Evolution of C₂B₄Carborane Chemistry:  from Early Years to the Present

Abstract

A review of the chemistry of polyhedral cluster complexes in which s-, p-, d- and f-block metals are incorporated mainly into C cage -trimethylsilyl-substituted C2B4 carborane cages is presented. While the main thrust of this review is on the results obtained in authors' laboratories, comparisons are made to similar systems and to those involving both large- and small-cage carboranes. In this way we hope to demonstrate the unique chemistry of the small-C2B4-cage systems that has emerged over the last 40 years. However, a full picture of the chemistry of these systems cannot be presented without heavy reference to the larger, more stable 11- and 12-vertex cages, which in many ways preceded their more diminutive cousins. The focus of much of the current research is directed toward systematizing the chemistry of metallacarboranes with the aim of promoting their use as possible electronic, ceramic, and/or catalytic materials. However, such endeavors are based on our knowledge about the fundamental interactions that are at work in determining the structures and properties of these cluster complexes. This review attempts to provide such an overview of small-cage heterocarborane chemistry.

Syntheses, Structures and Electronic Properties of Zwitterionic Iron(II) and Cobalt(II) Complexes Featuring Ambidentate Tris(pyrazolyl)methanide Ligands

“…Herein we describe the syntheses, structures and detailed bulk and spectroscopic studies of the electronic properties of homoleptic, zwitterionic 2 ] complexes. In addition, some density functional theory (DFT) studies have been used to support the findings of the presented experimental results.…”

“…So far, only the Au I compounds have been shown to be coordinated by the apical carbanion of R Tpmd to form a linear, two-coordinate complex containing a covalent AuÀC bond. [13,16] Since no homoleptic transition metal sandwich complex of the general formula [MA C H T U N G T R E N N U N G ( R Tpmd) 2 ] has been reported until now, it was of interest to check the accessibility, structure and electronic characteristics of such compounds and to compare them with the corresponding Tp R or R Tpm complexes. Previously, we concluded that k 3 N-Tp R and k 3 N-…”

“…The second possibility [Eqn. (2)] is only applicable if a suitable transition metal precursor consisting of a base (B, for example,…”

“…Alternatively, the zwitterionic 2 ] complexes can be prepared by pre-coordination of tris(pyrazolyl)methane ( Me Tpm) ligand to a divalent metal and subsequent deprotonation using suitable reagents such as sodium bis(trimethylsilyl)amide (NaNA C H T U N G T R E N N U N G (SiMe 3 ) 2 ), lithium diisopropylamide (LDA) or lithium alkyls (RLi). It should also be noted that as a result of the zwitterionic nature of the divalent M II complexes no counter-anions need to be involved, thus explaining the good solubility in non-polar solvents such as toluene.…”

“…The now familiar chemistry of metallacarboranes has its roots in the suggestion that the dicarbollide ligand, [h 5 -7,8-nido-C 2 B 9 H 11 ] 2À (A), and the corresponding carborane anions, are very similar to cyclopentadienide ligands. [2] The dicarbollide dianion has six delocalised electrons in pure p-type orbitals located on the open B 2 C 3 pentagonal face. [3] Typical ligands featuring both s-and p-donating properties include imides (B), [4] ketimides (C), [5] phosphorane iminates (D) [6] or the recently developed imidazolin-2-iminates (E), [7] which act as monodentate 2s-4p-electron donors.…”