THE USE OF HETEROMETALLIC BRIDGING MOIETIES TO GENERATE TRACTABLE LANTHANIDE COMPLEXES OF SMALL LIGANDS

COMMUNICATIONS P. L. Pauson. Z>rruhi,dron 1985. 41. 5855-5860, N. E. Schore. Org. ReucI. S. Shamhayati, W. E. C r o w . S. L. Schreiber, Tetrahedron L ~ I I 1990.3t. 5292. M. Thorninen, P. Gerber. R. Keese. Chbnia 1991. 45, 21 -24: N. Jeong, Y. K. Chung. B. Y Lee, S. H. Lee. S:E. Yoo, Sjnkerl 1991. 204-206: M. E. Kraflt. I . L. Scott, R. H. Romero. S. Feibelmann, C. E. Van Pelt, J. A m . Chrm. S i x . 1993. 115. 7199 7207. A. 7. Balabaii. M. Banciu, V. Ciorha, Annulenrs, Bunzo-, Helero-, Homo- D c r i w r i w s und rhrir fiilenre lsonwr. 3 u ( F ) were considered independent: R = 0.125, R, = 0.120. 9: Monoclinic, P2,!n, ( I = 685.X(2). h = 1796 7(4). c = 662.1(1) pm. [i = 90.08(2); 2090 measured re- llections. of which 1080 with F > 3 u ( F ) were considered Independent: R = 0.055. R , = 0.041. Further details of the crystal structure investigations ma) be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eg- genstein-Leopoldshafen ( F R G ) on quoting the depository numbers CSD- 400696 (8) and CSD-400697 (9). H. Irngartinger. K. L. Lukas. Angrw. Chrm. 1979. 91,750: Angriv. Chrm. Inr. Ed. D i R I . 1979. 18. 694-695: M. N. Paddon-Row, K. N. Houk, P. Dowd, P. Gamer, R Schappert, Erruhedron Lclf. 1981. 22, 4799 -4812; M. Eisenstein, F. L . Hirshfeld. Aclu CruluNogr. S K ~ . B 1983, 39. 61 -75: P. G. Gassman. M. 1.. Greenlee. D. A. Dixon, S. Richtsmeier. J. 2. Gougoutas, J Am. Chrm. sot,. 1983. ~ ( J J 5865-5874. R. Hoffrnaiin. fitrukrdron Lrrf. 1970. 2907-2909; H. Gunther. ihid. 1970, H . Irngarringer. W. Reimann. R. Lang, M. Christl, .4cru Cry3/a//ogr. Sect. B 1990. 46. 234-238. and references therein. T. J. Kati. E. J. Wang, N. Acton, J. Am. Chrm. Soc. 1971. 93, 3782-3783; T. J. Kati. R. J. Roth. N. Acton. E J. Carnahan. Org. S y l h . 1973, 53, 157. For many years it has been known that trialkylaluminum complexes will add to lanthanide complexes containing bulky stabilizing co-ligands to form bridged heterometallic species such as those given in Scheme 1 . Recently, we have shown that AIMe, will also bridge and stabilize metal ligand combinations such as Y(OCMe,), in [ y { ( p - OCMe,)(p-Me)A1Me,},][91 and [(C5H,SiMe,)Y(OCMe3),1 in [(C5H,SiMe,)Y{(p-OCMe,)(p-Me)A1Me,),l .[' I We now report that AIMe, will not only react with soluble reagents containing bulky ligands, but that it will also react with and solubilize the insoluble materials formed from reactions of lanthanide halides with small anionic hgands. The reaction of anhydrous NdCI, with three equivalents of LiNMe, (Z = NMe,) in THF at room temperature formed a blue material which was identified as 1 a [Eq. (a)]. Reaction of AIMe, or GaMe, with a suspension of 1 a in hexane yielded the hexane- soluble products 2a (violet-blue) and 2 b (blue), respectively. NdCI, + 3 LiNMe, la The Use of Heterometallic Bridging Moieties To Generate Tractable Lanthanide Complexes of Small Ligands ** William J. Evans,* Reiner Anwander, Robert J. Doedens, and Joseph W. Ziller Dedicated to Professor M . Frederick Hawthorne on the occasion of his 65th birrhduy Lanthanide chemistry is dominated by large, anionic ligands which meet the steric and electronic demands of these large electropositive metals. On the other hand, the chemistry of simple lanthanide complexes of general formula LnZ, in which Z is a small anionic ligand such as methanide, methanolate, ethanolate. or dimethylamido. is poorly developed since these complexes are either unstable or insoluble.