Synthesis and Structural Characterization of p-Carboranylamidine Derivatives

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R–N=C=N–R (R = iPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C2H10B10[C(NiPr)2Li(DME)]2 (2) and p-C2H10B10[C(NCy)2Li(THF)2]2 (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me3SiCl) provided the silylated neutral bis(amidine) derivatives p-C2H10B10[C{iPrN(SiMe3)}(=NiPr)]2 (4) and p-C2H10B10[C{CyN(SiMe3)}(=NCy)]2 (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.

Molecules 2023, 28 In 2010, we reported a novel type of o-carborane-based N-chelating ligands which were named carboranylamidinates. These were obtained in the form of their lithium derivatives by in situ lithiation of o-carborane using n-butyllithium followed by treatment with one equivalent of 1,3-diorganocarbodiimides, R-N=C=N-R (R = i Pr, Cy (= cyclohexyl)). As illustrated in Scheme 1, the resulting carboranylamidinate anions were quite unique as they combined the highly versatile amidinate ligand system, [RC(NR ) 2 ] − [36][37][38][39][40], with a σ-bond to the carborane cage. Subsequently, the lithium salts served as precursors for a variety of main-group and transition metal complexes comprising carboranylamidinate ligands [41][42][43][44][45]. In all these complexes, the carboranylamidinate ligands adopt the characteristic κC,κN-chelating coordination mode instead of the regular κN,κN -chelating mode of metal-coordinated amidinate anions. In 2010, we reported a novel type of o-carborane-based N-chelating ligands which were named carboranylamidinates. These were obtained in the form of their lithium derivatives by in situ lithiation of o-carborane using n-butyllithium followed by treatment with one equivalent of 1,3-diorganocarbodiimides, R-N=C=N-R (R = i Pr, Cy (= cyclohexyl)). As illustrated in Scheme 1, the resulting carboranylamidinate anions were quite unique as they combined the highly versatile amidinate ligand system, [RC(NR′)2] − [36][37][38][39][40], with a -bond to the carborane cage. Subsequently, the lithium salts served as precursors for a variety of main-group and transition metal complexes comprising carboranylamidinate ligands [41][42][43][44][45]. In all these complexes, the carboranylamidinate ligands adopt the characteristic κC,κN-chelating coordination mode instead of the regular κN,κN′-chelating mode of metal-coordinated amidinate anions.

Scheme 1. Preparation of lithium carboranylamidinates derived from o-carborane.
Thus far, the formation of κC,κN-chelating carboranylamidinate anions has been limited to compounds derived from o-carborane. In 2014, we reported that similar reactions starting from m-carborane take a completely different course. As illustrated in Scheme 2, successive treatment of m-carborane with n-butyllithium and 1,3-di-iso-propylcarbodiimide did not lead to the formation of a related carboranylamidinate anion. Instead, an unprecedented deboronation reaction of the m-carborane took place, in which a BH group was detached from the carborane cage and incorporated into a nido-carborane-anellated diazadiborepine ring system. 1,3-dicyclohexylcarbodiimide reacted in a very similar manner but afforded a slightly modified seven-membered diazadiborepine ring system [46]. CH CH C H ortho (1,2) meta (1,7) para (1,12)  Thus far, the formation of κC,κN-chelating carboranylamidinate anions has been limited to compounds derived from o-carborane. In 2014, we reported that similar reactions starting from m-carborane take a completely different course. As illustrated in Scheme 2, successive treatment of m-carborane with n-butyllithium and 1,3-di-iso-propylcarbodiimide did not lead to the formation of a related carboranylamidinate anion. Instead, an unprecedented deboronation reaction of the m-carborane took place, in which a BH group was detached from the carborane cage and incorporated into a nido-carborane-anellated diazadiborepine ring system. 1,3-dicyclohexylcarbodiimide reacted in a very similar manner but afforded a slightly modified seven-membered diazadiborepine ring system [46]. Until now, the question remained of how the third isomer, p-carborane (1), would behave in the same reaction sequence of lithiation and carbodiimide addition. Here, we present the answer to this question.

Synthesis and Characterization
In the first set of experiments, THF solutions of p-carborane were metalated with 1 Until now, the question remained of how the third isomer, p-carborane (1), would behave in the same reaction sequence of lithiation and carbodiimide addition. Here, we present the answer to this question.

