CRYSTAL GROWTH & DESIGN
Supramolecular Recognition Patterns - A Cause for Preferred Isomers in Phosphanylated 2-Amino-Thiazole Olaf Ku¨hl,* Bernhard Walfort, and Tobias Ru¨ffer TU Chemnitz, Anorganische Chemie, Strasse der Nationen 62, D-09111 Chemnitz, Germany
2006 VOL. 6, NO. 2 366-368
ReceiVed NoVember 14, 2005; ReVised Manuscript ReceiVed December 2, 2005
ABSTRACT: Supramolecular recognition patterns play a key role in the development of new materials but can also be employed as a methodology in the synthesis of amidophosphine ligands. In 2-phosphanylamino-thiazole, dimerization via hydrogen bonding acts as a template to establish the preferred stereochemistry of the second substitution reaction. Seemingly, the energetic advantage due to N-H hydrogen bonding amounting to 40-90 kJ/mol for a dimeric pattern is sufficient for regio- and stereoselective discrimination. Supramolecular recognition patterns play a key role in the development of new materials1 but can also be employed as a methodology in the synthesis of amidophosphine ligands. In 2-phosphanylamino-thiazole, dimerization via hydrogen bonding acts as a template to establish the preferred stereochemistry of the second substitution reaction. Seemingly, the energetic advantage due to N-H hydrogen bonding amounting to 40-90 kJ/mol for a dimeric pattern2-4 is sufficient for regio- and stereoselective discrimination. Reacting 2-amino-thiazole with 1 equiv of ClPPh2 in thf and an excess of triethylamine as auxiliary base yields the expected 2-phosphinoamino-thiazole (1) in high yield as a white powder after workup.5,6 Recrystallization from thf/hexane renders single crystals suitable for an X-ray crystal structure determination. Compound 1 crystalizes as a dimer in the monoclinic space group P2(1)/n with the monomer comprising the asymmetric unit and z ) 4 in the unit cell (see Figure 1).7-10 The dimers are formed from the monomers by a pair of N-H-N hydrogen bonds creating the well-known R22(8) structural motif.11 In theory, a tautomeric equilibrium can be formulated for compound 1 between a phosphinoamino (1a) and a phosphinoimino (1b) isomer as well as a prototopic shift equilibrium involving the phosphino group (1c) (see Figure 2).12 Compounds 1a and 1b are best described as the exocyclic amino and imino forms, respectively. Both isomers 1a and 1b are present in the platinum complex [PtCl(P,N-κ2-Me1a)(P-κ1-Me1b)] (Me1 ) compound 1 methylated in 4-position of thiazole).5 To decide on the true structure of 1, a closer inspection of the bond lengths and angles seems appropriate. The distinguishing features between the two tautomers are the C1-N1 and C1-N2 bond lengths as well as the location of the NH proton. The C1-N1 bond length is 130.2(2) pm and thus 6.1 pm shorter than C1-N2. This is in keeping with a C1-N1 double bond and a C1-N2 single bond in 1a and is supported by the position of the NH proton, which was found to be bonded to N2. The dimer is formed by a pair of N-H-N hydrogen bonds between N2-H2n and N1 with a N2-N1′ distance of 294.0(2) pm and a N-H-N angle of 177(3)° (see Figure 3). In the dimer, the tautomers are easily interconverted by movement of the proton H2n along the N-H-N hydrogen bond vector and readjustment of the C1-N1 and C1-N2 bond lengths, whereas the prototopic shift to phosphorus necessitates a serious rearrangement of the entire framework (see Figure 4). The reaction of 1 with a second equivalent of ClPPh2 is expected to yield the geminal N,N-bis(phosphanyl-2-amino) thiazole rather than any of the possible N,N′-isomers or the iminobiphosphine compound. However, reaction of 1 with another equivalent of ClPPh2 and triethylamine as auxiliary base in thf yields the bisphosphino compound 2 exclusively as the Z-isomer, with a small amount of * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. Molecular structure of 1. ORTEP plot showing the numbering scheme.
Figure 2. Synthesis and possible isomers of 1.
Figure 3. The hydrogen bond dimer of 1. C-H hydrogens omitted for clarity.
the unexpected iminobiphosphane product present (see Figure 5). This product distribution warrants a few comments regarding the possible mechanism of this reaction. The reaction between a chlorophosphine and an amine can be regarded as well known
10.1021/cg050603s CCC: $33.50 © 2006 American Chemical Society Published on Web 12/23/2005
Communications
Crystal Growth & Design, Vol. 6, No. 2, 2006 367
Figure 6. Phosphanylation on phosphorus. Table 1. Bond Lengths [pm] and Angles [°] bond lengths for 1 [pm] Figure 4. Hydrogen bonding in the dimer, a closer view.
