Review pubs.acs.org/Organometallics
Mercurophilic Interactions Hubert Schmidbaur*,†,‡ and Annette Schier† †
Department Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching, Germany Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia
‡
ABSTRACT: The widely confirmed abundance of aurophilic interactions in the structural chemistry of gold(I) compounds gives reason to scrutinize the evidence for analogous mercurophilic interactions in mercury(II) compounds more sporadically advanced in the literature. From the inventory of early observations and more recently accumulated data it appears that the equilibrium distances of intra- and intermolecular Hg- - -Hg contacts (ca. 3.5 Å) are generally much larger than those of Au- - -Au contacts (ca. 3.0 Å). There are very few estimations of the small energy contributions associated with the Hg- - -Hg contacts from experimental data, but quantumchemical calculations have confirmed that these energies also are generally much lower than those for the Au- - -Au contacts. Notwithstanding, there are several special cases where particularly short Hg- - -Hg distances, or the preferred modes of associations of molecules into oligo- or polymeric arrays, do indicate significant attractive mercurophilic interactions. Photophysical phenomena also suggest that for several groups of oligomeric organomercury(II) compounds there is a much stronger Hg−Hg interaction in the excited state, with interesting consequences for the emissive properties. Interestingly, the most intriguing observations that suggest mercurophilic interactions, like the polymercuration of oxygen, nitrogen, and carbon, date back to the 19th century (Roucher in 1844, Millon in 1839, Sakurai in 1880, and Hofmann in 1900, respectively) and still call for a rationalization.
I. INTRODUCTION
less common and hence there are fewer examples which provide unequivocal evidence for similar interactions. Up to the late last century, metallophilic bonding has not been part of one of the existing concepts of chemical bonding. Simple valence rules seemed to exclude interactions between metal centers such as gold(I) Au+ in any of its complexes: with its 5d106s0 configuration, there is no room for covalency, and the interaction must be Coulomb repulsive. And yet, there is clear evidence for an attraction among molecules LAuX, cations [LAuL]+, and anions [XAuX]−, with L and X being neutral and anionic ligands, respectively, associated with bond energies of up to 12 kcal mol−1.2,3,6,7 Most important, the closest contact between such molecules or ions is established between two or more metal centers (1a) and not between metal centers and any one of the ligands (1b). In contrast, the latter mode of interaction appears almost exclusively in the structures of the lighter homologues (1b) and may finally show a strong bending of the X−M−X axes (1c). It was only through sophisticated quantum chemical calculations that dispersion forces in combination with relativistic effects could be shown to lead to significant attraction.1,8,9 It is obvious that this weak interaction must not be hindered by the set of ligands present at the given metal centers, and therefore metallophilic interactions are most common for compounds with twocoordinate metal centers and much less so as the coordination number is increased to 3, 4, or beyond. Since two-coordination
“Metallophilic interactions” refer to the weak chemical bonding occurring between heavy-metal atoms with seemingly closed shell electronic configurations.1−3 As intramolecular forces, these interactions support the formation of unusual molecular compositions and influence significantly molecular structures and dynamics, leading e.g. to preferences for certain molecular configurations and conformations. Intermolecular metallophilc interactions codetermine the organization of molecules in solids, the mode of aggregation of polymers containing metal atoms, and the arrangement of the components of ionic crystals. In all cases, short sub van der Waals contacts between metal centers are established which are associated with low bond energies approaching those of other weak forces, including in particular standard hydrogen bonding, agostic interactions, halogen bonding, π−π stacking of arenes, and others. Metallophilic interactions are encountered mainly between metal centers with low coordination numbers, which are most common for the heavy late transition metals. The preference for low coordination numbers arises predominantly from relativistic effects,1,4 and therefore the concept of metallophilic bonding has always been closely associated with relativity and its consequences for the electronic structure of atoms.1−3 Since relativistic effects increase dramatically for the post-lanthanide elements in the periodic table, metallophilicity has been diagnosed preferentially for the heaviest transition metals (Pt0, Au+, and Hg2+ with 5d106s0) and the following heavy main-group metals (Tl+, Pb2+, and Bi3+ with 5d106s2). For the lighter homologues (e.g. Ag+),5 low coordination numbers are © XXXX American Chemical Society
Special Issue: Mike Lappert Memorial Issue Received: November 11, 2014
A
DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
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Organometallics is most abundant in the chemistry of gold(I), aurophilic interactions Au- - -Au in and between compounds with twocoordinate gold(I) centers have been taken as the prototypes for metallophilicity.1−3 Even though relativistic effects are smaller for silver, the dispersion forces between Ag+ centers (4d105s0) also give rise to significant Ag- - -Ag interactions, provided that the ligand sphere allows for a close metal−metal contact and exerts favorable electronic effects.5
