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Structure, Bonding, and Stability of Mercury Complexes with Thiolate and Thioether Ligands from High-Resolution XANES Spectroscopy and First-Principles Calculations Alain Manceau,*,† Cyprien Lemouchi,†,‡ Mauro Rovezzi,§ Martine Lanson,† Pieter Glatzel,§ Kathryn L. Nagy,∥ Isabelle Gautier-Luneau,‡ Yves Joly,‡ and Mironel Enescu⊥ †

ISTerre and ‡Institut Néel, Université Grenoble Alpes, CNRS, 38000 Grenoble, France European Synchrotron Radiation Facility (ESRF), 71 Rue des Martyrs, 38000 Grenoble, France ∥ Department of Earth and Environmental Sciences, University of Illinois at Chicago, MC-186, 845 West Taylor Street, Chicago, Illinois 60607, United States ⊥ Laboratoire Chrono Environnement, Université de Franche-Comté, CNRS, 25030 Besançon, France §

S Supporting Information *

ABSTRACT: We present results obtained from high energyresolution L3-edge XANES spectroscopy and first-principles calculations for the structure, bonding, and stability of mercury(II) complexes with thiolate and thioether ligands in crystalline compounds, aqueous solution, and macromolecular natural organic matter (NOM). Core-to-valence XANES features that vary in intensity differentiate with unprecedented sensitivity the number and identity of Hg ligands and the geometry of the ligand environment. Post-Hartree−Fock XANES calculations, coupled with natural population analysis, performed on MP2optimized Hg[(SR)2···(RSR)n] complexes show that the shape, position, and number of electronic transitions observed at high energy-resolution are directly correlated to the Hg and S (l,m)projected empty densities of states and occupations of the hybridized Hg 6s and 5d valence orbitals. Linear two-coordination, the most common coordination geometry in mercury chemistry, yields a sharp 2p to 6s + 5d electronic transition. This transition varies in intensity for Hg bonded to thiol groups in macromolecular NOM. The intensity variation is explained by contributions from next-nearest, low-charge, thioether-type RSR ligands at 3.0−3.3 Å from Hg. Thus, Hg in NOM has two strong bonds to thiol S and k additional weak Hg···S contacts, or 2 + k coordination. The calculated stabilization energy is −5 kcal/mol per RSR ligand. Detection of distant ligands beyond the first coordination shell requires precise measurement of, and comparison to, spectra of reference compounds as well as accurate calculation of spectra for representative molecular models. The combined experimental and theoretical approaches described here for Hg can be applied to other closed-shell atoms, such as AgI and AuI. To facilitate further calculation of XANES spectra, experimental data, a new crystallographic structure of a key mercury thioether complex, Cartesian coordinates of the computed models, and examples of input files are provided as Supporting Information.

1. INTRODUCTION

the MerP protein from the bacterial mercury detoxification system,1 the tricoordinate binding site of the metalloregulatory protein MerR,2,3 and several distorted tetrahedral coordination environments reported in Hg-substituted proteins.4−6 Because of relativistic effects,7,8 two is, however, the most common coordination number in mercury chemistry.9−12 This dominant binding mode does not exclude other ligands from being present beyond the first Hg(SR)2 coordination shell, at distances between the sums of the Hg and S covalent radii (2.37 Å) and the Hg and S van der Waals radii (3.35 Å). The

Divalent mercury (Hg2+) is most frequently bonded to deprotonated thiol donors [(SR)−] in natural organic matter (NOM) and biological systems. Precise knowledge of the coordination chemistry and stability of mercury thiolate complexes is essential for understanding the mechanisms by which mercury exerts toxic effects, undergoes biotransformation, and is transported. Linear two-coordinate [Hg(SR)2], trigonal-planar or T-shaped three-coordinate [(Hg(SR)3)−], and tetrahedral four-coordinate [(Hg(SR)4)2−] structures are typical for mercury complexes with monodentate thiolate ligands. Prototypical examples in biological systems are provided by linear coordination to two cysteine side chains of © XXXX American Chemical Society

Received: August 22, 2015

A

DOI: 10.1021/acs.inorgchem.5b01932 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Bonding environment of Hg in references 1−5 and computed complexes (MP2-RI/def2-TZVP-ecp) 6−15. Reference 16 and complexes 17−19 are represented in the Supporting Information. An aqueous complex is denoted as “aq”, crystal as “c”, and model as “m”. Thiolate S atoms [(SR)− functional group] are in yellow, and thioether-type (RSR) and thiophenic S atoms are in orange. Bond lengths are in angstroms.

infrared (IR) and Raman,15−17 199Hg NMR,17−20 and extended X-ray absorption fine structure (EXAFS),21−25 coupled with variation of the reactant concentrations and solution pH. As insightful as these methods are, with demonstrated sensitivity to the primary and secondary coordination environments of Hg in low-to-high-molecular-weight complexes, their detection limit is not low enough to address the in situ structural and chemical forms of Hg in natural matter and living organisms without addition of the metal. With a detection limit of about one-tenth that of EXAFS (∼2−5 vs 20−50 mg/kg or ppm26), X-ray absorption near-edge structure (XANES) spectroscopy is in this respect a potentially better tool. In addition, it generally

frequent occurrence of 2 + k coordination (two strong short bonds and k weak bonds) has led to the introduction of the terms “characteristic coordination number” (i.e., 2) and “effective coordination number” (i.e., 2 + k).7 A case in point is methionine thioether (RSR′), which can form a weak Hg···S contact in proteins.4,13 This interaction is the basis for the development of biologically inspired Hg sensors that incorporate thioether-containing side chains in de novo peptides.14 Similar Hg···S interaction may exist in NOM with inorganic sulfide (RSR) but is as yet unknown. Insight into the structure of mercury−sulfur complexes has been gained by means of spectroscopic techniques, such as B


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Inorganic Chemistry

2. RESULTS AND DISCUSSION 2.1. Evidence for Orbital Hybridization from HRXANES of Hg(SR)n Coordination. A change in the coordination of Hg (from 2- to 3- to 4-fold) with the same type of ligand [Hg(SR)2−4; Figure 2a] and a change of the

possesses good chemical-state and ligand sensitivity because of participation of unoccupied valence orbitals in the excited state and is also sensitive to distant atoms at low kinetic energy of the emitted photoelectron. Despite these positive attributes, XANES spectroscopy has been used less23,27,28 to elucidate Hgbinding environments for two main reasons. First, it has poor spectral resolution because the Hg core−hole lifetime broadening is as large as 5.5 eV at the L3-edge.29 This intrinsic effect severely degrades chemical-state and ligand sensitivity. Second, it is difficult and somewhat cumbersome to interpret in quantitative molecular structural terms. Therefore, insight into the coordination environment of Hg has been obtained so far by correlating phenomenologically broad XANES features with those of compounds with known structures.28 In contrast, EXAFS spectra are relatively straightforward to interpret and provide accurate distances and identities of the nearby atoms. A solution to the intrinsic broadening issue is to measure Xray fluorescence by means of perfect Bragg optics (E/ΔE > 5000) in analyzer crystals, instead of using a conventional solidstate detector (E/ΔE < 100).30−32 High energy-resolution XANES (HR-XANES) provides (1) better discrimination among reference compounds and the investigated samples, (2) the higher level of experimental detail required to achieve stable solutions in spectral modeling, thereby supporting more realistic multiatom structural models, and (3) a higher signalto-noise (s/n) ratio. In effect, unwanted X-ray scattering is eliminated with analyzer crystals, which potentially lowers the detection limit of Hg below the parts per million level. We present a first-principles analysis of HR-XANES data and thermochemical calculations for mercury thiolate, mercury thioether, and Hg-NOM complexes (Figure 1). Ground- and excited-state electron densities are calculated within the framework of the Schrödinger quantum mechanics by means of state-of-the-art ab initio methods, including the second-order Møller−Plesset perturbation theory (MP2)33 for geometry and energy calculations and the finite difference method (FDM)34 for XANES calculation.35 This theoretical level is required to obtain results of benchmark quality on heavy closed-shell atoms, such as Hg2+ 5d10.36,37 The outline of the article is as follows. In section 2.1, we show for reference compounds that the Hg(SR)n coordination (n = 2−4) can be determined from the intensity, width, and relative position of the 2p3/2 → 6s/5d electronic transitions as measured at the Hg L3-edge. Core-tovalence spectral features are interpreted accurately in section 2.2 from l-projected density of states (DOS), natural population analysis (NPA), and natural bond orbital (NBO)38−41 analysis for representative molecular models. Atomic charges calculated by NPA and the FEFF9 code are compared. Section 2.3 describes the Hg−S bond in NOM from (l,m)-DOS of Hg and S and NBO analysis in the context of the current Hg(SR)2 model. Then molecular orbital optimizations for a variety of 2 + k models are presented in section 2.4 and used to describe the predicted binding modes of Hg to organic S in NOM and to extract information on the character of the Hg−S bonds. Section 2.5 shows that the combined HR-XANES and modeling analysis presented here for HgII can be extended to other closed-d-shell atoms, such as monovalent Au (AuI) and Ag (AgI). Section 3 offers a summary, together with some outlooks on future studies performed at environmental Hg concentration and on other two-coordinate metal−ligand complexes.

