Generation of Annelated Dicarbenes and Their ... - ACS Publications

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Generation of Annelated Dicarbenes and Their Alkali-Metal Chelate Complexes in Solution: Equilibrium between Hetero- and Homoleptic NHC Lithium Complexes Kim S. Flaig, Benjamin Raible, Verena Mormul, Natalie Denninger, Cac̈ ilia Maichle-Mössmer, and Doris Kunz* Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany S Supporting Information *

ABSTRACT: Attempts to generate the free bis(N-heterocyclic carbene) vegiR (R = nPr, tBu; vegiR = 2,7-dihydro-2,7dialkyldiimidazo[1,5-b:5′,1′-f ]pyridazine-1,8-diylidene) from its imidazolium salts 1 with alkali-metal bases resulted exclusively in the formation of the respective lithium, sodium, and potassium complexes 2−4 due to the strongly chelating properties of the dicarbene. DFT calculations reveal pronounced dispersion interactions in the case of N-tertbutyl substituents as the reason for the formation of the homoleptic lithium species 2b-H in solution. The dynamic behavior of the lithium complexes in equilibrium was studied by NMR techniques. Attempts to liberate the free carbenes by addition of the respective crown ethers failed for the lithium and sodium complexes. Deprotonation of the imidazolium salts 1a,b with the strong metal-free phosphazene base P4-tBu ({(Me2N)3PN}3PNtBu) generated successfully the free dicarbenes vegiR (5) and monocarbene 6b in solution.

■

INTRODUCTION The synthesis of stable N-heterocyclic carbenes1 (NHCs) is typically carried out by deprotonation of the respective imidazolium salts with strong alkali-metal bases such as sodium hydride, alkyllithiums, and lithium amides or medium-strong bases such as potassium tert-butanolate and even sodium acetate. The free carbene can usually be isolated by removal of the alkali-metal salts by filtration; depending on the stability, distillation is also possible.2−6 The formation of alkali-metal complexes can be achieved on purpose by the addition of alkali-metal salts to free carbenes, as investigated by Alder (Chart 1, example A).7 However, in some cases it was observed that the alkali-metal ions remain coordinated to even neutral, nonfunctionalized carbenes during the generation of the carbenes by alkali-metal bases.8 A very prominent example is the so-called Weiss−Yoshida carbene B, a three-membered-ring isomer of an imidazolinylidene, which was first isolated and described as its lithium complex in the 1970s.9−11 In 2006 Bertrand and co-workers showed that the free carbene could not even be liberated upon addition of [12]crown-4 and that the coordination of the lithium ion is independent of its counterion (I−, ClO4− or BF4−). However, with bases containing the softer Lewis-acidic potassium cation, the free carbene could be isolated.12,13 Addition of a free NHC to a lithium-Cp complex yielded complex C, as shown by Arduengo and co-workers.14 Sodium or potassium NHC complexes are very rare and were also generated from the © XXXX American Chemical Society

free carbene by addition of an excess of potassium salt or potassium hexamethyldisilazide (KHMDS) in example A7,15 or by transfer of the HMDS anion to a more strongly coordinating counterion, as shown by Hill and Hevia for type D.16−18 Coordination of the alkali-metal ions to NHCs is enhanced if additional Lewis basic donor functionalities at the carbene moiety favor a chelating or bridging coordination mode19−21 or anionic substituents require the presence of the alkali-metal counterion.22−29 Chelation of lithium ions with a second NHC moiety was shown by Hofmann and co-workers with example E.30 In 2012 we reported on transition-metal complexes of the bidentate annelated dicarbene ligand vegiR 31 (Chart 1), which can be considered an analogue of 1,10-phenanthroline but shows a remarkable flexibility of the ring plane.32,33 This results in chelating as well as bridging coordination behavior, depending on the metal. The metal complexes have been synthesized by either in situ deprotonation−metalation with KOAc/metal salts or basic metal salts such as Ag2O and Cu2O and subsequent transmetalation. However, the deprotonation of veginPr·2HPF6 with alkyllithium bases to generate the free dicarbene was unsuccessful and has led to decomposition so far. We have now reinvestigated this deprotonation reaction and will report herein the formation of stable carbene complexes Received: January 29, 2018

A

DOI: 10.1021/acs.organomet.8b00053 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

chemical shift difference Δδ induced by the alkali-metal ions is largest for Li+, with a shift difference of typically ∼20 ppm,7 followed by Na+ (Δδ ≈ 10 ppm) and K+ (Δδ ≈ 6 ppm). This trend follows the decrease in Lewis acidity of the metals35 and in the covalent contribution to the metal-carbene bond36 and thus the expected bond strength of the primarily electrostatic carbene−metal bonds. To liberate the free carbene veginPr (5a) from the chelation, we added 2 equiv of [12]-crown-4 but still found the 13C{1H} NMR chemical shift of the carbene signal unchanged. This is in accordance with the observations made by Bertrand and coworkers for the Weiss−Yoshida carbene complex B, which was isolated as its crown ether complex.12 However, this is in contrast to the case for Bertrand and co-workers as well as for Hofmann and co-workers for the dicarbene ligand in E, who observed formation of the respective free carbenes using a potassium base (KHMDS). In our case the carbene signal is detected at 196.3 ppm upon deprotonation with KHMDS and at 191.9 ppm using NaHMDS. This indicates clearly the formation of the sodium and potassium complexes 3a and 4a. The free carbene 5a could not be generated by addition of 2 or 5 equiv37 of [15]-crown-5 to the sodium complex 3a but rather could be formed from the potassium complex 4a by addition of 2 equiv of [18]-crown-6. The identity of 5a was confirmed by 1 H and 13C{1H} NMR spectra and by independent generation of 5a (vide inf ra). Deprotonation of the N-tert-Butyl-Substituted Imidazolium Salt 1b. We then turned our attention to the N-tertbutyl-substituted imidazolium salt 1b, as we hoped for an additional steric effect of the bulkier substituents. However, upon deprotonation with 2.2 equiv of LiHMDS (or nBuLi or MeLi) in THF, we observed not only one new signal set in the 1 H NMR spectrum but also a second set in a 1:1 ratio. While the aromatic backbone signals are not baseline separated, it is remarkable that the tBu signals are separated by 0.17 ppm. Such a strong change of the environment around the tert-butyl groups could result from a species in which two carbene ligands are coordinated to the Li+ center, so that the tert-butyl groups would recognize the aromatic deshielding effect of the second ligand. Therefore, we conducted further NMR experiments to elucidate the molecular structure of the two species in solution. Cooling the sample to −80 °C led to sharp signals (Figure 1a). The ratio of the signal pairs changed to 1:0.8, which confirms the presence of two species in equilibrium. Upon heating of the sample the coalescence of signals H-3/6 (8.0 ppm) at 40 °C and of the signals H-4/5 (7.18 ppm) is observed, while the tBu signals still remain separated. The 7Li NMR spectrum shows one relatively sharp peak at 3.2 ppm and two broad peaks at 1.5 (smaller) and −0.4 ppm (larger) at room temperature. Upon cooling to −80 °C all peaks become sharp and baseline separated. The integrals of the peaks at 3.16, 1.51, 0.16, and −0.40 ppm show a ratio of 0.4:1:0.3:1.9. By

Chart 1. Literature-Known Alkali-Metal Complexes Bearing Neutral NHC Ligands

[M(vegiR)L2]PF6 of alkali metals and elucidation of their molecular structure and dynamic behavior in solution as well as the generation of the free dicarbenes vegiR under metal-free conditions.

■

RESULTS AND DISCUSSION Deprotonation of the N-n-Propyl-Substituted Imidazolium Salt 1a. Upon reinvestigation of the deprotonation reaction of the imidazolium salt 1a, we recognized that a stable deprotonation product can be generated when a slight excess of an alkali-metal alkyl or amide base or potassium tert-butanolate is used. Even slight substoichiometric amounts of the base led to hitherto unidentified decomposition products. Reacting bis(imidazolium) salt 1a with 2.2 equiv of LiHMDS in THF (Scheme 1) resulted in the absence of the imidazolium peaks in the 1H NMR spectrum and the presence of two broad signals at δ 7.03 ppm for the pyridazo (H-4/5) and at 7.56 ppm for the imidazo protons (H-3/6). In the 13C{1H} NMR spectrum a carbene signal was detected as a singlet at 185.3 ppm, which is about 20 ppm lower than the expected shift for the free carbene of between 200 and 210 ppm. The upfield shift of the signal is indicative of the coordination of the carbene to (alkali) metal ions as the shielding of the free carbene is increased.34 The

Scheme 1. Synthesis and Stability of Alkali-Metal Dicarbene Complexes 2−4

B


Article

Organometallics

Figure 1. Deprotonation of 1b with 2.2 equiv of LiHMDS in THF-d8: (a) VT 1H NMR spectra (detail) showing two signal sets that begin to coalesce at 40 °C; (b) proposed structure of the complexes in solution; (c) 7Li,7Li EXSY NMR spectrum (−40 °C) showing Li+ exchange for complex 2b but not for 2b-H; (d) both carbene signals in the 13C NMR spectrum showing 1JLiC couplings at −80 °C.

Table 1. Experimental and Calculated Characteristic NMR Chemical Shifts of the Two Compounds 2b and 2b-H signal 1

H C 7 Li 13

tBu carbene Li

δcalca 2b b

1.75 183.2b 1.68c

δexp 2bd

δcalca 2b-H-D2 b

1.69 182.1 1.51

1.60 185.8b 3.65c

δcalca 2b-H-C2 b

1.38 186.7b 3.06c

δexp 2b-Hd 1.53 184.3 3.16

a

Calculated chemical shifts (DFT, GIAO, BP86/def2-TZVP) based on a geometry optimization with DFT-D3 (BP86/def2-TZVP). bAverage of all respective nuclei, referenced to the signal of TMS; see the Supporting Information for individual atoms. cReferenced to [Li(THF)4]+ and the observed Li(THF)nPF6 signal in THF-d8. dMeasured at −80 °C in THF-d8.

comparison with pure samples, the peaks at −0.40 and 0.16 ppm can be assigned to LiPF6 that is formed during the reaction and residual LiHMDS. Lithium compounds with these chemical shifts are considered solvent-separated ion pairs, whereas higher chemical shifts indicate a more covalent character of the lithium−carbon bond.38 To link the 0.4:1 ratio of the 7Li NMR signals at 3.16 and 1.51 ppm with the respective 1:0.8 ratio observed for the 1H NMR signals at 1.68 and 1.51 ppm, a meaningful model would be first a complex 2b that contains one vegitBu ligand with the 1 H NMR tBu signal at 1.68 ppm and the 7Li NMR signal at 1.51 ppm and second the homoleptic complex 2b-H (H denoting homoleptic) containing two vegitBu ligands with the 1 H NMR tBu signal at 1.51 ppm and the 7Li NMR signal at 3.16 ppm (Figure 1b). This assignment results in a 1:0.4 ratio of the complexes 2b and 2b-H. The theoretical amount of LiPF6 formed is 1 equiv of LiPF6 per 2b and 1.5 equiv of LiPF6 per complex 2b-H, so that a total integral of 1 + 0.6 = 1.6 is expected, which fits reasonably well to the observed value of 2.0.

