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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Selective Three-Component Coupling for CO2 Chemical Fixation to Boron Guanidinato Compounds Sonia Moreno,† Alberto Ramos,‡,* Fernando Carrillo-Hermosilla,*,† Antonio Rodríguez-Dieǵ uez,§ Daniel García-Vivo,́ ∥ Rafael Fernań dez-Galań ,† and Antonio Antiñolo†

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Departamento de Química Inorgánica, Orgánica y Bioquímica-Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de Castilla-La Mancha, Campus Universitario, E-13071 Ciudad Real, Spain ‡ Departamento de Química Inorgánica, Orgánica y Bioquímica, Instituto Regional de Investigación Científica Aplicada, Universidad de Castilla-La Mancha, Campus Universitario, E-13071 Ciudad Real, Spain § Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Avenida de la Fuente Nueva S/N, 18071 Granada, Spain ∥ Departamento de Química Orgánica e Inorgánica/IUQOEM, Universidad de Oviedo, E-33071 Oviedo, Spain S Supporting Information *

ABSTRACT: A selective three-component coupling was employed to fix carbon dioxide to boron guanidinato compounds. The one-pot reaction of carbon dioxide, carbodiimides, and borylamines (ArNH)BC8H14 afforded the corresponding 1,2-adducts {R(H)N}C{N(Ar)}(NR)(CO 2 )BC 8 H 14 . Alternatively, the reaction with pMeOC6H4NC or 2,6-Me2C6H3NC gave the corresponding isocyanide 1,1-adducts {i-PrHN}C{N(p-Me-C6H4)}(Ni-Pr){CNAr}BC8H14. The molecular structures of products (2,6-iPr2C6H3NH)BC8H14 7, {i-Pr(H)N}C{N(p-MeC6H4)}(NiPr)(CO 2)BC 8 H14 9, {Cy(H)N}C{N(p-MeC 6H 4)}(Cy)(CO2)BC8H14 13, and {i-PrHN}C{N(p-MeC6H4)}(Ni-Pr){CNR″}BC8H14 (R″ = p-MeOC6H4, 2,6-Me2C6H3) 14 and 15 were established by X-ray diffraction. Density functional theory calculations at the M05-2X level of theory revealed that CO2 fixation and formation of the corresponding adduct is exothermic and proceeds via a nonchelate boron guanidinato intermediate.

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INTRODUCTION Chemical fixation and transformation of carbon dioxide (CO2), which is an inexpensive and renewable carbon source, is one of the greatest scientific and technological challenges.1 The development of both new reactions and new catalysts is needed to overcome the kinetic and thermodynamic stability of CO2.2 It is well-known that many transition-metal species react with CO2, whereas studies on the corresponding reactivity of CO2 with main-group molecular compounds are a subject of recent interest.3 Some of these examples correspond to systems named frustrated Lewis pairs (FLPs), a concept introduced by D. W. Stephan to explain the splitting of the dihydrogen molecule by sterically congested Lewis base and acid pairs, which are usually obtained from boron derivatives bearing C6F5 groups, which confer that atom with a stronger acid character.4 Guanidinato compounds of the main and transition-metal groups have been actively studied in recent years for their applications in a variety of attractive areas such as homogeneous catalysis (olefin polymerization, ring-opening polymerization, or amine guanylation) or materials science and © XXXX American Chemical Society

technology (as precursors for atomic layer deposition (ALD) or chemical vapor deposition (CVD) processes).5 Guanidinato compounds can be prepared in a straightforward way by three main methods: (i) carbodiimide insertion into M−N bonds; (ii) guanidine deprotonation with metal alkyls or hydrides; (iii) salt metathesis between metal halides and alkali metal guanidinates. The ease of substituent modification within the guanidinato framework and the different coordination modes of this ligand also enable the variation of the interesting steric and electronic features of these compounds. Although some examples of monodentate coordination are known, the coordination chemistry of these ligands is largely dominated by the chelating and bridging binding modes (Figure 1).6 Although some examples of FLP-CO2 adducts have been reported recently,7 it is noteworthy that the literature on CO2 fixation by intramolecular boron−nitrogen systems is rather limited. In particular, Stephan and co-workers7e described the FLP-type reactivity of related boron amidinates, of formula Received: April 19, 2018

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DOI: 10.1021/acs.inorgchem.8b01068 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

circumventing the use of C6F5 groups on boron led us to study an alternative pattern. On the basis of the information outlined above, carbodiimide insertion into N−B bonds of borylamines (also named aminoboranes) might offer a simple and valuable route to boron guanidinato compounds.10 Although these borylamines can usually be obtained by salt metathesis between lithium amides and haloboranes,11 therefore generating lithium halides as undesired side products, the dehydrocoupling of amines and hydridic boranes, including ammonia-borane and amine-borane adducts, appears to be an efficient alternative. Numerous catalysts have been reported to promote this dehydrogenation reaction,12 but it has also been shown that catalyst-free dehydrogenative processes can occur under thermal conditions.13 As part of our continued research on the chemistry of guanidine derivatives, in particular, that of main-group element derivatives,6a,9 we present here the three-component, highyield, one-pot reactions of borylamines, carbodiimides, and CO2 (or isocyanides) to give selectively boron guanidinato adducts {R(H)N}C{N(R′)}(NR)(CO2)BC8H14 (R = i-Pr; R′ = Ph, p-MeC6H4, o-MeC6H4, p-t-BuC6H4, p-CF3C6H4; R = Cy; R′ = p-MeC6H4) and {i-PrHN}C{N(p-MeC6H4)}(Ni-Pr){CNR″}BC8H14 (R″ = p-MeOC6H4, 2,6-Me2C6H3).

Figure 1. Guanidinato ligand and its coordination modes.