Figure 2. High-resolution Hg L3-edge XANES spectra (HR-XANES) for the references (a, b) and divalent Hg complexed to NOM (c). Hg is considered two-coordinate as a first approximation in complexes 3 and 5 because the first S and O shells are at 2.37 and 2.03 Å and the next-nearest shells at 2.96−3.00 Å (3) and 2.79−2.90 Å (5). HA and FA stand for humic acid and fulvic acid, respectively. CP is a carex peat and SP a sphagnum peat, both characterized previously,91 and PL (Pony Lake), NA (Nordic Aquatic), and ES (Elliott Soil) are references from the IHSS. The Hg concentration is 200 mg of Hg/kg of NOM, and the equilibration time is 15 h.

ligand [(SR)−, RSR′, O2−] with the same coordination to Hg (2-fold; Figure 2b) are reflected differently in the Hg L3-edge HR-XANES spectra of reference compounds. The clear spectral signatures of changes in the number and nature of ligands in the energy region below the edge maximum are not observed with conventional XANES as a result of the large core−hole lifetime broadening of the 2p3/2 core level (5.5 eV) .42 The C


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Figure 3. (a) Experimental and calculated Hg L3-edge HR-XANES spectra for the three main types of mercury thiolate coordinations. The energy positions of the calculated spectra, expressed in terms of the photoelectron energy (0 eV is the Fermi level), were shifted by the ionization energy (∼12278 eV) to match those of the experimental spectra, expressed in photon energy. No energy-dependent Lorentzian convolution48 was used. (b) Calculated near-edge region for dipole-allowed transitions to final states with s symmetry (blue line), d symmetry (red line), and s + d symmetry (black line). The calculated spectra in black are identical with those presented in part a. (c) Calculated unoccupied Hg s/d-DOS.

unoccupied valence orbitals. We show below that orbital hybridization depletes the Hg 5d closed shell, similarly to platinum. Valence orbitals of Hg have a 6s2−x6py5d10−z electron configuration in mercury complexes 4−15 as shown by the occupation numbers of the valence and Rydberg orbitals40 obtained by NPA (Table S1). In the same complexes, the electron configurations of S are 3s2−x3p4+y3dz and of O are 2s2−x2p4+y. The fractional charges for each atom result from orbital hybridization, and the effective atomic charge (designated “natural charge” in Table S1) is given by deviation of the charge sum of the valence and Rydberg orbitals from the electronic occupations for the free atom. Specifically and considering only valence orbital electrons, the deviation from 12e for Hg is x + z − y and the deviation from 6e for S and O is y + z − x. Hg has a natural electron configuration of

absorption edge is related to electronic transitions from the Hg 2p3/2 core level (L3-edge) to hybrid Hg−ligand orbitals in the continuum with overall 7s and 6d character based on an atomic orbital configuration of [Xe]4f146s25d10 for Hg and the dipole selection rule (Δl = ±1). As a first approximation, the HRXANES features below the edge maximum can be attributed to 2p3/2 → 6s transitions because the Hg 6s−O 2p and Hg 6s−S 3p σ bonds generate holes in the 6s orbitals of Hg. However, holes may also exist in the 5d orbitals of Hg in the ground-state because p → s dipolar transitions have a much lower probability than p → d transitions and quadrupolar transitions are extremely weak at the L3-edge.43 Hybridization of metal 6s and 5d valence orbitals, associated with charge transfer between the metal ion and ligand, was reported for platinum ([Xe]4f146s15d9)44 in the ground state and used to relate the intensity of X-ray absorption peaks to the density of D


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Inorganic Chemistry 6s0.7−1.06p0.15d9.7−9.8 with the thiolate ligand (6−15) and 6s0.96p0.15d9.65 with O (5). Thiol S has 3s1.83p4.5−4.73d0.1, and O has 2s1.92p5.2. Therefore, the Hg−ligand bond is between a hybrid 6s5d orbital of the Hg atom and a hybrid 3sp orbital of the S atom or a hybrid 2sp orbital of the O atom. The net charge on Hg amounts to +1.0e for the linear RS−Hg−SR bond (6−15) and to +1.3e for the linear O−Hg−O bond (5). O, which is more electronegative than S, draws a higher electron density away from the Hg atom. Our results are consistent with the reported atomic charges of +0.96e for Hg, −0.49e for Cl, and −0.97e for O in Cl−Hg−OH.45 In the case of 4, the Hg net charge (+1.3e) is compensated for by two distant nitrate anions [d(Hg−O) = 2.6−2.8 Å] in a plane perpendicular to the elongation axis of a short linear RSR′− Hg−RSR′ bonding [d(Hg−S) = 2.43 Å]. The S atom of the RSR′ thioether ligands is positively charged in 4 (+0.4e) in contrast to the O atom, which provides a strong covalent character to the Hg−RSR′ bond.46 Last, examination of the electron occupancy of each individual valence orbital shows that the Hg 5d holes (5d9.7−9.8) occur in the dz2 orbital and the negative charge on S (6p4.5−4.7) occurs in the 3pz orbital. Alignment of the s, pz, and dz2 orbitals permits formation of a collinear spd hybrid that is used to form a strong σ bond. The four Hg d lone pairs (dxy, dxz, dyz, dx2−y2) are stereochemically inactive. The sharp near-edge peak is less intense in compound 3, in which Hg is two-coordinate, because this complex has a reduced density of holes in the hybridized 6s5d orbital (corresponding molecular model 14 in Table S1). In contrast, higher intensity is observed in 4 and 5, which have more empty sd states, thereby increasing the probability of electron transition from the 2p3/2 core level. Other near-edge features are also distinctive of the 3- and 4-fold coordinations, as mentioned previously. Therefore, it is possible to obtain a first assessment of the nature and number of Hg ligands in unknown complexes by comparing their HR-XANES spectra to the spectra of reference compounds. This fingerprinting approach relies, however, largely on empirical knowledge. The Hg-NOM spectra show that meaningful interpretation is not as straightforward (Figure 2c). The Hg-NOM spectra show a sharp near-edge peak consistent with the known linear coordination of Hg to thiolates [Hg(SR)2 complex]. However, the near-edge heights vary significantly among NOM samples from different sources, suggesting that the Hg coordination is more complex. Also, the Hg(SR)2 reference 3 has two nextnearest thiolate residues at 2.96−3.00 Å from adjacent Hg(SR)2 units (2 + k coordination7), which may affect the height of its near-edge peak. Secondary Hg···S interactions are observed in most Hg(SR)2 structures,47 and it will be shown later in section 2.4 with FDM calculations that these weak ligands actually increase the intensity of the near-edge peak and are present in NOM. In the next section, the near-edge features specific to the Hg(SR)2, Hg(SR)3, and Hg(SR)4 coordinations will be interpreted electronically in terms of a final-state local lprojected DOS of the X-ray-absorbing Hg (s/d-DOS) and the surrounding thiolate ligands (s/p-DOS). 2.2. Assignment of Core-to-Valence Electronic Transitions to Spectral Features. The experimental HR-XANES spectra for Hg in 2-, 3-, and 4-fold coordinations with thiolate ligands (1−3) shown in Figure 2a are compared with those calculated for their respective molecular models 6−8 in Figure 3a. The three calculated spectra reproduce the general shape, position, and number of electronic transitions in the measured

