The surprising lability of bis(2,2’:6’,2’’-terpyridine)- chromium(III) complexes

The complex [Cr(tpy)(O3SCF3)3] (tpy = 2,2′:6′,2′′-terpyridine) is readily made from [Cr(tpy)Cl3] and is a convenient precursor to [Cr(tpy)2][PF6]3 and to [Cr(tpy)(4′-(4-tolyl)tpy)][PF6]3 and [Cr(tpy)(5,5′′-Me2tpy)][PF6]3 (4′-(4-tolyl)tpy = 4′-(4-tolyl)-2,2′:6′,2′′-terpyridine; 5,5′′-Me2tpy = 5,5′′-dimethyl-2,2′:6′,2′′-terpyridine); these are the first examples of heteroleptic bis(tpy) chromium(III) complexes. The single crystal structures of 2{[Cr(tpy)2][PF6]3}·5MeCN, [Cr(tpy)(4′-(4-tolyl)tpy)][PF6]3·3MeCN and [Cr(tpy)(5,5′′-Me2tpy)][PF6]3·3MeCN have been determined. Each cation contains the expected octahedral {Cr(tpy)2}3+ unit; in all three structures, the need to accommodate three anions per cation and the solvent molecules prevents the formation of a grid-like array of cations that is typical of many lattices containing {M(tpy)2}2+ motifs. Three reversible electrochemical processes are observed for [Cr(tpy)(4′-(4-tolyl)tpy)][PF6]3 and [Cr(tpy)(5,5′′-Me2tpy)][PF6]3, consistent with those documented for [Cr(tpy)2]3+. At pH 6.36, aqueous solutions of [Cr(tpy)2][PF6]3 are stable for at least two months. However, contrary to the expectations of the d3 Cr3+ ion being a kinetically inert metal centre, the tpy ligands in [Cr(tpy)2]3+are labile in the presence of base; absorption and 1H NMR spectroscopies have been used to monitor the effects of adding NaOH to aqueous and CD3OD solutions, respectively, of the homo- and heteroleptic complexes. Ligand dissociation is also observed when [Bu4N]F is added to CD3OD solutions of the complexes, but in aqueous solution, [Cr(tpy)2][PF6]3 is stable in the presence of fluoride ion

Editorial: Materials for Energy Conversion

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When Stereochemistry Raised Its Ugly Head in Coordination Chemistry-An Appreciation of Howard Flack

Chiral compounds have played an important role in the development of coordination chemistry. Unlike organic chemistry, where mechanistic rules allowed the establishment of absolute configurations for numerous compounds once a single absolute determination had been made, coordination compounds are more complex. This article discusses the development of crystallographic methods and the interplay with coordination chemistry. Most importantly, the development of the Flack parameter is identified as providing a routine method for determining the absolute configuration of coordination compounds

Isomers of Terpyridine as Ligands inCoordination Polymers and Networks Containing Zinc(II) and Cadmium(II)

The use of divergent 4,2′:6′,4″- and 3,2′:6′,3″-terpyridine ligands as linkers and/or nodes in extended coordination assemblies has gained in popularity over the last decade. However, there is also a range of coordination polymers which feature 2,2′:6′,2″-terpyridine metal-binding domains. Of the remaining 45 isomers of terpyridine, few have been utilized in extended coordination arrays. Here, we provide an overview of coordination polymers and networks containing isomers of terpyridine and either zinc(II) and cadmium(II). Although the motivation for investigations of many of these systems is their luminescent behavior, we have chosen to focus mainly on structural details, and we assess to what extent assemblies are reproducible. We also consider cases where there is structural evidence for competitive product formation. A point that emerges is the lack of systematic investigation

Supramolecular Approaches to Advanced Materials

Synthetic strategies based upon metal-directed self-assembly or upon the post-functionalization of dendritic and related systems have been used for the preparation of new nanodimensioned metal-containing species. These and related studies on the design of metallomesogens are reported in this paper

