed as bright-colored precipitates (2a orange, 2b yellow). To the best of our knowledge, both 2a and 2b are the first examples of ionic monoheteroborane clusters−carbene adducts, with the moiety being, like species A (Scheme 1), based on the parent but still elusive 13-vertex closo-[B13H13]2−. As reported by Fox, the reactions of ocarboranes with NHCs bearing small substituents (Me, Et) on the nitrogen atoms surprisingly proceeded as deboronations in the 1:2 ratio (Scheme 1D)6a while the 1:1 stoichiometry (for R = Me, Et, nBu) yielded deprotonation products (NHCRH)+ (closo-1,2-C2B10H11)−.17 In the case of thiaboranes 1a and 1b,

Scheme 2. Known Products of Other Heteroborane Reactions with NHCs

To the best of our knowledge, all the studies of reaction mechanisms involving a NHC have been focused on boranes and carboranes,1 without taking any other so-called heteroB

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 3. Reactivity of Thiaboranes 1a and 1b with NHCs

Figure 1. Molecular structure of 2a (left) and 2b (right): ORTEP viewthe 30% probability level; the hydrogen atoms of the NHC moieties as well as chloroform solvent molecules have been omitted for clarity.

are soluble in chlorinated solvents (DCM, chloroform), but slowly decompose (2a within weeks, 2b within hours), forming a mixture of deboronation and deprotonation products

the 1:1 ratio yields only 1:1 mixtures of starting thiaboranes and 2a or 2b, respectively. Both adducts 2a and 2b are stable in the solid state under an argon atmosphere. These complexes C

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry according to the 1H and 11B NMR spectra probably via a chlorine atom, from solvent molecule, abstraction. On the contrary, they are very stable in a solution of acetone (no decomposition was observed after one month). The structure of both compounds 2a and 2b was confirmed by the multinuclear NMR spectroscopy. The 1H and 13C spectra revealed one set of signals for two different NHC backbones. The 11B spectra exhibited almost identical patterns of 2:1:2:2:1:3 (2a) and 2:1:2:2:1:1:2 (2b) with comparable chemical shifts (the range from −23.0 to 5.0 ppm (2a) and −21.3 to 2.7 ppm (2b); for details, see the SI, experimental section) with the exception of the differently substituted B12 vertex, which is upfield-shifted in 2b (−3.0 ppm) with respect to 2a (5.0 ppm). For the structures of 2a and 2b (Figure 1), it is clear that the deprotonation takes place at the B2 atom, where also the carbene moiety is coordinated. The B2−S1 and B3−S1 bonds are cleaved, which results in interatomic distances about 1 Å longer than those found for derivatives of the parent closoSB11H11.10 A similar type of elongation has been found for all B−B bonds in the upper pentagon, where the extreme cases of B2−B6 and B3−B4 pairs of atoms are ca. 0.3 Å more distant than in the case of most of boranes and heteroboranes. In 2b, the I1 atom is unequivocally present at the B12 vertex located in the part opposite to the sulfur. The negative charge of the adduct is compensated by the imidazolium moiety, which only interacts with the cage through several noncovalent-like B− H···π, dihydrogen bonds, C−H···B−H, B−H···S, and dihalogen interactions. It should be noticed that B−H is an electron donor while the protonated imidazolium moiety is an electron acceptor in these interactions (see Figure S2). The reactions of 1a and 1b with more bulky carbene (NHCDip, Dip = 2,6-iPr2C6H3) in a 1:2 molar ratio were carried out in dry diethyl ether (Scheme 3). Two pairs of different products were isolated from both reaction mixtures. In each case, a colorless precipitate of a 1:1 adduct (3a or 3b) was filtered off the orange solution, which was further concentrated and left for crystallization at 4 °C overnight, yielding orange crystals of ionic species of the composition [NHCDipH]+[SB11H9X·NHCDip]− (5a or 5b) with thiaborane:carbene ratio 1:2. All compounds 3a, 3b, 5a, and 5b were prepared in moderate (48% for 5a) to high (82−92% for 3a, 3b, and 5b) yields by choosing the relevant reaction conditions and stoichiometry. Slow addition of a diluted solution of NHCDip to the solution of thiaboranes directs the reaction toward 1:1 adducts 3a and 3b according to the 1H and 11B NMR spectroscopy. In contrast, the addition of 1b to the solution of NHCDip leads to the direct formation of 5b. The same reaction protocol also works for 1a, but there is always a small amount of 3a (10−15% calculated to the parent thiaborane) as a byproduct present in the reaction mixture. The formation of 1:2 products is not in agreement with the literature findings, because o-carborane treated with NHCDip only gave 1:1 adducts6b for steric reasons, which is in strong contrast to the less hindered NHCs, where the formation of 1:2 adducts containing a nido-carborane (Scheme 1D)6a is observed. Poorly characterized ionic liquids of the claimed composition [NHCRR′]+[C2B10H11]− (R, R′ = Me, Et, nBu) were also reported.17 It is noteworthy that 3a and 3b do not react with a further equivalent of NHCDip, which means that the formation of these compounds is not an initial step for the preparation of 5a and 5b, respectively. This fact will be discussed further in the theoretical study that in other words

