Synthesis, Characterization, and Crystal Structures of Monomeric and Dimeric Palladium(II) Amide Complexes

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation

Thermodynamic knowledge of the metal-ligand (M-L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd-X (X=OR and NHAr) bond heterolysis energies (ΔG_het (Pd-X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N',N'-tetramethylethylenediamine) as the model complexes. The ΔG_het (Pd-X) scales span a range of 2.6-9.0 kcal mol^-1 for ΔG_het (Pd-O) values and of 14.5-19.5 kcal mol^-1 for ΔG_het (Pd-N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X-H bond activation reveal that the M-X bond metathesis is dominated by differences of the X-H and Pd-X bond strengths, the former being more influential. The ΔG_het (Pd-X) and pK_a (X-H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd-X complexes.

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.

Biaryl Phosphine Based Pd(II) Amido Complexes: The Effect of Ligand Structure on Reductive Elimination

Kinetic studies conducted under both catalytic and stoichiometric conditions were employed to investigate the reductive elimination of RuPhos (2-dicyclohexylphosphino-2',6'-diisopropoxybiphenyl) based palladium amido complexes. These complexes were found to be the resting state in Pd-catalyzed cross-coupling reactions for a range of aryl halides and diarylamines. Hammett plots demonstrated that Pd(II) amido complexes derived from electron-deficient aryl halides or electron-rich diarylamines undergo faster rates of reductive elimination. A Hammett study employing SPhos (2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl) and analogues of SPhos demonstrated that electron donation of the "lower" aryl group is key to the stability of the amido complex with respect to reductive elimination. The rate of reductive elimination of an amido complex based on a BrettPhos-RuPhos hybrid ligand (2-(dicyclohexylphosphino)-3,6-dimethoxy-2',6'-diisopropoxybiphenyl) demonstrated that the presence of the 3-methoxy substituent on the "upper" ring of the ligand slows the rate of reductive elimination. These studies indicate that reductive elimination occurs readily for more nucleophilic amines such as N-alkyl anilines, N,N-dialkyl amines, and primary aliphatic amines using this class of ligands.

Synthesis of an iridium porphyrin amido complex

A synthetic route for iridium porphyrin amido complexes has been established. Treatment of a THF solution of Ir(ttp)(CO)Cl (1) (ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) with an aqueous mixture of NaOH and NaBH4, followed by 2-iodoethanol leads to the clean formation of the β-hydroxyethyl complex Ir(ttp)(C2H4OH) (2) in 92% yield. Heating a pyridine solution of complex 2 in the presence of ca. 5–10 equiv. of phthalimide (HNC8H4O2; HPhth) leads to the formation of Ir(ttp)(C5H5N)(CH2CH2Phth) (3). The replacement of pyridine by THF as the reaction solvent leads to the formation of the THF adduct Ir(ttp)(THF)(Phth) (4), which has been characterized spectroscopically. Heating a solution of 4 in pyridine leads to the formation of Ir(ttp)(C5H5N)(Phth) (5), which has been isolated in 72% yield. Complexes 3 and 5 have been characterized by X-ray crystallographic studies. Complexes 4 and 5 are rare examples of monomeric late transition metal-amido complexes and are the first examples of iridium-amido complexes featuring a porphyrin as a supporting ligand.

Evolution of a fourth generation catalyst for the amination and thioetherification of aryl halides

