Ligand Fragments Research Articles

Triple oxidative addition of the diselenide PhSeSePh to the cluster [Os3(CO)10(MeCN)2] to give the osmium(II) compound [Os3(µ-Ph)(µ-PhCO)(µ3-Se)2(CO)8

The diselenide PhSeSePh reacts with [Os3(CO)10(MeCN)2] to give isomers of [Os3(CO)10(µ-SePh)2] which convert thermally to a compound of apparent formula [OS3(CO)9(SePh)2], which was shown by X-ray diffraction to be the triple oxidative addition product [Os3(µ-Ph)(µ-PhCO)(µ3-Se)2(CO)8] in which the diselenide has cleaved into four separate ligand fragments as all three osmium(0) atoms are oxidised to osmium(II) atoms by an overall six-electron oxidation.

ChemInform Abstract: Bridging the Fields of Organometallic and Classical Coordination Chemistry: Localized and Delocalized Bonding in Polynuclear Complexes of (C5R5)(CO)2Mn

Abstract The capability of the title fragment to undergo efficient π back bonding, to exist in two kinetically stable neighboring oxidation states, and to exhibit a relatively small ligand field splitting have allowed the formation of unusual coordination compounds. Several mono- and polynuclear examples with an odd or even electron count and different degrees and direction of electron delocalization between ligand and metal fragment(s) will be discussed.

ChemInform Abstract: Reactivity of the Iridium Nitrosyl Complex (Ir(NO)(paa)(PPh3)2) (PF6)2 (paa: 2‐Pyridinecarbaldehyde Azine) with Nucleophiles: Insertion of NO into a Methinic C‐H Bond; Crystal Structure of (Ir((2‐C5H4N)CH=N‐N=C(NOH)(2‐C5H4N))(Me(EtO)C=NH)(PPh3)2) (PF6)2.

AbstractThe title complex (I) reacts with nitriles such as (II) or Li salts (IV) with intramolecular insertion of the NO group into the methinic C‐H bond of the uncoordinated ligand fragment to give the cyclometalated compounds (III) and (V).

Cleavage of mixed tetrapnicogen trisulphide cage molecules by a cobalt(II)-ligand system. Crystal structure of the solid solution of [(tppme)Co(As 2S)]BF 4 and [(tppme)Co(PAsS)]BF 4

A solid solution of the mixed cage molecules PAs 3S 3 (50%) and P 2As 2S 2 (50%) reacts with Co(BF 4) 2·6H 2O in the presence of 1,1,1-tris(diphenylphosphinomethyl)ethane (tppme) yielding a crystalline material in good yield. The results of single crystal X-ray diffraction study and 31P NMR and analytical data show that the product consists of a solid solution of the [(tppme)Co(As 2S)]BF 4 and [(tppme)Co(PAsS)]BF 4 compounds in ca. 2.3 1 ratio. The crystals are monoclinic, space group P2 1/ n, with a 17.016(7), b 20.584(9), c 13.083(6) Å, β 104.86(6)°, Z = 4. The metal atom is in a six-coordinate environment formed by the tppme P atoms and the atoms of the heterocyclic As 2S or PAsS unit, which is η 3-bonded to the metal. The formation of these units from the solid solution of cage molecules provides insight into the process of cleavage of the latter molecules, assisted by the metalligand fragment.

Redox-rotational phenomena in organometallic chemistry. Unique features of the crystal structure and stereodynamic behaviour of acetylacetonato- and cyclopentadienylrhodium complexes with potentially dipolar two-ring semiquinoid ligands in solution

Replacement of the acac group in the 16-electron (metal oxidation state Rh 1) (acac)Rh(L) complex A (where L is 4-methyl-A-trichloromethyl-1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-2,5-cyclohexadiene) with Cp, under the action of CpTl, gives the 18-electron CpRh(L) complex B, in which, according to 1H and 13C NMR spectral data, the η 4-diene structure of the ligand L rearranges into the dipolar delocalized form of 1-(4,4-dimethyl-2,6-dioxocyclohexylide)-4-methyl-4-trichloromethylbenzolonium with η 5-coordination by the cyclohexadienyl ring to the Rh IIII atom, in a reaction we have termed a ligand redox-exchange. An X-ray diffraction has revealed that the dimedone ring of molecule B in the crystal is twisted around the central polarized ▪ bond by an angle of 28.9°, and 1H and 13C DNMR spectroscopy has shown that the dimedone fragments of the complexes A and B in solution undergo internal rotation about this band. The effects of solvent on this process, including the enhancing effect by the presence of H +, has been studied. The rotation in the case of complex B is faster than that in complex A (“redox-rotation”). The kinetic parameters of the stereodynamics of the process (including the accompanying dimedone ring inversion) have been measured (e.g. in acetone- d 6 Δ H ≠ 18.4 kJ/mol, δ S ≠ −125.8 J mol −1 K −1, Δ G ≠ 298 56.0 kJ/mol). Treatment of B with CF 5COOH gives a stable η 5-cyclohexadienyl complex of Rh III, CpRh(LH)OCOCF 3, with a protonated oxygen in the CO group of the ligand. A periodic system for virtually all the rotatable β-charged organometallic molecules has been proposed, which includes four structural types (and two classes) of models capable of undergoing either isovalent-odd or anisovalent-even internal rotations, depending on the evenness of the ligand fragment coordinated with the metal.

