Synthetic, Structural, Mechanistic, and Theoretical MO Studies of the

Jan 1, 1995 - Metal-Induced C–N Bond Cleavage in the Decomposition of Alkali (R,R)-Bis(α-methylbenzyl)amide Complexes. Emily C. Border , Melissa Ma...
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Organometallics 1996, 14, 427-439

427

Synthetic, Structural, Mechanistic, and Theoretical MO Studies of the Alkali-Metal Chemistry of Dibenzylamine and Its Transformation to 1,3-Diphenyl-2-azaallyl Derivatives Philip C. Andrews,? David R. Armstrong,? Daniel R. Baker,? Robert E. Mulvey,*yt William Clegg,$ Lynne Horsburgh,* Paul A. O’Neil,$ and David Reed$ Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G l 1XL U.K., and Departments of Chemistry, University of Newcastle, Newcastle upon Tyne, NE1 7RU U.K., and University of Edinburgh, Edinburgh, EH9 355 U.K. Received July 26, 1994@ Dibenzylamido anions ((PhCH2)2N-) can be transformed into 1,3-diphenyl-2-azadyl anions ({PhC(H*N-C(H)Ph}-) by the assistance of PMDETA- ((Me2NCH2CH2)2NMe)complexed Li+, Na+, or K+ cations. The heavier alkali-metal cations give only the trans,trans conformation of the azaallyl anion, in contrast to the lighter Li+ cation, which yields two crystalline conformers, the trans,trans and an unknown species. Ab initio MO geometry optimizations on model Li and Na complexes intimate that it is the relative tightness of the contact ion pair structures which dictates this distinction with Li+ having more influence on the conformation and stability of the anion than Na+, which forms a much looser contact ion pair more akin to the “free” anion. On the basis of kinetic lH NMR studies, combined with X-ray crystallographic data, the amido azaallyl conversion can be explained in terms of a two-step process involving B-elimination of a metal hydride followed by hydride metalation of the produced imine PhCH2N=C(H)Ph. This process appears to be initiated by deaggregation of the metallodibenzylamine to an intermediate monomeric structure, accomplished by solvation. The nature and degree of solvation required depend on the particular M+ cation involved. Three new crystal structures are revealed in the course of this study. All are based on familiar four-membered (N-M)2 rings, but whereas the sodium complex [{(PhCH2)2NNa*TMEDA}21 and the lithium complex C{(PhCH2)2NLi*THF}21are both discrete dimers, unique [{ [(PhCH2)2NLil2.(dioxane)},l, isolated as its toluene hemisolvate, is a polymer composed of linked dimeric units and so is the first dibenzylamido alkali-metal species to have a n infinitely extended structure.

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Introduction Over the past decade, dibenzylamine ((PhCH2)2NH), a nitrogen acid, has played a significant part in the rational development of lithium amide chemistry. Compounds belonging t o this class constitute one of the major tools of contemporary organic synthesis, primarily as strong selective deprotonating agents.l (Dibenzylamidollithim, [{(PhCH2)W},I, has contrastinglyfound use as a nitrogen nucleophile in the aminolysis of esters, producing high yields of the resulting organic amidesq2 However, the most instructive findings to date involving this ligand have been structural rather than synthetic. To begin with, the crystal structure of the unsolvated lithio derivative provided direct evidence of the cyclic, low-oligomeric (in this case, trimeric) constitution of certain lithium amides, while comparison with the dimeric, solvated structures of [{(PhCH2)2NLi*OEt2}21 and [{(PhCH2)2NLi-HMPA}21illustrated the competing + University of Strathclyde.

* University of Newcastle.

5 University of Edinburgh. a Abstract published in Advance ACS Abstracts, December 1,1994.

(1)(a) Wakefield, B. J. Organolithium Methods; Academic Press: London, 1987. (b) Brandsma, L.;Verkruijsse, H. Preparative Polar Organometallic Chemistry; Springer-Verlag: Berlin, 1987;Vol. 1. (2)Yang, IC-W.; Cannon, J. G.; Rose, J. G. Tetrahedron Lett. 1970, 21, 1791.

effects of aggregation and c~mplexation.~ From structural characterization of the lithium amidomagnesiates [ { ( P ~ C H ~ ~ N I ~and I J ~[{(PhCH2)2N}sLiMgpyI, ~M~I we learned that the stoichiometries and coordination geometries are largely dictated by the state (solvated or unsolvated) of the Li+ cations for both steric and electronic reason^.^ Another intermetallic dibenzylamide, this time combining Li+ and Na+, established for the first time that the ladder motif is a structural option for a mixed-alkali-metal comp~sition.~Collectively these structures testify to the coordinative versatility of the dibenzylamido ligand, as they show that its anionic N center can bond terminally, or bridge in p2 or p3 styles, t o metal centers. Besides these simple coordinative distinctions, there exists another facet of alkali-metal dibenzylamide chemistry. Under the mediation of a Na+ counterion complexed by tridentate PMDETA ((Me2NCHzCH2)2NMe) the amido anion ((PhCH&N-) is observed to convert to the 2-azaallyl formulation [PhC(HFN-N-C(H)Phl-K+.(18crown-6)I from an imineWcrown ether mixture.14 Significantly, this preparation required 12 h at reflux temperature. Clearly, therefore, dibenzylamine is superior as a source of the 2-azaallyl anion, as it produces this anion almost immediately at ambient temperature. Though the potassium complex was merely used for this purpose (specificallyto transfer the anion to a zirconium center), it was isolated and the resulting X-ray crystallographic study revealed a monomeric contact ion pair arrangement akin t o that of 2. For completeness, to confirm that NaH could similarly deprotonate the imine to generate the azaallyl species, we reacted such mixtures in the presence of PMDETA and of TMEDmHF. Both preparations were successful, giving the expected dichroic products 2 and 4, respectively, though only in very modest yields, reflecting the comparative inertness of and, linked to this, the poor solubility of the ionic hydride. This conflicts with the facile production of the azaallyl entities via the dibenzylamine route, which is seemingly quantitative. The key difference is that the hydride metalation step occurs in situ in the latter case, and so solubility problems are circumvented. Furthermore, the in situ NaH may be activated either by Lewis base solvation or by remaining attached to the imine produced simultaneously. 2. Lithium Compounds. Mixtures of (dibenzylamidollithium and the donor solvent HMPA or Et20 had previously been examined in some detail, but these only produced simple Lewis acid-Lewis base complexes with intact amido groups3 and gave no hint of possible azaallyl products. In light of the new developments, we subjected the lithium amide to a selection of donor solvents similar to that used in the sodium work. We also studied the lithiation of the imine PhCH2N=C(H)Ph. THF had the same effect as Et20, deaggregating the trimeric lithium amide to a dimeric complex, [{(PhCH2)2NLi*THF)21 (71, whose crystal structure is shown in Figure 7. The amount of excess THF present in the (14)Veya, P.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Chem. SOC.,Chem. Commun. 1991,991.

