C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 175–181© 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS. Tous droits reserve13871609(00)001377/FLA

Organic and organometallic synthesis / Synthese organique et organometallique

[MoO2(acac)2], a versatile precursor for diazenido-and hydrazido-complexesDavid Carrillo*

Laboratorio de Quımica Inorganica, Instituto de Quımica, Universidad Catolica de Valparaıso, Avenida Brasil 2950,Valparaıso, Chile

Received 24 November 1999, accepted 3 April 2000

Communicated by Francois Mathey

Abstract – In this review, we present the most significant aspects of the reactivity of [MoO2(acac)2] toward (i ) phenylhy-drazine and its p-substituted derivatives, R-p-C6H4NHNH2 (R=Me, MeO, Cl, NO2), (ii ) N,N-disubstituted hydrazines,PhRNNH2 (R=Me, Ph), and (iii ) N,N-disubstituted hydrazinium halides, PhRNNH3

+X– (R=Me, Ph; X=Cl, Br, I) in alcoholsand acetonitrile. In particular, we emphasize the versatility of [MoO2(acac)2] as a precursor for a number of mono-, di- andtetranuclear organodinitrogen molybdenum complexes containing diazenido, NNR, hydrazido(2-), NNRR%, and hydrazido(1-),NHNRR%, ligands. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

molybdenum / diazenido / hydrazido(2-) / hydrazido(1-) / alkoxo / dinuclear complexes / tetranuclear complexes

Resume – Version francaise abregee — [MoO2(acac)2], un precurseur versatile pour des complexes organodi-azenido et organohydrazido. Cette revue decrit les aspects les plus significatifs de la reactivite des hydrazines vis-a-vis ducomplexe [MoO2(acac)2]. Le comportement des arylhydrazines et des hydrazines N,N-disubstituees est tres varie, en raison dela diversite des produits de condensation avec la fonction cis-{MoO2} et de la possibilite de deplacer un des ligandsacetylacetonate, voire les deux. Des complexes bis(diazenido), oxo{hydrazido(2-)}, bis{hydrazido(2-)}, hydrazido(1-)-{hydrazido(2-)}, mono-, di- ou tetra-nucleaires ont ainsi ete obtenus, selon la nature de l’hydrazine, la valeur du rapportmolaire hydrazine:Mo et le solvant. Les arylhydrazines R-p-C6H4NHNH2 (R=MeO, Me, H, Cl, NO2) conduisent auxcomplexes [Mo(NNC6H4-p-R)2(acac)2] pour un rapport 2:1 dans le methanol ou l’acetonitrile, mais aux complexes dinucleaires[{Mo(NNC6H4-p-R)2(acac)(m-OR%)}2] (R=H, Me, F, MeO ; R%=Me, Et) pour un rapport 5:1 dans un alcool R%OH. Pour unrapport 50:1, les deux ligands acac sont deplaces et l’on obtient le complexe dinucleaire [{Mo(NNPh)2(NH2NHPh)(OMe)(m-OMe)}2]. Le deplacement des deux ligands acac peut eventuellement etre observe pour un rapport moins eleve : c’est ainsique l’on a obtenu les composes [HNEt3]2[{Mo(NNC6H4-p-R)2(m-OR%)(m-MoO4)}2] (R%=Me, Et), en utilisant cinq equivalents deMeO-p-C6H4NNH3

+Cl–, neutralises par la triethylamine. Les hydrazines N,N-disubstituees neutres PhRNNH2 conduisent auxcomplexes [MoO(NNRPh)(acac)2] dans l’acetonitrile et aux complexes [{MoO(NNRPh)(acac)((m-OR%)}2] dans R%OH (R=Me,Ph ; R%=Me, Et, n-Pr). Les complexes [Mo(NNRPh)2(acac)2] ne sont que tres difficilement obtenus, dans le toluene sec et enpresence de Na2SO4. Au contraire, les halogenures d’hydrazinium PhRNNH3

