Hydridoorganostannylene Coordination: Group 4 Metallocene Dichloride Reduction in Reaction with Organodihydridostannate Anions

Abstract Organodihydridoelement anions of germanium and tin were reacted with metallocene dichlorides of Group 4 metals Ti, Zr and Hf. The germate anion [Ar*GeH2]− reacts with hafnocene dichloride under formation of the substitution product [Cp2Hf(GeH2Ar*)2]. Reaction of the organodihydridostannate with metallocene dichlorides affords the reduction products [Cp2M(SnHAr*)2] (M=Ti, Zr, Hf). Abstraction of a hydride substituent from the titanium bis(hydridoorganostannylene) complex results in formation of cation [Cp2M(SnAr*)(SnHAr*)]+ exhibiting a short Ti–Sn interaction. (Ar*=2,6‐Trip2C6H3, Trip=2,4,6‐triisopropylphenyl).


Introduction
Since Lappert et al. invented the synthesis of germylenes and stannylenes in the early 1970s, the interaction of these Lewis basic and Lewis acidic molecules with transition metal fragments has been extensivelys tudied. [1] However,o ne of the first stannylene coordination compounds [Me 2 Sn(thf)Cr(CO) 5 ]w as already synthesized earlier,i n1 971, following as alt metathesis reactionb etween dimethyl tin dichloride and the dinuclear chromium carbonylate dianion [Cr 2 (CO) 10 ] 2À . [2] The coordination chemistry of diorgano-as well as diamidostannylenes became av ery attractive fieldo fr esearch and was further developed for av ariety of transitionm etal fragments.T he nature of the bondingb etween low valent Group 14 elements and transition metals was also studied intensively.D iamidostannylenes, for example, act as donorl igands, whereas dialkylstannylenes can also act as acceptor ligands, by providing an empty p-orbital.
The hydride chemistry of heavy Group 14 elements and, in particular, the chemistry of low valente lements have recently attracted major interest. [3] Power et al. used bulky terphenyl ligands for the synthesiso ft he first low valent hydrides of tin and germanium. [4] The coordination chemistry of low valent tin hydrides was investigated by Rivard et al. for the highly reactive SnH 2 ,w hich was coordinated at transition metals to produce aL ewis base stabilized adduct. [5] Tilley et al. reportedt he reactiono fa no smium benzyl complex with organotin trihydride tripSnH 3 (trip = 2,4,6-triisopropylphenyl) to give an organohydridostannylene complex upon toluene elimination. [6] By reactinga nN HC adduct of al ow valent tin hydride [Ar*SnH( Me NHC)] with the platinum complex [Pt(cod) 2 ]adimeric tin-platinum complexf eaturing bridging hydride ligands was characterized at low temperature. [7] We have been exploring the chemistry of organotin and organogermanium trihydrides:B oth, reductive elimination of hydrogen in reaction with various Lewis bases,a sw ell as hydridea bstractiont og ive highly reactive dihydridocationsw ere studied. [8] Furthermore deprotonation with Brønstedb ases, such as LiMe, LDA or KBn was investigated, which resulted in the formation of organodihydrido anionsofg ermanium and tin. [9] Al arge variety of Group 14 element ligandsw ere coordinated at metallocene fragments of Group 4m etals. [10] Nucleophilic substitution at metallocene dichlorides by using alkali metal salts of triphenyltin was reporteda se arly as 1968 to yield the bis triphenyltin substitution product A (Figure 1). [10a] Piers et al. published the coordination of Lappert's stannylene [Sn{CH(SiMe 3 ) 2 } 2 ]a ti ns itu generated zirconocene (B,F igure 1). [10c,d, 11]  plex (C,F igure 1) of Group 4m etallocenes was synthesized by reactiono fa lkaline metal salts of digermanes or silagermanes with respective metallocene dichlorides. [10e] By reacting ac omparable alkalines alt of adistannane and after stannylenetransfer,ametallocene derivative exhibiting af our-membered ring (D,F igure 1) was isolated.
[10e] Coordination of cyclic tetrylenes at metallocene fragments was achieved by reactingt he monomeric phosphine adduct of cyclic tetrylenes or ad imericp recursor of ac yclic stannyleneo rp lumbylene with the in situ reduced metallocene (E, F,F igure 1). [10f,g] Saito et al. studied the reactiono fd ilithio stannole with titanocene dichloride and found formation of aT iSn 2 moiety (G Figure 1). [10h] In comparison, dipotassium germole, which was investigated by Müller and co-workers, was reactedw ith titanocene or zirconocene dichloride under elimination of potassium cyclopentadienide, which led to the formation of dimericg ermole complexes (H Figure 1). [10i] Furthermore, diorganotin hydride coordination at hafnocene complexes was discussed, as an intermediate in catalytic dehydrocoupling of diorganostannanes. [12] Here, we present the reaction of metallocene dichlorides of titanium, zirconium and hafnium with organodihydrido anions of germanium and tin.

