Devin H.A. Boom, Andreas W. Ehlers, Martin Nieger and J. Chris Slootweg

Coordination of the ambiphilic phosphinoborane tBu2PCH2BPh2 to Cu(I)Cl

Accessible
De Gruyter | Published online: October 7, 2017

Abstract

A chloro-bridged dimeric copper(I) complex with the ambiphilic phosphinoborane ligand tBu2PCH2BPh2 is reported. The molecular structure was determined by single-crystal X-ray diffraction analysis, revealing a secondary η1-C interaction of the ligand with the metal center. The complex exhibits green fluorescence when exposed to UV light at 366 nm.

1 Introduction

Ambiphilic ligands are donor acceptor ligands bearing a Lewis basic site capable of σ donation and a σ-accepting Lewis-acidic moiety, which provide a rich coordination chemistry with unique M→L interactions [1], [2]. The variety of these Z-type ligands is dominated by phosphinoborane ligands, ranging from bidentate donor acceptors (DA) to tridentate (D2A) and tetradentate (D3A) systems [3]. The coordination chemistry of these ambiphilic ligands towards the coinage metals is well studied, in particular Cu(I)Cl, displaying a variety of Z-type interactions.

Bouhadir, Bourissou and co-workers reported ligand A (Fig. 1) to form a chloro-bridged dimeric structure with Cu(I)Cl, which does not contain any Z-type interaction [4]. Interestingly, changing the cyclohexyl groups on boron for phenyl groups led to complex B, which shows η3-BCC coordination of the BPh moiety to the copper center in the solid state. Switching to a tridentate, diphosphinoborane ligand, bearing phenyl and iso-propyl substituents on phosphorus, gave the monomeric Cu(I)Cl complexes C that display a unique η2-BC coordination mode with a significantly shorter Cu–B distance [Cu–B 2.396(5) (R=Ph), 2.379(5) (R=iPr) Å], indicating a stronger interaction compared to the one in complex B [Cu–B 2.555(2) Å] [4]. Such a Cu→B interaction was also observed for triphosphinoborane complex D [2.508(2) Å] in which the metal center is pentacoordinate and adopts a trigonal-bipyramidal geometry [5].

Fig. 1: Examples of Cu(I)Cl complexes with phosphinoborane-based ambiphilic ligands.

Fig. 1:

Examples of Cu(I)Cl complexes with phosphinoborane-based ambiphilic ligands.

Recently, we developed the phosphinoborane tBu2PCH2BPh2 (1), bearing a methylene linker between the donor and acceptor sites (Scheme 1), which displays frustrated Lewis pair reactivity [6], [7], [8] towards small molecules, such as H2, CO2, and isocyanates [9]. The presence of a strongly donating bis(tert-butyl)phosphine moiety and a Lewis acidic borane akin to the ambiphilic ligands of Bourissou et al. prompted us to explore the coordination chemistry of 1 toward copper(I) chloride.

Scheme 1: Synthesis of dimeric copper complex 2.

Scheme 1:

Synthesis of dimeric copper complex 2.

2 Results and discussion

Treatment of a suspension of Cu(I)Cl in dichloromethane (DCM) with 1 equiv. of tBu2PCH2BPh2 (1; δ31P{1H}=39.4 ppm) [9] at room temperature (in the absence of light) gave after 30 min a yellow solution, which afforded after work-up complex 2 in 65% isolated yield (δ31P{1H}=53.6 ppm; Scheme 1). The 11B{1H} NMR chemical shift of 2 at 70.7 ppm is diagnostic for tricoordinate diarylalkylboranes [4], and is only shifted 1.6 ppm from the free ligand (72.3 ppm) [9], indicating the absence of any Cu→B interaction. At room temperature, only three aromatic signals for the BPh2 group were observed in the 1H NMR spectrum of 2 and the 13C NMR signal for the ortho-carbons (δ13C{1H}(CD2Cl2)=136.2 ppm) was observed in the same range as for the free ligand (δ13C{1H}(C6D6)=136.8 ppm) [9], indicating that both phenyl groups rapidly exchange in solution at the NMR time scale, akin to what was reported for complex B (Fig. 1) [4].

