Coordination of the ester group - hydrido-rhodium(III) and iridium(III) complexes of orthometallated diphenylmethylene glycine esters [1]

Article abstract of DOI:10.1002/zaac.200900474

An overview on metal complexes that contain coordinated ester groups is given. A new example has been obtained by oxidative addition of the O'Donnell reagent Ph₂C=NCH₂CO₂R to [(cyclooctene) ₂MCl]₂ (M= Rh, Ir) which - in the presence of tricyclohexylphosphine - affords the complexes {(H)(Cl)[C₆H ¹¹)³P]²M(C₆H₄)C(Ph)= NCH₂CO₂R}. In these complexes, the chlorido ligand is replaced for the ester group by using AgPF₆ to give ((H)[C ₆H₁₁)₃P]₂M[C₆H ₄)C(Ph)=NCH₂CO₂R]⁺TF6^-.

Full text of DOI:10.1002/zaac.200900474

ARTICLE
DOI: 10.1002/zaac.200900474
Coordination of the Ester Group – Hydrido-Rhodium(III) and Iridium(III)
Complexes of Orthometallated Diphenylmethylene Glycine Esters [1]
Bernhard Schreiner[a] and Wolfgang Beck*[a]
Dedicated to Professor Wolf Peter Fehlhammer on the Occasion of His 70th Birthday
Keywords: Ester ligands; α-Amino acid esters; Rhodium; Iridium; Schiff bases
Abstract. An overview on metal complexes that contain coordinated ester complexes {(H)(Cl)[C6H11)3P]2M(C6H4)C(Ph)=NCH2CO2R}. In these
groups is given. A new example has been obtained by oxidative addition complexes, the chlorido ligand is replaced for the ester group by using
of the O'Donnell reagent Ph2C=NCH2CO2R to [(cyclooctene)2MCl]2 (M = AgPF6 to give {(H)[(C6H11)3P]2M[C6H4)C(Ph)=NCH2CO2R}+PF6 .
–
Rh, Ir) which – in the presence of tricyclohexylphosphine – affords the
E and Z isomers of the tungsten and rhenium complexes (or
Esters as Ligands
conformers, concerning metal atom and acyl alkyl group) and
Metal complexes
1 with carboxylic esters as ligands
their interconversion were observed by Faller [15] and Gla-
dysz [16]. The structures of conformers of the TiCl4 complexes
with carboxylic acid esters were discussed [18].
(Scheme 1) have a long tradition [2] since 1873 when the ad-
duct of titanium tetrachloride with ethyl acetate was reported
[3]. Another long known complex with acetic acid ethyl ester
is [Mn(PO2Cl2)2(CH3CO2Et)2]n [4]; its structure was deter-
mined by X-ray diffraction [5]. In 1961, Lappert [6] gave an
excellent survey on such adducts, e.g. with TiCl4, ZrCl4, FeCl3
and also with the tetrahalides of tin. E. g. the latter compounds
found the interest of the pioneers in coordination and organo-
metallic chemistry Paul Pfeiffer [7] and Walter Hieber [8].
Adducts of carboxylic acid esters with titanium and zirco-
nium halides and with SnCl4 have often been studied [9, 10]
and also their structures were determined by X-ray diffraction
[11]. Lappert [6], and Beyer, Kühn and Hoyer [12] thoroughly
discussed the assignment of the IR carbonyl absorptions of the
coordinated esters. The coordination of the ester group leads
to a remarkable decrease of the carbonyl stretching frequency
(50–150 cm–1) and to an increase of that of the C–OR vibra-
tion. This was also recognized by Aubin, Zackrisson, Mori et
al. [9] and since then the lowering of the carbonyl stretching
frequency has proven as an easy and perfect diagnosis for co-
ordination of the ester group.
Functional carboxylic acid esters form chelate complexes, as
was first observed for oxalic diester 2 [9] and for dicarboxylic
acid diesters
3 [7–9], for α-amino acid esters 4 in
Cl2M(NH2CHRCOOR') (M = Co, Ni, Cu, Pd, Pt) [19] and in
[(en)2Co(NH2CH2COOR')]3+ [20], for Beyer and Hoyer’s ver-
satile aminopolycarboxylic esters 5 (e.g. iminodiacetic acid es-
ter) [12,21], for 2-pyridylacetic acid ester 6 in X2M(OH2)2L2
(M = Zn, Ni, Co, Cu) [22], for N-[pyridyl-2-methyl]-1,2-diami-
noethan-N,N',N''-triacetic ester [21] and for diethyloxaloacetate
7 in ML2 [23]. 2:1 adducts of TiCl4 and dicarboxylic acid dies-
ters were obtained with a longer chain between the two ester
groups [8,9a].
