Syntheses, Crystal Structures, and Density Functional Theory Calculations of the closo-[1-M(CO)₃(η⁴-E₉)]^4- (E = Sn, Pb; M = Mo, W) Cluster Anions and Solution NMR Spectroscopic Characterization of [1-M(CO)₃(η⁴-Sn₉)]^4- (M = Cr, Mo, W)^†

The closo-[1-M(CO)₃(η⁴-E₉)]^4- (E = Sn, Pb; M = Mo, W) anions have been obtained by extracting the binary alloys KSn_2.05 and KPb_2.26 in ethylenediamine (en) in the presence of 2,2,2-crypt or in liquid NH₃ followed by reaction with M(CO)₃·mes (M = Mo, W) or Cr(CO)₃·tol in en or liquid NH₃ solution. Crystallization of the molybdenum and tungsten salts was induced by vapor diffusion of tetrahydrofuran into the en solutions. The salts [2,2,2-crypt-K]₄[1-M(CO)₃(η⁴-Sn₉)]·en (M = Mo, W) crystallize in the triclinic system, space group P1̄, Z = 4, a = 16.187(3) Å, b = 25.832(4) Å, c = 29.855(5) Å, α = 111.46(1)°, β = 102.84(2)°, γ = 92.87(2)° at −95 °C (M = Mo) and a = 17.018(3) Å, b = 27.057(5) Å, c = 28.298(6) Å, α = 66.42(3)°, β = 76.72(3)°, γ = 87.27(3)° at 20 °C (M = W). The salts (CO)₃M(en)₂[2,2,2-crypt-K]₄[1-M(CO)₃(η⁴-Pb₉)]·2.5en (M = Mo, W) crystallize in the triclinic system, space group P1̄, Z = 2, a = 16.319(3) Å, b = 17.078(3) Å, c = 24.827(5) Å, α = 71.82(3)°, β = 83.01(3)°, γ = 81.73(3)° at −133 °C (M = Mo) and a = 16.283(4) Å, b = 17.094(3) Å, c = 24.872(6) Å, α = 71.62(2)°, β = 82.91(2)°, γ = 81.35(2)° at −153 °C (M = W). The [1-M(CO)₃(η⁴-Sn₉)]^4- anions were also characterized in liquid NH₃ solution by ¹¹⁹Sn, ¹¹⁷Sn, and ⁹⁵Mo NMR spectroscopy. Unlike their fluxional precursor, nido-Sn₉^4-, NMR studies show that the [1-M(CO)₃(η⁴-Sn₉)]^4- anions are rigid on the NMR time scale. All possible inter- and intraenvironmental couplings, J(^119,117Sn−^119,117Sn), J(^119,117Sn−¹⁸³W), and one J(^119,117Sn−⁹⁵Mo) coupling, have been observed and assigned. Complete spin−spin coupling constant assignments were achieved by detailed analyses and simulations of all spin multiplets that comprise the ¹¹⁹Sn and ¹¹⁷Sn NMR spectra and that arise from natural abundance tin isotopomer distributions and from natural abundance ¹⁸³W, in the case of [1-W(CO)₃(η⁴-Sn₉)]^4-. Both the solid state and solution structures of the [1-M(CO)₃(η⁴-Sn₉)]^4- anions are based on a closo-bicapped square antiprismatic structure in which the transition metal occupies a cap position. The cluster structures are consistent with Wade's rules for 22 (2n + 2) skeletal electron systems. Electron structure calculations at the density functional theory (DFT) level provide fully optimized geometries that are in agreement with the experimental structures. Complete assignment of the NMR spectra was also aided by GIAO calculations. The calculated vibrational frequencies of the E₉^4- and [1-M(CO)₃(η⁴-E₉)]^4- anions are also reported and are used to assign the solid-state vibrational spectra of the [1-M(CO)₃(η⁴-E₉)]^4- anions.