Rh Coordination Research Articles

Synthesis of π-Ring-Fused Porphyrin(2.1.1.1)s and Their Rh(I) Complexes.

Stable and simplest expanded porphyrins, π-ring-fused porphyrin(2.1.1.1)s and Rh(I) complexes, have been obtained for the first time. Two free bases show chair-shaped molecular conformations, as if reassembled by the halves of porphyrin(1.1.1.1) and porphyrin(2.1.2.1). The insertion of Rh(CO)2 groups induced more twisted molecular conformations. The NMR spectra, X-ray structure, NICS, and ACID of obtained molecules all support their nonaromaticity due to chair-shaped molecular conformations. The protonated and Rh(I) coordination of porphyrin(2.1.1.1)s process red-shifted absorptions in the NIR region.

Nature of NMR Shifts in Paramagnetic Octahedral Ru(III) Complexes with Axial Pyridine-Based Ligands.

In recent decades, transition-metal coordination compounds have been extensively studied for their antitumor and antimetastatic activities. In this work, we synthesized a set of symmetric and asymmetric Ru(III) and Rh(III) coordination compounds of the general structure (Na+/K+/PPh4+/LH+) [trans-MIIIL(eq)nL(ax)2]- (M = RuIII or RhIII; L(eq) = Cl, n = 4; L(eq) = ox, n = 2; L(ax) = 4-R-pyridine, R = CH3, H, C6H5, COOH, CF3, CN; L(ax) = DMSO-S) and systematically investigated their structure, stability, and NMR properties. 1H and 13C NMR spectra measured at various temperatures were used to break down the total NMR shifts into the orbital (temperature-independent) and hyperfine (temperature-dependent) contributions. The hyperfine NMR shifts for paramagnetic Ru(III) compounds were analyzed in detail using relativistic density functional theory (DFT). The effects of (i) the 4-R substituent of pyridine, (ii) the axial trans ligand L(ax), and (iii) the equatorial ligands L(eq) on the distribution of spin density reflected in the "through-bond" (contact) and the "through-space" (pseudocontact) contributions to the hyperfine NMR shifts of the individual atoms of the pyridine ligands are rationalized. Further, we demonstrate the large effects of the solvent on the hyperfine NMR shifts and discuss our observations in the general context of the paramagnetic NMR spectroscopy of transition-metal complexes.

Rh-coordinated histidyl bolaamphiphile assembly: a catalyst for the isomerization of cis-stilbene and cis-alkene.

In this study, we present a colloidal assembly of histidyl bolaamphiphiles whose imidazoles coordinate with rhodium ions (HisC7[Rh]) to exhibit catalytic isomerization activity for cis-stilbene and cis-alkene molecules. The histidyl bolaamphiphiles self-assemble to form a soft scaffold that functions analogously to an apoenzyme. This scaffold exposes multiple histidyl imidazoles and carboxylates on its surface, to which rhodium ions bind, generating catalytically active sites. The Rh coordination with the biochemical functional groups was verified through comprehensive vibrational spectroscopy and calorimetry. The colloidal HisC7[Rh] demonstrated a significant catalytic effect on the isomerization of cis- to trans-stilbene under mild H2 conditions, resulting in 69% yield of trans-stilbene. In contrast, when Rh(cod)2BF4 was employed as a control catalyst, only the hydrogenated products of bibenzyl were obtained. These findings underscore the crucial role of histidyl motifs in exhibiting unique catalytic isomerization activity through the coordination with Rh. The catalytic activity of HisC7[Rh] is governed by several factors, such as rhodium content, solvent composition, temperature, and H2 pressure. Moreover, HisC7[Rh] displayed moderate isomerization activity towards not only stilbene but also unsaturated fatty acid isomers, highlighting its expansive potential as an isomerization catalyst.

Selective hydrogenation of phenols to cyclohexanols catalyzed by robust solid NHC–Rh coordination assemblies in water

Selective hydrogenation of phenols catalyzed by robust solid NHC–Rh coordination assemblies in water.

Successes and challenges in multiscale modelling of artificial metalloenzymes: the case study of POP-Rh2 cyclopropanase.

