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

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.