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 NO 2 À in the presence of competitive anions such as F À and NO 3 À . 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.

Appended Lewis Acids Enable Dioxygen Reactivity and Catalytic Oxidations with Ni(II)

“…We initiated our study by examining a series of borane-appended pyridine-pyrazole ligands that feature an identical Lewis acid (9-BBN) placed at different positions within the metal’s secondary coordination sphere. Previous efforts from our group established that cooperative binding of N 2 H 4 , a structurally related ligand to O 2 , is supported by both pendent ditopic and monotopic boranes. ,,, For the pendent monotopic boranes, N 2 H 4 binding affinities increased with decreasing distance from the metal: highest for a spacer containing two methylene (−CH 2 CH 2 –; Figure , n = 1), followed by three (−CH 2 CH 2 CH 2 –; Figure , n = 2) units. Because the binding mode of O 2 to Ni pyridine-pyrazole compounds was not known a priori , these previous studies provided a starting point from which to empirically optimize.…”

“…We prepared a series of NiBr 2 complexes containing an isostructural primary coordination sphere to the previously reported ditopic Lewis acid appended nickel complex ( (BBN)2 NN)NiBr 2 (1) (Figure 2). 71 We targeted monotopic borane complexes containing three and two methylene spacers ( an olefinic tether arm in 5 with an appended borane was confirmed through an SCXRD experiment (Figure 2). (5) or for the ditopic unit (1), steric crowding or deprotonation and deborylation pathways, reaction types previously reported for 1,1-gem-diborylalkanes.…”

Ligand Postsynthetic Functionalization with Fluorinated Boranes and Implications in Hydrogenation Catalysis

“…Among the wide variety of bifunctional ligands, those bearing a Lewis acidic site have enjoyed increasing popularity . Not surprisingly, ligands containing group 13 elements are the preferred choice, with boron as the more prevalent, , including the recent development of ligands with multiple borane sites . Accordingly, catalytic applications of boron-containing transition-metal catalysts are rapidly increasing (Figure a).…”

Reactivity of frustrated Lewis pairs with BOC protected diazocarboxylates: FLP capture of diazene

“…Subsequent reactions allowed deprotonation and interaction with the metal affording hydrazido and diazene ligands on Zn and Fe, retrospectively. 20 As FLPs combine electron donors and acceptors, we were encouraged by the above advances, to examine the reactions of FLPs with N 2 derivatives. Most recently, we have described the reactions of boranes with diazodiesters.…”

“…Subsequent reactions allowed deprotonation and interaction with the metal affording hydrazido and diazene ligands on Zn and Fe, retrospectively. 20…”