Quantum Yields over 80% Achieved in Luminescent Europium

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Quantum Yields over 80% Achieved in Luminescent Europium Complexes by Employing Diphenylphosphoryl Tridentate Ligands Chen Wei, Boxun Sun, Zelun Cai, Zifeng Zhao, Yu Tan, Weibo Yan, Huibo Wei,* Zhiwei Liu,* Zuqiang Bian, and Chunhui Huang Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China

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S Supporting Information *

and in some situations reduced energy-transfer efficiency.13 Nevertheless, the frequently used oxygen site (carboxyl/ phosphonate group) usually leads to the embedding of water molecules in the first/second coordination sphere because of its hydrophilic character.14 Consequently, the PLQYs of most tridentate europium(III) complexes reported so far are below 70% (Table S1). Herein, we introduce diphenylphosphoryl (DPPO), a hydrophobic oxygen-containing functional group, to a series of bidentate ligands, hence constructing tridentate ligands and their europium(III) complexes (Chart 1), with aim of high

ABSTRACT: Four tridentate europium(III) complexes containing a diphenylphosphoryl group are prepared with strong bonding between the ligands and centered ion, convinced by crystal structures. Compared to their parent bidentate complexes, the tridentate complexes display improved and exceptionally high photoluminescence quantum yields (PLQYs) in powder (all over 80%, best 91%), as well as in a CH2Cl2 solution and poly(methyl methacrylate) films, benefiting from compact, stable, and saturated coordination.

Chart 1. Tridentate Europium(III) Complexes Employing DPPO Group

L

uminescent lanthanide complexes have been widely used in bioimaging and sensing, solid-state lighting, and lightconversion devices because of their special photophysical properties.1High photoluminescence quantum yield (PLQY) is quite essential for these applications in order to get high signal intensity as well as high energy conversion efficiency, which requires the specific design of the complex structure, especially the ligands. Because the ligands serve as “antenna” to sensitize the luminescence of a lanthanide (Ln3+) ion, the proper energy level (usually a triplet state) and tight coordination (short distance between the ligands and Ln3+ ion) are preferred to achieve high energy-transfer efficiency.2 The ligands are also expected to occupy the lanthanide coordination sites as much as possible for eliminating quenching by the coordinated solvent.3 Other deactivation pathways such as low-lying ligand-to-metal charge-transfer (LMCT) states and O−H or N−H vibrations should be avoided when designing new lanthanide complexes.2 With these considerations, many types of ligands have been developed, among which bidentate ligands like β-diketone,4 1,10phenanthroline,5 and 2,2′-bipyridine6 are the most studied ones because of their ease of fabrication. Nevertheless, bidentate ligands, especially the neutral ones, suffer from weak chelating stability with the Ln3+ ion, which may lead to ease of destruction of the complex in solution or when heated. Introducing another coordination site onto bidentate ligands in order to prepare tridentate ligands is a simple way to increase the coordination stability. However, up to now, functional groups that can be grafted onto bidentate ligands are mostly N-coordination moieties, such as pyridine,7 benzimidazole,8 and tetrazole,9 while the groups containing coordinated oxygen sites are limited primarily to carboxyl10 and phosphinate/phosphonate derivatives.11 In fact, a nitrogen coordination site bonds less tightly with Ln3+ ion than an oxygen site, causing less stable coordination12 © XXXX American Chemical Society

PLQYs. Two methods were used to integrate DPPO with the bidentate ligands. HL1 (corresponding to the free ligand in Eu1, similarly hereinafter) and HL2 were initially synthesized by the nucleophilic substitution reaction between potassium diphenylphosphanide15 and a chloro/bromo precursor, followed by hydrogen peroxide oxidation (Scheme S1). However, potassium diphenylphosphanide is a very sensitive and strong nucleophile; thus, side reactions always take place. Alternatively, a modified nickel-catalyzed C−P cross-coupling method16 was adopted for HL1 by the reaction of a bromo precursor and diphenylphosphine oxide and proved to be a better route with a mild synthetic condition and good yield (Scheme S1). Therefore, L3 and L4 were both synthesized using a catalyzed reaction (Scheme S2). The corresponding complexes Eu(L)3 (Eu1 and Eu2 with L = L1 and L2, respectively) were prepared by the “self-assembly” of the ligand, NaOH, and EuCl3·6H2O in a ratio of 3:3:1 in methanol Received: April 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01028 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

