**6. Pyridine-based tetradentate platinum(II) complexes**

2-Phenylpyridine has been widely used as ligand for the iridium(III)- and platinum(II) based phosphorescent complexes, like Ir(ppy)<sup>3</sup> , due to its high stability and easy preparation. However, owing to the low T1 state level, the emitting colors are usually from green to red. So far, various types of pyridine-based tetradentate platinum(II) complexes have been reported; importantly, most of them are highly efficient, and some complexes are so stable that they can achieve the early stage of commercial applications. The pyridine-based tetradentate platinum(II) complexes are illustrated in **Figure 7**, their photophysical properties are summarized in **Table 8**, and some of the device performances based on these complexes are showed in **Table 9**.

In 2010, Huo's group reported three pyridine-based platinum complexes (**43**–**45**) using phenylamine as linking group [24]. Complex **43** exhibits a dominant emission peak at 512 nm in diluted 2-MeTHF solution, and the excimer emission at about 740 nm was observed at elevated concentration, due to the planar molecular configuration. The HOMO level can be stabilized by introducing fluorine atoms into the phenyl rings; thus, complex **44** has a blueshift of 24 nm with a peaking emission at 488 nm. Because of the electron-donating character of the phenylamine, complex **45** has a shallower HOMO level of −4.56 eV compared to that of complex **43** of −5.27 eV; therefore, significant redshift of 100 nm was observed for complex **45**. All the three complexes show strongly luminescence with ϕ of 14–75% in solution, and device doped with **43** achieved a peak EQE of 14.7% with coordinates of (0.32, 0.62). Unfortunately, the device stability was not studied in the literature.

performance was compared with that of *fac*-Ir(ppy)<sup>3</sup>

peak EQE could be increased to 21.5% if using Bebq<sup>2</sup>

OLEDs with an estimated operational lifetime LT97 of 62 h at 1000 cd/m<sup>2</sup>

**Figure 7.** Molecular structures of pyridine-based tetradentate platinum(II) complexes.

ptb (**50**), and an estimated operational lifetime LT97 of 638 h at 1000 cd/m<sup>2</sup>

A great progress has been made for the development of stable and efficient platinum(II) based red OLEDs in the past several years. In 2014, Li′s group demonstrated a stable red

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

of (0.60, 0.36) by employing PtON11Me (**49**) as emitter [48]. One year later, tenfold increase in the operational lifetime was achieved using more stable carbazole-linked emitter, PtN3N-

of 10.8%, which used CBP, TrisPCz, and BPyTP as host, electron-blocking, and transporting materials, respectively. Also, the turn on voltage was as low as about 2.0 eV. Moreover, the

adopting a bilayer emitting material layer (EML) with different dopant concentrations in the same device structure, PtN3N (**51**)-based orange OLED, could achieve a superior operational

the excision formation zone deep into the EML to suppress the potential fast degradation of

EQE of 22.3% could be achieved [23].

lifetime LT97 of 2057 h at 1000 cd/m<sup>2</sup>

in the same device structure, and peak

http://dx.doi.org/10.5772/intechopen.76346

91

as host material [49]. What's more is that

and peak EQE of 16.9%. This could be attributed to shift

with CIE coordinates

with peak EQE

In 2012, Fukagawa et al. developed two modified complexes TLEC-025 (**46**) and TLEC-027 (**47**) by incorporating δ-donating groups on the phenylamine to further destabilize the HOMO levels, resulting in redshift to about 620 nm, which were ideal emitters for red OLEDs [47]. TLEC-025-based device demonstrated an operational lifetime LT80 of 1290 h with a peak EQE of 18.5% and power efficiency (PE) of 20.7 lm/W at 100 cd/m<sup>2</sup> and still remained 14.4% and 25.2 lm/W at 1000 cd/m<sup>2</sup> . Encouragingly, device doped with TLEC-027 achieved further long operational lifetime LT80 of 3330 h with similar EQEs and even higher PEs of 25.5 and 30.3 lm/W at 100 and 1000 cd/m<sup>2</sup> , respectively. Unfortunately, the molecular structures of the hole-injecting material ND-1501 and electron-transporting material ETM-143 were unknown. Anyway, this was the first time to demonstrate that the platinum(II)-based devices could be as efficient and stable as the iridium(II)-based ones, opening a door for the development of efficient and stable OLEDs by employing platinum(II) complexes.

