Molecular Group 13 Metallaborates Derived from M–O–M Cleavage

Jul 3, 2017 - The use of BH3·THF with 1 and 2 led to the concomitant THF ring-opening reaction, while with BH3·SMe2 in THF no such reaction was obse...
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Molecular Group 13 Metallaborates Derived from M−O−M Cleavage Promoted by BH3 Erandi Bernabé-Pablo, Vojtech Jancik, Diego Martínez-Otero, Joaquín Barroso-Flores, and Mónica Moya-Cabrera* Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Carretera Toluca-Atlacomulco Km 14.5, Toluca, Estado de México 50200, México S Supporting Information *

ABSTRACT: The reaction of metalloxanes [{ Me LM(OH)}2(μ-O)] [M = Al (1), Ga (2); MeL = CH{CMe(NAr)}2−, Ar = 2,4,6-Me3C6H2, Me = methyl] with an excess of BH3·D (D = tetrahydrofuran (THF), SMe2) affords annular metallaborate systems achieved through M−O−M cleavage. Compound 1 led exclusively to the formation of eightmembered ring systems [{MeLAl(μ-O){B(OnBu)}(μ-O)}2] (3) and [{MeLAl(μ-O)(BH)(μ-O)}2] (6), while for 2 the unprecedented six-membered ring systems [{(MeLGa)2(μO)}(μ-O)2{B(OnBu)}] (4) and [(MeLGa)(μO)2{(BOnBu)2(μ-O)}] (5) were observed. The use of BH3· THF with 1 and 2 led to the concomitant THF ring-opening reaction, while with BH3·SMe2 in THF no such reaction was observed. The metallaborates [MeLAl{OB(pinacol)}(OH)] (7) and [{MeLGa(OB(pinacol))}2(μ-O)] (8) were synthesized from pinacolborane and the corresponding metalloxane, providing structural evidence that supports the reaction pathways proposed for the formation of 3−6. Density functional theory studies were performed on 3−5 to assess the effect of the metal exchange between aluminum and gallium atoms on the energy of the general ring structures.

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INTRODUCTION The widespread interest in group 13 metal boron-based materials is due to their optical and luminescent properties, as well as their use in the design of mineral-like materials.1 Moreover, aluminum boron compounds have proven to be active cocatalysts in the polymerization of olefins.2 Nonetheless, the preparation of molecular aluminum and gallium borates in pure crystalline form remains a synthetic challenge due mostly to the difficulty in assembling them in discrete arrangements. In fact, on the one hand, discrete compounds containing Ga−O− B units are limited to the gallium boroloxides [GaMe2(μMesB(OH)O)]2 (Mes = 2,4,6-Me3C6H2) and [R2GaOB(oCH2O)(C6H4)]2 (R = Me, tBu),3 while only one example of a molecular galloborate is known, namely, GaB5O8(OH)2(en)2· H2O (en = ethylenediamine), which was synthesized by solvoand hydrothermal techniques.1b,4 On the other hand, the synthetic methods for the preparation of aluminum boron compounds bearing Al−O−B units are restricted to protonolysis reactions between a boronic or borinic acid and the corresponding aluminum alkyl or hydride compound.5 In this regard, the number of examples of discrete aluminum boroloxides does not exceed by far that of their gallium analogues.3b,c,6 During our research with functionalized metalloxanes, [{MeLM(OH)}2(μ-O)] [M = Al (1), Ga (2)] these compounds proved to be unreactive in protonolysis reactions toward a wide variety of organometallic reagents. In contrast, 1 and 2 react © 2017 American Chemical Society

smoothly with metal amides M(NR2)4 (M = Ti, Zr, Hf) yielding annular hetero-bimetallic systems.7,8 Motivated by an interest in the reactivity patterns of 1 and 2 with inorganic bases soluble in organic solvents, namely, borane complexes BH3·D (D = donor), we now report on the preparation of annular alumo- and galloborate systems achieved by protonolysis and hydride transfer reactions. The molecular alumoborate [{MeLAl(μ-O){B(OnBu)}(μ-O)}2] (3) (MeL = CH[CMe(NAr)]2−; Ar = 2,4,6-Me3-C6H2), alumoborane [{MeLAl(μ-O)B(H)}(μ-O)}2] (6), and galloborate [(MeLGa)(μ-O)2{(BOnBu)2(μ-O)}] (5) were obtained from the cleavage of the M−O−M moiety promoted by BH 3 adducts. Furthermore, the unique galloxane borate [{(MeLGa)2(μO)}(μ-O)2{B(OnBu)}] (4) was prepared by the protonolysis reaction of 2 with BH3·THF exhibiting the reductive cleavage of THF molecules similar to the process that occurs in 3 and 5.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under a dry and oxygen-free atmosphere (N2) using Schlenk-line and glovebox techniques. The solvents were purchased from commercial sources and dried before use with an MBraun SPS using Grubb’s-type columns. Commercially available chemicals were purchased from Sigma-Aldrich and used without further purification. Compounds 1 Received: March 9, 2017 Published: July 3, 2017 7890

