Catalytic applications of keto-stabilised phosphorus ylides based on a macrocyclic scaffold: calixarenes with one or two pendant Ni(P,O)-subunits as ethylene oligomerisation and polymerisation catalysts.
Four calix[4]arenes containing either one or two ylidic -C(O)CH=PPh3 moieties anchored at p-phenolic carbon atoms were prepared starting from cone-25,27-dipropoxycalix[4]arene (1): 1,3-alternate-5,17-bis(2-triphenylphosphoranylideneacetyl)-25,26,27,28-tetrapropoxycalix[4]arene (12), 1,3-alternate-5-(2-triphenylphosphoranylideneacetyl)-25,26,27,28-tetrapropoxycalix[4]arene (13), cone-5-(2-triphenylphosphoranylideneacetyl)-25,27-dihydroxy-26,28-dipropoxycalix[4]arene (14), cone-5,17-bis(2-triphenylphosphoranylideneacetyl)-25,27-dihydroxy-26,28-dipropoxycalix[4]arene (15). All the ylides were shown to be suitable for the preparation of SHOP-type complexes, i.e. of molecules containing [NiPh{Ph2PCH=C(O)R}(PPh3)] subunits (R = calixarene fragment). The monometallic complexes, namely those obtained from the monophosphorus ylides 13 and 14, proved to be efficient ethylene oligomerisation or polymerisation catalysts. At 80 degrees C, they displayed significantly better activities than the prototype [NiPh{Ph(2)PCH=C(O)Ph}(PPh3)], hence reflecting the beneficial role of the bulky calixarene substituent. The systems derived from the two ylides 12 and 15, both containing two convergent ylidic moieties, resulted in lower activities, the proximity of the two catalytic centres facilitating an intramolecular deactivation pathway during the period of catalyst activation. For the first time, the solid-state structure of a complex containing two "NiPh(P,O)(PPh3)" units as well as that of a SHOP-type complex having two linked phosphorus units were determined.