Synthesis and Reactivity of o-Phosphane Oxide Substituted Aryl

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Synthesis and Reactivity of o‑Phosphane Oxide Substituted Aryl(hydro)borates and Aryl(hydro)boranes Jens Michael Breunig, Felix Lehmann, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner* Institut für Anorganische und Analytische Chemie, J. W. Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt (Main), Germany S Supporting Information *

ABSTRACT: The reaction of 1,2-C6H4(P(O)-t-Bu2)(Li) (Ph*Li) with B(OMe)3 furnishes a mixture of Li[Ph*B(OMe)3] (Li[2a]) and Ph*B(OMe)2 (2b). Further treatment with Li[AlH4] provides the trihydroborate Li[Ph*BH3] (Li[3]), which can subsequently be converted into the intramolecular PO−B adduct Ph*BH2 (4) through hydride abstraction with Me3SiCl. Addition of C6F5MgBr yields Mg[Ph*BH2(C6F5)]2 (Mg[5]2), which is inert toward Me3SiCl but reacts with water to give Ph*B(H)C6F5 (6). Upon addition of further C6F5MgBr, a mixture is formed, from which crystals of Mg[Ph*BH(C6F5)2]2 (Mg[7]2) were obtained. The reaction of Ph*Li with (C6F5)2BH·SMe2 provides access to Li[7], but again with limited product selectivity. The targeted acidic hydrolysis of Li[7] furnishes Ph*B(C6F5)2 (8), while Mg[7]2 reacts back to 6. The anions of the hydroborates Li[3] and Mg[5]2 act as BH2,O-chelating ligands toward their metal ions. Therefore, Li[Ph*2BH2] (Li[13]) was also synthesized to obtain the corresponding pincer-type species. 4, 6, and 8 exist as water-stable intramolecular PO−B adducts both in solution and in the solid state. X-ray crystallography and 31P NMR spectroscopy indicate an increase in the Lewis acidity of the boryl groups in the order 4 < 6 < 8.

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INTRODUCTION Alkyl(hydro)boranes, such as 9-borabicyclo[3.3.1]nonane, are among the best studied classes of compounds in main-group chemistry, primarily because of their widespread use as hydroboration reagents in organic synthesis.1 Aryl(hydro)boranes, in contrast, have long received considerably less attention. One important reason undoubtedly lies in their pronounced tendency to engage in dynamic substituent redistribution equilibria (ligand scrambling), a generally undesired process leading from initially pure RBH2 or R2BH species to mixtures RnBH3−n (n = 0−3; R = aryl) upon storage.2,3 However, when primary hydroboration products no longer serve exclusively as mere synthesis intermediates but become the actual synthesis targets, the availability of organyl(hydro)boranes RBH2 and R2BH with an as broad as possible variety of boron substituents R will be highly desirable and aryl(hydro)boranes can no longer be neglected. For instance, the rapidly growing interest in boron-doped πconjugated materials,4−12 some of which are conveniently accessible through the monohydroboration of arylalkynes with aryl(hydro)boranes,13−24 has recently made it inevitable to study both ArBH2 and Ar2BH in greater detail (Ar = aryl). As a result, the ligand-scrambling processes are now increasingly better understood3,25 and in certain cases have even been turned into powerful synthesis tools.26 As examples, we mention the polycondensation of in situ generated 1,1′fc(B(H)Br)2 (fc = (C5H4)2Fe), which leads to the boronbridged poly(ferrocenylene) I (Figure 1),25,27−30 the cyclocondensation of [1,2-C6H4(BH2)2], which gives the 9,10© XXXX American Chemical Society

Figure 1. Compounds I−III synthesized through controlled ligand scrambling.

dihydro-9,10-diboraanthracene adduct II,31 and the ringopening polymerization of 9H-9-borafluorene, which furnishes the main-chain boron-containing poly(1,2-phenylene) III.32,33 The controlled substituent redistribution of aryl(hydro)boranes is not only valuable when it comes to achieving a high degree of molecular complexity (as in the cases of I−III) but can also be exploited to facilitate the synthesis of important Received: April 11, 2014

A

dx.doi.org/10.1021/om500385h | Organometallics XXXX, XXX, XXX−XXX

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organoborane reagents such as Piers borane, (C6F5)2BH.34−36 Piers’ original, experimentally demanding synthesis of (C6F5)2BH starts from C6F5Li and Cl2SnMe2, proceeds via the reaction of the resulting stannane (C6F5)2SnMe2 with BCl3 to give the chloroborane (C6F5)2BCl, and ends with the reduction of (C6F5)2BCl to (C6F5)2BH by means of Me2Si(H)Cl. Our alternative synthesis sequence, which is based on a substituent redistribution strategy, first furnishes the dihydroborate [(C6F5)2BH2]− ([IV]−) from the easy-to-handle chemicals C6F5MgBr, BH3·SMe2, and Me3SiCl in a one-pot procedure (Scheme 1, top).37 Subsequent hydride abstraction from [(C6F5)2BH2]− with further Me3SiCl in SMe2 finally yields the adduct (C6F5)2BH·SMe2 (albeit not the free borane).37

devised to allow the successive buildup of a triarylborane from a corresponding aryl(dihydro)borane in a controlled fashion. As a key design element, the aryl substituent of the aryl(dihydro)borane is equipped with a Lewis basic ortho substituent to accomplish stabilization through intramolecular adduct formation.

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RESULTS AND DISCUSSION For the following reasons, a t-Bu2P(O) group was chosen as the Lewis basic ortho substituent. (i) The 31P nucleus provides a diagnostically helpful NMR probe, because its chemical shift value depends drastically on whether or not a PO−B adduct is established (cf. the use of Et3PO in the Gutmann−Beckett method39−42 for the NMR assessment of Lewis acidities). (ii) Phenyl(diorganyl)phosphane oxides easily undergo directed ortho metalation (formation of a favorable POMCC fivemembered ring),43 whereas phenyl(diorganyl)phosphanes do not. (iii) Two tert-butyl groups provide maximum steric protection of the aimed-for PO−B adducts. Synthesis of [ArB(C6F5)nH3−n]− Borates and ArB(C6F5)nH2−n Borane Adducts (n = 0−2). Our initial step of the triarylborane assembly essentially reproduced the synthesis of 1,2-C6H4(P(O)-t-Bu2)(B(OR)2) by Snieckus et al. ((HOR)2 = 1,3-propanediol).43 First, t-Bu2P(O)Ph (1)43 was ortholithiated with t-BuLi at −78 °C and then the phenyllithium intermediate was quenched with B(OMe)3 to obtain a mixture of the lithium phenylboronate Li[2a] and the phenylboronic acid ester 2b44 (Scheme 2; note that the product ratio Li[2a]:2b is poorly reproducible and, once Li[2a] has been formed, it does not easily eliminate LiOMe even in refluxing THF). The mixture Li[2a]/2b45 was used without further workup in a one-pot procedure for the preparation of the trihydroborate Li[3] (Scheme 2); the success of the subsequent

