Variable Coordination Mode of Chloranilic Acid. Synthesis, Structure, and Electrochemical Properties of Some Osmium Complexes

Abstract

Reaction of chloranilic acid (H2ca) with [Os(bpy)2 Br2] (bpy = 2,2'-bipyridine) affords a dinuclear complex of type [{Os(bpy)2}2 (ca)]2+, isolated as the perchlorate salt. A similar reaction of H2ca with [Os(PPh3)2 (pap)Br2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)2 (pap)}2 (ca)]2+ (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)2 (pap)(ca)]. Reaction of H2ca with [Os(PPh3)2(CO)2(HCOO)2] gives a dinuclear complex of type [{Os(PPh3)2(CO)2}2 (r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)2 (pap)}2 (ca)](ClO4)2, [Os(PPh3)2 (pap)(ca)], and [{Os(PPh3)2(CO)2}2 (r-ca)] complexes have been determined by X-ray crystallography. In the [{Os(bpy)2}2 (ca)]2+ and [{Os(PPh3)2 (pap)}2 (ca)]2+ complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [Os(PPh3)2 (pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)2(CO)2}2 (r-ca)] complex, the reduced form of the chloranilate ligand (r-ca(4-)) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)2 (pap)(ca)] complex shows an Os(II)-Os(III) oxidation, followed by an Os(III)-Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent Os(II)-Os(III) species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent Os(II)-Os(II) species. The mixed-valent Os(II)-Os(III) species show intense intervalence charge-transfer transitions in the near-IR region.

Synthesis, Characterisation and Properties of Diazoketiminato-Chelate Complexes of Os(II)

“…Studies on the chemistry of osmium complexes have drawn attention to properties related to electron transfer reactions [18][19][20][21][22][23][24][25][26] , metal-carbon bond formation, 18-20 aromatic ring deamination, 21 isomerism, 23 cytotoxicity toward cancer cells 24 and aromatic ring thiolation. 25 The IR spectra of 3 in KBr pellets displayed ν NH near 3330 cm −1 (sharp single band), in contrast to the ligands 2, where ν NH2 of the NH 2 function appears as a split band in the range 3360-3458 cm −1 , indicating the proton dissociation from the -NH 2 function of 2 upon coordination.…”

“…The observation of two multiplets (∼ δ 7.4 and ∼ δ 7.0), each for 12 equivalent protons and another multiplet (∼ δ 7.28) for six equivalent protons are in agreement with the two trans PPh 3 ligands. [18][19][20] The NMR spectra of 3 are deposited as ESI (Figs S7-S9) and data are collected in the Experimental section.…”

“…The Os-Br, Os-C(CO) and Os-P (PPh 3 ) bond lengths are within the normal ranges. [18][19][20][21][22][23]26 The C(2)-N( 1) bond (1.28 Å) is shorter than the C(7)-N(3) single bond (1.40 Å) in the same molecule and similar to an imine (-C=N-) length indicating delocalisation. [14][15][16][17]27 The quinonoid distortion in the phenyl ring (C2, C3, C4, C5, C6, C7) adjacent to the chelate ring with four longer (~1.43 Å) and two shorter (~1.34 Å) bonds, like other coordinated arylimines, [14][15][16][17][27][28][29] is consistent with delocalisation [14][15][16][17][27][28][29] and formation of diazoketiminato-chelate.…”

Radical ligand-containing single-molecule magnets

“…Efforts to generate benzosemiquinonoid-containing transition metal complexes with a nuclearity of greater than one have resulted in numerous dinuclear compounds, including those containing metal centers bridged by derivatives of 2,5-dihydroxy-1,4-benzoquinonoids [172][173][174][175][176][177][178][179][180][181][182][183][184][185][186][187][188][189], bis(pyrazolyl)benzoquinonoids [190][191][192][193], bis(phosphine)benzoquinonoids [194,195], bis(amino) benzoquinonoids [196][197][198][199][200][201][202], and bis(amino)diiminobenzoquinonoids [86,[203][204][205][206]. Indeed, the radical form of these ligands has shown enhanced magnetic coupling compared to their non-radical redox isomers [174,180,182].…”

Oxidation führt zu Reduktion – redoxinduzierter Elektronentransfer (RIET)

“…Ein entsprechendes Produkt, wie es bei den RIET‐Reaktionen der Tetraoxolatkomplexe 5 3+ , 6 3+ , 7 3+ , 8 3+ , 9 3+ und 10 3+ entsteht, wird bei strukturell verwandten Komplexen mit Übergangsmetallen der zweiten und dritten Reihe nicht beobachtet. Die elektrochemische Einelektronenoxidation von [(bipy) 2 Os II ( 1 Cl 2− )Os II (bipy) 2 ](ClO 4 ) 2 ,18 [(PPh 3 ) 2 (pap)Os II ( 1 Cl 2− )Os II (PPh 3 ) 2 (pap)](ClO 4 ) 2 [pap=2‐(Phenylazo)pyridin] ( 13 2+ ), 18 [(PPh 3 ) 2 (CO) 2 Os II ( 1 Cl 4− )Os II (PPh 3 ) 2 (CO) 2 ]18 und [(bipy) 2 Ru II ( 1 H 2− )Ru II (bipy) 2 ](PF 6 ) 2 ( 14 2+ )7 führt zu Kationen, deren gemischtvalente Erscheinungsform sich auf das Vorliegen eines M II ( 1 Cl n − )M III ‐Kerns beschränkt, wogegen aber kein radikalischer Tetroxolen‐Brückenligand gebildet wird. Anzumerken ist, dass das reduzierte Tetraoxolat – in Einklang mit den Befunden für 5 + – durch Reduktion von 14 2+ zu [(bipy) 2 Ru II ( 1 H .3− )Ru II (bipy) 2 ](PF 6 ) ( 14 + ) erzeugt werden kann.…”