Intense MLCT Transitions Research Articles

New Five Coordinate Ru(II) Carbonyl Complexes Containing 2‐Aminofluorene Schiff Base Derivatives: Synthesis, Structure and Catalytic Activity

AbstractThe monometallic five coordinated ruthenium(II) Schiff base complexes of the general type [Ru(CO)(EPh3)2(L1−3)]Cl (E=P or As) have been synthesized and characterized by analytical and spectral (FT−IR, UV−Vis and 1H NMR) methods. The electronic spectra of the complexes indicate the presence of intense MLCT transitions in the visible region. The molecular structure of the complex [Ru(CO)(PPh3)2(L3)]Cl 5 was characterized by single crystal X−ray diffraction studies, which revealed that the Schiff base ligands coordinated to the ruthenium(II) ion as mono anionic bidentate (O, N) donor forming six membered chelating rings. Further, the five coordinate [Ru(CO)(PPh3)2(L3)]Cl 5 complex was found efficiently to catalyze the formation of amides from various substituted aldehyde and showing higher catalytic activity in the presence of NH2OH.HCl and NaHCO3 and the yield was up to 97%.

Multichromophoric bimetallic Ru(II) terpyridine complexes based on pyrenyl-bis-phenylimidazole spacer: synthesis, photophysics, spectroelectrochemistry, and TD-DFT calculations.

A symmetrical bridging ligand, 5,11-bis(4-([2,2':6',2″-terpyridine]-4'-yl)phenyl)-4,12-dihydropyreno[4,5-d:9,10-d']diimidazole (tpy-H2PhImzPy-tpy), containing terpyridyl coordinating units connected via a pyrenyl-bis-phenylimidazole spacer have been designed to synthesize a new class of light harvesting bimetallic Ru(II) complexes. The electronic properties of this complexes can be fine-tuned by varying tridentate terminal ligands. Full characterization of the compounds has been done with the help of (1)H NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Geometry optimization of the complexes was also carried out with density functional theory (DFT). Electronic absorption spectra exhibit a number of very intense π-π* and n-π* transitions in the UV and moderately intense MLCT and ILCT transitions in the visible region. Interestingly, the present bimetallic complexes exhibit moderately strong luminescence in the range between 657 and 703 nm and lifetimes (long component) between 5.8 and 67.0 ns at room temperature showing the dependence of the emission characteristics upon the type of terminal ligand and solvent. Detailed temperature-dependent emission measurements showed that an overall enhancement of photoluminescence intensity and lifetime occur in all three cases upon lowering of temperature. The redox behavior of the compounds is characterized by a single reversible anodic wave corresponding to two closely spaced one-electron processes. The appearance of intervalence charge transfer transition (IVCT) bands in the NIR region on electrochemical generation of Ru(II)Ru(II)/Ru(II)Ru(III) species indicates the presence of substantial electronic communication among the two ruthenium centers in the bimetallic complexes. DFT and TDDFT calculations were also done for better understanding of the absorption and emission spectral characteristics of the complexes.

Diverse coordination of Schiff bases based on 2-(aminomethyl)pyridine or 2-acetylpyridine at Mo(IV) centre: Synthesis, crystal structures and physicochemical properties

The reaction of Schiff bases formed in reaction of 2-(aminomethyl)pyridine and aliphatic ketones (acetone, dihexyl ketone, 2- and 4-heptanone) or 2-acetylpyridine and 2-aminoethanol with K3Na[Mo(CN)4O2]·6H2O in water–ethanol solution results in isolation of two new complexes of formulae: (PPh4)[Mo(CN)3O(ampy)]·2H2O (1) (ampy=2-(aminomethyl)pyridine) and (PPh4)[Mo(CN)3O(aceamet)]·2H2O (2) (aceamet=2-{[1-(pyridin-2-yl)ethyl]amino}ethanol). The isolated salts were characterized by elemental analysis, single crystal X-ray structure measurements, IR and UV–Vis spectroscopy and cyclic voltammetry. The complexes crystalize in monoclinic (1) or triclinic (2) space group with very distorted geometry of the anion with almost identical MoO distance (1.674 and 1.673Å for 1 and 2, respectively). The energy of intense MLCT transitions in visible part of the spectra were found to be dependent on pyridine ring position (cis in 1 and trans in 2) to MoO bond and show strong solvatochromic effect dependent on the nature of the solvent. The very unusual position of pyridine ring in 2 as well as coordination of only 2-(aminomethyl)pyridine in 1 is discussed in terms of the reaction scheme in which for both salts only Schiff base ligand components [chelating 2-(aminomethyl)pyridine or 2-acetylpyridine] are coordinated at first reaction step. The results presented in paper indicate, that the Schiff bases cannot be treated as typical stable organic ligands, but rather as a complex system. The metal (or complex) determine which element of this system and in what sequence is coordinated.

