COMMUNICATION Trajectory of Approach of a Zn–H Bond to Transition Metals

Olga Ekkert, Andrew J. P. White and Mark R. Crimmin*

Dedication ((optional))

Abstract: Through a dramatic advance in the coordination chemistry recently isolated a rare example of a heterobimetallic complex of the – bond, we describe the trajectory for the approach possessing an unsupported Cu–H–Zn moiety.[15] Here we of this bond to transition metals. The dynamic reaction coordinate was describe an advance in the coordination chemistry of the zinc interrogated through analysis of a series of solid state structures and hydride bond to transition metals. Through analysis of a series of is one in which the TM–H–Zn angle becomes increasingly acute as solid state structures and calculations, we describe the reaction the TM---Zn distance decreases. Parallels may be drawn with the trajectory for the approach of a single zinc–hydride bond to a oxidative addition of boron–hydrogen and silicon–hydrogen bonds to transition metal. transition metal centers. Photolysis of a 1:1 mixture of the terminal zinc hydride 1 and 6 5 either [TM(CO)6] (TM = Cr, Mo), [(η -C6H6)Cr(CO)3], [(η - 5 C5H4Me)Mn(CO)3], or [(η -C5H5)Co(CO)2] in benzene, toluene or Appreciation of the trajectory for the approach of THF solution with a 400 W Hg lamp resulted in clean conversion carbon–hydrogen bonds to transition metals has led to a deeper to the corresponding heterobimetallic complexes (Figure 2, 2a-b, understanding of catalytic processes involving the breaking of 3a-b, 4a). While a similar protocol could not be used to synthesize carbon–hydrogen bonds.[1] As the C–H bond advances towards the tungsten pentacarbonyl complex 2c, photolysis of [W(CO)6] in the metal, formation of an intermediate σ-complex can preface d8-THF for 6h followed by addition of 1 gave the desired product. oxidative addition.[2] Many authors have described a continuum [2-3] The rhodium complex 5b was prepared by in situ generation of between the σ-complex and oxidative addition product. As the [10a] [Cp*Rh(H)2(SiEt3)(ZnBDI)] (5a) followed by photolysis in the M---C distance decreases, the H–M–C angle becomes i presence of excess PMe3 (BDI = {2,6- Pr2C6H3NCMe}2CH). A increasingly acute and electron density is transferred from both series of increasingly electron-rich ligands were included on the the d-orbitals of the metal and breaking C–H bond to the forming transition metal fragments to allow variation of the electron density M–C and M–H bonds. are not privileged in this regard, - σ at the TM center. borane and -silane complexes have an extensive chemistry and σ The new heterobimetallic complexes 2-5 posses a Zn–H–TM the relationship between them and the oxidative addition products functional group. Inspection of the solid state structures reveals a metal boryls and metal silyls is well understood (Figure 1).[4] number of distinct coordination modes of the zinc hydride which

may be differentiated by the formal shortness ratio (fsr) of the Zn---TM distance.[16] The formal shortness ratio normalizes the metal---metal distance and has been used to evaluate the intermetallic interaction in complexes containing two metals in close proximity.[16]

Figure 1. σ-Complexes and Oxidative Addition.

In comparison there is only limited precedent for the coordination of zinc to transition metal centers. Kubas and Shriver have commented on the nature of hydride-bridged zincate complexes in solution.[5] The proposition that these species could dimerize by 3-center 2-electron bonds seeded ideas that ultimately led to the discovery of dihydrogen complexes.[6] A handful of heterobimetallic complexes containing transition metal and zinc centers bridged by hydride ligands are known,[7-14] The majority however, include more than one bridging hydride ligand clouding analysis of the TM–H–Zn group. Through kinetic protection of a zinc center with a sterically demanding ligand, we

[a] Olga Ekkert, Andrew White, Mark Crimmin Department of Chemistry Imperial College London South Kensington, London, SW7 2AZ, UK E-mail: [email protected]

Supporting information for this article is given via a link at the end of Scheme 1. Synthesis of New TM–H–Zn Complexes 2-5 the document.

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Figure 2. The crystal structures of (a) 2b, 3a, 3b, 4a, and 5b. Along with tabulated data for the Zn–H, TM–Zn bond lengths and formal shortness ratio (fsr) .