reaction mixture did not appear to have any bearing on the nature of the product, but 10 molar equiv was necessary to completely dissolve the pink (dibenzylamido)lithium suspension in hexane. Retention of the (PhCH&N group was confirmed by the benzylic CH2 resonance at 6 3.71 in the 'H NMR spectrum of a C4D80 (THF-d8) solution. When the temperature was raised, the pink crystals of 7 melted at 135-137 "C t o a red liquid, with no sign of a dichroic hue, suggesting that the amido character is retained (i.e. it does not convert to an azaallyl form). Switching from THF to polydentate 1,4-dioxane promotes the joining up of the discrete O-complexed (NLi)2 rings into a polymeric chain structure of formula [{[(PhCH~)~NLi]~.(dioxane)),l,isolated as its toluene hemisolvate 8. Characterization was achieved by means of a crystal structure determination (Figures 8 and 9) and NMR spectroscopic studies. The bridging role played by the donor results in a 2/1 lithium amide/donor ratio overall in the structure, corresponding to the initial stoichiometry used in the reaction mixture (i.e., 0.5 molar equiv of dioxane/equiv of lithium amide, though dilute solutions (1mmol of amide in 30 mL of toluene) were necessary t o obtain crystals of 8 and t o avoid the rapid precipitation of a powdery form). However, this reaction proved to be sensitive to a change in stoichiometry with an excess of dioxane (10 molar equiv) producing the donor-rich, crystalline variant [{(PhCH2)2NLi.2(dioxane)},l (9). Distinguishable from 8 by the relative integrals of the benzylic and dioxane resonances on the IH NMR spectrum and by its lower melting point (86-88 "C; cf. dec from 260 "C), these crystals are notably less stable, degrading to an oil after 1 day of storage in an argon-filled glovebox, whereas 8 is stable indefinitely. For this reason the crystal structure of 9 could not be ascertained, though the above characteristics point toward an oligomeric structure, in which case the instability could be connected to dioxane molecules having one ligating end (terminally attached to Li+) and one unligating end. Whatever the precise structure, it is certain that amido character is maintained and that there is no azaallyl formation. Treating the lithium amide with excess TMEDA gave [{[(PhCH2)2NLi12TMEDA},J (lo),a red crystalline complex with the same 2/1 acceptor/donor stoichiometry displayed by 8. Therefore, as was the case with THF and dioxane, solvation by TMEDA fails to disrupt the amido character of the anionic ligand. If we draw comparisons to the sodium work, this failure is not unexpected given the empirical formula of 10,which, to have an integral number of donor molecules, requires dimeric (or higher associated) amido units and not monomeric units which appear essential to azaallyl formation. Though the structure of 10 has not yet been determined, it may be relevant that its 2/1 Li+/TMEDA stoichiometry fits another structural type referred to as an open dimer (see Figure 31, recently established for the lithio (TMEDA)derivative of 2,2,6,6-tetramethylpiperidine.l5 The complexed lithium amide structures proved more resilient than sodium congeners to variations in the quantity of donor solvent present, presumably as a consequence of the greater strength of NLi ring bonds versus NNa ones. This is particularly apparent when a mixed TMEDMHF medium is considered. Whereas ~

~~

(15)Williard, P. G.; Liu, &.-Y. J . Am. Chem. SOC.1993,115, 3380.

Alkali-Metal Chemistry of Dibenzylamine \

N

Organometallics, Vol.14,No. 1, 1995 431

/

I

Li

\/

I /

\

/ \ Figure 3. Idealized general structure of the open dimer adopted by TMEDA solvates of some lithium amides. 7

this combination causes the sodium amide to convert to the tris(so1vated)azaallyl derivative 4 containing both TMEDA and THF ligands, in contrast, the lithium amide remains as such due to the smaller Li+ center being merely monosolvated (by a THF molecule); Le., the mixture produces the same dimeric complex (7) as produced in THF alone, with TMEDA playing no part in complex formation. This preference for THF over TMEDA, which is also observed in solution when the solid TMEDA complex 10 is dissolved in C4DsO (THFds), could be indicative of an energetic preference for the closed-ring dimer over an open-ring alternative.16 Moreover, it is clear that aggregation (dimerization) is more important than solvation in this lithium system, as monosolvated dimers dominate in donor-rich media, with no evidence of bis-solvated (whether by two THF molecules or one TMEDA) monomers. THF being less sterically demanding obviously fits the exocyclic coordination site at Li+ in this dimeric (NLi)2ring structure better than does TMEDA. AnaIogous TMEDA-solvated dimers, which would formally contain four-coordinate Li+ centers, are rare, though they can occur with small and/or flat amido substituents as typified by [{Ph(Me)NLi*TMEDA}2I.l7 Bulkier ligands, on the other hand, force the adoption of monomeric arrangements with formally three-coordinate Lii centers, as demonstrated in [But&H2(H)NLi.TMEDAl. l8 It was decided to investigate the effect of the triamine PMDETA, in the expectation that dimerization (of any type) could no longer be sustained due to the increased steric requirement involved. Accordingly, 1 molar equiv of PMDETA was added to a hexane solution of (dibenzylamido)lithium, which was then heated gently. Slow cooling of the resulting deep red solution to ambient temperature afforded a large crop of purple-red crystals speckled with dichroic flakes. This crop was clearly a mixture; the dichroism suggested azaallyl formation, confirmed by a IH NMR spectrum (Figure 4), which showed no benzylic CH2 resonance and from which the crystals were formulated as [[PhC(HPN-C(H)PhI-Li+-PMDETA](111, though the two distinct isomers 1L4 and 11B could be distinguished. More specifically, the clutch of four signals characteristic of the (trans,trans) 2-azaallyl ligand is clearly visible between 6 6.95 and 6.08 (assigned to 11A) as are the four lower frequency signals between 6 2.37 and 2.10 due to complexed PMDETA. In addition, a clutter of multiplets lies toward the high-frequency end of the aromatic region

(in the range CS 7.7-7.1), assigned to 11B. Significantly, combining the relative integrals of 11A and 11B and comparing this total to the relative integral of the PMDETA signals give a 111 PhC(H)NC(H)Ph/PMDETA stoichiometricratio. Repeating this preparation several times always gave the same mixed product, though the relative proportions of 11A and 11B varied slightly. However, the overall PhC(H)NC(H)Ph/PMDETAstoichiometry remained constant at 111. Clearly, irrespective of the identity of 11B, which as yet remains to be established, the amido azaallyl conversion had taken place, initiated by PMDETA completely deaggregating the lithium amide to a monomeric form primed for @-hydridee1iminati0n.l~In this respect the lithium and sodium systems are in parity. Of course, where they differ is in the fact that only one azaallyl conformer (trans,trans) crystallizes in the latter case, as established by an X-ray analysis of 2, whereas the former system yields two distinct crystalline conformers (11A and 11B). Bracketing the alkali metals together as mere counterions “M+”is clearly erroneous in this case. Hence, this example illustrates why it is important to consider each alkali metal individually in synthetic applications seemingly concerning “anions”.20 To establish whether or not this cocrystallization phenomenon was specific to the dibenzylamido pathway, a 1/1/1 mixture of BunLi, the imine PhCHzN=C(H)Ph, and PMDETA was examined. The deeply colored complex produced by this lithiation reaction (though not involving PMDETA) has incidentally been proposed as an indicator for the standardization of organolithium reagents in ether or hydrocarbon media.21 Performing our lithiation at low temperature (-78 “C)in hexane and warming the solution to ambient temperature before introducing PMDETA also produced a mixture of 11A and 11B (as well as BunH), which, as in the previous case, could not be separated to allow them to be characterized by X-ray diffraction. The overall yield of 11 was high, but more importantly, the relative percentage of 1lA to 11B was found to be approximately 50/50 based on the integration in the lH N M R spectrum. Significantly,however, when PMDETA is present at the outset of the reaction (i.e., added directly to the cold Bun-

(16)For a discussion on effects of TMEDA vs those of THF in alkalimetal systems see: Collum, D. B. Acc. Chem. Res. 1992,25, 448. (17)Barr, D.; Clegg, W.; Mulvey, R. E.; Snaith, R.; Wright, D. S. J. Chem. Soc., Chem. Commun. 1987,716. (18) Fjeldberg, T.; Hitchcock, P. B.; Lappert, M. F.; “home, A. J. J. Chem. SOC.,Chem. Commun. 1984,822.