Pyridine-based tetradentate platinum(II) complex PtOO3 (**48**) with luminescent quantum efficiency of up to 97% in thin film was developed by Li′s group in 2013. PtOO3-based device

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes http://dx.doi.org/10.5772/intechopen.76346 91

**Figure 7.** Molecular structures of pyridine-based tetradentate platinum(II) complexes.

a color rendering index (CRI) of up to 80 and peak EQE of 12.5% [46]. Due to the strong emis-

was twice as long as that of Pt2O2 in the same device setting, and this could be attributed to

2-Phenylpyridine has been widely used as ligand for the iridium(III)- and platinum(II)-

red. So far, various types of pyridine-based tetradentate platinum(II) complexes have been reported; importantly, most of them are highly efficient, and some complexes are so stable that they can achieve the early stage of commercial applications. The pyridine-based tetradentate platinum(II) complexes are illustrated in **Figure 7**, their photophysical properties are summarized in **Table 8**, and some of the device performances based on these complexes are

In 2010, Huo's group reported three pyridine-based platinum complexes (**43**–**45**) using phenylamine as linking group [24]. Complex **43** exhibits a dominant emission peak at 512 nm in diluted 2-MeTHF solution, and the excimer emission at about 740 nm was observed at elevated concentration, due to the planar molecular configuration. The HOMO level can be stabilized by introducing fluorine atoms into the phenyl rings; thus, complex **44** has a blueshift of 24 nm with a peaking emission at 488 nm. Because of the electron-donating character of the phenylamine, complex **45** has a shallower HOMO level of −4.56 eV compared to that of complex **43** of −5.27 eV; therefore, significant redshift of 100 nm was observed for complex **45**. All the three complexes show strongly luminescence with ϕ of 14–75% in solution, and device doped with **43** achieved a peak EQE of 14.7% with coordinates of (0.32, 0.62). Unfortunately,

In 2012, Fukagawa et al. developed two modified complexes TLEC-025 (**46**) and TLEC-027 (**47**) by incorporating δ-donating groups on the phenylamine to further destabilize the HOMO levels, resulting in redshift to about 620 nm, which were ideal emitters for red OLEDs [47]. TLEC-025-based device demonstrated an operational lifetime LT80 of 1290 h with a peak

long operational lifetime LT80 of 3330 h with similar EQEs and even higher PEs of 25.5 and

hole-injecting material ND-1501 and electron-transporting material ETM-143 were unknown. Anyway, this was the first time to demonstrate that the platinum(II)-based devices could be as efficient and stable as the iridium(II)-based ones, opening a door for the development of

Pyridine-based tetradentate platinum(II) complex PtOO3 (**48**) with luminescent quantum efficiency of up to 97% in thin film was developed by Li′s group in 2013. PtOO3-based device

. Encouragingly, device doped with TLEC-027 achieved further

, respectively. Unfortunately, the molecular structures of the

operational lifetime LT80 could achieve over 400 h at an initial luminance of 1000 cd/m<sup>2</sup>

90 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements


, due to its high stability and easy prepara-

and still remained 14.4%

state level, the emitting colors are usually from green to


, which

sion of the excimer, 12% Pt1O2me<sup>2</sup>

tion. However, owing to the low T1

showed in **Table 9**.

the lack of high-energy blue emitters in the Pt1O2me<sup>2</sup>

based phosphorescent complexes, like Ir(ppy)<sup>3</sup>

the device stability was not studied in the literature.

and 25.2 lm/W at 1000 cd/m<sup>2</sup>

30.3 lm/W at 100 and 1000 cd/m<sup>2</sup>

EQE of 18.5% and power efficiency (PE) of 20.7 lm/W at 100 cd/m<sup>2</sup>

efficient and stable OLEDs by employing platinum(II) complexes.

**6. Pyridine-based tetradentate platinum(II) complexes**

performance was compared with that of *fac*-Ir(ppy)<sup>3</sup> in the same device structure, and peak EQE of 22.3% could be achieved [23].

