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Mo study of molecular nitrogen fixation II. Model systems: [N2 + H2], [N2 + H2]+, [N2 + H2]−
Jourcal of Moolecular Cafabsis. 6 (1979) 269 - 2B8 0 ELswier Secpoia SA., Iswxcae - Printed in tic
P. PELKiN,
M. CEPPAN,
Depa.rtment of (Czechoslowkia)
Physical
M. HARENCChemish-y,
269 Netherhnds
and M. EREZA
Slouak
Tecknicat
U~iuersity,
gg0
37 Bmtis!~ua
TechrGai
Urciuersity.
880
37 Bmfislaua
M. LL%KA Department of Inorga~ic (Czeckoslouakïu) L. TURL
Slouak
NAGY
Irrctifute of Experimental (Czechoslooakïa) (Received
Ckemistry.
Febnnry
Pkarmacology,
Sìooak Acadcmy
of Sciencca.
800 00 Bratrslaua
19,1979)
The model systems of molecullar nitrogen fmation ([Na f Ha], CNa + Hz] +, CNa + Ha] -) were studied usïng the semiempirical INDO method with automatie gradïent geometry optimisation. The analysis of energy and charge distribution characterïstics in ah possible reaction coordinates enabled us to propose the symmetrïes and donor-acceptor properties of suitable catelysts.
1. Introduction The synthesis of genuine molecular nitrogen fjxatïon catalytic systems represents one of the most important problems of chenústry today. Despite the many scientists werking in this field, the enormous complexïty of these systems makes a detailed determination of the active centre biocatalyst structure extremely difficuh. However, model quantum chemi& studies may be &uitful for solving these problems. The model systems IN, f H] -, INz +- H]‘, CN, f- H] - were studîed in a previous paper [ l] _ The present paper deals with the mcdel systems [Na + Ha], [Na + Ha]+, [Na f Ha]-.
2. Method Ah the resuhs present& were obtained using the semiempirical INDO method wïth automatie gradïent geometry optïmisation 121. The Fossible reaction coordinates were determined by a PFX&GI-I~~~ study of the shape of the energy hypersurfaces of the systems studied. The sarne energy and charge
distribution characterïstics as descríbed in the previo-us paper [l] were studied during the detaïled mapping (wïth full geometry optimïsation) of particular reaction coordïnates. The approprïate models of the fint step in molecular nitrogen fïxation were chosen on the basis of weakening of the N-N bond and a simultaneous creation of N-N honds in the reaction products. The reactions were classified as symmetry allowed or forbidden by the application of Woodward-Hoffmann rules arïd frontier orbitals theory, and by the presence of an energy banier in the total energy curve. The symmetñes and donor-acceptor properties of the necessary catalysts were proposed on the basis of a study of the electron dïstribution changes which take place at the extreme points of the reaction coordinates.
3. Results and drscussfon The symmetrïes of the possiblc reactlon coordinates of the systems [N, + H,], [N2 + Hz]+ and [N2 J- H,] ‘, determined by the prelinmrary mapping of their energy hypersurfaces, are shown in Table 1. The abbreviations used for the various geometñcal arrangements are gïven in Fig. 1. Figure 2 illustrates the dependences of total energy on the possible reaction coordinates of al1 the systems considered. The dependences were obtained from a detailed investigation of the chosen reaction coordmates. Geometries and charge distñbutions of the systems in the points correspondmg to the estremes of the energy curves are shown in Table 2. The suitability of particular models characteñzing the first step of molecular nitrogen fixation may be determined from the dependences of Wiberg’s indices on the reaction coordinates. The systems [N2 f Hz], [Na f Hz]+, [N2 + Ha]- (T structure, model A), [N, f H,] (non-planar, C?); [Na A Hz]‘, [Na + Hz]- (cis-configuration, Cz”); [N2 + Hs] * (non-planar, C,,) are promising from this point of view. The triple N-N bond is reduced to an ‘approximately’ double bond and two ‘rather’ single N- N bonds are sïmultaneously created in al1 these cases. However, al1 these reactions are forbidden by symmetry, as can be seen from TABLE
1
The symmetries
of the possible
reaction
System
Symmetrk
INZ -Hz1
Asymmetrie (C,); non-planar (Cz )
INz
+Hzl
[N2+ H21+
Tstructure non-planar
of the reactlon
- model (C,,)
T-structure non-planar
coordinates
T-structure
Llnear configuration (Cz,); (C2[,);
coordina+a
A (Cz,);
(C,,); - model (Cz,)
- model
A (Cti);
cisconfiguration
(C,,).
Tstructurc - model x\ B (Cz,); crsconfìguration
Fig. 1. The geometrïcal arraogements and theü 1. linear system (C,,); 2_ Tstructure system -model A (Cz”); 3. T&ructure system -model E (C,,); -I_ crsstructurc system (Cz,); 5. non-planar system (Cz,).
abbreviations.
the crossings in the correlation diagrams (Figs. 3 - 9)+ and from the pr-nee of (at least one) energy barriers on Fig. 1. The other reactïons are ailowed, but they do not affect the reduction of tbe N-N bond or the creation of the (N,)-(Ha) bond. The electron density transfer from the molecular orbital (MO) antibonding wSh respect to the N-X bcmds ztd simulkn-~eously bonding with respect to the N-N bond on the N-N antïbondïng MO takes This is a common feature of all the place at the energy barrier m aximum. appropriate systems. The symmetry restrictions may be removed by the catalytic systems which allow such electron transfer at the start of the reactïon. Our study enabled the symmetry and donor-acceptor properties of these catalysts to be described. The nomenclature in which the donor-acceptor properties are defined with respect to the ligand used is derived fkom ref. 3.
