Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η2-olefin)(NHC)(NHC’) have been synthesized by a stepwise reaction of [Rh(μ-Cl)(η2-olefin)2]2 with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η2-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η2,η2-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.

Mixed monodentate bis-NHC rhodium(I) complexes are quite scarce [26,27]. They have been prepared from dinuclear Rh-NHC precursors by CO-or chlorido-bridge cleavage with different NHC' moieties, being the marked difference in the stereolectronic properties of both carbenes the key for success. In this context, our research group has focused on the development of rhodium-NHC complexes and their applications in catalytic processes [48][49][50][51][52]. In particular, the dinuclear compounds [Rh(µ-Cl)(NHC)(η 2 -olefin)] 2 are useful starting materials for the preparation of a great variety of mononuclear complexes of type RhCl(NHC)(η 2 -olefin)(L) by simple bridge-cleavage with a nucleophilic ligand, which have proven to be efficient catalysts for gem-selective alkyne dimerization [48,51], hydrothiolation [50], or hydropyridonation [52]. We envisaged that nitrogen-centered nucleophiles could be substituted by carbon-centered NHCs, giving access to mixed bis-NHC species. Alternatively, we have synthesized a series of imidazolium salts functionalized with a pendant coumarin group, that have allowed for the synthesis of coumarin-functionalized NHC rhodium complexes by reaction with the internal base in the dinuclear precursor [Rh(µ-OCH 3 )(η 2 ,η 2 -cod)] 2 (cod = 1,5-cyclooctadiene) [53]. In the course of our investigation, we observed that in the presence of an external base, pentacoordinated bis-coumarin-NHC rhodium derivatives were obtained. Thus, we reasoned that a stepwise reaction using different azolium salt precursors could pave the way to mixed bis-coumarin-functionalized NHC complexes.

Scheme 2.
Steric relief as a key factor for the preparation of bis-NHC rhodium(I) complexes.
The NMR data of 2 agree with the presence of two carbene ligands in a mutual trans disposition. In the 1 H NMR spectrum, the characteristic signals of IPr integrate in a double ratio with respect to that of ethylene, which appears as a singlet at δ 2.01 ppm in agreement with the C2v symmetry of the complex. The appearance of two septuplets at 3.51 and 2.76 ppm, corresponding to the CH-isopropyl protons of the IPr, indicates a restricted rotation of the carbene ligands around the Rh-C axis [49]. In the 13 C{ 1 H}-APT spectrum, the most significant feature is the downfield shift of the carbene carbon atom to δ 191.8 ppm related to 1b (179.7 ppm), reflecting the increase of electron density at the metal center as a result of the coordination of a second powerful electron releasing IPr ligand [58]. Moreover, the reduction of the rhodium-carbon coupling (JRh-C = 41.6 Hz vs. 62.3 Hz in 1b) is a consequence of an opposite disposition of two ligands with a strong trans influence.
The steric pressure over the η 2 -olefin within a rhodium-bis-IPr architecture has been harnessed previously by Crudden et al., who described its facile decoordination under a Scheme 2. Steric relief as a key factor for the preparation of bis-NHC rhodium(I) complexes.
The NMR data of 2 agree with the presence of two carbene ligands in a mutual trans disposition. In the 1 H NMR spectrum, the characteristic signals of IPr integrate in a double ratio with respect to that of ethylene, which appears as a singlet at δ 2.01 ppm in agreement with the C 2v symmetry of the complex. The appearance of two septuplets at 3.51 and 2.76 ppm, corresponding to the CH-isopropyl protons of the IPr, indicates a restricted rotation of the carbene ligands around the Rh-C axis [49]. In the 13 C{ 1 H}-APT spectrum, the most significant feature is the downfield shift of the carbene carbon atom to δ 191.8 ppm related to 1b (179.7 ppm), reflecting the increase of electron density at the metal center as a result of the coordination of a second powerful electron releasing IPr ligand [58]. Moreover, the reduction of the rhodium-carbon coupling (J Rh-C = 41.6 Hz vs. 62.3 Hz in 1b) is a consequence of an opposite disposition of two ligands with a strong trans influence.
