Metal Complexes of Monocarbon Carboranes: A Neglected Area of Study?

Thomas D. McGratha; F. Gordon A. Stonea, *    a Department of Chemistry and Biochemistry, Baylor University, Waco, TX 76798-7348, USA
* Corresponding author. email address: gordon_stone@baylor.edu

I INTRODUCTION

The first metallacarboranes were isolated in M. F. Hawthorne's laboratory and contained a metal ion, two carbon and nine boron atoms forming an icosahedral {closo-MC2B9} cage structure.1,2 It was immediately recognized that these species may be viewed as having metal ions coordinated in a pentahapto manner by the open face of a [nido-7,8-C2B9H11]2– dianion. This was a useful formalism since it emphasized an isolobal relationship between the carborane dianion and the ubiquitous [C5H5]– ligand. Following isolation of metal dicarbollides from reactions between metal salts and salts of [nido-7,8-C2B9H11]2–, it was logical that the monocarbon trianion [nido-7-CB10H11]3– would react in a similar way to afford monocarbon metallacarboranes also with icosahedral frameworks. Indeed, a few complexes of this kind were isolated3 soon after the first dicarbon analogues were discovered. Importantly in the early work, two types of monocarbollide metal compound were characterized. In the first a metal ion is sandwiched between [nido-7-CB10H11]3– ligands, as in the FeIII complex [commo-2,2′-Fe-(closo-2,1-FeCB10H11)2]3– (1) (Chart 1). In the second the cage-carbon atom carries an NR3 or an NR2 group, with the metal ion sandwiched between [7-NR3-nido-7-CB10H10]2– groups, as in the FeIII complex [commo-2,2′-Fe-(1-NH3-closo-2,1-FeCB10H10)2]– (2), or [7-NR2-nido-7-CB10H10]3– groups as in [commo-2,2′-Ni-(1-NH2-closo-2,1-NiCB10H10)2]2– (3). The zwitterionic ligands [7-NR3-nido-7-CB10H10]2– are isolobal with the dicarbollide anion [nido-7,8-C2B9H11]2–, whereas the trianions [7-NR2-nido-7-CB10H10]3– are isolobal with [nido-7-CB10H11]3–.

In what has become a very large field of study during the past 40 years, metallacarborane chemistry has primarily focused on species with cages containing two or more carbon atoms,4,5 whereas monocarbollide metal complexes have received very little attention.4,6,7 The neglect of this area is somewhat surprising because monocarbollide metal complexes having [nido-7-CB10H11]3–, [7-NR3-nido-7-CB10H10]2–, or [7-NR2-nido-7-CB10H10]3– groups would be expected to display reactivity patterns different from those of the corresponding dicarbollide species. In particular, the higher formal negative charge associated with the trianions [nido-7-CB10H11]3– and [7-NR2-nido-7-CB10H10]3–, compared with the dianion [nido-7,8-C2B9H11]2–, renders metal complexes of the trianions more reactive towards electrophiles. As will be described later this feature provides avenues for introducing functional groups into a carborane cage, a topic of growing interest.

In metallacarborane chemistry it can be argued that it is more profitable to study molecules having half-sandwich ‘piano-stool’ structures than those with ‘full-sandwich’ structures. This is because in the piano-stool complexes the metal is ligated on one side by the carborane cage systems [{7,8-R2-nido-7,8-C2B9H9} (R=H or Me), {nido-7-CB10H11}, {7-NR2-nido-7-CB10H10}, or {7-NR3-nido-7-CB10H10}] and on the other side by the conventional ligands (CO, PR3, CNR, alkynes, etc.) of coordination chemistry. With a combination of different coordinated groups within the coordination sphere of the metal there is the probability of reactions occurring between the ligands and other substrate molecules, and also with the carborane cage itself, with the latter thus adopting a non-spectator role in the chemistry derived.8 Both mono- and di-carbollide metal carbonyl half-sandwich complexes are especially desirable as synthons. They have isolobal relationships with cyclopentadienide metal carbonyls that are known to function as precursors to numerous other species through the lability of their carbonyl groups. Dicarbollide metal carbonyls display a very extensive chemistry, as shown by [3,3,3-(CO)3-closo-3,1,2-RuC2B9H11] (4),9 thereby pointing to the desirability of obtaining related monocarbollide metal carbonyl species for use in synthesis (Chart 2).

