Ligand effects in chromium diphosphine catalysed olefin co-trimerisation and diene trimerisation

A series of symmetric and unsymmetric N,N-bis(diarylphosphino)amine ('PNP') ligands (Ar(2)PN(R)PNAr'(2): R = Me, Ar(2) = o-anisyl, Ar'(2) = Ph, 1, R = Me, Ar(2) = o-tolyl, Ar'(2) = Ph, 2, R = Me, Ar(2) = Ph(o-ethyl), Ar'(2) = Ph, 3, R = Me, Ar(2) = Ar'2 = o-anisyl, 4, R = (i)Pr, Ar(2) = Ar'(2) = Ph, 5) and symmetric N,N'-bis(diarylphosphino)dimethylhydrazine ('PNNP') ligands (Ar(2)PN(Me)N(Me)PAr(2): Ar(2) = o-tolyl, 6, Ar(2) = o-anisyl, 7) have been synthesised. Catalytic screening for ethene/styrene co-trimerisation and isoprene trimerisation was performed via the in situ complexation to [CrCl(3)(THF)(3)] followed by activation with methylaluminoxane (MAO). PNNP catalytic systems showed a significant increase in activity and selectivity over previously reported PNP systems in isoprene trimerisation. Comparing the symmetric and unsymmetric variants in ethene and styrene co-trimerisation resulted in a switch in selectivity, an unsymmetric catalytic (o-anisyl)(2)PN(Me)PPh(2) (1) ligand system affording unique incorporation of two styrenic monomers into the co-trimer product distribution differing from the familiar two ethene and one styrene omega-substituted alkenes. Complexes of the type [(diphosphine)Cr(CO)(4)] 8-11 were also synthesised, the single-crystal X-ray diffraction of which are reported. We propose the mechanisms of these catalytic transformations and an insight into the effect of the ligand series on the chromacyclic catalytic intermediates.