Synthesis of Cyclic and Cage Borosilicates Based on Boronic Acids

Article pubs.acs.org/IC

Synthesis of Cyclic and Cage Borosilicates Based on Boronic Acids and Acetoxysilylalkoxides. Experimental and Computational Studies of the Stability Difference of Six- and Eight-Membered Rings Aarón Torres-Huerta,†,§ Miriam de J. Velásquez-Hernández,†,§ Lillian G. Ramírez-Palma,‡ Fernando Cortés-Guzmán,*,‡ Diego Martínez-Otero,† Uvaldo Hernández-Balderas,† and Vojtech Jancik*,† †

Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM. Carretera Toluca-Atlacomulco, km. 14.5, 50200 Toluca, Estado de México México ‡ Instituto de Química, Ciudad Universitaria, Universidad Nacional Autónoma de México Circuito Exterior s/n, 04510 CDMX, México S Supporting Information *

ABSTRACT: A series of borosilicates was synthesized, where the structure of the borosilicate core was easily modulated using two strategies: blocking of condensation sites and controlling the stoichiometry of the reaction. Thus, on the one hand, the condensation of phenylboronic or 3-hydroxyphenylboronic acid with diacetoxysilylalkoxide [(tBuO)(Ph3CO)Si(OAc)2] led to the formation of borosilicates (tBuO)(Ph3CO)Si{(μ-O)BPh}2(μ-O) (1), [{(tBuO)(Ph3CO)Si(μO)BPh(μ-O)}2] (2), and [{(tBuO)(Ph3CO)Si(μ-O)B(3HOPh)(μ-O)}2] (3) with a cyclic inorganic B2SiO3 or B2Si2O4 core, respectively. On the other hand, the reaction of phenylboronic acid with triacetoxysilylalkoxide (Ph3CO)Si(OAc)3 in 3:2 ratio resulted in the formation of a cagelike structure [{(Ph3CO)Si(μ-O)2BPh(μ-O)}2] (4) with B4Si4O10 core, while the reaction of the boronic acid with silicon tetraacetate generated an unusual 1,3-bis(acetate)-1,3-diphenyldiboraxane PhB(μ-O)(μ-O,O′-OAc)2BPh (5). Additionally, compound 1 was used to evaluate the possibility to form N→B donor−acceptor bond between the boron atom in the borosilicates and a nitrogen donor. Thus, coordination of 1 with piperazine yielded a tricyclic [{(tBuO)(Ph3CO)Si(OBPh)2(μ-O)}2·C4H10N2] compound 6 with two borosilicate rings bridged by a piperazine molecule. Finally, the processes involved in the formation of the six- and eight-membered rings (B2SiO3 and B2Si2O4) in compounds 1 and 2 were explored using solution 1H NMR studies and density functional theory calculations. These molecules represent to the best of our knowledge first examples of cyclic molecular borosilicates containing SiO4 units.

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derivatives of [{(tBuO)3SiO}2BOH].16−18 Molecular borosilicates are usually obtained from boronic acids and either silanediols, dichlorosilanes, or dialkoxysilanes and contain 6-, 8-, 10-, or 12-membered cyclic cores based on B−O−Si units, where the eight-membered rings are the most common.19−26 On the one hand, it is noteworthy that the formation of the smaller or bigger rings requires the use of predesigned precursors such as ClSi(R)2OSi(R)2Cl or HOSi(R)2OSi(R)2OSi(R)2OH.23,24 However, only a handful of such compounds have been reported. On the other hand, few cagelike structures with B2Si6O9, B3Si2O6, and B4Si4O10 cores are also known and are based either on boric acid (B(OSiPh2OSiPh2O)3B) or boronic acids (tBuSi{O(BR)O}3SitBu or (tBuSi)4(BR)4O10; R = organic group).27−29 Also, despite the fact that borosilicate scaffolds are suitable

INTRODUCTION The borosilicates are solid tridimensional materials with characteristic properties such as chemical resistance, thermal stability, catalysis, ion exchange, and optoelectronic properties, and they are used for encapsulating organic molecules and fabrication of different sorts of glasses.1−6 The main component in the borosilicate glasses is silicon oxide in the form of the tetrahedral SiO4 unit, and in a smaller amount B2O3, Na2O, Al2O3, and CaO, where the B2O3 content is between 5 and 30%.7,8 The presence of the boron atoms is fundamental for the resulting properties of the material, due to its Lewis acid character and thus the possibility to change its coordination number and the geometry from trigonal planar to tetrahedral in the presence of basic oxides, nucleophiles, or Lewis bases.9,10 Despite the properties and applications of these materials there are very few examples of molecular borosilicates,11−15 and to the best of our knowledge, reports of structurally characterized borosilicates containing SiO4 units are limited to few acyclic © 2017 American Chemical Society