The steric pressure over the η 2 -olefin within a rhodium-bis-IPr architecture has been harnessed previously by Crudden et al., who described its facile decoordination un-der a nitrogen atmosphere to yield dinitrogen adducts [59]. Unfortunately, the mixed bis-NHC derivative RhCl(η 2 -ethylene)(IMes)(IPr) could not be cleanly obtained by the addition of IMes to 1b. In addition, attempts to use diolefin precursors of the type RhCl(η 2 ,η 2 -cod)(NHC) failed [60]. More recently, the group of Chaplin has disclosed that bis-NHC rhodium complexes bearing a η 2 -coe ligand could be prepared with a less sterically demanding IBiox carbene [61]. According to this, the treatment of 1a with less-bulky NHCs, such as IMe or ICy, prepared in situ by deprotonation of the corresponding azolium salts, resulted in the successful formation of the mixed bis-NHC complexes RhCl(η 2 -coe)(IPr)(NHC) (3) {NHC = 1,3-dimethylimidazolin-2-carbene (IMe) (3a) and 1,3-dicyclohexylimidazolin-2-carbene (ICy) (3b)}, which were isolated as yellow solids in good yields (Scheme 2). The η 2 -olefin ligand of complexes 3 remains coordinated in solution at room temperature, but can be smoothly replaced by CO to yield the carbonyl complexes RhCl(CO)(IPr)(NHC) (NHC = IMe, 4a; ICy, 4b). Single crystals of 3a suitable for X-ray structure determination were grown by slow diffusion of hexane into a saturated toluene solution of the complex. Figure 1 shows the molecular structure of 3a and the selected bond lengths and angles. nitrogen atmosphere to yield dinitrogen adducts [59]. Unfortunately, the mixed bis-NHC derivative RhCl(η 2 -ethylene)(IMes)(IPr) could not be cleanly obtained by the addition of IMes to 1b. In addition, attempts to use diolefin precursors of the type RhCl(η 2 ,η 2cod)(NHC) failed [60]. More recently, the group of Chaplin has disclosed that bis-NHC rhodium complexes bearing a η 2 -coe ligand could be prepared with a less sterically demanding IBiox carbene [61]. According to this, the treatment of 1a with less-bulky NHCs, such as IMe or ICy, prepared in situ by deprotonation of the corresponding azolium salts, resulted in the successful formation of the mixed bis-NHC complexes RhCl(η 2coe)(IPr)(NHC) (3) {NHC = 1,3-dimethylimidazolin-2-carbene (IMe) (3a) and 1,3-dicyclohexylimidazolin-2-carbene (ICy) (3b)}, which were isolated as yellow solids in good yields (Scheme 2). The η 2 -olefin ligand of complexes 3 remains coordinated in solution at room temperature, but can be smoothly replaced by CO to yield the carbonyl complexes RhCl(CO)(IPr)(NHC) (NHC = IMe, 4a; ICy, 4b). Single crystals of 3a suitable for X-ray structure determination were grown by slow diffusion of hexane into a saturated toluene solution of the complex. Figure 1 shows the molecular structure of 3a and the selected bond lengths and angles. The crystal structure of 3a shows a distorted square planar geometry at the metal center, with a trans disposition of the NHC ligands [C1-Rh-C30 169.65 (9) [49]}, reflecting the high trans influence of NHCs. The coordinated olefinic bond C37-C38, as well as the NHC cores of the IPr and IMe ligands, lie almost perpendicular to the coordination plane Rh-Cl-C1-C30-CT1 (79.6°, 85.3°, 69.6°, respectively). In addition, the pitch and yaw angles observed for each NHC ligand indicate a moderately distorted coordination to the metal center with respect to the corresponding rhodium-carbon bond.
The low temperature NMR data for complexes 3 agree with the structure reported in the solid state for 3a. Thus, in the 1 H NMR spectrum at 243 K, two septuplets ascribed to the CH-isopropyl protons of the IPr are observed around δ 4.3 and 2.7 ppm, which broaden at room temperature due to a Rh-IPr rotational process (see Figure S1 in Supporting Information). The presence of different NHC ligands in 3 is confirmed by the appearance of two resonances for the =CHN heterocyclic protons around δ 6.5 (IPr), 5.93 (IMe, 3a), and 6.34 ppm (ICy, 3b). In addition, two relatively deshielded doublets are displayed in the 13 C{ 1 H}-APT spectrum around δ 194 (IPr), 191.4 (IMe, 3a), and 188.0 (ICy, 3b), with relatively short couplings of around 42 Hz, as commented for 2. The presence of η 2 -olefin ligands is corroborated by a multiplet around δ 3 ppm in the 1 H NMR, and a doublet around 54 ppm (J C-Rh = 16 Hz) in the 13 C{ 1 H}-APT spectra. The substitution of coe by carbon monoxide in 4 results in the appearance of a new doublet in the 13 C{ 1 H}-APT spectrum. Resonance assignment is facilitated by 1 H-13 C HMBC correlation peaks between the imidazolinyl protons and the corresponding carbene-carbon atoms