Changes in Tricarbastannatrane Transannular N ... - ACS Publications

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Changes in Tricarbastannatrane Transannular N−Sn Bonding upon Complexation Reveal Lewis Base Donicities Petar Simidzija,† Michael J. Lecours,† Rick A. Marta,† Vincent Steinmetz,‡ Terrance B. McMahon,† Eric Fillion,*,† and W. Scott Hopkins*,† †

Department of Chemistry, University of Waterloo, Waterloo, OntarioN2L 3G1, Canada Laboratoire Chimie PhysiqueCLIO, Bâtiment 201, Porte 2, Campus Universitaire d’Orsay, Orsay, 91405, France

‡

S Supporting Information *

ABSTRACT: Hypercoordinated complexes involving tricarbastannatrane cation [N(CH2CH2CH2)3Sn]+ with various Lewis bases are investigated in the gas and solution phases using a combination of infrared multiple photon dissociation (IRMPD) spectroscopy, NMR spectroscopy, and density functional theory calculations. Coordination is found to occur at the apical position leading to a pentacoordinated Sn center. Strongly electron donating Lewis bases disrupt the N···Sn transannular interaction and induce higher degrees of geometric distortion at the metal center than weakly donating Lewis bases, an effect that manifests as anharmonic shifts in the vibrational spectra. Once characterized in the gas phase, [N(CH2CH2CH2)3Sn(Lewis base)]+ structures were embedded in a dichloroethane polarizable continuum model to investigate solution phase properties. Calculated 119Sn NMR chemical shifts were found to be in good agreement with those measured experimentally, thus suggesting that the bonding properties of [N(CH2CH2CH2)3Sn]+ are essentially the same in the gas and solution phases.

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INTRODUCTION Hypercoordinated 5-alkyl-1-aza-5-stannabicyclo[3.3.3]undecanes (alkyl-tricarbastannatranes) and derivatives were first reported by Jurkschat and Tzschach.1 The unusual structures of these pentacoordinated tetraorganotin compounds were extensively studied by NMR,2 IR and Raman,3 and Mössbauer spectroscopy.4 The tricarbastannatrane cage structure, N(CH2CH2CH2)3SnR, features a transannular N···Sn interaction, resulting in a pentacoordinated tin center with a distorted trigonal bipyramidal geometry, in which the nitrogen atom and alkyl group are located at the apical positions. Consequently, enhanced reactivity is typically observed for Sn− C cleavage of the apical alkyl group owing to the fact that the tin center is assisted by the nitrogen atom in the transition state. The donation of electron density from N to Sn produces a transannular N···Sn interaction that stabilizes the [N(CH2CH2CH2)3Sn]+ species that is subsequently released. The superior ability of alkyl-tricarbastannatranes in delivering metal alkyl groups has been exploited as an entry into Csp3− Csp2 σ bonds via Stille cross coupling, a challenging synthetic transformation. Vedejs pioneered this methodology,5 which is of considerable synthetic value.6 Biscoe and co-workers reported the stereoretentive Pd-catalyzed Stille cross-coupling reactions of secondary alkyl-tricarbastannatranes with aryl halides.7 Other related Pd-catalyzed transformations using alkyl-tricarbastannatranes have also been described in the literature.8 In 2015, as depicted in Scheme 1, Kavoosi and Fillion published the synthesis and characterization of tricarbastannatranes [N(CH2CH2CH2)3Sn](BF4), [N(CH2CH2CH2)3Sn](SbF6), [N(CH2CH2CH2)3Sn][B[3,5-(CF3)2C6H3]4], [N© XXXX American Chemical Society

Scheme 1. Formation and Complexation of [N(CH2CH2CH2)3Sn]+

(CH2CH2CH2)3Sn][MeB(C6F5)3], and [(N(CH2CH2CH2)3Sn)2OH][MeB(C6F5)3].9 Furthermore, the reaction of [N(CH2CH2CH2)3Sn](BF4) with neutral Lewis bases (LB), DABCO, MeCN, THF, and diphenylacetylene (DPA), for the formation of coordination complexes [N(CH2CH2CH2)3Sn(Lewis base)](BF4), was studied in the solution phase (Scheme 1). Interestingly, large shifts in 119Sn Received: May 13, 2016

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DOI: 10.1021/acs.inorgchem.6b01185 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Cartesian coordinates of all optimized structures, are provided in the Supporting Information accompanying this article.

