Evidence for Terminal Phosphinidene Oxide Complexes in O,P,C

Article pubs.acs.org/Organometallics

Evidence for Terminal Phosphinidene Oxide Complexes in O,P,CCage Complex Formation: Rearrangement of Oxaphosphirane Complexes Rainer Streubel,*,† Cristina Murcia-García,† Gregor Schnakenburg,† and Arturo Espinosa Ferao*,‡ †

Institut für Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany ‡ Departamento de Química Orgánica, Facultad de Química, Universidad de Murcia, Campus de Espinardo, E-30100 Murcia, Spain S Supporting Information *

ABSTRACT: A comparative study on the stereoselective synthesis of O,P,C-cage complexes is described using oxaphosphirane M(CO)5 complexes (M = Cr, Mo) and acetaldehyde or benzaldehyde. All products were unambiguously characterized by elemental analysis, multinuclear NMR, IR, MS, and single-crystal X-ray diffraction studies. DFT calculations provide evidence for initial C−O bond cleavage of the oxaphosphirane ring, most likely assisted by the selective bond weakening effect arising upon preliminary van der Waals complex formation. Subsequent P → C rearrangement of the Cp* group affords a phosphinidene oxide complex as key intermediate, which is responsible for the highly diastereoselective anchoring of the carbonyl compound.

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INTRODUCTION Recently, synthetic accessibility of three-membered-ring heterocycles I−IV (Scheme 1) was largely improved with the help of

ligand systems also created interest in theoretical studies such as reductive 15 and oxidative 16 SET ring opening of oxaphosphirane I and azaphosphiridine17 complexes II. It should also be noted that thermal ring cleavage of transient complexes I bearing an exocyclic CE double bond at the ring (E = O, NR) was reported recently.18 Due to our long-standing interest in the chemistry of strained P,E-heterocycles focusing largely on ring-opening and ring-expansion of P-ligated heterocycles I and II (E = O, NR), we have started to investigate the metal dependency of the thermal ring-expansion of complexes I bearing the P-bound C5Me5 substituent. Further motivation came from other reports showing the versatilility of the C5Me5 substituent in P-cage ligand chemistry.19,20 In a preliminary study,21 we had reported on the synthesis of tungsten(0) complexes bearing a polycyclic ligand (“O,P,Ccage ligand”) formed from I in a highly stereoselective process of unknown pathway for which only a very speculative explanation was provided. Herein, we report on the formation of chromium and molybdenum O,P,C-cage complexes starting from oxaphosphirane complexes. Theoretical calculations provide first time insight into the product formation, starting with an oxaphosphirane ring cleavage and subsequent P → C rearrangement of the P−C5Me5 substituent, thus forming a

Scheme 1. Oxaphosphirane I, Azaphosphiridine II, and Related Complexes III and IVa

a

R denote organic substituents; [M] = M(CO)5 (M = Cr, Mo, W).

a new protocol using the reaction of Li/Cl phosphinidenoid complexes1−3 with carbonyls1,4 or imines,5 affording oxaphosphirane I1,4,6 and azaphosphiridine II5,7 pentacarbonyl tungsten complexes in good yields. This is of particular interest, as stable oxaphosphiranes are still unknown and azaphosphiridines8 quite rare. Although their heavier homologues, the thiaphosphiranes9 and diphosphiranes10 as well as their complexes III11 and IV,12 are known as such, many more investigations have been carried out on the free ligands of III and IV, respectively. So far, especially tungsten complexes of I and II have received much attention, especially with regard to ligand transformations such as acid-induced ring-expansion reactions of I13 and II5 using nitriles. Interestingly, in the case of complex I the employed acid led to a valence isomerization giving alkylidene phosphonium complexes in the absence of nitriles.14 The aforementioned new synthetic methodology for small ring © XXXX American Chemical Society

Special Issue: Mike Lappert Memorial Issue Received: January 20, 2015

A

DOI: 10.1021/acs.organomet.5b00029 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

analogues by a Δδ of approximately 40−50 ppm in the case of chromium. The structural differences of the cages of 4a−c and 5a−c are clearly revealed through the phosphorus− hydrogen couplings with the C(R)H proton (R = Me, Ph), whereas the 31P and 1H NMR spectra of 4a−c clearly showed a 2 J(P,H) coupling constant magnitude of about 13 Hz and complexes 5a−c possess much smaller coupling constants of about 1 Hz. Complex 4b crystallizes in a triclinic lattice with space group P1̅ (Figure 1), while compound 5b crystallizes in a monoclinic

terminal phosphinidene oxide complex as the key intermediate able to react with aldehyde derivatives to yield O,P,C-cage ligands. A multistep reaction pathway is proposed.

