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. 2018 Nov 1;3(11):14538-14550.
doi: 10.1021/acsomega.8b01569. eCollection 2018 Nov 30.

"Weakly Ligated, Labile Ligand" Nanoparticles: The Case of Ir(0) n ·(H+Cl-) m

Joseph E Mondloch  1 , Saim Özkar  2 , Richard G Finke  1
Affiliations

"Weakly Ligated, Labile Ligand" Nanoparticles: The Case of Ir(0) n ·(H+Cl-) m

Joseph E Mondloch et al. ACS Omega. .

Abstract

It is of considerable interest to prepare weakly ligated, labile ligand (WLLL) nanoparticles for applications in areas such as chemical catalysis. WLLL nanoparticles can be defined as nanoparticles with sufficient, albeit minimal, surface ligands of moderate binding strength to meta-stabilize nanoparticles, initial stabilizer ligands that can be readily replaced by other, desired, more strongly coordinating ligands and removed completely when desired. Herein, we describe WLLL nanoparticles prepared from [Ir(1,5-COD)Cl]2 reduction under H2, in acetone. The results suggest that H+Cl--stabilized Ir(0) n nanoparticles, herein Ir(0) n ·(H+Cl-) a , serve as a WLLL nanoparticle for the preparation of, as illustrative examples, five specific nanoparticle products: Ir(0) n ·(Cl-Bu3NH+) a , Ir(0) n ·(Cl-Dodec3NH+) a , Ir(0) n ·(POct3)0.2n (Cl-H+) b , Ir(0) n ·(POct3)0.2n , and the γ-Al2O3-supported heterogeneous catalyst, Ir(0) n ·(γ-Al2O3) a (Cl-H+) b . (where a and b vary for the differently ligated nanoparticles; in addition, solvent can be present as a nanoparticle surface ligand). With added POct3 as a key, prototype example, an important feature is that a minimum, desired, experimentally determinable amount of ligand (e.g., just 0.2 equiv POct3 per mole of Ir) can be added, which is shown to provide sufficient stabilization that the resultant Ir(0) n ·(POct3)0.2n (Cl-H+) b is isolable. Additionally, the initial labile ligand stabilizer HCl can be removed to yield Ir(0) n ·(POct3)0.2n that is >99% free of Cl- by a AgCl precipitation test. The results provide strong support for the weakly ligated, labile ligand nanoparticle concept and specific support for Ir(0) n ·(H+Cl-) a as a WLLL nanoparticle.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Nanoparticle Formation Stoichiometry and Acetone Hydrogenation Activity of Ir(0)n·(Cl–H+)a(acetone)b WLLL Nanoparticles Starting from [Ir(1,5-COD)Cl]2 in Neat Acetone at 22 °C under an Initial 3.7 atm of H2
Scheme 2
Scheme 2. Formation and Possible Applications of Ir(0)n·(Cl–H+)a(acetone)b as a Weakly Ligated, Labile Ligand (WLLL) Precursor
The Ir(0)n·(Cl–H+)a(acetone)b is shown as a true synthon in the scheme to illustrate the concept; it should be noted, however, that, in the present work, the Ir(0)n·(Cl–H+)a(acetone)b is not premade or preisolated, but rather is formed in situ with the added ligands such as R3N and POct3, or solid support, γ-Al2O3.
Scheme 3
Scheme 3. Stoichiometry of Formation of Bu3NH+Cl–-Stabilized Ir(0)n Nanoparticles
Figure 1
Figure 1
(a) Hydrogen uptake vs time plot for the hydrogenation of cyclohexene plus acetone and concomitant formation of iridium(0) nanoparticles starting with [(1,5-COD)IrCl]2 (1.2 mM Ir) and cyclohexene (1.65 M) in acetone at 22.0 ± 0.1 °C. Both cyclohexene and acetone undergo hydrogenation; the former is faster than the latter. (b) Loss of cyclohexene versus time curve in the hydrogenation of 1.65 M cyclohexene and concomitant formation of Ir(0) nanoparticles starting with [(1,5-COD)IrCl]2 (1.2 mM Ir) and 1.2 mM Bu3N in acetone at 22.0 ± 0.1 °C. A linear hydrogenation of cyclohexene starts with an initial turnover frequency (TOF) of 1.1 s–1. The pressure rise in the initial part of both curves (not shown), due to the solvent vapor pressure reequilibration after 15 flushes of the reaction flask with H2, has been corrected (and thereby removed from this curve as) by the precedented procedure described elsewhere.
