With carbodiphosphoranes

Theoretical Studies of Metal Complexes with Carbodiphosphoranes and Related Ligands... 

The number of theoretical investigations of transition metal complexes with carbodiphosphoranes and related divalent carbon(O) ligands is rather small. Quantum chemical calculations of the nickel complexes (CO) Ni-C(PPh3)2 with n = 2, 3 have been pubhshed together with experimental work which describes the S3mthesis and X-ray structure analyses of the compounds [107]. The first systematic... 

Carbodiphosphoranes (R3P = C = PR3) are known,79 but ylides with a P-H bond are rare.80 Therefore, the spectroscopic characterization of 77 was unexpected. Even more surprising was the characterization of the carbodiphosphorane 79 featuring two P-H bonds.31 This compound, prepared by treatment of 2d with tert-butyllithium, rearranged in solution at room temperature over a period of 16 h to afford the phosphorus ylide 80 with one remaining P-H bond. This compound was also unstable and transformed completely into the diphosphinomethane 81 overnight. Note that calculations for the model compounds where R = NH2 predicted 79 to be 28 kcal/mol less stable than 80, which is also 34 kcal/mol above 81.16 The surprising stability of 79 and 80 is probably due to the presence of bulky substituents, since tetracoordinate phosphorus atoms can more readily accommodate the increased steric constraints than can their tricoordinate counterparts. 

In a less highlighted, but - perhaps - not less promising, manner, the story proceeded with the first insights into the organometallic chemistry of yUde and related carbodiphosphorane ligands, involving a pure simple metal-carbon bond (M-CR jP lls) [8-13]. 

Abstract The theoretical and experimental research on carbodiphosphoranes C(PR3)2 and related compounds CL2, both as free molecules and as ligands in transition metal complexes, is reviewed. Carbodiphosphoranes are examples of divalent carbon(O) compounds CL2 which have peculiar donor properties that are due to the fact that the central carbon atom has two lone electron pairs. The bonding situation is best described in terms of L C L donor acceptor interactions which distinguishes CL2 compounds (carbones) from divalent carbon(ll) compounds (carbenes) through the number of lone electron pairs. The stmctures and stabilities of transition metal complexes with ligands CL2 can be understood and predictions can be made considering the double donor ability of the carbone compounds. 

Transition Metal Complexes with Cyclic Carbodiphosphoranes. 82... 

Carbodiphosphoranes C(PR3)2 and related ligands CL2 which are also termed double ylides differ from the other carbon ligands which are discussed in this volume in the number of lone electron pairs at the carbon donor atom. Ylides, carbenes, allenylidenes, and cumulenylidenes have one lone electron pair but carbodiphosphoranes have two lone electron pairs with a and n symmetry. The bonding situation in a carbodiphosphorane (a special class of CL2 compounds, abbreviated as CDP) is best described in terms of donor-acceptor bonding between the phosphane ligands which serve as a donor and a naked carbon atom in an... 

Fig. 1 (a) Schematic representation of the bonding situation in carbodiphosphoranes. (b) Schematic representation of the donor-acceptor interaction in a divalent C(0) compound CL2 between a carbon atom in the electronic D state with the electron configuration 2pJ 2s°... 

The theoretical studies of carbodiphosphoranes and related divalent carbon(O) compounds suggest that the CL2 species should be very strong c donors but they should also possess a decent n donor strength. The theoretical and experimental investigations which are discussed below agree with the classification. 

A series of heteroatom substituted carbodiphosphoranes C PR2ECH(CF3)2 2 have been prepared in the last 10 years by various groups as shown in Fig. 6. The main synthetic approach consists of the reaction of hexafluoroacetone or thioacetone with the related diphosphines R2P-CH2-PR2 [25, 26]. The bent structure with a P-C-P angle (140°) confirms the double ylidic nature [27] and a related chemistry to C(PPh3)2 is expected however, no reports about coordination activities were reported so far. Theoretically, double alkylation at the heteroatoms of the dianion in Fig. 9 would lead to the substituted carbodiphosphoranes. The amino derivative C(P NMe2 3)2 has a linear structure and was not investigated further [28]. 

The carbodicarbenes C(NHC)2 constitute a new class of compounds, first theoretically predicted [8, 9] and shortly after synthesized and characterized by X-ray analyses [10, 11]. Carbon(O) atoms stabilized by two NHC ligands behave similar to carbodiphosphoranes and are also equipped with HOMO and HOMO-1 orbitals of a and n symmetry. The first carbodicarbene was prepared in a two-step reaction as depicted in Fig. 14. 

The majority of transition metal complexes [M]-CL2 are known with C(PPhs)2 as ligand CL2 while the coordination ability of other symmetrical carbodiphosphoranes remains unexplored as yet. General preparation methods to introduce this ligand consists in a reaction between the free double ylide and a transition metal complex with a labile bonded ligand or compounds with a vacant coordination site. 

New outstanding results were obtained in the preparation of compounds with either two C(PPh3)2 ligands at the same metal (Fig. 22) or of a complex with one carbodiphosphorane coordinated to two metal fragments as shown in Fig. 23. 

Unusual ortho metallation of one or two phenyl groups of C(PPh3)2 is observed by reacting with some second and third row group 9 and 10 transition metal compounds. As mentioned before, transition metal precursors were compounds with either vacant coordination sites or easily removable ligands. Doubly ortho metalated compounds can also be seen as pincer carbodiphosphoranes. 

Treatment of 77 with one equivalent of [AuCl(tht)] (tht = tetrahydrothiophene) leads to the type III AuCl bridged pincer complex 79 in high yield (counterion is Cl ) [149]. The unique complex 79 is the only heterodimetallic compound with two metal atoms attached at a carbodiphosphorane carbon(O) atom. The environment at the central carbon atom is planar in 77 and pyramidal in 78 and 79. P-C bond lengths and P-C-P angles in the dication 78 are closely related to those in the neutral Pt complex 36 (Fig. 25). The two metals of 79 attached to the carbon atom are coimected via a very short d -d ° pseudo closed shell interaction with a Pd-Au bond length of 2.8900(3) A a similar bonding situation (aurophilic attraction) is found in the gold complex 29 shown in Fig. 23. 

Transition metal complexes of coordination mode I with the cyclic carbodiphosphorane shown in Fig. 13 were obtained upon reacting the ligand with CuCl (46), [AuCl(SMe)2] (Cl exchange by Ot-Bu gives (47) and (48)) [138], [ClRh(CO)2]2 (49), [ClRh(nbd)]2 (50), or [ClPd(aUyl)]2 (51) [39 2]. A collection of the metal complexes is presented in Fig. 29. 

Carbodiphosphoranes of type 89 [209] have been known for many years. The bonding parameters and the reactivity indicate that the electronic situation in these compounds is reasonably described by the polar structure 89 with two electron pairs at the central carbon atom, although alternative representations (89" and 89 ") are also possible (Fig. 28) [210]. 

A rare example of this type is the metallacycle 4 which is formed by reaction of the carbodiphosphorane Ph3P=C=PPh3 with (Ph3P)2Pt CH2=CH2 <73JOM(47)391>. 

Cyclic double ylids with aliphatic ring members occur solely as carbodiphosphoranes (24, 25). The related benzo-heterocycle exists as a 1,3-bis-ylid IQ, however, and is easily transformed into a cyclic diphosphonium-triple-ylid 1A (26). 

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