An Air‐Stable Heterobimetallic Si2M2 Tetrahedral Cluster

Abstract Air‐ and moisture‐stable heterobimetallic tetrahedral clusters [Cp(CO)2MSiR]2 (M=Mo or W; R=SitBu3) were isolated from the reaction of N‐heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)2M=Si(SitBu3)NHC with a mild Lewis acid (BPh3). Alternatively, treatment of the NHC‐stabilized silylidene complex Cp(CO)2W=Si(SitBu3)NHC with stronger Lewis acids such as AlCl3 or B(C6F5)3 resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four‐membered ring is due to steric bulk.


Introduction
Te trahedral clusters that consist of main-group elements are attractive synthetic targets because of their high ring strain and reactivity. [1] Even white phosphorus (P 4 ), which has been known for centuries,h as recently been the subject of intense research (I;F igure 1). [2] Theheavier homologue,As 4 , is challenging to handle because of its thermal and photochemical instability.N evertheless,C ummins and co-workers even managed to isolate As 3 P, the first example of ah eteroatomic tetrahedrane. [3] Thearchetypical organic tetrahedrane (tBuC) 4 (II)was isolated by Maier and co-workers in 1978, [4] whereas Wiberg et al. reported the heavier analogue,t etrasilatetrahedrane (tBu 3 SiSi) 4 (III), in 1993. [5] One decade later, Sekiguchi and co-workers isolated af urther tetrasilatetrahedrane (R 4 Si 4 ,R= SiMe((CH(SiMe 3 ) 2 ) 2 )while trying to isolate ad isilyne with aS i Si triple bond. [6] Very recently,a nother neutral tetrahedron that contains two different heteroatoms, (tBuCP) 2 (IV), was reported to form upon dimerization of phosphaalkynes (RCP). [7] In addition to neutral tetrahedral complexes,anionic tetrahedral [E 4 ] 4À (E = Si, Ge,Sn) species, so-called Zintl-type polyanions,a re known. [8] Whilst binary combinations in Zintl tetrahedral clusters [E n M 4Àn ]have been reported, [9] there are no examples of neutral heterobimetallic tetrahedral clusters with heavier main-group elements and transition metals.
Numerous examples are known of M 2 C 2 -type bimetallic complexes (V)w ith bridging acetylene or acetylene derivatives that exhibit tetrahedral structures. [10] Such complexes find application in the Pauson-Khand synthesis of cyclopentanone derivatives and are catalysts in hydroboration reactions, [11] yet heavier congeners have remained unexplored to the best of our knowledge.A mong heavier analogues of Group 14 element compounds M 2 E 2 (E = Si, Ge,S n), particular interest has been devoted to silicon as bimetallic clusters with bridging silicon atoms are indeed alleged key intermediates in various transition-metal-catalyzed transformations. [12] Since the 1900s,v arious M 2 Si 2 binuclear transition-metal complexes have been reported and their catalytic activities have been exploited in the dehydrocoupling of hydrosilanes and the metathesis of olefins. [13] Thus far,however, all of these complexes feature ap lanar,d iamond-shaped, or butterflytype M 2 Si 2 core,w hereas tetrahedral structures remain elusive. [13a, 14] Generally,m onoatomic tetrahedral derivatives R 4 E 4 can be generated photochemically from the corresponding planar linear compounds by elimination or photoisomerization as reported for II. [1,15] Accordingly,tetrahedranes can form by dimerization of disilynes or nickel-mediated dimerization of phosphaalkynes. [7,16] Herein, we report the first isolable heterobimetallic M 2 Si 2 cluster with at etrahedral structure.I nspired by the previous reports from the groups of Wiberg and Sekiguchi, we based our synthetic strategy on the elimination of an N-heterocyclic carbene (NHC) from asilylidene complex (Si=M) to generate as ilylidyne complex (SiM), which was hypothesized to dimerize subsequently to at etrahedral M 2 Si 2 cage cluster (Scheme 1).