Molecular modelling applications in metalloenzyme design are still scarce due to a series of challenges. On top of that, the simulations of metal-mediated binding and the identification of catalytic competent geometries require both large conformational exploration and simulation of fine electronic properties. Here, we demonstrate how the incorporation of new tools in multiscale strategies, namely substrate diffusion exploration, allows taking a step further. As a showcase, the enantioselective profiles of the most outstanding variants of an artificial Rh2-based cyclopropanase (GSH, HFF and RFY) developed by Lewis and co-workers (Nat. Commun., 2015, 6, 7789 and Nat. Chem., 2018, 10, 318-324) have been rationalized. DFT calculations on the free-cofactor-mediated process identify the carbene insertion and the cyclopropanoid formation as crucial events, the latter being the enantiodetermining step, which displays up to 8 competitive orientations easily altered by the protein environment. The key intermediates of the reaction were docked into the protein scaffold showing that some mutated residues have direct interaction with the cofactor and/or the co-substrate. These interactions take the form of a direct coordination of Rh in GSH and HFF and a strong hydrophobic patch with the carbene moiety in RFY. Posterior molecular dynamics sustain that the cofactor induces global re-arrangements of the protein. Finally, massive exploration of substrate diffusion, based on the GPathFinder approach, defines this event as the origin of the enantioselectivity in GSH and RFY. For HFF, fine molecular dockings suggest that it is likely related to local interactions upon diffusion. This work shows how modelling of long-range mutations on the catalytic profiles of metalloenzymes may be unavoidable and software simulating substrate diffusion should be applied.

Perimeter Coordinated Diastereomeric Rh(I) Complex of Helically Twisted Weakly Aromatic Hybrid Singly N-Confused β-β Fused Ferrocenoporphyrinoids.

Expedient synthesis, spectroscopic, solid state structural proof, and theoretical study of helically twisted weakly aromatic hybrid singly N-confused ferrocenoporphyrinoids and the peripheral coordinated Rh(I) complex are reported. The X-ray crystal structure of the macrocycles reveals an ambiguously inverted pyrrole ring reinforcing regioselective β,β-linkage with the spatially adjacent N-confused N-methyl pyrrole ring leading to endocyclic extension of macrocyclic π-conjugation via tricyclic [5.5.5] moiety. The three-dimensional structure with built-in fused tricyclic [5.5.5] moiety has paved way to three-dimensional weak diatropicity with vis-NIR absorptions. The peripheral coordinated Rh(I) complex owing to helical chirality about the macrocyclic ring and planar chirality about the square planar Rh coordination site exists as a mixture of diastereomers (5:3) with well resolved 1H NMR spectra anticipating weak aromaticity. The experimental spectroscopic measurements are in agreement with theoretically determined electronic structure and properties strongly elucidating sustained weak diatropic ring currents in twisted macrocycles both in neutral form and in the metalated complex. Further fragment molecular orbital approach and molecular orbital theory gave insights on the stability of N-confused β-β fused oxo-ferrocenoporphyrinoids and formation of the selective peripheral coordinated Rh(I) complex.

Paddle Ball Dynamics during Conversion of a Rh–Methyl Hydride Complex to a Rh–Methane σ-Complex through Reductive Coupling

We used quasiclassical direct dynamics simulations to examine reaction pathways for protonation-induced reductive coupling of a pincer (PONOP)Rh–methyl hydride complex. These dynamics simulations revealed that the majority of trajectories (>90%) emerging from a vibrationally averaged velocity distribution of the Rh–methyl hydride reductive coupling transition state led to the Rh–methane σ-complex. Only a few trajectories (<10%) dynamically deviated and did not sample this σ-complex structure and resulted in direct methane reductive elimination/dissociation. This indicates that the Rh–methane σ-complex is directly dynamically connected to the Rh–methyl hydride reductive coupling transition state and is not the result of methane rebound due to a solvent cage. Unexpectedly, trajectories that resulted in a Rh–methane σ-complex occurred with paddle ball motion where methane began to leave the Rh coordination sphere but then returned to the σ-complex, and so the distinction between reductive coupling and elimin...