Received: June 20, 2017 Published: August 9, 2017 10032

DOI: 10.1021/acs.inorgchem.7b01580 Inorg. Chem. 2017, 56, 10032−10043

Article

Inorganic Chemistry Scheme 1. Synthesis of Compounds 1−5

Si(OAc)2 (a, OAc = CH3COO−) to obtain compounds with six-membered B2SiO3 (2:1) and BSi2O3 (1:2) rings. Ratio 1:1 was chosen for the synthesis of the eight-membered B2Si2O4 ring-containing species (Scheme 1). The reaction in a 2:1 ratio in anhydrous toluene resulted in a formation of the (tBuO)(Ph3CO)Si{(μ-O)BPh}2(μ-O) (1) borosilicate with the desired cyclic six-membered B2SiO3 inorganic core (vide infra) as a white solid highly soluble in polar solvents. The progress of the reaction was monitored by a complete disappearance of the signals of the acetate (δ = 1.88 ppm) and B−OH (δ = 7.99 ppm, deuterated dimethyl sulfoxide (DMSO-d6)) groups of the starting materials in the 1H NMR spectrum. After the reaction, the NMR spectra (1H,13C, 11B, 29 Si) confirm the formation of only one compound. When the reaction ratio between a and phenylboronic acid was changed to 1:1, another product was obtained as a highly crystalline white powder and was identified by X-ray diffraction studies as [{(tBuO)(Ph3CO)Si(μ-O)BPh(μ-O)}2] (2) with an eightmembered B2Si2O4 ring. However, as revealed by 1H, 13C, and 29 Si NMR analysis, compound 2 is even after recrystallization contaminated by compound 1, although repeated recrystallizations led to ∼95% purity of 2 (Figure S5 in the Supporting Information). Surprisingly, the 29Si NMR spectrum of 2 contains three signals (Figure S7 in the Supporting Information). On the one hand, there are two signals with a 1:1 intensity at δ = −106.7 and −107.1 ppm caused probably by the mutual orientation of the two different tBuO- groups present in the molecule with respect to the plane of the borosilicate ring. On the other hand, the third signal belongs to compound 1. A more exhaustive study about of the formation of compounds 1 and 2 with sixand eight-membered rings, respectively, is presented further ahead. Finally, in an intent to prepare compound with the BSi2O3 ring, phenylboronic acid and a were reacted in 1:2 ratio in the presence of 1 equiv of water (necessary to close the ring), but only the formation of a mixture of 2 with unreacted a was observed. With the intent to incorporate functional groups to the borosilicate molecule, 3-hydroxyphenylboronic acid was used instead of PhB(OH) 2 in the reactions with a. Independently of the reaction conditions or the ratio of the 3-hydroxyphenylboronic acid and a, only the 1:1 product

candidates for building blocks their use in supramolecular chemistry has been investigated only marginally.30−32 In this regard, Severin and Yaghi reported borosilicates with eightmembered rings used as nodes for the synthesis of macrocyclic and three-dimensional (3D) systems, respectively.33,34 More recently, the same strategy has been used for the synthesis of polymeric sensors for detection of volatile organic amines.35 Currently, despite their widespread use, only a few synthetic strategies for molecular borosilicates are known. The main reasons are (I) a limited number of commercially available silanols, (II) difficult structural modification of these compounds, (III) in the case of dichlorosilanes, the use of highly basic reaction conditions to neutralize the hydrochloric acid generated, and (IV) limited reactivity of the Si−C bonds in the siloxanes. To overcome all these difficulties, we propose a new synthetic route to cyclic and cagelike borosilicates from acetoxysilylalkoxides and boronic acids. The main advantages of this method are the mild reaction conditions, very easy modification of the silicon precursor by tertiary alcohols, and, thus, an exact control over the reactivity of the acetate groups. Therefore, herein we report a facile synthesis of molecular borosilicates containing either eight-membered B2Si2O4 rings, cagelike B4Si4O10 inorganic cores, or hitherto unknown sixmembered B2SiO3 cycle and a borosilicate adduct with piperazine. These molecules represent to the best of our knowledge first examples of cyclic molecular borosilicates containing SiO4 units. Finally, because of the competition observed in the formation of six- and eight-membered rings, theoretical calculations were performed to determine the probable routes of synthesis for each ring as well as their energy of formation. All compounds were obtained in high yields and were characterized by analytical methods and singlecrystal X-ray diffraction. Finally, the thermal stability of all compounds was investigated by thermogravimetric analysis (TGA).

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RESULTS AND DISCUSSION Preparation and Spectroscopic Characterization. For this study, we proposed three starting ratios between the phenylboronic acid and diacetoxysilylalkoxide (tBuO)(Ph3CO)10033


Article

Inorganic Chemistry

Figure 1. Perspective views of the molecular structures of compounds 1, 2, 4, and 5 with the thermal ellipsoids are drawn at the 50% probability level and the connectivity model of 3. Carbon ellipsoids and carbon-bound hydrogen atoms were eliminated for the sake of clarity.

[{(tBuO)(Ph3CO)Si(μ-O)B(3-HOPh)(μ-O)}2] (3) with the eight-membered B2Si2O4 ring was obtained. Next, to investigate the effect of the number of the acetate groups on the resulting borosilicate core, we used the triacetate (Ph3CO)Si(OAc)3 (b) instead of the diacetate a in the reaction with phenylboronic acid in a 2:3 ratio to assess the formation of bicyclic RSi(OBPh-O)3SiR (R = Ph3CO) species. However, the presence of three acetate groups per silicon atom resulted in a higher reactivity and in the formation of a cage-type borosilicate [{(Ph3CO)Si(μ-O)BPh(μ-O)}2(μ-O)]2 (4) with an inorganic B4Si4O10 core formed by two B2Si2O4 rings connected together by two Si−O−Si bridges, as determined by an X-ray diffraction analysis (vide infra). Nevertheless, the formation of these Si− O−Si units requires the presence of water in the reaction media, which is formed by a condensation of the extra equivalent of the phenylboronic acid to the corresponding boroxine that was identified in the reaction media. Furthermore, optimization of the reaction stoichiometry to 1:1:1 (PhB(OH)2/b/H2O) also leads to the formation of 4, albeit in a low yield (45% instead of 83%). Thus, the concentration of water in the reaction media plays an important role, and its generation through the slow condensation of boronic acid is preferred, as a higher amount of water leads to

the hydrolysis of b. The observed 11B NMR chemical shifts for compounds 1−4 are in the range for a trivalent boron atoms, while an upfield chemical shift (δ = from −95.3 to −116.9 ppm) in the 29Si NMR spectra was observed for the SiO4 unit when compared with similar borosilicates reported in the literature (δ = −3.3 to −45.0 ppm).22,26 Finally, the reactivity of the boronic acid with silicon tetraacetate was explored, to assess the possibility to form products with higher dimensionality. However, the NMR studies revealed that the obtained compound contained at least one remaining acetate group (1H NMR δ = 2.35 ppm), only one type of boron atom (11B NMR δ = 5.9 ppm), and suggested that no silicon atoms are present, as no silicon signal was observed in 29Si NMR spectrum. This was confirmed by single-crystal X-ray diffraction (vide infra), and the product was identified as 1,3-bis(acetate)1,3-diphenyldiboraxane (μ-O)(PhB) 2(μ-O,O′-OAc)2 (5). Although several compounds with the RB(μ-O)(μ-O,O′− O2CCH3)nBR (n = 1, R = F, C6F5; n = 2, R = F, C6F5, C2H5, C8H15, OCOCH3) unit are known, their synthesis requires predesigned compounds with B−O−B units and large quantities of acetic anhydride.36−43 Furthermore, all such reactions proceed at high temperatures, and none starts from a boronic acid. These facts clearly show the advantages of the 10034


