摘要:
1976Complexes of Organoaluminium Compounds. Part 1x2 Crystal andMolecular Structure of Hepta-p3-methylimido-heptakis( methylaluminium)a Cage Compound in the Series of Oligomers [ R1XYR2]n tBy Peter B. Hitchcock, J. David Smith,' and K. Mark Thomas, School of Molecular Sciences, University ofSussex, Brighton BN1 9QJThe compound (MeAINMe), crystallises in space group P2Jc with a = 14.059(7), b = 14.407(8), c = 14.435(8)8, = 93.1 0(5)", and Z = 4 ; the structure was solved by the symbolic addition method and least-squares refine-ment converged at R = 0.119 (1 179 reflections measured by diffractometer). The molecules have a cagestructure with CBU symmetry, in which each aluminium and each nitrogen atom is four-co-ordinate and bound toone methyl group and three atoms within the cage.Mean molecular dimensions are : AI-N 1.91 (4). AI-C 1.98(7),N-C 1.56(6) 8 : AI-N-AI 89(1) or 1 20(1)", N-AI-N 90(2) or 1 IO(2)". C-AI-N angles range from 11 2 to 129',and C-N-AI from 103 to 127". The structure found is one of a related series shown by oligomeric molecules(R1XYR2):(n = 1-8) ; a general discussion of these structures is given.THERE are many well characterised compounds withempirical formulae R1XYR2 in which X and Y are pairsTABLE 1Oligomeric compounds (RlXYRZ), an1 MeHgOSiMe,, PrlHgOPri2 ButBeOBut, MeBeOCHPh,, MeBeOCPh3,0 R1BeOCH,R2(R1 = Et or But, R2 = But or Ph),d MeCdOBut, ButZnOBut,PhZnOCPh,," ArBNAr (Ar = XC,H,, X = 2-Me.4-Me,4-C1, 4-Me0) f3 EtBeOCEt,, BuiBeOBu*,e BuiBeOCH2But,d BuMgOPr',EtZnOCHPh,, PhHgOR2 (R2 = Me or B u ) , ~ Borazenes4 MeBeOR2 (RZ = Me, Et, Pr, But, CH,Ph," or SiMe,g,h),PhBeOMe,C EtMgOR2 (R2 = Pri or But), Pr'MgOPr'MeZnOR2 (R2 = Me,gJ But, or SiMe, "), EtZnOBut,g,kMeCdOR2 (R2 = Me,g Et, Pr, Ph, or SiMeS6), EtBeSR2(R2 = Et, Pri, or But), MeBeSBut,C RMgSBut (R = Me orEt), MeCdSBut,o RBNBu' (R = P h or halogen),'lmPhAlNPh,9," PhAlNAr [Ar = 3-MeC8H,, 4-XC,H4 (X =Me, OMe, C1 or I)]," EtAlNPh,P EtAlNBut,q HAlNBut m-rh'56 MeCdSPr1,e HAlNR28 (R2 = P r i , p , u BuB, Bui, or cyclo-7 RAlNMe (R = Me or Et g ) , EtMgOPr*8 MeZnSPri,~~u PrjMgOR2 (R2 = Me or Et)R1BNR2 9RZnSBut (R = Me.9.' or Et ")C6H11)a Unless stated otherwise, established by cryoscopic measure-ments in benzene.G. E. Coates and G. L. Morgan,Adv.Organometallic Chem., 1970, 9, 195, and extensive refs.therein. d R. A. Anderson and G. E. Coates, J.C.S. Dalton,1974, 1171. * B. J. Wakefield, Adv. Inorg. Chem. and Radio-chem., 1968, 11, 341, which gives full refs. t o original literature.P. I. Paetzold, P. P. Habereder, and R. Mullbauer, J . Organo-metallic Chem., 1967, 7 , 51. 0 Established by X-ray structuredetermination. h D. Mootz, A. Zinnius, and B. Bottcher,Angew. Chem., 1969, 81, 398. 'G. E. Coates, J. A. Heslop,M. E. Redwood, and D. Ridley, J . Chem. SOC. ( A ) , 1968, 1118.Values in the range 7-8.4 were found by cryoscopic measure-ments for those species described as octamers. JH. M. M.Shearer and C. B. Spencer, Chem. Comm., 1966, 194. kY.Matsui, K. Kamiya, M. Nishikawa, and Y .Tomiie, Bull. Chem.SOC. Jafian, 1966,39, 1828. 