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 [Ta(C5Me5)Cl3(PMe3)] and [Ta(C5Me5)Cl2(CO)2(PMe3)

Abstract

The half-sandwich tertiary phosphine tantalum compounds [Ta(C5H5)Cl3(PMe3)](3), [Ta(C5H5)Cl2(PMe3)3](6), [Ta(C5Me5)Cl2(PMe3)2](7) and [Ta(C5Me5)Cl3(dmpe)](5)(dmpe = Me2PCH2CH2PMe2) have been prepared by treatment of [Ta(C5R5)Cl4](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 [Ta(C5R5)Cl4(PMe3)]. Compound (7) also decomposes in hydrocarbon solvents to give a mixture of products from which [Ta(C5Me5)Cl3(PMe3)](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 RCCPh (R = H or Ph) to give the known acetylene complexes [Ta(C5Me5)Cl2(η2-RCCPh)][R = Ph (9) or H (10)] and with butadiene to give [Ta(C5Me5)Cl2(η-C4H6)](11). By contrast, (6) reacts with butadiene to give the η2-butadiene complex [Ta(C5H5)Cl2(PMe3)2(η2-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 [Ta(C5H5)Cl2(η-C4H6)]. Carbon monoxide reacts with compounds (6) and (7) to give [Ta(C5H5)Cl2(CO)(PMe3)2](13) and [Ta(C5Me5)Cl2(CO)2(PMe3)](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-[Ta(C5Me5)(CO)2(PMe3)2](15) in 61% yield. Although stable indefinitely at room temperature, (15) converts upon warming at 100 °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)× 10–5s–1. Reduction of (14) with sodium amalgam in the presence of CO (1.5 atm) gives [Ta(C5Me5)(CO)4](17) in 65% yield. Complex (17) can also be obtained in 47% yield in a one-pot reaction by treatment of [Ta(C5Me5)Cl4] with 4 equivalents of sodium amalgam in the presence of 1 equivalent of PMe3. The oxide [Ta(C5Me5)(O)Cl2](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.