Disproportionation and Ligand Lability in Low Oxidation State Boryl‐Tin Chemistry

Abstract Boryltin compounds featuring the metal in the+1 or 0 oxidation states can be synthesized from the carbene‐stabilized tin(II) bromide (boryl)Sn(NHC)Br (boryl={B(NDippCH)2}; NHC=C{(N i PrCMe)2}) by the use of strong reducing agents. The formation of the mono‐carbene stabilized distannyne and donor‐free distannide systems (boryl)SnSn(IPrMe)(boryl) (2) and K2[Sn2(boryl)2] (3), using Mg(I) and K reducing agents mirrors related germanium chemistry. In contrast to their lighter congeners, however, systems of the type [Sn(boryl)] n are unstable with respect to disproportionation. Carbene abstraction from 2 using BPh3, and two‐electron oxidation of 3 both result in the formation of a 2 : 1 mixture of the Sn(II) compound Sn(boryl)2, and the hexatin cluster, Sn6(boryl)4 (4). A viable mechanism for this rearrangement is shown by quantum chemical studies to involve a vinylidene intermediate (analogous to the isolable germanium compound, (boryl)2Ge=Ge), which undergoes facile atom transfer to generate Sn(boryl)2 and trinuclear [Sn3(boryl)2]. The latter then dimerizes to give the observed hexametallic product 4, with independent studies showing that similar trigermanium species aggregate in analogous fashion.


Introduction
The organometallic chemistry of the group 14 elements in their lower oxidation states (� + 2) has been revolutionized by the development of a range of sterically encumbered ancillary ligands, which allows access to discrete molecular species that retain coordinative and electronic unsaturation.[3] In part, this is because these compounds (and related systems) posed a challenge to long-held ideas about the feasibility of multiple bonding between heavier main group elements. [4]In addition, due to their non-linear (trans-bent) structures and the resulting morphologies/energies of their frontier orbitals, dimetallynes can interact with a number of small molecule substrates in a manner comparable to transition metal complexes. [3,5]This chemistry includes seminal examples of the activation of dihydrogen, [6] and of organic substrates such as alkynes [7] and alkenes [6g,8] (in some cases reversibly). [9]mong the families of bulky monodentate ligands that have been employed to kinetically stabilize dimetallynes with respect to aggregation are strong σ-donors such as aryl, silyl or amido substituents (e. g., I and II; Figure 1). [1,2,6c-e,g,7b, 10,11] As an alternative, we have recently been interested in the use of boryl ligands, À {B(NDippCH) 2 } to access main group systems featuring unusual electronic or geometric structure, and unprecedented modes of reactivity. [12]12h] With this in mind, and given the varied structural and reaction chemistry reported for distannynes, we were interested in examining the consequences of the use of boryl ancillary ligands in the related chemistry of tin.
In contrast to the corresponding germanium system, which is inert to carbene abstraction under mild conditions, [12h] removal of the remaining NHC ligand from 2 by the use of BPh 3 proves to be synthetically viable (presumably due to the weaker nature of the SnÀ C bond).Moreover, the products of this reaction are found to be identical to those obtained from 3 by the action of two equivalents of a trityl oxidant (Scheme 1).In each case, the two species formed (in a ratio of 2 : 1) are the known diborylstannylene Sn{B(NDippCH) 2 } 2 [12f] and a new compound Sn 6 {B(NDippCH) 2 } 4 (4), the structure of which was determined unambiguously by X-ray crystallography (Figure 3).The molecular structure of 4 features four tin atoms arranged in distorted tetrahedral fashion, with opposite edges of the tetrahedron being elongated and bridged symmetrically by a [Sn{B(NDippCH) 2 } 2 ] unit. [15,16]The molecule sits on a C 2 axis, with the SnÀ Sn bonded distances within the tetrahedron (2.848(1), 2.831(1) Å) and the corresponding contacts involving the [Sn {B(NDippCH) 2 } 2 ] units (2.841(1), 2.834(1) Å) all falling in the range expected for SnÀ Sn single bonds.12h] With this in mind, the conversion of 3 to 4 was investigated by quantum chemical methods (Figure 4; see Supporting Information for computational details).A mechanism is proposed involving initial formation of the corresponding diboryldistannyne by oxidation of 3 (or removal of the carbene donor from 2) followed by 1,2migration of a boryl group to form a distannavinylidene (Int 1).12h] Unlike its germanium counterpart, however, Int1 cannot be isolated -an observation we ultimately attribute to the weaker nature of the Sn=Sn interaction.Tin atom transfer between two distannylvinylidene units can then occur to produce one of the experimentally observed products, Sn {B(NDippCH) 2 } 2 , and a second intermediate, Int 2, which features a triangular Sn 3 unit.The conversion of Int1 to Int2 is endergonic with a moderate barrier (ΔG = 7.5 kcal mol À 1 , ΔG � = 18.3 kcal mol À 1 ) and is likely rate limiting.However, subsequent dimerization of Int 2 to yield the other experimentally observed product (4) is a massively exergonic process and constitutes the thermodynamic driving force of the overall process (ΔG = À 75.5 kcal mol À 1 ).
The hypothesis that Sn 6 {B(NDippCH) 2 } 4 (4) is formed by the dimerization of a tri-tin system also gains some experimental credence from aspects of the corresponding germanium chemistry.While digermvinylidene {(HCDippN) 2 B} 2 GeGe does not undergo spontaneous disproportionation in the manner of its putative tin counterpart (presumably due to the stronger Ge=Ge bond, compared to Sn=Sn), the reaction of K 2 [Ge 2 {B(NDippCH) 2 } 2 ] with (IPrMe)GeCl 2 yields an oily green compound, which is proposed to be the trigermanium system Ge 3 {B(NDippCH) 2 } 2 (IprMe) (5; Scheme 2) primarily on the basis of multinuclear NMR measurements and subsequent reactivity (see below).A combination of 1 H and 11 B NMR measurements suggests that the structure of 5 possesses two (equivalent) boryl groups and a single IPrMe carbene ligand.While 5 proved too labile to be isolated as a pure bulk substance for definitive structural characterization, it is susceptible to carbene abstraction using BPh 3 to give a deep green product, which can be obtained as single crystals.Most revealingly (in the context of the tin chemistry outlined above) a combination of multinuclear NMR, micro-analytical and crystallographic studies shows that this compound is Ge 6 {B(NDippCH) 2 } 4 (6), i. e. the germanium analogue of Sn 6 {B(NDippCH) 2 } 4 (4) (Figure 3). [17]Structurally, 4 and 6 are very similar, differing primarily in the EÀ E and EÀ B bond lengths, at a level expected based on the differing covalent radii of germanium and tin (Δr = 1.39-1.20 = 0.19 Å). [18] The formation of 6 in this way, suggests more broadly that systems of the stoichiometry E 3 {B(NDippCH) 2 } 2 (E = Ge, Sn) are indeed prone to dimerization with accompanying boryl ligand migration.

