Co-reporter:David L. Davies; Charles E. Ellul; Stuart A. Macgregor; Claire L. McMullin;Kuldip Singh
Journal of the American Chemical Society 2015 Volume 137(Issue 30) pp:9659-9669
Publication Date(Web):June 26, 2015
DOI:10.1021/jacs.5b04858
A range of novel heterocyclic cations have been synthesized by the Rh(III)-catalyzed oxidative C–N and C–C coupling of 1-phenylpyrazole, 2-phenylpyridine, and 2-vinylpyridine with alkynes (4-octyne and diphenylacetylene). The reactions proceed via initial C–H activation, alkyne insertion, and reductive coupling, and all three of these steps are sensitive to the substrates involved and the reaction conditions. Density functional theory (DFT) calculations show that C–H activation can proceed via a heteroatom-directed process that involves displacement of acetate by the neutral substrate to form charged intermediates. This step (which leads to cationic C–N coupled products) is therefore favored by more polar solvents. An alternative non-directed C–H activation is also possible that does not involve acetate displacement and so becomes favored in low polarity solvents, leading to C–C coupled products. Alkyne insertion is generally more favorable for diphenylacetylene over 4-octyne, but the reverse is true of the reductive coupling step. The diphenylacetylene moiety can also stabilize unsaturated seven-membered rhodacycle intermediates through extra interaction with one of the Ph substituents. With 1-phenylpyrazole this effect is sufficient to suppress the final C–N reductive coupling. A comparison of a series of seven-membered rhodacycles indicates the barrier to coupling is highly sensitive to the two groups involved and follows the trend C–N+ > C–N > C–C (i.e., involving the formation of cationic C–N, neutral C–N, and neutral C–C coupled products, respectively).
Co-reporter:Dr. Andrés G. Algarra; David L. Davies;Dr. Qudsia Khamker; Stuart A. Macgregor;Dr. Claire L. McMullin;Kuldip Singh;Dr. Barbara Villa-Marcos
Chemistry - A European Journal 2015 Volume 21( Issue 7) pp:3087-3096
Publication Date(Web):
DOI:10.1002/chem.201405550
Abstract
Detailed experimental and computational studies have been carried out on the oxidative coupling of the alkenes C2H3Y (Y=CO2Me (a), Ph (b), C(O)Me (c)) with 3-aryl-5-R-pyrazoles (R=Me (1 a), Ph (1 b), CF3 (1 c)) using a [Rh(MeCN)3Cp*][PF6]2/Cu(OAc)2⋅H2O catalyst system. In the reaction of methyl acrylate with 1 a, up to five products (2 aa–6 aa) were formed, including the trans monovinyl product, either complexed within a novel CuI dimer (2 aa) or as the free species (3 aa), and a divinyl species (6 aa); both 3 aa and 6 aa underwent cyclisation by an aza-Michael reaction to give fused heterocycles 4 aa and 5 aa, respectively. With styrene, only trans mono- and divinylation products were observed, whereas with methyl vinyl ketone, a stronger Michael acceptor, only cyclised oxidative coupling products were formed. Density functional theory calculations were performed to characterise the different migratory insertion and β-H transfer steps implicated in the reactions of 1 a with methyl acrylate and styrene. The calculations showed a clear kinetic preference for 2,1-insertion and the formation of trans vinyl products, consistent with the experimental results.
Co-reporter:David L. Davies, Francesco Lelj, Mark P. Lowe, Karl S. Ryder, Kuldip Singh and Shalini Singh
Dalton Transactions 2014 vol. 43(Issue 10) pp:4026-4039
Publication Date(Web):13 Jan 2014
DOI:10.1039/C3DT52975D
Biscyclometallated iridium complexes [Ir(ppz)2(X^Y)][PF6] (X^Y = pyridine imine) have been synthesised. The pyridineimine ligands are prepared in situ during the complexation. The complexes show room temperature emission between 640 and 780 nm in CH2Cl2 solution. The emission is red shifted compared with the analogous bipyridine complex [Ir(ppz)2(bipy)][PF6]. DFT calculations have been used to shed light on the influence of the imine substituent on the electrochemical and photochemical properties. In particular, the calculations suggests that there is a significant change in geometry between the ground state and the first triplet excited state for arylimines but not for alkylimines, leading to much weaker emission for the arylimine complexes. The work demonstrates that pyridineimines can be used as a substitute for bipyridines in luminescent iridium complexes.
