P-Based Frustrated Lewis Pair

Article pubs.acs.org/Organometallics

Chalcogen Capture by an Al/P-Based Frustrated Lewis Pair: Formation of Al-E‑P Bridges and Intermolecular Tellurium−Tellurium Interactions Werner Uhl,*,† Philipp Wegener,† Marcus Layh,† Alexander Hepp,† and Ernst-Ulrich Würthwein‡ †

Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstraße 30, D-48149 Münster, Germany Organisch-chemisches Institut der Universität Münster, Corrensstraße 40, D-48149 Münster, Germany

‡

S Supporting Information *

ABSTRACT: Reaction of the Al/P-based frustrated Lewis pair (FLP) Mes2P-C(CH-C6H5)-Al(CMe3)2 1 with heavier chalcogens (E = S, Se and Te) yielded by oxidation of the P atoms the respective phosphorus(V) compounds Mes2P(E)-C(CHC6H5)-Al(CMe3)2 (2a, E =S, 2b, E = Se, 2c, E = Te) in good yield. The chalcogen atoms were coordinated to the Lewis-acidic Al atoms, which, in the case of the dark red Te compound 2c, resulted in a stabilization of the P−Te bond. Unique fourmembered, slightly puckered P-C-Al-E heterocycles were formed with P−E bond lengths in the normal range of terminal chalcogen atoms and comparatively long Al−E bonds, which are consistent with relatively weak Al−E interactions. Both the Se and more pronounced the Te compound formed dimers in the solid state as a result of closed-shell chalcogen−chalcogen interactions. While the Se···Se distance was only slightly shorter than the sum of the van der Waals radii (3.8 Å), the Te···Te separation (3.33 Å) was relatively short and in the characteristic range of significant intermolecular Te···Te interactions, which may result from a double donor−acceptor interaction. Quantum chemical calculations suggested a Te−Te bond energy of about −16.5 kcal/mol. The corresponding oxygen derivative could not be isolated in a pure form, but it may be formed by thermal decomposition of the new FLP adduct 1·ONMe3.

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INTRODUCTION Frustrated Lewis pairs (FLPs) are in the focus of current research interests.1 They have coordinatively unsaturated Lewis-acidic and -basic atoms in single molecules or bimolecular systems, and as a result of these opposite functionalities, they show a unique reactivity with respect to the coordination or activation of organic and inorganic substrates. In most cases, B/P-based FLPs have been applied, but many other element combinations showed impressively the importance and broad applicability of the FLP concept in stoichiometric or catalytic transformations.1 In some recent reports, several research groups independently verified the suitability of Al/P-based FLPs for a wider range of applications. The inherently high Lewis-acidity of Al atoms does not require an activation by electron-withdrawing groups, and many secondary reactions have been reported with various substrates, such as CO2,2−4 terminal alkynes,2,5 alkenes,6 hydrogen,7 carbonyl compounds,8 or phenyl isocyanate.4 As ambiphilic ligands, they were able to solubilize alkali metal hydrides,9 to act as a phase transfer catalyst in a hydride transfer reaction,9 or to form complexes with ambiphilic gold complexes10 or boron halides.11 They proved to be effective catalysts for the dehydrogenation of ammine boranes.12 A facile method for the generation of unimolecular Al/PFLPs is the hydroalumination of alkynylphosphines (1, Scheme 1),2 but steric shielding is necessary to prevent secondary © XXXX American Chemical Society

Scheme 1. Synthesis of the Al/P-FLP 1

reactions such as dimerization4a or adduct formation with the starting alkynes or dialkylaluminum hydrides.13 These FLPs should be able to capture effectively species with an electron sextet that are characterized by lone pairs of electrons and vacant orbitals at isolated main group element atoms or in main group element molecules. The AlR2 groups are highly reactive as a result of their strongly polar Al−C bonds and may initiate secondary reactions. Only recently, we reported on the reactions of 1 with boron halides,11 which, under mild conditions, formed adducts, followed by rearrangement with β-hydride elimination, migration of the hydrogen atom to boron, and shift of a halogen atom to aluminum. The mechanism suggested by quantum chemical calculations Special Issue: Mike Lappert Memorial Issue Received: November 28, 2014

A

DOI: 10.1021/om501206p Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Table 1. Selected NMR Parameters of 1,2 2a−c, and 3

comprises the occurrence of a borenium cation as the key intermediate. In continuation of these investigations, we treated FLP 1 with chalcogens.

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δ (31P) 3 JPH(vinyl) 1 JPC(Mes) 3 JPC(phenyl)

RESULTS AND DISCUSSION The conversion of 1 to the oxidation products 2 was, in the case of the elements S to Te, easily achieved by treatment of a solution of the starting material 1 in toluene with the respective chalcogen (E = S8, Se∞, Te∞) or in the case of 2a with propylene sulfide as an alternative sulfur source (Scheme 2).

