Copper(I) complexes of low-coordinate phosphorus(III) compounds

Chapter 1

Copper(I) complexes of low-coordinate phosphorus(III) compounds Christian Mu¨ller Institute of Chemistry and Biochemistry, Freie Universita¨t Berlin, Berlin, Germany

1.1 Introduction In low-coordinate phosphorus(III) compounds the number of substituents (coordination partners) at the phosphorus atom is, as per the definition, lower than its valency of three. Typically, organophosphorus compounds with phosphorus
carbon multiple bonds, but also phosphenium cations of the formula R2P1, belong to this class of compounds. In this respect, the term lowcoordinate should not be confused with low-valent phosphorus compounds, as in the case of, for example, phosphinidenes or diphosphenes with the phosphorus atom in a formal oxidation state of 11 and 12, respectively. This chapter exclusively deals with Cu(I) complexes of phosphaalkenes (I, Fig. 1.1) and derivatives thereof. These compounds are generally referred to as λ3,σ2-species, indicating the number of valence electrons involved in bonds (λ) and the number of σ-bonds between phosphorus and its substituents. The main focus is on unsaturated phosphorus heterocycles as ligands containing one or more phosphorus atoms. Also, the reaction of phosphaalkynes (II, Fig. 1.1) with Cu(I)-complexes is described briefly at the end of the chapter.

1.1.1 Electronic properties of low-coordinate phosphorus compounds Phosphorus, as a multifaceted element, has inspired many chemists to develop organophosphorus compounds, in which phosphorus mimics the chemical element carbon (“Phosphorus
the Carbon Copy”) [1]. This is particularly eminent in low-coordinate phosphorus compounds containing phosphorus
carbon multiple bonds. The difficulty of heavier elements to Copper(I) Chemistry of Phosphines, Functionalized Phosphines and Phosphorus Heterocycles. DOI: https://doi.org/10.1016/B978-0-12-815052-8.00001-4 © 2019 Elsevier Inc. All rights reserved.

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 1.1 General chemical structure of λ3,σ2-phosphaalkenes (I) and λ3,σ1-phosphaalkynes (II).

FIGURE 1.2 Frontier orbitals of the phosphaalkene HPQCH2.

undergo sp2 or sp hybridization leads to drastic consequences with respect to the electronic properties of such compounds, which also determine to a significant extent their reactivity and coordination chemistry. This is illustrated for the simplest phosphaalkene HPQCH2 in Fig. 1.2. Due to a weak 3s-3p overlap, the lone-pair at the phosphorus atom in HPQCH2 is represented by the energetically low-lying HOMO-1 and features a very high 3s character (66% 3s AO, 34% 3p AO) [2]. The HOMO has π-symmetry, while UV photoelectron spectroscopy reveal only a small energetic difference between the HOMO and HOMO-1 (210.3 vs 210.7 eV). This is in clear contrast to imines, which show an inverted orbital situation with the lone-pair (HOMO) highest in energy and with a classical sp2 hybridization of the nitrogen lone-pair (39% 2s AO, 61% 2p AO). It should be noted that also other lowcoordinate phosphorus compounds show similar properties. A consequence of the weaker hybridization of phosphorus also influences geometrical parameters: in phosphaalkenes, the R
P
C angle is about 100 degrees and therefore is considerably smaller compared to related, classical sp2 systems, such as imines or alkenes. A very important aspect, which again contrasts with classical P(III) compounds, is the fact that the presence of a phosphorus
carbon multiple bond leads to an LUMO of π symmetry with a large coefficient at the

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phosphorus atom (Fig. 1.2) [3]. This can be rationalized by the partial positive charge at the phosphorus atom due to the differences in electronegativities between P and C. Taking all these data into account, it is obvious that the coordination chemistry of low-coordinate phosphorus compounds is expected to be significantly different compared to classical P(III) species [4,5]. In fact, it will be strongly dictated by the weak σ-donor and strong π-acceptor properties of the phosphorus atom, while different coordination modes can be accessed due to the presence of the phosphorus lone-pair, as well as the π- and π -system. Moreover, it should be kept in mind that the phosphorus
carbon multiple bond is not chemically inert and can be reactive towards nucleophiles, electrophiles, and certain reagents.

