Formation and Structure of Rh (0) Complexes of Phosphinine-Containing Macrocycles: EPR and DFT Investigations Laurent Cataldo, Sylvie Choua, The´ o Berclaz, and Michel Geoffroy* Department of Physical Chemistry, 30 quai Ernest Ansermet, UniVersity of GeneVa, 1211 GeneVa, Switzerland Nicolas Me´ zailles, Narcis Avarvari, Franc¸ ois Mathey,* and Pascal Le Floch* Laboratoire “He´ te´ roe´ le´ ments et Coordination”, UMR CNRS 7653, Ecole Polytechnique, 91128 Palaiseau Cedex, France ReceiVed: NoVember 29, 2001 Electrochemical and chemical reductions of Rh (I) complexes of L P4 (a macrocycle containing four phosphinine rings) and of L P2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with 31 P nuclei. An additional coupling (27 MHz) with 103 Rh is detected, in the liquid state, for the spectrum obtained with [L P2S2 Rh (0) ]; moreover, resolved 31 P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh (I) complexes of model macrocycles L′ P4 and L′ P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for [L′ P2S2 Rh (0) ], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (F Rh )0.35 for [L′ P2S2 Rh (0) ]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle. Introduction Complexes of metals in low oxidation states present a considerable interest for reductive catalysis, and intense efforts are currently made to design new ligands able to complex electron-rich metal ions. In this context, compounds containing unsaturated carbon-phosphorus bonds have shown promising properties. 1 , 2 This is the case of phosphinines whose low lying π* LUMO is well suited to the accommodation of extra electrons. 3 Moreover, recent progress in synthetic chemistry, 45 has made possible the use of macrocycles such as L P4 and L P2S2 , which incorporate several phosphinine units and which are expected to provide thermodynamic stabilization 6 to the metal center. There are few complexes with rhodium centers in low oxidation states 7 and, as far as we know, only a single monomeric Rh (0) has been structurally characterized. 8 Because the chemical behavior of these paramagnetic complexes is mostly governed by the location of the unpaired electron, it is important to know how the nature of the ligand influences the contribution of the metal to the SOMO. However, due to the low stability of these complexes or, perhaps, to a too high fluxionality that limits the spectral resolution, the 103 Rh isotropic coupling of these rare complexes could never be directly observed in the liquid state. The geometry of Rh (0) complexes seems to be very sensitive to the nature of the ligands. For example, Chenier et al. 9 could trap [Rh(CO) 4 ] in hydrocarbon matrixes at 77 K and showed that this complex is only slightly distorted from tetrahedral geometry whereas Orsini and Geiger recently found that in [(COD) 2 Rh] “the ligands form a ligand field around Rh which is closer to square planar than to tetrahedral”. 10 In the present study, we used the two polydentate ligands L P4 and L P2S2 to form Rh (I) complexes and show that their chemical or electrochemical reductions easily lead to the macrocyclic Rh (0) complex. EPR spectroscopy and DFT calcula- tions were used to assess the structure of these formally d 9 - rhodium compounds and, in particular, to reveal how the nature of the heterocycles (phosphinines or thiophene) affects the spin repartition. Experimental Section Compounds. All reactions were routinely performed under an inert atmosphere of argon or nitrogen by using Schlenk and glovebox techniques and dry deoxygenated solvents. Dry THF and hexanes were obtained by distillation from Na/benzophen- one and dry CDCl 3 from P 2 O 5 . CD 2 Cl 2 was dried and stored, like CDCl 3 , on 4 Å Linde molecular sieves. Nuclear magnetic resonance spectra were recorded on a Bruker AC-200 SY spectrometer operating at 200.13 MHz for 1 H, 50.32 MHz for 13 C and 81.01 MHz for 31 P. Solvent peaks are used as internal reference relative to Me 4 Si for 1 H and 13 C chemical shifts (ppm); 31 P chemical shifts are relative to a 85% H 3 PO 4 external reference. Coupling constants are given in Hertz. The following abbreviations are used: s, singlet; d, doublet; t, triplet; m, 3017 J. Phys. Chem. A 2002, 106, 3017-3022 10.1021/jp014339z CCC: $22.00 © 2002 American Chemical Society Published on Web 02/28/2002