New, potentially chelating NHC ligands; synthesis, complexation studies, and preliminary catalytic evaluation

Abstract

Two new N-heterocyclic carbene (NHC) ligands bearing 2-morpholino and 2-piperidinyl naphthyl wingtips were synthesised (2-SIMorNap and 2-SIPipNap). Nuclear magnetic resonance studies, in conjunction with crystal structures and derivatisation of the NHC salts using a chiral counteranion, revealed that the ligand wingtips are oriented anti with respect to each other. From the free carbene, palladium, ruthenium and iridium complexes were prepared. NHC-iridium dicarbonyl complexes were made in order to extract the TEP values for these ligands. The study showed that these NHC ligands are more electron-donating than normal, aryl-substituted NHCs. The palladium complexes were tested in representative Suzuki-Miyaura cross-coupling reactions and compared to the state of the art systems. Ruthenium-catalysed ring-closing metathesis with these ligands was also performed. It was found that Grubbs' 2 generation catalyst incorporating 2-SIPipNap did not initiate at room temperature and required heating for RCM to occur.

N-Heterocyclic Carbene Complexes of Nickel, Palladium, and Iridium Derived from Nitron: Synthesis, Structures, and Catalytic Properties

“…The Ir–C bonds associated with the olefin moiety are longer than that of the Ir–C NHC bond, with those trans to the Nitron NHC ligand (2.162(5) and 2.182(5) Å) being longer than those cis to Nitron NHC (i.e. trans to Cl) (2.092(5) and 2.109(5) Å); as such, it is evident that the Nitron NHC ligand exerts a stronger trans influence than does Cl. ,,, These differences in Ir–C bond lengths are reproduced in the DFT geometry optimized structure (Figure ): namely, Ir–C NHC (2.027 Å), Ir–C trans (2.217 and 2.249 Å), and Ir–C cis (2.127 and 2.147 Å). One other noteworthy structural feature of (Nitron NHC )­Ir­(COD)Cl is that the N–H group participates in a hydrogen-bonding interaction with a molecule of acetonitrile, with an N···N distance of 3.017(7) Å, which is slightly shorter than the average value of 3.11 Å for compounds with N–H···NCMe interactions listed in the CSD …”

“…Specifically, square-planar ML 2 (CO)­X compounds may exhibit disorder between pairs of mutually trans CO and X ligands and this type of disorder can result in an artificial apparent lengthening of the Ir–CO bond that is cis to the NHC ligand. ,, An illustration of the ability of disorder to influence the Ir–CO bond lengths in this system is provided by the fact that there are several derivatives that possess more than one crystallographically independent molecule in the asymmetric unit and, for these structures, the Ir–CO bond lengths for the ligands that are trans to Cl are significantly different. For example, the Ir–CO bond lengths for the carbonyl groups that are cis to the NHC ligand in the two crystallographically independent molecules of (SIPr)­Ir­(CO) 2 Cl differ considerably, namely 1.855(6) and 1.959(4) Å, whereas those that are trans to the NHC ligand are much more similar, namely 1.872(5) and 1.883(5) Å .…”

“…The dicarbonyl complex (Nitron NHC )­Ir­(CO) 2 Cl is also of relevance because the LIr­(CO) 2 Cl platform has been introduced as a convenient alternative to LNi­(CO) 3 for evaluating the electron-donating properties of a donor ligand. ,,,− ,− Specifically, the Tolman electronic parameter (TEP), which is the frequency of the A 1 symmetry ν­(CO) vibrational mode of LNi­(CO) 3 complexes and is an indicator of the donor ability of the L ligand, , is empirically related to the average value of the ν­(CO) stretching frequencies (in cm –1 ) of LIr­(CO) 2 Cl via the expression TEP = 0.8475­[ν­(CO) av ] + 336.2. ,, On this basis, the value of ν­(CO) av for (Nitron NHC )­Ir­(CO) 2 Cl (2030.0 cm –1 ) predicts a TEP of 2056.6 cm –1 for Nitron NHC , which compares favorably to the value of 2057.4 cm –1 predicted by using the rhodium counterpart and the corresponding empirical relationship for the rhodium system LRh­(CO) 2 Cl . Both the ν­(CO) av values for (Nitron NHC )­Ir­(CO) 2 Cl and the TEP value indicate that the electronic properties of Nitron NHC are very similar to those of the structurally similar Enders carbene (2030.8 and 2057.3 cm –1 , respectively). , However, it is evident that Nitron NHC is less electron donating than imidazol-2-ylidenes with aryl substituents.…”

Crystal structures of [μ2-(Ra ,Sa ,3aR,7aR)-1,3-bis(2,7-dicyclohexylnaphthalen-1-yl)octahydro-1H-benzo[d]imidazolidin-2-ylidene]chlorido(η4-1,5-cyclooctadiene)iridium dichloromethane monosolvate and [μ2-(Sa ,Sa ,3aR,7aR)-1,3-bis(2,7-dicyclohexylnaphthalen-1-yl)octahydro-1H-benzo[d<

“…With respect to other complexes of general formula (NHC)Ir(COD)Cl, where COD is cyclooctadiene, these bond lengths are quite similar to 2.041 (3) Å in (SIMes)Ir(COD)Cl, 2.049 (5) Å in (SIPr)Ir(COD)Cl (Kelly et al, 2008), 2.034 (1) Å in (anti-2-SICyNap)Ir(COD)Cl, 2.052 (5) Å in (anti-2,7-SICyNap)Ir(COD)Cl (Sipos et al, 2016), 2.034 (3) Å in (anti-2-SICyOctNap)Ir(COD)Cl and 2.053 (5) Å [(R a ,R a ,S,S)-DiPh-2,7-SICyNap]Ir(COD)Cl (Gao et al, 2020). Whilst being shorter than the values of 2.072 (6) and 2.055 (6) Å for two independent molecules of (2-SIMor-Nap)Ir(COD)Cl (Ou et al, 2017), the bonds in the title compounds are slightly longer than in [(R a ,S a ,S,S)-DiPh-2-SICyNap]Ir(COD)Cl [2.028 (7) Å ; Gao et al, 2020]. DiPh (diphenyl) and FuCy (fused cyclohexyl) refer to the backbone of the NHC, and SI (saturated imidazolium) refers to the NHC type.…”