HR-XANES spectra. Better agreement would be obtained if the calculated spectra were convoluted by a Lorentzian function to reproduce the core−hole lifetime and experimental broadening,48 but details of the energy levels, which are unresolved in our experiment, would be less apparent. The reasonable agreement between the experimental spectra and the spectra calculated for corresponding simple mercury complex structural models indicates that we can rely on the optimized structures and the XANES theory used in this study. Next, spectra in the near-edge region were calculated considering either the 2p3/2 → 6s (Δl = −1) or 2p3/2 → 5d (Δl = +1) transitions (Figure 3b). Results show that the main near-edge peak A in Figure 3b is from transitions to both 6s and 5d final states. The final state has dominant 5d character in Hg(SR)2 and Hg(SR)3 and similar 6s and 5d character in Hg(SR)4. In the four-coordinate complex, the Hg 6s states are at lower energy than the 5d states, similar to the Cd 5s versus 4/5d states in CdO.49 Peak B, to the right of peak A, arises from 2p3/2 → 5d transitions only. The 2p3/2 → 6s transition of Hg bonded to thiolate, and also to thioether (Figure S1a), ligands is unusually strong for a metal L3-edge. For instance, the p → s transitions in rhenium and tungsten oxides were estimated to be an order of magnitude smaller than the p → d transitions,50 and the p → s transition in uranium oxide was about 2 orders of magnitude smaller.51 Yet, the Hg 2p3/2 → 6s transition is even more pronounced for [Hg(OH2)6]2+ than for the mercury thiolate/thioether complexes because of the high polarity of the Hg−(OH2) bond (Hg[6s0.35d9.87]; Figure S1b). Good agreement also was obtained between theoretical spectra and the Hg 6s and 5d DOS in the excited state (Figure 3c), as expected from previous L3-edge studies of Cd, Re, W, Pt, Au, and U.44,49−55 Three observations are noteworthy. First, the 6s final states have a much higher density above the Fermi level than the 5d final states, which is opposite to the relative strength of the corresponding p → s and p → d transitions described previously (Figure 3b). This difference comes from the radial integral in Fermi’s Golden Rule, which is higher for the p → d channel than for the p → s channel. Second, the 6s DOS above the Fermi level increases, and the 5d DOS decreases, in the final state when the Hg coordination increases from 2- to 3- to 4-fold. The same trend is observed in the ground state: the amount of s holes increases from 0.95 to 1.28 and the amount of d holes decreases from 0.26 to 0.17 with an increase in the coordination (Table S1). This evidence is consistent with theory: the lower the depletion of an orbital in the ground state of the complex, the lower the probability of filling it with a core electron. van der Veen et al.44 showed with a FEFF56 calculation for [Pt2(P2O5)4]12− at the Pt L3-edge ([Xe]4f146s15d9) that the ejected 2p3/2 electron (+1.0e) occupies primarily the partially filled d states at the absorbing Pt atom (+0.7e charge) and, with a lesser probability, the 6s and 6p valence states (+0.1e charge). The remaining charge (+0.2e) was suggested to be transferred to surrounding atoms.57 However, it is more likely transferred to other platinum states because the valence electron counts on the phosphorus ligand are the same (+0.7e) in the ground and excited states (Table S2). The 2p3/2 → 5d transition is expected to have a lesser probability at the Hg than the Pt L3-edge because the Hg d states above the Fermi level have lower density. Thus, the extra charge in outer shells brought by the photoexcited Hg 2p electron is most likely distributed predominantly over the 6s and 6p valence states. This assumption was confirmed with a FEFF calculation of E


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Figure 4. Calculated Hg s/dz2-DOS and S s/pz-DOS (from the thiolate ligand) for (a) 8 and (b) 11.

it is the reverse in Hg(SR)42−, in which only the low-energy 5d states are hybridized with the 6s states. The above results show that probing Hg unoccupied states with different orbital momentum at the L3 absorption edge and comparing the spectra with calculated DOS on realistic quantum structural models provide insight into the coordination of Hg. We now show how this modeling approach can be applied to Hg−S bonding in NOM, which consists of larger and more complicated organic molecules than the model compounds. 2.3. Hg−S Bonding in NOM: Current Hg(SR)2 Model. The height of the near-edge peak varies in Hg-NOM samples (Figure 2c). Because the DOS profiles of the unoccupied 6s and 5d states do not fully overlap when Hg is bonded to two thiolates (Figure 3c, right), this variation in the peak height may depend on the intensity, width, and relative position of the DOS peaks, thereby reflecting the nature and extent of mixing of the Hg and S atomic orbitals. Hybridization of these orbitals is examined from the (l,m)-DOS of Hg and S, and NBO analysis of the Hg−S bond, in the context of the current Hg(SR)2 model42,58,59 in this section. The m-projected DOS above the Fermi level for the Hg 5d and S 3p states in Hg(SR)2 are shown in Figure S2 and the (l,m)-DOS for the Hg sdz2 and S spz hybrid orbitals in Figure 4a. The unoccupied Hg 5d DOS is composed only of 5dz2 orbital character, which is consistent with the partially filled 5dz2 orbitals localized at the axial positions of the Hg atoms, as indicated by NPA. Similarly, both NPA and FDM consistently show that the S px, py, and pz orbitals are incompletely filled (Table S1 and Figure S2), which is explained by the presence of the S−C bond at 102° from the Hg−S bond. In contrast, Hg has three stereoinactive d lone pairs in the xy, xz, and yz planes and a fourth in the x and y directions (dx2−y2). Concerning orbital hybridizations, the first canonical orbital at 0.5−2.0 eV above the Fermi level is a mixture of the 6s5dz2 Hg hybrid and the 3s3pz S hybrid. The py and pz states of the S atom are at higher energy, excluding them from the Hg−S σ bond. In the theoretical case where the HgII−SII bonding is purely ionic, the 6s of Hg states would be empty ([Xe]4f146s05d10) and the 3p of S states would be fully occupied ([Ne]3s23p6), meaning that

Hg(SEt)2(c) (16 in the Supporting Information), which showed that the 6s and 6p states together gained +0.5e and the 5d states +0.3e, bringing the latter to 5d10 (Table S2). An important point to note is that FEFF yields a positive rather than a negative charge on the sulfur thiolate ligand, regardless of whether or not the protons are incorporated in the Hg(SEt)2 molecule (Table S2). The total charge on S calculated for the neutral [Hg5S10C20H50]0 cluster (16) is +0.2e and is +0.3e when protons are omitted ([Hg5S10C20]50− cluster), instead of −0.5e as calculated by NPA (Tables S1 and S2). Changing the self-consistent-field cutoff radius yields similar values. We see four reasons why charge counts calculated by FEFF are incorrect and thus calculations of transitions to bound states inaccurate. (1) Not only is the charge of the protons not well accounted for in the calculation, the total charge also cannot be specified in FEFF, implying that meaningful individual atomic charges are impossible to obtain. (2) In the muffin-tin approach used in FEFF, the charge is calculated by integration to an arbitrary radius; therefore, the charge located in any interstitial area is missed. (3) The atomic charge is calculated by integration up to the highest occupied molecular orbital, that is, at low kinetic energy of the emitted photoelectron, where the muffin-tin approximation is less valid. An incorrect potential is less of an issue at higher kinetic energy; therefore, the XANES spectra calculated with FEFF usually reproduce better the data above the main edge, in the multiple-scattering region. (4) The muffin-tin approximation (with a spherical potential used for each atom out to the muffin-tin radius) is rather poor for low-symmetry and openstructure compounds of the molecular type studied here. Because the approximation does not take into account the anisotropy of the real potential and its variation in the interstitial regions (in contrast to FDM), determination of the charge density is unreliable. Last, Figure 3c also shows that the Hg 5d and Hg 6s states have the same energy in Hg(SR)3− (2 and 7) and less so in Hg(SR)2 (3 and 8) and Hg(SR)42− (1 and 6) as a result of the split of the 5d states in two energy levels. Only the high-energy 5d states are hybridized with the 6s states in Hg(SR)2, whereas F


