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Organometallics 2004, 23, 3239-3245

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Unusual Hydrolysis Reactions of cis-Bis((2,2′-biphenylylene)phosphochloridite ester)tetracarbonylmolybdenum(0) Houston Byrd,*,† Jeremiah D. Harden,† Jennifer M. Butler,‡ Michael J. Jablonsky,‡ and Gary M. Gray*,‡ Department of Biology, Chemistry, and Mathematics, The University of Montevallo, Station 6480, Montevallo, Alabama 35115, and Department of Chemistry, The University of Alabama at Birmingham, 201 Chemistry Building, 1530 3rd Avenue South, Birmingham, Alabama 35204-1240 Received February 27, 2004

The hydrolysis of cis-Mo(CO)4(2,2′-C12H8O2PCl)2 (1) in the presence of excess triethylamine and water yields [(C2H5)3NH]2[cis-Mo(CO)4(2,2′-C12H8O2PO)2] (2). This complex is in equilibrium with [(C2H5)3NH][cis-Mo(CO)4(2,2′-C12H8O2PO)(2,2′-C12H8O2POH)] (3) and free triethylamine in solution. The hydrolysis of 1 with a stoichiometric amount of triethylamine and water yields only 3. A Scatchard plot for the reaction of 3 and triethylamine to form 2 at 25 °C gives an equilibrium constant of 5.8 × 10-3 for the reaction. Fitting the variabletemperature 31P{1H} NMR spectra of a CD2Cl2 solution of 3 and 0.5 equiv of triethylamine using the gNMR program gives an activation energy of 47.4 kJ/mol for the reaction. The diastereomers of 3 crystallize with very different morphologies, allowing the X-ray crystal structures of both enantiomers to be determined. The 3R*R* diastereomer crystallizes in the noncentrosymmetric P212121 space group, but the 3R*S* diastereomer crystallizes in the centrosymmetric P21/c space group. The most interesting feature of the structures of 3R*R* and 3R*S* is the strong, intramolecular hydrogen bonding between the 2,2′-C12H8O2POH and 2,2′-C12H8O2PO- ligands that result in short O-O distances in both enantiomers (2.520(6) Å in 3R*R* and 2.373(11) Å in 3R*S*). The hydrogen bonding in both diastereomers is asymmetric, in contrast to previous speculation in the literature. Introduction In a recent paper, we reported the preparation and structural characterization of a variety of cis-tetracarbonylbis([1,3,2]dioxaphosphepin)molybdenum(0) complexes.1 These complexes are of interest because of the wide applications of [1,3,2]dioxaphosphepin ligands in catalysis and the need for accurate data for computational studies. The complexes were prepared by the reactions of cis-Mo(CO)4(2,2′-C12H8O2PCl)2 (1) with nucleophiles of the type HXR (X ) NH, O, S; R ) alkyl, aryl). These reactions yielded both the desired disubstituted products, cis-Mo(CO)4(2,2′-C12H8O2PXR)2, and the unexpected hemihydrolysis products, [R3NH][cis-Mo(CO)4(2,2′-C12H8O2PXR)(2,2′-C12H8O2PO)]. The hydrolysis side product was unusual because it had not been observed in similar studies with the chlorodiphenylphosphines.2-11 * To whom correspondence should be addressed. E-mail: (G.M.G.) [email protected]; (H.B.) [email protected]. † The University of Montevallo. ‡ The University of Alabama at Birmingham. (1) Byrd, H.; Harden, J. D.; Butler, J. M.; Jablonsky, M. J.; Gray, G. M. Organometallics 2003, 22, 4198. (2) Kraihanzel, C. S. J. Organomet. Chem. 1974, 73, 137, and references therein. (3) Johanssen, G.; Stelzer, O.; Unger, E. Chem. Ber. 1975, 108, 1259. (4) Ledner, P. W.; Beck, W.; Theil, G. Inorg. Chim. Acta 1976, 20, L11. (5) Gray, G. M.; Kraihanzel, C. S. J. Organomet. Chem. 1978, 146, 23. (6) Gray, G. M.; Kraihanzel, C. S. J. Organomet. Chem. 1980, 187, 51.