NMR signals were observed upon addition of some of the Lewis basic solvents and additives, thus suggesting formation of tricarbastannatrane-containing Lewis acid/Lewis base complexes in solution. In this manuscript, we report on an investigation to elucidate the structures and properties of the tricarbastannatranecontaining Lewis acid/Lewis base complexes, and garner insight into the nature of the interactions at the Sn center in the formation of pentacoordinated Sn species. First, isolated, gas-phase [N(CH2CH2CH2)3Sn(Lewis base)]+ coordination complexes are studied computationally using high-level density functional theory calculations and experimentally with infrared multiple photon dissociation (IRMPD) spectroscopy. Following characterization of the gas-phase species, the complexes were further studied computationally by embedding the gasphase structures in a polarizable continuum and calculating 119 Sn NMR shifts for comparison with experimental solutionphase measurements. The Lewis acidic nature of [N(CH2CH2CH2)3Sn]+ is quantified, and it is found that the degree of anharmonicity exhibited in the IRMPD spectrum correlates strongly with the donicity of the Lewis base.

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RESULTS AND DISCUSSION [N(CH2CH2CH2)3Sn]+ Structure Determination. Electronic structure calculations identified two low-energy isomers of uncomplexed 1+. The global minimum isomer, which exhibits C3 symmetry about the N−Sn axis, is shown in Figure 1. A second, C1 symmetry isomer was identified 11.5 kJ mol−1

EXPERIMENTAL METHODS

Tricarbastannatrane cation [N(CH2CH2CH2)3Sn]+ (1+) was synthesized in acetonitrile as [N(CH2CH2CH2)3Sn(acetonitrile)](BF4) and [N(CH2CH2CH2)3Sn(acetonitrile)](SbF6) as described by Kavoosi and Fillion.9 Following filtration or centrifugation, the solution or supernatant was mixed with various Lewis bases, namely, DABCO, pyridine, acetamide, and DMSO to form electrospray ionization (ESI) solutions of ca. 100 μmol L−1 concentration, which were injected at 100 μL/h.10 The [N(CH2CH2CH2)3Sn(THF)](BF4) and [N(CH2CH2CH2)3Sn(acetone)](BF4) solutions were prepared in THF and acetonitrile:acetone (1:1), respectively, which were injected at 100 μL/h. Nascent complexes that were produced by ESI were transferred to a Bruker Esquire 3000+ ion trap mass spectrometer, where they were mass-selected and irradiated for 250 ms. Spectra were acquired over the 800−2000 cm−1 range, which corresponds to operation of the tunable output of the Centre Laser Infrarouge d’Orsay (CLIO) free electron laser (FEL) with an electron beam energy of ca. 46 MeV.11 The details of the experimental apparatus used in these experiments have been described previously.12 Vibrational spectra were recorded by monitoring the fragmentation efficiency of the complexes as a function of FEL wavenumber.

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Figure 1. Compound 1+: (A) skeletal formula showing N···Sn interaction, (B) on axis view of the C3 symmetric global minimum, (C) second highest occupied molecular orbital (HOMO−1) showing electron density in the N−Sn region, indicative of a bond, and (D) lowest unoccupied molecular orbital (LUMO) showing a large electron accepting lobe at the apical position of the distorted trigonal monopyramidal Sn center. Hydrogen atoms are removed for clarity in C and D.

above the global minimum structure. Owing to the relative energy of the C1 isomer, it is unlikely to be a major contributor to the observed chemistry and it was therefore not considered in subsequent calculations. The salient feature of the 1+ structure is the exceptionally short N···Sn distance. Electronic structure calculations yield a N−Sn distance of 2.24 Å (2.25 Å in a dichloroethane PCM), which accords well with the values of 2.21 and 2.22 Å measured by X-ray crystallography for 1+· SbF6 and 1+·BF4, respectively.9 In comparison, analogous calculations for the neutral derivatives 1-Cl and 1-CH3 yield transannular N···Sn distances of 2.55 and 2.74 Å, respectively.20 This is consistent with the expected elongation of the transannular N···Sn distance upon complexation at the apical site of the tin center. Owing to the variation in the transannular N−Sn upon complexation, the local geometry about the tin center undergoes significant distortion. It is of note that, while the framework of the tricarbastannatrane induces the N···Sn interaction, ring strain yields to distorted geometry at the tin center by preventing the methylene carbons from being perfectly equatorial (C−Sn−C angles of 120°) in the 4coordinated trigonal monopyramidal and 5-coordinated trigonal bipyramidal structures.1f The uncomplexed 1+ species exhibits a trigonal monopyramidal coordination (C−Sn−C = 119.20°) about the tin center (Table 1).21 For 1-Cl (C−Sn−C = 115.48°) and 1-Me (C−Sn−C = 112.43°), the Sn center shows a distorted geometry intermediate between 5-coordinate trigonal bipyramidal and 4-coordinated tetrahedral, the latter

