2182

Synthesis of meso-phenyl-4,6-dipyrrins, preparation of their Cu(II), Ni(II), and Zn(ll) chelates, and structural characterization of bis[meso-phenyl-4,6-d ipyrri nato] N i (II)

Christian Briickner, Veranja Karunaratne, Steven J. Rettig, and David Dolphin

Abstract: meso-Phenyldipyrromethanes can be oxidized by 2,6-dicyano-3,5-dichloro-para-benzoquinone (DDQ) to the corresponding meso-phenyldipyrrins. As expected, these novel, stable bipyrrolic pigments readily form metal chelates with copper(II), nickel(II), and zinc(II). Their UV -VIS spectra are compared with a series of known alkyl-substituted dipyrrin chelates and, based on the UV-VIS spectral analysis, the dihedral angle between the two ligands in the bis[meso­phenyldipyrrinato]Ni(II) complex was calculated to be 42°. The molecular structure of this complex was determined by X-ray crystallography, essentially confirming the calculation. Crystals of C30H22N4Ni are orthorhombic, a = 17.156(3), b = 35.217( 1), c = 7.886(1) A, Z = 8, space group Fddd. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.040 and Rw = 0.031 for 1058 reflections with I ~ 3u(F2

). The central nickel is coordinated in a distorted square-planar fashion by four nitrogens. The pair of the planar dipyrrinato ligands enclose a dihedral angle of 38.5°. This is the lowest angle reported for nickel(II) complexes of this kind. As a result of this, and in sharp contrast to previously described nickel(II) dipyrrin chelates, the central metal is diamagnetic.

Key words: meso-phenyldipyrromethanes, meso-phenyldipyrrins, meso-phenyldipyrrinato transition metal chelates, X-ray crystallography.

Resume: Sous l'influence de la 2,6-dicyano-3,5-dichloro-para-benzoquinone (DDQ), on peut oxyder les meso­phenyldipyrromethanes en meso-phenyldipyrrines correspond antes. Comme on pouvait s'y attendre, ces nouveaux pigments bipyrroliques stables forment des chelates metalliques avec Ie cuivre(II), Ie nickel(II) et Ie zinc(II). On a compare leurs spectres UV -VIS avec ceux d'une serie de chelates connus de dipyrrines substitutees par des groupes alkyles et, sur la base d'une analyse des spectres UV-VIS, on a calcule que l'angle diedre entre les deux coordinats du complexe bis[meso-phenyldipyrrinato]Ni(II) est de 42°. La structure moleculaire de ce complexe, telle que determinee par diffraction des rayons X, confirme essentiellement les conclusions obtenues par calculs. Les cristaux du C3oH22N4Ni sont orthorhombiques, groupe d'espace Fddd, avec a = 17,156(3), b = 35,217(1) et c = 7,886(1) A et Z = 8. La structure a ete resolue par des methodes directes et affinee par la methode des moindres carres jusqu' it des valeurs de R = 0,040 et Rw = 0,031 pour 1058 reflexions avec I ~ 3u(F2). Le nickel central est coordine d'une fa~on plan carre deformee par les quatre azotes. La paire de coordinats dipyrrinato plans forme un angle diedrdre de 38,5°. Cette valeur correspond it I' angle Ie plus faible rapporte pour des complexes de nickel(lII) de cette espece. II en resulte que, par opposition it ce qui a ete decrit anterieurement pour les chelates de nickel(II) dipyrrine, Ie metal central est diamagnetique.

Mots cMs : meso-phenyldipyrromethanes, meso-phenyldipyrrines, meso-phenyldipyrrinato, chelates des metaux de transition, diffraction des rayons X.

[Traduit par la redaction]

Introduction Their propensity to strongly chelate transition metals has long been recognized (1, 2). Their structure, atom numbering scheme, and the formal nomenclature for dipyrrins is shown below. Positions I and 9 are also referred to as a positions, positions 2, 3, 7, and 8 as 13 positions, and position 5 as the meso position.

Dipyrrins (1), also known as dipyrromethenes, are basic, brightly colored, fully conjugated flat bipyrrolic molecules.

Received December 1,1995.

This paper is dedicated to Professor Howard C. Clark in recognition of his contributions to Canadian chemistry.

C. Briickner, V. Karunaratne, S.J. Rettig, and D. Dolphin. l

Department of Chemistry, University British Columbia, 2036 Main Mall, Vancouver, BC V6T lZI, Canada.

1 Author to whom correspondence may be addressed. Telephone: (604) 822-7881. Fax: (604) 822-9678. E-mail: david@dolphin.chem.ubc.ca

Can. 1. Chern. 74: 2182-2193 (1996). Printed in Canada / Irnprirne au Canada

3 5 7

2~8 \""NHVNJ 1 10 11 9 1

4,6-Dipyrrin 2-(2-H-pyrrol-2-ylldenemethyl)pyrrole

2

5-Phenyl-4,6-dipyrrin 2-(2-H-pyrrol-2-ylidene-methyl­

phenyl)pyrrole

Bruckner et al. 2183

Scheme 1.

R' R"

)j R N

H

4

+ R" R' n OHC N R

H

5

R" R"

~,w, R' R' 1. base HBr 2. MXn, ~ N N-.- .-A R R Br

R \ / R 2 3 M

HCOOH Is ~';d";," 10, M = Co2+, Ni2+,

Cu2+, Zn2+

1. C'O 1 9, M = Ca2+

0 2. MXn,

R' R" R" R" R" H Br R" n R'~R Br2 R'~R R N C02C2Hs ~ NH HN !J 2

H

6

Scheme 2.

12

COCI

6 13

R 7

R

cr

14

Scheme 1 outlines the principal pathways for the synthesis of a- and l3-alkyldipyrrins (3) and their chelate-type mode of metal complex formation. Four main synthetic pathways can be distinguished: A: The "classic" acid-catalyzed reaction of an a,l3-alkyl-a'­free pyrrole (4) with a trialkylpyrrole-a-aldehyde (5) (1, 4); B: The reaction of an a,l3-alkyl-a'-ethyloxycarbonylpyrrole (6) in concentrated formic acid (5). C: The oxidation of hexaalkyl-dipyrromethanes (7) by ferrous chloride (1) or 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) (6). D: The meso-bromination of a dipyrromethane to yield the meso-bromo-dipyrromethane 8, and subsequent reaction with calcium oxide to form the calcium chelate (9), yields directly a metal chelate. The calcium chelate can easily be transmetal­lated with a variety of transition metals (7-9). In all other cases, reaction of dipyrrin (3) with a divalent tran­sition metal salt yields the corresponding dipyrrinato com­plexes 10. None of the methods A, B, or D has the potential to give access to meso-substituted, a,l3-free dipyrrins (2). Only

