N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.

http://aether.cmi.ua.ac.be/artikels/Artikels%20Gitte/Artikels/Reviews/NHC-%20liganden%20Herrmann.pdf

N-heterocyclic carbenes: a new concept in organometallic catalysis.

A decade ago we initiated research, the goal of which was isolation of a stable carbene. Our success has helped to catalyze a resurgence of interest in readily available and easily handled carbenes. Research on stable (nucleophilic) carbenes is again a popular theme worldwide. Efforts in the general area of stable carbenes now focus not only on chemistry of the carbenes themselves but also on their applications to other chemical systems, where their chemical properties create technical opportunities that are unavailable with other functional groups. The quest for isolable carbenes is a long saga whose origin can be traced back to the first half of the 1800s. A recently published history of this quest provides an important backdrop for the research described in this Account.1 It is the intent of this Account to delineate the events and environment that led to the report from DuPont laboratories of the first isolation of a stable crystalline carbene. Getting Started

Looking for Stable Carbenes: The Difficulty in Starting Anew

N-heterocyclic carbene analogues with low-valent group 13 and group 14 elements: syntheses, structures, and reactivities of a new generation of multitalented ligands.

In recent decades, it has generally been recognized that carbenes play an important role as transient intermediates. As a result of a number of stable carbenes having been isolated and investigated in detail, it is not an exaggeration to say that the chemistry of carbenes has been thoroughly investigated and is now well-understood.1 In addition, much attention has also been paid to the heavier analogues of carbenes, i.e., silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:). These so-called metallylenes are monomeric species of the polymetallanes. This is especially true of the silylenes, which are believed to be monomers of polysilane. The metallylenes could be expected to be of great importance in fundamental and applied chemistry as a result of their many differences and similarities to carbenes. The valency of the central atom of the heavier carbene analogues (R2M:, M ) Si, Ge, Sn, Pb) is two. That is, its oxidation state is MII and its stability increases as the principal quantum number (n) increases. In fact, dichloroplumbylene and dichlorostannylene, PbCl2 and SnCl2, respectively, are very stable ionic compounds. However, these dihalides exist as polymers or ion pairs both in solution and in the solid state. The dichlorogermylene complex GeCl2 · (dioxane)3 is also known to be stable and isolable, whereas the dihalosilylenes are barely isolable compounds.2 The early silylene research was concerned largely with comparing the chemistry of the dihalosilylenes with that of carbenes. Hence, the chemistry of the metallylenes has been considered mainly from the molecular chemistry point of view.4 In contrast to the carbon atom, the heavier group 14 atoms have a low ability to form hybrid orbitals. They therefore prefer the (ns)2(np)2 valence electron configurations in their divalent species.5 Since two electrons remain as a singlet pair in the ns orbital, the ground state of H2M: (M ) Si, Ge, Sn, Pb) is a singlet, unlike the case of H2C:, where the ground state is a triplet (Figure 1).1a On the basis of theoretical calculations, the singlet-triplet energy differences ∆EST for H2M, [∆EST ) E(triplet) E(singlet)], are found to be 16.7 (M ) Si), 21.8 (M ) Ge), 24.8 (M ) Sn), and 34.8 (M ) Pb) kcal/mol, respectively. That of H2C: is estimated as -14.0 kcal/mol.6 Furthermore, the relative stabilities of the singlet species of R2M: (M ) C, Si, Ge, Sn, Pb; R ) alkyl or aryl) compared to the corresponding dimer, R2MdMR2, are estimated to increase as the element row descends, C < Si < Ge < Sn < Pb. It follows, therefore, that one can expect that a divalent organolead compound such as plumbylene should be isolable as a stable compound. However, some plumbylenes, without any electronic or steric stabilization effects, are known to be thermally unstable and undergo facile disproportionation reactions, giving rise to elemental lead and the corresponding tetravalent organolead compounds.7 On this basis, it could be concluded that it might be difficult to isolate metallylenes as stable compounds under ambient conditions, since they generally exhibit extremely high reactivity toward other molecules as well as themselves. Metallylenes have a singlet ground state with a vacant p-orbital and a lone pair of valence orbitals. This extremely high reactivity must be due to their vacant p-orbitals, since 6 valence electrons is less than the 8 electrons of the “octet rule”. Their lone pair is expected to be inert due to its high s-character. In order to stabilize metallylenes enough to be isolated, either some thermodynamic and/or kinetic stabilization of the reactive vacant p-orbital is required (Figure 2). A range of “isolable” metallylenes have been synthesized through the thermodynamic stabilization of coordinating Cp* ligands, the inclusion of heteroatoms such as N, O, and P, * To whom correspondence should be addressed. Phone: +81-774-38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 3479–3511 3479

https://pubs.acs.org/doi/pdf/10.1021/cr900294f

Stable Heavier Carbene Analogues

Aromaticity as a cornerstone of heterocyclic chemistry.

