What Can We Learn from the Crystal Structures of Metallacarboranes?

Abstract

The determination of the molecular structures of metallacarboranes by X-ray diffraction remains critical to the development of the field, in some cases being the only viable way in which the overall architecture and the isomeric form of the molecule can be established. In such studies one problem frequently met is how to distinguish correctly {BH} and {CH} vertices, and this review begins by describing two relatively new methods, the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods, that have been developed to overcome the problem. Once the cage C atoms are located correctly, the resulting metallacarborane structure can frequently be analysed on the basis that cage B has a greater Structural Trans Effect (STE) than does cage C. In the absence of significant competing effects this gives rise to unequal M-L distances for a homogeneous ligand set and to a preferred Exopolyhedral Ligand Orientation (ELO) for a heterogeneous ligand set. ELO considerations can be used, amongst other things, to rank order the STEs of ligands and to identify suspect (in terms of cage C atom positions) metallacarborane structures.

Quinoline‐Conjugated Ruthenacarboranes: Toward Hybrid Drugs with a Dual Mode of Action

“…The p ‐cymene ligand in 3 is bent toward the boron atoms of the C 2 B 3 face, which results in higher deviation from coplanarity of the aromatic C 6 and the B 5 H 5 ring (6.84(7)°, Table ), and a higher tilt angle between C 6 and the C 2 B 3 ring (8.19(7)°), with respect to the corresponding unsubstituted complex [3‐( η 6 ‐ p ‐cymene)‐3,1,2‐RuC 2 B 9 H 11 ] (5.11(9)° and 6.25(7)°) . This is partially due to the stronger trans influence of the boron atoms compared to carbon, but also to electronic repulsion between O(1)/O(2) (carboxy group) and the π system of the arene ligand, rather than to steric crowding, as observed instead by Welch and co‐workers for ether‐substituted ruthenacarboranes (in the case of 3 , the lowest intramolecular H⋅⋅⋅H distances (H(17 A)⋅⋅⋅H(7) 2.668(1) Å; H(17 A)⋅⋅⋅H(8) 2.762(1) Å) are larger than the sum of their van der Waals radii).…”

Exploiting the Electronic Tuneability of Carboranes as Supports for Frustrated Lewis Pairs

“…For 2 and 3 , the cage C atoms were distinguished from B atoms by application of the Vertex-Centroid Distance (VCD) and Boron-Hydrogen Distance (BHD) methods [36,37,38].…”

Ruthenium carboranyl complexes with 2,2′-bipyridine derivatives for potential bimodal therapy application

“…[52][53][54][55] Furthermore, the weighted average 11 B NMR chemical shi, hd( 11 B)i, is À11.1 and À12.4 ppm for RuCB1 and RuCB2, respectively that were higher than À10 ppm and agree with a closo metallacarborane cluster. [56][57][58] In addition, the 1 J HB higher than 100 Hz were found in the 11 B NMR spectra giving a clear indication that all B-H protons from the C 2 B 9 H 11 cage remain intact.…”