Spin-state diversity in a series of Co(ii) PNP pincer bromide complexes

Abstract

We describe the structural and electronic impacts of modifying the bridging atom in a family of Co(ii) pincer complexes with the formula Co(t-Bu)PPyP(t-Bu)Br (Py = pyridine, E = CH, NH, and O for compounds 1-3, respectively). Structural characterization by single crystal X-ray diffraction indicates that compounds 1 and 3 are 5-coordinate complexes with both bromides bound to the Co(ii) ion, while compound 2 is square planar with one bromide in the outer coordination sphere. The reduction potentials of 1-3, characterized by cyclic voltammetry, are consistent with the increasing electron-withdrawing character of the pincer ligand as the linker (E) between the pyridine and phosphine arms becomes more electronegative. Magnetic property studies of compounds 1 and 2 confirm high- and low-spin behavior, respectively, through a broad temperature range. However, complex 3 features an unusual combination of high spin S = 3/2 Co(ii) and temperature dependent spin-crossover between S = 3/2 and S = 1/2 states. The different magnetic behaviors observed among the three CoBr pincer complexes reflects the importance of small ligand perturbations on overall coordination geometry and resulting spin state properties.

Electro‐ and Photocatalytic Generation of H2 Using a Distinctive CoII “PN3P” Pincer Supported Complex with Water or Saturated Saline as a Hydrogen Source

“…[54] In the case of 1,t he two molecules gave values of t = 0.03 and 0.003 confirming our geometric assignment and defining the basal plane as consisting of the tridentate PN 3 [50,55] In contrast, the structural report fort he 2,6-di(di-t butylphosphinomethyl)pyridine complex [Co(k 3 -2,6-{tBu 2 PCH 2 } 2 -(NC 5 H 3 )Br 2 ]c haracterized the geometry as being between trigonal bipyramidala nd square-based pyramidal. [55] The electronic structure of 1 was examined with DFTcomputations (B3LYP/def2-TZVP) and the optimizedm olecular structure ( Figure S2, Ta bles S4, S5) wasa ne xcellent match to the experimentally determined structure.I nt erms of frontier orbitals, the distorted square pyramidal geometry aroundt he low spin (S = 1 = 2 )C o II center was dominated by p*i nteractionsw ith the Br lone electron pairs and gave the anticipated d-orbital splitting array with an electron configuration of d xy The cyclic voltammogram (CV) for 1 (1.0 mm), obtained under N 2 in CH 3 CN with 0.1 m tetrabutylammoniumhexafluorophosphate (TBAHFP) supporting electrolyte at 100 mV s À1 using ag lassy carbon (GC) working electrode, displayed three reduction processes at À1.19 V, À1.90 Vand À2.10 Vv s. Fc/Fc + Figure 1A (Figure S7). The first reduction wave was reversible (Figure S8) and all three of these reduction peaks showed al inear dependence of peak currento nt he square root of the scan rate (n 1/2 )t hus indicatingf reely diffusing species( Figure S9 Addition of 1mLo fw ater to solutionso f1 resulted in an increase in the current, i,a tÀ1.9 Vc haracteristic of ac atalytic process ( Figure 1B,S 14).…”

“…[50][51][52][53] Single crystalso f1 were obtained from CH 2 Cl 2 and X-ray diffraction analysisr evealed two independentm olecules in the structure with distorted square-based pyramidal geometry at Co and with nearly identical bondingp arameters as shown in Scheme 1( Figure S1, Table S2, S3). [50,55] In contrast, the structural report fort he 2,6-di(di-t butylphosphinomethyl)pyridine complex [Co(k 3 -2,6-{tBu 2 PCH 2 } 2 -(NC 5 H 3 )Br 2 ]c haracterized the geometry as being between trigonal bipyramidala nd square-based pyramidal. [54] In the case of 1,t he two molecules gave values of t = 0.03 and 0.003 confirming our geometric assignment and defining the basal plane as consisting of the tridentate PN 3 P liganda nd one of the bromo ligands( Br bas ).…”

“…As imple metric,t he "tau" parameter,can be used to categorize fivecoordinate structures as trigonal bipyramidal (t = 1) or square-based pyramid (t = 0). [55] The electronic structure of 1 was examined with DFTcomputations (B3LYP/def2-TZVP) and the optimizedm olecular structure ( Figure S2, Ta bles S4, S5) wasa ne xcellent match to the experimentally determined structure.I nt erms of frontier orbitals, the distorted square pyramidal geometry aroundt he low spin (S = 1 = 2 )C o II center was dominated by p*i nteractionsw ith the Br lone electron pairs and gave the anticipated d-orbital splitting array with an electron configuration of d xy and green curve after additionof saturated aqueous NaCl solution.A ll of the currents have been normalized to current density. The second bromo group occupied the apical position( Br ap )o ft his geometry.T he Co-Br ap displayed al onger bond by 0.17 compared to CoÀ Br bas (average distances are 2.53 versus2 .36 respectively).…”

“…[27,43] As econd potential pathwayi sp rovidedb y an open question in the literature surrounding the redox activity of this class of pincerl igands and their ability to undergo one electron transferw ith first row metals. [42,43,55,58] We have attempted to obtain information on this pathway by preparing the analogue of 1 with ar edox inactive metal center,[ Zn(k 3 -2,6-{Ph 2 PNMe} 2 (NC 5 H 3 )Br 2 ]( 4)( Figure S25). The reduction of 4 appearsa tÀ2.95 Vv s. Fc/Fc + (Figure S26).…”

Enumeration of de novo inorganic complexes for chemical discovery and machine learning

“…In recent years, machine learning (ML) property prediction models trained on first-principles simulation data have further accelerated this discovery process 4,[11][12][13][14][15][16][17][18] throughout chemistry, [19][20][21][22][23][24] including for catalysis 15,16,25,26 and materials. 4,[27][28][29][30][31][32][33][34] Unique challenges arise in applying these tools to the discovery of open shell transition metal complexes, despite their importance as selective catalysts [35][36][37][38][39][40][41][42][43] and functional materials (e.g., molecular switches or sensors [44][45][46][47][48][49][50][51][52] ). The theoretical chemical space of inorganic complexes is diverse and relatively unexplored due to the variable spin states, oxidation states, and coordination num...…”