1.2 Phosphaalkenes as ligands The first Cu(I) complex containing a σ-coordinated phosphaalkene ligand was reported in 1994 by Geoffroy et al. [6]. The dimeric complex 1 was obtained from stoichiometric amounts of [(2,4,6-tri-tert-butylphenyl)phosphanediylmethyl]benzene and CuCl in CH2Cl2
CH3CN (Fig. 1.3). Inversely polarized phosphaalkenes with a Pδ2Cδ1 π-electron density rather than the normal Pδ1Cδ2 polarization react with CuBr or CuI to complexes of the type [Cu3X3{μ-P(tBu)C(NMe2)2}3] (2) as the formal result of a cyclotrimerization of a 1:1 adduct [7]. Interestingly, the phosphorus atom bridges to Cu(I) centers as a result of the unusual electronic situation at the phosphorus atom. With sterically more demanding Mes groups attached to the phosphorus atom, the mononuclear Cu(I) complex 3 is formed by reaction of the ligand with [Cu(CH3CN)4]BF4 in the ratio of 2:1 [8].

FIGURE 1.3 Phosphaalkenes with coordination of the lone-pair to a Cu(I) center.

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

Several bis-phosphaalkenes have also been described in the literature as. The coupling of alkynyl(2,4,6-tri-tert-butylphenyl)phosphane with Cu(I) chloride in the presence of n-BuLi leads to the quantitative formation of complex 4 (Fig. 1.4) [9]. The bidentate phosphaalkene 1,3-bis[(2,4,6-tritert-butylphenyl)phosphanediylmethyl]benzene forms with CuCl the heterocubane cluster 5 of the composition Cu4Cl4L2 (Fig. 1.4) [6]. In this complex, the two phosphaalkenes of the same ligand are σ-bonded to two Cu ions of two faces of the Cu4Cl4 core. Phosphaalkenes have also been used as donor sites in neutral P,N,P ligands. In contrast to P,N,P pincer-type complexes, in which the phosphorus donor consists of a classical trivalent phosphorus atom, the phosphaalkene moieties exhibit strong π-accepting properties. 2,6-Bis[1-phenyl-2-(2,4,6-tritert-butylphenyl)-2-phosphaethenyl]pyridine reacts readily with CuBr to form coordination compound 6 with the two phosphaalkene moieties and the central pyridyl group σ-bonded to the tetrahedrally coordinated Cu(I) center (Fig. 1.5) [10]. Reaction of a related P,N,P ligand with [Cu(CH3CN)4]PF6

FIGURE 1.4 Cu(I) complexes of bis-phosphaalkenes.

FIGURE 1.5 Cu(I) complexes with polydentate ligands based on phosphaalkene moieties.

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FIGURE 1.6 Cu(I)-induced rearrangement of 11 under formation of 12.

affords the corresponding acetonitrile complex 7 [11]. Interestingly, by reacting 6 with one equivalent NaBArF4 in CH2Cl2 complex 8 with a three-coordinated, T-shaped geometry around the Cu(I) center is formed [12]. Reaction of 6 with AgPF6 gives the corresponding complex [Cu(P,N,P)] PF6 [10]. Attempts to crystallize this compound from toluene led to the isolation and crystallographic characterization of the cationic, PF6-bridged dimer 9, having a PF62 counteranion. The strong affinity toward the PF62 anion can be explained by the presence of a highly electron-deficient Cu(I) center, caused by the strongly π-accepting phosphaalkene ligands. Other donor combinations have also been reported. Coordination compound 10 contains a chiral, tridentate ligand consisting of a phosphaalkene and a chiral oxazoline moiety, as well as a bridging pyridine unit [13]. 1,3,5Triphospha-1,4-pentadiene-2,4-diamine 11 contains P, N, and PQC double bonds as potential donors. Interestingly, the reaction with Cu(I) leads to an intramolecular rearrangement of the ligand with P
P bond formation and elimination of cyclohexyl isocyanide, giving coordination compound 12 (Fig. 1.6) [14].