+X– (R=Me, Ph ; X=Cl, Br, I) conduisent tresfacilement aux complexes mixtes [Mo(NHNRPh)(NNRPh)(acac)X2]. Ces complexes sont des precurseurs de nouveauxcomplexes bis(hydrazido) : ils reagissent avec les phosphines PMexPh3–x, les di-imines et les ligands trispyrazolylborate endonnant respectivement les complexes [Mo(NNRPh)2Cl2(PMe3Ph3–x)n ] (R=Me, n=2, x=0, 1, 2 ; R=Ph, n=1, x=0 ;R=Ph, n=2, x=1, 2) [Mo(NNRPh)2Cl2(N�N)] (N�N=bpy, o-phen) et [Mo(NNRPh)2Cl(Tp%)] (Tp%=HBpz3, HB(Me2pz)3). Lescomplexes [Mo(NNMePh)2Cl2(PPh3)2] et [Mo(NNPh2)2Cl2(PPh3)], traites par le triflate d’argent, AgOTf, conduisent auxintermediaires [Mo(NNRPh)2Cl(PPh3)3]

+OTf–, qui reagissent avec le cyclopentadienyle sodium en exces dans le THF, endonnant les complexes [(h5-Cp)Mo(NNRPh)2(PPh3)]

+OTf– (R=Me, Ph). L’influence des facteurs qui conditionnent lastructure, le mode de liaison et la stabilite des complexes hydrazido a ete etudiee par la methode de Huckel etendue. Cetteetude a montre que la liaison NN conserve un fort caractere multiple et que la charge sur le ligand hydrazido n’est guere

* Correspondence and reprints: dcarrill@ucv.cl

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D. Carrillo / C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 175–181

superieure a une unite, bien que les complexes soient generalement consideres comme des complexes hydrazido(2-). Dansles complexes bis(hydrazido), les deux ligands occupent invariablement des positions adjacentes et, lorsqu’ils sont consideresensemble, apportent dix electrons. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

molybdene / diazenido / hydrazido(2-) / hydrazido(1-) / alkoxo / complexes dinucleaires / complexes tetranucle-aires

1. Introduction

In 1985 Jeannin and co-workers [1] reported thesynthesis and characterization of the syn and antiforms (figure 1) of a new binuclear bis(phenyldi-azenido) complex, formulated as[{Mo(NNPh)2(acac)(m-OMe)}2], which was obtainedby reaction of [MoO2(acac)2] with phenylhydrazinein methanol. X-ray crystal structure analyses revealedthe presence of two {Mo(NNPh)2(acac)}+ moietieslinked by two methoxo ligands. The only differencebetween these stereoisomers lies in the orientationof the phenyl group trans to one oxygen atom of theacac ligand. They interconvert by syn-anti-isomer-ization at the outer (b) nitrogen atom of one di-azenido ligand. These unexpected results were thestarting point for the development of fruitful chem-istry based on the reactivity of hydrazines toward[MoO2(acac)2]. This led to the synthesis of a numberof organodinitrogen molybdenum complexes con-taining diazenido, NNR, hydrazido(2-), NNRR%, andhydrazido(1-), NHNRR%, ligands, which are of currentinterest as potential models of intermediates in theconversion of coordinated dinitrogen into ammonia.This chemistry of metal multiply bounded nitrogenligand has been the subject of an intense researchwork, starting from various precursors, and severalreviews on the topic have appeared [2]. Our purposein this article is to focus on the more relevant results

obtained from extensive studies of the reactivity of[MoO2(acac)2] toward organohydrazines. For com-parative purposes, the reactions of [MoO2(acac)2]with monosubstituted and disymmetrically disubsti-tuted hydrazines will be described separately.

2. Results and discussion

2.1. Reactivity of [MoO2(acac)2] towardphenylhydrazine and its p-substituted derivatives:diazenido-complexes

The following presentation is based on the resultsobtained by our group from 1985 to 1993 but departsfrom the chronological order. Mononuclear, dinu-clear and tetranuclear bis(diazenido) complexeshave been obtained depending on the nature andthe excess of hydrazine and on the solvent (scheme1).

For example, we have found that phenylhydrazineand its p-substituted derivatives react with[MoO2(acac)2] in a 2:1 ratio, in acetonitrile ormethanol, to give mononuclear complexes formu-lated as [Mo(NNC6H4-p-R)2(acac)2] (R=H, Me, MeO,Cl, NO2) [3]. The molecular structures of complexeswere determined by X-ray diffraction analyses forR=Me and R=MeO, which showed that both acac

Figure 1. ORTEP views of syn (a) and anti (b) isomers of complex [{Mo(NNPh)2(acac)(m-OMe)}2].

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Scheme 1.

ligands are retained in the coordination sphere ofmolybdenum [3] (scheme 1a).

