Characterization and Redox Chemistry

Homobimetallic Cobalt Phosphinoamides: Characterization and Redox Chemistry.

Master’s Thesis

Presented to

The Faculty of the Graduate School of Arts and Sciences Brandeis University Department of Chemistry Christine M. Thomas, Advisor

In Partial Fulfillment of the Requirements for the Degree

Master of Science in Chemistry

by Ramyaa Mathialagan

February 2014

Ramyaa Mathialagan

ü First of all, I want to thank my supervisor, Prof. Christine Thomas, for her excellent

guidance, critiques, advice and support. Thank you Chris, for teaching me everything

inside and outside chemistry.

ü I would like to record my sincere thanks to Prof. Casey Wade for his constant support.

ü I would like to convey my thanks to Prof. Bruce Foxman and Mark Bezpalko for their

kind help to solve all my structures.

ü My sincere thanks to my lab-mates for their co-operation and help during the course of

study.

ü My special thanks to faculties and chemistry department office staffs for their friendly

support during my stay.

ü My heartfelt thanks to all my friends who were encouraged me and gave me confidence

at the beginning of my course at Brandeis.

ü I would like to express my lovable thanks to my husband and my little MATHI, without

their support I can’t imagine the work accomplished throughout this period.

ü My parents and my brothers are the great source of inspiration for me. I would like to

express my gratitude for their encouragement, love, care and affection. They bestowed

and the freedom I enjoyed at every point in my life cannot be explainable with mere

word.

-Ramyaa Mathialagan

iii

Abstract

Homobimetallic Cobalt Phosphinoamides: Characterization and Redox Chemistry.

A thesis presented to the Department of Chemistry

Graduate School of Arts and Sciences Brandeis University Waltham, Massachusetts By Ramyaa Mathialagan

Homobimetallic dicobalt complexes featuring vastly different coordination environments have been synthesized, and their multielectron redox chemistry has been investigated. Treatment of CoX2 with K[MesNPiPr2] leads to self-assembly of

[(THF)Co(MesNPiPr2)2(µ-X)CoX] [X = Cl (1), I (2)], with one Co center bound to two amide donors and the other bound to two phosphine donors. Upon two-electron reduction, a ligand rearrangement occurs to generate the symmetric species (PMe3)Co(MesNPiPr2)2Co(PMe3) (3), where each Co has an identical mixed P/N donor set. One-electron oxidation of 3 to generate a mixed valence species promotes a ligand rearrangement back to an asymmetric configuration in

[(THF)Co(MesNPiPr2)2Co(PMe3)][PF6] (4). To construct rigid bimetallic cobalt complexes, a series of triply bridged phosphinoamide complexes been synthesized. Reaction of

K[MesNPiPr2] with CoCl2 in THF results in the formation of a brown crystalline solid, Co(µ- i 2 i PrNPPh2)3Co(η - PrNPPh2) (6). The one electron bulk chemical reduction in the absence and

i i presence of two-electron σ-donor ligands yields Co(µ- PrNPPh2)3Co( PrNHPPh2) (7) and Co(µ- i PrNPPh2)3Co(PMe3) (8). Treatment of 8 with organic azides results in the formation of two-

i electron oxidized Co(µ- PrNPPh2)3Co≡NMes (9, Mes = 2,4,6-trimethylphenyl). In contrast to 9, the reaction of 8 with Ph2CN2 led to the formation of unexpected two-electron phosphine

i oxidized product [Co(µ- PrNPPh2)2(µ-iPrNPPh2N2CPh2)Co] (11). Ligand substitution reactions

i of 8 with Et4N-N3 and KOH resulted to the formation of [Co(µ- PrNPPh2)3Co(N3)][Et4N] (12)

i and [Co(µ- PrNPPh2)3Co(OH)](KC12H24O6) (13), respectively.

Table of Contents

Chapter 1 Introduction: History and Recent Development on Late Transition 01

Metal Homobimetallic Complexes

Chapter 2 Metal−Metal Bonding in Low-Coordinate Dicobalt Complexes 12

Supported by Phosphinoamide Ligands

Chapter 3 Triply Bridged Homobimetallic Cobalt Phosphinoamides: Syntheses, 37

Characterization and Reactivity

List of Tables

1 MO diagrams of 3 and 4 22

2 X-ray Diffraction Experimental Details of 1 - 4 34

3 Interatomic distances (Å) and angles (degrees) of 6 - 8 47

4 Interatomic distances (Å) and angles (degrees) of 9 - 13 48

5 X-ray diffraction Experimental Details of Complexes 6 - 8 64

6 X-ray diffraction Experimental Details of Complexes 9 - 13 65

vii

List of Figures

1 Displacement ellipsoid (50%) representations of 2, 3 and 4 18

2 Frontier molecular orbital diagram of 3 and 4 21

1 3 H NMR of [(THF)Co(MesNPiPr2)2(µ-Cl)CoCl] (1) 29

1 4 H NMR of [(THF)Co(MesNPiPr2)2(µ-I)CoI] (2) 29

1 5 H NMR of (PMe3)Co(MesNPiPr2)2Co(PMe3) (3) 30

1 6 H NMR of [(THF)Co(MesNPiPr2)2Co(PMe3)]PF6 (4) 30

1 7 H NMR of [Co(MesNPiPr2)(PMe3)3]PF6 (5) 31

8 CV of [(THF)Co(MesNPiPr2)2(µ-I)CoI] (2) 31

9 CV of (PMe3)Co(MesNPiPr2)2Co(PMe3) (3) 32

10 EPR Spectrum of [(THF)Co(MesNPiPr2)2Co(PMe3)]PF6 (4) 32

11 UV- vis-NIR spectra of 3 and 4 in THF solution 33

12 ORTEP plots of molecular structures of 6 – 8 45

13 ORTEP plots of molecular structures of 9 and 11-13 46

14 Computed MO diagrams of complexes 8 and 9 50

viii

1 i 2 i 15 H NMR of Co(µ- PrNPPh2)3Co(η - PrNPPh2) (6) 58

1 i i 16 H NMR of Co(µ- PrNPPh2)3Co( PrNHPPh2) (7) 58

1 i 17 H NMR of Co(µ- PrNPPh2)3Co(PMe3) (8) 59

1 i 18 H NMR of Co(µ- PrNPPh2)3Co(NMes) (9) 59

1 i 19 H NMR of [Co(µ- PrNPPh2)2(µ-iPrNPPh2N2CPh2)Co] (11) 60

1 i 20 H NMR of [Co(µ- PrNPPh2)3Co(OH)](KC12H24O6) (13) 60

1 i 21 H NMR of [Co(µ- PrNPPh2)3Co(N3)][Et4N] (12) 61

i i 22 CV of Co(µ- PrNPPh2)3Co( PrNPPh2) (6) 61

i 23 CV of Co(µ- PrNPPh2)3Co(PMe3) (8) 62

i 24 CV of Co(µ- PrNPPh2)3Co(NMes) (9) 62

25 UV- vis-NIR spectra of 6-13 in THF solution 63

List of Schemes

1 The first quadruple and quintuple bonded Re2 and Cr2 complexes 3

2 Multi-metallic clusters with different donor ligands 4

3 Homobimetallic Co complexes with different ligand framework 6

4 Selected metal-ligand multiple bond complexes from literature 7

5 Electronic differences between homo and heterobimetallic complexes 8

6 Synthesis of complex 1 and 2 14

7 Reduction of 1 and 2 15

8 One and Two electron Oxidation of complex 3 16

9 Formation of mixed valence di-cobalt complexes 7 and 8 41

10 Synthesis of 9 and attempted nitrene transfer reaction 43

11 Attempted synthesis of di-cobalt carbene species 43

- - 12 Displacement of PMe3 from 8 by reaction with OH and N3 44

Chapter 1

Introduction: History and Recent Development on Late Transition Metal

Homobimetallic Complexes.

History of Metal-Metal Multiple Bond.

The first reported observation of metal-metal bonded complex was observed in

K3(W2Cl9) with W-W distance of 2.41 Å, which is significantly shorter than the elemental tungsten distance of 2.74 Å, by Brosset in 1935. The short distance between the metal atoms was thought to be due to a strong interaction, however, the actual bond order of this complex could not be determined.1 In 1956, Figgis and Martin suggested that the weak interaction between metal atoms in a well-known diamagnetic Cr2(µ-O2CMe)4(H2O)2 complex could possibly have metal-metal multiple bond character.2 In 1964, Cotton and coworkers reported the first structural

2- 3,4 characterization of a quadruple bond between rhenium atoms in [Re2Cl8] . Since then, a number of multiply bonded bimetallic complexes have been reported in the literature.5 The majority of the metal-metal multiply bonded complexes consist of two similar metal atoms in the

2nd and 3rd row, although a number of exceptions include vanadium and chromium complexes.6,7

A metal-metal bond order greater than four was not characterized in any isolated bimetallic

I 5 complexes until Power’s quintuply bonded Cr2 (Cr , d ) complex was reported in 2005 (Scheme

1). In this complex, all five d orbitals are involved in bonding since the sterically encumbered terphenyl ligand framework stabilizes the CrI centers with weak aryl π interactions.8 In recent years, quintuply bonded di-chromium complexes with diverse ligand frameworks have been reported and their reactivity towards small molecules has been explored.9 Unsupported metal- metal multiply bonded complexes are often targeted to achieve maximum bond order and to avoid complications from metal-ligand overlap.

2- Cl Cl Cl Cl

Re Re Cr Cr

Cl Cl Cl Cl

3,8 Scheme 1. The first quadruple and quintuple bonded Re2 and Cr2 complexes.

Cotton and coworkers introduced bridging amidinato, triazenato and carboxylato framework to construct D3h symmetric trigonal lantern and D4h tetragonal lantern complexes with transition metals across the periodic table, including multiply bonded di-iron, di-cobalt and di- nickel derivatives.10 Computational investigations of high spin metal-metal bonded complexes are challenging and yet to be explored. Multiple bonds between two different transition metals are uncommon, and this has become an emerging area of research in past few years.

Late Transition Metal Homobimetallic Complexes.

In contrast to the 2nd and 3rd row transition metals, multiple bonds between two first row transition metal atoms, except for vanadium and chromium complexes,5,6 are challenging targets for synthetic chemists. The intermediate spin state and lower electron count of early metal atoms

(vanadium and chromium) favor the arrangement of electrons only in bonding orbitals in the d- manifold, but in the case of late transition metals their high electron count and high spin nature populates anti-bonding orbitals, resulting in lower metal-metal bond orders. The interaction between metal atoms (in multi-metallic systems) plays an important role in various enzymatic transformations; therefore bimetallic complexes are simple models for complicated multi- metallic clusters.22 While a large number of dinuclear late transition metal complexes have been

3 reported, these are typically formed using bridging ligands that present similar (hard/hard or soft/soft) donors to each metal and, thus, lead to metals in similar electronic environments.23,24

An interesting class of polynuclear metal clusters composed of metal-metal interactions between one hard open-shell metal and a second soft metal, typically supported by carbonyls, has recently emerged in the literature.25,26 Theoretical investigations of these “xenophilic” complexes have suggested that the disparate coordination environments on two appended late transition metals may lead to unusual electronic properties and magnetic behavior (see Scheme 2).27

py thf CO Fe OC CO CO Mn CO OC CO CO Fe py Fe OC py Fe thf Fe CO OC thf CO Fe CO CO CO Mn CO py thf

Scheme 2. Multi-metallic clusters with different donor ligands.27

In 1994, Cotton and coworkers reported a high spin FeII/FeI complex with a short Fe-Fe distance (2.2318(8) Å).10a Recently, Lu and coworkers revisited this complex with additional spectroscopic characterization and computational methods to prove its high-spin electron configuration resulting in a bond order of 1.5.28 In addition, a tris(amidinato)amine motif has been employed as a ligand for the construction of bimetallic cobalt complexes. Due to the presence of an axial amine L-type donor, this molecule is asymmetric. The two different geometric environments of the metal atoms likely increases the interatomic distance as compared

29 to Cotton’s D3h symmetric cobalt bimetallic complexes (2.2943(7) Å vs 2.2318(8) Å). Ligand frameworks with electronically different donors have attracted attention for the preparation

4 homo-bimetallic complexes with bond polarity between the metal atoms. In this context, Thomas and coworkers have utilized bi-functional phosphinoamide ligands to synthesize a series of Mn and Fe complexes, and they have found that intermetallic distances are considerably longer

(3.1436(3) Å vs 2.354 Å and 2.8684(6) Å vs 2.346) than the covalent radii between two metal atoms, indicating weaker interaction.30

Homo-bimetallic Cobalt Complexes.

