Oxidation, Coordination, and Nickel‐Mediated Deconstruction of a Highly Electron‐Rich Diboron Analogue of 1,3,5‐Hexatriene

Abstract The reductive coupling of an N‐heterocyclic carbene (NHC) stabilized (dibromo)vinylborane yields a 1,2‐divinyldiborene, which, although isoelectronic to a 1,3,5‐triene, displays no extended π conjugation because of twisting of the C2B2C2 chain. While this divinyldiborene coordinates to copper(I) and platinum(0) in an η2‐B2 and η4‐C2B2 fashion, respectively, it undergoes a complex rearrangement to an η4‐1,3‐diborete upon complexation with nickel(0).


Introduction
Linear conjugated alkenes owe their intrinsic stability to the delocalization of their p electrons.F ound in many natural products and biologically relevant compounds, [1] they are also important building blocks in organic synthesis and materials chemistry.C onjugated trienes and higher oligoenes have attracted interest because of their photophysical properties, which enable applications in nonlinear optics and optical sensing. [2] In an industrial setting,c onjugated dienes (e.g. butadiene,i soprene) are mainly used as monomers for the Ziegler-Natta synthesis of synthetic rubbers. [3] In organic chemistry,they are principally employed in 1,4-addition [4] and Diels-Alder reactions, [5] as well as numerous other transformations. [6] Many of these reactions are metal-catalyzed and involve transition-metal (TM) 1,3-diene complexes as key reaction intermediates.S olution and solid-state analyses of such complexes show that the diene ligand can be found either in the cis or trans conformation and switch between the h 2 and h 4 coordination modes (Figure 1a), [7] which may determine its subsequent reactivity with incoming substrates.
Thesubstitution of one or more carbon atoms with more electronegative heteroatoms (e.g. N, O) has long been exploited to generate polar conjugated systems,w hich are employed in numerous organic reactions (e.g.M ichael additions [8] and hetero-Diels-Alder reactions). [9] In contrast, however, the chemistry of conjugated heterodienes or heterotrienes in which one or more carbon atoms have been substituted with am ore electropositive element is virtually unexplored.
One of the focuses of our research is on the synthesis, reactivity,a nd metal coordination of compounds displaying boron-element [10] and boron-boron multiple bonds. [11][12][13][14][15] Among the latter,d oubly base-stabilized diborenes,w hich are formally isoelectronic and isostructural to alkenes,h ave been relatively well studied since their first isolation by Robinson in 2007. [16] Unlike most alkenes,diborenes undergo 1,2-addition and [2+ +2] cycloaddition reactions without the need for ac atalyst, owing to their high-lying HOMO and relatively low-lying LUMO. [11] While the B=Bb ond coordinates to coinage metals in an h 2 fashion reminiscent of metalolefin p complexes, [12] it is also sufficiently electron-rich to bind to Zn II ,C d II , [13] and even Mg II centers, [17] which do not tend to form stable p-olefin complexes because of their limited capacity for p backdonation. DFT calculations have shown that the B 2 -M interaction in these complexes is mostly electrostatic in nature (ca. 60-70 %), with the electron-rich B = Bb ond donating into empty orbitals of the metal center, and no or little p backbonding from the metal to the diborene unit. [12,13,17] Theonly reactivity reported to date for such TMdiborene complexes is that of aP Me 3 -stabilized bis(9-anthryl)diborene,which upon complexation to copper triflate undergoes an intramolecular hydroarylation. [12b] Interested in expanding the coordination chemistry and reactivity of diborenes to conjugated systems isoelectronic to 1,3,5-trienes,w es et out to synthesize doubly base-stabilized 1,2-divinyl-substituted diborenes.I nt his contribution we describe the synthesis of a3 ,4-dibora-1,3,5-triene,e xplore its electronic configuration and its oxidation chemistry,a nd present its various coordination modes to Cu I and Pt 0 metal centers (Figure 1b), as well as its Ni 0 -mediated rearrangement into a1 -vinyl-1,3-diborete.
In contrast, the reduction of 1-tBu with 4equivalents KC 8 in benzene at room temperature led to the selective formation of the red-colored divinyldiborene 4,w hich was isolated in 72 %y ield as ab rown solid (Scheme 1). Thep resence of the sterically demanding b-tert-butyl substituent prevents the recombination of the intermediate b-carbon-centered radical, favoring instead further reduction of the boron center and boron-boron bond formation. Thed iborene 4 presents ab road 11 BNMR resonance at d = 25.1 ppm, in the range typical for NHC-stabilized diborenes. [11] The 1 HNMR spectrum displays acharacteristic 2H quartet at d = 4.80 ppm ( 4 J = 1.2 Hz) for the vinylic protons coupling to the tert-butyl protons and correlating by HSQC to a 13 C{ 1 H} NMR resonance at d = 132.4 ppm.
