Structural Similarities and Differences between N-Phenylureas and N

Jul 28, 1994 - with MMlV'.AF'-tetramethylurea have been determined by single-crystal X-ray diffraction. Both structures are stabilized by formation of...
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8888

J. Phys. Chem. 1995, 99, 8888-8895

Structural Similarities and Differences between N-Phenylureas and N-Phenylthioureas Krzysztof Woinisk,*tt Iwona Wawer,+and Dieter StrohF Chemistry Department, Warsaw University, ul. Pasteura I , 02 093 Warszawa, Poland, and Chemistry Department, Martin Luter Universitat, 06099 Halle(Saale), Weinbergweg 16, Germany Received: July 28, 1994; In Final Form: December 30, 1994@

Crystal structures of N-phenyl-N'-propylthiourea and the complex of N,N-bis(4,4'-difluoro-2-biphenyl)urea with N,N,N,N-tetramethylurea have been determined by single-crystal X-ray diffraction. Both structures are stabilized by formation of intra- and intermolecular hydrogen bonds. The first compound adopts an antisyn conformation whereas the second one has a syn-syn conformation. In both structures the nearest environment for the N atoms is planar. Structural similarities and differences between N-phenylureas and N-phenylthioureas are discussed together with their average geometry and relationships between changes of the structural parameters of these fragments. Hydrogen bonding and mesomerism involving ionic structures seem to be the dominant effects in changing the geometrical parameters of ureas and thioureas. The differences in the electronegativities of S and 0 atoms are responsible for a different mobility of x-electrons in the examined fragments and in consequence for the differences in the variation of structural parameters.

Introduction

Urea is one of the most important compounds in organic chemistry and biology. It is the final product synthesized in the urea cycle. Urea and its derivatives are used for the production of resins and as fertilizers. In general, the structures of N-phenylureas and N-phenylthioureas are good model compounds to investigate a wide variety of effects, e.g. their biological activity, possible internal rotation around the C-N bond, changes in conformation, and inter- and intramolecular hydrogen bonding. The use of ureido functions to create /3-pleated sheet models for conformational analysis in peptides] and the development of s u p r a m ~ l e c u l e s(hydrogen~~~ bonded donor-acceptor systems) make these compounds especially attractive to study. N-Phenylureas are also considered to be an activator as well as inhibitor of trypsin-catalyzed hydr~lysis.~ In the N-substituted N-aryl(thio)urea the substituents can adopt different conformations with respect to the central C=O (or C=S) bond (Figure 1). The preferred conformations were investigated using 'H,I3C, and I5N NMR spectroscopy in s ~ l u t i o n ~as- ~ well as in the solid state.* Molecular mechanics calculations were used to predict the stability of conformer^.^ It was concluded that among the four possible conformations only the anti-anti conformation should be destabilized due to steric reasons. In order to estimate the favored conformation in the solid state as well as to rationalize the results of solidstate NMR measurements, we have determined the structures of the complex of N,N'-bis(4,4'-difluoro-2-biphenyl)urea with N,N,","-tetramethylurea (hereafter abbreviated TMU x BBUFigure 2a) and N-phenyl-N'-propylthiourea(hereafter abbreviated NPPU-Figure 2b). The structures of other N-phenylurea and N-phenylthiourea derivatives can be found in the Cambridge Structural Database9 (CSD). It contains 23 N-phenylureas and 8 N-phenylthioureas refined with an R-factor less than 0.075. We shall use them as reference data.

* Author to whom correspondence should be addressed at Department

of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, U.K. Telephone: +44(1223) 336513. Fax: +44(1223) 336362.

E-mail: kwl [email protected],ac.uk. ' Warsaw University. Martin Luter Universitat. Abstract published in Advance ACS Abstracts, May I, 1995. @

H

H Ph

H

4

H N R anti-syn

H PhM

syn-syn

R

,'

H

H

0 or S syn-anti

.gars

Ph

@H

anti-anti

Figure 1. Possible conformations of N-phenyl-N-alkyl(thi0)ureas.

The aim of this paper is not only to present new structural data but also to identify and analyze what factors affect the geometry of basic urea and thiourea fragments of N-aryl derivatives, to examine the influence of hydrogen bonding on the conformations of these compounds and the variation of the basic N-CO(S)-N fragment, and to find out possible relationships between the structural parameters of these systems. Correlation analysis will be used to achieve all these aims. Experimental Section X-ray Diffraction. The X-ray measurements were made on a KUMA diffractometer with graphite-monochromatedCu K a radiation. The data were collected at room temperature using the w - 28 scan techniques. The intensity of the intensity control reflections for both compounds varied by less than 5%. A linear correction factor was applied to account for this effect. The data were also corrected for Lorentz and polarization effects. All structures were solved by direct methodsI0 and refined using SHELXL." The refinement was based on p for all reflections, except those with very negative p. The weighted R-factors wR and all goodness of fit S-values are based on p. Conventional R-factors are based on F with F set to zero for negative P . The observed criterion of F > 216(F) is used only for calculating R-factors and is not relevant to the choice of reflections for refinement. R-factors based on are statistically about twice as large as those based on F, and R-factors based on all the data will be even larger. Scattering factors and absorption coefficients were taken from the International Tables for crystallography'* (Tables 6.1.1.4 and 4.2.4.2, respectively).