A 7Li,7Li EXSY NMR experiment at −40 °C (Figure 1c) reveals that the Li ions of 2b, residual LiHMDS, and LiPF6 undergo a fast exchange, while no exchange on the NMR time scale is observed for the Li ion of the homoleptic complex 2b-H containing two vegitBu ligands. The 13C NMR spectrum at −80 °C (Figure 1d) confirms the formation of two carbene Li complexes not only by the two carbene signals found at the characteristic chemical shifts of 184.3 ppm (2b-H) and 182.2 ppm (2b) (assignment by 2D 1 13 H, C NMR experiments) but also by the resolved direct LiC coupling of 1JLiC = 22.7 Hz (2b-H) and 24.2 Hz (2b) leading to a 1:1:1:1 quartet for each carbene signal. At room temperature only the carbene signal of 2b-H shows the LiC coupling (1JLiC = 22.2 Hz), while the signal of complex 2b is detected as a broad singlet. This reconfirms the assignment of the signal sets, as a faster exchange of the Li+ ion is expected and was observed in the EXSY NMR experiment (vide supra) for complex 2b, which contains only one vegitBu ligand. In general, LiC coupling constants are not observed for LiNHC complexes for various reasons. To our knowledge, Braunstein, Danopoulos, and co-workers reported recently the C


Article

Organometallics

reorientation of the parent carbene is necessary to complex the lithium cation, a situation that was observed earlier for monodentate NHC lithium complexes.8 The negligible rearrangement of the bidentate vegitBu ligand upon coordination to the lithium ion can explain the stability of the lithium complex even in the presence of crown ether. On the basis of the 19F NMR spectrum in THF-d8 at −80 °C, which shows only one fluoride signal (δ 73.34 ppm), we assume that the solid-state structure 2b′ is not present in solution but that the PF6− counterion is substituted by a second THF molecule as in 2b. This is substantiated by fast Li−F equilibration observed at low temperature in literature.52 To elucidate the situation of the other alkali-metal ions, we deprotonated the vegitBu·2HPF6 salt (1b) with NaHMDS, KHMDS, and KOtBu (Scheme 1). In all cases only one signal set was observed at room temperature. The 13C NMR chemical shift of the carbene signal was determined 189.3 ppm for the Na complex 3b and 195.2 ppm for the K complex 4b. In the case of 3b the addition of 2 equiv of [15]-crown-5 did not liberate the free carbene from the chelation, while addition of 2 equiv of [18]-crown-6 to complex 4b released the free carbene 5b, as indicated by the 1H and 13C NMR spectra (vide inf ra). Reasons for the Formation of the Homoleptic Complex 2b-H. We ascribe the formation of the homoleptic complexes to London dispersion that should be enhanced in the case of N-tert-butyl substituents. These interactions were shown to contribute particularly to the stability of sterically demanding organic molecules and metal complexes.53−56 To gain more insight for our ligand system, we conducted DFT calculations (BP86/def-2 SVP or def-2 TZVP) of the homoleptic complexes 2b-H (N-tBu) and 2c-H (N-Me) with and without considering dispersion correction. Classic DFT calculations provide complexes of D2d symmetry in which the two ligand planes take an orthogonal conformation. However, applying the dispersion correction by Grimme (DFT-D3),57 the optimized geometry of 2b-H-D2 reveals a rotation of one ligand about the C2 axis by 9° (Figure 3, middle). In addition, we found a second minimum conformation whose SCF energy is 3.4 kJ/mol lower and in which the ligands coordinate in a C2symmetric butterfly confirmation (2b-H-C2) (Figure 3, right). The small energy differences indicate a very flat hypersurface in this region so that even at very low temperatures wobbling of the ligands interconverts these conformers. The conformation 2b-H-D2 lies close to the transition state that interconverts the two enantiomeric conformations of 2b-H-C2.58 As a consequence of the flat hypersurface, large-amplitude lowfrequency modes that contribute mainly to the entropy lead to larger errors in the calculated free Gibbs energy difference, which favors the tilted conformer 2b-H-D2 by 12.5 kJ/mol. Therefore, the entropy contributions should not be overinterpreted. For the N-methyl-substituted complex 2c-H only the D2d conformation with orthogonal ligand planes is obtained as a minimum with and without applying dispersion corrections. To estimate the energetic contribution of the dispersion, we calculated the free energy of the isodesmic reaction between the N-methyl- and N-tert-butyl-substituted carbenes 5c and 5b and their respective homoleptic lithium complexes 2b-H and 2c-H (Figure 4). The difference in the standard reaction enthalpies without and with dispersion corrections amounts to about 45 kJ/mol (298 K, 1 bar) (endothermic without and exothermic with dispersion correction) and differences in the Gibbs free energy of 17 kJ/mol (2b-H-C2) to 29 kJ/mol (2b-H-D2)

only example of a direct LiC coupling in an NHC complex (an amide PNC pincer complex with an imidazolin-2-ylidene moiety) with a coupling constant of 1JLiC = 32.0 Hz for the 13 C NMR signal at 198.6 ppm.39 The 7Li NMR signal was found at 3.5 ppm. The fact that the 1JLiC coupling can be observed in our complexes 2b and 2b-H is clear evidence for a covalent contribution to the Li−C bond, which is supported by the rather low field situated 7Li NMR signals at 3.16 and 1.51 ppm. Further evidence for the formation the two species 2b and 2b-H was obtained from DFT calculations40 (BP-86/def2TZVP)41−45 of the NMR chemical shift using the GIAO method46 (Table 1). For complex 2b-H, two minimum structures (D2 and C2 symmetry) of similar energy were found (vide inf ra) that might both be present and interconvert in solution. Single crystals of 2b were obtained by slow diffusion of benzene into a concentrated solution of the dicarbene−Li mixture generated in THF. The X-ray structure analysis reveals coordination of the PF6− counterion to the Li, as was also observed by Hofmann for complex E. Two independent molecules are in the asymmetric unit, one of which shows a stronger disorder of the PF6− counterion.47 The carbene−Li distances of 2.22−2.23 Å match those of complex E30 but are longer than those typically reported for NHC-Li complexes (2.09−2.20 Å).48 The bite angle C1−Li−C8 is 80.1°, which is a value also found for vegiR complexes of transition-metal complexes.32,49,50 The C1−C8 carbene distance of 2.89 Å is only slightly shorter than the value for the imidazolium salt 1b (3.04 Å) and the calculated value (DFT) for the free carbene 5b (2.98 Å) (see the Supporting Information). It lies right within the broad range of 2.67−3.26 Å observed for this ligand system so far.51 The lithium ion is not situated on the bisecting line of the N−C−N angles but lies 24.0° toward the center (Figure 2, right). Due to the large s character of the carbene σ orbital, which can be concluded from the very acute N−C−N angles (100.6°, average) of 2b′ as well as of the free carbene 5b (calculated 100.6°), this deviation should require only small amounts of energy. In summary, no significant geometric

Figure 2. Molecular structure of the Li(vegitBu) complex 2b′. Atoms are shown with anisotropic atomic displacement parameters at the 50% probability level. Hydrogen atoms as well as two cocrystallized benzene molecules and a second independent molecule of 2b′ (showing disorder) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Li1−C1 2.230(5), Li1−C8 2.223(5), O1−Li1 1.920(5), Li1−F1 1.902(5), N9−N10 1.386(3), C5−C4 1.347(4), C1−C8 2.892, C1−Li−C8 80.99(18), O1−Li1−F1 121.2(2), N2− C1−N10 100.7(2), N9−C8−N7 100.4(2), C3−C3A−C4 137.8(3), C5−C5a−C6 136.9(3). D


Article

Organometallics

Figure 3. DFT calculations (BP86/def2-SVP) on the stabilizing effect of dispersion in complex 2b-H: optimized geometries (BP86/def2-TZVP) without (left) and with (middle and right) dispersion corrections (DFT-D3). The C2-symmetric butterfly conformation 2b-H-C2 (right) is 3.4 kJ/ mol lower in energy than the D2-symmetric 2b-H-D2 (middle) that results from the rotation of one ligand about the C2 axis by 9°. With N-methyl substituents (2c-H) the orthogonal conformation (left) is found with and without dispersion correction.

Figure 4. Isodesmic equation to evaluate the effect of London dispersion by the tBu groups in the homoleptic complex 2b-H. The gain in enthalpy amounts to 44−45 kJ/mol and a gain in the Gibbs free energy of between 17 and 29 kJ/mol.

(endergonic without and exergonic with dispersion correction). This confirms the strong influence of the dispersive forces in the case of N-tert-butyl substituents and explains the formation of the homoleptic complex 2b-H. The homoleptic complexes are not observed for the sodium and potassium complexes 3b and 4b experimentally. This is likely due to the stronger ionic character of the carbene−metal bonds, which leads to a faster exchange rate. Longer carbene−metal bonds might in addition change the influence of dispersion, and the coordination of further solvent molecules should be also taken into account. Investigation of the Equilibrium. Analyzing the deprotonation of vegitBu·2HPF6 (1b) with 2.2 equiv of LiHMDS in THF-d8 at variable temperatures already revealed an equilibrium between 2b and 2b-H that favors complex 2b thermodynamically. To shift this neutral analogue of a Schlenk-type equilibrium59 toward the side of the homoleptic complex 2b-H, we added 2.2 equiv of [12]-crown-4 to the 2b/ 2b-H mixture generated in THF. The 1H NMR spectrum shows now a preference for the formation of the homoleptic complex 2b-H, as expected by the principle of Le Chatelier, in a 2b:2b-H ratio of 1:1.2. (Figure 5). The 13C NMR spectrum still shows a broad singlet at 184.1 ppm for the carbene signal of 2b and a 1:1:1:1 quartet at 185.7 ppm with a 1JLiC coupling of 22.5 Hz for complex 2b-H. This reveals that, despite complexation of LiPF6 by [12]-crown-4, the Li+ exchange in complex 2b is still fast on the NMR time scale.

Figure 5. Solvent dependence of the equilibrium between 2b and 2bH and the influence of crown ether.

DFT-D3 calculations reveal this reaction to be endothermic (ΔHR = 4−5 kJ/mol for 2b-H-D2 and 2b-H-C2) at 298 K, confirming the observation that 2b is favored at −80 °C. The Gibbs free energy varies between −2 kJ/mol (2b-H-C2) and 9 kJ/mol (2b-H-D2). The observed equilibrium constant of 2b:2b-H = 1:0.5 corresponds to a ΔGR value of approximately 0 kJ/mol, considering the overall concentration of LiPF6. Thus, the magnitude order of the calculated values matches well the experimental result. E


Article

Organometallics We also observed the formation of two species by deprotonation of 1b in benzene-d6 and toluene-d8 at room temperature. However, as THF-d8 is not present, the nature of the complex 2b must be different, while 2b-H could be formed. Indeed, the two observed species vary not only in the chemical shift of the tBu groups but also in the ligand backbone signals, which is not the case in THF. Therefore, we think that in addition to 2b-H the species 2b′′ is formed, which contains coordinated PF6−, solvent, or amine. The assignment is based on the tBu signal of the homoleptic complex at higher field. In both cases a larger amount of 2b-H is formed (ratios 2b′′:2b-H = 1:0.5 (benzene-d6) and 1:1.4 (toluene-d8)). It is not clear whether the increased concentration of 2b-H is due to the reduced stability of species 2b′′ or to the low solubility of the formed LiPF 6 in the nonpolar solvent. The stronger coordinating acetonitrile favors formation of the heteroleptic complex 2b′′ in a 7:1 ratio. At first sight the formation of the homoleptic vegiR carbene complex seems to be limited to the N-tert-butyl-substituted vegi ligand in the lithium complex. A closer examination of the npropyl-substituted species reveals at −80 °C a 7Li NMR signal at 2.42 ppm with a ratio of only 5% in comparison to the Li signal of complex 2a at 1.47 ppm. In analogy to the analytical data of complexes 2b-H this signal can tentatively be assigned to the homoleptic complex 2a-H. The EXSY spectrum at −40 °C reveals thatin contrast to the complex 2b-Halso the lithium ion of this complex undergoes a fast exchange with complex 2a and free LiPF6. The lower concentration and the faster exchange rate can be explained by a higher energy of this species. In Situ Generation of the Free Carbene vegiR. As the deprotonation of the bis(imidazolium) salts 1a,b led to the respective alkali-metal complexes and only in the case of potassium ions was it possible to liberate the free carbene by addition of crown ether, we investigated the use of metal-free bases. Combining vegitBu·2HPF6 (1b) with 2.2 equiv or 4.2 equiv of DBU (1,8-diazabicyclo[5.4.0]undec-7-en) (pKa = 24.3 (acetonitrile)) in THF-d8 only showed the formation of the monocarbene 6b. Therefore, we switched to the phosphazene base P4-tBu (“Schwesinger-Superbase”, {(Me2N)3PN}3PNtBu), one of the strongest metal free bases available (pKa of 42.1 (acetonitrile)) (Scheme 2). The reaction