The structure of 2 in the solid state was established by a single-crystal X-ray diffraction analysis of green crystals that were obtained by vapor diffusion of n-pentane into a solution of 2 in DCM at room temperature. The molecular structure of 2 revealed a chloro-bridged dimeric complex (Fig. 2, right) with crystallographic Ci symmetry and a Cu–P distance of 2.1887(5) Å, which is comparable to the Cu–P distances of complexes A and B (2.173(2) and 2.215(1) Å, respectively; Fig. 1). The Cu–B distance of 3.828(2) Å just exceeds the sum of the van der Waals radii (3.80 Å) [10] making any Cu→B interaction negligible, which is in agreement with the planar environment of the boron center (ΣC–B–C=359.9°). Interestingly, the distance between the copper center Cu1 and the ortho-phenyl carbon C15 is 2.876(2) Å, suggesting an η1-C interaction of the ortho-phenyl carbon with the copper center in the solid state.

Fig. 2: (Left) Photograph of the green fluorescence emission under UV irradiation at 366 nm. (Right) Molecular structure of 2 in the crystal (ellipsoids are set at 50% probability). Selected bond lengths (Å) and angles (deg): Cu1–Cu1A 3.0281(5), Cu1–Cl1 2.2906(5), Cu1–Cl1A 2.3337(5), Cu1–P1 2.1887(5), Cu1–B1 3.828(2), Cu1–C15 2.876(2); P1–Cu1–Cl1 131.86(2), P1–Cu1–Cl1A 128.34(2), Cu1–Cl1–Cu1A 81.81(2), Cl1–Cu1–Cl1A 98.19(2). Symmetry code: (A) −x+1, −y, −z+1.

Fig. 2:

(Left) Photograph of the green fluorescence emission under UV irradiation at 366 nm. (Right) Molecular structure of 2 in the crystal (ellipsoids are set at 50% probability). Selected bond lengths (Å) and angles (deg): Cu1–Cu1A 3.0281(5), Cu1–Cl1 2.2906(5), Cu1–Cl1A 2.3337(5), Cu1–P1 2.1887(5), Cu1–B1 3.828(2), Cu1–C15 2.876(2); P1–Cu1–Cl1 131.86(2), P1–Cu1–Cl1A 128.34(2), Cu1–Cl1–Cu1A 81.81(2), Cl1–Cu1–Cl1A 98.19(2). Symmetry code: (A) −x+1, −y, −z+1.

Interestingly, complex 2 shows green luminescent properties in the solid phase upon UV irradiation at 366 nm (Fig. 2, left). This photophysical behavior is known for Cu(I)Cl complexes bearing arylphosphine ligands [11], [12], [13], but to our knowledge has never been reported for Cu(I)Cl complexes with ambiphilic ligands. For the reported arylphosphine Cu(I)X complexes, emission of light occurs after metal-to-ligand charge transfer (MLCT) mixed with halide-to-ligand charge transfer (XLCT), where a metal d electron or halide p electron is excited into a delocalized π* orbital of the arylphosphine ligand. To gain more insight in the luminescent properties of alkylphosphine complex 2, we resorted to DFT calculations at the ωB97X–D/6-31G* (Def2-QZVP for Cu) level of theory [14]. These calculations have revealed that also for 2 the HOMO (–6.79 eV) is located at the dimeric copper(I)chloride core (Fig. 3, left), similar to the luminescent dimeric Cu(I) P-aryl systems [11], [12], [13]. The LUMO (0.20 eV) in complex 2, however, is a localized empty p orbital on boron (Fig. 3, right), whereas the B-aryl π* orbitals correspond to the higher lying LUMO –2 and LUMO –3 (1.74 eV and 1.75 eV, respectively). These HOMO and LUMO calculations indicate that the lowest excited state of 2 is to be attributed to the transition of an electron from the dimeric Cu(I)Cl core to the empty p orbital on boron, which is different from the MLCT transition found in the arylphosphine Cu(I)Cl complexes where an empty antibonding π* orbital of the ligand takes part [11], [12], [13].

Fig. 3: Plots of the HOMO (left) and LUMO (right) of 2 calculated at the ωB97X–D/6-31G* (Def2-QZVP for Cu) level of theory. Hydrogens are omitted for clarity.

Fig. 3:

Plots of the HOMO (left) and LUMO (right) of 2 calculated at the ωB97X–D/6-31G* (Def2-QZVP for Cu) level of theory. Hydrogens are omitted for clarity.

3 Conclusion

We have shown that the methylene-bridged phosphinoborane 1 coordinates to Cu(I)Cl, forming a dimeric species with an η1-C interaction of the ortho-phenyl carbon with the copper center, which possesses luminescent properties in the solid state. These results emphasize the potential of 1 as an ambiphilic ligand for the synthesis of unique coordination complexes with photophysical properties, which we are currently exploring in our laboratories.

