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DOI: 10.1055/s-0033-1340674
Post-Functionalization: A Useful Method for the Synthesis of Donor-Functionalized N-Heterocyclic Carbene–Transition-Metal Catalysts
Publication History
Received: 21 November 2013
Accepted after revision: 04 January 2014
Publication Date:
11 February 2014 (online)
Abstract
Functionalized N-heterocyclic carbene ligands are particularly useful and widely applied ligands in transition-metal chemistry and catalysis. In this article we introduce an emerging synthetic method for functionalized N-heterocyclic carbene ligands developed from the reactivity study on cyclometalated N-heterocyclic carbene compounds, namely postfunctionalization. The status quo of this method for the preparation of novel donor-functionalized N-heterocyclic carbene complexes and their application in catalysis are discussed.
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N-Heterocyclic carbenes (NHC) have emerged as a class of very useful ligands in organometallic chemistry and transition-metal catalysis.[1] Along with people’s efforts to develop new carbene ligands,[2] endeavors on NHC-based chelating ligands have also been exercised and have led to the emergence of a large body of such ligands bearing versatile chelating donors, such as alkoxide, amine, imine, thioether, phosphine, heterocycles, and so on.[3] Ideally, NHC-based chelating ligands could have their unique features beyond those of monodentate NHC in at least two aspects: i) enhanced metal–ligand interaction and consequently improving the stability of metal–NHC complexes, and ii) more readily tunable electronic and steric properties that enable handy control of the stereoelectronic environment of metal’s reactive sites. These features have endowed chelating NHC-supported metal-complex-catalyzed reactions with large turnover number, rapid reaction rate, and high selectivity.[1] [2] [3]
Several synthetic methods exist for NHC-based chelating ligands.[3] The most popular are the substitution reactions of donor-tethered electrophiles with imidazoles. As the substitution reactions require primary organic electrophiles, ring-closure reactions between donor-tethered amines and formal aldehyde or orthoformate could be an alternative route. The third method is the template-controlled synthesis that has been used mainly for macrocyclic NHC-based ligands.[4] These existing methods have enabled the preparation of most of the desired chelating NHC ligands. However, for chelating NHC with borane and silyl as the ancillary donors, which are potentially very useful ligands as inferred from the success of borane- and silylphosphine ligands in transition-metal chemistry and catalysis,[5] [6] the conventional methods might be invalid because of the inherent challenge of installing Lewis acidic boranes or silanes with nucleophilic carbenes in the same ligand scaffold.[7]
In this article, we introduce a new synthetic method for chelating NHC ligand scaffolds, namely postfunctionalization, by utilizing the reactivity of cyclometalated NHC complexes toward organic functionalities (Scheme [1]). This method is effective for the preparation of novel donor-functionalized chelating NHC ligands, including borane- and silyl-functionalized NHC ligands, on metal’s coordination sphere, and thus could directly furnish structurally well-defined metal catalysts for organometallic catalysis.
Liang Deng grew up in Shuangfeng, Hunan, P. R. of China. He is a graduate of Peking University (BSc in chemistry, 2002) and the Chinese University of Hong Kong (PhD in chemistry, 2006, supervisor: Prof. Zuowei Xie). He had his postdoc training in the research group of Professor Zuowei Xie for a brief period in 2006 and the research group of Professor Richard H. Holm at Harvard University from 2007 to 2009. In 2009, he started an independent research career at Shanghai Institute of Organic Chemistry, and is now a research professor of the State Key Laboratory of Organometallic Chemistry there.
