Oxidative Route to Abnormal NHC Compounds from Singly Bonded [M

Communication pubs.acs.org/Organometallics

Oxidative Route to Abnormal NHC Compounds from Singly Bonded [M−M] (M = Ru, Rh, Pd) Precursors Sayantani Saha, Prosenjit Daw, and Jitendra K. Bera* Department of Chemistry and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India S Supporting Information *

ABSTRACT: A new base-free entry to metal−aNHC compounds from metal−metal bonded bimetallic precursors and imidazolium salts is reported. Regioselective metalation proceeds via C−I oxidative addition of an annulated imidazo[1,2-a][1,8]naphthyridine system to [RuI−RuI], [RhII−RhII], and [PdI−PdI] single bonds, affording C5bound (abnormal) RuII−, RhIII−, and PdII−NHC compounds, respectively, at room temperature and in high yields.

T

alteration of the position of the X group on the imidazole unit, metal complexes with abnormal NHC (aNHC) ligands are also accessed.8 In addition to low-valent metal complexes, metal powder has also been successfully employed to obtain metal− NHC complexes from the imidazolium salts.9 The oxidative route to metal−NHC compounds is largely limited to low-valent late-transition-metal ions. To expand the scope of this chemistry, metal precursors based on metal−metal bonded compounds are used.10 Reaction of 2 equiv of [L1H]Br with [Ru I 2 (CO)4 (CH 3 CN) 6 ][OTf] 2 afforded [RuII (L 1 )(CO)2 Br(H 2 O)][OTf] (I) (Scheme 2). 10a Similarly, a pyridyl-functionalized imidazolium salt ([Py-NHC]Br) provided the corresponding mononuclear RuII−NHC compound [Ru(py-NHC)(CO)2Br2] (II).10b This oxidative methodology was extended to a singly bonded RhII−RhII system. Roomtemperature treatment of [Rh2(CO)4(acac)2][OTf]2 with 4 equiv of [L2H]Br in the presence of [nBu4N]OTf provided the

he chemistry of N-heterocyclic carbenes (NHCs) has assumed great importance in recent years owing to their rich applications in the areas of catalysis, organometallic materials, and metallodrugs.1 The synthesis of metal−NHC complexes involves the following protocols: (1) the reaction of the free, isolated NHC with the metal, (2) in situ deprotonation of the azolium salt followed by metalation, and (3) use of basic Ag2O or Cu2O to generate a Ag/Cu−NHC complex and subsequent NHC transfer to a late transition metal (transmetalation).2 In addition to these, oxidative addition of C−X (X = H, alkyl, halogen) of an imidazolium salt to a low-valent metal is an effective but less explored path (Scheme 1).3 The formation of metal−NHC compounds via Scheme 1. Oxidative Synthesis of Metal−NHC Compounds

Scheme 2. Synthesis of Metal−NHC Compounds from Metal−Metal Bonded Bimetallic Precursors

imidazolium C−H oxidative addition to Pd(0), Pt(0), or Ni(0) has been extensively studied.4 Donor-functionalized imidazolium salts are found to be particularly effective, as the donor group brings the imidazolium C−H in the vicinity of the metal center.5 Furthermore, the resultant chelate metal−NHC products are less prone to decomposition via a reductive elimination pathway. The Hahn group reported oxidative addition of 2-chlorobenzimidazoles to [M(PPh3)4] (M = Pd, Pt) yielding C2-metalated species, which upon protonation afford protic NHC complexes.6 Following the same principle, regioselective C8-metalation of bioactive caffeine and adenine are achieved, which on reaction with NH4BF4 gave the corresponding NH,NR−NHC (protic NHC) complexes.7 By © XXXX American Chemical Society

