Warren B. Cross

Variable coordination of amine functionalised N-heterocyclic carbene ligands to Ru, Rh and Ir: C-H and N-H activation and catalytic transfer hydrogenation.

Chelating amine and amido complexes of late transition metals are highly valuable bifunctional catalysts in organic synthesis, but complexes of bidentate amine-NHC and amido-NHC ligands are scarce. Hence, we report the reactions of a secondary-amine functionalised imidazolium salt 2a and a primary-amine functionalised imidazolium salt 2b with [(p-cymene)RuCl(2)](2) and [Cp*MCl(2)](2) (M = Rh, Ir). Treating 2a with [Cp*MCl(2)](2) and NaOAc gave the cyclometallated compounds Cp*M(C,C)I (M = Rh, 3; M = Ir, 4), resulting from aromatic C-H activation. In contrast, treating 2b with [(p-cymene)RuCl(2)](2), Ag(2)O and KI gave the amine-NHC complex [(p-cymene)Ru(C,NH(2))I]I (5). The reaction of 2b with [Cp*MCl(2)](2) (M = Rh, Ir), NaO(t)Bu and KI gave the amine-NHC complex [Cp*Rh(NH(2))I]I (6) or the amido-NHC complex Cp*Ir(C,NH)I (7); both protonation states of the Ir complex could be accessed: treating 7 with trifluoroacetic acid gave the amine-NHC complex [Cp*Ir(C,NH(2))I][CF(3)CO(2)] (8). These are the first primary amine- or amido-NHC complexes of Rh and Ir. Solid-state structures of the complexes 3-8 have been determined by single crystal X-ray diffraction. Complexes 5, 6 and 7 are pre-catalysts for the catalytic transfer hydrogenation of acetophenone to 1-phenylethanol, with ruthenium complex 5 demonstrating especially high reactivity.

Combined experimental and computational investigations of rhodium- and ruthenium-catalyzed C–H functionalization of pyrazoles with alkynes

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 κ(2)-κ(1) 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 k(H)/k(D) = 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 κ(2)-κ(1) displacement of acetate and so is still consistent with the lack of a significant experimental isotope effect (k(H)/k(D) = 1.1 ± 0.2).

C(sp3)H Activation without a Directing Group: Regioselective Synthesis of N‐Ylide or N‐Heterocyclic Carbene Complexes Controlled by the Choice of Metal and Ligand

N-Ylide complexes of Ir have been generated by C(sp(3))-H activation of α-pyridinium or α-imidazolium esters in reactions with [Cp*IrCl2]2 and NaOAc. These reactions are rare examples of C(sp(3))-H activation without a covalent directing group, which-even more unusually-occur α to a carbonyl group. For the reaction of the α-imidazolium ester [3H]Cl, the site selectivity of C-H activation could be controlled by the choice of metal and ligand: with [Cp*IrCl2]2 and NaOAc, C(sp(3))-H activation gave the N-ylide complex 4; in contrast, with Ag2O followed by [Cp*IrCl2]2, C(sp(2))-H activation gave the N-heterocyclic carbene complex 5. DFT calculations revealed that the N-ylide complex 4 was the kinetic product of an ambiphilic C-H activation. Examination of the computed transition state for the reaction to give 4 indicated that unlike in related reactions, the acetate ligand appears to play the dominant role in C-H bond cleavage.

Organo-palladium(II) complexes bearing unsymmetrical N,N,N-pincer ligands: synthesis, structures and oxidatively induced coupling reactions

The 2-(2′-aniline)-6-imine-pyridines, 2-(C6H4-2′-NH2)-6-(CMe=NAr)C5H3N (Ar = 4-i-PrC6H4 (HL1a), 2,6-i-Pr2C6H3 (HL1b)), have been synthesised via sequential Stille cross-coupling, deprotection and condensation steps from 6-tributylstannyl-2-(2-methyl-1,3-dioxolan-2-yl)pyridine and 2-bromonitrobenzene. The palladium(II) acetate N,N,N-pincer complexes, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)), can be prepared by reacting HL1 with Pd(OAc)2 or, in the case of 1a, more conveniently by the template reaction of ketone 2-(C6H4-2′-NH2)-6-(CMe=O)C5H3N, Pd(OAc)2 and 4-isopropylaniline; ready conversion of 1 to their chloride analogues, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C6H3 (2b)), has been demonstrated. The phenyl-containing complexes, [{2-(C6H4-2′-NH)-6-(CMe=NAr)C5H3N}PdPh] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)), can be obtained by treating HL1 with (PPh3)2PdPh(Br) in the presence of NaH or with regard to 3a, by the salt elimination reaction of 2a with phenyllithium. Reaction of 2a with silver tetrafluoroborate or triflate in the presence of acetonitrile allows access to cationic [{2-(C6H4-2′-NH)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NCMe)][X] (X = BF4 (4), X = O3SCF3 (5)), respectively; the pyridine analogue of 5, [{2-(C6H4-2′-NH)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NC5H5)][O3SCF3] (5′), is also reported. Oxidation of phenyl-containing 3a with one equivalent of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor™) in acetonitrile at low temperature leads to a new palladium species that slowly decomposes to give 4 and biphenyl; biphenyl formation is also observed upon reaction of 3a with XeF2. However, no such oxidatively induced coupling occurs when using 3b. Single crystal X-ray diffraction studies have been performed on HL1b, 1a, 1b, 2a, 2b, 3a, 3b and 5′.

N-heterocyclic carbene tethered amido complexes of palladium and platinum.

With a view to applications in bifunctional catalysis, a modular cross-coupling strategy has been used to prepare amine bis(imidazolium) salts (3a and 3b) and an amine mono(imidazolium) salt (6) as precursors to chelating amido-NHC ligands. Treating the pro-ligands 3 with 3 equivalents of the bulky base KHMDS and Pd(OAc)(2) or PtCl(2)(COD) gave the four amido bis(N-heterocyclic carbene) pincer complexes [CNC-R]M-I [M = Pd (7) or Pt (8); R = i-Pr (a) or n-Bu (b)], including the first examples of platinum complexes of a CNC ligand. The reaction of 7a with AgOTf in pyridine gave the cationic complex {[CNC-i-Pr]Pd-py}OTf (9a). Heating a mixture of amine mono(imidazolium) salt 6 with PdCl(2) or K(2)PtCl(4), K(2)CO(3) and KI in pyridine at 100 °C gave the complexes [C,NH]MI(2)py [M = Pd (10) or Pt (11)], in which the amine arm of the NHC ligand is not deprotonated and does not coordinate to the metal. For a solution of 10 in 1,4-dioxane, deprotonation of the amine occurred in a biphasic reaction with aqueous KOH at 40 °C, giving the dimeric amido complex {[C,N]Pd(μ-OH)}(2) (12). The more inert Pt analogue 11 was unreactive under the same conditions. Solid-state structures of the complexes 7a, 7b, 9a, 10, 11 and 12 have been determined by single crystal X-ray diffraction.

trans-Bis{1-[2-(2,6-diisopropyl-anilino)phenyl]-3-isopropyl-imidazolin-2-ylidenyl-κC}diiodidopalladium(II) benzene disolvate.

In the title complex, [PdI2(C24H31N3)2]·2C6H6, the Pd2+ ion is located on an inversion centre in a slightly distorted square-planar geometry. The angle between the I2C2 square plane and the mean plane of the N-heterocyclic carbene ring is 79.8 (2)°, with I—Pd—C—N torsion angles of −81.1 (6) and −78.2 (5)°. The Pd—carbene and Pd—I distances are 2.016 (6) and 2.5971 (10) Å, respectively.