Osmium and cobalt complexes incorporating facially coordinated N,N,O donor azo-imine ligands: Redox and catalytic properties

Reactions of 1-{[2-(arylazo)phenyl]iminomethyl}-2-phenol, HL [where H represents the dissociable phenolic protons of ligands and aryl groups of HL are C6H5 (HL1), p-CH3C6H5 (HL2), p-ClC6H5 (HL3)] with Os(H)(CO)(Br)(PPh3)3 in toluene and with Co(ClO4)3.6H2O in methanol afforded new facially coordinated complexes of composition [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4. The anionic ligand (L) − , binds the metal in tridentate (N, N, O) manner in [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4 complexes. All the Os(II) and Co(III) complexes are diamagnetic and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. The cyclic voltammograms exhibited one quasi reversible oxidative response near 0.10 V vs SCE for [(L)Os(CO)(Br)(PPh3)] and [(L)2Co]ClO4 complexes displayed serial reductive responses within − 0.06 to − 1.61 V vs SCE. Single point DFT calculation was carried out to examine the nature of redox orbitals.Conversion of ketones to corresponding alcohols has been studied using [(L)Os(CO)(Br)(PPh3)] as catalyst. New complexes of Os(II) and Co(III) have been synthesized where the tridentate ligands are held in facial manner about the metal centre. Redox properties of the metal complexes have been studied and nature of the redox orbitals were examined on the basis of DFT calculations. Catalytic transformation of secondary ketones to corresponding alcohols using the newly synthesized Os(II) complexes as catalyst have been outlined.

Copper(I) complexes of N-(aryl)pyridine-2-aldimines: Spectral, electrochemical and catalytic properties

The reactions of N-(aryl)pyridine-2-aldimines ( L– R; R = OCH 3, CH 3, H, Cl and NO 2), derived from pyridine-2-aldehyde and para-substituted anilines, with CuI in methanol under ambient conditions afford a series of brown complexes of the type [{Cu(L–R)I} 2]. The structure of the [{Cu(L–OCH 3)I} 2] complex has been determined by X-ray crystallography. In these dimeric complexes the two copper centers are linked through an iodo-bridge, and the L– R ligands are coordinated to the metal center through the pyridine-nitrogen and imine-nitrogen. All the complexes show characteristic 1H NMR signals and intense MLCT transitions in the visible region. These complexes also show an emission near 465 nm, whilst they are excited at 340 nm, with relatively poor quantum yields ( φ ∼0.002 at 298 K). Cyclic voltammetry on all the complexes shows two successive Cu(I)–Cu(II) oxidations on the positive side of SCE, and a reduction of the coordinated imine ligand on the negative side. These copper(I) complexes are found to efficiently catalyze Suzuki type C–C coupling reactions.

