Coordination properties of Q-alanine: new complexes with some transition metals cobalt(II), nickel(II), and copper(I1)

GIUSEPPE MARCOTRIGIANO Cartedra di Chimica, Facolrri di Medici~~a Veteritraria, Ut~icersiry of Buri, 70126 Bari, Italj,

AND

LEDI MENABUE AND GIAN CARLO PELLACANI Isriruro di Clzimica Getrerule e Itzorga12ica, Ut~icersiry of Modena, 41100 Modenu, Ifu11.

Received December 16, 1975

GIUSEPPE MARCOTRIGIANO, LEDI MENABUE, and GIAN CARLO PELLACANI. Can. J. Chem. 54, 2426 (1976).

The neutral six-coordinated tetragonal C ~ ( p - a l a ) ~ .2H20 complex (8-ala = NH2CH2CH2CO- 0-) was prepared. In this complex p-alanine acts as an 0,N-chelating agent, as in the known C ~ ( p - a l a ) ~ . 6 H ~ O and Ni(p-ala)2.3H20 complexes. We also prepared some complexes of a type that is new for p-alanine, with stoichiometries such as M(p-alaH)2X2, M(p-alaH)4(C104)2 (M = Co and Ni), C~(p-a laH)~(ClO~)~ , and Cu(p-alaH)CI2 (p-alaH = &H~CH~CH~COO-), in which the ligand acts as a zwitterion and coordinates toward the carboxy group as a formally neutral ligand. The possibility of formulating the complexes as H2[M(p-ala)2X2] is discussed. The electronic spectra and the magnetic moments of the halide complexes are in accord with pseudo- tetrahedral, octahedral, and tetragonal stereochemistry for the cobalt(II), nickel(Il), and copper(I1) ions, respectively, while those of the perchlorate complexes are in accord with pseudooctahedral stereochemistry. The p-alanine hydrohalide salts and the bis(p-alaninium)- tetrachlorocuprate complex were also prepared and are discussed.

GIUSEPPE MARCOTRIGIANO, LEDI MENABUE et GIAN CARLO PELLACANI. Can. J. Chem. 54, 2426 (1976).

On a prepare le complexe neutre hexacoordonnk tetragonal C ~ ( p - a l a ) ~ .2H20 (6-ala = NH2C- H2CH2COO-). Dans ce complexe, la p-alanine agit comme un agent chilatant par I'oxygkne et I'azote; ce comportement est aussi connu dans les complexes C ~ ( p - a l a ) ~ . 6H20 et Ni(p-ala)2. 3- H20. On a aussi prepare quelques complexes d'un type nouveau pour la p-alanine et possedant des stoechiometries telles que M(p-ala+H)2X2, M(p-alaH)4(C104)2 (M = Co et Ni), Cu(p-alaH)3- (C104)2 et Cu(p-alaH)C12 (p-alaH = NH3CH2CH2COO-), dans lesquels le ligand, neutre d'une f a ~ o n formelle et agissant comme un zwitterion, est coordonne par le groupe carboxyle. On discute de la possibilite de representer les complexes par la formule H2[M(p-ala)2X2]. Les spectres Clectroniques et les moments magnitiques des haloginures complexes sont en accord avec des stiriochimies pseudotitrabdrique, octaedrique et tetragonale respectivement pour les ions cobalt(II), nickel(I1) et cuivre(11); par ailleurs les propriktis des complexes perchlorates correspondent i des stCrCochimies pseudooctaCdriques. On a aussi prepark et on discute des complexes de sels halohydriques de la p-alanine et du bis(p-alaninium) titrachlorocuprate.

[Traduit par le journal]

Introduction Our understanding of the nature of metal-

binding sites in proteins owes much to the study of models involving metal complexes with amino acids and small peptides. The coordinating power of amino-acid side chains towards metals, particularly copper(II), has been the object of a great deal of investigation and has been taken into account in many discussions of metal- binding sites in proteins.

