Zinc(II) complexes based on sterically hindered hydrotris(pyrazolyl)borate ligands: Synthesis, reactivity and solid-state structures

The coordination chemistry and reactivity of zinc(II) complexes supported by monoanionic hydrotris(pyrazolyl)borate ligands substituted by 3,3,3-mesityl groups (Tp^Ms) and 3,3,5-mesityl groups (Tp^Ms∗) have been investigated. Salt metathesis of ZnCl₂, ZnEt₂, and Zn(OAc)₂ with Tl[Tp^Ms] or Tl[Tp^Ms∗] cleanly afforded the corresponding compounds Tp^MsZnCl (1), Tp^MsZnEt (2), Tp^Ms∗ZnEt (3), and Tp^MsZnOAc (5). Compound 3 slowly disproportionates in benzene solution to afford the bis(ligand) complex (κ²-Tp^Ms∗)₂Zn (4). Acetate complex 5 as well as Tp^MsZnOCOPh (6) and [Tp^Ms∗ZnOAc]₂ (7) were alternatively prepared by acidolysis of the parent ethyl complexes (2, 3) with the corresponding carboxylic acid. No reaction was observed between 2 and 3 and alcohols (ROH; R = Et, iPr, Bn), while salt metathesis reactions of ZnEt(OR) with Tl[Tp^Ms] led to 2 instead of the desired zinc-alkoxide complex. Compounds 1-7 were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy, as well as by X-ray diffraction studies for 1, 2, 4, 5 and 7. The former compounds adopt a monomeric structure in the solid state while [Tp^Ms∗ZnOAc]₂ (7) exists as an anti-syn bridged acetate dimer. Complex 4 is four-coordinated, featuring a rare bidentate coordination mode of the Tp^Ms∗ ligands. The results are rationalized in terms of the variable steric constraint around the zinc atom provided by the Tp^Ms and Tp^Ms∗ ligands.     

Diamidato-bis(diphenylphosphino) platinum(II) complexes: Synthesis, characterization, and reactivity in the presence of acid

The reaction of Pt(COD)Cl₂, where COD is 1,5-cyclooctadiene, with one equivalent of a diamidato-bis(phosphino) Trost ligand ((R,R)-2 = N,N′-bis(2-diphenylphosphino-1-benzoyl)-(1R,2R)-1,2-diaminocyclohexane, (R,R)-3 = N,N′-bis(2-diphenylphosphino-1-naphthoyl)-(1R,2R)-1,2-diaminocyclohexane, or (±)-4 = N,N′-bis(2-diphenylphosphino-1-benzoyl)-1,2-bis(aminobenzene)) in the presence of base afforded square planar diamidato-bis(phosphino) platinum(II) complexes (R,R)-2-Pt, (R,R)-3-Pt, (±)-4-Pt. Characterization of all complexes included the solution and solid state structure determination of each complex based on multinuclear NMR and X-ray analyses, respectively. Stability of the complexes in acid was examined on addition of HCl to (R,R)-2-Pt in chloroform and compared to the unreactive nature of the similar diamidato-bis(phosphino) complex 1-Pt (1 = 1,2-bis-N-[2′-(diphenylphosphino)benzoyl]diamino-benzene) in the presence of acid. Protonation of the bound amidato nitrogen atoms of (R,R)-2-Pt was observed along with decoordination of the nitrogen atoms from the platinum(II) center producing (R,R)-2-PtCl₂ in quantitative yield by NMR analysis. Confirmation of the product was made on comparison of the NMR spectra to that of authentic (R,R)-2-PtCl₂ prepared on reaction of Pt(COD)Cl₂ with (R,R)-2 in CH₂Cl₂ and characterized by single-crystal X-ray diffraction analysis and NMR spectroscopy. Results add to the knowledge of rich coordination chemistry of bis(phosphino) ligands with late transition metals, metal-amidato chemistry, and has implications in catalysis.     

Synthesis and characterization of Cu(II) and Ni(II) complexes of a tripodal ligand containing imidazoles

Two complexes of the formula [MH₃L](ClO₄)₂ [M = Cu(II) (1), Ni(II) (2)] have been prepared by the reaction of M(ClO₄)₂ · 6H₂O with the ligand (H₃L) formed by the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 4-methyl-5-imidazolecarboxaldehyde and structurally and magnetically characterized. The structures of 1 and 2 are isomorphous with each other and with the iron(II) complex of H₃L which has been reported previously. The ligand, while potentially heptadentate, forms six coordinate complexes with both metal centers forming three M-N_imine and three M-N_imidazole bonds. The tren central N atom is at a nonbonded distance from M of 3.261 Å for 1 and 3.329 Å for 2. The neutral complex CuHL 3 was prepared by reaction of H₃L with Cu(OCH₃)₂ and the ionic complex Na[NiL] 4 was prepared by deprotonation of 2 with aqueous sodium hydroxide. Magnetic measurements of 1-3 are consistent with the spin-only values expected for S = 1/2 (d⁹, Cu(II)) and S = 1 (d⁸, Ni (II)) systems.     

