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A Benchtop Method for Appending Protic Functional Groups to N‐Heterocyclic Carbene Protected Gold Nanoparticles

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A Benchtop Method for Appending Protic Functional Groups to N‐Heterocyclic Carbene Protected Gold Nanoparticles Angewandte Research Articles Chemie International Edition:DOI:10.1002/anie.202001440 Surface Functionalization German Edition:DOI:10.1002/ange.202001440 ABenchtop Method for Appending Protic Functional Groups to N-Heterocyclic Carbene Protected Gold Nanoparticles Joseph F. DeJesus+,Lindy M. Sherman+,Darius J. Yohannan, Jeffrey C. Becca, ShelbyL.Strausser,LeonhardF.P.Karger,Lasse Jensen, David M. Jenkins,* and JonP.Camden* Abstract: The remarkable resilience of N-heterocyclic carbene NHC–CO2 adducts or NHC bicarbonate salts are the most (NHC) gold bonds has quickly made NHCs the ligand of common method for coating noble metal surfaces;[1a, 4,5] choice when functionalizing gold surfaces.Despite rapid however, adding NHCs to nanoparticles in aqueous solutions progress using deposition from free or CO2-protected NHCs, is more challenging.Two complementary approaches for synthetic challenges hinder the functionalization of NHC NHC-protected nanoparticles have been developed. Thefree surfaces with protic functional groups,such as alcohols and NHCs can be added directly to preformed nanoparticles[6] or amines,particularly on larger nanoparticles.Here,wesynthe- gold NHC complexes can be reduced with borohydrides or sizeNHC-functionalized gold surfaces from gold(I) NHC boranes,forming NHC-functionalized gold nanoparticles complexes and aqueous nanoparticles without the need for (AuNPs) under 10 nm in diameter (Figure 1a).[7] Yet, many additional reagents,enabling otherwise difficult functional applications require larger nanoparticles than can be formed groups to be appended to the carbene.The resilience of the with these methods.For example,invivo biological imaging NHC Au bond allows for multi-step post-synthetic modifica- À tion. Beginning with the nitro-NHC,weform an amine-NHC terminated surface,whichfurther undergoes amide coupling with carboxylic acids.The simplicity of this approach, its compatibility with aqueous nanoparticle solutions,and its ability to yield protic functionality,greatly expands the potential of NHC-functionalized noble metal surfaces. Introduction While N-heterocyclic carbenes (NHCs) are known to form strong bonds with noble metals,[1] theextreme robust- ness of NHC self-assembled monolayers (NHC SAMs) was only recently recognized.[2] Following these initial studies, progress has been rapid with NHCs supplanting thiols in applications from catalysis and biosensing to drug deliver- y[1a,2a,3] because of their enhanced stability and accessibility for chemical modifications.[2d,4] NHC-coated materials are particularly appealing when non-ideal conditions are re- quired, such as large variations in pH or temperature.[1a,2b] [*] L. M. Sherman,[+] D. J. Yohannan, L. F. P. Karger,Dr. J. P. Camden Department of Chemistry and Biochemistry UniversityofNotre Dame South Bend, IN 46556 (USA) E-mail:[email protected] Figure 1. Synthetic schemes for NHC-functionalized surfaces and Dr.J.F.DeJesus,[+] S. L. Strausser,Dr. D. M. Jenkins nanoparticles. a) General methods for forming NHC-capped AuNPs Department of Chemistry,University of Tennessee with aprotic functional groups. b) Forming free NHCs with protic Knoxville, TN 37996 (USA) functionalgroups is problematic since the protic proton (blue) is more E-mail:[email protected] readily deprotonated than the benzimidazolium proton. c) Crudden has formed small NPs with carboxylic acids on the backbone.[7b] J. C. Becca, Dr.L.Jensen d) This work demonstrates ageneral method for formation of larger Department of Chemistry,The Pennsylvania State University NHC-NPs with protic functional groups on the NHC. The NHC UniversityPark, PA 16802 (USA) ligation is accomplished by adding aCH2Cl2 or CH3CN solution of the + [ ]These authors contributed equally to this work. NHC gold complex to an aqueous solution of citrate-capped nano- Supportinginformation and the ORCID identification number(s) for particles, resulting in spontaneous functionalization. The functional the author(s) of this article can be found under https://doi.org/10. groups on the NHC can be further derivatized, as shown by the nitro 1002/anie.202001440. reduction. Angew.Chem. Int.Ed. 2020, 59,7585 –7590 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 7585 Angewandte Research Articles Chemie require NPs greater than 10 nm to avoid rapid elimination by the kidneys[8] and surface-enhanced Raman scattering (SERS) is most efficient with nanoparticles between 20– 100 nm.[9] In contrast to thiols,functionalizing NHCs with protic groups is challenging.Such groups are essential as they are routinely applied in sensing,[10] biological applications,[11] and as aprecursor to amide couplings.