Rh Distance Research Articles

Nanosecond-Lived Excimer Observation in a Crystal of a Rhodium(I) Complex via Time-Resolved X-ray Laue Diffraction.

The rare observation of transient Rh···Rh excimer formation in a single crystal is reported. The estimated excited-state lifetime at 100 K is 2 ns, which makes it the shortest-lived small-molecule species caught experimentally using the laser-pump/X-ray-probe time-resolved Laue method. Upon excitation with 390 nm laser light, the intermolecular Rh···Rh distance decreases from 3.379(4) to 3.19(1) Å, and the metal-metal contact gains more bonding character. On the basis of the experimental results and theoretical modeling, the structural changes determined with 100 ps time resolution reflect principally the S0 → S1 electronic transition.

Two polymorphs of [Rh(μ-I)(COD)]2.

The solid-state structure of di-μ-iodido-bis-{[(1,2,5,6-η)-cyclo-octa-1,4-diene]rhodium(I)}, [Rh2I2(C8H12)2] or [Rh(μ-I)(COD)]2, was determined from two crystals with different morphologies, which were found to correspond to two polymorphs containing Rh dimers with significantly different mol-ecular structures. Both polymorphs are monoclinic and the [Rh(μ-I)(COD)]2 mol-ecules in each case possess C2 v symmetry. However, the core geometry of the butterfly-shaped Rh2I2 core differs substanti-ally. In the C2/c polymorph, the core geometry of [Rh(μ-I)(COD)]2 B is bent, with a hinge angle of 96.13 (8)° and a Rh⋯Rh distance of 2.9612 (11) Å. The P21/c polymorph features a more planar [Rh(μ-I)(COD)]2 P core geometry, with a hinge angle of 145.69 (9)° and a Rh⋯Rh distance of 3.7646 (5) Å.

Structures and Properties of 4-phpy, pyz, and 4,4′-bpy Adducts of Lantern-Type Dirhodium Complexes with µ-Formamidinato and µ-Carboxylato Bridges

Dinuclear and polymer complexes of 4-phenylpyridine (4-phpy), pyazine (pyz), and 4,4′-bipyridine (4,4′-bpy) were prepared by using cis-[Rh2(4-Me-pf)2(O2CR)2] (4-Me-pf- =N,N’-bis(4-methylphenyl)formamidinate anion; R = CF3 and CMe3) as precursor dinuclear units. The dinuclear structures of cis-[Rh2II,II(4-Me-pf)2(O2CR)2(4-phpy)2] and cis-[Rh2II,III(4-Me-pf)2(O2CCMe3)2(4-phpy)2]BF4 and polymer structures of [Rh2II,II(4-Me-pf)2(O2CR)2(L)]n (L = pyz and 4,4′-bpy) were confirmed by X-ray crystal structure analyses. In these complexes, the lantern-type dinuclear core structures with cis-(2:2) arrangement of formamidinato (4-Me-pf-) and carboxylato ligands are preserved with Rh–Rh distances of 2.44–2.47 Å, regardless of the difference in the axial ligand and oxidation state Rh2II,II or Rh2II,III. In the cyclic voltammograms (CVs) in CH2Cl2, the redox potentials for Rh2II,III/Rh2II,II were estimated as E1/2 = 0.07 V and −0.28 V (vs. Fc+/Fc) for cis-[Rh2(4-Me-pf)2(O2CCF3)2(4-phpy)2] and cis-[Rh2(4-Me-pf)2(O2CCMe3)2(4-phpy)2], respectively, negatively shifted by 0.16 and 0.12 V compared with those of corresponding parent dinuclear complexes. The results were interpreted that the axial interaction with 4-phpy ligands makes the Rh2II,II core oxidized easily. The oxidized complex cis-[Rh2(4-Me-pf)2(O2CCMe3)2(4-phpy)2]BF4 is paramagnetic, which was confirmed by effective magnetic moment value μeff = 1.90 μB at 300 K per Rh2II,III unit (S = 1/2). The polymer complexes [Rh2(4-Me-pf)2(O2CR)2(L)]n (L = pyz and 4,4′-bpy) showed Type II gas-adsorption properties for N2.

Unusual Structural Features in the Adduct of Dirhodium Tetraacetate with Lysozyme.

