Dehydrogenative Double C−H Bond Activation in a Germylene‐Rhodium Complex

Abstract Transition metal tetrylene complexes offer great opportunities for molecular cooperation due to the ambiphilic character of the group 14 element. Here we focus on the coordination of germylene [(ArMes2)2Ge :] (ArMes=C6H3‐2,6‐(C6H2‐2,4,6‐Me3)2) to [RhCl(COD)]2 (COD=1,5‐cyclooctadiene), which yields a neutral germyl complex in which the rhodium center exhibits both η 6‐ and η 2‐coordination to two mesityl rings in an unusual pincer‐type structure. Chloride abstraction from this species triggers a singular dehydrogenative double C−H bond activation across the Ge/Rh motif. We have isolated and fully characterized three rhodium‐germyl species associated to three C−H cleavage events along this process. The reaction mechanism has been further investigated by computational means, supporting the key cooperative action of rhodium and germanium centers.


Solution NMR analysis of compounds of type 2 and 3
As highlighted in the main text, we observed some slight but distinctive NMR spectroscopic differences in compounds 2 and 3 as a result of varying the counteranion. Thus, 1 H NMR signals associated to the activated mesityl ring in compounds 2 and 3 containing the triflimidate anion are similar, while differing from the corresponding resonances due to same species when a noncoordinating anion (BAr F or [B(C6F5)4] -) was used ( Figure S1). This would be consistent with triflimidate binding to rhodium accompanied by coordination rearrangement from η 3pseudoallylic to η 1 -methylenic forms. Nonetheless, these differences do not seem to be present in the solid-state, where in all cases η 3 -coordination of the flanking aryl ring in a pseudoallylic fashion was elucidated. Also, our prior investigations on a somewhat related cationic gold germylene compound revealed similar effects upon counteranion coordination. 4

X-Ray structural characterization of new compounds
Crystallographic details. Low-temperature diffraction data were collected on a Bruker APEX-II CCD diffractometer (1) or on a D8 Quest APEX-III single crystal diffractometer with a Photon III detector and a IμS 3.0 microfocus X-ray source (2, 3 and 5) at the Instituto de Investigaciones Químicas, Sevilla. Data were collected by means of ω and φ scans using monochromatic radiation λ(Mo Kα1) = 0.71073 Å. The diffraction images collected were processed and scaled using APEX-II or APEX-III software, respectively. The structures were solved with SHELXT and was refined against F 2 on all data by fullmatrix least squares with SHELXL. 5 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model, unless otherwise noted. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms to which they are linked (1.5 times for methyl groups). Despite many attempts, we could not obtain good quality crystals for compound 3 when pairing with the noncoordinating anion BArF ([B(C6H2-3,5-(CF3)2)4] -). However, we could grow crystals of enough quality for X-ray diffraction studies from the reaction between 1 and two equivalents of (PMe2Ar Dipp2 )Au(NTf2). Thus the structure of 3 contains triflimidate as the counteranion. Similarly, good quality crystals of 2 were obtained after anion exchange by triflate (OTf -).