Thec hemistry of transition-metal silylidyne complexes has as hort, yet spectacular history.T he arguably first silylidyne complex [Cp*(dmpe)(H)MoSiMes] (dmpe = PMe 2 CH 2 CH 2 PMe 2 )w as reported by Tilley and Mork in 2003. [17] Shortly after, when the role of the NHC in stabilizing low-valent silicon(II) species had been recognized, agenuine Mo Si triple-bonded complex was isolated by the group of Filippou using this elegant synthetic approach. [18] Following this achievement, ah andful of transition-metal silylidyne complexes and their reactivities were reported by further research groups. [17][18][19] Characteristically,all room-temperature isolable transition-metal silylidyne complexes bear very bulky ligands either on the silicon center (e.g., Ar Tr ip ,Eind) or on the metal center (e.g.,C p*, Tbb). We concluded that aM Si complex with comparably reduced bulk on both the silicon atom and the transition-metal center should be an excellent choice for the targeted tetrahedral cluster.

Results and Discussion
Very recently,w er eported the synthesis of the first silylsubstituted chlorosilylene (1)a nd studied the reactivity associated with its lone pair and chloride substituent. [20] Compound 1 is prone to salt metathesis reactions because of the presence of the chloride atom. In fact, heating an orange toluene solution of chlorosilylene 1 with Cp(CO) 2  The 29 Si NMR spectra of 2 and 3 show characteristic resonances,which are shifted strongly downfield to 278.8 ppm and 229.7 ppm ( 1 J Si-W = 261 Hz), respectively,i nr eference to those of 1 (d = 18.3 ppm). Similar chemical shifts were observed for the previously reported transition-metal silylidene complexes. [18,19m,21] Thes ilicon-bonded NHC carbene atoms resonate at 168.3 ppm (2)a nd 172.6 ppm (3)i nt he 13 CNMR spectroscopic analysis,w hich is similar to the chemical shift found for 1 (d = 169.7 ppm).
TheI Rs pectra of 2 and 3 each show two n(CO) absorption bands at 1782 and 1864 cm À1 (2), and at 1770 and 1849 cm À1 (3). Thep ositions of these bands agree well with previously reported metal arylsilylidene complexes and our predictions by density functional theory (DFT) calculations (3:1 837 and 1892 cm À1 ). [18,19l] Single crystals of 2 and 3 were obtained from at oluene/pentane (1:3) mixture at ambient temperature,a nd the structure in the solid state was determined by X-ray diffraction analysis ( Figure 2). Complex 2 features aM o = Si double bond (2.3499(10) ), which is shorter than that found for the donor-free molybdenum silylidene complex Cp*(CO) 2 (SiMe 3 )Mo=Si(Mes) 2 (2.3872 (7) )a nd lies in the range of previously reported molybdenum silylidene complexes (d(MoÀSi) = 2.288(2)-2.3872 (7) ). [22] Similarly, 3 exhibits aW = Si (2.346 (2) ) bond that is considerably shorter than in [Cp*W(CO) 2 ( = SiMes 2 )(SiMe 3 )] (2.3850(12) )a nd in the neutral alkyl(sily-  . Ellipsoid plots (set at 50 %probability) of the molecular structures of compounds 2 (one out of two independentmolecules in the asymmetric unit is shown), 3 (one out of three independent molecules in the asymmetric unit is shown) and 3' '. [39] Hydrogen atoms are omitted for clarity,a nd tert-butyl groups are depicted in wireframe for simplicity.  (11) ), but slightly longer than that of the anionic complex [Cp*(CO) 2 (17) ). [21,23] UV/Vis analysis ( Figure S6) revealed ac haracteristic absorption band at l max = 418 nm for the Mo=Si complex 2.F or the W=Si complex 3,aband at l max = 418 nm and av ery broad and weak band ranging from approximately l = 500 to 700 nm were found ( Figure S12). Time-dependent DFT (TD-DFT) calculations reproduced these values very well ( Figure S63). In addition, the Lçwdin population analysis indicates that 3 has ad 4 electron configuration with considerable negative partial charge at the tungsten atom, which is consistent with an oxidation state of + II. Both complexes 2 and 3 feature trigonal-planar-coordinated silicon centers,w ith the sum of bond angles at the silicon center being 3608 8.
