N-heterocyclic carbene analogues with low-valent group 13 and group 14 elements: syntheses, structures, and reactivities of a new generation of multitalented ligands.

This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

The Chemistry of Aluminum(I), Silicon(II), and Germanium(II)

This paper summarizes recent developments involving the preparation and reactivity of molecular species stabilized by terphenyl ligands that feature new bonding environments Highlights include the synthesis and characterization of dimetallenes and dimetallynes, ArEEAr [E = heavy group 13 (triel) or group 14 (tetrel) element, Ar = terphenyl ligand] and, more recently, the synthesis of a stable chromium(I) dimer, ArCrCrAr, that displays a 5-fold bonding interaction between the chromium centers

Multiple bonding in heavier element compounds stabilized by bulky terphenyl ligands.

The chemistry of univalent metal β-diketiminates

In contrast to carbon and its heavier congeners, group 13 elements are rarely observed to catenate into linear chains. Here, we find that treatment of indium(I) iodide with a protonated N-xylyl β-diketiminate and a strong potassium base in tetrahydrofuran yields a hexa-indium chain. X-ray crystallography revealed one β-diketiminate ligand bound to each indium center with no bridging ligands supporting the five indium-indium single bonds. The terminal indium centers were capped with iodine. Electronic spectroscopy and computations on a model compound offer preliminary support for σ delocalization along the chain.

https://science.sciencemag.org/content/sci/311/5769/1904.full.pdf

A Linear Homocatenated Compound Containing Six Indium Centers

The neutral monovalent Group 13 β-diketiminato complexes [CH{(CF3)2CN-2,6-iPr2C6H3}2In] and [CH{(Me)2CN-2,6-iPr2C6H3}2Tl] have been synthesised by analogous ‘one-pot’ procedures involving reaction of [KN(SiMe3)2] with the appropriate β-imino-enamine and Group 13 iodide. The structure of the indium complex reveals that replacement of the ligand backbone methyl groups of the previously reported complex [CH{(Me)2CN-2,6-iPr2C6H3}2In] with trifluoromethyl substituents results in only minor modifications to the dimensions of the NCCCNIn metallacycle. The electronic structures of both indium species were interrogated by DFT calculations to reveal similar frontier molecular orbital schemes. In agreement with calculations performed previously on the aluminium and gallium complexes, [CH{(Me)2CN-2,6-iPr2C6H3}2Al] and [CH{(Me)2CN-2,6-iPr2C6H3}2Ga], the HOMO in both indium species comprises a metal-based sp-hybrid while the LUMO is a ligand-based orbital of π symmetry. The vacant indium p-orbital is represented by the LUMO + 1. Although incorporation of the fluorinated substituents results in a stabilisation of the system overall, the stabilities and observed structural features of the complexes are reasoned to be primarily a result of the steric profile of the very bulky ligands and not through any redistribution of the electron density within the cyclic species. The thallium complex is isostructural to the analogous and previously reported aluminium, gallium and indium species. The greater stability of the monovalent state however is reflected in a reordering of the orbital energies and a stabilisation of the metal-based orbitals in the frontier region of the MO scheme.

Neutral carbene analogues of the heaviest Group 13 elements: Consideration of electronic and steric effects on structure and stability

Dimerization of indanediyl fragments: an alkene analogue for group 13?

Aggregation behaviour of thallium(I) β-diketiminates

Although the nuclearity of In(I) ‘carbenes’ is dependent upon the steric demands of the supporting β-diketiminate ligand, it is shown by solution NMR diffusion (DOSY) methods that the In–In interactions observed in the solid-state are very weak and not retained in solution.

Ruti Pongtavornpinyo

Papers

A Linear Homocatenated Compound Containing Six Indium Centers

Neutral carbene analogues of the heaviest Group 13 elements: Consideration of electronic and steric effects on structure and stability

Dimerization of indanediyl fragments: an alkene analogue for group 13?

Aggregation behaviour of thallium(I) β-diketiminates

Solid- and solution-state structures of indium ‘alkene analogues’