Chromium tetracarbonyl complex

Irradiation of the butadiene chromium tetracarbonyl complex again results in the loss of a carbonyl ligand as the primary photoprocess, and the formation of the tricarbonyl complex Cr(CO)3(j/ -C4H6) (Ref. 128) ... 

Scheme 12 S5mthesis of tetracarbonyl-chromium allenylidene complexes 48...

Some important reactions of chromium hexacarbonyl involve partial or total replacements of CO ligands by organic moieties. For example, with pyridine (py) and other organic bases, in the presence of UV hght or heat, it forms various pyridine-carbonyl complexes, such as (py)Cr(CO)5, (py)2Cr(CO)4, (py)3Cr(CO)3, etc. With aromatics (ar), it forms complexes of the type, (ar)Cr(CO)3. Reaction with potassium iodide in diglyme produces a potassium diglyme salt of chromium tetracarbonyl iodide anion. The probable structure of this salt is [K(diglyme)3][Cr(CO)4lj. 

The four-membered diazadiboretidine ring system has been formed by several routes including replacement of Sn with B in diazadistannetidines (144), thermolysis of diarylazidoboranes (145), reductive elimination of S from six-membered l,4-dithia-2,6-diaza-3,5-diborinane rings (146), and generally by dimerization of iminoboranes, which are intermediates formed by thermal elimination of chlorotrimethylsilane from aminoboranes as in equation 39 (147,148). Diazadiboretidines act as 4-electron donors forming chromium and tungsten tetracarbonyl complexes (149). 

E. O. Fischer, G. Kreis, C. G. Kreiter, J. Muller, G. Huttner, and H. Lorenz, trans-Halo[alkyl(aryl)carbyne]tetracarbonyl Complexes of Chromium, Molybdenum and Tungsten. A New Complex Type with Transition Metal-Carbon Triple Bond, Angew. Chem. Int. Ed. Engl. 12, 564-565 (1973). 

Only one example of a chelated tetracarbonyl chromium-containing complex has been reported during the period under review <1997T17297>. Molybdenum- and tungsten-containing complexes have also been studied. 

An efficient method for the formal abstraction of oxide from acyl ligands was developed in our laboratory 44). Oxalyl halides react directly with the pentacarbonylmetal acyl complexes of chromium, molybdenum, and tungsten to form the fra ,s-alkylidyne(halo)tetracarbonyl complexes [Eq. (5)]. Other suitable Lewis acids are COCI2, CI3COCI, CI3COCOCI, and... 

Although it is believed that the major photochemical reaction is loss of a carbonyl ligand trans to the alkene, an alternative photoprocess could involve conversion of the complex into a monoene chromium tetracarbonyl species/ ... 

There is also evidence for the intermediacy of alkene chromium carbonyl complexes in alkene hydrogenation. An example are the intermediacy of tranj,rrchromium tetracarbonyl in the photocatalytic hydrogenation of tronj,tra/tf-2,4-hexadiene. Substituted chromium tricarbonyl complexes that have ligands that can be replaced by alkenes also act as homogeneous... 

Similar pyrone complexes were isolated by Semmelhack97a as the products of the reaction between tetracarbonyl[ethoxy(alkyl)carbene]iron(0) complexes and various acetylenes. Vinylketene complexes are proposed as key intermediates in the mechanism of this conversion, which closely matches analogous reactions with cobalt carbenes51 (see Section V,B), while showing crucial differences with the analogous reaction of a chromium carbene (see Section II,B). 

Both conjugated and nonconjugated olefins form complexes with the transition-metal carbonyls. Despite the fact that the first known complex, Zeises salt K(PtC2H4Cl3), discovered in 1827, was that of a simple olefin, complexes of monoolefins are rather limited in number. However, nonconjugated diolefins (L) react with group-VI carbonyls to form complexes of the type LM(CO)4 an example is provided by tetracarbonyl-bicyclo-(2,2, l)hepta-2,5-diene chromium (2) (Fig. 1). In contrast, the iron carbonyls... 

