Trialkylsilyl-Substituted Silole and Germole Dianions as Precursors for Unusual Silicon and Germanium Compounds

Abstract

Conspectus Group 14 element heteroles are the heavier analogues of cyclopentadienes in which a heavier group 14 element atom replaces the sp3 carbon atom. In particular siloles and, to a somewhat smaller degree, germoles attracted considerable attention since the early 1990s due to their favorable photophysical properties which allowed the construction of OLEDs using group 14 element heteroles as emissive or electron-transport layers. Anions and in particular dianions derived from group 14 element heteroles have been of substantial interest due to the possible occurrence of Hückel aromaticity involving the heavier main group atom. Aromaticity is not the only notable electronic feature of silole and germole dianions; the spatial and energetic alignment of their frontier orbitals is equally remarkable. With a high lying lone pair at the heteroatom, which is orthogonal to a delocalized π-system, their frontier orbital sequence closely resembles that of N-heterocyclic carbene analogues. Despite these intriguing parallels between carbene analogues and silole and germole dianions, disappointingly little is known about their reactivity. The installation of trialkylsilyl substituents in the 2,5-positions of the heterocyclopentadiene ring as in K2[I] has a remarkable effect on the stability of silole and germole dianions and allows us to study their reactivity and to evaluate their synthetic potential in detail. Simple double salt metathesis reactions with different dihalides provided heterofulvenes. These were detected either as intermediates or, in the case of carbon dihalides, isolated in the form of their ylidic isomers II. In other cases, the heterofulvenes were the starting point for complex reaction sequences leading to novel binuclear complexes of titanium and zirconium III or for simple isomerization reactions that lead to bicyclohexene-type tetrylenes (BCH-tetrylenes) IV, a novel class of heavier carbenes. These bicyclic carbene analogues are significantly stabilized by homoconjugation between the electron deficient tetrel atom and the remote CC double bond. Compound IV with E′R2SiR2 and E = Si is a valence isomer of disilabenzene and is a stable derivative of the global minimum of the Si2C4H6 potential energy surface. With group 13 dihalides, as for example with boron dichlorides, topological closely related compounds V were isolated. These Ge­(II) complexes of borole dianions are isolobal to half-sandwich complexes of main group elements such as aluminum­(I) cyclopentadienide or can be viewed as nido-type clusters. These analogies already open a broad field for future investigations of their reactivity. Trialkylsilyl-substituted heterole dianions I provide a facile synthetic approach to several novel intriguing compound classes with the tetrel element in unusual coordination states. The reactivity and the synthetic potential of these new compounds is however widely unexplored and calls for future systematic studies. Gratifyingly, the periodic table of the elements stills holds a lot of potential for future ...

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

“…Since West et al reported on the first characterized delocalized germole dianion, [14] considerable progress has been made during the last two decades. Especially the groups of Tilley, [15] Boudjouk [16] and Müller [17] synthesized a variety of germole dianions and discussed their chemistry (compound 5, Scheme 1) The rare number of geminal dianions available and hence the undeniable interest in these derivatives drove our motivation to contribute to this field. Thereby our intention was the design of a reasonable preparative-scale approach for the synthetically highly attractive geminal bisgermenolate.…”

Radicals and Anions of Siloles and Germoles

“…The second mainspring for the development of the chemistry of tetroles was the possible occurrence of aromaticity in the negatively charged ions, II and III , and their applications as ligands in transition metal chemistry [13–16] . During the last years, the group of Saito and the Oldenburg team contributed to this field by demonstrating that tetrole dianions III are suitable precursors for a wide variety of novel compounds with the tetrel element in unusual coordination environments [13,16–19] . In view of the vast existing body of knowledge on tetrole's material chemistry, we asked ourselves if the favorable photophysical properties of tetroles might be combined with magnetic properties resulting from an unpaired spin as for example in tetrolyl radicals IV .…”

“…[13][14][15][16] During the last years, the group of Saito and the Oldenburg team contributed to this field by demonstrating that tetrole dianions III are suitable precursors for a wide variety of novel compounds with the tetrel element in unusual coordination environments. [13,[16][17][18][19] In view of the vast existing body of knowledge on tetrole's material chemistry, we asked ourselves if the favorable photophysical properties of tetroles might be combined with magnetic properties resulting from an unpaired spin as for example in tetrolyl radicals IV. An inspiration came from one report by the West group who communicated on the existence and stability of a silolyl-based biradical V with triplet ground state and its surprisingly low reactivity.…”

Three-membered cyclic digermylenes stabilised by an N-heterocyclic carbene

“…28,29 The digermylene isomer X is preferred when the substituent R is a multidentate ligand. [30][31][32][33][34][35][36][37][38][39] Following previous investigations from our laboratory, 40 we probed the potential of salt metathesis of group 14 element dianions XI with group 14 element dihalides XII followed by 1,2elimination for the synthesis of heavy vinylidenes III (Fig. 3).…”

“…Following previous investigations from our laboratory, 40 we probed the potential of salt metathesis of group 14 element dianions XI with group 14 element dihalides XII followed by 1,2-elimination for the synthesis of heavy vinylidenes III ( Fig. 3 ).…”