Coordination Environment Strategy for Improving Sensing Characteristics of Molybdenum(V) Porphyrin Complexes

Molybdenum(V) porphyrin complexes O=Mo(X)P (P is substituted porphin, X is singly charged anionic ligand) show affinity for practically significant organic bases in organic solvents and produce spectral response accessible for recording in the UV-visible region. Therefore, these compounds may be promising for the selection of optimal structures to be studied as active components of sensor devices. In order to achieve high sensing characteristics, the effect of the composition of the coordination sphere of O=Mo(X)P on these characteristics was analyzed by varying the axial ligand X and the substituent in the equatorial macrocyclic ligand. For this purpose, a complete kinetic description of the reaction with pyridine (Py, a classical example of highly volatile organic compound (VOCs)), proceeding as two consecutive two-way reactions, was developed in relation to (ethoxy)(oxo)[5,10,15,20-tetraphenylporphinato]molybdenum(V) (O=Mo(OEt)TPP) in toluene. The intermediates and final products were identified by UV-Vis, ¹H NMR, and IR spectroscopy and mass spectrometry, and sensing activity parameters (time, numerical spectral response to the presence of Py, and the lower limit of Py determination) were studied. Using similar data for O=Mo(OH)TPP, O=Mo(OEt)TTP, and O=Mo(OEt)T^tBuPP (TTP and T^tBuPP are tetra-4-methyl- and tetra-4-tert-butyl-substituted TPP dianions, respectively), it was shown that targeted design of the coordination sphere can be used for fine tuning of the sensing properties and that the O=Mo(OEt)TPP complex is most effective for this purpose.