Angewandte Zuschriften Chemie

Deutsche Ausgabe: DOI: 10.1002/ange.201608599 – Complexes Internationale Ausgabe: DOI: 10.1002/anie.201608599 Trajectory of Approach of a Zinc–Hydrogen Bond to Transition Metals Olga Ekkert, Andrew J. P. White, and Mark R. Crimmin*

Abstract: Through a dramatic advance in the coordination center with a sterically demanding ligand, we recently isolated chemistry of the zinc–hydride bond, we describe the trajectory a rare example of a heterobimetallic complex possessing an for the approach of this bond to transition metals. The dynamic unsupported Cu-H-Zn moiety.[15c] Here we describe an reaction coordinate was interrogated through analysis of advance in the coordination chemistry of the zinc–hydride a series of solid state structures and is one in which the TM- bond to transition metals. Through analysis of a series of solid H-Zn angle becomes increasingly acute as the TM–Zn distance state structures and calculations, we describe the reaction decreases. Parallels may be drawn with the oxidative addition trajectory for the approach of a single zinc–hydride bond to of boron–hydrogen and silicon–hydrogen bonds to transition a transition metal. metal centers. Photolysis of a 1:1 mixture of the terminal zinc hydride 6 1 and either [TM(CO)6] (TM = Cr, Mo), [(h -C6H6)Cr(CO)3], 5 5 Appreciation of the trajectory for the approach of carbon– [(h -C5H4Me)Mn(CO)3], or [(h -C5H5)Co(CO)2] in benzene, hydrogen bonds to transition metals has led to a deeper toluene or THF solution with a 400 W Hg lamp resulted in understanding of catalytic processes involving the breaking of clean conversion to the corresponding heterobimetallic com- carbon–hydrogen bonds.[1] As the CH bond advances plexes (Scheme 1, 2a,b, 3a,b, 4a). While a similar protocol towards the metal, formation of an intermediate s-complex can preface oxidative addition.[2] Many researchers have described a continuum between the s-complex and oxidative addition product.[2,3] As the M–C distance decreases, the H- M-C angle becomes increasingly acute and electron density is transferred from both the d-orbitals of the metal and breaking CH bond to the forming MC and MH bonds. are not privileged in this regard, s-borane and s-silane complexes have an extensive chemistry and the relationship between them and the oxidative addition products metal boryls and metal silyls is well understood (Figure 1).[4]

Figure 1. s-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 Scheme 1. Synthesis of new TM-H-Zn complexes 2–5. seeded ideas that ultimately led to the discovery of dihydro- gen complexes.[6] A handful of heterobimetallic complexes containing transition metal and zinc centers bridged by could not be used to synthesize the tungsten pentacarbonyl [7–15] hydride ligands are known. The majority however, include complex 2c, photolysis of [W(CO)6]in[D8]THF for 6 h more than one bridging hydride ligand clouding analysis of followed by addition of 1 gave the desired product. The the TM-H-Zn group. Through kinetic protection of a zinc rhodium complex 5b was prepared by in situ generation of [10a] [Cp*Rh(H)2(SiEt3)(ZnBDI)] (5a) followed by photolysis

in the presence of excess PMe3 (BDI = {2,6- [*] O. Ekkert, A. J. P. White, Dr. M. R. Crimmin i Department of Chemistry, Imperial College London Pr2C6H3NCMe}2CH). A series of increasingly electron-rich South Kensington, London, SW7 2AZ (UK) ligands were included on the transition metal fragments to E-mail: [email protected] allow variation of the electron density at the TM center. Supporting information for this article can be found under: The new heterobimetallic complexes 2–5 posses a Zn-H- http://dx.doi.org/10.1002/anie.201608599. TM functional group. Inspection of the solid state structures

Angew. Chem. 2016, 128, 1 – 5 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 These are not the final page numbers! Ü Ü Angewandte Zuschriften Chemie

Figure 2. The crystal structures of a) 2b, 3a, 3b, 4a, and 5b.b)ZnH and TMZn bond lengths and formal shortness ratio (fsr).

