Benzimidazole-diamide (bida) Pincer Chromium Complexes: Structures and Reactivity.

Serendipitous discovery of bida (i.e., N1-Ar-N2-((1-Ar-1-benzo[d]imidazol-2-yl)methyl)benzene-1,2-diamide; Ar = 2,6-iPr-C6H3), a potentially redox noninnocent, hemilabile pincer ligand with a methylene group that may facilitate proton/H atom reactivity, prompted its investigation. Chromium was chosen for study due to its multiple stable oxidation states. Disodium salt (bida)Na2(THF)n was prepared by thermal rearrangement of (dadi)Na2(THF)4 (i.e., (N,N'-di-2-(2,6-diisopropylphenylamine)phenylglyoxaldiimine)-Na2(THF)4). Salt metathesis of (bida)Na2(THF)n (generated in situ) with CrCl3(THF)3 or Cl3V═NAr (Ar = 2,6-iPr2C6H3) afforded (bida)CrCl(THF) (1-THF) and (bida)ClV═NAr, respectively. Substitutions provided (bida)CrCl(PMe2Ph) (1-PMe2Ph) and (bida)CrR(THF) (2-R, where R = Me, CH2CMe2Ph (Nph)). Oxidation of 1-THF with ArN3 (Ar = 2,6-iPr2C6H3) or AdN3 (Ad = 1-adamantyl) generated (bida)ClCr═NAr (3═NAr) and (bida)ClCr═NAd (3═NAd) and subsequent alkylation converted these to (bida)R'Cr═NR (R' = Me, R = Ad, Ar, 5═NR; R' = CH2CMe2Ph (Nph), R = Ad, Ar, 6═NR). In contrast, the addition of AdN3 to 2-Nph gave the insertion product (bida)Cr(κ2-N,N-ArN3Nph) (7). Addition of N-chlorosuccinimide to 1-THF produced (bia)CrCl2(THF) (8), where bia is the pincer derived via hydrogen atom loss from bida methylene. A similar HAT afforded (bia)ClCr(CNAr')2 (9, Ar' = 2,6-Me2C6H3) when 3═NAd was exposed to Ar'NC. An empirical equation of charge was applied to each bida species, whose metric parameters are unchanging despite formal oxidation state conversions from Cr(III) to Cr(V). Calculations and Mulliken spin density assessments reveal several situations in which antiferromagnetic (AF) coupling and admixtures of integer ground states (GSs) describe a complicated electronic structure.

Reversible C-C Bond Formation, Halide Abstraction, and Electromers in Complexes of Iron Containing Redox-Noninnocent Pyridine-imine Ligands.

The exploration of pyridine-imine (PI) iron complexes that exhibit redox noninnocence (RNI) led to several interesting discoveries. The reduction of (PI)FeX2 species afforded disproportionation products such as (dmpPI)2FeX (dmp = 2,6-Me2-C6H3, X = Cl, Br; 8-X) and (dippPI)2FeX (dipp = 2,6-iPr2-C6H3, X = Cl, Br; 9-X), which were independently prepared by reductions of (PI)FeX2 in the presence of PI. The crystal structure of 8-Br possessed an asymmetric unit with two distinct electromers, species with different electronic GSs: a low-spin (S = 1/2) configuration derived from an intermediate-spin S = 1 core antiferromagnetically (AF) coupled to an S = 1/2 PI ligand, and an S = 3/2 center resulting from a high-spin S = 2 core AF-coupled to an S = 1/2 PI ligand. Calculations were used to energetically compare plausible ground states. Polydentate diazepane-PI (DHPI) ligands were applied to the synthesis of monomeric dihalides (DHPI)FeX2 (X = Cl, 1-Cl2; X = Br, 1-Br2); reduction generated the highly distorted bioctahedral dimers (DHPA)2Fe2X2 ((3-X)2) containing a C-C bond formed from imine coupling; the monomers 1-X2 could be regenerated upon Ph3CX oxidation. Dihalides and their reduced counterparts were subjected to various alkyl halides and methyl methacrylate (MMA), generating polymers with little to no molecular weight control, indicative of simple radical-initiated polymerization.