Synthesis and Characterization
In the first set of experiments, THF solutions of p-carborane were metalated with 1 equiv. of n-butyllithium and then treated in situ with two different carbodiimides R-N=C=N-R (R = i Pr, Cy). Under these conditions, only small amounts (ca. 20% yield) of crystalline products could be isolated, which were difficult to separate from unreacted p-carborane (NMR control). This finding implied that the envisaged mono-amidinate derivatives shown in the upper equation in Scheme 3 were not formed as pure reaction products and that disubstitution was instead the preferred reaction pathway. This assumption was soon confirmed by adjusting the stoichiometric ratio of the reactants to 1:2:2 according to the second equation in Scheme 3. Under these conditions, the new compounds 2 (R = i Pr) and 3 (R = Cy) could be isolated as pure crystalline solids in significantly improved yields of 51% (2) and 46% (3), respectively. Both lithium amidinate salts are readily soluble in THF, DME, and diethyl ether. Crystallization from DME (2) and THF (3) afforded the nicely crystalline solvates depicted in Scheme 4.

Scheme 2. Formation of polycylic diazadiborepines from m-carborane.
Until now, the question remained of how the third isomer, p-carborane (1), would behave in the same reaction sequence of lithiation and carbodiimide addition. Here, we present the answer to this question.

Synthesis and Characterization
In the first set of experiments, THF solutions of p-carborane were metalated with 1 equiv. of n-butyllithium and then treated in situ with two different carbodiimides R-N=C=N-R (R = i Pr, Cy). Under these conditions, only small amounts (ca. 20% yield) of crystalline products could be isolated, which were difficult to separate from unreacted pcarborane (NMR control). This finding implied that the envisaged mono-amidinate derivatives shown in the upper equation in Scheme 3 were not formed as pure reaction products and that disubstitution was instead the preferred reaction pathway. This assumption was soon confirmed by adjusting the stoichiometric ratio of the reactants to 1:2:2 according to the second equation in Scheme 3. Under these conditions, the new compounds 2 (R = i Pr) and 3 (R = Cy) could be isolated as pure crystalline solids in significantly improved yields of 51% (2) and 46% (3), respectively. Both lithium amidinate salts are readily soluble in THF, DME, and diethyl ether. Crystallization from DME (2) and THF (3) afforded the nicely crystalline solvates depicted in Scheme 4. Both bis(anionic) title compounds 2 and 3 were fully characterized through the usual set of elemental analyses and spectroscopic methods. In the IR spectra, strong bands at 1523 cm -1 (2) and 1543 cm -1 (3) are typical for the stretching vibrations of the delocalized amidinate NCN units [36][37][38][39][40]. Medium strong bands in the range of 2590-2620 cm -1 could be assigned to the B-H stretching vibrations, while the νas(C-O-C) bands of the coordinated solvents appear around 1050 cm -1 as medium or strong bands. The 1 H and 13 C NMR spectra show the typical signals of the iso-propyl and cyclohexyl substituents, which do not need to be discussed here in detail (cf., Experimental Section and Supplementary Materials). In the 1 H NMR spectra, the B-H hydrogens give rise to broad multiplets extending over a range of ca. 1.5 ppm. The 13 C NMR chemical shifts of the carbon atoms of the NCN groups are 155.2 ppm (2) and 154.0 ppm (3), respectively. A 13 C resonance of the quaternary carbon atoms within the carborane cage could be detected only in the spectrum of 2 ( 93.3 ppm). All cage boron atoms give rise to a single resonance around −14 ppm in the 11 B NMR spectra of both amidinate salts. Apparently, the centrosymmetric structure leads to very similar chemical shifts of the boron atoms so that the signals could not be further resolved. Finally, 7 Li NMR spectra displayed only one signal around 0.1 ppm. As expected for salt-like compounds, the mass spectra of 2 and 3 did not show the respective molecular ions but only fragment peaks of the unsolvated carboranylamidinate anions (cf., Experimental Section).
Remarkably, the formation of the lithium carboranylamidiate represents the first incidence of a "normal" reactivity of a lithiated carborane with carbodiimides. This means that 1,12-dilithiocarborane behaves toward carbodiimides like any other organolithium reagents and adds to the central carbon atom of the N=C=N moiety under the formation of regular amidinate anion of the type [RC(NR′)2] − [36][37][38][39][40]. This finding reveals that all three C2B10H12 isomers behave differently in their reactivity toward 1,3-diorganocarbodiimides.
As an initial reactivity study involving the lithium carboranylamidinate salts 2 and 3, we investigated silylation reactions with chlorotrimethylsilane, Me3SiCl, which should lead to the formation of neutral bis-silylated amidine derivatives, as illustrated in Scheme 4.

