Chemical Exchange Saturation Transfer (CEST) as a Tool to Measure Ligand Flexibility of Chelating Chiral Di-N-heterocyclic Carbene Complexes

Abstract

A series of eight [(diNHC)Rh(CO)2][OTf] complexes, 1′-R, 2′-R, and 3′-R (where diNHC = DEAM, trans-9,10- d ihydro-9,10- e thano a nthracene-11,12-diyl m ethanediyl carbene, and DEA, trans-9,10- d ihydro-9,10- e thano a nthracene-11,12-diyl carbene ligands; R = R-methylphenylmethane (R-CHMePh), diphenylmethane (diPh), benzyl (Bn), methyl (Me), isopropyl ( i Pr), and o-methylbenzyl (o-MeBn); carbene heterocycle = imidazol-2-ylidene, and benzimidazol-2-ylidene), are synthesized and characterized. The purpose for the synthesis of these complexes was to examine the relationship between carbene N-heterocycle, R-group substitution, and metallocycle ring size (DEAM = 9, DEA = 7 atom ring) on ligand flexibility. The complexes were examined by chemical exchange saturation transfer to measure the rate of conformational isomerization. The results indicate that flexibility is not significantly altered by choice of carbene heterocycle and R group. However, the constrained DEA diNHC derivative does not undergo conformational isomerization.

A Chiral Macrocyclic Tetra‐N‐Heterocyclic Carbene Yields an “All Carbene” Iron Alkylidene Complex

“…[30,[37][38][39][40] The split resonances of both the 1 Ha nd 13 CNMR for 5 is ac ommont rait of constrained geometry C 2symmetric bidentate NHC complexes, but for 5 the difference between the 13 Cr esonances are more pronounced. [27,41,42] The solid state structure for 5 confirms this absolute C 2 symmetry and confirms that as ingle enantiomer is formed (Figure 2, top). Complex 5 has an average PdÀCd istance of 2.01 which is consistent with its isostructural complexes and a t 4 value of 0.07 showing that is close to the square planar ideal.…”

“…All previous diamagnetic macrocyclic tetra‐NHC complexes have only shown one resonance in solution implying that they are all equivalent on NMR time scale . The split resonances of both the 1 H and 13 C NMR for 5 is a common trait of constrained geometry C 2 ‐symmetric bidentate NHC complexes, but for 5 the difference between the 13 C resonances are more pronounced . The solid state structure for 5 confirms this absolute C 2 symmetry and confirms that a single enantiomer is formed (Figure , top).…”

Observing the three-dimensional dynamics of supported metal complexes

“…Dynamics have been shown to influence not only enzyme chemistry but also homogeneous catalysis where structural flexibility has been linked to enhanced activity. [22][23][24][25][26][27][28][29][30] Similar arguments have also been made in the case of single-site heterogeneous catalysts. 31 In supported metal catalysis, the rapid dynamics of metal clusters has also been shown to generate short-lived highlyactive species that are thought to dominate catalysis.…”

Synthesis of BINAM‐Based Chiral Di‐1,2,3‐triazolylidene Complexes and Application of the Di‐NHC RhI Catalyst in Enantioselective Hydrosilylation

“…Full conversion of the ligand was observed after 3 hours. [48] The rhodium(III) complexes reported as (pre)catalysts by Shi and co-workers contain two iodido ligands as well as a κ 2 -acetato ligand. Synthesis of rhodium(I) and iridium(I) cod complexes (R)-5 and -6.…”