For quite some time, reports on systems with unsupported Hg- - -Hg contacts were very rare and the observed long distances (>3.5 Å) have cast some doubt on the significance of the underlying metal−metal interaction. Since the turn of the century, the literature dedicated to mercurophilicity has grown considerably, and the present review is a critical summary of recent findings and the conclusions to be drawn from them. The terms “mercurophilicity” and “mercurophilic interactions” or “mercurophilic bonding” have been used for about 20 years, referring mostly to the parallels to aurophilicity as mentioned above. Strangely enough, both terms are a combination of the Latin word for the metal (aurum, mercurium) and the Greek word philein (having a distinct preference, love). (As with “auriophilicity”, the spelling “mercuriophilicity” used sporadically for some time has been abandoned.) It must be noted, however, that the term “mercurophilic” had previously been introduced in an entirely different area: it designated multifunctional ligands used in analytical chemistry for trapping or sequestering mercuric anions in aqueous or other solvent systems. These mercurophilic reagents include e.g. thioethers such as tetrathiocyclophanes.19−21 In the recent literature, this version is no longer much in use. It should also be remembered that in classical nomenclature organic thiols RSH were called “mercaptans” (captain, Greek: capture), because of their affinity for mercuric and mercurous cations, producing mercury thiolates as dark insoluble precipitates. Thioethers and thiols reflect the general affinity of mercury for sulfur. The present coverage of the pertinent literature first considers unsupported mercurophilic interactions between independent mononuclear units. The structures and molecular dynamics of these systems may offer the best chances to trace the characteristics of these contacts. This is followed by an investigation of the reports on ligand-supported di- and polynuclear mercury(II) compounds, where it is much more difficult to delineate the effects of any Hg- - -Hg contacts on the structures and properties. Throughout this review only homometallic contacts (Hg- - Hg) are considered. There is a growing wealth of literature on metallophilic interactions in mixed-metal systems including mercury in combination mainly with gold, platinum, and thallium (M- - -Hg), as reflected by recent summaries in the literature.22,23 1. Unsupported Hg- - -Hg Contacts between X−Hg−X Molecules. a. Mercury Dihydride, H−Hg−H. In endeavors to explore any possible Hg- - -Hg interactions between neutral triatomic molecules X−Hg−X with a linear structure, mercury dihydride has been the focus of several experimental and theoretical studies. This most simple candidate appeared to offer the chance to demonstrate the existence of even weak attractive forces between the metal atoms. Early preparative work by Wiberg and Henle had shown that a compound of the composition HgH2 may be obtained only at very low temperature. The white solid generated in several metathesis reactions decomposed above −125 °C.24 There are no other reports of successful synthetic approaches under standard
II. MERCUROPHILICITY Surprisingly, although two-coordination is also common for mercury(II) compounds, the number of structures that could provide evidence for Hg- - -Hg interactions is much smaller. This observation has been made early on and has recently become the subject of several discussions in the literature.4,9−13 The overview of the literature published up to the year 2000 has shown that there were less than 25 examples where short Hg- - -Hg contacts exist in solid-state structures.10 No experimental data on the gas or solution phase were available. Out of the listed examples, only very few feature conspicuously short Hg- - -Hg contacts near or even below 3.00 Å (Table 1 in ref 10), the most common range covering distances of standard aurophilic contacts Au- - -Au.2,3 In this context, estimations of van der Waals radii for the mercury(II) ion were published on the basis of either experimental data or theoretical calculations. Depending on the geometry of the models chosen for the calculations, the values cluster around 1.75 Å,10 in good agreement with the results of extrapolations from crystal structure data: 1.73 Å14 or 1.76 Å.15 Other values are often less discriminative regarding the oxidation state of the mercury atom16 and are not considered here. An intra- or intermolecular Hg- - -Hg distance larger than 3.50 Å therefore means that the metal−metal contact should not be associated with any major interaction energies significantly exceeding those of standard van der Waals contacts. With contributions from metallophilic bonding, attraction can result from dispersion forces, but it must be considered that there is an inverse exponential dependence (R−6) of the dispersion term VDISP of the interaction potential VINT on the distance R between two metal atoms.10 It has further been noted that most of the few welldocumented short Hg- - -Hg contacts are supported by one or even two ligands, and therefore their Hg- - -Hg distance is influenced by the ligand geometry, flexibility, and bonding directionality. A case in point is the product of the peri dimercuration of naphthalene (2a), where the observed short transannular Hg- - -Hg distance of only 2.80 Å is clearly a consequence of the directionality of the bonding at the carbon atoms at the 1,1′,8,8′-positions. It is therefore not surprising that the C−Hg−C angles (173.3°) deviate strongly from 180°, the metal atoms being clearly shifted away f rom each other to minimize Hg- - -Hg repulsion.17 The monobridged precursor 2b shows similar effects.18 Both observations indicate that Hg(II)- - -Hg(II) distances up to at least 3.1 Å imposed by the geometrical preferences of the ligands should be considered repulsive. B
DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
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pseudopotential structure optimizations of the dimer (HgH2)2 have shown that the model with C2h symmetry (3b) is lowest in energy as compared to the model with D2h symmetry (4), with a barrier as high as 95.5 kJ mol−1 for the interconversion. The dimerization energy for 2 HgH2 → (HgH2)2 is very low at −9 kJ mol−1, and the aggregation is associated with only small distortions of the structure of the monomer: H−Hg−H 179.2° for Hg−H 1.637 Å and Hg- - -Hg 3.821 Å, with the angle Hg− H−Hg of 98.5° being not very different from 90°.
