Transformations of Group 7 Carbonyl Complexes: Possible Intermediates in a Homogeneous Syngas Conversion Scheme

Abstract

A variety of C-H and C-C bond forming reactions of group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C 2þ compounds. The metal formyl complexes M(CO) 3 (PPh 3 ) 2 (CHO) (M=Mn, Re) are substantially stabilized by coordination of boranes BX 3 (X = F, C 6 F 5 ) in the form of novel boroxycarbene complexes M(CO) 3 -(PPh 3 ) 2 (CHOBX 3 ), but these boron-stabilized carbenes do not react with hydride sources to undergo further reduction to metal alkyls. The related manganese methoxycarbene cations [Mn(CO) 5-x (PPh 3 ) x (CHOMe)] þ (x=1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atmosphere of CO. The resulting acyls, cis-and trans-Mn(PPh 3 )(CO) 4 (C(O)CH 2 OMe), can be alkylated to form the cationic carbene complex [Mn(PPh 3 )(CO) 4 (C(OR)CH 2 OMe)] þ , which undergoes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or diethyl ether adducts of [Mn(PPh 3 )(CO) 4 ] þ . The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh 3 )(CO) 4 (CH 2 COOMe) and is protonolyzed to yield methyl acetate and [Mn(PPh 3 )(CO) 4 ] þ ; addition of L (L = PPh 3 , CO) to the manganese cation regenerates [Mn(PPh 3 )(CO) 4 (L)] þ . Since the original formyl complex can be obtained by the reaction of [Mn(PPh 3 )(CO) 5 ] þ with [PtH(dmpe) 2 ] þ , which in turn can be generated from H 2 , this set of transformations amounts to a stoichiometric cycle for selectively converting H 2 and CO into a C 2 compound under mild conditions.

Synthesis and Characterization of a Rhenabenzyne Complex

“…The C1-C2, C2-C3, C3-C4, and C4-C5 bond lengths are 1.310(11), 1.337(11), 1.426 (12), and 1.350 (14) , respectively. The Re-C1 bond length of 1.933 (7) is within the range of those reported for typical Re=CHR(carbene) bonds (1.850-2.143 ), [16,17] and shorter than those reported for typical Re-C(vinyl) bonds (1.996-2.305 ). [16,18] The Re-C5 bond length of 2.158 (8) is within the range of those reported for typical Re-C(vinyl) bonds, [16,18] and longer than those reported for typical Re=CHR(carbene) bonds.…”

“…[16,18] The Re-C5 bond length of 2.158 (8) is within the range of those reported for typical Re-C(vinyl) bonds, [16,18] and longer than those reported for typical Re=CHR(carbene) bonds. [16,17] Our preliminary DFT calculations show that the optimized structure agrees well with the structural features of 6 described above (see the Supporting Information for details).…”

“…[16,21] The Re-C5 bond length (2.107 (4) ) is within the range of typical Re-C(vinyl) bond lengths (1.996-2.305 ), [16,18] and in the high end of the reported range for typical Re=CH(carbene) bonds (1.850-2.143 ). [16,17] The structural data indicate that the metallacycle of 7 has a delocalized structure with contributions from resonance structures 7 and 7 a, with 7 being more important.…”

Synthesis and Characterization of Rhenabenzyne Complexes

“…[30,32] The Re À C5 bond length (2.107 (4) ) is within the range of typical Re À CA C H T U N G T R E N N U N G (vinyl) bond lengths (1.996-2.305 ) [30,33] and at the high end of those reported for typical Re=CHRA C H T U N G T R E N N U N G (carbene) bonds (1.850-2.143 ). [30,34] The ring CÀC distances in 8 a are in the range of 1.367-1.457 , which are intermediate between single and double carbon À carbon bonds. Due to the unsymmetrical nature of the rhenabenzyne complex 8 a, it is impossible for the metallacycle to have equal M À C or C À C bond lengths, and alternation in the bond lengths of C À C bonds of the metallacycles was noted.…”

Heterobimetallic Complexes of Rhodium Dibenzotetramethylaza[14]annulene [(tmtaa)Rh-M]: Formation, Structures, and Bond Dissociation Energetics

“…4–17 Several approaches attempting to advance the utilization of this type of metal-formyl fragment in the production of organic oxygenates invoke reactions that occur at the formyl group, such as hydrogenation using a second catalyst. 18,19 Bimetal catalyst systems for alkene hydroformylation that involve intermolecular processes illustrate the efficacy of this type of strategy. 20–22 Heterobimetallic complexes can be anticipated to occur in bimetal catalyst systems as nonproductive traps for active metal catalyst species.…”