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Inorganic Chemistry Table 1. Selected Bond Lengths [Å] and Angles [deg] for the Molecular Structure of Compounds 1, 2, and 4 bond lengths Si−OC Si−OB

B−OSi

B−C Si−OSi B−OB

bond angles

1

2

4·2CH2Cl2

1.601(2) 1.606(2) 1.640(2) 1.641(2)

1.602(1) 1.608(1) 1.609(1) 1.629(1)

1.373(3) 1.372(3)

1.345(1) 1.359(1)

1.558(4) 1.560(4)

1.594(5)

1.599(1) 1.602(1) 1.601(1) 1.604(1) 1.611(1) 1.613(1) 1.363(2) 1.362(2) 1.365(2) 1.365(2) 1.557(3) 1.554(3) 1.588(2)

CO−Si−OC CO−Si−OB

BO−Si−OB

2

103.3(1) 112.2(1) 111.7(1) 113.2(1) 112.1(1) 104.6(1)

109.6(1) 110.6(1) 104.0(1) 110.9(1) 111.2(1) 110.3(1)

SiO−B−OSi

B−O−B Si−O−B

1.377(3) 1.377(3)

Si−O−Si

4·2CH2Cl2

1

121.4(1) 124.6(2) 124.1(2) 124.0(2)

167.5(1) 138.1(1)

110.5(1) 111.2(1) 109.7(1) 112.7(1) 112.6(1) 110.4(1) 121.6(1) 121.7(1) 148.7(1) 150.7(1) 134.9(1) 131.0(1) 167.0(1)

H···π interactions, and one is nearly coplanar with the B2SiO3 ring (dihedral angle 23.3°, centroid-to-centroid distance = 3.55 Å). It is also noteworthy that this is to the best of our knowledge the first example of such six-membered B2SiO3 ring. The molecular structure of compound 2 corresponds to a molecule with an eight-membered B2Si2O4 ring located on an inversion center (Figure 1). The ring is formed by alternating SiO4 and BO2 units and is nearly planar with a mean deviation of the atoms from the plane of 0.034 Å. Because of the crystallographic symmetry, the tBuO groups are in a transorientation to each other. The Si−OC (1.608(1)−1.629(1) Å) and Si−OB (1.602(1)−1.608(1) Å) bond lengths are very similar and are comparable to those reported for compounds with the same eight-membered B2Si2O4 ring ([ArB(μ-O)SitBu2(μ-O)2]2 (Ar = Ph, p-BrC6H4, p-CHOC6H4, 3,4-ethylendioxythiophene), [p-BrC6H 4B(μ-O)SiPh2(μ-O)2 ]2 , and [PhB(μ-O)SiFc 2 (μ-O) 2 ] 2 (Fc = ferrocenyl) (Si−O B ; 1.626(1)−1.635(1) Å).19,21,25,29,34,35 It is noteworthy that the Si−OB bonds in 2 are shorter than those observed in 1 (av 1.640(2) Å). The presence of an additional SiO4 unit diminishes the ring strain, as demonstrated by the increased values of the O3−Si1−O4, Si1−O3−B1, Si1−O4−B1, and O3− B 1 −O 4 angles (110.3(0)°, 138.1(1)°, 167.5(1)°, and 121.4(1)°) in the inorganic ring, when compared to those in 1. In this case, the bridging oxygen atoms act as tensionreleasing elements. These values are comparable to those found in the literature for similar compounds with the B2Si2O4 rings (O−Si−O, 111.2°−112.0°; Si−O−B, 149.0°−166.7°; O−B−O, 120.7°−123.4°).19,21,25,29,34,35 As mentioned earlier, compound 4 contains a centrosymmetric cagelike B4Si4O10 inorganic core, composed of two B2Si2O4 rings connected via two Si−O−Si bridges. To the best of our knowledge, only two compounds with such a core have been reported: (tBuSi)4(ArB)4O10 (Ar = CH2CHC6H4, BrC6H4).28,29 Although 4 contains three different Si−O bond types (Si−OC, Si−OSi, and Si−OB), their average bond lengths around the two crystallographically independent silicon atoms (Si1 y Si2) are in the range from 1.598(1) to 1.623(1) Å and are comparable to values previously reported for B4Si4O10 systems (1.60−1.63 Å).27,28 The average values for the internal BO−Si−OB, Si1−O−B, Si2−O−B, and SiO−B−OSi angles within the eight-membered ring are 111.5(1)°, 132.9(1)°, 149.7(1)°, and 121.7(1)°, respectively, and are similar to those

