Experimental and Theoretical Investigations of the Formation of the Diazene PhSNC(H)NNC(H)NSPh from HCN2(SPh)3 by a Thiyl-Radical-Catalyzed Mechanism: Identification of the HC(NSPh)2• Radical and X-ray Structures of HCN2(SPh)3 and PhSNC(H)NNC(H)NSPh
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abstract
The reaction of HCN(2)(SiMe(3))(3) with benzenesulfenyl chloride in a 1:3 molar ratio produces HCN(2)(SPh)(3) (4) as thermally unstable, colorless crystals. The decomposition of (4) in toluene at 95 degrees C was monitored by UV-visible, (1)H NMR and ESR spectroscopy. The major final products of the decomposition were identified as PhSN=C(H)N=NC(H)=NSPh (5) and PhSSPh. The structures of 4 and 5 were determined by X-ray crystallography. The crystals of 4 are monoclinic, space group P2(1)/a, with a = 9.874(2) Å, b = 19.133(2) Å, c = 10.280(2) Å, beta = 113.37(1) degrees, V = 1782.8(5) Å(3), and Z = 4. The final R and R(w) values were 0.042 and 0.049, respectively. The crystals of 5 are monoclinic, space group P2(1)/n, with a = 5.897(6) Å, b = 18.458(10) Å, c = 7.050(8) Å, beta = 110.97(5) degrees, V = 716(1) Å(3), and Z = 2. The final R and R(w) values were 0.075 and 0.085, respectively. The diazene 5 adopts a Z,E,Z structure with weak intramolecular S.N contacts of 2.83 Å, giving rise to four-membered NCNS rings. During the thermolysis of 4 at 95 degrees C in toluene a transient species (lambda(max) 820 nm) was detected. It decomposes with second-order kinetics to give 5 (lambda(max) 450 nm). The ESR spectrum of the reaction mixture consisted of the superposition of a three-line 1:1:1 spectrum (g = 2.0074, A(N) = 11.45 G), attributed to (PhS)(2)N(*), upon a doublet of quintets (1:2:3:2:1) with g = 2.0070, A(N) = 6.14 G, A(H) = 2.1 G assigned to the radical HCN(2)(SPh)(2)(*). Density functional theory (DFT) calculations for the models of the radical showed the E,Z isomer to have the lowest energy. Thermochemical calculations indicate that the decomposition of HCN(2)(SH)(3) into the diazene (Z,E,Z)-HSN=C(H)N=NC(H)=NSH (and 2 HSSH) is substantially more exothermic (DeltaH = -176.1 kJ mol(-)(1)) than the corresponding formation of the isomeric eight-membered ring (HC)(2)N(4)(SH)(2) (DeltaH = -40.6 kJ mol(-)(1)). These calculations also indicate that the diazene is formed by a mechanism in which the RS(*) radical acts as a catalyst.