Hydrogen bonding was observed to play an important role in the crystal structures of the hydrolysis products. Because hydrogen bonding is becoming increasingly important in the assembly of nanostructures,12-15 gels,16 and polymers,17 further investigation of hydrogen bonding in cis-tetracarbonylbis([1,3,2]dioxaphosphepin)molybdenum(0) complexes is of interest. The hydrolysis of reactions of coordinated chlorodiphenylphosphines have been thoroughly studied.5,18-21 Perhaps the most interesting result from these studies is (7) Noth, H.; Reith, H.; Thorn, V. J. Organomet. Chem. 1978, 159, 165. (8) Lindner, E.; Wuhrmann, J. C. Chem. Ber. 1981, 114, 2272. (9) Wong, E. H.; Bradley, F. C. Inorg. Chem. 1981, 20, 2333. (10) Treichel, P. M.; Rosenheim, L. D. Inorg. Chem. 1981, 20, 1539. (11) Gray, G. M. Inorg. Chim. Acta 1984, 81, 157. (12) Han, Li; Luo, J.; Kariuki, N. N.; Maye, M. M.; Jones, V. W.; Zhong, C. J. Chem. Mater. 2003, 15, 29. (13) Paraschiv, V.; Crego-Calama, M.; Fokkens, R. H.; Padberg, C. J.; Timmerman, P.; Reinhoudt, D. N. J. Org. Chem. 2001, 66, 8297. (14) Klok, H.-A.; Jolliffe, K. A.; Schauer, C. L.; Prins, L. J.; Spatz, J. P.; Mo¨ller, M.; Timmerman, P.; Reinhoudt, D. N. J. Am. Chem. Soc. 1999, 121, 7154. (15) Fedin, A. P.; Virovets, A. V.; Sokolov, M. N.; Dybtsev, D. N.; Gerasko, O. A.; Clegg, W. Inorg. Chem. 2000, 39, 2227, and references therein. (16) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (17) Pei, J.; Liu, X.-L.; Chen, Z.-K.; Zhang, X.-H.; Lai, Y.-H.; Huang, W. Macromolecules 2003, 36, 323. (18) Gray, G. M.; Kraihanzel, C. S. J. Organomet. Chem. 1982, 238, 209. (19) Cotton, F. A.; Falvello, L. R.; Tomas, M.; Gray, G. M.; Kraihanzel, C. S. Inorg. Chim. Acta 1984, 82, 129. (20) Wong, E. H.; Bradley, F. C. Inorg. Chem. 1981, 20, 2333.

10.1021/om049851w CCC: $27.50 © 2004 American Chemical Society Publication on Web 05/20/2004

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the observation of a short hydrogen bond between the phosphinite and phosphinic acid ligands in the complexes. This type of hydrogen bonding is sufficiently strong to affect cis-trans isomerization.20 In contrast, no comprehensive study on the hydrolysis of chlorophosphites has been carried out. The cis-tetracarbonylbis(2-chloro-[1,3,2]dioxaphosphepin)molybdenum(0) complexes, such as 1, are ideal for these studies because they are stable and well characterized. In addition, 2-chloro-[1,3,2]dioxaphosphepin ligands derived from chiral diols such as 1,1′-bi-2-naphthol should allow the preparation of chiral, diastereomeric hydrolysis products. In this paper, we describe the hydrolysis of cis-Mo(CO)4(2,2′-C12H8O2PCl)2 (1). The solution conformations of the products have been characterized by multinuclear NMR spectroscopy, and crystal structures of both diastereomers of one of the hydrolysis products have been determined. Also, the solution proton exchange has been investigated using multinuclear NMR spectroscopy. Experimental Section All reactions and purifications were carried out under highpurity nitrogen. All starting materials were reagent grade and were purified by sublimation or distillation before use. All solvents were dried and distilled immediately prior to use. Tetrahydrofuran (THF) and triethylamine were distilled from sodium/benzophenone under high-purity nitrogen. Hexanes was distilled from calcium hydride under high-purity nitrogen. Deuterated NMR solvents (chloroform-d, dichloromethane-d2) were opened and stored under a nitrogen atmosphere at all times. The cis-Mo(CO)4(2,2′-C12H8O2PCl)2, 1, starting material was synthesized using literature procedures.1 All one-dimensional 31P{1H}, 13C{1H}, and 1H NMR spectra of the compounds were recorded using a Bruker ARX-300 NMR spectrometer with a quad (1H, 13C, 19F, 31P) 5 mm probe. The 31 P{1H} NMR spectra were referenced to external 85% phosphoric acid, and both the 13C and the 1H NMR spectra were referenced to internal TMS. Temperature-dependent spectra were recorded using a Bruker DRX-400 NMR spectrometer. Biphenoxy rings were equivalent in all disubstituted complexes and are numbered so that the carbon bonded to oxygen is C1 and the bridging carbon is C6. Elemental analyses were performed by Atlantic Microlabs, Norcross, GA. [(C2H5)3NH]2 [cis-Mo(CO)4(2,2′-C12H8O2PO)2], 2. Excess water (1.0 mL) and excess triethylamine (3.0 mL) were added via a syringe to a solution of 0.50 g (0.71 mmol) of 1 in 10 mL of dry THF. The mixture was allowed to stir at room temperature overnight and then was filtered to remove the triethylammonium chloride. The solution was rotovaped to dryness, leaving 0.54 g (0.62 mmol or 87 %) of a white solid. The crude solid was recrystallized from a dichloromethane-hexanes mixture to give analytically pure 2 as colorless crystals. Anal. Calcd for C40H48N2O10P2Mo: C, 54.92; H, 5.54; N, 3.20. Found: C, 55.02; H, 5.60; N, 3.29. 31P{1H} NMR (chloroformd): δ 173 (bs). In the presence of an excess of triethylamine, a sharp singlet is observed at δ 165.15. 1H NMR (dichloromethane-d2): δ 7.20-7.52 (m, 16H, Ar), 2.61 (q, 12H, CH2, |3J(HH)| ) 7 Hz) 1.00 (t, 18H, CH3, |3J(HH)| ) 7 Hz). [(C 2 H 5 ) 3 NH][cis-Mo(CO) 4 (2,2′-C 1 2 H 8 O 2 PO)(2,2′C12H8O2POH)], 3. Stoichiometric amounts of water (26 µL, 1.4 mmol) and dry triethylamine (220 µL, 2.1 mmol) were added to a solution of 0.50 g (0.71 mmol) of 1 in 20 mL of dry THF. The mixture was allowed to stir at room temperature (21) Throughout this paper, R* and S* are used to refer to the relative conformations of the 2,2′-biphenyl groups. The first conformation referred to in a molecule is designated R* by convention.