.-~ NH HN !J

R 8 R

route C offers access to the title compounds by oxidation of a meso-phenyldipyrromethane (10). As will be outlined later in detail, this route was, indeed, successful in providing the title compounds.

meso-Alkyl-substituted dipyrrins are less common (1, 11-13) and the synthesis of meso-phenyl-substituted dipyrrins is even rarer, in fact, we are only aware of four previous synthe­ses, two of them shown in Scheme 2. Rogers (14, 15) con­firmed in 1943 a finding of Gabriel from 1908 (16) that described the formation of 11 by reaction of 2,4-diphenylpyr­role (12) with in situ generated benzoyl chloride. In a similar approach, Treibs et al. reacted pyrrole 13 with benzoyl chlo­ride to yield the hexasubstituted meso-phenyldipyrrin hydro­chloride 14 (5). It is noteworthy that in both instances the pyrroles were substituted, particularly at one a and at least one 13 position. This prevents polymerization of the pyrroles dur­ing the harsh reaction conditions, consequently, these methods are not options to synthesize meso-phenyl, a-unsubstituted dipyrrins. A disadvantage of the protecting a-phenyl moieties is that they introduce severe steric interligand interactions

2184

Scheme 3.

CHO

¢ R

17, ~ = H 18, R = N02

TFA

19

R

15, R = H 16, R = N02

DDQ ..

upon metal complex formation. Moreover, meso-phenyl sub­stitution concomitant with [3-substituents also introduces intra-ligand steric interactions that can, for instance in the case of 14, lead to deviations from planarity and even chemical instability (17). No reports were made on the metal complex­ation properties of either 11 or 14. In 1985 the X-ray structure of bis[ 1-(2,6-dichlorobenzy 1)-5-(2,6-dichloropheny l)-dipyrri­nato]zinc(I1) was reported (18). This meso-phenyl and a-sub­stituted dipyrrin complex was the kinetic product in the Rothemund-type condensation of the sterically hindered 2,6-dichlorobenzaldehyde with pyrrole, and its isolation was unexpected and fortuitous. Similarly unanticipated, Cavaleiro et al. isolated and crystallized meso-aryl-substituted dibenzo­furanyldipyrrin in an attempted tetraarylporphyrin synthesis from o-acetoxybenzaldehyde and pyrrole (19). These syn­thetic pathways towards meso-phenyl dipyrrins and their metal complexes cannot be generalized. Recently two reports appeared in the literature in which meso-phenyl-substituted dipyrrin moieties were integral parts of larger molecules. The BF2 complex of an a-methyl-meso-phenyldipyrrin unit was the input unit of a molecular photonic wire (20) and an a­thiophenyl and [3-alkyl-substituted meso-phenyldipyrrin was synthesized in the course of research towards poly heterocyclic ligands; however, neither the complexing properties nor the conformation of this compound were reported (21).

The stereochemistry of pyrrin ligands around the metal is dependent on the bulkiness of the substituents in the a and a' positions, and has found interest (22) since Porter (23) called attention to this phenomenon; however, only a limited number of structural data are available (18, 22, 24, 25). Few a-unsub­stituted dipyrrinato complexes have been prepared (26, 27) and in no case has a crystal structure been described. There­fore, it was interesting to investigate the complex geometry of the a'-unsubstituted meso-phenyldipyrrin ligands of type 2 and to contrast these findings with published data. This and the general interest for novel ligand classes for use in transition metal catalysis (28), photometric metal detection (29), or bio­medical purposes (30) prompted us to investigate the synthesis and the metal complexing properties of a,[3-unsubstituted meso-pheny Idipyrrins.

Results and discussion

Synthesis of 5-phenyldipyrromethanes 15 and 16 The meso-phenyldipyrromethanes 15 and 16 were synthesized by the acid- catalyzed condensation of benzaldehyde (17) or p­nitrobenzaldehyde (18), with pyrrole (19). Pyrrole was also used as solvent according to a procedure of Lee and Lindsey

R

2, R = H 20, R = N02

Can. J. Chem. Vol. 74, 1996

(10) (see Scheme 3). The synthesis of 16 offers the great prac­tical advantage over the synthesis of 15 or other dipyr­romethanes described by Lee and Lindsey, in avoiding any chromatography during the work-up or purification of the compound; thus it is amenable to large-scale (2: 10.0 g product per experiment) preparations. The higher electrophilicity of p­nitrobenzaldehyde compared to benzaldehyde likely results in a faster reaction rate and a stabilization of the resulting dipyr­romethane towards acid-catalyzed decompositions. Both these aspects in combination with the simple work-up explain the high overall yield of 82% for 15 vs. the reported 49% (10) for 16.

Preparation and characterization of meso-phenyldipyrrins 2 and 20

Dehydrogenations with DDQ have found wide application in the synthesis of pyrrolic pigments (31). In particular, DDQ is useful in the conversion of any type of reduced porphyrins (e.g., porphyrinogens or chlorins) to the corresponding fully unsaturated porphyrins (32). Porphyrinogens are intermedi­ates (33) in the synthesis of meso-tetraarylporphyrins accord­ing to the methods of Adler et al. (34) or Lindsey and Wagner (35), i.e., the acid-catalyzed cyclization of pyrrole and benzal­dehydes. Hence, it was not unexpected that the reaction of meso-phenyldipyrromethanes 15 or 16 with one equivalent of DDQ smoothly formed the desired dipyrrins 20 and 2 (Scheme 3). p- and o-Chloranil are equally well suited to per­form the conversion. Reduction of 2 or 20 with NaBH4 in MeOH regenerates the leuko form 16 or 15. In dilute solution, the oxidation products are bright yellow in color. The optical spectrum of 2 under acidic and basic conditions is shown in Fig. I. The two-band pattern of the protonated species is anal­ogous to that of I, 2, 3, 7, 8, 9-hexamethyldipyrrin hydrobro­mide (11), but ~ 14 nm hypsochromic ally shifted, with slightly lower extinction coefficients. The bands have been assigned to 'IT* f- 'IT transitions and are indicative of the marked planarity of these fully conjugated aromatic systems. Addition of acid protonates the basic imine-type nitrogen of the 2H-pyrrole unit and this removal of non-degeneracy of the linear resonator in combination with the presence of a positive charge induces a bathochromic shift of 42 nm and a tripling of the extinction coefficient (36). For steric reasons, it can be inferred that the phenyl moiety is approximately perpendicu­lar to the plane of the dipyrrin. Consequently, the phenyl group is not in full conjugation with the pyrrolic system and substituents on the phenyl group minimally influence the 'IT­

cloud of the dipyrrin. This explains the close similarity of the optical spectrum of 2 and its p-nitro derivative 20; a similar

Bruckner et al.

