Tridentate Nitrogen Donor Ligand Research Articles

Trivalent Actinide Ions Showing Tenfold Coordination in Solution.

Trivalent actinides generally exhibit ninefold coordination in solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP), a tridentate nitrogen donor ligand, is known to form ninefold coordinated 1:3 complexes, [An(nPr-BTP)3]3+ (An = U, Pu, Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination in solution, [Cm(nPr-BTP)3(NO3)]2+. This species was identified using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Adding nitrate to a solution of the [Cm(nPr-BTP)3]3+ complex in 2-propanol shifts the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic shift is due to a higher coordination number of the Cm(III) ion in solution, in agreement with the formation of the [Cm(nPr-BTP)3(NO3)]2+ complex. The formation of this complex exhibits slow kinetics in the range of 5 to 12 days, depending on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)3(X)]2+ was not observed for anions other than nitrate (X- = NO2-, CN-, or OTf-). The formation of the [Cm(nPr-BTP)3(NO3)]2+ complex was studied as a function of NO3- and nPr-BTP concentrations, and slope analyses confirmed the addition of one nitrate anion to the [Cm(nPr-BTP)3]3+ complex. Experiments with varied nPr-BTP concentration show that [Cm(nPr-BTP)3(NO3)]2+ only forms at nPr-BTP concentrations below 10-4 mol/L whereas for concentrations greater than 10-4 mol/L the formation of the tenfold species is suppressed and [Cm(nPr-BTP)3]3+ is the only species present. The presence of the tenfold coordinated complex is supported by VSBS, XPS, and DFT calculations. The vibronic side band of the [Cm(nPr-BTP)3(NO3)]2+ complex exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)3]3+ complex. Moreover, XPS on [M(nPr-BTP)3(NO3)](NO3)2 (M = Eu, Am) yields signals from both non-coordinated and coordinated nitrate. Finally, DFT calculations reveal that the energetically most favored structure is obtained if the nitrate is positioned on the C2 axis of the D3 symmetrical [Cm(nPr-BTP)3]3+ complex with a bond distance of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides sound evidence for a unique tenfold coordinated Cm(III) complex in solution-a novelty in An(III) solution chemistry.

NiII formate complexes with bi- and tridentate nitrogen-donor ligands: synthesis, characterization, and magnetic and thermal properties.

The synthesis of four NiII formate complexes of the type [Ni(N∩N)n][O2CH]2 (2, adduct with 3/4 EtOH, N∩N = en, e[combining low line]thylen[combining low line]ediamine, n = 3; 4, N∩N = dien, N,N',N''-d[combining low line]i[combining low line]e[combining low line]thylen[combining low line]etriamine, n = 2), [Ni2(O2CH)4(H2O)(tmeda)2] (3, tmeda = N,N,N',N'-t[combining low line]etram[combining low line]ethyle[combining low line]thylened[combining low line]ia[combining low line]mine) and [{Ni(O2CH)2(pmdta)}2·H2O] (5, pmdta = N,N',N',N'',N''-p[combining low line]entam[combining low line]ethyld[combining low line]iethylenet[combining low line]ria[combining low line]mine) by a reaction of [{Ni(O2CH)2}·2H2O] (1) with the respective N-donor bases is reported. The structures of 2-5 in the solid state were determined by single X-ray structure analysis, revealing a discrete dinuclear structure of 3 and the formation of polymeric networks in the case of 2, 4 and 5 due to intermolecular hydrogen bonding. SQUID and ESR measurements of 3 evidenced a weak antiferromagnetic coupling between the NiII ions and an easy plane magnetic anisotropy. Accompanying quantum chemical studies of the magnetic properties and IR characteristics of 3 were performed to strengthen the conclusions drawn from experimentally obtained data. The thermal decomposition temperatures of 2-5 were determined by TG (thermogravimetry) and obtained residues were analyzed by PXRD (powder X-ray diffraction) measurements. The decomposition processes were completed at 207 (3), 215 (5), 250 (2) and 273 °C (4) and are shown to result in the formation of pure metallic nickel.

