Controlling Structure and Reactivity in Cationic Solid-State Molecular Organometallic Systems Using Anion Templating

Abstract

The role that the supporting anion has on the stability, structure, and catalytic performance, in solid-state molecular organometallic systems (SMOM) based upon [Rh(Cy2PCH2CH2PCy2)(η2η2-NBD)][BArX4], [1-NBD][BArX4], is reported (X = Cl, F, H; NBD = norbornadiene). The tetra-aryl borate anion is systematically varied at the 3,5-position, ArX= 3,5-X2C6H3, and the stability and structure in the solid-state compared with the previously reported [1-NBD][BArCF34] complex. Single-crystal X-ray crystallography shows that the three complexes have different packing motifs, in which the cation sits on the shared face of two parallelepipeds for [1-NBD][BArCl4], is surrounded by eight anions in a gyrobifastigium arrangement for [1-NBD][BArF4], or the six anions show an octahedral cage arrangement in [1-NBD][BArH4], similar to that of [1-NBD][BArCF34]. C–X···X–C contacts, commonly encountered in crystal-engineering, are suggested to be important in determining structure. Addition of H2 in a solid/gas reaction affords the resulting σ-alkane complexes, [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArX4] [1-NBA][BArX4] (NBA = norbornane), which can then proceed to lose the alkane and form the zwitterionic, anion-coordinated, complexes. The relative rates at which hydrogenation and then decomposition of σ-alkane complexes proceed are shown to be anion dependent. [BArCF34]– promotes fast hydrogenation and an indefinitely stable σ-alkane complex. With [BArH4]– hydrogenation is slow and the σ-alkane complex so unstable it is not observed. [BArCl4]– and [BArF4]– promote intermediate reactivity profiles, and for [BArCl4]–, a single-crystal to single-crystal hydrogenation results in [1-NBA][BArCl4]. The molecular structure derived from X-ray diffraction reveals a σ-alkane complex in which the NBA fragment is bound through two exo Rh···H–C interactions-different from the endo selective binding observed with [1-NBA][BArCF34]. Periodic DFT calculations demonstrate that this selectivity is driven by the microenvironment dictated by the surrounding anions. [1-NBA][BArX4] are catalysts for gas/solid 1-butene isomerization (298 K, 1 atm), and their activity can be directly correlated to the stability of the σ-alkane complex compared to the anion-coordinated decomposition products.