Dinuclear Molybdenum and Tungsten Complexes with Metal−Metal Triple Bonds Supported byp-tert-Butylcalix[4]arene Ligands

Abstract

In hydrocarbon solvents the compounds M2(NMe2)6 (M = Mo, W) react with p-tert-butylcalix[4]arene, H4L, to give the complexes (H2NMe2)2[M2(μ,η2,η2-L)2], 1 (M = Mo, W). In the molecular structure of 1·4THF, the calix[4]arene ligands span the Mo⋮Mo bond of distance 2.193(1) Å in a μ,η2,η2 manner such that each Mo atom is coordinated by four phenolic oxygen atoms that lie roughly in a plane with Mo−Mo−O angles that range from 92 to 102°. The calix[4]arene bowl encapsulates the H2NMe2 cations tightly as judged by 1H NMR spectroscopy, and heating to 100 °C under a dynamic vacuum in the solid state fails to remove HNMe2. However, upon refluxing in pyridine, compounds 1 are converted to (H2NMe2)2[M2(η4-L)2]·xpy, 2·xpy. The molecule of 2·4py (M = Mo) has a center of symmetry, and the Mo2−calix[4]arene unit has a dumbbell shape (Mo⋮Mo bond distance 2.226(1) Å). Each Mo atom has four Mo−O phenoxide bonds, one of which is hydrogen-bonded to a H2NMe2 cation, which in turn is hydrogen-bonded to a pyridine molecule. Each calix[4]arene bowl encapsulates one molecule of pyridine. Heating compounds 2 under a dynamic vacuum at 100 °C for 3 days forms compounds 3 of formula M2(η4-HL)2. A partial crystal structure determination of 3·xC6H6 (M = Mo) revealed Mo⋮Mo bonds, 2.226(6) and 2.214(7) Å, supported by η4-HL ligands. Compounds 3 are formed directly in the reactions between M2(OtBu)6 compounds and H4L in benzene at room temperature (1 or 2 days). Compounds 3 react in hydrocarbon solutions with HNMe2 to give the compounds (H2NMe2)2[M2(η4-L)2], 2. For tungsten, 3 and 2 react in benzene to give (H2NMe2)[W2(η4-L)(η4-HL)], 4, which crystallizes with seven molecules of benzene, two of which are encapsulated in the calix[4]arene bowls. The W⋮W bond, 2.304(1) Å, is spanned by one H2NMe2 cation that hydrogen-bonds to a pair of phenolic oxygen atoms while another pair of oxygen atoms in the trans position are directly hydrogen-bonded such that a short O···O distance results, 2.338(9) Å. The monomethylated p-tert-butylcalix[4]arene, H3L(1), reacts with Mo2(NMe2)6 to give 3, HNMe2, and Me3N whereas Mo2(OtBu)6 and H3L(1) yield Mo2(η4-L(1))2, 5, and tBuOH. The dimethylated p-tert-butylcalix[4]arene, H2L(2), failed to react with either Mo2(NMe2)6 or Mo2(OtBu)6 even under prolonged reflux. These results are discussed in terms of the mechanisms of substitution and isomerization reactions of M⋮M complexes and are compared with known reactions involving chelating biphenoxides and the coordination chemistry of calix[4]arene ligands.