Bridging phosphines

Zhang, H.-X. and Che, C.-M. (2001) Aurophilic Attraction and Luminescence of Binuclear Gold(I) Complexes with Bridging Phosphine Ligands ah initio Study. Chemistry - A European Journal, 7, 4887-4893. 

Pan, Q.-J. and Zhang, H.-X. (2003) Aurophilic attraction and excited-state properties of binuclear Au(I) complexes with bridging phosphine and/or thiolate ligands An ah initio study. Journal of Chemical Physics, 119, 4346-4352. 

The schematic (Figure 9) for a drum indicates how it is related to two cluster molecules. Formally, two bridging phosphinates rearrange, two oxygen atoms are added, four Sn-0 bonds are cleaved, the six Sn-O-Sn linkages become Sn(OH)Sn units, and two phosphinates are added to hydrogen bond to the hydroxyls. 

Examples are tending to be more sophisticated and complex in form. For example, a dinuclear complex featuring a bridging phosphinate and phenolate in addition to peroxide (221) has been reported,966 as a model for phosphodiester systems. Apart from dicobalt(III) systems, a mixed-valence CoII,ni di-/i-superoxo complex (222) has been prepared.967 Transition between the three redox states CoII,n, Co11,111, and Co111111 is electrochemically reversible. 

The versatile coordination of metal centers may result in a variable number of bridging phosphinate groups... 

Fig. 10 Schematic representation of the linear chain of poly metal(II) phosphinates, alternating triple and single bridging phosphinate groups, and double bridging phosphi-...

The linear chain of polymeric metal(II) phosphinates consists of metal atoms linked either by alternating single and triple bridging phosphinate groups [79-81], or by double bridging phosphinate groups [82-86] (Fig. 10). 

Fig. 11 Schematic representation of a linear chain of polymeric metal(III) phosphinates with triple bridging phosphinate groups...

Trivalent metal phosphinates have also been prepared. The chain structure was assumed to consist of triple-bridging phosphinate according to Fig. 11. 

The observed first-order rate constants, Kobs (s ) for the reaction of the rhodium] 11) complexes with diazo ester 34 varied over a range of > 10, in which the pivalate catalyst (entry 3) was almost two orders of magnitude faster than any of the other catalysts studied. The carboxamidate catalysts (entries 8-10) were slower than all the carboxy-lates, while the bridged phosphine catalyst (entry 7) behaved like most of the other car-boxy late s. 

Karsch described in 1985 the preparation of the related methylene-bridged phosphine-alane 2 by coupling Me2PCH2Li and Me2AlCl... 

Figure 3 31P and "B NMR data for the methylene-bridged phosphine-boranes 3a-c. 

Figure 6 31P and nB NMR data for the ethylene-bridged phosphine-boranes 40e and... 

Figure 7 3,P and 11B NMR data for the C2-bridged phosphine-boranes 40a-d (top). DFT-optimized structures for 40a (bottom) closed form (left) benefiting from 7t-stacking stabilization, open gauche (middle) and antiperiplanar (right) forms.

Figure 15 31P and lnB NMR data for the benzyl-bridged phosphine-boranes 26a-e. 

Figure 17 Key geometric and NMR data for the alkenyl-bridged phosphine-boranes 22e and 22g.

Fig. 2.26 The cover of the review summarizing our work on the chemistry of dinuclear rhodium complexes with bridging Phosphine, arsine and stibine ligands (from Angew. Chem. Int. Ed. 41, 938-954 (2004), reproduced with permission of Wiley-VCH)...

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

Complexes with bridging bidentate phosphines tend to be dimers or tetramers with bridging phosphines, as in (18-I-IV). Phosphines with a large bite angle, as in (18-1-V), may however form mononuclear chelates. 

Developments in the chemistry of multiply bonded dimetal complexes with bridging phosphine ligands. ... 

One major advantage offered by the dppf ligand in Rh-catalyzed olefin hydroformylation is exemplified in its higher linear aldehyde selectivity when present in a dppf Rh ratio of 1.5 or higher [37,242]. This result leads to the proposed key intermediate of a Rh dimer with both chelating and bridging phosphine in the catalytic cycle. It also confirms the significance of the tris (phosphine) moieties at the point when the aldehyde selectivity is determined, i.e., the step in which the hydride is inserted into the M-olefin bond. This involvement of a dinuclear or tris (phosphine) intermediate appears to differ from the intermediate RhH(CO)(PR 3)z (olefin) (which is converted into the square planar Rh(R)(CO)(PR 3)2 by hydride insertion) commonly accepted for hydroformylation catalyzed by monophosphine complexes. P NMR studies also established the existence of the equilibrium in which the disphosphine can be... 

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