Metal-assisted cleavage of the porphyrinogen skeleton: Reaction of meso-octaethylporphyrinogen complexes with benzaldehyde

Full text of DOI:10.1021/ic961446c

Inorg. Chem. 1997, 36, 2691-2695
2691
of view of a precursor of “artificial porphyrins”3,4 and as a very
versatile bifunctional ligand in organometallic reactions, when
assisting the C-H bond activation5 and the homologation of
the pyrrole to the pyridine ring.6 In the latter reaction pyrrole
is attacked by an electrophilic carbenium ion.7 We report here
the metal-assisted reactions of benzaldehyde on titanium- and
zirconium-meso-octaethylporphyrinogen which lead to the
modification and cleavage of the tetrapyrrolic structure as a
function of the oxophilicity of the metal.
Metal-Assisted Cleavage of the Porphyrinogen
Skeleton: Reaction of
meso-Octaethylporphyrinogen Complexes with
Benzaldehyde
Giovanna Solari,† Euro Solari,† Gilles Lemercier,†
Carlo Floriani,*,† Angiola Chiesi-Villa,‡ and
Corrado Rizzoli‡
Institut de Chimie Mine´rale et Analytique,
Universite´ de Lausanne, BCH, CH-1015, Lausanne,
Switzerland, and Dipartimento di Chimica,
Universita` di Parma, I-43100 Parma, Italy
Experimental Section
General Procedure. All reactions were carried out under an
atmosphere of purified nitrogen. Solvents were dried and distilled
before use by standard methods. The syntheses of 18 and 29 were
performed as reported. NMR spectra were measured on 200-AC and
400-DPX Bruker instruments.
ReceiVed December 4, 1996
Preparation of 3. A toluene (80 mL) solution of freshly distilled
benzaldehyde (1 mL, 9.7 mmol) was added under nitrogen to a dark-
green solution of 1 (4.75 g, 6.5 mmol) in toluene (300 mL) at -30 °C.
When the temperature was allowed to reach room temperature, the
solution turned red. After 48 h of stirring, the mixture was evaporated
to dryness. n-Hexane (150 mL) was added, and an ocre-red powder
was obtained (3.9 g, 85%). Anal. Calcd for C43H54N4OTi: C, 74.80;
H, 7.90; N, 8.10. Found: C, 74.20; H, 7.90; N, 7.80. 1H NMR (C6D6,
200 MHz, room temperature): δ 6.94-6.80 (m, 5H, ArH), 6.78 (d,
1H, J ) 5.3 Hz, C4H2N), 6.69 (s, 2H, C4H2N), 6.32 (d, 1H, J ) 3.4
Hz, C4H2N), 6.27 (d, 1H, J ) 3.4 Hz, C4H2N), 6.06 (d, 1H, J ) 3.4
Hz, C4H2N), 6.03 (d, 1H, J ) 3.4 Hz, C4H2N), 5.88 (s, 1H, PhCHO),
5.65 (d, 1H, J ) 5.3 Hz, C4H2N), 2.77 (m, 1H, CH2), 2.22 (m, 8H,
CH2), 1.92 (m, 4H, CH2), 1.59 (m, 3H, CH2), 1.03 (t, 3H, J ) 7.3 Hz,
CH3), 0.9 (t, 3H, J ) 7.3 Hz, CH3) overlapping with 0.89 (t, 3H, J )
7.3 Hz, CH3), 0.83 (t, 3H, J ) 7.3 Hz, CH3) overlapping with 0.82 (t,
3H, J ) 7.3 Hz, CH3), 0,72 (t, 3H, J ) 7.3 Hz, CH3), 0.64 (t, 3H, J )
7.3 Hz, CH3), 0.52 (t, 3H, J ) 7.3 Hz, CH3). Red crystals suitable for
X-ray analysis were obtained from a saturated benzene solution (1.3 g
in 60 mL) upon addition of hexane (100 mL). Compound 3 forms
independently of the Ti/PhCHO molar ratio.
Hydrolysis of 3. A toluene (40 mL) solution of 3 (0.64 g, 0.75
mmol) was hydrolyzed with degassed water. The mixture turned
orange-yellow and was stirred for 1 night under nitrogen. Extraction
with toluene and evaporation to dryness gave pure meso-octaethylpor-
phyrinogen (0.38 g, 75%).
Reactivity of 3 with LiMe. LiMe (1.3 mL, 1.56 M, 2.1 mmol)
was slowly added to a toluene (150 mL) solution of 3 (1.4 g, 2.0 mmol)
at -50 °C. Warming to room temperature resulted in the solution
turning gray-green (complex 1). After being stirred for 3 days, the
solution turned blue-green and was then evaporated to dryness and
hydrolyzed to give meso-octaethylporphyrinogen.
