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METMOD1 FORCE FIELD
by Lajos Bencze and Robert Szilagyi
ben016@almos.vein.hu szilagyi@miat0.vein.hu
University of Veszprem
Department of Organic Chemistry
Veszprem, HUNGARY
What is the METMOD1 force field ?
---------------------------------
The aim of this force field is to create a model for studying steric
effects governing the stereoselectivity of olefin metathesis throughout the
elementary steps of the catalytic process.
Molecular mechanical studies have been carried out [1] on penta-
coordinated tungsten-carbene complexes, alkene-carbene-complexes, metalla-cycles,
using PCMODEL commercial software (Serena Software) [2].
The PCMODEL is parametrized for transition metals in general, mainly using data
for iron derivatives with corrections applied for other metals. The bending
functions and all torsion potentials involving the metal atom are removed and
the 1,3-interaction and an 1/r2 attractive terms are introduced instead.
Rotational barriers are set to zero in each case, in contrast to the
experimental findings.
The original force field of the PCMODEL left unsatisfactory features in the
modelling of tungsten complexes even after major adjustment of the parameters.
For that reason, we abandoned the use of the general transition metal central
atom method and instead constructured a force field for the specific handling
of tungsten carbene species, termed METMOD1.
The energy terms used, identical with those for the MMX of the PCMODEL
[3].
These directories contain the nessesary files to install the METMOD1
force field into your own MM2 based molecular mechanical software. Please follow
the instructions and if you have any question or comment do not hesitate to
contact us!
WCl4CR1R2 complexes, Part I. [4]
--------------------------------
Some of our encouraging preliminary results on molecular mechanics
studies on ring-opening polymerisation of norbornene were briefly reported [1].
Now, in the first part of a series of publications we define and test a detailed
and refined version of the force field (termed METMOD1) of the WCl4CHR carbenes
formed in the WCl6/SnMe4-initiated metathesis reactions (and probably in hundred
of catalyst systems in which WCl6 is used as a catalyst precursor). This will be
followed in subsequent parts for the so-called well-defined carbene catalysts
[5], of the force fields of pi-complexes and metallacycles and by analysis of
metathesis reactions. We hope that METMOD1 will also provide an easily accessible
tool for experimental chemists working in the field of carbene chemistry.
{W[C(CH2)3CH2](OCH2tBu)2Br2}2, {W[C(CH2)3CH2](OCH2tBu)2Br2}.GaBr3, Part II. [6]
-------------------------------------------------------------------------------
The X-ray structures of two carbene complexes, {W[C(CH2)3CH2](OCH2tBu)2Br2}2
and {W[C(CH2)3CH2](OCH2tBu)2Br2}.GaBr3 were used to derive additional parameters
to the METMOD1 general tungsten-carbene force field. Three new atom types and
forty new interactions are defined. Pentacoordinate tunsgten atom is used and
the bridging bromine is coordinated by FIX DISTANCE (FXDIS) option. In optimizing
and testing the new parameters intermolecular and intramolecular fittings were
carried out for the corresponding units of the X-ray, METMOD1 and relevant MMX
structures.
However no X-ray structure on the pentacoordinate mononuclear species of
these kind of complexes has been published so far. The goal of this work is to
provide parameters for the construction of a general force field for the Osborn
carbenes acting also as unimolecular catalysts in solution.
"Well-defined" carbene complexes, Part III. [7]
----------------------------------------------
A new force fiedl termed METMOD had been originally constructed to
simulate the carbene complexes that initiate the metathesis of cyclic and acyclic
olefins with or without the application of a Lewis acid cocatalyst.
The largest and most studied group of them are the so-called Osborn carbenes.
They can be regarded as deribatives of the WX4CR2 (X=Cl, Br or I; R = H or alkyl
group) carbenes in whoch one or more W-X bonds are substituted to W-O bonds, e.g.
WX4-n(OY)nCR2, where n=1-4, Y= R, SO2CF3 or SiPh3. In the present part of the work
the force field is extended to all the mononuclear Osborn carbenes and some other
structures related to them. Trigonal bipyramidal (TBP) structures are proposed for
these compounds in which the carbene ligand occupies an equatorial site with its
two substitutents and with the two neopentoxo groups lying in the trigonal plane
of the TBP. Parameter development was based on experimental NMR, X-ray and IR data
as well as on ab-initio calculations.
Simulated structure fit ab initio and X-ray structures with high accuracy.
Experimental (NMR) rotational barriers for W=Cc bonds are simulated with an average
error of 0.563 kcal/mol.
REFERENCES
1. Bencze L. and Szilagyi R.: J. Mol. Catal., 1992, 76, 145-156
2. Serena Software, P.O. Box 3076, Bloomington, IN 47402-3076, USA
3. Gajewski J. J., Gilbert K. E. and McKelvey J.: in Liotta D.(ed.) Adcances in
Molecular Modelling, JAI Press, London, 1990, Vol. 2, Chapter 2.
