Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent

Article abstract of DOI:10.3390/molecules25163741

Mercury(II) complexes (Me-maloNHC_Dipp)HgCl (1b), (t-Bu-maloNHC_Dipp)HgCl (2b) and (t-Bu-maloNHC_Dipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl₂ and MeHgI. These molecules have been characterized by ¹H-NMR, ¹³C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHC_Dipp)HgCl}_n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHC_Dipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent.

Full text of DOI:10.3390/molecules25163741

molecules
Article
Mercury(II) Complexes of Anionic N-Heterocyclic
Carbene Ligands: Steric Effects of the
Backbone Substituent
Chandrakanta Dash *,†, Animesh Das ‡ and H. V. Rasika Dias *
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX 76019, USA;
adas@iitg.ac.in
* Correspondence: ckdash@curaj.ac.in (C.D.); dias@uta.edu (H.V.R.D.)
†
Current address: Department of Chemistry, School of Chemical Sciences and Pharmacy, Central University of
Rajasthan, Bandar Sindri, Ajmer-305817, Rajasthan, India.
‡ Current address: Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039,
Assam, India.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editor: Yves Canac
Received: 27 July 2020; Accepted: 13 August 2020; Published: 16 August 2020
Abstract: Mercury(II) complexes (Me-maloNHCDipp)HgCl (1b), (t-Bu-maloNHCDipp)HgCl (2b) and
(t-Bu-maloNHCDipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained
in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors
1
and mercury(II) salts, HgCl2 and MeHgI. These molecules have been characterized by H-NMR,
13C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are
also presented. Interestingly, complex 1b is polymeric {(Me-maloNHCDipp)HgCl}n in the solid
state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites
with a T-shaped arrangement, whereas the (t-Bu-maloNHCDipp)HgCl (2b) is monomeric and has
a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation
of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand
backbone substituent.
Keywords: ligands; N-heterocyclic carbene; mercury(II) complex; X-ray; T-shaped
1. Introduction
During the past three decades, considerable attention has been given to the chemistry of metal
complexes of N-heterocyclic carbenes [
1–
9]. N-heterocyclic carbenes are versatile ligands with extensive
applications in coordination chemistry and catalysis, as well as in bioinorganic chemistry [10
–
17].
Though traditional N-heterocyclic carbenes (NHCs) are neutral “L” type ligands, NHCs bearing tethered
anionic donor sites (e.g., alkoxide, aryloxide, amido etc.) have also been reported, along with their
metal complexes [18
–
27]. In contrast, N-heterocyclic carbene ligands with remote anionic functional
28 29].
groups/moieties within the heterocyclic ligand backbone have been less widely investigated [12
,
,
Reactions of these anionic NHCs with transition metals afford corresponding zwitterionic complexes.
Anionic, six-membered ring NHCs based on a malonate unit (maloNHC), introduced by Cesar, Lavigne
and co-workers [30], are particularly interesting as they are attractive ligands to stabilize late transition
metal ion complexes such as those involving AgI, AuI, RhI, CuI [30–35], as well as FeII [36].
The first known metal-NHC complex was a mercury(II) compound reported by Wanzlick and
Schonherr in 1968 [37]. The mercury(II)-NHC complexes have played an important role in the
development of N-heterocyclic carbene chemistry, but are still less explored compared to other
metals of the d-block [38–48]. Furthermore, to our knowledge, mercury(II) complexes of anionic
N-herterocyclic carbene ligands, such as maloNHC, have not been investigated. As a continuation
Molecules 2020, 25, 3741; doi:10.3390/molecules25163741
www.mdpi.com/journal/molecules

Molecules 2020, 25, 3741
2 of 10
of our interest in the NHCs and their metal chemistry [49
–
60], we have set out to probe the use of
anionic N-heterocyclic carbene ligands in mercury chemistry. In particular, we describe the synthesis
and spectroscopic data of three new mercury(II) complexes (Figure 1), (Me-maloNHCDipp)HgCl (1b),
(t-Bu-maloNHCDipp)HgCl (2b) and (t-Bu-maloNHCDipp)HgMe (2c), involving a bulkier maloNHC.
We have also studied the effects of backbone substituent in heterocyclic ring (malonate unit), on the
structure of the mercury complexes 1b and 2b.
−
]
Figure 1. Mercury(II) complexes supported by anionic N-heterocyclic carbenes [Me-maloNHCDipp
and [t-Bu-maloNHCDipp]−.
2. Results and Discussion
Synthesis and Characterization
The anionic, six-membered N-heterocyclic carbene ligand precursor 1a has been reported
earlier [32]. The anionic-NHC ligand precursors 1a and 2a were synthesized using the strategy
reported by C
monosubstituted malonic acid) [30]. Moreover, 1H NMR spectrum of 2a in CDCl3 at room temperature
exhibits a signal corresponding to the NCHN proton at the 8.07 ppm (compare for 1a 8.06 ppm [32]).
ésar et al. (from the condensation of N,N’-bis(2,6-diisopropylphenyl)formamidine and
δ
: δ
The mercury(II) complexes, (Me-maloNHCDipp)HgCl (1b) and (t-Bu-maloNHCDipp)HgCl (2b), were
prepared from in situ generated anionic N-heterocyclic carbene ligand, by using KHMDS as the base
and HgCl2 as the mercury source (Scheme 1). The complexes 1b and 2b are yellow crystalline solids,
which were characterized by NMR, IR spectroscopy, elemental analysis and X-ray crystallography.
