Comparison on the selectivity of the first and second attack of selected lithium organyls as nucleophiles on the carbonyls in the bioctahedron complexes M 2( μ-PPh 2) 2(CO) 8 (M=Mn, Re). Is there a neighbour group effect for the second attack?

Abstract

In thf solution the bioctahedron complex Re 2( μ-PPh 2) 2(CO) 8 1 reacts with one equiv of LiR (R=Ph, Me, nBu, tBu) at temperatures between −90 and −50°C to the acyl monoanion Li[Re 2( μ-PPh 2) 2(CO) 7(ax-C(R)O)] 3 and with a second equiv of LiR 1 (R 1=Ph, Me, nBu) at 20°C to the diacyl dianions Li 2[Re 2( μ-PPh 2) 2(CO) 6(ax-C(R)O)(ax-C(R 1)O)] 12 as trans or cis isomers. Both nearly quantitative reaction steps run selectively: First, both lithium nucleophiles attack exclusively axially coordinated carbonyl ligand. Second, the monoanion with R=Ph forms exclusively the cis diacyl dianions (R 1= nBu, Ph). Through this reaction an anchimeric effect was proved. Third, the monoanions with R= nBu, Me react only to the trans diacyl dianions. In general such mono- and dianions have been reacted with O-alkylating agens to the corresponding carbene and dicarbene derivatives (yield 50–70%) to get single crystals for X-ray structure analyses. All products were identified by means of 1H, 1P NMR and ν(CO)IR data. Additionally the molecular structures of the Fischer type carbene complexes Re 2( μ-PPh 2) 2(CO) 7(ax-C( tBu)OMe) 5b, cis-Re 2( μ-PPh 2) 2(CO) 6(ax-C(Ph)OEt) 2 14 and trans-Re 2( μ-PPh 2) 2(CO) 6(ax-C( nBu)OMe)(ax-C(Ph)OMe) 15b are presented. Opposite to 1 affords the homologous dimanganese complex 2 under the same reaction conditions at −50°C with one and two equiv of lithium organyl an unselective nucleophilic addition to axially and equatorially coordinated monoacyl monoanions and diacyl dianions. Our attempts to separate such isomers of the mono- and dianion type as lithium salt led to their decomposition. However, the carbene and dicarbene derivatives Mn 2( μ-PPh 2) 2(CO) 7(ax-C(Ph)OEt) 9ax and trans-Mn 2( μ-PPh 2) 2(CO) 6(ax-C( nBu)OEt) 2 16 could be crystallized as air-stable, yellow solids. Their molecular structures are presented.