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Π‐Complexes of Diborynes with Main Group Atoms

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Π‐Complexes of Diborynes with Main Group Atoms DOI: 10.1002/asia.202000185 Communication 1 2 3 π-Complexes of Diborynes with Main Group Atoms 4 [a, b] [a, b] [c] [d] 5 William C. Ewing, Theresa Dellermann, Y. T. Angel Wong, James D. Mattock, [d] [c] [a, b] [a, b] 6 Alfredo Vargas, David L. Bryce, Rian D. Dewhurst, and Holger Braunschweig* 7 8 metal to vacant π* orbitals on the multiple bond. In 1957, 9 Abstract: We present herein an in-depth study of com- Joseph Chatt discussed the use of this model as it pertains to 10 plexes in which a molecule containing a boron-boron triple the complexation of acetylenes to platinum.[3] Vibrational 11 bond is bound to tellurate cations. The analysis allows the spectroscopy performed on the studied complexes, triple-bond 12 description of these salts as true π complexes between the adducts of [Pt(PPh ) ], hinted at sizeable reductions in the bond 13 BÀ B triple bond and the tellurium center. These complexes 3 2 orders of the bound acetylenes. After noting this, Chatt 14 thus extend the well-known Dewar-Chatt-Duncanson mod- discussed two possible bonding schemes, a donation/back- 15 el of bonding to compounds made up solely of p block donation model where the contribution of the backdonation 16 elements. Structural, spectroscopic and computational (d !π* ) outweighed the σ-donation of the alkyne (π ! 17 evidence is offered to argue that a set of recently reported Pt C�C C�C d ) and a metallacyclopropene model featuring two PtÀ C two- 18 heterocycles consisting of phenyltellurium cations com- Pt center-two-electron bonds with a formal double bond between 19 plexed to diborynes bear all the hallmarks of π-complexes the carbon atoms. Though he favored the latter, he deftly 20 in the π-complex/metallacycle continuum envisioned by hedged his bet by commenting that the orbitals existed for 21 Joseph Chatt. Described as such, these compounds are “one structure to take on part of the character of the other”. In 22 unique in representing the extreme of a metal-free doing so, he allowed for the idea that the bonding in Pt-alkyne 23 continuum with conventional unsaturated three-membered complexes, and hence all such metal-alkyne complexes, may fall 24 rings (cyclopropenes, azirenes, borirenes) occupying the on a continuum ranging between pure π-complexes, where the 25 opposite end. bonding is primarily π !d in nature with negligible 26 (C�C) (M) backdonation, and metallacycles where backdonation domi- 27 nates and the orbitals at the alkynyl carbon may be appropri- 28 The side-on complexation of alkenes and alkynes to transition ately viewed as rehybridized toward the formation of two new 29 metals is generally described in the language of the Dewar- CÀ M bonds. 30 Chatt-Duncanson (DCD) bonding model[1,2] as an interplay Figure 1 depicts a few examples of organometallic com- 31 between σ-donation from the π-system of the multiple bond pounds favoring either the π-complex description or the 32 and π-symmetry backdonation from filled d-orbitals on the metallacyclic formulation. The π-complexes described here are 33 generally the result of the bonding of alkynes to high-valent 34 (e.g., Pt2+, Au3+)[4–5] metals or compact, hard metals (Ag+)[6] 35 [a] Dr. W. C. Ewing, Dr. T. Dellermann, Dr. R. D. Dewhurst, unlikely to redistribute d-electrons. The metallacycles[3,7–9] are 36 Prof. Dr. H. Braunschweig alternatively composed of electron rich, low-valent metals, 37 Institute for Inorganic Chemistry which readily participate in d !π* backbonding. 38 Julius-Maximilians-Universität Würzburg (M) (C�C) Am Hubland, 97074 Würzburg (Germany) Despite the current ubiquity of the continuum model for 39 [b] Dr. W. C. Ewing, Dr. T. Dellermann, Dr. R. D. Dewhurst, describing organometallic cyclics, it is not frequently employed 40 Prof. Dr. H. Braunschweig to describe metal-free systems. The smallest unsaturated three- 41 Institute for Sustainable Chemistry & Catalysis with Boron membered rings (cyclopropenes, azirenes, borirenes) are in- 42 Julius-Maximilians-Universität Würzburg Am Hubland, 97074 Würzburg (Germany) stead most commonly described by valence bond theory as the 43 E-mail: [email protected] product of overlapping hybrid orbitals, bent so as to accom- 44 [c] Y. T. Angel Wong, Prof. Dr. D. L. Bryce modate the imposed geometry of the small ring.[10] Though 45 Department of Chemistry and Biomolecular Sciences University of Ottawa useful in practice and pedagogy, this model is not without its 46 Ottawa, Ontario, K1N 6N5 (Canada) drawbacks, and as such, alternative views on the bonding 47 [d] Dr. J. D. Mattock, Dr. A. Vargas environment have been offered ranging from σ-bond 48 Department of Chemistry delocalization[11] to π-bridged σ-bonding.