Cr Ligand Research Articles

Novel Methods for the Formation of Free Carbyne Radicals in Solution.

Carbyne free radicals (RC) are usually associated with high-energy processes, thus research on their preparation, chemical reactivity, and prevalence under mild conditions is scarce. Recently, it was reported that metallo-complexes containing CR ligands may undergo spontaneous degradation in aqueous solutions to produce free RC radicals. These highly reactive species may react with each other to form the corresponding alkyne and other products. The reaction of 1,1,1 halo-carbo-hydrates with Cr(II) ions also forms RC radicals under mild conditions. Herein, we report two additional synthetic routes that produce free RC radicals under mild conditions. First, the reaction between metallic zinc and acetic anhydride produces 2-butyne and several other C2, C3, and C4 compounds. Isotopic-labeling experiments indicate that the formation of 2-butyne results from an inter-molecular reaction in which two RC moieties from two acetic anhydride molecules combine in solution. In addition, the degradation of the tri-molybdenum complex [Mo3(H3CC≡CCH3)(OAc)(H2O)2Br7], in which a 2-butyne ligand is coordinated to the Mo3 framework in a μ3-η2 (⊥) binding mode in aqueous solution produces several C2, C3, C4 and C5 molecules. This indicates the formation of free CH3C radicals by a homolytic cleavage of the carbon-carbon triple bond.

Photodynamics of the Molecular Ruby [Cr(ddpd)2]3.

The introduction of strong-field ligands can enable luminescence in first-row transition-metal complexes. In this way, earth-abundant near-infrared emitters can be obtained using early 3d metals. A prime example is the molecular ruby [Cr(ddpd)2]3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine) that can achieve high phosphorescence quantum yields at room temperature in aqueous solution. To understand these remarkable properties, here, we simulate its photodynamics in water using trajectory surface hopping on linear vibronic coupling potentials parametrized from multiconfigurational CASSCF/CASPT2 calculations. We find that after excitation to the second absorption band, a relaxation cascade through metal-centered states occurs. After an initial back-and-forth intersystem crossing with higher-lying doublet states, the complex relaxes through a manifold of quartet metal-centered states to the low-lying doublet metal-centered states which are responsible for the experimentally observed emission. These electronic processes are driven by an elongation of the Cr-ligand bond lengths as well as the twisting motion of the trans-coordinated pyridine units in the ddpd ligands. The low-lying doublet states are reached within 1-2 ps and are close in geometry to the doublet minima, thus explaining the high phosphorescence quantum yield of the molecular ruby [Cr(ddpd)2]3+.

Assessment of the mode of action for hexavalent chromium-induced lung cancer following inhalation exposures.

Inhalation of hexavalent chromium [Cr(VI)] is associated with increased lung cancer risk among workers in several industries, most notably chromate production workers exposed to high concentrations of Cr(VI) (≥100 μg/m(3)), for which clear exposure-response relationships and respiratory irritation and tissue damage have been reported. Data from this industry are used to assess lung cancer risk associated with environmental and current occupational exposures, occurring at concentrations that are significantly lower. There is considerable uncertainty in the low dose extrapolation of historical occupational epidemiology data to assess risk at current exposures because no published or well recognized mode of action (MOA) for Cr(VI)-induced lung tumors exists. We conducted a MOA analysis for Cr(VI)-induced lung cancer evaluating toxicokinetic and toxicological data in humans and rodents and mechanistic data to assess plausibility, dose-response, and temporal concordance for potential MOAs. Toxicokinetic data support that extracellular reduction of Cr(VI), which limits intracellular absorption of Cr(VI) and Cr(VI)-induced toxicity, can be overwhelmed at high exposure levels. In vivo genotoxicity and mutagenicity data are mostly negative and do not support a mutagenic MOA. Further, both chronic bioassays and the epidemiologic literature support that lung cancer occurs at exposures that cause tissue damage. Based on this MOA analysis, the overall weight of evidence supports a MOA involving deposition and accumulation of particulate chromium in the bifurcations of the lung resulting in exceedance of clearance mechanisms and cellular absorption of Cr(VI). Once inside the cell, reduction of Cr(VI) results in oxidative stress and the formation of Cr ligands. Subsequent protein and DNA damage lead to tissue irritation, inflammation, and cytotoxicity. These effects, concomitant with increased cell proliferation, result in changes to DNA sequences and/or methylation status that can lead to tumorigenesis. This MOA supports the use of non-linear approaches when extrapolating lung cancer risk occurring at high concentration occupational exposures to environmentally-relevant exposures.

