No abstract

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an R3m C2/m pseudoproper ferroelastic transition [1]. In that case, the group theory states that the order parameter associated with the structural transition must belong to a two dimensional irreducible representation Eg (x 2 − y 2 , xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 K and 290 K. Considering that the isostructural multiferroic compound CuCrO2 might show similar structural deformations at the antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are observed at ωE g = 352 cm −1 and ωA g = 692 cm −1 , while these modes are detected at ωE g = 457 cm −1 and ωA g = 709 cm −1 in CuCrO2. The analysis of the temperature dependence of modes shows that the frequency of all modes increases down to 5 K. This typical behavior can be attributed to anharmonic phonon-phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper ferroelastic transition observed at the Néel temperature TN1. Finally, a broad band at about 550 cm −1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be attributed to a magnon mode.

No abstract

Addition of conjugated diene to an equimolar mixture of palladium(I1) halides and a palladium(0) complex induces facile Pd-Pd bond formation to give anionic, neutral and cationic PdrPdI dinuclear complexes containing bridging 1,3-diene ligands.Increasing attention has been paid to organopalladium complexes containing a Pd-Pd bond bridged by 4e-donating hydrocarbon ligands, such as alkyne, ally1,2 allenyl/prop-2-yny1,3 cyclopentadienyl,2 or a~-ene.~ Another ubiquitous 4e donor, 1,3-diene, has been used only rarely as a bridging ligand at Pd-Pd, an example being [(isoprene)Pd2(p-PBut2)(PH-But&]+, recently prepared by exchange of a bridging PHBut2 ligand on Pd-Pd with the diene;5 by contrast the 1,3-diene chemistry of mononuclear palladium-(11) and -(O) complexes has been extensively developed.6 We report here a convenient method for the synthesis of 1,3-diene complexes containing Pd-Pd moieties, including two new fragments, [Pd2X2(pX)]-and [Pd2X(p-X)(PPh3)], which involves a directed Pd-Pd bond formation as a key step. This method is novel when compared to those for the known 1,3-diene complexes of di-or poly-nuclear metal systems7 employing a simple ligand substitution on the pre-formed metal-metal bond,? as above.5A mixture of [Pd2&(PPh3)2] (X = C1, Br, I) and [Pd2(dba)3] (dba = dibenzylideneacetone) was stirred in CH2C12, through which buta-1,3-diene was bubbled for 0.5 h at room temperature, to give a deep-red solution.

The first dipalladium µ-PPh(3) complex (1) was obtained by the facile loss of two CH(3)CN ligands from [Pd(2)(PPh(3))(2)(CH(3)CN)(4)](PF(6))(2) in CH(2)Cl(2). Coupling of p-tolylacetylene (p-TolC identical withCH) with 1 or its precusor afforded the µ-butenediylidene complex 2, treatment of which with CH(3)CN resulted in PPh(3) migration to give 3.