Introduction
A major perspective in metal-assisted processes is the
modification and reactivity of large and relevant structures. Our
target molecule in the present study is the porphyrinogen
skeleton, which is the chemical and biochemical precursor of
porphyrins.1 Its complexation to metals and the consequent
chemistry were unsuccessful because of its oxidative instability
and fast conversion to porphyrins.1 The metal-porphyrinogen
chemistry has been only recently developed using the meso-
octaalkylporphyrinogen,2 the fully alkylated form of the usual
meso-tetrahydrotetraalkylporphyrinogen. The metal-bonded
meso-octaalkylporphyrinogen3-7 was investigated from the point
* To whom correspondence should be addressed.
† University of Lausanne.
‡ University of Parma.
(1) (a) Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier:
Amsterdam, The Netherlands, 1975 The Porphyrins; Dolphin, D, Ed.;
Academic: New York, 1978. (b) Mashiko, T; Dolphin, D. In
ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R.
D., McCleverty, J. A., Eds.; Pergamon: Oxford, U.K., 1987; Vol. 2,
Chapter 21.1, p 855. (c) Kim, J. B.; Adler, A. D.; Longo, R. F. In The
Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 1,
Part A, p 85. Mauzerall, D. Ibid., Vol. 2, p 91. (d) Lindsey, J. S.;
Schreiman, I. C.; Hsu, H. C.; Kearney P. C.; Marguerettaz, A. M. J.
Org. Chem. 1987, 52, 827. Lindsey, J. S.; Wagner, R. W. J. Org.
Chem. 1989, 54, 828. (e) Biosynthesis of Tetrapyrroles, Jordan, P.
M., Ed.; Elsevier: New York, 1991.
(2) (a) Baeyer, A. Chem. Ber. 1886, 19, 2184. (b) Fischer, H.; Orth, H.
The Chemistry of Pyrroles; Akademische Verlagsgesellschaft: Leipzig,
Germany, 1934; p 20. (c) Dennstedt, M.; Zimmermann, J. Chem. Ber.
1887, 20, 850 and 2449; 1888, 21, 1478. (d) Dennstedt, D. Chem.
Ber. 1890, 23, 1370. (e) Chelintzev, V. V.; Tronov, B. V. J. Russ.
Phys. Chem. Soc. 1916, 48, 105, 127. (f) Sabalitschka, Th.; Haase, H.
Arch. Pharm. 1928, 226, 484. (g) Rothemund, P.; Gage, P. L. J. Am.
Chem. Soc. 1955, 77, 3340.
(3) Floriani, C. Pure Appl. Chem. 1996, 68, 1 and references therein.
(4) (a) Jubb, J.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1992, 114, 6571. (b) Piarulli, U.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. J. Chem. Soc., Chem. Commun. 1994, 895. (c) De Angelis,
S.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1994, 116, 5691. (d) De Angelis, S.; Solari, E.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1994, 116, 5702. (e)
Floriani, C. Transition Metals in Supramolecular Chemistry; Fabbrizzi,
L., Poggi, A., Eds; Nato ASI Series; Kluwer: Dordrecht, The
Netherlands, 1994; Vol. 448, pp 448, 191-209. (f) Piarulli, U.; Solari,
E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1996,
118, 3634-3642.
(5) (a) Jacoby, D.; Isoz, S.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J.
Am. Chem. Soc. 1995, 117, 2805. (b) Isoz, S.; Floriani, C.; Schenk,
K.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1996, 15, 337.
(6) Jacoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1993, 115, 7025. Jacoby, D.; Isoz, S.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. J. Am. Chem. Soc. 1995, 117, 2793. Floriani, C. In
StereoselectiVe Reactions of Metal-ActiVated Molecules; Werner, H.,
Sundermeyer, J., Eds.; Vieweg: Wiesbaden, Germany, 1995; p 97.
Jacoby, D.; Isoz, S.; Floriani, C.; Schenk, K.; Chiesi-Villa, A.; Rizzoli,
C. Organometallics 1995, 14, 4816.
Preparation of 6. Freshly distilled PhCHO (1.24 g, 11.7 mmol)
was added dropwise to a yellow toluene (100 mL) solution of 2 (4.09
g, 5.85 mmol) and the resulting deep red solution was stirred at room
temperature for 5 h. The solution was evaporated to dryness and the
residue triturated with n-hexane (80 mL) to give an orange-yellow
powder which was collected and dried in Vacuo (1.5 g, 30%). Crystals
suitable for X-ray analysis were grown in n-hexane. Anal. Calcd for
(7) Tatsumi, K.; Nakamura, A.; Hofmann, P.; Stauffert, P.; Hoffmann,
R. J. Am. Chem. Soc. 1985, 107, 4440. Martin, B. D.; Matchett, S.
A.; Norton, J. R.; Anderson, O. P. J. Am. Chem. Soc. 1985, 107, 7952.