Structural features of the mixed η 2:η 4 coordination of two transition metal atoms in binuclear and trinuclear π-complexes with semiquinoid ligands. Crystal and molecular structure of (η 2-ethylene)(4-methyl-4- exo-n-butyl-1- anti-η 2-methylene-[(2,5-η 4-cyclohexadiene)rhodium2,4- O,O′-pentanedionate])- rhodium-2,4- O,O′-pentanedionate

The crystal and molecular structure of (η 2-ethyelene)(4-methyl-4- exo-n-butyl-1- anti-η 2-methylene-[(2,5-η 4-cyclohexadiene)rhodium-2,4- O,O′-pentanedionate]- rhodium-2,4- O,O-pentanedionate has been determined by an X-ray diffraction study. Comparison of the data obtained with those of a previous study of a mononuclear η 4-coordinated rhodium(I) complex with the 4-methyl-4-trichloromethyl-2,5-cyclohexadiene-1-one ligand reveals that the most prominent feature of the mixed η 2:η 4 coordination in this series consists in the greater deviation (up to 30 and 40°, respectively) of the exo-unsaturated and the saturated geminal fragments of the semiquinoid ligand from the central plane of its cyclohexadiene ring. The shielding of the H A proton of the n-butyl group (-CH AH BC 3H 7) by the η 2-coordinated rhodium atom (as well as by other fragments of the molecule) is different from that of the H B proton of the same group, which accounts for the unusual features in the 1H NMR spectra of this complex. There is a pronounced transannular contact between the rhodium atom and the hydrogen atom of the peripheral CH A bond, practically at the face of the fragment being coordinated (the Rh…H A and Rh…CH A distances are 2.57 and 3.50 Å, respectively, and the Rh…X ACH B angle is 160°). Use of molecular modelling based on the results of the X-ray diffraction study shows that for the recently synthesized trinuclear rhodium(I) complexes with two exo-methylene-2,5-cyclohexadiene ligands, the anti-trans configuration is the most likely.

Bridging the Fields of Organometallic and Classical Coordination Chemistry: Localized and Delocalized Bonding in Polynuclear Complexes of (C5R5)(CO)2Mn

Abstract The capability of the title fragment to undergo efficient π back bonding, to exist in two kinetically stable neighboring oxidation states, and to exhibit a relatively small ligand field splitting have allowed the formation of unusual coordination compounds. Several mono- and polynuclear examples with an odd or even electron count and different degrees and direction of electron delocalization between ligand and metal fragment(s) will be discussed.

Receptor-linked proteolysis of membrane-bound glucagon yields a membrane-associated hormone fragment.

We have studied, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, glucagon-receptor complexes that arise from the incubation of canine hepatic plasma membranes with [[125I]iodo-Tyr10]glucagon. While a 54,000-dalton membrane protein was tentatively identified as the glucagon receptor by chemical cross-linking, an additional component having an apparent molecular weight of 30,000 was routinely identified as also resulting from glucagon-receptor interactions. The latter material, however, was not observed when gels were fixed prior to autoradiography and was not affected by the addition of cross-linking agents to membrane incubations. Subsequent analysis actually identified the material as a fragment of radiolabeled glucagon that contains at least residues 1-13, has no ability of its own to associate with plasma membranes, and remains tightly membrane bound once it has been formed by receptor-mediated processes. Formation of the fragment was inhibited by phenylmethylsulfonyl fluoride, glucagon, and GTP, but not by N-ethylmaleimide or by a variety of glucagon-related peptides. Overall, our results identify a proteolytic modification of glucagon this is linked to the binding of ligand to high affinity GTP-dependent receptors and the existence of a physically distinct state of receptor in which the binding site is tightly filled by a ligand fragment.