P1-N2 C1-N2 C1-N1 C3-N1 C2-S1 C1-S1 C2-C3 N2-N1′
bond lengths for Z-2)O [pm] 170.50(16) 136.3(2) 130.2(2) 138.4(2) 172.7(2) 173.62(18) 132.6(3) 294.0(2)
bond angles for 1 [°]
Figure 5. Phosphanylation of the dimer.
involving displacement of a proton at the nitrogen by the phosphino group. In the present case however there seem to exist some new facets. Evidence regarding the product distribution comes mainly from NMR spectroscopy, while the solid-state structures could be determined by X-ray crystallography. The minor product 2a could not be isolated but was unambiguously identified from its characteristic AB pattern (δ ) 34.89 and -23.48 ppm) and large PP coupling constant (227.6 Hz) in the 31P NMR spectrum in solution.12 The main product was identified as the N,N′-bisphosphino compound Z-2 rather than the expected geminal product by the presence of two signals in the 31P NMR spectrum with equal intensities as well as two sets of signals for the phenyl rings in the 13C-{1H} NMR spectrum. The low field signal at 67.84 ppm is typical for geminal bisphosphino compounds ArN(PPh2)2 (Ar ) aryl) and thus would indicate the synthesis of the expected geminal product.12 However, this would not explain the existence of the high-field signal at 49.04 ppm, which is in line with the exocyclic NP group in Z-2. Compound Z-2 requires another signal of identical intensity, which could be the signal at 67.84, if one compares it with the values of pyrrolyl phosphines that lie typically in the range between 60 and 90 ppm.13,14 If we take the structure of 2 as a starting point, then we would assume the reaction to take place from structure 1b. However, in 1b the exocyclic imino double bond is already formed and reaction of the chlorophosphine with the endocyclic N-H group would result in the exclusive formation of the Z-isomer only if formation of the Z-form is preferred for some reason. Before we counter this explanation, we ought to ask ourselves how likely is the existence of isomer 1b in solution, and why should it exist solely as the Z-isomer. A tautomeric equilibrium, apart from being unlikely,12 would strongly favor the E-isomer and might even prevent the second substitution in thf as with the monophosphino ureas.15,16 On the other hand, the Z-isomer once formed is likely to dimerize into the known solid-state structure as would the E-isomer on account of the stabilization provided by the R22(8) structural motif of the dimer. A prototopic shift equilibrium to the phosphorus is generally disfavored but possible to a limited extent.12 The existence of a PH species would explain the occurrence of the iminobiphosphane as a minor product (see Figure 6). If we take 1a (the structure in the solid state) as the starting point, then reaction of a chlorophosphine on the exocyclic N-H
P1-N2-C1 C3-N1-C1 N1-C1-S1 C2-S1-C1 N1-C1-N2 N2-C1-S1 P1-N2-H2n C1-N2-H2n N2-H2n-N1′
P1-N1 P2-N2 C1-N2 C1-N1 C3-N1 C2-S1 C1-S1 C2-C3 P1-O1
172.5(2) 170.68(18) 127.8(3) 139.1(3) 140.7(3) 174.2(3) 177.0(2) 132.0(4) 146.10(19)
bond angles for Z-2)O [°] 122.91(14) 110.17(17) 114.29(13) 89.01(10) 123.21(16) 122.50(14) 122.7(14) 114.0(17) 177(3)
P1-N1-C1 P2-N2-C1 C3-N1-C1 N1-C1-S1 C2-S1-C1 N1-C1-N2 N2-C1-S1
125.99(15) 119.11(16) 113.22(19) 109.08(16) 90.98(11) 123.1(3) 127.80(17)
bond would result in the formation of the N,N-bisphosphino product with both phosphino groups bonded to the exocyclic nitrogen. However, the preferred product is not the geminal N,N′-bisphosphino compound but Z-2. Reaction of monomeric 1a with a chlorophosphine moiety cannot explain this. We will have to assume that 1a actually reacts from its dimeric form (indistinguishable from dimeric 1b). The critical role of the dimer can be visualized best if it is considered as a molecule in its own right with a pair of strong hydrogn bonds N2-H2n and a pair of weak ones N1-H2n. Attack of the chlorophosphine on the endocyclic nitrogen would render the Z-bisphosphino product, and attack on the exocyclic nitrogen would yield the geminal bisphosphino product. The observed product distribution shows that substitution on the endocyclic nitrogen and thus the weaker hydrogen bond is preferred over substitution on the exocyclic one. As the simultaneous attack of two molecules of chlorophosphine on the dimer seems to be very unlikely, the reaction needs to proceed with breakage of the eight-membered ring and generation of the monomer as an intermediate. The monomer 1b generated as an intermediate in the reaction between 1 and ClPPh2 as the Z-isomer can either react with the ClPPh2 present to form Z-2 or reform the dimer of 1 and then react to the Z-product. It is therefore possible that the observed product distribution is largely due to the formation of Z-1b as an intermediate in the phosphonylation of the 1 dimer. It is even conceivable that 1b dimerizes via the phosphorus end to a 10-membered ring R22(10), thus enabling phosphonylation of phosphorus without the need for a formal proton shift from nitrogen to phosphorus. Evidence for the structure of compound 2 comes solely from spectroscopic data. However, the structure of a related compound could be determined by X-ray diffraction. When compound 2 was recrystallized from thf/hexane, virtually all the recovered product was a white powder, with the exception of very few white crystals.17 The crystals proved to be the monoxide of Z-2 (see Figure 7). The monoxide Z-2a is probably formed by residual ClP(O)Ph2 in the technical grade starting material ClPPh2.