Byrd et al. for 24 h and then was rotovaped to yield 0.36 g (0.46 mmol or 65%) of crude 3. The crude 3 was recrystallized from a dichloromethane-hexanes mixture to give analytically pure 3 as colorless crystals. Anal. Calcd for C34H33NO10P2Mo: C, 52.79; H, 4.31; N, 1.81. Found: C, 52.81; H, 4.23; N, 1.93. 31 P{1H} NMR (methylene chloride-d2): δ 175.65 (s). 13C{1H} NMR (carbonyl carbons, methylene chloride-d2): δ 213.51 (trans CO, aq, |2J(PC) + 2J(P′C)| ) 41 Hz), 209.03 (cis CO, t, |2J(PC)| ) 13 Hz). 1H NMR (dichloromethane-d2): δ 7.20-7.51 (m, 16H, Ar), 2.66 (q, 6H, CH2, |3J(HH)| ) 7 Hz), 1.05 (t, 9H, CH3, |3J(HH)| ) 7 Hz). 31 P{1H} NMR Titration of Compound 3 with Triethylamine. The 31P{1H} NMR spectrum of a solution of 0.011 g (0.014 mmol) of 3 in 5.0 mL of CDCl3 in a N2-filled NMR tube capped with a Teflon septum was first recorded. Aliquots of dry triethylamine were than added via a gastight 50 µL syringe, and the 31P{1H} NMR spectrum of the solution was completely acquired within 10 min after each addition. Additions of triethylamine were made to give solutions with 1:3, 1:5, 1:10, 1:15, and 1:20 mole ratios of 3 to triethylamine. Variable-Temperature 31P{1H} NMR Spectra. The 31 P{1H} NMR spectra of a solution of 0.017 g (0.022 mmol) of 3 and 1.53 µL (0.011 mmol) of triethylamine in 0.75 mL of dichloromethane-d2 were recorded at 206, 217, 228, 239, 250, 261, 272, 283, and 294 K. These spectra were transferred to the gNMR program22 and fit using a two-site exchange model. The chemical shifts of the resonances at each temperature were calculated from the temperature dependences of the resonances in the slow exchange region and were fixed. The line widths of the resonances were measured at slow exchange limit and were fixed. The variables in the fitting process were the rate constant and the concentrations of the two species. It was necessary to incorporate the concentrations of the species because some of the monocation, 3, precipitated from the solution at low temperatures. Despite the precipitation, the Arrhenius plot, shown in Figure 6, was linear over the temperature range with a correlation constant of 0.995 for the plot with 8 data points. X-ray Data Collection and Solution. Suitable single crystals of both diastereomers of 3 (3R*R* and 3R*S*)21 were mounted on glass fibers with epoxy cement and aligned on an Enraf Nonius CAD4 single-crystal diffractometer under aerobic conditions. The 3R*R* diastereomer crystallizes in the orthorhombic P212121 space group, and the crystals are trapezoidal prisms. The 3R*S* diastereomer crystallizes in the monoclinic P21/c space group, and the crystals are rectangular plates. These crystals were separated on the basis of their shapes. Standard peak search and automatic indexing routines followed by least-squares fits of 25 accurately centered reflections resulted in accurate unit cell parameters. The space groups of the crystals were assigned on the basis of systematic absences and intensity statistics. All data collection was carried out using the CAD4-PC software,23 and details of the data collections are given in Table 1. The analytical scattering factors of each complex were corrected for both ∆f′ and i∆f′′ components of anomalous dispersion. All data were corrected for the affects of absorption and for Lorentz and polarization effects. All crystallographic calculations were performed with the Siemens SHELXTL-PC program package.24 The Mo and P positions were located using the Patterson method, and the other non-hydrogen atoms were located in difference Fourier maps. Full matrix refinement of the positional and anisotropic thermal parameters for all non-hydrogen atoms versus F2 was carried out. The hydrogen atoms involved in hydrogen bonding (22) gNMR Ver. 4.1; Cherwell Scientific Ltd.: Oxford, United Kingdom, 1995. (23) CAD4-PC Ver. 1.2; Enraf-Nonius Co.: Delft, The Netherlands, 1988. (24) Sheldrick, G. M. SHELXTL NT ver. 5.10; Bruker AXS, Inc.: Madison, WI, 1999.

