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1. Hexahapto-lanthanide interconnects between the conjugated surfaces of single-walled carbon nanotubes

   https://doi.org/10.1039/c3dt53291g  

   Moser, Matthew L.; Tian, Xiaojuan; Pekker, Aron; Sarkar, Santanu; Bekyarova, Elena; Itkis, Mikhail E.; Haddon, Robert C. (January 2014, Dalton Trans.)

   We report the response of the electrical conductivity of semiconducting single-walled carbon nanotube (SWNT) thin films on exposure to metal vapors of the early lanthanides under high vacuum conditions. We attribute the strongly enhanced conductivities observed on deposition of samarium and europium to charge transfer from the metals to the SWNT backbone, thereby leading to the first examples of mixed covalent–ionic bis-hexahapto bonds [(η 6 -SWNT)M(η 6 -SWNT), where M = Sm, Eu]. 

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2. Higher spin states in some low-energy bis(tetramethyl-1,2-diaza-3,5-diborolyl) sandwich compounds of the first row transition metals: boraza analogues of the metallocenes

   https://doi.org/10.1039/C8NJ05517C  

   Chen, Jianlin; Feng, Hao; Xie, Yaoming; King, R. Bruce; Schaefer, Henry F. (March 2019, New Journal of Chemistry)

   The known sandwich compound [η 5 -(CH 2 ) 3 N 2 (BPh) 2 CMe] 2 Fe in which adjacent C 2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B 3 N 3 H 6 , considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me 4 B 2 N 2 CH) 2 M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me 4 C 5 H) 2 M. Low-energy sandwich structures having parallel B 2 N 2 C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η 5 -Me 4 B 2 N 2 CH) 2 M sandwich compounds relative to the corresponding metallocenes (η 5 -Me 4 C 5 H) 2 M. Thus (η 5 -Me 4 B 2 N 2 CH) 2 Cr has a quintet ground state in contrast to the triplet ground state of (η 5 -Me 4 C 5 H) 2 Cr. Similarly, the sextet ground state of (η 5 -Me 4 B 2 N 2 CH) 2 Mn lies ∼18 kcal mol −1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me 4 C 5 H) 2 Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η 5 -(CH 2 ) 3 N 2 (BPh) 2 CMe] 2 Fe. 

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3. Inverse potential scaling in co-electrocatalytic activity for CO ₂ reduction through redox mediator tuning and catalyst design

   https://doi.org/10.1039/D2SC03258A  

   Reid, Amelia G.; Moreno, Juan J.; Hooe, Shelby L.; Baugh, Kira R.; Thomas, Isobel H.; Dickie, Diane A.; Machan, Charles W. (August 2022, Chemical Science)

   Electrocatalytic CO 2 reduction is an attractive strategy to mitigate the continuous rise in atmospheric CO 2 concentrations and generate value-added chemical products. A possible strategy to increase the activity of molecular systems for these reactions is the co-catalytic use of redox mediators (RMs), which direct reducing equivalents from the electrode surface to the active site. Recently, we demonstrated that a sulfone-based RM could trigger co-electrocatalytic CO 2 reduction via an inner-sphere mechanism under aprotic conditions. Here, we provide support for inner-sphere cooperativity under protic conditions by synthetically modulating the mediator to increase activity at lower overpotentials (inverse potential scaling). Furthermore, we show that both the intrinsic and co-catalytic performance of the Cr-centered catalyst can be enhanced by ligand design. By tuning both the Cr-centered catalyst and RM appropriately, an optimized co-electrocatalytic system with quantitative selectivity for CO at an overpotential ( η ) of 280 mV and turnover frequency (TOF) of 194 s −1 is obtained, representing a three-fold increase in co-catalytic activity at 130 mV lower overpotential than our original report. Importantly, this work lays the foundation of a powerful tool for developing co-catalytic systems for homogeneous electrochemical reactions. 

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4. Dianionic Mononuclear Cyclo ‐P ₄ Complexes of Zero‐Valent Molybdenum: Coordination of the Cyclo ‐P ₄ Dianion in the Absence of Intramolecular Charge Transfer

   https://doi.org/10.1002/anie.201908885  

   Mandla, Kyle A.; Neville, Michael L.; Moore, Curtis E.; Rheingold, Arnold L.; Figueroa, Joshua S. (September 2019, Angewandte Chemie International Edition)

   Abstract Relative to other cyclic poly‐phosphorus species (that is,cyclo‐P_n), the planarcyclo‐P₄group is unique in its requirement of two additional electrons to achieve aromaticity. These electrons are supplied from one or more metal centers. However, the degree of charge transfer is dependent on the nature of the metal fragment. Unique examples of dianionic mononuclear η⁴‐P₄complexes are presented that can be viewed as the simple coordination of the [cyclo‐P₄]²⁻dianion to a neutral metal fragment. Treatment of the neutral, molybdenumcyclo‐P₄complexes Mo(η⁴‐P₄)I₂(CO)(CNAr^Dipp2)₂and Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)₂with KC₈produces the dianionic, three‐legged piano stool complexes, [Mo(η⁴‐P₄)(CO)(CNAr^Dipp2)₂]²⁻and [Mo(η⁴‐P₄)(CO)₂(CNAr^Dipp2)]²⁻, respectively. Structural, spectroscopic, and computational studies reveal a similarity to the classic η⁶‐benzene complex (η⁶‐C₆H₆)Mo(CO)₃regarding the metal‐center valence state and electronic population of the planar‐cyclic ligand π system. 

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5. Benchmarking the Fluxional Processes of Organometallic Piano-Stool Complexes

   https://doi.org/10.3390/molecules26082310  

   Frey, Nathan C.; Dornshuld, Eric Van; Webster, Charles Edwin (April 2021, Molecules)

   The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)3) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)3 (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)3 were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)3 complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)3 were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)3. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol−1. Notably, PBE0/BS1 describes the (CO)3 rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)3 complexes (except for (TMCOT)W(CO)3), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)3 complex (computed with the PBE0 functional) has a ΔG‡ of 12.6, 12.8, and 13.2 kcal mol−1 for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed 1H-NMR and 13C-NMR chemical shifts for (TMCOT)Cr(CO)3 and (TMCOT)Mo(CO)3 at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures. 

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