Synthesis and Characterization of Copper Complexes with a Tridentate Nitrogen-Donor Ligand: An Integrated Research Experiment for Undergraduate Students

An integrated laboratory experiment applying concepts and techniques developed in organic chemistry, inorganic chemistry, and instrumental analysis is presented for use by students interested in undergraduate research. The experiment incorporates some advanced laboratory practices such as multistep organic synthesis and purification, detailed reaction mechanism, air-sensitive metalation techniques, variable-temperature NMR experiments, cyclic voltammetry, and single crystal X-ray crystallography. Subsequently, students are presented with an experience that mirrors fundamental inorganic chemistry research practices found in laboratories throughout the world.

Toward Equatorial Planarity about Uranyl: Synthesis and Structure of Tridentate Nitrogen-Donor {UO2}2+ Complexes

The reaction of UO2Cl2·3THF with the tridentate nitrogen donor ligand 2,6-bis(2-benzimidazolyl)pyridine (H2BBP) in pyridine leads to the formation of three different complexes: [(UO2)(H2BBP)Cl2] (1), [(UO)2(HBBP)(Py)Cl] (2), and [(UO2)(BBP)(Py)2] (3) after successive deprotonation of H2BBP with a strong base. Crystallographic determination of 1-3 reveals that increased charge through ligand deprotonation and displacement of chloride leads to equatorial planarity about uranyl as well as a more compact overall coordination geometry. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra of 1-3 at the U-4d edges have been recorded using a soft X-ray Scanning Transmission X-ray Microscope (STXM) and reveal the uranium 4d5/2 and 4d3/2 transitions at energies associated with uranium in the hexavalent oxidation state. First-principles Density Functional Theory (DFT) electronic structure calculations for the complexes have been performed to determine and validate the coordination characteristics, which correspond well to the experimental results.

Synthesis, characterization and anti-proliferative activity of Cd(II) complexes with NNN type pyrazole-based ligand and pseudohalide ligands as coligand

Cd(II) complexes of tridentate nitrogen donor ligand, 2,6-bis(3,4,5-trimethylpyrazolyl)pyridine (btmpp), Cd(btmpp)X2 (X:Cl, ONO or N(CN)2) have been synthesized and characterized by elemental and spectral (FT-IR, 1H NMR, 13C NMR, UV–Vis) analyses, differential thermal analysis and single crystal X-ray diffraction studies. The molecular structure of reported complex 1, revealed distorted square-pyramidal geometry around Cadmium. Complexes 1–3 and corresponding ligand were tested for cytotoxic activity against the human carcinoma cell lines HEP3B (hepatocellular carcinoma), PC3 (prostate adenocarcinoma), MCF7 (breast adenocarcinoma) and Saos2 (osteosarcoma). The results show that, complexes are more cytotoxic than the free ligand and complex 2 is the most cytotoxic complex for PC3.

Fe‐Catalyzed Olefin Epoxidation with Tridentate Non‐Heme Ligands and Hydrogen Peroxide as the Oxidant

Do it with a tridentate ligand: The use of readily available tridentate nitrogen donor ligands gives highly active and selective Fe catalysts in the epoxidation of a series of alkenes.

Quantitative Assay for the Direct Comparison of Platinum Catalysts in Benzene H/D Exchange

This paper describes a protocol for the direct comparison of diverse Pt catalysts in the H/D exchange between C6H6 and TFA-d1, CD3CO2D, and TFE-d3 using turnover number (TON) as a standard metric. An initial survey of Pt complexes, including commercial Pt salts (PtCl2, K2PtCl4) and Pt chloride complexes containing bidentate and tridentate nitrogen donor ligands, has been conducted. These studies have established that the addition of AgOAc (in TFA-d1) or AgBF4 (in CD3CO2D and TFE-d3) displaces the Cl ligands on the Pt precatalyst, which leads to dramatically increased turnover numbers. In general, PtCl2 and K2PtCl4 provided the fewest turnovers, and species containing bidentate ligands afforded higher turnover numbers than those with tridentate ligands. A diimine Pt complex was found to be a top performing catalyst for H/D exchange with all deuterium sources examined. Interestingly, the relative reactivity of many of the catalysts varied dramatically upon changing the deuterium source, highlighting the nee...

