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Discovery of Os2(hp)4Cl2 was immediately followed by the isolation of Os2(O2CMe)4Cl2 from the reaction between a hydrochloric acid solution of OsCl62-, prepared by the reduction of OsO4 with FeCl2, and acetic acid/anhydride.2,3 Other Os2(O2CR)4Cl2 compounds (R = CH2Cl, Et, Prn, and 2-PhC6H4) have been synthesized from Os2(O2CMe)4Cl2 using carboxylate exchange reactions.3-5 Crystal structures of Os2(O2CR)4Cl2 with R = CH3, C2H5, n-C3H7 (Fig. 10.2) and 2-PhC6H4 have been determined,4-6 and the Os–Os bond lengths are within a narrow range of 2.301 – 2.318 Å. The axial chloro ligands in tetracarboxylates can be readily displaced with bromo ligands by treating Os2(O2CR)4Cl2 with anhydrous HBr at -78 oC.7

Fig. 10.2. The structure of Os2(O2CC3H7)4Cl2.

In addition to being the precursor of other tetracarboxylates, Os2(O2CMe)4Cl2 also serves as a convenient starting material for many diosmium compounds supported by other bridging bidentate ligands enumerated below. Os2(hp)4Cl2 was obtained from refluxing Os2(O2CMe)4Cl2 with excess 2-hydroxypyridine in methanol.3 Molten reaction between Os2(O2CMe)4Cl2 and benzamide resulted in Os2(PhCONH)4Cl2,8 which was converted to Os2(PhCONH)4Br2 when recrystallized in the presence of Me4NBr.9 X-ray diffraction studies revealed that the benzamidato ligands adopt the cis-(2,2) arrangement around the Os2 core in both cases and the Os–Os bond lengths are 2.369 and 2.383 Å for axial Cl and Br adducts, respectively, which are slightly elongated from that of Os2(hp)4Cl2. A molten reaction between Os2(O2CMe)4Cl2 and 6-chloro-2-hydroxypyridine in a sealed Pyrex tube resulted in Os2(chp)2Cl4(H2O) in addition to Os2(chp)4Cl (see below).10 Os2(chp)2Cl4(H2O) was converted to Os2(chp)2Cl4(py) and X-ray analysis revealed that two bridging chp ligands are trans to each other, four chloro ligands occupy the remaining equatorial sites, and H2O/py occupies the axial position.11 A molten reaction between Os2(O2CMe)4Cl2 and N,N'-di(p-tolyl)formamidine (HDTolF) furnished Os2(DTolF)4Cl2, which has the longest Os–Os bond length (2.467 Å) among all known Os26+ paddlewheel species, and an almost eclipsed arrangement of DTolF ligands (0.1o N–Os–Os'–N' torsion angle).12 Brief refluxing of Os2(O2CMe)4Cl2 with Me3SiCl and 2-anilinopyridine in toluene led to an unsymmetrical compound Os2(ap)3Cl3 (Fig. 10.3), where the Os–Os distance was determined to be 2.392 Å.13 Fully substituted Os2(ap)4Cl2 was obtained recently from the prolonged reflux of Os2(O2CMe)4Cl2 and 2-anilinopyridine with the aid of an acetic acid scrubbing apparatus. For single crystals obtained from CH3OH/CH2Cl2 solution, X-ray analysis revealed a cis-(2,2) arrangement of ap ligands (Fig. 10.4), an Os–Os distance of 2.396(1) Å, and an averaged N-Os–Os'-N' torsional angle of 5o.14 Surprisingly, crystals obtained from hexanes/CH2Cl2 solution contain the (3,1)-isomer instead, which exhibits similar dimensions.15 Os2(ap)4Cl2 undergoes facile reaction with LiC2Ph to yield Os2(ap)4(C2Ph)2, the first Os2-alkynyl complex, which was crystallized as either the (3,1)-isomer from hexanes/THF solution or the cis-(2,2)-isomer from CH3OH/CH2Cl2 (Fig. 10.5).15 Upon alkynylation, the Os–Os bond elon-

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gates about 0.06 Å in the cis-(2,2) isomer and 0.08 Å in (3,1) isomer. Similar to the original preparation of Os2(hp)4Cl2, Os2(hpp)4Cl2 was synthesized in 30% yield from refluxing OsCl3 with four equivalents of Hhpp in ethanol.16 X-ray structural analysis revealed an Os–Os bond length of 2.379 Å, the shortest among Os26+ compounds containing N,N'-bidentate ligands, and an eclipsed configuration of hpp ligands (0o N–Os–Os'–N' torsion angle as imposed by 4/mmm crystallographic symmetry).16,17

Fig. 10.3. The structure of Os2(PhNPy)3Cl3.

