Recent Highlights in Mixed-Coordinate Oligophosphorus Chemistry Cite This: Chem

Chem Soc Rev View Article Online REVIEW ARTICLE View Journal | View Issue Recent highlights in mixed-coordinate oligophosphorus chemistry Cite this: Chem. Soc. Rev., 2016, 45, 1145 Maximilian Donath,† Felix Hennersdorf† and Jan J. Weigand* This review aims to highlight and comprehensively summarize recent developments in the field of mixed-coordinate phosphorus chemistry. Particular attention is focused on the synthetic approaches Received 31st August 2015 to compounds containing at least two directly bonded phosphorus atoms in different coordination DOI: 10.1039/c5cs00682a environments and their unexpected properties that are derived from spectroscopic and crystallographic data. Novel substance classes are discussed in order to supplement previous reviews about mixed- www.rsc.org/chemsocrev coordinate phosphorus compounds. A tremendous development was reported and a great variety Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Introduction of combinations of bonding modes including multiple bonds Phosphorus is an intriguing element with several different bonding was discussed.2 However, charged and zwitterionic compounds modes. The combination of two phosphorus atoms connected were only considered very briefly. This review is aiming to give to each other multiplies the possibilities to gain compounds an overview of selected main-group compounds containing at with different P–P bonds displaying a great variety of bonding least two phosphorus atoms bonded to each other. Those com- modes. Such compounds have always attracted much attention pounds possess different coordination numbers, valence states and a first review in 1965 by Cowley covered the three basic and/or formal charges. Consciously excluding the wide field of types of substances containing phosphorus–phosphorus single metal-coordinated compounds, this report focuses on certain This article is licensed under a bonds that are derived from classical diphosphanes and their intriguing examples for most of the substance classes in order oxidation products.1 InthereportofDillonet al.,whichappeared to enable recognition of cross relations, recurring trends or pro- in 1995, the field of P–P bonded compounds was re-reviewed. perties. To keep the style of an overview, certain known substance classes or specific compounds which have been discussed exten- Open Access Article. Published on 08 February 2016. Downloaded 16/12/2016 11:45:22. sively in the past, will only be commented on briefly. Thus, the Department of Chemistry and Food Chemistry, TU Dresden, 01062 Dresden, Germany. E-mail: [email protected] more detailed discussions focus on cationic mixed-coordinate † Both authors contributed equally. phosphorus compounds for which, in the last two decades, Maximilian Donath studied at Felix Hennersdorf studied at WWU Mu¨nster (Germany) with a Leipzig University (Germany) with research stay in Palermo (Italy). research stays in Stellenbosch He received his diploma degree (South Africa), Singapore and in 2011 and is currently a PhD Melbourne (Australia). He received student in the Weigand group. his MSc degree in 2013 before he After starting his PhD thesis at joined the Weigand group as a the WWU Mu¨nster supported by PhD student and now holds a scholarship of the Fonds der scholarships of the FCI and the Chemischen Industrie (FCI) he Studienstiftung des Deutschen continued it at TU Dresden Volkes. Besides his interest in (Germany) since 2013. His crystallography his research is Maximilian Donath research interest is focused on Felix Hennersdorf focused on the synthesis of multinuclear NMR spectroscopy, polyphosphorus compounds by the synthesis of cationic polyphosphorus frameworks and the activation of white phosphorus. activation of white phosphorus with p-block compounds. This journal is © The Royal Society of Chemistry 2016 Chem.Soc.Rev.,2016, 45, 1145--1172 | 1145 View Article Online Review Article Chem Soc Rev the most rapid development with respect to neutral or anionic Table 1 Selected P–P bonded frameworks with non-symmetricala bond- b compounds was spotted. ing motifs Dillon et al. identified 170 hypothetical modes of P–P bond- s2 s3 s4 s5 ing which exceeds by far the scope of this review. Thus, the P–P bonding of the compounds in this article is classified only by 2 the number of s-bonds (vide infra). This allows to overcome the s problem of categorizing molecules differently due to distinct resonance structures. Secondary consideration of substituent s3 effects often reveals parallels and differences among a class, thus additional subdivisions are made. The term valency is only used if unambiguous. As an s4 example, the IUPAC definition3 gives no clear instruction how + to correctly entitle the valence state of an R4P cation that can be obtained by either protonation of the parent l3-phosphane s5 (phosphonium) or by hydride abstraction from the parent l5-phosphane/phosphorane (l5-phosphanylium).4 The nomen- 6 clature of catenated phosphorus compounds following the IUPAC s rules is thoroughly described in