Reactions of nickel(II) 2,21-dimethyl-2-aza-21-carbaporphyrin with phenyl grignard reagents, phenyllithium, and n-butyllithium

Article abstract of DOI:10.1021/ic000052p

Addition of a phenyl Grignard reagent to a toluene solution of the nickel(II) chloride complex of a dimethylated inverted porphyrin, (2-NCH₃-21-CH₃CTPP)Ni^IICl (1), at 203 K results in the formation of a rare paramagnetic (σ-phenyl)nickel(II) species, (2-NCH₃-21-CH₃CTPP)Ni^IIPh (2). The coordination of the σ-phenyl in 2 is determined by a unique pattern of three σ-phenyl resonances (ortho 375.0 ppm; meta 108.94 ppm, para 35.68 ppm (at 283 K)) in the ¹H NMR and ²H NMR spectra. The (σ-phenyl)nickel(II) compound 2 is in the high-spin ground electronic state (d_xy)²(d_xz)²(d_yz) ²(d_z²)¹(d_x² _-y²)¹, as confirmed by similarity of the NMR spectra of the equatorial ligand in 1 and 2. Titration of 1 with phenyllithium produces (2-NCH₃-21-CH₃CTPP)Ni^IIPh (2). One-electron reduction with excess PhLi yields [(2-NCH₃-21-CH₃CTPP)Ni^IIPh]^- (3), which can be also generated by independent routes, e.g., by reduction of (2-NCH₃-21-CH₃CTPP)Ni^IIPh using lithium triethylborohydride or tetrabutylammonium borohydride. The spectroscopic data indicate that (2-NCH₃-21-CH₃CTPP)Ni^IIPh (2) undergoes one-electron reduction without a substantial disruption of the molecular geometry. The presence of two paramagnetic centers in 3, i.e., the high-spin nickel(II) and the carbaporphyrin anion radical, produces remarkable variations in a spectral patterns, such as the upfield and downfield positions of pyrrole resonances (103.78, 96.66, -25.35, -50.97, -92.15, -114.83 ppm (at 253 K)) and sign alternations of the meso-phenyl resonances (ortho -77.81, -79.34 ppm; meta 48.77, 48.04 ppm; para -85.65, -86.46 ppm (at 253 K)). A single species, 4, is detected in the ¹H NMR titration of 1 with n-butyllithium. The formation of one- or two-electron-reduced species, [(2-NCH₃-21-CH₃CTPP)NiBu]^- or [(2-NCH₃-21-CH₃CTPP)NiBu]^2-, respectively, is considered to account for the spectroscopic properties of 4 (pyrrole 17.33, 15.45, -5.79, -7.74, -14.62, -58.14 ppm; 21-CH₃ 3 ppm (at 203 K)). The temperature dependence of the hyperfine shifts of 4 demonstrates pronounced anti-Curie behavior, interpreted in terms of a temperature-dependent spin equilibrium between diamagnetic and paramagnetic states with diamagnetic properties approached as the temperature is lowered. Warming of 2-4 results in complete decomposition via homolytic/heterolytic cleavage of an axial nickel-apical carbon bond. In the case of 2 or 3, the process yields a mixture of two compounds, 5 and 6, which are detected by EPR spectroscopy, demonstrating the anisotropy of the g tensor (5, g₁ = 2.237, g₂ = 2.092, g₃ = 2.090; 6, g₁ = 2.115, g₂ = 2.030, g₃ = 1.940 (in frozen toluene solution at 77 K)).

Full text of DOI:10.1021/ic000052p

Inorg. Chem. 2000, 39, 5639-5647
5639
Reactions of Nickel(II) 2,21-Dimethyl-2-aza-21-carbaporphyrin with Phenyl Grignard
Reagents, Phenyllithium, and n-Butyllithium
Piotr J. Chmielewski* and Lechosław Latos-Graz3yn´ski*
Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland
ReceiVed January 11, 2000
Addition of a phenyl Grignard reagent to a toluene solution of the nickel(II) chloride complex of a dimethylated
inverted porphyrin, (2-NCH3-21-CH3CTPP)NiIICl (1), at 203 K results in the formation of a rare paramagnetic
(σ-phenyl)nickel(II) species, (2-NCH3-21-CH3CTPP)NiIIPh (2). The coordination of the σ-phenyl in 2 is determined
by a unique pattern of three σ-phenyl resonances (ortho 375.0 ppm; meta 108.94 ppm; para 35.68 ppm (at 283
K)) in the 1H NMR and 2H NMR spectra. The (σ-phenyl)nickel(II) compound 2 is in the high-spin ground electronic
2
2
2
1
1
2
2
2
state (dxy) (dxz) (dyz) (dz ) (dx -y ) , as confirmed by similarity of the NMR spectra of the equatorial ligand in 1
and 2. Titration of 1 with phenyllithium produces (2-NCH3-21-CH3CTPP)NiIIPh (2). One-electron reduction with
excess PhLi yields [(2-NCH3-21-CH3CTPP)NiIIPh]- (3), which can be also generated by independent routes,
e.g., by reduction of (2-NCH3-21-CH3CTPP)NiIIPh using lithium triethylborohydride or tetrabutylammonium boro-
hydride. The spectroscopic data indicate that (2-NCH3-21-CH3CTPP)NiIIPh (2) undergoes one-electron reduction
without a substantial disruption of the molecular geometry. The presence of two paramagnetic centers in 3, i.e.,
the high-spin nickel(II) and the carbaporphyrin anion radical, produces remarkable variations in a spectral patterns,
such as the upfield and downfield positions of pyrrole resonances (103.78, 96.66, -25.35, -50.97, -92.15, -114.83
ppm (at 253 K)) and sign alternations of the meso-phenyl resonances (ortho -77.81, -79.34 ppm; meta 48.77,
1
48.04 ppm; para -85.65, -86.46 ppm (at 253 K)). A single species, 4, is detected in the H NMR titration of 1
with n-butyllithium. The formation of one- or two-electron-reduced species, [(2-NCH3-21-CH3CTPP)NiBu]- or
[(2-NCH3-21-CH3CTPP)NiBu]2-, respectively, is considered to account for the spectroscopic properties of 4 (pyrrole
17.33, 15.45, -5.79, -7.74, -14.62, -58.14 ppm; 21-CH3 3 ppm (at 203 K)). The temperature dependence of
the hyperfine shifts of 4 demonstrates pronounced anti-Curie behavior, interpreted in terms of a temperature-
dependent spin equilibrium between diamagnetic and paramagnetic states with diamagnetic properties approached
as the temperature is lowered. Warming of 2-4 results in complete decomposition via homolytic/heterolytic
cleavage of an axial nickel-apical carbon bond. In the case of 2 or 3, the process yields a mixture of two
compounds, 5 and 6, which are detected by EPR spectroscopy, demonstrating the anisotropy of the g tensor (5,
g1 ) 2.237, g2 ) 2.092, g3 ) 2.090; 6, g1 ) 2.115, g2 ) 2.030, g3 ) 1.940 (in frozen toluene solution at 77 K)).
