Synthesis and Isonitrile-Induced Isomerization of Manganese Pnictaketenes

Metal pnictaketenes (M–ECO; E = P, As) are known to readily decarbonylate to afford terminal metal pnictogenido complexes; however, alternative reactivity modes of these complexes remain comparatively unexplored. Herein, we report the synthesis of manganese(II) phosphaketene and arsaketene complexes supported by a tris(carbene)borate ligand through salt metathesis of a manganese(II) chloride precursor with the corresponding sodium salts of 2-pnictaethynolate anions, [Na(1,4-dioxane)x][OCE] (E = P, As). Single-crystal X-ray diffraction confirms κ1-E coordination of the phosphaketene and arsaketene ligands. The manganese arsaketene complex represents the first structurally characterized example a κ1-As-bound arsaketene supported by a 3d metal. Attempts to induce decarbonylation by heating or photolysis did not afford tractable products. However, treatment with isonitriles led to an isonitrile-induced linkage isomerization of the manganese pnictaketenes to give the corresponding manganese pnictaethynolate complexes (Mn–OCE). The linkage isomerization is supported by FT-IR spectroscopy, X-ray photoelectron spectroscopy, chemical derivatization with triisopropylsilyl triflate, and density functional theory calculations, which indicate that the phosphaethynolate isomer is thermodynamically favored. These results demonstrate that coordinated pnictaketenes can undergo linkage isomerization on a transition metal center, expanding the known reactivity of such complexes.