Many active pharmaceuticals, herbicides, conducting polymers, and components of organic light-emitting diodes contain arylamines. For many years, this class of compound was prepared via classical methods, such as nitration, reduction and reductive alkylation, copper-mediated chemistry at high temperatures, addition to benzyne intermediates, or direct nucleophilic substitution on particularly electron-poor aromatic or heteroaromatic halides. However, during the past decade, palladium-catalyzed coupling reactions of amines with aryl halides have largely supplanted these earlier methods. Successive generations of catalysts have gradually improved the scope and efficiency of the palladium-catalyzed reaction. This Account describes the conceptual basis and utility of our latest, "fourth-generation" palladium catalyst for the coupling of amines and related reagents with aryl halides. In the past five years, we have developed these catalysts using the lessons learned from previous generations of catalysts developed in our group and in other laboratories. The ligands on the fourth-generation catalyst combine the chelating properties of the aromatic bisphosphines of the second-generation systems with the steric properties and strong electron donation of the hindered alkylphosphines of the third-generation systems. The currently most reactive catalyst in this class is generated from palladium and a sterically hindered version of the Josiphos family of ligands that possesses a ferrocenyl-1-ethyl backbone, a hindered di-tert-butylphosphino group, and a hindered dicyclohexylphosphino group. This system catalyzes the coupling of aryl chlorides, bromides, and iodides with primary amines, N-H imines, and hydrazones in high yield. The reaction has broad scope, high functional group tolerance, and nearly perfect selectivity for monoarylation. It also requires the lowest levels of palladium that have been used for C-N coupling. In addition, this latest catalyst has dramatically improved the coupling of thiols with haloarenes to form C-S bonds. Using ligands that lacked one or more of the structural elements of the most active catalyst, we examined the effects of individual structural elements of the Josiphos ligand on catalyst activity. This set of studies showed that each one of these elements contributes to the high reactivity and selectivity of the catalyst containing the hindered, bidentate Josiphos ligand. Finally, we examined the effect of electronic properties on the rates of reductive elimination to distinguish between the effect of the properties of the M-N sigma-bond and the nitrogen electron pair. We have found that the effects of electronic properties on C-C and C-N bond-forming reductive elimination are similar. Because the amido ligands contain an electron pair, while the alkyl ligands do not, we have concluded that the major electronic effect is transmitted through the sigma-bond.

Electronic effects on reductive elimination to form carbon-carbon and carbon-heteroatom bonds from palladium(II) complexes

The electronic properties of reactive and ancillary ligands have a large impact on the rate and scope of reductive elimination reactions. The purpose of this review is to compare and discuss published data on the effect of ligand electronic properties on the rates and scope of reductive eliminations from palladium(II). An understanding of these effects is important because reductive elimination from palladium(II) is the product-forming step of a variety of catalytic processes. The scope of this review will encompass the effect of the electron-donating abilities of alkyl, aryl, amido, alkoxo, thiolato, and phosphido groups on the rate of reductive elimination, the relative importance of inductive and resonance effects on the rate of reductive elimination, the relative sensitivity of the different classes of reductive eliminations to electronic perturbations, and the effect of the differences in electronic properties between the two aryl groups of biaryl complexes undergoing reductive elimination. In addition, this review will include the effects of electronic properties of the ancillary ligands on the rate of reductive eliminations from palladium(II). The effect of the overall electron-donating ability of ancillary ligands and the effect of the relative orientation of ancillary ligands to the two reactive ligands on the rate of reductive elimination will be discussed. Where appropriate, electronic effects on reductive elimination from complexes of other metals are described.

Cyclometallated Pd(ii) azido complexes containing 6-phenyl-2,2′-bipyridyl or 2-phenylpyridyl derivatives: synthesis and reactivity toward organic isocyanides and isothiocyanates

Cyclometallated palladium(II) azido complexes containing C,N,N- or C,N-donor ligands, [Pd(N(3))L](HL = 6-phenyl-2,2'-bipyridine or 2-phenylpyridyl derivatives), showed different reactivities toward organic isocyanides and isothiocyanates. In particular, aryl isocyanides (CN-Ar) underwent insertion into the orthometallated Pd-C bond on the phenyl moiety of the supporting ligand (L) in [Pd(N(3))L] or [Pd(N(3))(PR(3))L] to selectively give carbodiimido [[Pd(N=C=N-Ar)L]], imidoyl [[Pd(N(3))(-C=N-Ar)(PR(3))L]], or imidoyl carbodiimido complexes [[Pd(N=C=N-Ar)(-C=N-Ar)L] or [Pd(N=C=N-Ar)(-C=N-Ar)(PR(3))L]], depending on reaction conditions. Interestingly, reactions of [Pd(N(3))(PR(3))L] with organic isothiocyanates gave unusual dinuclear complexes [(micro-SCN(4)-R)PdL](2), exhibiting the concurrent S- and N-coordinating thio-tetrazole bridge.