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Hg(SR)2.46 Geometry optimization shows that the two types of ligands yield similar coordination geometries with Hg (10 and 12 in Figure 1) and accordingly similar calculated near-edge HR-XANES spectra (Figure S5b). Considering the close correspondence between the two Hg environments and HRXANES spectra, only RSR is considered further. One, two, and four RSR groups were added to Hg(SR)2, featuring 2 + k Hg coordination with k = 1, 2, and 4 (Figure 1, models 9, 10a and 10b, and 11). To identify possible local energy minima, we considered several starting geometries (Figure S6). All of the starting structures converged to those shown in Figure 1. Additional bonds were nearly perpendicular to the original two-coordinate S−Hg−S configuration, as reported previously for solvated HgCl2 and HgClOH64−66 and Hg[(Cl)2+ring] complexes.62 The Hg[(SR)2+(RSR)2] adduct has two energetically equivalent forms: the 10a form approaches D4h symmetry and 10b planar C2h symmetry. The Hg···RSR distances range from 3.03 to 3.36 Å, similar to the secondary Hg···S interactions reported in dimethyl sulfide organomercurials,67,68 and increase with increasing number of exogenous RSR residues (i.e., Hg coordination number; Figure 1). The additional distances of 3.03−3.36 Å are secondary bonding interactions13,69 because they lie between the sum of the covalent radii (2.37 Å) and the sum of the van der Waals radii (3.35 Å) for Hg−S. These Hg···S interactions may feature a 2-center 2-electron (2c-2e) dative covalent bond,70 as in dimethyl sulfide organomercurials,67 palladium phosphane (PH3) complexes,71 and ruthenium phosphines/alkylidene complexes,72 and so may be described in terms of overlap of one S 3p lone pair with an empty Hg 6p orbital. Such bonding would correspond to σ donation of two electrons from the ligand to the metal center, resulting in a highly delocalized electron pair on Hg. Multinuclear complexes made by associating three (14) and five (15) Hg(SR)2 units were also considered for completeness and because crystals 3 and 16 feature 3[Hg(SR)2] and 5[Hg(SR)2] complexes, respectively. As when RSR groups are added, the linear geometry of the initial Hg(SR)2 complex is not affected by the formation of intermolecular Hg···S bonds with other neutral Hg(SR)2 complexes. The computed Hg coordination is orthogonal, and the long Hg···S bridging interactions have lengths of 3.12−3.21 Å for 14 and 3.19−3.31 Å for 15, which coincide with those obtained for Hg[(SR)2+(RSR)2−4]. The number of long Hg−S bonds that stabilize the n[Hg(SR)2] clusters is 4 for n = 3 and 12 for n = 5. Optimized structures are in good agreement with those reported7,8 for the n(HgX2) (n = 2, 3; X = F, Cl, Br, I) complexes. These reported and our optimized structures show that Hg atoms prefer a 2 + k coordination in contrast to Zn and Cd atoms, which prefer a tetrahedral coordination. This specificity of Hg for 2 + k coordination was explained by the relativistic contraction of the 6s orbital, which increases the 6s− 6p energy gap, thus making sp hybridization less probable.7 Along the same line, the preference of heavy transition metals, such as Au, for low coordination numbers was explained73,74 by the enhancement of 6s5d hybridization that is an additional consequence of the relativistic contraction of the 6s orbital. These results are consistent with the Hg 6s5d hybridization and the lack of Hg sp hybridization observed in this study. 2.4.2. Relative Thermodynamic Stability of Computed Structures. The relative thermodynamic stabilities of mononuclear and multinuclear mercury−sulfur complexes (Table 1) were calculated and considered in evaluating the structure(s)

the S p-DOS would not overlap the Hg sdz2 states at the Fermi energy. Evidence of covalent character of the Hg−S bond in the excited state is consistent with NPA results and NBO analysis (Table S1), which show that, in the ground state, the σ valence antibond (σ*) has 68% Hg 6s5d character and 32% S 3sp character. In a Lewis structure, which describes chemical bonds as shared electron pairs,60 the σHgS bond is 32% Hg and 68% S, which corresponds to a bond polarity of 68 − 32 = 36%. Thus, the height of the near-edge peak in NOM is related to the polarity of the Hg−SR bond. We show next with calculated HR-XANES and DOS profiles for molecular structures having more complex Hg−S bonding that the polarity can be increased by completing the Hg coordination with neutral RSR and RSR′ ligands. 2.4. Hg−S Bonding in NOM: New Hg[(SR)2 + (RSR)1−4] Model. 2.4.1. Computed Structures. The experimental HRXANES spectrum for compound 3 and its calculated counterpart 14 are the starting points for defining the structural models of the coordination environment of Hg in NOM. The near-edge intensity of Hg-NOM is similar or higher than that of 3 (Figure 2), which suggests that two-coordinate Hg may further complete its coordination sphere with other ligands. These ligands should not be bonded to other Hg atoms (as shown in 3 and 16, Figures 1 and S8); otherwise, Hg−Hg pairs would occur, and these are not observed in the regions near or beyond the edge (Figure S3).42 Because the formal 2+ charge of HgII is balanced in NOM by two (SR)− thiolate moieties and complexation of a third and fourth free thiolate would lead to trigonal-planar Hg(SR)3− (7) and tetrahedral Hg(SR)42− (6) adducts, any additional ligand should be neutral or near-neutral to retain the two-coordinate nearly linear S−Hg−S geometry of Hg(SR)2.12 Consideration of the chemical and functional group composition of NOM61 shows that organic sulfide (RSR), thioether (RSR′) (hereafter included with sulfide RSR), terahydrothiophene (THT), thiophene (SC4H4, denoted as Tph), and amine (RNH2) are the most likely types of ligands (Figure 1). Amine and other types of N ligands were disregarded as dominant secondary ligands because they yield a distinctive spectral signature (Figure S4). Although NH2 is neutral, the N atom is highly electronegative (3.04) and acquires the high negative charge of −0.9e in Hg[(SR)2+(NH2R)2] (18) compared to −0.5e for S in (SR)2 (Table S1), thereby causing a bisphenoidal bending of the S−Hg−S angle and diminution of the near-edge peak.12,18 In the computed Hg[(SR)2+(Tph)2] structure, the two thiophene residues are connected to Hg(SR)2 by four intermolecular Hg···C contacts at 3.1−3.2 Å (13 in Figure 1). The Hg atom interacts with the C−C bond of the thiophene ring rather than the S 3p lone pair. The geometry around Hg resembles that reported for intermolecular Hg π-back-bonding with aromatic rings.62,63 In the calculated HR-XANES spectrum, the four C ligands are reflected in a shift to the right of the near-edge relative to those of the Hg−S models and Hg-NOM (Figure S5a). The calculated displacement is in the same direction but smaller than that measured42 and herein calculated (Figure S5a) for the methylmercury (MeHgSR) spectrum, which features one short Hg−C bond of length 2.05 Å (19). These results indicate that the thiophenic complex is unsuitable as the major model for Hg-NOM. The two remaining possibilities are THT and RSR. S is nearneutral in THT (+0.1e) and RSR (+0.2e), implying that the two soft ligands would both form weak covalent bonds with G


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enthalpy ΔH. On the basis of the negative values of ΔH (Table 1), the three complexes are predicted to be stable under these circumstances. One notes also that the first complex is stabilized by one Hg−S long bond, the second further stabilized by two long bonds, and the third even more stabilized by four long bonds. When ΔH is divided by the number of long bonds, one obtains in each case almost the same value of −5 kcal/mol per bond. Coordinations 2 + 2 and 2 + 4 also may result from the association of several Hg(SR)2 complexes, as described previously (14 and 15). The formation of this third class of Hg−S adducts is described by the reaction

Table 1. Total Electronic Energy (ΔETot), Complexation Enthalpy (ΔH), and Complexation Gibbs Free Energy (ΔG) in kcal/mol of Mercury−Sulfur Complexesa mercury−sulfur complex

ΔE

ΔH

ΔG

[Hg(SR)4]2− (6) [Hg(SR)3]− (7) Hg(SR)2 (8) Hg[(SR)2+(RSR)] (9) Hg[(SR)2+(RSR)2] (10a) Hg[(SR)2+(RSR)4] (11) Hg[(SR)2+(THT)2] (12) Hg[(SR)2+(Tph)2] (13) 2[Hg(SR)2] 3[Hg(SR)2] (14) 5[Hg(SR)2] (15)

n.r. n.r. n.r. −7.0 −13.0 −24.6 −17.5 −16.6 −10.4 −28.4 −60.8

n.r. n.r. n.r. −5.3 −10.6 −21.8 −16.4 −15.0 −9.5 −25.8 −57.9

−22.6b −41.3b −54.9b 0.5 4.8 18.0 3.6 2.4 −0.5 −8.7 −13.7

nHg(SR)2 = n[Hg(SR)2 ]

All Hgn(SR)2n clusters have negative ΔG values (Table 1), in agreement with the known metallophilic attraction of metal atoms or ions with closed electron shells.78,79 However, the abundance of these clusters strongly depends on the concentration of linear complexes present in solution and the reaction time.42 At equilibrium, the concentration dependence can be evaluated from the equilibrium condition for reaction 2:

R radicals are methyl groups. ΔH includes thermal corrections to ΔETot, and ΔG includes entropic corrections to ΔH. ΔG values are corrected for the standard molar concentration in solution (c0). bFrom ref 75. ΔETot and ΔH were not reported (n.r.).

a

most likely in NOM. Previous energy calculations performed at the CCSD(T) level of theory75 showed that Hg(SR)2 (R = CH3) is more stable than Hg(SR)3− and Hg(SR)42− at acidic and neutral pH. Thus, the linear complex is predicted to be the most abundant of the three pure mercury thiolate complexes in NOM, in agreement with experiment.42,58,59 The Gibbs free energy (ΔG) of formation of Hg(SR)2 calculated for Hg2+ and SRH as free reactants is ΔG = −54.9 kcal/mol. Although ΔG is small, there is the possibility of decomposition by thiolate substitution with a stronger ligand, such as S2−, or by thiolate oxidation. Alternatively, the linear complex may be further stabilized by binding weaker S-containing ligands, as described next. The stability of this second class of Hg−S adducts was analyzed first by considering the model system [Hg(SR)2][RSR2]n with n = 1, 2, and 4 (9−11) and the complexation reaction Hg(SR)2 + n(RSR) = [Hg(SR)2 ][RSR]n

(2)

(n[Hg(SR)2 ])eq ⎛ −ΔG ⎞ n−1 ⎟ = c Keq, n = exp⎜ 0 ⎝ RT ⎠ ([Hg(SR)2 ]eq )n

(3)

where Keq,n is the equilibrium constant, R the gas constant, T the absolute temperature, and c0 the standard molar concentration in solution (1 mol/L), and [S]eq denotes the equilibrium molar concentration of species S. From eq 3, one can calculate for each type of cluster, the concentration of the free linear complex for which [Hg(SR)2]eq = (n[Hg(SR)2])eq. The equimolar concentration is given by ⎡ ⎤ ΔG [Hg(SR)2 ]eq = c0 exp⎢ ⎥ ⎣ (n − 1)RT ⎦

(4)

Carrying out the arithmetic, one obtains 0.4 mol/L for n = 2, 0.7 × 10−3 mol/L for n = 3, and 3.3 × 10−3 mol/L for n = 5, indicating that complexes with 2 + 2 and 2 + 4 coordination are more likely, if formed according to reaction 2. The preference for 2 + 2 and 2 + 4 coordination holds true even in macromolecular NOM, where the values of ΔG are dominated by the values of ΔH because the entropic correction is small. Overall, the results predict that the 2 + k coordination is thermodynamically favored in NOM. It should be noted that these modeled n[Hg(SR)2] multinuclear complexes have structures different from those of the nanoparticles of β-HgS formed when Hg-NOM is equilibrated for days to months.42

(1)

Gibbs free energies of complexation for reaction 1 are all positive, indicating that the three model complexes are predicted to be unstable (Table 1). However, the ΔG values are for cases where each reactant can move freely and independently. The situation is different in NOM given its macromolecular structure.76,77 If all reactants belong to the same molecular unit, the entropic correction to ΔG is small and the complex stability is mainly controlled by the complexation

Figure 5. Calculated near-edge region (a) and Hg s/dz2-DOS (b) for 8, 9, 10a, and 11. H


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Figure 6. Calculated Hg and S s/d/p-DOS for 8 and RSR separately (a), RSR coordinated to Hg(SR)2 (9) (b), and the mercury thioether reference 4 (c).

2.4.3. Computed HR-XANES and DOS Profiles. Important changes occur in calculated absorption spectra depending on the number of exogeneous S atoms (Figure 5a). Starting with the mononuclear complexes, the near-edge peak, which is a doublet in Hg(SR)2, becomes progressively narrower and more intense upon the addition of RSR residues. The evolution consists of a leftward shift of the high-energy shoulder and its resulting merger with the low-energy shoulder. In the (l,m)DOS profiles, the Hg 6s states, which dominate over the 5d states in the high-energy shoulder, shift to lower energies (Figure 5b). The total leftward shift of the 6s states is −1.1 eV in going from Hg(SR)2 (8) to Hg[(SR)2+(RSR)4] (11). Also, the initially split 5dz2 states merge, which together with the leftward shift of the 6s states contributes to greater overlap of the two DOS profiles, reinforcing 6s5dz2 orbital hybridization for Hg. The S s/p-DOS profiles from the two (SR)− ligands adapt to the new shape of the Hg states, optimizing the mixing of the metal−ligand empty valence states (Figure 4b). Similar modifications occur in the direction of the RSR ligands. Taking Hg[(SR)2+(RSR)] (9) as an example, the Hg 6s states overlap with the RSR 3spy states at 1.0−1.2 eV above the Fermi level, whereas the Hg 6s and RSR 3spy states have different energies in the absence of Hg···RSR contact (Figure 6a,b). Overall, Hg···RSR σ bonding lowers the energy and localizes the Hg vacant 6s orbitals, promoting favorable interactions (i.e., efficient mixing) with the donor orbitals of the RSR and (SR)− ligands. However, a major distinction between strong Hg−SR and weak Hg−RSR σ bonds is the lack of participation of Hg d states in the weak σ bonds. The DOS of Hg 5dx2−y2 is not observed in the Hg[(SR)2+(RSR)2−4] complexes (Figure 6b), whereas Hg dz2 is observed in the direction of the (SR)− ligands (Figure 5b). This finding is consistent with the dual character of Hg, which behaves as a post transition metal with filled d shells in organomercurials and a transition metal with involvement of d electrons in bonding in thiolate complexes.68,80−82

Complex 4 offers a rare example of Hg d orbital participation in chemical bonding with a neutral donor ligand, here a thioether (Figure 1). The Hg 6s states are hybridized with a small amount of the Hg d states, which both form a dominantly covalent bond with the RSR 3spz hybrid orbital (Figure 6c). As a result of polarization of the 5d10 shell, the Hg−RSR covalent bond has some electrostatic component, hence conferring Lewis acid properties to this adduct. Strong covalent bonding between the Hg sdz2 and RSR spz hybrid orbitals explains the stability of 4 because the 2+ formal charge of HgII is compensated by two weak (NO3)− anions, instead of the two strong (SR)− ligands in mercury thiolates. The extremely intense s-DOS and somewhat weak d-DOS, but high p → d transition strength, combined with the specific Hg binding of 4, provide a distinctive HR-XANES spectral signature for this type of complex. Next, we examine how the confinement in energy of the Hg s states and parallel augmentation in DOS induced by the primary (SR)− and secondary RSR ligands are reflected in NPA/NBO by a diminution of the Hg valence population and a corresponding increase of the Hg−SR bond polarity. The energy-integrated Hg s/d-DOS profiles in the [−1, 4] eV interval around the Fermi level show that the RSR ligands increase the empty density of the Hg 6s states from 0.024e to 0.028e and only marginally change the density of the Hg 5d states (Figure S7). This effect can be traced back to the electronic populations below the Fermi level in the ground state, as calculated by NPA (Table S1). Completing the Hg coordination with RSR groups decreases the 6s electronic population from 1.05e for 8 to 1.00e for Hg[(SR)2+(RSR)2] (10) to 0.97e for 11 and leaves unchanged the 5d population, which remains noticeably constant at 9.74−9.76e in all 2 + k complexes with thiolate ligands, either mononuclear or multinuclear. Accordingly, adding four RSR ligands increases the effective charge on Hg from +0.99e in 8 to +1.04e in 11. Analysis of the Hg−SR bond in terms of NBOs reveals a concomitant increase of the bond polarization with the number of RSR ligands from 38% in 8 to 43% in 11 (Table S1). Similar I


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Figure 7. Powder and polarized XANES calculations for the 2 + 2 coordination of Hg (a) with two secondary thioether-type ligands (Hg[(SR)2+(RSR)2] complex 10a) and (b) with two secondary thiolate ligands (3[Hg(SR)2] complex 14). (c and d) s/dz2/dx2−y2-DOS for the two complexes. Divalent Hg behaves as a transition metal with involvement of dz2 electrons in bonding with the two negatively charged thiolate ligands (SR−) and as a post transition metal with filled dx2−y2 orbitals (lone pairs) with the secondary neutral ligands [RSR and Hg(SR)2].