Luminescent Cyclometalated N-Heterocyclic Carbene-Containing Organogold(III) Complexes: Synthesis, Characterization, Electrochemistry, and Photophysical Studies

A new class of luminescent mononuclear and dinuclear N-heterocyclic carbene-containing gold(III) complexes has been synthesized and characterized. The X-ray crystal structures of most of the complexes have also been determined. Electrochemical studies reveal a ligand-centered reduction originated from the RC--N--CR moieties with no oxidation waves. Interestingly, one of the dinuclear complexes exhibited two distinct reduction couples instead of one with the first reduction occurring at less cathodic potential, probably related to the splitting of the pi* orbital-based LUMO resulting from intramolecular pi-pi interaction. The electronic absorption and luminescence behaviors of the complexes have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the intraligand pi-pi* transition, with mixing of a charge transfer character from the aryl ring to the pyridine moiety of the cyclometalating RC--N--CR ligand. The low-energy emission bands are ascribed to origins mainly derived from the intraligand pi-pi* states with an aryl to pyridine charge transfer character of the cyclometalating RC--N--CR ligand.

Do Metal–Metal Multiply‐Bonded “Ligands” Have a trans Influence? Structural and Magnetic Comparisons of Heterometallic CrCr···Co and MoMo···Co Interactions

AbstractReported here are two new compounds containing either a CrCr···Co [1, CrCrCo(dpa)4Cl2, dpa = 2,2′‐dipyridylamide] or a MoMo···Co [2, MoMoCo(dpa)4Cl2] framework both having a multiply‐bonded unit (CrCr in 1, MoMo in 2) in close proximity to the Co2+ ion and trans to a Co–Cl bond. Variable temperature magnetic susceptibility measurements reveal 1 to have a temperature‐dependent spin equilibrium between a low‐spin (S = 1/2) and high‐spin (S = 3/2) state, whereas the Co2+ ion in 2 exists solely in its high‐spin state. The crystal structures of 1 and 2 were determined. Variable temperature crystallographic data of 1 at 100 K and at room temperature reveal that the spin‐transition affects not only the Co–ligand bond lengths but also the terminal Cr–ligand bond lengths. Whereas the Cr···Co distance becomes shorter by 0.13 Å in the low‐spin form, the Co–Cldistance becomes longer by 0.2 Å. These observations,along with the crystal structure of 2, suggest that the multiply‐bonded MM group has a trans influence on the Co2+ ion.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A Class of Luminescent Cyclometalated Alkynylgold(III) Complexes: Synthesis, Characterization, and Electrochemical, Photophysical, and Computational Studies of [Au(C∧N∧C)(C⋮CR)] (C∧N∧C = κ3C,N,C Bis-cyclometalated 2,6-Diphenylpyridyl)

