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]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 trails 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, P2(1)/n, a = 13.7394(14) Angstrom, b = 20.421(2) Angstrom, c = 14.857(2) Angstrom, beta = 99.119(8)degrees, V = 4115.8(8) Angstrom(3), Z = 4, D-x = 1.469 g cm(-3), lambda(Mo K alpha) = 0.71073 Angstrom, mu (Mo K alpha) = 0.562 mm(-1), R(F) = 4.59%, R(wF(2)) = 110.77% for 4299 data with F-o > 4 sigma(F-o). The thermal stability of the cations decreases with increasing carbonyl Stretching frequencies in the order 1(+) > 2(+) much greater than 3(+). The decarbonylated product of thermolysis of 3(+) [CpMoClF(dppe)(MeCN)](PF6-)-P-+, [4]PF6, which is believed to arise via a 15-electron [CpMoCl(dppe)](+) intermediate, has been isolated and characterized by H-1, P-31, and F-19 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, P2(1)/n, a = 13.160(2) Angstrom, b = 10.942(2) Angstrom, c = 25.010(3) Angstrom, beta = 95.225(10)degrees, V = 3586.4(9) Angstrom(3), Z = 4, D-x = 1.557 g cm(-3), mu(Mo K alpha) = 0.71073 Angstrom, R(F) = 4.11%, R(wF(2)) = 11.81% for 5964 data with F-o > 4 sigma(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.