mentioned previously for Hg(SR)3− (7) and Hg(SR)42− (6). The resulting effect of the secondary ligands on the observed Hg L3-edge HR-XANES spectra is an increase of the near-edge peak intensity. Contrary to the current paradigm, the intensity of the first feature in L3-edge spectra therefore is not always a function of the number of d holes.53 The d-shell population remains the same in linear mercury complexes with two thiolate ligands [Hg(SR)2], independent of the number and identity (S or C) of the distant ligands that complete the Hg coordination sphere (Table S1). The reason lies in the fundamentally different nature of the primary Hg−S and secondary Hg···S bonds. The first Hg bond with two (SR)− donors compensates for the formal divalent charge of HgII, i.e., responds to the Lewis acidic character of Hg, and leads to participation of the Hg 5d shell in hybridization of Hg sdz2 valence orbitals with SSR spz orbitals, as seen from the d-DOS (Figures 4 and 5). The near-edge peak in Hg(SR)2 corresponds to absorption into this antibonding σ state. The four other Hg d orbitals are filled and stereochemically inactive. In contrast, longer intermolecular interactions between Hg and secondary S and C ligands indicate the behavior of Hg as a post transition metal bonded to Lewis bases with no involvement of 5d orbitals. However, these weak ligands are detected by HR-XANES because the Hg···S interaction modifies the Hg 6s orbital. Therefore, the intensity of the Hg L3-near-edge peak in Hg(SR)2 complexes is a

results are obtained when RSR is replaced by THT in Hg[(SR)2+(THT)2] (12) and by Hg(SR)2 in 3[Hg(SR)2] (14) and 5[Hg(SR)2] (15), which supports the calculations and validity of the trend. In this NBO-based Lewis description 68 of Hg−S bonding, increasing the metal coordination pulls the Hg 6s electrons toward the (SR)− ligands, thereby decreasing the (SR)− → Hg2+ ← (SR)− charge transfer. In summary, completing the Hg(SR)2 coordination with ancillary S or C (13) ligands decreases the occupancy of the 6s orbital and also narrows and shifts the s-DOS closer to the Fermi level without affecting the population of the Hg d shell. The decrease of the 6s occupancy is balanced by an increase of the electron density on the (SR)− ligands, thus reinforcing the polarity of the Hg−SR bond. The shift of s-DOS promotes the engagement of the otherwise diffuse 6s shell in bonding interactions with the secondary ligands. The two modifications occur whether or not the effective charge of S is neutral or nearneutral, as in the thioether ligand from the mononuclear complexes [Hg(SR)2...(RSR)1−4, 9−11], or negative, as in the intermolecular (SR)− ligands from the multinuclear complexes (Hg(SR)2...[Hg(SR)2]1−4), also denoted as n[Hg(SR)2], 14 and 15; Figure 7). In all cases, the RSR and Hg(SR)2 secondary residues should be neutral; otherwise, the Hg coordination would no longer be 2 + k but trigonal-planar or tetrahedral, as J


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Figure 8. (a) Calculated Au L3-near-edge region for [Au(SH)2]− and [AuCl2]− with the dipole selection rules Δl = ±1 (s + d final states) and Δl = −1 (s final states). (b) Experimental HgCl spectrum and calculated Ag L3-near-edge region for [Ag13Cl28]25− with the dipole selection rules Δl = −1 (s final states) and Δl = +1 (d final states).

Another example is Ag ([Kr] 5s14d10), which in monovalent Ag compounds, such as AgCl, shows a clear near-edge peak at 3360 eV in the middle of the L3-edge preceded by a distinctive shoulder at 3353 eV (peaks A and B in Figure 8b). The origin of the peak and shoulder is uncertain86,87 because of the 5s04d10 nominal electron configuration of Ag+ and weakness of the dipole-allowed 2p3/2 → 5s transition. NPA based on the geometry-optimized [AgCl6]5− model gives an electron configuration for the valence orbitals of 5s0.185p0.024d9.88. Consistent with this result, FDM calculation based on the [Ag13Cl38]25‑ rock salt cluster shows that the low-energy shoulder (peak A) arises from 2p3/2 → 5s transitions and the near-edge peak B from 2p3/2 → 4d transitions (Figure 8b). Quadrupolar transitions are also extremely weak at the Au and Ag L3-edge.

function of the density of holes in the Hg 6s states, which varies with the number of next-nearest ligands. Finally, we return to the major question addressed in this section, the coordination of Hg in NOM. Three conclusions can be drawn. (1) HgII predominantly forms a linear RS−Hg− SR Lewis acid−base complex with thiol S ligands at the investigated Hg concentration (200 ppm) and equilibration time (15 h). (2) The strong Hg−SR bonding is completed by weak Hg···RSR contacts with soft Lewis bases (e.g., organic sulfide RSR, thioether RSR′, possibly thiophene) perpendicular to the RS−Hg−SR bond direction. Because NOM is compositionally heterogeneous and polyfunctional,61,83 the secondary ligands are expected to be diverse in nature and variable in proportion among different macromolecules in one type of NOM. (3) No Hg−Hg pairs were detected. Multinuclear β-HgS nanoclusters, which are observed in aged NOM,42,59 take longer to form at this Hg concentration and have a local structure different from those of the multinuclear n[Hg(SR)2] model structures described here. The models have a molar ratio of thiol S to Hg of 2:1, compared to 1 in β-HgS. Elimination of the excess of thiolate ligands likely occurs by an alkylation reaction.42 2.5. Application of HR-XANES and Modeling Analysis to Other Closed-Shell Atoms. The combined experimental and theoretical approaches presented here and the new knowledge of the molecular binding of Hg to S, can be transposed to other closed-shell atoms. One example is Au ([Xe]4f146s15d10), which in its monovalent state features strong linear complexes with both S ([Au(SH) 2 ] − ) and Cl ([AuCl2]−).84,85 Ab initio calculation showed that the 6s and 5d shells are incomplete ([Xe]4f146s0.8−0.95d9.6) and the 2p3/2 → 6s/5d transition shifted to lower energy when Au is bonded to Cl relative to S, thus providing ligand sensitivity (Figure 8a and Table S3).85 In addition, this transition has a split profile, similar to that of Hg, which suggests that its intensity may also provide revealing insight into secondary ligands.

3. CONCLUSION XANES spectroscopy is a powerful tool for determining the chemical forms of metals in inorganic and organic materials. However, it has rarely been used to speciate Hg because its application has posed two major challenges. First, standard Hg L3-edge XANES spectra are essentially featureless because the dipole-allowed 2p3/2 → 5d electronic transition has a large natural width and the 5d shell is formally closed for HgII. Second, Hg has a low natural abundance, which requires an experimental mass sensitivity of typically 1 ppm, or below, for studies at environmental concentrations. We demonstrated that HR-XANES spectra, as measured with analyzer crystals, have more variable features than those measured with a conventional solid-state detector and provide the sensitivity needed for chemical characterization of Hg. Variable spectral features can be reproduced accurately with ab initio XANES calculations on energy-minimized structures, allowing the detection of small changes in molecular binding environments. A highlight has been the detection of weak ligands at 3.0−3.3 Å in the coordination sphere of Hg in NOM, which were previously K