They are air and light stable solids, and soluble in dichloromethane, THF and chloroform. The 1H
NMR spectra of 1b and 2b in CDCl3 at room temperature showed the absence of NCHN resonance
of the starting precursors (1a or 2a). The 13C{1H} NMR spectrum showed the appearance of a highly
downfield shifted mercury bound carbene carbon resonance at
2b, respectively. We did not observe Hg-Ccarbene coupling in the 13C NMR spectrum, although large
Ccarbene-Hg coupling in the range 2700–3300 Hz has been reported in the literature [61 62]. Yellow
δ 180.9 and 179.9 ppm, for 1b and
,
crystals of these molecules could be obtained from dichloromethane solution layering with hexane at
−10 ◦C.

Molecules 2020, 25, 3741
3 of 10
Scheme 1. Synthesis of mercury(II) complexes 1b and 2b supported by anionic NHC.
The molecular structures of the complexes 1b and 2b are illustrated in Figures 2 and 3. The complex
1b crystallizes in P-1 space group with two chemically similar molecules of 1b in the asymmetric
unit. X-ray structure of 1b shows the presence of three coordinated mercury atoms (and zig-zag
polymeric chains resulting from bridging Hg-O contacts involving neighboring molecules) having a
distorted T-shaped geometry with bond angles of 86.68(7)◦, 157.34(12)◦, 115.97(13)◦ at Hg1 and 86.52(7)◦,
156.92(12)◦, 116.55(13)◦ at Hg2. The inter-molecular Hg-O bond distances of 2.537(3) and 2.529(3) Å are
shorter than the sum of van der Waals radii of the respective elements, which is 3.07 Å for an Hg . . .
interaction. For comparison, interactions of similar magnitude have been observed in the complex
C6F5HgCl DMSO [Hg-O = 2.542(2) Å], reported by the Gabbai group [63]. The average Hg-Ccarbene
O
•
bond distance in 1b is 2.091(4) Å, which is in good agreement with the five membered imidazole
based mercury carbene complexes, i.e., [(IDipp)HgCl2] [2.090(4) Å], [(IMes)HgCl2] [2.084(6) Å] [64].
The average Hg-Cl bond distance in 1b (2.3154(11) Å) is comparable to the other three coordinated,
T-shaped mercury complexes, namely, [2-(Me2NCH2)C6H4]HgCl [2.319(2) Å], C6F5HgCl
[2.322(2) Å] [63,65].
•
DMSO
The complex (t-Bu-maloNHCDipp)HgCl (2b) crystallizes in P-1 space group with two independent
molecules of (t-Bu-maloNHCDipp)HgCl in the asymmetric unit. In contrast to the complex 1b, the
mercury complex 2b has a two-coordinate linear mercury center with Ccarbene-Hg-Cl bond angles of
177.50(15)◦ and 177.74(15)◦ for the two molecules. This is likely a result of having a bulky t-Bu moiety
in the six-membered ring at the remote, apical carbon, in addition to the bulky N-aryl group, which
inhibit the interaction of mercury atom with oxygen atom in an adjacent molecule. We should note
here that electronic donor properties at the carbene center have been modulated by using electrophiles
to interact with oxygen atoms of remote malonate group of Me-maloNHCMes and t-Bu-maloNHCMes
in rhodium complexes [35]. The linear geometry at mercury of (t-Bu-maloNHCDipp)HgCl is commonly
found in Hg-NHC complexes involving five membered neutral NHCs. The average Ccarbene-Hg bond
distance in 2b (2.059(5) Å) is similar to those observed in the other mercury complexes reported in
the literature. The Hg-Cl distances in 2b 2.2558(15) and 2.235(2) Å are shorter than in those found in
complex 1b [2.3158(10) Å]. This is not surprising, as complex 1b has three coordinate mercury sites,
whereas they are two-coordinate in 2b.

Molecules 2020, 25, 3741
4 of 10
Figure 2. Molecular structure of (Me-maloNHCDipp)HgCl (1b). Hydrogen atoms have been omitted for
clarity. Selected bond distances (Å) and angles (◦): Hg1-Cl1 2.3157(11), Hg1-O3 2.537(3), Hg1-C1 2.090(4),
Hg2-Cl2 2.3152(11), Hg2-O2 2.529(3), Hg2-C30 2.092(4), O1-C2 1.228(5), O2-C4 1.262(5), O3-C31 1.256(5),
O4-C33 1.227(5); Cl1-Hg1-O31 86.68(7); C1-Hg1-Cl1 157.34(12), C1-Hg1-O3 1 115.97(13), Cl2-Hg2-O2
86.52(7), C30-Hg2-Cl2 156.92(12), C30-Hg2-O2 116.55(13), C4-O2-Hg2 116.7(3), C31-O3-Hg1 2 117.3(3),
where 1 +X, 1 + Y,+Z; 2 +X, −1 + Y, +Z.
Figure 3. Molecular structure of (t-Bu-maloNHCDipp)HgCl (2b). The second molecule in the asymmetric
unit and hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (◦):
Molecule 1, Hg1-Cl1 2.2558(15), Hg1-C1 2.056(5), O1-C4 1.221(7), O2-C2 1.232(7); C1-Hg1-Cl1 177.50(15);
Molecule 2 (not shown), Hg2-Cl2 2.235(2), Hg2-C33 2.062(5), O3-C34 1.229(7), O4-C36 1.220(7),
C33-Hg2-Cl2 177.74(15).
The mercury(II) complex, (t-Bu-maloNHCDipp)HgMe (2c) was synthesized from a reaction
between in situ generated anionic N-heterocyclic carbene ligand and MeHgI as the metal precursor
(Scheme 2) in THF. It was isolated as a yellow colored solid in 70% yield. The 13C resonance of the
mercury(II)-bound carbene carbon (NCN) of (t-Bu-maloNHCDipp)HgMe (2c) in CDCl3 was observed
as a singlet at
δ 205.8 ppm. We have not attempted to obtain crystalline materials of 2c for an X-ray
crystallographic study.