0022-365419512099-8888$09.00/0 0 1995 American Chemical Society

J. Phys. Chem., Vol. 99, No. 21, 1995 8889

N-Phenylureas and N-Phenylthioureas

TABLE 1: Crystal Data and Structure Refinement Details for TMUxBBU and NPPU identification code empirical formula formula weight temperature (K) color wavelength (8,) crystal system space group unit cell dimensions

H(91

HilOll

volume (A31 Z density (calculated) (mg/m3) absorption coefficient (mm-I) F(0W crystal size (mm) 8 range for data collection (deg) index ranges reflections collected independent reflections refinement method datdrestraintdparameters final R indices (I > 2u4 R indices (all data)

Figure 2. Anisotropic displacement ellipsoids and the labeling of atoms for (a, top) TMUxBBU and (b, bottom) NPPU.

All positions of hydrogen atoms were obtained from differential electron density maps. All esd's were estimated using the full covariance matrix. The cell esd's were taken into account individually in the estimation of esd's in distances, angles, and torsion angles. Basic information conceming experimental details is given in Table 1. Full positional parameters are given in Tables 2 and 3. A full list of structural parameters (bond lengths and valence angles), thermal parameters, and tables of calculated structural factors are deposited as supplementary material.

Results and Discussion TMU x BBU was synthesized from 4,4'-difluoro-2-biphenylamine and dimethyl isocyanate in order to obtain an unsymmetrically substituted compound. NPPU was synthesized from propyl isothiocyanate and aniline. Both compounds were crystallized from an ethanol solution by slow evaporation. They were grown as well-formed colorless cubes. The overall view and labelling of atoms for both structures are shown in Figure 2. Selected bond lengths and angles of basic urea and thiourea systems as well as the shortest interatomic nonbonded distances are given in Tables 4 and 5, respectively. TMU x BBU. The complex of TMU x BBU is symmetric and the 0(1), C(13), 0(2), and C(16) atoms fit on the 2-fold axis. As a result only half of the total complex is independent. Both phenyl rings of the biphenyl are almost planar, but the first one (connected to a N atom) deviates more from planarity than the other one, the largest deviation being 0.02 8, for the C(2) atom.

goodness-of-fit on Fz extinction coefficient largest diff. peak and hole (e

Th4UxBBU

NPPU

293 colorless 1.541 78 monoclinic c2/c a = 13.612(1) 8, 6 = 10.963(1) 8, c = 18.470(1) 8, p = 97.00(1)' 2735.7(4) 8 1.342

293 colorless 1.541 78 monoclinic P2,lc a = 7.866(1) 8, 6 = 11.624(1)8, c = 11.345(2)8, p = 92.77(1)" 1036.2(2) 4 1.232

0.878

2.402

1152 408 0.35 x 0.25 x 0.30 0.30 x 0.20 x 0.20 4.5-60 4.5-60 -15 5 h 5 14 O5krll O S I S 19 1949 1878 [R(int) = 0.03011 full-matrix least-squares on Fz 1840/0/240 R1 = 0.045, R,2 = 0.126 R1 = 0.047, R,2 = 0.146 1.073 0.026(2) 0.145 and -0.145

-85h58 O5k512 0 5 I 5 12 1288 1218 [R(int) = 0.07751 full-matrix least-squares on Fz 1218/0/167 R1 = 0.079, R,2 = 0.221 R1 = 0.079, R,2 = 0.221 1.061 0.01 l(4) 0.701 and -0.604

