Zinc and cobalt(II) complexes of tripodal nitrogen ligands of the tris[2-substituted imidazol-4(5)-yl]-phosphane type. Biomimetic hydrolysis of an activated ester

Article abstract of DOI:10.1135/cccc20070492

Novel tris[2-substituted imidazol-4(5)-yl]phosphane (4-TIP^R) ligands 2b (R = Ph) and 2c (R = tBu) were prepared as model ligands for the tris(histidine) motif found in many zinc enzymes. They readily form cobalt and zinc nitrato and chloro comp

Full text of DOI:10.1135/cccc20070492

492
Kunz, Kläui:
ZINC AND COBALT(II) COMPLEXES OF TRIPODAL NITROGEN
LIGANDS OF THE TRIS[2-SUBSTITUTED IMIDAZOL-4(5)-YL]-
PHOSPHANE TYPE. BIOMIMETIC HYDROLYSIS OF
AN ACTIVATED ESTER
1,
2
Peter C. KUNZ * and Wolfgang KLÄUI
Heinrich-Heine-Universität, Institut für Anorganische Chemie, Universitätsstr. 1,
D-40225 Düsseldorf, Germany; e-mail: 1 peter.kunz@uni-duesseldorf.de, 2 klaeui@uni-duesseldorf.de
Received Decem ber 1, 2006
Accepted February 11, 2007
Dedicated to Dr Karel Mach on the occasion of his 70th birthday in recognition of his outstanding
contributions to the area of organometallic synthesis and catalysis.
Novel tris[2-substituted im idazol-4(5)-yl]ph osph an e (4-TIPR) ligands 2b (R = Ph) and 2c (R =
tBu) were prepared as m odel ligands for the tris(histidine) m otif found in m any zinc enzym es.
They readily form cobalt and zinc nitrato and chloro com plexes in protic solvents. The steric re-
quirem ents of the substituents R in 4(5)-position of the 4-TIPR ligan ds (R = iPr (2a), Ph (2b),
tBu (2c)) is discussed on th e basis of th eir UV/VIS spectra. Zin c com plexes of 2a an d 2c were
studied due to th eir ability to prom ote th e h ydrolysis of th e activated ester 4-n itroph en yl
pyridin e-2-carboxylate (pNpic).
Keyw ord s: Bioin organ ic ch em istry; En zym e m odels; N ligan ds; Tripodal ligan ds; Water
solubility; Zin c; Cobalt; Ph osph in es; Im idazoles; UV spectroscopy.
Zin c is foun d in m an y m on o- an d din uclear m etalloen zym es1. Carbon ic
an h ydrase (CA) was th e first iden tified zin c en zym e. Th e active site of th is
lyase is form ed by th e th ree h istidin e residues His-94, His-96 an d His-119.
CA is an exam ple of an en zym e in wh ich zin c is coordin ated tetrah edrally
by th e tris(h istidin e) m otif. Th is zin c bin din g m otif is also foun d in th e
fam ily of m etzin cin s. Due to th eir proteolytic poten tial th ese en zym es play
key roles in tum our gen esis, tum our in vasion , m etastasis, in flam m atory,
rh eum atoid arth ritis an d m an y oth er processes2.
Sin ce th e discovery of tris(pyrazolyl)borato (Tp) ligan ds (“scorpion ates”)
by Trofim en ko in th e early 60’s, tripodal n itrogen don or ligan d h ave at-
tracted ch em ists in th e organ om etallic an d bioin organ ic field. Th ese lig-
an ds h ave been used sin ce to stabilise m etal com plexes in catalytic reaction
cycles an d in th e m odellin g of active sites in m etalloen zym es3,4. Th e
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 4, pp. 492–502
© 2007 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20070492

Zinc and Cobalt(II) Complexes of Tripodal Nitrogen Ligands
493
‘Freiburg en zym e m odel’ an d related com poun ds by Vah ren kam p use
Tp-type ligan ds to m odel th e tris(h istidin e) m otif foun d in m an y zin c-
bin din g en zym es5. An in h eren t problem of th is class of tripodal ligan ds is
th e in stability of th e B–N bon ds to h ydrolysis. Tris(im idazolyl)ph osph an es
seem to be suitable N ligan ds as m odels for th e active site of th e facial His
triad in zin c-con tain in g en zym es. Th e tris(im idazol-2-yl)ph osph an e (2-TIP)
ligan ds usually bear alkyl an d/or aryl substituen ts in 1- an d 4- or 4- an d
5-position s an d sh ow on ly poor solubility in protic solven ts6,7
.