COMPUTATIONAL METHODS

The geometries of the 1+ complexes were optimized at the B3LYP/6311++G(d,p) level of theory with a Def2-TZVPPD/ECP-28 basis set and effective core potential for Sn using the Gaussian 09 suite for computational chemistry.13 Following geometry optimization, normalmode analysis was conducted to ensure that the complexes were local minima on the potential energy surface. This also served to produce harmonic spectra for comparison with experimental outcomes. To investigate bonding in detail, natural bond orbital (NBO) analyses, which also yielded atomic partial charges for the Lewis acid/base complexes, were performed on fully optimized structures.14 Following structural confirmation with IRMPD, solution phase properties were investigated by embedding the 1+·Lewis base complexes (Lewis base = DABCO, MeCN, THF, DPA) in a dichloroethane polarizable continuum model (PCM).15 Dichloroethane was chosen for these calculations because it was the solvent that was employed in NMR experiments reported by Kavoosi and Fillion.9 NMR chemical shifts for 119Sn were then calculated using the gauge independent atomic orbital (GIAO) method.16 These calculations were undertaken at the B3LYP/6-311++G(d,p) level of theory using an all electron DZVP basis set17 for Sn.18 The DZVP basis set has previously been shown to accurately predict NMR shifts for 119Sn containing compounds.19 All DFT results, including B


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Inorganic Chemistry Table 1. Calculated Transannular N···Sn Distances, Interaction Energies, and Dissociation Energies for 1+·Lewis Base Complexesa species

RN···Sn (Å)

C−Sn−C angleb (deg)

D0 (kcal mol−1)

Einteraction (kcal mol−1)

Edistortion (kcal mol−1)

ESn···LB (kcal mol−1)

1 1+·DABCO 1+·acetonitrile 1+·pyridine 1+·acetamide 1+·acetone 1+·THF 1+·DMSO

2.24 2.39 2.33 2.38 2.35 2.33 2.34 2.37

119.20 117.59 118.42 117.82 118.18 118.39 118.25 117.96

23.1 18.8 23.7 22.8 18.2 19.1 29.5

132.0 83.5 123.3 103.9 81.0 83.1 133.1

108.9 64.7 99.6 81.1 62.8 64.0 103.6

38.8 39.1 76.4 49.6 41.4 36.0 55.6

+

a

Calculations were performed at the B3LYP/6-311++G(d,p) level of theory with a Def2-TZVPPD/ECP-28 basis set and effective core potential for Sn(IV). bThe averages of the C−Sn−C angles are reported as the symmetry is not exact.

Figure 2. Experimental IRMPD (black) and B3LYP/6-311++G(d,p)/Def2-TZVPPD calculated (red) spectra of (A) DABCO, (B) acetonitrile, (C) pyridine, (D) acetamide, (E) acetone, (F) THF, and (G) DMSO Lewis base complexes with 1+. The Lewis bases in A, B, and C coordinate to Sn in 1+ via the lone pair electrons on the N atom, and the Lewis basic ligands D, E, F, and G coordinate to the Sn center in 1+ via the O atom. Hydrogen atoms are removed for clarity.

bound 1+ dimer where the N···Sn distance is measured to be 2.37 Å and C−Sn−C = 117.35°.9,24 Similar elongation of the N−Sn bond length and reduction of the N−Sn−C bond angles are calculated for each of the 1+·Lewis base complexes described below (see Table 1). [N(CH2CH2CH2)3Sn]+ Complexes. Figure 2A−C show the experimental IRMPD spectra for 1+ complexed with DABCO, acetonitrile, and pyridine, respectively. Also shown in Figure 2 are the calculated harmonic (unscaled) vibrational spectra for each of these three species. The IRMPD spectra for the 1+ complexes with DABCO and acetonitrile exhibit relatively small shifts (ca. 25 cm−1) with respect to the calculated harmonic spectra, whereas the spectral shifts for the 1+·pyridine complex are somewhat more pronounced (ca. 40 cm−1). In all three cases, the calculated spectra are an excellent match with experiment. Consequently, we are able to assign the geometries of the complexes (see Supporting Information) and we are able to easily identify [N(CH2CH2CH2)3Sn]+ vibrations at ca. 950 cm−1, ca. 1050 cm−1, and several in the 1200−1500 cm−1 range. It should be noted that we also attempted to record the IRMPD spectrum of uncomplexed 1+, however, we were unable to fragment the bare tricarbastannatrane cage with the FEL, indicating the superior stability of the cationic Sn species.