Scheme 1. One-Pot (Top) and Stepwise (Bottom) Synthesis of [IV]−

To gain further insight into the reaction mechanism, we have also performed the synthesis in a stepwise manner and monitored each of the individual steps by in situ NMR spectroscopy (Scheme 1, bottom). Thereby it became evident that the addition of the first equivalent of C6F5MgBr leads to a mixture of the three hydroborates [BH4]−, [(C6F5)BH3]−, and [(C6F5)2BH2]− ([IV]−). Most remarkably, in the second step of the sequence (i.e., H− abstraction/C6F5− addition) the initial blend transforms into a single product, namely the dihydroborate [IV]−. In this context, a related observation on the parent Piers borane is also revealing: although a dimer in the solid state, (C6F5)2BH establishes a monomer−dimer equilibrium in C6D6 solution.35 However, in our hands the compound gives rise to 11B and 19F NMR spectra containing not only the resonances of the monomer and the dimer but also two more 11B NMR signals and two additional sets of 19F resonances. After the addition of pyridine (py), the entire sample consists exclusively of the adduct (C6F5)2BH·py.38 We therefore conclude that the (minor) extra signals are not due to impurities but originate from either substituent scrambling products of (C6F5)2BH or from higher aggregates of the compound. If the first hypothesis is true, pyridine in the Piers borane solution would have the following effect: out of a variety of equilibrating aryl(hydro)boranes, one species is stabilized particularly well through adduct formation with the added Lewis base. The entire equilibrium is consequently shifted into the respective direction, which explains the resulting product selectivity. The question of how a mixture of different aryl(hydro)boranes can be transformed into one specific target species by addition of an appropriate Lewis base is of crucial importance to achieve the further development of substituent scrambling into a useful synthetic tool. In order to contribute more answers to this question, we now describe a molecular platform

Scheme 2. Synthesis of Compounds [2a]−/2b, [3]−, and 4a

a Conditions: (i) (1) t-BuLi, Et2O, −78 °C, 2 h, (2) B(OMe)3, Et2O, −78 °C → room temperature, 14 h; (ii) Li[AlH4], Et2O, −78 °C → room temperature, 14 h; (iii) Me3SiCl, toluene, room temperature, 14 h; (iv) Li[HBEt3], THF/Et2O, −78 °C → room temperature, 14 h.

B


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Scheme 3. Synthesis of Compounds [5]−, 6, and [7]− a

a

Conditions: (i) C6F5MgBr, Et2O, room temperature, 14 h; (ii) H2O, Et2O, room temperature, 2 h; (iii) C6F5MgBr, Et2O, room temperature, 2 h; (iv) H2O, Et2O, room temperature.

sequence starting from 6 (cf. Scheme 3) met with only partial success: 2 h after the addition of the Grignard reagent, NMR spectra (C6D6) showed that approximately 40% of unreacted 6 was still present in the sample. In addition, we observed several poorly resolved proton signals and two 11B{1H} resonances at −17.2 and −17.9 ppm, both of which split into doublets upon proton coupling (1JB,H = 72 Hz). These 11B NMR resonances are in agreement with the presence of tetracoordinated boron nuclei, each of them bearing one proton, as would be required for the target compound [7]− (the two 11B signals could then be due to two different aggregation states of the Mg([7]/Br)2 complexes). An X-ray crystallographic investigation of a few crystals, which grew from one NMR sample and turned out to consist of Mg[7]2·2C6D6, lends further support to the assumption that some [7]− was indeed generated in the experiment. Surprisingly, hydrolytic workup of the reaction mixture resulted in the almost quantitative recovery of the starting material 6. Thus, the elimination of C6F5H seems to be preferred over the liberation of H2 in this particular example. As an alternative approach to [7]− we tested the reaction between (C6F5)2BH·SMe2 and 1,2-C6H4(P(O)-t-Bu2)(Li) in a mixture of C6H6 and Et2O (Scheme 4). NMR spectroscopic monitoring revealed signal patterns (1H, 11B, 19F, 31P) similar to those assigned to Mg[7]2 in the paragraph above. We attribute minor differences in the chemical shift values to the different counterions present in the Grignard sample and the sample obtained by ortho lithiation. Again the spectra were poorly resolved and the number of resonances was higher than expected, which is due to either the generation of more than one product or to the presence of several complex aggregates. We refrained from a further investigation of [7]− and tried to directly transform the product blend into the triarylborane adduct 8 (Scheme 4). The introduction of a second C6F5 ring at the boron atom renders the removal of the remaining hydride substituent increasingly difficult: while an Et2O solution of [5]− hydrolyzes almost instantaneously, addition of H2O to Li[7] in Et2O leaves about half of the material unchanged after 1 week at room temperature (1H NMR spectroscopic control). For the clean and rapid transformation of Li[7] into the borane adduct 8, we had therefore to employ aqueous HCl (0.1 N). We emphasize that no B−C bond hydrolysis with formation of C6F5H was observed in this case. Apparently, the counterion (Mg2+ vs Li+) has a decisive influence on the outcome of the hydrolysis reaction, which could be due to the coordination of Mg2+ to [7]− being stronger than that of Li+ (cf. the solid-state structure of Mg[7]2· 2C6D6 in Figure 2, bottom). NMR Spectroscopic Characterization of [ArB(C6F5)nH3−n]− Borates (n = 0, 1) and ArB(C6F5)nH2−n Borane Adducts (n = 0−2). The lithium trihydroborate Li[3] shows an 11B{1H} NMR resonance at −24.3 ppm (C6D6

synthesis steps did not depend on the ratio Li[2a]:2b. Treatment of Li[2a]/2b with Li[AlH4] in Et2O furnished Li[3], which was purified by reprecipitation from C6H6 at 6 °C. Afterward, the trihydroborate was still contaminated with small amounts of aluminum salts, which turned out to be extremely hard to remove at this stage. Since the aluminum salt contaminants are no obstacle to the transformation of Li[3] into the borane adduct 4 with Me3SiCl in toluene, we next performed the hydride abstraction using crude Li[3] as the starting material. Exploiting the fact that 4 is water stable, separation of residual aluminum components was now easily performed through aqueous workup to give 4 in analytically pure form (yield 69% with respect to 1). With 4 in our hands, we were finally able to synthesize an authentic sample of pure Li[3] through hydride addition with Li[HBEt3] in THF/Et2O (62% yield; Scheme 2). Even though the PO−B adduct 4 withstands the attack of water, it readily reacts with the stronger nucleophile C6F5MgBr in Et2O to give the dihydroborate [5]− (Scheme 3). In most cases, this synthesis intermediate was immediately hydrolyzed to generate the corresponding water-stable borane adduct 6 with liberation of H2 (hydride abstraction using Me3SiCl failed in this case). Only for the purpose of NMR spectroscopic and structural characterization was the dihydroborate isolated once and recrystallized from Et2O to give the magnesium complex Mg(Et2O)[5]2·Et2O (cf. the Supporting Information for full details); X-ray-quality crystals of adduct 6 were grown from hexane at 6 °C. For the synthesis of the triarylborane adduct 8 (cf. Scheme 4), the repetition of the C6F5MgBr addition/hydrolysis Scheme 4. Synthesis of Compounds Li[7]a and 8b

a

Li[7] was obtained as part of a product mixture and was not characterized further. bConditions: (i) (1) t-BuLi, C6H6/Et2O, room temperature, 2 h, (2) (C6F5)2BH·SMe2, C6H6/Et2O, room temperature, 14 h; (ii) HCl, Et2O/H2O, room temperature, 14 h. C


