Structural Diversity in Paddlewheel Dirhodium(II) Compounds through

Article pubs.acs.org/crystal

Structural Diversity in Paddlewheel Dirhodium(II) Compounds through Ionic Interactions: Electronic and Redox Properties Pilar Amo-Ochoa,*,† Reyes Jiménez-Aparicio,*,‡ Josefina Perles,‡ M. Rosario Torres,§ Marcello Gennari,† and Félix Zamora† †

Departamento de Química Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain ‡ Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain § Centro de Asistencia a la Investigación de Rayos X, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain S Supporting Information *

ABSTRACT: Reactions of dinuclear rhodium(II) tetracarboxylates, [Rh2(O2CR)4] (R = Me, Et), with halides (Br− and I−) or pseudohalides (OCN−) yield dinuclear complexes with intriguing supramolecular architectures based on ionic interactions. The solid-state arrangement of the complexes presented here has been studied using single-crystal X-ray diffraction. Discrete anionic units with the axial positions occupied by isocyanate, Na2[Rh2(O2CMe)4(NCO)2]·4H2O (1), water and isocyanate, Na[Rh2(O2CMe)4(NCO)(H2O)] (2), iodide, {K2[Rh2(O2CEt)4I2]·H2O}n (3), and bromide ligands, {K 2 [Rh 2 (O 2 CEt) 4 Br 2 ]·H 2 O} n (4) and K[Rh 2 (O2CEt)4(Br)0.5]2[Rh2(O2CEt)4(H2O)2] (5), have been found. Complex 1 shows monodimensional polymeric chains stabilized through ionic interactions, while complexes 2−4 consist of two-dimensional layers. Finally, a three-dimensional network containing two kinds of dirhodium moieties has been found in complex 5. Speciation of the [Rh2(O2CR)4]/X− (R = Me, Et; X = OCN, Br, I) systems was investigated in aqueous solution by UV−visible titrations, helping us to rationalize the obtention of different Rh−X stoichiometries in the crystal state. By cyclic voltammetry, we have evaluated the effect of X− coordination on the oxidation properties of these dirhodium(II) units.

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Regarding the axial positions of these units, we find that, although, in some cases, these are occupied by the oxygen atom of a neighbor dimetallic unit to form chains (Scheme 1c), in the vast majority of the examples, O-, S-, N-, or P-donor solvent molecules are present (Scheme 1a). Yet, examples of dirhodium units with just one axial ligation are rare.6,12,13 Even more unusual is the formation of dirhodium carboxylate units with two different ligands in the axial positions (Scheme 1b). With the exception of the [Rh2(O2CCF3)4] complex, which has been widely studied due to its behavior as a Lewis acid,14,15 to our knowledge, only three other examples are described in the literature.12,13,16,17 In addition, the presence of anionic halide or pseudohalide ligands occupying the axial positions is not usual in dirhodium complexes.1,16,18−22 Although extensive investigations of [Rh2(O2CR)4] (R = alkyl group) have been carried out, virtually there is no research of anionic dirhodium(II) units, and just a few anionic

INTRODUCTION

The properties of the complexes containing metal−metal bonds have been extensively studied, showing a very rich and varied chemistry.1 From the supramolecular chemistry viewpoint, the use of dimetallic units is interesting because of the high versatility of these building blocks. These metal−metal bonded complexes have been used as subunit precursors and then linked by various equatorial and axial bridging groups, giving rise to a wealth of arrangements, from discrete species to one-, two-, or three-dimensional polymers.2−5 Molecular assemblies incorporating dinuclear metal species as rigid or semirigid corner units have also become an intensely active research area.6−10 Focusing on the case of dirhodium complexes with carboxylate ligands occupying the equatorial positions, a few hundred compounds with a variety of architectures, properties, and different scientific approaches have been reported.5−12 Most of these dirhodium units adopt the “paddlewheel” structure, where the dimetallic core is bridged by four monoanionic μ2-η2-carboxylato ligands (Scheme 1). © XXXX American Chemical Society

