Mononuclear complexes [Os(II)(bpy)2(H2L1)](ClO4)2 ([](ClO4)2) and [Os(II)(bpy)2(H2L2)](ClO4)2 ([](ClO4)2) incorporating two free NH protons at the back face of the coordinated H2L, and deprotonated L(2-) bridged symmetric dinuclear complexes [(bpy)2Os(II)(μ-L1(2-))Os(II)(bpy)2)](ClO4)2 ([](ClO4)2) and [(bpy)2Os(II)(μ-L2(2-))Os(II)(bpy)2)](ClO4)2 ([](ClO4)2) (bpy = 2,2'-bipyridine, H2L1 = 2,2'-biimidazole and H2L2 = 2,2'-bis(4,5-dimethylimidazole)) have been characterised. Crystal structures of [](ClO4)2 and the meso (ΔΛ) diastereomeric form of [](ClO4)2 have been determined. The crystal structure of [](ClO4)2 also reveals the hydrogen bonding interactions between its free acidic NH protons and the oxygen atoms of the perchlorate anion in the nearby asymmetric unit. Experimental and DFT/TD-DFT calculations have divulged the non-innocent feature of the doubly deprotonated L(2-) in (3+) and (3+), leading to the resonating formulation of {Os(II)(μ-L(2-))Os(III)} ↔ {Os(II)(μ-L˙(-))Os(II)}, instead of a simple mixed valent situation {Os(II)(μ-L(2-))Os(III)}. The dinuclear complexes (3+) and (3+) display one broad and moderately intense near-IR transition near 1000 nm corresponding to a mixed Os(dπ)/L(π) → Os(dπ)/L(π*) MLLMCT (metal/ligand to ligand/metal charge transfer) transition. Different experimental studies have also established the interaction of (2+) and (2+) with the selective anions.
The compounds Ru(acac)2(Q) (1), [Ru(bpy)2(Q)](ClO4)2 ([2](ClO4)2), and [Ru(pap)2(Q)]PF6 ([3]PF6), containing Q = N,N'-diphenyl-o-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac(-)), π-accepting 2,2(/)-bipyridine (bpy), or strongly π-accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system 1(+)-1(-) prefers trivalent ruthenium with corresponding oxidation states Q(0)-Q(2-) of the noninnocent ligand, the series 2(2+)-2(0) and 3(2+)-3(-) retain Ru(II). The bpy and pap co-ligands are not only spectators but can also be reduced prior to a second reduction of Q. The present study with new experimental and computational evidence on the influence of co-ligands on the metal is complementary to a report on the substituent effects in o-quinonediimine ligands [Kalinina et al., Inorg. Chem. 2008, 47, 10110] and to the discussion of the most appropriate oxidation state formulation Ru(II)(Q(0)) or Ru(III)(Q(• -)).