1 H. S. Turner and R. J. Warne,J . Chem. SOC., 1965, 6421. mEstablished by mass spectro-metric or other spectrometric methods. "Ref. 8. J. IdrisJones and W. S. McDonald, Proc. Chem. SOC., 1962, 366.p J. K. Gilbert and J. D. Smith J . Chem. SOG. ( A ) , 1968, 233.r H. Noth and P. Wolfgardt, personal communica-tion. 8 S . Cucinella, T. Salvatori, C. Busetto, G. Perego, andA. Mazzei, J . Organometallic Chem., 1974, 78, 185. ?3AlNEtand HAlNBut were obtained as mixtures of species withn = 6-9 m. t G. W. Adamson, H. M. M. Shearer, and C. B.Spencer, Acla Cryst., 1966, 21, A135. Ref. 11. G. W.Adamson and H. M. M. Shearer, Chem. Comm., 1969, 897.b Ref. 2.Ref. 6 .of main-group elements, either from Group 2 and Group6, or from Group 3 and Group 5, and R1, RZ are alkyl oraryl groups. A few of these compounds are mono-meric 2 in benzene solution, but the tendency of main-group elements to achieve co-ordination numbers >2results in oligomeric or polymeric structures in mostcases.Some of these (Table 1) are well established byspectroscopic or X-ray methods ; others are inferredrather less certainly, particularly since most of the sub-stances are very sensitive towards hydrolysis, frommolecular-weight measurements in solution. In thispaper, we describe an unusual heptameric species(MeAlNMe), and discuss the relation between its cagestructure and those shown by other oligomeric species(R1XYR2),. A preliminary account has been pub-l i ~ h e d .~EXPERIMENTALN.M.R. Spectra.-The n.m.r. spectrum of MeAlNMe hasbeen reported.* The spectrum of EtAlNMe5 in benzeneshows singlets (relative intensities in parentheses) atz 7.06(1), 7.10(3) and 7.34(3) [N-CH,], triplets centred a tT 8.33(1), 8.41(3), 8.54(3) [Al-CH,CH,], and quartets centreda t T 9.32(1), 9.47(3), and 9.57(3) [Al-CH,CH,]. Except forsmall changes in chemical shifts, the spectra in [2Hs]-toluene show little variation between 60 and -40 O C ,indicating that there is no interchange of alkyl groups onthe n.m.r. time scale.Crystal Data.-(a) (EtAlNMe),. C,lH,,A17N7, M =595.6, orthorhombic, a = 18.50, b = 34.94, c = 10.97 A,2 = 8, Do = 1.12 g ern-,. Space group Pbca from system-atic absences: OK1 for K odd, h01 for 1 odd, hkO for h odd.(b) (MeAlNMe),.C,,H,,Al,N,, M = 497.4, monoclinic,a = 14.059(7), b = 14.407(8), G = 14.435(8) A, @ = 93.10(5)"U = 2 919 fi3, 2 = 4, D, = 1.13 g ~ m - ~ , F(000) = 1064.~(Mo-K,) = 2.7 cm-l. Space group P2,/c, from systematicabsences: OK0 for K odd, and h01 for 1 odd.Crystallographic 1Measurements.White needles ofWeAlNMe],, which reacted rapidly with traces of moist air,were recrystallised from heptane and sealed in thin-walledt No reprints ttvailable.1 Part VIII, A. J. Conway, G. J. Gainsford, R. R. Schrieke,and J. D. Smith, J.C.S. Dalton, 1975, 2499.H. Schmidbaur and F. Schindler, Angew. Chem., 1965, 77,865; G. A. Razuvaev, S. F. Zhil'tsov, Yu. A. Aleksandrov, and0. N. Druzhkov, J . Gen. Chem. U.S.S.R., 1965,35,1164.3 P.B. Hitchcock, G. M. McLaughlin, J. D. Smith, and K. M.Thomas, J.C.S. Chem. Comm., 