within NHC moieties ( Figure 2). As expected, the activation barrier for IPr-Rh rotation is reduced for carbonyl complexes 4 [49]. Therefore, only one septuplet for CH-isopropyl protons is observed around δ 3.4 ppm. 3a), and 6.34 ppm (ICy, 3b). In addition, two relatively deshielded doublets are displayed in the 13 C{ 1 H}-APT spectrum around δ 194 (IPr), 191.4 (IMe, 3a), and 188.0 (ICy, 3b), with relatively short couplings of around 42 Hz, as commented for 2. The presence of η 2 -olefin ligands is corroborated by a multiplet around δ 3 ppm in the 1 H NMR, and a doublet around 54 ppm (JC-Rh = 16 Hz) in the 13 C{ 1 H}-APT spectra. The substitution of coe by carbon monoxide in 4 results in the appearance of a new doublet in the 13 C{ 1 H}-APT spectrum. Resonance assignment is facilitated by 1 H-13 C HMBC correlation peaks between the imidazolinyl protons and the corresponding carbene-carbon atoms within NHC moieties (Figure 2). As expected, the activation barrier for IPr-Rh rotation is reduced for carbonyl complexes 4 [49]. Therefore, only one septuplet for CH-isopropyl protons is observed around δ 3.4 ppm. Wingtip functionalized NHCs ligands can also participate in the formation of mixed bis-NHC rhodium(I) complexes. In this context, coumarin-functionalized-NHC metal complexes display interesting catalytic and luminescence properties [62][63][64][65][66]. In particular, we have prepared coumarin-NHC derivatives of type RhCl(NHC-Cou)(η 2 ,η 2 -cod) (5) in which, in contrast to the behavior observed for related IPr-or IMes-cod compounds, the diolefin ligand can be replaced in the presence of an excess of carbene to yield bis-NHC complexes [53]. Thus, a stepwise reaction using different carbene precursors enabled the synthesis of mixed-NHC species (Scheme 3). In this way, an imidazole-benzimidazole mixed-NHC complex RhCl(κC, (6) was obtained by refluxing a THF solution of the precursor RhCl(BzICou tol )(η 2 ,η 2 -cod)] (5a) and the azolium salt [HICou Bz ]Cl in the presence of sodium methoxide for 24 h. Under similar reaction conditions, RhCl(BzICou Bu )(IPr) (7) was formed starting from RhCl(BzICou bu )(η 2 ,η 2 -cod) (5b) and free IPr. Although the solid state structure of 6 could not be determined by X-ray diffraction methods, a trigonal bipyramidal structure with a trans disposition of the NHC ligands is assumed in an analogy to the related coumarin-functionalized bis-NHC complexes RhCl(BzICou R )2 and RhCl(ICou R )2 described previously [53]. Interestingly, the Xray single-crystal analysis of the mixed bis-NHC compound 7 revealed a cis disposition for the IPr and coumarin-BzI carbenes, an uncommon feature in square planar rhodium(I) complexes when a trans configuration is feasible [67,68] (Figure 3). Wingtip functionalized NHCs ligands can also participate in the formation of mixed bis-NHC rhodium(I) complexes. In this context, coumarin-functionalized-NHC metal complexes display interesting catalytic and luminescence properties [62][63][64][65][66]. In particular, we have prepared coumarin-NHC derivatives of type RhCl(NHC-Cou)(η 2 ,η 2 -cod) (5) in which, in contrast to the behavior observed for related IPr-or IMes-cod compounds, the diolefin ligand can be replaced in the presence of an excess of carbene to yield bis-NHC complexes [53]. Thus, a stepwise reaction using different carbene precursors enabled the synthesis of mixed-NHC species (Scheme 3). In this way, an imidazole-benzimidazole mixed-NHC complex (6) was obtained by refluxing a THF solution of the precursor RhCl(BzICou tol )(η 2 ,η 2 -cod)] (5a) and the azolium salt [HICou Bz ]Cl in the presence of sodium methoxide for 24 h. Under similar reaction conditions, RhCl(BzICou Bu )(IPr) (7) was formed starting from RhCl(BzICou bu )(η 2 ,η 2 -cod) (5b) and free IPr. Although the solid state structure of 6 could not be determined by X-ray diffraction methods, a trigonal bipyramidal structure with a trans disposition of the NHC ligands is assumed in an analogy to the related coumarin-functionalized bis-NHC complexes RhCl(BzICou R ) 2 and RhCl(ICou R ) 2 described previously [53]. Interestingly, the X-ray single-crystal analysis of the mixed bis-NHC compound 7 revealed a cis disposition for the IPr and coumarin-BzI carbenes, an uncommon feature in square planar rhodium(I) complexes when a trans configuration is feasible [67,68] (Figure 3).      (9) • ] brings about a severe deviation of the NHC core from the ideal arrangement with respect to the Rh-C30 bond (yaw angle, ψ 11.1 • ), similar to what is already observed in the related bis-chelate complexes RhCl(BzICou R ) 2 and RhCl(ICou R ) 2 [71].
