摘要:
J. CHEM. SOC. DALTON TRANS. 1992 527Studies on Bis(q4ndenyl)niobium Complexes *Malcolm L. H. Green and Andrew K. HughesInorganic Chemistry Laboratory, South Parks Road, Oxford OX I 3QR, UKThe reaction between [NbCl,(thf),] (thf = tetrahydrofuran) and Li[C,H,] or Na[C,H,]-thf gives[Nb(q-C,H,),CI,] 1, which reacts with NaBH, to give [Nb(q-C,H,),(p-H,BH,)] 2. The bridging andterminal tetrahydroborate protons in 2 exchange on the NMR time-scale (AG* = 55.2 -t 1.6 kJmol-l). Compound 2 reacts with PMe,Ph or pyridine via BH, extraction t o give [NbH(q-C,H,),(PMe,Ph)] 3 and [NbH (q-CgH7)2(NC5H5)] 4 respectively. The pyridine ligand in 4 undergoesrestricted rotation about the Nb-N bond (AG* = 52 & 1 kJ mol-l). The reaction of 2 with NMe, inthe presence of CO, H,C=C=CH,, trans,?rans- hexa-2,4-diene or styrene gives the new complexes[ N b (q - C,H,) (CO),] 5, [ N b (q - CgH,), (q - CH,C H C H,) ] 6, [ N b( q - C9H7), (q - MeC H C H C H C H,Me)] 7 andendo-[NbH(q-CgH,),(q-H,C=CHPh)] 9 respectively. The styrene hydride (complex 9) is fluxional andthis process has been studied using both one- and two-dimensional NMR techniques.The activationbarrier to olefin hydride exchange has been determined by coalescence methods t o be AG* =70.0 _+ 2 kJ mol-' at 50°C, this is significantly lower than that for the analogous q-C,H, andq-C,Me, complexes. A mechanism that involves a ring slipped intermediate is proposed.A range of q-C9H7 complexes have been described for a varietyof transition metals, and the replacement of q-C5R5 (R = H orMe) by q-C9H7 has been demonstrated to have a significanteffect on the chemistry of such complexes.The most notable ofthese effects is the enhancement of many reactions which involvering-slip to an q 3 bonding mode; this indenyl effect has beenreviewed. '*, Although Multani, Rastogi and co-workers3-' 'have described a number of bis(indeny1)niobium complexes,these are limited to compounds containing OR, SR or carbox-ylate as co-ligands. The same workers have also described anumber of tris(indeny1)niobium complexes.We set out to prepare and compare the properties of bis(q-indeny1)niobium analogues of several bis(q-cyclopentadieny1)-niobium derivatives for the reasons outlined below.Results and DiscussionAddition of Li[C9H7] to a tetrahydrofuran (thf) suspension of[NbCl,(thf),] rapidly produces a dark green solution whichbecomes red over 1 h and precipitates a green solid over 12 h.The green product, [Nb(q-C,H,),CI,] 1, can be isolated intypical yields of 3&35%, Scheme 1.This complex had beenpreviously described by Multani, Rastogi and co-workers usinga synthesis analogous to an early preparative method for [Nb-(q-CSHS)2C12].'6 The green colour contrasts with that of[Nb(q-C,H,),CI,] (dark red in solid and solution) l 6 and[Nb(q-C,Me,),Cl,] ( b r ~ w n ) . ' ~ The characterising data for 1and all other compounds described in this work appear in Table1, and will not be further discussed except where interpretationis not straightforward.The reaction between Na[C9H,]*thf and [NbCl,(thf),] wasalso performed in thf but no precipitate appeared, a dark redsolid was obtained when the solvent was removed underreduced pressure. This crude solid was found to be suitable forfurther reaction with NaBH, (see below), but it was found to beimpossible to obtain pure [Nb(q-C9H7)2C12] by the use of thesodium salt Na[C,H,]=thf.Addition of 1,2-dimethoxyethane (dme) to a mixture of solid[Nb(q-C9H7)2C12] 1 and an excess of NaBH, gave an intensepurple solution, from which [Nb(q-C9H7),(p-H2BH2)] 2 wasisolated as a dark red-purple amorphous solid.The IR spectrum* Non-St units ~ ~ r ~ p l o j * e d : atm = 101 325 Pa, mmHg z 133 Pa.of 2 shows bands at 2448, 2412 and 23 11 cm-' associated withthe BH, ligand.'8-'9 The 300 MHz 'H NMR spectrum at 25 "Cshows four resonances for the indenyl protons with integrals inthe ratio 2:2: 2: 1.This multiplicity indicates that the moleculehas CZ0 symmetry.The 'H NMR resonances of the BH, moiety in 2 are broad.Two mechanisms are responsible for this. First the interactionwith the quadrupolar "B ( I = 5, 80.4%), 'OB ( I = 3, 19.6%)and 93Nb ( I = f, 100%) nuclei causes broadening. The BH4protons are also broadened by a bridge-terminal exchangeprocess. This process has been studied previously for [Nb(q-CSH5),(p-H2BH2)] and [Nb(q-C5Mes)2(p-H2BH2)].'7 The200 MHz 'H-{"B}-'H difference spectrum reveals the BH,resonances; the terminal BH2 appears at 6 5.7, and the bridgingBH, at 6 - 14.2. The "B-('Hf spectrum is a singlet at 6 33.8and the ''B NMR spectrum reveals a quintet [J(H-B) = 82Hz].The bridge-terminal exchange rate at room temperature isfast compared with the 'H-"B coupling constants. Hence, theobserved coupling constant is an average of the typical bridging(30-60 Hz) and terminal (120-160 Hz) coupling constants.The AG * value for the bridge-terminal exchange process wasdetermined from the coalescence temperature of the bridge andterminal 'H NMR resonances.20 The value obtained is 55.2 &1.6 kJ mol-' at 331 & 5 K, which compares with the values for[Nb(q-C5H5),(p-H2BH2)] (61.0 0.8 kJ mol-' at 346 f 3K) and [Nb(q-C,Me,),(p-H2BH2)] (68.5 & 1.6 kJ mol-' at388 & 8K).I7A mechanism for this exchange process has been proposedfollowing a study of the related complex [Ta(q-C,Me,)(q-C5H5)(p-H2BH2)].21 The observed kinetics was interpreted interms of an q3-BH4 intermediate with a concomitant ring slipto retain an 18-electron configuration.The AG * value reportedhere for the q-C9H7 complex is lower than for the q-C5H5 andq-C,Me, complexes. The relative order of these values isconsistent with an exchange process that involves an associativemechanism and an intermediate with an q3-co-ordination ofone of the rings.Addition of PMe,Ph to a toluene solution of [Nb(q-C9H7)2(p-H2BH2)] gave a deep blue solution from which[NbH(q-C9H,),(PMe2Ph)] 3 was isolated. The 'H NMRspectrum of 3 shows seven indenyl resonances of equal integrals;the molecule thus has C, symmetry with the mirror planecontaining the Nb, P and H atoms. The 13C NMR spectrum of3 confirms the symmetry of the molecule as deduced from th528 J.CHEM. SOC. DALTON TRANS. 