Rh-Rh Bond Research Articles

RhNi Bimetallenes with Lattice-Compressed Rh Skin towards Ultrastable Acidic Nitrate Electroreduction.

Harvesting recyclable ammonia (NH3 ) from acidic nitrate (NO3 - )-containing wastewater requires the utilization of corrosion-resistant electrocatalytic materials with high activity and selectivity towards acidic electrochemical nitrate reduction (NO3 ER). Herein, ultrathin RhNi bimetallenes with Rh-skin-type structure (RhNi@Rh BMLs) are fabricated towards acidic NO3 ER. The Rh-skin atoms on the surface of RhNi@Rh BMLs experience the lattice compression-induced strain effect, resulting in shortened Rh-Rh bond and downshifted d-band center. Experimental and theoretical calculation results corroborate that Rh-skin atoms can inhibit NO2 */NH2 * adsorption-induced Rh dissolution, contributing to the exceptional electrocatalytic durability of RhNi@Rh BMLs (over 400 h) towards acidic NO3 ER. RhNi@Rh BMLs also reveal an excellent catalytic performance, boasting a 98.4% NH3 Faradaic efficiency and a 13.4mg h-1 mgcat -1 NH3 yield. Theoretical calculations reveal that compressive stress tunes the electronic structure of Rh skin atoms, which facilitates the reduction of NO* to NOH* in NO3 ER. The practicality of RhNi@Rh BMLs has also been confirmed in an alkaline-acidic hybrid zinc-nitrate battery with a 1.39V open circuit voltage and a 10.5mW cm-2 power density. This work offers valuable insights into the nature of electrocatalyst deactivation behavior and guides the development of high-efficiency corrosion-resistant electrocatalysts for applications in energy and environment.

C-H activation in bimetallic rhodium complexes to afford N-heterocyclic carbene pincer complexes.

The pro-ligands 1,8-bis(di-R-phosphinomethyl)-2,3-dihydroperimidine (RH2Pm, R = phenyl, cyclohexyl) react with [RhCl(CE)(PPh3)2] (E = O, S) to afford the bimetallic complexes [RhCl(CE)(μ-RH2Pm)]2 (E = O, S). Upon heating, these species undergo double C-H activation to afford the N-heterocyclic carbene (NHC) pincer complexes [RhCl(RPm)]. Reduction of [RhCl(CO)(μ-PhH2Pm)]2 with KC8 results in the bimetallic rhodium(0) complex, [Rh(μ-CO)(PhH2Pm)]2, with a formal Rh-Rh bond and a hydrogen-bonding interaction between rhodium and the central methylene group (C-H⋯Rh = 2.802 Å). Upon treatment with tritylium, ferrocenium or triphenylcyclopropenium tetrafluoroborates this species undergoes double C-H activation to afford a mononuclear NHC pincer complex salt, [Rh(CO)(PhPm)]BF4. Treatment of [RhCl(CO)(PhH2Pm)]2 with lithium (trimethylsilyl)acetylide provides another bimetallic species, [Rh(CCSiMe3)(CO)(PhH2Pm)]2, however heating this species does not proceed cleanly to the monomeric NHC complex, [Rh(CCSiMe3)(CO)(PhPm)] which may however be obtained from [RhCl(RPm)] and LiCCSiMe3.

Fine-tuning dirhodium compounds with bridging ligands: Synthesis, structure, catalytic efficiency

The structure of dirhodium compounds contains a unique Rh-Rh bond, two axial ligands and four bridging ligands. In the dirhodium(II) compounds, it is easy to migrate the coordinating atoms of the bridging ligand during the catalysis process, which leads to the degradation of the catalyst. Coordination atom migration was identified in bridging ligands. To improve the catalyst stability, we carried out a study on the effect of fine-tuning of the bridging ligand on the dirhodium compound. Several dirhodium compounds were designed and synthesized. During this process, we have successfully found Rh2(5-Br-esp)2 and Rh2(5-tBu-esp)2, which are closer to the ideal geometric configuration of the dirhodium(II) compounds. Rh2(5-Br-esp)2 has been applied in the oxidation of propargyl position and Rh2(5-tBu-esp)2 in the formation reaction of the C–N bond.