Thes olid-state structure of 4 is shown in Figure 3a.T he B À Bbond length of 1.601(2) lies in the upper range of B = B bonds [11] and is similar to that found in doubly IiPr-stabilized di(2-thienyl) [23] or ferrocenediyl-bridged diborenes, [24] for Thermal ellipsoids set at 50 %p robability. [39] Thermal ellipsoids of ligand periphery and hydrogen atoms omitted for clarity. Scheme 1. Reduction of 1-tBu to 4.

Angewandte Chemie
Research Articles 2,4,6-Me 3 C 6 H 2 ) [27] to form the radical cation/radical anion pair [(IiPr) 2 B 2 iPr 2 ] ·+ [MesBC 4 Ph 4 ]C À . [26b] Similarly, 4 reacted with the borole 5 to yield the radical cation/radical anion pair [4]C + [5]C À (Scheme 2b), which displays ab road EPR signal consisting of the overlap of the radical cation and radical anion resonances (Figure 4, right). Given the success of this single-electron transfer reaction, we can conclude that the redox potential of 4 must be lower than that of 5,t hat is, À1.69 V. Thel owest redox potential of ad iborene measured to date remains that of [(IiPr) 2 B 2 iPr 2 ]atÀ1.95 V. [26b] Since the oxidation potential of conjugated alkenes to radical cations occurs generally above 1V , [28] we can conclude that the oxidation of 4 occurs exclusively at the diboron core and not at the vinyl moieties.
DFT calculations show that the HOMO of 4-Pt is a p orbital mostly localized on the B = Bb ond donating into an empty d orbital at the platinum center,w ith only as mall contribution of the C = Cbond and anode at the B1 À C1 bond ( Figure 5a). TheHOMO-1 consists mainly of the p orbital of the nbe ligand donating to the Pt center,w ith as mall pbonding component localized on the B1ÀC1 bond, as already suggested by its shortened bond length in the solid-state structure ( Figure 6). Then ature of the Pt-C 2 B 2 bonding was further analyzed by energy decomposition analysis combined  deformation densities (D1 k ), at the same level of theory,ofthe orbital interactions of the C 2 B 2 fragment pdonating to the Pt 0 center (left) and the Pt center p-backdonating to the C 2 B 2 fragment( right). The j n k j values correspondt othe eigenvalues of the complementary eigenfunctions (y Àk , y k )inthe NOCV representation, while DE orb(k) is the k th orbital interaction energy (kcal mol À1 ), with the percentage contribution to the total orbital interactione nergy (DE orb )shown within parentheses. The electron density flows from yellow to purple.
with the natural orbitals for chemical valence theory (EDA-NOCV). [30] Ther esults suggest that the bonding in 4-Pt is dominated by electrostatics (65.0 %), with non-negligible orbital interaction contributions (35.0 %). These arise from ac ombination of equal amounts of the C 2 B 2 p-symmetrized fragment orbital (SFO), mostly centered on the B=Bb ond, donating into an empty platinum dSFO,and the platinum d z 2 SFO p-donating into an empty p*SFO of the C 2 B 2 fragment, with as trong B1 À C1 bonding component (Figure 5b). This bonding picture is also reflected in the calculated Mayer bond orders (MBOs) of 4-Pt and the metal-free optimized cis-h 4like structure of 4,namely cis-4.While the bond order of the B=Ba nd C1=C2 bonds decrease from 1.50 and 1.75, respectively,i ncis-4 to 1.13 and 1.28, respectively,i n4-Pt, only avery small increase from 0.87 in cis-4 to 0.89 in 4-Pt is observed in the MBO of B1-C1.