R
R
R
N
N
NH
N
R'
N
R'
H
N
P
P
R
N
N
R
R
N
N
HN
N
R
R
R'
R
CHART 1
Tris(im idazol-2-yl)ph osph an es (2-TIPR)
We th erefore developed recen tly th e tripodal tris(2-isopropylim idazol-
4(5)-yl)ph osph an e ligan d (Ch art 1), wh ich sh owed sufficien t solubility in
aqueous m eth an olic solution s an d is stable towards h ydrolysis8.
Th e 4-TIPR (R = iPr) ligan d form s octah edral as well as tetrah edral
m etal com plexes of com position [(4-TIPiPr)M(X)]X, som etim es with m etal-
coordin ated solven t m olecules. Th e coordin ation geom etry was sh own to
depen d on th e n ature of th e an ion X. Th e substituen t in 4(5)-position
sh ould also in fluen ce th e coordin ation n um ber an d geom etry. Th erefore we
design ed ligan ds of th e 4-TIP type with substituen ts R varyin g in bulkin ess.
RESULTS AND DISCUSSION
Synthesis and Spectroscopic Characterisation
Th e gen eral syn th esis of ligan ds of th e 4-TIPR type is sh own in Sch em e 1.
Th e easily rem ovable dieth oxym eth yl group h as been proven as ideal pro-
tective group for 2-isopropylim idazol8. Un fortun ately, n eith er 2-ph en yl-
im idazole n or 2-tert-butylim idazole could be protected usin g th is group.
Both th e im idazoles were protected by alkoxym eth yl groups accordin g to
th e procedure described by Breslow for 1-(eth oxym eth yl)-2-ph en ylim id-
azole9. In th e followin g step th e protected im idazole is lith iated in position
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494
Kunz, Kläui:
5 an d th en treated with PCl3 in THF at –78 °C. Tris(2-ph en ylim idazol-
4(5)-yl)ph osph an e (4-TIPPh ; 2b) an d tris(2-tert-butylim idazol-4(5)-yl)ph os-
ph an e (4-TIPtBu; 2c) were isolated as colourless solids after rem oval of th e
protective group.
R
N
R'
N
N
R'
1. BuLi
2. PCl3
H
N
NH protection
R'
P
R
R
N
N
N
N
R
N
R'
N
R
3. deprotection
R'
R
N
P
N
HN
NH
H
N
H
N
. . .
P
R
N
N
R'
N
NH
HN
N
R
R
SCHEME 1
Syn th esis of th e tris[2-substituted im idazol-4(5)-yl]ph osph an e ligan ds 4-TIPR (R = iPr (2a), Ph
(2b), tBu (2c))
Th e 1H NMR spectra of th e ligan ds in m eth an ol-d4 sh ow C3v sym m etry.
Th is is explain ed by fast equilibria of all rotam eric an d tautom eric form s
th ese ligan ds can adopt. Th e arom atic proton s in 2b give two m ultiplets in
th e region 7.6–8.2 ppm , th e sign al of th e proton s of th e im idazolyl rin gs is
foun d as a doublet at 8.23 ppm (3JPH = 1 Hz). 2c just sh ows two sin glets in
1
th e H NMR. Th e sign al of th e tert-butyl group sh ows at δ 2.30 an d of th e
1
im idazolyl at δ 7.90 ppm . In th e 31P{ H} NMR spectra, th e 31P sign al appears
aroun d δ ≈ –80 ppm as a sin glet (Table I). Th e FAB+ m ass spectra of 2b an d
2c sh ow th e correspon din g peak for th e m olecular ion at th e expected m/z
value, togeth er with a ch aracteristic fragm en tation pattern .