Synthesis of new platinum(II) compounds with several bidentate and tridentate nitrogen-donor ligands. Structural analyses by 1H, 13C{ 1H} and 195Pt{ 1H} NMR spectroscopy and X-ray crystal structure

The reaction of the N-alkylaminopyrazole ( NN′) ligands 1-[2-(ethylamino)ethyl]-3,5-dimethylpyrazole (deae), 1-[2-( tert-butylamino)ethyl]-3,5-dimethylpyrazole (deat), or ( NN′ N) ligands bis[(3,5-dimethylpyrazolyl)methyl]ethylamine (bdmae) and bis[(3,5-dimethylpyrazolyl)ethyl]ethylamine (ddae) with [PtCl 2(CH 3CN) 2] affords a series of square-planar Pt(II) complexes with formula [PtCl 2( NN′)] ( NN′ = deae ( 1); deat ( 2)), [PtCl 2(bdmae)] ( 3), or [PtCl(ddae)]Cl ( 4). Treatment of complex 4 in the presence of AgBF 4 in CH 2Cl 2/methanol (3:1) gives [PtCl(ddae)](BF 4) ( 5). These Pt(II) complexes have been characterised by elemental analyses, conductivity measurements and IR, 1H, 13C{ 1H}, and 195Pt{ 1H} NMR spectroscopies. The 1H NMR spectroscopic studies of the complexes prove the rigid conformation of the ligands when they are complexed. The solid-state structure of complex 1 was determined by single crystal X-ray diffraction methods. The deae ligand is coordinated through the N pz and N amino atoms to the metallic centre, which completes its coordination with two chlorine atoms in cis disposition.

Nickel and copper complexes based on tridentate nitrogen donor ligand 2,6-bis-(1-phenyliminoethyl) pyridine: Synthesis, spectral and structural characterization

The reaction of CuCl 2 · 2H 2O with 2,6-bis(1-phenyliminoethyl)pyridine (referred hereafter as L) in 1:1 molar ratio in methanol or acetronitrile at room temperature afforded distorted trigonal-bipyramidal complex [Cu(κ 3-L)Cl 2]. On the other hand, the reaction of NiCl 2 · 6H 2O with 2 equivalents of L gave an octahedral complex [Ni(κ 3-L) 2] 2+, which was isolated as [Ni(κ 3-L) 2][BF 4] 2 using NH 4BF 4. The complexes have been characterized by elemental analyses, FAB-MS, IR, EPR and electronic spectral studies. Molecular structures of both the [Cu(κ 3-L)Cl 2] ( 1) and [Ni(κ 3-L) 2](BF 4) 2 ( 2) have been determined by single crystal X-ray analyses. Weak interaction studies on 1 and 2 revealed stabilisation of the crystal packing by inter and intra-molecular C–H⋯X (X = F, Cl, π) interactions. In complex 2 ortho C–H bond from phenyl rings leads to unexpected C–H⋯π interaction with nickel α,α′-diimine chelate ring. This provides structural support for metalloaromaticity in the chelate ring of complex 2.

Transesterification and amide cis–trans isomerization in Zn and Cd complexes of the chelating amino acid ligand Boc–Asp(Dpa)–OBzl