Di-carbonyl-o-semiquinonato rhodium complexes

Two novel dicarbonyl-semiquinonato rhodium complexes were synthesized and studied. Complexes are analogues of (CO)2Rh(3,6-DBSQ) (3,6-DBSQ-3,6-di-tert-butyl-o-benzosemiquinone) which thread-like crystals can bend reversibly under light or heat activation (G.A. Abakumov, V.I. Nevodchikov, Dokl. Acad. Nauk SSSR 1982, 266, 1407–1410). Complexes differ from (CO)2Rh(3,6-DBSQ) by substituents in 3,6-positions of quinonato ring (cyclohexyl (1) and isopropyl (2) instead of tert-butyl). Single crystal structural study indicates that both complexes are the derivatives of Rh(I) with o-semiquinonato anion-radical. Molecules form pairs due the short contacts in contrast to (CO)2Rh(3,6-DBSQ) and other similar compounds which form the endless alternated chains with direct Rh-Rh bond. At that, complex 1 molecules form isolated pairs, whereas molecules of 2 can be united in the chain. Both compounds 1 and 2 have the array of intensive bands in IR spectrum in the region 1350 cm−1 which is closer to bond stretch vibrations of C–O ordinary bond but not sesquilateral. EPR of solutions of complexes indicates presence of monomeric complex molecules where the single unpaired electron is localized in o-semiquinonato ligand. The drop of EPR signal intensity with cooling of solutions of 1 and 2 evidence that the association process takes place.

Single Crystal X-ray Diffraction Studies of Two Polymorphic Modifications of the Dicarbonyl-o-Semiquinonato Rhodium Complex at Different Temperatures. Destruction Stimulated by Cooling Versus Stability.

It was found that the dicarbonyl-rhodium-o-semiquinonate complex (which thread-like crystals can bend reversibly under light/warm activation) can form two polymorphic modifications: isometric prisms (1) and sticks (2). Some thin sticks can bend as mentioned above. X-ray diffraction studies of polymorphic modifications at different temperatures were carried out. It was found that crystals 1 are destructed after cooling to 110 K as opposed to crystals 2. In turn, the reversible phase transition is detected in 2. In both polymorphic modifications, stack packaging motifs through the direct Rh–Rh bond are observed. The principal difference between packages of polymorphic modifications is that molecules 1 in the adjacent stacks are shifted relative to each other along the stack, in contrast to crystal 2. It was found that different packing of stacks leads to different anisotropic compression of crystals 1 and 2 during cooling, which is a key factor of their stability. Using the molecular invariom approach, the nature of the chemical bonds and charge distribution was investigated; the energy of the Rh–Rh bonds was estimated.

Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties.

The cis- and trans-dirhodium(II,II) complexes cis-[Rh2(μ-DTolF)2(μ-np)(MeCN)4][BF4]2 (1; DTolF = N,N'-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh2(μ-DTolF)2(μ-qxnp)(MeCN)3][BF4]2 [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)quinoxaline], and trans-[Rh2(μ-DTolF)2(μ-qxnp)2][BF4]2 (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh2(μ-DTolF)2(μ-qxnp)2][BF4]2 (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm-1 and exhibits shorter lifetimes of the 1ML-LCT and 3ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh-Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal-metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The 3ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.

Unbridged Rh(ii)-Rh(ii) complexes of N-heterocyclic carbenes and reactions with O2 to form dirhodium(μ-η1:η1-O2) complexes.

Dimeric rhodium(ii) complexes and rhodium(iii) peroxide complexes supported by N-heterocyclic carbenes were synthesized. The treatment of [Ag3(L)2](PF6)3 (L = bis(N-pyridylimidazolylidenyl)methane) with [Rh(COD)Cl] afforded [Rh(L)(CH3CN)]2(PF6)4 in which two [Rh(L)] moieties are bound together via an unsupported Rh-Rh bond. The substitution of axial acetonitrile by phosphines led to the insertion of O2 into the Rh-Rh bond and the isolation of [Rh(L)(PPh3)]2(μ-η1:η1-O2)(PF6)4 and [Rh(L)(PCy3)]2(μ-η1:η1-O2) (PF6)4.

Multifunctional Ionic Liquids from Rhodium(I) Isocyanide Complexes: Thermochromic, Fluorescence, and Chemochromic Properties Based on Rh-Rh Interaction and Oxidative Addition.

