A topic of interest consists on exploring the combination of robust bulky pincer ligands that present high stability and steric protection with redox active ligands like NO in the reactivity of {RhNO}n complexes. Nitrosyl complexes are described as {MNO}n where n stands for the number of electrons in the metal d and π* NO orbitals. In a previous work the activation of carbon-halogen bonds by a paramagnetic complex {RhNO}9 with a PCP pincer type ligand was reported [1]. Herein the synthesis and characterization of a new paramagnetic complex {RhNO}9 1+. with a PNP pincer type ligand is presented.
Reaction of {RhNO}8 1++ with 1 equivalent of Co(Cp)2 in trifluorotoluene resulted in quantitative formation of 1+. and [Co(Cp)2][BF4]. The paramagnetic nature of the one-electron reduced {RhNO}9 species 1+. was confirmed by EPR and by the broad resonances occurring in 1H NMR. Also by FTIR the expected change was observed in the NO stretching frequency upon the {RhNO}8/9 conversion from 1++ to 1+. (1911 to 1650 cm-1).
The molecular structure of both compounds 1++ and 1+. was confirmed by X-ray diffraction. In both structures the rhodium atom is located in the center of a square-planar geometry with the nitrosyl group occupying the position trans to the ipso carbon. In 1++ the Rh-N-O angle is 176.1(3)º while in 1+. is 145.5(2)º. This considerable change in the MNO angle is expected for a {RhNO}8/9 conversion in square-planar complexes.
Further reduction to the putative {RhNO}10 form was accomplished. This higly reduced species is not indefinitely stable in solution, and slowly decomposed to a paramagnetic {RhNO}9 complex with a dearomatized PNP ligand, through formal loss of an H radical.

[1] J. Pellegrino, C. Gaviglio, D. Milstein, F. Doctorovich, Organometallics, 32 (2013) 6555–6564.

REDOX BEHAVIOR AND STRUCTURAL CHARACTERIZATION OF NITROSYL PINCER-TYPE PNP RHODIUM COMPLEXES

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