J Mol Model (2015) 21:194 DOI 10.1007/s00894-015-2744-8

ORIGINAL PAPER

Construction of double- and triple-decker sandwich compounds from half-sandwich compounds: a theoretical assessment Mei Zhang 1,2 & Xueying Zhang 1 & Lingpeng Meng 1,2 & Qingzhong Li 3 & Xiaoyan Li 1,2

Received: 10 January 2015 / Accepted: 25 June 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract The viability and properties of double- (Cp2M) and triple-decker (Cp 3 M 2 ) sandwiches formed from halfsandwiches (CpM, Cp=C5H5; M=Li, Na, K, Be, Mg, Ca, Fe, Co, Ni, Cu, and Zn) are discussed based on the geometry, energy, HOMO-LUMO gap, and topological properties. The calculated results show that the alkali metals and transitional metals (Fe, Co, Ni) with more unpaired electron are more inclined to form high-symmetry sandwich complexes than the alkaline earth metals. The Cp2M and Cp3M2 symmetries for M=Cu and Zn are low. In Cp2M and Cp3M2, the electrostatic and π orbital interactions are dominant. For Cp3M2, the contributions of orbital interaction to the total M-C interaction and of σ-type interaction to the orbital interaction are larger than those in Cp2M. The nature of the M-C bond is well correlated to its bond length. The shorter the M-C bond, the more covalent it is.

Keywords Dissociation energy . Sandwich compounds . Topological analysis of electron density Electronic supplementary material The online version of this article (doi:10.1007/s00894-015-2744-8) contains supplementary material, which is available to authorized users. * Xiaoyan Li [email protected] 1

College of Chemistry and Material Science, Hebei Normal University, Road East of 2nd Ring South, Shijiazhuang 050024, China

2

Key Laboratory of Inorganic Nano-materials of Hebei Province, Shijiazhuang 050024, China

3

The Laboratory of Theoretical and Computational Chemistry, Science and Engineering College of Chemistry and Biology, Yantai University, Yantai 264005, China

Introduction The chemistry of sandwich complexes continues to attract a great deal of attention [1–12] due to their successful applications in many areas of chemistry such as olefin polymerization catalysis [13], asymmetric catalysis [14], C-H bond activation [15–18], and bioorganometallic chemistry [19]. Typically, transition metals can form a strong, covalent, and symmetric η5 bond to a Cp group. In contrast, main-group metals exhibit a bewildering variety of bonding, including electron-precise, electron-excess, and electron-deficient structures [20, 21]. The terms electron-precise, electron-excess, and electron-deficient mean that the compound has the precise number, too many, and too few valence electrons, respectively, for the connections between the atoms to be described as covalent bonds. Based on a large number of theoretical examinations and a comparison with experimental structures, Budzelaar and coworkers deduced that the electron-precise structure is caused by the preference for electronic saturation (8e rule, similar to the TM 18e rule). A high degree of ionicity leads to increased hapticity and hence to excess-electron structures [21]. Generally speaking, when two or more of the half-sandwich complexes CpM approach each other, various isomers can form, including the σ-type (CpM-M’Cp) and π-type (CpMCpM’) [20–22]. Due to their special structures, properties, and extensive applications, a variety of binuclear and multinuclear metallocenes have been intensively studied experimentally and theoretically since the identification of the ferrocene (Cp2Fe) structure in 1952 [4, 6, 23–30]. Frenking et al. reported the geometries and metal–ligand bond dissociation energies of an inverted sandwich [E-Cp-E’]+ complex (E, E’= group 13 element; Cp =cyclopentadienyl) [31]. Schaefer et al. [32] studied the electronic structure of (η5-C5H5)2 M2 (M=Ni, Cu, Zn). They obtained the molecular orbital diagram

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for the dimetallocene and compared the coaxial structure with the perpendicular structure. The orbital analysis and dissociation energies of [M2(η5-Cpx)2] (M=Zn, Cd, Cpx=C5Me5, C5H5) were calculated by Fang and co-workers[33]. The nature of the metal-ligand bonding in ferrocene and bis(benzene)chromium has been analyzed using an energy partitioning scheme by Frenking and co-workers. The calculated results show that bis(benzene)chromium is a δ-bonded molecule and ferrocene is a π-bonded molecule [34]. A series of donor-acceptor heteroleptic open sandwiches with the formula CpM-M’Pyl (M = B, Al, Ga; M’ = Li, Na; Cp = cyclopentadienyl; Pyl=pentadienyl) has been designed using density functional theory by Merino et al. [35] in 2006. Cerpa et al. [36] studied the structure and bonding in C5H7E (E=LiCs) and their analogues with density functional theory (DFT) calculations. In addition, the analogous metallocenes, such as CpMnCp (M=Be, Mg, Ca, and Zn with n=2–5) [37], Fe(η5E5)2 and FeCp(η5-E5) (E=N, P, As, Sb) [38], [Ti(η5-E5)2]2 − (E=CH, N, P, As, Sb) [39], Zn2(η5-Cp*)2 [40], Cp−Zn− Cd−Cp [41], (η5-P5)-MM’(η5-P5) and (η5-C5H5)MM’(η5-P5) (M, M’=Zn, Cd) [42], were investigated. Recently, main group metallocenes have become more important because of their structural fluxionality and synthetic utility in organometallic chemistry. Several good reviews of synthetic procedures, structural characterization, and other experimental features of main group metallocenes are currently available [6, 23–25]. Burkey et al. [25] summarized the computational studies on the main group half-sandwich and fullsandwich metallocenes. The bonding characters of main group metallocenes are different from those of the transition metal metallocenes due to the lesser involvement of the d orbitals. The triple-decker sandwich compound Cp3Ni2+ was subsequently confirmed by Werner and Salzer [43, 44]. However, studies of triple-decker sandwiches are still rare. Siebert synthesized the triple-decker sandwich complexes of Co, Ru, Rh bridged by C2B3/C3B2 ring [45] and Bib(cyclopentadienylmeta1)-p-1,3-diborolenyl complexes with 29–34 valence electrons [46]. Sitzmann synthesized the neutral triple-decker complexes of the heavy alkaline earth metals and ytterbium with tetraisopropylcyclopentadienide anions as terminal ligands and a cyclooctatetraene dianion as a middle deck [47]. By using the frontier orbital of MCp and M(CO)3 fragments, the electronic structures of the tripledecker sandwiches CpMCpMCp and (CO)3MCpM(CO)3 were analyzed by Lauher [48]. Canadell et al. optimized the structures of C5H5M (M=In, T1, Sn+) and suggested that the bonding between In/T1 and cyclopentadienyl has a large covalent component. The polymeric cyclopentadienylindium was analyzed using extended Hückel tight binding calculations [49]. Although numerous theoretical studies have been devoted to double- and triple-decker sandwiches [32–42, 48, 49], most of them focus on the electronic structures. The viability of