We investigated the replacement of the NHC moiety and treated the bulky IEt 2 Me 2 compound with more nucleophilic IMe 4 . [24] Indeed, treatment of 3 with excess IMe 4 resulted in quick conversion (60 %in30min) and eventually,after 12 h, quantitative exchange of IEt 2 Me 2 by IMe 4 (3' ';Scheme 3). As expected, only minor shifts in reference to the starting material were observed in all ( 1 H, 13 C, 29 Si)NMR experiments. Single crystals of 3' ' were obtained at ambient temperature from aC 6 D 6 /pentane (1:1) mixture,a nd the molecular structure was also confirmed by X-ray diffraction analysis ( Figure 2). TheW ÀSi bond is slightly elongated for 3' ' (2.3534(12) )incomparison to 3 (2.346(2) ); nevertheless, the structural parameters are very similar.
Theligand exchange ability of 3 encouraged us to abstract the NHC by treatment with Lewis acids.Indeed, treatment of 3 with the strong Lewis acid B(C 6 F 5 ) 3 or more oxophilic AlCl 3 resulted in an immediate color change from dark green to dark red in toluene (Scheme 3). Significant downfield shifts of the 29 Si NMR resonances for 4a (d = 322.0 ppm) and 4b (d = 323.2 ppm) compared to that of 3 (d = 229.7 ppm) were observed, whereas the 13 CNMR spectra of 4a and 4b displayed only av ery small change in shift (166.9 ppm for 4a and 167.3 ppm for 4b)f rom 3 ( 13 C d = 172.6 ppm). This suggests that the NHC remains coordinated to as ilylidene unit. Intriguingly,the addition of coordinating THF to the red solutions of 4a or 4b resulted in the regeneration of the darkgreen color,a nd the 1 HNMR spectroscopic analysis confirmed the regeneration of the initial starting material 3.This reaction corroborated the formation of apeculiar Lewis acidcarbonyl adduct, and was likewise confirmed by X-ray crystal structure analysis of 4a (Figure 3). Ac omparable terminal carbonyl ligand activation was observed by Cummins and coworkers during preparation of at erminal molybdenum carbide upon acylationo faM o II carbonyl complex with pivaloyl chloride. [25] TheW À Si bond in 4a (2.3630(18) )i se longated in reference to 3 (2.346(2) )b ecause of the reduced backdonation from tungsten to silicon and in line with the downfield-shifted resonance in the 29 Si spectrum (d = 322.0 ppm). TheW ÀCbond length for the AlCl 3 -coordinated CO ligand (1.840 (7) )i ss ignificantly shortened in comparison with that of the terminal carbonyl ligand (1.975 (7) ) and even in the range of tungsten carbyne complexes (1.82-1.87 ). [26] TheA l ÀOb ond length in 4a (1.777(8) )i s comparable with previously reported AlCl 3 coordinated to the oxygen atom of ac arbonyl ligand without bond rupture (1.812 (2) ). [27] Compound 4a has as imilar absorption band at 400 nm in the UV/Vis spectrum in toluene.Unfortunately, 4b could only be isolated as as ticky solid and not in crystalline form. TheI Rs pectrum of 4a in the solid state shows two n(CO) bands that appear as broad bands at 1813 cm À1 for AlCl 3 -coordinated CO and at 1901 cm À1 for the terminal CO.T wo peculiar carbonyl stretching frequencies are observed because of enhanced p-back-donation from tungsten to the carbonyl ligand and simultaneous weakening of the C À Ob ond (C27-O1 1.158(8) ,C 28-O2 1.255(9) ) that is coordinated to AlCl 3 .S imilarly, 4b also shows two n(CO) bands at 1906 cm À1 and 1873 cm À1 .T he latter can be assigned to CO···B(C 6 F 5 ) 3 ,w hich appears at ah igher wavenumber than in [Cp*(CO)À{C 6 F 5 ) 3 B···OC}W=Si(H)Tsi]-[H Me I i Pr] (n(CO···BCF) = 1535 cm À1 ). [19a] This observation indicates weaker coordination of the borane to the carbonyl group in comparison to that of the anionic silylidene complex.
In order to abstract the NHC from the silylidene complexes,w eh ence used am ilder Lewis acid, namely triphenylboron (BPh 3 ,S cheme 4). Indeed, heating toluene  solutions of 2 and 3 with one equivalent of BPh 3 for 30 min to 90 8 8Cafforded the desired tetrahedral clusters 5 and 6 in 40 % and 52 %y ield, respectively.T hese heterobimetallic compounds are well soluble in aromatic as well as aliphatic organic solvents.S urprisingly,t he heterobimetallic tetrahedral Si 2 M 2 clusters 5 and 6 are perfectly stable in moist air, as has been also reported for tetrasilatetrahedrane (tBu 3 SiSi) 4 (III). It is interesting to note that these complexes did not even react with methanol when heated to 70 8 8Cf or 24 h.
