Organometallic Chemistry: Volume 30 (Specialist Periodical Reports - Organometallic Chemistry, Volume 30) - Hardcover

Hardcover

ISBN 10: 0854043330 ISBN 13: 9780854043330

Publisher: Royal Society of Chemistry, 2002

Synopsis

Organometallic chemistry is an interdisciplinary science which continues to grow at a rapid pace. Although there is continued interest in synthetic and structural studies the last decade has seen a growing interest in the potential of organometallic chemistry to provide answers to problems in catalysis synthetic organic chemistry and also in the development of new materials. This Specialist Periodical Report aims to reflect these current interests reviewing progress in theoretical organometallic chemistry, main group chemistry, the lanthanides and all aspects of transition metal chemistry. Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research. Written by experts in their specialist fields the series creates a unique service for the active research chemist, supplying regular critical in-depth accounts of progress in particular areas of chemistry. For over 80 years the Royal Society of Chemistry and its predecessor, the Chemical Society, have been publishing reports charting developments in chemistry, which originally took the form of Annual Reports. However, by 1967 the whole spectrum of chemistry could no longer be contained within one volume and the series Specialist Periodical Reports was born. The Annual Reports themselves still existed but were divided into two, and subsequently three, volumes covering Inorganic, Organic and Physical Chemistry. For more general coverage of the highlights in chemistry they remain a 'must'. Since that time the SPR series has altered according to the fluctuating degree of activity in various fields of chemistry. Some titles have remained unchanged, while others have altered their emphasis along with their titles; some have been combined under a new name whereas others have had to be discontinued. The current list of Specialist Periodical Reports can be seen on the inside flap of this volume.

About this title

Excerpt. © Reprinted by permission. All rights reserved.

Organometallic Chemistry Volume 30

A Review of the Literature Published during 2000

By M. Green

The Royal Society of Chemistry

Copyright © 2002 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85404-333-0

Contents

Chapter 1 Theoretical Organometallic Chemistry By Adam J. Bridgeman and Stuart A. Macgregor, 1, 
Chapter 2 Groups 1 and 11: The Alkali and Coinage Metals By David J. Linton and Andrew E.H. Wheatley, 51, 
Chapter 3 Group 2 (Be–Ba) and Group 12 (Zn–Hg) By Richard A. Layfield and Dominic S. Wright, 75, 
Chapter 4 Scandium, Yttrium and the Lanthanides By John G. Brennan and Andrea Sella, 91, 
Chapter 5 Carboranes, Including Their Metal Complexes By Andrew S. Weller, 116, 
Chapter 6 Group 13: Boron, Aluminium, Gallium, Indium and Thallium By Matthew J. Almond, 128, 
Chapter 7 Group 15: Phosphorus, Arsenic, Antimony and Bismuth By Cameron Jones, 159, 
Chapter 8 Metal Carbonyls By John A. Timney, 172, 
Chapter 9 Complexes Containing Metal–Carbon σ-Bonds of the Groups Titanium and Manganese, Including Carbenes and Carbynes By Patrick C. McGowan and Jason M. Lynam, 189, 
Chapter 10 Organo-Transition Metal Cluster Compounds By Mark G. Humphrey and Marie P. Cifuentes, 219, 
Chapter 11 Complexes Containing Metal–Carbon σ-Bonds of the Groups Iron, Cobalt and Nickel, Including Carbenes and Carbynes By Philip J. King, 282, 
Chapter 12 Hydrocarbon Transition Metal π-Complexes Other Than η-C5H5 and η-Arene Complexes By Kevin R. Flower, 326, 
Chapter 13 Transition Metal Complexes of Cyclopentadienyl Ligands By Ian R. Butler, 385, 
Author Index, 435, 

CHAPTER 1

Theoretical Organometallic Chemistry

BY ADAM J. BRIDGEMAN AND STUART A. MACGREGOR

Part I: s- and p-Block Metalsby Adam J. Bridgeman

1 Introduction

This chapter aims to cover theoretical studies on organometallic molecules. Sections 2 and 3 cover the s- and p-block metals including clusters, carbonyls and metal–metal bonded systems containing M–C bonds. Standard abbreviations for computational methods and basis sets are employed.