Results and Discussion
Twoe quivalents of the lithium salt of organodihydridogermate salt 1 were reacted at À40 8Cw ith hafnocene dichloride (Scheme 1). The substitution product 2,w hich was isolated after crystallization from toluene,w as characterized by NMR spectroscopy and elementala nalysis. Crystals suitable for single crystal X-ray diffraction, however,w ere not obtained.
The metallocenec omplexes 4-6 were fully characterizedb y elemental analysis, NMRs pectroscopy and single crystal X-ray diffraction. Crystallographic data and refinementd etailsw ere placed in the Supporting Information. Since the molecular structures of 4-6 display almost identicalg eometriesi nt he solid state, only the molecular structure of 5 is shown in Figure 2, while representations of the molecular structures of 4 and 6 were placed in the SupportingI nformation. Selected interatomic distances and angles for all three systemsa re listed in Table 1. Despite the very bulky terphenyls ubstituent the TiÀ Sn bond lengths in 4 [2.677(2), 2.686(2) ]a re slightly shorter than the distances found in other TiÀSn complexes (see Figure 1): E 2.6940(9), F 2.7122(13), 2.7154 (14), G 2.6867(16) Scheme1.Synthesis of the hafnoceneb is(dihydridogermyl) complex 2.
However,t riply coordinated stannylene adducts Ar*SnH(L) (L = NHC, 1 J SnÀH 237.5, 227.6 Hz;L = py,9 9Hz; L= DMAP, 113.2 Hz) ( Et NHC = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene, DMAP = 4-dimethylaminopyridine) show relativelys mall 1 J Sn-H coupling constants. [8d,e] Müller et al. have characterized related NHC-stabilized hydridosilylenes [Ar'SiH( Me NHC)] and also identified small 1 J SiH coupling constants,f or example, 103 Hz [Ar' = 2,6-Mes 2 C 6 H 3 ,M es = 2,4, 6-trimethylphenyl].O nt he basis of quantum chemical calculations, the small coupling constant was shownt ob eac onsequence of as trongly reduced Fermi contact and, accordingly,asmalls -orbital participation in the SiÀHb ond of only 16 %. This, in turn, was rationalized with the lone pairo nt he silicon atom residingi na no rbitalw ith mainly sc haracter. [19] Following this argumentation, the larger 1 J SnÀH coupling constants in complexes 4-6 should be accompanied by ah igher s-orbital participation in the SnÀHb onds. In order to support this assumption we carried out DFT calculations on 4 and, for comparison, on the related complex [Ar*Sn-H( Et NHC)], [20] and analyzed their bonding using the natural bond orbital (NBO) approach [21] (for details, see the Supporting Information). In the NBO representing the SnÀHb onds in 4, the tin atoms display 22.1 %s-orbital participation.For comparison, in the adduct [Ar*SnH( Et NHC)] the 5s(Sn)-orbital character of the SnÀHb ond wasd etermined to be 12.3 %. This supports the above interpretation and indicates ac orrelation between the magnitude of the 1 J SnÀH coupling constant and s-orbital participation of Sn in the SnÀHb ond. However,i tw as shown for the higher homologue Pb, that the magnitude of 1 J(PbÀC) is not related to the sc haracter of the PbÀCb ond, as in this case relativistic effects playa ni mportantr ole. [22] Figure 2. ORTEP for the molecular structure of 5,with ellipsoidsdrawn at the 50 %p robability level. Hydrogen atoms H1 and H2 were found in difference Fourier maps and freely refined, iPr groups are omitted for clarity.  On the basis of the NBO analysis the bonding between the stannylene ligands and the titanocene fragment in 4 could be separated into two SnÀTi s-bonds [each composed of 42 %T i (16 %s ,8 4% dc haracter) and 58 %S n( 54 %s ,4 6% p-character) and a p-bond between the Ti center and both Sn atoms [Ti: 48 %( 97 %dcharacter), Sn:2 6% each (99 %p -character)], which is represented by at hree-centreN BO (for details, see the Supporting Information). The stannylene( Ar*SnH) therefore interactsw ith the titanocene fragment Cp 2 Ti as s-donor and p-acceptor ligand,w ith the p-backbonding from the Ti centre to the tin atoms distributed equally over both stannylene ligands. As imilar bonding scenariow as