Received: July 30, 2013 Revised: September 30, 2013

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dx.doi.org/10.1021/cg4011532 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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ligand or two different ones to a dimetallic unit. This should lead to the obtention of extremely rare examples of asymmetric dirhodium(II)-tetracarboxylates. Considering these aspects, in this work, we have focused on the synthesis and structural characterization of new dinuclear complexes, containing tetracarboxylatodirhodium(II) as basic units and halides (Br−, I−) and pseudohalides (OCN−) as axial ligands. The use of halides and pseudohalides produces anionic dirhodium(II) units, with alkali metals (Na+, K+) as counterions, affording a surprisingly broad structural diversity and a rich variety of supramolecular networks. Whereas complex Na2[Rh2(O2CMe)4(NCO)2]·4H2O (1) displays monodimensional chains, two-dimensional layers are found in complexes Na[Rh2(O2CMe)4(NCO)(H2O)](2) and {K2[Rh2(O2CEt)4X 2 ]·H 2 O} n (X = I, (3) and X = Br, (4)). Finally, K[Rh2(O2CEt)4(Br)0.5]2[Rh2(O2CEt)4(H2O)2] (5) shows a three-dimensional supramolecular network. Remarkably, the molecular architectures found in complexes 3−5 strongly differ from the 1D coordination polymers previously published by our group,19 obtained by reacting [Rh2(O2CMe)4] with KBr or KI. Here, speciation of the studied tetracarboxylatodirhodium(II)−(pseudo)halide (X−) systems was investigated in aqueous solution by UV−visible titrations in an attempt to rationalize the obtention of different Rh−X stoichiometries in the crystal state. By cyclic voltammetry, we corroborated the tendency of X− coordination to favor oxidation of dirhodium(II) units to the corresponding mixed-valence Rh(II)−Rh(III) forms.

Scheme 1. Paddlewheel Dimetallic Units with Identical (a) or Different Axial Ligands (b) or Axial M−O Interactions (c)

dirhodium(II) carboxylate units have been described.16,19,23 Most importantly, the use of axial anionic ligands, such as halides or pseudohalides, as ancillary ligands for neutral dirhodium(II)-tetracarboxylate units should be beneficial for different purposes. Primarily, the use of anionic units, in combination with different cationic ones, seems to be an interesting strategy to build new supramolecular architectures.24,25 As a second aspect, the introduction of anionic axial ligands should allow the tuning of the electronic and redox properties of the Rh(II) centers, with possible implications in the obtention of functional properties.26 Finally, the use of anions in combination with neutral donor ligands (like solvent molecules) could favor the selective binding of only one axial

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RESULTS AND DISCUSSION Synthesis and Charaterization of the Complexes. Complexes Na2[Rh2(O2CMe)4(NCO)2]·4H2O (1) and Na[Rh2(O2CMe)4(NCO)(H2O)]·(2) were obtained in aqueous solution by reacting 1:1 or 1:2 amounts of [Rh2(O2CMe)4] and

Table 1. Crystal Data and Refinement Data for 1−5 crystal data empirical formula formula weight crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/g/cm3 μ(Mo-Ka)/mm−1 F(000) Θ range/deg index ranges reflns collected unique reflns [R(int)] completeness to θ data/restraints/params goodness-of-fit on F2 R1 [I > 2σ(I)] wR2 (all data)

1

2

C10H20N2NaO14Rh2 644.06 monoclinic P21/c 7.7220(6) 9.1477(7) 15.474(1)

C9H14NNaO10Rh2 525.01 monoclinic P21/n 8.3851(2) 12.1694(3) 15.5635(4)

93.462(2)

105.263(1)

1091.1(2) 4 1.936 1.618 620.0 2.59−26.00 −9, −11 ,−16 to 9, 11, 19 8836 2145 [0.0371] 99.6% 2145/0/136 1.052 0.0297 0.0802