This is due to the steric interactions of two bulky substituents (the phenyl ring and the urea fragment) substituted in the ortho position. The second phenyl ring of the biphenyl is rotated 59.4( 1)" with respect to the best plane of the first one. This is because of weak intermolecular Caw *Fhydrogen type interactions. The network of hydrogen bonds in TMUxBBU is very uncommon for the N-phenylurea derivatives, and we will describe it in detail. The shortest H. *F(or 0) contacts are as follow^: H(9). .0(l), 0.5 X, y - 0.5, z = 2.92 A; H(11). * * F(I), X , y - 1, z = 2.64 A; and H(12).**F(l), 1 - X, -y, 1 z = 2.44 8, (Table 4). These interactions are illustrated in Figure 3. The 3D arrangement of the BBU moiety makes an ideal cage for a smaller guest molecule, Le. N,N,N"-tetramethylurea (Figure 4). The TMU moiety is also symmetric with the C=O group located on the 2-fold axis. Owing to that, a three-centered hydrogen bond is formed between N( 1)-H( 1N) (and symmetryrelated atoms) and O(2) from the TMU moiety. The structural parameters of this bond are as follows: the N(1)...0(2) distance equals 2.812(2) A, the N(l)H(lN) distance equals 0.85(2) A, the H(lN)*.0(2) distance equals 2.02(2) A, and the N(1)H(lN)0(2) angle equals 155(1)O. A similar type of hydrogen bond is often encountered in N-phenylureas, but usually hydrogen bonding is slightly asymmetric in this class of compounds and formed between the same molecular fragments shifted in the crystal lattice or related through the inversion center. In the TMUxBBU complex this is excluded because of the small size of the cage. A characteristic feature of packing of TMUxBBU molecules in the crystal lattice is stacking of C( l>C(2)C(3)C(4)C(S)C(6)rings of antiparallel moieties (Figure

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Woiniak et al.

8890 J. Phys. Chem., Vol. 99, No. 21, 1995 TABLE 2: Atomic Coordinates ( x 10") and Equivalent Isotropic Displacement Parameters (hiz x 103) for TMUxBBU atom

X

Y

Z

5850(1) 7447( 1) 5000 5000 5388(1) 4439(1) 5764(1) 6554(2) 6420(2) 5975(1) 5636(1) 6517(1) 7 128(2) 7446(2) 7 140(2) 6220(2) 473 l(3) 5000 3260(40) 4149(35) 3654(23) 4626(38) 42 13(31) 5439(39) 537 1(13) 6887( 16) 6664( 18) 5300( 19) 7333( 16) 7839(20) 6350(22) 5756( 17)

2450(1) -5903( 1) 1027(2) -3 107(2) -765(1) -4859(2) -314(1) -698(2) 498(2) 1257(2) 894(2) -2399(2) -265 l(2) -3837(2) -4740(2) -3364(2) -6053(3) -4230(2) -47 14(44) -3850(36) -3388(32) -5819(42) -6641(37) -6 199(41) -1538(21) - 1278(20) 785(22) 1445(24) -2015(21) -395 l(22) -5245(27) -3201 (22)

5307(1) 6647( 1) 7500 7500 6982(1) 697(1) 6359(1) 5289(1) 5059(1) 5503(1) 6136( 1) 6156( 1) 6793(1) 6962( 1) 6490( 1) 5697(1) 6708(3) 7500 62 16(29) 6018(26) 6653( 18) 6128(31) 6808( 19) 6902(27) 7033(11) 5004(13) 4639(15) 6413( 15) 7 1 17(13) 739 1( 16) 5558(17) 5247(13)

WeqY

U(eq) for nonhydrogen atoms is defined as one-third of the trace of the orthogonalized Uij tensor.

TABLE 3: Atomic Coordinates ( x 10") and Equivalent Isotropic Displacement Parameters (Azx 103) for NPPU 7855( 1) 10482(5) 9065(6) 11719(6) 13411(6) 14597(8) 14102(7) 12392(7) 11196(6) 9212(5) 7831(8) 7310(7) 7590(8) 10792(64) 9747(54) 13926(63) 1554l(85) 14846(66) 12135(76) 9999(65) 854 l(90) 6986(60) 6574(75) 7 115(87) 8142(89)

6054(1) 6562(3) 8102(4) 7209(4) 6984(4) 7568(5) 8381(5) 8598(5) 8015(4) 6967(3) 8737(5) 9836(4) 10207(6) 5887(50) 8466(38) 6482(46) 7470(45) 88 15(44) 9241(57) 8 150(40) 8817(58) 8233(43) 10267(53) 10905(62) 9714(70)

428( 1) -823(3) -1 15(4) - 1433(3) - 1215(4) - 1844(5) -2673(4) -2888(4) -2282(3) -202(3) 579(5) 380) -1004(6) -839(44) -352(35) -592(46) -1735(52) -3120(49) -3463(54) -2502(40) 1222(65) 649(36) 425(50) - 1384(61) - 1629(64)

U(eq) for nonhydrogen atoms is defined as one-third of the trace of the orthogonalized Uu tensor.

4). Such a packing helps to form intermolecular CH.**F hydrogen bonds. These hydrogen bonds form infinite chains of molecules. Such an infinite chain of hydrogen-bonded molecules is shown in Figure 3.

