Metal Complexes of Indigo and of Some Related Ligands [1]

. While the first coordination compounds of indigo were reported over 100 years ago, a systematic study of the coordination chemistry of this important dye and also its structural “relatives” re-mained silent for over half a century. Only in the last two decades of the 20th century research on this topic started again keeps on growing

ever since. It could be established, mainly by X-ray crystallography, but also by many other modern spectroscopic techniques, that deprotonated indigo, and its deprotonated oxidized and reduced forms act as mono-and bidentate chelate ligands in metal complexes conclusion that metal complexes of indigo are Werner type trans-N,O-chelates (Scheme 2), which was admitted by Kunz. [14] The product of the reaction of indigo with pentacarbonyliron was considered by Kunz [13] as model for hemoglobin to add oxygen reversibly. Larkworthy [15] could show, that O 2 uptake by the iron(II)-indigo complex is irreversible. For the extremely oxygen sensitive iron indigo complex a magnetic moment of 5.1 B.M. was determined. A structure for the "bisindigo iron" complex with a monoanionic indigo ligand and monoanionic reduced indigo ligand (indigo white) seems to be REVIEW Scheme 1. The first synthesis of an indigo zinc complex.

Scheme 2. Presumable structures of the first indigo bischelates.
consistent with the experimental data, which showed four active hydrogen atoms and gave indigo blue on addition of acids (Scheme 3), [15] which was observed also by Hieber and Sonnekalb [16] Scheme 3. Presumable structure of the reaction product from Fe(CO) 5 with indigo.

Deprotonated Trans-Indigo as a N,O Chelating Ligand
For a long time -about 40 years, with one exception [15] no attention has been paid to complexes of indigo. After many unsuccessful attempts -due to the very low solubility of indigo in common solvents -our group in Munich succeeded to obtain crystalline indigo complexes by introduction of tri(nbutyl)phosphine ligands and by use of chlorido bridged complexes in the presence of Ag-acetate or NaH. [17] In our groups chlorido bridged complexes have been proven most useful in many cases for the synthesis of novel metal complexes, e.g. of A series of similar complexes with the monoanion of disubstituted indigo derivatives IndR 2 (R = Cl, Br, OMe, tBu, SO 3 Na), using chlorido bridged Pd complexes [L n Pd(μ-Cl)] 2 [L n = (PR 3 )Cl, (η 3 -allyl), (azobenzene-H + ), (C 6 H 4 CH 2 NMe 2 ), (Cp)Fe(C 5 H 3 CH 2 NMe 2 ), (Ph 2 C=NCH 2 CO 2 Et-H + )], has been prepared in our group. [19] Dimers with the dianionic indigo bridge were obtained by reaction of indigo and substituted indigo with several chlorido bridged Pd and Pt complexes. [17,19] The structure of [(Et 3 P)(Cl)Pt(indigo-2H + )Pt(Cl)(PEt 3 )] has been determined by X-ray diffraction. [19] The NH protons in 4,4Ј-dichloroindigo could be exchanged by Schmidbaur's aurating agent [(Ph 3 P)Au] + [21] to give [(Ph 3 P)Au(4,4Ј-dichloroindigo-H + )] and [(Ph 3 P)Au] 2 [μ-(4,4Јdichloroindigo-2H + )] (Scheme 5). [22] The structure of the gold(I) complex with the monoanion of dichloroindigo was characterized by X-ray diffraction and shows centrosymmetric pairs of molecules with parallel arrangement of the two indigo ligands in the crystal. The same arrangement has been observed for [nBu 3 P)(Cl)Pd(indigo-H + )]. [17] Also, Ph 3 P-gold complexes with the "half molecules" of indigo, isatin, 5-bromoisatin, and 2-indoline have been obtained. [22] The structure of the isatin-gold complex was determined by X-ray diffraction. [23] Similar gold(I) complexes have been reported with the anions of hydantoin, [24] of PPP dyes [25] and of various other N-heterocycles. [26] In our group, experiments [27] have been carried out to use the benzene ring of indigo as π donor in metal complexes. For this purpose, indigo and some substituted indigo derivatives were treated with [Cp*Ru(CH 3 CN) 3 ] + OTf -. [28] REVIEW Scheme 5. Gold complexes of dichlorindigo, isatin and indoline. About 20 years after our studies on metal complexes of indigo, this chemistry again found attention and interest by various groups (Lu, Kaim, Lahiri, Konarev, Guo, Caulton, Hicks and their co-workers), who succeeded to synthesize and determine the structure of novel indigo metal complexes with special emphasis on their redox activity.
In a hexakis(tricarbonylrhenium) complex, in which the rhenium atoms are bridged by three trispyridyl-triazines three indigo dianions act as pillars to give a triangular metalloprism. Each rhenium atom is coordinated by three CO ligands, by one pyridyl N-atom and by the dianionic N,O-indigo chelate ligand in this novel compound, which absorbs strongly in the near infrared region. It also reveals interesting redox chemistry, as by reduction a series of highly stable (radical) species is formed. [29,30]
By reaction of allyl-bis(pentamethycyclopentadienyl)lanthanides with indigo and consecutive reduction with KC 8 the indigo bridged complexes [(Cp*)Ln(indigo-2H + )Ln(Cp*)] m (Ln = Gd, Dy; m = 0, -1, -2) (Scheme 7) could be obtained and characterized by X-ray structural analysis. The indigo radical bridged gadolinium complex, which was obtained through one electron reduction of the neutral complex, exhibits strong antiferromagnetic coupling of the lanthanide ion with the trianionic radical indigo. The interaction between Gd 3+ and the radical indigo ligand is the largest known for a lanthanide complex. The dysprosium complexes with m = 0 and 2 were shown to be single-molecule magnets in zero applied field. [36] Scheme 7. Indigo as redox active ligand bridging two lanthanoid ions.
Indigocarmine (5,5Ј-indigodisulfonic acid-disodium salt) was reacted with hexachlorocyclodiphosph(V)azane Cl 3 P-(NR) 2 PCl 3 to give P-bridged di-indigocarmines, from which Mn II , Co II . Ni II , Cu II , and Fe III complexes were obtained. It was proposed that in these complexes the metal ions are N,Oindigo coordinated. The magnetic moments and the antimicrobial activity of the metal compounds were studied. [37] Furthermore, metal compounds with the dianion of indigocarmine and with Cr III , Cu II , Ag I , Zn II , and Cd II were reported. [38]