Many simple amino-acids have been com- plexed with a large number of transition metal ions, and a recent review (1) collects the data

relating to interactions between transition metal ions and a-amino acids which, formally, act as tridentate ligands.

In this work we study the interactions of the cobalt(II), nickel(II), and copper(I1) ions with 0-alanine.

The crystal structure (2) and the esr spectra (3-5) of the C~(p -a l a )~ .6H~O complex and the electronic and vibrational spectra of the bis- hydrate and anhydrous Ni(p-ala)z complex (6) are known. Also, the rate constants for the formation of CoZ+, NiZ+ (7), and CuZ+ (8) com- plexes with p-alanine have been measured by

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MARCOTRIGIANO ET AL. 2427

temperature-jump and stopped-flow methods. This study represents an extension of our

previous research in which we have investigated the complexes of some transition metal ions with some amino-acids, such as N-acetylglycine and N-benzoylglycine, and their amine adducts (9).

Experimental All the reagents were of pure comn~ercial grade.

Preparation of the Complexes C0(/3-ala)~.2H~O was obtained by adding 4 X 10-3 mol

of freshly prepared C O ( O H ) ~ . ~ H ~ O to 10-2 mol of p-alanine in 10 ml of H 2 0 with stirring at 40-50 "C. The unreacted cobalt salt was filtered off and the wine-red solution was cooled at 4-5 "C overnight to yield a pink crystalline compound.

C0@-alaH)~(CI0~)2 and C~(p-alaH)~(ClO~)~. Solid P- alanine (8 X mol) were slowly added with stirring to a 60-70 "C methanolic solution containing 2 X 10-3 mol of M(C104)2.6H20. The undissolved p-alanine was rapidly filtered off. The lilac cobalt compound was separated by cooling the wine-red solution. The copper(I1) compound was separated by concentrating the blue solution, adding diethyl ether until a blue oil was ob- tained, and cooling the 011 at 4-5 "C overnight.

Ni(p-alaH)4(C104)2. p-Alanine (1 X 10-2 mol) was sus- pended in a methanolic solution (15 ml) of Ni(C10.J2. 6H20 (2.5 mol) and refluxed for2 h on a water bath. By concentrating (5 ml) the solution and adding an acetone - diethyl ether (1:l) solution a blue oil separated out. The oil crystallized by cooling in an ice bath overnight. This compound was also recrystallized by dissolving it in methanol and adding acetone as precipitating agent.

M(p-a l~H)~x~ (M = Cu, Ni and X = C1, Br; M = Co and X = C1, Br, I). The complexes were prepared by re- fluxing 4 X 10-3 mol of solid p-alanine suspended in a methanolic solution of anhydrous cobalt(l1) or nickel(l1) halide or in an ethanolic solution of anhydrous copper(l1) halide (2 X 10-3 mol in 30 ml) at 70-80 "C until a limpid solution was obtained. Acetone was added until incipient precipitation and then the solution was cooled in an ice bath. The cobalt(l1) halide complexes are blue, C ~ ( p - a l a H ) ~ I ~ is olive-green, and the nickel(1I) complexes are green.

Cu(p-alaH)Clz was prepared by the reaction of 2 X 10-3 rnol of CuClz.2H20 in absolute ethanol (20ml) with 2 X mol of p-alanine, as described above.

CuC14(p-a1aHt)Z. p-Alanine hydrochloride (2 X mol) in 10 rnl of absolute ethanol were added with stirring to a 50-60 "C ethanolic solution (15 ml? of CuC12.2H;0. By cooling the solution at 4-5 "C, a yellow crystalline product was obtained.

The compounds prepared cannot be recrystallized, they are hygroscopic, particularly the cobalt(I1) complexes, and decompose in air. They are insoluble in apolar solvents and fairly soluble, with decomposition, in methanol.