Diboron complexes of binucleating bis(amidinate) ligands

The synthesis and X-ray crystal structures of the following bis(amidinate)-substituted boron halides are reported: 1,3-C₆H₄[C{N(SiMe₃)}₂BCl₂]₂ (3), 1,4-C₆H₄[C{N(SiMe₃)}₂BCl₂]₂ (4), 1,4-C₆H₄[C{N(SiMe₃)}₂B(Ph)Cl]₂ (5), 1,4-C₆H₄[C{NCy}₂BCl₂]₂ (6), and 1,4-C₆H₄[C{NCy}₂B(Ph)Cl]₂ (7). Compounds 3-5 were prepared by trimethylsilyl chloride elimination, while 6 and 7 were prepared via salt metathesis reactions of the appropriate dilithium bis(amidinates) with BCl₃ or PhBCl₂. The molecular structures of complexes 3, 5, and 6 were determined by single-crystal X-ray diffraction, along with that of the free bis(amidine) 1a.     

Synthesis of substituted indazoles and their corresponding tris(indazolyl)borate tripodal ligands as key building blocks for molecular motors

The synthesis of functionalized indazoles at the 6-position of the indazole ring is developed. Such precursors give access to tris(indazolyl)borate ligands derived from the scorpionate ligands of Trofimenko. These tripodal ligands are truly bifunctional since they can coordinate a metal via the nitrogen centers of the indazolyl rings and be anchored on surfaces on the opposite side through their 6-functionalizations. Three pendant ester or thioether groups were selected to anchor metallic complexes onto, respectively, an oxide or a metallic surface in view of near-field microscopy experiments. These building blocks have been subsequently used as stator in a family of single molecular rotary motors. The architecture of such compounds is centered around half-sandwich complexes of the family of pentaphenylcyclopentadienyl hydrotris(indazolyl)borate ruthenium (II).     

Carboxyester hydrolysis promoted by Cu(II) complexes of pyridyl-amine carboxylate-pendant ligands

Multidentate ligands containing tripodal pyridyl-amine moieties tethered to a carboxylate group by alkyl linkers of varying lengths were synthesized to obtain a series of water-soluble ligands to elucidate the effects of the differing coordination environments on the properties of the resulting metal complexes. These new, water-soluble ligands, [bis-(2-pyridin-2-yl-ethyl)-amino]-acetic acid (L¹), 3-[bis-(2-pyridin-2-yl-ethyl)-amino]-propionic acid (L²), 4-[bis-(2-pyridin-2-yl-ethyl)-amino]-butyric acid (L³), and 6-[bis-(2-pyridin-2-yl-ethyl)-amino]-hexanoic acid (L⁴), were treated with copper(II) perchlorate hexahydrate to yield the corresponding Cu(II) complexes, which have all been characterized by X-ray crystallography. L¹ binds Cu(II) to form the tetrameric complex {[Cu(μ-¹)][ClO₄] · 4H₂O}₄ (1) in the solid state, whereas the Cu(II) complexes of ligands L²-L⁴ form long-chain one-dimensional polymeric complexes {[Cu(μ-L²)][ClO₄] · H₂O}_n (2), {[Cu(μ-L³)][ClO₄] · H₂O}_n (3), and {[Cu(μ-L²)][ClO₄]  · H₂O}_n (4), respectively, in the solid state. Complexes 1-4 dissolved in 10% (v/v) CH₃CN aqueous solution were tested for their ability to promote the hydrolysis of the activated ester compound 4-nitrophenylacetate (NA), with 3 being the most active complex and 1 being the least active, possibly due to differences in the ability of the carboxylate moiety to act as either a general base or a nucleophile in the hydrolysis of NA as dictated by the tether length. The pK_a values of the copper-bound aquo ligands in solution were measured by spectrophotometric titration.     