A new class of luminescent cyclometalated alkynylgold(III) complexes, [Au(RC=N(R')=CR)(CCR' ')], i.e., [Au(C=N=C)(C triple bond CR'')] (HC=N=CH = 2,6-diphenylpyridine) R' ' = C6H5 1, C6H4-Cl-p 2, C6H4-NO2-p 3, C6H4-OCH3-p 4, C6H4-NH2-p 5, C6H4-C6H13-p 6, C6H13 7, [Au(tBuC=N=CtBu)(C triple bond CC6H5)] 8 (HtBuC=N=CtBuH = 2,6-bis(4-tert-butylphenyl)pyridine), and [Au(C=NTol=C)(CCC6H4-C6H13-p)] 9 (HC=NTol=CH = 2,6-diphenyl-4-p-tolylpyridine), have been synthesized and characterized. The X-ray crystal structures of most of the complexes have also been determined. Electrochemical studies show that, in general, the first oxidation wave is an alkynyl ligand-centered oxidation, while the first reduction couple is ascribed to a ligand-centered reduction of the cyclometalated ligand with the exception of 3 in which the first reduction couple is assigned as an alkynyl ligand-centered reduction. Their electronic absorption and luminescence behaviors have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the pi-pi* intraligand (IL) transition of the cyclometalated RC=N(R')=CR ligand with some mixing of a [pi(C triple bond CR'') --> pi*(RC=N(R')=CR)] ligand-to-ligand charge transfer (LLCT) character. The low-energy emission bands of all the complexes, with the exception of 5, are ascribed to origins mainly derived from the pi-pi* IL transition of the cyclometalated RC=N(R')=CR ligand. In the case of 5 that contains an electron-rich amino substituent on the alkynyl ligand, the low-energy emission band was found to show an obvious shift to the red. A change in the origin of emission is evident, and the emission of 5 is tentatively ascribed to a [pi(CCC6H4NH2) --> pi*(C=N=C)] LLCT excited-state origin. DFT and TDDFT computational studies have been performed to verify and elucidate the results of the electrochemical and photophysical studies.

Antibody‐independent B cell‐intrinsic and ‐extrinsic roles for CD21/35

Mice lacking C3, C4 or complement receptor 1/2 (Cr) have defective germinal centers (GC). The requirement for C4 implicates complement fixation by immune complexes (IC) via the classical pathway. Yet, transgenic (Tg) mice that lack circulating antibody but still express membrane IgM (mIgM) have normal GC responses. We showed previously that cross-linking mIgM leads to the deposition of C3 on the B cell surface and that disruption of this pathway diminishes GC responses. Here, we investigate the role of Cr in this process by generating mIgM-Tg mice that lack Cr and serum Ig. These mIgM/Cr-/- mice have smaller, transient GC, with incomplete B cell receptor down-regulation and peanut agglutinin up-regulation, compared to mIgM/Crwt counterparts. BM chimera experiments showed that Cr on B cells is required for normal GC responses. These results establish that Cr ligands generated at the B cell surface are sufficient for normal GC responses and function by signaling Cr on B cells. Unexpectedly, chimera experiments also showed a critical role for Cr on follicular dendritic cells (FDC), even in the absence of IC, indicating novel functions for FDC-expressed Cr beyond the capture of C3-coated IC.

Contrasting Thermal and Photochemical Intramolecular Coupling in Alkynylphosphine Platinum(II) Complexes

The thermal and photochemical transformations of a series of alkynylphosphine platinum(II) complexes are described. Compounds [Pt](PPh2C⋮CR)2 ([Pt] = cis-Pt(C6F5)2, R = Ph, Tol) rearrange thermally to generate naphthalene-based diphenylphosphine complexes (1a, 1b) containing the fragment {C10H5-1-Ph-2,3-κPP‘(PPh2)2} or {7-CH3-C10H4-1-Tol-2,3-κPP‘(PPh2)2}, formed by intramolecular coupling of two adjacent PPh2C⋮CR ligands. By contrast, irradiation of these alkynylphosphine derivatives in toluene results in the formation of a mixture of 1a/1b and the 1,2-diphosphino-alk-1-ene complexes [Pt]{PPh2C(Ph)C(R)PPh(C⋮CR)} (R = Ph, 2a; Tol, 2b) in a final ratio of 60:40. However, irradiation of the mixed alkynylphosphine derivatives [Pt](PPh2C⋮CR)(PPh2C⋮Ct-Bu) gives selectively [Pt]{Ph2PC(Ph)C(R)PPh(C⋮Ct-Bu)} (R = Ph, 3a; Tol, 3b; t-Bu, 3c) as result of a P−C(Ph) activation of a tert-butylalkynylphosphine, PPh2C⋮Ct-Bu. Under thermolysis, bis(tert-butyl)alkynyl derivatives produce no evidence of any cyclization produ...

