Ligand design for site-selective installation of Pd and Pt centers to generate homo- and heteropolymetallic motifs

Abstract

The modular synthesis of a series of nitrogen-rich polydentate ligands that feature a common pincer-type framework is reported. These ligands allow for site-selective installation of palladium and platinum to give rise to bi-and trimetallic complexes that have d 8 -d 8 interactions.Natural and synthetic polymetallic reaction sites mediate a wide range of small molecule transformations that have not been realized in monometallic analogs. 1 For example, synthetic bimetallic complexes that contain d 8 -d 8 interactions display unusual photochemical reactivity 2 and have interesting electronic structures 3 owing to the metal-metal orbital interaction. Studies of these systems-and of polymetallic complexes more generally-are enhanced by the synthetic ability to rationally generate specific homo-and heteropolymetallics of interest. 4 We are beginning to explore the synthesis and siteselective metallation of polynucleating ligands in order to generate such complexes. Reported herein are initial efforts toward the preparation of nitrogen-rich ligands that feature a common pincer-type framework. We establish the utility of this family of ligands for the site-selective installation of Pd and Pt centers which can result in bi-and trimetallic systems featuring d 8 -d 8 interactions in the solid-state and in solution.The four ligands described in this communication feature two or three tridentate binding sites. Common to each ligand is a monoanionic, bis(quinolinyl)amide (BQA) pincer-type binding site. Decoration of the pincer scaffold with one or two neutral binding sites affords access to the polymetallic complexes of interest herein. We have chosen in this initial investigation to target Pd and Pt species due to our prior experience with the chemistry of group 10 BQA complexes, 5 and due to their low lability by comparison to first row late metals.The ligand synthesis of the dinucleating L1 R H series (Scheme 1) is accomplished in three steps from 8-bromo-2-methylquinoline (1). Oxidation of 1 with SeO 2 provides access to aldehyde 2. Reductive amination of 2 with NaHB(OAc) 3 and the appropriate secondary amine installs the neutral tridentate binding sites in amine intermediates 3, 4, and 5. The BQA binding pocket is generated by Pd-mediated C-N cross-coupling of 3, 4, and 5 with 8-aminoquinoline to give yellow ligands 6, 7, and 8, 5a respectively. The latter transformation proceeds to remarkably high conversion (>90%) given the potential for these ligands to poison the Pd cross-coupling catalyst. A nonadentate, trinucleating ligand (9) is prepared in an analogous synthesis from 5 and 8-amino-2-methylquinoline (see ESIw).The installation of Pd into the anionic binding site of 6 is readily achieved by reaction with (COD)Pd(Me)Cl and catalytic Et 3 N to form red, neutral L1 OMe PdCl 10 with concomitant loss of methane. Alternatively, 10 may be generated using (COD)PdCl 2 and stoichiometric Et 3 N; however, the use of (COD)Pd(Me)Cl obviates the need to purify 10 from Et 3 NÁ HCl. 1 H NMR spectroscopy of 10 shows the disappear...

PdII–PdII bonding interaction in dinuclear PdII complex with non-macrocyclic (O&N) chelates: Characterization, kinetics and DFT study

“…S13 and S14) (HOMO-LUMO energy separation) deduced from the DFT calculations, is significantly larger for 4 than for complex 1, the values being 2.28 eV and 1.64 eV, respectively, is closer to previously observed (HOMO-LUMO energy separation) by Mirica et al [23]. 1.84 Â 10 4 ± 14 35 2.41 Â 10 4 ± 12 45 2.70 Â 10 4 ± 09 15 k 2 (s À1 ) 8.88 ± 0.11 25 11.37 ± 0.09 35 13.84 ± 0.12 45 15.23 ± 0.24 …”

“…In the last several years, different research groups have targeted to synthesize Pd I and Pd II complexes persisting with metal-metal bonding interaction with different types of ligands by tuning the steric and electronic factors [2][3][4] using bridging or without bridging supports [5][6][7][8][9][10][11][12][13][14][15][16][17]. However, only a few Pd II dinuclear complexes with metal-metal [18][19][20][21][22][23][24][25] interactions have been described so far. In these cases, the weak d 8 -d 8 bonding interaction has been explained by symmetry allowed mixing of the Pd(5p z ) and (5s) orbitals into the 4dz 2 orbital of the Pd center [26][27][28][29], and DFT analysis have revealed Pd-Pd bond orders [22,24] of 0.10-0.15.…”

Methyl Complexes of the Transition Metals

“…115 Moving to structural and stoichiometric reactivity studies, Braunstein and Danopoulos described last year an unusual group interchange involving the rupture of Pd-CH3 and P-Ph bonds in palladium(II) complexes bearing non-symmetric diphosphines. 116 A unique transformation of a dinuclear palladium complex into a tetranuclear methylene bridged one was observed by Meyer and coworkers taking advantage of compartmental nitrogen ligands (Scheme 18): 117 Homo and heterobimetallic complexes of Pd and Pt exhibiting metal-methyl bonds have been generated by rational design of bridging structures, 118 Platinum methyl derivatives in both +2 and +4 oxidation states constitute a very ample family of organometallic compounds, mainly due to the availability of stable, easy-to-handle metal precursors such as PtMe2(DMSO)2, [PtMe2(dimethylsulfide)]2, [PtMe3I]4, etc. Aiming to offer a general and concise view on recent advances in this field, we will briefly mention some catalytic applications and focus the discussion on structural and synthetic aspects.…”

“…124b A number of heterobimetallic complexes including a Pt-CH3 unit appeared recently. 118,125 In a series of Pt-Zr dinuclear species, the influence of the electron-donating ability of the metalloligand RR'Zr(iPrNPPh2)2 on the Pt … Zr bond separation was discussed by Thomas and collaborators, who also reported on an unusual anagostic Zr … CH3-Pt interaction (Scheme 21a). 125a Chen et al were successful in isolating a rather elusive intermediate in transmetalation reactions performed with copper reagents and described a unique heterobimetallic Cu-Pt complex held together by a labile d 8 -d 10 bond supported by a bridging methyl group (Scheme 21b).…”

Mechanism of Evolution of Koneramine Complexes from One‐Pot Reactions: Snapshots of Intermediates Offer Facile Routes to New Dipicolylamines

“…[8][9][10][11] Anslyn and co-workers employed dpa-based compoundsi nd ynamic multicompo-nent covalenta ssemblies that are useful as chirality sensing agents. [12][13][14][15][16][17] Transition-metal complexeso fd pa-based ligands [18][19][20][21] are further seen in the literature frequently as new peptides, [22,23] artificial enzymes, [24] catalysts, [25][26][27][28][29] etc.T he strong chelating ability of tridentate dipicolylamine is often compromised as the centralNdonor (NH) of dpa is used as the substitution point for different functional componentst o create new ligands, thereby weakening the basicity of the N2 donor at times. The koneramines reported here have the fourth nitrogena tom that is not participating in coordination but available for any functional groups, which could make the koneramine complexes better than dipicolylamine complexes.…”