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A New Tetragonal Crystalline Polymorph of Lithium Octa-n-Butoxy-Naphthalocyanine (LiNc-BuO) Radical: Structural, Magnetic and Oxygen-Sensing Properties

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posted on 14.12.2010, 00:00 by Ramasamy P. Pandian, Nandyala P. Raju, Judith C. Gallucci, Patrick M. Woodward, Arthur J. Epstein, Periannan Kuppusamy
The single-crystal structure, paramagnetic, and oxygen-sensing characteristics of a new tetragonal polymorph of lithium octa-n-butoxynaphthalocyanine, LiNc-BuO (Tg), is reported. The compound was synthesized as fine, diamond-shaped, neutral radical crystals by reacting octa-n-butoxynaphthalocyanine (Nc-BuO) with lithium metal in n-butanol. Single-crystal X-ray diffraction analysis revealed a saddle-shaped conformation of the molecule with a crystallographic 2-fold rotation axis in a tetragonal unit cell. This represents the first single-crystal, structurally characterized, lithium naphthalocyanine complex. Unlike the triclinic polymorph of LiNc-BuO (Pandian, et al. J. Mater. Chem. 2006, 16, 3609), the tetragonal (Tg) form of LiNc-BuO did not show the presence of large open channels in the crystalline packing. The magnetic susceptibility and EPR spectroscopic properties were in agreement with the X-ray crystal structure showing the existence of a high degree of spin exchange in the crystalline system. The effective magnetic moment calculated for the LiNc-BuO (Tg) was 1.56 μB per formula unit, which was smaller than the expected value of 1.73 μB for a spin system with S = 1/2. The crystals showed a single, sharp EPR spectrum with an unusually low line width of 20 ± 5 mG under anoxic conditions. The EPR line width was linearly dependent on partial pressure of oxygen (pO2) in the range 0−760 mmHg, with a sensitivity of 4.54 ± 0.50 mG/mmHg. The LiNc-BuO (Tg) crystals possess several desirable properties for use as a probe for EPR oximetry.