Synthesis, Characterization, Interionic Structure, and Self-Aggregation Tendency of Zirconaaziridinium Salts Bearing Long Alkyl Chains

Zirconaaziridinium inner sphere ion pairs (ISIPs) bearing alkyl chains of different lengths, [Cp2Zr(η2-CH2NR1R2)+···X−] (R1 = Me, R2 = C2H5, X− = MeB(C6F5)3−, ZrNC2BN; R1 = Me, R2 = C16H33, X− = MeB(C6F5)3−, ZrNC16BN; R1 = R2 = C18H37, X− = MeB(C6F5)3−, ZrNC18BN; R1 = Me, R2 = Ph, X− = MeB(C6F5)3−, ZrNPhBN; R1 = R2 = C18H37, X− = B(C6F5)4−, ZrNC18BT; R1 = Me, R2 = Ph, X− = B(C6F5)4−, ZrNPhBT), have been synthesized by the reaction of [Cp2ZrMe+···MeB(C6F5)3−] or [Cp2ZrMe+···B(C6F5)4−] with the suitable tertiary amine followed by selective C−H activation of one Me group of the coordinated amine and methane elimination. Subsequent reaction of ISIPs with THF afforded the corresponding outer-sphere ion pairs (OSIPs) ([ZrNC2THF][BN], [ZrNC16THF][BN], [ZrNC18THF][BN], [ZrNPhTHF][BN], [ZrNC18THF][BT], [ZrNPhTHF][BT]) in which the anion has been displaced into the second coordination sphere. The interionic structure in solution (i.e., the relative cation−anion position) and the self-aggregation level of both ISIPs and OSIPs have been investigated by means of 1H NOESY, 19F,1H HOESY, and diffusion PGSE NMR methods, in low polar solvents. It is found that, independent of the nature or length of the alkyl chains, the anion prefers to pair with the cation from the side of the nitrogen atom and THF in ISIPs and OSIPs, respectively. Self-aggregation of ion pairs into higher aggregates is also weakly influenced by the nature of the alkyl chains, but it is strictly connected with the nature of the counterion: ISIPs and OSIPs bearing B(C6F5)4− showed an increased tendency to form higher aggregates with respect to those containing MeB(C6F5)3−. The level of the self-aggregation in benzene-d6 and cyclohexane-d12 has been quantified by fitting the hydrodynamic volumes obtained from NMR diffusion experiments with several models of indefinite self-association. The IK model, in which the equilibrium constants slightly increase on increasing the aggregation step, best describes the experimental trends in all cases. The standard Gibbs free energy of self-association at 297 K (ΔG0) for ZrNC18BN is −6.7 ± 0.2 kJ mol−1 and −12.6 ± 0.7 kJ mol−1 in benzene-d6 and cyclohexane-d12, respectively. In the same solvents, the values of ΔG0 are −17.9 ± 0.5 kJ mol−1 and −25.0 ± 0.3 kJ mol−1 for [ZrNC18THF][BT]. At room temperature, 80% and 4% of [ZrNC18THF][BT] are present in the form of ion pairs in cyclohexane-d12 at analytical concentrations of 10−5 and 10−3 M, respectively. The self-aggregation of [ZrNC18THF][BN] in cyclohexane-d12 is strongly depressed by increasing the temperature.