[',21 The only ho- moleptic lanthanide complexes with small ligands which are tractable enough to be definitively characterized by X-ray crys- tallography are the hexamethyl ate complexes [LnMe,13- of the late lanthanides Ho, Er, and L u . [ ~ ] We describe here a method for the synthesis of isolable, sol- uble lanthanide complexes of small ligands Z (Z = NMe,, Me) which also provides a convenient route to heterometallic com- plexes which have component combinations of interest with respect to polymerization and the formation of ma- terials from molecular precursors.[41 We also report details of the first structurally characterized molecular complexes con- taining a lanthanide metal and gallium.[51 [*I Prof W. J. Evans, Dr. R. Anwander. Prof. R. J. Doedens, Dr. J. W. Ziller Department of Chemistry, University of California Irwne. CA 92717 (USA) Telef:,ix. Int. code [**I We thank the Division of Chemical Sciences of the Office of Basic Energy Sciences of the Department of Energy for support for this research and the Deutsche Forschungsgemeinschaft for a Postdoctoral Fellowship (to R. A.j. Angeii,. Chmi. i n / . Ed. EngI. 1994. 33. No. 15/16 3 MMe, [Nd(NMe,),(LiCI),] [Nd(NMe,),(MMe,),] 2a, M Al; 2 b , M + 3 LiCl (a) Ga The intensely colored solutions of 2 are stable at room temper- ature. Evaporation of the solvent results in oily residues which solidify at - 35 C. These solids slowly decompose at room tem- perature to form large colorless crystals of [{ Me,NMMe,} 2] (M = Al, Ga (4))[ ] which sublime to the top of the container. Compounds 2 a and 2 b can be isolated in 80 % yield by crystal- lization of the initially isolated solids from hexane at - 35 C; the molecular structure of 2 b is shown in Figure 1 .[' 21 Qc2 Fig. 1. Molecular structure o f 2 b ; the numbering isidentical to that ofisostructural 2a. Thermal ellipsoids are drawn at the 50% probability level. Selected distances [.I Ndl-N1- M1 94.7(2) [96.0(1)], Ndt-Ct-M1 84.3(2) (85.7(1)], Nl-Ndl-C1 77.3(2) [74.7(1)], Nl-MI-CI 103.0(3) [102.5(2)], Ndl-N2-M2 94.5(4) [92.6(3)]. mc, VCH Verlugs~~,.sellschufl mhH, 0-69451 Weinheim, 1994 0570-0R33/94if51j-l64f $ iO.OO+ .2.i!O

Lanthanide chemistry is dominated by large, anionic ligands which meet the steric and electronic demands of these large electropositive metals. On the other hand, the chemistry of simple lanthanide complexes of general formula LnZ, in which Z is a small anionic ligand such as methanide, methanolate, ethanolate. or dimethylamido. is poorly developed since these complexes are either unstable or insoluble. [',21 The only homoleptic lanthanide complexes with small ligands which are tractable enough to be definitively characterized by X-ray crystallography are the hexamethyl "ate" complexes [LnMe,13-of the late lanthanides Ho, Er, and Lu. [~] We describe here a method for the synthesis of isolable, soluble lanthanide complexes of small ligands Z (Z = NMe,, Me) which also provides a convenient route to heterometallic complexes which have component combinations of interest with respect to polymerization and the formation of materials from molecular precursors.[41 We also report details of the first structurally characterized molecular complexes containing a lanthanide metal and gallium. For many years it has been known that trialkylaluminum complexes will add to lanthanide complexes containing bulky stabilizing co-ligands to form bridged heterometallic species such as those given in Scheme 1 .