Introduction
A complementary and relevant area of porphyrin modification
involves the replacement of one pyrrole ring by benzene,10
Interchange of a nitrogen and a â-methine group in a pyrrole
of 5,10,15,20-tetraphenylporphyrin (TPPH2) results in the
creation of the porphyrin-like skeleton 5,10,15,20-tetraphenyl-
2-aza-21-carbaporphyrin (21-CTPPH2, inverted tetraphenylpor-
phyrin) although with fundamentally changed electronic and
coordination properties.1,2 A peralkylated, meso-unsubstituted
inverted porphyrin has been synthesized as well.3,4 The inverted
porphyrin belongs to a larger group of recently emerged
porphyrin isomers5-9 although this particular molecule repre-
sents the only case where the pattern of the porphyrin framework
is preserved.
semiquinone,11,12 cycloheptatriene,13,14 indene,13,15 azulene,16
cyclopentadiene,17 an aliphatic bicyclic alkene,18 and 2,4-linked
thiophene19 moieties. Such an approach provides a CH unit in
the position of the pyrrole nitrogen, preserving the (N,NH,NH,CH)
central core as a common structural denominator of carbapor-
phyrin-like macrocycles. 2-Aza-21-carba-23-oxaporphyrin and
2-aza-21-carba-23-thiaporphyrin, i.e., isomers of 21-oxapor-
phyrin and 21-thiaporphyrin, combine features of both: inverted
porphyrin and 21-heteroporphyrin,20,21 forming definitely dif-
(9) Vogel, E.; Broring, M.; Erben, C.; Demuth, R. Angew. Chem., Int.
Ed. Engl. 1997, 36, 353.
* To whom correspondence should be addressed.
(1) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Rachlewicz, K.; Głowiak,
(10) Berlin, K.; Breitmaier, E. Angew. Chem., Int. Ed. Engl. 1994, 33, 1246.
(11) Lash, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 2533.
(12) Lash, T. D.; Chaney, S. T.; Richter, D. T. J. Org. Chem. 1998, 63,
9076.
(13) Berlin, K.; Steinbeck, C.; Breitmaier, E. Synthesis 1996, 336.
(14) Lash, T. D.; Chaney, S. T. Tetrahedron Lett. 1996, 37, 8825.
(15) Lash, T. D.; Hayes, M. J. Angew. Chem., Int. Ed. Engl. 1997, 35,
841.
T. Angew. Chem., Int. Ed. Engl. 1994, 33, 779.
(2) Furuta, H.; Asano, T.; Ogawa, T. J. Am. Chem. Soc. 1994, 116, 767.
(3) Liu, B. Y.; Bru¨ckner, C.; Dolphin, D. Chem. Commun. 1996, 2141.
(4) Lash, T. D.; Richter, D. T., Shiner, C. M. J. Org. Chem. 1999, 64,
7973.
(5) Vogel, E.; Ko¨cher, M.; Schmickler, H.; Lex, J. Angew. Chem., Int.
Ed. Engl. 1986, 25, 257.
(6) Vogel, E.; Koch, P.; Hou, X.-L.; Lex, J.; Lausman, M.; Kister, M.;
Aukauloo, M. A.; Richard, P.; Guilard, R. Angew. Chem., Int. Ed.
Engl. 1993, 32, 1600.
(7) Aukauloo, M. A.; Guilard, R. New J. Chem. 1994, 18, 1205.
(8) Sessler, J. L.; Brucker, E. A.; Weghorn, S. J.; Kisters, M.; Scha¨fer,
M.; Lex, J.; Vogel, E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2308.
(16) Lash, T. D.; Chaney, S. T. Angew. Chem., Int. Ed. Engl. 1997, 35,
839.
(17) Berlin, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1820.
(18) Hayes, M. J.; Lash, T. D. Chem.sEur. J. 1998, 4, 508.
(19) Sprutta, N.; Latos-Graz˘yn´ski, L. Tetrahedron Lett. 1999, 8457.
(20) Heo, P.-Y.; Shin, K.; Lee, C.-H. Tetrahedron Lett. 1995, 197.
10.1021/ic000052p CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/10/2000

5640 Inorganic Chemistry, Vol. 39, No. 25, 2000
Chmielewski and Latos-Graz˘yn´ski
ferent coordination cores, i.e., (N,CH,N,O) and (N,CH,NH,S).
The recent dicarbaporphyrinoid synthesis by Lash et al. provides
an important addition to the carbaporphyrinoid class of mac-
rocycles.22
Taking advantage of the unusual reactivity of the nickel
carbaporphyrin and of the 21-carbaporphyrin free base, we have
extended the group of inverted porphyrins with the correspond-
ing methylated derivatives 2-methyl-5,10,15,20-tetraphenyl-2-
aza-21-carbaporphyrin (2-NCH3CTPPH), 5,10,15,20-tetraphenyl-
21-methyl-2-aza-21-carbaporphyrin (21-CH3CTPPH2), and 2,21-
dimethyl-5,10,15,20-tetraphenyl-2-aza-21-carbaporphyrin (2-
NCH3-21-CH3CTPPH).23,24
CH3TPPH), and 21,23-dioxaporphyrin (O2TPP) environments.31-33
To search for novel paramagnetic organonickel(II) species
bearing one or two σ-phenyls, we had previously investigated
the reactions of (XTPP)NiIICl or (O2TPP)NiIICl2 with aryl
Grignard reagents, anticipating replacement of one or both
chloride ligands by σ-phenyl(s).31-33 Despite thermal instability,
the formation of paramagnetic organonickel(II) heteroporphyrins
has been directly confirmed by means of 1H NMR and 2H NMR
spectroscopy. Generally, the determined hyperfine shift patterns
provided several useful and potentially unique probes for
detecting (σ-phenyl)nickel heteroporphyrin derivatives.31-39
Here we report our efforts to obtain a novel class of
paramagnetic organonickel complexes. We had observed that
the structure of (2-NCH3-21-CH3CTPP)NiIII (1) contains the
Carbaporphyrins and methylated carbaporphyrins act as
mono- or dianionic ligands toward the nickel(II) and nickel(III)
ions.1,4,23-25 Thus one Ni-C and three Ni-N bonds are formed
within the inverted tetrapyrrole macrocycle of (CTPP)NiII or
(2-NCH3CTPP)NiII. Both macrocycles act as four-coordinate
dianionic ligands, enforcing the coordination of the pyrrole sp2
carbon to nickel(II).1,23 In the (21-CH3CTPP)NiII case, the
nickel(II) is four-coordinate with bonds to three pyrrole nitrogens
and the pyrrole carbon.24 The methylated pyrrole is bound to
the nickel via a pyramidal carbon in an η1 fashion. A similar
mode of coordination can be expected in a nickel(II) inverted
heptaalkyl porphyrin protonated at the C(21) carbon.4 In the
paramagnetic (2-NCH3-21-CH3CTPP)NiIII compound, the meth-
ylated pyrrole is linked to nickel in an η1 fashion but the
coordinating C(21) carbon preserves the features related to the
trigonal sp2 hybridization.24 Furuta and co-workers have dem-
onstrated that inverted porphyrins act as trianionic ligands,
stabilizing the silver(III) cation.26
essential features for coordination of σ-phenyl or σ-alkyl ligands
in addition to the NiII-C(21) bond already forced by the
equatorial macrocyle.24 Consequently, we investigated the
reactions of (2-NCH3-21-CH3CTPP)NiIICl with aryl (alkyl)
Grignard reagents and phenyl(alkyl)lithium, anticipating re-
placement of the apical ligand by σ-phenyl (σ-alkyl), to yield
paramagnetic (2-NCH3-21-CH3CTPP)NiIIR complexes, contain-
ing both equatorial and apical NiII-carbon bonds. The reactivity
of these new organometallic species has been a matter of our
interest as well.
