Extremely bulky copper(i) complexes of [HB(3,5-{1-naphthyl}2pz)3]- and [HB(3,5-{2-naphthyl}2pz)3]- and their self-assembly on graphene

Article abstract of DOI:10.1039/c6dt04851j

The synthesis and characterization, using NMR (¹H and ¹³C), infrared spectroscopy, and X-ray crystallography, of the ethene and carbon monoxide copper(i) complexes of hydridotris(3,5-diphenylpyrazol-1-yl)borate ([Tp(^Ph)₂]^-) and the two new ligands hydridotris(3,5-bis(1-naphthyl)pyrazol-1-yl)borate ([Tp(^(1Nt))₂]^-) and hydridotris(3,5-bis-(2-naphthyl)pyrazol-1-yl)borate ([Tp(^(2Nt))₂]^-) are described. X-ray crystal structures are presented of [Cu(Tp(^Ph)₂)(C₂H₄)] and [Cu(Tp(^(2Nt))₂)(C₂H₄)]. The compound [Cu(Tp^Ph)₂)(C₂H₄)] features interactions between the protons of the ethene ligand and the π-electron clouds of the phenyl substituents that make up the binding pocket surrounding the copper(i) center. These dipolar interactions result in strongly upfield shifted signals of the ethene protons in ¹H-NMR. [Cu(Tp(^(1Nt))₂)(CO)] and [Cu(Tp(^(2Nt))₂)(CO)] were examined using infrared spectroscopy and were found to have CO stretching vibrations at 2076 and 2080 cm^-1 respectively. The copper(i) carbonyl complexes form self-assembled monolayers when drop cast onto HOPG and thin multilayers of a few nanometers thickness when dip coated onto graphene. General macroscopic trends such as the different tendencies to crystallize observed in the complexes of the two naphthyl-substituted ligands appear to extend well to the nanoscale where a well-organized monolayer could be observed of [Cu(Tp(^(2Nt))₂)(CO)].

Full text of DOI:10.1039/c6dt04851j

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H. M. de Bruijn, T. G. Brevé, B. van Meijeren, M. A. Siegler and E. Bouwman, Dalton Trans., 2017, DOI:
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Volume 45 Number
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January 2016 Pages 1–398
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Efficient extraction of sulfate from water using
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Page 1 of 34
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–
Extremely bulky copper(I) complexes of [HB(3,5-{1-naphthyl} pz) ]
2
3
View Article Online
DOI: 10.1039/C6DT04851J
–
and [HB(3,5-{2-naphthyl} pz) ] and their self-assembly on graphene.
2
3
a
a
a
a
Thomas F. van Dijkman, Hans M. de Bruijn, Tobias G. Brevé, Bob van Meijeren, Maxime A.
b
a*
Siegler and Elisabeth Bouwman
a, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA, Leiden, the
Netherlands. b, Department of Chemistry, Johns Hopkins University, MD 21218, Baltimore, USA.
1
13
Abstract. The synthesis and characterization using NMR ( H and C), infrared spectroscopy,
X-ray crystallography of the ethene and carbon monoxide copper(I) complexes of hydridotris(3,5-
Ph2 –
diphenylpyrazol-1-yl)borate ([Tp ] ) and the two new ligands hydridotris(3,5-bis(1-
(1Nt)2 –
naphthyl)pyrazol-1-yl)borate ([Tp
] ) and hydridotris(3,5-bis-(2-naphthyl)pyrazol-1-yl)borate
(2Nt)2 –
Ph2
([Tp
] ) are described. X-ray crystal structures are reported of [Cu(Tp )(C H )] and
2 4
(2Nt)2
Ph2
[
Cu(Tp
)(C H )]. The compound [Cu(Tp )(C H )] features interactions between the protons of
2
4
2
4
the ethene ligand and the π-electron clouds of the phenyl substituents that make up the binding
pocket surrounding the copper(I) center. These dipolar interactions result in strongly upfield shifted
1
(1Nt)2
(2Nt)2
signals of the ethene protons in H-NMR. [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] were examined
–1
using infrared spectroscopy and were found to have CO stretching vibrations at 2076 and 2080 cm
respectively. The copper(I) carbonyl complexes form self-assembled monolayers when drop cast
onto HOPG and thin multilayers of a few nanometers thickness when dip coated onto graphene.
General macroscopic trends such as the different tendencies to crystallize observed in the
complexes of the two naphthyl-substituted ligands appear to extend well to the nanoscale where a
(2Nt)2
well-organized monolayer could be observed of [Cu(Tp
)(CO)].
1

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Page 2 of 34
Introduction
The properties of graphene can be altered and augmented through the application of self-assembled
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DOI: 10.1039/C6DT04851J
[1-3]
monolayers (SAMs) of functional materials and insulators.
Examples include the
functionalization of graphene with metal-organic frameworks and electro-catalysts through π-
[
4-6]
stacking interactions with the catalysts or with appended anchors.
SAMs can alter the properties
of graphene by insulating the graphene from harmful chemicals, shielding the graphene from
electrical interference or inducing a field effect to produce a chemically actuated graphene field
effect transistor (GFET). Because every atom in the graphene lattice is exposed to the influence of a
SAM GFETs can be used as extremely sensitive chemical sensors that respond to analytes such as
ethene gas and carbon monoxide even in trace concentrations. Ethene gas is an important analyte in
agricultural settings as it induces physiological responses in crops such as fruits, vegetables and
flowers. In agricultural settings and particularly in nature, ethene is typically found in extremely
low concentrations (10–1000 ppb). Carbon monoxide is often monitored at similarly low
concentrations for safety purposes. The potential for sensitive detection of ethene and carbon
monoxide using SAM-functionalized GFETs in this regard prompted us to study various aspects of
such devices. In this work we use two new complexes as models for such sensitizers and study their
self-assembly on graphene surfaces to pave the way towards eventual integration of such
functionalized graphene surfaces into GFET gas sensors.
An interesting class of sensitizers for GFETs intended to interact with ethene and carbon monoxide
consists of copper(I) complexes of hydridotrispyrazolylborate ligands. Copper(I)
hydridotrispyrazolylborate complexes are a natural starting point for ethene sensing as they mimic
the structure of the active sites of many biological systems. Example of such biological systems
include hemocyanin, responsible for reversible dioxygen uptake in mollusks; fungal galactose
oxidase, an extracellular copper enzyme capable of catalyzing the oxidation of primary alcohols to
aldehydes and ETR-1, the proposed active site of copper(I) containing ethene-sensing membrane
[
7-9]
proteins in climacteric plants.
Copper(I) is well known for its ability to reversibly bind alkenes
2

Page 3 of 34
Dalton Transactions
in general and ethene in particular. Many examples of hydridotrispyrazolylborate copper(I)
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DOI: 10.1039/C6DT04851J
complexes have been reported, with ligands containing electron-withdrawing substituents like
[
10-13]
trifluoromethyl or nitro groups or bulky substituents such as phenyl or mesityl groups.
By
carefully selecting the substituents on the pyrazolyl rings the coordination pocket surrounding the
copper(I) center can be modified electronically and structurally allowing significant control over the
electron density on the copper(I) center and the reactivity of the corresponding ethene
[12]
complexes.
So far, the interaction of the graphene plane with other molecules typically has been studied using
essentially two-dimensional molecules like pentacene, pyrene and porphyrins while more
[
14]
structurally complex compounds are avoided. In order to study the interaction and potential for
self-assembly of three-dimensional complexes such as the aforementioned copper(I) compounds on
the graphene surface we set out to synthesize and study a number of copper(I)
hydridotrispyrazolylborate complexes that were optimized for stacking on graphene. Current
examples of non-covalent SAMs on graphene typically exploit the ability of graphene to bind to
adsorbates by π-stacking interactions. For this reason the copper(I) complexes devised for the
purpose were modified with naphthyl-substituents in the 3- and 5-positions of the pyrazole rings.
The inclusion of 1-naphthyl and 2-naphthyl substituents offered not only the π-stacking interactions
desired for self-assembly on graphene but also allowed us to study the influence of the position in
which the naphthyl groups are anchored to the hydridotrispyrazolylborate core on the physical and
chemical properties of the resulting complexes. In addition to their potential for self-assembly on
1
Nt2 –
2Nt2 –
graphene the copper(I) complexes of new ligands [Tp ] and [Tp ] are some of the most
sterically bulky copper(I) hydridotrispyrazolylborate complexes ever reported. Other examples of
i
–
extremely bulky hydridotrispyrazolylborate ligands include [HB(3-Ad-5- Pr) ] containing the
3
–
–
extremely bulky adamantly group; [HB(3-{1-1Nt}) ] and [HB(3-{1-2Nt}) ] containing naphthyl
3
3
–
substituents on the 3 positions of the pyrazolyl groups; [HB(3-CF -5-{1Nt}) ] and [HB(3-CF -5-
3
3
3
–
–
{
2Nt}) ] which combine naphthyl groups and trifluoromethyl groups and [HB(3-Ms) ] comprising
3 3
3