The structures of the adducts formed upon reaction of the cytotoxic paddlewheel dirhodium complex [Rh2(μ-O2CCH3)4] with the model protein hen egg white lysozyme (HEWL) under different experimental conditions are reported. Results indicate that [Rh2(μ-O2CCH3)4] extensively reacts with HEWL:it in part breaks down, at variance with what happens in reactions with other proteins. A Rh center coordinates the side chains of Arg14 and His15. Dimeric Rh–Rh units with Rh–Rh distances between 2.3 and 2.5 Å are bound to the side chains of Asp18, Asp101, Asn93, and Lys96, while a dirhodium unit with a Rh–Rh distance of 3.2–3.4 Å binds the C-terminal carboxylate and the side chain of Lys13 at the interface between two symmetry-related molecules. An additional monometallic fragment binds the side chain of Lys33. These data, which are supported by replicated structural determinations, shed light on the reactivity of dirhodium tetracarboxylates with proteins, providing useful information for the design of new Rh-containing biomaterials with an array of potential applications in the field of catalysis or of medicinal chemistry and valuable insight into the mechanism of action of these potential anticancer agents.

Synthesis and structural determination of [Rh(opo)(CO)(PR3)] complexes (opo− = 2-oxopyridin-1-olate) and in situ isomeric behavior from preliminary kinetic study of iodomethane oxidative addition

A range of complexes, [Rh(opo)(CO)(PR3)], with opo− = 2-oxopyridin-1-olate, and [Rh(opo)(CO)2] (1), [Rh(opo)(CO)(PPh3)] (2AI/BI ), [Rh(opo)(CO)(PPh2Cy)] (3AI/BI ), [Rh(opo)(CO)(PPhCy2)] (4AI/BI ), [Rh(opo) (CO)(PCy3)] (5AI/BI ), [Rh(opo)(CO)(P(o-Tol)3)] (6AI/BI ), [Rh(opo)(CO) (P(m-Tol)3)] (7AI/BI ), [Rh(opo)(CO)(P(p-Tol)3)] (8AI/BI ), and cis-[Rh(opo)(CO)(CH3)(I)(PPh3)] (2AII ), were synthesized and characterized by IR and NMR spectroscopy. Two isomers (A = major, MaI; B = minor, MiI) were observed in ca. 2:1 ratios for complexes 2–8 due to the similar characteristics of the chelating O-donors, yet non-symmetric within the bidentate ligand. Single crystal X-ray diffraction yielded the solid-state structures of the major isomers of 4AI , 5AI , 6AI , and 2AII . Metallophilic interactions were observed between neighboring Rh(I) centers in [Rh(opo)(CO)2] (1) with an intermolecular Rh···Rh distance of 3.5780(5) Å. These interactions do not occur in the crystal structures of 4AI , 5AI , 6AI , and 2AII , attributed to the steric bulk of the phosphine ligands and the interactions observed in the crystal lattice of these structures. Preliminary iodomethane oxidative addition (OA) to the rhodium(I) complexes [Rh(opo)(CO)(PR3)] was investigated by IR, NMR, and UV-Vis spectroscopy, yielding corresponding alkyl intermediate species (nAII /BII ) or acyl products (nAIII /BIII ), respectively, for n = 2–8 and PR3 = PPh3 (2), PPh2Cy (3), PPhCy2 (4), PCy3 (5), P(o-Tol)3 (6), P(m-Tol)3 (7), P(p-Tol)3 (8). The rates of OA and assumed subsequent migratory carbonyl insertion (CI) reactions were identical within standard deviations, suggesting the CI to be fast relative to the OA. Moreover, the interconversion between the two isomers A and B seemed to be a slower process, governed by the relative steric strain induced by the tertiary phosphine ligands and not by the rates.

