Feuerstein, Thomas J.

Gamer, Michael T.

Jung, André P.

Kappes, Manfred M.

Köppe, Ralf

Lebedkin, Sergei

Roesky, Peter W.

Seifert, Tim P.

Uhlmann, Cedric

English

KITopen

DaltonTransactionsCOMMUNICATIONCite this: Dalton Trans., 2022, 51,10357Received 14th February 2022,Accepted 4th June 2022DOI: 10.1039/d2dt00458ersc.li/daltonLuminescent early-late-hetero-tetranuclear groupIV – Au(I) bisamidinate complexes†Cedric Uhlmann, a Thomas J. Feuerstein, a Tim P. Seifert,a André P. Jung, aMichael T. Gamer, a Ralf Köppe,a Sergei Lebedkin,b Manfred M. Kappesb,c andPeter W. Roesky *aThe versatile metalloligand [{HCuCC(NDipp)2}2Au2] (dipp = 2,6-diisopropylphenyl) was converted into early-late heterotetrametal-lic complexes [{ClCp2MCuCC(NDipp)2}2Au2] (M = Ti, Zr). Thesecompounds show photoluminescence with either remarkablydifferent (Ti) or similar (Zr) features as compared to related solelycoinage metal containing acetylide amidinate complexes.Complexes that contain both early and late transition metals,so-called early-late heterobimetallics (ELHBs), have been ofparticular interest for quite some time1,2 and have become aconstantly growing research topic.3,4 The main reason for thisinterest is their potential utility in homogeneous catalysis,since the pairing of electronically different metal cores in onecomplex may lead to cooperative effects.3,4 Such electroniccommunication is thought to play a role in heterogeneous cat-alysts, for instance, in those containing late transition metalslike Au which are dispersed on Lewis acidic supports like TiO2.These catalysts often show increased catalytic activity in com-parison to their precious metal free congeners.5This increase is attributed to the electronic interactionbetween the electron-poor and electron-rich metal atoms andtheir substrate activation via different coordination sites.6However, detailed evidence for this phenomenon is scarce andsuch cooperative interactions are not well understood so far.Hence, the assembly of two electronically different metalswithin one complex can act as a model system to investigateintermetallic communication and cooperative behaviour withrespect to luminescent or catalytic properties.7The synthesis of early-late heterobimetallic complexes(ELHBs) in principle requires a ligand system, which combinesdifferent donor sites with an affinity for the electron deficientand electron rich metal atoms, respectively.8–10 This is, forexample, a combination of a hard donor centre like an alkox-ide and a soft donor centre like a phosphine.11,12 However, bya stepwise complex synthesis and utilization of metalloligands,it is also possible to use ligands providing donor sites with anaffinity for both metal atoms. This requires that the firstimplemented metal is sufficiently stabilized by the ligandsystem. Such condition can be realized, for instance, withfunctionalized alkynes.13From a photophysical point of view, interesting metalsregarding the synthesis of ELHBs include gold and zirconium.Indeed, gold(I) complexes are well known for effective phos-phorescence due to the heavy atom effect which can be furthermodulated by metallophilic interactions.14–20 Zirconium(IV)complexes, particularly derivatives exhibiting cyclopentadienylligands,21 often show distinct PL in the visible spectral rangeas well.22Herein we report on the successful application of ethinyl-amidinates for the synthesis of tetranuclear group IV – gold(I)ELHB complexes. Since gold(I) amidinates come along withshort, fully supported aurophilic interactions23 and theimplemented ligand system furthermore exhibits a conjugatedπ-system which bridges the early and late metal atoms, thegroup IV – gold(I) ELHB compounds represent promisingobjects for photophysical studies. Following our previouslypublished reaction protocol, at first, the complex [{HCuCC(NDipp)2}2Au2] was lithiated in situ with two equivalents ofn-butyllithium at −78 °C in tetrahydrofuran.24 Subsequently,two equivalents of the precursors Cp2MCl2 (M = Ti, Zr) wereadded to obtain the complexes [{ClCp2MCuCC(NDipp)2}2Au2],1 (M = Ti) and 2 (M = Zr) (Scheme 1). Compound 1 wasobtained in form of a dark red solid. Remarkably, the driedcompound is air stable over at least one month. In comparisonto the starting material, in the 1H NMR spectrum of 1, similarresonances for the protons belonging to the 2,6-diisopropyl-phenyl (Dipp) moieties are observed.As expected, no resonance was detected in the range of theterminal alkyne protons and, a singlet arises for the cyclopen-†Electronic supplementary information (ESI) available. CCDC 2151930-2151931.For ESI and crystallographic data in CIF or other electronic format see DOI:https://doi.org/10.1039/d2dt00458eaInstitute for Inorganic Chemistry, Karlsruhe Institute of Technology (KIT),Engesserstr. 15, 76131 Karlsruhe, Germany. E-mail: roesky@kit.edubInstitute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, GermanycInstitute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, GermanyThis journal is © The Royal Society of Chemistry 2022 Dalton Trans., 2022, 51, 10357–10360 | 10357Open Access Article. Published on 01 July 2022. Downloaded on 7/27/2022 11:07:43 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuetadienyl protons at δ = 5.66 ppm. Furthermore, the Cp-carbonatoms are detected at δ = 116.2 ppm as the most intense reso-nance in the 13C{1H} NMR spectrum of 1. The alkyne carbonatoms are detected at δ = 154.8 and 120.2 ppm. These are sig-nificantly shifted in comparison to [{HCuCC(NDipp)2}2Au2](89.1 and 75.8 ppm) and comparable to the chemical shifts for[{(PPh3)AuCuCC(NDipp)2}2Au2] (155.3 and 123.7 ppm).24Moreover, in the IR spectrum of 1 only a very weak bandappears in the range of CuC bonds, but a very strongvibrational mode is detected at ν̃(CuC) = 2088 cm−1 in theRaman spectrum. Single crystals suitable for X-ray analysis ofcompound 1 were obtained in form of red prisms. Thecomplex crystallizes in the triclinic space group P1̄ featuringan inversion centre in the centre of the molecule in betweenthe Au atoms. The titanium atoms in the molecular structureof 1 in the solid state (Fig. 1) are terminally coordinated to theacetylide moieties. The Ti1–C3–C2 angle of 175.1(8)° is near tothe ideal value of 180°. The Cl1–Ti1–C3 angle of 95.4(2)° andthe Ti1–C3 bond length of 2.100(6) Å are in consistency withsimilar compounds.25 The Ti–Cl as well as the Ti–Cp distancesare comparable to the starting material Cp2TiCl2.26 The centralstructural motif, which consists of two NCN units and twogold(I) ions, only shows insignificant deviations from theparent compound and the CuC bond length C2–C3 of1.210(8) Å is in the typical range for a C–C triple bond. Thezirconium complex [{ClCp2ZrCuCC(NDipp)2}2Au2] (2) wasobtained as a yellow solid after a similar work-up as for 1. Incontrast to 1, however, 2 is extremely sensitive to moisture andpartially gets hydrolyzed to yield the starting material[{HCuCC(NDipp)2}2Au2] and the zirconium bridged oxide(Cp2ZrCl)2μ–O. The measured unit cell was in accordance withliterature27 and the NMR-spectra showed only the startingmaterial [{HCuCC(NDipp)2}2Au2] as shown in our previouswork.24 In consistency with 1, the 1H NMR spectrum of the Zrcomplex 2 also shows no resonance for terminal alkyneprotons. Therefore, a distinct singlet resonance appears for thecyclopentadienyl protons at δ = 5.46 ppm. In the 13C{1H} NMRspectrum the corresponding Cp–CH resonance is observed atδ = 113.2 ppm, while the alkyne carbon atoms are detected atδ = 149.3 and 115.8 ppm. The IR and Raman spectra appearvery similar to the spectra of 1. However, the stretching modeof the CuC bonds at ν̃(CuC) = 