Synthesis and Structure of a Stable Silylene

The He I and He II photoelectron spectra are reported and assigned for three closely related molecules, Si(ButNCHCHNBut), Si(ButNCH2CH2NBut) and SiH2(ButNCHCHNBut), providing evidence for an energetically significant contribution from the silicon 3pπ orbital to the ring π system of Si(ButNCHCHNBut). Theoretical calculations on the model compounds Si(HNCHCHNH), Si(HNCH2CH2NH) and SiH2(HNCHCHNH) have been carried out and there is good agreement between the calculated and experimental geometries. Trends in the orbital energies corresponded closely with the trends found for the ionization energies of the photoelectron bands. Calculated singlet–triplet splittings predict singlet ground states for the model silyienes. Calculated heats of hydrogenation at the ring carbons of Si(HNCHCHNH) and SiH2(HNCHCHNH) show the latter to be more exothermic by 13 kcal mol–1 providing evidence for aromatic cyclic (4n+ 2)-π delocalization in Si(ButNCHCHNBut). Estimates of Si–N double-bond character are obtained by ab initio calculations on rotamers of the model compound Si(NH2)2 and compared with analogous values for C(NH2)2.

Electronic structure of a stable silylene: photoelectron spectra and theoretical calculations of Si(NRCHCHNR), Si(NRCH2CH2NR) and SiH2(NRCHCHNR)

Synthesis of a silylene–borane adduct and its slow conversion to a silylborane

The reaction between B(C
 6
F
 5
)
 3
 and
[Fe(η-C
 5
H
 5
)(CO)
 2
Me] gave the
unexpected product
[Fe{C
 6
F
 4
C(O)Me-2}(η-C
 5
H

 5
)(CO)] 1 which reacts with the donor molecules
L = PMe
 3
, PPh
 3
 or Bu
 t
NC
giving
[Fe{C
 6
F
 4
C(O)Me-2}(η-C
 5
H

 5
)L].* Likewise
[{Fe[C
 6
F
 4
C(O)Me-2](η-C
 5
H
 
5
)}
 2
(µ-dppe)]* was formed from 1 and
Ph
 2
P(CH
 2
)
 2
PPh
 2
 (dppe). The
formation of the compound where L = PMe
 3
 is
shown to proceed via initial formation of
[Fe{C
 6
F
 4
C(O)Me-2}(η-C
 5
H

 5
)(CO)(PMe
 3
)]*. The reaction between
B(C
 6
F
 5
)
 3
 and
[M(η-C
 5
H
 5
)(CO)
 3
Me]
(M = Mo or W) gave the compounds
[M{C
 6
F
 4
C(O)Me-2}(η-C
 5
H

 5
)(CO)
 2
]. The compounds
[M{C
 6
F
 3
X-5-C(O)Me-2}(η-C
 5

H
 5
)(CO)
 2
]* (M = Mo or W) where
X = H were prepared from the compounds where
X = F via unusual C–F bond-activation
reactions. The compound
[Fe{C
 6
F
 3
H-5-C(O)Me-2}(η-C
 5

H
 5
)(PMe
 3
)] was prepared by photolysis of a
mixture of
[Fe{C
 6
F
 3
H-5-C(O)Me-2}(η-C
 5

H
 5
)(CO)] and PMe
 3
. The asterisk indicates
the crystal structure has been determined.

Synthesis of fluorophenyl derivatives of iron, molybdenum and tungstenvia B(C6F5)3 and unusualcarbon–fluorine bond reactions

1,2-Bis(dimethylamino)-1,2-di-9-fluorenyldiborane(4) (1) is doubly metallated with group 1 and 2 metals to yield metal 1,2-bis(dimethylamino)-1,2-di-9-fluorenyldiborate(4) 2-Li to 2-Ba. The conformations of these compounds in solution are investigated by NMR spectroscopy, and the solid-state structures of 2-Li · 5 THF, 2-K · 4 THF, 2-K · 2 DME, and 2-Ca · 2 THF are determined by X-ray diffraction. All four structures can be described as main group metallocenes. The potassium derivatives form contact ion pairs, leading to polymeric chain-like structures. In contrast, 2-Li · 5 THF exhibits the rare combination of a contact ion pair with a solventseparated ion pair, giving the monomeric structure [Li(THF)4][(fl2B2(NMe2)2)Li · THF]. 2-Ca · 2 THF, which is also monomeric, adapts a severely distorted geometry.

Nils Metzler

Papers

Synthesis and Structure of a Stable Silylene

Electronic structure of a stable silylene: photoelectron spectra and theoretical calculations of Si(NRCHCHNR), Si(NRCH2CH2NR) and SiH2(NRCHCHNR)

Synthesis of a silylene–borane adduct and its slow conversion to a silylborane

Synthesis of fluorophenyl derivatives of iron, molybdenum and tungstenvia B(C6F5)3 and unusualcarbon–fluorine bond reactions

Hetero‐ansa‐Bridged Main Group Metallocenes — a Structural and NMR Study