Data are consistent with formation of σ-zincane complexes of the complex 5b also contains exceptionally long Zn---H distances of

“M(CO)5” fragment in 2a-c. The Zn---M fsr is 1.08-1.09 and the 2.1-2.2 Å and a very acute TM–H–Zn angle. 4a may represent a Zn–H bond lengths, confirmed by DFT methods, range between stretched σ-complex, while 5b is reminiscent of the rhodium 1.7-1.8 Å. Analogous σ-alane and σ-gallane complexes have dihydride [Cp*Rh(H)2(SiEt3)(ZnBDI)] (5a). We have previously been reported by Aldridge and coworkers.[17] Soluble in described 5a as the product of complete hydride transfer from zinc [10a] 1 solvents the hydride ligands of 2a-c resonate to rhodium. The JRh–H coupling constant of 35 Hz and ν(Rh– 1 -1 between δ = -6 and -10 ppm in C6D6. The JW–H coupling constant H) stretch of 1976 cm for 5b suggests a similar structure to 5a. of 54 Hz in 2c is observed in 183W satellites of the hydride A plot of the experimentally determined fsr as a function of the resonance,[18] while both cis and trans carbonyl ligands could be TM–H–Zn angle for 2-5 is given in Figure 3. In all cases, the assigned based on HMBC NMR experiments. hydrogen atom was located in the fourier difference map during Upon modification of ligand to include a π-coordinating X-ray experiments and its position confirmed by DFT calculations hydrocarbon in 3a-b, a similar geometry of the Zn–H–M moiety is (Figure S3). Accepting that a series of static solid state structures observed. The fsr of this series of 1.02-1.04, however, is smaller can be used to interrogate a dynamic reaction coordinate,[3,21] the than that observed in 2a-c. In 3a-b both short Zn---CO distances data show that as the zinc–hydride bond approaches the [3a 2.423(3) Å; 3b, 2.548(3)] and the deviation of the M–C–O transition metal not only does the Zn---TM distance decrease but angle from 180o are consistent with the formation of semi-bridging the TM–H–Zn angle becomes increasingly acute. In the solid- carbonyl ligands. Infra-red spectroscopy supports the formulation state data, the angle decreases from 106o to 76o as the fsr and ν(CO) peaks may be assigned to terminal (3a, 1905 cm-1; 3b, decreases from 1.09 to 0.97. 1937 cm-1) and semi-bridging carbonyls (3a, 1799 cm-1; 3b, 1852 cm-1).[19] In 3a-b only a single 13C carbonyl resonance was observed suggesting fast exchange of terminal and semi-bridging carbonyl ligands on the NMR timescale. Comparison of the ν(CO) for 3b against the known series 5 [(η -C5H4Me)Mn(CO)2(L)] (L = CO, H–B, H–Al, H–Ga, H–Si, H– Ge, H–Sn) reveals that the carbonyl stretches are near identical to those reported for both σ-alane and four coordinate σ-borane complexes (Table S1). The latter σ-complexes have been described as having little or no back-bonding to the H–E bond from the transition metal center and based on vibration spectroscopy it appears 3b is no different.[17,20]

The cobalt and rhodium analogues 4a and 5b posses a fsr of <1: This is consistent with the metal---metal distance being Figure 3. Plot of the fsr versus the TM–H–Zn angle for complexes 2-5. smaller than that of the sum of the single bond radii. The rhodium

