Cooperativity‐Driven Reactivity of a Dinuclear Copper Dimethylglyoxime Complex

Abstract

In this report, we present the dinuclear copper(II) dimethylglyoxime (H2dmg) complex [Cu2(H2dmg)(Hdmg)(dmg)]+ (1), which, in contrast to its mononuclear analogue [Cu(Hdmg)2] (2), is subject to a cooperativity‐driven hydrolysis. The combined Lewis acidity of both copper centers increases the electrophilicity of the carbon atom in the bridging μ2‐O−N=C‐group of H2dmg and thus, facilitates the nucleophilic attack of H2O. This hydrolysis yields butane‐2,3‐dione monoxime (3) and NH2OH that, depending on the solvent, is then either oxidized or reduced. In ethanol, NH2OH is reduced to NH4+, yielding acetaldehyde as the oxidation product. In contrast, in CH3CN, NH2OH is oxidized by CuII to form N2O and [Cu(CH3CN)4]+. Herein are presented the combined synthetic, theoretical, spectroscopic and spectrometric methods that indicate and establish the reaction pathway of this solvent‐dependent reaction.

On the Equilibrium of Mono‐ and Dinuclear Cu(II) Dimethylglyoxime Complexes and its Exploitation for the Simple Preparation of Cu(I) Salts [Cu(CH3CN)4]X (X=ClO4−, BF4−, OTf−)

“…[15,39,41,43] In contrast, the usage of Cu(ClO 4 ) 2 • 6H 2 O or Cu(BF 4 ) • 6H 2 O (ratio 1 : 1 or 1 : 2, Cu 2 + : H 2 dmg) leads to the formation of the dinuclear species 2 a. [17][18][19]39,40] Thus, even though both anions used are weakly coordinating and accordingly, a similar reaction outcome could have been expected, there are clear preferences for the product formation. These preferences presumably are caused by the basicity of the anions as the basic anion OAc À yields mononuclear 1, whereas the neutral anions ClO 4 À and BF 4 À (as well as SO 4 2À ) favor formation of dinuclear 2 a.…”

“…As this exclusive formation of 5 starting from CuCl 2 (• 2H 2 O) is consistent with literature, we excluded it from further investigations. [16,18] As a side note, with regard to Cu(ClO 4 ) 2 • 6H 2 O and subsequent formation of 2 a, it was observed that excess H 2 dmg stabilizes 2 a against the reported hydrolysis [39] as these reaction mixtures required more time to yield the hydrolysis products than those with fewer amounts of H 2 dmg. In general, the results reflect the synthetic protocols, i.e.…”

“…[35] Additionally, complexes with Co(II) are used in hydrogen evolution reactions (HER). [36][37][38] Especially with regard to copper, the reactivity of H 2 dmg is very rich, which is reflected in the large number of binary Cu(II)-H 2 dmg complexes that range from mono- [15,16] over di- [17,18,39,40] to tetranuclear [18][19][20] compounds (Scheme 1). Among those complexes, [Cu(Hdmg ] motif of 2 b that is observed at higher pH values.…”

“…[18] As already indicated, little is known about the influences that govern the formation and especially the nuclearity of Cu(II) H 2 dmg complexes; however, it is known that in solution, 2 a, 2 b, and 3 can be formed starting from 1 by adding increasing amounts of Cu(ClO 4 ) 2 • 6H 2 O to 1. [17,18] Although such complexes are well known, [2,[15][16][17][18]20,21,25,[39][40][41][42] new aspects of their reactivity are still being discovered. E.g., just recently some of us reported the incorporation of BF 2 into Cu(II)-H 2 dmg complexes to yield O,O-bidentate difluoroboron complexes when using Cu(BF 4 ) 2 • 6H 2 O as Cu(II) source.…”

“…In contrast to the mononuclear analogue 1 (Scheme 1), 2 a is subject to a cooperativity-driven hydrolysis that yields NH 2 OH, which depending on the solvent either is oxidized or reduced. [39] Given these unexpected and species-specific reactivity, the knowledge of the species formed in solution through reaction of a Cu(II) salt with H 2 dmg is critical for analytical purposes as well as their reactivity in vitro or even in vivo. However, the prediction of the species formed is made difficult due to a lack of knowledge of the reaction conditions governing the products' nuclearity.…”