observedf or the higherh omologue Zr in the models ystem [Cp 2 Zr(SnHPh) 2 ]( for details, see the Supporting Information), which confirms earlier findings by Piers, Marschner,M üller who have analyzed the bondingi nb is(tetrylene) zirconocene complexes of type B and F (see Figure 1). [10d,f] Saito et al. reported as imilar structural motif for their TiSn 2 -ring complex (G in Figure 1). The nature of the bonding in this compound, however,i ss omewhat different and can be described by an electron donation from a s(SnÀSn) orbitalt owardsT il eading to ap artial aromatic character of the TiSn2 ring. [10h] Using the "Atoms in Molecules" (AIM) approach, [23] the atomic charges in 4 were determined to be + 1.25 for Ti and + 0.68 for both Sn1 and Sn2. It is interesting to note that, due to their closeness, the p-accepting p-orbitals on both Sn atoms seem to overlap to as ignificant extent in the aforementioned three-centre NBO, despite as lightly larger distance between both Sn atoms in the DFT-optimized geometry of 4 (3.474 vs. 3.370(1) in the X-ray diffraction study). At opological analysis of the electron density in the DFT model system for 4 revealed ar elatively high delocalization index (DI) of 0.37 between both Sn atoms.T he DI(A,B) is considered to measure the number of electron pairs shared between two atoms Aa nd B, and represents the bond order in case of equal atoms. [24] Therefore, this seems to point towards the existence of significant bonding interactions between the tin atoms in 4.A ne ven higher DI(Sn,Sn) of 0.52 was observed in the related DFT model system [Cp 2 Zr(SnHPh) 2 ]. However,t he SnÀSn distance in this case was significantly shorter than in the X-ray structure of 5, probably due to the sterically less demanding stannylene ligands (3.273 vs. 3.526(1) ).
Coordinationo fl ow valent Group 14 elementl igands at metallocene fragments was so far accomplished by reaction with the alkyne complex [Cp 2 Ti(btmsa)] [25] [btmsa:b is(trimethylsilyl)acetylene] or after reduction of the metallocene dichlorides with BuLi or Mg. [10c,d,f,g, 11] Furthermore, the reaction of ad ianionicm ain group compound, for example, dipotassiosilylgermane, with hafnocene dichloride also represents a methodt oc oordinate al ow valent compound, that is, as ilagermane, to am etallocene. [10e] In the following, we discuss two possible pathways for the formation of the characterizedm etallocenec omplexes [Cp 2 M(SnHAr*) 2 ]( 4-6). Titanoceneh ydridostannylene [Cp 2 Ti(SnHAr*) 2 ]( 4)f orms in ah ighy ield (96 %) reaction. The reactiono fC p 2 TiCl 2 with two equivalents of the anion [SnH 2 Ar*] À was monitored by 1 HNMR spectroscopy at room temperature. Immediately,f ormation of the organotin hydride[ Ar*SnH] 2 was observed making ah ydride transfer to the titanocene complex feasible. [4a] However,h ypothetically formed [Cp 2 TiH 2 ]i sh ighly reactive and should eliminate hydrogen (H 2 wasa lso detected in the 1 HNMR spectrum)t og ive titanocene [Cp 2 Ti], as af urtheri ntermediate. This should then react directly with the low valent organotin hydride to give the isolated product 4 (Scheme 3). [26] On the other hand, in the cases of zirconium andh afnium, we detected evolution of hydrogen by NMR spectroscopy and only slight amounts of organotin hydride [Ar*SnH] 2 .W e assume that in these cases, substitution of the chloride ligands took place first, resultingi nt he formation of [Cp 2 M(SnH 2 Ar*) 2 ], followed by evolution of hydrogen to give products 5 and 6 (Scheme 3). Potential dihydride complexes of Zr andH fa re known to be not as highly reactive as the homologous titanium dihydride. [27] Since no formation of Cp 2 MH 2 (M = Zr,H f) was observed, we propose the mechanism shown here, without the formation of an intermediate dihydride.
The Lewis acid [B(C 6 F 5 ) 3 ]reacted with bis(hydridoorganostannylene)titanocenec omplex 4 under abstraction of ah ydride substituent and led to formation of ac ationic complex 7.I n Figure 3t he moleculars tructure in the solid state is presented.
