Synthesis, Structure, and Luminescence of a Molecular Europium Tetracyanoplatinate Incorporating 4,5-Diazafluoren-9-One

Abstract

Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O represents a new member to the lanthanide tetracyanoplatinate family of materials. The synthesis, single-crystal X-ray structure, and photoluminescence properties of this compound are reported herein. The compound contains dimeric [Eu₂(Pt(CN)₄)₃(H₂O)₁₂] molecules in which the Eu³⁺ sites are connected with trans-bridging [Pt(CN)4] ²⁻ anions and co-crystallized along with 4,5-diazafluoren-9-one (dafone) and lattice water. Prominent noncovalent interactions that stabilize the structure include H-bonding as well as short stacking interactions involving the planar tetracyanoplatinate anions in conjunction with the planar dafone molecules. Photoluminescence measurements illustrate that Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O contains multiple emitting states and also displays enhanced Eu³⁺-based emission due to energy transfer within the compound.

1. Introduction

Cyanide-bridged, bimetallic compounds exhibit interesting structural and material properties, which have provided the basis for exploration of this research area [1,2,3,4]. Magnetic applications have dominated the work centering around transition-metal hexacyanides or Prussian blue type compounds [5,6,7], while optical applications have been the main research focus regarding the dicyanometallates of Au and Ag and the tetracyanometallates of Pd and Pt [8,9,10]. However, a multitude of research reports on the rare-earth cyanometallates focusing on the synthetic, structural, spectroscopic, and catalytic aspects of these materials are also available in the literature [11,12,13,14,15]. In many such compounds, metallophilic interactions are recognized as a major force determining structural dimensionality and properties [16,17]. For example, the square-planar [Pt(CN)₄]²⁻ anions readily associate into 1-D chains [18], or smaller geometric units [19], containing short Pt--Pt interactions and are prominent in the families of tetracyanoplatinate compounds.

Lanthanide ion luminescence is ideal for a number of applications, e.g., fluoroimmunoassays, [20,21] cellular imaging [22,23], and optical communications [24,25], due to its sharp f-f transitions and high color purity. However, because direct absorption of the f-f excited states is very inefficient, optimizing chromophoric ligands to enhance lanthanide-ion luminescence has been and continues to be an active area for researchers. Many organic [26,27] and inorganic [28,29] donors have been investigated over the years to efficiently transfer energy to acceptor lanthanide ions, including the aforementioned cyanometallate systems.

Cooperative enhancement of Ln³⁺ emissions by utilizing multiple donors has received more attention recently [30,31,32]. Efforts have primarily focused on preparing Ln³⁺ compounds that contain multiple light-harvesting moieties; for example, compounds containing multiple organic chromophores such as Ln(tfnb)₃dafone (Htfnb = 1-(2-naphthyl)-4,4,4-trifluoro-1,3-butanedionate; dafone = 4,5-diazafluoren-9-one) have been previously reported [30]. Our group has worked on preparing mixed ligand systems incorporating combinations of purely inorganic and inorganic/organic ligands [31,32,33,34,35]. Cyanometallate systems involving both linear d¹⁰ and square planar d⁸ cyanometallates and/or cyclic amines such as 2,2′-bipyridine (bpy) [33], 1,10-phenanthroline (phen) [34], or 2,2′:6′,2”-terpyridine [31,32,34] have been reported. In compounds such as Eu(C₁₅H₁₁N₃)(H₂O)₂(NO₃)(Pt(CN)₄)·CH₃CN [31,32] and K₂[Tb(H₂O)₄(Pt(CN)₄)₂]Au(CN)₂·2H₂O [33], where both donor groups have a direct pathway to a given Ln³⁺ cation, the emission from the Ln³⁺ has been shown to be enhanced relative to direct Ln³⁺ excitation via energy transfer by exciting either donor ligand. Continuing with this area of research, herein we report on a novel, molecular europium tetracyanoplatinate compound that incorporates both tetracyanoplatinate and 4,5-diazafluoren-9-one (dafone), [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O.

2. Materials and Methods

Eu(NO₃)₃·6H₂O (Strem, 99.9%), 4,5-diazafluoren-9-one (Alfa Aesar, 97%), and K₂Pt(CN)₄·3H₂O (Alfa Aesar, 99.9%) were used as received without further purification. The reactions reported produced the highest observed yields of the respective compounds. IR spectra were obtained on room-temperature samples pressed into KBr pellets using a ThermoScientific Nicolet iS50 FT-IR (Thermo Fisher Scientific, Madison, WI, USA) over the range of 400 to 4000 cm⁻¹.

