Synthesis, Structural Diversity, Dynamics, and Acidity of the M(II) and M(IV) Complexes [MH₃(PR₃)₄]⁺ (M = Fe, Ru, Os; R = Me, Et)

Abstract

The syntheses of complexes MH2L4 and their protonated analogues [MH3L4]+ (M = Fe (1), Ru (2), Os (3); R = Me (a), Et (b)) are described. The structures of 1a, 1b, and 3a were determined in X-ray diffraction studies. The solution structures of complexes 1−3 were established by detailed NMR investigations. 1a, 2b, and 3a form equilibrium mixtures of two isomers in solution. The iron (1a) and ruthenium (2b) complexes isomerize between six-coordinate M(II) dihydrogen cis-[MH(H2)L4]+ and seven-coordinate M(IV) trihydride [M(H)3L4]+ molecular geometries: the first is a distorted octahedron and the second can be viewed as a hydride-capped M(PR3)4 tetrahedron. Complex 3a is a pentagonal bipyramidal trihydride cis-[Os(H)3(PMe3)4]+ in equilibrium with the hydride-capped tetrahedral structural form. Trihydrides 1b and 3b are exclusively represented by the latter structural type. The cationic molecule 2a corresponds to a dihydrogen complex cis-[RuH(H2)(PMe3)4]+. The metal fragment [MH(PR3)4]+ is thus most reactive toward oxidative addition of H2 for osmium and iron, with a notably lower ability to reduce H2 for ruthenium. This trend and related properties are due to electronic rather than steric factors. All [MH3(PR3)4]+ species (1−3) are fluxional in solution. The intramolecular hydride exchanges and isomerizations were studied between 20 and −140 °C and quantitatively described in terms of their activation parameters. On the basis of these, mechanistic interpretations are provided. Finally the acid/base properties of the [MH3(PR3)4]+/M(H)2(PR3)4 systems were established in a series of NMR experiments in THF-d8. The pKa values range from 10.3 to 12.9 units and increase in the following order: 1a (10.3) < 2b (10.7) < 2a (10.9) < 3a (11.2) < 3b (12.9). This series demonstrates a higher acidity than that of the related [MH(H2)(PP)2]+ molecules with bidentate ligands. The complexes with monodentate phosphine ligands [MH3(PR3)4]+ (1−3) represent a new and distinguished family with structural, dynamic, and acid/base properties remarkably different from most of the other known [MH3L4]+ representatives.