Osmium assisted C–H activation and C [dbnd]N cleavage of N-(2′-hydroxyphenyl) benzaldimines. Synthesis, structure and electrochemical properties of some organoosmium complexes

Reaction of N-(2′-hydroxyphenyl)-4-R-benzaldimines ( L- R, R = OCH 3, CH 3, H, Cl and NO 2) with [Os(PPh 3) 3Br 2] in refluxing 2-methoxyethanol in the presence of triethylamine affords two families of organoosmium complexes ( 1- R and 2- R). In both 1- R and 2- R complexes a benzaldimine ligand is coordinated to the metal center as tridentate C,N,O-donor. In the 1- R complexes, a bidentate N,O-donor imionsemiquinonate ligand, derived from the hydrolysis of another benzaldimine, and a PPh 3 ligand are also coordinated to osmium. In the 2- R complexes, a carbonyl, derived from decarbonylation of 4-R-benzaldehyde (derived from the same hydrolysis stated above), and two PPh 3 ligands take up the remaining coordination sites on osmium. Structures of the 1- Cl and 2- OCH 3 complexes have been determined by X-ray crystallography. All the 1- R and 2- R complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the 1- R complexes shows a reversible Os(III)–Os(IV) oxidation within 0.47–0.67 V (vs SCE), followed by an irreversible oxidation of the imionsemiquinonate ligand within 1.10–1.36 V. An irreversible Os(III)–Os(II) reduction is also displayed by the 1- R complexes within −1.02 to −1.14 V. Cyclic voltammetry on the 2- R complexes shows a reversible Os(II)–Os(III) oxidation within 0.29–0.51 V, followed by a quasi-reversible oxidation within 1.04–1.29 V, and an irreversible reduction of the coordinated benzaldimine ligand within −1.16 to −1.31 V.

Iridium assisted S–H and C–H activation of benzaldehyde thiosemicarbazones. Synthesis, structure and electrochemical properties of the resulting complexes

Reaction of five 4- R-benzaldehyde thiosemicarbazones (R = OCH 3, CH 3, H, Cl and NO 2) with [Ir(PPh 3) 3Cl] in refluxing ethanol in the presence of a base (NEt 3) affords complexes of three different types, viz. 1- R, 2- R and 3- R. In the 1- R complexes the thiosemicarbazone is coordinated to iridium as a monoanionic bidentate N,S-donor forming a four-membered chelate ring. Two triphenylphosphines, a hydride and a chloride are also coordinated to the metal center. The 2- R complexes are very similar in composition and stereochemistry to the corresponding 1- R complexes, except that a second hydride is bound to iridium instead of the chloride. In the 3- R complexes, the thiosemicarbazones are coordinated to iridium as dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. Two triphenylphosphines and a hydride are also coordinated to the metal center. Structures of the 1- NO 2 , 2- NO 2 and 3- NO 2 complexes have been determined by X-ray crystallography. Reaction of the same 4- R-benzaldehyde thiosemicarbazones with [Ir(PPh 3) 3Cl] in refluxing toluene in the presence of NEt 3 affords complexes of two types, viz. 3- R and 4- R. The 4- R complexes are very similar in composition and stereochemistry to the corresponding 3- R complexes, except that a chloride is bound to iridium instead of the hydride. Structure of the 4- CH 3 complex has been determined by X-ray crystallography. In all the complexes the two PPh 3 ligands are trans. All the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on the complexes shows an Ir( III)–Ir( IV) oxidation on the positive side of SCE followed by an oxidation of the coordinated thiosemicarbazone. A reduction of the coordinated thiosemicarbazone is also observed on the negative side of SCE.

Iridium mediated phenolic O-H activation and cyclometalation of 2-(naphthyl-1′-azo)-4-methylphenol — Formation of organoiridium complexes

Reaction of 2-(naphthyl-1′-azo)-4-methylphenol with [Ir(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3) affords an organoiridium complex of type [Ir(PPh3)2(L)(H)], where L represents the coordinated 2-(naphthyl-1′-azo)-4-methylphenolate ligand. A similar reaction carried out in toluene affords the [Ir(PPh3)2(L)(H)] complex along with a similar complex of type [Ir(PPh3)2(L)Cl]. Structures of both the [Ir(PPh3)2(L)(H)] and [Ir(PPh3)2(L)Cl] complexes have been determined by X-ray crystallography. In both the complexes, 2-(naphthyl-1′-azo)-4-methylphenol is coordinated to iridium, via C-H activation at the 2′ position of the naphthyl ring, as a dianionic tridentate C, N, O-donor and the two triphenylphosphines are trans. The organoiridium complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ir(PPh3)2(L)(H)] and [Ir(PPh3)2(L)Cl] complexes shows a reversible Ir(III)-Ir(IV) oxidation respectively at 0·55 and 0·73 V vs SCE. An irreversible oxidation of the coordinated 2-(naphthyl-1′-azo)-4-methylphenolate ligand is observed above 1·0 V vs SCE and an irreversible reduction of the same is observed near −1·0 V vs SCE.

