Richard A. Andersen

Organometallic coordination complexes of the bis(pentamethylcyclopentadienyl)-alkaline earth compounds, (Me5C5)2MLn, where M is Mg, Ca, Sr, OR Ba and Me5C5BeCl

The ether complexes, (Me5C5)2Sr(OEt2) and (Me5C5)2Ba(thf)2, have been isolated from reactions of SrI2 in Et2O or BaI2 in thf with NaC5Me5. The base-free compounds were isolated by the “toluene-reflux” method. The coordination compounds (Me5C5)2M(bipy) (M = Mg, Ca, Sr, Ba), (Me5C5)2M(2,6-xylylisocyanide)2, and (Me5C5)2M(PEt3) (M = Ca, Sr, Ba) have also been obtained. The (Me5C5)2Be cannot be prepared from (Me5C5)2 Mg and BeCl2(OEt2)2; instead Me5C5BeCl is isolated.

Application of the E-C Approach to Understanding the Bond Energies Thermodynamics of Late-Metal Amido, Aryloxo and Alkoxo Complexes: An Alternative to pπ/dπ Repulsion

Abstract The concept of pπ/dπ repulsion in alkoxo and amido complexes of the late transition metals has been critically examined, especially as it relates to recent work by the authors on a series of nickel complexes. The thermodynamic preferences of late-metal M-X bonds (X=NR2, OR) can be explained by an alternative model which uses bond polarity as a starting point and E-C theory for quantification. In this model, the greater electrostatic character of the M-X bond (relative to the H-X bond) is sufficient to explain observed thermodynamic trends in ligand exchange reactions, without the inclusion of π-effects. The implications of adopting the polar bonding model have been examined, including discussion of metal alkyl complexes as well as oxo and imido complexes. In general, this polar model appears to lead to greater consistency with observation and deeper understanding, although more data are necessary to clearly determine the best way of describing bonds between late metals and X ligands.

The molecular structures of bis(pentamethylcyclopentadienyl)-calcium and -ytterbium in the gas phase; two bent metallocenes

Abstract Gas electron diffraction studies show that whereas the ligand rings in (η 5 -C 5 Me 5 ) 2 Mg, are essentially parallel, the thermal average structures of (η 5 -C 5 Me 5 ) 2 Ca and (η 5 -C 5 Me 5 ) 2 Yb are bent, the ring-centroid—metal—ring-centroid angles being 154(3)° and 158(4)°, respectively.

Synthesis of Chiral, Enantiopure Zirconocene Imido Complexes: Highly Selective Kinetic Resolution and Stereoinversion of Allenes, and Evidence for a Stepwise Cycloaddition/Retrocycloaddition Reaction Mechanism

We wish to report a series of highly enantioselective reactions using isolable imido complexes with reactive metal – nitrogen double bonds.[3–9] These involve the reactions of enantiopure (ebthi)(L)Zr=NR complexes with 1,3-disubstituted allenes (L = tetrahydrofuran; ebthi = bis (tetrahydroindenyl)ethane). Our observations a) demonstrate the operation of a general and highly selective kinetic resolution, b) provide the first example of a reaction that can be used to selectively invert the absolute configuration of one (symmetrically or unsymmetrically 1,3disubstituted) allene enantiomer within a racemic mixture, and c) provide strong mechanistic evidence for the stepwise nature of an organo-metallic cycloaddition reaction.

Gas-phase electron diffraction studies of two sterically hindered three-coordinate lanthanide amides: tris(bis(trimethylsilyl)amido)cerium(III) and -praseodymium(III), M(N(Si(CH3)3)2]3, M = Ce and Pr

Abstract Gaseous tris(bis(trimethylsilyl)amido)cerium(III) and -praseodymium(III), M{N-(SiMe 3 ) 2 } 3 (M = Ce and Pr) have been studied by electron diffraction at nozzle temperatures of ca. 435 K (M = Ce) and ca 430 K (M = Pr). The diffraction data for both compounds are consistent with monomeric molecules of C 3 symmetry. The effective configuration of the MN, skeletons is nonplanar (∠NMN = 112(3)° (M = Ce) and 113(3)° (M = Pr)) and the twist of the NSi 2 groups relative to the MN, unit is 42(5)° (M = Ce) and 47(5)° (M = Pr). Values of some selected bond lengths (τ a ) and valence angles are (parenthesized values refer to the Pr structure): τ(MN) = 2.33(4) A). τ(SiN) = 1.696(15) A (1.705(15) A), τ(SiC) = 1.887(8) A (1.890(8) A); ∠MNSi = 108(2) and 123(2)° (108(2) and 123(2)°), ∠SiNSi = 129(4)° (129(4)°), ∠NSlC = 117(2)° (116(2°) and ∠SiCH = 112(3_° (113(3°). The SiMe 3 groups are, in both compounds, twisted approximately 15° away from the reference positions typified by one SiC bond of each such group being anti to be adjacement SiN bond. The gas-phase structure of the Ce(III) and Pr(III) amides reported in this work is similar to the crystal structure of the analogous tris-silylamides of the lanthanides Nd(III), Eu(III) and Yb(III).