In con trast to th e in situ prepared tris[1-(dieth oxym eth yl)-2-isopropyl-
im idazol-5-yl]ph osph an e 1a, th e protected tris[1-(alkoxym eth yl)-2-substi-
tuted im idazol-5-yl]ph osph an es 1b an d 1c could be isolated an d ch aracter-
ised. An an alogous tris(im idazol-5-yl)ph osph an e was reported by Bell10 as
ligan d for an auran ofin an alog.
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Zinc and Cobalt(II) Complexes of Tripodal Nitrogen Ligands
495
Metal com plexes were prepared in high yields by stirring the tris(im idazolyl)
ligan ds 2b or 2c with th e correspon din g m etal salts in m eth an ol at am bien t
tem perature. In terestin gly, th e Co(II) n itrato com plex of 2b was soluble in
m eth an ol wh ereas th e correspon din g Zn (II) com plex 3b precipitated after a
1
wh ile an d could n ot be redissolved in m eth an ol11. Th e H NMR spectrum of
fresh ly prepared 3b sh ows two m ultiplets in th e region 8.5–8.7 ppm for th e
3
ph en yl proton s an d a doublet at δ 8.84 ppm with a JPH couplin g con stan t
of 2 Hz for th e im idazolyl proton s. In th e zin c ch loro com plex of 2c, th e
sign al of th e tert-butyl groups is sligh tly sh ifted down field to δ 2.49 ppm
upon coordin ation as is th e sign al of th e im idazolyl proton s (doublet at
3
1
δ 8.43 ppm with JPH = 2 Hz). Th e 31P{ H} NMR sign als of th e zin c com -
plexes 4 an d 6 in m eth an ol-d4 are sh ifted to h igh er field (δ –106 ppm ). Th e
NMR spectra of th e Co(II) com plexes 3 an d 5 sh ow exten sive param agn etic
broaden in g.
Table I
1
31P{ H} NMR ch em ical sh ifts (δ, ppm ) of protected (1) an d free (2) 4-TIPR
R
1
2
iPr
Ph
1a n ot isolated
–90 (1b)
–80 (2a)8
–79 (2b)
–84 (2c)
tBu
–87 (1c)
UV/VIS Spectra of 4-TIPR Cobalt(II) Complexes
In order to un derstan d th e solution beh aviour of th e zin c com plexes an d
get an im age of th e species presen t, we an alysed th e UV/VIS spectra of th e
correspon din g cobalt(II) com plexes. Th e coordin ation ch em istry of th e
‘colourless’ Zn (II) is well resem bled by th at of Co(II), som etim es referred to
as ‘spectroscopic ch am eleon ’. Because of th e sim ilar ion ic radius an d sim ilar
coordin ation ch em istry, cobalt(II) h as becom e th e substitute for zin c(II) in
zin c-con tain in g en zym es (isom orph ous substitution ), even th ough detailed
structural differen ces can occur12
.
