Sodium and Potassium Complexes of Anionic N‐Heterocyclic Carbenes

The reaction of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2ylidene (IDipp) with n-BuLi in the presence of sodium and potassium bis(trimethylsilyl)amide was studied, and the resulting polymeric dicarbene species were reacted with the fluoroboranes tris(pentafluorophenyl)borane, B(C6F5)3, and tris [3,5-bis(trifluoromethyl)phenyl]borane, B(m-XyF6)3 to produce sodium and potassium salts of an anionic N-heterocyclic carbene with a weakly coordinating borate moiety (WCA-NHC).


Introduction
The advent of modern organometallic chemistry in the middle of the 20th century had been sparked by the discovery and structural elucidation of bis(cyclopentadienyl)iron (ferrocene). [1] Ever since, cyclopentadienyl (Cp) derivatives have been among the most prominent ligands in transition metal as well as main group element chemistry, [2] including alkali metal complexes as the preferred transmetalation reagents. [3] The isolation and structural characterization of the first N-heterocyclic carbenes (NHCs) is arguably a similar important discovery, [4] which has made NHC ligands ubiquitous and indispensable in diverse research areas such as homogeneous catalysis, [5] medicinal chemistry, [6] and materials chemistry. [7] Nowadays, NHC complexes cover almost the whole periodic table of the elements, including the alkali metals despite the generally overall neutral charge of NHCs. [8] Early examples also comprise mixed Cp-NHC lithium half-sandwich complexes of the type [(η 5 -{1,2,4-(Me 3 Si) 3 C 5 H 2 }Li(NHC) (I), which bring these popular ligands systems together in one molecule ( Figure 1). [9] Ever since, a large number of lithium NHC complexes have been reported and used as transmetalation agents. [8] Numerous lithium salts have become available via the "anionic dicarbene" II, [10] which can be functionalized in the C4-position by treatment with electrophiles. [10,11] In our hands, the reaction with B(C 6 F 5 ) 3 afforded solvated lithium complexes of type III (solv. = THF, toluene) containing carbenes with a weakly coordinating fluoroborate moiety, so-called WCA-NHCs, which have been used extensively for the synthesis of transition metal complexes and homogeneous catalysts, [12,13] and more recently, also for the preparation of main group element compounds. [14] In general, such anionic N-heterocyclic carbenes should be particularly suitable for synthesizing unusual alkali metal carbene complexes. [15] In contrast to lithium, complexation of stable carbenes with heavier alkali metals is rare, and only a comparatively small number of well characterized sodium and potassium NHC complexes have been reported to date. [8] A remarkable exception are bis(NHC) complexes of the type [(IDipp) 2 M][M'{N-(SiMe 3 ) 2 } 3 ] (IV: M = Na, K; M' = Mg, Ca, Sr), which were obtained by combining equimolar mixtures of group 1 and group 2 bis (trimethylsilyl)amides with two equivalents of 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IDipp). [16] Complexes related to this study have been obtained by alkali-metal-mediated magnesiation, [17] zincation, [18] and ferration [19] reactions or by treatment of IDipp with NaR and KR followed by addition of GaR 3 (R = CH 2 SiMe 3 ). [20] Selected examples V and VI are shown in Figure 1. These reactions involve polymeric sodium or potassium species similar to II, and a potassium salt with briding K(THF) 2 units could be structurally characterized and used for the preparation of zincated and stannylated NHC potassium complexes. [21] Furthermore, potassium salts of abnormal NHCs could be generated by C4-metalation of imidazolium salts with n-butyl lithium (n-BuLi) and potassium bis(trimethylsilyl)amide (KHMDS, HMDS = hexamethyldisilazide). [22] Owing to our ongoing interest in exploiting the chemistry of WCA-NHC complexes, we aimed at the preparation of sodium and potassium congeners of the lithium complexes III, which might serve as advantageous carbene transfer and transmetalation reagents. Therefore, we have studied the reaction of IDipp with n-butyl lithium (n-BuLi) in the presence of sodium and potassium bis(trimethylsilyl)amide (MHMDS, M = Na, K; HMDS = hexamethyldisilazide) [23] and the reaction of the dicarbene intermediates similar to II with the fluoroboranes tris (pentafluorophenyl)borane, B(C 6 F 5 ) 3 , and tris[3,5-bis (trifluoromethyl)phenyl]borane, B(m-XyF 6 ) 3 . An initial account on these studies is given below.