4. Bencze L. and Szilagyi R.: J. Organometal. Chem., 1994, 465, 211-219
5. Bencze L. and Szilagyi R.: Xth Int. Symp. on Olefin Metathesis and Polym.
(ISOM-10), Tihany, June 27 - July 2, 1993 p. 77
6. Bencze L. and Szilagyi R.: J. Mol. Catal., 1994, 90, 157-170
7. Bencze L. and Szilagyi R.: J. Organometal. Chem., 1994, 475, 183-192
8. Aguero A., Kress J., Osborn J. A.: J. Chem. Soc., Chem. Commun., 1985, 793-794
Installation of METMOD1
------------------------
General informations:
Directory structure (~=/pub/chemistry/data/METMOD1)
- ~/carbenes = pentacoordinated tungsten-carbene complexes
- ~/picomplexes = tungsten-alkene-carbene complexes (in prep.)
- ~/metallacycles = tungsten metallacycles (in prep.)
*** - ~/carbenes/molecules = molecular structures for demonstrations
PART I:
methyl WCl4CH2 METMOD1 and ab initio structures
ethyl WCl4CHCH3
propyl WCl4CH(CH2CH3)
ethylH WCl4CH*CH3 alpha-H atom coordinated
propylH WCl4CH*(CH2CH3) to tungsten
PART II:
adduct X-ray structure of {W[C(CH2)3CH2](OCH2tBu)2Br2}.GaBr3,
dimer X-ray structure of the monomeric unit of
{W[C(CH2)3CH2](OCH2tBu)2Br2}2
dimer.corr X-ray structure of {W[C(CH2)3CH2](OCH2tBu)2Br2}2
adduct.min & dimer.min METMOD1 structure of above complexes
soft.build Reconstruction steps of dimer from METMOD1 starting
geometry using soft approximation mode (see Ref. [6]
for details)
PART III:
cats MMXTM structure of WCHtBu(OCH2tBu)nBr4-n, n=0, ..., 4
Fcats METMOD1 structure of WCHtBu(OCH2tBu)nF4-n, n=0, ..., 4
Clcats METMOD1 structure of WCHtBu(OCH2tBu)nCl4-n, n=0, ..., 4
Brcats METMOD1 structure of WCHtBu(OCH2tBu)nBr4-n, n=0, ..., 4
ICats METMOD1 structure of WCHtBu(OCH2tBu)nI4-n, n=0, ..., 4
Scats METMOD1 structure of WCHtBu(OCH2tBu)2Brn(SO2CF3)2-n
n=0, ..., 2
Sicats METMOD1 structure of WCHtBu(OCH2tBu)n(OSiPh3)2-nBr2
n=0, ..., 2
Osborn Molecular structure of complexes published in Ref. [8]
prop Molecular structure of some selected propagating
carbene form in the ring-opening polymerisation of
norbornene
collision Collison steps of two monomers (W[C(CH2)3CH2](OCH2tBu)2Br2)
to form a dimer {W[C(CH2)3CH2](OCH2tBu)2Br2}2
(used in pair-potential calculation)
Remark: The user can verify the proper installation of METMOD1 by
comparing his/her output files with the published ones!
(files with PCM extension are METMOD1 or MMX structures in PCM format,
with ABI extension are ab initio structures in XRA format,
with XRA extension are X-ray structures in XRA format,
with o extension are MINIMISATION OUTPUT files,
with s extension are MINIMISATION SUMMARY files)
*** - ~/carbenes/additional
additional parameter sets in mnemonic format. The user can very
easy incorporate into other softwares: MM2, Spartan, HyperChem,
Cerius2 etc., as well, instead of PCMODEL.
wclcarb = parameter set of WCl4CR1R2 type complexes
dimer_adduct = parameter set of {W[C(CH2)3CH2](OCH2tBu)2Br2}2
and {W[C(CH2)3CH2](OCH2tBu)2Br2}.GaBr3
complexes
osborn = parameter set of W(CR1R2)(OR3)nX4-n (Osborn)
type carbene complexes
*** - ~/carbenes/param.new
PARAM.NEW files. If you have PCMODEL latter version than 3.0,
do not need nothing else; download the files, rename them and
convert to binary code called DATA.STO by MMXDATA command.
wclcarb = parameter set of WCl4CR1R2 type complexes
dimer_adduct = parameter set of {W[C(CH2)3CH2](OCH2tBu)2Br2}2
and {W[C(CH2)3CH2](OCH2tBu)2Br2}.GaBr3
complexes
osborn = parameter set of W(CR1R2)(OR3)nX4-n (Osborn)
type carbene complexes
osborn.fluoride = additional parameter set to file
osborn.iodide (These files must be present in the same
osborn.Si_S directory as DATA.STO. They must not be
converted by MMXDATA command! The reason
why file does not contain these
files that the number of redefiniable atom
types was exceed.)
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