Nitrogen-based Ligands Research Articles

Five-Coordinate Dihydridosilyl Platinum(IV) Complexes Supported by a Chelating Monoanionic Nitrogen-Based Ligand

Complexes of Pt(IV) containing the bidentate ligand 3,5-diphenyl-2-(2-pyridyl)pyrrolide (PyPyr) were prepared. The ethylene complex PyPyrPt(C2H4)Cl (1) was treated with HSiEt3 or HSiEtMe2 to produce the Pt(IV) silyl dihydrides PyPyrPt(H)2SiEt3 (3) and PyPyrPt(H)2SiEtMe2 (4), respectively. The solid-state structure of 3, determined by X-ray crystallography, reveals a dimeric structure that forms via π-stacking between the PyPyr ligands. Addition of Lewis bases to 3 results in either coordination to generate an octahedral Lewis base adduct (with DMAP) or silane elimination to give square-planar Pt(II) Lewis base complexes (with phosphines). Complex 3 was also found to be an active hydrosilylation catalyst for the hydrosilylation of alkynes and terminal olefins with HSiEt3.

Palladium‐Catalyzed Intramolecular Hydroamination of Allenes Coupled to Aerobic Alcohol Oxidation

Taking the air! A Pd(II)-catalyzed intramolecular hydroamination of allenes coupled to alcohol oxidation has been developed. This reaction is performed by using a nitrogen-based ligand under aerobic conditions, under which the molecular oxygen is used as the terminal oxidant for the reoxidation of Pd(0) species to complete the catalytic cycle.

Synthesis, Characterization, and O2 Reactivity of Iridium(I) Complexes Supported by Guanidinato Ligands

Mononuclear [Ir{ArNC(NR(2))NAr}(C(8)H(12))] complexes (where R = Me or Et; Ar = Ph, 4-MeC(6)H(4), or 2,6-Me(2)C(6)H(3); and C(8)H(12) = 1,5-cyclooctadiene) were synthesized from the neutral N,N-dialkyl-N',N''-diarylguanidines via deprotonation and transmetalation. As confirmed by single-crystal structure determinations, the guanidinato(1-) ligands coordinate the low-valent d(8) Ir(I) center in an N,N'-chelating binding mode, and the (13)C NMR chemical shifts of the alkene carbon atoms establish that these ligands function as stronger donors than related monoanionic, bidentate nitrogen-based ligands. In the reactions of the complexes with O(2), the observed reactivity trends correlate with the electronic and steric influences of the substituents of the guanidinato ligands.

Effect of Initiator and Ligand Structures on ATRP of Styrene and Methyl Methacrylate Initiated by Alkyl Dithiocarbamate

Atom transfer radical polymerization (ATRP) of styrene (St) and methyl methacrylate (MMA) initiated by various alkyl diethyldithiocarbamate (DC) initiators was successfully carried out in the presence of copper catalysts with nitrogen-based ligands. Well-controlled polymerizations with narrow molecular weight distribution (Mw/Mn < 1.1 (St) and Mw/Mn < 1.2 (MMA)) were achieved in both polymerizations. The polymerization rate followed first-order kinetics with respect to monomer conversion, and the molecular weight of the polymers increased linearly up to high conversion. Initiation efficiency of both polymerizations was strongly dependent on the structure of a DC. The results of 1H NMR analysis of low-mass model compounds and chain extension confirmed that well-defined polystyrene bearing a DC group as the active chain end was obtained via ATRP of St with a DC initiator. Ligand structure and ligand/copper ratio also strongly affected the degree of control attained in the polymerization. Activation rate constants and equilibrium constants of ATRP with DC initiators and copper complexes were determined. The results of cyclic voltammetry with the CuIIDC2 complex indicated that it has more negative reduction potential and, consequently, higher (pseudo)halidophilicity than those of CuIIBr2 or CuIICl2 with the same ligand Me6TREN.

