Dinuclear molybdenum and tungsten complexes with metal-metal triple bonds supported by p-tert-butylcalix[4]arene ligands

In hydrocarbon solvents the compounds M-2(NMe2)(6) (M = Mo, W) react with p-tert-butylcalix[4]arene, H4L, to give the complexes (H2NMe2)(2)[M-2(mu,eta(2),eta(2)-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) Angstrom in a mu,eta(2),eta(2) 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 degrees. The calix[4]arene bowl encapsulates the H2NMe2 cations tightly as judged by H-1 NMR spectroscopy, and heating to 100 degrees 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)[M-2(eta(4)-L)(2)].xpy, 2.xpy. The molecule of 2.4py (M = Mo) has a center of symmetry, and the Mo-2-calix[4]arene unit has a dumbbell shape (Mo equivalent to Mo bond distance 2.226(1) Angstrom). 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 degrees C for 3 days forms compounds 3 of formula M-2(eta(4)-HL)(2). A partial crystal structure determination of 3.xC(6)H(6) (M = Mo) revealed Mo equivalent to Mo bonds, 2.226(6) and 2.214(7) Angstrom, supported by eta(4)-HL ligands. Compounds 3 are formed directly in the reactions between M-2(O'Bu)(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)[M-2(eta(4)-L)(2)], 2. For tungsten, 3 and 2 react in benzene to give (H2NMe2)[W-2(eta(4)-L)(eta(4)-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) Angstrom, 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) Angstrom. The monomethylated p-tert-butylcalix[4]arene, H3L(1), reacts with Mo-2(NMe2)(6) to give 3, HNMe2, and Me3N whereas Mo-2(O'Bu)(6) and H3L(1) yield Mo-2(eta(4)-L(1))(2), 5, and (BuOH)-Bu-t. The dimethylated p-tert-butylcalix[4]arene, H2L(2), failed to react with either Mo-2(NMe2)(6) or Mo-2((OBu)-Bu-t)(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.