Electron spin relaxation in chromium-nitrosyl complexes

A new method to prepare Cr(NO)(H2O)(5)(2+) from dichromate and NH2OH is rep orted. The chromium nitrosyls Cr(NO)(EHBA)(+) and Cr(NO)(EHBA)(2) (EHBA = 2 -ethyl-2-hydoxybutyrate) were prepared by a literature reaction and charact erized by continuous wave electron paramagnetic resonance and two-pulse ele ctron spin echo spectroscopy at X-band. The g values are characteristic of a single unpaired electron in a predominantly d(xy) orbital. In fluid and g lassy solutions Cr(NO)(EHBA)(2) is a mixture of cis and trans isomers. Rota tion of the methyl groups in the EHBA ligands causes an increased rate of s pin echo dephasing at temperatures between 40 and 120 K, For the EHBA compl exes echo envelope modulation is observed at temperatures below about 40 K that is attributed to inequivalent coupling to protons of the slowly rotati ng methyl groups. Both the effect of the methyl rotation on spin echo depha sing and the depth of the proton modulation are dependent on the number of ethyl groups in the ligand, and thus the spin echo experiments provide conf irmation of the number of EHBA ligands in the complexes. The spin-lattice r elaxation rates for the chromium-nitrosyl complexes at temperatures near 10 0 K are similar to values reported previously for Cr(V) complexes, which al so have a single unpaired electron in a predominantly d(xy) orbital. For Cr (NO)(H2O)(5)(2+), Cr(NO)(EHBA)(+), and Cr(NO)(EHBA)(2) the dominant contrib ution to spin-lattice relaxation between 12 and 150 K is the Raman process with a Debye temperature, theta(D), of 110-120 K. For Cr(NO)(CN)(5)(3-) the data are consistent with a Raman process (theta(D) = 135 K) and a contribu tion from a local mode, which dominates above about 60 K. The formally low- spin d(5) chromium nitrosyl complexes relax about 5 orders of magnitude mor e slowly than low-spin d(5) Fe(III) porphyrins, which is attributed to the absence of a low-lying excited state.