Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

Abstract The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non‐innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C−H activation at one methyl terminus of C5Me5 in [(η‐C5Me5)Rh(PMe3)2] to form a new Rh−H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C−H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E−H bonds (E=O, N), including the N−H bonds in ammonia, can be understood in terms of bimetallic frustration.


Compound 3a.
A solid mixture of compounds 1 (24 mg, 0.061 mmol) and 2a (50 mg, 0.061 mmol) was dissolved in toluene (5ml) and stirred at room temperature for 30 minutes. Reaction monitoring revealed that formation of 3a was immediate and proceeded quantitatively by NMR spectroscopy. The solution was concentrated to half of its volume and precipitated with pentane. The residue was then filtered and dried under vacum (49 mg, 66 %). To increase purity, compound 3a was crystallized by slow diffusion of pentane over a toluene solution to provide a brownish crystalline material. Compound 3b. A solid mixture of compounds 1 (19 mg, 0.049 mmol) and 2b (50 mg, 0.049 mmol) was dissolved in toluene (5ml) at −60 ºC and stirred for 30 minutes at that temperature. Reaction monitoring revealed that formation of 3b was immediate and proceeded almost quantitatively (ca. 90%) by NMR spectroscopy. The solution was gently warmed up to 25 ºC, concentrated to half of its initial volume and then precipitated with pentane. The residue was then filtered and dried under vacum (30 mg, 44 %). To increase purity, compound 3b was crystallized by slow diffusion of pentane over a toluene solution to provide a brownish crystalline material. 1 H NMR (300 MHz, THF-d8, 25 ºC) δ: 7.28 (t, 1 H, 3  Compound 4b. Compound 4b cannot be isolated in pure form as it represents the minor isomer during the reaction of 1 and 2b (maximun conversion of around 30%, c.f. major isomer 3b: ca. 70%) and exhibits a pronouced reactivity and thermal unstability. Nevertheless, multinuclear NMR analysis permitted identification of its 1 H and 31 P{ 1 3  Assignment of the resulting gold and rhodium products was made on the basis of the independently synthesized gold compounds (see below) and the known spectroscopic signatures of compound 5. 1 The following superimposed 31 P{ 1 H} NMR spectra due to the isomeric mixture 3b:4b evinces that only compounds of type 4 are reactive towards polar E-H bonds, while metal adducts 3 remains unaltered.  The resulting solution was evaporated and extracted with pentane (4 x 5 mL) to yield a mixture of two species in ca 3:1 ratio. While the major compound is attributed to 6 (whose spectroscopic resonances are reported and assigned below), the minor species is ascribed to cation [(PCyp2Ar Xyl2 )Au(NH3)] + (also described below). Although we did not separate the two species, we estimate an overall yield for 6 of around 64 %.

X-Ray structural characterization of new compounds
Crystallographic details. Crystals of compounds 3b and 4c were grown by slow diffusion of pentane into their benzene or THF solutions, respectively. Low-temperature diffraction data were collected on a Bruker APEX-II CCD diffractometer using monochromatic radiation λ(Mo Kα1) = 0.71073 Å at the Instituto de Investigaciones Químicas de Sevilla. Data collections were processed with APEX-W2D-NT (Bruker, 2004), cell refinement and data reduction with SAINT-Plus (Bruker, 2004) [5] and the absorption was corrected by multiscan method applied by SADABS. [6] The structures were solved with SHELXT and was refined against F 2 on all data by full-matrix least squares with SHELXL. [7] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model. 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). The hydride ligand directly bound to rhodium in structure 4c was located at the difference electron density map and its Rh-H bond distance restrained to typical values. The two structures contain solvent molecules in the unit cell (benzene and pentane in 3b and THF in 4c) with variable degrees of disorder to which several restraints were applied.
A summary of the fundamental crystal and refinement data are given in Table S1. Atomic coordinates, anisotropic displacement parameters and bond lengths and angles can be found in the cif files, which have been deposited in the Cambridge Crystallographic Data Centre with no. 2008917 and 2008918. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Computational details
General details.
DFT calculations were performed with the Gaussian software package. [8] Geometry optimization of minima was carried out without symmetry restrictions using the hybrid functional PBE0; [9] dispersion effects were accounted for by using Grimme's D3 parameter set with Becke−Johnson (BJ) damping. [10] The 6-31g(d,p) basis set [11] was used for non-metal atoms, Au and Rh atoms were described with the SDD basis and associated electron core potential (ECP). [12] Bulk solvent effects (dichloromethane) were included during optimization with the SMD continuum model. [13] Frequency calculations were carried out to ensure minima presented zero imaginary vibrational frequencies. Calculated geometries were in excellent agreement with crystallographic data. AIM charges, also know n as Bader charges, were computed according to QTAIM [14] by means of the Multiwfn program, [15] using high quality grid (grid spacing = 0.06 Bohr) to generate basins and locate attractors. Integration of real space functions in AIM basins was carried out with mixed type of grids, using exact refinement of basin boundary to improve accuracy.

AIM Charge analysis.
We Overall, these results along with the UV-vis spectra collected in Figure S5, highlight the difficulties of unambiguously assigning formal oxidation states in bimetallic species of type 3.