[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

Unexpected Vulnerability of DPEphos to C-O Activation in the Presence of Nucleophilic Metal Hydrides

C-O bond activation of DPEphos occurs upon mild heating in the presence of [Ru(NHC)2 (PPh3 )2 H2 ] (NHC=N-heterocyclic carbene) to form phosphinophenolate products. When NHC=IEt2 Me2 , C-O activation is accompanied by C-N activation of an NHC ligand to yield a coordinated N-phosphino-functionalised carbene. DFT calculations define a nucleophilic mechanism in which a hydride ligand attacks the aryl carbon of the DPEphos C-O bond. This is promoted by the strongly donating NHC ligands which render a trans dihydride intermediate featuring highly nucleophilic hydride ligands accessible. C-O bond activation also occurs upon heating cis-[Ru(DPEphos)2 H2 ]. DFT calculations suggest this reaction is promoted by the steric encumbrance associated with two bulky DPEphos ligands. Our observations that facile degradation of the DPEphos ligand via C-O bond activation is possible under relatively mild reaction conditions has potential ramifications for the use of this ligand in high-temperature catalysis.

Counterion Dependence of Dinitrogen Activation and Functionalization by a Diniobium Hydride Anion

We report the synthesis of anionic diniobium hydride complexes with a series of alkali metal cations (Li⁺ , Na⁺ , and K⁺ ) and the counterion dependence of their reactivity with N₂ . Exposure of these complexes to N₂ initially produces the corresponding side-on end-on N₂ complexes, the fate of which depends on the nature of countercations. The lithium derivative undergoes stepwise migratory insertion of the hydride ligands onto the aryloxide units, yielding the end-on bridging N₂ complex. For the potassium derivative, the N-N bond cleavage takes place along with H₂ elimination to form the nitride complex. Treatment of the side-on end-on N₂ complex with Me₃ SiCl results in silylation of the terminal N atom and subsequent N-N bond cleavage along with H₂ elimination, giving the nitride-imide-bridged diniobium complex.

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity

The photoexcitation of a triangular silver(I) hydride complex, [Ag₃ (μ₃ -H)(μ₂ -dcpm)₃ ](PF₆ )₂ ([P](PF₆ )₂ , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag₃ single and double electron transfer. The nature of the electronic transitions has been authenticated by absorption and photodissociation spectroscopy in parallel with high-level quantum-chemical computations utilizing the GW method and Bethe-Salpeter equation (GW-BSE). Specific photofragments of mass-selected [P]²⁺ ions testify to charge transfer and competing pathways resulting from the unique [Ag₃ (μ₃ -H)]²⁺ scaffold. This structural motif of [P](PF₆ )₂ has been unequivocally verified by ¹ H NMR spectroscopy in concert with DFT and X-ray diffraction structural analysis, which revealed short equilateral Ag-Ag distances (d_AgAg =3.08 Å) within the range of argentophilic interactions. The reduced radical cation [P]^{. +} exhibits strong oxophilicity, forming [P+O₂ ]^.+ ,which is a model intermediate for silver oxidation catalysis.

Facile Access to Unprecedented Electron-Precise Monohydrodiboranes(4), cis-1,2-Dihydrodiboranes(4), and a 1,1-Dihydrodiborane(5)

2,3-Bis(dimethylamino)-substituted B₂ N₂ C₂ heterocycles underwent selective dimethylamino/hydride exchange with either one or two equivalents of BH₃ ⋅SMe₂ to give the corresponding cyclic monohydrido- or (cis)1,2-dihydridodiboranes(4), respectively. Upon either heating or irradiation in solution, the latter underwent ring contraction to the corresponding five-membered BN₂ C₂ heterocycles, whereas irradiation of the 1,2-dimethylaminoethene-supported 1,2-dihydridodiborane(4) in the presence of PEt₃ gave an unprecedented unsymmetrical 1,1-dihydrodiborane(5) phosphine adduct.

Characterization of a palladium dihydrogen complex

The preparation and isolation of the first palladium dihydrogen complex is described. NMR spectroscopy reveals a very short H-H bond length, but the hydrogen molecule is activated toward heterolytic cleavage. An X-ray crystal structure suggests that proton transfer to the (tBu) PCP (κ(3)-2,6-((t)Bu2PCH2)2C6H3) pincer ligand is possible. The basicity of the ipso-carbon atom of the pincer ligand was investigated in a related complex.