Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

Enforcing unusually low coordination numbers on transition metals with sterically demanding ligands has long been an area of interest for chemists. Historically, the synthesis of these challenging molecules has helped to elucidate fundamental principles of bonding and reactivity. More recently, there has been a move towards exploiting these highly reactive complexes to achieve a range of transformations using cheap, earth-abundant metals. In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers that have been used in catalysis and the activation of small molecules featuring strong bonds (N2, CO2, and CO)

Evaluation of Several Molybdenum and Ruthenium Catalysts for the Metathesis Homocoupling of 3-Methyl-1-Butene

We synthesized Mo(NC[subscript 6]F[subscript 5])(CHCMe[subscript 2]Ph)(TPPO)(PPhMe[subscript 2])Cl (TPPO = 2,3,5,6‐tetraphenylphenoxide), Mo(NC[subscript 6]F[subscript 5])(CHCMe[subscript 2]Ph)(TTBTO)(PPhMe[subscript 2])Cl (TTBTO = 2,6‐di(3′,5′‐di‐tert‐butylphenyl)phenoxide), and Mo(NC[subscript 6]F[subscript 5])(CHCMe[subscript 2]Ph)(TPPO)(PPhMe[subscript 2])(CF[subscript 3]Pyr) (CF[subscript 3]Pyr = 3,4‐bistrifluoromethylpyrrolide), in order to evaluate them as catalysts for the homocoupling of 3‐methyl‐1‐butene. They were compared with Mo(NC[subscript 6]F[subscript 5])(CHCMe[subscript 2]Ph)(HMTO)(PPhMe[subscript 2])Cl (HMTO = 2,6‐dimesitylphenoxide), Mo(NC[subscript 6]F[subscript 5])(CHCMe[subscript 2]Ph)(HIPTO)(PPhMe[subscript 2])Cl (HIPTO = 2,6‐di(2′,4′,6′‐triisopropylphenyl)phenoxide), and several other Mo and Ru catalysts. In the best cases turnover numbers (TONs) of 400 – 700 were observed for the homocoupling of 3‐methyl‐1‐butene in a closed vessel (ethylene not removed). Keywords: Olefin metathesis, Homocoupling, Molybdenum, Ruthenium, 3‐Methyl‐1‐buteneNational Science Foundation (U.S.)National Science Foundation (U.S.). Center for Chemical Innovation (CCI Center for Enabling New Technologies through Catalysis. Grant CHE‐1205189

Syntheses of Molybdenum Adamantylimido and t‑Butylimido Alkylidene Chloride Complexes Using HCl and Diphenylmethylphosphine

Reactions between Mo(N-t-Bu)2(CH2-t-Bu)2 or Mo(NAdamantyl)2(CH2CMe2Ph)2 and 3 equiv of HCl in the presence of 1 equiv of PPh2Me yield Mo(NR)(CHR')(PPh2Me)Cl2 complexes, from which Mo(NR)(CHR')(PPh2Me)(OAr)Cl complexes (OAr = a 2,6-terphenoxide) can be prepared. The Mo(NR)(CHR')(PPh2Me)(OAr)Cl complexes were evaluated as cross-metathesis catalysts between cyclooctene and Z-1,2-dichloroethylene. The efficiencies of the test reaction for complexes in which OAr = OTPP, OHMT, OHIPT, or OHTBT (where OTPP is 2,3,5,6-tetraphenylphenoxide, OHMT is hexamethylterphenoxide, OHIPT is hexaisopropylterphenoxide, and OHTBT is hexa-t-butylterphenoxide) maximize when OAr is OHMT or OHIPT. Mo(N-t-Bu)(CH-t-Bu)(PPh2Me)Cl2 is essentially inactive for the reaction between cyclooctene and Z-1,2-dichloroethylene. X-ray structural studies were carried out on Mo(NAd)(CHCMe2Ph)(PPh2Me)Cl2, Mo(N-t-Bu)(CH-t-Bu)(PPh2Me)(OHMT)Cl, Mo(NAd)(CHCMe2Ph)(Cl)(OHTBT)(PMe3), and [Mo(NAd)(CHCMe2Ph)(PMe3)(Cl)]2(μ-O), the product of the reaction between Mo(NAd)(CHCMe2Ph)(Cl)(OHTBT)(PMe3) and 0.5 equiv of water

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex

Addition of one equiv of water to Mo(CAr)[OCMe(CF 3 ) 2 ] 3 (1,2 - dimethoxyethane) ( 2, Ar = o-(OMe)C 6 H 4 ) in the presence of PPhMe 2 leads to formation of Mo(O)(CHAr)[OCMe(CF 3 ) 2 ] 2 (PPhMe 2 ) ( 3(PPhMe 2 ) ) in 3 4 % yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, { Mo(CAr)[OCMe(CF 3 ) 2 ] 2 ( [mu]-OH)}2(dme) ( 4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an aryl methoxy group. Addition of PMe 3 to 4(dme) gives the alkylidene oxo complex, (3(PMe 3)), ananalog of 3(PPhMe 2)(95% conversion, 66% isolated). Treatment of 3(PMe 3) with two equiv of HCl gave Mo(O)(CHAr)Cl 2 (PMe 3)(5), which upon addition of LiO-2,6-(2,4,6-i-Pr 3 C 6 H 2) 2C 6 H 3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe 3)(6). Compound 6 in the presence of B(C6F5)3 will initiate the ring - opening metathesis polymeri- zation of cyclooctene, 5,6 - dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocouplin g of 1-decene to 9-octa decene. The poly(DCMNBD) has a cis,syndiotactic structure, whereas poly(DCMNBE) has a cis,syndiotactic,alt structure. X - ray structures were obtained for 3(PPhMe 2 ), 4(dme), and 6.National Institutes of Health (U.S.) (GM-58426)National Science Foundation (U.S.) (CHE-0946721