Publications

Abstract: A series of complexes is synthesized of the form [Os(NH3)5(2,3-jj1 2-pyrrole)]2+, with pyrrole and various alkylated

pyrroles. In contrast to the free ligands, these complexes can be protonated chemo- and stereoselectively at the ß carbon,

away from the metal, to produce pyrrolium species whose acidities range in pATa from 4.2 to 7.5. In the presence of a weak

base, two of these 3//-pyrrolium species can be converted to the corresponding 2//-pyrrolium tautomer, [Os(NH3)5(3,4-

?;2-2//-pynOlium)]3+. In the case of L= 2,5-dimethylpyrrole, the 2//-pyrrolium species can be deprotonated at nitrogen (pATa

= 7.9), rendering a neutral 2Z/-pyrrole ligand. When L= 1-methylpyrrole, the 2//-pyrrolium species can be deprotonated

at the a carbon, generating an unstable azomethine ylide complex bound through C3 and C4. Rapid rearrangement of this

species yields the neutral 1//-pyrrole complex, [Os(NH3)5(2,3-t?2-1 -methylpyrrole)]2+ (p/fcp= 7.8 (overall process)). Through

consideration of pATa and electrochemical data, the pyrrole/pyrrolenine isomerization energy (AG°) is found to decrease by

about 16 kcal on osmium(II), to the point where these tautomers become virtually isoergic. In contrast, the pyrrole/pyrrolenine

equilibrium is largely unaffected by coordination to Os(III), in comparison to the free ligand.

Abstract: The effect of 1

2 coordination on an arene is explored in the context of the phenol-ketodiene equilibrium. Whereas

for the free ligand this equilibrium heavily favors the phenol tautomer, we find that for the complexes [Os(NH3)5(2,3-i;2-arene)]2+

(arene= phenol; 4-, 5-, 6-methylphenol; 4,5-dimethylphenol) the corresponding equilibrium constants approach unity (20 °C).

Starting with the phenolic isomer in methanol, conversion to 2,4-cyclohexen-1 -one is kinetically favored over the formation

of the 2,5-analogue, although the latter is the thermodynamically favored product. All tautomerization processes are ster-

eocontrolled, with protonation and deprotonation occurring selectively at the ring face opposite the metal. Electrochemical

studies indicate that the phenol-2,5-dienone equilibrium is not profoundly influenced by complexation of osmium(III). In

addition to the mononuclear species, where L= phenol, two binuclear complexes are characterized of the form [|Os-

(NH3)5|2(m-j)2:»;2-L)]4+ where L= 2,4- or 2,5-cyclohexadien-1 -one.

Treatment of [0s(NH3)5(C3H5)20](0Tf)2 in methanol with triflic acid produces [Os(NH3)5(tj3-C3Hs)](OTf)3 in good yield. The

cation in solution was characterized by NMR spectroscopy. In cyclic voltammetry, no redox activity is revealed within the

solvent window, +1.5 to -1.5 V vs NHE. The complex readily undergoes addition at a terminal carbon by a variety of neutral

and negatively charged nucleophiles. Complexes of cyclic allyls are generated when either the complex of anisóle or of 1,4-di-

methoxybenzene is hydrogenated in acidic methanol. As is true of the benzene analogue, hydrogenation does not proceed beyond

two molecules of H2/molecule of ligand. In the reduction of the complex of 1,4-dimethoxybenzene, elimination of methanol leads

to the ij2-benzene complex as the reaction product. A proton is eliminated from [Os(NH3)5(tj3-C6H9)]3+, yielding [Os(NH3)5-

( 2-1,3-cyclohexadiene)]2+, and the reaction is governed by an equilibrium quotient of 0.3 M in methanol. This equilibrium quotient,

together with that for the addition of CH30" to coordinated allyl, leads to an estimate of 2 X 1015 for the equilibrium quotient

for elimination of CH3OH from [Os(NH3)5(?;2-3-methoxycyclohexene)]2+, to be compared to ca. 0.1 for the free organic ligand.

The comparison shows that the affinity of [Os(NH3)5]2+ for the carbon/carbon double bond in a conjugated diene is very much

greater than for the carbon/carbon double bond of 3-methoxycyclohexene.

Abstract: The reduction of 0s(NH3)5(CF3S03)3 in methanol or aqueous solution affords the dihydrogen complex [Os-

(NH3)5(H2)]2+, [1]2+. The one-electron oxidation of this species generates a trivalent dihydrogen complex [1]3+ which, unlike

its precursor, is capable of hydrogenating acetone to isopropyl alcohol. The use of methanol as a solvent offers important

advantages for the preparation of a number of Os(II) complexes over other nonaqueous solvents previously used.

Abstract: Several Abound arene complexes of pentaammineosmium(II) have been selectively hydrogenated to alkene products

in the presence of a heterogeneous catalyst (Pd, Rh). In all cases the coordinated osmium was shown both to activate the

arene ligand toward catalytic hydrogenation and to protect the alkene product toward further reduction. Hydrogenation can

be achieved both regio- and stereoselectivity and is shown to occur exclusively anti to the metal center. The structure of the

3-methoxycyclohexene complex [0s(NH3)5(j)2-C7H120)](0Tf)2 has been determined. The hydrogenation of eta2-bound anisóle

or 1,4-dimethoxybenzene is not concerted; the semihydrogenation product, a vinyl ether, rapidly hydrolyzes in acid medium

to form a cyclohexenone complex.

In the course of investigating 7/2-bound arene complexes of

pentaammineosmium(II),1 we have consistently observed the

formationoftraceamountsofa speciesinwhichthearene appears

to act as a bridging ligand. Recently we reported that [Os-

(NH3)5(i|2-C6H6)](OTf)2 (OTf= CF3S03') (1), when allowed

to stand in an inert solvent for24 h, condenses to form the bi-

nuclearspecies [(Os(NH3)5)2(j72:j/2-M-C6H6))](OTf)4 (2) andfree

benzene in high yield.2 'Hand 13C NMR data indicate three

inequivalentpositionsinthering,andchemical shiftdatasuggest

thatthemetalcentersengageseparatedoublebonds. Thisbonding

arrangementwould resultina single localizedolefinsitethatis

expected toshow enhanced reactivityrelativeto the free ligand.

However, whereas single-metalcoordinationhas been shown to

activatean arene towardhydrogenation3or furthercoordination,4

double-metal coordination appears to limit further reactivity:

attempts to add a third pentaammineosmium(II) moiety were

unsuccessful,as were our attemptsto hydrogenate3or epoxidize5

the complex. We attributed the failure of2 to undergo these

reactions to stericratherthan electroniceffects, reasoning that

stericconstraints requirethemetalcenterstolieon oppositefaces

ofthearene plane. Tosettlethequestionofgeometry, we have

determined the crystal structure for 2....