Dalton Transactions
Page 4 of 34
[
10, 15-19]
the highly bulky mesityl group.
The complexes described in this work have the unusual
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property that they include a pair of the bulky substituents in both the 3 and 5-positions of the
pyrazolyl groups. The inclusion of a bulky naphthyl-substituent on the 3-position forces the
naphthyl-substituents on the 3-positions towards each other which further constricts the already
crowded coordination pocket around the copper.
A less desirable aspect of the inclusion of the naphthyl-substituents is that it comes at the cost of
excluding more strongly electron withdrawing substituents such as trifluoromethyl groups that are
typically included to stabilize the copper(I) center against oxidation in air. To gain some initial
Ph2 –
Ph2 –
insight we used the structurally comparable ligand [Tp ] as our starting point. [Tp ] is a ligand
that has been used in biomimetic models of active sites of proteins and enzymes containing
[
7, 20-28]
Ph2 –
manganese, iron, nickel, cobalt and copper.
The copper chemistry of [Tp ] includes
Ph2
Ph2
Ph2
relevant examples such as [Cu(Tp )(CO)] (which is air-stable), [Cu(Tp )] , [Cu (Tp ) (µ-O )],
2
2
2
2
Ph2
Ph2
[7, 22, 29]
[
[
Cu(Tp )Cl] and [Cu(Tp )(S CNEt)].
Preliminary results indicated that the complex
2
Ph2
Cu(Tp )(C H )] fortunately is air-stable, indicating that this might also be the case for the
2
4
Ph2
naphthyl-substituted complexes. As to the best of our knowledge the compound [Cu(Tp )(C H )]
2
4
has not been reported to date; it is included in this work as it can serve as a useful reference for the
naphthyl-substituted complexes.
Ph2 –
2Nt2 –
1Nt2 –
Scheme 1. The ligands studied in this work. a [Tp ] , b [Tp ] , c [Tp ] , –[N-N] represents
the third substituted pyrazole ring.
Ph2 –
1Nt2 –
2Nt2 –
In this work the copper(I) chemistry of the three ligands [Tp ] , [Tp ] and [Tp ] (see
1
Nt2 –
2Nt2 –
Scheme 1) is explored including the synthesis of the new ligands [Tp ] and [Tp ] and the
4

Page 5 of 34
Dalton Transactions
characterization of the chemical properties of their ethene and CO complexes. Finally, the self-
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DOI: 10.1039/C6DT04851J
assembly on flat carbon surfaces of graphene and HOPG of the air-stable complexes
(1Nt)2
(2Nt)2
[
Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] is described.
Results
Syntheses of the ligands
Ph2
(2Nt)2
The ligands KTp and NaTp
were synthesized by slowly heating to reflux a small excess of
Ph2
the respective pyrazoles with KBH or NaBH in high-boiling solvents. The synthesis of KTp has
4
4
been reported previously by Kitajima et al. who employed a solventless reaction in molten 3,5-
[
7]
diphenylpyrazole. Using the polar aprotic solvent 1,2,4-trichlorobenzene allowed us to use lower
reaction temperatures and resulted in a considerably higher yield, even after an additional
(1Nt)2
purification step in which a byproduct insoluble in acetone is removed. KTp
was synthesized
by heating KBH in molten 3,5-di(1-naphthyl)pyrazole, as it has a relatively low melting point
4
(2Nt)2
(approx. 154 °C). For NaTp
the boiling point of 1,2,4-trichlorobenzene was found to be
insufficient for full conversion to the trisubstituted ligand. The more polar 1,3-
dimethyltetrahydropyrimidin-2(1H)-one (N,N’-dimethylpropyleneurea, DMPU, b.p. 246 °C) was
employed instead.
In general it is our observation that polar aprotic solvents such as N,N-dimethylacetamide (b.p.
1
65 °C), 4-methylanisole (b.p. 174 °C), 1,2,4-trichlorobenzene (b.p. 214 °C), 1,3-dimethyl-2-
imidazolidinone (b.p. 225 °C) and DMPU are excellent solvents for the synthesis of
hydridopolypyrazolylborate ligands due to their polar properties and high boiling points. The exact
choice of solvent depends on the solubility of the pyrazole and the alkali tetrahydridoborate in the
hot solvent with reactions proceeding faster, more homogeneously and more completely in solvents
capable of completely dissolving the reagents when hot. Not much difference was noted in the use
of NaBH and KBH except that NaBH appears to dissolve more readily in the hot organic solvents
4
4
4
leading to a reduction in reaction times. The products were isolated by distilling off the solvents in
vacuo and washing the solids with diethyl ether to remove remaining pyrazole. In the case of
5

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Page 6 of 34
(1Nt)2
KTp
the impure product was obtained as a thick liquid from which the pure product could be
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DOI: 10.1039/C6DT04851J
isolated by adding a small amount of acetonitrile causing the precipitation of an acetonitrile adduct
of the potassium salt of the ligand.
An important consequence of using polar aprotic solvents like DMPU and acetonitrile is that they
can become incorporated in the final product as ligands coordinating the alkali metal ion. Typical
examples of coordinated solvents include small molecules such as acetone, THF, water and diethyl
CF3,Me
ether which can occur as bridging ligands in dinuclear species as found in [Na (Tp
) (µ-H O) ]
2
2
2
2
CF3,4-CF3Ph
(CF3)2
and [Na (Tp
) (µ-H O) ] or as capping ligands in species such as [Cu(Tp
)(Et O)],
2
2
2
2
2
CF3,Ph
CF3,Ph
[12, 30]
(1Nt)2
[
Cu(Tp
)(H O)] and [Na(Tp
)(THF)].
In the NMR spectra of KTp
·MeCN and
2
(2Nt)2
NaTp
·DMPU one equivalent of DMPU and acetonitrile respectively were observed. In addition
1
(1Nt)2
the signals for water were found to be significantly broadened in the H NMR spectrum of KTp
1
(2Nt)2
and to integrate to 2H in the H NMR spectrum of NaTp
which indicates that DMPU and
acetonitrile are not the only coordinated small molecules in the alkali salts of these ligands and that
water likely completes the first coordination sphere of the alkali ions.
Synthesis of the carbonyl and ethene copper(I) complexes
Ph2
The acetone adduct [Cu(Tp )(Me CO)] has been reported previously and was synthesized by a
2
Ph2
[7]
Ph2
salt metathesis of KTp and CuCl in acetone. Unfortunately [Cu(Tp )(Me CO)] was found to
2
be too unstable in air limiting its usefulness as a convenient intermediate for further complex
(1Nt)2
(2Nt)2
synthesis. When the same synthetic procedure was applied using KTp
or NaTp
in DCM
deep red solutions were obtained upon even the briefest exposures of their DCM solutions to air.
Although no effort was made to identify these red products they are assumed to be copper(II)
complexes resulting from oxidation upon loss of the acetone ligand indicating that the use of
(
1Nt)2 –
“acetone-capped intermediates” is inconvenient for such sterically demanding ligands as [Tp
]
(2Nt)2 –
(1Nt)2 –
(2Nt)2 –
and [Tp
] . In contrast, the copper(I) acetonitrile complexes of [Tp
] and [Tp
] were
found to have reasonable oxidative stability in the solid state and solution with no discoloration
being seen in the solid state after several weeks of exposure to air. Solutions of the acetonitrile
6

Page 7 of 34
Dalton Transactions
complexes in solvents such as benzene and dichloromethane discolored to green only after several
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hours exposure to air. A downside of the use of acetonitrile as a capping ligand is that the
acetonitrile is difficult to remove using ligands such as CO and ethene because the acetonitrile-
adduct is always found in equilibrium with the other ligand and a small impurity of the acetonitrile
complex is always found in the final product.
For a more convenient synthesis of the complexes inspiration was found in the synthesis of the
Ph2
Ph2
carbonyl complex [Cu(Tp )(CO)] by a metathesis reaction between KTp and CuCl in DCM
[
7]
under 1 bar carbon monoxide as reported by Kitajima et al. We used a similar approach using CuI
(which, unlike CuCl, is air-stable) and higher pressures of carbon monoxide in steel autoclaves.
DCM is used as the solvent as KI and NaI are entirely insoluble in DCM which facilitates the work-
up. Using this approach undesired copper complexes are not formed; the by-products are insoluble
in DCM and typically the product can be purified simply through filtration and evaporation of the
solvent. The use of high pressure carbon monoxide is not strictly required but was merely
convenient as we had a high-pressure autoclave system available in which carbon monoxide could
(1Nt)2
be safely handled. For the synthesis of the ethene complexes [Cu(Tp
)(C H )] and
2 4
(2Nt)2
[
Cu(Tp
)(C H )] small excesses of CuI were stirred under 1 bar ethene atmospheres in the
2 4
presence of the sodium or potassium salts of the ligands in DCM.
Ph2
(Ph)2
To prepare [Cu(Tp )(C H )] ethene was bubbled through a clear solution of [Cu(Tp )] in THF;
2
4
2
as soon as the solution was exposed to ethene gas an equimolar mixture of KI and
Ph2
[
Cu(Tp )(C H )] precipitated. Filtration of the precipitate afforded a white air-stable powder
2 4
which was essentially insoluble in THF and diethyl ether and only slightly soluble in benzene,
chloroform and DCM. The KI impurity posed no hindrance for the intended analyses and thus no
Ph2
attempts were made to remove it. To grow X-ray quality crystals of [Cu(Tp )(C H )] without KI
2
4
we exploited the poor solubility of the complex by letting ethene gas diffuse slowly into a THF
Ph2
solution of KTp and CuI. As in this way the reaction proceeded considerably more slowly the
poorly soluble ethene complex and the KI byproduct had sufficient time to separate avoiding the
7

