Synthesis and Characterization of Heterobimetallic Carbonyl Clusters with Direct Au-Fe and Au···Au Interactions Supported by N-Heterocyclic Carbene and Phosphine Ligands

Article abstract of DOI:10.1002/ejic.201900537

The reaction of Collman's reagent Na₂[Fe(CO)₄]·2thf with one equivalent of Au(NHC)Cl (NHC = IMes, IPr; IMes = C₃N₂H₂(C₆H₂Me₃)₂; IPr = C₃N₂

Full text of DOI:10.1002/ejic.201900537

A Journal of
Accepted Article
Title: Synthesis and Characterization of Heterobimetallic Carbonyl
Clusters with Direct Au-Fe and Au···Au Interactions supported by
N-Heterocyclic Carbene and Phosphine Ligands
Authors: Beatrice Berti, Marco Bortoluzzi, Cristiana Cesari, Cristina
Femoni, Maria Carmela Iapalucci, Rita Mazzoni, Federico
Vacca, and Stefano Zacchini
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900537
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
Synthesis and Characterization of Heterobimetallic Carbonyl Clusters
with Direct Au-Fe and Au···Au Interactions supported by N-
Heterocyclic Carbene and Phosphine Ligands
Dr. Beatrice Berti,[a] Dr. Marco Bortoluzzi,[b] Dr. Cristiana Cesari,[a] Prof. Dr. Cristina Femoni,[a]
Prof. Dr. Maria Carmela Iapalucci,[a] Dr. Rita Mazzoni,[a] Dr. Federico Vacca,[a] and Prof. Dr.
Stefano Zacchini*[a]
[a] Dipartimento di Chimica Industriale "Toso Montanari", University of Bologna, Viale
Risorgimento 4, I-40136 Bologna Italy. E-mail: stefano.zacchini@unibo.it; Web:
https://www.unibo.it/sitoweb/stefano.zacchini/en; Tel: +39 051 2093711.
[b] Dipartimento di Scienze Molecolari e Nanosistemi, Ca’ Foscari University of Venice, Via Torino
155 – 30175 Mestre (Ve), Italy.
Abstract: The reaction of Collman's reagent Na2[Fe(CO)4]·2thf with one equivalent of Au(NHC)Cl
i
(NHC = IMes, IPr; IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3 Pr2)2) in dmso resulted in the
[Fe(CO)4(AuNHC)]– (NHC = IMes, 1; IPr, 2) mono-anions. 1-2 further reacted with Au(NHC)Cl or
Au(PPh3)Cl affording the neutral complexes Fe(CO)4(AuNHC)2 (NHC = IMes, 3; IPr, 4),
Fe(CO)4(AuIMes)(AuIPr) (5) and Fe(CO)4(AuNHC)(AuPPh3) (NHC = IMes, 6; IPr, 7). 1-7 have
1
13
31
been spectroscopically characterized by IR, H, C{1H} and P{1H} NMR techniques. Moreover,
the molecular structures of 1, 2, 4, 6 and 7 have been determined through single crystal X-ray
diffraction
as
their
[NMe4][Fe(CO)4(AuIMes)],
[NEt4][Fe(CO)4(AuIMes)],
Fe(CO)4(AuIPr)(AuPPh3),
[NEt4][Fe(CO)4(AuIPr)],
Fe(CO)4(AuIPr)2·1.5toluene,
Fe(CO)4(AuIMes)(AuPPh3)·0.5CH2Cl2 salts and solvates. The nature of the bonds in 1 and 2 was
elucidated on the basis of atoms-in molecules (AIM) analyses on the DFT-optimized structures and
compared with the corresponding compounds 3 and 4. 1-7 contained strong Fe-CO, Fe-Au, Au-P
and Au-NHC bonds as well as weak Au···Au interactions. The different stability and reactivity of
IMes-derivatives vs. IPr-ones was rationalized on the basis of steric effects.
Keywords: Heterometallic clusters / Carbonyl / Gold / Iron / N-Heterocyclic carbene
1
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
Introduction
Several heterobimetallic Au-Fe carbonyl clusters have been reported and these may be grouped into
five main categories:[1]
1) Iron carbonyl clusters surface-decorated by [AuL]+ fragments. This represents the largest class of
heterobimetallic Au-Fe carbonyl clusters, that comprises species such as Fe(CO)4(AuPPh3)2,[2]
i
Fe(CO)4(AuNHC)2 (NHC = IMes, IPr, IBu; IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3 Pr2)2;
IBu = C3N2H2(CMe3)2),[3] [Fe2(CO)8(AuPPh3)]–,[4] [Fe3(CO)11(AuPPh3)]–,[5] [Fe3E(CO)9(AuPPh3)]–
(E = O, S, Te),[6,7] Fe3E(CO)9(AuPPh3)2 (E = O, S),[8,9] [HFe4(CO)12(AuPPh3)3-n]n– (n = 0, 1),[10]
[13]
Fe4C(CO)12(AuPEt3)2,[11]
Fe5C(CO)14(AuPEt3)2,[12]
[Fe6C(CO)16(AuPPh3)]–
and
[Fe6C(CO)15(AuPPh3)2]2–.[14] These may be viewed as the result of the addition of [AuL]+ (L =
phosphine, N-heterocyclic carbene) fragments to the surface of an iron carbonyl cluster anion, i.e.,
[Fe(CO)4]2–, [Fe3(CO)11]2–, [Fe2(CO)8]2–, [Fe3E(CO)9]2– (E= O, S, Te), [HFe4(CO)12]3–,
[Fe4C(CO)12]2–, [Fe5C(CO)14]2–, [Fe6C(CO)16]2– and [Fe6C(CO)15]4–. These reactions have been
widely employed in order to selectively increase of a few units the nuclearity of the clusters.[1,15-17]
Moreover, based on the isolobal analogy between [AuL]+ and H+, the former fragments have been
used in order to have information on the protonation sites of anionic clusters.[18-20] When two or
more [AuL]+ fragments are present on the surface of the same cluster, because of aurophilicity,
often gold atoms prefer to gather together in order to form weak d10-d10 Au(I)···Au(I)
interactions.[21-24]
2) Iron carbonyl clusters with naked Au connecting atoms. At the moment, the only species
belonging to this category is [Fe4Au(CO)16]–, which may be viewed as composed of a central
Au(III) ion bonded to two [Fe2(CO)8]2– iron cluster anions.[25]
3) 1-D (chain) gold clusters stabilised by iron carbonyls, such as [Au3Fe2(CO)8(IMes)2]–,[3] and
[AuFe{Si(OMe)3}(CO)3(µ-dppm)].[26] These clusters contain linear 1-D [Aun]n+ chains protected by
[Fe(CO)4]2– carbonyl anions and miscellaneous ligands. They allow investigations about single,
direct bonds between metals.[26]
4) 2-D gold clusters stabilised by iron carbonyls. Representative examples are:
[Au3Fe(CO)4(dppm)2]+,[27] [Au3Fe2(CO)8(dppm)]–,[28] [Au3Fe3(CO)12]3–,[29] [Au4Fe4(CO)16]4–,[30]
[Au5Fe4(CO)16]3–,[31] [Au5Fe2(CO)8(dppm)2]+ [28] and Au8Fe4(CO)16(dppe)4.[32] These clusters are
composed of planar 2-D [Aun]n+ cores decorated on the surface by [Fe(CO)4]2– carbonyl anions and,
in some cases, by additional phosphine or NHC ligands.