fThc
folloting euampie ill&tcs the nbbreGtio= uzxd I =‘r?4 NbaNH rneans the orbibl, wbicb is of the antibonding TTtype with respect to the N-N bond and of the bocding o type wnith respect to the N-H bond. The symmetry propertïes of a given MO (S = syrometrical, A = antisymmetrïcal) with respect tn the symmetry opzrations (in the order denoted on particular Figures) are plaozd in parenth~. molecule
272
a
i
E
r r dl
(1)
-6l40
1
0-0
Yz) -035 B
0)
(3)
24
Lo
dClö?n]
Fig. 2. The total energy dependence Al CNZ+HZI meti+= Mm
BI
CNz+H?,l
Tstructure
-model
CI DI
:Na+&l IN, +
non-pIa.nar
(C,);
E/
F/
H21CN2 + H21-
cti6tructm (C,,); T’structure -model
IN2 +- H, J- nonMan=
(C2,);
on remtion
cc,);
A (C,,);
A (C,,);
coordinam
s (1
for:
273
4 E
ic
I
G
‘250
-b250
J (1)
-b30.0
(2)
ZO
íM
0,968 0,400 0,000 1,801 1511
maximum
minimum
maxlmum
minlmum
minlmum
Tmmodol A
Wz +H21 non.plonnr
1,213 1,900 1.060 0,778
minimum
maximum
1,782 1,202
mnrdmum
minlmum 1.587
1,844
minimum
P2 *Hz]+ T.modal A
mlnimum
2,170
[NO + Ha]+
0,450
mlnimum
[N2 *H21+ linour_
maximum
minimum
2,260 1.883
minimum maximum
minimum
P42 + H21non.plannr
[N2 + tI21Tmmadel A
1,003
mnximum minimum
mlnimum 1,96G 1,3GO
1.600
mlnimum
[Nz + Hz1
maximum
1,887 1,249
morimum
[N2 + H21cie conf.
2,272
mmimum
[Ne + Hz] (C,)
dWW,)
Extrcmnl point
System
1,745
0,973
1,826
1.126
0.860
0.917
1,669
1,377
2,431
2,380
1,835
o,a32 1.668
1.826 1.830
1.820
1,667
0,861
2.815
3.168
1,826 2.189
1.176
1.198
1.164
1.147
1.143
1.463
1.161 1.411
1.160
1.231
l.i72 l.lGS
1.222
1.201
1.196
1,178
1,170
1,205 1.229
1.198 1,222
1.173 1,222
1 149
0,771
1318
h-N
dH-11
2.660
2.186
2.403
2,837
2.911
0,716
0,920
2.901
2,91G
2,107
2,609 2 442
1.365
2.327 1,402
2,421
2,500
2.061
2,094
2,091
2.326
1.939
2,361
2.069
bl -N
0,808
0 003
0.010
0.694
0.367
0.010
0.000
0.201 0.190
0,001
0,630
l,G74
0.164 1,224
0,495
0,0b6
0.982
0,OSG
0,046
1,170
0,280 0.992
0.011
1.676 1,818
0,920
0 814
0,124
1,884
1.804
1,866
1,414
1,788
1408
0,069
\Y(N,k(II,)
0,676
0.039
0,080
0.002
0.016
0,623
0,049
0.032 0 008
0,260
0.028
0.103
0,927
~~tr-lr
Thc gcometricti ond chorgc distributionti In the cxtromul points ol the encrgy cwvcti*
TABLE 2
0,646
0.454
0.532
0.618
0.687
-1.366
-1,202
0,128
0.104
-0,328
-0,642 -0.l77
-0,664 -0.718
-0,274
-0,224
-0.682
-0.096
-0,120
-0.004
OJ96
-0.064
0,lSS
0,018
%N,)
0.464
0.646
0.468
0,481
0.413
0.300
0,202
-1,104 -1.128
-0,672
-0,823
-0,468
-0,436 -0.282
-0,726
-0,776
-0.418
; 0,096
0,120
0,064
-0,OBO
0,064
-0,166
-0,018
QHIJ
-827878
-633,42
-628,04
-629,42
-629,38
-f394,96
-632,1)6
-032,96 -013,06
-040,90
-636869
-636,26
-641.73
-637.60
-038.00
-635.85
-636,40
-644,64
-642.64
-644,84
-634 39
-043.63
-636.62
-641.63
Jp
1.301 1,099 Oh29
maxlmum
minlmum
non planer
*DistancesIn lO_” m; cnerglca in CV.
0.076
mlnlmum
minimum
1.403
mnximum
IN2 * 112 1’
1.4Gl
mlnimum
Wz * H2 1”
cia conf.
TmmadolB
1.696
1.070
0.768
1,890
0.720
0,832
1,176
1,226
1,19tl
1,176
1.190
1.183
l,IB8
1.760
2.676
2,389
2.690
2,047
0.018
0,(326
0,474
0.001
0.480
0,526
0.830
0.700
0.188
1.668
0.960
0.198
O.lGO
0.220
0.900
0,4GO
0,600
0.010
0,840
0.780
0.700
0.660
0.400
0,890
-uao,90
-027,Ol
-027,76
-cl93,Ql
-627.19
-027.11’7
3
cn
276
n,* Fig.