will distinguish between thermodynamically and kinetically driven pathways. Interestingly enough, 3a smoothly reacts with NHCiPr to give the 1:2 adduct 4a (Scheme 3). For an evaluation of such reactivity patterns and to account for the fact that thiaboranes do not interact with coordinating solvents or added weak bases, the hybrid NHCOMe containing one Dip and one methoxypropenyl substituent was reacted with 1a. This resulted in the fast and almost exclusive formation of 3a′, an analogue of 3a and 3b containing no notable interaction of the methoxy group. Mutually similar molecular structures of 3a, 3a′, and 3b (Figure 2) are in some respect analogous to 2a and 2b, respectively. They are composed of distorted SB11 cages, where even larger separations of B−B in the upper pentagonal rim and S1−B(2, 3) for 3a and 3a′ and S1−B(4, 5) for 3b were found. This further increase by ca. 0.2 Å in comparison to the situation in 2a and 2b is most likely caused by the fact that the hydrogen atom originating from the B−H bond of the B2 atom in the parent cage is now located in the bridging position between B2 and B3 atoms, which makes them more coordinatively and electronically saturated than in previous cases, where the second equivalent of the NHC acted as a hydrogen acceptor. Likewise the distances between the carbene donor carbon atom C13 and B2 in all discussed compounds are almost the same. On the other hand, a different type of isomer, is formed in the case of iodinated species 3b. The free carbene attack cleaves the upper pentagon of the cage in a similar way but remains coordinated to the boron atom which is still bound to the sulfur atom (Figure 2). This is virtually the opposite arrangement of the cage-carbene than in 3a or 3a′. This arrangement is most probably caused by different electronic properties of H and I substituents than the steric repulsion of the iodine atom and bulky NHCDip. As already noted, the sulfur atom in the 12-Ph-SB11H10 compound interacts through a chalcogen bond with the phenyl ring of the neighboring molecule. Although the geometry of the compounds 3a, 3a′, and 3b permits such interactions, the phenyl rings of the Dip substituents should be oriented with their centroids slightly closer (with the Cg−S1 distance about 0.5 Å longer than in 12-Ph-SB11H10) to the sulfur atom to be considered as a relevant contact. The structure of 5a (Figure 3) is similar to those of 2a and 2b, where less sterically demanding NHC is used. In this case, the significant elongation of the S1−B2,3 was again detected to be ca. 0.8 Å larger than ordinary bonding distances of this kind. Another difference is the fact that the hydrogen atom scavenger interacts in 5a with the diethyl ether solvent molecule instead of the thiaborane cage. Compounds 3, 4a, and 5 are stable in the solid state under an argon atmosphere. They are soluble in dichloromethane but decompose within several hours or days which is probably connected to the chlorine radical abstraction process. Their acetone solutions are stable for weeks with the exception of 3b, which undergoes deboronation almost quantitatively to give 6b (Scheme 3). Surprisingly enough, 3a in acetone solution remains completely intact after one month. Therefore, the structural differences in both nido--cages of 3a and 3b play a crucial role in the stability. On the contrary, a different type of deboronation of the o-carborane and NHCDip 1:1 adduct after exposure to air has been reported. The carbene was protonated from an external source upon the formation of the ionic compound [NHCDipH]+[C2B9H12]−.6a D

