Ferrocene-Bearing Homoleptic and Heteroleptic Paddlewheel-Type Dirhodium Complexes

: Two ferrocenecarboxylate (fca)-bridged dirhodium (Rh 2 ) complexes, [Rh 2 (fca) 4 ] ( 1 ) and [Rh 2 (fca)(piv) 3 ] ( 2 ; piv = pivalate), were prepared through the carboxylate-exchange reactions of [Rh 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ] and [Rh 2 (piv) 4 ], respectively, with fcaH and characterized by 1 H NMR, ESI-TOF-MS, and elemental analyses. Single-crystal X-ray di ﬀ raction analyses of [Rh 2 (fca) 4 (MeOH) 2 ] ( 1(MeOH) 2 ) and [Rh 2 (fca)(piv) 3 (MeOH) 2 ] ( 2(MeOH) 2 ), which are recrystallized from MeOH-con-taining solutions of 1 and 2 , revealed that (1) 1(MeOH) 2 and 2(MeOH) 2 possess homoleptic and heteroleptic paddlewheel-type dinuclear structures, respectively; (2) both complexes have a single Rh–Rh bond (2.3771(3) Å for 1(MeOH) 2 , 2.3712(3) Å for 2(MeOH) 2 ); and (3) the cyclopentadienyl rings of the fca ligands in 1(MeOH) 2 adopt an eclipsed conformation, whereas those in 2(MeOH) 2 are approximately 12–14° rotated from the staggered conformation. Density functional theory (DFT) calculations revealed that (1) the electronic con ﬁ gurations of the Rh 2 core in 1(MeOH) 2 and 2(MeOH) 2 are π 4 σ 2 δ 2 π * 2 δ * 2 π * 2 and π 4 σ 2 δ 2 δ * 2 π * 4 , respectively; and (2) the occupied molecular orbitals (MOs) localized on the fca ligands are energetically degenerate and relatively more unstable than those on the Rh 2 cores. Absorption features and electrochemical properties of 1 and 2 were investigated in a 9:1 CHCl 3 -MeOH solution and compared with those of fcaH and [Rh 2 (piv) 4 ]. Through examining the obtained results in detail using time-dependent DFT (TDDFT) and unrestricted DFT, we found that 1 and 2 exhibit charge transfer excitations between the fca ligands and Rh 2 cores, and 1 shows electronic interactions between ferrocene units through the Rh 2 core in the electrochemical oxidation process.


Introduction
In modern coordination chemistry, the synthetic strategy utilizing "metalloligands" or "metal-organic ligands," which possess substituents such as carboxylic acid that act as free coordination sites, are of great interest in the development of multinuclear complexes, supramolecular metal complexes, coordination polymers (CPs), and metal-organic frameworks (MOFs) [1][2][3][4][5][6].Rational photocatalysts [7,8], sensors [9,10], and functional magnetic materials [11,12] can be developed by appropriately combining photo-or redoxactive metalloligands with specific metal cations or clusters.Among the various metalloligands, ferrocenecarboxylate (fca; see Scheme 1a) and its derivatives [13], which are redox-active organometallic compounds, have been especially applied because of their structural accommodation, chemical stability, and reversible redox behavior [14][15][16].Multinuclear complexes, in which multiple fca ligands are connected to multinuclear building blocks, are expected to exhibit redox interactions between (1) a few fca ligands through a multinuclear core, or (2) a multinuclear core and fca ligands.For example, two fca-bearing square tetraplatinum (Pt4) complexes exhibit weak interactions between two ferrocene units through a Pt4 core in the electrochemical oxidation of the fca ligands [17].
Fca-bearing paddlewheel-type Rh2 complexes are considered to be particularly interesting because this type of Rh2 complex shows excellent functional properties such as catalysis, sensing, and antitumor activities [29][30][31][32][33][34][35][36][37].Homoleptic paddlewheel-type Rh2 complexes bridged with four fca derivative ligands, for example, [Rh2(fca)4], act as catalysts for carbene insertion, shape-selective alkane functionalization, and asymmetric intramolecular C-H insertion reactions [26,27,38]; however, the details of the synthesis, characterization, electrochemical and absorption properties, and X-ray crystal structures of the Rh2 complexes have not been previously reported.In addition, a heteroleptic paddlewheeltype Rh2 complex bridged with an fca ligand and three acetate ligands, [Rh2(fca)(O2CCH3)3], was recently reported as a catalyst for carbene C-H insertion reaction; however, its crystal structure has not yet been reported [28].It is important to investigate fca-bearing homoleptic and heteroleptic paddlewheel-type Rh2 complexes in detail not only for further development of these complexes as catalysts but also for a deep and systematic understanding of paddlewheel-type M2 complexes with metalloligands.Hence, in this study, we closely investigated the synthesis, characterization, crystal structure, and absorption and electrochemical properties of fca-bearing homoleptic and heteroleptic paddlewheel-type Rh2 complexes, [Rh2(fca)4] (1; Scheme 1b) and [Rh2(fca)(piv)3] (2; piv = pivalate, Scheme 1c), using a combination of experimental and theoretical techniques.

