Angewandte
Chemie
Based on previous investigations describing the selective
metalation of free-base porphyrins by using an atomic beam
[
19,20]
of Co or Fe at 300 K,
in situ Co metalation of a submo-
nolayer of 2H-TPCN was achieved on Ag(111). Consistent
with these earlier findings, Figure 2d,e evidences the appear-
ance of a new species with a central rodlike protrusion that we
identify as a Co-TPCN derivative. Importantly, the layer
organization is preserved in this step, despite the known
affinity of CN groups to Co atoms favoring the formation of
metal-organic networks at room temperature with related
[
7,24]
linkers.
We attribute this behavior to the increased
stability of the preorganized supramolecular network struc-
[
25]
ture stabilized by multiple noncovalent interactions.
Next we explored the coordination aptitude of 2H-TPCN
towards Gd atoms, which were also evaporated onto a sub-
monolayer of 2H-TPCN held at 300 K. In marked contrast to
the fivefold coordination nodes formed by dicyano-oligophe-
[
26]
nylene ligands and Ce or Gd on Ag(111),
an extended
porous gridlike array is formed with a mononuclear square-
planar motif (Figure 2g,h). No porphyrin metalation inter-
feres, and the Gd centers appear as bright round protrusions
regardless of the bias polarity. A statistical analysis of the
Gd···NC distances gives a projected average bond length of
Figure 3. Lanthanide-directed assembly and orthogonal interactions
yield domains of a heterobimetallic porphyrin network comprising Gd
and Co centers in specific coordination environments. a) Overview
STM image of the d-f assembly (V =1 V and I =100 pA). Scale bar:
2
.7 Æ 0.5 ꢀ, in agreement with recent STM observations on
b
t
[27]
Gd-dicyano-oligophenylene units.
The Gd nodes span
1
0 nm. b) Data recorded at V =1 V with similar appearance of the
b
a square network with a unit cell vector c = 24.5 Æ 0.5 ꢀ.
The TPCN orientation with respect to the atomic lattice of the
close-packed substrate is identical in the noncoordinated and
coordinated cases. The absence of macrocycle metalation by
Gd is tentatively assigned to a higher activation energy
metalated and free-base porphyrins, each showing bright lobes local-
ized at the four meso positions (I =75 pA). c) At negative bias voltage
t
(
V =À1 V and I =75 pA) free-base and Co-metalated macrocycles
b
t
appear as ring-shaped cores and elongated protrusions along two
different orientations, respectively (highlighted by light blue and dark
blue contours, respectively). Gd atoms are visualized as round dots
irrespective of the applied bias voltage. b,c) Scale bar: 2 nm. d) Atom-
istic model of (c). C, N, H, Gd, and Co atoms are depicted in green,
blue, white, brown, and red, respectively.
[28]
barrier (as also observed in the liquid phase), the larger
geometric footprint and adsorption energy of 2H-TPCN.
Interestingly, structural analogues exist in the solid state,
where lanthanide centers can steer the expression of two-
[29]
dimensional fourfold-reticulated sheets in ab planes. The
propensity of 2H-TCPN linkers towards lateral coordination
also prevails with increased substrate temperatures up to
between the 2D coordination superlattice and the hexagonal
substrate.
5
00 K during Gd deposition, and the same two-dimensional
We discard the possibility of Co atoms being attached to
the lanthanide vertexes for the following reasons: 1) the
aspect and size of the rare-earth centers is equivalent, within
the heterogeneity, before and after Co deposition, 2) no
superstructure is found without any evidence of metalation.
Taking advantage of the affinity of the tetrapyrrolic
macrocycle for Co atoms and the selective formation of
fourfold Gd···NC interactions, we thus probed the fabrication
of heterometallic networks containing both Gd and Co atoms.
To this end, Co atoms were evaporated onto a 1:1 2H-TPCN/
Gd network with the substrate held at 300 K. The STM data
depicted in Figure 3a,b clearly reveal that the reticulated
network and the fourfold Gd···NC complexes are preserved
following Co deposition. Inspecting the architectures at
negative bias (see Figure S5a,b in the Supporting Information
for an overview and Figure 2c,d for high-resolution data)
emphasizes the presence of metalated species, namely the
appearance of coexisting ring-shaped and rodlike macro-
cycles that are assigned to the free-base and Co porphyrins,
respectively. Importantly, all the Co porphyrins are in perfect
registry with the lanthanide-directed coordination network,
with no structural deviations of the gridlike assembly. We also
tried to realize more perfect architectures, however this could
not be achieved with the present system. This limitation is
presumably related to the symmetry and lattice mismatch
[
30]
clustering is observed at the vertexes,
thus keeping the
structure of the coordination network intact, and 3) within the
experimental error, the node to node distance after Co-
metalation (35.1 Æ 0.7 ꢀ, see Figure S6c in the Supporting
Information) is identical to the Gd-Gd distance in the
nonmetalated architecture (34.5 Æ 0.6 ꢀ, see Figure S6b in
the Supporting Information). This value is significantly larger
than the Co-Co distance (32.4 Æ 0.9 ꢀ) in Co-directed TPCN
networks observed after annealing the sample at 348 K to
promote metal–organic coordination in the absence of Gd
(see Figures S6a and S7 in the Supporting Information), thus
reflecting the larger size of the lanthanide atoms.
Importantly, the order of metal deposition is a decisive
parameter in the assembly procedure. Upon exposure of
previously Co-metalated arrays to Gd atoms there is a strict
preservation of the close-packed Co-TPCN network, with less
than 1% of the termini coordinated to rare-earth atoms. We
tentatively attribute this behavior to a change in the surface
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
These are not the final page numbers!
Urgel, José I.