Functionalizing NHC Ligands on Noble Metal’s Coordination Sphere
Many NHCs are steric demanding ligands and prone to cyclometalation. Since Lappert’s early reports on the cyclometalated NHC–ruthenium complexes in the 1970s,[8] more than 200 cyclometalated NHC complexes have been known.[9] Cyclometalation educes new C-based chelating NHC ligands. While their M–Ccarbene bonds are often inert under common reaction conditions, the M–Chydrocarbyl bonds could be reactive as demonstrated by the C–H bond-formation reaction, the retroreaction of cyclometalation, when treating cyclometalated NHC complexes with acids, H2, hydrosilanes, terminal alkynes, or even with arenes.[10]The activity of the M–Chydrocarbyl bonds of cyclometalated NHC ligands also provides opportunities to install functional groups on the N-bound wingtips. Prior to our studies, several studies have scattered in the literature. In 2011, Aldridge and co-workers reported an iridium-mediated C–H bond borylation reaction on 1,3-dimesitylimidazol-2-ylidene (IMes; Scheme [2]).[11]
By examining the stepwise reactions of IMes with [Ir(coe)Cl]2 and LiBH4, they found that the reaction between the cyclometalated NHC complex [(IMes)Ir(IMes′)HCl] (1) and LiBH4 is the key step of the C–B bond-formation reactions (Scheme [2]). The structure of the key intermediate [(IMes)Ir(IMesBH3)(H)2] (2), a borane-functionalized NHC complex, has been established by X-ray diffraction studies. Soon after Aldridge’s report, in the same year Sola and his co-workers reported their studies on a doubly cyclometalated IMes–iridium compound [(MeCN)2(Pi-Pr3)Ir(IMes′′)][PF6] (3).[12] Complex 3 can react with pinacolborane and phenylacetylene to yield 4 and 5, by which a pinacolboryl and anionic carbon chelate are tethered to the IMes ligand (Scheme [2]). Further interesting examples are the reactions of cyclometalated NHC–platinum complexes [(NHC)Pt(NHC′)][BArF 4] (6, NHC = IPr and IMes*) with Br2 or I2 reported by Conejero et al. in 2013 (Scheme [2]).[13] Wherein, four halogen-tethered NHC–platinum complexes [(NHC)(NHCX)PtX][BArF 4] (7, X = Br and I) are formed by C–X bond-forming reductive elimination on platinum(IV) intermediates [(NHC)Pt(NHC′)(X)2]+, and both X-ray structure data and solution 1H NMR studies indicate the tethered halogen atoms are coordinating to the platinum(II) centers.
Functionalizing NHC Ligands on Cobalt’s Coordination Sphere
The validity of tethering a donor moiety to a cyclometalated NHC compound necessitates a reactive M–Chydrocarbyl bond in the metallacycles. Since 3d metals usually have weaker M–C bonds than the ones of 4d and 5d metals, one could expect that the cyclometalated NHC complexes of 3d metals should be the ideal starting materials. With this understanding and also inspired by Ohki and Tatsumi’s work on cyclometalated NHC–iron(II) complexes promoted C–H bond boration of arenes,[10e] we started our study on a cyclometalated IMes–cobalt complex.[14] The four-coordinated cyclometalated IMes–cobalt(II) complex [Co(IMes′)2] (8) can be readily prepared from the reaction of [(IMes)2CoCl] or [(IMes)2CoCl2] with one or two equivalents of sodium amalgam in THF (Scheme [3]).[14] An X-ray diffraction study revealed that the metal center in the cyclometalated IMes–cobalt(II) complex has an unsual seesaw-type coordination geometry and long Co–Cbenzyl separations [2.025(2) Å]. The later structure feature hints the weak nature of the Co–Cbenzyl bonds.
Compound 8 was found very reactive toward some unsaturated organic substrates.[14] Its reactions with the diazo compound 2,6-dimethylphenyl-2-diazoacetate and the isocyanide 2,6-dimethylphenyl isocyanide yield the hydrazino- and imidoyl-functionalized NHC–cobalt(II) complexes 9 and 10 in moderate yields (Scheme [3]), which should result from the migratory insertion of the Co–Cbenzyl bond toward the unsaturated functionalities. Probably due to steric congestion, the remained cyclometalated IMes fragments in 9 and 10 are resistant to the further insertion. Different from the simple insertion reactivity pattern, the reaction of 8 with CO can furnish 11 featuring an unusual carbonyl-tethered bis-NHC ligand scaffold (Scheme [3]). The mechanism for the formation of 11 is not straightforward. A plausible route might involve the sequential steps of CO insertion into one of the Co–Cbenzyl bonds and a Cacyl–Cbenzyl bond-forming reductive elimination from the acyl intermediate.[14]
The further reactivity studies on 8 revealed that it can even react with hydrosilanes, for example, PhSiH3, PhMeSiH2, and Ph2SiH2, to give novel silyl-donor-functionalized NHC complexes [(IMesSi)Co(IMes′)] 12–14 in moderate yields (Scheme [4]).[15] These C–Si bond-formation reactions form a sharp contrast to the C–H bond-formation reactions of the cyclometalated complexes [(Cp*)Fe(IPr2Me2′)][16] (IPr2Me2: 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) or [(MeCN)2(Pi-Pr3)Ir(IMes′′)][PF6][12] with hydrosilanes. The different outcomes hint that the successful installation of the silyl donors should benefit from the coexistence of two cyclometalated IMes ligands in 8. As aforementioned, silyl-donor-functionalized NHC cannot be synthesized easily by the conventional methods[17] since nucleophilic carbenes can readily react with Lewis acidic silanes. Herein, with the postfunctionalization method, the direct interaction of NHC with silanes is avoided, and thus the construction of this unique chelating ligand scaffold can be achieved for the first time.
