Synthesis of N,N'-Chelate Organoboron Derivatives: Comparison
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N,N′-chelate organoboron compounds have been successfully applied in bioimaging, organic light-emitting diodes (OLEDs), functional polymer, photocatalyst, electroluminescent (EL) devices, and other science and technology areas. However, the concise and efficient synthetic methods become more and more significant for material science, biomedical research, or other practical science. Here, we summarized the organoboron-N,N'-chelate derivatives and showed the different routes of their synthesis. Traditional methods to synthesize N,N'-chelate organoboron compounds were mainly using bidentate ligand containing nitrogen reacting with trivalent boron reagents. In this review, we described a series of bidentate ligands, such as bipyridine, 2-(pyridin-2-yl)-1H-indole, 2-(5-methyl-1H-pyrrol-2-yl)quinoline, N-(quinolin-8-yl)acetamide, 1,10-phenanthroline, and diketopyrrolopyrrole (DPP).

  • organoboron
  • NN′-chelate
  • tetracoordinated
  • fluorescent materials

1. Introduction

N,N′-chelate tetracoordinated organoboron compounds have been widely applied in various science and technology areas. For example, boron dipyrromethene derivatives (BODIPY) were developed in luminescent materials [1][2][3][4][5], dyes [6][7][8][9], photosensitizers [10][11][12][13][14], molecular switches [15][16][17], photodynamic therapy [18][19][20][21][22], molecular probes [23][24][25][26], and bioimaging [27][28][29]. In recent decades, N,N′-chelate compounds become a popular topic and have gradually attracted the attention of scientists. Four-coordinated organoboron compounds (BAr2 (N, N)) have interesting luminescent properties that can be modulated by various substituents in the N, N′-chelating framework [30]. More and more similar structures were explored and exhibited wonderful results in fluorescent materials [30][31], cell bioimaging (A1) [32], organic light-emitting diodes (OLEDs) (A2) [33], functional polymer (A3) [34], photocatalyst (A8) [35], electroluminescent (EL) devices [36], and so on, as shown in Figure 1. Herein, various traditional methods for the formation of N,N′-chelate tetracoordinated organoboron complexes were exhibited in this review. The main highlights include that (a) the diversified synthetic methods were provided, (b) readily available trivalent boron compounds were used as boron reagents, (c) various complex starting materials were designed as good bidentate ligands, and (d) these reactions showed a wide range of tetracoordinated organoboron complexes and excellent optical properties.

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Figure 1. Application of tetracoordinate organoboron complexes in luminescent materials, organic light-emitting diode (OLED), functional polymer, and photocatalyst [30][31][32][33][34][35][36].

In recent years, the research on their syntheses falls into the following categories. At first, bipyridine derivatives could be used as bidentate ligand reacting with triphenylboron to form the desired products, as shown in Figure 2. The second, an indole connecting with pyridine derivatives (or other nitrogen heterocyclic molecule) reacted with triphenylboron to obtain the corresponding compounds, as shown in Figure 2. The third, pyridine attaching on pyrrole derivatives (or other nitrogen heterocyclic molecule) and trivalent boron could produce the fluorescent organoboron compounds, as shown in Figure 2. The fourth, a quinoline linking with pyrrole derivatives (or other nitrogen heterocyclic molecule) has been reported as a bidentate ligand to react with boron reagents. The different fluorescent compounds could be synthesized by N-(quinolin-8-yl)acetamide derivatives and boron reagents, as shown in Figure 2. The fifth, other bidentate ligands including 1,10-phenanthroline and diketopyrrolopyrrole (DPP) derivatives could also react with trivalent boron to obtain tetracoordinated organoboron compounds, as shown in Figure 2.

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Figure 2. The methods for the formation of N,N′-chelate organoboron derivatives [30][31][32][33][34][35][36].

2. Bipyridine-Based Derivatives as Bidentate Ligand

In this section, different traditional methods for the formation of N,N′-chelate organoboron derivatives will be displayed in detail from the following aspects.

2.1. Bipyridine as Bidentate Ligand

In 1985, Heinrich Noeth and co-workers reported that dibutyl(((trifluoromethyl)sulfonyl)oxy)borane (1) reacted with bipyridine (2) to complete desired product. The solution of the diorganylborane should be cooled to −78 °C in this reaction. Bipyridine-dibutylboronium(1+) triflate (3) was confirmed by 11B NMR in their lab [37], as shown in Scheme 1. This protocol realized the synthesis of bipyridine coordinated organoboron complex at low temperature. It played a certain role in promoting the deep study of tetracoordinated organoboron compounds.