In our group the N,O-chelated organometallic α-amino acid
ester complexes of type 4 Cp*(Cl)M(NH2C(Me)(H)COOMe
(M = Rh, Ir) [24], [(η3:η3-C10H18)Ru(NH2CHRCOOR')]+ [25]
and the complexes {Pt[NH2C(H)(R)COOMe]2}2+(BF4 )2 [26]
–
(from Pt[NH2(H)(R)CO2Me]2Cl2 with AgBF4) could be ob-
tained. The latter complexes are highly sensitive to water. They
are suggested as intermediates for the hydrolysis of α-amino
acid esters by [Pd(en)(OH2)2]2+ [27].
Quite stable organometallic adducts of carboxylic acid esters
(1) were obtained with the strong cationic Lewis acids
[CpFe(CO)2]+ [13, 14], [HC(py)3M(NO)2]2+ (M = Mo, W [15],
[CpRe(NO)(PPh3)]+ [16], [Re(CO)5]+ [17] and [CpRu(CO)-
(PPh3)]+ [17].
The phosphorus analogues of glycine esters, α-phosphinoa-
cetic acid esters R2PCH2CO2R', were introduced by Braunstein
[28] and Werner [29] as interesting hemilabile [30] bidentate
ligands in a series of organometallic complexes 8, e.g. [(di-
methylbenzylamine-H+)Pd(Ph2PCH2COOEt)]+
Cl2Os(R2PCH2COOR')2 [29a].
[28]
or
* Prof. Dr. W. Beck
Organometallic complexes with coordinated carboxylic acid es-
ters include cyclometallated cinnamic acid methyl ester
[(C5H5)(L)Ir–CPh=CHCOOMe] (9) [31], [(Ph3P)2Pt–CH2-
CH2COOMe]+ (10) [32] and ester group bridged dinuclear ru-
E-Mail: wbe@cup.uni-muenchen.de
[a] Department Chemie und Biochemie
Ludwig-Maximilians-Universität
Butenandtstr. 9–13
81377 München, Germany
Z. Anorg. Allg. Chem. 2010, 636, 499–505
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
499

B. Schreiner, W. Beck
ARTICLE
Scheme 1. Types of ligands with coordinating ester groups.
500
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Z. Anorg. Allg. Chem. 2010, 499–505

RhIII and IrIII Complexes of orthometallated Diphenylmethylene Glycine Esters
thena-cyclopentadienes (11) [33]. The reaction of the alkyl sul-
phide bridged complexes [Cp*M(μ-Cl)(μ-SR)2MCp*]+ (M = Rh,
Ir) with methyl propiolate afforded complexes with a RS–C=CH–
C(O)OR' bridge in which the carbonyl group is coordinated to
the metal atom [34]. Bruce and White and their co-workers [35]
synthesized a series of transition metal complexes 12 with the
interesting ligand penta(methoxycarbonyl)cyclopentadienide
[C5(CO2Me)5]–, which can act as bidentate O-donor through two
carbonyl groups or to “soft” metals through the C5 ring.
Often, α-amino acid esters are only N-coordinated as monoden-
tate ligands [e.g. 24, 36]. In metal complexes of α-amino acid
esters, which contain an additional donor group (e.g. histidine,
cysteine ester) usually the ester group is not coordinated [37]. Co-
ordination of the esters group occurs if the necessary steric re-
quirements and/or a vacant coordination site (by abstraction of a
chlorido ligand) are created. These tridentate ligands with coordi-
nated ester groups include dipeptide ester complexes 13 of PdII
[38] and of CuII [39], iminodiacetic diester complexes 14 [12] and
mono ester complexes [40] cyclopalladated [41, 42] and cycloru-
thenated complexes 15 [42, 43] of Schiff bases from benzophe-
none and α-amino acid esters (O'Donnell reagent [44]) and of
Schiff bases 16 and its reduced form 17 from 2-formylphenyldi-
phenylphosphine and α-amino acid esters [45].
