DOI: 10.1002/chem.201700103
Communication
& Luminescence | Very Important Paper|
Luminescent Ni0 Diisocyanide Chelates as Analogues of CuI Diimine Complexes Laura A. Beldt,[a, b] Christopher B. Larsen,[a] and Oliver S. Wenger*[a] their luminescence properties,[8] and to date, no luminescent homoleptic Ni0 isocyanide complexes seem to be known. Chelating diisocyanide ligands appeared to be particularly promising because they offer the possibility of obtaining structural analogs to [Cu(dpp)2] + (dpp = 2,9-diphenyl-1,10-phenanthroline), one of the prototypical emissive CuI complexes (Scheme 1 a).[9] Diisocyanide ligands have also been used to
Abstract: The first two homoleptic Ni0 isocyanide complexes that exhibit photoluminescence from long-lived excited states are presented. Electrochemical studies indicate that in one of the complexes significant geometrical distortion occurs upon metal oxidation. The observation of luminescence, even though currently restricted to low temperatures, is an important proof-of-concept in the search for earth-abundant alternatives to photoactive complexes made from precious metals. The prospect of using Ni0 isocyanide complexes as luminophores, photoredox catalysts, or dyes in solar cells, is highly attractive.
Emissive CuI complexes with a-diimine and phosphine chelating ligands are widely explored for applications in luminescent devices,[1] dye-sensitized solar cells,[2] and photoredox catalysis.[3] Until now, isoelectronic Ni0 has received practically no attention in these contexts, and thus it seemed worthwhile to explore the possibility of obtaining luminescent Ni0 complexes. Similar to CuI diimines, Ni0 isocyanides would be an attractive earth-abundant alternative to emissive and redox-active complexes made from precious metals, such as RuII or IrIII. Moreover, Ni0 complexes should exhibit substantially higher reducing power in their excited states. To stabilize Ni0, p-accepting ligands, such as carbonyls, isocyanides, or phosphines, are well suited. Isocyanides have been used for obtaining emissive d6 metal complexes with low oxidation states (in particular Cr0,[4] Mo0,[5] W0[5a, 6]), and therefore, we reasoned that this class of ligands would potentially be suitable for obtaining luminescent Ni0 complexes as well. Although Ni0 isocyanides have been explored for more than 60 years,[7] only a handful of studies have been reported on
[a] Dr. L. A. Beldt, Dr. C. B. Larsen, Prof. Dr. O. S. Wenger Department of Chemistry, University of Basel St. Johanns-Ring 19, 4056 Basel (Switzerland) E-mail:
[email protected] [b] Dr. L. A. Beldt Current address: Institute of Inorganic Chemistry University of Tebingen, Auf der Morgenstelle 18, 72076 Tebingen (Germany) Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.doi.org/10.1002/chem.201700103. It contains a description of equipment and methods, as well as ligand syntheses and product characterization data. Compounds [Ni(L1)2] and [Ni(L2)2] were synthesized by stirring two equivalents of ligands L1 or L2 with one equivalent of Ni(COD)2 in toluene at room temperature. Chem. Eur. J. 2017, 23, 8577 – 8580
Scheme 1. Chemical structures of (a) [Cu(dpp)2] + , (b) [Ni(L1)2], and (c) [Ni(L2)2].