Results and Discussion
General Procedures. Additions of the aryl Grignard reagents
PhMgBr and (Ph-d5)MgBr, butyllithium, and phenyllithium to
toluene-d8 solutions (203 K) of (2-NCH3-21-CH3CTPP)NiIICl
(1) resulted in the formation of a variety of paramagnetic
organonickel species, which were detected and subsequently
Paramagnetic organometallic nickel(II) complexes are rare
and typically unstable, although with some remarkable ex-
ceptions.24,27-30 Our contributions to the field, apart from (2-
NCH3-21-CH3CTPP)NiIII,24 include investigations on σ-phenyl
coordination to the nickel(II) ion located within 21-heteropor-
phyrin (XTPPH; X ) O, S, Se), N-methylporphyrin (N-
1
2
characterized by H and H NMR spectroscopy. The NMR
resonance assignments are based on relative intensities, line
width analysis, and site-specific deuteration using the following
derivatives of (2-NCH3-21-CH3CTPP)NiIICl: (2-NCD3-21-
CD3CTPP)NiIICl, (2-NCH3-21-CD3CTPP)NiIICl, (2-NCH3-21-
CH3CTPP-d7)NiIICl (deuterated in all pyrrole â positions), and
(2-NCH3-21-CH3CTPPH-d20)NiIICl (deuterated in all positions
(21) Lee, C.-H.; Kim, H.-J.; Yoon, O.-W. Bull. Korean Chem. Soc. 1999,
20, 276.
(22) Lash, T. D.; Romanic, J. L.; Hayes, M. J.; Spence, J. D. Chem.
Commun. 1999, 819.
(31) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1992, 31, 5231.
(32) Pacholska, E.; Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chim.
Acta 1998, 273, 184.
(33) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1998, 37, 4179.
(34) Latos-Graz˘yn´ski, L. Inorg. Chem. 1985, 24, 1681.
(35) Lisowski, J.; Latos-Graz˘yn´ski, L.; Szterenberg, L. Inorg. Chem. 1992,
31, 933.
(23) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. J. Chem. Soc., Perkin Trans.
2 1995, 503.
(24) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Głowiak, T. J. Am. Chem.
Soc. 1996, 118, 5690.
(25) Chmielewski, P. J.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1997, 36, 840.
(26) Furuta, H.; Ogawa, T.; Uwatoko, Y.; Araki, K. Inorg. Chem. 1999,
38, 2676.
(36) Chmielewski, P. J.; Latos-Graz˘yn´ski, L.; Pacholska, E. Inorg. Chem.
1994, 33, 1992.
(27) D’Anielo, M. J., Jr.; Barefield, E. K. J. Am. Chem. Soc. 1976, 98,
1610.
(28) Lin, S.-K.; Jaun, B. HelV. Chim. Acta 1991, 74, 1725.
(29) Kla¨ui, W.; Huhn, M.; Herbst-Irmer, R. J. Organomet. Chem. 1991,
415, 133.
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Rheingold, A. L.; Marchaj, A.; Norton, J. R. J. Am. Chem. Soc. 1997,
119, 1648.
(37) Latos-Graz˘yn´ski, L.; Pacholska, E.; Chmielewski, P. J.; Olmstead, M.
M.; Balch, A. L. Inorg. Chem. 1996, 35, 566.
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(39) Latos-Graz˘yn´ski, L. Core Modified Heteroanalogues of Porphyrins
and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000;
Vol. 2, p 361.

Nickel(II) Carbaporphyrin Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5641
of the meso-phenyl rings). In each case, the stoichiometry of
the σ-phenyl adduct was unambiguously determined by a careful
comparison of resonance intensities (integrated signal areas)
assigned to the equatorial macrocycle and σ-phenyl (ortho, meta,
and para) protons, respectively. The spectral patterns of the
formed σ-phenyl complexes strongly depend on the ligand
sources.
The half-wave potentials for the two one-electron reductions
of (2-NCH3-21-CH3CTPP)NiIICl, (1) -0.698 and (2) -0.970
V (vs SCE; THF solution, TBAP), are close to the corresponding
reduction potentials found for nickel(II) heteroporphyrins.39
Thus, the alternative reaction pathways of (2-NCH3-21-
CH3CTPP)NiIICl with aryl (alkyl) reagents may result in one-
or two-electron reductions with or without a formation of a
nickel-carbon bond and such a route will be included in our
analysis. Alkyllithium, aryllithium, and Grignard reagents are
known to act as one-electron-reducing reagents.40-46 The
reduction has been also demonstrated in several attempts to
generate metalloporphyrins with metal-carbon bonds.40,41,44-46
Reactions with Phenyl Grignard Reagents. Titration of 1
with PhMgBr (toluene-d8, 203 K) led to the substitution of one
chloride ligand by the phenyl anion, yielding the complex (2-
NCH3-21-CH3CTPP)NiIIPh (2) according to the reaction
(2-NCH3-21-CH3CTPP)NiIICl + PhMgBr f
1
(2-NCH3-21-CH3CTPP)NiIIPh + MgBrCl (1)
2
Figure 1. NMR spectra of (σ-phenyl)nickel(II) dimethylated inverted
The (2-NCH3-21-CH3CTPP)NiIIPh species 2 has been ob-
served in the temperature range 203-283 K although substantial
decomposition has been observed above 263 K. Representative
NMR spectra are shown in Figure 1. The coordination of the
apical σ-phenyl ligand has been unambiguously confirmed by
the “fingerprint”-type downfield pattern of the three resonances
in the 2H NMR spectrum (ortho 375.0 ppm; meta 108.94 ppm;
para 35.68 ppm (at 283 K)). The 1H (2H) NMR spectral
characteristics and accordingly the electronic structure of 2 are
similar to those determined for the analogous species which
were formed in the course of PhMgBr(Cl) reactions with
nickel(II) complexes of core-modified porphyrins, i.e., (NCH3-
TPP)NiIIPh, (OTPP)NiIIPh, (STPP)NiIIPh, (SeTPP)NiIIPh, and
(O2TPP)NiII(Ph)Cl.31-33 The characteristic paramagnetic shifts
are collected in Table 1, for the sake of comparison, with the
respective shifts of the parent complex 1.24 Patterns of NMR
spectra determined for (2-NCH3-21-CH3CTPP)NiIICl (1), (2-
NCH3-21-CH3CTPP)NiIII24 (1′), and (2-NCH3-21-CH3CTPP)-
NiIIPh (2) are similar, and observed differences reflect only the
fine effects due to the axial coordination. It was previously
shown for (2-NCH3-21-CH3CTPP)NiIII24 (1′) that the nickel(II)
ion is five-coordinated with equatorial bonds to three pyrrole
nitrogen atoms and to the pyrrole C(21) carbon. The nickel is
porphyrin (2,21-dimethyl-5,10,15,20-tetraphenyl-2-aza-21-carbaporphy-
1
rin) complexes 2: (A) (2-NCH3-21-CH3CTPP)NiIIPh, H NMR; (B)
2
(2-NCH3-21-CH3CTPP)NiII(Ph-d5), H NMR. Spectra were recorded
in toluene solutions (A) C7D8 and (B) C7H8 at 283 K. Peak labels follow
the systematic numbering of the carbaporphyrin ring or denote proton
groups: pyrr, pyrrole; o, ortho, m, meta, and p, para phenyl protons
(deuterons) of the σ-bonded phenyl ligand.
moved out from the three-nitrogen plane toward the iodo ligand.
The 21-CH3 group and iodide are located on the opposite sides
of the macrocycle. The substituted pyrrole is bound to nickel
in an η1 fashion, but the coordinating C(21) atom preserves
features related to trigonal sp2 hybridization. On the basis of
the paramagnetic shift similarity, one can presume that replace-
ment of the chloro or iodo ligand by the σ-phenyl ligand in 2
preserves the basic structural features of 1 or 1′.24
The Curie plots of the temperature dependence of the
chemical shifts of the pyrrole and methyl resonances of
(2-NCH3-21-CH3CTPP)NiIIPh (not shown), collected in a
limited temperature range, demonstrate linear behaviors with
extrapolated intercepts that do not correspond to the suitable
diamagnetic references, e.g., (21-CH3CTPP)NiII or the relevant
carbaporphyrin.24 Actually the deviations are a result of
curvatures of δ ) f(1/T) plots. These deviations, which are
typical for nickel(II) heteroporphyrins, indicate small contribu-
tions from dipolar shifts due to the anisotropies of zero-field
splittings.24,31-35,39 Generally, one can conclude that the (σ-
phenyl)nickel(II) derivative is in the high-spin state (dxy)2-
(40) Cloutour, C.; Debaig-Valade, C.; Gacherieu, C.; Pommier, J.-C. J.