Rare Examples of Fe(IV) Alkyl-Imide Migratory Insertions: Impact of Fe-C Covalency in (Me2IPr)Fe(═NAd)R2 (R = neoPe, 1-nor).

The iron(IV) imide complexes, (Me2IPr)-R2Fe=NAd (R = neoPe (3a), 1-nor (3b)) undergo migratory insertion to iron(II) amides (Me2IPr)RFe{NR(Ad)} (R = neoPe (4a), 1-nor (4b)) without evidence of imidyl or free nitrene character. By increasing the field strength about iron, odd-electron reactivity was circumvented via increased covalency.

Oxidatively Triggered Carbon-Carbon Bond Formation in Ene-amide Complexes.

Ene-amides have been explored as ligands and substrates for oxidative coupling. Treatment of CrCl2, Cl2Fe(PMe3)2, and Cl2Copy4 with 2 equiv of {(2,6-(i)Pr2C6H3)(1-(c)Hexenyl)N}Li afforded pseudosquare planar {η(3)-C,C,N-(2,6-(i)Pr2C6H3)(1-(c)Hexenyl)N}2Cr (1-Cr, 78%), trigonal {(2,6-(i)Pr2C6H3)(1-(c)Hexenyl)N}2Fe(PMe3) (2-Fe, 80%), and tetrahedral {(2,6-(i)Pr2C6H3)(1-(c)Hexenyl)N}2Co(py)2 (3-Co, 91%) in very good yields. The addition of CrCl3 to 1-Cr, and FeCl3 to 2-Fe, afforded oxidatively triggered C-C bond formation as rac-2,2'-di(2,6-(i)Pr2C6H3N═)2dicyclohexane (EA2) was produced in modest yields. Various lithium ene-amides were similarly coupled, and the mechanism was assessed via stoichiometric reactions. Some ferrous compounds (e.g., 2-Fe, FeCl2) were shown to catalyze C-arylation of {(2,6-(i)Pr2C6H3)(1-(c)Hexenyl)N}Li with PhBr, but the reaction was variable. Structural characterizations of 1-Cr, 2-Fe, and 3-Co are reported.

Nitrene Insertion into C-C and C-H Bonds of Diamide Diimine Ligands Ligated to Chromium and Iron.

The impact of redox non-innocence (RNI) on chemical reactivity is a forefront theme in coordination chemistry. A diamide diimine ligand, [{-CH=N(1,2-C6H4)NH(2,6-iPr2C6H3)}2](n) (n = 0 to -4), (dadi)(n), chelates Cr and Fe to give [(dadi)M] ([1Cr(thf)] and [1Fe]). Calculations show [1Cr(thf)] (and [1Cr]) to have a d(4) Cr configuration antiferromagnetically coupled to (dadi)(2-)*, and [1Fe] to be S = 2. Treatment with RN3 provides products where RN is formally inserted into the C-C bond of the diimine or into a C-H bond of the diimine. Calculations on the process support a mechanism in which a transient imide (imidyl) aziridinates the diimine, which subsequently ring opens.

Iron and chromium complexes containing tridentate chelates based on nacnac and imino- and methyl-pyridine components: triggering C-X bond formation.