Scheme 4. Preparation of the silylated bis(amidine) derivatives 4 and 5.
Both reactions were carried out in THF solutions at r.t. Work-up via extraction with toluene afforded the bis-silylated products 4 and 5 as colorless crystals in moderate yields (4: 54%, 5: 43%). Both compounds dissolve freely in diethyl ether and toluene, and are moderately moisture-sensitive due to the presence of Si-N bonds. Besides an X-ray structural analysis of 4 (see next paragraph), all analytical and spectroscopic data were in ex- Both bis(anionic) title compounds 2 and 3 were fully characterized through the usual set of elemental analyses and spectroscopic methods. In the IR spectra, strong bands at 1523 cm −1 (2) and 1543 cm −1 (3) are typical for the stretching vibrations of the delocalized amidinate NCN units [36][37][38][39][40]. Medium strong bands in the range of 2590-2620 cm −1 could be assigned to the B-H stretching vibrations, while the ν as (C-O-C) bands of the coordinated solvents appear around 1050 cm −1 as medium or strong bands. The 1 H and 13 C NMR spectra show the typical signals of the iso-propyl and cyclohexyl substituents, which do not need to be discussed here in detail (cf., Experimental Section and Supplementary Materials). In the 1 H NMR spectra, the B-H hydrogens give rise to broad multiplets extending over a range of ca. 1.5 ppm. The 13 C NMR chemical shifts of the carbon atoms of the NCN groups are 155.2 ppm (2) and 154.0 ppm (3), respectively. A 13 C resonance of the quaternary carbon atoms within the carborane cage could be detected only in the spectrum of 2 (δ 93.3 ppm). All cage boron atoms give rise to a single resonance around −14 ppm in the 11 B NMR spectra of both amidinate salts. Apparently, the centrosymmetric structure leads to very similar chemical shifts of the boron atoms so that the signals could not be further resolved. Finally, 7 Li NMR spectra displayed only one signal around 0.1 ppm. As expected for salt-like compounds, the mass spectra of 2 and 3 did not show the respective molecular ions but only fragment peaks of the unsolvated carboranylamidinate anions (cf., Experimental Section).
Remarkably, the formation of the lithium carboranylamidiate represents the first incidence of a "normal" reactivity of a lithiated carborane with carbodiimides. This means that 1,12-dilithiocarborane behaves toward carbodiimides like any other organolithium reagents and adds to the central carbon atom of the N=C=N moiety under the formation of regular amidinate anion of the type [RC(NR ) 2 ] − [36][37][38][39][40]. This finding reveals that all three C 2 B 10 H 12 isomers behave differently in their reactivity toward 1,3-diorganocarbodiimides.
As an initial reactivity study involving the lithium carboranylamidinate salts 2 and 3, we investigated silylation reactions with chlorotrimethylsilane, Me 3 SiCl, which should lead to the formation of neutral bis-silylated amidine derivatives, as illustrated in Scheme 4.
Both reactions were carried out in THF solutions at r.t. Work-up via extraction with toluene afforded the bis-silylated products 4 and 5 as colorless crystals in moderate yields (4: 54%, 5: 43%). Both compounds dissolve freely in diethyl ether and toluene, and are moderately moisture-sensitive due to the presence of Si-N bonds. Besides an X-ray structural analysis of 4 (see next paragraph), all analytical and spectroscopic data were in excellent agreement with the formation of bis(silylated) p-carboranyl-bis(amidines). Highly characteristic in the IR spectra are the ν C=N bands at 1622 cm −1 (4) and 1627 cm −1 (5), respectively. These bands clearly indicate the transition from the delocalized amidinate NCN units in the salt-like amidinate precursors 2 and 3 (ν NCN 1523 cm −1 (2) and 1543 cm −1 (3)) to N-C=N moieties with localized carbon-nitrogen double and single bonds. Bands at 2590 cm −1 (4) and 2606 cm −1 (5) can be assigned to the B-H stretching vibrations, while typical Si-C stretch bands of the SiMe 3 groups appear at ν as 726 cm −1 and 750 cm −1 as well as ν s 651 cm −1 and 658 cm −1 , respectively. The 1 H, 13 C, and 29 Si NMR spectra of 4 and 5 all showed only one singlet resonance for the SiMe 3 groups. This is in agreement with the centrosymmetric molecular structure found in the X-ray structural analysis of 4 (see following paragraph). As was observed for the anionic precursors 2 and 3, the 11 B NMR spectra of the bis-silylated derivatives also displayed only single resonances around −13.4 ppm.