Electron paramagnetic resonance characterization of the copper-resistance protein PcoC from Escherichia coli

Continuous-wave and pulsed electron paramagnetic resonance have been applied to the study of the Cu(II) site of the copper-resistance protein PcoC from Escherichia coli and certain variant forms. Electron spin echo envelope modulation (ESEEM) experiments confirm the presence of two histidine ligands, His1 and His92, at the Cu(II) site of wild-type PcoC, consistent with the available X-ray crystallographic data for the homolog CopC (67% sequence identity) from Pseudomonas syringae pv. tomato. The variants H1F and H92F each lack one of the histidine residues close to the Cu(II) site. The ESEEM data suggest that the surviving histidine residue remains as a ligand. The nA variant features an extra alanine residue at the N terminus, which demotes the His1 ligand to position 2. At least one of the two histidine residues is bound at the Cu(II) site in this form. Simulation of the (14)N superhyperfine structure in the continuous-wave spectra confirms the presence of at least three nitrogen-based ligands at the Cu(II) sites of the wild-type, H92F and nA forms, while the H1F variant has two nitrogen ligands. The spectra of wild-type form can be fitted adequately with a 3N or a 4N model. The former is consistent with the crystal structure of the CopC homolog, where His1 acts as a bidentate ligand. The latter raises the possibility of an additional unidentified nitrogen ligand. The markedly different spectra of the H1F and nA forms compared with the wild-type and H92F proteins further highlight the integral role of the N-terminal histidine residue in the high-affinity Cu(II) site of PcoC.

Synthesis, characterization and spectral studies of bis( O, O-dialkylmonothiophosphato)cobalt(II) complexes and their adducts with nitrogen base ligands: Crystal structures of [Co{O(S)P(OPr i) 2} 2(C 5H 5N) 4] and [Co{O(S)P(OPr n) 2} 2(C 10H 8N 2) 2

Bis( O, O-dialkylmonothiophosphato)cobalt(II) complexes of the type [Co{O(S)P(OR) 2} 2] (where R = Pr n , Pr i , Bu n ) were prepared under anhydrous conditions by the reaction of cobalt(II) dichloride hexahydrate with the sodium salts of dialkyl monothiophosphoric acids. The reactions of these blue tetrahedral complexes with nitrogen bases such as pyridine (C 5H 5N or py) and 2,2-bipyridyl (C 10H 8N 2 or bipy) resulted in the pink/orange octahedral adducts [Co{O(S)P(OPr i ) 2} 2(C 5H 5N) 4], [Co{O(S)P(OPr n ) 2} 2(C 10H 8N 2) 2] and [Co{O(S)P(OBu n ) 2} 2(C 10H 8N 2) 2]. These compounds were characterized by elemental analysis, IR spectroscopy, magnetic measurements and the single crystal X-ray diffraction structures of two of the adducts of the monothiophosphate complexes, [Co{O(S)P(OPr i ) 2} 2(C 5H 5N) 4] and [Co{O(S)P(OPr n ) 2} 2(C 10H 8N 2) 2], which are the first examples of this class of compounds. [Co{O(S)P(OPr i ) 2} 2(C 5H 5N) 4] crystallizes as triclinic in the space group P 1 ¯ with the cell parameters a = 9.0515(4) Å, b = 10.1629(5) Å, c = 11.8468(5) Å, α = 88.389(3)°, β = 71.984(4)°, γ = 69.181(3)°, Z = 1, V = 964.57(8) Å 3 and [Co{O(S)P(OPr n ) 2} 2(C 10H 8N 2) 2] crystallizes as monoclinic in the space group C2/ c with the cell parameters, a = 8.404(2) Å, b = 22.855(5) Å, c = 18.913(4) Å, β = 97.56(3)°, Z = 4, V = 3601(1) Å 3. These structures contain a unidentate bonding of the monothiophosphate ligands through oxygen, with the geometry of the CoO 2N 4 coordination sphere around cobalt being distorted octahedral.