1973, 934.4 K. J. Alford, K. Gosling, and J. D. Smith, J.C.S. Dalton,1972,2203.K. Gosling, J. D. Smith, and D. H. W. Wharmby, J . Chem.SOC. ( A ) , 1969, 17381434 J.C.S. Daltoncapillaries. Preliminary rotation , Weissenberg, and preces-sion photographs were used to find the space group andinitial cell dimensions, which were later adjusted by theleast-squares refinement of the angular settings of 12 reflec-tions (mean 8 14") determined on a Hilger and Watts Y 290four-circle diffractometer by use of Mo-K, radiation (p filter,A = 0.710 69 A). Intensity measurements were based onan 0-20 scan routine, with checks for instrumental stabilityand crystal alignment every 40 reflections.Of 3 356 crystallographically non-equivalent reflectionshaving 1" < 28 < 42", 1 179 reflections having I , >, 30(1,)were used in the subsequent structure analysis and refine-ment, with programs of ref.6(a) and atomic scatteringfactors from ref. 6(b). Intensities were corrected forLorentz and polarization effects but not for absorption.Structure Solutiort and Re$neunent.-Initial attempts tosolve the structure from a Patterson synthesis were un-successful.Normalised structure factors ]El were calculated frommeasured structure amplitudes and the statistical distribu-tion was found closely t o resemble the theoretical for thecentric case. Reflection phases were generated by asymbolic-addition procedure, using all reflections withTABLE 2Fractional atomic co-ordinates and thermal parameters,with estimated standard deviations in parenthesesAtom xla Y l b Z/G 102UlA2Al(1) 0.271 6(9) -0.159 8(8) 0.221 3(9) 6.8(5)Al(2) 0.239 0(9) -0.029 8(9) 0.090 8(9) 7.6(5)Al(3) 0.404 4(9) -0.029 5(9) 0.192 4(8) 6.5(4)Al(4) 0.305 5(8) -0.010 5(9) 0.395 4(8) 6.4(4)Al(5) 0.103 5(8) -0.013 3(9) 0.269 2(8) 6.0(4)0.264 3(9) 0.147 0(8) 0.234 5(9) 6.2(4) :$! 0.167 l(9) 0.118 4(8) 0.384 2(9) 5.2(4)N(l) 0.357 6(18) -0.087 6(18) 0.301 6(19) 4.7(9)N(2) 0.340 l(17) -0.118 9(16) 0.119 6(18) 3.7(8)N(3) 0.166 9(18) -0.089 O(17) 0.183 2(18) 4.1(8)0.319 l(18) 0.064 4(18) 0.145 9(18) 5.0(8)0.301 4(19) 0.114 8(18) 0.352 l(18) 5.6(9)N(7) 0.135 6(18) 0.114 9(19) 0.250 4(18) 5.6(9)0.257(3) -0.293(2) 0.267 (3) 8.5( 14) Eli! 0.185(3) -0.005(3) -0.027(3) 10.8(16)C(3) 0.543(3) 0.001 (3) 0.193(2) 6.8(12)C(4) 0.358(2) -0.033(2) 0.51 6( 2) 7.0(13)C(5) -0.340(2) -0.036(2) 0.275 (2) 6.3 (1 2)C(6) 0.285(2) 0.282 (2) 0.202(2) 6.6( 12)C(7) 0.115(3) 0.200(3) 0.480(3) 9.2(15)0.436( 2) - 0.151 (3) 0.335 (2) 5.9(12)0.381(3) -0.181(3) 0.044 (3) 8.3( 15)C(10) 0.094(3) -0.154(3) 0.122( 3) 8.0(13)C(11) 0.365(3) 0.11 6(3) 0.07 2 (3) 6.3(13)0.383(3) 0.17 6 (3) 0.409(3) 7.7(13) :[::I 0.132(3) -0.080(3) 0.466(3) 7.0( 13)C(14) 0.059(2) 0.1 7 1 ( 2) 0.202(2) 5.4(11)N(6) 0.167 8(19) -0.014 8(18) 0.384 4(18) 6.5(9)]El > 1.3 and an acceptance probability of 0.98.Thisprocedure gave the phases of 165 reflections which were usedin an electron-density synthesis, and revealed the positionsof all non-hydrogen atoms.A least-squares refinementwith isotropic Debye factors minimising the function