Structures and Magnetic Properties of Trinuclear Copper(II) Halide Salts

The crystal structures of a series of trinuclear copper(II) halide salts have been determined. These all contain pseudoplanar, symmetrically bibridged Cu₃X₈^2-(la) or Cu₃X₇L^-(lb) anions.acu₃C1₈or A₂Cu₃Cl₈salts are formed with A being the [formula omitted] N-methylpiperazinium (NMPZ), 3-methyl-2-aminopyridinium (3MAP), and 5-methyl-2-aminopyridinium (5MAP) cations. A Cu₃Br₈^3-analogue is found with the 5-bromo-6-methyl-2-aminopyridinium (5B6MAP) cation. With the methylphenethyl-ammonium (NMPH) cation, a (NMPH)Cu₃Cl₇•EtOH salt is isolated. The salts are all monoclinic with the following space group and lattice constants: NMPZ, C₅H₁₄N₂Cu₃Cl₈, P2_l/c, a = 6.840 (3) Å, b = 14.321 (9) Å, c = 9.890 (5) Å, β = 102.90 (5)°, Z = 4; 3MAP, C₁₂H₁₈N₄Cu₃Cl_s, C2/c, a = 26.05 (1) Å,b= 13.687 (4) Å, c = 7.099 (3) Å, β = 117.95 (3)°, Z = 4; NMPH, C₁₁H₂₀NOC₃Cl₇, P2₁/n, a = 11.843 (4) Å, b = 7.626 (3) Å, c = 23.840 (10) Å, β = 79.61 (3)°, Z = 4; 5B6MAP, C₁₂N₁₀N₄-Cu₃Br₁₀, P2₁/n, a = 13.216 (3) Å, b = 4.076 (1) Å, c = 24.614 (6) Å, β = 91.65 (2)°, Z = 2. Each copper ion extends its primary coordination by formation of two semicoordinate bonds to halide ions in adjacent oligomers, yielding the familiar 4+2 coordination geometry for copper(II) complexes. These additional linkages cause the oligomers to aggregate into stacks, yielding stacking patterns of type Ila (NMPZ, 3MAP salts) or type IIb (NMPH, 5MAP, and 5B6MAP), Cu-X distances are approximately 2.3 [figure omitted] Å (Cl) or 2.4Å (Br) within the oligomers and 2.7-3.3 Å (Cl) or 3.2-3.3 Å (Br) between oligomers. The bridging Cu-X-Cu angles within the trimer average near 94°. Magnetic susceptibility measurements have been made on the NMPZ, NMPH, 3MAP, and 5MAP chloride salts. In addition, measurements were performed on Cu₃Cl₆(CH₃CN)₂, which contains neutral trinuclear oligomers. All compounds have doublet ground states arising from antiferromagnetic exchange coupling between neighboring copper(II) ions. The value of J/k is typically -20 to -35 K, consistent with the structural characteristics of the trimeric species.