A Bidentate Ligand Featuring Ditopic Lewis Acids in the Second Sphere for Selective Substrate Capture and Activation

Abstract

We present a ligand platform featuring appended ditopic Lewis acids to facilitate capture/activation of diatomic substrates. We show that incorporation of two 9-borabicyclo[3.3.1]nonane (9-BBN) units on a single carbon tethered to a pyridine pyrazole scaffold maintains a set of unquenched nitrogen donors available to coordinate Fe^II , Zn^II , and Ni^II . Using hydride ion affinity and competition experiments, we establish an additive effect for ditopic secondary sphere boranes, compared to the monotopic analogue. These effects are exploited to achieve high selectivity for binding NO₂ ^- in the presence of competitive anions such as F^- and NO₃ ^- . Finally, we demonstrate hydrazine capture within the second-sphere of metal complexes, followed by unique activation pathways to generate hydrazido and diazene ligands on Zn and Fe, respectively.

Keywords: Chemoselective Anion Binding; Ditopic Boranes; Hydrazine Functionalization; Lewis Acids; Secondary Coordination Sphere.

Figure 1.

Selected examples of known ditopic boranes,^{[11, 12b, 14, 15e]} and design strategy outlining the ditopic ligand used in this work from prior work using a bidentate ligand with single second-sphere Lewis acid,^[9a-c] and a tridentate ligand with two Lewis acids,^{[4, 9d, 9e]}

Figure 2.

a) Synthesis of ^(ene)BBN^tBuFeBr₂ via post-metalation hydroboration and synthesis of 1 and 2–4 via pre-metalation hydroboration b) molecular structures (50% probability ellipsoids) of 1, ^(propargyl)BBN^tBuFeBr_2, ^(ene)BBN^tBuFeBr₂, and 2. H-atoms excluding (ene) and (propargyl) groups are omitted, and the 9-BBN substituents are displayed in wireframe for improved clarity.

Figure 3.

a) Anion capture mediated by 1 yielding ditopic boron containing heterocyles 1-OAc, 1-NO₂, and 1-H and their respective molecular structures b) Comparison vs. free ligand (yellow) of distortion for ditopic borane unit with each heterocycle (blue) c) Comparative thermodynamics for nitrite and hydride capture for 1 and 1-mono and d) Comparative pK_a values for hydrazine N-H protons. R represents the remainder of free ligand. Thermal ellipsoids displayed at 50% probability. H-atoms and counterions (in 1-OAc and 1-NO₂) are omitted, and the 9-BBN substituents and THF ligands in 1-H are displayed in wireframe for improved clarity.

Figure 4.

Anion competition reaction showing selective nitrite capture by 1 in the presence of fluoride and nitrate (top), selectivity of hydrazine binding to 1 in the presence of 1-mono (middle) and ¹H NMR of hydrazine competition experiment (bottom) (N-CH₂ and pyz-CH shown). BR₂ = 9-BBN and R represents the remainder of the free ligand.

Figure 5.

μ(1,2)-hydrazine capture using 2–4 to yield 2–4-N₂H₄ and hydrazine reactivity of 2-N₂H₄ and 4-N₂H₄ to generate hydrazido and diazene complexes 5-X and 6 (X = Br, Cl). Molecular structures of 2-N₂H₄, 3-N₂H₄, 5-Br and 6 (right). Thermal ellipsoids displayed at 50% probability. Non-diazene or hydrazine H-atoms are omitted, and the 9-BBN substituents are displayed in wireframe for improved clarity.