Metal-Dependent Ferro- Versus Antiferromagnetic Interactions in Molecular Crystals of Square Planar {M(II) Imino-Nitroxide Radical} Complexes (M = Pt, Pd)

The synthesis and structural, spectral, and magnetic characterizations of two new complexes of formula [Pt(IM2Py)Cl2] (A) and [Pd(IM2Py)Cl2] (B) are reported. IM2Py stands for the imino-nitroxide radical ligand 2-(ortho-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl. Their crystal structures were solved at room temperature and at 120 K revealing
structural phase transitions from pseudo-orthorhombic to monoclinic systems for the two compounds which remain
isostructural in the whole temperature range explored. Structural parameters for A:  T = 293 K [120 K], monoclinic
(P21/n) [P21/c], a = 7.906(2) [7.989(3)] Å, b = 17.872(9) [10.168(4)] Å, c = 10.357(3) [17.623(6)] Å, β = 90.732(13)° [95.940(2)]°, Z = 4 [4]. Structural parameters for B:  T = 293 K [120 K], monoclinic (P21/n) [P21/c], a =
7.900(3) [7.9730(2)] Å, b = 17.907(9) [10.1806(3)] Å, c = 10.299(3) [17.7171(4)] Å, β = 90.524(14)° [95.747(2)]°,
Z = 4 [4]. In both complexes, the metal coordination is essentially planar. The average Pt−N, Pt−Cl and Pd−N,
Pd−Cl bond lengths are 1.996(6) [1.88], 2.295(2) [2.248(8)] Å and 2.015(7) [2.029(8)], 2.287(3) [2.294(3)] Å,
respectively. The solid state structure is characterized by a pairlike molecular packing stacked in columns parallel
to the a axis; this dimer character is reinforced at low temperature. Despite their structural similarity, the investigation
of the magnetic properties revealed that dominant ferromagnetic interactions govern the behavior of the Pt derivative
A, whereas antiferromagnetic interactions take place for the Pd compound B. A rationalization for this rather intriguing
difference is proposed in light of the spin population deduced from density functional theory calculations. The
electronic absorption spectra of A and B present structured absorption bands in the visible which are attributed to
MLCT transitions. Both compounds are nonluminescent at room temperature. However, a weak emission is detected
for A in butyronitrile glasses at 77 K, indicating that the MLCT excited state is strongly quenched at low temperature.