Le e
Na u e o he me al–ca bon con ac s in ene-diamido d0me al
complexes
Galindo,*aAnd ea Iencoband Ca lo Mealli*bAgus •
n
aDepa amen o de Uni e sidad de Se illa,Ap do 553,41071 Se illa,Spain.Qu•mica Ino ga
nica,
E-mail: galindo=cica.es
bIs i u o pe lo S udio della S e eochimica ed Ene ge ica dei Compos i di Coo dinazione,CNR,
ViaJ.Na di 39,50132 Fi enze,I aly.E-mail: mealli=Ð.cn .i
Recei ed (in Mon pellie ,F ance)29 h No embe 1999,Accep ed 23 d Decembe 1999
Folding o ene-diamido chela es in d0me al complexes is gene -
ally a ibu ed o a sa u a ing pin e ac ion o he me al wi h
he cen al C2C linkage. A MO analysis o selec ed models
(DFT and EHMO le els) shows ha he la e in e ac ion is
negligible (nega i e M–C o e lap popula ions). Mo eo e , a
de ailed analysis o he o bi al in e ac ions highligh s a wo-
elec on Ñow om Ðlled o bi als o he ni ogen a omspn
owa d he me al consequen o ligand olding. Con e sely, i is
e iden ha he C2Cp-elec on cloud is only ba ely a†ec ed by
he con ac s wi h he me al a om.
G oup 4 de i a i es ha e wide applica ions inCp2MX2
ca alysis and hey ha e been ex ensi ely s udied.1Fo
example, o es new eac i i y and/o ca aly ic ac i i y, Cp
ings ha e been s epwise subs i u ed o o he ancilla y
ligands,2including amido g oups.3S ill, he agmen s
and (whe e M is a G oup 4,4G oup 55MCp(NR2) M(NR2)2
o G oup 66me al) a e conside ed me allocene-like and
p esen in e es ing eac i i ies. Some o us ha e epo ed MO
s udies on he bonding capabili ies o ene-diamido ligands in
b idging wo me als7and, p e iously, ela ed 1,2-ca echola es
we e also in es iga ed.8He e, we ocus ou a en ion on
me allocene-like d0complexes ha con ain a chela e ene-
diamido ligand.¤,9
A sea ch in he Camb idge da abase10 shows ha he
ligand is ben owa d he me al.”,11 In a iably, he ni ogen
a oms emain plana ( he bond angles sum up o 360¡) and he
ac ual olding occu s h ough he conce ed o sion o he NR
g oupings abou he CÈN bonds (ca.25È35¡). In his manne ,
he amoun o pconjuga ion in he chela e is educed wi h a
highe localiza ion o he in e media e C2C linkage. Impo -
an ly, he sho ened MÈC dis ances aise he ques ion as o
whe he hey co espond o ac ual bonding. In he li e a u e
(wi h he excep ion o some discussions by Boncella,6b,12
Rheingold and E ns ,13 Pe e sen,14 Dieck15 and hei
cowo ke s) i is almos aken o g an ed ha he cause o
bending is pco-o dina ion o he C2C bond o he me al (see
Scheme 1). In o he e ms, he classic desc ip ion ( 2,p)
adop ed o he g4bonding o bu adiene o ea ly ansi ion
Scheme 1
me als16 is ex ended o he ene-diamido and o he compa a-
ble ligands wi h wo addi ional elec ons in he psys em.
To unde s and be e he ben co-o dina ion, MO calcu-
la ions a di†e en le els o heo y (DFT°,17 and EHMOÒ,18)
we e pe o med o model compounds o selec ed d0com-
plexes o he G oup 4È6 me als. In pa icula , we ha e
conside ed he species 1,TiCl2[o-(iP 3SiN)2C6H4]19
(M Ti, Z ),20M(OA )2[(R@N)2C2R2]Cp
2
@Z [(R@N)2C2R2],21
(M Ta,5ahe,22 Nb23) andCp@MCl2[(R@N)2C2R2]
(X Cl12 2, and alkylW(NPh)X2[o-(Me3SiN)2C6H4]
g oups6ahb), all con aining a ben ene-diamido ligand. P e-
iously, MO calcula ions o complexes con aining he plana
1,4-diazabu adiene ligand24 and he me allacyclo e aazapen-
adiene moie y25 ha e been epo ed, while ela ed complexes
wi h olded di hiolene ligands ha e been s udied wi h EHMO
me hods.26
Full geome y op imiza ions we e pe o med o he modelp
compounds 1and 2, bo h ha ing symme y. Fo con e-Cs
nience, Scheme 2 epo s a selec ion o expe imen al and cal-
cula ed (in pa en heses) geome ical pa ame e s. Mo eo e o
1, he alues in i alics e e o he geome y o a ansi ion
s a e (one nega i e equency) cha ac e ized by a plana
ing. In a iably, he bisec o o he NÈMÈN angleMN2C2
coincides wi h he symme y axis o he me al agmen [C2
o in 1and o in 2].Cl2Ti pseudo-C3W(NR)Cl2
As seen, he DFT calcula ions p o ide ben geome ies o
he chela e ligand. Fo species 2in pa icula , he olding (q)is
on he opposi e side wi h espec o he single NR ligand.
