Rhodium Amidinate Dimers as Structural and Functional Hubs for Multimetallic Assemblies

The synthesis and characterization of multichromophore assemblies based on a dirhodium tetra-<i>N</i>,<i>N</i>′-diphenylisonicotinamidinate dimer are reported. The pyridyl moieties were used to coordinate up to four positively charged rhenium­(I) chromophores of the form <i>fac</i>-[Re­(bpy)­(CO)₃<b>L</b>]­PF₆ (bpy = 2,2′-bipyridine, <b>L</b> = a pyridyl group on the Rh₂ dimer). The mono-, bis-, tris-, and tetrarhenium assemblies were isolated by size-exclusion chromatography, and their spectroscopic and electrochemical properties were studied and compared with DFT and time-dependent (TD) DFT models of the original rhodium dimer and the mono- and tetrarhenium assembly. The rhenium chromophores modify the properties of the rhodium dimer: for example, the first oxidation of the Rh₂ dimer (Rh–Rh δ* orbital) increased from the original 210 mV versus SCE in acetonitrile, by 45 mV per rhenium complex added, finishing at 390 mV for the tetrarhenium complex. The rhodium dimers display solvatochromism with acetonitrile (MeCN) due to the formation of an axial adduct and has an association constant that increased by a factor of 3.8 when the dimer has four rhenium chromophores. The absorption data clearly exhibited the cumulative effect of the addition of rhenium chromophores in the 230 to 400 nm range. The main visible band, a metal-dimer-to-ligand charge transfer (¹M₂LCT) transition determined by TD-DFT, red-shifts from 541 nm to 603 nm, while the main near-IR band, a ¹Rh₂(π*→σ*) transition, has a small blue-shift (∼26 cm^–1/Re), varying from 837 to 831 nm upon addition of the four Re­(I) chromophores. This was observed in TD-DFT also with a total shift of 105 cm^–1 for the tetrarhenium assembly. In terms of emission, the rhenium excited state was completely quenched upon coordination to the dimer, suggesting fast electron transfer of the rhodium dimer. All other aspects of the rhenium chromophore are similar to the parent complex where <b>L</b> = pyridine, showing similar redox couples and additive spectral characteristics

Rhodium Amidinate Dimers as Structural and Functional Hubs for Multimetallic Assemblies

The synthesis and characterization of multichromophore assemblies based on a dirhodium tetra-<i>N</i>,<i>N</i>′-diphenylisonicotinamidinate dimer are reported. The pyridyl moieties were used to coordinate up to four positively charged rhenium­(I) chromophores of the form <i>fac</i>-[Re­(bpy)­(CO)₃<b>L</b>]­PF₆ (bpy = 2,2′-bipyridine, <b>L</b> = a pyridyl group on the Rh₂ dimer). The mono-, bis-, tris-, and tetrarhenium assemblies were isolated by size-exclusion chromatography, and their spectroscopic and electrochemical properties were studied and compared with DFT and time-dependent (TD) DFT models of the original rhodium dimer and the mono- and tetrarhenium assembly. The rhenium chromophores modify the properties of the rhodium dimer: for example, the first oxidation of the Rh₂ dimer (Rh–Rh δ* orbital) increased from the original 210 mV versus SCE in acetonitrile, by 45 mV per rhenium complex added, finishing at 390 mV for the tetrarhenium complex. The rhodium dimers display solvatochromism with acetonitrile (MeCN) due to the formation of an axial adduct and has an association constant that increased by a factor of 3.8 when the dimer has four rhenium chromophores. The absorption data clearly exhibited the cumulative effect of the addition of rhenium chromophores in the 230 to 400 nm range. The main visible band, a metal-dimer-to-ligand charge transfer (¹M₂LCT) transition determined by TD-DFT, red-shifts from 541 nm to 603 nm, while the main near-IR band, a ¹Rh₂(π*→σ*) transition, has a small blue-shift (∼26 cm^–1/Re), varying from 837 to 831 nm upon addition of the four Re­(I) chromophores. This was observed in TD-DFT also with a total shift of 105 cm^–1 for the tetrarhenium assembly. In terms of emission, the rhenium excited state was completely quenched upon coordination to the dimer, suggesting fast electron transfer of the rhodium dimer. All other aspects of the rhenium chromophore are similar to the parent complex where <b>L</b> = pyridine, showing similar redox couples and additive spectral characteristics

Optoelectronic Properties and Structural Effects of the Incremental Addition of Pyridyl Moieties on a Rhodium Dimer