Cis-Indigo as N,N and O,O Chelating Ligand
The indigo complexes, discussed so far, exhibit the indigo ligand in trans configuration. Some complexes are known with the indigo ligand in cis configuration: By reaction of (pentamethylcyclopentadieny) 2 Cr with indigo in the presence of a chloride donor the complex [(indigo-O,OЈ-)Cr(II)(Cl)Cp*)] could be isolated [39] (Scheme 8), which is the only example with indigo as neutral ligand and with coordination of cis configured carbonyl groups. Thus, the unstable cis-indigo is stabilized by coordination. The same group of Konarev [40] studied the reaction of reduced indigo with reduced Cp* 2 IrI 2 and a diamagnetic Ir Most notable, Kaim and Lahiri and their co-workers [42] succeeded to synthesize a pair of the isomeric diruthenium com- In the complex with cis configuration of the deprotonated indigo ligand a five-membered and a seven-membered chelate ring were ascertained by X-ray structure analysis. Again, the redox chemistry of these complexes with two oxidation and two reduction processes was studied.
In contrast to indigo and substituted indigo compounds, with pyrrolindigo always both NH-protons were exchanged by cationic metal fragments in reactions with chlorido bridged metal compounds to give indigo bridged N,O-chelated complexes. [17,20] Thus the polymeric Zn II and Cu II chelate complexes, the pyrrolindigo bridged Pd and Pt complexes and the pentamethylcyclopentadienyl rhodium and iridium complexes could be prepared (Scheme 13). The large bathochromic shift of the absorptions in the visible region of the complexes compared to the neutral ligands is in accordance with PPP calculations. [20]
The reaction of indigo-bis(phenyl)imine with [Ru(bipy) 2 (EtOH) 2 ] 2+ gave the complex with the monodeprotonated Nindigo ligand. Remarkably, in contrast to the reported complexes with trans-configured Nindigo ligands, reaction of the latter complex afforded with [Ru(acac) 2 a complex with cis-configuration of the indigo derivative, [(bipy) 2 Ru(cis-dehydro-nindigo)Ru(acac) 2 ] 2+ . The Nindigo ligand obviously is neutral in this complex [59] (Scheme 18). The complexes again are redox active. The parent imines of indigo, indigo-monoimine and indigodiimine and a zinc salt of the monoimine were already reported in 1913. [60,61] It appears of interest to study coordination of these indigo derivatives in the light of the new Nindigo complexes.

Thioindigo as Complex Ligand
The group of Konarev succeeded to synthesize the first metal complexes of thioindigo. [62] By reaction of bis(pentametylcyclopentadienyl)chromium with thioindigo one Cp* ligand is substituted by a dianionic reduced thioindigo, which adopts cis-configuration, allowing the coordination of both carbonyl  2 is present in a phthalocycanine indium complex, which is formed by reduction of indium(III) chloride phthalocycanine with KC 8 in the presence of cryptand-potassium and addition of thioindigo. [41] Scheme 19. Two thioindigo molecules acting as O,O chelate ligands bridged by two Cr III ions. [63] using [Ru(acac) 2 ] complexes (Scheme 20). Hicks and co-workers [64] reported the synthesis of mono and diruthenium complexes of the thioindigo-N,NЈ-diphenyldiimine, [(hfac) 2 Ru(thioindigo-N,N'-diphenylimine)Ru(hfac)] and [(hfac) 2 Ru(thioindigo-diimine)], which were characterized by X-ray crystallography and they emphasized the absence of thioindigo complexes. Coordination of the neutral thioindigoimine occurs through the S and the imine N atoms.

Cis-(O,O)-and trans-(O,S) coordination of neutral thioindigo could be verified by Kaim and Lahiri
Gompper and co-workers reported several novel derivatives of indigo and thioindigo, which may be suited as ligands in metal complexes. [65] It may be noted that Dube and co-workers, applied a derivative of thioindigo, hemithioindigo, for the development of a novel light driven molecular motor. [66]

Final Remark
When the draft of this article was finished, we became aware of a very interesting article bei Kaim and Lahiri on "The coordination potential of indigo, anthraquinone and related re-REVIEW dox-active dyes". [67] In this review the redox activity and the non-innocent behavior of the indigo ligand was emphasized; in our paper we tried to cover also the historical aspects of indigo metal complexes.