Conformational preferences of bis(acetonitrile)tetrachloro molybdenum(IV) and tungsten(IV). Crystal structure of WCl4(CH3CN)2 and DFT calculations

Abstract

Crystalline MoCl₄(NCCH₃)₂ and WCl₄(NCCH₃)₂ have been obtained by an electrocrystallization process and are shown to be isostructural. They crystallize in the monoclinic space group C2 with a=11.668(3), b=7.616(5), c=5.901(2) Å, β=103.04(3)° and a=11.679(2), b=7.631(3), c=5.896(2) Å, β=103.11(2)°, respectively. The structure of the tungsten compound determined on a single crystal was refined to a final R(F²) value of 0.046 [R_w(F²)=0.112] using 745 observed reflections. The metal is in a distorted octahedral environment with 4 chlorine atoms in the equatorial plane and 2 acetonitrile groups in axial positions. DFT optimization of the geometry was done at the B3LYP/LANLD1Z/LANLD2Z/6–31G*/6–31G** and BPW91/LANLD1Z/LANLD2Z/6–31G*/ 6–31G** levels of theory for singlet and triplet spin states for WCl₄(NCCH₃)₂ in cis and trans conformations, and for WCl₄(NCCH₃)_x (x=1 or 2) with an η² coordination of acetonitrile. At the two levels, the energy minimum corresponds to the trans structure with a triplet spin state. The electronic spectra were assigned and the vibrational, ¹³C and ¹H NMR spectroscopies confirm that the previously reported cis and η² structures are incorrect.