(19)For related LiH eliminations see: Richey, H. G., Jr.; Erickson, W. F. J. Org. Chem. 1963,48, 4349. (20) For a review of the effect of the gegenion on the structures and stabilities of alkali-metal compounds, see: Lambert, C.; Schleyer, P. v. R. Angew. Chem., Znt. Ed. Engl. 1994,33,1129. (21)Duhamel, L.;Plaquevent, J.4. J . Org. Chem. 1979,44, 3404.

PP9

7 0

5 3

Figure 4. lH NMR (THF-de, 40 “C)of [{[PhC(H)-N=C(H)Phl-Li+*PMDETA},I (11)showing clearly the presence of the two isomeric forms 11A and 11B.

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Andrews et al.

432 Organometallics, Vol. 14, No. 1, 1995

), ..___-‘ -A H

cis,trarzs

N

H

cis,&

trar is,(ratis Figure 5. Idealized conformations of the 1,3-dipheny1-2azaallyl anion. Lihmine mixture), the ratio changes to approximately 90/10. Metalation in this case is presumably via “BunLi-PMDETA, suggesting that sterically and/or electronically a complexed Li+*PMDETAcation acts more like a Na+ cation than a “free”Li+ cation, since it gives the trans,trans conformer (11A)predominately. T w o distinct types of PMDETA are not evident from variabletemperature IH NMR studies of 11, which is perhaps understandable given that 11A and 11B differ in conformation (of the anion) only and that in both isomers PMDETA will probably be found in its normal tridentate manner to a Li+ center attached to the azaallyl C-N-C unit via the central N atom. Idealized conformations of the anion are shown in Figure 5 . 3. Potassium Compounds. Having established that both Li and Na can facilitate the amido azaallyl conversion and having found significant differences between these systems, we decided to extend and complete our experimental survey by examining the effect of the larger K+ cation. Reaction of a 1/1 molar mixture of dibenzylamine and BunK with 2 molar equiv of PMDETA afforded a large crop of needlelike crystals. Their dichroic (greedred) appearance suggested an azaallyl formulation, and this was definitely confirmed by lH NMR spectroscopic studies, which revealed them to be [[PhC(H)-N-C(H)Phl-Kf*2PMDETA1(12). Note that in agreement with the reaction stoichiometry the complex contains two solvent molecules per metal center. This “excess” donor solvent was necessary to completely solubilize an apolar (hexane) solvent suspension of the intermediate amide [{(PhCH2)2NK},I (13). The stoichiometric solvation present in 12 proved insufficient to render the compound arene-soluble, in contrast to what is often the case with organic derivatives of the smaller alkali metals; therefore, its solution lH NMR spectrum had to be run in polar C4DsO (THF-ds). All three alkali-metal cations (Li+,Na+, K+)can thus bring about the dibenzylamido azaallyl transformation. What does differ, of course, and why each alkalimetal system should be individually assessed, is the number of ligating centers required to enforce a monomeric state, given that we are dealing with homoleptic systems. This general point is exemplified here by the fact that one PMDETA ligand is adequate for Na+, whereas K+ needs two such ligands, in keeping with their relative sizes. Complex 12 was examined by X-ray

C181

Figure6. Molecular structure of [{(PhCH2)2”a.TMEDA}21

(3) with atomic labeling scheme. Hydrogen atoms have been omitted.

-

-

Figure 7. Molecular structure of [{(PhCH2)2NLi*THF)21 (7) with atomic labeling scheme. Hydrogen atoms have been omitted. diffraction, but an accurate structure could not be obtained due to poor crystal quality. However, it was possible t o make out its monomer (contact ion pair) arrangement and bis(PMDETA1solvation. Aggregation would not have been expected in any event, given the precedent set by monomeric [[PhC(HPNnC(H)Phl-K+.(18-crown-6)1, discussed earlier.14 The only structural aspect open to debate is whether the K+ center in 12 binds to both types of atoms in the C-N-C linkage, as found in the crown analogue, or whether it binds to the central N alone. Given the multiple nature of the C-N-C bonds and the close proximity of the C and N atoms therein, the former option is preferred. X-ray Crystallographic Studies. Three new crystal structures have been determined during the course of this work. Two of them, the sodium complex [{(PhCH2)2NNa-TMEDA}21(3)and the lithium complex [{(PhCH2)2NLi*THF}2](7), are discretely dimeric (Figures 6 and 7,respectively),while the third, [{[(PhCH2)2NLil~.(dioxane)},l,isolated as its toluene hemisolvate 8 , exists as a polymer made up of two distinct dimeric units (Figures 8 and 9). Here the difference in aggrega-

Alkali-Metal Chemistry of Dibenzylamine

Organometallics, Vol. 14, No. 1, 1995 433 THF = dioxane ether (2.009 A). Clearly this trend is in keeping with the stereochemistries about the ligating 0 atoms, as HMPA is the least restricted, having an essentially linear P=O-Li linkage, and the cyclic ethers THF and dioxane are similar to each other in this regard, whereas the branched, acyclic nature of ether makes it the most sterically crowded. Benzylamido substituents are inherently flexible ligands, as regards the direction and tilt of their phenyl rings, but are relatively inflexible in the way they bridge to the metal centers in these dimeric rings. An exception is the endocycli,c MNM angle, which is mainly dictated by the relative size of the alkali metal M: the larger Na+ cation widens this by about 5-10' compared to that in the (NLi)2rings (i.e., 82.56' for 3; cf. 76.96' for 7 and 74.6 and 73.8' for 8). However, irrespective of the particular alkali metal attached, the anionic N atoms adopt four-coordinate,distorted-tetrahedral (mean bond angles 109.3, 108.9, and 107.8' in 3, 7, and 8, respectively) geometries, and the C-N-C bond angles are all similar (108.4, 109.5, and 109.8 (mean), respectively). The unique feature of 8 is its polymeric association. To link up the dimeric (NLi)2 units into infinite chains, the dioxane molecule adopts a chair conformation. Its four carbon atoms reside in a plane, while the two ligating oxygen atoms lie above and below this plane. The oxygen atoms have a three-coordinated distortedtriangular geometry. The 0(1),Li(l), C(l), and C(4) atoms are approximately coplanar, as are the 0(2), Li(21, C(21, and C(3) atoms. Within these units the smallest bond angles occur inside the ether ring (C(4)O(l)C(l), 110.3'; C(3)0(2)C(2), 110.4'; cf. 129.6, 119.1, 128.7, and 119.2 for C(4)0(1)Li(l),C(l)O(l)Li(l),C(3)O(2)Li(2), and C(2)0(2)Li(2), respectively). A similar pattern was recently observed in the alkylaluminum amide [(Me~Al[N(SiMe3)2l}~.(dioxane)lwith an endocyclic C-0-C angle of 111.4'.22 Theoretical Calculations. Ab initio MO geometry o p t i m l ~ a t i o n swere ~ ~ performed at the SCF level using the 6-31G basis set.24 Figure 5 shows the idealized conformations (trans,trans; cis,trans; cis,cis) of the free 1,3-diphenyl-2azaallyl anion. Simply by inspection, one would anticipate the first- and third-named isomers to be the most and least stable, respectively, as a consequence of the relative proximities of their phenyl substituents. Calculations confirm this is the case. Being considerably distorted from the idealized geometry and having a relative energy over 18 kcal mol-' less favorable than that of the trans,trans isomer, the cis,cis isomer is clearly unrealistic and does not warrant further discussion. Intermediate in stability with a relative energy of 6.4 kcal mol-', the cis,trans optimized (C,) structure is planar, like its trans,trans (C2J counterpart. Both therefore closely resemble their idealized representations in appearance. Delocalization is marginally more