In solution, the 11 BNMR spectrum of 4-Pt showed abroad resonance at d = 10.6 ppm, which is strongly upfield-shifted from that of 4 [d( 11 B) = 25.1 ppm] and 4-Cu [d( 11 B) = 19.5 ppm],p resumably owing to the strong p backdonation of the Pt 0 center. Ther oom-temperature 1 HNMR spectrum showed very broad signals and those for the vinylic protons were undetectable.A tl ow temperature (À90 to À40 8 8C) at least four different conformers are visible with vinylic proton resonances around d = 5ppm. These conformers could be rapidly exchanging cis/trans-h 4 -C 2 B 2 -Pt and h 2 -B 2 -Pt conformers,i nw hich the C1=C2 and the C21=C28 bonds are alternatingly bound to the Pt center, similarly to the bonding motifs found in 1,3-diene complexes (Figure 1a). Moreover, at temperatures above 40 8 8C 4-Pt decomposed rapidly in solution. An attempt to stabilize 4-Pt by replacing the remaining nbe ligand with IiPr resulted in complete release of free 4,a so bserved by 11 Ba nd 1 HNMR spectroscopic analyses (Scheme 3c;s ee Figures S34 and S35).
Unlike its reactions with CuCl and Pt(nbe) 3 ,the reaction of 4 with Ni(COD) 2 (COD = 1,5-cyclooctadiene) did not result in simple coordination to the metal center.I nstead ac omplex rearrangement of the B = Bu nit and one vinyl group took place,r esulting in the formation of the NiC 2 B 2 complex 6 [d( 11 B) = 13.3 ppm] as the major reaction product (Scheme 4). [31] Unlike for 4-Pt,t he addition of IiPr to 6 did not result in the liberation of the diborete ligand and no reaction was observed.
TheX -ray crystallographically derived structure of 6 ( Figure 6) shows the nickel center bound to all four atoms of a1-vinyl-1,3-diborete ligand, which displays abutterfly structure with the carbon atoms located at the tips of the wings, apuckering angle of 40.28 8 and aB -B distance of 1.890(2) . Furthermore,the b-vinyl hydrogen H2 has migrated from C2 to B1 [B1-H2 1.106 (17) ] [32] and the two IiPr ligands have migrated from boron to nickel, displacing the COD ligands. TheB À Cbond lengths are all relatively similar [1.5430 (19) to 1.5584 (19) ]a nd shorter by about 0.05 compared to typical BÀCbonds,suggesting some p delocalization over the C 2 B 2 ring. This delocalization is also confirmed by the 13 CNMR resonances of the C 2 B 2 ring, which appear in the Figure 6. Crystallographically derived molecularstructures of 4-Cu (the non-disordered one of the two distinct molecules present in the asymmetric unit), 4-Pt,and 6. [39] Thermal ellipsoids set at 50 %p robability.  (19),B 1-C2 1.5430 (19), B2-C1 1.5584 (19),B 2-C2 1.5506 (19), B1-H2 1.106 (17), B1-Ni1 2.2491(15), B2-Ni1 2.2466 (14), C1-Ni1 1.9710(12), C2-Ni1 2.0000(13), Ni1-C8 1.9194(13),N i1-C28 1.9111 (14), C21-C22 1.3396 (19). To assess the electronic situation of 6,t he nature of bonding was examined by EDA-NOCV.T wo distinct scenarios were assessed:a)The donor-acceptor interaction of aNi 0 fragment with an eutral 2p-electron 1,3-diborete ligand, and b) the interaction of aNi II center and adianionic 4p-electron [C 2 B 2 ] 2À ligand. Thec alculations reveal that, irrespective of the choice of fragments,the main bonding contribution arises from s interactions between Ni and the carbon atoms of the C 2 B 2 ring (Figure 7). Thescenario involving Ni 0 and aneutral C 2 B 2 1,3-diborete,h owever, yields al ower absolute value of the total orbital interaction energy, DE orb (Figure 7; see Table S5), [33] which indicates am ore appropriate choice of fragments.T hese data suggest that the bonding in 6 is best described as the result of the Ni 0 fragment donating into an empty p*S FO of the neutral diborete ligand located at the carbon centers.T his donor-acceptor interaction accounts for more than 80 %ofDE orb ,thereby suggesting that the valence electrons of the C 2 B 2 ring are bystanders.T he calculated MBOs of roughly unity for all endocyclic BÀCb onds in 6 suggest delocalization of the two p electrons over the C 2 B 2 ring despite the lack of planarity.F urthermore,t he MBO of only 0.25 for B1-B2 confirms the absence of B À Bb onding. Thec omplex 6 may also be viewed as a2 2e lectron C 2 B 2 Ni closo-cluster according to the Wade-Mingos rules and is the smallest nickel-carborane cluster reported to date.T he average bond lengths within the C 2 B 2 Ni fragment [Ni-C (avg) 1.99;Ni-B (avg) 2.25;B···B 1.890(2);B-C (avg) 1.55 ]are within the range of other nickel carborane clusters. [34] Considering the number