Osmium(
II) complexes of 2-[(arylamido)phenylazo]pyridines.
New examples of deamination reactions—X-ray structure
and redox propertiesy
Chayan Das,aaShie-Ming Peng,bbGene-Hsiang Leebband Sreebrata Goswami*aa
a
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India. E-mail: [email protected]
b
Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China Receivved (in Montpellier, France) 13th September 2001, Accepted 16th October 2001 First published as an Advvance Article on the web
Reaction of [NH4]2[OsBr6] with 2-[(arylamino)phenylazo]pyridine, NH4C5N=NC6H4N(H)C6H4(R) [R¼ H (HLa) or CH3(HLb)], in the presence of dilute NEt3affords multiple products. Five compounds of molecular formulas [Os(HL)(L)Br], 1, [Os(L)(pap)Br], 2, two isomers of [Os(pap)2Br2], 3 and 5, and[Os(HL)(pap)Br2], 4, where L andpap standfor the conjugate base of HL and2-(phenylazo)pyridine, respectively, have been separatedon a preparative TLC plate. The X-ray structures of the new andrepresentative complexes 1a, 2a and 4a have been solvedto characterise them. The complexes 3 and 5 were characterisedby comparing their spectral properties with those of the known andanalysedsamples of isomeric [Os(pap)2Br2]. Except for complex 1, the rest are formeddue to cleavage of an otherwise unreactive C(phenyl)–N(amine) bondwhich is promotedby the metal ion. The bidentate-tridentate combination of HL and L in 1 is due to electronic factors. Structural data of the compounds have revealedvery strong metal-ligandinteractions. Osmium(II)-ligandinteractions with the
neutral azo ligands, viz. HL or pap, are stronger than those with the anionic L ligand. All of the complexes display resolved1H NMR spectra. However, the spectral pattern is complex due to serious overlap of the resonances. There are multiple electronic transitions in the range 1200–220 nm. The lowest energy transition (HOMO! LUMO) is presumably due to metal-to-ligand charge transfer (MLCT). These complexes undergo multiple and successive one-electron redox processes. The lowest potential anodic response, in each case, has been assignedto the OsII=OsIIIcouple. E
1=2of this response in 1 and 2 is similar andoccurs near 0.45 V. Low oxidation potentials of the above couples allowed the generation of [1a]þand[2a]þin solution by exhaustive constant potential coulometry. The trivalent osmium complexes showedrhombic EPR spectra at 77 K. Distortion parameters using the observed g values have been computed.
The coordination chemistry of the anionic tridentate N3 ligand, 2-[(arylamido)phenylazo]pyridine, L of 3d metal ions has been rich andversatile.1,2 Its conjugate acid1–5 HL is obtainedby a cobalt promotedamination reaction1,2,4,5 at coordinated 2-(phenylazo)pyridine [pap]. The compound HL readily loses an Hþ (pKa 8.5) (Scheme 1) andthe anion L coordinates to metal in a bischelating tridentate fashion. The (phenylazo)pyridine part of L is soft6 andis capable of stabilising low oxidation states of metal ions, while the strong electron donor amido part prefers high valent metal ions. The tunable donor-acceptor properties of the chromophores in L help in stabilisation of the metal ions in variable valence states. Thus, the iron compound[Fe(L)2]nþis known1to exist both as [FeIII(L)
2]þ(n¼ 1) and[FeII(L)2] (n¼ 0). Another interesting feature of this ligandis that it stabilises the uncommon low-spin states of metal ions. Amongst these, the low-low-spin complex [Mn(L)2] deserves special mention.2Extensive delocalisation in the present anionic ligandalong its backbone is responsible for many of the uncommon properties of the complexes of L.
Our current interest7in the chemistry of heavier metal ions involving redox non-innocent ligands persuaded us to explore the osmium chemistry of HL. Osmium being kinetically inert,
different mixed ligand species were anticipated. Unlike the 3d metal ion chemistry of L, described above, the osmium chemistry with HL is not straight forward. The metal ion here promotes cleavage of an otherwise unreactive C(phenyl)– N(amine) single bond,8leading to deamination of the ligand.
Scheme 1 y Electronic supplementary information (ESI) available: partial energy
level diagrams and molecular orbitals of 1a, 2a and 4a, UV-vis spectra of 4 complexes andcyclic voltammograms of 1b, 2b and 4b. See http:==www.rsc.org=suppdata=nj=b1=b108507g=
Herein we report the isolation andcharacterisation of the multiple products that are obtained from a single reaction of [NH4]2[OsBr6] with HL. Single crystal X-ray structure analyses of the complexes are usedfor their authentication.
