Open coordination site

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

The impact of water on the differential behavior of the triflate and SbF6 catalysts has led to the postulate that catalyst turnover is facilitated by an open coordination site (apical) in four coordinate cationic Cu(II) complexes (200). Presumably, additional dienophile induces turnover through associative displacement of the neutral oxygen ligands (water or product). This rationale may explain why the triflate complex 266c is inactive as a catalyst prior to activation by dehydration. 

Many of these systems employ charged polymers or polyelectrolytes that confer on them particular properties due to the existence of electrical charges in the polymer structure. Oyama and Anson [14,15] introduced polyelectrolytes at electrode surfaces by using poly(vinylpiridine), PVP, and poly-(acrylonitrile) to coordinate metal complexes via the pyridines or nitrile groups pending from the polymer backbone. Thomas Meyer s group at North Carolina [16, 17[ also employed poly(vinylpyridine) to coordinate Ru, Os, Re and other transition-metal complexes by generating an open coordination site on the precursor-metal complex. 

Our collective mechanistic studies are consistent with the indicated catalytic cycle. Notably, the catalyst engages primary alcohols in rapid and reversible dehydrogenation, yet the coupling products, which are homoallylic alcohols, are not subject to oxidation as coordination of the homoallylic olefin to the catalyst provides a hexa-coordinate 18-electron complex lacking an open coordination site for p-hydride elimination (Scheme 14). 

In the met-Hr structure the two iron atoms differ in coordination number. Fe2 is pentacoordinate, yet has bond angles typical of octahedral iron with an open coordination site. This is in distinction to the pentavalent mononuclear iron proteins, which are better described as... 

Basdo etall00 found that the attachment of Fe(II)(TPP) to a rigid modified silica gel support produced an efficient oxygen carrier. The silica gel used contained a 3-imidazolylpropyl group bonded to the surface atoms of silicon. This then reacted with Fe(lI)(TPP)(B)2, and the axial base was removed by heating the silica gel. The five-coordinate Fe(II)(TPP) complex 41 was prepared. The open coordination site could reversibly bind molecular oxygen. It was concluded that... 

It is important to realize that for the inner-sphere or atom transfer pathways in reactions (4), (10) or (12) to appear at the expense of accessible outer-sphere pathways, there are heightened mechanistic demands on the reactants which must be met. For the cases cited there is a necessity for the substitutional lability of Cr11 in reaction (4), the existence of an open coordination site in Co CNJs3- in reaction (10), or the availability of the RuIV=0 group to act as an electron-proton acceptor in reaction (12). 

Like electrophilic addition to diazo compounds [7] from which diazonium ions and, subsequently, carbocations are generated, transition-metal compounds that can act as Lewis acids are potentially effective catalysts for metal carbene transformations. These compounds possess an open coordination site that allows the formation of a diazo carbon-metal bond with a diazo compound and, after loss of dinitrogen, affords a metal carbene (Scheme 5.2). 

Lewis bases (B ) that can occupy the open coordination site inhibit catalytic activity. The electrophilic nature of the metal carbene is seen in its subsequent reactions with nucleophiles (S ), which occur with the transfer of the carbene entity from the metal to a nucleophile without ever having generated or transformed an actual free carbene. 

Throughout this chapter, waters of coordination are omitted for clarity. However, the reader should be aware that any open coordination site on iron will be occupied by water. Iron typically has six coordination sites. 

The work on biomimetic models for [NiFe] and [FeFe] hydrogenase has been described in several review articles [15b 158]. In this work, many of the structural features important for proper function found in the native systems have been successfully incorporated for example, the bimetallic Ni-Fe or Fe-Fe core with rather short metal-metal distances and an open coordination site at one metal, the sulfur-rich environment (terminal and bridging thiolate ligands), CO/CN ligation of the iron(s), and the incorporation of a base for acceptance of the proton and, more recently, of hydride bridges. Still-existing problems of many model systems are the O2 sensitivity, the high overpotentials, and lack of activity (low turnover rates). 

FeFe-enzyme - proton or hydrogen substrate binding and also the hydride-proton reaction exclusively occurs at the iron distal to the [4Fe-4S] cluster, suggesting that mononuclear iron complexes might also be viable catalysts. Consequently, Ott and coworkers have synthesized and characterized some stable pentacoordinated Fe(II) complexes with five ligands that nicely mimic the native ones and exhibit an open coordination site [163, 164]. This approach avoids the formation of the less reactive bridging hydrides that are found in the dinuclear complexes [153]. Catalytic H2 formation from weak acids at low overpotentials with promising TOF and catalyst stability could be demonstrated [164]. 

How many open coordination sites are optimal to produce catalyt-ically active lanthanide complexes In the solid state (45), the... 

Most of the titanium metallocenes described above by Canned (128) formally start out as Ti(IV) however, he points out that the production of vinylcyclobutane is a general property of reduced organotitanium complexes having open coordination sites. In all probability, the catalysis is effected by low-valent titanium metallocenes which are produced during the reaction. 

In the most favored conformation, one side of the double bond has to be effectively blocked by the chiral auxiliary to allow an unambiguous attack of the reagent. By choosing the reaction conditions appropriately, the orientation of the carbonyl group can be influenced. Addition of a Lewis acid with two open coordination sites (e.g. TiCl4 or EtAlCl,) results in the formation of a chelate 4. It is important to note that the two oxygen atoms connected to sulfur are not equivalent but that one is positioned pseu-... 

Syn-, anti- and acetate aldol derivatives can be synthesized by choosing appropriate enolization protocols (Scheme 5) [20]. With lithium, boron and tin Lewis acids,. tyn-aldols can be obtained via (Z)-enolates [21]. If enolization is carried out with lithium or tin, there are enough open coordination sites available to position the aldehyde and the enolate in accordance with the chelate model for the sultam auxiliary and with the Zim-mermann-Traxler model. The combination of these models predicts the formation of 22, which is indeed experimentally obtained. If Lewis acids with only two open coordination sites are used... 

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