Synthesis and Characterization of a Sodium Carbene Complex
The carbene IDipp was suspended in diethyl ether and treated with equimolar amounts of NaHMDS dissolved in THF and n-BuLi (1.6 M solution in n-hexane). After stirring at room temperature for 24 h, the resulting precipitate was filtered off, washed with n-hexane and dried under high vacuum. Because of its insolubility, the precipitate was used without further characterization, assuming the formation of a polymeric sodium salt in analogy to II (Figure 1). A suspension of this material in toluene was treated with B(C 6 F 5 ) 3 , and the reaction mixture was stirred overnight. The resulting solid was isolated by filtration and recrystallized from THF solution to afford [(WCA-IDipp)Na(THF) 3 ] (1) in good yield (Scheme 1). The NMR spectroscopic data of 1 in THF-d 8 are in good agreement with those reported for the lithium congener [(WCA-IDipp)Li(THF) 2 ] of type III, which was isolated as a bis(THF) complex. [12] Thus, the 1 H NMR spectrum exhibits a characteristic singlet at 6.17 ppm for the backbone CH hydrogen atom, while the 13 C{H} NMR resonance for the carbene carbon atom is found at 213.7 ppm. The borate moiety gives rise to one sharp singlet in the 11 B{ 1 H} NMR spectrum at À 15.37 ppm and to three signals in the 19 F{ 1 H} NMR spectrum at À 127.9, À 164.5 and À 167.9 ppm for the fluorine atoms in ortho, para and meta position.

