Formazans: a Structural and Spectroscopic Study of 3-Substituted-1,5-Diphenylformazans
Authors:   , Cunningham Christopher William 
Published: November 2021

<p>A series of thirteen isomeric 1,5-diphenylformazans have been structurally characterised both in the solid state and in solution by the combined techniques of x-ray crystallography, nuclear magnetic resonance, Raman, mass and absorption spectroscopies. 1,5-Diphenylformazan is known to exist in the anti, s-trans configuration in the solid state and this is shown to be the solution dominant species. In aprotic solvents an equilibrium involving the anti, s-trans and syn, s-cis configurations is evidenced. 3-Methyl-1,5-diphenylformazan has been characterised by an x-ray crystal analysis. C14H14N4 belongs to the monoclinic space group P2/c, a = 8.133(1), b = 19.085(4), c = 9.364(2) A, beta = 105.93 degrees,U = 1397.6(5) A3, Z = 4. The anti, s-trans configuration of the solid state is also preferred in solution where it is in equilibrium with the syn, s-cis configuration. 3-Ethyl-1,5-diphenylformazan exists in two isomers in the solid state, both of which have been characterised by an x-ray crystal analysis. The red isomer of 3-ethyl-1,5-diphenylformazan belongs to the orthorhombic space group P2l2l2l and adopts the syn, s-trans configuration in the solid state. The orange, light stable isomer of 3-ethyl-1,5-diphenylformazan belongs to the monoclinic space group P2l/c and adopts the anti, s-trans configuration in the solid state. The rate of return of the photo-activated orange isomer to the dark-stable red isomer follows first order kinetics dependent upon the total concentration of the formazan and the water content of the solvent. 3-Tertiary-butyl-1,5-diphenylformazan has been characterised by an x-ray crystal analysis. C17H20N4, belongs to the monoclinic space group P2/c, a = 11.235(3), b = 20.117(5), c = 14.176(3) A, beta = 92.14(2) degrees, U = 3202(1) A3, Z = 8. The syn, s-cis configuration of the solid state is maintained in solution. 1,3,5-Triphenylformazan is shown to exist in two red forms in the solid state. The syn, s-cis and syn, s-trans isomers are both present in the crystalline sample. These isomers are also evident in solution with the syn, s-trans configuration becoming more dominant in aprotic solvents. 1,5-Diphenylformazan reacts with bromine in solution in a single reaction to give di(3-bromo-1,5-diphenyltetrazolium)-decabromide and 3-bromo-1,5-di-para-phenylformazan, both of which have been characterised by an x-ray crystal analysis. C13H10.6N4Br5.3 belongs to the triclinic space group Pl, a = 8.572(1), b = 9.711(1), c = 14.166(3) A, alpha = 75.18(1), beta = 89.84(1), gamma = 70.42(1) degrees, Z = 2. Stacks of anti-parallel pairs of 3-bromo-1,5-diphenyltetrazolium cations are interleaved by pairs of Br102- anions. The polybromide represents a new type of polyhalogen network for bromine, Br102-, the Raman spectrum of which has been recorded for the first time. C13H9N4Br3 belong to the orthorhombic space group Pnma, a = 7.343(2), b = 32.793(12), c = 5.912(1) A, Z = 4. The formazan adopts the anti, s-trans configuration in the solid state. 3-Chloro-1,5-diphenylformazan has been characterised by an x-ray crystal analysis. Preliminary results indicate that the formazan adopts the anti, s-trans configuration in the solid state. 3-Mercapto-1,5-diphenylformazan is shown to exist in the anti, s-trans configuration in CDCl3, solution. 3-Methylthio-1,5-diphenylformazan is shown to exist in an equilibrium mixture of syn, s-trans and anti, s-trans configurations in solution. The ratio of the two isomers is approximately equal. 3-Ethylthio-1,5-diphenylformazan exists in two isomers in the solid state, one of which has been characterised by an x-ray crystal analysis. Preliminary results indicate that the orange isomer of 3-ethylthio-1,5-diphenylformazan. C15H16N4S belongs to the monoclinic space group P2l/a, a = 11.027(6). b = 8.627(7), c = 15.487(8) A, b = 93.70(5) degrees, U = 1470 A3, Z = 4, and exists in the anti, s-trans configuration on the solid state. The orange and red isomers are both present in an equilibrium mixture in solution. The red isomer is shown to exist in the syn, s-trans configuration in the solid. 3-isopropylthio-1,5-diphenylformazan is shown to exist in an equilibrium mixture of anti, s-trans and syn, s-trans configurations in solution. 1-Methyl-1,5-diphenylformazan has been characterised by an x-ray crystal analysis. C14H14N4 belongs to the monoclinic space group 12/1, a = 28.402(7), b = 5.640(1), c = 15.688(4) A, beta = 97.34 degrees, U = 2493(1) A3, Z = 8. The formazan adopts the anti, s-trans configuration in the solid state. The formazan retains its configurational integrity in both protic and aprotic solutions. The excitation profile of the Raman active phonons based upon coupled vibrations of the formazan backbone indicate a maximum corresponding to the absorption spectra in both the solid state and in solution. Preliminary results of a kinetic investigation of some primary metal dithizonates indicate that the thermal-path return is strictly first order. The mechanism would appear to be essentially similar to that operating in 3-ethyl-1,5-diphenylformazan. The mass spectra of the series of formazan follow similar splitting schemes irrespective of the solid-state configuration.</p>