Synthesis, Characterization, and Application of Cationic Low-valent Group 14 and 15 Heavier Elements

Abstract

Low-valent main-group compounds, possessing at least one lone pair and a vacant p-orbital, have emerged as strong contenders to transition metals due to the low-lying energy separation between their frontier molecular orbitals. This unique electronic structure enables them to activate small molecules such as H₂, NH₃, and CO₂. In this context, new cationic main-group compounds have been synthesized, characterized, and investigated for their reactivity and catalytic applications. In the first part of this work, we synthesized and characterized the peri-substituted 5,6-(diisopropylphosphino)acenaphthalene, a valuable phosphine-based ligand, and utilized their strain-induced geometry and small bite angle to stabilize low-valent Ge(II) and Sn(II) monocations. The reactivity of these Ge(II) and Sn(II) species was explored through methylation and transmetallation. Additionally, the reactivity of germylene with the isopropylphosphine-substituted acenaphthene ligand was explored, culminating in the unprecedented activation of a P–C bond and oxidation of the germanium center. In the second part, we synthesized and characterized the elusive antimony(I) cation by 5,6-bis(diisopropylphosphino) acenaphthene (ligand), where ligand playing dual role of a reductant and supporting ligand that involve the forming of P-P bonded diphosphonium dication by two electron oxidation of ligand. We have explored the first-time nucleophilic behaviour of Antimony(I) cation towards coinage metals(M) (M = Au, Ag, Cu), yielding a series of novel Sb(I)–M complexes that provide insights into the coordination chemistry and electronic interactions. In the final part of the work, we extended our investigation into the reactivity of the Sb(I) cation with non-metals, including elemental sulfur (S₈), methyl triflate (CH₃OTf), gallium trichloride (GaCl₃), and phosphorus triiodide (PI₃). These reactions elucidate the diverse bonding modes and electronic transformations facilitated by the antimony center. Additionally, we explored the application of the diphosphonium dication as activator, as well as the catalytic potential of an Sb(I)–gold complex in the synthesis of oxazoles via the cycloisomerization of propargylic amides.