The syntheses, photochromism and aromaticity of dimethyldihydropyrene derivatives containing organometallic fragments and [e]-fused C7 and C8 aromatic systems




Zhang, Pengrong

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A series of cis- and trans-bis(ethynyl)platinum complexes containing dimethyldihydropyrene (DHP) photochromic compounds were synthesized from RDHPCCH (R = H, CH3CO, PhCO, 1-naphthoyl and benzo[e]) and platinum chloride with appropriate ancillary ligands (PEt3, PPh3, dppe, bipy and phen). The complexes were studied using mass spectrometry, NMR spectroscopy (1H, 13C, 31P and 195Pt) and IR. The X-ray structural information for bis(DHP-ethynyl) platinum complexes 44, 45, 47, 49, and 52 revealed that the Pt complexes possessed a square planar geometry at the metal centers. These platinum complexes are T type photochromic compounds. The BDHP-derived platinum complexes 48, 49, and 52 open completely when irradiated by visible light (λ > 590 nm). The photoopening rates for the platinum complexes are about 4 times slower than the corresponding DHP-alkynes. All the alkynyl platinum complexes close thermally faster than the corresponding free alkynes, and the closing rate is not affected substantially by changing the ancillary phosphine ligands. The BDHP-ethynyl platinum complexes with PEt3, PPh3 and dppe ligands have similar thermal closing half lives at 25°C of τ1/2 = 42 h, 38 h and 33 h, respectively, in contrast to the half life τ1/2 = 62 h of the BDHP-ethyne 40. The first cyclobutadienyl cobalt substituted dihydropyrenes were prepared by CpCo(CO)2 cyclization of a series of dihydropyrenyl ethynes. When the other alkyne substituent was small (methyl or carboxyethyl) only cis (head-to-head) isomers were obtained, but with larger sized groups, mixed head-to-head and head-to-tail isomers were obtained. The crystal structure of complex 21 indicated some unusually short bond distances were present. By comparison of the NMR and bond length data for complexes 21 or 69 with those for phenyl-DHP 60, the aromaticity of the cyclobutadienyl cobalt fragment was estimated quantitatively to be at least as large as that of benzene. The organometallic fragment [Cbd-Co-Cp] substantially slowed the DHP photoopening reaction of complex 72 relative to precursors 67 or 35. DHP[e]tropone, 17, DHP[e]tropylium cation, 18 and DHP[e]cyclooctatetraene dianion, 20 were synthesized to study the relative bond localizing abilities of COT dianion 80 (105 % relative to benzene), tropylium cation 15 (55 % relative to benzene), tropone 79 (13 % relative to benzene) using DHP as the NMR probe. The internal methyl protons of DHP resonated at δ -3.56 for 17, δ -2.61 for 18 and δ -1.38 for 20. Cycloheptatrienyl anions 111, 112 and 19 were made from cyclohepta-2,4,6-triene (CHT) isomers 107 and 108 (for two anions, 112 and 19). The internal methyl protons of their DHP resonated at δ -2.52, 2.67 for 111, δ -0.80, -0.84 for 112 and δ +2.10 for 19. The anion 19 was best taken as a 20π electron paratropic system. The homo-aromaticity of the CHT isomers 107, 108 and 117 was estimated based on the NMR data and X-ray structural data. Obvious anisotropic effects existed and the bond localization ability of the CHT (24% based on NMR data for 108) may have a large error. The ring inversion barriers of the COT 82 and CHT 77 were measured using variable temperature NMR spectroscopy as 13.9 and 8.7 kcal/mol, respectively. Five X-ray crystallography structures were obtained for 99B, 17, 108, 117 and 125 and the information was used in estimating the bond localization abilities and in establishing absolute stereochemistry



dimethyldihydropyrene, photochromic, bis(DHP-ethynyl) platinum complexes, cyclobutadienyl cobalt, tropone, tropylium cation, Cycloheptatrienyl anions, cyclooctatetraene dianion