The amino acid derivative Boc-Asp-OBzl (Boc=N-butyloxycarbonyl; Asp=aspartic acid; Bzl=benzyl) was functionalized by coupling its carboxylate side chain to dipicolylamine. This yielded the tridentate nitrogen donor ligand Boc-Asp(Dpa)-OBzl (-OBzl). The compound -OBzl contains three different carbonyl groups: a tertiary amide linkage between Asp and Dpa, a C-terminal benzyl ester function, and an N-terminal urethane protecting group. NMR spectra were used to compare the reactivity of these moieties. The Boc protecting group gives rise to two isomers, (E, 9%) and (Z, 91%). Coordination of Cd(NO3)2 and Zn(NO3)2 yielded the complexes and. These compounds have significantly reduced barriers to rotation about the tertiary amide C-N bond compared with the free ligand (-OBzl:18.5 kcal mol-1 in CDBr3;: 12.9 kcal mol-1 in (CD3)2CO;: 13.8 kcal mol-1 in (CD3)2CO). Both complexes readily undergo transesterification in methanol or CD3OD. Experimental pseudo-first order rate constants were determined in CD3OD and (CD3)2CO:CD3OD (3:1;). It was found that the zinc complex (k=(2.28+/-0.02)x10(-4) s-1) is significantly more reactive than the cadmium complex (k=(1.41+/-0.03)x10(-6) s-1). In order to study their tertiary amide cis-trans isomerization, the cadmium complex [(-OCH3)Cd(NO3)2] was synthesized, and the zinc complex [(-OCD3)Zn(NO3)2] was generated in situ in (CD3)2CO:CD3OD (3:1). The barriers to rotation were determined (:14.1 kcal mol-1 in CD3OD;: 13.4 kcal mol-1 in (CD3)2CO:CD3OD (3:1)). Our results show that the stronger Lewis-acid zinc(II) is significantly more active than cadmium(II) in the acceleration of the transesterification. This is in marked contrast to the tertiary amide bond rotation which is comparably fast with both metal ions.

Complexation of Lanthanide(III) and Actinide(III) Cations with Tridentate Nitrogen-Donor Ligands: A Luminescence and Spectrophotometric Study

The complexation of lanthanide(III) (lanthanum, europium, and lutetium) and americium(III) by four tridentate nitrogen-donor ligands was investigated in homogeneous methanol/water solutions by using structural and thermodynamic approaches. The stoichiometry and inner-sphere hydration state of the europium complexes formed were determined by time-resolved laser-induced fluorimetry. The stability constants and in some cases the thermodynamic parameters were measured by ultraviolet-visible spectrophotometry and the van’t Hoff method. The comparison of the stability constant of americium complexes with lanthanide complexes shows better stability for the actinide species. The strong affinity for these ligands toward Am3+ is confirmed by the formation of higher complexes, especially in the case of 2,6-bis-(5,6-dimethyl-1,2,4-triazin-3-yl)-pyridine (MeBTP), the only ligand able to form a 1:3 complex. The influence of the solvent composition on the complexation thermodynamics highlights the effect of the solvent reorganization on the reaction.

Five-coordinate iron(II) complexes bearing tridentate nitrogen donor ligands as catalysts for atom transfer radical polymerisation

A series of five-coordinate complexes of iron(II) containing tridentate nitrogen donor ligands has been synthesised and evaluated for the atom transfer radical polymerisation of styrene. The molecular structures of two derivatives are described. Cyclic voltammetric studies show that the redox potential for the Fe(II)/Fe(III) couple strongly depends on the donor capacity of the complexing ligand, and styrene polymerisations reveal that ligands derived from alkyl amine or pyridine groups are the most active for ATRP, although slower than for their four-coordinate relatives due to unfavourable steric interactions. In general, catalyst activity decreases in the order of donor group: alkyl amine ≈ pyridine > alkyl imine ≫ aryl imine > aryl amine, with the aryl derivatives being almost completely inactive in ATRP. The trend in polymerisation activity correlates with the redox potential of the corresponding complexes. Complexes derived from aryl-imine or -amine ligands afford catalytic chain transfer rather than ATRP catalysis.

Gas-phase ligand loss and ligand substitution reactions of platinum(II) complexes of tridentate nitrogen donor ligands.

The source of protons associated with the ligand loss channel of HX((n - 1)+) from [Pt(II)(dien)X](n+) (X = Cl, Br and I for n = 1 and X = NC(5)H(5) for n = 2) in the gas phase was investigated by deuterium-labelling studies. The results of these studies indicate that these protons originate from both the amino groups and the carbon backbone of the dien ligand. In some instances (e.g. X = Br and I), the protons lost from the carbon backbone can be even more abundant than the protons lost from the amino groups. The gas-phase substitution reactions of coordinatively saturated [Pt(II)(L(3))L(a)](2+) complexes (L(3) = tpy or dien) were also examined using ion-molecule reactions. The outcome of the ion-molecule reactions depends on both the ancillary ligand (L(3)) as well as the leaving group (L(a)). [Pt(II)(tpy)L(a)](2+) complexes undergo substitution reactions, with a faster rate when L(a) is a good leaving group, while the [Pt(II)(dien)L(a)](2+) complex undergoes a proton transfer reaction.