1

2a

2b

2c

3

−14.2 17.7 22.3 13.3

43.8 47.4 63.0 37.0

20.3 49.0 55.2 36.6

−51.0 50.8 46.0 35.7

1.9 27.3 57.0 14.2

atoms.2,8,9,11,12 The size of the 3JPH(vinyl) coupling constant is indicative of the cis-arrangement of the H and P atoms at the CC double bond. A trans-arrangement in related FLP adducts with four-coordinate P atoms resulted in coupling constants larger than 60 Hz.4a A ROESY experiment with the Se compound 2b confirmed the cis-configuration of H and P that was also found in the solid state for all three compounds (cf. Figures 1 and 2). Only one set of signals was observed in

Scheme 2. Reactions of 1 with S, Se, Te, and Trimethylamine Oxide

While the sulfur compound was quantitatively formed after only 5 min at room temperature, the polymeric polymorphs of the intrinsically less reactive heavier elements Se and especially Te required considerably higher reaction temperatures (100 °C) and times (8 and 16 h). The solid oxidation products 2a to 2c are moderately air-sensitive and thermally stable and can be stored indefinitely under an atmosphere of dry argon. Solutions of the Te compound 2c slowly precipitate elemental tellurium at room temperature. Decomposition is accelerated when the solutions are exposed to light. The S and Se compounds are pale yellow, while, surprisingly, the Te compound is dark red in the solid state and deeply orange in solution. The UV/vis spectra are relatively unspecific with absorptions (shoulders) of low intensity at about 400 nm. A red color was often observed for ditellurium compounds with intermolecular Te−Te interactions (intermolecular n → σ* transitions). The NMR spectra of compounds 2 were similar and differed characteristically from those of 1.2 The 31P NMR resonances of 2a and 2b were shifted by >35 ppm to a lower field, as usually observed for FLP adducts, while the P atom of the Te compound 2c resonated at an unusually high field (δ = −51). With the increasing atomic mass of the chalkogen atoms, these resonances show a continuous shift to higher field with a relatively large gap to the Te compound 2c. The strong shielding of the phosphorus nuclei in 2c may result from the bonding to the electron-rich Te atom (heavy atom effect14). The increased coordination (3 to 4) and oxidation numbers (+III to +V) caused a significant increase of the associated coupling constants 3JPH(vinyl), 1JPC(Mes), and 3JPC(phenyl) by 20−40 Hz as compared to 1 (Table 1). This behavior is welldocumented for other products of 1 with four-coordinate P

Figure 1. Molecular structure and atomic numbering scheme of compound 2a (the selenium derivative 2b is similar). Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12, arbitrary radius) have been omitted for clarity.

the 1H and 13C NMR spectra for the CMe3 and Mes substituents on Al and P, respectively, indicating the presence of a virtual mirror plane, which is consistent with dynamic behavior (e.g., ring fluctuation) in solution for the fourmembered, nonplanar P-C-Al-E heterocycles. The 1H NMR resonances of the tert-butyl groups at Al were slightly shifted to a lower field compared to 1 (δ = 0.96 vs about 1.35), which may indicate an increased coordination number at the metal atom. The 31P NMR signals of 2b and 2c show characteristic satellites due to the presence of 77Se, 123Te, and 125Te with 1J coupling constants of 430, 777, and 938 Hz. The size of the 1 31 125 J( P Te) coupling constants depends on the coordination number at tellurium and may be compared to RTePR2 or Te(PR2)2 (two-coordinate Te; 1JPTe = 250−680 Hz)15−19 and R3PTe derivatives (terminal Te; >1700 Hz).15,20−22 The value of 2c is intermediate and may be influenced by the Te−Al interaction and the increased coordination number at Te. (H5C6)2P(Te)-CH2CH2-P(C6H5)2 with a four- and threecoordinate P atom in the solid state does not fit in this series B


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Organometallics

Figure 2. Molecular structure and atomic numbering scheme of a dimeric molecule of 2c. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12, arbitrary radius) have been omitted for clarity.

