Unusual ligand design: A platinum(II) mediated [2+2] photocycloaddition followed by a nickel(II) induced methoxyactivation

The novel dimer of the composition [Pt2Cl4(μ-(κP1:κP2-o-MeO-trans-dppen))2] (1) (o-MeO-trans-dppen = 1,2-(bis(o-methoxyphenyl)phosphanyl)ethylene) has been prepared and characterized by a single crystal X-ray structure analysis, NMR spectroscopy, mass spectrometry and elemental analysis. This latter compound undergoes a [2+2] photocycloaddition reaction yielding the tetraphosphane all-trans-1,2,3,4-tetrakis(di(o-methoxyphenyl)phosphanyl)cyclobutane (o-MeO-dppcb). The X-ray structure of the dimeric Ni(II) complex that contains the latter ligand, of the formula [Ni2Cl4(μ-(κP1:κP2:κP3:κP4-o-MeO-dppcb))] (2) reveals that the apical coordination sites of both square pyramidal Ni(II) coordination spheres are occupied by methoxy-oxygen atoms of the ligand. As a consequence, this dimeric Ni(II) complex 2 is prone to a thermally induced regio- and diastereoselective metal-assisted methoxy-group cleavage. The stepwise formed new mono- and bis-phenolate complexes [Ni2Cl3(μ-(κO1,κP1:κP2:κP3:κP4-o-MeO-O-dppcb))] (3) and [Ni2Cl2(μ-(κO1,κP1:κP2:κO2,κP3:κP4-o-MeO-O,O′-dppcb))] (4), respectively, contain the novel chiral tetraphosphane ligands all-trans-1,2,3-tris((di-o-methoxyphenyl)phosphano)-4-((o-methoxy-phenyl)(o-phenolate)phosphano)cyclobutane (o-MeO-O-dppcb) and all-trans-1,2-bis((di-o-methoxyphenyl)phosphano)-3,4-bis((o-methoxyphenyl)(o-phenolate)phosphano)cyclobutane (o-MeO-O,O′-dppcb). Compounds 3 and 4 have been synthesized independently and are also fully characterized by both single crystal X-ray structure analyses, NMR spectroscopy, mass spectrometry and elemental analyses. The conversion of 2 into 3 and then further into 4 has been followed by a variable-temperature 31P{1H} NMR experiment with compound 2 in DMF-d7, revealing that the cleavage of the second methoxy group is kinetically disfavoured. This is in agreement with the X-ray structure analysis of 3, indicating the lack of any methoxy-oxygen atom coordination that could easily induce a further methoxy-group cleavage. o-MeO-O-dppcb and o-MeO-O,O′-dppcb are rare P-stereogenic tetraphosphine ligands and contribute to the synthetic field of new κ3-P,P,O-coordinating phosphanylphenolate ligands that are believed to be important for the SHOP process (SHOP, Shell Higher Olefin Process).