Th e UV/VIS spectra of th e th ree ligan ds 4-TIPiPr (2a), 4-TIPPh (2b) an d
4-TIPtBu (2c) in m eth an ol in th e presen ce of stoich iom etric am oun ts of co-
balt(II) n itrate or ch loride are sh own in Fig. 1. All th ree ch loro com plexes
sh ow th e ch aracteristic absorban ce in th e region 550–700 n m for tetrah ed-
rally coordin ated Co(II) cen tres. Th is is in agreem en t with th e tetrah edral
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496
Kunz, Kläui:
coordin ation en viron m en t in th e solid-state structure of th e com plex
[CoCl(4-TIPiPr)]Cl·MeOH·2.5H2O (ref.8). Th e spectra of 2a an d 2b in th e
presen ce of cobalt(II) n itrate sh ow weak absorption in th e region 450–
550 n m , ch aracteristic of octah edral coordin ation of th e Co(II) cen tre. Th is
is also in a good agreem en t with th e solid-state structure of th e cobalt(II)
n itrato com plex of 4-TIPiPr (ref.8). A sin gle crystal structure determ in ation
of th e cobalt(II) n itrato com plex of 4-TIPPh could n ot be obtain ed at satis-
factory R values due to twin n in g of th e crystals. Neverth eless, it is eviden t
from th e X-ray data, th at th ere are two com plexes in th e elem en tary cell of
3. On e com plex sh ows octah edral coordin ation of th e Co(II) by th e
N,N′,N′′-coordin ated 4-TIPPh ligan d, a diden tate n itrato ligan d an d a coordi-
n ated m eth an ol m olecule. Th e oth er com plex sh ows
a distorted
trigon al-bipyram idal coordin ation with out a coordin ated solven t m olecule
(Sch em e 2). In terestin gly, th e UV/VIS spectrum of cobalt(II) n itrate in th e
presen ce of 4-TIPtBu sh ows ban ds in th e region of 550–670 n m with th e typ-
ical h abitus of a tetrah edrally coordin ated Co(II) cen tre. Th is in dicates a
sufficien t steric pressure of th e tert-butyl groups to en force a low coordin a-
tion n um ber (at least at equilibrium ). Th at beh aviour is kn own in th e class
of Trofim en ko’s scorpion ate ligan ds, wh ere tert-butyl substituen ts in posi-
tion 3 are som etim es referred as tetrah edral en forcers13.
0.4
0.3
0.2
0.1
0.0
350
450
550
650
750
850
λ, nm
FIG. 1
UV/VIS spectra of 4-TIPiPr (2a; black), 4-TIPPh (2b; m edium grey) an d 4-TIPtBu (2c; ligh t grey)
in th e presen ce of stoich iom etric am oun ts of cobalt(II) n itrate (solid) or ch loride (dash ed) in
m eth an ol
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Zinc and Cobalt(II) Complexes of Tripodal Nitrogen Ligands
497
Th erefore, th e steric n ature of th e substituen ts R in th e 4(5)-position s in
4-TIPR can adjust th e coordin ation n um ber of th e boun d m etal. Th e coordi-
n ation n um ber is decreased from 6 to 4 as th e steric requirem en t in creased
in th e order iPr < Ph < tBu. It is likely, th at by ch oosin g th e righ t substitu-
en ts form ation of a h ydroph obic pocket of varyin g size is possible in com -
plexes of 4-TIPR type ligan ds.
P
P
NH
NH
HN
NH
HN
NH
N
N
Co
N
Co
N
N
O
N
N
O
O
N
OH
Me
O
O
O
SCHEME 2
Sch em atic drawin g of th e two com plex cation s foun d in th e elem en tary cell of th e solid 3
Mimicking Esterase Activity
Th e zin c n itrato com plexes of th e 4-TIPiPr an d 4-TIPtBu ligan ds were studied
in an esterase m odel reaction . In addition to h ydration of CO2, th e lyase
carbon ic an h ydrase also sh ows som e esterase activity. Th us we in vestigated
th e h ydrolysis of th e m odel com poun d 4-n itroph en yl pyridin e-2-carboxylate
(pNpic) in th e presen ce of th e n itrato zin c(II) com plexes of 2a an d 2c at
various pH. Th e com plexes of 2b were n ot con sidered due to th eir in suffi-
cien t solubility in m eth an ol. Sin ce picolin ate, th e product of h ydrolysis,
can coordin ate to th e m etal, at least ten -fold excesses of com plex to pNpic
were em ployed to m ain tain pseudo-first-order con dition s. Th e observed
rate con stan ts were th e sam e wh eth er an isolated com plex was used or
equim olar am oun ts of m etal salt an d ligan d. Th e h ydrolysis was perform ed
in 80% eth an ol/water m ixtures to com pare th e results with th ose obtain ed
by Brown on sim ilar tris(im idazol-2-yl)ph osph an e system s. Th e observed
acceleration of pNpic h ydrolysis in th e presen ce of 2a or 2c an d zin c n itrate
(Fig. 2) is in th e ran ge of th at reported for th e catalyst system s con tain in g
Zn (II) an d tris(im idazol-2-yl)ph osph an e (7) or bis(4,5-diisopropylim idazol-
2-yl)(im idazol-2-yl)ph osph an e (8)6d. Due to th e structure of th e 4-TIP lig-
an ds, th e solubility of 2a an d 2c allowed an in crease in water con ten t in
th e solven t m ixture, as expected. So, th e h ydrolysis was repeated usin g 50%
eth an ol/water m ixtures (Fig. 3). Th e reaction rates are very sim ilar, with th e
on es for h ydrolysis in th e solven t con tain in g 50% water bein g sligh tly
lower.