while the sharp bands at 580, 592, 615, 650, 692, and 812 nm are emissions of the EuIII ion, originating from the transitions of 5D0 to 7FJ (J = 0−5). Different intensity proportions (e.g., 592/615 nm) of the four complexes are ascribed to the various coordinating environments (symmetry) of the centered ion.21 Similar emission spectra were obtained in solution and powder compared to those in PMMA films for each individual complex (Figure S2), indicating favorable coordination stability without much dissociation in a dilute solution or PMMA film. This is in accordance with the luminescent lifetimes, which are all single exponential for the tridentate complexes (Table 1 and Figure S3). From the edge of the UV absorption spectra (Figure S4), we can obtain the singlet energy (Eg) of the ligands, which are listed in Table 1, together with the triplet-energy information (ET) obtained from the 0-phonon component of phosphorescence spectra of corresponding gadolinium(III) complexes in the glass state at 77 K (Figure S4). It is noteworthy that the triplet-energy levels are almost unchanged (for L1 and L4) or slightly lowered (ΔET < 850 cm−1 for L2 and L3) after grafting the DPPO group to their parent bidentate ligands (Table 1). This phenomenon is interpretable because the sp3 hybrid nature of the phosphoryl group does not significantly enlarge the conjugation system, different from the commonly used sp2 functional groups. This is of great importance considering that the introduction of a third coordination group usually changes the energy level of the parent ligand as well as the luminescent efficiency of the complex. For further illustration, two tridentate ligands were prepared by introducing two kinds of sp2 hybridization functional groups, acetyl and pyridine groups, onto the bidentate ligand L2′ (products named AcMCND and PyMCND, respectively), which showed much-reduced triplet-energy level (ΔET = 1850−2770 cm−1) and hence quite poor behavior in the sensitizing EuIII ion (Figure 3 and Table 1). Absolute PLQYs of the complexes were measured using a Hamamatsu C9920-02 PLQY measurement system with an integrating sphere.22 Exceptionally high PLQYs were obtained for all of the tridentate complexes (Eu1−Eu4; all over 80%, best 91%, in powder; Table 1), which are far better than those of the corresponding bidentate complexes (Eu1′−Eu4′, in the range of 10−73%) and are among the highest values of europium(III) complexes in the literature.23 To explain the substantial increase of PLQYs, we calculated the sensitization efficiencies (ηsens) and intrinsic quantum yields Eu ) of the corresponding complexes in PMMA films, (ΦEu according to eqs S1 and S2. The results in Table 1 show that two categories can be classified: one is the (O)^N^O ionic ligand-based complexes Eu1′ (Eu1) and Eu2′ (Eu2); the other is the (O)^N^N neutral ligand-based complexes Eu3′ (Eu3) and Eu4′ (Eu4). For the first category, both bidentate and tridentate complexes display very high ηsens (near unity), whereas ΦEu Eu dramatically increases after DPPO modification. This increased III ion by ΦEu Eu is attributed to the better protection of the Eu tridentate ligands than by bidentate ones. Taking Eu2 and Eu2′, for example, the tridentate complex Eu2 possesses a saturated complex structure without solvent coordination, as disclosed by the crystal structure shown above. However, the bidentate complex Eu2′ is very likely to contain quenching solvent molecules due to its complicated and unsaturated coordination feature, which, although not directly proven here, could be verified from the crystal structure of a similar bidentate complex that we reported earlier (Na[Eu(8mCND)4(H2O)]),24 possessing one coordinating water molecule (Figure S6). For the second

