Dimolybdenum bis((S,S,S)-triisopropanolaminate(3-)): A blue compound with an unusual Mo-Mo triple bonds

Mo-2((OBu)-Bu-t)(6) and Mo-2(NMe2)(6) each react with (S,S,S)-triisopropans lamine (2 equiv) in benzene to yield dimolybdenum bis((S,S,S)-isopropanolam inate(3-)), Mo-2[(OC-(S)-HMeCH2)(3)N](2) (M equivalent to M), as a blue cry stalline solid. Cell parameters at -160 degrees C: a =17.389(6) Angstrom, b = 10.843(3) Angstrom, c = 10.463(3) Angstrom, beta = 125.28(1)degrees, Z = 2 in space group C2. The molecular structure involves an Mo-2 unit inside an O6N2 distorted cubic box. The Mo-2 axis is disordered about three positi ons with occupancy factors of ca. 45%, 45% and 10%. Despite this disorder, the molecular structure is shown to contain a central Mo equivalent to Mo u nit of distance 2.15(3) A coordinated to two triolate ligands which each ha ve two chelating arms and one;that spans the Mo equivalent to Mo bond. The local Mo2O6N2 moiety has approximate C-2h symmetry, and the Mo-N distances are long, 2.4 Angstrom. The H-1 and C-13{H-1} NMR spectra recorded in benze ne-d(6) are consistent with the geometry found in the solid-state structure . The blue color arises from weak absorptions, epsilon similar to 150 dm(3) mol(-1) cm(-1), at 580 and 450 nm in the visible region of the electronic absorption spectrum: Raman spectra recorded in KCl reveal pronounced resona nce effects with excitation wavelengths of 488.0, 514.5, and 568.2 nm, part icularly for the 322 cm(-1) band, which can probably be assigned to v(Mo eq uivalent to Mo). The electronic structure of this compound is. investigated by B3LYP DFT calculations, and a comparison is made with the more typical ethane-like (D-3d) Mo-2(OR)(6) compounds is presented. The distortion impos ed on the molecule by the triisopropanolaminate(3-) ligands removes the deg eneracy of the M-M pi molecular orbitals. The HOMO and SHOMO are both M-M p i and M-O sigma* in character, while the LUMO is M-M pi* and-the SLUMO is p redominantly M-O sigma* with metal sp character. The calculated Single;-sin glet transition energies are compared with those implicit in the observed e lectronic spectrum.