Synthesis and Characterization of Bulky 1,3-Diamidopropane Complexes of Group 2 Metals (Be ‒ Sr).

Reaction of lithium 1,3-diamidopropane Li2(TripNCN) (TripNCN = [{(Trip)NCH2}2CH2]2-, Trip = 2,4,6-triisopropylphenyl) with BeBr2(OEt2)2 gave the diamido beryllium complex, [(TripNCN)Be(OEt2)]. Deprotonation reactions between the bulkier 1,3-diaminopropane (TCHPNCN)H2 (TCHPNCN = [{(TCHP)NCH2}2CH2]2-, TCHP = 2,4,6-tricyclohexylphenyl) and magnesium alkyls afforded the adduct complexes [(TCHPNCN)Mg(OEt2)] and [(TCHPNCN)Mg(THF)2], depending on the reaction conditions employed. Treating [(TCHPNCN)Mg(THF)2] with the N-heterocyclic carbene:C{(MeNCMe)2} (TMC) gave [(TCHPNCN)Mg(TMC)2] via substitution of the THF ligands. Reactions of (ArNCN)H2 (Ar = Trip or TCHP) with Mg{CH2(SiMe3)}2, in the absence of Lewis bases, yielded the N-bridged dimers [{(ArNCN)Mg}2]. Salt metathesis reactions between alkali metal salts M2(TCHPNCN) (M = Li or K) and CaI2 or SrI2 led to the THF adduct compounds [(TCHPNCN)Ca(THF)3] and [(TCHPNCN)Sr(THF)4], the differing number of THF ligands in which are a result of the different sizes of the metals involved. The described complexes hold potential as precursors to kinetically protected, low oxidation state group 2 metal species.

Enforcing Metal-Arene Interactions in Bulky p-Terphenyl Bis(anilide) Complexes of Group 2 Metals (Be-Ba): Potential Precursors for Low-Oxidation-State Alkaline Earth Metal Systems.

An extremely bulky p-terphenyl bis(aniline), p-C6H4{C6H4[N(H)TCHP]-2}2 (TCHP = 2,4,6-tricyclohexylphenyl) TCHPTerphH2, has been developed. Deprotonation of a less bulky analogue, DipTerphH2 (Dip = 2,6-diisopropylphenyl), with BePh2 affords the bimetallic system, [(BePh)2(µ-DipTerph)] 1. Treating either TCHPTerphH2 or DipTerphH2 with Mg{CH2(SiMe3)}2 gives the monomeric bis(anilide) complexes [Mg(ArTerph)] (Ar = Dip 2, TCHP 3) which display rare examples of η6-arene coordination to the metal center. Treating 2 with THF leads to partial dissociation of the Mg···arene interaction and formation of [Mg(DipTerph)(THF)] 4. Reactions of the bis(aniline)s with the group 2 metal amides [M{N(SiMe3)2}2] afford dimeric, structurally analogous compounds [{M(ArTerph)}2] (Ar = Dip, M = Ca 5, Sr 6, Ba 7; Ar = TCHP, M = Ca 8, Sr 9, Ba 10) which display intermolecular M···arene interactions in the solid state. Computational studies have shown that the intramolecular M···Î·6-arene interactions in models of the ether-free metal bis(anilide) compounds are largely electrostatic in nature. Reductions of these compounds with alkali metals led to mixtures of unidentified products.

Metal-metal cooperativity boosts Lewis basicity and reduction properties of the bis(gallanediyl) CyL2Ga2.

The interplay of two proximate gallium centres equips the bimetallic complex CyL2Ga2 (1, CyL2 = 1,2-trans-Cy[NC(Me)C(H)C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) with increased Lewis basicity and higher reducing power compared to the monometallic gallanediyl LGa (2, L = HC[MeCN(Dip)]2) as evidenced by cross-over experiments. Quantum chemical calculations were employed to support the experimental findings.

Homolytic C-H Bond Activation by Phosphine-Quinone-Based Radical Ion Pairs.