obtain emissive and photoredox-active CuI complexes.[10] A crucial aspect in CuI complexes is the geometry change occurring in the MLCT excited states, as a result of formal metal oxidation and the preference of d9 complexes for higher coordination numbers. These excited-state distortions (and possible addition of a fifth ligand) often lead to rapid nonradiative excited-state deactivation, and consequently, it is usually desirable to suppress large geometrical distortions by suitable ligand design.[11] Therefore, we decided to equip our diisocyanide chelators with peripheral phenyl substituents (Scheme 1 b/c), in analogy to the dpp ligand. The resulting homoleptic Ni0 diisocyanide complexes can be considered to be isoelectronic analogues of [Cu(dpp)2] + ·. The new ligands L1 and L2 were synthesized in five reaction steps, and their complexation to Ni0 readily occurred at room temperature when starting from Ni(COD)2 (COD = 1,5-cyclooctadiene; see the Supporting Information). By using either a benzene unit (L1) or a thiophene moiety (L2) as a central spacer, we expected to obtain somewhat different bite angles. Terphenyl-based diisocyanide ligands have been used for Cr0,[4] Mo0,[5b] and IrIII complexes,[12] but many other previously reported multidentate isocyanides have less p-conjugated backbones.[13] In the infrared spectra, the isocyanide C/N stretch vibrations of L1 and L2 appear at 2119 cm@1, whereas in the [Ni(L1)2] and [Ni(L2)2] complexes, these vibrations are observable at 2007
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Communication and 2004 cm@1, respectively, due to p backbonding (Figures S1 and S2 in the Supporting Information). A splitting into several bands that would indicate a lowering of the symmetry is not discernible,[7d, k] but the respective bands are substantially broader than in the free ligands. In cyclic voltammetry experiments (in THF with 0.1 m tetra-nbutylammonium hexafluorophosphate, TBAPF6), the [Ni(L1)2] complex exhibited a quasi-reversible wave at @0.32 V versus Fc + /Fc that can be attributed to the NiI/0 redox couple (Figure 1 a). Compared to [Ni(PPh3)4], the respective potential is shifted to less negative values by 0.65 V (Table S1 in the Supporting Information),[14] possibly due to stronger p backbonding with the isocyanide ligands and/or weaker s-donation.
The UV/Vis spectra of [Ni(L1)2] and [Ni(L2)2] in toluene exhibited absorption bands extending well into the visible spectral range (solid lines in Figure 2), rendering these complexes orange and deep red, respectively. The absorption bands
Figure 2. UV/Vis absorption spectra (solid lines) and luminescence spectra (dotted lines) of (a) [Ni(L1)2] and (b) [Ni(L2)2]. Absorption spectra were recorded in THF at 20 8C, luminescence spectra were obtained from frozen toluene glasses at 77 K following excitation at 430 nm.
Figure 1. Cyclic voltammograms of (a) [Ni(L1)2] and (b) [Ni(L2)2] in THF with 0.1 m TBAPF6. The potential sweep rate was 0.1 V s@1 in both cases.
In the oxidative potential sweep, the [Ni(L2)2] complex exhibits a wave at similar potential as [Ni(L1)2] (@0.30 V versus Fc + /Fc), which is attributable to the Ni0 !NiI oxidation (Figure 1 b), but the corresponding NiI !Ni0 reduction wave appears at much more negative potential (@0.76 V vs. Fc + /Fc). This large splitting between anodic (Epa) and cathodic peak potentials (Epc) for the NiI/0 couple is reminiscent of what is known for the CuII/I redox couple.[15] Upon oxidation from d10 to d9, geometrical changes in the coordination sphere occur, and binding of additional ligands can take place. In the case of copper, this has been exploited for motoring molecular machines, based on the principle that CuI favors four-coordinate environments, whereas CuII preferentially forms five- or six-coordinate complexes.[15] For [Ni(L2)2], Epa@Epc = 0.46 V, in line with prior studies on CuII/I that reported Epa@Epc values around 0.5–0.7 V.[15] An additional, quasi-reversible wave in the voltammogram of [Ni(L2)2] at 0.21 V versus Fc + /Fc is tentatively attributed to a thiophene-based oxidation. Chem. Eur. J. 2017, 23, 8577 – 8580
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around 310–320 nm are attributed to p–p* transitions on the L1 and L2 ligands, whereas the bands around 420–425 nm are assigned to MLCT transitions, in analogy to CuI diimine complexes.[1, 9, 11b, 16] The latter have similar extinction coefficients in both complexes (12 000–14 000 m@1 cm@1), whereas the p–p* transitions are more intense in L2 ( & 33 500 m@1 cm@1) than in L1 ( & 10 000 m@1 cm@1), possibly due to greater p-conjugation across the central thiophene unit. The MLCT absorptions of homoleptic CuI diimine complexes occur in a similar spectral range (440–480 nm) with comparable extinction coefficients (3000–15 000 m@1 cm@1), depending on substitution patterns and the possibility for distortion from tetrahedral geometry.