Computational details
Optimized geometries of Ge-Rh complexes and transition states have been obtained from DFT calculations with Gaussian 09. 6 Optimizations were carried out without symmetry restrictions in bulk solvent (benzene) using DFT methods: the ωB97xD functional 7 was chosen, which includes empirical dispersion corrections 8 and was used in conjunction with the 6-31g(d,p) basis set 9 for non-metal atoms, including Ge, and the SDD basis and associated electron core potential (ECP) 10 for Rh. Bulk solvent effects (benzene) were included during optimization with the SMD continuum model. 11 Topological analysis of the electron density within the Atoms In Molecules (AIM) formalism), 12 and NBO analysis were performed with the Multifwn code 13 and the NBO6.0 14 software respectively. The extended wavefunction [.wfx] and NBO [.47] files required for such analyses were calculated on the geometries previously optimized with the triple-ζ basis set Def2TZVP 15 for all atoms, which includes an ECP for Rh. 16 The CYLview visualization software has been used to create some of the figures. 17 • DFT-calculated free energy profile in benzene for the formation of species 3, 4, and 5 from the cationic germylene-rhodium A is shown below ( Figure S3). At variance with the profile discussed in the main text, herein intermediates A' and 3' en route to 3 are also shown. These intermediates are conformers of A and 3, which result from analysis of the minima connected by TSA→B and TSB→3 and their geometries can be found in the xyz coordinates file provided. Figure S3. Calculated free energy profile in bulk benzene for the formation of 3, 4, and 5.
Direct formation of 3 via C-H activation across the germanium-rhodium bond of A was explored and, although a transition state was not located, relaxed potential energy scans suggest a barrier in excess of 30 kcal·mol -1 for this transformation. Similarly, the calculations indicate that direct formation of 4 from A and B requires overcome barriers of 33.6 and 34.3 kcal·mol -1 respectively, which is at odds with the experiments since the formation of 5 from 4 should have a higher energy barrier than the steps leading to 4 from 3. Figure S4. DFT-optimized geometries of B and B' (most H atoms omitted). Notice the change in the coordination mode of the arene ligand. Further structural parameter can be retrieved from the xyz coordination file provided. Figure S5. DFT-optimized geometries dihydrogen complex 5·H2 (most H atoms omitted).
• QTAIM analysis of the electron density of relevant intermediates and transition states shows bond critical points (bcp) and the corresponding bond paths connecting their germanium and rhodium sites. Figures S6-S11 illustrate these bcps and bond paths overlaid on the Laplacian of the electron density of the corresponding species.     • Analysis of the electron density at the germanium-rhodium bcps of germylrhodium species 1 and 3 and germylenes A and B' (Table S2) reveal electron density values of approximately 0.1 e·bohr -3 and values of the Laplacian of their electron densities (∇ 2 ρb) at the same bcps close to 0. This, in addition to negative values for the total energy density (Hb) and values for the absolute electronic potential energy and kinetic energy densities ratio (│Vb│/Gb) close to 2 indicate a strong covalent character for the Ge-Rh interaction in these species. 18 Interestingly, the ellipticity of the Ge-Rh bond increases from 0.09 to ca. 0.2 from the parent germyl 1 to the germylene A, which is consistent with an increase in the double bond character (for cylindrical bonds, the ellipticity is expected to be 0). As we shall discuss later, this reflects back-donation from rhodium to germanium that may offset the effect of the loss of the chloride ligand. Table S2. QTAIM indicators at Ge-Rh BCPs. All data are in atomic units. ρb electron density (e·bohr -3 ); Hb total energy density (hartree·bohr -3 ); ∇ 2 ρb Laplacian of the electron density (e·bohr -5 ); |Vb|/Gb ratio between the absolute electronic potential energy and kinetic energy densities; εb ellipticity (ratio between the largest and smallest negative eigenvalues of the Hessian -1). • NBO analysis was used to complete the insight into the germanium-rhodium interaction of the species studied by AIM. Table S3 summarizes relevant donoracceptor interactions and Wiberg Bond Order (WBO) of the germanium-rhodium bonds. Typical NBO terminology has been used to name the different types of NBOs: LP for Lone Pair and LV for Lone Vacancy, which refers to an empty valence orbital localized on one atom. Also, the main Atomic Orbital contribution to the LVs has been included in parenthesis. The NLMO column indicates the percentage of non-Lewis orbitals from the acceptor atom that are mixed with the parent donor NBO. Second order perturbation theory analysis of the Fock matrix, reveals electron donation from 4d orbitals of the rhodium atoms to 4p orbitals of the germanium, both in the germyl and germylene species studied, as shown in Table S3. This is supported by the Ge contribution to the corresponding NLMOs. Notice that the contribution of Ge orbitals to the NLMO corresponding to the donation of electron density from Rh in complex 3 is by far the highest, but it must be recognized that in this case the germanium-rhodium bond is described in terms of electron delocalization (24.59% Ge; ∆Eij = 109.57 kcal·mol -1 ) between fragments ( Figure S12, right).

Ge-Rh bond
In complex B', the Ge atom also accepts electron density from the Rh-H bonding orbital, i.e. the bond is delocalized on the germanium (NLMO contribution 4.8%). Figure S12. NLMOs involved in the Rh→pGe back donation and in A (left) and the germanium-rhodium interaction in 3 (right).