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RESULTS AND DISCUSSION As the reaction course of the O,P,C-cage formation had remained unknown,21 it was decided to investigate this by employing the chromium and molybdenum complexes of I while keeping the same aldehydes as before. The starting material was obtained following the established Li/Cl phosphinidenoid complex protocol; that is, intermediate complexes 2a,b, obtained from complexes 1a,b18 via reaction with tert-butyllithium in the presence of 12-crown-4 (12-c-4) were treated with benzaldehyde at low temperature to yield oxaphosphirane complexes 3a,b after warming to ambient temperature (Scheme 2). Complexes 3a,b were then subjected Scheme 2. Synthesis of Oxaphosphirane Complexes 3a,b and Their Thermal Ring-Expansion Reactions to 4a,b and 5a,b

Figure 1. Molecular structure of complex 4b (Diamond 3.0, ellipsoids represent 50% probability level). Except C11−H and C18−H, all hydrogen atoms are omitted for clarity.

Figure 2. Reduced molecular structure of complex 4b (Diamond 3.0, ellipsoids represent 50% probability level). Except C11−H and C18− H, all hydrogen atoms, carbonyl groups at the metal, and Cp* methyl groups are omitted for clarity.

to thermal reactions with acetaldehyde and benzaldehyde in toluene. In all cases, the outcome was a highly diastereoselective reaction leading to complexes 4a,b and 5a,b (Scheme 2). The NMR data of the complexes 4a,b and 5a,b are closely related to those of the O,P,C-cage tungsten(0) complexes 4c and 5c21 (Table 1), except for the influence of the M(CO)5 groups exerted on the phosphorus chemical shifts; substituents at phosphorus are, in general, much less affected. The 31P NMR signals of the chromium and molybdenum complexes are shifted downfield relative to the tungsten Table 1. 31P NMR Data of Complexes 4a,b and 5a,b with Tungsten Complexes 4c21 and 5c21 Included for Comparison complex

δ31P (ppm)

4a 4b 4c 5a 5b 5c

201.3 180.1 155.1 197.5 176.1 152.7

2

JP,H (Hz)

mp (°C)

yield (%)

13 12 13 1 1 1

171 165 171 231 223 218

74 63 20 52 47 68

Figure 3. Molecular structure of complex 5b (Diamond 3.0, ellipsoids represent 50% probability level). Except C8−H and C3−H, all hydrogen atoms are omitted for clarity. B


Article

Organometallics Table 2. Bond Lengths and Angles of Complexes 4b,c and 5b,c 4c21

4b

5b

5c21

Bond Lengths [Å] M−P P−O(1) P−O(2) P−C(8/18)a O(1)−C(1) O(2)-C(16/3)b Bond Angles [deg] O(1)−P−O(2) O(1)−P−C(5) O(2)−P−C(5/8) C(1)−O(1)−P C(16/3)−O(2)−P a

2.456(9) 1.613(3) 1.603(4) 1.828(4) 1.455(6) 1.494(6)

2.457(4) 1.614(11) 1.609(10) 1.829(14) 1.451(15) 1.488(17)

2.456(9) 1.613(3) 1.603(4) 1.828(4) 1.455(6) 1.494(6)

2.457(4) 1.614(11) 1.609(10) 1.829(14) 1.451(15) 1.488(17)

107.54 99.6(2) 95.05(18) 125.5(3) 112.7(3)

106.80(6) 100.24(6) 94.29(6) 124.51(9) 112.58(8)

107.54 99.6(2) 95.05(18) 125.5(3) 112.7(3)

106.80(6) 100.24(6) 94.29(6) 124.51(9) 112.58(8)

C18 in complex 5b. bC16 in 4b and C3 in 5b.

lattice with space group P21/n (Figure 3). Selected data of 4b and 5b and the corresponding tungsten derivatives 4c and 5c,21 displayed in Table 2, unambiguously confirm the ligand cage constitutions. In order to provide a clearer view of the stereochemistry details, reduced structures are shown additionally in Figures 2 and 4.

