Project description:A rhodium(II) dibenzotetramethylaza[14]annulene dimer ([(tmtaa)Rh]2) undergoes metathesis reactions with [CpCr(CO)3]2, [CpMo(CO)3]2, [CpFe(CO)2]2, [Co(CO)4]2, and [Mn(CO)5]2 to form (tmtaa)Rh-M complexes (M = CrCp(CO)3, MoCp(CO)3, FeCp(CO)2, Co(CO)4, or Mn(CO)5). Molecular structures were determined for (tmtaa)Rh-FeCp(CO)2, (tmtaa)Rh-Co(?-CO)(CO)3, and (tmtaa)Rh-Mn(CO)5 by X-ray diffraction. Equilibrium constants measured for the metathesis reactions permit the estimation of several (tmtaa)Rh-M bond dissociation enthalpies (Rh-Cr = 19 kcal mol(-1), Rh-Mo = 25 kcal mol(-1), and Rh-Fe = 27 kcal mol(-1)). Reactivities of the bimetallic complexes with synthesis gas to form (tmtaa)Rh-C(O)H and M-H are surveyed.
Project description:The products of methane dehydrogenation by gas-phase Ta4 + clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H]+ reveal a dominance of vibrational bands of a H2 Ta4 C+ carbide dihydride structure over those indicative for a HTa4 CH+ carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M+ typically leads to the formation of either MCH2 + carbene or HMCH+ carbyne hydride structures, the observation of a H2 MC+ carbide dihydride structure implies that it is imperative to consider this often-neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.
Project description:Through relatively subtle changes in reaction conditions, we have been able to isolate four distinct Rh/Sn cluster compounds, [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and [Rh3@Sn24]5-, from the reaction of K4Sn9 with [(COE)2Rh(?-Cl)]2(COE = cyclooctene). The last of these has a hitherto unknown molecular topology, an edge-fused polyhedron containing three Rh@Sn10 subunits, and represents the largest endohedral Group 14 Zintl cluster yet to have been isolated from solution. DFT has been used to place these new species in the context of known cluster chemistry. ESI-MS experiments on the reaction mixtures reveal the ubiquitous presence of {RhSn8} fragments that may play a role in cluster growth.
Project description:Anthropogenic sources of xenobiotic metals with no physiological benefit are increasingly prevalent in the environment. The platinum group metals (Pd, Pt, Rh, Ru, Os, and Ir) are found in marine and plant species near urban sources, and are known to bioaccumulate, introducing these metals into the human food chain. Many of these metals are also being used in innovative cancer therapy, which leads to a direct source of exposure for humans. This paper aims to further our understanding of nontraditional metal metabolism via metallothionein, a protein involved in physiologically important metal homeostasis. The aggressive reaction of metallothionein and dirhodium(II) tetraacetate, a common synthetic catalyst known for its cytotoxicity, was studied in detail in vitro. Optical spectroscopic and equilibrium and time-dependent mass spectral data were used to define binding constants for this robust reaction, and molecular dynamics calculations were conducted to explain the observed results.
Project description:The dirhodium complex, [Rh(2)(C(18)H(15)P)(4)(CO)(2)]·2(CH(3))(2)CO, has crystallographic twofold symmetry and the Rh-Rh distance is 2.6266?(8)?Å. The four atoms proximate to each Rh atom [Rh-P = 2.3222?(7) and 2.3283?(8)?Å, and Rh-C = 1.961?(3) and 2.045?(3)?Å] form a distorted tetra-hedron with large deviations from the putative tetra-hedral angles [r.m.s. deviation = 23?(1)°]. The six angles more closely approximate those of a trigonal bipyramid [r.m.s. deviation = 14?(1)°] with one missing equatorial ligand. The two bridging carbonyl ligands are much more linearly coordinated to one Rh [Rh-C O = 151.0?(2)°] than to the other [127.0?(2)°], and the two Rh(2)CO planes form a dihedral angle of 45.43?(5)°. The two acetone solvent mol-ecules are disordered, and their estimated scattering contribution was subtracted from the observed diffraction data using the SQUEEZE routine in PLATON [Spek (2009 ?). Acta Cryst.D65, 148-155].
