Higher-valency ligands for COFs Metal-organic frameworks (MOFs) have exhibited more extensive connectivity (valency) and topological diversity than covalent organic frameworks (COFs), mainly because MOF linkers can connect from 3 to 24 discrete units or even infinity for one-dimensional rods. For COFs, linkers generally have a valency of 3 or 4 that reflect the valency of organic carbon. Gropp et al. created cubane-like linkers from 1,4-boronophenylphosphonic acid that could condense to make COFs with a valency of 8 or, by adding acid, could form large, single crystals with an infinite-rod topology. Science, this issue p. eabd6406 A polycube linker formed from 1,4-boronophenylphosphonic acid affords cubic and infinite-rod covalent organic frameworks. INTRODUCTION Valency is the connectivity of building units in reticular frameworks. Although metal-organic frameworks (MOFs) are known to have valencies of 3 to 24, covalent organic frameworks (COFs) are limited to the lower valencies of 3 and 4, principally owing to the heavy reliance of organic chemistry on sp2 and sp3 hybridization. We show that the diversity of COFs can be increased by finding new ways of linking simple organic molecules into building units of higher and even infinite valencies. RATIONALE Now, COFs are made by connecting preformed trigonal-planar, square-planar, and tetrahedral organic building units with linkages of low valency. Our strategy is to implement higher valency in COFs by designing molecules capable of forming higher-valency linkages through clustering. This is difficult to achieve by conventional organic methods. We therefore relied on the isoelectronic replacement of carbon-group elements by boron and phosphorus and demonstrated the feasibility of this chemistry by using borophosphonate for the silicate cube motif. We designed simple organic BPA linkers (BPA-1 to 5) based on boron and phosphorus, which self-condensed into cubic units and polycubane COFs of valency 8. The versatility of this chemistry was further exploited by cleaving the cubes in the polycubane COFs, leading to structures with rod units of infinite valency. RESULTS The BPA-1 linker combines a boronic acid and a phosphonic acid functionality in a single molecule, which was converged into the boron-phosphorus (BP) cube of a reticulated polycubane BP-COF-1. Specifically, eight BPA-1 linkers self-condensed to form the BP cube with the elimination of eight water molecules per cube. The crystallization of BP-COF-1 was realized through microscopic reversibility: The polarized B–O–P linkage can dynamically form and break at the B–O bond. This constituted new chemistry whereby simple linkers converged into frameworks composed of higher-valency clusters. The versatility of this strategy was demonstrated by the successful crystallization of isoreticular polycubane structures, BP-COF-2 to 5 from the BPA-2 to 5 linkers. This series of functionalized and expanded polycubane structures exhibited permanent porosity. We found that upon addition of acid to BP-COF-1, eight B–O bonds per cube were cleaved and rearranged into BP-COF-6, which exhibits rods of infinite valency. The structure of BP-COF-6 was obtained from single-crystal x-ray diffraction, making it one of the few COFs grown as large crystals. It revealed infinite B–O–P rods linked by phenyl units to form layers, which in the crystal were joined by interstitial water molecules acting to stabilize an otherwise unusual rod-within-layer arrangement. CONCLUSION The chemistry of BPA linkers and their demonstrated ability to form cubes and higher-valency structures expand the scope of COFs. In essence, BPA linkers were indispensable in making linkages in the form of cubes and rods of higher valency and their corresponding extended BP-COF structures, thereby opening opportunities for the diversification of COFs through clustering of organic molecules based on sp2 and sp3-hybridized atoms. The extension of this approach to COFs containing combinations of different shapes promises to uncover what we anticipate is a large, untapped structure space. Design of higher valency in COFs from single organic linkers. (A) BPA linkers (BPA-1 to 5) combine a boronic and a phosphonic acid functionality in a single molecule. (B and C) Self-condensation of BPA-1 (B) afforded BP cubes of valency 8 (C) and their reticulated polycubane BP-COF-1. Isoreticular forms (BP-COF-2 to 5) demonstrated the versatility of this chemistry. (D) Upon addition of acid, BP-COF-1 rearranged into the layered BP-COF-6, which exhibits rods of infinite valency. The valency (connectivity) of building units in covalent organic frameworks (COFs) has been primarily 3 and 4, corresponding to triangles and squares or tetrahedrons, respectively. We report a strategy for making COFs with valency 8 (cubes) and “infinity” (rods). The linker 1,4-boronophenylphosphonic acid—designed to have boron and phosphorus as an isoelectronic combination of carbon-group elements—was condensed into a porous, polycubane structure (BP-COF-1) formulated as (–B4P4O12–)(–C6H4–)4. It was characterized by x-ray powder diffraction techniques, which revealed cubes linked with phenyls. The isoreticular forms (BP-COF-2 to 5) were similarly prepared and characterized. Large single crystals of a constitutionally isomeric COF (BP-COF-6), composed of rod units, were also synthesized using the same strategy, thus propelling COF chemistry into a new valency regime.