In an attempt to redress the imbalance between studies on the dicarbollide and monocarbollide metal compounds we began a comprehensive study of the latter, concentrating our studies on the piano-stool-type complexes for the reasons given above. Our progress to date in this area will be the subject of this review. So far, most of the work has involved compounds in which the metal is one of the 12 vertexes in a {closo-2,1-MCB10} cage system, thereby complementing the host of studies made on their {closo-3,1,2-MC2B9} counterparts. However, as will also be described, preliminary investigation of 11-vertex {closo-1,2-MCB9} systems is revealing the existence of unprecedented molecular structures in the metallacarborane field. We do not review in this chapter recent developments in the chemistry of monocarbollide–metal complexes in which the cage-carbon carries an NR3 or NR2 group because we have recently given an account of such species.10

II SYNTHESIS

A Triruthenium and Triosmium Complexes

The methodologies used to prepare monocarbollide metal carbonyls resemble those used to obtain cyclopentadienide metal carbonyls. The latter are generally obtained by one of two methods: heating cyclopentadiene or a substituted cyclopentadiene with a metal carbonyl or a metal carbonyl anion, or treating the carbonyl or a halo derivative of it with a salt of the cyclopentadienide ion. Similarly, procedures involving either heating a nido-carborane with a metal carbonyl or treating a metal carbonyl or a carbonyl-metal halide with the salt obtained by deprotonating a nido-carborane have afforded monocarbollide metal carbonyl complexes.

Thus, in tetrahydrofuran (THF) at reflux temperatures [Ru3(CO)12] and [NHMe3][nido-7-CB10H13] react to give an anionic trinuclear ruthenium complex [PPh4][2,2-(CO)2-7,11-(μ-H)2-2,7,11-{Ru2(CO)6}-closo-2,1-RuCB10H9] (5)* (Chart 3) following the addition of [PPh4]Cl.11 Subsequent analysis of this system revealed evidence for traces of the mononuclear species [NHMe3][2,2,2-(CO)3-closo-2,1-RuCB10H11] (6) in the initial product mixture, but its isolation in a pure form proved impossible.12 In contrast, the corresponding reaction between [Os3(CO)12] and [N(PPh3)2][nido-7-CB10H13] in refluxing bromobenzene affords an approximately equimolar mixture of the analogous triosmium cluster [N(PPh3)2][2,2-(CO)2-7,11-(μ-H)2-2,7,11-{Os2(CO)6}-closo-2,1-OsCB10H9] (7) and the monoosmium complex [N(PPh3)2][2,2,2-(CO)3-closo-2,1-OsCB10H11] (8).12 The structures of the anions of 5 and 7 are similar to that of neutral [3,3-(CO)2-1,2-Me2-4,8-(μ-H)2-3,4,8-{Ru2(CO)6}-closo-3,1,2-RuC2B9H7] (9) obtained from the reaction between [Ru3(CO)12] and 7,8-Me2-nido-7,8-C2B9H11.13 In all the trimetal species a {nido-CB10} or {nido-C2B9} framework bridges a triangular arrangement of ruthenium or osmium atoms with the open or faces, respectively, coordinated in a pentahapto fashion to one metal atom while the carborane cage forms two exo-polyhedral B–HM bonds with the other two metal atoms.

Surprisingly, instead of affording a mixture containing the anions of the cluster compound 7 and the mononuclear species 8, the complex [12-NMe3-2,2,2-(CO)3-closo-2,1-OsCB10H10] (10) is obtained by heating [Os3(CO)12] with [NHMe3][nido-7-CB10H13] in refluxing bromobenzene. The NMe3 group is attached to a boron atom in the pentagonal belt lying above that of the ring 5-coordinated to the metal.12 The source of the trimethylamine group must be the cation [NHMe3]+, but the pathway by which its NMe3 fragment migrates to the cage is not clear.

B Mononuclear Compounds of Iron, Molybdenum, Tungsten, Rhenium, Platinum, Nickel and Cobalt

Reactions between salts of [nido-7-CB10H13]– and [Fe3(CO)12] afford the mononuclear anionic iron compound [2,2,2-(CO)3-closo-2,1-FeCB10H11]–, typically isolated as its [N(PPh3)2]+ salt (11) (Chart 4).14 No anionic triiron complex analogous to 5 and 7 is formed in this reaction. The anionic mononuclear iron, ruthenium and osmium complexes and the previously mentioned neutral mononuclear ruthenium dicarbollide complex 4, obtained from [Ru3(CO)12] and...