Co-reporter:Andrés G. Algarra, Warren B. Cross, David L. Davies, Qudsia Khamker, Stuart A. Macgregor, Claire L. McMullin, and Kuldip Singh
The Journal of Organic Chemistry 2014 Volume 79(Issue 5) pp:1954-1970
Publication Date(Web):February 24, 2014
DOI:10.1021/jo402592z
Detailed experimental and computational studies are reported on the mechanism of the coupling of alkynes with 3-arylpyrazoles at [Rh(MeCN)3Cp*][PF6]2 and [RuCl2(p-cymene)]2 catalysts. Density functional theory (DFT) calculations indicate a mechanism involving sequential N–H and C–H bond activation, HOAc/alkyne exchange, migratory insertion, and C–N reductive coupling. For rhodium, C–H bond activation is a two-step process comprising κ2–κ1 displacement of acetate to give an agostic intermediate which then undergoes C–H bond cleavage via proton transfer to acetate. For the reaction of 3-phenyl-5-methylpyrazole with 4-octyne kH/kD = 2.7 ± 0.5 indicating that C–H bond cleavage is rate limiting in this case. However, H/D exchange studies, both with and without added alkyne, suggest that the migratory insertion transition state is close in energy to that for C–H bond cleavage. In order to model this result correctly, the DFT calculations must employ the full experimental system and include a treatment of dispersion effects. A significantly higher overall barrier to catalysis is computed at {Ru(p-cymene)} for which the rate-limiting process remains C–H activation. However, this is now a one-step process corresponding to the κ2–κ1 displacement of acetate and so is still consistent with the lack of a significant experimental isotope effect (kH/kD = 1.1 ± 0.2).
Co-reporter:David L. Davies, Kuldip Singh, Shalini Singh and Barbara Villa-Marcos
Chemical Communications 2013 vol. 49(Issue 58) pp:6546-6548
Publication Date(Web):06 Jun 2013
DOI:10.1039/C3CC43121E
Reaction of [Ir(C⁁N)2Cl]2 with chiral bidentate N⁁OH ligands provides complexes [Ir(C⁁N)2(N⁁O)] as a 1:1 mixture of diastereomers which can be separated by crystallisation. A pure diastereomer can be converted to [Ir(C⁁N)2(bipy)][CF3CO2] with complete retention of stereochemistry at the metal.
Co-reporter:Carmen E. Castillo, David L. Davies, Anne-K. Duhme Klair, Kuldip Singh and Shalini Singh
Dalton Transactions 2012 vol. 41(Issue 2) pp:628-635
Publication Date(Web):24 Oct 2011
DOI:10.1039/C1DT11360G
Reactions of [Ir(C⁁N)2Cl]2 [HC⁁N = 2-(3-R-phenyl)pyridine, 2-(3-R-phenylpyrazole) R = H, Me] with Me2-phencat give luminescent complexes [Ir(C⁁N)2(Me2-phencat)][PF6] (Me2-2a, b, c)[PF6]. Deprotection of the methoxy groups with BBr3 is problematic as simultaneous bromination of the cyclometallated phenyl groups occurs. However, deprotection of Me2-phencat with BBr3 followed by complexation with [Ir(C⁁N)2Cl]2 gives luminescent complexes [Ir(C⁁N)2(H2-phencat)][PF6] (H2-3a, c)[PF6], which are luminescent sensors for molybdate.
Co-reporter:Glenn A. Burley, Youcef Boutadla, David L. Davies, and Kuldip Singh
Organometallics 2012 Volume 31(Issue 3) pp:1112-1117
Publication Date(Web):January 25, 2012
DOI:10.1021/om201157g
N-alkynylheterocycles (benzimidazole and indazole) are converted to triazoles by click chemistry, and the resulting triazoles react with [IrCl2Cp*]2. The benzimidazole-triazole coordinates in a monodentate fashion through the benzimidazole, whereas the indazole-triazole is bidentate through coordination of both heterocycles. Reaction of the benzimidazole-triazole with methyliodide gives a benzimidazolium salt that deprotonates on coordination to afford a rare example of a bidentate NHC–triazole.