Figure 6. Detail (aromatic region) of the 1H NMR spectra of the vegitBu complexes 2b−4b and the free carbene 5b in THF-d8.

at chemical shifts of 204.5 ppm (5a) and 202.6 ppm (5b), which lie in the range that is expected from the chemical shift differences of the alkali-metal complexes34 and from the DFT calculations (see the Supporting Information). Deprotonation of 1b with only 1 equiv of the P4-tBu base resulted in the monocarbene 6b, for which in the 1H NMR spectrum at room temperature is detected only half of the expected single signal set for the aromatic protons and one imidazolium signal. This indicates a fast exchange of the imidazolium proton between the carbene moieties. Therefore, we cooled the sample to −80 °C and found the expected four signals for the aromatic backbone signals. In the 13C NMR signal the carbene signal is found at 197.7 ppm, the high-field shift can be a consequence of interactions with the imidazolium hydrogen atom. Deprotonation of 1b with 0.4 and 1.5 equiv of P4-tBu led to formation of a symmetric species at room temperature, which shows that the imidazolium salt 1b and the dicarbene 5b stand in fast equilibrium with 6b. The chemical shifts differ according to the amount of base used, which is very pronounced for the imidazolium signal that shifts to low field with increasing amounts of base. While deprotonation of 1b with substoichiometric amounts of base resulted in the stable monocarbene 6b, deprotonation of 1a with substoichiometric amounts of metal base resulted in a complex unidentified product mixture. We can confirm this observation also with 1 equiv of the metal-free base P4-tBu. The bulky tert-butyl groups seem to prevent any undesired side reactions in the monocarbenes. To prove whether the formation of the alkali-metal complexes can also occur from the free carbene and whether the formation of 2b-H might be selectively possible, we generated carbene 5b with 2 equiv of P4-tBu and, after confirmation of the formation by NMR, added 0.5 equiv of LiPF6. The 1H NMR spectrum shows one single set of broad signals, whose chemical shifts (pronounced for the tBu signal) lie between those of the two complexes 2b and 2b-H. An identical experiment, but with 1 equiv of LiPF6, leads to observation of the tBu signals of 2b and 2b-H in a 0.5:1 ratio, but all peaks remain broad, which might be explained by a faster equilibrium due to the increased amount of PF6− present in solution. Applying 1.5 equiv of LiPF6 leads to a faster exchange, and only one signal set is observed, whose broad peaks lie between those of 2b and 2b-H. Repeating the experiment in benzene-d6 did not lead to the formation of Li-vegitBu complexes due to the poor solubility of LiPF6; the signals of 5b remain unchanged. The reaction of 5b with KPF6 in THF-d8 leads to the respective potassium complex 4b, but only with 1 equiv or more of KPF6. With 0.5 equiv of KPF6 only the signals of 5b are observed.

Scheme 2. Generation of Free vegiR Di- and Monocarbenes

of veginPr·2HPF6 (1a) as well as vegitBu·2HPF6 (1b) with 2.2 equiv of P4-tBu led to formation of the new species 5a,b that both show the absence of the imidazolium signal (H-1/8) in the 1H NMR spectrum and the strongest high field shift of the heteroaromatic proton signals H-3/6 and H-4/5 in comparison with the respective alkali-metal complexes (Figure 6).60 In the 13 C NMR spectrum (THF-d8) the carbene signals are detected F


Article

Organometallics

■

(NCH2), 50.9 (C(CH3)3), 126.6 (C4/5), 161.3 (C3/6). FT-ICR HRMS (ESI+) (CH3CN): m/z calcd for C14N4H26 + H+ [M + H]+ 251.22286, found 251.22302. Synthesis of 3,6-Bis(tert-butylformamidomethyl)pyridazine. 3,6Bis(tert-butylaminomethyl)pyridazine (5.91 g, 23.6 mmol) was added to a mixture of formic acid (42 mL) and acetic acid anhydride (42 mL). The temperature of the brown solution increased, and gas evolution was observed. After it was stirred for 1.5 h, the reaction mixture was hydrolyzed with water (50 mL). The solution was evaporated to dryness, and the brownish residue was washed with cold acetonitrile and dried in vacuo. The product was obtained in 74% yield (5.35 mg, 17.5 mmol) and stored below −30 °C. 1H NMR (CDCl3, 400.13 MHz): δ 1.36 (s, 18H, C(CH3)3), 4.93 (s, 4H, NCH2), 7.44 (s, 2H, H-4/5), 8.64 (s, 2H, CHO). 1H NMR (DMSO-d6, 400.13 MHz): δ 1.32 (s, 18H, C(CH3)3), 4.78 (s, 4H, NCH2), 7.41 (s, 2H, H-4/5), 8.58 (s, 2H, CHO). 13C{1H} NMR (CDCl3, 100.61 MHz): δ 29.9 (C(CH3)3). 44.4 (NCH2), 56.4 (C(CH3)3), 126.6 (C4/5), 160.0 (C3/ 6), 162.4 (CHO). FT-ICR HRMS (ESI+) (CH3CN): m/z calcd for C16N4H26O2 + Na+ [M + Na]+ 329.194488, found 329.194797. Synthesis of N,N′-Di-tert-butyldiimidazo[1,5-b:1′,5′-f ]pyridazindiiumbis(hexafluorophosphate) (vegitBu·2HPF6; 1b). To a mixture of 3,6-bis(tert-butylformamidomethyl)pyridazine (2.50 g, 8.21 mmol) was added dry toluene (300 mL) and POCl3 (3.00 mL, 32.3 mmol) under an argon atmosphere. The mixture was stirred overnight at 85 ◦C, whereupon a brown oil formed. The solvent was evaporated, and the oily residue was washed three times with diethyl ether and dried in vacuo. Then, the raw product was dissolved in water (70 mL) and treated with a saturated solution of KPF6 (3.02 g, 16.4 mmol) in water at 50 °C, upon which an off-white precipitate immediately formed. The mixture was slowly cooled to room temperature for completion. The precipitate was filtered off, washed with water, dichloromethane, and pentane (20 mL), and dried in vacuo to obtain the product as an off-white crystalline solid (3.74 g, 6.65 mmol, 81%). 1 H NMR (400.11 MHz, THF-d8): δ 1.79 (s, 18H, C(CH3)3), 7.46 (s, 2H, H-4/5), 8.35 (s, 2H, H-3/6), 10.22 (s, 2H, H-1/8). 1H NMR (400.11 MHz, CD3CN): δ 1.79 (s, 18H, C(CH3)3), 7.54 (s, 2H, H-4/ 5), 8.22 (d, 4JHH = 1.6 Hz, 2H, H-3/6), 9.74 (d, 4JHH = 1.6 Hz, 2H, H1/8). 1H NMR (400.11 MHz, DMSO-d6): δ 1.76 (s, 18H, C(CH3)3), 7.71 (s, 2H, H-4/5), 8.79 (s, 2H, H-3/6), 10.64 (s, 2H, H-1/8). 13 C{1H} NMR (DMSO-d6, 100.61 MHz): δ 29.0 (C(CH3)3). 63.0 (C(CH3)3), 114.9 (C4/5), 115.9 (C3/6), 124.0 (C3a/5a), 126.4 (C1/ 8). Anal. Calcd for C16H24N4F12P2: C, 34.17; H, 4.30; N, 9.96. Found: C, 34.14; H, 4.36; N, 9.79. FT-ICR HRMS (ESI+) (CH3CN): calcd for C16N4H24P2F12 − PF6 [M − PF6]+ 417.163470, found 417.163729. Deprotonation of veginPr·2HPF6 (1a) with Lithium Bases: in Situ Generation of 2a. In THF-d8. Lithium hexamethyldisilazide (LiHMDS; 7.90 mg, 47.2 μmol) was added to a suspension of veginPr· 2HPF6 (1a; 10.4 mg, 19.5 μmol) in 0.4 mL of THF-d8. The 1H, 13 C{1H}, and 7Li{1H} NMR spectra of the brown-red suspension confirms deprotonation of veginPr·2HPF6 and formation of 2a and 2aH. 1H NMR (400.11 MHz, THF-d8): δ 0.95 (t, 3J = 6.6 Hz, 6H, CH3), 1.90−1.95 (m, 4H, CH2), 4.19 (t, 3J = 6.2 Hz, 4H, NCH2), 7.03 (br s, 2H, H-4/5), 7.58 (br s, 2H, H-3/6). 7Li{1H} NMR (194.37 MHz, THF-d8): δ 0.10 (br s). 1H NMR (400.11 MHz, THF-d8, −80 °C): molecule A, δ 0.93 (br s), 1.91 (br), 4.26 (br s), 7.19 (br s), 7.77 (br s); molecule B, δ 0.82 (br s), 1.83 (br s), 4.11 (br s), two signals superimposed by the peaks at 7.19 and 7.77. 7Li{1H} NMR (194.37 MHz, THF-d8, −80 °C): δ 2.42 (s, 2a-H), 1.47 (s, 2a), 0.16 (s, LiHMDS), −0.38 (s, LiPF6). In Benzene-d6. LiHMDS (7.5 mg, 43 μmol) was added to a suspension of veginPr·2HPF6 (1a; 9.7 mg, 18 μmol) in 0.4 mL of benzene-d6. The 1H NMR spectrum of the brown solution shows a deprotonation of veginPr·2HPF6. 1H NMR (400.11 MHz, C6D6): δ 0.72 (t, 3J = 7.5 Hz, 6H, CH3), 1.49−1.56 (m, 4H, CH2), 3.98 (t, 3J = 7.1 Hz, 4H, NCH2), 5.87 (s, 2H, H-4/5), 5.97 (s, 2H, H-3/6). In Toluene-d8. LiHMDS (4.00 mg, 24.0 μmol, 2.4 equiv) was added to a suspension of veginPr·2HPF6 (1a; 5.2 mg, 10 μmol) in 0.4 mL of toluene-d8. The 1H NMR spectrum of the brown solution shows deprotonation of veginPr·2HPF6. 1H NMR (400.11 MHz, toluene-d8): 2a’’: δ 1.26 (br s, 6H, CH3), 1.54−1.58 (m, 4H, CH2), 3.99 (t, 3JHH =

CONCLUSION We have shown that deprotonation of the anellated bis(imidazolium) salts of type 1 with alkali-metal bases leads to very stable chelate complexes of lithium, sodium, and potassium ions that even withstand chelation by the respective crown ethers in the case of lithium and sodium. In the case of lithium and the N-tert-butyl substituted ligand vegitBu an equilibrium between the heteroleptic and the homoleptic complex is observed. This can be explained by enhanced dispersion in the homoleptic complex due to the bulky N-tert-butyl substituents and is supported by DFT calculations. The solvent- and temperature-dependent equilibrium can be shifted toward the homoleptic complex by addition of crown ether to remove free lithium ions. The observation of direct Li−C coupling constants in the 13C NMR and the low-field chemical shifts of the 7Li NMR signals point to an enhanced covalent contribution in the Li−carbene bond. The free carbenes can be generated using the strong metal-free phosphazene base P4tBu. This opens new possibilities for the synthesis of other vegiR metal complexes by simple coordination, instead of transmetalation.