Received: October 15, 2015

A

DOI: 10.1021/acs.organomet.5b00872 Organometallics XXXX, XXX, XXX−XXX

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Organometallics mononuclear RhIII−NHC compound [Rh(L2)2(H2O)Br][OTf]2 (III) (Scheme 2).10c Unlike the Ru complexes I and II, the Rh complex III contains two NHC ligands, possibly due to the presence of basic ligands in the precursor molecule. For all of the cases, the metal−metal bond is oxidatively cleaved to give a mononuclear metal−NHC compound accompanied by an increase in metal oxidation number of one unit. An NHC ligand that utilizes an imidazolium backbone (C4/ 5 C ) carbon for metalation is characterized as a mesoionic or abnormal NHC (aNHC).11 The aNHCs are kinetically and energetically less stable than their C2-bound analogues but are stronger electron donors to the metal. The demand for new metal−aNHC compounds continues to grow because of their extraordinary catalytic applications.12 Various synthetic methodologies have been developed for their targeted syntheses, which include blocking of the C2 carbon or increasing the bulk of the wingtip groups.13 The relatively lesser abundance of metal−aNHC compounds than their NHC counterparts is attributed to the lower acidity of the C4/C5−H proton, which renders the regioselective C−H bond activation a difficult task. Incorporation of a halide at the C4/C5 carbon raises the prospect of regioselective metalation via an oxidative activation pathway. The oxidative route to metal−NHC compounds is largely limited to low-valent late-transition-metal ions and zerovalent metals.14 Given the wider availability of metal−metal bonded compounds, it may be possible to gain access to metal−aNHC complexes, which are difficult to obtain otherwise, via an oxidative route. Herein, we report a convenient and high-yield synthetic route to RuII/RhIII/PdII− aNHC compounds accessed from an iodo-functionalized imidazolium salt and metal−metal bonded bimetallic precursor molecules of [RuI−RuI], [RhII−RhII], and [PdI−PdI], respectively. To evaluate the reactivity of the imidazole C5−H with a metal−metal bond, an initial reaction of a C2-blocked n a p h t h y r id i n e - s u b st i t ut e d i m id a z o l e ( L 3 H ) w i t h [Ru2(CO)4(CH3CN)6][BF4]2 was carried out, which afforded the mononuclear cyclometalated complex {[Ru(L 3 )(CO)2(CH3CN)](BF4)}6 (1), where the metal−metal bond is cleaved with concomitant increases in metal oxidation from RuI to RuII (Scheme 3). The molecular structure of 1 (Figure

Figure 1. X-ray structure of the cationic unit in 1 with metal coordination sites shown in the inset.

Information). Although analytically pure compound could be obtained easily by recrystallization, the 1H NMR spectrum of 1 invariably showed broad signals in CD3CN. The 13C NMR revealed the cyclometalated carbon at δ 169.7 ppm. Although the imidazole C 5 −H is readily added to [Ru2(CO)4]2+, the corresponding imidazolium C−H does not react.15 To overcome this apparent inactivity, we implemented two alterations in the ligand framework: (1) C−H was replaced with C−I to favor oxidative addition and (2) a planar annulated imidazonaphthyridine system12a,16 was designed to prevent rotational flexibility and thus impose the Ru···C−X interaction. Accordingly, 5′-iodo-3-phenyli midazo[1,2-a][1,8]naphthyridine was synthesized by the treatment of 3phenylimidazo[1,2-a][1,8]naphthyridine with N-iodosuccinamide (NIS), which upon quaternization with CH3I afforded the iodo-functionalized imidazolium salt [L4I]I (Scheme S1 in the Supporting Information). Room-temperature treatment of [Ru2(CO)4(CH3CN)6][BF4]2 with 2 equiv of [L4I]I in dichloromethane afforded the neutral complex [RuL4(CO)2I2] (2) (Scheme 4). The C−I added to the metal−metal bond, Scheme 4. Synthesis of Compounds 2−4

Scheme 3. Synthesis of Compound 1

1) reveals a hexanuclear cluster with circular topology. The asymmetric unit is comprised of a {Ru(L3)(CO)2(CH3CN)} unit, which is one-sixth of the hexameric molecule. The cyclometalated ligand chelates through imidazole carbon (C11) and naphthyridine nitrogen (N2). Two cis positions are occupied by two carbonyls, and an acetonitrile is coordinated trans to the cyclometalated carbon. The imidazole nitrogen N4 from the neighboring unit completes the pseudo-octahedral geometry around Ru. The Ru1−C11(cyclometalated) distance is 2.004(6) Å, and the Ru1−N2(naphthyridine) distance is 2.194(4) Å. Six Ru centers adopt a pseudo-chair conformation with the closest and farthest Ru···Ru distances being 6.198 and 12.208 Å, respectively (Figure S2 in the Supporting

causing its oxidative scission and finally affording regioselectively a C5-bound (abnormal) RuII−NHC compound. The 13C NMR signal corresponding to the carbene carbon appears downfield at δ 178 ppm, and two carbonyl carbons resonate at δ 195 and 187 ppm. The X-ray structure of 2 reveals a distorted-octahedral geometry around the Ru center (Figure 2a). The NHC ligand chelates the metal though the carbene carbon (C10) and the naphthyridine nitrogen (N1). The Ru1− C10 and Ru1−N1 bond distances are 2.030(7) and 2.186(5) Å, respectively. Two mutually cis carbonyl ligands and two iodides fulfill the remaining sites of the octahedral coordination around the metal. B

DOI: 10.1021/acs.organomet.5b00872 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 2. X-ray structures of 2 and 3.