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498
Kunz, Kläui:
In all cases at pH > 8.0 th e rate con stan t drops drastically (Fig. 2) an d ap-
proxim ates th e value for th e spon tan eous h ydrolysis. In gen eral, th is is due
to th e precipitation of Zn (OH)2. But as n o precipitate was observed, we as-
sum e th e form ation of soluble but in ert h ydroxo-bridged oligom ers.
FIG. 2
Pseudo-first-order kcat
obs
values for th e h ydrolysis of pNpic by 2a (᭿) an d 2c (᭹) in th e pres-
en ce of zin c(II) n itrate, com pared to 7 (ٗ) an d 8 (᭛) in 80% eth an ol/water (c(pNPic) = 5 × 10–5
m ol l–1, c(cat.) = 5 × 10–4 m ol l–1, c(buffer) = 0.05 m ol l–1, c(NaNO3) = 0.05 m ol l–1, T = 20 ± 0.1 °C)
FIG. 3
Comparison of the hydrolysis of pNpic in 80% ethanol/water (᭛ spontaneous, ᭿ [Zn(2a)](NO3)2)
an d 50% eth an ol/water (ٗ spon tan eous, ꢀ [Zn (2a)](NO3)2)
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Zinc and Cobalt(II) Complexes of Tripodal Nitrogen Ligands
499
CONCLUSION
We h ave syn th esised two n ew represen tatives of th e tris[2-substituted
im idazol-4(5)-yl]ph osph an e ligan d class: tris(2-ph en ylim idazol-4(5)-yl)-
ph osph an e 4-TIPPh (2b ) an d tris(2-tert-butylim idazol-4(5)-yl)ph osph an e
4-TIPtBu (2c). Th ey sh ow good solubility in protic solven ts as well as in
water/alcoh ol m ixtures with th e exception of th e zin c(II) com plex 4. As
sh own by UV/VIS spectroscopy th e h ydroph obic cavity built by th e sub-
stituen ts R in 4-TIPR in fluen ces th e coordin ation n um ber an d geom etry
aroun d th e coordin ated m etal ion . Th e coordin ation n um ber of cobalt(II) is
decreased from 6 to 4 as th e steric requirem en t in creased in th e order iPr <
Ph < tBu. Th e zin c(II) ch loro an d n itrato com plexes of 2a an d 2c sh ow
som e esterase activity. Th e solubility of th e zin c(II) com plexes of 2a al-
lowed a decrease in th e alcoh ol con ten t of th e reaction m ixture com pared
with th e an alogous tris(im idazol-2-yl)ph osph an e com plexes. Albeit th ese
com poun ds are good structural m odels for active sites of tris(h istidin e)zin c
en zym es, th eir catalytical activity is rath er disappoin tin g. At pH > 8 th e
esterase activity of th e zin c(II) com plexes of 2a an d 2c drops drastically.
Th e fate of th ese com plexes at elevated pH is curren tly in vestigated.
