Giant Spherical Cluster with I-C140 Fullerene Topology

We report on an effective cluster expansion of CuBr-linked aggregates by the increase of the steric bulk of the CpR ligand in the pentatopic molecules [CpRFe(η5-P5)]. Using [CpBIGFe(η5-P5)] (CpBIG=C5(4-nBuC6H4)5), the novel multishell aggregate [{CpBIGFe(η5:2:1:1:1:1:1-P5)}12(CuBr)92] is obtained. It shows topological analogy to the theoretically predicted I-C140 fullerene molecule. The spherical cluster was comprehensively characterized by various methods in solution and in the solid state.

SincethediscoveryoftheC 60 buckminsterfullerene [1] in 1985 by Curl and Smalley,t he synthesis of larger fullerenes continues to be the focus of research ( Figure 1). [2] Additionally,t he structural diversity,f unctionalization, and usage of fullerenes have been investigated extensively. [3] However,the number of larger fullerenes is still limited. Thel argest structurally characterized examples is the endohedral fullerene Sm 2 @C 104 of Balch et al. [4] and the chlorinated compounds C 104 Cl 16 and C 104 Cl 24 ,b oth reported by Yang et al. [5] Also much larger fullerenes up to C 418 have been detected by mass spectrometry. [6] Another strategy was pursued by Oshima and Takayanagi et al. who applied bias voltages on an amorphous carbon agglomerate between gold electrodes with at ransmission electron microscope/scanning tunneling microscope. [7] By size comparison they estimate that the fullerenes C 140 ,C 180 ,C 240 ,C 260 ,a nd C 620 were formed. However,t he structural identity of all these larger carbon clusters is unclear,s ince formation of dimers or aggregates (e.g. C 120 Q(C 60 ) 2 ), [8] onion-like structures, [9] and fullerene isomers (still fulfilling the isolated pentagon rule) [2,9] is possible.G enerally,i tw as calculated that the ball-shaped arrangements are energetically preferred over capsule-like structures. [10] In this context, the most stable isomer of C 140 fullerene was calculated to be icosahedral I-C 140 . [11] Carbon is not the only element suitable for the formation of spherical macromolecules.I np olyoxometalate chemistry, Keplerates [12] with the simplified formula [{(M VI )Mo VI 5 } 12 -(linker) 30 ]( M = Mo,W ;l inker = Mo 2 O 4 (acetate) + ,V O 2 + , Cr 3+ ,F e 3+ )a re well known. [13] In addition, pentasubstituted Cp R derivatives can also form nanoballs in combination with metal salts,a sW illiams et al. showed for the system [(C 5 H 4 POtBu 2 )Fe{C 5 (4-pyridyl) 5 }] and Cu I cations. [14] Furthermore,W right et al. obtained af ullerene-type metal-organic framework with the sodium salt of [C 5 (CN) 5 ] À . [15] In the field of coordination compounds,F ujita et al. succeeded in the preparation of spherical compounds from oligopodal pyridine linkers and Pd II moieties.H owever,n one of these materials display fullerene-like topology. [16] In contrast, the pentaphosphaferrocenes [Cp R Fe(h 5 -P 5 )] were shown to be excellent building blocks in combination with Cu I halides for the formation of fullerene-like supramolecules,f or example, [{Cp*Fe(h 5:1:1:1:1:1 -P 5 )} 12 {CuCl} 10 {Cu 2 Cl 3 } 5 {Cu(CH 3 CN) 2 } 5 ] (Cp* = C 5 Me 5 ), [17] which comprises 90 non-carbon scaffold atoms.D epending on the Cu I halide,t he template and the reaction conditions,different structural motifs were obtained following the fullerene topology predetermined by the cyclo-P 5 rings (e.g. scaffolds with a I h -C 80 related core,c f. Figure 1). [18] With [Cp Bn Fe(h 5 -P 5 )] (Cp Bn = C 5 (CH 2 Ph) 5 ) [19] as ab uilding block, products with good solubility are obtained. [20] Almost all the nanosized compounds described above (d = 2.1-2.8 nm) follow the fullerene topology,c ontaining 12 five-membered rings and (nÀ20)/2 six-membered rings. [18,20] Therefore the question arises whether the use of building blocks much larger in size results in the formation of samesized spheres with fewer cyclo-P 5 units or much larger spheres that arise from the self-assembly of 12 pentaphosphaferrocenes.The latter possibility might open the way to structurally unknown relatives of the largest known fullerenes.Therefore the very bulky building block [Cp BIG Fe(h 5 -P 5 )] (1;C p BIG = C 5 (4-nBuC 6 H 4 ) 5 )w as synthesized, [21] in which the radius of the Cp BIG ligand (r % 9)isabout three times larger than that of the Cp* ligand (r % 3). [22] Herein we report on the preparation and characterization of the "expanded" cluster [{Cp BIG Fe(h 5:2:1:1:1:1:1 -P 5 )} 12 Cu 70 Br 83 ] (2), which shows the same structural topology as the theoretically predicted I-C 140 fullerene ( Figure 1). This Br/ Cu/P scaffold is the non-carbon version of the I-C 140 fullerene, and its structural characterization reveals the potential of the building blocks as well as the fullerene building concept in supramolecular chemistry.