2.1. The Synthesis of [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O (C₁₁H₆N₂O = 4,5-Diazafluoren-9-One)

The synthesis of the title compound was carried out by mixing 1 mL of a 0.05 M solution of the appropriate lanthanide nitrate in THF, 1 mL of 0.05 M K₂[Pt(CN)₄] (aq), and 1 mL of 0.10 M 4,5-diazafluoren-9-one in THF. Slow evaporation of the solvent over a period of several days resulted in the crystallization of colorless needles with a yield of 87%. Elemental analysis: Calculated for C₅₆H₆₀Eu₂N₂₀O₂₂Pt₃: C, 29.8%; H, 2.68%; N, 12.4%. Found: C, 27.8%, H, 1.64%, N, 12.6%. Challenges associated with CHN analyses in organometallic compounds have been discussed previously [36,37]. IR (KBr, cm⁻¹): 3432 (s, br), 2165 (m), 2151 (s), 2130 (s), 1734 (s), 1669 (m, br), 1624 (m, br), 1596 (s), 1565 (s), 1470 (w), 1406 (s), 1384 (m), 1266 (m), 1161 (w), 1102 (m), 1087 (w), 1034 (w), 923 (m), 759 (s), 728 (w), 636 (w), 500 (w), 478 (w), 456 (w), 436 (w), 428 (w).

2.2. Single-Crystal X-ray Diffraction Studies

Single crystals of the title compound were selected, mounted on quartz fibers, and aligned with a digital camera on an Oxford Xcalibur E single-crystal X-ray diffractometer. Intensity measurements were performed using Mo Kα radiation from a sealed-tube Enhance X-ray source and an Eos area detector. CrysAlisPro [38] was used for the preliminary determination of the cell constants, for the data collection strategy, and for the data collection control. Following data collection, CrysAlisPro was also used to integrate the reflection intensities, apply an absorption correction to the data, and perform a global cell refinement.

For all structure analyses, the program suite SHELX (v. 2014/3) was used for the structure solution (XS) and least-squares refinement (XL) [39]. The initial structure solutions were carried out using direct methods, and the remaining atomic positions were located in difference maps. The final refinements included anisotropic displacement parameters for all non-hydrogen atoms. Some crystallographic details are listed in

Table 1. Crystallographic data for [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O.

Formula	C56H60Eu2N20O22Pt3
Formula weight (amu)	2254.43
Crystal System	Monoclinic
space group	P21/n
a (Å)	10.9678(3)
b (Å)	25.1612(6)
c (Å)	13.3381(3)
β (deg)	91.400(2)
V (Å3)	3679.72(16)
Z	2
T (K)	290
λ (Å)	0.71073
ρcalcd (g cm−3)	2.035
μ(Mo Kα) (mm−1)	7.446
R(Fo) for Fo2 > 2σ(Fo2) a	0.0261
Rw(Fo2) b	0.0476

^a. $R\left(F_{\mathrm{o}}\right)=\sum \left|\left|F_{\mathrm{o}}\right|-\left|F_{\mathrm{c}}\right|\right|/\sum \left|F_{\mathrm{o}}\right|$. ^b. $R_{\mathrm{w}}\left(F_{\mathrm{o}}^2\right)={\left[\sum \left[w{\left(F_{\mathrm{o}}^2-F_{\mathrm{c}}^2\right)}^2\right]/\sum w{F}_{\mathrm{o}}^4\right]}^{1/2}$.

, and additional details are available as Supplementary Materials. CCDC 2169999 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 29 August 2022, or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: [email protected])

2.3. Photoluminescence Studies

The luminescence measurements were conducted utilizing a Photon Technology International (PTI) spectrometer system. This instrument utilizes a 75 W Xe source and PMT detectors (Hamamatsu model 928 tube) operating in digital (photon counting) mode to measure steady-state emission and excitation spectra. Time-dependent measurements were performed by utilizing a variable, pulsed Xe source and gated photomultiplier tubes (Hamamatsu model 928 tube). The selection of excitation and emission wavelengths were conducted by means of computer-controlled, autocalibrated “QuadraScopic” monochromators, which are equipped with aberration-corrected emission and excitation optics. The emission monochromators were calibrated, using a NIST traceable tungsten light, to compensate for the wavelength-dependent variation in the system on the emission channel. The instrument operation, data collection, and handling were all controlled using the FeliX32 fluorescence spectroscopic package. All of the spectroscopic experiments are conducted on neat crystalline samples, individually screened for phase purity using single-crystal X-ray diffraction and held in sealed quartz capillary tubes.