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine-quinone systems and explore their potential for the activation of C-H bonds. PMes3 (Mes=2,4,6-Me3 C6 H2 ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes3 P-DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip=2,4,6-iPr3 C6 H2 ) afforded C-H bond activation product Tip2 P(H)(2-[CMe2 (DDQ)]-4,6-iPr2 -C6 H2 ) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3 ]⋅+ [DDQ]⋅- , and subsequent homolytic C-H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2 P(H)(2-[CMe2 {TCQ-B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip2 P(H)(2-[CMe2 {oQtBu -B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (8, oQtBu =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip3 with Lewis-acid activated quinones, TCQ-B(C6 F5 )3 and oQtBu -B(C6 F5 )3 , respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C-H bond activation by the synergistic action of radical ion pairs.

Schlenk-Type Equilibria of Grignard-Analogous Arylberyllium Complexes: Steric Effects.

The presence of complex Schlenk equilibria is a central property of synthetically invaluable Grignard reagents substantially affecting their reactivity and selectivity in chemical transformations. In this work, the steric effects of aryl substituents on the Schlenk-type equilibria of their lighter congeners, arylberyllium bromides, are systematically studied. Combination of diarylberyllium complexes Ar2 Be(OEt2 ) (1Ar, Ar=Tip, Tcpp; Tip=2,4,6-iPr3 C6 H3 , Tcpp=2,4,6-Cyp3 C6 H3 , Cyp=c-C5 H9 ), containing sterically demanding aryl groups, and BeBr2 (OEt2 )2 (2) affords the Grignard-analogous arylberyllium bromides ArBeBr(OEt2 ) (3Ar, Ar=Tip, Tcpp). In contrast, Mes2 Be(OEt2 ) (1Mes, Mes=2,4,6-Me3 C6 H3 ) and 2 exist in a temperature-dependent equilibrium with MesBeBr(OEt2 ) (3Mes) and free OEt2 . Ph2 Be(OEt2 ) (1Ph) reacts with 2 to afford dimeric [PhBeBr(OEt2 )]2 ([3Ph]2 ). Moreover, the influence of replacing the OEt2 donor by an N-heterocyclic carbene, IPr2 Me2 (IPr2 Me2 =C(iPrNCMe)2 ), on the redistribution reactions was investigated. The solution- and solid-state structures of the diarylberyllium and arylberyllium bromide complexes were comprehensively characterized using multinuclear (1 H, 9 Be, 13 C) NMR spectroscopic methods and single-crystal X-ray diffraction, while DFT calculations were employed to support the experimental findings.

X-Band Parallel-Mode and Multifrequency Electron Paramagnetic Resonance Spectroscopy of S = 1/2 Bismuth Centers.

The recent successes in the isolation and characterization of several bismuth radicals inspire the development of new spectroscopic approaches for the in-depth analysis of their electronic structure. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for the characterization of main group radicals. However, the large electron-nuclear hyperfine interactions of Bi (209Bi, I = 9/2) have presented difficult challenges to fully interpret the spectral properties for some of these radicals. Parallel-mode EPR (B1∥B0) is almost exclusively employed for the study of S > 1/2 systems but becomes feasible for S = 1/2 systems with large hyperfine couplings, offering a distinct EPR spectroscopic approach. Herein, we demonstrate the application of conventional X-band parallel-mode EPR for S = 1/2, I = 9/2 spin systems: Bi-doped crystalline silicon (Si:Bi) and the molecular Bi radicals [L(X)Ga]2Bi• (X = Cl or I) and [L(Cl)GaBi(MecAAC)]•+ (L = HC[MeCN(2,6-iPr2C6H3)]2). In combination with multifrequency perpendicular-mode EPR (X-, Q-, and W-band frequencies), we were able to fully refine both the anisotropic g- and A-tensors of these molecular radicals. The parallel-mode EPR experiments demonstrated and discussed here have the potential to enable the characterization of other S = 1/2 systems with large hyperfine couplings, which is often challenging by conventional perpendicular-mode EPR techniques. Considerations pertaining to the choice of microwave frequency are discussed for relevant spin-systems.

