Self-Assembly of Cu^II and Ni^II [2 × 2] Grid Complexes and a Binuclear Cu^II Complex with a New Semiflexible Substituted Pyrazine Ligand:  Multiple Anion Encapsulation and Magnetic Properties

With the new substituted pyrazine ligand pyrazine-2,3-dicarboxylic acid bis[(pyridin-2-ylmethyl)amide], H₂L, a binuclear complex [Cu₂(LH)(Cl₃)(H₂O)]·H₂O (1) and two [2 × 2]G grid complexes, {[Cu₄(LH)₄](ClO₄)₄}·5CH₃OH·4H₂O (2) and {[Ni₄(LH)₄]Cl₄}·5CH₃CN·13H₂O (3), have been synthesized and characterized spectroscopically and crystallographically. The ligand H₂L crystallized in the triclinic space group P1̄, with a = 4.9882(7) Å, b = 12.079(2) Å, c = 14.454(2) Å, α = 107.08(2)°, β = 98.61(2)°, γ = 97.54(2)°, V = 808.8(2) ų, Z = 2, R1 = 0.0747, and R_w = 0.1829 for 1319 observed reflections [I > 2σ(I)]. The molecule is L-shaped with a strong intramolecular bifurcated hydrogen bond in half of the molecule. In the crystal the molecules are linked by an intermolecular hydrogen bond to form a 1D polymer. The binuclear complex [Cu₂(LH)(Cl₃)(H₂O)]·H₂O (1) crystallized in the monoclinic space group P2₁/a, with a = 8.6859(7) Å, b = 28.060(2) Å, c = 9.5334(9) Å, β = 107.89(1)°, V = 2211.2(3) ų, Z = 4, R1 = 0.039, and R_w = 0.097 for 1408 observed reflections [I > 2σ(I)]. There are two independent copper atoms both having square pyramidal geometry. Both coordinate to a pyrazine, a pyridine, and an amide N atom. Two chlorines complete the coordination sphere of one of the copper atoms, while one chlorine atom and a water molecule complete the coordination sphere of the other. The copper(II) [2 × 2] grid complex {[Cu₄(LH)₄](ClO₄)₄}·5CH₃OH·4H₂O (2) crystallized in the triclinic space group P1̄, with a = 17.1515(14) Å, b = 17.7507(13) Å, c = 19.3333(15) Å, α = 67.34(1)°, β = 69.79(1)°, γ = 71.50(1)°, V = 4980.3(7) ų, Z = 2, R1 = 0.083, and R_w = 0.207 for 5532 observed reflections [I > 2σ(I)]. The four Cu^II atoms are octahedrally coordinated by two pyrazine, two pyridine, and two amide N atoms and occupy the corners of a [2 × 2] grid with edge lengths, Cu···Cu, varying from 7.01 to 7.39 Å. The nickel(II) [2 × 2] grid complex {[Ni₄(LH)₄]Cl₄}·5CH₃CN·13H₂O (3) crystallized in the monoclinic space group C2/c, with a = 16.3388(10) Å, b = 29.754(2) Å, c = 20.857(1) Å, β = 101.845(1)°, V = 9923.6(12) ų, Z = 4, R1 = 0.050, and wR2 = 0.101 for 3391 observed reflections [I > 2σ(I)]. Here the complex possesses C₂ symmetry and again each metal atom is octahedrally coordinated to two pyrazine, two pyridine, and two amide N atoms. They occupy the corners of a [2 × 2] grid with an average edge length, Ni···Ni, of 6.97 Å. Of the four anions (ClO₄^-'s in 2 and Cl^-'s in 3) required to equilibrate the charges in the grid complexes, two are encapsulated, one above and one below the plane of the four metal atoms. The remaining two anions are located between the “wings” of the ligands. Magnetic susceptibility measurements indicate that the binuclear complex 1 is antiferromagnetic, with a J value of −15.07 cm^-1. This is larger than the J values found for the Cu^II (2) and Ni^II (3) grid complexes, which were −5.87 and −2.64 cm^-1, respectively. DFT calculations have been carried out to explain the difference in the J values found for complexes 1 and 2.