368 Crystal Growth & Design, Vol. 6, No. 2, 2006
Communications Next to the off-white powder a small quantity of colorless crystals was found and identified as the monoxide of compound 2. NMR (CDCl3, δ ppm): 31P-{1H} 28.70 (s, endo), 54.55 (s, exo); 13C-{1H} 140.30 (s, C1), 132.95 (d, J PC ) 3.78 Hz, p-PhPO), 132.70 (d, JPC ) 11.17 Hz, m-PhPO), 131.59 (d, JPC ) 20.54 Hz, o-PhPO), 131.43 (d, JPC ) 20.54 Hz, i-PhPO), 129.79 (s, C2), 128.90 (s, C3), 128.73 (d, JPC ) 7.25 Hz, o-PhP), 128.46 (s, p-PhP), 128.17 (d, JPC ) 7.12 Hz, m-PhP), 68.12 (s, thf), 25.76 (s, thf),1H 7.936.97 (m, 20 H, Ph), 6.54 (d, JHH ) 3.61 Hz, 1 H, H3), 6.13 (m, 1H, H2), 3.75 (2 H, thf), 1.86 (2 H, thf). Elemental analysis for C29H26N2O1.5P2S (520.66): Calcd: C 66.90 H 5.03 N 5.40; found: C 67.18 H 5.14 N 5.27. Supporting Information Available: Crystallographic information files are available free of charge via the Internet at http://pubs.acs.org.
References Figure 7. Molecular structure of Z-2a. ORTEP plot showing the numbering scheme. Hydrogen atoms are omitted for clarity.
The most striking features in the structure of compound Z-2a are the carbon-nitrogen bond lengths. To accommodate the phosphino substituent on the endocyclic nitrogen atom N1, the C1N1 double bond has to become a single bond, and subsequently the C1-N2 single bond changes into a double bond with the loss of heteroaromatic character on the thiazole ring. However, comparison of these two C-N bonds with their counterparts in the dimer 1 reveals that there the C1-N2 single bond is already shortened and the C1-N1 bond is elongated and thus predisposed to form Z-2 upon reaction of the chlorophosphine on the endocyclic nitrogen. The stereospecific and regioselective formation of Z-2 from the monophosphino compound 1 is due to the dimerization of 1 via two hydrogen bonds from the NH-proton H2n to the endocyclic nitrogen of the thiazole ring. This dimerization enables the attack of the second chlorophosphine on the endocyclic nitrogen and fixation of the C1-N2 double bond in the Z-position as the first phosphino group is guided into the Z-position by the ring system of the dimer. Experimental Section. Crystal data was collected on a Bruker Smart CCD diffractometer at -85 °C using Mo KR radiation (λ ) 71.073 pm). Reflections were collected in the omega scam modus in 0.4° steps and an exposure time of 30 s per frame. N,N′-Bis-(diphenylphosphino)-2-amino-thiazole Z-2. To a solution of 1.42 g (5 mmol) of 1 in 50 mL thf were added 5 mL of triethylamine and 1.0 mL (5.5 mmol) of ClPPh2. Discarding of the white precipitate, concentration of the filtrate, and addition of n-hexane render 1.73 g (74%) of Z-2 as an off-white powder. NMR (CDCl3, δ ppm): 31P-{1H} 67.84 (s, endo), 49.04 (s, exo); 13C-{1H}138.83 (s, C1), 133.00 (d, J PC ) 23.1 Hz, i-PhPN-endo), 131.52 (d, JPC ) 10.6 Hz, o-PhPN-endo), 131.28 (s, p-PhPN-endo), 130.77 (d, JPC ) 9.2 Hz, m-PhPN-endo), 129.64 (d, JPC ) 19.7 Hz, i-PhPN-exo), 129.25 (d, JPC ) 7.2 Hz, o-PhPN-exo), 128.98 (d, JPC ) 8.8 Hz, m-PhPN-exo), 128.21 (s, p-PhPN-exo), 109.74 (s, C2), 106.11 (s, C3), 1H 7.84-7.00 (m, 20 H, Ph), 6.66 (d, JHH ) 4.13 Hz, 1 H, H3), 6.12 (m, 1 H, H2). Elemental analysis for C27H22N2P2S (468.61): Calcd C 69.20 H 4.73 N 6.00; found C 69.04 H 4.87 N 6.11.