3. Gx-relation
tiiagrarn for t5e reaction
The systems ture
-
model
hl+
R1 -
[Nz
+ Hz]
Tstructure
-r -model
n,sJ A (Cz,).
[N2 + Ha], [N, + Ha3 * and [N, f H21- with the T TrucFig. 1) requïre the followïng electron density transfer:
A (see -
aaNHboNN
boNr-i=NN
-
Four different catalyst geometnc arrangements allowing the necessary donoracceptor interaction may be suggested for these systems (Fig_ 10). Systems A and B represent the monocentric complexwïth the o-type donor interaction dr+’
-
aaNHboN?d
and the n-type acceptor interaction d =Y
-
bmNHanNN
_
The catalytïc effect of these systems may be increased hy the followïng geometrïc -gement M M,-N-N
/
\
M,.
‘H’ The bicentric complex a crdonor interactïon
with bridged molecular nitrogen (system
C) bas
277
Sl#MEIRl
CEOW-
C,
SlnM-ElEF OPLLATta13 : .
C,
aa
:;
$ R
El
n
q
-10
0
_Ft
-200
,-
-___‘yg?____--_____p _f#_L_____--_‘L__-----ft----_-_____ -3l
tt---T’____
s----____---ft
52 l
l6_._ --_ __
--=x-e=--_---------yq
__----
R._‘---____
04
d=-~5xsl
_--
WSJ
-
kc_
rn.IA,!.
EnzSJ
Y.
zo”
\
\
y/ \
\
I
lL,cw aCLW
1’ \\
,’
‘>” ,l
I
\ \
=
\ \
C;
-1oc
$
bh,
u y
lE.W
__----
-----Tg”“=_
bC,&-L!s
n
- 20.(
Fig. 4. CarrektÏon
diagram for the reaction [Nz + Hz]
nonpknar
(Cz).
hc_m
-110
1
Sr ISSQ
+--__ _++_------_
IC8 CsAd
--*=_=___
___------‘==-
.!
--a--
Fig.
5.
Con-elati~n
diagram
fo c
the
-
[UIn,];
[k- hl-reaction
[N2
+ Hz]-
[h
-___ ----H-•6dA3
[n,ff&
na-> (C,,).
cïsstructure
E [.T
2aa
G-ii%Ad
_/-_------
TG (AS3
5=------__--
__--
6
(ASJ
__--
la0
‘iTm-1&7
(511) _--
fG* ‘6.1
(trd -.________
; \ \\
/
\ ‘\
/
r’ /y
-
Fig. 6.
Corrdation
clïagmm
bS9
U’
lcm
(SAU
~-_-----2--4-.
IC,
(SSP
u-----
foor the reaction
[Nz
/
/
as, bSlsSr)
,k’
’ 1
\
\
\
,--*”
L&&_bG_~hd
/H
lor
101
/
\
/-+
/’
---*---__ __j+--cC
-
_--+t -----Fe
b~u3S5J b%z(AsJl
_---
+ Hn,]-
TQructure
-model
A (Cz,)
279
-300
wig_ 7. tin-elztion
diagram
for the reaction
[N2
2: [cl --:,____
-
c:
l#l
cf=A)
Tstructu~
-
model AICZ~)-
____-----aAd
100
II
E4
Y
+ Hu]’
C.
-200
.
-1OQ
-
Fig. 8_ Gxrdation
l
a
4
diagram
for the reaction
[N2
+ H2 ]* Cisstrudure
-
rnod&l A (C,,).
I diagram for
Fig. 9. Correlation
the
..-q-
-+-
EI.-]:
reection [Nz + Ff2 1’ non-planar (C,,!.
interaction
and a IT acceptor
1d:,
[m,-s]--[
+ d%]
-
b”,,,afl,,
-
T’he cr donor transfer
and the weak 6 acceptor d:,
[d$
-
=NHb’JNN
-
+ d,2,
*d$]
interaction baNHamNN
take place in system D. Only the first reaction step (interaction in the cr;Cconfïguration) is important in the non planar system [N, + H,] of C2 symmetry. The second step represents cnly the cis-tmrzs isomerisatlon which causes no significant change in the -bond charactetitics: N
N-H
H
N-H Three
arrangzments may be suggested for the geometic 11). A represents a monocentric catalyst with G donor electron
different
catalyst
first reaction step (Fig. System transfer
auNH
H-N
bWN
-
dr’_Y’
Fig. lO.C~talytlc eystems for[Nz+ Hz],[Nz+ Hz]"and[Nz+ Hz]-T-structuro -modo1A(cav).
I
d'rv
282
1
~x
u--N
k~._
d zy
d x ~- yZ
A
l~U
B
[d~-v~-d~v] ' [df'-V% d~V]~
[d~-~v~-d~d'-[d~!y~-~v]~
Fig. 11. Catalytic systems for IN 2 + H 2 ] non-planar (C2). arLd ~ a c c e p t o r i n t e r a c t i o n
dxy
> bONHaT¢NN.
