Tri‐ and Tetranuclear Metal‐String Complexes with Metallophilic d10–d10 Interactions

Abstract The reaction of 2,6‐F2C6H3SiMe3 with Ph2PLi provided 2,6‐(Ph2P)2C6H3SiMe3 (1), which can be regarded as precursor for the novel anionic tridentate ligand [2,6‐(Ph2P)2C6H3]− (PCP)−. The reaction of 1 with [AuCl(tht)] (tht=tetrahydrothiophene) afforded 2,6‐(Ph2PAuCl)2C6H3SiMe3 (2). The subsequent reaction of 2 with CsF proceeded with elimination of Me3SiF and yielded the neutral tetranuclear complex linear‐[Au4Cl2(PCP)2] (3) comprising a string‐like arrangement of four Au atoms. Upon chloride abstraction from 3 with NaBArF 4 (ArF=3,5‐(CF3)2C6H3) in the presence of tht, the formation of the dicationic tetranuclear complex linear‐[Au4(PCP)2(tht)2](BArF 4)2 (4) was observed, in which the string‐like structural motif is retained. Irradiation of 4 with UV light triggered a facile rearrangement in solution giving rise to the dicationic tetranuclear complex cyclo‐[Au4(PCP)2(tht)2](BArF 4) (5), which comprises a rhomboidal motif of four Au atoms. In 3–5, the Au atoms are associated by a number of significant aurophilic interactions. The atom‐economic and selective reaction of 3 with HgCl2 yielded the neutral trinuclear bimetallic complex [HgAu2Cl3(PCP)] (6) comprising significant metallophilic interactions between the Au and Hg atoms. Therefore, 6 may be also regarded as a metallopincer complex [ClHg(AuCAu)] between HgII and the anionic tridentate ligand [2,6‐(Ph2PAuCl)2C6H3]− (AuCAu)− containing a central carbanionic binding site and two “gold‐arms” contributing pincer‐type chelation trough metallophilic interactions. Compounds 1–6 were characterized experimentally by multinuclear NMR spectroscopy and X‐ray crystallography and computationally using a set of real‐space bond indicators (RSBIs) derived from electron density (ED) methods including Atoms In Molecules (AIM), the Electron Localizability Indicator (ELI‐D) as well as the Non‐Covalent Interaction (NCI) Index.


Introduction
Metallophilic interactions are structurally directing, attractive forces between two or more closed-shellm etal ions that prefer low coordination numbers. From aq uantum-mechanicalp oint of view,m etallophilic interactions are mostlyd ispersive forces that are significantly enhanced by relativistic effects. [1] Given that relativistic effectsdramatically increase for the post-lanthanide elements and reachamaximum for gold in the sixth period, linearly coordinated,1 2v alence-electron complexeso f Au I and Hg II both having a5 d 10 6s 0 electron configurationa re the most prominent closed-shellm etal species showingm etallophilic interactions. In addition to pure aurophilic [2] and mercurophilic [3] interactions, an increasing number of heteronuclear metal-metal contacts, [4] for example, of the Au···Hg type, [5,6] have been observed in recent years. Metallophilic interactions are often related to interesting photophysical phenomena such as luminescence. Since the first publication on a photoluminescent gold complex by the group of Dori in 1970, [7] numerous examples of photoluminescence based on aurophilic interactionsh ave been reported. [8] Given that bond energies associatedw ith metallophilic interactions rarely exceed5 0kJmol Àl ,m ultidentate substituentso rl igands often play ac riticalr ole to support multinuclear complexesi nw hich the metal atoms are fixed in close proximity to each other.I n this regard, the 2-diphenylphosphinophenyl ligand (I)a nd derivatives thereof have been frequently used to preparea number of dinuclear Au I ···Au I complexes,w hichw ere for instance the starting materials for the preparation of interesting Au II -Au II complexes through oxidative-addition reactions (Scheme 1). [9] In this work we reporto nt ri-and tetranuclear Au I and Hg II complexes derived from the related 2,6-bis(diphenylphosphino)phenyl ligand (II,S cheme 1), which werep repared through an ovel synthetic route using at rimethylsilyl substituent as protecting group. The 2,6-bis(diphenylphosphino)phenyl ligand II is isoelectronic to the 2,6-bis(diphenylphosphino)pyridine ligand (III), [10] which has been utilized previously for the preparation of interesting complexes including some metalstring complexes, which hold promise as molecular wire materials (Scheme 1). [11] The 2,6-bis(diphenylphosphino)phenyl ligand II is also closely relatedt ot he diphosphanyl NHC ligand IV and related ligand systems, which were introduced recently (Scheme 1) and were shown to stabilizep oly-homo and heterometallic complexes showing metallophilic interactions. [12] Results and Discussion

