DOI: 10.1002/chem.201103319 Charged Behaviour from Neutral Ligands: Synthesis and Properties of N- Heterocyclic Pseudo-amides Robert J. Thatcher, DavidG. Johnson, John M. Slattery,* and Richard E. Douthwaite* [a] Introduction As prototypical s and p donors, amido ligands, [NR 2 ]  , sup- port a wide range of metal complex reactivity and are also key intermediates in bond forming reactions, such as C N coupling. [1] Containing a formally anionic N III atom donor, amido ligands have two lone pairs available for metal– ligand bonding. Isoelectronic and broadly isostructural with amido ligands are neutral carbodicarbene compounds for- mally defined as C 0 . Examples, such as 1 (Scheme 1) have been isolated and have challenged our understanding of carbon chemistry. [2] Derived from coordination of carbenes or heteroatom moieties to a carbon atom, two filled carbon based orbitals are available for metal coordination. [3] Both amido and carbodicarbene compounds can be considered to possess localized lone pairs on the donor atom. In comparison, disubstituted, formally neutral N ligands exhibiting amido-like character can be envisaged, in which in lieu of a formal negative charge, N atom donation is sup- ported by conjugation with a substituent (Scheme 1). [4] Con- sequently p donation by the ligand can be modified to sup- port coordination and metal complex reactivity. [4, 5] For ex- ample compounds 2 and 3 (Scheme 1) exhibit canonical res- onance structures, and spectroscopic and X-ray structural data indicate a contribution to metal–ligand bonding from the azolium-amido forms 2’ and 3’. In related chemistry, the N donor ligand 4 exhibits chemistry reflective of an N I ligand, which incorporates a cyclopropenium substituent. [4a] Furthermore, a number of ligands incorporating the guani- dine motif have been developed, in which N atom donation is supported by conjugation to the amino moieties. [6] Com- plexes of some of these ligands have been investigated for a range of applications, including catalysis [7] and biomimetic chemistry. [8] The development of formally neutral analogues of amido ligands, which exhibit variable electron donation, could po- tentially open up fundamentally different catalytic pathways by ligand dissociation that are not available to anionic amido ligands. Catalytic applications of this novel class of li- gands are, as yet, rare. However, ligand 2 has recently been Abstract: Deprotonation of the 1-iso- propyl-3-(phenylamino)pyridin-1-ium iodide gives the corresponding neutral betaine, which is formalised as a pyridi- nium-amido ligand when coordinated to a metal. Spectroscopic, structural and theoretical methods have been used to investigate the metal–ligand bonding, ligand dynamics and electron distribution. Collectively, the data show that the ligand can be characterised as a pseudo-amide and is a strong donor akin to alkyl phosphines and N-hetero- cyclic carbenes. Furthermore, rotation about both N substituent C N bonds occurs, which is in contrast to the two alternative pyridinium positional iso- mers that exhibit neutral resonance structures. For comparison, compounds and complexes derived from norhar- man were prepared, which contain an additional C C bond supporting conju- gation and the accessibility of a neutral resonance structure. Notwithstanding the formal neutral structure, norhar- man-derived ligands are comparably strong donors, and have the additional advantage of exhibiting stability to di- oxygen and water. Keywords: aromaticity · coordina- tion chemistry · ligand design · N li- gands · nitrogen heterocycles [a] R. J. Thatcher, D. G. Johnson, Dr. J. M. Slattery, Dr. R. E. Douthwaite Department of Chemistry, University of York Heslington, York YO10 5DD (UK) Fax: (+ 44) 1904-322516 E-mail : richard.douthwaite@york.ac.uk john.slattery@york.ac.uk Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103319. Scheme 1. Comparison of carbodicarbene and neutral N donor ligands exhibiting amido-like character. Chem. Eur. J. 2012, 18, 4329 – 4336  2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 4329 FULL PAPER