Fig. 10.4. The structure of cis-(2,2)-Os2(PhNPy)4Cl2.

Fig. 10.5. The structure of cis-(2,2)-Os2(PhNPy)4(C2Ph)2.

In an attempt to prepare axial phosphine adducts having an Os2(O2CMe)4 core, gentle refluxing of Os2(O2CMe)4Cl2 and Ph3P in acetic acid resulted in cis-Os2(O2CCH3)2(Ph2PC6H4)2Cl2, where the ortho-metallated Ph2P(C6H4) group functions as a P,C-bidentate bridging ligand.18,19 cis- Os2(O2CC2H5)2(Ph2PC6H4)2Cl2 was prepared similarly. Crystal structures of both ortho-metal-

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lated products were determined, and very short Os–Os bond lengths (2.271 and 2.272 Å) were revealed.19 cis-Os2(O2CCH3)2(Ph2PC6H4)2Cl2 reacts with Me3SiCl to afford Os2Cl4(Ph2PC6H4)2 (Fig. 10.6) where the Os–Os bond (2.231 Å) was shortened further. This compound exhibits an unusually distorted geometry around the Os2 core that is best described as two trigonal bipyramidal (TBP) Os centers fused at the equatorial position (Fig. 10.6).

P Cl

Cl

C

OsOs

Cl

Cl

C P

Fig. 10.6. The structure of Os2Cl4(Ph2PC6H4)2.

While the Os–Os bond is retained in the aforementioned bridging ligand exchange reactions, Os2(O2CR)4Cl2 also undergoes facile Os–Os bond cleavage with many nucleophiles to yield a number of mononuclear Os complexes as summarized in Scheme 10.1.3,20-23 Reactions between Os2(O2CMe)4Cl2 and Grignard reagent MgRCl are most peculiar and yielded drastically different products depending on the nature of R. Cleavage products, OsR4, were isolated with R as cyclohexyl and 2-methylcyclohexyl.21,22 On the other hand, the partially alkylated dinuclear compounds Os2(O2CMe)2R4 were produced with R as CH2SiMe3 and CH2CMe3.24,25 Although these compounds were described as crystalline, structures were not determined. While Os2(O2CMe)2R4 could not be further alkylated with MgRCl in large excess, it reacts with Mg(C3H5)Br to yield Os2(δ3-C3H5)2R4. An X-ray diffraction study of Os2(δ3- C3H5)2(CH2CMe3)4 (Fig. 10.7) revealed the shortest Os–Os bond length known: 2.194 Å. Both Os2(O2CMe)2R4 and Os2(δ3-C3H5)2(CH2CMe3)4 are diamagnetic.

Table 10.1 The diosmium paddlewheel species and related compounds

(viii)

(ix)(x) (xi)

Scheme 10.1. Os–Os bond cleavage reactions. (i) aqueous HCl or HBr; (ii) bipy; (iii) acetylacetone; (iv) Na + CNBut; (v) CNBut; (vi) PMe3; (vii) vdpp, LiCl, reflux in toluene; (viii) (a) Pb(NO3)2, KPF6; (b) CNR; (ix) MgRCl, R = cyclohexyl; (x) MgRCl, R= CH2SiMe3 and CH2CMe3; (xi) (a) Mg(CH2CMe3)Cl; (b) Mg(C3H5)Br

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Fig. 10.7. The structure of Os2(δ3-allyl)2(CH2But)4; (a) labeled plot and (b) viewed along Os1–Os2 vector

The compound Os2(O2CMe)4Cl2 reacts with hydrohalic acids (HCl, HBr) to yield either [OsX6]2- in aqueous solution3 or [Os2X8]2- in anhydrous ethanol.33,34 [Os2I8]2- was obtained by treating (Bu4N)2[Os2Cl8] with gaseous HI in CH2Cl2, and crystallized via slow diffusion of toluene into a CH2Cl2 solution.35 More recently, [Os2Br8]2- was isolated from the reaction between H2OsBr6 and C5Me5H in the mixture of 48% HBr and ethanol (or methanol), representing the only example of [Os2X8]2- synthesis directly from a mononuclear source.36

4H2OsBr6 + 4C5Me5H + 3C2H5OH Α [(C5Me5)2OsH]2[Os2Br8] + 16HBr + 3CH3CHO While they resemble the quadruply bonded [Mo2X8]4- and [Re2X8]2- anions in formulation,

