Publications
Santanu Malakar*, Souvik Mandal*, Xiaoguang Zhou*, Quinton Bruch, Rachel Allen, Laurence Giordano, Nicholas Walker, Thomas Emge, Faraj Hasanayn, Alexander Miller and Alan Goldman
*Equal Contributors
The thioether-diphosphine pincer-ligated molybdenum complex, (PSP)MoCl3 (1-Cl3, PSP = 4,5-bis(diisopropylphosphino)-2,7-di-tert-butyl-9,9-dimethyl-9H-thioxanthene) has been synthesized as a catalyst-precursor for N2 reduction catalysis, with a focus on an integrated experimental/computational mechanistic investigation. The (PSP)Mo unit is isoelectronic with the (PNP)Mo (PNP = 2,6-bis(di-t-butylphosphinomethyl)pyridine) fragment found in the family of catalysts for the reduction of N2 to NH3 first reported in 2011 by Nishibayashi and co-workers. Under an atmosphere of N2 the reaction of 1-Cl3 with three reducing equivalents yields the dinuclear penta-dinitrogen Mo complex [(PSP)Mo(N2)2](-N2), 2. Electrochemical studies reveal that 1-Cl3 is significantly more easily reduced than (PNP)MoCl3 (with a potential ca. 0.4 eV less negative). The bridging-nitrogen complex 2 shows no indication of undergoing N2 cleavage to Mo nitride complexes. The reaction of 1-Cl3 with only two reducing equivalents, however, under N2 atmosphere and in the presence of iodide, affords the product of N2 cleavage, the nitride complex (PSP)Mo(N)(I). DFT calculations implicate another N2-bridged complex, [(PSP)Mo(I)]2(N2), as a viable intermediate in facile N2 cleavage to yield (PSP)Mo(N)(I). Conversion of the nitride ligand to NH3 has been studied. If considering sequential addition of H atoms to the nitride, formation of the first N-H bond is by far the thermodynamically least favorable of the three N-H bond formation steps. The first N-H bond was formed by reaction of (PSP)Mo(N)(I) with [LutH]Cl, where coordination of Cl– to Mo plays an essential role. Computations suggest that a second protonation, followed by a rapid and very favorable one-electron reduction, and then a third protonation, furnishes ammonia. In agreement with calculations, ammonia can be generated using either mild H-atom transfer reagents or mild reductants/acids. This comprehensive analysis of the elementary steps of ammonia synthesis and the role of the central pincer donor and halide association provides guidance for future catalyst designs.
14. Bifunctional Ligands: Evaluating the Role of Acidic Protons in the Secondary Coordination Sphere
Anant K. Jain, Santanu Malakar, Austin T. Cannon, Sophia M. M. Gonzalez, Taylor M. Keller, Patrick J. Carroll, Michael R. Gau, Jonathan L. Kuo, Karen I. Goldberg
To evaluate bifunctional ligand reactivity involving NH acidic sites in the secondary coordination sphere, complexes where the proton has been substituted with a methyl group (NMe) are often investigated. An alternative strategy involves substitution of the NH group for an O. This contribution considers and compares the merits of these approaches; the synthesis and characterization of cationic square-planar Rh carbonyl complexes bearing diprotic bispyrazole pyridine ligand L1, and the bis-methylated pyrazole pyridine ligand L1Me are described. The syntheses and characterization of the novel monoprotic pyrazole isoxazole pyridine ligand L2 and aprotic bisisoxazole pyridine ligand L3, and their corresponding Rh carbonyl complexes are also described. Comparison of the CO stretching frequencies of the four Rh complexes suggest that substitutions of NH with NMe, as well as with O, lead to significant electronic differences. These electronic differences result in different reactivities with respect to ligand addition/substitution of the Rh carbonyl complexes. Overall, the data suggest that electronic differences arising due to the NH substitutions can be significant and should be considered when the NH group is substituted in investigations of the participation of the NH proton in a reaction.