Ano he con o me , wi h he chela e o ien a ed owa d he
la e g oup, lies only 6.4 kcal mol~1 highe in ene gy. The
p obable ansi ion s a e, wi h an o e all plana ing,MN2C2
could no be de ec ed, p obably on accoun o a a he Ña
ene gy su ace. Fo he same eason, he compu ed olding o
he ligand abou he NÈN ec o is less p onounced han he
expe imen al one (compa e he q alues o 157 and 131¡ in
Scheme 2) while he gene al s uc u al ag eemen is sa is ac-
o y. Con e sely, he ansi ion s a e be ween he equi alen
ben con o me s was ully cha ac e ized o 1and lies 12.5
kcal mol~1 abo e wo symme ical minima. Since ou models
a e signiÐcan ly less hinde ed han he expe imen al species, i
may be sa ely s a ed ha s e ic ac o s a e deÐni ely o li le
ele ance o he bending o he chela e.
A quali a i e MO analysis helps o elucida e he majo
bonding componen s be ween he diamido and me al ag-
men s in bo h models 1and 2. Since he basic MO a gumen s
a e simila , we will concen a e mainly on he o me case.
Because o he Ðlled o bi als o he chlo ide ligands (6 elec-pn
on dono s), a agmen can be conside ed asTiCl2
me allocene-like. He e, he d0me al is cha ac e ized by h ee
low-lying on ie o bi als, one o and wo o sym-b1a1
me ies.27 While he o me has na u e (in he planedn
New J.Chem., 2000, 24,73È75 73
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Scheme 2
o hogonal o he ones a e z2and x2[y2-likeTiCl2), a1
o bi als, espec i ely (zcoincides wi h he axis o C2TiCl2).
The basic MO s uc u e o an isola ed ene-diamido ligand has
been desc ibed ecen ly.7I s bonding capabili ies a ise om
he wo ni ogen lone pai s (in-phase and ou -o -phase
combina ions) and he h ee Ðlled pMOs. Following he
obse ed o sions o he NR g oups abou CÈN linkages, he
lone pai s emain well o ien ed o dona ion in o he anddn
z2me al o bi als. Mos c i ical, howe e , is he HOMO o he
ligand (see Scheme 3), which is s ill dis ibu ed be ween he C
and N a oms bu which has componen s ha a e no longe pn
pa allel. This HOMO is e y close in ene gy o he x2[y2
me al o bi al and in e ac s ongly wi h i . The esul ing
bonding combina ion, which is also he HOMO o he
complex, is shown in Fig. 1 (no ice how he d awings gene -
a ed om DFT and EHMO calcula ions a e consis en ). E i-
den ly, he eo ien ed ni ogen lobes o e lap signiÐcan lypn
wi h he lobes o x2[y2on each side o he plane**TiCl2
hence he TiÈN bonds a e ein o ced. I is signiÐcan ha
in he plana TS s uc u e, whe e he la e in e ac ion is
missing, he TiÈN la e bonds a e sligh ly longe (see Scheme
2) and he co esponding o e lap popula ions a e smalle
(0.195 s. 0.213). As ano he poin , he ca bon o bi als o pn
he ene-diamido ligand a e only indi ec ly in ol ed in
bonding; hey a e s ill pa o he ligand HOMO bu do no
o e lap wi h he me al lobes. A ca e ul analysis o he ag-
men o bi al in e ac ions does no p o ide any e idence o
d i ing o he C2Cp-elec on cloud owa d he me al. In ac ,
he low-lying MO o he ligand, which is mainly esponsible
o C2C bonding, is o ally una†ec ed upon o ma ion o he
Scheme 3
Fig. 1 HOMO o model 1 Mbased on he s uc u e o TiCl2[o-
as i esul s om (a) DFT and (b) EHMO calcu-(iP 3SiN)2C6H4]
la ions.
complex. Nume ically, a ce ain elec on epulsion is con-
Ð med by he alues o he MÈC o e lap popula ion, which
a e sligh ly nega i e a any compu a ional le el. On he con-
a y, posi i e MÈC o e lap popula ion alues cha ac e ize
he 2,pbonding o bu adiene o ea ly ansi ion me als.28
Lauhe and Ho†mann29 poin ed ou long ago ha he p e -
e en ial up igh o ien a ion o single amido coligands in d0
complexes is due o impo an dona ion o he ÐlledCp2Mp
n
o bi al in o wo adjacen lobes o he x2[y2- ype o bi al.