Fig. 1. Optical spectra of 2 in CH2CI2 - 0.5% MeOH - trace NHpH (broken line) and in CHzCI2 - 0.5% MeOH - trace HCI (dotted line), and of 20 in CH2Clz - 0.5% MeOH - trace HCI (solid line).

6E+04

:=-- 4E+04 E o

~ W

2E+04

300 400 500 600 wavelength (nm)

situation is also found in variously phenyl-substituted meso­tetraphenylporphyrins (37). The nitro compound 20, and its metal complexes, exhibit a band at ~260 nm that we attribute to the p-nitro moiety. The optical spectrum of2 is closer to that of the hexamethyldipyrrin than to the spectrum of 11, which is about 110 nm bathochromically shifted (17), possibly reflect­ing the extended conjugation (and distortion) of this system by the a-phenyl groups. .

The signals in the lH and l3C NMR of the meso-phenyl­dipyrrins indicate a plane of symmetry. This is consistent with formulating the dipyrrins as adopting a planar conformation and a rapid tautomeric exchange of the NH proton between the two nitrogens. The lH NMR shifts for the ~-protons of 2 of 6.39 and 6.47 ppm and for the a-protons of7.78 ppm attest to the aromatic character of these compounds.

Alkyldipyrrins of type 3 are, owing to their basicity, gener­ally isolated and purified as their hydrobromide or hydrochlo­ride salts (1). Although conditions for thin-layer and column chromatography of dipyrrin hydrobromides have been described (mixture of formic acid, methanol, and chloroform -silica gel (38)), it is not common practice. Their free bases are also reportedly less stable. We were surprised to find that in case of the meso-phenyldipyrrins, column chromatography (CH2CI2 - silica gel) of their free bases posed no difficulty.

Formation and characterization of transition metal chelates of the meso-phenyldipyrrins

General data and synthesis A concentrated MeOH solution of the meso-phenylpyrrins 2 or 20, when mixed with a methanolic solution of the divalent metal ions N?+, Cu2+, and Zn2

+, as their acetates, yields the corresponding highly colored metal complexes (Scheme 4). The complexes are stable and do not require any special han­dling. Analyses confirmed the stoichiometry of the precipi­tates as M(ligandh. The metal complexes formed X-ray quality dichroic (metallic green-red) crystals.

The vibrational spectra of the metal complexes are similar to those of their ligands. This is not unexpected as conjugation is already attained in the planar ligand moiety before coordi­nation to a metal. Thus, intraligand vibrations will undergo only minor shifts upon metal chelation. This has been ratio-

Scheme 4.

2, R = H 20, R = N02

M(II)-acetate/

MeOH/base

23, R = N02, M = Zn 24, R = N02 M = eu 21, R = N02' M = Ni 22, R = H, rvi = Ni

Fig. 2. Optical spectra of 21 in CHCl3 (broken line) and 22 in CHCI3 (solid line).

6E+04

:=-- 4E+04 E o

~ W 2E+04

2185

OE+OO1---~--------r--------.----~~r----300 400 500 600

wavelength (nm)

nalized before for the metal complexes of hexaalkylpyrrins (39).

UV-VIS spectra The optical properties of the metal compounds are strongly dependent on the central metal but the nickel chelates 21 and 22 are very similar (Fig. 2). This similarity, in particular with respect to ~max and log E, is indicative of a very similar stere­ochemistry of these two compounds. The UV -VIS spectrum of the zinc chelate 23 resembles that of the protonated ligand, suggesting the absence of any metal f- metal transitions (see Fig. 3). However, metal f- ligand charge transfer transitions are generally observed in this energy region and they cannot be excluded (27), though other authors have assigned this band exclusively to intraligand 71"* f- 71" transitions and the bands in the 320-350 nm region to charge transfer transitions (40). The electronic spectra of the nickel and the copper che­lates 20, 21, and 24 follow the same general pattern as that of 23 (see Figs. 2 and 3). The UV-VIS spectra of the metal che­lates are nearly indistinguishable in non- or weakly coordinat­ing solvents such as benzene, methanol, methylene chloride, or chloroform but show changes in pyridine, most noticeable for the zinc chelate, as also shown in Fig. 3. Table 1 lists selected UV -VIS data of some known dipyrrinato-metal com­plexes (25-40) and of the novel compounds 21, 23, and 24.

When comparing the longest wavelength absorption of the Zn-chelate 23 at 486 nm against the equivalent transitions of

2186

Fig. 3. Optical spectra of 23 in CH2Cl2 (dotted line) and pyridine (dashed line), and 24 in CHCl3 (solid line).

E u

:. w

6E+04

4E+04

2E+04

.~ ... .I""""' .............. ..

OE+OO~---r------~;=------~~~~~~---300 400 500 600

wavelength (nm)

the alkyl-substituted analogues 25-32, it is remarkable that the meso-phenylpyrrin chromophore is distinguished by the high­est transition energy. An equivalent trend can be seen in the nickel (22 vs. 33-36) and copper (24 vs. 37-40) chelate series. Hyperconjugation effects have been suggested for the progres­sive bathochromic shift with increasing methyl substitution (27). Extended 1T-conjugation can be evoked for the batho­chromic ally shifted optical spectrum in case of dipyrrins 28 and 32. The absence of both effects in the zinc, nickel, and cop­per chelates 21-24 rationalize their relatively high transition energies. The introduction of a meso-methyl substituent in che­lates 29 and 30 leads, when compared to their meso-unsubsti­tuted analogues 27 and 28, to a relatively small change in the energy of the longest wavelength transition; however, their extinction coefficients significantly decrease. This, based on theoretical considerations, may be taken as a sign of distortion from planarity (17). Based on the foregoing, the high extinction

(RWJ R \ / R 2

~+

Compound no. R R' R" Rmeso

25 -H -Me -Me -H 26 -Me -H -Me -H 27 -Me -Me -Me -H 28 -Me -C02Et -Me -H 29 -Me -C02Et -Me -Me 30 -Me -Me -Me -Me 31 -C02Et -Cl -Cl -H 32 -Ph -H -H -H 33 -Me -H -Me -H 34 -Me -Me -Me -H 35 -Me -C02Et -Me -H 36 -Ph -H -H -H 37 -H -Me -Me -H 38 -Me -Me -Me -H 39 -Me -C02Et -Me -H 40 -Ph -H -H -H

Can. J. Chem. Vol. 74, 1996

coefficients of the metal chelates of the meso-phenyldipyrrins seem to indicate that the ligands are flat. In the absence of any l3-substituent and hence any intraligand steric crowding, and in analogy to the conformation of meso-tetraphenylporphyrins (42), this appears to be a reasonable assumption. As will be detailed later, the single crystal X-ray structure of 22 shows that the assumption of planarity is, in fact, valid.

