Structural Chemistry, Vol. 14, No. 4, August 2003 ( C  2003) Extended NMR Study of Spin-Crossover Compounds [Fe(1-alkyl-1 H -tetrazole) 6 ](BF 4 ) 2 and Their Zn II Analogs T. Marek, 1,6 M. Bokor, 2 K. Tompa, 2 A. V´ ertes, 1,3 K. S ¨ uvegh, 3 Zs. Nemes-Vet´ essy, 4 and K. Burger 4,5 Received January 28, 2002; accepted July 15, 2002 A systematic NMR study was performed on several alkyl–tetrazole complexes of iron(II) and zinc(II) in the 10–300 K temperature range. The experiments were designed to separate the electronic and reorientational phase transitions caused by the spin crossover of the iron compounds from those independent of unpaired electrons. The 19 F spectral data on the propyl-tetrazole compounds show that the electronic spin-transition has a strong effect on the spectra, and their behavior can be explained as a combined response to molecular reorientations and the spin transition. For these complexes, second-moment calculations revealed the strength of the interaction between resonant and nonresonant nuclei. Both of the applied NMR methods show irregularities at the temperature region between 70 and 120 K, suggesting the presence of a phase transition. The data also suggest two kinds of reorientational behavior for the BF − 4 counter ions. In the iron–ethyl–tetrazole compound, unlike in the propyl–tetrazole complex, a significant amount of unpaired electrons remains in their original high-temperature HS state. Above their effect, the behavior of the nuclear spins of the iron compound is basically governed by the same structural factors as in its zinc analog. The two-exponential behavior of the 1 H-T 1 in case of the zinc–methyl–tetrazole compound can be explained on the basis of cross relaxation with the 19 F nuclei due to the low 1 H/ 19 F ratio. The presence of the two types of methyl reorientation is assumed to be the sign of the two different lattice sites known to be present in the Fe II compound. The single-exponential T 1 above T c in the case of [Fe(mtz) 6 ](BF 4 ) 2 is consistently the sign of the strength of the paramagnetic relaxation observed in the ethyl and propyl compounds. KEY WORDS: Inorganic compounds; NMR spectroscopy. INTRODUCTION Extensive NMR spectroscopic study was carried out on compounds [M(Rtz) 6 ](BF 4 ) 2 (M = Fe, Zn; Rtz = 1- alkyl-1 H -tetrazole). The M = Fe compounds belong to 1 Research Group for Nuclear Techniques in Structural Chemistry, Hun- garian Academy of Sciences at E¨ otv¨ os University Budapest, pob. 32, H-1518 Budapest, Hungary. 2 Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest, Hungary. 3 Department of Nuclear Chemistry, E¨ otv¨ os University Budapest, Budapest, Hungary. 4 Department of Inorganic and Analytical Chemistry, J´ ozsef Attila Uni- versity, Szeged, Hungary. 5 Professor K´ alm´ an Burger deceased June 8, 2000. 6 Corresponding author, e-mail: marek@para.chem.elte.hu; tel.: +36 1 209 0555 ext.1680; fax: +36 1 209 0602 the large group of octahedrally coordinated iron(II) spin- crossover compounds exhibiting an entropy-driven spin transition from the 1 A 1 low-spin (LS) ground state at low temperatures to the 5 T 2 high-spin (HS) state at elevated temperatures. The substitution of the central iron(II) ions with zinc(II) has almost no effect on the structure, and so, a perfect model is gained, which lacks the very strong ef- fects of unpaired electrons. NMR spectroscopy provides a unique possibility to investigate both molecular motions (activation energies and correlation times of molecular group reorientations) and changes in the electronic struc- ture of iron(II), e.g., spin transition, the interdependence of which is not fully understood. NMR spectroscopic study of R = CH 2 CH 2 CH 3 compounds in our laboratory [1–4] has shown that this method can provide new information on the relation between molecular motions and spin tran- sition. In this work, we have chosen to extend our study 349 1040-0400/03/0800-0349/0 C  2003 Plenum Publishing Corporation