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

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; ε 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](PF6)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](PF6)2 has been determined:  C22H42F12MoN4O2P2, 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).