The synthesis and characterization of five C–C coupling products obtained from the reaction of a paddlewheel tetrakis 4-bromo-<i>N</i>,<i>N</i>′-diphenylbenzamidinate dirhodium dimer with 4-pyridineboronic acid pinacol ester are reported. The coupling reactions occur on one to four amidinate ligands, leading to rhodium dimers containing [tetrakis, tris, <i>cis</i>-bis, <i>trans</i>-bis, or mono]-<i>N</i>,<i>N</i>′-diphenyl-4-(pyridin-4-yl)­benzamidinate ligands, effectively creating new binding sites on the metal complexes. The new compounds were isolated by column chromatography, and the exact conformations were verified by X-ray crystallography. Redox processes showed only a small variation within the coupling products and included two oxidations (1.30 ± 0.02 V, 0.27 ± 0.01 V vs SCE) and one reduction (−1.55 ± 0.02 V vs SCE), all centered on the Rh–Rh core. Time-dependent density functional theory (TD-DFT) was used to analyze this series with four other fully characterized <i>N</i>,<i>N</i>′-diphenyl-aryl-amidinate rhodium dimers that were found in the literature. The two main absorption bands of these nine rhodium dimers were compared to TD-DFT calculations, both giving excellent correlation. The first, a metal-to-metal (MM) transition around 11800 cm^–1 (845 nm) was blue-shifted in the calculation, with an average difference of 1378 cm^–1 but had only a 15 cm^–1 standard deviation, showing a strong correlation despite the energy difference. The second, a metal-to-ligand charge transfer (MLCT) transition around 18900 cm^–1 (530 nm) was a near perfect match with only a 64 cm^–1 average difference and a 35 cm^–1 standard deviation. The electronic transition, redox potentials, and HOMO and LUMO energies of all dimers were plotted versus the Hammett parameter (σ) of the aryl group and Taft’s model with 2 components: field effects (σ_F) and resonance (σ_R). The properties involving only the Rh–Rh core (MM band, all oxidation potentials, HOMO and LUMO) were fit with a single set of σ_F and σ_R contributions (73% and 27%), with a goodness-of-fit (<i>R</i>²) value ranging from 90% to 99.7%. The metal-dimer to ligand charge-transfer band, involving the amidinate ligand, displayed different values of contribution with 45% and 55% for the σ_F and σ_R, respectively, with a fit of 94.8%. The accuracy of these fits enables the designed modification of amidinate-based dirhodium complexes to achieve desirable redox and spectroscopic properties

Rhodium Amidinate Dimers as Structural and Functional Hubs for Multimetallic Assemblies

The synthesis and characterization of multichromophore assemblies based on a dirhodium tetra-<i>N</i>,<i>N</i>′-diphenylisonicotinamidinate dimer are reported. The pyridyl moieties were used to coordinate up to four positively charged rhenium­(I) chromophores of the form <i>fac</i>-[Re­(bpy)­(CO)₃<b>L</b>]­PF₆ (bpy = 2,2′-bipyridine, <b>L</b> = a pyridyl group on the Rh₂ dimer). The mono-, bis-, tris-, and tetrarhenium assemblies were isolated by size-exclusion chromatography, and their spectroscopic and electrochemical properties were studied and compared with DFT and time-dependent (TD) DFT models of the original rhodium dimer and the mono- and tetrarhenium assembly. The rhenium chromophores modify the properties of the rhodium dimer: for example, the first oxidation of the Rh₂ dimer (Rh–Rh δ* orbital) increased from the original 210 mV versus SCE in acetonitrile, by 45 mV per rhenium complex added, finishing at 390 mV for the tetrarhenium complex. The rhodium dimers display solvatochromism with acetonitrile (MeCN) due to the formation of an axial adduct and has an association constant that increased by a factor of 3.8 when the dimer has four rhenium chromophores. The absorption data clearly exhibited the cumulative effect of the addition of rhenium chromophores in the 230 to 400 nm range. The main visible band, a metal-dimer-to-ligand charge transfer (¹M₂LCT) transition determined by TD-DFT, red-shifts from 541 nm to 603 nm, while the main near-IR band, a ¹Rh₂(π*→σ*) transition, has a small blue-shift (∼26 cm^–1/Re), varying from 837 to 831 nm upon addition of the four Re­(I) chromophores. This was observed in TD-DFT also with a total shift of 105 cm^–1 for the tetrarhenium assembly. In terms of emission, the rhenium excited state was completely quenched upon coordination to the dimer, suggesting fast electron transfer of the rhodium dimer. All other aspects of the rhenium chromophore are similar to the parent complex where <b>L</b> = pyridine, showing similar redox couples and additive spectral characteristics

Diimine Triscarbonyl Re(I) of Isomeric Pyridyl-fulvene Ligands: an Electrochemical, Spectroscopic, and Computational Investigation