CwA2,where A = IFo] - lFcl and w = 1/[0(F,)1~, converged withA final difference electron-density synthesis showed amaximum value of 0.96 eA-3 with 20 peaks > 0.8 eA-3.Hydrogen-atom positions could not be located clearly. Themaximum shift in positional and thermal parameters in the* See Notice to Authors No. 7 in J.C.S. Dalton, 1975, Indexissue.R = 0.119, R' 0.137.final cycle of refinement was 0.30, and mean shift 0.080.The error for an observation of unit weight was 3.2. Ananalysis of lFol vs. wA2 showed the weighting scheme to besatisfactory.The data were considered insufficiently pre-cise to warrant further refinement. Final atomic co-ordinates and thermal parameters are listed in Table 2, andobserved and calculated structure factor amplitudes aregiven in Supplementary Publication No. SUP 21660 (11 pp.,1 microfiche). *DISCUSSIONThe solid consists of discrete molecules (Figure 1).Within each (MeAlNMe), unit, aluminium and nitrogen(3171FIGURE 1 The molecule (MeAlNMe),FIGURE 2 The molecule (MeAlNMe), viewed along the C, axisatoms form a cage, with each nitrogen linked to onemethyl group and three aluminium atoms, and eachaluminium linked to three nitrogen atoms and one(a) ' X-Ray '70 ' system of programmes, University ofMaryland Technical Report; (b) D. T. Cromer and J.T. Waber,A d a Cryst., 1966, 18, 1041976 1435methyl group. All the methyl groups point outwardsfrom the cage. Interatomic distances and valencyangles are given in Table 3. No molecular symmetry isrequired by the space group, but displacements from themean planes (i)-(iii) (Table 4 and Figure 2) show thatTABLE 3Bond lengths (A) and angles ( O ) , with estimated standarddeviations in parentheses(a) DistancesAl(l)-N( 1)Al( 1)-N( 2)Al( 1)-N( 3)A1(2)-N(2)Al( 2)-N (3)A1 (2)-N (4)A1 (3)-C (1)A1(3)-N (2)Al( 3)-N (4)A1 (4)-N ( 1)A1(4)-N( 5)A1 (4)-N (6)Al( 5)-N (3)A1 (5)-N (6)Al( 5)-N (7)Al( 6)-N (4)Al( 6)-N (5)Al(G)-N(7)Al( 7 j-N (5)Al( 7)-N (6)A1 (7)-N (7)Al( 1)-C( 1)Al( 2)-C( 2)NNNNNNNNNNNNNNNNNNNNNCCCCCCCCCCCCCCCCCCCCC(b) Anglesl)-A1(3)-N(2)2)-Al(1)-N( 1)2) -A1 (2) -N (3)3)-Al(l)-N( 2)2)-A1(3)-N(4)4)-A1(2)-N( 2)3)-A1(5)-N(6)6)-A1(4)-N (1)1 )-Al( 1)-N (3)’l)-A1(4)-N (5)5)-Al(6)-N (4)‘4)-A1( 3) -N ( 1 )3)-A1( 2)-N (4)4)-A1( 6)-N ( 7)7)-A1( 5)-N (3):5)-A1(4)-N (6):6)-A1( 7)-N (5):7)-A1( 7)-N (6)‘5)-A1(7)-N(7)7)-A1( 6)-N( 5):6)-A1(5)-N(7)1)-Al(1)-N( 1)1 )-A1 (1) -N (2)l)-Al( 1)-N( 3)2)-A1(2)-N (2)2)-A1( 2)-N (4)2)-A1( 2)-N (3)3)-A1( 3)-N( 2)3)-A1(3)-N( 1)3)-A1(3)-N (4)4)-A1( 4) -N (5)4)-A1(4)-N( 1 )4)-A1(4) -N (6)5)-A1( 5)-N (3)5)-A1(5)-N(7)5) -A1 (5) -N ( 6)6)-Al(6)--N(4)6)-A1(6)-N( 5)6)-A1(6)-N(7)7)-A1(7)-N (5)7)-A1( 7)-N( 6)7)-A1(7)--N(7)1.93(3)1.89( 3)1.85( 3)1.94(3)1.92(3)1.9 l(3)1.93(3)1.87( 3)1.91 (3)1.93( 3)1.91 (3)1.91(3)1.85(3)1.92(3)1.94(3)1.81(3)1.89( 3)1.97(3)1.92(3)1.96(3)2.05 (4)1.8 5 (4)1.94(3)89.1 ( 12)88.2( 12)88.0(11)91.6( 12)88.4( 11)110.8( 11)1 0 9 4 12)108.7( 12)90.9( 12)110.1 (1 2)lll.O(l2)11 1.1 (12)1 10.4( 12)1 10.3( 12)109.6( 12)89.5 ( 1 3)88.2 (1 3)88.6( 13)86.3 (1 3)93.0( 14)9 1.8 (1 4)1 12.5( 14)1 27.5 (1 4)121.3( 14)125.4( 15)116.6( 15)121.1 (1 5)126.7 (1 4)1 17.8( 14)1 16.2 (1 4)1 1 8.2 (14)113.8(13)1 14.3 (1 4)1 15.6( 14)1 13.5( 14)113.2( 15)11 1.8( 14)11 5.2( 17)1 14.2 ( 16)125.1(15)1 26.4( 15)129.1 (15)A1 (3)-C (3)Al( 4)-C( 4)Al( 5)-C( 5)A1 (6)-C (6)Al( 7)-C( 7)N(l)-C(8)N(2)-C(9)N( 3)-C( 10)N (4)-C( 1 1)N (5)-C( 1 2)N(6)-C (1 3)N (7)-C( 14)C(1).