4 Experimental section

4.1 General considerations

All manipulations were carried out under an atmosphere of dry nitrogen, using standard Schlenk and drybox techniques, and were performed in the dark. Solvents were purified, dried and degassed according to standard procedures.

1H and 13C{1H} NMR spectra were recorded on a Bruker Avance 400 and internally referenced to the residual solvent resonances (CD2Cl2: 1H δ=5.32 ppm, 13C{1H} δ=53.8 ppm). 31P{1H} and 11B{1H} NMR spectra were recorded on a Bruker Avance 400 and externally referenced (85% H3PO4, BF3·OEt2, respectively). The melting point was measured on a sample in a sealed capillary and is uncorrected.

Mass spectra were collected on an AccuTOF GC v 4g, JMS-T100GCV Mass spectrometer (JEOL, Japan). FD Emitter, Carbotec or Linden (Germany), FD 10 μm. Current rate 51.2 mA min−1 over 1.2 min. Typical measurement conditions are: Counter electrode –10 kV, Ion source 37 V.

tBu2PCH2BPh2 (1) was prepared following a literature procedure [9] and Cu(I)Cl≥99% was purchased from Sigma-Aldrich and used without any further purification.

4.2 Preparation of complex 2

A solution of tBu2PCH2BPh2 (1; 0.193 g, 0.60 mmol, 1.0 equiv.) in DCM (4 mL) was added to a suspension of Cu(I)Cl (0.059 g, 0.60 mmol, 1.0 equiv.) in DCM (6 mL) at room temperature. The reaction mixture was stirred for 30 min at room temperature after which a yellow solution was obtained. Next, the solution was filtered and the solvent was removed in vacuo to yield a pale yellow-green solid, which was washed with pentane (3×5 mL) and dried in vacuo to yield a pale yellow-green powder (0.164 g, 65%). X-ray quality crystals were grown at room temperature by vapor diffusion of n-pentane into a solution of 2 in DCM. Melting point (nitrogen, sealed capillary): 117°C (decomp.). – 1H NMR (400.1 MHz, CD2Cl2, 297 K): δ=7.75 (d, 3JH−H=6.9 Hz, 4H; o-PhH), 7.59 (t, 3JH−H=7.3 Hz, 2H; p-PhH), 7.52 (t, 3JH−H=7.2 Hz, 4H; m-PhH), 2.36 (d, 2JH−P=13.9 Hz, 2H; PCH2B), 1.31 (d, 3JH−P=14.2 Hz, 18H; PC(CH3)3). – 13C{1H} NMR (100.6 MHz, CD2Cl2, 296 K): δ=141.4 (only observed in the HMBC spectrum, 2JC−H coupling with o-PhH, 3JC−H coupling with m-PhH and PCH2B; ipso-PhC), 136.2 (s; o-PhC), 132.6 (s; p-PhC), 129.1 (s; m-PhC), 34.2 (d, 1JC−P=17.3 Hz; PC(CH3)3), 29.9 (d, 2JC−P=7.3 Hz; PC(CH3)3), 16.6 (d, 1JC−P=~9 Hz; PCH2B; observed in the HSQC spectrum). – 31P{1H} NMR (162.0 MHz, CD2Cl2, 297 K): δ=53.6 (br. s). – 11B{1H} NMR (128.4 MHz, CD2Cl2, 297 K): δ=70.7 (br. s). – HRMS (FD): m/z=844.2319 (calcd. 844.2325 for C42H60B2P2Cl2Cu2, [M]+), 809.2682 (calcd. 809.2637 for C42H60B2P2Cl1Cu2, [M–Cl]+).

4.3 X-ray structure determination of 2

Data were collected on an Agilent Super Nova diffractometer with EOS CCD-detector using graphite-monochromatized Mo radiation (λ=0.71073 Å) at T=−100°C. The structure was solved by Direct Methods and refined by full-matrix least-squares on F2 [15], [16]. A semi-empirical absorption correction was applied. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located from difference Fourier maps and refined at idealized positions using a riding model. For details, see Table 1.

Table 1:

Crystal structure data for 2.

