Nickel(II) and Palladium(II) Complexes with Chelating N-Heterocyclic Carbene Amidate Ligands: Interplay between Normal and Abnormal Coordination Modes

Abstract

A series of six Ni(II) and Pd(II) complexes of two bidentate and two tetradentate N-heterocyclic carbene (NHC)/amidate ligands have been prepared. The complexes are uncharged, with square-planar coordination geometries, and the ligands are bound via the NHC groups and the deprotonated amide nitrogen atoms. Pd(II) complexes were prepared for the bidentate ligands, and in each case, two chelating bidentate ligands were bound to the metal center, yielding cis/trans geometric isomeric forms. The Pd(II) complexes of the tetradentate ligands were obtained as a series of constitutional isomeric forms that were separable by fractional crystallization. The constitutional isomers differed in the coordination mode of the NHC groups, which were bound as either “normal” (nNHC) or “abnormal” (aNHC) carbenes. Density functional theory (DFT) studies show that the energies of the isomeric forms increase in the order nNHC/nNHC < nNHC/aNHC < aNHC/aNHC and suggest that the “abnormal” NHC coordination mode occurs in kinetic rather than thermodynamic reaction products. The Ni(II) complexes of the tetradentate ligand showed only “normal” NHC coordination, suggesting that the mechanism by which aNHC binding occurs is metal dependent. The Ni(II) and Pd(II) complexes with nNHC donors displayed distorted-square-planar coordination geometries and axial chirality.

Selective Synthesis of Ni(II) and Pd(II) Complexes with either ‘Normal’ or ‘Abnormal’ N‐Heterocyclic Carbene Coordination Modes

“…In the case of pro‐ligand 7 the imidazolium groups were substituted with sterically demanding tert ‐butyl wingtip groups, while for 8 both imidazolium units were linked via o ‐xylene units to form a macrocyclic molecule. Pro‐ligand 5 was prepared by the reaction of 2‐chloroacetyl‐aniline with 1,2‐dimethylimidazole, while pro‐ligands 6 and 7 were prepared in a similar manner from N,N ′‐ bis (2‐chloroacetyl)‐1,2‐phenylenediamine and the appropriate substituted imidazole, 1,2‐dimethylimidazole or 1‐ t ‐butylimidazole (Scheme ) …”

“…1 H NMR analysis of 10 showed that both the imidazolium C4‐H and amide N–H proton signals of the pro‐ligand were absent, indicating deprotonation and coordination of these groups to the metal centre. These results are consistent with the complex being uncharged with two chelating ligands coordinated to the Pd(II) centre via the NHC groups and the deprotonated amide nitrogen atoms (amidate), as has been found previously for similar systems . The X‐ray crystal structure of 10 was obtained (Figure ) and this confirmed that the NHC groups were coordinated via the abnormal coordination mode.…”

“…Other researchers have also shown that incorporation of sterically demanding substituents on the wingtip of NHC groups, can result in the formation of aNHC−metal complexes . Previously, we reported a series of Ni(II) and Pd(II) complexes of tetradentate NHC / amidate ligands that displayed either ‘normal’ or ‘abnormal’ NHC coordination modes depending on the steric demands of the NHC wingtip group …”

Nickel N-heterocyclic carbene complexes and their utility in homogeneous catalysis

“…In particular, the (533−534), [163] (535), [164] (536), [165] (543−544), [166] (545−546), [167] (548 [168]−557), [169] (558), [51] (559), [71] (560), [161] (561), [170] (562−563), [141] (565), [171] (566−567), [172] (568), [173] (569), [126d] (570−573), [174] (574), [175] (575−576), [176] (577), [177] (578), [178] (579), [106] (580−581), [179] (582−583), [151] (584−585), [180] (586−588), [181] (595−596), [182] (597−599), [183] (600−603), [184] (604), [185] (605−608), [186] (609), [185] (610−613), [187] (614−618), [188] (619−621), [189] (622−623), [185] (624−630), [190] (631), [191] (633−635), [192] (636), [193] (637), [194] (647) [195] and (648) [196] complexes were obtained by this pathway from the treatment of the free or the in situ generated NHC The third route is the oxidation method (Scheme 15) using which the (589−590), [197] (638) …”

Ru–Ag and Ru–Au dicarbene complexes from an abnormal carbene ruthenium system

“…In the 1 H NMR spectrum, the signal of the NCHN proton of the abnormal carbene is exceptionally downfield shifted at δ = 9.75 ppm, close to that of the ligand precursor P-NHC × HBr (10.33 ppm), suggesting a relatively acidic proton. [42][43][44] Complex 2 can also be prepared directly from [Ru(OAc) 2 (PPh 3 ) 2 ] and 2 equiv. of P-NHC × HBr in THF in the presence of NaOAc (88% yield).…”