(Scheme S3), and [Eu(L)3](OTf)3 (Eu3 and Eu4 with L = L3 and L4; OTf = CF3SO3) were synthesized through 3:1 stoichiometry of the ligand and Eu(OTf)3 in CH2Cl2. For comparison, four europium(III) complexes based on the parent bidentate ligands were also synthesized with the formulas of Na[Eu(L)4] (Eu1′ and Eu2′, where L = L1′ and L2′) and [Eu(L)4](OTf)3 (Eu3′ and Eu4′, where L = L3′ and L4′). By slow evaporation of the complex solution, high-quality single crystals of Eu1, Eu2, and Eu4 were obtained. X-ray diffraction reveals the 3:1 ligand/Eu structures of the complexes with a coordination number of 9 (Figure 1 and Table S2). No

Figure 1. Perspective view of (a) Eu1, (b) Eu2, and (c) Eu4 at the 50% probability level, with Eu in cyan, N in blue, O in red, P in orange, and C in gray. Hydrogen atoms, counterions, and uncoordinated solvent are omitted for clarity.

solvent molecule is observed in the first coordination sphere, which can substantially eliminate the quenching of luminescence from O−H vibration. The polyhedra around the metal ion (Figure S1) are best categorized to a tricapped trigonal prism by calculating the degree of distortion of the title compounds from all possible nine-atom polyhedral structures17 using the continuous shape measures method (Table S3).18 The average coordination bond length of the complexes (2.48 Å for Eu1 and Eu2 and 2.54 Å for Eu4; Table S4) is shorter than that of the complexes based on tridentate N^N^N (2.55−2.58 Å)9,19 or bidentate N^N (2.62−2.67 Å)20 ligands. This compact coordination is beneficial for energy transfer from the ligands to centered ion. Eu1−Eu4 showed strong red emission in solution, poly(methyl methacrylate) (PMMA) film, and powder under UV irradiation (Figure 2a,b). Figure 2c displays the excitation and emission spectra of the four tridentate europium(III) complexes in PMMA films (1%, w/w). The broad bands ranging from 230 to 410 nm are excitation profiles of the corresponding ligands,

Figure 2. Photograph of (a) a PMMA film doped with Eu2 (5%, w/w) and (b) luminescent logo celebrating the 120th anniversary of Peking University employing anticounterfeiting inks containing Eu2, under 365 and 254 nm UV lamps, respectively. (c) Excitation and emission spectra of the tridentate europium(III) complexes in PMMA (1%, w/w) films. λem = 614 nm and λex = 270, 360, 300, and 320 nm for Eu1 to Eu4, respectively. B

DOI: 10.1021/acs.inorgchem.8b01028 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Table 1. Photophysical Properties of the Europium(III) Complexes PLQY (%)c complex Eu1 Eu2 Eu3 Eu4 Eu1′ Eu2′ Eu3′ Eu4′ Eu(PyMCND)3 Eu(AcMCND)3

Ega

−1

(cm )

35210 25130 29070 30670 35590 27400 29590 30580 23870 22220

ETb

−1

(cm )

27780 20750 21410 23260 27620 21460 22220 23360 19610 18690

powder

solution

PMMA

τobsd (ms)

τradd (ms)

f ΦEu Eu

ηsensg

Tdh (°C)

81 91 91 82 73 31 10 14 3.5 0.64

73 88 76 42 53 49 16 11 0.77 0.71

82 92 91 65 70 42 13 5.8 9.4 0.80

2.44 1.65 2.74 2.49 1.52 0.56 1.03e 0.46e 0.58e 0.47e

3.09 2.03 3.53 3.71 2.24 1.31 2.25 2.82 2.41 1.82

0.79 0.81 0.78 0.67 0.68 0.43 0.46 0.16 0.24 0.26

1.0 1.0 1.0 0.97 1.0 0.98 0.28 0.37 0.39 0.031

414 450 390 304 392 434 226 105 491 378

a

Singlet energy (0-phonon) of the ligand obtained from the edge of UV absorption spectra. bTriplet energy deduced from the 0-phonon component of the phosphorescence spectra. cAbsolute photoluminescence quantum yield measured by Hamamatsu C9920-02 with an integrating sphere. d Observed lifetime τobs and calculated radiative lifetime τrad in the PMMA film. eDual exponential decay. fIntrinsic quantum yields calculated from eqs S1 and S2. gSensitization efficiency calculated by the ratio of PLQYs and intrinsic quantum yields. hDecomposition temperature determined by the loss of 5% desolvated weight. Relative errors: τobs, ±2%; PLQY, ±3%; ΦEu Eu, ±12%; ηsens, ±15%.