Organomet. Chem. 1978, 161, 327.
(41) Cloutour, C.; Debaig-Valade, C.; Gacherieu, C.; Pommier, J.-C. J.
Organomet. Chem. 1984, 269, 239.
(42) Stasˇko, A.; Tka´cˇ, A.; Pˇrikryl, R.; Mal´ık, L. J. Organomet. Chem. 1975,
92, 253.
(43) Stasˇko, A.; Tka´cˇ, A.; Mal´ık, L. Adamcˇ´ık, V. J. Organomet. Chem.
1975, 92, 261.
2
2
1
1
2
2
2
(dxz) (dyz) (dz ) (dx -y ) . The spin transfer occurs via an interac-
2
tion of the unpaired electron in the σ-symmetric Ni dz atomic
orbital with the phenyl orbital of σ-symmetry. The large
downfield σ-contact shifts of the apical phenyl provide the
experimental evidence for such a description of the delocaliza-
tion mechanism.
Particularly, the downfield shifts of pyrrole and C(21)-methyl
resonances indicate a σ-delocalization of spin density. This
(44) Balch, A. L.; Hart, R. H.; Latos-Graz˘yn´ski, L.; Traylor, T. G. J. Am.
Chem. Soc. 1990, 112, 7382.
(45) Balch, A. L.; Latos-Graz˘yn´ski, L.; Noll, B. C.; Phillips, S. L. Inorg.
Chem. 1993, 32, 1124.
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1354.

5642 Inorganic Chemistry, Vol. 39, No. 25, 2000
Chmielewski and Latos-Graz˘yn´ski
Table 1. Chemical Shifts of Nickel(II) Complexes of Inverted Porphyrins (ppm)
pyrrole
H
inverted pyrrole
2,3-H
N- or C-CH3
H
axial ligand
H
complex
(2-NH-21-CH3CTPP)NiIICla
58.06, 58.06, 52.40, 48.25,26.53, 24.86
2.04 (3-H), -23.07 109.7 (21-CH3)
(2-NH)
(2-NCH3-21-CH3CTPP)NiIICla (1) 59.62, 55.95,50.91, 49.33, 28.15, 27.16
-1.67
98.81 (21-CH3),
28.30 (2-CH3)
(2-NCH3-21-CH3CTPP)NiIIPhb (2) 54.92, 51.52, 40.81, 39.75, 24.91, 23.89 -1.17
[(2-NCH3-21-CH3CTPP)NiIIpy]Clc 81.07, 78.07, 67.48, 63.34, 36.38, 36.44 -8.53
[(2-NCH3-21-CH3CTPP)NiIIPh]- d 103.78, 96.66, -25.35, -50.97, -92.15,
(3)
[(2-NCH3-21-CH3CTPP)NiBu]n- e 14.95, 13.48, -3.45, -4.37, -10.16,
(4) -46.87
115.43 (21-CH3), 375.0 (o), 108.94 (m),
21.19 (2-CH3) 35.68 (p)
157.70 (21-CH3), 215.18 (o), 71.18 (m),
59.98 (2-CH3) 20.28 (p)
217.20 (21-CH3), 123.30 (m), 111.61 (m),
86.80 (2-CH3)
19.17 (2-CH3),
3.32 (21-CH3)
-114.83
39.82 (p)
22.07, 21.36, 18.5,
-21.91
a Chloroform-d, 293 K; data from ref 24. b Toluene-d8, 283 K. c Toluene-d8, 223 K. d Toluene-d8, 253 K; the signals of meso-aryl protons were
observed at 48.77 (2H, m), 48.06 (2H, m), -77.81 (2H, o), -79.34 (2H, o), -85.65 (1H, p), and -86.46 ppm (1H, p). e Toluene-d8, 203 K.
1
Figure 2. 300 MHz H NMR spectrum of [(2-NCH3-21-CH3CTPP)-
NiII(py)]+ in toluene-d8 at 223 K. A total of 10 equiv of pyridine was
added with respect to starting (2-NCH3-21-CH3CTPP)NiIICl. Assign-
ment of the dimethylated inverted porphyrin resonances is as in Figure
1; o-py, m-py, p-py ) ortho, meta, para resonances of the pyridine
ligand.
Figure 3. NMR spectra of 3, formed by treating 1 with phenyllithium
mechanism has been described previously in detail for 1, and it
1
in toluene at 203 K: (A) [(2-NCH3-21-CH3CTPP)NiIIPh]-, H NMR,
is common for nickel core-modified systems.24,31-39 The shift
pattern of the carbaporphyrin equatorial ligand resembles that
found for the [(2-NCH3-21-CH3CTPP)NiII(py)]+ species, gener-
ated by titration of (2-NCH3-21-CH3CTPP)NiIICl with pyridine,
as shown in Figure 2 (toluene-d8, 223 K):
C7D8, 253 K; (B) [(2-NCH3-21-CH3CTPP-d7)NiIIPh]-, 1H NMR, C7D8,
2
253 K; (C) [(2-NCH3-21-CD3CTPP)NiIIPh]-, H NMR, C7H8, 223 K;
(D) [(2-NCD3-21-CD3CTPP)NiIIPh]-, 2H NMR, C7H8, 223 K. Assign-
ment of the dimethylated inverted porphyrin resonances is as in Figure
1; o-Ph, m-Ph, p-Ph ) resonances of strongly paramagnetic shifted
meso-phenyl substituents of the dimethylated inverted porphyrin. The
residual resonances of 6 are marked with asterisks.
(2-NCH3-21-CH3CTPP)NiIICl + py h
[(2-NCH3-21-CH3CTPP)NiII(py)]+ + Cl- (2)
complex 1H NMR picture than that determined in the analogous
experiment using PhMgBr. Initially, the H NMR spectrum of
1
As expected the shift patterns of axially coordinated pyridine
and the phenyl anion are alike, as both ligands are isoelec-
tronic.31
Reactions with Phenyllithium. Titration of 1 with PhLi
(toluene-d8) carried out at 203 K resulted unexpectedly in a more
(2-NCH3-21-CH3CTPP)NiIIPh (2) was accompanied by a well-
defined set of paramagnetic shifted resonances corresponding
to the novel compound 3. Compound 3 forms exclusively when
phenyllithium is used in excess (Figure 3). The 1H NMR
spectrum of 3 is different from that of any paramagnetic

Nickel(II) Carbaporphyrin Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5643
nickel(II) porphyrin or heteroporphyrin.24,31-39 On the basis of
its unusual NMR features (vide infra), the second monophenyl
adduct has been identified as the product of a one-electron
reduction of 2, i.e., [(2-NCH3-21-CH3CTPP)NiPh]- (3), formed
in a two-step process:
(2-NCH3-21-CH3CTPP)NiIICl + PhLi f
(2-NCH3-21-CH3CTPP)NiIIPh + LiCl (3)
(2-NCH3-21-CH3CTPP)NiIIPh + PhLi f
[(2-NCH3-21-CH3CTPP)NiIIPh]Li + Ph• (4)
To verify that the conversion of 2 to 3 is exclusively a
reduction, we demonstrated that 3 can been generated by
1
independent routes. The H NMR titration of 1 with PhMgBr
(toluene-d8, 203 K) to produce (2-NCH3-21-CH3CTPP)NiIIPh
(2) may be followed by the addition of a reducing reagent such
as lithium triethylborohydride or tetrabutylammonium borohy-
dride. The observation that 3 can be generated using a
combination of PhMgBr and tetrabutylammonium borohydride
excludes any special role of the lithium cations in the stabiliza-
tion of 3.