and showed a value of 1020 Hz.23 The electron impact mass spectra of all three compounds 2a−2c showed the molecular ion peak minus a CMe3 group. The reaction of 1 with molecular oxygen or Me3Si-O-OSiMe3 yielded mixtures of products that could not be separated. Treatment of 1 with Me3NO as an oxygen source afforded the corresponding Lewis acid−base adduct 3 in good yield. In contrast, the C5H5NO (pyridine-N-oxide) adduct was much less stable and decomposed already at room temperature to a mixture of unidentified products. It could not be isolated and characterized. The Me3NO adduct 3 displayed a typical lowfield shift in the 31P NMR spectrum and a slight increase in the 3 JPH coupling constant compared to the free FLP 1 (Table 1). Of particular interest was the EI mass spectrum, which did not show the molecular ion peak corresponding to the adduct, but instead an m/z value consistent with the loss of Me3N and the formation of Mes2P(O)-C(CH-C6H5)-Al(CMe3)2 2d. Attempts to obtain this compound in pure form by heating a larger sample of 3 to 100 °C in vacuo were only partially successful and yielded a mixture of products with compound 2d as a major species as evident from a comparison of its spectroscopic parameters with those of 2a−c (31P: δ = 57.8; 3 JPH = 40.3 Hz; c.f. Table 1). 2d is stable in benzene solution over months, but all attempts to separate the reaction mixture into its components by fractional crystallization failed. The molecular structures of compounds 2a (Figure 1), 2b, and 2c (Figure 2) feature four-membered, slightly puckered Al1-C11-P1-E1 heterocycles, which are formed by the addition of chalcogen atoms E to P and an interaction of a lone pair of electrons at E with the Lewis-acidic Al atom. The folding angles along the axis Al1−P1 increase from the S to the Te compound (14 to 22°, Table 2). The AlC2(CMe3) and PC2(Mes) groups are essentially perpendicular to the heterocycle. The Al and H atoms of the vinyl substituents are trans to each other, confirming the interpretation of the NMR data in solution. The endocyclic angles of the Al−C−P−S heterocycle of 2a are with

Table 2. Selected Bond Lengths (Å) and Angles (deg) of Compounds 2 (E = S, Se, Te) and 3 P1−E1 P1−C11 Al1-E1 Al1−C11 Al1−tBu(av.) E1···E1’ P1−C11−Al1 C11−P1−E1 P1−E1−Al1 C11−Al1−E1 folding anglea

2a

2b

2c

2.0320(4) 1.789(1) 2.3857(4) 2.047(1) 2.009 4.87 97.79(5) 98.08(4) 81.49(1) 80.98(3) 13.8

2.1894(3) 1.785(1) 2.5401(4) 2.045(1) 2.019 3.79 101.18(6) 98.03(4) 77.20(1) 81.36(4) 16.0

2.4651(4) 1.800(1) 2.7326(4) 2.069(1) 2.023 3.33 106.19(6) 95.22(4) 72.97(1) 81.75(4) 21.5

3 1.837(1) 2.055(2) 2.036 111.33(7)

97.16(5)

a Angle between the normals of the planes P1−C11−Al1 and P1−E1− Al1.

98° for the C11 and P1 centered angles much larger than those centered at Al1 and S1 (81°). This is consistent with the longer endocyclic bond lengths observed for Al1 and S1 as compared to C11 and P1. Replacing S with Se and Te in compounds 2 has little influence on the angles at Al and P, but the increasing size of the chalcogen atoms leads to an increase of the P1− C11−Al1 angle from 98 to 106° and a decrease of the C11− E1−Al1 angle from 81 to 73°. The Al−C, P−C, and C−C distances are unexceptional. P−E bond lengths of chalcogentriaryl- or -trialkylphosphorus(V) compounds R3PE21,22,24,25 are about 0.08 Å (E = S, Se) or 0.10 Å (E = Te) shorter than those in 2 or related compounds, such as [(H5C6)2P(S-AlMe3)CH2]2 (1.989(2) Å), (H5C6)3PSeAuBr (2.207 Å), or (iPr3PTe)4Ag2[N(SO2Me)2]2 (2.400 Å),26−32 in which the chalcogen atom acts as electron donor toward a metal atom. The latter compound and 2c are currently the only crystallographically characterized phosphine-tellurides of this type with a twoC


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Organometallics

the calculated intermolecular Te−Te distance was shorter (3.07 (TPSS) and 3.10 Å (B97D3)) compared to the experimental value (3.33 Å). The difference may depend on the relatively flat hypersurface for the dimer and may in the solid state be influenced by conformational changes in the molecule and molecular packing. Both theoretical methods afforded a comparable Te−Te bond energy (−15.9 and −17.4 kcal/ mol) that is between the values of very weak Te−Te interactions43 (see above) and a typical Te−Te single bond (about 30 kcal/mol).39 Wiberg bond indices58 show only negligible differences between both applied methods and indicate a significant bonding interaction between both Te atoms (WBI Te−Te 0.38; Al−Te 0.53; P−Te 0.79; Al−C(ring) 0.35). Calculated NBO charges58 are −0.19 (Te), +1.67 (Al), −0.93 (ring-C), and +1.16 (P). The bonding interaction between both Te atoms in the dimer may best be described by a double donor−acceptor interaction comparable to that of Lappert’s distannene.59 HOMO and donor orbital of the monomeric unit is a p-orbital at tellurium with a lone pair of electrons, which is perpendicular to the AlCPTe plane (Supporting Information). The acceptor orbital is a delocalized antibonding orbital that involves the σ*-orbital of the P−Te bond. In the structure of the Me3NO adduct 3 (Figure 3), the Me3NO ligand coordinates to the Al atom in such a way as to

