Stable Mononuclear, 17-Electron Molybdenum(III) Carbonyl Complexes. Synthesis, Structure, Thermal Decomposition, and Cl - Addition Reactions

International audienceThe first examples of stable, mononuclear 17-electron carbonyl complexes of Mo(III) have been synthesized, isolated, and characterized by IR and EPR spectroscopy. Oxidation of Cp*MoCl(CO)(PMe3)2 (1; E1/2 = −0.48 V vs Fc/Fc+), Cp*MoCl(CO)(dppe) (2; E1/2 = −0.44 V), and CpMoCl(CO)(dppe) (3; E1/2 = −0.25 V) with Fc+PF6- yields [1]PF6, [2]PF6, and [3]PF6, respectively. The IR stretching vibration of the 17-electron oxidation products are 136−153 cm-1 blue-shifted with respect to the corresponding stretching vibrations of the parent Mo(II) compounds. The room temperature EPR spectra show observable coupling to the Mo and P nuclei and indicate a trans geometry for 1+ and a cis geometry for 2+ and 3+. The four-legged piano stool geometry of 2+ with the phosphines atoms in relative cis positions has been confirmed by a single-crystal X-ray analysis. The X-ray data for [2]PF6·THF are the following: monoclinic, P21/n, a = 13.7394(14) Å, b = 20.421(2) Å, c = 14.857(2) Å, β = 99.119(8)°, V = 4115.8(8) Å3, Z = 4, Dx = 1.469 g·cm-3, λ(Mo Kα) = 0.71073 Å, μ (Mo Kα) = 0.562 mm-1, R(F) = 4.59%, R(wF2) = 10.77% for 4299 data with Fo > 4σ(Fo). The thermal stability of the cations decreases with increasing carbonyl stretching frequencies in the order 1+ > 2+ ≫ 3+. The decarbonylated product of thermolysis of 3+, [CpMoClF(dppe)(MeCN)]+PF6-, [4]PF6, which is believed to arise via a 15-electron [CpMoCl(dppe)]+ intermediate, has been isolated and characterized by 1H, 31P, and 19F NMR spectroscopies as well as a single-crystal X-ray analysis. The structure of 4+ shows a distorted pseudooctahedral geometry with the Cp ring and the F atom occupying the two axial coordination sites and the two phosphine atoms of the dppe occupying two cis equatorial sites. The data for [4]PF6·MeCN are as follows: monoclinic, P21/n, a = 13.160(2) Å, b = 10.942(2) Å, c = 25.010(3) Å, β = 95.225(10)°, V = 3586.4(9) Å3, Z = 4, Dx = 1.557 g·cm-3, μ(Mo Kα) = 0.71073 Å, R(F) = 4.11%, R(wF2) = 11.81% for 5964 data with Fo > 4σ(Fo). The nucleophilic addition of Cl- to 1+, 2+, or 3+ is followed by redox processes to afford mixtures of the parent Mo(II) carbonyl complexes 1−3 and Mo(IV) or Mo(V) products, depending on the nature of the ligands. The mechanism of these reactions has been elucidated through parallel chemical and electrochemical studies