60th Annual Report on Research 2015

Further progress on the first three of our specific aims is given for the final year of this grant.

Aim (i):  Synthesis of a representative group of metal-bound heteroarene-borane complexes

Our studies to date have focused on Cp*-containing azaferrocenes (Cp* = pentamethylcyclopentadienyl).  We have also now further examined a known derivative containing an unsubstituted Cp ring, DMAF (2,5-dimethylazaferrocene) – the synthesis of the borane-derivative, DMAF-BH₃ was previously reported by us.  We explored several alternate syntheses of DMAF, as the literature synthesis is a low-yielding cumbersome procedure, but were unable to improve upon it.  DMAF itself is an oil at room temperature but DMAF-BH₃ is a convenient-to-handle crystalline solid.  The attempted synthesis of the 9-bora[3.3.1]bicyclononane (9BBNH) derivative of DMAF was unsuccessful, presumably due to steric hinderance by the α-methyl groups as we have observed in other cases.  Attempts to form a Lewis pair of 9BBNH with our most sterically-hindered azaferrocene, containing a Cp* and a 2-methyl-5-trimethylsilylpyrrolyl ring (TMS-6MAF), yielded only TMS-6MAF-BH₃ suggesting that in this case, sterically-induced dehydroboration had occurred.  These observations are still under investigation.

Aim (ii):  Investigation of the formation of borenium ion species from these complexes

Hydride abstraction with trityl [B(C₆F₅)₄]^- from DMAF-BH₃ adducts yields the borenium ion [DMAF-BH₂]⁺, which like the Cp*-containing analogs is stable at room T in solution and was fully characterized by multinuclear NMR. 

 Aim (iii):  Survey of the reactivity of both –borane and –borenium complexes with respect to reduction, hydroboration and borylation chemistry

Reaction of [DMAF-BH₂]⁺ with 1,5-cyclooctadiene (COD) gives [DMAF-9BBN]⁺ as a mixture of [3.3.1] and [4.2.1] isomers, which was confirmed by quenching experiments with ammonium borohydride.  These observations are similar to those with the Cp*-containing borenium ions, and again we hypothesize that the second hydroboration is reversible leading to rapid equilibration of these species.  This area is still under investigation, as it points to interesting differences in hydroboration behavior between neutral boranes and borenium species.

Building on earlier results which have showed that triflimide (HNTf₂) gives very similar results to trityl cation as an activating agent in simple hydroboration studes, we have shown that only catalytic amounts (< 25 mol%) of HNTf₂ are required for 1-octene hydroboration in certain azaferrocene-BH₃ species.  We checked for background reactivity of the neutral azaferrocene-BH₃ adducts towards hydroboration activity, also using 1–octene as a substrate.  At room temperature in CH₂Cl₂ solution, only PMAF-BH₃ underwent any hydroboration yielding PMAF-B(oct)₃, but at a very slow rate, with the reaction complete after 24 hrs:  This contrasts with the rapid reaction of the borenium cations.  Those azaferrocenes with α-methyl groups (6MAF, DMAF and HMAF) were unreactive, implying that the increased strength of the B-N bond arising from pyrrolyl ring methylation is more important than any accompanying steric destabilization of the B-N bond.  This is opposite to what is observed in the pyridine/2,6-lutidine-BH₃ system. 

Impact of research

To date 5 undergraduate student researchers have worked on this project for credit as well as receiving summer research stipends for full time research.  One of these students is enrolled in a chemistry Ph. D. program, and a second is applying for chemistry graduate school in fall 2015.  We have published a full paper in Organometallics and have two further manuscripts in preparation.  The PI was part of a successful NSF-MRI grant that enabled purchase of a single crystal X-ray diffractometer to be used by a consortium of PUI’s in the Maryland/Mid-Atlantic region, in part based on some of the work presented here.