Synthesis, structures, and reactivity of weakly coordinating anions with delocalized berate structure: The assessment of anion effects in metallocenepolymerization catalysts

The formation of adducts of tris(pentafluorophenyl)borane with strongly coo rdinating anions such as CN- and [M(CN)(4)](2-) (M = Ni, Pd) is a synthetic ally facile route to the bulky, very weakly coordinating anions [CN{B(C6F5) (3)}(2)](-) and [M{CNB(C6F5)(3)}(4)](2-) which are isolated as stable NHMe2 Ph+ and CPh3+ salts. The crystal structures of [CPh3][CN{B(C6F5)(3)}(2)] (1 ), [CPh3][ClB(C6F5)(3)] (2), [NHMe2Ph](2)[Ni{CNB(C6F5)(3)}(4)]. 2Me(2)CO (4 b . 2Me(2)CO), [CPh3](2)[Ni{CNB(C6F5)(3)}(4)]. 2CH(2)Cl(2) (4c . 2CH(2)Cl(2 )), and [CPh3](2)[Pd{CNB(C6F5)(3)}(4)]. 2CH(2)Cl(2) (5c . 2CH(2)Cl(2)) are reported. The CN stretching frequencies in 4 and 5 are shifted by similar t o 110 cm(-1) to higher wavenumbers compared to the parent tetracyano comple xes in aqueous solution, although the M-C and C-N distances show no signifi cant change on B(C6F5)(3) coordination. Zirconocene dimethyl complexes L2Zr Me2 [L-2 = Cp-2, SBI = rac-Me2Si(Ind)(2)] react with 1, 4c or 5c in benzene solution at 20 degreesC to give the salts of binuclear methyl-bridged cati ons, [(L2ZrMe)(2)(mu -Me)] [CN{B(C6F5)(3)}(2)] and [(L2ZrMe)(2)(mu -Me)](2) [M{CNB-(C6F5)(3)}(4)] The reactivity of these species in solution was studi ed in comparison with the known [{(SBI)ZrMe}(2)(mu -Me)][B(C6F5)(4)]. While the latter reacts with excess [CPh3][B(C6F5)(4)] in benzene to give the mo nonuclear ion pair [(SBI)ZrMe+. . .B(C6F5)(4)(-)] in a pseudo-first-order r eaction, k = 3 x 10(-4) s(-1), [(L2- ZrMe)(2)(mu -Me)][CN{B(C6F5)(3)}(2)] r eacts to give a mixture of L2ZrMe(mu -Me)B (C6F5)(3) and L2ZrMe(mu -NC)B(C6 F5)(4)] Recrystallization of [Cp"Zr-2(mu -Me)(2)AlMe2] [CN{B(C6F5)(3)}(2)] affords Cp"2ZrMe(mu -NC)B(C6F5)(3) 6, the X-ray structure of which is repor ted. The stability of [L2ZrMe)(2)(mu -Me)]X-+(-) decreases in the order X [ B(C6F5)(4)] > [M{CNB(C6F5)(3)}(4)] > [CN{B(C6F5)(3)}(2)] and increases stro ngly with the steric bulk of L-2 Cp-2 << SBI. Activation of (SBI)ZrMe2 by 1 in the presence of AlBu3i gives extremely active ethene polymerization cat alysts. Polymerization studies at 1-7 bar monomer pressure suggest that the se, and by implication most other highly active ethene polymerization catal ysts, are strongly mass-transport limited. By contrast, monitoring propene polymerization activities with the systems (SBI)ZrMe2/1/AlBu3i and CGCTiMe(2)/ 1/AlBu3i at 20 degreesC as a function o f catalyst concentration demonstrates that in these cases mass-transport li mitation is absent up to [metal] approximate to 2 x 10(-5) mol L-1. Propene polymerization activities decrease in the order [CN{B(C6F5)(3)}(2)](-) > [ B(C6F5)(4)](-) > [M{CNB(C6F5)(3)}(4)](2-) >> [MeB(C6F5)(3)](-), with differ ences in activation barriers relative to [CN{B(C6F5)(3)}(2)](-) of Delta De ltaG double dagger = 1.1 (B(C6F5)(4)(-)), 4.1 (Ni{CNB(C6F5)(3)}(4)(2-)) and 10.7-12.8 kJ mol(-1) (MeB(C6F5)(3)(-)). The data suggest that even in the case of very bulky anions with delocalized negative charge the displacement of the anion by the monomer must be involved in the rate-limiting step.