N-bound Complexes Research Articles

Synthesis of copper(i) cyclic (alkyl)(amino)carbene complexes with potentially bidentate N^N, N^S and S^S ligands for efficient white photoluminescence.

The reaction of (Me2L)CuCl with either NaS2CX [X = OEt, NEt2 or carbazolate (Cz)] or with 1,3-diarylguanidine, 1,3-diarylformamidine or thioacetaniline in the presence of KOtBu affords the corresponding S- or N-bound copper complexes (Me2L)Cu(S^S) 1-3, (Me2L)Cu(N^N) 4/5 and (Me2L)Cu(N^S) 6 (aryl = 2,6-diisopropylphenyl; Me2L = 2,6-bis(isopropyl)phenyl-3,3,5,5-tetramethyl-2-pyrrolidinylidene). The crystal structure of (Me2L)Cu(S2CCz) (3) confirmed the three-coordinate geometry with S^S chelation and perpendicular orientation of the carbene and S^S ligands. On heating 3 cleanly eliminates CS2 and forms (Me2L)CuCz. The N-bound complexes show strongly distorted T-shaped (4) or undistorted linear (5) geometries. On excitation with UV light the S-bound complexes proved non-emissive, while the guanidinato and formamidinato complexes are strongly phosphorescent, with excited state lifetimes in the range of 11-24 μs in the solid state. The conformationally flexible formamidinato complex 5 shows intense green-white phosphorescence with a solid-state quantum yield of >96%.

Modulating the nitrite reductase activity of globins by varying the heme substituents: Utilizing myoglobin as a model system

Globins, such as hemoglobin (Hb) and myoglobin (Mb), have gained attention for their ability to reduce nitrite (NO2−) to nitric oxide (NO). The molecular interactions that regulate this chemistry are not fully elucidated, therefore we address this issue by investigating one part of the active site that may control this reaction. Here, the effects of the 2,4-heme substituents on the nitrite reductase (NiR) reaction, and on the structures and energies of the ferrous nitrite intermediates, are investigated using Mb as a model system. This is accomplished by studying Mbs with hemes that have different 2,4-R groups, namely diacetyldeuteroMb (-acetyl), protoMb (wild-type (wt) Mb, -vinyl), deuteroMb (-H), and mesoMb (-ethyl). While trends on the natural charge on Fe and O-atom of bound nitrite are observed among the series of Mbs, the FeII–NPyr (Pyr=pyrrole) and FeII–NHis93 (His=histidine) bond lengths do not significantly change. Kinetic analysis shows increasing NiR activity as follows: diacetyldeuteroMb<wt Mb<deuteroMb<mesoMb. Nitrite binding energy calculations of the different MbII–nitrite conformations demonstrate the N-bound complexes to be more stable than the O-bound complexes for all the different types of heme structures, with diacetyldeuteroMb having the greatest nitrite binding affinity. Spectral deconvolution on the final product generated from the reaction between MbII and NO2− for the reconstituted Mbs indicates the formation of 1:1 MbIII and MbII–NO. The electronic changes induced by the –R groups on the 2,4-positions do not alter the stoichiometric ratio of the products, resembling wt Mb.

Mechanistic Consideration of Asymmetric C–N and C–C Bond Formations with Bifunctional Chiral Ir and Ru Catalysts