On the other hand, as already stated above,phenylhydrazine reacts with [MoO2(acac)2] in a 5:1molar ratio in methanol to give the dinuclear com-plex [{Mo(NNPh)2(acac)(m-OMe)}2]. This reaction in-volves not only a condensation-type reaction of thehydrazine with the cis-{MoO2} function, but also thedisplacement of one acac ligand [1] (scheme 1b).This early study was later extended to p-substitutedphenylhydrazines in methanol or ethanol. A wholefamily of binuclear bis(aryldiazenido) complexes ofthe type [{Mo(NNC6H4-p-R)2(acac)(m-OR%)}2] (R=H,Me, MeO, F; R%=Me, Et) has been obtained in thisway [4].

In other respects, phenylhydrazine in quite largeexcess (50:1 ratio) reacts with [MoO2(acac)2] inmethanol or propanol to give the dinuclear com-plexes [{Mo(NNPh)2(OR%)(PhNHNH2)(m-OR%)}2] (R%=Me, n-Pr) [5]. In this case, both acac ligands havebeen displaced from the coordination sphere of themolybdenum atoms which achieve six-coordinationby two unidentate ligands, one end-on phenylhy-drazine and one terminal alkoxo ligand (scheme 1c).Contrary to the complex [{Mo(NNPh)2(acac)(m-OR%)}2], where the geometry around molybdenum isforced by the acac ring, two different geometrieswould be conceivable for[{Mo(NNPh)2(OR%)(PhNHNH2)(m-OR%)}2] according towhether the unidentate ligands are mutually cis ortrans. However, only the latter has been found [5].

Finally, the reaction of MeO-p-C6H4NHNH3+Cl–

with [MoO2(acac)2] in an alcohol in the presence ofNet3 (hydrazine:NEt3:Mo=5:5:1) affords a crudesolid that was found to consist mainly of the tetranu-clear compound [HNEt3]2[{Mo(NNC6H4-p-OMe)2(m-OR%)(m-MoO4)}2], with a small amount of thedinuclear complex [{Mo(NNC6H4-p-OMe)2(acac)(m-OR%)}2] (R%=Me, Et) [4]. The structure of the tetranu-clear species was determined by X-ray diffractionanalysis (figure 2). It consists of two {Mo(NNC6H4-p-OMe)2}

2+ moieties linked by two methoxo and twobidentate [MoO4]

2– ligands, and is quite similar to thecomplexes [{Mo(NNC6H4-p-R)2(m-OR%)(m-MoO4)}2]

2–

described by Zubieta and co-workers (R=H, R%=Me, Et, n-Pr [6]; R=NO2, R%=Me [7]).

Figure 2. ORTEP view of the tetranuclear complex [{Mo(NNC6H4-p-OMe)2(m-OMe)(m-MoO4)}2]

2–.

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Scheme 2.

These results show that (i ) the acac ligands canbe retained in the coordination sphere of molybde-num [3], or can be partly [1, 4] or fully [4, 5] dis-placed, depending on the hydrazine:Mo ratio, (ii )the nuclearity is dependent on the solvent, and (iii )the products invariably display the cis-{Mo(NNR)2}

2+

moiety, whatever the experimental conditions. Thebis{hydrazido(2-)} complexes [Mo(NNHAr)2(acac)2)]that would be expected from the reaction of[MoO2(acac)2] with ArNHNH2 have not been iso-lated. It is assumed that the initial condensation-typereaction is followed by an internal redox step whichremoves the remaining hydrogen atoms [8, 9].

2.2. Reactivity of [MoO2(acac)2] toward neutralN,N-disubstituted hydrazines:hydrazido(2-)-complexes

The study of the reactions of [MoO2(acac)2] withneutral N,N-disubstituted hydrazines, PhRNNH2 (R=Me, Ph) was initiated by our group in the early 1990s(scheme 2). The results proved to be less varied thanwith monosubstituted hydrazines. The reactions,when carried out in a 2:1 ratio of the reactants, giverise to mononuclear complexes, [MoO(N-NPhR)(acac)2], or dinuclear complexes, [{MoO-(NNPhR)(acac)(m-OR%)}2] (R=Me, Ph; R%=Me, Et,n-Pr), depending on whether the solvent is acetoni-trile or an alcohol. In refluxing R%OH, the complexes[MoO(NNPhR)(acac)2] transform into [{MoO(NNPhR)-(acac)(m-OR%)}2]. Thus, they are likely intermediatesin the formation of [{MoO(NNPhR)(acac)(m-OR%)}2]via the reaction of PhRNNH2 with [MoO2(acac)2] inR%OH. The molecular and crystal structures of themononuclear complexes have been determined byX-ray diffraction analyses for R=Me [10] (figure 3a),

while those of the dinuclear complexes have beendetermined for R=Me, R%=n-Pr [10] and for R=Ph,R%=Et [11] (figure 3b). All these complexes displaycis-{MoO(NNRPh)}2+ units, where the hydrazido lig-ands are nearly planar.