Synthesis and Characterization
Homoleptic complex 1 was easily prepared in a high yield (94.7%) using a facile reflux reaction of [Rh2(O2CCH3)4(H2O)2] with eight equivalents of fcaH in chlorobenzene, whereas heteroleptic complex 2 was obtained in a 29.1% yield via the partial carboxylateexchange reaction of [Rh2(piv)4] with one equivalent of fcaH in N,N-dimethylaniline at 433 K followed by purification with silica-gel column chromatography.The yield of 2 was higher than that of a 3:1-coordination arrangement of carboxylate-bridged Rh2 complexes such as [Rh2(O2CCH3)3(PABC)] (15.9% yield; PABC = p-aminobenzenecarboxylate) [39].One of the reasons for the relatively high yield of 2 is thought to be the strong coordination ability of the electron-donating pivalate ligands to the Rh2 core, which suppresses excessive unfavorable carboxylate-exchange reactions.In fact, unreacted [Rh2(piv)4] (recover yield: 31.4%) and scarce amounts of [Rh2(fca)x(piv)4-x] (x = 2-4) mixture were found in different fractions of the column chromatography.Both Rh2 complexes are air-stable compounds with diamagnetic closed-shell electronic structures, where 1 is less soluble in most organic solvents, but dissolves slightly in a mixed 9:1 CHCl3-MeOH solution.In contrast, 2 exhibits excellent solubility in common organic solvents.
To characterize 1 and 2, 1 H NMR, ESI-TOF-MS, and elemental analyses were performed.The 1 H NMR spectrum of 1 in DMSO-d6 showed three broad signals at 4.62, 4.22, and 3.85 ppm with an integral ratio of 8:8:20.As shown in Figure S1, because no additional proton signal was observed in the spectrum, only the homoleptic Rh2 complex was confirmed to be isolated.In 2, proton signals were observed at 4.61 (2H), 4.29 (2H), and 3.97 (5H) ppm, which can be attributed to an fca ligand, and at 0.90 (9H) and 0.89 (18H) ppm, which can be attributed to pivalate ligands at the trans and cis positions, respectively, relative to the fca ligand (see Figure S2).The ESI-TOF-MS spectra of 1 and 2 show positive ion peaks at 1144.7777 and 760.9762 m/z, respectively, in good agreement with their simulated [M+Na] + values (1144.7814and 760.9762 m/z, respectively).As shown in Figures S3  and S4, the shapes of the isotope patterns of 1 and 2 fit the corresponding simulated ones well.Furthermore, elemental analyses confirmed the purities of the obtained Rh2 complexes; the observed CHN values of 1 and 2 were very close to the calculated ones of desolvated 1 and 2 (see the experimental section).
In the structure of 1(MeOH)2, the asymmetric unit consists of one-half of a molecule comprising a Rh 2+ cation, two fca ligands, and a MeOH ligand.A crystallographic inversion center is located at the midpoint of the Rh-Rh bond, and the overall structure forms a paddlewheel unit, in which four fca and two MeOH ligands are coordinated to the equatorial and axial positions, respectively, of the Rh2 core, and is isostructural with the previously reported [Cu2(fca)4(MeOH)2] [20], of which the crystal system (triclinic, P-1) is different.In contrast, the asymmetric unit of 2(MeOH)2 contains two crystallographically independent molecules; both Rh2 molecules adopt a heteroleptic paddlewheel unit, in which the Rh2 cores are equatorially bridged with fca and three pivalate ligands and axially coordinated via two MeOH ligands.