The 1H NMR spectrum of 3 (Figure 3, trace A) revealed six
pyrrole resonances: two of them shifted downfield (103.78,
96.66 ppm) but four shifted remarkably upfield (-25.35,
-50.97, -92.15, -114.83 ppm (at 253 K)). The seventh pyrrole
resonance could not be located despite considerable effort. It
was likely obscured by the signal of the solvent (heptane or
THF) added with phenyllithium. The coordination of one
σ-phenyl ligand has been unambiguously determined. The
corresponding two meta resonances (123.30, 111.61 ppm) and
Figure 4. Plots of the chemical shifts versus 1/T for [(2-NCH3-21-
CH3CTPP)NiIIPh]- (3) in toluene-d8. Assignments of the resonances
follow those in Figure 3.
the 2H NMR spectrum of [(2-NCH3-21-CD3CTPP)NiPh]-
(Figure 3, trace C). The 2-NCD3 resonance is found at 104.8
ppm in the 2H NMR spectrum of [(2-NCD3-21-CD3CTPP)NiII-
Ph]- at 233 K (Figure 3, trace D). The remaining, paramagnetic
shifted resonances of 3 shown in Figure 3 have been assigned
to the meso-phenyls. The preliminary analysis, based on the C1
geometry of 3, requires four inequivalent meso-phenyls, each
presenting one para, two meta, and two ortho proton resonances.
The two ortho and the two meta protons on each of the meso-
phenyl rings are inequivalent, since the porphyrin plane bears
different substituents on the opposite faces and the rotation about
the Cmeso-Cphenyl bond may be restricted. Since all meso-phenyl
rings are structurally different and are exposed to different steric
hindrances, it is anticipated that there will be some differences
in their rates of rotation. Pairwise rotationally averaged features
of ortho and meta phenyl resonances have been seen for two
meso-phenyl rings. Two resonances of comparable line widths
at 48.77 and 48.06 ppm have been observed, each demonstrating
the intensity of two protons. These are assigned to the meso-
phenyl meta protons. Two other meso-phenyl resonances (each
revealing a two-proton intensity as well) have been identified
at -77.81 and -79.34 ppm (253 K) and are assigned to the
ortho protons originating from the same meso-phenyl rings.
Finally, the remaining two resonances (each of one-proton
intensity) at -85.65 and -86.46 ppm (253 K) are assigned to
the corresponding para protons. The sign alternation of the meso-
phenyl resonances and the sizes of the isotropic shifts indicate
very large π-spin densities at two out of four carbaporphyrin
meso positions. The resonances of the two other phenyls are
located in the rather typical 19-9 ppm region for meso-phenyls
of nickel(II) tetraphenylporphyrin derivatives.31-39 At this stage,
we cannot assign meso-phenyl resonances to the specific meso
positions in the macrocyclic structure.
1
one para resonance (39.08 ppm) have been assigned in the H
NMR spectrum (Figure 3, traces A and B). Their positions
parallel those recognized as typical for apical coordination of
σ-phenyl ligands in five-coordinate high-spin nickel(II) core-
modified porphyrins,31-33 including (2-NCH3-21-CH3CTPP)NiIIPh
(2) (Table 1). For 3, two meta resonances with intensities
corresponding to one proton were observed rather than the
expected single two-proton signal. The resolution of two
σ-phenyl meta resonances for 3 in contrast to 2 must result from
hindered rotation about the Ni-C bond that is slow on the NMR
time scale. Most likely, the σ-phenyl ligand binds with its plane
parallel to the N(2)-C(3) bond. Consequently, the carbapor-
phyrin asymmetry removes the intrinsic C2 symmetry of the
coordinated σ-phenyl, rendering the two meta protons and the
two ortho protons inequivalent. Other orientations are less
favorable because they may create a disadvantageous contact
between the σ-phenyl ligand and the methylated pyrrole ring.
This phenomenon, although feasible because of the identical
symmetries of the equatorial macrocycles, has been observed
neither for the related (2-NCH3-21-CH3CTPP)NiIIPh nor for
[(2-NCH3-21-CH3CTPP)NiII(py)]+ in the same temperature
range. This reflects the difference in the dynamic behaviors
related to the rotation with respect to the Ni-Cphenyl(Npyridine
)
bond. Likely, one-electron reduction of 2 to generate 3 causes
a structural rearrangement that increases this rotation barrier.
A similar restriction of rotation was determined in the case of
coordination of 4-methylimidazole by iron(III) N-methylpor-
phyrin.47
The downfield broad resonance at 257.9 ppm (223 K) has
been assigned to the 21-CH3 group. This resonance appears in
The Curie plots of the temperature dependence of the
chemical shifts of the pyrrole and methyl resonances of 3 (Figure
4) demonstrate linear behaviors over the limited temperature
(47) Balch, A. L.; Cornman, C. R.; Latos-Graz˘yn´ski, L.; Olmstead, M. M.
J. Am. Chem. Soc. 1990, 112, 7552.

5644 Inorganic Chemistry, Vol. 39, No. 25, 2000
Chmielewski and Latos-Graz˘yn´ski
range that could be examined. The extrapolated intercepts do
not correspond to the suitable diamagnetic references. The
observed deviations from the Curie law resemble those seen
for high-spin nickel(II) heteroporphyrins.24,31-38
ized on a porphyrin π-cation radical and on an oxophlorin radical
were previously reported.65,66 Consequently, we have concluded
that the observed paramagnetic shifts of 3 reflect simultaneous
contributions of two spin density sources, i.e., the high-spin
nickel(II) ion and a carbaporphyrin anion radical. For 3, there
are several electronic structures that need to be considered which
are differentiated by the distributions of the unpaired electron
spin densities and/or by the interactions between the magnetic
centers. Thus, a metal-centered reduction results in the formation
of a nickel(I) complex with a classical d9 electronic configu-
ration, e.g., PNiIPh (P ) (2-NCH3-21-CH3CTPP)-). On the other
hand, a ligand-centered reduction produces a carbaporphyrin
anion radical. The resulting electron configurations are as
follows: (a) (P•-)NiIIPh antiferromagnetically coupled (NiII(h.s),
The spectroscopic data reported here indicate that (2-NCH3-
21-CH3CTPP)NiIIPh (2) undergoes a one-electron reduction by
phenyllithium without a substantial disruption of the molecular
geometry. The 1H NMR data for 3 show a number of remarkable
features that must arise from its unusual molecular and electronic
structures. With regard to molecular structure, the observation
of paramagnetic shifted σ-phenyl resonances is of particular
importance. It is consistent with the presence of a five-coordinate
high-spin nickel(II) ion apically coordinated by a σ-phenyl.31-33
As a matter of fact, the isotropic shift of the σ-phenyl can be
used here as the definitive and selective probe of the high-spin
electronic state of the nickel(II) ion for the composite electronic
structure of 3. The large downfield shift of the 21-CH3 group
(Table 1) implies a preservation of the equatorial nickel(II)-
carbon(21) bond.
1H NMR spectroscopy has been shown to be the definitive
method for characterizing paramagnetic metalloporphyrins.48-51
The paramagnetic shifts of porphyrins and porphyrin substituents
are sensitive to the electronic structure of metalloporphyrins.
Consequently, similar spectra can be observed independently
of the metal ion coordinated by a porphyrin, showing the
similarity of the ground electronic states. Two classes of
metalloporphyrins may be relevant models expected to share
spectroscopic features with [(2-NCH3-21-CH3CTPP)NiPh]- (3)
and [(2-NCH3-21-CH3CTPP)Ni(n-Bu)]- (4). These are (a) iron
porphyrin π-cation radicals51-57 and (b) two-electron-reduced
high- or low-spin iron(III) porphyrins.50,58-61 In the second case,
the electronic structure can be described as the resonance hybrid
of two extreme canonical structures, i.e., an iron(I) porphyrin
and an iron(II) porphyrin π-anion radical. Apparently two
paramagnetic centers in the metalloporphyrin are required to
produce some of the NMR features detected for 3 and 4.