[Os2X8]2- anions are unique in that the majority adopt a staggered configuration,33-38 indicating the absence of a net β−bond. The Os–Os bond lengths in [Os2X8]2- (Fig. 10.8a) are generally short and within a narrow range of 2.182– 2.231 Å despite the large variation in the size of X. As with some other [M2X8]2- species, the Os2 core is sometimes disordered within the cage defined by eight halide ligands in several cases (see Table 2). A rare tetraosmium cluster [Os4I14]2- (Fig. 10.8b),38 where two [Os2I8]2- units were fused through edge-sharing, was obtained by recrystallizing [Os2I8]2- from ethanol/CH2Cl2.

Fig. 10.8. (a) The structure of [Os2Cl8]2-; (b) The structure of [Os4I14]2-.

The anion [Os2X8]2- readily reacts with various nitrogen and phosphorus donor ligands to yield either the mononuclear Os(III)/Os(II) complexes or face-sharing bioctohedral [Os2(µ-X)3(PR)6]+ complexes, as summarized in the scheme below.39 No simple substitution reaction to give, for example, an Os2X6L2 molecule has been observed. The crystal structure of [Os2(µ-Cl)3(PEt3)6]PF6 was determined, and the long Os···Os distance (3.47 Å) therein clearly indicates the absence of an Os–Os bond. [Os2Cl8]2- reacts with cyclic triaza ligands L to yield LOsCl3 (L = TACN and Me3TACN, where TACN is 1,4,7-triazacyclononane), which can be converted to [LOs(µ-Cl)3OsL]3+ upon refluxing in triflic acid.40 Os–Os bonding was deduced based on an Os–Os distance of 2.67 Å from a partially refined structure of [(TACN)Os(µ- Cl)3Os(TACN)](PF6)3.

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[OsX4(py)2]-

[Os2( -X)3P6]+

Scheme 10.2. (i) heat in DMF containing 5 equiv. py, X = Cl; (ii) reflux in neat py,

X = Cl; (iii) 10 equiv. bipy in methanol; (iv) bidentate phosphine (P–P) in ethanol; (v) 2.5 equiv. phosphine (P) in n-PrOH, 0 °C - room temp.; (vi) 5.5 equiv. P in methanol, reflux; (vii) n = 1, 3 equiv. P in methanol, room temp.; n = 0, 9 equiv. P in methanol reflux; (viii) 7.5 equiv. P in ethanol, reflux

Table 10.2. Compounds of the Os2X82- type

Edge sharing bioctahedral (ESBO) [Os2(µ-X)2X8]2- species with X as Cl- or Br- have been synthesized from OsX62-.42,43 While all ESBO W2 and Re2 compounds are metal–metal bonded, the Os–Os distance in Os2(µ-Br)2Br82- is 3.788(3) Å, consistent with the absence of an Os–Os bond.42 Reductive halide extrusion of Os2(µ-X)2X82- at -35 °C resulted in the face-sharing [Os2III(µ-X)3X6]3- species, and the X-ray structural analysis of a bromo complex revealed an Os–Os bond length of 2.779 Å,44 based on which the presence of a μ(Os–Os) bond is suggested.

10.2 Syntheses and Structures of Os25+ Compounds

Soon after their discoveries, both Os2(hp)4Cl2 and Os2(O2CR)4Cl2 were chemically reduced with cobaltocene to the corresponding monoanions [Os2(hp)4Cl2]- and [Os2(O2CR)4Cl2]-,45 but the structures of these Os25+ complexes were not determined. The Os26+ core was reduced also to an Os25+ core during the metathesis reactions between Os2(O2CCH3)4Cl2 and 6-X-2-hydroxy- pyridine (X = F or Cl) to result in Os2(Xhp)4Cl.10,28 Crystallographic analysis revealed that both compounds adopt the (4,0) arrangement: the Xhp ligands are so arranged that all X-atoms are placed around the axial position opposite to the one occupied by the chloro ligand. Clearly, the accommodation of four pyridine substituents X necessitates the loss of an axial Cl from the Os2 core, and consequently its reduction. The Os–Os distances are 2.341(1) and 2.348(1) Å for

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X = F and Cl, respectively, which are almost identical to that of Os2(hp)4Cl2. A plausible explanation is that the bond elongation due to the gain of an antibonding electron is cancelled out by the bond shortening caused by the reduction of electrostatic repulsion between two Os atoms in the Os25+ core. It is also interesting to note that the Os–Os distances in Os2(Xhp)4Cl are about 0.06 Å longer than the Ru-Ru distances for the isostructural Ru2(Xhp)4Cl compounds.46,47

Fig. 10.9. The structure of {[Os2(chp)4]2(µ-N,N'-pyrazine)}2+.