SYNTHESIS, MAGNETIC SUSCEPTIBILITY AND X-RAY CRYSTAL STRUCTURE OF (TBUNCHCHNTBU)3YB

Abstract The reaction of (C 10 H 8 )Yb(THF) 3 with t BuNCHCHN t Bu ( t Budad) in tetrahydrofuran at room temperature leads to Yb( t Budad) 3 ( 1 ), which is also obtained from the reaction of YbCl 3 and three molar equivalents of K( t Budad) in tetrahydrofuran or by metal vapor synthesis. 1 has been characterized by X-ray diffraction. The crystals are monoclinic, space group C 2, Z = 2 with a = 1034.8(9), b = 1710.4(6), c = 1020.4(8) pm, β = 113.28(2)°. The structure was refined to R = 0.0274 for 1466 observed reflections ( F O > 4 σ ( F O )). The structure shows that the empirical composition is Yb( t Budad) 3 and that the coordination number of Yb is six, but the X-ray data are not sufficiently accurate to judge if the oxidation state of ytterbium is zero, two or three. The magnetic susceptibility of solid 1 , prepared by metal vapor synthesis, was studied from 5 to 300 K. The magnetic results are explained by postulating that the bivalent ytterbium species [Yb II ( t Budad)] predominates at low temperature and as the temperature increases the trivalent ytterbium species [Yb III ( t Budad − ) 3 ] predominates in the solid state.

A general synthesis and crystal structure of [(Me3C)2C5H3]3Ce

Abstract The preparation of [(Me3C)2C5H3]3Ce from Ce(OSO2CF3)3 and [(Me3C)2C5H3]2Mg is described and compared to published routes to other Cp3Ln(Ln = lanthanide) compounds. The title compound is monomeric in the solid state with three η5-bound cyclopentadienyl rings which show some unusual distortions while maintaining idealized C3h symmetry. The crystals are monoclinic, in the space group P2 1 n , with the unit cell a 10.803(3) b 19.493(6) c 17.946(5)A, β 104.35(2)°, Z = 4. Anisotropic refinement of all heavy atoms with 361 parameters and 3420 reflections yielded R = 4.62%.

A phosphorus-carbon bond cleavage reaction of coordinated trimethylphosphine in (PMe3)4Ru(OC6H4Me)2

Abstract A product resulting from cleavage of the PC bond in a trimethylphosphine ligand forms upon thermolysis of (PMe 3 ) 4 Ru(OC 6 H 4 Me) 2 ( 1 ) or addition of p -cresol to the orthometallated complex (PMe 3 ) 4 Ru(η 2 -OC 6 H 3 Me) ( 2 ). The trimethylphosphine ligand has been transformed to a dimethylarylphosphinite ligand in the product (PMe 3 ) 3 (η 2 -PMe 2 OC 6 H 3 )Ru(OC 6 H 4 Me) ( 3 ). Although complex 1 exists in equilibrium with complex 2 and free p -cresol at 65° C, kinetic evidence is presented indicating that complex 1 undergoes the PC cleavage reaction. An X-ray crystal structure analysis was performed on 3 . Crystal data at 25° C: a 11.8875(11), b 36.000(4), c 14.1207(13) A, β = 90.428(8)°, Z = 8, D calc 1.33 g/cm 3 ; space group P 2 1 / n .

Arene C-H bond activation: reaction of (Me3P)3Rh(Me) with toluene to give (Me3P)3Rh(Ar) where Ar is o-, m- and p-tolyl

Abstract The square-planar rhodium (I) compound, (Me 3 P) 3 Rh(Me), reacts with benzene or toluene at 70° to give (Me 3 P) 3 Rh(Ph) or (Me 3 P) 3 Rh(Ar) where Ar is o -, m -, p -tolyl along with methane. These aryl compounds were prepared independently and characterized by 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy. The benzyl compound, prepared from PhCH 2 Li and (Me 3 P) 3 RhCl, is formulated as an η 3 -benzyl, is not formed in detectable amounts in the arene activation studies. The benzyl compound rearranges on heating to (Me 3 P) 3 Rh( o -tolyl) in near quantitative yield at 70°C. The four-coordinate compound, (Me 3 P) 3 Rh(Me), is fluxional at +20° though the fluxional process is stopped at −20°C. The five-coordinate compound, (Me 3 P) 4 Rh(Me), also is fluxional at +20°C though stereochemical rigid at −65°C. The geometry of the latter compound is based upon a trigonal bipyramid with the methyl group on the axial site.

The molecular structrure of gaseous tris(bis(trimethylsilyl) amido)scandium, Sc{N(Si(CH3)3)2}3, as determined by electron diffraction: A three-coordinate scandium(III) amide

Abstract Gaseous tris (bis(trimethylsilyl)amido)scandium, Sc{N(SiMe 3 ) 2 } 3 , has been studied by electron diffraction at a nozzle temperature of ca 450 K. The diffraction data are consistent with monomeric molecules of approximately D 3 - symmetry: the ScN, skeleton is essentially planar (∠NScN = 119.5(1.5)°), and the twist of the NSi 2 groups relative to the ScN 2 plane is 55(2)°. Other key parameters, including bond lengths ( r a ) and valence angles are: r (ScN) = 2.02(3) A, r (SiN) = 1.715(8) A, r (SiC) = 1.881(4) A; ∠ScNSi = 116.0(1.0)°, ∠SiNSi = 128.0(2.0)°, ∠NSiC = 110.7(1.0)° and ∠SiCH = 112.3(1.8)°. The SiMe 3 groups are twisted 12(3)° away from the positions typified by one SiC bond of each such group being anti to the adjacent SiN bond, and the methyl groups themselves may be twisted about 20° away from their staggered reference positions. The crystal structure, solved by Ghotra et al, is a variance with the gas-phase structure as it reveals a pyramidal ScN 3 skeleton; this probably illustrate the effects of crystalpacking forces in the solid phase.