An Old Crystallization Technique as a Fast, Facile, and Adaptable Method for Obtaining Single Crystals of Unstable Li2TCNQF4 and New Compounds of TCNQ or TCNQF4: Syntheses, Crystal Structures, and Magnetic Properties

Detailed structural information is essential for understandingthe properties of TCNQ and TCNQF(4) compounds (TCNQ = 7,7,8,8-tetracyanoquinodimethane;TCNQF(4) = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane).The ineludible requirement of obtaining crystals of a size and qualitysufficient to yield a successful X-ray diffraction analysis has beenchallenging to satisfy because of the instability of many of thesecompounds in solution. Crystals of two new complexes of TCNQ, [trans-M-(2ampy)(2)(TCNQ)(2)] [M = Ni (1), Zn (2); 2ampy = 2-amino-meth-yl-pyridine],as well as unstable [Li-2(TCNQF(4))-(CH3CN)(4)]center dot CH3CN (3), can be preparedin minutes by a horizontal diffusion technique and can be harvestedeasily for X-ray structural studies. Compound 3, previouslydescribed as Li(2)TCNQF(4), formsa one-dimensional (1D) ribbon. Compounds 1 and 2 can also be obtained as microcrystalline solids from methanolicsolutions of MCl2/LiTCNQ/2ampy. Theirvariable-temperature magnetic studies confirmed a contribution ofstrongly antiferromagnetically coupled pairs of TCNQ(center dot-) anion radicals at higher temperatures with exchange coupling J/k (B) = -1206 K and J/k (B) = -1369 K for 1 and 2, respectively, estimated using a spindimer model. The presence of magnetically active anisotropic Ni-(II)atoms with S = 1 in 1 was confirmed,and the magnetic behavior of 1, representing an infinitechain of alternating S = 1 sites and S = 1/2 dimers, was described by a spin-ring model suggesting ferromagneticexchange coupling between Ni-(II) sites and anion radicals.

Automated summary

This study successfully prepared crystals of TCNQ and TCNQF(4) compounds using a horizontal diffusion technique, and analyzed their structures using X-ray diffraction. The magnetic studies revealed that compounds 1 and 2 exhibit strongly antiferromagnetic coupling, while compound 3 showed a one-dimensional ribbon structure. Compound 1 also exhibited ferromagnetic exchange coupling between Ni-(II) sites and anion radicals.