[HC(NR)2B(C6F5)2] (R = i-Pr, t-Bu), toward small unsaturated molecules, including CO2 1,2-insertions (Scheme 1). Scheme 1. CO2 Fixation by a Boron Amidinato Compound

Cantat and co-workers7k described the formation of a CO2 adduct of a chelate boron guanidinato resulting from the dehydrocoupling of 9-borabicyclo[3.3.1]nonane (9-HBBN) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) during the reduction of CO2 with this borane in the presence of the bicyclic guanidine. Previous related work by Dorokhov and Mikhailov showed that N,N′-disubstituted boron amidinates of formula [PhC(NR)(NR′)BR″2] (R,R′ = alkyl, aryl; R″ = alkyl), with weaker electron-withdrawing groups on boron, were also able to react under mild conditions with unsaturated molecules such as carbodiimides, nitriles, or isocyanates.8 Additionally, we reported a systematic reactivity study toward unsaturated molecules (isocyanides, CO, benzaldehyde, and CO2) of dialkylboron guanidinates (Me2N)C(Ni-Pr)2BCy2 and {iPr(H)N}C(Ni-Pr){N(p-t-BuC6H4)}BCy2 or bisboron guanidinates(2−) {i-Pr(BCy2)N}C(NiPr){N(p-t-BuC6H4)}BCy2 and {i-Pr(C8H14B)N}C(Ni-Pr){N(p-MeC6H4)}BC8H14, obtained by salt metathesis from the corresponding lithium guanidinates and chloroboranes or by a dehydrocoupling reaction of a guanidine with a commercial secondary borane.9 All of these species were found to react with aromatic isocyanides, and some of them with CO, to give 1,1-insertion products. Likewise, the addition of benzaldehyde and CO2 led to 1,2-insertion products in an equilibrium with the reagents, although the result was somewhat disappointing in the latter case, probably due to the stability of the chelate form in the boron guanidinato precursor (Scheme 2). The idea of achieving more stable insertion products by using simple, straightforward synthetic methods for the preparation of new boron guanidinates, which can be described as intramolecular Lewis pairs, from inexpensive reagents and

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RESULTS AND DISCUSSION Inspired by the aforementioned success of the known guanidinato synthesis by carbodiimide insertion, we examined the reactions of common carbodiimides, namely, 1,3diisopropylcarbodiimide (DIC) and 1,3-dicyclohexylcarbodiimide, with different borylamines as an alternative to the reactions of 9-borabicyclo[3.3.1]nonane (9-HBBN) with preformed guanidines. In the first step, we synthesized the borylamine compounds by thermal dehydrocoupling of 9HBBN and anilines with different substituents. In this way we could observe the influence of both the steric and electronic effects of these substituents on the synthesis of borylamines and, subsequently, on the synthesis of CO2 guanidinato adducts. Therefore, C 6 H 5 NH 2 , p-MeC 6 H 4 NH 2 , p-tBuC 6 H 4 NH 2 , o-MeC 6 H 4 NH 2 , p-CF 3 C 6 H 4 NH 2 , 2,6Me2C6H3NH2, and 2,6-i-Pr2C6H3NH2 were reacted with 0.5 equiv of the 9-HBBN dimer at 80 °C in toluene (Scheme 3). The hydrogen evolution was complete within 2 h, and the NMR spectra of the reaction mixtures showed the clean formation of the respective amino-9-BBN derivatives 1−7.14 Scheme 3. Synthesis of Borylamines 1−7

Scheme 2. CO2 Fixation by a Boron Guanidinato Compound

A characteristic peak shifted downfield with respect to that in the aniline precursor was observed for the NH proton in the NMR spectra of these compounds. In addition, a broad signal was detected in the 11B NMR spectra, ca. 51 ppm, as expected for tricoordinated boron compounds (see Experimental Section and Supporting Information). Single-crystal X-ray analysis of compound 7 (Figure 2), obtained from a saturated solution in benzene/pentane, revealed a geometry around each B and N atom that corresponded to an sp2 hybridization of B


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Inorganic Chemistry

guanidinato compound by borylamine still present in solution, since evidence for free guanidine was also observed in the NMR spectra. This process would lead to the formation of a bisborylamine intermediate similar to the {PhCH2N(BBN)2} reported by Yalpani et al.,15 which in turn would give the bisboron guanidinate via carbodiimide insertion into an N−B bond. Theoretical calculations were performed using the hybrid functional M05-2X to rationalize the experimental observations. For this purpose, we optimized several structures of the possible products formed upon reaction of borylamine 2 with DIC. As shown in Figure 3, a family of monodentate, with slightly different orientations of the groups, and bidentate boron guanidinato compounds were located in the corresponding potential energy surface. All of these structures are almost isoenergetic with respect to the starting materials, a situation that agrees well with the complicated mixtures of species in equilibrium observed experimentally, as proposed in Scheme 4. In any case, for these system the most stable structure, both in solution and gas phase (see Supporting Information), is invariably the chelate-type structure. In contrast to compounds 1−5, borylamines 6 and 7 failed to react with carbodiimide at all, even after gentle warming. This finding can be explained by steric hindrance of the methyl or isopropyl radicals on the aryl group toward the plausible nucleophilic attack on the central carbon of the carbodiimide. Despite the poor performance of this process to obtain boron guanidinato compounds, the above mixed solution of borylamine 2, carbodiimide, and guanidinato compounds was allowed to react with 1 atm of CO2 in C6D6. Surprisingly, after a few minutes a significant quantity of crystals started to form on the tube walls. 1H and 11B NMR spectra of the soluble fraction showed the presence of a mixture of two main boron compounds, which correspond to the previously observed chelate guanidinato and a new CO2 adduct. After 24 h at room temperature, crystals were isolated and dissolved in CD2Cl2. The 1H NMR spectrum showed the presence of the CO2 adduct only, and this was characterized by two doublets, at δ 1.59 and 0.95 ppm, assigned to two inequivalent isopropyl groups, and a broad doublet at δ 4.11 ppm, assigned to the NH moiety of the guanidinato compound. In the 11B NMR spectrum, a broad signal was observed at δ 4.6 ppm, and this is characteristic of tetracoordinated boron. Finally, in the 13C NMR spectrum, two shifted peaks at δ 158.1 and 153.2 ppm

Figure 2. Molecular structure of compound 7: H atoms, except NH, are omitted for clarity, and ORTEP ellipsoids are plotted at the 50% probability level. Compound 7 crystallizes with two crystallographically independent molecules (distinguished with A and B in the Supporting Information) in the asymmetric unit. Only the molecular structure and numbering scheme of molecule A is depicted. Selected bond lengths (Å) and angles (deg): B1−N1 = 1.401(4), N1−C9 = 1.432(3), B1−N1−C9 = 127.9(2).