Scheme 3. Endocyclic Bond Cleavage Processes (i) and (ii) in Model Oxaphosphiranes 6a−d

Table 3. Computed (DLNPO-CCSD(T)/def2QZVPP) Relative Energies (kcal/mol) for Endocyclic Bond Cleavage Processes (i) and (ii) in Model Oxaphosphiranes 6 Figure 4. Reduced molecular structure of complex 5b (Diamond 3.0, ellipsoids represent 50% probability level). Except C8−H and C3−H, all hydrogen atoms are omitted for clarity, carbonyl groups of the metal and methyl groups of Cp* are omitted for clarity.

entry

ΔETSCO

ΔECO

ΔETSPC

ΔEPC

6a 6b 6c 6d

41.57 39.71 38.78 36.66

−11.49 −6.52 −14.43 −19.18

42.31 39.57 40.53 37.23

33.49 33.56 31.59 30.14

a slightly lower energy barrier than the very endergonic endocyclic P−C bond cleavage (ii). The introduction of particular substituents able to stabilize an adjacent positive charge at the ring carbon atom, i.e., a CH3 group in model derivatives 6b,d, and others, i.e., a SiH3 group in 6c,d, able to stabilize a negative charge at phosphorus, slightly favor both ring-opening processes (Table 3), but keep the C−O bond cleavage as the (almost) preferred pathway. It can be expected that the real system 3a behaves along this line, as it bears a C-phenyl substituent, which stabilizes the positive charge in both P−C and P−O endocyclic bond cleavage intermediates 7a and 8a, respectively (Scheme 4). Consequently, it would favor both ring-cleavage processes, whereas the presence of the metal fragment can additionally stabilize the negative charge developed at P after P−C bond cleavage, hence qualitatively favoring the latter pathway. This is further supported by the computed energetics for both processes in 3a (Figure 5).23 In consequence, this rules out the P−C bond cleavage route and points to a C−O cleavage as the rate-determining step in the O,P,C-cage complex formation, at least in the formation of 4a. Moreover, it can be assumed that the following reaction sequence starts with formation of a van der Waals complex

The structural data of O,P,C-cages in 4 and 5, distances of P−O and P−C bonds are shown in Table 2, reveal only small, not significant structural differences and, hence, shall not be discussed further. Nevertheless, they are similar to those of polycyclic phosphites and, certainly, to those of previously reported tungsten(0) analogues. Theoretical Calculations. Quantum chemical calculations were performed in order to get insight into mechanistic aspects of the complicated sequence leading to O,P,C-cage complexes 4 and 5. For the sake of simplicity only the potential energy surface (PES) for the reaction of chromium complex 3a with acetaldehyde was inspected. The first question to be solved was if the initial step is the cleavage of the P−C or the C−O endocyclic bond. Noteworthy might be here the recent report that related azaphosphiridine complexes undergo N,P,C-cage formation initiated by endocyclic P−C bond cleavage.22 In a preliminary approach and for the sake of performing high-level calculations at a reasonable computational cost, uncomplexed model oxaphosphirane species were evaluated with regard to the cleavage of the two weakest endocyclic bonds. At the DLPNO-CCSD(T)/def2-QZVPP level of theory the C−O bond cleavage (i) is rather exergonic for the parent oxaphosphirane ring 6a (Scheme 3, Table 3). It also possesses C


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Organometallics Scheme 4. Calculated Mechanism for the Formation of O,P,C-Cage Complexes 4a and 5a, as Well as Other Minima on the PES Starting from 3a and Acetaldehyde

Table 4. Computed (B3LYP-D3/def2-TZVPP) Bond Strength and Ring Strain Related Descriptors and Their Variation in Complexes 3a and 3a·OHCCH3 entry WBI

LBO

MBO

ρ(r), e/ao3

G(r), au

P−C P−O C−O P−C P−O C−O P−C P−O C−O P−C P−O C−O

3a

3a·OHCCH3

Δ (%)

0.843 0.696 0.947 1.047 1.318 1.384 0.967 0.924 0.912 0.1567 0.1572 0.2425 0.1594

0.849 0.704 0.940 1.053 1.331 1.375 0.999 0.941 0.892 0.1604 0.1588 0.2406 0.1653

