EPR Spectroscopic and Theoretical Study of Chromium(I) Carbonyl Phosphine and Phosphonite Complexes

Abstract

Isotropic and frozen-solution EPR spectra are reported for several Cr(I) carbonyl complexes of the type cis-[Cr(CO) 4 L 2 ] + , mer-and fac-[Cr(CO) 3 L 3 ] + , and trans-[Cr(CO) 2 L 4 ] + , where L is a monodentate (or L 2 a bidentate) phosphine or phosphonite ligand. Extended Hü ckel molecular orbital theory calculations were performed for cis-and trans-[Cr(CO) 4 L 2 ], merand fac-[Cr(CO) 3 L 3 ], and cis-and trans-[Cr(CO) 2 L 4 ] (L ) PH 3 , P(OH) 3 ). The EPR results are discussed in light of the electronic structure predictions, which also provide some insights on the oxidation potentials and the isomerization reaction energetics of these complexes. Isotropic 31 P hyperfine couplings can be understood in terms of spin densities in the Cr 3d, P 3s, and P 3p orbitals.

Stable salts of the hexacarbonyl chromium(I) cation and its pentacarbonyl-nitrosyl chromium(I) analogue

“…The paramagnetic 17 VE d 5 species gives rise to one single EPR-line corresponding to an isotropic g -value of g = 2.003 being close to the g -factor of the free electron ( g el = 2.0023) 55 . Similar g -values were reported for tetra-, tri-, and dicarbonyl chromium(I) phosphine and phosphite complexes 56 . In contrast to prior EPR investigations from electrochemical oxidations 33 , we did not observe any signal splitting due to hyperfine coupling to the 53 Cr nucleus ( I = 3/2, natural abundance: 9.55%).…”

“…Note, however, that a larger hyperfine coupling of 61 MHz was reported for a species assigned as [Cr(CO) 6 ] •+ and generated by in situ electrolysis of Cr(CO) 6 in saturated solutions of [NBu 4 ] + [ClO 4 ] − in CH 2 Cl 2 at −80 °C 33 . This 53 Cr hyperfine coupling is about three times larger than the coupling constants of chromium(I) carbonyl phosphine and phosphite complexes 56 . Furthermore, the g -value of g = 2.003 we identified for 1 is larger than the one previously reported ( g = 1.982) 33 .…”

Characterization of Cr-Hydrocarbyl Species via Pulse EPR in the Study of Ethylene Tetramerization Catalysis

“…In separate experiments utilizing either C 2 H 4 or C 2 D 4 , 5 was formed in flame-sealed quartz EPR tubes. The X-band CW-EPR spectra of these reaction samples (see Figure ) exhibit relatively narrow spectra centered at g = 2, consistent with S = 1/2 paramagnetic species …”

The dimerisation of carbon disulfide on Fe centres. Some new tetrathiolene complexes of iron

“…5, at 120 K. The isotropic spectrum is a 1 : 2 : 1 triplet with <g> = 2.0388 and <A> = 85.2 × 10 Ϫ4 cm Ϫ1 at both temperatures. Large couplings such as this are unusual 22 (20-30 × 10 Ϫ4 are more common 23 ) and generally signify that the phosphorus ligand(s) is (are) located essentially on a lobe of the SOMO, 22 whilst the triplet nature of the ESR signal suggests that the SOMO is confined to one Fe atom. As the iron The frozen solution spectrum is poorly resolved, but can be interpreted in terms of nearly axial g-matrix, g = 2.005, 2.048, 2.58 (all ± 0.001) and rhombic A-matrices, A 1 = 115, 62, 115, A 2 = 61, 121, 51 (all ± 2 × 10 Ϫ4 cm Ϫ1 ).…”