TABLE 4: Selected Bond Lengths (A), Valence Angles (deg), Torsion Angles, Geometry of Hydrogen Bonding, and the Shortest Nonbonded Distances for TMU x BBU O( 1)-C( 13) 0(2)-C(16) N( 1)-C( 13) N( l)-C( 1) N(1)-H(1N) N(2) -C( 16) N(2)-C(14) N(2)-C( 15) C(l)-C(6) C(l)-C(2) C(2)-C(3)

Bond Lengths 1.211(3) C(2)-C(7) 1.231(3) C(3)-C(4) 1.373(2) C(4)-C(5) 1.405(2) C(5)-C(6) 0.85(2) C(7)-C(8) 1.353(2) C(7)-C(12) 1.446(4) C(8)-C(9) 1.467(3) C(9)-C(10) 1.391(2) C(10)-C(11) 1.408(3) C(l1)-C(12) 1.386(3)

1.487(2) 1.383(3) 1.361(3) 1.366(3) 1.384(3) 1.384(3) 1.394(3) 1.350(4) 1.359(3) 1.386(3)

symmetry transformations C(13)-N(l)-C( 1) C(l3)-N(l)-H(lN) C( 1)-N( 1)-H( 1N) C( 16)-N(2)-C( 14) C( 16)-N(2)-C( 15) C( 14)-N(2)-C( 15) C(6)-C( 1)-N(l) C(6)-C(l)-C(2) N( 1)-C(1)-C(2) C(3)-C(2)-C( 1) C(3)-C(2)-C(7) C( 1)-c(2)-c(7) C(4)-C(3)-C(2) C(5)-C(4)-C(3) C(4)-C(5)-C(6) C(5)-C(6)-C( 1) C(8)-C(7)-C( 12) C(8)-C(7)-C(2) C( 12)-c(7)-c(2) C(7)-C(8)-C(9) C( 10)-C( 9)-C( 8) c(9)-c(lo)-c(ll) C(10)-C(11)-C(12) C(7)-C(12)-C(ll) O(l)-C(13)-N(l) N( 1)-C( 13)-N( 1)#1 O(2)-C( 16)-N(2) N(2)-C( 16)-N(2)#1

Bond Angles 125.9(2) 116.8(13) 117.2(13) 117.7(2) 122.8(3) 114.9(3) 122.2(2) 119.1(2) 118.7(2) 118.4(2) 118.0(2) 123.6(2) 122.8(2) 116.5(2) 124.0(2) 119.2(2) 117.9(2) 121.2(2) i20.6i2j 12 1.0(2) 118.4(2) 123.1(2) 118.1(2) 121.5(2) 123.32(10) 113.4(2) 1 = -X 120.66(12) 118.7(2) 1 = -X

Selected Torsion Angles - 10.62(24) C ( 1 3 N 1)C(1 17 1.40(13) C( 13" 1IC( 1)C(2) N( 1)C(1)C(2)CV) -7.73( 24) C(3)C(2)C(7)C(8) 117.5l(20) C(1)C(2)C(7)C(8) -60.09(24) C(3)C(2)C(7)C(12) -56.86(23) 125.53(19) C(1)C(2)C(7)C(12) C(l)N(1)C(13)0(1) -5.20( 17) C(l)N(l)C(13)N(l-$l) 174.80(18) 1 = -x C( 14)N(2)C(16)0(2) -6.48(23) C( 15)N(2)C(16)0(2) 148.32(23) 173.52(23) 1 = -X C(l4)N(2)C(lS)N(2-$1) C(lS)N(2)C(lS)N(2-$1) -31.68(23) 1 = -X

+ 1, y, - Z + + 1, y, -z +

312

312

+ l , y , -z + ' 1 2

+ 1, y, -Z -k + l , y , -Z +

'12

'12

Geometry of NH.. -0 Hydrogen Bonding 0(2)..-N(l) 2.812(2) 0(2).**H(lN) 2.02(2) N(1)WlN) 0.85(2) N( 1)H( W O ( 2 ) 155(2) Selected Shortest Nonbonded Distances 0.5 x , -0.5 y, z 2.44 F( 1).* *H(12) 1 - x , y , 1.5 - z 2.64 F( 1). * *H(11) 1 - x , -y. 1 - z 2.89 F( 1). * *H(142) F(2).*.H(8) 2.57 1 - x , y, 1.5 - z 1 - x , y, 1.5 - z F(2). * *H(4) 2.80 1.5 - X, -0.5 - y, 1 - z 2.28 O(2). * -H(142) 1.5 - x , -0.5 y, 1.5 - z 2.28 O(2). *-H(143) H( 153). * "(2) 2.54 +x, 1 + y, z 2.92 1 - x , -y, 1 - z H(9). * -0( 1)

+

+

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N-Phenylureas and N-Phenylthioureas

J. Phys. Chem., Vol. 99, No. 21, 1995 8891

TABLE 5: Bond Lengths (A) and Angles (deg) for NPPU Bond Lengths S(l)-C(7) ~(1)-~(7) N( 1)-C( 1) N(1)-H(1N) "-C(7) W)-C(8) C(l)-C(2) C(1)-C(6) c(2)-c(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C@)-C(9) C(9)-C( 10)