Crystal and Molecular Structures
The title compounds 3 and 4 could be structurally characterized through X-ray diffraction studies. The molecular structures are depicted in Figures 2 and 3. Colorless, prismshaped single-crystals of 3 were grown from concentrated solutions in THF at r.t., while compound 4 was obtained in the form of well-formed, colorless, block-like single-crystals upon slow crystallization from toluene at 4 • C. As illustrated in Figure 2, structure determination of compound 3 confirmed the presence of an anionic p-carboranylamidinate species formed by the addition of dilithiated p-carborane to the central C atom of the carbodiimide reagent. The overall molecular structure is centrosymmetric. With 1.953(3) Å, the Li-N2 bond length is typical for a coordinative bond. As in other typical lithium amidinates such as Li[MeC 6 H 4 C(NSiMe 2 ) 2 ](THF) 2 [36,47], the C-N distances in the amidinate NCN unit are quite similar (N(1)-C(2) 1.305 (2), N(2)-C(2) 1.340 (2)), indicating complete delocalization of the negative charge. However, the coordination of the lithium ion to the anionic amidinate moieties differs from the vast majority of other lithium amidinates in that it is not κN,κN -chelating. Instead, the lithium ions are coordinated to only one nitrogen atom of the NCN moiety, resulting in a nearly trigonal planar coordination geometry around Li. There are only very few examples of similar monodentate amidinate coordination to lithium [48,49], and all of them result from steric crowding around the NCN unit, e.g., through very bulky terphenyl or triptycenyl substituents. Thus it can be assumed that steric congestion is also the reason for the rather unusual trigonal planar coordination of the lithium ions in compound 3.
NMR spectra of the bis-silylated derivatives also displayed only single resonances around −13.4 ppm.

Crystal and Molecular Structures
The title compounds 3 and 4 could be structurally characterized through X-ray diffraction studies. The molecular structures are depicted in Figures 2 and 3. Colorless, prism-shaped single-crystals of 3 were grown from concentrated solutions in THF at r.t., while compound 4 was obtained in the form of well-formed, colorless, block-like singlecrystals upon slow crystallization from toluene at 4 °C. As illustrated in Figure 2, structure determination of compound 3 confirmed the presence of an anionic p-carboranylamidinate species formed by the addition of dilithiated p-carborane to the central C atom of the carbodiimide reagent. The overall molecular structure is centrosymmetric. With 1.953(3) Å, the Li-N2 bond length is typical for a coordinative bond. As in other typical lithium amidinates such as Li[MeC6H4C(NSiMe2)2](THF)2 [36,47], the C-N distances in the amidinate NCN unit are quite similar (N(1)-C(2) 1.305 (2), N(2)-C(2) 1.340 (2)), indicating complete delocalization of the negative charge. However, the coordination of the lithium ion to the anionic amidinate moieties differs from the vast majority of other lithium amidinates in that it is not κN,κN′-chelating. Instead, the lithium ions are coordinated to only one nitrogen atom of the NCN moiety, resulting in a nearly trigonal planar coordination geometry around Li. There are only very few examples of similar monodentate amidinate coordination to lithium [48,49], and all of them result from steric crowding around the NCN unit, e.g., through very bulky terphenyl or triptycenyl substituents. Thus it can be assumed that steric congestion is also the reason for the rather unusual trigonal planar coordination of the lithium ions in compound 3.  The neutral p-carboranyl-bis(amidine) derivative 4 crystallizes in the monoclinic space P21/n, and, like 3, the molecule also shows crystallographically imposed centrosymmetry. The transition from the delocalized anionic NCN moieties in the amidinate salts 2 and 3 to a neutral amidine is clearly evidenced by the change in the C-N bond lengths. With distances of C(2)-N(1) 1.265(2) Å and C(2)-N(2) 1.423(2) Å, compound 4 clearly contains N-C=N units with isolated single and double bonds. In this respect, the molecular structure of 4 is closely related to the oxygen analogue p-carborane-1,12-dicarboxylic acid [50]. The N1-C2-N2 angle is 128.9(1)°, and the Si-N2 distance is 1.751 (1) (9). Symmetry code to generate equivalent atoms: '1-x,1-y, 1-z.

