A Homologous Series of First-Row Transition-Metal Complexes of 2,2′-Bipyridine and their Ligand Radical Derivatives: Trends in Structure, Magnetism, and Bonding

Abstract

The organometallic first-row transition-metal complexes [M(2,2'-bipy)(mes)2] (M = Cr (1), Mn (2), Co (4), Ni (5); 2,2'-bipy = 2,2'-bipyridine; mes = 2,4,6-Me3C6H2) were reacted with potassium and a suitable alkali-metal sequestering agent to yield salts of the anionic species [M(2,2'-bipy)(mes)2](-). The neutral parent compounds and their corresponding anionic congeners were characterized by single-crystal X-ray diffraction in [Cr(2,2'-bipy)(mes)2]·1.5C6H6, [Mn(2,2'-bipy)(mes)2], [Co(2,2'-bipy)(mes)2]·THF, [Ni(2,2'-bipy)(mes)2], [K(dibenzo-18-crown-6)·THF][Cr(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·2THF, [K(18-crown-6)][Mn(2,2'-bipy)(mes)2]·0.67py·0.67tol, [K(2,2,2-crypt)][Co(2,2'-bipy)(mes)2], and [K(2,2,2-crypt)][Ni(2,2'-bipy)(mes)2]. These species, along with the previously reported neutral and anionic iron complexes [Fe(2,2'-bipy)(mes)2](0/-) (3/3(-)), form a homologous series of compounds which allow for an in-depth study of the interactions between metals and ligands. Single-crystal X-ray diffraction data, DFT calculations, and various spectroscopic and magnetic measurements indicate that the anionic complexes (1(-)-5(-)) can be best formulated as M(II) complexes of the 2,2'-bipyridyl radical anion. These findings complement recent studies which indicate that bond metric data from single-crystal X-ray diffraction may be employed as an important diagnostic tool in determining the oxidation states of bipyridyl ligands in transition-metal complexes.

Electronic Structures of “Low‐Valent” Neutral Complexes [NiL2]0 (S = 0; L = bpy, phen, tpy) – An Experimental and DFT Computational Study

“…[a] See bond labeling scheme in Figure 6. [14] Similar linear correlations between ligand redox state and bond lengths have been documented by us [15,16,18] and others [19] for N,NЈ-coordinated (bpy) n and (tpy) n ligands. It has been shown that an experimental assignment of the ligand oxidation level in a given coordination compound can be made from high-resolution X-ray structures alone by using these correlations.…”

Reduction of 2,2′-Bipyridine by Quasi-Linear 3d-Metal(I) Silylamides—A Structural and Spectroscopic Study

“…Thereby, it has been shown that the structural features of the central C-C bond within the bipyridine ligand can be a strong indicator of its redox state [32,34,35]. This has been showcased for four-coordinate [M(mes) 2 (bipy)] (mes = 2,4,6-trimethylphenyl), where reduction with potassium graphite in the presence of either 18-crown-6 or crypt.222 led to a mainly bipyridine-based reduction and formation of K{L}[M(mes) 2 (bipy)] (L = 18-crown-6 or crypt.222) [34,35].…”

“…The lengths of the central C-C bond of the bipyridine ligand increases slightly from 1.417(6) Å for chromium to 1.432(3) Å for cobalt. The C-C bond lengths are thereby in the range of ligated 2,2 -bipyridine radical anions within isolable complexes [32,34,35,40,41].…”

“…Within these complexes the exact determination of the electronic structure of the [M(bipy)] unit is usually not trivial leading to the generally accepted classification of 2,2 -bipyridine as a redox non-innocent ligand [32,33]. Thereby, it has been shown that the structural features of the central C-C bond within the bipyridine ligand can be a strong indicator of its redox state [32,34,35]. This has been showcased for four-coordinate [M(mes) 2 (bipy)] (mes = 2,4,6-trimethylphenyl), where reduction with potassium graphite in the presence of either 18-crown-6 or crypt.222 led to a mainly bipyridine-based reduction and formation of K{L}[M(mes) 2 (bipy)] (L = 18-crown-6 or crypt.222) [34,35].Given this background, we were curious if and how the above-mentioned two-coordinate metal(I) silylamides coordinate 2,2 -bipyridine.…”

“…Thereby, it has been shown that the structural features of the central C-C bond within the bipyridine ligand can be a strong indicator of its redox state [32,34,35]. This has been showcased for four-coordinate [M(mes) 2 (bipy)] (mes = 2,4,6-trimethylphenyl), where reduction with potassium graphite in the presence of either 18-crown-6 or crypt.222 led to a mainly bipyridine-based reduction and formation of K{L}[M(mes) 2 (bipy)] (L = 18-crown-6 or crypt.222) [34,35].Given this background, we were curious if and how the above-mentioned two-coordinate metal(I) silylamides coordinate 2,2 -bipyridine. Thereby it is important to note that the reported reduction potential of the free bipy 0 /bipy· − -redox couple is fairly negative (E 1/2 = −2.53 V vs Fc/Fc + in MeCN) [36] in comparison to those of the two-coordinate metal(I) complexes (E pc of −1.5 V for Co to −2.5 V for Mn) [6,13].…”