Figure 3. ORTEP views of the mononuclear complex [MoO(N-NMePh)(acac)2] (a) and the binuclear complex[{MoO(NNMePh)(acac)(m-OPrn)}2] (b).

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Figure 4. ORTEP view of the complex [Mo(NNMePh)2(acac)2].

provide the first examples of bis{hydrazido(2-)} com-plexes showing a reversible electrochemicalbehaviour.

The formation of the cis-{MoO(NNRR%)}2+ andcis-{Mo(NNRR%)2}

2+ units is assumed to occur viacondensation of the cis-{MoO2} function with thehydrazine like the first step in the formation of thebis(diazenido) complexes [13].

2.3. Reactivity of [MoO2(acac)2] towardprotonated N,N-disubstituted hydrazines:hydrazido(1-)-complexes

One of the most noteworthy results in our currentwork is the synthesis of mononuclear complexes[Mo(NHNRPh)(NNRPh)(acac)X2] containing bothorganohydrazido(1-) and organohydrazido(2-) lig-ands. Surprisingly, these complexes are readilyformed by reacting [MoO2(acac)2] with N,N-disubsti-tuted hydrazinium halides, PhRNNH3

+X– (R=Me,Ph; X=Cl, Br, I) in acetonitrile. These complexeshave been characterized by X-ray diffraction analysisfor R=Me, X=Cl and for R=Ph, X=Cl [14]. Al-though both hydrazido ligands occupy mutually cis-positions exhibiting end-on coordination, they areclearly different: location of the hydrogen atom atthe inner (a) nitrogen atom clearly shows up thehydrazido(1-) ligand (figure 5). The Mo�N�N linkagein the hydrazido(2-)-molybdenum moiety, MoN-NRPh, displays almost linear geometry (173.8(2) and171.9(4)° for R=Me and R=Ph, respectively), whilethe Mo�N�N linkage in the hydrazido(1-)-molybde-num moiety, MoNHNRPh, exhibits the bent ‘end-on’geometry (142.9(2) and 140.5(4) for R=Me andR=Ph, respectively) instead of the more common‘side-on’ bonding mode. These complexes could re-sult from the protonation of the intermediatebis{hydrazido(2-)} complexes (see 2.2). However, thedisplacement of one acac ligand under protonationwould leave two cis vacant coordination sites, whilethe two X ligands occupy trans positions in[Mo(NHNRPh)(NNRPh)(acac)X2].

The most salient feature of the complexes[Mo(NHNRPh)(NNRPh)(acac)X2] is their reactivity to-ward phosphines to give bis{hydrazido(2-)} com-plexes formulated as [Mo(NNRPh)2Cl2(PMexPh3–x)n ](R=Me, n=2, x=0, 1 or 2; R=Ph, n=1, x=0;R=Ph, n=2, x=1 or 2) [15] (scheme 3a). Like-wise, the reactions with diimines and trispyra-zolylborate ligands afford complexes formulated as[Mo(NNPhR)2Cl2(N�N)] (N�N=bpy or o-phen) [16](scheme 3b) and [Mo(NNPhR)2Cl(Tp%)] (Tp%=Tp orTp*) [17], respectively (scheme 3c) . The formation ofthese compounds could occur in two steps: (i )

Figure 5. ORTEP view of the complex [Mo(NHN-MePh)(NNMePh)(acac)Cl2].

Attempts to obtain bis{hydrazido(2-)} complexesby forcing the reaction of [MoO2(acac)2] withPhRNNH2 (hydrazine in large excess, high-boilingsolvent etc.) remained unsuccessful for a long time.Finally we reasoned that the condensation of theremaining oxo group with hydrazine could be hin-dered by the water generated in the first step of thereaction, and we succeeded in obtaining the targetedcomplexes [Mo(NNRPh)2(acac)2] by reactingPhRNNH2 (R=Me, Ph) on [MoO2(acac)2] in refluxingdry toluene in the presence of Na2SO4 [12]. Theircrystal and molecular structures were determined byX-ray diffraction analysis (figure 4). Cyclic voltam-mograms of both species at a platinum electrode inacetonitrile display an oxidation wave, which hasbeen recognized as a diffusion-controlled reversibleone-electron process possibly involving the hy-drazido(2-) ligand. Heretofore, these complexes