1992[ NbC14(thf)d(iii ) IH/'HI,NPoc &COoc5Scheme 1 The preparation and reactions of [N~(I&,H,)~CI,] and [Nb(q-CgH7),(p-H2BH2)]. Reagents and conditions: (i) Li[C9H7], thf, 15 h,34%; (ii) NaBH,, dme, 4 h, 80%; (iii) PMe,Ph, toluene, 80 h, 36%; (iu) C,H,N, toluene, 5 h, 41%; ( u ) CO and NMe,, toluene, 15 h, 33%; (ui) allene andNMe,, toluene, 36 h, 40%, R = R' = H, 6 rrans,rrans-hexa-2,4-diene and NMe,, toluene, 36 h, 51%, R =CH,, R' = CH2CH,, 7; (uii) styrene andNMe,, toluene, 15 h, 46%'H NMR data; the assignment of this spectrum was confirmedby a I3C-'H shift-correlation experiment.The 31P-('H) NMRspectrum at room temperature shows a broad singlet (Av+ =430 Hz) at 6 24.1, the broadness of this signal is due tointeraction with the 93Nb (1 = 9, 100%) nucleus.The reaction of [Nb(q-C,H,),(p-H,BH,)] with pyridineunder hydrogen was investigated. This reaction was promptedby the observation of Bercaw and co-workers ' that [Nb(q-C5Me,),(p-H,BH2)] reacts with pyridine in the presence ofhydrogen to give [NbH,(q-C,Me,),],'7 the preparation of[NbH,(q-C,H,),] being one of the initial targets of our work.Addition of an excess of pyridine to a toluene solution of [Nb-(q-C9H7)2(pHzBH2)] gave an identical deep green solutionunder both hydrogen (either at 1 or at 40 atm) and nitro-gen atmospheres. The green product was identified as [NbH(q-C9H7),(NC5H5)] 4, and was found to display fluxional 'H and ' 3C NMR spectra.The 'H NMR spectrum of 4 at room temperature shows sevensharp resonances, of equal integral, assigned to a diastereotopicindenyl ligand; there is also a slightly broad triplet at 6 5.8 and anumber of other broad low-field resonances.A broad resonanceat 6 -4.4 is assigned to a Nb-H hydride, broadened due to thequadrupolar 93Nb nucleus. The broad peak at 6 5.8 suggestedsome fluxional process involving the pyridine ligand; thiswas investigated by variable-temperature ' H NMR spectros-COPY.The 'H NMR spectrum of 4 in the low-temperature limit at210 K shows the seven indenyl resonances which are un-changed. The broad low-field resonance at 6 5.8 had decoal-esced to give two triplet resonances, and a number of doubletshave sharpened out of the baseline.A COSY-45 spectrum wasrecorded at 203 K and identified the presence of one dia-stereotopic pyridine ligand (integrals in the ratio l : l : l : l : l )and one non-diastereotopic pyridine ligand (integrals in theratio 2: 2: 1). The ratio of the integrals of the indenyl ligands andthe diastereotopic pyridine ligand is always ca. 14:5. The twosets of pyridine resonances are in the approximate ratio 1 : 1 (thisratio is sample dependent). On the basis of reference spectra thenon-diastereotopic pyridine was assigned to free pyridine whichco-crystallises with complex 4. Attempts to remove this pyridineby either sublimation in U ~ C U O or further crystallisations fromtoluene were only partially successful, and samples were foundalways to contain at least 20% free pyridine. In the 'H NMRspectrum at 210 K the resonances at 6 9.65 (d, ortho), 5.80 (t,meta), 6.30 (t, para), 5.55 (t, meta') and 4.20 (d, ortho') areassigned to the diastereotopic pyridine ligand.The very largechemical shift difference between the ortho and ortho'resonances is noteworthy.The fluxionality of 4 is interpreted in terms of restrictedrotation of the pyridine ligand about the Nb-N bond; this is be-lieved to arise as a consequence of steric effects. The coalescenceof the meta protons allows the activation barrier to the rotationto be calculated as ACs = 52 1 kJ mol-'.20 Alternativeprocesses involving dissociation of the pyridine or exchangewith the free pyridine can be discounted, since these would raisethe symmetry to CZv by inversion at Nb and reduce themultiplicity of the q-C9H7 resonances.Furthermore, there isno exchange between the free pyridine and the co-ordinatedpyridine ligand on the chemical time-scale; thus, the C5H5Nligand in [NbH(q-C9H7)2(NC,H,)] does not exchange with[2H,]pyridine.The 13C NMR spectrum of [NbH(q-C9H7),(NC,H5)] atroom temperature reveals seven indenyl C-H resonances, thering-junction quaternary resonances are coincident. The reson-ances due to the pyridine ligand are broad at room temperaturedue to the exchange process discussed above. Since the complexreadily crystallises from toluene at - 25 "C variable-tempera-ture 13C NMR experiments using direct observation were noJ.CHEM. SOC. DALTON TRANS. 1992 529attempted. Instead the low-temperature 3C NMR spectrumwas obtained using a 'H-detected 'H-13C shift correlationexperiment at 210 K using the pulse sequence of Bax andSubramanian.22 Although this technique has been used todetermine the chemical shifts of low-abundance and low-ynuclei such as '83W and 1870s,23 it has rarely been used for 13CNMR. In suitable cases ('H T , values of 1-2 s and molecularweights of less than a few thousand) this experiment is apowerful technique for obtaining 3C spectra when sensitivity islimited by the amount of sample available or, as here, solubility.The spectrum (Fig. 1) was recorded in less than 4 h and allowsthe unambiguous extraction of the 13C NMR chemical shifts forthe indenyl ligand and the co-ordinated pyridine ligand.Theconcentration of free pyridine in this sample was too low for theTable 1 Analytical and spectroscopic dataCompoundColour and analysis" (%)1 [Nb(q-C9H,)2C12]GreenC, 54.1 (54.8); H, 3.8 (3.6)Purple-mauveC, 60.7 (63.9); H, 5.2 (5.4)2 "b(q-C,H7)2(~-H2BH2)13 [NbH(q-C,H,),(PMe,Ph)]BlueC, 67.3 (67.5); H, 5.6 (5.7)5 CNb(rl-C9H,)(CO)J 'Light brownC, 47.8 (48.7); H, 2.9 (2.2)Nboc~'c>co6 "b(q-C,H 7 )2(q 3-CH 2CHCH2)lDark purpleC, 68.8 (69.2); H, 5.6 (5.3)NMR data'H: 7.14.5 (overlapping m, 13 H, phenyl and Ha,,,,,,), 5.30 [t, 2 H, J(HH) 3, H,], 4.24 [d, 2 H, J(HH)3, H, or He], 4.07 [d, 2 H, J(HH) 3, He or H J, 1.30 [d, 6 H, J(HP) 6.6, PMe], -4.55 [d, 1 H, J(HP)40.5, Nb-H]I3C: 131.8, 131.6, 128.6, 128.0, 127.9, 126.0, 125.2, 123.7, 122.1 (aromatic), 111.5 (2 overlapping s,Cqua,), 80.4 [d, J(CH) 