Designer ligands. Part 15. Synthesis and characterisation of novel Mn(lI), Ni(II) and Zn(II) complexes of 1,10-phenanthroline-derived ligands

Series of manganese(II), nickel(II) and zinc(II) complexes have been prepared using 1,10-phenanthroline-derived ligands, and their coordination geometries have been assigned using infrared data. It is apparent that, depending on the ligand, the metal centres adopt octahedral, tetrahedral and distorted tetrahedral coordination geometries. The catecholase activity of the manganese(II) complexes has also been investigated


Introduction
2][3][4] Mononuclear manganese centres have been found in the oxygen evolving center (OEC) of photosystem II (PS-II) 5 and in numerous enzymes, 6 including the superoxide dismutase, Mn-SOD, 7 and Mn-dioxygenase. 8Dinuclear manganese centres, on the other hand, have been found in various metalloenzymes, 14 including catalase, [9][10][11] Mn-ribonucleotide reductase 12 and arginase. 13Nickel has been found in ureases, 15 hydrogenases, CO dehydrogenases and coenzyme F 430 . 16Hydrogenases occur in many bacteria where they are responsible for catalyzing the oxidation of molecular hydrogen, while CO dehydrogenase interconverts carbon monoxide and carbon dioxide.Hydrogenases and CO dehydrogenases have nickel(III) (low spin d 7 ) as well as Fe-S clusters present in their structures. 16Nickel(II) exists in a square planar geometry in Coenzyme F 430 which is a cofactor of methyl coenzyme M reductase.Zinc is found in many enzymes, exhibiting active catalytic or structural roles.][19][20] We have previously described the preparation of ligands which are capable of chelating two metal ions and in which the coordinating moieties are separated by biphenyl 21 and 1,10phenanthroline 22 and, more recently, the preparation of manganese(II), nickel(II) and zinc(II) complexes using various benzamide-and biphenyl-derived ligands. 23In this paper, we now report:-the synthesis and structural assignment of three series of novel Mn(II), Ni(II) and Zn(II) complexes of 1,10-phenanthroline-derived multidentate ligands containing amido, amino and heterocyclic moieties, and the catecholase activity of the manganese(II) complexes.

