Rajendra S. Ghadwal

Neutral pentacoordinate silicon fluorides derived from amidinate, guanidinate, and triazapentadienate ligands and base-induced disproportionation of Si2Cl6 to stable silylenes.

Pentacoordinate silicon fluorides L(1)SiF(3) (2a), L(2)SiF(3) (2b), and (L(3)SiF(2))(2) (2c)(2) based on amidinate (L(1) = PhC(N(t)Bu)(2)), guanidinate (L(2) = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate), and triazapentadienate (L(3) = NC(NMe(2))NC(NMe(2))NAr; Ar = 2,6-(i)Pr(2)C(6)H(3)) ligands were prepared by fluorination of the corresponding chlorosilanes L(1)SiCl(3) (1a), L(2)SiCl(3) (1b), and L(3)SiCl(2) (1c) with Me(3)SnF at ambient temperature. Compounds 1b, 1c, 2a, 2b, and (2c)(2) were characterized by (1)H, (13)C, (19)F, and (29)Si NMR spectroscopic studies. Molecular structures of 1b, 1c, 2a, and (2c)(2) were determined by single crystal X-ray structural analysis. Invariom refinement involving non-spherical scattering factors of the Hansen-Coppens multipole model was performed for 1b. Compound L(3)SiF(2) (2c) is dimeric both in the solid state and in solution, whereas its chloro-analogue 1c is monomeric. The attempted synthesis of diamidinatotetrachlorodisilane by reaction of lithium amidinate with Si(2)Cl(6) led to the formation of the silane (1a) and the silylene L(1)SiCl (3). Reaction of Si(2)Cl(6) with N-heterocyclic carbenes (NHC) afforded NHC adducts of dichlorosilylene and SiCl(4). A one pot method for the preparation of base-stabilized silylenes from Si(2)Cl(6) is discussed.

A Facile Route to Functionalized N-Heterocyclic Carbenes (NHCs) with NHC Base-Stabilized Dichlorosilylene

Reaction of IPr x SiCl(2) (1) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] with 1-azidoadamantane leads to functionalized N-heterocyclic carbene (NHC) 2. Silyl-substituted NHC 2 reacts easily with 1-azidoadamantane to form triazene 3, in which the exocyclic C=N bond is slightly shorter than those of regular NHC-derived triazines. 2 could serve as a promising ligand for transition metals.

Carbon-based two electron σ-donor ligands beyond classical N-heterocyclic carbenes

N-Heterocyclic carbenes (NHCs) are an important class of compounds that are indeed regarded as most versatile carbon-donor ligands in transition metal and organometallic catalysis. In addition, NHCs are also capable of stabilizing a variety of highly reactive main group compounds with intriguing properties. The enormous success of NHCs prompted the investigation of other carbon-based neutral ligands with additional structural and electronic features. Reactivity studies of NHCs and their complexes unveiled routes to new carbon donor ligands. Such NHC-derived ligands can be grouped into three categories: (i) N-heterocyclic olefins (the donor site is extended by an intervening carbon atom), (ii) ditopic carbanionic-NHCs (an additional coordination site is generated by the deprotonation of an NHC), and (iii) abnormal-NHCs (the carbene center of an NHC is moved to different positions). This article summarizes the recent advances in NHC chemistry of compounds featuring the aforementioned ligands, which are exclusively derived by the functionalization of NHCs, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) in particular.

Crystalline Radicals Derived from Classical N-Heterocyclic Carbenes

One-electron reduction of C2-arylated 1,3-imidazoli(ni)um salts (IPrAr )Br (Ar=Ph, 3u2009a; 4-DMP, 3u2009b; 4-DMP=4-Me2 NC6 H4 ) and (SIPrAr )I (Ar=Ph, 4u2009a; 4-Tol, 4u2009b) derived from classical NHCs (IPr=:C{N(2,6-iPr2 C6 H3 )}2 CHCH, 1; SIPr=:C{N(2,6-iPr2 C6 H3 )}2 CH2 CH2 , 2) gave radicals [(IPrAr )]. (Ar=Ph, 5u2009a; 4-DMP, 5u2009b) and [(SIPrAr )]. (Ar=Ph, 6u2009a; 4-Tol, 6u2009b). Each of 5u2009a,b and 6u2009a,b exhibited a doublet EPR signal, a characteristic of monoradical species. The first solid-state characterization of NHC-derived carbon-centered radicals 6u2009a,b by single-crystal X-ray diffraction is reported. DFT calculations indicate that the unpaired electron is mainly located at the original carbene carbon atom and stabilized by partial delocalization over the adjacent aryl group.

