The first dinuclear sandwich complex Cp*Zn-ZnCp* (Cp*=C5Me5) was synthesized in 2004[1], which attracted intense attention and initiated a new field of organometallic chemistry. It is interesting that a homonuclear metal Zn-Zn bond is sandwiched by two Cp* rings in this remarkable D5h-symmetric compound. Stimulated by this pioneering finding, many novel dinuclear metallocenes have been investigated experimentally or theoretically[2~12]. Cyclopentadienyl (or substituents) is a well-known stabilizing ligand, however, it is not expected to be the only choice to stabilize the dinuclear metal atoms. Stable aromatic organic rings (such as C6H6, C4(BO)42-) [13~15], carbon-free E42- and E5- (E=N, P, As, Sb, Bi) inorganic rings [16~18], metalloaromatic rings (such as Ag4H4, Be32-, Al3R32-)[19~21] and spherical aromatic fullerene (C60, C70)[22] may also serve as the effective ligands to stabilize the metal-metal bond. Meanwhile, the central dinuclear metal atoms are extended from Zn to Cd, Hg, Be, Mg, Ca, Al, Ga[1~22]. The metal-metal bond in these dinuclear sandwich complexes is normal (single or multiple) bond. It is worth mentioning that Ding et al theoretically predicted the mixed dinuclear sandwich complex CpBe-LiCp associated by an “unaided” one-electron metal-metal bond in 2008[23]. Recently, the donor-acceptor sandwich complexes CpE-MCp (E=B, Al, Ga; M=Li, Na, K; Cp=η5-C5H5) and MQ (η4-E4) (M=Be or Mg; Q=C or Si; E=P, As, Sb, Bi) have been theoretically investigated[24, 25]. These dinuclear sandwich type complexes with novel metal-metal bonds enrich the field of the metallocenes. However, a key question arises: could more linear metals be sandwiched by two aromatic rings? In 2007, Gabriel Merino’s group firstly predicted multimetallocenes CpMnCp (M=Be, Mg, Ca, Zn; n=2~5)[26] and found that the beryllocenes CpBenCp (n>2) are the only species that could be synthesized. Recently, the triberyllium-benzene organometallic sandwich complexes have been studied at B3LYP/6-311+G** level [27]. As we all know, C4H42-is aromatic with six π electrons analogous to C5H5-. Its transition-metal complex [Fe (η4-C4H4)(CO)3] has been experimentally synthesized by Pettit and coworkers[28]. Recently, the inverse sandwich complexes E-C4H4-E (E=Al, Ga, In, Tl) and beryllium-cyclobutadiene multi-deck inverse sandwiches were also theoretically investigated [29, 30]. Very recently, synthesis and structural elucidation of the first stable representatives of pyramidanes compounds Ge[C4(SiMe3)4] and Sn[C4(SiMe3)4] were reported [31]. Can C4H42-function effectively as well to stabilize dinuclear Be-Be even to the -Be-Be-chain?
Herein, we performed DFT calculations on structures, bonding nature and the thermodynamic stabilities of a new class of sandwich complexes containing -Be-Be-chain with the formula [Ben(C4H4)2]2- and [Ben(C4H4)2]Li2 (n=2~8). As far as we know, there have been no theoretical or experimental investigations reported to date on the compounds [Ben(C4H4)2]2- and [Ben(C4H4)2]Li2 (n=2~8).
Two density functional theory (DFT) methods were used in this work. One is B3lYP[32, 33], the other is BP86[34, 35] method. The geometries of the titled complexes were fully optimized at both B3LYP/6-311+G (d, p) and BP86/6-311+G (d, p) levels. Vibrational frequencies of the optimized structures were calculated to characterize the nature of stationary points on the potential energy surface (PES). The natural bond orbital (NBO) analysis was performed at B3LYP level. All DFT calculations in this work were performed using the Gaussian 03 program package [36]. The topological bond characteristics of these sandwich complexes were analyzed using the AIM [37, 38] theory of Bader and carried out using the AIM2000 [39] and Multiwfn programs [40]. Molecular structures and molecular orbitals were visualized using the CYLview [41] and Molekel 5.4 program [42], respectively.