Electrochemical reduction of cobalt(II), palladium(II), and copper(II) chelates with ?-hydroxylamine oximes at the dropping mercury electrode

1. The degree of oxidation of the metal ion in the chelates of Co, Pd, and Cu with α-hydroxylamine oximes is 2+ according to electrochemial and EPR data. 2. In the first stage of the electroreduction of the investigated chelates of Co with nioxime and of Cu with α-hydroxylamine oxime the degree of oxidation of the metal ion in the complex changes, while in the case of the other compounds the first electrons are probably accepted by mixed molecular orbitals, to which the ligand fragment makes the largest contribution. 3. The electrode reactions of the Co(II) and Pd(II) chelates with α-hydroxylamine oximes are complicated by the catalytic evolution of hydrogen from solutions containing proton donors.

Iron atom discrimination by carbon during the fragmentation of a ligand around a FeFe bond. Crystal structure [μ-η 3-RCH 2OC(S)SMeFe 2(CO) 5P(OMe) 3

The substituted compound [μ-η 3RCH 2OC(S)SMeFe 2(CO) 5P(OMe) 3] ( 3) (the structure of which was determined by X-ray diffraction), undergoes a ligand fragmentation leading to the known substituted compound [μ-η 2(RCH 2OCS)Fe 2- (CO) 5P(OMe) 3μ-SMe] ( 4). Evidently during the rearrangement the carbon atom of the bridging ligand migrates from one iron atom to the adjacent iron that bears the phosphite ligand.

Correlative video enhanced DIC-LM, HVEM, and low-voltage high-resolution SEM in the study of surface and internal ultrastructure

Previously we have found high voltage electron microscopy (HVEM) and conventional scanning electron microscopy (SEM) to be useful in investigating relationships between surface and subsurface structure. The use of directly conjugated 5nm and 18nm colloidal gold-ligand or colloidal gold-antibody has been useful with HVEM and SEM to determine the location of surface membrane associated glycoproteins relative to surface and internal ultrastructure. Using the platelet as a model system, we have demonstrated cytoskeletal rearrangement coincident with the activation-associated shape change response. Furthermore the activity and location of the receptor (membrane glycoprotein complex IIb IIIa) for fibrinogen and several other RGD sequence-containing “adhesive” proteins is related to the structure and cytoskeletal organization of the platelet. The receptor is present, but unable to bind whole ligand, on discoid and spherical platelets (small ligand fragments and monoclonal antibody or peptide sequences specific for the active site can, however, bind).

Isolation, complementary DNA sequence, and regulation of rat hepatic lauric acid omega-hydroxylase (cytochrome P-450LA omega). Identification of a new cytochrome P-450 gene family.

Lauric acid omega-hydroxylase (cytochrome P-450LA omega) was purified from livers of rats that had been given the hypolipidemic drug clofibrate. Immunoblot analysis with anti-cytochrome P-450LA omega antibody revealed the presence of two clofibrate-induced cytochrome P-450 proteins in both liver and kidney. This antibody was used to screen a lambda gt11 expression library constructed from liver mRNA isolated from rats given clofibrate. The clone, pP-450LA omega, was isolated and found to contain 2062 base pairs with an open reading frame of 509 amino acids (Mr 58,222). The pP-450LA omega cDNA insert was placed in the yeast expression vector pAAH5, and the resultant plasmid expressed a protein of the same size and activity as cytochrome P-450LA omega. The mechanism of the regulation of the cytochrome P-450LA omega gene by hypolipidemic agents was examined. The rate of cytochrome P-450LA omega gene transcription was increased as early as 1 h after administration of clofibrate, and this increase was followed by an elevation of cytochrome P-450LA omega mRNA, immunochemically detectable protein, and cytochrome P-450LA omega activity. In contrast, no increase in transcriptional activity was detected after clofibrate administration for the cytochrome P-450PCN or P-450b/e genes. The cytochrome P-450LA omega has several structural features in common with other cytochrome P-450s, including a conserved cysteine-containing heme-binding fifth ligand fragment and a hydrophobic amino terminus. Overall, cytochrome P-450LA omega shared less than 35% cDNA nucleotide and amino acid similarity with cytochromes P-450c, P-450d, P-450e, and P-450PCN, indicating that it is a member of a new cytochrome P-450 gene family. Southern blot analysis indicates that this family contains two or three genes.