Tris-(1,3-diaryltriazenide) complexes of rhodium — Synthesis, structure and, spectral and electrochemical properties

Reaction of 1,3-diaryltriazenes (abbreviated in general as HL-R, where R stands for the para-substituent in the aryl fragment and H stands for the dissociable hydrogen atom, R = OCH3, CH3, H, Cl, NO2) with [Rh(PPh3)2(CO)Cl] in ethanol in the presence of NEt3 produces a series of tris-diaryltriazenide complexes of rhodium of type [Rh(L-R)3], where the triazenes are coordinated to rhodium as monoanionic, bidentate N,N-donors. Structure of the [Rh(L-OCH3)3] complex has been determined by X-ray crystallography. The complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. They also fluoresce in the visible region under ambient condition while excited at around 400 nm. Cyclic voltammetry on these complexes shows a Rh(III)-Rh(IV) oxidation (within 0·84–1·67 V vs SCE), followed by an oxidation of the coordinated triazene ligand (except the R = NO2 complex). An irreversible reduction of the coordinated triazene is also observed for all the complexes below −1·03 V vs SCE.

Rhodium complexes of 1,3-diaryltriazenes: Usual coordination, N–H bond activation and, N–N and C–N bond cleavage

Reaction of 1,3-diaryltriazenes (R–C 6H 4–N N–(NH)–C 6H 4–R, R = OCH 3, CH 3, H, Cl, NO 2 at the para position) with [Rh(PPh 3) 3Cl] in ethanol in the presence of a base (NEt 3) affords a family of yellow complexes ( 1- R) containing a PPh 3, two de-protonated triazenes coordinated as bidentate N,N-donors, and an aryl (C 6H 4–R) fragment coordinated in the η 1-fashion. A similar reaction in toluene yields a group of reddish-orange complexes ( 2- R) containing a PPh 3, two N,N-coordinated triazenes, and a chloride. Structures of the 1- CH 3 and 2- CH 3 complexes have been determined by X-ray crystallography. All the 1- R and 2- R complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. The 1- R and 2- R complexes also fluoresce in the visible region under ambient condition while excited at around 400 nm. Cyclic voltammetry on these complexes shows a Rh(III)–Rh(IV) oxidation (within 0.76–1.68 vs. SCE), followed by an oxidation of the coordinated triazene ligand (except the R = NO 2 complexes). An irreversible reduction of the coordinated triazene is also observed for all the complexes below −0.96 V vs. SCE. In the 1- R and 2- R complexes potential of the Rh(III)–Rh(IV) oxidation correlates linearly with the electron-withdrawing nature of the para-substituent (R).

Palladium and platinum complexes of 2-(2′-carboxyphenylazo)-4-methylphenol: Synthesis, structure and spectral properties

Reaction of 2-(2′-carboxyphenylazo)-4-methylphenol (H2L) with [M(PPh3)2Cl2] (M = Pd, Pt) affords mixed-ligand complexes of type [M(PPh3)(L)]. Structures of both the complexes have been determined by X-ray crystallography. Both the complexes are square planar, where the 2-(2′-carboxyphenylazo)-4-methylphenol is coordinated to the metal center, via dissociation of the two acidic protons, as a dianionic tridentate O,N,O-donor, and the fourth position is occupied by the triphenylphosphine. These complexes show intense MLCT transitions in the visible region.