Dalton Transactions
Page 8 of 34
Ph2
inclusion of KI in the product. Compared to [Cu(Tp )(C H )] the ethene and carbonyl complexes
2
4
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DOI: 10.1039/C6DT04851J
(2Nt)2 –
of [Tp
] have slightly higher solubilities whereas the ethene and carbonyl complexes of
(1Nt)2 –
[
Tp
] are quite soluble in most solvents.
In solution in DCM or benzene the carbonyl complexes appear to be stable allowing for short
exposures to air without obvious discoloration. However, the ethene complexes are less stable in
solution and the color of the solutions changes to reddish brown after exposure to air for more than
(1Nt)2
a few minutes. Attempts to crystallize [Cu(Tp
)(C H )] from DCM/pentane resulted in the
2 4
formation of a large amount of crystals that were found to consist entirely of pure 3,5-di(1-
naphthyl)pyrazole. In the solid state all complexes are reasonably stable with respect to oxidation
allowing for short exposures to air without apparent decomposition.
Descriptions of the X-ray crystal structures
Ph2
X-ray quality crystals were obtained for the compounds [Cu(Tp )(C H )] and
2
4
(2Nt)2
[
Cu(Tp
)(C H )]. The crystallographic data for both crystal structures are provided in Table 1;
2 4
Ph2
relevant bond distances and angles are given in Table 2. X-ray quality crystals of [Cu(Tp )(C H )]
2
4
were grown by slow diffusion of ethene gas into a filtered THF solution prepared from an
Ph2
Ph2
equimolar mixture of KTp and CuI. A projection of the crystal structure of [Cu(Tp )(C H )] is
2
4
shown in Figure 1. The copper(I) center is coordinated in a distorted tetrahedral geometry by three
Ph2 –
2
nitrogen atoms of the [Tp ] ligand and an ethene ligand which is coordinated in the typical η
Ph2
fashion. [Cu(Tp )(C H )] crystallized in π-stacked chains of head-to-tail rows of complexes with
2
4
the phenyl-rings alternatingly rotated clockwise and anticlockwise. The compound crystallized in a
trigonal space group with the copper and boron center located on an axis of threefold-rotational
symmetry. For refinement the ethene ligand was constrained to be distributed over three
orientations with occupancy factors of ⅓ each. The bond lengths Cu1–C1 and Cu1–C2 are 2.05(3)
and 2.02(3) Å respectively, which fall in the range of distances observed previously in similar
[11]
complexes. The ethene bond length C1–C2 of 1.381(18) Å is slightly longer than in free ethene
[
31]
Ms
(1.3384(10) Å).
The observed C1–C2 distance is comparable to those in [Cu(Tp )(C H )]
2 4
8

Page 9 of 34
Dalton Transactions
(
1.345(16) Å) and [(C H )Cu(µ-Tp)CuCl] (1.347(5) Å), but somewhat longer than the distances in
2 4 2
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The structure is
Me2
CF3,Ph
[11, 13, 32]
[
Cu(Tp )(C H )] (1.329(9) Å) and [Cu(Tp
)(C H )] (1.30(1) Å).
2 4
2
4
densely packed and features π-stacking interactions between the aromatic rings of adjacent
complexes. The protons of the ethene ligand and the carbon atoms in the surrounding phenyl rings
feature a number of short contacts with distances in the range of 2.7 – 3.0 Å. The phenyl rings are
rotated out-of-plane of the pyrazoles with torsion angles of 55° for the rings at the 3-positions and
5
1° for the rings in 5-positions.
Ph2
Figure 1. Displacement ellipsoid plot (50% probability level) of [Cu(Tp )(C H )] at 110(2) K.
2
4
Hydrogen atoms have been omitted and one of the arms of the trispyrazolylborate ligand is shown
as a wireframe projection for the sake of clarity, only one of the three orientations of the ethene
ligand is shown. Symmetry operation ‘ = [1–y, 2+x–y, z], symmetry operation for the wireframe
segment = [y–x–1, 1–x, z].
9

Dalton Transactions
Page 10 of 34
Ph2
(2Nt)2
(2Nt)2
Table 1. X-ray crystallographic data for [Cu(Tp )(C H )] and [Cu(Tp
)(C H )]
2 4
)(C H )]
2 4
DOI: 10.1039/C6DT04851J
2
4
Ph2
View Article Online
[
Cu(Tp )(C H )]
[Cu(Tp
2
6
4
Chemical formula
C H BCuN
2(C H BCuN )·3(DCM)
4
7
38
71 50 6
Mr
761.18
2377.81
Crystal system, space group
Trigonal, R3c:H
Monoclinic, C2/c
T (K)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
110
16.1559(3)
-
25.6568(6)
-
-
110
33.0952(9)
10.6204(3)
35.1414(10)
-
111.348(3)
-
11504.2(6)
120
5799.6(3)
3
V (Å )
Z
6
4
3
ρ (g/cm )
1.308
1.373
Radiation type
µ (mm )
Cu Kα
1.117
Cu Kα
2.21
-1
Crystal size (mm)
0.47 × 0.32 x 0.23
0.27 × 0.21 × 0.13
No. of measured, independent and
observed [I > 2s(I)] reflections
Rint
1
0907, 2459, 2452
34168, 11268, 10091
0.017
0.616
0.020, 0.054, 1.06
2459
190
27
0.026
0.617
0.035, 0.089, 1.04
11268
802
-1
(
sin θ/λ) (Å )
max
2
2
2
R[F > 2s(F )], wR(F ), S
No. of reflections
No. of parameters
No. of restraints
87
-3
residual e density (e Å )
0.15, –0.31
0.62, –0.62
2
2
2
2
o
2
w = 1/[s (F ) + (0.0381P) + 15.2884P] where P = (F + 2F )/3
o
c
Ph2
Table 2. Selected distances (Å) and angles (°) for [Cu(Tp )(C H )]
2
4
(2Nt)2
and [Cu(Tp
)(C H )].
2
4
Ph2
(2Nt)2
[
Cu(Tp )(C H )]
[Cu(Tp
)(C H )]
2 4
2
4
C1–C2
1.381(18)
2.02(3)
2.05(3)
2.0959(16)
1.356(3)
Cu1–C1
Cu1–C2
Cu1–N12
Cu1–N22
Cu1–N32
N12–Cu1–N22*
N22–Cu1–N32
N32–Cu1–N12
2.0379(15)
2.0626(16)
2.0720(13)
2.0216(13)
2.1440(13)
90.65(5)
89.66(5)
102.98(6)
3.052(2)
90.05(6)
Cu1···B1
3.066(1)
Ph2
*
For [Cu(Tp )(C H )] the angle N12–Cu–N12’ is given.
2 4
(2Nt)2
Colorless single crystals of [Cu(Tp
)(C H )] were obtained by diffusion of diethyl ether into a
2 4
DCM solution of the complex. The complex crystallizes in the space group C2/c with two
independent complex molecules and three solvent molecules in the asymmetric unit. The crystal
(2Nt)2
structure of [Cu(Tp
)(C H )] (Figure 2) shows a distorted tetrahedral copper(I) center
2 4
1
0

Page 11 of 34
Dalton Transactions
(2Nt)2 –
2
coordinated by three nitrogen atoms of the [Tp
] ligand and an η -coordinated ethene ligand.
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DOI: 10.1039/C6DT04851J
)(C H )] is found in one orientation sandwiched between two
2 4
(2Nt)2
The ethene ligand in [Cu(Tp
naphthyl groups. The Cu1–C1, Cu1–C2 and C1–C2 distances are 2.0379(15), 2.0626(16) and
(2Nt)2
1
.356(3) Å respectively. The Cu1–Cethene bond distances in [Cu(Tp
)(C H )] are comparable to
2 4
Ph2
those in [Cu(Tp )(C H )], while the C1–C2 bond is slightly shorter. The ethene ligand is oriented
2
4
approximately along the plane formed by Cu1, N21 and B1 and tilted away from the approximately
coplanar pyrazole ring. The angle between the axis Cu1-B1 and the centroid of the ethene ligand is
1
65°.
Figure 2. Displacement ellipsoid plot (50% probability level) of one of the independent molecules
(2Nt)2
[
Cu(Tp
)(C H )] at 110(2) K. Hydrogen atoms and solvent molecules were omitted for the sake
2 4
of clarity.
The dihedral angle between conjoined aromatic rings is typically ~45° as a result of the competition
[
33]
between steric and electronic effects. For instance, biphenyl has a dihedral angle of 44.4°.
In
(2Nt)2
[
Cu(Tp
)(C H )] the dihedral angles between the pyrazoles and the naphthyl groups are in the
2
4
range of 37°-52°. The deflections of the naphthyl rings away from the approximate 45° angle are
1
1