Mechanism study on rhodium(III)-catalyzed C[sbnd]H functionalization of o-vinylphenols with alkynes: Regioselectivity and chemoselectivity

The mechanism of rhodium(III)-catalyzed CH functionalization of o-vinylphenols (1a) with alkynes has been investigated with DFT calculations. The results suggest the whole reaction is comprised with four stages: (I) OH deprotonation, (II) CH activation induced by Rh(III) catalyst interacting with o-vinylphenols, (III) alkyne coordination and (IV) regeneration of Rh(III) catalyst. The CH activation step proceeds under the concerted metalation deprotonation mechanism. The regioselectivity and chemoselectivity of the reaction also have been discussed, respectively. The regioselectivity for different CH activation sites is depended on the coordination structure of the related Rh(III) complex. In addition, when the substrates are substituted with alkyl (or alkenes) at the terminal position of the alkene, such as (E)-2-(prop-1-en-1-yl)phenol 1b and 2-allylphenol 1c, the apparent activation energies (ΔG) are obviously higher than that of 1a, which indicates that they are unfavorable to occur. The effect of CO2Me substituent on the para position to the hydroxyl (1d) is also investigated.

The advantages of covalently attaching organometallic catalysts to a carbon black support: recyclable Rh(i) complexes that deliver enhanced conversion and product selectivity.

Pure carbon black (CB) was covalently attached to a bidentate nitrogen coordination motif with a carbon-carbon bond by spontaneous reaction with an in situ generated ligand precursor. The functionalized support was treated with [Rh(CO)2(μ-Cl)]2 to form a heterogeneous carbon-based support covalently linked to a well defined Rh(i) coordination complex. The hybrid material was characterized using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Infrared (IR) spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS). The CB-supported Rh(i) catalyst was active in both hydroamination and dihydroalkoxylation reactions achieving turnover numbers approaching 1000 and was readily recycled. The selectivity of an intramolecular dihydroalkoxylation reaction was significantly improved by covalently anchoring the catalyst to the CB surface.

ChemInform Abstract: Rh(I)‐Catalyzed Transformation of Propargyl Vinyl Ethers into (E,Z)‐Dienals: Stereoelectronic Role of trans Effect in a Metal‐Mediated Pericyclic Process and a Shift from Homogeneous to Heterogeneous Catalysis During a One‐Pot Reaction.

The combination of experiments and computations reveals unusual features of stereoselective Rh(I)-catalyzed transformation of propargyl vinyl ethers into (E,Z)-dienals. The first step, the conversion of propargyl vinyl ethers into allene aldehydes, proceeds under homogeneous conditions via a “cyclization-mediated” mechanism initiated by Rh(I) coordination at the alkyne. This path agrees well with the small experimental effects of substituents on the carbinol carbon. The key feature revealed by the computational study is the stereoelectronic effect of the ligand arrangement at the catalytic center. The rearrangement barriers significantly decrease due to the greater transfer of electron density from the catalytic metal center to the CO ligand oriented trans to the alkyne. This effect increases electrophilicity of the metal and lowers the calculated barriers by 9.0 kcal/mol. Subsequent evolution of the catalyst leads to the in situ formation of Rh(I) nanoclusters that catalyze stereoselective tautomerizatio...

Rh(I)-Catalyzed Transformation of Propargyl Vinyl Ethers into (E,Z)-Dienals: Stereoelectronic Role of trans Effect in a Metal-Mediated Pericyclic Process and a Shift from Homogeneous to Heterogeneous Catalysis During a One-Pot Reaction

The combination of experiments and computations reveals unusual features of stereoselective Rh(I)-catalyzed transformation of propargyl vinyl ethers into (E,Z)-dienals. The first step, the conversion of propargyl vinyl ethers into allene aldehydes, proceeds under homogeneous conditions via a "cyclization-mediated" mechanism initiated by Rh(I) coordination at the alkyne. This path agrees well with the small experimental effects of substituents on the carbinol carbon. The key feature revealed by the computational study is the stereoelectronic effect of the ligand arrangement at the catalytic center. The rearrangement barriers significantly decrease due to the greater transfer of electron density from the catalytic metal center to the CO ligand oriented trans to the alkyne. This effect increases electrophilicity of the metal and lowers the calculated barriers by 9.0 kcal/mol. Subsequent evolution of the catalyst leads to the in situ formation of Rh(I) nanoclusters that catalyze stereoselective tautomerization. The intermediacy of heterogeneous catalysis by nanoclusters was confirmed by mercury poisoning, temperature-dependent sigmoidal kinetic curves, and dynamic light scattering. The combination of experiments and computations suggests that the initially formed allene-aldehyde product assists in the transformation of a homogeneous catalyst (or "a cocktail of catalysts") into nanoclusters, which in turn catalyze and control the stereochemistry of subsequent transformations.