Optimized Geometries and Electronic Structures
Spin-restricted DFT calculations (B3LYP method) of 1(MeOH)2 and 2(MeOH)2 were performed to clarify the molecular geometries and electronic structures of 1 and 2 in their crystal states and in the CHCl3-MeOH solution.As shown in Figure S5, the optimized geometries of 1(MeOH)2 and 2(MeOH)2 reproduced the experimentally observed (X-ray) geometries without significant structural changes (Tables S1 and S2).The Rh-Rh bond lengths of the optimized geometries of 1(MeOH)2 and 2(MeOH)2 were calculated to be 2.407 and 2.405 Å, respectively, which are almost identical (~0.034Å difference) to those of observed geometries.In the primary coordination sphere of the Rh2 cores in 1(MeOH)2, although the averaged Rh-O(fca) bond length (2.066 Å) of the optimized geometry is close to that of the observed geometry, the averaged Rh-O(MeOH) bond length (2.389 Å) of the optimized geometry is 0.105 Å longer than that of the observed geometry.A similar tendency was found in 2; the averaged Rh-O(fca) (2.067 Å), Rh-O(trans-piv) (2.067 Å), and Rh-O(cispiv) (2.066 Å) bond lengths agree well with each other and those of the observed geometry, whereas the averaged Rh-O(MeOH) bond length (2.389 Å) of the optimized geometry is 0.094 Å longer than that of the observed geometry.The longer Rh-O(MeOH) bond lengths in the optimized geometries are presumably due to crystal packing stress [41].In the ferrocene moieties, the cyclopentadienyl rings in the optimized geometries of 1(MeOH)2 and 2(MeOH)2 adopt an eclipsed conformation.The average distances from the centroids of the cyclopentadienyl rings to the corresponding Fe atom in optimized geometries of 1(MeOH)2 and 2(MeOH)2 are 1.680 and 1.679 Å, respectively, which are also close to the distances in the observed geometries.
Figure 3 shows the electronic structure diagrams with selected molecular orbitals (MOs) of optimized geometries of 1(MeOH)2 and 2(MeOH)2.In the occupied orbital spaces, (1) electronic configurations of the Rh2 core in 1(MeOH)2 and 2(MeOH)2 are π 4 σ 2 δ 2 π* 2 δ* 2 π* 2 and π 4 σ 2 δ 2 δ* 2 π* 4 , respectively, indicating that a single bond is formed between two Rh ions in 1(MeOH)2 and 2(MeOH)2, similarly to other paddlewheel-type Rh2 complexes [41][42][43], and (2) the occupied MOs localized on fca moieties are energetically degenerate and relatively more unstable than those on Rh2 moieties.For instance, the HOMO to HOMO-7 in 1(MeOH)2 and HOMO and HOMO-1 in 2(MeOH)2 are mainly localized on the fca ligands (where, HOMO is the highest occupied MO).Two π* 2 (Rh2) and   excitation [44].The absorption spectral shape of 2 behaves like a superposition of those of [Rh2(piv)4] and fcaH; the band maxima of 2 are observed at 597 (ε = 400) and 443 nm (ε = 695), whereas the absorption coefficients are slightly higher than those of the corresponding bands of [Rh2(piv)4] and fcaH.Multiple coordination of fca ligands to the Rh2 core further increases the absorption coefficients; 1 has an intense absorption maximum at 423 nm whose absorption coefficient (ε = 4122) is apparently higher than the sum of the absorption coefficients of one [Rh2(piv)4] and four fcaH at 423 nm.In the longer-wavelength regions, the low-lying absorption bands due to the d-d excitations of the Rh2 core cannot be confirmed because they are hidden by an intense band at 423 nm.On the other hand, the solid-state diffuse reflectance spectrum of 1 in the visible light region shows a shoulder band at approximately 610 nm in addition to a higher-energy band at 444 nm (see Figure S6).

