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(NtBu)(2)(NHtBu)(2) Ia (M = Ma), Ib (M = W) or M(NMes)(2)Cl-2(dme) IIa (M = Mo), IIb (M = W) (Mes = 2,4,6-Me-3-C6H2) with p-tBu-calix[4]arene LH4 affords calix[4]arene metal complexes LM(NR) la, b (M = Mo, W; R = tBu) 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, tBu-isonitrile, or water within their macrocyclic pockets. The spectroscopic data of some inclusion compounds and the crystal structures of LMo(NtBu)(NCMe) la(NCMe), LW(NtBu)(OH2) Ib(OH2), LW(NtBu)(CNtBu) Ib(CNtBu), LMo(NMes)(NCMe) aa(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 pi 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.