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Molecular structure of 5a: ORTEP viewthe 50% probability level; the hydrogen atoms of the NHC moieties, the second positions of the disordered S and B−H atoms, as well as solvent molecules have been omitted for clarity. The longer distance between atoms is visualized by the dotted line.

three-coordinated sulfur of clusters 3a, 3a′, and 3b, along with the observation of three formally different isomers of these clusters in the solid state, has led us to the idea of the sigmatropic rearrangement in the suprafacial mode (wellestablished for 2D aromatics18) of the carbene ligand moving around the boron pentagon followed by a transfer of the hydrogen bridge between other boron atoms and the formation/release of the processes of sulfur−boron bonds. For an investigation of this process, several multinuclear and 2D NMR techniques have been applied including the variable temperature measurements. It is absolutely true that the dynamic process is taking place on the thiaborane part of the molecule where broadening and coalescence of signals were observed in coupled and decoupled 1H and 11B NMR spectra, while the carbene part of signals stayed nearly intact in 1H NMR spectra in the whole temperature range (see the SI, Figures S4−S6). Unfortunately, only a coalescence of some signals and broadening of others are detected at the lower temperature limit (−40 °C in acetone-d6) in 11B NMR which do not allow structural conclusions. The same is likely to be true for clusters 2a and 4a. These particular processes did not allow a proper comparison of calculated chemical shifts for optimized geometries based on solid-state molecular shapes and measured values as the latter ones are strongly influenced by the existence of such fast exchange processes hardly detectable by the “slow” technique of the nuclear magnetic resonance, which is able to establish only the mean values of the individual stationary states. Indeed, when computing

Figure 2. Molecular structure of 3a (top), 3a′ (middle), and 3b (bottom): ORTEP viewthe 30% probability level; the hydrogen atoms of the NHC moieties, the second positions of disordered S and B−H atoms, as well as solvent molecules have been omitted for clarity. Longer distances between atoms are visualized by dotted lines.

The compounds 3a, 3a′, and 3b could be understood as being on the path from closo to nido structures. Significant elongation of some S−B bonds associated with a subsequent transfer of one of the terminal hydrogen atoms to the bridging position could lead to the observation of different isomers of 1:1 adducts as the carbene attack proceeds from various directions. On the other hand, these isomers should be close in energy, and thus, the equilibrium is expected in solution because of cage conformational changes, fast motion of the hydrogen bridge, and the carbene ligand on the upper rim. The broadening of the signals for five boron atoms and appropriate hydrogen atoms from the upper B5 pentagon “bridged” by the E

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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vertices are in the normal range with the exception of the B9− B10 separation, which is slightly elongated as a result of the presence of the hydrogen bridge between them. To test the dispersion forces that are responsible for most of the chalcogen and pnictogen interactions (which are more pronounced for heavier elements and lower oxidation states), we reacted 1a with the antimony(I) complex20 8 (2,6-(tBuNCH)2-C6H3Sb) as an analogue of an unsuccessful reaction of 1a with stibanes. Surprisingly enough, both compounds were intact in solution even when warmed. The addition of one molar equivalent of NHCiPr to that solution caused the formation of a small amount of single-crystalline material of [NHCiPr(H)]+[SB10H11]− (7a, Scheme S1). Under the desired conditions, the very rich s-electron density of the carbene NHCiPr is apparently sufficient to attract one of the hydrogen atoms of the SB11H11 and deprotonate it. Under appropriate conditions, in terms of solubility and the presence of 8, most probably the crystallization of the 7a has a stabilization effect on that presumably very instable species. From the structural point of view, complex 7a crystallizes as a dimer {[NHCiPrH]+[SB10H11]−}2 (Scheme S1 and Figure S3, Supporting Information), exhibiting similar types of noncovalent interactions within the crystal lattice to 2a but a nido arrangement of the cage of anionic nature. When 7a is dissolved in common organic solvents, only a mixture of unidentified compounds is detected by NMR spectroscopy. Several attempts to repeat and modify the reaction to increase the yield of 7a have been performed, but without success.