N-Heterocyclic Vinylidene-Stabilized Phosphorus Biradicaloid

Four-membered biradicaloid compounds containing a N2 E2 (E=main group element) framework have been thoroughly investigated; however, the synthesis of stable analogues with a C2 P2 skeleton remains a challenge. Base-mediated double C-H functionalization of IPr=CH2 (1) (IPr=C[N(2,6-iPr2 -C6 H3 )CH]2 ) with PCl3 affords [{(IPr)CP}2 Cl]Cl (2) as a royal blue solid. Treatment of 2 with KC8 yields the stable phosphorus biradicaloid [(IPr)CP]2 (3) featuring a four-membered C2 P2 ring. Compound 3 is diamagnetic and shows sharp and temperature-independent NMR resonances, revealing its singlet biradicaloid nature. The stability of 3 is attributed to the σ- and π-electron-donating property of the N-heterocyclic vinylidene [(IPr)C] group.

The Viability of C5-Protonated- and C4,C5-Ditopic Carbanionic Abnormal NHCs: A New Dimension in NHC Chemistry

The first C5-protonated abnormal N-heterocyclic carbene (aNHC), PhC{N(2,6-iPr2 C6 H3 )}2 CHC: (4) is readily accessible by C4-deprotonation of [ArC{N(2,6-iPr2 C6 H3 )}2 CHCH]X (3u2009a-X) (Ar=Ph, X=Br or I) with a base. The aNHC 4 is stable at 298u2005K in [D8 ]THF solution and has been spectroscopically characterized. The facile availability of 4 enables the synthesis of a series of main-group compounds as well as transition-metal complexes featuring a new phosphorus-aNHC hybrid ligand. Double deprotonation of [ArC{N(2,6-iPr2 C6 H3 )}2 CHCH]X (Ar=Ph, 3u2009a-X (X=Br or I); 4-Tol, 3u2009b-Br; 4-DMP, 3u2009c-Br; Tol=MeC6 H4 , DMP=Me2 NC6 H4 ) with nBuLi yields the C4,C5-ditopic carbanionic aNHCs, [ArC{N(2,6-iPr2 C6 H3 )}2 (C:)2 ]Li(THF)n (Ar=Ph, 13u2009a; 4-Tol, 13u2009b; 4-DMP, 13u2009c), which on treatment with Ph2 PCl affords cationic vicinal bisphosphine derivatives [ArC{N(2,6-iPr2 C6 H3 )}2 {C(PPh2 }2 ]X (Ar=Ph, 14u2009a-X, X=Br or I; 4-Tol, 14u2009b-Br; 4-DMP, 14u2009c-Br).

Kekulé diradicaloids derived from a classical N-heterocyclic carbene

Two-electron reduction of bis(1,3-imidazolinium) salts 2 and 3 with KC8 gives rise to stable diradicaloids 4 and 5, respectively. Calculations reveal a very low singlet–triplet energy gap ΔES–T for 5 (10.7 kcal mol–1), while ΔES–T for 4 (29.1 kcal mol–1) is rather large.

N-Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons.

Stable N-heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6 H4 )(IPr)] and [(IPr)(C6 H4 )2 (IPr)] (4 and 5, respectively; IPr=C{N(2,6-iPr2 C6 H3 )}2 CHCH), are reported. In a nickel-catalyzed double carbenylation of 1,4-Br2 C6 H4 and 4,4-Br2 (C6 H4 )2 with IPr (1), [(IPr)(C6 H4 )(IPr)](Br)2 (2) and [(IPr)(C6 H4 )2 (IPr)](Br)2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8 . Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet-triplet energy gap ΔES-T of 10.7u2005kcalu2009mol-1 , whereas 4 features more quinoidal character with a rather large ΔES-T of 25.6u2005kcalu2009mol-1 . In view of the low ΔES-T , 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41u2009%) diradical character.

Diradical Character Enhancement by Spacing: N-Heterocyclic Carbene Analogues of Müller's Hydrocarbon

Two-fold C-C cross-coupling of N-heterocyclic carbenes [NHCs; SIPr=C(NArCH2 )2 , 1; IPr=C(NArCH)2 , 2; Me-IPr=C(NArCMe)2 , 3; Ar=2,6-iPr2 C6 H3 ] with 4,4-diiodo-p-terphenyl under Ni catalysis furnished [(SIPr)(C6 H4 )3 (SIPr)](I)2 (4), [(IPr)(C6 H4 )3 (IPr)](I)2 (5), and [(Me-IPr)(C6 H4 )3 (Me-IPr)](I)2 (6). Two-electron reduction of 4-6 with KC8 readily afforded NHC analogues of Müllers hydrocarbon (MH), [(SIPr)(C6 H4 )3 (SIPr)] (7), [(IPr)(C6 H4 )3 (IPr)] (8), and [(Me-IPr)(C6 H4 )3 (Me-IPr)] (9), respectively, as highly colored crystalline solids. Quantum chemical calculations suggested that the singlet ground state for 7-9 possesses a vertical singlet-triplet energy gap ΔES-T of -7.24 to -7.60u2005kcalu2009mol-1 , which is significantly lower compared to that of the NHC analogues of Thieles (TH) and Chichibabins (CH) (18-38u2005kcalu2009mol-1 ) hydrocarbons. Importantly, the calculated diradical character (y) of 7-9 (y≈0.6) is considerably higher compared to that of the related TH and CH (y=0.1-0.4), suggesting the open-shell singlet character of 7-9.