* C(8)C(2). C(10)C(2). C(11)C(3)- ’ -C(8)C(3). * *C(11)C(4)- C(8)C( 5) * .c (1 0)C(4). C(13)C(5). aC(14)C(6). * eC(11)A1 ( 1 )-N ( 1 )-A1 (3)A1 (3)-N ( 2)-A1( 1)Al( 1 )-N (2)-A1(2)A1 (2)-N (3)-A1( 1)A1 (2)-N( 2)-Al( 3)A1(3)-N(4)-A1(2)Al( 1)-N(3)-A1(5)Al( 5)-N (6)-Al( 4)Al( 4)-N ( 1 )-Al( 1)A1 (2)-N (4)-A1(6)Al( 6)-N( 7)-A1( 5)A1 (5)-N (3)-A1(2)A1 (3)-N ( 1)-Al(4)A1(4)-N (5)-A1( 6)A1 (6)-N (4)-A1(3)Al( 5)-N( 7)-A1( 7)A1 (7)-N (6)-A1( 5)Al( 4)-N (6)-A1( 7)Al(7)-N(5)-A1(4)A1(6)-N(7)-A1(7)Al( 7)-N( 5)-A1( 6)C( 8)-N (1 )-A1 ( 1)C (8)-N (1 )-A1 (4)C( 8)-N( l)-Al( 3)C ( 9)-N (2)-A1(1)C (9)-N (2)-A1(3)C( 9)-N (2)-A1(2)C( 1 0)-N (3)-A1(1)C ( 1 0) -N (3) -A1 (2)C ( 1 0)-N (3)-A1(5)C( 1 1)-N (4)-A1( 2)C (1 1 )-N (4)-A1( 6)C(ll)-N(4)-Al(3)C (1 2) -N (5)-A1(4)C( 12)-N (5)-A1(7)C (1 2) -N (5)-A1( 6)C ( 1 3)-N (6)-A1( 4)C (1 3)-N (6)-A1(5)C ( 13)-N( 6)-A1(7)C( 14)-N (7)-A1(5)C (1 4)-N (7)-A1(6)C ( 14)-N (7)-Al(7)1.99(4)1.88(4)2.05(4)2.03(4)1.98(4)1.50(5)1.55( 5)1.61 (5)1.48(5)1.63(5)1.62(5)1.50(4)3.35(5)3.34(5)3.33(5)3.35( 5)3.44(5)3.40( 5)3.40( 5)3.29(5)3.48( 5)3.28(5)88.5 ( 12)91.7(10)88.6(11)90.6 ( 10)89.5 ( 10)89.3( 11)1 20.9 (1 3)1 20.8( 13)118.9(10)1 17.7 (1 0)11 9.0( 13)11 8.8( 12)11 8.9( 12)1 2 3 4 11)117.9( 13)85.9( 16)89.1 (1 7)88.4( 16)87.8 (1 6)86.9(17)89.0( 17)107.0( 16)115.4(14)104.4(15)12 6.3 (1 6)1 2 7.4( 1 6)121.9( 17)108.2( 16)102.8 ( 15)112.6( 15)109.2 (1 6)1 1 1.8( 15)108.7 (1 4)109.8( 15)121.6 (1 5)1 19.4( 16)1 08.1 ( 1 5)120.4(15)125.7( 15)1 14.7( 14)1 1 8.8 (1 4)124.6(13)the symmetry is, within the limits of the present deter-mination, &,, with the three-fold axis through N(2) andAl(7).TABLE 4Displacements (A) from mean planesPlane (i) : N( 2), Al(2) ,N (7), A1 (7), A1(4), N (1)N(2) 0.003, Al(2) -0.006, N(7) 0.010, Al(7) -0.011, M(4)0.009, N(l) -0.006, C(9) -0.016, C(2) 0.036, C(14) -0.039,C(7) 0.042, C(4) -0.041, C(8) 0.031Plane (ii) : N (2) , Al( 1) ,N( 6), Al( 7), Al( 6) ,N( 4)N(2) -0.021, Al(1) 0.015, N(6) -0.004, Al(7) 0.000, Al(6)-0.010, N(4) 0.019, C(9) 0.067, C(l) -0.152, C(13) -0.196,C(7) -0.113, C(6) -0.008, C(11) 0.037Plane (iii) : N(2), Al( 3) ,N( 5),A1( 7), Al( 5),N( 3)N(2) -0.004, Al(3) 0.000, N(5) 0.006, Al(7) -0.007, Al(5)-0.138, 0.002, N(3) 0.002, C(9) 0.068, C(3) 0.003, C(12)C(7) -0.073, C(5) 0.014, C(10) 0.136Angles (”) between normals: (i)-(ii) 119.7, (i)-(iii) 61.0.