Norman, Timothy; (1995) Linking transition metal centres: Imido and phosphine bridged complexes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with the synthesis and characterisation of organometallic complexes in which metal centres are bridged by diimido, diphosphine and tetraphosphine ligands. The electronic communication of complexes thus synthesised has been further investigated using spectroscopic and electrochemical methods. Tungsten (IV) imido - bridged dimers have been prepared by oxidative addition of organic isocyanates to [WCI₂(PPh₂)₄]. The complexes 1,3- and 1,4-[{cis,trans-W(CO)Cl₂(PPh₂Me)₂}₂(μ-NC₆H₄N)] (2.1, 2.2) have been characterized by X-ray crystallography, and show rigid planarity of the aryl imido fragment which is likely to favour electronic communication. Low temperature cyclic voltammetry studies of 2.2 indicated some degree of electronic communication between the metal centres. Initial third order nonlinear opticality (χ⁽³⁾) measurements carried out on 2.2 indicate an approximate value of χ⁽³⁾ of 500 times the CS₂ standard employed. 1.4-[{Mo(S₂)(S₂CNEt₂)₂}₂(μ-NC₆H₄N)] arises on reaction of [Mo(O)₂S₂CNEt₂)₂] with 1,4-phenylene diisocyanate. 1.4-bis(diphenylphosphino)-2,5-(difluoro)benzene (FDPPB) and bis-(diphenylphosphino)acetylene (DPPA) react with cis-[Mo(CO)₄(pip)₂] at room temperature yielding respectively [Mo(CO)₄(μ-FDPPB)]₂ (3.6) and [Mo(CO)₄(μ-DPPA)]₂ (3.2). [W(CO)₄(μ-FDPPB)]₂ has also been prepared. Reflux of 3.2 yields [{Mo(CO)₃}₂(μ-DPPA)₃] (3.3). [{Mo(CO)₅}₂(μ-DPPA)] (3.1) is obtained by room temperature reaction of [Mo(CO)₅(THF)] with equimolar DPPA. [{Mo(CO)₄}₂(μ-DPPA)(μ-FDPPB)] (3.8) has been prepared by stoichiometric reaction of FDPPB, DPPA and cis-[Mo(CO)₄(pip)₂]. FDPPB, 3.2, 3.3, 3.6 and 3.8 have been crystallographically characterised. The ligand 1,2,4,5-tetrakis(diphenylphosphino)benzene (TPPB) has been used to prepare [{M(CO)₄)}₂{μ-η²-η²-TPPB}] (M: Cr, 4.1; Mo, 4.2; M: W, 4.3). TPPB, 4.2 and 4.3 have been characterised by X-ray crystallography. 1,2-bis(diphenylphosphino)ethane (DPPE) reacts with 4.2 and 4.3 in refluxing n-decane to give [{M(CO)₂(η²-DPPE)}₂{{μ-η²-η²-TPPB}] (M: Mo, W). [{Mo(CO)₃(PPh₃)}₂{{μ-η²-η²-TPPB}] and [{Mo(CO)₃(η¹-DPPE)}₂{{μ-η²-η²-TPPB}] have also been prepared. Spectroscopic evidence indicates the presence of oligomers from the reactions of 4.2 and 4.3 with TPPB, however, no pure product was obtained. Attempts to obtain these oligomers by microwave reflux, thermolytic and photolytic substitution of MeCN proved unsuccessful.

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