Abstract The mechanism of two enantioselective reactions, direct amination of α-cyanoacetates 3 with azodicarboxylates 4 and C–C bond formation reaction of α-cyanoacetates with acetylenic esters 6, catalyzed by chiral bifunctional Ir and Ru complexes, Cp*Ir[(S,S)-N-sulfonated dpen] 1 and Ru[(S,S)-N-sulfonated dpen](η6-arene) 2 (DPEN: 1,2-diphenylethylenediamine) was studied by NMR spectroscopic analysis combined with DFT analysis. Notably, these two reactions using the same chiral amide catalysts 1, 2 and pronucleophile, α-cyanoacetates 3 gave quantitatively the conjugate adducts bearing quaternary chiral carbon centers in excellent enantiomeric excess albeit with the opposite absolute configuration depending on the acceptor molecules 4 and 6. NMR investigation of the reactions between Ir complexes 1a–1c with α-cyanoacetates 3 showed that a stereoselective deprotonation reaction takes place to give an equilibrium mixture of N-bound amine complexes 8 and 9, the former with intramolecular hydrogen bonding and the latter without it, respectively. Computational study revealed the full details of the mechanism of the asymmetric C–N and C–C bond forming reactions catalyzed by the chiral Ir catalyst 1b. In the C–N bond forming reaction, the dimethyl azodicarboxylate 4a undergoes productive bifunctional activation by a non-hydrogen-bonded N-bound complex 9b(re) resulting in the formation of the R-product through the energetically favorable transition state. On the other hand, the linear geometry of the acetylenic ester molecule 6 allows its bifunctional activation with both types of the N-bound complexes: 8b and 9b with and without the intramolecular hydrogen bond respectively. The hydrogen-bond stabilized transition state for the C–C bond formation leading to the S-enantiomer is significantly lower in energy than the corresponding non-hydrogen-bonded transition state leading to the R-enantiomer. Thus, chiral induction of these two reactions is determined by the structures of the acceptor molecules.

Synthesis, characterization, and reaction pathways for the formation of a GMP adduct of a cytotoxic thiocyanato ruthenium arene complex

The organoruthenium complex [(eta(6)-hmb)Ru(en)(Cl)][PF6] (hmb is hexamethylbenzene, en is ethylenediamine) undergoes facile aquation and then reacts with KSCN in unbuffered solution to give the S-coordinated thiocyanato product [(eta(6)-hmb)Ru(en)(S-SCN)]+ which slowly converts to the thermodynamically favored N-bound complex [(eta(6)-hmb)Ru(en)(N-NCS)]+ (1+). Complex 1 was synthesized and characterized by X-ray crystallography and mass spectrometry. Despite its lack of hydrolysis over 24 h, complex 1 exhibits moderate cytotoxicity (IC(50) 24 microM) towards the human ovarian cancer cell line A2780, comparable with that of the chlorido analogue which is thought to be activated (towards potential target DNA) via a rapid aquation (Wang et. al. in Proc Natl Acad Sci USA 102:18269-18274, 2005). Detailed kinetic studies suggest that complex 1 binds to guanosine 5'-monophosphate (GMP) through direct N7 substitution of the N-bound SCN ligand. In the presence of a high concentration of chloride (104 mM), however, complex 1 may bind partly to GMP via Cl substitution.

Bifunctional transition metal‐based molecular catalysts for asymmetric CC and CN bond formation

This paper describes the recent advances in the conceptually new bifunctional Ir and Ru catalysts for asymmetric catalytic reactions. These reactions include the enantioselective Michael addition of 1,3-dicarbonyl compounds to cyclic enones and nitroalkenes, and the enantioselective direct amination of alpha-cyanoacetates with diazoesters. The outcome of these reactions in terms of reactivity and selectivity was delicately influenced by the catalyst structures and the reaction conditions including the solvents used. Even with a 1 : 1 molar ratio of donors to acceptors, the reactions proceeded smoothly to give the corresponding chiral adducts with an excellent yield and enantiomeric excess (ee). Preliminary mechanistic studies showed that the key stage of the catalytic cycle is the interaction of the bifunctional catalyst with a pronucleophilic reagent that leads to stereoselective formation of C-, O-, or N-bound complexes. The resulting protonated catalyst bearing metal-bound nucleophiles readily reacts with electrophiles to provide C--C and C--N bond formation products in a highly stereoselective manner.

Reactions of Indene and Indoles with Platinum Methyl Cations: Indene C-H Activation, Indole π vs. Nitrogen Lone-Pair Coordination.

Reactions of indene and various substituted indoles with [(diimine)PtII(Me)(TFE)]+ cations have been studied (diimine = ArN = C(Me) - C(Me) = NAr; TFE = 2,2,2-trifluoroethanol). Indene displaces the TFE ligand from platinum to form a stable π coordination complex that, upon heating, undergoes C-H activation with first order kinetics, ΔH‡ = 29 kcal/mol, ΔS‡ = 10 eu, and a kinetic isotope effect of 1.1 at 60 °C. Indoles also initially form coordination complexes through the C2=C3 olefin, but these undergo rearrangement to the corresponding N-bound complexes. The relative rates of initial coordination and rearrangement are affected by excess acid or methyl substitution on indole.

