Contrasting Thermal and Photochemical Intramolecular Coupling in Alkynylphosphine Platinum(II) Complexes

The thermal and photochemical transformations of a series of alkynylphosphine platinum(II) complexes
are described. Compounds [Pt](PPh2C⋮CR)2 ([Pt] = cis-Pt(C6F5)2, R = Ph, Tol) rearrange thermally to
generate naphthalene-based diphenylphosphine complexes (1a, 1b) containing the fragment {C10H5-1-Ph-2,3-κPP‘(PPh2)2} or {7-CH3-C10H4-1-Tol-2,3-κPP‘(PPh2)2}, formed by intramolecular coupling of two
adjacent PPh2C⋮CR ligands. By contrast, irradiation of these alkynylphosphine derivatives in toluene
results in the formation of a mixture of 1a/1b and the 1,2-diphosphino-alk-1-ene complexes [Pt]{PPh2C(Ph)C(R)PPh(C⋮CR)} (R = Ph, 2a; Tol, 2b) in a final ratio of 60:40. However, irradiation of the
mixed alkynylphosphine derivatives [Pt](PPh2C⋮CR)(PPh2C⋮Ct-Bu) gives selectively [Pt]{Ph2PC(Ph)C(R)PPh(C⋮Ct-Bu)} (R = Ph, 3a; Tol, 3b; t-Bu, 3c) as result of a P−C(Ph) activation of a
tert-butylalkynylphosphine, PPh2C⋮Ct-Bu. Under thermolysis, bis(tert-butyl)alkynyl derivatives produce
no evidence of any cyclization product, but the mixed alkynylphosphine derivatives [Pt](PPh2C⋮CR)(PPh2C⋮Ct-Bu) evolve giving small amounts of 1a/1b and trans-[Pt(C6F5)2(PPh2C⋮Ct-Bu)2], 4. Under
photolytic conditions, the diynyl phosphine derivatives [Pt](PPh2C⋮CC6H4C⋮CR)2 (R = Ph, t-Bu)
rearrange directly to the naphthalene complexes [Pt]{7-C⋮CR-C10H4-1-(C6H4-pC⋮CR)-2,3-κPP‘(PPh2)2}
(R = Ph, 5a; t-Bu, 5c), resulting from the intramolecular coupling of the two inner alkynyl fragments,
with no observable intermediates. Finally, site-selective activation takes place by photochemical or thermal
treatment of [Pt](PPh2C⋮CPh)(PPh2H), 6. Thus, while under photochemical conditions complex 6 yields
selectively [Pt]{Ph2PC(Ph)C(Ph)PPhH}, 7, by a ligand rearrangement coupling process involving
activation of a P−C(Ph) bond, the regioisomer [Pt]{Ph2PC(H)C(Ph)PPh2}, 8, is generated by a thermal
activation of the P−H bond.