Sequential Reduction of Borylstibane to an Electronically Nonsymmetric Diboryldistibene Radical Anion.

Understanding the formation of metal-metal bonds and their electronic structures is still a scientific task. We herein report on the stepwise synthesis of boryl-substituted antimony compounds in which the antimony atoms adopt four different oxidation states (+III, +II, +I, +I/0). Sb-C bond homolysis of Cp*[(HCNDip)2B]SbCl (1; Cp* = C5Me5; Dip = 2,6-iPr2C6H3) gave diboryldichlorodistibane [(HCNDip)2BSbCl]2 (2), which reacted with KC8 to form diboryldistibene [(HCNDip)2BSb]2 (3) and traces of cyclotetrastibane [(HCNDip)2B]3Sb4Cl (5). One-electron reduction of 3 yielded the potassium salt of the diboryldistibene radical anion [(HCNDip)2BSb]2̇-, [K(18-c-6)(OEt2)][{(HCNDip)2BSb}2] (4), which exhibits an unprecedented inequivalent spin localization on the Sb-Sb bond and hence an unsymmetric electronic structure. Compounds 1-4 were characterized by heteronuclear nuclear magnetic resonance (NMR) (1H, 13C, 11B), infrared (IR), ultraviolet-visible (UV-vis) spectroscopy (3, 4), and single crystal X-ray diffraction (sc-XRD, 1-5), while the bonding nature of 3 and 4 was analyzed by quantum chemical calculations. EPR spectroscopy resolves the dissimilar Sb hyperfine tensors of 4, reflecting the inequivalent spin distribution, setting 4 uniquely apart from all previously characterized dipnictene radical anions.

Bis-Phosphaketenes LM(PCO)2 (M=Ga, In): A New Class of Reactive Group 13 Metal-Phosphorus Compounds.

Phosphaketenes are versatile reagents in organophosphorus chemistry. We herein report on the synthesis of novel bis-phosphaketenes, LM(PCO)2 (M=Ga 2 a, In 2 b; L=HC[C(Me)N(Ar)]2 ; Ar=2,6-i-Pr2 C6 H3 ) by salt metathesis reactions and their reactions with LGa to metallaphosphenes LGa(OCP)PML (M=Ga 3 a, In 3 b). 3 b represents the first compound with significant In-P π-bonding contribution as was confirmed by DFT calculations. Compounds 3 a and 3 b selectively activate the N-H and O-H bonds of aniline and phenol at the Ga-P bond and both reactions proceed with a rearrangement of the phosphaethynolate group from Ga-OCP to M-PCO bonding. Compounds 2-5 are fully characterized by heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR and IR spectroscopy, elemental analysis, and single crystal X-ray diffraction (sc-XRD).

Geminal C-Cl and Si-Cl bond activation of chloromethanes and chlorosilanes by gallanediyl LGa.

The activation of relatively inert E-X σ-bonds by low-valent main group metal complexes is receiving increasing interest. We here confirm the promising potential of gallanediyl LGa (L = HC[C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) to activate E-Cl (E = C, Si) σ-bonds of group 14 element compounds. Equimolar reactions of LGa with chloromethanes and chlorosilanes EHxCl4-x (E = C, x = 0-2; E = Si, x = 0, 1) occurred with E-Cl bond insertion and formation of gallylmethanes and -silanes L(Cl)GaEHxCl3-x (E = C, x = 2 (1), 1 (2), 0 (3); E = Si, x = 1 (4)). In contrast, consecutive insertion into a geminal E-Cl bond was observed with two equivalents of LGa, yielding digallyl complexes [L(Cl)Ga]2EHxCl2-x (E = C, x = 2 (5); E = Si, x = 1 (6), 0 (7)). Compounds 1-7 were characterized by heteronuclear NMR (1H, 13C, 29Si (4, 6)), IR spectroscopy and elemental analysis, and their solid-state structures were determined by single-crystal X-ray diffraction (sc-XRD).

Observation of Discrete Valence Tautomers in Crystalline Cyclopentadienyl Radicals.