Molecules 2020, 25, 3741
5 of 10
Scheme 2. Synthesis of mercury(II) complex (t-Bu-maloNHCDipp)HgMe (2c).
3. Experimental Sections
3.1. Materials and General Methods
All manipulations were carried out under an atmosphere of dry nitrogen, using standard Schlenk
techniques, or in a glove box. Solvents were purchased from commercial sources, purified using an
Innovative Technology SPS-400 PureSolv solvent drying system (Innovative Technology, Inc., Galway,
Ireland), degassed by the freeze-pump-thaw method twice prior to use. Glassware was oven-dried at
150 ◦C overnight. NMR spectra were recorded at 298 K on JEOL Eclipse 500 and 300 spectrometers
(Jeol Ltd., Tokyo, Japan). NMR annotations used: br. = broad, d = doublet, m = multiplet, s = singlet,
t = triplet, sept = septet. Infrared spectra were recorded on a JASCO FT-IR 410 spectrometer (Jasco,
Tokyo, Japan) operating at 2 cm−1 spectral resolution. IR spectroscopic data were collected using KBr
pellets. Herein, we use abbreviations based on IUPAC guidelines, that is,
ν for frequency and ν for
wavenumber. Elemental analyses were performed using a Perkin Elmer Series II CHNS/O analyzer.
Methylmalonic acid, DCC were purchased from Sigma-Aldrich and used without further purification.
N,N’-bis(2,6-diisopropylphenyl)formamidine [66] and tert-butylmalonic acid [67] were synthesized as
reported, and anionic-NHC ligand precursor (1a) [32] was obtained via a modified literature procedure
(noted below). The data of NMR spectrum can be found in the Supplementary Materials.
3.1.1. Synthesis of 1,3-Bis(2,6-diisopropylphenyl)-5-methyl-6-oxo-6H-pyrimidinium-4-olate (1a)
A mixture of N,N’-bis(2,6-diisopropylphenyl)formamidine (1.81 g, 4.96 mmol) and
N,N’-dicyclohexylcarbodiimide (DCC) (2.04 g, 9.92 mmol) in dichloromethane (ca. 35 mL) was
placed in a 50 mL Schlenk flask. To this mixture, methylmalonic acid (0.586 g, 4.96 mmol) was added
as a solid at room temperature. The reaction mixture was stirred for 5 h. The solution was filtered and
the solvents were evaporated. The residue was purified by flash chromatography (SiO2, 30% EtOAc
in hexane), to obtain a yellow crystalline solid (1.62 g, 73% yield). The analytical data agree with the
reported values [32].
3.1.2. Synthesis of 1,3-Bis(2,6-diisopropylphenyl)-5-tert-butyl-6-oxo-6H-pyrimidinium-4-olate (2a)
A mixture of N,N’-bis(2,6-diisopropylphenyl)formamidine (5.18 g, 14.2 mmol) and
N,N’-dicyclohexylcarbodiimide (DCC) (5.86 g, 28.4 mmol) in dichloromethane (ca. 50 mL) was
taken in a 100 mL Schlenk flask. To this mixture, tert-butylmalonic acid (2.27 g, 14.2 mmol) was then
added as a solid at room temperature. The reaction mixture was stirred for 6 h. The solution was
filtered and the solvents were evaporated. The residue was purified by chromatography (SiO2, 10%
EtOAc in hexane) to give a yellow crystalline solid (4.71 g, 68% yield). Mp: 255–260 ◦C. 1H NMR
3
(CDCl3, 500.16 MHz, 298 K):
δ
8.07 (s, 1H, NCHN), 7.42 (t, 2H, JHH = 8.0 Hz, C6H3), 7.25 (d, 4H,
3JHH = 8.0 Hz, C6H3), 2.80 (sept, 4H, 3JHH = 6.9 Hz, CH(CH3)2), 1.46 (s, 9H, C(CH3)3), 1.30 (d, 12H,
3JHH = 6.9 Hz, CH(CH3)2), 1.20 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3, 125.77 MHz,
298 K):
3068, 2991, 2856, 1926, 1778, 1760, 1687, 1598, 1581, 1462, 1425, 1387, 1364, 1351, 1327, 1304, 1282, 1258,
δ :
158.3 (C-O), 146.9, 145.7, 132.5, 130.6, 124.2, 104.2, 34.5, 30.0, 29.2, 24.1, 23.9. IR (KBr) cm−1

Molecules 2020, 25, 3741
6 of 10
1218, 1183, 1149, 1107, 1059, 1018, 981, 937, 875, 803, 761, 741. Anal. Calcd. for C32H44N2O2: C, 78.65;
H, 9.07; N, 5.73. Found: C, 79.01; H, 9.03; N, 6.04%.
3.1.3. Synthesis of (Me-maloNHCDipp)HgCl (1b)
To a solution of 1a (0.491 g, 1.10 mmol) in THF (30 mL), KHMDS (0.5 M in toluene, 2.2 mL,
1.10 mmol, 1.0 equiv.) was added dropwise at 0 ◦C (ice bath). After 30 min at 0 ◦C, HgCl2 (0.299 g,
1.10 mmol) was added as a solid. The reaction mixture was stirred at room temperature for 6 h. The
solvent was removed under vacuum, and the crude residue was dissolved in dichloromethane (30 mL).
