Coordination of Sulfur Dioxide to Iridium(I) Centers. Sulfur Dioxide Insertion into a Methoxo-Metal Bond: Crystal and Molecular Structure of Carbonyl(methyl sulfito)(sulfur dioxide)bis(triphenylphosphine)iridium-Hemitoluene, the First Structurally Characterized Example of a Transition Metal Complex Containing an O-Coordinated Methyl Sulfito Ligand

Reactions of SO₂ with iridium(I) complexes have been examined. For trans-Ir(CO)L₂X (L = PPh₃, P(p-tolyl)₃, PCy₃; X = Cl, Br, Me), equilibrium constants for SO₂ binding have been evaluated. The binding of SO₂ is as a Lewis acid and is enhanced by electron-donating groups on the iridium. Increasing the size of the phosphine reduces the equilibrium constant for SO₂ binding. Reactions of SO₂ with the hydroxo or alkoxo complexes Ir(CO)(OR)L₂ (R = H, Me, t-Bu; L = PPh₃, P(p-tolyl)₃) result in insertion of the SO₂ to form an oxygen-coordinated sulfite ligand. The sulfur center of the sulfite ligand is pyramidal and asymmetric, resulting in a second-order ³¹P NMR spectrum at low temperature. A further rearrangement of the oxygencoordinated sulfite ligand to the sulfur-coordinated occurs in solution. For R = H, further decomposition of the HSO₃− ligand occurs. The products of each reaction are characterized by infrared and ¹H and ³¹P NMR spectroscopy. The product of SO₂ insertion into the iridiummethoxo bond, Ir(CO)(OS(O)OMe)(SO₂)(PPh₃)₂, is also characterized by X-ray crystallography. The complex Ir(CO)[OS(O)OMe](SO₂)(PPh₃)₂ crystallizes from toluene as the hemitoluene solvate, Ir(CO)[OS(O)OMe](SO₂)(PPh₃)₂·0.5(toluene), in the centrosymmetric triclinic space group P1 (No. 2) with a = 10.972(5) Å, b = 11.658(4) Å, c = 18.020(11) Å, α = 82.73(4)°, β = 77.76(4)°, γ = 63.25(3)°, V = 2007.5(17) ų, and Z = 2. The structure was solved and refined to R = 3.05% and R_w = 3.58% for all 5280 reflections with 2θ < 45° (Mo Kα) and R = 2.31% and R_w = 3.23% for those 4502 reflections with |F_o| > 6σ (F_o)· The iridium is in a squarepyramidal coordination environment with the SO₂ ligand in the apical site. Iridium-ligand bond lengths are as follows: Ir(1)-P(1) = 2.369(2) Å and Ir(1)-P(2) = 2.387(2) Å for the Ir-PPh₃ groups, Ir(1)-S(1) = 2.451(2) Å for the SO₂ ligand, Ir(1)-C(1) = 1.831(6) Å for the CO ligand, and Ir-O(23) = 2.080(4) Å for the 0S(=O)OMe ligand. The trans-basal angles are P(1)-Ir-(1)-P(2) = 170.3(1)° and C(1)-Ir(1)-O(23) = 176.7(2)°; angles to the apical SO₂ ligand are P(1)-Ir(1)-S(1) = 97.5(1)°, P(2)-Ir(1)-S(1) = 92.2(1)°, C(1)-Ir(1)-S(1) = 97.3(2)°, and O(23)- Ir(1)-S(1) = 85.8(1)°.