Intramolecular Apical C–H···M Interactions in Square‐Planar Nickel(II) Complexes with Dianionic Tridentate Ligands and 2‐Phenylimidazole

Abstract

Keywords: Nickel(II) complexes / Square-planar complexes / Apical C-H···Ni interactions / X-ray structures / Theoretical calculations and sp 2 N-donor 2-phenylimidazole (phim) as the ancillary ligand have been synthesised and characterised by analytical, spectroscopic, magnetic and electrochemical methods. The solid-state structures of both complexes have been determined by X-ray crystallography. The asymmetric unit of 1 contains a single complex molecule while that of 2 contains four complex molecules with different conformations. The molecular structures of both complexes reveal that one of the two ortho C-H groups of the pendant phenyl ring of the phim ligand is very close to the metal centre at the apical site indicating the presence of an intramolecular C-H···Ni interaction. In the structures of 1 and 2, the shape of the C-H···Ni interaction is determined by the twisting of the phim ligand and the extent of orthogo- IntroductionThe long standing interest in the interaction of transition metal ions with proximate C-H fragments is primarily due to the C-H activation processes involved in many catalytic reactions. Such interactions have been scrutinised intensively for a better understanding of their structural and bonding features. [1][2][3][4][5][6][7][8][9][10][11][12] The C-H···M interaction in coordinatively unsaturated and electron-deficient (16-electron) species is called an agostic interaction or agostic bond where the filled C-H σ orbital interacts with an empty metal orbital. In these agostic interactions, generally the C-H fragment is side-on to the metal ion. This T-shape of the agostic C-H···M interaction facilitates the involvement of the filled metal dπ orbital and the empty C-H σ* orbital in back bonding. However, the nature of the apical C-H···M interactions in square-planar 16-electron complexes of d 8 metal ions is not very clear. [9][10][11][12] In such species, these interactions can be agostic bond (three-centre two-electron) as well as non-classical hydrogen bond (three-centre four-elec- [a]

Syntheses and structures of nickel(II) and copper(II) complexes with biacetyl bis(benzoylhydrazone): one-dimensional self-assembly via π–π interaction and hydrogen bonding

“…In both complexes, babh 2-binds the metal centre through the two imine-N and the two deprotonated amide-O atoms forming three five-membered chelate rings and an N 2 O 2 squareplane around the metal centre. Table 2) are similar to the bond lengths observed in the corresponding bivalent metal ion complex having the same coordinating atoms [18][19][20][21][22][23][24][25][26][27][28]. In the latter complex, the apical Cu-O (methanol) bond length is relatively longer than the equatorial bond lengths.…”

“…These two bands are not observed in the spectra of the complexes. Absence of these bands is consistent with the iminolate form [18][19][20][21][22][23][24][25][26][27][28]37] Electronic spectra of the complexes were recorded in methanol solution. Both complexes display a strong band in the visible region.…”

“…Over the past few years we have been working on squareplanar/square-pyramidal complexes of nickel(II) and copper(II) with acid hydrazide-based tridentate Schiff bases and various monodentate ancillary ligands [18][19][20][21][22][23][24][25][26][27][28]. Structural investigations on these complexes revealed formation of a variety of self-assembled one-, two-, and three-dimensional networks via different types of intermolecular non-covalent interactions.…”

CH⋯Metal(II) axial interaction in planar complexes (metal = Cu, Pd) and implications for possible environmental effects of alkyl groups at biological copper sites

“…At the type 1 Cu center of these proteins a methyl group from the alkyl side chain is located above the Cu plane with the CuÁ Á ÁC distance of 3.51-3.76 Å [12,13,[16][17][18][19], and in none of the cases is the alkyl group considered as a ligand. Axial interactions between an electron-rich metal center (M) and the hydrogen atom of C-H bonds were reported earlier by Maitlis et al for a Pd(II) complex [20], and MÁ Á ÁH-X interactions (X = C, N, O) have been observed in various planar complexes of d 8 metal ions [21][22][23][24][25][26][27][28]. A similar axial contact was also reported for a Cu(II) complex with a hydrogen atom from a pendent aromatic ring above the coordination plane [29].…”

Agostic interactions in transition metal compounds

“…Complexes that contain anagostic M-H-C interactions are typically associated with d 8 transition metals centers that are square planar prior to the interaction, as illustrated by a variety of rhodium(I) phosphinate complexes synthesized by Bergman and coworkers (43). m Crabtree, Eisenstein, and coworkers (44) examined the structural characteristics of complexes that featured M-H-N and M-H-C interactions in otherwise square planar d 8 complexes and concluded that although the former are appropriately described as hydrogen bonds because the M⅐⅐⅐H-N bonds are close to the linear, the situation is ambiguous for the M-H-C interactions.…”