1533.3(2) 4 2.266 2.231 1016.0 2.71−25.39 −10, −14, −18 to 9, 14, 18 22 018 2798 [0.0612] 99.3% 2798/0/212 1.000 0.0301 0.0707

3 C24H44I4K4O18Rh4 1696.22 triclinic P1̅ 12.375(4) 13.372(3) 15.740(3) 97.41(2) 110.01(1) 102.37(2) 2332(1) 2 2.410 4.457 1592.0 1.41−25.01 −14, −12, −18 to 14, 15, 18 17 918 7985 [0.0457] 97.2% 7985/3/472 0.972 0.0569 0.1833 B

4 C24H44Br4K4O18Rh4 1508.22 triclinic P1̅ 12.588(1) 12.702(1) 15.812(2) 66.750(3) 72.652(4) 75.657(3) 2192.6(3) 2 2.224 5.562 1408.0 1.71−23.29 −13, −12, −17 to 13, 14, 12 6249 6249 [0.0610] 98.8% 6249/1/475 0.988 0.0586 0.1823

5 C36H64BrKO26Rh6 1649.31 triclinic P1̅ 13.291(1) 13.844(2) 16.453(2) 97.035(8) 108.81(1) 102.83(1) 2731.5(6) 2 2.000 2.657 1616.0 1.34−25.00 −14, −15, −19 to 15, 16, 19 21 109 9361 [0.0271] 97.3% 9361/16/631 0.973 0.0320 0.0875



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Figure 1. Ellipsoid plot of compound 1 (left) with thermal ellipsoids drawn at the 50% probability level, and schematic representation of a chain formed by the ionic interactions between Na+ cations and dimetallic units (right). In both representations, hydrogen atoms are removed for clarity.

Figure 2. Ellipsoid plot of compound 2 (left) with thermal ellipsoids drawn at the 50% probability level, and schematic representation of a 2D layer formed by the ionic interactions between Na+ cations and dimetallic units (right). In both representations, hydrogen atoms are removed for clarity.

NaOCN at 25 °C. In the infrared (IR) spectrum, compounds 1 and 2 display intense and sharp asymmetric cyanate stretching vibrations at 2196 and 2201 cm−1, respectively. These bands are consistent with N-bonding rather than O-bonding of NCO ligand to the Rh(II)27 ion (free cyanate absorbs at 2155 cm−1). Solid-state UV−visible absorption spectra of compounds 1 and 2 are very similar, showing two bands in the visible range (about 450 and 650 nm) and one well-defined band in the UV range at 276 nm (Figure S12, Supporting Information). It is well-known that the UV−visible spectra of [Rh2(O2CR)4L2] compounds show two principal absorption bands28,29 around 600−700 nm and about 450 nm. The first one has been assigned to a π*(Rh2)−σ*(Rh2) transition, whereas the second one is probably due to a π*(Rh2)−σ*(Rh−O) transition. However, the electronic transitions in the UV region have been less-studied. In this region, bands corresponding to σ(Rh2)−σ*(Rh2) or σ(RhL)−σ*(Rh2) transitions appear.29,30 On the other hand, the reaction of [Rh2(O2CEt)4], with potassium iodide or potassium bromide in a 1:2 molar ratio in water at 25 °C, leads to the formation of {K2[Rh2(O2CEt)4X2]· H2O}n (X = I, (3) and X = Br, (4)). Under 1:1 molar ratio (Rh2/KX) conditions, the same product (3) was isolated when using KI, whereas the new product K[Rh2(O2CEt)4(Br)0.5]2[Rh2(O2CEt)4(H2O)2] (5) was obtained when using KBr. Whereas it is difficult to discriminate between complexes 3−5 by IR spectroscopy, the solid-state UV−visible spectra show significant differences in the UV region (Figure S12, Supporting Information). Thus, compounds 3 and 4 show only an