DOI: 10.1021/acs.inorgchem.7b00634 Inorg. Chem. 2017, 56, 7890−7899

Article

Inorganic Chemistry and 2 were prepared according to the literature procedures.8,9 C6D6 was dried with a Na/K alloy and distilled through vacuum transfer (−196 °C) using a Swagelok system. 1H, 11B, and 13C NMR spectra were recorded at 25 °C on a Bruker Avance III 300 MHz spectrometer. 1H and 13C NMR spectra were referenced to the residual nondeuterated and deuterated solvent, respectively. A BF3· OEt2 solution in THF was used as a reference for the 11B NMR spectra. Infrared spectra were measured by using the attenuated total reflectance (ATR) on a Bruker Alpha FT-IR spectrometer under an inert atmosphere in the range of ṽ 4000−400 cm−1. Mass spectra (MS) were performed on a Shimadzu GCMS-QP2010 Plus using direct injection and electron impact (EI) as the ionization technique with a detection range from 20 to 1090 m/z. Elemental analyses (C, H, N) were performed on an Elementar MicroVARIO Cube analyzer. Melting points were measured in sealed glass tubes on a Bühi B-540 melting point apparatus. Crystallographic data for compounds 3−6 and 7a were collected on a Bruker SMART APEX DUO three-circle diffractometer equipped with an Apex II CCD detector using Mo Kα (Inotec sealed tube with a graphite monochromator). The crystals were coated with a hydrocarbon oil, picked up with a nylon loop, and immediately mounted in the cold nitrogen stream (−173 °C) of the diffractometer. Frames were collected by omega scans, integrated using SAINT program, and semiempirical absorption correction (SADABS) was applied.10 The structures were solved by direct methods (SHELXS) and refined by the full-matrix least-squares on F2 with SHELXL11 using the SHELXLE GUI.12 Weighted R factors, Rw, and all goodness-of-fit indicators are based on F2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in idealized geometrical positions and refined with Uiso tied to the parent atom with the riding model, whereas the hydrogen atoms of the BH moieties in 6 were localized from the difference electron-density map and refined isotropically with Uij tied to the boron atom. Preparation of [{MeLAl(μ-O){B(OnBu)}(μ-O)}2] (3). A solution of BH3·THF complex (1.0 M in THF, 0.32 mL, 0.32 mmol) was added to a toluene solution of 1 (0.25 g, 0.32 mmol) at −79 °C. The reaction mixture was allowed to warm to ambient temperature and stirred for 12 h after which the solution was filtered. All volatiles were removed under vacuum leaving a white residue, which was treated with cold pentane (3 × 2 mL). Yield 52% (0.080 g). mp 249 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ 0.98 [t, 3H, BO(CH2)3CH3], 1.44 (s, 6H, CH13), 1.47 (m, 4H, BOCH2(CH2)2CH3), 2.23 (s, 12H, o-Ar−CH3), 2.32 (s, 6H, p-Ar−CH3), 3.77 [t, 2H, BOCH2(CH2)2CH3], 4.84 (s, 1H, γ−CH), 6.84 ppm (s, 4H, m-Ar−H). 13C NMR (75 MHz, C6D6, 25 °C): δ 14.3 [BO(CH2)3CH3], 18.4 (CH3), 21.6 (o-Ar−CH3), 19.6 (p-Ar−CH3), 22.1 [BO(CH2)2CH2CH3], 34.8 (BOCH2CH2CH2CH3), 62.5 [BOCH2(CH2)2CH3], 96.8 (γ-CH), 129.2 (m-Ar−C), 133.8 (o-Ar−C), 134.4 (p-Ar−C), 140.2 (i-Ar−C), 169.5 ppm (CN). 11B NMR (96.2 MHz, C6D6, 25 °C): δ 17.5 ppm. IR (ATR): ṽ 2956 (w), 2918 (w), 2864 (w), 1610 (w), 1527 (sh), 1448 (b), 1372 (s), 1270 (s), 1248 (s), 1201 (m), 1145 (m), 1020 (m), 965 (w), 882 (m), 846 (m), 784 (b), 724 (w), 692 (sh), 654 (m), 627 (m), 553 (m), 522 (m), 435 (m) cm−1. MS-EI (70 eV) m/z: 952 [M−H]+. Anal. (%) calcd for C54H76Al2B2N4O6 (952.79): C 68.07, H 8.04, N 5.88; found: C 68.01, H 8.07, N 5.78. Preparation of [{(MeLGa)2(μ-O)}(μ-O)2{B(OnBu)}] (4). Compound 4 was synthesized using the same procedure outlined above for 3, starting from BH3·THF complex (0.29 mL, 0.29 mmol) and 2 (0.25 g, 0.29 mmol). Yield 44% (0.12 g). mp 243 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ 1.05 [t, 3H, BO(CH2)3CH3], 1.29 (s, 6H, CH3), 1.58 [m, 2H, BO(CH2)2CH2CH3], 1.72 (q, 2H, BOCH2CH2CH2CH3), 2.06, 2.29 (s, 12H, o-Ar−CH3), 2.24 (s, 6H, p-Ar−CH3), 3.96 [t, 2H, BOCH2(CH2)2CH3], 4.71 (s, 1H, γ−CH), 6.62, 6.83 ppm (s, 4H, mAr−H). 13C NMR (75 MHz, C6D6, 25 °C): δ 14.3 [BO(CH2)3CH3], 18.0 (CH3), 18.1, 22.4 (o-Ar−CH3), 20.9 (p-Ar−CH3), 22.1 [BO(CH 2 ) 2 CH 2 CH 3 ], 35.0 (BOCH 2 CH 2 CH 2 CH 3 ), 62.6 [BOCH2(CH2)2CH3], 95.6 (γ-CH), 129.1, 129.6 (m-Ar−C), 133.3, 134.1 (o-Ar−C), 133.8 (p-Ar−C), 141.4 (i-Ar−C), 169.3 ppm (C N). 11B NMR (96.2 MHz, C6D6, 25 °C): δ 23.2 ppm. IR (ATR): ṽ 2950 (w), 2916 (w), 2863 (w), 1607 (w), 1535 (sh), 1454 (b), 1380