Physical Measurements The electronic spectra of the solid compounds were

recorded on a Beckman DK 1A spectrophotometer. The samples were prepared by grinding the complexes using a filter paper as support and a filter paper as reference. Since these compounds are highly hygroscopic the instrument was flushed with dry nitrogen before and during use. The infrared spectra of KBr pellets were recorded with a Perkin-Elmer 521 (4000-250cm-1) spectrophotometer. The room-temperature magnetic moments were measured by the Gouy method using Ni(en)3S203 as calibrant and correcting for diamagnetism with the appropriate Pascal constants. The conductivities of the complexes in methanol were measured with a WTW conductivity bridge.

Analyses Nitrogen, carbon, and hydrogen were analysed by Mr.

Giuseppe Pistoni on a Perkin-Elmer 240 elemental analyzer. The halides were analysed by titration with the Volhard method after disgregation of the compound with the alkali metal fusion method (10).

Results and Discussion The compounds are listed in Table 1. Their

preparations and general properties are de- scribed in the Experimental section.

We were unable to determine molecular weights, because the compounds were insuffi- cientlv soluble or were unstable in suitable solvents (e.g. methanol).

The conductivity measurements of some com- pounds are reported in Table 1. The values obtained are in the range proposed for 2: 1 elec- trolytes (1 l), but the electronic spectra of the solution M) are indicative of a decomposi- tion of the compounds in solution. In particular very concentrated solutions of the blue cobalt(I1) complexes are pink, while M solution of all the compounds are colourless.

In the near-ir region p-alanine shows bands at 4310, 4650, 5130, 5710, and 6070 cm-l, which also appear in all the complexes in virtually the same positions with the same intensities, with the exception of the band at 4650cmW1, which is shifted to higher energies (495Ck4780cm-l) and is enhanced in intensity only in the M(p-ala)2.xHz0 (x = 2 for M = Co, x = 3 for M = Ni, and x = 6 for M = Cu) complexes. This band may thus be used to recognize this type of complexes.

The d-d bands of the complexes are reported in Table 2. The electronic spectra of the metal halide complexes are typical of octahedral nickel(I1) ion, six-coordinate copper(I1) ion, and pseudotetrahedral cobalt(I1) ion, respectively, and suggest in all these complexes halide co-

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2428 CAN. J. CHEM. VOL. 54. 1976

TABLE 1. Analytical, magnetic, and conductivity data5

c H N X AM Pert M

Compound Calcd. Found Calcd. Found Calcd. Found Calcd. Found (B.M.) (MeOH)

C0(p-ala)~.2H~O 26.56 26.48 5.95 6.10 10.33 10.06 5.10 C ~ ( p - a l a H ) ~ C l ~ 23.38 24.02 4.58 4.77 9.09 8.88 23.02 22.47 4.93 C~(b-a laH)~Br~ 18.14 18.05 3.56 4.24 7.06 6.33 40.27 39.99 4.75 C ~ ( p - a l a H ) ~ I ~ 14.79 14.80 2.90 3.63 5.75 5.13 4.61 C~(p-a laH)~(ClO~)~ 23.45 24.26 4.59 5.13 9.12 8.77 11.55 10.91 4.31 Ni(fl-alaH)zC12 23.39 23.88 4.59 5.69 9.10 8.24 23.04 22.69 3.20 123 Ni(p-alaH)2Br2 18.15 17.97 3.56 4.13 7.06 6.61 40.27 40.08 3.26 143 Ni(p-alaH)4(C104)2 23.46 23.58 4.60 4.68 9.13 8.34 11.52 10.93 138 Cu(p-alaH);Cl2 23.03 22.90 4.51 4.42 8.96 8.07 22.69 22.22 1.82 111 Cu(fl-alaH)C12 16.11 16.40 3.16 3.38 6.27 6.18 31.73 31.77 1.72 C ~ ( p - a l a H ) ~ B r ~ 17.93 17.96 3.51 3.68 6.98 6.57 39.81 39.37 1.99 139 C ~ ( p - a l a H ) ~ ( C l O ~ ) ~ 20.39 20.11 4.00 4.23 7.93 7.28 13.39 13.01 1.97 183 C U C ~ ~ ( P - ~ I ~ H ~ ) ~ 18.77 19.05 3.68 4.29 7.30 7.21 36.98 36.63 1.95

aAbbreviations: B-ala = NH2. CHzCHz. COO-; B-alaH = NHlf. CHZ . CH2. COO-; 8-alaH? = NH3+. CHZ . CH2. COOH *Not measured, for packing difficult, the compound being highly hygroscopic.

