Synthesis and Structural Characterization of Tris(2-mercapto-1-adamantylimidazolyl)hydroborato Complexes: A Sterically Demanding Tripodal [S3] Donor Ligand

Cationic Group 13 and 14 Element Clusters

Anionic and neutral clusters dominate the cluster chemistry of group 13 and 14 elements, many of which have become classic textbook examples of main group element clusters. However, facilitated by the development of unreactive, weakly coordinating anions, the number of known group 13 and 14 cationic cluster compounds has risen rapidly in recent years. Hence, this review aims to give an overview over this research field, which arouses increasing interest owing to the often unusual structures of the cationic clusters, as well as their application in bond activation chemistry. Challenges of the cluster formation are discussed and suitable starting materials are presented, as well as syntheses, structures and the rich follow-up chemistry of (also mixed) group 13 and 14 cluster cations.

Mono- and Hexanuclear Zinc Halide Complexes with Soft Thiopyridazine Based Scorpionate Ligands

Scorpionate ligands with three soft sulfur donor sites have become very important in coordination chemistry. Despite its ability to form highly electrophilic species, electron-deficient thiopyridazines have rarely been used, whereas the chemistry of electron-rich thioheterocycles has been explored rather intensively. Here, the unusual chemical behavior of a thiopyridazine (6-tert-butylpyridazine-3-thione, HtBuPn) based scorpionate ligand towards zinc is reported. Thus, the reaction of zinc halides with tris(6-tert-butyl-3-thiopyridazinyl)borate Na[TntBu] leads to the formation of discrete torus-shaped hexameric zinc complexes [TntBuZnX]6 (X = Br, I) with uncommonly long zinc halide bonds. In contrast, reaction of the sterically more demanding ligand K[TnMe,tBu] leads to decomposition, forming Zn(HPnMe,tBu)2X2 (X = Br, I). The latter can be prepared independently by reaction of the respective zinc halides and two equiv of HPnMe,tBu. The bromide compound was used as precursor which further reacts with K[TnMe,tBu] forming the mononuclear complex [TnMe,tBu]ZnBr(HPnMe,tBu). The molecular structures of all compounds were elucidated by single-crystal X-ray diffraction analysis. Characterization in solution was performed by means of 1H, 13C and DOSY NMR spectroscopy which revealed the hexameric constitution of [TntBuZnBr]6 to be predominant. In contrast, [TnMe,tBu]ZnBr(HPnMe,tBu) was found to be dynamic in solution.

Bis- and Tris(2-oxobenzimidazolyl)hydroborato Complexes of Sodium and Thallium: New Classes of Bidentate and Tridentate Oxygen Donor Ligands

A series of bis- and tris(oxobenzimidazolyl)hydroborato compounds, namely, [Bo^RBenz]Na and [To^RBenz]-Na (R = Me, Bu^t, Ad), which feature uncommon sterically demanding LX [O₂] and L₂X [O₃] donor ligands, have been obtained via the reactions of NaBH₄ with 1-R-1,3-dihydro-2H-benzimidazol-2-ones. Evidence that the alkyl substituents are suitably located to have a significant impact on the coordination environment is provided by the observation that the methyl derivative [To^MeBenz]Na(κ³-diglyme) exhibits κ³-coordination of the diglyme, whereas the t-butyl and adamantyl derivatives, [To^ButBenz]Na(κ²-diglyme) and [To^AdBenz]Na(κ²-diglyme), exhibit κ²-coordination. The [Bo^RBenz] and [To^RBenz] ligands also allow for isolation of discrete mononuclear thallium compounds, [Bo^RBenz]Tl and [To^RBenz]Tl, for which the steric demands of the ligands have been quantified in terms of both cone angle and buried volume concepts.

Dative Bonding between Group 13 Elements Using a Boron-Centered Lewis Base

An electron-rich monovalent boron compound is used as a Lewis base to prepare adducts with Group 13 Lewis acids using both its boron and nitrogen sites. The hard Lewis acid AlCl3 binds through a nitrogen atom of the Lewis base, while softer Lewis acids GaX3 (Cl, Br, I) bind at the boron atom. The latter are the first noncluster Lewis adducts between a boron-centered Lewis base and a main-group Lewis acid.