A)

C1281

Figure 8. Molecular structure of [([(PhCH&NLi]y(dioxane)},l (8) in its toluene hemisolvate with atomic labeling scheme. tion state is due to the bifunctionality of l,Cdioxane, which facilitates its monodentate, bridging role. TMEDA can also assume this role, but it is more commonly found in a bidentate, terminal role as in 3. Table 1 compares selected dimensions of the complexed (NM)2 four-memberedrings of 3,7,and 8. As is usual for azaalkali-metal rings, they are rhomboidal with acute angles at N and obtuse angles at M, with the latter being greater for Li+ (103.04(12)' in 7;105.8' (mean) in 8 ) than for Na+ (97.44(6)'). Due to crystallographic symmetry impositions, all these rings are strictly planar. Buckling the ring as in the related monobenzyl causes only slight amide [{PhCH2(Me)NNa*TMEDA}2110 changes in its dimensions (i.e., N-Na bond length cf. 2.40 A in 3; bond angle a t N 80.8', (mean) 2.37 cf. 82.4'; bond angle at Na 96.7', cf. 97.4'). Of greater significance is the closer, more symmetrical approach of TMEDA (N-Na bond lengths 2.496 and 2.511 (mean 2.504 A),cf. 2.489 and 2.678 8, (mean 2.584 A) in 3) which signifies that the dibenzyl units sterically shield the Na+ cation more so than the benzyl methyl units. Four-coordinate overall, the Na+ cation occupies an extremely distorted tetrahedral site (range of NNaN angles 71-139', mean 110.2'). Not surprisingly, the smaller Li+ centers can only attain coordination numbers of 3 in their dimeric setups, bonding to a single monodentate 0 donor ligand. The Li+ geometries are pyramidal, but only marginally so, as evidenced by the sums of their bond angles (i.e., 356.7' in 7; 352.6 and 358.9' in 8). There are no significant differences in the bond lengths involvin the Li+ centers (i.e., (mean) N-Li 2.043 and 1.983 1,O-Li 1.915 and 1.924 A, for 7 and 8, respectively), and when these are compared with corresponding bond lengths in the previously reported ether and HMPA complexes of (dibenzylamido)lithium, the 0-Li bond lengths follow the order HMPA (1.850

A,

A

Figure 9. Polymeric chain structure of 8.

434 Organometallics, Vol. 14,No.1, 1995

Andrews et al.

Table 1. Selected Dimensions of the Complexed ( N M ) 2 Four-Membered Rings of Compounds[{ (PhCH2)~dl'MEDA}21(31, [((PhCHz)zNLi.THF>21 (7), and [EI(PhCH~)2NLilz~(dioxane)),l(8) 3

(A)

7

8

Na-N(1) 2.397(2) Na-N(1a) 2.412(2)

Li-N(l) 2.028(3) Li-N(la) 2.058(3)

N-M-N(deg)

N(1)-Na-N(1a) 97.44(6)

N(1)-Li-N(la)

M-N-M(deg)

Na-N(1)-Na(a) 82.56(6)

Li-N(1)-Li(1a) 76.96(12)

N-M

Li(1)-N(1) 1.939(7) Li(1)-N(1a) 2.037(7) Li(2)-N(2) 1.978(7) Li(2)-N(2b) 1.978(7) N(1)-Li(1)-N(1a) 105.4(3) N(2)-Li(2)-N(2b) 106.2(3) Li( 1)-N(1)-Li( la) 74.6(3) Li(2)-N(2)-Li(2b) 73.8(3)

103.04(12)

1 1 s Figure 10. Ab initio structures of unsolvated (1,3-diphenyl-2-azaally1)sodiummodels. Here, and in Figures 1114, short metal-carbon contacts are represented by broken lines. efficient in the trans,trans arrangement, as evidenced by the lower charge on the central N atom (-0.48 e, cf. -0.54 e). There are also differences, mostly minor, in bond lengths and bond angles, a significant example being the azaallyl C-N-C bond angle, which is wider (135.3", cf. 126.5") in the cis,trans structure to offset increased steric crowding. Calculations on the contact (anion-cation) pair structures (cation = Na+ or Li+) are now discussed for each cation in turn. 1. Sodium Models. The relative stabilities of the trans,trans and cis,trans conformations remain the same (energy difference 5.3 kcal mol-l) in the presence of the larger, more polarizable Na+ cation. Disruption of the pn-pn orbital overlap dominates here, a t the expense of metal-anion bonding, for the preferred structure (14A)has (overall) longer and weaker, though more, Na+-anion interactions than 14B (Figure 10). In the former case, the anion hapticity is v7 (bond lengths: N-Na 2.248 A; C,-Na (x2), 2.964 A;CipsoNa (x2),3.083 Cortho-Na (x2), 2.819 A), whereas in the latter case this decreases to v4 (N-Na, 2.271 A; CaNa, 2.595 A;Cipso-Na, 2.626 A;Codho-Na, 2.590 A) or r5 if the long Ca-Na contact (3.205 A) is included. Note that short metal.*.H(C) contacts present in these calculated structures are disregarded, as it is taken that the bonding is primarily metal-C based and that these short contacts arise because of the close attachment of the H atoms (which carry a positive charge!) to the C atoms. Other notable features of 14A are the small metal-anion incline (37.1") (i.e., the dihedral angle between the N-Na slope and the CNC plane), compared to 50.1" in the crystal structure of 2, and the perceptible tilt (19") of the phenyl rings out of the CNC plane

A;

(22)Her, T.-Y.; Chang, C.-C.; Lee, G.-H.; Peng, S.-M.; Wang, Y. Inorg. Chem. 1994, 33, 99.

.'

