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 Cu3X82- (Ia) or Cu3X7L- (Ib) anions. A2Cu3Cl8 or A2Cu3Cl8 salts are formed with A being the (Chemical Equation Presented) N-methylpiperazinium (NMPZ), 3-methyl-2-aminopyridinium (3MAP), and 5-methyl-2-aminopyridinium (5MAP) cations. A Cu3Br83- analogue is found with the 5-bromo-6-methyl-2-aminopyridinium (5B6MAP) cation. With the methylphenethylammonium (NMPH) cation, a (NMPH)Cu3Cl7·EtOH salt is isolated. The salts are all monoclinic with the following space group and lattice constants: NMPZ, C5H14N2Cu3Cl8, P21/c, a = 6.840 (3) Å, b = 14.321 (9) Å, c = 9.890 (5) Å, β = 102.90 (5)°, Z = 4; 3MAP, C12H18N4Cu3Cl8, 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 IIa (NMPZ, 3MAP salts) or type IIb (NMPH, 5MAP, and 5B6MAP). Cu-X distances are approximately 2.3 (Figure Presented) Å (Cl) or 2.4 Å (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 specie