0.75 1.09 −0.64 0.63 0.98 −0.69 3.31 1.85 −2.16 2.37 1.01 −0.79 3.67

oxide complex 9a as the key intermediate. The expected electrophilicity for phosphinidene complexes, in general, is partly reduced in this particular case due to through-space electron density donation from a neighboring CC unit to a formally vacant 3p atomic orbital at P (dP···C2 = 2.724 Å; ∑WBIP−C2/C3 = 0.151; LBO = 0.187; MBO = 0.218),30 which results in a relatively stable complex. Furthermore, using the natural bond orbital (NBO) analysis31 this corresponds to a π(C2C3) → pP electron transfer with an associated stabilization of 18.82 kcal/mol in the second-order perturbation theory analysis of the Fock matrix in the NBO basis. The aldehyde reagent CH3CHO interacts via its basic O atom with the electrophilic P atom in 9a, affording adduct 10a (cf. Scheme 4) in a slightly exergonic and almost barrierless process (cf. Figure 5). Opposite the case of 3a·OHCCH3, the interaction in 10a cannot be considered as a van der Waals complex, as far as a moderately strong but genuinely covalent P−O(b) bond is formed (dP−O = 1.952 Å; WBI = 0.347; LBO = 0.696; MBO = 0.521; ρ(r) = 9.25 × 10−2 e/ao3) (compare to the values for 3a in Table 4; dP−O = 1.673 Å). The high diastereoselectivity in the formation of the final O,P,C-cage complexes 4a and 5a arises from the selective orientation of the aldehyde in 10a by means of a second anchoring point between the H atom and O(a) atom attached to P (dO···H = 2.299 Å; WBI = 0.007; ρ(r) = 1.66 × 10−2 e/ao3), in addition to a π-stacking (π-acceptor/donor) interaction between the carbonyl group and the Cp* mean plane (distance from the carbonyl C atom to Cp* mean plane: 2.784 Å; ∑WBI = 0.126; one BCP ρ(r) = 1.68 × 10−2 e/ao3, ε = 1.130). The above-mentioned interactions are easily visualized by means of reduced electron density (RDG) isosurfaces using the NCIplot technique (Figure 6).32,33 Such a docking fixes the acetaldehyde unit and enables an intramolecular 1,4-addition of the nucleophilic O(b) atom and the electrophilic carbonyl C atom across the Cp* diene moiety, affording exergonically the stable O,P,C-cage complex 4a. Finally, a 1,3-shift of the O(a) atom from C5 to C3 in the Cp* unit leads to the other most stable cage complex, 5a, after crossing an energy barrier slightly higher than the initial C−O cleavage step in 3a. It is worth mentioning that epimerization at the newly formed chiral center (the position arising from the carbonyl C atom) in 4a is a slightly exergonic process (relative energy Erel = −46.42 kcal/mol for 4a′) because 1,4-diaxial interactions in the boat-shaped six-membered ring decrease, but require passing a

Figure 5. Calculated (COSMOtoluene/B3LYP-D3/def2-TZVPP// COSMOtoluene/B3LYP-D3/def2-TZVP) minimum energy profile for the conversion of 3a into 4a and 5a.

between oxaphosphirane complex 3a and the aldehyde CH3CHO. Generation of this 3a·OHCCH3 complex entails a strengthening of the endocyclic C−P and P−O bonds, whereas the C−O bond is weakened, as shown by the variation of bond strength descriptors (VBSD)16,17,24 using several commonly used parameters such as the Wiberg bond index (WBI),25 Löwdin bond order (LBO),26 Mayer bond order (MBO),27 or the electron density at bond critical points (ρ(r)) in the framework of Bader’s atoms-in-molecules (AIM) theory28 (Table 4). Furthermore, the initial van der Waals complex also increases the ring strain as shown by the increase in the Lagrange of the kinetic energy density at ring critical points, G(r), which was recently reported to correlate with ring strain energies within related systems.24b,29 Worth mentioning is that complex 8a is the result of not only C−O bond cleavage in 3a but also an additional migration of the Cr(CO)5 metal fragment from P (end-on complex) to a side-on coordination at the CP bond. According to the calculations, complex 8a must undergo a P−Cp* bond rotation (8a′) to allow for a [2,3] shift of the Cp* group from P to the adjacent C atom. This exergonic, lowbarrier rearrangement is accompanied by a back-shift of the pentacarbonyl metal group to phosphorus, resulting in an endon coordination mode and leading to terminal phosphinidene D