2 s-Block Metals

2.1 Structural, Spectroscopic and Mechanistic Studies. – 2.1.1 Metal Alkyls. Photoelectron spectroscopy (PES), RHF and DFT studies at the B3LYP level of gas-phase alkyllithium [RLi]n clusters indicate that association leads to the formation of tetramers for R = Pri, Bus and But and to a mixture of tetramers and hexamers for R = Et, Prn and Bun. Geometry optimisations indicate that only the C–C distances are sensitive to the degree of aggregation with shorter distances predicted for the hexameric form. The photoelectron spectrum is assigned using ionisation energies calculated using Koopman's theorem and using Outer-Valence Green's Function (OVGF) method with the latter providing better agreement with experiment. The ionisation energies are found to be more sensitive to the nature of the alkyl group than the cluster size. The geometries of the [MeLi]n+ clusters along each of the Jahn–Teller active coordinates were obtained and indicate that a Cs structure is the most stable.

The formation energies of the aggregates formed between BunLi and lithium fencholate derivatives have been calculated from combined molecular mechanics (UFF) and DFT (B3LYP 6-31+ G*) ONIOM optimised geometries. A 1:3 complex is predicted between BunLi and lithium fencholate whereas a 2:2 ratio is predicted for fencholate ligand with a SiMe3 group ortho to the methoxy group, as found experimentally. The different behaviour of the fencholate systems is ascribed to unfavourable packing of the SiMe3 groups in the 1:3 tetramer.

The structures and bonding of the isostructural homologous series of mixed lithium–heavier alkali metal tertiary butoxides, [(ButO)8Li4M4] (M = Na, K and Cs) have been studied using HF/6-31G* and B3LYP/6-311G** calculations. The clusters have the distinctive 'breastplate' architecture shown in 1. To simplify the geometry optimisations, MeO was used in place of BuO and reasonable agreement with experimental structural parameters was obtained. By calculating the structures of [(MeO)6M6] (M = Li and Na) clusters, the heat of formation of models of 1 with respect to homonuclear components was predicted to be ca. 85 kJ mol-1 indicating that the formation of the heterometallic clusters is thermodynamically driven.

The mechanism of BunLi/TMEDA mediated arene ortholithiation has been studied by B3LYP/6-31G* optimisations of reactants and proposed transition structures. Activation energies using MeLi as a computational model for BunLi have been obtained for different proposed mechanisms including transition structures based upon triple ions of the form [R2Li]- [Li(TMEDA)2]+. It is shown that alkoxy–lithium interactions are of only minor importance.

The structures of intermediates and the transition state in the enantioselective deprotonation of N-boc-pyrrolidine with isopropyllithium/(—)-sparteine have been investigated using HF/3-21G and B3P86/6-31G* geometry optimisations. An activation enthalpy of ca. 46 kJ mol-1 is obtained for the lowest energy pathway for H atom transfer.

HF calculations of the structures and vibrational frequencies of monomers and dimers of lithium alkyl carbonates (methyl, ethyl, and propyl carbonate lithium) and lithium alkoxides (lithium methoxide, lithium ethoxide, lithium propoxide, and lithium butoxide) indicate that they adopt dimeric structures. Dimerisation energies of 214 kJ mol-1 for lithium alkyl carbonates and 266 kJ mol-1 for lithium alkoxides are calculated and are found to be approximately independent of the chain length.

The structures of MCH2OH and CH3OM (M = Li, Na) molecules have been studied at the MP2/6-31 + G* level. The most stable conformation of MCH2OH systems has a carbon–oxygen bridged structure.

DFT calculations at the BP86 level with triple-zeta quality STOs have been used to study the structures, vibrational frequencies, the solvent effect of diethyl ether on the Schlenk equilibrium and the aggregation of Grignard reagents RMgX with R = Me, Et and Ph. The Mg–C bond lengthens on replacing Ph for Me or Et and the Mg–X bonds are longer than in the MgX2 dihalide molecules. Coordination by one or two Et2O molecules lengthens the Mg–C and Mg–X bonds. Solvation energies decrease from MgX2 to RMgX to R2Mg but solvation of the phenyl derivatives is larger than that of the alkyl Grignard molecules. The tendency to dimerise reduces in the order [R2Mg]2< [RMgX]2< [MgX2]2 and is greater for chlorides than for bromides.

