[Au(CN)₄]^- as Both an Intramolecular and Intermolecular Bidentate Ligand with [(tmeda)Cu(μ-OH)] Dimers: from Antiferro- to Ferromagnetic Coupling in Polymorphs

Two polymorphic products, [{Cu(tmeda)(μ-OH)}2Au(CN)4][Au(CN)4] (1) and [Cu(tmeda)(μ-OH)Au(CN)4]2 (2), were synthesized from {Cu(tmeda)(μ-OH)}2X2 (tmeda = N,N,N‘,N‘-tetramethylethylenediamine, X = ClO4-, BF4-) and 2 equiv of K[Au(CN)4], and their X-ray structures were determined. Both compounds have {Cu(tmeda)(μ-OH)}22+ dimers with [Au(CN)4]- units bound in the axial positions. However, in 1, two trans N-donor cyanides of each [Au(CN)4]- unit bind to adjacent copper(II) dimers, forming a 1-D chain, whereas complex 2 is molecular, with two mono-coordinated [Au(CN)4]- units. The 1-D polymorph 1 is formed from aqueous solution, while the molecular polymorph 2 is obtained with X = BF4- in methanol. The polymorphs have slightly different Cu−O−Cu angles, a key magnetostructural parameter, such that the 1-D chain 1, with an angle of 96.6(2)°, shows ferromagnetic interactions with 2J = +57.5 cm-1 and g = 2.097, whereas the molecular complex 2, with an angle of 98.92(17)°, shows antiferromagnetic interactions with 2J = −143.6 cm-1 and g = 2.047. A similar Cu(II) complex, [{Cu(tmeda)(μ-OH)}2Au(CN)4][ClO4]·MeOH (3), was synthesized in methanol when X = ClO4-, in which the [Au(CN)4]- unit bridges the two Cu(II) centers within the dimer in an intramolecular fashion via cis N-donor cyanides. The average Cu−O−Cu angle of 98.4(2)° in 3 generates antiferromagnetic interactions with 2J = −64.8 cm-1 and g = 2.214. Complexes 1−3 represent the first examples of [Cu(tmeda)(μ-OH)]22+ dimers with Cu−O−Cu angles under 100°, thereby extending the range of 2J coupling constants for this moiety from 149 to 566 cm-1. The switch to ferromagnetic interactions in 1 as a result of the coordinating, bridging [Au(CN)4]- anion suggests that cationic, dinuclear moieties that are typically antiferromagnetically coupled may, with an appropriate coordinating counterion, become ferromagnetic units.