Review
Modern concepts and methods in the chemistry of polyhedral metallasiloxanes

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Highlights

  • New approaches to polyhedral metallasiloxanes are reviewed. Specific nature of each route is emphasized.

  • Prominent sensibility of structures of polyhedral metallasiloxanes toward steric factors, ligand surrounding and complexation is described.

  • Connection points between polyhedral metallasiloxanes, classical siloxanes, polyoxometalates and coordination polymers are marked out.

Abstract

The traditional and recent synthetic approaches to polyhedral metallasiloxanes (PHMSs) of different types are reviewed to demonstrate that each strategy yields a PHMS with a specific structure, which is typical of the particular synthetic approach selected. The factors governing the structures and transformations of PHMSs are analyzed, including examples of the formation of supramolecular aggregations of PHMSs.

Introduction

Metallasiloxanes are representatives of the vast family of organosilicon compounds that specifically contain a >Si(R)-O-M- fragment. However, their chemistry differs greatly from that of other types of organosilicon compounds because the transformations of metallasiloxanes usually involve a SiOM fragment and thus the coordination properties of metals play a vital role in their structural organization. The results of previous investigations of metallasiloxanes have been reported in previous reviews [1], [2], [3], [4], [5], such as the catalytic properties of polyhedral metallasiloxanes (PHMSs) in [2], [6], [7], [8], [9], [10]. The peculiarities of the syntheses and structures of PHMSs were reviewed in a traditional manner, where the compounds were classified according to the metal positions in the periodic table, and thus some regularities of structure formation were not considered. In the present review, the compounds are grouped according to the peculiarities of the starting materials and the common features of the PHMS structures. As a consequence, the basic approaches for PHMS synthesis and the principles of their structure formation are described better. In this review, we do not aim to provide a comprehensive listing of all the reported PHMSs, but instead we illustrate the basic approaches for PHMS synthesis and the principles of their structure formation, thereby allowing us to identify the factors that govern these processes. We focus on the most recently published PHMS structures and new synthetic procedures, as well as discussing recent examples of PHMS isomerization and the formation of supramolecular assemblies.

All of the structures described in this review are based on data obtained from X-ray diffraction analyses, although some simplifications are made to the drawings of molecules (which are often presented as Lewis structures) for make them easier to understand. The X-ray data for all the compounds discussed in the review (except 16 and 73) are provided as a cif file (see ESI).

Section snippets

Basic strategies for the synthesis of polyhedral metallasiloxanes from monosubstituted organosilicon compounds

The development of PHMS chemistry has resulted in various synthetic routes that utilize compounds bearing only one organic group at the silicon atom. There is a specific series of defined synthetic methods with their own scopes for each synthetic route. All of these approaches yield specific structures, which are characteristic for each method.

Polyhedral metallasiloxanes based on disubstituted organosilicon compounds

PHMS cages of diverse shapes can also be organized using -O-R2SiOSiR2-O- (R = Me, Ph, t-Bu) fragments. Various compounds based on main group or transition metal ions and lanthanides have been obtained in this manner [4], [61], [110], [111]. The presence of diorgano-substituted siloxane fragments in these compounds leads to some specific properties and allows the creation of a large variety of PHMS structures. Flexible disiloxane units -O-R2SiOSiR2-O- are used widely in these syntheses compared

Polyhedral metallasiloxanes based on trisubstituted organosilicon compounds

R3SiO- fragments cannot act as branching centers in polyhedral molecules, but PHMSs constructed based on these units do exist. The key feature of this group of compounds is the wide range of polyhedron architectures, but without any typical structure (the same was observed for the compounds considered in Sections 2.1 Introduction of metal atoms into pre-built siloxane structures, 2.2 Synthesis in the absence of a pre-built siloxane structure). Therefore, it is difficult to deduce general

Summary and future outlook

Many studies have investigated PHMSs and the various approaches employed in their synthesis and their structural diversity demonstrate that PHMSs comprise a vast, well-studied class of organoelement compounds.

Metallasiloxanes based on mono-substituted organosilicon compounds [RSiO1.5]n have been investigated most widely. Their synthesis is well developed and each method yields compounds with a well-defined specific structure (whereas other procedures usually do not produce the same compound).

Acknowledgments

The authors express their gratitude to RFBR (project 14-03-00713) and the Council for grants of the President of the Russian Federation (project MD-3589.2014.3) for financial support. The authors are grateful to Prof. Dr N.V. Belkova (INEOS RAS, Moscow, Russia) and Prof. F. Lamaty (IBMM, Montpellier, France) for helpful discussions.

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