(1) Roesky, H. W.; Andruh, M. Coord. Chem. ReV. 2003, 236, 91119; (b) Janiak, C. Angew. Chem. 1997, 109, 1499. (2) Dunitz, J. D.; Gavezzotti, A. Angew. Chem. 2005, 117, 1796-1819. (3) Gamez, P.; van Albada, G. A.; Mutikainen, I.; Turpeinen, U.; Reedijk, J. Inorg. Chim. Acta 2005, 358, 1975-1980. (4) Ku¨hl, O.; Goutal, S. Cryst. Growth Des. 2005, 5, 1875-1879. (5) For synthesis of compound 1, see Milton, H. L.; Wheatley, M. V.; Slawin, A. M. Z.; Woollins, J. D. Inorg. Chim. Acta 2005, 358, 1393-1400. (6) For recent reviews on phosphino amines see (a) Alajarı´n, M.; Lo´pezLeonardo, C.; Llamas-Lorente, P. Top. Curr. Chem. 2005, 250, 77106; (b) Fei, Z., Dyson, P. J. Coord. Chem. ReV. 2005, 249, 20562074; (c) Rodriguez i Zubiri, M.; Woollins, J. D. Comm. Inorg. Chem. 2003, 24, 189-232. (7) Crystallographic data for 1: C15H13N2PS, M ) 284.30, monoclinic, space group P21/n, a ) 777.36(4) pm, b ) 1181.33(7) pm, c ) 1622.83(9) pm, β ) 103.5490(10)°, V ) 1.44880(4) nm3, Z ) 4, µ ) 0.321 mm-1, T ) 293(2) K, 15809 reflections (Rint ) 0.0203, R1 ) 0.0382, wR2 ) 0.1045) and 2978 independent reflections with I > 2σ(1) (R1 ) 0.0448, wR2 ) 0.1119). (8) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467. (9) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of Go¨ttingen, 1997. (10) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838. (11) Davis, R. E.; Bernstein, J. Trans. Am. Crystallogr. Assoc. 1998, 33, 7-21. (12) Fei, Z.; Scopelliti, R.; Dyson, P. J. Dalton Trans. 2003, 2772-2779. (13) Burrows, A. D.; Mahon, M. F.; Varrone, M. Inorg. Chim. Acta 2003, 350, 152-162. (14) Huang, A.; Marcone, J. E.; Mason, K. L.; Marshall, W. J.; Maloy, K. G.; Serron, S.; Nolan, S. P. Organometallics 1997, 16, 33773380. (15) Ku¨hl, O.; Lo¨nnecke, P. Inorg. Chem. 2002, 41, 4315-4317. (16) Ku¨hl, O. Dalton Trans. 2003, 949-952. (17) Crystallographic data for Z-2a monoxide: C29H26N2O1‚5P2S, M ) 520.52, monoclinic, space group P21/n, a ) 984.97(8) pm, b ) 1790.16(15) pm, c ) 1453.51(12) pm, β ) 90.3330(10)°, V ) 2.5629(4) nm3, Z ) 4, µ ) 0.279 mm-1, T ) 188(2) K, 14662 reflections (Rint ) 0.0292, R1 ) 0.0439, wR2 ) 0.1140) and 5212 independent reflections with I > 2σ(1) (R1 ) 0.0598, wR2 ) 0.1228). The asymmetric unit contains a half occupied disordered THF solvent molecule and refined to a split occupancy of 0.74/0.26.
CG050603S