1.688(4) 1.336(6) 1.434(6) 0.82(6) 1.328(6) 1.477(6) 1.367(7) 1.392(6) 1.38l(8) 1.377(8) 1.379(8) 1.37l(7) 1.467(8) 1.288(8) Valence Angles

C(7)-N( 1)-C(l) C(7)-N(l)-H( IN) C( 1)-N( 1)-H( 1N) C(7)-N(2)-C(8) C(7)-N(2)-H(2N) C(8)-N(2)-H(2N) C(2)-C( 1)-C(6) C(2)-C( 1)-N( 1) C(6)-C( 1)-N( 1) C(l)-C(2)-C(3) C(4)-C(3)-C(2) C(3)-C(4)-C(5) C(6)-C(5)-C(4) C(5)-C(6)-C( 1) N(2)-C(7)-N( 1) N(2)-C(7)-S(1) N( l)-C(7)-S(l) C(9)-C(8)-N(2) C ( 10)-C(9)-C(8)

127.7(4) 125(4) 106(4) 126.7(4) 118 114(3) 120.4(4) 119.3(4) 120.2(4) 119.3(5) 121.0(5) 119.2(5) 120.5(5) 119.5(4) 117.3(4) 122.3(3) 120.4(3) 112.9(4) 128.2(5)

Selected Torsion Angles C(7)-N( l)-C(l)-C(2) C(7)-N( 1)-C( 1)-C(6) C(8)-N(2)-C(7)-N( 1) C(8)-N(2)-C(7)-S( 1) C( 1)-N( l)-C(7)-N(2) C(1)-N( l)-C(7)-S(l) C(7)-N(2)-C(8)-C(9) N(2)-C(8)-C(9)-C( 10)

127.44(0.47) -55.98(58) -176.15(47) 4.35(64) -0.90(5 8) 178.60(32) -147.72(49) 11.17(0.90)

Geometry of Hydrogen Bonding 2.53(6) S(l)-H(lN)-$l = 2 - X, 1 - y , -Z S( 1). -*N(1)-$1 3.333(4) N(1)-H(1N) 0.82(6) S(1)-H( lN)-$l -N( 1)-$1 166(5) Both urea fragments are planar within an experimental error. The angle between the best planes of these fragments is equal to 67.8(1)" (Figure 3b). The best plane of the second urea fragment (N(2), C(16), 0(2), and N(2)-$1) is almost parallel to the best plane of the second phenyl ring of the biphenyl (the angle between these planes is equal only to 9.4( l y ) . Considering the shortest interatomic contacts between the two parts of the complex (Table 3), it may be concluded that the whole structure is also stabilized by the hydrogen bond like interactions between some hydrogens from the methyl groups and F (and C) atoms (F(2), C(10), C(11), C(12)) from the parent BBU molecule as well as from the symmetry-related one (1 x, f z , 1 - y ; for F(1)). Discussing the bond lengths, we will use different types of reference bonds: a typical single bond, a typical double bond, and also the so-called optimal bonds.I3 The physical meaning of the optimal value of a given bond is that the energy of extension of a typical double bond to the optimal value is equal to the energy of compression of a typical single bond to the optimal one. The optimal CCoPt,CNoPt,COOP', and CSoPtbond lengths are 1.388, 1.334, 1.265, and 1.677 A, respectively. The

