Short Communication

DOI: 10.1002/zaac.200700254

Isolation of a Manganese Complex of a Tridentate Azo-aromatic Ligand froman Unusual Mn2(CO)10 Promoted Simultaneous Reductive Azo Cleavage andAromatic Ring Amination Reactions

Sudipta Chatterjeea, Anasuya Sanyala, Chen-Hsiung Hungb, and Sreebrata Goswami*,a

a Kolkata / India, Indian Association for the Cultivation of Science, Department of Inorganic Chemistryb Nankong, Taipei/ Taiwan, Republic of China, Academia Sinica, Department of Chemistry, Institute of Chemistry

Received May 14th, 2007.

Dedicated to Professor Alfonso Castineiras on the Occasion of his 65th Birthday

Abstract. The reaction of Mn2(CO)10 with pap [pap � 2-(phenyl-azo) pyridine, L1] in boiling n-octane affords a violet complex,[Mn(L2)2] (1, HL2 NH4C5N�NC6H4N(H)C6H5). This reactionexemplifies an unusual transformation wherein the anionic triden-tate ligand (L2)� is formed by the fusion of phenyl imide fragment,originated from the splitting of the �N�N- function of L1, to pen-

Metal complexes of the azoaromatic ligand 2-(phenylazo)pyridine(pap, L1) have been the subject of considerable interest [1, 2] pri-marily because of (i) their exceptional rich redox and spectroscopicbehavior, (ii) catalytic activities and (iii) unusual type of chemicalreactions that occur at the coordinated ligand due to C-H acti-vation. During the recent years there has been an upsurge in thestudy of the coordination chemistry of L1 involving 3d elementsafter the discovery that these complexes are suitable [3] for tailor-made synthesis of new ligands with multiple denticities via aro-matic ring amination reactions (Supplementary Scheme S1).

Herein we disclose our results of the reaction of Mn2(CO)10 andL1. Unusually the bidentate neutral ligand L1, in this reaction, hasundergone a chemical transformation producing an anionic triden-tate ligand [L2]� via simultaneous reductive azo cleavage and amin-ation reaction [4] of the coordinated L1 ligand. In the present con-text it is worth noting that the examples of reductive cleavage of acoordinated azo-function resulting in metal organoimido speciesare not common and belong to a class of useful chemical trans-formation. The reference reaction has been followed by the iso-lation and complete characterization of the product.

* Professor Sreebrata GoswamiDepartmentof Inorg. ChemistryIndian Association for the Cultivation of ScienceKolkata-700 032 / IndiaFax (�91)33 2473 2805E-mail: icsg@iacs.res.in

Supporting information for this article is available on theWWW under http://www.wiley-vch.de/home/zaac or from theauthor

Z. Anorg. Allg. Chem. 2007, 633, 1775�1777 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1775

dant phenyl ring of the coordinated azo ligand. X-ray structure ofthe manganese complex is reported.

Keywords: Mn2(CO)10 promoted reaction; Manganese complex;Azoaromatic ligand; Crystal structure

Results and Discussion

The two azoaromatic ligands, concerning the present work, areabbreviated as L1 and HL2 (Chart I).

Chart I

The reaction of Mn2(CO)10 and the ligand L1 in n-octane produceda dark violet crystalline compound of composition Mn(L2)2 (1) in35 % yield. The formation of the molecular compound 1 from theabove reaction was authenticated by X-ray structure analysis ofthe product. The reference organic transformation L1 � [L2]� ismediated by Mn2(CO)10 and exemplifies an uncommon type ofchemical reaction wherein an anionic tridentate N, N, N donor[L2]� is formed by the oxidative fusion of phenylimide fragment[PhN]2� to a pendant aromatic ring of coordinated L1. It is worthnoting here that regioselective aromatic ring amination [2a] of thependant aryl ring of L1 via its coordination to transition metal ionsdue to C-H activation has been studied by us during the recentyears. The imide fragment, [PhN]2� is formed [4a] by reductivecleavage of the diazo ligand L1 (Supplementary Scheme S2). Thecleavage of �N�N-function of L1 requires addition of four elec-trons at the π*(azo) orbitals, which are supplied by Mn2(CO)10.Difference of formal metal oxidation level in starting manganese

S. Chatterjee, A. Sanyal, C.-H. Hung, S. Goswami

Figure 1 An ORTEP plot and atom numbering scheme for[Mn(L2)2] (1). Hydrogen atoms are not shown for clarity.

carbonyl and in the product 1 is two units. This mismatch in elec-tron accounting in the above redox process may be responsible forthe low yield of the product.

The ORTEP representation of 1 with atom numbering scheme isdepicted in Figure 1. The two deprotonated tridentate ligands arecoordinated using pairs of pyridyl-N, azo-N, and deprotonated sec-ondary amine (anilido) N atoms. The configuration is bis-meri-dional, and the relative orientations within the aforesaid pairs ofcoordinated atoms are cis, trans and cis, respectively. X-ray crystal-lographic analysis (Table 1) reveals considerably distorted octa-hedral structure with trans positioned azo-N, cis positioned 2-pyri-dyl-N, and cis-oriented anilido nitrogen atoms. The Nazo-Mn-Nazo

angle is larger than 169°, however, the other two trans angles aresmaller at about 157° due to the overarching bite of the mer-triden-tate ligands. An examination of the bond parameters (Table 2) ofthe complex [Mn(L2)2] reveals that the N-N bonds [av. 1.345(4) A]are elongated considerably than that [5] in the uncoordinated andprotonated (2-pyridyl)-substituted ligand salt, [H2L�](ClO4)[HL� : NH4C5N�NC6H4N(H)C5H4N]. Coordination of the ligand(L2)� to a metal ion is typically associated with the delocalizationof the negative charge along the ligand backbone, resulting in con-traction of the C-N bonds on either side of the central o-phenylenering and in an elongation of the azo bond. In the present manga-nese complex though there have been contractions of theC-N bonds, the N-N bonds are distinctly longer than that observed[1.308(3) A] in other examples [6] of [M(L2)2]0/� complexes (M �

Co, Fe and Ni).