Co-reporter:David L. Davies, Mark P. Lowe, Karl S. Ryder, Kuldip Singh and Shalini Singh
Dalton Transactions 2011 vol. 40(Issue 5) pp:1028-1030
Publication Date(Web):21 Dec 2010
DOI:10.1039/C0DT01311K
Cyclometallated phenyls with substituents para to the metal have a larger impact on the redox potentials and emission of complexes [Ir(R-ppz)2(bipy)][PF6] than substituents at the meta position and hence enable tuning of emission wavelength over a wider range using the same substituent.
Co-reporter:Dr. Youcef Boutadla;Dr. David L. Davies;Rachel C. Jones ;Kuldip Singh
Chemistry - A European Journal 2011 Volume 17( Issue 12) pp:3438-3448
Publication Date(Web):
DOI:10.1002/chem.201002604
Abstract
CH activation by acetate-assisted cyclometallation of a phenyl group with half-sandwich complexes [{MCl2Cp*}2] (M=Ir, Rh) and [{RuCl2(p-cymene)}2] can be directed by a wide range of nitrogen donor ligands including pyrazole, oxazoline, oxime, imidazole and triazole, and X-ray structures of a number of complexes are reported. All the ligands tested cyclometallated at iridium, however ruthenium and rhodium fail to cause cyclometallation in some cases. As a result, the nitrogen donors have been categorised based on their reactivity with the three metals used. The relevance of these cyclometallation reactions to catalytic synthesis of carbocycles and heterocycles is discussed.
Co-reporter:Youcef Boutadla, David L. Davies, Omar Al-Duaij, John Fawcett, Rachel C. Jones and Kuldip Singh
Dalton Transactions 2010 vol. 39(Issue 43) pp:10447-10457
Publication Date(Web):06 Oct 2010
DOI:10.1039/C0DT00280A
The cyclometallated complexes [MCl(C⁁N)(ring)] (HC⁁N = 2-phenylpyrazole, M = Ir, Rh ring = Cp*; M = Ru, ring = p-cymene) readily undergo insertion reactions with RCCR (R = CO2Me, Ph) to give mono insertion products, the rhodium complex also reacts with PhCCH regiospecifically to give an analogous product. The products of the reactions of the cyclometallated imine complexes [MCl(C^N)Cp*] (HC⁁N = PhCHNR, R = Ph, CH2CH2OMe, Me; M = Ir, Rh) with PhCCPh depend on the substituent R; when R = CH2CH2OMe a monoinsertion is observed, however for R = Me the initial insertion product is unstable, undergoing reductive elimination with loss of the organic fragment, and for R = Ph no metal-containing product is isolated. With PhCCH the cyclometallated imine complexes can give mono or di-insertion products. The implications for catalytic synthesis of carbo- and heterocycles by a tandem C–H activation, alkyne insertion mechanism are discussed.
Co-reporter:David L. Davies, Omar Al-Duaij, John Fawcett and Kuldip Singh
Organometallics 2010 Volume 29(Issue 6) pp:1413-1420
Publication Date(Web):February 24, 2010
DOI:10.1021/om901088v
The ligand 4,4-dimethyl-2-oxazolinylbenzene is easily cyclometalated by [IrCl2Cp*]2 or [RuCl(MeCN)2(p-cymene)]PF6 in the presence of sodium acetate. In the case of iridium the resultant complex dissolves in acetonitrile in the presence of KPF6 to give an acetonitrile-coordinated cationic complex. The analogous complex is formed directly in the ruthenium cyclometalation reaction. These labile cationic complexes undergo insertion reactions with internal and terminal alkynes. Internal alkynes give only monoinsertion products, whereas terminal alkynes give mono- or di-insertion products. The cations will also react with CO, but no insertion occurs in this case.
Co-reporter:Youcef Boutadla, Omar Al-Duaij, David L. Davies, Gerald A. Griffith and Kuldip Singh
Organometallics 2009 Volume 28(Issue 2) pp:433-440
Publication Date(Web):December 19, 2008
DOI:10.1021/om800909w
Reactions of 2-substituted pyridines HL with [MCl2Cp*]2 (M = Ir, Rh) and [RuCl2(p-cymene)]2 have been carried out in the presence and absence of sodium acetate. 2-Phenylpyridine (HL1) is cyclometalated easily to form [MCl(L1)(ring)] 1a−c (M = Rh, Ir, ring = Cp*; M = Ru, ring = p-cymene). However, in the case of 2-acetylpyridine (HL2) sp3 C—H activation occurs cleanly with rhodium to form N,C chelate complex [RhCl(L2)Cp*] 2b, but the reactions with iridium and ruthenium give unseparable mixtures of products. The N,C cyclometalated products [MCl(L2)(ring)] 2a−c (M = Ir, Rh, ring = Cp*; M = Ru, ring = p-cymene) have been independently prepared from the lithium enolates of 2-acetylpyridine. Notably, in the absence of acetate, [RhCl2Cp*]2 shows no reaction with 2-acetylpyridine, whereas [IrCl2Cp*]2 and [RuCl2(p-cymene)]2 react to form equilibrium mixtures of the starting materials and N,O chelate complexes 4a,c, respectively. In the presence of KPF6 the N,O chelate complexes [MCl(HL2)(ring)][PF6] 4a,c,d (M = Ir, ring = Cp*; M = Ru, ring = p-cymene, mesitylene) can be isolated. These are not intermediates en route to the N,C cyclometalated products. These results suggest that for C—H activation to occur under these mild conditions acetate must coordinate to the metal prior to coordination of the ligand.