2
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
5) 3-D gold clusters embedded in iron carbonyls. This category includes Au-Fe-CO nanoclusters
such as [Au21Fe10(CO)40]5–, [Au22Fe12(CO)48]6–, [Au28Fe14(CO)52]8– and [Au34Fe14(CO)50]8–.[31]
These are metalloid nanoclusters where a Aun core is stabilised by Fe(CO)4 and Fe(CO)3 groups
present on their surface. Linear Fe-Au-Fe staple motives, reminiscent of the very well known S-Au-
S staple motives found in Au-thiolate nanoclusters, are present on the surface of these
organometallic Au-Fe carbonyl clusters.
Within category (1), most of the species reported to date contained phosphines as ancillary
ligands. More recently, we reported the Fe(CO)4(AuNHC)2 clusters, which displayed N-
heterocyclic carbenes bonded to gold.[3] Interestingly, heating a solution of Fe(CO)4(AuIMes)2 in
dmf at 100 °C resulted in [Au3Fe2(CO)8(IMes)2]–, whereas by performing its thermal decomposition
in dmso at 130 °C, [Au3Fe3(CO)12]3– was obtained.[29] This suggested that lower nuclearity Au-Fe
carbonyl clusters containing NHC ligands might be suitable precursors for the preparation of larger
clusters. In order to widen the scope of this study, we have started a systematic investigation on
heterobimetallic Au-Fe carbonyl clusters containing NHC as well as phosphine ligands. Herein, we
report the synthesis of the mono-anionic complexes [Fe(CO)4(AuNHC)]– (NHC = IMes, 1; IPr, 2)
as well as the heteroleptic neutral clusters Fe(CO)4(AuIMes)(AuIPr) (5) and
Fe(CO)4(AuNHC)(AuPPh3) (NHC = IMes, 6; IPr, 7). All the new species have been
spectroscopically characterized by IR and multinuclear NMR techniques, and their structures
determined by single crystal X-ray diffraction. The nature of the bonds in 1 and 2 was elucidated on
the basis of atoms-in molecules (AIM) analyses on the DFT-optimized structures and compared
with the corresponding compounds 3 and 4.
Results and Discussion
Synthesis and characterization of [Fe(CO)4(AuNHC)]– (NHC = IMes, 1; IPr, 2)
The reaction of Collman's reagent Na2[Fe(CO)4]·2thf with one equivalent of Au(NHC)Cl (NHC =
IMes, IPr) in dmso resulted in the [Fe(CO)4(AuNHC)]– (NHC = IMes, 1; IPr, 2) mono-anions in
accord to Scheme 1. It is noteworthy that, as previously reported,[3] by performing the same
reactions in thf, a mixture of the neutral clusters Fe(CO)4(AuNHC)2 (NHC = IMes, 3; IPr, 4) and
the starting Collman's reagent were formed. As expected, the reaction of Na2[Fe(CO)4]·2thf with
two equivalents of Au(NHC)Cl in dmso, as well as thf, resulted in 3, 4. Thus, it seemed that the
polarity of the solvent played a fundamental role in the reaction, when a 1:1 stoichiometry is
employed, whereas it was less relevant for a 1:2 stoichiometry. This might be related to the fact that
in polar solvents all ions are well solvated and, thus, [Fe(CO)4(AuNHC)]– was formed at first with a
3
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
1 : 1 stoichiometric ratio. Then, after the addition of the second equivalent of Au(NHC)Cl,
Fe(CO)4(AuNHC)2 was obtained. Conversely, in less polar solvents, the formation of neutral
products seemed to be favoured over anionc ones. This result was supported by the computed Gibbs
energy variations associated to the reaction [Fe(CO)4(AuNHC)]–
+
[AuNHC]+
→
Fe(CO4)(AuNHC)2, obtained using continuous models for the two solvents. The ∆Gdmso-∆Gthf
differences were 12.1 and 14.4 kcal mol-1 for NHC = IMes and NHC = IPr, respectively, in
agreement with the less favourable formation of the neutral trinuclear products in the presence of
solvents possessing quite high dielectric constants.
CO
R
CO
Au
OC
Au
CO
Fe
Fe
N
+
R'
+
R
Au(NHC)Cl
Au(NHC)Cl
Na
Au
]
CO
CO
2[Fe(CO)4
N
N
CO
N
OC
N
N
R
R'
R
R
Me
=
Me
1
2
R
R'
Me
iPr
+
Au(PPh3)Cl
Me
Me
Me
3
Me
Me
iPr
Me
iPr
iPr
4
iPr
Me
iPr
iPr
CO
CO
Au
OC
Fe
Me
5
R
Au
iPr
Me
N
PPh3
CO
Me
N
R
=
Me
6
R
Me
iPr
7
iPr
Scheme 1. Syntheses of 1-7
1
13
1 and 2 were characterized by means of IR, H and C{1H} NMR spectroscopy (Figures
S.1-S.4 in Supporting Information) and their structures determined by X-ray crystallography
(Figure 1 and Table 1). Compounds 1 and 2 displayed νCO bands in acetone solution at ca. 1924(s)
and 1820(vs) cm–1, considerably shifted towards lower wavenumbers compared to 3 and 4 (νCO
1974(s) and 1884(vs) cm–1) in view of their anionic charges. The Au-coordinated carbene resonated
in the 13C{1H} NMR spectra at δC 196.3 and 198.3 ppm for 1 and 2, respectively. For comparison,
the neutral complexes 3 and 4 displayed carbene resonances at very similar chemical shifts, that is
4
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
13
194.5 and 194.3 ppm. A singlet was present at all temperatures in the CO region of the C{1H}
NMR spectra of 1 and 2, suggesting a fluxional behaviour for the carbonyl ligands. This rapid
exchange process made the equatorial and apical CO ligands equivalent also at low temperature.
(a)
(b)
Figure 1. Molecular structures of (a) [Fe(CO)4(AuIMes)]– (1) and (b) [Fe(CO)4(AuIPr)]– (2). Au-
C(O) contacts [2.630(4)-2.900(4) Å for 1; 2.688(4)-2.805(3) Å for 2] are represented as fragmented
lines. Hydrogen atoms have been omitted for clarity (green Fe; yellow Au; blue N; red O; grey C)
Table 1. Main bond distances (Å) and angles (°) of [Fe(CO)4(AuIMes)]– (1) and [Fe(CO)4(AuIPr)]–
(2)
[Fe(CO)4(AuIMes)]– a [Fe(CO)4(AuIMes)]– b
[Fe(CO)4(AuIPr)]–
Fe-Au
Au-Ccarbene
Fe-C(O)
2.5168(5)
2.031(4)
2.5244(6)
2.029(4)
2.5015(4)
2.019(2)
1.749(4)-1.780(4)
2.630(4)-2.900(4)
173.66(10)
1.755(5)-1.780(4)
2.565(5)-2.931(4)
174.96(11)
1.757(3)-1.778(3)
2.688(4)-2.805(3)
175.21(7)
Au···C(O)
Fe-Au-Ccarbene
a As found in [NMe4][Fe(CO)4Au(IMes)].
b As found in [NEt4][Fe(CO)4Au(IMes)].
The monoanions 1 and 2 adopted a trigonal bipyramidal structure, with the AuNHC
fragment in an axial position. Similar structures were found in the cobalt complexes
Co(CO)4(AuPPh3) [33] and Co(CO)4(CuIPr).[34]
1 and 2 contained strong Au-Fe, Fe-C(O) and Au-Ccarbene interactions as well as some weak
Au···C(O) contacts (see Table 1). Regarding the latter contacts, their detection in the solid state
structures could merely be the consequence of the preferred arrangement of the CO ligands about
the Fe centre, which brings the CO ligands in closer proximity to the Au centre, rather than any
5
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
attraction (even van der Waals) between the carbonyls and Au. Indeed, their theoretical study (see
below) did not support bonding interactions between Au and CO (which could at best be a kind of
back-bonding from the d10 shell). The nature of such Au···C(O) contacts was already discussed in
the literature,[35] concluding that relatively short non-bonding contacts between Au(I) and
C(carbonyl) atoms must be analyzed with caution.