Dalton Transactions
Page 12 of 34
likely the effect of crystal packing interactions indicating relatively low rotational barriers in
View Article Online
DOI: 10.1039/C6DT04851J
[
33]
accordance with the low rotational barrier of 6.2(2.3) kJ/mol calculated for biphenyl. The crystal
packing is dominated by π-stacking interactions, but also includes a short contact of 3.05 Å between
the ethene ligand and a proton of a naphthyl group from a nearby complex.
NMR spectroscopy
The sodium or potassium salts of the ligands as well as the copper complexes were analyzed using
1
13
H and C NMR; relevant chemical shifts are collected in Table 3. The sodium and potassium salts
of the ligands are quite soluble in polar aprotic solvents like THF, acetone and acetonitrile and
somewhat soluble in DCM and chloroform. In protic solvents such as C D OD and CD OD
2
5
3
noticeable solvolysis of the B—H bond occurs within only a few minutes as evident from the
1
1
appearance of a new sharp peak in B NMR with a small downfield shift of 5–10 ppm compared to
1
1
the B signal of the original ligand. The sharpness of the peak observed after solvolysis indicates
the absence of J -coupling interactions and suggests that the initial reaction in the degradation
BH
pathway of the trispyrazolylborate ligands is protonation and subsequent solvolysis of the hydride
and not of the B–N bonds.
1
Ph2
(1Nt)2
(2Nt)2
The H-NMR spectra of [Cu(Tp )(C H )], [Cu(Tp
)(C H )] and [Cu(Tp
)(C H )] were
2 4
2
4
2
4
recorded in CD Cl , CDCl and C D . In all cases the spectra did not show the presence of free
2
2
3
6
6
Ph2
ethene. The ethene protons of [Cu(Tp )(C H )] are present as a sharp singlet at 3.53 ppm (CDCl ,
2
4
3
(2Nt)2
2
98 K). The ethene protons for [Cu(Tp
)(C H )] are located at 3.53 ppm in CDCl and THF-d
2 4 3 8
(1Nt)2
(
298 K) but at 3.78 ppm in C D (298 K). The ethene signal in [Cu(Tp
)(C H )] was observed at
2 4
6
6
the unexpectedly low chemical shifts of 2.64 ppm (C D , 297 K see Figure 3) and even at 2.15 ppm
6
6
(1Nt)2
in CD Cl (297 K). The surprisingly large shift for [Cu(Tp
)(C H )] even significantly exceeds
2 4
2
2
Ms
that reported for [Cu(Tp )(C H )] (2.72 ppm, CD Cl ), a complex featuring a somewhat similar
2
4
2
2
[
11]
coordination environment for the ethene molecule.
The unusually large upfield shifts of the
(1Nt)2
ethene protons in [Cu(Tp
)(C H )] is attributed to additional shielding by the naphthyl groups,
2
4
1
2

Page 13 of 34
Dalton Transactions
most likely enforced by restricted rotation of the naphthyl rings; the same explanation was offered
View Article Online
DOI: 10.1039/C6DT04851J
Ms
[11, 34]
for [Cu(Tp )(C H )].
2
4
1
(1Nt)2
Figure 3. Variable temperature H-NMR spectra of [Cu(Tp
)(C H )] in C D . The inset shows
2
4
6
6
1
3
the borohydride peak, the small singlet on top of the BH peak is a C-satellite peak of the DCM.
As may be expected, the upfield shifts are more pronounced than those observed in similar
CF3,Ph
complexes featuring strongly electron-withdrawing ligands such as [Cu(Tp
)(C H )] (4.91 ppm,
2 4
(CF3)2
[13]
CD Cl ) and [Cu(Tp
)(C H )] (4.96 ppm, CDCl ). More unexpected is that the shifts exceed
2 4 3
2
2
those observed in copper(I) complexes featuring electron-donating ligands like [Cu(Tp)(C H )] and
2
4
Me2
[32]
13
[
Cu(Tp )(C H )] (respectively 4.41 ppm and 4.43 ppm in CD Cl ). The C-shifts of the ethene
2 4 2 2
(1Nt)2
(2Nt)2
carbons in [Cu(Tp
)(C H )] and [Cu(Tp
)(C H )] in benzene-d were observed at 81.2 ppm
2 4 6
2
4
Ph2
and 81.9 ppm respectively which compare well with the signal observed for [Cu(Tp )(C H )]
2
4
(
81.6 ppm in CDCl ). The lack of a strong upfield shift in the carbon atoms of the ethene ligands
3
indicates that the cause of the upfield shifts of the proton signals is very local, which corroborates
the hypothesis that interaction between the ethene protons and the π-electrons of the aryl-
substituents on the ligands is responsible.
Ph2
(2Nt)2
(1Nt)2
In contrast to [Cu(Tp )(C H )] and [Cu(Tp
)(C H )] the ethene signal in [Cu(Tp
)(C H )]
2 4
2
4
2
4
is not observed as a sharp peak but rather as a broadened singlet which sharpens as the temperature
1
3

Dalton Transactions
Page 14 of 34
(1Nt)2
rises (see Figure 3). The peak broadening observed in [Cu(Tp
)(C H )] also extends to a number
2 4
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DOI: 10.1039/C6DT04851J
of the resonances for naphthyl protons, such as the prominent broad signal at ~5.5 ppm, indicating
restriction of movement of these protons in the complex. When the temperature was raised to 70 °C
the broad signals broadened further while the ethene signal sharpened indicating that the restricted
movement of the 1-naphthyl substituents does not become significantly less restricted at elevated
temperatures. A similar observation was reported by Rheingold et al. for the structurally
1
Nt –
1Nt
Me2
comparable ligand [Tp ] which in the complex [Co(Tp )(Tp )] produced NMR spectra aptly
[
17]
1
referred to as “baffling”.
The underlying cause for the complicated H-NMR spectrum of
1
Nt
Me2
[
Co(Tp )(Tp )] was found to be a lack of symmetry in the complex which is retained in
1
(1Nt)2
solution. The H-NMR spectrum of [Cu(Tp
)(C H )] is further complicated by the
2 4
decomposition of the ligand which releases free pyrazole upon hydrolysis of the B–N bonds.
1
13
R,R
Table 3. H and C NMR resonances of [Cu(Tp )(C H )] complexes (chemical shifts in ppm)* and
2
4
infrared CO stretching frequencies.
1
13
H NMR
C NMR
(ppm, L = C H )
123.20 (CD Cl )
ν(CO)
(cm , L = CO)
Ref.
–
1
(
ppm, L = C H )
2
4
2
4
[35]
Free C H
5.40 (CD Cl )
2
4
2
2
2 2
5
5
5
.40 (CDCl3)
123.13 (CDCl3)
122.96 (C D )
123.09 (THF-d8)
.25 (C D )
6
6
6
6
.36 (THF-d8)
Free CO
2143
2080
Ph2
[
Cu(Tp )(L)]
3.53 (CDCl3)
2.64 (C D , 297 K)
81.6 (CDCl3)
this work
[17]
(
1Nt)2
[
Cu(Tp
Cu(Tp
)(L)]
)(L)]
81.2 (C D , 343 K)
2076
2080
this work
6
6
6
6
2
2
.57 (C D , 343 K)
78.8 (CD Cl , 203 K)
6
6
2 2
.15 (CD Cl , 297 K)
2
2
(2Nt)2
[
[
3.53 (CDCl3)
81.91 (C D )
this work
6
6
3
3
.53 (THF-d8)
.78 (C D )
6
6
Ms
[11]
Cu(Tp )(L)]
2.72 (CDCl3)
.08 (C D )
77.4 (C D )
6 6
3
6
6
[
32, 36]
[
[
[
Cu(Tp)(L)]**
4.43 (CD Cl )
2083
2066
2103
2
2
Me2
[32, 36]
[13, 30]
Cu(Tp )(L)]
4.41 (CD Cl )
2 2
CF3,Ph
Cu(Tp
)(L)]
4.91 (CD Cl )
85.8 (CD Cl )
2
2
2
2
5
.20 (C D )
6
6
CF3
[13, 37]
[13, 38]
[
[
Cu(Tp )(L)]
4.98 (C D )
4
4.96 (CDCl3)
4
4
85.8 (C D )
2100
2137
6
6
6
6
.80 (CDCl3)
85.7 (CDCl3)
89.1 (CDCl3)
89.5 (C D )
(CF3)2
Cu(Tp
)(L)]
.98 (C D )
6 12
6
12
.94 (C D )
6
6
*
T = room temperature unless otherwise specified. ** [Cu(Tp)(C H )] was observed as
2 4
[
Cu(Tp)(C H )] ·[CuCl] .
2 4 2 2
1
4