Second- and inverse order pathways in the mechanism of orthopalladation of primary amines

The orthopalladation of benzylamine (baH) by Pd(II) acetate in acetonitrile occurs in two time-resolved steps. A first faster step taking place at 25 °C results in the formation of the N-bound complex [Pd(OAc) 2(baH) 2] and follows a second-order kinetics in the incoming amine according to the rate law k obs= k 1[baH] 2. The second order in baH arises from the successive substitution of the solvent in [Pd(OAc) 2(MeCN) 2]. The latter complex transforms slowly into the orthometalated derivative [Pd(ba)(OAc)] 2 at 60 °C. The initial rate of this step is inversely proportional to the concentration of baH and the following rate law holds: V 0=k m Pd( II) t/[ baH] . This is accounted for in terms of the mechanism involving a reversible dissociation of one baH ligand from [Pd(OAc) 2(baH) 2] followed by the electrophilic CH bond cleavage of N-bound baH by Pd(II).

Mechanism of the interconversions between C- and N-bound transition metal alpha-cyanocarbanions.

The first presentation of the intra- and intermolecular mechanisms of the C-N interconversions of transition metal alpha-cyanocarbanions is described. A pair of N- and C-bound isomers of isonitrile complex Ru+Cp(NCCH-SO2Ph)(PPh3)(CN-t-Bu) (1) and RuCp[CH(CN)SO2Ph](PPh3)(CN-t-Bu) (2) was synthesized for the mechanistic studies on the N-to-C isomerizations. Structural characterization by X-ray diffractions of 1 and 2 indicated their typical zwitterionic and alpha-metalated structures. The kinetic studies on the irreversible isomerization of 1 to 2 in benzene-d6 at 333-348 K were carried out using 1H NMR spectroscopy, affording the first-order rate constants k1 and the activation parameters DeltaH = 107 +/- 2 kJ.mol-1 and DeltaS = -22 +/- 5 J.K-1.mol-1. The almost identical values of k1 were obtained upon similar treatment of 1 with 4 equiv of external ligands such as PPh3, CH3CN, and t-BuNC at 333 K, indicating that the N-to-C isomerization proceeds in an intramolecular manner without dissociation of a ligand. As a model system for the C-to-N isomerization, the irreversible transformation of RuCp[CH(CN)SO2Ph](PPh3)2 (3) to Ru+Cp(NCCH-SO2Ph)(PPh3)2 (4) was investigated under various reaction conditions. The reaction of 3 at room temperature in THF affords the coordination dimers (RRu*,SC*,RRu*,SC*)-{RuCp[CH(CN)SO2Ph](PPh3)}2 (5) stereoselectively, and its distorted mu2-C,N-bound structure was determined by X-ray analysis. The reaction profiles for the isomerization of 3 includes the generation- and temperature-dependent decays of dimeric species 5 and its diastereomer 6, which strongly suggests that the intra- and intermolecular pathways are included in the C-to-N isomerization. The intramolecular process of the C-to-N isomerization of 3 has been confirmed by the kinetic studies on the isomerization of 3 with excess amount of PPh3 in benzene-d6 at 333-348 K which afford the first-order kinetics with the activation parameters of DeltaH = 121 +/- 1 kJ.mol-1 and DeltaS = 42 +/- 4 J.K-1.mol-1. Treatment of 5 with PPh3 in boiling benzene gives rise to the quantitative formation of N-bound complex 4. The controlled kinetic experiments on the cleavage of 5 with PPh3 have concluded that the cleavage of 5 with PPh3 proceeds via simultaneous C-Ru and N-Ru bond scissions, indicating the temperature-dependent participation of intermolecular process in the C-to-N isomerization of 3.

