ZWITTERIONIC METALATES OF Rh(I)

Transcription

1 ZWITTERIOI METALATE OF Rh(I) Massimiliano Delferro, Daniele auzzi, laudia Graiff, Antonio Tiripicchio Dipartimento di himica Generale ed Inorganica, himica Analitica, himica Fisica, Università di arma, Via Usberti 17/A, arma, Italy The reaction of 2 2 with in 1:2 molar ratio leads to the formation of the zwitterionic product () 2 = 2 () (). is protonable with X (X = l -, I, F 6 -, BF 4 - ) to afford ( 2 )X and deprotonable with a to form a(). ( 2 )l reacts with [Rh(cod)l] 2 to ontained [ 2 ][Rh(cod)l 2 ] (1), with - the complex [Rh(cod)()] (3) is achieved, while when was used [Rh(cod)()l] (2) is formed. The reaction of with [Rh(O) 2 l] 2 gives the metalate [Rh(O)()] (5); this complex can be protonated once or twice using for instance F 6 yielding [Rh(O)()](F 6 ) (6) and [Rh(O)( 2 )](F 6 ) 2 (7). The protonation constants in 2 l 2 of complex 5 were determined by a spectrophotometric titration using OTf as titrant. 7 was tested as catalyst for the hydroformylation of styrene at 50 and 50 atm of O/ 2 leading good results in term of activity (TOF) and selectivity. The zwitterionic ligand () 2 = 2 (, cheme 1) can be prepared in high yield by the double addition of ethylisothiocyanate () to bisdiphenylphosphinoamine ( 2 2, dppa) 1. The opposite charges can be formally located on the so-formed thioamidiyl-phosphonium, - () +, zwitterionic functional group. The observation of only one signal in the 31 -MR spectrum and the equivalence of the ethyl groups in the 1 MR spectrum can be explained by a rapid proton exchange between the thioamidic-thioamidyl functions. This tautomerism is facilitated by the templation due to an hydrogen bonding, as also found in the solid state. This proton exchange does not influence the charge separation, while acid-base reactions can take place on the molecule and vary its total charge. Thus, the positive charge can be distributed on the two atoms of the - - group and the negative one, because of the tautomerism, would be delocalized on the two thioamidic-thioamidyl functions (cheme 1).

2 cheme 1: Lewis structure, formal charge distribution and tautomerism of. The basicity of is due to the nitrogen atom of the thioamidyl moiety; protonation of with aqueous l or F 6 resulted in the formation of ( 2 ) + l - and ( 2 + ) (F - 6 ) respectively. Deprotonation of the thioamidic group with a yielded a + ( - ), containing the dianion-cation - (cheme 2). base acid cheme 2: formal charge distribution of deprotonated ( - ) and protonated ( 2 + ) forms of - can behave as,, tridentate or, bidentate ligand towards M I species (M = Rh, u, Ag, Au) 1,2. The -- group nitrogen can be coordinated, as in [Rh(O))] and [u()], weakly interacting with the metal centre, [Ag()] 2, or not coordinated as in the case of [Au()]. The [ ] + group is related to the classical [ 3 3 ] + cation (bis-triphenylphosphine-iminium, + ). When - coordinates in a,, tridentate fashion, the [ ] + coordination is promoted by the, chelation that brings the atom to a suitable geometry. The coordination properties of this ligand in its three forms: deprotonated zwitterionicanionic ( - ), zwitterionic-neutral () and protonated cationic ( 2 + ), were studied in the reactions towards Rh I species derived from [Rh(O) 2 l] 2 and [Rh(cod)l] 2 ; in the latter the presence of the chelating 1,5-cyclooctadiene (cod) does not allow the,, tricoordination. The reactivity of ( 2 )X, X = l, F 6, depends on the coordinative properties of the counter anion. In the reaction of ( 2 )l with [Rh(cod)l] 2, the chloride anion of the salt is coordinated by the rhodium atom of a Rh(cod)l moiety, forming a dichloro-rhodate;