+

comparison of bond lengths with their optimal values may be a measure of delocalization of n-electrons over a given molecular fragment. An HOMAI3 index may be used to estimate the aromatic character of the urea and thiourea fragments. This index is equal to zero for a typical resonance structure and to 1 for the system with all bonds optimal. The HOMA value for the BBU moiety is 0.40. As far as the C(13)=0(1) and C(16)=0(2) bonds are concerned, it may be said that their lengths are in the range of typical double bonds ((C#-NH)2-C=O = 1.241 &.I4 C( 16)=02 is 0.020 A longer than C( 13)=0( 1) due to the threecentered hydrogen bond formed. The values of the CO bonds are quite different from the optimal COOP'. The CN bond in the urea fragment is 0.026 longer than typical CN bonds in ureas ((C#-NH)2-C=0 = 1.347 and far shorter than a typical single CN bond. This suggests that the N-CO-N fragment is much delocalized. The delocalization of these bonds may be rationalized when we assume that the lone electron pairs of N atoms must interact strongly with n-electrons of the CO bond. In the case of the N(l) atom, additional interaction with the n-electrons of the phenyl ring diminishes the interactions of this atom with the urea fragment. The interactions within the urea fragment may be described using three resonance structures (Figure 5). As a result of the conjugation of the lone electron pairs of N atoms with the n-electrons of the C=O bond, an equilization of CN bonds may be expected and also partial charges at the C(7), N(1), N(2), and 0(1) atoms should appear. The planarity of the urea fragmentis very helpful when delocalized molecular orbitals are considered. Using the HOSE'5.'6 model, it is possible to calculate contributions of particular canonical structures in description of a real molecule. The contributions (c) of the structures (A, B, and C) defined in Figure 5 are CA = 68.3%, CB = 15.8%, and cc = 15.8%. The contribution of the neutral resonance structure is dominant in the description of the real geometry of the N-phenylurea fragment. The bond lengths in both phenyl rings do not altemate. C(4)C(5), C(5)C(6), C(lO)C(l l), and C(9)C(10) are significantly shorter than the other aromatic bonds. This is because of the substituent effect of F atoms attached to C(5) and C(10). Another consequence of this is an increase of the C(4)C(5)C(6) and C(9)C(lO)C(ll) (ipso) angles up to 124.0(2)" and 123.1(2)", respectively. The changes of valence angles in the biphenyl rings can be described in terms of the additivity rules." NPPU. The NPPU molecule consists of three planar parts: phenyl ring, thiourea fragment, and propyl group. The molecule as a whole is not planar. The angle between the best planes of the thiourea fragment and the phenyl group is 54.7(6)". The other angles between the best planes of the propyl group and the thiourea fragment and the phenyl group are equal to 41.3(6)" and 56.7(7)", respectively. The geometry of the phenyl ring is not significantly different from that of the unsubstituted benzene and only slightly modified due to the influence of the substituent. The deviation may be explained by the Walsh-Bent rule.I8 Also the geometry of the allyl group is typical. The conformation of the thiourea fragment is anti-syn. This enables an N-H.. .S hydrogen bond formation between the parent molecule and a symmetry-related one (2 - x, 1 - y , -z). As a result a characteristic dimer (Figure 6) is formed. The geometry of the hydrogen bond is as follows: S(l).*.N(l) = 3.333(4) A, S(l).*.H(lN) = 2.53(6) A, N(l)H(lN) = 0.82(6) A, and S(l)H(lN)N(l) = 166(5)". There is no significant difference between CN bond lengths [N(1)C(7) =

Woiniak et al.

8892 J. Phys. Chem., Vol. 99, No. 21, 1995

Y 0

Figure 3. Chains of C-H***Fhydrogen bonds in the TMUxBBU complex: (a, top) view perpendicular to the second biphenyl ring; (b, bottom) view along the y-axis.

1.336(6) A and N(2)C(7) = 1.328(6) A]. Both of them seem to be equal almost to the optimal13 value. The HOMA index for this structure is 0.96. It may be concluded that the equalization of the CN bonds suggests an important role of resonance in the description of changes in the thiourea fragment. This is because of a conjugation of lone electron pairs of N atoms with a double C-S bond. This conjugation may be illustrated by three resonance structures (Figure 5) similar to those for N-phenylureas. The contributions of particular canonical structures are 27.9%, 38.8%, and 33.2% for structures A, B, and C. It seems that the contribution of ionic structures is especially important to describe the real geometry. The differences in the contributions of resonance structures may be associated with different electr~negativities'~ of 0 and S atoms (3.5 and 2.5 for 0 and S, respectively). Electron-donating properties of the allyl group increase the contribution of structure B with respect to structure C. Average Geometry. The average geometry of N-phenylurea and N-phenylthiourea fragments is shown in Figure 7. This geometry was calculated using the structures found in the CSD and those presented in this paper. The REFCODEs and the tables of structural parameters are given in the supplementary material. In usually planar structures of N-phenylurea deriva-

tives, there is no significant difference between the average N(l)C(7) and N(2)C(7) bond lengths. A small difference between these bonds seems to exist in the N-phenylthioureas (0.014 A). As far as the valence angles are concerned the N( 1)C(7)0(1) (and N(l)C(7)S( 1)) angle is larger than the N(2)C(7)0( 1) (N(2)C(7)S(2)) angle, but there is no significant difference between these angles in both classes of compounds. The average CN bond lengths in N-phenyl derivatives are longer than those in the thioureas, but the values of the CN bond lengths in thioureas are closer to the optimal one. There are significant differences between C(2)C(1)N(1)C(7) torsions in N-phenylureas and N-phenylthioureas. On average, the urea derivatives are more planar (the average C(2)C(1)N(l)C(7) is 16.6'), whereas the same torsion for the thioureas is equal to 49.9'. The average angles between the best planes of the phenyl ring and the (thio)urea fragments are equal to 18.5' in the case of the N-phenylureas and 48.0" for the thioureas. Linear Correlation. Correlation analysis may be a very useful tool. to investigate hidden relations between different molecular parameters.20 All linear relationships discussed below are significant at the significance level a = 0.05 unless stated otherwise. Despite rather poor precision of H atom positions,

N-Phenylureas and N-Phenylthioureas

J. Phys. Chem., Vol. 99, No. 21, 1995 8893

Figure 4. Stacking of phenyl rings and projection of the cage formed by the BBU moiety.