General Procedures and Instrumentation
All reactions were carried out in oven-dried or flame-dried glassware under an inert atmosphere of dry argon employing standard Schlenk and glovebox (MBraun MBLab) techniques. The solvents n-pentane, toluene, DME, and THF were distilled from so-  (9). Symmetry code to generate equivalent atoms: '1-x,1-y, 1-z.

General Procedures and Instrumentation
All reactions were carried out in oven-dried or flame-dried glassware under an inert atmosphere of dry argon employing standard Schlenk and glovebox (MBraun MBLab) techniques. The solvents n-pentane, toluene, DME, and THF were distilled from sodium/benzophenone under nitrogen atmosphere prior to use. p-carborane was obtained from Katchem spol. s.r.o., 278 01 Kralupy nad Vltavou, Czech Republic (https://katchem.cz/en). Other starting materials were purchased from Sigma-Aldrich and used without further purification. All NMR spectra ( 1 H, 13 C, 29 Si, 11 B, and 7 Li) were recorded in THF-d 8 solutions on a Bruker DPX 400 spectrometer. IR spectra were measured with a Bruker Vertex 70V spectrometer equipped with a diamond ATR unit between 4000 cm −1 and 50 cm −1 . Mass spectra were measured on a MAT 95 apparatus (EI, 70 eV). Microanalyses (C, H, N) were performed using a VARIO EL cube apparatus. Melting/decomposition points were determined using a Büchi Melting Point B-540. A total of 0.50 g (3.5 mmol) p-carborane, dissolved in THF (50 mL), was treated at r.t. with 2 equiv. of n-butyllithium (7.0 mmol, 4.40 mL of a 1.6 M solution in n-hexane).
After stirring for 1 h, 0.88 g (7.0 mmol) of 1,3-di-iso-propylcarbodiimide was added via syringe, and stirring at r.t. was continued for 12 h. The resulting clear yellow solution was evaporated to dryness and the oily crude product was redissolved in a minimum volume of DME (ca. 10 mL). Product 2 was precipitated by the addition of n-pentane (ca. 50 mL) and isolated after drying under vacuum as a microcrystalline, pale yellow solid in 51% isolated yield (1.06 g). M.p. 215 • C (dec.). Elemental analysis calcd. for C 24  This compound was prepared in a similar manner as described for 2 but using 1.44 g (7.0 mmol) of N,N'-dicyclohexylcarbodiimide as a precursor. The resulting clear solution was concentrated to a total volume of ca. 30 mL which led to the formation of a white precipitate, which was then redissolved by brief heating. Colorless single crystals suitable

Conclusions and Future Outlook
In summary, we succeeded in the synthesis and full characterization of the first amidinate and amidine derivatives of para-carborane. Lithium carboranylamidinates based on p-carborane are readily accessible by the addition of in situ-prepared 1,12-dilithio-pcarborane to 1,3-diorganocarbodiimides, R-N=C=N-R (R = i Pr, Cy). This result showed that all three isomers of C 2 B 10 H 12 react in completely different manners with carbodiimides. An initial reactivity study involving treatment of 2 and 3 with 2 equiv. of Me 3 SiCl revealed that neutral bis-silylated amidine derivatives are also easily prepared. It should be noted here that the oxygen analogue p-carborane-1,12-dicarboxylic acid has been successfully utilized as a linker in the design of carborane-based MOFs (=metal-organic frameworks) [55,56]. One could easily foresee that the amidinate and amidine derivatives of p-carborane reported here will play a similar fruitful role in MOF chemistry in the future.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28093837/s1: IR, NMR, and mass spectra for all title compounds as well as X-ray diffraction data for 3 and 4.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.