Nacnac-based tridentate ligands containing a pyridyl-methyl and a 2,6-dialkyl-phenylamine (i.e., (2,6-R2-C6H3N═C(Me)CH═C(Me)NH(CH2py); R = Et, {Et(nn)PM}H; R = (i)Pr, {(i)Pr(nn)PM}H) were synthesized by condensation routes. Treatment of M{N(TMS)2}THFn (M = Cr, n = 2; M = Fe, Co, n = 1; TMS = trimethylsilane; THF = tetrahydrofuran) with {(i)Pr(nn)PM}H) afforded {(i)Pr(nn)PM}MN(TMS)2 (1-M(iPr); M = Cr, Fe); {Et(nn)PM}MN(TMS)2 (1-M(Et); M = Fe, Co) was similarly obtained. {R(nn)PM}FeBr (R = (i)Pr, Et; 2-Fe(R)) were prepared from FeBr2 and {R(nn)PM}Li, and alkylated to generate {R(nn)PM}Fe(neo)Pe (R = (i)Pr, Et; 3-Fe(R)). Carbonylation of 3-Fe(R) provided {(i)Pr(nn)PM}Fe(CO(neo)Pe)CO (4-Fe(iPr)), and carbonylations of 1-Fe(R) (R = Et, (i)Pr) and 1-Cr(iPr) induced deamination to afford {R(nn)PI}Fe(CO)2 (R = (i)Pr, 5-Fe(iPr); Et, 5-Fe(Et)), where PI is pyridine-imine, and {κ(2)-N,N-pyrim-pyr}Cr(CO)4 (6-Cr(iPr)), in which the aryl-amide side of the nacnac attacked the incipient PI group. Carbon-carbon bonds were formed at the imine carbon of the {R(nn)PI} ligand. Addition of [{(i)Pr(nn)PI}(2-)](K(+)(THF)x)2 to FeCl3 generated {(i)Pr(nn)CHpy}2Fe2Cl2 (7-Fe(iPr)), and TMSN3 induced the deamination of 1-Fe(Et), but with disproportionation to provide {[Et(nn)CHpy]2}Fe (8-Fe(Et)). Ph2CN2 induced C-C bond formation with 1-Fe(iPr) via its thermal degradation to ultimately afford {(i)Pr(nn)CHpy}2(FeN═CPh2)2 (9-Fe(iPr)). The compounds were examined by X-ray crystallography (1-M(iPr), M = Cr, Fe; 1-Co(Et); 2-Fe(iPr); 4-Fe(iPr); 5-Fe(iPr); 6-Cr(iPr); 7-Fe(iPr); 8-Fe(Et); 9-Fe(iPr)), Mössbauer spectroscopy, and NMR spectroscopy. Structural parameters assessing redox noninnocence are discussed, as are structural and mechanistic consequences of the various electronic environments.

Iron complexes derived from {nacnac-(CH2py)2}- and {nacnac-(CH2py)(CHpy)}n ligands: stabilization of iron(II) via redox noninnocence.

Nacnac-based tetradentate chelates, {nacnac-(CH2py)2}(-) ({nn(PM)2}(-)) and {nacnac-(CH2py)(CHpy)}(n) ({nn(PM)(PI)}(n)) have been investigated in iron complexes. Treatment of Fe{N(TMS)2}2(THF) with {nn(PM)2}H afforded {nn(PM)2}FeN(TMS)2 [1-N(TMS)2], which led to {nn(PM)2}FeCl (1-Cl) from HCl and to {nn(PM)2}FeN3 (1-N3) upon salt metathesis. Dehydroamination of 1-N(TMS)2 was induced by L (L = PMe3, CO) to afford {nn(PM)(PI)}Fe(PMe3)2 [2-(PMe3)2] and {nn(PM)(PI)}FeCO (3-CO). Substitution of 2-(PMe3)2 led to {nn(PM)(PI)}Fe(PMe3)CO [2-(PMe3)CO], and exposure to a vacuum provided {nn(PM)(PI)}Fe(PMe3) (3-PMe3). Metathesis routes to {nn(PM)(PI)}FeL2 (2-L2; L = PMe3, PMe2Ph) and {nn(PM)(PI)}FeL (3-L; L = PMePh2, PPh3) from [{nn(PM)(PI)}(2-)]Li2 and FeBr2(THF)2 in the presence of L proved feasible, and 1e(-) and 2e(-) oxidation of 2-(PMe3)2 afforded 2(+)-(PMe3)2 and 2(2+)-(PMe3)2 salts. Mössbauer spectroscopy, structural studies, and calculational assessments revealed the dominance of iron(II) in both high-spin (1-X) and low-spin (2-L2 and 3-L) environments, and the redox noninnocence (RNI) of {nn(PM)(PI)}(n) [2-L2, 3-L, n = 2-; 2(+)-(PMe3)2, n = 1-; 2(2+)-(PMe3)2, n = 0]. A discussion regarding the utility of RNI in chemical reactivity is proffered.

