Research Highlights

Spontaneous Dehydrocoupling in Peri-Substituted Phosphine-Borane Adducts
Laurence J. Taylor, Brian A. Surgenor, Piotr Wawrzyniak, Matthew J. Ray, David B. Cordes, Alexandra M. Z. Slawin and Petr Kilian*
Dalton Trans. 2016, 45, 1976–1986 (invited contribution to themed issue “Phosphorus Chemistry: Discoveries and Advances”)

Peri-substituted bis(borane) adducts undergo spontaneous intramolecular dehydrocoupling in solution, resulting in the formation of a P–B bond. This reaction is surprisingly facile, occurring at room temperature and in the absence of a catalyst (albeit in some cases at a slow rate). The ease with which the reaction proceeds can be attributed to the unique constraints of the peri-geometry; the two reactive moieties are held in close proximity and the repulsive interaction between them introduces considerable strain into the system, which is reduced on formation of a bridging P–B–P motif. This interesting reaction serves as a demonstration of the utility of peri-substitution for promoting unusual or unexpected reactivity.

2016 Dalton

For more details see DOI: 10.1039/C5DT02539G OPEN ACCESS

A Structural, Spectroscopic and Computational Examination of the Dative Interaction in Constrained Phosphine–Stibines and Phosphine–Stiboranes
Brian A. Chalmers, Michael Bühl, Kasun S. Athukorala Arachchige, Alexandra M. Z. Slawin, Petr Kilian*
Chem. Eur. J. 2015, 21, 7520–7531

Phosphine-stiboranes are normally redox unstable. Attaching their two pnictogen centres to peri-positions of an acenaphthene allowed isolation of the first examples of such σ4P→σ6Sb structural motif. This broadens the scope of motifs that are stabilised by peri-substitution towards hypervalent redox unstable motifs; expanding upon well known low-valent and (normally unstable) normal valence motifs.

Furthermore, this work supports the notion that chemical bonding entails a “spectrum” of interactions rather than a simple bonding/repulsion (binary situation). We observed that the strength of a dative phosphine–stiborane interaction increases with stepwise replacement of phenyl groups on antimony atom with chloride groups. As the Lewis acidity is increased in regular steps, essentially linear response is observed initially, however then a sudden change in the P–Sb distance takes place during one particular step. This is consistent with a sudden switch from a non-bonding to a bonding interaction, i.e. a discrete rather than continuum response  triggered by a relatively small change in electronic structure.
For more details see DOI: 10.1002/chem.201500281

Geometrically Enforced Donor-Facilitated De-hydrocoupling Leading to an Isolable Arsanylidene-Phosphorane
Brian A Chalmers, Michael Bühl, Kasun S. Athukorala Arachchige, Alexandra M. Z. Slawin, Petr Kilian*
J. Am. Chem. Soc. 2014, 136, 6247–6250 (communication)

A proximate -PiPr2 Lewis basic group facilitates mild dehydrogenative P–As intramolecular coupling in the phosphine-arsine peri-substituted acenaphthene, affording the first room temperature stable arsanylidene-phosphorane. The transformation shown in the figure below is thermoneutral due to the dehydrogenation being concerted with the donor coordination.


For more details see DOI: 10.1021/ja502625z OPEN ACCESS

Isolable Phosphanylidene Phosphorane with a Sterically Accessible Two-Coordinate Phosphorus Atom
Brian A. Surgenor, Michael Bühl, Alexandra M. Z. Slawin, J. Derek Woollins, Petr Kilian*
Angew. Chem. Int. Ed. 2012, 51, 10150 –10153 (communication)

Phosphanylidene phosphoranes are phosphorus analogues of familiarly known Wittig reagent. Only very few examples that are isolable (“bottleable”) are known. We report the first example of phosphanylidene phosphorane stabilized by a rigid peri-substituted backbone. The special peri-geometry results in the two co-ordinate phosphorus atom being sterically unshielded (“naked”), yet the compound is stable up to temperature of ca 100°C.

Reaction with Pd0 complex reveals that our compound acts as a source of phosphinidene (RP) moiety, the chemistry of which attracts much current attention as phosphorus analogue of carbene.


