( η 4-Tetraphenylcyclobutadiene )-( η 5-pentaphenylcyclopentadienyl )-cobalt

A one-pot reaction starting with C5Ph5Br, n-BuLi and [CoCl(PPh3)3] followed by the addition of diphenylethyne produces the title compound with 12% yield. Spectroscopic characterization involved 1H, 13C-NMR, UV-Vis and mass spectrometry. A crystal structure determination showed that the central aromatic rings are exactly parallel with the cyclobutadiene ring further apart from the metal as usual. The pentaphenyl–cyclopentadienyl ligand shows an usual paddlewheel orientation, whereas in the tetraphenyl–cyclobutadiene ligand, two of the phenyl groups are nearly coplanar with the four-membered ring. There are also numerous C–H. . . C(π) interactions between the phenyl groups on the same ring, as well as between rings.


Introduction
Penta-aryl-cyclopentadienyl complexes are a rather small subgroup of sandwich complexes. The last comprehensive review article of these complexes dates back to 2011 [1] and lists "over 250 publication" entries, with complexes known for s, p, d and f metals. An updated search in SciFinder n now lists 503 entries for an "M-C 5 Ph 5 " search mask (accessed on 2 November 2022). The continuing interest in this substance class is based on two of its properties: bulkiness, which allows for stabilization of unusual geometries and oxidation states [2][3][4][5], and inherent "propeller chirality", which is supposed to induce stereoselectivity in catalytic reactions [6][7][8]. Whereas most publications are devoted either to the synthesis of compounds, owing to the unusual structures or physicochemical properties of the compounds, some studies were devoted to investigating their dynamical properties with respect to phenyl-and cyclopentadienyl rotation [9,10]. These studies are closely related to the field of molecular machines and gears [11,12]. However, neither restricted rotation phenomena nor propeller chirality are restricted to penta-aryl-cyclopentadienyl systems; they also occur in tetra-aryl-cyclobutadienyl and hexa-aryl benzene derivatives [10]. A tetraphenyl-cyclobutadienyl cobalt complex, [Co(C 4 Ph 4 )(C 5 H 4 C≡CTrp)] (Trp = trypticenyl) was the first molecular gear with a mixed sandwich complex [13], whereas [Co(C 4 Ph 4 )(C 5 H 2 MeRCOOR')] were the first metallocenes with three distinct chiralities, including propeller chirality [14]. One other property that should, in principle, be common to all per-arylated [C n Ar n ] molecules but that has been studied only with respect to hexa-aryl benzenes is the possibility of "toroidal interactions", i.e., electron delocalization between the aryl substituents [15][16][17][18][19]. The possibility of such interactions is closely related to the canting angle of the peripheral aryls with respect to the central arene. It has been recognized that the degree of canting depends on the possibility of formation of C-H. . . π (C) hydrogen bonds, mainly between the ortho C-H bond of one peripheral aryl ring and the ipso C atom of the neighboring aryl substituent and, to a lesser extent, with the π clouds of the peripheral and central aryl π systems [1,2,4,5,20]. In the crystal structures of deca-aryl metallocenes, these interactions can also occur between the aryl rings of two distinct cyclopentadienyl rings and lead the larger central metal ions to a net attraction of the two substituted cp-rings [2,4,5]. The latter phenomenon has only been studied for symmetrical metallocenes with two penta-aryl-cyclopentadienyl rings. As we recently started, to study half-sandwich complexes of the [Fe(C 5 Ph 5 )(CO) 2 R] type, as well as mixed sandwich complexes of the [Co(C 4 Ph 4 )(C 5 X 5 )] type [21,22], we decided to look into the synthesis, as well as the spectroscopic and structural properties of [Co(C 5 Ph 5 )(C 4 Ph 4 )], the first perphenylated mixed-sandwich complex.

Synthesis
For the synthesis of the title compound, we used a procedure described in the literature [13], with slight modifications. Lithium pentaphenyl cyclopentadienide was prepared in situ from bromopentaphenyl cyclopentadiene and n-butyl lithium in THF and treated with a toluene solution of [CoCl(PPh 3 ) 3 ]. This produced a solution of [Co(C 5 Ph 5 )(PPh 3 ) 2 ], which was not isolated but treated with tolane under reflux. After chromatographic workup, the title compound was isolated with ca. 12% yield (Scheme 1). structures of deca-aryl metallocenes, these interactions can also occur between the aryl rings of two distinct cyclopentadienyl rings and lead the larger central metal ions to a net attraction of the two substituted cp-rings [2,4,5]. The latter phenomenon has only been studied for symmetrical metallocenes with two penta-aryl-cyclopentadienyl rings. As we recently started, to study half-sandwich complexes of the [Fe(C5Ph5)(CO)2R] type, as well as mixed sandwich complexes of the [Co(C4Ph4)(C5X5)] type [21,22], we decided to look into the synthesis, as well as the spectroscopic and structural properties of [Co(C5Ph5)(C4Ph4)], the first perphenylated mixed-sandwich complex.