Tripodal oxygen and tripodal nitrogen ligands in hydroformylation reactions: formation of novel rhodium(III) carbonyl bis(acyl) complexes

The rhodium(I) complexes TpmsRh(CO)₂ (1) and TpmsRh(cod) (2) of the tripodal nitrogen ligand tris(pyrazolyl)methanesulfonate, Tpms⁻=[(pz)₃CSO₃]⁻, catalyze the hydroformylation of 1-hexene. Addition of phosphine has a negative effect on the activity. The hydroformylation activity reaches a maximum at about 60 °C. At temperatures above 80 °C hydrogenation becomes an important secondary reaction. When the catalysis is performed at 60 °C in acetone with 1 or 2 as catalyst precursor all of the rhodium is recovered in the form of the rhodium(III) bis(acyl) complex TpmsRh(CO)(COC₆H₁₃)₂ (9). A similar behaviour is observed with rhodium(I) complexes bearing the tripodal oxygen ligand L_OMe⁻=[(cyclopentadienyl)tris(dimethylphosphito-P) cobalt O,O^′,O″]⁻. In this case all of the rhodium is transformed into L_OMeRh(CO)(COC₆H₁₃)₂ (10). These hitherto unknown bis(acyl) rhodium(III) complexes show the same catalytic activity as the rhodium(I) starting compounds.     

Synthesis and structural characterization of mixed-ligand silver(I) complexes containing diphosphazanes and bidentate N-donor ligands

The reactions of a self-assembled silver(I) coordination polymer, [Ag₂{μ-PrⁱN(PPh₂)₂}(μ-NO₃)₂]_n (1) with various bidentate N-donor ligands such as DABCO, 2,2′-bipyridyl and 1,10-phenanthroline yield 1-D helices or π-π stacked polymers, depending on the chelate vector of the N-donor ligand. The molecular structures of the resultant complexes, [Ag₂{μ-PrⁱN(PPh₂)₂}(DABCO)(NO₃)₂]_n (2), [Ag₂{μ-PrⁱN(PPh₂)₂}(2,2′-bipy)₂(NO₃)₂] (3) and [Ag₂{μ-PrⁱN(PPh₂)₂}(1,10-phen)₂](NO₃)₂ (4) have been confirmed by single-crystal X-ray diffraction. Complex 2 exists as an infinite helical polymer because of the exo-bidentate nature of DABCO. Complex 3 assumes a 2D grid motif as a result of intermolecular π-π stacking among adjacent bipyridine moieties. The phenanthroline complex 4 exhibits strong inter- and intramolecular π-π stacking interactions.     

Synthesis and characterization of sterically hindered tris(pyrazolyl)borate Ni complexes

Addition of KTp^Ph2 to a solution of NiX₂ (X = Cl, Br, NO₃, OAc and acac) or NiBr(NO)(PPh₃)₂ in THF yields the structurally characterized series [NiCl(Hpz^Ph2)Tp^Ph2] (1) and [NiXTp^Ph2] (X = Br 2, NO 3, NO₃4, OAc 5 and acac 6) including the first example of a tris(pyrazolyl)borate nickel nitrosyl complex. IR spectroscopy confirms that all the Tp^Ph2 ligands are κ³ coordinated and that the NO ligand in 3 is linearly bound. Electronic spectra are consistent with four- or five-coordinate species in solution. NMR spectroscopic studies indicate that the complexes are paramagnetic, with the exception of 3. This is confirmed by magnetic susceptibility studies, which suggest that complexes 1, 2 and 4-6 are paramagnetic with two unpaired electrons. X-ray crystallographic studies of 5 reveal a distorted trigonal bipyramidal nickel centre with a symmetrically coordinated acetate ligand.     

Monomeric, oligomeric, and polymeric copper(II) complexes of calix[4]arene-derived ligands

Deprotonation of the p-tert-butylcalix[4]arene disubstituted at alternate phenolic positions with picolyl groups 2 was achieved with alkali metal hydrides LiH, NaH, and KH. The dianionic calixarene derivatives were subjected to complete substitution at the phenolic rim with allyl bromide, providing the tetraalkylated derivatives in cone 3a and partial-cone conformations 3b; both compounds were crystallographically characterized. Compound 2, as well as 3a and 3b were tested as ligands towards CuCl₂, affording Cu²⁺ complexes in the first two cases. Polymeric [2·CuCl₂] was obtained from 2 and CuCl₂ in MeOH/CH₂Cl₂ solutions, and consists of chains of the ditopic calixarene acting as an N-donor towards Cu²⁺ ions outside the macrocyclic cavity. Employment of EtOH/CH₂Cl₂ mixtures results in the tricopper complex [(2)₂Cu₃Cl₆(EtOH)₂]. In contrast, reactions of ligand 3a with CuCl₂ afforded monomeric [3a·CuCl₂], while no Cu²⁺ complexes could be obtained when 3b was employed. The presence of intramolecular hydrogen bonds in 2 appears to control the formation of oligomeric or polymeric copper complexes, while the lack of such hydrogen bonds allows the proper alignment of N-donors to coordinate Cu²⁺ directly above the macrocyclic cavity.     

