Carbon nitride as a ligand: Edge-site coordination of ReCl(CO)3-fragments to g-C3N4

Article abstract of DOI:10.1039/c9cc02197c

IR spectroscopy and model structural studies show binding of ReCl(CO)₃-fragments to carbon nitride (g-C₃N₄) occurs via κ² N,N′ bidentate coordination.

Full text of DOI:10.1039/c9cc02197c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Carbon Nitride as a Ligand: Edge-Site Coordination of ReCl(CO)3-
fragments to g-C3N4
Ben Coulson,a Mark Isaacs,b Leonardo Lari,c Richard E. Douthwaite*a and Anne-K. Duhme-Klair *a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Direct coordination of ReCl(CO)3-fragments to carbon nitride (g- of these complex materials.5 Metal complexes have also been
C3N4) occurs via 2 N, N’ bidentate coordination at edge sites, and anchored to g-C3N4 using covalent linkers.3c, 6
NH2
IR spectroscopy of the carbonyl bands suggests that for ReCl(CO)3,
g-C3N4 acts as ligand donor comparable to a glyoxime.
N
N
N
N
N
N
N
N
N
N
N
N
N
NH2
N
NH2
N
N
N
N
N
N
N
N
N
O
, O2

The attachment of soluble metal complexes to a solid support
is a powerful strategy to combine the advantages of solution
and solid phase chemistry. For example, flow chemistry and
catalysis can benefit from a well-defined metal environment to
control activity and selectivity, which can be combined with
simple product separation, characteristic of an heterogeneous
system.1 The immobilisation of metal complexes on solid
supports is achieved using various methods, ranging from
simple adsorption to covalent linking of the metal complex to
the solid support.1-2 The most common solid supports by far are
metal oxides, but also metal halides and carbonaceous
materials have been investigated intensively. Of the latter class
of materials, carbon nitrides, primarily graphitic carbon nitride
(g-C3N4) have been of interest, as supports and catalysts for
light-driven redox catalysis.3
Structurally, g-C3N4 is characterised by repeating heptazine
moieties, which are condensed into chains and sheets,
depending on the synthesis conditions (Scheme 1).4 The
structure of g-C3N4 contains potential metal coordination sites
similar to motifs of N-heterocyclic ligands, both internally and
at edge sites (Scheme 1). Indeed the addition of metal ions and
atoms has been studied for a range of applications, although in
most cases detailed structural understanding of metal
coordination is limited due to the challenging characterisation

N
N
N
N
N
N
N
N
N
N
H2N
NH2
-NH3
-NH3
N
N
H2N
N
NH2
N
N
N
H
H
N
N
H
g-C3N4
melon
melem
Scheme 1. Structural motifs of carbon nitride arising from sequential condensation steps
We were motivated to study the direct coordination of
functional metal complex fragments to g-C3N4 with the ultimate
aim of studying light-driven and thermal catalytic applications
that may benefit from the established photoactivity and
acid/base behaviour of g-C3N4. Herein we describe the addition
of ReCl(CO)3 moieties to g-C3N4 and the characterisation of the
resulting material. Known molecular derivatives, such as
[ReCl(bpy)(CO)3] (bpy = 2, 2’-bipyridine) have attracted interest
for artificial photosynthesis and for the catalytic
photoreduction of CO2.7 Subsequent studies have found that
the replacement of bipyridine with, bulkier N-donor ligands
results in more stable active catalysts, because the steric bulk of
the ligand prevents the formation of catalytically-inactive
dimers.7b, 8 The distinct vibrational frequencies of the carbonyl
ligands allow their use as ‘reporter’ ligands for the identification
of catalytically-relevant carbonyl species using IR and Raman
spectroscopy.9
Steric considerations of g-C3N4 structural motifs (vide infra)
suggest that the direct coordination of metal complexes to g-
C3N4 is most likely to occur at the edge sites of g-C3N4 (Scheme
1), which contain both pyridyl and amino moieties. In addition,
to model direct coordination to g-C3N4, we have also
investigated coordination to the commercially available ligand
5, 7-dimethyl-[1, 8]-naphthyridin-2-amine (DMNA) (Scheme 2),
which contains a similar structural motif and hence has been
chosen as a model for g-C3N4 binding mode(s).10
a. Department of Chemistry, University of York, York, YO10 5DD, UK.
