The VCD method – a simple and reliable way to distinguish cage C and B atoms in (hetero)carborane structures determined crystallographically

Abstract

Replacing a boron vertex in a [B(n)H(n)](2-) cluster with a smaller atom, e.g. carbon, results in the distance from that atom to the polyhedral centroid being shorter. This forms the basis of a simple but very effective method of distinguishing between B and C atoms in (hetero)carboranes that have been studied crystallographically, the Vertex-to-Centroid Distance (VCD) method. Examples of well-characterised icosahedral and sub-icosahedral closo carboranes, nido carboranes, icosahedral metallacarboranes and supraicosahedral metallacarboranes are used to illustrate the generality of the VCD method. In this process a number of structures are identified in which the method suggests B/C disorder not previously recognised and some structures in which it appears that a cage C atom has been wrongly assigned. The largest sub-group of heterocarboranes is the family of 3,1,2-MC(2)B(9) compounds, and for these species consideration of Exopolyhedral Ligand Orientation (ELO) can sometimes be used to quickly suggest when a literature structure is suspect in terms of cage C atom positioning. The crystal structure of one such compound, 3,3-NO(3)-3-PPh(3)-3,1,2-closo-RhC(2)B(9)H(11), has been redetermined and the correct location of the cage C atoms established.

Hydridorhodathiaboranes: Synthesis, Characterization, and Reactivity

“…The reactions of picoline-ligated hydridorhodathiaboranes (3-5) with C 2 H 4 and CO in this report resemble those found for the pyridine analog, 2, giving products of ligand substitution and cluster dehydrogenation (12)(13)(14)(15)(16)(17)(18)(19). These reactions nicely illustrate the structural flexibility and chemical tunability of these 11-vertex clusters and demonstre that the parent rhodathiaborane, [8, ) 2 -nido-8,7-RhSB 9 H 10 ] (1), first reported in 1990, 4 is actually a rich source of organometallic chemistry.…”

The enhanced structural carborane effect

“…by metalecarborane distances. Nevertheless, clear evidence for this effect is available by consideration of metaleexopolyhedral ligand distances and exopolyhedral ligand orientations since the structural trans effect (trans influence) of cage C is less than that of cage B [7].…”

“…This arises because, as Mingos showed many years ago, the frontier molecular orbitals of a carborane ligand are relatively concentrated on the boron atoms in the ligand face [6]. However, because the atomic radius of C is less than that of B, the C atoms lie closer to the polyhedral centroid [7] and, everything else being equal, MÀC distances are actually shorter than MÀB distances, in spite of the C atoms being less strongly bound. Thus, in the archetypal species 3-Cp-3,1,2-closo-CoC 2 B 9 H 11 CoeC distances are 2.005 (2) and 2.009 (2) Å whilst CoeB distances are 2.069(2), 2.106(2) and 2.076 (2) Å [8].…”

“…by metalecarborane distances. Nevertheless, clear evidence for this effect is available by consideration of metaleexopolyhedral ligand distances and exopolyhedral ligand orientations since the structural trans effect (trans influence) of cage C is less than that of cage B [7].Given that fusing a {C 4 H 4 } diene fragment onto a Cp ligand (affording an indenyl ligand) transforms an otherwise evenly bound ligand (in terms of the strength of the MÀC links) into an unevenly bound one (the structural indenyl effect) and fusing the same fragment onto a h 6 -C 6 H 6 ligand has an analogous result (the structural naphthalene effect) we became interested in the consequences of fusing an exopolyhedral {C 4 H 4 } fragment onto the C atoms of an MC 2 B 9 species. Would this make the cage C atoms even…”

Carborane Substituents Promote Direct Electrophilic Insertion over Reduction–Metalation Reactions

“…Reduction [3] and boron-hydrogen distance [4] methods (see the Supporting Information for experimental, spectroscopic,a nd crystallographic details of all new compounds). Thes tructure of 2 is shown in Figure 1.…”