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from a clear-water lake in Antarctica (PLFA; 1R109F). One is a soil humic acid standard also from the IHSS (ESHA; 1S102H). They were completed by two humic acids from peat bogs, one (CPHA) derived from heath vegetation (mostly Carex sp.), and one (SPHA) from moss (mostly Sphagnum sp.), both of which have been characterized previously.91 Hg was complexed at pH 6 for 15 h as in previous work.42,59,92 The Hg concentration was 200 mg of Hg/kg of NOM (200 ppm of Hg), and the experimental molar ratio of Hg to exocyclic S [(SR)− + RSR + RSR′ + RSSR], as measured by S-XANES,61 was 1:25 for ESHA, 1:50 for SPHA, 1:60 for NAFA, 1:140 for CPHA, and 1:440 for PLFA. 4.2. Sample Preparation for XANES Spectroscopy. A total of 150 μL of 1 was placed in a poly(ether ether ketone) (PEEK) holder designed for solutions, then immediately frozen in liquid nitrogen (LN2), and stored in a LN2 Dewar until its transfer into the liquid helium (LHe) cryostat of the beamline. Solid references (2−5) were finely ground, mixed with boron nitride to a concentration of approximately 200 mg/kg to prevent overabsorption,93 and prepared as pressed pellets. After 15 h of contact time, the NOM samples were separated from their solutions by filtration on a stirred ultrafiltration cell (Millipore 8200) and the wet powders freeze-dried and pressed into pellets. Each pellet was mounted in a PEEK sample holder with a 2.5 mm path length and sealed with Kapton tape. 4.3. HR-XANES Measurements. All spectra were measured in fluorescence yield detection mode with analyzer crystals on beamline ID26 at the ESRF. Rejection of higher harmonics and reduction of the heat load were achieved with a front-end Pd-coated, flat mirror working under total reflection at 2.5 mrad deflecting angle. The incoming beam was monochromated by the 111 reflection of a Si double crystal monochromator and then focused horizontally by a second Pd-coated mirror and vertically by a third Pd-coated mirror. The flux on the sample was approximately 1 × 1013 photons/s in a beam footprint on the sample of ∼500 (H) × 80 (V) μm2 full width at half-maximum. The Hg Lα1 (3d5/2 → 2p3/2) fluorescence line was selected using the 555 reflection of five spherically bent (radius = 1 m) Si analyzer crystals (diameter = 100 mm) aligned at an 81.8° Bragg angle in a vertical Rowland geometry. The diffracted intensity was measured with a Si drift detector in single-photon-counting mode. The effective energy resolution, obtained by convolution of the total instrumental energy bandwidth (spreads of the incident and emitted rays) and the 3d5/2 core−hole width from the Lα1 line was about 3.0 eV, compared to about 6.1 eV in conventional fluorescence yield measurement with a solid-state detector. Spectra were collected at 10 K in quick-scan mode to reduce the exposure time and repeated at different pristine positions on the sample to increase the signal-tonoise ratio. Evidence of radiation damage was monitored carefully and the scan time varied from 15 to 30 s depending on the sample photosensitivity. The incident energy was scanned from 12260 to 12360 eV in 0.2 eV steps. The HR-XANES spectra were normalized to unity at E = 12360 eV. 4.4. Computational Methodology. Examples of input files used for the geometry optimizations, frequency calculations, and HRXANES calculations, as described below, are given in the Supporting Information. Cartesian coordinates of the 18 optimized complexes (3−19) represented in Figure 1 and the Supporting Information are also provided. 4.4.1. Geometry Optimization. Calculations were performed using the ORCA program94,95 at the second-order Møller−Plesset perturbation theory (MP2)33 level. Ahlrich’s polarized def2-TZVP basis sets and corresponding auxiliary basis sets def2-TZVP/C96,97 of triple-ζ quality98 were used throughout for all of the atoms, in combination with the auxiliary def2-TZVP/J99 Coulomb fitting basis to accelerate the MP2 calculations with the resolution of identity (RI) approximation.100,101 For Hg atoms, scalar relativistic effects were accounted for using the [OLD-SD(60,MDF)] effective core potential (ECP),102 as obtained from the Stuttgart pseudopotential library. The valence basis set used in connection with the ECP is based on a (8s8p6d1f)/[6s5p3d1f] contraction scheme. In all calculations, solvation effects were introduced using the conductor-like screening

unseen but are predictable from known mercury chemistry, NOM chemical composition, and S functionalities. In addition to providing better spectral resolution, analyzer crystals also provide superior signal-to-noise ratio, which reduces uncertainties associated with edge normalization and also lowers the detection limit. Still, the current detection limit at the L3-edge of Hg on the X-ray absorption spectrometer used in this study is typically 2 ppm for 6−8 h of counting time. Efforts are underway to increase the luminosity of the analyzer crystals used in this study while maintaining their small-energy bandwidth needed for high-resolution measurement. The objective is to bring the detection limit below the ppm level, allowing investigation of Hg and other trace metals in extremely diluted samples.

4. EXPERIMENTAL AND THEORETICAL METHODS 4.1. Materials. An aqueous Hg(Cys)42− complex (1), three crystallized mercury complexes (2−4), and a commercial HgO product (5) were selected as references for this study (Figure 1). Five Hg-NOM complexes were prepared. 4.1.1. Complex Hg(Cys)42−. This sample, a reference for a tetrahedral mercury thiolate complex [Hg(SR)42−, 1], was prepared according to a published procedure.22 All solutions were prepared with Milli-Q water, which had been degassed by boiling and bubbled with high-purity argon gas to prevent oxidation of cysteine to cystine (RSSR). The mercury cysteine complex was prepared at [Hg] = 1 mM and a Hg-to-ligand ratio of 1:10 at pH 11.9. 4.1.2. Crystals [NEt4][Hg(SC6H11)3] (2) and Hg(SC6H11)2 (3). These compounds served as references for trigonal [Hg(SR)3−, 2] and linear [Hg(SR)2, 3] mercury thiolate complexes. They were synthesized following a published procedures (VOXTOR for 2 and VOXTIL for 3),47 and their crystal structures were confirmed by single-crystal X-ray diffraction. 4.1.3. Crystal [Hg(C8H8OS)2][NO3]2 (4). This compound is a new mercury thioether complex, which served as the reference for Hg linearly coordinated to two neutral S-containing ligands (RSR′). Hg(NO3)2·H2O (200 mg, 0.60 mmol) was dissolved in methanol (5 mL) and added dropwise to 2-(methylthio)benzaldehyde of 90% purity (CAS 7022-45-9; 176 μL, 1.23 mmol) in methanol (5 mL). A fine yellowish precipitate formed after stirring at room temperature for 20 min. The mixture was centrifuged and the supernatant was isolated and then slowly evaporated at room temperature. Colorless plateletshaped crystals formed after 24 h. A small crystal (0.20 × 0.16 × 0.08 mm) was mounted on a Bruker Kappa CCD diffractometer and its structure obtained using monochromatic Ag Kα radiation (λ = 0.56087 Å). Crystal data at 295 K: C16H16HgN2O8S2, Mw = 629.02, space group P1̅ (No. 2), a = 7.8250(3) Å, b = 8.2100(4) Å, c = 8.2350(5) Å, α = 78.931(6)°, β = 68.325(7)°, γ = 84.565(4)°, V = 482.33(5) Å3, Z′ = 1, Dx = 2.166 g/cm3, μ = 45 cm−1, 2θmax = 62°, 9626 measured reflections and 2203 unique reflections (Rint = 0.06) and R1= 0.06 [with 2147 I > 2σ(I)] and wR2 = 0.11, 137 parameters refined, GOF = 1.06, and max/min residual peaks 1.51/−2.73 e Å3. Data were corrected for Lorentz and polarization effects and empirical absorption (SADABS from Bruker/Siemens). The crystal structure was solved by direct methods with the SIR9288 program and refined by full-matrix least squares, based on F2, using the SHELXL89 software through the WinGX90 program suite. The refinement was performed with anisotropic thermal parameters for all non-H atoms. H atoms were generated at idealized positions, riding on the carrier atoms, with isotropic thermal parameters. The crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (reference number CCDC 1409634). 4.1.4. Hg-NOM Complexes. Hg [from a 0.1 mM Hg(NO3)2 stock solution] was complexed to five types of NOM considered to represent the diversity of NOM in the environment. Two are fulvic acid references from the International Humic Substances Society (IHSS), one from a brown-water lake (NAFA; 1R105F), and the other L