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Organometallics

of benzaldehyde (6), and the mixture was stirred (at 75 °C in case of complexes c and at 85 °C for complexes a and b) for 4 h. The solvents were then removed in vacuo (ca. 0.01 bar), and the residues were extracted with 15 mL of petrol ether. After evaporation the crude product was washed with n-pentane at −20 °C and dried in vacuo. 4a: light green solid; yield 70 mg (0.130 mmol, 74%); mp 171 °C; selected NMR data: 1H NMR δ = 0.70 (s, 3H, Ccage-CH3), 0.91 (s, 3H, Ccage-CH3), 1.40 (s, 3H, Ccage-CH3), 1.81 (s, 3H, Ccage-CH3), 1.83 (s, 3H, Ccage-CH3), 4.20 (s, 1H,C(H)Ph), 5.22 (d, JP,H = 12.8 Hz, 1H, C(H)Ph), 7.28−7.45 (mc, 10H, Ph); 13C{1H} NMR δ = 9.1 (s, CcageCH3), 9.5 (s, Ccage-CH3), 18.0 (d, JP,C = 5.4 Hz, Ccage-CH3), 20.0 (s, Ccage-CH3), 20.1 (d, JP,C = 4.2 Hz, Ccage-CH3), 55.8 (s, Ccage), 56.1 (d, JP,C = 22.1 Hz, Ccage(H)-Ph), 59.1 (d, JP,C = 3.6 Hz, Ccage), 86.6 (d, JP,C = 13.1 Hz, Ccage(H)-Ph), 102.6 (d, JP,C = 9.5 Hz, Ccage), 126.8 (s, Ph), 127.5 (s, Ph), 127.8 (s, Ph), 130.1 (s, Ph), 133.9 (d, JP,C = 5.4 Hz, Ccage or Ph), 134.3 (s, Ccage or Ph), 138.4 (s, Ccage or Ph), 138.6 (d, JP,C = 1.2 Hz, Ccage or Ph), 200.1 (d, 2JP,C = 9.5 Hz, cis-CO), 203.6 (d, 2JP,C = 33.4 Hz, trans-CO); 31P NMR δ = 201.3 (s, 2JP,H = 12.9 Hz). MS: m/z (%) 570 (100) [(M)+]. IR (KBr; ν(CO)): ν̃̃ = (s, shoulder), 1935 (s), 1928 (s), 1912 (m, shoulder), 1780 (m, shoulder) cm−1. Anal. Calcd for C29H27CrO7P: C 61.05, H 4.77. Found: C 60.68, H 4.41. 4b: colorless solid; yield 63 mg (0.103 mmol, 63%); mp 165 °C; selected NMR data: 1H NMR δ = 0.67 (s, 3H, Ccage-CH3), 0.86 (s, 3H, Ccage-CH3), 1.41 (s, 3H, Ccage-CH3), 1.81 (s, 3H, Ccage-CH3), 1.83 (s, 3H, Ccage-CH3), 4.13 (s, 1H,C(H)Ph), 5.12 (d, JP,H = 12.8 Hz, 1H, C(H)Ph), 7.18−7.33 (mc, 10H, Ph); 13C{1H} NMR δ = 9.1 (s, CcageCH3), 9.5 (s, Ccage-CH3), 18.0 (d, JP,C = 5.4 Hz, Ccage-CH3), 20.0 (s, Ccage-CH3), 20.1 (d, JP,C = 4.2 Hz, Ccage-CH3), 55.8 (s, Ccage), 56.1 (d, JP,C = 22.1 Hz, Ccage(H)-Ph), 59.1 (d, JP,C = 3.6 Hz, Ccage), 86.6 (d, JP,C =13.1 Hz, Ccage(H)-Ph), 102.6 (d, JP,C = 9.5 Hz, Ccage), 126.8 (s, Ph), 127.5 (s, Ph), 127.8 (s, Ph), 130.1 (s, Ph), 133.9 (d, JP,C = 5.4 Hz, Ccage or Ph), 134.3 (s, Ccage or Ph), 138.4 (s, Ccage or Ph), 138.6 (d, JP,C = 1.2 Hz, Ccage or Ph(i)), 200.1 (d, 2JP,C = 9.5 Hz, cis-CO), 203.6 (d, 2JP,C = 33.4 Hz, trans-CO); 31P NMR δ = 180.1 (s, 