A large C 6D 6 solvent effect on the [formula omitted] NMR shifts of pyrazines coordinated to the M(CO) 5 series (M = W, Mo, Cr)

The pyrazine series of complexes M(CO) 5(pzR n ) with M = W, Mo or Cr; R = CH 3, pzR n = pyrazine ( n = 0), 2-methylpyrazine ( n = 1), 2,3-dimethylpyrazine and 2,6-dimethylpyrazine ( n = 2) and 2,3,6-trimethylpyrazine ( n = 3) have been prepared by UV photolyses in acetone. 1 H NMR spectra of the M(CO) 5(pzR n ) series, the free ligands, pzR n , and their conjugate acids, pzR n H +, were obtained in CDCl 3 and C 6D 6. The pzR n ring protons experience an upfield shift in C 6D 6 relative to other solvents which do not selectively associate with the ring C–H bonds. The shift difference, δ ̂ = δ(CDCl 3−C 6D 6), is attenuated with increasing methylation (∑ n) as a function of both molecular volume and an electronic factor proportional to the Hammett constant σ p for CH 3. Thus, δ ̂ = C o + (0.56 σ p + bV) ∑ n where C o = 0.58, σ p = −0.129, bV = −0.069 for pyrazines. Coordination of the Lewis acids M(CO) 5 (M = W, Mo, Cr) enhances the shift difference δ ̂ relative to the free pyrazine ligands, and increases the molecular volume such that ( bV complex/ bV free L) = 3.6 (W), 2.9 (Mo), and 2.9 (Cr). Compared to H + with a formal +1/2 charge at each ring N, the M(CO) 5 moieties exert a fractional charge influence in the order of 0.46 (W), 0.36 (Mo), and 0.34 (Cr) in the polarization of C–H bonds adjacent to the site of protonation or metallation. This action as a withdrawing group with W(CO) 5 > Mo(CO) 5 > Cr(CO) 5 is in harmony with 13 C NMR evidence on the degree of back-donation to the CO ligands (W > Mo > Cr). 1,4-Metallotropic shifts were not observed for any of the pzR n N-bound complexes (up to ca. 340 K) before ligand dissociation prevents further study. The M(CO) 5 series M = W, Mo, Cr promote downfield shifts of α-ring protons and upfield shifts of β-ring protons, thus behaving more like anionic ML 5 units (M(CN) 5 3−, M(hedta) −; Fe II and Ru II), toward N-heterocyclic ligands. An isomer with M(CO) 5 attached to the π-cloud, in addition to the N-bound complexes, was obtained only for Mo(CO) 5(2,6-Me 2pz). Reasons for the η 2 attachment near the C–3–N–C–4 arc of Mo(CO) 5(2,6-Me 2pz), and the absence of η 2 species for the remainder of the M(CO) 5(pzR n ) series, are suggested. A binuclear complex [W(CO) 5] 2(2,3-Me 2pz) forms more readily than for the unsubstituted pyrazine ligand in spite of larger steric factors. An elevation of the basicity of the terminal N of 2,3-Me 2pz provided by the methyl groups offsets the withdrawing influence of the W(CO) 5 center, raising the nucleophilicity of the W(CO) 5(2,3-Me 2pz) monomer compared to W(CO) 5(pz).

Linkage Isomerization of MSCN(CDOH)2(CDO)BMe to MNCS(CDOH)2(CDO)BMe (M = Tc, Re). Crystal Structures of TcNCS(CDOH)2(CDO)BMe, ReNCS(CDOH)2(CDO)BMe, and ReSCN(CDOH)2(CDO)BMe

The 7-coordinate technetium(III) and rhenium(III) monocapped tris(dioxime) complexes [MX(dioxime)3BR] have been prepared in which the seventh ligand, X, is thiocyanate or hydroxide (M = Tc, Re; R = Me, Et). Both the N- and S-bound thiocyanate linkage isomers are produced during the syntheses, however, the S-bound complex isomerizes to the N-bound analogue. The synthetic routes employed allowed isolation of sufficient quantities of the technetium and rhenium S-bound isomers to be identified; however, full characterization was not possible. The linkage isomerization reaction of TcSCN(CDOH)2(CDO)BMe to TcNCS(CDOH)2(CDO)BMe was monitored in dichloromethane by UV−visible spectrophotometry, with four isosbestic points observed, consistent with the formation of a single product. The N-bound isomers for both Tc and Re, and the S-bound isomer of Re were characterized by single-crystal X-ray diffraction analysis. The two N-bound complexes were found to be isostructural. TcNCS(CDOH)2(CDO)BMe crystallized in the orth...