Expression of mannose receptor and ligands for its cysteine-rich domain in venous sinuses of human spleen

The mannose receptor (MR) is a type I membrane molecule with two lectin activities. Mannose recognition takes place through the C-type lectin-like carbohydrate recognition domains, while recognition of sulphated glycans is mediated by the cysteine-rich domain (CR). In murine spleen CR ligands are present in a subpopulation of macrophages (Mφ) placed in the marginal zone whereas MR-expressing cells consisting of Mφ and nonvascular endothelia are located in the red pulp. No colocalisation of MR with CR ligands has been observed in murine tissues. In this manuscript we describe the distribution of MR and CR ligands in human spleen. In this organ we have detected a perfect colocalisation of MR with CR ligands in Lyve-1+ cells lining venous sinuses. These cells form a physical barrier for blood cells as they need to migrate through the sinuses in order to exit the splenic parenchyma and, in this way, contribute to the unique filtration function of this organ. Furthermore, unlike murine spleen, CD68+ red pulp Mφ lack MR expression. Our results suggest an unexpected contribution of MR to splenic function through the recognition of sulphated ligands that could influence the filtering capability of this organ.

Development of a specific system for targeting protein to metallophilic macrophages.

The cysteine-rich domain (CR) of the mannose receptor binds sulfated glycoprotein CR ligand (CRL) expressed by subpopulations of myeloid cells in secondary lymphoid organs (CRL(+) cells). In naïve mice, these CRL(+) cells, metallophilic macrophages (M) in spleen and subcapsular sinus M in lymph nodes, are located strategically for antigen capture and are adjacent to B cell follicles, but their role in the immune response is unknown. We have exploited the lectin activity of CR to develop a highly specific system for targeting protein to CRL(+) M. We demonstrate that the sulfated carbohydrates recognized by CR are exposed to the extracellular milieu and mediate highly specific targeting of CR-containing proteins. This model will allow the dissection of the role of metallophilic M in an immune response in vivo.

Uptake and reduction of Cr(VI) to Cr(III) by mesquite (Prosopis spp.): chromate-plant interaction in hydroponics and solid media studied using XAS.

Chromium (Cr) is a well-established carcinogen that is a contaminant at half of the EPA Superfund sites in the United States. Two separate studies were performed to investigate the possibility that mesquite (Prosopis spp.), which is an indigenous desert plant species, can remove Cr from the environment via active transport systems to the aerial portions of the plant. The first study was performed by growing mesquite on solid media (agar) at Cr(VI) concentrations of 75 and 125 ppm. The accumulation found in the leaves under the present conditions indicated that mesquite could be classified as a hyperaccumulator of chromium. The second study was conducted to investigate the differences between the type of Cr ligand involved in Cr uptake with agar and hydroponic cultures. We used X-ray absorption spectroscopy (XAS) to determine the mechanisms involved in the uptake and binding of Cr(VI) in live mesquite tissue. The XAS results for this study showed that some of the supplied Cr(VI) was uptaken by the mesquite roots; however, the data analyses of the plant tissues demonstrated that it was fully reduced to Cr(III) in the leaf tissues. Experiments are currently being performed to evaluate the behavior of the Mesquite plant using lower Cr concentrations.