absorption band at 370 and 306 nm, respectively, whereas complex 5 presents two features at 270 and 320 nm. However, in the visible region, compounds 3−5 show similar absorption bands at around 500 and 650 nm. The discrepancies observed between these spectra and those observed in solution (see below) are due to the different nature of the solvated species formed in solution. Description of the Structures. The crystal structures of complexes 1−5 have been determined by single-crystal X-ray diffraction. Table 1 collects the most remarkable experimental and structural data. The crystal structures of compounds 1 and 2 show dirhodium paddlewheel molecules with the axial positions occupied by isocyanate ligands or isocyanate and water, respectively. The structure of compound 1 contains paddlewheel dirhodium units joined equatorially by four bidentate acetate ligands and two monodentate OCN− axial ligands, bonded to the metal atoms by the nitrogen atom. The [Rh2(O2CEt)4(NCO)2]2− complex ions (Figure 1, left), give rise to one-dimensional chains parallel to the [100] direction taking into account the ionic interaction with Na+ cations (Figure 1, right). Each sodium atom is bonded to six atoms: four oxygen atoms (two from water molecules, and two from adjacent paddlewheel molecules), and two nitrogen atoms from axial isocyanate ligands. Neighbor chains are joined by hydrogen bonds that involve the two water molecules surrounding the sodium cation and the terminal oxygen atom of the isocyanate ligand. C



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Figure 3. Schematic view of the ionic network found in 3 and 4 with the embedded [Rh2(O2CEt)4] units (left, only the core of the dimetallic unit is shown) and view of the undulated two-dimensional ionic network of KX (right).

Figure 4. Schematic view of a {[Rh2(O2CEt)4]2Br}∞ chain (left) and packing of parallel chains through ionic interactions with K+ cations and [Rh2(O2CEt)4(H2O)2] paddlewheel units (right) in compound 5.

Figure 5. Schematic view of the simplified 3D net found in compound 5.

The crystal structure of compound 2 shows paddlewheel dirhodium units joined by four acetate ligands in the equatorial positions. The axial positions are occupied by one OCN− axial ligand bonded to the metal center through the nitrogen atom, and one water molecule. The [Rh2(O2CMe)4(NCO)(OH2)]2− complex ions (Figure 2, left), are ionically bonded to Na+ cations, giving rise to layers parallel to the (10̅ 1)̅ crystallographic plane (Figure 2, right). Each Na+ cation interacts with two nitrogen atoms from OCN ligands, three carboxylic oxygen atoms, and another oxygen atom from the axially coordinated water molecule. There are multiple hydrogen bonds between

layers involving the noncoordinating oxygen atoms from the OCN ligands and the water ligands (O−O distance = 2.629 Å). Compounds 3 and 4 display the same complex 2D polymeric arrangement of [Rh2(O2CEt)4]− paddlewheel moieties with the two axial positions occupied by halide atoms. This crystal structure can be described as a stacking of {K2[Rh2(O2CEt)4X2]·H2O}∞ layers parallel to the (111̅ )̅ crystallographic plane. Each layer consists of an undulated two-dimensional ionic network of KX (Figure 3, right) with embedded [Rh2(O2CEt)4] fragments. The KX network can be topologically described as a fes type 3-connected uninodal Shubnikov plane D



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Table 2. Comparison of the Topologies Found in Compounds 1−5 1 2 3 and 4 5

dimensionality

topology

1D: chains in the [100] direction 2D: layers parallel to the (101̅) plane 2D: layers parallel to the (111̅ )̅ plane 3D

2,4-connected binodal net (new) 3-connected uninodal fes net 3,6-connected binodal kgd net 2,3,6-connected trinodal net (new)

evaluated in 0.1 M Na2SO4 aqueous solution at T = 19 °C by spectrophotometric titrations. The aqueous medium has been used to reproduce the crystallization conditions31 of 1−5. Bisaquo adducts of the starting complexes, [Rh2(O2CR)4(H2O)2], are initially present in solution, with water molecules axially coordinated to each Rh(II) atom.32 The reactions with halides or pseudohalides have shown that the water molecules are sequentially substituted by the respective anions, as indicated in Scheme 2.