Results and discussion
Synthesis and structural characterisation
The 1 : 2 reaction of [NH4]2[OsBr6] with HL in boiling methanol in the presence of dilute NEt3 produced a brown solution in about 5 h. The crude mass, so formed, was a mixture of several products. Five pure compounds were successfully separated from the crude mass on a preparative TLC plate (Scheme 2). Except the first brown compound, 1, the rest (2–5) were formeddue to deamination of the ligand HL. Notably, this reaction is solvent dependent. For example, it does not proceed at all in other solvents like ethanol, t-butanol, 2-methoxyethanol, etc.
Brown (I), 1. A brown bandwas first elutedwith benzene. The compound, 1, was obtainedby evaporation of the benzene eluate in ca. 20% yield. The compound, 1a (HL¼ HLa), formedsuitable X-ray quality crystals for structure determi-nation.
Fig. 1 shows the ORTEP andatom numbering scheme for 1a. In this complex, the central osmium(II) is surrounded by a
distorted octahedral N5Br coordination environment. One of
the two extended ligands, La, is monoanionic, tridentate and chelates to osmium through N(5), N(7) andN(8) with depro-tonation of the amine nitrogen. The secondligand, HLa, on the other hand, is neutral and chelates to the metal ion only through the pyridine nitrogen N(1) and the azo nitrogen N(3). The amine nitrogen atom N(4) of this ligandbears a hydrogen andremains pendant. The sixth position is occupiedby a Br ion forming a hexacoordinated compound of formula [Os(HLa )-(La)Br]. The presence of a hydrogen at N(4) is confirmed based on its1H NMR spectrum (vide infra). It may be recalledhere that in all its known complexes with 3dmetal ions the ligand HL loses a proton spontaneously andbinds as an anionic tri-dentate donor to yield metal complexes1,2of general formula [ML2]nþ. Notably, the relative geometry of the coordinating atoms in 1 are different than those in [ML2]nþcomplexes. The orientations within the pairs of pyridyl-N, azo-N and amido-N atoms in [ML2]nþcomplexes are cis, trans and cis, respectively. In contrast, the geometry with respect to the pairs of pyridyl-N andazo-N in the osmium complex 1 is cis, cis (Scheme 3). In this orientation, the azo-N of the protonatedHL is trans to the coordinated Br. It implies that the amine N(4) is far from the sixth coordination site and hence remains protonated and uncoordinated. The relatively preferred cis, cis orientation in osmium(II) is due to electronic factors. Unlike the 3d metal
ions, the Os(II) (5d6) ion is known to enter into p-interactions
very effectively with the ligand p* orbitals. Thus, very strong dp-azo(p*) back-bonding is a persistent feature of M-pap chelates. In the trans-N(azo), N(azo) pair orientation, the p*(azo) orbitals compete for the same metal dp orbital, which is expectedto weaken back-bonding. In fact, such a trans orien-tation has never been observed9,10in [M(pap)2Cl2] (M¼ Ru, Os). Therefore, the tridentate and bidentate coordination modes of L and HL in [Os(HL)(L)Br] are understandable.
The bond distances in the osmium compound under con-sideration indicate considerable metal-ligand interactions. The N–N length [N(2)–N(3), 1.336(3) A] is appreciably elongated+ comparedto that observed11 in [papH]ClO
4. This is due to considerable Os–HL p-bonding with major involvement of the azo functionality. The shortest Os–N length (1.946 A) in this+ complex is the bondbetween Os andN(3) of neutral HLa, which further confirms the existence of strong Os–azo p bonding. The main features of bonding of the anionic tri-dentate [La] are similar to those observed1,2 in other M–L complexes. Extensive electron delocalisation along the ligand backbone has been noted. For the same reason the Os–N(7) bondis short andcomparable to the Os–N(3) bondlength, but is appreciably shorter than the three other Os–N bonds in the same molecule.
Brown (II), 2. The secondbrown band, which overlapped with the first brown band, was also eluted with benzene. Evaporation of benzene solution yielded the brown (II)
com-Fig. 1 Molecular structure andatom numbering scheme for [Os-(HLa)(La)Br], 1a. Selectedbondlengths (A): Os–N(1) 2.074(2), Os–+ N(3) 1.946(2), Os–N(5) 2.088(2), Os–N(7) 1.953(2), Os–N(8) 2.031(2), Os–Br 2.5301(3), N(2)–N(3) 1.336(3), N(6)–N(7) 1.319(3), N(7)–C(23) 1.386(3), N(8)–C(28) 1.369(4), N(8)–C(29) 1.440(4).