--+-( a )( C )(d 1x-Y( f 1FIGURE 3 Cage structures (R1XYR2), for (a) n = 4: cubanestructure trigonally distorted b y stretching bonds t o one corner;(b) n = 5, as in MeZnSBut ; (c) n = 7, as in MeAlNMe; (d) n =4: cubane structure with digonal distortion; (e) n = 6, asin HAlNPr’; and (f) n = 8, as in MeZnSPr’The mean A1-N bond distance [1.91(4) is compar-able to those found in aluminium nitride (1.893 A) and(PhAlNPh), [1.914(5) A]> and shorter than in the com-’ G.A. Jeffrey, G. S. Parry, and R. L. Mozzi, J. Chem. Phys.,T. R. R. McDonald and W. S. McDonald, Acta Cryst., 1972,1956, 25, 1024.B28, 16191436 J.C.S. Daltonpounds (Me,AlNMe,), [1.958(5) A], 9~10 and cis-(Me2-AlNHMe), [1.940(11) A].10 In the cage compound(HAlNPr'), l1 Al-N bond distances are 1.898(2) and1.956(2) A. A1-C [mean 1.98(7) A] andN-C [mean 1.56(6)A] distances are normal.Bond angles at both aluminium and nitrogen in thefour-membered rings are close to 90". In six-memberedrings (AlN),, the bond angles at nitrogen [mean111.8(4)"] are greater than those at aluminium [mean105.9(4)"], as has been found for six-membered ringswhich are not constrained in cages10J2 and in thehexamer (HAINPri),.ll In the ' capped ' ring formedby the at oms A1 ( 1) ,N (1) ,A1(3) ,N (4) ,A1 (2) ,N (3), the anglesat nitrogen are less than those at aluminium, so the p3-bridging rnethylimido-group results in considerable de-formation of this ring from the usual pattern.A similarsituation arises in the capped six-membered ring of thecubane structure outlined heavily in Figure 3(a). Ofthe five six-membered rings within the cage, three[A1 (I), N (3) ,A1(5), N (6) ,A1 (4), N (1) ; A1 (2), N (4) ,Al( 6) J ( 7 ) ,-A1(5),N(3) ; and A1(3),N(l),Al(4),N(5),A1(6),N(4)] are inthe symmetrical boat conformation and two [Al( 1) ,N( 1) ,-Al( 3) ,N (4) ,A1 (2) ,N (3) and 4 4 ) ,N (6) ,Al( 5) ,N (7) ,A1(6) ,-N(5)] are in the chair conformation.Various projectionsalong aluminium-nitrogen bonds, showing dihedralangles, are given in Figure 4. Bonds from adjacentatoms are staggered only when the aluminium andnitrogen atoms belong to six-membered rings in the chairconformation [Figure 4(d) and (e)]. Several intra-molecular C - . C distances (Table 3) are (3.5 A, show-ing that the molecule is crowded. In the open-ringcompound frans-(Me,AlNHMe), methyl-methyl repul-sions are relieved by distortion of the aluminium-nitrogen ring from a symmetrical to a twist-boat con-formation.l* This is not possible in the constrained ringsin the (MeAlNMe), cage.The molecules are well separated, with no inter-molecular C - * - C distance <3.5 A.Since the lengthsof the unit-cell axes are very nearly equal and the mole-cules are roughly spherical, it seems probable that thecrystal packing is based on a close-packed arrangement.The centre of the reference molecule in the monoclinicunit cell is at approximately &,O,g. With the symmetry-related molecules (at $,$,$; $,$,$; and $,O,$), the arrayis approximately f ace-centered cubic and the moleculesare oriented so that the molecular three-fold axes arealong body diagonals of the face-centred cubic unit cell.Mass spectra of the compounds RAlNMe (R = Me 4 orEt 6, show strong peaks corresponding to the loss of oneR group from the parent ions (RAlNMe),+; there arealso weak peaks due to the parent ions themselves.Itseems that heptameric structures for both compoundsare preserved