59th Annual Report on Research 2014

Since the start of the grant period we have made substantial progress on all of our specific aims.

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

Our preliminary studies focused on 1',2',3',4',5'-pentamethylazaferrocene (PMAF) and To date we have synthesized a variety of borane adducts of PMAF, 6MAF and HMAF.  These are shown in the scheme below and we have obtained crystal structures in several cases. The most striking results are the isolation and structural characterization of HMAF-B(C₆F₅)₃ which reveals considerable distortion of the HMAF moiety from a regular sandwich structure to accommodate the bulky triarylborane.  We have been unable to isolate HMAF-9BBNH presumably due to steric hinderance.  We have also found that lithiation of PMAF-BH₃ and 6MAF-BH₃ followed by addition of TMS-Cl leads to the silylated azaferrocene-boranes, TMS-PMAF-BH₃ and TMS-6MAF-BH₃.  The borane can also be easily removed to yield the free base.  Thus we have assembled a family of azaferrocenes with varying steric bulk at the α-positions of the pyrrolyl ring.

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

Hydride abstraction with trityl [B(C₆F₅)₄]^- from all of the azaferrocene-BH₃ adducts yields the borenium ions [azaferrocene-BH₂]⁺. Use of trityl BF₄^- leads to fluoride abstraction from the counterion. All of the [azaferrocene-BH₂]⁺ species are characterized by an ¹¹B NMR peak at around 40 ppm.  These ions have also been characterized by ¹H and ¹³C NMR spectroscopy.  Likewise, reaction of the 9-BBNH adducts of PMAF and 6MAF with trityl [B(C₆F₅)₄]^- form the corresponding borenium ions [azaferrocene-9BBN]⁺.  Both of these dialkylborenium ions are characterized by an ¹¹B NMR peak at around 70 ppm.  Chloride abstraction from azaferrocene-BCl₃ with AlCl₃ gave the corresponding [azaferrocene-BCl₂]⁺[AlCl₄]^- salts, and the PMAF-containing salt was characterized crystallographically.  All these –BCl₂⁺ containing borenium ions are characterized by peaks at 35-40 ppm in ¹¹B NMR.

We investigated the electronic structure of [PMAF-BH₂]⁺ by DFT calculations (performed by my colleague Prof. Shuhua Ma).  A dip angle of -23.7° of the B towards the Fe center was calculated and was shown to originate through quite delocalized interactions involving orbitals on the pyrrolyl and Cp rings mediated by Fe d-orbitals of appropriate symmetry.  Some calculated frontier orbitals are shown below (HOMO (left) through HOMO-4 right) with HOMO-2 and HOMO-4 illustrating the stabilization of the exocyclic B center.

Lewis acidity measurements were conducted using the Gutmann-Beckett method, and in a competition experiment to compare [PMAF-BH₂]⁺ with the neutral Lewis acid B(C₆F₅)₃ conducted using Et₃P=O, [PMAF-BH₂]⁺ was found to be more Lewis acidic.

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

Reaction of [PMAF-BH₂]⁺ with 1,5-cyclooctadiene (COD) gives [PMAF-9BBN]⁺ which was confirmed by NMR spectroscopy and independent synthesis of the product borenium ion as described above. When other [azaferrocene-BH₂]⁺ species with one or more α-pyrrolyl substituents react with COD, rapid dihydroboration occurs but ¹H (and in some cases ¹¹B NMR) spectroscopy revealed that two borenium ion species are formed.  We believe that these are the [3.3.1] and [4.2.1] isomers of 9-BBN resulting from the regioselectivity of the second hydroboration step.  There is a significant variation in isomer ratio with α-pyrrolyl substituent.  Further evidence of the products is provided by quenching experiments with ammonium borohydride.  Interestingly when 6MAF-9BBNH is subjected to hydride abstraction, the resulting borenium ion is also observed as a mixture of 2 isomers by NMR:  These results suggest that the barrier to dehydroboration is quite small allowing rapid thermal equilibration even at room temperature via a monohydroborated intermediate as shown in the scheme below.  Also, although the neutral 9-BBNH adduct of HMAF is not observable the [HMAF-9BBN]⁺ borenium ion can be observed by NMR as a mixture of isomers.

Related hydroboration reactions with isoprene were also performed and in these cases, dihydroboration does not appear to go to completion as a variety of mono- and di-hydroborated products are observed either directly after workup with ammonium borohydride, or indirectly after workup with methanol.  We have also found that HN(SO₃CF₃)₂ (triflimide or HNTf₂) will protonate the BH₃ adducts to form the tight ion pair species azaferrocene-BH₂-NTf₂ which also rapidly di hydroborate COD, to give the separated ion pair [azaferrocene-9BBN]+ NTf₂^-.  The isomer mixture obtained in the 6MAF case is very similar to that observed via trityl activation, suggesting that the active hydroborating agent is the same in each case.  We have also studied both modes of activation for the hydroboration of simple alkenes such as cyclooctene and α-methylstyrene.  The desired alcohols were observed after workup using alkaline hydropgen peroxide, however the complex product mixtures obtained suggest that electrophilic attack of the borenium ion on the alkene may compete with hydroboration in cases where a product carbocation may be electronically stabilized (for example α-methylstyrene or isoprene).  These systems are still under investigation particularly as to the nature of the byproducts.

 Aim (iv): Synthesis of a resolved, chiral azaferrocene, its -borane and –borenium species and investigation of asymmetric reactions

We have synthesized three azaferrocenes in racemic form, either as the free base or as the borane adduct – 6MAF, TMS-PMAF and TMS-6MAF.  We have utilized the method of Hansen et al. to resolve diastereomer of a chiral sulfoxide derivative of PMAF which we will then convert by known procedures into single enantiomer versions of these azaferrocenes.  We will then investigate their reactivity in asymmetric hydroboration based on some of our observations above.

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.  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.