Hydrolysis can occur also by nucleophilic attack of metal bound
hydroxide or water at the carbonyl group [51, 53, 54]. Also the
cleavage of peptides by coordination of metal ions at the oxygen
atom of the amide group is a most attractive subject of research
[39, 49–65]. Early, the cleavage of peptides by lanthanide and
zirconium metal ions has been studied by Bamann, Trapmann and
their co-workers [57] in Munich. Several complexes 21, 22 with
amino and (O) amide coordinated peptides and α-amino acid am-
ides were isolated, e.g. [(trien)Co(glyglyOEt)]3+ [58], and a series
of half sandwich complexes [(Cp*)(X)Co(NH2CHR'CONHR)]+
[24,
59a],
[(cymene)(Cl)Ru(NH2CHR'CONHR)]+
[59a],
(C6Me6)Ru(NH2CHR'CONHR) [59a] {(Cl)Pd[C6H4(C6H5)C=
N(gly-pro-alaOMe]} [59b] and [(triamine)Cu(glyglyO)] [60].
Here, as in other metal peptide complexes [61], the carbonyl oxy-
gen atom is preferred to the peptide nitrogen (amide) atom as a
metal binding site [58]. At higher pH, the peptide NH proton is
dissociated and coordinated to the metal ion 21 [62]. Artificial
metallopeptidases for the selective cleavage of peptides were de-
veloped [53, 55, 56, 63–65]. Interestingly, e.g. Kostić [56, 63] –
in a large series of studies – and others [64] anchored PtII and PdII
complexes at the side chain (cysteine, methionine, histidine) of
peptides to hydrolyze selectively the neighboring amide group.
Another approach by Suh et al. to synthetic metallopeptidases is
the use of polynuclear and polymer supported CuII complexes for
the selective hydrolysis of peptides [53a]. For other examples see
for example refs. [65].
Vrieze and co-workers [66] reported the formation of cyclome-
tallated rhodium and iridium complexes (H)(Cl)(R3P)2M-
(C6H3Me–CH=NMe) by oxidative addition of the imine MeC6H4–
CH=NMe to [(cyclooctene)MCl2]2 in the presence of phosphines
from which the chloride ligand could be replaced for CO or isocy-
anide with AgClO4 [67]. This method provides a convenient entry
for further cyclometallated imines.
In continuation of our studies with Schiff bases of α-amino acids
as ligands [41, 43, 68, 69b] we report in the following the reac-
tions of [(cyclooctene)2MCl]2 (M = Rh, Ir) with the Schiff base
from benzophenone and glycine ester (O'Donnell reagent [44])
which gives novel cyclometallated complexes.
In the past [69–71] and recently [72], cyclopalladated and cyclo-
platinated Schiff bases from α-amino acids and palladated amino
acid derivatives [73–75] attracted the interest of
several groups. Vicente [73], Albert, Granell and Solans [72] and
Urriolabeitia [76] mentioned these studies in the references of their
remarkable papers.
Non cyclometallated complexes (OC)4Re(Ph2C=NCH(R)-
CO2Me)+ (16) with the O'Donnell reagent as bidentate N,O-che-
late ligand are obtained by reaction of the orthometallated com-
plexes of type 18 with HBF4 [46].
Further remarkable ligands are Werner’s trifunctional phos-
phines RP(CH2CO2R')2 which are coordinated by two ester groups
in several complexes 19 in e.g. Cl2Ru[RP(CH2-
COOR')3] [29b]. In metal complexes of 3-carboxyethyl-substituted
trispyrazolylborates the ester carbonyl stretching frequency proved
to be highly diagnostic for the interaction of the ester groups with
coordinated water (hydrogen bonding) or with the metal ion [47].
A tetradentate or possibly pentadentate ligand is ethylenediamine-
tetraacetic ester 20 [12, 48] and pyridyl-2-methylethylenediamine
triacetic acid ester [21], which form a series of metal complexes
M(L)Cl2.
Particularly important, metal complexes with coordinated esters
are intermediates in the metal ion catalyzed hydrolysis of esters
(and also in the ester exchange), especially of α-amino acid and
peptide esters. As has been extensively demonstrated by various
groups (e.g. Buckingham, Angelici, Hay, Lim) [20, 38, 39, 48–53]
in many impressive kinetic studies, coordination of the carbonyl
oxygen atom at the metal ion promotes the attack of hydroxide at
the carbonyl group (see for example Scheme 2).
Scheme 2.
Z. Anorg. Allg. Chem. 2010, 499–505
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Schreiner, W. Beck
ARTICLE
Scheme 3.