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and THF-d8) and a 31P{1H} NMR signal at 69.1 ppm (C6D6; 60.2 ppm, THF-d 8 ). The former is characteristic of tetracoordinated boron nuclei46 and splits into the expected quartet when coupling to the three hydride substituents is admitted (1JH,B = 81 Hz). The latter signal is significantly shifted to lower field in comparison to the 31P resonance of the parent t-Bu2P(O)Ph (1; Table 1), thereby indicating that the

values of the respective adduct on the one hand and that of the free PO donor on the other. (ii) The angle at which the donor and the acceptor can approach each other plays an important role. If this is true, the Gutmann−Beckett method for the determination of Lewis acid strengths may lead to artifacts if the molecular shapes of the donor and/or acceptor impede an optimal mutual arrangement in the adduct complex. X-ray Crystal Structure Analyses of [ArB(C6F5)nH3−n]− Borates and ArB(C6F5)nH2−n Borane Adducts (n = 0−2). Selected crystallographic data for the structure analyses discussed in the following paragraphs are compiled in Tables S1−S4 (cf. the Supporting Information; note that in a crystallographic context we are, for reasons of simplicity, treating compounds containing deuterium atoms as if they contained exclusively hydrogen atoms). In the lithium trihydroborate (Li[3])2·C6H6 (Figure 2), each lithium ion is chelated by the PO donor and the BH3 moiety of a [3]− ligand. Using Edelstein’s47 correlation of metal··· boron distances as a measure of the denticity of a trihydroborate group, a value of 1.6 ± 0.1 Å is estimated for the ionic radius of bidentate trihydroborate ligands. Thus, B···Li distances of about 2.33 Å can be expected for RBH3-η2-Li coordination modes (ionic radius of tetracoordinated Li+:0.73 Å48).29,49 The B(1)···Li(1) distance of 2.226(3) Å in the Li[3]chelate therefore corresponds to an η2 coordination mode. The ligand environment of each lithium ion is completed by the PO donor of a second Li[3]-chelate to generate the centrosymmetric dimer (Li[3])2. It is revealing to compare the structure of (Li[3])2·C6H6 with that of the related dimeric species (Li(Et2O)[1,2-C6H4(BH3)(Bpin)])2 (HBpin = pinacolborane) featuring B−O instead of PO donor groups.50 In the latter compound the BH3 substituents are bridging the lithium ions, thereby generating a central Li2B2 ring as opposed to an Li2O2 ring; the Bpin units, in turn, take over the role of the BH3 groups in (Li[3])2·C6H6. In the solid-state structure of Mg(Et2O)[5]2·Et2O (Figure 2) the Mg2+ ion carries two BH2,O-chelating ligands [5]− and one Et2O ligand. The configuration of the molecule (τ51 = 0.36) lies between square pyramidal (τ = 0) and trigonal bipyramidal (τ = 1). In contrast to the case for Mg(Et2O)[5]2·Et2O, the monohydroborate salt Mg[7]2·2C6H6 (Figure 2) features homoleptic complexes with no additional solvent molecules being coordinated. The Mg2+ ion is located in a C2-symmetric ligand sphere established by two PO donors, two B−H units, and two F atoms. F(22) and F(22A) are occupying axial positions of the distorted octahedron, whereas O(1) and O(1A) (as well as B(1) and B(1A)) are located cis to each other. While the O−Mg bonds in Mg[7]2·2C6H6 (1.892(2) Å) are shorter than those in Mg(Et2O)[5]2·Et2O (average 1.954 Å), the B···Mg distances are elongated by 0.45 Å, which is in line with a switch in the coordination mode from η2 to η1. In the intramolecular PO−B adducts, the B(1)−O(1) bond lengths exhibit a small but continuous decrease from 4 to 6 to 8·0.5C6H6, which nicely fits the concomitant increase in the Lewis acidities of the respective free boranes (cf. the trend in the 31P NMR parameters discussed above). As already alluded to above, the intermolecular PO−B adduct 9 (two crystallographically independent molecules 9A and 9B in the asymmetric unit; cf. the Supporting Information) possesses an even shorter B−O bond than its intramolecular congeners (average 1.528 Å). The naphthalene-based intramolecular adduct V (Figure 3), in contrast, features a rather long B−O

Table 1. 11B and 31P Chemical Shift Values (Measured in C6D6) of 1, Li[3], 4, Mg[5]2, 6, 8, 9, and 10

a

compd

δ(11B)

δ(31P)

Li[3] Mg[5]2 4 6 8 9 10 1

−24.3 −24.1 4.0 2.5 4.8a −5.0 0.7

69.1 68.9 97.3 98.3 100.9a 74.6 77.7 49.4

Measured in CDCl3.