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Figure 1. Scheme for the synthesis of [(C2H5)3NH]2[cis-Mo(CO)4(2,2′-C12H8O2PO)2], 2, and [(C2H5)3NH][cis-Mo(CO)4(2,2′C12H8O2PO)(2,2′-C12H8O2POH)], 3, from the hydrolysis of cis-Mo(CO)4(2,2′-C12H8O2PCl)2, 1. that only one of the enantiomers was present in the crystal chosen for the X-ray data collection. The correct enantiomer for this crystal, 3R*R*, was chosen on the basis of its Flack parameter. Selected bond lengths are given in Table 2, selected bond angles are given in Table 3, and selected torsion angles are given in Table 4.

Results and Discussion

Figure 2. 31P NMR spectra of chloroform-d solutions of (a) [(C2H5)3NH]2[cis-Mo(CO)4(2,2′-C12H8O2PO)2], 2, (b) [(C2H5)3NH]2[cis-Mo(CO)4(2,2′-C12H8O2PO)2], 2, + 40 equiv of triethylamine, and (c) [(C2H5)3NH][cis-Mo(CO)4(2,2′C12H8O2PO)(2,2′-C12H8O2POH)], 3. were also located in difference Fourier maps, and their positional and isotropic thermal parameters were refined. The remainder of the hydrogen atoms were placed in calculated positions with the appropriate molecular geometry with δ(C-H) ) 0.96 Å. The isotropic thermal parameter of each hydrogen atom was fixed at 1.2 times the Ueq of the atom to which the hydrogen was bound. Although the 3R*R* diastereomer was crystallized from a mixture containing both 3R*R* and its 3S*S* enantiomer, the fact that the complex crystallized in the noncentrosymmetric P212121 space group meant

Hydrolysis Reactions of 1. The general reaction scheme for the hydrolysis of 1 is shown in Figure 1. The reaction of 1 with an excess of water and triethylamine in THF followed by removal of the solvent and recrystallization of the residue from a dichloromethanehexanes mixture yields a single product. Both the integration of the 1H NMR resonances and the elemental analyses are consistent with the formulation of this material as [(C2H5)3NH]2[cis-Mo(CO)4(2,2′-C12H8O2PO)2], 2. This result is surprising because the hydrolysis of cis-Mo(CO)4(Ph2PCl)2, 4, under identical reaction conditions yields only [(C2H5)3NH][cis-Mo(CO)4(Ph2PO)(Ph2POH)], 5.5 These differing results suggest that the analogous hydrolysis product of 1, [(C2H5)3NH][cis-Mo(CO)4(2,2′-C12H8O2PO)(2,2′-C12H8O2POH)], 3, is a significantly stronger acid than is 5. This is as expected given that the 2,2′-biphenoxy phosphorus substituents in 3 are more electron-withdrawing than are the phenyl phosphorus substituents in 5 and thus more able to stabilize the second negative charge in 2. The broad 31P{1H} NMR resonance of 2 suggests that 2 is in equilibrium with 3 and triethylamine in solution. Despite the equilibrium, analytically pure 2 can be obtained by recrystallization from dichloromethane-hexanes mixtures. This is most likely due to the fact that 2, which contains two monocations and a dianion, is less soluble in the dichloromethane-hexanes mixture than is 3, which contains a monocation and a monoanion, and precipitates first. It is possible to prepare 3 in high yields when the hydrolysis is carried out with stoichiometric amounts of water and triethylamine as shown in Figure 1.