179, C,], 79.8 [d, J(CH) 174, C, or C,], 79.5 [d, J(CH) 175, C, or C,], 20.2 [dq,J(CH) 131, J(CP) 21, PMe,]'H (at 293 K): 7.00 [d, 2 H, J(HH) 8, Ha or H,], 6.55 [t, 2 H, J(HH) 8, H, or H,], 6.51 [partially obsc.t, J(HH) 8, Hpcrra], 6.47 [t, 2 H, J(HH) 8, H, or H,], 5.80 (br t, 2 H, H,,,,), 5.60 [d, 2 H, J(HH) 8, H, orHa], 5.14 (virt.t, 2 H, Japp 2, H, or He), 4.58 (virt. q, 2 H, Japp 2, Hc), 3.91 (virt. t, 2 H, Japp 2, He or H,),-4.40 (br s, 1 H, Av, ~ 3 0 , Nb-H)13C (at 210 K, ['H,]toluene): 161.1 [d, J(CH) 195, Corlho], 152.9 [d, J(CH) 186, Conho], 136.9 [d,119.6 [d, J(CH) 161, c b or C,], 80.3 [d, J(CH) 185, C, or C,], 79.2 [d, J(CH) 180, C,], 76.6 [d, J(CH)174, C, or C,]J(CH) 166, Cparo].125.9 ( c , Or Cb), 125.2 ( c , Or cd), 123.1 (cd Or ca)+ 122.9 (crnetu,), 2 120.2 (c,,,,),'H: 6.99 [dd, 2 H, J(HH) 3, 6, Haas or Hbb.], 6.59 [dd, 2 H, J(HH) 3, 6, Hb,. or Has,], 5.35 [d, 2 H,J(HH) 3, Hcc,], 4.97 [t, 1 H, J(HH) 3, Hd]124.7 [d, J(CH) 161, caa, Or Cbb.1, 124.2 [d, J(CH) 160, ebb. Or caa,], 114.2 (S, Cqua,), 97.1 [d,J(CH) 174,Cd],83.8 [d,J(CH) 177,C,,,]'H: 7.10 [partially obsc., 2 H, Ha,a, ring 11, 6.92 [dd, 2 H, J(HH) 6, 3, H,.,. ring 13. 6.70 [dd. 2 H.J(HH) 6,3, H,.,, ring 2J6.57 [dd, 2 H, J(HH) 6,3, Ha.a, ring 2],5.61 [t, 1 H, J(HH) 4, H, ring 13.4.82[t, 1 H, J(HH) 4, H, ring 2J3.97 [d, 2 H, J(HH) 4, H,.,, ring I], 3.86 [d, 2 H, J(HH) 4.H,.c, ring 23,2.10 (m, 3 H, H, and H,,fr or HCec,), 1.10 (m, 2 H, Hc.c, or H,.,,)I3C-('H}: 126.6 (ebb? ring l), 125.3 (Caa, ring 2), 124.0 (C,,. ring 2), 123.6 (Cad, ring l), 91.0 (C, ring1). 88.8 (C,,, ring I), 86.7 (C,), 86.2 (Ccc. ring 2), 82.5 ( c d ring 2). 46.0 (CH2, C,. and C,, 530 J. CHEM. SOC. DALTON TRANS. 1992Table I (continued)CompoundColour and analysis" (%)7 [Nb(q-C,H,),(q3-MeCHCHCHCH,Me)]C, 70.9 (70.9); H, 6.0 (6.2)NMR datab'H: 7.08 (m, 1 H, Ha or H, up), 7.02 (m, 1 H, H, or Ha up), 6.90 (m, 2 H, H, and H, up), 6.80 (m, 2 H,(m, 1 H, H, or He up), 4.20 [t, 1 H, J(HH) 3, H, down], 4.07 (m, 1 H, He or H, down), 3.92 (m, 1 H, H,or Hedown), 3.73 (m, 1 H, He or H, up), 3.16 [t, 1 H, J(HH) 13, Hk], 2.00 (m, 1 H, H, or H,), 1.46 [d,3 H, J(HH) 5.5, Hh], 1.40-1.22 (overlapping m, 3 H, H, or H,, Hj and H,), 0.88 [apparent t, 3 H,Dark purple H, and Hd down), 6.75 (m, I H, Ha down), 6.64 (m, 1 H, H, down), 5.47 [t, 1 H, J(HH) 3, H, UP], 4.28CgH?JP)J(HH) 7, Hpl127.2 [d, J(CH) 160, Cd or c, up], 126.6 [d, J(CH) 159, c, or Cd down], 126.0 [d, J(CH) 157, c,or c d Up], 125.8 [d, J(CH) 158, cb or c, down], 123.9 [d, J(CH) 159, c, or c, down], 123.8 [d, CH3(h)J(CH) 160, C, or Cb or C, or Cd down], 123.7 [d, J(CH) 159, C, or C, up], 123.1 [d, J(CH) 160, C, orc b up], 1 1 1.5 (s, Cqual), 110.8 (two overlapping s, Cquat), 1 10.1 (s, Cqual), 92.2 [d, J(CH) 175, ck], 88.7[d, J(CH) 177, C, or C, up], 88.2 [d, J(CH) 179, C, up], 87.4 [d, J(CH) 178, C, or C, up], 84.6 [d,J(CH) 178, C, or C, down], 84.4 [d, J(CH) 178, C, or C, down], 79.7 [d, J(CH) 180, C, down], 66.7[d, J(CH) 145, CH,], 54.4 [d, J(CH) 148, CH,], 29.1 [t, J(CH) 130, CH,,,], 20.25 [q, J(CH) 131,CgH7idown)(P)H3CCHh], 18.3 [q, J(CH) 127, CH,]C9H7 = I'H: 7.47 [d, 2 H,J(HH) 7.5, Hortho], 7.24 [t, 2 H, J(HH) 7.5, H,,,,], 7.08 (br, 1 H, Ha,b,cord ring a orb),6.95 (br, 1 H, ring a or b), 6.93 [t, 1 H, J(HH) 7.5, Hpara], 6.79 (br q, 2 H, Ha,b,c,ord ring a orb), 6.67 (br app.d, 3 H, ring b or a), 6.55 (br app. d, 1 H, Ha,b.cord ring b or a), 5.07 [t, 1 H,J(HH) 25 3, H, ring a or b], 5.04 (sl br, 1 H, Hgor , ring b or a), 4.64 (sl br, 1 H, He,, , ring b or a), 4.409 [NbH(q-C,H,),(endo-q-H,C=CHPh)]Red-brownC, 72.9 (72.8); H, 5.4 (5.4)I III[t, 1 H, J(HH) 25 3, H, ring b or a], 4.37 (sl br, 1 H, Heor, ring a or b), 4.04 (sl br, 1 H, HBore ring a orb), 1.29 (m, 2 H, Hhork and HI), -0.97 (s, 1 H, Nb-H), - 1.33 [dd, 1 H, J(HH) 4.2, 8.6, Hkorh] l3c (at 283 K): 153.1 (s, cipso), 127.6 [d, J(CH) 160, Corrho], 126.6 [d, J(CH) 160, C,,,,], 125.8 [d,C9H7Hh C9H7 = 14ax?\> , /L f J(CH) 161, ring a or b], 125.5 [d, J(CH) 161, ring b or a], 125.2 [d, J(CH) 161, ring b or a], 124.7 [d,J(CH) 162, ring a orb], 124.5 [d, J(CH) 161, ring a orb], 124.0 [d, J(CH) 163, ring b or a], 123.9 [d,J(CH) 160, ring a or b], 122.7 [d, J(CH) 160, Cpora], 122.0 [d, J(CH) 163, ring b or a], 115.7 (Cqua1),114.6 (C,,,,), 112.0 (Cqua,), 110.7 (Cqual), 99.3 [d, J(CH) 177, C, ring a or b], 94.5 [d, J(CH) 174, C,ring b or a], 92.5 [d, J(CH) 179, Cgore ring a or b], 85.9 [d, J(CH) 177, Ceorg ring a or b], 83.8 [d,J(CH) 177, C,,,, ring b or a], 82.8 [d, J(CH) 178, C,,,, ring b or a], 51.5 [d, J(CH) 154, CH,], 30.7c\d -eC9H7uI - um - urIII[d, J(CH) 150, CHIdl -dm -dr" Required values are given in parentheses.Unless otherwise stated, all 'H and 3C NMR data were recorded at 300 MHz ('H) or 75.43 MHz (I3C)at 293 K for 'H and 298 K for I3C in ['H,]benzene. The following abbreviations are used: s (singlet), d (doublet), t (triplet), q (quartet), qnt (quintet),m (multiplet), sl (slight), br (broad), obsc. (obscured), virt. (virtual), Japp (apparent coupling constant) and C,,,, (quaternary carbon). CI 17.6 (18.0)%.Electron impact (EI) mass spectrum m/z 278 [P - CI]', 208 [P - 2C1]+.'H-("B}- 'H (200 MHz, 282 K, ['H,]benZene) 6 5.7 (br s, Nb-H,),- 14.2 (br s, B-H,-B); "B (64.17 MHz, 283 K, [%,]benzene) 6 33.79 [qnt, J(HB) 821. Selected IR data (CsI pellet) 2448m, 2412m and 231 lm cm-'v(B-H). 