Reduction of (pddi)Cr reveals redox noninnocence via C-C bond formation amidst competing electrophilicity: [(cpta)CrMen]- (n = 0, 1) and [(pta)Cr].

Reversible cyclopropane formation is probed as a means of redox noninnocence in diimine/diamide chelates via reduction and complex anion formation. Competition from imine attack renders complications in the latter approach, and electrochemical measurements with calculational support provide the rationale.

Selective extraction of N2 from air by diarylimine iron complexes.

Treatment of cis-(Me3P)4FeMe2 with ortho-substituted diarylimines afforded 2 equiv of MeH, PMe3, and {mer-κC,N,C'-(Ar-2-yl)CH2N═CH(Ar'-2-yl)}Fe(PMe3)3 (Ar = 3,4,6-(F)3-C6H, Ar' = 3,5-(CF3)2-C6H2, 1a; Ar = 3,4,6-(F)3-C6H, Ar' = 3,4,5-(F)3-C6H, 1b; Ar = 4,5,6-(F)3-C6H, Ar' = 3,5-(CF3)2-C6H2, 1c; Ar = C6H4, Ar' = 3-(OMe)-C6H3, 1d; Ar = 4,5,6-(F)3-C6H, Ar' = 3,6-Me2-C6H3, 1e; Ar = C6H4, Ar' = 3,6-Me2-C6H2, 1f). Exposure of 1a-f to O2 caused rapid degradation, but substitution of the unique PMe3 with N2 occurred when 1a-f were exposed to air or N2 (1 atm), yielding {mer-κC,N,C'-(Ar-2-yl)CH2N═CH(Ar'-2-yl)}Fe(PMe3)2L (L = N2, 2a-f); CO, CNMe, and N2CPh2 derivatives (L = CO, 3a-d,f; L = CNMe, 8b; L = N2CPh2, 9b) were prepared. Dihydrogen or NH3 binding to {mer-κC,N,C'-(3,4,6-(F)3-C6H-2-yl)CH2N═CH-(3,4,5-(F)3-C6H-2-yl)}Fe(PMe3)2 (1b', S = 1 (calc)) to provide 5b (L = H2) or 6b (L = NH3) was found comparable to that of N2, while PMe3 (1b) and pyridine (L = py, 7b) adducts were unfavorable. Protolytic conditions were modeled using HCCR as weak acids, and trans-{κC,N-(3,4,5-(F)3-C6H2)CH2N═CH(3,4,6-(F)3-C6H-2-yl)}Fe(PMe3)3(CCR) (R = Me, 4b-Me; R = Ph, 4b-Ph) were generated from 1b. Exposure of 1b to N2O or N3SO2tol generated 2b and Me3PO or Me3P═N(SO2)tol, respectively. Calculations revealed 2b to be thermodynamically and kinetically favored over the calculated Fe(III) superoxide complex, (3)[FeO2], relative to 1b' + N2 + O2. The correlation of 1b' + (3)O2 to (3)[FeO2] is likely to have a relatively high intersystem crossing point (ICP) relative to 1b' + N2 to 2b, thereby explaining the dinitrogen selectivity.

C-C bond formation and related reactions at the CNC backbone in (smif)FeX (smif = 1,3-di-(2-pyridyl)-2-azaallyl): dimerizations, 3 + 2 cyclization, and nucleophilic attack; transfer hydrogenations and alkyne trimerization (X = N(TMS)2, dpma = (di-(2-pyridyl-methyl)-amide)).

