The preparation and structural elucidation of uranium (VI) complexes and salts of the phosphorus ylides Ph3PCHCOPh, Ph3PC(COMe)(COPh) and Ph3PCHCOOCH2CH3

Inorganica Chimica Acta 359 (2006) 35–43 www.elsevier.com/locate/ica

The preparation and structural elucidation of uranium (VI) complexes and salts of the phosphorus ylides Ph3PCHCOPh, Ph3PC(COMe)(COPh) and Ph3PCHCOOCH2CH3 Elinor C. Spencer a,*, Balasubramanian Kalyanasundari b, Mahimaidoss Baby Mariyatra c, Judith A.K. Howard a, Krishnaswamy Panchanatheswaran c a

Department of Chemistry, Durham University, South Rd, Durham DH1 3LE, United Kingdom b Department of Chemistry, National College, Tiruchirappalli 620 001, India c Department of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India

Received 16 February 2005; received in revised form 22 August 2005; accepted 11 September 2005 Available online 8 November 2005

Abstract The reaction in methanol of the phosphorus ylides Ph3PCHCOPh, benzoylmethylenetriphenylphosphorane (BPPY), and Ph3PC(COMe)(COPh), a-acetyl-a-benzoylmethylenetriphenylphosphorane (ABPPY) with UO2(NO3)2 Æ 6H2O at 273 K leads to the formation of O-coordinated bis(ylide)-uranium (VI) complexes of the type [UO2(ylide)2(NO3)2], whereas the reaction of BPPY and UO2(NO3)2 Æ 6H2O under reflux in benzene yields the salt ½H-BPPY
2 þ ½U2 O4 ðNO3 Þ4 ðOHÞ2
2 . The reaction of Ph3PCHCOOCH2CH3, carbethoxymethylenetriphenylphosphorane (EPPY) with UO2(CH3COO)2 Æ 2H2O produces the salt [H-EPPY]+[UO2(CH3COO)3]. The structures of the free ylides ABPPY and EPPY are also discussed.
2005 Elsevier B.V. All rights reserved. Keywords: Uranium complexes; Phosphorus Ylide; Uranium (VI); Dodecahedral; Resonance stabilisation

1. Introduction The preparation and characterisation of a-stabilised phosphorus ylides, and metal complexes incorporating these ylides, has attracted much attention in recent years. This interest has been driven primarily by the necessity to develop new reagents for chemical synthesis that exhibit enhanced properties [1–3]. It is the requirement to satisfy this need that has provided the impetus for our work, which been directed towards evaluating the ligating behaviour of phosphorus ylides [4,5]. As part of this ongoing study we have chosen to investigate the bonding modes adopted by ylides when ligated to U(VI).

*

Corresponding author. Tel.: +44 0191 3342004; fax: +44 0191 3844737. E-mail address: [email protected] (E.C. Spencer). 0020-1693/$ - see front matter
2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.09.006

The field of actinide-ylide chemistry is well established, yet much of the original work concerned with uraniumylide materials has been directed towards the synthesis of uranium (IV) complexes [6–10]. Herein, we report on the preparation and characterisation of the uranium (VI) phosphorus ylide complexes [UO2(BPPY)2(NO3)2] (1), and [UO2(ABPPY)2(NO3)2] (3). These structures represent the first examples of uranium-ylide complexes in which the uranium metal is present in the VI oxidation state. Phosphorus ylides are known to demonstrate rich coordination chemistry. One of the significant aspects of our work has been to ascertain the preferred coordination modes of BPPY and ABPPY to the uranium metal. Due to the resonance delocalisation of the ylide electron density (Scheme 1) it is viable for ylides to ligate to uranium metal via the carbanion (i), or the carbonyl oxygen atom (iii).

36

E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43 O

O + (Ph3)P

R' R

i

R'

(Ph3)P R

R'

4.84 (d, 2JP–H: 21.5), 5.61(b), 7.10–8.09(m). (162 MHz, CDCl3): 14.0(s), 20.7(s).

iii

2.2. ABPPY (2)

O

ii

+ (Ph3)P R

Scheme 1. Accessible resonance modes of keto-stabilised phosphorus ylides.

