Triazolyl- vs Pyridyl-Functionalized N-Heterocyclic Carbene Complexes: Impact of the Pendant N-Donor Ligand on Intramolecular C–C Bond Formation

Organometallic Rh(Cp*) (Cp* = η5-pentamethylcyclopentadienyl) complexes with monodentate N-heterocyclic carbene (NHC) ligands bearing a pendant anthracenyl substituent have been shown to undergo intramolecular C–C coupling reactions. Herein, two bidentate NHC ligands substituted with pyridyl or triazolyl donor groups were prepared along with the corresponding MII/III (M = RuII, OsII, RhIII, IrIII) complexes. While the Rh(Cp*) complex featuring an NHC-triazole bidentate ligand underwent the equivalent reaction as the monodentate Rh(NHC) complex, i.e., it formed a polydentate ligand, the pyridyl-pendant derivative was unequivocally shown to be unreactive. This contrasting behavior was further investigated by density functional theory (DFT) calculations that highlighted significant differences between the two types of Rh(III) complexes with pendant pyridyl or triazolyl N-coordinating groups. Modeling of the reaction pathways suggests that the initial formation of a dicationic Rh(III) species is unfavorable and that the internal ligand transformation proceeds first by dissociation of the coordinated N atom of the pendant group from the Rh center. After the formation of a neutral η4-fulvene ligand via combined proton/single electron transfer, a cycloaddition occurs between the exo-ene bond of fulvene and the 9′ and 10′ positions on the pendant anthracenyl group. The resulting experimental UV–visible spectrum recorded in methanol of the polydentate triazolyl-based Rh species revealed the loss of the vibronic coupling typically associated with an anthracenyl functional group. Moreover, TD-DFT modeling indicates the presence of an equilibrium process whereby the N-coordination of the pendant triazolyl group to the RhIII center appears to be highly labile. Charge decomposition analysis (CDA) of the DFT-modeled species with the dissociated triazolyl group revealed a pseudo-η3-allylic interaction between the π-type MOs of the transformed anthracenyl group and the RhIII center; thus, the singly attached chelating ligand is classified as having rare nonadenticity.

. Observed and calculated m/z values, and peak assignment of the most abundant peaks in the positive ion-mode ESI-mass spectra of compounds 3-5, 6a-d, 7a-d and 8', and the adduct of 8' with pta (8 pta

Reaction 2. Reaction of 7c with Ag2O
S5 Figure S2. Molecular structure of the (protonated) pro-carbene 4 drawn at 50% probability level. The PF6counterion was omitted for clarity. Only one of the two independent molecules is shown and the disorder of the benzimidazolium moieties and the pyridyl groups was omitted.

Reaction of 8' with 1,3,5-triaza-7-phosphaadamantane (pta)
A mixture of 8' (5.0 mg, 5.8 µmol, 1.0 eq.) and pta (1.0 mg, 5.8 µmol, 1.0 eq.) in methanol (10 mL) was stirred for 24 h at room temperature. ESI-mass ( Figure S3), and 1 H and 31 P{ 1 H} NMR spectra ( Figures S4 and S5) of the reaction mixture were recorded.   The reaction pathways were constructed from geometry-optimized structures representing energy minima or saddle points (transition states) using the ωB97xD level of theory, which has S8 an established reputation for producing highly accurate structural parameters for organometallic complexes. [3][4][5][6] This method incorporates a modified version of Grimme's D2   algorithm for calculating damped long-range dispersion corrections, which are beneficial in   providing realistic energies for complexes where strong internal interactions between substituents are present. 6,7 Calculated gas-phase energies were corrected through an applied approximated solvent potential using the SMD protocol, using MeCN as the solvent. 8 For each step of the geometry optimizations, self-consistent iterations were performed until a convergence criterion of 10 -8 was obtained. A double zeta Gaussian-type basis set of Petersson was used and augmented with an additional set of d-orbitals for C, N and Cl and an additional p-orbital for hydrogen (6-31G(d,p)). 9 In order to successfully locate the transition states involving hydroxide or water, a more polarized basis set (6-31+G(d,p)) for O was employed.
The rhodium center was modelled with the Stoll and Preuss SDDAll basis set of double zeta quality, substituting the core electron wave-functions with an effective pseudopotential. 10 Optimized structures were verified to be stationary minima as indicated by the absence of imaginary frequencies or transition states as indicated by a single imaginary frequency. All transition states were verified to be connected to the appropriate minima by performing IRC analyses using the geometry and frequency analysis of the transition state as the starting point.
Population analysis was performed using CM5 which employs the charge fitting methodology (Charge Model 5) as developed by Truhlar and Cramer et al. 11 Post-quantum analyses including both VIIIa and VIIIb were performed using the MULTIWFN program. Where appropriate, the density gradient was analyzed for covalent and non-covalent interactions and was performed using Density Overlap Regions Indicator algorithm (IRI) as developed by Hen and Lu, 12 and implemented using the MULTIWFN program version 1.38. 13 IRI plots were generated using VMD. 14