Metal complexes of new scorpionate ligands: 2,2′-Bis(pyrazolyl)ethylamine and its derivatives

The new bis(pyrazolyl)amine ligand NH₂CH₂CH(pz)₂ (1) was prepared from the reaction of N-[2,2-bis(pyrazolyl)ethyl]-1,8-naphthalimide with hydrazine monohydrate. A substituted derivative, C₆H₅CH₂NHCH₂CH(pz)₂ (2), was prepared by the reaction of 1 with benzaldehyde followed by reduction with NaBH₄. Ligand 1 was also converted by two methods to the new bitopic, para-linked bis(pyrazolyl)amine ligand p-C₆H₄(CH₂NHCH₂CH(pz)₂)₂, (3). The reactions of the ligands 1-3 with [Cu(PPh₃)₂]NO₃ yields {(PPh₃)Cu[(pz)₂CHCH₂NH₂]}NO₃, {(PPh₃)Cu[(pz)₂CHCH₂NHCH₂C₆H₅]}NO₃ and {[(PPh₃)Cu]₂[p-((pz)₂CHCH₂NHCH₂)₂C₆H₄]}(NO₃)₂·solvate, respectively. Complex {(N₃)₂Cu[(pz)₂CHCH₂NHCH₂C₆H₅]} was obtained from a methanol solution of 2, copper(II) acetate monohydrate and sodium azide. The complex {Cd[(pz)₂CHCH₂NHCH₂C₆H₅]₂}(PF₆)₂·3C₃H₆O was synthesized by reaction of the protonated form of ligand 2, [(pz)₂CHCH₂NH₂CH₂C₆H₅]PF₆, with Cd(acac)₂. In all of the structures the ligands are tridentate, bonding to the metal through the lone pair on the amine group as well as through the pyrazolyl rings - they act as true scorpionates. The solid state structures all have extensive non-covalent interactions, with the N-H functional groups of the amines participating in both N-H?π and N-H?O or N-H?N hydrogen bonding interactions.     

Examining the impact of steric and electronic variation in N2S scorpionate ligands on the properties of zinc(II) and cadmium(II) complexes

A series of LZn(II)Br (1-4) and LCd(II)Cl complexes (9-11) has been prepared by the reaction of metal halide precursors with the lithium salts of the N₂S⁻ ligands bis(3,5-diisopropylpyrazol-1-yl)dithioacetate (L¹), bis(3,5-di-tert-butylpyrazol-1-yl)dithioacetate (L²), N-phenyl-2,2-bis(3,5-diisopropylpyrazol-1-yl)thioacetamide (L³) and N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide (L⁴). Characterization by X-ray crystallography and DOSY NMR studies indicate that LZnBr complexes 1-4 are mononuclear both in the solid state and in solution. Steric differences between ligands L¹-L⁴ result in distortion from an ideal tetrahedral geometry for each complex, with the degree of distortion depending on the bulk of the ligand substituents. In contrast, the related complex L³CdCl was shown by X-ray crystallography to dimerize in the solid state to form the chloride-bridged five-coordinate complex [L³CdCl]₂ (10). Despite 10 having a dinuclear structure in the solid state, DOSY NMR studies indicate 9-11 exist as mononuclear LCdCl species in solution. In addition, Zn(II) cyanide complexes of the form LZnCN [L = L¹ (5), L³ (7), L⁴ (8)] have been characterized and the X-ray structure of 8 determined. Moreover, density functional theory calculations have been conducted which yield important insight into the bonding in 1-4 and 5-8 and the electronic impact of ligands L¹-L⁴ on the zinc(II) ion and its ability to function as a Lewis acid catalyst.     