Email:richard.douthwaite@york.ac.uk
b. HarwellXPS, R92 Research Complex at Harwell, Rutherford Appleton Laboratories,
Harwell, Didcot, OX11 0QS, UK, and Department of Chemistry, University College
London, 20 Gordon Street, London, WC1H 0AJ, UK.
c. Department of Physics, University of York, Heslington, York, YO10 5DD, UK.
† Electronic Supplementary Information (ESI) available: Experimental details and
additional characterisation of all compounds and materials. CCDC 1897421. See
DOI: 10.1039/x0xx00000x
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The preparation of g-C3N4 was achieved using a literature
procedure via the thermal degradation of urea (see
experimental section in the ESI†).11 Characterisation by powder
X-ray diffraction (PXRD), elemental analysis, solid state 13C NMR
and scanning electron microscopy (SEM) (see details in the ESI†)
is consistent with that reported for g-C3N4 comprised primarily
of the melon structural motif (Scheme 1). The reaction between
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DOI: 10.1039/C9CC02197C
o
[ReCl(CO)5] and g-C3N4 in toluene at 80 C gave [ReCl(CO)3(g-
C3N4)] as a yellow solid. PXRD of [ReCl(CO)3(g-C3N4)] (Fig. S1,
ESI†) shows broad peaks at 13.1 and 27.4° 2θ, corresponding to
an interlayer spacing of 0.325 nm for the aromatic heptazine
units.12 The data are indistinguishable from g-C3N4 indicating
that intercalation of a metal complex has not occurred. A
comparison of the SEM images of [ReCl(CO)3(g-C3N4)] with
those of g-C3N4 (Fig. S2, ESI†) confirms that the reaction with Reaction between [ReCl(CO)5] and DMNA in toluene at 80 C
[ReCl(CO)5] does not significantly change the morphology of g- gave [ReCl(CO)3(DMNA-κ2N, N’)] (Scheme 1). 1H and 13C NMR,
C3N4. The rhenium loading of [ReCl(CO)3(g-C3N4)] was mass spectrometry, and elemental analysis data are consistent
determined by ICP-MS and chlorine loading by combustion with the proposed formulation (see ESI†). A single crystal
analysis, giving 7.35 and 1.17 wt %, respectively, which indicates diffraction study of [ReCl(CO)3(DMNA-κ2 N, N’)] confirms the
Fig. 1 XPS spectra a) C 1s [ReCl(CO)3(g-C3N4)]; b) C 1s g-C3N4.
o
a Re : Cl ratio of 1:0.85.
formulation and the molecular structure (Fig. 2). The geometry
X-ray photoelectron spectroscopic (XPS) characterisation of about the rhenium atom is pseudo-octahedral with facial
[ReCl(CO)3(g-C3N4)] and g-C3N4 produced very similar N 1s carbonyl ligands, as commonly observed for complexes of
spectra (Fig. S3a and b, ESI†), consistent with urea-derived g- ReCl(CO)3(NN) (where NN = bidentate N-donor ligand). The
C3N4.13 In contrast, the C 1s spectra show significant differences DMNA ligand is coordinated to rhenium via both [1, 8]-
with an additional signal at 286.6 eV observed for [ReCl(CO)3(g- naphthyridineyl nitrogen atoms N1 and N2, whilst the adjacent
C3N4)], consistent with carbonyl ligands (Fig. 1a and b).14 The Re amine group is uninvolved in metal coordination. This binding
4f spectrum of [ReCl(CO)3(g-C3N4)] (Fig. S3c, ESI†) shows two mode is also observed for several other DMNA complexes
peaks at 42.2 and 44.6 eV, fitted to Re 4f7/2 and 4f5/2 signals, including [MCl2(DMNA)2] (where M = Mn, Co and Ni). 17b-d
respectively. The peak width (fwhm = 1.25 eV) and binding
For [ReCl(CO)3(DMNA)], the Re-N bond lengths are 2.212(2)
energies are consistent with a single rhenium(I) environment.15 and 2.218(2) Å, compared to 2.168(7) and 2.173(8) Å,
Collectively, the PXRD and XPS data indicate that the respectively, for [ReCl(bpy)(CO)3].19 The N1-Re1-N2 bite angle
coordination of the rhenium moiety most likely occurs at the of [ReCl(CO)3(DMNA)] is 60.41(8)°, which lies within the range
edge sites of g-C3N4. The insolubility of g-C3N4 presents 54.6-62.6o found for other κ2 N, N’ complexes of DMNA, but as
challenges for the direct determination of the exact metal expected, is smaller than observed for [ReCl(bpy)(CO)3]
coordination environment. Hence, to model the coordination of (74.6(3)°). Other metrical data are enumerated in the ESI†.