■

EXPERIMENTAL SECTION

General Methods. All experiments were carried out under an argon atmosphere using a glovebox or standard Schlenk techniques. Nondeuterated solvents were purchased from Sigma-Aldrich, dried, and degassed using an MBraun-SPS-800 solvent purification system. All other chemicals used were purchased from commercial suppliers and used without further purification. Deuterated solvents were dried with standard techniques. Methyllithium was bought as a diethyl solution that was dried in vacuo to remove the solvent. 3,6Bis(chloromethyl)pyridazine and veginPr·2HPF6 (1a) were synthesized according to the literature.32 All analytics were carried out at the University of Tübingen. NMR spectra were recorded using a Bruker AVII+400 or 500 spectrometer at 26 °C. 1H and 13C{1H} NMR spectra were calibrated to TMS on the basis of the relative chemical shift of the solvent as an internal standard. 7Li{1H} NMR chemical shifts are reported in ppm and calibrated to a LiCl solution (H2O) as external standard. Elemental analyses were carried out using a varioMICRO cube by the EA section at the Institut für Anorganische Chemie. HR-ESI mass spectra were measured using a Bruker Daltonics APEX II FT-ICR instrument by the MS section. Compounds 2a,b− 6a,b were only characterized by NMR measurements. The salts forming as byproducts in the syntheses of compounds 2a,b−4a,b were not removed, as they are a reservoir for the equilibrium reaction studied in solution by NMR spectroscopy. Due to the challenging removal of the highly soluble P4-tBu·HPF6 salt formed during syntheses of compounds 5a,b and 6a,b, we refrained from it. All conclusions are based on NMR experiments (see the Supporting Information for original spectra) and DFT calculations and in the case of 2b′ on X-ray structure analysis. Three-Step Synthesis of N,N′-Di-tert-butyldiimidazo[1,5b:1′,5′-f ]pyridazindiiumbis(hexafluorophosphate) (vegitBu· 2HPF6; 1b). Synthesis of 3,6-Bis(tert-butylaminomethyl)pyridazine. To a precooled solution (0 °C) of 3,6-bis(chloromethylpyridazine) (1.81 g, 10.2 mmol) in 50 mL of acetonitrile was added a 5-fold excess of tert-butylamine (10.8 mL, 102 mmol). The reaction mixture was stirred for 40 h at room temperature. The beige precipitate was filtered off, and the brown filtrate was concentrated to dryness in vacuo. The brown residue was extracted with diethyl ether and the solvent evaporated in vacuo. This was repeated with pentane as solvent, resulting in a 91% yield (2.33 g, 9.31 mmol) of the product as a light brown residue. 1H NMR (CDCl3, 400.13 MHz): δ 1.19 (s, 18H, C(CH3)3), 1.88 (br s, 2H, NH), 4.05 (s, 4H, NCH2), 7.52 (s, 2H, H4/5). 1H NMR (CD3CN, 400.13 MHz): δ 1.12 (s, 18H, C(CH3)3), 3.97 (s, 4H, NCH2), 7.58 (s, 2H, H-4/5), (N−H not observed). 13 C{1H} NMR (CDCl3, 100.61 MHz): δ 29.1 (C(CH3)3), 46.8 G


Article

Organometallics 7.2 Hz, 4H, NCH2), 5.95 (s, 2H, H-4/5), 6.05 (s, 2H, H-3/6). 2a-H: δ 0.86−0.87 (m, 6H, CH3), 1.34−1.36 (m, 4H, CH2), 3.69−3.77 (m, 4H, NCH2), 6.20 (br s, 4H, H-4/5 and H-3/6). On a small preparative scale 2a was prepared as a mixture with LiPF6 suspending veginPr·2HPF6 (1a; 60.4 mg, 113 μmol) in 2 mL of THF and addition of a 2.5 M solution of n-butyllithium in n-hexane (93 μL, 0.23 mmol) at −70 °C. After 10 min the pale yellow solution was warmed to RT over a period of 30 min upon which it turned red. After 1 h at RT, the solvent was removed in vacuo to yield the product mixture. 1 H NMR (400.11 MHz, THF-d8): δ 0.93 (t, 3J = 7.4 Hz, 6H, CH3), 1.91 (ps sxt, 3J = 7.2 Hz, 4H, CH2), 4.18 (t, 3J = 7.1 Hz, 4H, NCH2), 6.99 (s, 2H, H-4/5), 7.56 (s, 2H, H-3/6). 1H NMR (400.11 MHz, CD3CN): δ 0.91 (t, 3J = 7.2 Hz, 6H, CH3), 1.88 (ps sxt, 3J = 7.2 Hz, 4H, CH2), 4.11 (t, 3J = 7.0 Hz, 4H, NCH2), 6.98 (s, 2H, H-4/5), 7.39 (s, 2H, H-3/6). 13C{1H} NMR (100.61 MHz, THF-d8): δ 11.2 (CH3), 25.6 (CH2), 54.5 (NCH2), 113.0 (C4/5), 117.0 (C3/6), 124.5 (C3a/ 5a), 185.3 (C1/8). 7Li{1H} NMR (97.21 MHz, THF-d8): δ 0.3 (s br, 2a+[Li(THF)n]+). Deprotonation of veginPr·2HPF6 (1a) with NaHMDS. In THFd8. NaHMDS (3.9 mg, 21 μmol) was added to a suspension of veginPr· 2HPF6 (1a; 4.7 mg, 8.8 μmol) in 0.4 mL of THF-d8 and stirred for 15 min at room temperature. The 1H NMR spectrum of the light brown suspension indicated formation of the Na complex 3a. 1H NMR (400.13 MHz, THF-d8): δ 0.93 (t, 3J = 7.4 Hz, 6H, CH3), 1.88 (ps sxt, 3 J = 7.2 Hz, 4H, CH2), 4.17 (t, 3J = 7.2 Hz, 4H, NCH2), 6.91 (s, 2H, H-4/5), 7.43 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 11.4 (CH3), 24.8 (CH2), 54.6 (NCH2), 112.6 (C-4/5), 116.0 (C-3/ 6), 125.1 (C-3a/5a), 191.9 (C-1/8). In Benzene-d6. NaHMDS (3.9 mg, 21 μmol) was added to a suspension of 1a (4.7 mg, 9 μmol) in 0.4 mL of benzene-d6 and stirred for 15 min at room temperature. The 1H NMR spectrum of the brown-red suspension indicated formation of 3a′. 1H NMR (400.13 MHz, C6D6): δ 0.80 (t, 3J = 7.4 Hz, 6H, CH3), 1.60 (ps sxt, 3J = 7.6 Hz, 4H, CH2), 3.81 (t, 3J = 7.7 Hz, 4H, NCH2), 5.97 (s, 2H, H-4/5), 6.04 (s, 2H, H-3/6). In Toluene-d8. NaHMDS (3.6 mg, 6.7 μmol) was added to a suspension of 1a (3.0 mg, 16 μmol) in 0.4 mL of toluene-d8 and stirred for 15 min at room temperature. The signals in the 1H NMR spectrum of the brown-red suspension indicated formation of the Na complex 3a′. 1H NMR (400.13 MHz, toluene-d8): δ 0.87 (t, 3J = 7.1 Hz, 6H, CH3), 1.68−1.72 (m, 4H, CH2), 3.93 (t, 3J = 7.4 Hz, 4H, NCH2), 6.06 (s, 2H, H-4/5), 6.14 (s, 2H, H-3/6). Deprotonation of veginPr (1a) with KHMDS. In THF-d8. KHMDS (3.6 mg, 6.7 μmol) was added to a suspension of 1a (3.0 mg, 16 μmol) in 0.4 mL of THF-d8 at −78 ◦C. The reaction mixture was stirred for 15 min, warmed to room temperature within 30 min, and stirred for 1 h. The signals in the 1H NMR spectrum indicated formation of the K complex 4a. 1H NMR (400.13 MHz, THF-d8): δ 0.95 (br s, 6H, CH3), 1.89 (ps sxt, 3J = 7.3 Hz, 4H, CH2), 4.16 (br s, 4H, NCH2), 6.86 (s, 2H, H-4/5), 7.37 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 11.4 (CH3), 26.0 (CH2), 54.6 (NCH2), 112.5 (C-4/5), 115.5 (C-3/6), 125.4 (C3a/5a), 196.3 (C-1/8). In Benzene-d6. KHMDS (8.0 mg, 40 μmol) was added to a suspension of 1a (10.4 mg, 19 μmol) in 0.4 mL of benzene-d6 and stirred for 15 min at room temperature. The 1H NMR spectrum of the brown-red suspension showed the signals of complex 4a′. 1H NMR (400.13 MHz, C6D6): δ 0.82 (t, 3J = 7.4 Hz, 6H, CH3), 1.63 (ps sxt, 3J = 7.9 Hz, 4H, CH2), 3.90 (t, 3J = 8.0 Hz, 4H, NCH2), 6.02 (s, 2H, H4/5), 6.10 (s, 2H, H-3/6). Deprotonation of vegitBu·2HPF6 (1b) with Li Bases: Generation of [Li(vegitBu)(thf)2]PF6 (2b) and [Li(vegitBu)2]PF6 (2b-H). In THF-d8, with LiHMDS. LiHMDS (5.4 mg, 32 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 7.6 mg, 14 μmol) in 0.5 mL of THFd8 and stirred for 20 min at room temperature. The 1H NMR spectrum of the brown-red solution showed deprotonation of vegitBu· 2HPF6 (1b). In all cases the formation of complexes 2b and 2b-H in a ratio of 1:0.5 was observed. Data for 2b are as follows. 1H NMR (500.13 MHz, THF-d8, −80 °C): δ 1.69 (s, 18H, C(CH3)3), 7.16 (s, 2H, H-4/5), 8.03 (s, 2H, H-3/6). 1H NMR (500.13 MHz, THF-d8): δ

