Dppe bidentate ligand

The pyridine ligand in the products shown in Scheme 126 is labile and can be readily replaced by bidentate ligands such as 1,10-phenanthroline (phen) or l,2-bis(diphenylphosphino)ethane (dppe). Scheme 127 illustrates the reaction with phen. ... 

The adducts involving bidentate ligands are all of type [M2C16(LL)2]. The structures of those with bidentate phosphines (M = Nb, LL = dppe 602 M = Ta, LL = dmpe595) are related to that of (31), but with the chelate rings spanning two equatorial positions. The metal-metal distances are consistent with either a double or a single bond (Nb—Nb 2.712(2) A Ta—Ta 2.710(1) A). 

The domino carbonylation and Diels-Alder reaction proceed only as an intramolecular version. Attempted carbonylation and intermolecular Diels-Alder reaction of conjugated 2-yne-4-enyl carbonates 101 in the presence of various alkenes as dienophiles give entirely different carbocyclization products without undergoing the intermolecular Diels-Alder reaction. The 5-alkylidene-2-cyclopenten-4-onecarboxy-lates 102 were obtained unexpectedly by the incorporation of two molecules of CO in 82% yield from 101 at 50 °C under 1 atm [25], The use of bidentate ligands such as DPPP or DPPE is important. The following mechanism of the carbocyclization of 103 has been proposed. The formation of palladacyclopentene 105 from 104 (oxidative cyclization) is proposed as an intermediate of 108. Then CO insertion to the palladacycle 105 generates acylpalladium 106. Subsequent reductive elimination affords the cyclopentenone 107, which isomerizes to the cyclopentenone 108 as the final product. 

The oxidation of substituted dicarbonyl complexes of chromium(I) featuring the bidentate ligands dppm and dppe has been examined. Chromium(O) starting materials, Cr(CO)2(dppm)2 and Cr(CO)2(dppe)2, both with cis carbonyl configurations, were oxidized to trans cations [Cr(CO)2(dppm)2]+ and [Cr(CO)2(dppe)2]" chemical and electrochemical methods were employed. Chemical oxidation of the dppm compound proved more facile than that of the dppe analogue this had been predicted on the basis of electrochemical studies. The dicarbonyl hydride, [Cr (CO)2(dppm)2H]+, was isolated from the oxidation reaction of [Cr(CO)2(dppm)2] with O2/HCIO4, while the same reaction conditions oxidized the neutral dppe complex to the cation. 

The chromium carbonyl complexes with one monodentate and one bidentate ligand in System 8 show systematic changes in F ,i and F ,2 (but in opposite directions), when the ligands are changed from a P—P to a P—As and an As—As type (for -dpadpe the coordination to Cr is through P ). The first, chemically reversible oxidation is made easier in going from dppe to dpae, while the second, chemically irreversible oxidation becomes increasingly more difficult. The one-electron oxidation products slowly decompose to trons-[Cr(CO)2( -L-L)2]+ - . 

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