Russell N. Grimes

Two-dimensional boron-11−boron-11 nuclear magnetic resonance spectroscopy as a probe of polyhedral structure: application to boron hydrides, carboranes, metallaboranes, and metallacarboranes

Une technique generale est mise en œuvre pour la determination directe des connectivites bore-bore dans tous les types de composes cages du bore, basee sur la spectroscopie RMN 11 B- 11 B a deux dimensions correlee en J. Discussion des resultats obtenus par cette methode

Metallacarboranes in the new millennium

Abstract The field of metallacarborane chemistry has grown in many different directions and now interfaces with several other disciplines including metal cluster and organometallic chemistry, transition-metal catalysis, materials science, environmental science, and biomedical applications. In this turn-of-the-century overview, some of the more significant recent developments and trends in this area are selectively examined and an attempt is made to project some useful directions in which this chemistry might develop over the next decade or two.

Metal-carborane multidecker sandwich complexes as building blocks for new materials

Transition-metal sandwich complexes incorporating bifacially coordinated C 2 B 3 planar carborane rings form a large and varied family of generally air-stable, highly robust compounds that feature multidecker stacking. As a group, these complexes offer a number of advantages that are potentially of interest in the development of new electronic, magnetic and/or optical materials. They are remarkably versatile, accommodating a wide range of metals and organic substituents ; they are soluble in organic solvents and are typically resistant to air and moisture ; they can be reversibly oxidized and reduced ; in many cases they are paramagnetic, exhibiting substantial electron delocalization of the unpaired electrons between metal centers. Moreover, the small carborane starting materials can now be prepared in large (ca 100 g) quantities, making the complexes readily accessible. A brief overview of this area is presented with emphasis on the systematic utilization of metallacarborane sandwich complexes in the construction of large multimetallic systems and studies of their electronic and molecular structures.

Direct insertion of transition metals into polyhedral carboranes. Structurally novel mono-, di-, and trimetallic small cage systems

Abstract : Metallocarboranes of iron, cobalt and nickel have been prepared by the direct reaction of the small polyhedral carboranes 1,5-C2B3H5, 1,6-C2B4H6, or 2,4-C2B5H7 with organometallic and metal carbonyl reagents in the gas phase or in solution, without the use of a prior cage-opening step. Novel 6-vertex cages as well as 7-vertex species were obtained, including mono-, di-, and trimetallocarborane species. Molecular structures were assigned to the new compounds on the basis of boron-11 and proton nmr spectra. Fine structure in the carborane CH proton nmr signals exhibited by several of the cobaltacarborane species was interpreted on the basis of H-C-B-H proton-proton coupling. (Modified author abstract)

Carbon-Rich Carboranes and their Metal Derivatives

Publisher Summary This chapter discusses the carbon-rich carboranes and their metal derivatives. The boron hydrides, with their peculiar, unconventional cage-like structures, have intrigued theorists for years, even as the degree of interest exhibited in them by industry and government has waxed and waned. Because boron has four valence orbitals but only three electrons, its binary hydrides have less than one electron pair per bonding interaction, yet they hold together very nicely by the trick of electron delocalization; that is, some of the electrons are not constrained to ordinary Lewis-type bonds between pairs of atoms, but can extend over three atoms. In order to facilitate this electron delocalization, borane molecules adopt cluster shapes that permit high connectivities between boron atoms and thus make the most efficient use of the available electrons. Hydrocarbons, in contrast, have exactly the right numbers of electrons for localized two-center bond networks of carbon and hydrogen atoms.

Bridge insertion reactions of the 2,3-C2B4H7− ion with aluminum, gallium, and transition metal reagents

Abstract The reaction of lithium or sodium salts of the 2,3-C 2 B 4 H 7 − anion with organometallic reagents of aluminum, gallium, rhodium, gold and mercury effects insertion of the metal atom into a bridging position on the base of the pyramidal carborane cage, with the metal apparently linked to the cage by a BMB three-center, two-electron bond. μ-Dimethylaluminum-2,3- nido -dicarbahexaborane(8) is extremely unstable in solution or in the liquid phase with respect to decomposition to 2,3-C 2 B 4 H 8 , but in the gas phase forms CH 3 AlC 2 B 4 H 6 . The gallium-bridged analog is thermally stable but reacts readily with HCl to generate C 2 B 4 H 8 and (CH 3 ) 2 GaCl; however, under no conditions studied did this material rearrange to the previously characterized closo -gallacarborane, 1-CH 3 -1,2,3-GaC 2 B 4 H 6 . The compounds μ-[(C 6 H 5 ) 3 P] 3 RhC 2 B 4 H 7 and μ-[(C 6 H 5 ) 3 P]AuC 2 B 4 H 7 were prepared and characterized as moderately air-sensitive crystalline solids, and a partially characterized, unstable material believed to be μ-C 6 H 5 HgC 2 B 4 H 7 was obtained.

