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-d 8. The pK a 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.

Synthesis, Structure, and Reactivity of (Dihydrogen)(hydrido)iron(II) Complexes Bearing Chiral Diphos­phanes

“…The phosphorus–iron bonds (see Table 3) of the two equatorial phosphorus atoms [2.2400 Å for Fe–P(3), and 2.2226 Å for Fe–P(4)] are slightly longer than those for the two axial atoms, P(1) and P(2) (2.2188 and 2.2161 Å, respectively). This structure is similar to the reported X‐ray structures of cis ‐[FeH(η 2 ‐H 2 )(PMe 3 ) 4 ]BPh 4 16 and cis ‐[FeH(η 2 ‐H 2 ){P(CH 2 CH 2 PPh 2 ) 3 }]BPh 4 · THF, in which the tripodal polyphosphane P(CH 2 CH 2 PPh 2 ) 3 is coordinated to the metal center 18. The short iron–phosphorus bonds observed in cis ‐[FeH(η 2 ‐H 2 ){P(CH 2 CH 2 PPh 2 ) 3 }] + could be attributed to the geometrical disposition of the polyphosphane.…”

“…A remarkable asymmetry of the bonding in the Fe–(η 2 ‐H 2 ) distances has been observed in the refined model. This asymmetry has been reported for cis ‐[Fe(H)(η 2 ‐H 2 )(PR 3 ) 4 ] + (R = H or Me) from X‐ray and computational results, and it has been described as a weak attractive interaction between the hydrido ligand and the H(b) atom in a cis disposition 16,27. The influence of the neighboring BF 4 – counteranion on the elongation of one iron–hydrogen bond has been discarded, because the interatomic H(a) ··· F (2.905 Å) and H(b) ··· F (3.203 Å) distances are too long; the characteristic fluorine signal associated with the BF 4 – counteranion was observed in the 19 F NMR spectrum, and no hydrogen–fluorine interactions were observed in solution.…”

Dihydrogen complexes as prototypes for the coordination chemistry of saturated molecules

“…The intermediate is a ''trihydrogen'' complex (91,92). Although not isolated, evidence exists for its intermediacy in facile H-atom exchange in ReH 2 (H 2 )(CO)(PR 3 ) 3 (93), which can be exceedingly fast even at Ϫ140°C in hydrido(H 2 ) complexes (94)(95)(96)(97)(98)(99). The barrier for hydrogen exchange in IrClH 2 (H 2 )(P i Pr 3 ) 2 is only 1.5 kcal/mol even in the solid state (95,96).…”

Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

“…Another remarkable reaction that occurs even in the solid involves incorporation of deuterium into the PMe 3 ligands of [Fe(D 2 )H(PMe 3 ) 4 ] + , which may involve agostic interactions in which associative (r-bond metathesis as in equation 2) or dissociative C-H bond activation occurs [96].…”