Trans Influence on the Rate of Reductive Elimination. Reductive Elimination of Amines from Isomeric Arylpalladium Amides with Unsymmetrical Coordination Spheres

To determine the trans effect on the rates of reductive eliminations from arylpalladium(II) amido complexes, the reactions of arylpalladium amido complexes bearing symmetrical and unsymmetrical DPPF (DPPF = bis(diphenylphosphino)ferrocene) derivatives were studied. THF solutions of LPd(Ar)(NMeAr') (L = DPPF, DPPF-OMe, DPPF-CF3, DPPF-OMe,Ph, DPPF-Ph,CF3, and DPPF-OMe,CF3; Ar = C6H4-4-CF3; Ar' = C6H4-4-CH3, Ph, and C6H4-4-OMe) underwent C-N bond forming reductive elimination at -15 C to form the corresponding N-methyldiarylamine in high yield. Complexes ligated by symmetrical DPPF derivatives with electron-withdrawing substituents on the DPPF aryl groups underwent reductive elimination faster than complexes ligated by symmetrical DPPF derivatives with electron-donating substituents on the ligand aryl groups. Studies of arylpalladium amido complexes containing unsymmetrical DPPF ligands revealed several trends. First, the complex with the weaker donor trans to nitrogen and the stronger donor trans to the palladium-bound aryl group underwent reductive elimination faster than the regioisomeric complex with the stronger donor trans to nitrogen and the weaker donor trans to the palladium-bound aryl group. Second, the effect of varying the substituents on the phosphorus donor trans to the nitrogen was larger than the effect of varying the substituents on the phosphorus donor trans to the palladium-bound aryl group. Third, the difference in rate between the isomeric arylpalladium amido complexes was similar in magnitude to the differences in rates resulting from conventional variation of substituents on the symmetric phosphine ligands. This result suggests that the geometry of the complex is equal in importance to the donating ability of the dative ligands. The ratio of the differences in rates of reaction of the isomeric complexes was similar to the relative populations of the two geometric isomers. This result and consideration of transition state geometries suggest that the reaction rates are controlled more by substituent effects on ground state stability than on transition state energies. In addition, variation of the aryl group at the amido nitrogen showed systematically that complexes with more electron-donating groups at nitrogen undergo faster reductive elimination than those with less electron-donating groups at nitrogen.

Platinum and palladium imido and oxo complexes with small natural bite angle diphosphine ligands

Treatment of L(2)MCl(2) (M = Pt, Pd; L(2) = Ph(2)PCMe(2)PPh(2) (dppip), Ph(2)PNMePPh(2) (dppma)) with AgX (X = OTf, BF(4), NO(3)) in wet CH(2)Cl(2) yields the dinuclear dihydroxo complexes [L(2)M(mu-OH)](2)(X)(2), the mononuclear aqua complexes [L(2)M(OH(2))(2)](X)(2), the mononuclear anion complexes L(2)MX(2), or mixtures of complexes. Addition of aromatic amines to these complexes or mixtures gives the dinuclear diamido complexes [L(2)Pt(mu-NHAr)](2)(BF(4))(2), the mononuclear amine complexes [L(2)M(NH(2)Ar)(2)](X)(2), or the dinuclear amido-hydroxo complex [Pt(2)(mu-OH)(mu-NHAr)(dppip)(2)](BF(4))(2). Deprotonation of the Pd and Pt amine or diamido complexes with M'N(SiMe(3))(2) (M' = Li, Na, K) gives the diimido complexes [L(2)M(mu-NAr)](2) associated with M' salts. Structural studies of the Li derivatives indicate association through coordination of the imido nitrogen atoms to Li(+). Deprotonation of the amido-hydroxo complex gives the imido-oxo complex [Pt(2)(mu-O)(mu-NAr)(dppip)(2)].LiBF(4).LiN(SiMe(3))(2), and deprotonation of the dppip Pt hydroxo complex gives the dioxo complex [Pt(mu-O)(dppip)](2).LiN(SiMe(3))(2).2LiBF(4).

