Synthesis and Characterization of Mixed-Phosphine Osmium Polyhydrides:  Hydrogen Delocalization in [OsH₅P₃]⁺ Systems

Abstract

The hexahydrido complex OsH6(PiPr3)2 (1) reacts with PHPh2 to give molecular hydrogen and the tetrahydride OsH4(PHPh2)(PiPr3)2 (2). However, the formation of OsH2(PHPh2)2(PiPr3)2 (3) as a consequence of the substitution of a second hydrogen molecule from 2 by PHPh2 does not occur. The treatment of 2 with 1.0 equiv of PHPh2 in toluene at 80 °C leads after 3 days to OsH2(PHPh2)3(PiPr3) (4). The preparation of 3 requires the previous acidolysis of 2 with HBF4, which gives [OsH5(PHPh2)(PiPr3)2]BF4 (5). In contrast to 2, the addition of PHPh2 to 5 affords [OsH3(PHPh2)2(PiPr3)2]BF4 (6) and molecular hydrogen. Deprotonation of 6 with Et3N yields 3. The skeleton of the cation of 5 has been determined by X-ray diffraction. The configuration is consistent with a Y-shaped OsP3 disposition with the osmium atom in the common vertex. Complex 5 also reacts with methanol and water to give [OsH5{P(OMe)Ph2}(PiPr3)2]BF4 (7) and [OsH5{P(OH)Ph2}(PiPr3)2]BF4 (8), respectively. The addition of Et3N to 7 affords OsH4{P(OMe)Ph2}(PiPr3)2 (9). A theoretical study on the OsH5(PH3)3 + model complex reveals that although a static description is fully consistent with a classical pentahydride assignment, the formation of a dihydrogen is a very low energy costing process, on both thermodynamic and kinetic grounds. Thus, these polyhydride systems might be better described as possessing delocalized hydrogen atoms. A further QM/MM IMOMM study on the actual OsH5(PR3)+ systems indicates that the inclusion of bulky phosphine substituents plays a role against the stability of dihydrogen forms, because of the higher steric congestion of lower coordination number complexes arising from repulsions between bulky phosphines. Although IMOMM calculations improve significantly the agreement with experimental structures, they do not change the validity of the aforementioned statement concerning delocalization.

Primary and Secondary Phosphane Complexes of Metalloporphyrins: Isolation, Spectroscopy, and X‐ray Crystal Structures of Ruthenium and Osmium Porphyrins Binding Phenyl‐ or Diphenylphosphane

“…Since only the coupling between the two trans P nuclei of the axial phosphanes occurs here, we can obtain the 2 J P,P value from this spectrum. It was found to be 467 Hz, a value much larger than that observed between trans PHPh 2 and P i Pr 3 in [OsH 2 (PHPh 2 ) 3 (P i Pr 3 )] ( 2 J P,P =209.3 Hz) 21…”

“…Hydroxylphosphane complexes of transition metals are rare. Prior to this work, only a few non‐porphyrin metal hydroxyphosphane complexes had been reported,21, 23b, 25, 26 and, to the best of our knowledge, complex 6 is the first structurally characterized example of an osmium complex of a hydroxyphosphane.…”

Coordination chemistry and catalysis with secondary phosphine oxides

“…Further addition of a water molecule to the Os(IV)=PPh2 moiety affords the complex in high yield. The same procedure has been successfully applied to the preparation of pentahydride-diphenylphosphine Os(IV) complexes and the detected intermediates support the proposed mechanism 119. Although other less common oxidation states have been explored in osmium-SPO complexes, such as the porphyrin-Os(VI) derivatives described by Che and coworkers,120 an increasing interest have appeared in the design of bidentate and…”

Reduction of Benzonitriles via Osmium–Azavinylidene Intermediates Bearing Nucleophilic and Electrophilic Centers

“…Complex 1 is a saturated compound. However, it easily releases a hydrogen molecule to afford the unsaturated species OsH4(P i Pr3)2, which has been previously trapped with diphenylphosphine, 19 pyridines, 20 and boranes. 18g 2,6-Dimethylbenzonitrile is also able to capture this species.…”

“…Hydrogen atoms (except hydrides) have been omitted for the sake of clarity. Selected bond lengths (angstroms) for the X-ray and DFT-optimized (in brackets) structures: Os−H(01), 1.583(13) [1.685]; Os−H(02), 1.584(13) [1.626]; Os−H(03), 1.583(13) [1.626]; Os−H(04), 1.586(13) [1.687]; Os−P(1), 2.3193(18)[2.365]; Os−P(2), 2.3164(19) [2.364]; Os−N(1), 2.080(5)[2.081]; C(1)−N(1), 1.136(7) [1.163]; H(01)−H(02),[1.87]; H(02)−H(03),[1.70]; H(03)−H(04),[1.84]. Selected bond angles (degrees) for the X-ray and optimized (in brackets) structures: P(1)− Os−P(2), 173.64(7) [174.7]; H(01)−Os−H(02), 61(3) [68.8]; H(02)−Os−H(03), 61(3) [63.2]; H(03)−Os−H(04), 57(3) [67.0]; H(01)−Os−H(04), 175(3) [160.6]; H(01)−Os−N(1), 86(2) [79.1]; H(04)−Os−N(1), 94(2) [81.5]; Os−N(1)−C(1), 177.7(5) [178.1].…”