Structures of Prostacyclin Synthase and Its Complexes with Substrate Analog and Inhibitor Reveal a Ligand-specific Heme Conformation Change

Prostacyclin synthase (PGIS) is a cytochrome P450 (P450) enzyme that catalyzes production of prostacyclin from prostaglandin H(2). PGIS is unusual in that it catalyzes an isomerization rather than a monooxygenation, which is typical of P450 enzymes. To understand the structural basis for prostacyclin biosynthesis in greater detail, we have determined the crystal structures of ligand-free, inhibitor (minoxidil)-bound and substrate analog U51605-bound PGIS. These structures demonstrate a stereo-specific substrate binding and suggest features of the enzyme that facilitate isomerization. Unlike most microsomal P450s, where large substrate-induced conformational changes take place at the distal side of the heme, conformational changes in PGIS are observed at the proximal side and in the heme itself. The conserved and extensive heme propionate-protein interactions seen in all other P450s, which are largely absent in the ligand-free PGIS, are recovered upon U51605 binding accompanied by water exclusion from the active site. In contrast, when minoxidil binds, the propionate-protein interactions are not recovered and water molecules are largely retained. These findings suggest that PGIS represents a divergent evolution of the P450 family, in which a heme barrier has evolved to ensure strict binding specificity for prostaglandin H(2), leading to a radical-mediated isomerization with high product fidelity. The U51605-bound structure also provides a view of the substrate entrance and product exit channels.

Dynamic Axial Ligand-Site Exchange in Facially Discriminated Ruthenium(II) Carbonyl and Rhodium(III) Halide Metalloporphyrins

For a series of six-coordinate RuII(CO)L or RhIII(X-)L porphyrins, which are facially differentiated by having a naphthoquinol- or hydroquinol-containing strap across one face, we show that ligand migration from one face to the other can occur under mild conditions and that ligand-site preference is dependent on the nature of L and X-. For bulky nitrogen-based ligands, the strap can be displaced sideways to accommodate the ligand on the same side as the strap. For the ligand pyrazine, we show 1H NMR evidence for monodentate and bridging binding modes on both faces, dependent on ligand concentration and metalloporphyrin structure, and that interfacial migration is rapid under normal conditions. For monodentate substituted pyridine ligands, there is a site-dependence on structure, and we show clear evidence of dynamic ligand migration through a series of ligand-exchange reactions.

The First Structurally Determined Ruthenium Nitrate Complex of Chelating Diphosphine Ligand

Among the reported functionalized complexes, oxo and nitrido ruthenium or osmium complexes containing multidentate nitrogen-based ligands have proven to be especially valuable in mechanistic and oxidative catalytic studies. During the last decade the chemistry of inorganic and organometallic complexes with phosphine ligands has been well developed to apply them to catalysts. Therefore it is of interest to combine these two characteristics to prepare oxo complexes surrounded by the phosphine ligands. A general way to synthesize Ru-oxo complexes can be started from the related Ru-chloro species via the corresponding Ru-aquo complexes. Oxidation of the Ru-aquo complexes by a powerful one-electron oxidant like (NH4)2Ce(NO3) (CAN) can lead to the formation of the final products. Accordingly we prepared and characterized ruthenium di-chloro complexes, [Ru(dppm)2Cl2] and [Ru(dppe)2Cl2], by using the reaction of [Ru(PPh3)2Cl2] with the dppm or dppe ligand (PPh3: triphenylphosphine, dppm: 1,1-bis(diphenylphosphino)methane, dppe: 1,2-bis(diphenylphosphino)ethane). A ruthenium di-aquo complex, [Ru(dppm)2(H2O)2](PF6)2, can be obtained by adding AgClO4 to the [Ru(dppm)2Cl2] complex followed by precipitating with saturated NH4PF6 solution. CAN in HClO4 was added into the di-aquo complex and the residue was put into the refrigerator. The crystals obtained by diffusion method from acetone/water show the formation of an unexpected complex rather than the desired ruthenium di-oxo species. In organic chemistry CAN has been known to catalyze the ring opening reaction of different epoxides through the formation of nitrate compound in aprotic solvent, which is a key intermediate in the stereoselective syntheses and is useful for a variety of oxidative transformations. The photochemical reaction of CAN with olefins also produced 1,2nitrate adduct via the initial formation of nitrate radical. It is quite interesting to show that CAN can be used to prepare the nitrate complexes in coordination chemistry. Here we found out that the reaction of CAN in perchloric acid with the di-aquo complex gave the unusual nitrate complex rather than the di-oxo complex. In the reaction it is presumed that the more labile aquo group in the di-aquo complex can be quickly substituted by the nitrate anion in a bi-dentate fashion, which is more favorable in entropy. The structure of the nitrate complex, which turned out to be [Ru(dppm)2(O2NO)][ClO4], has been determined from X-ray diffraction. The perspective view with the numbering scheme of the atoms is shown in Figure 1. The complex consists of discrete [Ru(dppm)2(O2NO)] and ClO4, which contains a ruthenium atom surrounded by two chelating diphosphine ligands and the nitrate ligand. The presence of the nitrate ligand coordinated to the ruthenium metal was confirmed by a singlet resonance at 13.8 ppm in the N NMR spectrum and that of the perchlorate anion by a sharp singlet peak at 1.01 ppm in the Cl NMR spectrum. The magnitude of the splitting of peaks at 1435 and 1232 cm−1 observed in the FT IR spectrum re-confirmed that the nitrate ligand coordinated to the metal in a bi-dentate way. The coordination polyhedron around the ruthenium metal is described as a severely distorted octahedron. Selected bond distances and angles listed in Table 1 are comparable with other structurally characterized complexes containing the dppm ligand. The Ru-P bond distances range from 2.277(1) to 2.393(1) A. The phosphorous atoms coordinated to ruthenium metal deviate strongly from a square planar arrangement as noted by the P1-Ru-P2 and P3-Ru-P4 bond angles of less than 90 (70.76(3) and 70.35(3), respectively) for two phosphorous atoms bonded to the ruthenium metal cis to one another and by the P1-Ru-P4 and P2-Ru-P3 angles of less than 180 (103.8(1) and 102.7(1), respectively) for those in the opposite position.

