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Abstract We introduce the heterocumulene ligand [(Ad)NCC(^tBu)]⁻(Ad=1‐adamantyl (C₁₀H₁₅),^tBu=tert‐butyl, (C₄H₉)), which can adopt two forms, the azaalleneyl and ynamide. This ligand platform can undergo a reversible chelotropic shift using Brønsted acid‐base chemistry, which promotes an unprecedented spin‐state change of the [V^III] ion. These unique scaffolds are prepared via addition of 1‐adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡C^tBu(OTf)] (A) (BDI⁻=ArNC(CH₃)CHC(CH₃)NAr), Ar=2,6‐ⁱPr₂C₆H₃) and [(dBDI)V≡C^tBu(OEt₂)] (B) (dBDI²⁻=ArNC(CH₃)CHC(CH₂)NAr). ComplexAreacts with C≡NAd, to generate the high‐spin [V^III] complex with a κ¹‐N‐ynamide ligand, [(BDI)V{κ¹‐N‐(Ad)NCC(^tBu)}(OTf)] (1). Conversely,Breacts with C≡NAd to generate a low‐spin [V^III] diamagnetic complex having a chelated κ²‐C,N‐azaalleneyl ligand, [(dBDI)V{κ²‐N,C‐(Ad)NCC(^tBu)}] (2). Theoretical studies have been applied to better understand the mechanism of formation of2and the electronic reconfiguration upon structural rearrangement by the alteration of ligand denticity between1and2. 

Abstract Decarbonylation along with P‐atom transfer from the phosphaethynolate anion, PCO⁻, to the Nb^IVcomplex [(PNP)NbCl₂(N^tBuAr)] (1) (PNP=N[2‐PⁱPr₂‐4‐methylphenyl]₂⁻; Ar=3,5‐Me₂C₆H₃) results in its coupling with one of the phosphine arms of the pincer ligand to produce a phosphanylidene phosphorane complex [(PNPP)NbCl(N^tBuAr)] (2). Reduction of2with CoCp*₂cleaves the P−P bond to form the first neutral and terminal phosphido complex of a group 5 transition metal, namely, [(PNP)Nb≡P(N^tBuAr)] (3). Theoretical studies have been used to understand both the coupling of the P‐atom and the reductive cleavage of the P−P bond. Reaction of3with a two‐electron oxidant such as ethylene sulfide results in a diamagnetic sulfido complex having a P−P coupled ligand, namely [(PNPP)Nb=S(N^tBuAr)] (4). 

The reactivity of a [V^V] alkylidyne with unsaturated substrates such as NCR (R = Ad or Ph) and PCAd leads to the formation of rare transition metal complexes featuring an α-aza-vanadacyclobutadiene and a β-phospha-vanadacyclobutadiene moieties.