Formula C42H60B2Cl2Cu2P2
Mr 846.44
Cryst. size, mm3 0.32×0.24×0.16
Crystal system Triclinic
Space group P1̅ (no. 2)
a, Å 9.0624(7)
b, Å 9.1088(8)
c, Å 14.4241(13)
α, deg 79.065(7)
β, deg 75.929(7)
γ, deg 66.417(7)
V, Å3 1052.75(17)
Z 2
Dcalcd, g cm−3 1.34
μ(MoKα ), mm−1 1.2
F(000), e 444
hkl range –7≤h≤+12

–12≤k≤+12

–20≤l≤+19
max, deg 60
Refl. measured/unique/Rint 8522/5244/0.026
Param. refined 226
R1 (for 4551 I>2 σ(I)) 0.033
wR(I) (all refl.) 0.085
GoF (I) 1.05
Δρfin (max/min), e Å−3 0.35/–0.38

CCDC 1548516 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

5 Supporting information

1D and 2D NMR spectra as well as the Cartesian coordinates for complex 2 are given as Supporting Information available online (DOI: 10.1515/znb-2017-0078).

Acknowledgment

This work was supported by the Council for Chemical Sciences of The Netherlands Organization for Scientific Research (NWO/CW) by a VIDI grant (J.C.S.) and benefitted from interactions within the COST Action CM1205 CARISMA (Catalytic Routines for Small Molecule Activation). We thank Ed Zuidinga for the exact mass determination and Dr. Feriel Rekhroukh for proofreading this manuscript.

References

[1] G. Bouhadir, D. Bourissou in Ligand Design in Metal Chemistry, (Eds.: M. Stradiotto, R. J. Lundgren), John Wiley & Sons, Chichester, 2016, p. 237. Search in Google Scholar

[2] G. Bouhadir, D. Bourissou, Chem. Soc. Rev.2016, 45, 1065. Search in Google Scholar

[3] G. Bouhadir, D. Bourissou in The Chemical Bond III, (Eds.: D. Michael, P. Mingos), Springer International Publishing, Berlin, 2016, p. 141. Search in Google Scholar

[4] M. Sircoglou, S. Bontemps, M. Mercy, K. Miqueu, S. Ladeira, N. Saffon, L. Maron, G. Bouhadir, D. Bourissou, Inorg. Chem.2010, 49, 3983. Search in Google Scholar

[5] M. Sircoglou, S. Bontemps, G. Bouhadir, N. Saffon, K. Miqueu, W. Gu, M. Mercy, C.-H. Chen, B. M. Foxman, L. Maron, O. V. Ozerov, D. Bourissou, J. Am. Chem. Soc.2008, 130, 16729. Search in Google Scholar

[6] D. W. Stephan, Science2016, 354, 1248. Search in Google Scholar

[7] D. W. Stephan, Acc. Chem. Res.2015, 48, 306. Search in Google Scholar

[8] D. W. Stephan, G. Erker, Angew. Chem. Int. Ed.2010, 49, 46. Search in Google Scholar

[9] F. Bertini, V. Lyaskovskyy, B. J. J. Timmer, F. J. J. de Kanter, M. Lutz, A. W. Ehlers, J. C. Slootweg, K. Lammertsma, J. Am. Chem. Soc.2012, 134, 201. Search in Google Scholar

[10] S. S. Batsanov, Inorg. Mater.2001, 37, 871. Search in Google Scholar

[11] L. Qi, Q. Li, X. Hong, L. Liu, X.-X. Zhong, Q. Chen, F.-B. Li, Q. Liu, H.-M. Qin, W.-Y. Wong, J. Coord. Chem.2016, 69, 3692. Search in Google Scholar

[12] A. Tsuboyama, K. Kuge, M. Furugori, S. Okada, M. Hoshino, K. Ueno, Inorg. Chem.2007, 46, 1992. Search in Google Scholar

[13] M. Osawa, M. Hoshino, M. Hashimoto, I. Kawata, S. Igawaa, M. Yashimaa, Dalton Trans. 2015, 44, 8369. Search in Google Scholar

[14] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09 (revision D.01), Gaussian, Inc., Wallingford CT (USA) 2013. Search in Google Scholar

[15] G. M. Sheldrick, Acta Crystallogr.2008, A64, 112. Search in Google Scholar

[16] G. M. Sheldrick, Acta Crystallogr.2015, C71, 3. Search in Google Scholar

Supplemental Material:

The online version of this article offers supplementary material (https://doi.org/10.1515/znb-2017-0078).

Received: 2017-5-10
Accepted: 2017-6-2
Published Online: 2017-10-7
Published in Print: 2017-11-27

©2017 Walter de Gruyter GmbH, Berlin/Boston