Low-Temperature Spectral Observation of the First Six-Coordinate Nitrosyl Complexes of Cobalt(II)meso-Tetratolylporphyrin with Trans Nitrogen Base Ligands

Low-temperature interaction of nitrogen base ligands with layered Co(TTP)(NO) (TTP = meso-tetratolylporphyrinato dianion) as well as its toluene solution leads to the formation of the first six-coordinate species of the general formula (B)Co(TTP)(NO) (where B = piperidine and pyridine). The nu(NO) stretching bands of these species appear at lower frequencies compared with the five-coordinate nitrosyl derivative and depend on the nature of the trans axial ligand. The equilibrium constants and enthalpies of formation of these new species are determined. Fairly stable at low-temperature conditions in the solid state, they slowly dissociate the nitrogen base ligands upon warming to restore the five-coordinate nitrosyl complex Co(TTP)(NO).

Oxidative palladium(II) catalysis: A highly efficient and chemoselective cross-coupling method for carbon-carbon bond formation under base-free and nitrogenous-ligand conditions.

We report herein the development of a general and mild protocol of oxygen-promoted Pd(II) catalysis resulting in the selective cross-couplings of alkenyl- and arylboron compounds with various olefins. Unlike most cross-coupling reactions, this new methodology works well even in the absence of bases, consequently averting undesired homo-couplings. Nitrogen-based ligands including dimethyl-phenanathroline enhance reactivities and offer a highly efficient and stereoselective methodology to overcome challenging substrate limitations. For instance, oxidative palladium(II) catalysis is effective with highly substituted alkenes and cyclic alkenes, which are known to be incompatible with other known catalytic conditions. Most examined reactions progressed smoothly to completion at low temperatures and in short times. These interesting results provide mechanistic insights and utilities for a new paradigm of palladium catalytic cycles without bases.

Synthesis of poly(triazinylstyrene) containing nitrogen-based ligand and function as metal ion adsorbent and oxidation catalyst

A set of poly(triazinylstyrene)s containing ethylenediamine (PEd1 and PEd2) and pyridine (PAp2) moieties was prepared by the radical polymerization, in which PEd1 and PEd2 were synthesized at the lower temperature to avoid the gel formation. PEd2 having two hydrophilic ethylenediamine side chains could selectively bind copper(II) ion from an aqueous buffer solution containing individual metal(II) sulfate; on the other hand, PEd1 and PAp2 showed a poor chelating ability. Polymer catalysts prepared from copper(II) nitrate were subjected to the oxidation reaction of p-hydroquinone to analyze whether the complex bearing ethylenediamine moiety showed a higher catalytic activity. The activity of the catalyst improved with increasing pH of the reaction media, especially in the catalyst prepared from PEd2.

