Interplay of bite angle and cone angle effects. A comparison between o-C6H4(CH2PR2)(PR′2) and o-C6H4(CH2PR2)(CH2PR′2) as ligands for Pd-catalysed ethene hydromethoxycarbonylation

Abstract

The following unsymmetrical diphosphines have been prepared: o-C₆H₄(CH₂P^tBu₂)(PR₂) where R = P^tBu₂ (L_3a3a3a); PCg (L_3b3b); PPh₂ (L_3c3c3c); P(o-C₆H₄CH₃)₂ (L_3d3d3d); P(o-C₆H₄OCH₃)₂ (L_3e3e) and o-C₆H₄(CH₂PCg)(PCg) (L_3f3f) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions has been investigated in the presence of Pd complexes of each of the ligands L_3a–f3a–f and the results compared with those obtained with the commercially used o-C₆H₄(CH₂P^tBu₂)₂ (L_1a1a). The Pd complexes of the bulkiest ligands L_3a3a3a, L_3b3b and L_3f3f are highly active catalysts but the Pd complexes of L_3c3c3c, L_3d3d3d and L_3e3e are completely inactive. The crystal structures of the complexes [PtCl₂(L_1a1a)] (1a) and [PtCl₂(L_3a3a3a)] (2a) have been determined and show that the crystallographic bite angles and cone angles are greater for L_1a1a than L_3a3a3a. Solution NMR studies show that the seven-membered chelate in 1a is more rigid than the six-membered chelate in 2a. Treatment of [PtCl(CH₃)(cod)] with L_3a–f3a–f gave [PtCl(CH₃)(L_3a–f3a–f)] as mixtures of 2 isomers 3a–f and 4a–f. The ratio of the products 4 : 3 ranges from 100 : 1 to 1 : 20, the precise proportion is apparently governed by a balance of two competing factors, steric bulk and the antisymbiotic effect. The palladium complexes [PdCl(CH₃)(L_3b3b)] (5b/6b) and [PdCl(CH₃)(L_3c3c3c)] (5c/6c) react with labelled ¹³CO to give the corresponding acyl species [PdCl(¹³COCH₃)(L_3b3b)] (7b/8b) and [PdCl(¹³COCH₃)(L_3c3c3c)] (7c/8c). Treatment of [PdCl(¹³COCH₃)(L)] with MeOH gave CH₃¹³COOMe rapidly when L = L_3b3b but very slowly when L = L_3c3c3c paralleling the contrasting catalytic activity of the Pd complexes of these two ligands.