Unlocking metal coordination of diborylamides through ring constraints†

W. Kice Brown , Kevin K. Klausmeyer and Brian M. Lindley *
Department of Chemistry and Biochemistry, Baylor University, Waco, Texas 76798, USA. E-mail: brian_lindley@baylor.edu

First published on 16th December 2021

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Imposing cyclic structures on main group fragments can dramatically alter the bonding properties and reactivity of these functionalities. This strategy is particularly salient in the development of cyclic allenes. In contrast to the familiar allene structure with adjacent carbon–carbon double bonds, cyclic allenes feature a dominant zwitterionic resonance structure with a dianionic central carbon and C–C single bonds (Scheme 1, species A).^1,2 This perturbation in the bonding properties of cyclic allenes is manifest in their reactivity with proton sources and transition metals. Cyclic allenes can undergo double protonation of the central carbon and are among the most strongly donating ligands.^1,2 Cyclic allene complexes of Rh, Ru, Fe, and U have been reported, with Stephan and coworkers demonstrating high catalytic activity of Ru cyclic allene complexes for olefin hydrogenation.^1–6 Applying similar cyclic constraints to other main group motifs could unlock new classes of ligands for transition-metal-catalyzed reactions.

The diborylamide (⁻N(BR₂)₂) functionality, which is isoelectronic and isostructural with allene, is an attractive candidate for exploring the effects of ring constraints on their bonding properties. Nöth and Power previously showed that deprotonation of tetra-tert-butyl and tetramesityl diborylamines resulted in a significant contraction of the B–N bonds of nearly 0.1 Å, consistent with B–N double bonds.^7,8 These lithium diborylamide compounds adopt linear heteroallene structures, with no nitrogen lone pairs available for metal coordination. Cyclic diborylamides are expected to favor a resonance structure with fewer formal charges than their acyclic analogs, with lone pairs and a formal 1⁻ charge on nitrogen (Scheme 1, B). These properties should enable cyclic diborylamides to coordinate strongly with transition metals, allowing for the synthesis of the first metal diborylamide complexes.

Cyclic diborylamides would represent a new addition to the growing scope of amide ligands.⁹ The electronic and steric properties of amides can be readily tuned by installing different aryl, alkyl, and silyl substituents, but the effect of boron substitution remains underexplored. In the late 1980s, Power and coworkers developed monoboryl arylamide ligands.^10,11 These ligands were found to have strong N–B π-bonding interactions, which effectively precluded these borylamides from serving as bridging ligands and thus allowed for the synthesis of remarkable 2-coordinate 1st-row transition metal complexes. Compared to monoborylamides, cyclic diborylamides are expected to be even stronger σ-donors due to the σ-inductive effects imparted by the additional boron substituent. These properties may make diborylamides useful ligands for future catalytic applications.

In this study, we report the synthesis of 5-membered cyclic diborylamine and diborylamide compounds. We characterize these compounds using single-crystal X-ray diffraction and compare them to the previously reported acyclic compounds. We also report the synthesis of the first Fe diborylamide complexes.

Given the precedent for synthetic elaboration of 1,2-bis(dichloroboryl)benzene (1) using amine and aryl nucleophiles,^12–16 we set out to synthesize diborylamide complexes based on the 1,2-diborylphenylene core. We targeted cyclic diborylamide compounds with aryl substituents to allow for meaningful structural comparisons to the acyclic tetramesityl diborylamide compound, [Li(OEt₂)₃][N(BMes₂)₂].⁸ We chose bulky 2,4,6-trimethylphenyl (mesityl, Mes) substituents to limit potential complications from Lewis base coordination to boron.

Following the synthetic protocols of Kaufmann and Wegner,^12,16 we were able to generate compound 1 on multigram scale and in sufficient purity (>90%) for subsequent reactions. Treatment of 1 with 2.2 equiv. MesLi at 40 °C in toluene for 20 h gave the crude product as a tan solid after removal of LiCl. Washing the crude material with pentane afforded the desired 1,2-bis(chloromesitylboryl)benzene (2) as a white solid in 55% yield (Scheme 2). Compound 2 is sparingly soluble in pentane but highly soluble in benzene and toluene. Characterization of 2 by ¹¹B NMR spectroscopy revealed a broad singlet at 66.4 ppm, in the usual range for chlorodiarylboranes.¹⁷ The ¹H NMR spectrum of 2 features a diagnostic AA′XX′ splitting pattern for the 1,2-diborylphenylene backbone, consistent with the expected C_2v molecular symmetry.

A mixture of 2 and NH₃ (3 equiv., 0.5 M in 1,4-dioxane) was stirred for 2 h at 20 °C in toluene solvent, followed by heating at 85 °C for 16 h to give a cloudy, colorless mixture. After removal of the volatiles in vacuo, the diborylamine 3 was extracted with pentane to remove NH₄Cl. Compound 3 was obtained as a sticky white solid in 80% yield. The ¹H NMR spectrum of 3 features an AA′XX′ splitting for the phenylene backbone as well as a broad singlet at 5.26 ppm, assigned to the diborylamine NH. An upfield shift in the ¹¹B NMR resonance from 66.4 for 2 to 55.6 ppm for 3 is consistent with substitution of chloride for amine. The structure of 3 was verified by single-crystal X-ray diffraction (Fig. 1). The boron-nitrogen bond distances of 1.437(2) and 1.426(2) Å are in line with the previously reported acyclic tetramesityl diborylamine, HN(BMes₂)₂ (d_B–N = 1.435 Å), and a 6-membered cyclic diborylamine recently reported by Wagner and coworkers (d_B–N = 1.425(4), 1.427(4) Å).^8,18