Page 15 of 34
Dalton Transactions
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Initially the free pyrazole was assumed to be an impurity carried over from the ligand, but further
(1Nt)2
purification of the ligand did not result in pyrazole-free NMR spectra for [Cu(Tp
)(C H )]. This
2 4
(1Nt)2
observation is in agreement with the crystallization experiments with [Cu(Tp
)(C H )], which
2 4
resulted in the formation of crystals of the free pyrazole and confirm that even over long periods of
(1Nt)2
time [Cu(Tp
)(C H )] is unstable in solution even in the absence of air. Only when the
2 4
integration was corrected for the presence of free pyrazole satisfactory values were obtained.
1
(1Nt)2
Peak broadening is not seen in the H-NMR spectrum of [Cu(Tp
)(CO)] nor in
(2Nt)2
(1Nt)2
[
Cu(Tp
)(CO)]; clearly the structural asymmetry present in [Cu(Tp
)(C H )] is not present in
2
4
1
3
13
1
the carbonyl complexes. The CO signals were not observed on C{ H}-NMR, their absence is
attributed to peak broadening of the carbonyl signal. The absence of clear signals for the carbonyl
1
3
ligands is not unusual; unless C-labeled CO is used it is generally very difficult to observe these
[
39]
signals.
In all cases the hydride was observed as a broadened singlet around 4.5 ppm. The BH signals
1
0
showed no visible splitting patterns as a consequence of high quadrupole moments of the B (S =
1
1
3
) and B (S = 3/2) nuclei. Typically splitting of the BH peaks in polypyrazolylborate ligands and
their complexes is only observed if the local symmetry around the B nuclei is high, as asymmetry
increases the quadrupole moments of the boron nuclei. Though the BH signals in copper(I)
hydridotrispyrazolylborate complexes are a typical feature of such complexes they are rarely
[
12]
discussed in much detail.
Infrared spectroscopy
Previously we reported a series of tunable copper(I) hydridotrispyrazolylborate complexes which
showed a strong correlation between the CO stretching frequency and the electronic properties of
R,R
the copper(I) ion, a trend that has been observed for a wide range of [Cu(Tp )(CO)]-type
[12, 39-41]
(1Nt)2
complexes.
To gauge the electronic properties of the copper(I)-centers in [Cu(Tp
)(CO)]
(2Nt)2
and [Cu(Tp
)(CO)] IR spectra of solid samples were recorded; their CO-stretching frequencies
1
5

Dalton Transactions
Page 16 of 34
–
1
were found to be 2076 and 2080 cm respectively. This places the electronic properties of
View Article Online
DOI: 10.1039/C6DT04851J
)(CO)] close to the structurally similar compound
(1Nt)2
(2Nt)2
[
[
Cu(Tp
)(CO)] and [Cu(Tp
Ph2
–1
[42]
Cu(Tp )(CO)] for which a frequency of 2080 cm has been reported.
These CO stretching
frequencies are significantly lower than those reported for electron-poor copper carbonyl complexes
(CF3)2
CF3,Ph
–1
like [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] (2137 and 2103 cm ) and higher than the CO
Me2
[28, 36, 38]
frequency reported for an electron-rich carbonyl complex such as [Cu(Tp )(CO)].
The CO
(1Nt)2
(2Nt)2
stretching frequencies of [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] are therefore best considered to
indicate copper(I) centers that are neither particularly electron rich nor electron poor. The CO
stretching frequencies of a number of carbonyl copper(I) hydridotrispyrazolylborate complexes are
listed in Table 3.
Self-assembly on HOPG and graphene
To study the behavior of the carbonyl complexes on graphene surfaces two approaches were taken.
In the first approach we exploited the characteristic ability of carbon allotropes such as graphite and
amorphous carbon (e.g. charcoal) to adsorb dissolved compounds. Our aim was to adsorb the
carbonyl complexes as monolayers on the surface of graphene by immersing samples of graphene
(on silicon wafer substrates) in concentrated solutions of the carbonyl complexes in
(1Nt)2
dichloromethane. In the second approach a dilute (~0.5 µM) solution of either [Cu(Tp
)(CO)] or
(2Nt)2
[
Cu(Tp
)(CO)] was drop casted on freshly cleaved highly-oriented pyrolytic graphite (HOPG) in
the hope of forming domains of monolayers on the HOPG surface. The concentration of the
solutions was chosen so as to result in a less-than-complete surface coverage of the complexes on
the HOPG surface.
1
6

Page 17 of 34
Dalton Transactions
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DOI: 10.1039/C6DT04851J
Figure 4. a. Schematic exploded view of the graphene samples. CVD graphene is placed atop a
(Nt)2
silicon dioxide layer on a silicon wafer and covered in a layer of [Cu(Tp )(CO)]. b. Estimated
(1Nt)2
(2Nt)2
layer thicknesses as determined with ellipsometry of the [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)]
layers on graphene obtained by dip-coating in concentrated solutions of the complexes in DCM.
The layer thicknesses obtained by dip coating were estimated using ellipsometry. The obtained
layers were found to be of homogeneous thickness throughout each sample with only small
variations (see Figure 4b), the obtained layer thicknesses signify layers of 3–5 complex molecules.
(
1Nt)2
Ellipsometry could not be used for the samples prepared by drop casting of [Cu(Tp
)(CO)] and
)(CO)] as HOPG is not sufficiently flat. The HOPG samples and the graphene samples
were studied using atomic force microscopy (AFM). A clear difference was observed between the
(2Nt)2
[
Cu(Tp
(2Nt)2
two compounds in the HOPG samples. The sample drop casted with [Cu(Tp
)(CO)] showed
domain-like areas delineated by steps of approximately 10 Å high while the sample drop casted
(1Nt)2
with [Cu(Tp
)(CO)] showed no obvious surface structures (see Figure 6). The step height of the
(2Nt)2
domains corresponds with the expected height of a monolayer of [Cu(Tp
)(CO)]. The angles
between the domains are approximately 60° or 120° indicating that they are possibly aligned with
the underlying graphene lattice.
1
7

Dalton Transactions
Page 18 of 34
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DOI: 10.1039/C6DT04851J
(2Nt)2
Figure 5. Impression of the proposed stacking of the complex [Cu(Tp
)(C H )] onto graphene.
2
4
(2Nt)2
The X-ray crystal structure of [Cu(Tp
)(CO)] has not been determined but the compound is
assumed to have roughly the same dimensions.
(1Nt)2
(2Nt)2
Figure 6. AFM scans of [Cu(Tp
)(CO)] (a) and [Cu(Tp
)(CO)] (b) drop casted on HOPG.
The domains visible in (b) have lengths and widths of hundreds of nm and a step height of
approximately 10 Å corresponding to monolayer coverage.
(1Nt)2
(2Nt)2
The graphene samples dip coated with [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] did not show the
domains observed in the HOPG samples. Instead, fibrous structures were observed with average
heights that correspond well with the layer thicknesses determined using ellipsometry. The larger
(1Nt)2
structures on the dip-coated sample of [Cu(Tp
)(CO)] appear to align along similar axes, the
(2Nt)2
structures are much smaller than those observed in the dip-coated sample of [Cu(Tp
Figure 7).
)(CO)] (see
1
8

Page 19 of 34
Dalton Transactions
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(1Nt)2
(2Nt)2
Figure 7. AFM scans of dip-coated samples of [Cu(Tp
graphene.
)(CO)] (a) and [Cu(Tp
)(CO)] (b) on
As AFM does not normally offer atomic resolution scanning tunneling microscopy (STM) was used
to study the ordering on the surface in greater detail. To maximize the odds of observing self-
(1Nt)2
assembly on HOPG the use of the typically non-crystalline [Cu(Tp
)(CO)] was forgone in favor
(
2Nt)2
of [Cu(Tp
)(CO)]. In order to improve the ordering on the surface the solvent used to drop cast
)(CO)] was not DCM but cyclohexane. Cylcohexane offers the advantage of a lower
(
2Nt)2
[
Cu(Tp
rate of evaporation compared to DCM, which potentially leads to the formation of larger, more
(2Nt)2
ordered domains as the self-assembly of [Cu(Tp
)(CO)] has more time to proceed. Cyclohexane
was not used for dip coating experiments as the solubility of the complex in cyclohexane is quite
(2Nt)2
low, i.e. the dissolution of [Cu(Tp
)(CO)] in cyclohexane to form a 0.5 mM proceeded only very
slowly and a 1 mM solution could not be made due to saturation.
(2Nt)2
The domain of [Cu(Tp
)(CO)] on HOPG in the lower half of Figure 8b shows square-packed
regions. The maxima of the regions have a lattice constant of approximately 2.6 nm which
(2Nt)2
corresponds to slightly more than two widths of [Cu(Tp
)(CO)] (based on the crystal structure of
(
2Nt)2
[
Cu(Tp
consist of [Cu(Tp
rotated to accommodate π-stacking interactions with the graphene substrate.
)(C H )]) which would be 2.4 nm. We assume the regions that make up the domain to
2
4
(2Nt)2
)(CO)] molecules packed anti-parallel to each other with naphthyl groups
1
9