assuming complete loss of the Sn−N interaction (the ideal angles for tetrahedron being 109.5°).22 In examining the frontier molecular orbitals of 1+ (see Figure 1C), one can see qualitatively that the HOMO−1 (i.e., second highest energy occupied molecular orbital) should have a significant effect on stabilization of the nominally tin-centered positive charge and contraction of the N···Sn distance owing to the bonding electron density between the N and Sn centers. Indeed, NBO analysis yields a bonding orbital between the N and Sn centers with an occupancy of 1.95e, which predominantly arises from donation of electron density from N to electron-deficient Sn center. NBO analysis also indicates that there is a charge of ca. + 1.6e on the Sn center. This, in combination with the large Sn-centered lobe of the LUMO (see Figure 1D), accords well with the propensity for Lewis bases to coordinate to the Sn apical site (vide inf ra). Close examination of the 1+ LUMO reveals a node in the wave function between the N and Sn centers, thus suggesting that a concomitant elongation of the transannular N−Sn bond should be expected upon complexation of 1+ with a Lewis base. This would also lead to a change of geometry at the Sn center, from 4coordinated trigonal monopyramidal to a distorted 5coordinate trigonal bipyramidal.23 These expectations are borne out in, for example, the X-ray structure of the hydroxide C


Article

Inorganic Chemistry Figure 2D−G shows the IRMPD spectra for the 1 + coordination complexes with oxygen-containing Lewis bases: acetamide, acetone, THF, and DMSO. Again, the calculated harmonic spectra match well with those recorded experimentally via IRMPD. In all cases, the Lewis base was found to bind to the Sn center via the oxygen atom. For the 1+·acetamide complex, coordination of 1+ with the sp2-hybridized oxygen atom occurs anti to the NH2 group. A second, higher energy (nitrogen bound) isomer was found computationally for 1+· acetamide, but it was discounted on the basis of comparison of the calculated and measured IR spectra. As was the case with 1+·pyridine, a relatively large (ca. 45 cm−1) spectral shift is observed for each of the oxygen-containing Lewis bases when comparing the calculated and measured vibrational frequencies. Qualitatively, these spectral shifts suggest that the O-containing Lewis bases and pyridine bind relatively strongly with the tricarbastannatrane cationso much so that Sn center geometry is significantly affected (and consequently, so too is the vibrational energy level structure of the cage). To further investigate the Lewis acid/Lewis base interactions, a NBO analysis was conducted for each of the complexes. Table 1 also gives the energies for electron donation from the Lewis bases to the accepting orbital of 1+. In all cases, we find significant interaction energies associated with electron density donation from the Lewis base to [N(CH2CH2CH2)3Sn]+ LUMO (i.e., ESn···LB in Table 1). Note also that upon complexation and donation of electron density to the tricarbastannatrane LUMO, the transannular N···Sn distance, RN···Sn, elongates by 0.1−0.2 Å and the Sn center converts from a 4-coordinated trigonal monopyramidal to a distorted 5-coordinated trigonal bipyramidal geometry (i.e., a reduction of C−Sn−C angle is observed). This is consistent with the molecular orbital picture discussed above (see Figure 1D) and with the donicity of the base in the order N-sp3 hybridized, N-sp2, and N-sp. Table 1 also gives the calculated D0 values (zero-point corrected dissociation energies) and interaction energies (Einteraction) for each of the 1+·Lewis base coordination complexes. In every case, the complex dissociation energies are significantly different from the 1+·Lewis base interaction energies calculated by NBO analysis. These differences can be rationalized by considering the geometry of the complexes. The calculated D0 values consider the energies of the geometrically optimized complex and optimized dissociation products; in all of the complexes studied, upon complexation with a Lewis base, high degrees of geometric distortion at the metal center are induced in the conversion from a 4-coordinated trigonal monopyramidal to a distorted 5-coordinated trigonal bipyramidal geometry (see Table 1). Thus, the energy of interaction between 1+ and the Lewis bases is the sum of D0 and the energy of geometric distortion (Edistortion) upon complexation. This is shown diagrammatically in Figure 3. The ESn···LB donation energies, on the other hand, are the energies associated with electron density donation from the Lewis base to the LUMO of the 1+ moiety within the geometrically optimized complexes. ESn···LB accounts for approximately 45% of the total interaction energy in the 1+ complexes (ranging from 32% to 62%). The remainder of the intermolecular interaction energy is predominantly partitioned between contributions from donation to the valence Sn Rydberg orbitals and to the antibonding Sn−C σ* MOs. A detailed breakdown of charge-transfer interaction energies is provided in the Supporting Information. Since the electron density donation from the Lewis base populates the 1+ LUMO (see Figure 1D), one might expect