coordinate Te atom. The Al−E distances in 2 are rather long (S, 2.3857(4) Å; Se, 2.5401(4) Å; Te, 2.7326(4) Å) and in the upper range of related compounds with four-coordinate Al and two-coordinate chalcogen atoms.32−37 They are consistent with a relatively weak Al−chalcogen interaction. Bond lengths in monomeric Al−chalcogenides R2Al-E-AlR2 [R = CH(SiMe3)2] with two-coordinate chalcogen and three-coordinate Al atoms are expectedly significantly shorter (S, 2.187(4) Å; Se, 2.319(1) Å; Te, 2.549(1) Å).38 There is an increasing degree of association between the chalcogen atoms of adjacent molecules. While the S−S contacts in compound 2a are larger than 4 Å and the closest contacts between two molecules are from a CMe3 group to a mesityl group, there is a weak interaction (3.79 Å) between two Se atoms, but a relatively narrow contact between Te atoms (3.33 Å) of adjoining molecules, resulting in the formation of dimers in the solid state (2c, Figure 2). The two molecules in the dimers are related to each other by an inversion center localized between the Se or Te atoms. The two C-Al-E-P heterocycles are parallel to each other with interplanar distances of 2.18 Å (2b) or 1.23 Å (2c). The P1−Te1···Te1’ group approaches linearity (P1−Te1···Te1’ 149.4°; P1−Se1···Se1’ 137.9°). The Se···Se distance corresponds to the sum of the van der Waals radii (3.80 Å,39 Se−Se single bond length: 2.34 Å40) and may be compared to intermolecular contact distances of 3.35−3.78 Å reported for the solid-state structures of Se8,40 P4Se3,41 and some diselenides RSeSeR42 (closed-shell interactions43). The Te−Te distance in 2c (3.33 Å) is much shorter and indicates a relatively strong intermolecular Te−Te bonding interaction with the formation of dimeric species. It is longer than typical Te−Te single bond lengths between two-coordinate Te atoms or in coordination compounds of ditellanes (∼2.7 Å, Te∞, MeTe-Te-Me, (CO)5M←(H5C6)Te-Te(C6H5)→M(CO)5, M = Cr, W),42,44−46 but significantly shorter than observed for typical closed-shell interactions.42,43 Distances similar to those of 2c were found for bonds between three-coordinate Te atoms as in Mes2Te-Te(I)Mes or compounds with strained geometries.47−51 The bonding situation in these compounds is completely different from that of 2c. The short Te−Te distances in these cases are forced by an 1,8-position of the Te atoms on a naphthalene backbone50 or favored by electronwithdrawing iodine atoms bonded to Te47−49 or a cationic tellurium species in the bridging position between two Te atoms.51 The tetrameric compound (ITeC6H5)4 has fourcoordinate Te atoms and a Te4 ring in the solid state with Te− Te distances of about 3.15 Å.52 The dimeric structure of compound 2c in the solid state (Figure 2) is expected to result in a characteristic splitting pattern in the 31P and 125Te NMR spectra. The absence of 2JPTe satellites even at low temperature is consistent with the dissociation of 2c into monomers in solution. The nature of the Te···Te bonding interaction in the solidstate structure of 2c was evaluated by DFT calculations for monomeric and dimeric model compounds in the gas phase (2,6-dimethylphenyl instead of mesityl groups attached to phosphorus). The DFT functionals TPSS53 (supplemented by dispersion correction D3 as developed by Grimme) and B97D354 were used as implemented in the Gaussian 09 program55 with the basis set def2-TVZP(ECP) (including relativistic contributions for the Te atoms)56 from EMSL.57 The structural parameters were reproduced in an appropriate accuracy (e.g., Al−Te 2.751 and 2.783 vs 2.733 Å; P−Te 2.532 and 2.541 vs 2.465 Å; Al−Te−P 72.0 and 71.6 vs 73.0°), but

Figure 3. Molecular structure and atomic numbering scheme of the adduct 3. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms and a cocrystallized CH2Cl2 molecule have been omitted for clarity.

minimize steric interactions and electrostatic repulsion of the lone pairs of electrons on P and O (P1−C11−Al1−O1 −44.2°, Al−O−N 140.2(1)°). One of the ipso-C atoms (C41) of the mesityl groups, the P atom, and the vinyl group (C11, C12, H12) are essentially coplanar (largest deviation from plane H12 0.06 Å) and approximately perpendicular (76.1°) to a mesityl group. This results in a close contact between H12 and the ipsoC atom C41 (2.42 Å). The N−O bond length is with 1.391(2) Å only marginally longer than in the free base as determined by gas-phase electron diffraction [1.379(3) Å]60 and comparable to the distances found in the solid-state structure of Me3NO D