Dalton Transactions
Page 20 of 34
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(2Nt)2
Figure 8. STM scan of [Cu(Tp
)(CO)] on HOPG. In the lower half of the image a discrete
(2Nt)2
domain of self-assembled [Cu(Tp
in the top half of the image.
)(CO)] is clearly distinguishable from the bare HOPG planes
Discussion
We set out in this work to study the properties of copper(I) complexes of the new, extremely bulky
(1Nt)2 –
(1Nt)2 –
naphthyl-substituted hydridotrispyrazolylborate ligands [Tp
] and [Tp
] . The resulting
complexes were expected to have properties similar to those of structurally related complexes
Ph2
Ph2
Ms
[
Cu(Tp )(C H )], [Cu(Tp )(CO)] and [Cu(Tp )(C H )]. Instead, an apparent mismatch was
2 4 2 4
found between the observations from infrared spectroscopy and NMR. Whereas the IR spectra for
(1Nt)2
(2Nt)2
[
Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] conformed reasonably to expectations with CO
Ph2
1
frequencies close to those reported for [Cu(Tp )(CO)], the large upfield shifts in the H NMR
(1Nt)2
(2Nt)2
spectra of [Cu(Tp
)(C H )] and [Cu(Tp
)(C H )] for the ethene protons seemingly indicate
2 4
2
4
the copper(I) centers to be more electron rich. This apparent mismatch between the results from the
IR and NMR experiments is ascribed to shielding of the ethene protons by the the π-electrons of
Ph2
(2Nt)2
nearby aromatic groups. Indeed the crystal structures of [Cu(Tp )(C H )] and [Cu(Tp
)(C H )]
2 4
2
4
show the ethene protons to be in close proximity to the nearby π-systems. Further supporting this
explanation are the observations that the protons of the ethene ligand show as a broadened singlet in
1
(1Nt)2
the H-NMR spectrum of [Cu(Tp
)(C H )], whereas that peak broadening is essentially absent in
2 4
2
0

Page 21 of 34
Dalton Transactions
1
(2Nt)2
the H-NMR spectrum of [Cu(Tp
)(CO)] while it dominates the spectrum of
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DOI: 10.1039/C6DT04851J
(2Nt)2
(1Nt)2
[
Cu(Tp
)(C H )]. The peak broadening observed in the ethene signal in [Cu(Tp
)(C H )] is
2
4
2
4
likely the result of the limited ability of 1-naphthyl groups to rotate with respect to the adjacent
pyrazole plane, an effect that was observed before in other naphthyl-substituted
CF3,1Nt –
1Nt – [17]
hydridotrispyrazolylborate ligands such as [Tp
] and [Tp ] .
(1Nt)2 –
(2Nt)2 –
The influence of the conformational freedom of the [Tp
] and [Tp
] ligands extends
1
(2Nt)2 –
beyond electronic effects as observed in H-NMR spectra. Notably, the complexes of [Tp
]
show a pronounced tendency to crystallize and have low solubilities in most solvents while the
(1Nt)2 –
complexes containing the ligand [Tp
] show barely any tendency to crystallize. Although the
(
1Nt)2 –
complexes of [Tp
inability of complexes containing the ligand [Tp
from the formation of a number of different conformational isomers of the complexes due to the
] form solids (rather than oils) all attempts at crystallization failed. The
(1Nt)2 –
] to form well-defined crystals likely stems
(1Nt)2 –
limited rotational freedom of the naphthyl groups in the [Tp
] ligand.
This important difference between the naphthyl-substituted complexes reported in this work can be
observed even on the nanoscale in the self-assembled layers on HOPG and graphene using AFM
(2Nt)2
and STM. Whereas [Cu(Tp
)(CO)] formed clearly visible domains the surface of a similarly
(1Nt)2
prepared sample bearing [Cu(Tp
)(CO)] showed no evidence of ordered assembly. When
samples were prepared using dip coating the resulting layers were found to be quite even in
thickness forming layers of between three and five monolayers. The difference in the layer
(1Nt)2
(2Nt)2
thicknesses observed in the samples of [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] may have been
(1Nt)2
caused by the difference in the concentrations of the solutions of [Cu(Tp
)(CO)] and
(
2Nt)2
2Nt)2
[
[
Cu(Tp
Cu(Tp
)(CO)] used to dip the samples. Drop casting dilute solutions of the complexes
)(CO)] on HOPG was found to be an effective means by which to apply a monolayer.
(
(2Nt)2
The fact that discrete, ordered monolayers in drop-cast samples of [Cu(Tp
)(CO)] were
(2Nt)2
observed on STM indicates that for [Cu(Tp
)(CO)] on graphene Volmer-Weber growth does not
(2Nt)2
apply. The epitaxial growth on graphene of complex molecules such as [Cu(Tp
)(CO)] is
2
1

Dalton Transactions
Page 22 of 34
probably described best as Stranski-Krastanov growth in which the formation of two-dimensional
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DOI: 10.1039/C6DT04851J
and three-dimensional islands occur simultaneously after a certain number of monolayers has built
[
43]
up.
(1Nt)2
(2Nt)2
On AFM multilayer surfaces of [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] were observed with
undulating structures where both the thinner and thicker parts appeared to be amorphous. As the
multilayers have average thicknesses corresponding to approximately 3-5 monolayers the thinner
zones observed on AFM were likely only two or three monolayers in thickness which probably
(2Nt)2
places the critical layer thickness of the graphene/[Cu(Tp
)(CO)] composite at two or three
monolayers. The critical layer thickness is a result of lattice mismatch between the substrate and the
(2Nt)2
adsorbate which in the case of [Cu(Tp
)(CO)] adsorbed on graphene must be substantial as
(2Nt)2
graphene has a hexagonal lattice while [Cu(Tp
)(CO)] adsorbed in a square packing.
Conclusions
Ph2
(1Nt)2
(2Nt)2
In this work the complexes [Cu(Tp )(C H )], [Cu(Tp
)(C H )], [Cu(Tp
)(C H )],
2 4
2
4
2
4
(1Nt)2
(2Nt)2
[
Cu(Tp
)(CO)], and [Cu(Tp
)(CO)] were synthesized. The new naphthyl-substituted
(1Nt)2 –
(2Nt)2 –
hydridotrispyrazolylborate ligands [Tp
] and [Tp
] , constitute some of the bulkiest
hydridotrispyrazolylborate ligands reported to date. The ethene complexes were found to show
particularly strong shielding of the ethene protons in NMR spectra due to the limited rotational
freedom of the naphthyl groups combined with the steric pressure caused by the 3,5-disubstitution
of the pyrazole rings which forces the naphthyl groups in the 3-positions towards the ancillary
ligands. The conformational freedom of the naphthyl groups in the carbonyl complexes is
considerably larger than in the ethene complexes which indicates, together with the information
Ph2
(2Nt)2
gained from the X-ray crystal structures of [Cu(Tp )(C H )] and [Cu(Tp
)(C H )], that the
2 4
2
4
interactions between the π electron clouds of the naphthyl groups around the copper(I) center and
the ethene protons play an important role in the properties of the resulting ethene complexes. The
carbonyl complexes were used to prepare monolayers or thin multilayers on graphene and HOPG
and showed that a bulk property such as a pronounced ability or inability to crystallize on the
2
2

Page 23 of 34
Dalton Transactions
macroscale translates well to the nanoscale. The findings presented in this work offer insight into
View Article Online
DOI: 10.1039/C6DT04851J
the exciting chemistry and behavior of bulky, π-stacking copper(I) complexes that likely also apply
to other complex molecules incorporating π-stacking ligands and opens the way for further
application-oriented studies of such complexes in combination with graphene.
Experimental
General considerations
All manipulations of air-sensitive compounds were performed in an atmosphere of purified argon
gas using standard Schlenk techniques. All solvents were purchased from commercial sources and
were reagent grade. Solvents used for air-sensitive manipulations were dried and deaerated using a
PureSolv MD 5 Solvent Purification System and stored on 3 Å molecular sieves under argon. When
appropriate, glassware was flame dried in vacuo immediately prior to use. 3,5-Diphenyl-1H-
[
7]
1
13
pyrazole was synthesized following a literature procedure.
H and C NMR spectra were
1
13
recorded on a Bruker DPX300 spectrometer (300 MHz for H, 75.44 MHz for C), a Bruker
1
11
13
DMX400 spectrometer (400 MHz for H, 128.3 MHz for B and 100.6 MHz for C) or Bruker
1
13
Avance AV500 spectrometer (500 MHz for H and 126 MHz for C). Chemical shifts are given in
1
13 [35] 13
ppm and referenced using the deuterated solvents as internal references for H and C.
C
1
spectra were recorded using H-decoupling. Elemental analyses were performed by Mikrolab Kolbe
in Germany. IR spectra were recorded on a Perkin Elmer UATR Two FT-IR spectrometer set to a
–1
resolution of 2 cm . ESI-MS spectra were recorded on a Thermo Finnigan AQA ESI-MS system in
MeCN. HRMS spectra were recorded on a Thermo Scientific LTQ Orbitrap XL high resolution FT-
MS system in MeCN. AFM experiments were performed using a Nanoworld USC-FO.3-KO.3 tip
in a JPK NanoWizard 4a NanoScience AFM. STM experiments were performed using a freshly cut
Pt/Ir wire tip at 298 K. The samples of graphene on SiO @Si were prepared by wet-transfer of
2
[
44]
CVD graphene which was purchased from Graphenea.
2
3

