A thermochemical study of 5,10,15,20-tetraphenylporphine zinc(II) by rotating bomb combustion calorimetry and by Knudsen effusion experiments

The standard molar enthalpies of combustion {ΔcHmo(cr)} and formation {ΔfHmo(cr)} at T= 298.15 K ( po= 0.1 MPa) for the 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP, Chemical Abstracts registry number 14074-80-7), were determined by rotating bomb combustion calorimetry. To dissolve the zinc oxides formed during the combustion reaction, an amount of aqueous solution of nitric acid was put into the bomb at the beginning of each experiment; in this way, when the bomb is rotated, a well defined final state is achieved. The corresponding enthalpy of sublimation (ΔcrgHmo) at T= 559 K was determined by the Knudsen effusion method using a quartz-crystal microbalance. The derived enthalpies of formation {ΔfHmo(g)} and atomization {ΔatHmo(g)} for gaseous ZnTPP were computed from the experimental measurements. A summary of results ( T= 298.15 K; po= 0.1 MPa) is shown below: With the previously reported enthalpy of formation value for the free porphyrin, the mean dissociation enthalpy of the N–Zn co-ordination bond was determined as D(N–Zn) = (185 ± 6) kJ · mol−1in the studied compound.