Synthetic aspects
The reaction of 2,6-F 2 C 6 H 3 SiMe 3 with Ph 2 PLi provided 2,6-(Ph 2 P) 2 C 6 H 3 SiMe 3 (1)a sacolorless solid in 54 %y ield (Scheme 2). The 31 P{ 1 H} NMR spectrum of 1 dissolvedi nC DCl 3 shows ac haracteristic singlet at d = À6.4 ppm. The corresponding 29 Si NMR resonance is observed as at riplet ( 4 J Si-P = 7.8 Hz) in the 29 Si{ 1 H} NMR spectrum at d = À3.0 ppm. The complex [2,6-(Ph 2 PAuCl) 2 C 6 H 3 SiMe 3 ]( 2)w as readily prepared through the reactiono f1 with two equivalents of [AuCl(tht)] (Scheme 2). The reactiono ccurred instantaneously at ambient temperature when both reactants were suspended in dichloromethane. Subsequent crystallization gave 2 as colorless crystals in 90 %y ield. The coordination of the two Pa toms to Au gives rise to as ignificant shift of the 31 PNMR resonance to higher frequencies than 1.T he 31 P{ 1 H} NMRs pectrumo f2 dissolved in [D 6 ]DMSO reveals as inglet at d = 33.8 ppm for two chemically equivalent 31 Pn uclei. Note that the poor solubility of 2,e ven in DMSO, precludedt he acquisition of a 29 Si{ 1 H} NMR spectrum with sufficient signal intensity.H owever,t he 1 HNMR resonance associated with the (CH 3 ) 3 Si group is observed at d = 0.58 ppm. Heating am ixture of 1,[ Au(tht)Cl],a nd anhydrous CsF as a suspension in THF/CH 3 CN (1:1)a t6 08Cp roceededw ith elimination of Me 3 SiF and gave rise to the formation of the neutral tetranuclear complex linear-[Au 4 Cl 2 (PCP) 2 ]( 3, PCP = 2,6-(Ph 2 P) 2 C 6 H 3 )t hat was obtained as yellow prisms in 60 %y ield after recrystallisation from CH 2 Cl 2 /n-hexane (Scheme 2). Inspection of the 31 P{ 1 H} NMR spectrumo f3 in CD 2 Cl 2 shows two 31 PNMR singlet resonances at d = 36.6 and 36.0 ppm associated with two sets of two chemically inequivalent phosphorus nuclei. That is, two Ph 2 Pm oieties coordinate to Au atoms with Cl À ligandsi nm utual trans-position (inorganic coordination site) and the remaining two Ph 2 Pd onors coordinate to the two carbon-bound Au atoms in mutual trans-position (organometallicc oordination site). The 29 Si{ 1 H} NMR spectrum shows no signal indicating the loss of the Me 3 Si group in complex 3. Coherently,n or esonance corresponding to the CH 3 group of the (CH 3 ) 3 Si group is observed in the 1 HNMR spectrum.M oreover,w hen the reaction was monitored in situ in ac losed NMR tube, the formationo fM e 3 SiF was observed indicated by the characteristic multiplet resonance in the 19 FNMR spectrum at À159.6 ppm. [13] Chloride abstraction from complex 3 employing NaBAr F 4 (Ar F = 3,5-(CF 3 ) 2 C 6 H 3 )i nt he presence of tht readily gave the dicationic tetranuclearc omplex linear-[Au 4 (PCP) 2 (tht) 2 ](BAr F 4 ) 2 (4)w ithf ull conversion. Complex 4 is light sensitive and consequently,t he reaction was performed in the dark. With respect to 3,t he 31 P{ 1 H} NMR spectrum of 4 similarly reveals two sets of characteristics inglet resonances at d = 39.5 and 35.1 ppm, each associated witht wo chemically inequivalent 31 Pn uclei. One set resides in the internal organometallic coordination site and one set remains in the terminal inorganic coordination site. When the reaction mixture was subsequently exposed to UV light (l max = 366 nm), an rearrangement and thus quantitative formation of the dicationic tetranuclear complex cyclo-[Au 4 (PCP) 2 (tht) 2 ](BAr F 4 )( 5)w as observed. The rearrangementc oincidesw ith an increase in symmetry as signaled by the presence of as inglet resonance at d = 44.4 ppm in the 31 P{ 1 H} NMR spectrum (CD 2 Cl 2 ). All 31 Pn uclei in 5 reside in an indistinguishable chemical environment with all Ph 2 Pd onors in mutual cis-position coordinated to at erminal Au atom. Complex 5 was isolated as colorless crystals after recrystallization from CH 2 Cl 