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Inorganic Chemistry model (COSMO) optimized for water solvation,103 as implemented in ORCA.104 The accuracy of the modeling approach, in particular the applicability of the relativistic pseudopotentials for Hg, was assessed with respect to the prediction of three crystallographic structures chosen for the diversity of Hg coordination (3 and 4 in Figure 1 and 16 in Figure S8). Adduct 16 is bis(ethanethiolato)mercury [Hg(SEt)2(c)].105 The metrical and angular details of the computed and crystallographic structures are in good agreement (Figure S8). Differences arise, in part, from the loss of periodicity in the molecular models. Because geometry optimizations for transition-metal compounds most often are performed using density functional theory (DFT),106−115 including that for Hg,23,46,116−118 comparative tests were conducted on the three mononuclear complexes mercury bis(methanethiolate) [Hg(SMe)2, 8], mercury bis(ethanethiolate) [Hg(SEt)2, 17; Figure S9), and mercury bis(thiolate) + mercury bis(tetrahydrothiophene) (denoted as Hg[(SR)2+(THT)2)], 12) and on the multinuclear cluster 5[Hg(SEt)2] derived from 16 (Figure S8). The initial coordinates of 5[Hg(SEt)2] were taken from the crystal structure of 16.105 Specifically, the performance of the two DFT hybrid functionals PBE0119,120 and B3LYP121,122 and of the nonhybrid functional BP86122,123 was assessed. For consistency, the same Hg ECP and def2-TZVP all-electron basis sets as those for the MP2 calculations were used. Table S4 summarizes the Hg−S distances, and Figure S9 shows the computed structures at various theoretical levels. The three functionals provide similar geometry predictions for the three mononuclear complexes, which are also similar to those obtained with accurate benchmark MP2 calculations. However, the DFT bond lengths are all overestimated by several hundredths (Hg−SR) to tenths (Hg−RSR from THT) of an angstrom, especially with the B3LYP and BP86 functionals.66,72,117,124−126 The performance of the PBE0 functional is the best of those tested, which is in line with a previous test set for third-row mononuclear transition-metal complexes, which ordered the performance of the three DFT functionals as follows: PBE0 > B3LYP ≈ BP86.127 Overestimation of the bond lengths by DFT has been attributed to neglect (B3LYP and BP86) or partial correction (PBE0) of dispersion forces between atoms and molecules in the region of the van der Waals minimum.72,125,126,128,129 Thiolate bonds to large metal atoms, including Au, were shown to be particularly prone to this effect.85,117,129 This pitfall can be problematic when Hg coordination is high because the optimized complex is not sufficiently compact. A case in point is 5[Hg(SEt)2], which features six-coordinate Hg in approximate D4h symmetry in both the crystal105 and the MP2 structure and incorrect 3/4-fold (PBE0/BP86) and 2-fold (B3LYP) coordination in the DFT structure. The RS−Hg−SR angle is 176− 177° at the PBE0−B3LYP level, which is similar to the 177−179° values for the crystal and MP2 structure (Figure S8) but bent to 173° for BP86. The computed Hg(SMe)2 and Hg(SEt)2(m) structures, when obtained with the same method, are practically identical. Therefore, for simplicity of computation and display, methyl (Me) groups were used throughout to represent a generalized alkyl group (R) attached to S [(SR)−]. Because the starting Cartesian coordinates were taken from crystallographic structures, the RS−Hg−SR and Hg−RSR bond directions of the molecular models could have any orientation with respect to the dx2−y2 and dz2 orbitals of Hg and the px, py, and pz orbitals of S. Optimized structures were oriented in the Cartesian coordinate system as follows. Atomic coordinates were translated to bring the central Hg atom to the origin and then rotated to align the shortest Hg−SR bond along the Z axis and other Hg−ligand bonds along the X and Y axes if orthogonal to the Hg−SR bond. The as-oriented structures were used for electronic calculation in the ground-state (NPA and NBO) and excited-state (l,m)-DOS and for polarized XANES calculation. 4.4.2. Electronic Ground-State Calculation. Atomic orbital populations and atomic charges were deduced from NPA and the polarity of the Hg−S bonds by NBO analysis.38−41 The NPA and NBO analyses were carried out with NBO 6.0,130 as implemented in ORCA. The impact of the level of theory (MP2 and DFT), nature of

the functional (PBE0, B3LYP, and BP86) used in DFT to describe each atomic orbital, and length of the side R chain on the quality of the NPA and NBO results was assessed with Hg(SMe)2 (8) and Hg(SEt)2(m) (17). All calculations used the same Gaussian basis sets (i.e., contraction scheme; Table S5). Essentially the same results were obtained with the two complexes, again supporting the use of methyl to describe R groups. The MP2 and DFT/PBE0 calculations yielded the same effective charges on Hg (+0.99e) and on S (−0.44e) but slightly different 5d and 5f orbital occupations for Hg and 3p and 3d orbital occupations for S. The Hg−S bond polarities were also close in value: 35.8% (MP2) vs 35.1% (DFT/PBE0) for 8 and 36.2% vs 35.9% for 17. The charge transfer and degree of bond polarity are in good agreement with the relative difference in the Pauling electronegativity of the two atoms (2.00 for Hg vs 2.58 for S). Somewhat similar electronic properties were obtained with B3LYP but not with its nonhybrid variant BP86. The differences between the charges on the Hg atom, relative to the MP2 and DFT/PBE0 values, are −0.03e (B3LYP) and −0.08e (BP86). The Hg−S bond polarities are also lower: ∼ 33% with B3LYP and 28−29% with BP86 versus ∼35% with MP2 and DFT/PBE0. We conclude that both MP2 and DFT/PBE0 methods yield reliable NPA and NBO results on mercury thiolate complexes. Independent of the overall accuracy of estimation, electronic trends between structures computed with the same method are expected to be precise because NPA and NBO analyses are known to be sensitive to small changes in the bond characteristics.131 4.4.3. Thermochemical Calculations. The relative stabilities of the complexes were determined by calculating their Gibbs free energies (G) of complexation in aqueous solutions:

ΔG = G(C) − ΣG(R i)

(5)

where C denotes the complex and Ri the free reactants. For each complex, G was evaluated from G = E Tot + ΔGcorr

(6)

Here ETot is the single-point total electronic energy (including the COSMO solvation energy) of the optimized structure, and ΔGcorr is the sum of the thermal and entropic corrections for T = 298 K and p = 1 bar. These corrections were evaluated analytically from a separate frequency calculation on the previously optimized structure. The ascalculated ΔG values were corrected for the difference between the standard concentrations in the gas and solution phases. On the basis of our previous ab initio study on mercury thiolate complexes,75 we estimate energetic variations to be precise to 2−3 kcal/mol. 4.4.4. Calculation of the HR-XANES Spectra. Hg L3-edge XANES spectra were calculated with the ab initio FDM, as implemented in FDMNES.35 The code calculates the final state potential in real space from a cluster of atoms. The form of the potential is not approximated, in contrast to the alternative muffin-tin approach,48 thus providing a better estimate of the scattering phenomena in asymmetric Hg(SR)2 complexes. Computations are, however, much more time-expensive. A three-dimensional grid is constructed, and the final-state wave functions and their projections with s and d symmetry around the absorbing atom for the L3-edge simulation are calculated by solving the Schrödinger equation on each node point.132 Dynamic electronic effects of relativity are especially important for the valence electrons of Hg133 and are not accounted for in the Schrödinger equation. These effects are treated in FDMNES with the relativistic Dirac−Fock approach in the evaluation of the spin−orbit coupling of atomic states and with a semirelativistic variant for the scattering and propagation of the photoelectron. Calculations were performed on a grid with a radius of 6 Å centered on the absorbing Hg atom, regardless of the number of atoms in the cluster. Atomic potentials, Fermi level, and charge transfer were calculated self-consistently, i.e., recursively, until convergence was achieved. Because the simulation code uses an arbitrary photoelectron zero energy, the calculated spectra were shifted in energy to match the experimental spectra. In this way and because a monoelectronic approximation is valid at the Hg L3-edge, this alignment causes the electron Fermi level to correspond to the ionization energy from the X-ray. It also has the advantage of M


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correcting errors in estimation of the self-consistent Fermi level and the somewhat arbitrary calibration in energy of the X-ray monochromator. We used the usual zero energy origin at the Fermi level when working with the electronic structure and the photon energy scale when working with absorption spectra. To better assign the spectral features, the as-calculated theoretical spectra were compared directly to the data without convolution by the core−hole and final state lifetime broadening and experimental resolution. FDM calculation also provides the partial DOS of the excited atom resolved over the (l, m) quantum numbers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01932. Tables S1−S3 containing partial atomic charges and electronic occupations of valence orbitals calculated by MP2/NPA and FEFF, Tables S4 and S5 comparing the performances of MP2 and DFT calculations, additional XANES (S1 and S3−S5), DOS (S2 and S7), and structural (S6, S8, and S9) figures, examples of input files, and Cartesian coordinates for the computed structures (PDF) X-ray crystallographic data in CIF format for 4 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Tel.: +33 4 76 63 51 93. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ragnar Björnsson for his advice on the use of ORCA, Kiyotaka Asakura for providing the AgCl XANES spectrum, and Christophe Lapras (ESRF) for his technical assistance during synchrotron measurement. Support was provided to A.M., I.G.-L., M.E., and C.L. by the French National Research Agency (ANR) under Grant ANR-12-BS06-0008-01, to A.M., M.R., and P.G. by the ANR under Grant ANR-10-EQPX-27-01 (EcoX Equipex), to K.L.N. and A.M. by the Office of Science (BER), U.S. Department of Energy, under Grant DESC0001730, and to K.L.N. by the U.S. National Science Foundation under Grant EAR-0952311. The Froggy platform of the CIMENT infrastructure (ANR Grant ANR-10-EQPX29-01) provided computing resources.

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REFERENCES

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