Roddick, D. M.; Bercaw, J. E. Chem. Ber. 1989, 122, 1579. Hofmann,
P.; Stauffert, P.; Frede, M.; Tatsumi, K. Chem. Ber. 1989, 122, 1559.
Hofmann, P.; Stauffert, P.; Tatsumi, K.; Nakamura, A.; Hoffmann,
R. Organometallics 1985, 4, 404. Tatsumi, K.; Nakamura, A.;
Hofmann, P.; Hoffmann, R.; Moloy, K. G.; Marks, T. J. J. Am. Chem.
Soc. 1986, 108, 4467.
(8) (a) De Angelis, S.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli,
C. Angew. Chem., Int. Ed. Engl. 1995, 34, 1092. (b) De Angelis, S.;
Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics
1995, 14, 4505.
(9) (a) Jacoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1993, 115, 3595. (b) Jacoby, D.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. J. Chem. Soc., Chem. Commun. 1991, 790.
S0020-1669(96)01446-2 CCC: $14.00 © 1997 American Chemical Society

2692 Inorganic Chemistry, Vol. 36, No. 12, 1997
Notes
Table 1. Experimental Data for the X-ray Diffraction Studies on
Crystalline Complexes 3, 6, and 7
complex
3
6
7
formula
a, Å
b, Å
C43H54N4OTi
11.447(3)
18.842(4)
17.642(2)
90
101.68(1)
3726.3(13)
4
C50H60N4O2Zr
18.552(3)
25.784(4)
17.561(3)
90
90
8400(2)
8
C82H76N4O8Zr2
12.205(3)
22.265(4)
13.041(3)
90
105.15(2)
3420.7(13)
2
c, Å
R, γ, deg
â, deg
V, Å3
Z
fw
690.8
840.3
1428.0
space group
t, °C
P21/n (No. 14)
-140
1.541 78
1.231
22.23
0.809-1.000
0.044
Pbca (No. 61)
-140
1.541 78
1.329
24.96
0.720-1.000
0.045
P21/c (No. 14)
-140
1.541 78
1.386
30.04
0.708-1.000
0.049
λ, Å
F
calc, g cm-3
µ, cm-1
transm coeff
Ra
Figure 1. ORTEP view of the structure of complex 3 (50% probability
wR2a
0.123
0.124
0.142
ellipsoids).
a R ) ∑|∆F|/∑|Fo|calculated on the unique observed data [I > 2σ(I)].
2 2
wR2 ) [∑w|∆F2|2/∑w|Fo | ]1/2 calculated on the unique data having I
for neutral atoms were taken from ref 14a for non-hydrogen atoms
and from ref 15 for H. Among the low-angle reflections, no correction
for secondary extinction was deemed necessary.
> 0.
C50H66N4O2Zr: C, 70.96; H, 7.86; N, 6.62. Found: C, 70.99; H, 7.45;
N, 6.67. 1H NMR (C6D6, 200 MHz, 298 K): δ 7.62 (m, 2H, ArH),
7.10 (m, 10H, ArH) overlapping with 7.07 (d, 2H, C4H2N), 6.84 (m,
2H, C4H2N), 6.64 (d, J ) 4.88 Hz, 1H, C4H2N), 6.32 (s, 2H, C4H2N),
6.22 (d, J ) 3.90 Hz, 1H, C4H2N), 6.01 (d, J ) 4.88 Hz, 2H), 4.95 (s,
2H, PhCH2O), 3.03 (m, 1H, CH2), 2.46 (m, 7H, CH2), 2.10 (m, 4H,
CH2), 1.76 (m, 4H, CH2), 1.37 (t, J ) 7.32 Hz, 3H, CH3), 1.21 (t, J )
7.32 Hz, 3H, CH3), 1.05 (m, 6H, CH3), 0.77 (t, J ) 7.32 Hz, 3H, CH3)
overlapping with 0.67 (m, 9H, CH3).