2094 cm−1 in the Ramanspectrum of 2 is slightly shifted to higher wavenumbers incomparison with 1. Single crystals of compound 2 wereobtained from toluene/n-pentane. Thereby the compound crys-tallizes analogously to 1 in the triclinic space group P1̄ (Fig. 2).The asymmetric unit contains half of a molecule with acrystallographic inversion centre in the middle of the two goldatoms. Like for complex 1, a terminal acetylide–metal coordi-nation with a Zr1–C3–C2 angle of 178.6(6)° is observed. TheCl1–Zr1–C3 angle of 99.7(2)° as well as the Zr1–C3 bond lengthof 2.236(6) Å are expectedly slightly larger in comparison to 1.All parameters for the central motif are in consistency with thestarting material24 and the C2–C3 bond length of 1.193(8) Å isstill in the typical range for a CuC triple bond.Subsequently, we also intended to synthesize an analogoushafnium compound. However, although the colour andluminescence of the reaction solution appeared like those forcomplex 2, we were not able to isolate a neat product.Following the synthesis and characterization, we thoroughlyinvestigated the PL behaviour of 1 and 2 in the solid (polycrys-talline) state at variable temperatures between 20 and 295 K.Of particular interest was the influence of the group 4 metallo-cene chloride moieties on the PL properties with regard to theScheme 1 Synthesis of the compounds [{ClCp2TiCuCC(NDipp)2}2Au2](1) and [{ClCp2ZrCuCC(NDipp)2}2Au2] (2).Fig. 1 Molecular structure of compound 1 in the solid state. Hydrogenatoms are omitted for clarity. Structural parameters are given in the ESI.†(Cg = ring centroid).Fig. 2 Molecular structure of compound 2 in the solid state. Hydrogenatoms are omitted for clarity. Structural parameters are given in the ESI.†(Cg = ring centroid).Communication Dalton Transactions10358 | Dalton Trans., 2022, 51, 10357–10360 This journal is © The Royal Society of Chemistry 2022Open Access Article. Published on 01 July 2022. Downloaded on 7/27/2022 11:07:43 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineclosely related di- and tetranuclear gold(I) bisethinylamidi-nates we recently reported.24The zirconium complex 2 emits bright yellow phosphor-escence with the maximum at 545 nm (Fig. 3 and Table 1),high quantum yield ΦPL = 41% and lifetime of 5.9 μsec atambient temperature (Fig. S13 and Table S2†). Its intensitymoderately increases upon cooling to 20 K (with ΦPL ≈ 60% asestimated from the temperature-dependent PL spectra),roughly correlating to the increased PL lifetime (9.8 μsec).These PL properties are similar to those of the related di- andtetranuclear gold(I) bisethinylamidinates,24 with the exceptionof a ca. 50 nm blueshift of the emission and PL excitation(PLE) onset observed for the latter.In contrast, absorption and phosphorescence of the tita-nium complex 1 are strongly redshifted clearly indicating theinfluence of the group 4 metal coordinated to the acetylidefunction. The PLE onset at ca. 620 nm corresponds to a darkred visual appearance of the solid compound. Such increase ofthe energy gap from Ti to Zr (and further to Hf) appears to becommon for homologous complexes with the lowest unoccu-pied molecular orbitals (MOs) mainly contributed by d orbitalsof these metals (see below).28 The red emission of 1 has amaximum at 720/705 nm (at 20/295 K), decays on a sub-micro-second timescale (Fig. S13†) and shows a quantum efficiencyof only 0.6% at ambient temperature (Table S2†). The differentemission colours of 1 and 2 are illustrated in the CIE 1931diagram (Fig. S14†). In addition, complex 1 features a rare typeof temperature dependence of the (integral) PL intensity, mod-erately decreasing upon cooling below 250 K (Fig. 3). Theseobservations suggest that the PL properties of 1 are cruciallydetermined by the titanocene terminal moieties.The similarities and differences