Ruthenium Azocarboxamide Half-Sandwich Complexes: Influence of the Coordination Mode on the Electronic Structure and Activity in Base-Free Transfer Hydrogenation Catalysis

Abstract

Azocarboxamides were used as chelating ligands in ruthenium half-sandwich complexes. The synthesis and characterization of two new complexes with an unprecedented coordination motif are presented together with an in-depth investigation of two recently published complexes. Three different coordination modes of the ligands were realized, as evident by NMR spectroscopy and single-crystal X-ray diffraction. The use of base during the synthesis leads to a coordination of a deprotonated ligand, while the introduction of additional donor atoms results in a noncoordinated amide group. The first systematic experimental (cyclic voltammetry and UV–vis–NIR and EPR spectroelectrochemistry) and theoretical (DFT) investigation of the electronic structure of metal complexes bearing this redox-active ligand class is presented, revealing redox processes with ligand contribution. The absorption spectra and electrochemistry are mainly determined by the protonation state of the ligand. While complexes 2­[PF 6 ], 3­[PF 6 ], and 4­[PF 6 ] with neutral azocarboxamides show similar electronic spectra and cyclovoltammograms, the incorporation of a deprotonated monoanionic ligand in complex 1 leads to significant changes of these properties. In contrast, the catalytic activity in the base-free transfer hydrogenation reaction is mainly dependent on the coordination of the amide group, with only minor effects of the protonation state. While complexes 3­[PF 6 ] and 4­[PF 6 ], with an uncoordinated amide group, are inactive without the addition of base, complexes 1 and 2­[PF 6 ], with a metal-bound amide group, show activity under base-free conditions. The impact of the position of the amide group together with the detection of metal hydride species in 1H NMR spectroscopy suggests the operation of metal–ligand bifunctional catalysis to take place when no base is added.

Synthesis and X-ray Structural Analysis of the Ruthenium(III) Complex Na[trans-RuCl4(DMSO) (PyrDiaz)], the Diazene Derivative of Antitumor NAMI-Pyr

“…31 Combining complex 2 with an excess of ligand 1 in acetone solution resulted in displacement of one dimethyl sulfoxide ligand in the ruthenium coordination sphere and the formation of a new compound 3 shown in Scheme 3 (left). In contrast to our previous findings 19,20 for arene-ruthenium(II) compounds from Figure 2, in complex 3 diazene moiety did not participate in coordination to the metal centre. Instead, the pyridine part of the molecule replaced one axial dimethyl sulfoxide ligand.…”

“…18 Redox-active diazenecarboxamides were also combined with organometallic [Ru(II)-Arene] to generate complexes with interesting coordination modes and chemical reactivity ( Figure 2). 19,20 In the context of our endeavour in the field of potential anti-cancer agents, complex B was identified as highly cytotoxic against tumour cell lines with IC 50 values in the low micro-molar range. 19,21 ure 3), we were prompted to examine the coordination ability of diazenedicarboxamide N 1 -(4-isopropylphenyl)-N 2 -(pyridin-2-ylmethyl)diazene-1,2-dicarboxamide (1) (Scheme 1).…”

Versatile Coordination of Azocarboxamides: Redox‐Triggered Change of the Chelating Binding Pocket in Ruthenium Complexes

“…Azocarboxamides are as pecial class of azo ligands [7] that are largely underrepresented in coordination chemistry. [8,9] This is surprising, as azocarboxamides are truly versatile ligands and offer several functionalities and many dif-ferent coordination modes ( Figure 1). The azo group in azocarboxamides is surrounded by the carbonyl group of the amide and am odifiable substituent, whichc an be used to introduce an additional donor atom (Figure1).…”

“…The azo group in azocarboxamides is surrounded by the carbonyl group of the amide and am odifiable substituent, whichc an be used to introduce an additional donor atom (Figure1). Moreover,t he amide nitrogen atom (in its deprotonated form) can also participate in coordination to am etal center, [8,9] and the acidic protono pens up the possibility of investigating the complexes in terms of proton-coupled electron transfer.The ligand architecture therefore allows am etal centert ob ec oordinated in three different modes:O ,N(azo), N(azo),N(heterocycle), and N(azo),N-(amide). [8,9] Additionally,a zocarboxamides offer two distinct binding pockets for metal coordination, af acet that is absent in symmetrically substitutedl igands such as abpy and adc-R (Scheme 1).…”

“…[8,9] It was shown that the redoxb ehavior and the electronic spectra are pre- dominantly influenced by the protonation state, whereas the catalytic activity in base-free transfer hydrogenation strongly dependso nt he coordination mode. [8] Furthermore, the complex with an amide-deprotonated, phenyl-substituted azocarboxamide ligand shows remarkable cytotoxic properties. [9] Deprotonation is as traightforward way to force azocarboxamides to coordinate in aN (azo),N(amide) fashion, and choosing as ubstituent withouta dditional donor atom predetermines an O,N(azo) coordination mode.…”