Dynamic Exchange in Intramolecular Lewis Pairs with Multiple Lewis

Article pubs.acs.org/Organometallics

Dynamic Exchange in Intramolecular Lewis Pairs with Multiple LewisAcidic Functions Leif A. Körte, Sebastian Blomeyer, Jan-Hendrik Peters, Andreas Mix, Beate Neumann, Hans-Georg Stammler, and Norbert W. Mitzel* Centre for Molecular Materials CM2, Chair of Inorganic and Structural Chemistry, Faculty of Chemistry, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany S Supporting Information *

ABSTRACT: Hydroboration of allyldimethylamine, diallylmethylamine, triallylamine, and diallylmethylphosphane with dimeric 9-BBN yielded the corresponding singly, doubly, and triply Lewis-acid-functionalized intramolecular Lewis pairs. For the singly Lewis-acid-functionalized derivative Me2N(CH2)3-9-BBN no evidence for the existence of an equilibrium involving an open-chain form was found in solution. For the doubly and triply Lewis-acid-functionalized compounds Me3−xE[(CH2)3-9-BBN]x [E = N (x = 2, 3), P (x = 2)] a dynamic exchange of the free Lewis-acid functions with an intramolecular Lewis acid base complex was observed and investigated by variable-temperature NMR spectroscopy. The free energies of activation of the exchange processes were determined by the coalescence method and found to be lower for the Lewis-base component nitrogen than for phosphorus. To further understand the exchange process of the Lewis acids at the central Lewis base, transition states for two different exchange mechanisms were considered and searched for.

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INTRODUCTION The description of bases as electron-pair donors and acids as electron-pair acceptors, Lewis’s fundamental theory,1 is one of the most widely used concepts in the description of chemical reactivity. Usually Lewis acids and bases form more or less strong adducts. The strong adduct of trimethylamine and boron trifluoride Me3N−BF3, for example, shows no dynamic exchange of its components in solution. However, Me3N− BMe3, containing the weaker Lewis-acidic trimethylborane, shows a dynamic exchange between acid and base components; this has been proven in solution by line-broadening NMR methods. The exchange rate has been found to be independent of both base and acid concentration, which is consistent with a dissociative exchange mechanism.2 The exchange processes of different types of Lewis pairs containing boron, gallium, or indium Lewis acids and ether or amine Lewis bases have been investigated.3 The exchange at Me3N−GaMe3, with gallium being capable of adopting a hypercoordinate transition state, is dependent on the concentration of NMe3, but independent of the concentration of GaMe3. This is consistent with a mechanism of the SN2-type exchange at the gallium Lewis acid, but a dissociative mechanism at nitrogen.4 Different compounds with a central Lewis-acid function located between two amine Lewis-base functions were investigated (Scheme 1) and show a rapid exchange of the amine bases at the borane center.5,6 Determination of the free enthalpy of activation and comparison with the monoacid-functionalized analogues indicate an associative SN2-type exchange mechanism.5 In the solid state, the Lewis-acid functions appeared to be bonded to only one Lewis acid.5,6 In contrast, replacement of the amino © XXXX American Chemical Society

Scheme 1. Examples for a Boron Lewis Acid Located between Two Amine Groups

Lewis-base group by an oxygen function in the anthracenebased compounds leads to pentacoordinated boron compounds showing two weak interactions with the adjacent oxygen atoms in the solid state.7 The inverse situation, the intramolecular competitive behavior of two or more Lewis acids for a central Lewis base, has not been investigated in detail. Recently we communicated that doubly Lewis-acid-functionalized PhN[(CH2)3B(C6F5)2]2 shows an intramolecular exchange of the two Lewis acids at the central base moiety.8 Further, frustrated Lewis pair (FLP) reactivity in terms of H/D scrambling and catalytic hydrogenation was observed. In contrast, the singly acid-functionalized analogue Ph(Me)N(CH2)3B(C6F5)2 is found to be nearly inactive. Therefore, the FLP activity could be attributed Received: December 15, 2016

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DOI: 10.1021/acs.organomet.6b00935 Organometallics XXXX, XXX, XXX−XXX

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Organometallics to the presence of the second Lewis-acid arm, and quantumchemical investigations revealed a cooperative hydride binding motif by both boron acids to stabilize the hydrogen splitting product. The analogous phosphorus derivatives PhP[(CH2)3B(C6F5)2]2 and tBuP[(CH2)3B(C6F5)2]2 showed no exchange of the acid functions in solution and also were found catalytically inactive. Both observations could be attributed to a high B−P bond dissociation energy, as calculations indicated.9 In this contribution we present the syntheses of multiple Lewis-acid/single-base systems, containing a central Lewis base, nitrogen, or phosphorus, linked by 1,3-propandiyl spacers to one, two, or three 9-borabicyclo[3.3.1]nonane (9-BBN) Lewisacid functions. These compounds are easily accessible by hydroboration of corresponding unsaturated precursors with dimeric 9-BBN.10 In the case of one Lewis-acid function, the trivial formation of a Lewis pair is expected. For more than one Lewis-acid function linked to the central base, all present acid functions are capable of bonding to the central base. The focus of this work is to investigate this competitive situation by variable-temperature NMR spectroscopy. In this context, the effect of the type of central Lewis base on the dynamic behavior will be analyzed.