Scheme 2
poundin ca. 20% yield. The colour and the physical properties of this compoundare similar to those of compound1. The compound 2a (HL¼ HLa
) also formedsuitable X-ray quality crystals for structure determination. Fig. 2 shows the ORTEP andatom numbering scheme for 2a. Analysis of the crystal data indeed authenticated the formation of a mixed ligand compound, [Os(L)(pap)Br], from the reaction as shown in Scheme 2. One of the two HL ligands here has undergone a deamination reaction with cleavage of a C–N bond to form a bidentate neutral 2-(phenylazo)pyridine (pap) ligand. Com-pounds 1 and 2 are similar with the exception that in 1 an ortho-substituted ligand, HL, replaces the pap ligand. The important bonddistances in 2a and 1a are collectedin the respective figure captions, which indeed showed structural similarities between the two compounds.
Violet, 3. Working up of the eluate followedby crystal-lisation of the thirdviolet bandyieldedthe known12trans, cis-Os(pap)2Br2. Thus, compound 3 has formeddue to complete deamination of HL.
Pink, 4. A pink bandmovedafter the above violet band, the yieldof which was ca. 10%. By repeatedtrials we were able to grow suitable X-ray quality crystals of 4a. The ORTEP and atom numbering scheme of the compoundare shown in Fig. 3. In this compoundthere are two types of chelating ligands. One of the extended ligands is HLa, which acts as a bidentate chelate as in 1 (vide supra) andthe secondligandis pap, which is generated due to deamination of HLa. There are two cis bromides to complete the hexacoordination of the central osmium(II) in [Os(HLa)(pap)Br2]. Very strong Os(dp)-azo(pp) interactions are the general bonding features in this com-pound. As a result, the N–N lengths are elongatedandOs– N(azo) distances are shorter than Os–N(py) lengths. Selected bondlengths in this compoundare collectedin the Fig. 3 caption. The relative orientations within the coordinating pairs, N(py), N(py) andN(azo), N(azo), are cis and cis.
Red-violet, 5. This last red-violet band showed12 identical spectral properties to those of known cis,cis-Os(pap)2Br2. Thus, compounds 3 and 5 are the two isomers of Os(pap)2Br2 and these are formed due to complete deamination of the ligandHL usedin this work.
Rationale for the deamination reaction. Besides these above five bands there were a few minor overlapping bands, which were observedon the TLC plate. We have not been successful in isolating these in the pure state. Out of the five compounds, four (2–5) were obtaineddue to cleavage of at least one C(phenyl)–N(amine) bond. The reaction occurs only in boiling methanol andthe mechanism8of this bondcleavage couldnot be establishedso far. However, the overall reaction may be viewed as hydrogenation of HL across a C–N bond, leading to the formation of pap andAr–NH2. We note here that the ligandHL reacts spontaneously with several 3dmetal ions, where no such C–N bondcleavage was observed. Moreover, the composition of the metal complex (ML2nþ) involving the 3delements is different than that observedfor the osmium complex. In the osmium complex 1, one of the two coordinated ligands binds as a neutral bidentate donor with a pendant aryl amine group, which may be responsible for the C–N bond cleavage reaction. It is now known that methanol in the pre-sence of suitable metal complex catalysts can be a useful source of hydrogen for reduction of organic substrates.13Oxidative addition of methanol led to the formation of metal complex hydrido intermediates that were shown to be the active species for the hydrogenation reactions. Unfortunately, we have yet to identify any hydrido intermediate from our reaction. Furthermore, we note here that the compounds 1 and 4, once formed, cannot be converted to 2 and 3 (5), respectively, in boiling methanol. Hence it is reasonable that the present deamination reactions occur via some transient osmium hydrido intermediates, which were not isolable. Thus, given the obvious limitations in our above proposal, we prefer not to speculate on this point but just to note that it is a possibility. Spectral properties
There are two notable features in the IR spectra of the osmium complexes: (i) the complexes containing bidentate HL sys-tematically showed14 moderately intense nN–H in the range 3410–3285 cm1and(ii) the nN=Nstretching frequencies in the complexes appearedbetween 1290 and1220 cm1, which are appreciably lower6than those in the free ligands. These further confirm strong Os(dp)-azo(pp) interactions in the present Fig. 2 Molecular structure andatom numbering scheme for
[Os(La)(pap)Br], 2a. Selectedbondlengths (A): Os–N(1) 2.079(6),+ Os–N(3) 1.941(5), Os–N(4) 1.998(6), Os–N(5) 2.084(5), Os–N(7) 1.938(5), Os–Br 2.5321(8), N(2)–N(3) 1.313(7), N(3)–C(6) 1.369(7), N(4)–C(11) 1.395(8), N(4)–C(12) 1.426(9), N(6)–N(7) 1.345(6).