net {4.82}, where four- and eight-membered rings can be found. The dirhodium units are located inside the eight-membered rings, displaying Rh−X and K−O ionic interactions with the inorganic framework (Figure 3, left). There are weak hydrogen bonds between adjacent layers involving the oxygen atoms from the water molecules located around the cations. In this case of complex 5, the crystal structure shows a completely different assembly of the dirhodium units (see Figure S4 in the Supporting Information). The structure consists of a complicated three-dimensional arrangement through ionic interactions and comprises two kinds of dirhodium moieties. The compound, with the formula {K[Rh2(O2CEt)4(Br)0.5]2[Rh2(O2CEt)4(H2O)2]}n, contains [Rh2(O2CEt)4]2Br dimeric units of dirhodium paddlewheel fragments joined by O−Rh interactions, in which the oxygen atom from the neighbor dirhodium unit occupies the axial position of a paddlewheel fragment and conversely (Scheme 1b). The two remaining axial positions of these tetramers are occupied by shared bromide ligands to create one-dimensional zigzag {[Rh2(O2CEt)4]2Br}∞ chains parallel to the [011]̅ direction (see Figure 4, left). In the second type of dirhodium units, [Rh2(O2CEt)4(H2O)2], the axial positions are occupied by water molecules (Figure 5, blue and red spheres). The {[Rh2(O2CEt)4]2Br}∞ chains (green spheres) and one-half of the [Rh2(O2CEt)4(H2O)2] molecules (blue spheres) form a three-dimensional framework through K−Br and K−O interactions (Figure 4, right, and Figure 5). The remaining half of the [Rh2(O2CEt)4(H2O)2] molecules are located in the channels formed in the resulting three-dimensional arrangement (red spheres in Figure 5). The underlying net (Figure 5), considering the paddlewheel units and K+ cations as nodes, presents a new topology. It is a 2,3,6-connected trinodal net with symbol (4.82)2(42.68.83.102)(8), where the 2-connected nodes are one-half of the [Rh2(O2CEt)4(H2O)2] molecules depicted by blue spheres, 3connected nodes are the potassium cations, and 6-connected nodes are the [Rh2(O2CEt)4]2Br dimeric units. The remaining half of the [Rh2(O2CEt)4(H2O)2] molecules do not take part in the underlying net, as they are not playing a role in the construction of the framework due to the fact that they do not display ionic interactions (0-connected nodes). A comparative topological study of the four new structural types found in this work has been performed. Considering both the anionic paddlewheel fragments and the cations as nodes, and the ionic interactions between them as connectors, we find structures with 1D (compound 1), 2D (compounds 2, 3, and 4), and 3D (compound 5) dimensionality, as summarized in Table 2. Depictions of the four underlying nets are collected in the Supporting Information (Figures S1−3 and S5). Formation of Anionic Dirhodium(II) Species in Aqueous Solution. As stated above, in complexes 1−5, the UV bands are sensitive to the change of the axial ligands, and therefore, they have been used to carry out the speciation studies. Thus, speciation of the dirhodium(II)/X− anion system [Rh2(O2CR)4]/X− (R = Me, Et; X = OCN, Br, I) has been

Scheme 2. Speciation of the [Rh2(O2CR)4]/X− Systems in 0.1 M Na2SO4 Aqueous Solution

A summary of the global formation constants (log β) for the substitution reactions of Scheme 2 is shown in Table 3. Table 3. Global Formation Constants (log β) for the Reactions of Scheme 2 (0.1 M Na2SO4 Aqueous Solution, T = 19 °C) system

log β1

log β2

[Rh2(O2CEt)4]/I− [Rh2(O2CEt)4]/Br− [Rh2(O2CMe)4]/OCN−

1.44(2) 0.69(2) 1.23(4)

