No abstract

Herein, we describe the syntheses and structural characterization of bis(carbene)-and tris(carbene)-stabilized organomagnesium cations. The reaction of the N-heterocyclic carbene (NHC) stabilized Grignard reagent ( iPr NHC) 2 Mg(Me)(Br) (1) and Na [BAr F 4 ] ( iPr NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, Ar F = 3,5-bis(trifluoromethyl)phenyl) in chlorobenzene yields exclusively the bis(NHC). These monomeric cations 3[A] and 4[A] (A = BAr F 4 , BPh 4 ) can be independently prepared as single pure products in high yields using common hydrocarbon solvents. The electronic influence of tris(carbene) stabilization is further evidenced by an NHC-mediated ionization of magnesium bromide in the absence of abstraction reagents. The reaction between the sterically unencumbered 1,3,4,5tetramethylimidazol-2-ylidene ( Me NHC) ligand and ( Me NHC) 2 MgBr 2 (7) resulted in two geometrically unique cations of the type [( Me NHC) 3 MgBr][Br]: complex 8a bearing a weakly coordinating bromide anion resulting in a trigonal bipyramidal magnesium center, and complex 8b bearing a noncoordinating bromide anion where the magnesium atom resides in a tetrahedral coordination environment. All isolated complexes were characterized by NMR spectroscopy and single-crystal X-ray diffraction, and their bonding was investigated by density functional theory (DFT).

Durable catalysts based on abundant metals are needed for the photocatalytic CO2 reduction reaction (PCO2RR). Thus, we synthesized a series of low-valent cobalt­(I) complexes, [(CNC)­Co­(CO)2]+[Co­(CO)4]−, with H (1Co‑ ) or OMe (2Co‑ ) in the 4-position of the pyridyl N donor group (where CNC = L1 and L2 from double deprotonation of the [CNC]2+ preligands L1­(HOTf)2 = 1,1′-(pyridine-2,6-diyl)­bis­(3-methyl-1H-imidazol-3-ium) ditriflate and L2­(HOTf)2 = 1,1′-(4-methoxypyridine-2,6-diyl)­bis­(3-methyl-1H-imidazol-3-ium) ditriflate). Anion exchange for [BArF24]− (tetrakis­(3,5-trifluoromethyl)­phenyl)­borate) produced 1 and 2 and phosphine substitution produced 1PMe3 , 1PPh3 , and 2PPh3 complexes with the structure [(CNC)­Co­(CO)­(PR′3)]+[BArF24]−. In 1DPPP , the DPPP ligand bridges two Co­(I) centers (DPPP = 1,3-bis­(diphenylphosphino)­propane). All complexes were fully characterized, and electrochemical measurements suggest that for most of the phosphine complexes, CO2 binding by the complex occurs prior to reduction due to a vacant coordination site. Intriguingly, the introduction of a phosphine ligand resulted in a geometry change from trigonal bipyramidal to square pyramidal which correlates to preassociation of CO2 to the complex and higher reactivity in the PCO2RR. Complexes 1, 1PMe3 , 1PPh3 , 1DPPP , 2, 2PPh3 , and Na­[Co­(CO)4] are PCO2RR catalysts with a methoxy substituent deactivating and a phosphine ligand activating. With monodentate phosphines, catalyst 1PPh3 (1 μM) had the highest turnover frequency (TOFM = 3.9 h–1) and turnover number (TON = 199). The dinuclear 1DPPP complex was the most active and robust catalyst with TON = 278 and TOF = 21.1 h–1 at 1 μM loading. Under dilute conditions (1 nM), 1PPh3 produced up to 36,000 TON with TOF = ∼800 h–1 over 6 days, which shows that this is a durable molecular catalyst acting with fast rates in the PCO2RR. Thus, stabilizing low-valent cobalt can offer a unique entry point to highly active PCO2RR catalysts. While cobalt­(I) has been proposed as a catalytic species, catalysts that start from Co­(I) have not been made previously and the use of phosphine co-ligands has allowed these catalysts to achieve high activity.

The first examples of pyrene-fused Janus-type N-heterocyclic germylenes (NHGe) are reported.

A combined synthetic and theoretical investigation of N-heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valent s-block element. The reaction of (NHC)Mg(N(SiMe 3 ) 2 ) 2 (1) and dimethylamine borane yields the tris(amide) adduct (NHCÀ BN)Mg(NMe 2 BH 3 )(N(SiMe 3 ) 2 ) (2; NHCÀ BN = NHCÀ BH 2 NMe 2 ). In addition to Me 2 N=BH 2 capture at the NHC CÀ Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe 2 BH 2 NMe 2 BH 3 ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe 2 BH 3 ) 2 ( 8) and (NHC)Mg(NMe 2 BH 2 NMe 2 BH 3 ) 2 (9) were synthesized, and a dynamic migration of Me 2 N=BH 2 between MgÀ N and NHC CÀ Mg bonds was observed in 9. This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the À {NMe 2 BH 2 NMe 2 BH 3 } anion, as well as the capacity of NHCs to reversibly capture Me 2 N=BH 2 in the presence of Lewis acidic magnesium species.