Sandwiching the linear Ben2+ center with two C4H42- ligands along the four-fold axis of the complexes may form the staggered D4d and eclipsed D4h [Ben(C4H4)2]2-(n=2~8). Both D4h and D4d structures are proved to be true minima on their potential surfaces without imaginary frequencies. It should be noted that D4d structures are slightly lower than the D4h structures in the total energies (including zero point energy). However, the energy differences between them (D4d and D4h) are negligible ( < 0.1 kcal/mol), which suggests that there is probably no internal barrier for rotation of the two C4H42- ligands about the C4 axis. Hence, only the optimized equilibrium structures of D4d [Ben(C4H4)2]2- (n=2~8), along with the Be-Be, Be-C and C-C bond lengths at both B3LYP and BP86 levels are illustrated in Fig. 1. As shown in Fig. 1, B3LYP and BP86 approaches produced essentially the same structures with minor bond length differences (within 0.04 Å). To be clear, the following discussion will mainly focus on B3LYP geometrical and electronic properties until otherwise stated.
All the Be-Be distances in D4d [Ben(C4H4)2]2- are close to the normal Be-Be σ single-bond. For example, the Be-Be distance in D4d [Be2(C4H4)2]2- is predicted to be 2.160Å, which is much shorter than the experimental value of diatomic Be2(2.45Å). Influenced by the ligands, the terminal Be-Be distances are slightly shorter than the central Be-Be distances in [Ben(C4H4)2]2- (n=4~8). For example, in Ben(C4H4)22-, the terminal Be-Be distance is 2.060 Å, while the central Be-Be distances are 2.181, 2.178 and 2.173 Å, respectively. Comparing the Be-Be distances in [Ben(C4H4)2]2- (n=2~8) with those in the counterparts CpBenCp, the Be-Be distances in the former are slightly longer than that in the latter. It’s worth mentioning that the Be-C distance gradually decreases from 1.878 Å (n=2) to 1.845 Å (n=8), suggesting that the bonding interactions between C4H42- ligands and Ben2+(n=2~8) nuclear increase as the n number rises for these sandwich complexes. As shown in Fig. 1, D4h C4H42- units in [Ben(C4H4)2]2- (n=2~8) complexes are well preserved. The C-C distance in [Ben(C4H4)2]2- (n=2~8) is slightly longer than that in free C4H42-, indicating C-C interaction weaken within ligand as interaction between ligands and Be-Be chain strengthen. However, the C-C distance in C4H42-ligand changes slightly and amounts to 1.462 Å throughout the series, which suggests that there are mainly ionic bonding between the C4H42-ligands and Ben2+ core. To check the local ring-current effect of the C4H42- ligands when incorporated into these sandwich complexes, the total isotropic nucleus independent chemical shifts (NICS) and their out-of-plane components (NICSzz)[43~45] are calculated at point 1.0 Å above the ring centers along the molecular axes perpendicular to the ligand planes. As shown in Tab. 1, the large negative NICSzz(1) values (-23.3~-24.9) indicate that the aromaticity of C4H42- are well maintained in these sandwich complexes.
In fact, two C4H42-ligands and the Ben2+ chain can assemble to form some possible sandwich types including face-face, face-side, side-side, face-corner, side-corner, corner-corner modes[46, 47]. The “homo-decked sandwich” form is probably the most important but not expected to be the only choice for D4d [Ben(C4H4)2]2- complexes. Here we only take [Be2(C4H4)2]2- as an example, possible sandwich isomers are systematically depicted in Fig. 2. It should be noted that both perpendicular structures and coaxial structures to the four-fold axis of the C4H42- ring are all considered. Each Be atom is bonded to a portion of both C4H42- rings in the perpendicular structure, while each Be atom is bonded to only one of the C4H42- rings in axial structures. Similar to the well-known Cp*Zn-ZnCp*, two C4H42-ligands of D4d [Ben(C4H4)2]2- favor to adopt the face-face type. We can find that the traditional face-face sandwich form is the most stable, which is lower in energy than the other structures at B3LYP/6-311+G (d, p) level.