PO donor engages in Li+ coordination (cf. the solid-state structure of (Li[3])2·C6H6; Figure 2 below). Replacement of one of the three hydride ligands with a C6F5 substituent leaves the 31P and 11B chemical shift values essentially unchanged (cf. [5]−; Table 1) but slightly reduces 1JH,B by 7 Hz to a value of 74 Hz (triplet resonance). In the 1H NMR spectra, 1:1:1:1 quartets assignable to the BH3 (Li[3]) and BH2 (Mg[5]2) groups appear at 1.24/2.14 ppm (THF-d8/C6D6) and 2.83 ppm (C6D6), respectively. Upon going from the hydroborates to the PO−B adducts, both the 11B and the 31P signals experience downfield shifts of approximately 30 ppm (Table 1). A closer inspection of the 31P NMR data reveals that the phosphorus nucleus becomes more deshielded as more C6F5 substituents are attached to the boron atom. In analogy to the Gutmann−Beckett NMR method, this is the expected trend if we assume an increasing boron Lewis acidity along the sequence 4 < 6 < 8. Remarkably, the 31P resonances of 4, 6, and 8 are shifted strongly downfield in comparison not only to the signal of the free phosphane oxide 1 but also to the resonances of the closely related intermolecular PO−B adducts 9 and 10 (Figure 3). A deceptively simple first explanation would be that steric repulsion in the intermolecular case prevents the optimal close approach between the donor and the acceptor in 9 and 10, which could result in a weaker PO−B interaction. Being aware that bond lengths are not strictly correlated with bond strengths and that the significance of solid-state structures for the interpretation of solution properties is limited, we find that this explanation is not supported by our X-ray crystallographic data: the B−O bond length in the intermolecular adduct 9 is even somewhat shorter than the corresponding lengths in 4, 6, and 8 and the P−O bonds are almost equally long (cf. Table 2 below). A distinct difference, however, is obvious for the B− O−P bond angle, which possesses an average value of 137.9° in 9 but is significantly reduced in 4, 6, and 8 (114.2(2)− 115.9(2)°; Table 2). The peculiar 31P NMR data of the intramolecular adducts are therefore likely attributable to the following effects. (i) In contrast to the case for 9 and 10, the compounds 4, 6, and 8 cannot take part in dynamic association−dissociation equilibria. Thus, their 31P chemical shift values are not the weighted averages between the shift D


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Figure 3. Intramolecular borane adduct 6 together with the four related systems 9, 10, V, and VI used for comparison.

Figure 2. Molecular structures of (Li[3])2·C6H6 (top), Mg(Et2O)[5]2· Et2O (middle), and Mg[7]2·2C6H6 (bottom). Hydrogen atoms (except on boron), methyl groups, carbon atoms of coordinated Et2O (cf. O(3) in Mg(Et2O)[5]2·Et2O), and cocrystallized solvent molecules are omitted for clarity. Selected bond lengths (Å) and atom···atom distances (Å): (Li[3])2·C6H6, O(1)−Li(1) = 1.852(3), O(1)−Li(1A) = 1.869(3), B(1)···Li(1) = 2.226(3) (symmetry transformation used to generate equivalent atoms (A) −x + 1, −y + 1, −z + 1); Mg(Et2O)[5]2·Et2O, O(1)−Mg(1) = 1.957(2), O(2)− Mg(1) = 1.951(2), O(3)−Mg(1) = 2.087(2), B(1)···Mg(1) = 2.555(4), B(2)···Mg(1) = 2.550(4); Mg[7]2·2C6H6, O(1)−Mg(1) = 1.892(2), F(22)−Mg(1) = 2.075(1), B(1)···Mg(1) = 3.002(2) (symmetry transformation used to generate equivalent atoms (A) −x + 1, y, −z + 1/2).

Figure 4. Molecular structures of 4 (top), 6 (middle), and 8·0.5C6H6 (bottom). Hydrogen atoms (except on boron) and the cocrystallized solvent molecule are omitted for clarity. For key structural parameters see Table 2.

1.514 Å to 1.544 Å (Table 2). Remarkably, however, the aforementioned differences in the B−O bond lengths between individual adducts (cf. entries 1−5 in Table 2) are not accompanied by any appreciable variations of the PO bond lengths in spite of the existing B−O/PO synergism. Finally, the formal relationship between the intramolecular PO−B adducts and the methylene-bridged phosphonium borate VI53,54 (Figure 3) is worth mentioning, even though both classes of compounds differ in details of their molecular scaffolds.

distance (1.621(6) Å),52 because of either the highly congested environment around the boron atom or the slightly twisted conformation of the 1,8-substituents (again for steric reasons). In comparison to the corresponding hydroborate complexes, the adducts 4, 6, and 8·0.5C6H6 (Figure 4) experience a significant stretch of the PO bonds from an average value of E


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obtained through the aforementioned reaction between 1,2C6H4(PPh2)(Li) and (C6F5)2BH·SMe2, the alternative approach was not pursued further. 12 gives rise to an 11B NMR resonance at −26.9 ppm (1JB,H not resolved) and to a multiplet in the 31P{1H} NMR spectrum at −0.8 ppm (cf. (C6F5)2BH·PPh3:57 δ(11B) −23.9; δ(31P) 12.0). The resonance assignable to the boron-bonded hydrogen atom appears as a broad hump at δ(1H) 3.80. Two sets of signals in the 19F NMR spectrum can be attributed to the C6F557 and C6F4 rings. According to X-ray crystallography (Figure 5), the sixmembered heterocycle of 12 contains a B(1)−P(1) bond

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of 4, 6, 8·0.5C6H6, 9, V, (Li[3])2·C6H6, Mg(Et2O)[5]2·Et2O, and Mg[7]2·2C6H6 compd 4 6a 8·0.5C6H6 9b V (Li[3])2·C6H6 Mg(Et2O)[5]2·Et2O Mg[7]2·2 C6H6

B(1)−O(1)

P(1)−O(1)

B(1)−O(1)−P(1)

1.584(3) 1.574(2) 1.550(2) 1.524(3) 1.531(3) 1.621(6)

1.542(2) 1.545(2) 1.546(2) 1.531(2) 1.534(2) 1.532(3) 1.518(2) 1.513(2) 1.512(1)

114.2(2) 114.4(1) 115.9(2) 138.4(2) 137.5(2) 119.7(2)

a

6 crystallizes also as the polymorph 6* (bond lengths and angles of 6 and 6* are the same within the experimental error margins; cf. the Supporting Information). bTwo crystallographically independent molecules, 9A and 9B, in the asymmetric unit.

Synthesis and Characterization of a 9,10-Dihydro-9phospha-10-boraphenanthrene. Having successfully employed the 1,2-C6H4(P(O)-t-Bu2)(B(C6F5)nH2−n) scaffold for the sequential synthesis of the series n = 0−2, we next modified the Lewis basic side arm. Since the phosphane 1,2-C6H4(P-tBu2)(Br) has recently been made available,55 the obvious next step would have been to switch from a P(O)-t-Bu2 to a P-t-Bu2 donor. However, in light of the pronounced oxygen sensitivity of di-tert-butylphosphanes we selected a PPh2 group instead. Lithium−bromine exchange on 1,2-C6H4(PPh2)(Br) (11) by means of n-BuLi at room temperature, followed by the addition of (C6F5)2BH·SMe2, did not establish a new B−C bond but furnished the 9,10-dihydro-9-phospha-10-boraphenanthrene 12 instead (Scheme 5). The air- and water-stable compound Scheme 5. Synthesis of Compound 12

Figure 5. Molecular structure of 12. Hydrogen atoms (except on boron) are omitted for clarity. Selected bond lengths (Å), bond angles (deg), and torsion angles (deg): B(1)−P(1) = 1.951(2), B(1)−C(1) = 1.617(3), P(1)−C(11) = 1.796(2); B(1)−P(1)−C(11) = 105.9(1), P(1)−B(1)−C(1) = 103.2(2); C(1)−B(1)−P(1)−C(11) = 46.0(2), C(1)−C(2)−C(12)−C(11) = 30.3(3).

