Remarkable Effect of 4-Substituted 2,2′-Bipyridine Ligands on the Stereochemistry of Ruthenium(II) Complexes

Microwave-Assisted Synthesis: Can Transition Metal Complexes Take Advantage of This “Green” Method?

Microwave-assisted synthesis is considered environmental-friendly and, therefore, in agreement with the principles of green chemistry. This form of energy has been employed extensively and successfully in organic synthesis also in the case of metal-catalyzed synthetic procedures. However, it has been less widely exploited in the synthesis of metal complexes. As microwave irradiation has been proving its utility as both a time-saving procedure and an alternative way to carry on tricky transformations, its use can help inorganic chemists, too. This review focuses on the use of microwave irradiation in the preparation of transition metal complexes and organometallic compounds and also includes new, unpublished results. The syntheses of the compounds are described following the group of the periodic table to which the contained metal belongs. A general overview of the results from over 150 papers points out that microwaves can be a useful synthetic tool for inorganic chemists, reducing dramatically the reaction times with respect to traditional heating. This is often accompanied by a more limited risk of decomposition of reagents or products by an increase in yield, purity, and (sometimes) selectivity. In any case, thermal control is operative, whereas nonthermal or specific microwave effects seem to be absent.

Solvent-dependent fac/mer-isomerization and self-assembly of triply helical complexes bearing a pivot part

Tris-chelate metal complexes of unsymmetrical bidentate ligands can form two geometric stereoisomers, facial (fac) and meridional (mer) isomers. Due to the small difference in their properties, the highly-selective synthesis of one of the isomers is challenging. We now designed a series of tripodal ligands with a tris(3-(2-(methyleneoxy)ethoxy)phenyl)methane pivot. Surprisingly, the ratio of the fac/mer isomers of the triply helical Fe^II complexes significantly changed depending on the solvents. To the best of our knowledge, this is the first example of fac/mer isomerism of a labile tris(2,2'-bipyridine) Fe^II complex governed by the solvent. Furthermore, well-defined self-assemblies were quantitatively produced by imine bond formation with a suitable diamine. The supramolecular assemblies contained only the fac isomer even though a mixture of the two isomers existed in solution before the condensation reaction. Namely, the self-assembly formation effectively adjusted the geometries of the building unit that results in the suitable supramolecular structure.

Alternative Technologies That Facilitate Access to Discrete Metal Complexes

Organometallic complexes: these two words jump to the mind of the chemist and are directly associated with their utility in catalysis or as a pharmaceutical. Nevertheless, to be able to use them, it is necessary to synthesize them, and it is not always a small matter. Typically, synthesis is via solution chemistry, using a round-bottom flask and a magnetic or mechanical stirrer. This review takes stock of alternative technologies currently available in laboratories that facilitate the synthesis of such complexes. We highlight five such technologies: mechanochemistry, also known as solvent-free chemistry, uses a mortar and pestle or a ball mill; microwave activation can drastically reduce reaction times; ultrasonic activation promotes chemical reactions because of cavitation phenomena; photochemistry, which uses light radiation to initiate reactions; and continuous flow chemistry, which is increasingly used to simplify scale-up. While facilitating the synthesis of organometallic compounds, these enabling technologies also allow access to compounds that cannot be obtained in any other way. This shows how the paradigm is changing and evolving toward new technologies, without necessarily abandoning the round-bottom flask. A bright future is ahead of the organometallic chemist, thanks to these novel technologies.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Synthesis of three series of ruthenium tris-diimine complexes containing acridine-based π-extended ligands using an efficient “chemistry on the complex” approach

Novel multi-step chemistry on the complex strategy.

mer and fac isomerism in tris chelate diimine metal complexes

In this perspective, we highlight the issue of meridional (mer) and facial (fac) orientation of asymmetrical diimines in tris-chelate transition metal complexes. Diimine ligands have long been the workhorse of coordination chemistry, and whilst there are now good strategies to isolate materials where the inherent metal centered chirality is under almost complete control, and systematic methodologies to isolate heteroleptic complexes, the conceptually simple geometrical isomerism has not been widely investigated. In systems where the two donor atoms are significantly different in terms of the σ-donor and π-accepting ability, the fac isomer is likely to be the thermodynamic product. For the diimine complexes with two trigonal planar nitrogen atoms there is much more subtlety to the system, and external factors such as the solvent, lattice packing and the various steric considerations play a delicate role in determining the observed and isolable product. In this article we discuss the possibilities to control the isomeric ratio in labile systems, consider the opportunities to separate inert complexes and discuss the observed differences in their spectroscopic properties. Finally we report on the ligand orientation in supramolecular systems where facial coordination leads to simple regular structures such as helicates and tetrahedra, but the ability of the ligand system to adopt a mer orientation enables self-assembled structures of considerable beauty and complexity.