Interaction of N-(aryl)picolinamides with iridium. N–H and C–H bond activations

Reaction of N-(4-R-phenyl)picolinamide (R = OCH 3, CH 3, H, Cl and NO 2) with [Ir(PPh 3) 3Cl] in refluxing ethanol in the presence of a base (NEt 3) affords two yellow complexes ( 1-R and 2-R). The 1-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines, a chloride and a hydride. The 2-R complexes contain an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphines and two hydrides. Similar reaction of N-(naphthyl)picolinamide with [Ir(PPh 3) 3Cl] affords two organometallic complexes, 3 and 4. In complex 3 the amide ligand is coordinated to the metal center, via C–H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor, along with two triphenylphosphines and one chloride. Complex 4 is similar to complex 3, except a hydride is bonded to iridium instead of the chloride. Structures of the 1-OCH 3 , 2-Cl and 4 complexes have been determined by X-ray crystallography. All the complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ir III–Ir IV oxidation within 0.50–1.16 V vs. SCE and a reduction of the coordinated amide ligand within −1.02 to −1.25 V vs. SCE.

1-(2′-Pyridylazo)-2-naphtholate complexes of ruthenium: Synthesis, characterization, and DNA binding properties

Reaction of 1-(2′-pyridylazo)-2-naphthol (Hpan) with [Ru(dmso) 4Cl 2] (dmso = dimethylsulfoxide), [Ru(trpy)Cl 3] (trpy = 2,2′,2″-terpyridine), [Ru(bpy)Cl 3] (bpy = 2,2′-bipyridine) and [Ru(PPh 3) 3Cl 2] in refluxing ethanol in the presence of a base (NEt 3) affords, respectively, the [Ru(pan) 2], [Ru(trpy)(pan)] + (isolated as perchlorate salt), [Ru(bpy)(pan)Cl] and [Ru(PPh 3) 2(pan)Cl] complexes. Structures of these four complexes have been determined by X-ray crystallography. In each of these complexes, the pan ligand is coordinated to the metal center as a monoanionic tridentate N,N,O-donor. Reaction of the [Ru(bpy)(pan)Cl] complex with pyridine (py) and 4-picoline (pic) in the presence of silver ion has yielded the [Ru(bpy)(pan)(py)] + and [Ru(bpy)(pan)(pic)] + complexes (isolated as perchlorate salts), respectively. All the complexes are diamagnetic (low-spin d 6, S = 0) and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ru(II)–Ru(III) oxidation on the positive side of SCE. Except in the [Ru(pan) 2] complex, a second oxidative response has been observed in the other five complexes. Reductions of the coordinated ligands have also been observed on the negative side of SCE. The [Ru(trpy)(pan)]ClO 4, [Ru(bpy)(pan)(py)]ClO 4 and [Ru(bpy)(pan)(pic)]ClO 4 complexes have been observed to bind to DNA, but they have not been able to cleave super-coiled DNA on UV irradiation.

The role of 5-R-1,10-phenanthroline (R = CH 3, NO 2) on the emission properties and second-order NLO response of cationic Ir(III) organometallic chromophores

[Ir(cyclometallated 4,5-diphenyl-2-methyl-thiazole) 2(5-R-1,10-phenanthroline)][PF 6] (R = CH 3, NO 2) complexes were prepared and fully characterized, the structure of the complex with 5-CH 3-1,10-phenanthroline being also determined by X-ray diffraction. The emission properties of both complexes have been investigated and their second-order nonlinear optical (NLO) response has been determined experimentally by the EFISH technique and found to be similar but slightly lower than that of related [Ir(ppy) 2(5-R-1,10-phenanthroline)][PF 6] (ppy = cyclometallated 2-phenylpyridine), characterized by one of the highest second-order NLO response ever reported for a metal complex. In the complexes, SOS/TDDFT calculations show that the large and negative sign of the measured hyperpolarizability is mainly due to the significant contribution of rather intense MLCT transitions involving the phenanthroline as acceptor ligand.