both in solution and in the vapour. Theoctameric formulae previously reported 435 are clearlynot correct.9 H. Hess, A. Hinderer, and S. Steinhauser, 2. anorg. Chem.,10 G. M. McLaughlin, G. A. Sim, and J. D. Smith, J.C.S.1970, 377, 1.Dalton, 1972, 2197.It is of interest to inquire how the structure of theheptameric compound (MeAlNMe), is related to thoseof other oligomeric species (R1XYR2),. For values ofn < 4, where the (XY). system is linear or planar, andfor the compounds (RlBNRZ),, which have open-ringrather than cage structures, it is necessary to postulatemultiple bonding, if X and Y are to have completeoctets. In compounds with n >, 4, the X and Y atomsAl, NAl(elFIGURE 4 Projections along A1-N bonds, showing mean dihedralangles ('), averaged over the bonds related by the C , axis, for(a) Al(l)-N(2), A1(2)-N(2), and A1(3)-N(2) ; (b) A1(7)-N(5),A1(7)-N(6), and A1(7)-N(7) : (c) A1(4)-N(l), A1(5)-N(3), andA1(6)-N(4); (d) Al(1)-N(l), AI(I)-N[3), A1(2)-N(3), Al(2)-N(4), A1(3)-N(4), and A1(3)-N(1) ; and (e) A1(4)-N(5), Al(4)-N (6), Al( 6)-N (6), Al( 5)rN (7), Al( 6)-N (7), and Al( 6)-N ( 6 ) .For (d) and (e), the projections along half the bonds are themirror images of those shownform cages, with four- or six-membered rings.Eight-membered rings are in principle possible, but they havenot so far been established in cages of this kind.If theatoms X and Y have complete octets, the number ofX-Y bonds in the cage oligomer (R1XYR2), is 3n and,since each bond is shared between two rings, the numbersof four-membered rings p and six-membered rings q arerelated by 4p + 6q = 6n. Further, the numbers ofrings p + q, vertices 2n, and bonds are related byl1 M. Cesari, G. Perego, G. Del Piero, S. Cucinella, and E.l2 J. L. Atwood and G. D. Stucky, J . Amer. Chem. Soc., 1970,Cernia, J . Organometallic Chew., 1974, 78, 203.92. 2851976 1437Euler’s theorem, so that fi + q = n + 2. Integralvalues of p and Q which satisfy these conditions are givenin Table 5. The compounds already established (k.TABLE 5rings (q) in cage compounds [R2XYR2],n 4 6 6 7 86 6 6 6 60 1 2 3 4P4Numbers of four-membered rings ( p ) and six-memberedwith n = 4-8) form a related series, in which the geo-metrical relations are shown in more detail in Figure 3.Removal of one corner of the cubane, leaving a fragment(R1XYR2),YR2 [Figure 3(a)], followed by the insertionof a fragment (R1X),YR2 yields the pentameric structureof (MeZnSBut), [Figure 3(b)].Combination of twofragments (R1XYR2),YR2 and (R1XYR2),R1X yields theheptameric structure of (MeAlNMe),. Breaking twobonds of the cubane structure gives the fragment(R1XYR2), [Figure 3(d)]. Insertion of a fragment(RlXYR2), gives the hexameric structure of (HAlNPr’),[Figure 3(e)], and combination of two fragments ofFigure 3(d) gives the octameric structure of the com-pound (MeZnSPri)* [Figure 3(f)].It is possible toenvisage other members of this series of oligomers. Forexample, another six-membered ring could be inserted inthe structure of (MeAlNMe),, e.g. between the cappingatom Al(7) and