3 2 + acts as counter cation yielding [ 2 ][Rh(cod)l 2 ] (1). Instead, when ( 2 )F 6 was added to [Rh(cod)l] 2, no reaction could be observed. omplex [Rh()(cod)l] (2) (Figure 1), in which behaves as monodentate through the thioamidyl sulfur atom, can be prepared by reacting with [Rh(cod)l] 2 and, in chloroform solution, is in equilibrium with isomer 2a, not isolable, (cheme 3), in which behaves as an, chelating ligand. The equilibrium is shifted to the left, as evaluated by 31 MR spectroscopy. The non - equivalence of the atoms in 2, can be appreciated by the presence of two doublets, suggesting the absence of the fast tautomeric equilibrium found in the free ligand. Figure 1: View of the molecular structure [Rh()(cod)l]. l Rh l Rh 2 2a cheme 3: Tautomeric equlibrium When t-buok was added to a solution of 2, 2a was first consumed, slowly followed by the disappearance of the signals of 2 and accompanied by the formation of [Rh(cod)()] (3).

4 3 can be also prepared by the bis-deprotonation of 1, or more efficiently by reaction of and [Rh(cod)l] 2 in the presence of a stoichiometric amount of t-buok (Figure 2). Figure 2: View of the crystal structure [Rh()(cod)]. Reaction of 3 with [Rh(cod)(TF) 2 ][OTf] results in the formation of a purple dinuclear compound, namely [{Rh(cod)} 2 (µ-, -)][OTf] (4) (Figure 3). In the molecular structure of 4, both sulfur atoms of - bridge two -Rh(cod) moieties, thus forming a Rh 2 2 butterfly core. Figure 3: View of the crystal structure of [{Rh(cod)} 2 (µ-, -)][OTf]. TF solvents and OTf- are omitted for clarity.

5 reacts with [Rh(O) 2 l] 2 to afford the metalate [Rh(O)()] (5). In the solid state this complex is found in,,-κ 3 coordination fashion. omplex 5 can be protonated once or twice using for instance F 6 yielding [Rh(O)()](F 6 ) (6) (Figure 4) and [Rh(O)( 2 )](F 6 ) 2 (7). As expected, in the solid state FTIR spectra, the O stretching frequency increase with the protonation. This suggests that the protonation results in a increased electron density on the metal centre and thus in a more efficient back donation. Figure 4: View of the crystal structure of [Rh(O)()](F 6 ). In order to study the acid-base properties of 5, titration with OTf was monitored by UV- VI spectroscopy in 2 l 2 solution. Four isosbestic points were detected: two at 438 and 353 nm for acid to complex ratios from 0 to 1 and two at 509 and 385 nm for acid to complex ratios from 1 to 2 (Figure 5). These results are in agreement with the expected diprotic nature of [Rh(O)(,, - 2 )] 2+. Two log K a values of 6.5 (3) and 4.8 (4) (σ = ) were determined for processes: [RhO()] + + [RhO()] + [RhO()] [RhO( 2 )] 2+ Therefore, [Rh(O)( 2 )] 2+ behaves in dichloromethane as a weak biprotic acid.

6 [RhO] [RhO] + [RhO 2] % Rh / RhO a b Figure 5: a)uv-visible titration of 5 ( = 1x10-3 M) with OTF (OTF = trifluormethansulphonic acid); b) Distribution daigram for the system [RhO()] + + [RhO()] + and [RhO()] [RhO( 2 )] 2+ omplex 7 was tested as catalyst for the hydroformylation of styrene (cheme 4) and was found effective. tyrene conversion, TOF and compositions are listed in Table 1. O O + O/ 2 at. Toluene * 3 + Linear Branched (R,) cheme 4: ydroformylation of styrene Table 1: tyrene conversion, TOF and compositions; T = 50, t = 10 h, = 50 atm. mmol tyrene onvertion % TOF electivity h -1 b/l = 86/5 1 M. Asti, R. ammi, D. auzzi,. Graiff, R. attacini, G. redieri, A. tercoli, A. Tiripicchio; hem. Eur. J. 2005, 11, R. attacini, L. Barbieri, A. tercoli, D. auzzi, L. Elviri,. Graiff, M. Lanfranchi, A. Tiripicchio; J. Am. hem. oc.,2006, 128,