Co-reporter:David L. Davies, Sukhvinder K. Kandola, Raj K. Patel
Tetrahedron: Asymmetry 2004 Volume 15(Issue 1) pp:77-80
Publication Date(Web):12 January 2004
DOI:10.1016/j.tetasy.2003.10.011
Cyclopropanation of styrene with ethyl diazoacetate catalysed by copper triflate and a bis(oxazoline) occurs with good yield and enantioselectivity in a variety of ionic liquids. The catalyst solution can be recycled four times with little drop in yield or selectivity. The purity of the ionic liquid has a marked effect on the yield and enantioselectivity.Graphic
Co-reporter:David L. Davies, Omar Al-Duaij, John Fawcett, Marco Giardiello, Stephen T. Hilton and David R. Russell
Dalton Transactions 2003 (Issue 21) pp:4132-4138
Publication Date(Web):22 Sep 2003
DOI:10.1039/B303737A
N,N-Dimethylbenzylamine, alkyl and aryl imines derived from benzaldehyde, and 2-phenyl-4,4-dimethyloxazoline all undergo cyclometallation with [IrCl2Cp*]2
(Cp*
=
η-C5Me5) when treated with NaOAc in dichloromethane at room temperature. The imines are also cyclometallated by [RhCl2Cp*]2 under the same conditions whilst only N-alkyl imines are cyclometallated by [RuCl2(p-cymene)]2. The role of acetate in the cyclometallation is more than just as a base. X-Ray structures of cyclometallated complexes [MCl{C6H4-2-C(H)NCH2CH2OMe-κC,N}(η-ring)](M = Ir, Rh ring = Cp*; M = Ru, ring =
p-cymene), [MCl{C6H4-2-C(H)NCH2CH2OMe-κC,N}Cp*](M = Ir, Rh), [RuCl(η2-O2CMe)(p-cymene)] and [IrCl2(NH2Ph)Cp*] are reported.
Co-reporter:Andrew P. Abbott, Glen Capper, David L. Davies, Ray K. Rasheed and Vasuki Tambyrajah
Green Chemistry 2002 vol. 4(Issue 1) pp:24-26
Publication Date(Web):16 Jan 2002
DOI:10.1039/B108431C
Synthetically important Diels–Alder reactions occur in high yield in novel Lewis acidic ambient temperature ionic liquids composed of choline chloride–MCl2 (1:2) (M = Zn or Sn). The liquids are not moisture-sensitive and after separation of the products they can be reused with no noticeable drop in activity.
Co-reporter:Andrew P. Abbott, Glen Capper, David L. Davies, Helen L. Munro, Raymond K. Rasheed and Vasuki Tambyrajah
Chemical Communications 2001 (Issue 19) pp:2010-2011
Publication Date(Web):19 Sep 2001
DOI:10.1039/B106357J
A range of novel, moisture-stable, Lewis-acidic ionic liquids
has been prepared by mixing appropriate molar ratios of MCl2 (M
= Zn and/or Sn) and quaternary ammonium salts of formula
[Me3NC2H4Y]Cl (Y = OH, Cl, OC(O)Me,
OC(O)Ph); the influence of substituent Y and metal M on the physical
properties of the melts has been investigated.