Reactivity and Catalytic Application of the Silyl-Donor-Fuctionalized NHC–Cobalt Complexes
The silyl-donor-functionalized NHC compounds 12–14 still possess a reactive cyclometalated IMes fragment that can be exploited for further reactivity and catalytic application studies. Our explorations showed that 12 can react with BH3·THF to yield 15 bearing a newly formed IMes–BH3 ligand scaffold that was observed in 2 (Scheme [5]).[15] Complex 12 is also reactive toward PhSiH3. However, rather than forming (IMesSi)2Co, the reaction produces oligomers of polyphenylsilanes and H2, and 12 is fully recovered (Scheme [5]).[15] These results suggest the potential of the silyl-donor-fuctionalized NHC–cobalt complexes as catalysts for dehydrogenative polymerization of hydrosilanes.
The more creditable performance of the silyl-donor-fuctionalized NHC complexes was found in catalytic olefin hydrosilylation reactions. Cobalt complexes with carbonyl, phosphine, and cyclopentadienyl ligands have long been known to catalyze the hydrosilylation of olefins, but suffer from low catalytic activity, requiring harsh reaction conditions, and promoting side reactions of alkene isomerization and dehydrogenative hydrosilylation.[18] In contrast, the silyl-donor-fuctionalized NHC complexes 12–14 are very active in catalyzing the hydrosilylation of 1-octene with PhSiH3 with fast initial rates, large turnover number, and high selectivity (Scheme [6]).[15] The reaction employing 12 (0.1 mol%) in five minutes can produce n-C8H17SiPhH2 and n-C6H13CHMeSiPhH2 in 89% total yield with the ratio of 14:1, along with only trace amounts of n-C8H18 (3%) and 2-C8H16 (2%). More intrigueingly, when the catalyst loading is reduced to 0.005 mol%, the reaction can still operate and afford the hydrosilylation products in 75% total yield in 24 hours, which corresponds to a turnover number of 15000. The presence of different substituents on the silyl donors of 12–14 makes the evaluation on the effect of the steric property on the rates possible. The yields in five minutes are found decreasing in the order of 12 > 13 > 14 (Scheme [6]). The trend is in parallel with the increased bulk of the substituents.
The fine performance of the cobalt complexes has proved to be related to their silyl-donor-functionalized NHC ligands. The comparisons with other potential cobalt catalysts revealed that the reaction using 1 mol% Co2(CO)8 can only afford the hydrosilylation product in 26% yield in 24 hours. In sharp contrast, another trial using monodentate NHC-coordinated cobalt(II) compound [trans-(IPr2Me2)2CoPh2] (1 mol%) as the catalyst can not produce the hydrosilylation products (Scheme [6]).[15] While the reaction mechanism is not clear at this stage, we believe that the strong electron-donating ability and sterically demanding nature of the silyl-functionalized carbene ligands are the key factors endowing the cobalt catalysts unique performance.
Summary and Outlook
Cyclometalation is a commonly occurred reaction for organic ligands.[19] Different from some peoples’ view that the cyclometalation of a spectator ligand is detrimental as it sacrifices the open coordination site of the metal center, the chemistry described above has shown that it can be exploited for the construction of new chelating NHC ligands that are difficult to access with the conventional synthetic methods. As such, we and others have achieved the buildup of a series of donor-functionalized NHC ligands from the reactions of cyclometalated NHC–metal compounds with hydrosilanes, hydroboranes, halogens, CO, and some unsaturated organic functionalities. These novel donor-functionalized NHC complexes can be harnessed to catalysis. Our preliminary explorations showed that the silyl-donor-functionalized cobalt(II) compounds can serve as catalysts for the hydrosilylation of 1-octene with PhSiH3 with very fast initial rates, high turnover number, and high selectivity.
