A New Generation of All-Carbon Bridged Bimetallic Complexes featuring Redox-Active, Group 6 Metal End-Caps.

Lead Research Organisation: University of Manchester
Department Name: Chemistry

Abstract

Metal-stabilised carbon chains [M-(CC)x-M]n+ have attracted increasing interest over the last 10 years, mainly due to the remarkable changes in electronic structure which accompany changes in the formal oxidation state of the compounds. While much of this work has focussed on homobimetallic examples, recent synthetic advances make heterometallic examples accessible. This is a particularly exciting development as much of the work with homometallic polycarbon complexes has shown that distinctly different properties are imposed by different supporting metal fragments.The objective of this work is to synthesise and investigate the electronic properties of a series of compounds of the type [M-(CC)x-M']n+ in which the linking group Cx is an unsaturated carbon chain of four or more carbons and the metal end-caps M and M' are redox active, organometallic centres. A range of oxidation states (n = 0-4) is accessible of which some are stable, isolable systems. The electronic structures of this fascinating class of complexes are strongly dependent upon the identity of M and M'. However previous work has been restricted essentially to end-caps of group 7 and 8 transition metal centres of iron, ruthenium and rhenium in which the 3d metal centre Fe is much more strongly donating than the 4d/5d centres of Ru and Re. The innovation in this work is to include an organometallic end-cap featuring the group 6 metal molybdenum which is both strongly electron donating and based upon a 4d centre - this provides a unique opportunity to delineate the effect of end-cap electron donor capacity vs. metal transition series upon electronic structure. Further advantages that can be enjoyed through the introduction of Mo based end-cap group are the availability of a highly resolved spectroscopic handle (EPR) with which to investigate electronic structure by a direct experimental probe and the potential to control electronic structure in oxidised forms of the mixed group 6/group 8 heterobimetallics [M-(CC)x-M']n+ by control of molecular configuration.The work is of importance because it will lead to an enhanced understanding of the electronic structure of the unsaturated all-carbon bridge, and how this can be controlled as a function of the metal, the supporting ligands and the length of the carbon fragment. Although many talk of the need for such detailed understanding in the design of molecular components for future molecular based electronics, our goal is to arrive at a full understanding of the underlying chemistry in these metal-stabilised carbon fragments, free of the restrictions imposed by the reliance on one signature metal end-cap.

Publications

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Description Metal-stabilised carbon chains [M-(CC)x-M]n+ are important for the remarkable modifications in electronic structure which accompany changes in the formal oxidation state, n of the compounds. Understanding the electronic structure of the unsaturated all-carbon bridge as a function of the metal auxiliary M and the length of the carbon chain x, permits the design of molecules with tailored magnetic and electronic properties. This grant, linked with EP/E02582X/1, provided a two year PDRA appointment to investigate the characteristics of various metal auxiliaries with an emphasis on complexes of the cycloheptatrienyl molybdenum, Mo(dppe)(C7H7) [={Mo}] system.



An examination of the structure of the alkynyl complexes [{Mo}(CCR)]n+ (n = 0 or 1) by spectroelectrochemical and DFT theory methods revealed that the HOMO features a symmetry constrained interaction between a metal dz2 orbital and the filled alkynyl 'pi' level. This finding is consistent with the strongly metal based redox chemistry of the group 6 {Mo} system and provides a rationalisation for the contrast in properties with the group 8 complexes [Ru(CCR)(dppe)Cp*]n+ in which the redox orbital has substantial carbon chain character. This key result provided the basis for much of the subsequent molecular design work.



Investigation of the redox chemistry of monometallic, extended carbon chain complexes has resulted in the observation of unique examples of stable 17-electron radicals [{Mo}(CC)xR]+ (x = 2, 3). The stability of these complexes is attributed to the relatively limited participation of the carbon chain ligand to the redox orbital. Secondly the {Mo} auxiliary has been incorporated as an end cap into carbon chain-bridged bimetallic complexes [{Mo}2(mu-B)]n+ (B = 1,12-(CC)2-1,12-carbaborane; butadiynydiyl or 1,4-diethynylbenzene). These systems are characterised by relatively weak interactions between metal centres; increased electronic coupling along the series allows investigation of the progressive deviation from the Hush two-state model in mixed valence systems [{Mo}2(mu-B)]+.



The heterobimetallic [{Mo}(mu-1,4-diethynylbenzene){Fe(dppe)Cp*}]n+ has been investigated in an international collaboration with Professors Lapinte and Halet. This system features a molecular design in which the metal end caps exhibit similar redox potentials but diverse electronic structures. In the monocation (n = 1), spin density may be located at either the Mo or Fe terminus as demonstrated experimentally by EPR spectroscopy, however the distribution of spin density between Fe and Mo is sensitive to the torsion angle between the metal end caps across the bridging ligand. This result may provide a model for conformation dependent, gated electron transfer.



Finally, the correlation between the electronic structure of the metal support group and the redox chemistry of carbon chain complexes was explored further by combined experimental and DFT theory investigations on [W(CCPh)(dppe)(C7H7)]n+, [Mo(CCPh)(CO)(dppe)Cp*]n+ and trans-[Mo(CCPh)(CO)(PMe3)2Cp*]n+. In the case of the MoCp* complexes, the composition and symmetry of the HOMO is dependent upon the cis vs. trans phosphine ligand arrangement and this is reflected in differences in the experimental redox chemistry.
 
Description The Nuffield Foundation
Amount £1,440 (GBP)
Funding ID URB/37028 
Organisation Nuffield Foundation 
Sector Charity/Non Profit
Country United Kingdom
Start  
 
Description The Nuffield Foundation
Amount £1,440 (GBP)
Funding ID URB/37028 
Organisation Nuffield Foundation 
Sector Charity/Non Profit
Country United Kingdom
Start  
 
Description The Nuffield Foundation
Amount £1,360 (GBP)
Funding ID URB/34287 
Organisation Nuffield Foundation 
Sector Charity/Non Profit
Country United Kingdom
Start  
 
Description The Nuffield Foundation
Amount £1,360 (GBP)
Funding ID URB/34287 
Organisation Nuffield Foundation 
Sector Charity/Non Profit
Country United Kingdom
Start