A great progress has been made for the development of stable and efficient platinum(II) based red OLEDs in the past several years. In 2014, Li′s group demonstrated a stable red OLEDs with an estimated operational lifetime LT97 of 62 h at 1000 cd/m<sup>2</sup> with CIE coordinates of (0.60, 0.36) by employing PtON11Me (**49**) as emitter [48]. One year later, tenfold increase in the operational lifetime was achieved using more stable carbazole-linked emitter, PtN3Nptb (**50**), and an estimated operational lifetime LT97 of 638 h at 1000 cd/m<sup>2</sup> with peak EQE of 10.8%, which used CBP, TrisPCz, and BPyTP as host, electron-blocking, and transporting materials, respectively. Also, the turn on voltage was as low as about 2.0 eV. Moreover, the peak EQE could be increased to 21.5% if using Bebq<sup>2</sup> as host material [49]. What's more is that adopting a bilayer emitting material layer (EML) with different dopant concentrations in the same device structure, PtN3N (**51**)-based orange OLED, could achieve a superior operational lifetime LT97 of 2057 h at 1000 cd/m<sup>2</sup> and peak EQE of 16.9%. This could be attributed to shift the excision formation zone deep into the EML to suppress the potential fast degradation of


b Doped in Bebq<sup>2</sup>

c Estimated from the emission spectrum.

.

**Table 8.** Photophysical properties of pyridine-based tetradentate platinum(II) complexes.

the device [50]. These device performances indicate that the platinum(II)-based complexes are more appealing as phosphorescent emitters in the display applications.

In fact, a synthetic challenge still remains for the gram-scale preparation of the PtNON and the PtON1 series complexes. Recently, our group developed an efficient approach for the CuCl-catalyzed C-N band cross coupling of carbazoles and 2-bromopyridine derivatives to synthesize 2-bromo-*N*-(hetero)arylcarbazoles. It was found that base t-BuOLi could accelerate the reaction significantly and just a few hours needed to complete the reaction [55]. However,

**Dopant λmax/nm CIE CRI ηEQE Device LT**

[24] 512 (0.32, 0.62) — 14.7 — —

 [23] 500 — — 22.3 17.6 — 6% **49**<sup>d</sup> [48] — (0.60, 0.36) — 4.7 4.6 LT97: 62

[47] — (0.662, 0.337) — 18.5 14.4 LT80: 1920

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

[47] — (0.657, 0342) — 18.2 14.5 LT80: 3330

[49] — (0.63, 0.37) — 10.8 7.8 LT97: 638

[50] — (0.55, 0.45) — 16.9 15.3 LT97: 2057

[49] — (0.58, 0.42) — 21.5 13.5 LT97: 25

6% **52**h [33] — (0.17, 0.32) — 10.7 9.1 LT70: 624

[52] — (0.41, 0.44) 75 11.6 5.5 —

[52] — (0.41, 0.45) 74 17.0 12.4 —

[52] — (0.41, 0.45) 76 9.6 8.4 —

[52] — (0.29, 0.63) — 9.7 9.5 —

[53] 555 (0.44, 0.55) — 26.0 23.1 —

[53] — (0.31, 0.64) — 27.6 25.6 —

/ETM-143/LiF/Al.

/BAlq/BPyTP/LiF/Al.

Device structure: ITO/CFx/NBP/TCTA/dopant: TPBI:TCTA/TPBI/Alq/LiF/Mg:Ag.

Device structure: ITO/PEDOT:PSS/TAPC/dopant: 26mCPy/PO15/BmPyPB/LiF/Al. dDevice structure: ITO/HATCN/NPD/dopant:mCBP:BAlq/BAlq/Alq/LiF/Al.

Device structure: ITO/HATCN/NPD/TrisPCz/dopant: CBP/BAlq/BPyTP/LiF/Al.

<sup>h</sup>Device structure: ITO/HATCN/NPD/dopant:mCBP/mCBT/BPyTP/LiF/Al.

Device structure: ITO/PEDOT:PSS/dopant:PVK:OXD-7/TmPyPb/TPBi/LiF/Al.