3. Results and Discussion

3.1. Structural Studies

The structure of the europium complex in [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O is molecular, and contains a dimeric core with the two Eu(III) sites joined by a trans-bridging Pt(CN)₄²⁻ anion. A thermal ellipsoid plot of the asymmetric unit with the included atomic labeling scheme is illustrated in Figure 1, and the dimer is illustrated in Figure 2. This dimeric core is reminiscent of that contained in [{Eu(DMSO)₂(C₁₂H₈N₂)(H₂O)₃}₂Pt(CN)₄]²⁺ complex cations [40]. The slightly distorted, square-antiprismatic coordination geometry around each Eu(III) center results from one monodentate N-bound Pt(CN)₄²⁻ anion, one trans-bridging N-bound Pt(CN)₄²⁻ anion, and six water molecules. Pt1, occupying the trans-bridging Pt(CN)₄²⁻, resides on an inversion center, which relates the two halves of the dimeric complex cation by symmetry.

In addition to the metal-centered complexes, dafone molecules and waters of hydration are co-crystallized within the lattice and add to its structural stability with many and varied H-bonding and pi-stacking interactions. The number of H-bonds for the complex waters is maximized via the formation of two H-bond donor interactions, as described in detail in

Table 2. Hydrogen-bond geometries (Å, ^o) for [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O.

D–H···A	D–H	H···A	D···A	D–H···A
O(1)–H(1A)···N(3 i)	0.85	1.88	2.700(6)	160.5
O(1)–H(1B)···N(10)	0.85	2.06	2.893(5)	168.7
O2–H2A···O11 ii	0.85	2.14	2.898(5)	148.1
O2–H2B···O9	0.85	1.81	2.635(5)	165.0
O3–H3A···N9	0.85	1.90	2.735(5)	165.5
O3–H3B···O8 iii	0.85	1.86	2.714(5)	179.3
O4–H4A···N2 iv	0.85	2.06	2.881(6)	161.3
O4–H4B···O7	0.85	3.10	3.798(5)	140.8
O5–H5A···O7	0.85	1.91	2.740(5)	166.1
O5–H5B···O7 v	0.85	1.95	2.795(5)	169.9
O6–H6A···N7	0.85	2.15	2.807(5)	134.3
O6–H6B···N8	0.85	1.87	2.703(5)	165.5
O9–H9A···N4 v	0.85	2.03	2.884(7)	178.9

Symmetry transformation used to generate equivalent atoms: (i) x, y, z + 1; (ii) −x + ½,y − ½, −z + ½; (iii) x + ½, −y + ½, z − ½; (iv) x − ½, −y + ½,z + ½; (v) −x, −y, −z.

. All of the following acceptors are involved in at least one H-bonding interaction in the structure: uncoordinated N-terminated cyanide, lattice water, dafone-N, or dafone-O. Furthermore, every non-coordinated cyanide from each tetracyanoplatinate anion is involved in an H-bonding acceptor interaction with an O−H donor from either a coordinated or uncoordinated water molecule. In addition to these common types of van der Waals interactions mentioned above, [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O contains additional intermolecular interactions as shown in Figure 2. The structure contains stacks of alternating pairs of dafone molecules and tetracyanoplatinate anions. The pi-stacking distances between dafone molecules are ~3.42 Å, and the plane-centroid distances between dafone and tetracyanoplatinate are ~3.55 Å within these 1-D stacks. Previous structural studies containing square planar Pt(II) and organic aromatics have described similar interactions [41]. The structure of [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O does not contain metallophilic (Pt—Pt) interactions which are present in many structures containing tetracyanoplatinate [18].

Select bond distances for [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O can be found in

Table 3. Selected Bond Distances (Å) for [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O.

Distances (Å)
Eu1–N1	2.513 (4)	Pt1–C5	1.989 (5)
Eu1–N5	2.501 (4)	Pt1–C6	2.034 (8)
Eu1–O1	2.386 (3)	Pt1–C5 i	1.989 (5)
Eu1–O2	2.411 (3)	Pt1−C6 i	2.034 (8)
Eu1–O3	2.373 (3)	Pt2–C1	1.989 (5)
Eu1–O4	2.456 (3)	Pt2−C2	1.989 (6)
Eu1–O5	2.397 (3)	Pt2−C3	2.007 (5)
Eu1–O6	2.335 (3)	Pt2−C4	1.994 (6)

Symmetry transformation used to generate equivalent atoms: (i) −x + 1, −y, −z.