As mentioned earlier, cyclometalated NHC complexes are abundant in the literature.[9] However, up to now only a handful of them have been utilized to approach donor-functionalized NHC ligands, and the majority of the studies focused on the cyclometalated IMes compounds. Hence, the application of this method to other cyclometalated NHC or even to the cyclometalated compounds of phosphine- and pyridine-based ligands could be a meaningful future direction.[20] In addition to this, efforts should also be exercised to deepen our knowledge on the reactivity of the cyclometalated compounds, by which new methods to effect C–X bond formation (X denotes for functionalities) could be developed. Another aspect deserving particular attention is to apply the resulting donor-functionalized NHC, phosphine, or pyridine complexes to catalysis. The postfunctionalization method enables the access to a series of metal complexes with gross geometrical similarity but differentiated in metal centers’ microcoordination environments. Consequently, it creates opportunities to examine the electrosteric effect of the tethered donors on the performance of the metal catalysts and should provide meaningful information for new catalyst design. Intrigued by these challenges and also by the bright future of this synthetic method for new catalyst discovery, the Deng research group is now enthusiastically pursuing all these goals.
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Acknowledgment
We thank the financial support from the National Basic Research Program of China (973 Program, No. 2011CB808705) and the National Natural Science Foundation of China (Nos. 21002114, 21121062, and 21222208).
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References and Notes
- 1a N-Heterocyclic Carbenes in Transition Metal Catalysis: Topics in Organometallic Chemistry. Vol. 21. Glorius F. Springer; Berlin: 2007
- 1b N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis. In Catalysis by Metal Complexes. Vol. 32. Cazin CS. J. Springer; Heidelberg: 2011
- 1c Díez-Gonzalez S, Marion N, Nolan SP. Chem. Rev. 2009; 109: 3612
- 2a Hahn MC, Jahnke MC. Angew. Chem. Int. Ed. 2008; 47: 3122
- 2b Schuster O, Yang L, Raubenheimer HG, Albrecht M. Chem. Rev. 2009; 109: 3445
- 2c Benhamou L, Chardon E, Lavigne G, Bellemin-Laponnaz S, César V. Chem. Rev. 2011; 111: 2705
- 3a Kühl O. Functionalized N-Heterocyclic Carbene Complexes. Wiley; Chichester: 2010
- 3b Poyatos M, Mata JA, Peris E. Chem. Rev. 2009; 109: 3677
- 3c Pugh D, Danopoulos AA. Coord. Chem. Rev. 2007; 251: 610
- 5a Bontemps S, Gornitzka H, Bouhadir G, Miqueu K, Bourissou D. Angew. Chem. Int. Ed. 2006; 45: 1611
- 5b Anderson JS, Rittle J, Peters JC. Nature (London) 2013; 501: 84
- 6a Mankad NP, Whited MT, Peters JC. Angew. Chem. Int. Ed. 2007; 46: 5768
- 6b Morgan E, MacLean DF, McDonald R, Turculet L. J. Am. Chem. Soc. 2009; 131: 14234
- 7a Frey GD, Masuda JD, Donnadieu B, Bertrand G. Angew. Chem. Int. Ed. 2010; 49: 9444
- 7b Schmidt D, Berthel JH. J, Pietsch S, Radius U. Angew. Chem. Int. Ed. 2012; 51: 8881
- 7c Stephan DW, Erker G. Angew. Chem. Int. Ed. 2010; 49: 46
- 8a Hitchcock P, Lappert MF, Pye PL. J. Chem. Soc., Chem. Commun. 1977; 196
- 8b Hitchcock P, Lappert MF, Pye PL, Thomas S. J. Chem. Soc., Dalton Trans. 1979; 1929
- 9 Data based on the searching results on Cambridge Crystallographic Data Centre (CCDC) in November 2013.