Cluster forming and cage fusion in metallacarborane chemistry

Abstract Multidecker sandwich complexes in which transition metal centers are bridged by C 2 B 3 carborane rings are organometallic species by definition, and are close electronic and structural relatives of metallocenes and metal-arene sandwiches. At the same time, these complexes can be described as 7-vertex MC 2 B 3 M′ or MC 2 B 4 metallacarborane clusters and manifest chemical properties typical of that class, such as cage rearrangement and metal insertions into the polyhedral framework. Their “boron cluster” character is particularly evident in metal-promoted oxidative fusion reactions in which two 6-vertex nido -MC 2 B 3 cages are fused face to face to form a 12-vertex M 2 C 4 B 6 cluster. Such fusions often occur as competitive processes in the synthesis of multidecker sandwich complexes via metal stacking reactions. In this article we discuss the propensity of small metal-boron units to undergo clustering, centering on the factors that promote such reactions and strategies that can be employed to control and exploit them for synthetic purposes. Specific examples of unusual cluster geometries, some of them unique, that have been obtained via fusion will be treated from both synthetic and structural perspectives.

SMALL METALLOBORON CAGE COMPOUNDS AS ANALOGUES OF METAL CLUSTERS. UNIFYING CONCEPTS OF BONDING

The transition metal clusters and the boron cage compounds (boranes, carboranes, metalloboranes, and metallocarboranes) are closely related structurally and electronically, particularly to the extent that the bonding entails delocalization of electrons over the surface of a polyhedral (or polyhedral fragment) cage framework. Although this relationship has been tacitly acknowledged on occasion, until recently there has been no serious attempt to describe the polyhedral boranes and the metal clusters in terms of a uniform, generally applicable bonding scheme. Aside from the considerable differences in the synthesis and chemistry of these two large classes, one major distinction in the past has been the fact that the metal clusters rarely exceed six cage atoms, whereas prior to about 1970 most of the known carborane and metallocarborane polyhedra were much larger, the 12-atom icosahedral geometry being predominant. A second factor undoubtedly is the extraordinary variety in metal cluster structures, which has tended to discourage the formulation of broadly generalized bonding schemes and has led many workers to deal with the bonding in such compounds as individual cases. Thus, while boron chemistry is replete with isoelectronic and isostructural families, such themes have been less frequently emphasized in transition metal cluster chemistry. However, the recent rapid development of the metalloborane-metallocarborane field, including the preparation of species featuring metal-metal bonding in the polyhedral boron cage, has underlined the intrinsic relationship between the two classes. Indeed, Hawthorne’ has suggested that metallocarboranes may eventually be used as templates for the synthesis of metal clusters. Two recent developments form the basis for this discussion paper. The first is a highly useful theoretical contribution by Wade,’ who has demonstrated that a few simple bonding considerations based on the number of skeletal bonding electrons in cage and cluster compounds can be applied successfully, with considerable predictive power, to boranes, carboranes, and a wide variety of metal clusters and metallocenes. The second development is the preparation in the author’s laboratory of a number of small metalloboron compounds having seven-, sixand five-membered cages, which, despite their synthetic derivation from boranes and carboranes, are direct structural and electronic analogues of known metal clusters and can be considered as straightforward extensions of the latter class of compounds. We shall first outline the essential principles, and then present specific examples from recent synthetic work.

Cobalt-boron clusters

Abstract : The novel compounds (eta-C5H5)3C03B3H5 (I), (eta-C5H5)3C053B4H4 (II), and (eta-C5h5)4C04B4H4 (III) have been isolated as air-stable crystalline solids from the reaction of Na(+)B5H8(-) with CoCl1 and NaC5H5 below 20 degrees, and characterized from mass spectra and NMR spectra. The proposed structures of I, II, and III are respectively octahedral, capped octahedral, and dodecahedral. Compounds I and III are the first metalloboron species to have as many metal as boron atoms in the cage, III is the first tetrametallic metalloborane, and II is the only polyhedral boron cage molecule having a boron isolated from other boron atoms. The three compounds are closely related electronically and structurally to both the metal clusters and the boranes, and may be regarded as hybrids of the two classes.

Substituent NMR effects in triple-decked sandwich metallocarboranes

Abstract The electronic structure of the triple-decked metallocarborane complexes 1,7,2,3- and 1,7,2,4-(η5-C5H5)2Co2C2B3H5 has been examined via 11B and 1H pulse Fourier transform nuclear magnetic resonance spectroscopy of a series of derivatives containing substituents on the central (C2B3H54-) or end (C5H5-) rings. The results suggest that the 1,7,2,4 complex contains a highly electron-delocalized, metallocene-like central ring, while the 1,7,2,3 system is dominated by strong localπ-interactions between the metals and an ethylenic CC bond in the central ring. In both isomers, the substitution of CH3, C6H5, or Si(CH3)3 at a carboranyl ring carbon atom, or of Br or I at B(5), produces moderate to large changes in chemical shift (relative to the parent molecule) at all boron positions and at all hydrogens in each ring. End-ring substitution by CH3, C2H5, or Si(CH3)3 in each isomer generates strong chemical shift effects in the central ring, and the Si(CH3)3 species also show a small effect in the unsubstituted C5H5 ring. A direct trans-polyhedral electronic interaction between the metal atoms is proposed to account for the observed antipodal effects.