Synthesis and dynamic nuclear magnetic resonance studies of pentafluorobenzenethiolate complexes of molybdenum and tungsten. The crystal and molecular structures of [W(SC6F5)3(CO)(η5-C5H5)]·0.5CH2Cl2 and [N(PPh3)2][Mo(SC6F5)4(η5-C5H5)]

Abstract

The reaction between [WBr₃(CO)₂(η⁵-C₅H₅)] and excess of Tl(SC₆F₅)(CH₂Cl₂, 20 °C) affords Tl[W(SC₆F₅)₄(η⁵-C₅H₅)](2b) as the major product and [W(SC₆F₅)₃(CO)(η⁵-C₅H₅)](3) as the minor product. Complex (3) has been structurally characterised as its 0.5 CH₂Cl₂ solvate by X-ray diffraction. Chiral molecules of [W(SC₆F₅)₃(CO)(η⁵-C₅H₅)] adopt a distorted square-based ‘piano-stool’ geometry with the trans W–S bond [2.443 6(15)Å] longer than the cis W–S bonds (mean 2.358 Å). The SC₆F₅ ligands lie with S–C bonds approximately in the plane of the square base and, all C₆F₅ groups have the same rotational orientation about the central W atom. However, dynamic ¹⁹F n.m.r. studies of (3) reveal that two isomeric forms exist at low temperature due to different orientations of the SC₆F₅ ligands but these undergo exchange at ambient temperature as a result of rotation about the W–SC₆F₅ bonds or inversion at sulphur. Similar studies of complex (2b) established that apparent rotation of the SC₆F₅ groups is solvent dependent and occurs in conjunction with ionic dissociation into Tl⁺ and [W(SC₆F₅)₄(η⁵-C₅H₅)]^–. The Tl^I in (2b) and related complexes Tl[Mo(SC₆F₅)₄(η⁵-C₅H₅)] and Tl[Mo(SC₆F₅)₂(CO)₂(η⁵-C₅H₅)] can be replaced by non-co-ordinating cations to give X⁺[M(SC₆F₅)₄(η⁵-C₅H₅)]^–[M = Mo, X = NBuⁿ₄(4a) or N(PPh₃)₂(4b); M = W, X = NMe₄] and N(PPh₃)₂⁺[Mo(SC₆F₅)₂(CO)₂(η⁵-C₅H₅)]^– which also exhibit fluxional behaviour according to ¹⁹F n.m.r. studies. The structure of complex (4b), determined in the solid state by X-ray diffraction, comprises discrete ions [(Ph₃P)N(PPh₃)]⁺ and [Mo(SC₆F₅)]₄[(η⁵-C₅H₅)]^–. The complex anion has a ‘piano-stool’ geometry with a square base defined by four SC₆F₅ ligands (mean Mo–S 2.420 Å) and the C₆F₅ groups form a ‘swastika-like’ arrangement about the square plane. Reactions of (2b) with tertiary phosphines (L) in CH₂Cl₂ at 20 °C afford 1:2 adducts Tl[W(SC₆F₅)₄L₂(η⁵-C₅H₅)][L = PPh₃(6b), PMe₂Ph (6c), or PEt₃(6d)]. Dynamic ¹⁹F n.m.r. studies established a different structure for (6b) in solution compared to (6c) and (6d).