sp3 hybrid feature of the phosphoryl group, the energy level of the sensitizing chromophore remains almost unchanged. As a result, the title complexes displayed exceptionally high PLQYs. The DPPO modification strategy can also be applied to other ligands and other lanthanide ions, which are being investigated in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01028. Synthesis protocols and characterization data, photophysical measurements, and thermogravimetric analysis curves (PDF)

Figure 3. Triplet-energy levels and PLQYs (in powder) of Eu2′, Eu2, and control samples Eu(PyMCND)3 and Eu(AcMCND)3, showing much-reduced luminescent efficiency after the introduction of sp2 hybrid pyridine and acetyl groups, due to the reduced energy level. The dashed lines are the locations of the EuIII 5D1 and 5D0 levels.

category, both ηsens and ΦEu Eu are noticeably increased for O^N^Nbased complexes Eu3 and Eu4 compared with bidentate N^Nbased complexes Eu3′ and Eu4′. Weak coordination and relatively long bond lengths are considered to be the main reason for the poor sensitizing ability of neutral bidentate N^N ligands (ηsens = 0.28 and 0.37 for Eu3′ and Eu4′, respectively). Therefore, adding an oxygen-containing DPPO group to such a N^N ligand can bring about much shortened distance between the ligands and centered ion and better suppression of possible dissociation, leading to an increase of ηsens for the tridentate complexes (near unity for both Eu3 and Eu4).25 The improved coordination stability can also be perceived from the enhanced thermal stability of Eu3 and Eu4 compared with that of bidentate Eu3′ and Eu4′, showing an increase of the decomposition temperature (Td) over 160 °C (Table 1 and Figure S7). One may notice that, for Eu4, much reduced PLQY values in a PMMA film and solution compared to that in powder are obtained, which is probably due to the comparably weak rigidity of the bipyridyl group, leading to nonradiative quenching in solution and PMMA with more degrees of freedom. In summary, we have developed a strategy of preparing highly emissive europium(III) complexes by introducing a hydrophobic oxygen-containing functional DPPO group to bidentate ligands. The tridentate ligands and EuIII ion form mononuclear complex structures with ligands occupying all of the coordination sites. The DPPO group draws the ligands closer to the ion and improves the coordination stability of the complex. Thanks to the

Accession Codes

CCDC 1508105, 1814238, and 1814264 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhiwei Liu: 0000-0003-3001-5310 Zuqiang Bian: 0000-0003-4788-6032 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Jiajia Liu and Dr. Xiaonan Yao for their help in crystal refinement. This research is supported through grants from the National Key R&D Program of China (Grants 2016YFB0401001 and 2017YFA0205100), National Natural Science Foundation of China (Grant 21621061), and Beijing Natural Science Foundation (Grant 2172017). C