(s), 1332 (s), 1308 (m), 1288 (m), 1263 (b), 1248 (b), 1224 (m), 1206 (m), 1148 (m), 1120 (m), 1098 (m), 1021 (b), 962 (w), 935 (w), 907 (w), 867 (m), 843 (m), 790 (w), 754 (sh), 690 (m), 637 (w), 610 (m), 570 (w), 550 (m), 508 (m), 453 (w) cm−1. MS-EI (70 eV) m/z: 938 [M−H]+. Anal. (%) calcd for C50H67BGa2N4O4 (938.35): C 64.00, H 7.20, N 5.97; found: C 64.09, H 7.21, N 5.82. Preparation of [(MeLGa)(μ-O)2{(BOnBu)2(μ-O)}] (5). Compound 5 was synthesized using the same procedure outlined above for 3, starting from BH3·THF complex (2.33 mL, 2.33 mmol) and 2 (0.25 g, 0.29 mmol). Yield 95% (0.087 g). mp 127 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ 0.83 [t, 3H, BO(CH2)3CH3] 1.30 [m, 2H, BO(CH 2 ) 2 CH 2 CH 3 ], 1.43 (s, 6H, CH 3 ), 1.48 (q, 2H, BOCH2CH2CH2CH3), 2.02 (s, 12H, o-Ar−CH3), 2.33 (s, 6H, pAr−CH3), 3.87 [t, 1H, BOCH2(CH2)2CH3], 4.75 (s, 1H, γ−CH), 6.72 ppm (s, 4H, m-Ar−H). 13C NMR (75 MHz, C6D6, 25 °C): δ 14.1 [BO(CH2)3CH3], 18.3 (CH3), 20.8 (o-Ar−CH3), 19.5 (p-Ar−CH3), 22.3 [BO(CH 2 ) 2 CH 2 CH3 ], 34.4 (BOCH 2 CH 2CH 2 CH3 ), 62.7 [BOCH2(CH2)2CH3), 95.6 (γ-CH), 129.8 (m-Ar−C), 133.4 (o-Ar− C), 136.5 (p-Ar−C), 139.0 (i-Ar−C), 171.3 ppm (CN). 11B NMR (96.2 MHz, C6D6, 25 °C): δ 20.2 ppm. IR (ATR): ṽ 2957 (m), 2926 (m), 2872 (m), 1608 (w), 1534 (sh), 1479 (m), 1444 (m), 1429 (m), 1409 (m), 1365 (b), 1295 (m), 1255 (b), 1205 (m), 1150 (m), 1104 (m), 1077 (m), 1027 (m), 963 (w), 938 (w), 875 (m), 845 (m), 802 (m), 722 (m), 702 (sh), 642 (w), 601 (w), 568 (m), 515 (w), 504 (w), 473 (w), 441 (w) cm−1. MS-EI (70 eV) m/z: 618 [M−CH3]+. Anal. (%) calcd for C32H50B2GaN2O5 (634.09): C 60.61, H 7.95, N 4.42; found: C 60.00, H 7.73, N 4.38. Preparation of [{MeLAl(μ-O)(BH)(μ-O)}2] (6). A solution of BH3· SMe2 (2.0 M in THF, 0.26 mL, 1.29 mmol) was added dropwise to a solution of 1 (0.25 g, 0.32 mmol) in toluene (20 mL) at −79 °C. The reaction mixture was allowed to warm to ambient temperature and stirred for 12 h, after which the solution was filtered. All volatiles were removed under vacuum leaving a white residue, which was treated with cold pentane (3 × 2 mL). Yield 48% (0.062 g). mp 368 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ 1.41 (s, 6H, CH3), 2.20 (s, 12H, o-Ar− CH3), 2.31 (s, 6H, p-Ar−CH3), 4.83 (s, 1H, γ−CH), 6.79 ppm (s, 4H, m-Ar−H). 13C NMR (75 MHz, C6D6, 25 °C): δ 18.2 (CH3), 21.0 (oAr−CH3), 22.1 (p-Ar−CH3), 96.7 (γ-CH), 129.3 (m-Ar−C), 133.8 (oAr−C), 134.5 (p-Ar−C), 139.9 (i-Ar−C), 169.7 ppm (CN). 11B NMR (96.2 MHz, C6D6, 25 °C): δ 30.2 (br) ppm. IR (ATR): ṽ 2998 (w), 2956 (w), 2917 (w), 2857 (w), 2393 (m, B−H), 1527 (m), 1390 (b), 1372 (b), 1296 (s), 1248 (w), 1221 (m), 1202 (m), 1149 (m), 1021 (m), 909 (w), 882 (m), 845 (m), 771 (w), 739 (w), 729 (w), 693 (w), 667 (w), 647 (w), 586 (m), 554 (m), 464 (w), 420 (w) cm−1. MS-EI (70 eV) m/z: 807 [M+−H]. High-resolution (HR) MS (APCI+, toluene): m/z 807.436 289 (calcd 807.435 319) [M−H]+ (1.2 ppm deviation). Preparation of [{MeLAl(OB(pinacol))}2(μ-OH)2] (7a). Pinacolborane (4,4,5,5-tetramethyl-1,2,3-dioxaborolane; 0.046 mL, 0.32 mmol) was added slowly to a solution of 1 (0.25 g, 0.32 mmol) in 20 mL of toluene at −79 °C. The reaction was stirred at room temperature for 1 h, and a 1.0 M solution of H2O in THF (0.32 mmol) was added dropwise followed by the addition of a second portion of pinacolborane. The reaction mixture was allowed to stir overnight. After this time, the formation of a white precipitate was observed, which was isolated by filtration and washed with hexane (3 × 5 mL). Yield 75% (0.125 g). mp 219 °C. 1H NMR for the dimeric structure 7a (300 MHz, C6D6, 25 °C): δ 1.21 (s, 12H, B{O(C)CH3}2), 1.35 (s, 6H, CH3), 1.49 [s, 1H, μ-OH], 2.18, 2.23 (s, 12H, o-Ar−CH3), 2.31 (s, 6H, p-Ar−CH3), 4.99 (s, 1H, γ−CH), 6.66, 6.83 ppm (s, 4H, m-Ar− H); 1H NMR (300 MHz, CDCl3, 25 °C): δ 1.12 (s, 12H, B{O(C)CH3}2), 1.30 (s, 6H, CH3), 1.37 [s, 1H, μ-OH], 1.80, 2.37 (s, 12H, o-Ar−CH3), 1.94 (s, 6H, p-Ar−CH3), 4.97 (s, 1H, μ−CH), 6.54, 6.74 ppm (s, 4H, m-Ar−H). 1H NMR for the monomeric structure 7 (300 MHz, CDCl3, 25 °C): δ 0.65 [s, 1H, AlOH], 1.02 (s, 12H, B{O(C)CH3}2), 1.71 (s, 6H, CH3), 2.23 (s, 6H, p-Ar−CH3), 2.26 (s, 12H, o-Ar−CH3), 5.14 (s, 1H, γ−CH), 6.88 ppm (s, 4H, mAr−H). 11B NMR (C6D6, 25 °C): δ 19.7 ppm. IR (ATR): ṽ 3676 (m, AlO−H), 2917 (w), 2857 (w), 1524 (w), 1478 (b), 1244 (m), 1224 (m), 1203 (b), 1155 (m), 1120 (m), 1097 (m), 1027 (b), 922 (m), 7891

DOI: 10.1021/acs.inorgchem.7b00634 Inorg. Chem. 2017, 56, 7890−7899

Article

Inorganic Chemistry 859 (m), 741 (m), 690 (m), 654 (w), 568 (w), 523 (w), 514 (w), 465 (w) cm−1. MS-EI (70 eV) m/z: 1023 [M+−OH], 520 [1/2M+]. Anal. (%) calcd for C58H84B2Al2N4O8 (1040.89): C 66.93, H 8.13, N 5.38; found: C 67.05, H 7.97, N 5.24. Preparation of [{MeLGa(OB(pinacol))}2(μ-O)] (8). Pinacolborane (0.035 mL, 0.25 mmol) was added to a solution of 2 (0.100 g, 0.116 mmol) in toluene (20 mL) at −79 °C. The reaction mixture was allowed to warm to ambient temperature and stirred for 16 h. The solution was filtered, and all volatiles were removed under vacuum. The residual colorless solid was highly soluble in cold hexane and pentane. Yield 81% (0.104 g). mp 58 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ 0.96 (s, 12H, B{O(C)CH3}2), 1.49 (s, 6H, CH3), 2.10, 2.46 (s, 12H, o-Ar−CH3), 2.15 (s, 6H, p-Ar−CH3), 4.77 (s, 1H, γ−CH), 6.71, 6.77 ppm (s, 4H, m-Ar−H). 13C NMR (75 MHz, C6D6, 25 °C): δ 19.5 (CH3), 17.5, 23.2 (o-Ar−CH3), 21.0 (p-Ar−CH3), 23.6 (B{O(C)CH3}2), 78.8 (B{O(C)CH3}2), 93.8 (γ-CH), 128.0, 128.7 (m-Ar−C), 133.3, 134.1 (o-Ar−C), 130.8 (p-Ar−C), 139.6 (i-Ar−C), 167.1 ppm (CN). 11B NMR (96.2 MHz, C6D6, 25 °C): δ 21.8 ppm. IR (ATR): ṽ 2974 (w), 2919 (w), 1941 (w), 1377 (b), 1257 (m), 1202 (m), 1150 (b), 867 (m), 630 (w), 569 (w), 506 (w) cm−1. MS (ESI+, THF): m/z 1109.35 [M+H]+. Anal. (%) calcd for C58H82B2Ga2N4O7 (1108.36): C 62.85, H 7.46, N 5.05; found: C 63.05, H 7.57, N 5.24.