present synthetic route. In electron ionization mass spectrometry (EI-MS) spectra, the molecular ions were observed for all compounds (m/z = 584 (1), 945 (2), 992 (3), and 312 (5)) except for compound 4, whose molecular mass (1693 g·mol−1) is out of the range of our equipment (max m/z 1090). Single-Crystal X-ray Diffraction Analysis. The molecular structures of the borosilicates 1, 2, and 4 and the 1,3bis(acetyloxy)-1,3-diphenyldiboraxane 5 were determined by single-crystal X-ray diffraction experiments. Compounds 1 and 5 were recrystallized from saturated hexane or tetrahydrofuran (THF) solutions, respectively, and crystallize in orthorhombic P212121 and Pca21 space groups with one molecule in the asymmetric unit. Slow recrystallization of compounds 2 and 4 from hexane or CH2Cl2/hexane mixture, respectively, yielded triclinic (P1)̅ crystals with half a molecule of the compound in the asymmetric unit accompanied in the case of 4 by a dichloromethane molecule. Only very small and weakly diffracting crystals of 3 were obtained, and although the measured data had sufficient quality to corroborate the connectivity of 3, they were not sufficient for full structure refinement. Thus, only X-ray data-based connectivity model for 3 is presented, and no bond lengths and angles are discussed (Figure 1). Table S1 in the Supporting Information contains selected data collection and structure refinement details. The molecular structure of 1 confirmed the formation of a B2SiO3 ring, where the silicon atom has a tetrahedral geometry with the tetrahedral character (THC)44 of 75.2% pointing to a considerable ring strain, while the boron atoms have a trigonal planar environment (Figure 1). Two different Si−O bonds (Si−OC and Si−OB) were observed. On the one hand, the average (av) Si−OC bond length is 1.603(2) Å, while the av Si− OB distance is with 1.640(2) Å slightly longer. On the other hand, the B−O bond lengths are nearly identical with the average value of 1.374(3) Å. The B1O2−Si1−O3B2, Si1−O2−B1, Si1−O3−B2, and B1−O−B2 angles have values of 104.6(1)°, 124.1(2)°, 124.0(2)°, and 124.6(2)°, respectively, and are similar to the angles observed in related compounds with the six-membered BSi2O3 ring (105.3° to 129.4°).20−22,25 The values for the O−B−O angles fall into a very narrow range from 119.1(2)° to 120.5(2)°. Although the silicon atom in the B2SiO3 ring in 1 has a tetrahedral geometry, the ring is nearly planar with a mean deviation of the atoms from the plane of 0.044 Å. Finally, the phenyl rings are involved in several C− 10035


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Inorganic Chemistry in 2. The Si−O−Si angle is with 167.1(1)° very close to the ideal value for an sp hybridization (180°; Figure 1), but it is more obtuse than Si−O−Si angles in (tBuSi)4(ArB)4O10 (Ar = CH2CHC6H4, (154.1(1)°; Ar = BrC6H4; 147.5(2)°),28,29 which have smaller steric protection around the silicon atom. Compound 5 has a bicyclic core with a B−O−B unit and tetracoordinated boron atoms with B−OB bond length of 1.402(2) Å and B−O−B angle of 111.7(2)°. The acetate groups are bridging the B−O−B unit in an O,O′-isobidentate mode with average B−OC bond lengths of 1.569(2) Å. The geometry around the boron atoms can be best described as distorted tetrahedral with angles between 101.1(1)° and 118.6(1)° and the THC of 77.9%. The C−O bond lengths in the acetate units are identical within the estimated standard deviation (esd; 1.269(2)−1.275(2) Å) pointing to a complete delocalization of the electron density over the O−C−O unit (Figure 1). These parameters are similar to those observed for other systems with the RB(μ-O)(μ-O,O′−OOCCH3)2BR core.36,38,40,42 However, these values are different from those found in systems where only one acetate group is bridging the two boron atoms of the B−O−B unit as the values for the B−O and B−OC bonds and B−O−B angle are in the ranges of 1.49− 1.52 and 1.48−1.62 Å and 118.4°−128.9°, respectively.37,39,41,43 Selected bond lengths and angles for compounds 1−4 are listed in Table 1. Recently, the donor−acceptor N→B bonds were successfully used for the construction of macrocycles, cagelike compounds, or one-dimensional (1D), two-dimensional (2D), or 3D networks.45−54 However, a clear majority of these adducts is based on boronic esters or boroxine rings, and only a handful of compounds with B−O−Z (Z ≠ B, C, H) moiety were reported containing N→B bonds. These are (N,O-κ2-H2N(CH2)3O)(RO)B(μ-O)SiPhnMe3−n (R = Ph, n = 0, 1, 2, 3; R = tBu, n = 1, 2, 3),12,55 [tBuSi{O(PhB)O}3SitBu]n·L (L = pyridine, n = 1; L = Me2NCH2CH2NMe2, n = 2),56 and {2,6-(CH2NMe2)2-C6H3}Sn(OBPy)2(μ-O) (py = pyridine).57 Therefore, we became interested in the coordination of boron atoms in the borosilicate rings with an organic diamine. The borosilicate 1 was selected in combination with piperazine due to the lower steric hindrance around the boron atoms and the similarity of the six-membered B2SiO3 ring with the B3O3 ring in boroxines. Although a 1:1 initial ratio between 1 and piperazine was used, 2:1 adduct [{(tBuO)(Ph3CO)Si(OBPh)2(μ-O)}2·C4H10N2] (6) was generated. Recrystallization from THF gave monoclinic (P21/n) crystals that contain half of the molecule of 6 in the asymmetric unit (Table S2 in the Supporting Information; Figure 2). The two six-membered borosilicate rings are coordinated in a trans-orientation to the piperazine molecule via N→B donor−acceptor bonds, and this coordination changes the trigonal planar environment of the boron atom to a distorted tetrahedral with THC of 69.1%.44 This tetrahedral nature of the boron atom within the B2SiO3 ring in 6 causes elongation of the B−O (0.08 Å) and B−C (0.05 Å) bonds compared to those in 1. Furthermore, slight shortening (0.03 Å) of the neighboring Si(1)−O(3) and B(2)−O(5) bonds was observed. The N(1)−B(1) bond distance is 1.681(2) Å, which is slightly longer than in the monoadducts of boroxines (ArBO)3L, where L is an aliphatic or aromatic Ndonor ligand (1.61 to 1.67 Å).58−66 Surprisingly, this boron nitrogen interaction does not affect the planarity of the B2O3Si ring, and in fact, the values for the SiOBO torsion angles decreased from 11.3° and 13.1° (1) to 1.5° and 12.3° (6). Additionally, the N−H protons are involved in a N−H···π

Figure 2. Perspective view of the molecular structure of compound 6.