each atom. This finding suggests a significant contribution of π-bonding between the two atoms. Thus, the distance of 1.401(4) Å falls in the range observed for other aminoboranes, and the sum of angles around the B or N atom was found to be ∼360°.11 However, the C1−N−B angle is higher than the theoretical 120° (127.9(2)°), and this may be explained by the steric effect of the bulky substituents. As a model, the reaction of borylamine 2 with DIC to obtain the desired guanidinato compound was followed by multinuclear NMR in C6D6. Disappointingly, variable amounts of different species were formed (Figure S22). Careful analysis of the spectra indicated that, in addition to the expected guanidinato compound, unreacted borylamine and carbodiimide were also present at high levels (Scheme 4). Monodentate guanidinato isomers seem to be in a fast equilibrium with the major species, the chelate isomer, as observed previously.9 Several attempts to improve the yield of the desired product by gentle warming resulted in an increase in the levels of starting materials in solution. This kind of deinsertion process from boron guanidinato compounds has been observed previously.9 Similar results were obtained for compounds 1, 3, 4, and 5. Over time, solutions of 2 and carbodiimide also showed the formation of a small amount of bisboron compound {i-Pr(C 8 H 14 B)N}C(Ni-Pr){N(pMeC6H4)}BC8H14.9b In an effort to understand this process, we proposed a protonolysis of the N−B bond of the

Scheme 4. Proposed Mechanism for the Formation of Boron and Bisboron Guanidinato Compounds

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Inorganic Chemistry

Figure 3. DFT-M05-2X computed structures for the possible products formed upon reaction of 2 with DIC. H atoms are omitted. Their relative Gibbs free energies at 298 K (in kJ mol−1) in benzene solution are indicated between brackets. See the Supporting Information for the corresponding data in gas phase.

were assigned by 1H−13C HMBC experiment to the carbon atom of the CN3 core and the CO2 molecule 1,2-inserted in the boron guanidinato compound, respectively (see Figures S26−S28). On the basis of these results, the same CO2 capture reaction was performed using borylamines 1, 3, 4, and 5. In these cases, different relative concentrations of the CO2 adduct and guanidinato precursor were observed in the NMR spectra of the crude reaction mixtures. In fact, derivative 5 with the CF3 electron-withdrawing group and, to a lesser extent, the more hindered compound 4 showed higher ratios of the guanidinato precursor and starting materials in solution. Reactions performed with the bulky borylamines 6 and 7 with a mixture of carbodiimide and CO2 resulted in recovery of the starting materials. Given that the solubility profile of these compounds seems to play an important role in improving the yield in the CO2 adducts, the reactions were performed in a Schlenk tube using a small amount of toluene as solvent to form the borylamine in a one-pot procedure, with carbodiimide and CO2 added consecutively. In all cases, a white precipitate started to form after a few minutes. Addition of pentane after 24 h at room temperature produced, after the appropriate workup, copious amounts of white solids characterized as the CO2 adducts (see Experimental Section). As one might expect, the crude product obtained using the less nucleophilic CF3 derivative was a complex mixture containing ca. 4:1 mixture of guanidinato compounds with and without CO2 inserted in the B−N bond, respectively (see Figure S35). Attempts to isolate the CO2 adduct (or the boron guanidinato intermediate) failed due to decomposition reactions during the purification process. Fortunately, X-ray quality crystals of 9 were obtained from a saturated solution in C6D6. The molecular structure and the atomic numbering scheme are shown in Figure 4. Compound 9 has a six-membered boracycle (oxadiazaborininone) with a half-chair configuration. The new N2−C2 bond formed after CO2 addition had a distance of 1.425(3) Å, which falls in the range of C−N single bonds. The resulting B1−O1 bond distance was 1.528(2) Å, that is, slightly shorter than those found in phosphane/borane FLP systems [1.54−1.55 Å]7d or guanidine/borane [1.537 Å]7k FLP systems and similar to that found for the six-membered boron amidinato CO2 adduct

Figure 4. Molecular structure of compound 9: H atoms are omitted for clarity, and ORTEP ellipsoids are plotted at the 50% probability level. Selected bond lengths (Å) and angles (deg): C1−N1 = 1.316(2), C1−N2 = 1.376(2), C1−N3 = 1.361(2), N2−C2 = 1.425(3), C2−O1 = 1.308(2), C2−O2 = 1.207(2), B1−N1 = 1.599(2), B1−O1 = 1.528(2), N1−C1−N3 = 121.7(2), N3−C1−N2 = 118.6(2), N1−C1−N2 = 119.6(2), O1−C2−O2 = 124.5(2).

mentioned above [1.493 Å].7e Likewise, the CO and C−O bond lengths of 1.207(2) and 1.308(2) Å in 9 exhibit the expected double- and single-bond character, respectively.7 It is noteworthy that, in the presence of both heterocumulene compounds, carbodiimide, and CO2, these borylamines chemoselectively reacted with the former to form a chelate guanidinato intermediate that subsequently, at the boundary of classical and FLP reactivity, evolved to open and regioselectively trap a CO2 molecule by formal insertion in a boron-alkylic nitrogen bond (vs arylic nitrogen one) to give the corresponding six-membered heterocycles 8−10 (Scheme 5). This process was also assumed by Stephan for FLP-type reactions of boron amidinates.7e This selectivity toward the carbodiimide insertion was in contrast to the reported reaction of borylamines with CO216 and analogous isocyanates,17 which proceeded with remarkable ease at room temperature to obtain the insertion products (carbamates) or the [2 + 2] cycloaddition adducts (diazaboretidinones), respectively. This reaction was not observed for the borylamines described in this work. Recently, a comparable selective multicomponent D


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Inorganic Chemistry Scheme 5. Proposed Pathway for the Synthesis of CO2 Boron Guanidinato Adducts

product in this reaction was that derived from the selective insertion of CO2 into the N-alkyl bond, which is the most stable product from a thermodynamic point of view. These adducts slowly lose CO2 in solution after several days (ca. 10% of decomposition after 4 d) to give the original mixture of borylamine, carbodiimide, and boron guanidinate. However, the compounds are stable indefinitely in the solid state under a protective atmosphere of dry nitrogen. In a similar way to 1,3-diisopropylcarbodiimide, 1,3dicyclohexylcarbodiimide reacted with borylamine 2 and CO2 to give the corresponding adduct 13. Crystals of compound 13 suitable for X-ray diffraction were grown from a concentrated C6D6/pentane solution, and the molecular structure is represented in Figure 6.

coupling of CO2, bis(pinacolato)diboron, LiOt-Bu, and aldehydes to obtain lithium cyclic boracarbonate ion pair compounds has been reported, but an NHC-copper catalyst was required to achieve success.18 The energetic profile of the reaction with CO2 was also studied computationally for the borylamine 2 (Figure 5). In

Figure 6. Molecular structure of compound 13: H atoms are omitted for clarity, and ORTEP ellipsoids are plotted at the 50% probability level. Selected bond lengths (Å) and angles (deg): C1−N1 = 1.323(1), C1−N2 = 1.382(2), C1−N3 = 1.350(1), N2−C2 = 1.426(1), C2−O1 = 1.302(1), C2−O2 = 1.212(2), B1−N1 = 1.600(1), B1−O1 = 1.532(2), N1−C1−N3 = 121.5(1), N3−C1−N2 = 119.15(9), N1−C1−N2 = 119.32(9), O1−C2−O2 = 124.0(1).