In order to understand the electronic structures and the bonding characteristics of these sandwich structures, natural bond orbital (NBO) and atom in molecular (AIM) analysis were performed. As shown in Tab. 1, the natural atomic charge on each Be atom bonded with the C4H42- are between +0.59|e| and +0.80|e|. The total natural charges of all Be atoms in these complexes are in the range of +1.38|e| to +1.60|e|. Moreover, the Wiberg bond indexes (WBIs) for the Be-C linkage are in the range of 0.09 to 0.12, which indicate that the Be-C4H42- binding in [Ben(C4H4)2]2- (n=2~8) is mainly ionic bonding. This may be due to the relatively large electro-negativity difference between C and Be. The WBIs 0.40~0.93 for Be-Be in [Be2(C4H4)n]2-(n=2~8) clearly indicate that all Be-Be bonds in these complexes are close to covalent single bond. The C-C WBIs of [Ben(C4H4)2]2- (n=2~8) species are equal to 1.23, suggesting the existence of electronic delocalization in the C4H42- ligands.
The nature of the Be-Be and Be-C bonding can also be revealed by AIM analysis. Here, we only take D4d [Be2(C4H4)2]2- as an example. The molecular graphs of total electron density for D4d [Be2(C4H4)2]2- is shown in Fig. 3. As depicted in Fig. 3, the Be-Be bond of [Be2(C4H4)2]2- is different from classical metal-metal bond. Similar to Be2(C5H5)2[48], there is a nonnuclear attractor (NNA), or (3, -3) CP close to the midpoint of the Be-Be internuclear vector in electron density of [Be2(C4H4)2]2-. It is not bound by one metal-metal bond but rather by two metal-NNA bonds.
Tab. 2 lists the topological properties at the NNA and BCPs of [Be2(C4H4)2]2-. The Laplacian of the density (▽2ρ(NNA)) at the NNAs is negative, indicating the local concentration of electron density. Between the Be atom and the NNA, there is a bond critical point (BCP). As shown in Tab. 2, both the ▽2ρ(rc) and E (rc) values of the Be-Be bond are negative, and the -Gc/Vc value is far below 0.5; all of these values suggest that the Be-Be bond possess covalent character. Small ρ(rc) and positive ▽2ρ(rc) indicate that the interactions between the ligands and Be atom are mainly ionic.
In addition, molecular orbital (MO) analysis can also help us to further understand the bonding characters of these complexes. For the sake of clarity, only seven selected occupied molecular orbitals of [Be2(C4H4)2]2- are depicted in Fig. 4. As shown in Fig. 4, the highest occupied molecular orbital (HOMO) is symmetrical about the Be-Be axis and corresponds to the Be-Be σ single bond, which is mainly from the 2s orbitals interactions of two Be atoms. HOMO-1, HOMO-2 and HOMO-7 MOs are mainly directly inherited from three delocalized π orbitals of the C4H42- ligands, which only had a little contribution from the Be atom. MO analysis is consistent with our above argument that the Be-Be bond of [Be2(C4H4)2]2- is covalent σ single bonds, while the bonding between ligands and Be22+ center is mainly ionic.