GIAO-MP2 shielding tensors for the MP2/TZP geometries of the 2a (in the computed interval between −29.0 and 26.3 ppm, with the fit not much improved by the addition of the counterion) and 4a solid-state-based molecular shapes, the agreement with the experimental values was very poor, the 3a geometry (in the computed interval between −27.5 and 21.1 ppm) behaving slightly better. In addition, molecular dynamics (MD) simulations were not able to reveal any structure or a mixture of them that would account for the experimental 11B NMR chemical shifts. The 1H and 13C NMR spectra of 4a, 5a, and 5b revealed one set of signals for each of the two different NHC backbones (coordinated and imidazolium), while a single set of signals was detected for NHC moieties in 3a, 3a′, 3b, and 6b. The 11B spectra of 4a, 5a, and 5b resembled those found for 2a and 2b, which indicates that the steric properties of the carbene moiety do not have a crucial impact on the overall structure of the [12In-2-NHC-1-SB11H10−n]− anion (n = 0, 1). In contrast, the 11B NMR spectra of 1:1 adducts 3a, 3a′, and 3b revealed patterns of 2:2:1:2:2:1:1 for 3a and 3a′ and 2:2:1:2:1:2:1 for 3b in the wide range from −19.7 to 20.8 ppm. The only difference is again the upfield shift of the B5 (B−I) vertex in 3b (6.9 ppm) when compared to 3a and 3a′ (20.8 ppm). It is interesting that the signals corresponding to the vertices B2, B3, B9, and B12 located in the opening of the nido-cage are very broadened, which implies fluxional behavior in solution. This fluxionality is already described in the solid state by the differences in the structure of the nido-thiaborane cages in 3a and 3b. The 11B NMR spectrum of 6b has revealed a more complex pattern of 1:1:1:1:1:2:1:1:1 of broad signals. The overall thiaborane structure resembles the anionic compound [HNEt3]+[7-nidoSB10H11]−19 which both have signals in the same range (between −33.5 and −3.0 ppm for 6b and between −36.6 and −6.6) ppm for [HNEt3]+[7-nido-SB10H11]−. Figure 4 shows the molecular structure of the neutral 5-ISB10H9 adduct with NHCDip 6b. Despite the fact that there are

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COMPUTATIONAL STUDIES The enthalpies of formation (ΔHf) in the diethyl ether environment have been computed for all the reported structural motifs. The results are summarized in Table S1 and Schemes 4 and 5. The reaction of 1a with two molar equivalents of a carbene species to the kinetic products (Scheme 4, 2a, or 5a) is about ∼15 kcal mol−1 more favored for smaller carbenes. On the other hand, some of the hypothetical products of the thermodynamic pathway are even more pronounced, but Scheme 4. Proposed Reaction Pathways for 1a with Calculated ΔHf in the Diethyl Ether Environment (kcal mol−1)

Figure 4. Molecular structure of 6b: ORTEP viewthe 50% probability level; the hydrogen atoms of the NHC moieties have been omitted for clarity.

numerous known structures of different boron compounds containing an NHC, only two papers6a,b deal with borane/ heteroborane−NHC adducts. Complex 6b is the first example of an 11-vertex nido-borane/heteroborane−carbene adduct. In comparison to previous compounds, 6b exhibits a nearly perfect polyhedral shape, where all the distances between the F

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 5. Proposed Reaction Pathways for 1b with Calculated ΔHf in the Diethyl Ether Environment (kcal mol−1)

except for 3a, these have not been identified in reaction mixtures. These findings have shown that the activation energy barrier of the thermodynamic pathway should be significantly higher for smaller carbene. In addition, the 12-vertex nido-3a is the final product in the case of a larger carbene reactant. A similar situation occurs for the conversion of closo-1b (Scheme 5), where the additional transformation of 12-vertex nido-3b to 11-vertex nido-6b proceeds essentially quantitatively. This particular reaction has been calculated to be similar in ΔHf to the same type of 3a conversion, where the only difference is the presence of the iodine atom in the b series. To gain a deeper insight into the closo−nido conversion pathway, we have modeled this reaction for a simpler case, i.e., the reaction of 1a with NHCiPr. The mutual orientation of both reactants is influenced by the charge distribution of both reaction components. Conceivably, the 1a compound has a large dipole moment (experimental value of 3.6 D)11 pointing from the partially positively charged S atom, with the large σholes, to the center of the cluster (Figure 5B). As indicated in the LUMO (lowest unoccupied molecular orbital) of 1a (Figure 5), both S and B12 atoms are of antibonding nature, and therefore, the reaction further proceeds in terms of direct contact of the carbene with the bonding character of the B−B pairs of pentagonal belt adjacent to the S atom. The first transition state, TS1, corresponds to the formation of the B−C chemical bond (the one imaginary frequency of −165.5 cm−1 shown in Figure 5, TS1). This transition state yields the intermediate C, which has already a nido electron count and is attacked with the second NHCiPr (this contact is the essence of the imaginary frequency of −188.2 cm−1, Figure 5, TS2). This promotes the intermediate D and eventually, through the TS3 transition state, the 2a product (Figure 5). The TS3 indicates the exchange of the position of the C−H···H−B interactions (the single imaginary frequency of −47.7 cm−1).