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Scheme 1. Reactions of bipyridine with dibutyl(((trifluoromethyl)sulfonyl)oxy)borane [37].

2.2. 2,5-Di(pyridin-2-yl)pyrazine and 2′, 2′: 4′, 4′: 2″, 2‴-Quaterpyridine as Bidentate Ligand

In 2002, Matthias Wagner’s group adopted ferrocenylboranes (4) to react with 2,5-bis(pyridyl)pyrazine (5) and 2′, 2′: 4′, 4′: 2″, 2‴-quaterpyridine (6) to obtain charge-transfer complexes, respectively [38]. The organoboron adducts possess green color, which is charge transfer from the electron-rich ferrocene skeleton to their electron-poor aromatic structures, as shown in Scheme 2. This strategy provided a new route to acquire more ferrocenylborane derivatives with high yield and simple operation.

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Scheme 2. Reactions of 2,5-di(pyridin-2-yl)pyrazine (or 2′, 2′: 4′, 4′: 2″, 2‴-quaterpyridine) with FcB(Me)Br [38].

2.3. Bipyridine in {Fc(Bbipy)2O}(PF6) and {Fc(Bbipy}2(OH)2}(PF6)

In 2002, Matthias Wagner’s group studied the synthesis of ferrocene complexes {Fc(Bbipy)2O}(PF6)2 (11) and {Fc(Bbipy)2(OH)2}(PF6)2 (12) [39][40], as shown in Scheme 3. They successfully synthesized tetracoordinated organoboron compounds with two boron centers. All of these compounds have the property of charge transfer. They could realize ring-closing and ring-opening products using bromide (FcMeBr) for the formation of 11 and 12 by adding different amounts of water in the reaction.

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Scheme 3. The methods for the synthesis of {Fc(Bbipy)2O}(PF6)2 and {Fc(Bbipy)2(OH)2}(PF6)2 [39][40].

2.4. 4,4′-Di(but-3-en-1-yl)-2,2′-bipyridine as Bidentate Ligand

In 2005, Matthias Wagner’s team reported other N,N′-chelate organoboron complexes in this reaction. They used dibromo(phenyl)borane, bromo(methyl)(phenyl)borane, bromo(ethoxy)(phenyl)borane to give bipyridine adducts with satisfactory yields (15–18, 76–89%) [41], as shown in Scheme 4. This reaction indicated that different N,N′-chelate organoboron compounds could be obtained by adding distinct boron reagents under ambient temperature.

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Scheme 4. Reactions of 4,4′-di(but-3-en-1-yl)-2,2′-bipyridine with trivalent boron [41].

2.5. Bipyridine Reacting with 5-Chloro-5,10-dihydrodibenzo[b,e]borinine

In 2009, Warren E. Piers’s lab prepared a series of neutral radicals, which had significant spin density on boron [42]. The scaffold of 2,2′-bipyridyl-stabilized boronium ions was interesting and demonstrated bipyridine adducts persistent neutral radical. They added AgBF4 in this reaction and offered moderate yields (20–23, yield 53–77%), as shown in Scheme 5. It was proved that this protocol could easily get the target 2,2′-bipyridyl boronium ions and neutral radicals.

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Scheme 5. Reactions of bipyridine with 5-chloro-5,10-dihydrodibenzo[b,e]borinine [42].

2.6. Bipyridine for the Formation of N,N′-Chelate Organoboron and Ferrocene Derivatives

In 2010, Matthias Wagner’s group reported a more powerful synthetic route of ferrocene complexes [43]. They used bromide (26) to obtain FcBBr polymers (27, 28) and continued to form the bipyridine organoboron polymers with main chain charge-transfer structure (30, 32, 33, 80%, 65%, 76%), as shown in Scheme 6. This method provided a new opportunity for the synthesis of multifunctional organoboron polymers.

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Scheme 6. Bipyridine for the formation of N,N′-chelate organoboron and ferrocene derivatives [43].

2.7. Bipyridine Reacting with 5-Bromo-10-mesityl-5,10-dihydroboranthrene

In 2011, Matthias Wagner’s group has been committed to the development of more diversified N,N′-chelate organoboron chemistry for many years [44]. It always showed wonderful results in bipyridine organoboron adducts. They realized cleavage of the B-O-B bridge in this reaction and got the desired product with moderate yield (36, 51%), as shown in Scheme 7. In this protocol, they prepared 5-bromo-10-mesityl-5,10-dihydroboranthrene (35) as a boron reagent. This building block revealed a novel synthetic route for us.

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Scheme 7. Bipyridine reacting with 5-bromo-10-mesityl-5,10-dihydroboranthrene [44].

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