[1, 11b] Thus, despite the fact that the metal center in [Ni(L1)2] and [Ni(L2)2] is much easier to oxidize than CuI in homoleptic diimine complexes, the MLCT bands occur in a similar spectral range. This observation is in line with that made recently for Cr0 and Mo0 isocyanide complexes, which exhibit MLCT absorption bands at similar energies as FeII and RuII diimine complexes.[4, 5b] Presumably, this is because L1 and L2 are difficult to reduce, but attempts to measure their reduction potentials have been unsuccessful. In solution at room temperature, no photoluminescence was detectable from [Ni(L1)2] and [Ni(L2)2], but in frozen matrices at 77 K they readily exhibit emission following excitation at 430 nm. In toluene glasses, the luminescence spectra of [Ni(L1)2] and [Ni(L2)2] consist of unstructured bands with maxima at 511 and 554 nm, respectively (dotted traces in Figure 2). The emission-band shape of [Ni(L2)2] (Figure 2 b) is
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Communication substantially broader than that of [Ni(L1)2] (Figure 2 a). L1 and L2 were expected to exhibit somewhat different bite angles when coordinated to Ni0, because of the change from a six-membered to a five-membered central ring unit in the ligand backbone. Furthermore, it seems to be possible that the extent of p conjugation on the ligand backbone is somewhat different in L1 and L2, and this in turn could also have an influence on the emission band shapes. The luminescence decays in frozen toluene at 77 K are bi-exponential for both complexes (Figure S4 in the Supporting Information). Compound [Ni(L1)2] exhibits lifetimes of t1 = 200 ns (53 %) and t2 = 1100 ns (47 %), whereas for [Ni(L2)2], one finds t1 = 230 (57 %) and t2 = 1200 ns (43 %; Table S2 in the Supporting Information). These long lifetimes are compatible with 3 MLCT luminescence, similar to what is observed for CuI diimine complexes under analogous conditions.[1, 11b] The observation of two lifetimes for both complexes likely reflects the presence of different conformers (Figure S5 in the Supporting Information). Similar to solid-state IR spectra, IR spectra recorded in THF solution exhibited only a single band for the C/N stretch (Figure S3 in the Supporting Information), making the possibility of partial ligand de-coordination as a contributor to the bi-exponential luminescence less likely. However, these IR bands are substantially broader than for the free ligands (Figure S2 in the Supporting Information), compatible with the presence of multiple conformers (Figure S5 in the Supporting Information). It seems plausible that the lack of luminescence in fluid solution at room temperature is due to geometrical distortions and, in the case of [Ni(L2)2], coordination of additional ligands in the MLCT excited states. In the case of CuI diimines, pentacoordinate complexes have been observed even with very poor donor solvents, such as toluene.[17] One of the most successful strategies leading to CuI diimines with high luminescence quantum yields and long excited state lifetimes is the introduction of steric bulk around its ligands, to suppress geometrical distortion upon MLCT excitation.[1, 11] Given the larger size of the ligands L1 and L2, as well as the greater distance between metal and ligand backbone compared to 1,10-phenanthroline (phen), it is possible that peripheral phenyl substituents are simply not bulky enough. Moreover, the classic cases of emissive CuI diimine complexes have rigid phen ligands (Scheme 1 a), whereas the backbones of L1 and L2 are fully flexible (more like 2,2’-bipyridine), and this could be an additional reason for the efficient non-radiative relaxation of [Ni(L1)2] and [Ni(L2)2] in fluid solution. Lastly, in [Cu(dpp)2] + and related congeners, interligand p-stacking interactions have been invoked to explain their favorable luminescence properties,[1] but [Ni(L1)2] and [Ni(L2)2] lack analogous interactions. With regard to photoluminescence studies, the field of homoleptic Ni0 isocyanide complexes is now at a point, which was reached in 1978 for CuI diimine complexes, when emission in frozen glasses at 77 K (but not yet at room temperature) was measurable.[18] With nearly 40 years of backlog, there is yet much important photophysics and photochemistry to discover for Ni0 isocyanides, but an important proof-of-principle has now been made with the present study. The use of chelating Chem. Eur. J. 2017, 23, 8577 – 8580
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diisocyanide ligands seems crucial for the observation of lowtemperature luminescence in [Ni(L1)2] and [Ni(L2)2], similar to our prior studies of emissive Cr0 and Mo0 diisocyanide complexes.[4, 5b] Promising future research directions that directly emerge from our work include the exploration of heteroleptic (mixedligand) Ni0 isocyanide complexes (e.g., POP/ diisocyanide combinations),[16, 19] diisocyanide ligands with more rigidified backbones (similar to phen), and isocyanide chelators bearing even more sterically demanding peripheral substituents. Possible applications of emissive Ni0 isocyanide complexes range from luminophores in light-emitting devices to photoredox catalysis and dyes in solar cells.