2JP,H =12 Hz). MS: m/z (%) 614 (100) [(M)+]. IR (KBr; ν(CO)): ν̃ = 1989 (s, shoulder), 1835 (s), 1828 (s), 1912 (m, shoulder), 1780 (m, shoulder) cm−1. Anal. Calcd for C29H27MoO7P: C 56.69, H 4.43. Found: C 56.08, H 4.31. X-ray crystallographic analysis: Suitable colorless single crystals were obtained from concentrated diethyl ether solutions. C24H25O7PMo, crystal size 0.36 × 0.12 × 0.12 mm, triclinic, space group P1,̅ a = 12.0254(5) Å, b = 13.6346(8) Å, c = 17.2713(8) Å, α = 89.966(3)°, β = 85.428(3)°, γ = 83.274(3)°, V = 2803.3(2) Å3, Z = 2, 2θmax 61°, collected (independent) reflections 29 990 (12 087), Rint = 0.0898, μ = 12.1 mm−1, 695 refined parameters, R1 (for I > 2σ(I)) = 0.0581, wR21 (for all data) = 0.1475, max./min. residual electron density =0.83/−1.20 e Å−3. 5a: light green solid; yield 52 mg (0.102 mmol, 52%); mp 231 °C (dec); selected NMR data: 1H NMR δ = 0.71 (q, JH,H = 1.1 Hz, 3H, Ccage-CH3), 0.85 (s, 3H, Ccage-CH3), 1.35 (d, JP,H = 0.9 Hz, 3H, CcageCH3), 1.47 (s, 3H, Ccage-CH3), 1.65 (d, JH,H = 6.9 Hz, 3H, C(H)CH3), 1.71 (q, JH,H = 1.1 Hz, 3H, Ccage-CH3), 2.87 (d, JP,H = 4.5 Hz, 1H, C(H)Ph), 4.66 (dq, JP,H = 3.0 Hz (d), JH,H = 6.9 Hz (q), 1H, C(H)Me), 7.26−7.48 (mc, 5H, Ph); 13C{1H} NMR δ = 8.9 (s, CcageCH3), 10.9 (s, Ccage-CH3), 16.2 (d, JP,C = 1.9 Hz, Ccage-CH3), 16.9 (d, JP,C = 2.3 Hz, Ccage-CH3), 18.4 (d, JP,C = 2.3 Hz, C(H)-CH3), 18.7 (d, JP,C = 4.2 Hz, Ccage-CH3), 49.0 (d, JP,C = 22.3 Hz, Ccage), 54.3 (d, JP,C = 4.2 Hz, Ccage), 54.7 (d, JP,C = 21.7 Hz, Ccage(H)Ph), 74.9 (d, JP,C = 6.8 Hz, Ccage(H)Me), 93.2 (d, JP,C = 6.5 Hz, Ccage), 126.2 (d, JP,C = 1.6 Hz, m-Ph), 127.1 (s, p-Ph), 127.5 (d, JP,C = 1.3 Hz, m-Ph), 129.3 (d, JP,C = 9.4 Hz, o-Ph), 130.9 (d, JP,C = 10.7 Hz, o-Ph), 133.0 (s, Ccage), 134.9 (d, JP,C = 6.8 Hz, i-Ph), 141.5 (s, Ccage), 197.5 (dSat, JP,C = 9.7 Hz, cisCO), 202.5 (d, JP,C = 32.7 Hz, trans-CO); 31P NMR δ = 197.5 (br sSat, 2 JP,H = 1). MS: m/z = 508 [M•+, 62]. IR (KBr; (CO)): ν̃̃ = 2090 (m), 1964 (m), 1957 (s), 1861 (s) cm−1. Anal. Calcd for C24H25CrO7P: C 56.70, H 4.96. Found: C 57.03, H 4.43. 5b: colorless solid; yield 47 mg (0.085 mmol, 47%); mp 223 °C (dec); selected NMR data: 1H NMR δ = 0.69 (q, JH,H = 1.1 Hz, 3H, Ccage-CH3), 0.82 (s, 3H, Ccage-CH3), 1.35 (d, JP,H = 0.9 Hz, 3H, CcageCH3), 1.44 (s, 3H, Ccage-CH3), 1.65 (d, JH,H = 6.9 Hz, 3H, C(H)CH3), 1.69 (q, JH,H = 1.1 Hz, 3H, Ccage-CH3), 2.83 (d, JP,H = 4.5 Hz, 1H,

