Copper(I), Silver(I), and Gold(I) Ethylene Complexes of Fluorinated and Boron-Methylated Bis- and Tris(pyridyl)borate Chelators

Abstract

Bis- and tris-pyridyl borate ligands containing pyridyl donor arms, a methylated boron cap, and a fluorine-lined coordination pocket have been prepared and utilized in coinage metal chemistry. The tris­(pyridyl)­borate ligand has been synthesized using a convenient boron source, [NBu4]­[MeBF3]. These N-based ligands permitted the isolation of group 11 metal–ethylene complexes [MeB­(6-(CF3)­Py)3]­M­(C2H4) and [Me2B­(6-(CF3)­Py)2]­M­(C2H4) (M = Cu, Ag, Au). The gold complexes display the largest coordination-induced upfield shifts of the ethylene 13C resonance relative to that of the free ethylene in their NMR spectra, while the silver complexes show the smallest shift. Solid-state structures of five of these metal–ethylene complexes as well as the related free ligands were established by X-ray crystallography. Surprisingly, all three [MeB­(6-(CF3)­Py)3]­M­(C2H4) adopt the rare κ2 coordination mode rather than the typical κ3 coordination mode of facial capping tridentate ligands. Computational analyses indicate that κ2 coordination mode is favored over the κ3-mode in these coinage metal–ethylene complexes and point to the effects CF3-substituents have on κ2/κ3-energy difference. The M–C and M–N bond distances of [MeB­(6-(CF3)­Py)3]­M­(C2H4) follow the trend expected based on covalent radii of M­(I) ions. The calculated ethylene–M interaction energy of κ2-[MeB­(6-(CF3)­Py)3]­M­(C2H4) indicated that the gold­(I) forms the strongest interaction with ethylene. A comparison to the related poly­(pyrazolyl)­borates is also presented.

Tris(pyridyl)borates: an emergent class of versatile and robust polydentate ligands for catalysis and materials applications

“…The convenient synthesis routes that are available nowadays to generate Tpyb ligands and their complexes enable a host of different applications in materials sciences and catalysis. Of particular significance is that trifluoroborate precursors pioneered by Dias 25 should allow researchers to install a broader range of functional groups onto the ligands by avoiding the need to go through a highly reactive aryldibromoborane intermediate. Combined with the increased scope of pyridyl derivatives that are now being explored, we expect a wide variety of Tpyb ligands to emerge over the coming years.…”

“…Recently, the Dias group has demonstrated the synthesis of Tpyb ligands L4 from bench-stable trifluoroborate salts, further simplifying the handling of the starting materials and thereby facilitating access to and expanding the scope of this ligand class. 25 Polymer-supported Tris( pyridyl)borate ligands While polymer modification reactions had been explored to attach tris(pyrazolyl)borates and other borane functional groups to polymers, 35,36 our efforts toward polymer-supported Tpyb ligands have focused on the preparation of pre-functionalized ligands that incorporate a polymerizable group (Fig. 2).…”

“…Recently, the Dias group has demonstrated the synthesis of Tpyb ligands L4 from bench-stable trifluoroborate salts, further simplifying the handling of the starting materials and thereby facilitating access to and expanding the scope of this ligand class. 25…”

“…24 Most recently, the Dias group has generated trifluoromethylpyridine-functionalized ligands from methyltrifluoroborate and dimethylboron bromide, yielding trispyridyl- and bispyridylborates. 25 When incorporated into coinage metal ethylene complexes ( C10 ), they again adopted a κ 2 -coordination geometry. FTIR studies of the silver and copper isocyanide and carbonyl complexes allow the electronics of the complexes to be investigated and compared with previously reported analogues.…”

“…The Dias group has reported pyridyl ring-substituted, sterically demanding Tpyb ligand versions and explored their coordination chemistry with coinage metals. 18,24,25 The Comito group has explored the catalytic activity of heteroleptic Vanadium complexes in ethylene polymerization 26,27 and of group 2 and group 12 complexes in the polymerization of lactones and carbonates. 28 The Hikichi group has explored Typb complexes in catalytic oxidations of alkanes, employing sterically hindered ligands to generate catalytically relevant Ni(II) bromide complexes.…”