31P-(1H) (121.49 MHz, 298 K, ['HJbenzene) 6 24.1 (br s, Avt 430 Hz). Selected IR data (Nujol mull, KBr plates) 1633m cm-' Nb-H.Horrho not observed at 293 K; only selected J(CH) data available. Additional 'H NMR data (300 MHz, 203 K, [2H8]toluene), 6 9.65 [d, 2 H, J(HH) 4,Horrho], 7.10 (obsc., Ha or H,), 6.65 [t, 2 H, J(HH) 8, H, or H,], 6.52 [t, 2 H, J(HH) 8, H, or H,], 6.30 [t, 1 H, J(HH) 4, Hpara], 5.80 [t, 1 H, J(HH) 4,H,,,,], 5.70 [d, 2 H, J(HH) 8, H, or Ha], 5.64 [virt. t, 2 H, Japp 252, H, or He], 5.55 [t, 1 H, J(HH) 4, H me,0.], 5.21 (virt.q, 2 H, Japp z 2 , H,), 4.20 [d, 1 H,J(HH) 4, Horrho,], 3.80 (virt. t, 1 H, Japp 252 Hz, He or HJ, -4.40 (br s, AV! z 2 0 Hz, Nb-HI. Additional "C NMR data (75.43 MHz, 191 K,[2H,]toluene); 6 252.0 (br s, AV: 30 Hz, CO). Selected IR data [light petroleum (b.p. lO(r-120 "C) solution, KBr cell] 2034m, 1947(sh) and 1931sv(C=O) cm-' .correlations due to this to be observed. Free pyridine wasobserved on another occasion in a distortionless enhancementby polarisation transfer (DEPT) 3C NMR experiment. Themost notable feature is that the separation of the resonances dueto the ortho and ortho' sites of the pyridine ligand is consider-ably smaller in the I3C spectrum (7.1 ppm or 536 Hz) than inthe 'H spectrum (5.45 ppm or 1635 Hz).The 'H-coupled I3CDEPT spectrum at 210 K provides access to some of the'J(C-H) coupling constants, those for the orrho resonances are195 and 186 Hz. These data mitigate against a structure with anortho agostic interaction, i.e. [NbH(q-C,H 7)2(q 2-NC5H!)],where the indenyl ligands are undergoing rapid q5-q3 ringshifts. The reaction of [Nb(q-C9H7)2(p-H2BH2)] with Lewisbases, L (L = PMe,Ph or pyridine) results in the formation of[NbH(q-C,H,),L], this is presumed to occur via a [NbH(q-C,H,),] intermediate; the adduct BH,L is believed to be theother product. With the aim of trapping the [NbH(q-C,H,),]fragment with alternative reagents a number of reactions wereperformed using NMe, in the presence of potential ligands.The reaction of [Nb(q-C9H7)2(p-H2BH2)] with excess CO inthe presence of an excess of NMe, gave a brown-yellow solutionfrom which polycrystalline needles of [Nb(q-C,H,)(CO),] 5,were isolated.The 'H NMR spectrum of 5 indicates thepresence of a non-diastereotopic indenyl ring, i.e. there are fourresonances in the ratio 2:2:2: 1. The I3C NMR spectrum of 5shows two CH resonances for the C, ring, one quaternary ringresonance and two CH resonances for the C5 ring in theapproximate ratio 2: 1. Additionally, on cooling the sample to-82 "C, a broad carbonyl resonance at 6 252 is observed. Thesolution IR spectrum shows two strong bands at 2034 and 1931cm-I and a weak shoulder at 1947 cm-'. These are consistentwith the complex [Nb(q-C,H,)(CO),] having approximatelJ.CHEM. SOC. DALTON TRANS. 1992 53 110 9 8 7 6 5 4S('H)Fig. 1 The ' H-detected inverse-mode H-' 3C heteronuclear shiftcorrelation spectrum of compound 4 at 210 K in [ZH,]toluene. The 'Hprojection is shown at the bottom of the figure, the resonances assignedto the pyridine ligand are labelled, the peak marked with an asterisk isdue to an impurityC4u local symmetry, the E and A, symmetry stretches aresymmetry allowed, and the weak shoulder is assigned to thesymmetry-forbidden (under true C4u) B, band.As further confirmation of the identity of the product, the re-action of [Nb(q-C9H7),(p-H2BH2)] with NMe, in the presenceof excess CO was carried out in a NMR tube. After 12 h, the 'HNMR spectrum showed signals assigned to free indene bycomparison with a genuine sample.The ratio of free indene tocomplexed indenyl was ca. 1 : 1 by integration.Trogler and co-workers 24 have reported that the reaction of[V(q-C,H,),] with CO forms the dicarbonyl complex [V(q-C9H ,)(q '-C,H,)(CO),]; this represents one of a growingnumber of q3-C9H7 complexes that have been structurallycharacterised. On warming this complex under excess CO, anindenyl ligand is lost to give [V(q-C,H,)(CO),], which hadbeen previously prepared via another route.,, This suggests thatthe reaction of [Nb(q-C9H7),(p-H2BH2)] with CO mayproceed uiu mono- and di-carbonyl intermediates, but no suchintermediates were observed.The reaction of a toluene solution of [Nb(q-C,H,),(p-H,BH2)] with a mixture of allene (H,C=C=CH,) and tri-methylamine gave a very deep purple solution from whichamorphous [Nb(q-C,H,),(q-C,H,)] 6 was obtained by crys-tallisation from light petroleum. The 'H NMR spectrum showsfour indenyl resonances with integrals in the ratio 4:4:4:2 aswell as two complex multiplets centred at 6 2.10 and 1.10 withintegrals in the ratio 3:2.The multiplicity of the indenylresonances indicates that the molecule has C, symmetry withthe mirror plane perpendicular to the indenyl ligands andcontaining the Nb and allyl C-H atoms. The I3C NMRspectrum of 6 confirms the symmetry of the molecule as deducedfrom the ' H NMR data.Addition of trimethylamine to a toluene solution of [Nb(q-C9H7),(p-H2BHZ)] containing trans~rans-hexa-2~4-diene gavea deep purple solution from which [Nb(q-C9H,),(q3-MeCH-CHCHCH,Me)] 7 was obtained by crystallisation frompentane.The substitution of the allyl ligand in this complexresults in C1 symmetry. As a consequence, compound 7 contains17 separate CH groups, one diastereotopic CH, group, twoCH, groups and four indenyl quaternary carbons. The 'HNMR spectrum of 7 shows a number of complex multiplets inthe aromatic region (6 6.6-7.1), these are assigned to the eightinequivalent aromatic C,-ring indenyl protons. The resonancesare heavily overlapped, but the phase-sensitive double-quantum-filtered 'H-'H COSY (COSY-DQF) allowed a tentativeassignment to be made. The C,-ring resonances for the twoinequivalent rings are clearly distinguished.A COSY optimisedfor long-range coupling (COSYLR) was also run, and thisshows weak correlations between the C,- and C,-indenyl ringswhich allow the connections between the rings to be elucidated.The remaining 'H resonances were assigned to the substitutedallyl ligand on the basis of the COSY-DQF experiment. Theheavily overlapped region around 6 1.30 contains resonancesassigned to H, (or H,) and Hj and H1. Due to the overlap ofthese resonances, the full assignment of the 'H spectrum wasonly obtained with the assistance of the I3C-DEPT and 13C-'Hcorrelation experiments.A phase-sensitive ' H-'H nuclear Overhauser effect (NOESY)experiment showed weak correlations between the allyl reson-ances HjIl and He,, for one indenyl ring and also between the allylresonance H, and He,, for the other indenyl ring.This result wasused to assign the indenyl rings as 'up7 and 'down' relative to theallyl ligand.The reaction of [Nb(q-C9H7),(p-H2BH2)] with isoprene anda mixture of pyridine and triethylamine gave a red-brownsolution from which no crystalline material could be