The mixture was filtered through a pad of Celite, and the filtrate was evaporated under vacuum to
obtain a yellow solid. The compound was further washed with hexane and vacuum dried (0.405 g,
54% yield). Mp: 287 ◦C. 1H NMR (CDCl3, 500.16 MHz, 298 K):
δ
7.53 (t, 2H, 3JHH = 8.0 Hz, C6H3),
7.33 (d, 4H, 3JHH = 8.0 Hz, C6H3), 2.80 (sept, 4H, 3JHH = 6.9 Hz, CH(CH3)2), 2.02 (s, 3H, CH3), 1.27
(d, 12H, 3JHH = 6.9 Hz, CH(CH3)2), 1.22 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3,
125.77 MHz, 298 K):
δ 180.9 (Hg-NCN), 160.1 (C-O), 146.2, 136.7, 131.8, 125.4, 94.4, 29.1, 24.6, 24.4, 10.1.
IR (KBr) cm−1: 2963, 2928, 2871, 1727, 1678, 1645, 1610, 1546, 1466, 1447, 1386, 1365, 1345, 1322, 1259,
1222, 1181, 1147, 1107, 1062, 1039, 934, 795, 755. Anal. Calcd. for C29H37N2O2HgCl
6.02; N, 3.72. Found: C, 52.63; H, 5.81; N, 4.13%.
•
THF: C, 52.58; H,
3.1.4. (t-Bu-maloNHCDipp)HgCl (2b)
To a solution of 2a (0.752 g, 1.54 mmol) in THF (30 mL), KHMDS (0.5 M in toluene, 3.4 mL,
1.70 mmol, 1.1 equiv.) was added dropwise at 0 ◦C (ice bath). After 30 min at 0 ◦C, HgCl2 (0.418 g,
1.54 mmol) was added as a solid. The reaction mixture was stirred at room temperature for 6 h. The
solvent was removed under vacuum, and the crude residue was dissolved in dichloromethane (40 mL).
The mixture was filtered through a pad of Celite, and the filtrate was evaporated under vacuum to
obtain a yellow solid. The compound was further washed with hexane and vacuum dried (0.657 g,
59% yield). Mp: 250 ◦C. 1H NMR (CDCl3, 500.16 MHz, 298 K):
δ
7.51 (t, 2H, 3JHH = 8 Hz, C6H3), 7.33
(d, 4H, 3JHH = 8 Hz, C6H3), 2.85 (sept, 4H, 3JHH = 6.9 Hz, CH(CH3)2), 1.45 (s, 9H, C(CH3)3), 1.31 (d,
12H, 3JHH = 6.9 Hz, CH(CH3)2), 1.26 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3, 125.77
MHz, 298 K):
δ 179.9 (Hg-NCN), 159.1 (C-O), 146.1, 136.9, 131.6, 125.3, 104.1, 34.5, 30.0, 29.2, 24.4, 24.3.
IR (KBr) cm−1: 3069, 2990, 2863, 1644, 1465, 1410, 1387, 1365, 1345, 1325, 1301, 1257, 1215, 1181, 1148,
1111, 1056, 1014, 979, 936, 879, 797, 759, 749. Anal. Calcd. for C32H43N2O2HgCl: C, 53.11; H, 5.99; N,
3.87. Found: C, 53.41; H, 6.52; N, 3.87%.
3.1.5. Synthesis of (t-Bu-maloNHCDipp)HgMe (2c)
To a solution of 1,3-diisopropyl-5-tert-butyl-6-oxo-6H-pyrimidinium-4-olate (0.376 g, 0.77 mmol)
in THF (20 mL), KHMDS (0.5 M in toluene, 1.92 mL, 0.96 mmol, 1.25 equiv.) was added dropwise at 0 ◦C
(ice bath). After 30 min at 0 ◦C, MeHgI (0.264 g, 0.77 mmol) was added as a solid. The reaction mixture
was stirred at room temperature for 12 h. The solvent was removed under vacuum, and the crude
residue was dissolved in dichloromethane (20 mL). The mixture was filtered through a pad of Celite,
and the filtrate was evaporated under vacuum to obtain a yellow solid. The compound was washed
1
with hexane and vacuum dried (0.380 g, 70% yield). H NMR (CDCl3, 500.16 MHz, 298 K):
δ
7.43 (t,
2H, 3JHH = 8 Hz, C6H3), 7.27 (d, 4H, 3JHH = 7.5 Hz, C6H3), 2.88 (sept, 4H, 3JHH = 6.9 Hz, CH(CH3)2),
1.45 (s, 9H, C(CH3)3), 1.29 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2), 1.21 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2),
0.21 (s, 3H, CH3). 13C{1H} NMR (CDCl3, 125.77 MHz, 298 K):
δ 205.8 (HgNCN), 160.3 (C-O), 146.3,
136.2, 130.5, 124.6, 104.0, 34.5, 30.2, 29.0, 24.3, 24.2, 4.9 (HgCH3). Anal. Calcd. for C33H46N2O2Hg: C,
56.35; H, 6.59; N, 3.98. Found: C, 55.98; H, 6.37; N, 4.11%.
3.2. Crystallographic Data Collection and Refinement
A suitable crystal covered with a layer of hydrocarbon/Paratone-N oil was selected and mounted
with in a Cryo-loop, and immediately placed in the low-temperature nitrogen stream. Diffraction

Molecules 2020, 25, 3741
7 of 10
data were collected at T = 100(2) K. The data sets were collected on a Bruker SMART APEX II CCD
detector diffractometer with graphite monochromated Mo K radiation ( = 0.71073 Å). The cell
α
λ
parameters were obtained from a least-squares refinement of the spots (from 60 collected frames),
using the SMART program. Intensity data were processed using the Bruker ApexII program suite.