metal fragments to g-C3N4, the commercially available ligand, 5,
Comparison of the IR spectra of g-C3N4, [ReCl(CO)3(g-C3N4)],
7-dimethyl-[1 ,8]-naphthyridine-2-amine (DMNA) was selected, and [ReCl(CO)3(DMNA-κ2 N, N’)] allow the identification of
because DMNA contains both [1, 8]-naphthyridine and primary carbonyl-containing metal fragments and support structural
amine functionalities, analogous to the heptazine moieties at assignment. The solid state ATR-IR spectra of g-C3N4 and
the edges of g-C3N4. Comparison of the characterisation data, [ReCl(CO)3(g-C3N4)] (Fig. S4, ESI†) show that the bands assigned
in particular the infrared spectra (vide infra) of [ReCl(CO)3(g-
C3N4)] and [ReCl(CO)3(DMNA)], could provide insight into the
coordination modes. Reported DMNA complexes exhibit a
range of binding motifs including monodentate N-
heterocyclic,16 2-N, N’-heterocyclic,16-17 and 3 N, N’, N’’-
heterocyclic/amido coordination.18
N
N
N
N
N
Toluene
N2, 80oC
[ReCl(CO)3(g-C3N4)]
[ReCl(CO)5]
+
N
N
N
H2N
N
H
Heptazine (g-C3N4)
Toluene
N2, 80oC
NH2
[ReCl(CO)5]
+
H3C
[ReCl(CO)3(DMNA)]
N
N
Fig. 2 Molecular structure of [ReCl(CO)3(DMNA-κ2 N, N’)]. Thermal ellipsoids are at 50 %
DMNA
probability level and hydrogen atoms have been omitted for clarity.
Scheme 2: Synthesis of [ReCl(CO)3(g-C3N4)] and [ReCl(CO)3(DMNA).
2 | J. Name., 2012, 00, 1-3
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and shape of the CO bands of [ReCl(CO)3(g-C3N4)] resemble
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DOI: 10.1039/C9CC02197C
2
those of [ReCl(CO)3(DMNA-κ N, N’)], thereby indicating that a
facial (fac) Re(CO)3 moiety is present in [ReCl(CO)3(g-C3N4)] (Fig.
3). The higher energy carbonyl bands of [ReCl(CO)3(g-C3N4)]
compared to those of [ReCl(CO)3(DMNA-κ2 N, N’)] reveal a
lower electron density at the metal centre, suggesting either
that g-C3N4 donates less electron density than the DMNA ligand,
or that g-C3N4 is a very strong π-acceptor. To investigate the
ligand properties and binding strength of g-C3N4 further,
[ReCl(CO)3(g-C3N4)] was stirred for 3 hours in solvents of varying
metal-coordinating ability, followed by centrifugation and
subsequent IR analysis of both the supernatant and the solid
residue (Fig. S6 and S7, ESI†). The IR spectra show that in
strongly-coordinating solvents, such as MeCN and ethanol,
significant leaching occurs, indicating that g-C3N4 is a weak
donor ligand. Other ReCl(CO)3 complexes with common N-
donor ligands such as triazole,21 imidazole,22 and imines,23 show
carbonyl bands at lower energies broadly similar to those
obtained for [ReCl(bpy)(CO)3] (Table 1). However, similar CO
band positions are observed for the weak κ2 N, N’ donor ligand
dimethylglyoxime (DMG), in the complex [ReCl(CO)3(DMG)]
which exhibits CO bands at 2032, 1938, 1916 cm-1,24 similar to
those of [ReCl(CO)3(g-C3N4)] (Table 1).
Fig. 3 Infrared spectra of [ReCl(CO)3(g-C3N4)] and [ReCl(CO)3(DMNA)].