laboratory conditions. However, in the mid-1990s LegaySommaire and Legay prepared the molecules H−Hg−H, H− Hg−D, and D−Hg−D by irradiation (KrF laser, at 249 nm) of atomic mercury in argon or nitrogen matrices containing various concentrations of hydrogen (and/or deuterium) at 5.7 K. The authors identified the matrix-trapped products by their vibrational spectra, with major bands in the ranges 1880−1950/ 1350−1400 (ν3, stretch) and 730−790/540−580 cm−1 (ν2, bend) for H/D, respectively.25,26 Interestingly, upon annealing of the matrices by heating up to 45 K significant changes in the number and intensities of the IR bands were observed (prominent bands for ν3 at 1943/1395 cm−1 and for ν2 at 781/561 cm−1 for Hg(H/D)2), which in part were tentatively assigned to a dimerization and oligomerization of the molecule, yielding (HgH2)x aggregates (x = 2, 3) “with two or more Hg atoms as nearest neighbors”.25 In 2004, similar experiments were carried out by Wang and Andrews, who irradiated atomic mercury (mercury lamp, 240− 380 nm) in a matrix of pure solid hydrogen at 4.5 K.27 Two sets of bands were again observed in the regions already mentioned, but more specific assignments were made after annealing, also on the basis of data calculated for various (HgH2)n model systems. ν3 was thus localized at 1902.3/1368.4 cm−1 for the monomer, at 1886.6/1357.3 cm−1 for the dimer, and at 1878.3/ 1350.5 cm−1 for the trimer of Hg(H/D)2. The corresponding values for ν2 are 772.8/555.9, 764.3/550.0, and 753.4/541.6 cm−1. The structures of the models (DFT and MP2 calculations) are shown in formula 3a for the monomer with a Hg−H distance of 1.656 Å, which is nearly unchanged in the dimer 3b (1.652 and 1.659 Å) and the trimer 3c (1.652 and 1.662 Å) for terminal and bridging H atoms. Vibrational frequencies calculated for these models are in good agreement with the experimental data. Precise ν3 fundamentals for the HgH2 monomer with the various isotopes of mercury have been measured by IR emission spectra, and these data confirm the values of the absorption measurements.28 The solid product left after sublimation of the hydrogen matrix (ν3 1802 cm−1, ν2 673 cm−1) decomposed at 150−170 K, in accord with the result of Wiberg and Henle.24
In a theoretical study carried out in 2000 by Pyykkö and Straka the energy associated with the oligomerization of HgH2 was examined by ab initio quantum chemical methods up to the CCSD(T) level.10 The formation of dimers (HgH2)2 was investigated for four different models (designated as parallel, perpendicular, T-shaped, and slipped), as shown in 5a−d. The shortest Hg- - -Hg distance (R = 3.363 Å) was found for the perpendicular array (5b) with a depth of the potential well V(R) at −7.8 kJ mol−1. Interestingly, a very similar value of V(R) = −7.4 kJ mol−1 was found for the slipped array (5d), where R is much longer at 3.708 Å. The stability of the latter is largely due to electrostatic contributions VEL, while for the former the dispersion term VDISP is the dominant factor. Note that the geometry of the slipped model 5d is similar to those proposed by Kaupp and von Schnering and by Wang and Andrews (3b) except for a very significant difference in the Hg- - -Hg distance (3.708 Å vs 3.821 and 4.072 Å).4,10,27 Notwithstanding, the results of both theoretical studies are generally in satisfactory agreement.