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Byrd et al.

Figure 3. Equilibrium between [(C2H5)3NH][cis-Mo(CO)4(2,2′-C12H8O2PO)(2,2′-C12H8O2POH)], 3, + triethylamine and [(C2H5)3NH]2[cis-Mo(CO)4(2,2′-C12H8O2PO)2], 2.

Figure 4. Scatchard plot and 31P NMR titration curve for the titration of [(C2H5)3NH][cis-Mo(CO)4(2,2′-C12H8O2PO)(2,2′-C12H8O2POH)], 3, with triethylamine. The inset shows the difference in chemical shift as the monoanion, 3, is completely converted to the dianion, 2. The ∆ ppm is obtained by subtracting the chemical shift of each titration point from the chemical shift of 3.

Unlike 2, 3 has a sharp 31P{1H} NMR resonance. It is also possible to observe two 13C resonances for the CO ligands of 3, indicating that the [1,3,2]dioxaphosphepin ligands are oriented in a cis geometry. The 13C{1H} NMR resonance of the carbonyls cis to both [1,3,2]dioxaphosphepin ligands (cis-COs) is a triplet (A portion of an AX2 spin system) and is the upfield resonance. The 13C{1H} NMR resonance of the carbonyls trans to one of the [1,3,2]dioxaphosphepin ligands (trans-COs) is an apparent quintet (A portion of an AXX′ spin system26) and is the downfield resonance. 31P NMR Studies of the Equilibrium between 2 and 3. To better understand the acid-base equilibrium involving 2 and 3, solutions of 2 were treated with either acid or base. The addition of a stoichiometric amount of acetic acid to a dichloromethane-d2 solution of 2 caused the broad 31P{1H} NMR resonance to sharpen and shift downfield to 175.64 ppm, which is where the resonance of 3 is observed (Figure 2c). In contrast, the addition of a stoichiometric amount of triethylamine only caused the broad 31P{1H} NMR resonance to shift upfield by a few tenths of a ppm. Only when a large excess of triethylamine (40 equiv) was added did the signal sharpen and shift significantly upfield to 165.15 (25) Scatchard, G. Ann. N. Y. Acad. Sci. 1949, 51, 660. (26) Redfield, D. A.; Nelson, J. H.; Cary, L. W. Inorg. Nucl. Chem. Lett. 1974, 10, 727.

Figure 5. Observed and calculated variable-temperature 31P NMR spectra for a dichloromethane-d solution that is 2 0.029 M in [(C2H5)3NH][cis-Mo(CO)4(2,2′-C12H8O2PO)(2,2′C12H8O2POH)], 3, and 0.014 M in triethylamine.

ppm. The fact that a large excess of triethylamine is needed to produce only compound 2 suggests that there is an equilibrium between 2 and 3 and that the equilibrium lies strongly toward 3. To obtain an equilibrium constant for the reaction shown in Figure 3, 3 was titrated with dry triethylamine at ambient temperature, and the reaction was followed by monitoring the shift in the position of the 31P{1H} NMR resonance as a function of the equivalents of triethylamine added. The addition of the triethylamine caused the sharp signal at 175.64 ppm due to 3 to broaden and shift upfield. As additional triethylamine was added, the resonance continued to shift upfield and slowly began to sharpen until a single sharp peak at 165.56 ppm due to 2 was observed. Figure 4 (inset)

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Organometallics, Vol. 23, No. 13, 2004 3243 Table 2. Selected Bond Distances (Å) with Their ESDs for 3R*R* and 3R*S*

Figure 6. Arrhenius plot using the rate constants obtained from the fitting of the variable-temperature 31P NMR spectra for a dichloromethane-d2 solution that is 0.029 M in [(C2H5)3NH][cis-Mo(CO)4(2,2′-C12H8O2PO)(2,2′C12H8O2POH)], 3, and 0.014 M in triethylamine to a twosite exchange model using gNMR. Table 1. Experimental Data for Crystallographic Studies of 3R*R* and 3R*S* formula MW temperature (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) volume (Å3) Z dcalc (g/cm3) absorp coeff (mm-1) F(000) cryst size (mm) hmax, hmin kmax, kmin lmax, lmin 2θ limits (deg) no. of reflns measd no. of indep reflns measd Rint (%) scan type abs corr abs coeff (mm-1) no. of variables refinement method no. of data/restraints/ params extinction coeff R, % Rw, % GOF

3R*R*

3R*S*

C34H33MoNO10P2 773.49 293(2) 0.71073 orthorhombic P212121 11.994(2) 16.128(3) 18.710(4) 90 90 90 3619.1(12) 4 1.605 0.517 1816 0.4 × 0.6 × 0.4 0, 12 -1, 17 0, 20 2.02-22.48 2882 2851