Absorption corrections were applied by using SADABS. Initial atomic positions were located by direct
methods using XS, and the structures of the compounds were refined by the least-squares method
using SHELXL [68]. All the hydrogen atoms were refined anisotropically. Hydrogen positions were
input and refined in a riding manner, along with the attached carbons. X-ray structural figures were
generated using Olex2 [69]. The CCDC 2019022–2019023 contains the supplementary crystallographic
data. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or
from the Cambridge Crystallographic Data Centre (CCDC) (12 Union Road, Cambridge, CB2 1EZ, UK).
Crystal data for (Me-maloNHCDipp)HgCl (1b): C58H74Cl2Hg2N4O4 (M = 1363.29 g/mol): triclinic,
space group P
1
(no. 2), a = 10.7845(8) Å, b = 13.9152(11) Å, c = 19.1697(15) Å,
α
= 106.9910(10)◦,
β = 91.2040(10)◦, γ = 90.0910(10)◦, V = 2750.5(4) Å3, Z = 2, T = 100.15 K,
µ(MoK
α
) = 5.721 mm−1
,
Dcalc = 1.646 g/cm3, 22,195 reflections measured (3.06◦
≤
2
Θ ≤ 50.996◦), 10,213 unique (Rint = 0.0257,
R
sigma = 0.0369), which were used in all calculations. The final R1 was 0.0313 (I > 2σ(I)) and wR2 was
0.0797 (all data).
Crystal data for (t-Bu-maloNHCDipp)HgCl (2b): C32H43ClHgN2O2 (M = 723.72 g/mol): triclinic,
space group P (no. 2), a = 10.9912(9) Å, b = 14.9118(13) Å, c = 19.9787(17) Å,
= 99.9700(10)◦,
β = 98.6110(10)◦, γ = 90.2430(10)◦, V = 3187.1(5) Å3, Z = 4, T = 100.15 K, ) = 4.942 mm−1
,
1
α
µ(MoK
α
Dcalc = 1.508 g/cm3, 27,908 reflections measured (2.774◦
≤
2
Θ ≤ 53.464◦), 13,462 unique (Rint = 0.0260,
R
sigma = 0.0407), which were used in all calculations. The final R1 was 0.0405 (I > 2σ(I)) and wR2 was
0.1067 (all data).
4. Conclusions
In summary, we report the isolation of three new mercury(II) complexes supported by anionic
maloNHC ligands. Solid state structures of 1b and 2b illustrate the effects of backbone substituent
in maloNHC, supplemented by the N-aryl substituent steric demands have on the aggregation of
these mercury complexes. The (Me-maloNHCDipp)HgCl (1b) bearing methyl substituent in the
malonate unit forms a polymeric mercury(II) complex, with a rare, distorted T-shaped structure,
whereas (t-Bu-maloNHCDipp)HgCl (2b) having a t-butyl substituent on the ligand backbone, remains
monomeric. The [t-Bu-maloNHCDipp]− is a sterically demanding and useful supporting ligand to
stabilize low-coordinate metal complexes.
Supplementary Materials: The following are available online. Crystal data, structure refinement, bond distances
and angles for (Me-maloNHCDipp)HgCl (1b) and (t-Bu-maloNHCDipp)HgCl (2b), additional figures, and NMR
spectra of new molecules.
Author Contributions: Conceptualization, H.V.R.D.; funding acquisition, H.V.R.D.; Data curation, C.D., A.D.;
Methodology, C.D. and A.D.; writing and editing, C.D. and H.V.R.D.; All authors have read and agreed to the
published version of the manuscript.
Funding: This research was supported by the Robert A. Welch Foundation (Grant Y-1289).
Acknowledgments: We thank Muhammed Yousufuddin for collecting the X-ray data.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Hopkinson, M.N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014
510, 485–496. [CrossRef] [PubMed]
,
2.
D
íez-González, S.; Marion, N.; Nolan, S.P. N-Heterocyclic carbenes in late transition metal catalysis. Chem. Rev.
2009, 109, 3612–3676. [CrossRef] [PubMed]

Molecules 2020, 25, 3741
8 of 10
3.
4.
Doddi, A.; Peters, M.; Tamm, M. N-Heterocyclic carbene adducts of main group elements and their use as
ligands in transition metal chemistry. Chem. Rev. 2019, 119, 6994–7112. [CrossRef] [PubMed]
Zhao, Q.; Meng, G.; Nolan, S.P.; Szostak, M. N-Heterocyclic carbene complexes in C–H activation reactions.
Chem. Rev. 2020, 120, 1981–2048. [CrossRef] [PubMed]
5.
6.
Peris, E. Smart N-heterocyclic carbene ligands in catalysis. Chem. Rev. 2017, 118, 9988–10031. [CrossRef]
Wang, W.; Cui, L.; Sun, P.; Shi, L.; Yue, C.; Li, F. Reusable N-heterocyclic carbene complex catalysts and
beyond: A perspective on recycling strategies. Chem. Rev. 2018, 118, 9843–9929. [CrossRef]
Cheng, J.; Wang, L.; Wang, P.; Deng, L. High-Oxidation-State 3d metal (Ti–Cu) complexes with N-heterocyclic
carbene ligation. Chem. Rev. 2018, 118, 9930–9987. [CrossRef]
Weskamp, T.; Böhm, V.P.; Herrmann, W.A. N-Heterocyclic carbenes: State of the art in transition-metal-complex
synthesis. J. Organomet. Chem. 2000, 600, 12–22. [CrossRef]
Herrmann, W.A.; Elison, M.; Fischer, J.; Köcher, C.; Artus, G.R.J. Metal complexes of N-heterocyclic
carbenes—A new structural principle for catalysts in homogeneous catalysis. Angew. Chem. Int. Ed. 1995, 34,
2371–2374. [CrossRef]
7.
8.
9.