reveals a number of distinct coordination modes of the zinc reveals that the carbonyl stretches are near identical to those hydride which may be differentiated by the formal shortness reported for both s-alane and four coordinate s-borane ratio (fsr) of the Zn–TM distance (Figure 2).[16] The formal complexes (Table S1 in the Supporting Information). The shortness ratio normalizes the metal–metal distance and has latter s-complexes have been described as having little or no been used to evaluate the intermetallic interaction in back-bonding to the HE bond from the transition metal complexes containing two metals in close proximity.[16] center and based on infrared spectroscopy it appears 3b is no Data are consistent with formation of s-zincane com- different.[17,20]

plexes of the “M(CO)5” fragment in 2a–c. The Zn–M fsr is The cobalt and rhodium analogues 4a and 5b posses a fsr 1.08–1.09 and the ZnH bond lengths, confirmed by DFT of < 1: This is consistent with the metal–metal distance being methods, range between 1.7–1.8 . smaller than that of the sum of the single bond radii. The Analogous s-alane and s-gallane complexes have been rhodium complex 5b also contains exceptionally long Zn–H reported by Aldridge and co-workers.[17] Soluble in hydro- distances of 2.1–2.2 and a very acute TM-H-Zn angle. 4a carbon solvents the hydride ligands of 2a–c resonate between may represent a stretched s-complex, while 5b is reminiscent 1 d = 6 and 10 ppm in C6D6.The JW-H coupling constant of of the rhodium dihydride [Cp*Rh(H)2(SiEt3)(ZnBDI)] (5a). 54 Hz in 2c is observed in 183W satellites of the hydride We have previously described 5a as the product of complete [18] [10a] 1 resonance, while both cis and trans carbonyl ligands could hydride transfer from zinc to rhodium. The JRh-H coupling be assigned based on HMBC NMR experiments (see the constant of 35 Hz and n(Rh–H) stretch of 1976 cm1 for 5b Supporting Information). suggests a similar structure to 5a. Upon modification of ligand to include a p-coordinating A plot of the experimentally determined fsr as a function in 3a,b, a similar geometry of the Zn-H-TM of the TM-H-Zn angle for 2–5 is given in Figure 3. In all cases, moiety is observed. The fsr of this series of 1.02–1.04, the hydrogen atom was located in the Fourier difference map however, is smaller than that observed in 2a–c.In3a,b both during X-ray experiments and its position confirmed by DFT short Zn–CO distances [3a, 2.423(3) ; 3b, 2.548(3)] and the calculations (Figure S3). Accepting that a series of static solid deviation of the M-C-O angle from 1808 are consistent with state structures can be used to interrogate a dynamic reaction the formation of semi-bridging carbonyl ligands. Infrared coordinate,[3,21] the data show that as the zinc–hydride bond spectroscopy supports the formulation and n(CO) peaks may approaches the transition metal not only does the Zn–TM be assigned to terminal (3a, 1905 cm1; 3b, 1937 cm1) and distance decrease but the TM-H-Zn angle becomes increas- semi-bridging carbonyls (3a, 1799 cm1; 3b, 1852 cm1).[19] In ingly acute. In the solid-state data, the angle decreases from 3a,b only a single 13C carbonyl resonance was observed 1068 to 768 as the fsr decreases from 1.09 to 0.97. suggesting fast exchange of terminal and semi-bridging Calculations provide insight to the reaction trajectory. carbonyl ligands on the NMR timescale. Comparison of the Inspection of the charges for the ground state structures of 2– 5 n(CO) for 3b against the known series [(h -C5H4Me)Mn- 5 reveals that the hydrogen atom becomes less negatively

(CO)2(L)] (L = CO, H-B, H-Al, H-Ga, H-Si, H-Ge, H-Sn) charged as it approaches the transition metal. In conjugation,

2 www.angewandte.de 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2016, 128,1–5 Ü

Ü These are not the final page numbers! Angewandte Zuschriften Chemie

Acknowledgements

We are grateful to the European Research Council for provision of a Marie Curie Fellowship (O.E.) and starting grant (FluoroFix:677367). M.R.C. is grateful to the Royal Society for a University Research Fellowship.