Ligand Tethering by Ion-Exchange for the Immobilization of Homogeneous Catalysts

A Rh phosphine complex, derived from the Wilkinson’s catalyst, has been immobilized by ion-exchange on the ammonium form of a Al-MCM-41 sample. Ammonium ions have been exchanged by cholamine ions, which act as an amine ligand, and then the Wilkinson’s catalyst has been immobilized by substitution of a phosphine ligand by the anchored amine. This is a novel immobilization procedure, as a ligand, instead of the whole complex, is tethered to the support by ion exchange. The obtained hybrid catalyst has been characterized by Elemental Analysis, DRIFTS and XPS. The quantitative exchange of ammonium by cholamine and coordination of Rh to amines has been observed. Most of the anchored Rh is considered to be coordinated to the ligand tethered to the support and a small proportion seems to be interacting with the protonated ligand or with the support surface. The catalyst has been tested in the hydrogenation of cyclohexene and in the hydroformylation of 1-octene. In the first case the catalyst is active and reusable, while a strong Rh leaching takes place in the second one. Keywords: Immobilization, ion-exchange, hybrid catalysts, Al-MCM-41, Rh complex, BET, Soxhlet, NMR, FTIR, MCT, DRIFTS, rhodium, silanol groups, XPS analysis, chelating ligand

Rh(I) coordination chemistry of hexacationic Dendriphos ligands and their application in hydroformylation catalysis

The coordination chemistry of hexacationic Dendriphos ligands L∗ with respect to Rh(I), as well as their application in Rh-catalyzed hydroformylation reactions, is described. Complexes of the type RhCl(CO)(L∗) 2 were synthesized and characterized. The results show that Dendriphos ligands are weaker σ-donors and/or stronger π-acceptors compared to PPh 3. The reaction of L∗ with [Rh(cod) 2]BF 4 in MeCN afforded monophosphine-Rh(I) complexes of the type Rh(cod)(MeCN)(L∗), which points to the tendency of these ligands to form coordinatively unsaturated metal complexes. The catalytic performance of Dendriphos ligands in the Rh-catalyzed hydroformylation appeared to be dominated by steric effects arising from the large dendritic shells of these ligands. Furthermore, the possibility of tuning the catalytic activity and selectivity of the catalytic species, by changing the six counteranions of the hexacationic Dendriphos ligand, has been investigated. Changing the anions from BF 4 - to the chiral anions camphorsulphonate or Δ-Trisphat did not render the hydroformylation reaction of styrene enantioselective, albeit small changes in its regioselectivity were observed.

Increasing the Curvature of a Bowl-Shaped Polyarene by Fullerene-like η2-Complexation of a Transition Metal at the Interior of the Convex Surface

The first transition metal complex of monoindenocorannulene, [{Rh2(O2CCF3)4}2·(C26H12)] (1), has been synthesized by gas-phase deposition and has been structurally characterized by X-ray crystallography. In the solid state, it forms a 2D organometallic network based on intermolecular Rh−C interactions and rare tetra-bridged coordination of a π-bowl. In addition to η2-rim binding, one Rh(II) center interacts exclusively with interior carbon atoms on the convex surface, exhibiting an η2-coordination type previously observed only in closed, all-carbon buckyballs. The latter unique coordination of Rh(II) accentuates the pyramidalization of the C atoms of monoindenocorannulene. Thus, in contrast to all other reported Rh(II) complexes with buckybowls, metal complexation leads to a curvature increase of the C26H12 core in 1. DFT calculations (PBE0) reveal the preferred coordination sites of C26H12 to be the rim of the corannulene core, followed by the interior spoke and then the rim CC bonds of the indeno site. This calculated trend is nicely followed by the average Rh−C bond distances in the solid-state structure of 1: 2.567 (rim) < 2.687 (spoke) < 2.715 Å (indeno site). The nature of Rh(II)−π interactions was quantitatively evaluated in terms of ligand-to-metal and metal-to-ligand contributions, showing the consistently greater role of the former in all computed complexes.

