Dynamic stereochemical rearrangements in chiral organometallic complexes

Abstract

Organometallic complexes have long been known to display a wide variety of dynamic stereochemical processes. Classic examples of such processes include the exchange of axial and equatorial environments in trigonal bipyramidal complexes, such as Fe(CO)(5), and the migration of the metal moiety round the periphery of the cyclopentadiene ring in eta(1)-bound Cp complexes. The systematic study of fluxional processes is of interest because it can not only help provide a detailed, quantitative 'picture' of the bonding between the metals and ligands involved, but it can also help to rationalise chemical reactivity patterns. The introduction of chirality into organometallic complexes, usually in the form of a non-racemic chiral ligand, has led to an explosion in the importance such species, particularly with regard to their applications in organic functional group transformations. The presence of a chiral centre can also provide an excellent spectroscopic handle on the complex in question, enabling both novel fluxional processes to be observed and new light to be shed on old (unresolved) problems. In this critical review (101 references) the literature on metal-centred fluxional rearrangements in chiral transition and main group organometallic complexes is reviewed, complementing the recent review by Faller (see reference 8).

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

“…The fluxionality of k 2 species has been studied in depth for complexes of hemilabile ligands with redundant donor atoms. [48] Moreover, k 2 coordination of the related PNP ligand was recently observed for uranium halide complexes [40,49] and, in the case of phosphido pincer zinc complexes, it is supported by DFT calculations.…”

Dynamic Stereochemical Behaviour of Congested Ruthenium(II) Complexes Containing Asymmetric Thioether Ligands Based on Pyridine and Pyrimidine

“…[1] On the other hand, the bonding modes of the polydentate ligands can promote stereochemical nonrigidity of the transition-metal complexes. [2][3][4] Dynamic stereochemical rearrangements can be observed if all of the donor groups coordinated, but it can occur reversibly in an on/off coordination mode for one of the donor groups. [4,5] In contrast, if at least one donor atom remains uncoordinated, it is possible to observe the exchange between different coordination modes.…”

“…[2][3][4] Dynamic stereochemical rearrangements can be observed if all of the donor groups coordinated, but it can occur reversibly in an on/off coordination mode for one of the donor groups. [4,5] In contrast, if at least one donor atom remains uncoordinated, it is possible to observe the exchange between different coordination modes. When tridentate ligands such as 2,2Ј:6,2ЈЈ-terpyridine bond in a bidentate mode they display dynamic 4807 AB systems are assigned to dinuclear species d and e containing two bridging L that act as N pyridine ,S-or N pyridine , N pyrimidine -donor ligands.…”

“…[3][4][5][6][7] Sometimes, restricted rotation of the pendant ring is observed. [3,7,8] Hybrid ligands often display dynamic behaviour.…”

“…[10] Mechanisms proposed for the exchange between coordinated and uncoordinated donor atoms include the "tick-tock twist" mechanism, which involves a pseudotridentate bonding mode of the ligand in the transition state, the "rotation" mechanism, which involves the loosening of the outer metal-donor atom bond followed by a 180°r otation about the central metal-donor atom bond, and a mechanism involving cleavage of the outer metal-donor atom bond followed by the formation of a T-shaped intermediate, in which the polydentate ligand is bound to the metal in a monodentate fashion. [4,[6][7][8]11] Some years ago, studying the coordination chemistry of thioethers based on pyridine or pyrimidine, we found that these usually N,N-bidentate donors [12] often show N,S che-lation towards congested ruthenium substrates. [13][14][15][16][17] The resultant complexes display dynamic stereochemical rearrangements associated with inversion at the sulfur atom coordinated to the ruthenium, [14] the restricted rotation of the pendant ring [15] and the exchange between different coordination modes.…”