1.68 (s, 18H, C(CH3)3), 7.02 (s, 2H, H-4/5), 7.77 (s, 2H, H-3/6). 1H NMR (400.11 MHz, CD3CN): δ 1.64 (s, 18H, C(CH3)3), 6.96 (s, 2H, H-4/5), 7.57 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8, −80 °C): δ 31.1 (s, C(CH3)3), 58.9 (s, C(CH3)3), 113.3 (C-4/5), 115.0 (C-3/6), 124.3 (C-3a/5a), 182.2 (q, 1JCLi = 24.2 Hz, C-1/8). 13 C{1H} NMR (125.76 MHz, THF-d8): δ 31.3 (s, C(CH3)3), 58.9 (s, C(CH3)3), 113.2 (C-4/5), 114.5 (C-3/6), 124.6 (C-3a/5a), 184.1 (br s C-1/8). 7Li{1H} NMR (194.37 MHz, THF-d8, −80 ◦C): δ 1.51 (s). Data for 2b-H are as follows. 1H NMR (500.13 MHz, THF-d8, −80 ◦ C): δ 1.53 (s, 18H, C(CH3)3), 7.18 (s, 2H, H-4/5), 7.99 (s, 2H, H-3/ 6). 1H NMR (500.13 MHz, THF-d8): δ 1.51 (s, 18H, C(CH3)3), 7.04 (s, 2H, H-4/5), 7.75 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8, −80 °C): δ 31.1 (s, C(CH3)3), 58.9 (s, C(CH3)3), 113.3 (C4/5), 115.0 (C-3/6), 123.9 (C-3a/5a), 184.3 (q, 1JCLi = 22.7 Hz, C-1/ 8). 13C{1H} NMR (125.76 MHz, THF-d8): δ 31.3 (s, C(CH3)3), 58.9 (s, C(CH3)3), 113.2 (C-4/5), 114.2 (C-3/6), 124.4 (C-3a/5a), 185.7 (q, 1JCLi = 22.6 Hz, C-1/8). 7Li{1H} NMR (194.37 MHz, THF-d8, −80 ◦C): δ 3.16 (s). Only one signal for PF6− was found, even at −80 °C. 19F{1H} NMR (470.59 MHz, THF-d8, −80 °C): δ 73.34 (d, JPF = 710.8 Hz). 19F{1H} NMR (470.59 MHz, THF-d8): δ 73.68 (d, JPF = 710.3 Hz). 31P{1H} NMR (161.97 MHz, THF-d8): δ 144.10 ppm (hept, JPF = 708.7 Hz). Single crystals of 2b′ suitable for X-ray crystallography were grown by standard techniques from a solution of a mixture containing 2b, 2bH, and [Li(THF)4]PF6 in benzene at ambient temperature. See the Supporting Information for further information. In THF-d8, with MeLi. Solid methyllithium (1.1 mg, 50 μmol) was added at −30 ◦C to a suspension of 1b (13.0 mg, 23.1 μmol) in 0.4 mL of THF-d8. The 1H NMR spectrum of the brown-red solution showed formation of 2b and 2b-H (1:0.5). In THF-d8, with n-BuLi. A hexane solution of n-butyllithium (2.5 M, 90 μL) was added to a stirred suspension of vegitBu·2HPF6 (1b; 61.6 mg, 109 μmol) in 3 mL of THF at −78 °C to give a dark red solution which was stirred for 30 min and another 30 min at room temperature. After drying in vacuo, a product mixture with LiPF6 was obtained as a dark red solid. The 1H NMR spectrum in THF-d8 showed the product signals for 2b and 2b-H in a 1:0.5 ratio. In Benzene-d6. LiHMDS (3.9 mg, 23 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 6.0 mg, 11 μmol) in 0.4 mL of benzene-d6 and stirred for 20 min at room temperature. The 1H NMR spectrum indicates deprotonation of vegitBu·2HPF6 by absence of the imidazolium peak and formation of the signal sets of 2b′′ and 2b-H (ratio 1:0.5). Data for 2b-H are as follows. 1H NMR (400.11 MHz, C6D6): δ 1.22 (s, 18H, C(CH3)3), 6.37 (s, 2H, H-4/5), 6.64 (s, 2H, H3/6). 7Li{1H} NMR (194.37 MHz, C6D6): δ 2.92 (br s). 2b’’: 1H NMR (400.11 MHz, C6D6): δ 1.36 (s, 18H, C(CH3)3), 6.06 (s, 2H, H-4/5), 6.34 (s, 2H, H-3/6). 7Li{1H} NMR (194.37 MHz, C6D6): δ 1.43 (broad signal due to fast exchange with lithium salts. In Toluene-d8. LiHMDS (3.9 mg, 23 μmol) was added to a suspension of vegitBu· 2HPF6 (1b; 6.0 mg, 11 μmol) in 0.4 mL of toluene-d8 and stirred for 20 min at room temperature. The 1H NMR spectrum indicated deprotonation of vegitBu·2HPF6 by the absence of the imidazolium peak and formation of the signal sets of two complexes 2b′′ and 2b-H in a ratio of 1:1.4. Data for 2b-H are as follows. 1H NMR (400.11 MHz, toluene-d8): δ 1.23 (s, 18H, C(CH3)3), 6.38 (s, 2H, H-4/5), 6.66 (s, 2H, H-3/6). 7Li{1H} NMR (194.37 MHz, toluene-d8): δ 2.94 (br s). Data for 2b′′ are as follows. 1 H NMR (400.11 MHz, toluene-d8): δ 1.37 (s, 18H, C(CH3)3), 6.15 (s, 2H, H-4/5), 6.43 (s, 2H, H-3/6). 7Li{1H} NMR (194.37 MHz, toluene-d8): δ 1.28 (broad signal due to fast exchange with lithium salts). In Acetonitrile-d3. LiHMDS (7.1 mg, 43 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 10.0 mg, 17.8 μmol) in 0.4 mL of CD3CN and stirred for 20 min at room temperature. The 1H NMR spectrum indicated the formation of two strongly superimposed signal sets for the two Li complexes 2b′′ and 2b-H (ratio 1:0.15). 1H NMR (400.11 MHz, CD3CN): δ 1.64 (s, 18H, C(CH3)3), 6.95/6.99 (s, 2H, H-4/5), 7.56 (s, 2H, H-3/6). Deprotonation of vegitBu·2HPF6 (1b) with NaHMDS: Formation of Complex 3b. In THF-d8. NaHMDS (3.1 mg, 17 μmol) H


Article

Organometallics was added at −30 °C to a stirred suspension of vegitBu·2HPF6 (1b; 3.9 mg, 6.9 μmol) in 0.4 mL of THF-d8 to give a light orange solution which was stirred for 15 min. 1H NMR (400.11 MHz, THF-d8): δ 1.64 (s, 18H, C(CH3)3), 6.92 (s, 2H, H-4/5), 7.65 (s, 2H, H-3/6). 13C{1H} NMR (100.61 MHz, THF-d8): δ 31.4 (s, C(CH3)3), 58.7 (s, C(CH3)3), 112.7 (C-4/5), 113.6 (C-3/6), 125.0 (C-3a/5a), 189.3 (C-1/8). In Benzene-d6. NaHMDS (3.4 mg, 19 μmol) was added to a stirred suspension of 1b (4.40 mg, 7.82 μmol) in 0.4 mL of benzene-d6 to give a gray suspension which was stirred for 40 min. The 1H NMR spectrum shows a signal set for the Na complex 3b′. 1H NMR (400.11 MHz, C6D6): δ 1.35 (s, 18H, C(CH3)3), 6.08 (2H, H-4/5), 6.39 (2H, H-3/6). Deprotonation of vegitBu·2HPF6 (1b) with KHMDS. In THF. KHMDS (27.3 mg, 137 μmol) was added at −30 °C to a stirred suspension of vegitBu·2HPF6 (1b; 37.5 mg, 66.7 μmol) in 2 mL of THF to give a brown solution which was stirred for 30 min at −30 °C and at room temperature for 1.5 h. After drying in vacuo, the product 4b was obtained as a light brown solid. 1H NMR (400.11 MHz, THFd8): δ 1.65 (s, 18H, C(CH3)3), 6.85 (s, 2H, H-4/5), 7.57 (s, 2H, H-3/ 6). 1H NMR (400.11 MHz, CD3CN): δ 1.62 (s, 18H, C(CH3)3), 6.89 (s, 2H, H-4/5), 7.52 (s, 2H, H-3/6). 13C{1H} NMR (100.61 MHz, THF-d8): δ 31.6 (s, C(CH3)3), 58.5 (s, C(CH3)3), 112.4 (C-4/5), 113.0 (C-3/6), 125.2 (C-3a/5a), 195.2 (C-1/8). In Benzene-d6. KHMDS (5.6 mg, 28 μmol) was added to a stirred suspension of vegitBu·2HPF6 (1b; 7.5 mg, 13 μmol) in 0.4 mL C6D6. The 1H NMR spectrum of the beige suspension shows the signal set for the K complex 4b′. 1H NMR (400.11 MHz, C6D6): δ 1.43 (s, 18H, C(CH3)3), 6.18 (s, 2H, H-4/5), 6.54 (s, 2H, H-3/6). Deprotonation of veginPr·2HPF6 (1a) with nBuLi + 2.05 equiv of [12]-crown-4. To a suspension of veginPr·2HPF6 (1a; 60.4 mg, 0.113 mmol) in 1 mL of THF was added a solution of n-butyllithium in n-hexane (2.5 M, 93 μL, 0.23 mmol) at −70 °C. After 10 min, the light green solution was warmed to room temperature and kept for 1 h, upon which it turned red. The solvent was removed in vacuo and the residue dissolved in 0.4 mL THF-d8. After the identity of the carbene was proven by 1H NMR spectroscopy, [12]-crown-4 (37.5 μL, 0.232 μmol, 2.05 equiv) was added. The colorless single crystals that formed were identified as [Li([12]-crown-4)] by an X-ray structure analysis, and in solution the Li complex 2a was confirmed by NMR spectroscopy. 7Li{1H} NMR (194.37 MHz, THF-d8): δ 0.58 (br s). Deprotonation of veginPr·2HPF6 (1a) with LiHMDS + 4.8 equiv of [12]-crown-4. In THF-d8. LiHMDS (8.4 mg, 50 μmol, 2.4 equiv) was added to a suspension of veginPr·2HPF6 (1a; 11.2 mg, 21.0 μmol, 1 equiv) in 0.4 mL of THF-d8. A 1H NMR spectrum confirms formation of the Li complex 2a. [12]-crown-4 (16.3 μL, 100 μmol, 4.8 equiv) was then added to the dark brown solution, which lightened upon addition. The 1H and 13C{1H} NMR spectra showed no change in the chemical shifts of the resonances of 2a. In the 7Li{1H} NMR spectrum the signal remained broad and was slightly shifted. Deprotonation of veginPr ·2HPF6 (1a) with NaHMDS + [15]crown-5. In THF-d8 with 2.4 equiv of Crown Ether. NaHMDS (5.9 mg, 32 μmol) was added to a suspension of veginPr·2HPF6 (1a; 7.2 mg, 13 μmol) in 0.4 mL of THF-d8. A 1H NMR spectrum confirmed formation of the Na complex 3a. A solution of [15]-crown-5 in toluene (1.14 M, 28 μL, 32 μmol, 2.4 equiv) was then added to the dark brown solution, which lightened upon addition. The 1H NMR and the 13C{1H} NMR spectra showed no shift of the resonances of 3a. In THF-d8 with 4.8 equiv of Crown Ether. NaHMDS (6.4 mg, 35 μmol) was added to a suspension of veginPr·2HPF6 (1a; 7.8 mg, 15 μmol) in 0.4 mL of THF-d8. The solution turned orange immediately and dark brown after 5 min. The 1H NMR spectrum confirmed formation of the Na complex 3a. A solution of [15]-crown-5 in toluene (1.14 M, 62 μL, 70 μmol, 4.8 equiv) was then added to the dark brown solution, which lightened upon addition. The 1H NMR spectrum showed no shift of the resonances of 3a. Deprotonation of veginPr·2HPF6 (1a) with KHMDS + 2.1 equiv of [18]-crown-6. In THF-d8. KHMDS (17.3 mg, 86.7 μmol) was added to a suspension of veginPr·2HPF6 (1a; 22.1 mg, 41.4 μmol)

