Rhodium dinuclear intermediates

The consequence G ig. 6a) is that the rate(s) can be expressed as a multilinear summation in the sum total of intermediates in each disjoint unicyclic mechanism (Eq. 8). Next assume one more precursor, for example, a chelated dinuclear rhodium complex [32], is added to the system. This dinuclear rhodium complex has been shown to facilitate hydroformylation via a unicycUc mechanism having exclusively dinuclear intermediates. This new system is illustrated in Fig. 6b where again it is assumed that the individual mechanisms do not have common intermediates and the intermediates in different mechanisms do not interact. In terms of canonical variables in the system, the rate(s) is still multilinear as shown in Eq. 9 ... 

Reaction of one equivalent of the isolated mononuclear species 73 with one equivalent of [Rh(f 5-C9H7)(f72-C2H4)2] leads to clean formation of the dinuclear species 88 (151), presumably via an intermediate in which both rhodiums are coordinated to the same side of the perfluorinated ring. This cis... 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

Increasing the number of electrons reduces the activation of N2, because the electrons occupy the orbitals which are bonding with respect to the NN bond, and actually stabilize it. In agreement with this prediction dinitrogen is sufficiently activated to be reduced by protonation by dinuclear complexes of titanium(II), zirco-nium(Il), niobium(III), tantalum(III), molybdenum(IV), and tungsten(IV), whereas it is not reduced by protonation by certain d -d complexes, such as those of molybdenum(O), ruthenium(II), or rhodium(I). Apparently dinuclear complexes M-N=N-M in which M has the d electronic configuration can be intermediates in dinitrogen reduction in protic media, particularly if they represent part of polynuclear complexes (vide infra). 

The mechanism proposed in the original paper, [2] however, proved to be inconsistent with further mechanistic studies by Stanley et al. The original catalytic cycle was based on neutral dinuclear rhodium intermediates, the first of which, the neutral hydrido-carbonyl complex rac-[Rh2(CO)2( / 7 -L)], was believed to form from the dicationic rac-1 under syngas (CO/H2) conditions with liberation of and norbomadiene. In order to model the intermediacy of this neutral species, Stanley et al. designed the neutral allyl (C3H5) analogue rac-[Rh2(7/ -C3H5)2( / / -L)] (rac-... 

It is tempting to speculate that only for the dinuclear complexes of iridium is there initial formation of a dihydrido-complex, which subsequently reacts with an alkene to form an alkyl intermediate, or with an alkyne to form an alkenyl intermediate. If such is the case, the activity of dinuclear rhodium complexes must depend on initial formation of an alkyne or alkene complex, which would then react with hydrogen. There exists some evidence for such a scheme. The successive hydrogenation of alkynes and alkenes " suggests that activation of an alkene is inhibited by an alkyne, probably by preferential coordination of the latter. Further, complexes (VII, X = H) or (IX) do not alone react with hydrogen, but do so after reaction with an alkyne (acetylene or phenylactylene). ... 

Fig. 6 (a) A homogeneous catalytic system containing exactly two independent unicyclic mechanisms for hydroformylation of a symmetric alkene. X, Y, Z are organometallic present which do not participate in a catalytic mechanism and do not interact with any intermediates. They are just spectator species, (b) The system after addition of a chelated dinuclear rhodium complex which also performs hydroformylation... 

Certain dinuclear Rh(II) carbene complexes react with alkanes to generate products from insertion of the carbene imit into the alkane C-H bond with high diastereo- and enantioselectivity (Equation 6.59). ° These reactions occur by mechanisms distinct from those of the reactions of C-H bonds witti the tungsten alkylidene and alkylid5me complexes just described. The reactions of the dinuclear Rh(II) carbene complexes appear to occur by a mechanism that involves direct reaction of the carbene at the C-H bond without coordination of the alkane and addition across the M=C bond of the carbene. Such rhodium carbene complexes have not been isolated, but the absence of an open coordination site cis to the carbene ligand in the accepted carbene intermediate is thought to preclude initial reaction of tire substrate at the metal center to form a new metal-carbon bond. The catalytic chemistry that occurs via these carbene complexes is presented in more detail in Chapter 18 (catalytic C-H bond functionalization). 

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