Square-planar rhodium(I) isocyanide complexes exhibit unique chemical reactivities such as the formation of Rh-Rh bonds and oxidative addition. This paper details the syntheses and properties of multifunctional ionic liquids containing RhI isocyanide complexes [Rh(nBuNC)4 ]X (X=Tf2 N (=N(SO2 CF3 )2- ), Nf2 N (=N(SO2 C4 F9 )2- ), FSA (=N(SO2 F)2- ), CF3 BF3- ). Salts with Tf2 N and Nf2 N were liquids, whereas those with FSA and CF3 BF3 were solids at room temperature. The salts exhibited thermochromism in the liquid state, changing from orange at high temperatures to blue-purple at lower temperatures. This is based on the equilibrium between monomer, dimer, and other oligomers associated with Rh-Rh bond formation. The salts also exhibited fluorescence. Exposure of the Tf2 N salt to methyl iodide vapor produced ionic liquid mixtures [Rh(nBuNC)4 ]x [Rh(nBuNC)4 IMe](1-x) [Tf2 N], concomitant with a color change from purple to red, orange, and yellow, extending the thermochromic color range. The reaction of the Tf2 N salt and iodine produced mononuclear and polynuclear iodine adducts. Thus, these liquids exhibit thermochromism, fluorescence, vapochromism, chemical reactivities, and characteristic properties of ionic liquids.

A van der Waals Density Functional Investigation on the Improved Adsorption Properties of NO on the Rhn/MgO (100) Interface.

Using spin-polarized density functional theory calculations, we have studied the interaction and morphology of small Rhn clusters (n≤5) deposited on a MgO (100) surface, as well as the ability of Rhn/MgO systems to trap NO molecules. We show that Rhn clusters can tightly adhere to a MgO (100) surface because of hybridization of O 2p and Rh 4d orbitals. The most energetically preferred structure for each adsorbed Rhn cluster looks similar to the structure of the isolated cluster, except for an increment in the length of base edge(s) close to the substrate. We find that the Rhn/MgO system exhibits a much stronger ability to trap NO, especially for Rh2 and Rh4 clusters, than do the bare Rhn clusters. The hybridization of N 2p and Rh 4d orbitals is responsible for the strong adsorption of NO on Rhn/MgO. This can be explained by the increased Rh-Rh bond length that results in a decrease in the effective coordination number, and the presence of extra electrons obtained from the substrate, causing the N and O atoms to bond strongly to the Rh atoms, which in turn weakens the N-O molecular interaction. Additionally, binding to NO induces a reduction in the total magnetic moment of the Rhn/MgO system. Thus, putting Rhn on the MgO (100) surface can decrease competition between bonding and magnetism. However, whether it can be related to improve trapping of NO deserves further investigations.

Amide-functionalized naphthyridines on a Rh(II) -Rh(II) platform: effect of steric crowding, hemilability, and hydrogen-bonding interactions on the structural diversity and catalytic activity of dirhodium(II) complexes.

Ferrocene-amide-functionalized 1,8-naphthyridine (NP) based ligands {[(5,7-dimethyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene (L(1) H) and {[(3-phenyl-1,8-naphthyridin-2-yl)amino]carbonyl}ferrocene (L(2) H) have been synthesized. Room-temperature treatment of both the ligands with Rh2 (CH3 COO)4 produced [Rh2 (CH3 COO)3 (L(1) )] (1) and [Rh2 (CH3 COO)3 (L(2) )] (2) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho-methyl group in L(1) H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh(II) -Rh(II) } unit. The use of the more labile Rh2 (CF3 COO)4 salt with L(1) H produced a cis bis-adduct [Rh2 (CF3 COO)4 (L(1) H)(2) ] (3), whereas L(2) H resulted in a trans bis-adduct [Rh2 (CF3 COO)3 (L(2) )(L(2) H)] (4). Ligand L(1) H exhibits chelate binding in 3 and L(2) H forms a bridge-chelate mode in 4. Hydrogen-bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen-bonding interaction, both ligands bind axially as evident from the X-ray structure of [Rh2 (CH3 COO)2 (CH3 CN)4 (L(2) H)2 ](BF4 )2 (6). However, the axial ligands reorganize at reflux into a bridge-chelate coordination mode and produce [Rh2 (CH3 COO)2 (CH3 CN)2 (L(1) H)](BF4 )2 (5) and [Rh2 (CH3 COO)2 (L(2) H)2 ](BF4 )2 (7). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7, which features hemilabile amide side arms that occupy sites trans to the Rh-Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the CH bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen-bonding functionalities to govern the structure and catalytic efficacyof dirhodium(II,II) compounds.