EXPERIMENTAL
Gen eral
Th e com poun ds 2-tert-butylim idazole14 an d 1-(eth oxym eth yl)-2-ph en ylim idazole9 were syn -
th esised accordin g to publish ed procedures. All oth er reagen ts were com m ercial sam ples an d
were used as received. 1H an d 31P NMR spectra (δ, ppm ; J, Hz) were recorded with a Bruker
1
DRX 200 spectrom eter. Th e 1H an d 13C{ H} NMR spectra were calibrated again st th e proton
an d th e carbon sign als of th e solven ts as in tern al referen ces (CDCl3: δH 7.30 ppm an d δC
1
77.0 ppm ; m eth an ol-d4: δH 3.30 ppm (quin tuplet) an d δC 49.1 ppm ), wh ile th e 31P{ H} NMR
spectra were referen ced to extern al 85% H3PO4. Th e EI m ass spectra were recorded with a
double focussin g m ass spectrom eter, Varian MAT m odel 311 A, ion isation en ergy 70 eV. Th e
FAB m ass spectra were recorded with a Fin n igan m ass spectrom eter, m odel MAT 8200, in an
3-n itroben zyl alcoh ol (NBA) m atrix. In frared spectra were recorded with a Bruker IFS 66 FT-IR
spectrom eter.
Kin etic Measurem en ts
Th e rate of appearan ce of 4-n itroph en ol or 4-n itroph en olate was followed by m on itorin g
th e in crease in absorban ce at 320 or 405 n m depen din g on th e pH of th e buffered solution
with an An alytik Jen a Specord S 100 UV/VIS spectroph otom eter at 20.0 ± 0.1 °C. Th e pH
was m ain tain ed usin g th e n on -coordin atin g buffers 3-(N-m orph olin o)propan esulfon ic acid
(MOPS, pH 6.1–8.0)15 an d 2-(N-cycloh exylam in o)eth an e sulfon ic acid (CHES, pH 8.5–9.2)15
,
c = 0.05 m ol l–1 an d th e ion ic stren gth was I(NaNO3) = 0.05 m ol l–1. pH was m easured usin g
a Metroh m 744 pH m eter with Metroh m pH-electrode 6.0232.100 (3 M KCl) calibrated with
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500
Kunz, Kläui:
stan dard solution s of pH 4 an d 9 (Riedel–de Haën ), n o correction for solven t m ixtures was
applied. Such correction s are sm all an d am oun t to a reduction from th e observed readin gs
by ~0.2 un its16. Reaction s were m on itored un der pseudo-first-order con dition s with excess
of com plex, c = 5 × 10–4 m ol l–1, or equim olar ligan d an d zin c(II) salt. Th e pseudo-first-order
rate con stan ts (kobs) were determ in ed by NLLSQ fittin g of th e absorban ce versus tim e traces
to a stan dard expon en tial m odel im plem en ted in th e software Aspect Plus (Spektralanalyti-
sche Software 1997, An alytik Jen a). All observed rate con stan ts were determ in ed at least in
duplicate.
Tris[1-(eth oxym eth yl)-2-ph en ylim idazol-5-yl]ph osph an e (1b)
To a solution of 5.0 g (24 m m ol) of 1-(eth oxym eth yl)-2-ph en ylim idazole in 100 m l of abso-
lute THF were added 17 m l (27 m m ol) of 1.6 M butyllith ium in h exan e at –78 °C. Th e solu-
tion was stirred at –78 °C for 30 m in an d addition ally at room tem perature for 30 m in . After
coolin g to –78 °C, 1.49 m l (16.7 m m ol) of PCl3 in 10 m l of absolute THF was added. Th e re-
action m ixture was kept at –78 °C for 1 h . Th e solven t was rem oved in vacuo an d th e resi-
due was dissolved in 100 m l CH2Cl2. To th e wh ite suspen sion , 50 m l of con cen trated
aqueous am m on ia were added, th e organ ic layer separated, wash ed with water an d dried
over an h ydrous MgSO4. Th e solven t was rem oved in vacuo an d th e oily residue
recrystallised from toluen e (2.1 g, 50%). 1H NMR (200 MHz, CDCl3): 0.89 t, 9 H (J = 7,
CH2CH3); 3.27 q, 6 H (J = 7, CH2CH3); 5.37 d, 6 H (J = 7, CH2); 7.18 s, 3 H (Him ); 7.3–7.4 m ,
1
9 H (Ph ); 7.6–7.7 m , 6 H (Ph ). 31P{ H} NMR (200 MHz, CDCl3): –90. EI MS (70 eV, 250 °C),
m/z (%): 634 (18) [M]+. For C36H39N6O3P (634.7) calculated: 68.0% C, 6.3% H, 13.2% N;
foun d: 67.9% C, 6.1% H, 13.2% N.