Co-reporter:David L. Davies, Francesco Lelj, Mark P. Lowe, Karl S. Ryder, Kuldip Singh and Shalini Singh
Dalton Transactions 2014 - vol. 43(Issue 10) pp:NaN4039-4039
Publication Date(Web):2014/01/13
DOI:10.1039/C3DT52975D
Biscyclometallated iridium complexes [Ir(ppz)2(X^Y)][PF6] (X^Y = pyridine imine) have been synthesised. The pyridineimine ligands are prepared in situ during the complexation. The complexes show room temperature emission between 640 and 780 nm in CH2Cl2 solution. The emission is red shifted compared with the analogous bipyridine complex [Ir(ppz)2(bipy)][PF6]. DFT calculations have been used to shed light on the influence of the imine substituent on the electrochemical and photochemical properties. In particular, the calculations suggests that there is a significant change in geometry between the ground state and the first triplet excited state for arylimines but not for alkylimines, leading to much weaker emission for the arylimine complexes. The work demonstrates that pyridineimines can be used as a substitute for bipyridines in luminescent iridium complexes.
Co-reporter:Youcef Boutadla, David L. Davies, Omar Al-Duaij, John Fawcett, Rachel C. Jones and Kuldip Singh
Dalton Transactions 2010 - vol. 39(Issue 43) pp:NaN10457-10457
Publication Date(Web):2010/10/06
DOI:10.1039/C0DT00280A
The cyclometallated complexes [MCl(C⁁N)(ring)] (HC⁁N = 2-phenylpyrazole, M = Ir, Rh ring = Cp*; M = Ru, ring = p-cymene) readily undergo insertion reactions with RCCR (R = CO2Me, Ph) to give mono insertion products, the rhodium complex also reacts with PhCCH regiospecifically to give an analogous product. The products of the reactions of the cyclometallated imine complexes [MCl(C^N)Cp*] (HC⁁N = PhCHNR, R = Ph, CH2CH2OMe, Me; M = Ir, Rh) with PhCCPh depend on the substituent R; when R = CH2CH2OMe a monoinsertion is observed, however for R = Me the initial insertion product is unstable, undergoing reductive elimination with loss of the organic fragment, and for R = Ph no metal-containing product is isolated. With PhCCH the cyclometallated imine complexes can give mono or di-insertion products. The implications for catalytic synthesis of carbo- and heterocycles by a tandem C–H activation, alkyne insertion mechanism are discussed.
Co-reporter:Carmen E. Castillo, David L. Davies, Anne-K. Duhme Klair, Kuldip Singh and Shalini Singh
Dalton Transactions 2012 - vol. 41(Issue 2) pp:NaN635-635
Publication Date(Web):2011/10/24
DOI:10.1039/C1DT11360G
Reactions of [Ir(C⁁N)2Cl]2 [HC⁁N = 2-(3-R-phenyl)pyridine, 2-(3-R-phenylpyrazole) R = H, Me] with Me2-phencat give luminescent complexes [Ir(C⁁N)2(Me2-phencat)][PF6] (Me2-2a, b, c)[PF6]. Deprotection of the methoxy groups with BBr3 is problematic as simultaneous bromination of the cyclometallated phenyl groups occurs. However, deprotection of Me2-phencat with BBr3 followed by complexation with [Ir(C⁁N)2Cl]2 gives luminescent complexes [Ir(C⁁N)2(H2-phencat)][PF6] (H2-3a, c)[PF6], which are luminescent sensors for molybdate.
Co-reporter:David L. Davies, Mark P. Lowe, Karl S. Ryder, Kuldip Singh and Shalini Singh
Dalton Transactions 2011 - vol. 40(Issue 5) pp:NaN1030-1030
Publication Date(Web):2010/12/21
DOI:10.1039/C0DT01311K
Cyclometallated phenyls with substituents para to the metal have a larger impact on the redox potentials and emission of complexes [Ir(R-ppz)2(bipy)][PF6] than substituents at the meta position and hence enable tuning of emission wavelength over a wider range using the same substituent.
Co-reporter:David L. Davies, Kuldip Singh, Shalini Singh and Barbara Villa-Marcos
Chemical Communications 2013 - vol. 49(Issue 58) pp:NaN6548-6548
Publication Date(Web):2013/06/06
DOI:10.1039/C3CC43121E
Reaction of [Ir(C⁁N)2Cl]2 with chiral bidentate N⁁OH ligands provides complexes [Ir(C⁁N)2(N⁁O)] as a 1:1 mixture of diastereomers which can be separated by crystallisation. A pure diastereomer can be converted to [Ir(C⁁N)2(bipy)][CF3CO2] with complete retention of stereochemistry at the metal.