The binding of small nitrogen-containing heterocycles to neutral and cationic (pentamethylcyclopentadienyl) Ir(III) complexes and the X-ray crystal structure of an unexpected diaquo complex, [(Cp ∗)IrCl(H 2O) 2][PF 6

Cleavage of the chloro-bridged [Cp ∗IrCl 2] 2 dimer by halide abstraction using [NH 4][PF 6] leads to the formation of the [Cp ∗IrCl] + cation which reacts with 2-picoline, 2,6-lutidine, and 2-methylquinoline to form stable N-bound complexes of the type [Cp ∗Ir(L) 2Cl] +. Halide abstraction using AgBF 4 results in complete removal of all halides from the dimer to produce the [(Cp ∗)Ir] 2+ dication. Both N and π-bound complexes can be obtained, depending on the nature of the nitrogen-containing heterocycle. Direct reaction of the [(Cp ∗)IrCl 2] 2 dimer with 2,6-lutidine results in cleavage of the dimer to form a Cp ∗Ir(L)Cl 2 complex containing one N-bound 2,6-lutidine ligand. An iridium complex having the formula [(Cp ∗)IrCl(H 2O) 2][PF 6] was obtained fortuitously during the course of attempts to grow crystals of the cationic 2-picoline complex. The presence of two molecules of water in the coordination sphere of the Ir(III) ion was verified cyrstallographically. The complex crystallizes in the orthorhombic Pnma space group with a = 20.260(7) A ̊ , b = 8.700(2) A ̊ , c = 9.656(3) A ̊ , and V = 1702.0(9) A ̊ 3 . Convergence to conventional R values of R = 0.0417 and R W = 0.0560 with a goodness-of-fit of 0.72 was obtained for 110 variable parameters and 1205 reflections with I > 0 σ( I).

Metal ion biomolecule interactions. VI: Synthesis and spectroscopic properties of methylmercury(II) complexes of xanthosine

The interaction of MeHg(II) with xanthosine (Xanth H 2, 1) in aqueous medium has been found to lead to several methylmercurated complexes depending on the reactant stoichiometries and the pH. The N-bound complexes [(MeHg)(Xanth H)] ( 2), [(MeHg) 2Xanth] ( 3), [(MeHg) 3(Xanth)]NO 3 ( 4), [(MeHg)(Xanth H 2)]NO 3 ( 5), and the N- and C-bound complex [(MeHg) 4(Xanth)]NO 3 ( 6) have thus been prepared. The complexes were characterized by means of 1H and 13C nuclear magnetic resonance and infrared as well as elemental analysis. Formation of the carbon-bound methylmercurated species 6 is in accord with our previous results obtained with inosine and imidazole derivatives, thus substantiating our proposal that activation through electrophilic coordination at N 7 is a requirement for C 8-H abstraction. Correlations are drawn between 2J( 1H- 119Hg) values and p K a as well as 13C chemical shifts.

The synthesis and characterization of mono- and dinuclear cobaloxime selenocyanate and thiocyanate complexes containing phosphines, arsines and stibines

Complexes of the type trans-[Co(DH) 2(L)X] and trans-[(L)Co(DH) 2-X-Co(DH) 2-X] have been synthesized by the reaction between trans-[Co(DH) 2(L)Y] and X in a variety of solvents (DH = monoanion of dimethylglyoxime; L = triphenylstibine, triphenylarsine, triphenylphosphine, tri- n-butylstibine; X = SeCN −, SCN −; Y = Cl −, Br −). The formation of mononuclear complexes resulted in solvents of high dielectric, while dinuclear complexes usually resulted in solvents of low dielectric. All of the terminal SeCN − groups exhibit coordination via the selenium atom, regardless of the nature of L. Several of the terminal SCN − groups in the mononuclear complexes were N-bound. Little or none of the dinuclear complexes could be produced from N-bound complexes, regardless of solvent dielectric. These results are in accord with the dissociative mechanism proposed for the formation of the dinuclear complexes, i.e., dissociation of the neutral L ligands would be expected to be facilitated by solvents of low dielectric and retarded by the lower trans-effect of the N-bound thiocyanate. The bonding modes adopted by the NCS − and NCSe − ions in these complexes are discussed in the context of competing anti-symbiotic and symbiotic effects, respectively.