The Au-Fe distances [2.50-2.52 Å] found in these monoanionic complexes were very similar
to those previously reported for the neutral complexes Fe(CO)4(AuNHC)2 [2.51-2.53 Å] (Table
2).[3] Conversely, a slight elongation of the Au-Fe contact [2.56-2.57 Å] was observed in the
cationic complexes [Fe(CO)5(AuNHC)]+ and [Fe(CO)5(AuPR3)]+ that contained a Fe(CO)5 rather
than a Fe(CO)4 fragment.[36]
Synthesis
and
characterization
of
Fe(CO)4(AuIMes)(AuIPr)
(5)
and
Fe(CO)4(AuNHC)(AuPPh3) (NHC = IMes, 6; IPr, 7)
The mono-anions 1-2 further reacted with Au(NHC)Cl or Au(PPh3)Cl affording the neutral
complexes Fe(CO)4(AuNHC)2 (NHC = IMes, 3; IPr, 4), Fe(CO)4(AuIMes)(AuIPr) (5) and
Fe(CO)4(AuNHC)(AuPPh3) (NHC = IMes, 6; IPr, 7). Compounds 3-4 were identified by
comparison of their IR and NMR spectroscopic data with those previously reported in the literature
(Figures S.5-S.8 in Supporting Information).[3] Moreover, the nature of 4 was further corroborated
by single crystal X-ray diffractometry on its new solvate Fe(CO)4(AuIPr)2·1.5toluene. The
molecular structure was comparable to that previously reported, and the crystallographic data were
included as supplementary material for sake of completeness (Figure S.17 in the Supporting
Information).
The mixed complex 5 was spectroscopically characterized (Figures S.9 and S.10 in the
Supporting Information), and it displayed an IR spectrum in the νCO region very similar to that of 3
13
and 4. An unique resonance was observed in the C{1H} NMR spectrum at δC 217.8 ppm for the
CO ligands, whereas two distinct resonances were observed in the Au-Ccarbene region (δC 195.1 and
193.7 ppm) in view of the presence of both AuIMes and AuIPr fragments.
Compounds 6 and 7, containing both an AuNHC and AuPPh3 fragments, displayed very
similar spectroscopic features (see experimental and Figures S.11-S.16 in Supporting Information).
Moreover, their molecular structures were determined by single crystal X-ray diffraction (Figures 2
and 3, and Table 2). They are composed of a C2v-Fe(CO)4 sawhorse/seesaw unit coordinated to one
AuNHC and one AuPPh3 fragment in relative cis position. The structures of 6 and 7 were similar to
those previously reported for the homoleptic complexes 3, 4 and Fe(CO)4(AuPPh3)2 (8).[2,3] The
Au···Au contacts [3.0471(4) Å for 6, 3.0479(2) Å for 7] were indicative of weak aurophilic
6
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
interactions, which were intermediate between those found in 8 [2.8750(3)-3.0698(2) Å, depending
of the polymorph] and 3 [3.2015(8) Å]. Conversely, the Au···Au distance of 4 [3.984(1) and
4.082(1) Å, depending of the polymorph] was completely non-bonding. The fact that the Au···Au
distance was strongly affected by the steric properties of the Au-bonded ligands as well as packing
effects pointed out that such aurophilic interactions were rather deformable. In particular, the
Au···Au distance increased with the increasing bulkiness of the ligands when passing from PPh3 to
IMes and eventually IPr. All complexes 3, 4 and 6-8 displayed also some sub-van der Waals
Au···C(O) contacts.
Figure 2. Molecular structure of Fe(CO)4(AuIMes)(AuPPh3) (6). Two different views as well as its
core are reported. Au-C(O) contacts [2.621(8)-2.885(7) Å] are represented as fragmented lines.
Hydrogen atoms have been omitted for clarity (green Fe; yellow Au; purple P; blue N; red O; grey
C)
Figure 3. Molecular structure of Fe(CO)4(AuIPr)(AuPPh3) (7). Two different views as well as its
core are reported. Au-C(O) contacts [2.573(3)-2.955(3) Å] are represented as fragmented lines.
Hydrogen atoms have been omitted for clarity (green Fe; yellow Au; purple P; blue N; red O; grey
C)
7
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
Table 2. Main bond distances (Å) and angles (deg) of Fe(CO)4(AuIMes)2 (3), Fe(CO)4(AuIPr)2 (4),
Fe(CO)4(AuIMes)(AuPPh3) (6), Fe(CO)4(AuIPr)(AuPPh3) (7) and Fe(CO)4(AuPPh3)2 (8).
3 a
4 a
4 b
6
7
8 c
8 d
Au(1)-Fe(1)
Au(2)-Fe(1)
Au(1)-Au(2)
Au(1)-X(1)
Au(2)-X(2)
Fe(1)-CO
2.5158(15)
2.5312(15)
3.2015(8)
2.008(10)
2.020(10)
1.756(13)-
1.783(13)
2.609(12)-
2.764(13)
177.8(3)
2.512(2)
2.524(2)
4.082(1)
2.012(11)
1.949(14)
1.737(15)-
1.781(15)
2.572(14)-
2.873(18)
168.3(3)
2.5203(9)
2.5326(9)
3.984(1)
2.018(6)
2.023(6)
1.727(8)-
1.769(8)
2.629(7)-
2.827(8)
2.5471(9)
2.5089(9)
3.0471(4)
2.020(6)
2.5371(4)
2.5107(4)
3.0479(2)
2.011(3)
2.5344(6)
2.5555(6)
2.8750(3)
2.5329(4)
2.5181(4)
3.0698(2)
2.2696(10) 2.2756(7)
2.2661(16) 2.2621(8) 2.2707(10) 2.2646(7)
1.768(8)-
1.777(8)
2.621(8)-
2.885(7)
1.772(3)-
1.786(3)
2.573(3)-
2.955(3)
1.787(4)-
1.800(4)
2.604(6)-
3.069(5)
172.02(3)
1.772(3)-
1.790(3)
2.638(3)-
3.050(3)
177.55(2)
Au···C(O)
Fe(1)-Au(1)-
X(1)
172.75(18) 175.77(17) 174.65(8)
Fe(1)-Au(2)-
X(2)
165.9(3)
78.74(4)
50.84(4)
50.42(3)
168.4(5)
107.90(9)
36.15(5)
35.95(5)
168.16(17) 170.15(5)
171.12(2)
173.45(3)
173.71(2)
Au(1)-Fe(1)-
Au(2)
104.08(3)
38.07(2)
37.85(2)
74.12(3)
52.37(2)
53.51(2)
74.284(12) 68.781(14) 74.855(12)
52.462(10) 55.956(13) 52.354(10)
53.254(10) 55.263(13) 52.791(9)
Fe(1)-Au(1)-
Au(2)
Fe(1)-Au(2)-
Au(1)
Fe(1)
Fe(1)
Fe(1)
Au(2)
Au(2)
P(2)
Au(1)
Au(1)
Au(2)
P(2)
Au(1)
P(1)
N
N
N
C(2)
C(1)
C(1)
N
N
N
a
b
c
d
�
See ref. [3]. As found in Fe(CO)4(AuIPr)2·1.5toluene. As found in Fe(CO)4(AuPPh3)2 (Pꢀ). As found in
Fe(CO)4(AuPPh3)2 (P21/n).