Molecular orbital analysis depicts the CNC(nb) backbone of the smif (1,3-di-(2-pyridyl)-2-azaallyl) ligand as having singlet diradical and/or ionic character where electrophilic or nucleophilic attack is plausible. Reversible dimerization of (smif)Fe{N(SiMe3)2} (1) to [{(Me3Si)2N}Fe]2(µ-κ(3),κ(3)-N,py2-smif,smif) (2) may be construed as diradical coupling. A proton transfer within the backbone-methylated, and o-pyridine-methylated smif of putative ((b)Me2(o)Me2smif)FeN(SiMe3)2 (8) provides a route to [{(Me3Si)2N}Fe]2(µ-κ(4),κ(4)-N,py2,C-((b)Me,(b)CH2,(o)Me2(smif)H))2 (9). A 3 + 2 cyclization of ditolyl-acetylene occurs with 1, leading to the dimer [{2,5-di(pyridin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN(SiMe3)2]2 (11), and the collateral discovery of alkyne cyclotrimerization led to a brief study that identified Fe(N(SiMe3)2(THF) as an effective catalyst. Nucleophilic attack by (smif)2Fe (13) on (t)BuNCO and (2,6-(i)Pr2C6H3)NCO afforded (RNHCO-smif)2Fe (14a, R = (t)Bu; 14b, 2,6-(i)PrC6H3). Calculations suggested that (dpma)2Fe (15) would favorably lose dihydrogen to afford (smif)2Fe (13). H2-transfer to alkynes, olefins, imines, PhN═NPh, and ketones was explored, but only stoichiometric reactions were affected. Some physical properties of the compounds were examined, and X-ray structural studies on several dinuclear species were conducted.

Synthetic approaches to (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) reveal redox non-innocence and C-C bond-formation.

Attempted syntheses of (smif)(2)Ti (smif =1,3-di-(2-pyridyl)-2-azaallyl) based on metatheses of TiCl(n)L(m) (n = 2-4) with M(smif) (M = Li, Na), in the presence of a reducing agent (Na/Hg) when necessary, failed, but several apparent Ti(II) species were identified by X-ray crystallography and multidimensional NMR spectroscopy: (smif){Li(smif-smif)}Ti (1, X-ray), [(smif)Ti](2)(µ-κ(3),κ(3)-N,N(py)(2)-smif,smif) (2), (smif)Ti(κ(3)-N,N(py)(2)-smif,(smif)H) (3), and (smif)Ti(dpma) (4, dpma = di-2-pyridylmethyl-amide). NMR spectroscopy and K-edge XAS showed that each compound possesses ligands that are redox noninnnocent, such that d(1) Ti(III) centers AF-couple to ligand radicals: (smif){Li(smif-smif)(2-)}Ti(III) (1), [(smif(2-))Ti(III)](2)(µ-κ(3),κ(3)-N,N(py)(2)-smif,smif) (2), [(smif(2-))Ti(III)](κ(3)-N,N(py)(2)-smif,(smif)H) (3), and (smif(2-))Ti(III)(dpma) (4). The instability of (smif)(2)Ti relative to its C-C coupled dimer, 2, is rationalized via the complementary nature of the amide and smif radical dianion ligands, which are also common to 3 and 4. Calculations support this contention.

TEMPO coordination and reactivity in group 6; pseudo-pentagonal planar (η2-TEMPO)2CrX (X = Cl, TEMPO).

The exposure of CrCl2(THF)2 to 1 equiv. of TEMPO and 1 equiv. [TEMPO]Na afforded (η2-O,N-TEMPO)2CrCl (1, 67%); addition of [TEMPO]Na to 1 yielded (η2-O,N-TEMPO)2Cr(TEMPO) (2). Both 1 and 2 exhibit pseudo-pentagonal planar (PPP) geometry, instead of myriad alternatives. Calculations and spectral studies suggest the solid-state geometry persists in solution.

Carbon-carbon bond formation from azaallyl and imine couplings about metal-metal bonds.

Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation.

A theoretical study of the 3d-M(smif)2 complexes: structure, magnetism, and oxidation states.

We carry out a theoretical investigation of the recently reported M(smif)(2) series1,2 and find a number of interesting phenomena. These include complex potential energy surfaces with near-degenerate stationary points, low-lying states, non-trivial electron configurations, as well as non-innocent ligand behavior. The M(smif)(2) exhibit a delicate balance between geometry and electronic structure, which has implications not only for their reactivity but also for controlling their properties through ligand design. We address methodological issues and show how conceptual quantities such as oxidation states and electronic configurations can be extracted through a simple analysis of the electron and spin densities-without a complicated examination of the underlying orbitals.