between 1, 2 and therelated tetranuclear gold(I) bisethinylamidinate complex[{(PPh3)AuCuCC(NDipp)2}2Au2]24 were further elucidated byDFT calculations (for details see the ESI†). The character ofthe triplet (emitting) state is similar in 1 and 2, but distinct inthe Au2Au2 complex. The non-relaxed density plots reveal exci-tations predominantly promoted by occupied MOs of theAu2N4 core and unoccupied d orbitals of Ti and Zr in 1 and 2,whereas a contribution of the terminal Au atoms in the Au4complex is negligible, only unoccupied MOs of the phosphineterminal groups are involved (Fig. 4). The different charactersof the triplet state may have connection with the observationof vibronically well-structured emission of the Au2Au2complex,24 whereas this is absent in 1 and 2 (Fig. 3). Namely,the vibronic coupling might be contributed by the phosphinegroups in the former compound. The calculations underesti-mated the emission (phosphorescence) energies by about 0.3eV for 1 and Au2Au2 complexes and by 0.5 eV for 2 as com-pared with the experimental values obtained for solid (crystal-line) samples (Table S3†). A trend of the relative emission is,however, reproduced much better, within 0.2 eV.Fig. 3 Left: Normalized photoluminescence excitation (PLE) and emis-sion (PL) spectra of compounds 1 and 2 at 20, 100 and 295 K. The PL/PLE spectra were excited/recorded at 350/710 nm (1) and 360/550 nm(2). Right: The integral PL intensity vs. temperature.Fig. 4 Non-relaxed transition densities calculated for the T1 tripletexcitations of tetranuclear Ti2Au2 (1) and Zr2Au2 (2) complexes in com-parison to the structurally related tetranuclear gold(I) bisethinylamidi-nate [{Ph3PAu2MCuCC(NDipp)2}2Au2].24 The contributions of occupiedorbitals are plotted in pale blue and those of unoccupied orbitals inviolet.Table 1 Absorption onset and emission maximum for solid compounds1, 2 and the binuclear Au2 precursor complex [{HCuCC(NDipp)2}2Au2](ref. 24)Compound Absorption (nm) 295 K Emission (nm) 20/295 KAu2 430 500a/490aTi2Au2 (1) 620 720/705Zr2Au2 (2) 490 545/550a Centre wavelength.Dalton Transactions CommunicationThis journal is © The Royal Society of Chemistry 2022 Dalton Trans., 2022, 51, 10357–10360 | 10359Open Access Article. Published on 01 July 2022. Downloaded on 7/27/2022 11:07:43 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineConclusionsTwo new early-late heterobimetallic complexes [{ClCp2MCuCC(NDipp)2}2Au2] (M = Ti, Zr) and their photophysical characteristicsare presented. This was achieved via direct lithiation of a pre-viously used ethynyl compound [{HCuCC(NDipp)2}2Au2] followedby the reaction with Cp2MCl2 (M = Ti, Zr). The heavier hafniumanalogue could not be isolated, even after several attempts.The zirconium complex emits yellow phosphorescence withquantum yields of 41% in the solid state under ambient temp-erature and 60% upon cooling to 20 K. In contrast, the tita-nium complex emits red phosphorescence with a quantumyield of only 0.6% at ambient conditions. Additionally, it fea-tures a rare type of the temperature dependence of the PLintensity, moderately decreasing upon cooling below 250 K.This distinction in the otherwise structurally very similarcompounds is likely due to the different electronic interactionbetween the group IV metals and the gold(I) moieties as indicatedby the TDDFT calculations. Along similar lines, substitution ofthe remaining chloride atom on the group IV metal may lead to aseries of compounds with adjustable photoluminescent pro-perties and therefore could bring more insight into the role ofmetal–metal interactions in determining the photoluminescentbehaviour of early-late-heterobimetallic complexes.Experimental sectionExperimental details are found in the ESI.