Absorption Features
In the ultraviolet region, 1 and 2 show unique absorption features similarly to fcaH, with a shoulder band around 350 nm, but their absorption coefficients are relatively higher than those of fcaH.These results indicate that the absorption features of 1 and 2 may possess new characteristics, such as charge transfer (CT) excitation, in addition to the respective absorption characteristics of the Rh2 cores and fca ligands. in the visible light region are relatively higher than those of 2(MeOH)2, and this tendency is consistent with the experimental results.In the ultraviolet region, 1(MeOH)2 and 2(MeOH)2 possess CT excitation characteristics from the Rh2 core to the fca ligands in addition to d-d excitations of the fca ligands and the Rh2 core and CT excitations from the fca ligands to the Rh2 core.One of the reasons why the absorption coefficients of 1(MeOH)2 and 2(MeOH)2 are higher than those of fcaH and [Rh2(piv)4] may be the occurrence of CT excitations of 1(MeOH)2 and 2(MeOH)2.

Electrochemical Properties
To investigate the electrochemical properties of 1 and 2, cyclic voltammetry (CV) analyses were performed in a degassed 9:1 CHCl3-MeOH solution.As shown in Figure 5, the fcaH and [Rh2(piv)4] exhibit reversible one-electron oxidation waves at redox potentials E1/2 = 0.620 and 0.822 V vs. SCE, respectively, in which the separation of the cathodic (Epc) and anodic (Epa) peak potentials (∆E) is estimated to be 59 and 94 mV, respectively.The CV diagram of 2 shows two reversible one-electron redox waves at 0.574 V (∆E = 90 mV) and 0.984 V vs. SCE (∆E = 92 mV), which are ascribed to oxidation processes of the fca ligand and Rh2 core, respectively.The reason for the positive shift of potential for Rh2 oxidation in 2 compared to [Rh2(piv)4] may be due to the electron-withdrawing effect of the oxidized fca ligand (fca + ).In the CV diagram of 1, only one reversible and broad redox wave is observed at 0.597 V vs. SCE.The reason is inferred that (1) the observed redox wave is an aggregate of several waves of fca oxidation processes because the ∆E value was estimated to be 180 mV, which is obviously larger than those of fcaH (∆E = 59 mV) and 2 (∆E = 90 mV for the fca ligand) and ( 2) the potential for the Rh2 oxidation process was not found in the observed window because of the strong electron-withdrawing effects of the four fca (or fca + ) ligands on the Rh2 core.The result of the differential pulse voltammetry (DPV) measurement of 1 supported the first explanation above; the DPV diagram of 1 took a shape that merged two consecutive potential peaks (See Figure S8).Furthermore, these explanations are consistent with the unrestricted DFT (uDFT) calculation results of oneelectron oxidation species of 1(MeOH)2 and 2(MeOH)2; the spin density distributions of 1(MeOH)2 + and 2(MeOH)2 + are predominantly localized on the fca ligands (see Figure 6).Remarkably, this calculation result also clearly indicates that 1(MeOH)2 + exhibits redox interactions between four fca ligands through a Rh2 core.