Results and Discussion
The reactions of [(cyclooctene)2MCl]2 (M = Rh, Ir) [76] with (Ir) cyclometallated chlorido-hydrido complexes 24a–24f
the O'Donnell reagent Ph2C=NCH2CO2R [44] in the presence of (Scheme 3). From these complexes the chlorido ligand could be
tricyclohexylphosphine lead to the yellow (Rh) and to orange abstracted with AgPF6 to give 25c and 25d which contain the
1
Table 2. H NMR spectroscopic data of 24 and 25 in CDCl3 (δ in ppm,
Table 1. Characteristic IR absorptions /cm–1 of 24 and 25 in Nujol.
3
relative to internal TMS, J = J(H–H) in Hz).
ν(M–H)
ν(C=O)
ν(C=N)
ν(C=C)
ν(PF6–)
ν(M–Cl)
ortho-H
arom. H
α-CH2
O–R'
M–H
[intensity]
5.25s; 2H 3.31s; 3H –14.83ψq; 1H
24a 6.84dd; 1H 6.71–7.68; 8H
24a 2142m
1754s
1600vw
1581w
1569m
1536m
1578w
1568m
1532m
1597vw
1578sh
1568m
1534w
1597vw
1578w
1565m
1531m
1578m
1567m
1538m
1575m
1563m
1529m
1627vw
1571m
1539vw
1580m
1564w
1536w
–
253m
J = 7.9
[1:3:3:1]
JRh–N ≈15
2095vw
4J = 1.7
J
P–H ≈15
b
c
6.81d; 1H 6.68–7.70; 8H
J = 7.4
5.32s; 2H 3.29s; 3H –18.88t; 1H
P–H = 21
b
c
2261m
1758s
1752s
–
–
258m
268w
J
6.84dd; 1H 6.71–7.67; 8H
J = 7.7
5.23s; 2H 3.72q; 2H –14.83ψq; 1H
0.96t; 3H [1:3:3:1]
2154m
2132w
4J = 1.5
J = J = 7.0 JRh–H ≈14
JP–H ≈14
d
e
6.70–7.68; 9H
5.30s;2H 3.71q; 2H –18.87t; 1H
0.94t; 3H P–H = 21
J = J = 7.0
J
d
2253m
2155m
1752s
1749s
–
243m
6.82dd; 1H 6.70–7.60;
5.14s; 2H 1.07s; 9H –14.87ψq; 1H
J = 8.2
14H
[1:3:3:1]
4J = 1.8
JRh–H ≈14
e
f
–
258w
262m
–
JP–H ≈14
f
6.70–7.69;9H
5.20s;2H 1.06s;9H
–18.90t;1H
P–H = 21
J
2259m
2272w
1744s
1734m
–
25ca) 6.99dd;1H 6.88–7.72;13H 4.97s;2H 4.37q;2H
–11.56ψq; 1H
[1:3:3:1]
J = 5.9
1.37t;3H
4J = 1.8
J = J = 7.2 JRh–H ≈13
P–H ≈13
–14.34t; 1H
P–H = 20
25c 2120br
1662s
838vs
840vs
J
Δ = 90
1755vw
1643s
da)
6.85–7.71;9H
5.07s;2H 4.44q;2H
1.41t;3H
J
d
2221m
–
J = J = 7.1
Δ = 109
a) in CD2Cl2
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Z. Anorg. Allg. Chem. 2010, 499–505

RhIII and IrIII Complexes of orthometallated Diphenylmethylene Glycine Esters
coordinated ester group. Analogous reactions with Schiff bases of
Experimental Section
alanine and phenylalanine were not successful.
[(Cyclooctene)2MCl2]2 (M = Rh, Ir) [76] and Ph2C=NCH2CO2R [44] were
prepared as described. The NMR spectra were recorded on a Jeol GSX-
270 spectrometer. Schlenk technique was used for the synthesis of 24 und
25. For analytic data and yields of 24 and 25 see Table 5.
The IR spectra of 24 and 25 (Table 1) exhibit the characteristic
metal–hydride absorption at 2000 cm–1; the ν(M–H) frequency is
considerably higher for iridium complexes than for the rhodium
compounds. This shift which corresponds to a stronger Ir–H bond
was also observed with analogous complexes from the imine
MeC6H3CH=NMe [66, 67]. For 24 the ν(M–Cl) absorption is
found at 250 cm–1. Clear evidence for the coordination of the ester
group in 25 is the shift of the carbonyl absorption by 90 or
109 cm–1 to lower frequencies than that of the free ester.
Table 5. Analytical data and yields of 24 and 25.
