Cooperating ligands in catalysis.

Well-known hybrid ligands are P,N-oxazolines, short PHOX, introduced by the groups of Pfaltz and Helmchen about 15 years ago, which bind by a soft (phosphorus) and hard (nitrogen) donor to the metal center. The distinct trans effects of both donor groups control by electronic effects, for example, the selectivity in metal-catalyzed allylations. Hemilabile ligands—a special form of hybrid ligands—consist of one strongly bound donor group and one which binds weakly and reversibly to the metal center. Phosphanyl ether, R2P(CR2)nOR’, or ester R2P(CR2)nCOOR’, or phosphanyl amines, R2P(CR2)nNR’2, are established hemilabile ligands. These may mask a coordination site which is liberated during the reaction and thus stabilize a reactive intermediate, which is of advantage for a number of homogeneous metalcatalyzed transformations. The term innocent ligand was coined 40 years ago by Jørgensen. An innocent ligand allows the unambiguous determination of the oxidation state of the metal center. Conversely, for a non-innocent ligand, usually with a delocalized p system, simple electron counting rules cannot be applied. Modern theory and spectroscopy, however, can substitute the intuitive and often misleading weighing of the resonance forms A$ B$ C (Scheme 1) and reveals the true electronic structure of metal complexes with non-innocent ligands. Recent discoveries make the introduction of a further ligand class meaningful, which could be useful guidance for new developments. Cooperating ligands may be defined as those which participate directly in a bond activation reaction and undergo a reversible chemical transformation. Thus the metal and the ligand cooperate in a synergistic manner and their interplay facilitates a chemical process. An example of a non-innocent ligand that is also cooperative is found in nature. The enzyme galactose oxidase, GOase, contains a tyrosinyl radical, tyrOC, which is coordinated to a copper(II) center. With impressive activity (turnover frequencies, TOF, of up to 19 10 h ) this enzyme converts chemoselectively primary alcohols into aldehydes (Scheme 2A). In the key step (b) of the catalytic cycle, the oxygen atom of the tyrosinyl radical abstracts a hydrogen atom from the a-CH2 group of the alcohol substrate which is bonded to the Lewis-acidic Cu center. The resulting tyrosine (c) is subsequently converted back into the tyrosine radical through coupled redox and proton-transfer steps (d,e). Numerous model complexes have been developed which successfully mimic GOase reactivity, and a wide range of substrates (alcohols, amines) can be oxidized, sometimes even under aerobic conditions. The imido group acts as an cooperating ligand in early transition metal imido complexes of the form [{M}=NR] ({M}=metal fragment with Ti, Zr, Ta, W). Remarkably, even unactivated C H bonds can be reversibly added across the M=N double bond (Scheme 2B). A recent study investigated the relative kinetic and thermodynamic selectivity for this 1,2-R H addition. The H H bond or the Si H bond of silanes can also be reversibly cleaved across M=X multiple bonds. In this case, even a sulfido ligand X=S may act as an cooperating ligand. For example, [Cp*2(py)Ti=S] cleaves reversibly H2 to give [Cp*2HTi-SH] (Cp*= pentamethylcyclopentadienyl, py= pyridine). The activation of H2 is also Scheme 1. Examples of hybrid ligands (P,N ligands, such as PHOX), hemilabile ligands (P,O ligands, such as phosphanyl ether or ester), and non-innocent ligands, such as 1,4-heterodienes (X=NR, O, S) or quinone derivatives.