of strong bonds broken (one C= Cbond and the B=Bbond, one CÀHand two BÀCbonds,as well as four Ni-COD p interactions) and reformed (three BÀ C, one B À H, two Ni À B, and four Ni À Cb onds) during the formation of 6,t he reaction is surprisingly selective. [31] We therefore decided to undertake ac omputational analysis of the mechanism of formation of 6 at two different levels of theory,t he results of which are shown in Figure 8( see the Supporting Information for details). We propose that in the first step,N ic oordinates to the divinyldiborene in an analogous manner to Pt, yielding the slightly favorable intermediate 4-Ni.T he next step,which is the rate-determining one,c onsists of the migration of the first NHC ligand to the Ni center and liberation of one molecule of COD.T his step is followed by an intramolecular [2+ +2] cycloaddition of the alkene moiety to the B = Bbond, starting from intermediate (4-Ni)b and leading to the 1,2-diborete complex (4-Ni)c. While ahandful of cycloaddition reactions of alkynes to B-B multiple bonds accompanied by C 2 B 2 rearrangements have been reported, [35] this is the first example of cycloaddition of an alkene to aB -B multiple bond. Ther earrangement of (4-Ni)c to its 1,3-diborete isomer (4-Ni)d may be expected: extensive experimental studies in the 1980s [36,37] and later computational investigations [38] have shown that in the absence of electronic stabilization 1,2-diboretes rearrange to their thermodynamically more stable 1,3-isomers.T he final formation of 6 by migration of the second NHC to Ni and of H2 from C2 to B1 is calculated to be highly favorable from athermodynamic point of view (DG = À28.2 kcal mol À1 at the B3LYP-D3(BJ)/def2-TZVPP level), and the barrier heights obtained are consistent with areaction temperature of 80 8 8C.

Conclusion
Thes ynthesis of 4 from the reductive coupling of two NHC-stabilized (dibromo)vinylboranes was only rendered possible by suppressing b-carbon radical recombination through the introduction of as terically hindering tert-butyl group in this position. While formally isoelectronic to a1,3,5hexatriene,e xperimental and theoretical data show that 4 does not display any delocalization of p electron density over the C 2 B 2 C 2 core.C alculations show that this lack of delocalization is mainly aresult of the sterics of the methyl groups at the a-vinyl positions preventing planarization.
We have shown that the coordination mode of such a3,4dibora-1,3,5-hexatriene is strongly dependent on the nature of the metal used, unexpectedly resulting in three different outcomes with three different late transition metals.Whereas with CuCl, 4 forms at ypical p-diborene complex, it coordinates to Pt 0 in af ashion reminiscent of 1,3-dienes by forming a cis-h 4 -vinyldiborene complex, the coordination of which is fluxional in solution. EDA-NOCV calculations show that, despite as tronger degree of planarization in the metalbound C 2 B 2 unit, there still is little delocalization of the p electron density: p donation to platinum occurs mostly from the B=Bdouble bond, while the Pt center p-backdonates into the empty p*o rbital of the C 2 B 2 ligand. In contrast, coordination of the vinyldiborene unit to aN i 0 complex induces ac omplex rearrangement into an h 4 -1,3-diborete complex, which proceeds by an ovel metal-templated cycloaddition of the alkene moiety to the adjacent diborene.
This study demonstrates once more that the replacement of aC =Cb ond by an isoelectronic,y et much more electronrich B=Bb ond considerably alters the chemistry of the resulting olefin analogue,o pening up new avenues for Figure 7. Plot of the main deformation densities of 6 (B3LYP/TZV2P) consideringa )Ni 0 and neutral C 2 B 2 fragments (total DE orb = À175.9 kcal mol À1 )a nd b) Ni II and [C 2 B 2 ] 2À fragments (total DE orb = À221.3 kcal mol À1 ). The j n k j values correspond to the eigenvalues of the complementary eigenfunctions (y Àk , y k )inthe NOCV representation, DE orb(k) is the k th orbital interactione nergy (kcal mol À1 ), with the percentage contribution to the total orbital interaction energy (DE orb )s hown in parentheses. The electron density flows from yellow to purple. reactivity.F urthermore,t he hitherto undocumented coordination of B=Bb onds to group 10 metals known for their catalytic performance in olefin functionalization is promising for future applications in catalytic diborene functionalization reactions.