length of 1.951(2) Å, which is not too much shorter than the corresponding bond in the intermolecular adduct (C6F5)2BH· PPh3 (1.986(2) Å57). The interannular bond angles at boron and phosphorus do not deviate appreciably from the ideal tetrahedral angle (B(1)−P(1)−C(11) = 105.9(1)°, P(1)− B(1)−C(1) = 103.2(2)°), thereby indicating a largely unstrained ring system. The hydrogen atom at B(1) was located in the difference Fourier map and freely refined. Synthesis and Characterization of a Diaryl(dihydro)borate with Two (t-Bu)2PO Donor Groups. Given that the dihydroborate [5]− showed no tendency to undergo substituent redistribution, we deemed it an attractive goal to replace the C6F5 ring by a second C6H4(P(O)-t-Bu2) group. The result would be an anionic pincer ligand possessing an unusual dihydroborate moiety instead of a classical Lewis basic atom at the central donor site. The ability of hydroborates to coordinate (transition) metal ions has ample precedence in the literature58,59 and also becomes evident from an inspection of the crystal structures of (Li[3])2·C6H6, Mg(Et2O)[5]2·Et2O, and Mg[7]2·2C6H6. Applying essentially the protocol developed for the synthesis of Mg[5]2, the tridentate ligand [13]− was accessible in a straightforward manner starting from the borane adduct 4 and ortho-lithiated 1 (Scheme 6). The compound crystallizes from Et2O/pentane at −40 °C as the dimeric complex (Li[13])2 (Figure 6). Different from the hydroborates discussed so far, an attempted hydride elimination using Me3SiCl, H2O, or HCl in Et2O did not provide a clean PO−B adduct but resulted in a complex product mixture. The 11B{1H} (−17.8 ppm) and 31P{1H} (59.8 ppm) chemical shift values of Li[13] in C6D6 are comparable to

a

a

Conditions: (i) (1) n-BuLi, C6H6/Et2O, room temperature, (2) (C6F5)2BH·SMe2, C6H6/Et2O, room temperature, 14 h.

obviously results from the nucleophilic attack of the aryllithium species on an ortho carbon atom of the C6F5 ring;53,56 the electron deficiency of the boron atom, in turn, is alleviated by a dative P−B bond. We note in passing that the addition pattern underlying 12 is inverse to the addition pattern leading to VI (Figure 3) via B−C and ortho C−P bond formation between (C6F5)2BOEt and LiCH2P(t-Bu)2.53,54 Compound 12 was obtained in a moderate yield of 45%. In an attempt to increase the yield, we first prepared the adduct (2-Br-C6H4)Ph2P· B(H)(C6F5)2 with the aim to carry out a subsequent lithium− bromine exchange reaction, followed by intramolecular C−C bond formation. However, since the isolated yield of the adduct (55%; cf. the Supporting Information for more details) was already comparable to the yield of the target compound 12 F


Organometallics

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Scheme 6. Synthesis of Compound Li[13]a

8, whereas water addition to Mg[7]2 leads to the cleavage of a B−C6F5 bond with formation of 6. In all three molecules 4, 6, and 8, the presence of the PO donor shuts down any substituent scrambling. In the cases of the hydroborates Li[3] and Mg[5]2, the respective anion acts as a BH2,O-chelating ligand toward its metal ion. Examples of such chelating ligands, in which only one donor is a classical Lewis base and the other a hydroborate ion, are virtually unknown.61 With the synthesis of Li[Ph*2BH2] (Li[13]) from Ph*BH2 (4) and Ph*Li we even extended the palette of these new ligands in the direction of pincer-type species. Work is currently in progress in our laboratories to thoroughly investigate the coordination chemistry of these promising new chelators.

Conditions: (i) (1) t-BuLi, Et2O, −78 °C, 2.5 h, (2) 4, Et2O, −78 °C → room temperature, 4 days.

a

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EXPERIMENTAL SECTION

General Considerations. All reactions and manipulations were carried out in an argon-filled glovebox or with application of standard Schlenk techniques under a nitrogen atmosphere. Pentane, hexane, toluene, C6H6, C6D6, Et2O, THF, and THF-d8 were carefully dried over Na/benzophenone; Me3SiCl was stirred over CaH2 and freshly distilled prior to use. NMR: Bruker Avance II 300, Avance 400, Avance III HD 500. Chemical shifts are referenced to (residual) solvent signals (1H and 13C{1H}: C6D6, δ 7.16/128.06; CDCl3, δ 7.26/77.16; THFd8, δ 3.58/67.21), external BF3·Et2O (11B and 11B{1H}), CFCl3 (19F), or 85% H3PO4 (31P{1H}). Abbreviations: s = singlet, d = doublet, t = triplet, vtr = virtual triplet, q = quartet, m = multiplet, br = broad, n.r. = not resolved, n.o. = not observed. t-Bu2P(O)Ph (1),43,62 1,2C6H4(PPh2)(Br) (11),63 C6F5MgBr,37 and (C6F5)2BH·SMe237 were synthesized according to literature procedures. Synthesis of 4. A solution of t-Bu2P(O)Ph (1; 1.16 g, 4.87 mmol) in Et2O (40 mL) was cooled to −78 °C. t-BuLi in pentane (1.7 M, 3.5 mL, 6.0 mmol) was added dropwise via syringe, whereupon the solution turned yellow. The mixture was stirred for 2 h at −78 °C, and B(OMe)3 (0.66 mL, 5.8 mmol) in Et2O (15 mL) was slowly added, whereupon the solution turned colorless; the mixture was warmed to room temperature and stirred overnight. The vessel was again cooled to −78 °C, and a calibrated solution of Li[AlH4] in Et2O (1 M, 7.5 mL, 7.5 mmol) was added dropwise. The reaction mixture was slowly warmed to room temperature overnight and filtered, the filter cake was washed with Et2O (15 mL), and the filtrate was evaporated to dryness under vacuum. C6H6 (8 mL) was added, and the resulting suspension was stored at 6 °C overnight. The supernatant was removed via cannula, and the solid residue of crude Li[3] was dried under vacuum (1.26 g). The material was suspended in toluene (50 mL), and neat Me3SiCl (0.65 mL, 5.1 mmol) was added, which resulted in an immediate change of the external appearance of the colorless solid. The mixture was stirred overnight at room temperature, followed by aqueous workup using a NaHCO3 solution (1 M, 40 mL). The aqueous phase was extracted with toluene (3 × 50 mL), the combined organic phases were dried over MgSO4 and filtered, and the filtrate was evaporated under vacuum to yield analytically pure 4 (841 mg, 69% with respect to 1). Colorless block-shaped crystals suitable for X-ray crystallography were obtained by slow evaporation of a solution of 4 in Et2O. 1 H NMR (300.0 MHz, C6D6): δ 0.92 (d, 3JH,P = 15 Hz, 18H; CCH3), 4.64 (br m, 2H; BH2), 6.97−7.03 (m, 1H; ArH), 7.09−7.13 (m, 1H; ArH), 7.27−7.32 (m, 1H; ArH), 7.92−7.95 (m, 1H; ArH). 11 B NMR (96.3 MHz, C6D6): δ 4.0 (t, 1JH,B = 103 Hz). 11B{1H} NMR (96.3 MHz, C6D6): δ 4.0 (h1/2 = 100 Hz). 31P{1H} NMR (121.5 MHz, C6D6): δ 97.3 (h1/2 = 11 Hz). 13C{1H} NMR (75.4 MHz, C6D6): δ 25.8 (d, 2JC,P = 1 Hz; CCH3), 36.4 (d, 1JC,P = 54 Hz; CCH3), 124.7 (d, 1 JC,P = 89 Hz; PC), 124.8 (d, JC,P = 11 Hz; ArC), 127.5 (d, JC,P = 16 Hz; ArC), 130.4 (d, JC,P = 14 Hz; ArC), 131.6 (d, JC,P = 3 Hz; ArC), 171.9 (BC)*. Anal. Calcd for C14H24BOP [250.11]: C, 67.23; H, 9.67. Found: C, 67.43; H, 9.54. The asterisk indicates that this signal was only observed in the 2D HMBC NMR experiment.