Complexes with bent or linear M-E-R (M = transition metal, E = Group 14 element, R = organic substituent) arrangements were investigated by Power et al. and Filippou et al. [28] Depending on the bond order between transition metal and low valent Group 14 element, either al inear metal tetrylidyne, M E-R, or am etallo tetrylene, M-E-R, with ab ent geometry,w as found. [18,28,29] To explore the nature of the bonding in the cationic complex 7,q uantum chemical calculations were carried out. The DFT-optimized geometry of 7 (Figure 4) was in good agreement with the solid-state structure (for details, see the Supporting Information). An NBO analysis revealed that both tin ligands exhibit aT i ÀSn s-bond. In addition, a p-interaction between the titanium atom and Sn2 wasi dentified (for figures and details, see the Supporting Information), however with a clearly reduced occupation (1.64) of the respective NBO. This indicates as ignificantd elocalization towards other atoms in the complex. The corresponding natural localized molecular orbital (NLMO)r evealed that the atom Sn1 of the hydridostannylene ligand also receives some p-bonding from the Ti centre, althought oam uch smaller extent than Sn2 (see Figure 4). Therefore, only 6.2 %o ft he electrons reside on Sn1, whereas 25.5 %a re found on Sn2;t he Ti centre comprises 56 %o ft he electrons.T his suggestst hat due to the highere lectrophilicity of the cationic Ar*Sn ligand, tin atom Sn2 abstracts ah igher amount of the available electron pair from the titanium atom into its emptyp -orbital. The higher degree of p-bonding then leads to ac learly shorter TiÀSn2 bond and al arger angle C2-Sn2-Ti. This interpretation is also corroborated by the values for the delocalization index (DI), which can be obtained from the electron density distribution:F or TiÀSn1 and TiÀSn2 aD I of 0.53 and 0.77, respectively,w as calculated. For comparison, in the neutral complex 4,aD I(TiÀSn) of 0.70 waso btained, which again demonstrates as hift of the p-bonding towards Sn2 upon abstraction of the hydride substituent. It is interesting to note that the increased back-bonding leads to as maller atomic charge (using the AIM approach) on Sn2, when compared to Sn1 (+ 0.80 vs. + 0.95;T i: + 1.40). Finally,t he remaining empty p-orbitals of both tin atomsa re part of the LUMO, LUMO + 1a nd LUMO + 2o rbitals (for details, see the Supporting Information).
The signal of the tin hydride proton of 7 was found at low field (16.88 ppm, 1 J SnÀH = 550 Hz) exhibiting ar eduction of the SnÀHc ouplingc onstant in comparison to the starting material (4:1 3.27 ppm, 1 J SnÀH = 750 Hz). This low-field shift might be due to the cationic character of 7.F or comparison, osmium complex [Cp*(iPr 3 P)HOs=SnH(trip)] showeda 1 HNMR signal for the SnÀHm oiety at 19.4 ppm. In this example, the low-field shift could be explained with the influence of the heavy atom on the light atom chemical shift. [31] In the 119 Sn NMR spectrum of 7,t wo signals, ad oublet (1484 ppm, 1 J 119SnÀH = 550 Hz) and as inglet (1789 ppm), were found. The cationic Ar*Sn moiety exhibits the signala tl ower field, compared to the signalo ft he coordinated stannylene.  . DFT optimized geometry of 7,superimposed with the NLMO representing the back-bonding from af illed titanium d-orbital into the empty porbitalsoft in atoms Sn1 and Sn2. [30] The chemical shift of the coordinated stannylene lies close to the signal of the startingm aterial 4 at 1250ppm. On the basis of the optimizedg eometry of cation 7,N MR calculations were performed using ADF with the implemented GGA revPBE-D3(BJ) functional and ZORA TZ2P basis set. [32] The obtained values are in excellent accordance with the measuredv alues: Sn1 1484 (1425);S n2 1789 (1754) ppm (calculated chemical shift in brackets).
Among as eries of metal stannylidyne complexes, Filippou et al. also reported as ynthesis, in which am anganese chlorostannylidene complex was transformed into as tannylidyne complexb ya bstraction of the chloride substituent from the coordinated chloridoorganostannylene. [28m] For the synthesis of the Ti-Sn-Ar* moiety we presented here, we employed ac losely relateda pproach, by abstracting ah ydride substituent from ac oordinated hydridoorganostannylene. In contrast to Filippou et al.,h owever,w edid not observe formation of aT i ÀSn triple bond, although another empty p-orbital would be available at the cationict in substituent. Thisc an be explained by the fact that af urther electron pair is missing in the Cp 2 Ti II fragment, which would be necessary for increased backdonation. Therefore, hydride abstraction only leads to formationo f ap artial double bond between the titaniumc entre and the cationic tin atom.

Conclusion
In contrast to an organodihydridogermate anion,w hich reacts under nucleophilic substitution with hafnocene dichloride, the homologouso rganodihydridostannate anion reacts as ar educing agent with Group 4m etallocene dichlorides. The formed metallocenes of titanium, zirconium and hafnium weres tabilized by hydridostannylene coordination.H ydridoorganostannylene ligands coordinate via aS n ÀM s-bond and both Sn atoms share the p-backdonation of the metal electron pair into the empty tin p-orbitals. Hydride abstraction from ah ydridoorganotin complexo ft itanocene represents as ynthetic pathway to increase the bond order of the TiÀSn interaction.