Scheme 3. Hydroboration of Singly, Doubly, and Triply Unsaturated Lewis-Base Precursors with Dimeric 9-BBN

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RESULTS AND DISCUSSION Syntheses and Characterizations. Diallylmethylamine (2) was prepared in a modified Eschweiler−Clarke reaction from diallylamine (1), aqueous formaldehyde solution, and formic acid according to a protocol of Döbbelin and Odriozola (Scheme 2).11 After distillation, the identity of 2 was proven by Scheme 2. Syntheses of the 2-Fold Unsaturated Precursor for the Hydroboration Reactions resonances of 10 was hampered by overlapping signals of the two 1,3-propandiyl groups and the two 9-BBN units. Anyhow, regioselective hydroboration was confirmed by 13C DEPT 135 spectroscopy, which shows only two negative resonances, one doublet at 5.8 ppm (2JP,H = 9 Hz) of the CH3 group bonded to the central phosphorus and one broad resonance of the CH groups of the 9-BBN units at 31.5 ppm. Several attempts to grow crystals of 10 suitable for X-ray diffraction failed. Molecular Structures in the Solid State. The molecular structure in the solid state of the singly Lewis-acid-functionalized compound 6 is depicted in Figure 1 and shows a fivemembered-ring structure with a half-chair conformation. The torsion angle C(3)−C(2)−C(1)−N(1) is 36.0(1)°. The boron−nitrogen bond [1.726(1) Å] is shorter than comparable boron−nitrogen bonds for 9-BBN adducts in five-membered rings R2N−CH2−o-C6H4−BBN with a more rigid aromatic backbone [1.74−1.77 Å].13 The molecular structure of the doubly Lewis-acid-functionalized compound 7 in the solid state is depicted in Figure 2. It shows one boron Lewis acid to be bonded to nitrogen in a fivemembered heterocycle with half-chair conformation. The torsion angle C(3)−C(2)−C(1)−N(1) is 41.6(1)°. The 4fold-bonded boron atom is in a distorted tetrahedral coordination sphere. The other boron Lewis acid is tricoordinate in a trigonal planar coordination sphere. The bridging 1,3-propandiyl group adopts an all-anti conformation. The crystal structure of the triply Lewis-acid-functionalized compound 9 is depicted in Figure 3 and shows one of the three Lewis-acid groups to be bonded to the central nitrogen atom, again forming a five-membered heterocycle with a half-chair

NMR spectroscopy and mass spectrometry. Diallylmethylphosphane (4) was prepared in a salt elimination reaction from dichloromethylphosphane (3) and allylmagnesium chloride in THF. Phosphine 4 was purified by distillation and characterized by NMR spectroscopy. The NMR spectra of 4 agree well with those described in the literature.12 Selective anti-Markovnikov hydroboration of allyldimethylamine (5), diallylmethylamine (2), triallylamine (8), and diallylmethylphosphane (4) with 9-BBN was achieved in nhexane or n-pentane at ambient temperature either overnight or within a few days (Scheme 3). The singly, doubly, and triply Lewis-acid-functionalized amines 6, 7, and 9 were crystallized from n-hexane and characterized by NMR spectroscopy, highresolution mass spectrometry, CHN elemental analyses, and Xray crystallography. Hydroboration of diallylmethylphosphane (4) led to formation of compound 10, which was characterized by NMR spectroscopy, high-resolution mass spectrometry, and CHN elemental analyses. Full assignment of the NMR B


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Organometallics

Figure 1. Molecular structure in the crystal (P21/n) of Lewis pair 6. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: B(1)−N(1) 1.726(1), N(1)−C(1) 1.500(1), C(1)− C(2) 1.515(1), C(2)−C(3) 1.544(1), C(3)−B(1) 1.648 (1), B(1)− C(4) 1.631(1), B(1)−C(8) 1.621(1); B(1)−N(1)−C(1) 102.2(1), N(1)−C(1)−C(2) 106.7(1), C(1)−C(2)−C(3) 105.5(1), C(2)− C(3)−B(1) 109.4(1), C(3)−B(1)−N(1) 97.0(1), N(1)−B(1)−C(4) 112.3(1), N(1)−B(1)−C(8) 113.6(1), C(3)−B(1)−C(4) 115.4(1), C(3)−B(1)−C(8) 113.8(1), C(4)−B(1)−C(8) 105.0(1); torsion angle C(3)−C(2)−C(1)−N(1) 36.0(1).

Figure 3. Molecular structure of compound 9 in the crystal (P1̅). Displacement ellipsoids are drawn at the 50% probability level. The structure shows a disorder at C(21) as well as at C(32) and C(33) with a ratio of 68:32. Minor part and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: B(1)−N(1) 1.757(2), N(1)−C(1) 1.504(2), C(1)−C(2) 1.517(2), C(2)−C(3) 1.541(2), C(3)−B(1) 1.648(2), B(1)−C(4) 1.630(2), B(1)−C(8) 1.622(2), C(14)−B(2) 1.568(2), C(25)−B(3) 1.572(2); B(1)− N(1)−C(1) 99.4(1), N(1)−C(1)−C(2) 107.7(1), C(1)−C(2)− C(3) 105.1(1), C(2)−C(3)−B(1) 109.5(1), C(3)−B(1)−N(1) 95.2(1), N(1)−B(1)−C(4) 111.5, C(3)−B(1)−C(4) 115.3(2), C(4)−B(1)−C(8) 104.9(1); torsion angle C(3)−C(2)−C(1)−N(1) 35.1(1).