中文翻译：

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共价有机框架中更高价的设计

COF 的高价配体 金属有机框架 (MOF) 比共价有机框架 (COF) 表现出更广泛的连接性（价）和拓扑多样性，主要是因为 MOF 接头可以连接 3 到 24 个离散单元，甚至可以无限连接一个-维杆。对于 COF，接头通常具有 3 或 4 价，这反映了有机碳的价数。格罗普等人。由 1,4-硼苯基膦酸创建了类立方烷的连接体，可以缩合形成 8 价的 COF，或者通过添加酸，可以形成具有无限棒拓扑结构的大单晶。科学，这个问题 p。eabd6406 由 1,4-硼苯基膦酸形成的多立方体接头提供立方体和无限棒共价有机框架。引言 价是网状框架中建筑单元的连通性。尽管已知金属有机骨架 (MOF) 的价数为 3 到 24，但共价有机骨架 (COF) 仅限于较低的 3 和 4 价，这主要是由于有机化学严重依赖 sp2 和 sp3 杂化。我们表明，通过寻找将简单有机分子连接成更高甚至无限价的构建单元的新方法，可以增加 COF 的多样性。基本原理 现在，COF 是通过将预制的三角平面、方形平面和四面体有机建筑单元与低价键连接起来制成的。我们的策略是通过设计能够通过聚类形成更高价键的分子，在 COF 中实现更高价。这是通过传统的有机方法难以实现的。因此，我们依靠硼和磷对碳族元素的等电子置换，并通过使用硼膦酸盐作为硅酸盐立方体基序来证明这种化学的可行性。我们设计了基于硼和磷的简单有机 BPA 接头（BPA-1 至 5），它们自缩合为立方单元和 8 价的聚立方 COF。通过切割聚立方 COF 中的立方，进一步利用了这种化学的多功能性，导致具有无限价棒单位的结构。结果 BPA-1 接头在单个分子中结合了硼酸和膦酸官能团，其会聚成网状聚立方烷 BP-COF-1 的硼磷 (BP) 立方体。具体来说，八个 BPA-1 接头自缩合形成 BP 立方体，每个立方体消除了八个水分子。BP-COF-1 的结晶是通过微观可逆性实现的：极化的 B-O-P 键可以在 B-O 键处动态形成和断裂。这构成了新的化学反应，其中简单的接头会聚成由更高价簇组成的框架。该策略的多功能性通过等网状聚立方结构、BP-COF-2 至 5 从 BPA-2 至 5 接头的成功结晶得到证明。这一系列功能化和膨胀的聚立方结构表现出永久的孔隙率。我们发现在 BP-COF-1 中加入酸后，每个立方体的 8 个 B-O 键被裂解并重新排列成 BP-COF-6，其表现出无限价棒。BP-COF-6 的结构是从单晶 X 射线衍射获得的，使其成为少数生长为大晶体的 COF 之一。它揭示了由苯基单元连接以形成层的无限 B-O-P 棒，这些层在晶体中由间隙水分子连接，以稳定一种不寻常的棒内层排列。结论 BPA 接头的化学性质及其已证明的形成立方体和更高价结构的能力扩展了 COF 的范围。从本质上讲，BPA 接头在以更高价的立方体和棒状形式连接以及它们相应的扩展 BP-COF 结构中是必不可少的，从而通过基于 sp2 和 sp3 杂化原子的有机分子的聚类为 COF 的多样化提供了机会. 将这种方法扩展到包含不同形状组合的 COF 有望揭示我们预期的大型未开发结构空间。从单一有机接头设计更高价的 COF。(A) BPA 接头（BPA-1 至 5）在单个分子中结合了硼酸和膦酸功能。(B 和 C) BPA-1 (B) 的自缩合得到 8 价的 BP 立方体 (C) 及其网状聚立方 BP-COF-1。等网状形式（BP-COF-2 到 5）证明了这种化学的多功能性。(D) 加入酸后，BP-COF-1 重排成层状 BP-COF-6，呈现无限价棒。共价有机框架 (COF) 中构建单元的价数（连接性）主要为 3 和 4，分别对应于三角形和正方形或四面体。我们报告了一种制造具有 8 价（立方体）和“无穷大”（棒状）的 COF 的策略。链接器 1, 4-硼苯基膦酸——设计为将硼和磷作为碳族元素的等电子组合——缩合成多孔聚立方烷结构 (BP-COF-1)，公式为 (–B4P4O12–)(–C6H4–)4。它通过 X 射线粉末衍射技术进行表征，该技术揭示了与苯基相连的立方体。类似地制备和表征等网状形式（BP-COF-2 至 5）。由棒单元组成的结构异构 COF（BP-COF-6）的大单晶也使用相同的策略合成，从而推动 COF 化学进入新的价态。类似地制备和表征等网状形式（BP-COF-2 至 5）。由棒单元组成的结构异构 COF（BP-COF-6）的大单晶也使用相同的策略合成，从而推动 COF 化学进入新的价态。类似地制备和表征等网状形式（BP-COF-2 至 5）。由棒单元组成的结构异构 COF（BP-COF-6）的大单晶也使用相同的策略合成，从而推动 COF 化学进入新的价态。