σ-Alkynyl complexes of manganese(I) as η2-bonding ligands for Group 1B metal–ligand fragments. X-Ray crystal structure of [Mn2Cu(µ-CCBut)2(CO)6(dppe)2]PF6·0.5CH2Cl2

Several compounds of the types [MnML(µ-CCR)(CO)3(dppe)]n+{ML = CuCl (n= 0), Au(C6F5)(n= 0), or M[P(C6H4Me-2)3](n= 1; M = Cu, Ag, or Au); R = CH2OMe, But, or Ph; dppe = Ph2PCH2CH2PPh2} and [Mn2M(µ-CCR)2(CO)6(dppe)2]+ have been prepared from the alkynyl complexes fac-[Mn(CCR)(CO)3(dppe)] and the appropriate reagents to generate the metal–ligand fragments ML. The salt [Mn2Cu(µ-CCBut)2(CO)6(dppe)2] PF6·0.5CH2Cl2 has been characterized by X-ray diffraction. Several equilibria in solution involving the cationic species with free σ-alkynyl complex and P(C6H4Me-2)3 have been observed and their reactions with Cl– have been examined.

ChemInform Abstract: Complexes of the Platinum Metals. Part 30. Fragmentation Reactions of Rhodium and Iridium Trichloro‐ and Tribromo‐acetates.

AbstractDie Nitrosyl‐Komplexe (I) reagieren mit Trichloressigsäure, der Rh‐Komplex auch mit Tribromessigsäure, in Aceton bei Raumtemp. unter Bildung der Dihalogeno‐Komplexe (II) in ausgezeichneten Ausbeuten.

Electrophilic attack on unsubstituted phosphorus units in transition metal compounds: synthesis and characterization of the methyltriphosphirene unit stabilized by transition metal–ligand fragments

The reaction of trimethyloxonium tetrafluoroborate or methyl trifluoromethanesulphonate with the [(triphos)M(P3)] complexes [triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane; M = Co, Rh, or lr] in which the triphosphirene ring is η3-bonded to the metal, yields the compounds [(triphos)M(MeP3)]Y (Y = BF4, CF3SO3) which contain the novel methyltriphosphirene unit η3-linked to the M(triphos) moiety.

Complexes of the platinum metals. Part 30. Fragmentation reactions of rhodium and iridium trichloro-and tribromo-acetates

The nitrosyl complexes [M(NO)(PPh3)3](M = Rh or Ir) react readily with trichloroacetic acid in acetone solution at ambient temperature to afford the dichloro-complexes [MCl2(NO)(PPh3)2] in excellent yield. The rhodium-based reaction performed at ca. 0 °C and quickly worked-up affords the carboxylate complex [Rh(O2CCCl3)2(NO)(PPh3)2] which is stable in pure acetone, but rapidly converts to the dichloride [RhCl2(NO)(PPh3)2] when free trichloroacetic acid and triphenylphosphine are introduced to the solution. The complexes [MCl(CO)(PPh3)2], [MH(CO)(PPh3)3], mer-[IrH3(PPh3)3], and [RhCl(PPh3)3] also react with trichloroacetic acid to form trichloroacetates which undergo similar ligand fragmentation reactions. Reaction pathways involving formation of CCl3–, Cl–, :CCl2, and CO2 fragments are outlined; hydrolysis of dichlorocarbene affords carbonyl ligands. Similar reactions have been observed with tribromoacetic acid.

The crystal and molecular structure of the copper(II) bis(dialyldithiocarbamate) complex

The crystal structure of {Cu[S2CN(C3H5)2]2}2 was solved by the single crystal method of X-ray structural analysis. The substance crystallized as a dimer in the triclinic system with space group of PI and latice parameters a = 1.0161 (4), b =0.9294(4), c = 1.0518(3) nm, α = 77.46(3), β = 77.10(3), γ = 89.02(3)°. The structure was refined by the least squares method to a final value of R = 4.9% using all the 1 713 observed reflections. The crystal structure consists of dimeric molecules, where each pair of centrosymmetrically dependent Cu atoms lies at a distance of 0.3742 nm. The coordination polyhedron of the Cu atom is a tetragonal pyramid, where the four sulphur atoms lie at distances of Cu-S1 0.2314, Cu-S2 0.2309, Cu-S3 0.2324, Cu-S4 0.2328 and are approximately in a place from which the Cu atoms lies at a distance of 0.026 nm. The fifth, longer bond, Cu-S'4 0.2888 nm forms the apex of the tetragonal pyramid. In the streochemistry of the dithiocarbamate ligands of the studied substances there are no marked differences in the bond lengths and corresponding angles compared with the values for the solvent structures of the other dialkyl-dtc complexes. The lengths of the sulphur-carbon bonds lie in the range from 0.170 to 0.173 nm and both lengths of the C(sp)2 - N(sp2) bonds equal to 0.134 and 0.133 nm indicate marked double bond character of the C-N bond. The S2CN ligand fragment is planar. In the alyl part of the ligand, the N-C bond lengths lie in the range 0.147-0.149 nm, the average C-C bond length is 0.149 nm and C=C bond length is 0.132 nm.

