Crystal Structure and Exchange Coupling of a Novel Binuclear Palladium(I) Complex

G. A. ECHEVERRÍA; F. SIVES; O. E. PIRO; S. H. TARULLI; O. V. QUINZANI; R. M. FERULLO; N. CASTELLANI.

Bahía Blanca (BA), Argentina

Congreso; IV Reunión de la Asociación Argentina de Cristalografía (AACr); 2008

Asociación Argentina de Cristalografía (AACr)

Abstract. For a long time in the field dealing with the chemistry of Pd-dppm complexes [dppm: bis(diphenylphosphine) methane], there have been known three typical coordination modes for dinuclear species of Pd(I)/(II) ions: i) side-by-side, ii) A-frame, and iii) face-to-face. The first structural prototype with the Pd(I) atoms bridged by two dppm molecules, having the d9-d9 “M2(dppm)22+” fragment, has been intensely studied as a model compound and the interest was renewed in the last decade due to the ability of complexes of this kind to react with small molecules, namely CO or SO2, making them useful for catalytic applications. Despite the continuous interest in molecular or polymeric compounds containing the “Pd2(diphos)22+” fragment, there is scarce information on LPd(μ-dppm)2PdL’ structures, where the L, L’ ligands are 4-mercapto-pyridine molecules [1], Br [2], or Cl and 2-vinylpyridine [3], and to the best of our knowledge no magnetic studies have been so far reported in the literature. We report here the crystal structure of bis(thiosaccharinato)bis{bis (diphenylphosphine) methane)}dipalladium(I), where the Pd(I) ions are in the unpaired spin state 4d9 (s=1/2).The complex crystallizes in the monoclinic space group C2/c with a=13.937(3), b=19.024(2), c=23.039(4) Å, b=102.25(2)º, and Z=4 molecules per unit cell. The structure was solved from 4537 X-ray reflections with I>2s(I) by Patterson and Fourier methods and refined to an agreement R-factor of 0.0631. The Pd(I) atoms are doubly bridged by two dppm molecules through the P-atoms and the square planar environment of each metal is completed with the S-atoms of a thiosaccharinate anion and a Pd-Pd single bond, forming a Pd2P4C2 framework in the side-by-side conformation. Based on the well-studied binuclear Cu(II): 3d9 (s=1/2) complexes, we infer Pd(I)…Pd(I) exchange interaction giving rise to a diamagnetic (S=0) ground state and to a paramagnetic (S=1) excited state. We attempted to test this hypothesis by magnetic susceptibility measurements in the 10-390 K range, employing a SQUID magnetometer. The results showed diamagnetic behaviour up to 390 K hence suggesting a relatively large exchange coupling, much larger than the values of about 400 K found for lantern-like Cu(II) binuclear complexes. This is sustained by preliminary DFT calculations of the singlet-triplet energy splitting.