1.3 Phosphorus heterocycles as ligands 1.3.1

Four-membered heterocycles with two phosphorus atoms

Diphosphacyclobutadienes are formed in the coordination sphere of transition metals by a metal-mediated cyclodimerization of phosphaalkynes [15]. Typically, 1,3-diphosphacyclobutadienes are formed with tBu-CP, while 1,2-diphosphacyclobutadienes can be formed with sterically less demanding phosphaalkynes. Formally these phosphorus heterocycles are described as aromatic (6-π-electrons), dianionic systems, while the corresponding sandwich complexes are typically termed diphosphete complexes. Thus, the neutral Co-sandwich-complex 13 (Fig. 1.7) consists formally of a Co(III) center and a cationic [Cu(PPh3)2]1 fragment, coordinated to one of the lowcoordinate phosphorus atoms [15]. It can be prepared by salt metathesis

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 1.7 Cu(I) complexes of diphosphetes.

reaction of [K(thf)3{(CoP2C2tPent2)2}] and [K(thf)3{(CoP2C2tBu2)2}] with Cu(I) salts, such as [{CuCl(PPh3)}4]. The formally neutral heteroleptic diphosphete complex with one 1,3,4-tBu-Cp ligand forms coordination polymers of type 14 with a large excess of CuX (X 5 Cl, Br, I) [16]. These complexes contain integrated heterocubane Cu4X4 clusters.

1.3.2

Five-membered heterocylces with one phosphorus atom

The coordination chemistry of 1,3-benzazaphospholes was intensively investigated by Heinicke et al. Reaction of the ligand with copper(I) acetate led to the formation of coordination compound 15 (Fig. 1.8), consisting of a distorted tetrahedral Cu4 core containing two μ2-P benzazaphosphole binding ligands and a total of four bridging carboxylate ligands [17]. The μ2-P bridging coordination of the benzazaphosphole ligand can be regarded as typical for π-electron-rich, aromatic low-coordinate phosphorus compounds. The same group reported on the reaction of a 1,3-benzaphosphole with excess CuBr in THF under formation of 16 [18]. In this complex, two benzazaphospholes coordinate to two Cu(I) ions in a μ2-P fashion, while two additional benzazaphospholes coordinate in a bent η1-P mode, with the bromides in terminal positions. The coordination chemistry of diphosphaferrocenes has also been explored to a significant extent. Interestingly, octa (ethyl)diphosphaferrocene can serve as a chelate ligand. Upon reaction with

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FIGURE 1.8 Cu(I) complexes of benzazaphospholes and phosphaferrocenes.

[Cu(CH3CN)4]PF6, the bis-chelate complex 17 is formed (Fig. 1.8) [19]. 2-(2-Pyridyl)phosphaferrocenes have been investigated by Mathey et al. Reaction of the ligand with [Cu(CH3CN)4]BF4 gives instantaneously the corresponding chelate complex 18 [20]. The first example of a π-bound PQC unit at a Cu(I) ion was reported by Gudat et al. The formally neutral, but zwitterionic benzo[c]phospholide readily forms coordination compound 19 upon reaction with CuI in CH2Cl2 [21]. This complex contains an η1-Pbound benzophospholide as well as an η2 coordinated PQC double bond of a second phospholide, while the iodide completes a Ψ-trigonal geometry at the copper(I) center.

1.3.3

Five-membered heterocycles with two phosphorus atoms

1,3-Diphosphaferrocenes have been used by Scheer et al. for the construction of supramolecular assemblies. The 1D coordination polymer 20 was obtained by diffusion experiments of the ligand with CuI in acetonitrile (Fig. 1.9) [22]. The chains are formed from moieties of the 1,3-diphosphaferrocenes linked by {Cu2(μ-X)2} four-membered rings. Zenneck et al. reported on the use of 1-trimethylstannyl-3,4,5-tri-para-chlorophenyl-1,2-diphosphole as a precursor for 1,2-diphosphacyclopentadienide ligands. Upon reaction of the stannylated 1,2-diphosphole with excess copper(I) 2,20 -bipyridine bromide dimer the unusual Cu(I)-cluster 21 is formed [23]. It consists of five copper atoms, which are linked together by two bridging 1,2-diphosphacyclopentadienide ligands and one bridging CuBr. A tribromide anion acts as the counteranion.

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

FIGURE 1.9 1,3-Diphosphaferrocenes and 1,2-diphosphacyclopentadienides as ligands for Cu(I).