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products and that for the reagents at each step is reported in relative values with respect to that of the starting materials.

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RESULTS AND DISCUSSION With the possibility of synthesizing annular six-membered metalloxane boranes, the functionalized metalloxanes [{MeLM(OH)}2(μ-O)] [M = Al (1), Ga (2)] were devised as appropriate precursors to participate in protonolysis reactions with boranes. Conversely, the reaction of [{MeLAl(OH)}2(μO)] (1) and BH3·THF (1.0 M in THF) under mild conditions gave the eight-membered alumoborate [{MeLAl(μ-O){B(OnBu)}(μ-O)}2] (3) (Scheme 1). Compound 3 originates from the splitting of the Al−O−Al unit promoted by the BH3·THF complex and the reductive ring-opening reaction of two THF molecules to form OnBu substituent groups. In this regard, boranes BxHy are known to promote THF ring-opening reactions at high temperatures (BH3) or using a mixture of B2H6/LiAlH4 at ambient temperature.15 In addition, 1H NMR analysis of the crude reaction mixture shows the alumoxane hydroxide−hydride [{MeLAl(OH)}(μO){MeLAl(H)}]16 as the other major product accompanying the formation of 3. This compound stems from the autocondensation reaction of the transient monometallic product [MeLAl(OH)(H)], thus suggesting that the Al−O−Al cleavage is promoted by a hydride transfer from the BH3·THF complex to the alumoxane moiety. The proposed reaction pathway for the formation of 3 is depicted in Scheme 2. The first step involves the splitting of the alumoxane unit assisting the B−H hydride transfer to the THF α-carbon forming the OnBu groups (intermediate A). The second step involves the condensation reaction of two molecules of A with the consequent H2 evolution and the formation of 3. Furthermore, to gain further insight on the feasibility of the reaction pathway depicted in Scheme 2, free-energy changes (ΔΔG) were calculated for intermediates A and 3 using the thermochemistry data obtained with Gaussian 09 at a B3LYP/ 6-31G(d,p) level of theory (see Figure S1 in Supporting Information). These results indicate that the formation of 3 from 1 and BH3·THF via intermediate A is an energy-favorable process with a 151.21 kcal mol−1 gain. However, the protonolysis reaction between galloxane [{MeLGa(OH)}2(μ-O)] (2) and BH3·THF maintaining a 1:1 molar ratio led to the formation of the anticipated annular galloxane borate [{(MeLGa)2(μ-O)}(μ-O)2{B(OnBu)}] (4), where the Ga−O−Ga unit is retained. In contrast, the reaction of 2 with an excess of BH3·THF led to the isolation of the unprecedented six-membered galloborate [(MeLGa)(μ-O)2{(BOnBu)2(μ-O)}] (5) (Scheme 1). This monometallic borate 5 stems from the cleavage of the Ga− O−Ga moiety by a hydride transfer reaction from BH3·THF to the initially formed galloxane borate 4. A similar hydride transfer mechanism as observed in 3 leads to the Ga−O−Ga splitting; however, due to the annular nature of 4, the reaction pathway produces the six-membered ring GaO3B2 instead of an eight-membered ring system like in 3. The only subproduct identified in the formation of 5 corresponds to the gallium hydride MeLGaH2,8 which is in agreement with the mechanism proposed for the formation of 5 starting from 4 (Scheme S1, Supporting Information). This reaction pathway accounts also for the low yield of 4 and the observation of 5 as the only other subproduct in the reaction mixture.

COMPUTATIONAL METHODS

All calculations were performed with the Gaussian09 revision D.01 suite of programs.13 Geometry optimizations at the B3LYP/631G(d,p) level of theory were undertaken for compounds 3−5 from their crystallographic structures to eliminate all stress due to the crystal field. Vibrational frequency analysis yielded all real eigenvalues that indicated the presence of a minimum energy structure. Upon optimization, the metallic elements in all three compounds were switched by the corresponding element yielding the hypothetical compounds 3′, 4′, and 5′. Wiberg bond indexes14 were calculated on the optimized structures as well as on the crystalline configurations. The proposed mechanisms depicted in Schemes 1 and S1 were computationally assessed by calculating the thermochemistry data of

Scheme 1. Synthesis of Compounds 3−6

every species in them at the B3LYP/6-31G(d,p) level of theory, which was previously used to optimize each one of them. The thermochemistry data are obtained by performing a frequency analysis with the keyword freq, from which we extract the values reported under the “Sum of electronic and thermal Free Energies” for each compound and appropriately multiplied according to their respective stoichiometry. The difference between the sum of those values for the 7892

DOI: 10.1021/acs.inorgchem.7b00634 Inorg. Chem. 2017, 56, 7890−7899

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Inorganic Chemistry Scheme 2. Reaction Pathway Proposed for the Formation of 3

Scheme 3. Formation of 7, 7a, and 8

The viability of the reaction pathway in Scheme S1 was also assessed by calculating with the free-energy changes (ΔΔG) for intermediate B and 5 starting from 4 and BH3·THF (see Figure S2 in Supporting Information), leading to a 70.25 kcal mol−1 energy gain. Further confirmation of the reaction pathway proposed for the formation of 3 was achieved by reacting 1 with a dioxaborane, namely, pinacolborane, to preclude the formation of the eight-membered ring, thus promoting the isolation of a product with a structure similar to that of intermediate A in

Scheme 2. Indeed, the monometallic aluminum borate [{MeLAl{OB(pinacol)}(OH)] (7) (Scheme 3) was isolated in 43% yield, and [{MeLAl(OH)}(μ-O){MeLAl(H)}] was also identified as the other major product in the reaction mixture. Furthermore, considering that the latter stems from the transient [MeLAl(OH)(H)], the yield of 7 was increased (75%) by adding 1 equiv of H2O and an additional equivalent of pinacolborane in a two-stage reaction. Under these conditions once the initial 7 is formed, the in situ [MeLAl(OH)(H)] is transformed to the dihydroxide aluminum 7893

DOI: 10.1021/acs.inorgchem.7b00634 Inorg. Chem. 2017, 56, 7890−7899

Article

Inorganic Chemistry Table 1. Crystallographic Data and Refinement Details for Compounds 3−6 and 7a chemical formula formula weight space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z temp, K λ, Å μ, mm−1 ρcalc, g cm−3 R1 (I > 2σ(I))a wR2 (all data)b a

3

4·CH2Cl2

5

6

7a

C54H76Al2B2N4O6 952.76 P1̅ 10.4617(4) 12.3037(5) 12.3440(5) 113.2832(7) 100.9887(8) 103.2423(7) 1348.62(9) 1 100(2) 0.710 73 0.105 1.173 0.0318 0.0860

