Angewandte Communications Chemie International Edition:DOI:10.1002/anie.201907749 Main-GroupComplexes German Edition:DOI:10.1002/ange.201907749 ANeutral“Aluminocene” SandwichComplex: h1-versus h5- CoordinationModes of aPentaarylborole with ECp* (E = Al, Ga; Cp* = C5Me5) Christian P. Sindlinger* and Paul Niklas Ruth F Abstract: The pentaaryl borole (Ph*C)4BXyl [Ph* = 3,5- complexes,inparticular, is that borolediide salts act as F tBu2(C6H3); Xyl = 3,5-(CF3)2(C6H3)] reacts with low-valent reducing agents rather than as aligand source in metathesis Group 13 precursors AlCp* and GaCp* by two divergent reactions with p-block halides. routes.Inthe case of [AlCp*]4,the borole reacts as an oxidising We recently reported the synthesis of aset of novel, highly agent and accepts two electrons.Structural, spectroscopic,and soluble tert-butyl-decorated pentaphenyl boroles (Ph*C)4BR [12] computational analysis of the resulting unprecedented neutral [Ph* = 3,5-tBu2(C6H3)]. We are interested in further 5 5 F III h -Cp*,h -[(Ph*C)4BXyl ]complex of Al revealed astrong, expanding the chemical scope of boroles as ligands to the p- ionic bonding interaction. The formation of the heteroleptic block elements.Tocircumvent salt metathesis reactions,we F borole-cyclopentadienyl “aluminocene” leads to significant treated borole (Ph*C)4BXyl (A)with the established, changes in the 13CNMR chemical shifts within the borole unit. potentially reductive monovalent Group 13 reagents F In the case of the less-reductive GaCp*, borole (Ph*C)4BXyl (AlCp*)4 and GaCp* (Scheme 1). reacts as aLewis acid to form adynamic adduct with adative 2-center-2-electron Ga Bbond. The Lewis adduct was also À studied structurally,spectroscopically,and computationally. Fifty years ago,Eisch reported the first authentic isolation of pentaphenyl borole.[1] Free boroles are weakly anti-aromatic cyclic 4p-electron compounds.[2] Among avariety of intrigu- ing reactivities,including the activation of hydrogen[3] or Si H À bonds,[4] Diels–Alder reactions,and ring expansions,[1b, 5] boroles can be readily reduced by two electrons to form Hückel-aromatic borolediides[6] or they can react as potent Lewis acids.[7] In recent years,variation of the boron-bound substituent allowed for an extension of the library of known Scheme 1. Divergentreaction pathways of free borole A with AlCp* [2b, 6b,8] boroles with substantially altered optical gaps. and GaCp*. Thecoordination chemistry of boroles toward transition metals has been studied since the late 1970s.[6a,9] However, despite the isoelectronic nature of borolediide with the—in When GaCp* was added to borole A an immediate colour organometallic chemistry—ubiquitous and iconic cyclopenta- change from dark green to bright orange was observed. NMR dienyl anion, very few complexes other than with d-block spectroscopic examination of the reaction mixture confirmed metals or very electron-positive s-block metals are known. aclean conversion and the formation of asingle product. The Recently Müller, Albers,and co-workers reported aGeII- 1HNMR spectrum revealed no substantial changes in the borole complex that resulted from arearrangement during shifts compared to the individual starting materials.However, the reaction of agermole dianion with amidoborane diha- the 11BNMR signal drastically shifts from abroad signal in [10] lides. Although only afew comments are found in the the typical range of tricoordinate boron atoms at d11B = [9d, 11] literature, alikely reason for the scarcity of p-block 71 ppm (w1/2 = ca. 3250 Hz) in A to anarrower signal at d11B = 7.6 ppm (w1/2 = ca. 1550 Hz) in 2.The shift to higher [*] Dr.C.P.Sindlinger,M.Sc. P. N. Ruth field is aclear indication of ahigher coordination number at Institut fürAnorganische Chemie the boron atom.[13] Major changes ( 2ppm) in the Æ Georg-August-UniversitätGçttingen 13C{1H} NMR spectrum of the borole framework are observed Tammannstrasse 4, 37077 Gçttingen (Germany) for the a-and b-carbon atoms of the C4Bcycle as well as the E-mail:[email protected] ipso-and para-positions of the boron-bound aryl moiety Supportinginformation (including Experimental Details) and the (Table 1). ORCID identification number for one of the authors of this article can An interaction of the GaCp* fragment with the boron- be found under:https://doi.org/10.1002/anie.201907749. centred LUMO is also in line with the change in colour from 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. an intense green (stemming from p/p*excitation in free KGaA. This is an open access article under the terms of the Creative Commons AttributionLicense, which permits use, distribution and boroles) to abright orange.The colour of 2 is unique among I reproduction in any medium, provided the original work is properly the otherwise colorless (Cp/R)Ga adducts with Lewis-acidic cited. boranes.