Tris(2-ph en ylim idazol-4(5)-yl)ph osph an e (4-TIPPh ) (2b)
1.28 g (2.00 m m ol) of 1b were h eated to reflux in 40 m l of a 1:1 m ixture of eth an ol an d
20% HCl for 5 h . Th e solution was poured in to 100 m l of 1 M KOH solution , th e precipitate
was filtered an d dried in vacuo. Yield 0.81 g (88%) of wh ite powder. 1H NMR (200 MHz,
1
m eth an ol-d4): 7.6–7.8 m , 9 H (Ph ); 8.1–8.2 m , 6 H (Ph ); 8.23 d, 3 H (JPH = 1, Him ). 31P{ H}
NMR (200 MHz, m eth an ol-d4): –79. FAB+ MS (NBA-m atrix), m/z (%): 460 (39) [M]+. For
C
27H21N6P·3HCl·6H2O (460.5) calculated: 47.8% C, 5.4% H, 12.4% N; foun d: 47.6% C,
5.1% H, 12.2% N.
2-tert-Butyl-1-(m eth oxym eth yl)im idazole
A m ixture of 12.4 g (100 m m ol) of tert-butylim idazole an d 2.4 g (100 m m ol) of NaH was
stirred in THF till gas evolution ceased. 0.8 g (0.1 m ol) of ch lorom eth yl m eth yl eth er in THF
were added dropwise an d th e reaction m ixture was stirred overn igh t, filtered an d th e solven t
rem oved in vacuo. Distillation (46 °C, 2.0 × 10–2 m bar) yielded 33 g (71%) of th e product as
a colourless oil. 1H NMR (200 MHz, CDCl3): 1.35 s, 9 H (C(CH3)3); 3.23 s, 3 H (OCH3);
5.23 s, 2 H (CH2); 6.80 d, 1 H (Him ); 6.82 d, 1 H (Him ).
Tris[2-tert-butylim idazol-5-yl-1-(m eth oxym eth yl)]ph osph an e (1c)
To a solution of 4.0 g (24 m m ol) of 2-tert-butyl-1-(m eth oxym eth yl)im idazole in 100 m l of
absolute THF were added 17 m l (27 m m ol) of 1.6 M butyllith ium in h exan e at –78 °C. Th e
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Zinc and Cobalt(II) Complexes of Tripodal Nitrogen Ligands
501
solution was stirred at –78 °C for 30 m in an d addition ally at room tem perature for 30 m in .
After coolin g to –78 °C, 1.49 m l (16.7 m m ol) of PCl3 in 10 m l of absolute THF were added.
Th e reaction m ixture was kept at –78 °C for 1 h . Th e solven t was rem oved in vacuo an d th e
residue was dissolved in 100 m l of CH2Cl2. To th e wh ite suspen sion , 50 m l of con cen trated
am m on ia were added, th e organ ic layer was separated, wash ed with water an d dried over
an h ydrous MgSO4. Th e solven t was rem oved in vacuo an d th e oily residue redissolved in 100
m l of h exan e. Th e product (2.0 g, 47%) crystallised after a few days. 1H NMR (200 MHz,
CDCl3): 1.42 s, 27 H (C(CH3)3); 3.15 s, 9 H (OCH3); 5.36 d, 6 H (4JPH = 2, CH2); 6.79 s, 3 H
1
(Him ). 31P{ H} NMR (200 MHz, m eth an ol-d4): –87. EI MS (70 eV, 250 °C), m/z (%): 532 (7)
[M]+.
