Solid-State Structures and Magnetic Properties of Halide-Bridged, Face-to-Face Bis-Nickel(II)-Macrocyclic Ligand Complexes:  Ligand-Mediated Interchanges of Electronic Configuration

Abstract

Dramatic differences are found between the ambient and 100 K X-ray structures of [L(2)Ni2Br2](ClO4)2 (L(2) = alpha,alpha'-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene), in which the bromide-bridged, bimetallic, macrocyclic ligand complexes of nickel(II) are held face-to-face and in which each bimetallic complex has a net triplet spin multiplicity. The ambient structure of this complex consists of very highly ordered, infinite chains of alternating R and S isomers in which the identical Ni(II) coordination spheres are near to the average expected for the high- and low-spin Ni(II) coordination sites, and there is appreciable stereochemical strain in the linkage of the macrocyclic ligands to the phenyl ring. In contrast, every other dinickel complex of the 100 K structure is displaced about 40 pm along the infinite chains to form tetrameric repeat units (pairs of dinickel complexes), in which each dinickel complex has well-defined high-spin and low-spin Ni(II) coordination sites; the high-spin sites are adjacent in the tetramers, and the stereochemical strain in the linkage to the phenyl spacer is relaxed. The molecular magnetic moments and structural contrasts are similar for the 100 K structure and the previously reported ambient structure of [L(2)Ni2Br3](ClO4) complex for which the molecular magnetic moments also correspond to a single triplet state per complex. The halide-bridged, monochloro- and monobromo dinickel complexes also have triplet spin multiplicity, and they crystallize with a coordinated perchlorate completing the axial coordination of the high-spin Ni(II) site, while the other Ni(II) site of these halide-bridged complexes has equatorial Ni-N bond lengths typical of low-spin Ni(II) coordination. The bridging halide is sandwiched between the face-to-face macrocyclic ligand Ni(II) moieties and slightly off the Ni-Ni axis in all of the complexes. The temperature dependence of the magnetic moments of the series of complexes indicates that their singlet-triplet energy gaps are small, with zero point energy differences that are generally less than 10(3) cm(-1). The very weak metal-metal electronic coupling, the triplet state spin multiplicity of each dinickel complex, and the averaged high-spin/low-spin coordination environments of the ambient structure implicate a vibronic mechanism for the electronic configurational exchange in the dibromo and tribromo complexes. The single molecular vibrational mode that correlates with the configurational exchange in these complexes includes the concerted motion of the bridging bromide between the Ni(II) centers. Activation of this vibrational mode is sufficient to effect the configurational exchange. These complexes present especially clear examples of the effects of the coupling of nuclear vibrational motions to the interchange of electronic configuration between two different centers.

Anion Recognition in Water, Including Sulfate, by a Bicyclam Bimetallic Receptor: A Process Governed by the Enthalpy/Entropy Compensatory Relationship

“…In any case, metal-ligand interactions are typically more intense than electrostatic/hydrogen-bond interactions and afford the formation of more stable receptor/anion complexes. [23] More recently,w eh ave studied the behaviour of the [Ni II 2 (bicyclam)] 4 + complex as an anion receptor in DMSO, with special reference to the effect of anion inclusion on the Ni II -to-Ni III oxidation processes taking place at the two metal centres. Such an inconvenience can be removed by using metallocyclam subunits: in particular, transition-metali ons, if encircled by the 14-membered cyclic tetraamine cyclam, achieve an extreme inertness towards demetallation, even under severe conditions (due to the kinetic macrocyclic effect).…”

“…Dinickel(II) complexes of bicyclam were investigated for their uniquem agnetic properties by Ito and co-workers, [21,22] and later,v ery thoroughly,b y Endicott and co-workers. [23] More recently,w eh ave studied the behaviour of the [Ni II 2 (bicyclam)] 4 + complex as an anion receptor in DMSO, with special reference to the effect of anion inclusion on the Ni II -to-Ni III oxidation processes taking place at the two metal centres. [24] Herein, we report ac omplete thermodynamic investigation on the interaction of the [Cu II 2 (bicyclam)] 4 + receptor with anions in pure water.T he [Cu II (cyclam)] 2 + subunit exhibits a defined tendency to bind furtherl igands( e.g.,a nions) to achieve five coordination; [25] ac ircumstancet hat should favour anion inclusion inside the intermetallic space of [Cu II 2 (bicyclam)] 4 + .A nion binding equilibriah ave been investigated through isothermal titration calorimetry (ITC), which allowed the determination of association constants (and DG8), as well as of enthalpy, DH8,a nd entropy contributions, TDS8.I t has been shown that knowledge of these thermodynamic quantities may allow the interpretation of the solutions tability of the anion complexes in terms of inner effects (receptoranion bonding, conformational rearrangement of the framework of the receptor) and outer effects (releaseo fw ater molecules to the solution due to anion and receptor dehydra-tion).…”

“…Because Cu II ions are displaced towards the inside of the receptors, the two Cu II ÀBr distances are slightly different:C u II À N in = 2.92(1) ,C u II ÀN out = 2.98(1) .T he Cu II ···Cu II distance (5.83(1) )i sc omparable to the intermetallic distance observed in the analogous [Ni II 2 (bicyclam)Br 3 ] + complex, which contains ah igh-spinN i II (bound to inner Br À )a nd al ow-spinN i II (not bound). [23] In the crystal,both bromide ions in the inner and outer positions are positioned on two crystallographic mirror planes and bridget he two Cu II metal centres,w ith almost linear two-fold coordination.T he Cu II ÀBr À distance, Cu II -Br-Cu II angle and Cu II ···Cu II separation are 2.92(1) ,1 71.7(1)8 and5 .83(1) ,r espectively,i nside the dimera nd 2.98(1) ,1 70.9(1)8 and 5.94(1) ,r espectively,o utside the dimer.T hese interactions result in infinite molecularchains of dimetallic subunits extending alongt he directiono ft he b crystallographic axis (see Figure 1). Figure 2.…”

Ligand Association/Dissociation Paths and Ill‐Defined Coordination Numbers

“…Although it would constitute an exciting research topic, it will encounter difficulties, because in many instances the magnetic behavior has gone unreported, and often the high-or low-spin nature of a complex is deduced from interatomic distance criteria without magnetic or spectral evidence. [37] Nevertheless, our shape analysis has allowed us to identify cases of significantly dissociated octahedra for which a lowspin behavior has been reported (see entries PTOCNI10 [38] and LATXOU [39] in Table 1). …”

Halide-bridged polymers of divalent metals with donor ligands – structures and properties

“…For the bromo-bridged Ni complex of the macrocyclic ligand ␣,␣ -2-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene} a phase transition between room temperature and 100 K with marked changes both in the structure and in the magnetic properties has been observed [50].…”