**Table 9.** Device performances of pyridine-based tetradentate platinum(II) complexes.

Device structure: ITO/HATCN/NPD/TrisPCz/20 wt%**51**: CBP/6 wt%**51**:CBP/BAlq/BPyTP/LiF/Al.

/TAPC/dopant:TCTA/TmPyPB/LiF/Al.

Device structure: ITO/ND-1501/α-NPD/dopant:Bebq<sup>2</sup>

Device structure: ITO/HATCN/NPD/TrisPCz/dopant:Bebq<sup>2</sup>

29]. Moreover, a directly hydroxylation of the 2-bromo-*N*-(hetero)arylcarbazoles catalyzed by CuCl was also developed [56]. Both of the approaches are suitable for large-scale synthesis and have been successfully applied in the gram-scale synthesis of PtNON and PdNON, demonstrating its practicability in organic synthesis methodology and materials science [56]. Early in 2013, Che's group had developed a series of symmetric (**53**) [51] and asymmetric (**54–62**) [52–54, 57] phenoxyl-pyridine containing tetradentate platinum(II) complexes. All asymmetric

as base according to the previous reported method [28,

**1000 cd/m2 Peak (%) 1000 cd/m (h) <sup>2</sup> (%)**

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93

CO<sup>3</sup>

3–6 days were needed if using K<sup>2</sup>

Device structure: ITO/MoO<sup>3</sup>

4% **43**<sup>a</sup>

6% **46**<sup>b</sup>

6% **47**<sup>b</sup>

8% **48**<sup>c</sup>

10% **50**<sup>e</sup>

2% **50**<sup>f</sup>

10–6% **51**<sup>g</sup>

10% **54**<sup>i</sup>

16% **55**<sup>i</sup>

20% **56**<sup>i</sup>

4% **57**<sup>i</sup>

10% **58**<sup>j</sup>

10% **59**<sup>j</sup>

**a**

b

c

e

f

g

i

j

For the development of iridium(III)-based blue emitters containing phenylpyridine moiety, generally, electron-withdrawing groups, like fluorine, are needed to be introduced into the phenyl group to stabilize the HOMO level [21]. However, this would result in electrochemical stability problems to accelerate the device degradation, which was unfavorable to the development of the stable blue OLEDs. In 2016, Li′s group developed a new rout for stable and efficient blue OLEDs through breaking the conjugation of the phenylpyridine with a sixmembered chelating rings (**52**, PtNON) [33]. Therefore, an operational lifetime LT70 of 624 h at 1000 cd/m<sup>2</sup> with peak EQE of 10.7% and CIE coordinates of (0.17, 0.32) was achieved for the PtNON-based blue OLED. This device performance was comparable to the best iridium(III) based blue OLEDs reported in literatures [34].

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes http://dx.doi.org/10.5772/intechopen.76346 93


c Device structure: ITO/PEDOT:PSS/TAPC/dopant: 26mCPy/PO15/BmPyPB/LiF/Al.

dDevice structure: ITO/HATCN/NPD/dopant:mCBP:BAlq/BAlq/Alq/LiF/Al.

e Device structure: ITO/HATCN/NPD/TrisPCz/dopant: CBP/BAlq/BPyTP/LiF/Al.

f Device structure: ITO/HATCN/NPD/TrisPCz/dopant:Bebq<sup>2</sup> /BAlq/BPyTP/LiF/Al.

g Device structure: ITO/HATCN/NPD/TrisPCz/20 wt%**51**: CBP/6 wt%**51**:CBP/BAlq/BPyTP/LiF/Al.

<sup>h</sup>Device structure: ITO/HATCN/NPD/dopant:mCBP/mCBT/BPyTP/LiF/Al.

i Device structure: ITO/PEDOT:PSS/dopant:PVK:OXD-7/TmPyPb/TPBi/LiF/Al.

j Device structure: ITO/MoO<sup>3</sup> /TAPC/dopant:TCTA/TmPyPB/LiF/Al.

the device [50]. These device performances indicate that the platinum(II)-based complexes are

For the development of iridium(III)-based blue emitters containing phenylpyridine moiety, generally, electron-withdrawing groups, like fluorine, are needed to be introduced into the phenyl group to stabilize the HOMO level [21]. However, this would result in electrochemical stability problems to accelerate the device degradation, which was unfavorable to the development of the stable blue OLEDs. In 2016, Li′s group developed a new rout for stable and efficient blue OLEDs through breaking the conjugation of the phenylpyridine with a sixmembered chelating rings (**52**, PtNON) [33]. Therefore, an operational lifetime LT70 of 624 h at

PtNON-based blue OLED. This device performance was comparable to the best iridium(III)-

with peak EQE of 10.7% and CIE coordinates of (0.17, 0.32) was achieved for the

more appealing as phosphorescent emitters in the display applications.