Ionospheric total electron content of comet 67P/Churyumov-Gerasimenko

We study the evolution of a cometary ionosphere, using approximately two years of plasma measurements by the Mutual Impedance Probe on board the Rosetta spacecraft monitoring comet 67P/Churyumov-Gerasimenko (67P) during August 2014–September 2016. The in situ plasma density measurements are utilized to estimate the altitude-integrated electron number density or cometary ionospheric total electron content (TEC) of 67P based on the assumption of radially expanding plasma. The TEC is shown to increase with decreasing heliocentric distance (rh) of the comet, reaching a peak value of ~(133 ± 84) × 109cm−2averaged around perihelion (rh< 1.5 au). At large heliocentric distances (rh> 2.5 au), the TEC decreases by ~2 orders of magnitude. For the same heliocentric distance, TEC values are found to be significantly larger during the post-perihelion periods compared to the pre-perihelion TEC values. This “ionospheric hysteresis effect” is more prominent in the southern hemisphere of the comet and at large heliocentric distances. A significant hemispheric asymmetry is observed during perihelion with approximately two times larger TEC values in the northern hemisphere compared to the southern hemisphere. The asymmetry is reversed and stronger during post-perihelion (rh> 1.5 au) periods with approximately three times larger TEC values in the southern hemisphere compared to the northern hemisphere. Hemispheric asymmetry was less prominent during the pre-perihelion intervals. The correlation of the cometary TEC with the incident solar ionizing fluxes is maximum around and slightly after perihelion (1.5 au <rh< 2 au), while it significantly decreases at larger heliocentric distances (rh> 2.5 au) where the photo-ionization contribution to the TEC variability decreases. The results are discussed based on cometary ionospheric production and loss processes.

Intramolecular hydrogen bonding, π-π stacking interactions, and substituent effects of 8-hydroxyquinoline derivative supermolecular structures: a theoretical study

Quantum chemical calculations have been performed to gauge intramolecular hydrogen bonding and π-π stacking interactions of 8-hydroxyquinoline (8-HQ) derivatives (substituents X = OH, CH3, H, F, Cl, CF3, CN, NO2) supermolecular structures. The effects of substituents on 8-HQ derivatives have also been studied with the M06-2X method and 6-311++G** basis set. The basis set superposition error (BSSE)-corrected binding energies (ΔE), hydrogen bonding distances (rH···N), and intermolecular distance (d┻) were calculated with corresponding Hammett electronic parameters (σ). The topological parameters (ρ, ∇ρ2) and energy of intramolecular hydrogen bonding interaction (EH···N) of these π-π stacking 8-HQ derivatives have been studied by using atoms in molecules (AIM) theory. The electrostatic potential (ESP) analysis has been applied to gain the most negative and the most positive electrostatic potential V(r) values (Vs,min and Vs,max). It can be found that there are good relationships between the Hammett constants and the Vs,min values.

μ-Carbonyl-bis(carbonyl{η5-[tricarbonyl(η6-2-methylindenyl)chromium(0)]rhodium(III)})(Rh—Rh)

The structure of the title complex, [Cr2Rh2(C10H9)2(CO)9], was solved by single-crystal X-ray diffraction analysis in the P21/c space group. The crystal shows two units of the tetrametallic complex in the asymmetric unit. The most remarkable features of this new compound are the asymmetric bonding of the RhIII atom to the indenyl ligand, which can be formulated in terms of an η2;η3 coordination rather than η5, and the Rh—Rh distances of 2.6756 (6) and 2.6737 (6) Å that are characteristic of an intermetallic interaction supported by a bridging carbonyl ligand. The infrared spectroscopic signature informs of its clear bridging feature characterized by a significantly elongated C=O bond length compared to those in the two Rh-bound `terminal' carbonyl ligands.

Rare earth-rhodium-plumbides RE 2Rh2Pb with RE = La–Nd, Sm, Gd and Tb

Abstract The plumbides RE 2Rh2Pb (RE = La–Nd, Sm, Gd, Tb) were synthesized in sealed niobium ampoules by induction melting of the pure elements or directly via arc-melting. The characterization of the samples by X-ray powder diffraction confirmed their Mo2B2Fe-type structure (space group P4/mbm) at room temperature. The Sm2Rh1.924Pb structure was refined from single-crystal X-ray diffractometer data: a=760.02(5), c=378.20(3) pm, wR=0.0387, 292 F 2 values, 13 variables. The rhodium site shows small defects. The RE 2Rh2Pb plumbides are simple 1:1 intergrowth structures of AlB2 and CsCl related slabs of compositions RERh2 and REPb. The Rh2 dumbbell in the SmRh2 slab of Sm2Rh2Pb shows a Rh–Rh distance of 281 pm. Temperature-dependent magnetic susceptibility measurements of La2Rh2Pb, Pr2Rh2Pb and Nd2Rh2Pb showed that the rhodium atoms carry no localized magnetic moments. La2Rh2Pb exhibits Pauli-paramagnetic behavior induced by the conduction electrons. The ground state of the praseodymium compound is ferromagnetic below T C=3.3 K while the neodymium compound shows a transition to an antiferromagnetic state at T N=6.1 K and a metamagnetic transition at a critical field of ca. 1000 Oe.