Generally speaking, the electronic structure of dianions is usually unstable due to large electronic repulsion, such as [Ti (P5)2]2-[49]. It should be pointed out that D4d [Ben(C4H4)2]2-(n=2~8) sandwich complexes are no exception, which possess positive HOMO energies (+0.91~+3.37eV) and appears to be unstable towards detachment of the outermost valence electrons. In corporation of Li+ cations while keeping the basic structures of the dianions unchanged, was an effectively approach to stabilize these novel sandwich complexes. When two Li+ cations approach the D4d [Ben(C4H4)2]2- along the four-fold molecular axis in opposite directions, the sandwich series D4d [Ben(C4H4)2]Li2 (n=2~8) are produced. The optimized geometries of D4d [Ben(C4H4)2]Li2(n=2~8)，along with the bond lengths at B3LYP level are depicted in Fig. 5. The calculated electronic properties are summarized in Tab. 3. These high-symmetry sandwich complexes all proved to be stable true minima with real frequencies. As shown in Tab. 3, these lithium salt compounds are confirmed to have the negative HOMO energies (-3.38~-4.05 eV). The Be-Be and Be-C bonds of [Ben(C4H4)2]Li2 are shorter than that of the [Ben(C4H4)2]2-, only except for the terminal Be-Be bonds in [Ben(C4H4)2]Li2(n=7, 8). Two Li+ cations located at 1.82~1.93 Å above the center of C4H42- rings in the [Ben(C4H4)2]Li2 (n=2~8). The Li+ ionic interactions are clearly demonstrated by the fact that Li atoms in these sandwich complexes carry high calculated nature atomic charges (equal to +0.93|e|).
The stabilities of these novel -Be-Be-chain sandwich complexes are supported by the negative thermodynamic quantity changes of the following processes starting from stable Ben(C5H5)2.
Ben(C5H5)2+2C4H4Li2=[Ben(C4H4)2]Li2 + 2C5H5Li
With zero-point corrections, the disproportion reactions have the negative total energy changes (-24.20~-27.16 kcal/mol) at the B3LYP level, indicating that the replacements of C5H5- ligands in D5h Ben(C5H5)2 with D4h C4H4Li2 are overall favored in energy. Based upon this observation, we propose the possibility to synthesize these novel sandwich complexes through C4H4Li2/C5H5- ligands exchange reactions. It should be noted D4h C4H4Li2has not been experimentally reported. Fortunately, its substituted dilithium salt C4(Si (CH3)4Li2[50, 51] has been synthesized and characterized in experiments. Thus, using the C4(Si (CH3)4Li2 as reactants to synthesize these -Be-Be-chain sandwich complexes will be more effective. Detailed kinetic studies are out of the reach of available resources at this stage.
In order to further investigate the possibility of these -Be-Be-chain sandwich complexes, we calculated the change in energies for the metal extrusion reaction [Ben(C4H4)2]Li2=[Ben-1(C4H4)2]Li2 + Be. As depicted in Fig. 6, the removal of a metal from [Be2(C4H4)2]Li2 is an endothermic process (58.24 kcal/mol), suggesting that the formation of [Be2(C4H4)2]Li2 is a favorable process. It should be noted that the energies changes drastically when the numbers of beryllium atoms sandwiched by two C4H4Li-rings are higher than two. However, the ΔE for the reaction are more than 27.69 kcal/mol, indicating that the formation of [Ben(C4H4)2]Li2 (n>2) is favorable in experiment under standard condition. As shown in Tab. 3, [Ben(C4H4)2]Li2 (n=2~8) possess large HOMO-LUMO energy gaps (>1.88 eV), which further support their stability.
In summary, we have presented in this work theoretical evidences for the possible existence of these novel -Be-Be-chain complexes [Ben(C4H4)2]2- and [Ben(C4H4)2]Li2 (n=2~8). NBO and AIM analysis indicate the covalent Be-Be bonds and mainly ionic bonding between the Be and the C4H42- ring in these sandwich complexes. The proposal of replacing C5H5- in Ben(C5H5)2 with C4H4Li2 to form D4d [Ben(C4H4)2]Li2 may represent a step forward towards the synthesis and characterization of these sandwich complexes. [Ben(C4H4)2]2- and [Ben(C4H4)2]Li2 (n=2~8) complexes are favored in energy and could be targeted in future experiments.
2016,
Vol. 79 Issue (8): 760-767
PDF
2016,
Vol. 79 Issue (8): 760-767 PDF