Synthesis, structures and spectroscopic properties of palladium(II) complexes with tridentate piperidyl-containing pincer ligands

A new series of square planar palladium(II) complexes with pincer ligands, pip₂NCN⁻ (pip₂NCNH = 1,3-bis(piperidylmethyl)benzene) and pip₂NNN (2,6-bis(piperidylmethyl)pyridine), has been prepared: Pd(pip₂NCN)X (X = Cl, Br, I), [Pd(pip₂NCN)(L)](BF₄) (L = pyridine, 4-phenylpyridine), and [Pd(pip₂NNN)Cl]Cl. The X-ray crystal structures of Pd(pip₂NCN)Br, [Pd(pip₂NCN)(L)]BF₄, and [Pd(pip₂NNN)Cl]Cl confirm the tridentate coordination geometries of the pincer ligands. For the pip₂NCN⁻ complexes, each piperidyl ring adopts a chair conformation with the metal center at an equatorial position on the N(piperidyl) atom. However, one of the piperidyl groups of Pd(pip₂NNN)Cl⁺ adopts a previously unobserved coordination geometry, effectively placing the metal center at an axial position on the N(piperidyl) atom. ¹H NMR and UV-Vis absorption measurements provide additional insight into the electronic structures of these complexes. The ¹H NMR spectra of Pd(pip₂NCN)X (X = Cl, Br, I) are consistent with deshielding of the pip₂NCN⁻ ligand resonances along the Cl < Br < I series, in opposition to the relative halogen electronegativities. It is suggested that this trend is consistent with decreasing filled/filled repulsions between the dπ orbitals of the metal center and the lone pair orbitals of the halide ligands along this series. Electronic absorption spectra support the notion that ligand-to-metal charge-transfer states are stabilized in these palladium(II) complexes relative to their platinum(II) analogues.     

Synthesis, characterization and cytotoxic activity of gallium(III) complexes anchored by tridentate pyrazole-based ligands

Reactions of GaCl₃ with pyrazole-containing ligands of the pyrazole-imine-phenol (HL¹-HL³) or pyrazole-amine-phenol (HL⁴-HL⁶) types led to the synthesis of well-defined [GaL₂]⁺ homoleptic complexes (1-6). Complexes 1-6 were characterized by elemental analysis, ESI-MS (electrospray ionization-mass spectrometry), IR and NMR spectroscopies, and in the case of Complex 1 also by X-ray diffraction analysis. In complexes 1-3, the pyrazole-imine-phenolate ligands act as monoanionic chelators that coordinate to the metal in a meridional fashion, while 4-6 contain monoanionic and facially coordinated pyrazole-amine-phenolate ligands. Complexes 1-3 have a greater stability in solution compared to 4-6, which have shown a more pronounced tendency to release the respective ancillary ligands. The cytotoxicity of 1-6 and of the respective ligands (HL¹-HL⁶) was evaluated against human prostate cancer cells PC-3 and human breast cancer cells MCF-7. The substituents of the phenolate rings strongly influenced the cytotoxicity of the compounds. Complexes 3 and 6 that contain chloride substituents at the phenolate rings have shown the highest cytotoxicity, including in the cisplatin-resistant PC-3 cell line. The cytotoxic profile of 3 and 6 is very similar to the one displayed by the respective anchor ligands, respectively HL¹ and HL⁶. The cytotoxic activity of 3 and 6 is slightly increased by the presence of transferrin, and both complexes provoke cell death mainly by induction of apoptotic pathways.     

Synthesis, characterization and properties of iron(II) complexes with a series of tripodal ligands based on the parent ligand tris(2-pyridylmethyl)amine

Iron(II) complexes of the type [Fe(L)(NCS)₂] with the tripodal ligand apme (apme = N¹-(2-aminoethyl)-N¹-(2-pyridyl-methyl)-1,2-ethanediamine) as well as with its derivatives were prepared and structurally characterized. The bond distances thus obtained showed that all complexes investigated were high-spin at the respective temperature. Furthermore [Fe(Me₄apme)(NCS)₂] was analyzed using Mößbauer spectroscopy that showed that this complex remains in its high-spin state over the entire temperature range.     

Complexation of silver(I) cation by a series of heterosubstituted dipyridyl ligands

The synthesis of a series of dipyridyl ligands based on 1,2-bis(2′-pyridylethynyl)benzene and their complexation of silver cation is described. NMR binding studies confirm that the incorporation of thioether appendages results in an increased binding constant while ether appendages result in similar or lower binding constants as compared to the unsubstituted ligand. X-ray crystallographic analysis confirms that steric hinderance is critical.     