Tris(2-tert-butylim idazol-4(5)-yl)ph osph an e (4-TIPtBu) (2c)
1.03 g (1.93 m m ol) of 1c were h eated to reflux in 40 m l of a 1:1 m ixture of eth an ol an d
20% HCl for 5 h . Th e solution was poured in to 100 m l of 1 M KOH solution , th e precipitate
was filtered an d dried in vacuo. Yield 0.65 g (84%) of wh ite powder. 1H NMR (200 MHz,
1
m eth an ol-d4): 2.30 s, 27 H (C(CH3)3); 7.90 s, 3 H (Him ). 31P{ H} NMR (200 MHz, m eth an ol-d4):
–84. FAB+ MS (NBA), m/z (%): 401 (55) [M]+, 277 (100) [M – im tBu]+.
[Co(MeOH)(NO3)(4-TIPPh )]NO3 (3)
A m eth an olic solution of 0.23 g (50 m m ol) of 2b an d 0.15 g (50 m m ol) Co(NO3)2·6H2O was
stirred for 2 h an d th en layered with dieth yl eth er. Th e product (0.23 g, 85%) was obtain ed
as violet crystals. FAB+ MS (NBA-m atrix), m/z (%): 581 (100) [Co(NO3)(4-TIPPh )]+, 534 (9)
[Co(4-TIPPh ) – H]+. For C27H21CoN8O6P·2MeOH (707.5) calculated: 49.2% C, 4.1% H, 15.9% N;
foun d: 49.3% C, 3.9% H, 15.8% N.
[Zn (4-TIPPh )](NO3)2 (4)
A m eth an olic solution of 0.23 g (50 m m ol) of 2b an d 148 m g (50.0 µm ol) Zn (NO3)2·6H2O
was stirred for 2 h . Durin g th is tim e a wh ite precipitate was form ed. Th e suspen sion was
stirred for addition al 2 h , filtered an d th e wh ite solid was dried in vacuo. Yield 0.27 g (81%).
1H NMR (200 MHz, m eth an ol-d4): 8.5 m , 9 H (Ph ); 8.7 m , 6 H (Ph ); 8.84 d, 3 H (JPH = 2,
1
H
im ). 31P{ H} NMR (200 MHz, m eth an ol-d4): –106. For C27H21N8O6PZn ·MeOH (681.9) calcu-
lated: 49.3% C, 3.7% H, 16.4% N; foun d: 49.3% C, 3.4% H, 16.1% N.
[Co(4-TIPtBu)]Cl2 (5)
A m eth an olic solution of 0.22 g (50 m m ol) of 2c an d 65 m g (50 m m ol) CoCl2 was stirred
for 2 h an d layered with dieth yl eth er. Th e product (0.20 g, 70%) was obtain ed as deep blue
crystals. FAB+ MS (NBA), m/z (%): 494 (100) [CoCl(4-TIPtBu)]+. For C22H37Cl2CoN6OP (562.4)
calculated: 47.0% C, 6.6% H, 14.9% N; foun d: 46.3% C, 6.7% H, 14.6% N.
[Zn (4-TIPtBu)]Cl2 (6)
A m eth an olic solution of 0.20 g (50 m m ol) of 2c an d 68 m g (50 m m ol) Zn Cl2 was stirred
for 2 h an d layered with dieth yl eth er. Th e product (0.11 g, 40%) was obtain ed as colourless
crystals. 1H NMR (200 MHz, m eth an ol-d4): 2.49 s, 27 H (C(CH3)3); 8.43 d, 3 H (JPH = 2, Him ).
Collect. Czech. Chem. Commun. 2007, Vol. 72, No. 4, pp. 492–502

502
Kunz, Kläui:
31P{1H} NMR (200 MHz, m eth an ol-d4): –106. FAB+ MS (NBA), m/z (%): 499 (100)
[Zn Cl(4-TIPtBu)]+. For C22H37Cl2N6OPZn (562.4) calculated: 46.5% C, 6.6% H, 14.8% N;
foun d: 46.7% C, 7.0% H, 12.7% N.
We thank the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie for
financial support.
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