Unexpected polymorphism during a catalyzed mechanochemical Knoevenagel condensation

Abstract

The transformation of a base-catalyzed, mechano-assisted Knoevenagel condensation of mono-fluorinated benzaldehyde derivatives (p-, m-, o-benzaldehyde) with malonodinitrile was investigated in situ and in real time. Upon milling, the para-substituted product was found to crystallize initially into two different polymorphic forms, depending on the quantity of catalyst used. For low catalyst concentrations, a mechanically metastable phase (monoclinic) was initially formed, converting to the mechanically stable phase (triclinic) upon further grinding. Instead, higher catalyst concentrations crystallize directly as the triclinic product. Inclusion of catalyst in the final product, as evidenced by mass spectrometric analysis, suggests this complex polymorphic pathway may be due to seeding effects. Multivariate analysis for the in situ Raman spectra supports this complex formation pathway, and offers a new approach to monitoring multi-phase reactions during ball milling.

Metastability and Seeding Effects in the Mechanochemical Hybrid Lead(II) Iodide Formation

“…Most studies focus on organic polymorphs. By adding either a small amount of a molecule which is not involved in the reaction or a small amount of the wanted product, the reaction can be guided to produce the designated polymorph [55–58] . In our case we have compounds of different stoichiometry not polymorphs and the seeding does not change the end‐product.…”

“…By adding either as mall amount of am olecule which is not involved in the reaction or as malla mount of the wanted product, the reaction can be guided to produce the designated polymorph. [55][56][57][58] In our case we have compounds of different stoichiometry not polymorphs and the seeding does not change the end-product. Although 3 is not stable under normal conditions, it is the product of the synthesis with ar atio of 1:3, even when ah uge amount of 4 is produced.…”

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

“…Mechanochemical techniques have now been applied to an enormous array of materials syntheses, well beyond what can be covered here. For example, mechanochemical covalent chemical reactions have been reported ( Andersen and Mack, 2018b ; Bolm and Hernández, 2019 ; Tan and García, 2019 ), spanning classical condensation ( Haferkamp et al, 2019 ) or cyclization ( Andersen and Mack, 2017 ) reactions through to metal-catalyzed ( Pickhardt et al, 2020 ) or piezocatalyzed reactions ( Kubota et al, 2019 ). Regarding inorganic materials, syntheses and modifications have been described for hydrated oxides ( MacKenzie et al, 1999 ; Mehrotra et al, 2016 ; Alex et al, 2020 ), mixed oxides and ceramics ( Balaz et al, 1994 ; Mackenzie et al, 2000 ; Temuujin et al, 2000 ; MacKenzie and Barneveld, 2006 ), silicates ( Temuujin et al, 1998a ; Temuujin et al, 1998b ; MacKenzie et al, 2007 ), and high entropy alloys ( Kumar et al, 2017 ; Balcerzak et al, 2019 ; Kamalakannan et al, 2019 ), amongst many others.…”

Zeitaufgelöste In‐Situ‐Untersuchungen von mechanochemischen Reaktionen

“…Thermische Messungen während mechanisch‐chemischer Experimente werden seit vielen Jahren durchgeführt. Die Daten wurden mithilfe von Thermoelementen und Sensoren, die am Mahlgefäß angebracht waren, [56–59] oder durch Messung der Temperatur des Gefäßes und der Mahlkugeln direkt nach Beendigung des Experiments gewonnen [64, 67] …”

“…Thermische Messungen während mechanisch-chemischer Experimente werden seit vielen Jahren durchgeführt. Die Daten wurden mithilfe von Thermoelementen und Sensoren, die am Mahlgefäß angebracht waren, [56][57][58][59] oder durch Messung der Temperatur des Gefäßes und der Mahlkugeln direkt nach Beendigung des Experiments gewonnen. [64,67] James Mack und seine Mitarbeiter stellten ein elegantes Experiment vor, bei dem sie die Geschwindigkeit einer Diels-Alder-Reaktion in Lösung mit der Geschwindigkeit derselben Reaktion unter mechanochemischen Bedingungen korrelierten.…”