The Influence of Ligand Substitution at the Electron Donor Center in Molecular Cyano‐Bridged Mixed‐Valent CoIII/FeII and CoIII/RuII Complexes

AbstractIn this paper we report a new variety of dinuclear cyano‐bridged CoIII/MII (M = Fe or Ru) mixed‐valent complexes of the general formula [LnCoIII(μ‐NC)MII(CN)x(bpy)y]z+ (where Ln represents a pentaamine macrocycle and M = Fe or Ru with x = 4, y = 1 and z = 1 or x = 2, y = 2 and z = 3)]. The incorporation of the bpy ligand in place of cyano ligands within the coordination sphere raises the MIII/II redox potential significantly relative to the hexacyanometalate analogs we have reported previously. However, the metal‐to‐metal (MII to CoIII) charge‐transfer transitions are shifted to lower energies in accordance with Hush theory as a result of a decreased reorganizational energy component, which result in them overlapping with more intense MLCT (MII to bpy) transitions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

N, N′-Bis(aryl)pyridine-2,6-dicarboxamide complexes of ruthenium: Synthesis, structure and redox properties

Reaction of five N, N′-bis(aryl)pyridine-2,6-dicarboxamides (H 2L-R, where H 2 denotes the two acidic protons and R (R = OCH 3, CH 3, H, Cl and NO 2) the para substituent in the aryl fragment) with [Ru(trpy)Cl 3](trpy = 2,2′,2″-terpyridine) in refluxing ethanol in the presence of a base (NEt 3) affords a group of complexes of the type [Ru II(trpy)(L-R)], each of which contains an amide ligand coordinated to the metal center as a dianionic tridentate N,N,N-donor along with a terpyridine ligand. Structure of the [Ru II(trpy)(L-Cl)] complex has been determined by X-ray crystallography. All the Ru(II) complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru II(trpy)(L-R)] complexes shows a Ru(II)–Ru(III) oxidation within 0.16–0.33 V versus SCE. An oxidation of the coordinated amide ligand is also observed within 0.94–1.33 V versus SCE and a reduction of coordinated terpyridine ligand within −1.10 to −1.15 V versus SCE. Constant potential coulometric oxidation of the [Ru II(trpy)(L-R)] complexes produces the corresponding [Ru III(trpy)(L-R)] + complexes, which have been isolated as the perchlorate salts. Structure of the [Ru III(trpy)(L-CH 3)]ClO 4 complex has been determined by X-ray crystallography. All the Ru(III) complexes are one-electron paramagnetic, and show anisotropic ESR spectra at 77 K and intense LMCT transitions in the visible region. A weak ligand-field band has also been shown by all the [Ru III(trpy)(L-R)]ClO 4 complexes near 1600 nm.

Synthesis, structure and electrochemical properties of a family of organoruthenium complexes

Reaction of N-(2′-hydroxyphenyl)benzaldimines (abbreviated in general as H 2L-R, where R stands for the para-substituent in the benzaldehyde fragment and H stands for the dissociable hydrogen atoms) with [Ru(PPh 3) 2(CO) 2Cl 2] affords a family of organoruthenium complexes of the type [Ru(PPh 3) 2(CO)(L-R)] where the N-(2′-hydroxyphenyl)benzaldimine ligand is coordinated to the metal center as tridentate C,N,O-donor. Structure of a representative complex has been determined by X-ray crystallography. All the [Ru(PPh 3) 2(CO)(L-R)] complexes are diamagnetic, and show characteristic 1H NMR signals and moderately intense MLCT transitions in the visible region. Cyclic voltammetry of the [Ru(PPh 3) 2(CO)(L-R)] complexes shows a reversible Ru(II)–Ru(III) oxidation within 0.38–0.68 V versus SCE, followed by an irreversible oxidation of the coordinated benzaldimine ligand within 1.09–1.27 V versus SCE. An irreversible reduction of the coordinated benzaldimine ligand is also observed near −1.1 V versus SCE. Potential of the Ru(II)–Ru(III) oxidation is observed to be sensitive to the nature of para-substituent R.