Single crystal X-ray (sc XRD) analyses of three symmetrically substituted cyclopentadienyl radicals (1, 2, 5) containing sterically demanding aryl groups showed that they crystallize as discrete valence tautomers (Jahn-Teller distortion) in the solid state with the unpaired electron either located in the b1 orbital (type I, state 2B1), resulting in a localized radical with two adjacent double bonds, or the a2 orbital (type II, state 2A2), leading to an allyl-type radical. Their properties in solution were examined by EPR spectroscopy as well as cyclovoltammetry and UV/vis spectroscopy including two additional cyclopentadienyl radicals (1-5). The electronic nature of 1-5 was further investigated by quantum chemical calculations.

Synthesis of heteroleptic gallium-substituted antimony hydrides by stepwise ß-H elimination.

Heteroleptic stibanes Cp*(R)SbCl (R = Dip 1, N(SiMe3)22, OB(NDipCH)23; Cp* = C5Me5; Dip = 2,6-i-Pr2-C6H3) react with monovalent gallanediyl LGa (L = HC[C(Me)N(Dip)]2) with elimination of 1,2,3,4-tetramethylfulvene, yielding heteroleptic metal-stabilized Sb hydrides [L(Cl)Ga](R)SbH (R = Dip 4, N(SiMe3)25, OB(NDipCH)26). Compounds 1-6 were characterized by heteronuclear NMR (1H, 11B, 13C) and IR spectroscopy, and the solid-state structures of 4-6 were determined by single-crystal X-ray diffraction. A close correlation between the 1H NMR chemical shift of the hydride ligand and the electronegativity of the Sb-coordinating atoms was revealed.

A Mechanistic Study on Reactions of Group 13 Diyls LM with Cp*SbX2 : From Stibanyl Radicals to Antimony Hydrides.

Oxidative addition of Cp*SbX2 (X=Cl, Br, I; Cp*=C5 Me5 ) to group 13 diyls LM (M=Al, Ga, In; L=HC[C(Me)N (Dip)]2 , Dip=2,6-iPr2 C6 H3 ) yields elemental antimony (M=Al) or the corresponding stibanylgallanes [L(X)Ga]Sb(X)Cp* (X=Br 1, I 2) and -indanes [L(X)In]Sb(X)Cp* (X=Cl 5, Br 6, I 7). 1 and 2 react with a second equivalent of LGa to eliminate decamethyl-1,1'-dihydrofulvalene (Cp*2 ) and form stibanyl radicals [L(X)Ga]2 Sb. (X=Br 3, I 4), whereas analogous reactions of 5 and 6 with LIn selectively yield stibanes [L(X)In]2 SbH (X=Cl 8, Br 9) by elimination of 1,2,3,4-tetramethylfulvene. The reactions are proposed to proceed via formation of [L(X)M]2 SbCp* as reaction intermediate, which is supported by the isolation of [L(Cl)Ga]2 SbCp (11, Cp=C5 H5 ). The reaction mechanism was further studied by computational calculations using two different models. The energy values for the Ga- and the In-substituted model systems showing methyl groups instead of the very bulky Dip units are very similar, and in both cases the same products are expected. Homolytic Sb-C bond cleavage yields van der Waals complexes from the as-formed radicals ([L(Cl)M]2 Sb. and Cp*. ), which can be stabilized by hydrogen atom abstraction to give the corresponding hydrides, whereas the direct formation of Sb hydrides starting from [L(Cl)M]2 SbCp* via concerted ß-H elimination is unlikely. The consideration of the bulky Dip units reveals that the amount of the steric overload in the intermediate I determines the product formation (radical vs. hydride).

Ligand Effects on the Electronic Structure of Heteroleptic Antimony-Centered Radicals.

