Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization

Abstract

We outline the generality and requirements for cooperative N₂H₄ capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH₂ stabilization. While the Lewis acids are required to bind N₂H₄, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

Figure 1.

Conceptual design to probe multiple aspects of N₂H₄ capture and reduction as well as properties of resulting bis(amido) compounds.

Figure 2.

A) Previous report demonstrating product/substrate equilibration for compounds derived from 1-Fe. B) Weaker Lewis acid/base adduct enables facile release. C) Molecular structure of 1-Fe-NH₂Ph displayed with 50% probability ellipsoids. 9-BBN substituents are displayed in wireframe for clarity.

Figure 3.

A) Capture of N₂H₄ to form complexes 2-M. B) Molecular structures of 2-Mn and 2-Co displayed with 50% probability ellipsoids. 9-BBN substituents are displayed in wireframe for clarity. C) Cyclic voltammograms of 2-M (0.2 M [Bu₄N][PF₆], THF, 200 mV/s). D) NH₄⁺ quantification from reduction of complexes 2-M.

Figure 4.

Synthesis of vanadium compounds discussed in this manuscript. The molecular structures are displayed with 50% probability ellipsoids. 9-BBN substituents are displayed in wireframe for clarity.

Figure 5.

Synthesis of ammonia-borane complexes 3-M and molecular structures displayed with 50% probability ellipsoids. 9-BBN substituents are displayed in wireframe for clarity.

Figure 6.

Synthesis of amido-borane complexes 4-M and molecular structures displayed with 50% probability ellipsoids. 9-BBN substituents are displayed in wireframe for clarity. Cyclic voltammograms of 4-M: 0.2 M [Bu₄N][PF₆], THF, 100 mV/s.

Figure 7.

A) Oxidation of 4-M. B) Reduction of 4-M. C) Anionic portion of molecular structures of 5-Co and 5-Fe displayed with 50% probability ellipsoids. 9-BBN substituents are displayed in wireframe for clarity.

Figure 8.

Alkylation of 4-Fe to produce 6 and subsequent oxidation to form 7. Molecular structures of 6 (30% probability ellipsoids; cation omitted for clarity) and 7 (50% probability ellipsoids). 9-BBN substituents are displayed in wireframe for clarity.