COMMUNICATION Grellier, S. Sabo-Etienne, Acc. Chem. Res. 2009, 42, 1640. (e) J. C. Green, M. L. H. Green, G. Parkin, Chem. Commun. 2012, 48, 11481. [5] (a) G. J. Kubas, D. F. Shriver, J. Am. Chem. Soc. 1970, 92, 1949. (b) G. J. Kubas, D. F. Shriver, Inorg. Chem. 1970, 9, 1951. (c) For a recent review on molecular zinc hydrides see: A.-K. Wiegand, A. Rit, J. Okuda, Coord. Chem. Rev. 2016, 314, 71. [6] G. J. Kubas in Metal Dihydrogen and s-Bond Complexes: Structure, Theory and Reactivity, (Eds: J. P. Fackler), Kluwer Academic/Plenum Publishers, New York, 2001, pp. 20-21. [7] W. A. Skupinski, J. C. Huffman, J. W. Bruno, K. G. Caulton, J. Am. Chem. Soc. 1984, 106, 8128. [8] (a) M. D. Fryzuk, D. H. McConville, S. J. Rettig, Organometallics 1993, 12, 2152. (b) M. D. Fryzuk, D. H. McConville, S. J. Rettig, Organometallics 1990, 9, 1359. [9] M. Molon, C. Gemel, R. A. Fischer, Eur. J. Inorg. Chem. 2013, 3616. [10] (a) O. Ekkert, A. J. P. White, H. Toms, M. R. Crimmin, Chem. Sci. 2015, 6, 5617. (b) M. J. Butler, A. J. P. White, M. R. Crimmin, Angew. Chem., Figure 4. Selected data from (a) NBO and (b) QTAIM calculations on 2-5. Int. Ed. 2016, 55, 6951. [11] M. A. Porai-Koshits, A. S. Antsyshkina, A. A. Pasynskii, G. G. Sadikov, Calculations provide insight to the reaction trajectory. Inspection Y. V. Skripkin, V. N. Ostrikova, Inorg. Chim. Acta 1979, 34, L285. [12] (a) P. H. Budzelaar, K. H. den Haan, J. Boersma, G. J. M. van der Kerk, of the charges for the ground state structures of 2-5 reveals that A. L. Spek, Organometallics 1984, 3, 156. (b) P. H. Budzelaar, A. A. H. the hydrogen atom becomes less negatively charged as it van der Zeijden, J. Boersma, G. J. M. van der Kerk, A. L. Spek, A. J. M. approaches the transition metal. In conjugation, the Wiberg bond Duissenberg, Organometallics 1984, 3, 159. index of both TM–H and TM–Zn increase as that of Zn–H [13] I. M. Riddlestone, N. A. Rajabi, J. P. Lowe, M. F. Mahon, S. A. Macgregor, – + decreases and the magnitude of the (TM–H) àZn donor- M. K. Whittlesey, J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b05243. acceptor interaction increases as assessed by second-order [14] (a) M. Ohashi, K. Matsubara, T. Iizuka, M. Suzuki, Angew. Chem., Int. perturbation theory. QTAIM calculations support a progression Ed. 2003, 42, 937. (b) M. Ohashi, K. Matsubara, H. Suzuki, from a σ-zincane complex to a transition metal-zinc hydride Organometallics 2007, 26, 2330. [15] (a) M. Plois, R.Wolf, W. Hujo, S. Grimme, Eur. J. Inorg. Chem. 2013, complex. While curved bond critical paths along with bond critical 3039. (b) M. Plois, W. Hujo, S. Grimme, C. Schwickert, E. Bill, B. de Bruin, points (bcp) can be identified between Zn–H and TM–H for 3a and R. Pöttgen, R. Wolf, Angew. Chem., Int. Ed. 2013, 52, 1314. 4a no Zn–TM bcp was apparent. For 5b, no Zn–H bcp is found [15] A. E. Nako, Q. W. Tan, A. J. P. White, M. R. Crimmin, Organometallics while both TM–H and Zn–TM are apparent (Figure 4). 2014, 33, 2685.