in equilibrium. The 11B NMR resonance shows only a small temperature-related shift of 2 ppm downfield, but no line broadening. The structural dynamics of the multiply Lewis-acid-functionalized compounds 7, 9, and 10 were investigated by 1H and 11B variable-temperature (VT) NMR spectroscopy. In contrast to the singly Lewis-acid-functionalized intramolecular Lewis pair 6, a dynamic exchange of the two or three Lewis-acid sites in their bonding to the central Lewis-base atom was found. The VT 1H NMR spectra of compound 7 are depicted in Figure 4. Due to fast exchange on the NMR time scale at 363 K only one averaged set of signals for both 1,3-propandiyl units and 9-BBN units is obtained. This allows a full assignment of the resonances in the 1H and 13C NMR spectra recorded at high temperature (see Experimental Section). The resonances of the three CH2 groups are highlighted and assigned in Figure 4. With decreasing temperature the resonances broaden. At 283 K the resonance for the CH2 (▲) groups bonded to the central nitrogen atom disappears, while all other resonances become very broad. At this temperature coalescence is reached. With further decreasing temperature many signals arise from the baseline, due to the formation of a stereocenter at the tetrahedrally coordinated nitrogen atom. This results in four diastereotopic protons for the two CH2 groups bonded directly to the central nitrogen atom. Due to a severe overlap of these resonances with those of the two 9-BBN units, a reliable assignment of the resonances as well as the evaluation of rate constants or of the free energy of activation based on line broadening methods failed. Further, the coalescence method is not applicable to this compound, as kinetic and thermodynamic data can be obtained only for two-site systems from coalescence

Figure 2. Molecular structure of compound 7 in the crystal (P21/c). Displacement ellipsoids are drawn at the 50% probability level. The structure shows a disorder at C(17) and C(21) with a ratio of 85:15. Minor part and hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: B(1)−N(1) 1.733(2), N(1)−C(1) 1.500(2), C(1)−C(2) 1.512(2), C(2)−C(3) 1.542(2), C(3)−B(1) 1.657(2), B(1)−C(4) 1.637(2), B(1)−C(8) 1.620(2), C(14)−B(2) 1.570(2), B(2)−C(15) 1.570(2); B(1)−N(1)−C(1) 101.4(1), N(1)− C(1)−C(2) 105.6(1), C(1)−C(2)−C(3) 104.8(1), C(2)−C(3)− B(1) 109.1(1), C(3)−B(1)−N(1) 96.7(1), C(3)−B(1)−C(4) 115.1(1), C(3)−B(1)−C(8) 113.8(1), N(1)−B(1)−C(4) 112.5(1), C(4)−B(1)−C(8) 104.6(1), C(14)−B(2)−C(15) 125.0(1), C(14)− B(2)−C(19) 122.9(1), C(15)−B(2)−C(19) 111.8(1); torsion angle C(3)−C(2)−C(1)−N(1) 41.6(1).

conformation. The torsion angle C(3)−C(2)−C(1)−N(1) is 35.1(1)°. The boron−nitrogen bond [1.757(2) Å] is longer than those in compounds 6 and 7 [1.726(1) and 1.733(1) Å]; this might be rationalized by the increasing steric demand at the nitrogen Lewis base. The other two Lewis acid groups are tricoordinate at boron and connected via the 1,3-propandiyl spacer in all-anti conformation. Structural Dynamics in Solution. Expectedly, the NMR spectra of the singly Lewis-acid-functionalized compound 6 reveal a five-membered-ring structure in solution. The 11B NMR shift of 4.8 ppm confirms this and is typical for 4-foldbonded boron atoms. For full NMR assignment, 1H and 13C NMR spectra were recorded at 373 K to diminish line broadening due to flipping of the 9-BBN unit. Consequently only averaged resonances were observed for the protons. Variable-temperature measurements from 298 to 383 K provided no evidence for the existence of an open-chain form C


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Organometallics

the temperature dependence of quadrupolar broadening.15,16 All temperature-related changes observed in VT NMR measurements appeared to be completely reversible, disregarding some decomposition at high temperature. A similar dynamic exchange of the two Lewis-acid groups alternatingly bonded to the central Lewis-base site was found for phosphorus analogue 10, but coalescence was reached at higher temperature compared to the amine 7. Consequently, VT 11B NMR spectra (Figure 6) were recorded at lower

Figure 4. Excerpt of the variable-temperature 1H NMR spectra (600 MHz, Tol-d8) of compound 7. Spectra were recorded in steps of 10 K in the range between 193 and 363 K; only five spectra are presented for clarity.

methods.14 Therefore, VT 11B NMR spectra (192 MHz, Told8) of compound 7, which contains two boron sites, were measured and are depicted in Figure 5. At 273 K exchange is Figure 6. Excerpt of the variable-temperature 11B NMR spectra (96 MHz, Tol-d8) of compound 10. The trace impurity with the monohydroboration product appears relatively large due to strong line broadening of the signals of 10 at elevated temperature. At 393 K the coalescence signal arises from the baseline. Measurements at higher temperatures were limited by the high vapor pressure of the deuterated solvent near the boiling point.