There are only a few cobalt bimetallic complexes with short metal-metal bonds reported in the literature.29,31-35 The majority of these dicobalt complexes were reported by Cotton and workers. These complexes were supported by amidinato ligands and display diverse oxidation states of CoII/CoI, CoII/CoII and CoII/CoIII, with strong metal-metal bonds (see Scheme 3).31-33

Jones and coworkers showed that a sterically encumbered amidinato ligand framework could even support a low-coordinate (three-coordinate) planar dicobalt(I) complex with a short intermetallic distance of 2.13-2.14 Å.34 Mindiola and coworkers reported a dicobalt(I) complex supported by PNP pincer type ligands with a Co-Co distance of 2.35 Å.35 Very recently, Lu and coworkers designed a unique double decker ligand framework to construct a mixed valent

CoII/CoI complex with the second shortest Co-Co distance ever reported (2.29 Å).29

Metal ligand multiple bonded complexes are postulated intermediates in many chemical and enzymatic reactions.36,37 Monometallic complexes with metal ligand multiple bonds are relatively well-known across the periodic table, whereas their multi-metallic counterparts are uncommon.38 However, metal-metal interactions could be a driving force to increase turnover of substrate activation in enzymes, and this is also true in the case of synthetic dirhodium catalysts. Simple dirhodium acetate catalyzes a diverse series of reactions, in particular group

5 transfer (carbene and nitrene) and C-H activation and insertion reactions.39 Despite the fact that superelectrophilic 3-center-4-electron dirhodium imide and carbene intermediates are highly unstable, these metastable species have recently been spectroscopically characterized.

Scheme 3. Homobimetallic Co complexes with different ligand framework.29,31-35

In this context, Berry and coworkers began to investigate the unusual metal-metal and metal-ligand multiple bonded complexes with 2nd and 3rd row transition metals.40 Thomas and coworkers have shown that metal-metal bonded bimetallic iron complexes are capable of stabilizing imide fragments, and are the first reported metal-metal bonded and metal-ligand multiple bonded complexes involving 1st row transition metals.31 One electron oxidation of the diiron imide disrupts the metal-metal bond via fluoride ion abstraction (when ferrocenium hexa-

6 flurophosphate is used as an oxidant) by the oxidized iron center. Disruption of the Fe-Fe interaction increases the nucleophillicity of the Fe-imide functionality (Scheme 4).41

i Pr iPr O O O N PPh2 PPh O O N PPh 2 2 PPh2 OMe Rh Rh C Fe Fe NR Ph B Co N O O O O N PPh2 PPh2 O iPr

Scheme 4. Selected metal-ligand multiple bond complexes from literature.30,37,39

Peters and coworkers have reported monometallic cobalt imide fragments supported by tris(phosphino) borate ligand systems in which group transfer reactions only proceeded under relatively harsh conditions over a prolonged period of time (Scheme 4).42 The introduction of additional metal-metal interaction could possibly enhance the reactivity of the resulting complex as compared to monometallic analogues. Thus, we were curious to study homobimetallic cobalt complexes.

Phosphinoamide Ligand Framework.

A large number of homobimetallic complexes reported in the literature contain bridging ligands with similar donors (hard/hard or soft/soft), which offer the same electronic structure and geometry to both metals.24,25 As discussed in the above sections, bifunctional ligands have received significant attention toward the design of homobimetallic complexes with increased polarity. In heterobimetallic systems, polarity can be induced by the electronegativity of different metals and the ligand character is less likely to play a role. In the case of homo-bimetallic complexes, a certain degree of polarity may be achieved by the presence of different donors in

7 the same ligand. Phosphinoamide ligands have hard (amide) and soft (phosphine) functionalities that offer different coordination environments for two metals in homobimetallic complexes.43,44

R R R R N PR'2 N PR' N PR' 2 2 N PR'2 M M M M Early Metal N PR'2 N PR' Late Metal Late Metal 2 Late Metal R R

Scheme 5. Electronic differences between homo and heterobimetallic complexes.12,30

Scheme 5 shows a general depiction of hetero and homobimetallic systems with bridging phosphinoamide ligands. In heterometallic systems, an electron rich late metal atom donates electron density to electrophilic early metal atom (Scheme 5, left),12 whereas in the case of homobimetallic complexes, the electron density moves from the electron rich tris-amide coordinated metal to tris-phosphine coordinated metal atom (Scheme 5, right).30

Objective of the Thesis:

(i) Synthesis of homobimetallic cobalt complexes with bridging phosphinoamides.

(ii) Characterization of newly isolated bimetallic cobalt complexes and study of their

electrochemistry, electronic structure, and reactivity.

(iii) Synthesis of low coordinate planar dicobalt complexes and step-wise chemical redox

reactions to make mixed valence complexes.

(iv) To tune the electronics and sterics, different substituted phosphinoamides are utilized.

(v) Attempts to isolate reactive intermediates and study the chemical reactivity of

dicobalt complexes.

References. 1. Brosset, C. Nature, 1935, 135, 874. 2. Figgis, B. N.; Martin, R. L. J. Chem. Soc., 1956, 3837. 3. Cotton, F. A.; Curtis, N. F.; Harris, C. B.; Johnson, B. F. G.; Lippard, S. J.; Mague, J. T.; Robinson, W. R.; Wood, J. S. Science 1964, 145, 1305; 4. Cotton, F. A. Inorg. Chem., 1964, 4, 334. 5. F. A. Cotton, C. A. Murillo and R. A. Walton, Multiple Bonds Between Metal Atoms, Springer Science and Business Media, Inc., New York, 2005. 6. (a) Cotton, F. A.; Hillard, E. A.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 6063; (b) Cotton, F. A.; Hillard, E. A.; Murillo, C. A. J. Am. Chem. Soc. 2003, 125, 2026; (c) Cotton, F. A.; Millar, M. J. Am. Chem. Soc. 1977, 99, 7886; (d) Cotton, F. A.; Daniels, L. M.; Murillo, C. A. Inorg. Chem. 1993, 32, 2881. 7. Kundig, P. E.; Moskovits, M.; Ozin, A. G. Nature 1975, 254, 503. 8. Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J. Power, P. P. Science 2005, 310, 844. 9. Wagner, F. R.; Noor, A.; Kempe, R. Nat. Chem., 2009, 1, 529. 10. (a) Cotton, F. A.; Daniels, L. M.; Murillo, C. A. Inorg. Chim. Acta, 1994, 224, 5. (b) Cotton, F. A.; Poli, R. Inorg. Chem., 1987, 26, 3652. (c) Arnold, D. I.; Cotton, F. A.; Maloney, D. J.; Matonic, J. H.; Murillo, C. A. Polyhedron, 1997, 16, 133. 11. (a) Stephan, D. W. Coord. Chem. Rev. 1989, 95, 41; (b) Wheatley, N.; Kalck, P. Chem. Rev. 1999, 99, 3379; (c) Bullock, R. M.; Casey, C. P. Acc. Chem. Res. 1987, 20, 167. 12. Thomas, C. M. Comments on Inorganic Chemistry, 2011, 32, 14. 13. Slaughter, L. M.; Wolczanski, P. T. Chem. Comm. 1997, 2109. 14. (a) Sue, T.; Sunada, Y.; Nagashima, H. Eur. J. Inorg. Chem. 2007, 2897. (b) Nagashima, H.; Sue, T.; Oda, T.; Kanemitsu, A.; Matsumoto, T.; Motoyama, Y.; Sunada, Y. Organometallics 2006, 25, 1987. (c) Tsutsumi, H.; Sunada, Y.; Shiota, Y.; Yoshizawa, K.; Nagashima, H. Organometallics 2009, 28, 1988. 15. Greenwood, B. P.; Rowe, G. T.; Chen, C.-H.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc. 2010, 132, 44. 16. Zhou, W.; Napoline, J. W.; Thomas, C. M. Eur. J. Inorg. Chem. 2011, 2029. 17. Krogman, J. P.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc. 2011, 133, 14582. 18. Napoline, J. W.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Chem. Commun. 2013, 49, 4388. 19. Marquard, S. L.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc. 2013, 135, 6018. 20. Clouston, L, J.; Siedschlag, R. B.; Rudd, P. A.; Planas, N.; Hu, S.; Miller, A. D.; Gagliardi, L.; Lu, C. C. J. Am. Chem. Soc., 2013, 135, 13142. 21. Kuppuswamy, S.; Powers, T. M.; Krogman, J. P.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Chem. Sci. 2013, 4, 3557. 22. Lindahl, P. A. J. Inorg. Biochem. 2012, 106, 172. 23. Simona, M. Coord. Chem. Rev. 2009, 253, 1793. 24. Rosenthal, J.; Bachman, J.; Dempsey, J. L.; Esswein, A. J.; Gray, T. G.; Hodgkiss, J. M.; Manke, D. R.; Luckett, T. D.; Pistorio, B. J.; Veige, A. S.; Nocera, D. G. Coord. Chem. Rev. 2005, 249, 1316.

25. Gade, L. H. Angew. Chem., Int. Ed. Engl. 1996, 35, 2089. 26. Whittlesey, B. R. Coord. Chem. Rev. 2000, 206, 395. 27. Xu, Z.; Lin, Z. Chem.-Eur. J. 1998, 4, 28. 28. Zall, C. M.; Zherebetskyy, D.; Dzubak, L. A.; Bill, E.; Gagliardi, L.; Lu, C. C. Inorg. Chem., 2012, 51, 728. 29. Zall, C. M.; Clouston, L. J.; Young, G. V., JR.; Ding, K.; Kim, H. J.; Zherebetskyy, D.; Chen, Y.; Bill, E.; Gagliardi, L.; Lu, C. C. Inorg. Chem. 2013, 52, 9216. 30. Kuppuswamy, S.; Bezpalko, M. W.; Powers, T. M.; Turnbull, M. M.; Foxman, B. M.; Thomas, C. M. Inorg. Chem. 2012, 51, 8225. 31. Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Matonic, J. H.; Murillo, C. A. Inorg. Chim. Acta 1997, 256, 283. 32. Cotton, F. A.; Daniels, L. M.; Feng, X.; Maloney, D. J.; Matonic, J. H.; Murilio, C. A. Inorg. Chim. Acta 1997, 256, 291. 33. Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Murillo, C. A. Inorg. Chim. Acta 1996, 249, 9. 34. Jones, C.; Schulten, C.; Rose, R. P.; Stasch, A.; Aldridge, S.; Woodul, W. D.; Murray, K. S.; Moubaraki, B.; Brynda, M.; La Macchia, G.; Gagliardi, L. Angew. Chem. Int. Ed., 2009, 48, 7406. 35. Fout, A. R.; Basuli, F.; Fan, H.; Tomaszewski, J.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. Angew. Chem. Int. Ed. 2006, 45, 3291. 36. Berry, F. J. Comments on Inorganic Chemistry, 2009, 30, 28. 37. Saouma, C. T.; Peters, J. C. Coord. Chem. Rev. 2011, 255, 920. 38. Verma, A. K.; Nazif, T. N.; Achim, C.; Lee, S, C. J. Am. Chem. Soc. 2000, 122, 11013. 39. Kornecki, K. P.; Briones, J. F.; Boyarskikh, V.; Fullilove, F.; Autschbach, J.; Schrote, K. E.; Lancaster, K. M.; Davies, H. M. L.; Berry, J. F. Science 2013, 342, 351. 40. (a) Long, A. K. M.; Timmer, G. H.; Pap, J. S.; Snyder, J. L.; Yu, R. P.; Berry, J. F. J. Am. Chem. Soc. 2011, 133, 13138; (b) Long, A. K. M.; Yu, R. P.; Timmer, G. H.; Berry, J. F. J. Am. Chem. Soc. 2010, 132, 12230; (c) Pap, J. S.; George, D.; Berry, J. F. Angew. Chem. Int. Ed. 2008, 47, 10102. 41. Kuppuswamy, S.; Krogman, J. P.; Powers, T. M.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. Inorg. Chem. 2014, DOI: ic-2013-03039x. 42. Sisler, H.; Smith, N. J. Org. Chem. 1961, 26, 611. 43. Poetschke, N.; Nieger, M.; Khan, M. A.; Niecke, E.; Ashby, M. T. Inorg. Chem. 1997, 36, 4087.