A 158 Figure 11. Ab initio structures of monosolvated (1,3diphenyl-2-azaally1)sodium models. toward the centrally disposed Na+ cation. Forcing the metal to lie in the same plane as the trans,trans anion (between Ph rings) results in a loss in stability of 13.3 kcal mol-', advertising the N atom's preference for a pyramidal geometry over a planar one, in its interaction with the metal. Solvation is taken into consideration in models 15A and 15B (Figure 11)with each Naf cation solvated by a single ammonia molecule. Again, the trans,trans conformer is energetically preferred, by a similar margin (5.4 kcal mol-l). The energy gain on solvation is 21.0 kcal mol-l, due to the formation of the N-Na dative bond (length 2.399 A;NNaN bond angle 155.9"). Corresponding values for the cis,trans conformer are 20.8 kcal mol-l, 2.404 A, and 132.8", respectively. The asymmetric anion in this structure (15B)binds to the solvated cation in a v4 arrangement (N-Na, 2.311 A; C,-Na, 2.628 A;Cipso-Na, 2.684 A; Cortho-Na, 2.675 A) or in a v5 arrangement if the long C,,-Na contact (3.203 b) is included. In contrast, 15A displays a looser r7 interaction (N-Na, 2.279 A; Ca-Na (~2),2.980 A; CipsoA;Cortho-Na (x2), 2.936 A),if the long Na ( ~ 2 13.149 , ipso carbon-metal separations are included. Comparison with 14A reveals that, as expected, solvation weakens and loosens the polyhapto anion-cation con~~

~~~~~

(23) Dupuis, M.; Spangler, D.; Wendolowski, J. NRCC Software Catalog; Vol. 1, Program No. QGOl (GAMESS),Daresbury, U.K. (b) Guest, M. F.; Fantucci, P.; Harrison, R. J.; Kendrick, J.; van Lenthe, J. H.; Schoeffel, K.; Sherwood, P. GAMESS-UK; CFS Ltd., 1993. (24)Hehre, W. J.; Ditchiield, R.; Pople, J. A. J . C h e m Phys. 1972, 56, 2257. (b) Hariharan, P. C., Pople, J. A. Theor. Chim. Acta 1973, 28, 213.

Alkali-Metal Chemistry of Dibenzylamine

1

Organometallics, Vol. 14, No. 1, 1995 435

t

T

*

T

2 Figure 12. Ab initio structures of trisolvated sodium and lithium 1,3-diphenyl-2-azaallylderivatives.

tact pair, steepens the metal-anion incline (to 41.0'1, and reduces the tilt of the phenyl rings (to 16"). Model 16 (Figure 12), the tris(ammonia)-solvated trans,trans sodium complex, provides the closest analogy to the crystal structure of the PMDETA solvate 2. However, the three model monodentate ligands are clearly more efficient at solvating, and are more compactly organized about, the metal center than the sole tridentate ligand. This is reflected in a substantial elongation of the (anion) N-Na bond (length 2.501 A; cf. 2.384 A in Zl7> and an exaggerated metal-anion incline (77.8"; cf. 50.1" in 2). Rarely can the actual solvating ligands used in the synthetic experiments be considered theoretically, as the number of atomic orbitals usually has to be restricted in a calculated structure (since NH3 or H2O molecules have fewer, they are often used to simulate the solvent molecules);therefore, such difTerences between real and theoretical solvates are to be expected. On account of the extra solvation in 16, the tilt of the phenyl rings (8.2") becomes less significant compared to that in 15A. 2. Lithium Models. The experimental survey implied that there is an approximately equal chance of the 2-azaallyl anion adopting either the trans,trans or unknown form when it is in contact with the smaller, less polarizable Li+ cation in the absence of solvation. On the other hand, the trans,trans conformer forms exclusively with Na+. At the simplest level, this suggests that in the latter case the contact ion pair is loose, effectively approaching the "free" anion, whereas in the former case, the contact ion pair is much tighter to the extent that the metal ion now exerts a much greater influence on the stability of the anion. Hence, in effect, the close attachment of the Li+ cation reduces the energetic preference for the trans,trans conformation over other possible conformations. The relative energies of the calculated structures strongly support this idea. Model 17A, the trans,trans Li (C,) structure, is more stable than 17B, the cis,trans ((21) counterpart, but by a mere 1.7 kcal (Figure 13). This is in contrast with the much larger differentials of 6.4 and 5.3 kcal found for the free anion and Na+ complex analogs, respectively. Interestingly, however, model 17A is not the most stable arrangement. Model 17C (Figure 131, another trans,trans structure but an asymmetrical one

17c Figure 13. Ab initio structures of unsolvated (1,3-diphenyl-2-azaally1)lithiummodels.

with the Li+ cation oriented toward, and thus strongly interacting with, one side of the anion, has the lowest energy of all, 1.9 kcal more favorable than 17A. This Li+ cation makes four short contacts (to N, 1.944 A; to Ca, 2.163 A; to Cipso, 2.250 A; to Coda, 2.265 A) with the shortest contact to the remote side of the anion being considerably longer (C,, 3.048 A). In relation to the CNC plane, the metal is inclined at an angle of 31.9". On monosolvation, the energy gap between the C, trans,trans structure (la) and cis,trans (18B)minima (Figure 14) increases (to 2.9 kcal), but not significantly so. In the favored structure (MA) the anion assumes a mode toward the metal (bond lengths: to N, 1.955 to C,, Cat, 2.738 A; to Codo,Codo,,2.693 A; to Cipso, Cipso', 2.932 A), which is inclined a t an angle of 29.1" toward the CNC plane. As in the analogous Na model (15A), the phenyl rings tilt out of this plane in the direction of the metal center, a t an angle of 20". Solvation in 18A provides an additional stability of 27.5 kcal mol-l when compared against the unsolvated analog 17A, due to the formation of the (ammonia)N-Li bond (length 2.031 A). Significantly, however, as in the unsolvated series 17A-C, again the lowest minimum of all (by 1.4 kcal mol-l; cf. 18A) is an asymmetric variant of the trans,trans structure (18C, Figure 14) in which the lithium coordination is biased toward one side of the anion. The Li+ cation interacts strongly with the (anionic) N (1.985 A), Ca (2.269 A), Cipso (2.354 A), and C o d o(2.357 A) as well as with NH3 (N-Li, 2.036 A), while the nearest C (C,,) of the remote side of the anion lies 3.035 A away from the metal. As a result, the C-N bonds of the central CNC unit (bond angle 127.3") are inequivalent, being 1.361 and 1.290k The longer bond involves the C, atom strongly attached to the Li+ center. Lying 36.6" out of the CNC plane, the Li+ center's attraction for one side of the anion can be gauged by a comparison of the CaNLi and Ca*NLibond angles, which are 83.2 and 134.7", respectively. It is significant that