C51H69BCl2Ga2N4O4 1023.25 P1̅ 12.0610(2) 13.4194(2) 17.3269(3) 106.8270(10) 93.5000(10) 103.1060(10) 2590.1(1) 2 100(2) 0.710 73 2.582 1.312 0.0418 0.1120

C31H47B2GaN2O5 619.04 P21/n 10.2227(3) 17.1285(5) 18.3962(5) 90 97.4277(12) 90 3194.1(2) 4 100(2) 0.710 73 1.497 1.287 0.0303 0.0845

C46H60Al2B2N4O4 808.56 P21/n 11.6705(6) 11.0006(6) 18.0376(9) 90 98.5281(11) 90 2290.1(2) 2 100(2) 0.710 73 0.109 1.173 0.0422 0.1216

C58H84Al2B2N4O8 1040.87 P21/c 20.6579(6) 21.2971(6) 13.9347(4) 90 103.3111(6) 90 5965.9(3) 4 100(2) 0.710 73 0.103 1.159 0.0389 0.1107

R1 = ∑∥F0| − |Fc∥/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑(F02)2]1/2.

derivative [MeLAl(OH)2] that then reacts with pinacolborane to increase the yield of 7. It is noteworthy that, in solution, 7 associates producing the dimeric structure [{MeLAl(OB(pinacol))}2(μ-OH)2] (7a) (Scheme 3). However, the reaction of galloxane 2 with an excess of pinacolborane (Scheme 3) leads to the galloxane borate [{MeLGa(OB(pinacol))}2(μ-O)] (8) in 85% yield. Compound 8 represents the straightforward acid−base product, which maintains the Ga−O−Ga unit and thus supports the reactivity pattern also observed in the formation of 4, favoring the protonolysis reaction over the hydride-transfer reaction that leads to Ga−O−Ga cleavage. Moreover, to examine the influence of the donor molecule in BH3 complexes on the M−O−M cleavage and the THF ringopening reactions, 1 was reacted with BH3·SMe2 (2.0 M in THF) under similar reaction conditions as for 3, producing the eight-membered aluminoborane [{MeLAl(μ-O)(BH)(μ-O)}2] (6) without the presence of reductive-opening of THF moieties (Scheme 1). However, equimolar and excess molar-ratio reactions of BH3·SMe2 and 2 led to the isolation of the gallium dihydride MeLGaH2, albeit with no evidence of THF ringopening products, as shown by 1H NMR analysis of the crude reaction products. Overall, these results with BH3·SMe2 suggest that the M− O−M cleavage and the reductive ring-opening of THF are independent reactions, where the latter only takes place if THF is coordinated to the BH3 group. Furthermore, the cleavage of the M−O−M unit by the hydride transfer from the BH3 donor complex is facilitated by the formation of a strong B−O (787 kJ mol−1)17 bond in the resulting M−O−B metallaborate systems. However, conversely to the foregoing borane adducts, compounds 1 and 2 are unreactive toward the borane− ammonia complex BH3·NH3. Compounds 3−6 were obtained as moderately air-sensitive solids highly soluble in toluene, THF, and CH2Cl2 and moderately soluble in hexane and pentane. The high solubility of the compounds accounts in part for their low isolation yields (44−54%). It is noteworthy that during the crystallization of 6 in THF, adventitious polymerization of the latter was observed. Consequently, the polymerization reaction of THF was investigated with different concentrations of 6 in THF under

strict anhydrous conditions, albeit showing no evidence of the polymer formation. However, the addition of trace amounts of H2O to the above solutions was followed by the rapid polymerization of THF. This behavior is consistent with reports on the ability of aluminum compounds to act as catalysts for cationic ring-opening polymerization of THF when H2O is used as a cocatalyst.18 Structural and Spectroscopic Characterization of 3−8. The IR spectra of compounds 3−5, 7a, and 8 exhibit the characteristic pattern owing to the deprotonated β-diketiminate ligand. Additionally, compound 6 exhibits a medium-intensity band ascribed to the B−H vibration mode observed at ṽ 2393 cm−1, while 7a shows a band at ṽ 3676 cm−1 owing to the vibration of the AlO−H group. The 1H and 13C NMR spectra of 3, 5, and 6 show only one set of signals for the β-diketiminate ligand. In the cases of 4 and 8, their 1H NMR spectra show a pattern corresponding to the M−O−M spectroscopic signature, which is evidenced by the signal splitting of the o-Me groups and m-Ar hydrogen atoms of the ligand. Additionally, the spectra of compounds 3−5 exhibit the signals corresponding to methyl and methylene groups from the n-butoxide moieties. It is noteworthy that B−H group is not observed in the 1H NMR spectrum of 6, albeit IR spectroscopy and X-ray diffraction analysis unequivocally confirm its presence. Furthermore, the dimeric structure in 7a is evidenced by the presence of signal splitting due to the M−O−M unit, and the signal owing to the bridging μ-OH groups appear at δ 1.37 ppm. However, the monomeric structure of 7 shows only one signal for the o-Me groups and m-Ar hydrogen atoms from the ligand, and the nonassociated AlOH group is displayed at δ 0.58 ppm. The equilibrium is affected by the nature of the solvent used, C6D6 exhibits only the dimeric structure in solution, while in CDCl3 an equilibrium is reached within 2 h. In addition, the 11B NMR spectra of 3−6, 7a, and 8 exhibit broad single signals in the range of δ 17.5−30.2 ppm, which are in agreement with the structures proposed. The EI-MS of 3, 4, and 6 shows the peak at m/z 954, 938, and 807, respectively, corresponding to [M−H]+, while 5 exhibits a peak at m/z 618 owing to [M+−Me]. Compound 7a displays peaks at m/z 1123 and 520 corresponding to the [M− 7894

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Inorganic Chemistry OH]+ and [1/2M]+ fragments, respectively, whereas the EI-MS spectrum of 8 shows only peaks owing to fragments from the ligand. Nonetheless, the MS(ESI+) spectrum of 8 exhibits at m/ z 1109.35 the corresponding molecular ion [M+H]+. Single-crystal X-ray diffraction studies were performed for compounds 3−6 and 7a that crystallized from their saturated toluene (3, 5, and 6) and CH2Cl2 (4 and 7a) solutions at 25 °C. The crystal data and refinement details for these compounds are listed in Table 1, and selected bond lengths and angles for these complexes are presented in Tables 2 and 3. In compounds 3−6, the metal center shows a slight distorted tetrahedral geometry, while the boron atom exhibits a planar trigonal geometry.

