Size-Selective Hydroformylation by a Rhodium Catalyst Confined in a Supramolecular Cage

Abstract

Size-selective hydroformylation of terminal alkenes was attained upon embedding a rhodium bisphosphine complex in a supramolecular metal-organic cage that was formed by subcomponent self-assembly. The catalyst was bound in the cage by a ligand-template approach, in which pyridyl-zinc(II) porphyrin interactions led to high association constants (>10⁵ m^-1 ) for the binding of the ligands and the corresponding rhodium complex. DFT calculations confirm that the second coordination sphere forces the encapsulated active species to adopt the ee coordination geometry (i.e., both phosphine ligands in equatorial positions), in line with in situ high-pressure IR studies of the host-guest complex. The window aperture of the cage decreases slightly upon binding the catalyst. As a result, the diffusion of larger substrates into the cage is slower compared to that of smaller substrates. Consequently, the encapsulated rhodium catalyst displays substrate selectivity, converting smaller substrates faster to the corresponding aldehydes. This selectivity bears a resemblance to an effect observed in nature, where enzymes are able to discriminate between substrates based on shape and size by embedding the active site deep inside the hydrophobic pocket of a bulky protein structure.

Keywords: cage compounds; hydroformylation; porphyrins; substrate selectivity; supramolecular chemistry.

Scheme 1

(a) Formation of the encapsulated supramolecular bidentate phosphine ligand through selective encapsulation of two trispyridylphosphine ligands (L3) inside cage Fe₄(Zn‐L)₆ driven by tritopic pyridyl–zinc porphyrin coordination. (b) Hydroformylation of mixtures of substrates by a non‐encapsulated rhodium phosphine catalyst. (c) Size‐selective hydroformylation of mixtures of substrates by an encapsulated rhodium phosphine catalyst. ee=equatorial‐equatorial isomer, ea=equatorial‐apical isomer.

Scheme 2

(a) Formation of the tetrahedral cage Fe₄(Zn‐L)₆ through metal‐directed self‐assembly of the corresponding building blocks and a schematic representation of the resulting nanocage. For clarity, only one of the six ligands occupying the edges of the cage is shown in the molecular structure. OTf=trifluoromethanesulfonate. (b) xTB‐optimized structure of cage Fe₄(Zn‐L)₆.

Figure 1

Molecular structures of the various pyridine‐functionalized guests.

Figure 2

(a) A shift in equilibrium from a mixture of ee and ea isomers in the bulk solution to solely the ee isomer during encapsulation of the active species. (b) xTB‐optimized structure of the encapsulated active species Rh1@Fe₄(Zn‐L)₆.

Figure 3

High‐resolution CSI‐MS spectra of the empty host and the catalyst–cage assembly. (Top) Full mass spectrum of Fe₄(Zn‐L)₆ showing the different charged species, with an inset showing the theoretical and experimental isotopic distribution of the 8+ signal. The peak with the m/z ratio of 900 belongs to a single ligand of the cage, and the peak with the m/z ratio of 993 belongs to a symmetrically fragmented cage. (Bottom) Full mass spectrum of Rh4@Fe₄(Zn‐L)₆ showing the different charged species, with an inset showing the theoretical and experimental isotopic distribution of the 5+ signal. The peak with the m/z ratio of 661 belongs to the fragment [Rh(CO)(L3)₂]¹⁺.

Figure 4

(a) Overlay of UV/Vis spectra of the titration of Fe₄(Zn‐L)₆ (host) and L3 (guest) at a constant host concentration of 8.8 μm in acetonitrile at 298 K. (b) Variation in the absorption at the Q bands versus the logarithm of the equivalents of added guest. H=host, G=guest, HGG=1:2 host–guest complex.

Figure 5

(a) ¹H NMR (400 MHz, 298 K) spectrum of Fe₄(Zn‐L)₆ (top) and (L2)₂@Fe₄(Zn‐L)₆ (bottom) in CD₃CN. (b) Variable‐temperature ¹H NMR (400 MHz) spectrum of (L2)₂@Fe₄(Zn‐L)₆ in CD₃CN.

Figure 6

¹H NMR (500 MHz, 298 K) spectrum of Fe₄(Zn‐L)₆ (top) and Rh4@Fe₄(Zn‐L)₆ (bottom) in CD₃CN. The bottom spectrum contains a 1:2:1 mixture of Rh(acac)(CO)₂, L3 and Fe₄(Zn‐L)₆.

Figure 7

Strategies for the formation of the encapsulated active species.

Figure 8

High‐pressure IR spectrum of the rhodium hydrido species (a) in the absence of the cage and (b) in the presence of the cage at 20 bar syngas at 298 K in a 3:2 mixture of acetonitrile and dichloromethane.

Figure 9

High‐pressure IR spectrum of Rh1@Fe₄(Zn‐L)₆ at 20 bar H₂/CO (green trace) and at 20 bar D₂/CO (red trace) at 298 K in a 3:2 mixture of acetonitrile and dichloromethane.

Figure 10

Results of the hydroformylation of alkenes ranging from 1‐hexene to 1‐decene (left) with the encapsulated catalyst and (right) with the catalyst in the bulk solution. All of the experiments were performed in duplicate, and estimated errors are 10 % and indicated with error bars.

Figure 11

Results of the xTB and DFT calculations with the encapsulated catalyst–substrate complexes. (a) xTB‐optimized structures for catalyst–substrate complexes with 1‐hexene (left), 1‐heptene (middle) and 1‐octene (right). The folded alkene is shown in red. (b) Energies and structures of the alkenes calculated by DFT. Δ(dG) refers to the folding energy obtained by taking the difference in the Gibbs free energy of the folded alkene versus that of the linear alkene. (c) Odd‐even effect in the folding energy.

Figure 12

Structure of the bifunctional substrate Sub1.