The dynamics of double proton transfer in 7-azaindole (7-AI) dimers, a model DNA base pair, are investigated in real time using femtosecond transient absorption and fluorescence upconversion techniques. In nonpolar solvents we examine the isotope effect, the excitation energy dependence, and the structure analogue of the tautomer (7-MeAI). A detailed molecular picture of the nuclear dynamics in the condensed phase emerges with the relationship to the dynamics observed in molecular beams:  Following the femtosecond excitation there are three distinct time scales for structural relaxation in the initial pair, proton (hydrogen) transfers, and vibrational relaxation or cooling of the tautomer. The molecular basis of tunneling and concertedness are elucidated by careful examination of the isotope effect and the time resolution. Comparison with the results in the isolated pair indicates the critical role of the N−H and N···N nuclear motions in determining the effective potential, and the thermal excitation in solution. Because the barrier is small, ∼1.3 kcal/mol, both are important factors and experiments at much higher energies will be unable to test either tunneling or concertedness. Finally, we compare the experimental results and the dynamical picture with detailed ab initio and molecular dynamics simulations.

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Abstract:The first representatives of phosphino(stibin0)methanes R,PCH,Sb-R; ( 3 -5) with bulky alkyl or cycloalkyl groups R and R' were prepared in two steps from Ph,SnCH,I via the isolated stannylated phosphanes Ph,SnCH2PR, (1, 2) as intermediates. X-ray structural analysis of 5 (R = C,H,,, R' = tBu) reveals that the lone pairs and the substituents R and R at phosphorus and antimony and the hydrogen atoms of the CH, bridge adopt staggered conformations. Treatment of [{C8H1,RhC1},] with 3-5 affords the neutral compounds [RhCl(~4-C,H,2)(~-P-R2PCH,SbR2)] (6--8), of which 7 and 8 react with CH,MgI to give the corresponding methylrhodium derivatives [RhCH,(q4-C,H,,)(h--P-R,-PCH,SbR;)J (9, 10). Cationic complexes [Rh(q4-C8H,,)(~2-P,Sb-R,PCH2SbR2)]-X (X = PF,: l l a , 12a, 13; X = BPh,: 11 b, 12b) containing the phosphino(stibino)methanes as chelating ligands were obtaincd either from [{C,H,,RhCI},], Keywords antimony -arene complexes * diene complexes rhodium -ylides lntroduction Recent work in our laboratory has shown that the replacement of triisopropylphosphane by triisopropylstibane as a ligand leads to remarkable differences in the reactivity of low-valent metal complexes of the iron and cobalt triads." -3 1 While some of these differences are probably steric in nature, they mainly reflect the unequal o-donor and n-acceptor capabilities of trialkylphosphane and -stibane ligands. In almost all cases studied to date, the M -SbiPr, bond is more labile than its M-PiPr, counterpart, and this can be put to advantage for synthetic purposes."' 3b.41 A disadvantage ofthe stibane-metal complexes, however, is their tendency to decompose under the conditions of subsequent conversions.In order to combine the favorable aspects of the ligand behavior of bulky trialkylphosphanes on the one hand and of related trialkylstibanes on the other, we set out to prepare mixed P/Sb donor systems. Here we describe the synthesis of peralkylated phosphino(stibino)methanes, the molecular and crystal structure of one representative, and the use of the unsymmetrical ligands to prepare square planar as well as half-sandwich-type organometallic rhodium complexes. Results and DiscussionSynthesis of the P/Sb donor ligands: The bulky phosphino(stibino)methanes 3-5 were prepared by a two-step procedure (Scheme 1). The first step is the inetalation of Ph,SnCH,I by BuLi in toluene/hexane at low temperature, which, in the presence of TMEDA (tetramethylethylenediamine), probably yields solvated Ph,SnCH,Li. This in situ generated intermediate reacts with R,PCI at -90 to -80°C to give a clear solution from which, upon warming to room temperature and addition of water, an oily air-sensitive product is obtained. Chromatographic workup for R = cyclohexyl (Cy) affords white crystals (1) and for R = iPr a colorless liquid (2), both in 85-90% yield. Following a similar route, Kauffmann and Kriegesmann prepared the stannylated arsanes R,SnCH,AsPh, (R = Me, Ph) from R,SnCH,I, BuLi, and Ph,AsCI.

The π-allyl complexes [Rh(η3-2-RC3H4)(κ2-R‘2PCH2PiPr2)] (R = H, Me; R‘ = iPr, Cy, Ph) (2−7) were prepared from [RhCl(η4-C8H12)]2, 2-RC3H4MgX, and R‘2PCH2PiPr2 via [Rh(η3-2-RC3H4)(η4-C8H12)] as the intermediate. Reaction of 2−4 (R = H) with Broensted acids HX (X = Cl, CF3CO2, CF3SO3) led to cleavage of the allyl−metal bond and to the formation of the monohydridorhodium(III) compounds [RhHX2(κ2-R‘2PCH2PiPr2)] (8−12). Variable-temperature NMR measurements of 8−12 confirm that these compounds are fluctional in solution. The reaction of 2−4 with CO gave in the initial step the 1:1 adducts [Rh(η3-C3H5)(CO)(κ2-R‘2PCH2PiPr2)] (16−18), of which that with R‘ = iPr was characterized by X-ray structure analysis. Compounds 16 (R‘ = iPr) and 18 (R‘ = Ph) reacted with excess carbon monoxide via migratory insertion of CO into the allyl−metal bond to yield the five-coordinate acylrhodium(I) complexes [Rh{C(O)CH2CHCH2}(CO)2(κ2-R‘2PCH2PiPr2)] (19, 20). This insertion reaction is reversible. The analogous acyl compound [Rh{C(O)CH2Ph}(CO)2(κ2-iPr2PCH2PiPr2)] (22) was obtained from [Rh(η3-CH2Ph)(κ2-iPr2PCH2PiPr2)] and CO. Acid cleavage of the acyl−metal bond of 19 (R‘ = iPr) afforded the aldehyde CH2CHCH2CHO (26) and a mixture of 8 and [RhHCl2(CO)(κ2-iPr2PCH2PiPr2)] (25).