C34H33MoNO10P2 773.49 293(2) 0.71073 monoclinic P2(1)/c 17.918(4) 10.479(2) 19.944(4) 90 100.45(3) 90 3682.6(13) 4 1.395 0.497 1584 0.17 × 0.43 × 0.17 -21, 1 0, 11 -19, 19 2.08-25.14 5304 4807

2.95 ω-2θ none 0.517 445 full-matrix leastsquares on F2 2851/0/445

5.95 ω-2θ none 0.497 445 full-matrix leastsquares on F2 4807/0/445

0.0130(6) 2.82 (2σ) 7.01 (2σ) 1.150

0.00000(14) 5.91 (2σ) 12.67 (2σ) 0.988

shows a plot of the change in signal position versus the equivalents of triethylamine added. Because the proton exchange occurs in the moderately fast exchange region at room temperature, the observed chemical shift, δav, is a weighted average of the chemical shift of the dianion, δ2, and the chemical shift monoanion, δ3, as shown in eq 1. The equilibrium concentrations of 2, 3, and triethylamine in the equilibrium constant expression (eq 2) can be expressed using the value of n

Mo-P1 Mo-P2 X-P1 X-P2 O1-P1 O2-P1 O3-P2 O4-P2 O9-H O10-H-O9 O10-H-N C25-Mo C26-Mo C27-Mo C28-Mo C25-O5 C26-O6 C27-O7 C28-O8

3R*R*

3R*S*

2.4379(13) 2.4667(12) 1.559(4) 1.528(3) 1.620(3) 1.631(3) 1.654(3) 1.635(3) 0.89(6) 1.63(6) 1.75(5) 2.019(6) 2.013(5) 2.036(6) 2.013(6) 1.156(6) 1.131(6) 1.130(6) 1.146(6)

2.445(2) 2.472(3) 1.631(7) 1.623(6) 1.683(6) 1.512(5) 1.702(6) 1.482(5) 0.67(9) 1.74(9) 1.930(9) 2.058(11) 2.030(13) 1.992(11) 2.055(11) 1.184(11) 1.134(11) 1.152(10) 1.149(10)

Table 3. Selected Bond Angles (deg) with Their Esds for 3R*R* and 3R*S* C25-Mo-C26 C25-Mo-C27 C25-Mo-C28 C26-Mo-C27 C26-Mo-C28 C27-Mo-C28 C25-Mo-P1 C25-Mo-P2 C26-Mo-P1 C26-Mo-P2 C27-Mo-P1 C27-Mo-P2 C28-Mo-P1 C28-Mo-P2 P1-Mo-P2 O9-H-O9-O10

3R*R*

3R*S*

90.09(19) 93.31(19) 90.7(2) 91.0(2) 89.2(2) 176.0(2) 177.25(13) 89.03(13) 92.40(14) 179.09(14) 87.83(14) 88.85(14) 88.19(15) 91.07(16) 88.48(4) 176.2(3)

82.8(5) 79.7(4) 102.34(4) 92.5(4) 91.9(4) 175.4(4) 177.2(3) 98.2(3) 96.9(3) 177.7(3) 97.5(3) 89.8(3) 80.5(3) 85.8(13) 82.14(8) 158.1(3)

Table 4. Selected Torsion Angles (deg) and Their Esds for 3R*R* and 3R*S* C6-C1-O1-P1 C7-C12-O2-P1 C18-C13-O3-P2 C19-C24-O4-P2 P1-Mo-P2-X P2-Mo-P1-X

3a

3b

-78.1(5) -76.9 (5) -77.4(5) -78.5(5) 5.73(16) -7.57(19)

-77.7(9) -72.4(7) 81.6(12) 74.5(9) -4.2(3) 1.0(4)

calculated from the average chemical shift and the initial concentrations of 3 and triethylamine, as shown in eq 3. An equilibrium constant of 5.8 × 10-3 was obtained from the Scatchard plot25 shown in Figure 4.