Despite the structural analogies between 6 and 7, their chemical behaviour was rather
different. 7 was very stable and could be easily obtained in a very pure form, as indicated by the
presence of a single resonance at δP 40.8 ppm in the 31P{1H} NMR spectrum (Figure S.16 in
Supporting Information). Conversely, 6 was less stable and its synthesis was not so straightforward.
Thus, it was often obtained in mixtures with minor amounts of 8 and a, yet, unknown species 9, as
31
evidenced by the presence of three distinct resonances in the P{1H} NMR spectrum (Figure S.13
in Supporting Information) at δP 40.8 ppm (6), 40.1 ppm (8) and 38.5 ppm (9).
8
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
More in details, the addition of increasing amounts of Au(PPh3)Cl to an acetone solution of
1 resulted in the partial substitution of the AuIMes fragment with AuPPh3 (Scheme 2). Thus, after
the addition of one equivalent of Au(PPh3)Cl a mixture of [Fe(CO)4(AuPPh3)]– (10) (major) and 1
(minor) was present in solution. Further addition of Au(PPh3)Cl resulted in the formation of 8 as the
major product. Compound 8 was spectroscopically detected and, moreover, its molecular structure
was corroborated by X-ray crystallography on two new polymorphs (Figure S.18 and S.19 in
�
Supporting Information), that is Fe(CO)4(AuPPh3)2 (Pꢀ) and Fe(CO)4(AuPPh3)2 (P21/n). The new
crystal data have been included as Supporting Information, for sake of completeness.
Concerning the ability of the AuPPh3 fragment to replace AuIMes in the coordination sphere
of Fe(CO)4, this suggested that the AuPPh3 fragment was more electrophilic than AuIMes. To
demonstrate this point, 3 was reacted with increasing amounts of Au(PPh3)Cl. As a result, 6 and,
then,
8
were formed in sequence. During this study,
a
few crystals of
[Au(IMes)2][Fe2(CO)8(AuPPh3)]·CH2Cl2 were obtained and characterized by single crystal X-ray
crystallography (Figure S.20 in Supporting Information). It is likely that some [Fe2(CO)8]2– was
formed by oxidation of [Fe(CO)4]2– at some stage of the reaction that, eventually, reacted with
Au(PPh3)Cl. The structure of the [Fe2(CO)8(AuPPh3)]– mono-anion was previously reported as
[NEt4]+ salt,4 displaying almost identical geometry and bonding parameters. It must be remarked
that 8 did not react with Au(IMes)Cl even when used in large excess.
CO
Fe
R
CO
Fe
CO
Fe
CO
Au
OC
Au
N
+
+
Au(PPh3)Cl
acetone
Au(PPh3)Cl
Au
CO
CO
+
Ph3P Au
CO
CO
Au(IMes)Cl
acetone
N
Ph3P
PPh3
CO
OC
OC
10
R
8
1
Me
Me
Vaccum
CH Cl
1)
2)
=
Me
R
2
2
+
Au(PPh3)Cl
CO
Fe
CO
Fe
CO
Au
OC
CO
Au
OC
+
Au(PPh3)Cl
CH2Cl2
R'
R
R
N
Au
Au
N
N
PPh3
CO
CO
N
N
N
R'
R
R
6
3
Scheme 2. Reactivity of 1 and 3 with Au(PPh3)Cl.
Therefore, the best way in order to obtain 6 was to mix 1 and Au(PPh3)Cl in a 1:1 ratio in
acetone, remove the solvent in vacuum and let the reaction to continue in CH2Cl2. This allowed to
obtain 6 in a pure crystalline form, after slow diffusion of n-pentane. Conversely, the reaction of 2
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
with Au(PPh3)Cl directly afforded 7, without any evidence of the formation of 10. Only after the
addition of a slight excess of Au(PPh3)Cl, 7 was eventually transformed into 8 (Scheme 3).
CO
Fe
R
CO
Fe
CO
Fe
CO
Au
OC
Au
N
CO
Au
OC
Au
+
+
Au(PPh3)Cl
acetone
Au(PPh3)Cl
Au
CO
CO
R
acetone
N
N
Ph3P
PPh3
CO
PPh3
OC
CO
R
N
8
2
R
iPr
iPr
7
=
R
Scheme 3. Reactivity of 2 with Au(PPh3)Cl.
From a mechanistic point of view, the substitution reaction, which transformed 1 into 10
(Scheme 2), was likely to proceed via an associative mechanism involving 6 as an intermediate.
Since 6 was not very stable in polar solvents such as acetone, it rapidly dissociated a [Au(IMes)]+
fragment, affording 10 as the major product. Therefore, in order to favour the formation of 6, a less
polar solvent such as CH2Cl2 must be employed in order to hamper dissociation. Conversely,
because of the greater stability of 7 compared to 6, addition was observed rather than substitution,
regardless of the solvent employed.
To address the point of whether in some cases the whole AuL (L = NHC, PPh3) fragment
migrates or only the ligand L, 3 and 4 were reacted with increasing amounts of PPh3 and the
31
reaction monitored via IR and P NMR spectroscopies. No reaction was observed, suggesting that
substitution of the whole Au(NHC) fragment by AuPPh3 occurred.
The different behaviour of the IMes-derivatives compared to the IPr-derivatives was also
evidenced by investigating the reactions of 6 and 7 with Au(Et2S)Cl. Thus, 6 readily reacted with
Au(Et2S)Cl affording Au-Fe-CO nanoclusters of the type [Au22Fe12(CO)48]6–, [Au21Fe10(CO)40]5–,
[Au28Fe14(CO)52]8– and [Au34Fe14(CO)50]10–, as evidenced by comparison of their IR spectra with
those reported in the literature.[31] Conversely, 7 did not react with Au(Et2S)Cl. Overall, it seemed
that the IPr-derivatives were always far more stable than the analogous IMes-derivatives.
IMes and IPr ligands possessed very similar Tolman Electronic Parameter (TEP) values,[37]
as also evidenced by the almost identical νCO frequencies displayed by their complexes. Thus, the
different behaviour of IMes and IPr-containing species should be attributed mainly to steric effects.
This prompted a computational investigation, whose results are described in the next section.
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Computational studies
AIM analyses were performed of the DFT-optimized ground-state structures of the compounds 1, 2,
3 and 4. (3,-1) bond critical points (b.c.p) were localized for the metal-carbon interactions and the
Fe-Au bonds. Selected data are collected in Tables 3-6. The comparison of the data indicated
negligible changes on the properties at b.c.p. on changing the solvent from dmso to thf. More
important, quantities related to the strength of the bonds such as the electron density ρ and the
potential energy density V remained almost constant on changing the carbene ligand from IMes to
IPr. The negative values of energy densities E and the positive values for the Laplacian of electron
density ∇2ρ agreed with Bianchi's definitions of dative M-M bonds.[38,39]
Table 3. Selected electron densities ρ (a.u.) at b.c.p. for compounds 1-4; dmso as continuous
medium, data for thf in parenthesis.
[Fe(CO)4(AuNHC)]-
Fe(CO)4(AuNHC)2
ρC-Au
ρAu-Fe
ρFe-C
ρFe-C
ρC-Au
ρAu-Fe
ρFe-C
ρFe-C
trans-Au
cis-Au, aver.
trans-Au, aver.
cis-Au, aver.
IMes
IPr
0.127
(0.126)
0.128
0.605∙10-1
(0.617∙10-1)
0.606∙10-1
0.162
(0.162)
0.162
0.161
(0.162)
0.161
0.129
(0.128)
0.128
0.573∙10-1
(0.577∙10-1)
0.578∙10-1
0.157
(0.157)
0.157
0.155
(0.155)
0.157
(0.128)
(0.613∙10-1)
(0.162)
(0.161)
(0.127)
(0.580∙10-1)
(0.156)
(0.156)
Table 4. Selected potential energy densities V (a.u.) at b.c.p. for compounds 1-4; dmso as
continuous medium, data for thf in parenthesis.