DOI: 10.1021/acs.inorgchem.8b01028 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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(17) Ruiz-Martinez, A.; Casanova, D.; Alvarez, S. Polyhedral structures with an odd number of vertices: nine-atom clusters and supramolecular architectures. Dalton Trans. 2008, 2583−2591. (18) Pinsky, M.; Avnir, D. Continuous Symmetry Measures. 5. The Classical Polyhedra. Inorg. Chem. 1998, 37, 5575−5582. (19) Chen, J.-L.; Luo, Y.-S.; Gao, G.-P.; Zhao, J.-L.; Qiu, L.; Liu, N.; He, L.-H.; Liu, S.-J.; Wen, H.-R. Luminescent mononuclear Eu(III) and Tb(III) complexes with bipyridyl-tetrazolate tridentate ligands. Polyhedron 2016, 117, 388−393. (20) (a) Shi, S.-M.; Chen, Z.-F.; Liu, Y.-C.; Mao, L.; Liang, H.; Zhou, Z.-Y. Synthesis and crystal structures of lanthanide complexes with foliage growth regulator: phenoxyalkanoic acid. J. Coord. Chem. 2008, 61, 2725−2734. (b) Wang, S.; Zhang, J.; Hou, Y.; Du, C.; Wu, Y. 4,5Diaza-9,9′-spirobifluorene functionalized europium complex with efficient photo- and electro-luminescent properties. J. Mater. Chem. 2011, 21, 7559−7561. (21) Kirby, A. F.; Foster, D.; Richardson, F. S. Comparison of 7FJ←5D0 emission spectra for Eu(III) in crystalline environments of octahedral, near-octahedral, and trigonal symmetry. Chem. Phys. Lett. 1983, 95, 507−512. (22) The instrument was calibrated by the PLQYs of both the PMMA film of Eu(TTA)3(phen) (measured/literature reported 70%/72% (ref 5); TTA = 2-thienoyltrifluoroacetonate; phen = phenantroline) and 1 N H2SO4 solution of quinine sulfate (10−5 M; measured/literature reported 54%/55% (Melhuish, W. H. J. Phys. Chem. 1961, 65, 229−235.)), and each value is given as the average of three measurements. (23) (a) de Bettencourt-Dias, A.; Viswanathan, S.; Rollett, A. Thiophene-Derivatized Pybox and Its Highly Luminescent Lanthanide Ion Complexes. J. Am. Chem. Soc. 2007, 129, 15436−15437. (b) Moudam, O.; Rowan, B. C.; Alamiry, M.; Richardson, P.; Richards, B. S.; Jones, A. C.; Robertson, N. Europium complexes with high total photoluminescence quantum yields in solution and in PMMA. Chem. Commun. 2009, 6649−6651. (c) Gangan, T. V. U.; Sreenadh, S.; Reddy, M. L. P. Visible-light excitable highly luminescent molecular plastic materials derived from Eu3+-biphenyl based β-diketonate ternary complex and poly(methylmethacrylate). J. Photochem. Photobiol., A 2016, 328, 171−181. (24) Wei, H.; Zhao, Z.; Wei, C.; Yu, G.; Liu, Z.; Zhang, B.; Bian, J.; Bian, Z.; Huang, C. Antiphotobleaching: A Type of Structurally Rigid Chromophore Ready for Constructing Highly Luminescent and Highly Photostable Europium Complexes. Adv. Funct. Mater. 2016, 26, 2085− 2096. (25) The near-unity ηsens also suggests that low-lying charge-transfer state (especially the LMCT state) is avoided in the tridentate complexes. Because the bidentate ligands and corresponding tridentate ligands share similar LMCT state levels (the weak electron-withdrawing DPPO group has little effect on the singlet energy and oxidation potential of the ligands), the LMCT state will not quench the luminescence of bidentate complexes either in this study.

