[3+2] Cycloadditions of Molybdenum(II) Azide Complexes with Nitriles and an Alkyne

A series of products of [3+2] cycloaddition reactions of molybdenum azide complexes with nitriles and an alkyne were prepared and characterized. These include the tetrazolate complexes Mo(η3-C3H5)(CO)2(en)(N4CR) (R = CH3 (1), C6H5 (2), C(CH3)3 (3), CHCHCN (4)) and the triazolate complexes Mo(η3-C3H5)(CO)2(L2){N3C2(CO2CH3)2} (L2 = en (6), dppe (7)). They are the first examples of a complex with a heterocyclic ligand prepared via a reaction of a group VIB metal azide with an unsaturated dipolarophile. All these complexes are fluxional in solution, with those containing en ligands displaying broad unresolved proton NMR signals at room temperature. With the exception of 1, their solution behavior was studied by low-temperature NMR spectroscopy. In contrast, the dppe complexes display average NMR signals at room temperature due to a fast exchange. The solid-state structures of these cycloaddition products were also determined by single-crystal X-ray diffraction analysis. The tetrazolate complexes 1, 2, and 4 adopt an asymmetric endo-conformation, while complex 3 adopts a symmetric endo-conformation. The tetrazolate ligand is N(1)-bonded in 1 and N(2)-bonded in complexes 2−4. The steric effect is the main reason for the N(2)-bonding mode found in 3. Complex 4 is the first structurally characterized product of a [3+2] cycloaddition reaction involving the CN bond of fumaronitrile. The triazolate complexes 6 and 7 adopt an asymmetric endo-conformation, with their heterocyclic ligands being bonded to the metal via N(2).