[13,14] Angew.Chem. Int.Ed. 2019, 58,15051 –15056 2019 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 15051 Angewandte Communications Chemie Table 1: Diagnostic NMR chemical shifts in C6D6 at 298 KofA, 1,and 2. B1-Ga1 92.60(11)8.The Ga Bbond (2.1382(19) )issimilar Calculated averaged values in brackets. À to those in B(C6F5)3 adducts of GaCp derivatives (2.154(3), [b] [b] [b] [b] 11 [13a, 14b] Compound Cb Ca i-CXylF p-CXylF B 2.155(6), 2.161(2) ). Thebond lengths within the [a] borole ring clearly reveal isolated C=Cand C Cbonds.The A 166.2 140.6 135.9 125.3 71.6 F À 1 128.4 118.0 144.2 119.1 24.6/17.3[c] Xyl residue at the tetracoordinate boron centre noticeably [126.1] [117.9] [144.8] [18.6] bends out of the borole plane away from the GaCp* cone.A 2 151.2 149.6 150.7 119.4 7.6/ 0.4[d] related structural motif and reactivity was also observed for À [151.7] [149.9] [151.6] [ 0.9] AlCp* adducts of 9-borafluorenes.[11] À [a] See Ref. [12].[b] 13CNMR shift in ppm in C D .[c] At 758Cin Over the course of afew weeks,small amounts of afine 6 6 À toluene. [d] At 508Cintoluene. grey solid deposited from solutions of 2 along with the À formation of unassigned decomposition products.[15] Clearly,the monovalent GaICp* was too reluctant to At ambient temperature,nofurther signal for free GaCp* transfer electrons and reduce the borole.Wetherefore turned I was observed after addition of afurther 0.5 equiv of GaCp* to to (AlCp*)4,asAl is astronger reductant. AlCp derivatives solutions of 2,thus indicating adynamic exchange of GaCp*. can also form base adducts with boranes.[16] Suspending the Variable-temperature NMR experiments of solutions of 2 in poorly soluble (AlCp*)4 in green solutions of A leads to avery toluene with aslight excess of GaCp* reveal hindered slow decolourisation over the course of three days to finally rotation of the C -bound Ph* groups starting at 408C. At yield pale yellow solutions.Monitoring the process by NMR b À 308C, the Cp* signal significantly broadens and gradual spectroscopy revealed avery clean conversion into asingle À cooling from 408Cto 758Cleads to two increasingly sharp product 1.Crystals of 1 readily form from concentrated À À separate Cp* signals of GaCp* and 2 being observed. The solutions in various hydrocarbons.Inall cases,and despite 1HNMR chemical shifts all lie in the range of pure GaCp*, numerous attempts,weobtained poorly resolved diffraction which is reported to likely form hexamers at low temper- data.[17] Examination of the data, however, allowed the key ature.[15] However,the intense orange colour does not change structural feature to be clearly identified:the anticipated 5 5 F III upon cooling,thus rendering apotential equilibrium between quasi h -Cp*,h -[(Ph*C)4BXyl ]Al sandwich complex 1. 2 and A + 1/6[GaCp*]6 unlikely.Orange-red crystals suitable This represents the first neutral “aluminocene” and the for X-ray diffraction grew from benzene solutions.The second borole complex of ap-block element.[10,18] Hetero- molecular structure clearly confirms the formation of leptic Cp/borole sandwich complexes are known for various aboron-centred Lewis-base adduct, with donation of the transition metals.[8e, 19] GaI lone pair of electrons into an empty porbital on boron Thequality of the data limits extensive structural dis- (Figure 1). TheGa1–Cp*centroid vector is virtually aligned with cussion;however, some key features can clearly be identified. the Ga1 B1 bond (175.58), and the Ga1–B1 vector is almost Compared to A and 2,which both feature localized cyclic 1,3- À perpendicular to the C4Bplane (C4-B1-Ga1 95.04(11)8,C1- butadiene systems,the atomic distances within the (C4B) ring in 1 are much more uniform. Shortened B C and C C À a bÀ b bonds together with an elongated C C bond are in line with aÀ b substantial p-delocalization, as expected for aHückel-aro- [6b] matic boroldiide. TheAl1-(C4B)centroid distance is approx- imately 1.80 ,which is slightly shorter than the Al1- Cp*centroid distance of approximately 1.86 .This is rational- ized by greater electrostatic attraction between the dianionic borole and AlIII compared to the simple monoanionic Cp*. TheCp* and borole units adopt adistorted staggered conformation. TheCp*-Al contacts range from 2.17(1) to 2.27(1) ,thus indicating aslight deviation of the Al atom from an ideal central localisation. Thedisorder in the X-ray structure prevents discussion of individual Al1 (C B) dis- À 4 tances.The DFT-optimised structure (Figure 2) reveals acen- tered Al atom with comparatively short Al C and Al B À a À contacts.All other experimental structural features are in general good agreement with the gas-phase DFT-optimised [20] Figure 1.