T w o 7r b o n d s r e q u i r e t h e d o n o r i n t e r a c t l o n
affNHb~'NN
)
[d~
in t h e b i c e n t r ~ c s y s t e m B
--dy=]
w h i l e t h e a c c e p t o r e l e c t r o n t r a n s f e r is a c h i e v e d w i t h t w o w e a k 8 b o n d s . { d ~ y + d2y]
.
baNHa~rNN
"
T h e l a s t o f t h e s u g g e s t e d c a t a l y s t s ( s y s t e m C) is t h e b i c e n t ~ i c s y s t e m i n which both donor and accepbor interactions
283
require a proper hybrÏdisation of the atunüc orbitals of the tmnsition metals. It is interesting that almost all authors studying biological molecular nikogen transition met& ín the active centers fïxation assume such an arrangement of ferments [4,5]_ An analogous caLMyst geometric arrangement is required for the first reaction step in the system [Nz ’ f Hz] - (Ca, cti). Further, the orbital occupa tion of the transition metals must al!ow the electron transfer
of
aoNHb%N
-
atïNpi
the second reaction step. This electron density transfer may occur in two ways (Fig. 12). Both donor and acceptor interactions are of the n type in system A
in
anNH
bnKN
d YZ
-
d XZ -
arNN_
The u donor interaction d,x_,z
+
aoN_XbnNN
and the weak 6 acceptor ïnteraction d TZ -
axNN
take place in system B. The system [ET2 + Hz]+ aaNHbgNNbaHz+
-
(Cz”, CLS) requires the eiectron density transfer anNN
-
Two different catalyst geometrïc arrangements may be suggested for this system (Fig. 13). Eoth these caklysts are bicentrïc. Two correspondïng monocentric cakalytic systems may be obtained when one of the metal atoms is omitzed. It is obvious that the catiytic influence is amplified in the bicentrïc arrangements. The u donor and 0 acceptor interactions aaNHh%NhugH
-
1dfz + d%
-
d:+Z L
+ d,Zz_,l
=NN
1
1 1 present in system A. The catalyst causes the (T donor interaction rd,‘Z +d$ aGNuboNNb%H 1 and the m aceptor interaction
are
1
in
system B.
284
ì
a-
b
d TE A
Fig. 12. Catalytio systems for [Nz + Hz]-
‘The Een pknar density uansfer +
aoNHbNN
system
[N,
cisconfiguration
+ Hz; + (C,,)
(C,,).
requires
the following
electron
mNN-
The catiysts can act in three different geometrie axrangements (Fig. 14). The 5rst two cases (A and B) are bicentzïc systems which can be reduced a monocentric syskm as in the previous case. The 0 donor ïnteraction mNH
bil
NN
and the 6 acceptor
-
d&a -d$_,1 [ electron transfer
1
to
285
d’r’
d'.'
in system A. System B ïnvolves both donor and acceptir ïnteractions wïth the R bonds
are present
=NHhNN
d ==
-
d,,
-
=NN
are present in the monocentrïc
system C.
286
C Fig. 14. Ca’dytic
systems
for [N2
+ + @ba n
du
+ Hz]+
non-planar
(C2,).
A fairly complicated reaction mechanism consistïng of two steps is found in the non-planar systzm [N, + H2] - (C,,). In constructing a scheme for the catalytïc t;ystem, there is a requiremerrt for the same symmetry for the catalyst’s geometic arrangement in each of the reaction steps (Fig. 15).
287
,/
Tg
~ -
A
d~z y
~i'... b c i ' ~
- el' xV
-d2ffiy
7*
~
~r..
- d z ~V
E
Qkf~'~ ~-'~,
d']LZ
• ~'.,. b~"..
- d2r.~
d'zz
I~'--
dz i]:
B Fig. 1 5 . C a t a l y t i c s y s t e m s f o r I N 2 ÷ H2 ] - n o n - p l a n a ~ (C2~)_
288
In the first reaction step (scheme taITNH
b~NN~~HH)2
A) the electron transfer
+ (a%Hb~NNb”dl
(aoNtIb~NNbuHH)2
-
+ (arNHbmNNaGHH)’
is realized through the x bonds. The complex system of TFbonds causes the electron (amNH
hNN taTTNN
+ (amNHbGN)l
)’ 1’
+ (~~NN)I
+ (amNN)O + (aUNtsbnNN
+ (aTTNN)’ 1’
transfer
-
+ tarrNHbTTNN
1’
in the second reaction step (scheme B). This mechanism can be supported by the chelate structure of the remruning ligands, which can serve as both donor and acceptor of electron density. It must be pointed out here, that al1 discussion of the suggested catalysts concerns only the geometry and symmetry properties of thc orbitals which act as electron density donors or acceptors. On the other hand, these orbitzk must have the necessary wïth the chosen orbitds
energy
and occupancy
(important
so that they can ïnteract
only
from our point of tiew). These orbital
catalyst properties are maïnly determmed by two factors : (i) the properties of the transition metai used; (ii) the symmetry and strength of the ligand field of ligands which are not direccly involved in the reaction. An optimum requïrement for donor-acceptor properties can therefore be arrived at by mode!ling the ligand sphere in a particular geometrïcal arrangement. Thls IS the aïm of our future work, and first qualitative results may be found in ref. 6.
References 1 P. Pelikán, XI. LXka, hl. Haring, M. &ppan, M Brc~ Und L Turi Nagy, J- Mol. Cxtal., a (1979) 3-$9. 2 J_ Pancíi, Theor. Chim. Acts, 29 (1973) 21. 3 F. D. hfango, Fortschr. Chem. Forsch., $1 (1973) 39. 4 M. M. rnqul Khan and A. E. Martell, in Homogeneous Catalysis by Metal Complexes, Vol. l., Academie PW%. New York, 1974. 5 J Chatt and G. J_ Leigh, Q. Rev_. 26 (1972) 121. 6 L. Turi Nagy, P. Pel:k3n, M. LiEka, M. Haring. M. &ppan and hl. Breza, Int_ J. Quantum Chcm., in prcss.