The ylide ABPPY also has the potential to bond through either of its carbonyl atoms in a monodentate fashion. This ylide is also capable of chelating to the uranium metal via both carbonyl oxygen atoms in a bidentate manner, possibly forming a eight-coordinate complex analogous to the structure of (2,2 0 -bipyridine N,N 0 -dioxide)dinitratodioxouranium (VI) [11]. The final coordination mode adopted by the ylide is expected to be influenced by the steric requirements of the ylide molecule itself, and of the UO2(NO3)2 entity. The formation of bis(ylide)-uranium (VI) complexes is likely to necessitate O- rather than C-coordination, as this mode is less sterically demanding. However, C-coordination could result if a single ylide molecule binds to the UO2(NO3)2 unit, despite being coordinatively unsaturated, the steric bulk of ligand may assist in stabilising the complex. The electronic influence of the metal centre must also be considered. The high charge density of the uranium in the UO2 2þ ion (U6+), suggests that this cation will be poorly polarisable, and as such can be regarded as a
hard centre. In accordance with the Hard Acid Soft Base theory proposed by Pearson [12] the ylide must be expected to bind to the metal centre via the
hard carbonyl oxygen as opposed to the softer carbanion. In addition to the uranium (VI)-ylide complexes 1 and 3, we discuss the structures of the free phosphorus ylides ABPPY (2) and EPPY (4), and the synthesis and structures of two uranium (VI) salts, namely, [H-EPPY]+[UO2(CH3COO)3] 2 (5) and ½H-BPPY
2 þ ½U2 O4 ðOHÞ2 ðNO3 Þ4
ð6Þ. 2. Experimental 2.1. [UO2(BPPY)2(NO3)2] (1) Over the course of 3 h a solution of the phosphorus ylide BPPY (0.5 g, 1.55 mmol) in methanol was added to a methanolic solution of UO2(NO3)2 Æ 6H2O (0.39 g, 0.78 mmol) at 273 K. The solution was then stirred overnight at room temperature. A yellow solid formed on removal of the solvent in vacuo; this was washed with petroleum ether and benzene and then dried under vacuum. Bright yellow crystals were obtained by crystallisation from chloroform using the vapour diffusion method with petroleum ether as the diffusing solvent. Yield: 0.78 g (94%); m.p. 516–517 K. Anal. Calc. for C52H42N2O10P2U1: C, 54.08; H, 3.67; N, 2.43. Found: C, 55.37; H, 3.62; N, 2.59%. IR (cm1): 3550, 3033, 1677, 1663, 1587, 1507, 1484, 1438, 1382, 1358, 1342, 1274, 1254, 1203, 1175, 1109, 1025, 994, 933, 921, 850, 745, 718,688, 513. 1H NMR (400 MHz, DMSO-d6):

31

P NMR

Acetylation of BPPY afforded the ylide ABPPY [13]. The BPPY used for this synthesis was prepared by reacting triphenylphosphine with phenacyl bromide. The resulting phosphonium salt was deprotonated using anhydrous Na2CO3. IR (cm1): 1587, 1565, 1535, 1482, 1439, 1420, 1361, 1332, 1304, 1187, 1172, 1160, 1147, 1112, 1104, 1069, 1023, 999, 926, 866, 787, 750, 728, 696, 669, 635, 619, 597, 563, 519, 510, 496, 483, 466, 446, 436. 1H NMR (400 MHz, CDCl3): 1.80(s). 31P NMR (162 MHz, CDCl3): 16.5(s). 2.3. [UO2(ABPPY)2(NO3)2] (3) To a solution of ABPPY (0.25 g, 0.59 mmol) in methanol a methanolic solution of UO2(NO3)2 Æ 6H2O (0.30 g, 0.59 mmol) was added drop-wise, this was performed at 273 K. The solution was then stirred overnight. On removal of the solvent in vacuo, a yellow solid was obtained; this was re-crystallised from CH2Cl2 using the vapour diffusion method with petroleum ether as the diffusing solvent. Yield: 44.6%; m.p. 513–515 K. Anal. Calc. for C56H46N2O12P2U1: C, 54.28; H, 3.68; N, 2.26. Found: C, 54.97; H, 3.74; N, 2.81%. IR (cm1): 3446, 1636, 1587, 1480, 1435, 1406, 1384, 1332.7, 1293, 1166, 1107, 1026, 997, 931, 864, 779, 742, 717, 691, 565, 520,436. 1H NMR (400 MHz, CDCl3): 1.64(s), 7.41–7.72(m). 31P NMR (162 MHz, CDCl3): 18.8(s). 2.4. EPPY (4) This phosphorus ylide was prepared by the action of KOH on a phosphonium salt that had been previously prepared by reacting triphenylphosphine with a-chloroethyl acetate [14]. The resultant solid was crystallised from benzene by the vapour diffusion method using petroleum ether as the diffusing solvent. IR (cm1): 3436, 3055, 2976, 2929, 2900, 2360, 1608 (mC@O), 1521, 1482, 1437 (mC–O), 1397, 1371, 1331, 1186, 1159, 1123, 1105, 1066, 1024, 997, 929, 890, 863, 755, 723, 695, 545, 522, 512, 455. 1H NMR (400 MHz, CDCl3): 1.13(b), 2.87(b), 3.96(b), 7.32– 7.69(m). 31P NMR (162 MHz, CDCl3): 16.5(s). 2.5. [H-EPPY]+[UO2(CH3COO)3] (5) An equimolar mixture of EPPY and UO2(CH3COO)2 Æ 2H2O (0.2435 g, 0.57 mmol) in 95% ethanol was stirred at 298 K for 24 h. The resulting yellow solution was allowed to evaporate at 298 K. After several days yellow crystals suitable for X-ray diffraction were obtained. Yield: 67.8%; m.p. 498–499 K. Anal. Calc. for C28H31O13P1U1: C, 39.81; H, 3.67. Found: C, 40.27; H, 3.88%. IR (cm1): 3436, 2894, 1721(mC@O), 1546, 1458,