Preparation of 7. Freshly distilled PhCHO (5.25 g, 49.0 mmol)
was added dropwise to a yellow toluene (100 mL) solution of 2 (4.22
g, 6.0 mmol), and the resulting deep red solution was stirred at room
temperature for 5 h. The solvent was partially evaporated, and n-hexane
(100 mL) was added. An orange-yellow powder formed and was then
collected and dried in Vacuo (3.4 g, 40%). Crystals suitable for X-ray
analysis were grown in a mixture of toluene/Et2O. Anal. Calcd for
C82H76N4O8Zr2: C, 68.97; H, 5.36; N, 3.92. Found: C, 69.25; H, 5.35;
N, 3.77. 1H NMR (C6D6, 200 MHz, 298 K): δ 7.79 (m, 8H, ArH),
7.27 (m, 10H, ArH), 7.12 (m, 10H, ArH), 6.93 (m, 8H, ArH), 6.75 (s,
8H, C4H2N), 6.34 (m, 4H, ArH), 5.85 (s, 4H, PhCH2O), 4.42 (s, 4H,
PhCH2O), 1.82 (q, J ) 7.3 Hz, 4H, CH2), 1.76 (q, J ) 7.3 Hz, 4H,
CH2), 0.61 (t, J ) 7.3 Hz, 6H, CH3), 0.36 (t, J ) 7.3 Hz, 6H, CH3).
The best way to run the reaction is to use a 1:4 Zr/PhCHO ratio.
Compound 7 can be equally well obtained reacting 6 with 2 equiv of
PhCHO.
The structures were solved by the heavy-atom method (Patterson
and Fourier syntheses). Refinement was first done isotropically and
then anisotropically for all the non-H atoms using the unique data with
I > 0. In complex 7 the C62-C67 aromatic ring was found to be
disordered over two positions called A and B which were isotropically
refined with a site occupation factor of 0.5 by applying a rigid body
constrain (D6h symmetry). The H atoms except those associated with
the disordered benzylic group, which were ignored, were located in
∆F maps and introduced in the refinements as fixed contributors with
isotropic U values fixed at 0.05 Å2 for all complexes. The final
difference map showed no unusual features, with no significant peak
above the general background. Selected bond distances and angles
are given in Tables 2-4 for complexes 3, 6, and 7, respectively.16
Results and Discussion
The reaction of 18 and 25,6,9 with PhCHO was carried out in
toluene at room temperature, with various stoichiometric ratios
depending on the metal. In the case of 1, regardless of the
stoichiometric ratio, reaction conditions, and time, we obtained
3 exclusively and almost quantitatively. The genesis of 3 can
be easily understood by assuming precoordination of benzal-
dehyde to titanium, which would enhance its electrophilicity,
followed by the attack of benzaldehyde on the R-pyrrole carbon
of one of the pyrrolyl anions of the macrocycle. The coordina-
tion of benzaldehyde to the metal center is a general event in
such reaction (Vide infra the reactivity of zirconium). In the
case of titanium we did not observe any further reaction with
PhCHO, as expected due to the lower Lewis acidity of the metal
on 3 Vs 1. The addition of PhCHO to 1 gave rise to a novel
pentadentate tetraanionic N4O macrocyclic ligand. The reaction
of 1 with PhCHO introduced two chiral centers, though a single
diastereoisomer has been observed in the case of 3. The steric
hindrance of the meso-carbon and the phenyl group prevents,
very probably, the formation of the other diastereoisomer. The
red crystals of 3 have been characterized, including an X-ray
analysis. The structure of 3 is reported in Figure 1, with
structural parameters in Table 2.
X-ray Crystallography for Complexes 3, 6, and 7. Intensity data
were collected at low temperature on a single-crystal four-circle
diffractometer. Crystal data and refinement details for complexes 3,
6, and 7 are given in Table 1. The reduced cells were obtained with
use of TRACER.10 For intensities and background individual reflection
profiles were analyzed.11 Intensity data were corrected for Lorentz and
polarization effects and for absorption.12 The function minimized during
the full-matrix least-squares refinement was ∑w|∆F2|2 using a weighting
scheme based on counting statistics.13 Anomalous scattering corrections
were included in all structure factor calculations.14b Scattering factors
(10) Lawton, S. L.; Jacobson, R. A. TRACER, A Cell Reduction Program;
Ames Laboratory, Iowa State University of Science and Technology:
Ames, IA, 1965.
(11) Lehmann, M. S.; Larsen, F. K. Acta Crystallogr., Sect. A: Cryst. Phys.,
Diffr., Theor. Gen. Crystallogr. 1974, A30, 580.
(12) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1968, A24, 351.
(13) Sheldrick, G. M. SHELX92 Gamma Test: Program for Crystal
Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany,
1992.
(14) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol IV: (a) p 99; (b) p 149.