Chapter 2

Metal−Metal Bonding in Low-Coordinate Dicobalt Complexes

Supported by Phosphinoamide Ligands

Introduction.

Thomas and coworkers have recently synthesized a series of heterobimetallic Zr/Co complexes and studied the effects of metal-metal interactions on redox properties and reactivity.1

The vastly different electronic nature of the two metals in these systems leads to highly polar metal−metal interactions, shifting the CoI/0 and Co0/-I potentials by ca. +1 V with respect to monometallic Co analogues and facilitating reactivity with polar bonds in substrates such as

2,3 CO2. We were curious to ascertain whether similar effects on redox properties and reactivity could be observed for homobimetallic dicobalt complexes in disparate coordination environments. We have recently found that treatment of late metal salts (FeX2, MnX2) with phosphinoamide ligands leads to self-assembly of homobimetallic species with each metal center in a different (polyamide vs polyphosphine) coordination environment.4 Herein, we extend this chemistry to dicobalt systems and uncover some interesting low-coordinate geometries and ligand rearrangements in metal−metal bonded species.

Results and Discussion.

The construction of the bimetallic framework was achieved via treatment of cobalt halide salts CoX2 (X = Cl, I) with an equimolar ratio of the phosphinoamide potassium salt

i i MesNKP Pr2. In this manner, the green complexes [(THF)Co(MesNP Pr2)2(µ- X)CoX] [X = Cl

(1), I (2)] were obtained in high yield (Scheme 6). Complexes 1 and 2 have similar paramagnetically shifted 1H NMR spectra with 11 distinct resonances. The apparent inequivalence of many of the phosphinoamide substituents (e.g., four distinct isopropyl-methyl resonances) is indicative of asymmetric complexes (later confirmed by X-ray diffraction).

iPr N PiPr KN P N 2 i 0.5 P Pr2 CoX2 iPr THF, - KX Co Co O X X X = Cl, 1; I, 2

Scheme 6. Synthesis of complex 1 and 2.

The solution magnetic moments of complexes 1 and 2 were determined using the Evans

5,6 method [µeff = 3.0 (1) and 3.3 µB (2)] and are lower than would be expected for two high spin

CoII centers (spin only value expected for two uncoupled S = 3/2 CoII centers = 6.48 µB), implying some degree of magnetic interaction or bonding between the two Co centers leading to an intermediate spin state. The redox behavior of 2 was investigated using cyclic voltammetry revealing multiple reductive events. The cyclic voltammogram of complex 2 revealed a well- defined quasi-reversible one-electron reduction at −1.2 V, followed by a series of broad irreversible features at −2.0 and −2.3 V (vs ferrocene). While the reduction at −1.2 V appears fully reversible at scan rates greater than 0.4 V/s, the relative intensity of the return oxidative

14 wave decreases as scan rate is decreased, and at a scan rate of 0.01 V/s, this reduction wave appears entirely irreversible. The quasi-reversible nature of this reductive feature suggests a substantial chemical change in the composition of complex 2 upon reduction.

To investigate these redox processes further, complex 2 was treated with excess Na/Hg amalgam in THF. While this reaction led to an intractable mixture of products, treatment of 2 with Na/Hg in the presence of excess PMe3 resulted in clean formation of a new red complex,

i 1 later identified as (PMe3)Co(MesNP Pr2)2Co(PMe3) (3), in 63% yield (Scheme 2). The H NMR spectrum of 3 remained broad and paramagnetically shifted, but in this case, only six resonances were observed, suggesting a more symmetric structure. Much like 1 and 2, the solution magnetic moment of complex 3 is also significantly lower than would be expected for two high spin CoI

I centers (µeff = 2.9 µB versus the expected value of 5.66 µB for two non-interacting Co centers or 4.90 for a delocalized S = 2 system). This implies that the two CoI centers are either antiferromagnetically coupled or that the electrons on the two metal centers in this highly symmetric complex are delocalized throughout a metal−metal bonded d orbital manifold. The latter explanation is supported by computational results (vide infra). The cyclic voltammogram of complex 3 revealed multiple irreversible one-electron redox events, including an oxidation at

−1.1 V and a further reduction at −2.7 V (vs Fc/ Fc+).

iPr iPr2 N P iPr P Na/Hg, PMe3 N i N P Pr2 Me3P Co Co PMe3 THF, - NaI Co Co iPr P N O X X iPr

X=Cl, 1; X=I, 2 3 Scheme 7. Reduction of 1 and 2.

On the basis of the mild oxidation potential of 3, attempts were made to carefully oxidize this complex to obtain a mixed valence complex. Treatment of 3 with 1 equiv of [Cp2Fe][PF6]

i resulted in a mixed valence complex [(THF)Co(MesNP Pr2)2- Co(PMe3)][PF6] (4) in which the phosphinoamide had again unexpectedly rearranged to adopt a structure in which one Co center was bound to two phosphines and the other was ligated by two amides (Scheme 3).

i i Pr i Pr PF6 Pr P N P iPr N

O Co Co PMe3 3 2 Co PMe3 Fc PF6 2 Fc PF6 i THF, - Fc THF, - 2 Fc Me3P N P Pr PMe3 iPr 5 PF6 4 Fc PF6 THF, - Fc

Scheme 8. One and Two electron Oxidation of complex 3.

Notably, treatment of 3 with 2 equiv of [Cp2Fe][PF6] in attempts to generate a halide-free dicobalt(II) complex led to isolation of the monometallic CoII complex

i 1 [Co(MesNP Pr2)(PMe3)3]PF6 (5), (see Appendix). The paramagnetically shifted HNMR of 4 revealed nine distinct resonances, suggesting a relatively symmetric structure. As might be expected based on the magnetic moment of precursor complex 3, the solution magnetic moment of complex 4 is sufficiently low to indicate distribution of the metal d electrons throughout a metal-metal bonding manifold (µeff = 1.4 µB). Furthermore, the solution EPR spectrum of 4 (77

K, X-band) is indicative of an S = 1/2 system (see appendix). Again, it is difficult to speculate on how the ligand rearrangement occurs upon oxidation from 3 to 4, but the driving force may be that the oxidized CoII center would be more likely to prefer π-donating amide donors than a CoI center. This implies an assignment of the amide-ligated Co center as CoII in this mixed valence

CoIICoI complex, and such an assignment is consistent with computational predictions (vide infra).

Solid-state molecular structures of 2-4.

The solid state structures of both 1 and 2 were obtained via X-ray structure analysis of single crystals (Figure 1). Interestingly, the dicobalt complex adopts an asymmetric structure in which one CoII center is coordinated by two amide donors, while the other is bound by two phosphines. This preference was also observed in several other phosphinoamide-linked bimetallic complexes that we have recently reported.4,7 The geometry at each Co center in 2 is distorted pseudotetradrahedral. As a result of the asymmetric coordination environments of the two Co centers, the iodide bridges somewhat asymmetrically, with a slightly longer distance to the amide-bound Co center [2.7294(3) Å vs 2.6768(4) Å]. The short Co-Co distance in 2 is suggestive of a metal-metal bond [2.5939(4) Å and 2.6142(4) Å in two independent molecules in the asymmetric unit of 2]. Other nonorganometallic CoIICoII complexes with similar metal- metal

8,9 distances include Co2(NR2)4 [R = SiMe3, 2.583(1) Å; R = Ph, 2.566(3) Å], while the 3- and 4-

+ 10 fold symmetric amidinato- and triazenato-bridged complexes Co2(amidinato)3 [2.885(1) Å],

11 12 Co2(amidinato)4 [2.3735(9) Å], and Co2(triazenato)4 [2.265(2) Å] complexes have much longer and shorter interatomic distances, respectively. Notably, these complexes were also reported to have room temperature magnetic moments indicative of low or intermediate spin states via antiferromagnetic exchange, although no further explanation of the electronic structure was provided.8-13 X-ray diffraction structure analysis of single crystals of 3 confirmed this observation and revealed that a ligand rearrangement had occurred to generate a complex in which both CoI centers are in identical coordination environments with one amide and one phosphine donor from the two bridging phosphinoamides (Figure 1). Each Co center is also

17 coordinated by a PMe3 ligand, and the six-membered Co2N2P2 core is rigorously planar. On the basis of the isolated product 3, ligand rearrangement as well as halide loss may be responsible for the quasi-reversibility of the first reductive feature observed in the CV of 3. Although it remains unclear how this ligand rearrangement occurs, it seems reasonable that the preferences for a reduced CoI center for soft phosphine donors might lead to this phenomenon. Indeed, the requirement of excess PMe3 to promote clean product formation may imply that PMe3 coordination plays a key role in the ligand rearrangement process.

Figure 1. Displacement ellipsoid (50%) representations of 2, 3 and 4. For clarity, only one of - two independent molecules in the asymmetric unit of 2 is shown, only one of two disordered PF6 positions in 4 is shown, and hydrogen atoms have been omitted. Relevant interatomic distances (Å) and angles (°): 2 (parameters for one of two independent molecules are listed): Co-Co2, 2.5939(4); Co1-N1, 1.9124(18); Co1-N2, 1.9254(19); Co2-P1, 2.3298(6); Co2-P2, 2.3429(7); Co1-I1, 2.7294(3); Co2-I1, 2.6768(4); Co2-I2, 2.5264(4); Co1-O1, 2.0540(16), Co1-I1−Co2, 57.335(10). 3: Co1-Co1, 2.5536(3); Co1-P1, 2.1763(3); Co1-P2, 2.2699(3); Co1-N1, 1.9569(8); Co1-Co1−P2, 160.245(11). 4: Co1-Co2, 2.4864(6); Co1-N1, 1.889(2); Co1-N2, 1.883(2); Co2- P1, 2.2255(9); Co2-P2, 2.2283(9); Co2-P3, 2.2554(9); Co1-O1, 2.063(2).

While the Co2N2P2 core of the molecule is planar, the terminal PMe3 ligands lie 0.93 Å out of this plane. Upon closer inspection, a weak agostic interaction is observed between the Co center and one of the mesityl-methyl hydrogen atoms, and it is likely this interaction that leads to distortion. The hydrogen atom was located in the Fourier difference map and refined, revealing a

Co1-H153 distance = 2.327(17) Å and a Co-C-H angle of 120.8°, both indicative of an agostic

18 interaction.14 Further support for this weak Co-C-H interaction comes from a medium intensity

µ(C-H) stretch at 2714 cm-1 in the solid state IR spectrum of 3 (see Appendix). Similar weak agostic interactions have been documented in at least one other low-coordinate cobalt complex15 and are an indicator of the coordinative unsaturation of the two Co centers of 3. The metal-metal distance in 3 [2.5536(3) Å] is shorter than that in complexes 1 and 2, despite the absence of constraints imposed by a monatomic bridging ligand (Table 1). The Co-Co distance is, however, longer than that observed in the limited number of nonorganometallic CoICoI complexes in the

16 literature, including Co2(PMe3)4(µ-SPh)2 [2.3997(5) Å]. the amidinate and guanidate dimers

t 17 Co2(ArN)2CR)2 [R = Bu, 2.1404(10) Å; R = NCy2, 2.1345(7) Å], and (PNP)2Co2 [2.254(1)

18 Å]. The solid state structure of 4 (Figure 1) reveals that PMe3 remains coordinated to the phosphine-bound Co center, while THF coordinates to the amide-ligated Co ion. In this mixed valence complex, the Co-Co separation is contracted even further to 2.4864(6) Å, in line with

II I other nonorganometallic Co Co complexes in the literature: Co2(amidinato)3 [2.385(1) Å,

10 - 19 t 20 2.3201(9) Å], [Co2(SAr)5] [2.511(4) Å], and Co2(µ- Bu2P)2Cl(PMe3)2 [2.508(2) Å].