Deprotonation of diborylamine 3 was carried out by treating 3 with ⁿBuLi in Et₂O at −35 °C for 3 h. After removal of volatiles in vacuo, the product (4) was isolated as a white solid in 63% yield by washing the crude product with pentane. X-ray diffraction quality crystals of 4 were grown by slow evaporation of a pentane/Et₂O solution. Diborylamide 4 adopts a C₂-symmetric dimer structure with a Li₂N₂ core, as commonly observed for lithium amides in the solution and solid state (Fig. 1).¹⁹ Despite crystallization in the presence of Et₂O, 4 is unsolvated, though one of the Li atoms has weak interactions with two mesityl ipso carbons (d_Li–C = 2.427 Å). The Li–N bond distances of 1.987(3) and 1.990(3) Å are on the shorter end for lithium amides (d_Li–N ∼ 1.96–2.1 Å), indicative of strong Li–N bonding. The B–N distances in diborylamide 4 and diborylamine 3 are virtually identical, with B–N bond lengths of 1.432(2) and 1.436(2) Å (d_B–N(av) = 1.434) for 4 compared to 1.432(2) Å for 3.

A structural comparison of compounds 3 and 4 to acyclic diborylamine and diborylamide compounds, HN(BMes₂)₂ and [Li(OEt₂)₃][N(BMes₂)₂], is informative. Ring constraints result in significantly reduced B–N–B bond angles of 112.4(1)° and 105.9(1)° for 3 and 4, respectively, compared to 139.3° and 176.2° for the acyclic analogs. Though the B–N bond distances of diborylamines 3 and HN(BMes₂)₂ are statistically identical, the B–N distances for the corresponding lithium diborylamides differ by 0.09 Å. While [Li(OEt₂)₃][N(BMes₂)₂] clearly features B–N double bonds, the bonding in the cyclic diborylamide appears unperturbed relative to the diborylamine. The dimeric structure and the short N–Li bonds in 4 support a major diborylamide resonance structure with two nitrogen lone pairs participating in bonding to Li (Scheme 1, species B).

Encouraged by Li–N bonding in 4, we set out to synthesize the first transition metal diborylamide complexes. Given the steric bulk of the diborylamide ligand, we targeted a 2-coordinate Fe bis(diborylamide) complex. FeCl₂ was treated with 2 equiv. 4 in Et₂O at −78 °C for 4 h, followed by warming to 20 °C overnight (Scheme 3). After the removal of LiCl, the crude material was isolated as an orange solid. The ¹H NMR spectrum of the solid showed 2 paramagnetic products in an ∼8:1 ratio.

Both Fe products were structurally characterized by single-crystal X-ray diffraction. The major product, obtained as yellow crystals from slow evaporation of a pentane solution, was the desired 2-coordinate bis(diborylamide) Fe(II) complex 5 (Fig. 2). Notable structural features include a N–Fe–N angle of 162.49(9)° and Fe–N distances of 1.894(2) and 1.898(2) Å. These Fe–N bond distances are on the shorter end for homoleptic Fe amide complexes, indicative of strong bonding between iron and the diborylamide ligands.²⁰ Complex 5 adds to the growing list of 2-coordinate Fe complexes, some of which have been shown to be active for reactions ranging from CO₂ activation to ketone hydrosilylation.²¹

The minor species was obtained as a red crystalline solid by slow evaporation of a Et₂O solution. Single crystal X-ray diffraction revealed the minor product to be a diiron complex containing terminal diborylamide and bridging mesityl ligands, {(BNB)Fe(μ-Mes)}₂ (6). Similar complexes have been reported previously, including the classic diiron tetramesityl complex, {(Mes)Fe((μ-Mes))}₂.^22–24 The bridging mesityl ligands in 6 must be formed via boron–carbon bond cleavage of the diborylamide ligand, possibly via a radical process (see ESI† Scheme S1), thus representing a new pathway to a bis(μ-Mes) diiron complex. A more rational synthesis of diiron complex 6 from {(Mes)Fe((μ-Mes))}₂ and 2 equiv. diborylamine 3 was attempted but ultimately proved unsuccessful.

In conclusion, we report the synthesis and characterization of the first cyclic diborylamide complexes of lithium and iron. The lithium diborylamide adopts a dimeric Li₂N₂ structure in the crystalline state, consistent with B–N single bonds and retention of electron lone pairs on nitrogen, in stark contrast to the heteroallene structures of acyclic diborylamides. We leveraged the bonding properties of this novel diborylamide compound to synthesize a 2-coordinate bis(diborylamide) Fe(II) complex, along with a mesityl-bridged diiron species as a minor product. Together, these studies demonstrate that cyclic diborylamides can bind strongly to alkali and transition metals. Future efforts will focus on probing the magnetic properties and reactivity of the Fe complexes, as well as the mechanism of formation of the unexpected diiron product.

Funding for this research was provided by Baylor University and the Welch Foundation (AA-2037).

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic details, NMR spectra, and X-ray crystallographic data. CCDC 2122484–2122487. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc06458d

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