Table 3. Selected Bond Distances (Å) and Angles (deg) for Compounds 5, 6, and 7a Ga(1)−O(1) Ga(1)−O(2) Ga(2)−O(1) Ga(2)−O(3) B(1)−O(2) B(1)−O(3) B(1)−O(4)

Ga(1)−O(1) Ga(1)−O(3) B(1)−O(1) B(2)−O(3) B(1)−O(2) B(2)−O(2) B(1)−O(4) B(2)−O(5)

Table 2. Selected Bond Distances (Å) and Angles (deg) for Compounds 3 and 6 Al(1)−O(1) Al(1)−O(3A) B(1)−O(1) B(1)−O(2) B(1)−O(3) Al(1)−N(1) Al(1)−N(2)

1.693(1) 1.720(1) 1.347(2) 1.385(2) 1.357(2) 1.877(1) 1.881(1)

Al(1)−O(1) Al(1)−O(2) B(1)−H(1) B(1)−O(1) B(1)−O(2A) Al(1)−N(1) Al(1)−N(2)

1.709(1) 1.726(1) 1.15(2) 1.342(2) 1.349(2) 1.877(1) 1.877(1)

compound 3 O(1)−Al(1)−O(3A) O(1)−B(1)−O(2) O(2)−B(1)−O(3) O(1)−B(1)−O(3) B(1)−O(1)−Al(1) B(1)−O(3)−Al(1A) N(1)−Al(1)−N(2) compound 6 O(1)−Al(1)−O(2) O(1)−B(1)−O(2A) O(1)−B(1)−H(1) O(2A)−B(1)−H(1) B(1)−O(1)−Al(1) B(1A)−O(2)−Al(1) N(1)−Al(1)−N(2)

117.1(1) 115.3(1) 119.9(1) 124.8(1) 162.3(1) 131.8(1) 97.2(1)

Al(1)−O(1) Al(1)−O(2) Al(1)−O(3) Al(2)−O(1) Al(2)−O(2) Al(2)−O(6) Al(1)···Al(1) O(1)−H(1) O(2)−H(2)

118.3(1) 124.7(2) 118.4(10) 116.9(10) 156.1(1) 132.0(1) 97.8(1)

The molecular structures of 3 and 6 (Figures 1 and 2) show a three-fused ring system sharing the metal atoms. The bimetallic eight-membered Al2B2O4 ring exhibits a nonplanar conformation with a mean deviation from the plane (Δ) of 0.12 Å for 3 and 0.10 Å for 6. The Al−O bond distances in 3 [1.693(1) and 1.720(1) Å] and 6 [1.709(1) and 1.726(1) Å] are shorter that those in the cyclic alumoboroxine [tBu2Al(O)BAr(OLi)]2 [1.744(2) and 1.821(2) Å],2 but they are comparable with those in [ iPr LAlO 2 B(3-OHCH 2 CH 6 H 4 )] 2 [ iPr L = HC{CMeNAr}2−, Ar = 2,6-iPr2C6H3] [1.700(3) and 1.726(3) Å]5c and in [tBu2AlOB(o-CH2O)(C6H4)]2 [1.727(1) Å].3b The B−O(endocyclic) distances [1.342(2)−1.357(2) Å] in 3 and 6 are within the ranges expected for the eight-membered Al−O−B compounds [ t Bu 2 Al(O)BAr(OLi)] 2 [1.319(3) Å] 2 and [iPrLAlO2B(3-OHCH2CH6H4)]2 [1.347(6) and 1.359(6) Å]19 but shorter than the B−O(exocyclic) bond in 3 [1.385(2) Å]. The O−Al−O and O−B−O(endocyclic) angles in 3 [117.1(1) and 124.8(1)°] and 6 [118.3(1) and 124.7(2)°] are comparable to those found in [ t Bu 2 Al(O)BAr(OLi)] 2 [117.2(1) and 123.7(2)°]2 and [{iPrLAlO2B(3-OHCH2CH6H4)}2] [115.0(1) and 124.0(5)°].5c The B−H bond distance [1.15(2) Å] in 6 is comparable with those observed in the trigonal-boron compounds: HBO2C6H2O2BH [1.100 Å]19 and [ZnCl3(HBcatechol)]2(μ-Cl)2 [1.054 Å].20 The molecular structures of 4 and 5 feature spirocyclic arrangements with three and two fused six-membered rings, respectively (Figures 3 and 4). The gallium-borate rings Ga2BO2 (4) and GaB2O3 (5) both exhibit planar arrangements with a mean deviation from the plane (Δ) corresponding to

compound 4 1.792(2) Ga(1)−O(1)−Ga(2) 1.827(2) O(1)−Ga(1)−O(2) 1.805(2) O(1)−Ga(2)−O(3) 1.824(2) O(2)−B(1)−O(3) 1.359(4) O(3)−B(1)−O(4) 1.359(4) O(2)−B(1)−O(4) 1.386(4) N(1)−Ga(1)−N(2) N(3)−Ga(2)−N(4) compound 5 1.832(1) O(1)−Ga(1)−O(3) 1.829(1) O(1)−B(1)−O(2) 1.345(2) O(2)−B(2)−O(3) 1.346(2) B(1)−O(1)−B(2) 1.391(2) Ga(1)−O(1)−B(1) 1.387(2) Ga(1)−O(3)−B(2) 1.365(2) N(1)−Ga(1)−N(2) 1.379(3) compound 7a 1.840(1) Al(1)−O(1)−Al(2) 1.874(1) Al(1)−O(2)−Al(2) 1.733(1) O(1)−Al(1)−O(2) 1.873(1) N(1)−Al(1)−N(2) 1.841(1) N(1)−Al(1)−O(2) 1.734(1) N(1)−Al(1)−O(1) 2.960(1) O(1)−Al(2)−N(2) 0.82(2) N(3)−Al(2)−N(4) 0.83(2) N(3)−Al(2)−O(2) N(4)−Al(2)−O(1)

115.6(1) 112.4(1) 111.9(1) 125.6(2) 120.7(2) 113.7(2) 96.5(1) 96.3(1) 103.3(1) 124.4(2) 124.4(2) 127.0(1) 119.9(1) 119.9(1) 99.4(1)

105.7(1) 105.7(1) 74.1(1) 90.7(1) 153.9(1) 90.6(1) 146.3(1) 90.9(1) 144.7(1) 155.5(1)

Figure 1. Molecular structure of 3 with thermal ellipsoids at 30% probability. Hydrogen atoms and solvent molecules are omitted for clarity.

0.04 Å. The dihedral angle between the planes of the GaB2O3 and the GaN2C3 rings in 5 corresponds to 88.3°, while those between the plane of Ga2BO3 and the GaN2C3 rings are 87.6 and 89.4°. In the molecular structure of 4, the Ga−(μ-O) bond distances [1.827(2) and 1.824(2) Å] are comparable to those in the hetero-bimetallic galloxanes [{(MeLGa)2(μ-O)}(μO)2{M(NEt2)2}] (M = Zr, Hf) [1.825(2) and 1.833(2) Å]8 7895

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Figure 2. Molecular structure of 6 with thermal ellipsoids at 30% probability. Hydrogen atoms (except those from the BH group) and solvent molecules are omitted for clarity.