To further address the metal-metal interactions in complex 4, UV-vis-NIR data were collected. The UV-vis-NIR spectrum of 4 has a number of low intensity transitions in the

650−950 nm range, including a somewhat broad low intensity band at 949 nm (ε = 140 M-1 cm-

1). Because complex 4 is formally a mixed valence CoIICoI system, this band could reasonably be assigned as an intervalence charge transfer (IVCT) band, and the asymmetric coordination environments of the two cobalt centers might be expected to increase the energy of this transition into the visible range. Given the metal-metal bonding and the degree of orbital mixing in complex 4, however (vide infra), such an interpretation may be oversimplified in this case. For comparison, the UV-vis-NIR spectrum of the symmetric CoICoI complex 3. This spectrum is less

19 complex than that of 4, with a very broad low intensity feature centered at 1476 nm (ε = 64 M-1 cm-1), which can likely be attributed to a d-d transition within the metal-metal bonding orbital manifold. (See Appendix).

Computational Investigation.

To further investigate the electronic structure and metal-metal bonding in the planar dicobalt complexes 3 and 4, a computational investigation was conducted using density functional theory (DFT) methods [BP86/LANL2TZ(f)/6-311+G(d)/D95 V]. Solution magnetic moment data for complexes 3 and 4 indicate triplet and doublet configurations, respectively.

Nonetheless, geometry optimizations were performed on both molecules in all possible spin states (triplet and quintet for 3; doublet, quartet, and sextet for 4). Comparison of the optimized geometries computed for these various spin states with the solid state optimized geometries obtained for these two complexes using X-ray crystallography reveals that the best prediction is obtained with the S = 1 (3) and S = 1/2 (4) configurations.

The frontier molecular orbital diagram of 3 reveals both metal-metal σ and π interactions, as shown in Figure 2. As a result of substantial phosphinoamide ligand contributions, 12 MOs with metal orbital contributions are shown in the figure. There are three orbitals comprised of metal-metal σ interactions, including a low-lying metal-metal σ bonding orbital (-5.06 eV) that is also σ bonding with respect to the metal-amide bonds, a metal-metal σ bond that is σ* with respect to the metal-amide bonds (-4.27 eV), and the LUMO (-1.64 eV), which is σ* with respect to the metal-metal interaction as well as the metal-ligand interactions. Additional δ and π interactions between the two Co centers are also present, with the symmetry partially disrupted by the two different ligand donors on each Co ion. The LUMO is sufficiently higher in energy

20 than the remaining occupied orbitals in the metal-metal bonding manifold, resulting in an intermediate spin (S = 1) ground state. On the basis of the MO diagram shown in Figure 2, the

Co-Co bond order is estimated to be ca. 1.

Figure 2. Frontier molecular orbital diagram of 3 and 4.

More information about the bonding between the cobalt centers was obtained via natural bond orbital (NBO)21 analysis and Mayer population analysis.22 The Co-Co Wiberg bond indices calculated for 3 and 4 (0.54 and 0.51, respectively) are nearly identical (Table 1), while calculations suggest that the Mayer bond order for 3 is slightly higher than that of 4. While the absolute values of these computed bond orders are difficult to interpret, comparison of the

Co−Co bond orders calculated for these two molecules is a useful exercise and indicates that the metal−metal bond in 3 is stronger than that in 4. NBO analysis reveals that the Co-Co σ bond in

3 is covalent in nature, with 50% orbital contributions from each Co center (for both α and β

NBOs, see Appendix). This is consistent with the identical coordination environments and

21 equivalent natural charges of the two metal centers (Table 2). In contrast, the Co centers in the asymmetric mixed valence molecule 4 have vastly different natural charges, with the amide- bound Co center significantly more positively charged than either the phosphine bound Co atom or the two Co centers in starting material 3. In this case, there is poorer orbital overlap between the two different Co centers because of mismatched orbital energies, as illustrated in the calculated frontier MO diagram shown in Figure 2. There are no apparent δ interactions, and the

π interactions are weaker and more polarized. Nonetheless, the Co−Co σ bond remains relatively covalent (α NBO: 29% CoN, 71% CoP; β NBO: 75% CoN, 28% CoP). As a result, the overall

Co−Co bond order in 4 remains ~1, although the bond may be slightly weaker than that in 3 as a result of mismatched orbital energies.

Table 1. Natural charges and Wiberg bond indices (WBIs) calculated for 3 and 4 using NBO calculations and Calculated Mayer Bond Orders (MBOs).

Natural Charges Calculated Bond Orders

Co1 Co2 Co-Co WBI Co-Co (MBO)

3 -0.16 -0.16 0.54 0.98

4 0.62(CoN) -0.23(CoP) 0.51 0.80

i - In summary, the [MesNP Pr2] ligand supports low-coordinate dicobalt dimers with significant metal-metal bonding. The orientation of the ligands in these complexes is highly dependent on the overall redox state of the dicobalt unit. Future studies will focus on the reactivity of the reduced complex 3 toward small molecule activation.

Experimental Section.

General Considerations. Unless specified otherwise, all manipulations were performed under an inert atmosphere using standard Schlenk or glovebox techniques. Glassware was oven- dried before use. Benzene, pentane, diethyl ether, tetrahydrofuran, and toluene were dried using a

Glass Contours solvent purification system. All solvents were stored over 3 Å molecular sieves prior to use. Benzene-d6 (Cambridge Isotopes) was degassed via repeated freeze-pump-thaw cycles and dried over 3 Å molecular sieves. THF-d8 was dried over CaH2, vacuum-transferred,

i and degassed via repeated freeze-pump-thaw cycles. MesNKP Pr2 was synthesized using

2,4 literature procedures. Anhydrous CoCl2 and CoI2 were purchased from Strem Chemicals and used after 12 h of drying at 100 °C under vacuum. NMR spectra were recorded at ambient temperature on a Varian Inova 400 MHz instrument. Chemical shifts are reported in δ (ppm). For

1Hand 13C{1H} NMR spectra, the solvent resonance was used as an internal reference, and for

31 1 P{ H} NMR spectra, 85% H3PO4 was referenced as an external standard (0 ppm). IR spectra were recorded on a Varian 640-IR spectrometer controlled by Resolutions Pro software. UV-vis spectra were recorded on either a Cary 50 UV-vis or Cary 5000 UV-vis-NIR spectrophotometer using Cary WinUV software. Elemental analyses were performed at Complete Analysis

Laboratory Inc. (Parsippany, NJ). Solution magnetic moments were measured using Evans’ method.5,6

X-ray Crystallography.

All operations were performed on a Bruker-Nonius Kappa Apex2 diffractometer, using graphite monochromated Mo Kα radiation. All diffractometer manipulations, including data collection, integration, scaling, and absorption corrections, were carried out using the Bruker

Apex2 software.23 Preliminary cell constants were obtained from three sets of 12 frames.

Computational Details.

All calculations were performed using Gaussian09, Revision A.02, for the Linux operating system.24 Density functional theory calculations were carried out using a combination of Becke’s 1988 gradient-corrected exchange functional25 and Perdew’s 1986 electron correlation functional26 (BP86). A mixed basis set was employed, using the LANL2TZ(f) triple-ζ basis set with effective core potentials for cobalt,27,28 Gaussian09’s internal 6-311+G(d) for heteroatoms (nitrogen, oxygen, phosphorus), and Gaussian09’s internal LANL2DZ basis set

(equivalent to D95 V29) for carbon and hydrogen. Using crystallographically determined geometries as a starting point, the geometries were optimized to a minimum, followed by analytical frequency calculations to confirm that no imaginary frequencies were present. Mayer bond analysis was performed with the routines included in the Gaussian09 software package,22 and Wiberg bond indices and NBO calculations were carried out using Gaussian NBO Version

3.1.21

Electrochemistry.

CV measurements were carried out in a glovebox under a dinitrogen atmosphere in a one- compartment cell using a CH Instruments electrochemical analyzer. A glassy carbon electrode and platinum wire were used as the working and auxiliary electrodes, respectively. The reference

n electrode was Ag/AgNO3 in THF. Solutions of electrolyte (0.40 M [ Bu4N][PF6] in THF) and analyte (2 mM) were also prepared in the glovebox. All potentials are reported versus an internal ferrocene/ferrocenium reference.

i i Synthesis of [(THF)Co(MesNP Pr2)2(µ-Cl)CoCl] (1). A solution of MesNKP Pr2 (0.28 g, 1.0 mmol) in THF (3 mL) was cooled to -32 °C and this was added to CoCl2 (0.130 g, 1.00 mmol) in THF (2 mL) drop-wise over 5 min. The resulting mixture was gradually allowed to warm to room temperature and continuously stirred for 12 h. The insoluble materials were removed by filtration through Celite, and all volatiles were subsequently removed from the filtrate in vacuo. The resulting green material was extracted with diethyl ether (4 × 2 mL) and filtered to remove the byproduct, KCl, and other insoluble impurities. Upon standing at room temperature, the concentrated ether solution of 1 yielded analytically pure 1 as green blocks

1 i (0.36 g, 95%). H NMR (400 MHz, C6D6): δ 26.3 (6H, Pr-Me), 17.0 (2H, Mes), 12.3 (2H, Mes),

9.3 (6H, Mes-Me), 6.7 (6H, Mes-Me), 4.3 (6H, iPr-Me), 2.9 (6H, Mes-Me), -4.2 (6H, iPr-Me), -

4.9 (4H, THF), -8.0 (6H, iPr-Me), -11.4 (4H, THF) (isopropyl-methine proton is not observed because of its close proximity to the paramagnetic Co center, tentative assignments based on

-1 -1 relative integration). UV-vis (C6H6) λmax, nm (ε, L mol cm ): 450 (390), 506 (230), 365 (910),

670 (730). Evans’ method (C6D6): 2.98 µB. Anal. calcd for C34H58C02N2P2OCl2: C, 53.62; H,

7.68; N, 3.69. Found: C, 53.53; H, 7.79; N, 3.75.

i i Synthesis of [(THF)Co(MesNP Pr2)2(µ-I)CoI] (2). A solution of MesNKP Pr2 (0.85 g,

3.0 mmol) was cooled to -32 °C in THF (15 mL) and this was added to CoI2 (0.94 g, 3.0 mmol) in THF (10 mL) dropwise over 5 min. The resulting mixture was gradually allowed to warm to room temperature and continuously stirred for 12 h. The insoluble materials were removed by filtration through Celite, and all volatiles were subsequently removed from the filtrate in vacuo.

The resulting green material was extracted with diethyl ether (4 × 5 mL) and filtered to remove the byproduct, KI, and other insoluble impurities. Concentration of this diethyl ether solution and storage at −32 °C afforded analytically pure 2 as green blocks (0.92 g, 66%). 1H NMR (400

i MHz, C6D6): δ 24.5 (6H, Pr-Me), 15.8 (2H, Mes), 11.7 (2H, Mes), 10.0 (6H, Mes-Me), 7.9 (6H,

Mes-Me), 5.3 (6H, iPr-Me), 1.9 (6H, Mes-Me), −2.9 (6H, iPr-Me), -5.7 (4H, THF), -10.6 (6H, iPr-Me), -11.7 (4H, THF) (isopropyl-methine proton is not observed because of its close proximity to the paramagnetic Co center, tentative assignments based on relative integration).

−1 −1 UV-vis (C6H6) λmax, nm (ε, L mol cm ): 364 (440), 608 (640), 688 (860), 746 (610). Evans’ method (C6D6): 3.29 µB. Anal. calcd for C34H58Co2N2P2I2: C, 43.24; H, 6.19; N, 2.97. Found: C,

43.30; H, 6.29; N, 3.04.

i Synthesis of (PMe3)Co(MesNP Pr2)2Co(PMe3) (3). A 0.5% Na/Hg amalgam was prepared from 0.003 g of Na (0.1 mmol) and 0.6 g of Hg. To this vigorously stirred amalgam in

10 mL of THF was added a cold (-32 °C) solution of 2 (0.047 g, 0.050 mmol) in THF (5 mL).