The synthesis and characterization of a novel family of positively charged <i>fac</i>-[Re­(bpy)­(CO)₃(<b>L</b>)]­PF₆ (bpy = 2,2′-bipyridine) complexes are reported, where <b>L</b> is a pyridine functionalized in <i>para</i> or <i>meta</i> position with a fulvene moiety, namely, 4-fluoren-9-ylidenemethyl-pyridine (<i><b>p</b></i><b>Fpy</b>) and 3-fluoren-9-ylidenemethyl-pyridine (<i><b>m</b></i><b>Fpy</b>). The complexes were prepared in high yield (86%) by direct addition at room temperature of the corresponding pyridine to the tetrahydrofuran (THF) adduct <i>fac</i>-[Re­(bpy)­(CO)₃(THF)]­[PF₆] precursor. Both ligand and complex structures were fully characterized by a variety of techniques including X-ray crystallography. The complexes did not exhibit the expected triplet mixed metal–ligand-to-ligand charge transfer (MLLCT) emission, because of its deactivation by the non-emissive triplet excited state of fulvene. The absorption profile shows that the MLLCT is overshadowed by the fulvene centered π–π* transition of higher molar absorptivity as shown by time dependent density functional theory (TD-DFT) calculations. The position of the fulvene on the pyridyl ring has a large effect on this transition, the <i>para</i> position displaying a much higher absorption coefficient (21.3 × 10³ M^–1 cm^–1) at lower energy (364 nm) than the meta position (331 nm, 16.0 × 10³ M^–1 cm^–1

Self-Assembled Molecular Squares as Supramolecular Tectons

Concentration-dependent equilibria of molecular squares [Pd₄(L′)₄(L)₄]­(NO₃)₈ and triangles [Pd₃(L′)₃(L)₃]­(NO₃)₆ were obtained when cis-protected Pd­(II) units [PdL′(NO₃)₂] (L′ = tmeda, 2,2′-bpy, and phen) were combined independently with 4,4′-bipyridine (L) in water. However, complexation of [PdL′(OTs)₂] with L resulted in exclusive formation of the corresponding molecular squares. The addition of AgOTs to each mixture of square and triangle led to a shift in the equilibrium, resulting in the disappearance of the triangles and exclusive formation of the corresponding squares. The crystal structures of the molecular squares [Pd₄(L′)₄(L)₄]­(OTs)₈ revealed a pair of tosylate anions encapsulated in the hydrophobic cavity of the square. Further, [Pd₄(2,2′-bpy)₄­(L)₄]­(OTs)₈ and [Pd₄(phen)₄(L)₄]­(OTs)₈ exhibited solvatomorphism, yielding two crystalline forms each, respectively. The cationic units in these crystals associate through intermolecular π<b>···</b>π stacking interactions wherein the cis-protecting units (i.e., 2,2′-bpy and phen) of adjacent molecules overlap via side-on or end-on modes. Thus, the cations may be considered as “tectons”, each of which contains four peripheral 2,2′-bpy/phen units, which behave as “supramolecular synthons” in the self-assembly of the squares. The tosylates interact with the cations through C–H<b>···</b>O and C–H<b>···</b>π interactions and play a role in the packing of the molecular squares

Palladium(II)-Directed Self-Assembly of a Neutral Molecular Triangle as a Heteroditopic Receptor for Ion Pairs

A molecular triangle, based on the self-assembly of 4,7-phenanthroline by a neutral palladium complex, has been synthesized and characterized by a combination of techniques: ¹H NMR and UV–vis absorption spectroscopies, mass spectrometry, elemental analysis, and gel permeation chromatography. This new <i>neutral metallocavitand</i> has demonstrated the capacity to host <i>both</i> anionic and cationic guests, thus acting as an open-shaped heteroditopic receptor. Density functional theory calculations have shown that (i) there is no overtension in the assembly of the discrete triangle, which is more stable than open-chain oligomers, (ii) the adducts formed between the triangle and some salts (modeled in the gas phase) are thermodynamically stable, and (iii) two types of cavities coexist in the triangle, which host ions and ion pairs. This easily accessible triangular unit extends further the rational design of model nanoarchitectures in host–guest chemistry with applications in analytical chemistry and multifunctional molecular materials

Exploring Polymorphism: Hydrochloride Salts of Pitolisant and Analogues

Pitolisant hydrochloride is used to treat excessive daytime sleepiness in adults with narcolepsy. The drug is formulated as a crystalline solid, and a monoclinic P21 form has been claimed in patents, but little additional information about the structure and polymorphism of the compound has been published. No new forms were obtained when we grew crystals from solution under various conditions. Re-examination of the crystals revealed a disordered and partially hydrated structure that resembles the one reported earlier but is not identical. Further insight was obtained by synthesizing analogues of pitolisant with its Cl substituent replaced by Me, F, and Br, followed by structural analysis of the hydrochloride salts by X-ray diffraction. Pitolisant hydrochloride and its three analogues showed very similar solid-state behavior, and each compound yielded new metastable forms when crystallized from melts. The lifetime of metastable form III of pitolisant hydrochloride could be extended significantly by adding small amounts of the fluoro analogue, but none of the metastable forms could be obtained as single crystals suitable for structural analysis. Computational predictions of the polymorphic landscapes of pitolisant hydrochloride and its analogues identified possible structures of the metastable forms. Dual experimental and computational approaches are already widely used in polymorphic screening, but our work shows the value of broadening these searches to include sets of structural analogues