E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43

1315, 1112, 915, 850, 752, 691, 674, 516. 1H NMR (400 MHz, CDCl3): 0.92(t, 3JH–H 6.8), 2.25(bs), 2.73(d, 2 JP–H 13.2), 3.9 (q, 3JH–H 7.3), 4.87(d, 2JP–H 13.7), 7.3– 7.7(m). 31P NMR (162 MHz, CDCl3): 19.6(s), 20.4(s), 28.3(s). 2.6. [H -BPPY ]2 þ [U 2 O4 (OH )2 (NO3 )4 ]2 (6) A mixture of UO2(NO3)2 Æ 6H2O (0.68 g, 1.36 mmol) and BPPY (0.52 g, 1.36 mmol) in benzene (15 ml) was refluxed for 1 h. On allowing the solution to cool to room temperature a lemon-yellow solid crystallised out. The product was isolated and dried under high vacuum. Yield: 0.92 g (64.9%); m.p. 154–155 C. Anal. Calc. for C26H23N2O10PU: C, 39.41; H, 2.93; N, 3.54. Found: C, 39.38; H, 2.98; N, 3.56%. IR (cm1): 1674, 1595, 1507, 1439, 1384, 1327, 1298, 1279, 1210, 1109, 1034, 993, 917, 857, 809, 790, 746, 718, 686, 509, 438. 1H NMR (400 MHz, CDCl3): 2.24(b), 5.58 (d, 2JP–CH 12.21), 7.42– 8.14(m). 31P NMR (162 MHz, CDCl3): 20.28(s). 2.7. Instrumentation Solid-state IR spectra in the region of 4000–400 cm1 were obtained using KBr pellets on a Perkin–Elmer FT-IR spectrophotometer. 1H, 13C and 31P NMR spectra were obtained at 300 K using a Jeol 400 MHz instrument at the Indian Institute of Technology Madras, Chennai, India and using a Bruker 400 MHz instrument at the Indian Institute of Science, Bangalore, India. Elemental

37

analyses were performed at the Central Drug Research Institute, Lucknow, India. 2.8. Crystallography All data were collected at 120 K using graphite mono˚ ) on a three-circle chromated X-radiation (k = 0.71073 A Bruker SMART 6K diffractometer (see Table 1). Data processing was performed using standard Bruker software [15]. Structure solutions were via Direct Methods. Refinement was on F2 using full-matrix least-squares techniques. Data for 1, 2, 5 and 6 were corrected for absorption using SADABS [16]. Data for 3 were corrected for absorption using numerical methods (psi-scans). All hydrogen atoms in compounds 2, 3, and 5 are located at calculated positions (Uiso = 120% Uiso (parent atom)). For compound 1 all hydrogen atoms have been geometrically added to the model with Uiso values equal to 120% that of the parent atom, the exception being the hydrogen atoms bound to the ylide carbons (C8 and C34) which were located in the difference Fourier map and their position and Uiso parameters refined. The hydrogen atoms in the crystallographic model for 4 were located in the difference Fourier map and their position and Uiso parameters were refined. For compound 6 all hydrogen atoms were placed at calculated positions with Uiso values equal to 120% that of the parent atom, except for the hydrogen atoms associated with the anion, these were located in the difference Fourier map and their positional and Uiso parameters were refined.

Table 1 Crystallographic data and refinement details for compounds 1–6

Chemical formula Crystal size (cm3) Crystal colour/habit Formula weight (g) Crystal system Space group Z ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A l (Mo Ka) (mm1) Tmin/Tmax ratio 2h Range () Collected reflections Unique reflections Rint Goodness-of-fit R1 [I > 2r(I)] wR2 [I > 2r(I)]

1

2

3

4

5

6

C52H42N2O10P2U, 3(CCl2) 0.37 · 0.28 · 0.20 yellow/block 1403.58 triclinic P 1 2 13.1276(6) 14.5793(7) 16.1758(8) 80.677(2) 67.458(2) 75.911(2) 2765.2(2) 3.342 0.768194 2.74–50.14 26 193 9821 0.0257 1.059 0.0373 0.0981

C28H23O2P

C56H46N2O12P2U, 2(C6H6) 0.22 · 0.19 · 0.17 yellow/block 1395.13 monoclinic P21/n 2 11.2237(3) 17.2428(4) 16.0559(5) 90 90.446(1) 90 3107.2(2) 2.727 0.850782 3.46–50.00 28 023 5476 0.0736 0.929 0.0274 0.0642

C22H21O2P

C22H22O2P, C6H9O8U 0.16 · 0.12 · 0.04 colourless/plate 796.53 monoclinic P21 2 9.5596(5) 14.1265(7) 10.9521(5) 90 93.183(2) 90 1476.7(1) 5.604 0.658255 3.72–52.00 9501 5670 0.0293 0.795 0.0246 0.0525

2(C26H22OP), (H2N4O18U2) 0.20 · 0.17 · 0.14 yellow/block 1584.93 monoclinic P21/n 4 18.5771(8) 14.0779(6) 20.8976(9) 90 93.496(2) 90 5455.1(4) 6.070 0.574171 2.84–50.04 50 383 9642 0.0405 1.016 0.0212 0.0504