The pyrrole rings containing the N1, N2, N3, and N4 nitrogen
atoms are referred as A, B, C, and D, respectively. The
increased electron demand by titanium in 3 along with the steric
(15) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys. 1965,
42, 3175.
(16) See paragraph at the end of paper regarding Supporting Information.

Notes
Inorganic Chemistry, Vol. 36, No. 12, 1997 2693
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for
Complex 3
Scheme 1
Ti1-O1
Ti1-N1
Ti1-N2
Ti1-N3
Ti1-N4
Ti1-C16
Ti1-C17
Ti1-C18
Ti1-C19
O1-C41
N1-C1
N1-C4
N2-C6
N2-C9
N3-C11
N3-C14
N4-C16
N4-C19
C1-C2
1.850(2)
2.105(2)
2.139(2)
2.042(2)
2.295(2)
2.371(2)
2.459(2)
2.465(2)
2.336(2)
1.422(3)
1.393(3)
1.398(3)
1.299(3)
1.483(3)
1.381(3)
1.396(3)
1.365(3)
1.384(3)
1.361(5)
1.416(5)
C3-C4
1.370(4)
1.504(4)
1.504(4)
1.456(4)
1.331(5)
1.501(3)
1.590(4)
1.574(3)
1.523(3)
1.366(6)
1.412(5)
1.357(5)
1.507(3)
1.515(4)
1.413(3)
1.399(3)
1.405(3)
1.504(4)
1.519(3)
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C9-C10
C9-C41
C10-C11
C11-C12
C12-C13
C13-C14
C14-C15
C15-C16
C16-C17
C17-C18
C18-C19
C19-C20
C41-C42
C2-C3
N3-Ti1-N4
N2-Ti1-N4
N2-Ti1-N3
N1-Ti1-N4
N1-Ti1-N3
N1-Ti1-N2
Ti1-O1-C41
Ti1-N1-C4
Ti1-N1-C1
C1-N1-C4
Ti1-N2-C9
Ti1-N2-C6
C6-N2-C9
Ti1-N3-C14
98.2(1)
146.3(1)
79.7(1)
80.7(1)
135.5(1)
78.0(1)
123.5(1)
132.1(2)
120.3(2)
105.5(2)
109.9(1)
137.2(2)
109.1(2)
124.5(2)
Ti1-N3-C11
C11-N3-C14
Ti1-N4-C19
Ti1-N4-C16
C16-N4-C19
N2-C9-C8
C8-C9-C41
C8-C9-C10
N2-C9-C41
N2-C9-C10
C10-C9-C41
O1-C41-C9
C9-C41-C42
O1-C41-C42
127.9(2)
106.7(2)
74.2(1)
76.1(1)
106.6(2)
102.8(2)
113.3(2)
111.2(2)
101.5(2)
110.8(2)
116.0(2)
108.8(2)
113.8(1)
109.7(2)
Scheme 2
hindrance of the novel ligand causes a change in the bonding
mode of the porphyrinogen skeleton from η1-η1-η1-η1 in 18 to
η1-η1-η1-η5 in 3. The Ti-centroid distance to the D pyrrolic
ring, 2.071(2) Å, is quite close to those found in [(cp)2Ti]
derivatives.17 Titanium shows a distorted square pyramidal
coordination with the N4 set defining the basal plane. The Ti-
O1 direction forms a dihedral angle of 30.2(1)° with the normal
to the basal plane. The N4 core shows tetrahedral distortions
ranging from -0.081(2) to 0.084(2) Å, with titanium displaced
by 0.685(1) Å toward O1. The macrocycle assumes a confor-
mation with the opposite B and D pyrrole rings tilted upward
and the opposite A and C rings downward with respect to the
N4 core (“up” means the side of the N4 plane tilted toward the
O1 oxygen), the dihedral angles formed with the N4 core being
44.7(1), 41.7(1), 19,9(1), and 80.6(1)° for the A, B, C, and D
rings, respectively. The Ti-N bond distances associated with
uncharged N1 [Ti-N1, 2.105(2) Å] and N2 [Ti-N2, 2.139(2)
Å] forming N···H-C hydrogen bonding [N1···C43, 3.583(4) Å;
N1···H43, 2.62 Å; N1···H43···C43, 165°] are particularly long
compared to those in 1 and Ti-N3 [2.042(2) Å]. The five-
membered metallacycle [Ti,O1,C41,C9,N2] has an envelope
conformation. The C9 is displaced by 0.645(2) Å from the mean
plane through N4 and is almost perpendicular to it [dihedral
angle 97.01(1)°]. The C41-O1, C41-C9, N2-N6, C7-C8,
and Ti-O1 distances (Table 2) support the bonding sequence
given in Scheme 1.
