Cationic Group 4 Metallocene-(o-Phosphanylaryl)oxido Complexes: Synthetic Routes to Transition-Metal Frustrated Lewis Pairs

Synthetic routes to cationic group 4 metallocene(o-phosphanylaryl)oxido compounds of the type [CpR2M(OPR2)][WCA] (M = Ti, Zr, Hf; WCA = weakly coordinating anion) are described. The neutral mono-methyl complexes [CpR2ZrMe(OPR2)] 16 [CpR = Cp (13) or Cp* (4); OPR2 = o-OC6H4(PtBu)2 (1 and 4), OCMe2CH2(PtBu)2 (2) or OC(CF3)2CH2(PtBu)2 (3)] are prepared by protonolysis of [CpR2ZrMe2] by the parent alcohol. The remaining methyl group in such complexes is best removed by protonolysis with [DTBP][B(C6F5)4] (DTBP = 2,6-di-tert-butylpyridinium) to yield the desired cationic complexes 7 and 8 in the case of 1 and 4. In the case of 2 and 3, this method leads to side reactions. Treatment with B(C6F5)3 yields the desired cations in all cases; however, side reactions with the generated [MeB(C6F5)3] anion in subsequent reactions leads to problems. Hafnium analogues may be synthesised by similar routes. In the case of titanium, a different method must be adopted: chloride abstraction using [Et3Si][B(C6F5)4] from the parent complex [Cp2TiCl(OPR2)]. Such cationic group 4 metallocene(o-phosphanylaryl)oxido compounds exhibit reactivity that is best described by the frustrated Lewis pair concept.