The stereochemistry of the ligands around the central metal is strongly dependent on the metal type. The preference of zinc(lI) for a tetrahedral, and of nickel(II) and, even more so, of copper(II) for a square-planar coordination sphere is well documented (43). Regardless of the a substituents present in the dipyrrin ligands, the realization of a tetrahedral coordina­tion sphere poses no interligand steric interactions. Conse­quently, and in analogy to the stereochemistry of the zinc chelates of alkyldipyrrins, compound 23 can be assigned a tet­rahedral structure (9, 22, 27,40). The picture is more complex for the nickel and copper chelates. It has been found in previ­ous studies that a substituents prevent square-planar coordina­tion due to interligand crowding. This forces the complex into a distorted tetrahedral structure in which the two approxi­mately planar a-methyldipyrrinato ligands are inclined (as determined by X-ray crystal structure analysis) at an angle (referred to as dihedral angle) of 76.3° for nickel chelate 33 (24) and 66° for copper chelate 39 (25). With hydrogen as the sole a substituents no a priori statement can be made about the stereochemistry around the metal. It has been suggested that some electronic interaction exists between the 1T-systems of the two dipyrrin units coordinated to the same metal ion in the "tetrahedral" Co(ll) and Cu(I1) complexes (44). Motekaitis and Martell presented an MO theory model and derived a rela­tionship (eq. [I]) in which the intensity of the longest wave­length transition is assumed to change with the tetrahedral angle e between the ligands:

M2+

Zn Zn Zn Zn Zn Zn Zn Zn Ni Ni Ni Ni Cu Cu Cu Cu

Bruckner et al.

Table 1. Selected UV-VIS data and dihedral angles of dipyrrinato chelates.

Compound Am" (log £)" no. (nm)

23 486 (4.97)d

25 500 (5.08/ 483 (4.97)

26 488 (4.22)' 469, sh

27 505 (4.93)' 487 (5.07)

28 490 (5.33)' 446, sh

29 501 (5.051 30 505 (4.061 31 537 (5.13)' 32 552 (4.93)'

510 (5.06)

22 484 (4.63)d

33 512 (4.85)' 34 531 (4.70)UIe

462 (4.57) 35 495 (4.99)' 36 540 (4.85)'

24 474 (4.72)'

37 471 (4.85)' 409 (4.61)

38 525 (4.65)' 471 (4.76)

39 495 (5.05)' 40 564 (4.93)'

509 (4.70)

"Longest wavelength n* (,-- n transition. bBetween planes formed by the ligands. 'p-N02-pheny \. dIn CH,CI,. 'In CHCI3•

fSolvent not specifIed. g Assumed angle.

Dihedral angle CO)b

90"

90"

9(F

90"

90"

90"

42/' 3805'

76.3'

39

60! 66h

4810

50,10 6Y

6Si

66' 90,h 7Y

"Calculated according to Motekaitis et al. (9). 'From X-ray single crystal structure. iBased on ligand field transition analysis (26).

[1] E8

. 2 e -::-go = sm E

Reference

This work

27

27

27

22

II 11 9,41 27

This work

24, 39

40 26

This work

26

39

25,40 26

e is the tetrahedral angle, E8 is the extinction coefficient of a reference compound known to be tetrahedral, i.e., e = 90°, and E8 is the extinction coefficient of a similar compound whose geometry is to be determined. According to eq. [1], the calculated tetrahedral angle in the meso-phenyldipyrrin nickel complex 22 would be 42°, and in the copper complex 24, 48°.

2187

There are, however, precautions to be taken when applying Martell's methodology for determination of the interligand dihedral angle. The prerequisite that the ligands are flat and coordinate in exactly the same MN4 fashion to the complexes to be compared must be strictly fullfilled. Fergusson and co­workers (22), for instance, published the crystal structure anal­ysis of the palladium chelate of 3,3',5,5'-tetramethyl-4,4'­diethoxycarbonyldipyrrin in which the dipyrrin unit was not planar. The tendency for palladium to achieve square-planar coordination geometry is strong enough to distort the planar ligand and to enforce a stepped arrangement of the ligands around the metal centre. With little change in transition energy, the extinction coefficients were reduced compared to the analogous tetrahedral cadmium, mercury, or zinc com­plexes. However, the application of eq. [I] gives incorrect results when compared to the actual structures. A second example can be derived from examination of the literature. Based on the extinction coefficient of 31, Martell determined the dihedral angle of the copper analogue to be 40° (9). How­ever, considering the steric requirement of the ethoxycarbonyl moiety and setting it against the crystallographically deter­mined dihedral angle of 66° for 39 or values determined for the complexes 37-40, this value appears to be considerably too low. Murakami et al. investigated the IR spectrum and the ligand-field bands of this complex and proposed the involve­ment of the carbonyl oxygen in this copper chelate, giving a CuN40 2 coordination (41). In light of this it becomes apparent that the dihedral angle predicted by Martell' smethod had to be in error. In the present case, however, the prerequisites of sim­ilarly flat ligands forming in all cases a MN4 coordination sphere are most likely fullfilled and, consequently, the theoret­ically determined values may be significant. Indeed, the X-ray crystal structure analysis for nickel complex 22 proved the value determined by Martell's method to be fairly accurate (3.5° deviation, Table 1). As for the copper chelate, a final experimental proof of the calculated value still awaits, but the value of 48° is in agreement with the calculated value of one other a-unsubstituted copper chelate 37 and, as expected, is significantly smaller than for the a-alkyl-substituted chelates. The calculated values for the copper chelates have to be taken with some reservation as shown by the discrepancies of the values determined by ligand field transition band analysis and by Martell's method.

The change of the optical spectrum of 23 upon the addition of pyridine (Fig. 3) results from an expansion of the Zn-coor­dination sphere from tetrahedral ZnN4 to a distorted tetragonal pyramid ZnNs' This forces the dipyrrinato ligands to take up a smaller dihedral angle, which probably accounts for the observed spectrum. The nickel chelate UV-VIS spectrum shows only a slight change upon addition of pyridine. This is analogous to, for instance, the reluctance of the square- planar nickel(II) porphyrins to expand their coordination sphere and the small changes in their optical spectrum associated with any additional coordination (45). This effect is even more pro­nounced in the case of the Jahn-Teller ion copper(II).