in 0.4 mL of THF-d8. A 1H NMR spectrum confirmed formation of the potassium complex 4a. A solution of [18]-crown-6 in toluene (0.95 M, 93 μL, 87.0 μmol, 2.1 equiv) was then added to the brown-red solution. The 1H and 13C{1H} NMR spectrum showed the signals of the dicarbene 5a. 1H NMR (400.11 MHz, THF-d8): δ 0.95 (br s, 6H, CH3), 1.90 (br s, 4H, CH2), 4.10 (br s, NCH2), 6.75 (s, 2H, H-4/5), 7.24 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 11.7 (CH3), 26.0 (CH2), 54.7 (NCH2), 112.2 (C-4/5), 115.2 (C-3/6), 125.5 (C3a/5a), 202.1 (C-1/8). Deprotonation of vegitBu·2HPF6 (1b) with LiHMDS and Addition of [12]-crown-4. In THF-d8 with 2.4 equiv of Crown Ether. LiHMDS (8.9 mg, 54 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 12.5 mg, 22 μmol) in 0.4 mL of THF-d8 to give a light brown solution. The 1H NMR spectrum confirmed formation of the two species 2b and 2b-H in the ratio 2:1. Then [12]-crown-4 (9.3 μL, 54 μmol, 2.4 equiv) was added, and the color of the solution lightened. The 1H NMR spectrum showed the formation of the two species 2b and 2b-H in the ratio 1:1.2 along with signals for the Li crown ether. Data for 2b are as follows. 1H NMR (500.13 MHz, THFd8): δ 1.68 (s, 18H, C(CH3)3), 7.04 (s, 2H, H-4/5), 7.78 (s, 2H, H-3/ 6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 31.4 (s, C(CH3)3), 58.9 (s, C(CH3)3), 113.3 (C-4/5), 114.3 (C-3/6), 124.4 (C-3a/5a), 184.1 (br s, C-1/8). Data for 2b-H are as follows. 1H NMR (500.13 MHz, THF-d8): δ 1.51 (s, 18H, C(CH3)3), 7.05 (s, 2H, H-4/5), 7.76 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 31.4 (s, C(CH3)3), 58.9 (s, C(CH3)3), 113.3 (C-4/5), 114.3 (C-3/6), 124.4 (C-3a/5a), 185.7 (q, 1JCLi = 22.5 Hz, C-1/8). In THF with 4.8 equiv of Crown Ether. LiHMDS (2.7 mg, 16 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 3.8 mg, 6.8 μmol) in THF to give a light brown solution. The 1H NMR spectrum confirmed formation of the two species 2b and 2b-H in the ratio 2:1. Then [12]-crown-4 (5.2 μL, 32 μmol, 4.8 equiv) was added, and the color of the solution lightened. The 1H NMR spectrum showed the formation of the two species 2b and 2b-H in the ratio 1:1.2 along with signals for the Li crown ether. Data for 2b are as follows. 1H NMR (500.13 MHz, THF-d8): δ 1.69 (s, 18H, C(CH3)3), 7.02 (s, 2H, H-4/ 5), 7.75 (s br, 2H, H-3/6). Data for 2b-H are as follows. 1H NMR (500.13 MHz, THF-d8): δ 1.51 (s, 18H, C(CH3)3), 7.05 (s, 2H, H-4/ 5), 7.75 (br s, 2H, H-3/6). Deprotonation of vegitBu·2HPF6 (1b) with NaHMDS + [15]crown-5. In THF-d8 with 2.4 equiv of Crown Ether. NaHMDS (8.20 mg, 44.8 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 10.5 mg, 18.7 μmol) in 0.4 mL of THF-d8, and a brown solution formed. 1 H NMR spectroscopy confirmed formation of the Na complex 3b. Upon addition of a solution of [15]-crown-5 in toluene (1.14 M, 39.4 μL, 44.8 μmol, 2.4 equiv) a suspension formed. The 1H NMR spectrum confirmed the presence of the Na complex 3b. In the 13 C{1H} NMR spectrum only the C(CH3)3 signal could be detected at 31.5 ppm due to the precipitate. In THF-d8 with 4.8 equiv of Crown Ether. NaHMDS (2.9 mg, 16 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 3.7 mg, 6.6 μmol) in 0.4 mL of THF-d8, and a brown solution formed. 1H NMR spectroscopy confirmed formation of the Na complex 3b. Upon addition of a solution of [15]-crown-5 in toluene (1.14 mol/L, 28 μL, 32 μmol, 4.8 equiv) a suspension formed. The 1H NMR spectrum showed no change in the signals of Na complex 3b. The intensity of the product signals is very low due to the precipitate. Deprotonation of vegitBu·2HPF6 (1b) with KHMDS + 2.1 equiv of [18]-crown-6. In THF-d8. KHMDS (7.6 mg, 38 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 10.4 mg, 18.5 μmol) in 0.4 mL of THF-d8. A 1H NMR spectrum confirms formation of the Kcomplex 4b. A solution of [18]-crown-6 in toluene (0.95 M, 40 μL, 38 μmol, 2.1 equiv) was then added to the brown suspension. The formation of more solid is recognized and the 1H and 13C{1H} NMR spectra show the peaks of dicarbene 5b. 1 H NMR (400.11 MHz, THF-d8): δ 1.65 (s, 18H, C(CH3)3), 6.75 (s, 2H, H-4/5), 7.42 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 31.7 (s, C(CH3)3), 57.5 (s, C(CH3)3), 112.0 (C-4/5 and C-3/6), 125.2 (C-3a/5a), 202.9 (C-1/8). The signal for: C(CH3)3 was detected via a 1H,13C HMBC NMR spectrum. I


Article

Organometallics Generation of the Free Dicarbene veginPr (5a). In THF-d8. P4tBu (30.3 mg, 47.8 μmol) was added to a suspension of veginPr·2HPF6 (1a; 12.8 mg, 22.8 μmol) in 0.4 mL of THF-d8. The suspension turned dark brown immediately. The 1H NMR showed full conversion of the imidazolium salt 1a into the dicarbene 5a and the signals of the protonated P4-tBu base. 1H NMR (400.11 MHz, THF-d8): δ 0.94 (t, 3 J = 7.4 Hz, 6H, CH3), 1.90 (ps sxt, 3J = 7.1 Hz, 4H, CH2), 4.10 (t, 3J = 7.1 Hz, 4H, NCH2), 6.79 (s, 2H, H-4/5), 7.33 (s, 2H, H-3/6). 13 C{1H} NMR (125.76 MHz, THF-d8): δ 11.7 (CH3), 54.7 (NCH2), 112.1 (C-4/5), 114.8 (C-3/6), 125.7 (C3a/5a), 204.5 (C-1/8). The signal for methylene carbon (CH2) was covered by the THF-d8 signal at 25 ppm. The carbene signal at 204.5 ppm was detected via a 1H,13C HMBC NMR experiment. 1H NMR (400.11 MHz, THF-d8, −80 °C): δ 0.92 (br s, 6H, CH3), 1.86 (br s, 4H, CH2), 4.16 (br s, 4H, NCH2), 6.97 (br s, 2H, H-4/5), 7.73 (br s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8, −80 °C): δ 11.9 (CH3), 54.3 (NCH2), 112.3 (C-4/5), 115.6 (C-3/6), 125.5 (C-3a/5a), 202.3 (C-1/8). The signal for methylene carbon (CH2) was covered by the THF-d8 signal at 25 ppm. In Benzene-d6. P4-tBu (14.5 mg, 23.0 μmol) was added to a suspension of veginPr·2HPF6 (1a; 5.8 mg, 11 μmol) in 0.4 mL of C6D6, and a dark brown solution of 5a containing brown oily droplets formed. 1H NMR (400.11 MHz, THF-d8): δ 0.71 (t, 3J = 7.4 Hz, 6H, CH3), 1.64 (m, 3J = 7.4 Hz, 4H, CH2), 3.83 (t, 3J = 7.1 Hz, 4H, NCH2), 6.25 (s, 2H, H-4/5), 6.36 (s, 2H, H-3/6). In Toluene-d8. P4-tBu (25.3 mg, 39.9 μmol) was added to a suspension of veginPr·2HPF6 (1a; 9.7 mg, 18 μmol) in 0.4 mL of toluene-d8, and a dark brown oil formed. The 1H NMR spectrum confirmed full conversion of the imidazolium salt 1a into the dicarbene 5a. 1H NMR (400.13 MHz, toluene-d8): δ 0.74 (t, 3J = 9.1 Hz, 6H, CH3), 1.67 (m, 4H, CH2), 3.81 (t, 3J = 7.2 Hz, 4H, NCH2), 6.25 (s, 2H, H-4/5), 6.37 (s, 2H, H-3/6). Generation of the Free Dicarbene vegitBu (5b). In THF-d8. P4tBu (29.8 mg, 47.0 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 12.6 mg, 22.4 μmol) in 0.4 mL of THF-d8, and the reaction mixture turned immediately into a light brown solution of 5b. 1H NMR (400.11 MHz, THF-d8): δ 1.64 (s, 18H, C(CH3)3), 6.77 (s, 2H, H-4/5), 7.47 (s, 2H, H-3/6). 13C{1H} NMR (125.76 MHz, THF-d8): δ 31.7 (s, C(CH3)3), 58.0 (s, C(CH3)3), 112.1 (C-4/5 and C-3/6), 125.3 (C-3a/5a), 202.6 (C-1/8). In Benzene-d6. P4-tBu (21.1 mg, 33.0 μmol) was added to a suspension of 1b (7.8 mg, 14 μmol) in 0.4 mL of C6D6, and a dark brown solution of 5b containing brown oily droplets formed which was stable for 3 days at room temperature. 1H NMR (400.11 MHz, THF-d8): δ 1.49 (s, 18H, C(CH3)3), 6.33 (s, 2H, H-4/5), 6.71 (s, 2H, H-3/6). In Toluene-d8. P4-tBu (11.9 mg, 18.8 μmol) was added to a suspension of 1b (4.8 mg, 8.5 μmol) in 0.4 mL of toluene-d8, and a colorless solution of 5b formed. 1H NMR (400.13 MHz, toluene-d8): δ 1.48 (18H, C(CH)3), 6.31 (s, 2H, H-4/5), 6.71 (s, 2H, H-3/6). Generation of the Monocarbene veginPr (6a). In THF-d8 with 0.5 equiv of Base. P4-tBu (2.0 mg, 3.2 μmol, 0.5 equiv) was added to a suspension of veginPr·2HPF6 (1a; 3.7 mg, 6.9 μmol) in 0.4 mL of THF-d8. The 1H NMR spectrum of the orange mixture showed the formation of two species: the monocarbene 6a and one (possibly decomposition) byproduct. Data for 6a are as follows. 1H NMR (400.11 MHz, THF-d8): δ 1.04 (t, 3J = 7.5 Hz, 6H, CH3), 2.06 (ps sxt, 3 J = 7.1 Hz, 4H, CH2), 4.49 (t, 3J = 7.1 Hz, 4H, NCH2), 7.57 (s, 2H, H-4/5), 8.21 (s, 2H, H-3/6), 10.67 (s, 1H, H-1 or H-8). In THF-d8 with 1 equiv of Base. P4-tBu (5.2 mg, 8.2 μmol, 1 equiv) was added to a suspension of veginPr·2HPF6 (1a; 4.4 mg, 8.2 μmol) in 0.4 mL of THF-d8. The 1H NMR spectrum of the brown solution showed the formation of monocarbene 6a and one unknown side product. 1H NMR (400.11 MHz, THF-d8): 6a, δ 0.91 (s, 6H, CH3), 1.02 (s, 4H, CH2), 4.49 (s, 4H, NCH2), 7.59 (s, 2H, H-4/5), 8.19 (s, 2H, H-3/6); side product, δ 0.91 (s, 6H, CH3), 1.02 (s, 4H, CH2), 4.49 (s, 4H, NCH2), 6.30 (s, 2H, H-4/5), 7.37 (s, 2H, H-3/6). All peaks are very broad. Generation of the Monocarbene vegitBu (6b). In THF-d8 with 0.4 equiv of Base. P4-tBu (4.1 mg, 6.5 μmol, 0.4 equiv) was added to