**Table 8.** Photophysical properties of pyridine-based tetradentate platinum(II) complexes.

**Comp. In solution at RT In PMMA at RT**

92 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

**Solvent λmax/nm ϕ/% τ/μs λmax/nm ϕ/% τ/μs**

Cl<sup>2</sup> 512 63 2.0 — 97 4.5

Cl<sup>2</sup> 614 — 3.6 — 40 —

Cl<sup>2</sup> 502 34 — — — —

Cl<sup>2</sup> 582 63 7.3 — — —

Cl<sup>2</sup> 508 31 2.6 474 83 3.8

Cl<sup>2</sup> 595 12 1.9 — — —

Cl<sup>2</sup> 479, 510, 624 60 5.8 — — —

Cl<sup>2</sup> 480, 510, 616 66 5.4 — — —

Cl<sup>2</sup> 482, 512, 624 75 17.7 — — —

Cl<sup>2</sup> 503 76 4.1 — — —

Cl<sup>2</sup> 551 90 4.3 — 74 —

Cl<sup>2</sup> 517 80 5.1 — 91 —

Cl<sup>2</sup> 553, 587 86 6.6 — — —

Cl<sup>2</sup> 526 47 5.9 — — —

Cl<sup>2</sup> 527 49 8.8 — — —

 [24] 2-MeTHF 512, 548 74 7.6 514, 551, 595<sup>a</sup> — — [24] 2-MeTHF 488, 523 75 11.4 541, 583<sup>a</sup> — — [24] 2-MeTHF 613 14 7.6 741, 782<sup>a</sup> — — [47] — — — — 621b 58 — [47] — — — — 620b,<sup>c</sup> — —

1000 cd/m<sup>2</sup>

**48** [23] CH<sup>2</sup>

**49** [48] CH<sup>2</sup>

**50** [49] CH<sup>2</sup>

**51** [50] CH<sup>2</sup>

**52** [33] CH<sup>2</sup>

**53** [51] CH<sup>2</sup>

**54** [52] CH<sup>2</sup>

**55** [52] CH<sup>2</sup>

**56** [52] CH<sup>2</sup>

**57** [52] CH<sup>2</sup>

**58** [53] CH<sup>2</sup>

**59** [53] CH<sup>2</sup>

**60** [54] CH<sup>2</sup>

**61** [54] CH<sup>2</sup>

**62** [54] CH<sup>2</sup>

Doped in Bebq<sup>2</sup>

.

Estimated from the emission spectrum.

a Solid state.

b

c

based blue OLEDs reported in literatures [34].

**Table 9.** Device performances of pyridine-based tetradentate platinum(II) complexes.

In fact, a synthetic challenge still remains for the gram-scale preparation of the PtNON and the PtON1 series complexes. Recently, our group developed an efficient approach for the CuCl-catalyzed C-N band cross coupling of carbazoles and 2-bromopyridine derivatives to synthesize 2-bromo-*N*-(hetero)arylcarbazoles. It was found that base t-BuOLi could accelerate the reaction significantly and just a few hours needed to complete the reaction [55]. However, 3–6 days were needed if using K<sup>2</sup> CO<sup>3</sup> as base according to the previous reported method [28, 29]. Moreover, a directly hydroxylation of the 2-bromo-*N*-(hetero)arylcarbazoles catalyzed by CuCl was also developed [56]. Both of the approaches are suitable for large-scale synthesis and have been successfully applied in the gram-scale synthesis of PtNON and PdNON, demonstrating its practicability in organic synthesis methodology and materials science [56].