[Fe(CO)4(AuNHC)]-
Fe(CO)4(AuNHC)2
VC-Au
VAu-Fe
VFe-C
VFe-C
VC-Au
VAu-Fe
VFe-C
VFe-C
trans-Au
cis-Au, aver.
trans-Au, aver.
cis-Au, aver.
IMes
IPr
-0.179
(-0.183)
-0.181
-0.532∙10-1
(-0.540∙10-1)
-0.533∙10-1
(-0.542∙10-1)
-0.301
(-0.301)
-0.300
-0.289
(-0.289)
-0.289
-0.180
(-0.181)
-0.179
-0.497∙10-1
(-0.502∙10-1)
-0.502∙10-1
(-0.505∙10-1)
-0.284
(-0.284)
-0.283
-0.270
(-0.269)
-0.272
(-0.182)
(-0.299)
(-0.289)
(-0.179)
(-0.283)
(-0.271)
Table 5. Selected energy densities E (a.u.) at b.c.p. for compounds 1-4; dmso as continuous
medium, data for thf in parenthesis.
[Fe(CO)4(AuNHC)]-
Fe(CO)4(AuNHC)2
EC-Au
EAu-Fe
EFe-C
EFe-C
EC-Au
EAu-Fe
EFe-C
EFe-C
trans-Au
cis-Au, aver.
trans-Au
cis-Au, aver.
IMes
IPr
-0.569∙10-1
(-0.554∙10-1)
-0.572∙10-1
(-0.572∙10-1)
-0.180∙10-1
(-0.179∙10-1)
-0.181∙10-1
(-0.183∙10-1)
-0.786∙10-1
(-0.783∙10-1)
-0.783∙10-1
(-0.781∙10-1)
-0.780∙10-1
(-0.782∙10-1)
-0.780∙10-1
(-0.780∙10-1)
-0.581∙10-1
(-0.580∙10-1)
-0.575∙10-1
(-0.571∙10-1)
-0.169∙10-1
(-0.170∙10-1)
-0.172∙10-1
(-0.172∙10-1)
-0.735∙10-1
(-0.734∙10-1)
-0.732∙10-1
(-0.730∙10-1)
-0.727∙10-1
(-0.721∙10-1)
-0.738∙10-1
(-0.733∙10-1)
Table 6. Selected Laplacian of electron densities ∇2ρ (a.u.) at b.c.p. for compounds 1-4; dmso as
continuous medium, data for thf in parenthesis.
[Fe(CO)4(AuNHC)]-
Fe(CO)4(AuNHC)2
2ρ Au-Fe 2ρ Fe-C
2ρ C-Au
2ρ Au-Fe
2ρ Fe-C
2ρ Fe-C
2ρ C-Au
2ρ Fe-C
trans-Au
cis-Au, aver.
trans-Au, aver.
cis-Au, aver.
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
0.259
(0.290)
0.264
0.688∙10-1
(0.730∙10-1)
0.685∙10-1
0.575
(0.578)
0.573
0.533
(0.531)
0.532
0.254
(0.259)
0.256
0.637∙10-1
(0.650∙10-1)
0.632∙10-1
0.549
(0.548)
0.548
0.499
(0.498)
0.498
IMes
IPr
(0.270)
(0.702∙10-1)
(0.573)
(0.532)
(0.258)
(0.629∙10-1)
(0.547)
(0.499)
The most noticeable difference, that was detected from the AIM analysis of the bonds, was
the lowering of ρ and V at Fe-Au b.c.p. moving from anionic dinuclear species to trinuclear neutral
compounds. The coordination of a second [AuNHC]+ fragment caused, also, the expected lowering
of electron densities at Fe-C b.c.p. The Au-NHC interactions appeared instead roughly comparable
among all the complexes here considered. The lowering of Fe-Au and Fe-C bond strengths in
Fe(CO4)4(AuNHC)2 complexes was confirmed by the computed Wiberg bond orders, collected in
Table 7. The electronic structure of the gold centres was comparable among [Fe(CO4)4(AuNHC)]-
and Fe(CO4)4(AuNHC)2 compounds, as suggested by the Hirshfeld charges, ranging between 0.032
and 0.096 a.u. The coordination of a second [Au(NHC)]+ ligand slightly reduced the partial
negative charge on iron, the average Hirshfeld charge being -0.304 a.u. in [Fe(CO4)4(AuNHC)]- and
-0.246 a.u. in Fe(CO4)4(AuNHC)2.
Table 7. Selected Wiberg bond orders for compounds 1-4; dmso as continuous medium, data for thf
in parenthesis.
[Fe(CO)4(AuNHC)]-
Fe(CO)4(AuNHC)2
Au-C
Au-Fe
Fe-C
Fe-C
Au-C
Au-Fe
Fe-C
Fe-C
trans-Au
cis-Au, aver.
trans-Au, aver.
cis-Au, aver.
0.921
(0.954)
0.916
0.487
(0.512)
0.481
1.405
(1.387)
1.403
1.416
(1.406)
1.415
0.903
(0.905)
0.889
0.444
(0.449)
0.439
1.318
(1.305)
1.306
1.297
(1.296)
1.306
IMes
IPr
(0.923)
(0.489)
(1.398)
(1.414)
(0.890)
(0.444)
(1.301)
(1.302)
No b.c.p was found between the gold centres in 3 and 4, in agreement with the lack of any
localized interaction. Moreover, no (3,+1) ring critical point (r.c.p) was found in the [Au2Fe]
triangles. Finally, no b.c.p. was found also for the Au···CO contacts, suggesting a dispersion-driven
nature or no interaction at all. The electron density map of compound 3, reported as an example in
the Supporting Information, showed relatively poor electron density between the gold centres and
between Au and CO. The ρ values at the middle of the Au---Au distances in the Fe(CO)4(AuNHC)2
complexes here considered were around 0.25·10-1 a.u. for NHC = IMes and 0.19·10-1 a.u. for the
bulkier IPr carbene derivative. For what concerns Au···CO, the electron density minima between
gold and carbon are comprised between 0.35·10-1 and 0.39·10-1 a.u. in 1 and 2, with no meaningful
difference between the two compounds. The highest density values for the Au···CO contacts in 3
and 4 fall in the same range.
The population analyses on the ground-state structures did not highlight any appreciable
electronic difference able to explain the different reactivity of the compounds on changing the
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
substituents on the carbene ligands, in agreement with the similar TEP values reported for IMes and
IPr.[35] Steric effects were therefore tentatively invoked. The space occupied by the NHC ligands in
the first coordination sphere of gold can be described by the buried volume, %VBur. The calculated
%VBur values for IMes and IPr in the DFT-optimized structures of 1 and 2 were respectively 36.0%
and 42.9%, and the different reactivity of the IPr derivatives was probably related to the higher
%VBur. Figure 4 showed plots of the electron density surfaces for the [Fe(CO)4(AuNHC)]-
complexes. The LUMO of the complexes were localized on the carbene donor atom and on gold,
and in the case of NHC = IPr the substituents seemed to better protect the unoccupied orbital from
incoming nucleophiles. We might therefore conclude that the different reactivity experimentally
observed could be ascribed to the different reactivity of the Au-NHC groups on changing the
bulkiness of the carbene substituents.