Synthesis
For the synthesis of the title compound, we used a procedure described in the literature [13], with slight modifications. Lithium pentaphenyl cyclopentadienide was prepared in situ from bromopentaphenyl cyclopentadiene and n-butyl lithium in THF and treated with a toluene solution of [CoCl(PPh3)3]. This produced a solution of [Co(C5Ph5)(PPh3)2], which was not isolated but treated with tolane under reflux. After chromatographic workup, the title compound was isolated with ca. 12% yield (Scheme 1).
The only other identified product was hexaphenylbenzene. The low yield was reproducible; however, all attempts to identify further products were met with failure.

Spectroscopic Characterization
In the room temperature 1 H NMR spectra, five multiplet signals were observed for the phenyl protons in an approximate integral ratio of 8:4:13:10:10 ( Figure S1 of the Supporting Information). We assigned the first two multiplets to the cyclobutadiene phenyls and the last two to the cyclopentadiene phenyls, whereas the signal group in the middle belongs to both rings. From the integrals, it can be assumed that both the "doublets" at the highest and lowest fields belong to the meta protons and that the para protons of the C5Ph5 ligands and the ortho protons of the C4Ph4 ligands coincide. This assignment is supported by the 1 H-COSY spectrum ( Figure S2). We also undertook a VT-NMR study between 25 °C and −80 °C ( Figure S3). Despite several observable changes in the spectra, such as merging of the two low-field signals and an impressive shift of the high-field doublet to an even higher field, there were no clear indications of a possible concerted rotation of the phenyl rings on both sides of cobalt.
In the 13 C-NMR spectra, (at room temperature) only eight signals of the phenyl carbon atoms and two of the π-co-ordinated rings can be observed ( Figure S4), suggesting unrestricted rotations of both phenyl rings and π-co-ordinated rings.
UV-Vis spectra were measured in CH2Cl2 solution at concentrations between 6 × 10 −4 and 2 × 10 −5 mol/L. λmax was ca. 279 nm at the lowest concentration. Three broad shoulders at λ ≈ 331, 364 and 416 nm were also identified ( Figure S6). Irradiation of an ethereal solution at λ = 360 nm produced a weak blue fluorescence. The only other identified product was hexaphenylbenzene. The low yield was reproducible; however, all attempts to identify further products were met with failure.

Spectroscopic Characterization
In the room temperature 1 H NMR spectra, five multiplet signals were observed for the phenyl protons in an approximate integral ratio of 8:4:13:10:10 ( Figure S1 of the Supporting Information). We assigned the first two multiplets to the cyclobutadiene phenyls and the last two to the cyclopentadiene phenyls, whereas the signal group in the middle belongs to both rings. From the integrals, it can be assumed that both the "doublets" at the highest and lowest fields belong to the meta protons and that the para protons of the C 5 Ph 5 ligands and the ortho protons of the C 4 Ph 4 ligands coincide. This assignment is supported by the 1 H-COSY spectrum ( Figure S2). We also undertook a VT-NMR study between 25 • C and −80 • C ( Figure S3). Despite several observable changes in the spectra, such as merging of the two low-field signals and an impressive shift of the high-field doublet to an even higher field, there were no clear indications of a possible concerted rotation of the phenyl rings on both sides of cobalt.
In the 13 C-NMR spectra, (at room temperature) only eight signals of the phenyl carbon atoms and two of the π-co-ordinated rings can be observed ( Figure S4), suggesting unrestricted rotations of both phenyl rings and π-co-ordinated rings.
UV-Vis spectra were measured in CH 2 Cl 2 solution at concentrations between 6 × 10 −4 and 2 × 10 −5 mol/L. λ max was ca. 279 nm at the lowest concentration. Three broad shoulders at λ ≈ 331, 364 and 416 nm were also identified ( Figure S6). Irradiation of an ethereal solution at λ = 360 nm produced a weak blue fluorescence.