Neat PMe3 (26 µL, 0.20 mmol) was added to the reaction mixture immediately, and the solution rapidly changed from green to brick red in color. After it was stirred for 2.5 h, the resulting red solution was decanted from the amalgam and filtered through Celite. Volatiles were removed from the filtrate in vacuo. The resulting red material was extracted with diethyl ether (4 × 2 mL) to remove NaI and other insoluble impurities. Upon concentration of this diethyl ether solution

26 of 3 followed by storage at room temperature for 12 h, dark reddish brown single crystals of 3

1 were obtained (0.024 g, 63%). H NMR (400 MHz, C6D6): δ 10.6 (6H, Mes-Me), 7.9 (18H,

i i PMe3), 4.2 (12H, Mes-Me), 0.3 (4H, Mes), -6.2 (12H, Pr-Me), -9.0 (12H, Pr-Me) (isopropyl- methine proton is not observed because of its close proximity to the paramagnetic Co center,

−1 −1 tentative assignments based on relative integration). UV-vis (C6H6) λmax, nm (ε, L mol cm ):

471 (2500), 623 (470), 1476 (64). Evans’ method (C6D6): 2.86 µB. Anal. calcd for

C36H68Co2N2P4: C, 56.10; H, 8.89; N, 3.63. Found: C, 56.11; H, 8.94; N, 3.73.

i Synthesis of [(THF)Co(MesNP Pr2)2Co(PMe3)]PF6 (4). A solution of 3 (0.039 g, 0.050 mmol) in THF (3 mL) was cooled to -32 °C for 30 min, and this solution was added to a THF (2

1 mL) solution of FcPF6 (0.017 g, 0.050 mmol). The reaction progress was monitored by H NMR spectroscopy, and after 2 h of stirring the mixture at rt, the starting material had cleanly converted to a new compound. At this point, the solution was filtered through Celite to remove insoluble materials. The volatiles were removed from the filtrate in vacuo, and this material was washed with pentane to remove ferrocene and other byproducts. The remaining orange material was redissolved in THF (2 mL), layered with pentane (3 mL), and stored at room temperature, resulting in orange blocks of 4 along with a small amount of [Co(PMe3)4]PF6 as a minor byproduct. Complex 4 was isolated by manual separation from the mixture (0.039 g, 29%). 1H

NMR (400 MHz, C6D6): δ 80.5 (4H, THF), 32.3 (Mes-Me), 20.3 (4H, THF), 17.0 (9H, PMe3),

1.2 (12H, Mes-Me), -7.6 (12H, iPr- Me), -10.0 (12H, very broad, Mes), −17.4 (iPr-Me) (isopropyl methine proton is not observed because of its close proximity to the paramagnetic Co center,

−1 −1 tentative assignments based on relative integration). UV-vis (C6H6) λmax, nm (ε, L mol cm ):

454 (1300), 682 (150), 728 (140), 949 (140). Evans’ method (C6D6): 1.45 µB. Complex 4 is thermally unstable in both solution and the solid state it is likely for this reason that the solution

27 magnetic data are artificially low and satisfactory combustion analysis data could not be obtained.

i Synthesis of [Co(MesNP Pr2)(PMe3)3]PF6 (5). A solution of 3 (0.039 g, 0.050 mmol) in

THF (3 mL) was cooled to -32 °C for 30 min, and this was added to a THF (2 mL) solution of

1 FcPF6 (0.033 g, 0.10 mmol). The reaction progress was monitored by H NMR spectroscopy; the starting materials were completely converted to a new compound after 2h of stirring the reaction mixture at rt. The solution was filtered through Celite to remove insoluble materials. Volatiles were removed from the filtrate, and the orange material was subsequently washed with pentane to remove ferrocene and other byproducts. The remaining orange material was redissolved in

THF (2 mL), layered with pentane (3 mL), and stored at room temperature, resulting in orange

1 blocks of 5 (0.038 g, 40%). H NMR (400 MHz, C6D6): δ 18.9 (2H, Mes), 1.8 (27H, PMe3), 1.3

(3H, Mes-Me), -1.7 (6H, iPr-Me), -2.4 (6H, Mes-Me), -8.0 (6H, iPr-Me) (isopropylmethine proton is not observed because of its close proximity to the paramagnetic Co center, tentative

−1 −1 assignments based on relative integration). UV-vis (C6H6) λmax, nm (ε, L mol cm ): 450

(1400), 671 (380). Evans’ method (C6D6): 3.53 µB. Anal. calcd for C37H67Co2N2P5F12: C, 42.70;

H, 6.49; N, 2.69. Found: C, 43.23; H, 7.96; N, 1.85.

Appendix: Supporting Figures and Tables.

1 i Figure 3. H NMR of [(THF)Co(MesNP Pr2)2(µ-Cl)CoCl] (1).

1 Figure 4. H NMR of [(THF)Co(MesNPiPr2)2(µ-I)CoI] (2).

1 Figure 5. H NMR of (PMe3)Co(MesNPiPr2)2Co(PMe3) (3).

1 Figure 6. H NMR of [(THF)Co(MesNPiPr2)2Co(PMe3)]PF6 (4).

1 Figure 7. H NMR of [Co(MesNPiPr2)(PMe3)3]PF6 (5).

Figure 8. Cyclic voltammogram of [(THF)Co(MesNPiPr2)2(µ-I)CoI] (2) (2 mM in 0.4 M n [ Bu4N][PF6] in THF, scan rate = 100 mV/s).

Figure 9. Cyclic voltammogram of (PMe3)Co(MesNPiPr2)2Co(PMe3) (3) (2 mM in 0.4 M n [ Bu4N][PF6] in THF, scan rate = 100 mV/s).

Figure 10. EPR Spectrum of 4 (Frozen THF/Tolune solution, 77 K, X-band, frequency = 9.33 GHz). Spectrum collected at this temperature was too broad to stimulate as anything other than a broad isotropic signal centered at g = 2.059.

Figure 11. UV- vis-NIR spectra of 3 and 4 in THF solution. The inset shows an expanded view of the near-infrared region of the spectrum.

Table 2. X-ray Diffraction Experimental Details of 1 - 4

1 2 3 4

Chemical C34 H58 Cl2 Co2 N2 C34 H58 Co2 I2 N2 C36 H68 Co2 N2 C37 H67 Co2 F6 formula O1 P2 O1 P2 P4 N2 O1 P4 fw 761.57 944.47 770.71 911.70

T (K) 120 120 120 120

λ (Å) 0.71073 Å 0.71073 Å 0.71073 Å 0.71073 Å a (Å) 8.4435(7) 9.2673(3) 8.7710(5) 11.0817(16) b (Å) 19.6022(15) 21.1143(7) 11.5687(6) 12.3347(16) c (Å) 22.6710(18) 21.1748(7) 12.0272(6) 18.134(3)

α (deg) 90 71.024(2) 106.184(2) 87.776(9)

β (deg) 90.807(5) 88.342(2) 105.840(2) 78.776(9)

γ (deg) 90 80.705(2) 110.519(2) 64.197(8)

V (Å3) 3751.9(5) 3865.5(2) 1000.13(10) 2186.0(5) space group P21/c1 P-1 P -1 P -1

Z 4, 1 4, 2 1, 0.5 2, 1

3 Dcalcd (g/cm ) 1.348 1.623 1.280 1.385 µ (cm–1) 1.140 2.556 1.015 0.961 R1, wR2a (I > 0.0372, 0.0847 0.0285, 0.0582 0.0203, 0.0508 0.0421, 0.0718 2σ)

aR1 = Σ(||Fo| – |Fc||) / Σ|Fo|, wR2 = {Σ[w(Fo2 – Fc2)2/Σ[w(Fo)2}1/2.

References.

1. Thomas, C. M. Comments Inorg. Chem. 2011, 32, 14. 2. Greenwood, B. P.; Forman, S. I.; Rowe, G. T.; Chen, C.-H.; Foxman, B. M.; Thomas, C. M. Inorg. Chem. 2009, 48, 6251. 3. Krogman, J. P.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc. 2011, 133, 14582. 4. Kuppuswamy, S.; Bezpalko, M. W.; Powers, T. M.; Turnbull, M. M.; Foxman, B. M.; Thomas, C. M. Inorg. Chem. 2012, 51, 8225. 5. Sur, S. K. J. Magn. Reson. 1989, 82, 169. 6. Evans, D. F. J. Chem. Soc. 1959, 2003. 7. Kuppuswamy, S.; Cooper, B. G.; Bezpalko, M. W.; Foxman, B. M.; Powers, T. M.; Thomas, C. M. Inorg. Chem. 2012, 51, 1866. 8. Murray, B. D.; Power, P. P. Inorg. Chem. 1984, 23, 4584. 9. Hope, H.; Olmstead, M. M.; Murray, B. D.; Power, P. P. J. Am. Chem. Soc. 1985, 107, 712. 10. Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Matonic, J. H.; Murillo, C. A. Inorg. Chim. Acta 1997, 256, 283. 11. Cotton, F. A.; Daniels, L. M.; Feng, X.; Maloney, D. J.; Matonic, J. H.; Murilio, C. A. Inorg. Chim. Acta 1997, 256, 291. 12. Cotton, F. A.; Poli, R. Inorg. Chem. 1987, 26, 3652. 13. Theopold, K. H.; Silvestre, J.; Byrne, E. K.; Richeson, D. S. Organometallics 1989, 8, 2001. 14. Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6908. 15. Olmstead, M. M.; Power, P. P.; Sigel, G. Inorg. Chem. 1986, 25, 1027. 16. Jiao, G.; Li, X.; Sun, H.; Xu, X. J. Organomet. Chem. 2007, 692, 4251. 17. Jones, C.; Schulten, C.; Rose, R. P.; Stasch, A.; Aldridge, S.; Woodul, W. D.; Murray, K. S.; Moubaraki, B.; Brynda, M.; La; Macchia, G.; Gagliardi, L. Angew. Chem., Int. Ed. 2009, 48, 7406. 18. Fout, A. R.; Basuli, F.; Fan, H.; Tomaszewski, J.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. Angew. Chem., Int. Ed. 2006, 45, 3291. 19. Ruhlandt-Senge, K.; Power, P. P. J. Chem. Soc., Dalton Trans. 1993, 649. 20. Jones, R. A.; Stuart, A. L.; Atwood, J. L.; Hunter, W. E.; Rogers, R. D. Organometallics 1982, 1, 1721. 21. Glendening, E. D.; Reed, A. E.; Carpenter, J. E. NBO Version 3.1. 22. Mayer, I. Int. J. Quantum Chem. 1986, 29, 477. 23. Apex 2: Version 2 User Manual, M86-E01078; Bruker Analytical X-ray Systems: Madison, WI, 2006. 24. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.;

Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.2; Gaussian, Inc.: Wallingford, CT, 2009. 25. Becke, A. D. Phys. Rev. A 1988, 38, 3098. 26. Perdew, J. P. Phys. Rev. B 1986, 33, 8822. 27. Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. 28. Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. 29. Dunning, T. H.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer, H. F., Ed.; Plenum: New York, 1976; Vol. 3, pp 1.

Chapter 3

Triply Bridged Homobimetallic Cobalt Phosphinoamides: Syntheses,

Characterization and Reactivity

Introduction.

Despite the greater abundance of first row transition metal atoms versus many second and third row metals, thermodynamically stable small molecule activation of these complexes has not been thoroughly explored. Metal complexes with direct bonding interactions between two metal atoms are prevalent in many enzymatic transformations. For example, Carbon monoxide dehydrogenases and heme-copper oxidases,1,2 utilize metal-metal interactions between the metal atoms to achieve bio-chemical reactions in their protein active sites.3 Recently, the early/late metal combinations of hetero-bimetallic complexes have proven to be active catalysts in many organic transformations and small molecule activations.4-6 It is well known that many second and third row transition metal homo-bimetallic complexes facilitate the multi-electron redox

7-9 reactions under milder conditions. Rh2(OAc)4 is a commercially available catalyst for many catalytic and group transfer (carbene and nitrene) reactions,10 but its high cost and low abundance has turned researchers’ attention towards more earth abundant and environmental benign 1st row transition metals. First row transition metal complexes typically undergo one- electron redox processes. However, bimetallic first row transition metal complexes, particularly those with electronically assymetric donor sets and metal-metal bond polarization, may be poised to undergo two electron redox processes. Recently, Thomas and coworkers have reported the structural characterization, electronic structure interpretation and magnetic studies of a series of

11 II I zwitterionic homo-bimetallic Fe and Mn complexes. The reaction of Fe /Fe complex with RN3 allowed for the isolation and characterization of the first stable metal-metal and metal-ligand multiple bonded species;13 however, these complexes are inert for group transfer reactions.