Heterometallic complexes with gold(I) metalloligands: self-assembly of helical dimers stabilized by weak intermolecular interactions and solvophobic effects

New gold(I) alkynyl metalloligands bpylCCAuL, bpyl'CCAuPPh3 , and PPN[Au(CCbpyl')2 ] (bpyl or bpyl'=2,2'-bipyridin-5-yl or -4-yl, respectively; L=PMex Ph3-x (x=1-3), P(C6 H3 Me2 -3,5)3 , PCy3 , XyNC) have been synthesized. Ligands bpylCCH and metalloligands bpylCCAuL (L=PPh3 , PMePh2 , PCy3 , CNXy) react with MX2 (M=Fe, Zn, X=ClO4 ; M=Co, X=BF4 ) to give complexes [M(bpylCCZ)3 ]X2 (Z=H or AuL). In most cases, these complexes are mixtures of fac and mer isomers in a statistical distribution, in both CH2 Cl2 and MeCN. However, for L=PPh3 , the fac isomer is dominant in MeCN. NMR and ESI-MS studies, together with the crystal structure of [Co(bpylCCAuPPh3 )3 ](BF4 )2 , suggest that this solvent dependence is originated by the formation of helical dimers between two fac complexes in MeCN. These dimers are stabilized by solvophobic effects and multiple intermolecular interactions. Complex [Fe(Ph3 PAuCCbpdiylCCAuPPh3 )3 ](ClO4 )2 (bpdiyl=2,2'-bipyridin-5,5'-diyl) was obtained by reaction of three diauro diethynylbipyridines and Fe(ClO4 )2 .

Thermodynamic N-donor trans influence in labile pseudo-octahedral zinc complexes: a delusion?

While the forces responsible for the chelate effect are well-established in coordination chemistry, the origin and implementation of the related thermodynamic trans influence remains debatable. This work illustrates a simple approach for quantifying this effect in labile pseudo-octahedral [Zn(Lk)3](2+) complexes lacking stereochemical preferences (Lk = L1–L4 are unsymmetrical didentate α,α′-diimine ligands). In line with statistics, the triply degenerated meridional isomers mer-[Zn(Lk)3](2+) are stabilized by 0.8 ≤ ΔGexch(mer→fac) ≤ 4.2 kJ/mol over their nondegenerated facial analogues fac-[Zn(Lk)3](2+) and therefore display no apparent trans influence at room temperature. However, the dissection of the free energy terms into opposite enthalpic (favoring the facial isomers) and entropic (favoring the meridional isomers) contributions reveals a trans influence assigned to solvation processes occurring in polar solvents. Altogether, the thermodynamic trans influence operating in [Zn(α,α′-diimine)3](2+) complexes is 1–2 orders of magnitude smaller than the chelate effect. A weak templating effect provided by a noncovalent lanthanide tripod is thus large enough to produce the wanted facial isomer at room temperature.

Origins of stereoselectivity in optically pure phenylethaniminopyridine tris-chelates M(NN')3(n+) (M = Mn, Fe, Co, Ni and Zn)

One-pot reactions of 2-pyridinecarboxaldehyde, chiral phenylethanamines and Fe(II) give single diastereomer fac diimine complexes at thermodynamic equilibrium so that no chiral separations are required (d.r. > 200 : 1). The origins of this stereoselectivity are partly steric and partly a result of the presence of three sets of inter-ligand parallel-offset π-stacking interactions. Mn(II), Co(II), Co(III), Ni(II) and Zn(II) give similar fac structures, alongside the imidazole analogues for Fe(II). While most of the complexes are paramagnetic, the series of molecular structures allows us to assess the influence of the π-stacking present, and there is a strong correlation between this and the M-N bond length. Fe(II) is close to optimal. For the larger Zn(II) ion, very weak π-stacking leads to poorer measured stereoselectivity (NMR) but this is improved with increased solvent polarity. The mechanism of stereoselection is further investigated via DFT calculations, chiroptical spectroscopy and the use of synthetic probes.

Self-assembling optically pure Fe(A-B)3 chelates

Optically pure, single diastereomer fac-tris(diimine) complexes of Fe(II) are available from a remarkably facile one-pot procedure using a range of readily available (R)-2-phenylglycinol derivatives.

The comparison of fac and mer ruthenium(ii) trischelate complexes in anion binding

The synthesis of three new homoleptic trischelate ruthenium(ii) complexes bearing new 2,2'-bipyridine ligands, 5,5'-dibenzylamido-2,2'-bipyridine () and 5-benzylamido-2,2'-bipyridine (L1) has been achieved. In the case of [Ru(L2)(3)](2+), the mer and fac isomers have been separated. (1)H NMR spectroscopic anion binding studies indicate that the two C(3)-symmetric pockets provided by [Ru(L1)(3)](2+) is conducive to receive a range of anions, although this is not readily reflected in the photophysical behaviour. The fac-isomer of [Ru(L2)(3)](2+) does appear to have an enhancement in the binding interactions over the mer form with dihydrogenphosphate salts, although the difference is much less marked with the spherical chloride ions. From X-ray crystallographic evidence, the ability to hold water in the "anion" binding cleft can inhibit the strength of the interactions with anions, giving rise to the observed selectivity for directional oxoanions such as dihydrogen phosphate.