P. PELKiN,
M. CEPPAN,
Depa.rtment of (Czechoslowkia)
Physical
M. HARENCChemish-y,
269 Netherhnds
and M. EREZA
Slouak
Tecknicat
U~iuersity,
gg0
37 Bmtis!~ua
TechrGai
Urciuersity.
880
37 Bmfislaua
M. LL%KA Department of Inorga~ic (Czeckoslouakïu) L. TURL
Slouak
NAGY
Irrctifute of Experimental (Czechoslooakïa) (Received
Ckemistry.
Febnnry
Pkarmacology,
Sìooak Acadcmy
of Sciencca.
800 00 Bratrslaua
19,1979)
The model systems of molecullar nitrogen fmation ([Na f Ha], CNa + Hz] +, CNa + Ha] -) were studied usïng the semiempirical INDO method with automatie gradïent geometry optimisation. The analysis of energy and charge distribution characterïstics in ah possible reaction coordinates enabled us to propose the symmetrïes and donor-acceptor properties of suitable catelysts.
1. Introduction The synthesis of genuine molecular nitrogen fjxatïon catalytic systems represents one of the most important problems of chenústry today. Despite the many scientists werking in this field, the enormous complexïty of these systems makes a detailed determination of the active centre biocatalyst structure extremely difficuh. However, model quantum chemi& studies may be &uitful for solving these problems. The model systems IN, f H] -, INz +- H]‘, CN, f- H] - were studîed in a previous paper [ l] _ The present paper deals with the mcdel systems [Na + Ha], [Na + Ha]+, [Na f Ha]-.
2. Method Ah the resuhs present& were obtained using the semiempirical INDO method wïth automatie gradïent geometry optïmisation 121. The Fossible reaction coordinates were determined by a PFX&GI-I~~~ study of the shape of the energy hypersurfaces of the systems studied. The sarne energy and charge
distribution characterïstics as descríbed in the previo-us paper [l] were studied during the detaïled mapping (wïth full geometry optimïsation) of particular reaction coordïnates. The approprïate models of the fint step in molecular nitrogen fïxation were chosen on the basis of weakening of the N-N bond and a simultaneous creation of N-N honds in the reaction products. The reactions were classified as symmetry allowed or forbidden by the application of Woodward-Hoffmann rules arïd frontier orbitals theory, and by the presence of an energy banier in the total energy curve. The symmetñes and donor-acceptor properties of the necessary catalysts were proposed on the basis of a study of the electron dïstribution changes which take place at the extreme points of the reaction coordinates.
3. Results and drscussfon The symmetrïes of the possiblc reactlon coordinates of the systems [N, + H,], [N2 + Hz]+ and [N2 J- H,] ‘, determined by the prelinmrary mapping of their energy hypersurfaces, are shown in Table 1. The abbreviations used for the various geometñcal arrangements are gïven in Fig. 1. Figure 2 illustrates the dependences of total energy on the possible reaction coordinates of al1 the systems considered. The dependences were obtained from a detailed investigation of the chosen reaction coordmates. Geometries and charge distñbutions of the systems in the points correspondmg to the estremes of the energy curves are shown in Table 2. The suitability of particular models characteñzing the first step of molecular nitrogen fixation may be determined from the dependences of Wiberg’s indices on the reaction coordinates. The systems [N2 f Hz], [Na f Hz]+, [N2 + Ha]- (T structure, model A), [N, f H,] (non-planar, C?); [Na A Hz]‘, [Na + Hz]- (cis-configuration, Cz”); [N2 + Hs] * (non-planar, C,,) are promising from this point of view. The triple N-N bond is reduced to an ‘approximately’ double bond and two ‘rather’ single N- N bonds are sïmultaneously created in al1 these cases. However, al1 these reactions are forbidden by symmetry, as can be seen from TABLE
1
The symmetries
of the possible
reaction
System
Symmetrk
INZ -Hz1
Asymmetrie (C,); non-planar (Cz )
INz
+Hzl
[N2+ H21+
Tstructure non-planar
of the reactlon
- model (C,,)
T-structure non-planar
coordinates
T-structure
Llnear configuration (Cz,); (C2[,);
coordina+a
A (Cz,);
(C,,); - model (Cz,)
- model
A (Cti);
cisconfiguration
(C,,).
Tstructurc - model x\ B (Cz,); crsconfìguration
Fig. 1. The geometrïcal arraogements and theü 1. linear system (C,,); 2_ Tstructure system -model A (Cz”); 3. T&ructure system -model E (C,,); -I_ crsstructurc system (Cz,); 5. non-planar system (Cz,).
abbreviations.
the crossings in the correlation diagrams (Figs. 3 - 9)+ and from the pr-nee of (at least one) energy barriers on Fig. 1. The other reactïons are ailowed, but they do not affect the reduction of tbe N-N bond or the creation of the (N,)-(Ha) bond. The electron density transfer from the molecular orbital (MO) antibonding wSh respect to the N-X bcmds ztd simulkn-~eously bonding with respect to the N-N bond on the N-N antïbondïng MO takes This is a common feature of all the place at the energy barrier m aximum. appropriate systems. The symmetry restrictions may be removed by the catalytic systems which allow such electron transfer at the start of the reactïon. Our study enabled the symmetry and donor-acceptor properties of these catalysts to be described. The nomenclature in which the donor-acceptor properties are defined with respect to the ligand used is derived fkom ref. 3.
