Facilitating Constructive Discussions of Difficult Socio-Scientific Issues.

Discussion can be an important and powerful tool in efforts to build a more diverse, equitable, and inclusive future for STEM (i.e., science, technology, engineering, and mathematics). However, facilitating discussions on difficult, complex, and often uncomfortable issues, like racism and sexism, can feel daunting. We outline a series of steps that can be used by educators to facilitate productive discussions that empower everyone to listen, contribute, learn, and ultimately act to transform STEM.

Promoting proton coupled electron transfer in redox catalysts through molecular design.

Most bond-forming and -breaking redox reactions require the concomitant transfer of protons. Unassisted proton movement can result in kinetic and thermodynamic barriers that inhibit the rate of these reactions, leading to slow and/or inefficient catalysis. These barriers can be circumvented by effective proton management through molecular design. Different strategies for managing proton movement are discussed with examples from biological and synthetic systems. As proton management is particularly important in redox reactions for chemical fuel generation and utilization, the focus will be on catalysts for H-H and O-O bond formation and cleavage. However, we expect the approaches discussed herein will be general to most multi-electron, multi-proton reactions.

Crystal structure of NiFe(CO)5[tris(pyridyl-meth-yl)aza-phosphatrane]: a synthetic mimic of the NiFe hydrogenase active site incorporating a pendant pyridine base.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC5H4)CH2]3P(NC2H4)3N} with Fe(CO)5 resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)5 complex di-µ-carbonyl-tricarbon-yl[2,8,9-tris-(pyridin-2-yl-meth-yl)-2,5,8,9-tetra-aza-1-phosphabi-cyclo-[3.3.3]undeca-ne]ironnickel, [FeNi(C24H30N7P)(CO)5]. Characterization of the complex by 1H and 31P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)5 reveals three terminally bound CO mol-ecules on Fe0, two bridging CO mol-ecules between Ni0 and Fe0, and TPAP coordinated to Ni0. The Ni-Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)5 is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol-ecules are linked by C-H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π-π [inter-centroid distance = 3.2739 (5) Å] and C-H⋯π inter-actions present. The layers are linked by further C-H⋯π inter-actions, forming a supra-molecular framework.

Adaptable ligand donor strength: tracking transannular bond interactions in tris(2-pyridylmethyl)-azaphosphatrane (TPAP).

Flexible ligands that can adapt their donor strength have enabled unique reactivity in a wide range of inorganic complexes. Most examples are composed of flexible multi-dentate ligands containing a donor that can vary its interaction through its distance to the metal center. We describe an alternative type of adaptable ligand interaction in the neutral multi-dentate ligand tris(2-pyridylmethyl)-azaphosphatrane (TPAP), which contains a proazaphosphatrane unit. Prozaphosphatranes are intrinsically strong phosphorus donors; upon coordination to more Lewis acidic atoms the phosphorus can accept additional electron density from a tertiary nitrogen to form a transannular bond, increasing its donor strength. An experimental and computational investigation of the varying degree of transannular interaction in TPAP when coordinated to late transition metals is reported. The synthesis and characterization of the complexes M(TPAP), where M = Co(i)Cl, Ni(0)(1,5-cyclooctadiene), Ni(ii)(CH3CN)(BF4)2, Pd(ii)(CH3CN)(BF4)2, or Pt(ii)Cl(PF6) is described. Structural characterization and density functional theory calculation of these complexes, along with the previously reported [Co(ii)(TPAP)(CH3CN)](BF4)2 establish significant increases in the degree of transannular interaction of the proazaphosphatrane unit when coordinated to more electron deficient metal ions.

Electronic and steric Tolman parameters for proazaphosphatranes, the superbase core of the tri(pyridylmethyl)azaphosphatrane (TPAP) ligand.

The Tolman electronic parameters (TEP) and cone angles were experimentally measured for a series of substituted proazaphosphatrane ligands by synthesizing their respective Ni(L(R))(CO)3 complexes, where L = P(RNCH2CH2)3N and R = Me, iPr, iBu and Bz. The complexes Ni(L(Me))(CO)3 (), Ni(L(iPr))(CO)3 (), Ni(L(iBu))(CO)3 () and Ni(L(Bz))(CO)3 () display CO vibrational frequencies (A1 mode) at 2057.0, 2054.6, 2054.9 and 2059.1 cm(-1), respectively. The TEPs for the phosphine ligands in are among the lowest measured, with values close to P(tBu)3 the most donating phosphine measured by Tolman. The cone angles of L(R) measured in are 152, 179, 200 and 207° for R = Me, iPr, iBu and Bz, respectively. The substituted proazaphosphatranes have larger cone angles compared to the analogous trialkyl subsituted monophosphines. Our study demonstrates that while the cone angles have a significant dependence on R, all of the substituted proazaphosphatranes are strong electron donors. Additionally, in order to determine the electronic donor strength of our previously reported multidentate ligand, TPAP, Ni(TPAP)(CO)2 () (TPAP = tris(2-pyridylmethyl)azaphosphatrane) and Ni(L(Me))2(CO)2 () were also synthesized and evaluated in a similar fashion.

Flexibility is Key: Synthesis of a Tripyridylamine (TPA) Congener with a Phosphorus Apical Donor and Coordination to Cobalt(II).

Tripyridylamine (TPA), a tetradentate ligand that forms 5-membered chelate rings upon metal coordination, has demonstrated significant utility in synthetic inorganic chemistry. An analogue with a phosphorus apical donor is a desirable target for tuning electronic structure and enhancing reactivity. However, this congener has been synthetically elusive. Prior attempts have resulted in tridentate coordination to transition metal ions due to a lack of ligand flexibility. Herein, we report the successful synthesis of tris(2-pyridylmethyl)proazaphosphatrane (TPAP), a more accommodating tripyridyl ligand containing an apical phosphorus donor. The TPAP ligand forms 6-membered chelate rings upon coordination and binds in the desired tetradentate fashion to a Co(II) ion. Structural studies elucidate the importance of ligand flexibility in tripodal ligands featuring phosphorus donors. Cyclic voltammetry, UV-vis, and solution magnetic susceptibility experiments of [Co(TPAP)(CH3CN)](2+) are also reported and compared to [Co(TPA)(CH3CN)](2+). Notably, magnetic susceptibility measurements of [Co(TPAP)(CH3CN)](2+) indicate a low spin electronic configuration, in contrast to [Co(TPA)(CH3CN)](2+), which is high spin.

Ligand-based reduction of CO2 and release of CO on iron(II).

A synthetic cycle for the CO(2)-to-CO conversion (with subsequent release of CO) based on iron(II), a redox-active pydridinediimine ligand (PDI), and an O-atom acceptor is reported. This conversion is a passive-type ligand-based reduction, where the electrons for the CO(2) conversion are supplied by the reduced PDI ligand and the ferrous state of the iron is conserved.