Synthesis and characterization of the 1:1 adducts of copper(I) halides with bidentate N,N′-bis(benzophenone)-1,2-diiminoethane Schiff base: Crystal structures of [Cu(bz2en)2][CuX2] (X = Br, I) complexes

1:1 adducts of N,N′-bis(benzophenone)-1,2-diiminoethane (bz₂en) with copper(I) chloride, bromide and iodide, [Cu(bz₂en)₂][CuX₂] (X = Cl, Br, and I), have been synthesized and the structures of the solid bromide and iodide adducts were determined by X-ray crystallography from single-crystal data. The solid-state structure reveals ionic complexes containing a cation of copper(I) ion coordinated to four nitrogen atoms of two bz₂en molecules (distorted tetrahedron) and a linear dibromocuprate(I) and a di-μ-iodo-diiododicuprate(I) anion for the bromo and iodo adducts, respectively. The bromo adduct structure contains CH?Br intermolecular hydrogen bonds. The complexes are very stable towards atmospheric oxygen in the solid state. The spectral properties of the above complexes are also discussed.     

Steric and counterion effects on the structure of dipicolylamine nickel complexes

Four nickel complexes each containing an R-2,2′-dipicolylamine ligand species (RDPA; R = benzyl, isopropyl, or tert-butyl) were synthesized and structurally characterized. In the absence of an interfering coordinating counterion, BzDPA and iPrDPA form 1:2 nickel:ligand complexes, with two facial ligands completing an pseudooctahedral nickel(II) coordination environment. In contrast, the sterically hindered tBuDPA ligand instead forms 1:1 metal:ligand complexes, even in the absence of associating counterions. Two novel tBuDPA nickel complexes with different counterions are described: nickel(II) chloride gives rise to an unusual 2Ni-3Cl dimer complex, while nickel(II) nitrate affords a 1:1 nickel:ligand complex which crystallizes with both fac and mer conformations in the same unit cell.     

N-Alkylation of tripodal iron(III) imidazolate complexes: Reactivity and structure

The N-alkylation of iron(III) complexes of the tripodal imidazolate complexes derived from the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 2-imidazolecarboxaldehyde (2ImH), 4-imidazolecarboxaldehyde (4ImH) or 4-methyl-5-imidazolecarboxaldehyde (5-Me4ImH) was investigated. While each complex possesses three nucleophilic imidazolate nitrogen atoms, only the complex derived from 2-imidazolecarboxaldehyde, Fetren(2Im)₃, was completely alkylated under the ambient conditions used in this work. Using methyl iodide as the alkylating agent, a correlation between spin state of the product and degree of methylation was observed. Low spin iron complexes were more nucleophilic than high spin systems. The structure reactivity relationship was exploited in the reaction of Fetren(2Im)₃ with methyl iodide and allyl iodide to give [Fetren(N-Me2Im)₃]²⁺ and [Fetren(N-allyl2Im)₃]²⁺. The products are iron(II) due to reduction of the iron(III) by iodide ion which builds up in the reaction mixture as the alkylation reaction proceeds. These complexes were characterized by a number of methods including EA, IR, ES-MS, Mössbauer spectroscopy, magnetic susceptibility and X-ray diffraction.     

Hydrido iridium(III) complexes of azoaromatic ligands. Isolation, structure and studies of their physicochemical properties

Reactions of 2-(arylazo)pyridine (L^a-c) with [IrCl₃(PPh₃)₂] in two different solvents, viz. ethanol and toluene are reported. In refluxing toluene two new isomeric (mer and fac geometries) iridium complexes, having molecular formula [IrCl₃(PPh₃)(L)] (1 and 2) have been isolated. The reaction in refluxing ethanol yielded two new hydrido complexes of molecular formula [IrHCl₂(PPh₃)(L)] (3) and [IrHCl(PPh₃)₂(L)]Cl (4) along with the compound 2. All the complexes have been thoroughly characterized by NMR, UV-Vis spectroscopy, cyclic voltammetry and X-ray crystallographic analysis. The ¹H NMR spectra of the hydrido complexes 3 and 4 showed a doublet and a triplet signals at δ −20.43 and −14.82 respectively due to coupling with magnetically equivalent phosphorous nuclei. Strong trans influence of the π-acceptor ligands guided the X-ray structural parameters; bonds trans to the these ligands are unusually long. Similar elongation effect was also noted for the bonds trans to the coordinated hydrido ligand. UV-Vis-NIR spectrum consisted of multiple transitions in the UV and visible regions. Cyclic voltammetry of each of these complexes has exhibited a reductive response between −0.25 and −0.55 V, which has been assigned to azo-ligand reduction. The compound 3, however, showed a quasireversible oxidative wave near 1.45 V, due to Ir^III/Ir^IV couple.