Mikhail V. Butovskii

Molecules containing rare-earth atoms solely bonded by transition metals

Although metal-metal bonding is important in the chemistry of both solid-state intermetallic compounds and molecular species, the study of this bonding is limited by the compounds available and it is rarely possible to identify connections between these two areas. In this study, molecular intermetalloids [Ln(ReCp(2))(3)] (Ln = Sm, Lu and La) have been synthesized that contain lanthanoid metals bound only to transition metals. Although they are highly reactive species, such lanthanoid-core transition-metal-shell compounds can be stable in solution. They mimic the bonding situation of intermetallic compounds, as revealed by a direct comparison of molecular and solid state lanthanoid-transition metal bonding.

f-Element–Metal Bonding and the Use of the Bond Polarity To Build Molecular Intermetalloids

Metal-metal bonding in heterobimetallic complexes is of fundamental interest due to its implications to both bonding theory and new reactivities. In this Concept, structurally authenticated molecular compounds with direct bonds between rare-earth metals or actinoids and transition or main group metals are summarized. Special attention is given to the use of bond polarity as a tool for designing molecular intermetalloids incorporating rare-earth atoms and transition metals.

Molecular Lanthanoid–Transition-Metal Cluster through CH Bond Activation by Polar Metal–Metal Bonds

Metal–metal bonds have been fascinating scientists for long time and nowadays a lot of enthusiasm is devoted to unsupported metal–metal bonds. Until now unsupported Ln–TM bonds (Ln = lanthanoid, TM = transition metal) could only be found in a few compounds. These bonds are rather polar 11] and are important for the fundamental understanding of bonding phenomena between these metals. An improved understanding of a Ln–TM bond is important because intermetallic compounds of these metals play an important role in everyday life. The high bond polarity should allow a systematic approach towards highly aggregated systems. 15] To date there has been little exploration of the reactivity of such Ln–TM bonds. Herein we show how metal clusters can be prepared by multiple C H bond activations at Ln–TM bonds, which leads to the formation of doubly deprotonated Cp ligands. (Cp = cyclopentadienyl). The starting point of this reaction sequence is the fourcoordinate rare-earth-metal compound 2 which has a chiral lanthanoid atom. We recently explored the reaction of tris(alkyl) Ln compounds with [Cp2ReH] and ascertained that in addition to triply Re-bonded Ln complexes, polymeric insoluble byproducts are formed in bulk (66–99 %). Since the reaction of [Cp2Y(thf)(CH2SiMe3)] (Me = methyl) with the above-mentioned rhenium hydride proceeds in very good yields, it was suspected, that the presence of one Ln–carbon bond brings about side reactions of the Ln–TM bond leading to those polymeric materials. Now, if one wants to understand and to use such (side) reactions purposefully, a bis(alkyl) Ln compound which allows the substitution of one of the two alkyl ligands by Cp2Re-ligands should be exploited. The reaction of [Lu(thf)2(CH2SiMe3)3] [16] with one equivalent of 2,6-di-tertbutylphenol affords bis(alkyl) 1 in high yields (Scheme 1). The new complex reacts selectively with one equivalent of [Cp2ReH] to yield compound 2 (Scheme 1). The molecular structure of 2 as determined by X-ray structural analysis is shown in Figure 1. The lutetium ion in 2 is four-coordinated, in a tetrahedral environment, and is chiral owing to the different substituents. The selective introduction of four different substituents appears to be complicated for rare earth ions, which have a tendency for very high coordination numbers. The Lu–Re distance is 2.8498(6) and is significantly shorter than the Lu–Ru distance of 2.955(2) in [Cp2(thf)Lu-Ru(CO)2Cp] [6] and almost identical with the average value of the Lu–Re bonding distances in [Lu(ReCp2)3] [9] [2.886(1)]. The Lu C bond in 2 is 2.359(10) and complies with the expected value for such a bond (2.3781 ). The H NMR spectrum of 2 shows strong temperature dependence (at 188–295 K; see the Supporting Information). By virtue of the chirality, the signal belonging to the protons of the CH2-group of the alkyl ligand appears as AB spin system. By analogy the protons of the coordinated THF ligand should display more than two groups of signals. At room temperature, however, only two broad signals are observed for the H atoms of THF ligand and for the CH2 group merely one broad signal. Upon cooling to 253 K signal separation occurs and for the CH2 group (typical) geminal Scheme 1. Synthesis of 2.

Lanthanoid-transition-metal bonding in bismetallocenes.

Bismetallocenes [Cp2 LuReCp2 ] and [Cp*2 LaReCp2 ] (Cp=cyclopentadienyl; Cp*=pentamethylcyclopentadienyl) were prepared using different synthetic strategies. Salt metathesis-performed in aromatic hydrocarbons to avoid degradation pathways caused by THF-were identified as an attractive alternative to alkane elimination. Although alkane elimination is more attractive in the sense of its less elaborate workup, the rate of the reaction shows a strong dependence on the ionic radius of Ln(3+) (Ln=lanthanide) within a given ligand set. Steric hindrance can cause a dramatic decrease in the reaction rate of alkane elimination. In this case, salt metathesis should be considered the better alternative. Covalent bonding interactions between the Ln and transition-metal (TM) cations has been quantified on the basis of the delocalization index. Its magnitude lies within the range characteristic for bonds between transition metals. Secondary interactions were identified between carbon atoms of the Cp ligand of the transition metal and the Ln cation. Model calculations clearly indicated that the size of these interactions depends on the capability of the TM atom to act as an electron donor (i.e., a Lewis base). The consequences can even be derived from structural details. The observed clear dependency of the LuRu and interfragment LuC bonding on the THF coordination of the Lu atom points to a tunable Lewis acidity at the Ln site, which provides a method of significantly influencing the structure and the interfragment bonding.

Rare earth–metal bonding in molecular compounds: recent advances, challenges, and perspectives

Metal–metal bonding in heterometallic complexes is of fundamental interest due to its implications in both bonding theory and new reactivities. In this review, structurally authenticated molecular compounds with direct bonds between rare earth metals and transition or main group metals are summarized. Novel aspects of their syntheses, properties and reactivities are highlighted. Special attention is given to the synthetic challenges and the use of bond polarity as a tool for designing molecular intermetalloids incorporating rare earth atoms and transition metals.