The Base Side of Noble Metal Clusters: Efficient Route to Captamino

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Clusters, Radicals, and Ions; Environmental Chemistry

The Base Side of Noble Metal Clusters: Efficient Route to Captamino-Gold, Au(-S(CH)N(CH)), n = 25 – 144 n

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M. Mozammel Hoque, David M. Black, Kathryn M Mayer, Amala Dass, and Robert L. Whetten J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00886 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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The Journal of Physical Chemistry Letters

The Base Side of Noble Metal Clusters: Efficient Route to CaptaminoGold, Aun(-S(CH2)2N(CH3)2)p, n = 25 – 144 M. Mozammel Hoque,a David M. Black,a Kathryn M. Mayer,a Amala Dass,b and Robert L. Whettena* a

Department of Physics & Astronomy, The University of Texas, San Antonio, TX 78249, USA

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Department of Chemistry & Biochemistry, University of Mississippi, Oxford, MS 38677, USA

ABSTRACT: Monolayer protected clusters (MPCs), typified by the (Au, Ag)thiolates, share dimensions and masses with aqueous globular proteins (enzymes), yet efficient bioanalytical methods have not proved applicable to MPC analytics. Here we demonstrate that direct facile ESI(+)MS analysis of MPCs succeeds, at the few-picomol level, for aqueous basic amino-terminated thiolates. Specifically, captamino-gold clusters, Aun(SR)p, wherein -R = (CH2)2N(CH3)2, are prepared quantitatively via a direct 1-phase (aq/EtOH) method, and are sprayed under weakly acidic conditions, to yield intact 6.8kDa complexes, (n,p) = (25, 18), with up to 5 H+ adducts, or 34.6-kDa MPCs (144, 60) at charge-state z = 8+. These exceed all prior reports of positive charging of MPCs except for those bearing per-cationized (quat) ligands. pHmediated reversible phase transfer (aqueous to/from DCM-rich phases) are consistent with peripheral exposure of all tertiary amino-groups to solutions. This surprising development opens the way to all manner of modifications or extensions, as well as to advanced analyses inspired by those applied to intact biomolecules. KEYWORDS: Base metal cluster, 2-(dimethylamino)ethane thiolate, DMAET, DCM-rich, ESI-MS, Ion pairing, and HFIP

solution and other phases. By making the terminal groups similar, in all respects (spatial extent, organization and density; polarity and acid-base chemistry) to the biomolecules, it should therefore be possible to analyze them by the now standard methods of molecular biology. That this objective has not yet been realized is evident when one considers that the vast majority of precision MPC analyses performed only on the hydrophobic (non-aqueous) systems. Of the aqueous minority (hydrophilic MPCs), essentially all have been acidic (negative charge in solution) or, at best, zwitterionic, and their solution and phase-transfer properties have been analyzed at high pH, which pose extra difficulties.11-14 Recent advances in the high performance liquid chromatography electrospray ionization mass spectrometry (HPLC-ESI-MS) analysis of such polyanionic MPCs, attained only by employing conditions optimized for oligonucleotides (special ionpairing agents), illustrate the nature of the challenge.15 1 ACS Paragon Plus Environment

Certain larger noble-metal molecules, as exemplified by thiolated gold clusters, Aun(SR)p (n >> p), have attracted great recent interest. Often called “monolayer protected clusters” (MPCs),1-3 they combine key aspects from three distinct fields: (i) from the molecular chemistry of the pendant Rgroups; (ii) from the surface chemistry of the Au(I)-S coordinative bonding; and (iii) from the solid-state chemistry of the incipient crystalline-metallic core.4 New applications of distinct cluster compounds {n, p} in various technical fields are under exploration.5-7 Bioconjugates for medical research products have steadily advanced over the past quarter-century.8-10 However, the analytical characteristics of known MPCs have continued to pose great challenges that are similar in some respects to those faced earlier for biomolecules of similar dimension (mass), such as globular proteins or oligonucleotides. It is generally assumed that the handling and related analytical properties are set largely by the termini of the Rgroups, as these form the periphery exposed to