Figure 5. Relative energy profile of the reaction between the aminoborane, CO2, and DIC.

the first place, we found that the formation of a hypothetical insertion product of CO2 into the B−N bond of 2 to form a carbamate compound is an uphill process, with the resulting product being nearly 21 kJ mol−1 above the corresponding starting materials in benzene solution (29 kJ mol−1 in gas phase). In contrast, reaction of the guanidinato intermediates with CO2 was found to be an overall exergonic process. The two possible outcomes for this reaction were computed, namely, insertion of CO2 into the N-alkyl or N-aryl bond of the guanidinato intermediate to form the corresponding products. While both processes are favorable from a thermodynamic point of view, the product of insertion into the N-alkyl bond was found to be slightly more stable (by ca. 14 kJ·mol−1) than its analogue derived from insertion into the N-aryl bond. Although the corresponding reaction barriers were not computed, note that the overall energetic picture agrees well with the experimental observations, as the major

Compound 13 has a similar configuration to that found for 9, with similar distances and angles. In both compounds there is no large π delocalization along the C−N bonds of the guanidinato moiety, with distances C1−N1 1.323−1.316 Å, C1−N2 1.382−1.376 Å, and C1−N3 1.350−1.361 Å, which suggests greater localization of the π electrons on the C1−N1 bond, with the others being essentially single bonds. The reactivity of these borylamines encouraged us to examine the reaction toward isocyanides to probe the generality of this process also in 1,1-insertion reactions. Reference compound 2 reacted with aromatic isocyanides pMeOC6H4NC and 2,6-Me2C6H3NC in the presence of DIC to give the expected 1,1-insertion adducts {i-PrHN}C{N(pMeC6H4)}(Ni-Pr){CNR″}BC8H14 (R″ = p-MeOC6H4 11, 2,6-Me2C6H3 12) in almost quantitative yield (Scheme 6). A E


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Inorganic Chemistry Scheme 6. Synthesis of Isocyanide Boron Guanidinato Adducts

mixture of the two possible regioisomers appeared over time for solutions of compound 12, with the major isomer being the one in which the isocyanide was inserted into the i-PrN−B bond, with a ratio of 12a/12b of ca. 10:1, probably due to the steric volume of the isocyanide substituent. Apart from this evolution, and in contrast to CO2 compounds, the isocyanide adducts are stable in solution for prolonged periods with respect to the reversible decomposition to the starting materials. This finding demonstrates the great stability of the five-membered heterocycle. In the 11B NMR spectrum, a broad signal was observed at ca. δ −3 ppm for the boron atoms in a tetracoordinated disposition. The expected resonances for the guanidinato ligand were also observed in the NMR spectra, with the most significant features being a doublet at ca. δ 4 ppm, attributed to the NH proton in the 1H NMR spectrum, and two downfield peaks, one for the central CN3 carbon at ca. δ 161 ppm and a weak signal at ca. δ 180 ppm for the isocyanide carbon in the 13 C NMR spectra. The spectra also showed the chemical inequivalence of all the methyl and methynic units of the isopropyl substituents. In an effort to obtain more information on the real structure of these new compounds, X-ray quality crystals were obtained from solutions in toluene/hexane (14) or C6D6/pentane (15). Diffraction studies (Figure 7) revealed five-membered heterocycles (diazaboroles) and confirmed the insertion of the isocyanide into the B−N bond of the alkylic nitrogen of the guanidinato ligand. Whereas compound 15 is roughly planar, with a sum of internal angles of ca. 540°, compound 14 has a puckered ring in which the borane moiety is slightly out of the plane formed by the other four atoms (guanidinato core/ N1B1C2 angle 21.19°). The different steric effects of the C8H14 group on the surrounding arylic groups could explain this half-chair configuration of 14 when compared to compound 15. In addition, a different orientation of the substituent on the exocyclic CN moiety was found for each of these compounds, near or far from the boron atom, respectively. Nevertheless, in both compounds the C−N double bond of the guanidinato moiety is well-localized on the C1−N1 bond, with distances of 1.333(3) and 1.325(2) Å, respectively, whereas the other two C−N bonds fall in the range of ca. 1.35 Å. Similarly, the C−N bond distance of ca. 1.27 Å for the inserted isocyanide indicates the double-bond character. The new compounds 14 and 15 are reminiscent of the bisboron compounds {i-Pr(BC 8 H 1 4 )N}C{N(p-MeC6H4)}(Ni-Pr)(CNAr)BC8H14 (Ar = p-MeOC6H4, 2,6Me2C6H3) described recently by us.9 These latter compounds were found to be in a temperature-dependent equilibrium with the reagents, with the equilibrium shifted toward the insertion products at room temperature. Such a process did not take place for compounds 14 and 15.

Figure 7. Molecular structures of compounds 14 (left) and 15 (right): H atoms are omitted for clarity, and ORTEP ellipsoids are plotted at the 50% probability level. Compound 14 crystallizes with two crystallographically independent molecules (distinguished with A and B in the Supporting Information) in the asymmetric unit. In Figure 7 only the molecular structure and numbering scheme of molecule A is depicted. Selected bond lengths (Å) and angles (deg): 14: C1−N1 = 1.333(3), C1−N2 = 1.354(2), C1−N3 = 1.342(4), N2−C2 = 1.449(4), C2−N4 = 1.276(4), B1−N1 = 1.639(5), B1−C2 = 1.640(4), N1−C1−N2 = 113.7(3), N2−C1−N3 = 121.7(3), N3− C1−N1 = 124.5(3), C2−N4−C26 = 121.7(3). 15: C1−N1 = 1.325(2), C1−N2 = 1.373(2), C1−N3 = 1.350(2), N2−C2 = 1.445(2), C2−N4 = 1.274(2), B1−N1 = 1.628(2), B1−C2 = 1.629(3), N1−C1−N2 = 114.4(1), N2−C1−N3 = 119.4(1), N3− C1−N1 = 126.1(1), C2−N4−C24 = 134.3(1).