H 10.91

Figure 5. Resonance structures of basic urea and thiourea fragments.

I I.370 H

Figure 7. Average geometry of the (a, top) N-phenylurea fragment and (b, bottom) N-phenylthiourea fragment.

Figure 6. NPPU dimer and geometry of the hydrogen bond.

one of the strongest linear structural relationships exists between (a) N(l)H(lN) and N(l)C(l) (correlation coefficient r = 0.71 for 25 data points for N-phenylureas but only 0.21 for 9 data points for thioureas), (b) C(7)N(2) and N(l)C(7)0( 1 with correlation coefficient r = 0.83 for the urea and thiourea derivatives, and (c) C(7)0(1) and C(7)N(2) (r = -0.89 for thioureas). Additionally, C(7)0( 1) correlates very well with the difference C(7)N(2)-C(7)N(l) (I = -0.82 for thioureas). This means that the rotation of the thiourea fragment with respect to the best plane of the phenyl ring makes the interactions of this fragment with the phenyl ring hardly possible. The C(l)N(l) and C(7)N(1) bonds are under the influence of variation of H(1N) (even though the coordinates of this atom are less precise and accurate than in the case of any nonhydrogen atom). The variation of C(7)N(2) is caused by the

changes of C(7)0(1) bond length. It seems that the influence of the H(1N) position on C(7)N(1) is more important (larger correlation coefficient) than the influence of C(7)0( 1) changes on this bond. The values of the HOMA index have been calculated to estimate the aromatic character of the N-phenylurea and N-phenylthiourea fragments. These values for the N-phenylurea derivatives are in the range 0.07-0.84, whereas for the thioureas they are in the range 0.75-0.97. The variation of this index may be correlated with structural parameters of these fragments as well as with the properties of hydrogen bonds. It appears that in the case of the thiourea derivatives there is a strong correlation between the HOMA index and N( 1)H(1N) (correlation coefficient r = -0.94; Figure 8). A similar correlation for the N-phenylureas is not significant at the significance level a = 0.05, but for these compounds there exists a far stronger (than for thioureas) relationship between the values of the HOMA index and C(l)N(l) bond length. This confirms the existence of conjugation between the urea fragment and the

Woiniak et al.

8894 J. Phys. Chem., Vol. 99, No. 21, I995 0.ssL.

HOMA

166

167

168

169

170'

C(7)=S(1) (')''.O

Figure 10. Correlation between S(1) .-Ndistance and C(7)S(1) bond length. Confidence interval for the slope and the tolerance interval at the significance level a = 0.05. 74

a4

94

104

N(I)-H(lN)

114 (~0.01)

Figure 8. Linear correlation between the HOMA index and the N(1)H( 1N) distance for N-phenylthioureas. Confidence interval for the slope and the tolerance interval at the significance level a = 0.05.

H 6.92 I

H

0.24' I

Figure 9. "Maps" of correlation coefficients between (a, top) HOMA values and the bond lengths of N-phenylthioureas (index s) and N-phenylureas (without any index) and (b, bottom) donor-acceptor distance and the bond lengths of both fragments.

phenyl ring in the N-phenylureas. The correlation coefficients between the values of the HOMA index and the bond lengths of both urea and thiourea fragments are shown in Figure 9a. Such a "map" of correlation coefficients clearly shows (a) the differences in conjugation of the urea and thiourea fragments with the phenyl ring, (b) the different role of NH and CO (or CS) groups in both classes of compounds, and (c) the different influence of these two groups on the N(l)C(7)bond length. To discuss the effect of the variation of the donor-acceptor distance on the changes in both the urea and thiourea fragments, the shortest donor-acceptor distances have been calculated. There is a very strong correlation between this parameter and the C(7)S(1) bond length (Figure 10;r = - O M ) , but in the case of the urea derivatives the equivalent relationship is not important [more significant relations exist between C(7)0(1) and N(1)H(1N)or C(1)N(l)]. A very characteristic scatter of the data points on this plot seems to be small for the short S***N distances (for strong hydrogen bonds) and larger for the longer S.*.N distances (for weak hydrogen bonds). Also for the donor-acceptor distance its correlation coefficients show some regular trends (Figure 9b). The trends seem to be opposite in both classes of compounds. If a correlation with a given structural parameters in the thiourea derivatives is strong, the

