Coordination of Unsaturated Molecules to Transition Metals

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

The crucial experiment suggesting that the H2 molecule might act as a dihapto ligand to transition metals was the dramatic observation that toluene solutions of the deep purple coordinatively unsaturated 16-electron complexes [Mo(CO)3(PCy3)2] and [W(CO)3-(PCy3)2l (where Cy = cyclohexyl) react readily and cleanly with Ha (I atm) at low temperatures to precipitate yellow crystals of [M(CO)3H2(PCy3)2] in 85-95% yield. The... 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

The encapsulation of classical and organometallic transition-metal complexes to yield molecules of the type complex in a complex is a very attractive research area. A variety of inclusion complexes of this type has been reported. Of relevance for this review are reports on the encapsulation of coordinatively unsaturated transition-metal complexes inside self-assembled coordination cages (120), cyclodextrins (121,122), and cucurbiturils (123). 

In spite of some declining industrial interest, the last 5 years have seen an unusual academic interest in the catalytic properties of the metal carbonyls. This has been part of a wider surge of interest in the organometallic chemistry of the transition metals and its application to homogeneous catalysis. Reactions such as Ziegler polymerization, the Oxo reaction, and the Wacker process are but a few of the many reactions of unsaturated molecules catalyzed in the coordination sphere of transition metal complexes (20). These coordination catalyses have much in common, and the study of one is often pertinent to the study of the others. 

The excellent coordination properties of alkynes with transition metals led to their use as partners for the coupling with a large variety of unsaturated molecules. Two partners such as alkynes and alkenes can produce various modes of C-C bond formation. Linear or cyclic couplings can occur via different pathways, similar to those reported for two C=C bonds couplings (Scheme 1). 

The reactivity implications of -coordination of unsaturated organic molecules to a transition metal... 

It involves the addition of a covalent molecule to the metal with cleavage of a covalent bond and can be considered a two-electron oxidation of the metal. One of the necessary conditions for this reaction is that the metal complexes to be oxidized should be coordinatively unsaturated. With the transition metal complexes, a saturated coordination number is determined by the configuration of the metal (/-electrons (five-coordination for ds metals, and four-coordination for d ° metals. If the complexes with these dn have less than these coordination numbers, they are said to be coordinatively unsaturated, and oxidative addition reaction may occur. In other words, co-ordinative unsaturation means that there are vacant sites on the complexes. 

Catalysis of the [2+2+2] cycloaddition of alkynes by transition metal complexes has been extensively exploited for the synthesis of complex organic molecules [30-34]. The accepted mechanism for this transformation, shown in Scheme 10, involves coordination of two alkyne molecules to the metal centre followed by oxidative coupling to form the coordinatively unsaturated metallocyclo-pentadiene 49, which can coordinate a third molecule of alkyne to afford 50. Insertion of the alkyne in a metal-carbon bond of this complex leads to met-allocycloheptadiene 51, and reductive elimination then affords cyclotrimer 52 and regenerates the catalytic species. Alternatively, the transformation of 49 into 52 might involve a Diels-Alder reaction giving intermediate 53, followed by reductive elimination [35]. 

Most transition metal ions can be coordinated into a polymeric structure. These metal ions have empty or unsaturated d or f atom orbitals (receptors) that can accept electrons from ligand molecules (donors) to form coordination bonds. The first-row transition metal ions, such as, Fe +, Co +, Ni +, and Zn +, usually form labile coordination bonds with ligands in coordinating solvents such as water [23], whereas those of the second- and third-row transition metal ions often form irreversible coordination bonds [24], There are also exceptions, such as some lanthanoid... 

Not long after transition-metal catalyzed hydrosilation took root in the 1950s as a new and powerful tool for Si-C bond formation, based on parallels drawn from TM-catalyzed hydrogenation and on the intrinsic redox properties of these metals. Chalk and Harrod proposed a simple and yet very logical catalytic cycle for TM-catalyzed hydrosilation (Scheme 1). The key steps of this cycle are (1) oxidative addition of Si-H to the metal center, (2) coordination of the olefin or other unsaturated species to the metal, (3) insertion of this usually-t/ -coordinated molecule into the M-H bond and finally, (4) reductive elimination of the Si-C pair to regenerate the catalytically active metal center. 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

---