Chemistry of Iridium(I) Cyclooctadiene Compounds with Thiapentadienyl, Sulfinylpentadienyl, and Butadienesulfonyl Ligands

The metathesis reaction of [(η⁴-COD)­Ir­(μ-Cl)]₂ (4) with two equivalents of the sodium thiapentadienide (1Na) or potassium sulfinylpentadienide salt (2K) led to the formation of the corresponding dimers [(η⁴-COD)­Ir­(μ₂-1-2,5-η-CH₂CHCHCHS)]₂ (5) and [(η⁴-COD)­Ir­(μ₂-1-2,5-η-CH₂CHCHCHSO)]₂ (9). The single-crystal analysis of 5 and 9 reveals the presence of the thiapentadienyl or sulfinylpentadienyl ligands bridging through the sulfur atoms and the terminal double bonds to both iridium centers. Treatment of 5 with two equivalents of PMe₃ produces [(η⁴-COD)­Ir­(1-2,5-η-CH₂CHCHCHS)­PMe₃] (6), while compound Ir­(1-2,5-η-CH₂CHCHCHS)­(CO)­(PPh₃)₂ (8) is obtained from reaction of Ir­(CO)­(Cl)­(PPh₃)₂ (7) with potassium thiapentadienide (1K). The ¹H and ¹³C NMR support the preferred U conformation and the same η^2,1-bonding mode of the thiapentadienyl ligand in each case. The reaction of 4 with butadienesulfinate salts M­[CH₂CHCHCHSO₂] (3M) (M = Li, K) affords the ion-pair complexes [(η⁴-COD)­IrCl­(1-2,5-η-CH₂CHCHCHS­(O₂^–M⁺)] (M = Li, 10; M = K, 11). Compound (η⁴-COD)­Ir­(μ-Cl)­(1-2-η-S,O-μ-OSOCHCHCHCH₂)­Ir­(η⁴-COD) (12) can be isolated if the reaction of 4 with 3K is carried out at low temperature and after a short period of time in solution. The crystal structure of 12 shows a dinuclear compound where the butadienesulfonyl is bridging through the S and one of the O atoms to the iridium center. In solution, 12 dissociates in the presence of coordinating solvents, such as DMSO-d₆ or THF-d₈, while the dinuclear asymmetric structure of 12 remains in CDCl₃. The series of pentacoordinated Ir­(I) complexes of general formula [(η⁴-COD)­Ir­(1-2,5-η-CH₂CHCHCHSO₂)­L] (L = PMe₃, 14; PMe₂Ph, 15; PMePh₂, 16; PPh₃, 17; DMSO, 18; and CO, 19) can be obtained, under mild conditions, from 11 and the corresponding ligand L, which shows different σ or π donor–acceptor properties. The disubstituted phosphine derivative [(η⁴-COD)­Ir­(5-η-CH₂CHCHCHSO₂)­(PMe₃)₂] (20) can be prepared directly from 14 and an excess of PMe₃. A comparative study of these derivatives was carried out through the analysis of the IR, mass spectrometry, and ¹H, ¹³C, and ³¹P NMR spectroscopy, as well as through the crystalline structures of 12, 14, 15, and 17–20, and allowed establishing trends among them. The presence of the butadienesulfonyl ligand in complexes 14–19 induces a total asymmetry that is reflected through the ¹H and ¹³C NMR. The preferred coordination mode (1-2,5-η-) in the butadienesulfonyl ligand for complexes 14–19 was confirmed. A better synthetic procedure for 14 is described if [(η⁴-COD)­IrClPMe₃] (21) reacts with 3K. In contrast, no synthetic advantage was found in the formation of 17 or 20 when [(η⁴-COD)­IrClPPh₃] (22) or [(η⁴-COD)­IrCl­(PMe₃)₂] (23) is used as a precursor. Monitoring reactions through ¹H and ³¹P NMR of 11, 12, and 14 in the presence of PMe₃ and 23 with 3K afforded mixtures of compounds, from which an equilibrium in the reaction mixture is proposed.