Table 1 Infrared spectroscopic data associated with [ReCl(CO)3(g-C3N4)] and related
compounds.
Material (IR mode)
[ReCl(CO)5] (solution, DCM)
[ReCl(CO)3(DMNA)] (solution, DCM)
[ReCl(CO)3(bpy)](solution, DCM)9a
[ReCl(CO)3(DMNA)] (ATR)
νCO / cm-1
1986, 2047
1899, 1916, 2025
1899, 1921, 2024
1886 (br), 2018
1916 (br), 2034
[ReCl(CO)3(g-C3N4)] (ATR)
Collectively, the data suggest that direct coordination of
metal complex fragments to g-C3N4 can occur, with g-C3N4
acting as a bidentate N-donor ligand. Spectroscopic evidence
and the leaching observed in coordinating solvents indicate that
g-C3N4 is a weak donor with electronic properties that resemble
those of a bidentate glyoxime ligand. On a solid support, steric
interactions between the support and a metal complex moiety
will be significant. Comparison between the size of a ReCl(CO)3
moiety (ca. 0.55 nm diameter), estimated from the crystal
structure of [ReCl(CO)3(DMNA-κ2N, N’)], and the interplanar
distance of 0.325 nm for g-C3N4, determined from PXRD,
suggest ReCl(CO)3 could only bind to exposed edge sites. Non-
covalent interactions will likely prevent coordination at plane
edge sites of g-C3N4 with coincident plane termination or
stepped planes (Fig. 4).
To directly image and analyse the distribution of ReCl(CO)3
moieties in [ReCl(CO)3(g-C3N4)], (Scanning)Transmission
Electron Microscopy (S)TEM with Energy Dispersive X-ray (EDX)
was used High Angle Annular Dark Field (HAADF) imaging (Fig. 5
and Fig. S9, ESI†) shows the concentration of bright features
along edges of g-C3N4 particles, and EDX (Fig. S10, ESI†) shows
that Re and Cl are co-located in these regions. At the greatest
magnification (Fig. S11, ESI†), some isolated bright features 200
pm in diameter can be identified, which are larger than
to carbon nitride are unchanged as a result of metal complex
coordination, presumably due to the low loading. Nevertheless,
[ReCl(CO)3(g-C3N4)] shows strong vibrational bands in the
carbonyl region, with a broad band centred at 1916 cm-1 and a
sharper band at 2034 cm-1. Using diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS) (Fig. S5, ESI†), which
has higher surface sensitivity, the metal-carbonyl vibrational
band intensity increases compared to g-C3N4, indicating that the
carbonyl-containing species is concentrated at the surface,
which is consistent with the PXRD and XPS data.
The solution IR spectrum of [ReCl(CO)3(DMNA-κ2 N, N’)] in
dichloromethane (Fig. 3) shows three carbonyl bands at 1899,
1916 and 2025 cm-1, respectively, as expected for an octahedral
rhenium tricarbonyl species with facial geometry, assigned to
the totally symmetric in-phase A’(1) vibration, the equatorially
asymmetric A’’ and totally symmetric out-of-phase A’(2) modes,
20
respectively.9b,
These data can be compared to other
complexes containing bidentate nitrogen ligands including
[ReCl(bpy)(CO)3], which show bands at very similar frequencies
(Table 1), suggesting that the ligand donor properties of bpy and
DMNA are very similar.9a In the solid state, the vibrational bands
of [ReCl(CO)3(DMNA-κ2 N, N’)] broaden, causing an overlap of
the two lower energy bands observed in solution and all bands
are shifted to lower energy by 6-13 cm-1 (Table 1). For
[ReCl(CO)3(g-C3N4)], comparing the number and position of CO
vibrational bands with the precursor [ReCl(CO)5] confirms that
simple physisorption of [ReCl(CO)5] onto g-C3N4 has not
occurred; instead the data are consistent with direct
coordination. Due to the insolubility of [ReCl(CO)3(g-C3N4)] in
common solvents, a solution IR spectrum of [ReCl(CO)3(g-C3N4)]
could not be obtained. However, in the solid state the number
Fig. 4. Illustration of ReCl(CO)3 coordination to g-C3N4.
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and Cl; b) higher magnification of the region within the red box of a)
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In conclusion, direct coordination of metal complex
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