Syntheses, Molecular Structures, and Bonding of Molybdenum and Tungsten Calix[4]arene Imido Complexes

Syntheses, spectroscopic properties, molecular structures, and bonding of novel calix[4]arene imido compounds are described. Treatment of M(N t Bu) 2 (NH t Bu) 2 Ia (M = Mo), Ib (M = W) or M(NMes) 2 Cl 2 (dme) IIa (M = Mo), IIb (M = W) (Mes = 2,4,6‐Me 3 ‐C 6 H 2 ) with p ‐ t Bu‐calix[4]arene LH 4 affords calix[4]arene metal complexes LM(NR) 1a , b (M = Mo, W; R = t Bu) and 2a , b (M = Mo, W; R = Mes). Analytical and spectroscopic data are consistent with monomeric structures for 1 and 2 , retaining a local C 4v symmetry for the calix[4]arene metal fragment. These complexes are well‐suited to bind small molecules like acetonitrile, t Bu‐isonitrile, or water within their macrocyclic pockets. The spectroscopic data of some inclusion compounds and the crystal structures of LMo(N t Bu)(NCMe) 1a(NCMe) , LW(N t Bu)(OH 2 ) 1b(OH 2 ) , LW(N t Bu)(CN t Bu) 1b(CN t Bu) , LMo(NMes)(NCMe) 2a(NCMe) , and LW(NMes)(NCMe) 2b(NCMe) are reported. All complexes contain a group VI metal imido [M=NR] moiety mounted on the phenoxide rim of the calix[4]arene ligand as well as an incorporated guest molecule within the cavity. Some insights into the structures of complexes of the type L′W(NR′) (L′ = p ‐H‐calix[4]arene; R′ = H, Me) and into bonding in these compounds are provided by density functional theory, applying the B‐P86 density functional and an all SVP basis set within the RI‐J‐DFT approximation. At least one π bond is of importance for calix[4]arene‐metal bonding in these compounds. The metal‐imido bond can be described as a triple bond. A geometrically optimized minimum structure of L′W(NMe) 4 shows a calix[4]arene ligand only slightly distorted from a local C 4v symmetry and an almost linear tungsten‐imido moiety.