δav ) (n)δ2 + (1 - n)δ3 δav - δ3 δ2 - δ 3

(1)

[2] [3][NEt3]

(2)

∴n) Keq ) Keq )

n[3]i (1 - n)[3]i(1 - n)[NEt3]i

(3)

Both the dianion, 2 (in the presence of a large excess of base), and the monoanion, 3, exhibit 31P{1H} NMR resonances that are sharp singlets. This suggests either

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Organometallics, Vol. 23, No. 13, 2004

Figure 7. ORTEP drawing of the molecular structure of 3R*R*. Thermal ellipsoids are drawn at 50%, and hydrogen atoms are omitted for clarity. Hydrogen bonding is shown by the dotted lines.

that the R* and S* conformations21 of the rings are rapidly interconverting in solution or that only the R*S* conformation is present in solution. The former explanation seems most likely based on previous work1 and because both R*S* and R*R*/S*S* conformations are found in the solid-state conformations of the complexes. Variable-Temperature 31P{1H} NMR Spectroscopic Studies of the Equilibrium between 2 and 3. The 31P{1H} NMR spectrum of a CD2Cl2 solution that was 0.029 M in the monoanion, 3, and 0.014 M in triethylamine was measured at various temperatures. Representative spectra at various temperatures are shown in Figure 5. The 31P spectra were fit to a twosite exchange process using the gNMR program,22 and the calculated spectra are also shown in Figure 5. An Arrhenius plot using the calculated rate constants, shown in Figure 6, was linear and gives a reasonable activation energy of 47.4 kJ/mol for the exchange process. X-ray Crystal Structures of Compounds of 3R*R* and 3R*S*. Recrystallization of 3 from dichloromethane-hexanes mixtures yields X-ray quality crystals with two very different morphologies. The 3R*R* diastereomer crystallizes in the orthorhombic P212121 space group, and the crystals are trapezoidal prisms. The 3R*S* diastereomer crystallizes in the monoclinic P21/c space group, and the crystals are rectangular plates. The molecular structures of 3R*R* and 3R*S* are shown in Figures 7 and 8. The twist of the 2,2′-biphenoxy groups of the [1,3,2]dioxaphosphepin ligands in 3R*R* and 3R*S* causes each of the ligands to be chiral and the complexes, each of which contains two of these ligands, to be diastereomeric. The chirality of the ligands can be determined from the torsion angles about the C-O bonds of the 2,2′biphenoxy groups, given in Table 4. The conformation of the [1,3,2]dioxaphosphepin ring requires the torsion angles about the two C-O bonds to have the same sign, and the similar conformations of the [1,3,2]dioxaphosphepin ligands requires the torsions angles to have approximately the same magnitudes. When both ligands in a complex have the same signs for the C-O torsion

Byrd et al.

Figure 8. ORTEP drawing of the molecular structure of 3R*S*. Thermal ellipsoids are drawn at 50%, and hydrogen atoms are omitted for clarity. Hydrogen bonding is shown by the dotted lines.

angles, as is the case for 3R*R*, the complex is the R*R*/S*S* diastereomer. In contrast, when the torsion angles for the C-O bonds of the two ligands have opposite signs, as is the case for 3R*S*, the complex is the R*S* diastereomer. It is not surprising that one of the two enantiomers of the R*R*/S*S* diastereomer, 3R*R*, crystallizes in a noncentrosymmetric space group, while the meso diastereomer, 3R*S*, crystallizes in a centrosymmetric space group. However, the opposite behavior has been observed with other cis-Mo(CO)4(phosphepin)2 complexes.1 The R*S* diastereomer of cis-Mo(CO)4(2,2′C12H8O2PCl)2, 1, crystallizes in the noncentrosymmetric P212121 space group, while both enantiomers of the R*R*/S*S* diastereomers of cis-Mo(CO)4(2,2′-C12H8O2PNHCH2CH2CH3)2, 6, [CH3CH2CH2NH3][cis-Mo(CO)4(2,2′-C12H8O2PNHCH2CH2CH3)(2,2′-C12H8O2PO)], 7, and cis-Mo(CO)4(2,2′-C12H8O2POCH3)2, 8, crystallize in centrosymmetric space groups (6, P1 h ; 7, P1 h ; 8, P21/c).1 These results demonstrate the importance of crystal packing forces in the determination of the space groups of crystals. It is important to note that, because the [1,3,2]dioxaphosphepin groups of all of the complexes racemize rapidly in solution, the observation of a particular diastereomer in the solid state does not necessarily reflect the distribution of diastereomers in solution. This is illustrated by the fact that the relative amounts of the two diastereomers of 3 that were obtained by recrystallization, as qualitatively determined by the numbers of crystals with the different morphologies that were observed, varied from batch to batch. Perhaps the most interesting feature in the structures of 3R*R* and 3R*S* is the hydrogen bonding that occurs between the OH and O- substituents of the phosphepins. The hydrogen bonding results in very short O9-O10 distances in both diastereomers (2.520(6) in 3R*R* and 2.373(11) in 3R*S*). These O-O distances are significantly shorter than the sum of the van der Waals radii of the oxygens and are very similar to those observed in the crystal structures of complexes containing the closely related cis-coordinated diphenylphosphinous acid/diphenylphosphinite ligand

