Four-versus five-co-ordinate bis(imido) alkene complexes of molybdenum: the contrasting effects of tert-butyl- and 2,6-diisopropylphenyl-imido substituents

Abstract

The five-co-ordinate complex [Mo(NC₆H₃Prⁱ₂-2,6)₂(PMe₃)₂(C₂H₄)]2a has been prepared via treatment of [Mo(NC₆H₃Prⁱ₂-2,6)₂Cl₂(dme)]1a(dme = 1,2-dimethoxyethane) with 2 equivalents of EtMgCl in Et₂O in the presence of excess PMe₃; its propene analogue 2b is unstable at reduced pressure, converting to [Mo(NC₆H₃Prⁱ₂-2,6)₂(PMe₃)₂]3 with loss of propene. Complex 2a can also be synthesised, and 2b generated in situ, by treatment of 3 with ethene and propene respectively in hydrocarbon solvent; the analogous η²-butadiene complex [Mo(NC₆H₃Prⁱ₂-2,6)₂(PMe₃)₂(η²-C₄H₆)]2c has also been generated in solution and identified by NMR spectroscopy. The molecular structure of 2a shows a distorted trigonal-bipyramidal co-ordination geometry with axial phosphines and equatorial imido ligands: the ethene ligand aligns in the P–Mo–P plane and perpendicular to the N–Mo–N plane. Complex 3 can also be prepared directly through the reaction of 1a with magnesium turnings in tetrahydrofuran (thf) in the presence of excess PMe₃. By contrast, the reaction of [Mo(NBu^t)₂CI₂(dme)]1b under identical conditions affords the binuclear imido-bridged complex [{Mo(NBu^t)(µ-NBu^t)(PMe₃)}₂]4. Complex 4 does not react with ethene or propene to give the known mononuclear bis(tert-butylimido) derivatives, [Mo(NBu^t)₂(PMe₃)(C₂H₃R)](R = H 5a or Me 5b) reflecting the robust nature of the imido bridges.