2.35(2) 0.86(1) very low

In the case of [Rh2(O2CEt)4]/X− (X = Br, I) systems, three species in equilibria were found (Figure 6 and Figure S6, Supporting Information), involving the initial [Rh2(O2CEt)4(H2O)2], and the [Rh2(O2CEt)4X(H2O)]− [Rh2(O2CEt)4X2]2− adducts. In particular, when the halide concentration was increased, the appearance of two UV bands was observed: the first one appeared at 297 and 271 nm, and the second one, at 333 and 292 nm, for X = I and Br, respectively. These bands were assigned to [Rh2(O2CEt)4X(H2O)]− and [Rh2(O2CEt)4X2]2−, respectively. These attributions match very well with the data previously reported29,30 for [Rh2(O2CMe)4X2]2−, displaying absorption bands at 332 and 291 nm for X = I and Br, respectively. The transitions have been assigned to a predominantly σ(RhX)−σ*(Rh2) in character transition.27,28 On this basis, the red shifts observed from [Rh2(O2CEt)4X(H2O)]− to [Rh2(O2CEt)4X2]2−, and from [Rh2(O2CEt)4Br2]2− to [Rh2(O2CEt)4I2]2− species, are in accordance with the substitution of water molecules by halide ligands and with the change from bromide to iodide, respectively. Concerning the [Rh2(O2CMe)4]/OCN aqueous system, only slight variations of the visible spectrum were observed with the addition of NaOCN to the initial dimer (Figure S7, Supporting Information). A two-species equilibrium was found, involving the initial species [Rh2(O2CMe)4(H2O)2] and the E



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formation constants for our [Rh2(O2CR)4]/X− (R = Me, Et; X = OCN, Br, I) adducts are much lower (Table 3), primarily due to the presence of water as a solvent, which strongly competes with X− anions to occupy axial positions of dirhodium(II) units. To our knowledge, only the speciation of one RhII2-L system has been reported in aqueous solution, containing water-soluble acid phosphines as ligands. Again, the formation constants in the published study are quite higher than those of Table 3: log β1 ≥ 6 and log β2 ≤ 12. This can be explained with the high πacceptor character of the used phosphines, which reinforces their affinity to RhII, compared to weakly π-donor (pseudo)halides. Concerning the formation constants from the different [Rh2(O2CR)4]/X− (R = Me, Et; X = OCN, Br, I) adducts, they could be compared neglecting the effect of different substituents on the carboxylate groups of [Rh2(O2CR)4] (R = Me, Et). A precise trend is found for log β2 values: (log β2[Rh2(O2CEt)4I2]2− > log β2[Rh2(O2CEt)4Br2]2− > log β2[Rh2(O2CMe)4(NOC)2]2−). This behavior is in agreement with the greater affinity of Rh2+ for sof t iodide and, to a minor extent, bromide anions, compared to hard nitrogen donors.39−41 A deviation from this trend is encountered for log β1 values: (log β1[Rh2(O2CEt)4I(H2O)]− > log β1[Rh2(O2CMe)4(NCO)(H2O)]− > log β1[Rh2(O2CEt)4Br(H2O)]−. The particular stabilization of [Rh2(O2CMe)4(NCO)(H2O)]− can be tentatively explained with (a) a weak interaction of the carboxylate-RhII-NCO− moiety with a Na+ ion, as found in the crystal structure of 2, or (b) an intramolecular H-bond between a water molecule, the coordinated −NCO ligand, with N as a better H-bond acceptor than Br and I, and an adjacent carboxylate group to form a six-membered cycle. Electrochemical Properties of the Anionic Dirhodium(II) Units. The redox properties of dirhodium(II) acetate and propionate have been previously reported using different solvents.32,42 These species are reversibly oxidized by a single electron process to yield stable Rh(II)−Rh(III) dimers,43 whereas they are irreversibly reduced in several steps. The halfwave potentials were found to depend on the nature of the solvent and the substituent present on the carboxylate groups. To our knowledge, no examples of dirhodium(II)-tetracarboxylates with anionic axial ligands have been previously studied by electrochemical methods. However, coordination of solvent molecules (like CH3CN, DMSO, or py) or other neutral ligands in the axial positions of [Rh2(O2CR)4] paddlewheels is known to affect their redox properties.32,44 In particular, as the metal−ligand σ bond strength increases, the metal orbital nonbonding electrons are driven higher in energy and the adducts are more readily oxidized.37 In this work, we have investigated the effects of axial coordination of halides or pseudohalides on the oxidation of these Rh(II) dimers by cyclic voltammetry (CV). Concerning the CV of [Rh2(O2CMe)4], in water 0.1 M Na2SO4, the reversible oxidation peak of the RhII2/RhIIRhIII system (E1/2 = +1.04 V vs Ag/AgCl, KCl 3 M, ΔEp = 70 mV) becomes fully irreversible and is slightly shifted to a less positive potential (Epa = +0.91 V) after addition of a big excess (100 equiv) of NaOCN (Figure 7a). The corresponding cathodic peak45 is visible only at lower concentrations of NaOCN (Figure S11, Supporting Information), even if it disappears progressively by decreasing the scan rate. In other words, coordination of one isocyanate anion to the initial dirhodium(II) moiety, fast exchanging with a water molecule in a chemical equilibrium process, provokes progressively (1) the shift of the