In summary, we have prepared and crystallographically [16] The formal shortness ratio (fsr) is defined as the d(TM---M) / [rM(TM) + characterized seven new zinc---transition metal heterobimetallic rM(M)] where rM is the single-bond radius of the atom defined by (a) L. hydrides. Through variation of the transition metal fragment a Pauling, J. Am. Chem. Soc. 1947, 69, 542. For applications see: (b) series of “snap-shots” of the reaction coordinate of the approach Eisenhart, R. J.; Clouston, L. J.; Lu, C. C. Acc. Chem. Res. 2015, 48, 2885. (c) Cotton, F. A., Murillo, C. A., Walton, R. A., Eds. Multiple Bonds of a Zn–H bond toward a transition metal have been obtained. Between Metal Atoms, 3rd ed.; Springer: New York, 2005. Calculations on each of the isolated complexes show that the [17] (a) I. M. Riddlestone, J. A. B. Abdalla, S. Aldridge, Advances in reaction trajectory is characterized by the transfer of electron Organometallic Chemistry 2015, 63, p1. (b) I. M. Riddlestone, S. density from the breaking largely ionic Zn–H bond to forming, Edmonds, P. A. Kaufman, J. Urbano, J. I. Bates, M. J. Kelly, A. L. increasingly covalent, TM–H and TM–Zn bonds. The reaction Thompson, R. Taylor, S. Aldridge, J. Am. Chem. Soc. 2012, 134, 2551. trajectory is analogous to the oxidative addition of silanes and (c) J. Turner, J. A. B. Abdalla, J. I. Bates, R. Tirfoin, M. J. Kelly, N. Phillips, to a transition metal. S. Aldridge, Chem. Sci. 2013, 4, 4245. (d) J. A. B. Abdalla, I. M. Riddlestone, R. Tirfoin, N. Phillips, J. I. Bates, S. Aldridge, Chem. Commun. 2013, 49, 5547. (e) I. M. Riddlestone, J. Urbano, N. Phillips, M. J. Kelly, D. Vidovic, J. I. Bates, R. Taylor, S. Aldridge, Dalton. Trans. Acknowledgements 2013, 42, 249. (f) J. A. B. Abdalla, I. M. Riddlestone, J. Turner, P. A. Kaufman, R. Tirfoin, N. Phillips, S. Aldridge, Chem. Eur. J. 2014, 20, We are grateful to the European Research Council for provision 17624. of a Marie Curie Fellowship, MSCA IF action (OE) and starting [18] The coupling constant is reminiscent of that found in tungsten hydride clusters containing 3-center 2-electron hydride bridges, e.g [Cp*WH4]2 grant (FluoroFix:677367). MRC is grateful to the Royal Society for 1 1 o JW–H = 48 Hz and [Cp*WH3]3 JW–H = 43 Hz at 60 C: J. Okuda, R. C. a University Research Fellowship. Murray, J. C Dewan, R. R. Schrock, Organometallics 1986, 5, 1681. [19] S. R. Parmelee, N. P. Mankad, Dalton. Trans. 2015, 44, 17007. Keywords: Heterobimetallic • Hydrides • Zinc • σ-Complexes [20] (a) P. D. Lee, J. L. King, S. Seebald, M. Poliakoff, Organometallics 1998, 17, 524. (b) S. Schlecht, J. F. Hartwig, J. Am. Chem. Soc. 2000, 122, [1] (a) J. F. Hartwig, Acc. Chem. Res. 2012, 45, 864. (b) F. Kakiuchi, N. 9435. (c) T. Kakizawa, Y. Kawano, M. Shimoi, Organometallics 2001, 20, Chatani, Adv. Synth. Catal. 2003, 345, 1077. 3211. (d) W. Jetz, W. A. G. Graham, Inorg. Chem. 1971, 10, 4. (e) F. [2] (a) R. H. Crabtree, Angew. Chem., Int. Ed. 1993, 32, 789. (b) M. Carré, E. Colomer, R. J. P. Corriu, A. Vioux, Organometallics 1984, 3, Brookhart, M. L. H. Green, G. Parkin, PNAS 2007, 104, 6908. 1272. (f) U. Schubert, E. Kunz, B. Harkers, J. Willnecker, J. Meyer, J. Am. [3] R. H. Crabtree, E. M. Holt, M. Lavin, S. M. Morehouse, Inorg. Chem. Chem. Soc. 1989, 111, 2572. 1985, 24, 1986. [21] (a) H. B. Bürgi, J. D. Dunitz, Acc. Chem. Res. 1983, 16, 153. (b) D. [4] (a) G. I. Nikonov, Adv. Organomet. Chem. 2005, 53, 217. (b) S. Lachaize, Scheschkewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D. Bourissou, G. S. Sabo-Etienne, Eur. J. Inorg. Chem. 2006, 2115. (c) G. Alcaraz, S. Bertrand, Angew. Chem., Int. Ed. 2004, 43, 585. (c) T. R. Cundari, J. Am. Sabo-Etienne, Coord. Chem. Rev. 2008, 252, 2395. (d) G. Alcaraz, M. Chem. Soc. 1994, 116, 340. (d) J. L. Vincent, S. Luo, B. L. Scott, R.

COMMUNICATION Butcher, C. J. Unkefer, C. J. Burns, G. J. Kubas, A. Lledós, F. Maseras, J. Tomàs, Organometallics 2003, 22, 5307.

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The reaction trajectory for the Olga Ekkert, Andrew J. P. White and approach of a zinc–hydride bond to a Mark R. Crimmin* transition metal is described Page No. – Page No.

Trajectory of Approach of a Zn–H Bond to Transition Metals