Tetraborane

Abstract: boron gives little hint of the comparative wasteland making up much of the hydride estate of the heavier Group 13 elements., At a recent count2 about 100 binary boranes are now known, typically as discrete molecules remarkable for their stoicheiometries and structures which have done much to challenge and reshape our understanding of chemical bonding at large. By contrast, aluminium forms only one binary hydride stable under normal conditions as a polymeric solid, [AIH,],, the a-form of which is isostructural with AlF,, featuring 6-coordinate aluminium atom^.^^^ Attempts to prepare the analogous gallium compound have a chequered hist~ry,~ and it has taken nearly 50 years from the first reported sighting to establish the true credentials of gallane, [GaH,],,6 which now emerges as showing obvious affinities to diborane in the vapour state (i.e. n = 2) while being relatively short-lived under normal conditions. Despite some claims, however, it is unlikely that the hydrides [JnH,], and [TlH,], have yet materialized. In this account we review the current status of the hydrides formed by the Group 13 metals aluminium, gallium, indium, and thallium. Coordinatively saturated derivatives like MH, (M = Al, Ga, In, or T1) and Me,N. MH, (M = A1 or Ga) having been known for some year^,^?^ we are concerned primarily with the parent hydrides, [MH,], (m = 1, 2, or 3; n = 1, 2...), and related unsaturated derivatives. The last category includes species with more than one Group 13 element, for example tetrahydroborate derivatives like Al(BH,), and H,Ga(BH,),-, (m = 1 or 2) and tetraborane(l0) derivatives like 2-R2MB,H, (M = Al, R = Me; M = Ga, R = H or Me). It is appropriate first to consider the physical properties of the binary hydrides [MH,],. Hence it is possible to identify not only feasible methods of synthesizing compounds with M-H bonds, but also the origins of the thermal lability and reactivity besetting such compounds. 1.1 Theoretical Modelling Exploration of the Group 13 metal hydrides has been spurred by the greatly enhanced sophistication of modern computational methods which now admit the use of relatively elaborate basis sets, as well as making due allowance for factors like configuration interaction and relativistic correction^.^ Where comparisons can be made, such calculations typically yield dimensions and energetics which reproduce closely the experimental findings, and in some cases improve upon those findings. Such is the case, for example, with the monohydride molecules MH (M = B, Al, Ga, In, or Tl), which are short-lived under normal condition^.^,^ Accordingly we can place some trust in such results to anticipate the likely equilibrium molecular structures, vibrational properties, and binding energies of Group 13 hydrides, including numerous species whose existence has yet to be authenticated. Just what inferences are to be drawn will be discussed in Section 2.

Abstract: Supplementing gas-phase electron-diffraction data with restraints derived from a graded series of ab initio calculations makes possible refinement of all geometrical parameters and amplitudes of vibration. By avoiding the need to fix some parameters, this technique yields structures which are more completely refined and thus have more reliable standard deviations than procedures used previously. It has been applied to the gas-phase structure of the arachno boron hydride tetraborane(10), B4H10. Salient structural parameters (rα0 structure) were found to be: r[B(1)–B(2)] 186.6(2), r[B(1)–B(3)] 173.7(5), r[B(1)–H(1,2)] 123.0(15), r[B(2)–H(1,2)] 141.7(8), r[B(1)–H(1)] 119.8(8), r[B(2)–H(2)]endo] 121.0(8) and r[B(2)–H(2)exo] 120.5(8) pm; butterfly angle 117.2(4)°. The crystal structure was also redetermined at 100 K. All gas-phase, crystallographic and ab initio structural parameters were found to be in good agreement.

Abstract: Building upon our earlier results on the synthesis of electron-precise transition-metal-boron complexes, we continue to investigate the reactivity of pentaborane(9) and tetraborane(10) analogues of ruthenium and rhodium towards thiazolyl and oxazolyl ligands. Thus, mild thermolysis of nido-[(Cp*RuH)2B3H7] (1) with 2-mercaptobenzothiazole (2-mbtz) and 2-mercaptobenzoxazole (2-mboz) led to the isolation of Cp*-based (Cp* = η(5)-C5Me5) borate complexes 5 a,b [Cp*RuBH3L] (5 a: L = C7H4NS2; 5 b: L = C7H4NOS)) and agostic complexes 7 a,b [Cp*RuBH2(L)2], (7 a: L = C7H4NS2; 7 b: L = C7H4NOS). In a similar fashion, a rhodium analogue of pentaborane(9), nido-[(Cp*Rh)2B3H7] (2) yielded rhodaboratrane [Cp*RhBH(L)2], 10 (L = C7H4NS2). Interestingly, when the reaction was performed with an excess of 2-mbtz, it led to the formation of the first structurally characterized N,S-heterocyclic rhodium-carbene complex [(Cp*Rh)(L2)(1-benzothiazol-2-ylidene)] (11) (L = C7H4NS2). Furthermore, to evaluate the scope of this new route, we extended this chemistry towards the diruthenium analogue of tetraborane(10), arachno-[(Cp*RuCO)2B2H6] (3), in which the metal center possesses different ancillary ligands.