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Synthesis and Characterisation of Half-sandwich Tantalum Compounds in Oxidation States (I)-(V): Tertiary Phosphine, Acetylene, Butadiene, Carbonyl, and Oxo Derivatives. X-ray Crystal Structures of and

DOI: 10.1039/DT9900003199

Source and publish data:

Journal of the Chemical Society, Dalton Transactions p. 3199 - 3210 (1990)

Update date:2022-08-05

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Authors:

Gibson, Vernon C.Gibson, Vernon C.

Kee, Terence P.Kee, Terence P.

Clegg, WilliamClegg, William

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Article abstract of DOI:10.1039/DT9900003199

The half-sandwich tertiary phosphine tantalum compounds (3), (6), (7) and (5) (dmpe = Me2PCH2CH2PMe2) have been prepared by treatment of (R = H or Me) with magnesium in tetrahydrofuran (thf) in the presence of the phosphine.Compounds (3), (6), and (7) decompose in chlorocarbons to give the tetrachlorides .Compound (7) also decomposes in hydrocarbon solvents to give a mixture of products from which (4) may be crystallized selectively.The X-ray crystal structure of (4) has been determined in the orthorhombic space group P212121 and reveals a four-legged piano-stool geometry.Compound (7) reacts with RC2Ph (R = H or Ph) to give the known acetylene complexes 2-RC2Ph)> and with butadiene to give 2-C4H6)> (11).By contrast, (6) reacts with butadiene to give the η2-butadiene complex 2-C4H6)> (12) which is unstable in the absence of an excess of butadiene but does not eliminate PMe3 upon prolonged heating at elevated temperatures to form the known compound 2-C4H6)>.Carbon monoxide reacts with compounds (6) and (7) to give (13) and (14), respectively.The X-ray structure of (14) reveals a pseudo octahedral geometry with mutually trans ring and phosphine ligands and a cis-dichloro/cis-dicarbonyl ligand arrangement.There is also a considerable ring-slip distortion present in the C5Me5 ring.Reduction of (14) with sodium amalgam in thf in the presence of PMe3 affords cis- (15) in 61 percent yield.Although stable indefinitely at room temperature, (15) converts upon warming at 100 deg C to the trans isomer.The reaction has been monitored by 1H n.m.r. spectroscopy giving a pseudo first-order rate constant of 9.9(1)*10-5 s-1.Reduction of (14) with sodium amalgam in the presence of CO (1.5 atm) gives (17) in 65 percent yield.Complex (17) can also be obtained in 47 percent yield in a one-pot reaction by treatment of with 4 equivalents of sodium amalgam in the presence of 1 equivalent of PMe3.The oxide (18) has been obtained by treatment of (7) with CO2.A dimeric structure with bridging oxo ligands is suggested on the basis of i.r. spectroscopy and mass spectrometry.

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