Evolution of H2O production in comet C/2012 S1 (ISON) as inferred from forbidden oxygen and OH emission

We present H2O production rates for comet C/2012 S1 (ISON) derived from observations of [O I] and OH emission during its inbound leg, covering a heliocentric distance range of 1.8–0.44 AU. Our production rates are in agreement with previous measurements using a variety of instruments and techniques and with data from the various observatories greatly differing in their projected fields of view. The consistent results across all data suggest the absence of an extended source of H2O production, for example sublimation of icy grains in the coma, or a source with spatial extent confined to the dimensions of the smallest projected field of view (in this case < 1000 km). We find that ISON had an active area of around 10 km2 for heliocentric distances Rh > 1.2 AU, which then decreased to about half this value from Rh = 1.2–0.9 AU. This was followed by a rapid increase in active area at about Rh = 0.6 AU, corresponding to the first of three major outbursts ISON experienced inside of 1 AU. The combination of a detected outburst in the light curve and rapid increase in active area likely indicates a major nucleus fragmentation event. The 5–10 km2 active area observed outside of Rh = 0.6 AU is consistent with a 50–100% active fraction for the nucleus, larger than typically observed for cometary nuclei. Although the absolute value of the active area is somewhat dependent on the thermal model employed, the changes in active area observed are consistent among models. The conclusion of a 50–100+% active fraction is robust for realistic thermal models of the nucleus. However the possibility of a contribution of a spatially unresolved distribution of icy grains cannot be discounted. As our [OI]-derived H2O production rates are consistent with values derived using other methods, we conclude that the contribution of O2 photodissociation to the observed [O I] emission is at most 5–10% that of the contribution of H2O for ISON. This is consistent with the expected contribution of O2 photodissociation if O2/H2O ∼ 4%, meaning [O I] emission can still be utilized as a reliable proxy for H2O production in comets as long as O2/H2O ≲ 4%, similar to the abundance measured by the ROSINA instrument on Rosetta at comet 67P/Churyumov–Gerasimenko.

Rhodium-rhodium interactions in [Rh(β-diketonato)(CO)2] complexes

The solid state single crystal structures of [Rh(β-diketonato)(CO)2], where β-diketonato = (R1COCHCOR2)−, with R1, R2 = CF3, C4H3S (1), C4H3S, C4H3S (2), Ph, C4H3S (3) and CF3, Ph (4), show that these complexes in some cases form dinuclear units, which stack in chains with weak metallophilic rhodium-rhodium interactions, while in other cases they produce continuous polymeric units, with equal intermolecular Rh⋯Rh distances. Different solid state structural data is reported herein for these four complexes, including a low temperature comparison with ambient data for (4). In the latter case, weak intermolecular halogen bonding has also been identified, which additionally contributes to the stability of (4) in the solid state. Computational evaluation of the frontier molecular orbitals of both dinuclear and tetranuclear models of complexes (1)–(4), show Rh(dz2)-Rh(dz2) σ bonding and σ* anti-bonding orbitals. An NBO analysis of the dinuclear units, revealed a donor-acceptor interaction between the two rhodium atoms in such a unit, while a QTAIM study identified a bonding path between the two rhodium atoms therein.

EN ROUTE TO DESTRUCTION: THE EVOLUTION IN COMPOSITION OF ICES IN COMET D/2012 S1 (ISON) BETWEEN 1.2 AND 0.34 AU FROM THE SUN AS REVEALED AT INFRARED WAVELENGTHS*