Comparative behaviours of phospha-alkynes and alkynes at electron-rich phosphinic metal centres

The coordination chemistry of phospha-alkynes (PCR) at electron-rich and sterically hindered {M(diphosphine) 2} [M=Mo(0), W(0), Re(I), Fe(II)] centres, in which the PCR ligand adopts the very rare and electronically unfavoured end-on (η 1) coordination mode, as well as at the less sterically demanding {Rh(triphos)} + site, are described and compared with those exhibited by alkynes at the same metal centres. At the former sites, the η 1-PCR ligand behaves as a weak π-acceptor and is typically activated to α-nucleophilic addition to give phospha-alkene, phosphine and phosphinidene oxide products, although activation towards electrophilic (protic) addition has also been recognized upon hydrometalation (insertion into a metal–H bond to give a phospha-alkenyl species). At the same metal sites, the η 2-coordination of alkynes is unfavoured (on both electronic and steric grounds) and these substrates undergo rearrangements (H-shifts) towards η 1-bonded alkynyl and vinylidene derivatives, or towards a less sterically demanding η 2-allene species, and these products are activated towards β-electrophilic addition (protonation) on account of their π-electron withdrawal ability which contrasts with the behaviour of the η 1-P-ligated phospha-alkyne and derivatives. The alkyne and phospha-alkyne insertions into an Fe–H bond are also compared, as well as their cycloaddition reactions (cyclotri- and cyclodimerisations, respectively) at the {Rh(triphos)} + centre which exhibits open coordination sites for η 2-ligation. The above reactions are discussed in terms of both stereochemical and electronic effects.

Solvatochromism in ligand-field absorption bands and solvent-dependent deuteron nuclear magnetic resonance spectra of [CrF 3(tri(2-pyridyl)amine)]: implication of trigonal splitting for the first spin-allowed d–d band and evaluation of π bonding peculiarities

The correlation of the remarkable solvatochromism and solvent-dependent deuteron NMR spectra of fac-[CrF 3(tpa)] (tpa=tri(2-pyridyl)amine) with the acceptor numbers ( A Ns) for solvents demonstrated the sensitivity of the Crligand bond length or the angular overlap model (AOM) parameters to the embedding solvent, and also suggested a significant trigonal splitting in the first 4T 2← 4A 2 ligand-field band. The inverse correlation of the 2H NMR shifts for 4- and 3-deuterons of the pyridyl in the tpa complex with the solvent electron A N gives direct evidence for the π interaction with the pyridyl groups in the tpa ligand that varies from strong acceptor to weak donor behavior depending upon the solvent.

Stability and Ligand Exchange Reactions of Chromium(IV) Carboxylato Complexes in Aqueous Solutions1

The quantitative in situ generation of a range of Cr(IV) carboxylato complexes in aqueous media has been achieved by a combination of the newly-developed Cr(IV) ligand exchange chemistry together with the existing methods of reduction of Cr(VI) or Cr(V) complexes. The reactions Cr(VI) + As(III) and Cr(V) + V(IV) in buffer solutions of the corresponding ligands were used for generation of Cr(IV) complexes with 2-hydroxy-2-methylbutanoate (hmba), 2-ethyl-2-hydroxybutanoate (ehba), and (−)-quinate (qa) ligands. Addition of oxalate (ox), malonate (mal), or 2-picolinate (pic) to the generated Cr(IV) complexes led to the quantitative formation of the new Cr(IV) species. Spectral and chemical properties of Cr(IV) complexes with the mentioned ligands have been described for the first time (except for the known Cr(IV)−ehba complexes). In excess ligand, Cr(IV) appears to exist mainly as bis-chelated oxo complexes, on the basis of UV−visible and CD spectroscopic data. All of the studied Cr(IV) complexes exhibit bell...

Scanning tunneling microscopy of Cr(III) chemisorbed on α-Fe 2O 3 (001) surfaces from aqueous solution: Direct observation of surface mobility and clustering