Furthermore, both the large spread of meso-phenyl resonances
and the shift alternation of the pyrrole resonance signs have
precedent, particularly in systems with intrinsic asymmetries
of porphyrin skeletons.62-64 Similarly, the pertinent couplings
between high-spin nickel(II) ions and unpaired electrons local-
1
3
S ) /2); (b) (P•-)NiIIPh ferromagnetically coupled (S ) /2);
1
(c) (P•-)NiIIPh uncoupled or weakly coupled (S ) 1, S ) /2).
In the case of [(2-NCH3-21-CH3CTPP)NiPh]-, which is EPR
silent in frozen toluene solution at 77 K, nickel-centered
reduction can be excluded on the basis of the fact that nickel(I)
in a macrocyclic environment is expected to display character-
1
istic EPR spectra.32,36,67-69 Typically, H NMR spectra of d9
metalloporphyrin ions are beyond detection, as T1e is relatively
long. Detectable NMR signals could be observed solely using
deuterium NMR spectroscopy.36,70 Similarly, porphyrin radicals
typically reveal well-resolved EPR but not 1H NMR spectra.58,71
However once the interaction between two paramagnetic centers
is accessible, the mutual interaction leads to the shortening of
T1e, affording well-resolved paramagnetic shifted 1H NMR
resonances, even for d9 metal ions or anion porphyrin radi-
cals.49,58,60 Hence the 1H NMR meso-phenyl resonance pattern
of 3 is clearly consistent with the presence of the large π-spin
density on the carbaporphyrin. This reflects the anion radical
electronic structure of the equatorial ligand. Presumably the
unpaired carbaporphyrin electron is localized in the π orbital
having its spin density at the four selected pyrrole positions
and two of the four meso-carbons. The remaining pyrrole shifts
are due to the high-spin nickel(II) ion, as in 2.
The origin of the direction of the meso-phenyl shifts in the
1H NMR spectra of metal complexes containing porphyrin
radicals has been a matter of interest. It appears that a correlation
exists between these shifts and the coupling of porphyrin and
metal spins.52-57,62,64 For metalloporphyrin radicals in which
the porphyrin and metal spins are antiferromagnetically coupled,
the phenyl rings show downfield shifts for ortho and para
protons and upfield shifts for meta protons. For complexes in
which there is no antiferromagnetic coupling, the pattern is
reversed; i.e., ortho and para resonances are located upfield and
meta resonances downfield. Thus the meso-phenyl NMR
features determined for 3 resemble the second situation, which
excludes strong antiferromagnetic coupling in the ground
electronic state.
(48) La Mar, G. N.; Walker, F. A. In The Porphyrins; Dolphin, D., Ed.;
Academic Press: New York, 1979; pp 61-312.
(49) Bertini, I.; Luchinat, C. NMR of Paramagnetic Molecules in Biological
Systems; The Benjamin/Cummings Publishing Co.: Reading, MA,
1986.
(50) Bertini, I.; Luchinat, C. Coord. Chem. ReV. 1996, 150, 131.
(51) Walker, F. A.; Simonis, U. In NMR of Paramagnetic Molecules;
Berlinger, L. J., Reuben, J., Eds.; Biological Magnetic Resonance,
Vol. 12; Plenum Press: New York, 1993; p 133.
(52) Gans, P.; Buisson, G.; Duwe, E.; Marchon, J.-C.; Erler, B. S.; Scholz,
W. F.; Reed, C. A. J. Am. Chem. Soc. 1986, 108, 1223.
(53) Goff, H. M.; Phillippi, M. A. J. Am. Chem. Soc. 1983, 105, 7567.
(54) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.; Evans,
B. J. J. Am. Chem. Soc. 1981, 103, 284.
(63) Balch, A. L.; Latos-Graz˘yn´ski, L.; Noll, B. C.; Olmstead, M. M.;
Szterenberg, L.; Zovinka, E. P. J. Am. Chem. Soc. 1993, 115, 11846.
(64) Rachlewicz, K.; Latos-Graz˘yn´ski, L. Inorg. Chem. 1996, 35, 1136.
(65) Renner, M. W.; Barkigia, K. M.; Melamed, D.; Smith, K. M.; Fajer,
J. Inorg. Chem. 1996, 35, 5120.
(55) Balch, A. L.; Latos-Graz˘yn´ski, L.; Renner, M. W. J. Am. Chem. Soc.
1985, 107, 2983.
(56) Mandon, D.; Weiss, R.; Jayaraj, K.; Gold, A.; Terner, J.; Bill, E.;
Trautwein, A. X. Inorg. Chem. 1992, 31, 4404.
(57) Fujii, H. J. Am. Chem. Soc. 1993, 115, 4641.
(66) Balch, A. L.; Noll, B. C.; Phillips, S. L.; Reid, S. M.; Zovinka, E. P.
Inorg. Chem. 1993, 32, 4730.
(67) Busch, D. H. Acc. Chem. Res. 1978, 11, 392.
(58) Hickman, D. L.; Shirazi, A.; Goff, H. M. Inorg. Chem. 1985, 24, 563.
(59) Donohoe, R. J.; Atamian, M.; Bocian, D. F. J. Am. Chem. Soc. 1987,
109, 5593.
(60) Yamaguchi, K.; Morishima, I. Inorg. Chem. 1992, 31, 3216.
(61) Kadish, K. M.; Van Caemelbecke, E.; D’Souza, F.; Lin, M.; Nurco,
D. J.; Medforth, C. J.; Forsyth, T. P.; Krattinger, B.; Smith, K. M.;
Fukuzumi, S.; Nakanishi, I.; Shelnutt, J. A. Inorg. Chem. 1999, 38,
2188.
(68) Chmielewski, P. J.; Grzeszczuk, M.; Latos-Graz˘yn´ski, L.; Lisowski,
J. Inorg. Chem. 1989, 28, 3546.
(69) Renner, M. W.; Furelind, L. R.; Barkigia, K. M.; Forman, A.; Sim,
H.-K.; Simpson, D. J.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1991,
113, 6891.
(70) Godziela, G. M.; Goff, H. M. J. Am. Chem. Soc. 1986, 108, 2237.
(71) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; p 198.
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Am. Chem. Soc. 1992, 114, 2230.

Nickel(II) Carbaporphyrin Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5645
Figure 6. Plots of the chemical shifts versus 1/T for 4 in toluene-d8.
Assignments of the resonances follow those in Figure 5.
biguously identified at 19.17 ppm, since it is present in the
spectrum of [(2-NCD3-21-CD3CTPP)NiBu]n- (Figure 5, trace
D). The remaining, paramagnetic shifted resonances shown in
Figure 5 are assigned to meso-phenyl groups.
The Curie plots of the temperature dependence of the
chemical shifts demonstrate pronounced anti-Curie behavior
(Figure 6), which is unique for the whole collection of nickel(II)
heteroporphyrins and nickel(II) carbaporphyrins,31-39 including
the new organometallic derivatives 2 and 3 described above.
Qualitatively, the temperature dependence of the hyperfine shifts
of 4 can be interpreted in terms of a temperature-dependent spin
equilibrium between diamagnetic and paramagnetic states with
diamagnetic properties approached as the temperature is low-
ered.