0.12 · 0.10 · 0.09 colourless/block 422.43 monoclinic P21 4 8.6186(5) 27.494(2) 9.8621(6) 90 114.142(2) 90 2132.5(2) 0.152 0.611688 4.52–52.00 13 345 8058 0.0532 1.047 0.0584 0.1143

0.29 · 0.20 · 0.18 colourless/block 348.36 monoclinic P21/n 4 9.8513(3) 14.5488(4) 12.8581(4) 90 101.443(1) 90 1806.25(9) 0.164 4.28–54.22 12 950 3994 0.0356 1.038 0.0358 0.0910

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E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43

3. Results and discussion 3.1. Structure of 1 The asymmetric unit of 1 is composed of a single [UO2(BPPY)2(NO3)2] unit and three disordered CH2Cl2 molecules. The uranium-ylide complex is shown in Fig. 1. As expected the ylide ligands are O-coordinated to the metal atom. The uranium atom is eight-coordinate, and resides in a triangulated dodecahedral environment (Fig. 2). The dihedral angle between the two planes defined by atoms U1, O1, O2, O6, O7 and U1, O3, O5, O8, O10, respectively, is 90.0(1). Both ylide components of the complex exhibit a cis arrangement of the phosphorus and oxygen atoms about their respective partial double bonds (C7–C8 and C33–C34). Selected bond parameters for complex 1 are given in Table 2. The U–O bond lengths are similar to those found for complexes in which the uranium atom possesses the same triangulated dodecahedron geometry observed in 1. The inequality of the U–O bond lengths and small bite angles for the bidentate nitrate groups are characteristic of this type of uranyl complex [11,18–20]. The P1–C8 and P2–C34 bonds of the ylide ligands are slightly elongated with respect to the free ylide (1.716(5), ˚ ; two molecules in the asymmetric unit) [21]. 1.725(4) A The C7–C8 and C33–C34 bonds are shorter than expected ˚ ), but longer than expected for a for a C–C bond (1.530 A ˚ ); this is consistent with the partial C@C bond (1.331 A double bond character of these bonds [22]. 3.2. Spectra for 1 The 1H and 31P NMR spectra of 1 in CDCl3 (Table 3) reveal the presence of two isomers in solution at ambient temperatures and exhibit no change in signal shape when the temperature is lowered. The 2JP–H of 21.5 Hz observed in

Fig. 2. Triangulated dodecahedron environment of uranium observed in complexes 1 and 3 [17]. Table 2 Selected bond geometries for 1 Bond

Length ˚) (A

Bond

Length ˚) (A

Atoms

Angle ()

U1–O1 U1–O2 U1–O3 U1–O5 U1–O6 U1–O7 U1–O8 U1–O10 N1–O3 N1–O4 N1–O5

2.358(3) 2.358(3) 2.549(3) 2.524(3) 1.777(3) 1.774(3) 2.535(3) 2.525(3) 1.268(5) 1.225(5) 1.261(5)

N2–O8 N2–O9 N2–O10 P1–C8 C8–C7 C7–O1 P2–C34 C34–C33 C33–O2

1.269(5) 1.224(5) 1.269(5) 1.749(5) 1.377(6) 1.287(5) 1.739(5) 1.377(6) 1.287(5)

O1–U1–O2 O3–U1–O5 O3–N1–O5 O6–U1–O7 O8–U1–O10 O8–N2–O10 P1–C8–C7 O1–C7–C8 O1–C7–C1 P2–C34–C33 O2–C33–C34 O2–C33–C27

178.74(9) 49.9(1) 115.7(4) 179.8(1) 50.2(1) 115.5(4) 127.9(4) 124.9(4) 119.3(4) 127.2(4) 124.4(4) 119.6(4)

the 1H NMR of 1 is within the range of values reported for O-coordinated complexes of BPPY [23,24]. At 228 K, a yellow solid separates from the solution, and the spectrum

Fig. 1. Molecular structure of 1. Thermal displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity.

E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43

39

shows the presence of only one isomer. Theoretically, the Ocoordination of the BPPY ligands to uranium metal centre can lead to isomeric cis and trans forms (with respect to the orientation of the P and O atoms within the ylide ligands), and these are shown as forms I and II in Fig. 3. Such isomerism was previously observed in the palladium complex of acetylmethylenetriphenylphosphorane [25]. The 2JP–CH for recorded for the peak at 4.84 ppm is close to that observed in the parent ylide (24.5 Hz), for which a cis arrangement of the P and O atoms has been confirmed by X-ray diffraction.

Therefore, the upfield peaks in the 1H NMR and 31P NMR spectra of 1, 4.84 and 14.0 ppm, respectively, are ascribed to the cis isomer of O-coordinated complex (form I in Fig. 3). The lower field signals in the 1H NMR and 31P NMR spectra, 5.61 and 20.7 ppm, respectively, are very similar to the values observed in the spectra of compound 6. We therefore assign these peaks to compound 6 that is present as an impurity in the sample used for this analysis.

Table 3 Spectroscopic results for compounds 1–6

The molecular structure of the free ylide 2 is shown in Fig. 4, and selected bond parameters are given in Table 4. There are two symmetry independent ylide molecules in the asymmetric unit. The variation in bond lengths and angles between the two ylide molecules will be due to differences in their packing environments within the crystal structure. The spectroscopic results for compound 2 (Table 3) are in agreement with the molecular structure deduced from the crystallographic data.