Acknowledgements This work was supported by the Swiss National Science Foundation through grant number 200021 156063/1. Support from COST action CM1202 (PERSPECT-H2O) is gratefully acknowledged. The authors would like to thank Dr. Daniel H-ussinger and Mr. Thomas Mentener for NMR support. C.B.L. would like to acknowledge a Swiss Government Excellence Postdoctoral Scholarship.
Conflict of interest The authors declare no conflict of interest. Keywords: copper · isocyanide ligands · luminescence · nickel · UV/Vis spectroscopy
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[1] a) N. Armaroli, Chem. Soc. Rev. 2001, 30, 113 – 124; b) R. D. Costa, E. Ort&, H. J. Bolink, Pure Appl. Chem. 2011, 83, 2115 – 2128. [2] C. E. Housecroft, E. C. Constable, Chem. Soc. Rev. 2015, 44, 8386 – 8398. [3] O. Reiser, Acc. Chem. Res. 2016, 49, 1990 – 1996. [4] a) L. A. Beldt, X. Guo, R. Vogel, A. Prescimone, O. S. Wenger, J. Am. Chem. Soc. 2017, 139, 985 – 992; b) L. A. Beldt, O. S. Wenger, Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201701210. [5] a) K. R. Mann, H. B. Gray, G. S. Hammond, J. Am. Chem. Soc. 1977, 99, 306 – 307; b) L. A. Beldt, X. W. Guo, A. Prescimone, O. S. Wenger, Angew. Chem. Int. Ed. 2016, 55, 11247 – 11250. [6] a) W. Sattler, M. E. Ener, J. D. Blakemore, A. A. Rachford, P. J. LaBeaume, J. W. Thackeray, J. F. Cameron, J. R. Winkler, H. B. Gray, J. Am. Chem. Soc. 2013, 135, 10614 – 10617; b) W. Sattler, L. M. Henling, J. R. Winkler, H. B. Gray, J. Am. Chem. Soc. 2015, 137, 1198 – 1205; c) H. Kvapilov#, W. Sattler, A. Sattler, I. V. Sazanovich, I. P. Clark, M. Towrie, H. B. Gray, S. Zalis, A. Vlcek, Inorg. Chem. 2015, 54, 8518 – 8528. [7] a) W. Hieber, Z. Naturforsch. B 1950, 5, 129 – 130; b) F. Klages, K. Mçnkemeyer, Chem. Ber. 1950, 83, 501 – 508; c) R. S. Nyholm, Chem. Rev. 1953, 53, 263 – 308; d) F. A. Cotton, F. Zingales, J. Am. Chem. Soc. 1961, 83, 351 – 355; e) H. Behrens, K. Meyer, Z. Naturforsch. B 1966, 21, 489 – 490; f) S. Otsuka, A. Nakamura, Y. Tatsuno, J. Am. Chem. Soc. 1969, 91, 6994 – 6999; g) Y. Yamamoto, Coord. Chem. Rev. 1980, 32, 193 – 233; h) R. J. Angelici, M. H. Quick, G. A. Kraus, D. T. Plummer, Inorg. Chem. 1982, 21, 2178 – 2184; i) Y. Ito, K. Kobayashi, T. Saegusa, J. Organomet. Chem. 1986, 303, 301 – 308; j) L. Weber, Angew. Chem. Int. Ed. 1998, 37, 1515 – 1517; Angew. Chem. 1998, 110, 1597 – 1599; k) F. E. Hahn, M. Mender, R. Frçhlich, Z. Naturforsch. B 2004, 59, 850 – 854; l) B. J. Fox, M. D. Millard, A. G. DiPasquale, A. L. Rheingold, J. S. Figueroa, Angew. Chem. Int. Ed. 2009, 48, 3473 – 3477; Angew. Chem. 2009, 121, 3525 – 3529; m) C. L. Perrine, M. Zeller, J. Woolcock, T. M. Styranec, A. D. Hunter, J. Chem. T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
