[(η⁶‐B₆X)₂M] (X=C, N; M=Mn, Fe, Co, Ni): A New Class of Transition‐Metal Sandwich‐Type Complexes

Abstract

Metallocenes, including ferrocene, [(h 5 -C 5 H 5 ) 2 Fe], and its 18-electron analogues [(h 5 -E 5 ) 2 M] (E = P, N, As, Sb, and Bi; M = transition-metal atom), have found important applications in both fundamental research and materials science.[ (n = 4, 6). [9][10][11] In these sandwich-type structures, the transition-metal center, M, is coordinated between two aromatic h n -E n monocycles (n = 4-6), each of which has 6-p electrons. The p-d interactions between the delocalized p molecular orbitals (MOs) of the ligands and the partially occupied d orbitals of the transitionmetal center play a crucial role in stabilizing the systems. Designing new forms of metallocenes and their sandwich-type analogues requires the right match between the monocyclic ligands and transition-metal centers, both geometrically and electronically. Inspired by the proposed 6-p electron aromatic D 6h B 6 C 2À ion featuring a carbon atom in a planar hexacoordinate environment (denoted phC) at the center of a perfect B 6 hexagon, [12] we present herein an investigation by density functional theory (DFT) of a new class of sandwich-type complexes, D 6d [(h 6 -B 6 X) 2 M] (X = C, N; M = Mn, Fe, Co, Ni). These complexes are unique in that they contain two parallel h 6 -B 6 X hexagons centered with two nearly planar hexacoordinate carbon or nitrogen atoms (phN) located along the sixfold molecular axis. The results obtained in this work provide an important extension to the traditional concept of sandwich-type complexes by incorporating hexacoordinate carbon or nitrogen atoms in the systems and present a viable possibility to stabilize and characterize phC or phN atoms in future experiments. To the best of our knowledge, there have been no investigations reported to date on B 6 C 2À ligands in metallocene-like complexes.DFT structural optimizations at the B3LYP/6-311 + G-(3df) level [13][14][15] were performed on the sandwich complexes under investigation and imaginary frequencies and DFT wavefunction instabilities checked at the same theoretical level. Natural bond orbital (NBO) [16] analyses were implemented to gain insight into the bonding pattern of the complexes. Figure 1 and Figure 2 depict the optimized structures of Fe-containing complexes, and Figure 3 shows the MO pictures of the top 13 occupied energy levels of D 6d [(h 6 -B 6 C) 2 Fe] 2À (with one orbital shown for two degenerate MOs). These optimum structures are well-maintained when symmetry constraints are removed. Co-, Ni-, and Mncentered complexes have the same geometries and similar MOs but have different energy levels. Table 1 tabulates the calculated bond lengths, natural atomic charges, lowest vibrational frequencies (ñ min ), highest occupied molecular orbital (HOMO) energies, and total Wiberg bond indices (WBIs) [12,17] of the constituent atoms for the complexes under investigation. Detailed geometrical and electronic properties of 28 neutral and charged complex ions that contain B 6 X 2À

Four Decades of the Chemistry of Planar Hypercoordinate Compounds

“…3) linked with alkali-metal cations and the hexagonal B 6 centered with Co rNmay be modified to form various heterocyclic ligands." [150] Similar sandwich-type motifs of main-group metals,K [(h 6 -B 6 C)Ca] n (h 6 -B 6 C)K (n = 1-3) and [(h 6 -B 6 C)Ca] n (h 6 -B 6 C) 2À (n = 1,2), were predicted in 2007 by Li and co-workers. [77] However,Liand co-workers transition-metal (Fe, Co,Ni) sandwich-type complexes may be difficult to prepare,s ince the configuration with two fused B 6 X( X= C, B, N) units is energetically much lower than that of sandwich-type complexes.F or example,t he energy of the fusion isomer of (B 6 N) 2 Fe is much lower than that of the sandwich-like form by 183.9 kcal mol À1 at the B3LYP/6-311 + G(d) + ZPVE level.…”

[Ta3O3]A (A = Li, Na, K) and [Ta3O3]B[Ta3O3] (B = Ca, Sr, Ba): Sandwich‐Type Complexes Containing Ta3O3– δ and π Double Aromatic Ligands

“…[11] The calculated NICS (2) values are close to the value of NICS(1) = -9.9 ppm calculated for C 5 H 5 -at the same theoretical level. Considering that fact that transition metals possess much bigger atomic sizes than carbon with expanded orbital distributions, we suggest that NICS (2) [13,14] As shown in Figure 1 and Table 1, the Ta 3 O 3 -units in these D 3h sandwich-type complexes have also been well preserved, and the O atoms are only slightly off-planed and the TaTa and Ta-O single bonds are well maintained. The alkaline-earth metal centers in these complexes are highly positively charged: q Ca = +1.28 |e|, q Sr = +1.38 |e|, and q Ba = +1.49 |e|, which indicates that there is increasing ionicity in the [ Ta 3 O 3 ] --B 2+ -[Ta 3 O 3 ] -series from B = Ca, Sr, to Ba.…”

“…As clearly shown in Figure 3, the top five MOs of free D 3h Ta 3 O 3 - [4] were essentially maintained in C 3v [ Ta [13,14] in which the Fe-C coordination bonds have bond orders of WBI Fe-C = 0.29 at the B3LYP level). As shown in Figure 3, -.…”

Transition metal–boron complexes BnM: From bowls (n = 8–14) to tires (n = 14)

“…In stark contrast with the three-dimensional structural units (like icosahedral B 12 and octahedral B 6 ) that dominate boron chemistry, small boron neutrals B n and anions B n À (n ¼ 3-15) have been confirmed to possess planar or quasi-planar geometries in recent photoelectron spectroscopy and density functional theory (DFT) investigations. [1][2][3][4][5][6][7] The planarity of B n clusters originates from their multiple aromaticity ( and ) and antiaromaticity in molecular orbital (MO) theory and the 2D-3D structural transition occurs to bare B n at tubular B 20 .…”

“…11 In these systems, however, the A-B n interactions are basically ionic. In a recent communication, 12 our group proposed a new class of sandwich-type complexes (B n X) 2 M (n ¼ 6, 7; X ¼ B, C, N; M ¼ Mn, Fe, Co, Ni), in which the transition metal center M is directly coordinated to two parallel [B 6 C] 2À or [B 7 B] 2À ligands. 13 In this work, we explore the possibility at DFT level to form transition metal-boron complexes B n M with bigger B n ligands in the size range of n ¼ 8-14.…”