Oxidation of these diiron complexes, which disrupts bonding between the metal atoms, results in increased nucleophillicity at the imide functionality and promotes nitrene transfer reactions.14 In

38 order to design active first row transition metal catalysts and as an extension of our group’s interest on bimetallic complexes, we have turned our attention towards homobimetallic cobalt complexes. The synthesis, characterization, electronic structure, and chemical reactivity of a series of cobalt complexes are discussed in this chapter.

Results and Discussion:

i Salt metathesis of K[ PrNPPh2] with CoCl2 in THF at room temperature results in the

i 2 i formation of a brown crystalline solid, Co(µ- PrNPPh2)3Co(η - PrNPPh2) (6), in 69% yield. The

1H NMR spectrum of complex 6 exhibits 11 resonances between +19 and -10 ppm, indicative of the presence of two-phosphinoamide ligand environments in solution. The analytical purity and elemental composition of 6 were confirmed by combustion analysis, and the molecular structure of 6 has been established by a single crystal X-ray diffraction study (vide-infra). The disparate coordination environments of the cobalt atoms in 6 make this complex unique in terms of structure, bonding, electronic properties, and chemical reactivity. The cyclic voltammogram of complex 6 shows multiple redox events, including two quasi-reversible oxidations and one quasi-reversible reductions at Epa=0.02 V, E1/2 =-0.5 V and E1/2 =-2.1 V (vs. ferrocene/ferrocenium), respectively. We were particularly interested to investigate the reversible reduction to create a reactive mixed valence complex. The solution magnetic moment of 6 (µeff =

4.42 µB measured by Evans method) is lower than the expected value for two high-spin cobalt centers, which likely indicate an antiferromagnetic interaction between the metal centers. Based

i i on the iso-structural diiron complex, Fe(µ- PrNPPh2)3Fe( PrNPPh2), reported recently, the oxidation of 6 can tentatively assigned to be a zwitterion CoII/CoII (vide infra).

i 1.5 Na/Hg, PMe3, THF, - PrNHPPh2

i i i i Pr Pr Pr Pr iPr iPr N PPh2 N PPh2 N PPh N iPr 2 PPh2 N PPh2 N PPh2 1.5 Na/Hg PMe3, THF N PPh Co Co Co Co 2 Co Co PMe i 3 THF - PrNHPPh2 N PPh N iPr 2 PPh2 N PPh2 N PPh2 H iPr iPr iPr 6 7 8

Scheme 9. Formation of mixed valence di-cobalt complexes 7 and 8.

The bulk chemical reduction of 6 with excess Na/Hg amalgam in the absence of σ-donor

i i ligands leads to the formation of reddish brown Co(µ- PrNPPh2)3Co( PrNHPPh2) (7) in 62% yield. The reaction progress was monitored by 1H NMR spectroscopy. The disappearance of resonances attributed to the starting material and appearance of new set of resonances suggests the formation of one electron reduced complex 7 without any detectable intermediate species.

The solid-state structure of 7, confirmed by X-ray diffraction analysis, reveals that the phosphine donor of a neutral phophinoamine ligand coordinates to tris-phosphine cobalt atom (vide infra).

The presence of the N-H functionality in 7 was confirmed by the appearance of an infrared stretching vibration at 3400 cm-1. The origin of the amine proton was unclear; either may come from the solvent or ligand. When the above reaction was performed in deuterium-labeled solvent, the appearance of N-H strectching frequency in IR spectroscopy confirms that the proton should come from the ligand decomposition.11 The presence of 11 broad proton resonances in the NMR spectrum of complex 7 suggests two in equivalent ligand environments (triply bridged phosphinoamide and terminal phosphinoamine). The reaction of 7 with PMe3 results in displacement of the neutral phosphinoamine to generate a C3-symmetric complex Co(µ-

41 i PrNPPh2)3Co(PMe3) (8) as a red crystalline solid. Complex 8 can also be synthesized using a straightforward one-pot procedure by reducing complex 6 with excess Na/Hg amalgam in the

1 presence of PMe3 at room temperature. The presence of six resonances in the H NMR spectrum of 8 indicates a C3-symmetric structure in solution. The chemical formulation and purity of both

7 and 8 were confirmed by combustion analysis. Electrochemical studies of 8 revealed two reversible oxidations at E1/2 = -0.6 and -2.1 V (vs. ferrocene/ferrocenium). Thus, we are interested to study the oxidation of complex 8.

i Reactivity studies of Co(µ- PrNPPh2)3Co(PMe3) (8).

Cobalt (I) complexes supported by trisphosphino borate ligands have been shown to exhibit two-electron group transfer reactivity (nitrene transfer) with organic azides.15 Peters’ trisphosphino borate cobalt (I) complex has no interaction between B and Co. In the case of complex 8, the additional interaction from the second cobalt atom trans to PMe3 ligand, which may facilitate reactivity with group transfer reagents. Therefore, the reactions of imide and carbene group transfer reagents with complex 8 were investigated. Addition of two equivalents of 2,4,6 trimethylphenylazide to a solution of 8 gives rise to a green, two-electron oxidized

i complex, Co(µ- PrNPPh2)3Co≡NMes (9, Mes = 2,4,6-trimethylphenyl), with concomitant elimination of dinitrogen and the formation of one equivalent of the phosphoranimine byproduct,

1 Me3P=NMes. The H NMR spectrum of 9 shows seven paramagnetic resonances in the 19.9 to -

12.4 ppm range, which suggests a C3-symmetric structure in solution. The cyclic voltammogram of 9 exhibits irreversible oxidation at Epa = -0.8 V. Similar to the diiron complex, Fe(µ- i PrNPPh2)3Fe≡NR, complex 9 does not transfer its nitrene functionality to CO or RNC at room temperature. However themolysis at 70 °C leads to the formation of the decomposition product

i [Co(iPrNHPPh2)( PrN(CO)PPh2)(CO)2] (10) along with unidentified impurities.

Scheme 10. Synthesis of di-cobalt imide 9 via two-electron oxidation and attempted nitrene transfer reaction. Complex 8 exhibits two-electron oxidation chemistry as evident from the formation of 9.

Diazodiphenylmethane (Ph2CN2) is a well-known organic reagent for generation of metal- carbene species, M=CPh2. The reaction of 8 with Ph2CN2 in THF solution affords an red [Co(µ- i PrNPPh2)2(µ-iPrNPPh2N2CPh2)Co] (11) complex with insertion of Ph2CN2 into one of Co-P bonds of the bridging ligands, The structure of 11 was confirmed by X-ray diffraction (vide

1 infra). Eleven paramagnetic H NMR resonances indicate that Ph2CN2 insertion breaks the C3- symmetry in this complex. Photolysis or thermolysis of 11 did not result in formation of the desired di-cobalt carbene complex.

Scheme 11. Attempted synthesis to generate a di-cobalt carbene species.

i Ligand displacement reactions of Co(µ- PrNPPh2)3Co(PMe3) (8).

In addition to group transfer, we investigated ligand exchange reactions of PMe3 with

- - OH and N3 in complex 8 in attempts to isolate highly reactive functionalities at the dicobalt framework that might be useful precursors to additional M-ligand multiply-bonded species. The reaction of two equivalents of [Et4N][N3] with 8 results in the orange complex [Co(µ- i PrNPPh2)3Co(N3)][Et4N] (12) in 39% yield. The presence of the M–N3 functionality of 12 was established by a very strong infrared stretching vibration at 2360 cm-1. Furthermore, the solution structure (based on 1H NMR) of complex 12 is consistent with the solid-state structure (vide- infra). Similarly, the treatment of complex 8 with KOH in the presence of a crown ether (18-

I i crown-6) generated a anionic Co -hydroxide complex [Co(µ- PrNPPh2)3Co(OH)](KC12H24O6)

II t i (13). A similar monometallic Co -hydroxide complex [{HB(3- Bu-5- Prpz)3}Co(OH)] was

t i 12 obtained upon reaction of [{HB(3- Bu-5- Prpz)3}Co(NO3)] with NaOH. The characteristic O-H

-1 stretching frequency is apparent in the IR spectrum of 13 at 3371 cm . Complex 13 is C3- symmetric in solution, as confirmed by seven broad paramagnetic proton resonances in the range of 28.21 to -12.35 ppm.

- - Scheme 12. Displacement of PMe3 from complex 8 by reaction with OH and N3 .

Solid-state molecular structures of 6 – 13.

X-ray quality single crystals of 6 were grown from concentrated ethereal solution at room temperature. The solid-state structure of complex 6 was further confirmed by single crystal X-ray crystallography. The dinuclear cobalt complex 6 consists of three bridging and one η2- phosphinoamide ligand. The molecular structure of 6 and pertinent geometric parameters are given in Figure 1 and Table 1. The tris(amide)-bound Co1 adopts a trigonal planar geometry; whereas Co2 has a distorted trigonal bipyramidal geometry with the Addison parameter of τ =

0.6.15 The core structure of 6 resembles that of the recently reported homo-bimetallic Cr, Fe and

Mn complexes.11,16 The intermetallic distance between Co centers in 6 is significantly longer

17 than the sum of the covalent radii (2.8919(3) Å vs 2.338 Å). The avg. Co-N and Co-Pbridging distances are 1.9151 Å and 2.2945 Å, respectively, and are comparable to those in tris(2-

18,19 amidinato ethyl)amine-Co2 and [PhBP3]CoI. In addition, P4 is strongly coordinated with

Co2, with a Co-P distance of 2.1922(4) Å.

6 7 8

Figure 12. ORTEP plots of molecular structures of 6 - 8. Hydrogen atoms are omitted for clarity.

One electron-reduced complexes 7 and 8 were also analyzed by X-ray crystallography and the structures are presented in Figure 12. The only difference between two complexes is the terminal ligand attached to the Co2 atom. This tris(amide)-bound Co1 center adopts a trigonal pyramidal geometry and the phosphine-coordinated Co2 center has a pseudotetrahedral geometry. The core structures of 7 and 8 resemble the recently reported Cr and Fe complexes.13,16 The Co1-Co2 distances for 7 and 8 are 2.5603(3) Å and 2.5320(5) Å, respectively, which are significantly shorter than the corresponding precursor complex 6; however, the Co1-Co2 distance remains substantially longer than that in other reported di-cobalt complexes.19-23 The avg. Co-N distance of 7 and 8 are 1.9212 Å and 1.9162 Å, respectively, which suggests that there is no considerable structural change at Co1 center. The geometrical changes on the Co2 center compared to precursor 6, supports the reduction. (see Table 1).

9 11 12 13

Figure 13. ORTEP plots of molecular structures of 9 and 11-13. Hydrogen atoms are omitted for clarity.

Table 3. Interatomic distances (Å) and angles (degrees) of 6 - 8.