Addition of aCH 2 Cl 2 solution of 1 to aCH 3 CN solution of CuBr results in the formation of the novel giant supramolecule [{Cp BIG Fe(h 5:2:1:1:1:1:1 -P 5 )} 12 Cu 70 Br 83 ]( 2). [23] Compound 2 has poor solubility in CH 2 Cl 2 and is insoluble in hexane,t oluene,C H 3 CN,a nd Et 2 O. By diffusion of toluene into CH 2 Cl 2 solutions,i tc rystallizes in the monoclinic space group C2/c as black cubes and can be isolated in ay ield of 25 %, which is astonishing for ac ompound of this type and size.Single crystals could also be grown when hexane is used instead of toluene.H owever,i nt he latter case they quickly lose crystallinity when the crystals are removed from the mother liquor.
Theunprecedented scaffold of 2 consists of 92 copper and 92 bromide positions as well as 12 units of 1.T he supramolecule has an outer diameter of about 3.5 nm and shows athree-shell core structure.Ifthe Cp BIG ligands are taken into account even af our-shell structure is present. This is in contrast to the compounds constructed from [Cp*Fe(h 5 -P 5 )], which exclusively show single-shell aggregates with cavities occupied by atemplate or asolvent molecule. [17][18][19] Thespacefilling view of 2 ( Figure 2a)e xhibits at ight arrangement of the twelve Cp BIG ligands forming ad istorted pentagonal dodecahedron as the outer shell of the giant molecule.T he supramolecules are ordered in aslightly distorted cubic close packing ( Figure S4), which is typical for the packing of spheres. [24] In 2 all Patoms of the pentaphosphaferrocene units 1 are coordinated to Cu ions resulting in a1 ,2,3,4,5 coordination mode (Figure 2b-d). Each Cu ion of the outer shell additionally binds to three bromide ions:one points inward, one is "in-plane", and one "out-of-plane" (see Figure 2c). These Cu and Br ions in combination with the Pa toms of 1 form the outer shell. Interestingly,w ith 12 five-membered cyclo-P 5 rings and 60 six-membered P 2 Cu 3 Br rings based on the "inplane" Br atoms,t he outer scaffold of 2 shows topological analogy to the theoretical structure of I-C 140 fullerene (Figure 2d). In addition, 30 "out-of-plane" Br atoms bridge the 30 Cu-Cu edges of the outer framework. This results in overall 170 scaffold positions of the outer shell (P 60 Cu 60 Br 50 ). In the aggregate 2,i cosahedral symmetry is violated by small distortions caused by the presence of different types of atoms.Incontrast, the structural motif of the I-C 140 fullerene has not yet been proven experimentally,a lthough numerous calculations on the structure and physical properties have been performed for the carbon cluster. [9][10][11]25] Thediameter of the outer scaffold of 2 of roughly 18.0 i sm uch larger than the predicted diameter of I-C 140 (10.5 ; see Ref. [2b]).
Thes caffold of the next shell shows ap entagon-dodecahedral shape (d ø % 14 ). Thenodes of the dodecahedron are occupied by 20 Cu atoms and all edges are bridged in an almost linear fashion (Cu-Br-Cu % 1708 8)b y3 0B ri ons (Figure 2c,e). Theouter and middle shells are linked together by numerous Cu-Br bonds (Figure 2c). Tw elve additional CuBr fragments are connected to two adjacent Br ions such that each pentagonal {Cu 5 Br 5 }cycle bears only one these bromides (Figure 2e). Every Cu ion of this additional CuBr unit coordinates one pentaphosphaferrocene 1 in a h 2 -mode and therefore further links these two shells (Figure 2f). This also implies that the cyclo-P 5 rings are located directly above the faces of the dodecahedron.
