Elusive Phosphine Copper(I) Boryl Complexes: Synthesis, Structures

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Elusive Phosphine Copper(I) Boryl Complexes: Synthesis, Structures, and Reactivity Corinna Borner, Lisa Anders, Kai Brandhorst, and Christian Kleeberg* Institut für Anorganische und Analytische Chemie, Technische Universität Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106 Braunschweig, Germany S Supporting Information *

ABSTRACT: We report the first isolation of phosphine copper boryl complexesspecies pivotal to numerous copper-catalyzed borylation reactions. The reaction of diboron(4) derivatives with copper tertbutoxide complexes of phosphine ligands allows the isolation of the dimeric μ-boryl-bridged Cu(I) complexes [(iPr3P)Cu−Bdmab]2 (4) and [(C6H4(Ph2P)2)Cu−Bpin]2 (6) with Cu···Cu distances of 2.24−2.27 Å (dmab = (NMe)2C6H4, pin = (OCMe2)2)). A slightly more sterically demanding boryl ligand furnishes the unprecedented multinuclear copper boryl complex [(iPr3P)2Cu8(B(iPrEn))3(OtBu)3] (5), a potential intermediate of the decomposition of an initial Cu(I) boryl complex (iPrEn = (NiPr)2C2H4). All complexes were characterized by singlecrystal X-ray diffraction, NMR spectroscopy, and elemental analysis. DFT computations support the nature of these unique complexes and give insight into their electronic structures.

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boryl complexes, such sterically encumbered complexes are typically not involved in catalytic processes. Attempts to isolate and characterize sterically less encumbered and in particular phosphine-containing copper boryl complexes haveto the best of our knowledgeso far been unsuccessful. Presumably this is a consequence of the high reactivity and low stability of these complexes.2,7,8a In order to investigate further the chemistry of copper(I) boryl complexes, we combined phosphine ligands, a ligand class frequently employed in catalytic reactions but not yet known in isolated boryl complexes, with diaminoboryl ligands with reduced steric demand, in comparison with those derived from boryl lithium derivatives. The reaction of the dimeric alkoxidebridged complex [(iPr3P)Cu−OtBu]2 (2) with the unsymmetrical diborane(4) derivative pinB-Bdmab (3a; dmab = (NMe)2C6H4), recently introduced by us as a versatile precursor for otherwise inaccessible diaminoboryl ligands, results in the formation of the unprecedented bis-μ-boryl-bridged complex [(iPr3P)Cu−Bdmab]2 (4) in good yields (Scheme 1).8,9 The sterically more demanding diboron compound pinB−B(iPrEn) (3b; iPrEn = (NiPr)2C2H4), however, furnishes under similar reaction conditions the unprecedented octanuclear boryl complex [(iPr3 P)2 Cu8(B(iPrEn)) 3(OtBu) 3] (5) (Scheme 1).9,10 The sterically more encumbered pinB−BtBuEn (3c; tBuEn = (NtBu)2C2H4) does not react with 2, whereas B2pin2 (1) undergoes facile B−B bond activation.9 While for the latter no boryl complex could be isolated, the observed (decom-