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In this regard, thiolate ligands that are basic (positive in solution) would seem to offer great advantages, but a direct route to basic gold-thiolate MPCs, as cluster compounds of definite composition, has been elusive despite two decades of work. In 2005, Ackerson, Jadzinsky, and Kornberg stated categorically: “Small positively charged ligands do not support16 the production of MPCs in the Brust synthesis.”17 The few claims made of direct production appear poorly characterized and not to show the properties expected of systems bearing terminal aminogroups.18-19 Rather, exchange reaction procedures, involving complex protected amino groups, have been necessary.20 Cationic-termini MPCs had also remained elusive, until recently when Ishida et al. reported success with a modified Brust method to obtain MPCs from thiolates with pendant quaternary-ammonium groups, which produce readily analyzable clusters, albeit only with special counter-anions, e.g. PF6-.21-23 Based on this assessment of the recent literature, it is apparent that the quest to “make MPCs behave like proteins” has yet to meet success, despite many efforts. For this reason, we were extremely surprised when one of us (MMH) found that a simple extension of Ishida’s method yields reproducibly the desired amino-terminated gold-thiolate clusters in robust forms that show every indication of fulfilling the analytical promise of the base side of noble-metal clusters, as we report hereby present. In particular, we present for a series of size-tunable captaminogold clusters {n, p} our early evidence of facile acidbase chemistry; reversible phase-transfer; and record high positive charge states, as generated and detected by electrospray mass-spectrometry; as well as preliminary indications of tertiary-amino coordination chemistry. The simplicity of these procedures and robust nature of the products suggest that these and closely related systems should be widely and immediately applicable to all. In this report, we present as evidence some typical results (optical spectra, and especially ESI mass spectrometry) indicating the identity, the relative abundances and relevant analytical properties of the products obtained over a wide range of reaction conditions. To be noted here, after this work completed, we became aware of work from Ishida and Huang for 4-pyridyl-ethane-thiolate-gold clusters. Captamine, or 2-(dimethylamino) ethane thiol (DMAET), is a short-chain (C2) bifunctional ligand that results conceptually from joining the simplest alkyl thiol, CH3SH, to the minimal tertiary amine, (CH3)3N, via a single C-C bond: HS-CH2-CH2-N(CH3)2. The affinity of thiolate RS- for Au(I) ensures that reduced captamino-gold clusters will have gold-cluster

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surfaces saturated by Au-S coordination, leaving each amino N-site lone-pair free to act in the following generic ways: (i) hydrogen bonding to protic agents and solvent; (ii) acid-base chemistry as a proton acceptor (pKa ~ 10.8); (iii) co-ordination to nonaurous metal ions; and (iv) covalent-bond formation, e.g. the quaternization as thiocholine. Below we describe briefly our range of synthetic procedures and then provide results to establish that the above picture and properties {(i), (ii), (iii)} are indeed obtained. A full account of the detailed procedures to produce captamino-gold clusters is presented in the SI section. Briefly, we adopted initially those conditions described by Ishida22 for a long-chain quaternary thiolate, but employed instead the hydrogen-chloride salt of DMAET (captamine-H+Cl-). A typical procedure (see SI section for experimental details) for a 3:1 (RS:Au) ratio involves first combining 20-µmol of HAuCl4, in (1.0:2.5 v/v) H2O:EtOH, with 60-µmol of aqueous DMAET salt, then equilibrating to allow formation of soluble (Au(I)SR)-oligomers (colorless), after which pH ~ 12 is established by dropwise addition of 1M KOH (aq). The second stage starts with addition of a stoichiometric quantity (~32-µmol) of chilled NaBH4 (aq., 0.2-M KOH), to initiate the reduction of the oligomers to metallic clusters, as indicated by profound color changes and ultimate darkening of the solution. (See SI section for photographs) The reaction is allowed to proceed for several hours, prior to cleanup (below) and analysis. The conversion is quantitative (~100% yield), in the sense that all starting material is converted to colored products (Figure 1), with no insoluble byproducts or unreduced starting material. We tested scaling of the reaction quantities from 2-mg to 16-mg (Au content), without hint of any limitation. This basic procedure was extended in several directions, in order to ensure complete reaction, or to obtain a selected size, from essentially all-Au25 product to exclusively Au144: (i) subsequent (sub)stoichiometric additions of the same NaBH4 solution; (ii) gradual heating from room-temperature to final temperatures of 55 or ~ 67 oC (not beyond 70 oC); (iii) varied RS: Au ratios; (iv) varied reactionequilibration times and co-solvents (e.g. CH3CN vs. EtOH). We found that this procedure is robust, in the sense of generating only the dark soluble products, over a wide range of these conditions, provided that the pH range is well established and the reducing agent (NaBH4) is introduced in a gradual, or stepwise stoichiometric manner. In other words, the Ishida conditions applied to this tertiary ammonium chloride thiolate permit the modified Brust method to yield

