Synthesis and Characterization of Structurally Diverse Alkaline-Earth Salen Compounds for Subterranean Fluid Flow Tracking

A family of magnesium and calcium salen-derivatives was synthesized and characterized for use as subterranean fluid flow monitors. For the Mg complexes, di-n-butyl magnesium ([Mg­(Bu n )2]) was reacted with N,N′-ethylene bis­(salicylideneimine) (H2-salen), N,N′-bis­(salicylidene)-1,2-phenylenediamine (H2-saloPh), N,N′-bis­(3,5-di-t-butylsalicylidene)-ethylenediamine (H2-salo-Bu t ), or N,N′-bis­(3,5-di-t-butylsalicylidene)-1,2-phenylenediamine (H2-saloPh-Bu t ), and the products were identified by single-crystal X-ray diffraction as [(κ3-(O,N,N′),μ-(O′)­saloPh)­(μ-(O),(κ2-(N,N′),μ-(O′)­saloPh)2­(μ-(O),κ3-(N,N′,O′)­saloPh′)­Mg4]·2tol (1·2tol; saloPh′ = an alkyl-modified saloPh derivative generated in situ), [(κ4-(O,N,N′,O′)­saloPh)­Mg­(py)2]·py (2·py), [(κ4-(O,N,N′,O′)­salo-Bu t )­Mg­(py)2] (3), [(κ4-(O,N,N′,O′)­saloPh-Bu t )­Mg­(py)2]·tol (4·tol), and [(κ3-(O,N,N′),μ-(O′)­saloPh-Bu t )­Mg]2 (5), where tol = toluene; py = pyridine. For the Ca species, a calcium amide was independently reacted with H2-salo-Bu t and H2-saloPh-Bu t to generate the crystallographcially characterized compounds: [(κ4-(O,N,N′,O′)­salo-Bu t )­Ca­(py)3] (6), [(κ4-(O,N,N′,O′)­saloPh-Bu t )­Ca­(py)3]·py (7·py). The bulk powders of these compounds were further characterized by a number of analytical tools, where 2–7 were found to be distinguishable by Fourier transform infrared and resonance Raman spectroscopies. Structural properties obtained from quantum calculations of gas-phase analogues are in good agreement with the single-crystal results. The potential utility of these compounds as taggants for monitoring subterranean fluid flows was demonstrated through a series of experiments to evaluate their stability to high temperature and pressure, interaction with mineral surfaces, and elution behavior from a loaded proppant pack.