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The synthesis, characterization, and reactivity of paramagnetic (or open‐shell) organometallic species are described. Many stable complexes featuring transition metals, lanthanides, and actinides have been reported. These systems, which are exceptions to the 18‐electron rule (or 16‐electron rule), are increasingly being used in the catalytic realm, and also have different reactivity patterns compared to their diamagnetic counterparts due to the presence of unpaired electrons. The formation and subsequent stability of paramagnetic organometallic complexes in violation of the 18‐electron rule can be explained via molecular orbital (MO) theory in terms of (i) the use of π‐donor ligands (as opposed to the ubiquitous π‐acceptor ligands usually found in organometallic chemistry); (ii) kinetic stabilization with sterically demanding ligands; and (iii) partially filled MOs being non‐bonding or slightly antibonding/bonding. Although metallocene‐based complexes are probably the most well studied of all paramagnetic organometallic complexes, homoleptic σ‐alkyl and σ‐aryl complexes (particularly with bulky groups such as CH 2 SiMe 3 and meta ‐terphenyl respectively) and other non‐Cp systems have also been explored. Such non‐Cp ancillary ligands include amido‐, alkoxo‐, and thiolato‐ligands, β‐diketiminate ligands, tris(pyrazolylborate) and N ‐heterocyclic carbene ligands. f‐Electron organometallic systems also utilize a similar range of ligands, but because of their larger size, they can accommodate more sterically congested coordination spheres; for example, Cp* 3 M complexes can be prepared, and they show unusual reactivity pathways. Open‐shell organometallic molecules are known to span a range of electron counts from 8 to 20. Nineteen‐electron organometallic complexes are of particular interest with respect to the location of the extra electron residing primarily on the metal or on the ligand (in this case, the complex is referred to as an 18 + δ system). Redox‐active ligands in general are now recognized to play a significant role in organometallic chemistry and catalysis and are discussed briefly. The discussion of short‐lived paramagnetic organometallic systems revolves around 17‐ and 19‐electron radicals, and their use in catalytic processes and as “super reducing agents.” Many reactivity pathways for such organometallic radicals (or “metalloradicals”) have been identified. Characterization tools for paramagnetic organometallic complexes, such as nuclear magnetic resonance (NMR) and electron spin resonance spectroscopy, density functional theory calculations, and magnetic measurements are presented. Variable‐temperature NMR studies and 2 H NMR can be of significant use in interpreting paramagnetic NMR spectra. Solid‐state NMR spectroscopy of paramagnetic organometallic systems is still in its infancy. The applications of paramagnetic organometallic complexes span many areas including catalysis (e.g., olefin polymerization), synthetic organic chemistry, and the development of molecule‐based magnetic materials.
The synthesis, characterization, and reactivity of paramagnetic (or open‐shell) organometallic species are described. Many stable complexes featuring transition metals, lanthanides, and actinides have been reported. These systems, which are exceptions to the 18‐electron rule (or 16‐electron rule), are increasingly being used in the catalytic realm, and also have different reactivity patterns compared to their diamagnetic counterparts due to the presence of unpaired electrons. The formation and subsequent stability of paramagnetic organometallic complexes in violation of the 18‐electron rule can be explained via molecular orbital (MO) theory in terms of (i) the use of π‐donor ligands (as opposed to the ubiquitous π‐acceptor ligands usually found in organometallic chemistry); (ii) kinetic stabilization with sterically demanding ligands; and (iii) partially filled MOs being non‐bonding or slightly antibonding/bonding. Although metallocene‐based complexes are probably the most well studied of all paramagnetic organometallic complexes, homoleptic σ‐alkyl and σ‐aryl complexes (particularly with bulky groups such as CH 2 SiMe 3 and meta ‐terphenyl respectively) and other non‐Cp systems have also been explored. Such non‐Cp ancillary ligands include amido‐, alkoxo‐, and thiolato‐ligands, β‐diketiminate ligands, tris(pyrazolylborate) and N ‐heterocyclic carbene ligands. f‐Electron organometallic systems also utilize a similar range of ligands, but because of their larger size, they can accommodate more sterically congested coordination spheres; for example, Cp* 3 M complexes can be prepared, and they show unusual reactivity pathways. Open‐shell organometallic molecules are known to span a range of electron counts from 8 to 20. Nineteen‐electron organometallic complexes are of particular interest with respect to the location of the extra electron residing primarily on the metal or on the ligand (in this case, the complex is referred to as an 18 + δ system). Redox‐active ligands in general are now recognized to play a significant role in organometallic chemistry and catalysis and are discussed briefly. The discussion of short‐lived paramagnetic organometallic systems revolves around 17‐ and 19‐electron radicals, and their use in catalytic processes and as “super reducing agents.” Many reactivity pathways for such organometallic radicals (or “metalloradicals”) have been identified. Characterization tools for paramagnetic organometallic complexes, such as nuclear magnetic resonance (NMR) and electron spin resonance spectroscopy, density functional theory calculations, and magnetic measurements are presented. Variable‐temperature NMR studies and 2 H NMR can be of significant use in interpreting paramagnetic NMR spectra. Solid‐state NMR spectroscopy of paramagnetic organometallic systems is still in its infancy. The applications of paramagnetic organometallic complexes span many areas including catalysis (e.g., olefin polymerization), synthetic organic chemistry, and the development of molecule‐based magnetic materials.
In the burgeoning efforts of collecting solar energy for power generation, [1] the development of rechargeable solar thermal batteries containing photochromic molecules capable of reversible photoisomerization is receiving increasing scrutiny. [2] Among them, organometallic compounds show particular promise, because of their complementary potential for steric and electronic tunability. [3] In this regard, an intriguing system with which to illustrate the concept is the robust photothermal fulvalene (Fv) diruthenium couple 1Q2 (Scheme 1). [4] Initially assumed to occur by a concerted pathway, a recent study of the heat releasing step 2 a!1 a pinpointed a stepwise trajectory. Its salient features (Figure 1, black solid line) consist of a pre-equilibrium of 2 a (20.8 kcal mol À1 ) with anti biradical B (38.8 kcal mol À1 ) by initial cyclopentadienyl (Cp) coupling (transition state, TS, C, 43.2 kcal mol À1 ), subsequent rate-determining CpRu(CO) 2 rotation (TS A, 50.5 kcal mol À1 ), and Ru-Ru bond formation to give 1 a (0.0 kcal mol À1 ). [5] The relative difficulty of anti to syn biradical rotation and the ease with which B proceeds to 2 a (DH°= 4.4 kcal mol À1 ) prompted a reconsideration of the mechanism of the photostorage step. Originally, [4a] the normally expected Ru-Ru photodissociation [6] was discounted, because added CCl 4 (1m) had no effect on the outcome of the photorearrangement, leading again to the postulate of a concerted process. However, the effortless step B!2 a makes B a viable photointermediate, provided that it Scheme 1. Photoisomerization of fulvalene(tetracarbonyl)diruthenium at ! 350 nm and its thermal reversal. Irradiation 300 nm causes decarbonylation. [4a] [*] Dr.
Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.
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