Platinum Complexes of Alkynyl-Substituted Dimethyldihydropyrenes

A series of photochromic dimethyldihydropyrenes (DHP)
bearing ethyne substituents in the 4-position (RDHP-CCH;
R = H (4), acetyl (5), benzoyl (6), 1-naphthoyl (7), benzo­[e] (8)) were prepared for use as precursors to ethynyl ligands.
Dehydrohalogenation of the adduct of these ethynes with various PtCl2(L)2 complexes afforded a series of cis and trans
square-planar bis­(DHP-ethynyl) platinum complexes, (RDHP-CC)2Pt­(L)2 (cis, R = H, L2 = PEt3 (9), acetyl, PEt3 (10), benzoyl,
PEt3 (11), 1-naphthoyl, PEt3 (12), benzo­[e], PEt3 (13), benzoyl, PPh3 (14), benzo­[e], PPh3 (15); trans, R = H, L2 = dppe (16), benzo­[e], dppe (17), acetyl, bipy (18), 1-naphthoyl, bipy (19), H, phen (20)). The DHP-ethynes bearing acyl
and benzo­[e] substituents on the DHP (5–8) and their platinum complexes (10–19) are photochromic and undergo ring opening
to the related cyclophanediene (CPD) isomer on irradiation with visible
light (λ >550 nm). In the case of the benzo­[e]­DHP 8, adding a 4-ethynyl substituent increases the
rate of photo-opening 4-fold; however, in the acylDHP series 5–7, addition of a 4-ethynyl group slows
the rate of photo-opening by a factor of about 4. The platinum complexes
generally open more slowly than the corresponding alkyne precursors,
again by a factor of about 4. The rate of photo-opening does not appear
to be affected significantly by the nature of the ancillary ligands
or the metal geometry, suggesting that there is poor electronic communication
between the platinum center and the DHP π system. The thermal
back-reaction from the open CPD to closed DHP form is about 50% faster
for the metal complexes than for the DHP-ethyne precursor. The platinum
complexes and the DHP-ethynes were characterized by NMR and IR spectroscopy
and MS and by X-ray crystallography for metal complexes 10, 11, 14, 15, and 17.