Fig. 3 Molecular structure andatom numbering scheme for [Os (HLa)(pap)Br
2], 3a. Selectedbondlegnths ( +
A): Os–N(1) 2.086(3), Os–N(3) 1.962(3), Os–N(5) 2.046(3), Os–N(7) 1.990(3), Os–Br(1) 2.5505(4), Os–Br(2) 2.5454(4), N(2)–N(3) 1.311(4), N(6)–N(7) 1.307(5).
complexes. The IR spectra containedall other expected vibrations1,2due to the coordinated ligands.
The 1H NMR spectra of the complexes were recorded in CDCl3. Except for the violet trans, cis-Os(pap)2Br2 (3) pound, the rest are unsymmetrical and their spectra are com-plex in nature with a very large number of overlapping proton resonances. However, most of the pyridyl proton resonances appearedin the low fieldregion (d 9.5–7.3). Notably, the NH resonance in the complexes of HL shiftedupfieldappreciably. For example, the d(N–H) of the brown [Os(HL)(L)Br], 1, appearednear d 6.00. In comparison, in the free HL this appearedat d 10.58. The N–H proton in the above complex is shielded by the p-cloudof the phenyl ring of the co-ligand. The two relatedcomplexes of [HLb], [Os(HLb)(Lb)Br], 1b, and [Os(Lb)(pap)Br], 2b, showedtwo andone methyl resonances, respectively, near d 2.25 (Fig. 4).
In order to gain some insight into the nature of redox and spectroscopically relevant orbitals, in the new osmium com-plexes, standard extended Hu¨ckel MO calculations using the crystallographic parameters were performedfor 1a, 2a and 4a using the semi-empirical CACAO programme by Mealli and Proserpio.15The MOs of the complexes have some common features: (i) the HOMO is primarily a metal orbital where the ligandcontribution is <10%, (ii) the two closely spaced molecular orbitals LUMO andLUMOþ 1 are strongly delo-calised. These have ca. 35–40% metal d-character with sig-nificant contributions from the ligand p-acceptor (–N=N–) orbitals, and(iii) the filled HOMO-1 is also delocalised, where the metal contribution is a little over 25%. Partial energy level diagrams of the representative complexes 1a, 2a and 4a, along with a pictorial presentation of the representative MOs, are submittedas ESI (Fig. S1–S6).
The complexes 1, 2 and 4 displayed multiple bands and shoulders in the region 250–1400 nm in dichloromethane solution (Table 1). Notably, spectral patterns of the complexes 1 and 2 are similar but are complex in nature with a large number of transitions.16The spectrum of 4, on the other hand, is relatively simpler. For example, each of 1 and 2 displays four transitions in the visible range while the complex 4 shows only one intense transition near 515 nm anda broadbandnear 820 nm, which is similar12to what is observedfor cc-Os(pap)2Br2. We note here that the two coordinated organic ligands in the former two complexes 1 and 2 are totally different. In contrast, the two ligands in 3, though not identical are similar. The ligandHL may be viewedas a substitutedpap ligandwith a– N(H)Ar substitutedon the ortho carbon of the phenyl ring of pap. This ortho-substitution appears to have very little
elec-tronic effects andhence the physicochemical properties of the complexes of HL andthose of metal-pap complexes are simi-lar. The lowest energy transitions in the present complexes are broad(e > 1000 M1cm1) and are due to nonbonding HOMO! LUMO transitions. Hence these transitions are formally metal-to-ligandcharge transfer (MLCT) transi-tions.1,7dThe next higher transitions are more intense andare assignedto transitions between heavily mixedmetal-ligand orbitals, LUMO, LUMOþ1, HOMO-1 andHOMO-2 orbi-tals. The UV-range transitions in these complexes are very intense andare assignedto intraligandp-p* transitions. Representative spectra of the complexes are submittedas ESI (Fig. S7).
Redox properties and EPR spectra
Redox properties of the osmium complexes were studied by cyclic voltammetry (CV) with a platinum working electrode. Voltammetric data are collected in Table 2 and representative voltammograms are deposited as ESI (Fig. S8). In all cases dichloromethane was used as solvent. The nature of the vol-tammograms of the complexes does not change in acetonitrile. However, due to the relatively low solubility of the compounds in acetonitrile, dichloromethane was commonly used for the voltammetric studies. All the potentials are referenced to the SCE (SCE¼ saturatedcalomel electrode).
