Lanthanides with Unusually Low Oxidation States in the PrB₃^– and PrB₄^– Boride Clusters

Abstract

Lanthanide elements typically exhibit a +III oxidation state (OS) in chemical compounds with a few in +IV or even +V OS. Although lanthanides with +II OS have been observed recently in organometallic compounds, +I OS is extremely rare. Using a joint photoelectron spectroscopy and quantum theoretical study, we have found two low OS lanthanides in doped boron clusters, PrB3 – and PrB4 –. These two clusters are shown to have planar structures, in which the Pr atom is bonded to the aromatic boron clusters via two Pr–B σ bonds. Chemical bonding and electronic structure analyses reveal that the Pr atom is in a very low OS in the two boride clusters: +II in PrB3 – and +I in PrB4 –. The current finding suggests that there should exist a whole class of boride complexes featuring rather low-valent lanthanides and expands the frontier of lanthanide chemistry.

High‐Resolution Photoelectron Imaging of IrB3−: Observation of a π‐Aromatic B3+ Ring Coordinated to a Transition Metal

“…Joint PES and theoretical studies of lanthanide‐doped boron clusters have been reported recently, revealing both half‐sandwich and inverse‐sandwich complexes, as well as several transition‐metal diboride clusters . Most recently, a small lanthanide‐doped boron cluster, PrB 3 − , was found to be planar with Pr II [η 2 ‐B 3 3− ] coordination . The question we were trying to address in this study is whether three‐membered aromatic boron rings can coordinate with a transition metal in η 3 ‐B 3 coordination while retaining σ or π aromaticity.…”

Structural transformations in boron clusters induced by metal doping

“…Recently, some reports of clusters formed by three B atoms showed intriguing bonding and aromaticity. 35–40,44,46 However, since doping B 3 does not lead to any significant modification but rather to an energetic exchange between its two isomers, the linear and the triangular, we begin by describing clusters from four B atoms and, from the lowest to the highest, in our perspective, structural transformation degree. While most experimental techniques have favored the characterization of mainly anionic boron clusters, and in most cases, preceded the characterization of the neutral or cationic counterparts, the order shown for the bare systems is from cations to anions, since the former are the first to show a distortion of the boron skeleton, namely, the 2D to 3D transition.…”

“…43 Multiple B 4 clusters doped with one or two heteroatoms are described in the literature. In most cases, the resulting structure maintains a rhomboidal shape of the B moiety with one heteroatom on one edge, bridging two B atoms ( 4-3 ), as in the case of B 4 Pr − , 44 B 4 C − , 45 and bridging two opposite B atoms, as in the case of B 4 Bi 2 − , 46 or directly attached to a B atom, as in B 4 Mg ( 4-4 ). 47 For these examples, the modification in B skeleton is mild, as a result of the electron transfer from dopant to B 4 cluster.…”

Expanded Inverse-Sandwich Complexes of Lanthanum Borides: La2B10– and La2B11–

“…Boron forms a wide range of important boride materials, from the superconducting MgB 2 and superhard transition-metal borides to borides with ultrahigh thermal conductivity. − Lanthanide (Ln) borides constitute another interesting class of boride materials with unique magnetic, optical, superconducting, mechanical, and thermoelectric properties. − Over the past two decades, the structures and chemical bonding of size-selected bare boron clusters have been studied by joint experimental and theoretical methods, − discovering planar clusters, nanotubular structures, graphene-like borophenes, and fullerene-like borospherenes. − Transition-metal-doped boron clusters have led to a new direction of boron nanoclusters, giving rise to new structures and bonding, such as the metal-centered aromatic borometallic wheels − and tubular metal-centered drums. − However, lanthanide boride clusters have been rarely explored until recently, − despite the importance of bulk lanthanide boride materials.…”