Exchange of Alkyl and Tris(2-mercapto-1-t-butylimidazolyl)hydroborato Ligands Between Zinc, Cadmium and Mercury

The tris(2-mercaptoimidazolyl)hydroborato ligand, [Tm^But ], has been used to investigate the exchange of alkyl and sulfur donor ligands between the Group 12 metals, Zn, Cd and Hg. For example, [Tm^But ]₂Zn reacts with Me₂Zn to yield [Tm^But ]ZnMe, while [Tm^But ]CdMe is obtained readily upon reaction of [Tm^But ]₂Cd with Me₂Cd. Ligand exchange is also observed between different metal centers. For example, [Tm^But ]CdMe reacts with Me₂Zn to afford [Tm^But ]ZnMe and Me₂Cd. Likewise, [Tm^But ]HgMe reacts with Me₂Zn to afford [Tm^But ]ZnMe and Me₂Hg. However, whereas the [Tm^But ] ligand transfers from mercury to zinc in the methyl system, [Tm^But ]HgMe/Me₂Zn, transfer of the [Tm^But ] ligand from zinc to mercury is observed upon treatment of [Tm^But ]₂Zn with HgI₂ to afford [Tm^But ]HgI and [Tm^But ]ZnI. These observations demonstrate that the phenomenological preference for the [Tm^But ] ligand to bind one metal rather than another is strongly influenced by the nature of the co-ligands.

Influence of Benzannulation on Metal Coordination Geometries: Synthesis and Structural Characterization of Tris(2-mercapto-1-methylbenzimidazolyl)hydroborato Cadmium Bromide, {[TmMeBenz]Cd(μ-Br)}2

The tris(2-mercapto-1-methylbenzimidazolyl)hydroborato cadmium complex, {[TmMeBenz]Cd(μ-Br)}2, may be synthesized via the reaction of [TmMeBenz]K with CdBr2. X-ray diffraction demonstrates that {[TmMeBenz]Cd(μ-Br)}2 exists as a dimer, which is in marked contrast to the monomeric structure of the non-benzannulated counterpart, [TmMe]CdBr, and thereby demonstrates that benzannulation of tris(2-mercapto-1-methylbenzimidazolyl)hydroborato ligands can have a distinct impact on the molecular structure of their metal complexes. In accord with this observation, density functional theory calculations indicate that the benzannulated dimers, {[TmMeBenz]Cd(μ-X)}2 (X = Cl, Br, I), are more stable with respect to dissociation than are their non-benzannulated counterparts, {[TmMe]Cd(μ-X)}2. Furthermore, the calculations also indicate that the stability of the dimer depends on the nature of X, such that the dimer becomes more stable in the sequence I < Br < Cl.

Synthesis and structural characterization of tris(2-mercapto-1-methylbenzimidazolyl)hydroborato cadmium halide complexes, {[Tm(MeBenz)]Cd(μ-Cl)}2 and [Tm(MeBenz)]CdI: a rare example of cadmium in a trigonal bipyramidal sulfur-rich coordination environment

The tris(2-mercapto-1-methylbenzimidazolyl)hydroborato cadmium complexes, {[Tm(MeBenz)]Cd(μ-Cl)}2 and [Tm(MeBenz)]CdI, have been synthesized via the reactions of [Tm(MeBenz)]K with CdCl2 and CdI2, respectively. While X-ray diffraction studies demonstrate that the iodide derivative, [Tm(MeBenz)]CdI, is a monomer, the chloride derivative, {[Tm(MeBenz)]Cd(μ-Cl)}2, exists as a dimer, which is unprecedented for Group 12 [Tm(R)]MX (X = Cl, Br, I) compounds. Furthermore, the cadmium centers of {[Tm(MeBenz)]Cd(μ-Cl)}2 are trigonal bipyramidal, which is an uncommon motif for cadmium complexes with a [S3Cl2] coordination sphere.

Molecular structures of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato and tris(2-mercapto-1-adamantylimidazolyl)hydroborato sodium complexes: analysis of [Tm(R)] ligand coordination modes and conformations

The tris(mercaptoimidazolyl)hydroborato complexes, [κ(3)-S2H-Tm(Bu(t))]Na(THF)3 and [κ(3)-S2H-Tm(Ad)]Na(THF)3, which feature t-butyl and adamantyl substituents, have been synthesized via the reactions of the respective 1-R-1,3-dihydro-2H-imidazole-2-thiones with NaBH4 in THF (R = Bu(t), 1-Ad). X-ray diffraction studies indicate that the compounds are monomeric and that the [Tm(R)] ligands coordinate to the metal in a κ(3)-S2H manner via two of the sulfur donors and the hydrogen attached to boron, a combination that is unprecedented for sodium derivatives. Analysis of the tris(mercaptoimidazolyl)hydroborato compounds that are listed in the Cambridge Structural Database has allowed for the formulation of a set of criteria that enables κ(x)-S(x) and κ(x+1)-S(x)H coordination modes to be identified. Furthermore, the various κ(x)-S(x) and κ(x+1)-S(x)H coordination modes have also been analyzed with respect to the conformations of the [Tm(R)] ligands, which differ by rotation of the imidazolethione moieties about the B-N bond.