Magnetic properties The magnetic properties of nickel (II) complexes with N-donor ligands may permit conclusions regarding their coordination geometry. Square-planar complexes are typically diamag­netic, and tetrahedral complexes paramagnetic (43). All

2188 Can. J. Chem. Vol. 74, 1996

Fig. 4. ORTEP plot (stereoview) of 22; 33% probability thennal ellipsoids are shown for the non-hydrogen atoms.

22 41

nickel(II) chelates of dipyrrins have been described as para­magnetic and, therefore, their description as distorted tetrahe­dral rather than distorted square planar is plausible regardless of the actual dihedral angle between the ligands. To our sur­prise, the nickel chelates 21 and 22, as judged by the sharp 1 H and l3C NMR, proved to be diamagnetic. This suggests that they are (distorted) square planar. On the basis of a compari­son of the steric interactions in the cyclic and planar [meso-tet­raphenylporphyrinato]nickel(II) (41) or the cyclic and saddle­shaped [5, 15-dimethyl-5, 15-dihydrooctaethylporphyrinato]­nickel(II) (42) (46), it becomes clear that a planar coordination can be excluded since the two a-hydrogens of the opposing ligands would occupy the same space (given standard Ni-N bond lengths) in case of square- planar coordination. To unam­biguously answer the question about the dihedral angles in the nickel chelate, an X-ray crystal structure analysis of a single crystal of 22 was undertaken.

42

Crystal structure analysis An ORTEP representation of 22 as it exists in the crystal together with the numbering system employed in Tables 2 and 3 is shown in Fig. 4. Atom coordinates and Beq are listed in Table 2. Selected bond parameters are listed in Table 3, and the crystallographic data are listed in Table 4.

The molecule has D2 symmetry, which makes the two ligands equivalent and endows a C2-axis passing through the p-hydrogens of the meso substituent, the methine carbons, and the central metal. The planes of the two essentially planar dipyrrin ligands enclose a dihedral angle of 38.5°, in close agreement with the calculated value of 42°. The equivalent angle in [3,3',5,5'-tetramethylpyrrinato]nickel(II) (33) is, as mentioned above, 76.3° (24). The small angle results directly from the smaller size of the a-H as compared to the a-methyl group. The bite angle N-Ni-Na of the ligand is 94.3°, which is, within the experimental uncertainty, equal to the bite angle

Bruckner et al.

Table 2. Atom coordinates and BC'J° for 22.

Atom x y z Beq

Ni(l) 0.12500 0.12500 0.62500 3.07(1) N(1) 0.0990(1) 0.16291(5) 0.4630(2) 3.61(5) C(l) 0.0985(1) 0.20166(7) 0.4808(3) 3.91(6) C(2) 0.0592(2) 0.21708(8) 0.3404(4) 6.72(9) C(3) 0.0340(2) 0.18773(9) 0.2426(4) 8.0(1) C(4) 0.0592(2) 0.15443(8) 0.3215(3) 5.29(7) C(5) 0.1250 0.22034(9) 0.6250 3.50(7) C(6) 0.1250 0.26307(9) 0.6250 4.18(8) C(7) 0.1658(2) 0.28288(8) 0.5040(4) 5.70(8) C(8) 0.1666(2) 0.32263(8) 0.5060(6) 7.5(1) C(9) 0.1250 0.3416(1) 0.6250 8.6(2)

"Beq = 8/3rc'(,i,'·£'Uija;*a;*(a:a).

Table 3. Selected bond distances and bond angles for 22.0

Atoms

Ni(1)-N(l) N(1)-C(4) C(1)-C(5) C(3)-C(4) C(6)-C(7) C(8)-C(9) N(l)-C(1) C(1)-C(2) C(2)-C(3) C(5)-C(6) C(7)-C(8)

Distance (A)

1.879(2) 1.336(3) 1.390(3) 1.396(4) 1.374(3) 1.355(4) 1.404(3) 1.405(4) 1.360(4) 1.505(4) 1.400(4)

Atoms

N(1)-Ni(1)-N(1). N(1)-Ni(l)-N(1\ Ni(1)-N(1)-C(4) N(1)-C(1)-C(2) C(2)-C(1)-C(5) C(2)-C(3)-C(4) C(1)-C(5)-C(1)e C(5)-C(6)-C(7) C(6)-C(7)-C(8) C(8)-C(9)-C(8\ N(1)-Ni(1)-N(1)b Ni(1)-N(l)-C(l) C(1)-N(1)-C(4) N(1)-C(1)-C(5) C(1)-C(2)-C(3) N(1)-C(4)-C(3) C(1)-C(5)-C(6) C(7)-C(6)-C(7)e C(7)-C(8)-C(9)

94.3(1) 92.1(1)

123.4(2) 108.0(2) 128.2(2) 106.7(3) 123.5(3) 120.5(2) 120.3(3) 120.9(4) 152.5(1) 128.6(2) 106.2(2) 123.4(2) 107.8(2) 111.3(3) 118.2(1) 118.9(3) 119.7(3)

"Symmetry operations: (a) 114 - x, 114 - y, z; (b) x, 114 - y, 5/4 - z; (c) 114 - x, y, 5/4 - z.

observed for 33. The N-Ni-Nb angle is 152.5°. Unlike in the latter structure, no distortion in the sense of a deviation of colinearity of the two local twofold axes of each Ni-ligand group can be detected. The four Ni-N distances are equal (1.879(2) A), and unusually short for complexes of this kind. We regard this effect to be partially due to the reduced ionic radius of the d8 low spin ion vs. the high-spin congener in 33 (47) and partially due to the reduced interligand steric interac­tions. The extent of the steric effect becomes perceptible if the bond length is compared to those in related nickel complexes 33, nickel porphyrin (41), a ruffled nickel porphyrin (42), and a 5,1O-dihydroporphyrin system (43) as listed in Table 5. The quasi-rigid porphyrin core (in complex 41) resists undue radial expansions or contractions in the equatorial plane of the core. Therefore, the metal - porphyrin nitrogen bond lengths are

2189

Table 4. Crystallographic data for compound 22."