Iron‐based catalysts bearing bis(imido)–pyridine ligands for the polymerization of tert‐butyl acrylate

AbstractIron(II) dichloride complexes bearing a tridentate nitrogen donor ligand were investigated for the homopolymerization of tert‐butyl acrylate after activation with methylaluminoxane. Two new complexes were synthesized, 2,6‐bis[1‐(cyclohexylimido)ethyl]pyridine iron chloride (FeCl2) and 2,6‐bis[1‐(isopropylimido)ethyl]pyridine FeCl2, and the single‐crystal X‐ray structure of the latter one was determined. Turnover frequencies of the catalysts during polymerization ranged from 36 to 241 cycles/mol/h. The obtained polymers exhibited weight‐average molecular weights ranging from 24,000 to 438,500 g/mol, and the molar mass distribution varied between 1.8 and 4.3. Activity of the catalytic system and the molar mass of the polymer are influenced by the ligand structure of the complexes as well as other polymerization conditions such as monomer concentration and temperature. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1380–1389, 2003

Binding of nitrite and its reductive activation to nitric oxide at biomimetic copper centers.

The reactivity of nitrite towards the copper(II) and copper(I) centers of a series of complexes with tridentate nitrogen donor ligands has been investigated. The ligands are bis[(1-methylbenzimidazol-2-yl)methyl]amine (1-bb), bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-bb), and bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (ddah) and carry two terminal benzimidazole (1-bb, 2-bb) or pyrazole (ddah) rings and a central amine donor residue. While 2-bb and ddah form two adjacent six-membered chelate rings on metal coordination, 1-bb forms two smaller rings of five members. The binding affinity of nitrite and azide to the Cu(II) complexes (ClO4- as counterion) has been determined in solution. The association constants for the two ligands are similar, but nitrite is a slightly stronger ligand than azide when it binds as a bidentate donor. The X-ray crystal structure of the nitrite complex [Cu(ddah)(NO2)]ClO4 (final R=0.056) has been determined: triclinic P1space group, a=8.200(2) A, b=9.582(3) A, c=15.541(4) A. It may be described as a perchlorate salt of a "supramolecular" species resulting from the assembly of two complex cations and one sodium perchlorate unit. The copper stereochemistry in the complex is intermediate between SPY and TBP, and nitrite binds to Cu(II) asymmetrically, with Cu-O distances of 2.037(2) and 2.390(3) A and a nearly planar CuO2N cycle. On standing, solutions of [Cu(ddah)(NO2)]ClO4 in methanol produce the dinuclear complex [Cu(ddah)(OMe)]2(ClO4)2, containing dibridging methoxy groups. In fact the crystal structure analysis (final R=0.083) showed that the crystals are built up by dinuclear cations, arranged on a crystallographic symmetry center, and perchlorate anions. Electrochemical analysis shows that binding of nitrite to the Cu(II) complexes of 2-bb and ddah shifts the reduction potential of the Cu(II)/Cu(I) couple towards negative values by about 0.3 V. The thermodynamic parameters of the Cu(II)/Cu(I) electron transfer have also been analyzed. The mechanism of reductive activation of nitrite to nitric oxide by the Cu(I) complexes of 1-bb, 2-bb, and ddah has been studied. The reaction requires two protons per molecule of nitrite and Cu(I). Kinetic experiments show that the reaction is first order in [Cu(I)] and [H+] and exhibits saturation behavior with respect to nitrite concentration. The kinetic data show that [Cu(2-bb)]+ is more efficient than [Cu(1-bb)]+ and [Cu(ddah)]+ in reducing nitrite.