magnetic field strength (corresponding to 96 MHz, Tol-d8), resulting in a lower coalescence temperature. Two resonances are observed at 303 K, one for the tricoordinate boron atom at 88 ppm and the other for the tetracoordinate boron at −7 ppm. Coalescence is found at about 368 K, and the resonances disappear. At 393 K only one signal can be observed at 40 ppm, the averaged shift of tetra- and tricoordinate boron atoms. Similar to amine 7 evaluation of thermodynamic data from VT 1 H NMR data by line-broadening methods failed due to severe overlap of resonances. Gutowsky and Holm have solved the Bloch equation and obtained eq 1. It affords the rate constant kc, describing the exchange at the coalescence temperature Tc, as a function of Δν, the difference in the resonance frequency of the two sites. Substitution of kc in the Eyring equation and combination of all constants leads to eq 2 as a description of the free energy of activation for the exchange process.14,17 Kinetic and thermodynamic data for the dynamic exchange of the two Lewis-acid sites in compounds 7 and 10 were determined from their 11B VT NMR spectra. The results are listed in Table 1. Errors were calculated by Gaussian error propagation. Due to a broad coalescence region the error in the coalescence temperature was estimated to be ΔTc = 15 K. The error in the chemical shift difference was estimated to be Δν = 200 Hz.

Figure 5. Excerpt of the variable-temperature 11B NMR spectra (192 MHz, Tol-d8) of compound 7. Spectra were recorded in steps of 10 K in the range between 193 and 363 K; only seven spectra are presented for clarity.

slow on the NMR time scale and two broad boron resonances are observed: one showing the tetracoordinate boron atom at 7 ppm and one the tricoordinate boron atom at 88 ppm. With increasing temperature these signals broaden and disappear between 313 and 333 K; at 363 K a single signal at 48 ppm is obtained, representing the averaged shift of both boron nuclei (see Figure 5). Spectra measured at temperatures below 273 K do not show two sharp resonances for the different boron sites, but again line broadening is observed. This might result from D


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Organometallics

for phosphine 10 than for amine 7 is consistent with the generally higher inversion barrier of phosphines. Therefore, at least for phosphine 10, an associative mechanism can be ruled out. For the dissociative mechanism no transition state but a steady increase of energy with lengthening of the boron nitrogen distance up to 4 Å was found. Their further separation results in a variety of different minima. We suppose that the energetic profile of this bond dissociation process is, in first approximation, shaped like a Morse potential, resulting in possible transition states being only slightly higher in energy than the surrounding structures on the potential hypersurface. Therefore, we considered the respective thermodynamic data, i.e., the energy differences between ring and open-chain minimum structures ΔRGdiss, as the best approximation for the kinetic barrier of a dissociative mechanism. These energy differences at calculated values of 37 (7) and 46 kJ mol−1 (10) differ by about 15 kJ mol−1 from the experimental results, but the energy difference between amine and phosphine at 9 kJ mol−1 is in very good agreement with that obtained from the VT NMR studies. Therefore, we suggest a dissociative exchange mechanism of the boron acid functions at the Lewis-base site for both amine 7 and phosphine 10. For amine 7 we cannot reject the associative mechanism as a possible pathway of Lewis-acid exchange. Nevertheless, the calculated barrier for the dissociative mechanism results only from thermodynamic data and is therefore likely to be underestimated, making the corresponding mechanism more likely. The triply Lewis-acid-functionalized amine 9 also shows a dynamic exchange of the Lewis-acid sites at the Lewis-base moiety. The 1H NMR spectrum at ambient temperature (600 MHz) shows only slightly broadened resonances and only one set of signals for the 1,3-propandiyl spacers and the 9-BBN units, respectively. At 243 K coalescence is reached and the signals become broad (Figure 7). Similar to compound 7, at

Table 1. Calculated Rate Constants (kc) at the Coalescence ⧧ Temperature (Tc) and Free Energy of Activation (ΔGexp) for the Dynamic Exchange of the Two Boron Lewis-Acid Side Arms at the Central Lewis Base, Nitrogen, or Phosphorus, in Compounds 7 (N) and 10 (P) ν [MHz]

Tc [K]

Δν [Hz]

kc [kHz]

ΔG‡exp [kJ/mol]

192 96

323 ± 15 368 ± 15

15 800 ± 200 9150 ± 200

35.1 ± 0.4 20.3 ± 0.4

51.2 ± 2.5 60.4 ± 2.6

7 10

kc =

π Δν 2

(1)

ΔGc⧧ = 1.914 × 10−2

⎛ T ⎞⎫ kJ ⎧ Tc⎨10.32 + log10⎜ c ⎟⎬ mol ⎩ ⎝ kc ⎠⎭ ⎪

⎪

⎪

⎪

(2)