ABSTRACT We report production rates for H2O and eight trace molecules (CO, C2H6, CH4, CH3OH, NH3, H2CO, HCN, C2H2) in the dynamically new, Sun-grazing Comet C/2012 S1 (ISON), using high-resolution spectroscopy at Keck II and the NASA IRTF on 10 pre-perihelion dates encompassing heliocentric distances R h = 1.21–0.34 AU. Measured water production rates spanned two orders of magnitude, consistent with a long-term heliocentric power law Q(H2O) &prop; R h ( &minus; 3.1 &PlusMinus; 0.1 ) ?> . Abundance ratios for CO, C2H6, and CH4 with respect to H2O remained constant with R h and below their corresponding mean values measured among a dominant sample of Oort Cloud comets. CH3OH was also depleted for R h &gt; 0.5 AU, but was closer to its mean value for R h ≤ 0.5 AU. The remaining four molecules exhibited higher abundance ratios within 0.5 AU: for R h &gt; 0.8 AU, NH3 and C2H2 were consistent with their mean values while H2CO and HCN were depleted. For R h &lt; 0.5 AU, all four were enriched, with NH3, H2CO, and HCN increasing most. Spatial profiles of gas emission in ISON consistently peaked sunward of the dust continuum, which was asymmetric antisunward and remained singly peaked for all observations. NH3 within 0.5 AU showed a broad spatial distribution, possibly indicating its release in the coma provided that optical depth effects were unimportant. The column abundance ratio NH2/H2O at 0.83 AU was close to the “typical” NH/OH from optical wavelengths, but was higher within 0.5 AU. Establishing its production rate and testing its parentage (e.g., NH3) require modeling of coma outflow.

Preparation, structure, and dynamic and electrochemical behaviors of dinuclear rhodium(I) complexes with bridging formamidinato ligands

Dinuclear rhodium(I) complexes, [Rh(4-Me-pf) (cod)]2 (1), [Rh(3,5-Me2-pf) (cod)]2 (2), [Rh(4-Me-pf) (nbd)]2 (3), [Rh(3,5-Me2-pf) (nbd)]2 (4), and [Rh(2,6-F2-pf) (nbd)]2 (5), have been synthesized and characterized by X-ray structure analysis, 1H, 13C, and 19F NMR, UV–vis, ESI-TOF-MS, and elemental analysis. In these complexes, two rhodium atoms are bridged by two formamidinato ligands and each rhodium atom is coordinated by one chelating cod or nbd ligand to form an approximately square planar coordination structure with two nitrogen atoms and two double bonds. The Rh··Rh distances are in the range of 3.2668 to 2.9726 Å, suggesting a direct bonding interaction between two rhodium atoms. Variable temperature NMR studies in CD2Cl2 solution have revealed that 1–5 exhibit a novel dynamic behavior, that is, an interconversion between two enantiomers. The activation parameters for racemization have been determined by the line shape analyses of the 1H and 19F NMR spectra taken at various temperatures. Variable temperature NMR studies have also revealed that the rotation rates of the four aryl groups around N–C(aryl) bonds are extremely different in each complex. The cyclic voltammetry study has shown that the oxidation potentials corresponding to Rh23+/Rh2+ are 0.41 V in the cod complexes (1 and 2) while those in the nbd complexes (3–5) have shown negative shift by ca. 0.2 V. The reasons for the difference in dynamic behaviors and redox properties among these complexes have been discussed.

{μ-PbSe}: a heavy CO homologue as an unexpected ligand.

Reactions of [K(18-crown-6)]2 [Pb2 Se3 ] and [K([2.2.2]crypt)]2 [Pb2 Se3 ] with [Rh(PPh3 )3 Cl] in en (ethane-1,2-diamine) afforded ionic compounds with [Rh3 (PPh3 )6 (μ3 -Se)2 ](-) and [Rh3 (CN)2 (PPh3 )4 (μ3 -Se)2 (μ-PbSe)](3-) anions, respectively. The latter contains a PbSe ligand, a rather uncommon homologue of CO that acts as a μ-bridge between two Rh atoms. Quantum chemical calculations yield a significantly higher bond energy for PbSe than for CO, since the size of the ligand orbitals better matches the comparably rigid Rh-Se-Rh angles and the resulting Rh⋅⋅⋅Rh distance. To rationalize the bent coordination of the ligand, orbitals with significant ligand contributions and their dependence on the bonding angle were investigated in detail.

Bistable multifunctionality and switchable strong ferromagnetic-to-antiferromagnetic coupling in a one-dimensional rhodium(I)-semiquinonato complex.