Unusual Hydrolysis Reactions

system,19,27-29 and to those found in acid salts.30 The closest analogue, [(C2H5)4N][cis-Mo(CO)4(Ph2PO)(Ph2POH)], 6, has an O-O distance of 2.415(4) Å, which is intermediate between those of 3R*R* and 3R*S*. The very short O-O distances in the complexes containing cis-coordinated R2POH/R2PO- ligand systems could result from strong symmetric hydrogen bonds31 or instead could result from the cis orientation of the ligands. Because the bridging hydrogen was not located in any of the crystal structures of the complexes that had been reported in the literature, it was not possible to determine which was the case. In contrast, the bridging hydrogens were located in the crystal structures of both 3R*R* and 3R*S* and are asymmetric in both diastereomers. The O9-H-O10 angles are close to the expected 180° angles for asymmetric hydrogen bonds. This suggests that the hydrogen bonds in the complexes containing cis-coordinated diphenylphosphinous acid/diphenylphosphinite ligand systems are also asymmetric, and thus the cis coordination of the ligands to the metal centers allows for the very short O-O distances, even with asymmetric hydrogen bonds. The coordination environment of the molybdenum is a slightly distorted octahedron in both 3R*R* and 3R*S*. The most interesting feature of the distortion is that the P1-Mo-P2 angles are both smaller than 90° and are significantly different in the two diastereomers. We have previously noted that the P1-Mo-P2 angle in cis-Mo(CO)4(2,2′-C12H8O2PXR)2 complexes is quite variable and seems to depend on the orientations of the substituents of the two [1,3,2]dioxaphosphepin ligands.1 The P1-Mo-P2 angle in 3R*R* is within the range previously observed for the cis-Mo(CO)4(2,2′-C12H8O2PXR)2 complexes (97.74(6)-87.23(3)°),1 and this sug(27) Naik, D. V.; Palenik, S.; Jacobson, S. E.; Carty, A. J. J. Am. Chem. Soc. 1974, 96, 2286. (28) Carty, A. J.; Jacobson, S. E.; Simpson, R. T.; Taylor, N. J. J. Am. Chem. Soc. 1975, 97, 7254. (29) Cornock, M. C.; Gould, R. O.; Jones, C. L.; Stephenson, T. A. J. Chem. Soc., Dalton Trans. 1977, 1307. (30) Wells, A. F. Structural Inorganic Chemistry, 4th ed.: Clarendon Press: Oxford, U.K., 1975; pp 301-302. (31) Pimentel, G. C.; McClellan, A. L. Annu. Rev. Phys. Chem. 1971, 22, 347.

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gests that the hydrogen bonding does not significantly affect this angle in 3R*R*. In contrast, the P1-Mo-P2 angle is significantly smaller in 3R*S* than in any of the previously studied cis-Mo(CO)4(2,2′-C12H8O2PXR)2 complexes. It is interesting to note that the P-Mo-P angles in the complexes containing the closely related cis-coordinated diphenylphosphinous acid/diphenylphosphinite ligand system19,27-29 were smaller than observed for complexes with other cis-coordinated phosphine ligands, and these smaller angles were attributed to the hydrogen bonding. As previously observed in structures of molybdenum carbonyl complexes with related phosphorus-donor ligands, the carbonyls trans to the [1,3,2]dioxaphosphepin ligands in 3R*R* and 3R*S* have shorter Mo-C bonds than do the carbonyls trans to carbonyls.32 This is consistent with the [1,3,2]dioxaphosphepin ligands being better σ-donors and/or poorer π-acceptors than the carbonyl ligands. However, there is no corresponding difference in the C-O bond lengths of the carbonyl ligands. These observations have been reported by a number of authors, and one example is given in ref 32. Acknowledgment. The authors thank the NSFREU Summer Research Program (CHE-9820282) at UAB and the ACS Petroleum Research Fund (35349AC3) for funding this research. J.M.B. thanks the Chemistry Department of the University of Alabama at Birmingham for a graduate teaching assistantship. H.B. thanks the University of Montevallo for a sabbatical semester. Supporting Information Available: Tables giving experimental data for the crystallographic studies, bond distances, bond angles, positional parameters, anisotropic thermal coordinates, and hydrogen atom coordinates for compounds 3R*R* and 3R*S*. This material is available free of charge via the Internet at http://pubs.acs.org. OM049851W (32) Gray, G. M.; Fish, F. P.; Srivastava, D. K.; Varshney, A.; van der Woerd, M. J.; Ealick, S. E. J. Organomet. Chem. 1990, 385, 49.