A Zwitterionic Heterobimetallic Gold–Iron Complex Supported by Bis(N‐Heterocyclic Imine)Silyliumylidene

Abstract The facile synthesis of the first bis‐N‐heterocyclic imine‐stabilized chlorosilyliumylidene 1 is reported. Remarkably, consecutive reaction of 1 with PPh3AuCl and K2Fe(CO)4 gives rise to the unique heterobimetallic complex 1,2‐(MesNHI)2‐C2H4‐ClSiAuFe(CO)4 (4). The overall neutral complex 4 bears an unusual linear Si−Au−Fe structure and a rare anagostic interaction between the d 10‐configured gold atom and a CH bond of the mesityl ligand. According to the computational analysis and 57Fe Mössbauer spectroscopy, the formal Fe‐oxidation state remains at −II. Thus, the electronic structure of 4 is best described as an overall neutral—yet zwitterionic—heterobimetallic “Si(II)+‐Au(I)+‐Fe(‐II)2−”‐silyliumylidene complex, derived from double anion exchange. The computational analysis indicates strong hyperconjugative back donation from the gold(I) atom to the silyliumylidene ligand.


Introduction
Iron and gold engage in strong metallophilic interactions [1] and show-both in homogeneous and heterogeneous phaseremarkable catalytic activity in industrial relevant processes, such as valorization of CO or hydrogenation of olefins. [2] However,h eterogeneous catalysts,i ncluding active clusters and nanoparticles,o ften suffer from agglomeration, nonuniform size distribution, and alloy segregation. [2a] Thus,t he identification of novel preparative building blocks that ensure the ideal mixing of the alloy metals is of significant current importance.H eterobimetallic complexes arguably represent monomeric subunits of alloy clusters and feature ab ifunctional metal core.C ooperativity between these two welldefined sites is believed to engender unusual chemical transformations,a nd may serve as models;h ence,i mproving our understanding of heterogeneous reaction mechanisms. [3] Still, defined small clusters or even heterobimetallic complexes with am onomeric Au À Fe unit are rare. [1a, 2b,4] So far, sterically demanding donor ligands,s uch as NHCs (Nheterocyclic carbenes) or phosphines,w ere applied for the isolation of these smallest units of the gold-iron alloy ( Figure 1). [5] Siliconsa bility to partially mimic its lighter congener carbon, and to show metal-like behavior at the same time,has been ap owerful concept for small molecule activation and preparation of novel materials. [6] In recent years,silicon-based ligands beyond conventional silyl-ligands have been highlighted. Besides the nowadays well-known silylene ligands, [7] silyliumylidenes are equally fascinating;y et, remain comparatively unexplored. [8] Silyliumylidenes can serve as ligands in transition metal chemistry through their accessible lone pair of electrons,a nd their cationic nature offers coordination chemistry complementary to silylenes ( Figure 2). [9] Hitherto known silyliumylidene metal complexes are mono-(J Au , J Fe , K W )o rh omobimetallic (I, K Rh )b ut, so far, no heterobimetallic silyliumylidene complex is reported. [10] Even in silylene chemistry,e xamples are scarce and are mainly represented by bissilylenes or other multinuclear silylene systems. [11] Due to the unique electronic properties of silyliumylidenes,w eh ypothesize them to be promising candidates to stabilize uncommon bonding motifs,s uch as monomeric heterobimetallic AuÀFe complexes. [10a-c, 12] Recently,w eh ave shown that bis-NHIs (bis-N-heterocyclic imines/ bis-imidazoline-2-imines) [13] are suitable for the stabilization of electron deficient main group complexes. [14] Hence,w ee xpect the strong donor abilities in combination with the chelating effect of bridged bis-NHIs to provide as ignificant advantage in the formation of reactive silyliumylidenes and their metal complexes. [13a, 15] Herein, we present the synthesis of ab is-NHI-stabilized silyliumylidene ion and its reactivity towards the heavier chalcogens and coinage metals.Furthermore,anoverall neutral, heterobimetallic silyliumylidene-gold complex bearing ac oordinated Fe(CO) 4 2À dianion (4)i sreported for the first time.
With silyliumylidene 1 in hand, we investigated the reactivity of its lone pair of electrons,b yo xidation with heavier chalcogenes.Indeed, elemental sulfur, selenium, and tellurium could be activated by stirring with 1 in acetonitrile at room temperature,f urnishing the heavier silaacylium ions 2a-2c in nearly quantitative yields (Scheme 2, top reaction pathway).
