Determining the Ligand Properties of N‐Heterocyclic Carbenes from <sup>77</sup>Se NMR Parameters

Verlinden, Kathrin; Buhl, Hannes; Frank, Walter; Ganter, C.

doi:10.1002/ejic.201500174

Cited by 250 publications

(280 citation statements)

References 90 publications

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“…The Ag atom is in an almost linear arrangement between the two (8) is also in line with the coordination to transition metals. 37 It was found that poor σ-donors reveal high coupling constants for the C-H bond, which can be explained by a higher s-orbital character of the corresponding C-H bond 38 25[OTf ] is extremely sensitive and decomposes readily in solution which prevents its isolation. S2.10 †).…”

Section: Resultsmentioning

confidence: 99%

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

et al. 2016

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2-Phosphanyl-substituted imidazolium salts 2-PR2(4,5-Cl-Im)[OTf] (9a,b[OTf]) (4,5-Cl-Im = 4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolium) (a: R = Cy, b: R = Ph) are prepared from the reaction of R2PCl (R = Cy, Ph) with NHC 8 (4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) in the presence of Me3SiOTf. 5-Phospanyl-substituted imidazolium salts 5-PR2(2,4-Cl-Im)[OTf] (10a,b[OTf]) are obtained in quantitative yield when a slight excess of the NHC 8 is used. 5-Phosphonio-substituted imidazolium salts 5-PR2Me(2,4-Cl-Im)[OTf]2 (14a,b[OTf]2) and 5-PR2F(2,4-Cl-Im)[OTf]2 (16a,b[OTf]2) result from methylation reaction or oxidation of 10a,b[OTf] with XeF2 and subsequent fluoride abstraction. According to our quantum chemical studies the Cl1 atom at the 2-position at the imidazolium ring of dication 14b(2+) carries a slightly positive charge and is therefore accessible for nucleophilic attack. Accordingly, the reaction of 14a,b[OTf]2 and 16a,b[OTf]2 with R3P (R = Cy, Ph) affords cationic 5-phosphonio-substituted NHCs 5-PR2Me(4-Cl-NHC)[OTf] (17a,b[OTf]) and 5-PR2F(4-Cl-NHC)[OTf] (18a,b[OTf]) via a SN2(Cl)-type reaction. A series of transition metal complexes such as [AuCl(5-PPh2Me(4-Cl-NHC))][OTf] (19[OTf]), [CuBr(5-PPh2Me(4-Cl-NHC))][OTf] (20[OTf]), [AuCl(5-PPh2F(4-Cl-NHC))[OTf] (21[OTf]) and [RhCl(cod)(5-PPh2Me(4-Cl-NHC))][OTf] (23[OTf]) are prepared to prove the coordination abilities of carbenes 17b[OTf] and 18b[OTf]. The isolation of a rare example of a tricationic bis-carbene silver complex [Ag(5-PPh2Me(4-Cl-NHC))2][OTf]3 (22[OTf]3) is achieved by reacting 14b[OTf] with Cy3P in the presence of AgOTf. NHC 17b[OTf] represents a very effective dehydrocoupling reagent for secondary (R2PH, R = Ph, Cy, (i)Bu) and primary (RPH2, R = Ph, Cy) phosphanes to give diphosphanes of type R4P2 (R = Ph, Cy, (i)Bu) and oligophosphanes R4P4, R5P5 (R = Ph, Cy), respectively. Methylation of 17b(+) and subsequent deprotonation reaction with LDA affords the cationic NHO (N-heterocyclic olefin) 35(+) of which the gold complex 36(+) is readily accessible via the reaction with AuCl(tht).

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Section: Resultsmentioning

confidence: 99%

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

et al. 2016

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“…In 2013, Ganter presented the 77 Se NMR chemical shift of the respective selenoureas as a suitable probe to determine directly the π-influence of the carbene [42–43], as the paramagnetic shift tensor has the largest influence on the 77 Se NMR chemical shift. This method is so far redundant [44] to the method of determining the 31 P NMR chemical shifts of the respective NHC–phosphinidene adducts [45–46]. …”

Section: Introductionmentioning

confidence: 99%

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Nonnenmacher¹,

Buck²,

Kunz³

2016

Beilstein J. Org. Chem.

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SummaryRh(CO)2Cl(NHC) complexes of dipyrido-annelated N-heterocyclic carbenes were prepared. From the C–H coupling constant of the respective imidazolium salts and the N–C–N angle of the N-heterocyclic carbene (NHC), a weaker σ-donor character than that of typical unsaturated NHCs is expected. However, the IR stretching frequencies of their Rh(CO)2Cl complexes suggest an electron-donor character even stronger than that of saturated NHCs. We ascribe this to the extremely weak π-acceptor character of the dipyrido-annelated NHCs caused by the conjugated 14 πe− system that thus allows for an enhanced Rh–CO backbonding. This extremely low π-acceptor ability is also corroborated by the 77Se NMR chemical shift of −55.8 ppm for the respective selenourea, the lowest value ever measured for imidazole derived selenoureas. DFT-calculations of the free carbene confirm the low σ-donor character by the fact that the σ-orbital of the carbene is the HOMO−1 that lies 0.58 eV below the HOMO which is located at the π-system. Natural population analysis reveals the lowest occupation of the pπ-orbital for the saturated carbene carbon atom and the highest for the pyrido-annelated carbene. Going from the free carbene to the Rh(CO)2Cl(NHC) complexes, the increase in occupancy of the complete π-system of the carbene ligand upon coordination is lowest for the pyrido-annelated carbene and highest for the saturated carbene.

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“…However, the correlation coefficient with the electron donating indices TEP is worse (R 2 = 0.833). 262 An improved correlation of -electron donation with the NMR coupling constant 1 JCSe has been reported. 262 However, the data for this index are considerably scarcer than those for the chemical shifts of 31 Р and 77 Se.…”

Section: 251mentioning

confidence: 98%

“…262 An improved correlation of -electron donation with the NMR coupling constant 1 JCSe has been reported. 262 However, the data for this index are considerably scarcer than those for the chemical shifts of 31 Р and 77 Se. Obviously, the chemical shift of 77 Se, as well as other NMR indices, are under the influence not only of electronic but also anisotropic shielding effects.…”

Section: 251mentioning

confidence: 98%

“…As a criterion of the -electron accepting properties, the measurement of the chemical shifts of hetero groups bonded to the carbene ( 31 Р and 77 Se) have been proposed; [259][260][261][262][263][264][265][266] these have been found to correlate, one with the other, moderately well (R 2 = 0.9051). However, the correlation coefficient with the electron donating indices TEP is worse (R 2 = 0.833).…”

Section: 251mentioning

confidence: 99%

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Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

Коротких¹,

Cowley²,

Clyburne³

et al. 2017

Arkivoc

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Determining the Ligand Properties of N‐Heterocyclic Carbenes from ⁷⁷Se NMR Parameters

Cited by 250 publications

References 90 publications

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

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Determining the Ligand Properties of N‐Heterocyclic Carbenes from 77Se NMR Parameters

Cited by 250 publications

References 90 publications

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

Cationic 5-phosphonio-substituted N-heterocyclic carbenes

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

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Determining the Ligand Properties of N‐Heterocyclic Carbenes from ⁷⁷Se NMR Parameters