Dalton Transactions
Page 24 of 34
X-ray crystallography
View Article Online
DOI: 10.1039/C6DT04851J
All reflection intensities were measured at 110(2) K using a SuperNova diffractometer (equipped
with Atlas detector) with Cu Kα radiation (λ = 1.54178 Å) under the program CrysAlisPro (Version
1
.171.36.32 Agilent Technologies, 2013). The same program was used to refine the cell dimensions
and for data reduction. The structure was solved with the program SHELXS-2013 and was refined
2
[45]
on F with SHELXL-2013.
Analytical numeric absorption correction based on a multifaceted
crystal model was applied using CrysAlisPro. The temperature of the data collection was controlled
using the system Cryojet (manufactured by Oxford Instruments). The H atoms were placed at
calculated positions (unless if specified) using the instructions AFIX 13 or AFIX 43 with isotropic
displacement parameters having values 1.2 or 1.5 times Ueq of the attached B or C atoms.
Ph2
[
Cu(Tp )(C H )]
2 4
The H atoms attached to the ethene molecule (C1/C2) were found from difference Fourier maps,
…
and the C−H and H H distances were restrained using the DFIX restraints.
(2Nt)2
[
Cu(Tp
)(C H )]·1.5DCM
2 4
The structure is mostly ordered. The lattice DCM solvent molecules are disordered over two
orientations. One of the two solvent molecules is found at sites of twofold axial symmetry, and its
occupancy factor is constrained to 0.5. The occupancy factor of the major component of the other
disordered solvent molecule refines to 0.913(5). The H atoms attached to B1, C1 and C2 were
found from difference Fourier maps, and their coordinates and isotropic temperature factors were
refined freely.
Sample preparation for AFM and STM
(1Nt)2
(2Nt)2
Solutions of [Cu(Tp
)(CO)] and [Cu(Tp
)(CO)] (0.5 mM in cyclohexane) were drop-casted
in drops of 5 µL on freshly cleaved HOPG mounted on steel sample holding plates. The samples
were left to dry in a stream of filtered argon for at least thirty minutes before being scanned. For the
(1Nt)2
(2Nt)2
samples that were dip coated 11 mg (10 µmol) of [Cu(Tp
)(CO)] or [Cu(Tp
)(CO)] were
2
4

Page 25 of 34
Dalton Transactions
(1Nt)2
added to 1 mL dry, degassed DCM. [Cu(Tp
)(CO)] dissolved completely while
View Article Online
DOI: 10.1039/C6DT04851J
(2Nt)2
[
Cu(Tp
)(CO)] did not, the solution was filtered (0.45 µm PTFE) before use. Samples of 288 nm
thick SiO2 on Si coated with CVD graphene were then immersed completely in the
solution/suspension for 10 minutes after which the samples were removed from the solutions and
thoroughly rinsed using a flow of DCM from a syringe (2 × 20 mL). The samples were left to dry in
air for at least 60 minutes before being used for ellipsometry and AFM. The STM sample was
(2Nt)2
prepared by drop casting 10 µL of a 0.5 µM solution of [Cu(Tp
)(CO)] in cyclohexane onto a
2
~
25 mm freshly cleaved HOPG surface. After drying in argon, the sample was dried further in a
stream of argon for 10 minutes and used immediately afterwards.
Syntheses
(
1Nt)2
3
,5-Di-(1-naphthyl)-1H-pyrazole (HPz
-Acetylnaphthalene (8.93 mL, 58.8 mmol) and ethyl 1-naphthoate (9.0 mL, 58.8 mmol) were
added simultaneously to a rapidly stirred solution of potassium tert-butoxide (15.0 g, 134 mmol) in
50 mL dry THF in a flame-dried 500 mL round bottom flask. The flask was fitted with an oven-
)
1
2
dried reflux condenser and a drying tube and the reaction was heated to reflux for 48 hours. The
THF was then removed in vacuo and 230 mL water, 20 mL 37% HCl and 250 mL diethyl ether
were added. The flask was agitated until two clear layers had formed which were separated. The
organic layer was washed with 1 M HCl (3 × 100 mL) and dried over anhydrous Na SO before
2
4
being filtered and evaporated to dryness to yield 1,3-bis(1-naphthyl)propane-1,3-dione as a yellow
1
solid of sufficient purity for further synthesis. Yield 18.3 g (56.4 mmol, 96%). H NMR (300 MHz,
CDCl ) δ 8.58 (d, J = 1.7 Hz, 2H), 8.11 – 7.85 (m, 8H), 7.59 (tt, J = 7.0, 5.1 Hz, 4H), 7.14 (s, 1H),
3
1
3
4
1
.91 (s, 1H). C NMR (75 MHz, CDCl ) δ 185.66, 135.47, 132.96, 132.90, 129.52, 128.63, 128.51,
3
28.29, 127.92, 126.94, 123.42, 93.91. 1,3-Di-(1-naphthyl)propane-1,3-dione (18.28 g, 56.4 mmol)
was suspended in 57 mL absolute ethanol, hydrazine hydrate (3.3 mL, 68.4 mmol) was added and
the mixture was refluxed overnight. The next morning the reaction was cooled to room temperature
and HCl (10 mL, 37%) was added, the mixture was heated to reflux for 10 minutes before being
2
5

Dalton Transactions
Page 26 of 34
poured into 600 mL aqueous Na CO solution (12 g, 143 mmol). The resulting slurry was extracted
2
3
View Article Online
DOI: 10.1039/C6DT04851J
with CHCl (3 × 200 mL), the combined organic fractions were washed with water (3 × 200 mL)
3
before being dried over anhydrous Na SO . The solution was evaporated to dryness in vacuo to
2
4
yield a brown oil from which the product was crystallized by adding 57 mL ethanol and water to
1
yield the product as a tan solid. Yield 9.03 g (28.2 mmol, 50%). M.p. = 153–155 °C. H NMR (300
MHz, CD Cl ) δ 10.73 (s, 1H), 8.49 (s, 2H), 8.00 – 7.85 (m, 4H), 7.75 (dd, J = 7.1, 1.0 Hz, 2H),
2
2
1
3
7
.56 (dt, J = 5.1, 2.8 Hz, 7H), 6.94 (s, 1H). C NMR (126 MHz, CDCl ) δ 147.12, 133.89, 131.32,
3
1
29.43, 128.83, 128.46, 127.20, 126.58, 126.03, 125.68, 125.30, 107.77, 77.16.
(2Nt)2
3
,5-Di(2-naphthyl)-1H-pyrazole (HPz
)
1
,3-Di-(2-naphthyl)propane-1,3-dione was prepared according to the same method used for 1,3-di-
(2-naphthyl)propane-1,3-dione using 2-acetylnaphthalene (5.0 g, 26.9 mmol) and methyl 2-
naphthoate (4.57 g, 26.9 mmol). All other reagents were used to scale. Yield 8.35 g (97%).
[
46]
Analyses were in agreement with those reported in the literature.
Hydrazine hydrate (1.5 mL,
3
0.8 mmol) was added to a suspension of 1,3-di(2-naphthyl)propane-1,3-dione (8.35 g, 26.0 mmol)
in 100 mL n-propanol and heated to reflux overnight. The following day the reaction mixture was
cooled to room temperature and 2.2 mL HCl 37% was added to complete the dehydration reaction.
The reaction was heated to reflux for 3 hours before being poured carefully into 250 mL saturated
sodium bicarbonate solution. The precipitate was filtered as a tan solid which was recrystallized
from ethanol with a small amount of water to yield the product as light-yellow flakes. Yield 5.23 g
1
(
16.3 mmol, 63%). M.p. = 243–245 °C. H NMR (400 MHz, DMSO-d ) δ 13.60 (s, 1H), 8.42 (s,
6
1
2
=
=
H), 8.24 – 7.79 (m, 8H), 7.64 – 7.41 (m, 5H). H NMR (500 MHz, C D ) δ 8.04 (s, 1H), 7.76 (d, J
6
6
8.0 Hz, 2H), 7.69 (d, J = 8.4 Hz, 2H), 7.66 (d, J = 8.0 Hz, 2H), 7.35 – 7.30 (m, 4H), 7.28 (ddd, J
1
3
8.0, 6.8, 1.4 Hz, 2H), 7.00 (s, 2H), 6.87 (s, 1H). C NMR (126 MHz, DMSO-d ) δ 151.40,
6
1
1
43.56, 133.27, 133.09, 132.54, 131.16, 128.72, 128.18, 128.00, 127.78, 127.68, 126.90, 126.77,
26.43, 125.86, 123.81, 123.59, 123.50, 123.44, 100.50.
2
6