The reaction of anhydrous NdCI, with three equivalents of LiNMe, (Z = NMe,) in THF at room temperature formed a blue material which was identified as 1 a [Eq. (a)]. Reaction of AIMe, or GaMe, with a suspension of 1 a in hexane yielded the hexanesoluble products 2a (violet-blue) and 2 b (blue), respectively. The intensely colored solutions of 2 are stable at room temperature. Evaporation of the solvent results in oily residues which solidify at -35 "C. These solids slowly decompose at room temperature to form large colorless crystals of [{ Me,NMMe,} 2] (M = Al, Ga (4))["] which sublime to the top of the container. Compounds 2 a and 2 b can be isolated in 80 % yield by crystallization of the initially isolated solids from hexane at -35 "C; the molecular structure of 2 b is shown in Figure 1 X-ray crystallography revealed that compounds 2a and 2b are isostructural, which is somewhat surprising considering the structural differences of the aluminum and gallium starting reagents and their different bonding patterns towards transition metal cent e r~. ' '~] The overall structure shown in Figure 1 lacks the threefold symmetry found in ~{(p-OCMe,)(p-Me)AIMe,)] .[91 Instead, there are only two symmetrically identical [ (,u-NMe,)(p-Me)MMe,] chelating units (involving N1 and Nla) which have planar four-membered Nd-C-M-N metallacyclic rings. The third NMe,MMe, unit involving N2 is disordered about the twofold symmetry axis and the methyl groups attached to aluminum or gallium in this unit are not as close to the Nd center as those in the other two units. A planar four-membered ring is not formed because neither of the two closest carbon atoms, C6 and C7, lie in a plane with Ndl, N2, and M.
The formation of 3 is likely to occur through an intermediate 2 b, which can be seen clearly by the disappearance of M-NMe, vibrational modes in the IR spectra. The formation of the very stable 4["] undoubtedly helps drive the reaction to completion. Complex 3 is not sublimable and decomposes above 50 'C under high vacuum. Both 3 and 4 are very soluble in hexane and can be separated by fractional crystallization at -35 "C. Complex 3 crystallizes first as large blue pyramids (edge lengths ca. 5 mm).
after which 4 crystallizes as colorless cubes. Complex 3 was characterized by IR spectroscopy, elemental analysis. and X-ray structural analysis (Fig. 2). [19] The methyl groups in 3 are arranged in an octahedral geometry around Nd and a tetrahedral geometry around Ga. Each Nd-C-Ga-C ring is planar to within < 0.07 A. The fact that octa- 2.619(10). Ndl-C6 Z.S97(11), Ndl-C9 2.581(10). Ndl-C10 2.599(10). Ga-(/i-C) hedral lanthanide complexes exist which contain larger bridging groups, for example 2a, and smaller metals, for example p { ( p -OCMe,),(p-Me) AIMe,] suggests that the coordination environment around neodymium in 3 is sterically unsaturated. Complex 3 differs from the -ate complexes mentioned earlier in that the lanthanide metal is more electropositive than the other metallic component (electronegativities : Nd z 1.1, Ga z 1.8[201). Therefore 3 is better described as a gallate ([GaMeJ) rather than a neodymate ([NdMe,13-) complex, although the most important aspect of this difference will be in the reactivity.
The The exchange of NMe, groups for methyl in reaction (b) is likely to occur in a stepwise fashion; examination of the analogous lanthanum system 1 b with four equivalents of GaMe, provided the partially exchanged complex 5 [Eq. (c)]. which was characterized by elemental analysis, NMR spectroscopy. IR spectroscopy, and X-ray structural analysis (Fig. 3) .Iz4]  The isolation and characterization of complexes 2, 3, and 5 demonstrates that insoluble, oligomeric lanthanide complexes of small ligands can be "cracked" under very mild conditions (room temperature, hexane) by the attack of strong, soluble Lewis acids to form alkane-soluble, tractable compounds. This approach to developing the chemistry of lanthanide complexes of small ligands should be general; preliminary results show that species such as "Ln(OMe)," and InR, fit in this scheme.