Structural and spectral diversities in copper(II) complexes of 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine ‡

Copper(II) complexes of a tridentate nitrogen donor ligand, 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L), CuLX2 (X = Cl, Br, NO3, NCS or ClO4) have been synthesized and characterized using UV-Vis and EPR spectra and electrochemistry. The EPR spectra reveal significant variations in a polycrystalline medium. Copper in all the compounds exhibits a stereochemistry close to a square based pyramid with trigonal bipyramidal distortion and the distortion is more pronounced in CuL(NO3)2. CuLCl2 and CuLBr2 crystallize in space group C2/c and the molecules are arranged in the lattice such that they form columnar packing along the crystallographic a axis. CuL(NO3)2 crystallizes with space group P21/c and forms discrete dimeric units through inter- and intra-molecular hydrogen bonding involving the nitro group and methyl and methylene groups. The polycrystalline EPR spectrum is characteristic of a triplet state. The electrochemical properties of these compounds are also discussed.

Methyltris(pyrazolyl)silanes: new tripodal nitrogen-donor ligands

Two tridentate nitrogen-donor ligands, the methyltris(pyrazolyl)silanes MeSi(3,5-RR′pz) 3 (R=R′=Me; R=Bu t, R′=H), have been synthesized and the X-ray structure of the latter has been determined. The first transition metal derivatives of these ligands, the cationic copper(I) complexes [LCu(NCMe)] + (where L=MeSi(3,5-Me 2pz) 3 or MeSi(3-Bu tpz) 3), have been prepared and characterized analytically and spectroscopically.

Tetranuclear Manganese-Oxo Aggregates Relevant to the Photosynthetic Water Oxidation Center. Crystal Structure, Spectroscopic Properties and Reactivity of Adamantane-Shaped [Mn4O6(bpea)4]4+ and the Reduced Mixed-Valence Analog [Mn4O6(bpea)4]3+

The tetranuclear MnIV adamantane-like complex [Mn4O6(bpea)4](ClO4)4(1(ClO4)4) was isolated from a comproportionation reaction of Mn(ClO4)2·6H2O and [n-Bu4N][MnO4] with the ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) in acetonitrile. Characterization by X-ray crystallography reveals that the [Mn4O6(bpea)4]4+ cation approaches S4 point symmetry. There are three distinct types of Mn−N bonds and two types of bridging oxo ligands in 1. 1H NMR protonation studies of 1 in acetonitrile reveal that each type of oxo bridge renders a different protonation isomer and that the isomers are readily distinguished by their differences in solution structural symmetry and oxo bridge acidity. In addition, pH-dependent aqueous electrochemical studies show that the proton-coupled electron-transfer behavior of 1 is significant because it is the first example of a Mn-oxo aggregate exhibiting a e-/2H+ stoichiometry and because it displays pH-dependent e-/H+ stoichiometry. Remarkably, reaction of 1(BF4)4 with the tridentate nitrogen donor ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn) in acetonitrile affords the one-electron reduced complex [Mn4O6(bpea)4](BF4)3 (2(BF4)3), rather than the anticipated ligand substituted product. Like 1, the [Mn4O6(bpea)4]3+ cation also contains the adamantane skeleton. The (MnIII)(MnIV)3 cluster of 2 contains a crystallographically distinguishable MnIII ion, as noted by elongation along the Nalkyl-Mn−Ooxo axis. The 1H NMR solution spectrum of 2 is consistent with a valence-delocalized (MnIII)(MnIV)3 cluster, indicating fast intramolecular electron transfer on the NMR time scale, and redox titration of 1 to 2 indicates slow intermolecular electron transfer on the same time scale. Solution magnetic susceptibility measurements in acetonitrile show that conversion of 1(ClO4)4 to 2(ClO4)3 is attendant with a change from net ferromagnetic coupling to overall moderate antiferromagnetic coupling within the manganese-oxo core. Isolation of the {Mn4O6} core in the (IV)4 and (III)(IV)3 oxidation states is facilitated by the relatively weak donor nature of the bpea ligand, in marked contrast to the strongly basic donor ligands 1,4,7-triazacyclononane (tacn) and 1,1,1-tris(aminomethyl)ethane (tame), which stabilize the higher oxidation state of the {Mn4O6} cluster, making the one-electron reduced form less accessible. The novel protonation and electrochemical properties of 1 are discussed in the context of the Kok cycle of photosynthetic water oxidation.