d

436 Organometallics, Vol. 14, No. 1, 1995

. 14

4

i

L. i

Figure 14. Ab initio structures of monosolvated (1,3diphenyl-2-azaally1)lithiummodels.

this asymmetrical bridging is exclusive to Li; corresponding Na models when freely optimized always reverted to the C, trans,trans arrangements (e.g., 14A and 15A). To convert a trans,trans structure to a cis,trans alternative requires rotation of one of the PhC(H) units about the N-C axis. With Li having two inequivalent N-C "axes" in 18C,the implication is that there will be two distinct energy barriers, depending on which PhC(H) unit (metal coordinated or metal noncoordinated) is rotated, i.e., a lower and a higher one. Irrespective of which half of the anion is involved, rotation should be distinctly easier than in the Na structure, where both PhC(H) units would have the same energy barrier to rotation because of the symmetrical nature of the CNC metal bridge. Hence, it should be easier to form the two conformers (the trans,trans and cis,trans) in the Li case, though whether the cis,trans species is produced in the synthetic experiments is still open to debate. The tris(ammonia1 solvate 19 (Figure 12) provides some indication of the likely structure of the crystalline trans,trans Li PMDETA solvate 11A, for which no crystal structure is available. It also enables a comparison to be made with 16,from which the influence of the identity of the metal cation (Li+ or Na+) in the structure can be ascertained. In both cases, the ammonia ligands form shorter bonds to the metal cation (2.11912.129A in 19;2.45912.470A in 16)than do the central nitrogen atoms of the CNC linkages (2.327A in 19;2.501 A in 16). Solvation by PMDETA would not be as efficient as that achieved by the less sterically inhibited ammonia molecules. Therefore, the cationanion separations are overestimated in the calculated structures (by approximately 5%, when the (anion) N-Na bond length in 16 is compared with that in crystalline 2 (2.384A)). Size dictates that Li+ cations are more strongly solvated than Na+ cations. Here, the

Andrews et al. respective total enthalpies of solvation are 57.7and 50.6 kcal mol-', which corresponds to 19.2 and 16.9 kcal mol-l, respectively, per ammonia molecule. Geometrically, the complexes are very similar in the vicinity of the cation-anion attachment. For 19,the cation-anion inclination angle is 77.9",the CNC bond angle is 127.2", and the tilt of the phenyl rings toward the cation is 9.0". Corresponding dimensions in 16 are 77.8, 127.7,and 8.2",respectively. This finding is consistent with the experimental NMR data of the lithium (11A) and sodium (2)complexes, which give essentially identical lH spectra. The similarity extends to the anion hapticity ( q 3 ) , as aside from the central N, the cations interact with the pair of equivalent C, atoms (bond lengths: for Li+, 2.780A; for Na+, 2.934A) but not with the Corthoatoms, which lie 3.758 and 3.783 A away, respectively. However, in reality, these latter atoms lie significantly closer to the metal center (e.g., 3.133A in the crystalline Na+ complex 21, due to the poorer solvating power of the sterically bulkier PMDETA ligand. The conclusion of this theoretical part of the study is that it is the tightness of the metal-anion contact pair (greater for Li+ than Na+) and, linked to this, the ability of the stronger interacting metal center to lean toward one particular side of the anion, which offers the possibility of a second stable anion conformation in addition to the symmetrical trans,trans one-the preferred choice in the absence of a metal cation. Experimental Section All manipulations were carried out under an argon atmosphere using standard Schlenk techniques. Solvents were distilled from an appropriate drying agent prior to use. All chemicals were obtained from Aldrich with the exception of BunNaZ5and Bu"K,~~ which were synthesized according to reported methods. lH and 13CNMR spectra were recorded on a Bruker AMX400 spectrometer operating at 400 and 100.6 MHz, respectively. Products 1-13 were all shown t o be airand moisture-sensitivebut were stable (with the exception of 9) indefinitely when stored under argon. Notably, the product yields reported are first-batchfigures (basedupon consumption of the starting metalating reagent) with a view t o obtaining single crystals suitable for X-ray diffraction and are not optimized. Preparation of [{(PhCH&NNa),l (1). Dibenzylamine (1.92 mL, 10 mmol) was added dropwise t o a chilled, stirred suspension of BunNa (0.80 g, 10 mmol) in hexane (-8 mL). Warming to room temperature deposited a red solid, which was washed with chilled hexane and dried in vacuo. Yield: 1.71g, 78%. Mp: 89-91 "C to a dichroic melt. Anal. Calcd for C14HIdNNa: C, 76.7; H, 6.4; N, 6.5; Na, 10.5. Found: C, 75.8; H, 6.3; N, 6.8; Na, 10.9. lH NMR (benzene-&,25 "C) in ppm: 6 7.29-7.07 (m,0-, m - , p - H , lOH), 3.67 (s, PhCHz, 4H). Preparation of t{tPhC(H)~N.-.C(H)Phl-Na+.PMDETA)] (2). Freshly prepared 1 (2.19 g, 10 mmol) was stirred in hexane (-8 mL). Dropwise addition of PMDETA (2.08 mL, 10 mmol) and toluene (-5 mL) completely dissolved the red precipitate with gentle heating. Cooling the deep purple solution to room temperature afforded a large crop of red-green dichroic needles which were washed with chilled toluene and dried in vacuo. Yield: 1.76 g, 45%. Mp: 169 "C to a dichroic melt. Anal. Calcd for C&3&": C, 70.8; H, 9.0; N, 14.4; Na, 5.9. Found: C, 71.1; H, 8.6; N, 13.9; Na, 5.9. lH NMR (THF-ds,25 "C) in ppm: 6 7.04 (br s, 0 - H , 4H), 6.95 (t, m - H , 4H), 6.79 (s, PhCH, 2H), 6.34 (t,p-H, 2H); PMDETA signals 2.53 and 2.45 (t, NCH2, 4H), 2.31 (s, NCH3, 12H), 2.24 (s, (25) Lochmann, L.; Pospisil, J.; Lim, D. Tetrahedron Lett. 1966,2, 257. (26) Lochmann, L.; Lim, D. J. Organomet. Chem. 1971,28, 153.