Figure 6. Computed (COSMOtoluene/B3LYP-D3/def2-TZVPP// COSMOtoluene/B3LYP-D3/def2-TZVP) most stable structure for 10a with NCIplot highlighting key stabilizing NCIs. The RDG s = 0.3 au isosurface is colored over the range −0.07 < sign(λ2)ρ< 0.07 au: blue denotes strong attraction, green stands for moderate interaction, and red indicates strong repulsion.

very high energy barrier (ΔETS = 70.19 kcal/mol), which in practice is not affordable. Some other minima were also found on the PES corresponding to these molecular systems (see the Supporting Information), but will not be discussed any further.

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CONCLUSIONS Herein, facile transformation of P-Cp*-substituted oxaphosphirane chromium and molybdenum complexes in thermal reactions with aldehydes is described, which led to the highly stereoselective formation of O,P,C-cage complexes. On the basis of theoretical findings a reaction pathway for the cage formation is proposed for the first time: the initial and ratedetermining step is the C−O bond cleavage reaction in oxaphosphirane complex 3, leading to 8, which rearranges into the phosphinidene oxide complex 9. This key intermediate fixes the aldehyde molecule in a diastereoselective manner in 10 by virtue of a set of noncovalent interactions (NCIs) that enables reaction of the carbonyl C atom and the P-oxide O atom (Oa) with the terminal positions of the diene moiety at the Cp* substituent leading to O,P,C-cage complex 4, whose epimerization to 4a′ is energetically unfavored. Finally, a [1,3] shift of the same Oa atom affords isomer 5.

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EXPERIMENTAL SECTION

General Considerations. All operations were performed in an atmosphere of purified and dried argon. Solvents were distilled from sodium/benzophenone. NMR data were recorded on a Bruker Avance 300 spectrometer at 30 °C using C6D6 (3) or CDCl3 (5, 6) as solvent and internal standard; shifts are given relative to tetramethylsilane (13C: 75.5 MHz) and 85% H3PO4 (31P: 121.5 MHz). Mass spectra were recorded on a Kratos MS 50 spectrometer (EI, 70 eV); only m/z values are given. Elemental analyses were performed using an Elementa (Vario EL) analytical gas chromatograph. Infrared spectra were collected on an FT-IR Nicolet 380. Melting points were obtained on a Büchi 535 capillary apparatus. X-ray crystallographic analysis of 4 and 5. Data were collected on a Nonious Kappa CCD diffractometer at 123 K using Mo Kα radiation (λ = 0.710 73 Å) (3, 5) and a Bruker SMART diffractometer at 133 K (6). The structures were refined by full-matrix least-squares on F2 (SHELXL-9722). All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were included in calculated positions using a riding model. General Procedure for the Synthesis of Complexes 5a,b and 4a,b. To a solution of 100 mg of 3 in 1.5 or 2.8 mL of toluene was added 1.5 mL (26.5 mmol) of acetaldehyde (5) or 39 μL (0.38 mmol) E