10. Hahn, F.E.; Jahnke, M.C. Heterocyclic carbenes: Synthesis and coordination chemistry. Angew. Chem. Int. Ed.
2008, 47, 3122–3172. [CrossRef]
11. Herrmann, W.A. N-Heterocyclic carbenes: A new concept in organometallic catalysis. Angew. Chem. Int. Ed.
2002, 41, 1290–1309. [CrossRef]
12. Nasr, A.; Winkler, A.; Tamm, M. Anionic N-heterocyclic carbenes: Synthesis, coordination chemistry and
applications in homogeneous catalysis. Coord. Chem. Rev. 2016, 316, 68–124. [CrossRef]
13. Peris, E.; Crabtree, R.H. Recent homogeneous catalytic applications of chelate and pincer N-heterocyclic
carbenes. Coord. Chem. Rev. 2004, 248, 2239–2246. [CrossRef]
14. Mercs, L.; Albrecht, M. Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal,
luminescent, and functional materials applications. Chem. Soc. Rev. 2010, 39, 1903–1912. [CrossRef]
15. Zou, T.; Lok, C.-N.; Wan, P.-K.; Zhang, Z.-F.; Fung, S.-K.; Che, C.-M. Anticancer metal- N-heterocyclic carbene
complexes of gold, platinum and palladium. Curr. Opin. Chem. Biol. 2018, 43, 30–36. [CrossRef]
16. Ibáñez, S.; Poyatos, M.; Peris, E. N-heterocyclic carbenes: A door open to supramolecular organometallic
chemistry. Acc. Chem. Res. 2020, 53, 1401–1413. [CrossRef]
17. Santini, C.; Pellei, M.; Gandin, V.; Porchia, M.; Tisato, F.; Marzano, C. Advances in copper complexes as
anticancer agents. Chem. Rev. 2013, 114, 815–862. [CrossRef]
18. Aihara, H.; Matsuo, T.; Kawaguchi, H. Titanium N-heterocyclic carbene complexes incorporating an
imidazolium-linked bis(phenol). Chem. Commun. 2003, 17, 2204–2205. [CrossRef]
19. Arduengo, A.J., III; Dolphin, J.S.; Gurău, G.; Marshall, W.J.; Nelson, J.C.; Petrov, V.A.; Runyon, J.W. Synthesis
and complexes of fluoroalkoxy carbenes. Angew. Chem. Int. Ed. 2013, 52, 5110–5114. [CrossRef]
20. Arnold, P.L.; Mungur, S.A.; Blake, A.J.; Wilson, C. Anionic amido N-heterocyclic carbenes: Synthesis of
covalently tethered lanthanide–carbene complexes. Angew. Chem. Int. Ed. 2003, 42, 5981–5984. [CrossRef]
21. Arnold, P.L.; Rodden, M.; Wilson, C. Thermally stable potassium N-heterocyclic carbene complexes with
alkoxide ligands, and a polymeric crystal structure with distorted, bridging carbenes. Chem. Commun. 2005
13, 1743–1745. [CrossRef] [PubMed]
,
22. Arnold, P.L.; Scarisbrick, A.C. Di- and trivalent Ruthenium complexes of chelating, anionic N-heterocyclic
carbenes. Organometallics 2004, 23, 2519–2521. [CrossRef]
23. Asay, M.; Donnadieu, B.; Baceiredo, A.; Soleilhavoup, M.; Bertrand, G. Cyclic (amino)[bis(ylide)]carbene as
an anionic bidentate ligand for transition-metal complexes. Inorg. Chem. 2008, 47, 3949–3951. [CrossRef]
[PubMed]
24. Hameury, S.; de Frémont, P.; Breuil, P.-A.R.; Olivier-Bourbigou, H.; Braunstein, P. Synthesis and
characterization of palladium(II) and nickel(II) alcoholate-functionalized NHC complexes and of mixed
nickel(II)–lithium(I) complexes. Inorg. Chem. 2014, 53, 5189–5200. [CrossRef] [PubMed]
25. Ketz, B.E.; Cole, A.P.; Waymouth, R.M. Structure and reactivity of an allylpalladium N-heterocyclic carbene
enolate complex. Organometallics 2004, 23, 2835–2837. [CrossRef]
26. Legault, C.Y.; Kendall, C.; Charette, A.B. Structure and reactivity of a new anionic N-heterocyclic carbene
silver(i) complex. Chem. Commun. 2005, 30, 3826–3828. [CrossRef]

Molecules 2020, 25, 3741
9 of 10
27. Spencer, L.P.; Beddie, C.; Hall, M.B.; Fryzuk, M.D. Synthesis, reactivity, and DFT studies of tantalum
complexes incorporating diamido-N-heterocyclic carbene ligands. facile endocyclic C
Am. Chem. Soc. 2006, 128, 12531–12543. [CrossRef]
−
H bond activation. J.
28. Forster, T.D.; Krahulic, K.E.; Tuononen, H.M.; McDonald, R.; Parvez, M.; Roesler, R. A
σ
-donor with a planar
six-
Chem. Int. Ed. 2006, 45, 6356–6359. [CrossRef]
29. Vujkovic, N.; C sar, V.; Lugan, N.; Lavigne, G. An ambidentate janus-type ligand system based on fused
carbene and imidato functionalities. Chem. Eur. J. 2011, 17, 13151–13155. [CrossRef]
sar, V.; Lugan, N.; Lavigne, G. A Stable Anionic N-heterocyclic carbene and its zwitterionic complexes. J.
