Crystallographic snapshots of the bond-breaking isomerization reactions involving nickel(II) complexes with hemilabile ligands.

Hemilabile ligands have been used to prepare a wide variety of complexes, which are important in many fields ranging from catalysis to biomimetic chemistry. These ligands allow coordination complexes to be prepared where one end of the hemilabile ligand is anchored to a metal center, while the other often is involved in fluxional processes. Indeed, in certain cases, researchers refer to these structures as “windshield-wiper” ligands, since the weakly binding portion of the ligand can often dissociate and recoordinate to the metal center, undergoing exchange in the presence of a coordinating solvent. Consequently, many researchers employ hemilabile ligands to stabilize what effectively are highly reactive solvent adducts or coordinatively unsaturated species. As a result, complexes with certain types of hemilabile ligands have been made and characterized in both monodentate and multidentate states with different ancillary ligands. To our knowledge, however, there is no example where a pair of isomeric structures, which represent the two “end states” that define a hemilabile ligand exchange process (where no displacement reaction has taken place), have been crystallographically characterized. The reason for this observation is that invariably in such systems one state is significantly more stable than the other in a given environment, or the exchange process occurs dynamically under rapid exchange conditions and the identities of the complexes exchanging can only be inferred through in situ spectroscopic means. Herein, we report the synthesis, isolation, and crystallographic characterization of a series of octahedral and square planar structurally isomeric nickel(II) complexes 2 a–2c, which effectively define the two key “end states” in a well-defined bondbreaking isomerization reaction and a likely intermediate complex. While these structures are stable in the solid state, they rapidly interconvert in solution at room temperature. The reaction of complex 1 with two equivalents of sodium thiocyanate (NaSCN) in a mixture of CH2Cl2 and EtOH yields a mixture of complexes 2a–2 c (Scheme 1). In solution at room temperature, these complexes are rapidly interconverting, as evidenced by a single broad P{H} NMR resonance observed at d = 32.6 in CD2Cl2. Variable-temperature (VT) P{H} NMR spectroscopy confirmed that this broadening is a result of rapid exchange between multiple complexes. When the temperature of the solution was lowered to 213 K in a stepwise manner, the spectrum changed significantly. At 238 K, a sharp resonance at d = 32.6 ppm assigned to 2a is observed along with a broad one at d = 8.7 ppm assigned to 2c (Supporting Information, Figure S1). The assignments for these resonances are based upon literature precedent 29] involving model complexes and crystallographic data (see below). Interestingly, when red-orange CH2Cl2 solutions containing these rapidly interconverting complexes were layered with diethyl ether, both purple and orange block-like crystals formed, along with red plates. Single-crystal X-ray diffraction studies of the purple, orange, and red crystals showed that they were 2a, 2 b, and 2c, respectively (Figure 1). The structure of 2a consists of a nickel(II) center in an octahedral geometry, defined by two chelating P,S Me ligands coordinated in a trans configuration to form the equatorial plane. Furthermore, it has two apical nitrogen-bound SCN ligands, which are bent relative to the equatorial plane (150–1608 ; Figure 1a and Supporting Information, Table S1). The strucScheme 1. Synthesis of complexes 2a–c. a) 2 equiv NaSCN in 1:1 CH2Cl2/EtOH for 1 h.