Attempts to demetalate 3 and to free the corresponding
pentadentate macrocycle Via hydrolysis led, instead, to the
original meso-octaethylporphyrinogen and benzaldehyde revers-
ing the condensation process of 1 to 3. The same process from
3 to 1 can be reversed by reacting 3 with strong nucleophiles
such as LiMe which removes PhCHO from 3 and re-forms 1.
Due to its higher oxophilicity and coordination number,
zirconium drives the reactions in Scheme 1 much further than
titanium; thus, 4 might be not isolated. Our investigation
focused, however, on the subsequent reaction products. The
following step requires 2 equiv of PhCHO and proceeds to 6,
without any possibility of intercepting intermediates like 4 and
5.
The coordination of a second molecule of PhCHO on
zirconium, which achieves the coordination number 7, results
in the juxtapositioning of an alcoholic and a carbonylic function.
As in the case of Al3+, zirconium assists the so-called Meer-
wein-Ponndorf reduction,18 with the hydrogen transfer from
the alcoholic to the carbonylic function Via a cyclic transition
state similar to that shown schematically for 5. This causes
the cleavage of the C-C between the pyrrole and the meso-
carbon and affords the structure shown for 6 (see also Figure
2).
The R-carbon in the pyrrolyl anion is particularly sensitive
to electrophilic attack, as shown by the synthesis of the starting
macrocycle,2 and redox chemistry emphasized by its transfor-
mation into artificial porphyrins3,4 (Scheme 2).
Complex 6 has been isolated as yellow-orange crystals. Its
structure allows us to understand the further transformation in
the reaction with benzaldehyde.
(17) Bochmann, M. In ComprehensiVe Organometallic Chemistry II;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
U.K., 1995; Vol. 4; Chapter 5.
(18) March, J. AdVanced Organic Chemistry, 3rd ed.; Wiley: New York,
1985; p 913.

2694 Inorganic Chemistry, Vol. 36, No. 12, 1997
Notes
Figure 2. ORTEP view of the structure of complex 6 (50% probability
ellipsoids).
Table 3. Selected Interatomic Distances (Å) and Angles (deg) for
Complex 6
Zr1-O1
Zr1-O2
Zr1-N1
Zr1-N2
Zr1-N3
Zr1-N4
Zr1-C16
Zr1-C17
Zr1-C18
Zr1-C19
O1-C41
O2-C51
N1-C1
2.236(4)
1.923(4)
2.192(3)
2.222(3)
2.616(3)
2.445(3)
2.491(5)
2.598(5)
2.628(4)
2.509(4)
1.282(6)
1.384(6)
1.408(5)
1.402(6)
1.328(6)
1.391(6)
1.427(6)
1.321(6)
1.386(6)
1.366(5)
1.374(7)
C2-C3
1.411(8)
1.374(7)
1.530(7)
1.508(7)
1.430(7)
1.373(8)
1.404(7)
1.407(7)
1.356(7)
1.441(7)
1.351(7)
1.451(7)
1.504(7)
1.514(7)
1.405(7)
1.402(7)
1.409(5)
1.516(6)
1.477(6)
1.506(8)
1.392(8)
C3-C4
C4-C5
Figure 3. ORTEP view of the structure of complex 7 (50% probability
ellipsoids). Prime denotes a transformation of 1 - x, - y, 1 - z. The
B position of the disordered C62-C67 aromatic ring has been omitted
for clarity.
C5-C6
C6-C7
C7-C8
C8-C9
C9-C41
C10-C11
C11-C12
C12-C13
C13-C14
C14-C15
C15-C16
C16-C17
C17-C18
C18-C19
C19-C20
C41-C42
C51-C52
C52-C53
Scheme 3
N1-C4
N2-C6
N2-C9
N3-C11
N3-C14
N4-C16
N4-C19
C1-C2
distance [2.236(4) Å] in agreement with the values of the O-C
bond distances and Zr-O-C bond angles indicating a single
bond character of the O2-C51 bond [1.384(6) Å] and a partial
double bond character of the O1-C41 bond [1.282(6) Å].
As far as the latter point is concerned, the structural
parameters (Table 3) related to the benzoyl group R-bonded to
the pyrrole B allow us to write down equally well the two
bonding sequences shown in Scheme 3.