2 /petroleum ether.N oteworthy,c omplex 5 is stable towards moisture and air and only decomposes above 218 8C. The transmetalationo fo rganogold compounds with Cu [12c] or Hg [14] salts has been previously reported.T he atom-economic reactiono f3 with HgCl 2 in CH 2 Cl 2 proceeded with disaggregation of the tetranuclear complex and formation of HgÀCa nd AuÀCl bonds and produced the neutralt rinuclear complex [HgAu 2 Cl 3 (PCP)] (6)i n9 6% yield (Scheme 2). A 199 Hg{ 1 H} NMR spectrum of 6 was recordedi nC D 2 Cl 2 .T he observed triplet resonance at d = À841.5 ppm shows as ignificant HgÀPc oupling with a 3 J Hg-P coupling constant of 327 Hz. Conversely, the 31 P{ 1 H} NMR spectrum has as inglet resonancea t d = 42.3 ppm, signifying two Ph 2 Pm oietiesw ith identical chemicale nvironment, with 199 Hg satellites exhibiting a 3 J P-Hg coupling constant of 327 Hz.

Molecular structures
Precise structurali nformationw as obtainedf rom X-ray crystallography. The molecular structures of 1-6 are shown in Figure 1- Figure 5. Selected bond lengths are collected in the captiono ft he figures. In all structures the Au I and Hg II atoms adopt almost linear spatial arrangements as anticipated for complexes with a14v alence-electron count. The related AuÀC, AuÀP, AuÀS, AuÀCl, HgÀC, and HgÀCl bond distances exhibit typical lengths. [5,6,9] In the structure of 2 the Me 3 Si group is still present after the complexation of the two Au atoms ( Figure 1). However,t he displacement of the Si atom from the plane defined by the central phenylr ing increases notably upon going from 1 (0.072(1)) to 2 (0.219 (1) ), which might be as ign for the SiÀCb ond activation in 2.H owever,t he SiÀCb ond lengths of 1 (1.8721(1)) and 2 (1.878 (6) )a re indistinguishable within the experimental error.T he molecular structures of 3 and 4 ( Figure 2a nd Figure 3) reveal as tring arrangement of four Au atoms, which are associated by three Au-Au contacts (2.8280(5)t o3 .0567 (4) ). The contact distances between the inner Au1 and Au3 atoms (2.8746(4) in 3,2 .8280(5) )a re significantly shorter than the intermetallic distances in similar cationic or neutral string gold complexes that display aurophilic interactions (e.g. for [Au 4 (dpmp) 2 (SCN) 2 ] 2+ + the distance between the inner Au atoms is 3.0049(8) (dpmp = bis(diphenylphosphinomethyl)phenylphosphine). [15] The neutral complex 3 entails two sets of two Au atoms in equivalent chemical environment:t he organometallic coordination site encompasses two Au atoms each coordinatedb yaphenylate-and the Ph 2 P donor moiety in mutual trans-position (Au1 and Au3). This pattern gives rise to al inear coordination sphere aroundA u atomsl ocatedi nt he center of the Au 4 string.T he second set Scheme2.Synthesis and 31 PNMR chemicalshifts in ppmof1-6. Chem. Eur.J.2020, 26,275 -284 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim of Au I atoms terminatet he Au 4 gold string and reside likewise in al inear coordination motif. APdonor and aC ll igand in mutual trans-position build this linear coordinationp attern (Au2 and Au4). The related dicationic complex 4 is centrosym-metric but retains the string-like Au 4 motif with its internal organometallic coordination sitea roundA u1 andA u1a and the terminal inorganic coordination site. Due to the abstractiono f both Cl ligands and the introduction of tht,t he terminalA u moieties (Au2/Au2a) are coordinated by the Pd onor of the PCP ligand and the sulfur donor stemming from the thioether (tht), both in mutual trans-arrangement. Twow eakly coordinating [BAr F 4 ] À anions maintain the chargec ompensation for the dicationic complex. In contrast, the molecular structure of 5 ( Figure 4) shows aplanar rhomboidal motif with five significant Au···Au interactions (2.9980(3)t o3 .1356 (3) )i nt he cycle defined by Au1, Au2, Au1a, and Au2a, that are comparable to other similar complexes stabilized by tripodal phosphine ligands (dpmp) reported recently. [16,17] The increase in number of aurophilic contacts is due to the rhomboidal arrangement and the additional trans-annular contact Au1···Au1a (3.1118 (5) ), which might be the thermodynamic drivingf orce for the rearrangemento f4 into 