SC-XRD analysis of compound 2a was carried out on colorless crystals grown from aM eCN/THF mixture.U pon addition of sulfur,t he silicon atom adopts at etrahedral coordination ( Figure 4). TheS i = Sb ond length (1.9740 (6) ) is in good agreement with other donor stabilized Si=Sdouble bonds (1.96-2.08 ) [17, 21a, 23] reported before and matches the Si=Sb ond in G S (1.984(2) ). Them olecular structure of 2a serves as arepresentative for compounds 2 by replacing Sfor Se or Te.C ompounds 2b and 2c were characterized by multinuclear NMR techniques,i ncluding 77 Se (for 2b)a nd 125 Te (for 2c)aswell as mass spectrometry (ESI). All data are consistent and, therefore,s uggest that 2b and 2c are isostructural with 2a.
SC-XRD analysis confirmed the heterobimetallic structure of 4.T he silicon atom remains in adistorted tetrahedral coordination environment with the NÀSiÀNa ngle being strained to 87.80(13)8 8,a ss hown in Figure 5. As common for Au I complexes,t he gold atom is nearly linearly coordinated with aSi À Au À Fe angle of 173.65(3)8 8.The Si À Au bond length (2.2676 (9) )f alls within the range of 2.246-2.363 for aS i(II)ÀAu complex. [25] TheA u ÀFe distance of 2.5305(6) matches related compound A (2.5168 )a nd is within the range of AuÀFe single bond lengths in small molecular goldiron clusters (A-E:2.516-2.567 ). Moreover,comparatively short C À H···Aud istances can be detected for the mesityls ortho-methyl groups pointing towards the gold atom, indicating rare anagostic interactions.T he shortest contact is detected for CH42···Auw ith 2.70(3) ,w hile two more are slightly elongated (CH17···Au2 .87 (6), CH33···Au2 .97 (5) ) but still shorter than S vdW (H,Au) = 3.3 . [26] Also computationally,a na nagostic interaction is found between the gold ion and am ethyl group of the mesityl ligand (BP:2 .361 ; PBE0:2 .445 ;F igure 8). Although agostic interactions of Au III complexes have been recently evidenced, CÀH···Au interactions remain rare in general. [27] Also,a nagostic CÀ H···Aui nteractions are at opic of current interest. [28] In contrast to agostic interactions that are understood as threecenter-two-electron bonds between C À Ha nd vacant dorbitals of transition metals with rather short H À Mdistances and narrow CÀHÀMa ngles ( % 1.8-2.3 ; % 90-1408 8), the weaker anagostic interactions are associated with filled dorbitals,l onger HÀMd istances and larger CÀHÀMa ngles ( % 2.3-2.9 ; % 110-1708 8). [29] Consequently,M I (M = Cu, Ag, Au)c omplexes,t hat exclusively feature occupied d-orbitals, are good candidates to observe such contacts.T he 29 Si NMR spectrum of 4 displays as ingle resonance at d( 29 Si) =+ 67.6 ppm. This is further downfield shifted compared to starting material 1 (+ 1.5 ppm) and precursor compound 3c (+ 18.2 ppm). Infrared (IR) vibrational spectroscopic measurements reveal the carbonyl stretching frequencies of 4 to occur at 1924, 1835, 1811, and 1796 cm À1 (Table 1). The positions of these bands,w hich were reproduced by calculations at the ZORA-BP86-D3/def2-SVP level of theory, [30] is indicative for the donor properties,that is,combined s-donor/ p-acceptor abilities of the "ligand" 3c,w hich coordinates anionic Fe(CO) 4 in 4.Compared to related compounds (cf. A, J Fe ,I Mes ÀFe(CO) 4 ,N a 2 Fe(CO) 4 ,s ee Table 1) these bands are shifted to higher wavenumbers in case of zwitterionic 4. [31] Prompted by its unexpected polar solubility properties and CO stretching IR bands,w ea imed for further understanding of the bonding situation of 4.Whereas the structural data are consistent with gold in the formal oxidation state of + I, the iron center presents itself electron-rich, compared to Fe 0 (CO) 4 -containing complexes,such as I Mes ÀFe(CO) 4 .Zerofield 57 Fe Mçssbauer spectroscopy was performed to assess the complexse lectronic structure, ( Figure 6a nd Figure 7). Specifically,the Mçssbauer isomer shift, d,reflects the total sorbital electron density at the nucleus,which is determined by the oxidation state,the coordination geometry,spin state,and, consequently,the metal-ligand distance as well as the degree of covalency. [32] Thes tarting material K 2 Fe(CO) 4 ,w hich is commonly assigned af ormal oxidation state of ÀII, was reinvestigated under the exact same conditions,f or comparison. [33] Intriguingly,K 2 Fe(CO) 4 (d = À0.19(1) mm s