equivalent correlation in the urea derivatives is rather weak. In general, the absolute values of correlation coefficients in ureas seem to be smaller than those in thioureas. This may reflectthe fact that the thiourea fragment seems to be more sensitive to the outside stimulation than the urea one. On the other hand the reaction of the urea fragment for a perturbation (e.g. hydrogen bonding) seems to be more complex than in the case of the thiourea group and pointed out rather outside of the fragment. This is also confirmed by values of partial correlation coefficientscalculated for bond lengths in these fragments. There is no partial correlation between bond lengths in the urea derivatives and quite a strong one in thioureas. There is a very interesting correlation between contributions of different canonical structures (see Figure 5) and other structural parameters. The contribution of structure A in thioureas correlates well with the contribution of structure B ( r = -0.85) but not with that of a structure C. A more symmetrical situation is found in the urea derivatives. In this case the contribution of structure A correlates well with the contributions of the other structures (correlation coefficients -0.91 and -0.86 for the correlation with structures C and B, respectively). It also correlates with the C(7)0(1)bond length ( r = -0.69). Such a correlation is also present in the thioureas ( r = -0.88). In general, the contributions of resonance structures correlate well with the variation of angles in the N-phenylthioureas ( I ' S > 0.8) but the equivalent relationships in N-phenylureas are not significant.

Conclusions N-Phenylureas and N-phenylthioureas differ significantly not only in their conformations but also in the amount of delocalization of the lone electron pairs of N atoms over the basic urea and thiourea systems. Urea and thiourea fragments also differ in the degree of conjugation with the phenyl ring. The conjugation is weaker for the thiourea fragment than for the other one. Mesomerism involving ionic structures seems to be useful in the description of the geometry of these fragments. The contribution of ionic structures is larger for N-phenylthioureas than for N-phenylureas. There is a correlation between structural parameters of the discussed systems and contributions of resonance structures. The properties of hydrogen bonds formed play an important role in changing the geometry of N-phenylurea and N-phenylthiourea fragments. The differences in the electronegativities of S and 0 atoms are responsible for a different mobility of n-electrons in the examined fragments and in consequence for the differences in the variation of structural parameters.

J. Phys. Chem., Vol. 99, No. 21, 1995 8895

N-Phenylureas and N-Phenylthioureas Acknowledgment. We thank the Department of Chemistry, Warsaw University, for financial support in the frame of Project 12-501/BWNII-936/17/93. Supplementary Material Available: Tables of bond lengths, bond angles, anisotropic displacement parameters, and structural parameters (8 pages). Tables of observed and calculated structure factors (8 pages). Ordering information is given on any current masthead page. References and Notes (1) Kemp, D. S . ; Bowen, B. R.; Muendel, C. C. J. J. Org. Chem. 1990, 55, 4650. (2) Lehn, J. M.; Pascal, M.; Decian, D.; Fisher, J. J . Chem. Soc., Chem.

Commun. 1990, 479.

(3) Brienne, M. J.; Gobard, J.; Lehn, J. M.; Stibor, I. J . Chem. Soc., Chem. Commun. 1990, 1868. (4) Kashino, S.; Haisa, M. Acta Crystallogr. 1977, €333, 855. ( 5 ) Wawer, I.; Koleva, V. Magn. Res. Chem. 1993, 31, 375. (6) Sudha, G. L. V.; Sathyanarayama, D. N. J . Mol. Struct. 1984, 125, 89.

(7) Jean-Claude, B. J.; Just, G. Magn. Res. Chem. 1992, 30, 571.

(8) Kolodziejski, W.; Wawer, I.; Wozniak, K.; Klinowski, J. J . Phys. Chem. 1993, 97, 12147.

(9) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, 0.;Kennard, 0.; Macrae, C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson, D. G. J . Chem. In$ Comput. Sci. 1991, 31, 187. (10) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (11) Sheldrick, G. M. J . Appl. Crystallogr., in preparation. (12) Intemational Tables for Crystallography; Kluwer Academic Publishers: Dordrecht, 1992; Vol. C. (13) Krygowski, T. M. J . Chem. In5 Comput. Sci. 1993, 33, 70. (14) Allen, F. H.; Kennard, 0.; Watson, D. G.; Brammer, L.; Orpen, G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1. (15) Krygowski, T. M.; Anulewicz, R.; Kruszewski, J. Acta Crystullogr. 1983, B39, 732. (16) Wozniak, K.; Krygowski, T. M. J . Mol. Struct. 1989, 193, 81. (17) Domenicano, A,; Murray-Rust, P. Tetrahedron Lett. 1979, 2283. (18) Bent, H. A. Chem. Rev. 1961, 61, 275. (19) Pauling, L. The Nature of the Chemical Bond; Come11 University Press: Ithaca, NY, 1960. (20) Krygowski, T. M.; Wozniak, K. Similarity Models: Statistical Tools and Problems in Using Them. In Similarity Models in Organic Chemistry, Biochemistry and Related Fields; Zalewski, R. I., Krygowski, T. M., Shorter, J., Eds.; Elsevier: Amsterdam, 1991. JP941972U