We report on the structures of three unprecedented heteroleptic Sb-centered radicals [L(Cl)Ga](R)Sb. (2-R, R=B[N(Dip)CH]2 2-B, 2,6-Mes2 C6 H3 2-C, N(SiMe3 )Dip 2-N) stabilized by one electropositive metal fragment [L(Cl)Ga] (L=HC[C(Me)N(Dip)]2 , Dip=2,6-i-Pr2 C6 H3 ) and one bulky B- (2-B), C- (2-C), or N-based (2-N) substituent. Compounds 2-R are predominantly metal-centered radicals. Their electronic properties are largely influenced by the electronic nature of the ligands R, and significant delocalization of unpaired-spin density onto the ligands was observed in 2-B and 2-N. Cyclic voltammetry (CV) studies showed that 2-B undergoes a quasi-reversible one-electron reduction, which was confirmed by the synthesis of [K([2.2.2]crypt)][L(Cl)GaSbB[N(Dip)CH]2 ] ([K([2.2.2]crypt)][2-B]) containing the stibanyl anion [2-B]- , which was shown to possess significant Sb-B multiple-bonding character.

Synthesis of a Ga-Stabilized As-Centered Radical and a Gallastibene by Tailoring Group 15 Element-Carbon Bond Strengths.

A convenient synthetic route to Ga-stabilized pnictogen-centered radicals and gallapnictenes by manipulation of pnictogen-carbon bond strengths is presented. Two equivalents of LGa (L = HC[C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) react with CpArAsCl2 (CpAr = C5(4-t-BuC6H4)5) with formation of the arsenic-centered radical [L(Cl)Ga]2As·1. In contrast, the analogous reaction with TerSbCl2 (Ter = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) yields the gallastibene LGa═SbTer (2) containing a Ga-Sb double bond, whereas reactions of DipSbCl2 with one and two equivalents of LGa give the monoinsertion and bisinsertion products L(Cl)GaSb(ClDip) (3) and [L(Cl]Ga]2SbDip (4), respectively. 1-4 were structurally characterized by single crystal X-ray diffraction. The description of 1 as an arsenic-centered radical is consistent with results of electron paramagnetic resonance and density functional theory (DFT) studies. The π-bonding in LGa═SbTer (2) is estimated to 10.68 kcal mol-1 by variable-temperature (VT) NMR spectroscopy, and DFT studies reveal a significant π-bonding interaction between Sb and Ga.

Comprehensive Study on Reactions of Group 13 Diyls with Tetraorganodipentelanes.

L1Ga {L1 = HC[C(Me)N(2,6- iPr2C6H3)]2} reversibly reacts with E2Ph4 (E = Sb, Bi) in a temperature-dependent equilibrium reaction with insertion into the E-E bond and formation of L1Ga(EPh2)2 (E = Sb 1, Bi 2). Analogous findings were observed in the reactions of L2Ga {L2 = (C6H11)2NC[N(2,6- iPr2C6H3)]2} with E2R4 (R = Ph, Et), yielding L2Ga(EPh2)2 (E = Sb 3, Bi 4) and L2Ga(EEt2)2 (E = Sb 5, Bi 6). 1-3 and 5 were isolated by fractional crystallization at low temperature, whereas 4 and 6 could not be isolated in their pure form even at low temperature. In contrast, reactions of [Cp*Al]4 (Cp* = C5Me5) with Sb2R4 (R = Ph, Et) and Bi2Et4 did not proceed with insertion into the E-E bonds but with formation of (Cp*Al)3E2 (E = Sb, 7; Bi, 8), whereas the reaction with Bi2Ph4 yielded metallic bismuth. 8 was also formed in the reaction of [Cp*Al]4 and BiEt3 at ambient temperature, whereas the analogous reaction of [Cp*Al]4 with SbEt3 did not yield 7 even under drastic reaction conditions (120 °C, 3 days). In contrast, Cp*Ga and Sb2R4 (R = Ph, Et) were found to react only at elevated temperature (120 °C) with formation of antimony metal.

Synthesis of a Gallaarsene {HC[C(Me)N-2,6- i-Pr2-C6H3]2}GaAsCp* Containing a Ga═As Double Bond.

