AMMONIACAL SYNTHESIS OF LANTHANUM ARYLOXIDE COMPLEXES AND OBSERVATION OF A UNIQUE INTERCONVERSION BETWEEN OXYGEN-BRIDGED AND ETA-ARENE-BRIDGED DIMERIC SPECIES - X-RAY CRYSTAL-STRUCTURES OF LA-2(OAR)(6)(NH3)(N) (N=0, 2), LA(OAR)(3)(NH3)(4), AND LA(OAR)(3)(THF)(2) (AR=2,6-I-PR2C6H3)

Reaction of LaCl3 with 3 equiv of sodium metal in liquid ammonia at -78 degrees C, followed by addition of 3 equiv of HOAr (Ar=2,6-i-Pr2C6H3) in toluene produces La(OAr)(3)(NH3)(x) (1) after warming to room temperature. Dissolution of 1 in toluene followed by 6 h reflux provides La-2(OAr)(6) (2) in good yield. An X-ray crystallographic study revealed that 2 contains a dimeric unit held together with two eta(6)-arene bridges formed from aryloxide ligands with La-C(av)=3.062(10) Angstrom. The La-O distances average 2.193(5) and 2.273(5) Angstrom for terminal and bridging aryloxide ligands, respectively. Dissolution of 1 in toluene followed by vacuum filtration and crystallization from toluene without reflux yields La-2(OAr)(6)(NH3)(2)(3) in high yield. The X-ray crystal structure of 3 reveals an alternate dimeric structure bridged by oxygen atoms of the aryloxide ligands. X-ray crystallography has shown that a third compound may be isolated by gravity filtration and crystallization of 1 to produce La(OAr)(3)(NH3)(4) (4) 4 displays a capped pseudo-octahedral geometry around the metal center with facial aryloxide and NH3 ligands. A fourth NH3 ligand caps the face defined by the other three. La-O distances for the aryloxide ligands average 2.252(6) Angstrom. The La-N distances for the NH3 ligands average 2.750(9) Angstrom with no significant lengthening of the capped La-N bond. The monomeric five- and six-coordinate Lewis base adducts, La(OAr)(3)(THF)(2) (5) and La(OAr)(3)(py)(3) (6) (py = pyridine), are prepared by reaction of 2 with an excess of the appropriate Lewis base. An X-ray crystal structure for 5 displays a distorted trigonal bipyramidal La metal center with two axial THF and three equatorial aryloxide ligands. La-O distances for aryloxide ligands average 2.21(1) Angstrom while La-O distances for the THF ligands average 2.52(1) Angstrom, Crystal data for 2 (-70 degrees C): monoclinic space group P2(1)/c, a=17.550(4) Angstrom, b=10.225(2) Angstrom, c=21.837(4) Angstrom, beta=111.57(3)degrees, V=3644.2 Angstrom(3), Z=2, d(calc)=1.310 g cm(-3), R(F)=0.0394, R(W)(F)=0.0447. Crystal data for 4 (-100 degrees C): trigonal space group R ($) over bar 3, a=14.220(2) Angstrom, c=32.821(4) Angstrom, V=5748.8 Angstrom(3), Z=6, d(calc)=1.259 g Crystal data for 5 (-70 degrees C): monoclinic space group P2(1), a=9.870(2) Angstrom, b=19.367(4) Angstrom, c=12.128(2) Angstrom, beta=109.88(3)degrees, V=2180.1 Angstrom(3), Z=2, d(calc)=1.241 g cm(-3), R(F)=0.0.449, R(W)(F)=0.0583.