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Organometallics [1.388(5)] 61 or in Lewis acid base adducts such as Me3NOAlMe3 [1.410(2) Å],62 [(Me3Si)2CH]2Al-O-Al[CH(SiMe3)2]2(ONMe3) [1.393(5)],63 and [(Me3Si)2CH]2Al-OAl[CH(SiMe3)2]2(ONMe3)2 [1.393(4)].63 The Al−O bond length and other pertinent structural parameters are unexceptional (Table 2).

and may reflect a new type of strengthening of intermolecular interactions between Te atoms. It is probably the polarization of the soft Te atoms by an intramolecular coordination to the hard Lewis-acidic aluminum atom that influences, in this case, the bond strength and causes the formation of a dimer with a relatively short Te−Te distance. Coordination of Lewis acids to unsaturated systems is well-known to decrease the energy of HOMOs and LUMOs.66 In view of the postulated double donor−acceptor interaction between the Te atoms in the dimer, this effect may help to stabilize the intermolecular Te··· Te interaction and the formation of dimeric formula units. This concept should allow the generation of further oligotellurium compounds with relatively short Te···Te contacts via the coordination of tellurium atoms by alkylaluminum or -gallium groups and a systematic investigation into the structural and electronic properties of such aggregates.

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CONCLUSION The Al/P-based FLP 1 reacted as an ambiphilic ligand with the electron-sextet chalcogen atoms (E = S, Se, Te) by the oxidation of the P atoms and formation of P−E bonds. Fourmembered AlCPE heterocycles were formed in highly selective reactions via an interaction of the Lewis-acidic aluminum with the Lewis-basic chalcogen atoms. These compounds differ markedly in their color. While the S and Se compounds (2a and 2b) are pale yellow, the Te compound 2c is dark red. Arylphosphorus tellurides R3PTe have been reported to be too unstable to be isolated in a preparative scale,21,22,64 and accordingly, the few structurally authenticated aryl derivatives of this type based on ethylene bridged diphosphines have been obtained only in very low yields.25 The ambiphilic character of 1 and the intramolecular Al−Te interaction clearly help to stabilize the P−Te bond, and the Te compound 2c was isolated in 65% yield. It is stable at room temperature in the solid state, but decomposes slowly in solution in the dark, faster in the light. The oxygen compound 2d could not be isolated in a pure form. It was obtained together with some impurities by treatment of 1 with ONMe3 as an oxygen donor, but could not be purified by recrystallization. These results verified that FLP 1 is an effective ambiphilic ligand for the coordination of ambivalent electron sextet particles and suitable for the stabilization of unstable structural motifs such as an arylphosphine telluride. This specific reactivity is highly stimulating for future investigations and the treatment of 1 with other reactive substrates such as phosphinidenes or carbenes. It is not only the exceptional stability and color of the Te compound that is of particular importance but also its unique molecular structure in the solid state. It forms dimers with a relatively short Te−Te contact of 3.33 Å. This is longer than the Te−Te single bond length in ditellurium compounds R-TeTe-R (∼2.7 Å),42 but shorter than that observed for intermolecular Te−Te distances in elemental tellurium or some ditellurium compounds (>3.7 Å).42−44 Phosphorus(V) compounds of the type R 3 PTe do not show short intermolecular Te···Te distances in the solid state.21,22,25 Replacement of an aryl group at the tellurium atoms of TeR2 molecules (without short Te···Te contacts) by a soft iodine atom resulted in relatively strong intermolecular Te···Te interactions. Accordingly, compounds such as Mes2Te-Te(Mes)I,47 I(Me4HC6)Te-Te(C6HMe4)I,49 or (ITeC6H5)452 show relatively short Te−Te distances of 3.0−3.3 Å and are often deeply colored. It seems that the P−I bond provides a suitable acceptor orbital for the formation of a relatively stable donor−acceptor interaction. Short Te−Te distances have also been reported for the cationic species [Mes2Te-Te(Mes)TeMes2]+.51 The bonding situation in these compounds has been compared to that of the triiodide anion, [I-I-I]− (I−I distances: I2, 2.72 Å; [I3]−, 2.93 Å)65 with a 3c−4e bond, while n−σ*, dispersive, and inductive interactions seem to determine the bonding in molecules with longer Te···Te distances.43 Electron-withdrawing groups were reported to increase the dispersive interactions.43 The situation in 2c is clearly different