The brown complexes 1 and 2 showedalmost identical voltammograms. The complexes exhibitedmultiple redox responses in the range þ1.8 to 1.5 V. There were two electrochemically reversible anodic responses near 0.40 and 0.90 V. The first oxidation in both 1 and 2 occurs from a primarily metal orbital, the HOMO. The secondredox active orbital in these complexes is a ligandorbital, HOMO-1. Thus, the secondoxidative response may be assignedas ligandoxi-dation of the amido function. It may be noted here that free HLa displayed1 two irreversible anodic responses and a reversible cathodic response at 1.30, 1.00 and 1.15 V, respectively. It is expectedthat the anodic responses would shift cathodic in the anion La. The multiple irreversible cathodic responses in the present complexes are assigned to ligandreductions.
Low potentials for the OsII=OsIIIcouple in the complexes 1
and 2 persuaded us to try and isolate the corresponding osmium(III) complexes [1]þ and[2]þ in their pure states. Accordingly, we tried to oxidise them both chemically and Fig. 4 1H NMR spectra of (a) [Os(HLb](Lb)Br] and(b) [Os(Lb
)(pap)-Br] in CDCl3. Inset: Resonance due to methyl protons.
Table 1 Electronic spectral data in CH2Cl2
Compound lmax=nm (e=M1cm1)
Os(HLa)(La)Br (1a) 930(1970)a, 730(5870), 620(6290), 520(9760)a, 420(11 620), 290(49 680), 240(42 090) Os(HLb)(Lb)Br (1b) 890(2300)a, 730(4390), 620(4630), 490(7310)a,
420(8600), 290(34200), 230(32000) Os(La)(pap)Br (2a) 910(1960)a, 730(4650)a, 620(5030)a,
520(8240)a, 420(10 120), 360(18 390)a, 310(26 540), 250(68 090), 230(31 214) Os(Lb)(pap)Br (2b) 920(1770)a, 730(4700)a, 620(5190)a, 520(7870)a, 420(9870), 310(24 820), 230(28 090) tc-Os(pap)2Br2(3) 980(900), 580(7980) a , 525(11 990), 320(27 010)
Os(HLa)(pap)Br2(4a) 820(1380), 515(11 970), 370(11 960)a, 290(30 370), 230(31 980) Os(HLb)(pap)Br2(4b) 810(1090), 510(9330), 370(10 350)a, 290(29 640), 230(26 930) cc-Os(pap)2Br2(5) 850(1200), 518(11 790), 320(17 860), 230(26 160) a Shoulder.
electrochemically. Unfortunately, the oxidised complexes were not stable enough for their isolation, as they revert to the parent bivalent complexes rapidly. However, we were able to generate two representatives of the cationic complexes, viz. [1a]þ and[2a]þ, in solution by the controlledpotential bulk electrolysis of 1a and 2a.
In frozen (77 K) dichloromethane–toluene solutions the above low-spin osmium(III) ( t25) complexes displayed well-resolved rhombic spectra. The EPR data are collected in Table 3 and the spectra are shown in Fig. 5. The observedrhombicity of the EPR spectra in the present case17is understandable in terms of the gross molecular symmetry of these complexes, containing three non-equivalent axes. The rhombic distortion can be thought of as a combination of axial distortion (D, which splits t2into a ande) andrhombic distortion (V, which splits e). The splitting are illustratedas insets in Fig. 5. Spin-orbit coupling causes further changes in the energy gaps. Thus, two electronic transitions, of transition energies DE1and DE2(DE1<DE2) are, in principle, probable within these three levels. All these energy parameters for the above low-spin osmium(III) complexes have been computed18
using the observed g values. The axial distortion is indeed much stronger than the rhombic one. The analyses of EPR spectral data thus indicate that both [1a]þand[2a]þare significantly distorted from ideal octahedral arrangements, which indeed is in line with the observedstructures of the parent bivalent complexes.