. A clear trend in the Eu−O and Eu−N distances among the various coordinating ligands is observed; Eu−N(TCP) distances are longer (average of 2.507 Å) by over a tenth of an Angstrom relative to the Eu−O(H₂O) distances (average of 2.393 Å). The Pt−C distances vary little, ranging from 1.989(5) to 2.034(8) Å with an average value of 2.003 Å, with no clear trend in bridging versus terminal groups. All of these bond distances are within normal ranges as found in previously reported Eu(III) tetracyanoplatinate structures [31,32,35].

3.2. Photoluminescence Studies

The solid-state emission spectra of [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O are shown in Figure 3, and band locations and assignments are given in

Table 4. Band locations (most intense) and assignments for the emission transitions of [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O.

Assignment	λ (nm)
5D1 → 7F2	552
5D0 → 7F0	579
5D0 → 7F1	590
5D0 → 7F2	617.5
5D0 → 7F3	649.5
5D0 → 7F4	699

. These data illustrate that [Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O displays a number of sharp emission bands, some strong, stretching across the visible region, in addition to a weak, broad emission band centered at approximately 510 nm. The assignment of the sharp emission bands in the spectrum is straightforward as these originate from f−f emissions of two different Eu(III) excited states. Expectedly, direct Eu(III) excitation, e.g., the ⁵D₂←⁷F₀ transition at 467 nm, provides relatively weak Eu-based emission due to low absorption of the f-element ion, however, excitation with higher energy UV radiation in the range of 270 to 450 nm leads to similar emission profiles with much greater intensities.

The ⁵D₀→⁷F₀ transition, oftentimes just referred to as the 0−0 transition, is observed at 579 nm and signifies a strong electric dipole contribution. The observance of this 0−0 transition with only a single band is in line with the crystallographic structure analysis that shows a C₁ site symmetry for the single Eu(III) site in the compound. The breaking of the degeneracy of the ⁵D₀→⁷F₁ transition resulting in the observance of multiple Stark components is also consistent with a low site symmetry for Eu(III) (C₁ symmetry) [42]. The ⁵D₀→⁷F₂ transition shows splitting into five Stark components even at room temperature. Although the two outer bands are not completely resolved, this splitting is estimated at 100 cm⁻¹. The Eu³⁺ ion emission has a lifetime of 120 μs, measured for the ⁵D₀→⁷F₁ transition, in the title compound. While this value is in line with the reported Eu³⁺ lifetimes in other systems, the differences can be attributed to the number of H₂O molecules directly coordinated to the Eu(III) ion in the different compounds [43] or to the relative amount of hydrogen bonding involved in systems containing charge transfer interactions with Eu(III) acceptors [44].

The assignment of the broad emission band centered at approximately 510 nm is not as straightforward. Performing time-delay emission experiments, with pulsed excitation, reveals that the lifetime of this emission is much shorter than the Eu³⁺ f-f emission bands. Figure 4 shows the decoupled emission from the title compound at different time delays. This indicates that the emission originates from different emitting species within the compound. Since dafone and tetracyanoplatinate can both independently present broad emission bands within this spectral region [30,31], the emission spectra alone is inconclusive. However, the excitation spectra shown in Figure 5 are not reminiscent of the absorption features of either dafone or tetracyanoplatinate alone. Further, the similarity of the excitation bands upon monitoring the Eu(III) emission or the ligand-based emission band is direct evidence for the existence of energy transfer in the system. All of this together indicates the existence of exciplex formation within this compound.

4. Conclusions

Structural and spectroscopic features of Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O), a new member of the lanthanide tetracyanoplatinate family, are presented. The structural analyses reveal that this compound contains a molecular structure with [Eu₂(Pt(CN)₄)₃(H₂O)₁₂] molecules in which the Eu³⁺ sites are connected with trans-bridging [Pt(CN)4] ²⁻ anions. The structure is stabilized by multiple intermolecular interactions, including H-bonding and stacking interactions involving the square planar tetracyanoplatinate anions and dafone molecules. The solid-state photoluminescence characterization illustrates that multiple emitting states are present for Eu₂(Pt(CN)₄)₃(H₂O)₁₂]·4C₁₁H₆N₂O·6H₂O, and the compound also displays enhanced Eu³⁺-based emission as a result of energy transfer within the compound.