For more details see DOI: 10.1002/anie.201204998 OPEN ACCESS

1,2-Diphosphaacenaphthene 1,2-Dications: Synthetic, Stereochemical and Computational Study of the Stabilising Role of Naphthalene-1,8-diyl Backbone
Upulani K. Somisara, Michael Bühl, Tomas Lebl, Neville V. Richardson, Alexandra M. Z. Slawin, J. Derek Woollins and Petr Kilian*
Chem. Eur. J. 2011, 17, 2666 – 2677 (full paper)

1,2-Dications (compounds with two positive charges on adjacent atoms) are still considered as chemical curiosities. We report synthesis and full characterisation (incl. single crystal X-ray diffraction) of two diastereomeric forms of 1,2-diphosphonium, a phosphorus 1,2-dication containing peri-substituted naphthalene motif. The reported compounds are the first examples of chiral 1,2-diphosphoniums. We have shown that their stereochemistry can be manipulated by the choice of (coordinating) anion, and that the epimerisation is accelerated in the presence of fluoride anion. Computational studies highlight the significant contribution of the naphthalene ring in stabilising the 1,2-diphosphoniums with respect to related (non-rigid) C3-aliphatic backbone.

Assessment of the energy cost of the steric clash of a variety of groups attached to the peri-positions of the naphthalene backbone is included in the paper. This allows making predictions which per-substituted species are unlikely to be synthesised by standard P-C coupling reactions.


Figure shows rac– and meso-forms of peri-naphthalene 1,2-diphosphoniums.

For more details see DOI: 10.1002/chem.201002259

Phosphorus Peri-Bridged Acenaphthenes: Efficient Syntheses, Characterisation and Quaternization Reactions
Piotr Wawrzyniak, Alexandra M. Z. Slawin, Amy L. Fuller, J. Derek Woollins and Petr Kilian*
Dalton Trans. 2009, 7883-7884 (communication)

 Only very few phosphorus (single-atom) peri-bridged naphthalenes (and acenaphthenes) are known. In this paper, we describe a new synthetic pathway to these intriguing compounds, and report several crystal structures. These peri-bridged species contain four-membered heterocycle C3P in which all three carbon atoms are part of the aromatic system (i.e. they are sp2 hybridised). The sp2 hybridisation is normally synonymous with approximately 120 degrees bond angles. In our species these angles are much distorted, to less than 90 degrees. This angular strain results in significant reactivity of these species, with propensity to ring expansion and ring opening reactions.

Dalton 2009

Figure shows phosphorus single-atom peri-bridged acenaphthene with the strained four membered ring highlighted in red.

For more details see DOI: 10.1039/b916166j

Intramolecular Phosphine-Phosphine Donor-Acceptor Complexes
Piotr Wawrzyniak, Amy L. Fuller, Alexandra M. Z. Slawin and Petr Kilian*
Inorg. Chem. 2009, 48, 2500-2506 (full paper).

Phosphines (PR3) are considered prototypical donors because of the presence of the lone pair which is readily accepted by a range of transition metals as well as many main group Lewis acids. Although much less common, molecules in which phosphine acts as an acceptor do exist.

Compounds containing a combination of phosphine donor – phosphine acceptor are usually highly unstable and decompose well below room temperature. The main achievement described in this paper is the preparation of a room temperature stable intramolecular complex, in which two different phosphine functionalities (ArPiPr2 and ArPCl2) act as a donor and as an acceptor, respectively. The fixed peri-region geometry appears to contribute to stabilization of this unusual bonding situation.

1new combined

Figure shows resonance structures and X-ray crystal structure of the principal compound. Left: phosphine-phosphine structure. Middle: Structure depicting the donor-acceptor interaction (phosphonium-phosphoranide structure), this structure was found to be present in the crystal.

For more details see DOI: 10.1021/ic801833a

1,8,9-Substituted Anthracenes, Intramolecular Phosphine Donor Stabilized Metaphosphonate and Phosphenium
Petr Kilian* and Alexandra M. Z. Slawin
Dalton Trans. 2007, 3289-3296 (full paper).

A series of 1,8,9-tris(phosphorus) substituted anthracenes was synthesised and fully characterised. The substitution pattern in these molecules is a good template for introducing unusual P→P interactions. Starting from highly crowded bis(amino) phosphine (pictured below), metaphosphonato (single phosphine donor stabilised) as well as phosphenium (doubly phosphine donor stabilised) environments were synthesised.

Dalton2007 Figure shows X-ray structure of the crowded bis(amino) phosphine precursor (top), phosphine donor stabilised metaphosphonate (bottom left) and doubly phosphine donor stabilised phosphenium (bottom right).

For more details see DOI: 10.1039/b705286c