Phosphino and amino borates: Stronger field donors for coordination chemistry

Abstract

Our group has developed useful protocols to (phosphino)borate ligands featuring synthetic flexibility at both the borate and the phosphine donor positions. Expanding on these methods, we have also developed routes to (amino)borates. Representative examples of some ligands that we have prepared thus far are shown in the figure below. In this talk I plan to highlight interesting structural, electronic, and reactivity properties of complexes supported by these types of ligands, each of which is characterized by a borate anion that is partially insulated from the coordinated metal center. For example, an understanding of subtle structural and electronic properties of a class of tris(phosphino) borate cobalt(II) complexes enables us to assign pseudo-tetrahedral cobalt(II) complexes that are either high or low spin, in addition to examples that display temperature dependent spin-crossover. From a reactivity standpoint, we have uncovered unusual group transfer processes that lead to molecular iron and cobalt imides, each of which undergoes subsequent nitrene release to CO as an acceptor substrate. In the case of iron in particular, the Fe(I)/Fe(III) nitrene group transfer cycle to CO (liberating isocyanate) can be made catalytic and both Fe(I) and Fe(III) species can be identified as intermediates in the cycle. Our continued efforts to explore these systems, and to expand this chemistry to other first row metals including nickel and copper, will be discussed.