Conclusions
In conclusion we have shown that boryltin compounds featuring the metal in the formal oxidation states + 1 and 0 can be accessed from a carbene-stabilized tin(II) bromide precursor by the use of strong reducing agents.The formation of the mono-carbene stabilized distannyne and donor-free distannide systems (boryl)SnSn(IPrMe)(boryl) (2) and K 2 [Sn 2 (boryl) 2 ], (3) mirrors related germanium chemistry occurring under similar conditions.In contrast to their lighter congeners, however, systems of the type [Sn(boryl)] n are unstable with respect to disproportionation.Carbene abstraction from 2 using triphenylborane and two-electron oxidation of 3 both result in the formation of a 2 : 1 mixture of the diborylstannylene, Sn(boryl) 2 , and the hexatin cluster, Sn 6 (boryl) 4 (4).A viable mechanism for this rearrangement is shown by quantum chemical studies to proceed via a vinylidene intermediate (analogous to the isolable germanium compound, (boryl) 2 Ge=Ge), which undergoes facile atom transfer in the case of tin to generate Sn(boryl) 2 and trinuclear [Sn 3 (boryl) 2 ].The latter then dimerizes to give the observed hexametallic product 4. Independent studies also show that similar trigermanium species aggregate in analogous fashion.

for 2 and 3, respectively (Figure
B NMR resonance at δ B = 55.2 ppm (see two CH signals and two 11 B resonances at δ B = 43.5, 54.2 ppm for 2).