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

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.