Figure 4. Electron density surfaces (light blue, isovalue = 0.1 a.u.) and LUMOs (yellow tones,
isovalue = 0.03 a.u.) for compounds 1 and 2. The high isovalue for electron density surfaces was
chosen to make the plot clear.
Conclusions
The synthesis, spectroscopic and structural characterization as well as theoretical investigation of
new heterobimetallic carbonyl clusters supported by NHC and phosphine ligands were reported.
Despite the fact that N-heterocyclic carbenes were widely employed as ligands in coordination and
organometallic chemistry,[40-43] molecular clusters containing NHC ligands were more scarce.[3,29,44-
49]
The compounds herein reported demonstrated the ability of obtaining heteroleptic clusters
containing CO, NHC and PR3 ligands by the stepwise addition of [AuNHC]+ and [AuPPh3]+
fragments to anionic carbonyl clusters. This might be a general strategy in order to obtain molecular
clusters supported by NHC ligands.
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
1-7 contained strong Fe-CO, Fe-Au, Au-P and Au-NHC bonds as well as weak Au···Au
interactions. AIM analyses and DFT studies pointed out that the Au···Au interactions in such
heterobimetallic clusters were not covalent bonds but mainly dispersion-driven.[21] In addition, the
different behaviour of IMes and IPr derivatives was essentially due to steric effects, since no
appreciable electronic difference was evidenced by population analyses performed at DFT level.
This was also in keeping with the very similar TEP values reported for IMes and IPr.[37] In
particular, it seemed that the bulkier IPr ligand provided a better steric protection to the LUMO
localized on the AuNHC fragment than IMes. Overall, molecular carbonyl clusters might be
considered as valuable platforms for the study of M-M and M-ligand interaction as well as their
related reactivities.[1,15,20-22]
Experimental Section
General experimental procedures
All reactions and sample manipulations were carried out using standard Schlenk techniques under
nitrogen and in dried solvents. All the reagents were commercial products (Aldrich) of the highest
purity available and used as received, except Na2[Fe(CO)4]·2thf,[50] Au(NHC)Cl (NHC = IMes, IPr)
[51] and Fe(CO)4(AuNHC)2 [3] which were prepared according to the literature. Analyses of C, H and
N were obtained with a Thermo Quest Flash EA 1112NC instrument. IR spectra were recorded on a
Perkin Elmer Spectrum One interferometer in CaF2 cells. Structure drawings have been performed
with SCHAKAL99.[52]
Synthesis of [NEt4][Fe(CO)4(AuIMes)] ([NEt4][1])
Na2[Fe(CO)4]·2thf (0.180 g, 0.500 mmol) and Au(IMes)Cl (0.270 g, 0.500 mmol) were charged
in a Schlenk tube under a nitrogen atmosphere and dmso (15 mL) was added. The mixture was
stirred for 2 h at room temperature and, then, a saturated solution of [NEt4]Br in H2O (40 mL)
was added up to complete precipitation. The resulting yellow solid was recovered by filtration,
washed with H2O (3 × 15 mL), toluene (3 × 15 mL), and extracted with acetone (15 mL).
Crystals of [NEt4][Fe(CO)4(AuIMes)] suitable for X-ray crystallography were obtained by slow
diffusion of n-hexane (35 mL) on the acetone solution (yield 0.232 g, 58 % based on Fe, 58 %
based on Au).
[NMe4][Fe(CO)4(AuIMes)] was obtained following a similar procedure and employing
[NMe4]Cl instead of [NEt4]Br.
C33H44AuFeN3O4 (799.53): calcd. (%): C 49.55, H 5.55, N 5.26; found: C 50.04, H 5.34, N 5.07. IR
(nujol, 293 K) νCO: 1975(w), 1927(s), 1830(vs), 1790(vs) cm–1. IR (dmso, 293 K) νCO: 1923(s),
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
1814(vs) cm–1. IR (CH3CN, 293 K) νCO: 1927(s), 1821(vs) cm–1. IR (acetone, 293 K) νCO: 1924(s),
1820(vs) cm–1. 1H NMR (CD3CN, 298 K): δ 7.20 (s, 2H, CHimid), 7.05 (s, 4H, CHAr), 3.19 (q, 2JHH
=
2
7.2 Hz, 8H, NCH2CH3), 2.35 (s, 6H, CH3), 2.18 (s, 12H, CH3), 1.22 (t, JHH = 7.2 Hz, 12H,
NCH2CH3). 13C{1H} NMR (CD3CN, 298 K): δ 226.5 (CO), 196.3 (C-Au), 138.7, 135.9, 135.1,
128.9, 121.4 (CAr and CHimid), 52.2 (NCH2CH3), 20.3, 17.2 (CH3), 6.8 (NCH2CH3).
Synthesis of [NEt4][Fe(CO)4(AuIPr)] ([NEt4][2])
Na2[Fe(CO)4]·2thf (0.210 g, 0.580 mmol) and Au(IPr)Cl (0.360 g, 0.580 mmol) were charged in
a Schlenk tube under a nitrogen atmosphere and dmso (15 mL) was added. The mixture was
stirred for 2 h at room temperature and, then, a saturated solution of [NEt4]Br in H2O (40 mL)
was added up to complete precipitation. The resulting yellow solid was recovered by filtration,
washed with H2O (3 × 15 mL), toluene (3 × 15 mL), and extracted with acetone (15 mL).
Crystals of [NEt4][Fe(CO)4(AuIPr)] suitable for X-ray crystallography were obtained by slow
diffusion of n-hexane (35 mL) on the acetone solution (yield 0.287 g, 56 % based on Fe, 56 %
based on Au).
C39H56AuFeN3O4 (883.68): calcd. (%): C 52.98, H 6.39, N 4.76; found: C 53.12, H 6.21, N 4.54. IR
(nujol, 293 K) νCO: 1972(w), 1924(s), 1810(vs) cm–1. IR (dmso, 293 K) νCO: 1923(s), 1815(vs) cm–
1. IR (CH3CN, 293 K) νCO: 1926(s), 1820(vs) cm–1. IR (acetone, 293 K) νCO: 1924(s), 1821(vs) cm–
1. 1H NMR (CD3CN, 298 K): δ 7.49 (t, 2JHH = 7.4 Hz, 2H, CHAr), 7.34 (d, 2JHH = 7.4 Hz, 4H, CHAr),
2
2
7.32 (s, 2H, CHimid), 3.19 (q, JHH = 7.2 Hz, 8H, NCH2CH3), 2.75 (sept, JHH = 6.8 Hz, 4H,
2
CH(CH3)2), 1.39 (d, 2JHH = 6.8 Hz, 12H, CH(CH3)2), 1.22 (t, JHH = 7.2 Hz, 12H, NCH2CH3), 1.21
2
(d, JHH = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (CD3CN, 298 K): δ 226.4 (CO), 198.3 (C-Au),
145.9, 135.5, 129.6, 123.7, 122.4 (CAr and CHimid), 52.1 (NCH2CH3), 28.5 (CH(CH3)2), 23.4, 23.0
(CH(CH3)2), 6.8 (NCH2CH3).
Synthesis of Fe(CO)4(AuIMes)2 (3)
Au(IMes)Cl (0.176 g, 0.328 mmol) was added as a solid in small portions to a solution of
[NEt4][Fe(CO)4(AuIMes)] (0.260 g, 0.325 mmol) in acetone (15 mL) and the reaction
monitored by IR spectroscopy. The resulting mixture was stirred for 2 h at room temperature
and, then, the solvent removed under reduced pressure. The residue was washed with H2O (3 ×
20 mL), isopropanol (3 × 20 mL), and extracted with acetone (15 mL). A microcrystalline
powder of Fe(CO)4(AuIMes)2 was obtained after removal of the solvent under reduced pressure
(yield 0.213 g, 56 % based on Fe, 56 % based on Au).