Cis-Tetrakis(μ-N-phenylacetamidato)-κ4N:O;κ4O:N-bis[(benzonitrile-κN)rhodium(II)](Rh—Rh)

The complex molecule of the title compound, [Rh2{N(C6H5)COCH3}4(C6H5CN)2], exhibits crystallographically imposed centrosymmetry. The four acetamide ligands bridging the dirhodium core are arranged in a 2,2-cis manner, with two N atoms and two O atoms coordinating to the unique RhII atom cis to one another. The Neq—Rh—Rh—Oeq torsion angles on the acetamide bridges vary between 1.62 (4) and 1.78 (4)°. The Rh—Rh bond length is 2.4319 (3) Å. The axial nitrile ligand completes the distorted octahedral coordination sphere and shows a non-linear coordination with an Rh—N—C bond angle of 167.14 (15)°, while the N—C bond length is 1.135 (3) Å.

A new structural motif for photoinitiated electron collection: Ru,Rh bimetallics providing insight into H2 production via photocatalysis of water reduction by Ru,Rh,Ru supramolecules

Ru,Rh,Ru complexes are photocatalysts for the reduction of H(2)O to H(2)via a Rh(I) intermediate. The herein reported Ru,Rh bimetallics undergo PEC but do not catalyze the reduction of H(2)O, establishing intact supramolecules are photoactive in the Ru,Rh,Ru systems. The Ru,Rh(I) photoproduct dimerizes via Rh-Rh bond formation, deactivating the Rh(I) center sterically prohibited in the Ru,Rh,Ru trimetallic systems.

Chiral Crown Conformation of Rh2(S-PTTL)4: Enantioselective Cyclopropanation with α-Alkyl-α-diazoesters

Herein, we provide crystallographic and computational evidence that Hashimoto's Rh(2)(S-PTTL)(4) catalyst adopts a "chiral crown" conformation with a reactive chiral face and an unreactive achiral face. In Rh(2)(S-PTTL)(4), all four tert-butyl groups are aligned on the same face of the catalyst, and each C-(t-Bu) bond is roughly parallel to the central Rh-Rh bond. This orients the four phthalimido groups on the opposite face of the catalyst. Also described is an enantioselective and diastereoselective protocol for intermolecular Rh(2)(S-PTTL)(4)-catalyzed cyclopropanation using alpha-alkyl-alpha-diazoesters. Intermolecular cyclopropanation is selective over two competing intramolecular pathways: C-H insertion and beta-hydride elimination. On the basis of density functional theory calculations and the Davies-Singleton model for cyclopropanation, a model for asymmetric induction is proposed.

Adsorption properties and vibrational spectra of propyne adsorbed on Rh(111). Comparison with other (111) metal surfaces

We have studied the adsorption properties of propyne on the Rh(111) surface by means of the generalized gradient approach of density functional theory using periodic slab models. The simulation of the vibrational spectra has permitted us to corroborate and complete the experimental band assignment and to confirm the adsorption site preference. Propyne prefers to sit on a 3-fold hollow site, with the C[triple bond]C axis parallel to a Rh-Rh bond and the molecular plane tilted away from the surface normal. The comparison between the adsorption behaviour of propyne on Rh(111) and on other (111) metal surfaces allows one to provide an explanation for the different reactivity observed experimentally.

Synthesis, structure, and catalytic properties of polymer-immobilized rhodium clusters

Immobilized Rh6 clusters (cyclohexene hydrogenation catalysts) were prepared by the polymer-analogous transformations or copolymerization of cluster-containing monomers and characterized. Intermediates formed in the course of a catalytic reaction were studied using IR spectroscopy, XPS, and atomic force microscopy. It was found that the relative intensity of a low-energy line in the Rh3d 5/2 spectrum of the initial polymer-immobilized cluster in the XPS spectrum of Rh6 increased in the course of hydrogenation. The catalytic activity of the immobilized complex changed symbatically with both the number of Rh atoms bound to the H(CO) group and the number of Rh atoms, the charge on which was greater than that in the parent cluster. Some experimental evidence was obtained in favor of the hypothesis of cluster fragmentation in the course of hydrogenation with the formation of highly active, most likely, nanosized particles, which are true catalysts, in low concentrations. The surface of macrocomplex particles after hydrogenation became more homogeneous and hydrophilic; this fact is also indicative of an increase in the concentration of polar functional groups in surface layers. This was likely due to Rh-Rh bond cleavage in the polymer-immobilized cluster.