TABLE 2. d-d transitions (cm-1) of the complexes in the solid state

Compound Transition (cm-I)

ordination, as the energy order of the d-d bands is C1 > Br > I, in accordance with the spectro- chemical series. Their magnetic moments agree with the proposed stereochemistries.

The absorption peaks in the nickel-complex spectra (Fig. 1) indicate a fairly symmetrical octahedral field. The low energy band, which represents the average ligand field strength (12), is broad.

In the spectra of the cobalt(I1) halide com- plexes (Fig. 2) the splitting for the 4T1(F) term suggests a marked effect of a low symmetry component of the crystal field (13).

The electronic spectrum of the nickel(I1) perchlorate complexes (Fig. 1) is similar to those of the nickel(I1) halide complexes, suggesting an octahedral environment. The energy of the

d-d bands of the halide complexes which is lower than that of the perchlorate complex depends on the donor strength of the halides.

The electronic spectra of the C ~ ( p - a l a ) ~ 2H20 and of the Co(O-alaH)4- (C104)2 complexes (Fig. 2) show a weak intensity band split into two components in the near ir region and a more intense multiple band in the 18 000-22 000 cm-I region. The absorption band regions are charac- teristic of six-coordinate cobalt(I1) derivatives, as is confirmed by their magnetic moments, but the splitting of the near-ir band also suggests the existence of distortion from a regular octahedral structure in both the complexes (14).

In the C0(p-ala)~-2H~O complex, as in the hydrate nickel and copper p-alaninate com- plexes, the b-ala ligands have a trans, square-

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MARCOTRIGIANO ET AL.

FIG. 1. Examples of electronic spectra of the Ni- (p-alaH)4(C104)2 (I), Ni(p-alaH)2C12 (2), and Ni(p-alaH)2- Br2 (3) con~plexes.

planar arrangement about the metal atom, and the octahedral coordination completed by two water molecules (1 5).

The assignment of all the bands of the ir spectra of the p-alanine has not been made, but those of NH stretching and antisymmetric and symmetric carboxy stretching frequencies are straightforward. Of primary concern for deter- mining the nature of coordination is the posi- tions of the antisymmetric and symmetric carboxy stretching frkquencies and their separa- tions. These are listed in Table 3.

The bands at 1720-1689 and 1236-1 190 cm-I and their separation in the 6-alanine hydrohalide salts are consistent with those of other amino acids and their hydrohalide salts in which the -COOH group is present (16, 17). The infrared spectra of all the other compounds show a single peak, except the cobalt(I1) and copper(I1) perchlorate complexes, in the 16 10-1545 cm-I region. The absence of absorption at higher wave numbers indicates that no uncomplexed -COOH groups are present (18-20).

In the M(P-ala);!. xH20 (M = Co, Ni, and Cu) complexes of known structure (15), the asym-

FIG. 2. Examples of electronic spectra of the p-alanine (I) and of the C0(p-ala)~.2H~O (2), C0(p-alaH)~(CI0~)2 (3) , C ~ ( p - a l a H ) ~ X ~ (X = Cl (I), Br (5) and 1 (6)).

metric carboxy stretching vibration and the Av separation are assumed to typify a monodentate carboxyl group coordination, being also in the range found for other monocoordinated car- boxyl group amino acid (6). The energy order of the v(COO), is Ni > Co > Cu.

In the C ~ ( p - a l a H ) ~ X ~ complexes, which show Av and v(COO), similar to those of the above mentioned complexes, a similar monodentate coordination of the carboxyl group may be proposed.