- 10a Huang J, Stevens ED, Nolan SP. Organometallics 2000; 19: 1194
- 10b Jazzar RF. R, Macgregor SA, Mahon MF, Richards SP, Whittlesey MK. J. Am. Chem. Soc. 2002; 124: 4944
- 10c Torres O, Martín M, Sola E. Organometallics 2009; 28: 863
- 10d Choi G, Tsurugi H, Mashima K. J. Am. Chem. Soc. 2002; 135: 13149
- 10e Ohki Y, Hatanaka T, Tatsumi K. J. Am. Chem. Soc. 2008; 130: 17174
- 10f Rivada-Wheelaghan O, Ortunño MA, Díez J, Lledόs A, Conejero S. Angew. Chem. Int. Ed. 2012; 51: 3936
- 11 Tang CY, Smith W, Thompson AL, Vidovic D, Aldridge S. Angew. Chem. Int. Ed. 2011; 50: 1359
- 12 Navarro J, Torres O, Martín M, Sola E. J. Am. Chem. Soc. 2011; 133: 9738
- 13 Rivada-Wheelaghan O, Ortunño MA, Dίez J, Garcia-Garrido SE, Maya C, Lledόs A, Conejero S. J. Am. Chem. Soc. 2012; 134: 15261
- 14 Mo Z, Chen D, Leng X, Deng L. Organometallics 2012; 31: 7040
- 15 Mo Z, Liu Y, Deng L. Angew. Chem. Int. Ed. 2013; 52: 10845
- 16 Hatanaka T, Ohki Y, Tatsumi K. Eur. J. Inorg. Chem. 2013; 3966
- 17a Aksin Ö, Türkmen H, Artok L, Çetinkaya B, Ni C, Büyükgüngör O, Özkal E. J. Organomet. Chem. 2006; 691: 3027
- 17b Dastgir S, Coleman KS, Green ML. H. Dalton Trans. 2011; 40: 661
- 17c Albright A, Eddings D, Black R, Welch CJ, Gerasimchuk NN, Gawley RR. J. Org. Chem. 2011; 76: 7341
- 18a Chalk AJ, Harrod JF. J. Am. Chem. Soc. 1967; 89: 1640
- 18b Archer NJ, Haszeldine RN, Parish RV. J. Chem. Soc., Chem. Commun. 1971; 524
- 18c Archer NJ, Haszeldine RN, Parish RV. J. Chem. Soc., Dalton Trans. 1979; 695
- 19 Albrecht M. Chem. Rev. 2010; 110: 576
- 20a Estevan F, García-Bernabé A, Lahuerta P, Sanaú M, Ubeda MA, Galán-Mascarós JR. J. Organomet. Chem. 2000; 596: 248
- 20b Minato M, Zhou D.-Y, Zhang L.-B, Hirabayashi R, Kakeya M, Matsumoto T, Harakawa A, Kikutsuji G, Ito T. Organometallics 2005; 24: 3434
- 20c Djukic J.-P, Sortais J.-B, Barloy L, Pfeffer M. Eur. J. Inorg. Chem. 2009; 817
- 20d Scherl P, Wadepohl H, Gade LH. Organometallics 2013; 32: 4409
Silyl-substituted NHC ligands could be synthesized by the conventional synthetic method. But the silyl groups on these NHC ligands only function as innocent substituents, rather than chelating donors. For examples, see:
Constructing phosphine- and pyridine-based chelating ligands from cyclometalated phosphine and pyridine compounds has precedents scattered in literature. For examples, see:
-
References and Notes
- 1a N-Heterocyclic Carbenes in Transition Metal Catalysis: Topics in Organometallic Chemistry. Vol. 21. Glorius F. Springer; Berlin: 2007
- 1b N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis. In Catalysis by Metal Complexes. Vol. 32. Cazin CS. J. Springer; Heidelberg: 2011
- 1c Díez-Gonzalez S, Marion N, Nolan SP. Chem. Rev. 2009; 109: 3612
- 2a Hahn MC, Jahnke MC. Angew. Chem. Int. Ed. 2008; 47: 3122
- 2b Schuster O, Yang L, Raubenheimer HG, Albrecht M. Chem. Rev. 2009; 109: 3445
- 2c Benhamou L, Chardon E, Lavigne G, Bellemin-Laponnaz S, César V. Chem. Rev. 2011; 111: 2705
- 3a Kühl O. Functionalized N-Heterocyclic Carbene Complexes. Wiley; Chichester: 2010
- 3b Poyatos M, Mata JA, Peris E. Chem. Rev. 2009; 109: 3677
- 3c Pugh D, Danopoulos AA. Coord. Chem. Rev. 2007; 251: 610
- 5a Bontemps S, Gornitzka H, Bouhadir G, Miqueu K, Bourissou D. Angew. Chem. Int. Ed. 2006; 45: 1611
- 5b Anderson JS, Rittle J, Peters JC. Nature (London) 2013; 501: 84
- 6a Mankad NP, Whited MT, Peters JC. Angew. Chem. Int. Ed. 2007; 46: 5768
- 6b Morgan E, MacLean DF, McDonald R, Turculet L. J. Am. Chem. Soc. 2009; 131: 14234
- 7a Frey GD, Masuda JD, Donnadieu B, Bertrand G. Angew. Chem. Int. Ed. 2010; 49: 9444
- 7b Schmidt D, Berthel JH. J, Pietsch S, Radius U. Angew. Chem. Int. Ed. 2012; 51: 8881
- 7c Stephan DW, Erker G. Angew. Chem. Int. Ed. 2010; 49: 46
- 8a Hitchcock P, Lappert MF, Pye PL. J. Chem. Soc., Chem. Commun. 1977; 196
- 8b Hitchcock P, Lappert MF, Pye PL, Thomas S. J. Chem. Soc., Dalton Trans. 1979; 1929
- 9 Data based on the searching results on Cambridge Crystallographic Data Centre (CCDC) in November 2013.