opper-catalyzed borylation reactions of e.g. CO2, carbonyls, α,β-unsaturated carbonyls, olefins, and alkynes as well as aryl and alkyl halides employing diboron(4) reagents (in particular B2pin2 (1), pin = (OCMe2)2) are a currently flourishing field of research.1−3,4a−c Since the first independent reports on those reactions by Miyaura, Hosomi, and co-workers copper(I) boryl complexes have been proposed as the central reactive intermediates.5 However, only in 2005 did Sadighi and co-workers report the first isolation of a Cu(I) boryl complex as well as initial reactivity studies. This complex of the type (NHC)Cu−Bpin (I), was prepared, in analogy to the proposed catalytic reaction pathway, by σ-bond metathesis reaction of a copper alkoxide precursor and the diborane(4) derivative B2pin2 (1).3,4a,b This σ-bond metathesis reaction was later extended to a variety of related NHC-Cu complexes, including a remarkable cationic μ-catecholatoboryl complex [((IDipp)Cu)2Bcat]+ (II) by Sadighi in 2016. As a common feature all of these complexes employ sterically demanding ancillary NHC ligands to provide kinetic stabilization.4c,d Cu(I) boryl complexes with ancillary ligands having little steric demand (e.g., halide, cyanide) have been obtained by Yamashita and co-workers using sterically demanding diaminoboryl ligands, by reaction of the respective boryl lithium species with an appropriate copper precursor, e.g., [(C2H4((2,6-iPr2C6H3)N)2B)2Cu4Br2] (III).6 It has to be emphasized that the known copper boryl complexes are stabilized either by sterically demanding NHC ligands or by the steric demand of the boryl ligand itself; nonetheless, despite the steric protection it has been stated that these copper boryl complexes are highly reactive and decompose readily to elemental copper.2,4a,c While these complexes are useful to establish the general reactivity and structural properties of copper © XXXX American Chemical Society

Received: October 18, 2017

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DOI: 10.1021/acs.organomet.7b00775 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of the Boryl Complexes 2, 5, and 69

of 6 and one molecule of cocrystallized PhMe; however, as for 4, each molecule of 6 is situated on a center of inversion (Figure 1, right).9 Moreover, the THF solvate 6(thf)4 was obtained from THF solution, exhibiting geometrical properties very similar to those of 6(PhMe)2.9 Complexes 4 and 6 both show as a pivotal structural feature, despite the difference in coordination number of the copper ion, a planar, slightly unsymmetrical Cu2B2 moiety exhibiting a μ-boryl coordination and an extremely short Cu···Cu distance of 2.24−2.27 Å, significantly shorter than twice the van der Waals radius (2.8 Å) or the interatomic distance in elemental copper (2.56 Å).11 While μ-boryl-bridged copper complexes have been reported, they have not been considered as readily formed, possible intermediates in borylation reactions. In fact, the reported μ-bridged boryl complexes II and III are structurally quite distinct from 4 and 6.4d,6a,7 The Cu···Cu distances in II and III are at 2.4082(2) and 2.312(1) Å, respectively, significantly longer than those in 4 and 6, whereas the slightly unequal Cu−B distances are significantly shorter in II (2.051(6), 2.041(6) Å) and III (2.073(5), 2.093(4) Å), although the Cu−B−Cu angles are wider in the latter (II 72.1(2)°, III 67.4(1)°).4d,6a These difference in B−Cu distances are rationalized by assuming a mutual weakening of the B−Cu bond by the two competing boryl ligands in 4 and 6, similar to a trans influence,12 whereas the shorter Cu···Cu distance may be considered a direct result of the more effective bonding within the B2Cu2 core (vide infra). It is worth noting that a very similar structural motif, Cu2Si2, including short Cu···Cu distances, is also found in copper silyl complexes, once more emphasizing the diagonal relationship between boron and silicon.13 Complex 5 crystallizes from a toluene/n-pentane mixture at −40 °C as small hexagonal bronze prisms. Each entity of 5 is situated on a 3-fold axis through the atoms P1/Cu3 and on a mirror plane perpendicular to this axis (P6̅2c, Z = 1, Z′ = 1/3, Figure 2).9