Synthesis and characterization of (smif)2M(n) (n = 0, M = V, Cr, Mn, Fe, Co, Ni, Ru; n = +1, M = Cr, Mn, Co, Rh, Ir; smif =1,3-di-(2-pyridyl)-2-azaallyl).

A series of Werner complexes featuring the tridentate ligand smif, that is, 1,3-di-(2-pyridyl)-2-azaallyl, have been prepared. Syntheses of (smif)(2)M (1-M; M = Cr, Fe) were accomplished via treatment of M(NSiMe(3))(2)(THF)(n) (M = Cr, n = 2; Fe, n = 1) with 2 equiv of (smif)H (1,3-di-(2-pyridyl)-2-azapropene); ortho-methylated ((o)Mesmif)(2)Fe (2-Fe) and ((o)Me(2)smif)(2)Fe (3-Fe) were similarly prepared. Metatheses of MX(2) variants with 2 equiv of Li(smif) or Na(smif) generated 1-M (M = Cr, Mn, Fe, Co, Ni, Zn, Ru). Metathesis of VCl(3)(THF)(3) with 2 Li(smif) with a reducing equiv of Na/Hg present afforded 1-V, while 2 Na(smif) and IrCl(3)(THF)(3) in the presence of NaBPh(4) gave [(smif)(2)Ir]BPh(4) (1(+)-Ir). Electrochemical experiments led to the oxidation of 1-M (M = Cr, Mn, Co) by AgOTf to produce [(smif)(2)M]OTf (1(+)-M), and treatment of Rh(2)(O(2)CCF(3))(4) with 4 equiv Na(smif) and 2 AgOTf gave 1(+)-Rh. Characterizations by NMR, EPR, and UV-vis spectroscopies, SQUID magnetometry, X-ray crystallography, and DFT calculations are presented. Intraligand (IL) transitions derived from promotion of electrons from the unique CNC(nb) (nonbonding) orbitals of the smif backbone to ligand π*-type orbitals are intense (ε ≈ 10,000-60,000 M(-1)cm(-1)), dominate the UV-visible spectra, and give crystals a metallic-looking appearance. High energy K-edge spectroscopy was used to show that the smif in 1-Cr is redox noninnocent, and its electron configuration is best described as (smif(-))(smif(2-))Cr(III); an unusual S = 1 EPR spectrum (X-band) was obtained for 1-Cr.

Pnictogen-hydride activation by (silox)3Ta (silox = (t)Bu3SiO); attempts to circumvent the constraints of orbital symmetry in N2 activation.