Figure 6. (a) Spectrophotometric data (d = 0.2 cm; CRh2 range: 0.43− 0.34 mM), calculated spectra (inset), and (b) distribution diagram for the [Rh2(O2CEt)4]/I system (0.1 M Na2SO4 aqueous solution, T = 19 °C).

[Rh2(O2CMe)4(NCO)(H2O)]− monoadduct. This means that the bis-adduct [Rh2(O2CMe)4(NCO)2]2− was not detected in solution under the applied experimental conditions,33 in agreement with a value of log β2 much lower than the one corresponding to log β1. Coherently, the ESI-MS spectrum of a 2 mM aqueous solution of [Rh2(O2CMe)4] in the presence of 0.07 M NaOCN after a 1:40 dilution in CH3CN shows a major peak at 483.87 m/z in negative ionization mode (Figure S9, Supporting Information). This corresponds to a [Rh2(O2CMe)4(NCO)]− monoadduct, whereas the peak at 990.72 m/z is attributed to a sodium-bridged dimer of the latter, [Na{Rh2(O2CMe)4(NCO)}2]−. Conversely, the bisadduct seems to be present in very low concentration, as indicated by the minor peak at 548.86 m/z, corresponding to [Na{Rh2(O2CMe)4(NCO)2}]−.34 Anyway, the formation of [Rh2(O2CMe)4(NCO)2]2− is supposed to become significant at much higher CRh2 (CRh2 = total concentration of dirhodium units). In fact, the latter species crystallized, as complex 1, after complete evaporation of the solvent. Only few examples of speciation studies of Rh(II) paddlewheel dimers in the presence of exogenous ligands are reported in the literature.35−38 However, a direct comparison of the reported data with the [Rh2(O2CR)4]/X− (R = Me, Et; X = OCN, Br, I) systems reported here is difficult. In fact, most of these studies36−38 were performed in noncoordinating solvents by adding neutral ligands.28,29 All of these systems concern three-species equilibria that involve {RhII2},{RhII2L}, and{RhII2L2} species, with log β1 ≥ 3 and log β2 ≤ 13. The F