Formula CJoH22N.Ni fw 497.23 Crystal system Orthorhombic Space group Fddd a, A 17.156(3) b, A 35.217(1) c, A 7.886(1) v,N 4764.4(9) z 8 Dead' g/cm

3 1.386 F(OOO) 2964 j..l. cm-1 8.41 Crystal size, mm 0.20 x 0.30 x 0.40 Transmission factors 0.877-1.000 Scan type 0>-28 Scan range, deg in 0) 0.94 + 0.35 tan 8 Scan speed, deg/min 16 (up to 8 rescans) Data collected +h, +k, +1 28max, deg 70 Crystal decay, % 2.3 Total reflections 2874 Total unique reflections 2874 Reflections with I :;; 3cr(F2) 1058 No. of variables 82 R 0.040 Rw 0.031 gof 2.00 Max /!Jcr (final cycle) 0.0002 Residual density e/A3 -0.44 to 0.42

"Temperature 294 K, Rigaku AFC6S diffractometer, Mo Ko: radiation (A = 0.71069 A), graphite monochromator, takeoff angle 6.0°, aperture 6.0 x 6.0 mm at a distance of 285 mm from the crystal, stationary background counts at each end of the scan (scan/background time ratio 2: I); cr'(F2

) = [S,(C + 4B)]lLp, (S = scan rate, C = scan count, B = normalized background count), function minimized ~w (lFol - IF,I), where w = 4Fo'/cr'(Fo'), R = ~IIFol - IF,II~IFol, R" = (~w(lFo - IF,I)'~wIFi)ll2, and gof = [~w(IFol - IF,I)2/(m - n)]II2. Values given for R, R", and gof are based on those reflections with I S; 3cr(F').

Table S. Ni-N bond lengths of selected tetrapyrrolic nickel complexes.

Compound no.

22 27 42 43 41

Ni-N bond length (A)

1.879(2) 1.952(7) (averaged) 1.904(5) (3 out of 4) 1.929 1.960

"Typical value for nickel porphyrins.

Reference

This work 24 46 48 49, 51

restricted relative to the normal range of values that are found in metal - monodentate nitrogen ligand bond lengths, which results in a "stretched" bond length of ~ 1.96 A. Ruffled nickel porphyrins can reduce the bond length by about 0.03 A; in the saddle-shaped 42, which is essentially a strapped bisdipyrri-

2190

Fig. 5. Intraligand bond lengths, and limiting resonance structures of the dipyrrinato ligand.

nato nickel(II) compound, the bond length is a further 0.025 A shorter. The removal of the ligand strap concomitant with the introduction of a-methyl groups introduces severe steric inter­actions in 33 but allows for a large dihedral angle, nonetheless, a long Ni-N bond length is recorded for this class of com­plexes. Removal of a large portion of this interaction by the replacement of the methyl groups with hydrogens in 22 allows the two ligands to achieve "pseudo-planarity" and results in the shortest Ni-N bond length of its class.

Inspection of the intraligand bond lengths reveals that two types of C-N bonds exist, a short Crt -N bond and a long Crt,-N bond. The differences can be accounted for in terms of a resonance description of the 'IT-electrons in the ligand mole­cule. Figure 5 shows the two limiting resonance structures and the associated bond lengths. According to this simplified pic­ture, the Crt -N bond would receive partial 'IT-contribution, the Crt,-N bond would not. The difference in double bond character of these bonds explains the observed bond length differences in a qualitative way. The deviation of the Crt,-N bond length from the expected 1.42 A for a C-N single bond reflects the aromatic character of the pyrrole unit itself, albeit the analogous bond length in pyrrole is about 0.04 A shorter (50). The Crt,-Cmeso bond and the Crt -Ci3 bond have a for­mal bond order of 1.5, and hence their bond lengths are as expected. The mean plane of the meso-phenyl group is tilted 58.1 ° with respect to the mean plane of the dipyrrin unit. This deviation from the, perhaps, expected orthogonal finds its par­allels in the structure of meso-tetraphenylporphyrins (38, 51).

NMR spectroscopy The IH and l3C NMR data of the diamagnetic metal chelates 21-23 are largely as expected, and similar to the spectra of the protonated ligands. One noticeable exception is a large low field shift of the a-protons in the nickel chelates 21 and 22, i.e., a shift of +2.48 ppm for 22 as compared to the zinc analog 23. This also is evidence of the small dihedral angle of the ligand mean planes in the nickel complex. The a-protons experience shielding effects of both the aromatic dipyrrinato systems and thus are more shielded than in the tetrahedral zinc complex, where such "double" shielding cannot occur.

Conclusion

The meso-phenyl-4,6-pyrrins can be conveniently prepared from the corresponding dipyrromethanes. They exhibit prop­erties similar to previously described alkyl-substituted dipyr­rins with the exception that they exhibit a significantly higher 'IT* f-- 'IT transition energy as judged by their hypsochromically shifted UV-VIS spectra. The meso-phenyldipyrrins fonn

Can. J. Chem. Vol. 74,1996

metal complexes with nickel(II), copper(II), and zinc(II). Their spectroscopic data can be rationalized in the context of the previously described dipyrrinato complexes; however, the lack of a bulky a-substituent allows unique properties to this class of ligands. Based on optical and IH NMR data, the zinc complex can be assigned a tetrahedral structure. Both the nickel and the copper complex can be described as distorted square-planar complexes. The distortion from planarity of the nickel complex was predicted to be 42°, based on UV-VIS data analysis. An X-ray crystal structure analysis determined the angle to be 38S and the Ni-N distance to be 1.879(2) A. This is the smallest angle and the shortest Ni-N bond length recorded for dipyrrinato complexes. As a consequence of this, and in contrast to previously described [dipyrrinato ]nickel(II) complexes, the [meso-phenyldipyrrinato ]nickel(II) complexes are diamagnetic. The stereochemistry of the copper complex is assumed to be similar to that of the nickel complex. Studies to utilize the unique steric requirement of the meso-phenyl­dipyrrin ligand, e.g., to form coordination polyhedra with no precedent in the dipyrrinato field, such as octahedral M(III)(ligand)3 complexes, are currently under way in our lab­oratories.

Experimental section

Instrumentation and materials Melting points were determined on a Thomas model 40 Micro Hot Stage and are uncorrected. The infrared spectra were mea­sured with a Perkin-Elmer model 834 FT-IR instrument. The NMR spectra were measured with a Bruker AC 200 FT spec­trometer and are expressed in parts per million (0) relative to the external standard TMS. The low- and high-resolution mass spectra were obtained by Dr. G. Eigendorf and co-workers of this department using an AEI MS9 and a Kratos MS50 spec­trometer, respectively. The electronic spectra were measured on a HP 8452A photodiode array spectrophotometer. Elemen­tal analyses were performed by Mr. P. Borda of this depart­ment on a Fisons CHN/O Analyzer, model 1108.

meso-Phenyldipyrromethane 15 was synthesized according to the procedure of Lee and Lindsey (10). All other reagents and solvents were commercially available and of reagent grade or higher, and were, unless otherwise specified, used as recieved. The silica gel used in flash chromatographies was Merck Silica Gel 60, 230-400 mesh.

