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

The 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)(PMe 3 ) 2 ( 1; E 1/2 = −0.48 V vs Fc/Fc + ), Cp*MoCl(CO)(dppe) ( 2; E 1/2 = −0.44 V), and CpMoCl(CO)(dppe) ( 3; E 1/2 = −0.25 V) with Fc + PF 6 - yields [ 1 ]PF 6, [ 2 ]PF 6, and [ 3 ]PF 6, 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 ]PF 6 ·THF are the following: monoclinic, P 2 1 / n, a = 13.7394(14) Å, b = 20.421(2) Å, c = 14.857(2) Å, β = 99.119(8)°, V = 4115.8(8) Å 3, Z = 4, D x = 1.469 g·cm - 3, λ(Mo Kα) = 0.71073 Å, μ (Mo Kα) = 0.562 mm - 1, R ( F ) = 4.59%, R ( wF 2 ) = 10.77% for 4299 data with F o > 4σ( F o ). 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)] + PF 6 -, [ 4 ]PF 6, which is believed to arise via a 15-electron [CpMoCl(dppe)] + intermediate, has been isolated and characterized by 1 H, 31 P, and 19 F 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 ]PF 6 ·MeCN are as follows: monoclinic, P 2 1 / n, a = 13.160(2) Å, b = 10.942(2) Å, c = 25.010(3) Å, β = 95.225(10)°, V = 3586.4(9) Å 3, Z = 4, D x = 1.557 g·cm - 3, μ(Mo Kα) = 0.71073 Å, R ( F ) = 4.11%, R ( wF 2 ) = 11.81% for 5964 data with F o > 4σ( F o ). 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.