Effect of Ligand Structure on Activation Rate Constants in ATRP

Activation rate constants (kact) for Cu complexes with various nitrogen-based ligands for Cu-mediated ATRP were determined at 35 °C in acetonitrile. The ratio of kact for complexes with different ligands exceeds 1 million times. The general order of Cu complex activity for ligands is tetradentate (cyclic-bridged) > tetradentate (branched) > tetradentate (cyclic) > tetradentate (linear) > tridentate > bidentate ligands. The nature of nitrogen atoms in ligands also plays a significant role in the activity of the Cu complexes and follows the order pyridine ≥ aliphatic amine > imine.

The Structural Systematics of Protonation of Some Important Nitrogen-base Ligands. II. Some Univalent Anion Salts of Singly Protonated Bis(2-pyridyl)amine

Syntheses and single crystal X-ray structure determinations are recorded for a number of normal and ‘acid’ salts of bis(2-pyridylamine), ‘dpa’, with univalent anions, X, variously hydrated, i.e. [dpaH]X·nH2O, and [dpaH]X·HX·nH2O. The ‘normal’ salt arrays so characterized are for X = Br− (n = 2, isomorphous with the previously described chloride compound) and, I−, ClO4−, ‘tca−’ (≡ Cl3CCO2)− (all n = 1); acid salt arrays are described for X = NO3− and tca (both n = 0). In all cases except those of X = ClO4−, NO3−, there is one independent formula unit devoid of crystallographic symmetry comprising the asymmetric unit of the structure. In all cases, the proton associated with the cation is ‘chelated’ by the pair of ring nitrogen atoms, disposed ‘endo’; in the tca adducts and the nitrate salt, the total cation is disordered in each case by inversion about a real or putative inversion centre between the rings. In the perchlorate compound, the (ordered) cation lies on a crystallographic 2-axis, as does the water molecule, and the perchlorate ion, which is disordered about such an axis; in the nitrate compound, the acid hydrogen atom is modelled as disposed on a crystallographic inversion centre between a pair of symmetry-related nitrate groups, containing, like the Htca adduct, the [XHX]− moiety rather than a diprotonated cation.

Stoichiometric and Catalytic Arene Activations by Platinum Complexes Containing Bidentate Monoanionic Nitrogen-Based Ligands

New platinum complexes based on incorporation of the bidentate, monoanionic 2-(2‘-pyridyl)indolide (PyInd) ligand, or two closely related ligands, have been prepared. Reaction of the dimethyl complex K[(PyInd)PtMe2] (K[2a]) in benzene at 150 °C produces the diphenyl species K[(PyInd)PtPh2] (K[3a]), a transformation that appears to proceed via direct oxidative addition of benzene to the 16-electon Pt(II) center. Complex K[2a] also undergoes oxidative addition of Me3SiOTf (OTf = triflate) to give (PyInd)Pt(SiMe3)Me2 (4), a rare five-coordinate Pt(IV)-silyl species. Additionally, reactions that involve stereoselective formation of products of the type (PyInd)Pt(X)(L), including the structurally characterized complexes (PyInd)Pt(Ph)(MeCN) (6) and (PyInd)Pt(Cl)(C2H4) (7), have been observed. These results indicate that the sites trans to the pyridinyl and indolyl fragments of the PyInd ligand are electronically differentiated. Studies of catalytic reactions that presumably proceed via the 14-electron fragment ...

Uniqueness and Versatility of Iminopyrrolyl Ligands for Transition Metal Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural diversity in the coordination of amidines and guanidines to monovalent metal halides.