The reaction of 2,2'-bipyridine-3,3'-diol (H2L) and cis-Os(II)(bpy)2Cl2 (bpy = 2,2'-bipyridine) results in isomeric forms of [Os(II)(bpy)2(HL(-))]ClO4, [1]ClO4 and [2]ClO4, because of the varying binding modes of partially deprotonated HL(-). The identities of isomeric [1]ClO4 and [2]ClO4 have been authenticated by their single crystal X-ray structures. The ambidentate HL(-) in [2]ClO4 develops the usual N,N bonded five-membered chelate with a strong O-H···O hydrogen bonded situation (O-H···O angle: 160.78°) at its back face. The isomer [1]ClO4 however represents the monoanionic O(-),N coordinating mode of HL(-), leading to a six-membered chelate with the moderately strong O-H···N hydrogen bonding interaction (O-H···N angle: 148.87°) at its backbone. The isomeric [1]ClO4 and [2]ClO4 also exhibit distinctive spectral, electrochemical, electronic structural, and hydrogen bonding features. The pKa values for [1]ClO4 and [2]ClO4 have been estimated to be 0.73 and <0.2, respectively, thereby revealing the varying hydrogen bonding interaction profiles of O-H···N and O-H···O involving the coordinated HL(-). The O-H···O group of HL(-) in 2(+) remains invariant in the basic region (pH 7-12), while deprotonation of O-H···N group of HL(-) in 1(+) estimates the pKb value of 11.55. This indeed has facilitated the activation of the exposed O-H···N function in [1]ClO4 by the second {Os(II)(bpy)2} unit to yield the L(2-) bridged [(bpy)2Os(II)(μ-L(2-))Os(II)(bpy)2](ClO4)2 ([3](ClO4)2). However, the O-H···O function in [2]ClO4 fails to react with {Os(II)(bpy)2}. The crystal structure of [3](ClO4)2 establishes the symmetric N,O(-)/O(-),N bridging mode of L(2-). On the other hand, the doubly deprotonated L'(2-) (H2L' = 2,2'-biphenol) generates structurally characterized twisted seven-membered O(-),O(-) bonded chelate (torsion angle >50°) in paramagnetic [Os(III)(bpy)2(L'(2-))]ClO4 ([4]ClO4). The electronic structural aspects of the complexes reveal the noninnocent potential of the coordinated HL(-), L(2-), and L'(2-). The Kc value of 49 for 3(3+) reveals a class I mixed-valent Os(II)Os(III) state.
The varying coordination modes of the ambidentate ligand 2,2'-bipyridine-3,3'-diol (H2L) in a set of ruthenium complexes were demonstrated with special reference to the electronic features of the coligands, including σ-donating acac(-) (= acetylacetonate) in Ru(III)(acac)2(HL(-)) (1), strongly π-accepting pap (= 2-phenylazopyridine) in Ru(II)(pap)2(L(2-)) (2)/[(pap)2Ru(II)(μ-L(2-))Ru(II)(pap)2](ClO4)2 ([4](ClO4)2), and reported moderately π-accepting bpy (= 2,2'-bypiridine) in [Ru(II)(bpy)2(HL(-))]PF6 ([5]PF6)/[(bpy)2Ru(μ-L(2-))Ru(bpy)2](PF6)2 ([7](PF6)2). The single-crystal X-ray structures reveal that, in paramagnetic and electron paramagnetic resonance active 1 and reported diamagnetic [5]PF6, nearly planar monoanionic HL(-) coordinates to the metal ion via the N,N donors forming a five-membered chelate ring with hydrogen-bonded O-H···O function at the backbone of the ligand framework, as has also been reported in other metal complexes. However, structurally characterized diamagnetic 2 represents O(-),O(-) bonded seven-membered chelate of fully deprotonated but twisted L(2-). The nonplanarity of the coordinated L(2-) in 2 does not permit the second metal fragment {Ru(pap)2} or {Ru(bpy)2} or {Ru(acac)2} to bind with the available N,N donors at the back face of L(2-). Further, the deprotonated form of the model ligand 2,2'-biphenol (H2L') yields Ru(II)(pap)2(L'(2-)) (3); its crystal structure establishes the expected O(-),O(-) bonded seven-membered chelate of nonplanar L'(2-) as in reported Ru(II)(bpy)2(L'(2-)) (6), although {Ru(acac)2} metal precursor altogether fails to react with H2L'. All attempts to make diruthenium complex from {Ru(acac)2} and H2L failed; however, the corresponding {Ru(pap)2(2+)} derived dimeric [4](ClO4)2 was structurally characterized. It establishes the symmetric N,O(-)/N,O(-) bridging mode of nonplanar L(2-) as in reported [7](PF6)2. Besides structural and spectroscopic characterization of the newly developed complexes, the ligand (HL(-), L(2-), L'(2-), pap)-, metal-, or mixed metal-ligand-based accessible redox processes in 1(n) (n = +2, +1, 0, -1), 2(n)/3(n) (n = +2, +1, 0, -1, -2), and 4(n) (n = +4, +3, +2, +1, 0, -1) were analyzed in conjunction with density functional theory calculations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.