The exchange rate of the two compounds is higher for compound 7, bearing nitrogen as the central Lewis base, than for 10 even at lower temperature. Therefore, for nitrogen compound 7 the free energy of activation of the exchange process is 9.2 kJ mol−1 lower than for the corresponding phosphorus compound 10. In order to get more insight into systems 7 and 10, we searched for transition states of two different exchange mechanisms at the PBEh-3c level of theory: an associative mechanism, with two equal boron−nitrogen distances and a planar nitrogen atom, and a dissociative mechanism by subsequent elongation of the boron−nitrogen bond without the other boron atom approaching (compare Scheme 4). An Scheme 4. Two Considered Exchange Mechanisms

associative transition state was found for both amine 7 and phosphine 10, with the boron atoms being equidistant from the central trigonal planar coordinated pnictogen. The vibrational mode corresponding to the imaginary frequency shows the inversion of the Lewis base coordination sphere resulting in hopping of the pnictogen between the two Lewis-acid functions. The relative Gibbs free energies ΔG‡ass of 66 and 178 kJ mol−1, respectively (for details see the Supporting Information), differ significantly from the experimentally derived ΔG‡exp values of 51 (N) and 60 kJ mol−1 (P) (Table 2). The significantly higher barrier of the associative exchange Table 2. Calculated Relative Energies for the Barrier of Lewis Acid Exchange in the Dissociative and Associative Mechanisms, ΔRGdiss (open−closed) and ΔG‡ass (TS− closed), for Compounds 7 and 10 at the PW6B95-D3BJ/ def2-TZVP/COSMO(toluene)//PBEh-3c Level of Theory and the Experimental Values Lewis base

ΔRGdiss [kJ/mol]

ΔG‡ass [kJ/mol]

ΔG‡exp [kJ/mol]

N P

36.7 45.6

65.7 178

51.2 ± 2.5 60.4 ± 2.6

Figure 7. Excerpt of the variable-temperature 1H NMR spectra (600 MHz, Tol-d8) of compound 9. Spectra were recorded every 10 K in the range between 193 and 333 K, and only six spectra are presented for clarity. E


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Quantum-chemical analyses revealed that for phosphine 10 a dissociative mechanism can be assumed, whereas for amine 7 both an associative and a dissociative mechanism are possible exchange pathways, with the latter being more likely.

lower temperatures, many resonances were observed that could not be assigned due to a mutual overlap. Consequently, the evaluation of rate constants and thermodynamic data by linebroadening methods failed. 11B NMR spectra were recorded between 203 and 363 K (192 MHz) and are depicted in Figure 8. At low temperature the exchange is slow on the NMR time

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EXPERIMENTAL SECTION

General Procedures. All operations involving air- and moisturesensitive compounds were performed applying conventional Schlenk techniques or within a glovebox. Volatile compounds were handled in a vacuum line. Tetrahydrofuran (THF) was dried over potassium; nhexane and n-pentane were dried over LiAlH4 and distilled prior to use. Toluene-d8 and benzene-d6 were dried over Na/K alloy; dichloromethane-d2 was dried over CaH2. Allylmagnesium bromide in THF, diallylamine (1), and triallylamine (8) were purchased from Sigma-Aldrich. Allyldimethylamine (5) was purchased from Fluorochem. Dichloromethylphoshane (3) was purchased from Alfa Aesar. 9-BBN was prepared by the procedure described by Brown et al.18 NMR spectra were recorded on Bruker AV 300, Bruker DRX 500, Bruker Avance III 500, and Bruker AV 600 spectrometers at ambient temperature if not stated otherwise. NMR spectroscopic chemical shifts were referenced to the residual proton and carbon peaks of the used solvents19 (1H, 13C) or externally (11B: BF3·OEt2, 31P: 85% H3PO4 in H2O). Figure 9 shows the atom numbering used to assign

Figure 8. Excerpt of the variable-temperature 11B NMR spectra (600 MHz, Tol-d8) of compound 9.

Figure 9. Atom numbering within the 9-BBN groups in the NMR spectra description. For H-3 two different protons are expected to be observed, one axial and one equatorial proton.

scale, but no resonances of the tri- and tetracoordinated boron nuclei can be observed probably due to quadrupolar broadening effects at low temperatures as stated above.15,16 At 303 K a broadened resonance at 62 ppm represents the coalescence signal of the three exchanging Lewis-acid sites. With increasing temperature the signal sharpens. The observed chemical shift of 62 ppm corresponds to the average of the shifts of two tricoordinated boron nuclei (88 ppm in compound 7) and one tetracoordinate boron nucleus (7 ppm in compound 7). Kinetic and thermodynamic data cannot be obtained from these VT measurements. The method described by Gutowsky and Holm applies only for two-state exchange systems. Consequently, a direct comparison of the three-site system 9 with compound 7 is not possible.

