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Title: Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons

Abstract

Here we present a new toolset of precursors for semiconductor nanocrystal synthesis, N-heterocyclic carbene (NHC)-metal halide complexes, which enables a tunable molecular platform for the preparation of coinage metal chalcogenide quantum dots (QDs). Phase-pure and highly monodisperse coinage metal chalcogenide (Ag 2E, Cu 2-xE; E = S, Se) QDs are readily synthesized from the direct reaction of an NHC-MBr synthon (where M = Ag, Cu) with alkylsilyl chalcogenide reagents at room temperature. We demonstrate that the size of the resulting QDs is well tailored by the electron-donating ability of the L-type NHC ligands, which are further confirmed to be the only organic capping ligands on the QD surface, imparting excellent colloidal stability. Local superstructures of the NHC-capped Ag 2S QDs are observed by TEM, further demonstrating their potential for synthesizing monodisperse ensembles and mediating self-assembly.

Authors:
 [1]; ORCiD logo [1]
  1. Univ. of Southern California, Los Angeles, CA (United States). Dept. of Chemistry
Publication Date:
Research Org.:
Univ. of Southern California, Los Angeles, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1406130
Grant/Contract Number:
SC0006812; FG02-11ER46826
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Chemistry of Materials
Additional Journal Information:
Journal Volume: 29; Journal Issue: 3; Journal ID: ISSN 0897-4756
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Lu, Haipeng, and Brutchey, Richard L. Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons. United States: N. p., 2017. Web. doi:10.1021/acs.chemmater.6b05293.
Lu, Haipeng, & Brutchey, Richard L. Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons. United States. doi:10.1021/acs.chemmater.6b05293.
Lu, Haipeng, and Brutchey, Richard L. Mon . "Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons". United States. doi:10.1021/acs.chemmater.6b05293. https://www.osti.gov/servlets/purl/1406130.
@article{osti_1406130,
title = {Tunable Room-Temperature Synthesis of Coinage Metal Chalcogenide Nanocrystals from N -Heterocyclic Carbene Synthons},
author = {Lu, Haipeng and Brutchey, Richard L.},
abstractNote = {Here we present a new toolset of precursors for semiconductor nanocrystal synthesis, N-heterocyclic carbene (NHC)-metal halide complexes, which enables a tunable molecular platform for the preparation of coinage metal chalcogenide quantum dots (QDs). Phase-pure and highly monodisperse coinage metal chalcogenide (Ag2E, Cu2-xE; E = S, Se) QDs are readily synthesized from the direct reaction of an NHC-MBr synthon (where M = Ag, Cu) with alkylsilyl chalcogenide reagents at room temperature. We demonstrate that the size of the resulting QDs is well tailored by the electron-donating ability of the L-type NHC ligands, which are further confirmed to be the only organic capping ligands on the QD surface, imparting excellent colloidal stability. Local superstructures of the NHC-capped Ag2S QDs are observed by TEM, further demonstrating their potential for synthesizing monodisperse ensembles and mediating self-assembly.},
doi = {10.1021/acs.chemmater.6b05293},
journal = {Chemistry of Materials},
number = 3,
volume = 29,
place = {United States},
year = {Mon Jan 23 00:00:00 EST 2017},
month = {Mon Jan 23 00:00:00 EST 2017}
}

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  • A free energy map interconnecting nine tungsten complexes has been determined based on chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies (BDFEs) are 59.3(3) kcal mol -1 for CpW(CO)2(IMes)H and 59(1) kcal mol -1 for the dihydride [CpW(CO)2(IMes)(H) 2]+. For the radical cation, [CpW(CO)2(IMes)H]•+, W-H bond homolysis to generate the 16-electron cation [CpW(CO)2(IMes)]+ is followed by MeCN uptake, with free energies for these steps being 51(1) kcal mol -1 and -16.9(5) kcal mol -1 respectively. Based on these two steps, the free energy change for conversion of [CpW(CO)2(IMes)H]•+ to [CpW(CO)2(IMes)(MeCN)]+more » in MeCN is 34(1) kcal mol -1. The pKa of CpW(CO)2(IMes)H in MeCN was determined to be 31.9(1), significantly higher than the 26.6 reported for the related phosphine complex, CpW(CO)2(PMe3)H. This difference is attributed to the electron donor strength of IMes far exceeding that of PMe3. The pKa values for [CpW(CO)2(IMes)H]•+ and [CpW(CO)2(IMes)(H)2]+ were determined to be 6.3(5) and 6.3(8), much closer to the pKa values reported for the PMe3 analogs. The free energy of hydride abstraction from CpW(CO)2(IMes)H is 74(1) kcal mol -1, and the resultant [CpW(CO)2(IMes)]+ cation is significantly stabilized by binding MeCN to form [CpW(CO)2(IMes)(MeCN)]+, giving a hydride donor ability of 57(1) kcal mol -1. Electrochemical oxidation of [CpW(CO)2(IMes)] - shows a fully reversible wave (E° = -1.65 V vs. Cp2Fe+/0 in MeCN), and CpW(CO)2(IMes)H is reversibly oxidized (E° = -0.13(3) V) at high scan rates (800 V s -1). High pressure NMR experiments provide an estimate of ΔG° = 10.3(4) kcal mol -1 for the displacement of MeCN by H2 for [CpW(CO)2(IMes)(MeCN)]+ to give [CpW(CO)2(IMes)(H)2]+.« less
  • A series consisting of a tungsten anion, radical and cation, supported by the N-heterocyclic carbene IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and spanning formal oxidation states W(0), W(I) and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)2(IMes)H with KH and 18 crown 6 gives the tungsten anion [CpW(CO)2(IMes)]-[K(18 crown 6)]+. The crystal structure of this complex shows that the K+ interacts not only with the oxygen atoms in the crown ether, but also with the carbonyl oxygens. The electrochemical oxidation of [CpW(CO)2(IMes)]- in acetonitrile is fully reversible (E½ = -1.65 V vs Cp2Fe+•/0) at all scan rates, indicating that CpW(CO)2(IMes)•more » is a persistent radical. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+PF6 affords [cis-CpW(CO)2(IMes)(MeCN)]+PF6 . Comproportionation of [CpW(CO)2(IMes)]- with [CpW(CO)2(IMes)(MeCN)]+ gives the 17-electron tungsten radical CpW(CO)2(IMes)•. This complex shows paramagnetically shifted resonances in 1H NMR spectra and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)2(IMes)• is very stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)2(IMes)• and [CpW(CO)2(IMes)]- are reported. DFT calculations were carried out on CpW(CO)2(IMes)H, as well as on related complexes bearing NHC ligands with N,N´ substituents Me [CpW(CO)2(IMe)H] or H [CpW(CO)2(IH)H] to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal W H homolytic bond dissociation energies (BDEs) to decrease with increasing steric bulk of the NHC ligand, from 67 for CpW(CO)2(IH)H to 64 for CpW(CO)2(IMe)H to 63 kcal/mol for CpW(CO)2(IMes)H. The calculated spin density at W for CpW(CO)2(IMes)• is 0.63. The W radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)• are calculated to form weak W W bonds. The weakly bonded complexes [CpW(CO)2(IMe)]2 and [CpW(CO)2(IH)]2, are predicted to have W-W BDEs of 6 and 18 kcal/mol, respectively, and to dissociate readily to the W-centered radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)•. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less