Cp*AsCl2 (Cp* = C5Me5) reacts with one equivalent of LGa (L = HC[C(Me)N(2,6- i-Pr2C6H3)]2) with formation of L(Cl)GaAs(Cl)Cp* 1, whereas the reaction with two equivalents of LGa yielded gallaarsene LGaAsCp* 2 containing a Ga═As double bond and (η1-Ga(Cp*)L(η2-GaL)(µ-As3) 3. Compounds 2 and 3 were structurally characterized by single crystal X-ray diffraction, and the π-bonding contribution in 2 was analyzed by temperature-dependent 1H NMR spectroscopy (9.65 kcal mol-1) and by quantum mechanical computation.

From stable Sb- and Bi-centered radicals to a compound with a Ga=Sb double bond.

Neutral stibinyl and bismuthinyl radicals are typically short-lived, reactive species. Here we show the synthesis and solid-state structures of two stable stibinyl [L(Cl)Ga]2Sb· 1 and bismuthinyl radicals [L(I)Ga]2Bi· 4, which are stabilized by electropositive metal centers. Their description as predominantly metal-centered radicals is consistent with the results of NMR, EPR, SQUID, and DFT studies. The Lewis-acidic character of the Ga ligands allow for significant electron delocalization of the Sb- and Bi- unpaired radical onto the ligand. Single-electron reduction of [L(Cl)Ga]2Sb· gave LGaSbGa(Cl)L 5, the first compound containing a Ga=Sb double bond. The π-bonding contribution is estimated to 9.56 kcal mol-1 by NMR spectroscopy. The bonding situation and electronic structure is analyzed by quantum mechanical computations, revealing significant π backdonation from the Sb to the Ga atom. The formation of 5 illustrates the high-synthetic potential of 1 for the formation of new compounds with unusual electronic structures.

A General Route to Metal-Substituted Dipnictenes of the Type [L(X)M]2 E2.

Two equivalents of LGa (L=HC[C(Me)N(2,6-iPr2 C6 H3 )]2 ) reacted with PX3 (X=Cl, Br) with insertion into two P-X bonds and formation of [L(X)Ga]2 PX (X=Cl 1, Br 2), whereas the analogous reaction with AsCl3 occurred with twofold insertion and subsequent elimination of LGaCl2 and formation of the Ga-substituted diarsene [L(Cl)Ga]2 As2 (3). Analogous findings were observed in the reactions with Me2 NAsCl2 , yielding the unsymmetrically-substituted diarsene [L(Cl)Ga]As=As[Ga(NMe2 )L] (4). The reaction of As(NMe2 )3 with LGa gave [L(Me2 N)Ga]2 As2 (5) after heating at 165 °C for five days, whereas the reaction with LAl gave [L(Me2 N)Al]2 As2 (6) after heating at only 80 °C for one day. Finally, two equivalents of LGa reacted with Bi(NEt2 )3 to give [L(Et2 N)Ga]2 Bi2 (7). Complexes 1-7 were characterized by NMR spectroscopy (1 H, 13 C, 31 P), elemental analysis, and single-crystal X-ray diffraction (except for 1 and 5). The bonding situations in 4, 6, and 7 were analyzed by quantum chemical calculations.

Synthesis, Structure, and Reactivity of Ga-Substituted Distibenes and Sb-Analogues of Bicyclo[1.1.0]butane.

Monovalent gallanediyl LGa {L=HC[C(Me)N(2,6-iPr2 C6 H3 )]2 } reacts with SbX3 to form the Ga-substituted distibenes [(LGaX)2 Sb2 ] (X=NMeEt 1, Cl 2). Upon heating, 2 reacts to the bicyclo[1.1.0]butane analogue [(LGaCl)2 (µ,η1:1 -Sb4 )] 3 containing a [Sb4 ]2- dianion. Moreover, 2 reacts with Li amides LiNR2 in salt elimination reactions that form the corresponding amido-substituted compounds 1 and [(LGaNMe2 )2 Sb2 ] 4, whereas reactions of 4 and [(LGaNMe2 )2 (µ,η1:1 -Sb4 )] 5 with two equivalents of GaCl3 resulted in the formation of 2 and 3, respectively. 1, 2 and 3 were characterized by 1 H and 13 C NMR spectroscopy, elemental analysis, and single crystal X-ray diffraction. In addition, their bonding situation was analyzed by quantum chemical calculations.