An integrated thermodynamic and kinetic study was carried out to clarify how the formation of ionic aggregates higher than ion pairs affects the rate of olefin insertion into the Zr-C bond. To this aim, the reaction with olefin of three-member zirconaziridium metallacycles [Cp2Zr(η2-CH2-N(R)2)]+[X]- [R = C18H37; X- = MeB(C6F5)3- (1BN) or B(C6F5)4- (1BT)], models for tight (t-ISIP, 1BN) and loose (l-ISIP, 1BT) inner sphere ion pair (ISIPs), was studied at different levels of concentration and thus self-aggregation. Both 1BN and 1BT undergo a regioselective insertion of a single α-olefin molecule (1-hexene or 2-Me-1-heptene) into the Zr-C bond to form the corresponding five-member metallacycles outer sphere ion pairs (2BN, 2BT, 3BN, and 3BT), with no evidence for further olefin insertions. Under comparable experimental conditions, where monomeric ion pairs dominate (aggregation number N ≈ 1), the apparent rate constant (kapp) of 1-hexene insertion into 1BT is about 200 times higher than that in 1BN, in agreement with the weaker strength of cation-anion interaction in the former. For 1BT, kapp is only marginally affected by the self-aggregation level in all investigated experimental conditions. For example, kapp = 1.1 M-1 s-1 (N ≈ 1.1) and 1.3 (N ≈ 4.8) for the insertion reaction of 1-hexene in toluene at 223 K and range from 19 × 10-3 M-1 s-1 (N ≈ 1.5) to 10 × 10-3 (N ≈ 39) for the insertion of 2-Me-1-heptene in methylcyclohexane at 258 K. On the contrary, kapp is markedly dependent on self-aggregation in the case of 1BN. For instance, when 1BN is reacted with 1-hexene in methylcyclohexane at 258 K, kapp increases more than 1 order of magnitude, from 6 × 10-3 to 93 × 10-3 M-1 s-1, upon increasing the aggregation level from N ≈ 1 ([1BN] = 6 × 10-5 M) to N ≈ 4.6 ([1BN] = 12.8 × 10-3 M). Although the insertion of 1-hexene in 1BN is faster in the slightly more polar solvent toluene (kapp = 39 × 10-5 M, N ≈ 1), the relative reactivity difference between ion pairs and larger ionic aggregates is similar in both solvents (about 4× and 3× increment, on passing from N ≈ 1.0 to N ≈ 1.6, in methylcyclohexane and toluene, respectively). The addition of an inert external salt with a common counterion (2BN) to solution of 1BN dramatically raises the olefin insertion rate constant: at [1BN] = 6 × 10-5 M, kapp increases from 6 × 10-3 M-1 s-1, in the absence of 2BN, to 42 × 10-3, 90 × 10-3, and 166 × 10-3 M-1 s-1 in the presence of 1, 3, and 5 equiv of 2BN, respectively. All of the thermodynamic and kinetic data can be rationalized by taking into account the strength of cation-anion interaction in the two kinds of ion pairs. In t-ISIPs, the cation-anion interaction energy is relatively strong, resulting in a relatively small dipole moment and a little thermodynamic tendency to self-aggregate. However, under conditions where homo (or mixed) ion aggregates form, an additional weakening of the cation-anion interaction occurs, causing a substantial increase of the intrinsic reactivity of each t-ISIP within aggregates. On the other side, the cation-anion interaction is already relatively weak in l-ISIPs and ionic aggregates forms much more easily from the thermodynamic point of view; in this case, however, the extra weakening of cation-anion interaction only marginally alters the overall barrier for olefin insertion in each l-ISIP.
Understanding the Role of Metallocenium Ion-Pair Aggregates on the Rate of Olefin Insertion into the Metal-Carbon Bond
Sian L.;Macchioni A.;Zuccaccia C.
2020
Abstract
An integrated thermodynamic and kinetic study was carried out to clarify how the formation of ionic aggregates higher than ion pairs affects the rate of olefin insertion into the Zr-C bond. To this aim, the reaction with olefin of three-member zirconaziridium metallacycles [Cp2Zr(η2-CH2-N(R)2)]+[X]- [R = C18H37; X- = MeB(C6F5)3- (1BN) or B(C6F5)4- (1BT)], models for tight (t-ISIP, 1BN) and loose (l-ISIP, 1BT) inner sphere ion pair (ISIPs), was studied at different levels of concentration and thus self-aggregation. Both 1BN and 1BT undergo a regioselective insertion of a single α-olefin molecule (1-hexene or 2-Me-1-heptene) into the Zr-C bond to form the corresponding five-member metallacycles outer sphere ion pairs (2BN, 2BT, 3BN, and 3BT), with no evidence for further olefin insertions. Under comparable experimental conditions, where monomeric ion pairs dominate (aggregation number N ≈ 1), the apparent rate constant (kapp) of 1-hexene insertion into 1BT is about 200 times higher than that in 1BN, in agreement with the weaker strength of cation-anion interaction in the former. For 1BT, kapp is only marginally affected by the self-aggregation level in all investigated experimental conditions. For example, kapp = 1.1 M-1 s-1 (N ≈ 1.1) and 1.3 (N ≈ 4.8) for the insertion reaction of 1-hexene in toluene at 223 K and range from 19 × 10-3 M-1 s-1 (N ≈ 1.5) to 10 × 10-3 (N ≈ 39) for the insertion of 2-Me-1-heptene in methylcyclohexane at 258 K. On the contrary, kapp is markedly dependent on self-aggregation in the case of 1BN. For instance, when 1BN is reacted with 1-hexene in methylcyclohexane at 258 K, kapp increases more than 1 order of magnitude, from 6 × 10-3 to 93 × 10-3 M-1 s-1, upon increasing the aggregation level from N ≈ 1 ([1BN] = 6 × 10-5 M) to N ≈ 4.6 ([1BN] = 12.8 × 10-3 M). Although the insertion of 1-hexene in 1BN is faster in the slightly more polar solvent toluene (kapp = 39 × 10-5 M, N ≈ 1), the relative reactivity difference between ion pairs and larger ionic aggregates is similar in both solvents (about 4× and 3× increment, on passing from N ≈ 1.0 to N ≈ 1.6, in methylcyclohexane and toluene, respectively). The addition of an inert external salt with a common counterion (2BN) to solution of 1BN dramatically raises the olefin insertion rate constant: at [1BN] = 6 × 10-5 M, kapp increases from 6 × 10-3 M-1 s-1, in the absence of 2BN, to 42 × 10-3, 90 × 10-3, and 166 × 10-3 M-1 s-1 in the presence of 1, 3, and 5 equiv of 2BN, respectively. All of the thermodynamic and kinetic data can be rationalized by taking into account the strength of cation-anion interaction in the two kinds of ion pairs. In t-ISIPs, the cation-anion interaction energy is relatively strong, resulting in a relatively small dipole moment and a little thermodynamic tendency to self-aggregate. However, under conditions where homo (or mixed) ion aggregates form, an additional weakening of the cation-anion interaction occurs, causing a substantial increase of the intrinsic reactivity of each t-ISIP within aggregates. On the other side, the cation-anion interaction is already relatively weak in l-ISIPs and ionic aggregates forms much more easily from the thermodynamic point of view; in this case, however, the extra weakening of cation-anion interaction only marginally alters the overall barrier for olefin insertion in each l-ISIP.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