Figure 3. A schematic energy level diagram showing how the tricarbastannatrane cage geometry changes upon dissociation of the 1+·Lewis base complexes. ZPE = zero-point energy.

that ESn···LB should correlate with geometric distortion and a higher degree of anharmonicity in the 1+ moiety. Indeed, the calculated transannular distances (RN···Sn) and C−Sn−C angle values do exhibit a strong correlation with ESn···LB (see Table 1; plotted in the Supporting Information). The 1+·DABCO complex is a notable outlier from this trend, and this can be rationalized by considering the cage structure of the Lewis base; owing to the fact that DABCO is bulkier than the other Lewis bases, steric hindrance hampers the interaction of the DABCO nitrogen lone pair with the 1+ LUMO. Consequently, even though DABCO has the highest gas phase basicity of all the Lewis bases studied,25 it exhibits the second lowest value for ESn···LB (see Table 1). Indeed, of the complexes studied here, the ESn···LB/Einteraction ratio for 1+·DABCO is the lowest. In other words, electron density donation to the 1+ LUMO is not as important to overall stability in 1+·DABCO as it is in the other complexes. Instead, the NBO calculations show that the dominant contributions (49.1 kcal mol−1) to charge transfer in 1+·DABCO arise from donation to the valence Rydberg orbitals of Sn (20.0 kcal mol−1) and to the Sn−C σ* antibonding molecular orbitals (29.1 kcal mol−1). This latter interaction, in particular, affects the local geometry of the Sn center, leading to an opening of the Sn−C−C bond angle from 103.4° in 1+ to 106.9° in 1+·DABCO, which then leads to the geometric changes reported for 1+·DABCO in Table 1. Consequently, a significant change is also observed at the Sn center, with a C−Sn−C angle of 117.59°, and an elongated N··· Sn interaction with a distance of 2.39 Å (Table 1). Electron density donation to the valence Rydberg orbitals of Sn has a negligible effect on bonding and, therefore, the geometries of the complexes. One can further test the idea that electron density donation from the Lewis base is associated with structural changes in 1+ by comparing ESn···LB with the differences between the observed and calculated vibrational spectra for the tricarbastannatrane moiety in the complexes. Based on the argument outlined above, the observed vibrational transition wavenumbers should exhibit a greater shift from the calculated harmonic wavenumbers with increasing ESn···LB. Figure 4 plots the average wavenumber shift (νc̅ alcd − ν̅obsd) for the three 1+ CH2 twisting vibrational transitions in the 1200−1300 cm−1 region versus ESn···LB for each complex. These three vibrational transitions D


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Inorganic Chemistry

Figure 4. Spectral shifts (νc̅ alcd − ν̅obsd) plotted as a function of calculated distortion energies for 1+ complexes with (red) oyxgen- and (blue) nitrogen-containing Lewis bases. Calculations were performed at the B3LYP/6-311++G(d,p) level of theory with a Def2-TZVPPD/ ECP-28 basis set and effective core potential for Sn center.

Figure 5. A comparison of measured 119Sn NMR chemical shifts of tricarbastannatrane compounds9 with those calculated using the GIAO method at a B3LYP/6-311++G(d,p) level of theory with a DZVP all electron basis set for Sn.16 CH3 and Cl are covalently bound to 1+, while the remaining Lewis bases are coordinately bound to 1+. The numbers in parentheses indicate the partial charge on the tricarbastannatrane moiety in the complex.

were chosen because they were easily identified and free from overlap with other transitions in all of the IRMPD spectra. Note that electron donation into the 1+ LUMO is expected to affect the methylene moieties (see Figure 1D). It is immediately apparent from Figure 4 that there is a positive correlation between the anharmonic spectral shift and ESn···LB. Once again, though, 1+·DABCO is a noticeable outlier from the observed trend. This discordant behavior arises from the inconsonant (among this data set) charge-transfer interactions in 1+· DABCO. Although electron density donation to σ*Sn−C weakens the Sn−C bond, this interaction does not significantly impact the bonding of any of the functional groups giving rise to the vibrational peaks in the IRMPD spectrum. Likewise, charge transfer to the valence Sn Rydberg orbitals has only a minor effect on the bonding of the functional groups probed by IRMPD. Consequently, and due to the relatively minor degree of charge transfer to the 1+ LUMO, the observed IR bands of 1+·DABCO exhibit very minor anharmonic shifts from their calculated harmonic band positions. A linear regression of the spectral shift against the calculated ESn···LB (excluding the 1+· DABCO data point) yields a standard error of ±10 kcal mol−1, thus suggesting that by measuring anharmonicity one can quantify the Lewis acidity of 1+ and the donicity of the Lewis bases. Structures in Solution. The complexes [N(CH2CH2CH2)3Sn(Lewis base)](BF4) where Lewis base = DABCO, THF, acetonitrile, and diphenylacetylene (DPA) were investigated in the solution phase using 1H, 13C, and 119Sn NMR spectroscopy.9 In their study, Kavoosi and Fillion added 1 equiv of the Lewis bases to a dichloroethane solution containing 1+ and recorded the 119Sn chemical shifts. To compare with the solution phase work, 1+ complexes (as characterized by IRMPD) were embedded in a dichloroethane PCM, geometries of the Lewis acid/base complexes were reoptimized, and 119Sn NMR chemical shifts were calculated using the GIAO method.15 Reoptimization of the complexes within the PCM yielded negligible changes to the geometry of the coordination complex. Also included in the set of molecules is a 1+ complex with diphenylacetylene (DPA),26 chlorotricarbastannatrane (1-Cl), and methyl-tricarbastannatrane (1Me). Figure 5 plots a comparison of calculated versus measured 119 Sn chemical shifts. Given that alkyl 119Sn(IV) species typically exhibit chemical shifts over the range δ = −700 to