Figure 4. Molecular structure of 5 with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity.

[1.907(1) and 1.914(1) Å], which are shorter than those found in 4 [1.936(2)−1.945(2) Å]. Compound 7a exhibits a dimeric-type of structure achieved by means of two μ-OH groups. The Al atoms are fivecoordinated exhibiting a square-based pyramidal geometry. The basal positions are occupied by two nitrogen and the two bridging oxygen atoms, while the apical position is occupied by the oxygen atom from the borate group (Figure 5).

Figure 3. Molecular structure of 4, hydrogen atoms are omitted for clarity. Thermal ellipsoids are set at 30% probability.

and shorter than those in the galloborate [GaB5O8(OH)2(en)2· H2O] (en = ethylenediamine) [1.924(1)−1.941(1) Å],1b where the gallium center has a higher coordination number. The O− B−O angle [125.6(2)°] is more obtuse than the O−M−O angle in [{(MeLGa)2(μ-O)}(μ-O)2{M(NEt2)2}] (M = Zr, Hf) [103.4(1) and 104.1(1)°] due to the smaller size of the boron atom compared to the zirconium and hafnium atoms. The Ga−O bond distances in 5 [1.829(1) and 1.832(1) Å], as well as those for the B−O bond lengths [1.345(2)−1.391(2) Å], are within the expected ranges. The B−O−B unit has an angle of 127.0(1)° and is comparable with those from the aluminum benzoboroxol oxides [ iPr LAl{(OBPh) 2 O}] [122.4(7)°]5a and [iPrLAl(OBAr)2(μ-O)] (Ar = 3-CH3C6H4, 3-FC6H4) [125.7(2)°].5b The substitution of a MeLGa unit by a B(OnBu) group in 5 has an effect on the Ga−N bond lengths

Figure 5. Molecular structure of 7a where the hydrogen atoms (except those for the AlOH group) are omitted for clarity. Thermal ellipsoids are set at 30% probability.

The two borate groups are in an anti conformation with a dihedral angle between O(3)−Al(1)···Al(2) and Al(1)···Al(2)− O(6) planes corresponding to 179°. The Al(1)···Al(2) separation (2.960(1) Å) is significantly longer than the sum of the covalent radii for the aluminum atoms [∑rcov(Al, Al) 2.42 Å].21 The Al−O bond distances of the bridging oxygen 7896

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Figure 6. Structures for the hypothetical compounds 3′−5′.

hypothetical compound 3′, while the Ga/Al substitution for compounds 4 and 5 yields the more stable compounds 4′ and 5′, respectively. Overall, the eight-membered aluminum borate 3 and the hypothetical six-membered ring systems 4′ and 5′ are significantly more stable than their gallium analogues. This leads to conflicting results regarding the exclusive formation of eight-membered ring systems for aluminum borates and sixmembered rings for gallium borate derivatives when our synthetic approach is used. However, it is noteworthy that the six-membered aluminum boroloxides [iPrLAl{OB(C5H9)}2(μO)] (iPrL = CH{CMe(NAr)}2−, Ar = 2, 6-iPr2C6H3)5c and [iPrLAl{OB(3-XC6H4)}2(μ-O)] (X = H, F, CH3),5a,b as well as with a Al2BO3 ring in [(iPrLAlO)2B(2,6-OCH3C6H4)(μ-O)],22 are accessible from the reaction of aluminum hydrides or alkyls and boronic or borinic acids but not from aluminum hydroxides and boron hydrides as studied in this work. On the one hand, the latter can be explained in terms of the reactivity of Al−O− Al moiety in the presence of BH3 complexes leading to the cleavage of Al−O bonds and the consequent formation of eight-membered ring systems. On the other hand, the exclusive formation of six-membered ring for gallium borates is a direct consequence of the reaction mechanism, which precludes the formation of eight-membered rings, thus discarding reasons related to the stability of the products. Wiberg bond indexes were also calculated for all n/n′ pairs, and the analysis of the bond orders were performed (Tables S1 and S2, Supporting Information). As anticipated, in all compounds the bonds around the gallium atoms are more covalent that those in the aluminum derivatives supporting the higher stability of the Ga−O−Ga over the Al−O−Al moiety. Additionally, the frontier orbitals for the n/n′ pairs exhibit identical HOMO and LUMO plots (Figures S3−S5, Supporting Information)

atoms are slightly asymmetric 1.840(1) and 1.874(1) Å for Al(1), and 1.841(1) and 1.873(1) Å for Al(2). In contrast to the eight-membered ring scaffolds observed in the aluminum borates in 3 and 6, gallium borates 4 and 5 exhibit exclusively six-membered rings, regardless of the reaction conditions. The inorganic cores in 4 and 5 are unprecedented for molecular structures of gallium boron compounds, and only one example featuring a gallium borate material is known to have such arrangements.1b,4 In this regard, while eight-membered rings are a common feature in the scaffold of gallium phosphates materials, these structural arrangements are unknown in molecular gallium boron compounds and in their corresponding materials. Theoretical Calculations. Density functional theory (DFT) calculations were performed with the aim of clarifying if the preference in the formation of an eight-membered ring system for the aluminum borate 3 and six-membered rings for the gallium borates 4 and 5 are due to an energetic component. Upon optimization, the metallic elements in all three compounds were switched by the corresponding element yielding the hypothetical compounds 3′, 4′, and 5′ (Figure 6). A second geometry optimization under the same conditions as before was performed on them to account for any distortions on the rings. However, no significant changes were observed given the fact that the covalent radii of aluminum (1.21 Å) and gallium (1.22 Å) are similar.21 A single point calculation at the same level of theory was performed for the isolated Al and Ga atoms for a reference. An energy comparison between the existent (n) and nonexisting (n′) compounds was performed according to expression (1): ΔE = E(MnBxOy) − nE M + nE M′ − E(M ′ nBxOy)

(1)

An ideal or seamless substitution would be implied by ΔE = 0 and would be interpreted as the substitution of one atom by the other as having no effect in the overall energy of compound n. Consequently, negative values of ΔE indicate that the existing n compound is energetically favored over the nonexisting n′, while the opposite is true for a positive ΔE value. The ΔE values for the three pairs of compounds n/n′ are collected in Table 4. The ΔE negative value obtained from the Al/Ga substitution indicates that 3 is 81 kcal mol−1 more stable than the

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CONCLUSIONS The higher covalent character of the Ga−O−Ga unit compared to the aluminum analogue together with the higher acidity of the Ga−OH groups favors the protonolysis reaction over the hydride-transfer reaction, which accounts for the difference between the structural outcome of compounds 4 and 8 for gallium against the compounds 3, 6, and 7 for aluminum. However, galloxane borate 4 showed a similar behavior to 1 for hydride-transfer reactions leading to 5 from the Ga−O−Ga cleavage. The concomitant THF ring-opening reaction was observed when this Lewis base was coordinated to BH3 and the borane complex reacted to form M−O−B units. Finally, the unique structures of the galloxane borate 4 and gallium borate 5 provide information about the structure of gallium borates at a molecular level.

