Electronic Absorption Spectra and Phosphorescence of Oxygen-Containing Molybdenum(IV) Complexes

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Electronic absorption and emission spectra are reported for salts of two oxomolybdenum(IV) cations, [MoOCl-(CN-t-Bu)4]+ and [MoOCl(Ph2PCH2CH2PPh2) 2]+. and for the new Mo(IV) complex [trans-Mo(OCH3)2(CN-t-Bu)4]2+. All three ions show absorption bands (λmax.abs 550-570 nm; ∈ 45-120 M-1 cm-1) attributable to the 1A1[(dxy)2] → 1E[(dxy)1(dxz,yz)1] (C4v) transition, and the last two show weak shoulders in the 700-750 nm range due to the analogous spin-forbidden (1A1 → 3E) transition. Phosphorescence (λmax,em 850-960 nm) occurs in the solid state for all three compounds at both room temperature and 77 K, and for [MoOCl(CN-t-Bu)4]+ in CH2Cl2 at room temperature. These are the first phosphorescences recorded for molybdenum(IV) complexes. [MoOCl-(CN-t-Bu)4](BPh4) precipitates quickly if NaBPh4 is added to the Mo(IV) solution prepared from MoCl5 and tert-butyl isocyanide in CH3OH. However, if NaPF6 is used instead, [trans-Mo(OCH3)2(CN-t-Bu)4](PF 6)2 (formed by reaction of [MoOCl(CN-t-Bu)4]+ with methanol) crystallizes over a period of ca. 24 h. The crystal structure of [trans-Mo(OCH3)2(CN-t-Bu)4](PF 6)2 has been determined: C22H42F12MoN4O2P 2, monoclinic; space group P21/ c; a = 9.1538(8) Å, b = 15.709(2) Å, c = 13.456(2) Å; β= 103.31(1)°; Z = 2; R(F) = 0.063, RW(F) = 0.056 for 2719 reflections with I > σ(I).

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Inorganic Chemistry

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