isolated.The 'H NMR spectrum of the crude oil 8 showed a multitude ofresonances spanning the region from 6 8.2 to -2.2 and clearlyindicates that more than one isomer is present; no conclusionshave been reached about the molecular structure of any of theseisomers. We note that, although the related complex [Ta(q-C,H,),(q3-CH,CHCHMe)] was prepared by the reaction of[Ta( q-C,H ,),C1,] with Mg(CH ,CHCHCH,), at tempts toperform the corresponding reaction with Mg(CH,CHC-MeCH,) gave more than one isomer.26Addition of NMe, to a toluene solution of [Nb(q-C9H,),(p-H,BH,)] containing an excess of styrene slowly produced adeep red solution from which the styrene hydride complex endo-[NbH(q-C,H,),(q-H,C=CHPh)] 9 was obtained as an amor-phous red solid by crystallisation from pentane. Compound 9was found to be thermally stable to 100 "C in solution, and todisplay fluxional 'H and I3C NMR spectra.The 'H NMR spectrum of 9 shows resonances assigned to thestyrene phenyl ring and the indenyl C6 ring in the region 6 6.5-7.1; the six C, resonances are clearly resolved between 6 4.0 and5.1. There is a complex multiplet at 6 1.29, which is assigned tothe olefin CH overlapping with one of the diastereotopic CH,protons, the other CH, proton appears at 6 - 1.33 as a doubletof doublets.The hydride resonance appears as a slightlybroadened singlet at 6 - 0.97. This assignment was assisted by a ' H-'H COSY-DQF spectrum. The connection between the C,and C6 rings was successfully determined from a COSYLRexperiment.The 13C NMR spectrum of 9 was assigned on the basis of a'C-' H shift-correlation spectrum. The ,C-' H correlationexperiment was run on a very strong sample (350 mg ofcompound) and shows minor peaks due to a second compound.These are very tentatively assigned to the e.w isomer of thestyrene complex, although no data will be presented tocharacterise this minor isomer.At ambient probe temperature (294 K) the C,-ring and c6-ring resonances in the 300 MHz 'H NMR spectrum of 9 arebroadened, and on slight warming the CH2 and CH resonance532 J.CHEM. SOC. DALTON TRANS. 1992Table 2 Activation parameters for some styrene hydride complexes ofniobium"ComplexAG(at 50 "C) Ref.endo-[NbH(q-C,H,),(q-H,CSHPh)] 70.0 f 2 This workendo-[NbH(q-C,H,),(q-H,C==CHPh)] 85.7(4)' 28endo-[ NbH( q -C MeS),(q -H ,C=CHPh)] 76.5(4) 27" All values are in kJ mol-' corrected to 50 "C assuming that AS* = + 28 ( _+ 20) J K-' mol-'. From coalescence of the olefin resonances. Bymagnetisation transfer. From coalescence of the C,Me, ('H)resonances.also become broadened. The process which causes this broaden-ing is well understood from the analogous q-C,H, and q-C,Me, c o m p l e ~ e s , ~ ~ * ~ ~ and involves insertion of the olefin intothe Nb-H bond followed by rotations in the resulting 'alkyl' andregeneration of the olefin hydride.We, and others, have recentlyre-investigated the mechanism of this process for the q-C,H,complexes.29930 Complex 9 provided an opportunity further toinvestigate these dynamic processes.The most simple approach to studying the fluxionality in 9involved the use of the coalescence method for the 'H NMRspectra. The variable-temperature ' H NMR spectra wererecorded at 300 MHz in [2H10]o-xylene. The resonancesassigned to the CH,, CH and Nb-N protons were observed tocoalesce at 364 10 K, the exact position of the coalescence isnot clear since there are two sets of overlapping resonances inexchange (i.e., Nb-H +-+ C-H and CH, - CH,).Sinceboth CH, and CH olefin resonances are observed to be involvedin the coalescence, it is clear that the isomer involved is the endoisomer. For the e m isomer, we would observe exchange be-tween the Nb-H and both CH, resonances, but no exchangewith the CH. The olefin-hydride exchange process also resultsin exchange of the indenyl ligand resonances. In the 'H NMRspectrum, the six resonances assigned to the C, ring at lowtemperature coalesce to give two resonances (at 6 4.61 and 4.49)in the ratio 1 : 2 at 374 K. Above 375 K the complex decomposesto give ethylbenzene (identified by reference spectra); thus, thehigh temperature limiting spectrum for the olefin resonances wasnot observed.From the approximate coalescence temperature of the olefinresonances the activation energy can be estimated to be AG =67.5 f 2 kJ mol-' at 364 K.In order to make comparisons withthe data for the q-C5H, and q-C,Me, analogues this value iscorrected to 70.0 f 2 kJ mol-' at 50 "C (ASt = +28 J K-'mol-').28 The AGt values at 50 "C for the q-C5H5, q-C,Me,and q-C9H7 endo-styrene hydride complexes are presented inTable 2. The rate of insertion of these complexes is in the orderq-C9H7 > q-C5Me, > q-C5H5.Bercaw and co-workers28 have interpreted the relative orderof the olefin insertion rates of the q-C,H, and q-C,Me, styrenehydride complexes as being due to steric influences. Thus, theactivated state in the exchange process involves a linear Nb-CH,CH,Ph fragment (this gives the CZv symmetry observed forthe 'H NMR spectra of [NbH(q-C9H7),(q-H2C=CHPh)] inthe fast-exchange limit) which is less sterically demanding thanthe olefin hydride ground state.28 In contrast, in the case of thefour ethylene hydride complexes [MH(q-C,R5),(q-H2C=CH,)] (M = Nb or Ta; R = H or Me) the insertion rates aredetermined by the enhanced donor ability of q-C,Me, relativeto q-C,H,, this stabilises the formally MV metallacyclopropaneground state more for the q-CSMe5 complex.Thus, the order ofinsertion rates is q-C,H, > q-C5Me5 for both Nb and Ta. Inthe case of the propene hydride complexes and the styrenehydride complexes (as discussed above) the influence of stericfactors is dominant.The enhanced rate of olefin insertion for the indenyl complexmight be attributed to steric factors, although this implies thatthe q-C9H7 is larger than the q-C,Me, ligand by approximatelythe same proportion as the q-C,Me, ligand is larger than the q-C5H5 ligand. This assertion appears unlikely.There is noevidence that the rate of olefin insertion in the styrene hydridecomplexes is controlled by electronic factors. Furthermore,photoelectron spectroscopic data have demonstrated that theq-C9H7 ligand is intermediate between the q-C5Hs and q-C,Me, ligands in donor a b i l i t ~ . ~ *We suggest an explanation based on the mechanism we haveproposed to account for the observed kinetics in the exo-[MH-(q-C,H,),(q-H,C=CHMe)] (M = Nb of Ta) c o m p l e ~ e s .