The magnetic moment of [Mn(L2)2] is 1.65 µB indicating aS�1/2 ground state. In dichloromethane-toluene solution, the com-plex exhibits six-line (55Mn, I� 5/2) spectrum with g � 2.045. Thecyclic voltammogram of the reference manganese compound dis-plays three reversible one-electron transfer waves. Characterizationdata of 1 are collected in Table 3.

www.zaac.wiley-vch.de 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2007, 1775�17771776

Table 1 Crystallographic data and refinement results of 1

Emperical formula C34H26MnN8 Z 4Formula weight 601.57 Dcalc. [Mg/m3] 1.418Temp /K 293(2) Cryst dimensions /mm3 0.28 x 0.10 x 0.08Cryst syst monoclinic GOF 0.942Space group P21/n Reflns collected 5381a /A 10.3746(17) Unique reflns 4901b /A 16.061(6) Final R indices [I>2σ(I)] R1 � 0.0332c /A 16.726(6) wR2 � 0.0740β /deg 90.15(2)

Table 2 Selected bond distances /A and angles /° for [Mn(L2)2].The estimated standard deviations are shown in parentheses.

Distances

Mn-N(1) 2.003(2) N(8)-C(29) 1.424(3)Mn-N(3) 1.888(2) C(23)-C(24) 1.395(3)Mn-N(4) 1.957(2) C(24)-C(25) 1.369(4)Mn-N(5) 1.997(2) C(25)-C(26) 1.394(4)Mn-N(7) 1.889 (2) C(26)-C(27) 1.368(4)Mn-N(8) 1.950(2) C(27)-C(28) 1.411(4)N(5)-C(22) 1.361(3) N(5)-C(18) 1.351(3)N(6)-C(22) 1.362(3) C(18)-C(19) 1.370(4)N(6)-N(7) 1.324(3) C(19)-C(20) 1.383(4)N(7)-C(23) 1.388(3) C(20)-C(21) 1.359(4)C(23)-C(28) 1.410(3) C(21)-C(22) 1.399(4)C(28)-N(8) 1.357(3) N(2)-N(3) 1.344(4)

Angles

N(1)-Mn-N(3) 77.04(8) N(5)-Mn-N(7) 76.75(8)N(3)-Mn-N(4) 81.25(8) N(7)-Mn-N(8) 81.26(8)

Table 3 Characterization data of [Mn(L2a)2].

ESIMS 602.73 (MH�)

Elemental Analysisa) C, 67.58 (67.89); H, 4.42 (4.32);N, 18.70 (18.63) %

UV-Vis {λmax/nm(ε/ M�1cm�)}b) 1170(2060), 760(10360), 575(10940),525 c)(9360), 375(15030), 322 c)(25300),290(32320), 240(40430)

Electrochemical Data d), 1.10(70), 0.38(65), �0.96(80)[E1/2/V](∆Ep/mV)

Infrared Data e), ν(cm�1) 1580 (νC�C � νC�N), 1230 (νN�N)

Magnetic Moment, µeff 1.65 µB

a) Calculated values are in parenthesis. b) Solvent : acetonitrile. c) Shoulder.d) Conditions : Solvent CH3CN; working electrode, platinum; solute concen-tration, 10�3M; scan rate, 50 mVs�1. e) In KBr disc.

In summary we have reported a rare example of simultaneous azocleavage and ortho amination of phenyl moiety of the coordinated2-(phenylazo)pyridine ligand. This reaction is mediated by manga-nese carbonyl. The formal oxidation state of Mn in the presentcomplex is 2�, however, the N-N lengths in it are unusually longand the possibilities of diradical [7] description of the complex cannot be totally ruled out. Spectroeectrochemical experiments, in thisregard, should be helpful.

Manganese Complex of a Tridentate Azo-aromatic Ligand

Experimental SectionBis{phenyl[2-(2-pyridylazo)phenyl]amido}manganese[Mn(L2)2].

A mixture of the ligand L1 (200 mg, 1.1 mmol) and Mn2(CO)10

(40 mg, 0.18 mmol) in 30 ml n-octane was refluxed for 4 hours. Thecrude mass, thus obtained by solvent evaporation, was crystallizedseveral times from dichloromethane-hexane solvent mixture. X-rayquality sample was grown by slow diffusion of dichloromethanesolution of the compound into hexane. Yield of the dark violetcrystals of 1 was 35 %.

Crystallographic data for the structure have been deposited withthe Cambridge Crystallographic Data Centre under the depositorynumber CCDC 647426. These data can be obtained free ofcharge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/datarequest/cif.

Acknowledgements. Financial support received from the Depart-ment of Science and Technology (Project SR/S1/IC-24/2006) andthe Council of Scientific and Industrial Research, New Delhi isgratefully acknowledged. S.C. thanks the Council of Scientific andIndustrial Research for his fellowship. We are grateful to Dr.G. Mostafa, Jadavpur University for his help.

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