Molecular and Crystal Structure
The title compound crystallizes in the monoclinic space group P2 1 /n with one molecule in the asymmetric unit ( Figure 1). Both co-ordinated rings show nearly perfect delocalization, with the C-C bonds in the cyclopentadienyl ring ranging from 1.433(4) to 1.451(4) Å and from 1.460(4) to 1.472(4) Å in the cyclobutadiene part. The phenyl rings at the cyclopentadienyl ring show the expected paddlewheel orientation, with canting angles ranging from 37 to 73 • (average 54.9 • ). The phenyl rings on the cyclobutadiene ring do not have a paddlewheel orientation, with one phenyl ring (C91-C96) nearly coplanar with the cyclobutadiene ring (interplanar angle, 6.6(2) • ) and another (C71-C76) with a small canting angle of 23.5(2) • . The distances between the cyclopentadienyl carbon atoms and the phenyl ipso carbons are longer than the corresponding distances at the cyclobutadiene ring (average distance, 1.484(4) vs. 1.469(4) Å). Whereas all phenyl rings are bent out of the planes of the co-ordinated rings away from the Co atom, this effect is stronger in the cyclobutadiene ring (average bending angle [2,4], 13.1 • vs. 7.1 • in the cyclopentadienyl ring). The centroid CT cp of the cyclopentadienyl ring is slightly closer to the Co atom (by ca. 0.02 Å) than the centroid (CT Cb ) of the cyclobutadiene, with an angle of CT Cb -Co-CT cp of 179.3(7) • .
As mentioned in the Introduction, the observed phenyl ring conformations are the consequence of C-H. . . π interactions. Analysis of the structure of 1 shows the presence of intramolecular (C-H) ortho . . . C i and (C-H) ortho . . . C ortho interactions, both between phenyls on the same (Cp or Cb) ring (Tables S2 and S3) and between phenyls on different rings (Table S4). Most of these distances are significantly shorter than the sum of van der Waals radii of C and H (2.90 Å) (a quantitative comparison with the "superbulky" metallocenes of references [4,5] is not possible, as in these studies, the C-H distances were artificially set to 1.08 Å (in our study, this distance was set to 0.95 Å). Figure 2 shows an overview of these interactions. In addition, there are also some intra-and intermolecular interactions of the C-H. . . CT type (Table S5), one of which involves the π-system of the cyclopentadienyl ring, whereas the others occur only between phenyl rings.
Another possible approach to this kind of interaction is Hirshfeld analysis [17,20]. We therefore undertook a Hirshfeld analysis of compound 1, using the CrystalExplorer program [32]. Figure S7 shows the Hirshfeld Surface, Figure S8 shows a "fingerprint plot" and Figure S9 shows views of the HOMO and LUMO of 1. Careful analysis of the fingerprint plot shows that C. . . C, C. . . H and H. . . H interactions contribute 0.6%, 29.2% and 70.2%, respectively. In a comparable study on hexaphenylbenzenes it was found that, "...cohesion is maintained primarily by diffuse H. . . H contacts, and well-defined directional forces such as C-H. . . π and π. . . π interactions are less important", with hexaphenylbenzene values of 1%, 32% and 67% reported [20]. These values are similar to our values, with ours being slightly superior if properties "like inefficient packing, high solubility, and molecular cohesion" are desired for particular applications [17]. Another possible approach to this kind of interaction is Hirshfeld analysis [17,20]. We therefore undertook a Hirshfeld analysis of compound 1, using the CrystalExplorer program [32]. Figure S7 shows the Hirshfeld Surface, Figure S8 shows a "fingerprint plot" and Figure S9 shows views of the HOMO and LUMO of 1. Careful analysis of the fingerprint plot shows that C…C, C…H and H…H interactions contribute 0.6%, 29.2% and 70.2%, respectively. In a comparable study on hexaphenylbenzenes it was found that, "..cohesion is maintained primarily by diffuse H…H contacts, and well-defined directional forces such as C-H…π and π…π interactions are less important", with hexaphenylbenzene values of 1%, 32% and 67% reported [20]. These values are similar to our values, with ours being slightly superior if properties "like inefficient packing, high solubility, and molecular cohesion" are desired for particular applications [17].
This brings us to the last part of our study: the crystal packing of 1. Figure 3 shows a packing diagram along the crystallographic b axis. The planes of both co-ordinated rings are oriented perpendicular to the projection plane. This brings us to the last part of our study: the crystal packing of 1. Figure 3 shows a packing diagram along the crystallographic b axis. The planes of both co-ordinated rings are oriented perpendicular to the projection plane. Another possible approach to this kind of interaction is Hirshfeld analysis We therefore undertook a Hirshfeld analysis of compound 1, using the CrystalE program [32]. Figure S7 shows the Hirshfeld Surface, Figure S8 shows a "fingerprin and Figure S9 shows views of the HOMO and LUMO of 1. Careful analysis of the print plot shows that C…C, C…H and H…H interactions contribute 0.6%, 29.2 70.2%, respectively. In a comparable study on hexaphenylbenzenes it was foun "..cohesion is maintained primarily by diffuse H…H contacts, and well-defined tional forces such as C-H…π and π…π interactions are less important", with phenylbenzene values of 1%, 32% and 67% reported [20]. These values are similar values, with ours being slightly superior if properties "like inefficient packing, hig bility, and molecular cohesion" are desired for particular applications [17].
This brings us to the last part of our study: the crystal packing of 1. Figure 3 s packing diagram along the crystallographic b axis. The planes of both co-ordinate are oriented perpendicular to the projection plane. Analysis of the structure by the subroutine VOID of PLATON shows the presence of a potential solvent accessible volume of ca 121 Å 3 corresponding to 2.7% of the cell volume. Figure S10 shows a PLATON cavity plot, showing that 121 Å 3 are spread over eight places with individual volumes of 13 Å 3 or 17 Å 3 , making them all too small to take up a solvent molecule.

