Synthesis, antimony-121 mössbauer and infrared spectral characterization of antimony trifluoride and catecholatoantimony(III) fluoride adducts with tridentate nitrogen donors

121/123Sb Nuclear Quadrupole Resonance Spectroscopy: Characterization of Non-Covalent Pnictogen Bonds and NQR Crystallography

Pnictogen (or pnicogen) bonding is an attractive interaction between the electrophilic region of group 15 elements (N, P, As, Sb, Bi) and a nucleophile. This interaction for which unique applications in catalysis have recently been uncovered continues to gain popularity. Here, we investigate a series of pnictogen-bonded cocrystals based on SbF₃ and SbCl₃, prepared via mechanochemical ball milling, with ^121/123Sb ( I = ⁵/₂ and ⁷/₂, respectively) nuclear quadrupole resonance (NQR) spectroscopy. Observed NQR frequency shifts upon cocrystallization are on the order of 0.1 to 10 MHz and are clearly diagnostic of the formation of pnictogen bonds to antimony. Further evidence for pnictogen bonding is obtained by complementary ¹³C cross-polarization magic-angle spinning solid-state NMR experiments. DFT calculations of NMR parameters as well as natural localized molecular orbital analyses support the experimental findings and elucidate the electronic origins of the experimental NQR frequency shifts. This work provides insights into the changes in the antimony quadrupolar coupling constant upon pnictogen bonding: strikingly, the decreases noted here parallel those known for hydrogen bonds, but contrast with the increases reported for halogen bonds. The utility of the observed antimony nuclear quadrupolar coupling constants in constraining structural models of cocrystals for which diffraction-based structures are unavailable, i.e., a rudimentary implementation of NQR crystallography, is established. Overall, this work offers a new approach to understand emerging classes of electrophilic interactions and to contextualize them in the broader landscape of established chemical bonding paradigms.

Pentadentate terpyridine-catechol linked ligands and their cobalt(III) complexes

A series of novel pentadentate terpyridine-catechol linked ligands were prepared, in which the terpyridine and catechol moieties were linked together by (CH2)n chains of different lengths (n = 4-6). Together with 1-methylimidazole, these ligands formed low-spin, six-coordinate Co(III) complexes. Two of the complexes (n = 4, 5) were characterized by X-ray crystallography [n = 4, monoclinic, P2(1)/c, a = 14.957(7) A, b = 10.585(9) A, c = 23.033(7) A, beta = 106.01(3) degrees, V = 3505(3) A3, Z = 4, R = 0.063; n = 5, monoclinic, P2(1)/c, a = 8.848(7) A, b = 15.78(1) A, c = 25.455(7) A, beta = 93.90(5) degrees, V = 3544(3) A3, Z = 4, R = 0.056], which revealed similar structures around the Co(III) centers but different conformations for the (CH2)n linkers. The (CH2)4 linker showed a straight, symmetric conformation whereas the (CH2)5 linker showed a curved conformation that allowed the accommodation of one extra CH2 unit, suggesting that the (CH2)4 linker presents the "best-fit" length for these complexes.