S.J. Langford et al. / Inorganic Chemistry Communications 8 (2005) 920–923
921
Sn(IV) porphyrin 2 with two equivalents of silanol 3 in
refluxing toluene proceeded to yield the bis-product
2.32 (Scheme 1) in essentially quantitative yield.
ature by 1H NMR spectroscopy. No changes to the
spectrum were observed compared to 2 or 3 separately
supporting the coordinate covalent nature of the bond-
ing in 2.32 rather than non-covalent. Prolonged reaction
times (days) revealed some spectral changes indicative of
complexation and after four days no resonances corre-
sponding to 2 were observed suggesting condensation
was driven toward 2.32.
Characterisation of the complex 2.32 was possible by
1H, 119Sn and 29Si NMR spectroscopy. As shown in
Fig. 1(b), the 1H NMR spectrum of complex 2.32 shows
a minor but significant upfield shift (Dd À0.1) of the
resonance corresponding to the b-pyrrolic protons of 2
upon complexation. Chemical shift changes of compara-
ble magnitude have been reported for 2 reflecting the
exchange of axial ligands from the bis(hydroxy) complex
[11]. A more significant change in chemical shift
(Dd À1.8) was observed for the peak corresponding to
the tert-butyl protons of 3. The large change in chemical
shift is attributed to the close proximity of these protons
to the shielding anisotropy of the porphyrin macrocycle
upon binding. This shift is not as significant as for other
ligands but consistent with the average distance the
tert-butyl protons reside from the plane of the porphy-
rin. Associated with this Dd was the disappearance of
the axial hydroxy proton resonance at d À7.4 as these
protons are exchanged for silanol ligands. Satellite sig-
119Sn NMR spectroscopy revealed a resonance at
d À645.4 (Fig. 2(a)) which was assigned to the Sn(IV)
centre also diagnostic of 2.32. The value was shifted
upfield by 76 ppm from the reported Sn(IV) chemical shift
of porphyrin 2 (d À569.6) [10]. The minor resonance
observed at d À607.9 is thought to be attributable to a
minor amount of the mono-hydroxy product (HO)2.3.
29Si NMR spectroscopy was also used to probe the
nature of the complex 2.32. The resonance observed at
d À101 (Fig. 2(b)) is attributable to the presence of
silicon in complex 2.32. This value was shifted upfield
by 11 ppm compared to the silicon resonance of uncom-
plexed 3 (À90 ppm). By acquiring in excess of 50 000
scans, the appearance of satellites due to Sn(IV)–Si
coupling (ca. 2J = 16 Hz) were also evident in the
spectrum (Fig. 2(b)).
4
nals due to Sn(IV)–H coupling (e.g., 2.32 Jav = 12.4 Hz)
were also evident for the b-pyrrolic resonances (see
expansions in Fig. 1) although not sufficiently resolved
to observe separate coupling to the two tin nuclei
Mass spectrometry (electrospray and laser desorption
ionisation) confirmed the presence of the condensation
product 2.32. The molecular ion was observed at m/e
995 which compares well to the calculated value less
one silanol ligand 3. The observation of the mono-
complexed species in the gas phase is acceptable due
(
117Sn and 119Sn).
To confirm the formation of 2.32 and not just a
hydrogen bonded complex, porphyrin 2 and silanol 3
were mixed in CDCl3 and monitored at ambient temper-
Ph
Ph
HO
N
N
N
N
NH
N
N
(i)
Ph
Sn
Ph
Ph
Ph
(ii)
HN
OH
Ph
Ph
2
1
(iii)
O
(tBuO)3Si
Ph
Sn
Ph
Sn
Ph
O
N
O
N
N
N
N
N
N
(iv)
Ph
Ph
Ph
Ph
N
O
O
Si(OtBu)3
Ph
O
2.42
2.32
Scheme 1. Reagents and conditions: (i) SnCl2, pyridine; (ii) K2CO3, THF, H2O; (iii) (tBuO)3SiOH 3 (2 equivalents), toluene, D, Dean–Stark; (iv)
benzoic acid 4 (2 equivalents), CDCl3, D.