Peak IR

1 2 3 4 5

6 1

H NMR

1 2 3 4

5

6 31

P NMR

1 2 3 4 5

6

Assignment 1

1484 cm 1535 cm1 1565 cm1 1480 cm1 1636 cm1 1608 cm1 1437 cm1 1721 cm1 1458 cm1 1546 cm1 1674 cm1

m(C@O) COPh m(C@O) COMe m(C@O) COPh m(C@O) COMe m(C@O) COPh m(C@O) m(C–O) m(C@O) m(C–O) acetate group m(C@O)

4.84 ppm (doublet) 5.61 ppm (broad) 1.80 ppm (singlet) 1.64 ppm (singlet) 5.8 ppm (doublet) 3.8 ppm (quartet) 0.5 ppm (triplet) 0.92 ppm (triplet) 2.25 ppm (singlet) 2.73 ppm (doublet) 3.90 ppm (quartet) 4.87 ppm (doublet) 5.58 ppm (doublet)

P–CH (Form I) P–CH (6) –CH3 –CH3 P–CH –CH2 –CH3 –CH3 (ylide)a –CH3 (acetate)a P–CHa –CH2a P–CH2 P–CH2

14.0 ppm (singlet) 20.7 ppm (singlet) 16.5 ppm (singlet) 18.8 ppm (singlet) 16.5 ppm (singlet) 19.6 ppm (singlet) 20.4 ppm (singlet) 28.3 ppm (singlet) 20.28 ppm (singlet)

P P P P P P P P P

atom (Form I) atom (6) atom atom atom atom atom atom atom

See Section 2 for coupling constants and experimental details. a Merged signal.

O Ph (Ph)3P

O

O

N O U

O

O (Ph)3P

O O

O O Ph N O

Form I: Trans-cisoid

P(Ph)3

Ph

O

O

N O U

O

O O

O O Ph N

P(Ph)3

O

Form II: Trans-transoid

Fig. 3. The two isomeric forms that were observed in the NMR spectra of compound 1. In form I the O and P centres within the ylide ligands are in a cis arrangement, in form II they are trans.

3.3. Structure of 2

Fig. 4. Molecular structure of 2. Thermal displacement ellipsoids are drawn to the 50% probability level.

Table 4 Selected bond geometries for 2 Bond

Length ˚) (A

Bond

Length ˚) (A

Atoms

Angle ()

P1–C8 P1–C11 P1–C17 P1–C23 C8–C9 C8–C7 C7–O1 C1–C7 C9–O2 C9–C10

1.751(4) 1.810(4) 1.811(4) 1.813(4) 1.430(5) 1.453(5) 1.232(5) 1.502(6) 1.268(5) 1.507(6)

P2–C36 P2–C39 P2–C45 P2–C51 C36–C37 C36–C35 C35–O3 C29–C35 C37–O4 C37–C38

1.770(4) 1.801(4) 1.801(4) 1.812(4) 1.440(5) 1.435(5) 1.251(5) 1.496(5) 1.249(5) 1.498(6)

P1–C8–C9 P1–C8–C7 C9–C8–C7 O1–C7–C8 O1–C7–C1 O2–C9–C8 O2–C9–C10 P2–C36–C37 P2–C36–C35 C37–C36–C35 O4–C37–C36 O4–C37–C38 O3–C35–C36 O3–C35–C29

109.6(3) 120.0(3) 127.2(4) 121.9(4) 118.4(3) 117.8(4) 116.6(4) 111.7(3) 117.7(3) 126.4(4) 119.7(4) 118.4(3) 122.8(4) 117.6(4)

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E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43

The C–O bond lengths are longer than expected for ˚ ) [22], and the C8–C and C36–C bond ketones (1.210 A distances are greater than expected for C@C bonds, yet shorter than for a C–C bond. This is strongly suggestive of resonance delocalisation with in the ylide molecules. The geometries about atoms C8 and C36 are distorted trigonal planar, indicating that the molecules remain unprotonated at these sites. The torsion angles about atoms P1, C8, C9, O2 and P1, C8, C7, O1 are 0.2(5) and 17.8(5), respectively. The torsion angles assumed by atoms P2, C36, C37, O4 and P2, C36, C35, O3 are 11.5(5) and 14.1(5), respectively. The non-bonding distances between the P+ and O centres ˚ ; P1–O1, 3.046(3) A ˚ ; P2–O4, are: P1–O2, 2.661(3) A ˚ ˚ 2.785(3) A; P2–O3, 2.997(3) A. These distances are significantly less than the sum of the van der Waals radii for ˚ ) [26], and are suggestive phosphorus and oxygen (3.3 A of strong interactions between the P and O atoms, accounting for their near cis orientation. This type of short contact is a well-known feature of ylide chemistry [27,28]. 3.4. Structure of 3 Complex 3 crystallises in the centrosymmetric space group P21/n. The uranium atom resides on an inversion centre, and as such the asymmetric unit comprises half a [UO2(ABPPY)2(NO3)2] molecule and a single benzene molecule. The uranium complex is shown in Fig. 5. The molecular structure of 3 is analogous to the structure of 1. The ABPPY O-coordinates to the uranium in a monodentate fashion through the COMe group. The geometric configuration surrounding the uranium atom in 3 is also triangulated dodecahedral (Fig. 2), and the bond lengths and angles in complexes 1 and 3 are comparable (Tables 2 and 5). The dihedral angle between the two planes defined by atoms U1, O4, N1, O6 and U1, O2, O3 is 88.8(1). The near cis orientation of atoms O2 and P1 is demonstrated by the 4.3(5) torsion angle adopted by atoms P1, C8, C9 and O2. The non-bonding distance between atoms

Fig. 5. Molecular structure of 3. Thermal displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity. Symmetry equivalent atoms have been shown for completeness [symmetry code: x, y, 1  z].