- 10a Huang J, Stevens ED, Nolan SP. Organometallics 2000; 19: 1194
- 10b Jazzar RF. R, Macgregor SA, Mahon MF, Richards SP, Whittlesey MK. J. Am. Chem. Soc. 2002; 124: 4944
- 10c Torres O, Martín M, Sola E. Organometallics 2009; 28: 863
- 10d Choi G, Tsurugi H, Mashima K. J. Am. Chem. Soc. 2002; 135: 13149
- 10e Ohki Y, Hatanaka T, Tatsumi K. J. Am. Chem. Soc. 2008; 130: 17174
- 10f Rivada-Wheelaghan O, Ortunño MA, Díez J, Lledόs A, Conejero S. Angew. Chem. Int. Ed. 2012; 51: 3936
- 11 Tang CY, Smith W, Thompson AL, Vidovic D, Aldridge S. Angew. Chem. Int. Ed. 2011; 50: 1359
- 12 Navarro J, Torres O, Martín M, Sola E. J. Am. Chem. Soc. 2011; 133: 9738
- 13 Rivada-Wheelaghan O, Ortunño MA, Dίez J, Garcia-Garrido SE, Maya C, Lledόs A, Conejero S. J. Am. Chem. Soc. 2012; 134: 15261
- 14 Mo Z, Chen D, Leng X, Deng L. Organometallics 2012; 31: 7040
- 15 Mo Z, Liu Y, Deng L. Angew. Chem. Int. Ed. 2013; 52: 10845
- 16 Hatanaka T, Ohki Y, Tatsumi K. Eur. J. Inorg. Chem. 2013; 3966
- 17a Aksin Ö, Türkmen H, Artok L, Çetinkaya B, Ni C, Büyükgüngör O, Özkal E. J. Organomet. Chem. 2006; 691: 3027
- 17b Dastgir S, Coleman KS, Green ML. H. Dalton Trans. 2011; 40: 661
- 17c Albright A, Eddings D, Black R, Welch CJ, Gerasimchuk NN, Gawley RR. J. Org. Chem. 2011; 76: 7341
- 18a Chalk AJ, Harrod JF. J. Am. Chem. Soc. 1967; 89: 1640
- 18b Archer NJ, Haszeldine RN, Parish RV. J. Chem. Soc., Chem. Commun. 1971; 524
- 18c Archer NJ, Haszeldine RN, Parish RV. J. Chem. Soc., Dalton Trans. 1979; 695
- 19 Albrecht M. Chem. Rev. 2010; 110: 576
- 20a Estevan F, García-Bernabé A, Lahuerta P, Sanaú M, Ubeda MA, Galán-Mascarós JR. J. Organomet. Chem. 2000; 596: 248
- 20b Minato M, Zhou D.-Y, Zhang L.-B, Hirabayashi R, Kakeya M, Matsumoto T, Harakawa A, Kikutsuji G, Ito T. Organometallics 2005; 24: 3434
- 20c Djukic J.-P, Sortais J.-B, Barloy L, Pfeffer M. Eur. J. Inorg. Chem. 2009; 817
- 20d Scherl P, Wadepohl H, Gade LH. Organometallics 2013; 32: 4409
Silyl-substituted NHC ligands could be synthesized by the conventional synthetic method. But the silyl groups on these NHC ligands only function as innocent substituents, rather than chelating donors. For examples, see:
Constructing phosphine- and pyridine-based chelating ligands from cyclometalated phosphine and pyridine compounds has precedents scattered in literature. For examples, see:
Liang Deng grew up in Shuangfeng, Hunan, P. R. of China. He is a graduate of Peking University (BSc in chemistry, 2002) and the Chinese University of Hong Kong (PhD in chemistry, 2006, supervisor: Prof. Zuowei Xie). He had his postdoc training in the research group of Professor Zuowei Xie for a brief period in 2006 and the research group of Professor Richard H. Holm at Harvard University from 2007 to 2009. In 2009, he started an independent research career at Shanghai Institute of Organic Chemistry, and is now a research professor of the State Key Laboratory of Organometallic Chemistry there.