Rhodium Dimers with 2,2-Dimethyl-1,3-diisocyano and Bis(diphenylphosphino)methane Bridging Ligands

Four rhodium dimers have been synthesized with a bridging diisocyanide ligand, dmb (2,2-dimethyl-1,3-diisocyanopropane): [Rh2(dmb)4](BPh4)2, [Rh2(dmb)4Cl2]Cl2, [Rh2(dmb)4I2](PF6)2, and [Rh2(dmb)2(dppm)2](BPh4)2 (dppm = bis(diphenylphosphino)methane). The complexes have been characterized by elemental analysis and mass spectrometry, as well as UV-visible, IR, and 1H NMR spectroscopies. X-ray crystal structures of the rhodium(I) complexes, [Rh2(dmb)4](BPh4)2 . 1.5CH3CN (3.2330(4), 3.2265(4) A) and [Rh2(dmb)2(dppm)2](BPh4)2.0.5CH3OH . 0.2H2O (3.0371(5) A), confirm the existence of short Rh...Rh interactions. The metal-metal separation for the rhodium(II) adduct, [Rh(2)(dmb)4Cl2]Cl2.6CHCl3 (2.8465(6) A), is consistent with a formal Rh-Rh bond. For the two luminescent rhodium(I) dimers and six previously investigated diisocyano-bridged dimers with and without dppm ligands, the intense spin-allowed dsigma-->psigma absorption band maximum shifts to longer wavelengths with decreasing Rh...Rh separation, and there is an approximate correlation between band energy and the inverse of the metal-metal separation cubed. Both [Rh2(dmb)4]2+ and [Rh2(dmb)4(dppm)2]2+ undergo oxidative addition in the presence of iodine. In the conversion of [Rh2(dmb)4]2+ to [Rh2(dmb)4I2]2+, the observed intermediate is tentatively assigned to a tetramer composed of two rhodium dimers. In the case of [Rh2(dmb)2(dppm)2]2+, no intermediate was detected.

Dinuclear rhodium(II) pivalate complexes with N-donor ligands

Eight dinuclear rhodium(II) complexes containing various, (primarily, polyfunctional) N-donor ligands in the trans position with respect to the Rh-Rh bond were synthesized and characterized by X-ray diffraction. In the Chinese-lantern dinuclear rhodium(II) pivalates, RhII2 (μ-OOCCMe3)4(L)2 (L is 2,3-diaminopyridine (2), 7,8-benzoquinoline (4), 2,2′:6′,2″-terpyridine (5), N-phenyl-o-phenylenediamine (7)), and RhII2 (μ-OOCCMe3)4L1L2 (3, L1 is 2-phenylpyridine, L2 = MeCN), the steric effects of the axial ligands are most strongly reflected in the Rh-N(L) and Rh-Rh bond lengths. The introduction of chelating ligands containing a conformationally rigid chelate ring leads to the cleavage of two carboxylate bridges to form the dinuclear double-bridged structure RhII2 (μ- OOCCMe3)2(OCCMe3)2(η2-L3)2, where L3 is 8-amino-2,4-dimethylquinoline (6). The reaction of complex 7 containing coordinated N-phenyl-o-phenylenediamine with pyrrole-2,5-dialdehyde afforded the new RhII2(μ-OOCCMe3)4(L4)2 complex (8) containing 5-(1-phenyl-1-H-benzimidazol-2-yl)-1H-pyrrole-2-carbaldehyde (L4) in the axial positions of the dirhodium tetracarboxylate fragment. The coordinated diamine differs in reactivity from the free diamine. The reaction of the former with the above dialdehyde affords the [1+1]-condensation product, viz., 5-{(E)-[(2-anilinophenyl)imino]methyl}-1-H-pyrrole-2-carbaldehyde, whereas the reaction of unsubstituted o-phenylenediamine gives 5-{(E)-[(2-aminophenyl)imino]methyl}-1-H-pyrrole-2-carbaldehyde (L5) . The reaction of the latter with RhII2(μ-OOCCMe3)4(H2O)2 affords the dinuclear complex RhII2(μ-OOCCMe3)2(OOCCMe3)2(η2-L5)2 (9), which is an analog of complex 6 containing only two bridging carboxylate groups.