Theoretical studies on C–heteroatom bond formation via reductive elimination from group 10 M(PH3)2(CH3)(X) species (X = CH3, NH2, OH, SH) and the determination of metal–X bond strengths using density functional theory

Density functional calculations have been used to investigate C-C, C-N and C-O bond forming reactions via reductive elimination from Group 10 cis-M(PH3)2(CH3)(X) species (X = C-I3, NH2, OH). Both direct reaction from the four-coordinate species and a three-coordinate mechanism involving initial PH3 loss have been considered. For the four-coordinate pathway the ease of reductive elimination to give M(PH3)2 and CH3-X follows the trend M = Pd < Pt < Ni. The reaction of the cis-M(PH3)2(CH3)(NH2) species is promoted by the formation of methylamine adducts. Non-planar transition states are located and the C-heteroatom bond forming processes are characterised by migration of CH3 onto the cis-heteroatom ligand. For a given ligand, X, activation energies follow the trend M = Ni < Pd < Pt. Formation of the three-coordinate M(PH3)(CH3)(X) species is promoted by a labilisation of the cis-PH3 ligand in the four-coordinate reactants when X = NH2 or OH. For the three-coordinate pathway the energy change for reductive elimination to give M(PH3) and CH3-X again follows the trend M = Pd < Pt < Ni and in all cases the initial product is an M(PH3)(XCH3) adduct. The three-coordinate transition states again involve migration of the CH3 ligand onto the cis-X ligand and for X = NH2 or OH activation energies follow the trend Ni > Pd < Pt. For a given metal activation energies in both the four- and three-coordinate pathways increase along the series CH3 < NH2 < OH. These trends in activation energy can be rationalised in terms of the strength of M-CH3/M-X bonding as long as the extent of geometrical distortion required to obtain the transition state geometry is taken into account. Further calculations on cis-Pd(PH3)2(CH3)(SH) suggest that the more common experimental observation of C(sp3)-S compared to C(sp3)-O reductive elimination arises from the greater kinetic accessibility of the former process rather than an intrinsic thermodynamic preference for C-S bond formation. By comparison, the calculations indicate that C(sp3)-N reductive elimination should be feasible from Ni and Pd systems. DF calculations are shown to reproduce the relative homolytic bond strengths determined experimentally for Pt-X bonds. In the cis-M(PH3)2(CH3)(X) systems the M-CH3 homolytic bond strength increases down the group while for M-NH2 and M-OH bonds the trend is M = N approximately equal to Pd < Pt. M-NH2 and M-OH are considerably stronger than M-CH3 bonds and the presence of a heteroatom ligand serves to weaken M-CH3 bonds even further.

Dinuclear rhodium and iridium complexes with mixed amido/methoxo and amido/hydroxo bridges

The reactions of [[M(mu-OMe)(cod)](2)] (M = Rh, Ir; cod = 1,5- cyclooctadiene) with p-tolylamine, alpha-naphthylamine, and p-nitroaniline gave complexes with mixed-bridging ligands, [[M(cod)](2)(mu-NHAr)(mu-OMe)]. Similarly, the related complexes [[Rh(cod)](2)(mu-NHAr)(mu-OH)] were prepared from the reactions of [[Rh(mu-OH)(cod)](2)] with p-tolylamine, alpha-naphthylamine, and p-nitroaniline. The reactions of [[Rh(mu-OR)(cod)](2)] (R = H, Me) with o-nitroaniline gave the mononuclear complex [Rh(o-NO(2)C(6)H(4)NH)(cod)]. The syntheses of the amido complexes involve a proton exchange reaction from the amines to the methoxo or hydroxo ligands and the coordination of the amide ligand. These reactions were found to be reversible for the dinuclear complexes. The structure of [[Rh(cod)](2)(mu-NH[p-NO(2)C(6)H(4)])(mu-OMe)] shows two edge-shared square-planar rhodium centers folded at the edge with an anti configuration of the bridging ligands. The complex [[Rh(cod)](2)(mu-NH[alpha-naphthyl])(mu-OH)] cocrystallizes with [[Rh(mu-OH)(cod)](2)] and THF, forming a supramolecular aggregate supported by five hydrogen bridges in the solid state. In the mononuclear [Rh(o-NO(2)C(6)H(4)NH)(cod)] complex the o-nitroamido ligand chelates the rhodium center through the amido nitrogen and an oxygen of the nitro group.