+200 ppm,27 the agreement between experiment and theory is quite satisfactory. This suggests that the gas phase structures determined by IRMPD and DFT are representative of the solution phase species. Note that the complex with the most weakly coordinating Lewis base (DPA) exhibits the least shielded 119Sn NMR signal, whereas the neutral, covalently bound species exhibit the most shielded signals. This trend is reflected in the calculated partial charges for the tricarbastannatrane cages in these complexes (provided in parentheses in Figure 5). DPA donates only ca. 0.10e of electron density to the 1+ LUMO, while the more strongly coordinating O- and Ncontaining Lewis bases donate ca. 0.15e−0.19e of density. The Cl and CH3 moieties in compounds 1-Cl and 1-CH3 donate ca. 0.42e and 0.53e, respectively. On first glance, the relatively large step change in calculated partial charges for the 1+·DABCO and 1-Cl complexes might appear to be inconsistent with the relatively narrow range of observed 119Sn chemical shifts for these species (δDABCO = 61.4 ppm; δCl = 17.6 ppm). These similar 119Sn chemical shifts arise despite differences in electron density donation from the complexing ligand owing to transannular N to Sn electron donation within the tricarbastannatrane moiety, resulting in a pentavalent Sn(IV) center of distorted trigonal bipyramidal geometry. In the 1-Cl and 1-CH3 complexes, a lower degree of transannular electron donation is observed owing to the reduced Lewis acidity of the Sn center, as evidenced by elongation of the transannular N··· Sn distance to 2.45 and 2.72 Å, respectively (in dichloroethane PCM). In comparing the calculated N−Sn distances for the solvated 1-Cl and 1-CH3 complexes to those measured by Xray crystallography, we find relatively good agreement with the crystal structures of 1-Cl (RN···Sn = 2.36 Å) and 1-CH3 (RN···Sn = 2.62 Å).1b,c Of course, our solution phase calculations do not account for the packing effects that may be experienced by 1-Cl and 1-CH3 in the solid state.

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SUMMARY AND CONCLUSIONS A combination of DFT calculations and IRMPD spectroscopy was used to study the gas phase structures and properties of the tricarbastannatrane cation complexed with a series of Lewis bases. The Lewis bases were found to coordinate to the 1+ cage at the apical position of the Sn center. Electron donation from E


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Inorganic Chemistry the Lewis bases to the 1+ LUMO induced an elongation of the transannular N···Sn distance, a reduction of the C−Sn−C angle (relative to [N(CH2CH2CH2)3Sn]+), and a conversion of the geometry at the Sn center from 4-coordinated trigonal monopyramidal to distorted 5-coordinated trigonal bipyramidal. The degree of change at the Sn center upon complexation and the observed anharmonic shift of 1+ vibrational transitions correlate with the electron density donation from the Lewis base to the 1+ LUMO. We will explore this phenomenon in more detail in a future study. By embedding the gas phase complexes in a dichloroethane PCM, we were able to calculate solution phase properties. We find that the calculated geometries, bonding, and chargetransfer properties for the 1+·Lewis base complexes are essentially the same in the gas phase and in solution. Moreover, calculated 119Sn NMR chemical shifts for tricarbastannatrane coordination complexes are in very good agreement with those determined experimentally. Close examination of the DFT results shows that transannular N···Sn electron density donation provides a compensation mechanism for charge stabilization at the Sn center; complexes with weakly interacting Lewis bases exhibit a higher degree of transannular donation than do complexes with strongly interacting bases.