Upon inspection of the molecular geometries shown in 3b,c it appears that for both the dimer and the trimer the Hg- - -Hg distances (4.072 and 4.449 Å) are well beyond the threshold for which significant mercurophilic attraction could be assumed. In the discussion of their results, Wang and Andrews have concluded from comparisons with the structures of ZnH2 and CdH2, where hydride bridging between the metal atoms is the dominating mode of interaction, that the distant Hg- - -Hg contacts contribute much less to the bonding between the monomers.27 The linear structure of H−Hg−H favored by relativistic effects (3a) is largely retained in the oligomers, whereas the H−Zn−H and H−Cd−H units are strongly bent in their oligomers and in the polymers (1c).29 These experimental investigations of the possible oligomerization of HgH2 were preceded, already in 1994, by a computational study by Kaupp and von Schnering, which also included the four mercury(II) halides.4 Quasi-relativistic
As a reference for the data of oligomers of HgH2, computational and experimental data for the molecular structure of the monomer have been available early on. In all cases a structure with D∞h symmetry and a H−Hg distance typically in the range 1.615−1.650 Å have been proposed,9,10,27 in comparison to an experimental value of 1.63324(1) Å.28 b. Diorganomercury(II) Compounds R−Hg−R′. The dimerization of dimethylmercury was also the subject of quantum-chemical studies by Pyykkö and Straka.10 Four different models with arrangements as also chosen for the (HgH2)2 dimers (6a−d) were investigated, of which the parallel and perpendicular ones were found to be lowest in energy with V(R) in both cases at −12.67 kJ mol−1, but with different Hg- - -Hg distances R of 3.279 and 3.408 Å, respectively. For both models, the dispersion term VDISP appears to have an even C
DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
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Organometallics more dominant influence, because the polarizibility of HgMe2 is much larger than that of HgH2. There are no experimental data for comparison. The solid-state structure of HgMe2 is not known, and no association was observed in the solution or gas phase. The structure of the monomer has been determined by electron diffraction (D3d, Hg−C 2.23 Å).30
However, crystals of bis(trimethylsilyl)mercury were found to be built from dimeric molecules with a perpendicular arrangement of the monomers (D2d symmetry, 10) and with a short Hg- - -Hg contact of only 3.1463(6) Å.41,42 To date, this is the shortest unsupported Hg- - -Hg contact reported in the literature, much closer to standard distances of aurophilic contacts. It calls for an explanation, because it is a more unexpected result, considering the bulkiness of the Me3Si groups, which must cause steric hindrance. Two arguments can be offered. One is based on the higher polarity of Hg−Si bonds as compared to Hg−C bonds with significant consequences for the dispersion term VDISP that dominates the mercurophilic interaction. The other argument refers to a support of the Hg- - -Hg interaction by an extended system of van der Waals interactions between the methyl groups, which are held in a conformation providing ample contacts between the two ends of the dumbbell monomers. The former explanation may find support in the fact that an only slightly longer Hg- - -Hg contact of 3.286 Å (11) has been encountered in dimercuracyclic compounds with two Si−Hg−Si units facing each other for a transannular contact.43 In contrast, bis(triphenylsilyl)mercury is a Ci-symmetric monomer in the crystal state, where the packing of the monomers appears to be determined by various π interactions between the phenyl groups.43
In crystals of dibenzylmercury the monomers are arranged in chains with Hg- - -Hg contacts of 3.54 Å (7). This feature was not commented on by the authors31 but recognized in later studies.32 The distance coincides perfectly with the proposed sum of the van der Waals radii (above). In contrast, bis(1naphthylmethyl)mercury is a monomer in the crystal, with Hg- - -Hg distances larger than 5.6 Å. 1Naphthylmethylmercury(II) chloride C10H7CH2HgCl could not be crystallized.33
The crystal structures of diphenyl-, di-p-tolyl-, bis(pentafluorophenyl)-, and bis(8-quinonyl)mercury have not provided any evidence for conspicuously short Hg- - -Hg contacts,34−37 even though the latter compound exists in the form of several polymorphs.37 Diarylmercury molecules with a variety of functional substituents in ortho positions also show no mutual approach in the crystals that would indicate Hg- - Hg attraction.38 Very surprisingly, however, macrocyclic cations with two −C6H4−Hg−C6H4− spacers between Schiff base units engaged in complexation with a Cu+ center (8, the Schiff base ligand being represented by a bold face bar) were found to be aggregated in the crystals of the perchlorate salt to give dimers with a very short unsupported Hg- - -Hg contact of only 3.203 Å. This unusual contact may be influenced by additional metallophilic contacts with the central copper atom (Hg- - -Cu 2.921 Å) in an almost linear transannular bridge: Hg- - -Cu- - Hg 177.9°.39 The same is true for the crystal structure of methylmercury cyanide, MeHgCN, where Hg- - -Hg- - -Hg zigzag chains have Hg- - -Hg distances of 3.77 Å (9).32,40 The structure resembles very closely some of the arrangements found in the crystals of gold(I) compounds, but the Au- - -Au contacts are generally much shorter (by about 0.5 Å).2,3