Kinetic studies of some substituted hexarhodium carbonyl clusters

Reactions of the halides X- (X- = chloride, bromide or iodide) with the substituted cluster Rh6(CO)15(PPh3) in oxygen-free chloroform lead to [Rh5(CO)14(PPh3)]-, Rh(CO)2(PPh3)2X and [Rh(CO)2X2]- in the molar ratios 2:1: approximately 13. Oxidation by the solvent is assumed to lead to most of the Rh(I) product, and the stoichiometry for reactions with I- can be defined as 4Rh6(CO)15(PPh3) + 27I- + 12CHCl3 --> 2[Rh5(CO)14(PPh3)]- + Rh(CO)2(PPh3)2I + 13[Rh(CO)2I2]- + 6C2H2Cl4 + 4CO + 12Cl-. This can be rationalized quite simply with the aid of a few generally justifiable assumptions. Rate constants for reactions with bromide increase to a limiting value with increasing [Br-] in a way that shows that breaking of one Rh-Rh bond, with an unusual closo to nido structural change, is rate determining. This opening of the cluster might be spontaneous or solvent induced. To complete the reaction, the bromide has to compete with the reverse nido to closo change. The same closo to nido change is also a major rate determining step for reactions with P(OPh)3 in oxygen-free solutions, and for reactions with bromide in oxygenated solutions in the presence of trifluoroacetic and some other acids. The limiting rates increase slightly with increasing basicity of the ligands P(p-XC6H4)3 along the series X = F3C, Cl, F, H and MeO. Activation parameters for these reactions are reported.

A hardwon dirhodium paddlewheel with guanidinate type (hpp) bridging ligands

We report a dirhodium paddlewheel compound which has an Rh2(5+) core enveloped by four guanidinate ligands and an axially coordinated bromine atom; Rh2(hpp)4Br crystallizes as a 1-D chain with a Rh-Rh distance of 2.430(3) è and a Rh-Rh bond order of 1.5 (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine).

Binding of DNA purine sites to dirhodium compounds probed by mass spectrometry.

The adducts formed between the antitumor active compounds [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), Rh(2)(O(2)CCH(3))(4), and Rh(2)(O(2)CCF(3))(4) with DNA oligonucleotides have been assessed by matrix-assisted laser desorption ionization (MALDI) and nanoelectrospray (nanoESI) coupled to time-of-flight mass spectrometry (TOF MS). A series of MALDI studies performed on dipurine (AA, AG, GA, and GG)-containing single-stranded oligonucleotides of different lengths (tetra- to dodecamers) led to the establishment of the relative reactivity cis-[Pt(NH(3))(2)(OH(2))(2)](2+) (activated cisplatin) approximately Rh(2)(O(2)CCF(3))(4) > cis-[Pt(NH(3))(2)Cl(2)] (cisplatin) >> [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) > Rh(2)(O(2)CCH(3))(4) approximately Pt(C(6)H(6)O(4))(NH(3))(2) (carboplatin). The relative reactivity of the complexes is associated with the lability of the leaving groups. The general trend is that an increase in the length of the oligonucleotide leads to enhanced reactivity for Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) and Rh(2)(O(2)CCH(3))(4) (except for the case of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+), which reacts faster with the GG octamers than with the dodecamers), whereas the reactivity of Rh(2)(O(2)CCF(3))(4) is independent of the oligonucleotide length. When monitored by ESI, the dodecamers containing GG react faster than the respectiveAA oligonucleotides in reactions with Rh(2)(O(2)CCF(3))(4) and Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), whereas AA oligonucleotides react faster with Rh(2)(O(2)CCH(3))(4). The mixed (AG, GA) purine sequences exhibit comparable rates of reactivity with the homopurine (AA, GG) dodecamers in reactions with Rh(2)(O(2)CCH(3))(4). The observation of initial dirhodium-DNA adducts with weak axial (ax) interactions, followed by rearrangement to more stable equatorial (eq) adducts, was achieved by electrospray ionization; the Rh-Rh bond as well as coordinated acetate or acetonitrile ligands remain intact in these dirhodium-DNA adducts. MALDI in-source decay (ISD), collision-induced dissociation (CID) MS-MS, and enzymatic digestion studies followed by MALDI and ESI MS reveal that, in the dirhodium compounds studied, the purine sites of the DNA oligonucleotides interact with the dirhodium core. Ultimately, both MALDI and ESI MS proved to be complementary, valuable tools for probing the identity and stability of dinuclear metal-DNA adducts.