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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.)
Modified: Mon Aug 22 16:00:00 1994 GMT
Page accessed 10602 times since Sat Apr 17 21:25:59 1999 GMT