Keywords: heterobimetallic complexes · hydrides · zinc · s-complexes

Figure 3. Plot of the fsr versus the TM-H-Zn angle for complexes 2–5. [1] a) J. F. Hartwig, Acc. Chem. Res. 2012, 45, 864; b) F. Kakiuchi, N. Chatani, Adv. Synth. Catal. 2003, 345, 1077. [2] a) R. H. Crabtree, Angew. Chem. Int. Ed. Engl. 1993, 32, 789; the Wiberg bond index of both TMH and TMZn bonds Angew. Chem. 1993, 105, 828; b) M. Brookhart, M. L. H. Green, increase as that of ZnH decreases and the magnitude of the G. Parkin, Proc. Natl. Acad. Sci. USA 2007, 104, 6908. (TMH) !Zn+ donor–acceptor interaction increases as [3] R. H. Crabtree, E. M. Holt, M. Lavin, S. M. Morehouse, Inorg. assessed by second-order perturbation theory. QTAIM cal- Chem. 1985, 24, 1986. culations support a progression from a s-zincane complex to [4] a) G. I. Nikonov, Adv. Organomet. Chem. 2005, 53, 217; b) S. a transition metal–zinc hydride complex. While curved bond Lachaize, S. Sabo-Etienne, Eur. J. Inorg. Chem. 2006, 2115; c) G. Alcaraz, S. Sabo-Etienne, Coord. Chem. Rev. 2008, 252, 2395; critical paths along with bond critical points (bcp) can be d) G. Alcaraz, M. Grellier, S. Sabo-Etienne, Acc. Chem. Res. identified between ZnH and TMH for 3a and 4a no Zn 2009, 42, 1640; e) J. C. Green, M. L. H. Green, G. Parkin, Chem. TM bcp was apparent. For 5b,noZnH bcp is found while Commun. 2012, 48, 11481. both TMH and ZnTM are apparent (Figure 4). [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/ Plenum, 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. Int. Ed. 2016, 55, 6951; Angew. Chem. 2016, 128, 7065. Figure 4. Selected data from a) NBO and b) QTAIM calculations on [11] M. A. Porai-Koshits, A. S. Antsyshkina, A. A. Pasynskii, G. G. 2–5. Sadikov, 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 In summary, we have prepared and crystallographically der Kerk, A. L. Spek, Organometallics 1984, 3, 156; b) P. H. characterized seven new zinc–transition metal heterobime- Budzelaar, A. A. H. van der Zeijden, J. Boersma, G. J. M. van tallic hydrides. Through variation of the transition metal der Kerk, A. L. Spek, A. J. M. Duissenberg, Organometallics fragment a series of “snapshots” of the reaction coordinate of 1984, 3, 159. [13] I. M. Riddlestone, N. A. Rajabi, J. P. Lowe, M. F. Mahon, S. A. the approach of a ZnH bond toward a transition metal have Macgregor, M. K. Whittlesey, J. Am. Chem. Soc. 2016, 138, been obtained. Calculations on each of the isolated complexes 11081. show that the reaction trajectory is characterized by the [14] a) M. Ohashi, K. Matsubara, T. Iizuka, M. Suzuki, Angew. Chem. transfer of electron density from the breaking, largely ionic Int. Ed. 2003, 42, 937; Angew. Chem. 2003, 115, 967; b) M. ZnH bond to forming, increasingly covalent, TMH and Ohashi, K. Matsubara, H. Suzuki, Organometallics 2007, 26, TMZn bonds. The reaction trajectory is analogous to 2330. the oxidative addition of silanes and to a transition [15] a) M. Plois, R. Wolf, W. Hujo, S. Grimme, Eur. J. Inorg. Chem. 2013, 3039; b) M. Plois, W. Hujo, S. Grimme, C. Schwickert, E. metal. Bill, B. de Bruin, R. Pçttgen, R. Wolf, Angew. Chem. Int. Ed. 2013, 52, 1314; Angew. Chem. 2013, 125, 1352; c) A. E. Nako, Q. W. Tan, A. J. P. White, M. R. Crimmin, Organometallics 2014, 33, 2685.