NMR spectra of compounds containing the 9-BBN group. Elemental analyses were carried out using a EuroEA elemental analyzer. EI mass spectra were recorded using an Autospec X magnetic sector mass spectrometer with EBE geometry (Vacuum Generators, Manchester, UK) equipped with a standard EI source. Samples were introduced by push rod in aluminum crucibles. Ions were accelerated by 8 kV in EI mode. Diallylmethylamine (2). Diallylmethylamine (1) was prepared by a modified procedure described by Döbbelin and Odriozola.11 Diallylamine (20.3 g, 0.21 mol) and aqueous formaldehyde solution (37%, 25.9 g, 0.32 mol) were subsequently added dropwise to formic acid (51.6 g, 1.12 mol) at 0 °C. The resulting yellow solution was stirred at ambient temperature for 3 h and then refluxed for 3 d. Water (80 mL) and concentrated hydrochloric acid (37%, 40 mL) were added. Water and formic acid distilled off (80 °C, 60 mbar) until the solution was concentrated to 50 mL. The brown solution was basified with NaOH (15 g in 150 mL of water) and extracted with dichloromethane (3 × 100 mL), the combined organic extracts were dried over Na2SO4, and the solvents were removed under reduced pressure. Diallylmethylamine (2) (9.39 g, 84.4 mmol, 40% yield) was obtained after distillation (bp 112 °C) as a colorless liquid. 1H NMR (500 MHz, CDCl3): δ [ppm] = 5.84 (m, 2H, CHCH2), 5.12 (m, 4H, CHCH2), 2.96 (d, 3JH,H = 6.7 Hz, 4H, N−CH2), 2.17 (s, 3H, CH3). 13C{1H} NMR (126 MHz, C6D6): δ [ppm] = 135.8 (CH CH2), 117.6 (CHCH2), 60.6 (N−CH2), 42.0 (CH3). GC-MS (70 eV): m/z = 111.0 ([M]+•), 84.0 ([M − C2H3]+), 82.0, 68.0, 42.1, 41.1 ([C3H5]+), 39.1. Diallylmethylphosphane (4). A solution of allylmagnesium bromide in THF (2 m, 28 mL, 56 mmol) was diluted in THF (100 mL) and degassed. Dichloromethylphosphane (3) (2.48 g, 21.4 mmol) was condensed onto the solution at −196 °C. The mixture was allowed to reach ambient temperature and was refluxed for 1 h. THF was distilled off, and the remaining suspension was diluted in npentane (50 mL). The magnesium salts were filtered off and washed

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CONCLUSION The intramolecular competitive behavior of multiple Lewis-acid functions linked to a common central Lewis-base group via 1,3propandiyl spacers was studied. For this purpose the singly, doubly, and triply unsaturated amines and the doubly unsaturated phosphine Me3−xE(CH2−CHCH2)x (E = N (x = 1, 2, 3), P (x = 2)) were treated with dimeric 9-BBN to selectively yield the anti-Markovnikov hydroboration products. The singly Lewis-acid-functionalized compound 6 showed no equilibrium open-chain form in solution. However, the doubly and triply functionalized amines 7 and 9 as well as the doubly functionalized phosphine 10 revealed dynamic exchange of the acid functions in solution; this was proven by 1H and 11B NMR spectra recorded at various temperatures. The free energies of activation of the exchange processes of 7 and 10 were determined using the coalescence method and were found to be 9.2 kJ mol−1 lower than for the phosphorus compound 10. F