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(X−) leads to several supramolecular architectures in which ionic interactions play a crucial role. In particular, the charge of the [Rh2(O2CR)4(X)n]n− units (n = 1, 2), as well as the nature of the axial ligands (X = OCN, Br, I), of the corresponding cations (Na+, K+), and of the R groups on the carboxylate ligands (Me, Et) are key factors in the formation of these supramolecular networks. The structural determination by single-crystal X-ray diffraction of all the compounds has allowed the detection of singular arrangements. The speciation studies allow the understanding of the formation of different Rh−X stoichiometries, showing that the formation of monoadducts is favored when log K2 is much lower than log K1 (log β1 = log K1, log β2 = log K1 + log K2). Thus, the monoadduct species [Rh2(O2CMe)4(NCO)(H2O)], and the more peculiar {K[Rh2(O2CEt)4(Br)0.5]2[Rh2(O2CEt)4(H2O)2]}n complex, have been isolated. The electrochemical studies show that the oxidation of the dirhodium(II) compounds, to yield the corresponding mixed-valence Rh(II)−Rh(III) species, is favored by the coordination of the halide to the axial position of the dimetallic units.

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EXPERIMENTAL SECTION

General Procedures. KBr, RhCl3·xH2O, carboxylic acids, and solvents were purchased and used as received. Dirhodium(II) acetate and dirhodium(II) propionate were obtained by a method described before.46,47 IR spectra were recorded on a PerkinElmer spectrum 100 spectrophotometer using a universal ATR sampling accessory. Electronic absorption spectra were recorded on an Agilent 8452 diode array spectrophotometer over a 190−1100 nm range in 0.1, 0.2, and 1 cm quartz cuvettes. Samples for solid-state UV−visible absorption were prepared by dispersion in paraffin oil of the crystals of compounds 1−5 and [Rh2(O2CR)4] (R = Me, Et), previously triturated to a very fine powder. Elemental analyses were carried out by the Microanalytical Service of the Complutense University of Madrid. Electrospray mass (ESI-MS) spectra were recorded on a QSTAR Pulsar mass spectrometer from Applied Biosystems, equipped with a hybrid analyzer Q-TOF (quadrupole time-of-flight). The samples were analyzed in negative ionization mode by direct perfusion in the ESI source, using a syringe pump at a flow rate of 20 μL/min (ion spray voltage: −4500 V; focusing potential: −210; declustering potential: −30 V; declustering potential 2: −15 V). Synthesis of Na2[Rh2(O2CMe)4(NCO)2]·4H2O (1) and Na[Rh2(O2CMe)4(NCO)(H2O)] (2). Solid NaOCN (0.014 g, 0.226 mmol) was added to an aqueous solution (15 mL) of dirhodium(II) acetate (0.05 g, 0.113 mmol). The solution was stirred for 18 h at 25 °C. The resulting blue-violet solution was filtered to remove a small amount of undissolved solid. A mixture of green, violet, and red crystals was obtained by slow evaporation of the filtrate at room temperature after a few days. The different crystals were mechanically separated and collected. Whereas the green product corresponded to the starting complex, the red and violet X-ray suitable crystals correspond to complexes 1 and 2, respectively. Product 1 (10 mg, 17%): IR (cm−1): 3409(mb), 2196(s), 1635(w), 1585(s), 1418(vs), 1353(m), 1317(m), 1043(w), 705(s). UV (dispersion in paraffin oil): 276 nm. Anal (%) Calcd for C10H20N2Na2O14Rh2: C, 18.65; H, 3.13; N, 4.35. Found: C, 18.41; H, 2.99; N, 4.47. Product 2 (12 mg, 16.4%): IR(cm−1): 3413(mb), 2201(s), 1678(w), 1585(s), 1413(vs), 1354(w), 1324(w), 1044(w), 698(s). UV (dispersion in paraffin oil): 276 nm. Anal (%) Calcd for C9H14NNaO10Rh2: C, 20.59; H, 2.67; N, 2.67. Found: C, 20.25; H, 2.89; N, 2.58. By carrying out the same reaction in the presence of equimolar amounts of dirhodium(II) acetate and NaOCN, the yield of 1 was

Structural Diversity in Paddlewheel Dirhodium(II) Compounds through

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