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MPCs, in direct contradiction of Ackerson-JadzinskyKornberg's report.16 From the product as obtained (aq-EtOH or -CH3CN solution), reaction byproducts are removed by repeated centrifugation dialysis, using a Millipore Amicon Ultra-Centrifugal Filter; NMWL: 3kDa that passes all small molecules and ions (colorless) while retaining the deeply colored metallic clusters. Second, quantitative transfer of product (color) to an immiscible phase (dichloromethane, DCM), occurs simply by increasing the aqueous phase pH (addition of aq-NH4OH); this permits one to discard the byproducts retained in the aqueous phase. [See Figure S-2 for photographs.] Remarkably, the phasetransfer can be reversed by contact with an excess of low-pH phase (aq-acetate): the color returns to the aqueous solution. Naturally, these manifestations are consistent with the acid-base chemistry of free (exposed) amino-groups on the periphery of the MPCs. Finally, from a highly concentrated DCM solution, one obtains (via evaporation of volatile solvent) a reversible precipitation of the MPC material; this facilitates complete purification via (fractional) recrystallization. We found that a simple, reliable method to explore the electrospray properties comprises the following: From the entire phase-transferred product, ~ 5-mg cluster product in 10-mL of DCM-rich solution, 200-µL (2%) is extracted and combined with a mixture of 100-µL DCM and 100-µL of a co-solvent (usually CH3OH or CH3CN) containing one of several electrolytes or weak acids {(NH4-AcO), TEAH-AcO, formic acid, acetic acid, or HFIP}, at the concentrations indicated in the Figures 2 and 3. [See the SI Section for Table indicating the range explored. Several of the ESI-MS results presented are on mixed samples, i.e. ones that show several members of the series in the same mass spectra. But this is normal and sufficient for early work on an entirely new line of compounds. Other mass spectra are so clean of such 'byproducts' that they could be declared size-purified. Separation by size has never been a major issue, or barrier, in this field.] The combined 400-µL solution, containing no more than ~100-µg-Au, infuses directly into the ESI needle at a rate of 400-µL/hr, or ~ 7µL/min, or ~ 1.0-µg-Au per minute. In most cases, high S/N spectra could be obtained by signal averaging over a single-minute period, for clusters ranging from 25 to 144 Au atoms. For clusters of molecular mass ~ 40-kDa (Au144) this corresponds to a sensitivity no worse than 25-pmol. We expect this sensitivity could be greatly improved by HPLC injection. Acidic conditions (FA, AA, HFIP) yielded higher (+) charge states, consistent with enhanced

protonation of exposed base (amino) sites, as is well established for globular proteins and peptides. Optical absorbance spectra in the UV-visible-NIR regions are obtained on highly diluted solution-phase samples (typically aqueous or DCM solvents). The optical spectra (Figure 1) show absorption with discrete features at 1.8, 2.75, and 3.1 eV as shown in Figure 1 (a, b, c) due to strong quantum size effects. Aikens et al.24 have reported that the calculated optical absorption peak at 1.6 eV is due to the HOMOLUMO transition, a signature mark of the gold core and that it does not depend on the thiolate.25

Figure 1. Optical absorbance spectra of captamino-gold. (A) Synthesized in room temperature with different thiol to Au ratio, 2:1 (black color), 3:1 (purple), 4:1 (red). At elevated temperature, 65 ℃ (green), 55 ℃ (blue) using thiol to Au ratio 3:1. Insets showing the chemical structure of the 2dimethylaminoethanethiol (DMAET) and the color of the captamino-gold clusters in cuvette. Spectra are acquired for clusters in aqueous phase and plotted in terms of normalized absorbance/squared-eV versus photon energy and all the spectra are normalized at 3.45 eV for better comparison purpose. (B) Captamino-gold (Au-DMAET) clusters’ spectra acquired in organic (DCM) phase (black) and aqueous phase (red) of the sample (thiol to Au ratio 4:1) are compared with reported purified phenylethanethiol protected Au25 (Au25PC2)26 clusters (green) in organic (toluene) phase. See Figure S-3 for linear plot of the same result. These are consistent with the spectra of prior neutral or acidterminated alkyl-thiolated gold MPCs of the same sizes (compositions) as identified here by ESI-MS+ methods. ESIMS results obtained on the aqueous-alcohol phases by direct infusion of highly diluted solution (with added electrolyte/ionpairing agents) gave the results indicated, wherein adduct peaks (Cl/HCl) predominate upon the anticipated compounds (25, 18) and (38, 24) shown in Figure S-4. Perhaps the strongest evidence that these clusters can solve the “analytical challenge” is provided in the Figure 3 which shows the (25, 18) cluster in the 3+ and even 4+ charge states––with the isotope pattern––consistent with 4H+ and 5H+ levels of protonation, far exceeding the (-1, 0, +1) states observed for neutral ligands and approaching the record (-6H+) for negative charging of acidic-ligand (glutathionate) (25, 18) clusters.27

Similarly, in the case of (144, 60)z+ shown in Figure S10, the positive charging observed using the weak


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volatile acid HFIP can extended far beyond the usual limit of 3+ or (weakly) 4+, obeyed even with the use of cesium (Cs+) adducts, to a distribution that peaks around 5+ or 6+, and extends at least to 8+. Ishida et al. obtained even higher positive chargestates but only by use of the long-chain (undecyl) quaternary ammonium terminated ligands, which reduce the electrostatic repulsion. 19 The extended ‘tailing’ observed on these and other (137, 56) clusters may indicate strong adduction (mass shifts) of small cations & retained solvent, which will need to be reduced in order to extend this work further.