N3-Zr1-N4
N2-Zr1-N4
N2-Zr1-N3
N1-Zr1-N4
N1-Zr1-N3
N1-Zr1-N2
O1-Zr1-O2
Zr1-O1-C41
Zr1-O2-C51
Zr1-N1-C4
Zr1-N1-C1
C1-N1-C4
Zr1-N2-C9
Zr1-N2-C6
C6-N2-C9
Zr1-N3-C14
65.0(1)
139.8(1)
82.2(1)
86.3(1)
104.6(1)
79.9(1)
Zr1-N3-C11
C11-N3-C14
Zr1-N4-C19
Zr1-N4-C16
C16-N4-C19
N2-C9-C8
C8-C9-C41
N2-C9-C41
C25-C10-C27
C11-C10-C27
C11-C10-C25
O1-C41-C9
C9-C41-C42
O1-C41-C42
O2-C51-C52
133.1(3)
106.5(4)
76.6(2)
75.5(2)
106.9(3)
108.9(4)
135.3(4)
115.0(3)
116.3(4)
122.2(5)
121.4(5)
118.5(4)
123.2(4)
118.2(4)
114.7(4)
Accordingly, this system is nearly planar, the displacements
from the planarity ranging from -0.119(5) to 0.100(5) Å. The
dihedral angle between the pyrrole and the metallacycle is 9.7-
(2)°. The cleavage of C-C at the meso-position occurs with
the formation of the C10-C11 [1.356(7) Å] double bond and
as a consequence the bonding sequence shown for the C pyrrole,
which interacts very weakly with zirconium at a distance of
2.616(3) Å. The weakly bonded nitrogen, N3, can be easily
displaced by an incoming molecule of PhCHO which, through
the same pathway, cleaves a second meso-bridge and leads to
the formation of 7 and to the extrusion of 8. Complex 7 can
be obtained either by reacting 2 with 4 equiv, eventually an
excess of PhCHO, or by reacting 6 with 2 equiv of PhCHO
under the same conditions. The rearrangement of the mono-
meric precursor to a dimeric form like 7 (see Figure 3) is not
chemically relevant. The reaction ends up with the formation
of a quadridentate dianionic N2O2 chelating ligand which is the
deprotonated form of the dibenzoyl-R,R′-dipyrromethane. The
synthesis of diacyl-R,R′-dipyrromethanes can be performed
85.7(1)
118.0(3)
167.1(3)
130.5(3)
121.7(3)
105.5(4)
116.2(3)
135.1(3)
108.0(3)
110.6(3)
The coordination polyhedron around zirconium is a distorted
tetragonal bipyramid, with the apical sites occupied by the O2
and N3 atoms and the best equatorial plane defined by O1, N1,
N2 and the centroid of the η5-bonded D pyrrole ring [Zr-Cp4,
2.240(4) Å]. The metal is displaced by 0.166(1) Å from the
equatorial plane which shows significant tetrahedral distortions
ranging from -0.050(4) to 0.062 Å. The Zr-O2 bond distance
[1.923(4) Å] is significantly shorter than the Zr-O1 bond

Notes
Inorganic Chemistry, Vol. 36, No. 12, 1997 2695
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Complex 7a
dihedral angle they form is 11.4(1)°. The two monomeric units
are bonded in centrosymmetric dimers by the bridging alkoxo
oxygens O3 and O4. The coordination polyhedron around
zirconium could be described as a distorted pentagonal bipyra-
mid with the equatorial plane defined by the nitrogen and
oxygen atoms of the N2O2 core and by the O4 oxygen atom
[displacements in the range (0.299(4) Å]. The axial positions
are occupied by the O3 and O4′ oxygen atoms (′ ) 1 - x, -
y, 1 - z), the Zr-O3 and Zr-O4′ lines forming dihedral angles
of 2.9(1) and 7.9(1)°, respectively, with the normal to the
equatorial plane. Zirconium is displaced from it by 0.114(1)
Å toward O3. The Zr,N1,C1,C51,O2 and Zr,N2,C9,C41,O1
five-membered chelate rings are nearly planar [displacements
ranging from -0.058(5) to 0.043(5) and from -0.066(5) to
0.074(5) Å, respectively] and coplanar with the adjacent A and
B pyrrole rings, the dihedral angles they form being 8.5(2) and
7.1(1)°, respectively. The Zr,N1,C4,C5,C6,N2 six-membered
chelate ring assumes a flattened half-boat conformation, the C5
meso-carbon atom being displaced by 0.200(5) Å from the mean
plane through the other atoms. As a possible consequence of
the seven-coordinate metal, the Zr-N bond distances [mean
value 2.294(5) Å] are significantly longer than the corresponding
Zr-N2 bond distance in complex 6. The Zr-O1 [2.223(4) Å]
and Zr-O2 [2.236(4) Å] bond distances involving the oxygen
atoms of the benzoyl groups as well as the Zr-O3 bond distance
[1.906(4) Å] involving the oxygen atoms of the alkoxo ligands
agree well with the corresponding values observed in 6. The
Zr-O4 bond distance [2.201(4) Å] is remarkly longer than Zr-
O3 as expected by the bridging role of the O4 oxygen atom.