Packing Motifs in [M(bpy)₂X₂] Coordination Compounds (bpy = 2,2′-bipyridine; X = F, Cl, Br, I)

Packing motifs within structurally characterized cis-[M(bpy)2X2] (M = any metal, bpy = 2,20-bipyridine, X = F, Cl, Br, I) coordination compounds have been investigated using data from the Cambridge Structural Database. Compounds fall into two classes: non-solvated cis-[M(bpy)2X2] moieties and those with additional lattice molecules (solvent or other molecules). A recurring packing motif is a dimeric unit involving intermolecular face-to-face -stacking of bpy ligands and CHbpy...X contacts, although in several cases, slippage of the stacked bpy units reduces the effectiveness of the face-to-face interaction leaving the CHbpy...X contacts as the dominant crystal-packing interaction. The prevalence of the dimeric unit versus the assembly of 1D-chains in the solid state is described

The terpyridine isomer game: from chelate to coordination network building block

The first 4,2':6',4''-terpyridine (4,2':6',4''-tpy) containing coordination polymer was reported over 20 years ago and in the last decade, there has been increased interest in the use of ditopic 4,2':6',4''-tpy ligands as linkers in coordination polymers and 2D-networks. Functionalization in the 4'-position of 4,2':6',4''-tpy is synthetically straightforward, giving access to a large suite of building blocks. Less well explored is the coordination chemistry of 3,2':6',3''-tpy ligands which exhibit greater conformational flexibility than 4,2':6',4''-tpy. One approach to making the transition from 2D- to 3D-networks is to utilize tetratopic bis(4,2':6',4''-tpy) and bis(3,2':6',3''-tpy) ligands which act as 4-connecting nodes. In this highlight, we survey recent progress towards a better understanding of the design principles associated with the use of ditopic and tetratopic 4,2':6',4''-tpy and 3,2':6',3''-tpy containing ligands and their roles both as linkers and nodes in coordination assemblies

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S-1) and excited triplet (T-1) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P boolean AND P)(N boolean AND N)](+) (P boolean AND P = large-bite angle bisphosphane, and N boolean AND N is typically a diimine), [Cu(P)(2)(N boolean AND N)](+) (P = monodentate phosphane ligand), [Cu(P)(tripodal-N-3)](+), [Cu(P)(N boolean AND N)(N)](+) (N = monodentate N-donor ligand), [Cu(P boolean AND P)(N boolean AND S)](+) (N boolean AND S = chelating N,S-donor ligand), [Cu(P boolean AND P)(P boolean AND S)](+) (P boolean AND S = chelating P,S-donor ligand), [Cu(P boolean AND P)(NHC)](+) (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P boolean AND P and N N ligands, three-coordinate [Cu(N boolean AND N)(NHC)](+) and two-coordinate [Cu(N)(NHC)](+) complexes. We pay particular attention to solid-state structural features, e.g. pi-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations

Halide Ion Embraces in Tris(2,2'-bipyridine)metal Complexes

An analysis of the [M(bpy)3]n+ (bpy = 2,2′-bipyridine) complexes with halide counterions in the Cambridge Structural Database reveals a common structural motif in two thirds of the compounds. This interaction involves the formation of 12 short C–H…X contacts between halide ions lying within sheets of the cations and H-3 and H-3′ of six [M(bpy)3]n+ complex cations. A second motif, also involving 12 short contacts, but with H-6 and H-5, is identified between halide ions lying between sheets of the [M(bpy)3]n+ cation

Chemical bonding: the journey from miniature hooks to density functional theory

Our modern understanding of chemistry is predicated upon bonding interactions between atoms and ions resulting in the assembly of all of the forms of matter that we encounter in our daily life. It was not always so. This review article traces the development of our understanding of bonding from prehistory, through the debates in the 19th century C.E. bearing on valence, to modern quantum chemical models and beyon