Alkali-Metal Chemistry of Di benzylamine NCH3,3H). 13CNMR (THF-ds, 25 "C) in ppm: 6 144.27 (ipso114.35 (PhCH), 110.06 @-C); C), 127.91 (m-C), 117.50 (0-0, PMDETA signals 57.42 and 55.48 (CHd, 44.76 (CH3, terminal), 41.94 (CH3, central). Preparation of [{(PhCH2)2NNaTMEDA}al(3). Freshly prepared 1 (2.19 g, 10 mmol) was stirred in hexane (-8 mL). Dropwise addition of TMEDA (1.51 mL, 10 mmol) and toluene (-1 mL) with gentle heating completely dissolved the red precipitate. Cooling the red solution t o room temperature afforded a large crop of red crystals which were washed with chilled toluene and dried in vacuo. Yield 1.41 g, 42%. Mp: 83-84 "C to a red liquid. Anal. Calcd for C~oH30N3Na:C, 71.6; H, 9.0; N, 12.5; Na, 6.9. Found: C, 74.3; H, 8.4; N, 11.9; Na, 7.5. lH NMR (benzene-&, 25 "C) in ppm: 6 7.44 (d, 0-H, 4H), 7.31 (t, m-H, 4H), 7.15 (t,p - H , 2H), 4.48 (br s, PhCH2, 4H); TMEDA signals 1.93 (s, CHZN,4H), 1.83 (5, NCH3, 12H). 13CNMR (benzene-&, 25 "c) in ppm: 6 149.50 (ipso-c), 128.39 (0-C), 128.03 (m-C), 125.46 (p-C), 62.77 (PhCHz); TMEDA signals 57.48 (CHz), 45.45 ( C H 3 ) . Preparation of [[{PhC(H)nNW(H)Phl-Na+*TMEDA. THF}](4). Freshly prepared 1 (2.19 g, 10 "01) was stirred in hexane (-8 mL). Dropwise addition of TMEDA (1.51 mL, 10 mmol) and THF (0.8 mL, 10 mmol) completely dissolved the red precipitate with gentle warming. Cooling the deep purple solution to room temperature afforded a large crop of red-green dichroic crystals which were washed with chilled toluene and dried in vacuo. Yield: 1.86 g, 46%. Mp: 120122 "C to a dichroic melt. Anal. Calcd for C~H36N30Na:C, 71.1; H, 8.9; N, 10.4; Na, 5.7. Found: C, 71.0; H, 7.8; N, 4.3; Na, 6.1. IH NMR (THF-ds, 25 "C) in ppm: 6 6.81 (br s, 0-H, 4H), 6.78 (t, m-H, 4H), 6.64 (s, PhCH, 2H), 6.18 (t, p - H , 2H); TMEDA signals 2.29 (8,CHzN, 4H), 2.12 (8,NCH3,12H); THF signals 3.60 (m, a-CH2, 4H), 1.75 (m, B-CHz, 4H). 13C NMR (THF-ds, 25 "C) in ppm: 6 145.47 (ipso-C), 129.16 (m-C), 118.76 (0-C), 115.54 (PhCH), 111.43 (p-C); TMEDA signals 58.84 (CHz), 46.24 (CH3); THF signals 68.38 (a-CHd, 26.50 (B-CHd. Preparation of [{ (P~CH~)~WWTHF'}~I (5). Freshly prepared 1 (2.19 g, 10 mmol) was stirred in hexane (-8 mL). Dropwise addition of THF (1.60 mL, 20 mmol) completely dissolved the red precipitate with gentle heating. Cooling the dark red solution to room temperature afforded a large crop of red crystals which were washed with chilled toluene and dried in vacuo. Yield: 1.19 g, 41%. Mp: 78-80 "C to a dichroic melt. Anal. Calcd for ClsH22NONa: C, 74.2; H, 7.6; N, 4.8; Na, 7.9. Found: C, 73.9; H, 7.3; N, 4.9; Na, 8.2. lH NMR (THF-&,25 "C) in ppm: 6 7.23-7.16 (m, 0-,m-H, 8H), 7.04 (t, p - H , 2H), 3.96 (9, PhCH2, 4H); THF signals 3.63 (m, a-CH2, 4H), 1.77 (m, B-CH2, 4H). 13C NMR (THF-de, 25 "C) in ppm: 6 149.93 (ipso-C),128.78 (0-C),128.41(m-C), 125.42@C), 63.51 (PhCH2);THF signals 63.38 (a-CHz), 26.49 (B-CHz). Preparation of [{ [PhC(H)nN-C(H)Phl-Na+*3THF} 1 (6). Freshly prepared 1 (2.19 g, 10 mmol) was stirred in hexane (-8 mL). Dropwise addition of excess THF (2.4 mL, 30 mmol) completely dissolved the red precipitate with gentle heating. Cooling the deep purple solution t o 3 "C for 24 h afforded a large crop of red-green dichroic crystals which were washed with chilled toluene and dried in vacuo. Yield: 2.13 g, 49%. Mp: 62-64 "C to a dichroic melt. Anal. Calcd for C26H3803Na: C, 72.1; H, 8.3; N, 3.2; Na, 5.3. Found C, 71.0; H, 6.4; N, 3.3; Na, 5.0. IH NMR (THF-de, 25 "C) in ppm: 6 6.88 (br s, 0-H, 4H), 6.79 (t, m-H, 4H), 6.34 (s, PhCH, 2H), 6.18 (t,p-H, 2H); THF signals 3.60 (m, a-CH2, 4H), 1.77 (m, P-CHZ,4H). 13C NMR (THF-ds, 25 "C) in ppm: 6 145.49 (ipso-C), 129.12 (m-C), 121.66 (0-C), 115.50 (PhCH), 111.40 (p-C); THF signals 63.38 (a-CHd,26.49 (P-CHZ). Preparation of [{ (PhCH&NLi*THF)al(7). Dibenzylamine (1.92 mL, 10 mmol) was added dropwise to a chilled, stirred solution of BunLi (10 mmol in hexane, 6.9 mL of a 1.45 M solution), which resulted in the formation of a pink solid. Addition of THF (8 mL, 100 mmol) completely dissolved the pink precipitate with gentle heating. Cooling the red solution to ambient temperature afforded a large crop of pink crystals

Organometallics, Vol. 14,No. 1, 1995 437 which were washed with chilled THF and dried in vacuo. Yield: 1.84 g, 67%. Mp: 135-137 "C to a red liquid. Anal. Calcd for ClsH22NOLi: C, 78.6; H, 8.0; N, 5.1; Li, 2.6. Found: C, 78.6; H, 7.8; N, 6.1; Li, 2.3. IH NMR (THF-de, 25 "C) in ppm: 6 7.31-6.97 (m, 0-,m-, p-H, lOH), 3.71 ( 6 , PhCH2,4H); THF signals 3.63 (m, a-CH2, 4H), 1.77 (m, B-CHz, 4H). 13C NMR (THF-ds, 25 "C) in ppm: 6 148.50 (ipso-C), 129.22 (0-0, 128.47 (m-C), 125.80 (p-C), 61.88 (PhCH2);THF signals 68.29 (a-CHZ), 26.43 (P-CHz). Preparation of [{ [(PhCH2)2NLil2*(dioxane)}J (8). Dibenzylamine (1.92 mL, 10 mmol) was added dropwise to a chilled, stirred solution of BunLi (10 mmol in hexane, 6.9 mL of a 1.45 M solution), which resulted in the formation of a pink solid. Dropwise addition of 1,Cdioxane (0.43 mL, 5 mmol) resulted in the formation of a red solid. Single crystals (red) were obtained by preparing 8 (in its toluene hemisolvate form) on a 1mmol scale in 30 mL of toluene, heating to 100 "C, and cooling to room temperature over several hours. Both crystals and solid gave identical analyses. Yield: 0.26 g, 52% (crystals). Mp: >360 "C. Crystals do not melt below this temperature but turn from red to white above 250 "C. Anal. Calcd for C32H&202Li: C, 77.7; H, 7.3; N, 5.7; Li, 2.8. Found: C, 79.0; H, 6.2; N, 5.8; Li, 2.9. 'H NMR (THF-&, 25 "C) in ppm: 6 7.19-7.04 (m, 0-,m-,p-H, 20H), 3.62 (6, PhCH2,8H); dioxane signal 3.56 (s,OCH2, 8H); toluene signals 7.13 (m, 0-,m-,p-H, 5H), 2.31 (5, PhCH3, 3H). Integration indicates variable amounts of toluene in crystals, as this is easily lost when the product is dried in vacuo. 13CNMR (THF-de, 25 "C) in ppm: 6 148.51 (ipso-C), 129.22 (0-C), 128.47 (m-C), 125.79 (p-C), 61.67 (PhCH2); dioxane signal 67.87 (OCH2); toluene signals 138.27 (ipso-C), 129.62 (0-C), 128.90 (m-C), 126.03 (p-C), 21.51 (PhCH3). Preparation of [{ (PhCHdzNLi*2dioxane),1(9). Dibenzylamine (1.92 mL, 10 mmol) was added dropwise to a chilled, stirred solution of BunLi (10 mmol in hexane, 6.9 mL of a 1.45 M solution). Dropwise addition of excess 1,4-dioxane (8.5 mL, 100 mmol) and toluene (-10 mL) completely dissolved the precipitate with gentle warming. Cooling the purple solution t o room temperature afforded a large crop of pink crystalline flakes which were washed with chilled toluene and dried in vacuo. Yield: 2.16 g, 57%. Mp: 86-88 "C to a red liquid. Anal. Calcd for C22H30N04Li: C, 69.7; H, 7.9; N, 3.7; Li, 1.9. Found C, 70.0; H, 7.6; N, 4.3; Li, 1.7. 'H NMR (THF-ds, 25 "C) in ppm: 6 7.16-7.09 (m, 0-,m-,p-H, lOH), 3.58 (8,PhCH2, 4H); dioxane signal 3.56 (8,OCH2, 16H). 