position) products agree with an intermediate formation of an (unstable) copper boryl species (vide infra). Replacing the monodentate phosphine in 2 by the bidentate 1,2-bis(diphenylphosphino)benzene (dppbz) resulted in the isolation of the complex [(C6H4(Ph2P)2)Cu−Bpin]2 (6) upon reaction with 1 (Scheme 1).9 It has to be emphasized that the complexes 4−6 are highly sensitive species and are not stable at ambient temperature in solution (vide infra). Their isolation was successful as conditions were found under which crystallization is competitive with decomposition. However, as solids the compounds may be stored for several months at −40 °C under an inert nitrogen atmosphere. Single crystals of 4 were obtained from an n-pentane/PhMe solution at −40 °C as light orange hexagonal plates. 4 crystallizes with half a molecule in the asymmetric unit (Pbca, Z = 4, Z′ = 1/ 2), each molecule being situated on a center of inversion (Figure 1, left).9 Single crystals of 6(PhMe)2 were obtained from a PhMe solution at −40 °C as orange rhombohedral plates. The asymmetric unit (P1,̅ Z = 1, Z′ = 1/2) comprises half a molecule

Figure 2. Two selected views of the molecular structure of 5. Hydrogen atoms and disordered parts are omitted for clarity.9 Selected distances (Å) and angles (deg): P1−Cu3 2.300(4), Cu1−B1−Cu2 64.3(4), Cu1− O1−Cu2″ 86.9(4).

The molecular structure of 5 comprises an octanuclear copper cluster consisting of a slightly irregular hexagon of copper atoms with both hexagonal faces capped by two additional copper atoms. This cluster is decorated with three diaminoboryl and three alkoxide ligands in the equatorial plane; the apical copper atoms are coordinated by additional phosphine ligands. The B− Cu distances are slightly shorter than those observed in the dinuclear complexes 4 and 6 but longer than those in I (R = dipp) and III.4d,6a The O−Cu distances, however, are significantly shorter than those in its precursor 2 (1.983(3) and 1.994(3) Å),

Figure 1. Molecular structures of (left) 4 and (right) 6. Hydrogen atoms and disordered parts are omitted for clarity.9 Selected distances (Å) and angles (deg): 4, Cu1−Cu1′ 2.2359(4), B1−Cu1 2.179(2), B1−Cu1′ 2.263(2), P1−Cu1 2.1989(4), B1−Cu1−P1 123.52(4), B1−Cu1′−P1′ 116.89(4), Cu1−B1−Cu1′ 60.41(4), B1−Cu1−B1′ 119.59(4); 6, Cu1−Cu1′ 2.2730(5), B1−Cu1 2.203(3), B1−Cu1′ 2.311(3), P1− Cu1 2.2706(6), P2−Cu1 2.2826(6), P1−Cu1−P2 84.42(2), B1−Cu1− B1′ 119.59(7), Cu1−B1−Cu1′ 60.41(7). B

