phinoethane) seemed mainly to stabilize intermediates, and often the catalytic reactions were slower when dppe was used instead of the most common monodentate triphenylphosphine. We will briefly review the history of “ligand effects” in catalysis before discussing a range of reactions for which a notable effect has been observed. It has taken quite some time before the positive effect that bidentates can have on selectivities and rates of catalytic reactions was fully recognized. Most of the established examples of bite angle effects involve diphosphine ligands. Therefore, many important catalysts containing a chelate ligand such as bipyridine and diimine will fall outside the scope of this review. The connecting bridge in these bidentates does play a dominant role in the performance of these catalysts, but systematic studies have not been published. The effects of phosphine ligands in catalysis have been known for quite some time. One of the first reports involves the use of triphenylphosphine in the “Reppe” chemistry, the reactions of alkynes, alcohols, and carbon monoxide.1 It was found that formation of acrylic esters was much more efficient using NiBr2(PPh3)2 than NiBr2 without ligand. In the commercial system, though, a phosphine-free catalyst is used. While the reaction was not yet understood mechanistically, the use of phosphines in catalysis attracted the attention of the petrochemical industry worldwide. An early example of a phosphine ligand modified catalytic process is the Shell process for alkene hydroformylation using a cobalt catalyst containing a trialkylphoshine.2 The reaction requires higher temperatures, but it leads to more linear product as compared to the unmodified catalyst. The general mechanism of the hydroformylation reaction has been known for a long time.3 Hydrocyanation as used by Du Pont is another early example of an industrially applied catalytic reaction employing ligands.4 It is a nickel-catalyzed reaction in which aryl phosphite ligands are used for the production of adiponitrile. The development of this process has played a key role in the introduction of the now very common study of “ligand effects” in the field of homogeneous catalysis by organometallic complexes.5 While several industries were working on new homogeneous catalysts, important contributions to the new field were made in academia in the early 1960s with the appearance of the first phosphinemodified hydrogenation catalysts. An early example of a phosphine-free ruthenium catalyst was published by Halpern.6 Triphenylphosphine-modified platinumtin catalysts for the hydrogenation of alkenes were reported by Cramer from Du Pont in 1963.7 In the same year Breslow (Hercules) included a few phosFigure 1. Bite angle: The ligand-metal-ligand angle of bidentate ligands. 2741 Chem. Rev. 2000, 100, 2741−2769

Ligand Bite Angle Effects in Metal-catalyzed C−C Bond Formation

Catalytic reactions of C–C and C–X bond formation are discussed in this critical review with particular emphasis on cross coupling reactions catalyzed by palladium and wide bite angle bidentate diphosphine ligands. Especially those studies have been collected that allow comparison of the ligand bite angles for the selected ligands: dppp, BINAP, dppf, DPEphos and Xantphos. Similarities with hydrocyanation and CO/ethene/MeOH reactions have been highlighted, while rhodium hydroformylation has been mentioned as a contrasting example, in which predictability is high and steric and electronic effects follow smooth trends. In palladium catalysis wide bite angles and bulkiness of the ligands facilitate generally the reductive elimination thus giving more efficient cross coupling catalysis (174 references).

Bite angle effects of diphosphines in C-C and C-X bond forming cross coupling reactions.

The NHC-stabilized complex [Ni 2 (iPr 2 Im) 4 (cod)] (1) was isolated in good yield from the reaction of [Ni-(cod) 2 ] with 1,3-diisopropylimidazole-2-ylidene (iPrJm). Compound 1 is a source of the [Ni(iPr 2 Im) 2 ] complex fragment in stoichiometric and catalytic transformations. The reactions of 1 with ethylene and CO under atmospheric pressure or with equimolar amounts of diphenylacetylene lead to the compounds [Ni(iPr 2 Im) 2 (η 2 -C 2 H 4 )] (2), [Ni(iPr 2 Im) 2 (η 2 -C 2 Ph 2 )] (3), and [Ni(iPr 2 Im) 2 (CO) 2 ] (4) in good yields. In all cases the [Ni(iPr 2 Im) 2 ] complex fragment is readily transferred without decomposition or fragmentation. In the infrared spectrum of carbonyl complex 4, the CO stretching frequencies are observed at 1847 and 1921 cm - 1 , and are significantly shifted to lower wave-numbers compared with other nickel(0) carbonyl complexes of the type [NiL 2 (CO) 2 ]. Complex 1 activates the C-F bond of hexafluorobenzene very efficiently to give [Ni(iPr 2 Im) 2 (F)-(C 6 F 5 )] (5). Furthermore, [Ni 2 (iPr 2 Im) 4 -(cod)] (1) is also an excellent catalyst for the catalytic insertion of diphenylacetylene into the 2,2' bond of biphenylene. The reaction of 1 with equimolar amounts of biphenylene at low temperature leads to [Ni(iPr 2 Im) 2 (2,2'-biphenyl)] (6), which is formed by insertion into the strained 2,2' bond. The reaction of diphenylacetylene and biphenylene at 80 °C in the presence of 2 mol% of 1 as catalyst yields diphenylphenanthrene quantitatively and is complete within 30 minutes.

Efficient C-F and C-C activation by a novel N-heterocyclic carbene-nickel(0) complex.

Quadratic Hyperpolarizability Enhancement of para-Substituted Pyridines upon Coordination to Organometallic Moieties: The Ambivalent Donor or Acceptor Role of the Metal

Publisher Summary This chapter presents a brief review on the chemistry of fluorocarbon derivatives of metals. A great majority of these compounds are those in which a metal is bonded to a carbon atom, which is, in turn, linked to one or more fluorine atoms. Some evidence was obtained for the formation of perfluoroalkyllithium and perfluoroalkyl Grignard reagents. This work was, particularly valuable, indicating strongly that the fluorocarbon derivatives of metals would possess a distinctly different chemistry from that of their hydrocarbon analogs. Compounds of metals of the main groups are discussed systematically according to the Periodic Table, which is followed by a survey of the fluorocarbon derivatives of the transition metals and the zinc subgroup. Compounds of boron and silicon are considered even though these elements are not metals in the real sense of the term. Studies on the fluorocarbon derivatives of boron and silicon established important principles applicable to fluorocarbon–metal compounds in general, and the surveys of organometallic chemistry discuss boron and silicon compounds.

J. Chatt

Papers

276. Reactions of tertiary diphosphines with nickel and nickel carbonyl