The presence of two different NHC ligands in 6 and 7 is confirmed by the NMR data. The 13 C{ 1 H}-APT NMR spectrum of 6 display two doublets at δ 184.8 and 172.4 ppm with Rh-C coupling constants around 33 Hz, ascribed to carbene carbon atoms. Correlations of the imidazolinyl protons at δ 6.45 and 6.40 ppm with the carbon signal at 172.4 ppm in the 1 H-13 C-HMBC spectrum of 6 helps to ascribe that signal to the I-Cou ligand (Figure 4). In addition, four doublets with J H-H~1 3 Hz are observed for the N-methylene protons in the 1 H NMR spectrum. The coordination of the olefinic group of coumarin moieties is reflected in the appearance of high field doublets at 72.0 ppm (J C-Rh = 12.1 Hz) and 50.8 ppm (J C-Rh = 7.2 Hz) for BzI-Cou and 74.0 ppm (J C-Rh = 11.7 Hz) and 52.5 ppm (J C-Rh = 7.6 Hz) ascribed to the I-Cou ligand. Regarding 7, the presence of two different NHC ligands is corroborated by the observation of two low field doublets at 196.1 ppm (J C-Rh = 52.2 Hz) and 191.1 ppm (J C-Rh = 60.2 Hz) for BzI-Cou and IPr, respectively, in the 13 C{ 1 H}-APT NMR spectrum. The deshielding of these resonances compared to those of 6 could be ascribed to a more electron-rich rhodium center due to the coordination of only one electron-acceptor olefin ligand, whereas the higher J C-Rh values reflect Rh-C shorter separations.  [71]. The presence of two different NHC ligands in 6 and 7 is confirmed by the NMR data. The 13 C{ 1 H}-APT NMR spectrum of 6 display two doublets at δ 184.8 and 172.4 ppm with Rh-C coupling constants around 33 Hz, ascribed to carbene carbon atoms. Correlations of the imidazolinyl protons at δ 6.45 and 6.40 ppm with the carbon signal at 172.4 ppm in the 1 H-13 C-HMBC spectrum of 6 helps to ascribe that signal to the I-Cou ligand (Figure 4). In addition, four doublets with JH-H⁓13 Hz are observed for the N-methylene protons in the 1

Materials and Methods
All reactions were carried out with the rigorous exclusion of air and moisture, using Schlenk-tube techniques and a dry box when necessary. The reagents were purchased from commercial suppliers and used as received. Organic solvents were dried by standard procedures and distilled under argon prior to use, or obtained oxygen-and water-free from a Solvent Purification System (Innovative Technologies). Deuterated solvents were deoxygenated and dried over sodium metal (C6D6 and toluene-d8) or activated molecular sieves (CDCl3). The organometallic precursors [Rh(μ-Cl)(η 2 -coe)(IPr)]2 (1a) [54], [Rh(μ-Cl)(η 2 -ethylene)(IPr)]2 (1b) [57], and RhCl)(η 2 ,η 2 -cod)(NHC-Cou) (5) [53] were prepared as previously described in the literature. Chemical shifts (expressed in parts per million) are referenced to residual solvent peaks. Coupling constants, J, are given in Hz. Spectral assignments were achieved by a combination of 1 H-1 H COSY, 13 C{ 1 H}-APT and 1 H-13 C HSQC/HMBC experiments. The attenuated total reflection infrared spectra (ATR-IR) of

Materials and Methods
All reactions were carried out with the rigorous exclusion of air and moisture, using Schlenk-tube techniques and a dry box when necessary. The reagents were purchased from commercial suppliers and used as received. Organic solvents were dried by standard procedures and distilled under argon prior to use, or obtained oxygen-and water-free from a Solvent Purification System (Innovative Technologies). Deuterated solvents were deoxygenated and dried over sodium metal (C 6 D 6 and toluene-d 8 ) or activated molecular sieves (CDCl 3 ). The organometallic precursors [Rh(µ-Cl)(η 2 -coe)(IPr)] 2 (1a) [54], [Rh(µ-Cl)(η 2 -ethylene)(IPr)] 2 (1b) [57], and RhCl)(η 2 ,η 2 -cod)(NHC-Cou) (5) [53] were prepared as previously described in the literature. Chemical shifts (expressed in parts per million) are referenced to residual solvent peaks. Coupling constants, J, are given in Hz. Spectral assignments were achieved by a combination of 1 H-1 H COSY, 13 C{ 1 H}-APT and 1 H-13 C HSQC/HMBC experiments. The attenuated total reflection infrared spectra (ATR-IR) of solid samples were run on a PerkinElmer Spectrum 100 FT-IR spectrometer. C, H, and N analyses were carried out in a Perkin-Elmer 2400 CHNS/O analyzer. Highresolution electrospray mass spectra (HRMS) were acquired using a MicroTOF-Q hybrid quadrupole time-of-flight spectrometer (Bruker Daltonics, Bremen, Germany).