Article

Organometallics Notes

C(H)Ph), 4.66 (dq, JP,H = 3.0 Hz (d), JH,H = 6.9 Hz (q), 1H, C(H)Me), 7.06−7.28 (mc, 5H, Ph); 13C{1H} NMR δ = 8.7 (s, CcageCH3), 10.4 (s, Ccage-CH3), 16.1 (d, JP,C = 1.9 Hz, Ccage-CH3), 16.9 (d, JP,C = 2.3 Hz, Ccage-CH3), 18.2 (d, JP,C = 2.3 Hz, C(H)-CH3), 18.7 (d, JP,C = 4.2 Hz, Ccage-CH3), 49.1 (d, JP,C = 22.3 Hz, Ccage), 54.3 (d, JP,C = 4.2 Hz, Ccage), 54.0 (d, JP,C = 21.7 Hz, Ccage(H)Ph), 74.9 (d, JP,C = 6.8 Hz, Ccage(H)Me), 93.2 (d, JP,C = 6.5 Hz, Ccage), 126.7 (d, JP,C = 1.6 Hz, m-Ph), 126.9 (s, p-Ph), 127.5 (d, JP,C = 1.3 Hz, m-Ph), 128.8 (d, JP,C = 9.4 Hz, o-Ph), 130.9 (d, JP,C = 10.7 Hz, o-Ph), 132.2 (s, Ccage), 134.9 (d, JP,C = 6.8 Hz, i-Ph), 141.5 (s, Ccage), 197.5 (d, JP,C = 9.7 Hz, cisCO), 202.5 (d, JP,C = 32.7 Hz, trans-CO); 31P NMR δ = 176.1 (br s, 2 JP,H = 1). MS: m/z (%) 552 (62) [(M)+]. IR (KBr; ν(CO)): ν̃ = 2090 (m), 1964 (m), 1957 (s), 1861 (s) cm−1. Anal. Calcd for C24H25MoO7P: C 52.18, H 4.56. Found: C 51.39, H 4.12. X-ray crystallographic analysis of 4b: Suitable colorless single crystals were obtained from concentrated n-pentane/diethyl ether (3:1) solutions upon slow cooling to 4 °C. C29H27O7PMo; crystal size 0.32 × 0.12 × 0.12 mm3, monoclinic, P21/n, a = 10.4830(2) Å, b = 15.8099(3) Å, c = 14.8749(3) Å, α = 90°, β = 99.6410(10)°, γ = 90°, V = 2430.48(8) Å3, Z = 4, 2θmax = 28°, collected (independent) reflections = 22 941 (5856), Rint = 0.0330, μ = 4.873 mm−1, 304 refined parameters, R1 (for I > 2σ(I)) = 0.0224, wR21 (for all data) = 0.0438; max./min. residual electron density = 0.69/−0.39 e Å−3. Computational Details. DFT calculations were performed with the ORCA program.34 All geometry optimizations were run in redundant internal coordinates and using the B3LYP functional35 together with the def2-SVP basis set set.36 The latest Grimme’s semiempirical atom-pairwise London dispersion correction (DFT-D3) was included in all calculations.37 Harmonic frequency calculations verified the nature of ground states or transition states (TSs) having all positive frequencies or only one imaginary frequency, respectively. All minima were subsequently reoptimized using the def2-TZVP basis set,38 which was used for the harmonic frequency calculations and for obtaining the zero-point energy (ZPE) correction, whereas for TSs the ZPE correction at the B3LYP-D3/def2-SVP level was employed, unless otherwise stated (see ref 23). From these optimized geometries all reported data were obtained by means of single-point calculations using the B3LYP-D3 functional and the more flexible and polarized def2-TZVPP38 basis set, which constitutes the working level of theory in the text. When stated, the spin-component-scaled second-order Möller−Plesset perturbation theory (SCS-MP2)39 and the recently developed near linear scaling domain-based local pair natural orbital (DLPNO) method40 to achieve coupled cluster theory with single− double and perturbative triple excitations (CCSD(T)) were also used to compute energies. Solvent effects (toluene) were taken into account as single point via the COSMO solvation method.41 AIM-derived topological analysis of the electron density was conducted with AIM2000.42 Pro-molecular densities were used for the NCIplot program. Figure 6 was obtained with VMD.43

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge financial support of the Deutsche Forschungsgemeinschaft (STR 411/29-1) and the cost action CM1302 “SIPs”; G.S. thanks Prof. A. C. Filippou for support.

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DEDICATION In memory of the inspiring personality of Prof. M. Lappert.

ASSOCIATED CONTENT

S Supporting Information *

All crystallographic data, additional description of the PES for the 3a + CH3CHO system, detailed NCIplot analysis, and energy and Cartesian coordinates for all computed species are included in the Supporting Information, as well as a text file of all computed molecule Cartesian coordinates in a format for convenient visualization (.xyz). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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*Fax: (49)228-739616. Tel: (49)228-735345. E-mail: r. [email protected] (R. Streubel). *Fax: (34)868 88 7449. Tel: (34)868 88 7489. E-mail: [email protected] (A. Espinosa). F


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Organometallics

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