The complex Os2Cl4(Ph2Ppy)2(O2CMe) was the unexpected product (30% yield) from the reaction between Os2(O2CMe)4Cl2 and Ph2Ppy in the presence of Me3SiCl,29 and its yield was significantly improved by reacting Os2(O2CMe)4Cl2 and Ph2Ppy in the presence of an excess of LiCl.30 The species Os2Cl4(Ph2Ppy)2(O2CMe) crystallized as both CH2Cl2 and acetone solvates, and Os–Os distances are 2.395 and 2.388 Å, respectively.30 Reaction between Os2(chp)4Cl and [Ag(NCMe)4](BF4) resulted in [Os2(chp)4(NCMe)](BF4). The axial acetonitrile in the latter complex ion was displaced by either pyridine to yield [Os2(chp)4(py)](BF4), or pyrazine to yield {[Os2(chp)4]2(µ-N,N'-pyrazine)}(BF4)2 (Fig. 10.9),31 and nearly identical Os–Os distances were found for [Os2(chp)4(py)]+ (2.336 Å) and {[Os2(chp)4]2(µ-N,N'-pyrazine)}2+ (2.334 Å).

10.3 Syntheses and Structures of Other Os2 Compounds

The compounds [Os(Porp)]2 were synthesized from the pyrolysis of Os(Porp)(py)2 (Porp = TPP, TTP, OEP, and OETAP),48,49 while heterometallic dimer [(Porp)OsMo(OEP)] was isolated from the mixture produced from the pyrolysis of Os(Porp)py2 and Mo(OEP)(δ2-PhCCPh).50,51 Using a cofacial bis(porphyrin) linked with biphenylene (DPB), an heterometallic dimer OsRu(DPB) was isolated as a dark brown solid from the pyrolysis of Os(py)2(DPB)Ru(py)2.52 Later, [(OEP)OsRu(OETAP)] was isolated from the mixture produced via the co-pyrolysis of Ru(OETAP)(py)2 and Os(OEP)(py)2,49,53 and [(OEP)OsW(OEP)] from the co-pyrolysis of Os(OEP)(py)2 and W(OEP)(PEt3)2. Structural details of these compounds would be very interesting since the Os–Os and Os–M' bonds are not sustained by bridging ligands. The only reported structure, however, is that of [(TPP)OsMo(OEP)]+(PF6)- (Fig. 10.10), where the Os–Mo bond length is 2.238(3) Å.51 While the Os–Mo bond order should be 3.5 based on the valence electron count, the single β-type electron is probably nonbonding, judging from the nearly staggered configuration adopted by the Os–N4 and Mo–N'4 cores (N–Os–Mo–N' = 42.1°).

The Os–Os bonds in [Os(Porp)]2 can be readily cleaved by a nucleophilic ligand. Os2(OEP)2 reacts with a simple nucleophilic ligand L (L = CO, py, and THF) to yield mononuclear trans- Os(OEP)L2 and the reaction rate is proportional to the ligand field strength of L: the reaction with CO is complete in seconds, py in minutes, and THF in days.54 The compound Os2(OEP)2 reacts with several linear bidentate linkers L-L (L-L = pyrazine, 4,4'-bipyridine and 1,4-diazab icyclo[2.2.2]octane) to yield insoluble polymers {Os(OEP)(µ-L-L)} , which can be oxidatively

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doped with either I2 or NOPF6 resulting in conductive polymers.54,55 The cation [Os2(TTP)2]2+ was also used as precursor to mononuclear OsIV(TTP) complexes.56

Fig. 10.10. The structure of [(TPP)OsMo(OEP)]+ viewed from the side (left) and along the Os–Mo bond (right, Os–N4 plane at the front and labeled).