13C NMR (THF-de, 25 "C) in ppm: 6 148.49 (ipso-C), 129.22 (0-C), 128.48 (m-C), 125.81 (p-C), 61.68 (PhCHZ); dioxane signal 67.88 (OCH2). Notably, 9 was stable only for a few days under argon before converting into a pink oil, although the oil retains the analytical makeup. Preparation of [{[(PhCH2)aNLilz.TMEDA},,l (10). Dibenzylamine (1.92 mL, 10 mmol) was added dropwise to a chilled, stirred solution of BunLi (10 mmol in hexane, 6.9 mL of a 1.45 M solution), which resulted in the formation of a pink precipitate. Dropwise addition of TMEDA (1.51 mL, 10 "01) and toluene (-15 mL) completely dissolved the pink solid on gentle heating. Cooling the red solution to room temperature afforded a large crop of red crystals which were washed with chilled toluene and dried in vacuo. Yield: 2.56 g, 49%. Mp: 117-119 "C to a red liquid. Anal. Calcd for C34Hd4Li~:C, 78.2; H, 8.4; N, 10.7; Li, 2.7. Found: C, 78.9; H, 8.6; N, 7.3; Li, 2.9. lH NMR (THF-ds, 25 "C) in ppm: 6 7.18-7.03 (m, 0 - , m-,p - H , 20H), 3.59 (8, PhCH2, 8H); TMEDA signals 2.32 (br s, CH2N, 4H), 2.17 (br s, NCH3, 12H). I3C NMR (THF-ds, 25 "C) in ppm: 6 148.50 (ipso-C), 129.22 (0-C), 128.47 (m-C), 125.80 (p-C), 61.78 (PhCH2); TMEDA signals 59.10 (CHZ), 46.36 (CH3).

Preparation of [{[PhC(H)-NmC (H)Phl-Li+.PMDETA},,](11). Dibenzylamine (1.92 mL, 10 "01) was added dropwise to a chilled, stirred solution of BunLi (10 mmol in hexane, 6.9 mL of a 1.45 M solution), which resulted in the precipitation of a pink solid. Dropwise addition of PMDETA (2.08 mL, 10 mmol) completely dissolved the pink solid. Cooling the deep

Andrews et al.

438 Organometallics, Vol. 14, No. 1, 1995 Table 2. Structure Determination Summary 3 I formula color, habit cryst size (mm) cryst syst space group a (A)

b (A) c

(A)

a (de&

B (deg) Y (deg)

v (A3)

Z fw density (calcd) (g/cm3) abs coeff (mm-')

WW temp (K) 28 range (deg) max indices hkl no. of rflns collected no. of indep rflns weighting parameters a, b no. of params refined R, (all data) R (obsd data) goodness of fit max, min electron dens (&A3)

CaoHnaN$rlaz

redprism 0.58 x 0.54 x 0.52 trislinic P1 9.674(3) 11.183(3) 11.373(3) 114.02(2) 111.952(8) 91.339(14) 1019.7(5) 1 670.92 1.093 0.083 364 240 4-50 11,13,13 7103 3604 0.0811, 0.1390 222 0.1668 0.0546 (2162) 1.059 +0.20, -0.23

purple solution to room temperature afforded a large crop of microcrystalline product (purple-red crystals speckled with dichroic flakes) which was washed with chilled toluene and dried in vacuo. Yield: 1.23 g, 33%. Mp: 112-114 "C to a dichroic melt. Anal. Calcd for C Z ~ H W N ~ C, L ~73.8; : H, 9.4; N, 15.0; Li, 1.9. Found: C, 69.5; H, 9.9; N, 15.9; Li, 1.9. 'H NMR (THF-de, 40 "C) in ppm: 6 7.80-7.06 (m, 0-, m-, p - H and PhCH, 12H), 6.93-6.84 (br s, 0-H, 4H), 6.68 (t,m-H,4H), 6.50 (s, PhCH, 2H), 6.03 (t, p-H, 2H); PMDETA signals 2.35 and 2.27 (dm,CHzCHz, 8H), 2.13 ( 8 , NCH3,6H), 2.10 (9, NCH3, 24H).

Preparation of [{ [PhC(H)nN 2a(Fa2),the latter for comparison with conventional refinements based on F values. Programs were standard Stoe control software, members of the SHEW family,29and locally written routines, running on personal computers and UNM workstations.

+

+

+

(29)Sheldrick, G. M. SHELXTLIPC Manual;Siemens Analytical X-Ray Instruments, Inc.: Madison, WI,1990;SHE=-93, Program for Crystal Structure Refinement; University of mttingen, mttingen, Germany, 1993.

Table 7. Atomic Coordinates ( x lo4) and Equivalent Isotropic Displacement ParameteH (A2x 1oJ) for 8 X Y z Wes) O(1) O(2) C(1) C(2) C(3) C(4) Li(1) NU) C(5) C(6) C(7) C(8) C(9) C(10) C( 11) C(12) C(13) ~(14) ~(15) C( 16) C(17) C(18) Li(2) N(2) C(19) C(20) C(21) C(22) C(23) C(24) C(25) C(26) ~(27) C(28) C(29) C(30) ~(31) ~(32)

2947(3) 1829(3) 1693(5) 1807(5) 3073(5) 2962(5) 4079(7) 3720(3) 3808(5) 4813(4) 423 l(5) 5175(5) 6731(6) 7331(5) 6380(5) 2711(5) 2261(5) 967(5) 512(7) 1398(9) 2713(8) 3135(6) 618(8) 531(3) 2022(5) 2804(5) 3724(5) 4427(5) 4207(7) 3284(7) 2592(5) -313(5) 359(4) 1165(5) 1798(5) 1631(6) 836(6) 214(5)

3754(2) 1876(2) 3303(4) 2111(4) 2351(3) 3527(3) 4740(5) 4896(2) 3915(3) 3953(3) 4321(3) 4395(3) 4080(4) 3702(4) 3650(3) 5733(3) 6650(3) 6640(4) 7478(6) 8330(15) 8357(4) 7514(4) 784(5) 7W) -411(3) - 1106(3) -687(4) -1339(5) -24 1O(5) -2824(4) -2183(4) 616(3) 1613(3) 1637(4) 2564(4) 3478(4) 3481(4) 2558(3)

1930(2) 3448(2) 1832(3) 2412(3) 3547(3) 2955(3) 752(4) -544(2) -732(3) -1692(3) -2582(3) -3463(3) -3464(4) -2586(4) -1711(3) -1161(3) -825(3) -139(4) 185(5) -187(5) -859(4) -1185(3) 4457(4) 5921(2) 6239(3) 5699(3) 4787(3) 4281(4) 4694(6) 5585(6) 6079(4) 6560(3) 64433) 7138(3) 6991(4) 6161(4) 5463(4) 5603(3)

53.0(7) 54.2(7) 61.5(12) 63.4(12) 53.9(11) 56.9(11) 48(2) 42.6(7) 50.9(10) 44.5(9) 52.8(10) 60.1(11) 65.4(12) 64.5(12) 55.8(11) 53.2(10) 50.5(10) 69.2(13) 94(2) 99(2) 89(2) 64.5(12) 48(2) 42.6(7) 58.1(11) 50.0(10) 58.2(11) 82(2) 94(2) 92(2) 67.8(13) 51.8(10) 48.4(10) 61.9(11) 73.2(14) 71.9(13) 67.1(13) 58.8(11)

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

Table 8. Selected Bond Lengths (A) and Angles (deg) for 8a 0(2)-Li(2) 1.922(6) O(1)-Li( 1) 1.926(6) Li(1)-N( 1) Li(2)-N(2) C( l)-O(l)-C(4) C(4)-O(l)-Li(l) C(3)-0(2)-Li(2) O(1)-Li(1)-N(1) N(1)-Li(1)-N(1a) C(l2)-N(l)-Li(l) C(l2)-N(l)-Li(la) Li(1)-N(1)-Li(1a) 0(2)-Li(2)-N(2b) C(19)-N(2)-C(26) C(26)-N(2)-Li(2) C(26)-N( 2)-Li(2b)

1.939(7) 1.978(7)

Li( 1)-N( la) Li(2)-N(2b)

110.3(3) 129.6(3) 128.7(3) 118.9(3) 105.4(3) 125.6(3) 115.1(3) 74.6(3) 120.2(3) 109.7(3) 120.4(3) 118.1(3)

C(1)-O(1)-Li(1) C(3)-0(2)-C(2) C(2)-0(2)-Li(2) O(l)-Li(l)-N(la) C(12)-N(l)-C(5) C(5)-N(l)-Li(l) C(5)-N(l)-Li(la) 0(2)-Li(2)-N(2) N(2)-Li(2)-N(2b) C(19)-N(2)-Li(2) C(19)-N(2)-Li(2b) Li(2)-N(2)-Li( 2b)

2.037(7) 1.978(7) 119.1(3) 110.4(3) 119.2(3) 128.3(3) 109.9(3) 118.0(3) 106.9(3) 132.5(3) 106.2(3) 113.7(3) 117.5(3) 73.q 3)

Symmetry transformations used to generate equivalent atoms: (a) -x (b) -x, - y , - Z 1.

+ 1, -y + 1, -z;

+

Refined coordinates are given in Tables 3, 5, and 7 and selected bond lengths and angles in Tables 4,6, and 8.

Acknowledgment. We thank the EPSRC for financial support and Professor C . L. Raston (Griffith University, Brisbane, Australia) for useful discussions. Supplementary Material Available: Tables of bond lengths and angles, anisotropic displacement parameters, and hydrogen atom coordinates and isotropic displacement parameters for the crystal structures of 3, 7 , and 8 (10 pages). Ordering information is given on any current masthead page. OM940597P