Zr1-O1
Zr1-O2
Zr1-O3
Zr1-O4
Zr1-O4′
Zr1-N1
Zr1-N2
O1-C41
O2-C51
O3-C61
O4-C71
N1-C1
N1-C4
N2-C6
N2-C9
2.223(4)
2.236(4)
1.906(4)
2.201(4)
2.159(4)
2.296(5)
2.292(5)
1.274(8)
1.278(7)
1.411(9)
1.425(6)
1.405(9)
1.343(7)
1.334(7)
1.405(8)
C1-C2
1.409(7)
1.402(8)
1.357(11)
1.415(9)
1.506(10)
1.516(9)
1.407(10)
1.376(8)
1.417(9)
1.403(7)
1.491(8)
1.480(10)
1.654(10)
1.406(9)
1.508(8)
C1-C51
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C9-C41
C41-C42
C51-C52
C61-C62A
C61-C62B
C71-C72
N1-Zr1-N2
O3-Zr1-O4′
O2-Zr1-N1
O2-Zr1-O4
O1-Zr1-N2
O1-Zr1-O4
O1-Zr1-O2
Zr1-O1-C41
Zr1-O2-C51
Zr1-O3-C61
Zr1-O4-Zr1′
Zr1′-O4-C71
Zr1-O4-C71
Zr1-O4′-Zr1′
Zr1-N1-C4
Zr1-N1-C1
C1-N1-C4
73.5(2)
171.1(2)
70.8(2)
74.5(1)
70.3(2)
Zr1-N2-C6
C6-N2-C9
138.7(4)
106.3(5)
115.8(5)
109.0(5)
134.8(6)
109.4(4)
135.2(5)
114.8(5)
118.1(5)
122.9(5)
118.7(5)
118.5(5)
122.7(5)
118.7(5)
125.0(6)
103.7(5)
113.8(4)
N1-C1-C51
N1-C1-C2
C2-C1-C51
N2-C9-C8
73.7(2)
145.6(1)
121.0(3)
119.9(3)
166.9(4)
107.6(2)
126.3(3)
122.0(3)
107.6(2)
138.2(4)
114.3(4)
106.3(5)
114.9(3)
C8-C9-C41
N2-C9-C41
O1-C41-C9
C9-C41-C42
O1-C41-C42
O2-C51-C1
C1-C51-C52
O2-C51-C52
O3-C61-C62B
O3-C61-C62A
O4-C71-C72
The metal-assisted electrophilic activation of the porphyrino-
gen skeleton sheds light on some of the important processes in
which porphyrinogen is involved, such as the macrocyclic ring
closure and opening and its degradation to dipyrromethane
moieties. From a synthetic point of view, such reactions can
be adapted to major synthetic purposes: (i) the functionalization
of the porphyrinogen skeleton; (ii) the synthesis of novel
polydentate ligands derived from R,R′-dipyrromethane.
Zr1-N2-C9
a Prime denotes a transformation of 1 - x, -y, 1 - z.
either from the direct acylation of the dipyrromethane19 or from
the reaction reported here, using 2 as starting material. The
latter method can be a quite convenient synthesis, due to the
availability of the meso-octaalkylporphyrinogen complexes. In
addition, it leads directly to the corresponding metalated form.
The structural parameters (Table 4) support the bonding scheme
given in the picture for each monomeric unit of 7, with a charge
delocalization, as shown in Scheme 3, between nitrogen and
oxygen for each half of the quadridentate ligand.
Acknowledgment. We thank the “Fonds National Suisse de
la Recherche Scientifique” (Grant No. 20-40268.94) and Ciba-
Geigy SA (Basel, Switzerland) for financial support.
Supporting Information Available: Tables of experimental details
associated with data collections and structure refinements, final atomic
coordinates for non-H atoms, hydrogen atom coordinates, thermal
parameters, and bond distances and angles for 3, 6, and 7 (17 pages).
Ordering information is given on any current masthead page.
The two N···O systems are nearly planar [displacements
ranging from-0.059(5) to 0.052(6) and from -0.075(5) to
0.046(5) Å for N1···O2 and N2···O1, respectively], and the
(19) Lee, C.-H.; Li, F.; Iwamoto, K.; Dadok, J.; Bothner-By, A. A.; Lindsey,
J. S. Tetrahedron 1995, 51, 11645-11672.
IC961446C