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CONCLUSIONS In summary, we have developed a new strategy for the synthesis of boron heterocyclic compounds from readily available starting materials. The one-pot coupling of CO2 (or isocyanides), carbodiimide, and borylamine was employed for the successful synthesis of a family of boron guanidinato adducts, which might be of interest as potential candidates for catalytic processes for the activation and transformation of CO2. The novel boron cyclic guanidinato molecules were constructed by the carbodiimide insertion into the N−B bond of a borylamine and the subsequent CO2 (or isocyanide) insertion into a N−B bond of a guanidinato intermediate followed by ring closing through bond formation between an oxygen atom of the CO2 and the boron atom. We assume that, in the first stage, an open intermediate is required to react with the incoming unsaturated molecule. These transformations took place sequentially and selectively by competing against a number of possible side reactions. The present threecomponent coupling reaction constitutes a new and efficient process for CO2 chemical fixation. Studies on the stoichiometric and catalytic properties of the compounds obtained in this work are currently underway in our laboratory.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under dry nitrogen using standard Schlenk and glovebox techniques. Solvents were distilled from appropriate drying agents and stored under N2 in Schlenk tubes equipped with J. Young-type Teflon stoppers and containing activated molecular sieves (4 Å). Microanalyses were performed on a LECO CHNS-932 analyzer. NMR spectra were recorded on Bruker Avance Neo 400 and 500 spectrometers. Chemical shifts (δ) are given in parts per million, and coupling constants (J) are in hertz. All reagents were purchased from commercial suppliers. F


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Inorganic Chemistry Procedure for the Preparation of Borylamines 1−6. The corresponding aniline (2.46 mmol) and 9-borabicyclo[3.3.1]nonane dimer (H-BC8H14)2 (300 mg, 1.23 mmol) were dissolved in toluene (10 mL). The resulting solution was stirred for 2 h at 80 °C, after which time the solvent was removed under vacuum. The remaining solid was washed with pentane (3 × 10 mL) and dried under vacuum to give compounds 1, 2, and 4−7 as white solids, or as a sticky white solid in the case of 3. Compounds 1 and 7 have been described previously.13d,e Data for (p-CH3C6H4NH)BC8H14 2. Isolated yield 97% (542 mg). 1 H NMR (400 MHz, C6D6) δ 6.92 (m, 2H, C6H4), 6.85 (m, 2H, C6H4), 5.61 (s, 1H, NH), 2.12 (s, 3H, CH3), 2.02−0.96 (m, 14 H, BC8H14) ppm. 13C{1H} NMR (101 MHz, C6D6) δ 140.8 (s, ipso-CC6H4), 132.1 (s, p-C-C6H4), 129.3, 122.6 (2 × s, o,m-C-C6H4), 33.6, 32.9, 27.6 (3 × s, CH2−BC8H14), 23.5, 22.3 (2 × br s, CH-BC8H14), 20.4 (CH3) ppm. 11B NMR (128 MHz, C6D6) δ 50.89 ppm. Anal. Calcd for C15H22BN: C, 79.31; H, 9.76; N, 6.17. Found: C, 79.20; H, 9.70; N, 6.13%. Data for (p-C(CH3)3C6H4NH)BC8H14 3. Isolated yield 98% (662 mg).1H NMR (500 MHz, C6D6) δ 7.19 (m, 2H, C6H4), 6.89 (m, 2H, C6H4), 5.66 (s, 1H, NH), 2.04−1.00 (m, 14 H, BC8H14) 1.24 (s, 9 H, C(CH3)3), ppm. 13C{1H} NMR (126 MHz, C6D6) δ 145.4 (s, ipso-CC6H4), 140.8 (s, p-C-C6H4), 125.5, 122.2 (2 × s, o,m-C-C6H4), 33.9, 33.6, 32.9, 23.4 (5 × s, CH2−BC8H14), 31.2 (C(CH3)3), 27.2, 22.4 (2 × br s, CH-BC8H14) ppm. 11B NMR (160 MHz, C6D6) δ 51.11 ppm. Anal. Calcd for C18H28BN: C, 80.30; H, 10.48; N, 5.20. Found: C, 80.10; H, 10.45; N, 5.18%. Data for (o-CH3C6H4NH)BC8H14 4. Isolated yield 97% (540 mg). 1 H NMR (400 MHz, C6D6) δ 7.13−6.89 (m, 4H, C6H4), 5.37 (s, 1H, NH), 2.01−1.07 (m, 14 H, BC8H14), 1.95 (s, 3 H, CH3) ppm. 13 C{1H} NMR (101 MHz, C6D6) δ 141.5 (s, ipso-C-C6H4), 130.5 (s, p-C-C6H4), 130.2, 126.3, 124.7, 123.96 (4 × s, o,m-C-C6H4), 33.7, 32.9, 23.4 (4 × s, CH2−BC8H14), 27.0, 22.2 (2 × br s, CH-BC8H14), 17.6 (CH3) ppm. 11B NMR (128 MHz, C6D6) δ 50.8 ppm. Anal. Calcd for C15H22BN: C, 79.31; H, 9.76; N, 6.17. Found: C, 79.17; H, 9.65; N, 6.09%. Data for (p-CF3C6H4NH)BC8H14 5. Isolated yield 95% (655 mg). 1 H NMR (500 MHz, C6D6) δ 7.28 (m, 2H, C6H4), 6.61 (m, 2H, C6H4), 5.52 (s, 1H, NH), 2.01−0.93 (m, 14 H, BC8H14) ppm. 13 C{1H} NMR (126 MHz, C6D6) δ 146.5 (s, ipso-C-C6H4), 125.9 (m, CF3), 121.8 (s, o,m-C-C6H4), 33.5, 32.7, 23.2 (3 × s, CH2−BC8H14), 27.4, 22.5 (2 × br s, CH-BC8H14) ppm. 19F{1H} NMR (471 MHz, C6D6) δ −61.53 ppm. 11B NMR (160 MHz, C6D6) δ 52.4 ppm. Anal. Calcd for C15H19BF3N: C, 64.09; H, 6.81; N, 4.98. Found: C, 63.89; H, 6.70; N, 4.95%. Data for (2,6-(CH3)2C6H3NH)BC8H14 6. Isolated yield 96% (570 mg). 1H NMR (400 MHz, C6D6) δ 6.98 (m, 3H, C6H3), 4.69 (s, 1H, NH), 2.12 (s, 6 H, CH3), 2.10−1.04 (m, 14 H, BC8H14) ppm. 13 C{1H} NMR (101 MHz, C6D6) δ 140.3 (s, ipso-C-C6H4), 134.2 (s, p-C-C6H4), 127.9 (s, m-C-C6H4), 124.9 (s, o-C-C6H4), 33.7, 32.9, 23.5 (4 × s, CH2−BC8H14), 26.2, 22.4 (2 × br s, CH-BC8H14), 18.7 (CH3) ppm. 11B NMR (128 MHz, C6D6) δ 50.4 ppm. Anal. Calcd for C16H24BN: C, 79.68; H, 10.03; N, 5.81. Found: C, 79.35; H, 9.98; N, 5.78%. Procedure for the Preparation of CO2 Adducts 8−13. Compounds 1−5 (0.15 mmol) were dissolved in C6D6 (ca. 0.8 mL) and charged to an NMR tube equipped with a J. Young valve. 1,3-Diisopropylcarbodiimide (0.15 mmol) was added. Finally, CO2 (ca. 1 atm) was added through the freeze−pump−thaw procedure. Monitoring of the reaction by 1H NMR revealed that an equilibrium was reached between the corresponding adducts and starting materials (see text). In some cases, the CO2 adduct crystallized on the tube walls. Alternatively, in a 50 mL Schlenk tube equipped with a J. Young valve, the corresponding aniline (1.23 mmol) and 9borabicyclo[3.3.1]nonane dimer (H-BC8H14)2 (150 mg, 0.61 mmol) were dissolved in toluene (5 mL). The resulting solution was stirred for 2 h at 80 °C. 1,3-Diisopropylcarbodiimide (191 μL, 1.23 mmol) or 1,3-dicyclohexylcarbodiimide (250 mg, 1.23 mmol) was added. CO2 (ca. 1 atm) was added through the freeze−pump−thaw procedure,