Indium(III) complexes with tridentate nitrogen donor ligands. Synthesis, characterization and crystal structure of complexes with pyridine-2,6-bis(acetyloxime) and 2,6-bis(1-phenyliminoethyl)pyridine

2,6-Bis(acetyloxime)pyridine, 2,6-C 5H 3N(C 2H 3NOH) 2 ( L 1), reacts with indium(III) chloride in methanol to give the seven-coordinate complex [InCl 3(L 1)(MeOH)]. The structurally similar Schiff base ligand 2,6-bis(1-phenyliminoethyl)pyridine ( L 2), 2,6-C 5H 3N(C 2H 3NC 6H 5) 2, however, forms a six-coordinate indium(III) complex of the composition [InCl 3(L 2)]. The InN bond distances are longer by 0.03–0.12 Å in the seven-coordinate compound compared with those in the Schiff base complex. The MeOH ligand in [InCl 3(L 1)(MeOH)] can be replaced by Cl − or H 2O to give the complex anion [InCl 4(L 1)] − and [InCl 3(L 1)(OH 2)], respectively. The pentagonal-bipyramidal coordination environment of the metal is preserved during these reactions.

High-Pressure NMR and ab Initio Computational Studies on the Insertion Mechanism of Carbon Monoxide into Cationic Monoorganopalladium(II) Complexes Bearing Tridentate Nitrogen Donor Ligands

A number of methyl and arylpalladium(I1) cations bearing tridentate nitrogen-donor ligands has been prepared, and their reactivity in the insertion of carbon monoxide has been studied. The resulting acetyl and aroylpalladium complexes of the ligands N-(2-picolyl)N,”,”-trimethylethylenediamine ( pico) and NJVJV’,iV”,N”-pentamethyldiethylenetriamine (pmdeta) were isolated and characterized. In the case of the 4-nitrophenylpalladium and 2,4,64rimethylphenylpalladium pico- and pmdeta complexes, no carbonylation products could be isolated. In these cases, either the equilibrium of the carbonylation reaction did not lie fully to the side of the insertion product even at 10 atm of CO or the insertion product decarbonylated upon attempted isolation. The methylpalladium complex of the 2,6-bis[(dimethylamino)methyl]pyridine (NN”) ligand reacted quantitatively with CO in CD3COCD3 to give acetic anhydride, palladium metal, and the protonated ligand; a mechanism for the anhydride formation is proposed. With the exception of the l-naphthoyl derivative, the aroyl complexes of the NN” ligand could not be isolated due to decarbonylation upon attempted isolation. Most of the unstable insertion products could, however, be characterized by IR and high-pressure NMR. Crystals of the l-naphthoylpalladium complex with the NNT ligand were obtained from acetone/pentane under a CO atmosphere. This complex is the first example of an aroylpalladium(I1) cation. Two reaction pathways for the carbonyl insertion reaction, Le., dissociative and associative, have been evaluated using lH NMR studies and ab initio calculations. The insertion reaction at 10 atm of CO pressure in CD3COCD3 is complete within 2.5 min for most complexes, with the exception of those bearing strongly electron-withdrawing- para substituents (e.g., NO2) or sterically demanding ortho substituents (e.g., 2,4,6-trimethyl) on the aryl ring. Ab initio calculations at the RHF, MP21 /SCF, and CAS-SCF/CI levels on the cationic model system [Pd(CH3)(NH3)31+ + CO and the neutral system [Pd(CH3)2(NH3)21 + CO show that the carbonylation reaction follows a hybrid pathway, Le., a concerted replacement of NH3 by CO followed by migratory insertion of CO into the Pd-C bond instead of a purely dissociative or associative mechanism. For both the neutral and the cationic systems the rate-determining step is the migratory insertion. The insertion process is enhanced by coordination of the dissociated amine and is slightly more favorable in the neutral system. Together with the low-energy replacement of NH3 by CO, this implies that in both systems the rate of carbonyl insertion should be independent of the applied CO pressure.