1.3.4

Five-membered heterocycles with three phosphorus atoms

The reaction of K(P3C2tBu2) with Cu2I2 in the presence of PMe3 affords the dinuclear Cu(I) complex 22 (Fig. 1.10) [24]. The two adjacent phosphorus atoms of each ligand bridge the two Cu(I) centers, which possess the usual tetrahedral coordination geometry. Similar to the preparation of 21 (Fig. 1.9), [(3,5-di-tert-butyl-1,2,4-triphospholyl)Cu(PPh3)] (23) is formed by reaction of the stannylated 1,24-triphosphole with the tetrameric copper complex [ClCu(PPh3)]4 [25]. The NMR spectroscopic data are in line with a symmetrical η5-coordinated triphospholyl ligand. Interestingly, 23 can serve as a ligand for the formation of copper
tungsten triphospholyl complexes of type 24 by reaction with [W(CO)5(thf)]. Layering a solution of 1,2,4-triphosphaferrocene [CpFe(η5-P3C2tBu2)] in CH2Cl2 with a solution of CuBr in acetonitrile the formation of the 1D coordination polymer 25 is observed [26]. This compound consists of three different types of Cu atoms, which link two triphosphaferrocene moieties. All three phosphorus atoms of the triphosphaferrocene participate in coordination. Hexaphosphaferrocene [Fe(η5-P3C2tBu2)2] was used by Scheer et al. as the connecting moiety in oligomeric and polymeric Cu(I) compounds. Upon reaction of the hexaphosphaferrocene with Cu(I) halides in a 1:1 ratio and in a mixture of CH2Cl2 and CH3CN, 1D polymeric (CuX)n ladder structures of type 26 are obtained, in which [Fe(η5P3C2tBu2)2] acts as a chelating ligand in a 1,10 -coordination mode [27]. The reaction of [Cp(Fe(η5-P3C2tBu2)2)] with CuCl in a 1:1 ratio leads to the formation of the dimeric complex 27. In this coordination compound, the two adjacent phosphorus atom of each 1,2,4-triphosphaferrocene are

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FIGURE 1.10 Cu(I) complexes of five-membered heterocycles containing three phosphorus atoms.

bridging two Cu(Cl)CH3CN moieties [28]. The remaining phosphorus atom remains uncoordinated, which might suggest that it can further participate in a further coordination toward a Cu(I) center under formation of polymeric chains. Also 1,2,3-triphosphaferrocenes have been used in coordination chemistry. Reaction of [Cp000 Fe(η5-P3C2PhH)] (Cp000 5 η5-C5H2tBu3) with CuBr in CH3CN and CH2Cl2 produces the polymeric complex 28. In this compound a [(CuBr)4(CH3CN)2] unit is doubly bridged by the 1,2,3-triphosphaferrocene moieties in a 1,3-coordination mode [29].

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

1.3.5

Five-membered heterocycles with five phosphorus atoms

Cyclo-P52 is the all-phosphorus analog of the cyclopentadienyl anion and isolobal to Cp2 [30]. In cyclo-P52 each phosphorus atom bears a lone-pair, potentially available for coordination to a metal center. Over the last decade, pentaphosphaferrocenes of the type [CpRFe(η5-P5)] (CpR 5 substituted cyclopentadienides) have been investigated by the group of Scheer for the preparation of a large variety of one- and two-dimensional polymers, as well as discrete spherical compounds. In the latter ones, almost all phosphorus atoms coordinate to a metal center and the topology of fullerenes can be achieved. Reaction of [Cp Fe(η5-P5)] (Cp 5 pentamethyl cyclopentadienide) with CuX (X 5 Cl, Br, I) in solvent mixtures of CH2Cl2 and CH3CN leads to linear 1D chains of the type 29 (Fig. 1.11) [31]. This coordination polymer consists of alternating planar six-membered Cu2P4 and four-membered Cu2Cl2 rings, which are arranged in an alternating orthogonal manner. The coordination environment of Cu(I) is, as expected, tetrahedral. Moreover, each cycloP52 ring coordinates in a 1,2 fashion to two different metal ions. By using Cu(OSO2CF3) as a Cu(I) source and [Cp Fe(η5-P5)], the formation of the 2D

FIGURE 1.11 Cu(I) complexes of pentaphosphaferrocenes.

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polymer [{Cp Fe(μ4,η5:1:1:1-P5)}{Cu(CF3SO3)}]n (30) is observed [32]. In this polymer a 1,2,4-coordination mode of the cyclo-P52 ring is observed, giving rise to the formation of six-membered {Cu2P4} rings and distorted 16-membered {Cu4P12} rings with a porphyrine-like structural motif.