the ring A1(4),N(6),A1(5),N(7),A1(6),N(5).The molecule would have the formula (R1XYR2),, with6 six-membered and 6 four-membered rings.The factors which determine the values of n shown byparticular oligomeric molecules ( R1XYR2), are presum-ably similar to those discussed earlier for the series ofoligomers (R12XYR2,),.13 As n increases (Table 5), fi/n(the number of four-membered rings per XY unit)decreases, and qln (the number of six-membered ringsper XY unit) increases. The thermodynamic drivingforce for the formation of oligomers with larger values ofn from oligomers with smaller values of n is probably therelief of ring strain, or, in other words, the more favour-able orbital overlap possible, as six-membered replacefour-membered rings.This driving force must be suffi-cient to balance the unfavourable entropy contributionto the free energy of formation of oligomers with largern from those with smaller 12, and must also not be offsetby steric hindrance between substituents R1 and R2. Itis probable that, as for the compounds (R1,A1NR2&,steric interactions between N-substituted groups aremore important than those between AZ-substitutedgroups. Thus the compounds (MeAlNMe) and (EtAlN-Me) are heptameric, but the t-butylimido-derivatives(EtAlNBut) and (HAINBut) and the compounds (PhAlN-Ar) (Table 1) are tetrameric.In magnesium-oxygen(isoelectronic with aluminium-nitrogen) cages, (Et MgO-Pr*) and (PriMgOR2) (R2 = Me or Et) are hepta- or octa-meric, but (EtMgOR2) (R2 = But or Pri) and (PriMgOPri)are tetrameric. Thus, although the higher oligomer isformed with a branched substituent R1 at magnesium,branching in the substituent R2 at oxygen leads to thelower oligomers. Steric interactions between substitu-ents R2 may account for the higher values of n shownby sulphur compounds, compared with correspondingoxygen derivatives.The formation of oligomeric aluminium-nitrogencompounds requires careful control of the reaction condi-tions. Indeed, the compound (MeAlNMe) was firstisolated as an involatile, glassy material.14 The glassyforms of the compounds R1A1NR2 dissolve with difficultyin hydrocarbons and contrast strongly with the highlysoluble crystalline forms. They are best made in reac-tions without solvents and are probably poly- rather thanoligo-meric, with irregular cross-linked aluminium-nitrogen chains. Further work is necessary to elucidatethe process whereby the trimeric precursors cis- andtrans-(Me,AlNHMe), are converted into the heptamericcompound (MeAlNMe),.We thank Miss C. Battrick and G. Mclaughlin fortechnical assistance, Professor H. Noth and Herr P. Wolf-gardt, of the Institut fur anorganische Chemie der Universi-tat Munchen, for helpful discussions, and the S.R.C. and theAlexander von Humboldt Stiftung for financial support.[6/1669 Received, 8th A u p s t , 19761l3 0. T. Beachley and G. E. Coates. J . Chenz. SOL, 1965, 3241.14 G. Bahr in ‘ F.I.A.T. Review of German Science, 1939-45,’Inorg. Chem., Part 11, p. 159
ISSN:1477-9226
DOI:10.1039/DT9760001433
出版商:RSC
年代:1976
数据来源: RSC