We present a comprehensive study of the synthesis, heat capacity, crystal structures, UV-vis-NIR and mid-IR spectra, DFT calculations, and magnetic and electrical properties of a one-dimensional (1D) rhodium(I)-semiquinonato complex, [Rh(3,6-DBSQ-4,5-(MeO)2)(CO)2]∞ (3), where 3,6-DBSQ-4,5-(MeO)2(•-) represents 3,6-di-tert-butyl-4,5-dimethoxy-1,2-benzosemiquinonato radical anion. The compound 3 comprises neutral 1D chains of complex molecules stacked in a staggered arrangement with short Rh-Rh distances of 3.0796(4) and 3.1045(4) Å at 226 K and exhibits unprecedented bistable multifunctionality with respect to its magnetic and conductive properties in the temperature range of 228-207 K. The observed bistability results from the thermal hysteresis across a first-order phase transition, and the transition accompanies the exchange of the interchain C-H···O hydrogen-bond partners between the semiquinonato ligands. The strong overlaps of the complex molecules lead to unusually strong ferromagnetic interactions in the low-temperature (LT) phase. Furthermore, the magnetic interactions in the 1D chain drastically change from strongly ferromagnetic in the LT phase to antiferromagnetic in the room-temperature (RT) phase with hysteresis. In addition, the compound 3 exhibits long-range antiferromagnetic ordering between the ferromagnetic chains and spontaneous magnetization because of spin canting (canted antiferromagnetism) at a transition temperature T(N) of 14.2 K. The electrical conductivity of 3 at 300 K is 4.8 × 10(-4) S cm(-1), which is relatively high despite Rh not being in a mixed-valence state. The temperature dependence of electrical resistivity also exhibits a clear hysteresis across the first-order phase transition. Furthermore, the ferromagnetic LT phase can be easily stabilized up to RT by the application of a relatively weak applied pressure of 1.4 kbar, which reflects the bistable characteristics and demonstrates the simultaneous control of multifunctionality through external perturbation.

WIDE-FIELDHUBBLE SPACE TELESCOPEOBSERVATIONS OF THE GLOBULAR CLUSTER SYSTEM IN NGC 1399

We present a comprehensive high spatial-resolution imaging study of globular clusters (GCs) in NGC1399, the central giant elliptical cD galaxy in the Fornax galaxy cluster, conducted with HST/ACS. Using a novel technique to construct drizzled PSF libraries for HST/ACS data, we accurately determine the fidelity of GC structural parameter measurements from detailed artificial star cluster experiments. The measurement of rh for the major fraction of the NGC1399 GC system reveals a trend of increasing rh versus galactocentric distance, Rgal, out to about 10 kpc and a flat relation beyond. This trend is very similar for blue and red GCs which are found to have a mean size ratio of rh(red)/rh(blue)=0.82+/-0.11 at all galactocentric radii from the core regions of the galaxy out to ~40 kpc. This suggests that the size difference between blue and red GCs is due to internal mechanisms related to the evolution of their constituent stellar populations. Modeling the mass density profile of NGC1399 shows that additional external dynamical mechanisms are required to limit the GC size in the galaxy halo regions to rh~2 pc. We suggest that this may be realized by an exotic GC orbit distribution function, an extended dark matter halo, and/or tidal stress induced by the increased stochasticity in the dwarf halo substructure at larger Rgal. We match our GC rh measurements with radial velocity data from the literature and find that compact GCs show a significantly smaller line-of-sight velocity dispersion, <sigma(cmp)>=225+/-25 km/s, than their extended counterparts, <sigma(ext)>=317+/-21 km/s. Considering the weaker statistical correlation in the GC rh-color and the GC rh-Rgal relations, the more significant GC size-dynamics relation appears to be astrophysically more relevant and hints at the dominant influence of the GC orbit distribution function on the evolution of GC structural parameters.

The Bimetallic Activation of White Phosphorus by trans‐[RhCl(CO)(dppm)]2 Results in a Tetrahedro‐Rh2P2 Moiety

AbstractThe reaction of trans‐[RhCl(CO)(dppm)]2 with P4 carried out at room temperature showed that bimetallic cooperation resulted in the cleavage of P4, giving a μ,η2‐P2 (μ‐κ2:κ2‐P2) ligand coordinated to the two metal centres. The new complex cis‐[Rh2(CO)2(μ‐dppm)2(μ,η2‐P2)] has been fully characterised in solution by NMR spectroscopy and in the solid state by single‐crystal X‐ray diffraction. The Rh–Rh distance of 2.723(18) Å is consistent with the presence of a metal–metal single bond, which was supported by theoretical analysis showing that the HOMO of the complex has metal–metal bond character. A literature survey revealed that, in most cases, the P2 unit acts as a P24– ligand, and also in the case at hand the diphosphenyl ligand behaves as an eight‐electron donor with all the π* combinations occupied and donates into four empty metal combinations.

