Publisher Summary This is one of two articles in this volume that is concerned with the borane-carborane structural pattern. In the other, Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon 7∼ complexes, and various neutral or charged hydrocarbons. Boranes and carboranes may be regarded as cluster compounds in the sense defined by Cotton; they contain a finite group or skeleton of atoms held together entirely, mainly, or at least to a significant extent by bonding directly between those atoms, even though some other atoms may be associated intimately with the cluster. Examples of their structural pattern, however, can be found far beyond the confines of what is normally regarded as cluster chemistry, so this survey includes many systems not commonly referred to as clusters.

Structural and Bonding Patterns in Cluster Chemistry

Driven by the depletion of crude oil, the direct oxidation of methane to methanol has been of considerable interest. Promising low-temperature activity of an oxygen-activated zeolite, Cu-ZSM-5, has recently been reported in this selective oxidation and the active site in this reaction correlates with an absorption feature at 22,700 cm−1. In the present study, this absorption band is used to selectively resonance enhance Raman vibrations of this active site. 18O2 labeling experiments allow definitive assignment of the observed vibrations and exclude all previously characterized copper-oxygen species for the active site. In combination with DFT and normal coordinate analysis calculations, the oxygen activated Cu core is uniquely defined as a bent mono-(μ-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu2O]2+ core, which is directed into the zeolite channel, upon approach of CH4. This induces significant oxyl character into the bridging O atom leading to a low transition state energy consistent with experiment and explains why the bent mono-(μ-oxo)dicupric core is highly activated for H atom abstraction from CH4. The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction.

https://www.pnas.org/content/pnas/106/45/18908.full.pdf

A [Cu2O]2+ core in Cu-ZSM-5, the active site in the oxidation of methane to methanol

Chemical bonding models based on graph theory or tensor surface harmonic theory demonstrate the analogy between the aromaticity in two-dimensional planar polygonal hydrocarbons such as benzene and that in three-dimensional deltahedral borane anions of the type BnHn2- (6 < or = n < or = 12). Such models are supported both by diverse computational studies and experimental determinations of electron density distribution. Related methods can be used to study the chemical bonding in the boron polyhedra found in other structures including neutral binary boron hydrides, metallaboranes, various allotropes of elemental boron, and boron-rich solid-state metal borides.

Three-dimensional aromaticity in polyhedral boranes and related molecules.

The theoretical study of spin-crossover compounds is very challenging as those parts of the experimental findings that concern the electronic structure of these compounds can currently hardly be reproduced because of either technical limitations of highly accurate ab initio methods or because of inaccuracies of density functional methods in the prediction of low-spin/high-spin energy splitting. However, calculations with reparametrized density functionals on molecules of the thermal spin-crossover type can give improved results when compared with experiment for close-lying states of different spin and are therefore important for, e.g., transition metal catalysis. A classification of transition metal compounds within hybrid density functional theory is given to distinguish standard, critical, and complicated cases. From the class of complicated cases we choose the prominent spin-crossover compound Fe(phen)2(NCS)2 and show in a first step how the electronic contribution to the energy splitting can be calcul...

Theoretical study of the Fe(phen)(2)(NCS)(2) spin-crossover complex with reparametrized density functionals.

Spectres i.r. et Raman d'hétéropolyanions α - XM12O40n− de structure de type Keggin (X = BIII, SiIV, GeIV, PV, AsV et M = WVI et MoVI)

Publisher Summary Metallocarboranes are only been known for ten years, and research into their synthesis and characterization has involved a small number of workers. Practical applications of these unique compounds are not been rapidly forthcoming. Recent work has shown catalytic activity in certain of these compounds and may signify future commercial value and industrial importance of metallocarboranes. The preparative methods of metallocarboranes are discussed in this chapter, for the synthesis of all the known metallocarboranes has been accomplished by one or more of these routes. This chapter focuses on the synthesis, structures, properties, and reactions of η-bonded metallocarboranes. Complexes of 2-carbon carboranes and species that have between nine and fourteen total polyhedral vertices are described in the chapter. The approach to the subject has been to divide the metallocarboranes according to the size of the polyhedron–starting with twelve-vertex compounds, which constitute the majority of the effort, to the larger polyhedral, so far unknown in the B n H n 2 - and C 2 B n-2 H, series, and then to the lower polyhedral. Further subdivisions within each polyhedral size include synthesis, structures, and properties of monometallic complexes, reactions of monometallic, bimetallic preparations and reactions, and, in two instances, trimetallic compounds. This catalyst systemis extremely stable and may be recovered quantitatively from alkene isomerizations and hydrogenation reactions.

Ten Years of Metallocarboranes

Characterization of metal-oxygen bridge systems

: The polyhedral expansion of 4,5-C2B7H9 with ferrous chloride and sodium cyclopentadienide produced a new ferracarborane formulated as (C5H5)2-Fe2C2B6H8, which exists in paramagentic and diamagnetic forms. The crystal and molecular structure of the diamagnetic form has been determined by a three-dimensional x-ray diffraction study at - 160C. The compound crystallized in the monoclinic centrosymmetric space group P2(1)/n. The polyhedral geometry observed in this compound is a new 10-vertex species which is derived from a bicapped square antiprism. Structures of the title compound and of its cobalt analog are discussed in terms of the bonding theory of metallocarboranes.

Structures of metallocarboranes. V. Synthesis and crystal and molecular structure of the closo 20-electron bimetallocarborane 1,6-bis(.eta.-cyclopentadienyl)-1,6-diferra-2,3-dicarba-closo-decaborane(8), 1,6-(.eta.-C5H5)2-1,6,2,3-Fe2C2B6H8

: Biscarborane has now been prepared in 55-60% yield from B10H12:2S(CH2CH3)2 and diacetylene (previously 21%) An improved synthesis (70%) of ethynylcarborane has also been developed (Author)

An Improved Synthesis of Biscarborane and Its Precursor Ethynylcarborane.

: An x-ray crystal structure determination of ((CH3)4N)2(Ti(C2B10H10(CH3)2)2)-2(CH3)2CO was carried out at -160C. This investigation provides the first unambiguous structural evidence for a symmetrical sandwich complex of titanium. The geometry of the Ti(C2B10H10-(CH3)2)2 dianion can be viewed as two thirteen-vertex polyhedra fused through the titanium atom.

Structures of metallocarboranes. VI. Titanium sandwich complex. Crystal and molecular structure of [(CH3)4N]2[(1,6-C2B10H10(CH3)2)2Ti], bis(tetramethylammonium) 4,4'-commo-bis(decahydro-1,6-dimethyl-1,6-dicarba-4-titana-closo-tridecaborate)(2-) at -160.deg.

Kenneth P. Callahan

Papers

Ten Years of Metallocarboranes

Characterization of metal-oxygen bridge systems

Structures of metallocarboranes. V. Synthesis and crystal and molecular structure of the closo 20-electron bimetallocarborane 1,6-bis(.eta.-cyclopentadienyl)-1,6-diferra-2,3-dicarba-closo-decaborane(8), 1,6-(.eta.-C5H5)2-1,6,2,3-Fe2C2B6H8

An Improved Synthesis of Biscarborane and Its Precursor Ethynylcarborane.

Structures of metallocarboranes. VI. Titanium sandwich complex. Crystal and molecular structure of [(CH3)4N]2[(1,6-C2B10H10(CH3)2)2Ti], bis(tetramethylammonium) 4,4'-commo-bis(decahydro-1,6-dimethyl-1,6-dicarba-4-titana-closo-tridecaborate)(2-) at -160.deg.