The voltammetric response of the pink dibromo compound [Os(HL)(pap)Br2], 4 consistedof two closely spacedquasi-reversible anodic responses near 0.9 and 1.1 V. A number of irreversible cathodic responses in the range0.40 to 1.50 V were also observed. For comparison, the corresponding com-plex cis, cis-Os(pap)2Br2undergoes two successive oxidations at 1.03 and1.80 V, which are assignedto OsII=OsIII and
OsIII=OsIV couples, respectively. The lowest potential anodic
response near 0.9 V in 4 is thus assignedto a OsII! OsIIIcouple
occurring at the metal orbital (HOMO) andthe secondwave at 1.1 V is due19to ligandoxidation. Interestingly, the E1=2for the OsII=OsIIIresponse in 4 is similar to that in 5 but is far more
anodic than those in 1 and 2. The coordination of a hard amido donor function in the latter complexes is responsible for this cathodic shift.
Conclusion
Our present study shows that the coordination chemistry of HL involving osmium(II) is rich with many novel features.
Table 2 Electrochemical data in CH2Cl2a
Compound Anodic responsesb E1=2=V(DEp=mV) Epa=V Cathodic responsesb E1=2=V(DEp=mV) Epc=V 1a 0.44(70), 0.91(100), 1.19c 0.88d ,1.03(60) 1b 0.41(80), 0.84(80), 1.2c 0.89d, 1.07(50) 2a 0.44(100), 1.0(80) 0.77(310)e ,1.07 2b 0.40(80), 0.95(80) 0.78(300)e, 1.1d 3 1.04(80), 1.81(210) 0.53d, 0.80d, 1.50d, 1.77d 4a 0.96(100)f, 1.14(100)f 0.70(100), 1.01d 4b 0.93(100)f, 1.09(80)f 0.41g,0.67(90), 0.97d 5 1.03(60), 1.81(280) 0.64d, 0.87d, 1.51d, 1.79d a Supporting electrolyte TBAP; reference electrode SCE; scanrate: 50 mV s1. b E
1=2¼ 0.5(Epaþ Epc) where Epa and Epcare the anodic and cathodic peak potentials, respectively; DEp¼ EpaEpc.
c
Irreversible, Epa; a cathodic peak of unclear origin near 1.0 V appeared. d
Irreversible, Epc. e Quasi-reversible ipc> ipa. f Quasi-reversible, ipa> ipc. g Anodic responses generated in situ when the scan was allowedbeyond1.5 V.
Table 3 EPR g valuesa andderivedparametersb of the complexes [1a]þand[2a]þ
Compd.
Derivedenergy parameters=cm1 [1a]þ [2a]þ
g1 2.2299 2.1862 g2 2.0778 2.0743 g3 1.8747 1.9165 D 15 670 18 880 V 8780 10 030 DE1 11 625 14 125 DE2 20 720 24 455
a In 1 : 1 dichloromethane–toluene solution at 77 K. b Taking the value of the spin-orbit coupling constant (l) for low-spin osmium(III) as equal to 3000 cm1.
Fig. 5 EPR spectra of (a) [Os(HLa)(La)Br]þ and(b) [Os(La )-(pap)Br]þin frozen 1 : 1 dichloromethane–toluene solution at 77 K, showing the computedsplitting of the t2orbitals. DPPH¼ diphenyl-picrylhydrazyl.
Osmium promoteddeamination of HL has resultedin the formation of 2-(phenylazo)pyridine [pap], in situ, which is responsible for the several mixedligandproducts that were formedfrom the reaction of [NH4]2[OsBr6] andHL. It may be notedhere that the ligandHL is obtained2by amination of coordinated pap to a Co(II) centre. Both C–N bondformation
andbondcleavage processes of the above kindare otherwise not achievable. Electronic reasons that govern the tridentate and bidentate coordination modes of the ligand have been noted. Presently we are pursuing the synthesis of monoaquo compounds from the monobromo compounds, 1 and 2, by their hydrolysis. It is anticipated that these would be suitable mediators20for the study of oxo transfer processes.
Experimental
Materials and physical measurements
The starting compounds [NH4]2[OsBr6]21 and2-[(arylamino)-phenylazo]pyridine2 were preparedfollowing the reported procedures.
Caution!
Caution! Perchlorate salts of metal complexes can be explosive. Although no detonation tendencies have been observed, care is advised and handling of only small quantities recommended.
