Responsive Iron Neighboring Group Participation in Amino-Substituent-Stabilized [3]Ferrocenophane α-Carbenium Ions:  A Combined Theoretical and Experimental Study

Treatment of the dimethylamino-[3]ferrocenophane <BO>1a</BO> with B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF> results in hydride abstraction at the −CH(NMe<INF>2</INF>) moiety in α-position to the ferrocene nucleus to yield the corresponding cation <BO>4a</BO>. The X-ray crystal structure analysis of the salt <BO>4a</BO>·[HO{B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF>}<INF>2</INF>^-] has revealed a residual Fe−C<INF>α</INF> interaction even in this case of a strongly substituent-stabilized α-ferrocenyl carbenium ion. A DFT calculation of a series of derivatives containing nitrogen substituents of a decreasing ability for carbenium ion stabilization (<BO>4a</BO>, −NMe<INF>2</INF>; <BO>4b</BO>, −pyrrolyl; <BO>4c</BO>, −triazolyl) revealed a markedly increased Fe−C interaction with decreasing C⁺ stabilization by the hetarene substituents.