Page 27 of 34
Dalton Transactions
(1Nt)2
Potassium hydridotris(3,5-di{1-naphthyl}pyrazol-1-yl)borate, (KTp
·MeCN·H O)
2
View Article Online
DOI: 10.1039/C6DT04851J
3
,5-Di(1-naphthyl)pyrazole (9.00 g, 28.1 mmol) and finely ground KBH (433 mg, 8.0 mmol) were
4
added to a flame-dried 100 mL round bottom flask fitted with an oven-dried reflux condenser and
stir bar and placed under argon using a Schlenk apparatus. The mixture was heated to 160 °C and
stirred until completely molten and homogeneous after which the melt was allowed to cool to room
temperature. The solidified reaction mixture was broken up and the stir bar was removed. The
mixture was again placed under argon and heated to 250 °C for 20 hours after which vacuum was
applied for seven days to remove the excess pyrazole by sublimation. The crude product was then
cooled to room temperature; the solids were dissolved in 50 mL acetone and the solution was
filtered through celite to remove an insoluble byproduct. The acetone was removed in vacuo and
replaced with 50 mL MeCN. Approximately half of the MeCN was evaporated in vacuo and the
solution was left to stand upon which a tan solid formed. After 30 minutes the resulting solids were
collected by filtration and washed with 2 × 10 mL MeCN. The product was then air dried to yield
1
the product as a tan solid (3.2 g, 39%, the filtrate contained additional impure product). H NMR
(
(
(
500 MHz, CD Cl ) δ 8.77 (s, 3H), 7.90 – 7.80 (m, 8H), 7.56 (t, J = 7.6 Hz, 6H), 7.48 (s, 3H), 7.39
2 2
t, J = 7.5 Hz, 6H), 7.15 (d, J = 27.7 Hz, 6H), 6.84 – 6.26 (m, 6H), 5.27 (s, 2H), 3.47 (bs, 1H), 1.94
s, 3H, MeCN), 1.46 (bs, 2H, H O). A satisfactory integration is only obtained when the range
2
7
.00–8.00 ppm is integrated in its entirety; significant peak broadening of some signals is the cause
1
3
as deduced from the integration and the heightened baseline. C NMR (126 MHz, acetone-d ) δ
6
1
1
51.23, 147.89, 135.17, 134.60, 134.07, 133.99, 132.87, 132.68, 129.10, 128.15, 127.96, 127.91,
1
3
27.41, 127.25, 126.96, 126.47, 126.37, 125.96, 125.85, 125.31, 117.65, 108.10, 1.10. C NMR
(
126 MHz, CD Cl ) δ 150.83, 147.70, 134.65, 133.48, 131.95, 131.76, 128.92, 127.82, 127.34,
2 2
1
27.07, 126.72, 126.53, 126.37, 126.14, 126.09, 125.72, 125.51, 124.62, 107.30, 2.06. HRMS (ESI
–
–
neg.) m/z calcd. for [M ] (=C H BN ) 969.38715 found 969.38715.
6
9
46
6
2
7

Dalton Transactions
Page 28 of 34
Ph2
Potassium hydridotris(3,5-diphenylpyrazol-1-yl)borate, (KTp )
View Article Online
DOI: 10.1039/C6DT04851J
3
,5-Diphenylpyrazole (28.1 g, 128 mmol) and KBH (2.079 g, 38.5 mmol) were suspended in 10
4
mL 1,2,4-trichlorobenzene and placed under argon using a Schlenk apparatus fitted with a H SO
2
4
bubbler. The reaction was heated to reflux (approx. 220 °C) with stirring for 17 hours upon which a
solid precipitated. The reaction mixture was cooled to room temperature and 100 mL petroleum
ether 40-60 was added. The resulting mixture was then stored overnight at – 20 °C before being
filtered. The residue was washed with boiling toluene (300 mL) to remove most of the unreacted
pyrazole and then dissolved in acetone and filtered over celite. The acetone was removed in vacuo
and the resulting product was washed with pentane (20 mL) and dried in vacuo to yield the product
as a white solid (17.7 g, 65%). The toluene filtrate contained some additional product. The analyses
[
4]
agreed with values reported by Kitajima et al.
Sodium hydridotris(3,5-di{2-naphthyl}pyrazol-1-yl)borate, (NaTp
,5-Di-(2-naphthyl)pyrazole (4.50 g, 14.0 mmol) and NaBH (158 mg, 4.20 mmol) were suspended
(2Nt)2
·DMPU )
3
4
in 18 mL dry, degassed DMPU in a flame-dried 100 mL round bottomed flask. The flask was fitted
with a glass pipe (~30 cm in length, NS29 adapters, oven dried) to serve as an air-cooled condenser.
The apparatus was placed under argon using a Schlenk apparatus fitted with a H SO bubbler to
2
4
release overpressure. The reaction mixture was stirred gently and heated to 180 °C until the bubbler
showed only limited bubbling (3 hours) before being heated to reflux (246 °C) overnight. The next
day the reaction mixture was cooled to 150 °C and the air-cooled condenser was replaced with a 90°
glass elbow fitted to a 250 mL 2-necked round bottomed flask which was connected to the Schlenk
apparatus. The DMPU was then removed in vacuo at 150 °C until completely dry. It appeared to be
very important to completely remove all solvent before proceeding. The remaining brown solids
were dissolved in acetone (50 mL) and filtered over celite. The celite cake was washed with acetone
(2 × 50 mL). The combined acetone fractions were evaporated to dryness in vacuo to yield the
crude product as a brown solid which was purified by repeated recrystallization from
1
DCM/petroleum ether to yield a light brown microcrystalline solid. Yield 3.0 g (72%). H NMR
2
8

Page 29 of 34
Dalton Transactions
(
500 MHz, CD Cl ) δ 8.34 (s, 3H), 8.02 (d, J = 8.5 Hz, 3H), 7.91 – 7.83 (m, 9H), 7.70 (s, 3H), 7.52
2 2
View Article Online
DOI: 10.1039/C6DT04851J
–
3
7.38 (m, 15H), 7.33 (t, J = 7.5 Hz, 3H), 7.03 (d, J = 8.4 Hz, 3H), 6.90 (s, 3H), 6.76 (d, J = 8.4 Hz,
1
3
H), 5.01 (bs, 1H), 2.64 (t, J = 5.8 Hz, 4H), 2.26 (s, 6H), 1.54 (p, J = 5.8 Hz, 2H). C NMR (126
MHz, CD Cl2) δ 156.44, 152.83, 151.68, 134.12, 133.18, 132.91, 132.70, 131.10, 128.50, 128.45,
2
1
4
9
28.41, 128.12, 127.92, 127.65, 127.03, 126.35, 126.09, 126.03, 125.97, 125.33, 124.75, 105.01,
–
–
7.58, 35.28, 21.77. HRMS (ESI neg.) m/z calcd. For [M ] (=C H BN ) 969.38715 found
69
46
6
69.38928.
Synthesis of the copper compounds
Ph2
[
Cu(Tp )(C H )]·KI
2 4
Potassium hydridotris(3,5-diphenylpyrazolyl)borate (360 mg, 0.508 mmol) and CuI (89 mg, 0.47
mmol) were dissolved in dry and degassed THF (10 mL) under argon. Ethene was bubbled through
the solution upon which immediately a white precipitate formed. The resulting suspension was
filtered in air; the residue was washed with n-pentane (2 × 5 mL) and dried in a stream of argon.
Ph2
This yielded [Cu(Tp )(C H )] mixed with an equimolar amount of KI as a white solid (yield 272
2
4
1
mg, 62%). H NMR (400 MHz, CDCl ) δ = 7.59 (d, J = 8 Hz, 6H), 7.38 (m, 9H), 7.23 (t, J = 7 Hz),
3
7
.03 (d, J = 8 Hz, 6H), 6.95 (t, J = 8 Hz, 6H), 6.36 (s, 3H,), 4.80 (bs, 1H, BH), 3.53 (s, 4H).
1
3
C{1H} NMR (100 MHz, CDCl ) δ = 152.8, 149.7, 134.8, 132.4, 130.0, 128.6, 128.1, 128.0,
3
1
27.7, 127.6, 105.6, 81.6 (ethene). Elemental analysis (%) calculated for C H BCuN ·KI (found):
4
7
38
6
–
1
–1
C 60.88 (61.21), H 4.13 (3.91), N 9.06 (9.08). IR ν (cm ): 2660 cm (B-H).
(1Nt)2
[
Cu(Tp
)(C H )]
2 4
(1Nt)2
KTp
(430 mg, 426 µmol) was dissolved in 15 mL DCM in a Schlenk flask (50 mL) and
bubbled with ethene for 5 minutes. The bubbling was interrupted and CuI (100 mg, 525 µmol) was
added. Bubbling was then resumed for another minute before the Schlenk flask was sealed. The
reaction mixture was stirred overnight during which the slightly off-white color of suspended CuI
disappeared and a fine white precipitate of KI formed. The next day stirring was halted and the
suspension was allowed to settle before the supernatant was transferred by cannulation through a
2
9