Mechanism of Molybdenum Nitrogenase.

Transition metal complexes are indispensable tools for any synthetic chemist. Ideally, any metal-mediated process should be fast, clean, efficient, and selective and take place in a catalytic manner. These criteria are especially important considering that many of the transition metals employed in catalysis are rare and expensive. One of the ways of modifying and controlling the properties of transition metal complexes is the use of appropriate ligand systems, such as pincer ligands. Usually consisting of a central aromatic backbone tethered to two two-electron donor groups by different spacers, this class of tridentate ligands have found numerous applications in various areas of chemistry, including catalysis, due to their combination of stability, activity, and variability. As we focused on pincer ligands featuring phosphines as donor groups, the lack of a general method for the preparation of both neutral (PNP) and anionic (PCP) pincer ligands using similar precursor compounds as well as the difficulty of introducing chirality into the structure of pincer ligands prompted us to investigate the use of amines as spacers between the aromatic ring and the phosphines. By introduction of aminophosphine and phosphoramidite moieties into their structure, the synthesis of both PNP and PCP ligands can be achieved via condensation reactions between aromatic diamines and electrophilic chlorophosphines (or chlorophosphites). Moreover, chiral pincer complexes can be easily obtained by using building blocks obtained from the chiral pool. Thus, we have developed a modular synthetic strategy with which the steric, electronic, and stereochemical properties of the ligands can be varied systematically. With the ligands in hand, we studied their reactivity towards different transition metal precursors, such as molybdenum, ruthenium, iron, nickel, palladium, and platinum. This has resulted in the preparation of a range of new pincer complexes, including various iron complexes, as well as the first heptacoordinated molybdenum pincer complexes and several pentacoordinated nickel complexes by using a controlled ligand decomposition pathway. In addition, we have investigated the use of some of the complexes as catalysts in different C-C coupling reactions: for example, the palladium PNP and PCP pincer complexes can be employed as catalysts in the well known Suzuki-Miyaura coupling, while the iron PNP complexes catalyze the coupling of aromatic aldehydes with ethyl diazoacetate under very mild reaction conditions to give selectively 3-hydroxyacrylates, which are otherwise difficult to prepare. While this Account presents an overview of current research on the chemistry of P-N bond containing pincer ligands and complexes, we believe that further investigations will give deeper insights into the reactivity and applicability of aminophosphine-based pincer complexes.

Modularly designed transition metal PNP and PCP pincer complexes based on aminophosphines: synthesis and catalytic applications.

Structural and functional evolution of metallacrowns.

Complexes of heterocyclic thionates. Part 1. Complexes of monodentate and chelating ligands

Aromatic nitrogen heterocycles as bridging ligands; a survey

The synthesis and spectral properties of some highly reactive new seven-coordinate molybdenum(II) and tungsten(II) bisacetonitrile dihalogenotricarbonyl complexes

Synthesis and spectral properties of [MI2(CO)(NCMe)(.eta.2-RC2R')2] and [Mo(.mu.-I)I(CO)(.eta.2-MeC2Me)2]2 bis(alkyne) complexes. X-ray crystal structures of [WI2(CO)(NCMe)(.eta.2-MeC2Me)2] and [WI2(CO)(NCMe)(.eta.2-PhC2Ph)2]

Oxidation of [Mo(CO)3(NCMe)3] with SnCl4 to give the seven-coordinate compound [MoCl(SnCl3)(CO)3(NCMe)2]. Reactions of [MoCl(SnCl3)(CO)3(NCMe)2] with phosphine donor ligands

The reactions of the highly versatile bis(alkyne) compounds (WI/sub 2/(CO)(NCMe)(/eta//sup 2/-RC/sub 2/R)/sub 2/) (R = Me, Ph) with pyridine and related ligands to give neutral and cationic alkyne complexes are reported. The x-ray structure of (WI/sub 2/(CO)(bpy)(/eta//sup 2/-MeC/sub 2/Me)/sub 2/)(BPh/sub 4/) was also measured and is reported. 10 references, 1 figure, 6 tables.

Reactions of (WI/sub 2/(CO)(NCMe)(/eta//sup 2/-RC/sub 2/R)/sub 2/) (R = Me, Ph) with pyridines and related ligands, X-ray crystal structure of (WI(CO)(bpy)(/eta//sup 2/-MeC/sub 2/Me)/sub 2/)(BPh/sub 4/)

The complexes [MI2(CO)3(NCMe)2] (M=Mo or W) react with one molar equivalent of L in CH2Cl2 at room temperature initially to afford the mononuclear sevencoordinate complexes [MI2(CO)3(NCMe)L] which have been isolated for L-PPh3, AsPh3, SbPh3, PPh2Cy or P(OPh3)3. Many of these complexes dimerise to give the iodide bridged compounds [{M(μ−I)I(CO)3L}2]via displacement of acetonitrile. When L=PPhCy2, PCy3, PEt3 or P(OMe)3 only the dimeric complexes have been isolated. The ease of dimerisation of the mononuclear complexes [MI2(CO)3(NCMe)L] is discussed in terms of the electronic and steric effects of the ligands, L. Low temperature13C n.m.r. spectroscopy of the mononuclear [Wl2(CO)3(NCMe)(EPh3)](E=P or As) complexes are interpreted as suggesting the likely stereochemistry of these seven-coordinate complexes.

The reactions of [MI2(CO)3(NCMe)2] (M=Mo or W) with one equivalent of L(L-phosphorus, arsenic and antimony donor ligands) to afford [MI2(CO)3(NCMe)L] and [M(μ-I)I(CO)3L]2

Paul K. Baker

Papers

The synthesis and spectral properties of some highly reactive new seven-coordinate molybdenum(II) and tungsten(II) bisacetonitrile dihalogenotricarbonyl complexes

Synthesis and spectral properties of [MI2(CO)(NCMe)(.eta.2-RC2R')2] and [Mo(.mu.-I)I(CO)(.eta.2-MeC2Me)2]2 bis(alkyne) complexes. X-ray crystal structures of [WI2(CO)(NCMe)(.eta.2-MeC2Me)2] and [WI2(CO)(NCMe)(.eta.2-PhC2Ph)2]

Oxidation of [Mo(CO)3(NCMe)3] with SnCl4 to give the seven-coordinate compound [MoCl(SnCl3)(CO)3(NCMe)2]. Reactions of [MoCl(SnCl3)(CO)3(NCMe)2] with phosphine donor ligands

Reactions of (WI/sub 2/(CO)(NCMe)(/eta//sup 2/-RC/sub 2/R)/sub 2/) (R = Me, Ph) with pyridines and related ligands, X-ray crystal structure of (WI(CO)(bpy)(/eta//sup 2/-MeC/sub 2/Me)/sub 2/)(BPh/sub 4/)

The reactions of [MI2(CO)3(NCMe)2] (M=Mo or W) with one equivalent of L(L-phosphorus, arsenic and antimony donor ligands) to afford [MI2(CO)3(NCMe)L] and [M(μ-I)I(CO)3L]2