and this process was performed three times to ensure CO2 saturation. After 24 h at room temperature pentane (10 mL) was added to the obtained suspension. The mixture was placed in a refrigerator at −20 °C for 16 h, and the corresponding products were isolated as white solids. Data for {iPr(H)N}C{N(C6H5)}(N-iPr)(CO2)BC8H14 8. Isolated yield 89% (420 mg). 1H NMR (400 MHz, CD2Cl2) δ 7.46 (m, 2H, C6H5), 7.22 (m, 3H, C6H5), 4.09 (d, JHH = 10.8 Hz, 1H, NH), 4.01 (sept, JHH = 6.7 Hz, 1H, N−CH-iPr), 3.57 (dsept, JHH = 6.2 Hz, 10.8 Hz, 1H, NH−CH-iPr), 2.02−0.64 (m, 14 H, BC8H14), 1.60 (d, 6H, JHH = 6.7 Hz, CH3-iPr), 0.94 (d, 6H, JHH = 6.2 Hz, CH3-iPr) ppm. 13C{1H} NMR (101 MHz, CD2Cl2) δ 157.6 (s, CN3), 152.6 (s, NCO2), 141.3 (s, ipso-C-Ar), 138.4 (s, p-C-Ar), 129.7 (s, m-C-Ar), 127.8 (s, o-CAr), 125.2 (s, p-C-Ar), 52.6, 48.1 (2 × s, 2 × CH-iPr), 31.8, 30.6, 24.3, 23.9 (4 × s, 4 × CH2−BC8H14), 23.4 (br, CH-BC8H14), 23.0, 21.2 (2 × s, 2 × CH3-iPr) ppm. 11B NMR (128 MHz, CD2Cl2) δ 4.72 ppm. Anal. Calcd for C22H34BN3O2: C, 68.93; H, 8.94; N, 10.96. Found: C, 68.89; H, 8.90; N, 10.95%. Data for {iPr(H)N}C{N(p-CH3C6H4)}(N-iPr)(CO2)BC8H14 9. Isolated yield 88% (420 mg). 1H NMR (400 MHz, CD2Cl2): δ 7.24, 7.07 (AA′XX′, 4H, C6H4), 4.11 (d, 1H, JHH = 9.6 Hz, NH), 3.99 (sept, 1H, JHH = 6.7 Hz, N−CH-iPr), 3.57 (dsept, 1H, JHH = 9.6, 6.4 Hz, NH− CH-iPr), 2.39 (s, 3H, p-CH3-C6H4), 2.0−0.6 (m, 14H, C8H14), 1.59 (d, 6H, JHH = 6.7 Hz, CH3-iPr), 0.95 (d, 6H, JHH = 6.4 Hz, CH3-iPr). 13 C{1H} NMR (101 MHz, CD2Cl2): δ 158.1 (s, CN3), 153.2 (s, NCO2), 139.0, 138.4 (2 × s, ipso-C-Ar), 130.7 (s, m-C-Ar), 127.9 (s, o-C-Ar), 53.0, 48.5 (2 × s, 2 × CH-iPr), 32.2, 31.1, 24.7, 24.3 (4 × s, 4 × CH2−BC8H14), 23.9 (br, CH-BC8H14), 23.5, 21.7 (2 × s, 2 × CH3-iPr), 21.2 (s, p-CH3−Ar). 11B NMR (128 MHz, CD2Cl2): δ 4.6 ppm. Anal. Calcd for C23H36BN3O2: C, 69.52; H, 9.13; N, 10.57. Found: C, 69.15; H, 9.04; N, 10.51%. Data for {iPr(H)N}C{N(p-C(CH3)3C6H4)}(N-iPr)(CO2)BC8H14 10. Isolated yield 72% (381 mg). 