Multiple Hg- - -Hg contacts have also been observed between dialkynylmercurials with their linear chain −CCR substituents. For R = Ph, SiMe3 the monomers are associated into oligomers with five or eight Hg atoms.32 The metal atoms are assembled in small puckered sheets with the phenylethynyl or trimethylsilylethynyl groups sticking out above and below these sheets (12). A mercury atom may be engaged in contacts with up to four or five other mercury atoms and with Hg- - -Hg distances in the range 3.75−4.0 Å. This type of organization of two-coordinate metal atoms in sheets with the M- - -M contacts as the closest approach between the molecules is again reminiscent of the related examples observed in gold(I) coordination chemistry with alkynyl, carbonyl, cyanide, or isocyanide ligands,2,3 but again the Hg- - -Hg distances are longer than the Au- - -Au distances in their counterparts. Moreover, with alkynyl ligands the positioning of the D
DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
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Organometallics approaching Hg atom is often shifted toward one of the CC functions, where it may also entertain π complexation (13). The units −CC−Hg−CC− have also been integrated into polymers with various fluorene spacers, but it is unclear if Hg- - -Hg contacts between the chains are of any significance in building and stabilizing the materials, which have been tested as cathode interlayers in solar cells.44 Hg- - -Hg contacts have also been established in mixed-metal complexes with alkynyl ligands.45
Heteroleptic alkyl(alkynyl)mercurials show similar modes of aggregation. An example is [(fluoren-2-yl)ethynyl]methylmercury, which exists in pentameric packages aggregated via rather long Hg- - -Hg contacts in the range 3.99−4.29 Å along a zigzag chain (14).46 Comparable interactions are present in the fluorenone-2,7-diethynylidene bis(methylmercury) (15)47 and the 9,9-dialkyl derivatives (16, R = CnH2n+1, n = 6, 8, 16).48 The Hg- - -Hg distances are in the range 3.70−4.25 Å. It appears that these contacts have an influence on the emissive properties of the compounds in solution (CH2Cl2) at 290 and 11 K (λem 381 vs 427 nm). Evaporation of the solvent affords films with comparable properties. The emission can be quenched in Langmuir− Blodgett films by heteropolyacids of molybdenum and tungsten of the Keggin and Anderson type (nanohybrid films).49 The authors admit that Hg- - -Hg distances near 4.0 Å probably mean only very weak interactions but nevertheless may represent “more than just van der Waals forces” and assume that the large number of such interactions present throughout the multidimensional array (as a π-conjugated organometallic composite) leads to significant contributions to the stability of the structures. The excited states of these systems are assumed to be based on Hg−Hg bonding stronger than that in the ground state, drawing the strings together and influencing strongly the emissive properties. There is extensive evidence for this phenomenon in gold(I) chemistry.2,3 Studies of related aryl(alkynyl)mercurials have also afforded several compounds which show aggregation of the molecules in the crystal through Hg- - -Hg contacts. These examples include several ortho-mercurated Schiff bases with a phenylethynyl group as the second ligand for the metal atom (17). In the zigzag chain of mercury atoms (Hg- - -Hg- - -Hg 135.9°), the Hg- - -Hg distances are 3.959 Å. Because the Hg−Hg−C angles are different from 90°, contributions from Hg- - -η2 (CC) π interactions must also be considered.50 This mode of aggregation is also resembled by diphenyl-4,4′diethynylidene bis(methylmercury) (18).51 These dinuclear monomers are aligned to form chains propagated via Hg- - -Hg contacts of 3.83 and 4.22 Å. There is also an influence of the association on the emissive behavior of solutions, where a strong temperature dependence is observed. For CH2Cl2 the emission maxima at 290 and 77 K are found at λem 367 and 464 nm, respectively. The photophysical properties are similar for
