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-(Cy 2 PCH 2 CH 2 PCy 2)(η 2 η 2-NBD)][BAr X 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, Ar X = 3,5-X 2 C 6 H 3 , 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 H 2 in a solid/gas reaction affords the resulting σ-alkane complexes, [Rh(Cy 2 PCH 2 CH 2 PCy 2)(η 2 η 2-NBA)][BAr X 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 σalkane 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.

Selectivity of Rh⋅⋅⋅H−C Binding in a σ‐Alkane Complex Controlled by the Secondary Microenvironment in the Solid State

“…In both cases, more localized disc‐like features reflect the presence of stabilizing nonclassical C−H δ+ ⋅⋅⋅F δ− −C hydrogen bonds. Underscoring the importance of this local anion environment, the exo ‐regioselectivity for NBA binding is also observed in [Rh(dcpe)‐ exo ‐(NBA)][BAr Cl 4 ], [Cy‐ exo ‐NBA][BAr Cl 4 ] (Ar Cl =3,5‐Cl 2 ‐C 6 H 3 ) [22] . Despite the [BAr Cl 4 ] − adopting a very different periodic arrangement of anions (Figure 4 B), the microenvironment around the NBA ligand is broadly similar to that of [ t Bu‐ exo ‐NBA][BAr F 4 ] .…”

MicroED characterization of a robust cationic σ-alkane complex stabilized by the [B(3,5-(SF5)2C6H3)4]− anion, via on-grid solid/gas single-crystal to single-crystal reactivity

“…These data combine to show that the [S-BAr F 4 ] − anion supports the formation of a stable σ-alkane complex, unlike alternative anions such as [B(3,5-Cl 2 -C 6 H 3 ) 4 ] − where loss of NBA and anion-coordination results. 23 …”

“…These data combine to show that the [S-BAr F 4 ] − anion supports the formation of a stable σ-alkane complex, unlike alternative anions such as [B(3,5-Cl 2 -C 6 H 3 ) 4 ] − where loss of NBA and anion-coordination results. 23 As the formation of suitably sized crystals for SC X-ray diffraction was challenging and stochastic, we were interested if microED could provide a complementary structural solution of [1-NBA][S-BAr F 4 ]. MicroED has recently been used for the structural determination of microcrystalline organometallic 24 and coordination complexes, 25 as well as complex organic molecules 19,26 and MOFs.…”

Metathesis by Partner Interchange in σ‐Bond Ligands: Expanding Applications of the σ‐CAM Mechanism

“…[8,[22][23][24] Most notably,s everal rhodium complexes and one cobalt complex have been characterized by single crystal X-ray diffraction, using single-crystal to single-crystal reactivity of molecular alkene precursors by simple addition of H 2 ,providing the long sought geometric proof of structure (Figure 1a). [25][26][27][28][29][30][31] The majority of these complexes contain an alkane ligand coordinated by two CÀHb onds on different carbon atoms to the metal, each in aM(h 2 -CÀH) mode (Figure 1). M(h 1 -CÀ H) coordination modes are also reported depending on the identity of the metal/ligand/alkane.Many of these complexes can be observed at room temperature,aconsequence of the stabilizing non-covalent interactions provided by the secondary anion microenvironment in the solid state.…”