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.