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undecane, a 2,8,9-Tricarbastannatrane. J. Organomet. Chem. 1985, 290, 285−289. (c) Jurkschat, K.; Tzschach, A.; Meunier-Piret, J. Crystal and Molecular Structure of 1-Aza-5-stanna-5-methyltricyclo[3.3.3.01,5]undecane. Evidence for a Transannular Donor−acceptor Interaction in a Tetraorganotin Compound. J. Organomet. Chem. 1986, 315, 45−49. (d) Tzschach, A.; Jurkschat, K. Pentacoordinated Tetraorganotin and Silicon Compounds - Synthesis and Structures. Pure Appl. Chem. 1986, 58, 639−646. (e) Jurkschat, K.; Schmid, B.; Dybiona, M.; Baumeister, U.; Hartung, H.; Tzschach, A. Zur Struktur und Reaktivität von Stannylierten Propylaminen und -Sulfiden. K r i s t a l l - u n d M o l e k ü l s t r u k t u r v o n B i s ( 3 dimethylchlorostannylpropyl)sulfid S(CH2CH2CH2SnMe2Cl)2. Z. Anorg. Allg. Chem. 1988, 560, 110−118. (f) Kolb, U.; Dräger, M.; Dargatz, M.; Jurkschat, K. Unusual Hexacoordination in a Triorganotin Fluoride Supported by Intermolecular Hydrogen Bonds. Crystal and Molecular Structures of 1-Aza-5-stanna-5halotricyclo[3.3.3.01.5]undecanes N(CH2CH2CH2)3SnF·H2O and N(CH2CH2CH2)3SnX (X = Cl, Br, I). Organometallics 1995, 14, 2827− 2834. (2) Mügge, C.; Pepermans, H.; Gielen, M.; Willem, R.; Tzschach, A.; Jurkschat, K. NMR Investigations on Stannabicycloundecanes of the Type RSn(CH2CH2CH2)3N. Z. Anorg. Allg. Chem. 1988, 567, 122− 130. (3) (a) Schenzel, K.; Kolbe, A.; Reich, P. Zum Schwingungsverhalten pentakoordinierter Zinn(IV)-organischer Verbindungen. IR- und Raman-Untersuchungen an 2,8,9-Tricarbastannatranen. Monatsh. Chem. 1990, 121, 615−623. (b) Ignatyev, I. S.; Schenzel, K.; Sundius, T. Normal Coordinate Analysis of the Vibrational Spectra of Stannatranes. Spectrochim. Acta, Part A 1992, 48, 489−493. (c) Kolb, U.; Dräger, M. Hypervalent Tin-organic Compounds: Vibrational Spectroscopy in the Solid as a Tool for Structure Determination. Spectrochim. Acta, Part A 1997, 53, 517−529. (4) Jurkschat, K.; Tzschach, A.; Weichmann, H.; Rajczy, P.; Mostafa, M. A.; Korecz, L.; Burger, K. Mössbauer Spectroscopic Investigations of Intra- and Intermolecularly Coordinated Organotin Compounds. Inorg. Chim. Acta 1991, 179, 83−88. (5) Vedejs, E.; Haight, A. R.; Moss, W. O. Internal Coordination at Tin Promotes Selective Alkyl Transfer in the Stille Coupling Reaction. J. Am. Chem. Soc. 1992, 114, 6556−6558. (6) (a) Jensen, M. S.; Yang, C.; Hsiao, Y.; Rivera, N.; Wells, K. M.; Chung, J. Y. L.; Yasuda, N.; Hughes, D. L.; Reider, P. J. Synthesis of an Anti-Methicillin-Resistant Staphylococcus aureus (MRSA) Carbapenem via Stannatrane-Mediated Stille Coupling. Org. Lett. 2000, 2, 1081− 1084. (b) Yang, C.; Jensen, M. S.; Conlon, D. A.; Yasuda, N.; Hughes, D. L. A Simple and Scalable Method to Prepare 1-Aza-5-chloro-5stannabicyclo[3.3.3]undecane. Tetrahedron Lett. 2000, 41, 8677−8681. (c) Forngren, T.; Samuelsson, L.; Långström, B. A 11C-Methyl Stannane (5-[11C]Methyl-1-aza-5-stanna-bicyclo[3.3.3]undecane) for Use in Palladium-mediated [11C]C−C Bond Forming Reactions With Organohalides. J. Labelled Compd. Radiopharm. 2004, 47, 71−78. (d) Jones, P.; Atack, J. R.; Braun, M. P.; Cato, B. P.; Chambers, M. S.; O’Connor, D.; Cook, S. M.; Hobbs, S. C.; Maxey, R.; Szekeres, H. J.; Szeto, N.; Wafford, K. A.; MacLeod, A. M. Pharmacokinetics and Metabolism Studies on (3-tert-Butyl-7-(5-methylisoxazol-3-yl)-2-(1methyl-1H-1,2,4-triazol-5-ylmethoxy) pyrazolo[1,5-d][1,2,4]triazine, a Functionally Selective GABAA α5 Inverse Agonist for Cognitive Dysfunction. Bioorg. Med. Chem. Lett. 2006, 16, 872−875. (e) Parker, D. L., Jr.; Meng, D.; Ratcliffe, R. W.; Wilkening, R. R.; Sperbeck, D. M.; Greenlee, M. L.; Colwell, L. F.; Lambert, S.; Birzin, E. T.; Frisch, K.; Rohrer, S. P.; Nilsson, S.; Thorsell, A.-G.; Hammond, M. L. Triazolo-tetrahydrofluorenones as Selective Estrogen Receptor beta Agonists. Bioorg. Med. Chem. Lett. 2006, 16, 4652−4656. (f) Theddu, N.; Vedejs, E. Stille Coupling of an Aziridinyl Stannatrane. J. Org. Chem. 2013, 78, 5061−5066. (7) Li, L.; Wang, C.-Y.; Huang, R.; Biscoe, M. R. Stereoretentive Pdcatalysed Stille Cross-coupling Reactions of Secondary Alkyl Azastannatranes and Aryl Halides. Nat. Chem. 2013, 5, 607−612. (8) (a) Sebahar, H. L.; Yoshida, K.; Hegedus, L. S. Effect of Adjacent Chiral Tertiary and Quaternary Centers on the Metal-Catalyzed Allylic