Am. Chem. Soc. 2008, 130, 11286–11287. [CrossRef]
31. Bastin, S.; Barthes, C.; Lugan, N.; Lavigne, G.; C sar, V. Anionic N-heterocyclic carbene complexes of gold(I) as
precatalysts for silver-free cycloisomerization of enynes. Eur. J. Inorg. Chem. 2015, 13, 2216–2221. [CrossRef]
sar, V.; Barthes, C.; Farr , Y.C.; Cuisiat, S.V.; Vacher, B.Y.; Brousses, R.; Lugan, N.; Lavigne, G. Anionic and
π-electron B2N2C2 framework: Anionic N-heterocyclic carbene or heterocyclic terphenyl anion? Angew.
é
30.
Cé
é
32.
C
é
é
zwitterionic copper(I) complexes incorporating an anionic N-heterocyclic carbene decorated with a malonate
backbone: Synthesis, structure and catalytic applications. Dalton Trans. 2013, 42, 7373–7385. [CrossRef]
[PubMed]
33. Hobbs, M.G.; Knapp, C.J.; Welsh, P.T.; Borau-Garcia, J.; Ziegler, T.; Roesler, R. Anionic N-heterocyclic carbenes
with N,N0-bis(fluoroaryl) and N,N0-bis(perfluoroaryl) substituents. Chem. Eur. J. 2010, 16, 14520–14533.
[CrossRef] [PubMed]
34. Chotard, F.; Romanov, A.S.; Hughes, D.L.; Linnolahti, M.; Bochmann, M. Zwitterionic mixed-carbene coinage metal
complexes: Synthesis, structures, and photophysical studies. Eur. J. Inorg. Chem. 2019, 2019, 4234–4240. [CrossRef]
35.
C
é
sar, V.; Lugan, N.; Lavigne, G. Electronic Tuning of a carbene center via remote chemical induction, and
relevant effects in catalysis. Chem. Eur. J. 2010, 16, 11432–11442. [CrossRef] [PubMed]
sar, V.; Misal Castro, L.C.; Dombray, T.; Sortais, J.-B.; Darcel, C.; Labat, S.; Miqueu, K.; Sotiropoulos, J.-M.;
36.
C
é
Brousses, R.; Lugan, N.; et al. (Cyclopentadienyl)iron(II) complexes of N-heterocyclic carbenes Bearing a
malonate or imidate backbone: Synthesis, structure, and catalytic potential in hydrosilylation. Organometallics
2013, 32, 4643–4655. [CrossRef]
37. Wanzlick, H.-W.; Schönherr, H.-J. Direct synthesis of a mercury salt-carbene complex. Angew. Chem. Int. Ed.
1968, 7, 141–142. [CrossRef]
38. Yu, M.-H.; Yang, H.-H.; Gu, Y.-C.; Wang, B.-H.; Liu, F.-C.; Lin, I.J.; Lee, G.-H. Formation of anionic NHC
complexes through the reaction of benzimidazoles with mercury chloride. Subsequent protonation and
transmetallation reactions. J. Organomet. Chem. 2019, 887, 12–17. [CrossRef]
39. Baker, M.V.; Brown, D.H.; Haque, R.A.; Simpson, P.V.; Skelton, B.W.; White, A.H.; Williams, C.C. Mercury
complexes of N-heterocyclic carbenes derived from imidazolium-linked cyclophanes: Synthesis, structure,
and reactivity. Organometallics 2009, 28, 3793–3803. [CrossRef]
40. Haque, R.A.; Budagumpi, S.; Choo, S.Y.; Choong, M.K.; Lokesh, B.E.; Sudesh, K. Nitrile-functionalized Hg(II)-
and Ag(I)- N-heterocyclic carbene complexes: Synthesis, crystal structures, nuclease and DNA binding
activities. Appl. Organomet. Chem. 2012, 26, 689–700. [CrossRef]
41. Budagumpi, S.; Haque, R.A.; Salman, A.W.; Ghdhayeb, M.Z. Mercury(II)- and silver(I)-N-heterocyclic
carbene complexes of CNC pincer-type ligands: Synthesis, crystal structures and Hofmann-type elimination
studies. Inorg. Chim. Acta 2012, 392, 61–72. [CrossRef]
42. Budagumpi, S.; Endud, S. Group XII metal– N-heterocyclic carbene complexes: Synthesis, structural diversity,
intramolecular interactions, and applications. Organometallics 2013, 32, 1537–1562. [CrossRef]
43. Scheele, U.J.; Dechert, S.; Meyer, F. Bridged dinucleating N-heterocyclic carbene ligands and their double
helical mercury(II) complexes. Inorg. Chim. Acta 2006, 359, 4891–4900. [CrossRef]
44. Lee, K.-M.; Chen, J.C.; Lin, I.J. Helical mono and dinuclear mercury(II) N-heterocyclic carbene complexes. J.
Organomet. Chem. 2001, 617, 364–375. [CrossRef]
45. Liu, Q.-X.; Yin, L.-N.; Feng, J.-C. New N-heterocyclic carbene silver(I) and mercury(II) 2-D supramolecular
layers by the π–π stacking interactions. J. Organomet. Chem. 2007, 692, 3655–3663. [CrossRef]
46. Haque, R.A.; Salman, A.W.; Guan, T.S.; Abdallah, H.H. New N-heterocyclic carbene mercury(II) complexes:
Close mercury–arene interaction. J. Organomet. Chem. 2011, 696, 3507–3512. [CrossRef]
47. Liu, Y.; Wan, X.; Xu, F. An arene
5590–5592. [CrossRef]
−mercury(II) N-heterocyclic carbene complex. Organometallics 2009, 28,

Molecules 2020, 25, 3741
10 of 10
48. Liu, Q.-X.; Yin, L.-N.; Wu, X.-M.; Feng, J.-C.; Guo, J.; Song, H.-B. New N-heterocyclic carbene mercury(II)
and silver(I) complexes. Polyhedron 2008, 27, 87–94. [CrossRef]
49. Adiraju, V.A.K.; Jin, W.; Yousufuddin, M.; Dias, H.V. Copper(I) complexes of anionic tridentate CNC pincer
ligands. Z. Anorg. Allg. Chem. 2020, 646, 215–219. [CrossRef]
50. Dash, C.; Wang, G.; Muñoz-Castro, A.; Ponduru, T.T.; Zacharias, A.O.; Yousufuddin, M.; Dias, H.V.R.
Organic azide and auxiliary-ligand-free complexes of coinage metals supported by N-heterocyclic carbenes.