A Perkin–Elmer 240C elemental analyser was usedto collect microanalytical data (CHN). The IR spectra were obtained with a Perkin–Elmer 783 spectrophotometer andthe1H NMR spectra with a Bruker Avance DPX 300 spectrophotometer using SiMe4 as an internal standard. Electrochemical mea-surements were performed at 298 K under a dry nitrogen atmosphere on a PC controlledPAR model 273A electro-chemistry system. A platinum disc, a platinum wire auxiliary electrode and an aqueous saturated calomel reference electrode were usedin a three-electrode configuration. The E1=2for the ferrocenium-ferrocene couple under our experimental condi-tions is 0.42 V. Electronic spectra were recorded on a JASCO V-570 spectrophotometer. EPR spectra were recorded on a Varian model 109C E-line X-band spectrometer fitted with a quartz Dewar for measurement at 77 K (liquidnitrogen) and all spectra were calibratedagainst the spectrum of DPPH. Syntheses
The complexes 1–5 were isolatedfrom the reaction of [NH4]2[OsBr6] with 2-[(arylamino)phenylazo]pyridine taken in a 1 : 2 molar ratio, in methanol, in the presence of dilute NEt3. Details are given below.
Reaction of [NH4]2[OsBr6] with HLa. A mixture of [NH4]2 -[OsBr6] (130 mg, 0.183 mmol), 2-[(arylamino)phenyl-azo]pyr-idine (100 mg, 0.365 mmol) and 2–3 drops of NEt3taken in 50 ml methanol was refluxedfor 5 h on a steam bath. The initial red colour changed to dark brown during this period. The solution was cooledandconcentratedto one-thirdof its initial volume and precipitated by the addition of diethyl ether. The crude product was then subjected to chromatography on a preparative TLC plate. It was first elutedwith benzene; two brown bands (1 and 2) were separated. A small amount of unreactedligandwas observedat the junction of the two brown bands. The unmoved dark band was again loaded on a fresh preparative TLC plate anda benzene–chloroform mix-ture (3 : 1) was then usedas the eluent. This resultedin the separation of three distinct bands, viz. violet (3), pink (4) and red-violet (5). All these compounds (1–5) were collectedby the complete evaporation of eluates. Recrystallisation of these from a dichloromethane–hexane solvent mixture yielded the products in their crystalline states.
1a. Yield18%, IR (KBr): n¼ 3410 (N–H), 1225, 1265 (N=N) cm1; calcdfor C34H27BrN8Os: C 49.94, H 3.33, N 13.70; found: C 49.79, H 3.06, N 13.40%. 2a. Yield22%, IR (KBr): n¼ 1220, 1265 (N=N) cm1; calcdfor C28H22BrN7Os: C 46.28, H 3.05, N 13.49; found: C 46.19, H 3.16, N 13.27%. 3. Yield 5%, its spectra exactly corresponded to those of the known sample of tc-Os(pap)2Br2; calcdfor C22H18Br2N6Os: C 36.88, H 2.53, N 11.73; found: C 35.61, H 2.34, N 11.12%. 4a. Yield 10%, IR (KBr): n¼ 3285 (N–H), 1230, 1285 (N=N) cm1; calcdfor C28H23Br2N7Os: C 41.64, H 2.87, N 12.14; found: C 41.52, H 2.91, N 12.18%. 5. Yield10%, its spectra exactly corresponded to those of the known sample of cc-Os(pap)2Br2; calcdfor C22H18Br2N6Os: C 36.88, H 2.53, N 11.73; found: C 35.52, H 2.32, N 11.21%.
Reaction of [NH4]2[OsBr6] with HLb. This reaction was performedas describedabove, except that an equivalent amount of HLb was usedin place of HLa. The yields and characterisation data are given below.
1b. Yield20%, IR (KBr): n¼ 3410 (N–H), 1220, 1265 (N=N) cm1; calcdfor C36H31BrN8Os: C 51.12, H 3.69, N 13.25; found: C 51.54, H 3.52, N 13.32%. 2b. Yield20%, IR (KBr): n¼ 1220, 1265 (N=N) cm1; calcdfor C29H24BrN7Os: C 47.03, H 3.27, N 13.24; found: C 46.85, H 3.26 N 13.13%. 4b. Yield 12%, IR (KBr): n¼ 3285 (N–H), 1240, 1280 (N=N) cm1; calcdfor C29H25Br2N7Os: C 42.40, H 3.07, N 11.93; found: C 42.28, H 3.02, N 11.81%. Yields of the compounds 3 and 5 were similar to those of the previous reaction.
Electrochemical generation of [1a]þand [2a]þ
The complexes [1a]þand[2a]þwere generatedin solution by constant potential coulometric oxidation of solutions of 1a and 2a, respectively. Details of a representative example are notedbelow.