Alternatively, 3 may be obtained from the reaction of Na2[Fe(CO)4]·2thf with two equivalents
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Au(IMes)Cl in thf, as described previously.[3]
C46H48Au2FeN4O4 (1170.67): calcd. (%): C 47.17, H 4.13, N 4.79, Fe 4.78, Au 33.66; found: C
46.89, H 4.34, N 4.98, Fe 5.02, Au 33.35. IR (nujol, 293 K) ν(CO): 1980(vs), 1898(sh), 1875(s) cm–
1. IR (CH2Cl2, 293 K) ν(CO): 1979(m), 1894(s) cm–1. IR (acetone, 293 K) ν(CO): 1977(m), 1895(s)
1
cm–1. IR (DMF, 293 K) ν(CO): 1975(m), 1890(s) cm–1. H NMR (CD2Cl2, 298 K): δ 7.04 (s, 4H,
13
CHimid), 6.96 (s, 8H, CHAr), 2.35 (s, 12H, CH3), 2.07 (s, 24H, CH3). C{1H} NMR (CD2Cl2, 298
K): δ 218.2 (CO), 194.5 (C-Au), 138.7, 135.2, 134.7, 128.9 (CAr), 121.0 (CHimid), 20.8, 17.7 (CH3).
Synthesis of Fe(CO)4(AuIPr)2 (4)
Au(IPr)Cl (0.204 g, 0.328 mmol) was added as a solid in small portions to a solution of
[NEt4][Fe(CO)4(AuIPr)] (0.287 g, 0.325 mmol) in acetone (15 mL) and the reaction monitored
by IR spectroscopy. The resulting mixture was stirred for 2 h at room temperature and, then, the
solvent removed under reduced pressure. The residue was washed with H2O (3 × 20 mL),
isopropanol (3
×
20 mL), and extracted with toluene (15 mL). Crystals of
Fe(CO)4(AuIPr)2·1.5toluene, suitable for X-ray crystallography were obtained by slow diffusion
of n-pentane (35 mL) on the toluene solution (yield 0.265 g, 61 % based on Fe, 61 % based on
Au). Alternatively, 4 may be obtained from the reaction of Na2[Fe(CO)4]·2thf with two
equivalents Au(IPr)Cl in thf, as described previously.[3]
C58H72Au2FeN4O4 (1338.99): calcd. (%): C 52.03, H 5.42, N 4.18, Fe 4.17, Au 29.42; found: C
51.89, H 5.64, N 4.03, Fe 4.38, Au 29.19. IR (nujol, 293 K) ν(CO): 1974(vs), 1901(sh), 1890(s) cm–
1
1. IR (CH2Cl2, 293 K) ν(CO): 1974(m), 1884(s) cm–1. H NMR (CD2Cl2, 298 K): δ 7.51, 7.33 (br,
12H, CHAr), 7.16 (s, 4H, CHimid), 2.69 (br, 8H, CH(CH3)2), 1.30 (br, 48H, CH(CH3)2). 13C{1H}
NMR (CD2Cl2, 298 K): δ 216.9 (CO), 194.3 (C-Au), 145.7, 134.8, 129.9, 123.8 (CAr), 122.3
(CHimid), 28.7 (CH(CH3)2), 24.0 (CH(CH3)2).
Synthesis of Fe(CO)4(AuIPr)(AuIMes) (5)
Au(IMes)Cl (0.176 g, 0.328 mmol) was added as a solid in small portions to a solution of
[NEt4][Fe(CO)4(AuIPr)] (0.287 g, 0.325 mmol) in acetone (15 mL) and the reaction monitored
by IR spectroscopy. The resulting mixture was stirred for 2 h at room temperature and, then, the
solvent removed under reduced pressure. The residue was washed with H2O (3 × 20 mL),
isopropanol (3 × 20 mL), and extracted with acetone (15 mL). A microcrystalline powder of
Fe(CO)4(AuIPr)(AuIMes) was obtained after removal of the solvent under reduced pressure
(yield 0.208 g, 51 % based on Fe, 51 % based on Au).
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European Journal of Inorganic Chemistry
C52H60Au2FeN4O4 (1254.33): calcd. (%): C 49.75, H 4.82, N 4.47; found: C 49.97, H 5.02, N 4.11.
IR (acetone, 293 K) νCO: 1971(s), 1885(vs), 1864(sh) cm–1. 1H NMR (CD3COCD3, 298 K): δ 7.53,
2
2
7.38 (s, 4H, CHimid), 7.41 (t, JHH = 7.4 Hz, 2H, CHAr), 7.23 (d, JHH = 7.4 Hz, 4H, CHAr), 6.96 (s,
4H, CHAr), 2.70 (m, 4H, CH(CH3)2), 2.31 (s, 6H, CH3), 2.09 (s, 12H, CH3), 1.25 (d, 2JHH = 6.5 Hz,
2
13
12H, CH(CH3)2), 1.17 (d, JHH = 6.5 Hz, 12H, CH(CH3)2). C{1H} NMR (CD3COCD3, 298 K): δ
217.8 (CO), 195.1, 193.7 (C-Au), 145.5, 138.2, 135.5, 135.0, 134.6, 129.7, 128.9, 123.6, 122.7,
121.6 (CHAr+CHimid), 28.5 (CH(CH3)2), 23.6, 23.4 (CH(CH3)2), 20.4, 17.4 (CH3).
Synthesis of Fe(CO)4(AuIMes)(AuPPh3)·0.5CH2Cl2 (6·0.5CH2Cl2)
Au(PPh3)Cl (0.161 g, 0.326 mmol) was added as a solid in small portions to a solution of
[NEt4][Fe(CO)4(AuIMes)] (0.260 g, 0.325 mmol) in acetone (15 mL) and the reaction
monitored by IR spectroscopy. The resulting mixture was stirred for 5 min at room temperature
and, then, the solvent removed under reduced pressure. The residue was dissolved with CH2Cl2
(15 mL), filtered and crystals of Fe(CO)4(AuIMes)(AuPPh3)·0.5CH2Cl2 suitable for X-ray
crystallography were obtained by slow diffusion of n-pentane (35 mL) on the CH2Cl2 solution
(yield 0.171 g, 45 % based on Fe, 45 % based on Au).
Fe(CO)4(AuIMes)(AuPPh3) is less stable than Fe(CO)4(AuIPr)(AuPPh3), and rapidly
decomposes in acetone solution even at room temperature.
C43.5H40Au2ClFeN2O4P (1170.98): calcd. (%): C 44.61, H 3.45, N 2.39; found: C 44.87, H 3.12, N
2.16. IR (nujol, 293 K) νCO: 1982(s), 1906(sh), 1896(s), 1873(ms) cm–1. IR (acetone, 293 K) νCO:
1
1988(ms), 1912(ms), 1886(s) cm–1. IR (CH2Cl2, 293 K) νCO: 1989(ms), 1893(s) cm–1. H NMR
(CD3COCD3, 298 K): δ 7.77-7.35 (m, 21H, CHAr+CHimid+Ph), 2.24 (s, 6H, CH3), 2.04 (s, 12H,
CH3). 13C{1H} NMR (CD3COCD3, 298 K): δ 218.4 (CO), 193.3 (C-Au), 138.6-121.8
(CHAr+CHimid+Ph), 20.3, 17.0 (CH3). 13C{1H} NMR (CD3COCD3, 298 K): δ 218.4 (CO), 193.3 (C-
Au), 138.6, 134.6, 134.0, 133.9, 131.2. 131.1, 129.1, 129.0, 121.8 (CHAr+CHimid+Ph), 20.3, 17.0
(CH3). 31P{1H} NMR (CD3COCD3, 298 K): δ 40.8.
