Chelating Bis-silylenes As Powerful Ligands To Enable Unusual Low-Valent Main-Group Element Functions

CONSPECTUS: Silylenes are divalent silicon species with an unoccupied 3p orbital and one lone pair of electrons at the SiII center. Owing to the excellent sigma-donating ability of amidinato-based silylenes, which stems from the intramolecular imino-N donor interaction with the vacant 3p orbital of the silicon atom, N-heterocyclic amidinato bis(silylenes) [bis(NHSi)s] can serve as versatile strong donating ligands for cooperative stabilization of central atoms in unusually low oxidation states. Herein, we present our recent achievement on the application of bis(NHSi) ligands with electronically and spatially different spacers to main-group chemistry, which has allowed the isolation of a variety of low-valent compounds consisting of monatomic zero-valent group 14 E-0 complexes (named metallylones, E = Si, Ge, Sn, Pb); monovalent group 15 EI complexes (E = N, P, isoelectronic with metallylones); and diatomic low-valent E2 complexes (E = Si, Ge, P) with intriguing electronic structures and chemical reactivities. The role of the Si-II center dot center dot center dot Si-II distance was revealed to be crucial in this chemistry. Utilizing the pyridine-based bis(NHSi) (Si center dot center dot center dot Si distance: 7.8 angstrom) ligand, germanium(0) complexes with additional Fe(CO)(4) protection at the Ge-0 site have been isolated. Featuring a shorter Si center dot center dot center dot Si distance of 4.3 angstrom, the xanthene-based bis(NHSi) has allowed the realization of the full series of heavy zero-valent group 14 element E-0 complexes (E = Si, Ge, Sn, Pb), while the o-carborane-based bis(NHSi) (Si center dot center dot center dot Si distance: 3.3 angstrom) has enabled the isolation of Si-0 and Ge-0 complexes. Remarkably, reduction of the o-carborane-based bis(NHSi)-supported Si0 and Ge-0 complexes induces the movement of two electrons into the o-carborane core and provides access to SiI-SiI and Ge-I-Ge-I species as oxidation products. Additionally, the o-carborane-based bis(NHSi) reacts with adamantyl azide, leading to a series of nitrogen(I) complexes as isoelectronic species of a carbone (C0 complex). Moreover, cooperative activation of white phosphorus gives bis(NHSi)-supported phosphorus complexes with varying and unexpected electronic structures when employing the xanthene-, o-carborane-, and aniline-based bis(NHSi)s. With the better kinetic protection provided by the xanthene-based bis(NHSi), small-molecule activation and functionalization of the bis(NHSi)-supported central E or E-2 atoms (E = Si, Ge, P) are possible and furnish several novel functionalized silicon, germanium, and phosphorus compounds. With knowledge of the ability of chelating bis(NHSi)s in coordinating and functionalizing low-valent group 14 and 15 elements, the application of these ligand systems to other main-group elements such as group 2 and 13 is quite promising. To fully understand the role of the NHSi in a bis(NHSi) ligand, introducing a mixed ligand, i.e., the combination of an NHSi with other functional groups, such as Lewis acidic borane or Lewis basic borylene, in one chelating ligand could lead to new types of low-valent main-group species. Furthermore, the development of a genuine acyclic silylene, without an imino-N interaction with the vacant 3p orbital at the silicon(II) atom, as part of a chelating bis(acyclic silylene) has the potential to form very electronically different main-group element complexes that could achieve even more challenging bond activations such as N-2 or unactivated C-H bonds.