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EXPERIMENTAL SECTION

General Considerations. All procedures were carried out under an atmosphere of purified argon in dried solvents (n-hexane, c-pentane with LiAlH4; toluene with Na/benzophenone). NMR spectra were recorded in C6D6 at ambient probe temperature using the following Bruker instruments: Avance I (1H, 400.13; 13C, 100.62; 31P, 161.98; 77 Se, 76.30; 125Te, 126.2 MHz) or Avance III (1H, 400.03; 13C, 100.59; 31 P, 100.60 MHz) and referenced internally to residual solvent resonances (chemical shift data in δ). 13C NMR spectra were all proton decoupled. IR spectra were recorded as paraffin mulls between CsI plates on a Shimadzu Prestige 21 spectrometer. The Al/P-based FLP Mes2P-C(CH-C6H5)-Al(CMe3)2 (1) was obtained according to a literature procedure.2 The assignment of NMR spectra is based on HMBC, H,H-ROESY, HSQC, and DEPT135 data. Synthesis of Mes2P(S)-C(CH-C6H5)-Al(CMe3)2 (2a). Method 1: An excess of propylene sulfide (0.052 g, 0.74 mmol) was added to a solution of 1 (0.34 g, 0.66 mmol) in toluene (10 mL). The mixture was stirred for 5 min at room temperature. The solvent was removed in vacuo to afford 2a as a pale yellow, analytically pure solid (0.355 g, 99%). Method 2: A mixture of 1 (0.27 g, 0.53 mmol) and excess sulfur (0.017 g, 0.076 mmol based on S8) was treated with toluene (10 mL). The resulting suspension was stirred until sulfur had dissolved completely. The solvent was removed in vacuo and the residue was recrystallized from toluene to yield compound 2a as a pale yellow solid (0.26 g, 90%). mp (argon, sealed capillary): 194 °C. Anal. Calcd for C34H46AlPS (544.8): C, 75.0; H, 8.5. Found: C, 74.6; H, 8.6. 1H NMR (C6D6, 300 K): δ 1.32 (s, 18H, CMe3), 1.95 (s, 6H, p-Me), 2.54 (s, 12H, o-Me), 6.58 [d, 4JPH = 4.0 Hz, 2H, m-H(Mes)], 7.04 [m, 1H, pH(C6H5)], 7.12 [m, 2H, m-H(C6H5)], 7.41 (d, 3JPH = 47.4 Hz, 1H, CCH), 7.44 [d, 3JHH = 7.4 Hz, 2H, o-H(C6H5)]. 13C NMR (C6D6, 300 K): δ 17.5 (br., AlCMe3), 20.8 (p-Me), 24.6 (d, 3JPC = 4.5 Hz, oMe), 31.7 (AlCMe3), 128.3 [overlap, o-C(C6H5)], 128.4 [overlap, 1JPC = 63.0 Hz, ipso-C(Mes)], 129.2 [m-C(C6H5)], 129.9 [p-C(C6H5)], 132.2 [d, 3JPC = 10.7 Hz, m-C(Mes)], 140.6 [d, 3JPC = 37.0 Hz, ipsoC(C6H5)], 141.4 [d, 4JPC = 2.8 Hz, p-C(Mes)], 141.6 [d, 2JPC = 9.9 Hz, o-C(Mes)], 150.3 (d, 2JPC = 7.7 Hz, CCH), 152.6 (d, 1JPC = 11.4 Hz, CCH). 31P{1H} NMR (C6D6, 300 K): δ 43.8. IR (CsI plates, paraffin, cm−1): 1601 s, 1582 s, 1553 s νCC, phenyl; 1460 vs, 1377 vs (paraffin); 1304 vw, 1246 vw δCH3; 1163 w, 1023 w, 996 vw, 930 w, 891 vw, 853 m, 810 m νCC; 723 s (paraffin); 692 m, 665 vw, 635 m phenyl; 559 m, 517 vw, 496 w, 459 m νAlC, νPC, νAlS, νPS, δCC. MS (EI, 20 EV, 343 K): m/z (%) = 487 (100) [M+ − CMe3], 431 (7) [M+ − CMe3 − butene], 403 (10) [M+ − Al(CMe3)2]. Synthesis of Mes2P(Se)-C(CH-C6H5)-Al(CMe3)2 (2b). Selenium (0.040 g, 0.51 mmol) was added to a solution of 1 (0.26 g, 0.51 mmol) in toluene (10 mL). The resulting suspension was stirred for 8 h at 100 °C. The clear solution was concentrated and cooled to yield yellow crystals of 2b (0.26 g, 86%). mp (argon, sealed capillary): 207 °C. Anal. Calcd for C34H46AlPSe (591.7): C, 69.0; H, 7.8. Found: C, 69.0; H, 7.8. 1H NMR (C6D6, 300 K): δ 1.34 (s, 18H, CMe3), 1.93 (s, E