We report scanning tunneling microscope (STM) observations of Cr(III) monomer and oligomer complexes chemisorbed to α-Fe 2O 3 (001) surfaces from aqueous solution. Monomers appear to prefer a particular surface site, one for each surface unit cell. This site is probably associated with an octahedral vacancy in the underlying oxide structure. Monomers were mobile; although “hops” from one site to a neighboring site were observed, most commonly adsorbates appeared upon or disappeared from particular surface sites. This probably represents adsorption-desorption coupled with outer-sphere surface diffusion. Activation energies for Cr(III) monomer motion estimated from the observed lifetimes of Cr complexes in surface sites are lower than those for ligand substitution in aqueous Cr(III) ions or its hydrolysis products. This may reflect an artifactual influence of the STM upon adsorbed Cr(III), but could also suggest that Cr(III) ligand exchange is not rate-limiting for Cr(III) adsorption-desorption or surface diffusion. A dimerization event was recorded, and larger surface clusters were observed. Clusters were immobile within the time-scale of our observations.

Interactions between murine B lymphocyte surface membrane molecules. Loaded but not free receptors for complement and the Fc portion of IgG co-cap independently with cross-linked surface Ig.

We have investigated the possible physical interactions between CR, receptors for the Fc gamma R and surface Ig (sIg) on the surface membrane of murine B lymphocytes. We used the rat mAb to murine CR, 8C12, and 7G6, as CR ligands, and soluble Ag-antibody complexes as FcR ligands; and F(ab')2 fragments of rabbit antibodies specific for mouse IgM and IgD as sIg ligands. We have found that: 1) sIg, CR, and Fc gamma R are not directly linked, because capping of any one did not affect the expression of the others; 2) the mAb 8C12 and 7G6 failed by themselves to cross-link CR; 3) soluble Ag-antibody complexes crosslinked some, Fc gamma R on a minority of Fc gamma R+ lymphocytes; 4) once loaded with anti-CR mAb, CR co-capped with sIg when sIg was cross-linked; 5) once loaded with Ag-antibody complexes, Fc gamma R also co-capped with sIg when sIg was sIg was cross-linked; 6) loading of Fc gamma R did not affect the co-capping of surface CR with cross-linked sIg and conversely, loading of CR did not affect the co-capping of Fc gamma R with cross-linked sIg; only loaded CR or Fc gamma R co-capped with sIg regardless of the status of the other surface molecule; 7) neither loaded nor free CR co-capped with cross-linked Fc gamma R, and neither loaded nor free Fc gamma R co-capped with cross-linked CR. These results demonstrate that both Fc gamma R and CR independently become associated with sIg when either receptor is loaded and sIg is cross-linked.

Stability and the gas chromatography of alkyl-substituted η 6-benzenetricarbonylchromium(O) complexes

Thermoanalytical and gas chromatographic data are reported for benzenetricarbonylchromium(0) and nine of its methyl- and t-butyl-substituted derivatives. The distinct trends found for the volatilities, thermal stabilities, air stabilities and gas chromatographic retentions with increasing alkyl substitution are interpreted in terms of Cr ligand bond energies and polarity and steric effects. Satisfactory chromatographic behaviour was observed only when highly inert columns were used, and when the solutions were deaerated and stored in the dark.

Chemistry of di- and tri-metal complexes with bridging carbene or carbyne ligands. Part 31. Synthesis and crystal structures of the compounds [AuW(µ-CHR)(CO)2(PPh3)(η-C5H5)]·CH2Cl2and [AuPtW(µ3-CR)(CO)2(PMe3)3(η-C5H5)][PF6](R = C6H4Me-4)