Early in 2013, Che's group had developed a series of symmetric (**53**) [51] and asymmetric (**54–62**) [52–54, 57] phenoxyl-pyridine containing tetradentate platinum(II) complexes. All asymmetric complexes have high ϕ of 49–90% and τ of 4.1–17.7 μs in CH<sup>2</sup> Cl<sup>2</sup> solutions, and the OLEDs doped these complexes that showed very high brightness, even up to 66,000 cd/m<sup>2</sup> at 10.5 eV. Moreover, the planar rigid molecular configuration enabled the fluorine-containing complexes **54**–**56** to have strong excimer emission at 616–624 nm, making them serve as ideal emitters for singledoped white OLEDs with CRI of up to 76 [52]. However, after introducing sterically bulky 3,5-di-*tert*-butylphenyl group to the pyridine ring, excimer formation was suppressed for the complexes **57**–**62**. Devices doped with complex **58** bridging phenylamine or complex **59** with a spiro linkage demonstrated peak EQEs over 25% and maximal PEs up to 109.4 lm/W using TmPyPB as ETL. The maximal PE of complex **59** could further be improved to 126.0 lm/W if Tm3PyBPZ as ETL, which were the highest among the reported platinum(II)-based OLEDs [53]. In the same year, Che's group developed another series of tetradentate platinum(II) complexes containing carbazole (**60**), phenoxazine (**61**), and phenothiazine (**62**) moieties, which served as yellow phosphorescent emitters combined with blue emitter to make white OLEDs [54].

potential applications for the next-generation full-color display and solid-state lighting. Owing to the square planar and rigid molecular configurations, platinum(II) complexes have many unique and exciting photophysical properties. On the one hand, easy molecular modification enables tunable emission spectra, and the FWHM of the pyrazole- or NHC-carbene-based complexes can achieve no more than 20 nm and can be as narrow as 15 nm. This facilitates them to serve as efficient deep-blue emitters, and device-doped NHC-carbene-based complex successfully realized "pure" blue emitting with CIE coordinates of (0.148, 0.079) and peak EQE of 24.8%.

Tetradentate Cyclometalated Platinum(II) Complexes for Efficient and Stable Organic Light-Emitting Diodes

to achieve 18e structure in their excited state, making them serve as single-doped white OLEDs with high CRI values. Besides, tetradentate platinum(II)-based green, especially for the red OLEDs, demonstrated superlong operational lifetime and satisfied the requirements of the initial commercialization. What's more is that sky-blue OLEDs also achieved encouraging performances, indicating their bright future for the development of the efficient and stable blue OLEDs. Despite great progress that has been made for the tetradentate platinum(II) complexes, a challenge remains for the development of the stable deep-blue OLEDs, and more work still be needed. To overcome this challenge, it is important to develop stable host materials with a

tinued efforts of the academia and industry, we believe that these critical issues can be solved and the platinum(II)-based OLEDs will be one candidate for display and lighting applications.

The authors thank the National Natural Science Foundation of China (21602198, 21776259, 21476270), the "Qianjiang Talents Plan" (QJD1602017), and AAC Technologies for their financial support. The authors also thank Dr. Tyler Fleetham from the University of Southern California for the measurements of the quantum efficiency and luminescent lifetime of the PtON3.

state level and highly balanced charge carrier ability. However, through con-

platinum(II) complexes can form intermolecular Pt-Pt bond

http://dx.doi.org/10.5772/intechopen.76346

95

On the other hand, some planar d8

high enough T1

**Acknowledgements**

**Abbreviations**

Alq<sup>3</sup> tris(8-hydroxyquinolinato)aluminium

BmPyPB 1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene

DPPS diphenyl-bis[4-(pyridin-3-yl)phenyl]-silane

BPyTP 2,7-di(2,2′-bipyridin-5-yl)triphenylene

CBP 4,4′-bis(*N*-carbazolyl) biphenyl

BAlq bis(2-methyl-8-quinolinolato) (biphenyl-4-olato)aluminum

CzSi 9-(4-(*tert*-butyl)phenyl)-3,6-bis(triphenylsilyl)-9*H*-carbazole

Bebq<sup>2</sup> bis(benzo[h]quinolin-10-olato-κN,κO)beryllium(II)