Discussion
The synthesis according to a synthetic protocol that usually produces cyclopentadienylcyclobutadiene-cobalt complexes in high yields afforded the title compound with a low yield (12%). An alternative might be the use of the known [Co(C 5 Ph 5 ) (CO) 2 ] as a starting material; however, steric congestion in the desired product might favor the formation of a tetraphenyl-cyclopentadienone ligand instead. The formed unknown side products, which could not completely be removed by chromatography, also prevented characterization by elemental analysis. However, full spectroscopic characterization by 1 H and 13 C { 1 H}-NMR spectroscopy was possible, as well as mass spectrometry, including HRMS and UV-Vis spectroscopy. At room temperature, the 13 C NMR spectrum showed only one set of signals (five signals for each of the two ligands). VT-NMR spectra showed several changes in the appearance of signals; however, no clear tendency toward coalescence of signals was observable. This parallels the observations made in related complexes reported in the literature.
The molecular structure does not show many significant deviations from the structures of related compounds. However, the steric influence exerted by the C 5 Ph 5 group pushes both the ring atoms of the cyclobutadiene and the ipso carbons of the attached phenyl rings further away from the metal than usual, allowing for a nearly coplanar orientation of two phenyl rings on the ligand. It would be interesting to investigate whether ortho methyl or even larger substituents were introduced at the phenyl substituents of the cyclobutadiene moiety. This should prevent the coplanarity of the phenyl rings with the cyclobutadiene ring. Alternatively, ortho substituents at the phenyl rings of the C 5 Ph 5 ligands might push the cyclobutadiene ligand even further away. Moreover, such substituents would prevent the formation of (C-H) ortho . . . C(π bonds), which are structure-determining in the present compound.
Furthermore, a "bond valence analysis" (performed with one of the subroutines of platon) produces a bond valence sum of 2.807, suggesting Co(III). However, the reliability of such calculations with organometallic compounds is dubious.

Materials and Methods
All solvents used in the present study, as well as reagents n-BuLi (2.5 M solution in hexane), CoCl 2 ·6H 2 O, PPh 3 and tolane, were obtained commercially and used as such.

Synthesis
A solution of [C 5 BrPh 5 ] (0.25 g, 0.48 mmol) in THF (5 mL) was treated at −78 • C with n-BuLi solution (0.19 mL, 0.48 mmol) and stirred for 1 h. The obtained solution of [LiC 5 Ph 5 ] was added to a solution of [CoCl(PPh 3 ) 3 ] (0.44 g, 0.50 mmol) in toluene (30 mL) at 0 • C, and the mixture was then heated for 30 min to 60 • C. Then, solid PhC≡CPh (0.20 g, 1.10 mmol) was added, and the mixture was heated to reflux for 6 h. After cooling down to r.t., ca. 75% of the solvents were evaporated in vacuo. After the addition of an equal volume of Et 2 O, the obtained mixture was filtered through a plug of silica gel, and the filtrate was completely evaporated in vacuo. The residue was taken up in the minimum amount of petroleum ether and chromatographed on silica gel using a petroleum ether: dichloromethane 9:1 mixture as eluent. The obtained yellow fraction was evaporated in vacuo to afford the title compound as a yellow solid (0.050 g, 0.060 mol, 12%). 1

Crystal Structure Determinations
The crystal was measured on a BRUKER D8VENTURE system. The experimental details of the structure determinations are collected in Table S1 of the Supporting Information. The WINGX software package [35] was used for structure solution (SHELXT [36]), refinement (SHELXL 2018/3, [37]), evaluation (PLATON) and graphical representation (ORTEP3 and MERCURY). Carbon-bound hydrogen atoms were treated with a riding model using the AFIX command of SHELXL.