Mass spectral analysis of schiff bases derived from 1,1,1‐trifluoro‐2,4‐pentanedione and 1,2‐diaminoethane, 1,3‐diaminopropane, 1,4‐diaminobutane and 1,2‐diaminobenzene and their copper(II) and nickel(II) complexes

AbstractThe mass spectra of the title compounds have been determined and special attention has been given to metastable transitions and ion composition assignments. The interpretation of the mass spectra of N,N'‐ethylenebis(5,5,5‐trifluoro‐4‐oxopentan‐2‐imine) and its metal complexes is relatively straightforward. Bond breaking α and β (with H transfers) to nitrogen in the ethylenediamine portion of the molecule overwhemingly dominates the spectra. Changing the diamine bridge adds new features to the spectra in both expected and unexpected ways. Ligand fragmentation changes little when complexed to nickel as compared to hydrogen. Chelation of Cu(II) on the other hand leads to new fragmentation channels which increase in importance as electron availability in the diamine bridge increases. These new reactions are attributed to a reduction of the copper centre in the molecule.

Studies of lambert's reaction: The formation of [Mn 2(CO) 8(μ-AsR 2) 2] complexes from tertiary arsines and [Mn 2(CO) 10] at high temperatures

Although very bulky ligands e.g.( o-MeC 6H 4) 3E or (μ-C 10H 7) 3E (E = P or As) are inert, the normal photochemical or thermal reaction of tertiary phosphines or arsines, L, with [Mn 2(CO) 10] is CO substitution with the formation of [Mn 2(CO) 8(L) 2] derivatives (I). At elevated temperatures some triarylarsines, R 3As, undergo Lambert's reaction with ligand fragmentation to give [Mn 2(CO) 8(μ-AsR 2) 2] complexes (II) (R = Ph, p-MeOC 6H 4, p-FC 6H 4, or p-CIC 6H 4) even though, in the absence of [Mn 2(CO) 10] R 3As are stable under the same conditions. Exceptional behaviour is exhibited by ( p-Me 2NC 6H 4) 3- As which forms a product of type I; by some HN(C 6H 4) 2AsR which give a product of type II as a result of loss of the non-aryl groups R = PhCH 2, cyclo-C 6H 11, or MeO; and by Ph(α-C 10H 7 2P which is the only phosphine to form a product of type II, albeit in trace amounts only. The thermal decomposition of a n-butanol solution of [Mn 2(CO) 8(AsPh 3) 2] in a sealed tube gives C 6H 6 and [Mn 2(CO) 8(α-AsPh 2) 2], whilst in an open system in the presence of various tertiary phosphines, L, [Mn(H)(CO) 3(L) 2] are obtained. It is suggested that Lambert's reaction is a thermal fragmentation of [Mn(CO) 4(AsR 3] * radicals, the first to be recognised. They lose the radical R * which abstracts hydrogen from the solvent. The resulting [Mn(CO) 4(AsR 2)] moiety dimerises to [Mn 2(CO) 8-(α-AsR 2) 2]. the reaction is facilitated by the stability of the departing radical (e.g. PhCH 2 or MeO) and, as the crowding about As is relieved, by its size (e.g. Ph, cyclo-C 6H 11, o-MeC 6H 4, or α-C 10H 7). In general, phosphine-substituted radicals [Mn(CO) 4(PR) 3] * do not undergo this decomposition, probably because the PC bonds are much stronger than AsC.

High-spin, five-coordinate complexes of cobalt(II)with 2-substituted imidazoles

A series of new five-coordinate complexes of cobalt(II) of the type, dihalotris-(2-substituted imidazole)cobalt(II), of formulae Co(2-MeIm)3Cl2, Co(2-EtIm)3X2 (X = Cl, Br, I) and Co(2-i-PrIm)3I2, were synthesized and characterized by elemental analyses, electrolytic conductivity, diffuse reflectance as well as by solution spectra, i.r. and far i.r. spectra, x-ray powder pattern and magnetic susceptibility measurements. The mass spectra of the complexes did not exhibit peaks due to the molecular ions, but only showed the ligand fragmentation patterns. The t.g.a. and d.t.a. measurements were also carried out for the complexes.