a suspension of vegitBu·2HPF6 (1b; 90 mg, 16 μmol) in 0.4 mL of THF-d8. The 1H NMR spectrum of the beige suspension showed the formation of two species: one main species, the monocarbene 6b, and one byproduct, a decomposed species. 1H NMR (400.11 MHz, THFd8): monocarbene, δ 1.76 (s, 18H, C(CH3)3), 7.18 (s, 2H, H-4/5), 8.17 (s, 2H, H-3/6), 10.02 (s, 1H, H-1 or H-8). In THF-d8 with 1.0 equiv of Base. P4-tBu (11.7 mg, 18 μmol, 1.0 equiv) was added to a suspension of vegitBu·2HPF6 (1b; 10.4 mg, 18 μmol, 1.0 equiv) in 0.4 mL of THF-d8. The 1H NMR spectrum of the light pink solution showed the formation of the monocarbene 6b. The solution is stable over several days. At room temperature, a fast exchange of the imidazolium proton leads to a symmetric species. 1H NMR (400.11 MHz, THF-d8): δ 1.75 (s, 18H, C(CH3)3), 7.31 (s, 2H, H-4/5), 8.17 (s, 2H, H-3/6), 10.08 (s, 1H, H-1 or H-8). 1H NMR (400.11 MHz, THF-d8, −80 °C): δ 1.75 (br s, 18H, C(CH3)3), 7.22 (s, 1H, H-4 or H-5), 7.48 (s, 1H, H-4 or H-5), 8.14 (s, 1H, H-3 or H6), 8.36 (s, 1H, H-3 or H-6), 10.34 (br s, 1H, H-1 or H-8). 13C{1H} NMR (125.76 MHz, THF-d8, −80 °C): δ 29.6 (s, C(CH3)3), 31.2 (s, C(CH3)3), 59.5 (s, C(CH3)3), 62.4 (s, C(CH3)3), 111.1, 115.8, 117.4, 123.9, 125.7, 127.4, 197.7 (C-1/8). In THF-d8 with 1.5 equiv of Base. P4-tBu (16 mg, 26 μmol, 1.5 equiv) was added to a suspension of vegitBu·2HPF6 (1b; 9.7 mg, 17 μmol) in 0.4 mL of THF-d8. The 1H NMR spectrum of the brown solution showed the formation of the monocarbene 6b. The solution was stable over the weekend at room temperature. Only one signal set was observed, whose peaks lie between those of monocarbene 6b and dicarbene 5b. 1H NMR (400.11 MHz, THF-d8): δ 1.70 (s, 18H, C(CH3)3), 7.15 (s, 2H, H-4/5), 8.04 (s, 2H, H-3/6), 10.27 (s, 1H, H1 or H-8). In THF-d8. DBU (4.8 mg, 32 μmol, 2.1 equiv) was added to a suspension of vegitBu·2HPF6 (1b; 8.5 mg, 15 μmol) in THF-d8 to give a beige solution. The 1H NMR spectrum showed the formation of monocarbene 6b. 1H NMR (400.11 MHz, THF-d8): δ 1.75 (s, 18H, C(CH3)3), 7.30 (2H, H-4/5), 8.15 (2H, H-3/6), 10.50 (2H, H-1/8). In THF-d8 with 4.8 equiv of Base. DBU (6.9 mg, 45 μmol, 4.8 equiv) was added to a THF suspension of vegitBu·2HPF6 (1b; 5.3 mg, 9.4 μmol, 1 equiv) to give a beige solution. The 1H NMR spectrum showed only the formation of the monocarbene 6b. 1H NMR (400.11 MHz, THF-d8): δ 1.75 (s, 18H, C(CH3)3), 7.30 (2H, H-4/5), 8.14 (2H, H-3/6), 10.79 (2H, H-1/8). Generation of the Free Dicarbene vegitBu (5b) and Addition of LiPF6. In THF-d8 with 0.5 equiv of LiPF6. P4-tBu (19.6 mg, 31 μmol) was added to a suspension of 1b (8.3 mg, 15 μmol) in 0.4 mL of THF-d8 to give a light brown solution, and LiPF6 (11 mg, 7.2 μmol, 0.5 equiv) was added. The 1H NMR spectrum showed one signal set, whose broad peaks lie between those of complexes 2b and 2b-H. 1H NMR (400.11 MHz, THF-d8): δ 1.62 (s, 18H, C(CH3)3), 6.78 (br s, 2H, H-4/5), 7.49 (br s, 2H, H-3/6). In THF-d8 with 1.0 equiv of LiPF6. P4-tBu (8.9 mg, 14 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 3.3 mg, 5.9 μmol) in 0.4 mL of THF-d8 to give a light brown solution, and LiPF6 (0.9 mg, 5.9 μmol, 1 equiv) was added. The 1H NMR spectrum showed full conversion into the Li complexes 2b and 2b-H in a ratio of 0.6:1. The backbone signals were broad and were not baseline separated. H NMR (500.13 MHz, THF-d8): 2b, δ 1.69 (s, 18H, C(CH3)3), 7.05 (s, 2H, H4/5), 7.77 (s, 2H, H-3/6); 2b-H, δ 1.51 (s, 18H, C(CH3)3), 7.05 (s, 2H, H-4/5), 7.77 (s, 2H, H-3/6). In THF-d8 with 2.0 equiv of LiPF6. P4-tBu (15.7 mg, 24.8 μmol) was added to a suspension of vegitBu·2HPF6 (1b; 5.8 mg, 10 μmol) in 0.4 mL of THF-d8 to give a light brown solution, and LiPF6 (31 mg, 20 μmol, 2 equiv) was added. The 1H NMR spectrum showed one signal set of broad peaks that lie between those of 2b and 2b-H. 1H NMR (400.11 MHz, THF-d8): δ 1.56 (br s, 18H, C(CH3)3), 7.01 (br s, 2H, H-4/5), 7.75 (br s, 2H, H-3/6). In benzene-d6. P4-tBu (21.1 mg, 33.3 μmol) was added to a suspension of 1b (7.8 mg, 14 μmol) in C6D6 to give a colorless solution and a brown oil. LiPF6 (22 mg, 14 μmol) was added to the solution. The 1H NMR spectrum showed no shift of the signals. 1H NMR (400.11 MHz, C6D6): δ 1.49 (s, 18H, C(CH3)3), 6.32 (s, 2H, H-4/5), 6.70 (s, 2H, H-3/6). J


Article

Organometallics Generation of the Free Dicarbene vegitBu (5b) and Addition of KPF6. In THF-d8 with 0.5 equiv of KPF6. P4-tBu (10.6 mg, 16.7 μmol) was added to a suspension of 1b (3.9 mg, 6.9 μmol) in 0.4 mL of THF-d8 to give a light brown solution, and KPF6 (0.6 mg, 3.3 μmol, 0.5 equiv) was added. The 1H NMR spectrum showed the signals for the dicarbene 5b. 1H NMR (400.11 MHz, THF-d8): δ 1.63 (s, 18H, C(CH3)3), 6.75 (s, 2H, H-4/5), 7.43 (s, 2H, H-3/6). In THF-d8 with 1.0 equiv of KPF6. P4-tBu (17.1 mg, 27.0 μmol) was added to a suspension of 1b (6.9 mg, 12 μmol) in THF-d8 to give a light brown solution, and KPF6 (2.3 mg, 12 μmol, 1.0 equiv) was added. The 1H NMR spectrum showed full conversion into the K complex 4b. 1H NMR (400.11 MHz, THF-d8): δ 1.64 (s, 18H, C(CH3)3), 6.86 (s, 2H, H-4/5), 7.57 (s, 2H, H-3/6). In THF-d8 with 1.5 equiv of KPF6. Carbene 5b was generated from P4-tBu (10.6 mg, 16.7 μmol) and 1b (3.9 mg, 6.9 μmol) in 0.4 mL of THF-d8, and KPF6 (1.8 mg, 9.8 μmol, 1.5 equiv) was added. The 1H NMR spectrum showed full conversion to complex 4b. 1H NMR (400.11 MHz, THF-d8): δ 1.64 (s, 18H, C(CH3)3), 6.85 (s, 2H, H-4/ 5), 7.57 (s, 2H, H-3/6).

■

(3) Arduengo, A. J., III Looking for Stable Carbenes: The Difficulty in Starting Anew. Acc. Chem. Res. 1999, 32, 913−921. (4) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Stable Carbenes. Chem. Rev. 2000, 100, 39−91. (5) Hahn, F. E.; Jahnke, M. C. Heterocyclische Carbene − Synthese und Koordinationschemie. Angew. Chem. 2008, 120, 3166−3216; Heterocyclic Carbenes: Synthesis and Coordination Chemistry. Angew. Chem., Int. Ed. 2008, 47, 3122−3172. (6) de Frémont, P.; Marion, N.; Nolan, S. P. Carbenes: Synthesis, properties, and organometallic chemistry. Coord. Chem. Rev. 2009, 253, 862−892. (7) Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali, S.; Butts, C. P.; Linehan, E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. J. Complexation of stable carbenes with alkali metals. Chem. Commun. 1999, 241−242. (8) Boche, G.; Hilf, C.; Harms, K.; Marsch, M.; Lohrenz, J. C. W. Struktur des dimeren (4-tert-Butylthiazolato)(glyme)lithiums: CarbenCharakter eines Formyl-Anion-Ä quivalents. Angew. Chem. 1995, 107, 509−511; Crystal Structure of the Dimeric (4-tert-Butylthiazolato)(glyme)lithium: Carbene Character of a Formyl Anion Equivalent. Angew. Chem., Int. Ed. Engl. 1995, 34, 487−489. (9) Yoshida, Z.; Konishi, H.; Sawada, S.; Ogoshi, H. Synthesis and Characterization of the Tetra-aminotriafulvalene Dication. J. Chem. Soc., Chem. Commun. 1977, 850−851. (10) Konishi, H.; Matsumoto, S.; Kamitori, Y.; Ogoshi, H.; Yoshida, Z. Synthesis and Properties of Diaminocyclopropenylidene Transition Metal Complexes. Chem. Lett. 1978, 7, 241−244. (11) Weiss, R.; Priesner, C.; Wolf, H. SE-Reaktionen am C3+-Ring: σAngriff. Angew. Chem. 1978, 90, 486−487; SE Reactions on the C3+Ring: σ-Attack. Angew. Chem., Int. Ed. Engl. 1978, 17, 446−447. (12) Lavallo, V.; Ishida, Y.; Donnadieu, B.; Bertrand, G. Isolation of Cyclopropenylidene−Lithium Adducts: The Weiss−Yoshida Reagent. Angew. Chem. 2006, 118, 6804−6807; Angew. Chem., Int. Ed. 2006, 45, 6652−6655. (13) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Cyclopropenylidenes: From Interstellar Space to an Isolated Derivative in the Laboratory. Science 2006, 312, 722−724. (14) Arduengo, A. J., III; Tamm, M.; Calabrese, J. C.; Davidson, F.; Marshall, W. J. Carbene-Lithium Interactions. Chem. Lett. 1999, 28, 1021−1022. (15) Koch, A.; Görls, H.; Krieck, S.; Westerhausen, M. Coordination behavior of bidentate bis(carbenes) at alkali metal bis(trimethylsilyl)amides. Dalton Trans. 2017, 46, 9058−9067. (16) Hill, M. S.; Kociok-Köhn, G.; MacDougall, D. J. N-Heterocyclic Carbenes and Charge Separation in Heterometallic s-Block Silylamides. Inorg. Chem. 2011, 50, 5234−5241. (17) Armstrong, D. R.; Baillie, S. E.; Blair, V. L.; Chabloz, N. G.; Diez, J.; Garcia-Alvarez, J.; Kennedy, A. R.; Robertson, S. D.; Hevia, E. Alkali-metal-mediated zincation (AMMZn) meets N-heterocyclic carbene (NHC) chemistry: Zn−H exchange reactions and structural authentication of a dinuclear Au(I) complex with a NHC anion. Chem. Sci. 2013, 4, 4259−4266. (18) Maddock, L. C. H.; Cadenbach, T.; Kennedy, A. R.; Borilovic, I.; Aromí, G.; Hevia, E. Accessing Sodium Ferrate Complexes Containing Neutral and Anionic N-Heterocyclic Carbene Ligands: Structural, Synthetic, and Magnetic Insights. Inorg. Chem. 2015, 54, 9201−9210. (19) Arnold, P. L.; Mungur, S. A.; Blake, A. J.; Wilson, C. Anionic Amido N-Heterocyclic Carbenes: Synthesis of Covalently Tethered Lanthanide−Carbene Complexes. Angew. Chem. 2003, 115, 6163− 6166; Angew. Chem., Int. Ed. 2003, 42, 5981−5984. (20) Wang, Y.; Xie, Y.; Abraham, M. Y.; Wei, P.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. A Viable Anionic N-Heterocyclic Dicarbene. J. Am. Chem. Soc. 2010, 132, 14370−14372. (21) Jürgens, E.; Buys, K.; Schmidt, A.-T.; Furfari, S. K.; Cole, M. L.; Moser, M.; Rominger, F.; Kunz, D. Optimised synthesis of monoanionic bis(NHC)-pincer ligand precursors and their Licomplexes. New J. Chem. 2016, 40, 9160−9169. (22) Wacker, A.; Pritzkow, H.; Siebert, W. Borane-Substituted Imidazol-2-ylidenes: Syntheses, Structures, and Reactivity. Eur. J. Inorg. Chem. 1998, 1998, 843−849.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00053. NMR spectra of all new compounds and NMR experiments as well as the experimental and crystal data for 1b, 2b′, and [Na([15]-crown-5)]PF6 and the calculated NMR chemical shifts and thermodynamic data (PDF) Cartesian coordinates of all calculated structures (XYZ) Accession Codes