In the Cu(6-alaH)X2 and M(P-alaH)2X2 (M =

Co and Ni) complexes the position of the v(COO),, which is in the order Ni > Co > CU, similar to that found for the M(p-ala)2.xH20

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CAN. J. CHEM. VOL. 54. 1976

TABLE 3. Some ir absorption frequencies (cm-1) of the complexesa

Compound v(NH) v(COO),, v(COO), AV

p-ala p-alaH2. C1 p-alaHz .Br p-alaH2 1 C ~ C l ~ ( p - a l a H ~ ) ~ C~(p-ala)~. 2H20 Ni(p-ala)2. 3H2Ob C~(p-ala)~. 6H2O C~(p-alaH)~Cl~ C~(p-alaH)~Br~ C~(p-alaH)~I C~(p-alaH)~(ClO~)~ Ni(p-alaH)2C12 Ni(p-alaH)2Br2 Ni(p-alaH)4(C104)2 Cu(p-alaH)C12 C~(p-alaH)~Cl2 C~(p-alaH)~Brz C~(p-alaH)~(ClO.,)~

aThe values reported for the M(B-alaH)Xz (M=Co and Ni) complexes are the average of a rather broad band probably containing also the S(NH) vibration. These complexes and the perchlorate ones show a broad band at 3480-3350 which may be due to the absorption of moisture when the KBr pellets of these compounds are prepared.

bReference 6.

complexes, and the Av separation are reduced compared with those of the complexes contain- ing a monodentate coordinated carboxyl group. This reduction was also found in the acetate (21) and in the haloacetate (22) complexes of cobalt(II), nickel(II), and copper(II), for which a bidentate coordination of the carboxyl group is proposed.

The v(NH) bands of these complexes, lower than those of the M(p-ala)2-xH20 complexes which are also coordinated toward the nitrogen atom (15), are in the range of the v(NH) found in the p-alanine hydrohalide salts and in other hydrochloride salts of amino acids in which the NH3+ group is present (16, 17).

All these data indicate that the ligand may be zwitterionic and coordinate as formally neutral ligand, as

M = Co, Ni, or Cu X = CI, Br for Ni and Cu

CI, Br, I for Co

The presence of the 0 and X atom in the coordination sphere of the metal ion is justified by the low energy d-d bands of these complexes.

An alternative hypothesis is suggested by the fact that in these complexes the presence of uncomplexed -COO- groups is not precluded, since they absorb in the same region as carboxyl groups which are coordinated (20) and by the fact that the v(NH) bands are also in the range expected for coordinated NH2 groups (23). This would permit the formulation of the halide complexes as H2[M(p-ala)2X2], in which the ligand is only coordinated toward the nitrogen atom.

We prefer the first hypothesis which also per- mits the interpretation of the perchlorate com- plexes. If the ligand is in the zwitterionic form coordinated toward the oxygen atom, the two v(CO0) bands in the cobalt(I1) and copper(1I) perchlorate complexes suggest the presence of mono and bidentate carboxyl groups. The v(NH) bands, higher than those of the halide complexes, may be explained as due to the interaction of the NH3+ group with the C104- anions. In fact the ir bands of the C104- ions correspond to a slight deformation of this ion from T, symmetry (24).

The electronic spectrum of the CuC14(p-alaH2)2 complex, for which no thermochromic behaviour was observed in the 290-370K temperature range, suggests near square-planar configuration

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MARCOTRIGI LANO ET AL. 243 1

of the CuCL2- anion (25, 26), which indicates the prevailing effect of the crystal field stabiliza- tion, since extensive N-H- . C1 hydrogen bonding reduces the effective charge on the chlorine atoms (27, 28). The red shift of the v(NH) band of this complex, as distinct from that of the 0-alanine hydrohalide supports this hypothesis. The ir and far-ir spectra of this complex closely resemble those of the palanine hydrochloride salt, except for three bands at 270vs, 282sh and 200s, assignable to metal- chlorine vibration in D,, symmetry (28).

Acknowledgement The authors are grateful to the Istituto di

Chimica Generale e Inorganica of the University of Modena which has supplied us the instru- ments.

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