Theoretical nuclear hyperfine coupling constants for the CH3Mg radical and its isotopomers have been calculated using a variety of correlated methods. The results have been compared with the experimental spectra obtained from CHMg isolated in a neon matrix. A MP2 geometry optimisation using a cc-pVDZ basis set for Mg and the Dunning DZP basis sets for C and H yields a Mg–C bond length of 2.114 Å. This is in better agreement with the experimental bond length of 2.11 Å than the values of 2.138 Å and 2.126 Å derived from DFT calculations with the B3LYP and B3PW91 hybrid functionals respectively and the same basis sets. Reasonable agreement with the experimental Aiso(25Mg), Adip(25Mg) and Adip(13C) magnetic hyperfine coupling constants is obtained from HF single and double excitation configuration interaction (HFSDCI) and multi-reference single and double excitation configuration interaction (MRSDCI) methods. However, the HFSDCI and MRSDCI calculated values for Aiso(13C) are 50% and 32% too low respectively. This is due to an overestimation of the ionic character of the radical arising from the limited reference space in the calculations. B3LYP and B3PW91 calculations yield good agreement for these A values despite the poorer geometries. All methods give poor agreement with the experimental Aiso(1H) value.

The performance of DFT hybrid functionals, GGAs and post-HF methods in calculating the geometries and electronic structure of MCH3 (M = Li, Na) and MCCH (M = Li, Na, K) have been compared. The hybrid functionals B1LYP, mPW1PW91 and PBE0 perform extremely well whilst the BLYP GGA shows larger errors. Larger basis sets are required for post-HF treatments to achieve comparable results.

The geometry of the ground state of the MgCH3 radical has also been studied in conjunction with an investigation of the structure of the Jahn–Teller active first excited state. At the MP2 level the Mg–C bond length is calculated to be 2.113 Å and the Mg–C stretching mode is calculated to lie at 527 cm-1 for the ground state. Vertical transitions calculated at the multiconfigurational SDCI-1 level lead to an assignment of the first excited state at 2.650 eV to a Rydberg state of 2E symmetry. This state suffers a Jahn–Teller distortion to give a 2A' state and a 2A" state with small stabilisations of 16 cm-1 and 21 cm-1 with respect to the 2E state and small changes to the H–C–Mg bond angles.

The coordination of Mg+ with straight chain alkanes has been studied using B3LYP/6-311++G(2d,2p) calculations on the Mg+-n-pentane system with counterpoise, zero-point and thermal corrections. The most stable conformer 2 has the Mg+ cation attached to four hydrogen atoms. The enthalpy of ligation is calculated to be ca. 72 kJ mol-1 with Mg+–H bond lengths of 2.317 Å. The barrier of interconversion of the possible conformers and rotamers is predicted to be very low.

2.1.2 Interactions with Unsaturated Organic Systems. The geometries and bonding of the pentadienyl anion and its complexes with Na+ and Li+, studied at the HF/3-21G, HF/6-311 +G* and DFT B3LYP/6-311 +G* levels, show that η5-complexation is preferred. This bonding mode leads to a U-shaped structure which is reported to maximise electrostatic interactions. Although this conformation is high in energy for the free ligand, it is strongly stabilised by the primarily ionic interaction with the alkali metal cation.

DFT B3LYP/6-311+G(d) calculations on the sulfur stabilised allyllithium compounds 3, 4 and 5 have been used to investigate their structures in conjunction with experimental studies. For 3, the stability of the conformers increases with the coordination of the ligand. The η3 systems that allow additional coordination by the heteroatom are further stabilised. Coordination by up to three THF molecules is favourable and leads to increasing carbanion character at the Cα carbon and decreased Li–Cγ bonding. The high natural charge (NPA) for the Cα atom is stabilised by the adjacent SPh group. Comparison of the experimental 13C NMR chemical shifts and those calculated using the Gauge-Independent Atomic Orbital (GIAO) approach indicates the importance of including solvent THF molecules in the computational modelling. For 4, the most stable conformer has both η3-allyl-Li+ and PhSO–Li+ interactions. The importance of the O–Li interaction reduces the probable solvent coordination to two THF molecules. A similar structure is found for 5 with η3-allyl–Li+ and PhS(O)O–Li+ interactions. Coordination by two THF molecules leads to a number of conformers of similar energy. The predicted 13C NMR spectra of 4 and 5 are unable to distinguish between the possible conformers within the likely accuracy of the GIAO approach.