Results and Discussion
The synthesis of the 1,10-phenanthroline-derived ligands 1a-d has been reported previously, 24 and Schemes 1-3 outline the use of these ligands in the formation of the corresponding Mn(II), Ni(II) and Zn(II) complexes.The structural assignments are based on a consideration of the microanalysis and infrared data and, in the case of the Zn(II) complexes 6b,c and 7 (Scheme 3), on the 1 H-and 13 C-NMR data.Microanalysis data for the various complexes are summarized in Table 1.It is apparent that complexes 2a and 2c are mononuclear while complexes 2b and 3 are dinuclear.All of the Mn complexes contain two chloride ions except complex 2b, which contains four (Scheme 1).The nickel complexes 4a-c and 5 all appear to be dinuclear and each complex contains four chloride ions (Scheme 2).Similarly, the zinc complexes 6b,c and 7 also all appear to be dinuclear and each complex contains four chloride ions except complex 6b which contains two (Scheme 3).The orientation of the chloride atoms within the coordination sphere(s) in each complex was deduced from the far-IR spectroscopic data.The mid-IR data provided a basis for determining whether the metal ions coordinate through the amide nitrogen or oxygen atoms, a negative shift for the amide carbonyl band (amide I) indicating coordination via the amide oxygen, a positive shift coordination via the amide nitrogen.The relevant mid-and far-IR data for all of the complexes examined are summarised in Table 2.For the mononuclear Mn(II) complex 2a, a positive shift of the amide carbonyl band and the negative shift of the amide NH band (relative to the free ligand) indicate coordination through the amide nitrogens, while a band at 3137 cm -1 , assigned to the imidazole NH stretch, suggests coordination with the imidazole tertiary nitrogen atoms.The microanalysis data suggest the presence of two chloride anions in this mononuclear complex, supporting the assumption that the amide protons were not removed on formation of the complex.In the case of the dinuclear Mn(II) complex 2b, an amide NH band also appears to be present, although the benzimidazole NH band is believed to be masked by water; the presence of four chloride atoms suggest that the benzamidazole moieties have not been deprotonated.The positive shift of the amide carbonyl band and the negative shift of the amide NH band again indicate coordination through the amide nitrogens.The negative shifts observed for the amide carbonyl and NH bands of the mononuclear Mn(II) complex 2c, on the other hand, indicate coordination through the amide oxygen.The secondary amine NH band could not be seen in the spectrum of the Mn(II) complex 3 and deprotonation is supported by the presence of only two chloride anions for this dinuclear complex.The bands at 245, 270 and 277 cm -1 in the far-IR spectra of complexes 2a-c indicate apical octahedral orientations of the chloride ligands, 25 while the two bands observed at 281 and 220 cm -1 for complex 3 is characteristic of chloride bridging. 26he Ni(II) complexes 4a-c all exhibit negative shifts of the amide NH band indicating coordination through the amide nitrogens − a conclusion supported by the positive amide C=O band shifts in complexes 4a,b; the microanalysis data also provide evidence for the presence of four chloride ligands in these complexes.The strong bands in the region 370-390 cm -1 for complexes 4a-c are attributed to trans-coordinated chloride ions in an octahedral environment, while others at 253, 283 and 310 cm -1 , are indicative of chloride ions coordinated in a tetrahedral environment. 27,28Similar bands are evident for complex 5 at 278 and 352 cm -1 , but the location of the respective metal centres within all four Ni(II) complexes is not clear (as indicated in Scheme 2).Interestingly, the structure of the microbial urease from Klebsiella aerogenes contains pseudotetrahedrally coordinated nickel, 29 while Meyer has reported ethanolysis of urea by an asymmetrical dinuclear nickel complex containing one nickel atom having octahedral and the other trigonal bipyramidal geometry. 30In the absence of X-ray crystal structures, computer modelling at the Molecular Mechanics level was used to explore the possible 3-D structures of the dinuclear nickel complexes.For complex 4c, for example, it seems that isomer (a) is the favored structure.In both cases, one of the nickel(II) ions is octahedral and the other distorted tetrahedral.The positive shifts observed for the amide carbonyl bands in complexes 6b,c are also indicative of coordination through the amide ntrogens, although the amide NH shifts are positive in both cases.The benzimidazole NH band for complex 6b also cannot be seen but the microanalysis data indicate the presence of only two chloride atoms and, hence, that deprotonation of the benzimidazole moiety had, in fact, occurred.An IR band at 3237 cm -1 for complex 7 suggests that complexation does not require deprotonation of the secondary amine, a shift of 120 cm -1 for this NH band (relative to the free ligand) reflecting coordination with the aliphatic amine nitrogen.Two Zn-Cl bands (283 and 313 cm -1 ) characteristic of tetrahedral coordination geometry for zinc are observed for complex 6b, 27 but comparable bands are not observed for the other Zn(II) complexes.Instead, a very strong, broad band, with a lower and higher frequency shoulder, is observed at ca. 300 cm -1 for complexes 6c and 7, respectively.This may be due to accidental degeneracy of the symmetric and anti-symmetric Zn-Cl stretches and is indicative of distorted tetrahedral zinc arrangements within these complexes.
Dinuclear copper(II) complexes are the most reported functional mimics of catechol oxidase, but catalytically active complexes containing other transition metal ions such as Mn, Co or Ni 31,32 are also known.As part of our research on the development of complexes that mimic the active site of the enzyme tyrosinase, 33 we also explored the phenolase and catecholase activity of the Mn(II) complexes 2a-c and 3.For a model to successfully mimic the enzyme tyrosinase it must be capable of oxidizing a phenol and/or catechol.5][36] 3,5-DTBP 8 is oxidized to 3,5-DTBC 9 (phenolase activity) which is subsequently oxidized to 3,5-DTBQ 10 (catecholase activity).DTBP 8 can also be oxidized to the coupled product 11.These reactions were conducted in DMF in the presence of triethylamine (Et 3 N) and the oxidation product(s) were detected using 1 H NMR spectroscopy.It is apparent from an examination of the data in Table 3 that none of the complexes exhibited phenolase activity.The axial binding of the phenol to the metal centre and Berry pseudorotation of the trigonal bipyramidal complex, required to expose the equatorial substrate to ortho-hydroxylation appears to be inhibited in all four cases.Catecholase activity only requires a transfer of electrons and is, in fact, observed for three of the complexes (2a,b and 3), with the dinuclear complexes 2b and 3 exhibiting higher conversion rates than the mononuclear complex 2a and excellent re-cyclability.The conversion rates (Table 3) are higher than those observed previously for biphenyl dinuclear cobalt(II) 37 and macrocyclic dinuclear copper(I) and copper(II) complexes, 38 but similar to those exhibited by macrocyclic dinuclear cobalt(II), 38 mononuclear Mn(II) benzamide and dinuclear biphenyl Mn(II) manganese complexes. 23atalase, itself, has two manganese ions in its active site and catalyzes the disproportionation of two hydrogen peroxide molecules to two molecules of water and one of dioxygen, and the catalytic activity of complexes 2b and 3 may be attributed to the fact that they are dinuclear and have similar structures to that of the complex reported by Dismukes. 39The apparent inactivity of complex 2c may be attributed to strong binding of the metal to the amide oxygens rather than the nitrogens and to the relative basicity of the pyridine nitrogen; without the presence of weakly coordinated H 2 O or amide NH, dissociation of the complex to form the necessary trigonal bipyramidal intermediate is not possible.The catalytic oxidation of 3,5-DTBC 9 to 3,5-DTBQ 10 by a series of mononuclear manganese complexes has been reported previously. 40

Conclusions
The 1,10-phenanthroline ligands 1a-d clearly form complexes with manganese(II), nickel(II) and zinc(II) and, depending on the ligand, the metal centres in these complexes adopt octahedral, tetrahedral or distorted tetrahedral coordination geometries − patterns observed in our earlier communication. 23Microanalytical, mid-and far-infrared and, where appropriate, NMR spectroscopic data have all been used to assign structures to the complexes and computer modelling has been used to explore structural preferences between isomeric possibilities.The manganese(II) complexes 2a, 2b and 3 have been shown to exhibit significant catecholase activity (67 -100% conversion within 24 hours), while complexes 2b and 3 also exhibit excellent recyclability.

Biomimetic studies
The substrates DTBP and DTBC were added to solutions of the Mn(II) complexes in DMF, containing Et 3 N (see Table 3), to give substrate:complex molar ratios of 100:1.The resulting mixtures were aerated by stirring vigorously for 24h.At the conclusion of each reaction period, the mixture was concentrated to dryness in vacuo and the residue analysed by 1 H NMR spectroscopy to determine the substrate:product ratio.Recyclability was established by adding fresh substrate, DMF and Et 3 N to the residue from the initial reaction, and stirring for 24 h.The solvent was evaporated in vacuo and the residual material analyzed as before.

Table 1 .
Microanalytical data for the manganese(II), nickel(II) and zinc(II) complexes followed, in parentheses, by the calculated values Complex

Table 2 .
Summary of the IR frequencies (ν M-Cl ) and the amide frequency shifts (∆ν NH and ∆ν C=O ) on formation of the metal complexes, together with residual water bands