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containing both a iPr3P and a Bdmab moiety, indicating the presence of 4. The 31P and 13C NMR data are in full agreement with this interpretation; unfortunately no 11B NMR signal could be detected. This may be due to the fast relaxation of the quadrupolar 11B nucleus in an environment of low symmetry and the proximity of quadrupolar Cu nuclei.4d,9 Moreover, from the NMR data two major decomposition products of 4 were identified: the symmetrical diborane(4) derivative B2dmab2 and free iPr3P (Figures S1−S5 in the Supporting Information).9,16 In addition, in working with 4 we observed tiny amounts of a very fine black precipitate and/or a blackish metallic luster at the vessels used, which are assigned, in agreement with earlier reports, to elemental copper formation.4a,c Unfortunately, all attempts to directly characterize this material, e.g. by X-ray powder diffraction, failed due to experimental reasons (the small amounts available, fine nature, etc.). However, the isolation of the boryl complex 5 suggests that copper clusters, including lowvalent copper atoms, are also possible decomposition products or intermediates in the reductive decomposition of boryl complexes. The solution structure of 4 has not been straightforwardly elucidated; however, the phosphine-bound isopropyl group exhibits apparent triplet signals in the 13C NMR spectrum (Figure S3 in the Supporting Information). Possible explanations include coupling to two phosphorus atoms or the diastereotopicity of the isopropyl groups, both tentatively supportive of a dimeric (or higher aggregated) rather than a mononuclear linear solution structure.9 In solution 6 exhibits behavior similar to that for complex 4; it decomposes at ambient temperature in solution instantaneously. At −45 °C in THF-d8 and PhMe-d8 solution NMR data can be obtained along with indications for decomposition even at low temperatures. However, again no 11B NMR signal was detected (vide supra) and a complete set of 13C NMR data was not obtained due to the low solubility of this compound.9 The symmetrical diborane(4) B2pin2 (1) was unambiguously identified by NMR spectroscopy as a decomposition product of 6, suggesting a reductive decomposition pathway analogous to that described for 4. However, additionally Ph−Bpin along with PPh3 (and further unidentified phosphorus containing decomposition products) were observed as decomposition products of 6, indicating the presence of additional decomposition pathways besides reductive B−B bond formation (Figures S7−S12 in the Supporting Information).9 Due to its extremely low solubility 5 could not be comprehensively characterized by NMR spectroscopy. However, 31 P NMR spectroscopic data are in agreement with the presence of a copper phosphine complex in solution at −45 °C and its decomposition upon heating to ambient temperature (Figure S6 in the Supporting Information).9 An initial study on the reactivity of complexes 4−6 as sources of a nucleophilic boron moiety was conducted. The copper boryl complexes were reacted with excess 4-iodotoluene as an electrophile established as a substrate in borylation catalysis and used in reactivity studies of [(IDipp)Cu−Bpin] (type I).2 Indeed, for all three boryl complexes 4−6, the expected boronic acid derivatives 4-Me(C5H4)-Bdmab, 4-Me(C5H4)-B(iPrEn) and 4-Me(C5H4)-Bpin, respectively, were identified by NMR spectroscopy and/or GCMS analysis as a major reaction product (Figures S19−S28 in the Supporting Information).9 It has to be noted that the borylation of the aryl iodide is competitive with decomposition of the boryl complexes (vide supra) and hence difficult to quantify.9 Nonetheless, a mixture of CuOtBu and

whereas the P−Cu distance is longer than in 4 and 6 as well as in 2.9 The Cu···Cu distances within 5 vary greatly in the range of 2.28−2.72 Å depending on the ligands at the copper atoms (Figure 2). The μ-boryl-bridged Cu1···Cu2 unit exhibits the shortest Cu···Cu distance, in the range observed for 4 and 6, whereas the Cu1···Cu2″ distance, bridged by a μ-alkoxide ligand, is significantly shorter than in the μ-alkoxide precursor complex 2 (2.8565(6) Å). 9 The unbridged Cu1/2···Cu3 distances, however, are only slightly shorter than double the van der Waals radius of copper (2.8 Å).11 Complex 5 comprises three anionic diaminoboryl and three anionic alkoxide ligands as well as two neutral phosphine ligands and eight copper atoms; hence, formally an averaged oxidation state of each copper atom of +3/4 has to be assigned. It should be emphasized that, as polyhydrido copper clusters are well established,14 no indication of the presence of hydride ligands was obtained by an X-ray diffraction study or by elemental analysis.9 Hence, it may be concluded, in agreement with computational data (vide infra), that 5 comprises an unprecedented multinuclear copper boryl complex with an average formal Cu oxidation number of lower than 1. The bonding situation in the boryl complexes 4−6 was analyzed using DFT calculations. For all compounds, starting from the solid-state structures, the geometries were fully optimized and were confirmed to be minima on the energy hypersurface.9 Generally, for 4 as well as for 6, the computed molecular structure resembles the solid-state structures well.4d However, the small differences found are more pronounced for 6 than for 4, possibly due to solid-state effects not considered in the gas-phase computations. To evaluate the bonding in 4−6 the Laplacians were calculated and analyzed using Bader’s atoms in molecules method (AIM).15 For complexes 4 and 6 bond critical points on all B···Cu as well as Cu···Cu bond paths were found (Figure S35, S40), while quite low electron densities (4, 0.063− 0.070; 6, 0.045−0.080) and positive Laplacians (4, 0.063−0.117; 6, 0.059−0.111) indicate closed-shell interactions (Table S2 in the Supporting Information).9,15 These findings, indicating a Cu−Cu bond, are in agreement with the data reported for the related complex II.4d A qualitative examination of the computed orbitals of 4 and 6 shows that some of the energetically high lying orbitals (Figures S34 and S39 in the Supporting Information) exhibit substantial boron and copper contributions (4, HOMO, HOMO-5, -11, -16; 6, HOMO, HOMO-2, -32).9 This is in agreement with a simple rationalization assuming multicentered bonds within the B2Cu2 core, essentially composed of an occupied σ-type orbital at the boryl ligands and vacant orbitals at the copper atoms, a bonding situation similar to that described for II.4d The computed structure of 5, however, is of lower symmetry (C3) than its solid-state structure (approximately D3h).9 The computed Cu···Cu distances, in particular those bridged by alkoxide ligands, are longer, whereas the Cu···B distances are shorter than those observed in the solid state.9 For a deeper understanding of the bonding situation in the octanuclear cluster 5, a more detailed analysisbeyond the scope of this communicationis required. It is noted that while no bond critical points within the equatorial Cu6 unit in 5 were found, the HOMO of this molecule includes substantial boron and copper contributions as found for 4 and 6 (Figure S37 in the Supporting Information).9 Compound 4 is, in solution, unstable at room temperature, but NMR spectra could be recorded at −47 °C. Nonetheless even at this temperature 4 shows considerable decomposition within a few hours (Figures S1 and S2 in the Supporting Information).9,16 However, NOESY NMR contacts prove the presence of a species C