Experimental studies of the ionisation threshold energies of clusters of Li and Na atoms solvated by acetone have been augmented by B3LYP/6-311G** and MP2/6-311G** calculations of M–acetone complexes (M = Li, Na). Both metals coordinate to the oxygen of the carbonyl group. The Li–O–C bond angle is essentially linear whilst the Na bonds more weakly with a Na–O–C bond angle of ca. 152°. Complexation by Li leads to a greater length carbonyl bond due to electron transfer from Li into the C–O π* orbital. The carbonyl bond in the Na complex is essentially unchanged from that in free acetone. The NPA charges of + 0.76e for Li and +0.11e for Na are consistent with the structural results. The molecular and cationic complexes formed between Na and Na2 and the organic carbonyls, formaldehyde, acetaldehyde and acetone have been studied using MP2/6-31 + G(d,p) geometries and MP2, CCSD and B3PW91/6-311 + G(2d,p) single point energies. For each monosodium carbonyl, three minima are found. Two are of the ion-pair type with coordination of Na+ to carbonyl following metal to CO π* electron transfer. The third type is a complexation pair with donation from carbonyl oxygen to Na. The disodium complexes are related to the complexation pair type with Na2 linearly coordinated to carbonyl.

HF/6-31G calculations have been performed to determine thermodynamic parameters for the dissociation of dimeric associates of crotyllithium and 1-lithium-2,6-octadiene and for crotyllithium complexes of butadiene. An equilibrium constant within an order of magnitude of the experimentally determined value for the dissociation was obtained.

The origin of the ligand conformation in α,α-bis(trimethylsilyl)benzyl potassium and calcium complexes has been studied using MP2 calculations with 6-31+ G+* geometries and 6-311+ G*single point energies. The observed rotation of the C(SiMe3)2 group from the plane of the phenyl ring of the PhC(SiR3)2- group is rationalised using a model system with R = H. The inductive effect of the silyl group reduces the delocalisation of the anionic charge on the phenyl ring, reducing the double bond character of the Cα-Cipso group allowing the distortion.

A gas-phase lithium cation basicity scale has been calculated using G2 and G2(MP2) on 37 compounds and B3LYP/6-311+G** calculations of 63 compounds including Lewis bases and saturated and unsaturated organic molecules. Good agreement with experimental basicities is found for all three computational methods.

The structure and binding energies and entropies of the Na+ cation with a wide range of 30 organic molecules have been calculated at the MP2/6-311G(2d,2p) level, including saturated, polar and aromatic groups. Counterpoise corrections and zero-point and thermal corrections using vibrational frequencies obtained at the MP2/6-31G were included leading to an absolute basis for the relative experimental free energy scale. The structures obtained illustrate the importance of electrostatic and polarisation interactions and the small role of covalency in determining the structure of the complexes. For the Na+ complexes of π ligands, charge–quadrupole interactions are found to be important.

The interaction of Na atoms with small molecules and organic ligands has been used to select small basis sets able to predict accurate geometries, dipole moments and binding energies with small basis set superposition error (BSSE). To model the interaction of metals with large organic systems, it is necessary to reduce basis sets to a minimum size to increase computational efficiency and to reduce hardware requirements. Basis sets based on the small 6-31G set have been augmented with polarisation and diffuse functions on different fragments of the molecules to test the performance of the B3LYP method in predicting the geometries and binding energies of complexes between Na atoms and water, ammonia, methylamine and 8-quinolinol. With strong donors, the metal atom may become negatively charged so that it is necessary to include polarisation and diffuse functions on the metal and on electronegative atoms directly bonded to the metal. With this selected and fairly small basis set, accurate structures and energies are obtained.

Large basis sets and MP2 calculations have been used to calculate the structures and binding energies between Na+ and the π-ligands ethylene and benzene. The binding energies obtained with diffuse function augmented correlation consistent basis sets of double-zeta, triple-zeta and quadruple-zeta quality have been used to extrapolate to the complete basis set (CBS) limit. At the quadruple-zeta level, the bond lengths between the Na+ ion and the centre of gravity of the π-ligand are predicted to be 2.641 Å and 2.390 Å for ethylene and benzene respectively and are shorter than previously reported values. The CBS binding energies are predicted to be 56.9 kJ mol-1 for Na+-ethylene and 102.0 kJ mol-1 for Na+-benzene. The distortion of the coordinated benzene ring from planarity is less than 1°.

(Continues...)

Excerpted from Organometallic Chemistry Volume 30 by M. Green. Copyright © 2002 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2002
• Language: English
• ISBN 10: 0854043330
• ISBN 13: 9780854043330
• Binding: Hardcover
• Edition number: 1
• Number of pages: 430
• Editor: Green M