Synthesis, characterisation, and molecular and electronic structure of CpMoCl2(R1CCR2) (R1, R2 = Ph, Et, Me): a new class of half-sandwich 17-electron molybdenum(III) organometallics

Abstract

Addition of alkyne to [CpMoCl₂]₂ affords compounds CpMoCl₂(η²-alkyne) (alkyne = EtCCMe, 1; EtCCEt, 2; PhCCMe, 3; PhCCPh, 4) in good yields. The compounds have been characterised by C,H analyses, IR, EPR and mass spectroscopies, magnetic susceptibility, and cyclic voltammetry. In addition, a single crystal X-ray diffraction analysis has been carried out for compound 4. The alkyne ligand adopts an almost parallel conformation relative to the Cp ring, essentially identical with that of previously reported Nb, Ta, and W analogues. Geometry optimisations on the CpMCl₂(HCCH) (M = Nb, Mo) model compounds show that the total energy is nearly independent of the alkyne orientation. The SOMO for M = Mo is an essentially metal-based orbital with a slight Mo-Cl π* component, in agreement with the observed trends in M–Cl bond lengths on going from Group 5 to Group 6. The cyclic voltammetric behaviour of 1–4 is similar to the analogous diene complexes CpMoCl₂(η⁴-diene). A reaction between 1–4 and excess alkyne takes place only for the dialkylsubstituted alkyne complexes under forcing conditions. Compound CpMoCl₂(η⁴-C₄Et₄H), 5, has been isolated from the reaction between 2 and excess EtCCEt and crystallographically characterized.