Figure 5. Energetic (kcal mol−1) and structural details of the modeled reaction of 1a with NHCiPr, including LUMO of 1a. The occurrence of the imaginary frequencies is indicated by arrows. The TS3 transition state is not shown as the corresponding barrier is very low. In the case of B, the electrostatic potential (ESP) molecular surface of the thiaborane is shown. The ESP color coding is the following: blue− positive and red−negative.

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thiaboranes are subjected to superior s-electron density of the NHC moiety, different types of products are isolated depending on the stoichiometry and steric hindrance of the NHC as well as the presence of an iodine atom at the thiaborane cage. Both thiaboranes react with smaller NHC to form kinetic products−anionic adducts of open deprotonated

CONCLUSIONS Neither of the closo-thiaboranes under investigation, SB11H11 and 12-I-SB11H10, interacts in solution or the solid state with various bases under an inert atmosphere. When these G

DOI: 10.1021/acs.inorgchem.8b03037 Inorg. Chem. XXXX, XXX, XXX−XXX

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Å) or Mo (Mo Kα radiation; λ = 0.710 73 Å) microfocus X-ray (IμS) sources and a photon CMOS detector, and an Oxford Cryosystems cooling device was used for data collection. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for absorption effects using the Multi-Scan method (SADABS). Obtained data were treated by XT-version 2014/5 and SHELXL-2014/7 software implemented in the APEX3 v2016.5-0 (Bruker AXS) system.24 Hydrogen atoms were mostly localized on a difference Fourier map; however, to ensure uniformity of the treatment of the crystal, all hydrogens were recalculated into idealized positions (riding model) and assigned temperature factors Hiso(H) = 1.2Ueq (pivot atom) or 1.5Ueq (methyl). H atoms in methyl, methylene, methine, and vinylidene moieties and hydrogen atoms in aromatic rings were placed with C−H distances of 0.96, 0.97, 0.98, 0.93, 0.93, and 1.1 Å for terminal and 1.25 Å for bridging B−H bonds. Crystallographic data for structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 1860368− 1860375. In the case of crystal of 3b, only poor quality material which revealed a weak diffraction pattern was obtained, which was the reason that only a low completeness to desired θ angle was obtainedan alert in the checkcif procedure. In this particular case, the result confirms an analogous composition as found for 3a, and it had no further influence on the quality of the final model. For 3a′ and 5a, the positions of the sulfur atom in the cage are disordered to the two positions with nearly equal occupancy. Moreover, the presence of the additional diethyl ether molecule in 5a is masked by the SQUEEZE procedure. Disorder of one of the boron atoms in the molecule of nido-7a is treated by standard methods. Computational Section. NMR Spectroscopy. For the purpose of magnetic shielding computations, the structures of 2a and 3a were optimized at the MP2/TZP level of theory. Since 2a and 4a resemble each other, 4a was not tackled computationally, also to reduce the computational problem to manageable dimensions. Magnetic shieldings were calculated using the GIAO-MP2 method utilizing the IGLO-II basis25 with the MP2 optimized structures and frozen core electrons. Iodine derivatives were not also computed, mainly because of obvious spin−orbit coupling between the iodine and boron to which the halogen is bound. All of these computations were run with Gaussian09.26 The ensemble of geometries of 2a for additional NMR calculations was obtained from 10 ps molecular dynamic (MD) simulation in 300 K at the DFT-D3/BLYP/DZVP level. The MD simulation was performed using Turbomole6.627 and Cuby428 program packages. Heat of Formation (Hf). The gradient optimization of isolated compounds was performed at the DFT/BLYP/DZVP level using the LBFGS algorithm with the strict optimization criteria (energy change