Figure 2. Effect of temperature on synthesis: ESI-MS spectra of captamino-gold clusters synthesized at (A) 55 ℃ and (B) at 25 ℃ and equilibrate for 6 hours. ESI-MS performed by direct infusion in organic (DCM) phase (aq. phase, Figure S-8). Proposed major components are Au25(SR)18, Au38(SR)24, Au67(SR)35, and Au137(SR)56. Both samples are synthesized using RS: Au (3:1) in acetonitrile (MeCN): H2O (1.25:1). TEAA buffer diluted with MeCN, as ion-pairing agent used for direct infusion electrospray. See Figure S-7 for time evolution clusters growth process.

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Figure 3. Effect of ion pairing agent shown in the ESI-MS spectra obtained using (A) HFIP in MeCN solvent, (B) CH3COOH in MeOH is used as ion-pairing agent for direct infusion. Reaction carried out at 65 ℃ using RS: Au, 3:1. Inset showing Isotope pattern of charge state 3+ and 4+ for Au25(SR)18. Experimental isotope pattern was shifted right to 0.6 Da for 3+ and 4+ respectively to eliminate the offset due to instrument calibration. See Figure S-9 for the equivalent mass spectrum of different charge state of the same analysis. The full range of ESI-MS results presented here (and in the SI), showing the effects of—(i) ratio (RS: Au) variations (Figure S-5); (ii) reaction temperature (Figure 2, S-7); (iii) reaction/equilibration time (Figure 2B); and (iv) other variations (co-solvents) (Figures S-5 and S-6)—these are all indicative of a robust and well controlled reaction procedure that should undoubtedly be extended in a number of new directions. All results of the analysis are summarized in table S-1.

Our key results and conclusions may be summarized as follows: Aqueous amino-terminated gold-thiolate clusters, of sizes ranging from 25-144 Au atoms (and beyond), have been obtained in high yield and their key physio-chemical properties have been investigated. A direct Ishida-type method to obtain the elusive basic (amino-group terminated) gold thiolate clusters is disclosed, using the commonplace [(N, N)-dimethyl] amino-ethane-thiolate ligand, (CH3)2N-CH2CH2S-, aka DMAET or captamine, pKa = 10.8, a terminal tertiary amino-group. Evidences are described of their advantageous analytical characteristics, namely: (a) acid-base properties consistent with all amino-groups exposed and active; (b) reversible pH-dependent phase-transfer between aqueous-alcoholic and non-aqueous (e.g. dichloromethane DCM, acetonitrile) solutions, enabling removal of byproducts & impurities; and (c) facile electrospray ionization, particularly from highvolatility solvent mixtures (DCM/MeCN) and employing very weak acid (electrolyte), such as HFIP. These advantageous characteristics have enabled a broad but yet preliminary investigation of several aspects of the formation, yield, and other sizedependent characteristics of the various self-selecting clusters. The following items enumerate the dependences explored to date: dependence of products identified upon preparation procedures & parameters, e.g. ratios, times of reaction and rate of heating, temperature (25-65 oC), phases, co-solvent, ion pairing agent. Special procedures used to obtain selectively the larger ones such as Au144+. Ionization: typical & maximum [+] charge-states detected for different ‘magic numbers’ (self-selecting compositions) and conditions. Optical spectroscopic and other properties appear to be similar to other non-aromatic R-groups. Special attention has been devoted here to the rather simple cases of (25, 18),


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(38, 24), and (144, 60), as these are among the bestknown of aliphatic R-group gold-thiolate clusters. The efficiency and simplicity of the methods described herein should render them adaptable to a wider variety of metal and ligand combinations, 250-2000 µL or 2-16 mg of Au. The formation processes appear adaptable to microscale and automated high-speed reactors, with integrated LC-X-ESI-MS detection, also appear within reach. ASSOCIATED CONTENT Supporting Information Supporting Information Available: Experimental procedure, clean-up, and instrument procedures and parameters, Table S-1. Captamino-gold analysis summary, Figures S-1 to S-10 show captamino-gold synthesis procedures, photographs of phase transfer of the captamino-gold, supplemental optical spectra, and mass spectra identifying discrete captamino-gold clusters before and after phase transfer, time-evolution of the products of lowtemperature and ele-vated-temperature reactions, in the aqueous phase, as equivalent mass spectra for different charge state, and from the special gradual heating that selects for Au144(SR)60. This material is available free of charge via the internet at http://pubs.acs.org.

Corresponding Author * [email protected], [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors thank Dr. Lorenzo Brancaleon for use of the UV-Vis spectrometer. RLW acknowledges support from the Welch Foundation AX-1857.

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The Base Side of Noble Metal Clusters: Efficient Route to Captamino

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