Angew. Chem. 2016, 128, 1 – 5 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.de 3 These are not the final page numbers! Ü Ü Angewandte Zuschriften Chemie

[16] The formal shortness ratio (fsr) is defined as the d(TMM)/[rM- 608C: J. Okuda, R. C. Murray, J. C. Dewan, R. R. Schrock,

(TM) + rM(M)] where rM is the single-bond radius of the atom Organometallics 1986, 5, 1681. defined by a) L. Pauling, J. Am. Chem. Soc. 1947, 69, 542; For [19] S. R. Parmelee, N. P. Mankad, Dalton Trans. 2015, 44, 17007. applications see: b) R. J. Eisenhart, L. J. Clouston, C. C. Lu, Acc. [20] a) P. D. Lee, J. L. King, S. Seebald, M. Poliakoff, Organometallics Chem. Res. 2015, 48, 2885; c) Multiple Bonds Between Metal 1998, 17, 524; b) S. Schlecht, J. F. Hartwig, J. Am. Chem. Soc. Atoms, 3rd ed. (Eds.: F. A. Cotton, C. A. Murillo, R. A. Walton), 2000, 122, 9435; c) T. Kakizawa, Y. Kawano, M. Shimoi, Springer, New York, 2005. Organometallics 2001, 20, 3211; d) W. Jetz, W. A. G. Graham, [17] a) I. M. Riddlestone, J. A. B. Abdalla, S. Aldridge, Adv. Organo- Inorg. Chem. 1971, 10, 4; e) F. Carr, E. Colomer, R. J. P. Corriu, met. Chem. 2015, 63, 1; b) I. M. Riddlestone, S. Edmonds, P. A. A. Vioux, Organometallics 1984, 3, 1272; f) U. Schubert, E. Kaufman, J. Urbano, J. I. Bates, M. J. Kelly, A. L. Thompson, R. Kunz, B. Harkers, J. Willnecker, J. Meyer, J. Am. Chem. Soc. Taylor, S. Aldridge, J. Am. Chem. Soc. 2012, 134, 2551; c) J. 1989, 111, 2572. Turner, J. A. B. Abdalla, J. I. Bates, R. Tirfoin, M. J. Kelly, N. [21] a) H. B. Brgi, J. D. Dunitz, Acc. Chem. Res. 1983, 16, 153; b) D. Phillips, S. Aldridge, Chem. Sci. 2013, 4, 4245; d) J. A. B. Scheschkewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D. Abdalla, I. M. Riddlestone, R. Tirfoin, N. Phillips, J. I. Bates, S. Bourissou, G. Bertrand, Angew. Chem. Int. Ed. 2004, 43, 585; Aldridge, Chem. Commun. 2013, 49, 5547; e) I. M. Riddlestone, Angew. Chem. 2004, 116, 595; c) T. R. Cundari, J. Am. Chem. J. Urbano, N. Phillips, M. J. Kelly, D. Vidovic, J. I. Bates, R. Soc. 1994, 116, 340; d) J. L. Vincent, S. Luo, B. L. Scott, R. Taylor, S. Aldridge, Dalton Trans. 2013, 42, 249; f) J. A. B. Butcher, C. J. Unkefer, C. J. Burns, G. J. Kubas, A. Lleds, F. Abdalla, I. M. Riddlestone, J. Turner, P. A. Kaufman, R. Tirfoin, Maseras, J. Toms, Organometallics 2003, 22, 5307. N. Phillips, S. Aldridge, Chem. Eur. J. 2014, 20, 17624. [18] The coupling constant is reminiscent of that found in tungsten hydride clusters containing 3-center 2-electron hydride bridges, Manuscript received: September 2, 2016 1 1 && && &&&& e.g [Cp*WH4]2 JW-H = 48 Hz and [Cp*WH3]3 JW-H = 43 Hz at Final Article published: ,

4 www.angewandte.de 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2016, 128,1–5 Ü

Ü These are not the final page numbers! Angewandte Zuschriften Chemie Zuschriften

Zink-Hydrid-Komplexe Sieben neue Zink-bergangsmetall(TM)- Hydride wurden hergestellt und kris- O. Ekkert, A. J. P. White, tallographisch charakterisiert. Durch M. R. Crimmin* &&&& — &&&& Variieren des TM-Fragments wurde eine Serie von Momentaufnahmen der Reak- Trajectory of Approach of a Zinc– tionskoordinate fr die Annherung der Hydrogen Bond to Transition Metals Zn-H-Bindung an das TM erhalten. Die Reaktionstrajektorie ist durch den Trans- fer von Elektronendichte von der weitge- hend ionischen Zn-H-Bindung zu den kovalenter werdenden TM-H- und TM-Zn- Bindungen gekennzeichnet.

Angew. Chem. 2016, 128, 1 – 5 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.de 5 These are not the final page numbers! Ü Ü