the polymeric mercury 4,4′-dialkynyldiphenyl (no terminal methyl groups, 19).51 The same concept has been applied in extended photophysical studies of the aggregation of α,ωdimethylmercurated 2,5-oligothiophenes with ethynyl functions in the terminal 2,5′-positions (20). With one or two thiophene units between the mercury atoms, the dinuclear monomers crystallize as layers containing parallel Hg- - -Hg- - -Hg zigzag chains with Hg- - -Hg distances between 3.777 and 3.935 Å. The details of the structures also suggest Hg- - -η2(CC) π interactions.52 Finally, 4,4′-diphenyl ether, sulfide, sulfoxide, and sulfone spacers have been introduced between the two ethynyl groups to yield methylmercury compounds of the type shown in formula 21 (E = O, S, SO, SO2). In crystals of these compounds the intermolecular Hg- - -Hg contacts are in the broad range 3.621−4.343 Å. Again, it has been assumed that the weak forces associated with these contacts may contribute significantly to the mode of aggregation and the stability of the arrays. Investigations of the absorption and emission properties have shown that the heavy-metal atoms enhance the intersystem crossing rates to harvest the triplet energy provided in the absorption. The effect is comparable to that of analogous gold(I) compounds but is poor in comparison to that in some Pt(II) complexes.53 Mononuclear complexes of the formula F3CCC−Hg−R with R = Ph, Fc (ferrocenyl) were also found to be associated into chains establishing a line of mercury atoms, but the Hg- - Hg distances are long (up to 4.09 Å). Nevertheless, the crossedswords type of array of neighboring molecules (22) brings the mercury atoms closest together.54 c. Alkylmercury(I) Halides, Alkoxides, and Thiolates R− Hg−ER′ (E = O, S). The crystal structures of the methylmercury(II) halides Me−Hg−X (except for X = F) have been determined.55−57 The chloride, bromide, and iodide E
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Cl- - -Cl diagonals (3.70 Å average), indicating a slight preference for the former of the two contacts.61 Obviously, in this case the structure is not greatly governed by other weak forces. The crystal structure of 8-quinolinylmercury(II) chloride has been determined as an example for an arylmercury(II) halide. The molecules have a distant intramolecular Hg- - -N contact and also a long intermolecular Hg- - -Cl contact but show no mutual approach of Hg atoms (>4.0 Å).37
Very few compounds of the type R−Hg−OR have been structurally investigated. None of the alkoxy compounds MeHgOR′ obtained in preparative studies with R′ = benzyl, 1-naphthylmethyl could be crystallized. However, NMR data have shown that in solution a rapid ligand scrambling leads to an exchange of OR′ units, which may proceed via dimeric intermediates (25a). This assumption was confirmed in mass spectrometric studies, where cations [R′O(HgMe)2]+ (25b) were observed as prominent ionic fragments. 33 The methylmercury(II) compounds obtained in the reaction of MeHgOH with 8-azahypoxanthene and its 9-benzyl derivative have the MeHg group(s) attached to nitrogen atoms, where they are also coordinated to the residual functions of the heterocycles, precluding any close Hg- - -Hg approaches.62 exist in tetragonal (P4/nmm or P42̅ 1m) and orthorhombic (Pbcm) polymorphs with the molecules assembled in layers and the molecular axes alternating (parallel and antiparallel, 23) such that neighboring monomers are in the slipped-antiparallel arrangement (1b). This array provides a quasi-octahedral environment for the mercury centers with a methyl group and a unique halogen atom in the apical positions and four more distant halogen atoms in the equatorial positions. The contact between the layers is established by Hg−CH3 - - -X−Hg contacts. The X-ray diffraction studies have been complemented by temperature- and pressure-dependent NQR, microwave, and Raman investigations.58−60 Even under extreme conditions, there are no signs of an influence of mercurophilic interactions. At this point it is particularly important to note that gold(I) complexes of the type Me−Au− L are all assembledsteric effects permittingvia short Au- - Au contacts (1a).2,3
However, in monomers are schematically in Hg diagonals of
Methylmercury(I) trimethylsilanolate, MeHgOSiMe3, is a monomer in benzene solution63,64 but crystallizes as a tetramer with interpenetrating tetrahedra of oxygen and mercury atoms (26).65 The long Hg−O distances of 2.75 Å indicate that the aggregation is rather loose, as also suggested by the degradation of the tetramer in solution. The cations of [MeHgOSMe2]+BF4−, in which the DMSO ligand is O-coordinated to the mercury atom, are not aggregated in the crystals but are well separated by the anions.18
crystals of benzylmercury(II) chloride the indeed aggregated into chains, as shown 24. The Hg2Cl2 parallelograms have Hg- - 3.50 Å (average), which are shorter than the
An example from the methylmercury thiolate series is the compound obtained with 4-methylpyrimidine-2-thiol (or its tautomer 4-methylpyrimidine-2-thione, 27).66 In the crystal, the monomers are associated into dimers with an Hg- - -Hg F