[8]
[9] [10]
[11]
[12]
Crystallogr. 2010, 40, 289 – 295; n) B. M. Emerich, C. E. Moore, B. J. Fox, A. L. Rheingold, J. S. Figueroa, Organometallics 2011, 30, 2598 – 2608; o) B. R. Barnett, J. S. Figueroa, Chem. Commun. 2016, 52, 13829 – 13839. a) R. F. Ziolo, S. Lipton, Z. Dori, J. Chem. Soc. Chem. Commun. 1970, 1124 – 1125; b) J. V. Caspar, J. Am. Chem. Soc. 1985, 107, 6718 – 6719; c) R. C. G. Frem, A. C. Massabni, A. M. G. Massabni, A. E. Mauro, Inorg. Chim. Acta 1997, 255, 53 – 58; d) H. Kunkely, A. Vogler, Inorg. Chem. Commun. 2000, 3, 143 – 144; e) H. Kunkely, A. Vogler, J. Organomet. Chem. 2003, 684, 113 – 116. C. O. Dietrich-Buchecker, P. A. Marnot, J. P. Sauvage, J. R. Kirchhoff, D. R. McMillin, J. Chem. Soc. Chem. Commun. 1983, 513 – 515. a) D. B. Bagal, G. Kachkovskyi, M. Knorn, T. Rawner, B. M. Bhanage, O. Reiser, Angew. Chem. Int. Ed. 2015, 54, 6999 – 7002; Angew. Chem. 2015, 127, 7105 – 7108; b) M. Knorn, T. Rawner, R. Czerwieniec, O. Reiser, ACS Catal. 2015, 5, 5186 – 5193. a) M. K. Eggleston, D. R. McMillin, K. S. Koenig, A. J. Pallenberg, Inorg. Chem. 1997, 36, 172 – 176; b) N. Armaroli, G. Accorsi, F. Cardinali, A. Listorti, Top. Curr. Chem. 2007, 280, 69 – 115. F. Monti, A. Baschieri, E. Matteucci, A. Mazzanti, L. Sambri, A. Barbieri, N. Armaroli, Faraday Discuss. 2015, 185, 233 – 248.
Chem. Eur. J. 2017, 23, 8577 – 8580
www.chemeurj.org
[13] a) F. E. Hahn, Angew. Chem. Int. Ed. Engl. 1993, 32, 650 – 665; Angew. Chem. 1993, 105, 681 – 696; b) F. E. Hahn, M. Tamm, Angew. Chem. Int. Ed. Engl. 1991, 30, 203 – 205; Angew. Chem. 1991, 103, 213 – 215. [14] T. T. Tsou, J. K. Kochi, J. Am. Chem. Soc. 1979, 101, 6319 – 6332. [15] a) A. Livoreil, J. P. Sauvage, N. Armaroli, V. Balzani, L. Flamigni, B. Ventura, J. Am. Chem. Soc. 1997, 119, 12114 – 12124; b) F. Durola, J. P. Sauvage, O. S. Wenger, Coord. Chem. Rev. 2010, 254, 1748 – 1759. [16] D. G. Cuttell, S. M. Kuang, P. E. Fanwick, D. R. McMillin, R. A. Walton, J. Am. Chem. Soc. 2002, 124, 6 – 7. [17] L. X. Chen, G. Jennings, T. Liu, D. J. Gosztola, J. P. Hessler, D. V. Scaltrito, G. J. Meyer, J. Am. Chem. Soc. 2002, 124, 10861 – 10867. [18] M. T. Buckner, D. R. McMillin, J. Chem. Soc. Chem. Commun. 1978, 759 – 761. [19] R. Molteni, K. Edkins, M. Haehnel, A. Steffen, Organometallics 2016, 35, 629 – 640.
Manuscript received: January 9, 2017 Version of record online: March 29, 2017
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