Peter T. Wolczanski

CHEMISTRY OF ELECTROPHILIC METAL CENTRES COORDINATED BY SILOX (TBU3SIO), TRITOX (TBU3CO) AND RELATED BINFUNCTIONAL LIGANDS

Abstract Bulky siloxide and alkoxide ligands, notably silox ( t Bu 3 SiO), and tritox ( t Bu 3 CO), have been employed as ancillary ligands in the chemistry of low-coordinate early transition metal complexes. Related sterically hindered difunctional ligands (e.g., alkoxyalkylphosphines) are utilized to link disparate early and late transition metal centres together in a quest for cooperative reactivity. This review focuses on our groups efforts in this area over the past 14 years. Emphasis is placed on the rationale for using bulky, hard, anionic donor ligands, and the synthesis and reactivity studies of this class of compounds.

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.

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.

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.

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.

Synthesis and Characterization of (smif)2Mn (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 = tBu3SiO); 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(β))(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.

Carbon-carbon bond activation via formal β-methyl-elimination from [η5-6,6-dimethylcyclohexadienyl)Ru(DPPE)(CH2Cl2)]PF6☆☆☆

Abstract Carbon-carbon bond activation has been observed through a formal β-methyl elimination from a 6,6-dimethylcyclohexadienyl (dmCh) ligand. Reflux of an EtOH solution of RuCl 3 · 3H 2 O, (dmCh)H (15 equiv.), and Zn dust (15 equiv.) afforded (dmCh) 2 Ru ( 1 , 65–77%). Protonation of 1 with HBF 4 · Et 2 O in either provided [(dmCh) 2 RuH][BF 4 ] ( 2 ) in 77% yield; NMR spectra were consistent with either a terminal hydride or rapidly equilibrated agostic ground-state structure. Addition of CH 3 CN to 2 , or protonation of 1 in CH 3 CN, gave [( η 5 -dmCh)Ru(NCCH 3 ) 3 ][BF 4 ] ( 3 , 70%). Treatment of 3 with 2.0 equiv. PMe 3 or 1.0 equiv. dppe produced [(dmCh)RuL 2 (NCCH 3 )][BF 4 ] ( 4 , L = PMe 3 ; 5 , L = dppe), which were poor precursors to halide derivatives. Treatment of 1 with 12 M aqueous HCl in acetone generated [(dmCh)RuCl] n ( 6 ) in 55% yield. Addition of excess norbornadiene to 6 in hexane yielded (dmCh)Ru(NBD)Cl ( 7 , 90%), which proved to be a ready precursor to (dmCh)RuL 2 Cl ( 8 , L = PMe 3 , 90%; 9 , L 2 = dppe, 53%) upon addition of the appropriate phosphine. Chloride abstraction from 8 with TlPF 6 afforded numerous [(dmCh)Ru(PMe 3 ) 2 (solvent)]PF 6 [( 10 -solvent), solvent = CD 2 Cl 2 , CD 3 NO 2 , THF, 2-Me-THF] derivatives, but β-methyl elimination was not observed in subsequent thermolyses. A similar chloride abstraction from 9 produced [(dmCh)Ru(dppe)(CD 2 Cl 2 )]PF 6 ([ 11 -CD 2 Cl 2 ]PF 6 ); thermolysis of 11 -CD 2 Cl 2 at 91°C for 12 h generated [( η 6 -C 7 H 8 )Ru(dppe)(CH 3 )]PF 6 ( 12 ), presumably via the coordinatively unsaturated precursor, [(dmCh)Ru(dppe)]PF 6 ([ 11 ]PF 6 ). The molecularity of the β-methyl elimination pathway remained elusive. Addition of 1.0 equiv. of [Cp 2 Fe][PF 6 ] to 1 in CD 3 CN gave 3 -PF 6 , while oxidation in CD 2 Cl 2 provided [(dmCh)Ru( η 6 -toluene)]PF 6 ( 13 -PF 6 ); cyclic voltammetry pinpointed the irreversible oxidation at ±0.85 V vs Ag/AgCl in THF. Three critical factors are responsible for β-methyl elimination from [ 11 ]PF 6 : (1) coordinative/electronic unsaturation; (2) the compatability of ruthenium to both dmCh (precursor) and toluene (product) ligation; (3) an orbital with directionality appropriate to accept the migrating methyl group.

Molybdenum and Tungsten Structural Differences are Dependent on ndz2/(n + 1)s Mixing : Comparisons of (silox)3MX/R (M = Mo, W; silox = tBu3SiO)