Activation of N(2) by (silox)(3)Ta (1, silox = (t)Bu(3)SiO) to afford (silox)(3)Ta═N-N═Ta(silox)(3) (1(2)-N(2)) does not occur despite ΔG°(cald) = -55.6 kcal/mol because of constraints of orbital symmetry, prompting efforts at an independent synthesis that included a study of REH(2) activation (E = N, P, As). Oxidative addition of REH(2) to 1 afforded (silox)(3)HTaEHR (2-NHR, R = H, Me, (n)Bu, C(6)H(4)-p-X (X = H, Me, NMe(2)); 2-PHR, R = H, Ph; 2-AsHR, R = H, Ph), which underwent 1,2-H(2)-elimination to form (silox)(3)Ta═NR (1═NR; R = H, Me, (n)Bu, C(6)H(4)-p-X (X = H (X-ray), Me, NMe(2), CF(3))), (silox)(3)Ta═PR (1═PR; R = H, Ph), and (silox)(3)Ta═AsR (1═AsR; R = H, Ph). Kinetics revealed NH bond-breaking as critical, and As > N > P rates for (silox)(3)HTaEHPh (2-EHPh) were attributed to (1) ΔG°(calc)(N) < ΔG°(calc)(P) ∼ ΔG°(calc)(As); (2) similar fractional reaction coordinates (RCs), but with RC shorter for N < P∼As; and (3) stronger TaE bonds for N > P∼As. Calculations of the pnictidenes aided interpretation of UV-vis spectra. Addition of H(2)NNH(2) or H(2)N-N((c)NC(2)H(3)Me) to 1 afforded 1═NH, obviating these routes to 1(2)-N(2), and formation of (silox)(3)MeTaNHNH2 (4-NHNH(2)) and (silox)(3)MeTaNH(-(c)NCHMeCH(2)) (4-NH(azir)) occurred upon exposure to (silox)(3)Ta═CH(2) (1═CH(2)). Thermolyses of 4-NHNH(2) and 4-NH(azir) yielded [(silox)(2)TaMe](µ-N(α)HN(ß))(µ-N(γ)HN(δ)H)[Ta(silox)(2)] (5) and [(silox)(3)MeTa](µ-η(2)-N,N:η(1)-C-NHNHCH(2)CH(2)CH(2))[Ta(κ-O,C-OSi(t)Bu(2)CMe(2)CH(2))(silox)(2)] (7, X-ray), respectively. (silox)(3)Ta═CPPh(3) (1═CPPh(3), X-ray) was a byproduct from Ph(3)PCH(2) treatment of 1 to give 1═CH(2). Addition of Na(silox) to [(THF)(2)Cl(3)Ta](2)(µ-N(2)) led to [(silox)(2)ClTa](µ-N(2)) (8-Cl), and via subsequent methylation, [(silox)(2)MeTa](2)(µ-N(2)) (8-Me); both dimers were thermally stable. Orbital symmetry requirements for N(2) capture by 1 and pertinent calculations are given.

Unusual electronic features and reactivity of the dipyridylazaallyl ligand: characterizations of (smif)2M [M = Fe, Co, Co+, Ni; smif = {(2-py)CH}2N] and [(TMS)2NFe]2(smif)2.

Application of the dipyridylazaallyl ligand (2-py)CHNCH(2-py) (smif) to a series of first-row transition metals afforded (smif)(2)M(n) [n = 0, M = Fe (1), Co (2), Ni (3); n = +1, M = Co (2+)] and {(TMS)(2)NFe}(2)(smif)(2) (4(2)) via metathetical procedures. The Mossbauer spectrum of 1 (S = 0) and TDDFT calculations, including a UV-vis spectral simulation, reveal it to be a covalent, strong-field system with Delta(o) estimated as approximately 18,000 cm(-1) and B approximately 470 cm(-1). (smif)(2)Co (2) has S = 1/2 according to SQUID data at 10 K. DFT calculations suggest that the odd electron is localized in a smif pi* orbital, i.e., smif is redox-active. EPR-silent (smif)(2)Ni (3) has S = 1 (SQUID), and calculations show that the unpaired spins reside in the d(z(2)) and d(x(2))(-y(2)) orbitals. X-ray structural parameters suggest that low-spin d(6) 1 and 2+ are relatively symmetric D(2d) species, but 2 and 3 manifest a distortion in which one smif is canted in the plane perpendicular to the other. (smif)FeN(TMS)(2) (4) is principally monomeric in solution, but reversibly dimerizes (K(eq) approximately 10(-4) M(-1)) via C-C bond formation in the azaallyl backbone to crystallize as {(TMS)(2)NFe}(2)(smif)(2) (4(2)). The azaallyl compounds possess extraordinary UV-vis absorptivities (epsilon approximately 18,000-52,000) at 580 +/- 15 nm and 406(25) nm that have been identified as intraligand bands with C(nb) --> smif pi* character.

Structure and reactivity studies of transition metals ligated by tBuSi3X (X = O, NH, N, S, and CC).

Use of the (t)Bu(3)Si group can sterically protect O, N, S, and acetylide donors, enabling chemistry at low-coordinate transition metal centers. This article provides examples from these laboratories on the utilization of (t)Bu(3)SiO(-), (t)Bu(3)SiNH(-), (t)Bu(3)SiN(2-), (t)Bu(3)SiS(-) and (t)Bu(3)SiCC(-) as ligands in exploratory synthesis and reactivity studies of transition metal complexes.

