2011 Trends in Inorganic Chemistry Coordination Chemistry: Key Advances

In 2011, coordination chemistry saw significant advances focused on metal-organic frameworks (MOFs), multifunctional ligands, and the role of metal complexes in catalysis and materials science. Researchers moved from simply characterizing new complexes to engineering them for specific properties like gas storage, magnetic behavior, and catalytic activity.

Porous Frameworks and Gas Storage

Metal-organic frameworks were a dominant topic in 2011. These crystalline materials consist of metal ions or clusters connected by organic linkers, forming porous structures with very high surface areas. The trend was toward making MOFs with larger pores and better stability, especially in the presence of water or other solvents.

A key advance was the development of MOFs that could selectively capture carbon dioxide from gas mixtures. Researchers focused on tuning the pore size and the chemical groups lining the pores to improve selectivity for CO₂ over methane or nitrogen. Introducing polar functional groups like amine or nitro groups into the organic linkers increased the affinity for CO₂ molecules. The Royal Society of Chemistry published several studies showing that these modified MOFs could adsorb CO₂ at low pressures, making them candidates for carbon capture applications Royal Society of Chemistry.

Another area was hydrogen storage. While earlier MOFs had shown promise for storing hydrogen at cryogenic temperatures, the 2011 work concentrated on improving uptake at room temperature. This involved using lighter metals like magnesium and aluminum to reduce framework weight, and incorporating open metal sites that bind hydrogen molecules more strongly.

Multifunctional Ligands and Complex Topologies

Ligand design became more sophisticated in 2011. Instead of simple monodentate or bidentate ligands, chemists used ligands that could coordinate in multiple ways or respond to external stimuli.

One example was the use of ligands containing both a hard donor atom (like oxygen) and a soft donor atom (like sulfur or phosphorus). This allowed the same ligand to bind different metals selectively, or to change its binding mode depending on the metal’s oxidation state. Such ligands were used to build heterometallic complexes, where two different metals sit in the same molecule. These complexes showed interesting magnetic and electronic properties.

Another development was the synthesis of metallosupramolecular structures, like cages and grids. By using rigid ligands with specific angles, chemists could direct the self-assembly of metal ions into predictable shapes. For instance, a square-planar metal like palladium(II) combined with a bent ligand could form a molecular square. These structures had internal cavities that could host small molecules, acting as reaction vessels or sensors.

Catalysis with Well-Defined Metal Complexes

Homogeneous catalysis using coordination compounds continued to be a major focus in 2011. The emphasis was on understanding the mechanism at a molecular level, so that catalysts could be rationally improved.

Palladium-catalyzed cross-coupling reactions, such as Suzuki and Heck couplings, were already well-known, but 2011 saw work on more active and selective catalysts. The trend was toward using bulky, electron-rich phosphine ligands that stabilize the active palladium(0) species and accelerate the rate-determining step. These catalysts allowed reactions to proceed at lower temperatures and with smaller amounts of catalyst.

There was also progress in oxidation catalysis using non-precious metals like iron and copper. For example, iron complexes with tetradentate nitrogen ligands were shown to catalyze the oxidation of alkanes to alcohols using hydrogen peroxide as the oxidant. The selectivity and efficiency of these catalysts depended heavily on the ligand structure.

Magnetism and Molecular Electronics

Coordination complexes with unpaired electrons, known as single-molecule magnets (SMMs), were an active area of research in 2011. The goal was to create molecules that retain magnetization at higher temperatures, which would be useful for data storage and quantum computing.

Progress was made by using lanthanide ions like dysprosium and terbium. These ions have large magnetic moments and strong spin-orbit coupling. By designing ligands that create a specific coordination geometry, researchers could control the magnetic anisotropy of the lanthanide ion. A linear or near-linear coordination environment around dysprosium was found to produce very high energy barriers for magnetization reversal.

Another magnetic phenomenon studied in 2011 was spin crossover, where an iron(II) complex switches between a low-spin and a high-spin state in response to temperature, pressure, or light. These materials are candidates for molecular switches and sensors. The 2011 trend was to incorporate spin crossover complexes into solid materials, like nanoparticles or thin films, to make them more practical for devices.

Bioinorganic Chemistry and Metalloenzyme Models

Bioinorganic chemistry in 2011 focused on mimicking the active sites of metalloenzymes with small coordination complexes. These models help chemists understand how enzymes work and can lead to new catalysts.

Hydrogenases are enzymes that produce or consume hydrogen gas. Their active sites contain iron and nickel atoms with unusual ligands, including carbon monoxide and cyanide. In 2011, several research groups reported synthetic models that could catalyze proton reduction to hydrogen. The challenge was to make these models stable in water and air, and to achieve high turnover numbers. Some progress was made by using bulky ligands that protect the metal center from decomposition.

Copper and iron complexes that activate dioxygen were also studied. These models help explain how enzymes like methane monooxygenase and tyrosinase work. In 2011, researchers characterized new copper-oxygen intermediates, such as bis(μ-oxo) dicopper complexes, using spectroscopic methods like resonance Raman and X-ray absorption spectroscopy.

Photophysical and Photochemical Properties

Coordination complexes with luminescent properties were developed for use in light-emitting devices and solar energy conversion. The 2011 trends focused on using earth-abundant metals instead of expensive and toxic ones like ruthenium or iridium.

Copper(I) complexes with diimine ligands were known to be luminescent, but their efficiency was often low due to non-radiative decay. In 2011, researchers improved the quantum yields by using bulky ligands that prevent flattening distortions in the excited state. These complexes emit in the visible range and were tested as emitters in organic light-emitting diodes (OLEDs).

Coordination complexes were also used as dyes in dye-sensitized solar cells (DSSCs). While ruthenium-based dyes had the highest efficiencies, 2011 saw work on cheaper alternatives like copper(I) and zinc(II) complexes. These dyes absorb light and inject electrons into a semiconductor, generating electricity. The challenge was to achieve broad absorption across the visible spectrum and stable attachment to the semiconductor surface.

Key Takeaways

• 2011 was a year of applied coordination chemistry, with emphasis on designing materials for gas storage, catalysis, and electronics.
• Metal-organic frameworks advanced through functionalization of linkers and control of pore chemistry for selective gas adsorption.
• Multifunctional ligands enabled the construction of heterometallic complexes and supramolecular structures with specific shapes and functions.
• Homogeneous catalysis saw progress in palladium cross-coupling with bulky ligands and iron-based oxidation catalysts.
• Single-molecule magnets using lanthanide ions achieved higher energy barriers through careful control of coordination geometry.
• Bioinorganic models of hydrogenase and oxygen-activating enzymes provided insight into natural catalysis and guided synthetic catalyst design.
• Luminescent copper(I) complexes emerged as potential alternatives to precious metal emitters in light-emitting devices.