High Valent Early Transition Metal Complexes as Catalysts for Organic Transformations

Abstract

A series of oxo-phenoxide complexes with the general formula [WO(OAr)Cl3]X (Ar = C6H4tBu-4, C6H3Me2-2,6, C6H3Me2-2,5, C6H2tBu2-2,6-Me-4, C6H3tBu-2-Me-4, C6H3Cl2-2,6,C6H3iPr2-2,6) have been prepared by addition of HOAr to WOCl4. The chloro bridged complex [WOCl2(µ-Cl)(OC6H3Me2-2,6)]2 (2) has been structurally characterised and the oxo and phenoxide ligands shown to be in a mutually cis arrangement. Debutylation occurs with 2,6-di-tert-butylphenol however a monophenoxide, [WOCl3(OC6H2tBu2-2,6-Me-4)]x (4), was prepared with LiOC6H2tBu2-2,6-Me-4. Variable temperature NMR studies of 4 show the tert-butyl groups lock the aryl group preventing free rotation. The ethylene polymerisation ability of the complexes was investigated (activities of 0.22-0.96 g mmol-1 hr-1 atm-1). Titanium aryl imido complexes, Ti(NAr)Cl2(py)3, [Ti(NAr)Cl2(py)2]2 and Ti(NAr)Cl2(tmeda)]x were prepared via tert-butylimido/arylamine exchange reactions. Structural characterisation of the dimeric bis-pyridine adducts (Ar = C6H4Me-2, C6H4tBu-2 and C6H4Ph-2) showed an interesting balance of steric and electronic effects. Conversion of the tris-pyridine adducts to the dimeric bis-pyridine adducts is accomplished by recrystallisation which results in loss of the labile trans pyridine from the former complexes. Complexes substituted with the tmeda ligand, [Ti(NAr)Cl2(tmeda)]x (when Ar = C6H4tBu-2 x = 1 while when Ar = C6H4Ph-2, x = 2), display solid state structures where crystal packing forces are important. Dimerisation results with the phenyl substituted arylimide, possibly as a result of electronic factors. In solution these complexes appear to exist in a monomer-dimer equilibrium. The precursor to these complexes, [Ti(NtBu)C1(µ-C1)(NH2tBu)2]2 (22), was also structurally characterised and shown to be a chloro bridged dimer rather than the previously suggested imido bridged species. Supporting these complexes on functionalised polystyrene beads results in poorly characterised products. The 1-alkene polymerisation ability of these complexes was investigated (activities of 3.2-12.7 g mmol-1 hr-1 atm-1) and found to be similar to previously tested imido species. Addition of benzoyl isocyanate to solutions of WOCl4 affords WOCl4(N≡CPh) in quantitative yield. Nitrile adducts of WOCl4 are also formed upon reaction with other acyl isocyanates (4-tert-butylbenzoyl isocyanate and 2-chlorobenzoyl isocyanate). WOCl4(N≡CPh) was structurally characterised showing the oxo and nitrile ligands to be in a mutually trans arrangement. Smooth conversion of acyl isocyanates to nitriles can be effected with catalytic amounts of WOCl4 or WOCl4(N≡CAr) (Ar = C6H5, C6H3tBu-4). Theoretical calculations of the reaction pathway show that entropic, rather than thermodynamic, considerations are important. WOCl4(N≡CPh) was also prepared from addition of benzamide to solutions of WCl6, a formal dehydration reaction. WO2Cl2(N≡CPh) can be prepared by addition of benzamide to WOCl4 in a similar manner. The mechanism of formation probably involves O-coordination of the amide followed by dehydrohalogenation. [Me2NCH2CH2NMe2CH2Cl]Cl, the non-cyclised product of nucleophilic attack of tmeda on CH2Cl2, was isolated and structurally characterised. Spectroscopic evidence for a metallacyclic amide species produced from the reaction of [WOCl4] and 2-tert-butylisocyanate suggested that imido ligand substituents are not necessarily innocent. A range of tungsten imido complexes, [W(NAr)Cl4]x (Ar = C6H4Me-2,C6H4tBu-2, C6H4Ph-2, C10H7), was produced and further functionalisation was carried out. The complex W(NC6H4Ph-2)(NHC6H4Ph-2)Cl3(C14H9NH2-9) (54), formed in the reaction of [W(NC6H4Ph-2)Cl4]x with N-trimethylsilylbiphenylamine, was structurally characterised and the presence of the imido, amido and amine ligands confirmed. Reduction of the monomeric imido complexes with sodium-mercury amalgam in the presence of trimethylphosphine afforded complexes of the type W(NAr)Cl2(PMe3)3 (Ar = C6H4Me-2, C6H4tBu-2, C6H4Ph-2,C10H7). Structural characterisation was obtained for the complex W(NC6H4tBu-2)Cl2(PMe3)3. Exchange of the labile cis trimethylphosphine ligands in the complexes W(NAr)Cl2(PMe3)3 with π-acceptor ligands (ethylene and diphenylacetylene) can be effected upon thermolysis or, in the case of ethylene, under irradiation with intense visible light to afford complexes of the type W(NAr)Cl2(CH2=CH2)(PMe3)2 (Ar = C6H4tBu-2, C6H4Ph-2) and W(NC6H4tBu-2)Cl2(PhC=CPh)(PMe3)2. The latter complex contains the alkyne as a 2-electron donor ligand. The complex [Ta(NC6H4tBu-2)Cl4(py)][pyH] (63), formed in the first tert-butylimido/arylamine exchange reaction involving a tantalum species, was structurally characterised and shown to contain the four chloride ligands cis to the imido ligand. The structure of ZrCl4(tmeda) (64) was also determined, confirming the pseudo-octahedral coordination environment of the zirconium atom.