A series of structurally characterised, monovalent metal-halide complexes incorporating neutral amidine and guanidine ligands is reported. N,N'-diphenylbenzamidine reacted with copper(I) chloride to afford the bis-ligand complex [CuCl(PhC[NPh][NHPh])2]2 (1), that exists as a chlorine bridged dimer in the solid state, with a non-symmetrical distribution of NH...Cl interactions within the 'Cu2Cl2' metallacycle. In contrast, only one equivalent of the guanidine, Me2NC[NiPr][NHiPr] (2), is coordinated in the copper(I) iodide complex [CuI(Me2NC[NiPr][NHiPr])]2 (3), which was also isolated as the dimer with bridging halide atoms. The molecular structure of the bicyclic guanidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-alpha]pyrimidine (hppH), is reported, revealing a hydrogen bridged dimer with extensive delocalisation throughout the ligand framework. Coordination of hppH to lithium chloride afforded the dimeric bis-ligand complex [LiCl(hppH)2]2 (4) in which each hppH molecule interacts with a different chlorine atom of the central 'Li2Cl2' core of the molecule via NH...Cl hydrogen bonding. In contrast the 2:1 ligand to metal complex is formed with silver(I) chloride to afford AgCl(hppH)2 (5), a unique example of a monomeric, three-coordinate silver chloride supported by nitrogen-based ligands. The series of mixed ligand complexes [CuX(hppH)(PPh3)]n (6, X = Cl, n= 1; 7, X = Br, n= 2; 8 X = I, n= 2) have also been synthesised and structurally characterised, allowing comparisons of the relative coordinating behaviour of hppH and PPh3 as neutral donors at copper(I) centres to be made.

Spectroscopic and photophysical properties of hexanuclear rhenium(III) chalcogenide clusters.

The electronic, vibrational, and excited-state properties of hexanuclear rhenium(III) chalcogenide clusters based on the [Re(6)(mu(3)-Q)(8)](2+) (Q = S, Se) core have been investigated by spectroscopic and theoretical methods. Ultraviolet or visible excitation of [Re(6)Q(8)](2+) clusters produces luminescence with ranges in maxima of 12 500-15 100 cm(-)(1), emission quantum yields of 1-24%, and emission lifetimes of 2.6-22.4 microseconds. Nonradiative decay rate constants and the luminescence maxima follow the trend predicted by the energy gap law (EGL). Examination of 24 clusters in solution and 14 in the solid phase establish that exocluster ligands engender the observed EGL behavior; clusters with oxygen- or nitrogen-based apical ligands achieve maximal quantum yields and the longest lifetimes. The excited-state decay mechanism was investigated by applying nonradiative decay models to temperature-dependent emission experiments. Solid-state Raman spectra were recorded to identify vibrational contributions to excited-state deactivation; spectral assignments were enabled by normal coordinate analysis afforded from Hartree-Fock and DFT calculations. Excited-state decay is interpreted with a model where normal modes largely centered on the [Re(6)Q(8)](2+) core induce nonradiative relaxation. Hartree-Fock and DFT calculations of the electronic structure of the hexarhenium family of compounds support such a model. These experimental and theoretical studies of [Re(6)Q(8)](2+) luminescence provide a framework for elaborating a variety of luminescence-based applications of the largest series of isoelectronic clusters yet discovered.

Mixed-ligand complexes of cadmium(II) containing bulky polydentate nitrogen-based ligands

A series of monomeric complexes of cadmium(II) bonded to bulky ligands, such as [H 2B(3-Bu tpz) 2]Cd(S 2CNEt 2), [HB(3-Bu tpz) 3]Cd(S 2CNEt 2) (pictured), [HB(3-Phpz) 3]CdCH 3, [HB(3-Phpz) 3]CdS( p-C 6H 4CH 3) (pz=pyrazolyl) and 2,6-bis[(2,6-dimethylphenylimino)methyl]pyridineCdCl 2 have been prepared and in some cases characterized crystallographically.

Tridentate Nitrogen-Based Ligands in Cu-Based ATRP: A Structure−Activity Study

The kinetic parameters for the activation and deactivation steps in ATRP for Cu-based catalysts were determined with eight different tridentate nitrogen-based ligands. Additionally, the redox properties of these Cu complexes were measured by cyclic voltammetry. By correlating the kinetic parameters of the activation and deactivation steps with the reduction potential of the Cu(II) complexes, it was found that more reducing Cu catalysts form faster activating Cu(I) and slower deactivating Cu(II) species. The rate of activation depends on the nature of the N-binding site of the ligand. Ligands with alkyl amine or pyridine binding sites form the fastest activating catalysts. The phenyl-substituted ligands form very slowly activating and very rapidly deactivating catalysts. Slower deactivation rates were found for catalysts with a central pyridine unit in the ligand than for catalysts derived from ligands with a central amine unit. In general, the activity of the ligands decreases in the following order: alk...