Inorg. Chem. 2019, 59, 2188–2199. [CrossRef]
51. Wang, G.; Pecher, L.; Frenking, G.; Dias, H.V. Vinyltrifluoroborate complexes of silver supported by
N-heterocyclic carbenes. Eur. J. Inorg. Chem. 2018, 2018, 4142–4152. [CrossRef]
52. Adiraju, V.A.K.; Yousufuddin, M.; Dias, H.V. Copper(I), silver(I) and gold(I) complexes of N-heterocyclic
carbene-phosphinidene. Dalton Trans. 2015, 44, 4449–4454. [CrossRef]
53. Dash, C.; Yousufuddin, M.; Cundari, T.R.; Dias, H.V.R. Gold-mediated Expulsion of dinitrogen from organic
azides. J. Am. Chem. Soc. 2013, 135, 15479–15488. [CrossRef] [PubMed]
54. Dash, C.; Das, A.; Yousufuddin, M.; Dias, H.V.R. Isolable, copper(I) dicarbonyl complexes supported by
N-heterocyclic carbenes. Inorg. Chem. 2013, 52, 1584–1590. [CrossRef] [PubMed]
55. Dash, C.; Kroll, P.; Yousufuddin, M.; Dias, H.V.R. Isolable, gold carbonyl complexes supported by
N-heterocyclic carbenes. Chem. Commun. 2011, 47, 4478–4480. [CrossRef] [PubMed]
56. Arduengo, A.J.; Dias, H.V.R.; Harlow, R.L.; Kline, M. Electronic stabilization of nucleophilic carbenes. J. Am.
Chem. Soc. 1992, 114, 5530–5534. [CrossRef]
57. Arduengo, A.J.; Davidson, F.; Dias, H.V.R.; Goerlich, J.R.; Khasnis, D.; Marshall, W.J.; Prakasha, T.K. An Air
stable carbene and mixed carbene “dimers”. J. Am. Chem. Soc. 1997, 119, 12742–12749. [CrossRef]
58. Arduengo, A.J.; Dias, H.V.R.; Dixon, D.A.; Harlow, R.L.; Klooster, W.T.; Koetzle, T.F. Electron distribution in
a stable carbene. J. Am. Chem. Soc. 1994, 116, 6812–6822. [CrossRef]
59. Dias, H.V.R.; Jin, W. A stable tridentate carbene ligand. Tetrahedron Lett. 1994, 35, 1365–1366. [CrossRef]
60. Arduengo, A.J.; Dias, H.V.R.; Calabrese, J.C.; Davidson, F. Homoleptic carbene-silver(I) and carbene-copper(I)
complexes. Organometallics 1993, 12, 3405–3409. [CrossRef]
61. Buron, C.; Stelzig, L.; Guerret, O.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Synthesis and structure of
1,2,4-triazol-2-ium-5-ylidene complexes of Hg(II), Pd(II), Ni(II), Ni(0), Rh(I) and Ir(I). J. Organomet. Chem.
2002, 664, 70–76. [CrossRef]
62. Arduengo, A.J., III; Harlow, R.L.; Marshall, W.J.; Prakasha, T.K. Investigation of a mercury(II) carbene
complex: Bis(1,3-dimethylimidazol-2-ylidene) mercury chloride. Heteroat. Chem. 1996, 7, 421–426. [CrossRef]
63. Tschinkl, M.; Schier, A.; Riede, J.; Gabbaï, F.P. Complexation of DMF and DMSO by a monodentate
organomercurial lewis acid. Organometallics 1999, 18, 2040–2042. [CrossRef]
64. Pelz, S.; Mohr, F. “Oxide route” for the preparation of mercury(II) N-heterocyclic carbene complexes.
Organometallics 2011, 30, 383–385. [CrossRef]
65. Bumbu, O.; Silvestru, C.; Gimeno, M.C.; Laguna, A. New organomercury(II) compounds containing
intramolecular N Hg interactions: Crystal and molecular structure of [2-(Me2NCH2)C6H4]HgCl and
→
[2-(Me2NCH2)C6H4]Hg[S(S)PPh2]. J. Organomet. Chem. 2004, 689, 1172–1179. [CrossRef]
66. Kolychev, E.L.; Portnyagin, I.A.; Shuntikov, V.V.; Khrustalev, V.N.; Nechaev, M.S. Six- and seven-membered
ring carbenes: Rational synthesis of amidinium salts, generation of carbenes, synthesis of Ag(I) and Cu(I)
complexes. J. Organomet. Chem. 2009, 694, 2454–2462. [CrossRef]
67. Leung-Toung, R.; Wentrup, C. Flash vacuum pyrolysis of tert-butyl
β-ketoesters: Sterically protected
α-oxoketenes. Tetrahedron 1992, 48, 7641–7654. [CrossRef]
68. Sheldrick, G.M. A short history of SHELX. Acta Crystallogr. Sect. A Found. Crystallogr. 2007, 64, 112–122. [CrossRef]
69. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A complete structure
solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339–341. [CrossRef]
Sample Availability: Samples of the compounds are not available from the authors.
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).