À1 )and 4 (d = À0.14(1) mm s À1 )feature rather similar isomer shifts,which is indicative of similar electronic structures and, thus,p hysical oxidation states.T he computational simulation of the 57 Fe Mçssbauer isomer shift at the DKH2-TPSSh/def2-TZVPP Figure 5. Solid-state plot of the moleculars tructure of 4.Thermal ellipsoids are set at 50 %probability.Hydrogen atoms are omitted for clarity;mesityl-substituents are depicted as wireframe for simplicity. [39] Selected bond lengths []and angles [8 8]: Au1-Si1 2.2676(9), Au1-Fe1 2.5305 (6)  (Fe: CP(PPP)) level of theory matches the experimental finding almost perfectly,w ith ac alculated isomer shift d of À0.16 mm s À1 for 4.T he isomer shift can be rationalized by the removal of d-electron density from the iron center by the four CO ligands,r esulting in al ess-shielded positive charge and concentrated s-electron density at the iron nuclei. [34] In this regard, experimental characteristics suggest 4 to be best described as zwitterionic L 2 ClSi + !Au + ! Fe 2À (CO) 4 (L 2 = bis-NHI). [35] Electronic structure analysis was carried out using Intrinsic Bond Orbitals (IBOs [36] )o btained by the BP86 (Figure S37) and PBE0 (Figure 8) functionals to further pinpoint the d-orbital population of the gold and iron atoms in 4.Both methods give consistent results corresponding to formally d 10configured gold(+ I) and iron(ÀII) ions.Adative interaction between the silicon lone pair (LP) and the gold metal is found with the weight of silicon amounting to 0.70 (Au: 0.30). This value,w hich is in the typical range of late transition metal NHC complexes,i ndicates ac oordinative ligand-metal interaction of considerable covalency. [37] TheIBOs relating to the gold d(xy) and d(x 2 -y 2 )o rbitals suggest orbital overlap with the methyl groups of the congesting mesityl substituents.The non-bonding Au d(z 2 )orbital shows large admixture of the 6s orbital, whereas the Au d(xz) and d(yz) orbitals indicate strong p-backbonding with the silyliumylidene ligand. This hyperconjugative interaction, which is also reflected by ac alculated Lçwdin bond order of 1.56, is analogous to pbackbonding of isoelectronic phosphine ligands commonly applied in homogeneous transition metal catalysis.Note that considerable research effort has been directed recently towards engineering cationic phosphine ligands for exceedingly strong p-backbonding. [38] Also,a ll d-orbitals of the iron ion are fully populated, which gives rise to af ormal oxidation state of ÀII. TheF e d(xy), d(xz), and d(yz) orbitals engage in strong backbonding (contribution Fe % 0.75) with the CO ligands,w hich is in excellent agreement with the 57 Fe Mçssbauer spectroscopic data (vide supra). Further, the Fe d(z 2 )o rbital, associated with ad ative bond to the gold atom, is quite covalent (contribution Fe :0 .58;c ontribution Au :0 .26). Both, the Hirshfeld as well as Lçwdin ( Figure S39) population analysis indicate accumulation of negative partial charge at the iron atom, whereas the positive partial charge is delocalized across the silyliumylidene,g old ion, and the NHC moieties.W e conclude that the computational analysis further supports az witterionic Si(II) + !Au(I) + ! Fe(ÀII) 2À (4)e lectronic structure with considerably covalent Au À Si and, especially, AuÀFe (4' ')bonds,asrepresented by the mesomeric structures 4 and 4' '.

Conclusion
In summary,the bis-NHI-stabilized silyliumylidene 1 was isolated and studied for reactivity.The activation of elemental heavier chalcogens through the silyliumylidenesl one pair resulted in complexes 2a-2c,w hereas coordination to group   11 metals led to silyliumylidene-metal complexes 3a-3c.T he first over-all neutral heterobimetallic silyliumylidene complex 4 was isolated via nonstandard anion exchange reaction using K 2 Fe(CO) 4 .T he solid-state structure,s pectroscopic analysis, and calculations of 4 reveal arare anagostic interaction of the gold ion with the mesityl ligand, as well as strong donor and considerable p-backbonding capabilities of the silyliumylidene ligand 1.C omputational analysis and 57 Fe Mçssbauer spectroscopy indicate 4 to feature az witterionic L 2 ClSi + ! Au + ! Fe 2À (CO) 4 electronic structure.Investigations on bond activation reactions and catalytic applications of 1, 3 and 4 are currently ongoing in our group.