Project description:Single-atom catalysts often show exceptionally high performance per metal loading. However, the isolated atom sites tend to agglomerate during preparation and/or high-temperature reaction. Here we show that in the case of Rh/Al2O3 this deactivation can be prevented by dissolution/exsolution of metal atoms into/from the support. We design and synthesise a series of single-atom catalysts, characterise them and study the impact of exsolution in the dry reforming of methane at 700-900 °C. The catalysts' performance increases with increasing reaction time, as the rhodium atoms migrate from the subsurface to the surface. Although the oxidation state of rhodium changes from Rh(iii) to Rh(ii) or Rh(0) during catalysis, atom migration is the main factor affecting catalyst performance. The implications of these results for preparing real-life catalysts are discussed.
Project description:A dianionic, square planar cobalt(II) complex reacts with O2 in the presence of acetonitrile to give a cyanomethylcobalt(III) complex formed by C-H bond cleavage. Interestingly, PhIO and p-tolylazide react similarly to give the same cyanomethylcobalt(III) complex. Competition studies with various hydrocarbon substrates indicate that the rate of C-H bond cleavage greatly depends on the p Ka of the C-H bond, rather than on the C-H bond dissociation energy. Kinetic isotope experiments reveal a moderate KIE value of ca. 3.5 using either O2 or PhIO. The possible involvement of a cobalt(IV) oxo species in this chemistry is discussed.
Project description:C-F activation of 2,3,5,6-tetrafluoropyridine at [Rh{Si(OEt)3}(PEt3)3] (1) yields [Rh{2-(3,5,6-C5F3HN)}(PEt3)3] (2) and FSi(OEt)3, but in an unprecedented consecutive reaction FSi(OEt)3 acts as a fluoride source to give [Rh(4-C5F4N)(PEt3)3] (4) by regeneration of the C-F bond and C-H activation. Analogous refluorination steps were observed for other 2-pyridyl rhodium complexes. NMR spectroscopic studies revealed a delicate balance between the feasibility for C-F bond formation accompanied by a C-H activation and the occurrence of competing reactions such as hydrodefluorinations induced by the intermediary presence of H2.
Project description:Silicon carbide (SiC) formation plays an important role during the production of elemental silicon. SiC forms through a high temperature reaction between silicon monoxide gas (SiO) and carbon. Currently, the carbon sources are solids, however finding a way of substituting the solid carbon with methane could have several advantages. SiC formation was studied in argon, hydrogen and methane containing atmospheres at 1650 °C and 1750 °C. SiO gas was generated from pellets of a 1:2 molar ratio of SiC and silica (SiO2). The reactions were investigated through CO off-gas analysis in conjunction with measuring the weight change. After each experiment, the reaction products were examined in a scanning electron microscope with secondary electrons and through energy-dispersive X-ray spectroscopy. It was confirmed that SiC may form from SiO and methane. Increasing the methane content to 5% caused a significant increase in SiC formation. Furthermore, the SiC structure was also highly sensitive to the methane content that was used. In addition, the SiO producing reaction was affected by hydrogen. The hydrogen lead to an increased rate of SiO formation relative to what was seen in argon. The effect of hydrogen was most pronounced at 1750 °C which is right after the melting of silica.
Project description:Bearing the versatility of N-heterocyclic carbene (NHC) ligands, here density functional theory (DFT) calculations unravel the capacity of coordination of a deprotonated NHC ligand (pNHC) to generate a doubly C2,N3-bridged dinuclear complex. Here, in particular the discussion is based on the combination of the deprotonated 1-arylimidazol (aryl = mesityl (Mes)) with [M(cod)(?-Cl)] (M = Ir, Rh) generated two geometrical isomers of complex [M(cod){µ-C3H2N2(Mes)-?C2,?N3}]2). The latter two isomers display conformations head-to-head (H-H) and head-to-tail (H-T) of C S and C 2 symmetry, respectively. The isomerization from the H-H to the H-T conformation is feasible, whereas next substitutions of the cod ligand by CO first, and PMe3 later confirm the H-T coordination as the thermodynamically preferred. It is envisaged the exchange of the metal, from iridium to rhodium, confirming here the innocence of the nature of the metal for such arrangements of the bridging ligands.