Figure 5. NMR spectra of 4, formed by treating 1 with n-butyllithium
in toluene at 203 K: (A) 1 ) (2-NCH3-21-CH3CTPP)NiIICl, C7D8, 1H
NMR; (B) 1 ) (2-NCH3-21-CH3CTPP-d7)NiIICl, C7D8, 1H NMR; (C)
1 ) (2-NCH3-21-CD3CTPP)NiIICl, C7H8, 2H NMR; (D) 1 ) (2-NCD3-
21-CD3CTPP)NiIICl, C7H8, 2H NMR. Assignment of the dimethylated
inverted porphyrin resonances is as in Figures 1 and 4; Bu ) resonances
of the coordinated σ-butyl ligand.
Reactions with Butyllithium. 1H NMR titration of 1
(toluene-d8) with n-butyllithium carried out at 203 K resulted
in a spectroscopic pattern different from that determined in the
analogous experiments using PhLi and PhMgBr (Figure 5). The
novel species 4 does not decompose if kept in solution at 203
K. However, decomposition is observed when the temperature
reaches 253 K. Under these experimental conditions, the process
goes to completion in 0.5 h. Species 4 has also been detected
using di-n-butylmagnesium as a carbanion source.
All resonances of 4 are located in the +30 to -50 ppm region
(203 K). The species reveals six pyrrole resonances: two of
them are shifted downfield (14.95, 14.38 ppm), but four of them,
remarkably upfield (-3.45, -4.37, -10.16, -46.87 ppm (at
203 K)). We could not locate the seventh pyrrole resonance.
With regard to the molecular structure, the observation of the
paramagnetic shifted σ-butyl resonances (22.07, 21.36, 18.5,
-21.91 ppm (at 203 K)) is of particular importance (Figure 5).
It is consistent with the presence of a five-coordinate nickel
ion coordinated to a σ-butyl ligand. The resonance of the 21-
CH3 (21-CD3) group has been detected in the 2H NMR spectrum
of [(2-NCH3-21-CD3CTPP)NiBu]n- at 3.32 ppm (Figure 5, trace
C). Its position is remarkable when compared to the cases of
1-3, where large downfield shifts in the range 110-260 ppm
have been observed. The 2-NCH3 resonance has been unam-
Unlike the case of the (2-NCH3-21-CH3CTPP)NiIICl-PhLi
system, there is no direct NMR evidence for the formation of
several organometallic intermediates in the course of the titration
of (2-NCH3-21-CH3CTPP)NiIICl with n-butyllithium at 203 K.
The new species could be a product of the replacement of the
chloride ligand by the n-butyl anion to form (2-NCH3-21-
CH3CTPP)NiIIBu (4′), i.e., the n-butyl analogue of 2. Alterna-
tively, the ligand exchange process could be accompanied by
one-electron reduction by BuLi to generate the n-butyl analogue
of 3, i.e., [(2-NCH3-21-CH3CTPP)NiBu]- (4′′). Taking into
account the reducing properties of n-butyllithium, the following
reaction has been also considered:
[(2-NCH3-21-CH3CTPP)NiIIBu]- + BuLi f
4′′
[(2-NCH3-21-CH3CTPP)NiBu]2- + Li+ + Bu• (5)
4′′′
The three hypothetical forms 4′, 4′′, and 4′′′ occur at three
levels of oxidation, and in principle, they can be interconverted
by one-electron-redox reactions by analogy to one-electron
reduction of 2 to 3 using (TBA)BH4. Consequently an attempt

5646 Inorganic Chemistry, Vol. 39, No. 25, 2000
Chmielewski and Latos-Graz˘yn´ski
was made to detect one of the missing forms of 4 by considering
alternative pathways: 4′′ f 4′; 4′ f 4′′; 4′′′ f 4′′. However,
oxidation of 4 with I2 or reduction of 4 with (TBA)BH4 failed
to generate well-resolved paramagnetic shifted 1H NMR spectra,
which could be ascribed to the presence of another species
analogous to [(2-NCH3-21-CH3CTPP)NiIIPh]n-. Taking into
account the fact that butyllithium is considered to be a stronger
reducing reagent than phenyllithium, we suggest that, under
these reaction conditions, [(2-NCH3-21-CH3CTPP)NiBu]- or
the two-electron-reduced species [(2-NCH3-21-CH3CTPP)NiBu]2-
can be trapped. A detailed comparison of the NMR spectra of
3 and 4 reveals a meaningful similarity in the patterns of the
pyrrole resonances, at least at 263 K. However, the meso-phenyl
groups of 4 show noticeably smaller shifts in comparison to
those of 3, and a striking difference is observed in the positions
of the 21-CH3 resonances. In light of the noticeable distinction
in the extents of the paramagnetic shifts of 3 and 4 and the
remarkable differences in the temperature dependences of the
hyperfine shifts, we favor the formation of the two-electron-
reduced species [(2-NCH3-21-CH3CTPP)NiBu]2-
.
The thermal equilibrium between two electronic states
accessible for this overall oxidation state
(P•-)NiI(adz2 + bdx2-y2)(S ) 0) a
(P•-)NiI(adz2 + bdx2-y2)(S ) 1)
is consistent with the reverse temperature dependence of the
paramagnetic shift vs 1/T. The hyperfine shifts of 4 are related
to the (P•-)NiIBu(S ) 1) electronic state. On the basis of our
previous conclusion, it is reasonable to expect that the overall
downfield shifts of the pyrrole resonances are associated with
delocalization through a σ-type molecular orbital. This implies
2
2
2
the population of (adz + bdx -y ) by an unpaired electron and
a downfield shift due to a σ-mechanism. However, the hypo-
Figure 7. EPR results (toluene, 77 K): (A) spectra of 5; (A′) second
derivatives of spectra A showing hyperfine structure due to three 14N
nuclei; (B) spectra of 6. The solid lines correspond to experimental
spectra, and the simulated spectra are shown as dashed lines.
•-
I
2
2
thetical contribution (P )Ni (dz )(S ) 1), if dominating (a f
1), would decrease the spin density on the macrocycle due to
the nickel(I) ion, because the crucial for the transfer dx -y orbital
is fully occupied. The very small paramagnetic shift of 21-CH3
is directly accounted for by this model insofar as the downfield
shift of this particular resonance can be considered a measure
2
2
smaller than ge, is particularly characteristic. Compound 6 could
be prepared by independent synthetic routes, generating the same
EPR spectrum. Reduction of 1 with zinc amalgam and the
reaction of 1 with butyllithium or sec-butyllithium gave the same
form 6 (vide infra). Thus, we conclude that 6 does not contain
an apically bound aryl or alkyl ligand and is most likely four-
coordinate. Importantly, a spectrum resembling that of 5 but
with slightly different values for the g tensor components
(g| ) 2.253, g ) 2.106) was detected in the course of the
decomposition of [(2-NCH3-21-CH3CTPP)NiBu]n- (4).
One-electron reduction of nickel(II) porphyrinoids can result
in three different limiting electronic structures, i.e., a nickel(I)
macrocycle, a low-spin nickel(II) π-macrocyclic anion radical,
and high-spin nickel(II) anion radical (see 3). The description
of the electron distribution in low-valent nickel porphyrins
requires a molecular orbital which allows substantial spin density
to be localized simultaneously on the nickel ion and the
macrocycle. It is generally accepted that larger values of the g
tensor and gi > ge reflect a dominant contribution of the nickel(I)
electronic structure. On the other hand, the gi components
approach ge as the extent of localization of the electron density
on the macrocycle increases.36,38,39,67-69,72-80 Some one-electron-
2
2
of the unpaired electron spin delocalization from the dx -y
orbital. In the second extreme, the carbaporphyrin anion radical
structure has to be considered analogous to that of 3. This
contribution, although smaller than for 3, accounts for the upfield
shifts of four pyrrole protons.