[12] While protic thiols can be added to noble metal surfaces directly,many protic functional groups,such as alcohols and amines,are incom- patible with the free carbene,either due to reactions with the strong bases that are used to deprotonate the imidazolium or reactions with the free carbene itself (Figure 1b).[13] To date,only carboxylic acids have been employed for protic NHC-AuNPs.[3d,7b,14] Crudden and co-workers[7b] pre- pared apendant carboxylic acid on the back of the NHC gold complex precursor prior to reduction with sodium borohy- dride (Figure 1c). Clearly if NHCs are to reach their full potential as versatile surface functionalization ligands,agen- eral, straightforward method for including protic functional groups on NHC-functionalized surfaces is essential. Results and Discussion Figure 2. Summary of NHC complexesand NHC functionalized surfa- ces and their corresponding identifiers. Herein, we report ageneral method for (1) creating NHC- functionalized nanoparticles (d > 15 nm) and (2) adding protic functional groups to NHC-coated surfaces (Figure 1d). correspond to the isopropyl and aryl stretching frequencies.[16] 1 Addition of NHC gold(I) complexes to citrate-capped AuNPs These SERS peaks are red-shifted approximately 20 cmÀ (> 15 nm diameter) leads to aNHC ligand replacement on relative to the Raman spectra, indicating binding to the gold the particles.Asboth functionalized and unfunctionalized surface.[5b] Several variations of our procedure all yielded 1- NHCs can be transferred, this eliminates the need for NHC– AuNP:water/dichloromethane (Method 1);water/acetoni- CO2 adducts,bicarbonate salts,orexternal reductants.Using trile (Method 2);using the mono-NHC ligated gold(I) apost synthetic modification sequence,wecreate NHC– complex 2-Cl with biphasic water/dichloromethane (Method amines in situ, thereby opening up amide coupling to 3). In addition, complexes with non-coordinating anions like acarboxylic acid. Each step in the synthetic sequence is hexafluorophosphate (PF6)result in the same transfers of the followed by SERS,combined with isotope labeling and NHC (See Figure S37).[17] Last, we observed equivalent theoretical calculations,toconfirm the progress of the spectra for an array of nanoparticle sizes (d = 20–100 nm), chemical reactions on the AuNP. indicating formation of 1-AuNP for avariety of initial We previously employed CO2-protected NHCs to make nanoparticle substrates (See Figures S44, S45). SERS-active NHC-functionalized gold film-over-nano- Deposition from the gold(I) complexes is still not spheres (AuFONs);[4b,5b] however,this method of deposition amenable to the synthesis of an amine-functionalized surface is not effective for nanoparticles,except with very small due to the reactivity of the protic R-group.Wereasoned that atomic gold clusters.[15] TheAuFONs NHCs were deposited anitro-NHC-functionalized gold surface could instead serve on gold surfaces by heating the solid NHC–CO2 compound in as the intermediate for an in situ reduction of the nitro-NHC vacuum;therefore,this method cannot be transferred to an to the amine-NHC.We, therefore,synthesized the benzimi- aqueous environment for Au nanoparticles.[4b,5b] Instead, we dazolium iodide precursor, 6-I,modifying our previously hypothesized that gold(I) complexes might transfer the NHC reported method with other functional groups,in70% yield to the gold surface (Figure 2). Indeed, stirring adilute over two steps from 5-nitro-benzimidazole (See SI).[4b] The solution of the gold(I) salt (1-I/Cl)mixture of dichloro- bis-NHC gold complexes, 3-I/Cl and 3-I,are prepared by both methane and water with citrate-capped nanoparticles suc- astrong base (Figure 4A)and amild base approach (Fig- cessfully appended the NHC to the colloids without the need ure 4B)in95% and 71%yield, respectively.[7b] Thecom- for an external reductant, forming 1-AuNPs. plexes were characterized by 1HNMR (Figures S17 and S21), TheNHC binding to the AuNP was confirmed using HRMS (Figure S26), and X-ray crystallography (Figure S35). SERS (Figure 3), XPS (Figure S51), and TGA (Figure S55) Additionally,the IR spectra show the characteristic 1523 and 1 and the particle morphology was measured with TEM. The 1341 cmÀ N Ostretches (Figure S25), confirming retention À resulting SERS spectra are consistent with those obtained of the NO2 moiety. from NHC surfaces created using the NHC–CO2 adduct Given the effectiveness of the unfunctionalized mono- [4b, 5b] 1 1 method (Figure S36). Peaks at 1296 cmÀ and 1403 cmÀ NHC complex, 2-Cl,for NHC deposition, we also prepared 7586 www.angewandte.org 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem.Int. Ed. 2020, 59,7585 –7590 Angewandte Research Articles Chemie Figure 3. a) Neat Raman spectrum of 1-I/Cl (black) and SERS of 1-AuNPs.b)Neat Raman spectrum of 3-I/Cl (black) and SERS of 2-AuNPs.Three methods were used to append NHCs to the gold
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