A solution of 9.2 mg of 1a in 25 ml dichloromethane solvent containing 35 mg of TBAP was oxidised coulometrically. The oxidation was performed at 0.65 V; n¼ 1.108=1.086 ¼ 1.02, n¼ Q=Q0, where Q0 is the calculatedcoulomb count for one
electron transfer and Q is the coulomb count foundafter exhaustive electrolysis. A part of the electrogeneratedsolution (5 ml) of [1a]þwas mixedwith an equal volume of toluene and the mixture was quickly frozen at 77 K, andthen usedfor an EPR measurement. The oxidised complex [1a]þwas not stable at room temperature. Most of it (> 90%) underwent reduction to produce 1a, associatedwith some insoluble material of unknown composition. The oxidation of 1a also may be per-formedchemically using an aqueous solution of Ce4þ. In this case the quantity of the byproducts was more than 25%. To date, we were not successful in isolating the trivalent osmium compound[1a]þin its pure state.
The trivalent complex [2a]þ was generatedin solution similarly as described above.
Crystallography
X-Ray quality crystals of [Os(HLa)(La)Br] (1a) were obtained by the slow diffusion of a dichloromethane solution of 1a into hexane. Crystals of [Os(La)(pap)Br] (2a) were obtainedby the slow diffusion of a solution of 2a in toluene into hexane. Crystals of [Os(HLa)(pap)Br
2](4a) were obtainedby slow evaporation of a solution of 4a in acetonitrile. Relevant crys-tallographic data are collected in Table 4. Intensity data of 1a and 4a were collectedon a Siemens SMART diffractometer, equippedwith a graphite-monochromatedMo-Ka radiation source, l¼ 0.71073 A. These were correctedfor Lorentz-+ polarisation effects. The structures were solvedby using the SHELXS-86 package of programmes22 andrefinedby full-matrix least-squares basedon F2 (SHELXL-93).23 All the
hydrogen atoms were located in calculated positions. The data of 2a were collectedon an Enraf-Nonius CAD4 diffractometer (Mo-Ka radiation, l¼ 0.71073 A) These were correctedfor+ Lorentz-polarisation effects. The structure was solvedby using the SHELXS-97 package of programmes24andrefinedby full-matrix least-squares basedon F2(SHELXL-97).25 Hydrogen atoms were added in the calculated positions.
CCDC reference numbers 157836–157838. See http:== www.rsc.org=suppdata=nj=b1=b108507g= for crystallographic data in CIF or other electronic format.
Acknowledgement
Financial support receivedfrom the Council of Scientific and Industrial Research, New Delhi, is acknowledged.
References
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Crystal Structures, University of Go¨ttingen, Germany, 1997. Table 4 Crystallographic data and refinement details of Os(HLa)(La)Br (1a), Os(La)(pap)Br (2a) andOs(HLa)(pap)Br
2(4a)
1a 2a 4a
Chemical formula C34H27BrN8Os C28H22BrN7Os C28H23Br2N7Os
Formula weight 817.75 726.64 807.55
T=K 150(1) 295(2) 150(1)
Crystal system Monoclinic Monoclinic Rhombohedral
Space group P21=n P21=n R3 a=A+ 11.0940(2) 13.643(2) 38.4934(7) b=A+ 19.9527(3) 14.1915(18) 38.4934(7) c=A+ 14.5400(3) 13.850(3) 9.7379(2) a= 90 90 90 b= 110.873(1) 91.36(2) 90 g= 90 90 120 u=A+3 3007.28(9) 2680.8(8) 12 495.9(4) Z 4 4 18 m=mm1 5.608 6.278 7.501 Coll. reflect. 21 518 4731 24 176 Unique reflect. 6877 4731 6362 Rint 0.0264 0.0000 0.0445 R1[I > 2s(I)] 0.0227 0.0353 0.0285 wR2[I > 2s(I)] 0.0499 0.0650 0.0611
![Fig. 3 Molecular structure andatom numbering scheme for [Os (HL a )(pap)Br 2 ], 3a. Selectedbondlegnths ( +](https://thumb-ap.123doks.com/thumbv2/9libinfo/8874802.249761/3.897.461.812.50.371/fig-molecular-structure-andatom-numbering-scheme-os-selectedbondlegnths.webp)
![Fig. 5 EPR spectra of (a) [Os(HL a )(L a )Br] þ and(b) [Os(L a )- )-(pap)Br] þ in frozen 1 : 1 dichloromethane–toluene solution at 77 K, showing the computedsplitting of the t 2 orbitals](https://thumb-ap.123doks.com/thumbv2/9libinfo/8874802.249761/5.897.72.433.920.1082/spectra-frozen-dichloromethane-toluene-solution-showing-computedsplitting-orbitals.webp)