Cooperative Catalysis: Large Rate Enhancements with Bimetallic Rhodium Complexes

A large increase in reaction rate was observed for the catalyzed mono- and dihydroalkoxylation of alkynes when structurally constrained bimetallic Rh(I) catalysts with imidazolyl-imine ligands were compared to their monometallic analogues. The “cooperative” enhancement was vastly improved by restricting the conformational freedom of the bimetallic structure and minimizing the intermetallic Rh···Rh distance.

Neutral one-dimensional metal chains consisting of alternating anionic and cationic rhodium complexes

The metallophilic interactions were investigated within chains of oppositely charged rhodium carbonyl complexes. The cationic [Rh(CO)(2)(L)](+) (L = 2,2'-bipyridine and 1,10-phenanthroline) and anionic [RhCl(2)(CO)(2)](-) units were self-assembled into one dimensional rhodium chains supported by electrostatic interactions. The array of Rh centers in {[Rh(CO)(2)(2,2'-bpy)][RhCl(2)(CO)(2)]}(n) was found to be nearly linear with a Rh···Rh···Rh angle of 170.927(11)° and Rh···Rh distances of 3.3174(5) Å and 3.4116(5) Å. The crystal structure of {[Rh(CO)(2)(1,10-phen)][RhCl(2)(CO)(2)]} consisted of two sets of crystallographically independent chains with slightly different Rh···Rh···Rh angles (170.275(9)° and 159.573(9)°). The higher linearity allowed closer packing of the rhodium complexes. The Rh···Rh distances were 3.2734(3) Å and 3.3155(3) Å for the more linear and 3.3498(3) Å and 3.3211(3) Å for the less linear system. The existence of metallophilic interactions was confirmed computationally by TD-DFT and QTAIM analysis. The computational results also indicated that the intermolecular charge transfer from the cation to the anion had a significant contribution to the absorption properties of the chain compounds.

Unprecedented partial paddlewheel dirhodium methyl isocyanide compounds with unusual structural and electronic properties: a comprehensive experimental and theoretical study

A series of dirhodium acetonitrile compounds, namely cis-[Rh2(DTolF)2(CH3CN)6][BF4]2 (1) cis-[Rh2(F-form)2(CH3CN)6][BF4]2 (2), cis-[Rh2(NNN)2(CH3CN)6][BF4]2 (3) ([F-form]−: p-difluorophenylformamidinate, [NNN]−: p-ditolyltriazenide, [DTolF]−: p-ditolylformamidinate), cis-[Rh2[Ph2P(C6H4)]2(CH3CN)6][BF4]2 (4) and their unprecedented methyl isocyanide analogs 5–8, respectively, were synthesized and characterized by X-ray crystallography, cyclic voltammetry, NMR, electronic and infrared spectroscopies. The elongation of the Rh–Rh distances (∼0.07 A) and the bonds trans to eq CH3NC in 5–8vs.1–4 are in accord with the strong trans influence of isocyanide. The short Rh–C (CH3NC) distances in 5–8 are attributed to π-backbonding, whereas the short CN distances in CH3NC and the high energy ν(CN) stretches are attributed to appreciable σ-donation of CH3NC. Of particular note are the longer Rh–Rh bonds (∼2.76 A) that 8 exhibits and unprecedented short ax Rh–C distances in the same range as the eq Rh–C bonds (2.01–2.04 A) for CH3NC. TD-DFT calculations predict the lowest-energy LMCT transitions in the order 1 < 2 < 5 < 6, 3 < 7 and the HOMO–LUMO energy gaps to be greater in 5–7vs.1–3, respectively, findings corroborated by electrochemical and electronic spectral data. This trend is attributed to the stabilization of the HOMOs and destabilization of the LUMOs in 5–7. The π-backbonding in 5–8 stabilizes the Rh2(π*) orbitals and σ-donation from CH3NC destabilizes the Rh2(σ) orbitals, with the extreme case being 8 where Rh2(σ) becomes the HOMO. This fact accounts for the ∼20-fold increase in intensity of the HOMO Rh2(σ) → LUMO Rh2(σ*) transition for 8 (∼370 nm) vs.4, also in accord with TD-DFT calculations.