Quantifying and understanding the steric properties of N-heterocyclic carbenes

Abstract

This Feature Article presents and discusses the use of different methods to quantify and explore the steric impact of N-heterocyclic carbene (NHC) ligands. These include (a) the percent buried volume (%V), which provides a convenient single number to measure steric impact and (b) steric maps, which provide a graphical representation of the steric profile of a ligand using colour-coded contour maps. A critical discussion of the scope and limitations of these tools is presented, along with some examples of their use in organometallic chemistry and catalysis. This Article should provide all users of NHCs, from organic, organometallic, and inorganic chemistry backgrounds, with an appreciation of how these tools can be used to quantify and compare their steric properties.

Increasing the Reactivity of Diborenes: Derivatization of NHC‐Supported Dithienyldiborenes with Electron‐Donor Groups

“…À1.7 ppm), [8] whereas the dibromo derivatives appeared furtheru pfield, around À8ppm (Table 2), as expected from the less electron-withdrawing character of Br versus Cl. The 13 CNMR resonances of the boron-bound NHC and thienyl carbon atoms, broadened by coupling to the quadrupolar [25] d ( 31 P) [ppm] [22,26] %V bur [24] I Me Me H2 054. [8] andt ypicalo fC NHC ÀB sdonor interactions in sp 3 boranes.…”

“…The electronic properties of NHCs are commonly quantified by the Tolman electronic parameter (TEP), [21] which corresponds to the IR stretching frequency of the carbonyll igands in complexes of the form [Ni(CO) 3 (NHC)]: the more s-donating and/or less p-accepting the NHC ligand, the highert he value of the TEP.F urthermore,B ertrand and co-workersh ave shown that the 31 PN MR shifts of NHCp henylphosphinidene adducts provide ar eliable indicator of the relative p-acceptors trength of NHCs:t he more p-accepting the carbene ligand, the further downfield the 31 PN MR shift. [22] The steric properties of NHCs can be conveniently compared using the percentb uried volume( % V bur ), [23] defined here as the percentage of a3 .5 radius sphere around the metal center that is occupied by the ligand,a ssuming am etal-ligand distance of 2 .I Me ,I MeMe ((1,3,4,5-Me 4 )imidazol-2-ylidene), I iPr andI Mes were thus selected for their increasing steric bulk [24] and decreasing TEPs, [25] as well as fort he increased p-acceptor strength going from I Me to I MeMe to I iPr , [22,26] as summarized in Table 1.…”

Accessing Low‐Valent Inorganic Cations by Using an Extremely Bulky N‐Heterocyclic Carbene

“…[5] To our knowledge,t his represents the most sterically encumbering NHC to date and for comparison, the %V bur for the AuCl complexes of the widely used bulky NHCs IPr and IPr* are 45.4 and 50.4 %, respectively (Scheme 1). [6] The coordination profile of ITrw as investigated by the synthesiso favariety of 1:1a dducts (3)(4)(5), as summarized in Scheme 3. The Rh I carbonyl complex [(ITr)Rh(CO) 2 Cl] (5)w as used to extract an estimated donor strength of ITrr elative to previously reported carbenes by determination of the To lman electronic parameter (TEP).…”

Coordination Behavior of a Planar Chiral Cyclic (Amino)(Ferrocenyl)Carbene Ligand in Iridium Complexes

“…[19] According to the observed values at 2057 and 1975 cm À1 (CH 2 Cl 2 ; n av (CO) = 2016cm À1 ), the Tolman electronic parameter( TEP) of CAFeC was estimated to be 2044 cm À1 . [7b, 9a, 16] Finally,t he percentb uried volume (%V bur ) [21] and the steric map were calculated from the solid-states tructure of (R a ,R p )-8 to investigate the steric properties of 3 ( Figure 3). [7b, 9a, 16] Finally,t he percentb uried volume (%V bur ) [21] and the steric map were calculated from the solid-states tructure of (R a ,R p )-8 to investigate the steric properties of 3 ( Figure 3).…”