Kinetics and thermodynamics of small molecule binding to pincer-PCP rhodium(I) complexes

Handle URI:
http://hdl.handle.net/10754/562722
Title:
Kinetics and thermodynamics of small molecule binding to pincer-PCP rhodium(I) complexes
Authors:
Doherty, Mark D.; Grills, David C.; Huang, Kuo-Wei ( 0000-0003-1900-2658 ) ; Muckerman, James T.; Polyansky, Dmitry E.; Van Eldik, Rudi V.; Fujita, Etsuko
Abstract:
The kinetics and thermodynamics of the binding of several small molecules, L (L = N2, H2, D2, and C2H 4), to the coordinatively unsaturated pincer-PCP rhodium(I) complexes Rh[tBu2PCH2(C6H3)CH 2PtBu2] (1) and Rh[tBu 2P(CH2)2(CH)(CH2)2P tBu2] (2) in organic solvents (n-heptane, toluene, THF, and cyclohexane-d12) have been investigated by a combination of kinetic flash photolysis methods, NMR equilibrium studies, and density functional theory (DFT) calculations. Using various gas mixtures and monitoring by NMR until equilibrium was established, the relative free energies of binding of N2, H2, and C2H4 in cyclohexane-d12 were found to increase in the order C 2H4 < N2 < H2. Time-resolved infrared (TRIR) and UV-vis transient absorption spectroscopy revealed that 355 nm excitation of 1-L and 2-L results in the photoejection of ligand L. The subsequent mechanism of binding of L to 1 and 2 to regenerate 1-L and 2-L is determined by the structure of the PCP ligand framework and the nature of the solvent. In both cases, the primary transient is a long-lived, unsolvated species (τ = 50-800 ns, depending on L and its concentration in solution). For 2, this so-called less-reactive form (LRF) is in equilibrium with a more-reactive form (MRF), which reacts with L at diffusion-controlled rates to regenerate 2-L. These two intermediates are proposed to be different conformers of the three-coordinate (PCP)Rh fragment. For 1, a similar mechanism is proposed to occur, but the LRF to MRF step is irreversible. In addition, a parallel reaction pathway was observed that involves the direct reaction of the LRF of 1 with L, with second-order rate constants that vary by almost 3 orders of magnitude, depending on the nature of L (in n-heptane, k = 6.7 × 10 5 M-1 s-1 for L = C2H4; 4.0 × 106 M-1 s-1 for L = N2; 5.5 × 108 M-1 s-1 for L = H2). Experiments in the more coordinating solvent, THF, revealed the binding of THF to 1 to generate 1-THF, and its subsequent reaction with L, as a competing pathway. © 2013 American Chemical Society.
KAUST Department:
Biological and Environmental Sciences and Engineering (BESE) Division; KAUST Catalysis Center (KCC); Chemical Science Program; Physical Sciences and Engineering (PSE) Division; HCL
Publisher:
American Chemical Society
Journal:
Inorganic Chemistry
Issue Date:
15-Apr-2013
DOI:
10.1021/ic300672g
Type:
Article
ISSN:
00201669
Appears in Collections:
Articles; Physical Sciences and Engineering (PSE) Division; Chemical Science Program; KAUST Catalysis Center (KCC); Biological and Environmental Sciences and Engineering (BESE) Division

Full metadata record

DC FieldValue Language
dc.contributor.authorDoherty, Mark D.en
dc.contributor.authorGrills, David C.en
dc.contributor.authorHuang, Kuo-Weien
dc.contributor.authorMuckerman, James T.en
dc.contributor.authorPolyansky, Dmitry E.en
dc.contributor.authorVan Eldik, Rudi V.en
dc.contributor.authorFujita, Etsukoen
dc.date.accessioned2015-08-03T11:03:11Zen
dc.date.available2015-08-03T11:03:11Zen
dc.date.issued2013-04-15en
dc.identifier.issn00201669en
dc.identifier.doi10.1021/ic300672gen
dc.identifier.urihttp://hdl.handle.net/10754/562722en
dc.description.abstractThe kinetics and thermodynamics of the binding of several small molecules, L (L = N2, H2, D2, and C2H 4), to the coordinatively unsaturated pincer-PCP rhodium(I) complexes Rh[tBu2PCH2(C6H3)CH 2PtBu2] (1) and Rh[tBu 2P(CH2)2(CH)(CH2)2P tBu2] (2) in organic solvents (n-heptane, toluene, THF, and cyclohexane-d12) have been investigated by a combination of kinetic flash photolysis methods, NMR equilibrium studies, and density functional theory (DFT) calculations. Using various gas mixtures and monitoring by NMR until equilibrium was established, the relative free energies of binding of N2, H2, and C2H4 in cyclohexane-d12 were found to increase in the order C 2H4 < N2 < H2. Time-resolved infrared (TRIR) and UV-vis transient absorption spectroscopy revealed that 355 nm excitation of 1-L and 2-L results in the photoejection of ligand L. The subsequent mechanism of binding of L to 1 and 2 to regenerate 1-L and 2-L is determined by the structure of the PCP ligand framework and the nature of the solvent. In both cases, the primary transient is a long-lived, unsolvated species (τ = 50-800 ns, depending on L and its concentration in solution). For 2, this so-called less-reactive form (LRF) is in equilibrium with a more-reactive form (MRF), which reacts with L at diffusion-controlled rates to regenerate 2-L. These two intermediates are proposed to be different conformers of the three-coordinate (PCP)Rh fragment. For 1, a similar mechanism is proposed to occur, but the LRF to MRF step is irreversible. In addition, a parallel reaction pathway was observed that involves the direct reaction of the LRF of 1 with L, with second-order rate constants that vary by almost 3 orders of magnitude, depending on the nature of L (in n-heptane, k = 6.7 × 10 5 M-1 s-1 for L = C2H4; 4.0 × 106 M-1 s-1 for L = N2; 5.5 × 108 M-1 s-1 for L = H2). Experiments in the more coordinating solvent, THF, revealed the binding of THF to 1 to generate 1-THF, and its subsequent reaction with L, as a competing pathway. © 2013 American Chemical Society.en
dc.publisherAmerican Chemical Societyen
dc.titleKinetics and thermodynamics of small molecule binding to pincer-PCP rhodium(I) complexesen
dc.typeArticleen
dc.contributor.departmentBiological and Environmental Sciences and Engineering (BESE) Divisionen
dc.contributor.departmentKAUST Catalysis Center (KCC)en
dc.contributor.departmentChemical Science Programen
dc.contributor.departmentPhysical Sciences and Engineering (PSE) Divisionen
dc.contributor.departmentHCLen
dc.identifier.journalInorganic Chemistryen
dc.contributor.institutionChemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000, United Statesen
dc.contributor.institutionDepartment of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germanyen
dc.contributor.institutionGeneral Electric Global Research, One Research Circle, Niskayuna, NY 12309, United Statesen
kaust.authorHuang, Kuo-Weien
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