Seminar of the School of Physical Sciences
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Title: Boron Beyond the Icosahedral Barrier

Speaker: Sundargopal Ghosh
(IIT-Madras, Chennai)

Date: May 24, 2013 (Friday)
Time: 16:00 hrs (4:00 pm)
Venue: Seminar Room, First Floor,
School of Physical Sciences (SPS), JNU

Abstract: While large metal clusters are known with of the order of 100 atoms, for decades the search for boron clusters with more than 12 atoms met with little success. Hawthorne's work in the development of polyhedral expansion chemistry pioneered the way to the synthesis of large metallacarboranes. Ever since the chemistry of supraicosahedral clusters has been an active research area experimentally and computationally. The first 13-vertex metallacarborane was described 40 years ago and there only about a hundred such compounds identified. In addition, only a handful of 14- and 15-vertex metallacarboranes, no examples of species with 16 or more vertices in the polyhedron have been reported. This is unfortunate, as the applications of polyhedral borane-cluster compounds include "least-coordinating anions", electronics, ceramics, polymers, nanomaterials, immunodiagnosis and boron-neutron-capture therapy where the option of higher nuclearity clusters containing boron is important. For these reasons we have sought to isolate clusters possessing more than 15-vertices. For boranes theoretical study on [BnHn]2-, n > 12 reveal valuable structural
trends. That is, Lipscomb showed that for deltahedral structures for [BnHn]2-, n = 13-24, the number of degree-six vertices increases with increasing n. However, later computations suggested that [BnHn]2-, n ≥ 16, have energetically accessible alternative structures not fully triangulated that are progressively more favorable. The lack of larger metallaboranes has led us to renewed experimental efforts and for example, we have recently reported the synthesis and structural characterization of dirhodium analogue of [B8H12] and [B10H14]. An extension of these studies resulted in the isolation and characterization of the first 16-vertex closo-metallaborane. With a geometry based on a supraicosahedral framework, it provides a carbon-free example which demonstrates an additional structure type. The key results of this work will be described.