Article

Organometallics two times with n-pentane (50 mL each). n-Pentane was condensed off, and diallylmethylphosphane (4) (1.78 g, 13.9 mmol, 65%) was obtained after vacuum distillation (bp 70 mbar, 70 °C). NMR data are in agreement with literature values.12 1H NMR (300 MHz, C6D6): δ [ppm] = 5.70 (m, 2H, CHCH2), 4.94 (m, 4H, CHCH2), 2.01 (m, 4H, P−CH2), 0.78 (d, 2JP,H = 4.0 Hz, 3H, CH3). 13C{1H} NMR (75.6 MHz, C6D6): δ [ppm] = 133.4 (d, 2JP,C = 6 Hz, CHCH2), 115.9 (d, 3 JP,C = 8 Hz, CHCH2), 33.3 (d, 1JP,C = 16 Hz, P−CH2), 9.3 (d, 1JP,C = 19 Hz, CH3). 31P{1H} NMR (121 MHz, C6D6): δ [ppm] = −43.8. Me2N−CH2−CH2−CH2−BBN (6). Dimeric 9-BBN (114 mg, 0.47 mmol) was added to a solution of allyldimethylamine (5) (80 mg, 0.93 mmol) in n-pentane, and the solution was stirred at ambient temperature for 3 d. The solvent was removed in vacuo (allowing recording NMR spectra of the crude product), and the resulting colorless solid was dissolved in n-hexane (5 mL). The solution was decanted off the solid residuals, and the hydroboration product was crystallized at −80 °C in the form of colorless needles. The supernatant solution was removed by a syringe, and the crystalline solid dried in vacuo to yield hydroboration product 6 (120 mg, 62%). The identity of 6 was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The purity was confirmed by NMR spectroscopy and elemental analysis. 1H NMR (500 MHz, Told8, 373 K): δ [ppm] = 2.22 (t, 3JH,H = 6.9 Hz, 2H, N−CH2), 2.05 (m, 2H, H-3), 2.02 (s, 6H, CH3), 1.94 (m, 4H, H-2), 1.82 (m, 4H, H-2), 1.64 (m, 2H, H-3), 1.44 (quint., 2H, N−CH2−CH2−CH2B), 0.81 (m, 2H, CH2−B), 0.69 (br, 2H, H-1). 11B{1H} NMR (160 MHz, C6D6, 298 K): δ [ppm] = 5 (τ1/2 = 60 Hz). 13C{1H} NMR (126 MHz, Told8, 373 K): δ [ppm] = 66.4 (N−CH2), 48.1 (CH3), 33.8 (C-2), 26.7− 25.2 (broad, C-1), 24.7 (C-3), 21.3 (N−CH2−CH2−CH2−B), 19.4− 18.6 (broad, N−CH2−CH2−CH2−B). HR-MS (EI, 70 eV): calcd for C13H26BN+• 207.21528, found 207.21446. Anal. Calcd for C13H26BN: C 75.4, H 12.7, N 6.8. Found: C 75.0, H 13.0, N 6.6. MeN(CH2−CH2−CH2−BBN)2 (7). To a solution of diallylmethylamine (2) (100 mg, 0.90 mmol) in n-hexane (5 mL) was added 9BBN dimer (221 mg, 0.91 mmol), and the solution was stirred at ambient temperature for 2 d. The solvent was removed in vacuo to record NMR spectra of the crude product. The residue was dissolved in n-hexane (5 mL), and the hydroboration product 7 was crystallized at −80 °C. The supernatant was decanted off and removed via a syringe. The crystalline solid was dried in vacuo to yield pure product 7 (236 mg, 0.66 mmol, 73%). The identity of 7 was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The purity was confirmed by NMR spectroscopy and correct values in hydrogen and nitrogen content in elemental analysis. Carbon values are too low, as organoboranes tend to form boron carbides as well as glassy boric acid with carbon enclaves during combustion. Addition of WO3 reduces this effect.20 1H NMR (500 MHz, Tol-d8, 373 K): δ [ppm] = 2.65 (t, 3JH,H = 7.7 Hz, 4H, N−CH2), 2.22 (s, 3H, CH3), 1.99 (m, 4H, H-3), 1.89 (m, 16H, H-2), 1.54 (quint, 3JH,H = 7.7 Hz, 4H, N−CH2−CH2−CH2−B), 1.45 (m, 4H, H-3), 1.26 (broad, 4H, H-1), 1.05 (t, 3JH,H = 7.6 Hz, 4H, CH2−B). 11B NMR (160 MHz, Tol-d8, 273 K): δ [ppm] = 88 (τ1/2 = 850 Hz, tricoord), −7 (τ1/2 = 740 Hz, tetracoord); (353 K): δ [ppm] = 48 (τ1/2 = 290 Hz). 13C{1H} NMR (126 MHz, Tol-d8, 373 K): δ [ppm] = 60.6 (N−CH2), 43.2 (CH3), 34.0 (C-2), 29.1 (broad, C-1), 24.2 (C-3), 22.3 (CH2−B), 19.6 (N− CH2−CH2−CH2−B). HR-MS (EI, 70 eV): calcd for C23H43B2N+• 355.35761, found 355.35720. Anal. Calcd for C23H43B2N: C 77.8, H 12.2, N 3.9. Found: C 77.1, H 12.2, N 3.9, under addition of WO3 and C 76.0 and 75.4 without addition of WO3 due to boron carbide formation;20 all four measurements show precise values for H and N. N(−CH2−CH2−CH2−BBN)3 (9). To a solution of triallylamine (8) (57 mg, 0.42 mmol) in n-hexane (5 mL) was added dimeric 9-BBN (152 mg, 0.62 mmol), and the colorless solution was stirred at ambient temperature for 3 d. The solvent was removed in vacuo (allowing recording NMR spectra of the crude product), and the residue dissolved in n-hexane (5 mL). The hydroboration product (9) was crystallized at −80 °C. The supernatant was decanted off and removed via a syringe, and the crystalline solid was dried in vacuo (95 mg, 0.23 mmol, 43%). The identity of 9 was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The purity was