Decomposition. The observation of the solution behaviors
of 2-4 revealed that these complexes were thermally stable at
203 K. At higher temperatures, their stabilities varied. While
the NMR spectrum of 2 can be observed even at room
temperature for about 20 min, warming of 3 resulted in its
complete decomposition at 263 K after 30 min. In the case of
4, decay of the NMR spectrum was observed over 15 min at
253 K. The decomposition process of 2 or 3 seems to be rather
complex. The process yielded the two new compounds 5 and
6, detected by EPR spectroscopy, which belong to the small
class of organonickel(I) compounds.33 Their relative concentra-
tions varied in different experiments. Representative EPR spectra
of 5 and 6 in frozen toluene solutions at 77 K are shown in
Figure 7. The spectrum of species 5 (Figure 7, trace A) reveals
the anisotropy of the g tensor g1 ) 2.237, g2 ) 2.092, g3 )
2.090 and hyperfine coupling to three pyrrolic nitrogens
(72) Barefield, E. K.; Krost, D. A.; Edwards, D. S.; Van Derveer, D. G.;
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(A|N ) 9 G, A ) 10 G). The second product, 6 (Figure 7,
trace B) is characterized by markedly different effective g values
(g1 ) 2.115, g2 ) 2.030, g3 ) 1.940). The g3 value, which is
N

Nickel(II) Carbaporphyrin Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5647
reduced nickel(II) chlorins and nickel(II) porphyrins yield a
g ) 2 signal, but the spectra are anisotropic with high-field
shoulders at g ) 1.99, accounted for by metal character added
to the π-anion radicals.69,75 Therefore, a low-spin nickel(II)
porphyrin anion radical with some nickel(I) character gives
spectra with g1 > g2 > g3 and g3 < ge.75 According to these
criteria, the EPR spectrum of 5 reveals the NiI carbaporphyrin
rather than the low-spin NiII anion radical electronic structure.
On the other hand, the electronic structure of 6 can be considered
as a low-spin nickel(II) carbaporphyrin anion radical with
essential nickel(I) character. The spectral parameters of 6 are
consistent with a report in the literature for one-electron-reduced
nickel(II) meso-tetrakis(o,o,m,m-tetrafluoro-p-(dimethylami-
no)phenyl)porphyrin where similar g tensor values were noted.75
Alternatively, we can rationalize the spectrum of 6 by two-
electron reduction below [(2-NCH3-21-CH3CTPP)NiII]+ to form
(2-NCH3-21-CH3CTPP)Ni. The species can be considered as
the biradical (P•-)NiI with S ) 1. Such a description allows the
satisfactory simulation of the experimental EPR spectrum using
by the σ-phenyl ligand. This is the quite uncommon casesapart
from the previously reported (O2TPP)NiIIPh2swhere two σ-car-
bon atoms are directly involved in coordination to the para-
magnetic nickel(II) ion.33 In the course of our investigation, we
have trapped reduction products of (2-NCH3-21-CH3CTPP)NiIICl
and (2-NCH3-21-CH3CTPP)NiIIPh. Of particular importance is
the fact that the [(2-NCH3-21-CH3CTPP)NiIIPh]- form preserves
the coordination to two carbon donors. This compound reveals
a remarkable electronic structure simultaneously combining
features of the high-spin nickel(II) σ-organometallic species and
the carbaporphyrin anion radical.
Experimental Section
Materials. 2,21-Dimethyl-5,10,15,20-tetraphenyl-2-aza-21-carba-
porphyrin (2-NCH3-21-CH3CTPPH), its deuterated derivatives 2-NCD3-
21-CD3CTPPH, 2-NCH3-21-CD3CTPPH, 2-NCH3-21-CH3CTPPH-d7
deuterated at the pyrrole positions, and 2-NCH3-21-CH3CTPPH-d20
deuterated at the meso-phenyl positions, and their nickel(II) complexes
were synthesized as described previously.24
the parameters g| ) 2.025, g ) 2.016, D ) 144 × 10-4 cm-1
,
The Grignard reagents (C6H5MgBr, C6D5MgBr), phenyllithium,
n-butyllithium, sec-butyllithium, lithium triethylborohydride (Super-
Hydride), and tetrabutylammonium borohydride were purchased from
Aldrich.
and E ) 48 × 10-4 cm-1 in the SimFonia program. However,
a half-resonant-field transition typical for a triplet state has not
been observed in the experimental spectra.81
Characterization of the compositions of 5 and 6 is less
complete than that for the precursors 2 and 3 investigated by
NMR spectroscopy. Initially, the thermally unstable, paramag-
netic organonickel(II) complexes 2 and 3 can decompose via
one-electron internal reductions or by dissociations of the anionic
aryl ligands to produce the EPR-active [(2-NCH3-21-CH3CTPP)-
Ni] and [(2-NCH3-21-CH3CTPP)Ni]- species. We have previ-
ously suggested the anion radical electronic structure for the
five-coordinate complex 3, which gave well-resolved NMR
spectra and was EPR silent. However, modification of the
carbaporphyrinoid macrocycle via a reaction with phenyllithium
or butyllithium as found for regular metalloporphyrins82 or with
generated radicals cannot be excluded as well. This aspect is
currently under further investigation.
1
Instrumentation. H NMR (300 MHz) and 2H NMR (46 MHz)
spectra were recorded on a Bruker AMX 300 spectrometer operating
in a quadrature detection mode. Usually 1000-10 000 scans were
accumulated over a 15-kHz bandwidth with 16K data points and with
a delay time of 100 ms. The signal-to-noise ratio was improved by
apodization of the free induction decay, which typically induced 25-
Hz broadening. The peaks were referenced against solvent resonances.
EPR spectra were obtained with a Bruker ESP 300E spectrometer.
The magnetic field was calibrated with a proton magnetometer and
EPR standards. The EPR spectra were simulated by assuming ortho-
rhombic symmetry. The details of the EPR simulation for S ) 1/2 have
been given elsewhere.36,68 The simulation for the triplet ground state
was carried out using the SimFonia program supplied by Bruker.
Electrochemical measurements were performed in THF with tetra-
butylammonium perchlorate as the supporting electrolyte. Cyclic
voltammograms were recorded for a potential scan rate ranging from
0.02 to 0.5 V/s using an EA9C apparatus (MIM, Krako´w, Poland). A
Pt-disk working electrode, a Pt-wire auxiliary electrode, and an Ag/
AgCl reference electrode were employed.
Conclusion
Porphyrin core modification has been demonstrated to be
crucial in the relative stabilization of the unprecedented orga-
nometallic compounds of nickel(III), nickel(II), and nick-
el(I).24,31-33 Here we have shown novel examples of paramag-
netic organometallic nickel derivatives. Thus, the dimethylated
inverted porphyrin 2-NCH3-21-CH3CTPPH stabilizes the or-
ganonickel complex (2-NCH3-21-CH3CTPP)NiIIPh, where the
nickel-carbon fragment is efficiently protected in the macro-
cyclic environment but, in addition, the axial position is occupied
Sample Preparation. Typically, a 3-mg sample of (2-NCH3-21-
CH3CTPP)NiIICl (1) was dissolved in 0.5 cm3 of oxygen-free toluene-
d8 directly in an NMR tube capped with a rubber septum cap. The
sample was cooled to 193 K in an acetone slush bath, and the respective
arylating (alkylating) reagent (the Grignard reagent, phenyllithium,
butyllithium, or sec-butyllithium) was titrated into the NMR tube using
a microsyringe. The sample was shaken vigorously and then im-
mediately transferred to the NMR spectrometer which was maintained
at 203 K. Samples of the reduced (2-NCH3-21-CH3CTPP)Ni complex
for EPR measurements were prepared by stirring 1 with a suspension
of fine-powdered zinc amalgam for 2 h under inert atmosphere. The
reductant was then removed by filtration, and the filtrate was used as
a stock solution.
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Acknowledgment. The financial support of the Polish State
Committee for Scientific Research (Grant 3 09A 155 15) and
the Foundation for Polish Science (L.L.G.) is gratefully
acknowledged.
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