Gargi
Kundu
ab,
P. R.
Amrutha
a,
K. Vipin
Raj
bc,
Srinu
Tothadi
d,
Kumar
Vanka
bc and
Sakya S.
Sen
*ab
aInorganic Chemistry and Catalysis Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pashan, Pune 411008, India. E-mail: ss.sen@ncl.res.in
bAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
cPhysical and Material Chemistry Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pashan, Pune 411008, India
dAnalytical and Environmental Sciences Division and Centralized Instrumentation Facility, CSIR-Central Salt and Marine Chemicals Research Institute, Gijubhai Badheka Marg, Bhavnagar-364002, India
First published on 26th April 2023
Despite recent advancements in the chemistry of multiply bound boron compounds, the laboratory isolation of the parent oxoborane moiety, HBO has long remained an unsolved and well-recognized challenge. The reaction of 6-SIDipp·BH3 [6-SIDipp = 1,3-di(2,6-diisopropylphenyl)tetrahydropyrimidine-2-ylidene] with GaCl3 afforded an unusual boron–gallium 3c–2e compound (1). The addition of water to 1 resulted in the release of H2 and the formation of a rare acid stabilized neutral parent oxoborane, LB(H)O (2). Crystallographic and density functional theory (DFT) analyses support the presence of a terminal BO double bond. Subsequent addition of another equivalent of water molecule led to hydrolysis of the B–H bond to the B–OH bond, but the ‘BO’ moiety remained intact, resulting in the formation of the hydroxy oxoborane compound (3), which can be classified as a monomeric form of metaboric acid.
Scheme 1 Examples of previously reported oxoborane and a compound with a B–O triple bond. The parent oxoborane is still elusive. |
Recently, a wide range of N-heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs) have been utilized to form stable adducts with BH3, thanks to the initial works of Robinson13 and later from the groups of Fensterbank, Lacôte, Malacria, Curran, Braunschweig, and others.14,15 This has resulted in applications in the borylation of aromatic or aliphatic C–H bonds, the hydroboration of alkenes, and many other organic transformation.16 Beginning of the last year, we became interested in employing 6-SIDipp as a ligand in borane chemistry. We demonstrated substitution reactions of 6-SIDipp·BH3, ring expansion with 6-SIDipp·9BBN, and activation of the B–H bond of HBpin with 6-SIDipp.17–19 These studies have indicated that 6-SNHC has a notably higher donating capacity than 5-NHC. Our hypothesis was that by incorporating the 6-SNHC framework, along with bulky Dipp groups, and using a Lewis acid, we could potentially achieve the isolation of a single, parent oxoborane molecule.
Scheme 2 Diverse reactivity of NHC·BH3. The upper scheme describes the generation of 5-IDipp·BCl2 cation. The lower scheme shows the synthesis of boron–gallium 3c–2e complex (1). |
In order to investigate the electronic structure of compound 1, density functional theory (DFT) calculations were performed. The B3LYP functional and the def2-TZVP basis set have been employed in the calculations (please see the ESI† for further details). The NBO analysis shows that there are two significant electron donations from the BD orbital (which denotes a bonding orbital) of the B1–H2 bond to the vacant s and p orbitals of gallium in 1, with stabilization energies of 24.3 kcal mol−1 and 27.6 kcal mol−1 (see Fig. S17†). Similarly, the BD orbital of the second B–H bond (which is identically denoted as B1–H2 in Fig. 2) also shows two significant electron donations to the vacant s and p orbitals of gallium, with identical (to that of the first B–H bond) stabilization energies of 24.3 kcal mol−1 and 27.6 kcal mol−1 (see Fig. S17†). It should be noted that the NBO analysis in this case resulted in a misinterpretation of the nature of the Ga–Cl bonds as ionic instead of covalent, due to the limitations in NBO in accurately locating polar bonds. Conceptually, the B–H bonding orbitals can donate electron density to either an initially vacant p orbital located on Ga or to a Ga–Cl σ* antibonding orbital, in which the lobe is predominantly located on Ga. This corroborates the experimental observation of two identical B–H bonds and also suggests a 3c–2e bond character in B–H–Ga interactions.