intramolecular interaction with one of the Ph rings of the Ph3CO group, where the H···centroid distance is 2.719 Å, and the N−H···centroid angle is nearly linear (178.8°; Table 2). Study of the Formation of Borosilicates 1 and 2 in Solution. The fact that compound 2 is always contaminated by compound 1, and the fact that from a thermodynamic standpoint, the eight-membered B2Si2O4 ring is the most stable structure in borosilicates,22 prompted us to follow the synthesis of compound 2 via periodic 1H NMR measurements in toluene-d8 (Figure 3). Surprisingly, this NMR study revealed that compound 1 is formed immediately after adding the deuterated solvent to the mixture of a and the phenylboronic acid and that compound 2 appears only after 30 min. Furthermore, when 2 starts to form, the quantity of 1 diminishes suggesting a transformation from 1 to 2. However, after 3 h, the proportion between 1 and 2 remains almost constant. Finally, after 24 h, nearly all a is consumed. This is consistent with the isolation of a mixture of 1 and 2 from a 1:1 reaction (a/phenylboronic acid) in toluene. To explain the transformation of 1 to 2, we explored the stability of compound 1 in solution, but no changes were observed even after one week in a dry solvent excluding redistribution. Nonetheless, on the one hand, when 1 was mixed with (tBuO)(Ph3CO)Si(OH)2 to encourage the splitting of the B−O−B unit in 1, ∼80% of 1 was converted to 2 after 12 h (Figure S17 in the Supporting Information). On the other hand, compound 1 does not react with a; therefore, conversion from 1 to 2 requires the presence of a protic reagent. However, the addition of 1 equiv of water into the equimolar reaction of the phenylboronic acid with a led to decomposition of a to polysilicate derivatives, and only small amount of 1 was formed, while compound 2 was absent. Independently, water is being generated in the reaction mixture in the condensation of the phenylboronic acid necessary to form the B−O−B unit present in 1. This water molecule most probably reacts with a to form the (tBuO)(Ph3CO)Si(OH)(OAc) intermediate. This is corroborated by the presence of a weak signal at (δ = 1.21 ppm, Figure 3, visible between the 15th and 45th minutes of the reaction) that is between the values for the parent compound a (tBuO)(Ph3CO)Si(OAc)2 (δ = 1.19 ppm) and the fully hydrolyzed silanodiol (tBuO)(Ph3CO)Si(OH)2 (δ = 1.23 ppm).67 However, this compound is consumed during the first hour of reaction and is not detectable again later in the reaction mixture. It is thus possible that its higher initial concentration is caused by traces of water originated from the recrystallization of the phenylboronic acid. 10036


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Inorganic Chemistry Table 2. Selected Bond Lengths [Å] and Angles [deg] for the Molecular Structure of Compound 6 bond lengths Si−OC 1.622(1) 1.610(1)

Si−OB 1.615(1) 1.640(1)

Bsp3−OSi 1.452(2)

Bsp3−OB 1.451(2)

Bsp2−OSi 1.384(2)

Bsp2−OB 1.350(2)

Bsp3−N 1.681(2)

bond angles BO−Si−OB

107.0(1)

Si−O−Bsp3 125.0(1)

O−Bsp3−O 114.6(1)

Si−O−Bsp2 122.5(1)

O−Bsp2−O 122.4(1)

B−O−B 127.2(1)

partial condensation to form B is the first step in the formation of 1. This intermediate would react with a molecule of a upon the elimination of two molecules of acetic acid resulting in the cyclic structure of 1. B cannot be isolated, but the formation of the B−O−B unit is further supported by the formation of 5 in the reaction between the phenylboronic acid and silicon tetraacetate. It is noteworthy that 5 was not observed in the reaction mixture at any time. Furthermore, compound 1 can react with the water molecule generated during the formation of B causing ring opening and formation of C (−4.49 kcal· mol−1). Such hydrolysis of B−O−B units has been reported earlier.68,69 Because of the low steric bulk around the OH groups in C and their syn orientation, a reaction with a would close the eight-membered ring in 2. As mentioned earlier, 1 can be transformed in 2 in a reaction with (tBuO)(Ph3CO)Si(OH)2, and from the thermodynamic point of view, the reaction between 1 and (tBuO)(Ph3CO)Si(OH)(OAc) (L, which was identified in the reaction mixture) would also lead to 2 and thus constitutes an alternative route from 1 to 2. Thus, we believe that, initially, 2 equiv of phenylbodonic acid condense to form B, which reacts with 1 equiv of a giving 1. In the presence of water, 1 transforms to C, which is converted by another equivalent of a to the thermodynamically more stable 2 or 1 reacts with (tBuO)(Ph3CO)Si(OH)(OAc) (formed from a and water) to form directly 2 and acetic acid. The free energy of formation of 2 is more than twice that of 1 (−35.04 vs −16.28 kcal·mol−1). This reaction pathway can also be used to explain why in the case of 3-hydroxyphenylboronic acid only compound 3 with the eight-membered ring is formed. First, the solubility of the 3-hydroxyphenylboronic acid is even lower than that of the phenylboronic acid, and even though the free energy of formation of compound G is only 2.6 kcal·mol−1 lower than that of 1, the reaction of G with water is quite exergonic (−10.62 kcal·mol−1). That is more than twice that for the analogous reaction of 1 (−4.49 kcal·mol−1). The same situation is valid also for the reaction of (tBuO)(Ph3CO)Si(OH)(OAc) with 1 (−12.43 kcal·mol−1) or G (−18.53 kcal· mol−1), respectively. This difference can be attributed to the electronic effects of the OH group in the meta position to the boron atom, as it increases the acidity of the B(OH)2 protons and thus decreases the stability of the B−O−B bridges toward hydrolysis. Therefore, the compounds 1 and G are the kinetic products, while 2 and 3 are the thermodynamic species. This also explains why compounds with the six-membered B2SiO3 ring have not been observed in reactions between boronic acids and silanols, as these usually proceed under reflux, and large quantities of water and silanols are present.19−26 b. Bonding Differences between Compounds 1 and 6. Next, we decided to use the topological descriptors as the delocalization index (DI) to describe better the different bonds inside the B2SiO3 rings in 1 and 6.70 The DI accounts for the number of electrons shared by two different atoms, as a measure of bond stability,71 and correlates with the exchange

Figure 3. 300 MHz 1H NMR spectra (toluene-d8) showing the reaction between phenylboronic acid with 1 equiv of diacetoxysilylalkoxide (Ph3CO)(tBuO)Si(OAc)2 a.