double- and triple-decker sandwiches from half-sandwiches needs to be investigated; the results of such investigations can provide useful information for chemical syntheses and applications of these complexes. In this work, the viability and properties of double- (Cp2M, M=Li, Na, K, Be, Mg, Ca, Fe, Co, Ni, Cu, Zn) and triple-decker (Cp3M2) sandwich compounds are assessed from the standpoint of theoretical approaches. The nature of the interactions between M and the Cp ring is investigated using the topological analysis of electron density [50]. Moreover, the regularity of the same group and same period elements, as well as the similarities and differences between the transition metals (TM) and main group metals (MM) in different types of sandwich complexes are compared.

Computation methods For all of the studied compounds, quantum chemical calculations are performed at the B3LYP/6-311++G(d, p) and M062X/6-311++G(d, p) levels with the Gaussian 09 package of programs [51]. Vibration analysis was used to check the nature of the stationary points at the same level of theory. The decomposition of interaction energies was carried out using the ADF2008.1 program [52]. TZP basis sets were used for Fe, Co, and Ni, and DZP basis sets were used for the other elements. The bond characters were analyzed by using the atoms in molecules (AIM) theory of Bader and were carried out with AIMALL [53] programs.

Results and discussion Equilibrium geometry The optimized geometries of CpM, Cp2M and Cp3M2 (M=Li, Na, K, Be, Mg, Ca, Fe, Co, Ni; Cp=C5H5) are shown in Fig. S1 (Supporting information), and the molecular graphs are shown in Fig. 1. For convenience, the half-sandwich, sandwich, and triple-decker sandwich compounds are labeled as H-, S-, and T- sandwich, respectively. The optimized geometry parameters, charges, and spin multiplicities of the studied compounds are listed in Table 1. As shown in Table 1, the geometries calculated at the B3LYP and M06-2X levels are similar, with all CpM exhibiting C5v symmetry. Most of the Cp2M and Cp3M2 exhibit D5h symmetry, except for M= Be, Cu, and Zn. In these compounds, the two ligands adapt a parallel and eclipsed configuration. The metal atoms M (M= Li, Na, K, Mg, Fe, Co, and Ni) and the two centers of the Cp ring lie along the same line. The Cp2M symmetries for M=Be, Cu, and Zn are C2h, C2h, and C1, respectively. The Cp3M2 compounds (M=Be, Cu, Zn) exhibit C2 symmetry. In summary, the alkali cations tend to form the high symmetry S- and T-

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Fig. 1 Molecular graphs of the studied complexes (a. CpM; b. Cp2M; c. Cp3M2; d. Cp2Cu; e. Cp2Zn; and f. Cp3M2’(M’ =Be, Cu, Zn). BCP in green)

sandwiches. The symmetries of the Cp2M and Cp3M2 for M= Fe, Co, and Ni are higher than those for M=Cu and Zn. That is, the alkali main group cations and the transition metal cations with high unpaired electrons are more inclined to form the high symmetry S- and T-sandwich compounds than the alkaline earth metal cations. Because Cu and Zn cannot form the D5h symmetry S- and T-sandwich complexes, the Cp2M and Cp3M2 for M=Cu and Zn are not included in the following discussion. Comparison of the B3LYP and M06-2X results shows that although the calculated distances between the metal to the center of Cp at M06-2X are slightly smaller than those calculated at the B3LYP level, the trends in dMM-π and dTM-π value

changes in H-, S-, and T-sandwiches are the same. Both methods obtain values that are close to the experimental results. Therefore, the following discussions are based on the geometries calculated at the B3LYP level. For CpM, as shown in Table 1, the distances between MM and the center of Cp increase (dMM-π) in the order of Li

Construction of double- and triple-decker sandwich compounds from half-sandwich compounds: a theoretical assessment.

The viability and properties of double- (Cp2M) and triple-decker (Cp3M2) sandwiches formed from half-sandwiches (CpM, Cp = C5H5; M = Li, Na, K, Be, Mg...
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