Upon UV irradiation in hydrocarbon solution, the hexacarbonyls of chromium, molybdenum, and tungsten react differently with conjugated dienes like 1,3-butadiene (la), ( )-l,3-pentadiene (lb), 2-methyl-1,3-butadiene (lc), ( , )-2,4-hexadiene (Id), ( )-2-methyl-l,3-pentadiene (le), 2-ethyl-1,3-butadiene (If), or 1,3-cyclohexadiene (Ig). Chromium hexacarbonyl (2) yields, with the acyclic dienes la-lf, tetracarbonyl-r/2-dienechromium(0) complexes (3a-3f) in a smooth reaction (8-10). With 1,3-cyclohexadiene, in addition to 3g, dicarbonylbis(>/4-l,3-cyclohexadiene)chromium(0) (4g) is obtained [Eqs. (7) and (8)j. During chromatography on silica gel, the 1,3-cyclohexadiene complex 3g dismutates readily to [Cr(CO)6] and 4g [Eq. (9)]. Under the same conditions with 2 1,3-cyclopentadiene (lh) yields, in a hydrogen-transfer reaction, the stable dicarbonyl- / 5-cyclopentadienyl-r/ 3-cyclopent-enylchromium (5) (11-13) [Eq. (10)]. 

Insertion of the alkyne into the chromium carbene bond in intermediate B affords vinyl carbene complex D, in which the C=C double bond may be either (Z) or (E). A putative chromacydobutene intermediate resulting from a [2+2] cydoaddition of the alkyne across the metal-carbene bond on the way to chromium vinylcarbene D, as was sometimes suggested in early mechanistic discussions, has been characterized as a high energy spedes on the basis of theoretical calculations [9c]. Its formation and ring-opening cannot compete with the direct insertion path of the alkyne into the chromium-carbene bond. An example of an (E)-D alkyne insertion product has been isolated as the decarbonylation product of a tetracarbonyl chromahexatriene (4, Scheme 4) [14], and has been characterized by NMR spectroscopy and X-ray analysis. 

The stepwise coupling of two cis ligands as depicted in Scheme 3 has been verified as involving a sequence of three discrete steps at low temperatures, allowing the isolation of the relevant intermediates as individual compounds [18]. When a chelated tetracarbonyl amino-vinyl carbene complex (chelated analogue of intermediate B in Scheme 3) was reacted with an electron-deficient alkyne under controlled conditions, a l,4,5- 3-dienylcarbene tetracarbonyl chromium complex (corresponding to intermediate D in Scheme 3) was formed. It underwent thermal decomposition to give phenol derivatives as the final products. 

Richard followed the course of the reactions of Cr(CO)6 and Mo(CO)6 with hexamethylborazine by UV-vis spectroscopy but although he observed that the intensity of the absorption maximum of the hexacarbonyls at around 290 nm decreased and a new band at around 350 nm appeared, he was unable to identify the new product. Experiments with other starting materials such as norborna-diene chromium and molybdenum tetracarbonyl or tris(aniline) molybdenum tricarbonyl which readily react with arenes by ligand exchange, also failed. The key to success was to use tris(acetonitrile) chromium tricarbonyl as the precursor, which in dioxan under reduced pressure afforded the desired hexamethylborazine chromium tricarbonyl as a stable crystalline solid in 90% yield. This was the breakthrough and, after we had communicated the synthesis and spectroscopic data of the complex in the January issue 1967 of Angewandte Chemie, Richard finished his work and defended his Ph.D. thesis in June 1967. Six months before, in December 1966,1 defended my Habilitation thesis in front of the faculty and became Privatdozent (lecturer) on the 1st of January 1967. 

Insertion of isocyanide carbon atoms into the Cr—carbene bond of [(CO)5CrC(OMe)Me] gave aziridinylcarbene complexes (CIV), some reactions of which are summarized in Scheme 2 28, 198). Cyclic carbene groups (CV)-(CVIII), in which the carbene carbon atom is part of an aromatic six-electron Tr-system, have been reported to form pentacarbonyl chromium and tetracarbonyl iron complexes 383, 384). Related to carbene... 

Cr, Mo and W Fe, Ru and Os The reaction of norbomadiene(tetracarbonyl)-chromium and -tungsten with aryUithium reagents and subsequent alkylation afforded the dialkene-caibene complexes [M( n -C7Hg)(CO)3(=C(OEt)Ar)], stmcturally characterised for M = Cr and Ar = 4-CH3C6H4. 

Fischer obtained tetracarbonyl(diethylaminocarbyne)triphenylstannylchromium by spontaneous rearrangement of the carbene ligand within the following chromium complex ... 

Treatment of the carbene complex [Cr C(OMe)Ph (CO)6] with certain alkenes yields cyclopropanes by transfer of the carbene entity. The reaction of diethyl fumarate with (- )-(/ )-methyIphenylpropylphosphine(phenylmethoxycarbene)tetracarbonyl-chromium produces the optically active cyclopropane (19), the formation of which demonstrates that no free carbene is involved in the mechanism for the reaction. ... 

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