Table 5 Selected bond geometries for 3 ˚ ) Bond Bond Length (A U1–O2 U1–O3 U1–O4 U1–O6 N1–O4 N1–O5 N1–O6

2.340(2) 1.768(3) 2.563(3) 2.511(2) 1.267(4) 1.219(4) 1.274(4)

P1–C8 C7–O1 C8–C7 C9–C8 C9–C10 C9–O2

˚) Length (A

Atoms

Angle ()

1.771(4) 1.223(4) 1.490(5) 1.383(5) 1.512(5) 1.283(4)

O2–U1–O3 O4–U1–O6 O4–N1–O6 P1–C8–C7 O1–C7–C8 O1–C7–C1 P1–C8–C9 C10–C9–C8 C10–C9–O2

87.8(1) 50.04(8) 115.3(3) 119.9(3) 122.1(3) 119.2(3) 116.2(3) 122.8(3) 116.9(3)

˚ , indicating that a strong intraP1 and O2 is 2.851(3) A molecular interaction is present between these two atoms. The bond lengths for the ylide components of 3 are akin to those reported for the chloride salt and O-coordinated mercury (II) complexes of ABPPY [29,30]. Comparison of the bond lengths with those of the parent ylide (2) show that, as expected, the resonance stabilisation of the ABPPY ligands is lost when bound to the uranium metal. The spectroscopic data for compound 3 are given in Table 3 and are consistent with the crystallographic model proposed. In particular, the 31P NMR data are indicative of the ylide molecules being O-coordinated to the uranium metal. Further evidence for this coordination mode is provided by the upward shift of the COMe mC@O peak by 55 cm1, and downward shift of 71 cm1 for the COPh group in the IR spectrum. 3.5. Structure of 4 The molecular structure of 4 is shown in Fig. 6 and principle geometric parameters are listed in Table 6. The asymmetric unit of 4 contains a single EPPY molecule. The O1–C20 bond length is longer than expected for ketones, and the C19–C20 distance lies between the lengths

Fig. 6. Molecular structure of 4. Thermal displacement ellipsoids are drawn at the 50% probability level.

E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43 Table 6 Selected bond geometries for 4 ˚) Bond Length (A P1–C1 P1–C7 P1–C13 P1–C19 C19–C20 C19–H19 O1–C20 O2–C20 O2–C21 C21–C22

1.809(1) 1.811(2) 1.809(2) 1.706(2) 1.408(2) 0.95(2) 1.23(2) 1.380(2) 1.443(2) 1.504(2)

41

Table 7 Selected bond geometries for 5 Atoms P1–C19–C20 P1–C19–H19 C20–C19–H19 O1–C20–C19 O1–C20–O2 C19–C20–O2 C20–O2–C21

Angle () 119.5(1) 118(1) 122(1) 126.9(1) 120.9(1) 112.2(1) 116.5(1)

expected for either a C@C or C–C bond, therefore, as in the case of ABPPY (2), compound 4 exhibits resonance delocalisation. The torsion angle about atoms P1, C19, C20 and O1 is 10.7(2), which suggests a near cis orientation about the C19–C20 partial double bond. The bond angles surrounding the C19 atom signify that the environment about this carbanion is distorted trigonal planar, this confirms that the molecule remains unprotonated at this site. The spectroscopic results for compound 4 (Table 3) are congruent with the crystallographic model proposed. The non-bonding distance between the P+ and O ˚ , implying that an intra-molecular centres is 3.039(1) A interaction is present between these two atoms. 3.6. Structure of 5 The molecular structure of this salt is shown in Fig. 7. Table 7 lists selected bond parameters for 5. The asymmetric unit consists of a single formula unit. The EPPY entity exhibits C-protonation (Table 4) and as such is deprived of its ylide character. The protonation of the C19 site is reflected in the change in the bond angles surrounding this atom with respect to the parent ylide (4). The significant shortening of the C20–O1 bond and elongation of the C19–C20 bond in 5 when compared with 4 is indicative of the expected loss of resonance stabilisation that accompanies protonation of EPPY. The triacetatouranylate (VI) anion possesses triangulated dodecahedron geometry. Due to lack of bulky mole-

Fig. 7. Molecular structure of [H-EPPY]+[UO2(CH3COO)3] (5). Thermal displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.

