DISPROPORTIONATION OF SO2 TO S8 AND SO3 ON CP-ASTERISK-2CR2S5 - CRYSTALLOGRAPHIC AND S-34-LABELING EVIDENCE FOR AN OXYGEN-TRANSFER MECHANISM AND OBSERVATION OF FACILE EXCHANGE OF CR2S5 CORE SULFURS WITH S-34 - STRUCTURES OF CP-ASTERISK-2CR2(MU-S)(MU-ETA-2-S2)(MU-ETA-1-SS.SO2).SO2 AND CP-ASTERISK-2CR2(MU-S)(MU-ETA-2-S2)(MU-SSO3)

The syntheses, structures, and reactivities of Cp*2Cr2(mu-S)(mu-S2)(mu-SS.SO2).SO2, 1.2SO2, and CP*2Cr2(mu-S)(mu-S2)(mu-SSO3).(CD3OD)lattice, 2, are reported. Reversible addition of SO2 to Cp*2Cr2S5, 1, to give 1.2SO2 was found to occur at room temperature. 1 possesses a novel mu-eta-1-S2 ligand, and X-ray crystallography of 1.2SO2 showed that SO2 binds as a Lewis acid to the terminal sulfur of this ligand with an acute S-S-S angle (94-degrees). The S-SO2 distance of 2.41 angstrom is about 0.2 angstrom shorter than in the previously known Cp*2Mo2(mu-S)(mu-S2)(mu-S.SO2). Consistent with this greater activation, 1.2SO2 further reacts in the presence of excess SO2 to give the thiosulfate 2 (isomorphous to the Mo analogue) at a much faster rate than in the Mo system. Significantly, the crystal structure of 1.2SO2 shows the presence of associated SO2 molecules unsymmetrically bridging (via sulfur) the oxygen atoms of each sulfide-bound SO2 molecule to give a cyclic aggregate of four SO2 molecules (intermolecular S...O distances of 2.78 and 2.95 angstrom). Infrared studies demonstrate that these interactions further activate (weaken) the S=O bonds of the sulfide-bound SO2. Thus, it is proposed that the mechanism of SO2 disproportionation in solution involves base-assisted oxygen atom transfer from the sulfide-bound SO2 molecule to the associated SO2 to produce SO3. The SO3 then displaces S2O (which decomposes to S8 and SO2) from the bridging sulfide ligand to give the observed thiosulfate product. The reactivities of 1.2SO2 and Cp*2Cr2S4.SO2 vs Cp*2Mo2S4.SO2 and of 2 vs Cp*2Mo2S4.SO3 are discussed. A mechanism for the oxygen atom transfer reaction is proposed on the basis of the crystal structure of 1.2SO2 and S-34-labeling studies. Facile reaction of Cp*2Cr2S4 with S8 was found to give 1, and all five of the core sulfurs in the latter incorporated S-34 if excess S-34(8) was used. 1.2SO2 and 2 crystallize in the space groups C2/c and P1BAR, respectively. The cell dimensions are the following: 1.2SO2, a = 16.087 (4) angstrom, b = 15.926 (3) angstrom, c = 21.650 (3) angstrom, beta = 105.29 (2)-degrees, and Z = 8; 2, a = 10.336 (2) angstrom, b = 10.891 (2) angstrom, c = 13.219 (3) angstrom, alpha = 85.15 (3)-degrees, beta = 78.85 (3)-degrees, gamma = 62.39 (3)-degrees, and Z = 2. R = 4.85% for 1.2SO2, and R = 4.84% for 2.