5-( 4-Nitrophenyl)dipyrromethane (16) Nitrobenzaldehyde (18) (3.0 g, 19.87 mmol) was dissolved in freshly distilled pyrrole (19) (44.0 g, 0.66 mol). The mixture was degassed by bubbling with Nz for 10 min. TFA (0.15 mL, 0.1 equiv based on the benzaldehyde) was added and the mix­ture was stirred under Nz until no starting aldehyde could be detected by TLC (ca. 15 min). The volume of the slightly yel­low mixture was reduced under high vacuum at 50°C to a vis­cous oil. This oil was dissolved in CHzClz (100 mL) and cyclohexane (50 mL) was added. Without heating, the mixture was reduced on the rotary evaporator until precipitation just began. Scratching with a glass rod caused rapid crystallization of an slightly greenish solid which, after drying at 50°C/0.2 Torr (1 Torr = 133.3 Pa) for 24 h gave 3.85 g (71.2%) of ana­lytically pure compound 16. A second crop of lesser purity was obtained from the mother liquor upon further evaporation

Bruckner et al.

(0.55 g, 10.3%); mp 158°C; UV-VIS (MeOH) Amax nm (reI. intensity): 222 (1.0), 266 (0.77); IH NMR (200 MHz) 0: 5.58 (s, lH), 5.87 (m, 2H), 6.18 (dd, J = 11.8,2.5 Hz, 2H), 6.74 (m, 2H), 7.37 (d, J = 11.8 Hz, 2H), 7.95 (br s, 2H), 8.14 (d, J = 11.8 Hz, 2H); l3C NMR (50 MHz) 0: 43.8, 107.8, 108.8, 118.0, 123.8, 129.2, 130.8, 146.9, 149.7; LR-MS(EI) mle: 267 (100.0, M+), 220 (9.7, MW - N02), 201 (16.3, M+ - C4H4N), 154 (9.7), 145 (47.3, MW - Ph - N02). Exact Mass calcd. for CISHl3N302: 267.10078; found: 267.10080. Anal. calcd. for CISHl3NP2: C 67.41, H 4.9, N 15.72; found: C 67.23, H 4.98, N 15.62.

5-Phenyl-4,6-dipyrrin (2) meso-Pheny1dipyrromethane (15) (500 mg, 2.25 mmo1) was dissolved with the help of a heat gun in benzene (25 mL). A solution of 2,3-dich10ro-5,6-dicyano-1 ,4-benzoquinone (537 mg, 2.35 mmo1) dissolved in benzene (5 mL) was added and the mixture stirred until no starting material could be detected by tlc (1 h). The black precipitate was filtered off and air-dried to provide 440 mg (85 %) of crude 2, which was used for metal complex formation. An analytical sample was purified by col­umn chromatography (silica gel, 25 g, 1 % MeOH in CHCi3). The bright yellow main fraction was collected and evaporated to dryness. The yellow film was dissolved in acetone (20 mL) and precipitated by diffusion of cyclohexane into this solution to yield a yellow-brown precipitate (55.0 % based on crude material); mp 184°C; IR (neat): 1555, 1450, 1435, 1340, 1055 cm-I; UV-VIS (CH2Ci2 - 0.5% MeOH - trace NH40H) Amax

(log E): 310 (3.75), 434 (4.10) nm; (CH2Ci2 - 0.5% MeOH­trace HC1) Amax (log E): 354 (4.02),466 (4.65) IH NMR (200 MHz, acetone-d6) 0: 6.39 (m, 2H), 6.47 (d, J = 3.5 Hz, 2H), 6.39-6.47 (m, 5H), 7.78 (s, 2H), ~ 12.5 (s, very broad, lH); l3C NMR (50 MHz, acetone-d6 ) 0: 118.3, 128.5, 129.2, 129.7, 131.3,135.0, 138.0, 144.6; nm; LR-MS (EI, 180°C) mle: 220 (77, M+), 219 (100, M+ - H). Exact Mass calcd. for CISH12N2: 220.10005; found: 220.10012. A consistent elemental analysis (deviation 3.0% from the calculated values) could not be achieved, possibly due to varying amounts of solvation and (or) salt formation.

5-( 4-Nitrophenyl)-4,6-pyrrin (20) This compound was prepared from 16 by a method analogous to that used for the preparation of compound 2. Yield after chromatography: 59%; mp 189-191°C; IR (neat): 1555,1520, 1515,1510,1450,1340, 1050cm- l ; UV-VIS (CH2Ci2 -0.5% MeOH - trace NH40H) Amax (log E): 264 (3.99), 300 (4.08), 434 (4.38) nm; (CH2Ci2 - 0.5% MeOH - trace HCl) Amax (log E): 258 (4.16), 336 (4.17),470 (4.74) nm;IH NMR (200 MHz, acetone-d6) 0: 6.21 (m, 2H), 6.40 (m, 2H), 7.36 (d, J = 8 Hz, 2H), 7.55 (s, 2H), 7.96 (d, J = 8 Hz, 2H), ~ 12.0 (very broad, lH); l3C NMR (50 MHz, acetone-d6) 0: 119.0, 123.7, 128.8, 128.9, 132.4, 139.5, 140.9, 144.6, 145.6; LR-MS (EI, 150°C) mle: 265 (100, M+), 234 (18.8, MW - 20),228 (68.2), 218 (96.5, M+ - HN02). Exact Mass ca1cd. for CIsHu02N3: 265.0851; found: 265.08501.

Bis[5-( 4-nitrophenyl)-4,6-pyrrinato ]Zn(II) (23) To a solution of dipyrrin 20 (100 mg, 3.77 x 10-4 mol) in MeOH (10 mL) was added zinc acetate dihydrate (420 mg, 5 equiv.) in MeOH (10 mL) and the mixture was heated on a water bath for 4 h. The mixture was evaporated to dryness on

2191

the rotary evaporator and the remaining solids were tritrurated with CHCI3. The resulting bright orange-yellow solution was filtered though a short plug of silica gel and allowed to slowly evaporate. Compound 23 was obtained as dark orange lumps with a bright green metallic lustre, which were, after drying (0.1 Torr/50°C), analytically pure (180 mg, 81 %); mp > 300°C; IR (neat): 1590, 1541, 1515, 1405, 1372, 1335, 1243, 1190, 1025, 995 cm-I; UV-VIS (CH2Ci2) Amax (log E): 486 (4.97),352 (4.08), 308 (4.30), 272 (4.42) nm; (pyridine) Amax

(reI. intensity): 440 (0.2), 316 (1.0); IH NMR (200 MHz, CDC13) 0: 6.45 (dd, J = 0.8, 4.2, lH), 6.59 (dd, J = 0.8, 4.2, lH), 7.59 (s, lH), 7.75 (d, 9.2 Hz, lH), 8.35 (d, 9.2 Hz, lH); LR-MS (EI) mle: 592 (40.3, M+), 295 (88.8), 280 (48.7), 265 (100, ligand+). Anal. ca1cd. for C30H20N604Zn: C 60.63, H 3.39, N 14.1; found: C 60.71, H 3.3, N 14.00.