Bond

Length ˚) (A

Bond

Length ˚) (A

Atoms

Angle ()

U1–O3 U1–O4 U1–O5 U1–O6 U1–O7 U1–O8 U1–O9 U1–O10 O3–C23 O4–C23 C23–C24

2.457(4) 2.458(4) 2.495(3) 2.491(4) 2.458(4) 2.451(4) 1.775(3) 1.777(3) 1.254(7) 1.263(7) 1.498(7)

O5–C25 O6–C25 C25–C26 O7–C27 O8–C27 C27–C28 P1–C19 C19–C20 O1–C20 O2–C20 O2–C21

1.272(7) 1.259(7) 1.501(8) 1.271(7) 1.258(7) 1.511(8) 1.810(5) 1.508(7) 1.196(2) 1.337(6) 1.466(8)

O9–U1–O10 O3–C23–O4 O5–C25–O6 O7–C27–O8 P1–C19–C20 O1–C20–C19 O1–C20–O2 O2–C20–C19 C20–O2–C21

179.4(2) 119.8(5) 119.4(5) 119.8(5) 115.8(4) 125.0(5) 125.3(5) 109.7(4) 114.7(4)

cules ligating to the uranium atom the dodecahedron is notably less distorted than in the case of complexes 1 and 3. The structure of the triacetatouranylate (VI) anion in 5 is consistent with the structure reported by Navaza et al. [31] for sodium triacetatouranylate. The acetate ligands are bidentate and positioned equatorially to the linear O@U@O unit, the dihedral angle between the two planes defined by atoms O9, U1, O10 and U1, C23, C25, C27, respectively, is 87.1(1). The acetate bite angles are similar to those seen for the nitrate ligands in 1. The U–O(acetate) bonds are shorter than the U–O(nitrate) bonds observed in 1 and 3; this can be attributed to the reduced steric crowding surrounding the metal centre in 5. A possible mechanism for the formation of the triacetatouranylate (VI) anion is given below. The acidic proton produced in step 2 will participate in the protonation of the EPPY species to give the [H-EPPY]+ ion. Step 1: UO2(CH3COO)2 + 2H2O ! UO2(OH)2 + 2CH3COOH Step 2: UO2(CH3COO)2 + CH3COOH ! [UO2(CH3COO)3] + H+

3.7. Spectra for 5 The mC@O peak in the IR spectrum of 5 is shifted upward when compared to the spectrum of 4 (Table 3). This is to be expected if the ylide molecule is either C-protonated or Ccoordinated. The spectroscopic data indicate the presence of more than one isomer of the ylide species in solution and these are shown in Fig. 8. The appearance of doublet peaks at 2.73 and 4.87 ppm in the 1H NMR spectrum can be assigned to the P–CH and P–CH2 groups in the O-protonated (both cis and trans) and C-protonated species, respectively. It is worth noting that signals for the –CH3 and –CH2 groups of the three species in solution are merged in the 1H NMR spectrum, this has been

42

E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43 +

+

Ph3P H

O H

OEt

C-protonation

Ph3P H

+

O H OEt

O-protonation (cis)

Ph3P H

OEt

Table 8 Principle bond geometries for 6 Bond

Length ˚) (A

Bond

Length ˚) (A

Atoms

Angle ()

U1–O3 U1–O5 U1–O6 U1–O8 U1–O9 U1–O10 U1–O11 U1–O12 N1–O3 N1–O4 N1–O5 N2–O6 N2–O7 N2–O8

2.558(2) 2.563(2) 2.541(2) 2.546(2) 1.773(2) 1.774(2) 2.307(3) 2.332(2) 1.270(3) 1.216(3) 1.272(3) 1.280(3) 1.212(4) 1.263(3)

U2–O13 U2–O14 U2–O15 U2–O17 U2–O18 U2–O20 U2–O11 U2–O12 N3–O15 N3–O16 N3–O17 N4–O18 N4–O19 N4–O20

1.772(2) 1.773(2) 2.544(2) 2.534(2) 2.568(2) 2.564(2) 2.331(3) 2.309(2) 1.269(4) 1.209(4) 1.274(4) 1.274(3) 1.218(3) 1.263(3)

O3–U1–O5 O3–N1–O5 O6–U1–O8 O6–N2–O8 O9–U1–O10 O11–U1–O12 O11–U2–O12 O13–U2–O14 O15–U2–O17 O15–N3–O17 O18–U2–O20 O18–N4–O20

49.45(7) 114.9(3) 49.99(7) 115.4(3) 175.0(1) 65.68(9) 65.68(9) 174.9(1) 50.01(7) 115.1(3) 49.32(7) 115.1(3)

O H

O-protonation (trans)

Fig. 8. The three isomeric forms that were observed in the NMR spectra of compound 5.