Treatment of a mixture of the salts [N(PPh3)2][W2{µ-CH(C6H4Me-4)}(CO)7(η-C5H5)] and TIPF6 in tetrahydrofuran (thf) with [AuCl(PPh3)] affords the bridged p-tolylmethylidene complex [AuW{µ-CH(C6H4Me-4)}(CO)2(PPh3)(η-C5H5)], the structure of which has been established by a single-crystal X-ray diffraction study. As expected, the Au–W bond [2.729(1)A] is spanned by the CH(C6H4Me-4) group. The µ-C–W separation is relatively short [2.058(14)A] whereas the µ-C–Au distance is relatively long [2.268(14)A]. To account for these features a three-centre two-electron µ-C–Au–W interaction is postulated. Addition of [Au(PR′3)]+[PR′3= PPh3 or P(cyclo-C6H11)3] fragments, generated in situ from [AuCl(PR′3)] and TIPF6 in dichloromethane, to the alkylidynetungsten compounds [W(CR)(CO)2(η-C5H5)](R = C6H4Me-4 or Me) affords the salts [AuW(µ-CR)(CO)2(PR′3)(η-C5H5)][PF6][R = C6H4Me-4, PR′3= PPh3or P(C6H11)3; R = Me, PR′3= PPh3]. N.m.r. studies (1H, 31P-{1H}, and 13C-{1H}) on solutions of these salts, however, reveal that the cations dissociate, affording equilibrium mixtures containing the species [Au{W(CR)(CO)2(η-C5H5)}2]+ and [Au(PR′3)2]+. The trimetal compound [AuPtW(µ3-CR)(CO)2(PMe3)3(η-C5H5)][PF6](R = C6H4Me-4) has been prepared by two routes: from the reaction between [Au{W(CR)(CO)2(η-C5H5)}2][PF6] and [Pt(C2H4)(PMe3)2], and by addition of [Au(thf)(PMe3)][PF6] to [PtW(µ-CR)(CO)2(PMe3)2-(η-C5H5)]. The structure of [AuPtW(µ3-CR)(CO)2(PMe3)3(η-C5H5)][PF6] has been established by X-ray crystallography. A triangular array of metal atoms [Au–W 2.801 (2), Au–Pt 2.956(2), and Pt–W 2.770(2)A] is asymmetrically bridged by the CR ligand [µ3-C–Au 2.21 (4), µ3-C–Pt 1.97(4), and µ3-C–W 2.01 (4)A]. The Au–Pt separation suggests that there is little or no direct metal–metal bonding between these two metal atoms. The tungsten atom carries the cyclopentadienyl ligand and two CO groups, but the latter are appreciably non-linear. The gold and platinum atoms are ligated by one and two PMe3 groups, respectively.

The heats of iodination of chromium tricarbonyl complexes of arenes and cycloheptatriene

Zusammenfassung The heats of iodination of chromium tricarbonyl complexes of benzene, toluene, mesitylene, and cycloheptatriene have been measured by solution calorimetry in tetrahydrofuran at 25°C. The order of the Cr—ligand strength is: mesitylene > cycloheptatriene > toluene > benzene. This order differs from that determined by high temperature decomposition and iodination and is confirmed by chemical equilibration studies.

Group IB organometallic chemistry : XIX. Synthesis and characterization of mixed-organocopper cluster compounds R 4R′ 2Cu 6 containing aryl and acetylide ligands

Mixed-organocopper cluster compounds Ar 4Cu 6(CCR) 2 (Ar = 2-Me 2NC 6H 4, R = phenyl, 4-tolyl, 2,4-xylyl or mesityl) have been prepared in high yield by the ligand-substitution reaction of Ar 4Cu 6Br 2 with two equivalents of LiCCR. Ar 4Cu 6(CCC 6H 4CH 3-4) 2 has also been prepared via the aryl—arylacetylide exchange reaction of Ar n Cu n with HCCC 6H 4CH 3-4 as well as via the reaction of Ar n Cu n with polymeric CuCCC 6H 4CH 3-4. IR and NMR spectroscopy reveal that Ar 4Cu 6(CCR) 2 and Ar 4Cu 6Br 2 have the Ar 4Cu 6 skeleton as a common structural feature. Dynamic NMR spectroscopy provides information about the occurrence of intra-cluster CuN coordination. The position of the ν(CC) band (≈ 2040 cm −1) and considerations concerning the number of cluster electrons in Ar 4Cu 6X 2 clusters (84 e) suggest that the CCR ligand acts as a three electron ligand bridging two equatorial Cu atoms of the Cu 6 core. In solution the Cu 2CCR interaction is probably fluxional (σ,π σ(2 e–3 c) π,σ).