“Third-Generation”-Type Functional Tris(2-pyridyl)borate Ligands and Their Transition-Metal Complexes

Abstract

Phenyltris(2-pyridyl)borates (Tpyb) are a promising class of tripodal "scorpionate"-type ligands with potential utility in the development of transition-metal complexes with interesting optical, electronic, or magnetic properties and as building blocks to metallosupramolecular polymers. We report here a new class of "third-generation"-type Tpyb ligands that contain different functional groups attached to the boron-bound aryl moiety. The synthesis, characterization, and metal-ion complexation behavior of ligands with iodo and trimethylsilyl groups are discussed. The electrochemical and absorption characteristics of the corresponding low-spin iron(II) and ruthenium(II) complexes are compared. We demonstrate the further elaboration of iodo derivatives with alkynes via Sonogashira-Hagihara coupling, a process that proceeds with high yield for the iron(II) and ruthenium(II) complexes but not for the free ligand. Borylation of the silyl-substituted ruthenium(II) complex with BBr3 was also investigated. In addition to the expected borylation product Ru(Tpyb-Bpin)2, the replacement of one (major product) or two phenyl groups is observed, suggesting that electrophilic borylation occurs at both the C(Ph)-Si and the C(Ph)-B aromatic carbon atoms. The successful attachment of a range of different functional groups at the periphery of the Tpyb metal complexes is expected to provide opportunities to access new polymeric materials via C-C coupling or click-type reactions.

A Robust Vanadium(V) Tris(2-pyridyl)borate Catalyst for Long-Lived High-Temperature Ethylene Polymerization

“…We hypothesized that these distinctions would promote the redox stability and structural integrity of vanadium­(V) tris­(2-pyridyl)­borate complexes, thus promoting their reactivity at high temperatures. Prior to our work, the only reported tris­(2-pyridyl)­borate metal complexes were coordinatively saturated homoleptic complexes of divalent ions (Figure B), which were studied for applications other than catalysis . Also, the tris­(2-pyridyl)­borates published prior to this work featured aryl substituents distal to the reaction center, which we anticipated would not be relevant for catalysis.…”

Tris(2-pyridyl) Bismuthines: Coordination Chemistry, Reactivity, and Anion-Triggered Pyridyl Coupling

“…One further potential consequence of incorporating more electropositive (metallic and semimetallic) bridgehead atoms is an increase in the polarity of the bridgehead atom–C bonds to the pyridyl groups. In contrast to more robust tris (2-pyridyl) borate ligands, − tris (2-pyridyl)­aluminates [RAl­(2-py) 3 ] − (Figure c) react as bases with H 2 O, alcohols, or carboxylic acids in a controlled manner, providing an interesting route to new aluminates containing additional donor functionality. − This behavior has led to new applications as thermally stable pyridyl-transfer reagents as well as reagents for the rapid NMR spectroscopic determination of the enantiomeric excess ( ee ) of chiral alcohols. − …”

Cation- and Anion-Mediated Supramolecular Assembly of Bismuth and Antimony Tris(3-pyridyl) Complexes

“…; 2-py = 2-pyridyl; Figure b) . However, incorporating heavier and more metallic main-group bridgehead atoms has been shown to provide an important tool for tuning the ligand character, enabling systematic modification of the bite angle, donor/acceptor properties, and reactivity. − A case in point is the series of Group 15 tris­(2-pyridyl) ligands E­(6-Me-2-py) 3 (6-Me-2-py = 6-methyl-2-pyridyl; E = As, Sb, Bi) for which changing the bridgehead can be used to provide incremental change in the σ-donor character and (thereby) the catalytic activity and behavior …”