Figure 2
Figure 2
TEM image of Ir(0)n nanoparticles sample harvested after complete cyclohexene hydrogenation starting with [(1,5-COD)IrCl]2 (1.2 mM Ir) and 1.2 mM Bu3N in acetone at 22.0 ± 0.1 °C. The average particle size is 1.8 ± 0.6 nm.
Figure 3
Figure 3
TGA of the Bu3NH+Cl–-stabilized Ir(0)n nanoparticles formed during the reduction of 3.6 mM [Ir(1,5-COD)Cl]2 in the presence of 7.2 mM Bu3N in acetone at 22 °C and under an initial pressure of 4.7 atm H2 along with concomitant hydrogenation of an initial 1.65 M cyclohexene. The observed 8.6 wt % loss from 25 to 120 °C is attributable to the removal of ≥98% of the HCl present (i.e., 8.6% weight loss vs a theoretical 8.8% loss).
Scheme 4
Scheme 4. Stoichiometry and Theoretical Weight Loss Percentages for the Removal of Volatile H+Cl– from Ir(0)n·Cl–Bu3NH+ by Mild, ≤120 °C Heating
Figure 4
Figure 4
Loss of cyclohexene vs time curve in the H2 reduction of [(1,5-COD)IrCl]2 (1.2 mM Ir) in the presence of 1.2 mM Dodec3N in acetone at 22.0 ± 0.1 °C along with concomitant complete hydrogenation of 1.65 M cyclohexene. The pressure rise observed in the initial part of the curve, due to the solvent vapor pressure reequilibration after 15 flushes of the reaction flask with H2, has been corrected (and hence removed from this curve) by the precedented procedure described elsewhere.
Figure 5
Figure 5
TEM image of Ir(0)n nanoparticles harvested after complete cyclohexene hydrogenation starting with [(1,5-COD)IrCl]2 (1.2 mM Ir) and 1.2 mM tris(dodecyl)amine, Dodec3N in acetone at 22.0 ± 0.1 °C. The average particle size is 1.8 ± 0.6 nm.
Figure 6
Figure 6
(a) Hydrogen uptake vs time plot for the hydrogenation of cyclohexene/acetone and concomitant formation of Ir(0)n nanoparticles starting with [(1,5-COD)IrCl]2 (1.2 mM Ir) plus 0.2 equiv POct3 and cyclohexene (1.65 M) in acetone at 22.0 ± 0.1 °C. The pressure rise observed in the initial part of curve, due to the solvent vapor pressure reequilibration after 15 flushes of the reaction flask with H2, has been corrected (and hence removed from this curve) by the precedented procedure described elsewhere. (b) Bright-field scanning transmission electron microscopy (STEM) image and (c) the corresponding particle size histogram of Ir(0)n·(POct3)a(Cl–H+)b nanoparticles in the sample harvested after 22 h hydrogenation (1.0 ± 0.1 equiv cyclooctane had evolved as determined by GC). The mean diameter of Ir(0) nanoparticles is 1.5 ± 0.2 nm (i.e., ±15%).
Figure 7
Figure 7
(a) Hydrogen consumption vs time plot for the hydrogenation of cyclohexene (the fast part of the H2 pressure loss curve) then acetone (the slower, subsequent part of the hydrogenation curve) using the redispersed Ir(0)n·(POct)0.2n nanoparticles formed from a first run of cyclohexene/acetone hydrogenation (i.e., formed starting with [(1,5-COD)IrCl]2 (1.2 mM Ir) plus 0.2 equiv POct3 and cyclohexene (1.65 M) in acetone at 22.0 ± 0.1 °C). The Ir(0) nanoparticles were isolated by removing the volatiles including HCl in vacuum and then redispersed in acetone for this second hydrogenation. The pressure rise observed in the initial part of the curve, due to the solvent vapor pressure reequilibration after 15 flushes of the reaction flask with H2, has been corrected (and hence removed from this curve) by the precedented procedure described elsewhere. (b) Bright-field STEM image of the sample harvested after 22 h hydrogenation and (c) the corresponding particle size histogram of Ir(0)n·(POct3)0.2n nanoparticles. Mean diameter of Ir(0) nanoparticles is 1.4 ± 0.4 nm (i.e., ±28%).
Scheme 5
Scheme 5. Stoichiometry, Synthesis Conditions, and Cyclohexene Hydrogenation Reaction of Interest for Ir(0)n/(γ-Al2O3)(Cl–H+)-Supported Nanoparticle Heterogeneous Catalyst Formation
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