Treatment of trans-(Et 2O) 2MoCl 4 with 2 or 3 equiv of Na(silox) (i.e., NaOSi (t) Bu 3) afforded (silox) 3MoCl 2 ( 1-Mo) or (silox) 3MoCl ( 2-Mo). Purification of 2-Mo was accomplished via addition of PMe 3 to precipitate (silox) 3ClMoPMe 3 ( 2-MoPMe 3), followed by thermolysis to remove phosphine. Use of MoCl 3(THF) 3 with various amounts of Na(silox) produced (silox) 2ClMoMoCl(silox) 2 ( 3-Mo). Alkylation of 2-Mo with MeMgBr or EtMgBr afforded (silox) 3MoR (R = Me, 2-MoMe; Et, 2-MoEt). 2-MoEt was also synthesized from C 2H 4 and (silox) 3MoH, which was prepared from 2-Mo and NaBEt 3H. Thermolysis of WCl 6 with HOSi ( t )Bu 3 afforded (silox) 2WCl 4 ( 4-W), and sequential treatment of 4-W with Na/Hg and Na(silox) provided (silox) 3WCl 2 ( 1-W, tbp, X-ray), which was alternatively prepared from trans-(Et 2S) 2WCl 4 and 3 equiv of Tl(silox). Na/Hg reduction of 1-W generated (silox) 3WCl ( 2-W). Alkylation of 2-W with MeMgBr produced (silox) 3WMe ( 2-WMe), which dehydrogenated to (silox) 3WCH ( 6-W) with Delta H (double dagger) = 14.9(9) kcal/mol and Delta S (double dagger) = -26(2) eu. Magnetism and structural studies revealed that 2-Mo and 2-MoEt have triplet ground states (GS) and distorted trigonal monopyramid (tmp) and tmp structures, respectively. In contrast, 2-W and 2-WMe possess squashed-T d (distorted square planar) structures, and the former has a singlet GS. Quantum mechanics/molecular mechanics studies of the S = 0 and S = 1 states for full models of 2-Mo, 2-MoEt, 2-W, and 2-WMe corroborate the experimental findings and are consistent with the greater nd z (2) /( n + 1)s mixing in the third-row transition-metal species being the dominant feature in determining the structural disparity between molybdenum and tungsten.

Carbon-oxygen and related RX bond cleavages mediated by (silox)3Ti and other Group 4 derivatives (silox = tBu3SiO)

Abstract Halogen atom abstractions by (silox) 3 Ti ( 1 ) from CCl 4 , ClRh(PPh 3 ) 3 , Br 2 and I 2 produced (silox) 3 TiCl ( 2 ), (silox) 3 TiBr ( 3 ) and (silox) 3 TiI ( 4 ), respectively. Treatment of 1 with MeI afforded a 1:1 mixture of 4 and (silox) 3 TiMe ( 5 ), regardless of [MeI], implicating a rough I abstraction rate constant of k a 5 M −1 s −1 . Exposure of 2 to NaI (THF) or MeMgBr(Et 2 O) provided independent syntheses of 4 and 5 , respectively. Br abstraction by 1 from the radical clock H 2 CCH(CH 2 ) 3 CH 2 Br yielded 3 and (silox) 3 TiCH 2 (CH 2 ) 3 CHCH 2 ( 6 ), according to 1 H NMR spectroscopy, and trapping of 1 by hexenyl radical is roughly k t >2×10 7 M −1 s −1 . A rationalization of the formation of (silox) 3 TiCH 2 CH 2 Ti(silox) 3 ( 7 ) from 1 and C 2 H 4 is presented. Na/Hg reduction of (silox) 2 TiCl 2 ( 9 ) generated [(silox) 2 Ti] 2 (μ-Cl) 2 ( 10 ) ( μ eff = 0.75 μ B /Ti at 300.6 K). Quenching of 10 with CCl 4 and C 6 h 4 O 2 produced 9 and [( silox ) 2 TiCl ] 2 -( m :η 1 ,η 1 -p- OC 6 H 4 O ( 11 ), respectively. Upon treatment of 10 with RCCR (REt, Ph) or C 2 H 4 , disproportionation to 9 and (silox) 2 TiCRCRCRCR (REt ( 12 ); Ph ( 13 )), also prepared via Na/Hg reduction of 9 in the presence of alkyne, or (silox) 2 TiCR 2 (CH 2 ) 2 CH 2 ( 14 ) occurred. According to 1 H NMR spectroscopy, exposure of 12 to C 2 H 4 gave 14 , and 10 catalytically hydrogenated Me 2 CCH 2 . Addition of THF to 1 yielded (silox) 3 TiOCH 2 (CH 2 ) 2 CH 2 Ti(silox) 3 ( 17 ) via metallaradical ring-opening, while inclusion of ≈ 10 equiv. of HSnPh 3 provided a mixture of 17 and (silox) 3 TiO n Bu ( 19 ). Addition of PhCH 2 MgCl to (silox) 3 MCl (MTi ( 2 ); Zr ( 22 )) and (silox) 2 TiCl 2 ( 9 ) produced (silox) 3 MCH 2 Ph (MTi ( 21 ); Zr ( 23 )) and (silox) 2 Ti(CH 2 Ph) 2 ( 24 ), respectively, but (silox) 2 Zr(CH 2 Ph) 2 ( 26 ) was synthesized from addition of (silox)H to Zr(CH 2 Ph) 4 . While 21 and 23 were photolytically inactive, photolysis of 24 in THF produced dibenzyl and [(silox) 2 TiOCH 2 (CH 2 ) 2 CH 2 ] n ( 27 , n = 2 (tentative)), while related photolysis of 26 afforded [(silox) 2 ZrOCH 2 (CH 2 ) 2 C H 2 ] 2 ( 28 21 ) and dibenzyl. Mass spectral analysis on dibenzyl derived from a 26 :(silox) 2 Zr(CD 2 Ph) 2 ( 26 -d 4 ) mixture showed that benzyl scrambling occurred. (Silox) 2 Zr(CH 2 - m -tolyl) 2 ( 36 ) was prepared from Zr(CH 2 - m -tolyl) 4 and H(silox). Crossover, i.e., detection of (silox) 2 Zr(CH 2 Ph)(CH 2 - m -tolyl) ( 38 ), occurred when a mixture of (silox) 2 Zr(CH 2 Ph) 2 ( 26 ) and (silox) 2 Zr(CH 2 - m -tolyl) 2 ( 36 ) was photolyzed, showing that benzyl scrambling, presumably via PhCH 2 , preceded THF scission. The mechanisms of THF ring-opening by 1 and, plausibly, (silox) 2 ZrCH 2 Ph ( 32 ), are discussed.