1H NMR (400 MHz, CD2Cl2): δ 7.49, 7.15 (AA′XX′, 4H, C6H4), 4.13 (d, 1H, JHH = 9.6 Hz, NH), 4.03 (sept, 1H, JHH = 6.7 Hz, N−CH-iPr), 3.60 (dsept, 1H, JHH = 9.6, 6.4 Hz, NH−CH-iPr), 2.39 (s, 3H, p−CH3-C6H4), 2.0−0.6 (m, 14H, C8H14), 1.62 (d, 6H, JHH = 6.7 Hz, CH3-iPr), 1.38 (s, 9H, C(CH3)3), 0.97 (d, 6H, JHH = 6.4 Hz, CH3-iPr). 13C{1H} NMR (101 MHz, CD2Cl2): δ 157.8 (s, CN3), 152.8 (s, NCO2), 151.3, 138.5 (2 × s, ipso-C-Ar), 138.4 (s, p-C-Ar), 127.2 (s, m-C-Ar), 126.5 (s, o-C-Ar), 52.5, 48.1 (2 × s, 2 × CH-iPr), 34.6 (C(CH3)3), 31.8, 30.7, 24.3, 23.9 (4 × s, 4 × CH2−BC8H14), 31.1 (C(CH3)3), 23.3 (br, CH-BC8H14), 23.0, 21.3 (2 × s, 2 × CH3-iPr). 11B NMR (128 MHz, CD2Cl2): δ 4.7 ppm. Anal. Calcd for C26H42BN3O2: C, 71.06; H, 9.63; N, 9.56. Found: C, 70.66; H, 9.47; N, 9.57%. Data for {iPr(H)N}C{N(o-CH3C6H4)}(N-iPr)(CO2)BC8H14 11. Isolated yield 75% (365 mg). 1H NMR (500 MHz, CD2Cl2): δ 7.32 (m, 4H, C6H4), 4.09 (d, 1H, JHH = 9.3 Hz, NH), 3.99 (sept, 1H, JHH = 6.7 Hz, N−CH-iPr), 3.53 (m, 1H, NH−CH-iPr), 2.23 (s, 3H,o−CH3-C6H4), 2.1−0.3 (m, 14H, C8H14), 1.68 (d, 3H, JHH = 6.7 Hz, CH3-iPr), 1.51 (d, 3H, JHH = 6.6 Hz, CH3-iPr), 0.94 (d, 6H, JHH = 6.4 Hz, CH3-iPr). 13 C{1H} NMR (126 MHz, CD2Cl2): δ 156.9 (s, CN3), 152.7 (s, NCO2), 140.4, 134.8 (2 × s, ipso-C-Ar), 132.0, 128.9, 127.9, 126.6 (s, o,m-C-Ar), 52.2, 47.6 (2 × s, 2 × CH-iPr), 32.6, 31.8, 31.2, 30.2, 24.3, 22.6 (6 × s, 6 × CH2−BC8H14), 23.9 (s, 2 × CH3-iPr), 24.1, 21.9 (2 × br, 2 × CH-BC8H14), 21.9, 20.5 (2 × s, 2 × CH3-iPr), 21.2 (s, oCH3−Ar). 11B NMR (160 MHz, CD2Cl2): δ 5.0 ppm. Anal. Calcd for C23H36BN3O2: C, 69.52; H, 9.13; N, 10.57. Found: C, 69.38; H, 8.96; N, 10.54%. Partial Spectroscopic Data for {iPr(H)N}C{N(p-CF3C6H4)}(N-iPr)(CO2)BC8H14 12. 1H NMR (400 MHz, CD2Cl2): δ 7.74, 7.38 (AA′XX′, 4H, C6H4), 4.03 (m, 1H, N−CH-iPr), 3.53 (m, 1H, NH− CH-iPr), 1.60 (d, 6H, JHH = 6.7 Hz, CH3-iPr), 0.96 (d, 6H, JHH = 6.3 Hz, CH3-iPr). 19F{1H} NMR (376 MHz, CD2Cl2) δ −62.82 ppm. 11B NMR (128 MHz, CD2Cl2): δ 3.5 ppm. Data for {Cy(H)N}C{N(C6H5)}(NCy)(CO2)BC8H14 13. Isolated yield 89% (522 mg). 1H NMR (400 MHz, CD2Cl2) δ 7.25, 7.08 (AA′XX′, 4H, C6H4), 4.25 (d, 1H, JHH = 9.6 Hz, NH), 3.51 (m, 1H, N−CHCy), 3.18 (m, 1H, NH−CH-Cy), 2.5−0.6 (m, 34H, CH2-Cy, C8H14), 2.40 (s, 3H, p-CH3-C6H4). 13C{1H} NMR (101 MHz, CD2Cl2) δ G


