conditions.2 Note that this difference between Co and Zn is in
agreement with recent DFT calculations, which predict that the
metallation has an activation barrier for the late transition metals
Zn and Cu, but not for Co, Fe and Ni.7
In a second experiment, we reversed the order of deposition of
the two reactants, i.e., we first deposited zinc atoms (hZn = 0.11,
i.e., three times the stoichiometric amount) and then approxi-
mately one monolayer of H2TPP. As in the first experiment, the
reaction is not complete at 300 K, but can be induced by heating to
550 K. Evidence for the metallation is again provided by the
disappearance of the two N 1s signals of H2TPP, which are
replaced by a single peak at 398.6 eV (Fig. 2B). This peak position
is almost identical to that of a monolayer of ZnTPP that was
directly deposited on the Ag surface as a reference (398.7 eV). (The
very small difference of 0.1 eV may be attributed to a work
function change caused by some residual Zn atoms on the Ag
surface). We conclude that H2TPP is able to ‘‘pick up’’ and
coordinate Zn atoms from the Ag(111) surface. The respective Zn
2p3/2 XP spectra are displayed in Fig. 3. They confirm the
formation of ZnTPP by showing the higher binding energy of
the coordinated Zn ions (1022.0 eV) as compared to that of the
adsorbed Zn atoms (1021.6 eV). The Zn 2p3/2 spectra also show
that the excess of Zn atoms (those not coordinated by the
porphyrin molecules) has almost completely vanished from the
surface after heating, probably by diffusion into the bulk of the Ag
crystal. (Ag dissolves up to 40 atomic percent Zn at 550 K.8)
What are the driving forces for this unusual reaction?
Apparently, the sum of the bond energies of the four coordinate
bonds in ZnTPP exceeds the bond energy of Zn to the Ag(111)
surface. A value for the latter has not been reported yet. However,
it is probably similar to that of Zn on Cu(111), y150 kJ mol21, as
was estimated from temperature programmed desorption (TPD)
data.9 The bond energy of Zn in ZnTPP can be calculated from
the heats of combustion or formation of H2TPP and ZnTPP.
These values, however, are not available for ZnTPP. A recent
DFT calculation of the reaction between H2TPP and Zn atoms in
Fig. 3 Zn 2p3/2 spectra of (A) Zn on Ag(111) (hZn = 0.11) and (B) after
deposition of Zn (hZn = 0.11) followed by approximately one monolayer of
H2TPP and subsequent heating to 550 K. (C) Zn 2p3/2 spectrum of a
directly deposited monolayer of ZnTPP on Ag(111) for comparison.14
which is indeed much larger than our estimated Zn–surface bond
energy of y150 kJ mol21
.
We note that there is apparently no reaction between H2TPP
and Ag atoms of the substrate. Heating to higher temperatures
than needed to start the Zn metallation reaction (.600 K) leads, in
the absence of Zn, eventually to decomposition of H2TPP (as, for
example, seen by a shift and broadening of the C 1s signal), but no
metallation of the porphyrin with Ag is observed. This result is not
surprising, because metal ions in metalloporphyrins usually adopt
the +2 oxidation state (if no additional axial ligand is present),
which is chemically not favourable for Ag (although Ag2+
compounds do exist).10 This fact is also reflected by the gas phase
ionisation potentials of the two elements, 17.96 eV for Zn2+ and
the gas phase predicted a total reaction energy of 318 kJ mol21 7
,
21.48 eV for Ag2+ 11
.
In conclusion, we have demonstrated that adsorbed meso-
tetraphenylporphyrin molecules on an Ag(111) surface are able to
react with Zn atoms, which were deposited on this surface prior to
the adsorption of the porphyrin molecules, forming tetraphenyl-
porphyrinato-zinc(II). To our knowledge, this is the first example
of a reaction in which a large adsorbed organic ligand removes a
metal atom from a metal surface to coordinate and incorporate it
in a metal complex.
This work was supported by the Deutsche Forschungs-
gemeinschaft through SFB 583.
Notes and references
1 J. K. M. Sanders, N. Bampos, Z. Clyde-Watson, S. L. Darling,
J. C. Hawley, H.-J. Kim, C. C. Mak and S. J. Webb, in The Porphyrin
Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, San Diego, 2000.
2 J. M. Gottfried, K. Flechtner, A. Kretschmann, T. Lukasczyk and
H.-P. Steinru¨ck, J. Am. Chem. Soc., 2006, 128, 5644.
Fig. 2 N 1s XP spectra of (A) a monolayer of H2TPP on Ag(111) and
(B) after deposition of Zn (hZn = 0.11) followed by approximately one
monolayer of H2TPP and subsequent heating to 550 K. (C) N 1s spectrum
of a directly deposited monolayer of ZnTPP on Ag(111) for comparison.14
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 568–570 | 569