Heterolytic H-H and H-B Bond Cleavage Reactions of {(IPr)Ni(µ-S)}2.

Kinetic and DFT computational studies reveal that the reaction of {(IPr)Ni(µ-S)}2 (1, IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazolin-2-ylidene) with dihydrogen to produce {(IPr)Ni(µ-SH)}2 (2) proceeds by rate-limiting heterolytic addition of H2 across a Ni-S bond of intact dinuclear 1, followed by cis/trans isomerization at Ni and subsequent H migration from Ni to S, to produce the bis-hydrosulfide product 2. Complex 1 reacts in a similar manner with pinacolborane to produce {(IPr)Ni}2(µ-SH)(µ-SBPin) (3), showing that heterolytic activation by this nickel µ-sulfide complex can be generalized to other H-E bonds.

Synthesis and reactivity of NHC-supported Ni2(µ(2)-η(2),η(2)-S2)-bridging disulfide and Ni2(µ-S)2-bridging sulfide complexes.

The (IPr)Ni scaffold stabilizes low-coordinate, mononuclear and dinuclear complexes with a diverse range of sulfur ligands, including µ(2)-η(2),η(2)-S2, η(2)-S2, µ-S, and µ-SH motifs. The reaction of {(IPr)Ni}2(µ-Cl)2 (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with S8 yields the bridging disulfide species {(IPr)ClNi}2(µ(2)-η(2),η(2)-S2) (2). Complex 2 reacts with 2 equiv of AdNC (Ad = adamantyl) to yield a 1:1 mixture of the terminal disulfide compound (IPr)(AdNC)Ni(η(2)-S2) (3a) and trans-(IPr)(AdNC)NiCl2 (4a). 2 also reacts with KC8 to produce the Ni-Ni-bonded bridging sulfide complex {(IPr)Ni}2(µ-S)2 (6). Complex 6 reacts with H2 to yield the bridging hydrosulfide compound {(IPr)Ni}2(µ-SH)2 (7), which retains a Ni-Ni bond. 7 is converted back to 6 by hydrogen atom abstraction by 2,4,6-(t)Bu3-phenoxy radical. The 2,6-diisopropylphenyl groups of the IPr ligand provide lateral steric protection of the (IPr)Ni unit but allow for the formation of Ni-Ni-bonded dinuclear species and electronically preferred rather than sterically preferred structures.

Metal binding to ligand-containing peptide nucleic acids.

The substitution of nucleobases in nucleic acid duplexes with ligands that have high affinity for transition metal ions creates metal-binding sites at specific locations within the duplexes. Several studies on the incorporation of metal ions into DNA and peptide nucleic acid (PNA) duplexes have suggested that the stability constant of the metal complex formed within the duplexes is a primary determinant of the thermal stability of the duplexes. To understand this relationship, we have synthesized two PNA monomers that carry the same ligand, namely 8-hydroxyquinoline, but have this ligand attached differently to the PNA backbone. The PNA monomers have been incorporated into PNA duplexes. UV and CD spectroscopy and calorimetric studies of the 8-hydroxyquinoline-PNA duplexes showed that the effect of the stability of the metal complex on the PNA duplexes was significantly modulated by the steric relationship between the complex and the duplex. This information is useful for the construction of hybrid inorganic-nucleic acid nanostructures.