On the Failure To Observe Isotropic Electron Paramagnetic Resonance Spectra for Certain Chromium(I) Carbonyl Complexes

Abstract

Although isotropic EPR spectra have often been reported for Cr(I) complexes with degenerate ground states and consequent Jahn−Teller distortions, such spectra have not been reported for cis-dicarbonyl or fac-tricarbonyl complexes of Cr(I). The most likely explanation is that ground states are subject to an approximate accidental degeneracy, probably modulated by a ligand bending vibrational mode, thus providing an efficient spin−lattice relaxation pathway.

First experimental evidence for a bis-ethene chromium(I) complex forming from an activated ethene oligomerization catalyst

“…S1) were simulated by a mixture of the three species using the spin Hamiltonian parameters given in table S1. Two of the room temperature spectra were in agreement with the previously reported species A (labeled A) with two magnetically equivalent 31 P nuclei (nuclear spin, I = ½ and isotropic hyperfine coupling, P A iso = 52.3 MHz) resulting in a 1:2:1 triplet (indicated by dashed lines in green) and Cr I -bisarene with 10 near-equivalent protons ( 1 H; I = ½; H A iso = 9.5 MHz) resulting in an undecet with 11 superhyperfine lines (with a binomial intensity distribution of 1:10:45:120:210:252:210:120:45: 20:1), with the exception of the two outer lines being too weak to be observed. 53 Cr (I = 3/2; natural abundance, 9.5%; Cr A iso = 50.5 MHz) causes quartet satellites where the undecet is further split into a 1:1:1:1 quartet.…”

“…A iso = 50.5 MHz), (b) X ( 13 CO) 0 (two 31 P and eight 1 H nuclei), (c) X ( 13 CO) 1 (two 31 P, eight 1 H, and one 13 C nuclei), (d) X ( 13 CO) 2 (two 31 P, eight 1 H, and two 13 C nuclei), (e) d 8 -X ( 13 CO) 0 (two 31 P and eight 2 H nuclei), (f) d 8 -X ( 13 CO) 1 (two 31 P, eight 2 H, and one 13 C nuclei), and (g) d 8 -X ( 13 CO) 2 (two 31 P, eight 2 H, and two 13 C nuclei). Additional spin Hamiltonian parameters: g iso = 1.9970; 31…”

Unravelling the Photochemical Transformations of Chromium(I) 1,3 Bis(diphenylphosphino), [Cr(CO)4(dppp)]+, by EPR Spectroscopy

“…Although the EPR spectrum of [Cr(CO) 6 ] + has only once been reportedly observed at 4 K, 23 Bond et al gave a very comprehensive explanation on the failure to observe isotropic EPR signals for such a species. 24 Therefore, if [Cr(CO) 6 ] + is indeed formed, it would prove exceedingly difficult to detect by conventional X-band EPR. All attempts to detect [Cr(CO) 6 ] + in our experiments by EPR, and additionally by chemical oxidation of [Cr(CO) 6 ] with Ag[Al(OC(CF 3 ) 3 ) 4 ], proved to be unsuccessful, even at low temperatures (5 K).…”

“…One possible explanation is that [Cr­(CO) 6 ] + is formed during the photochemical transformation of [Cr­(CO) 4 (dppp)] + into trans -[Cr­(CO) 2 (dppp) 2 ] + or that phosphine-free Cr­(I) ions are released into solution and not observed by EPR. Although the EPR spectrum of [Cr­(CO) 6 ] + has only once been reportedly observed at 4 K, Bond et al gave a very comprehensive explanation on the failure to observe isotropic EPR signals for such a species . Therefore, if [Cr­(CO) 6 ] + is indeed formed, it would prove exceedingly difficult to detect by conventional X-band EPR.…”

EPR Study of Photochemical Reactions of fac- and mer-[Cr(CO)3(η1-L2)(η2-L2)]+ (L2 = Bidentate Phosphine, Arsine, or Phosphonite Ligand)

“…Several mysteries remained from the work of Bond and Colton. We have recently provided an explanation for the failure to observe isotropic EPR spectra of fac -[Cr(CO) 3 L 3 ] + or cis -[Cr(CO) 2 L 4 ] + , but there are still a number of unsolved problems related to the mechanisms of Cr(I) reactions.…”