Elucidation of Metal‐Ion Accumulation Induced by Hydrogen Bonds on Protein Surfaces by Using Porous Lysozyme Crystals Containing RhIIIIons as the Model Surfaces

Metal-ion accumulation on protein surfaces is a crucial step in the initiation of small-metal clusters and the formation of inorganic materials in nature. This event is expected to control the nucleation, growth, and position of the materials. There remain many unknowns, as to how proteins affect the initial process at the atomic level, although multistep assembly processes of the materials formation by both native and model systems have been clarified at the macroscopic level. Herein the cooperative effects of amino acids and hydrogen bonds promoting metal accumulation reactions are clarified by using porous hen egg white lysozyme (HEWL) crystals containing Rh(III) ions, as model protein surfaces for the reactions. The experimental results reveal noteworthy implications for initiation of metal accumulation, which involve highly cooperative dynamics of amino acids and hydrogen bonds: i) Disruption of hydrogen bonds can induce conformational changes of amino-acid residues to capture Rh(III) ions. ii) Water molecules pre-organized by hydrogen bonds can stabilize Rh(III) coordination as aqua ligands. iii) Water molecules participating in hydrogen bonds with amino-acid residues can be replaced by Rh(III) ions to form polynuclear structures with the residues. iv) Rh(III) aqua complexes are retained on amino-acid residues through stabilizing hydrogen bonds even at low pH (approximately 2). These metal-protein interactions including hydrogen bonds may promote native metal accumulation reactions and also may be useful in the preparation of new inorganic materials that incorporate proteins.

The dual role of Cu(i) as a protective group and a template in the synthesis of a tetra-rhodium(iii)porphyrin [2]catenane

A new [2]catenane containing four Rh(III) porphyrins has been prepared following a three-step strategy: (i) Cu(I)-driven entwining of two free base porphyrin-bearing bidentate chelates, followed by (ii) insertion of rhodium in the porphyrin nuclei, and finally (iii) cyclisation via the formation of N–Rh coordination bonds. Direct metalation of porphyrinic compounds with [Rh(CO)2Cl]2 was not met with success when the bidentate chelate used as a connector was metal free, thus making the Cu(I) protecting approach very attractive.

Di-, tetra- and polynuclear RhI complexes containing phenylene-1,4-diaminotetra(phosphonite), p-C6H4[N{P(OC6H4OMe-o)2}2]2 and their catalytic investigation towards transfer hydrogenation reactions

The reactions of phenylene-1,4-diaminotetra(phosphonite), p-C(6)H(4)[N{P(OC(6)H(4)OMe-o)(2)}(2)](2) (P(2)N(Ph)NP(2)) () with [Rh(COD)Cl](2) in 1 : 2 and 1 : 1 molar ratio produce tetra- and polymetallic chelate complexes, [Rh(4)(COD)(2)(micro-Cl)(4)(P(2)N(Ph)NP(2))] () and [Rh(2)(micro-Cl)(2)(P(2)N(Ph)NP(2))](n) (), respectively. Similar reaction of with [Rh(COD)Cl](2) in dichloromethane-acetonitrile mixture furnishes a dinuclear complex, [Rh(2)Cl(2)(CH(3)CN)(2)(P(2)N(Ph)NP(2))] (). The reaction of or with CO affords a dinuclear carbonyl derivative, [Rh(2)Cl(2)(CO)(2)(P(2)N(Ph)NP(2))] (). Treatment of with 2 equivalents of pyrazine or 4,4'-bipyridine produce one-dimensional Rh(I) coordination polymers, [Rh(2)Cl(2)(C(4)H(4)N(2))(P(2)N(Ph)NP(2))](n) () and [Rh(2)Cl(2)(C(10)H(8)N(2))(P(2)N(Ph)NP(2))](n) in quantitative yields. The catalytic activity of Rh(I) complexes have been investigated in transfer hydrogenation reactions.