Boranes have been identified as promising candidates for facilitating hydrogen release from water, as documented in several early microwave spectroscopic studies, and theoretical calculations.26 It is known that BH3 reacts with H2O to give B(OH)3 with concomitant dihydrogen liberation. Recently, Kraka and co-workers investigated the mechanism of H2 release from BH3 in water and demonstrated that one H2O molecule interacting with BH3 led to an activation energy of 24.9 kcal mol−1.27
We have reacted 1 with 1.1 equivalent of H2O, which resulted in the formation of 2 with simultaneous elimination of dihydrogen (Scheme 3). 2 is a heterodinuclear compound that can be considered as the acid-stabilized parent oxoborane containing an unusual H(BO) linkage. The elimination of dihydrogen was detected using NMR spectroscopy and GC-MS. The amount of simultaneous hydrogen production is quite large (2.1 mmol h−1 g−1). Most importantly, the NHC moiety remained intact during the reaction, which was not the case for 5-IDipp·BH2I and 5-IDipp·BH2OTf as their reactions with water (or methanol) resulted in fast hydrolysis (or methanolysis) to the corresponding imidazolium salts.28 The 11B{1H} NMR spectrum of 2 displays a broad signal at 21.6 ppm for the BO moiety, which falls in the range of 19.7–32.3 ppm, typically found in oxoboranes. The characteristic terminal ν(B–O) stretch frequency appeared at 1665 cm−1 for 2, which is in good agreement with the previously reported oxoboranes (1558–1646 cm−1).5,6
Scheme 3 Stepwise synthesis of -hydrido and -hydroxy oxoboranes, 2 and 3 from the reaction of 1 with water. |
Attempts to crystallize 2 resulted in two types of colorless single crystals: one is block shaped, and the other one is needle shaped. While the needle shaped crystals belong to 2, the block shaped crystals account for 2·GaCl3. Though the constitution and the connectivity of 2 can be unambiguously established from the single crystal X-ray data (Fig. S16, ESI†), we refrain from a discussion of bonding parameters because of the low quality of the data. The molecular structure of 2·GaCl3 is displayed in Fig. 2. One of the GaCl3 molecule cocrystallizes with 2. The main feature of 2·GaCl3 is that the central boron atom has bonded to one terminal hydrogen atom and oxygen atom which is connected to the gallium center. The boron atom exhibits a highly distorted trigonal planar coordination site with the carbon atom of NHC, the hydride ligand, and the (μ-O) atom. The gallium atom has a distorted tetrahedral coordination sphere comprised of the (μ-O) atom and the three chloride ligands. The B–H [1.09 (3) Å] and B–C [1.621(4) Å] bond lengths compare well with their typical bond lengths. It is noted that the BO distance in 2·GaCl3 [1.307(3) Å] is in good agreement with the reported acid stabilized oxoboranes,3–6 slightly longer compared to Aldridge's acid free anionic oxoboranes [1.256(3) Å, 1.273(8) Å and 1.287 (2) Å],7 and substantially longer than Braunschweig's BO triply bound complex.11 The Ga–O bond length in 2·GaCl3 [1.807(2) Å] is in good agreement with that in Roesky's Ga(Me)–O–Bi linkage [1.815(13) Å].29
Having found that water undergoes facile dehydrocoupling with the bridging hydride of 1, it seemed plausible that further addition of H2O may lead to the release of another molecule of H2 and convert the terminal B–H bond to a B–OH bond to generate a (hydroxy)oxoborane. Gratifyingly, the reaction of 2 with another molecule of H2O led to the formation of 3 containing a B(OH)O–GaCl3 motif with concomitant elimination of H2 (Scheme 3). The NHC moiety and the (BO)–GaCl3 remained intact, while the B–H bond underwent hydrolysis with H2O to generate the terminal B–OH bond. 3 possesses a rare unit of [B(OH)O], and it is the second example of a monomeric boracarboxylic acid complex that has been isolated, followed by Rivard's discovery of D.5 The [B(OH)O] unit is transient and can only be observed in the gas phase during the thermal conversion of orthoboric acid (B(OH)3) to metaboric acid [(HOBO)3].30 The boron atom resonates at 23.6 ppm. The solid-state structure of 3 is displayed in Fig. 3. The terminal B–O bond length [1.334(10) Å] in 3 is very similar with the B–O double bond [1.320(9)] Å as they overlap within three times of estimated standard deviation. For a note, the B–O double bond in 3 is also comparable to that in 2 [1.307(3) Å]. The characteristic terminal ν(BO) stretching frequency of 3 appeared at 1595 cm−1.
To investigate the electronic structure of 2 and 3, density functional theory (DFT) calculations were performed. The B3LYP functional and the def2-TZVP basis set have been employed in the calculations (see the ESI†for further details). The Wiberg Bond Index (WBI) of the B–O bond in 2 was determined to be 1.19. Moreover, the natural population analysis (NPA) charges on B and O were found to be +0.77 and −1.04, respectively, indicating a multiple bond character of the B–O bond, which is polarized towards oxygen. Similarly, the Wiberg Bond Index (WBI) of the B–O bond in compound 3 is 1.09, which is slightly lower than that of compound 2, indicating a weaker B–O bond in compound 3. The natural population analysis (NPA) charges on B and O were determined to be +1.08 and −1.06, respectively. The higher positive charge on the boron atom in compound 3 is likely due to its bonding with two oxygen atoms, in contrast to one oxygen and one hydrogen in compound 2.
Furthermore, the NBO analysis shows a significant electron donation from the LP orbital (which denotes a lone pair orbital) of oxygen to the vacant p orbital of boron in 2, with a stabilization energy of 85.4 kcal mol−1 (see Fig. 4). This indicates significant B–O π bonding. In the case of 3, the corresponding interaction was weaker, with a stabilization energy of 66.1 kcal mol−1 (see Fig. S18†), indicating a weaker π bonding interaction between boron and oxygen in 3 compared to 2.
Fig. 4 The NBO plot of the desired intramolecular donor–acceptor interactions (LP orbital of oxygen to vacant p orbital of boron) in 2. |
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, spectroscopic data, theoretical calculations, and representative NMR spectra. CCDC 2234583 (1), 2234584 (2) and 2234585 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc01544k |
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