We performed detailed DFT calculations at the PBE0-D3BJ(SMD)/ZORA-def2-TZVPP//PBE0-D3BJ/def2-SVP level of theory in order to understand the electronic structures of 6 and 3 ( Figures S57-S62). [34,35] Indeed, Lçwdin population analysis of the DFT-optimized structure of 6 also indicated aS i ÀSi bond order of 1.1, indicative of only very small multiple bond character (Figure 4, right), which is in line with the short SiÀSi bond length in 6.Interestingly,the calculations suggest ah igher Si-Si multiple bond character of 1.3 for the molybdenum complex 5 ( Figure S61), which is in line with the relative 29 Si NMR shifts of 5 and 6 (see above). Plotting the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) corroborates delocalization of both orbitals over the whole cluster ( Figure S58). Thei ntrinsic bond orbitals (IBOs) [36] show two different, yet quite covalent, s-interactions between Si1 and W1 or W2, respectively ( Figure 5, top). TheS i1 À W1 s-bond is polarized towards Si, whereas the longer Si1 À W2 bond features additionally a p-back-bonding interaction with the CO p*orbitals. Besides,w ef ound aS i1ÀSi2 s-bond as well as considerable bonding interactions between the W1 and W2 atoms (  ure 5, bottom). Overall, the calculations confirmed atetrahedral structure with strong and covalent interactions between all silicon and tungsten atoms.
Furthermore,the reaction mechanism for the formation of 6 was modeled in order to understand the peculiar Si-Si dimerization ( Figure 6). [37] Ther estricted DFT calculations suggest that the formation of the intermediate silylidyne complex 7 proceeds essentially in isoergic fashion (DG = + 1.7 kcal mol À1 ). Thes ubsequent dimerization (DG = À43.1 kcal mol À1 )f eatures ab arrier of DG°=+26.6 kcal mol À1 ,which agrees well with areaction occurring at elevated temperatures.T he transition state ( Figure 6) shows av ery large separation of the two tungsten atoms (5.05 )and hence is indicative of only weak interactions between these two atoms.Nevertheless,avery small orbital overlap in HOMO-3 substantiates av ery asynchronous,y et still concerted formation of the SiÀSi and WÀWbonds. [38] Most importantly,t he transition state reveals that the steric bulk associated with the supersilyl groups and the Cp substituents allows only for ap erpendicular arrangement of the two silylidyne groups.T his orientation consequently determines the formation of the tetrahedral cluster instead of four-membered rings as would be expected by simplifying polarity considerations.I ndeed, attempts to geometry-optimize four-membered rings with either SiÀSi/WÀWo ra lternating Si À Wb onds led to isomeric tetrahedra (DG = À34.0 kcal mol À1 ;F igure S70), three-membered rings (DG = À15.8 kcal mol À1 ; Figure S70), or quadrangles of much higher energy (Si-W-Si-W quadrangle 8: DG = À10.2 kcal mol À1 ). Attempts to model dimeric compounds with decoordination of only one NHC also did not meet with success.W econclude that the steric bulk in 6 prevents the formation of quadrangles or triangles and allows only for the formation of atetrahedral cluster subsequent to abstraction of the NHC ligand.

Conclusion
We have reported the isolation of the heteroatomic, bimetallic M 2 Si 2 tetrahedral clusters 5 and 6.T hese compounds form after NHC abstraction from the respective NHC-stabilized silylidene complexes 2 and 3 by dimerization of transient transition-metal silylidyne complexes.T hese tetrahedral clusters are air-a nd moisture-stable unlike many other main-group organometallic compounds.F urthermore, we have shown that the NHC (IEt 2 Me 2 )incomplex 3 can be exchanged for am ore nucleophilic NHC (IMe 4 ). Contrarily, the addition of stronger Lewis acids such as AlCl 3 or B(C 6 F 5 ) 3 resulted in reversible activation of the carbonyl ligands (4a, b). Calculations confirm the covalent bonding in the cluster and indicate that steric bulk is crucial for the formation of the tetrahedron-type structure.