ORGANOMETALLIC OXIDES - PREPARATION AND MOLECULAR AND ELECTRONIC-STRUCTURE OF ANTIFERROMAGNETIC [(ETA-C5H5)CR(MU-3-O)]4 AND [(ETA-C5H5)CR]4(MU-3-ETA-2-C5H4)(MU-3-O)3

The reaction between (η-C5H5)2Cr and Me3NO gave, as the major product, [(η-C5H5)Cr(µ3-O)]4 (I) (18%) and, as the minor product (overall isolated yield 2%), [η-Cβ5H5)Cr]4(µ3-η2-C5H4)(µ3-O)3 (II). I and II were separated by chromatography on glass beads (120-200 mesh). At 295 K I has a distorted (D2) cubane structure, as determined by X-ray diffraction. The Cr-Cr distances are 2.7114 (6) and 2.7004 (6), 2.8099 (5) and 2.8369 (5), and 2.8933 (5) and 2.8987 (5) Å. The Cr-O-Cr and O-Cr-O angles fall into three sets to conform to the D2 symmetry, but the Cr-O distances lie in a narrow range, 1.922 (2).1.947 (2)Å. Crystal data: monoclinic, P21/n, a = 10.0465 (7) Å, b = 20.9200 (13) Å, c = 10.4345 (4)Å, β = 115.015 (5)°; R = 0.042 for 274 variables and 6022 observed (I > 2.5σ(I)) reflections with 2θ < 70°. The structure was verysimilar at 100 K. The Cr-Cr distances were 2.6930 (7) and 2.7024 (7), 2.8010 (7) and 2.8270 (7), and 2.8992 (7) and 2.9031 (7) Å; the Cr-O distances were 1.919-1.945 (2) Å. Crystal data: a = 9.9834 (7) Å, b = 20.7828 (23) Å, c = 10.3521 (10) Å, β = 115.359 (7)°; R = 0.028 for 2830 observed reflections with 29 < 50°. In the solid state I is diamagnetic below 50 K and exhibits antiferromagnetic behavior at higher temperatures, the magnetic moment increasing to 3.75 µg at 500 K. The magnetic behavior of I has been studied in toluene solution by1H and13C NMR spectroscopy. The observed shifts and their temperature dependence over the temperature range 215-385 K clearly show intramolecular antiferromagnetism. The electron spin delocalization points to magnetic orbitals that are mainly metal in character. The He I photoelectron spectrum in the gas phase at 440 K and the electronic absorption spectrum (450-1550 nm) of a CC14 solution of I have been measured; both show considerable low-energy structure. Extended Hückel molecular orbital calculations indicate that the ordering of the 12 nonbonding cluster orbitals for a Td cubane is e < a1 < 1t2 < t1 ≃ 2t2. The spectra and magnetism of I are interpreted on the basis of a static distortion of a [(η-C5H5)M(µ3-O)]4 cubane. The structure of II was previously established by X-ray diffraction as a butterfly cubane derived from I by replacement of a corner oxygen atom with the η2 ligand C5H4 [inline-formula omitted] II shows antiferromagnetism between 4 and 120 K. It has a magnetic moment of ca. 2.3 µB at 4 K. The magnetic behavior is explained with use of extended Hückel calculations on various forms of I and II; the ground state of the 12 cluster orbitals of II is la22la12lb22lb222a122b113a11 (in C2v symmetry). The ground state is markedly influenced by the Cr-Cr distance spanned by the η2-C5H4 ligand, because the 2b1 orbital is antibonding and the 3a1 orbital bonding with respect to this Cr-Cr interaction. II cannot be reduced but is readily oxidized to I by reagents containing oxygen. © 1990, American Chemical Society. All rights reserved.