Synthesis and Characterization of Potassium Carbene Complexes
Attempts to isolate a potassium complex analogous to 1 proved more difficult. It has been noted previously that the use of sodium or potassium bis(trimethylsilyl)amides together with n-BuLi for the deprotonation of IDipp is complicated by the generation of LiHMDS. [21] In our hands, the reaction of IDipp with KHMDS and n-BuLi was performed in diethyl ether (with a small amount of THF), affording a colorless precipitate. Elemental analysis suggested the formation of a polymeric complex [K(THF) x ][IDipp] (2, x = 1, Scheme 2), while the crystal structure has been reported for x = 2. [21] Single crystals were obtained by diffusion of n-hexane into a concentrated THF solution of the isolated precipitate, and X-ray diffraction analysis afforded the molecular structure of the mixed lithium-potas-Scheme 1. Synthesis of the sodium complex 1.  (17) sium complex 3 ( Figure 3). 3 crystallizes in the monoclinic space group P2 1 /n and reveals one-dimensional chains, in which bis (carbene) potassium units are bridged by Li(THF) via the deprotonated carbene backbone. The potassium atom is primarily bound to the carbene carbon atoms with KÀ C1 = 2.8394(14) Å and KÀ C28 = 2.9045(14) Å and completes its coordination sphere by interaction with the ipso-and orthocarbon atoms of the Dipp substituents. These potassium-carbon contacts range from 3.0839(16) to 3.6019(15) Å and are typical for potassium-π-aryl contacts. [24] As a result, the carbene ligands are strongly tilted, which is indicated by very different KÀ CÀ N angles, e. g., KÀ C1À N1 = 105.34(8)°and KÀ C1À N2 = 151.35(10)°. In contrast to bis(NHC) potassium complexes of type IV, [16] the C1À KÀ C28 angle of 128.57(4)°shows a cis rather than a typical trans arrangement of the NHC units. The lithium ion adopts a distorted trigonal-planar geometry and is bound to one THF ligand with LiÀ O = 1.985(3) Å and to the carbene backbone with LiÀ C2 = 2.092(3) Å and LiÀ C29 = 2.097(3) Å, which is shorter than reported for the related complex II · THF, viz. Li1À C1 = 2.175(6) Å and Li1À C30 = 2.125(6) Å. [10] Treatment of the lithium-potassium complex 3 with two equivalents of the borane B(C 6 F 5 ) 3 in toluene afforded the complex [Li(THF) 4 ][(WCA-IDipp) 2 K] (4) after recrystallization from THF/n-hexane solution. The 1 H NMR spectrum (in THF-d 8 ) shows the characteristic signals for the backbone CH hydrogen atoms at 6.13 ppm; the 13 C{ 1 H} NMR resonance of the carbene carbon atom is found at 217.4 ppm. Crystals of 4 · THF suitable for X-ray crystal structure analysis were obtained at room temperature by diffusion of n-hexane into a concentrated THF solution. The molecular structure was refined as a racemic twin (with a twin ratio of 43 to 57 %) in the triclinic space group P1 (Figure 4 15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57 hexane crystallized in the triclinic space group P � 1, and the molecular structure is shown in Figure 5. In both complexes 4 and 5, the anionic components each consist of a centrosymmetric [(WCA-IDipp) 2 (Figure 1). [16] Thus, the potassium-carbene distances of 2.828(4)/2.852(4) Å in 4 and 2.9080(12) Å in 5 fall in the same range (2.8210(2)-2.8614(17) Å), and the coordination is augmented by the same asymmetric η 3 -interaction with the ipsoand ortho-carbon atoms of the Dipp substituents adjacent to the borate moiety (C ipso distances ca. 3.19 Å, C ortho distances ca. 3.31À 3.43 Å). [16] In contrast to 3, the CÀ KÀ C angles of 179.19(16)°(4) and 180° (5)  The reaction of the solid obtained from the reaction of IDipp with KHMDS/n-BuLi (presumably a mixture of 2 and 3) was also treated with tris[3,5-bis(trifluoromethyl)phenyl]borane, B(m-XyF 6 ) 3 . [25] After crystallization from THF/n-hexane solution, the complex [WCA-IDipp)K(THF) 3 ] (6, WCA = B(m-XyF 6 ) 3 ) was isolated in moderate yield. (Scheme 3). The 1 H NMR spectrum (in THF-d 8 ) displays characteristic signals at 6.30 ppm and at 7.55/7.43 ppm for the carbene and m-XyF 6 CH hydrogen atoms, respectively. The 13 C{ 1 H} NMR resonance is found in the expected range at 218.5 ppm. In the 11 B{ 1 H} and 19 F{ 1 H} NMR spectra, the borate unit gives rise to signals at À 8.71 and at À 61.9 ppm, which is in agreement with the values reported for related systems. [25] Crystals suitable for X-ray diffraction analysis could be obtained at room temperature by diffusion of nhexane into a concentrated THF solution. 6 crystallized in the monoclinic space group P2 1 /n, and the molecular structure is shown in Figure 6. The obtained bond lengths and angles are in agreement with the already discussed potassium complexes. Again, the WCA-NHC ligand is bound in tilted fashion with a KÀ C1 bond length of 2.9430(15) Å and additional short contacts in the range 3.2646(14)-3.4995(14) Å, involving the ipso-and ortho-carbon atoms of the borate-flanking IDipp substituent. As a result, different KÀ CÀ N1/N2 angles of 106.99(9)°and 150.81(10)°are observed. The potassium coordination sphere is completed by three THF ligands, and the overall geometry around the metal ion is similar to that found for the potassium complex of type VI (M = K, Figure 1). [20]

