In-plane coordinated double bonds. Molecular structures, spectroscopy, and stability of 5-methylenecyclooctene and 5-methylenecycloheptene complexes of platinum(II)

Article abstract of DOI:10.1021/om00149a015

A series of platinum(II) complexes having coordinated olefin constrained to lie in the square plane of coordination has been synthesized and characterized. The diolefin chelates used in this work are 5-methylenecycloheptene (MCH) and 5-methylenecyclooctene (MCOT). Since this class of compounds is of marginal stability, the diolefins, synthesis conditions, and metal substrates must be carefully chosen [e.g., all attempts to prepare Pd(II) analogues result in facile chloropalladation]. Analysis of X-ray and NMR data leads to the conclusion that the in-plane olefin is more weakly bound to platinum than is the out-of-plane olefin, and the principal source of instability is the close contact of terminal exocyclic carbon and gem-cis atom [2.82 (1) ? for C9 and C12 in (MCOT)PtCl₂ (4a), for example]. Several lines of evidence indicate that MCH forms more stable complexes than does MCOT [e.g., MCH displaces MCOT from (MCOT)PtCl₂ to yield (MCH)PtCl₂]. Reaction of (MCH)PtCl₂ with (C₆H₅)Sn(CH₃)₃ leads to either (MCH)PtClPh (6) or (MCH)PtPh₂ (7). The phenyl ligand in the former complex has large cis and trans influences (seen in X-ray and NMR results) of opposite sign. Virtually any diene displaces MCOT from (MCOT)PtCl₂. Even bridging bromide is a better ligand than the exocyclic olefin in MCOT. Thus (η⁴-MCOT)PtBr₂ (4b) readily dimerizes to [(η²-MCOT)PtBr(η-Br)]₂. The kinetics and thermodynamics of this dimerization/dedimerization are reported. In the course of the present work, the molecular structures of the following five compounds were determined by X-ray diffraction: 4a, 4b, 6, 7, and dichloro[(1,2,5,6-η⁴)-2,5-dimethyl-1,5-hexadiene]platinum(II) (9). Crystal data for 4a: C₉H₁₄Cl₂Pt, space group C2/c with a = 14.954 (2) ?, b = 7.233 (2) ?, c = 18.726 (5) ?, β = 94.01 (2)°, V = 2021 (1) ?³, Z = 8. Crystal data for 4b: C₉H₁₄Br₂Pt, space group C2/c with a = 15.317 (3) ?, b = 7.427 (2) ?, c = 18.920 (3) ?, β = 93.68 (1)°, V = 2148 (1) ?³, Z = 8. Crystal data for 6: C₁₄H₁₇ClPt, space group Pbca with a = 12.380 (2) ?, b = 12.757 (4) ?, c = 16.559 (3) ?, V = 2615 (2) ?³, Z = 8. Crystal data for 7: C₂₀H₂₂Pt, space group P2₁2₁2₁ with a = 6.250 (3) ?, b = 14.694 (4) ?, c = 17.796 (5) ?, V = 1634 (2) ?³, Z = 4. Crystal data for 9: C₈H₁₄Cl₂Pt, space group P2₁/n with a = 7.633 (3) ?, b = 12.147 (1) ?, c = 11.048 (1) ?, β = 97.84 (2)°, V = 1014.8 (6) ?³, Z = 8.