deduced by the comparison the relative intensities of these peaks with respect to P–CH and P–CH2 signals. The peak at 28.3 ppm in the 31P NMR spectrum corresponds to the C-protonated species, and the peaks at 19.6 and 20.4 ppm can be assigned to the cis and trans (with respect to the Ph3P+ and –OH groups) O-protonated isomers, respectively. The presence of three cation species in solution is supported by the 13C NMR spectrum of 5. In addition to the signals for the carbon atoms of the phenyl and carbonyl groups, two peaks are observed at 128.8 and 133.2 ppm, these are assigned to the vinylic carbon atoms of the cis and trans O-coordinated species, respectively (Fig. 8) [32]. Two further signals at 32.0 and 164.5 ppm are ascribed to the carbon atoms of P–CH2 and C@O groups in the C-protonated cation (Fig. 8). 3.8. Structure of 6 The asymmetric unit of 6 is composed of two protonated BPPY cations and a single di-l-hydroxo-bis[dinitratodioxouranate(VI)] anion. The BPPY cations exhibit no unusual structural features and are equivalent to the structure of HBPPY+ reported by Antipin and Struchkov [33]. It is worth noting, however, that both cations have exceptionally short ˚ , respectively. P  O contacts of 2.980(1) and 2.922(2) A The uranate (VI) anion is shown in Fig. 9. Key bond lengths and angles are listed in Table 8, these agree with the values previously reported for this comparatively rare

anion [34]. The spectroscopic results for 6 reported in Table 3 are in agreement with the structure determined from the X-ray diffraction data. Materials 1 and 6 are synthesised from the same reactants but under different reaction conditions. Although a precise reaction mechanism for the formation of the [UO2(NO3)4(OH)2]2 anion is unknown, it is likely that some form of hydrolysis reaction occurs: Step 1: UO2(NO3)2 Æ 6H2O + BPPY ! [UO2(BPPY)(NO3)2] + 6H2O Step 2: 2½UO2 ðBPPYÞðNO3 Þ2
þ H2 O 2 ! ½H-BPPY
2 þ ½U2 O4 ðOHÞ2 ðNO3 Þ4
ð6Þ The protic nature of the solvent in which these reactions take place appears to be inconsequential, Antipin and Struchkov prepared the [UO2(NO3)4(OH)2]2 anion in ethanol whereas 6 was prepared in benzene. However, in both cases heat was applied the reaction mixture and it is possible that this is an essential requirement for the hydrolysis reaction to proceed (step 2). This is logical, as the hydrolysis of 1 to give 6 would necessitate the breaking of two U–O bonds. In comparison, complexes 1 and 3 are prepared at 273 K. However, further work is required before any reaction pathway can be deduced conclusively. 4. Summary

Fig. 9. [UO2(NO3)4(OH)2]2 anion observed in the structure of 6. Thermal displacement ellipsoids are drawn at the 50% probability level.

The product from the reaction of BPPY with UO2 (NO3)2 Æ 6H2O is highly dependent on the reaction conditions imposed, by judicious choice of conditions, different uranium (VI)-BPPY compounds can be selectively synthesised. Both the BPPY and ABPPY bis(ylide)-uranium (VI) complexes reported herein exhibit monodentate O-coordination of the ylide ligands to the metal centre.

E.C. Spencer et al. / Inorganica Chimica Acta 359 (2006) 35–43

In the case of ABPPY coordination is via the –COMe carbonyl oxygen. In the salts [H-EPPY]+[UO2(CH3COO)3] 4 2 and ½H-BPPY
2 þ ½U2 O4 ðOHÞ2 ðNO3 Þ
the EPPY and BPPY molecules are protonated, and subsequently have lost their ylide status. These compounds, and the free ylides EPPY and ABPPY, have been structurally characterised by spectroscopic techniques and X-ray diffraction. It has been shown that the metal atoms in all four uranium compounds are eight-coordinate and display triangulated dodecahedral geometry. 5. Supplementary materials All CIFs are deposited with the Cambridge Structural Database (CSD), and can be accessed via the website http://www.ccdc.cam.ac.uk (deposition codes CCDC 263472 (1), 236874 (2), 263473 (3), 263474 (4), 263475 (5), and 263476 (6)). Acknowledgements E.C.S. thanks the EPSRC for funding. J.A.K.H. thanks the EPSRC for a Senior Research Fellowship. E.C.S. thanks V.A. Money for assistance. K.P. thanks the Department of Science and Technology, New Delhi, India for financial assistance (SERC-SR/S1/IC-29/2003). References [1] O.I. Kolodiazhnyi, Tetrahedron 52 (1996) 1855, Report Number 389. [2] M. Taillefer, H.-J. Cristau, Top. Curr. Chem. 229 (2003) 41. [3] R. Navarro, E.P. Urriolabeitia, J. Chem. Soc., Dalton Trans. 4111 (1999). [4] M. Kalyanasundari, K. Panchanatheswaran, W.T. Robinson, H. Wen, J. Organomet. Chem. 491 (1995) 103. [5] E.C. Spencer, M.B. Mariyatra, J.A.K. Howard, K. Panchanatheswaran, J. Organomet. Chem. (accepted). [6] R.E. Cramer, R.B. Maynard, J.W. Gilje, J. Am. Chem. Soc. 100 (1978) 5562. [7] R.E. Cramer, R.B. Maynard, J.W. Gilje, Inorg. Chem. 19 (1980) 2564. [8] R.E. Cramer, R.B. Maynard, J.C. Paw, J.W. Gilje, J. Am. Chem. Soc. 103 (1981) 3589. [9] R.E. Cramer, R.B. Maynard, J.W. Gilje, Inorg. Chem. 20 (1981) 2466.

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