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According to the results of a vibrational spectroscopic analysis, mercury arc irradiation of Hg atoms in solid neon or argon matrices containing both hydrogen (and/or deuterium) and oxygen afforded mercury(II) dihydroxide molecules HO− Hg−OH, HO−Hg−OD and DO−Hg−OD trapped in the matrix cavities. The bands have been assigned by theoretical calculations to a monomeric model system with C2 symmetry. There is as yet no evidence for aggregation which may occur through both hydrogen bonding and mercurophilic interactions.73 Structural information on complexes of mercury(II) dihalides with various donor ligands L of the general composition HgX2(L)n is vast and cannot be presented in the present context. In all cases the mercury atoms are at least monobridged by the halogen atoms. Recent examples obtained from HgCl2 or HgBr2 and 5,5-bipyrimidine as shown in 28 (ligand shown as a bar) may serve to illustrate the most common principle of the aggregation. There are chains (HgX2)n in which the different Hg−X distances (2 × 2.41 Å and 2 × 2.90 Å for X = Cl) and the presence of almost linear X−Hg−X axes both indicate the inherent preference for mercury two-coordination. The difunctional ligands connect the chains into layers, where the coordination number of the metal atoms is finally increased to 6. In the Hg 2 X 2 parallelograms (see also 24), the Hg- - -Hg diagonal is 3.77 (X = Cl) and 4.02 Å (X = Br).
contact of 3.101 Å. The axes S−Hg−C are crossed in the staggered conformation. The authors have noted a marked difference of this structure from that of the unsubstituted pyrimidine-2-thione, for which no mercurophilic interactions have been documented.67 The methylated compound dissolves as a monomer in benzene, as shown by ebullioscopic molecular mass determinations. This observation indicates that the Hg- - Hg interaction is weak and easily overcome by solvation. The dipole moments of the two homologues determined in solution are very similar, at 3.2 and 3.6 D.66
d. Mercury(II) Dihalides X−Hg−X. The crystal structures of compounds HgX2 with X = F, Cl, Br, I are well-known.4,68 Only HgF2 has a typical salt structure (coordination number 8 for Hg in the CaF2 type), but in the isomorphous crystals of HgCl2, HgBr2, and HgI2 (yellow polymorph) the mercury atoms are in a very strongly distorted octahedral coordination (two very short, four much longer Hg−X distances) by chlorine, bromine, or iodine atoms, and there are no short Hg- - -Hg contacts (4.0 Å. Between two molecules NC−Hg− CN a shorter Hg- - -Hg contact of only 3.825 Å was found in a distantly supported array (30). In its counterpart with [N C−Au−CN]− anions the Au- - -Au distance is again much shorter at 3.305 Å in spite of the Coulomb repulsion between
A similar supramolecular aggregation has been observed for the 2:1 adduct of [C6F4Hg]3 and ferrocene. The ferrocene is sandwiched between two organomercurials, and these packages are stacked cofacially on both sides in such a way that pairs of mercury atoms have close contacts of only 3.416 Å to another pair of mercury atoms (63, replacing acetone by ferrocene). The structure of the nickel analogue is similar (3.370 Å).194 6. Mercury Derivatives of Carboranes: Mercuroborands. The local geometry of C,C-dimercurated o-carboranes (64) is very similar to that in ortho-dimercurated benzene. The structures of the corresponding cyclic oligomers were studied extensively by the Hawthorne group.195,196 Their results have been summarized in a review in 2003,197 concentrating in particular on the tri- and tetramercuracycles as multifunctional Lewis acids. The mercury atoms were found to act as potent acceptors, in particular for halides and other anionic guests. In fact, the halide anions also act as templates which induce the formation of specific oligomers. A determination of the structures of these adducts has shown that the halides become attached above the cavities of the metallamacrocycles. In the present context the question arises if there is any significant interaction between the mercury atoms in the absence of these hosts. In the trimer [(1,2-C2B10H10)Hg]3 the central ninemembered ring [CCHg]3 has the same symmetry and dimensions as in the trimer [1,2-C6H4Hg]3, with linear C− Hg−C spacers between the carborane clusters (65). Therefore, the Hg- - -Hg distances are also near 3.60 Å with small variations if the carboranes are bearing substituents. The dimensions are even unchanged when the carboranes are not C,C-difunctional but become B,B-attached in an “inside-out” fashion, which is found for the products with m-carboranes (1,3-C2B10H12). The tetramer (66; the carborane linkers are drawn as loops) has its four mercury atoms arranged in a butterfly (S4) conformation with longer Hg- - -Hg contacts (3.654 Å), even though the C−Hg−C linkages are bent to 167.7°. The bulkiness of the carborane units precludes intermolecular contacts between mercury atoms which are common in crystals of the flat [1,2-C6H4Hg]3 molecules (above). O
DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
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judged from the observations made for the model compound 2a. Intermolecular Hg- - -Hg contacts are excluded by the shielding of the atoms by their ligands. However, there is clear evidence for multiple intermolecular mercurophilic interactions in stacks of the phenylenemercury oligomers [1,2-C6X4Hg]n (X = H, F; 61, 62), suggested not only by the modes of stacking and the resulting short Hg- - -Hg distances but also by the photophysical properties. In contrast, considering the metal−metal distances, there is doubt whether intramolecular Hg- - -Hg contacts at the inner circle of the monomers add to the special characteristics of these molecules. The same is true for the cyclic mercury derivatives of the carboranes (65).
