Difference in the Formation of Two Structural Types of V-Shaped M^II₃ Clusters: Diffraction, Mass Spectrometry, and...

A way to understand kinetics and explore mechanism of reactions is to identify the intermediates and their relative energetics. In this respect, low-energy electrospray ionization mass spectrometry is providing information on possible intermediates that can be tandemly verified using crystallography of the products. This has been extended to the study of the formation of functional clusters of transition metals under varying conditions. The reaction of NiL₂ (L = 2-ethoxy-6-( N-methyliminomethyl)phenolate) with M^II(H₂O)₆(ClO₄)₂ in the presence of Et₃N base leads to [Ni _xM_{7- x}(μ₃-OH)₆L₆]²⁺ according to NiL₂ → [M₂L₃]⁺ → [M₄(OH)₂L₄]²⁺ → [M₇(OH)₆L₆]²⁺. In contrast, its reaction with M^II(H₂O)₆(NO₃)₂ in the absence of base leads to two crystallographic structural types [M^II₃L₄(NO₃)₂(H₂O)₂]·CH₃CN for M = Ni (I-Ni₃) or Co (I-Co _xNi_{3- x}, x = 0-3) and [M^IINi^II₂L₄(NO₃)₂] for M = Zn (II-ZnNi₂) or Co (II-CoNi₂). Interestingly, ESI-MS suggests three slightly different formation processes: for I-Ni₃, {NiL₂ → [Ni₂L(NO₃)₂]⁺ → [Ni₂L₂(NO₃)]⁺ → [Ni₂L₃]⁺ → [Ni₃L₄(NO₃)]⁺}; for II-ZnNi₂, {NiL₂ → [ZnNiL₂(NO₃)]⁺ → [ZnNi₂L₄(NO₃)]⁺}; for II-CoNi₂ and I-Co _xNi_{3- x}, {NiL₂ → [M₂L₂(NO₃)]⁺ → [M₂L₃]⁺ → [M₃L₄(NO₃)]⁺}. Magnetization measurements reveal the site of each metal ionin II-ZnNi₂ and the number of single electrons within different clusters. Without the base, there is an interplay between the weak coordinating nitrate and water stabilizing the two structural types via the different formation processes. The results indicate that not only the strength of the ligand matters but also the ionic sizes and possibly softness of the metals may be implicated.