Solution Structures and Dynamic Properties of Chelated d⁰ Metal Olefin Complexes {η⁵: η¹-C₅R₄SiMe₂N^tBu}Ti(OCMe₂CH₂CH₂CHCH₂)⁺ (R = H, Me):  Models for the {η⁵: η¹-C₅R₄SiMe₂N^tBu}Ti(R‘)(olefin)⁺ Intermediates in “Constrained Geometry” Catalysts

To model the Ti−olefin interaction in the putative {η⁵: η¹-C₅R₄SiMe₂N^tBu}Ti(R‘)(olefin)⁺ intermediates in “constrained geometry” Ti-catalyzed olefin polymerization, chelated alkoxide olefin complexes {η⁵: η¹-C₅R₄SiMe₂N^tBu}Ti(OCMe₂CH₂CH₂CHCH₂)⁺ have been investigated. The reaction of {η⁵: η¹-C₅R₄SiMe₂N^tBu}TiMe₂ (1a,b; R = H, Me) with HOCMe₂CH₂CH₂CHCH₂ yields mixtures of {η⁵-C₅R₄SiMe₂NH^tBu}TiMe₂(OCMe₂CH₂CH₂CHCH₂) (2a,b) and {η⁵: η¹-C₅R₄SiMe₂N^tBu}TiMe(OCMe₂CH₂CH₂CHCH₂) (3a,b). The reaction of 2a/3a and 2b/3b mixtures with B(C₆F₅)₃ yields the chelated olefin complexes [{η⁵: η¹-C₅R₄SiMe₂N^tBu}Ti(OCMe₂CH₂CH₂CHCH₂)][MeB(C₆F₅)₃] (4a,b; 71 and 89% NMR yield). The reaction of 2b/3b with [Ph₃C][B(C₆F₅)₄] yields [{η⁵: η¹-C₅Me₄SiMe₂N^tBu}Ti(OCMe₂CH₂CH₂CHCH₂)][B(C₆F₅)₄] (5b, 88% NMR yield). NMR studies establish that 4a,b and 5b exist as mixtures of diastereomers (isomer ratios:  4a/4a‘, 62/38; 4b/4b‘, 75/25; 5b/5b‘, 75/25), which differ in the enantioface of the olefin that is coordinated. NMR data for these d⁰ metal olefin complexes show that the olefin coordinates to Ti in an unsymmetrical fashion primarily through C_term such that the CC π bond is polarized with positive charge buildup on C_int. Dynamic NMR studies show that 4b/4b‘ undergoes olefin face exchange by a dissociative mechanism which is accompanied by fast inversion of configuration at Ti (“O-shift”) in the olefin-dissociated intermediate. The activation parameters for the conversion of 4b to 4b‘ (i.e., 4b/4b‘ face exchange) are:  ΔH^⧧ = 17.2(8) kcal/mol; ΔS^⧧ = 8(1) eu. 4a/4a‘ also undergoes olefin face exchange but with a lower barrier (ΔH^⧧ = 12.2(9) kcal/mol; ΔS^⧧ = −2(3) eu), for the conversion of 4a to 4a‘.