REFERENCES

(1) Wei, C.; Ma, L.; Wei, H.; Liu, Z.; Bian, Z.; Huang, C. Advances in luminescent lanthanide complexes and applications. Sci. China: Technol. Sci. 2018, DOI: 10.1007/s11431-017-9212-7. (2) Eliseeva, S. V.; Bunzli, J.-C. G. Lanthanide luminescence for functional materials and bio-sciences. Chem. Soc. Rev. 2010, 39, 189− 227. (3) Haas, Y.; Stein, G. Pathways of radiative and radiationless transitions in europium(III) solutions. The role of high energy vibrations. J. Phys. Chem. 1971, 75, 3677−3681. (4) Raj, D. B. A.; Francis, B.; Reddy, M. L. P.; Butorac, R. R.; Lynch, V. M.; Cowley, A. H. Highly Luminescent Poly(Methyl Methacrylate)Incorporated Europium Complex Supported by a Carbazole-Based Fluorinated β-Diketonate Ligand and a 4,5-Bis(diphenylphosphino)9,9-dimethylxanthene Oxide Co-Ligand. Inorg. Chem. 2010, 49, 9055− 9063. (5) Freund, C.; Porzio, W.; Giovanella, U.; Vignali, F.; Pasini, M.; Destri, S.; Mech, A.; Di Pietro, S.; Di Bari, L.; Mineo, P. Thiophene Based Europium β-Diketonate Complexes: Effect of the Ligand Structure on the Emission Quantum Yield. Inorg. Chem. 2011, 50, 5417−5429. (6) Wang, D.; Luo, Z.; Liu, Z.; Wang, D.; Fan, L.; Yin, G. Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated βdiketone and nitrogen heterocyclic ligands. Dyes Pigm. 2016, 132, 398− 404. (7) Wei, H.; Yu, G.; Zhao, Z.; Liu, Z.; Bian, Z.; Huang, C. Constructing lanthanide [Nd(III), Er(III) and Yb(III)] complexes using a tridentate N,N,O-ligand for near-infrared organic light-emitting diodes. Dalton Trans. 2013, 42, 8951−8960. (8) Shavaleev, N. M.; Eliseeva, S. V.; Scopelliti, R.; Bünzli, J.-C. G. Influence of Symmetry on the Luminescence and Radiative Lifetime of Nine-Coordinate Europium Complexes. Inorg. Chem. 2015, 54, 9166− 9173. (9) Andreiadis, E. S.; Imbert, D.; Pecaut, J.; Demadrille, R.; Mazzanti, M. Self-assembly of highly luminescent lanthanide complexes promoted by pyridine-tetrazolate ligands. Dalton Trans. 2012, 41, 1268−1277. (10) D’Aléo, A.; Picot, A.; Beeby, A.; Gareth Williams, J. A.; Le Guennic, B.; Andraud, C.; Maury, O. Efficient Sensitization of Europium, Ytterbium, and Neodymium Functionalized Tris-Dipicolinate Lanthanide Complexes through Tunable Charge-Transfer Excited States. Inorg. Chem. 2008, 47, 10258−10268. (11) Andres, J.; Chauvin, A.-S. 6-Phosphoryl Picolinic Acids as Europium and Terbium Sensitizers. Inorg. Chem. 2011, 50, 10082− 10090. (12) Shavaleev, N. M.; Eliseeva, S. V.; Scopelliti, R.; Bünzli, J.-C. G. Tridentate Benzimidazole-Pyridine-Tetrazolates as Sensitizers of Europium Luminescence. Inorg. Chem. 2014, 53, 5171−5178. (13) Petoud, S.; Bünzli, J.-C. G.; Schenk, K. J.; Piguet, C. Luminescent Properties of Lanthanide Nitrato Complexes with Substituted Bis(benzimidazolyl)pyridines. Inorg. Chem. 1997, 36, 1345−1353. (14) (a) Wei, C.; Wei, H.; Yan, W.; Zhao, Z.; Cai, Z.; Sun, B.; Meng, Z.; Liu, Z.; Bian, Z.; Huang, C. Water-Soluble and Highly Luminescent Europium(III) Complexes with Favorable Photostability and Sensitive pH Response Behavior. Inorg. Chem. 2016, 55, 10645−10653. (b) Harbuzaru, B. V.; Corma, A.; Rey, F.; Jordá, J. L.; Ananias, D.; Carlos, L. D.; Rocha, J. A Miniaturized Linear pH Sensor Based on a Highly Photoluminescent Self-Assembled Europium(III) Metal− Organic Framework. Angew. Chem., Int. Ed. 2009, 48, 6476−6479. (15) Reisser, M.; Maier, A.; Maas, G. Propyne Iminium Salts and Phosphorus(III) Nucleophiles: Synthesis of (3-Morpholinoallenyl)phosphanes and -phosphane Oxides or 1-(Morpholinopropargyl)phosphanes and -phosphonates. Eur. J. Org. Chem. 2003, 2003, 2071− 2079. (16) Zhao, Y.-L.; Wu, G.-J.; Li, Y.; Gao, L.-X.; Han, F.-S. [NiCl2(dppp)]-Catalyzed Cross-Coupling of Aryl Halides with Dialkyl Phosphite, Diphenylphosphine Oxide, and Diphenylphosphine. Chem. Eur. J. 2012, 18, 9622−9627. D

DOI: 10.1021/acs.inorgchem.8b01028 Inorg. Chem. XXXX, XXX, XXX−XXX