1H NMR Studies on the Motion of Guanidinium Ions and Dipolar‐Quadrupolar Cross Relaxations in Crystalline Guanidinium Tetrachloro‐ and Tetrabromometallates Including Au(III), Pt(II), or Pd(II) as Central Metal Ions

Abstract

AbstractThe temperature dependences of proton spin‐lattice relaxation times T1 at various Larmor frequencies and of 1H NMR second moments at 40 MHz were determined for (guH)AuCl4, (guH)AuBr4, (guH)2PtCl4, (guH)2PtBr4, (guH)2PdCl4, and (guH)2PdBr4, where guH indicates a guanidinium cation [C(NH2)3]+. A solid‐solid phase transition between stable phases was located for (guH)AuCl4, (guH)AuBr4, and (guH)2PtBr4 at 363, 387, and 359 K, respectively. The high‐temperature phases of the three complexes were easily supercooled. For (guH)AuBr4, another phase transition between substable phases was found at 218 K. From the measurements of 1H T1, the motional parameters for the C3 reorientation of the D3h cation in the all complexes studied were evaluated. Below room temperature, (guH)2PtCl4 and (guH)2PdCl4 yielded unusual temperature dependences of 1H T1 unexplainable by the usual BPP theory. Moreover, the 1H T1 values in this temperature region depended strongly and in an anomalous manner on the 1H Larmor frequency employed. These unusual temperature and frequency dependences of 1H T1 can be explained as that the 1H T1 values are drastically affected by the temperature‐variable quadrupolar relaxation of chlorine nuclei in the complex anions through dipolar‐quadrupolar cross relaxation between 1H and chlorine nuclei. The frequency dependence of 1H T1 of (guH)2PtCl4 and (guH)2PdCl4 determined at various temperatures was analyzed by the Kimmich method and it revealed that the onset of anionic motion, presumably the 90° reorientation of the anion approximately as a whole about its C4 axis occurs near 200 and 250 K, respectively, with a rather small activation energy of ca. 30 kJ mol−1 for both complexes. The correlation time of chlorine nuclei for the C4 reorientation of the anions obtained at various temperatures indicated that the fade‐out phenomenon of 35Cl NQR signals observed for these complexes can be explained by the life‐time broadening. From the frequency dependence of 1H T1, the quadrupole coupling constant of chlorine nuclei for both complexes could be roughly estimated at higher temperatures where no signals could be detected by the usual NQR technique because of the fade‐out phenomenon. For (guH)AuCl4, unusual temperature and frequency dependences of 1H T1 attributable to the cross relaxation between 1H and chlorine nuclei were also observed above ca. 300 K and below ca. 230 K.