Acetonitrile and cyanide compounds containing metal-metal bonds: syntheses, structures and applications to solid-state chemistry

Abstract Compounds with acetonitrile and cyanide ligands have been prepared from mild substitution reactions of well- known metal-metal bonded precursors. Facile non-redox substitution of the carboxylate ligands in Mo2(O2CCH3)4 occurs in chlorinated solvents treated with excess [R4N][CN] (R=Et, Bun) to produce the salts [R4N]4[Mo2(CN)8] (R=Et (3); Bun (1)) in high yields. These reactions were found to be solvent dependent, with the major product isolated from reactions performed in THF being [Bun4N]3[Mo2(O2CCH3)(CN)6] (2). The chloride ligands in multiply-bonded dirhenium compounds are readily displaced by cyanide as demonstrated by the reaction between Re2Cl4(dppm)2 and [Bun4N]CN to give [Bun4N]2(Re2(CN)6(dppm)2] (4). Compounds 1–4 were characterized by IR and electronic spectroscopies; single crystal X-ray data were collected for 1, 2 and 4. 1·8CHCl3: orthorhombic, Pbca, a=20.526(8), b=28.122(5), c=19.855(7) A, V=11461(4) A3, R=0.082, Rw=0.083; 2: monoclinic, P21, a=12.046(3), b=16.050(10), c=16.854(3) A, β=94.11(2)°, V=3250(2) A3, R=0.067, Rw=0.067; 4·8CH2Cl2: triclinic, P 1 , a=13.835(2), b=18.172(2), c 12.261(1) A, α=106.788(8), β=107.850(9), γ=93.894(9)°, V=2767.5(6) A3, R=0.041, Rw=0.045. The molecular structure of [Bun4N]4[Mo2(CN)8] contains dimolybdenum(II) centers ligated by eight CN− ligands in the familiar unbridged M2L8 arrangement, with the two Mo(CN)4 units being perfectly eclipsed as expected for a molecule with a δ component to the MM bond. The MoMo distance of 2.122(2) A is the shortest reported distance for an unbridged homoleptic Mo2II,IIL8 compound. The related salt, (Bun4N]3[Mo2(O2CCH3)(CN6] (2), contains an anion displaying a structure similar to 1, but with one bridging acetate ligand, the presence of which results in a slight contraction of the MoMo bond to 2.114(2) A. The hexacyanodirhenium compound, 4, is reminiscent of M(III) complexes of the type M2X6(dppm)2 with one important exception, namely it is of the type [M2X6(dppm)2]2− with divalent metal centers. The equatorial plane in 4, bisected by two trans dppm ligands, consists of four unidentate terminal cyanide ligands and two cyanide groups that adopt an unusual η2-(σ,π) bridging arrangement. The ReRe distance of 3.0505(6) A is formally a single bond based on the conventional overlap scheme for a d5d5 M2L10 compound, viz. σ2π2(σ2π*2)π*2 this assignment is supported by the diamagnetism of the compound as revealed by NMR and magnetic susceptibility studies. With the exception of the bridging cyanides in 4, the v(CN) stretches in the new compounds 1–4 occur at higher frequencies than free CN−, indicating that they are acting purely as donors. In related chemistry, the alkylcyanide CH3CN is used as the sole supporting ligand in dinuclear metal cations. The compounds [Bun4N]2[Re2Cl8] and Re2Cl4(PPrn3)4 react with HBF4 etherate solutions to yield the hitherto unknown compound [Re2(MeCN)10][BF4]4 (5), containing a triply-bonded solvated cation. Compound 5 and the previously reported salts [M2(MeCN)10][BF4]4 (M=Mo, Rh) were metathesized with [Bun4N]2[M6O19] (M=Mo, W) and [Bun4N]4[Mo8O26] in MeCN to give [M2(MeCN)10][M′6O19]2 (M=Re, M′=Mo (6); M=Rh, M′=Mo (7); M=Rh, M′=W (9); M=M′=Mo (10); M=Mo, M′=W (12)) and [M2(MeCN)10][M′8O26] (M=Rh (8); M′=Mo (11)). The products were isolated directly from acetonitrile solutions as insoluble salts and characterized primarily by IR and UV-Vis spectroscopies which are diagnostic tools for the presence of the MM bonded chromophore with coordinated nitrile ligands as well as the intact polyoxometallate anions. Layering of the two separate salt solutions led to the isolation of single crystals in two cases. Crystal data for representative salts are: 6·4CH3CN·2H2O: monoclinic C2/c, a=18.36(2), b=22.67(2), c=17.45(2) A, β=90.31(9)°, V=7263(13) A3, R=O.066, Rw=0.083; 7·4CH3CN: monoclinic C2/c, a=18.457(2), b=22.723(2), c=17.190(2) A, β=90.07(1)°, V=7209(2) A3, R=0.079, Rw=0.059. Compounds 6 and 7 were subjected to thermal gravimetric analysis and found to undergo quantitative loss of acetonitrile solvent ligands to give amorphous metal oxide materials of empirical composition MMo6O19 (M=Rh (13); Re (15)). The anaerobic thermal treatment of [Mo2(MeCN)10][Mo6O19]2 (10) leads to a crystalline material of empirical formula Mo′Mo6O19 (14) whose powder X-ray pattern revealed tetragonal symmetry with unit cell dimensions a=b=10.811(1), c=2.819(8) A, V=329.56(6) A3. These results will be discussed along with the cyanide chemistry in light of the design of new ordered materials starting from metal-metal bonded molecules.