anions which should be absent between electroneutral NC− Hg−CN molecules. Supported Hg- - -Hg contacts are generally shorter than unsupported contacts, but these reduced distances are largely determined by the directionality of the bonds of the bridging atoms. Thus, the zigzag chains in the polymorphs of HgO and HgS have the usual tetrahedral angles Hg−E−Hg at the chalcogen atoms E, which leads to intrachain Hg- - -Hg distances of 3.30 and 3.75 Å, respectively. There is no safe way to decide if mercurophilic bonding has an influence on these structures. There are also short interchain Hg- - -Hg contacts in the range 3.57−3.75 Å, which may indicate an attraction that codetermines the packing of the chains. Similar chalcogen-bridging characteristics are observed e.g. in mercury(II) acetate or in mercury(II) thiocarbamates or thiolates. The structural analogies between e.g. HgO and AuCl are particularly noteworthy, but the evidence for metallophilic interactions in the latter, with its smaller, subtetrahedral Au−Cl−Au angles, is much stronger. A very special phenomenon in mercury(II) chemistry is the pronounced tendency for a clustering of mercury atoms at small elements such as C, N, and O, noticed already in the 19th century by Strohmeyer, Millon, Dimroth, Hofmann, and others. The reactions proceed under mild conditions with no special auxiliary reagents, and yet full mercuration occurs, producing units of the formulas C(HgX)4 and [N(HgX)4]+ in various arrays and combinations. With oxygen as the metallization center, oxonium cations [O(HgX)3]+ are reached. All of these products have their analogues in gold(I) chemistry, where even higher degrees of auration have been realized with examples such as [C(AuL)6]2+, [C(AuL)5]+, and [N(AuL)5]2+, along with [O(AuL)4]2+. To our knowledge, no attempts have been made to clearly identify the driving force that leads to these phenomena. Metallophilic bonding may be only one component of a diverse set of contributions, but as the number of metallophilic contacts increases in a multinuclear array, the effect can lead to unexpected structural preferences such as the clustering at a given central element. To date it has gone largely unnoticed that the lengths of Hg−C, Hg−N, and Hg−O bonds on the one hand and Au−C, Au−N, and Au−O bonds on the other (for two-coordinate Hg(II) and Au(I), respectively) are all in the very narrow range of 2.0−2.2 Å. Model tetrahedra such as NHg4 in Millon’s base (44) and NAu4 in [N(AuL)4]+ thus have almost exactly the same dimensions, including the metallophilic contacts. This is surprising, since the standard ionic and covalent radii of Hg(II) and Au(I) given in handbooks call for shorter bonds with mercury than with gold, and this is also intuitively expected owing to the doubling of the positive charge. Contacts between cations [O(HgX)3]+, as in 36 with X = uracil nucleobases were found to be remarkably short, indicating that these η2 interactions may be relevant in condensed multinuclear systems. Mercury atoms of XHg+ units (X = alkyl, halogen) have also been encountered clustering at transition-metal centers in the same way as the gold atoms in the isolobal units LAu+ (L = neutral donor ligand). In examples such as (OC)4Fe(HgBr)2 (52) with an octahedral six-coordination of the iron atom, the intramolecular Hg- - -Hg contacts are of course short, but there is again no way to clearly decide if mercurophilic interactions are exerting a significant effect on the overall bonding situation. In fact, the mercury atoms of 52 are held at a distance (ca. 3.0 Å) which must be associated with Hg- - -Hg repulsion, as
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DEDICATION Dedicated to the memory of Professor Michael F. Lappert. REFERENCES
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DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/om501125c Organometallics XXXX, XXX, XXX−XXX
