ReviewStereoelectronic parameters associated with N-heterocyclic carbene (NHC) ligands: A quest for understanding
Introduction
Carbenes are electron-deficient two-coordinate carbon compounds that have two non-bonding electrons on that carbon. In the ground state, the two unshared electrons may be either in the same orbital with antiparallel spins (singlet state), or in two different orbitals with parallel spins (triplet state).
The quest for a stable carbene was long considered an unreasonable target, until Wanzlick showed that the stability of carbenes could be dramatically increased by vicinal amino substituents [1]. However, no isolation of a ‘monomeric’ carbene was achieved at that time. In 1964, Fischer reported the first stable transition metal complexes bearing carbene ligands [2]. The so-called Fischer-carbene complexes were characterized as having a electrophilic carbenic carbon. Ten years later, Schrock isolated a different type of carbenic complex in which the polarization of the metal-carbon bond is inverted and the carbenic carbon was nucleophilic [3].
Between these two groundbreaking discoveries, Wanzlick [4] and Öfele [5] reported independently the first (NHC)-transition metal complexes. In 1988, Bertrand and co-workers succeeded in isolating the first stable carbene [6]. Unfortunately, the reported (phosphino)(silyl)carbene did not show any ability as a ligand for transition metals. The isolation of a free imidazol-2-ylidene by Arduengo et al. in 1991 provided access to numerous transition metal carbene complexes by simple complexation of a stable carbene [7]. The dogma that carbenes were only transient species disappeared and a new and exciting field of research unfolded for synthetic chemists.
The unexpected stability of the N-heterocyclic carbenes (NHCs) has prompted several groups to carry out studies in order to better understand these unusual species. The nature and strength of the (NHC)–metal bond are key information to rational catalyst design. Studies aimed at quantifying phosphine steric and electronic effects have had a major impact on the development of new and improved ligands for catalysis [8].
First considered as simple phosphine mimics [9], there is increasing experimental evidence that (NHC)–metal catalysts surpass their phosphine-based counterparts in both activity and scope [10]. Despite the existence of several families of stable carbenes, only the five-membered cyclic diamino carbenes have found numerous applications so far. Even if exceptions have been reported [11], free acyclic carbenes including diamino carbenes [12] are far more fragile than these NHCs and are poorer ligands, so far, for transition metal complexes [13]. Although some representative examples will be presented, their discussion will not be exhaustive. Structures for the most commonly employed carbene ligands are shown in Fig. 1.
Section snippets
Electronic structure of NHCs: the aromaticity question
To date, all theoretical and experimental evidence indicates that, in order to form a stable carbene, the carbenic carbon needs to be bonded to strong π-donor atoms [14]. However, the remarkable stability of the first isolated carbene, IAd, was totally unexpected at the time. Ab initio studies led Dixon and Arduengo to postulate that p(π)–p(π) delocalization is not extensive and that the bonding in these ligands should be considered carbenic since ylidic resonance structures are not dominant
Electronic factors
Chemical computations often provide good approximations for bond strength values, but the experimental determination by thermodynamic means results in unquestionable values.
A major breakthrough in the importance of NHCs in metal-catalyzed reactions came from the preparation of NHC–ruthenium complexes. These highly thermally stable catalysts C [23] and D [24] (Fig. 2) have allowed the preparation of functionalized carbocycles and heterocycles from the corresponding acyclic diene precursors that
Nature of the (NHC)–metal bond
For decades it has been accepted that divalent carbon species: CR1R2 exhibit σ-donor and π-acceptor properties upon binding to transition metals. The coordination of conventional carbenes depends mainly on π back-bonding since they are weak σ-donor, but early studies suggested that the π-acceptor ability of NHCs, lying between those of nitriles and pyridine, was negligible [57]. However, more recent results point to a more flexible behavior of NHCs where back-donation might importantly
Final remarks
Fifteen years have passed since the isolation of the first NHC. In spite of the true revolution that these ligands have brought to metal-catalyzed reactions, we are still far from entirely understanding the factors governing their reactivity. We are confident that further experimental exploration as well as improved computational methods will continue to highlight the unique features associated with these very versatile ligands.
Acknowledgements
SPN is an ICREA Research Professor. SDG thanks the Education, Research and Universities Department of the Basque Government (Spain) for a postdoctoral fellowship.
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