C
H
N
calcd.
found
color m.p./°C yield /%
24a C52H81ClNO2P2Rh
65.57 8.57 1.47 yellow 150–154 58
65.77 8.93 1.58
1
The H NMR spectra (Table 2) show the typical high field
(952.5)
b
c
C52H81ClNO2IrP2
(1041.8)
C53H83ClNO2P2Rh
(966.6)
59.95 7.84 1.34 orange 148–150 55
59.98 7.96 1.53
65.86 8.66 1.45 yellow 161–165 67
65.97 8.66 1.49
shift for the (metal) hydride, which is coupled with two equivalent
phosphorus atoms and with the rhodium atoms [65, 66]. The coor-
dination of the ester group in 25 causes a downfield shift for the
CH2 protons of the ethyl group compared with 24c and 24d,
which is also observed in the 13C NMR spectra (Table 3). Again,
the coordination of the ester group is proved by the considerable
down field shift of the signal for the carbonyl carbon atom in
compound 25 to that of 24. The appearance of only one 31P NMR
spectroscopic signal in 24 and 25 (Table 4) shows the equivalence
of the two Pcy3 ligands in trans-positions which confirms the mer-
coordination of the tridendate ligand.
d
e
C53H83ClNO2IrP2
(1055.9)
C55H87NO2ClP2Rh
–benzene
(1072.7)
C55H87ClNO2IrP2
(1083.9)
60.29 7.92 1.33 orange 158–161 62
60.31 8.08 1.48
68.30 8.74 1.31 yellow 129–133 41
69.18 8.74 1.13
f
60.94 8.09 1.29 orange 148–152 51
59.57 8.10 1.38
25c C53H83F6NO2P3Rh
–1/2 toluene
63.38 8.19 1.31 yellow 181–182 83
63.09 8.01 1.30
(1168.1)
d
C53H83F6NO2IrP3
(1165.4)
54.63 7.18 1.20 orange 190–192 85
54.30 7.50 1.04
Table 3. 13C NMR spectroscopic data of 24 and 25 in CDCl3 (δ in ppm,
relative to internal TMS, 1H decoupled; J = J(13C–31P) in Hz).
C=N
C–M
C=O arom.
α-C O–R'
Complex 24: Ph2C=NCH2CO2Me (280 mg, 1.1 mmol) and
[(C8H14)2RhCl]2 (359 mg, 0.5 mmol) were added under nitrogen to ben-
zene (20 mL) through which nitrogen was passed for 15 min before use.
To the deep red solution tricyclohexylphosphine (561 mg, 2 mmol) was
added. The solution was heated under reflux for 3 h and an orange brown
suspension was obtained. The liquid was removed with a pipette and the
residue was washed twice with n-pentane (9 mL) and with benzene
(5 mL). The product was dried at 60 °C in vacuo.
24a
b
182.0m
184.4
181.2m
169.5 119.7–146.6
169.5 118.5–147.0
169.0 119.6–146.5
52.9 51.2
53.2 51.2
53.2 60.2
14.0
c
175.5dt
JRh–C = 33.0
J = 8.6
d
e
f
184.2t
J = 2.1
180.7
155.3t
169.0 118.5–146.9
167.5 119.5–146.6
167.5 118.3–147.0
177.8 122.3–146.1
182.3 121.3–145.9
53.6 60.3
14.1
54.2 80.0
27.8
54.5 80.1
27.8
56.1 65.0
14.4
J = 5.0
175.2m
Complexes 24b–f were prepared by the same procedure.
183.8t
154.9
Complex 25c: A suspension of 24c (174 mg, 0.18 mmol) and of AgPF6
(61 mg, 0.24 mmol) in dichloromethane (11 mL) was stirred for 2 h at
room temperature. The gold yellow solution was removed with a pipette
and from the residue the solvent was removed in vacuo.
J = 2.1
25ca) 185.0
da)
190.2
178.5t
J = 2.8
58.2 65.8
14.4
The residue was dried in vacuo for 30 min at 60 °C and dissolved in
toluene (3 mL). Crystals of 25c were obtained at –30 °C, which were again
recrystallized from toluene and finally dried in vacuo at room temperature.
Complex 25d was synthesized analogously.
a) in CD2Cl2
Table 4. 31P NMR spectroscopic data of 24 and 25 in CDCl3 (δ relative
1
to external H3PO4, H decoupled).
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δ /ppm
1J(103Rh–31P) /Hz
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b
23.8d
–4.1s
23.8d
–4.1s
23.9d
–3.9s
31.5d
10.6s
108
–
108
–
108
–
110
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Z. Anorg. Allg. Chem. 2010, 499–505
© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Schreiner, W. Beck
ARTICLE
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Received: October 2, 2009
Published Online: January 7, 2010
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