Di-<i>tert-</i>butylbiphenylphosphinimide Titanium and Zirconium Complexes:  Pendant Arene−Metal Interactions

Reaction of t- Bu 2 (2-C 6 H 4 Ph)PNSiMe 3 ( 1 ), with TiCl 4 affords the product t- Bu 2 (2-C 6 H 4 Ph)PNTiCl 3 ( 4 ). This species is readily alkylated with MeMgBr to give t- Bu 2 (2-C 6 H 4 Ph)PNTiMe 3 ( 5 ). X-ray crystallographic studies reveal that both 4 and 5 adopt conformations in which the biphenyl substituent is oriented toward the metal center. In contrast, the species derived from alkylation or amination and subsequent chemistry including t- Bu 2 (2-C 6 H 4 Ph)PNTi(CH 2 Ph) 3 ( 6 ), t- Bu 2 (2-C 6 H 4 Ph)PNTi(NMe 2 ) 3 ( 7 ), t- Bu 2 (2-C 6 H 4 Ph)PNTi(NMe 2 ) 2 Cl ( 8 ), and t- Bu 2 (2-C 6 H 4 Ph)PNTi(CH 2 Ph)(NMe 2 ) 2 ( 9 ) exist in solution as two isomers in which the biphenyl fragment is oriented toward and away from the metal center. The barriers to interconversion of these isomers were determined by variable-temperature NMR studies to be ca. 92 kJ/mol. The analogous Zr complexes t- Bu 2 (2-C 6 H 4 Ph)PNZr(NEt 2 ) 3 ( 10 ), t- Bu 2 (2-C 6 H 4 Ph)PNZrCl 3 ( 11 ), and t- Bu 2 (2-C 6 H 4 Ph)PNZrMe 3 ( 12 ) showed behavior similar to 6 − 9 . Reduction of 4 with Mg powder prompted disproportionation, affording [ t- Bu 2 (2-C 6 H 4 Ph)PN] 2 TiCl 2 ( 13 ). This species also exists as two isomers: in the minor isomer one of the biphenyl substituents is oriented toward and the other away from the Ti center, while in the major isomer both biphenyl units are oriented away from the metal center. A similar observation was made for [ t- Bu 2 (2-C 6 H 4 Ph)PN] 2 TiMe 2 ( 14 ) and [ t- Bu 2 (2-C 6 H 4 Ph)PN] 2 ZrMe 2 ( 15 ). The latter species was formed in a second-order ligand redistribution reaction from 12 with a rate constant of 4 × 10 - 4 s - 1 at 333 K and Δ H ⧧ = 69(2) kJ/mol and Δ S ⧧ = −108(6) J/(mol K). Reaction of 5 with B(C 6 F 5 ) 3 generates [ t- Bu 2 (2-C 6 H 4 Ph)PNTiMe 2 ][MeB(C 6 F 5 ) 3 ] ( 16 ). 16 is a remarkably stable salt in which the Ti-cation is arene-stablized, although the arene is readily displaced by THF to give [ t- Bu 2 (2-C 6 H 4 Ph)PNTiMe 2 (THF)][MeB(C 6 F 5 ) 3 ] ( 18 ). In a similar fashion two isomers of [( t- Bu 2 (2-C 6 H 4 Ph)PN) 2 TiMe][MeB(C 6 F 5 ) 3 ] ( 19 ) are formed. The biphenyl substituents of the major isomer of 19 are oriented in opposing directions, while in the minor isomer both biphenyl substituents are oriented toward the Ti center. In the latter isomer the arene rings are involved in an intramolecular exchange process (Δ H ⧧ = 69(2) kJ/mol, Δ S ⧧ = 60(2) J/(mol K)).

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