Distance 6 7 8

Co1-Co2 2.8919(3) 2.5603(3) 2.5320(5)

Co1-N(avg) 1.9151 1.9212 1.9162(14) Co2-N4 1.9593(12) - -

Co2-P(avg) 2.2945 2.3204 2.2795(4) Co2-P4/P2 2.1922(4) 2.2289(5) 2.2144(7) N1-Co1-N2 120.69(5) 119.78(6) 119.342(11) N1-Co1-N3 36.10(3) 118.37(6) 119.342(11) N2-Co1-N3 120.78(5) 120.02(6) 119.342(11) P1-Co2-P2 100.370(14) 106.634(18) 108.861(13) P1-Co2-P3 99.486(14) 105.782(18) 108.861(13) P1-Co2-P4/P2 105.468(15) 114.095(18) 110.075(12) P1-Co2-N4 137.30(4) - - P2-Co2-P3 101.106(14) 113.019(19) 108.861(13) P2-Co2-P4/P2 102.860(14) 113.541(18) 110.075(12) P2-Co2-N4 114.50(4) - - P3-Co2-P4/P2 141.326(15) 103.643(19) 110.075(12) P3-Co2-N4 97.23(4) - - N4-Co2-P4 44.97(4) - -

Single crystals of 9 suitable for X-ray analysis were obtained by slow evaporation of an ethereal solution at room temperature. X-ray analysis of revealed that this complex has a C3- symmetric structure, in which the imide group is terminally attached to the Co2 atom (Figure 2,

Table 2). Notably, the intermetallic distance in 9 is significantly longer than that in the C3-

47 symmetric precursor complex 8 (2.8626(3) Å vs 2.5320(5) Å). Based on bond distances the oxidation state assignments can be made as follows: The Co1 center remains in the +2 oxidation state, since there are no appreciable structural changes on this side of molecule as compared with complexes 6-8. The average Co2-P distance of 9 is significantly shorter than the Peter’s tris(phosphino)borate cobalt-imide complex (2.250 Å vs 2.174 Å).18a Importantly, the distance between Co2 and the imido nitrogen (Co2-N4 = 1.6723(12) Å) is slightly longer compared to C3 symmetric CoIII(imide) complexes reported in the literature (1.65 Å -1.66 Å) confirms that Co2 center is in +3 oxidation state.24,25

Table 4. Interatomic distances (Å) and angles (degrees) of 9–13.

Distances 9 11 12 13

Co1-Co2 2.8626(3) 2.7203(3) 2.7484(6) 2.7837(6)

Co1-N 1.9257 1.9423 1.9384 1.9212 (avg) Co2-N4/O1 1.6723(12) 1.9849(13) 1.950(3) 1.906(2)

Co2-P(avg) 2.2499 2.2896 2.2704 2.2837

N1-Co1-N2 119.32(5) 119.66(6) 120.14(12) 119.57(10) N1-Co1-N3 123.26(5) 119.03(6) 118.41(12) 118.20(10)

N2-Co1-N3 113.435(6) 121.07(6) 119.27(12) 120.16(10)

P1-Co2-P2 99.811(16) 106.817(16) 103.37(4) 103.46(3) P1-Co2-P3/N4 103.319(5) 115.91(4) 103.39(4) 102.65(3)

P2-Co2-P3/N4 98.154(15) 106.24(4) 102.85(4) 101.24(3)

P1-Co2-N4/O1 115.37(4) - 115.44(10) 111.53(8)

P2-Co2-N4/O1 119.06(4) - 115.05(10) 115.56(7)

P3-Co2-N4/O1 117.99(4) - 115.07(11) 120.32(7)

The unusual diazodiphenylmethane insertion into a bridging phosphine ligand of 11 was confirmed by X-ray crystallography. In complex 11, the Co1 atom remains in the same geometry as the precursor complex 8, however, substantial changes occur at Co2 as a result of the diazodiphenylmethane insertion into one of the bridging phosphine via two-electron oxidation.26

A similar two-electron phosphine oxidation was reported in a NiI-PNP complex26 and the oxidized phosphine P=N distance of 11 is comparable with that reported (1.6495 (13) Å vs 1.678

Å). The P3-N3 distance is significantly shorter than that of P1-N1 and P2-N2 distances (1.6029

(13) Å vs 1.6426 Å and 1.6434 (13) Å), whereas Co1-N3 distance is considerably longer than the

Co1-N1 and Co1-N2 distances (1.9730 (13) Å vs 1.9342 (13) Å and 1.9197 (13) Å). Also the longer Co2-N4 distance (1.9848 (13) Å) of 11 suggests that N4 acts as an L-type ligand rather than an X-type ligand. In addition to these structural changes, one of the aryl rings of diphenyldiazomethane strongly interacts with Co2 (Co2-C53 = 2.4169(15) Å and Co2-C54 =

2.2418 (16) Å), to stabilize the complex. The intermetallic distance (2.7203(3) Å) of 11 is considerably longer than the Co-Co distance in starting complex 8.

Solid-state structures of ligand-substituted complexes 12 and 13 were characterized by single crystal X-ray diffraction studies. Both complexes are C3-symmetric and display unique structural features. Terminal N3-and OH-coordinated cobalt complexes are uncommon and only few structurally characterized neutral CoI and CoII complexes are known.12,27,28 Replacement of

- - the L type PMe3 ligand by X-type OH or N2 ligands results in elongation of intermetallic distances in 12 and 13 (2.7484(6) Å and 2.7837(6) Å, respectively). The Co2–N4 distance of 12 is 1.950(3) Å, which is significantly longer than the neutral CoI complex of [{HB(3-tBu-5- i II 27,28 Prpz)3}Co(N3)] (1.911 (2) Å) and Meyer’s cationic Co complex (1.938(2) Å). In the case of

13, the terminal hydroxide functionality stabilized by a weak interaction with a potassium

49 countercation capped by a crown ether. Co2–O1 distance (1.906(2) Å) in 13 is shorter than the

I t i 12 neutral Co -OH distance of [{HB(3- Bu-5- Prpz)3}Co(OH)] (1.854 (3) Å).

Theoretical calculations of complexes 8 and 9.

To better understand the metal-metal interactions in complexes 8 and 9, computational investigations using Density Functional theory (DFT) were performed.

Figure 14. Computed MO diagrams of complexes 8 and 9.

Although geometry optimizations were performed on multiple spin states of complex 8, the DFT-derived geometry that best matched the solid state structure was the S = 3/2 spin state.

Both cobalt atoms contribute a total of fifteen electrons to construct the frontier molecular orbital

(MO) diagram of 8, in which six electrons occupy metal-metal bonding orbitals (σ, π) and four electrons occupy metal-metal anti-bonding orbitals. The remaining five electrons reside in orbitals that are non-bonding with respect to the metal-metal interaction. The bond order in this

50 complex is therefore one, which explains the relatively short Co-Co distance in this complex

(Figure 3). Complex 9 has an S = ½ spin state, which is consistent with the solution magnetic moment (1.81 µB). In this complex, a total of thirteen electrons are involved to construct MO.

Two electrons are present in each of σ and σ* orbitals, resulting in a net bond order of 0; the remaining nine electrons occupy metal-metal non-bonding orbitals. The bond order of zero is strongly supported by the longer intermetallic distance.

Summary.

A series of homobimetallic Co complexes has been synthesized and structurally characterized to understand the bonding between metal atoms. The one-electron reduced complex 8 has further been utilized for reactivity studies. Mesitylazide cleanly reacts with complex 8 at room temperature to produce the two-electron oxidized dicobalt imide complex 9.

Complex 9 did not show any promising group transfer reactivity. Attempts to install a carbene fragment onto the cobalt center were unsuccessful. The terminal hydroxide and azide species were synthesized by ligand substitution reactions. Complexes 8 and 9 would be the potential precursors for small molecule activations.

Experimental Section.

General considerations. Unless specified otherwise, all manipulations were performed under an inert atmosphere using standard Schlenk or glovebox techniques. Glassware was oven- dried before use. Benzene, pentane, diethyl ether, tetrahydrofuran, and toluene were degased via sparging with ultra-high purity argon and dried using a Glass Contours solvent purification system. All solvents were stored over 3 Å molecular sieves prior to use. Benzene-d6 (Cambridge

Isotopes) was degassed via repeated freeze-pump-thaw cycles, and dried over 3 Å molecular

i i sieves. PrNHPPh2, K[ PrNPPh2] and diazodiphenylmethane (Ph2CN2) were prepared using

11,29-31 literature procedures. Anhydrous CoCl2 was purchased from Strem Chemicals and used after 12 h drying at 100 °C/1 Torr. NMR spectra were recorded at ambient temperature on a

Varian Inova 400 MHz instrument. Chemical shifts are reported in δ (ppm). For 1H NMR spectra, the solvent resonance was used as an internal reference. IR spectra were recorded on a

Varian 640-IR spectrometer controlled by Resolutions Pro software. UV-vis and UV-vis-NIR spectra were recorded on Cary 5000 UV-vis spectrophotometer using Cary WinUV software.

Elemental analyses were performed at Complete Analysis Laboratory Inc., Parsippany, NJ.

Solution magnetic moments were measured using Evans’ method.32,33

Electrochemistry. CV measurements were carried out in a glove box under a dinitrogen atmosphere in a one-compartment cell using a CH Instruments electrochemical analyzer. A glassy carbon electrode and platinum wire were used as the working and auxiliary electrodes, respectively. The reference electrode was Ag/AgNO3 in THF. Solutions of electrolyte (0.40 M n [ Bu4N][PF6] in THF) and analyte (2 mM) were also prepared in the glove box. All potentials are reported versus an internal ferrocene/ferrocenium reference.

X-ray crystallography procedures. All operations were performed on a Bruker–Nonius

Kappa Apex2 diffractometer, using graphite monochromated Mo Kα radiation. All diffractometer manipulations, including data collection, integration, scaling, and absorption corrections were carried out using the Bruker Apex2 software.34 Preliminary cell constants were obtained from three sets of 12 frames. All crystal structure refinements were performed on F2.

Fully labelled diagrams and data collection and refinement details are included in the ESI.

Computational details. All calculations were performed using Gaussian09-E.0135 for the

Linux operating system. Density functional theory calculations were carried out using a combination of Becke's 1988 gradient-corrected exchange functional36 and Perdew's 1986 electron correlation functional37 (BP86). For open shell systems, unrestricted wave functions were used in energy calculations. A mixed-basis set was employed, using the LANL2TZ(f) triple zeta basis set with effective core potentials for Cobalt,38,39 Gaussian09's internal 6-311+G(d) for atoms bonded directly to the metal centers (nitrogen, phosphorus), and Gaussian09's internal

LANL2DZ basis set (equivalent to D95 V40) for carbon and hydrogen. Basis sets and functionals were chosen based on methods that had previously proven successful with other homobimetallic complexes and were optimized for computational time and accuracy.11,13,41 Starting with crystallographically determined geometries as a starting point, the geometries were optimized to a minimum, followed by analytical frequency calculations to confirm that no imaginary frequencies were present.

i 2 i i Synthesis of Co(µ- PrNPPh2)3Co(η - PrNPPh2) (6). A solution of K[ PrNPPh2] (1.4 g,

5.0 mmol) in THF (5 mL) was cooled to -32 °C and this was added to CoCl2 (0.325 g, 2.5 mmol) in THF (2 mL). The reaction mixture was stirred at room temperature for 12 h to ensure completion of reaction. The insoluble materials were removed via filtration. Volatiles were

53 removed in vacuo. The crude materials were extracted with ether (4 x 2 mL) to remove KCl and other insoluble impurities. Dark brown blocks of single crystals were obtained by allowing a

1 concentrated Et2O solution of 6 to stand at room temperature (0. 95 g, 69%). H NMR (400

- MHz, C6D6): δ 19.4, 8.4, 7.2, 5.1, 3.9, 2.4, 1.9, -1.0, -8.9, -10.4. UV-vis (C6H6) λmax nm (ε, Lmol

1 -1 cm ): 499 (1040), 668 (390). Evans’ method C6D6: 4.42 µB, Anal. Calcd for C60H68Co2N4P4: C,

66.72; H, 6.59; N, 5.02. Found: C, 66.37; H, 6.34; N, 5.24.

i i Synthesis of Co(µ- PrNPPh2)3Co( PrNHPPh2) (7). A 0.5 % of Na/Hg amalgam was prepared from 0.012 g of Na (0.6 mmol) and 2.32 g of Hg in THF (30 mL). A solution of 6 (0.37 g, 0.4 mmol) in THF (10 mL) was chilled at -32 °C, and added drop-wise to the above mixture.

The resulting mixture was stirred at room temperature for 12 h. The reaction mixture was carefully decanted from the amalgam and the insoluble materials were removed by filtration.