Figure 6. Molecular structure of (Li[13])2. Hydrogen atoms (except on boron) and t-Bu groups on phosphorus atoms are omitted for clarity. Selected bond lengths (Å) and atom···atom distances (Å): B(1)···Li(1) = 2.378(11), B(1)···Li(2) = 3.435(11), B(2)···Li(2) = 2.429(11), B(2)···Li(1) = 3.223(11), P(1)−O(1) = 1.498(3), P(2)− O(2) = 1.485(4), P(3)−O(3) = 1.490(4), P(4)−O(4) = 1.493(4), O(1)−Li(1) = 1.839(8), O(3)−Li(1) = 1.841(9), O(2)−Li(2) = 1.826(9), O(4)−Li(2) = 1.809(10).

those of the trihydroborate Li[3] (−24.3 and 69.1 ppm, respectively). Moreover, a triplet multiplicity in the protoncoupled boron spectrum together with a well-resolved 1:1:1:1 quartet in the 1H NMR spectrum (integrating 2H; 1JB,H = 79 Hz) testify to the presence of an [R2BH2]− unit. In the solid state, the C1-symmetric dimer (Li[13])2 consists of contact ion pairs in which all lithium ions are exclusively coordinated by [13]− ligands. As already discussed for (Li[3])2, each lithium cation is chelated by one [13]− anion through the central BH2 moiety (η2 coordination29,47,49) and one of the two PO donors. The second PO side arm acts as a bridge holding the dimer together; the complex is further supported by two η1-BH2···Li interactions.

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CONCLUSION (PhBH2)2, like other aryl(dihydro)boranes, is prone to substituent redistribution.60 We have now prepared a PhBH2 derivative equipped with an o-P(O)-t-Bu2 substituent (i.e., Ph*BH2; 4), which exists as an intramolecular PO−B adduct both in solution and in the solid state. The Lewis basic side arm renders 4 air and water stable, but the PO−B adduct is weak enough to be cleaved by strong carbon nucleophiles. We were therefore able to apply a repetitive hydride abstraction/ C6F5MgBr addition sequence to prepare the series of compounds Mg[Ph*BH2(C6F5)]2 (Mg[5]2), Ph*B(H)(C6F5) (6), and Mg[Ph*BH(C6F5)2]2 (Mg[7]2). An alternative access route to [7]− is provided by the reaction between Ph*Li and (C6F5)2BH·SMe2 to give Li[7]. An interesting counterion effect became apparent when we tried to prepare the final adduct Ph*B(C6F5)2 (8) of the above series via hydride abstraction from [7]−: only the targeted acidic hydrolysis of Li[7] furnishes G