Soumyadipa Das, Souvik Mandal, Santanu Malakar, Thomas J. Emge, Alan S. Goldman
Iridium dibromide complexes of the phenyldiimine ligand 2,6-bis(1-((2,6-dimethylphenyl)imino)ethyl)phenyl, trans-(XyPhDI)IrBr2L, have been synthesized, and relative Ir-L BDFEs have been experimentally determined for a wide range of corresponding adducts of ligands L. An estimate of the absolute enthalpy of Ir-L binding has been obtained from dynamic NMR measurements. The results of DFT calculations are in very good agreement with the relative and absolute experimental values. Computational studies were extended to the formation of adducts of (XyPhDI)IrH2 and (XyPhDI)IrI, as well as other (pincer)IrI fragments, (Phebox)IrI and (PCP)IrI, to enable a comparison of electronic and steric effects with these archetypal pincer ligands. Attempts to reduce (XyPhDI)IrBr2(MeCN) to a hydride or an IrI complex yielded a dinuclear CN-bridged complex with a methyl ligand on the cyanide-C-bound Ir center (characterized by scXRD), indicating that C-CN bond cleavage took place at that Ir center. DFT calculations indicate that the C-CN bond cleavage occurs at one Ir center with strong assistance by coordination of the CN nitrogen to the other Ir center.
Quinton Bruch, Santanu Malakar, Alan S. Goldman and Alexander J. M. Miller
Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N2 binding and splitting by a molybdenum(III) pincer complex, (pyPNP)MoBr3 (pyPNP = 2,6-bis(tBu2PCH2)-C5H3N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction–diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e– and generates a bimetallic molybdenum(I) Mo2(μ-N2) species capable of N–N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N2 cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(pyPNP)Mo(N2)2](μ-N2) in N2 splitting pathways. The same “over-reduced” molybdenum(0) species was also found to cleave N2 upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.
Santanu Malakar, Benjamin M. Gordon, Souvik Mandal, Thomas J. Emge, Alan S. Goldman
The reaction of [(p-cymene)RuCl2]2 with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (tBuPHPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged (“H–P/Ru–Cl exchange”) to give the (chlorophosphine)ruthenium hydride complex (tBuPClPP)RuHCl [1Cl-HCl; tBuPClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (tBuPHPP)RuCl2 (1H-Cl2), undergoes an H–P/Ru–Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (tBuPPP)RuHCl2, followed by Ru-to-P α-Cl migration to give the observed product 1Cl-HCl (crystallographically characterized). Dehydrochlorination of 1Cl-HCl under a H2 atmosphere gives (tBuPClPP)RuH4 (1Cl-H4), which then can undergo a second dehydrochlorination and addition of H2 to give (tBuPHPP)RuH4 (1H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1H-Cl2, i.e., loss of H2 from 1Cl-H4 to give 1Cl-H2, which could undergo Cl–P/Ru–H exchange to give (tBuPHPP)RuHCl (1H-HCl). Accordingly, the thermodynamics of Cl–P/Ru–H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (RPXPP)RuHCl (X = H, Cl; R = Me, tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d6 complexes, both pincer- and nonpincer-ligated.
Xiaoguang Zhou, Santanu Malakar, Thomas Dugan, Thomas J. Emge, Karsten Krogh-Jespersen, Alan S. Goldman
We report an iridium acetate complex with a fluorinated Phebox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3,5-bis(trifluoromethyl)phenyl) that is a highly effective catalyst for acceptorless dehydrogenation of alkanes. Under typical acceptorless dehydrogenation conditions, a high turnover frequency is obtained, which is limited by the rate of expulsion of H2 from the reaction solution. Rates and turnover numbers for acceptorless dehydrogenation are significantly greater than found for the nonfluorinated analogue. As in the case of the nonfluorinated analogue, Na+ acts as a cocatalyst with the fluorinated catalyst again yielding greater rates and total turnovers. Computational studies shed light on the possible mechanistic pathways. The initial alkane activation is a net Ir–H/C–H bond metathesis leading to the formation of an Ir–alkyl bond and loss of H2; this is the slowest chemical step in the cycle. The lowest-energy pathway is calculated to proceed via concerted metalated deprotonation (CMD) of the alkane. Pathways proceeding via transition states with oxidative addition (Ir(V)) character, however, are calculated to be only slightly higher in energy. These transition states can lead either to Ir(V) intermediates, which then lose H2, or connect directly to a dihydrogen complex. The role of Na+ is largely to promote dechelation by coordinating to an acetate oxygen, opening a vacant coordination site that allows reaction with the alkane. This coordination by Na+ prevents the CMD mechanism from operating, but it significantly lowers the energy of the Ir(V) TSs. NBO analysis shows a net transfer of charge from the alkane atoms to the metal complex in the Ir(V) TSs, with and without coordinated Na+. Thus, the oxidative addition is actually reductive in nature, driven in part by electrophilicity of the metal center. The Na+cation further increases electrophilicity in addition to promoting dechelation. The greater activity of the fluorinated catalyst compared with the parent complex can also be explained in terms of the electrophilic nature of the reaction. The fluorinated catalyst is also more resistant to decomposition than the nonfluorinated analogue.