[12] Though lacking 49 School of Life Sciences University of Sussex accessible d-orbitals with which to participate in d(M)!π*(C�C) 50 Brighton BN1 9QJ, Sussex (UK) backbonding, main group elements in such unsaturated three 51 Supporting information for this article is available on the WWW under membered rings have a p-orbital lying in the plane of the 52 https://doi.org/10.1002/asia.202000185 adjacent multiple bond with the correct symmetry for p!π* 53 This manuscript is part of a special collection for the 20th Anniversary of the Tateshina Conference. backbonding.[13] If viewed as part of the DCD-continuum, small 54 © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This rings such as cyclopropene would constitute the extreme re- 55 is an open access article under the terms of the Creative Commons Attri- hybridized end of the spectrum; however, such a supposition 56 bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. requires examples of the other extreme, a metal-free π-complex 57 Chem Asian J. 2020, 15, 1553–1557 1553 © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA Wiley VCH Donnerstag, 07.05.2020 2010 / 163086 [S. 1553/1557] 1 Communication 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Figure 1. Examples of three-membered cyclics exhibiting either π-complex Figure 2. a) Solid-state structure of cationic portions of 1 and 2. Thermal 27 or metallacyclic characteristics. IDip=1,3-di(2,6-diisopropylphenyl)imidazol- ellipsoids represent 50% probability, and have been omitted from the ligand 28 2-ylidene. R=alkyl, aryl, H. See text for related references. periphery. For clarity, all hydrogen atoms are likewise omitted. Selected bond lengths (Å) and angles (°): (1) B1–B2 1.490(6), C1À B1À B2 165.0(4), 29 C2À B2À B1 164.1(3); (2) B1À B2 1.494(10), C1À B1À B2 164.7(7), C2À B2À B1 30 163.6(7). b) Structural metrics relevant to complexation in the DCD model. c) 31 Table of structural measures of compounds on the π-complex/metallacycle where the bound alkyne maintains the majority of its triple- continuum. IMeMe =1,3,4,5-tetramethylimidazol-2-ylidene. For references, 32 bond character. see text. 33 A number of recent examples have emerged of three- 34 membered heterocycles formed via the reaction of elemental 35 chalcogens with disilenes (Si=Si double bonds) with structural Figure 2b) and substantial cis-bending away from the linear 36 features eliciting their description as π-complexes,[14] but such structure (α, Figure 2b). When the distances between the boron 37 complexes formed with triple bonds are still unknown. Upon atoms in 1 and 2 were measured (1, 1.490(6) Å; 2, 1.494(10) Å) 38 complexation to neutral metals (Pt and Pd), disilynes (RSi�SiR) and compared to the same distance in the uncomplexed 39 perhaps unsurprisingly form compounds identified as metalla- diboryne (1.449(3) Å),[17] difference (Δd) values of 0.041 Å (1) 40 cycles by the substantial structural differences between bound and, 0.045 Å (2) were obtained. Figure 1c compares these values 41 and free disilyne.[9] The closest examples of metal-free alkynyl π- (using 1 only, due to the similarity of 1 and 2) to other relevant 42 complexes come from the complexation of bulky acetylenes to complexes. As Pt2+ and Pt0 compounds have frequently been 43 organo-tellurirenium and -selenirenium fragments,[15] but even used as examples demonstrating the differences between π- 44 in these cases, as will be discussed below, the geometrical complex (Pt2+) or metallacyclic (Pt0) bonding, examples of both 45 features of the compounds put them somewhere between the are also included in Figure 1c. While Pt2+ complexes of di-tert- 46 π-complex and rehybridized extremes of the DCD continuum. butylacetylene such as [PtCl (toluidine)(C tBu )] and 47 2 2 2 We recently reported the syntheses of two new three- [PtCl (C tBu )]À showed limited elongation of the C�C bond 48 3 2 2 membered heterocycles formed in the reactions of diborynes (3.2%) in comparison to the much more drastic elongation of 49 with diarylditellurides (Figure 1, bottom left).[16] These com- the C�C bond in the zerovalent platinum complex [Pt(C Ph ) 50 2 2 pounds, in a departure from conventional metal-free cyclics, (PPh ) ] (10.5%), the elongation found in 1 (2.8%) was even less. 51 3 2 clearly displayed the signatures of bonding dominated by a The bend-back angles (α) tell the same story with the Pt2+ 52 π !Te donation. The first hints were provided by X-ray compounds showing comparatively mild bending (15°–20°) 53 (B�B) crystallography (Figure 2a).
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