literature.5 Crystal structure a analysis permits a precise determination of coordination environ- Rare symmetrical bonding motifs I, L and P are considered for reasons of completeness. b The overall charge of the resulting systems ments and PÁÁÁP distances allowing a statement on bonding strongly depends on the nature of substituents attached to the P-atoms. interactions. Appropriate computational calculations often give further insights into the strength of the respective bond. The P–P 6 8 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. distance of 2.2 Å in P4 is often used as a benchmark in order to proof in certain cases. Table 1 displays selected P–P bonded classify the bonding situation in the title compounds. Although frameworks with non-symmetrical bonding motifs A–H, J–K the sum of the van der Waals radii of the P–P bonded framework and M–O. In contrast to the well-known symmetrical combina- strongly depends on the oxidation states of the corresponding tions s2–s2 (diphosphenes)9 and s3–s3 (diphosphanes),1 rare phosphorus atoms (e.g. 3.6 Å),7 a value beyond 3 Å is considered examples exhibiting the symmetrical bonding situation I, as very weak interaction, if not repulsion, in cases where a L and P were included for reasons of completeness. The overall bridging ligand prevents a longer distance. 31PNMRchemical charge of the resulting systems strongly depends on the nature shifts and 31P–31P-coupling constants are often indicative for a of the substituents attached to the P-atoms. For clarification certain bonding situation and coordination numbers of the reasons, a short comment about the s-notation and the drawing This article is licensed under a respective phosphorus atoms, but appeared to be of no ultimate of structures in this review is required. There are several ways to indicate the bonding situation of a P–P bonded system which might seem ambiguous. However, they require certain consid- Open Access Article. Published on 08 February 2016. Downloaded 16/12/2016 11:45:22. eration such as following the octet rule and minimizing the Jan J. Weigand obtained his number of formal charges or extended coordination numbers diploma in chemistry in 2002 that are exceptions from the octet rule. For most of the depicted and his Dr. rer. nat. in 2005 from systems we decided to draw the simplest Lewis-type structure the LMU in Munich (Germany). In which primarily follows the octet rule in combination with 2005 he was awarded the Bavarian formal charges (bonding modes: A–F, I, J and L). For a better culture prize and obtained a Lynen recognition of lone pairs of electrons they are depicted (as two Scholarship from the AvH founda- dots) only where relevant. We are aware that in many cases, tion for postdoctoral research at these formal drawings do not display the actual electronic Dalhousie University in Halifax structures of the systems but indicate the actual number of (Canada). He returned to bonding partners as denoted with the s-notation. The number Germany with a Lynen Return of bonding partners at phosphorus can exceed four in certain Fellowship and started his habili- cases, thus representing hyper-coordinated bonding situa- Jan J. Weigand tation at the WWU Mu¨nster at the tions (bonding modes: G, K, and M–P). As Kilian et al. stated: end of 2007 under the supervision ‘‘Hyper-coordination in phosphorus chemistry is not unusual, of Prof. Hahn. He was awarded shortly after the Liebig scholarship penta- and hexa-coordinate compounds are stable when highly of the FCI which allowed him to start his independent career in electronegative substituents are attached to the phosphorus 2008. In April 2010 he became a fellow of the Emmy Noether atom (as e.g.inPF5). On the other hand, hyper-coordination research program awarded by the DFG and recently obtained the with less electronegative atoms around the central phosphorus Wo¨hler research award for young scientists. In 2012, he obtained is less common and more sophisticated approaches (resonance an ERC starting grant from the European Council. Since 2013 he stabilization, favorable ring size formation) are needed to stabilize has been Professor at the TU University Dresden. such species’’.10 1146 | Chem.Soc.Rev.,2016, 45, 1145--1172 This journal is © The Royal Society of Chemistry 2016 View Article Online Chem Soc Rev Review Article The discussion in the following section is divided in the combination of the s-matrix as depicted in Table 1. Only for the combination of s2–s3, s2–s4, s3–s3 and s4–s4 a subdivision into classes of related compounds has been included. In general, for compound classes that are derived from related reactions, only one distinct synthetic approach is discussed. Although Scheme 1 Resonance structures of triphosphaallyl and triphosphapenta- dienyl compounds. some of the related compounds have been reviewed in previous articles, we wish to emphasize on certain reactivity patterns, structural features or physico-chemical properties on the base of The chemical shift of the central, dicoordinate phosphorus their connectivity.

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