~ ~ . ~ ~The mechanism as applied to the endo isomer of a styrenehydride is depicted in Fig. 2. The initial step involves olefininsertion into the Nb-H bond to give an agostic alkyl A, thereverse of this insertion step creates no net exchange ofhydrogen nuclei. In order to exchange Nb-H with C-H, it isnecessary to perform an in-place rotation to give species C. Thereverse of the olefin insertion step from this species will result inan olefin hydride where Nb-H and C-H have exchanged, alsothe CH, sites have exchanged. The 'transition state' to the in-place rotation is proposed to be the 'doubly agostic' species B,which might be formally considered as a 20-electron inter-mediate. In order to maintain an 18-electron count for thisintermediate we propose a ring slip to give an q3-ligand.Wehave previously observed that this in-place rotation is the ratelimiting step for exo-[NbH(q-C,H,),(q-H,CSHMe)] whilstthe olefin insertion step is rate limiting for the Ta anal~gue.~'Thus, the substitution of q-C5R, (R = H or Me) by the q-C9H7ligand is likely to result in a faster rate for this exchange processas a consequence of the 'indenyl effect'.As a further complicating factor we note that the exchangeprocess described above is only one of the two possibleprocesses which result in the observed NMR exchange. Thusthis process results in time-averaged C, symmetry. That is, the'up' and 'down' q-C9H7 ligands are exchanged, but the 'left'and 'right' sides of the ligands are not exchanged.A secondprocess is required to account for the observed C2" symmetryof the system in the high-temperature limiting 'H NMRspectrum. This second process is depicted in Fig. 3 andinvolves dissociation of the agostic alkyl A to give a 16-electron alkyl intermediate B. This alkyl group may thenswing through the wedge defined by the q-C9H7 ligandsbefore reassociating to give the agostic alkyl (C or D). Thisprocess may achieve no exchange of Nb-H with C-H (or ofthe CH, groups) as seen for species C, or may exchange thesesites (species D). Both of these exchange events also achievesome combination of exchange of 'up' with 'down' and of 'left'with 'right' within the Nb(q-C9H,), fragment. This processwould not be expected to be significantly affected by thesubstitution of q-C9H7 for q-C,R, (R = H or Me).Clearly,the presence of exchange involving the diastereotopic indenylligands potentially provides further kinetic data on thisprocess. Thus, we attempted to obtain further quantitativedata.Two-dimensional exchange experiments were attemptedusing the ' H-'H nuclear Overhauser enhancement spectros-copy (NOESY) [or phase-sensitive exchange correlationspectroscopy (PSEXSY)] technique. Although these clearlyshow which C,-ring resonances are undergoing exchange it wasnot possible quantitatively to analyse the data from theseexperiments, due to peak overlap. However, the existence of across-peak (indicating exchange) between the Nb-H resonanceand the CH resonance and between the CH, resonancesprovides further evidence that the complex is the endo isomer.The coincidence of resonances also frustrated attempts tomeasure exchange rates using saturation transfer in the 'HNMR spectrum.32The exchange between the indenyl ligand resonances wasstudied in the ' 3C NMR spectrum using magnetisation transfer(using the DANTE sequence33), the greater dispersion of the13C resonances eliminates overlap.The data from thesJ. CHEM. SOC. DALTON TRANS. 1992 533Bi n -place- rot at io nI \J3-elimination olefin insertionAInitial stateFig. 2 The non-dissociative mechanism, involving in-place rotation of an agostic alkyl, proposed to account for part of the exchange processes incompound 9p-elimination4inversion at Nb and t bond rotationdissociation of D-aaostic olefin insertionPh i PhAinversion at Nb andbond rotation ip-elimination -Fig.3 The dissociative mechanism, involving rotation of a non-agostic alkyl, proposed to account for part of the exchange processes in compound 9.The figures use a modified Newman-type projection for clarity, the q-C,H, rings are represented as horizontal linesexperiments were found to display large error bars, and the T ,values determined by this technique differed greatly from thosedetermined by an inversion-recovery experiment. These datawere thus disregarded as suspect534 J. CHEM. SOC. DALTON TRANS. 1992urn, ur ?' dlul-urn- urH &\HH VPhdl-drn-drI Q d100 90 856Fig. 4 The C,-ring C-H region of the 'H-decoupled I3C-l3CPSEXSY spectrum of compound 9 at 288 KFig.5 The four distinct exchange rates that are observed in theexchange of the C,-ring C-H resonances of compound 9Table 3H,),(q-H,C=CHPh)] 9 obtained using 3C PSEXSY experimentsP.bMagnetisation transfer (kMT) rates for endo-[NbH(q-C,-Rates '/s-~~~ ~T/K k, k b k c kd283 2.15(11) 1.49(11) 0.44( 13) 0.4 1 (28)288 4.29(2) 2.98( 6) 1.05(6) 0.4 I (1 8)298 10.23( 10) 7.66( 72) 1.7 1 (23) 0.90( 66)308 26.67(21) 18.51(99) 3.50( 32) 4.45(85)All data at 75.43 MHz, [12H,]benzene. Analysis was performed usingthe D2DNMR program. Rates are labelled as discussed in the text.The PSEXSY experiment and the D2DNMR program weredeveloped to study exchange in heteronuclear NMR; 34 thisexperiment is well suited to 13C since I3C is a dilute spin andthere are no complications due to 3C-1 3C nuclear Overhauserenhancement (NOE) or scalar coupling.The most seriousdisadvantage is that a very strong sample is required so that atwo-dimensional data set can be obtained in reasonable time.Fig. 4 shows the region of the 3C-1 3C PSEXSY containing theC,-ring C-H resonances of end~-[NbH(q-C~H~)~(q-H,C=CHPh)] at 288 K.Analysis of the data shows that there are four distinct rates(Fig. 5), this is consistent with four possible exchange events(u = up, d = down, r = right, 1 = left, m = middle): (i) thesimple up-down exchange (um - dm) labelled k,, (ii) the up-down exchange of the diastereotopic carbons (ul - dl) and(ur t--) dr) labelled k,, (iii) the cross-exchange (ul - dr) and(ur - dl) labelled k,, and (iv) the left-right exchange withinone ring (ur - ul) and (dr t+ dl) labelled k d .The 13C PSEXSY experiment was performed for 9 at fourtemperatures and the magnetisation transfer rates (kMT) thusobtained are presented in Table 3.The rates can be placed in order, it is clearly the case that k , isthe fastest exchange rate followed by k,, and that k , and k , areslower than either k, or k,.However, the order of k, and k , isnot clear, indeed at all temperatures except 288 K these rates arethe same within experimental error (3 e.s.d.s). Attempts weremade to investigate the relationship between the experimentallymeasured rates k,, k,, k,, kd and the mechanistic rates byconsidering certain limiting cases.These investigations did notgive satisfactory results and it did not prove possible to analysethe data in this way. Thus, we have not been able to investigatethe relative activation barriers to the in-place rotation anddissociative exchange processes in endo-[NbH(q-C,H,),(q-H,C=CHPh)].ExperimentalAll preparations, manipulations and reactions were carried outunder an inert atmosphere of dinitrogen (< 10 ppm oxygen,< 20 ppm water) using standard Schlenk-tube and vacuum-linetechniques, or in a dry-box. Dinitrogen was purified by passagethrough a column containing BTS catalyst and 5 8, molecularsieves.Solvents were pre-dried over activated molecular sieves andthen distilled from potassium [tetrahydrofuran (thf), 1,2-di-methoxyethane (dme)], sodium [toluene, light petroleum (b.p.100-1 20 "C)], sodium-potassium alloy [light petroleum (b.p.40-60 "C unless otherwise stated), diethyl ether, pentane], orphosphorus pentaoxide (dichloromethane), under an inertatmosphere of dinitrogen before use.Deuteriated solvents forNMR samples were used as received (Aldrich), or after dryingover Na-K alloy, samples being prepared in the dry-box. Celite545 filtration aid (Koch-Light) was pre-dried in an oven at80 "C before use.NMR spectra were recorded on either a Briiker AM-200 ['H(200 MHz), IlB (64.17 MHz)] or a Briiker AM-300 ['H (300MHz), 13C (75.43 MHz), 31P (121.44 MHz)] spectrometer.Spectra were referenced internally using the residual solvent ( 'Hand I3C) resonances relative to tetramethylsilane (6 = 0), orexternally using either trimethyl phosphate [P(O)(OMe),] inD 2 0 ( 3 1 P) or BF3-Et20 ( B).All chemical shifts are quoted in6, high-field shifts being taken as negative, and coupling con-stants are in Hertz (Hz). NMR samples were either preparedunder nitrogen in screw-capped tubes (Wilmad), or undervacuum in sealed soda-glass NM R tubes. Two-dimensionalNMR experiments were acquired using standard Briiker soft-ware, and processed using an ASPECT 3000 computer.Low-resolution mass spectra were obtained on an AEI MS302 mass spectrometer, updated by a data handling system sup-plied by Mass Spectrometry Services Ltd. Elemental analyseswere performed by the Analysis Department in this laboratoryor, in the cases of very air-sensitive compounds, by AnalytischeLaboratorien, Elbach.Infrared spectra were recorded as CsI orKBr pellets on a Masston Polaris FT-IR interferometer.The compound [NbCl,(thf),] was prepared by a literaturemethod.35 Indene and styrene (Aldrich) were distilled prior touse, all other reagents were used as receivedJ. CHEM. SOC. DALTON TRANS. 1992 535The salt Li[C,H7] was prepared by the addition of LiBu (100cm3 of 1.6 mol dm-, solution in hexanes, 0.16 mol) to a solution ofindene (20 g, 0.17 mol) in light petroleum (600 cm3). After stirringfor 48 h the product was collected on a frit, washed with lightpetroleum (2 x 50 cm3) and dried in uacuo. Yield 17.8 g,91%.The salt Na[C,H,]*thf was prepared by slowly (1 h) adding athf (200 cm3) solution of indene (30 g, 0.25 mol) to a suspensionof washed NaH (10 g, 0.4 mol) in thf (100 cm3).After stirring for15 h the solution was filtered and the solvent removed underreduced pressure to give a light brown solid which was washedwith a little light petroleum (50 cm3). The stoichiometry wasconfirmed by 'H NMR spectroscopy in CD,CN before use.Yield 90%.Preparafions.-[Nb(q-C,H,),CI,I 1. A vigorously stirredsuspension of [NbCl,(thf),] (14.47 g, 38.2 mmol) in thf (80 cm3)was treated with a solution of Li[C9H7] (9.325 g, 76.4 mmol) inthf (1 20 cm3). During the addition the yellow solid turned greenand then gave a red solution. The mixture was stirred overnight.The green solid was isolated on a bed of Celite, then extractedinto warm CH,Cl, (250 cm3), filtered and cooled to -80 "Cgiving green crystals which became red on drying in uacuo. Asecond crop was obtained by reducing the volume of the mother-liquors to 60 cm3 and cooling to - 80 "C.Yield 5.15 g, 34%.[Nb(q-C,H,),(p-H,BH2)] 2. A stirred mixture of [Nb(q-C9H7),CI,] (1.8 g, 4.6 mmol) and NaBH, (1.0 g, 26 mmol) wastreated with dme (80 cm3). After 30 s gas was evolved and thesolution became purple. After 4 h the solution was filtered andthe solvent removed under reduced pressure. The mauve residuewas extracted into toluene (40 cm3), filtered and the volume ofthe filtrate was reduced to 30 cm3 and pentane (15 cm3) added.Cooling the solution to - 80 "C gave purple crystals which werewashed with cold pentane.Yield 1.23 g, 80%. An analyticallypure sample was obtained after two further recrystallisationsfrom toluene-pentane.One-pot preparation of[Nb(q-C,H,),(p-H,BH,)] 2. A stirredsuspension of [NbCl,(thf),] (21.1 g, 55.8 mmol) in thf (200cm3)was treated with a solution of Na[C,H,]-thf (23.5 g, 101.6mmol) in thf (300 cm3), instantly giving a dark red-brownsuspension which was stirred overnight. The solvent wasremoved under reduced pressure to give an oily solid. Sodiumtetrahydroborate (4 g, 105 mmol) was added, followed by dme(600 cm3), slight effervescence being observed. After stirringovernight, the solvent was removed under reduced pressurefrom the purple solution and the product recrystallised fromeither toluene-light petroleum (1 : l), hot light petroleum (b.p.100-1 20 'C), or used without further purification.Typical yield4.8 g, 14.2 mmol, 25% based on [NbCl,(thf),].