N‐(Aryl)picolinamide Complexes of Ruthenium: Usual Coordination and Strategic Cyclometalation

AbstractReaction of five N‐(4‐R‐phenyl)picolinamides (R = OCH3, CH3, H, Cl, and NO2) with [Ru(PPh3)2(CO)2Cl2] in refluxing 2‐methoxyethanol in the presence of a base (NEt3) affords two geometrical isomers of a group of complexes (1‐R and 2‐R), each of which contains an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphanes, a carbonyl, and a hydride. Similar reaction of N‐(naphthyl)picolinamide with [Ru(PPh3)2(CO)2Cl2] affords an organometallic complex, 3, in which the amide ligand is coordinated to the metal center, by C–H activation of the naphthyl ring at the 8‐position, as a dianionic tridentate N,N,C donor along with two triphenylphosphanes and one carbonyl. Structures of the 1‐OCH3, 2‐CH3, and 3 complexes have been determined by X‐ray crystallography. In all the complexes the two triphenylphosphanes are trans. In the 1‐R complexes the hydride is trans to the pyridine nitrogen and in the 2‐R complexes it is trans to the amide‐nitrogen. All the complexes are diamagnetic, and show characteristic 1H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a RuII–RuIII oxidation within 0.71–0.93 V versus SCE. An oxidation and a reduction of the coordinated amide ligand are also observed within 1.29–1.69 V versus SCE and –1.02 to –1.21 V versus SCE respectively.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis, structure and electrochemical properties of some thiosemicarbazone complexes of iridium

Reaction of thiosemicarbazones of salicylaldehyde and 2-hydroxyacetophenone (H2L1 and H2L2) with [Ir(PPh3)3Cl] affords complexes of type [Ir(PPh3)2(L)(H)] (L = L1 or L2) in ethanol. A similar reaction carried out in toluene affords the [Ir(PPh3)2(L)(H)] complexes along with complexes of type [Ir(PPh3)2(L)Cl], where a chloride is coordinated to iridium instead of the hydride. The structure of the [Ir(PPh3)2(L2)(H)] and [Ir(PPh3)2(L2)Cl] complexes has been determined by X-ray crystallography. Crystal data for [Ir(PPh3)2(L2)(H)]: space group, P21/c; crystal system, monoclinic; a=12.110(2) A, b=17.983(4) A, c=18.437(4) A, β=103.42(3)°, Z=4; R1=0.0591, wR2=0.1107. Crystal data for [Ir(PPh3)2(L2)Cl]: space group, P21/c; crystal system, monoclinic; a=17.9374(11) A, b=19.2570(10) A, c=24.9135(16) A, β=108.145(5)°, Z=4; R1=0.0463, wR2=0.0901. In all the complexes the thiosemicarbazones are coordinated to the metal center as dianionic tridentate O, N, S-donors and the two triphenylphosphines are trans. The complexes are diamagnetic (low-spin d 6, S=0) and show intense MLCT transitions in the visible region. Cyclic voltammetry on all the [Ir(PPh3)2(L)(H)] and [Ir(PPh3)2(L)Cl] complexes shows a quasi-reversible Ir(III)–Ir(IV) oxidation within 0.55–0.78 V vs. SCE followed by an irreversible oxidation of the thiosemicarbazone within 0.91–1.27 V vs. SCE. An irreversible reduction of the thiosemicarbazone is also observed within −1.10 to −1.23 V vs. SCE.

Interaction of 2-(arylazo)phenols with rhodium. Usual coordination vs. C–H and C–C activation

Reaction of 2-(2′-hydroxyphenylazo)phenol with [Rh(PPh 3) 3Cl] in refluxing benzene in presence of triethylamine afforded a red complex in which the ligand is coordinated to rhodium as a tridentate O,N,O-donor. However, similar reaction of [Rh(PPh 3) 3Cl] with 2-(2′-carboxyphenylazo)-4-methylphenol yielded two complexes, viz. a blue one and a green one. In both the complexes the ligand is coordinated as C,N,O-donor. However, in the blue complex orthometallation takes place from the ortho-carbon atom, which bears –COOH group via decarboxylation and in green one orthometallation occurs from the other ortho-carbon. Structures of all the three complexes were determined by X-ray crystallography. In all the three complexes rhodium is sharing the equatorial plane with the tridentate ligand and a chloride, and the two triphenylphosphines are axially disposed. All of the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on these complexes shows a Rh(III)–Rh(IV) oxidation on the positive side of SCE and a reduction of the coordinated azophenolate ligand on the negative side.