5.T his rearrangement results into ar edistributiono ft he ligandsi no rganometallic and inorganic coordination sites. The gold atoms Au2 andA u2a show ah omoleptic linear coordination of two Ph 2 Pd onors in mutual trans-position, whereas the organometallic coordinations ite shows am utual trans-arrangement of ap henylate and at ht moiety to each gold atom of this site (Au1 and Au1a). The crystal structure of 6 comprises two crystallographicallyi ndependent conformers, in which the two Au atoms chelate the central Hg atom (Figure 5). In this way, the two conformers adopt Au-Hg-Au transoid and cisoid arrangements, respectively.O verall, the Au···Hg contacts of 6 (3.0253(4)t o3 .4082(4) )a re somewhat longert han the Au-Au contacts in 3-5,b ut are close to intermolecular Au···Hg distances (3.097(2)-3.498 (3) )o bserved recently for an umber of so called molecular Au-Hg amalgams, [5f,g, 18] as well as the intramolecular Au···Hg distances (3.112(1)-3.2940(9) )o bserved for some similarc omplexes. [5h, 14] Complex 6 may be also regarded as am etallo-pincer  complex [ClHg(AuCAu)] between Hg II andt he anionic tridentate ligand [2,6-(Ph 2 PAuCl) 2 C 6 H 3 ] À (AuCAu) À containing ac entral carbanionic binding site and two "gold-arms" contributing pincer-type chelation through metallophilic interactions. In a more general way,t he (PCP)-ligand II is extended by two metal units, which are coordinated to the PPh 2 moieties in al inear fashion giving rise to an (MCM)-metallo pincer ligand V comprised of ac entral carbanionic donor and two "metal-arms" providing pincer-typec helation through metallophilic interactions (Scheme 1).

Photophysicalp roperties
Only in case of 3,b right luminescence can be observed upon exposure to UV-A light. The UV/Vis absorption spectrum and the emission spectrum of 3 in CH 2 Cl 2 solution are shown in Figure 6a.A na bsorption maximum at 365 nm and another strong absorption band reaching into the UV region below 250 nm are observed. The emission spectra in CH 2 Cl 2 solution and in the solid state revealm axima at 538 and 539 nm, respectively,w hich is associated with the emission of yellow-  Chem. E ur.J.2020, 26,275 -284 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim green light. Photoluminescence quantum yields of 4.3 %i nd ichloromethanes olutiona nd 17 %i nt he solid state were found. Similar emissive properties although at higherq uantum yields in solution or solid state, have been reported for linear polynuclear string gold complexes. [17] The photoluminescence spectrumo f3 in the solid state is shown in Figure 6b.

DFT analysis
Complementing the interpretation of the structuralp arameters, electronic bond characteristics of the metallophilic Au···Au and Au···Hg interactions were examined in terms of computed real-space bondingi ndicators (RSBI). All calculations are based on the experimentally obtained XRD structures with CÀHd istances corrected in order to obey neutron diffraction results. [19] The RSBI set comprises parameters extracted from topological analysiso ft he electron and pair densities according to the Atoms In Molecules (AIM) [20] and Electron Localizability Indicator (ELI-D) [21] space-partitioning schemesa sw ell as as urface study within the framework of the recently introduced Non-Covalent Interactions (NCI) index, [22] which is based on the reduced density gradient (s)o ft he electron density (ED) and unravels noncovalenti nteraction areas. Thus, the NCI transcend topological approaches, which mainly rely on stationary point analysisa si ta lso detects weak intra-and intermoleculari nteractions,s uch as London dispersion, [23] for which not essentially bond-critical points (bcp)a re detectable in the underlying ED. By mapping the second Eigenvalue (l 2 )o ft he ED-Laplacian (r 2 1 = l 1 + l 2 + l 3 )o ns,b onding( l 2 < 0) can be distinguished from weak Vand er Waals (VdW) forces (l 2 % 0), or steric repulsion (l 2 > 0). The topological AIM bond paths' motifs, which are typicallyr eferred to resemble the molecular structures, are displayed in Figures 7-Figure 9f or compounds 3, 4, 5,a sw ell as the transoid and cisoid conformers of 6.F or all cases, metallophilic