Rhodium(I)-Catalyzed Nucleophilic Ring-Opening Reactions of Oxabicyclo Adducts Derived from the [4 + 2]-Cycloaddition of 2-Imido-Substituted Furans

A series of 2-imido-substituted furans containing tethered unsaturation were prepared by the addition of the lithium carbamate of furan-2-ylcarbamic acid tert-butyl ester to a solution of the mixed anhydride of an appropriately substituted 3-butenoic acid. The initially formed imido furans undergo a rapid intramolecular [4 + 2]-cycloaddition at room temperature to deliver the Diels-Alder cycloadducts in good to excellent yield. Isolation of the highly labile oxabicyclic adduct is believed to be a consequence of the lower reaction temperatures employed as well as the presence of the extra carbonyl group, which diminishes the basicity of the nitrogen atom, thereby retarding the ring cleavage/rearrangement reaction generally encountered with related systems. By using a Rh(I)-catalyzed ring opening of the oxabicyclic adduct with various nucleophilic reagents, it was possible to prepare highly functionalized hexahydro-1H-indol-2(3H)-one derivatives in good yield. The major stereoisomer obtained possesses a cis-relationship between the nucleophile and hydroxyl group in the ring-opened product. The stereochemistry was unequivocally established by X-ray crystallographic analysis. Coordination of Rh(I) to the alkenyl pi-bond followed by a nitrogen-assisted cleavage of the carbon-oxygen bond occurs to furnish a pi-allyl rhodium(III) species. Addition of the nucleophile then occurs from the least hindered terminus of the resulting pi-allyl rhodium(III) complex. Proton exchange followed by rhodium(I) decomplexation ultimately leads to the cis-diastereomer.

Hexarhodium Clusters on Lanthana: Synthesis, Characterization, and Catalysis of Ethene Hydrogenation

Rhodium carbonyl clusters were prepared on the surface of La2O3 powder (calcined at 673 K) by a surface-mediated synthesis from La2O3-supported Rh(CO)2(acac) in the presence of CO at 1 atm and 373 K. The cluster preparation and subsequent decarbonylation by treatment in He were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment in He at 573 K removed the carbonyl ligands, giving site-isolated La2O3-supported clusters that are well approximated as Rh6 octahedra, being characterized by a first-shell Rh−Rh coordination number of 3.9 ± 0.4 at a distance of 2.64 ± 0.02 Å. The supported clusters were characterized by IR and EXAFS spectroscopies in the presence of ethene and H2 reacting catalytically to give ethane. The EXAFS first-shell Rh−Rh coordination number was found to be about 4, consistent with the presence of Rh6 octahedra, which are inferred to be the catalytically active species. IR spectra show that both hydrocarbons and hydride ligands were present on the working cluster catalyst, including π-bonded ethene and others, inferred to be ethyl, ethylidyne, and di-σ-bonded ethene. The concentration of hydride on Rh6 increased during the initial induction period in a flow reactor as the catalytic activity increased almost proportionately; hydrides are inferred to be reactive intermediates. 1H NMR spectroscopy showed that hydride remained on the clusters following catalysis. The results suggest that the hydrogenation of ethene on Rh6/La2O3 proceeds by insertion of π-bonded ethene into a Rh−H bond to form ethyl, which is subsequently hydrogenated to give ethane. Rh6/La2O3 is about 50 times more active for ethene hydrogenation catalysis than Rh6/γ-Al2O3, and we suggest that the difference is related to the electron-donor properties of the supports.

A new mixed-metal MnRh coordination polymer assembled from Mn-containing molecular building blocks and Rh 2(OAc) 4 dimers

The manganese-containing molecular building block Mn(2-methylpyrazine-5-carboxylato) 2·(H 2O) 2 ( 1, triclinic, P-1, a=7.2042(6) Å, b=7.4067(6) Å, c=13.9982(12) Å, α=87.205(2)°, β=81.162(2)°, γ=80.517(2)°, Z=2) was prepared and used in the synthesis of a new mixed-metal organic-inorganic polymeric material. The reaction between 1 and Rh 2(OAc) 4 resulted in the formation of catena-poly{[Mn(MePyzca) 2(MeOH) 2][Rh 2(OAc) 4]}·2MeOH ( 2, monoclinic, C2/c, a=29.711(3) Å, b=7.9124(8) Å, c=15.5028(15) Å, β=109.212(2)°, Z=4), where the Mn(MePyzca) 2(MeOH) 2 building blocks are linked by Rh 2(OAc) 4 dimers into a one-dimensional zigzag chain structure. The magnetic susceptibility for 2 follows the Curie law, with no indication of long range magnetic order. Synthesis, X-ray structure determination and magnetic properties are presented.