1.3.6

Formally cationic five-membered phosphorus heterocycles

The bis-phosphonio-isophosphindolide ligand shown in compound 31 (Fig. 1.12) is formally cationic but can be regarded as a hybrid between a phosphenium ion R2P1 (two-electron donating ligand) and a (μ2,η1-P)-coordinated phospholide anion (four-electron donor). This compound forms a dinuclear Cu(I) complex, while the Cu2P unit displays features of a twoelectron three-center L(σ)-M bond with the phosphorus lone-pair mainly involved [33]. Theoretical studies on the isophosphindolylium ligands of compound 32 indicate an aromaticity around the five-membered PC4 phosphole moiety [34]. Even though this compound is formally cationic in nature, the phosphorus atom can serve as a good σ-donor and forms, in the presence of CuCl, the neutral copper(I) phosphenium complex 32 (Fig. 1.12). In the presence of CuBr  SMe2, the triazaphospholenium cation, which is a phosphorus analog of a triazolylidene (mesoionic carbene) according to the principle of valence isoelectronicity, forms a neutral coordination compound containing a [Cu2Br4]22 core (33) [35]. In this compound, the phosphorus atom adopts a pyramidal geometry, with the [CuBr2]2 fragment pointing into the direction above the plane created by the five atoms of the

FIGURE 1.12 Cu(I) complexes of formally cationic phosphorus heterocycles.

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

˚ are slightly longer heterocycle. The P(1)
Cu(1) bond lengths at 2.5251(5) A than an average “normal” P(III)-Cu(I) dative bond, but shorter than the sum of the van der Waals radii of the phosphorus and copper atoms. Theoretical investigations on 33 revealed a partial ionic interaction between the ligand and the metal fragment. However, second-order perturbation theory analysis on the NBO basis also showed interaction energies of 35 and 15 kcal/mol from the in-plane phosphorus lone-pair, and a π-orbital of the PQC double bond to the Cu-based d-orbitals, respectively, augmented by a back-bonding interaction of 6 kcal/mol from the metal to the LUMO of the ligand. This demonstrated that the Cu
P bonding also has some covalent character.

1.3.7

Six-membered heterocycles: phosphinines

Phosphinines, the higher homologs of pyridines, have been described in the literature since 1966 [36]. Similar to the situation in phosphaalkenes, the phosphorus lone-pair is represented by the energetically low-lying HOMO-2, while the LUMO has π -symmetry with a large coefficient at the phosphorus atom (Fig. 1.13). Consequently, the aromatic phosphinines are relatively weak σ-donors, but good π-acceptor ligands. The synthesis, reactivity, and coordination chemistry of phosphinines have been highlighted in recent years by several review articles [2
5,37]. Kanter and Dimroth reported on the first Cu(I) complex of 2,4,6-triphenylphosphinine (34, Fig. 1.14) in 1975 [38]. However, the structure was not reported at that time. Later, it turned out that 2,4,6-triarylphosphinines can form heterocubane clusters of the type Cu4Br4L4 (35) [39]. These compounds show an orange phosphorescence at room temperature. With Cu(I), 2-phenyl-3,4-dimethylphosphinine (dmppn) forms an infinite stair-like structure of the type [Cu(dmppn)I]N (36) with the Cu(I) centers possessing the usual tetrahedral coordination geometry [40]. Also, other structures are accessible with phosphinines. 2-TMSphosphinine forms Cu2Br2L4 dimers, as shown for compound 37 [41]. 2-TMSphosphinine can be used as a starting material for the synthesis of the parent phospinine C5H5P by means of a protodesilylation reaction with HCl [41]. C5H5P forms with CuBr  SMe2 infinite CuBr chains with two ligands per Cu(I) center completing the tetrahedral coordination sphere at the metal atom, as found for complex 38. 2-Hydroxyphosphinines are phosphorus derivatives of phenols. 2-Phosphaphenol acts as a pure phosphorus ligand toward FIGURE 1.13 HOMO-2 und LUMO of phosphinine C5H5P.

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FIGURE 1.14 Cu(I) complexes of phosphinines.

FIGURE 1.15 Cu(I) complexes of hydroxyl-functionalized phosphinines.

copper(I) chloride. Interestingly, the dimeric complex 39 with a Cu2P2 core and Cu
Cu interactions shows both η1 and μ2-phosphorus bound phosphinines (Fig. 1.15) [42]. The bridging μ2-P mode is not uncommon for phosphinines and has also been observed for tricoordinate λ3,σ3-phosphorus compounds. The reaction of 2-phosphaphenol with CuBr and CuI is, however, completely