Volatiles were removed from the filtrate in vacuo. The remaining crude materials were extracted with ether and solvent removal resulted in a brown crystalline solid. Complex 7 was further purified by recrystallization of crude materials in ether at room temperature (0.27 g, 62%). 1H

NMR (400 MHz, C6D6): δ 15.1, 8.4, 7.4, 7.2, 4.6, 0.9, -0.4, -1.0, -15.7. IR (KBr solution cell,

-1 benzene): 3400 cm . UV-vis (C6H6) λmax nm (ε, L mol-1 cm-1): 497 (1760), 671 (530). Evan’s method C6D6: 3.88 µB. Anal. Calcd for C60H69N4P4Co2: C, 66.24; H, 6.39; N, 5.15. Found: C,

66.12; H, 6.44; N, 5.09.

i Synthesis of Co(µ- PrNPPh2)3Co(PMe3) (8). Route 1: A solution of 7 (0.37 g, 0.4 mmol) in THF (5 mL) was chilled to -32 °C and to this neat PMe3 (0.06 mL, 0.6 mmol) was added. Then the reaction mixture was stirred at room temperature for 12 h. Volatiles were removed in vacuo, and subsequently washed with pentane to remove soluble impurities. The crude materials were re-dissolved in ether and a concentrated ether solution of 8 was left at room

54 temperature to obtain the reddish-brown single crystals of 8 (0.21 g, 57%). Route 2: A 0.5 %

Na/Hg amalgam was prepared from 0.016 g of Na (0.7 mmol) and 3.2 g of Hg in THF (30 mL).

This was added to a THF (10 mL) solution of 6 (0.5 g, 0.46 mmol) and PMe3 (0.12 mL, 0.13 mmol). The resulting mixture was stirred at room temperature for 3.5 h to ensure completion of reaction. The reaction mixture was carefully decanted from the amalgam and the insoluble materials were removed by filtration. Volatiles were removed from the filtrate in vacuo. The remaining crude materials were extracted with ether and solvent removal resulted in brown crystalline solid. Allowing a concentrated ether solution of 8 to stand at room temperature

1 resulted in crystallization of analytically pure 8 (0.3 g, 71 %). H NMR (400 MHz, C6D6): δ

-1 -1 29.2, 11.3, 3.7, 0.8, -1.1, -7.0. UV-vis (C6H6) λmax nm (ε, Lmol cm ): 505 (1580), 677 (520).

Evan’s method C6D6: 3.33 µB. Anal. Calcd for C48H60N3P4Co2: C, 62.61; H, 6.57; N, 4.56.

Found: C, 62.55; H, 6.63; N, 4.41.

i Synthesis of Co(µ- PrNPPh2)3Co(NMes) (9). A solution of 8 (0.15 g, 0.17 mmol) in

THF (6 mL) was chilled to -32 °C, and was added to a solution 2,4,6-trimethylphenylazide (0.53 g, 0.33 mmol) drop-wise over a period of 5 min. The reaction mixture was stirred at room temperature overnight. Volatiles were removed in vacuo, and the crude materials were washed with pentane to remove the byproducts and soluble impurities. The crude green materials were dissolved in ether and the concentrated ether solution of 9 was left at room temperature to crystallize. Green blocks of single crystals of 9 were deposited in 2-3 days (0.14 g, 84%). 1H

NMR (400 MHz, C6D6): δ 19.9, 12.9, 3.3, 2.4, 1.2, -1.8, -5.7, -12.4. UV-vis (C6H6) λmax nm (ε,

-1 -1 Lmol cm ): 505 (1580), 677 (520). Evan’s method C6D6: 1.81 µB. Anal. Calcd for

C54H62N4P3Co2: C, 60.77; H, 8.64; N, 4.34. Found: C, 60.73; H, 8.53; N, 4.29.

i i Synthesis of Co( PrNHPPh2)(COPPh2NH Pr)(CO)2 (10). A 50 mL Schlenk tube was charged with a THF solution (20 mL) of complex 9 (0.05 g, 0.05 mmol). The resulting mixture was degassed with three freeze-pump-thaw cycles, and pressurized with CO (1 atm) at room temperature. The green reaction mixture was heated at 70 °C for 7 days to ensure completion of reaction or consumption of starting materials. The insoluble materials were removed via filtration. All volatiles were removed in vacuo from the filtrate. The crude golden-yellow materials were extracted with ether (4 x 2 mL) to remove insoluble impurities. The concentrated

Et2O solution of 10 was left at room temperature for crystallization. Golden-yellow single

1 crystals of 10 were obtained in 1-2 days. H NMR (400 MHz, C6D6): δ 30.19, 20.42, 16.87,

12.72, 7.16, 1.65, -12.96. IR (KBr solution cell, benzene): 1986 cm-1, 1927 cm-1, 1898 cm-1.

i i Synthesis of [Co(µ- PrNPPh2)2(µ- PrNPPh2N2CPh2)Co] (11). A solution of 8 (25 mg,

0.03 mmol) in THF (5 mL) was chilled to -32 °C, and was added to diazodiphenylmethane (10 mg, 0.06 mmol). The reaction mixture was stirred at room temperature for 12 h to ensure consumption of starting materials. Insoluble materials were removed via filtration. Volatiles were removed from the filtrate in vacuo. The crude materials were extracted with Et2O, and the concentrated Et2O solution of 11 was left at room temperature for crystallization. Red blocks of

1 single crystals of 11 were obtained in 1-2 days (22 mg, 71%). H NMR (400 MHz, C6D6): δ 33.7,

-1 - 23.4, 21.9, 17.6, 6.2, -2.0, -5.2, -7.7 -10.1, 11.9, -15.7 ppm. UV-vis (C6H6) λmax nm (ε, Lmol cm

1 ): 550 (900). Evan’s method C6D6: 3.14 µB. Anal. Calcd for C58H61Co2N5P3: C, 67.05; H, 5.92;

N, 6.74. Found: C, 66.97; H, 5.83; N, 6.63.

i Synthesis of [Co(µ- PrNPPh2)3Co(N3)][Et4N] (12). A solution of 8 (45 mg, 0.05 mmol) in THF (5 mL) was chilled to -32 °C, and was added to solid tetraethyl ammonium azide (17 mg,

0.10 mmol). The resulting mixture was continuously stirred at room temperature for 48 h. The

56 insoluble byproducts and impurities were removed via filtration. All volatiles were removed from the filtrate in vacuo. The orange crude materials were thoroughly washed with pentane and ether. Again the orange materials were extracted with THF. Pentane was layered over the concentrated THF solution of 12 at room temperature for crystallization. Single crystals of 12

1 were obtained in 3-4 days (20 mg, 34%). H NMR (400 MHz, C6D6): δ 28.5, 14.9, 11.3, 8.3, -

-1 -1 0.6, -3.8, -5.9, -12.8. UV-vis (C6H6) λmax nm (ε, Lmol cm ): 322 (1440), 480 (230). IR (KBr

-1 solution cell, benzene): 2360 cm . Anal. Calcd for C53H71Co2N7P3: C, 63.09; H, 7.55; N, 8.44.

Found: C, 62.51; H, 7.11; N, 9.56.

i Synthesis of Co(µ- PrNPPh2)3Co(OH)(KC12H24O6) (13). A solution of 8 (50 mg, 0.05 mmol) in THF (5 mL) was chilled to -32 °C, and was added to a solid KOH (3 mg, 0.05 mmol), and 18-C-6 (14 mg, 0.05 mmol). The reaction mixture was stirred at room temperature overnight.

The insoluble materials were removed via filtration. All volatiles were removed in vacuo. The crude materials were extracted with Et2O, and the concentrated Et2O solution of 13 was left at room temperature for crystallization. Red blocks of single crystals of 13 were obtained in 1-2

1 days (43 mg, 74%). H NMR (400 MHz, C6D6): δ 28.2, 14.9, 8.4, -2.8, -10.9, -12.8. UV-vis

-1 -1 (C6H6) λmax nm (ε, Lmol cm ): 405 (1120), 479 (460). IR (KBr solution cell, benzene): 3371

-1 cm . Evan’s method C6D6: 4.81 µB. (Owing to the instability of 13, repeated combustion analyses of single crystals were not consistent).

Appendix: Supporting Figures and Tables.

1 i 2 i Figure 15. H NMR of Co(µ- PrNPPh2)3Co(η - PrNPPh2) (6).

1 i i Figure 16. H NMR of Co(µ- PrNPPh2)3Co( PrNHPPh2) (7)

1 i Figure 17. H NMR of Co(µ- PrNPPh2)3Co(PMe3) (8).

1 i Figure 18. H NMR of Co(µ- PrNPPh2)3Co(NMes) (9).

1 i Figure 19. H NMR of [Co(µ- PrNPPh2)2(µ-iPrNPPh2N2CPh2)Co] (11).

1 i Figure 20. H NMR of [Co(µ- PrNPPh2)3Co(OH)](KC12H24O6) (13).

1 i Figure 21. H NMR of [Co(µ- PrNPPh2)3Co(N3)][Et4N] (12).

i i Figure 22. Cyclic voltammogram of Co(µ- PrNPPh2)3Co( PrNPPh2) (6) (2 mM in 0.4 M n [ Bu4N][PF6] in THF, scan rate = 100 mV/s). Open Circuit Potential = -0.95 V.

i Figure 23. Cyclic voltammogram of Co(µ- PrNPPh2)3Co(PMe3) (8) (2 mM in 0.4 M n [ Bu4N][PF6] in THF, scan rate = 100 mV/s). Open Circuit Potential = -2.37 V.

i Figure 24. Cyclic voltammogram of Co(µ- PrNPPh2)3Co(NMes) (9) (2 mM in 0.4 M n [ Bu4N][PF6] in THF, scan rate = 100 mV/s). Open Circuit Potential = -1.83 V.

Figure 25. UV- vis-NIR spectra of 6-13 in THF solution. The inset shows an expanded view of the near-infrared region of the spectrum

Table 5. X-ray diffraction Experimental Details of Complexes 6 – 8.

6 7 8

Chemical C60H68Co2N4P4 C60H69N4P4Co2 C48H60N3P4Co2 formula fw 1124.05 1087.99 920.79

T (K) 120 120 120

λ (Å) 0.71073 Å 0.71073 Å 0.71073 Å a (Å) 12.0049(6) 13.1374(4) 16.2529(5) b (Å) 12.3781(6) 14.4929(4) 16.2529(5) c (Å) 22.0973(11) 28.4813(8) 23.0307(8)

α (deg) 86.181(2) 90 90

β (deg) 85.929(2) 96.264(2) 90

γ (deg) 61.881(2) 90 120

V (Å3) 2886.7(3) 5390.4(3) 5268.7(3) space group P-1 P21/c1 P3/c1

Z 2 4 4

3 Dcalcd (g/cm ) 1.293 1.341 1.161

µ (cm–1) 0.728 0.777 0.783

R1, wR2a (I > 2σ) 0.0298, 0.0759 0.0334, 0.0822 0.0305, 0.0880 aR1 = Σ(||Fo| – |Fc||) / Σ|Fo|, wR2 = {Σ[w(Fo2 – Fc2)2/Σ[w(Fo)2}1/2.

Table 6. X-ray diffraction Experimental Details of Complexes 9 – 13.

9 11 12 13

Chemical C54H62N4P3Co2 C58H61Co2N5P3 C53H71Co2N7P3 C57H76Co2KN3O7P3 formula fw 977.90 1038.94 1161.19 1165.13

T (K) 120 120 120 120

λ (Å) 0.71073 Å 0.71073 Å 0.71073 Å 0.71073 Å a (Å) 15.6021(8) 10.5946(4) 13.0378(8) 12.6994(5) b (Å) 18.6443(10) 23.2922(9) 13.6173(8) 13.3588(6) c (Å) 16.9912(8) 21.2518(8) 19.0658(12) 19.8731(9)

α (deg) 90 90 71.036(4) 78.366(2)

β (deg) 91.749(3) 102.0740(10) 80.571(4) 79.363(2)

γ (deg) 90 90 69.944(4) 63.410(2)

V (Å3) 4940.3(4) 5108.4(3) 3001.9(3) 2935.7(2) space group P21/c1 P21/c1 P-1 P-1

Z 4 4 2 2

3 Dcalcd (g/cm ) 1.315 1.351 1.285 1.318

µ (cm–1) 0.809 0.787 0.680 0.769

R1, wR2a (I > 2σ) 0.0300, 0.0767 0.0329, 0.0888 0.0509, 0.1309 0.0425, 0.0839

aR1 = Σ(||Fo| – |Fc||) / Σ|Fo|, wR2 = {Σ[w(Fo2 – Fc2)2/Σ[w(Fo)2}1/2.

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