8

Scheme 10.3. Reactions between [Os(Porp)]2 and nucleophiles

In a related example, the reaction of OsCl3 with molten o-cyanobenzamide in excess yielded (Pc)OsLx, which produced a peak corresponding to [(Pc)Os]2 (m/e = 1407) in a FD mass spectrometer.57 Subsequently, the structure of “Os(Pc)” prepared from the pyrolysis of Os(Pc)(py)2 was analyzed with a wide angle X-ray scattering technique and a dimeric structure was deduced with an estimated Os–Os bond length of 2.38 Å.58

Although the ease of undergoing one-electron oxidation has been established for many Os26+ species through voltammetric studies, it is not until recently that the first Os27+ complex, [Os2(hpp)4Cl2](PF6), was isolated from the chemical oxidation of Os2(hpp)4Cl2 by ferrocenium.32 The Os–Os bond lengths determined for the acetone and hexane solvates are 2.331(1) and 2.329(1) Å, respectively, and the shortening from that of the neutral parent Os2(hpp)4Cl2 (2.379(2) Å) is consistent with the loss of a β* electron.

10.4 Magnetism, Electronic Structures, and Spectroscopy

While the most common dinuclear species of other 5d metals, namely those having W24+ and Re26+ cores, are typically diamagnetic, paramagnetism has been the hallmark for the majority of the Os26+ species, especially those having paddlewheel motifs. Paramagnetism of Os2n+ species was first uncovered in Os2(O2CR)4Cl2, where µeff measured using the Evans technique decreases from 1.15 B.M. per Os (1.6 per Os2) at 300 K to 1.0 B.M. per Os at c. 200 K.3 While the diamagnetic ground state μ2/4β2β*2 (Scheme 10.4) was clearly ruled out, data obtained were insufficient to distinguish between two possible S = 1 configurations: μ2/4β2/*2 and μ2/4β2(β*/*)2.3 A later study of the magnetic susceptibility of Os2(O2CC6H4-2-C6H5)4Cl2 over a temperature range of 5 – 300 K ruled out the possibility of μ2/4β2/*2, but modeling based on the μ2/4β2(β*/*)2 configuration was not performed.5 Subsequently, a detailed analysis of the magnetic properties for Os2(O2CCMe3)4Cl2 was accomplished based on the μ2/4β2(β*/*)2 configuration, for which the temperature dependence of the effective magnetic moment µeff was derived:26

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where x = D/kBT, and D is the zero-field splitting parameter for the 3Eu state derived from the μ2/4β2(β*/*)2 configuration. This deceptively simple equation yielded a satisfactory fit of data between 30 – 350 K for Os2(O2CCMe3)4Cl2. This study, together with the short Os–Os bond lengths observed, firmly establishes the existence of Os–Os triple bonds in Os2(O2CR)4Cl2 compounds.

Scheme 10.4. Possible ground state configurations for Os26+ paddlewheel species.

All three paddlewheel Os26+ compounds supported by the N,N'-bidentate ligands (DTolF, hpp and ap) exhibit elongated Os–Os bonds in comparison with those of Os2(O2CR)4Cl2 compounds, and are paramagnetic. Temperature-dependence of the measured µeff for Os2(DTolF)4Cl2 resembles that reported for diruthenium(II) compounds supported by both carboxylates and hydroxypyridinates,59,60 and a satisfactory fit according to the following relationship was achieved (Fig. 10.11):5

2eff = 2geff2[e-x + (2/x)(1-e-x)] 1 + 2e-x

where x = D/kBT, and D is the zero-field splitting parameter for the 3A1g state derived from the μ2/4β2/*2 configuration. A very long Os–Os bond is also consistent with the μ2/4β2/*2 assignment. SCF-X_ calculations, both nonand relativistic, performed on the model compound Os2[HNC(H)NH]4Cl2 revealed a HOMO(/*)-LUMO(β*) gap of 1.13 eV, which is attributed to the substantial destabilization of β*(Os–Os) by the /nb(N-C-N) orbitals.12 Os2(hpp)4Cl2, on the other hand, exhibits a very small, temperature-independent paramagnetism (TIP, 4.1 x 10-5 emu/mol; 0.3 B.M. per Os2) over the temperature range of 10 – 300 K, which is best explained by a singlet ground state μ2/4β2β*2 with a low-lying triplet excited state.16 Consistent with the weak antibonding nature of β* orbital, the Os–Os bond length in Os2(hpp)4Cl2 is 0.08 Å shorter than that of Os2(DTolF)4Cl2. The one-electron oxidation product of Os2(hpp)4Cl2 has an effective magnetic moment of 1.3 B.M., and a very small g value (0.79) determined from the X-band EPR spectrum.32 The recently reported Os2(ap)4Cl2 has an Os–Os bond length ca. 0.02 Å longer than that of Os2(hpp)4Cl2, and an effective magnetic moment of 2.76 B.M. per Os2 unit. Effective magnetic moments of diosmium compounds tend to be much lower