To widen the scope of metal−metal bonded compounds for accessing metal−aNHC compounds, an unsupported RhII−RhII system was employed. Room-temperature treatment of [Rh2(CO)4(acac)2][OTf]2 with [L4I]I in the presence of [nBu4N]I provided the mononuclear Rh(III) compound [Rh(L4)(CO)I3] (3) in high yield (Scheme 4), whereas the unsubstituted imidazolium salt ([L4H]I) (Figure S17 in the Supporting Information) remained inactive. The 13C NMR signal corresponding to the carbene carbon appears downfield at δ 172 ppm, and the carbonyl carbon resonates at δ 201 ppm. The X-ray structure of 3 reveals a distorted-octahedral geometry around the Rh center (Figure 2b). Similarly to complex 2, the NHC ligand chelates the metal though the carbene carbon (C10) and the naphthyridine nitrogen (N1). The Rh1−C10 and Rh1−N1 bond distances are 2.028(9) and 2.134(7) Å, respectively. One carbonyl ligand and three iodides fulfill the remaining sites of the octahedral coordination sphere around the metal. A range of PdII−NHC complexes are accessed via oxidative addition of an imidazolium salt to Pd0 precursors such as Pd(PPh3)4 and Pd2(dba)3.4 The use of dipalladium(I) precursors in bond activation chemistry, however, is limited. The C−H/Br bond of heteroaryl-substituted naphthyridines is oxidatively added to the metal−metal bonded dipalladium(I) solvento complex Pd2(CH3CN)6(BF4)2 to give a series of biand trinuclear palladium complexes.17 The Pd−Pd bond is oxidatively cleaved in the initiation step in the alkyne dimerization or polymerization reaction.18 Attempts to activate the C−H bond of the imidazolium salt ([L4H]I (Figure S17 in the Supporting Information), [L1H]Br, [L2H]Br) were unsuccessful and invariably ended in black metallic precipitation. Much to our delight, however, the reaction of [L4I]+ with Pd2(CH3CN)6(BF4)2 afforded the corresponding PdII− aNHC compound in high yield. [Pd2(CH3CN)6](BF4)2, dissolved in a small amount of acetonitrile, was treated with a dichloromethane solution of anion exchanged ligand [L4I]BF4 in a ratio of 1/2 (i.e., 1 equiv per Pd) at room temperature to give the PdII−aNHC complex [Pd(L 4 )(CH 3 CN) 2 ](BF 4 ) 2 (4) (Scheme 2). The C−I activation was evident in the ESI-MS, revealing a signal at m/ z (z = 2) 224.8 (Figure S18 in the Supporting Information) assigned to the [PdL4(CH3CN)2] unit. The molecular structure of complex 4 was established by X-ray crystallography. The core structure of 4 (Figure 3) consists of one Pd center and one ligand, where the metalation occurs at the C5 position of the imidazo moiety and chelate through the naphthyridine nitrogen. The Pd1−C10 and Pd1−N1 bond distances are 1.962(4) and 2.066(3) Å, respectively. Two mutually cis acetonitrile molecules fulfill the remaining sites of a squareplanar geometry around the metal, where the Pd1−N4 (trans

Figure 3. X-ray structure of the cationic unit in complex 4.

to carbene carbon) bond is longer (2.075(3) Å) in comparison to the other bond (1.986(4) Å), which is due to the strong trans effect. In summary, the metal−aNHC compounds are readily and reliably accessed from the metal−metal singly bonded precursors via a base-free oxidative route. The oxidative protocol is largely applied to late transition metals such as Ag, Pd, and Pt, but the present methodology gives access to the metal−aNHC products involving Ru and Rh. The annulated ligand system and the iodo substituent clearly favor oxidative reaction. Regioselective metalation proceeds via C−I oxidative addition to the metal−metal single bond in [RuI−RuI], [RhII− RhII], [PdI−PdI] precursors, resulting in RuII−, RhIII−, and PdII−aNHC compounds, respectively. The metal oxidation state in the final product is increased by one unit from that in the precursor molecule. High yields of products confirm the utilization of both metals. The annulated ligand system is an aNHC analogue of phenanthroline, and its metal complexes are of interest for organometallic catalysis. Efforts are ongoing to extend this chemistry to other metal−metal bonded systems, including metal clusters and multiply bonded compounds, and gain insight into the underlying mechanism of the process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00872. Experimental details, crystallographic data, and characterization data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.K.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dedicated to Professor T. K. Chandrashekar on the occasion of his 60th birthday. Financial support from the Department of Science and Technology (DST), India, and the Department of Atomic Energy (DAE) is gratefully appreciated. J.K.B. thanks the DAE for an SRC-OI fellowship. S.S. and P.D. thank CSIR, India, for fellowships. C

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