Organometallics

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Synthesis of Li[3]. A solution of 4 (0.10 g, 0.40 mmol) in THF (14 mL) was cooled to −78 °C, and Li[HBEt3] in Et2O (1 M, 0.4 mL, 0.4 mmol) was added. The stirred reaction mixture was warmed to room temperature overnight. After all volatiles had been removed under vacuum, the resulting colorless residue was dissolved in a mixture of C6H6 (5 mL) and Et2O (2 mL). The solution was concentrated until it turned slightly turbid and then stored at 6 °C overnight, whereupon colorless crystals formed. The mother liquor was removed via cannula, and the crystals were dried under vacuum. Yield of Li[3]: 64 mg (62%). Colorless rod-shaped crystals of (Li[3])2· C6H6 suitable for X-ray crystallography were obtained by storing a concentrated solution of Li[3] in C6H6 at room temperature. Note: crystals grown from C6H6 and dried under vacuum to constant mass lose their incorporated C6H6 and the integrity of their crystal lattice. 1 H NMR (400.1 MHz, THF-d8): δ 1.24 (q, 1JH,B = 81 Hz, 3H; BH3), 1.27 (d, 3JH,P = 13 Hz, 18H; CCH3), 6.84−6.88 (m, 1H; ArH), 7.00−7.04 (m, 1H; ArH), 7.30−7.35 (m, 1H; ArH), 7.69 (br, 1H; ArH). 11B NMR (128.4 MHz, THF-d8): δ −24.3 (q, 1JH,B = 81 Hz). 11 1 B{ H} NMR (128.4 MHz, THF-d8): δ −24.3 (h1/2 = 11 Hz). 31 1 P{ H} NMR (162.0 MHz, THF-d8): δ 60.2 (h1/2 = 23 Hz). 13C{1H} NMR (100.6 MHz, THF-d8): δ 28.3 (d, 2JC,P = 0.5 Hz; CCH3), 37.7 (d, 1JC,P = 60 Hz; CCH3), 121.3 (d, JC,P = 13 Hz; ArC), 129.2 (s; ArC), 130.6 (d, JC,P = 18 Hz; ArC), 131.8 (d, 1JC,P = 88 Hz; PC), 142.2 (d, JC,P = 14 Hz; ArC), 169.6 (BC)*. The asterisk indicates that this signal was only observed in the 2D HMBC NMR experiment. Synthesis of 6. 4 (0.82 g, 3.3 mmol) in Et2O (50 mL) was added at room temperature to a freshly prepared solution of C6F5MgBr (3.7 mmol) in Et2O (40 mL), and the mixture was stirred overnight. After filtration, the filtrate was quenched with H2O (0.05 mL) and a colorless suspension formed. More H2O (40 mL) was added, the mixture was stirred for 2 h, and the two phases were separated. The aqueous layer was extracted with Et2O (3 × 30 mL). The combined organic phases were dried over MgSO4 and filtered, and the filtrate was evaporated to dryness under vacuum. Yield: 773 mg (57%). Colorless block-shaped crystals of 6 suitable for X-ray crystallography were obtained by storing a concentrated solution in hexane at 6 °C. 1 H NMR (300.0 MHz, C6D6): δ 0.85 (d, 3JH,P = 15 Hz, 9H; CCH3), 0.86 (d, 3JH,P = 15 Hz, 9H; CCH3), 4.94 (br m, 1H; BH), 6.98−7.07 (m, 2H; ArH), 7.28−7.34 (m, 1H; ArH), 7.81−7.83 (m, 1H; ArH). 11 B NMR (96.3 MHz, C6D6): δ 2.5 (br d, 1JH,B = ca. 60 Hz). 11B{1H} NMR (96.3 MHz, C6D6): δ 2.5 (h1/2 = 160 Hz). 31P{1H} NMR (121.5 MHz, C6D6): δ 98.3 (h1/2 = 11 Hz). 19F{1H} NMR (282.3 MHz, C6D6): δ −165.0 (m, 2F; F-m), −159.8 (m, 1F; F-p), −131.1 (m, 2F; F-o) [Δδ(19Fm,p) = 5.2]. 13C{1H} NMR (75.4 MHz, C6D6): δ 25.5 (d, 2 JC,P = 1 Hz; CCH3), 25.6 (d, 2JC,P = 1 Hz; CCH3), 35.3 (d, 1JC,P = 52 Hz; CCH3), 36.8 (d, 1JC,P = 53 Hz; CCH3), 124.9 (d, 1JC,P = 86 Hz; PC), 125.7 (d, JC,P = 11 Hz; ArC), 127.3 (d, JC,P = 15 Hz; ArC), 131.0 (dt, JC,P = 14 Hz, JC,F = 2 Hz; ArC), 132.0 (d, JC,P = 3 Hz; ArC), 167.0 (BC)*, n.o. (FC). Anal. Calcd for C20H23BF5OP [416.16]: C, 57.72; H, 5.57. Found: C, 57.69; H, 5.69. The asterisk indicates that this signal was only observed in the 2D HMBC NMR experiment. Conversion of 6 with C6F5MgBr. 6 (0.28 g, 0.67 mmol) in Et2O (10 mL) was added to a freshly prepared solution of C6F5MgBr (0.75 mmol) in Et2O (10 mL) at room temperature. After 2 h, an NMR sample was taken (C6D6; colorless block-shaped crystals of Mg[7]2· 2C6D6 grew upon storage of the NMR tube at ambient temperature; in a crystallographic context we will treat the material as Mg[7]2·2C6H6) and the remaining reaction mixture was quenched with H2O (20 mL). The two phases were separated, and the aqueous layer was extracted with Et2O (3 × 15 mL). The combined organic phases were dried over MgSO4, filtered, and evaporated to constant mass under vacuum. According to NMR spectroscopy, the solid residue consisted of the starting material 6. Yield: 245 mg (88%). Synthesis of 8. t-BuLi in pentane (1.72 M, 0.6 mL, 1 mmol) was slowly added at room temperature to a solution of t-Bu2P(O)Ph (1; 0.20 g, 0.84 mmol) in a mixture of C6H6 (15 mL) and Et2O (3 mL). The resulting orange solution was stirred for 2 h, (C6F5)2BH·SMe2 (0.34 g, 0.83 mmol) in C6H6 (10 mL) was added, and the reaction mixture was stirred overnight. All volatiles were removed under

