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Chem. Commun., 2005, 784–786

Molecular N 2complexes of iron stabilised by N -heterocyclic ‘pincer’dicarbene ligands {

Andreas A.Danopoulos,*a Joseph A.Wright a and William B.Motherwell b

Received (in Cambridge,UK)7th October 2004,Accepted 15th November 2004First published as an Advance Article on the web 22nd December 2004DOI:10.1039/b415562a

The first N 2complex stabilised by N -heterocyclic carbene ligands,Fe(C–N–C)(N 2)2,has been obtained by the reduction of Fe(C–N–C)Br 2where C–N–C 52,6-bis(aryl-imidazol-2-ylidene)pyridine,aryl 52,6-Pr i 2C 6H 3,with Na(Hg);it serves as a convenient precursor for other iron NHC ‘pincer’complexes of the type Fe(C–N–C)(N 2)L where L 5C 2H 4,PMe 3and Fe(C–N–C)(CO)2.

Since their discovery,N 2complexes of the transition metals have been considered as important intermediates in plausible artificial systems for the fixation and activation of molecular nitrogen under mild conditions.1It is generally accepted that fixation of N 2is accompanied by some ‘activation’,which can be probed by structural and spectroscopic means and may lead to increased reactivity.The degree of ‘activation’is dependent on the type of the metal and the electronic properties of co-ligands present in the complex.Most of the coordinated N 2cleavage reactions have been observed with low oxidation state phosphine complexes of group 5and 6metals.1a However,recent work with versatile higher oxidation state Mo amido complexes gave remarkable systems capable of N 2cleavage and reduction.1b ,2

In general,most synthetic Fe–N 2complexes are considered as ‘unactivated’,even though iron plays an important role in biological and industrial nitrogen derivatisation.3,4

Recent work by Holland and Peters has given iron complexes with ‘activated’bridging dinitrogen.5However,the search for new ligand topologies and donor environments which could give rise to iron complexes capable of dinitrogen fixation and activation is still an active area of research.We envisaged that dinitrogen complexes of iron with preorganised co-ligands incorporating strongly s -donating NHCs could provide strongly p -basic metal centres capable of N 2activation.In addition,they may serve as useful precursors to other reactive iron NHC complexes.

In this communication,we wish to report the first iron(0)dinitrogen complexes stabilised by NHC ligands.We also describe preliminary results on their use as convenient precursors for other iron NHC complexes.

Reduction of Fe(C–N–C)Br 26with an excess of Na(Hg)in thf under N 2{gave red-brown solutions from which,after filtration and crystallisation from pentane,green-brown crystals of 1can be isolated (see Scheme 1).

The structure of 1in the solid state determined by X-ray diffraction is shown in Fig.1.The geometry is best described as

distorted square pyramidal with one apical and one basal end-on dinitrogen ligand.The N M N bond lengths are very similar and

slightly elongated from the free N M N value (1.0968A

˚);they,as well as the Fe–N bond lengths,are comparable to those reported for other mono-N 2complexes of iron.4The iron atom is positioned

0.3556(12)A

˚above the basal http://www.doczj.com/doc/ccced81aa1c7aa00b42acb82.html plex 1is diamagnetic and is stable in C 6D 6solutions for a limited time (ca .1day);the 1H-NMR spectrum indicates that in solution it adopts a C 2v symmetric structure,supporting a trigonal bipyramidal geometry or fluxionality.The intense infra-red absorptions {at 2109and 2031cm 21can be assigned to either square pyramidal or trigonal bipyramidal geometry.However,the presence of a weaker band at 2044cm 21could be assigned as part of a partially obscured pair suggesting the presence of an equilibrium between trigonal bipyramidal and square pyramidal structures in solution.The position of the absorptions indicate reduced backbonding to the coordinated N 2compared with other Fe(N 2)(P–P)2complexes (1950–2068cm 21).4Complex 1is the first structurally characterised bis-dinitrogen complex of iron and the first dinitrogen complex stabilised by NHC ligands.§

{Electronic supplementary information (ESI)available:experimental procedures and spectroscopic characterisation data.See http://www.doczj.com/doc/ccced81aa1c7aa00b42acb82.html /suppdata/cc/b4/b415562a/

Chem. Commun., 2005, 784–786

*ad1@soton.ac.uk

Scheme 1The Fe complexes described in this paper;reagents and conditions:(i)Na(Hg),thf,N 2;(ii)C 2H 4,thf;(iii)CO,thf;(iv)Na(Hg)–CO;(v)PR 3,thf;(vi)Na(Hg),PR 3,R 5Me,4a ;cyclohexyl,4b .

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P u b l i s h e d o n 22 D e c e m b e r 2004. D o w n l o a d e d b y L a n z h o u I n s t i t u t e o f C h e m i c a l P h y s i c s , C A S o n 22/01/2016 08:42:07.

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