title: Modulating the Bonding Properties of N-Heterocyclic Carbenes (NHCs): A Systematic Charge-Displacement Analysis.
abstract: In view of their intensive use as ligands in many reactions catalyzed by transition-metal complexes, modulation of the bonding properties of N-heterocyclic carbenes (NHCs) on a rational basis is highly desirable, which should enable optimization of current applications or even promote new functions. In this paper, we provide a quantitative analysis of the chemical bond between a metal fragment AuCl and a series of 29 different NHCs in [(NHC)AuCl] complexes. NHCs electronic properties are modified through: i) variation of the groups attached to the NHC nitrogen atoms or backbone; ii) change of unsaturation/size of the NHC ring; iii) inclusion of paracyclophane moieties; or iv) heteroatom substitution on the NHC ring. For evaluating the donation and back-donation components of the Dewar-Chatt-Duncanson (DCD) model in the NHC-AuCl bond, we apply the charge-displacement (CD) analysis within the NOCV (natural orbitals for chemical valence) framework, a methodology that avoids the constraint of using symmetrized structures. We show that modulation of the NHC bonding properties requires substantial modification of their structure, such as, for instance, insertion of two ketone groups into the NHC backbone (which enhances the π back-donation bond component and introduces an effective electronic communication within the NHC ring) or replacement of a nitrogen atom in the ring with an sp3or sp2carbon atom (which increases and decreases the π back-donation bond component, respectively). We extend our investigation by quantitatively comparing the NHC electronic structures for a subset of 13 NHCs in [(NHC)PPh] adducts, the31P NMR chemical shift values of which are experimentally available. The latter have been considered as a suitable tool for measuring the NHCs π acceptor properties [Bertrand et al., Angew. Chem. Int. Ed. 2013, 52, 2939-2943]. We show that information obtained using the metal fragment can be transferred to the PPh moiety and vice versa. However, the31P NMR chemical shift values only qualitatively correlate with the π acceptor properties of the NHCs, with the stronger π acidic carbenes as the most outliners.
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.
authors.forename: Carlo Alberto
authors.initials: CA
authors.lastname: Gaggioli
authors.affiliation: Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany.
authors.forename: Giovanni
authors.identifier: http://orcid.org/0000-0003-4849-1323
authors.initials: G
authors.lastname: Bistoni
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Giuseppe Moruzzi 13, 56124, Pisa, Italy.
authors.forename: Gianluca
authors.identifier: http://orcid.org/0000-0001-5113-2351
authors.initials: G
authors.lastname: Ciancaleoni
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.|Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM) c/o Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto 8, 06123, Perugia, Italy.
authors.forename: Francesco
authors.identifier: http://orcid.org/0000-0002-1285-0606
authors.initials: F
authors.lastname: Tarantelli
authors.affiliation: Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM) c/o Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto 8, 06123, Perugia, Italy.
authors.forename: Leonardo
authors.identifier: http://orcid.org/0000-0002-2888-4990
authors.initials: L
authors.lastname: Belpassi
authors.affiliation: Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto 8, 06123, Perugia, Italy.|Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM) c/o Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, via Elce di Sotto 8, 06123, Perugia, Italy.
authors.forename: Paola
authors.identifier: http://orcid.org/0000-0002-1286-9294
authors.initials: P
authors.lastname: Belanzoni
keywords: N-heterocyclic carbenes (NHCs); NMR spectroscopy; bond theory; coordination chemistry; density functional calculations