Synthesis of Fe(CO)4(AuIPr)(AuPPh3) (7)
Au(PPh3)Cl (0.161 g, 0.326 mmol) was added as a solid in small portions to a solution of
[NEt4][Fe(CO)4(AuIPr)] (0.287 g, 0.325 mmol) in acetone (15 mL) and the reaction monitored
by IR spectroscopy. The resulting mixture was stirred for 2 h at room temperature and, then, the
solvent removed under reduced pressure. The residue was washed with H2O (3 × 20 mL),
isopropanol (3
×
20 mL), and extracted with CH2Cl2 (15 mL). Crystals of
Fe(CO)4(AuIPr)(AuPPh3) suitable for X-ray crystallography were obtained by slow diffusion of
17
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
n-pentane (35 mL) on the CH2Cl2 solution (yield 0.226 g, 57 % based on Fe, 57 % based on
Au).
C49H51Au2FeN2O4P (1212.67): calcd. (%): C 48.51, H 4.24, N 2.32; found: C 48.32, H 3.97, N 2.51.
IR (nujol, 293 K) νCO: 1984(s), 1911(s), 1892(s), 1867(s) cm–1. IR (acetone, 293 K) νCO: 1989(s),
1
1914(s), 1882(ms) cm–1. IR (CH2Cl2, 293 K) νCO: 1990(s), 1913(s), 1878(ms) cm–1. H NMR
(CD3COCD3, 298 K): δ 7.74-7.14 (m, 23H, CHAr+CHimid+Ph), 2.70 (m, 4H, CH(CH3)2), 1.28 (d,
2
2JHH = 6.8 Hz, 12H, CH(CH3)2), 1.15 (d, JHH = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(CD3COCD3, 298 K): δ 217.9 (CO), 194.7 (C-Au), 145.5, 134.2, 134.1, 131.1, 129.9, 129.1, 129.0,
123.7, 123.0 (CHAr+CHimid+Ph), 28.9 (CH(CH3)2), 23.6, 23.5 (CH(CH3)2). 31P{1H} NMR
(CD3COCD3, 298 K): δ 40.8.
X-ray Crystallographic Study.
Crystal data and collection details for [NMe4][Fe(CO)4(AuIMes)], [NEt4][Fe(CO)4(AuIMes)],
[NEt4][Fe(CO)4(AuIPr)],
Fe(CO)4(AuIPr)(AuPPh3),
Fe(CO)4(AuIMes)(AuPPh3)·0.5CH2Cl2,
(P21/n), Fe(CO)4(AuIPr)2·1.5toluene,
�
Fe(CO)4(AuPPh3)2
(P1), Fe(CO)4(AuPPh3)2
[Au(IMes)2][Fe2(CO)8(AuPPh3)]·CH2Cl2, are reported in Table S.1. The diffraction experiments
were carried out on a Bruker APEX II diffractometer equipped with a PHOTON100 detector using
Mo–Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical
absorption correction SADABS).[53] Structures were solved by direct methods and refined by full-
matrix least-squares based on all data using F2.[54] Hydrogen atoms were fixed at calculated
positions and refined by a riding model. All non-hydrogen atoms were refined with anisotropic
displacement parameters, unless otherwise stated.
The [NEt4]+ cation in the structures of [NEt4][Fe(CO)4(AuIMes)] and
[NEt4][Fe(CO)4(AuIPr)] is disordered and, thus, it has been split into two positions and refined
using one occupancy factor per disordered group. The atoms of the disordered cations have been
restrained to have similar U parameters (SIMU line in SHELXL, s.u. 0.01).
The CH2Cl2 molecule in the structure of Fe(CO)4(AuIMes)(AuPPh3)·0.5CH2Cl2 is
disordered over two symmetry related (by a 2-fold axis) positions. The disordered CH2Cl2 molecule
has been refined isotropically and its atoms have been restrained to have similar U parameters
(SIMU line in SHELXL, s.u. 0.01). Restraints to bond distances were applied as follow (s.u. 0.02):
1.75 Å for C–Cl in CH2Cl2.
One toluene molecule in the structure of Fe(CO)4(AuIPr)2·1.5toluene is disordered over two
symmetry related (by an inversion centre) positions. The other toluene molecule (located on a
general position) is also disordered and, thus, it has been split into two positions and refined using
18
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
one occupancy factor per disordered group. All disordered atoms have been refined isotropically.
All C, N and O atoms have been restrained to have similar U parameters (SIMU line in SHELXL,
s.u. 0.01) and to isotropic behaviour (ISOR line in SHELXL, s.u. 0.01). The aromatic C-atoms of
the toluene molecules have been constrained to fit regular hexagons (AFIX 66 line in SHELXL).
The CH2Cl2 molecule in the structure of [Au(IMes)2][Fe2(CO)8(AuPPh3)]·CH2Cl2 is
disordered and, thus, it has been split into two positions and refined isotropically using one
occupancy factor per disordered group. The atoms of the disordered CH2Cl2 molecule have been
restrained to have similar U parameters (SIMU line in SHELXL, s.u. 0.01). Restraints to bond
distances were applied as follow (s.u. 0.02): 1.75 Å for C–Cl in CH2Cl2.
CCDC 1911648-1911656 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.
Computational details
Geometry optimizations of the complexes were carried out using the range-separated hybrid DFT
functional ωB97X.[55] The basis set used was the Ahlrichs' def2 split-valence, with polarization
functions and relativistic ECP for Au.[56] The C-PCM implicit solvation model was added to
ωB97X calculations. considering dmso and thf as solvents.[57] The "restricted" approach was used in
all the cases. The stationary points were characterized by IR simulations (harmonic approximation),
from which zero-point vibrational energies and thermal corrections (T = 298.15 K) were
obtained.[58] Calculations were performed with the Gaussian 09 software.[59] Single point
calculations were then carried out on the optimized geometries with the ORCA 4.0.1.2 software [60]
with the same method, but including diffusion functions and non-local correlation by the VV10
functional.[61] The output, converted in .molden format, was used for AIM, Wiberg and Hirshfeld
analyses, performed with the software Multiwfn, version 3.5.[62] Cartesian coordinates of the DFT-
optimized structures are collected in a separated .xyz file. The buried volumes (%VBur) of IMes and
IPr in compounds 1 and 2 were calculated from the Cartesian coordinates of 1 and 2 with the web
application SambVca,[63] using the calculated Au-C distances (Au-IMes, 2.066 Å; Au-IPr, 2.064 Å).
The radius of the sphere built around the metal atom was 3.5 Å. Bondi radii scaled by 1.17 were
used for the atoms of the NHC ligands and hydrogen atoms were included in the calculations.
Supporting Information
NMR spectra of 1-7, molecular structures of 4, 8 and 9, electron density map of compound 3,
crystal data and experimental details, Cartesian coordinates of the DFT-optimized structures as pdf
file.
19
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10.1002/ejic.201900537
European Journal of Inorganic Chemistry
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Table of contents
NHC ligands protect and tune the
properties of heterobimetallic Fe-Au
carbonyl clusters.
Organometallic clusters
Beatrice Berti, Marco Bortoluzzi,
Cristiana Cesari, Cristina Femoni, Maria
Carmela Iapalucci, Rita Mazzoni,
Federico Vacca, and Stefano Zacchini*
Page No. – Page No.
Synthesis and Characterization of
Heterobimetallic Carbonyl Clusters
with Direct Au-Fe and Au···Au
Interactions supported by N-
Heterocyclic Carbene and Phosphine
Ligands
24
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