Benchmarking of the employed DFT level of theory and basis set
To validate the use of -B97xD and the employed basis set, an optimization of the Ru(cym) complex 6a was performed and compared to the experimentally obtained molecular structure (Tables S3 and S4). In this case, the unit cell contains two crystallographic independent molecules in the asymmetric unit (6a' and 6a''), where the arrangement of the bidentate ligand occurs in two orientations. One of the orientations matches the structure modeled by the DFT calculations. A graphical comparison ( Figure S7) shows an excellent match of all metal-ligand bond lengths (Table S3) with the major differences being the rotation of the p-cymene group S9 and pendant anthracenyl unit. The RMS value was obtained by comparing the bond distance average of the two crystallographic independent molecules and the DFT calculated model (Table S3). A small RMS difference of 7.7% was found, thus indicating the chosen set of parameters for the DFT model is valid. Additionally, our previous study on the Rh complex B using the identical level of theory and the exact basis set including the SDDAll pseudo potential for Rh also indicated a high level of agreement. 15 Calculation of the simulated UV-Visible absorption spectra for 8 The gas phase geometries of the triazolyl metal-coordination and uncoordinated complexes were reoptimized using SMD solvent corrections, employing the dielectronic constant for methanol. The same basis set as described previously was employed. Upon confirming the absence of imaginary frequencies, the UV-visible spectra were calculated using 100 states.
Analysis of the UV-visible transitions and determination of the % metal character for the MOs was preformed using the AOMIX program. 16 Figure S7. Overlay of one of the crystallographic independent molecules found in the unit cell (orange) and the DFT-calculated model (yellow) for Ru(cym) complex 6a with a pendant pyridyl group. S10 Table S3. Comparison of DFT modelled and experimental (two crystallographically independent structures 6a' and 6a'') bond lengths for complex 6a.
Bond length X-ray 6a'    A rationale for the difference between complexes XVIIIb (pyridyl) and XVIIIa (triazolyl) extend from the constrained geometry imposed by the cycloaddition and fusion of the pendant anthracenyl group with the Cp* ligand. The five-membered triazolyl ring is able to bind weakly to the Rh center, while the 6-membered pyridyl XVIIIb is too strained to form a successful orbital overlap with the metal. This was confirmed by several unsuccessful attempts to optimize a complex analogous to XVIIIa whereby the pendant pyridyl group was bound to Rh via the N center. Moreover, no convergence to a stable minimum during the optimization process was found when the Rh-N bond was fixed. To compensate for the lack of electron density offered by the 4 th ligand in XVIIIb, an overall stronger interaction with the anthracenyl group is calculated. IRI analysis of the reduced electron density gradient (RDG; Figure S13) reveals one strong covalent between Rh and C9', while two weaker flanking covalent interactions are noted as shown by the yellow arrows in Figure S13. In contrast, the additional electron density imparted by coordination of the triazolyl groups, reduces the anthracenyl interaction, restricting it to a single covalent bond. S16 Figure S13.

UV-visible spectroscopy
The compounds were dissolved in methanol (1.7 × 10 -4 mol/L) and the UV-vis spectra were recorded on a Shimadzu UV-3600 Plus spectrophotometer.

Fluorescence spectroscopy
The compounds were dissolved in methanol (1.7 × 10 -4 mol/L) and the fluorescence emission spectra were collected on a JASCO FP-8600 NIR spectrofluorometer.    Figure S24. 13