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

different: both complexes consist of infinite chains of (P2CuBr/I) units (40), most likely to reduce steric repulsion [42]. The related sodium salt of 2phosphanaphthalene reacts with [CuBr(PPh3)] to the disodium salt 41 [43]. 2-Hydroxyphosphinines can further be converted to phosphininephosphinites (POP0 ). Such ligands can form dimeric [Cu2(X)2(μ2-POP0 )2] complexes of type 42 (X 5 Cl, Br, I) with Cu
Cu interactions (Fig. 1.16) [44]. These coordination compounds show red phosphorescence at room temperature, with relatively long decay times. The tetranuclear [CuI]4-cluster 43 contains the chelating 2-diphenylphosphino-3-methylphosphinine ligand. This cluster may be described as an octahedron having a nearly square planar Cu4 base [45]. Pyridyl-bridged bisphosphinines, the phosphorus derivatives of terpyridines, were reported by Mu¨ller et al. In contrast to its terpyridine analog, facile coordination of this tridentate ligand toward a neutral Cu(I) center was observed [46]. The corresponding CuBr complex 44 was characterized crystallographically and revealed a distorted tetrahedral coordination geometry of the metal center as a result of an unusual coordination mode of the two phosphinine ligands. The cationic copper(I) complex of 4,40 ,5,50 -tetramethyl bisphosphinine (tmbp) [Cu(tmbp)(L2)]1 (45) is accessible using the precursor [Cu(tmbp)(MeCN)2]1, which is obtained by ligand exchange from [Cu(MeCN)4]1[BF4]2 [47]. Interestingly, using [Cu(MeCN)4]1[BF4]2, 2,20 bipyridine and tmbp, the polymeric helix 46 is formed, in which tmbp acts as a linker between [Cu(bpy)]1 fragments, while 2,20 -bipyridine acts as a chelating ligand [47]. The coordination compound 47 is composed of a phosphinine-based macrocycle, consisting of three phosphinine units and Me2Si
O
SiMe2 linker, and contains a tricoordinated Cu(I) ion [48].

FIGURE 1.16 Cu(I) complexes of polydentate phosphinines.

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1.4 Reactions of phosphinines with Cu(I) Metal complexes of phosphinines can be very sensitive toward protic reagents (e.g., H2O, alcohols, amines) and oxygen [49
51]. Reaction of 2,4,6-triphenylphosphinine 48 (2,4,6-TTP) with [Cu(CH3CN)4]ClO4 leads in the first instance to the formation of the corresponding [Cu(2,4,6-TTP)2] ClO4 complex [52]. However, upon recrystallization of this compound, a cofacial oxidative coupling of the two phosphinine ligands was observed and the cage compound 49 was generated (Fig. 1.17). It turned out that the ClO42 anion is most likely required for this reaction. Moreover, the oxidation of the Cu(I)-coordinated 2,4,6-TPP by ClO42 to the radical cation 2,4,6TPP1 is suggested to be the initial step in the subsequent coupling reaction. No other coupling products were detected, which indicates that the stoichiometry of the coupling is determined by the number of 2,4,6-TPP molecules in the coordination sphere of Cu(I).

FIGURE 1.17 Cu(I)-mediated oxidative coupling of 2,4,6-triphenylphosphinine.

FIGURE 1.18 Reactions of phosphaalkynes in the presence of Cu(I).

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Phosphines, Functionalized Phosphines, and Phosphorus Heterocycles

1.5 Reactions of phosphaalkynes with Cu(I) Similar to phosphaalkenes, phosphaalkynes display a rich and versatile coordination chemistry. Also, in their case, the σ-, π-, and σ/π-coordination modes are known. However, no Cu(I) complexes of phosphaalkynes have been reported to date. On the other hand, the reaction of phosphaalkynes with Cu(I) complexes has been studied to a certain extent. Interestingly, the reaction of CuCl with the aminophosphaalkyne 50 leads to the trimer (iPr2NCP)3 51, along with several byproducts (Fig. 1.18) [53]. In the presence of oxygen and water, the 1λ3,3λ5-diphosphetene 52 is isolated in reasonable amounts. In the presence of Cu(I) and acetonitrile, tert-butylphosphaalkyne 53 undergoes oligomerization under formation of C4P5 phosphorus cages, which are stabilized in a matrix of Cu(I) (54, Fig. 1.18) [54].

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[14] A.K. Adhikari, T. Grell, P. Lo¨nnecke, E. Hey-Hawkins, Versatile coordination modes of triphospha-1,4-pentadiene-2,4-diamine, Inorg. Chem. 57 (2018) 3297
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