2CH₃CN⊂(n-Bu₄N)₂[{Ir(1,5-COD)}₆W₄O₁₆]·2CH₃CN:  A Hybrid Inorganic−Organometallic, Flexible Cavity Host, Acetonitrile−Guest Complex Composed of a [W₄O₄]n⁺ Tetratungstate Cube and Six Polyoxoanion-Supported (1,5-COD)Ir⁺ Organometallic Groups

The tetratungstate polyoxoanion-supported organometallic compound 2CH 3 CN⊂( n -Bu 4 N) 2 [{Ir(1,5-COD)} 6 W 4 O 16 ]·2CH 3 CN (2CH 3 CN⊂ 1 ·2CH 3 CN) is reported, a complex which exhibits an unusual “methyl-first” CH 3 CN inclusion (“⊂”) chemistry. The synthesis of 2CH 3 CN⊂ 1 ·2CH 3 CN was accomplished by the reaction of 3 equiv of [Ir(1,5-COD)Cl] 2 with 4 equiv of ( n -Bu 4 N) 2 WO 4; an X-ray diffraction single-crystal structure shows that the four tungsten atoms in of 2CH 3 CN⊂ 1 ·2CH 3 CN are located in tetrahedral positions and combine with four bridging oxygens to yield a tungsten-oxygen cubane structure. The cubane core tungsten atoms are capped by six surrounding [Ir(1,5-COD)] + groups in an octahedral arrangement relative to the cube, with each [Ir(1,5-COD)] + group coordinated by two terminal oxygen atoms from the tetratungstate unit. Two sets of flexible acetonitrile-binding cavities, formed between each three of the six [Ir(1,5-COD)] + groups, form a roughly tetrahedral array surrounding the cube. This host-guest complex binds acetonitrile methyl-first, that is, with the methyl group rather than the nitrile group oriented toward a cubane core oxygen atom. The C Me ···O nonbonded distances fall in the range 3.10−4.46 Å for C Me ···(μ 2 -O) and 3.08−4.02 Å for C Me ···(μ 3 -O). Acetonitrile-free 1 was also prepared, and its possible acetonitrile and toluene complexation in benzene and methylene chloride solution was studied by NMR. No binding between acetonitrile or toluene and 1 in solution is observed, even when using greater than 300 mol equiv of guest relative to 1 . Therefore, the host-guest interaction between 1 and acetonitrile appears to be weak ( K eq < 10 M - 1 ), possibly being limited to the solid state.