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01185. Thermodynamic energies, NBO calculation results, Cartesian coordinates, correlation plots, IRMPD spectra, calculated IR data, spectral shifts, and HRMS data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: efi[email protected]. Tel: 001-519-888-4567, ext 32470. *E-mail: [email protected]. Tel: 001-519-888-4567, ext 33022. Funding

Funding was provided by Natural Sciences and Engineering Research Council (NSERC) of Canada, Compute Canada, and University of Waterloo. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge high performance computing support from the SHARCNET consortium of Compute Canada. We are also grateful to the Centre Laser Infrarouge d’Orsay (CLIO) team and technical support staff for the valuable assistance and hospitality. We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada and the University of Waterloo for funding.

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REFERENCES

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have been reported for Sn(IV) complexes; see: (a) Shustorovich, E. M.; Buslaev, Yu. A. Mutual Influence of Ligands in Main Group Element Coordination Compounds. Inorg. Chem. 1976, 15, 1142− 1147. (b) Britton, D.; Dunitz, J. D. Chemical Reaction Paths. 7. Pathways for SN2 and SN3 Substitution at Tin(IV). J. Am. Chem. Soc. 1981, 103, 2971−2979. (c) Paseshnitchenko, K. A.; Aslanov, L. A.; Jatsenko, A. V.; Medvedev, S. V. Trans-Strengthening in Tin(IV) Anionic Complexes. J. Organomet. Chem. 1985, 287, 187−194. (d) Kolb, U.; Beuter, M.; Dräger, M. Heterocyclic Systems Containing Tin(IV). 10. Control of Three-Center Interactions X···Sn-Hal in Stannocanes by Halide Type. Inorg. Chem. 1994, 33, 4522−4530. (e) Beuter, M.; Kolb, U.; Zickgraf, A.; Bräu, E.; Bletz, M.; Dräger, M. Heterocyclic Systems Containing Tin(IV)XIII [1]. Possible Ceasing or Inversion of the Structural trans Influence During the Course of a Bimolecular Nucleophilic Attack of a Donor Group in a Series of SnIV Rings. Polyhedron 1997, 16, 4005−4015. (f) Anderson, K. M.; Orpen, A. G. On the Relative Magnitudes of cis and trans Influences in Metal Complexes. Chem. Commun. 2001, 2682−2683. (g) See also refs 1f and 3c. (24) The averages of the C−Sn−C angles are reported as the symmetry is not exact. (25) Hunter, E. P. L.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update. J. Phys. Chem. Ref. Data 1998, 27, 413−656. (26) This complex was too labile to be detected by mass spectrometry and characterized by IRMPD. (27) Vivas-Reyes, R.; De Proft, F.; Biesemans, M.; Willem, R.; Geerlings, P. DFT Calculations of 119Sn Chemical Shifts Using GaugeIncluding Atomic Orbitals and Their Interpretation via Group Properties. J. Phys. Chem. A 2002, 106, 2753−2759.

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