Complete and partial 1,2-additions across transition metal-boron double bonds

The first 1,2-additions across a metal-boron double bond are reported, one a definitive chlorogallation of a Fe═B bond, the other a partial chlorogallation of a Mo═B bond that leads to a highly unusual, planar Mo-B-Ga-Cl rhombus. The two reactions occur with opposite regiochemistry, with the Ga atom bound to the Fe atom in the former and to both the Mo and B atoms in the latter. The bonding in the Mo adduct and the reasons for the differing regiochemistry of the reaction are explored computationally.

Benzannulated tris(2-mercapto-1-imidazolyl)hydroborato ligands: tetradentate κ4-S3H binding and access to monomeric monovalent thallium in an [S3] coordination environment

The benzannulated tris(mercaptoimidazolyl)borohydride sodium complex, [Tm(Bu(t)Benz)]Na, has been synthesized via the reaction of NaBH4 with 1-tert-butyl-1,3-dihydro-2H-benzimidazole-2-thione, while [Tm(MeBenz)]K has been synthesized via the reaction of KBH4 with 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione. The molecular structures of the solvated adducts, {[Tm(Bu(t)Benz)]Na(THF)}2(μ-THF)2 and [Tm(MeBenz)]K(OCMe2)3, have been determined by X-ray diffraction, which demonstrates that the [Tm(R)] ligands in these complexes adopt different coordination modes to that in {[Tm(MeBenz)]Na}2(μ-THF)3. Specifically, while the [Tm(MeBenz)] ligand of the sodium complex {[Tm(MeBenz)]Na}2(μ-THF)3 adopts a κ(3)-S3 coordination mode, the potassium complex [Tm(MeBenz)]K(OCMe2)3 adopts a most uncommon inverted κ(4)-S3H coordination mode in which the potassium binds to all three sulfur donors and the hydrogen of the B-H group in a linear KH-B manner. Furthermore, the [Tm(Bu(t)Benz)] ligand of {[Tm(Bu(t)Benz)]Na(THF)}2(μ-THF)2 adopts a κ(3)-S2H coordination mode, thereby demonstrating the flexibility of this ligand system. The monovalent thallium compounds, [Tm(MeBenz)]Tl and [Tm(Bu(t)Benz)]Tl, have been obtained via the corresponding reactions of [Tm(MeBenz)]Na and [Tm(Bu(t)Benz)]Na with TlOAc. X-ray diffraction demonstrates that the three sulfur donors of the [Tm(RBenz)] ligands of both [Tm(MeBenz)]Tl and [Tm(Bu(t)Benz)]Tl chelate to thallium. This coordination mode is in marked contrast to that in other [Tm(R)]Tl compounds, which exist as dinuclear molecules wherein two of the sulfur donors coordinate to different thallium centers. As such, this observation provides further evidence that benzannulation promotes κ(3)-S3 coordination in this system.

Synthesis and structural characterization of bis and tris(2-mercapto-1-methylbenzimidazolyl)hydroborato complexes: benzannulation promotes κ³-coordination

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [BmMeBenz]Na and [TmMeBenz]Na, have been synthesized via the reactions of NaBH4 with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[BmMeBenz]Na(THF)₂}₂ possesses bridging [BmMeBenz] ligands and terminal THF ligands, while {[TmMeBenz]Na}₂(μ-THF)₃ possesses terminal [TmMeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[TmMeBenz]Na}₂(μ-THF)₃ coordinates in a κ³-manner, which is in marked contrast to the κ²-, κ¹- and κ⁰-modes that have been reported for various [TmMe]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [TmMeBenz]⁻ and [TmMe]⁻ in the gas phase indicate that the conformation required for κ³-S₃ coordination, i.e. one in which the three sulfur donors point away from the B-H group, is relatively more stable for [TmMeBenz]⁻ than for [TmMe]⁻, and thus provides a rationalization for the observation that benzannulation enables κ³-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[TmMeBenz]Na}₂(μ-THF)₃. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [TmMeBenz]Re(CO)₃ with those of [TmMe]Re(CO)₃ indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃ react with [Me₃PCuCl]₄ to give [BmMeBenz]CuPMe₃ and [TmMeBenz]CuPMe₃, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.