Conclusions
Deprotonation of the N-heterocyclic carbene IDipp with n-BuLi in the presence of sodium or potassium bis(trimethylsilyl) amides (NaHMDS/KHMDS) and the reactivity of the intermediate dicarbene species towards fluoroboranes (WCA = B(C 6 F 5 ) 3 , B(m-XyF 6 ) 3 ) was studied. While the sodium complex [(WCA-IDipp)Na(THF) 3 ] (1, WCA = B(C 6 F 5 ) 3 ) could be conveniently obtained, the reactivity of the polymeric dicarbene species isolated from the reaction of IDipp with KHMDS/n-BuLi was not unambiguous, but provided evidence for the formation of a mixture of polymeric potassium and lithium-potassium complexes. The latter combination was confirmed crystallographi-    6 ][(WCA-IDipp) 2 K] (5) on different occasions, revealing that the combination of KHMDS/n-BuLi is not ideally suited for providing access to pure potassium WCA-NHC salts. However, the reaction with B(m-XyF 6 ) 3 gave [WCA-IDipp)K(THF) 3 ] (6, WCA=B(m-XyF 6 ) 3 ), which in contrast to 5 did not disproportionate into a bis(NHC) potassium complex and a solvated potassium counterion. For future applications of WCA-NHC ligands, sodium and potassium complexes such as 1 and 6 can serve as convenient carbene transfer and transmetalation reagents, in addition to their widely employed lithium congeners. For a reliable metalation of the NHC ligands such as IDipp, however, the combination of n-BuLi with other bases such as alkoxides (Lochmann-Schlosser superbases) [26] might lead to a more straightfoward formation of lithium-free sodium and potassium complexes.

Experimental Details
[(WCA-IDipp)Na(THF) 3 ] (1): IDipp (750 mg, 1.9 mmol, 1 eq) and NaHMDS (353 mg, 1.9 mmol, 1 eq or 1 M in THF, 1.923 mL) were treated with Et 2 O (30 mL) and THF (0.47 ml, 5.85 mmol, 3 eq) as needed. The suspension was stirred for 10 min and reacted with n-BuLi (1.6 M in n-hexane, 1.21 mL, 1.9 mmol, 1 eq). The suspension darkened and was stirred overnight. The precipitate was filtered off, washed with n-hexane (4 × 10 mL) and dried under high vacuum. Only 400 out of 600 mg of the isolated precipitate were further reacted and we asumed a connectivity of the type [Na(THF) n ][IDipp] (n = 1: M = 482.69 g/mol, 0.82 mmol) and calculated the amount of borane based on that. The precipitate was suspended in toluene (15 mL) and stirred at room temperature. B(C 6 F 5 ) 3 (424 mg, 0.82 mmol) was dissolved in toluene (4 mL) and added dropwise to the stirred solution. The reaction mixture was stirred overnight and the resulting precipitate was filltered off, washed with toluene (3 × 10 mL) and n-hexane (2 × 5 mL), and dried under high vacuum. Crystals suitable for X-ray diffraction analysis were obtained by diffusion of n-hexane into a concentrated THF solution. The product 1 was isolated as colorless solid (726 mg, 0.72 mmol, 88 %) after recrystallization out of THF/n-hexane. Elemental analysis calc. (%) for C 57 H 59 BF 15

[Li(THF)][(IDipp) 2 K] 1 (3):
In a Schlenk flask, IDipp (1.0 g, 2.6 mmol, 1 eq) and KHMDS (540 mg, 2.6 mmol, 1 eq) were suspended in diethyl ether (60 mL) and THF (0.63 mL, 7.8 mmol, 3 eq) was added. The yellow suspension was stirred for 10 min at room temperature and n-BuLi (1.6 M in n-hexane, 1.61 mL, 2.6 mmol, 1 eq) was added dropwise. The reaction mixture became clear and orange. The solution was stirred overnight and the resulting colorless precipitate was filtered off and was washed with n-hexane (3 × 20 mL). After drying under high vacuum the desired product 3 was isolated as a colorless solid (760 mg, 1.8 mmol, 70 %). Crystals suitable for Xray diffraction analysis were obtained by diffusion of n-hexane into a concentrated THF solution of 3.