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

• Published in: Organometallics Vol. 37; no. 20; pp. 3524 - 3532
• Main Authors: McKay, Alasdair I, Martínez-Martínez, Antonio J, Griffiths, Hannah J, Rees, Nicholas H, Waters, Jordan B, Weller, Andrew S, Krämer, Tobias, Macgregor, Stuart A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22-10-2018
• DOI: 10.1021/acs.organomet.8b00215
• EISSN: 1520-6041
• ISSN: 0276-7333

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)]­[BArX 4], [1-NBD]­[BAr X 4 ], 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]­[BAr CF3 4 ] 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]­[BAr Cl 4 ], is surrounded by eight anions in a gyrobifastigium arrangement for [1-NBD]­[BAr F 4 ], or the six anions show an octahedral cage arrangement in [1-NBD]­[BAr H 4 ], similar to that of [1-NBD]­[BAr CF3 4 ]. 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)]­[BArX 4] [1-NBA]­[BAr X 4 ] (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. [BAr CF3 4 ] – promotes fast hydrogenation and an indefinitely stable σ-alkane complex. With [BAr H 4 ] – hydrogenation is slow and the σ-alkane complex so unstable it is not observed. [BAr Cl 4 ] – and [BAr F 4 ] – promote intermediate reactivity profiles, and for [BAr Cl 4 ] – , a single-crystal to single-crystal hydrogenation results in [1-NBA]­[BAr Cl 4 ]. 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]­[BAr CF3 4 ]. Periodic DFT calculations demonstrate that this selectivity is driven by the microenvironment dictated by the surrounding anions. [1-NBA]­[BAr X 4 ] 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.