Crystallography
Single crystals of [Rh2(fca)4(MeOH)2] (1(MeOH)2) suitable for SCXRD analysis were grown using a slow diffusion of hexane into MeOH/CH2Cl2 solution containing 1, whereas those of [Rh2(fca)(piv)3(MeOH)2] (2(MeOH)2) were obtained using a slow diffusion of water into MeOH solution containing 2. Diffraction data were collected on a Rigaku HyPix-6000 detector system (Mo Kα radiation; λ = 0.71073 Å) at 150 K. Data collection and reduction were performed with CrysAlisPro (version 1.171.39.43a) software [47].The structures were solved with the SHELXT program [48], and the full-matrix least-square refinements on F 2 were performed using the SHELXL program [49] via the Olex2 (version 1.5) software [50].All nonhydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed in calculated positions and refined as a riding model.In the refinement of 2(MeOH)2, the residual electron density of disorder solvents was removed using the solvent mask routine of the Olex2.Crystallographic data of final refined structures are summarized in Table 1, and selected bond lengths and angles are given in Tables S5 and S6 in the Supplementary Materials.These crystallographic data can be obtained free of charge from Cambridge Crystallographic Data Centre (CCDC); deposition numbers of 1(MeOH)2 and 2(MeOH)2 are CCDC-2324331 and 2324332, respectively.

Theoretical Calculation Method
All density functional theory (DFT) calculations were performed using the B3LYP functional [51] in conjunction with the LANL08f for Rh atom, SVP for Fe atom, aug-cc-pVDZ for O atom, and cc-pVDZ for C and H atoms on the Gaussian 16 program package [52].The solvent effect of CHCl3 was taken into account using the self-consistent reaction field (SCRF) through the polarizable continuum model (PCM) theory [53].The initial structures for geometry optimizations were produced from the CIF files, and obtained optimized structures were confirmed as the minima via frequency analyses.The singlet vertical excitations were calculated using the time-dependent density functional theory (TDDFT).Optimized geometries, molecular orbitals, and spin density distributions were drawn using a GaussView 5.0 [54].

Conclusions
This study described the synthesis, characterization, structural determination, and absorption and electrochemical properties of fca-bearing homoleptic and heteroleptic paddlewheel-type Rh2 complexes, 1 and 2. SCXRD analyses of 1(MeOH)2 and 2(MeOH)2, of which crystals were obtained using recrystallization from MeOH-containing solutions of 1 and 2, clearly proved that the fca ligands are connected to the equatorial positions of the Rh2 core.To the best of our knowledge, this study is the first result of the structural determination of fca-bearing paddlewheel-type Rh2 complexes.Optimized geometries of 1(MeOH)2 and 2(MeOH)2, which were obtained using DFT calculations, reproduced the experimentally observed geometries without significant structural changes, and their electronic structure analyses revealed that (1) a single bond is formed between two Rh ions in 1(MeOH)2 and 2(MeOH)2, similarly to other paddlewheel-type Rh2 complexes, and (2) the occupied MOs localized on fca moieties are energetically degenerate and relatively more unstable than those on Rh2 moieties.Complexes 1 and 2 showed unique absorption features in the 9:1 CHCl3-MeOH solution; absorption coefficients of 1 and 2 are apparently higher than those of [Rh2(piv)4] and fcaH.TDDFT calculations indicated that the higher absorption coefficients of 1 and 2 are due to the CT excitations between fca ligands and Rh2 cores in 1 and 2. CV results and uDFT calculations clarified that 2 undergoes stepwise oxidation in the order of a fca ligand and a Rh2 core, whereas 1 exhibited redox interactions between four fca ligands through a Rh2 core in the oxidation process.It is necessary to improve the solubilities of homoleptic Rh2 complexes coordinated by metalloligands with ferrocene units for further investigation of absorption and electrochemical properties as well as applications as the catalysts and building blocks for the CPs and MOFs.These studies are currently being conducted by introducing various substituents on Rh2 and fca units in our laboratory.

Table 1 .
Crystallographic data of 1