The binding of a Si–H bond to a single metal center is of particular interest. Understanding binding modes of a Si-H-M moiety is crucial in metal-catalyzed transformations involved in various hydrosilylations. Along with a well-known oxidative addition process of a Si-H bond, a silyl coordination to a metal center can be achieved via a nonclassical σ-SiH interaction. To explore the details of a hydrosilylation mechanism, a Si-H bond was introduced to a series of divalent metal ions by preparing nickel and cobalt complexes supported by an anionic diphosphinosilyl RSiP2 ligand (RSiP2 = RSiH[2-PiPr2-C6H4]2; R = Me or Ph). While a clean conversion occurs in the reaction of a silane ligand with nickel(II) bromide, the corresponding cobalt reaction to prepare a cobalt congener (MeSiP2)CoBr reveals the formation of a low-temperature stable cobalt-SiH intermediate species (MeSiHP2)CoBr2. The green intermediate was carefully investigated by employing low-temperature UV-Vis and EPR spectroscopies. Using various tools including X-ray diffraction, ENDOR and high-level DFT calculations, the exact position of the hydrogen atom in the paramagnetic cobalt intermediate species was detected and revealed a Co–H bond distance of ~1.53 Å. Surprisingly, we found evidence that the Si–H bond activation proceeds via deprotonation followed by Co–silyl coordination along with dehalogenation, rather than in the classical oxidative addition pathway. In fact, the same moiety was also introduced to a copper(I) ion to minimize geometry relaxation in metal-to-ligand charge transfer (MLCT) excited states by lifting the degeneracy of relevant Cu d-based orbitals. Such effect can be evaluated by studying the emission property of copper complexes and ultimately lead to further development of efficient copper emitters. A series of corresponding cuprous complexes (RSiHP2)Cu(CbzR) were prepared and their emission properties studied. A blue/green emission band at λmax = 474–513 nm was established with a high quantum efficiency (φ = 0.40–0.59) and a long lifetime of 15–23 μs. This emissive behavior contrasts with tetrahedral copper analogues, which do not display the blue emission due to a significant Jahn-Teller distortion after the MLCT transition from nearly degenerate Cu d-based orbitals. Details of the syntheses and photophysical properties of copper complexes will be discussed.

σ-Complexes of cobalt and copper

253rd National Meeting (2017)

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