Relative Hydride, Proton, and Hydrogen Atom Transfer Abilities of [HM(diphosphine)₂]PF₆Complexes (M = Pt, Ni)

Abstract

A series of [M(diphosphine)2]X2, [HM(diphosphine)2]X, and M(diphosphine)2 complexes have been prepared for the purpose of determining the relative thermodynamic hydricities of the [HM(diphosphine)2]X complexes (M = Ni, Pt; X = BF4, PF6; diphosphine = bis(diphenylphosphino)ethane (dppe), bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)ethane (dmpe), bis(dimethylphosphino)propane (dmpp)). Measurements of the half-wave potentials (E 1/2) for the M(II) and M(0) complexes and pK a measurements for the metal hydride complexes have been used in a thermochemical cycle to obtain quantitative thermodynamic information on the relative hydride donor abilities of the metal−hydride complexes. The hydride donor strengths vary by 23 kcal/mol and are influenced by the metal, the ligand substituents, and the size of the chelate bite of the diphosphine ligand. The best hydride donor of the complexes prepared is [HPt(dmpe)2](PF6), a third-row transition metal with basic substituents and a diphosphine ligand with a small chelate bite. The best hydride acceptors have the opposite characteristics. X-ray diffraction studies were carried out on eight complexes: [Ni(dmpe)2](BF4)2, [Ni(depe)2](BF4)2, [Ni(dmpp)2](BF4)2, [Pt(dmpp)2](PF6)2, [Ni(dmpe)2(CH3CN)](BF4)2, [Ni(dmpp)2(CH3CN)](BF4)2, Ni(dmpp)2, and Pt(dmpp)2. The cations [Ni(dmpp)2]2+ and [Pt(dmpp)2]2+ exhibit significant tetrahedral distortions from a square-planar geometry arising from the larger chelate bite of dmpp compared to that of dmpe. This tetrahedral distortion produces a decrease in the energy of the lowest unoccupied molecular orbital of the [M(dmpp)2]2+ complexes, stabilizes the +1 oxidation state, and makes the [HM(dmpp)2]+ complexes poorer hydride donors than their dmpe analogues. Another interesting structural feature is the shortening of the M−P bond upon reduction from M(II) to M(0).

Homogeneous CO Hydrogenation: Dihydrogen Activation Involves a Frustrated Lewis Pair Instead of a Platinum Complex

“…NMR analysis after 20 minutes revealed complete conversion to a new symmetric species ( 31 P δ 2.1). IR spectroscopy (THF solution) showed a single CO stretch at 1990 cm -1 (s), shifted slightly from 1998 cm -1 in [1][BF 4 ], consistent with a tetracarbonyl structure (PhO -stretches were observed at 1585 and 1490 cm -1 ). 11 B NMR showed a broad resonance at δ -0.1, but neither a downfield resonances consistent with 3-coordinate boron, nor a [BF 4 ] -resonance was observed.…”

“…IR spectroscopy (THF solution) showed a single CO stretch at 1990 cm -1 (s), shifted slightly from 1998 cm -1 in [1][BF 4 ], consistent with a tetracarbonyl structure (PhO -stretches were observed at 1585 and 1490 cm -1 ). 11 B NMR showed a broad resonance at δ -0.1, but neither a downfield resonances consistent with 3-coordinate boron, nor a [BF 4 ] -resonance was observed. Taken together, these data are consistent with phenoxide addition to the borane of [1] + to give a zwitterionic borate species, which would be anionic (accounting for the slight IR shift to lower energy).…”

“…S4 Figure 4 ], 11.3 mg (0.085 mmol) KOPh, and ~0.6 mL THF-d 8 . Some fine precipitates were visible, although the bulk of the KOPh (mostly insoluble in THF) seemed to dissolve.…”

The roles of the first and second coordination spheres in the design of molecular catalysts for H2production and oxidation

“…11,21,22,28,29 Fig. 2 shows a plot of the hydride donor ability for a series of [HNi(diphosphine) 2 ] + complexes vs. the sum of the inductive and resonance parameters 30 for the different substituents R. A plot of the pK a values for this series of complexes versus the sum of the same ligand parameters shows less scatter and a higher r 2 value.…”

“…However, more polar M-H bonds will exhibit larger substituent effects for homolytic bond dissociation free energies than less polar M-H bonds. 11,29 2.2 Relationships between natural bite angles, dihedral angles, and hydride donor/acceptor abilities Studies of the influence of the steric properties of the diphosphine ligand on reaction (6) have shown that the geometry of the four-coordinate, sixteen-electron product, 5, plays a major role in determining the hydride donor ability of [HM(diphosphine) 2 ] + complexes. One geometric parameter is the natural bite angle (NBA) of the diphosphine ligand, which is the P-M-P angle the ligand would prefer to impose on the metal (b in view 1 of structure 5).…”