DOI: 10.1021/acs.organomet.7b00775 Organometallics XXXX, XXX, XXX−XXX

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dppbz was shown to be an effective catalyst system for the copper-catalyzed borylation of 4-iodotoluene, under conditions virtually identical with those reported elsewhere (53% isolated yield).2,9 In particular it should be noted that this system performs better than that in the absence of a phosphine ligand.9 Further details on the reactivity of the presented copper boryl complexes in stoichiometric as well as catalytic reactions will be the subject of forthcoming studies. The findings presented may be summarized in three major points, all immediately relevant to the vibrant field of coppercatalyzed borylation reactions. (a) Phosphine copper boryl complexes are isolable compounds that may be used for mechanistic studies. Nonetheless, those complexes are intrinsically highly unstable toward decomposition: e.g., via reductive B−B coupling. (b) μ-Boryl coordination appears to be the preferred coordination mode of phosphine copper boryl complexes. While this does not exclude mononuclear copper boryl complexes as catalytic relevant intermediates, dimeric and possibly also higher aggregated species also have to be considered. (c) So far unprecedented, multinuclear copper boryl complexes have been observed and are possible intermediates in the reductive decomposition of Cu(I) boryl complexes. Moreover, those complexes are formed under conditions similar to those for Cu(I) boryl complexes and are sources of boron nucleophiles and, as such, have to be considered as potential reactive intermediates in borylation reactions.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00775. Experimental, analytical, computational and crystallographic data (PDF) Cartesian coordinates for the calculated structures (MOL) Accession Codes

CCDC 1572689−1572693 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

Corresponding Author

*C.K.: fax, +49 (0) 531 391 5387; e-mail, [email protected]. ORCID

Christian Kleeberg: 0000-0002-6717-4086 Notes

The authors declare no competing financial interest.

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REFERENCES

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ACKNOWLEDGMENTS

C.K. and C.B. thank the Fonds der Chemischen Industrie for the generous support by a PhD and a Liebig Fellowship. C.K. gratefully acknowledges support by a Research Grant (KL 2243/ 5-1) of the DFG. The authors thank BASF SE for a gift of B2(NMe2)4. D

DOI: 10.1021/acs.organomet.7b00775 Organometallics XXXX, XXX, XXX−XXX