Bis[5-( 4-nitrophenyl)-4,6-pyrrinato ]Ni(II) (21) Compound 20 was prepared using the procedure for the prep­aration of 23. Slow evaporation of a CHC13 - 1 % MeOH solu­tion of 21 yielded a dark brown-orange microcrystalline material, which was, after drying (0.1 Torr/50°C) analytically pure; mp, 230°C; IR (neat): 1595, 1555, 1520, 1335, 1370, 1240, 1040, 1020,995 cm-I; UV-VIS (CH2C2) Amax (log 274 (4.58),318 (4.47), 484 (4.63) nm; (MeOH) Amax (reI. intensity): 272 (0.47), 292, sh (0.45), 466, sh (0.96), 482 (1.0) nm; (pyri­dine) Amax (reI. intensity): 316 (1.0),456, br (0.43), 486 (0.48); I H NMR (200 MHz) 0: 6.66 (d, J = 4.1 Hz, lH), 7.52 (d, J = 8.6 Hz, lH), 8.26 (2 overlapping d, 2H), 10.83 (s, lH); LR-MS (EI, 180°C) mle: 586 (48.3, M+), 539 (15.6, M+ - HN02), 464 (15.3, M+ - Ph - N02). Exact mass ca1cd. forC30H2oN6Ni04: 586.08997; found: 586.08992. Anal. ca1cd. for C30H2oN6Ni04: C 61.36, H 3.43, N 14.31; found: C 6.57, H 3.36, N 14.20.

Bis[5-( 4-nitrophenyl)-4,6-pyrrinato ]Cu(II) (24) Dipyrrin (20) (100 mg, 3.77 mmol) was dissolved in minimal warm MeOH (~5 mL) and, with stirring, a solution of copper acetate monohydrate (380 mg, 5 equiv) in MeOH (5 mL) and concentrated ammonia (0.5 mL) was added. The metal com­plex precipitated from the dark orange solution almost instan­taneously. The precipitate was filtered off after stirring for 12 h at room -temperature, then dried and chromatographed on a short (10 x 2.5 cm, 1 % MeOH - CHCI3) column of silica gel. The first intensely orange band was collected and slow evap­oration of the solvent furnished 160 mg (72%) of the metal complex 24 as black needles with a green metallic lustre. Alternatively, repeated recrystallization from CHClrMeOH yields dark green, dichroic microcrystals with a metallic lus­tre. After drying (0.1 Torr/50°C) an analytically pure sample was obtained; mp> 300°C; UV-VIS (CHCI3) Amax (log E): 274 (4.52), 314 (4.21), 368 (4.26),474 (4.72) nm; (MeOH) Amax (reI. intensity): 268 (0.72), 308 (0.47), 374 (0.39), 468 (1.0), 502, sh (0.53); LR-MS (EI) mle: 591 (6.6, M+), 295 (100), 280 (68.6),264 (44.3), 248 (26.5), 234 (49.0). Exact Mass ca1cd. for C30H2004N66SCu: 593.08240; found: 593.08305. Anal. ca1cd. for C30H20CuN604: C 60.63, H 3.39, N 14.15; found: C 60.70, H 3.54, N 14.03.

Bis[5-phenyl-4,6-pyrrinato]Ni(II) (22) Prepared in 76% yield in an analogous fashion as described for 23. Slow evaporation of an acetone solution gave 22 as dich­roic crystals; mp (dec.) 240°C; IR (neat): 1575, 1535, 1505,

2192

1410, 1380, 1345, 1245, 1035, 1030, 1000 cm- I; UV-VIS

(CHC13) Amax (log E): 324 (4.33), 472 (462) nm; (MeOH) Amax

(reI. intensity): 264 (0.18), 314 (0.23), 458 (0.83), 478 (1.0); IH NMR (300 MHz, CDC13) 0: 6.73 (d, J = 4.6 Hz, 4H), 7.38-7.46 (m, lOH), 7.60 (d, J = 4.5 Hz, 4H), 9.63 (s, 4H); l3C NMR (50 MHz, CDCI3) 0: 127.4,129.1,130.7,134.5,136.8,139.4, 143.7,147.7,173.0; LR-MS (EI, 200°C) mle: 496 (100, M+), 430 (18.8), 419 (18.2), 219 (74.8). Exact Mass calcd. for C30H22N4Ni: 496.11978; found: 496.12071. Anal. ca1cd. for C30H22N4Ni: C 72.47, H 4.46, N 11.27; found: C 72.33, H 4.70, N 11.3.

X-ray crystallographic analysis of bis[S-phenyl-4,6-pyrrinato)Ni(II) (22)

Crystallographic data appear in Table 4. The final unit-cell parameters were obtained by least squares on the setting angles for 25 reflections with 29 = 23.3°-33.6°. The intensities of three standard reflections, measured every 200 reflections throughout the data collection, decayed linearly by 2.3%. The data were processed, corrected for Lorentz and polarization effects, decay, and absorption (empirical, based on azimuthal scans for three reflections). The structure was solved by direct methods, the coordinates of the non-hydrogen atoms being determined from an E-map or from subsequent difference Fourier syntheses (52). The molecule has exact D2 (crystallo­graphic 222) symmetry. Non-hydrogen atoms were refined with anisotropic thermal parameters and hydrogen atoms were fixed in calculated positions with C-H = 0.98 A and BH = 1.2 Bbonded atom' No correction for secondary extinction was neces­sary. Neutral atom scattering factors for all atoms and anoma­lous dispersion corrections for the non-hydrogen atoms were taken from the International Tables for X-Ray Crystallogra­phy (53). Hydrogen atom parameters, anisotropic thermal parameters, torsion angles, intermolecular contacts, scheme of the unit cell, symmetry operators, and least-squares planes are included as supplementary material. 2

Acknowledgements

We thank the Natural Sciences and Engineering Council of Canada for financial support.

Note added in proof: Since the submission of the manuscript, another report describing meso-phenyl-4,6-dipyrrin and its BF2 complex has appeared in the literature: R.W. Wagner and J.S. Lindsey. Pure Appl. Chern. 68, 1373 (1996).

2 Supplementary material metioned in the text may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research Council of Canada, Ottawa, Canada, KIA OS2. Tables of hydrogen atom coordinates and bond lengths and angles involving hydrogen atoms have also been deposited with the Cambridge Crystallographic Data Centre and can be obtained on request from The Director, Cambridge Crystallographic Data Centre, University Chemical Laboratory, 12 Union Road, Cambridge CB2 lEZ, U.K.

Can. J. Chem. Vol. 74, 1996

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