Preparation of RhCl(CO)(IMe)(IPr) (4a).
Carbon monoxide was bubbled through a yellow solution of 3a (100 mg, 0.14 mmol) in 20 mL of toluene at room temperature for 30 min. After filtration through Celite, the solvent was evaporated to dryness. The addition of n-hexane induced the precipitation of a yellow solid, which was washed with n-hexane  Figure S17. 1H NMR spectrum of 4a in C6D6 at 298 K. Figure S18. 13C{1H}-APT NMR spectrum of 4a in C6D6 at 298 K. Figure S19. 1H-1H COSY NMR spectrum of 4a in C6D6 at 298 K. Figure S20. 1H-13C HSQC NMR spectrum of 4a in C6D6 at 298 K. Figure S21. 1H-13C HMBC NMR spectrum of 4a in C6D6 at 298 K.

Conclusions
Mixed bis-NHC rhodium(I) complexes of type RhCl(η 2 -olefin)(NHC)(NHC') have been synthesized following a stepwise procedure. It has been revealed that the steric hindrance imparted by the wingtips of the carbene ligands as well as that of the η 2 -olefin is critical. Thus, bis-NHC derivatives containing both bulky IPr or IMes could be accessed only for smaller η 2 -ethylene, whereas the steric relief in IMe or ICy allows for the preparation of mixed bis-NHC complexes containing a η 2 -coe ligand. Regarding the use of RhCl(NHC)(η 2 ,η 2 -cod) precursors, in contrast to the stability observed for IPr derivative, the coumarin-functionalized NHC ligands facilitates decoordination of the diolefin, enabling the straightforward introduction of a second NHC. Thus, the pentacoordinated mixed bis-NHC complexes bearing coumarin functionalities adopt a trans-NHC disposition, whereas the square planar IPr-BzIcou species presents an uncommon cis-NHC configuration, exhibiting an anagostic interaction between the metal atom and one IPr methyl group. Crystal Structure Determination: Single crystals of 3a and 7 for the X-ray diffraction studies were grown by slow diffusion of hexane into a saturated toluene solution (3a) or slow evaporation of CDCl 3 solution in a NMR tube (7). X-ray diffraction data were collected at 100(2) K on a Bruker APEX DUO CCD diffractometer with graphitemonochromated Mo−Kα radiation (λ = 0.71073 Å) using ω rotations. The intensities were integrated and corrected for absorption effects with SAINT-PLUS [72] and SADABS [73] programs, both included in the APEX2 package. The structures were solved by the Patterson method with SHELXS-97 [74] and refined by full matrix least-squares on F 2 with SHELXL-2014 [71], under WinGX [75]. CCDC 2204568 (3a) and 2204569 (7) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, accessed on 17 October 2022 (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-12-2333-6033; E-mail: deposit@ccdc.cam.ac.uk).
Crystal data and structure refinement for 3a.

Conclusions
Mixed bis-NHC rhodium(I) complexes of type RhCl(η 2 -olefin)(NHC)(NHC') have been synthesized following a stepwise procedure. It has been revealed that the steric hindrance imparted by the wingtips of the carbene ligands as well as that of the η 2 -olefin is critical. Thus, bis-NHC derivatives containing both bulky IPr or IMes could be accessed only for smaller η 2 -ethylene, whereas the steric relief in IMe or ICy allows for the preparation of mixed bis-NHC complexes containing a η 2 -coe ligand. Regarding the use of RhCl(NHC)(η 2 ,η 2 -cod) precursors, in contrast to the stability observed for IPr derivative, the coumarin-functionalized NHC ligands facilitates decoordination of the diolefin, enabling the straightforward introduction of a second NHC. Thus, the pentacoordinated mixed bis-NHC complexes bearing coumarin functionalities adopt a trans-NHC disposition, whereas the square planar IPr-BzIcou species presents an uncommon cis-NHC configuration, exhibiting an anagostic interaction between the metal atom and one IPr methyl group.