Yansong J. Lu, Xiawei Zhang, Santanu Malakar, Karsten Krogh-Jespersen, Faraj Hasanayn, Alan S. Goldman
Di-isopropylphosphino-substituted pincer-ligated iridium catalysts are found to be significantly more effective for the dehydrogenation of simple tertiary amines to give enamines than the previously reported di-t-butylphosphino-substituted species. It is also found that the di-isopropylphosphino-substituted complexes catalyze dehydrogenation of several β-functionalized tertiary amines to give the corresponding 1,2-difunctionalized olefins. The di-t-butylphosphino-substituted species are ineffective for such substrates; presumably, the marked difference is attributable to the lesser crowding of the di-isopropylphosphino-substituted catalysts. Experimentally determined kinetic isotope effects in conjunction with DFT-based analysis support a dehydrogenation mechanism involving initial pre-equilibrium oxidative addition of the amine α-C–H bond followed by rate-determining elimination of the β-C–H bond.
Bangaru Bhaskararao, Sukriti Singh, Megha Anand, Pritha Verma, Prafull Prakash, Athira C, Santanu Malakar, Henry F. Schaefer, Raghavan B. Sunoj
In the contemporary practice of palladium catalysis, a molecular understanding of the role of vital additives used in such reactions continues to remain rather vague. Herein, we disclose an intriguing and a potentially general role for one of the most commonly used silver salt additives, discovered through rigorous computational investigations on four diverse Pd-catalyzed C–H bond activation reactions involving sp2 aryl C–H bonds. The catalytic pathways of different reactions such as phosphorylation, arylation, alkynylation, and oxidative cycloaddition are analyzed, with and without the explicit inclusion of the silver additive in the respective transition states and intermediates. Our results indicate that the pivotal role of silver salts is likely to manifest in the form of a Pd–Ag heterobimetallic species that facilitates intermetallic electronic communication. The Pd–Ag interaction is found to provide a consistently lower energetic span as compared to an analogous pathway devoid of such interaction. Identification of a lower energy pathway as well as enhanced catalytic efficiency due to Pd–Ag interaction could have broad practical implications in the mechanism of transition metal catalysis and the current perceptions on the same.
Soumik Biswas, Michael J. Blessent, Benjamin M. Gordon, Tian Zhou, Santanu Malakar, David Y. Wang, Karsten Krogh-Jespersen, Alan S. Goldman
PCP-pincer (κ3-2,6-C6H3(CH2PR2)2) iridium complexes have been reported to catalyze the transfer dehydrogenation of n-alkanes with high regioselectivity for the terminal position. We find that the very closely related PCOP (κ3-2,6-C6H3(CH2PR2)(OPR2)) and POCOP (κ3-2,6-C6H3(OPR2)2) complexes, in contrast, afford no such regioselectivity. The difference is a true kinetic phenomenon, i.e., it is not a result of isomerization subsequent to the formation of free α-olefin. In addition to direct observation of the distribution of n-alkane dehydrogenation products over time, the pronounced difference in regioselectivity is confirmed through intermolecular competition studies of the reverse reaction (olefin transfer hydrogenation) and of the dehydrogenation of cycloalkane vs n-alkane. Electronic structure (DFT) calculations indicate that the rate- and selectivity-determining step for dehydrogenation by the (PCP)Ir complexes is β-H transfer. C–H activation at the primary position is much more favorable than at secondary positions, but this is not responsible for the terminal regioselectivity; indeed, the formation of α-olefin via C2–H addition and transfer of the C1–H bond is calculated to be slightly more favorable than dehydrogenation proceeding via C1–H addition. For both PCP and POCOP complexes, the formation of the α-olefin iridium dihydride complex is more facile than the formation of internal-olefin complexes. The next step in the catalytic pathway, loss of olefin, is calculated to have an activation energy that is significantly greater than the metal–ligand (thermodynamic) bond energy. In the case of POCOP complexes, the loss of olefin, rather than β-H transfer, is the rate- and selectivity-determining step. The hydrocarbon moiety in the transition state for olefin loss has the character of a fully formed olefin; this favors the formation of internal olefin. The different regioselectivity of (POCOP)Ir vs (PCP)Ir catalysts is thus attributable to the different rate-determining steps of their respective catalytic cycles; this in turn can be explained in terms of different electronic effects of O versus CH2 linker exerted through the pincer aromatic ring.