Analogously, he ene-diamido chela e engages all o he
x2[y2lobes in he p esen case. Since such a wo-elec on
dona ion om ni ogen con ibu es mos o me al sa u a ion,
he con ibu ion o he C2Cpcloud is negligible.
In conclusion, he p esen analysis o he bonding o he
ene-diamido ligand ag ees wi h a 2,pdesc ip ion whe e he
pcomponen is also la gely con ibu ed by he ni ogen
a oms. Thus, he alence bond pic u e o Scheme 1 is inade-
qua e. A be e o mal desc ip ion is ha o Scheme 4, whe e
he MÈN bonds a e o o de 1.5 and whe e he C2C linkage o
he chela e is localized and la gely p ese ed.
Scheme 4
Acknowledgemen s
AG app ecia es he Ðnancial suppo om he Royal Socie y
o Chemis y (RSC Jou nal G an o In e na ional Au ho s)
ha made possible he de elopmen o his wo k a ISSECC
(I aly). We hank P o esso James M. Boncella o aluable
discussions. Thanks a e also due o he ““A ea della Rice ca
CNR di Fi enzeÏÏ (in pa icula , D A. T onconi and M S.
Ce e i) o p o iding compu a ional suppo .
No es and e e ences
¤ O he nomencla u e used o his ““non-innocen ÏÏ ype o ligand,
which can be conside ed as he educed o m o diimine ligands, is
ha de i ed om he 1,4-diaza-1,3-bu adiene moie y. See, o
example, e . 9.
” Plana diamido ligands suppo ed by addi ional coo dina ion a e
encoun e ed in e . 11(aÈc). Also, a Ti complex con aining a plana
1,8-diamidonaph halene ligand was epo ed in e . 11(d). Fo d0com-
plexes con aining classical plana diimine ligands see e . 11(e, ).
°Ab ini io calcula ions we e pe o med using he Gaussian-94
p og am. Molecula geome ies and ha monic ib a ional equencies
ha e been de e mined using he densi y unc ional app oach (DFT).
In hese calcula ions we ha e used BeckeÏs 1988 exchange unc ional
wi h he co ela ion unc ional o Pe dew (BP86). The s anda d
double-ze a basis se (LANL2DZ) and e†ec i e co e po en ial (ECP)
was used o he me al a om. Fo he o he a oms, DunningÏs basis
se , augmen ed wi h one d pola iza ion unc ion o he non-hyd ogen
a oms, was used.
ÒAll he EHMO calcula ions we e o he ex ended ype usingHu
ckel
a weigh ed-modiÐed Wol sbe ge ÈHelmholz o mula. The EH pa am-
e e s we e aken om CACAO. The bond dis ances and angles used
in he calcula ions ha e been aken di ec ly om he X- ay esul s
h ough he new ou ines o he CACAO p og am ( e . 18).
pIn he ab ini io calcula ions o models 1and 2, wi h he aim o
speed he calcula ions, he o-phenylene g oup (o- ha links heC6H4)
wo N a oms o he diamido ligand was simpliÐed o a ÈCH2CHÈ
ca bon chain. Also, he o ganic g oups on he ni ogen a oms
( ialkylsilyl subs i u en s o amido and phenyl g oup o imido) we e
eplaced by hyd ogen a oms o simplici y.
74 New J.Chem., 2000, 24,73È75
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** Al hough cha ac e ized by a di†e en ancilla y agmen ,L3M
compound 2has a HOMO e y simila o ha in Fig. 1 wi h x2[y2
o bi al in ol ed. The la e can be desc ibed as a membe o he ““ 2g ÏÏ
se ypical o agmen s ( e . 27). No ice ha he o he woL3M
2g
membe s, wi h cha ac e , a e al eady much des abilized by hednpn
o bi als o he imido ligand (hence he o mula ion o W3NR as a
iple bond).
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L e e a909433d
New J.Chem., 2000, 24,73È75 75
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