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Scheme 3.

substitution of one oxygen atom of the acac ligandby the incoming ligand and (ii ) concerted elimina-tion of Hacac with the addition of a second incomingligand. From the synthetic point of view, the mostattractive complexes obtained from mixed hy-drazido(1-) hydrazido(2-) complexes are the phos-phine derivatives. In particular, the treatment of[Mo(NNPhMe)2Cl2(PPh3)2] or [Mo(NnPh2)2Cl2(PPh3)]with silver triflate, AgOTf, in CH2Cl2/CH3CN, inthe presence of PPh3 for the pentacoordinatedcomplex, yields ionic intermediates formulated as[Mo(NNRPh)2Cl(PPh3)2]

+Otf– (R=Me [18, 19], Ph[19]) which give ionic products formulated as [(h5-

Cp)Mo(NNRPh)2(PPh3)]+Otf– [19] under reaction

with excess NaCp in THF. These complexes are thefirst two members of a new class of organometalliccompounds. Particularly, the compound [(h5-Cp)Mo(NNPh2)2(PPh3)]

+OTf– has been authenti-cated by X-ray diffraction analysis [19] (figure 6).Using Na(acac) instead of NaCp affords the ioniccomplex [Mo(NNPh2)2(acac)(PPh3)2]

+OTf– [20].

3. The M-NNR and M-NNRR% bonding

During more than two decades, the nature of thechemical bond in transition metal diazenido andhydrazido complexes has been described using sim-ple Lewis structures. Considering the growing inter-est in this chemistry and the particularly scarcetheoretical data on this type of complexes [21, 22],we have studied the electronic factors governing thestructure, bonding and stability of the NNR2 com-plexes with the help of ab initio and Extended-Huckel Molecular Orbital (EHMO) calculations onthe free (neutral and anionic) NNH2 ligand, as wellas on different model complexes [23]. Our principalresults indicate that: (i ) the hydrazido ligand hasthree Frontier Molecular Orbitals (FMO) available forbonding to a metal atom: two of them, ps and sn,are associated with lone pairs on Na, the third beingthe p*NN orbital, (ii ) the pNN orbital is never signifi-cantly involved in the interaction with the metal,

Figure 6. ORTEP view of the complex [(h5-Cp)Mo(NNPh2)2-(PPh3)]

+.

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leaving almost untouched its bonding electron pair;this is one of the reasons why the NN bond retainssome multiple-bond character associated with analmost invariable sp2 hybridization of Nb, (iii ) al-though the interaction with the metal leads to apartial occupation of the p*NN FMO, which tends todecrease the N�N bond order, calculations indicatethat the ligand is reluctant to accept a full occupationof this FMO; moreover, this effect is partially bal-anced by depopulation of the somewhat N�N anti-bonding ps FMO and the overall N�N bond-orderweakening is not strong enough to force Nb topyramidalize, (iv) although usually considered ashydrazido(2-), the NNRR% oxidation state in most ofthe studied compounds is close to or slightly largerthan 1-, corresponding to the following FMO formaloccupation: (s)2(pNN)2(ps)2(p*NN)1 and (v) if inmono(hydrazido) complexes the formally hy-drazido(2-) ligand is a 6-electron donor, in the cis-bis(hydrazido) the symmetry allows the two formallyhydrazido(2-) ligands, taken as a whole, act as a10-electron only ligand system.

4. Conclusion

The complex [MoO2(acac)2] has proved to be aversatile precursor for a variety of organodinitrogenmolybdenum complexes. Condensation of the cis-{MoO2} function with RNHNH2, RR%NNH2 orRR%NNH3

+X–, followed by an internal redox reactionin the case of RNHNH2, affords complexes contain-ing cis-[Mo(NNR)2]

2+, cis-[MoO(NNRR%)]2+, cis-[Mo(NNRR%)2]

2+ or cis-[Mo(NHNRR%)(NNRR%)]3+

units. Although the condensation mechanism israther speculative, it has undoubtedly directed a lotof research work toward the synthesis of organodini-trogen complexes from high-oxidation state Mo, W,Tc and Re oxo precursors. Further work is clearlyneeded to establish the intimate mechanism ofthese reactions and to clarify the respective influ-ences of the hydrazine, the ancillary ligands and thesolvent.

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