Density Functional Theory (DFT) Calculations. a. Reaction Mechanism. To shed more light on the reaction mechanism and the formed intermediates and to be able to explain why in the case of the 3-hydroxyphenylboronic acid, no compound homologous to 1 is observed, theoretical calculations were performed. The thermochemistry of several reaction paths was investigated (Figure 4). The most logical step would be the condensation of a with phenylboronic acid to give (tBuO)(Ph3CO)(AcO)Si(μ-O)BPh(OH) (D). However, the very low solubility of the boronic acid in toluene speaks against this reaction pathway, as the transformation of D into 1 would have to proceed via the (tBuO)(Ph3CO)Si(OBPhOH)2 (C) intermediate that would cyclize to 1. Nonetheless, the dehydration of C to 1 is slightly endergonic (+4.49 kcal·mol−1), and additionally the formation of C from D is highly unlikely, because the high concentration of a in the reaction media (and low solubility of the phenylboronic acid in toluene) would convert D to PhB({(μ-O)Si(OCPh3)(OtBu)(OAc)}2 (E), and E cannot be converted to 1. Therefore, to obtain 1, the B−O− B unit must be formed first. Taking into account that, while the boronic acid is virtually insoluble in toluene, the corresponding trimeric anhydride (PhBO)3 (A) is highly soluble in this solvent; therefore, the solubility of the partial anhydride (μO)(BPhOH)2 (B) should be higher than that of the boronic acid. Also, such condensation of phenylboronic acid is known to take place in solution, and the formation of B is slightly exergonic (−4.21 kcal·mol−1). Thus, we hypothesize, that this 10037


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Figure 4. Possible reaction routes in the formation of 1−3 (I) and the stepwise hydrolysis of a to the corresponding silanediol (II) with the free energies of formation. Blue text corresponds to derivatives based on phenylboronic acid, while red labels belong to species with 3hydroxyphenylboronic acid.

energy between two atoms, which is the main energetic component of covalent bonds (For details, see Section 3 in the Supporting Information). In compound 1, the B−OSi bond is slightly more stable than the B−OB bond based on DI and the electron density at the bond critical points (ρ). In the case of compound 6, the most stable bonds are around the trigonal boron atom, where B−OB is the strongest one. In the case of the tetragonal boron atom, the dative N→B bond is the weakest interaction, while the B− OB and B−OSi bonds have nearly the same strength. On the one hand, with structures A and 2 as references for B−OB and B−OSi bonds, respectively, it is possible to observe that the B− OB bond increases its stability in 6 while it decreases in 1. On the other hand, B−OSi bonds are weaker in both 1 and 6 (Table S3 in the Supporting Information). Finally, to see if there are any intramolecular interactions between the nearly coplanar phenyl and B2SiO3 rings in 1, we performed a topological study of the electron density in 1.72

Figure 5 depicts the molecular graph of 1, where just one weak C···O interaction, between the B2SiO3 and phenyl rings, is observed. The interaction energy of this contact is only ∼2 kcal· mol−1, as estimated using the Espinosa-Molins-Lecomte approximation.73 Other properties of bond critical points are presented in Table S4 in the Supporting Information. Therefore, it can be concluded that the mutual orientation of the two rings is rather the result of crystal packing. Thermal Stability. The TGA of compounds 1−6 under a nitrogen atmosphere revealed that all compounds are stable below 200 °C, but a partial mass loss of 5.7% and 8.4% respectively, was observed for compounds 3 and 4 at ∼100 °C and is attributed to the loss of lattice solvent. Their presence was confirmed by 1H and by 13C NMR spectra and by X-ray crystallography (vide supra). The highest thermal stability was observed for compound 4, as it presents a weight loss of 48.7% at 310 °C. Furthermore, compound 2 with B2Si2O4 borosilicate ring decomposes at 275 °C, which is higher than the 10038


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temperatures. In this case, the silicon acetates are easily synthesized from silicon tetraacetate and alcohols, and this reaction pathway allows for a straightforward modification of the properties of the silicate precursor. Also, the reaction between the silicon acetate moiety and the boronic acid is smooth and proceeds even at low temperature and offers high yields. Furthermore, although the vast majority of chemical reactions are done in borosilicate glassware, compounds 1−4 are the first structurally characterized examples of molecular compounds with borosilicate rings containing SiO4 units. Additionally, we confirmed the possibility of the boron atoms in such a borosilicate ring to be coordinated by a base, which represents an appealing route for the preparation of supramolecular systems with controlled structure and dimensionality. Such studies are currently underway in our laboratory.

