Multiple complexation of CO and related ligands to a main-group element.

The ability of an atom or molecular fragment to bind multiple carbon monoxide (CO) molecules to form multicarbonyl adducts is a fundamental trait of transition metals. Transition-metal carbonyl complexes are vital to industry, appear naturally in the active sites of a number of enzymes (such as hydrogenases), are promising therapeutic agents, and have even been observed in interstellar dust clouds. Despite the wealth of established transition-metal multicarbonyl complexes, no elements outside groups 4 to 12 of the periodic table have yet been shown to react directly with two or more CO units to form stable multicarbonyl adducts. Here we present the synthesis of a borylene dicarbonyl complex, the first multicarbonyl complex of a main-group element prepared using CO. The compound is additionally stable towards ambient air and moisture. The synthetic strategy used--liberation of a borylene ligand from a transition metal using donor ligands--is broadly applicable, leading to a number of unprecedented monovalent boron species with different Lewis basic groups. The similarity of these compounds to conventional transition-metal carbonyl complexes is demonstrated by photolytic liberation of CO and subsequent intramolecular carbon-carbon bond activation.

Trihapto ligation of a borirene to a single metal atom: a heterocyclic analogue of the η³-cyclopropenyl ligand.

The first example of a complex containing an η(3)-coordinated borirene ligand bound to a single metal atom was prepared by photolytic transfer of an arylborylene ligand to diphenylacetylene and loss of three CO ligands from the precursor. The η(3)-borirene complex possesses a chromium(0) atom which is also bound to one phenyl group in an η(6) fashion. The complex was isolated in 15% yield along with the corresponding metal-free borirene, which was isolated in 62% yield. The bonding between the chromium center and the borirene ring was studied computationally by DFT methods.

Reversible intramolecular coupling of the terminal borylene and a carbonyl ligand of [Cp(CO)2Mn=B-tBu].

Cyclic complex 2 with bridging carbonyl ligands was synthesized from a facile and reversible intermolecular carbonyl-borylene ligand coupling reaction at room temperature. Complex 2 exhibits an unprecedented coordination mode for boron-metal complexes, which is also reflected in its remarkable (11)B NMR chemical shift of -57.2 ppm. Findings from spectroscopic, X-ray, and computational studies are presented, along with a proposed mechanism.

Reductive borylene-CO coupling with a bulky arylborylene complex.

Partial metal-boron bond cleavage and coupling of a borylene with two CO ligands was observed upon reduction of a new bulky arylborylene complex. Both the borylene precursor and dianionic product were structurally and spectroscopically characterized. In contrast, reduction of an aminoborylene complex led to complete loss of the borylene ligand and classical Hieber reduction. A rationale for these differences based on DFT methods is presented.

Cyclic (amino)(imino)carbene complexes by borylene transfer to isocyanides.

Here we report the facile formation of boron-containing cyclic (amino)(imino)carbene complexes by reaction of a chromium borylene complex [(OC)(5)Cr=B=N(SiMe(3))(2)] with isocyanides.

Hemilabile and reversible carbon monoxide binding properties of iron(II), cobalt(II) and nickel(II) complexes containing a new tridentate P-S-N ligand.

Several first-row transition metal complexes of the formulation [M(1)(2)](X)(2) {where 1 = Ph(2)PCH(2)CH(2)S(2-C(6)H(4)NH(2)); M = Fe(II), X = BF(4)(-) (2); M = Co(II), X = BF(4)(-) (3), Ni(II), X = ClO(4)(-) (4)} have been prepared by reaction of two equivalents of the new P-S-N ligand Ph(2)PCH(2)CH(2)S(2-C(6)H(4)NH(2)) 1 with one equivalent of the appropriate [M(OH(2))(6)](X)(2) precursor in acetonitrile. In the solid state, complexes 2-4 exist as distorted centrosymmetric octahedral structures featuring facially capping ligands in an all-trans arrangement. Reaction of 2 and 3 with a stream of carbon monoxide (1 atm.) for 5 min in acetonitrile generates iron(II) monocarbonyl species of formulation [Fe(CO)(1)(2)](BF(4))(2)2a, and a cobalt(II) dicarbonyl complex, [Co(CO)(2)(1)(2)](BF(4))(2)3a, which can be isolated in the solid state. Complete removal of CO is achieved by either heating to reflux samples of 2a in acetonitrile for 5 min or by heating solid samples of 3a at 120 °C in vacuo over a period of 4 h. The binding of carbon monoxide is fully reversible for 2 and 3 and can be repeated over multiple cycles. When the same trapping reactions were carried out with very low radiochemical (11)CO concentrations, metal carbonyl species were no longer formed. It is likely that the kinetics of (11)CO adduct formation are too slow to allow for effective trapping under the applied radiochemical conditions.

Encapsulation of hydride by molecular main group metal clusters: manipulating the source and coordination sphere of the interstitial ion.

The sequential treatment of Lewis acids with N,N'-bidentate ligands and thereafter with ButLi has afforded a series of hydride-encapsulating alkali metal polyhedra. While the use of Me3Al in conjunction with Ph(2-C5H4N)NH gives Ph(2-C5H4N)NAlMe2 and this reacts with MeLi in thf to yield the simple 'ate complex Ph(2-C5H4N)NAlMe3Li.thf, the employment of an organolithium substrate capable of beta-hydride elimination redirects the reaction significantly. Whereas the use of ButLi has previously yielded a main group interstitial hydride in which H- exhibits micro6-coordination, it is shown here that variability in the coordination sphere of the encapsulated hydride may be induced by manipulation of the organic ligand. Reaction of (c-C6H11)(2-C5H4N)NH with Me3Al/ButLi yields [{(c-C6H11)(2-C5H4N)N}6HLi8]+[(But2AlMe2)2Li]-, which is best viewed as incorporating only linear di-coordination of the hydride ion. The guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) in conjunction with Me2Zn/ButLi yields the micro8-hydride [(hpp)6HLi8]+[But3Zn]-.0.5PhMe. Formation of the micro8-hydride [(hpp)6HLi8]+[ButBEt3]- is revealed by employment of the system Et3B/ButLi. A new and potentially versatile route to interstitial hydrides of this class is revealed by synthesis of the mixed borohydride-lithium hydride species [(hpp)6HLi8]+[Et3BH]- and [(hpp)6HLi8]+[(Et3B)2H]- through the direct combination of hppLi with Et3BHLi.