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Organometallics

895 m, 851 vs, 808 vs, 770 s, 737 vs νCC, νNC, νNO; 719 vs (paraffin); 704 sh m, 617 m phenyl; 559 vs, 541 sh, 519 sh, 486 m, 461 s, 438 s νAlC, νPC, νAlO, δCC, δCN. MS (EI, 25 EV, 373 K): m/z (%) = 528 (13) [M+ − NMe3, FLP-O, 2d], 471 (30) [2d+ − CMe3], 415 (29) [2d+ − CMe3 − butene]. X-ray Crystallography. Crystals suitable for X-ray crystallography were obtained from toluene (2a, 2b), n-hexane (2c), or CH2Cl2 (3). Intensity data were collected on Bruker Quazar or D8-Venture diffractometers with monochromated MoKα radiation. The collection method involved ω scans. Data reduction was carried out using the program SAINT+.67 The crystal structures were solved by direct methods using SHELXTL.68 Non-hydrogen atoms were first refined isotropically, followed by anisotropic refinement by full-matrix leastsquares calculation based on F2 using SHELXTL.68 Hydrogen atoms were positioned geometrically and allowed to ride on their respective parent atoms. Further details of the crystal structure determinations are available from the Cambridge Crystallographic Data Centre on quoting the depository number CCDC-1036153 (2a), -1036152 (2b), -1036151 (2c), and -1036154 (3).