TheB ri ons of the CuBr fragments point inside the pentagon-dodecahedron. With twelve of these fragments,the Br atoms form aB r 12 icosahedron as central inner shell. It shows ad iameter of about 8.6 a nd an inner cavity of roughly 4.7 . An icosahedral core structure is reminiscent of Mackay topology. [26] However,these concepts cannot fully be applied in the present case due to the presence of directed covalent bonds rather than ac lose packing of spheres. Nevertheless,s imilar arrangements of multiple-shell clusters can be observed for late transition metal clusters.F or instance,D ahl et al. synthesized large nanosized Pd clusters also exhibiting an icosahedral core structure and an outer dodecahedral structure. [27] Another multishell cluster has been reported by Eichhorn et al. who synthesized an As-Ni cluster that also shows the structural feature of an icosahedron interpenetrating apentagonal dodecahedron. [28] In general, the whole structure of the aggregate can be described as an icosahedron@pentagon-dodecahedron@"I-C 140 ". Interestingly,t wo of the inner shells are dual to one another, since the vertices of the icosahedron correspond to the faces of the pentagon-dodecahedron (Figure 2e) and vice versa. Note,this is only the case with full occupation of all atom positions (see the Supporting Information). However,s ome of the Cu and Br positions in all shells are vacant. X-ray crystallography establishes the cluster as [{Cp BIG FeP 5 } 12 Cu 70 Br 83 ]and the best description of the crystal structure is asolid solution of different possible isomers.This is acommon feature of fullerene-like spheres [20,29] and is also known for huge coinage-metal chalcogen clusters [30] as well as the polyoxomolybdate giant anions. [31] Thed ifferent Cu and Br content has also been proven by the elemental analysis.A possible oxidation of the building block 1 could be ruled out by zero-field 57 Fe Mçssbauer measurements,indicating exclusively Fe II centers.T he EPR and SQUID measurements confirm that cluster 2 is diamagnetic.This makes the presence of Cu II improbable which could explain the "missing" positive charges. Va rious NMR investigations also exclude the possibility of am ultiply protonated aggregate. (For ad etailed description of the performed experiments,t he data analyses,and the description of the structure see the Supporting Information.) It was not possible to detect the molecular ion peak of 2 by mass spectrometry,which is usually not the case for such clusters. [17][18][19]  In summary,t he change to the sterically highly demanding Cp BIG ligand resulted in an effective expansion of the aggregate. Thes ynthesized giant spherical cluster 2 has ad iameter of about 3.5 nm. It exhibits structural analogy to the hitherto unknown I-C 140 fullerene,astructural motif that that has not Angewandte Chemie yet been observed experimentally.I nc ontrast to the supramolecules obtained from pentaphosphaferrocenes,c ompound 2 shows at hree-shell molecular structure which can strikingly be described as icosahedron@pentagonal-dodecahedron@"I-C 140 ". Thec luster shows poor solubility,b ut it could be characterized by NMR spectroscopy. Because of the successful concept of increasing the steric bulk for the formation of larger supramolecules,i tr emains open where the boundaries of this concept lie.

Experimental Section
All experiments were carried out under an atmosphereo fd ry argon or nitrogenu sing gloveboxa nd Schlenk techniques.S olvents were purified, dried, and degassed prior to use.CuBr was used as obtained by commercial suppliers,[Cp BIG Fe(h 5 -P 5 )] [21] was preparedaccording to literature procedures.The NMR spectra in solution were measured on Bruker Avance 300, 400, and 600 spectrometers.T he MAS NMR spectrum was recorded on aB ruker Avance 300 spectrometer by using ad ouble-resonance 2.5 mm MAS probe ( 31 P: 121.495MHz). Thespectrum was acquired at MAS rotation frequencies up to 20 kHz with a908 8 pulse length of about 2.3 msand relaxation delays of 120 s ( 31 P). ESI mass spectra were measured on aT hermoQuest Finnigan TSG 7000 mass spectrometer. Theelementalanalysis was determined by Mikroanalytisches Labor,Lehrbereich Anorganische Chemie,TU Munich.
Synthesis of 2:Asolution of CuBr (100 mg,6 .7 mmol) in 5mL CH 3 CN was treated with as olution of [Cp BIG Fe(h 5 -P 5 )] (60 mg, 64 mmol) in 5mLC H 2 Cl 2 and the reaction mixture was stirred for 30 min resulting in ab rown solution. After the solvent was removed in vacuum the residue was triturated with 10 mL CH 2 Cl 2 and filtered through acannula into athin Schlenk tube.The reaction mixturewas layered with 20 mL toluene.After complete diffusion black crystals of 2 were obtained (32 mg,2 5% yield for the idealized cluster