Table 4. Calculated Energy Difference between the Existing 3−5 and the Hypothetical Structures of 3′−5′ compared compounds

ΔE (kcal mol−1)

3/3′ 4/4′ 5/5′

−81.38 40.61 70.98 7897

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(c) Anulewicz-Ostrowska, R.; Lulinski, S.; Serwatowski, J. Synthesis and Characterization of Dialkylmetal Boryloxides [(μ-9-BBN-9O)MMe2]2, M = Al, Ga, In. Inorg. Chem. 1999, 38, 3796−3800. (4) Cheng, L.; Yang, G.-Y. A Series of Novel Gallium and Indium Borates Constructed from [B5O8(OH)2]3− Clusters and Metal Complex Linkers. J. Solid State Chem. 2013, 198, 87−92. (5) (a) Yang, Z.; Ma, X.; Oswald, R. B.; Roesky, H. W.; Noltemeyer, M. Synthesis of an Aluminum Spirocyclic Hybrid with an Inorganic B2O3 and an Organic C3N2 Core. J. Am. Chem. Soc. 2006, 128, 12406− 12407. (b) Ma, X.; Yang, Z.; Wang, X.; Roesky, H. W.; Wu, F.; Zhu, H. Synthesis of Boroxine-Linked Aluminum Complexes. Inorg. Chem. 2011, 50, 2010−2014. (c) Yang, Z.; Hao, P.; Liu, Z.; Ma, X.; Roesky, H. W.; Li, J. Synthesis of an eight-membered Al2B2O4−Ring with two aluminum atoms each in a spiro center. J. Organomet. Chem. 2014, 751, 788−791. (6) (a) Synoradzki, L.; Bolesławski, M.; Lewínski, J. Reaction of diethylhydroxyborane with trialkylaluminium. J. Organomet. Chem. 1985, 284, 1−4. (b) Anulewicz-Ostrowska, R.; Lulinski, S.; Serwatowski, J.; Suwinska, K. Diverse Reactivity of Dialkylaluminum Dimesitylboryloxides [(μ-Mes2BO)AlR2]2. Synthetic and Structural Study. Inorg. Chem. 2000, 39, 5763−5767. (c) Gibson, B. C.; Mastroianni, S.; White, A. J. P.; Williams, D. J. Formation and Unexpected Catalytic Reactivity of Organoaluminum Boryloxides. Inorg. Chem. 2001, 40, 826−827. (7) Hidalgo-Bonilla, S.; Peyrot, R.; Jancik, V.; Barroso-Flores, J.; Reyes-Lezama, M.; Moya-Cabrera, M. Molecular Heterobimetallic Aluminoxanes and Aluminoxane Sulfides Containing Group 4 Metals. Eur. J. Inorg. Chem. 2013, 2013, 2849−2857. (8) Bernabé-Pablo, E.; Jancik, V.; Moya-Cabrera, M. A Synthetic Route to a Molecular Galloxane Dihydroxide and its Group 4 Heterobimetallic Compounds. Inorg. Chem. 2013, 52, 6944−6950. (9) González-Gallardo, S.; Jancik, V.; Cea-Olivares, R.; Toscano, R. A.; Moya-Cabrera, M. Preparation of Molecular Alumoxane Hydrides, Hydroxides, and Hydrogensulfides. Angew. Chem., Int. Ed. 2007, 46, 2895−2898. (10) SAINT and SADABS; Bruker AXS Inc.: Madison, WI, 2007. (11) SHELX: Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (12) SHELXLE: Hübschle, C. B.; Sheldrick, G. M.; Dittrich, B. ShelXle: a Qt Graphical User Interface for SHELXL. J. Appl. Crystallogr. 2011, 44, 1281−1284. (13) Frisch, M. J.; et al. Gaussian 09, revision D.01; for the full reference, see the Supporting Information. (14) Wiberg, K. B. Application of the Pople-santry-segal CNDO Method to the Cyclopropylcarbinyl and Cyclobutyl Cation and to Bicyclobutane. Tetrahedron 1968, 24, 1083−1096. (15) (a) Kollonitsch, J. Reductive Ring-cleavage of Tetrahydrofurans by Diborane. J. Am. Chem. Soc. 1961, 83, 1515. (b) Brown, H. C.; Weissman, P. M. Selective Reductions. VII. Reaction of Lithium Trimethoxyaluminohydride with Selected Organic Compounds Containing Representative Functional Groups. J. Am. Chem. Soc. 1965, 87, 5614−5620. (c) Brown, H. B.; Krishnamurthy, S.; Coleman, R. A. Remarkably Facile Reductive Opening of Tetrahydrofuran and Related Ethers by Lithium tri-tert-butoxyaluminohydride in the Presence of Triethylborane. J. Am. Chem. Soc. 1972, 94, 1750−1751. (16) González-Gallardo, S.; Jancik, V.; Delgado-Robles, A.; MoyaCabrera, M. Cyclic Alumosiloxanes and Alumosilicates: Exemplifying the Loewenstein Rule at the Molecular Level. Inorg. Chem. 2011, 50, 4226−4228. (17) National Standard Reference Data Series, No. 31; National Bureau of Standards: Washington, DC, 1970. (18) (a) Meerwein, H.; Delfs, D.; Morschel, H. Die Polymerisation des Tetrahydrofurans. Angew. Chem. 1960, 72, 927−934. (b) Furukawa, J. Ionic Polymerization of Polar Monomers. Polymer 1962, 3, 487− 509. (19) Aldridge, S.; Calder, R. J.; Rossin, A.; Dickinson, A. A.; Willock, D. J.; Jones, C. M.; Evans, D. J.; Steed, J. W.; Light, M. E.; Coles, S. J.; Hursthouse, M. B. Linking of metal centres through boryl ligands: synthesis, spectroscopic and structural characterisation of symmetri-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00634. ΔΔG energy diagrams, illustrated reaction pathway, Wiberg bond indexes, HOMO and LUMO frontier molecular orbital plots (PDF) Accession Codes

CCDC 1552673−1552677 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

Vojtech Jancik: 0000-0002-1007-1764 Joaquín Barroso-Flores: 0000-0003-0554-7569 Mónica Moya-Cabrera: 0000-0002-5531-7799 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 The authors acknowledge the CONACyT (0220625) for financial support, and E.B.-P. thanks the CONACyT for the Ph.D. fellowship (227105). S. P. Hidalgo is acknowledged for the exploratory work, and A. Núñez, L. Triana, and N. Zavala are acknowledged for their technical assistance.

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DOI: 10.1021/acs.inorgchem.7b00634 Inorg. Chem. 2017, 56, 7890−7899

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DOI: 10.1021/acs.inorgchem.7b00634 Inorg. Chem. 2017, 56, 7890−7899