6H, p-Me), 2.53 (s, 12H, o-Me), 6.55 [d, 4JPH = 3.9 Hz, 2H, mH(Mes)], 7.03 [m, 1H, p-H(C6H5)], 7.12 [m, 2H, m-H(C6H5)], 7.36 (d, 3JPH = 49.0 Hz, 1H, CCH), 7.44 [m, 2H, o-H(C6H5)]. 13C NMR (C6D6, 300 K): δ 17.3 (br., AlCMe3), 20.7 (p-Me), 24.9 (d, 3JPC = 4.7 Hz, o-Me), 31.9 (AlCMe3), 127.0 [d, 1JPC = 55.2 Hz, ipso-C(Mes)], 128.3 [overlap, o-C(C6H5)], 129.2 [m-C(C6H5)], 129.9 [p-C(C6H5)], 132.1 [d, 3JPC = 10.5 Hz, m-C(Mes)], 140.4 [d, 3JPC = 36.6 Hz, ipsoC(C6H5)], 141.4 [d, 4JPC = 3.0 Hz, p-C(Mes)], 141.8 [d, 2JPC = 9.4 Hz, o-C(Mes)], 150.9 (d, 2JPC = 8.9 Hz, CCH), 153.7 (d, 1JPC = 17.9 Hz, CCH). 31P{1H} NMR (C6D6, 300 K): δ 20.3 (1JSeP = 430 Hz). 77Se NMR (C6D6, 300 K): δ 89.4 (1JSeP = 430 Hz). IR (CsI plates, paraffin, cm−1): 1591 s, 1551 s νCC, phenyl; 1458 vs, 1373 m (paraffin); 1285 vw, 1242 w δCH3; 1169 w, 1032 m, 964 vw, 928 m, 889 vw, 853 s, 804 s, 741 s νCC; 729 s (paraffin); 692 m, 627 m phenyl; 563 m, 494 w, 457 s νAlC, νPC, νAlSe, νPSe, δCC. UV/vis (CH2Cl2): λmax (ε; L mol−1cm−1) 235 (30800), 275 (sh, 25700), 400 (sh, 300). MS (EI, 20 EV, 353 K): m/z (%) = 535 (100) [M+ − CMe3], 479 (8) [M+ − CMe3 − butene]. Synthesis of Mes2P(Te)-C(CH-C6H5)-Al(CMe3)2 (2c). Tellurium (0.12 g, 0.94 mmol) was added to a solution of 1 (0.48 g, 0.94 mmol) in toluene (15 mL). The resulting suspension was heated to 100 °C and stirred overnight. The mixture was filtered, and the solvent of the filtrate was removed in vacuo. The obtained residue was dissolved in n-hexane and left at room temperature to yield dark red crystals of compound 2c (0.39 g, 65%), which were no longer soluble in n-hexane. mp (argon, sealed capillary): 179 °C. Anal. Calcd for C34H46AlPTe (640.3): C, 63.8; H, 7.2. Found: C, 63.8; H, 6.9. 1H NMR (C6D6, 300 K): δ 1.36 (s, 18H, CMe3), 1.95 (s, 6H, p-Me), 2.49 (s, 12H, o-Me), 6.53 [d, 4JPH = 3.8 Hz, 2H, m-H(Mes)], 7.02 [t, 3JHH = 7.3 Hz, 1H, p-H(Ph)], 7.13 [pseudo-t, 3JHH = 7.6 Hz, 2H, m-H(Ph)], 7.23 (d, 3JPH = 50.8 Hz, 1H, CCH), 7.42 [d, 3JHH = 7.4 Hz, 2H, oH(Ph)]. 13C NMR (C6D6, 300 K): δ 17.2 (br., AlCMe3), 20.7 (p-Me), 25.5 (d, 3JPC = 5.7 Hz, o-Me), 32.2 (AlCMe3), 125.1 [d, 1JPC = 46.0 Hz, ipso-C(Mes)], 128.3 [overlap, o-C(C6H5)], 129.3 [m-C(C6H5)], 129.8 [p-C(C6H5)], 132.0 [d, 3JPC = 10.2 Hz, m-C(Mes)], 140.1 [d, 3JPC = 35.7 Hz, ipso-C(C6H5)], 141.4 [d, 4JPC = 2.9 Hz, p-C(Mes)], 141.9 [d, 2 JPC = 8.8 Hz, o-C(Mes)], 153.1 (d, 2JPC = 9.8 Hz, CCH), 158.2 (d, 1 JPC = 21.7 Hz, CCH). 31P{1H} NMR (C6D6, 300 K): δ −51.0 (1JTeP = 777.0, 937.5 Hz). 125Te NMR (C6D6, 300 K): δ 310.2 (1JTeP = 937.5 Hz). IR (CsI plates, paraffin, cm−1): 1946 w, 1890 vw, 1805 vw, 1767 w, 1734 w, 1640 vw, 1599 vs, 1537 vs νCC, phenyl; 1458 vs, 1375 vs (paraffin); 1285 vs, 1248 vs δCH3; 1211 m, 1173 m, 1026 vs, 963 m. 926 vs, 885 m, 853 vs, 800 vs, 746 vs νCC; 720 sh (paraffin); 696 s, 627 s phenyl; 586 s, 565 vs, 517 sh, 486 m, 451 s νAlC, νPC, νAlTe, νPTe, δCC. UV/vis (CH2Cl2): λmax (ε; L mol−1cm−1) 236 (51600), 268 (sh, 47540), 400 (sh, 1340). MS (EI, 20 EV, 353 K): m/ z (%) = 585 (8) [M+ − CMe3]. Synthesis of Mes2P-C(CH-C6H5)-Al(CMe3)2(ONMe3) (3). Me3NO (0.076 g, 1.01 mmol) was added to a solution of compound 1 (0.52 g, 1.01 mmol) in c-pentane (15 mL). After stirring the mixture for 10 min, a white solid started to precipitate. The suspension was stirred overnight and filtered. The colorless solid was dried in vacuo to yield 3 (1·ONMe3) directly in high purity (0.46 g, 78%). 3 slowly decomposes at room temperature in solution and the solid state; therefore, we were not able to get a satisfactory elemental analysis. mp (argon, sealed capillary): 134 °C (dec.). 1H NMR (C6D6, 300 K): δ 1.26 (s, 18H, CMe3), 2.11 (s, 9H, NMe3), 2.13 (s, 6H, p-Me), 2.63 (s br., 12H, o-Me), 6.75 [d, 4JPH = 1.0 Hz, 2H, m-H(Mes)], 7.05 [t, 3JHH = 7.4 Hz, 1H, p-H(C6H5)], 7.17 [pseudo-t, overlap, 3JHH = 7.6 Hz, 2H, m-H(C6H5)], 7.45 [d, 3JHH = 7.3 Hz, 2H, o-H(C6H5)] 7.66 (d, 3JPH = 27.3 Hz, 1H, CCH). 13C NMR (C6D6, 300 K): δ 16.3 (br., AlCMe3), 20.9 (p-Me), 24.1 (d, 3JPC = 13.0 Hz, o-Me), 33.5 (AlCMe3), 58.7 (NMe3), 126.2 [p-C(C6H5)], 127.9 [m-C(C6H5)], 128.9 [oC(C6H5)], 129.8 [d, 3JPC = 2.0 Hz, m-C(Mes)], 136.6 [p-C(Mes)], 136.7 [1JPC = 57.0 Hz, ipso-C(Mes)], 144.1 [d, 2JPC = 14.0 Hz, oC(Mes)], 145.9 [d, 3JPC = 14.2 Hz, ipso-C(C6H5)], 146.3 (CCH), 152.7 (d, 1JPC = 93.0 Hz, CCH). 31P NMR (C6D6, 300 K): δ 1.9. IR (CsI plates, paraffin, cm−1): 1728 w, 1680 w, 1599 s, 1570 m, 1537 m νCC, phenyl; 1466 vs, 1374 vs (paraffin); 1308 s, 1290 s, 1263 s, 1246 s δCH3; 1200 m, 1171 s, 1123 m, 1069 s, 1030 s, 997 vs, 930 s,

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AUTHOR INFORMATION

S Supporting Information *

Crystallographic data (CIF files) and results of quantum chemical calculations. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (W.U.). Notes

The authors declare no competing financial interest.

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DEDICATION This manuscript is dedicated to the memory of the late Prof. Michael F. Lappert, a pioneering researcher and inspiring mentor (M. L.).

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REFERENCES

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