Au···Au or Au···Hg attraction is disclosed by formation of correspondingb cp in the ED,t he topological parameters of which are typical for this kind of interactions (Table 1). [6,24] The low value of the ED at the bcp (1 bcp approx.0 .1-0.3 e À3 ), the positive but close to zero value of the Laplacian (r 2 1 bcp approx.1 -3 e À5 ), as wella st he dominance of the kinetic energy density over ED ratio (G/1 bcp approx.0 .6-0.8 h e À1 ) against the total energy density over ED ratio (H/1 bcp approx. À0.2-0.0 h e À1 )u ncovers these contacts to be mainly noncova-   (Table 1). This is supported by the integrated delocalization index, d(A,B), which quantifies the number of electron pairs shared between two adjacent or distant atoms and lies in the range of 0.2 to 0.4. Similar values are typically observed for ionic atom-atomc ontacts with the only differencet hat r 2 1 bcp and G/1 bcp values are larger positivef or the latter.C onsequently,t he NCI surfacea nalysiso f3-6 shows pronounced features along the Au···Au andA u···Hg interaction axes in terms of discshaped reduced density gradient basins with highly negative l 2 values on the surface suggesting attractive metallophilic interactions (Figures 7), whereas no corresponding ELI-D basins are formed (Figure 8, Figure9). [6,24] In the NCI, weaker attractive or even repelling H···H and H···p interactions are also observed, which determine the spatialo rientation of the differentm olecular fragments (e.g. phenyl groups) and thus the three-dimensional appearance of the molecule in the crystal.With aconsiderably higher ED at the bcp of about 0.7-0.9 e À3 but Laplac-ian values (approx.1 -5 e À5 )s imilar to the Au···Au andA u···Hg bonds the Au/HgÀCl/P/C bonds combine covalenta sw ell as noncovalent bondinga spects and may thus be regarded as polarized covalent. Accordingly,b oth G/1 bcp (approx. 0.6-0.9 h e À1 )a nd H/1 bcp (approx. À0.5-0.4 h e À1 ), show strongly positive and negative values, respectively.The covalent character is further supported by d(A,B) being close to or even above 1a nd the formation of AuÀCl/P and HgÀCl/C bondingb asins in the ELI-D ( Figure 8). Due to the higherP auling electronegativity of Au atoms (2.4) compared with Hg atoms (1.9) the AIM atomic charges are close to zero for the former (Q AIM (Au) = À0.09-0.07 e), but positive for the latter (Q AIM (Hg) = 0.64-0.65 e), which confirms previous results. [6,24] As anticipated, Cl atomic charges are negative (approx. À0.5 e), whereas Pa tomic charges are highly positive (approx.1 .8 e) within AIM space-partitioning (see the Supporting Information, Ta bles S3-S6 for afull list).  1 bcp :e lectron density, r 2 1 bcp :L aplacian, d 1 /d:r atio, e:b onde llipticity, G/1 bcp and H/1 bcp :k inetic and total energyd ensity over 1 bcp ratios, d:d elocalization index. [BAr F 4 ] 2 (4), cyclo-[Au 4 (PCP) 2 (tht) 2 ][BAr F 4 ] 2 (5), and the trinuclear bimetallic complex [HgAu 2 Cl 3 (PCP)] (6)w erer eported, whereby (PCP) À comprises the novel tridentate carbanionic ligand [2,6-(Ph 2 P) 2 C 6 H 3 ] À (II). Compounds 4 and 5 are metal-string complexes in which four Au atoms are associated by three aurophilic interactions in very similar linear chain arrangements. Compound 3 shows yellow-green photoluminescence both in dichloromethane solution and in the solid state (l max = 538 and 539 nm, respectively). UV-light triggersa ni rreversible rearrangementf rom 4 into the isomer 5 in which four Au atoms are associated by five aurophilic interactions in ar homboidal arrangement. Compound 6 can be regarded as metallo-pincer complex [ClHg(AuCAu)],w hereby (AuCAu) À comprises the novel tridentate carbanionic ligand [2,6[2,6-(Ph 2 PAuCl) 2 C 6 H 3 ] À (V)c ontaining ac entral carbanionic binding site and two "gold-arms" contributing pincer-type chelation through two metallophilic AuÀHg interactions (Scheme 1). All metallophilic interactions of 3-6 give rise to AIM bond pathsa nd bond critical points. Typically for noncovalent interactions, the NCI showsc ontact patches where the Au···Au and Au···Hg interac-tions occur,w hile the ELI-Dr emains featureless demonstrating the complementarity of these real-space bond indicators. [25]