fThc
folloting euampie ill&tcs the nbbreGtio= uzxd I =‘r?4 NbaNH rneans the orbibl, wbicb is of the antibonding TTtype with respect to the N-N bond and of the bocding o type wnith respect to the N-H bond. The symmetry propertïes of a given MO (S = syrometrical, A = antisymmetrïcal) with respect tn the symmetry opzrations (in the order denoted on particular Figures) are plaozd in parenth~. molecule
272
a
i
E
r r dl
(1)
-6l40
1
0-0
Yz) -035 B
0)
(3)
24
Lo
dClö?n]
Fig. 2. The total energy dependence Al CNZ+HZI meti+= Mm
BI
CNz+H?,l
Tstructure
-model
CI DI
:Na+&l IN, +
non-pIa.nar
(C,);
E/
F/
H21CN2 + H21-
cti6tructm (C,,); T’structure -model
IN2 +- H, J- nonMan=
(C2,);
on remtion
cc,);
A (C,,);
A (C,,);
coordinam
s (1
for:
273
4 E
ic
I
G
‘250
-b250
J (1)
-b30.0
(2)
ZO
íM
0,968 0,400 0,000 1,801 1511
maximum
minimum
maxlmum
minlmum
minlmum
Tmmodol A
Wz +H21 non.plonnr
1,213 1,900 1.060 0,778
minimum
maximum
1,782 1,202
mnrdmum
minlmum 1.587
1,844
minimum
P2 *Hz]+ T.modal A
mlnimum
2,170
[NO + Ha]+
0,450
mlnimum
[N2 *H21+ linour_
maximum
minimum
2,260 1.883
minimum maximum
minimum
P42 + H21non.plannr
[N2 + tI21Tmmadel A
1,003
mnximum minimum
mlnimum 1,96G 1,3GO
1.600
mlnimum
[Nz + Hz1
maximum
1,887 1,249
morimum
[N2 + H21cie conf.
2,272
mmimum
[Ne + Hz] (C,)
dWW,)
Extrcmnl point
System
1,745
0,973
1,826
1.126
0.860
0.917
1,669
1,377
2,431
2,380
1,835
o,a32 1.668
1.826 1.830
1.820
1,667
0,861
2.815
3.168
1,826 2.189
1.176
1.198
1.164
1.147
1.143
1.463
1.161 1.411
1.160
1.231
l.i72 l.lGS
1.222
1.201
1.196
1,178
1,170
1,205 1.229
1.198 1,222
1.173 1,222
1 149
0,771
1318
h-N
dH-11
2.660
2.186
2.403
2,837
2.911
0,716
0,920
2.901
2,91G
2,107
2,609 2 442
1.365
2.327 1,402
2,421
2,500
2.061
2,094
2,091
2.326
1.939
2,361
2.069
bl -N
0,808
0 003
0.010
0.694
0.367
0.010
0.000
0.201 0.190
0,001
0,630
l,G74
0.164 1,224
0,495
0,0b6
0.982
0,OSG
0,046
1,170
0,280 0.992
0.011
1.676 1,818
0,920
0 814
0,124
1,884
1.804
1,866
1,414
1,788
1408
0,069
\Y(N,k(II,)
0,676
0.039
0,080
0.002
0.016
0,623
0,049
0.032 0 008
0,260
0.028
0.103
0,927
~~tr-lr
Thc gcometricti ond chorgc distributionti In the cxtromul points ol the encrgy cwvcti*
TABLE 2
0,646
0.454
0.532
0.618
0.687
-1.366
-1,202
0,128
0.104
-0,328
-0,642 -0.l77
-0,664 -0.718
-0,274
-0,224
-0.682
-0.096
-0,120
-0.004
OJ96
-0.064
0,lSS
0,018
%N,)
0.464
0.646
0.468
0,481
0.413
0.300
0,202
-1,104 -1.128
-0,672
-0,823
-0,468
-0,436 -0.282
-0,726
-0,776
-0.418
; 0,096
0,120
0,064
-0,OBO
0,064
-0,166
-0,018
QHIJ
-827878
-633,42
-628,04
-629,42
-629,38
-f394,96
-632,1)6
-032,96 -013,06
-040,90
-636869
-636,26
-641.73
-637.60
-038.00
-635.85
-636,40
-644,64
-642.64
-644,84
-634 39
-043.63
-636.62
-641.63
Jp
1.301 1,099 Oh29
maxlmum
minlmum
non planer
*DistancesIn lO_” m; cnerglca in CV.
0.076
mlnlmum
minimum
1.403
mnximum
IN2 * 112 1’
1.4Gl
mlnimum
Wz * H2 1”
cia conf.
TmmadolB
1.696
1.070
0.768
1,890
0.720
0,832
1,176
1,226
1,19tl
1,176
1.190
1.183
l,IB8
1.760
2.676
2,389
2.690
2,047
0.018
0,(326
0,474
0.001
0.480
0,526
0.830
0.700
0.188
1.668
0.960
0.198
O.lGO
0.220
0.900
0,4GO
0,600
0.010
0,840
0.780
0.700
0.660
0.400
0,890
-uao,90
-027,Ol
-027,76
-cl93,Ql
-627.19
-027.11’7
3
cn
276
n,* Fig.
3. Gx-relation
tiiagrarn for t5e reaction
The systems ture
-
model
hl+
R1 -
[Nz
+ Hz]
Tstructure
-r -model
n,sJ A (Cz,).