Interactions of aziridines with nickel complexes: oxidative-addition and reductive-elimination reactions that break and make C-N bonds

Reaction of the N-tosylaziridines (p-CH(3)C(6)H(4)SO(2))NCH(2)CHR (1a, R = H; 1b, R = Me; 1c, R = n-Bu; 1d, R = i-Pr) with (bpy)Ni(cod) (2; bpy = 2,2'-bipyridine; cod = 1,5-cyclooctadiene) or (bpy)NiEt(2) (3) results in elimination of cod or butane from 2 and 3, respectively, and oxidative addition of an aziridine C-N bond to give the azametallacyclobutane complexes (bpy)Ni(NTosCHRCH(2)) (4a, R = H; 4b, R = Me; 4c, R = n-Bu; 4d, R = i-Pr) as maroon solids in 50-70% isolated yields. The structure of 4b exhibits a puckered four-membered azametallacycle containing a pyramidal nitrogen and with Ni-N(1) = 1.911(5) A; the tosyl group on N and the methyl substituent on the adjacent C are disposed in an anti conformation. The monodeuterated aziridine syn-(p-CH(3)C(6)H(4)SO(2))NCHDCH-n-Bu (1e) reacts with either 2 or 3 to give (bpy)Ni[NTosCH(n-Bu)CHD] (4e) in 60-65% yield, having an anti arrangement of the methine and methylene protons in the azametallacycle, and indicates that >95% inversion of stereochemistry has occurred at the methylene carbon during the oxidative-addition reaction. When the azametallacyclobutane complexes 4a-e are exposed to oxygen, oxidatively induced reductive elimination ensues, giving the free aziridines in 30-60% isolated yields. In the oxidation of 4e, the product aziridine is spectroscopically identical to its parent, 1e, indicating the elimination that forms the C-N bond also proceeds with inversion of stereochemistry (approximately 92% by (1)H NMR) at the methylene carbon.

Low-valent chemistry of cobalt amide. Synthesis and structural characterization of cobalt(II) amido, aryloxide, and thiolate compounds

The binuclear cobalt(II) amide complex [(CoL2)2-(TMEDA)] (1) [L = N(Si(t)BuMe2)(2-C5H3N-6-Me); TMEDA = Me2NCH2CH2NMe2] has been synthesized by the reaction of anhydrous CoCl2 with 2 equiv of [Li(L)(TMEDA)]. X-ray crystallography revealed that complex 1 consists of two [CoL2] units linked by one TMEDA ligand molecule, which binds in an unusual N,N'-bridging mode. Protolysis of 1 with the bulky phenol Ar(Me)OH (Ar(Me) = 2,6-(t)Bu2-4-MeC6H2) and thiophenol ArSH (Ar = 2,4,6-(t)Bu3C6H2) gives the neutral monomeric cobalt(II) bis(aryloxide) [Co(OAr(Me))2(TMEDA)] (2) and dithiolate [Co(SAr)2(TMEDA)] (3), respectively. Complexes 1-3 have been characterized by mass spectrometry, microanalysis, magnetic moment, and melting-point measurements, in addition to X-ray crystallography.

Structural correlations and conformational preference in edge-sharing binuclear d8 complexes with XR2 bridges. A theoretical study

The binuclear complexes of d8 transition metal ions of the type [M2(mu-XR2)2L4] (where M = RhI, IrI, NiII, PdII, PtII, or AuIII; X = S, N, P, or As) appear in a variety of molecular conformations in which the coordination planes around the two metal atoms are sometimes coplanar, sometimes bent. For the bent compounds with asymmetric bridges, XR1R2, the substituents adopt different orientations relative to the metal framework and to each other. Ab initio theoretical studies on the different conformers of 30 representative complexes, complemented with a structural database analysis, have allowed the establishment of structural correlations in this family of compounds. The conformational choice results from a delicate balance of different interactions which are qualitatively analyzed, such as the changes in bond angles around the bridging atoms, the existence of weak metal...metal bonding in the bent structures, and steric interactions involving the terminal ligands and the substituents at the bridging atoms.