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EXPERIMENTAL SECTION

Instrumental. NMR spectroscopic data were recorded on a Bruker Avance III 300 MHz and Varian NMRSystems 500 MHz spectrometer and referenced to residual signals of the deuterated solvent for 1H and 13 C nuclei or tetramethylsilane (TMS) and F3B·OEt2, respectively, as external standards for the 29Si and 11B spectra. Electron impact mass spectrometry (EI-MS) were performed on a Shimadzu GCMSQP2010 Plus using direct injection in the detection range of 20−1090 m/z. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker ALPHA FTIR spectrometer placed inside a glovebox using the attenuated total reflectance (ATR) technique with a diamond window in the range of ν̃ 500−4000 cm−1. Melting points were measured in sealed capillaries on Büchi B-540 melting point apparatus. Elemental analyses (C, H, N) were determined on an Elementar MicroVario Cube analyzer. For all compounds, the experimental value for carbon content was lower than calculated, which is common in compounds with boron and silicon due to the formation of silicon and boron carbides difficult to pyrolyze. Compounds a and b were synthesized according to literature procedures.67 (tBuO)(Ph3CO)Si{(μ-O)BPh}2(μ-O) (1). In a Schlenk flask, diacetyloxysilylalkoxide (Ph3CO)(tBuO)Si(OAc)2 (0.20 g, 0.42 mmol) and phenylboronic acid (0.10 g, 0.83 mmol) (1:2 ratio) were dissolved in toluene (20 mL) and stirred overnight. Afterward, all volatiles were removed under reduced pressure. Crystals of compound 1 were grown by a slow evaporation (one week) of saturated hexane solution at room temperature. Yield: 75% (0.18 g, 0.31 mmol). mp 143−144 °C. Anal. (%) Calcd for C35H34B2O5Si (584.35 g·mol−1): C 71.94, H 5.86; Found: C 68.71, H 5.88. FT-IR (ATR) ν̃ 2963 (w, −CH3), 1259 (s, B−O), 1017 (s, Si−O) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.40 (s, 9H, CCH3), 7.15−7.83 (m, 25H, ArH) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) δ 31.6 (CCH3), 75.2 (CCH3), 86.7 (CPh3), 127.4, 127.7, 127.9, 128.6, 131.8, 135.4, 145.7 (p, m, o, i, C of Ar, the signal for the ipso carbon bound to the boron atom, was not observed) ppm. 29Si NMR (59.63 MHz, CDCl3, 25 °C) δ −93.5 ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 28.9 ppm. EI-MS: m/z (%) 584 (25) [M]+, 527 (100) [M − tBu]+, 507 (78) [M − Ph]+, 451 (63) [M − tBu − Ph]+. [{(tBuO)(Ph3CO)Si(μ-O)BPh(μ-O)}2] (2). In a Schlenk flask, diacetyloxysilylalkoxide (Ph3CO)(tBuO)Si(OAc)2 (0.20 g, 0.42 mmol) and phenylboronic acid (0.05 g, 0.42 mmol) (1:1 ratio) were dissolved in toluene (20 mL) and stirred overnight. Then, all the volatiles were removed under reduced pressure. Crystals of compound 2 were grown by slow evaporation form hexane at room temperature. Yield: 75% of ∼70% purity (0.15 g, 0.15 mmol). Repeated recrystallizations from hexane led to a 95% pure sample. mp 187− 189 °C. Because of the contamination by ∼5% of compound 1, the elemental analysis was not performed. FT-IR (ATR) ν̃ 2962 (w, −CH3), 1259 (s, B−O), 1014 (s, Si−O) cm−1. The following NMR data contain only signals for compound 2. All signals for 1 are omitted; however, full spectra can be found in the Supporting Information. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.18 (s, 18H, CCH3), 7.13− 7.41 (m, 40H, ArH) ppm. 29Si NMR (59.63 MHz, CDCl3, 25 °C) δ

Figure 5. Molecular graph of compound 1.

decomposition temperature observed for compound 1 with the six-membered B2SiO3 ring (250 °C) and 3 (200 °C). Finally, compound 5 presents one stage decomposition (87.3% weight loss) above 300 °C leaving a B2O residue.74,75 Finally, compound 6 starts to decompose at 125 °C, but a larger weight loss is observed only after reaching 275 °C (70.2% weight loss). In compounds 1−4 and 6, the weight loss corresponds to partial or total elimination of organic groups attached to silicon leaving PhB2O3Si (% calc = 29.8; % found = 32.3) for 1, (PhBSiO2)2 (% calc = 30.5; % found = 28.7) for 2, [(C6H4OH)BSiO2]2OC(Ph)3, (% calc = 45.7; % found = 48.8) for 3, (PhBSiO3)4O2 (% calc = 37.2; % found = 42.9) for 4, and [(B2O3Si)O(OtBu)]2 (% calc = 26.2; % found = 29.8) for 6, respectively (Figure 6).

Figure 6. TGA trace for compounds 1−6.

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CONCLUSIONS An easy method for the synthesis of cyclic borosilicate compounds with different ring size under mild conditions is presented. The method starts from substituted silicon acetates and organoboronic acids and allows for an easy modulation of the final inorganic core. The major advantage of the method is the large diversity of possible silicate precursors eliminating the main restriction of current synthetic methods, which is the very limited availability of silanol precursors stable at high 10039


