Further spectroscopic and electrochemical studies of pyrazolato-bridged dirhodium complexes that undergo a thermochromic isomerization

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Paramagnetic Rh 2 3+complexes of the type [Rh 2(μEE′) 2(μ-Z)L 2](PF 6) 2 (Z=3-methyl-5-trifluoromethylpyrazolate (mtmpz), 3,5-bis(trifluoromethyl)pyrazolate, (btmpz); EE′=bis(diphenylphosphino)methane (dppm), (diphenylarsino)(diphenylphosphino)methane (dapm); L=t-BuNC, CO) have been generated via electrochemical oxidation of the Rh 2 2+ parent complexes [Rh 2(μ-EE′) 2(μ-Z) L 2]PF 6. Except for the diisocyanide complex containing dppm and btmpz, the complexes undergo isomerization which can be monitored by ESR spectroscopy down to −196 °C in CH 2Cl 2. For a given pyrazolate ligand the dppm complex undergoes isomerization at a lower temperature than the dapm complex, consistent with previous results that indicated that the dppm complex containing the 3,4,5- tribromopyrazolate (345Bpz) ligand did not isomerize down to −196 °C. Here we show that the low isomerization temperature for the complex containing the btmpz and dppm ligands is associated with the electronegativity of the CF 3 substituents on the pyrazolate ring. The ESR spectrum of the complex that contains the mtmpz and dppm ligands indicates that both the high-temperature and low-temperature isomers are present at −196 °C, supporting the correlation between low isomerization temperature and electronegativity of the substituents on the pyrazolate ring.