confirmed by NMR spectroscopy and correct values in hydrogen and nitrogen content in elemental analysis. Carbon values are too low, as organoboranes tend to form boron carbides as well as glassy boric acid with carbon enclaves during combustion.20 1 H NMR (300 MHz, C6D6): δ [ppm] = 2.73 (m, 6H, N−CH2), 1.93 [m-overlap, 30H, H-2 (24H) and H-3 (6H)], 1.67 (quint, 3JH,H = 7.7 Hz, 6H, N−CH2−CH2−CH2−B), 1.50 (broad, 6H, H-1), 1.44 (m, 6H, H-3), 1.13 (t, 3JH,H = 7.7 Hz, 6H, CH2−B). 11B NMR (96 MHz, C6D6): δ [ppm] = 65 (τ1/2 = 220 Hz, av boron nuclei). 13C{1H} NMR (75.6 MHz, C6D6): δ [ppm] = 58.1 (N−CH2), 33.8 (C-2), 29.7 (broad, C-1), 24.1 (C-3), 23.3 (broad, CH2−B), 20.2 (N−CH2−CH2− CH2−B). HR-MS (EI, 70 eV): calcd for C33H60BN3+• 503.49994, found 503.50134. Anal. Calcd for C33H60BN3: C 78.8, H 12.0, N 2.8. Found: C 75.3 due to boron carbide formation,20 H 12.0, N 2.6. MeP(CH2−CH2−CH2−BBN)2 (10). To a solution of diallylmethylphosphane (4) (69 mg, 0.54 mmol) in n-hexane (5 mL) was added 9BBN dimer (133 mg, 0.54 mmol), and the colorless solution was stirred at ambient temperature for 3 d. The solvent was removed in vacuo to yield the hydroboration product (10) (0.16 g, 0.43 mmol, 80%). The missing yield is addressed to the sample used for NMR measurements performed for reaction control. The identity of 10 was confirmed by NMR spectroscopy and mass spectrometry. The purity was confirmed by NMR spectroscopy and a correct value in hydrogen content in the elemental analysis. Carbon values are too low, as organoboranes and phosphanes tend to form boron carbides as well as glassy boric acid or phosphoric acid, respectively, with carbon enclaves during combustion.20 1H NMR (500 MHz, C6D6): δ [ppm] = 2.6−0.9 (broad signals at 2.31, 2.24, 2.11, 1.95, 1.84, 1.68, 1.41, 1.11, 1.00, total of 40H, CH2CH2CH2-9-BBN), 0.75 (d, 2JP,H = 9 Hz, CH3). 11B{1H} NMR (160 MHz, C6D6): δ [ppm] = 88 (τ1/2 = 420 Hz, tricoord), −7 (τ1/2 = 220 Hz, tetracoord). 13C-DEPT135 NMR (76 MHz, C6D6): δ [ppm] = 34.9, 33.2, 32.3, 31.1 (neg, broad, C-1), 27.0, 26.8, 26.2, 25.9, 25.0, 23.9, 23.6, 23.3, 18.8 (all peaks broad, more than 11 due to P−C coupling), 5.5 (neg, d, 2JP,C = 22 Hz, CH3). 31P{1H} NMR (202 MHz, C6D6): δ [ppm] = 3.1. HR-MS (EI, 70 eV): calcd for C23H43B2P+• 372.32830, found 372.32738. Anal. Calcd for C23H43B2P: C 74.2, H 11.7. Found: C 72.5 due to boron carbide formation,20 H 11.7. Quantum-Chemical Calculations. Density functional theory (DFT) calculations were performed using the Turbomole 7.0 software package.21 Structures were optimized at the PBEh-3c level of theory.22 Stationary points were characterized by harmonic frequency analyses. Herein, minima exhibited no imaginary frequency, and transition points exactly one imaginary frequency. Thermochemical corrections to Gibbs free energies were calculated at the temperatures of coalescence obtained from the VT-NMR experiments. In addition, single-point energies at the PW6B95-D3BJ(abc)/def2-TZVP level of theory23 were calculated, both applying the COSMO approach24 (toluene, ε = 2.37) to include solvation effects and in the gas phase. To accelerate the geometry optimizations and frequency calculations, the density-fitting RI-J approach was used.25 The absolute values for Gibbs free energy as well as Cartesian coordinates for all compounds are provided in the Supporting Information. Crystallography. Crystal structures were determined at 100.0(1) K by X-ray diffraction using Cu Kα radiation (λ = 1.541 Å) on an Agilent SuperNova diffractometer. Using Olex2,26 the structures were solved with the ShelXS27 structure solution program using direct methods and refined with the ShelXL27 refinement package using least-squares minimization. Data are listed in Table S1. CCDC 1441278−1441280 contain the supplementary crystallographic data for this paper. These data can be obtained via www.ccdc.cam.ac.uk/ data_request/cif free of charge from the Cambridge Crystallographic Data Center.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00935. G


Article

Organometallics

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(12) Antberg, M.; Prengel, C.; Dahlenburg, L. Inorg. Chem. 1984, 23, 4170−4174. (13) Toyota, S.; Oki, M. Bull. Chem. Soc. Jpn. 1992, 65, 1832−1840. (14) (a) Sandström, J. Dynamic NMR Spectroscopy; Academic Press: London, 1982. (b) O̅ ki, M. Applications of Dynamic NMR Spectroscopy to Organic Chemistry; VCH: Weinheim, 1985. (15) Brown, H. C.; Racherla, U. S.; Pellechia, P. J. J. Org. Chem. 1990, 55, 1868−1874. (16) Kintzinger, J. P.; Lehn, J. M. Mol. Phys. 1968, 14, 133−145. (17) Gutowsky, H. S.; Holm, C. H. J. Chem. Phys. 1956, 25, 1228. (18) Soderquist, J. A.; Brown, H. C. J. Org. Chem. 1981, 46, 4599− 4600. (19) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176−2179. (20) Roth, H. Angew. Chem. 1937, 50, 593−604. (21) (a) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Chem. Phys. Lett. 1989, 162, 165−169. (b) TURBOMOLE V7.0; TURBOMOLE GmbH, 2015. (22) Grimme, S.; Brandenburg, J. G.; Bannwarth, C.; Hansen, A. J. Chem. Phys. 2015, 143, 054107. (23) (a) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (b) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119−124. (c) Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2005, 109, 5656−5667. (d) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. (e) Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comput. Chem. 2011, 32, 1456−1465. (24) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799−805. (25) Deglmann, P.; May, K.; Furche, F.; Ahlrichs, R. Chem. Phys. Lett. 2004, 384, 103−107. (26) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341. (27) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

NMR spectra, crystallographic data, computational details (PDF) Crystallographic data (CIF) Cartesian coordinates (XYZ)

AUTHOR INFORMATION

Corresponding Author

*E-mail (N. W. Mitzel): [email protected]. ORCID

Norbert W. Mitzel: 0000-0002-3271-5217 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Deutsche Forschungsgemeinschaft. We thank Klaus-Peter Mester and Gerd Lipinski for recording the NMR spectra, Brigitte Michel for performing the elemental analyses, and Dr. Jens Sproß, Heinz-Werner Patruck, and Sandra Heitkamp for recording the mass spectra. Calculations leading to the results presented here were performed on resources provided by the Regionales Rechenzentrum der Universität zu Köln (RRZK) as well as the Paderborn Center for Parallel Computing (PC2).

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REFERENCES

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Dynamic Exchange in Intramolecular Lewis Pairs with Multiple Lewis

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