vacuum, the orange waxy residue was dissolved in Et2O (15 mL)/H2O (5 mL), HCl in H2O (0.1 N, 6 mL) was added, and the mixture was stirred overnight. The two phases were separated, the aqueous layer was extracted with Et2O (2 × 20 mL), and the combined organic phases were dried over MgSO4. After filtration, all volatiles were removed from the filtrate under vacuum and the solid residue was recrystallized from C6H6 (5 mL) to give colorless, block-shaped, X-rayquality crystals of 8·0.5C6H6. Yield: 128 mg (25%). 1 H NMR (500.2 MHz, CDCl3): δ 1.28 (d, 3JH,P = 15 Hz, 18H; CCH3), 7.38−7.41 (m, 1H; ArH), 7.51−7.54 (vtr, 1H; ArH), 7.59− 7.62 (vtr, 1H; ArH), 8.27−8.29 (d, 3JH,H = 8 Hz, 1H; ArH). 11B{1H} NMR (160.5 MHz, CDCl3): δ 4.8 (h1/2 = 170 Hz). 31P{1H} NMR (202.5 MHz, CDCl3): δ 100.9 (h1/2 = 16 Hz). 19F{1H} NMR (470.6 MHz, CDCl3): δ −164.5 (m, 4F; F-m), −159.3 (m, 2F; F-p), −132.0 (m, 4F; F-o) [Δδ(19Fm,p) = 5.2]. 13C{1H} NMR (125.8 MHz, CDCl3): δ 26.3 (d, 2JC,P = 1 Hz; CCH3), 36.1 (d, 1JC,P = 52 Hz; CCH3), 125.6 (d, 1JC,P = 83 Hz; PC), 127.1 (d, JC,P = 15 Hz; ArC), 127.2 (d, JC,P = 11 Hz; ArC), 132.5 (d, JC,P = 2 Hz; ArC), 133.9 (br; ArC), 162.0 (BC)*, n.o. (FC). Anal. Calcd for C26H22BF10OP·0.5C6H6 [621.27] (the amount of C6H6 present in the sample was confirmed by X-ray crystallography and 1H NMR spectroscopy): C, 56.06; H, 4.06. Found: C, 56.38; H, 4.47. The asterisk indicates that this signal was only observed in the 2D HMBC NMR experiment. Synthesis of 12. 1,2-C6H4(PPh2)(Br) (11; 0.18 g, 0.53 mmol) was dissolved in a mixture of C6H6 (10 mL) and Et2O (2 mL). n-BuLi in hexane (1.59 M, 0.35 mL, 0.56 mmol) was added dropwise via syringe at room temperature, whereupon the solution turned orange. (C6F5)2BH·SMe2 (0.21 g, 0.51 mmol) in C6H6 (10 mL) was added over a period of 20 min, and the reaction mixture was stirred overnight and quenched with H2O (15 mL). The organic phase was separated, the aqueous layer was extracted with CH2Cl2 (3 × 15 mL), and the combined organic phases were dried over MgSO4. After filtration, all volatiles were removed from the filtrate under vacuum to give a yellow oil. The crude product was purified first by flash column chromatography (silica gel; Et2O) and then by recrystallization from hexane. Yield: 139 mg (45%). Colorless block-shaped crystals suitable for X-ray crystallography were obtained by slow evaporation of a solution of 12 in Et2O. 1 H NMR (500.2 MHz, CDCl3): δ 3.80 (br, 1H; BH), 6.88−6.92 (m, 2H; PhH-o), 7.22−7.26 (m, 1H; ArH), 7.32−7.36 (m, 2H; PhHm), 7.37−7.41 (m, 1H; ArH), 7.46−7.49 (m, 2H; PhH-m), 7.51−7.55 (m, 1H; PhH-p), 7.55−7.58 (m, 1H; PhH-p), 7.67−7.72 (m, 3H; ArH and PhH-o), 8.01−8.04 (m, 1H; ArH). 11B NMR (160.5 MHz, CDCl3): δ −26.9 (n.r., h1/2 = 250 Hz). 11B{1H} NMR (160.5 MHz, CDCl3): δ −26.9 (h1/2 = 170 Hz). 31P{1H} NMR (202.5 MHz, CDCl3): δ −0.8 (m). 19F{1H} NMR (470.6 MHz, CDCl3): δ −164.0 (m, 2F; F-m), −158.6 (m, 1F; ArF), −157.9 (m, 1F; F-p), −155.7 (m, 1F; ArF), −141.9 (m, 1F; ArF), −129.7 (m, 2F; F-o), −128.4 (m, 1F; ArF) [Δδ(19Fm,p) = 6.1]. 13C{1H} NMR (125.8 MHz, CDCl3): δ 121.1 (d, JC,P = 59 Hz; ArC-ipso), 122.6 (d, JC,P = 64 Hz; PhC-ipso), 123.2 (d, JC,P = 61 Hz; PhC-ipso), 128.3 (d, JC,P = 8 Hz; ArC), 129.0 (d, JC,P = 11 Hz; PhC-m), 129.5 (d, JC,P = 11 Hz; PhC-m), 131.7 (dd, J = 14 Hz, 8 Hz; ArC), 132.4 (d, JC,P = 3 Hz; PhC-p), 132.8 (d, JC,P = 3 Hz; PhC-p), 132.9 (d, JC,P = 2 Hz; ArC), 133.0 (d, JC,P = 3 Hz; ArC), 133.3 (d, JC,P = 9 Hz; PhC-o), 133.9 (d, JC,P = 9 Hz; PhC-o), 139.0 (m; ArC-ipso), n.o. (BC), n.o. (FC). Anal. Calcd for C30H15BF9P [588.20]: C, 61.26; H, 2.57. Found: C, 61.45; H, 2.91. Synthesis of Li[13]. t-Bu2P(O)Ph (1; 0.2 g, 0.8 mmol) in Et2O (20 mL) was cooled to −78 °C. t-BuLi in pentane (1.7 M, 0.64 mL, 1.1 mmol) was added dropwise to give a yellow solution, which was stirred for 2.5 h at −78 °C. 4 (0.21 g, 0.84 mmol) in Et2O (20 mL) was slowly added, and the reaction mixture was warmed to room temperature and stirred for 4 days (the progress of the reaction was continuously monitored by 1H NMR spectroscopy). All volatiles were removed under vacuum to give an orange solid residue. Hexane (50 mL) was added, the resulting suspension was filtered, and the filtrate was concentrated to a volume of 5 mL and stored at −40 °C for 7 days. A colorless precipitate formed, the supernatant was removed via cannula, and the precipitate was dried under vacuum. Yield: 0.20 g (48%). Colorless block-shaped crystals suitable for X-ray crystallogH


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raphy were obtained by storing a concentrated solution of Li[13] in Et2O/pentane (1/1) at −40 °C. 1 H NMR (300.0 MHz, C6D6): δ 1.27 (d, 3JH,P = 13 Hz, 36H; CCH3), 3.18 (q, 1JH,B = 79 Hz, 2H; BH2), 6.91−6.97 (m, 2H; ArH), 7.13−7.19 (m, 2H; ArH), 7.36−7.43 (m, 2H; ArH), 7.76 (br, 2H; ArH)*. 11B NMR (96.3 MHz, C6D6): δ −17.8 (t, 1JH,B = 79 Hz). 11 1 B{ H} NMR (96.3 MHz, C6D6): δ −17.8 (h1/2 = 17 Hz). 31P{1H} NMR (121.5 MHz, C6D6): δ 59.8 (h1/2 = 25 Hz). 13C{1H} NMR (75.4 MHz, C6D6): δ 28.2 (s; CCH3), 37.3 (d, 1JC,P = 59 Hz; CCH3), 121.0 (d, JC,P = 14 Hz; ArC), 128.8 (s; ArC), 130.3 (d, JC,P = 17 Hz; ArC), 132.2 (d, 1JC,P = 88 Hz; PC), 140.6 (d, JC,P = 15 Hz; ArC), 169.3 (q, 1JB,H = 52 Hz; BC). The asterisk indicates that the proton shifts of this compound (especially the shift of the broad signal) are unusually concentration dependent, which may be due to cation−anion association effects.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving NMR data/syntheses of Li[2a], 2b, Mg[5]2, 9, 10, and (2-Br-C6H4)Ph2P·B(H)(C6F5)2, NMR spectra of Li[3], Mg[5]2, and Li[13], details of the X-ray crystal structure analyses of (Li[2a])2, (Li[3])2·C6H6, (Li[3])2, 4, Mg(Et2O)[5]2·Et2O, Mg(Et2O)[5]2·1.5Et2O, 6, 6*, Mg[7]2·2C6H6, 8·0.5C6H6, 9, 12, (2-Br-C6H4)Ph2P·B(H)(C6F5)2·0.5(toluene), (Li[13])2, 1,2-C6H4(P(O)-t-Bu2)(I), and 1,2-C6H4(P-t-Bu2)(Br), and crystallographic data of (Li[2a])2, (Li[3])2·C6H6, (Li[3])2, 4, Mg(Et2O)[5]2·Et2O, Mg(Et2O)[5]2·1.5Et2O, 6, 6*, Mg[7]2·2C6H6, 8·0.5C6H6, 9, 12, (2-Br-C6H4)Ph2P·B(H)(C6F5)2·0.5(toluene), (Li[13])2, 1,2-C6H4(P(O)-t-Bu2)(I), and 1,2-C6H4(P-t-Bu2)(Br). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.W.: fax, +49 69 798 29260; e-mail, Matthias.Wagner@ chemie.uni-frankfurt.de. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.W. and J.M.B. gratefully acknowledge financial support by the Beilstein Institute, Frankfurt/Main, Germany, within the research collaboration NanoBiC.

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REFERENCES

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