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Inorganic Chemistry −106.7, −107.1 ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 26.1 ppm. EI-MS: m/z (%) 945 (4) [M − Me]+, 903 (37) [M − tBu]+, 883 (60) [M − Ph]+. [{(tBuO)(Ph3CO)Si(μ-O)B(3-HOPh)(μ-O)}2] (3). Compound 3 was obtained following the methodology described for compound 2, using 3-hydroxyphenylboronic acid (0.056 g; 0.42 mmol) instead of phenylboronic acid. Yield: 72% (1.49 g, 0.15 mmol). mp 208−210 °C. Anal. (%) Calcd for C58H58B2O10Si2 (992.87 g·mol−1): C 70.16, H 5.89; Found: C 68.11, H 5.89. FT-IR (ATR) ν̃ 3352 (w, O−H), 2973 (w, −CH3), 1311 (s, B−O), 1074 (s, Si−O) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.19 (s, 18H, CCH3), 4.32 (s, 2H, OH), 6.61− 7.43 (m, 38H, ArH) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 31.4 (CCH3), 74.0 (CCH3), 86.1 (CPh3), 118.0, 121.8, 125.3, 126.8, 127.6, 128.2, 128.6, 129.0, 146.2, 154.6 ppm (p, m, o, i, C of Ar). [{(Ph3CO)Si(μ-O)2BPh(μ-O)}2] (4). Method A: In a Schlenk flask, triacetyloxysilylalkoxide (Ph3CO)Si(OAc)3 (0.50 g, 1.10 mmol) and phenylboronic acid (0.20 g, 1.61 mmol) (2:3 stoichiometric ratio) were dissolved in toluene (20 mL) and stirred overnight. Then, all the volatiles were removed under reduced pressure, and the remaining solid was washed with hexane to give white solid. Crystals of 4 were grown using a mixture of dichloromethane/hexane 1:1 at room temperature. Yield: 83% (0.37 g, 0.22 mmol). Method B: In a Schlenk flask, triacetyloxysilylalkoxide (Ph3CO)Si(OAc)3 (0.30 g, 0.646 mmol) and phenylboronic acid (0.08 g, 0.646 mmol) (1:1 stoichiometric ratio) were dissolved in toluene (20 mL), and the reaction mixture was stirred for 2 h. Afterward 1 equiv of water was added (0.01 g, 0.646 mmol) and stirred overnight. Then, all the volatiles were removed under reduced pressure, and the remaining solid was washed with hexane to give white solid. Yield: 43% (0.12 g, 0.073 mmol). mp 270− 272 °C. Anal. (%) Calcd for C100H80B4O14Si4·2CH2Cl2 (1831.09 g· mol−1): C 66.90, H 4.62; Found: C 66.68, H 4.71. FT-IR (ATR) ν̃ 2962 (w, −CH3), 1304 (s, B−O), 1068 (s, Si−O) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 6.72−7.46 ppm (m, 80H, H of Ar). 13 C{1H} NMR (75.57 MHz, CDCl3, 25 °C) δ 86.3 (CPh3), 126.8, 127.1, 127.9, 128.5, 130.9, 135.8, 145.8 (p, m, o, i, C of Ar, the signal for the ipso carbon bound to boron was not observed) ppm. 29Si NMR (59.63 MHz, CDCl3, 25 °C) δ −106.9 ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 29.6 ppm. (μ-O)(PhB)2(μ-O,O′-OAc)2 (5). In a Schlenk flask, silicon tetraacetate (1.00 g, 3.78 mmol) and phenylboronic acid (0.46 g, 3.77 mmol) were suspended in toluene (20 mL), and the mixture was stirred overnight. All the volatiles were removed under reduced pressure, and the remaining solid was washed with hexane to remove the oily silicon subproducts. Compound 5 was isolated as a white solid. Crystals were grown from a saturated solution in THF at −36 °C. Yield: 80% (0.47 g, 1.51 mmol). mp > 220 °C (dec). Anal. (%) Calcd for C16H16B2O5 (309.92 g·mol−1): C 62.01, H 5.20; Found: C 58.82, H 5.04. FT-IR (ATR) ν̃ 2962 (w, −CH3), 1258 (s, B−O) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 2.35 (s, 6H, CCH3), 7.38 (m, 6H, p, m, H of Ar), 7.78 ppm (m, 4H, o, H of Ar). 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) δ 22.7 (CCH3), 127.6, 128.4, 131.6, 138.8 (p, m, o, i, C of Ar), 184.1 (COO) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 5.9 ppm. EI-MS: m/z (%). 312 (100) [M]+. [({(tBuO)(Ph3CO)Si(μ-O)BPh(μ-O)}2)2·C4H10N2] (6). In a vial, 2 equiv of compound 2 (0.10 g, 0.17 mmol) and piperazine (0.07 g, 0.09 mmol) were dissolved in diethyl ether (2 mL). After the solution was stirred for 12 h, all volatiles were removed under reduced pressure. Crystals were grown by slow evaporation of THF at room temperature. Yield: 87% (0.09 g, 0.07 mmol); mp 225−226 °C. Anal. (%) Calcd for C74H78B4N2O10Si2 (1254.83 g·mol−1): C 70.83, H 6.27, N 2.23; Found: C 68.64, H 6.39, N 2.22. FT-IR (ATR) ν̃ 3063 (w, −NH), 2978 (w, −CH3), 1341 (s, B−O), 1063 (s, Si−O) cm−1. Because of a very limited solubility, it was not possible to acquire meaningful NMR spectra. Single-Crystal X-ray Diffraction Analysis. Single crystals of all compounds were mounted on nylon loops and placed in the cold nitrogen stream (100 K) inside a Bruker APEX DUO diffractometer equipped with an Apex II CCD detector. Frames were collected using omega scans and integrated with SAINT.76 Multiscan absorption correction (SADABS) was applied.76 The structures were solved by

using direct methods (SHELXT)77 and refined using full-matrix leastsquares on F2 with SHELXL78 using the ShelXle GUI.79 Weighted R factors, wR, and all goodness-of-fit indicators are based on F.2 All nonhydrogen atoms were refined anisotropically. The hydrogen atoms of the C−H bonds were placed in idealized positions, whereas the hydrogen atom from the NH moiety in 6 was localized from the difference electron density map, and its position was refined with Uiso tied to the parent atom with distance restraint (DFIX). The disordered groups and solvent molecules (2, 2 × Ph, 1 × tBu; 4·2CH2Cl2, 1 × CH2Cl2, 6, 1 × tBu) were refined using geometry (DFIX, SADI, SAME) and Uij restraints (SIMU, RIGU) implemented in SHELXL.78 The sum of the three refined positions of the dichloromethane molecule in 4·2CH2Cl2 was controlled using SUMP instruction. The molecular graphics were prepared using GRETEP, POV-RAY, and GIMP.80−82 Computational Methods. The DFT and QTAIM were performed at M06-2X83/SDD theoretical level. For the calculations of the thermochemistry, all compounds were fully optimized, while for the analysis of the contacts between the phenyl and the B3O3 rings in compound 1, single-point calculations based on the crystal structure were used. Frequency calculations ensured that minima were found. The molecular optimizations and thermochemistry calculations were performed with Gaussian 0984 program, and the local properties of electron density of compound 1 were obtained with the AIMAll software.85

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01580. 1 H, 13C, 11B, and 29Si NMR spectra, tabulated crystallographic data, delocalization indices, molecular graph, local properties of bond critical points, optimized structures (PDF) Accession Codes

CCDC 1551306−1551310 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 Authors

*E-mail: [email protected]. (V.J.) *E-mail: [email protected]. (F.C.-G.) ORCID

Vojtech Jancik: 0000-0002-1007-1764 Author Contributions §

Both authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Dirección General de Asuntos del Personal Académico from the UNAM (PAPIIT Grant No. IN205115) and CONACyT (Grant No. 179348). N. ZavalaSegovia, A. Núñez-Pineda, and L. Triana-Cruz are acknowledged for their technical assistance. We thank DGTIC-UNAM for the supercomputer time (Grant No. LANCAD-UNAMDGTIC-194). M.J.V.-H., L.G.R.-P., and U.H.-B. are grateful to CONACyT for their Ph.D. and posdoctoral fellowships (Grant 10040


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Nos. 262860, 308338, and 14045, respectively). A.T.-H. acknowledges DGAPA-UNAM for postdoctoral fellowship.

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Synthesis of Cyclic and Cage Borosilicates Based on Boronic Acids

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