[NbH(q-C,H,),(PMe,Ph)] 3. A solution of [Nb(q-C,H7)2(p-H,BH,)] (500 mg, 1.5 mmol) in toluene (40 cm3) wastreated with PMe,Ph (0.5 cm3, z 2 equivalents) giving a darkblue solution which was left to stand for 80 h. Volatiles wereremoved under reduced pressure, and any borane productssublimed onto a - 196 "C probe at lo-, mmHg. The residue wasextracted into diethyl ether (60 cm3) giving a blue-greensolution. Filtration and cooling to - 80 "C gave green crystals.Yield 250 mg, 36"/',.[NbH(q-C,H,),(NC,H5)] 4. A frozen ( - 196 'C) solution of[Nb(q-C9H,)2(p-H2BH,)1 (1.23 g, 3.64 mmol) in toluene (100cm3) was treated with an excess of pyridine (7 cm3).The vesselwas evacuated and back-filled with hydrogen, and allowed towarm to room temperature. A dark green solution resulted,which was stirred for 5 h. The solvent was removed underreduced pressure, and C,H,N-BH, removed by vacuum sub-limation (2 h, 40 'C, lo-, mmHg) to a probe at - 196 "C. Theresidue was extracted into toluene (100 em3), filtered and thevolume reduced to 20 cm3. Dark green needles were producedon cooling to -25 -C. Despite repeated attempts at puri-fication, these needles were shown by NMR always to contain atleast 20",, pyridine. Yield 0.6 g, 41%.[Nb(q-C,H,)(CO),] 5. A solution of [Nb(q-C9H7),(p-H2BH2)] (600 mg, 1.8 mmol) in toluene (60 cm3) was evacuatedand NMe, ( % 10 mmol) distilled into the vessel and CO (1 atm)was admitted.Over 1 h the solution became orange and wasstirred overnight. The solvent was removed under reducedpressure and the residue was extracted into pentane (50 cm3)and filtered. The filtrate was cooled to -80 "C to give theyellow-brown microcrystalline product. Yield 190 mg, 33%.[Nb(q-C9H7),(q3-CH2CHCH2)] 6. Allene (z 6 mmol) andNMe, ( z 4 mmol) were vacuum transferred using a calibratedvacuum manifold into a 200 cm3 Young's ampoule containing asolution of [Nb(q-C9H7),(p-H2BH2)] (500 mg, 1.5 mmol) intoluene (40 cm3) cooled to - 196 "C. The solution was warmedto room temperature and stirred at 20 "C for 36 h, producingvery little colour change. The solvent was removed underreduced pressure, and the residue extracted with light petroleum(50 cm3), filtered and cooled to - 80 "C to give a dark powder.Yield 40%.[Nb(q-C9H,),(q3-MeCHCHCHCH,Me)] 7.A solution of[Nb(q-C,H,),(p-H,BHJ (1.15 g, 3.4 mmol) in toluene (80cm3) in a Young's ampoule was treated with transpans-hexa-2,4-diene (0.6 cm3, 6 mmol). The solution was cooled to- 196 "C and NMe, (z 12 mmol) added by vacuum transfer.The solution was warmed to room temperature and stirred for36 h giving a very deep mauve solution. The solvent wasremoved under reduced pressure and the residue extracted intolight petroleum (3 x 70 cm3), filtered through Celite and cooledto -80 "C giving purple polycrystallites. Yield 0.7 g, 51%.endo-[NbH(q-C9H7),(q-H2C=CHPh)]9. A solution of [Nb-(q-C,H7)2(p-H2BH2)] (1.5 g, 4.5 mmol) in toluene (90 cm3) wastreated with an excess of styrene ( 3 4 cm3) and NMe, (z 1 cm3)added by vacuum transfer.The solution soon became orangeand was stirred overnight. The solvent was removed underreduced pressure and the residue was extracted into diethylether-pentane (1 : 1, 300 cm3) and filtered. The volume of thefiltrate was reduced to 60 cm3 under reduced pressure. Coolingto - 80 "C gave the dark orange polycrystalline product. Yield900 mg, 46%.Analysis of the ' 3C-1 3C PSEXS Y Data Jbr 9.-The BriikerPSEXSY.AUR microprogram 36 was modified to give hetero-nuclear spectra using composite pulse decoupled ' H aecoupling.The spectral width was made as narrow as possible by folding.The mixing times were selected carefully to provide satisfactorycross-peak intensities for both the slow- and fast-exchange rates.The recycle delay was set to 5 x T I , the I3C NMR T I valueswere determined by inversion-recovery and are in the range 1-1.3 s. Prior to Fourier transformation the data were treated withsuitable window functions, typically Lorentzian (LB = 6 Hz) int2 with shifted sine-bell squared in tl.The unsymmetrisedmatrix was examined before the data were symmetrised, noappreciable artifacts were found; the positive levels were foundto be insignificant.Standard Briiker software was used to measure volumeintegrals for the diagonal- and cross-peaks in the C, region ofthe 13C-{1H} PSEXSY spectrum of 9 at four differenttemperatures.When these integrals were measured more thanonce the errors were found to be relatively small. Hence, it wasdecided not to record such integrals more than once or to applystatistical methods to these data. The volume integrals wereused as inputs to the D2DNMR program. The rates in Table 3are the mean values for those rates which are equivalent bysymmetry together with standard deviations. No attempts weremade to produce more rigorous estimated errors by recordingeach data set more than once.ConclusionWe have presented a facile synthesis of the indenyl complexes[Nb(q-C,H,),CI,] and [Nb(q-C,H7),(p-H,BH2)] and de-monstrated that the tetrahydroborate complex is a usefu536 J. CHEM. SOC. DALTON TRANS. 1992precursor to a range of Nb(q-C,H,), complexes.The chemistryof these complexes reflects the chemistry of their Nb(q-CsR5)2(R = H or Me) analogues, although significant differences areobserved. The reasons for these differences have been investi-gated together with a number of fluxional processes in thesecomplexes. These investigations have provided further evidencefor enhancement of exchange processes which are associatedwith ring-slipped q3-indenyl intermediates or transition states.AcknowledgementsWe thank the SERC for a research studentship to A. K. H. Wealso wish to thank A. Sella and L.-L. Wong for helpful dis-cussions and E. W. Abel and D. 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ISSN:1477-9226
DOI:10.1039/DT9920000527
出版商:RSC
年代:1992
数据来源: RSC