Aryl-containing chelates and amine debenzylation to afford 1,3-di-2-pyridyl-2-azaallyl (smif): structures of {kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}FeBr and (smif)CrN(TMS)(2).

Aryl-bromide ligand precursors have been prepared with the potential to afford tetradentate chelates (2-pyridylmethyl)(3-x)N(CH(2)-2-Aryl)(x) (x = 1, 2) containing metal-aryl linkages that promise to impart stronger fields about first row transition metals. Oxidative addition to Ni(COD)(2) afforded two diamagnetic Ni(II) complexes, {kappa-C,N,N(py)-(2-pyridylmethyl)N(CH(2)(4-(t)Bu-phenyl-2-yl))(CH(2)(4-(t)Bu-phenyl-2-Br))}NiBr (1-Ni) and {(kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}NiBr (2-Ni) in 96% and 67% yield, respectively. Extending these synthetic efforts to iron provided {kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}FeBr (2-Fe, X-ray) in 91% yield via reduction of an adduct, {kappa-N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-Br))}FeBr(2) (3-Fe). 5-Coordinate 2-Fe possessed a pseudo-tbp structure, and SQUID magnetometry showed it to be S = 2 with significant zero field splitting (ZFS). 2-Fe was initially prepared via oxidative addition to Fe{N(TMS)(2)}(2)(THF) upon disproportionation to "Fe(0)" and 2 Fe{N(TMS)(2)}(3), but when this approach was attempted with Cr{N(TMS)(2)}(2)(THF)(2), the azaallyl complex {kappa-N,N(py)(2)-1,3-dipyridyl-2-azaallyl}CrN(TMS)(2) ((smif)CrN(TMS)(2), 4-Cr, X-ray), formed instead (>50%) via amine debenzylation. An alternative route consisting of addition of 1,3-di-2-pyridyl-2-azapropene to Cr{N(TMS)(2)}(2)(THF)(2) afforded 4-Cr in 74% yield. Pseudo-square planar 4-Cr was also S = 2 (SQUID) with marked ZFS. The dipyridylazaallyl ligand "smif" imparts a remarkable optical density to 4-Cr via intraligand bands at 675 nm (epsilon approximately 15,000 M(-1)cm(-1)) and 396 nm (epsilon approximately 27,000 M(-1)cm(-1)). The effective fields of the chelate complexes are discussed, and a comparison of smif to isoelectronic NHC ligands is given.

Olefin substitution in (silox)3M(olefin) (silox = (t)Bu3SiO; M = Nb, Ta): the role of density of states in second vs third row transition metal reactivity.

The substitution chemistry of olefin complexes (silox)3M(ole) (silox = (t)Bu3SiO; M = Nb (1-ole), Ta (2-ole); ole = C2H4 (as 13C2H4 or C2D4), C2H3Me, C2H3Et, cis-2-C4H8, iso-C4H8, C2H3Ph, cC5H8, cC6H10, cC7H10 (norbornene)) was investigated. For 1-ole, substitution was dissociative (deltaG(double dagger)(diss)), and in combination with calculated olefin binding free energies (deltaG(o)(bind)), activation free energies for olefin association (deltaG(double dagger)(assoc)) to (silox)3Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement to alkylidenes. Instead, activation free energies for olefin association to (silox)3Ta (2) were measured, and when combined with deltaG(o)(bind) (calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despite stronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than that from 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships (with respect to deltaG(o)(bind)) characterize olefin dissociation from 1-ole, but not olefin dissociation from 2-ole, and olefin association to 2, but not olefin association to 1. Calculated transition states for olefin dissociation from (HO)3M(C2H4) (M = Nb, 1'-C2H4; Ta, 2'-C2H4) are asymmetric and have orbitals consistent with either singlet or triplet states. The rearrangement of (silox)3Nb(trans-Vy,Ph-cPr) (1-VyPhcPr) to (silox)3Nb=CHCH=CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transition state for substitution. The disparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater density of states (DOS) for the products (i.e., (silox)3M + ole) where M = Nb, leading to intersystem crossing events that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vs the 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of 4d vs 5d elements.