†Conflicts of interestThere are no conflicts to declare.AcknowledgementsFinancial support by the DFG funded transregional collabora-tive research centre SFB/TRR 88 “Cooperative Effects in Homoand Heterometallic Complexes (3MET)” is gratefully acknowl-edged (projects C1 and C7). The authors acknowledge supportby the state of Baden-Württemberg through bwHPC and theGerman Research Foundation (DFG) through grant no INST40/575-1 FUGG (JUSTUS 2 cluster).Notes and references1 D. W. Stephan, Coord. Chem. Rev., 1989, 95, 41–107.2 N. Wheatley and P. Kalck, Chem. Rev., 1999, 99, 3379–3420.3 P. Buchwalter, J. Rosé and P. Braunstein, Chem. Rev., 2015,115, 28–126.4 E. Bodio, M. Picquet and P. Le Gendre, in Homo- andHeterobimetallic Complexes in Catalysis: CooperativeCatalysis, ed. P. Kalck, Springer International Publishing,Cham, 2016, pp. 139–186. DOI: 10.1007/3418_2015_161.5 S. J. Tauster, Acc. Chem. Res., 1987, 20, 389–394.6 S. S. Langer, E. Chatt, B. T. Heaton, D. M. P. Mingos,J. R. Dilworth, C. D. Garner, G. Frenking, R. L. Richards,G. A. Gamlen and J. Lewis, Modern Coordination Chemistry:The Legacy of Joseph Chatt, Royal Society of Chemistry,Cambridge, 2007.7 U. Helmstedt, S. Lebedkin, T. Höcher, S. Blaurock andE. Hey-Hawkins, Inorg. Chem., 2008, 47, 5815–5820.8 P. Braunstein and F. Naud, Angew. Chem., Int. Ed., 2001, 40,680–699.9 S. Fritzsche, P. Lönnecke, T. Höcher and E. Hey-Hawkins,Z. Anorg. Allg. Chem., 2006, 632, 2256–2267.10 F. Völcker, F. M. Mück, K. D. Vogiatzis, K. Fink andP. W. Roesky, Chem. Commun., 2015, 51, 11761–11764.11 R. Ruiz, J. Faus, F. Lloret, M. Julve and Y. Journaux, Coord.Chem. Rev., 1999, 193–195, 1069–1117.12 S. Bestgen, C. Schoo, C. Zovko, R. Köppe, R. P. Kelly,S. Lebedkin, M. M. Kappes and P. W. Roesky, Chem. – Eur.J., 2016, 22, 7115–7126.13 V. Fernández-Moreira, I. Marzo and M. C. Gimeno, Chem.Sci., 2014, 5, 4434–4446.14 M. Olaru, J. F. Kögel, R. Aoki, R. Sakamoto, H. Nishihara,E. Lork, S. Mebs, M. Vogt and J. Beckmann, Chem. – Eur. J.,2020, 26, 275–284.15 H. Schmidbaur and A. Schier, Chem. Soc. Rev., 2012, 41,370–412.16 H. Schmidbaur and A. Schier, Chem. Soc. Rev., 2008, 37,1931–1951.17 V. W.-W. Yam and E. C.-C. Cheng, Chem. Soc. Rev., 2008,37, 1806–1813.18 J. M. López-de-Luzuriaga, M. Monge and M. E. Olmos,Dalton Trans., 2017, 46, 2046–2067.19 T. P. Seifert, V. R. Naina, T. J. Feuerstein, N. D. Knöfel andP. W. Roesky, Nanoscale, 2020, 12, 20065–20088.20 V. R. Naina, F. Krätschmer and P. W. Roesky, Chem.Commun., 2022, 58, 5332–5346.21 G. V. Loukova and V. A. Smirnov, Chem. Phys. Lett., 2000,329, 437–442.22 L. Kuhlmann, R. Methling, J. Simon, B. Neumann,H.-G. Stammler, C. A. Strassert and N. W. Mitzel, DaltonTrans., 2018, 47, 11245–11252.23 T. J. Feuerstein, M. Poß, T. P. Seifert, S. Bestgen,C. Feldmann and P. W. Roesky, Chem. Commun., 2017, 53,9012–9015.24 T. J. Feuerstein, T. P. Seifert, A. P. Jung, R. Müller,S. Lebedkin, M. M. Kappes and P. W. Roesky, Chem. – Eur.J., 2020, 26, 16676–16682.25 H. Lang, S. Blau, H. Pritzkow and L. Zsolnai,Organometallics, 1995, 14, 1850–1854.26 A. Clearfield, D. K. Warner, C. H. Saldarriaga-Molina,R. Ropal and I. Bernal, Can. J. Chem., 1975, 53, 1622–1629.27 J. T. Spletstoser, J. M. White, A. R. Tunoori and G. I. Georg,J. Am. Chem. Soc., 2007, 129, 3408–3419.28 X. Wang, L. Chen, A. Endou, M. Kubo and A. Miyamoto,J. Organomet. Chem., 2003, 678, 156–165.Communication Dalton Transactions10360 | Dalton Trans., 2022, 51, 10357–10360 This journal is © The Royal Society of Chemistry 2022Open Access Article. Published on 01 July 2022. Downloaded on 7/27/2022 11:07:43 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

Luminescent early-late-hetero-tetranuclear group IV – Au( i ) bisamidinate complexes

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Luminescent early-late-hetero-tetranuclear group IV – Au( i ) bisamidinate complexes

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