Article

Inorganic Chemistry 157.9 (s, CN3), 153.1 (s, NCO2), 138.7, (s, m-C-Ar), 137.9 (2 × s, ipso-C-Ar), 130.3 (s, m-C-Ar), 127.5 (s, o-C-Ar), 61.9, 55.0 (2 × s, 2 × CH-Cy), 33.7, 31.2, 31.3, 30.7, 26.8, 25.3, 24.8, 24.5, 24.3, 23.9, 23.4 (CH2−Cy, CH2−BC8H14), 20.8 (s, p-CH3−Ar). 11B NMR (128 MHz, CD2Cl2) δ 4.2 ppm. Anal. Calcd for C29H44BN3O2: C, 72.95; H, 9.29; N, 8.80. Found: C, 72.87; H, 9.22; N, 8.65%. Procedure for the Preparation of Isocyanide Adducts 14 and 15. In a 50 mL Schlenk tube equipped with a J. Young valve, compound 2 (280 mg, 1.23 mmol) was dissolved in toluene (5 mL). 1,3-Diisopropylcarbodiimide (191 μL, 1.23 mmol) and p-methoxyphenylisocyanide (164 mg, 1.23 mmol) or 2,6-dimethylphenylisocyanide (161 mg, 1.23 mmol) was added. After 24 h at room temperature, the solvent was removed under vacuum, and pentane (10 mL) was added. The mixture was placed in a refrigerator at −20 °C for 16 h to afford the corresponding products as white solids. Data for {iPrHN}C{N(p-MeC6H4)}(N-iPr){p-MeOC6H4NC}BC8H14 14. Isolated yield 88% (526 mg). 1H NMR (500 MHz, CD2Cl2) δ 7.18, 7.10 (AA′XX′, 4H, p-MeC6H4), 6.74, 6.67 (AA′XX′, 4H, pMeOC6H4), 4.36 (sept, 1H, JHH = 6.7 Hz, N−CH-iPr), 4.01 (d, 1H, JHH = 9.6 Hz, NH), 3.75 (s, 6H, CH3O), 3.44 (dsept, 1H, JHH = 9.6, 6.4 Hz, NH−CH-iPr), 2.37 (s, 3H, p-CH3-C6H4), 1.7−0.5 (m, 14H, C8H14), 1.50 (d, 6H, JHH = 6.7 Hz, CH3-iPr), 0.97 (d, 6H, JHH = 6.4 Hz, CH3-iPr). 13C{1H} NMR (126 MHz, CD2Cl2) δ 183.8 (br, C6H4NC), 159.3 (s, CN3), 154.5 (s, ipso-MeO-C-Ar), 146.0 (s, ipsoNC-C-Ar), 141.3 (s, ipso-N-C-Ar), 136.4 (s, ipso-Me-C-Ar), 129.7, 128.4, 120.7, 113.6 (s, o,m-C-Ar), 55.4 (s, CH3O), 46.7, 46.6 (2 × s, 2 × CH-iPr), 31.5 31.3, 24.5, 23.9 (4 × s, 4 × CH2−BC8H14), 24.8 (br, CH-BC8H14), 23.2, 20.9 (2 × s, 2 × CH3-iPr), 20.7 (s, p-CH3−Ar). 11 B NMR (160 MHz, CD2Cl2): δ 2.5 ppm. Anal. Calcd for C30H43BN4O·0.5C6H14: C, 74.84; H, 9.52; N, 10.58. Found: C, 74.34; H, 9.32; N, 10.56%. Data for {iPrHN}C{N(p-MeC6H4)}(N-iPr){2,6-Me2C6H3NC}BC8H14 15. Isolated yield 96% (566 mg). 1H NMR (500 MHz, CD2Cl2) δ 7.22, 7.16 (AA′XX′, 4H, p-MeC6H4), 7.00, 6.78 (AXX′, 3H, 2,6Me2C6H3), 4.18 (d, 1H, JHH = 8.8 Hz, NH), 4.13 (sept, 1H, JHH = 7.0 Hz, N−CH-iPr), 3.27 (dsept, 1H, JHH = 6.5, 8.8 Hz, NH−CH-iPr), 2.43 (s, 3H, p-CH3-C6H4), 2.22 (s, 6H, 2,6-(CH3)-C6H4), 2.2−0.7 (m, 14H, C8H14), 1.30 (d, 6H, JHH = 7.0 Hz, CH3-iPr), 1.02 (d, 6H, JHH = 6.5 Hz, CH3-iPr). 13C{1H} NMR (126 MHz, CD2Cl2) δ 181.6 (br, C6H4NC), 159.5 (s, CN3), 149.5 (s, ipso-NC-C-Ar), 141.3 (s, ipso-N-C-Ar), 136.4 (s, ipso-Me-C-Ar), 129.68, 128.15 (s, o,m-C-Ar), 127.5 (s, m-C-Ar), 125.7 (s, ipso-Me2-C-Ar), 120.4 (s, p-C-Ar), 46.9, 46.1 (2 × s, 2 × CH-iPr), 32.0, 30.8, 24.7, 24.1 (4 × s, 4 × CH2− BC8H14), 24.3 (br, CH-BC8H14), 22.9, 21.1 (2 × s, 2 × CH3-iPr), 20.6 (s, p-CH3−Ar), 18.9 (s, (CH3)2-Ar). 11B NMR (160 MHz, CD2Cl2): δ 2.6 ppm. Anal. Calcd for C31H45BN4: C, 76.84; H, 9.36; N, 11.56. Found: C, 76.44; H, 9.15; N, 11.53%. X-ray Diffraction Studies. X-ray data collection of suitable single crystals of compounds 7, 9, 13, 14, and 15 were done at 100(2) K on a Bruker VENTURE area detector equipped with graphite monochromated Mo Kα radiation (λ = 0.710 73 Å) by applying the ω-scan method. The data reduction was performed with the APEX219 software and corrected for absorption using SADABS.20 Crystal structures were solved by direct methods using the SIR97 program21 and refined by full-matrix least-squares on F2 including all reflections using anisotropic displacement parameters by means of the WINGX crystallographic package.22 All hydrogen atoms were included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2 or 1.5 times those of their parent atoms for the organic ligands. Details of the structure determination and refinement of compounds are summarized in Table S1. Crystallographic data (excluding structure factors) for the structures reported in this paper were deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. CCDC 1835576−1835580. Additional crystallographic information is available in the Supporting Information. Theoretical Calculations. All the calculations were performed by using the Gaussian09 suite of programs.23 The M05-2X functional of Truhlar and co-workers24 was employed to optimize all the structures. This functional was parametrized for organic systems with non-

covalent interactions, and we9b,25 and others7k have recently demonstrated the accuracy of this functional for the computation of the structures and reaction mechanisms of highly related compounds. A 6-31+G** basis set including polarization and diffuse functions for heavy atoms and polarization functions for the hydrogen atoms was used for all the computations.26 Geometry optimizations were performed under no symmetry restrictions, and frequency analyses were performed for all the stationary points to ensure that minimum structures with no imaginary frequencies were achieved. Gibbs free energies for the different species were calculated within the harmonic approximation for vibrational frequencies. Solvent effects (benzene, ε = 2.2706) were modeled using the polarized continuum model (PCM) of Tomasi and co-workers,27 by using the gas-phase optimized structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01068. Preparation and characterization of borylamines 1−6, CO2 adducts 8−13, and isocyanide adducts 14 and 15, X-ray diffraction studies, tabulated crystallographic data and structure refinement details for all compounds, illustrated molecular structures (PDF) Illustrated molecular structure (XYZ) Accession Codes

CCDC 1835576−1835580 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fernando Carrillo-Hermosilla: 0000-0002-1187-7719 Antonio Rodríguez-Diéguez: 0000-0003-3198-5378 Daniel García-Vivó: 0000-0002-2441-2486 Antonio Antiñolo: 0000-0002-4417-6417 Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Ministerio de Economiá y Competitividad (MINECO), Spain (Grant Nos. CTQ2016-77614-P, CTQ2016-81797REDC, and CTQ2015-63726-P). A.R. acknowledges a postdoctoral contract funded by the “Plan Propio de I + D + i” of the Universidad de Castilla-La Mancha.

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DEDICATION Dedicated to Prof. Ernesto Carmona, in occasion of his 70th birthday, for a life dedicated to Inorganic Chemistry. H


Article

Inorganic Chemistry

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