[N2 + Ha], [N, + Ha3 * and [N, f H21- with the T TrucFig. 1) requïre the followïng electron density transfer:
A (see -
aaNHboNN
boNr-i=NN
-
Four different catalyst geometnc arrangements allowing the necessary donoracceptor interaction may be suggested for these systems (Fig_ 10). Systems A and B represent the monocentric complexwïth the o-type donor interaction dr+’
-
aaNHboN?d
and the n-type acceptor interaction d =Y
-
bmNHanNN
_
The catalytïc effect of these systems may be increased hy the followïng geometrïc -gement M M,-N-N
/
\
M,.
‘H’ The bicentric complex a crdonor interactïon
with bridged molecular nitrogen (system
C) bas
277
Sl#MEIRl
CEOW-
C,
SlnM-ElEF OPLLATta13 : .
C,
aa
:;
$ R
El
n
q
-10
0
_Ft
-200
,-
-___‘yg?____--_____p _f#_L_____--_‘L__-----ft----_-_____ -3l
tt---T’____
s----____---ft
52 l
l6_._ --_ __
--=x-e=--_---------yq
__----
R._‘---____
04
d=-~5xsl
_--
WSJ
-
kc_
rn.IA,!.
EnzSJ
Y.
zo”
\
\
y/ \
\
I
lL,cw aCLW
1’ \\
,’
‘>” ,l
I
\ \
=
\ \
C;
-1oc
$
bh,
u y
lE.W
__----
-----Tg”“=_
bC,&-L!s
n
- 20.(
Fig. 4. CarrektÏon
diagram for the reaction [Nz + Hz]
nonpknar
(Cz).
hc_m
-110
1
Sr ISSQ
+--__ _++_------_
IC8 CsAd
--*=_=___
___------‘==-
.!
--a--
Fig.
5.
Con-elati~n
diagram
fo c
the
-
[UIn,];
[k- hl-reaction
[N2
+ Hz]-
[h
-___ ----H-•6dA3
[n,ff&
na-> (C,,).
cïsstructure
E [.T
2aa
G-ii%Ad
_/-_------
TG (AS3
5=------__--
__--
6
(ASJ
__--
la0
‘iTm-1&7
(511) _--
fG* ‘6.1
(trd -.________
; \ \\
/
\ ‘\
/
r’ /y
-
Fig. 6.
Corrdation
clïagmm
bS9
U’
lcm
(SAU
~-_-----2--4-.
IC,
(SSP
u-----
foor the reaction
[Nz
/
/
as, bSlsSr)
,k’
’ 1
\
\
\
,--*”
L&&_bG_~hd
/H
lor
101
/
\
/-+
/’
---*---__ __j+--cC
-
_--+t -----Fe
b~u3S5J b%z(AsJl
_---
+ Hn,]-
TQructure
-model
A (Cz,)
279
-300
wig_ 7. tin-elztion
diagram
for the reaction
[N2
2: [cl --:,____
-
c:
l#l
cf=A)
Tstructu~
-
model AICZ~)-
____-----aAd
100
II
E4
Y
+ Hu]’
C.
-200
.
-1OQ
-
Fig. 8_ Gxrdation
l
a
4
diagram
for the reaction
[N2
+ H2 ]* Cisstrudure
-
rnod&l A (C,,).
I diagram for
Fig. 9. Correlation
the
..-q-
-+-
EI.-]:
reection [Nz + Ff2 1’ non-planar (C,,!.
interaction
and a IT acceptor
1d:,
[m,-s]--[
+ d%]
-
b”,,,afl,,
-
T’he cr donor transfer
and the weak 6 acceptor d:,
[d$
-
=NHb’JNN
-
+ d,2,
*d$]
interaction baNHamNN
take place in system D. Only the first reaction step (interaction in the cr;Cconfïguration) is important in the non planar system [N, + H,] of C2 symmetry. The second step represents cnly the cis-tmrzs isomerisatlon which causes no significant change in the -bond charactetitics: N
N-H
H
N-H Three
arrangzments may be suggested for the geometic 11). A represents a monocentric catalyst with G donor electron
different
catalyst
first reaction step (Fig. System transfer
auNH
H-N
bWN
-
dr’_Y’
Fig. lO.C~talytlc eystems for[Nz+ Hz],[Nz+ Hz]"and[Nz+ Hz]-T-structuro -modo1A(cav).
I
d'rv
282
1
~x
u--N
k~._
d zy
d x ~- yZ
A
l~U
B
[d~-v~-d~v] ' [df'-V% d~V]~
[d~-~v~-d~d'-[d~!y~-~v]~
Fig. 11. Catalytic systems for IN 2 + H 2 ] non-planar (C2). arLd ~ a c c e p t o r i n t e r a c t i o n
dxy
> bONHaT¢NN.