CCDC 1590036−1590038 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*D.K.: tel, +49 7071 29-72063; fax, +49 7071 29-2436; e-mail, [email protected]. ORCID

Doris Kunz: 0000-0002-4388-6804 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This paper is dedicated to Bo Arduengo on the occasion of his 65th birthday and his retirement in 2017. We are grateful to the Hambrecht-Voscherau-Stiftung (fellowship for B.R.) for financial support. We thank Prof. Dr. Reinhold Fink for helpful discussions, Dr. Klaus Eichele for special NMR experiments and discussions, and Eric Moinet for the preparation of a single crystal of 2b′ suitable for X-ray structure analysis.

■

REFERENCES

(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. A Stable Crystalline Carbene. J. Am. Chem. Soc. 1991, 113, 361−363. (2) Herrmann, W. A.; Köcher, C. N-Heterocyclische Carbene. Angew. Chem. 1997, 109, 2256−2282; N-Heterocyclic Carbenes. Angew. Chem., Int. Ed. Engl. 1997, 36, 2162−2187. K


Article

Organometallics (23) Shishkov, I. V.; Rominger, F.; Hofmann, P. New Structural Motifs of Lithium N-Heterocyclic Carbene Complexes with Bis(3-tertbutylimidazol-2-ylidene)dialkylborate Ligands. Organometallics 2009, 28, 3532−3536. (24) Uzelac, M.; Hernán-Gómez, A.; Armstrong, D. R.; Kennedy, A. R.; Hevia, E. Rational synthesis of normal, abnormal and anionic NHC−gallium alkyl complexes: structural, stability and isomerization insights. Chem. Sci. 2015, 6, 5719−5728. (25) Garner, M. E.; Hohloch, S.; Maron, L.; Arnold, J. A New Supporting Ligand in Actinide Chemistry Leads to Reactive Bis(NHC)borate-Supported Thorium Complexes. Organometallics 2016, 35, 2915−2922. (26) Phillips, N.; Tirfoin, R.; Aldridge, S. Anionic N-heterocyclic carbenes (NHCs): a versatile route to saturated NHCs bearing pendant weakly coordinating anions. Dalton Trans. 2014, 43, 15279− 15282. (27) Kolychev, E. L.; Kronig, S.; Brandhorst, K.; Freytag, M.; Jones, P. G.; Tamm, M. Iridium(I) Complexes with Anionic N-Heterocyclic Carbene Ligands as Catalysts for the Hydrogenation of Alkenes in Nonpolar Media. J. Am. Chem. Soc. 2013, 135, 12448−12459. (28) Danopoulos, A. A.; Monakhov, K. Yu.; Braunstein, P. Anionic N-Heterocyclic Carbene Ligands from Mesoionic Imidazolium Precursors: Remote Backbone Arylimino Substitution Directs Carbene Coordination. Chem. - Eur. J. 2013, 19, 450−455. (29) El-Hellani, A.; Lavallo, V. Fusing N-Heterocyclic Carbenes with Carborane Anions. Angew. Chem. 2014, 126, 4578−4582; Angew. Chem., Int. Ed. 2014, 53, 4489−4493. (30) Brendel, M.; Wenz, J.; Shishkov, I. V.; Rominger, F.; Hofmann, P. Lithium Complexes of Neutral Bis-NHC Ligands. Organometallics 2015, 34, 669−672. (31) Named after the coauthor Verena Gierz, who managed to prepare this ligand system first. (32) Gierz, V.; Maichle-Mössmer, C.; Kunz, D. 1,10-Phenanthroline Analogue Pyridazine-Based N-Heterocyclic Carbene Ligands. Organometallics 2012, 31, 739−747. (33) Gierz, V.; Seyboldt, A.; Maichle-Mössmer, C.; Törnroos, K. W.; Speidel, M. T.; Speiser, B.; Eichele, K.; Kunz, D. Dinuclear CoinageMetal Complexes of Bis(NHC) Ligands: Structural Features and Dynamic Behavior of a Cu−Cu Complex. Organometallics 2012, 31, 7893−7901. (34) Tapu, D.; Dixon, D. A.; Roe, C. 13C NMR Spectroscopy of “Arduengo-type” Carbenes and Their Derivatives. Chem. Rev. 2009, 109, 3385−3407. (35) Herrmann, W. A.; Runte, O.; Artus, G. Synthesis and structure of an ionic beryllium-‘‘carbene” complex. J. Organomet. Chem. 1995, 501, C1−C4. (36) Arduengo, A. J., III; Davidson, F.; Krafczyk, R.; Marshall, W. J.; Tamm, M. Adducts of Carbenes with Group II and XII Metallocenes. Organometallics 1998, 17, 3375−3382. (37) As also 2:1 complexes of crown ether with alkali-metal ions are known, we repeated this reaction with 4 equiv of crown ether but still could not observe any shift in the carbene signal of 3a. Colorless single crystals formed were analyzed by X-ray structure analysis and were identified as the [Na([15]-crown-5)]PF6 complex. As the published data contains strong disorders, we report our analysis in the Supporting Information. See: Weller, F.; Borgholte, H.; Stenger, H.; Vogler, S.; Dehnicke, K. Synthesen und Kristallstrukturen der Kronenether-Komplexe (18-Krone-6)·2 CH3CN, [Na-15-Krone-5][ReO4]·CH3CN und [Na-15-Krone-5]PF6. Z. Naturforsch., B: J. Chem. Sci. 1989, 44, 1524−1530. (38) Elschenbroich, C. Organometallchemie; Teubner, B. G., Ed.; GWV: Wiesbaden, Germany, 2008. (39) Simler, T.; Karmazin, L.; Bailly, C.; Braunstein, P.; Danopoulos, A. A. Potassium and Lithium Complexes with Monodeprotonated, Dearomatized PNP and PNCNHC Pincer-Type Ligands. Organometallics 2016, 35, 903−912. (40) TURBOMOLE V6.3.1 2011, a development of the University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007,

TURBOMOLE GmbH, since 2007; available from http://www. turbomole.com. (41) Becke, A. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (42) Perdew, J. P. Density-functional approximation for the correlation energy of the inhomogeneous electron gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (43) Schäfer, A.; Horn, H.; Ahlrichs, R. Fully optimized contracted Gaussian basis sets for atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571− 2577. (44) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (45) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (46) Schreckenbach, G.; Ziegler, T. Calculation of NMR Shielding Tensors Using Gauge-Including Atomic Orbitals and Modern Density Functional Theory. J. Phys. Chem. 1995, 99, 606−611. (47) As the geometries of both independent complexes are identical within the standard deviation, only the geometry of the nondisordered complex is discussed in the following. (48) Allen, F. H. The Cambridge Structural Database: a quarter of a million crystal structures and rising. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380−388. (49) Gierz, V.; Urbanaite, A.; Seyboldt, A.; Kunz, D. Rhodium Complexes Bearing 1,10-Phenanthroline Analogue Bis-NHC Ligands Are Active Catalysts for Transfer Hydrogenation of Ketones. Organometallics 2012, 31, 7532−7538. (50) Raible, B.; Gierz, V.; Kunz, D. Identifying and Rationalizing the Conditions for the Isomerization of 1,5-Cyclooctadiene in Iridium Complexes by Experimental and Theoretical Mechanistic Studies. Organometallics 2015, 34, 2018−2027. (51) The shortest distance was observed for an Ir−chelate complex (2.667 Å50) and the longest for the dichloroimidazolium salt (3.256 Å): Raible, B.; Kunz, D. Selective di- and monochlorination of pyridazine-annelated bis(imidazolium) salts. Z. Naturforsch., B: J. Chem. Sci. 2016, 71, 659−666. (52) Stalke, D.; Klingenbiel, U.; Sheldrick, G. M. Bicyclische Lithiumt-butylaminofluorsilane, Moleküle mit zwei- und vierfachkoordiniertem Lithium. J. Organomet. Chem. 1988, 344, 37−48. (53) Wagner, J. P.; Schreiner, P. R. London‘sche Dispersionswechselwirkungen in der Molekülchemie − eine Neubetrachtung sterischer Effekte. Angew. Chem. 2015, 127, 12446−12471; London Dispersion in Molecular ChemistryReconsidering Steric Effects. Angew. Chem., Int. Ed. 2015, 54, 12274−12296. (54) Wagner, J. P.; Schreiner, P. R. London Dispersion Decisively Contributes to the Thermodynamic Stability of Bulky NHCCoordinated Main Group Compounds. J. Chem. Theory Comput. 2016, 12, 231−237. (55) Liptrot, D. J.; Power, P. P. London dispersion forces in sterically crowded inorganic and organometallic molecules. Nat. Rev. Chem. 2017, 1, 0004. (56) Hawley, A. L.; Ohlin, C. A.; Fohlmeister, L.; Stasch, A. Heavier Group 13 Metal(I) Heterocycles Stabilized by Sterically Demanding Diiminophosphinates: A Structurally Characterized Monomer−Dimer Pair For Gallium. Chem. - Eur. J. 2017, 23, 447−455. (57) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (58) The geometry optimization applying the def2-TZVP basis set and starting from the minimum structure 2b-H-D2, obtained with the def2-SVP basis set, leads to the butterfly conformation when very tight convergence criteria are used and stays in the tilted conformation when regular convergence criteria are applied. L


Article

Organometallics (59) Schlenk, W.; Schlenk, W., Jr Ü ber die Konstitution der Grignardschen Magnesiumverbindungen. Ber. Dtsch. Chem. Ges. B 1929, 62, 920−924. (60) σ-Donation of the carbene to a metal center is considered to induce a low-field shift to the imidazolylidene backbone signals due to a reduced electron density, while a π back-donation is considered to increase the electron density in the π system of the ylidene, so that an upfield shift is observed: Arduengo, A. J., III; Camper, S. F.; Calabrese, J. C.; Davidson, F. Low-Coordinate Carbene Complexes of Nickel(0) and Platinum(0). J. Am. Chem. Soc. 1994, 116, 4391−4394.

M


Generation of Annelated Dicarbenes and Their ... - ACS Publications

Recommend Documents