A new family of flexible scorpionate ligands based on 2-mercaptopyridine is reported. The tris- and bis-substituted ligands, K[HB(mp)(3)] (1) and Na[H(2)B(mp)(2)] (2) (mp = 2-mercaptopyridine) have been prepared and fully characterised. The structural characterisation of 1 reveals an unprecedented mu(3)-kappa(3)-SS'H-eta(1)eta(1)eta(2)-kappa(2)-S''C-eta(1)eta(1)-kappa(1)-S'-eta(1) coordination mode. The coordination of both 1 and 2 to copper(I) complexes containing triphenylphosphine and tricyclohexylphosphine co-ligands is investigated suggesting kappa(3)-SSS coordination modes for [Cu{HB(mp)(3)}(PR(3))] {where R = Ph (3); R = Cy, (4)} and kappa(3)-SSH coordination modes for [Cu{H(2)B(mp)(2)}(PR(3))] {where R = Ph (5); R = Cy (6)} the latter confirmed by structural characterisation of 5. A structural comparison with the sulfur based scorpionates, HB(mt)(3) and H(2)B(mt)(2) (mt = methyl-2-mercaptoimidazole) is made in terms of the degree of tautomerisation of the heterocyclic rings.
A series of palladium(II) complexes incorporating di-NHC-amine ligands has been prepared and their structural, dynamic and catalytic behaviour investigated. The complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))PdCl(2)] (12) and [trans-(kappa(2)-(Mes)CN(H)C(Mes))PdCl(2)] (13) do not exhibit interaction between the amine nitrogen and palladium atom respectively. NMR spectroscopy between -40 and 25 degrees C shows that the di-NHC-amine ligand is flexible expressing C(s) symmetry and for 13 rotation of the mesityl groups is prevented. In the related C(1) complex [(kappa(3)-(tBu)CN(H)C(tBu))PdCl][Cl] (14) coordination of NHC moieties and amine nitrogen atom is observed between -40 and 25 degrees C. Reaction between 12-14 and two equivalents of AgBF(4) in acetonitrile gives the analogous complexes [trans-(kappa(2)-(tBu)CN(Bn)C(tBu))Pd(MeCN)(2)][BF(4)](2) (15), [trans-(kappa(2)-(Mes)CN(H)C(Mes))Pd(MeCN)(2)][BF(4)](2) (16) and [(kappa(3)-(tBu)CN(H)C(tBu))Pd(MeCN)][BF(4)](2) (17) indicating that ligand structure determines amine coordination. The single crystal X-ray structures of 12, 17 and two ligand imidazolium salt precursors (tBu)C(H)N(Bn)C(H)(tBu)][Cl](2) (2) and [(tBu)C(H)N(H)C(H)(tBu)][BPh(4)](2) (4) have been determined. Complexes 12-14 and 15-17 have been shown to be active precatalysts for Heck and hydroamination reactions respectively.
The synthesis and characterization of the first platinum
group
metal complexes of the recently reported ligand [H2B(mp)2]− (where mp = 2-mercaptopyridyl) are presented
herein. Addition of 2 equiv of Na[H2B(mp)2]
to [MCl(COD)]2 (where M = Rh, Ir; COD = 1,5-cyclooctadiene)
leads to the hydride migration products [Rh{κ3-SSB-BH(mp)2}(η3-C8H13)] and [Ir(H){κ3-SSB-BH(mp)2}(η4-C8H12)], respectively. Structural characterization of the rhodium complex
reveals a notably short rhodium–boron distance of 2.054(2)
Å. The reactivity observed for the rhodium complex is different
from that of all known scorpionate ligands, suggesting a higher propensity
for hydride migration within the 2-mercaptopyridine-based ligands.
The complex [Ir(Cl){κ3-SSB-BH(mp)2}(η4-C8H12)], which
is formed via hydride/halide exchange in chloroform, is also structurally
characterized. The new complexes provide rare examples of metallaboratrane
complexes where one hydrogen substituent remains at the boron center.
Rhodium and iridium complexes of chiral NHC-phenolimine and NHC-amine ligands have been prepared and studied for asymmetric transfer hydrogenation. X-ray and NMR spectroscopy show that for NHC-phenolimine complexes abstraction of chloride results in a change in ligand coordination from NHC only to chelating NHC-imine. Complexes of NHC-amines are inactive for transfer hydrogenation, whereas complexes of NHC-phenolimines are active at room temperature for a range of aryl containing ketones. Enantioselectivity is very sensitive to the NHC N-substituent resulting in a switch in the predominant enantiomer.
A new class of chiral N-heterocyclic carbene (NHC) precursors has been prepared incorporating a phenoxyimine moiety. Reaction between 3-((1R,2R)-2-{[1-(3,5-di-tert-butyl-2-hydroxyphenyl)meth-(E)-ylidene]amino}cyclohexyl)-1-isopropyl-4-phenyl-3H-imidazol-1-ium bromide [C(H)NO(H))][Br] (2a) and Pd(OAc)2 gives the complex [Pd(κ3-CNO)Br] (3), which exhibits tridentate ligand coordination via double deprotonation of 2a. The NHC ligand transfer agents [Ag(κ1-CNO(H))X] (X = Br (5a), Cl (5b)) are prepared from 2a, chloride analogue 2b, and Ag2O. Reaction between 5b and [Rh(COD)Cl]2 gives the NHC complex [Rh(κ1-CNO(H))Cl] (6) and, on chloride abstraction, the bidentate NHC-imine-coordinated complex [Rh(κ2-CNO(H))][B{3,5-(CF3)2C6H3}4] (7). For complexes 5 and 6 coordination of the phenoxyimine and deprotonation of the phenoxy moiety appear to be modified by intramolecular hydrogen bonding between hydroxyl and imine nitrogen groups. Reaction between 5a and Fe{N(SiMe3)2} gives an unusual bimetallic complex, [Fe(κ2-C(AgBr)NO)2] (8), that exhibits a 1-D polymeric chain motif in the solid state. Preliminary catalytic properties of an acetonitrile adduct of 3, [Pd(κ3-CNO)MeCN][BF4] (4), and 7 are reported for hydroamination and phenylation of enones, respectively, that show good activity but poor enantioselectivity.
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