T w o 7r b o n d s r e q u i r e t h e d o n o r i n t e r a c t l o n
affNHb~'NN
)
[d~
in t h e b i c e n t r ~ c s y s t e m B
--dy=]
w h i l e t h e a c c e p t o r e l e c t r o n t r a n s f e r is a c h i e v e d w i t h t w o w e a k 8 b o n d s . { d ~ y + d2y]
.
baNHa~rNN
"
T h e l a s t o f t h e s u g g e s t e d c a t a l y s t s ( s y s t e m C) is t h e b i c e n t ~ i c s y s t e m i n which both donor and accepbor interactions
283
require a proper hybrÏdisation of the atunüc orbitals of the tmnsition metals. It is interesting that almost all authors studying biological molecular nikogen transition met& ín the active centers fïxation assume such an arrangement of ferments [4,5]_ An analogous caLMyst geometric arrangement is required for the first reaction step in the system [Nz ’ f Hz] - (Ca, cti). Further, the orbital occupa tion of the transition metals must al!ow the electron transfer
of
aoNHb%N
-
atïNpi
the second reaction step. This electron density transfer may occur in two ways (Fig. 12). Both donor and acceptor interactions are of the n type in system A
in
anNH
bnKN
d YZ
-
d XZ -
arNN_
The u donor interaction d,x_,z
+
aoN_XbnNN
and the weak 6 acceptor ïnteraction d TZ -
axNN
take place in system B. The system [ET2 + Hz]+ aaNHbgNNbaHz+
-
(Cz”, CLS) requires the eiectron density transfer anNN
-
Two different catalyst geometrïc arrangements may be suggested for this system (Fig. 13). Eoth these caklysts are bicentrïc. Two correspondïng monocentric cakalytic systems may be obtained when one of the metal atoms is omitzed. It is obvious that the catiytic influence is amplified in the bicentrïc arrangements. The u donor and 0 acceptor interactions aaNHh%NhugH
-
1dfz + d%
-
d:+Z L
+ d,Zz_,l
=NN
1
1 1 present in system A. The catalyst causes the (T donor interaction rd,‘Z +d$ aGNuboNNb%H 1 and the m aceptor interaction
are
1
in
system B.
284
ì
a-
b
d TE A
Fig. 12. Catalytio systems for [Nz + Hz]-
‘The Een pknar density uansfer +
aoNHbNN
system
[N,
cisconfiguration
+ Hz; + (C,,)
(C,,).
requires
the following
electron
mNN-
The catiysts can act in three different geometrie axrangements (Fig. 14). The 5rst two cases (A and B) are bicentzïc systems which can be reduced a monocentric syskm as in the previous case. The 0 donor ïnteraction mNH
bil
NN
and the 6 acceptor
-
d&a -d$_,1 [ electron transfer
1
to
285
d’r’
d'.'
in system A. System B ïnvolves both donor and acceptir ïnteractions wïth the R bonds
are present
=NHhNN
d ==
-
d,,
-
=NN
are present in the monocentrïc
system C.
286
C Fig. 14. Ca’dytic
systems
for [N2
+ + @ba n
du
+ Hz]+
non-planar
(C2,).
A fairly complicated reaction mechanism consistïng of two steps is found in the non-planar systzm [N, + H2] - (C,,). In constructing a scheme for the catalytïc t;ystem, there is a requiremerrt for the same symmetry for the catalyst’s geometic arrangement in each of the reaction steps (Fig. 15).
287
,/
Tg
~ -
A
d~z y
~i'... b c i ' ~
- el' xV
-d2ffiy
7*
~
~r..
- d z ~V
E
Qkf~'~ ~-'~,
d']LZ
• ~'.,. b~"..
- d2r.~
d'zz
I~'--
dz i]:
B Fig. 1 5 . C a t a l y t i c s y s t e m s f o r I N 2 ÷ H2 ] - n o n - p l a n a ~ (C2~)_
288
In the first reaction step (scheme taITNH
b~NN~~HH)2
A) the electron transfer
+ (a%Hb~NNb”dl
(aoNtIb~NNbuHH)2
-
+ (arNHbmNNaGHH)’
is realized through the x bonds. The complex system of TFbonds causes the electron (amNH
hNN taTTNN
+ (amNHbGN)l
)’ 1’
+ (~~NN)I
+ (amNN)O + (aUNtsbnNN
+ (aTTNN)’ 1’
transfer
-
+ tarrNHbTTNN
1’
in the second reaction step (scheme B). This mechanism can be supported by the chelate structure of the remruning ligands, which can serve as both donor and acceptor of electron density. It must be pointed out here, that al1 discussion of the suggested catalysts concerns only the geometry and symmetry properties of thc orbitals which act as electron density donors or acceptors. On the other hand, these orbitzk must have the necessary wïth the chosen orbitds
energy
and occupancy
(important
so that they can ïnteract
only
from our point of tiew). These orbital
catalyst properties are maïnly determmed by two factors : (i) the properties of the transition metai used; (ii) the symmetry and strength of the ligand field of ligands which are not direccly involved in the reaction. An optimum requïrement for donor-acceptor properties can therefore be arrived at by mode!ling the ligand sphere in a particular geometrïcal arrangement. Thls IS the aïm of our future work, and first qualitative results may be found in ref. 6.
References 1 P. Pelikán, XI. LXka, hl. Haring, M. &ppan, M Brc~ Und L Turi Nagy, J- Mol. Cxtal., a (1979) 3-$9. 2 J_ Pancíi, Theor. Chim. Acts, 29 (1973) 21. 3 F. D. hfango, Fortschr. Chem. Forsch., $1 (1973) 39. 4 M. M. rnqul Khan and A. E. Martell, in Homogeneous Catalysis by Metal Complexes, Vol. l., Academie PW%. New York, 1974. 5 J Chatt and G. J_ Leigh, Q. Rev_. 26 (1972) 121. 6 L. Turi Nagy, P. Pel:k3n, M. LiEka, M. Haring. M. &ppan and hl. Breza, Int_ J. Quantum Chcm., in prcss.