Reactivity of a Borane-Appended 1,1′-Bisphosphinoferrocene Ligand: Transition Metal and Lewis Acid Coordination, Cyclopentadienyl Ring Borylation, and Boronium Cation Generation

Synthesis and characterization of NiAl-hydride heterometallics: perturbing electron density within Al-H-Ni subunits

Heterometallic hydride complexes are of growing interest due to their potential to contribute to highly active insertion-based catalysis; however, methods to modulate electron density within this class of molecules are underexplored. Addition of ancillary ligands to heterotrimetallic NiAl2H2 species (1) results in the formation of heterobimetallic NiAl-hydride complexes with varying phosphine donors (2-(L)2). Incorporation of sigma donating ancillary ligands of increasing strength led to contractions of the Ni-Al distances correlated to a strengthening of a back donation interaction to the Al-H sigma antibonding orbital, most prominently present in 2-(PMe3)2. Demethylation of the aryl ether from 2-(PMe3)2 provides access to a novel anionic nickel-aluminum complex (3) with a maintained bridged hydride moiety between Ni and Al. Increased negative charge in complex 3 results in an elongation of the Ni-Al interaction. Combined crystallographic, spectroscopic, and computational studies support a 3-center interaction within the Al-H-Ni subunits and were used to map the degree of Ni-H character of the series within the Al-H-Ni bonding continuum.

Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C-Cl bond activation

Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd⁰ complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (pSpR, meso-isomer) and 1b (pSpS-isomer) differ in the orientation of the ferrocene moieties relative to the central Ph₂PC₅H₃-Hg-C₅H₃PPh₂ bridging entity. The structurally distinct trigonal LPd⁰(dba) complexes 2a (meso) and 2b (pSpS) are obtained upon treatment with Pd(dba)₂. A competition reaction reveals that 1b reacts faster than 1a with Pd(dba)₂. Unexpectedly, catalytic interconversion of 1a (meso) into 1b (rac) is observed at room temperature in the presence of only catalytic amounts of Pd(dba)₂. Both Pd⁰ complexes, 2a and 2b, readily undergo oxidative addition into the C-Cl bond of CH₂Cl₂ at moderate temperatures with formation of the square-planar trans-chelate complexes LPd^IICl(CH₂Cl) (3a, 3b). Kinetic studies reveal a significantly higher reaction rate for the meso-isomer 2a in comparison to (pSpS)-2b.

The core of the matter - arene substitution determines the coordination and catalytic behaviour of tris(1-phosphanyl-1'-ferrocenylene)arene gold(I) complexes

Changing the aromatic core of C3-symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound effects on their coordination behaviour towards gold(i). Depending on the arene (s-triazine, benzene, or trifluorobenzene), four different coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed fluctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.

Isolation of ligand-centered borocations in molybdenum complexes containing a triaminoborane-bridged diphosphorus ligand

Metal complexes that form isolable, ligand-centered borenium ions (i.e. reactive three-coordinate boron cations) are rare, especially with highly-versatile diphosphorus ligands. Here we report the first structurally-characterized examples of ligand-centered borocations in a class of diphosphorus ligands derived from the bicyclic triaminoborane 1,8,10,9-triazaboradecalin (TBD). Treating (PhTBDPhos)Mo(CO)4 (1) with HOTf or HNTf2 resulted in protonation of the bridgehead α-nitrogen on the TBD backbone and formation of ligand-centered borenium ions (1-HOTf and 1-HNTf2, respectively), whereas reaction of 1 with HBF4·Et2O resulted in protonation of two α-nitrogen atoms and fluoride abstraction from BF4- to form a four-coordinate borocation in [1-H2F][BF4]. Single-crystal XRD data confirmed the formation of the borocations, and multinuclear NMR and IR spectroscopy studies were used to interrogate the electronic environment at molybdenum and boron. The 11B NMR resonances for the borenium ions in 1-HOTf and 1-HNTf2 (δ 29.5 and 29.6 ppm) were more deshielded than the resonance for 1 (δ 25.9 ppm), consistent with the decreased electron density at boron. The 31P NMR data revealed similar trends in response to increasing protonation on the TBD backbone, and aligned well with small, stepwise increases in Mo-CO stretching frequencies that followed the order borane (1) < borenium (1-HOTf and 1-HNTf2) < boronium ([1-H2F][BF4]). Density functional theory calculations conducted on 1, 1-HOTf, and [1-H2F][BF4] revealed subtle changes in boron and nitrogen atomic charges consistent with those calculated for more well-established borenium ions in metal-free systems. Overall, the results confirm previous observations of latent borenium ion reactivity in TBDPhos complexes.