3 research outputs found
Reactivity of (Triphos)FeBr<sub>2</sub>(CO) towards sodium borohydrides
<p>The addition of CO to (Triphos)FeBr<sub>2</sub> (Triphos = PhP(CH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>) resulted in formation of six-coordinate (Triphos)FeBr<sub>2</sub>(CO). This coordination compound was found to have <i>cis</i>-bromide ligands and a <i>mer</i>-Triphos ligand by single crystal X-ray diffraction. Once characterized, the reactivity of this compound toward NaEt<sub>3</sub>BH and NaBH<sub>4</sub> was investigated. Adding 1 eq. of NaEt<sub>3</sub>BH to (Triphos)FeBr<sub>2</sub>(CO) resulted in formation of (Triphos)FeH(Br)(CO), while the addition of 2.2 eq. afforded previously described (Triphos)Fe(CO)<sub>2</sub>. In contrast, adding 2.2 eq. of NaBH<sub>4</sub> to (Triphos)FeBr<sub>2</sub>(CO) resulted in carbonyl dissociation and formation of diamagnetic (Triphos)FeH(<i>η</i><sup>2</sup>-BH<sub>4</sub>), which has been structurally characterized. Notably, efforts to prepare (Triphos)FeH(<i>η</i><sup>2</sup>-BH<sub>4</sub>) following 2.2 eq. NaBH<sub>4</sub> addition to (Triphos)FeBr<sub>2</sub> were unsuccessful. The importance of these observations as they relate to previously reported (Triphos)Fe reactivity and recent developments in Fe catalysis are discussed.</p
Mechanistic Investigation of Bis(imino)pyridine Manganese Catalyzed Carbonyl and Carboxylate Hydrosilylation
We
recently reported a bisÂ(imino)Âpyridine (or pyridine diimine, PDI)
manganese precatalyst, (<sup>Ph2PPr</sup>PDI)Mn (<b>1</b>),
that is active for the hydrosilylation of ketones and dihydrosilylation
of esters. In this contribution, we reveal an expanded scope for <b>1</b>-mediated hydrosilylation and propose two different mechanisms
through which catalysis is achieved. Aldehyde hydrosilylation turnover
frequencies (TOFs) of up to 4900 min<sup>–1</sup> have been
realized, the highest reported for first row metal-catalyzed carbonyl
hydrosilylation. Additionally, <b>1</b> has been shown to mediate
formate dihydrosilylation with leading TOFs of up to 330 min<sup>–1</sup>. Under stoichiometric and catalytic conditions, addition of PhSiH<sub>3</sub> to (<sup>Ph2PPr</sup>PDI)Mn was found to result in partial
conversion to a new diamagnetic hydride compound. Independent preparation
of (<sup>Ph2PPr</sup>PDI)ÂMnH (<b>2</b>) was achieved upon adding
NaEt<sub>3</sub>BH to (<sup>Ph2PPr</sup>PDI)ÂMnCl<sub>2</sub> and single-crystal
X-ray diffraction analysis revealed this complex to possess a capped
trigonal bipyramidal solid-state geometry. When 2,2,2-trifluoroacetophenone
was added to <b>1</b>, radical transfer yielded (<sup>Ph2PPr</sup>PDI<b>·</b>)ÂMnÂ(OC<b>·</b>(Ph)Â(CF<sub>3</sub>)) (<b>3</b>), which undergoes intermolecular C–C bond
formation to produce the respective MnÂ(II) dimer, [(μ-<i>O</i>,<i>N</i><sub>py</sub>-4-OCÂ(CF<sub>3</sub>)Â(Ph)-4-H-<sup>Ph2PPr</sup>PDI)ÂMn]<sub>2</sub> (<b>4</b>). Upon finding <b>3</b> to be inefficient and <b>4</b> to be inactive, kinetic
trials were conducted to elucidate the mechanisms of <b>1</b>- and <b>2</b>-mediated hydrosilylation. Varying the concentration
of <b>1</b>, substrate, and PhSiH<sub>3</sub> revealed a first
order dependence on each reagent. Furthermore, a kinetic isotope effect
(KIE) of 2.2 ± 0.1 was observed for <b>1</b>-catalyzed
hydrosilylation of diisopropyl ketone, while a KIE of 4.2 ± 0.6
was determined using <b>2</b>, suggesting <b>1</b> and <b>2</b> operate through different mechanisms. Although kinetic trials
reveal <b>1</b> to be the more active precatalyst for carbonyl
hydrosilylation, a concurrent <b>2</b>-mediated pathway is more
efficient for carboxylate hydrosilylation. Considering these observations, <b>1</b>-catalyzed hydrosilylation is believed to proceed through
a modified Ojima mechanism, while <b>2-</b>mediated hydrosilylation
occurs via insertion
Carbon Dioxide Promoted H<sup>+</sup> Reduction Using a Bis(imino)pyridine Manganese Electrocatalyst
Heating a 1:1 mixture of (CO)<sub>5</sub>MnBr and the phosphine-substituted pyridine diimine ligand, <sup>Ph2PPr</sup>PDI, in THF at 65 °C for 24 h afforded the diamagnetic
complex [(<sup>Ph2PPr</sup>PDI)ÂMnÂ(CO)]Â[Br] (<b>1</b>). Higher
temperatures and longer reaction times resulted in bromide displacement
of the remaining carbonyl ligand and the formation of paramagnetic
(<sup>Ph2PPr</sup>PDI)ÂMnBr (<b>2</b>). The molecular structure
of <b>1</b> was determined by single crystal X-ray diffraction,
and density functional theory (DFT) calculations indicate that this
complex is best described as low-spin MnÂ(I) bound to a neutral <sup>Ph2PPr</sup>PDI chelating ligand. The redox properties of <b>1</b> and <b>2</b> were investigated by cyclic voltammetry (CV),
and each complex was tested for electrocatalytic activity in the presence
of both CO<sub>2</sub> and Brønsted acids. Although electrocatalytic
response was not observed when CO<sub>2</sub>, H<sub>2</sub>O, or
MeOH was added to <b>1</b> individually, the addition of H<sub>2</sub>O or MeOH to CO<sub>2</sub>-saturated acetonitrile solutions
of <b>1</b> afforded voltammetric responses featuring increased
current density as a function of proton source concentration (<i>i</i><sub>cat</sub>/<i>i</i><sub>p</sub> up to 2.4
for H<sub>2</sub>O or 4.2 for MeOH at scan rates of 0.1 V/s). Bulk
electrolysis using 5 mM <b>1</b> and 1.05 M MeOH in acetonitrile
at −2.2 V vs Fc<sup>+/0</sup> over the course of 47 min gave
H<sub>2</sub> as the only detectable product with a Faradaic efficiency
of 96.7%. Electrochemical experiments indicate that CO<sub>2</sub> promotes <b>1</b>-mediated H<sub>2</sub> production by lowering
apparent pH. While evaluating <b>2</b> for electrocatalytic
activity, this complex was found to decompose rapidly in the presence
of acid. Although modest H<sup>+</sup> reduction activity was realized,
the experiments described herein indicate that care must be taken
when evaluating Mn complexes for electrocatalytic CO<sub>2</sub> reduction