Franck Ulm

Yann Cornaton

Jean-Pierre Djukic

Michael J Chetcuti

Vincent Ritleng

Hydroboration of alkenes catalysed by a nickel N-heterocyclic carbene complex: reaction and mechanistic aspects

The pentamethylcyclopentadienyl N-heterocyclic carbene nickel complex [Ni(η 5 -C5Me5)Cl(IMes)] (IMes = 1,3-dimesitylimidazol-2-ylidene) efficiently catalyses the anti-Markovnikov hydroboration of alkenes with catecholborane in the presence of a catalytic amount of potassium tert-butoxide, and joins the very exclusive club of nickel catalysts for this important transformation. Interestingly, the regioselectivity can be reversed in some cases by using pinacolborane instead of catecholborane. Mechanistic investigations involving control experiments, 1 H and 11 B NMR spectroscopy, cyclic voltammetry, piezometric measurements and DFT calculations suggest an initial reduction of the Ni(II) precursor to a Ni(I) active species with the concomitant release of H2. The crucial role of the alkoxo-catecholato-borohydride species resulting from the reaction of potassium tert-butoxide with catecholborane in the formation of an intermediate nickel-hydride species that would then be reduced a Ni(I) active species, is highlighted.

Introduction

Boronic acid and ester derivatives are important starting materials in molecular synthesis and are well-known partners for classical transformations, such as the Petasis reaction, 1 Chan-Lam coupling, 2 conjugate addition reactions, [START_REF] Berthon-Gelloz | Boronic Acids. Rhodium-and Palladium-Catalyzed Asymmetric Conjugate Addition of Organoboronic Keywords: nickel • hydroboration • alkene • N-heterocyclic carbene • Ni(I) active species Acid[END_REF] and, most impressively, Suzuki-Miyaura cross-couplings, [START_REF] Suzuki | [END_REF] but their traditional production methodology involves a metal-halogen exchange with an organoalkali reagent and trialkylborates, [START_REF] Hall | Boronic Acids. Structures, Properties, and Preparation of Boronic Acid Derivatives: Overview of their Reactions and Applications[END_REF] which is far from being satisfying from an environmental perspective. Subsequently, new methods were developed through the direct borylation of aryl halides, mostly catalysed by Pd. 6 More recent advances have allowed the hydroboration of alkenes, 7 a reaction which is of great interest since the substrates are readily available and it creates new C-B bonds by the most straightforward and atom-thrifty synthetic route.

The main drawback of this methodology is that it is principally based on noble metals, such as rhodium and iridium. [START_REF] Fu | Transition Metal-Catalyzed Hydroboration of Olefins[END_REF] The decrease in non-renewable natural resources has indeed become a global concern for the future, and precious transition metals, are, just like to fossil fuels, limited and thus strategic resources. This can be monitored by the tremendous increase of their prices over the last decade. New directions for homogeneous catalysis should thus not only address energy issues by developing highly reactive catalysts, but should also favour the use of earth-abundant materials as catalysts.

In this context, complexes based on first-row transition metals, [START_REF] Obligacion | [END_REF] most notably copper, 9 cobalt, 10 iron, 11 and manganese 12 have recently emerged as alternative catalysts for this process. In contrast, although it has shown promise for the hydroboration of alkynes, 13 the 1,4-hydroboration of 1,3-dienes, 14 or more recently for formal hydroborations making use of a diboron reagent with a proton source, 15 there are to our knowledge only three reports of the nickel-catalysed hydroboration of alkenes. 16 The first one involved an activated nickel powder that showed relatively low activity and selectivity. 16a In the two other examples, well-defined nickel(II) pre-catalysts were used, which showed excellent activity and selectivity. Hence, in 1996, Srebnik et al. reported that the half-sandwich triphenylphosphine nickel complex, [Ni(Cp)Cl(PPh3)] (Cp = η 5 -C5H5), catalyses the selective hydroboration of trans-4-octene and styrene with HBPin to give 4pinacolboryloctane and 1-phenyl-2-pinacolborylethane in high yields (Scheme 1, eq. 1). 16b In contrast, Schomaker et al. recently reported that the heteroleptic NHC-phosphine nickel complex, [NiCl2(IMes)(PCy3)] (IMes = 1,3-dimesitylimidazol-2-ylidene), catalyses the Markovnikov-selective hydroboration of styrenes with good efficiency in the presence of KOt-Bu (Scheme 1, eq. 2). 16c Remarkably, no dehydroborylation product was observed in both cases, but the reaction scopes were rather limited, and no mechanistic study was carried out. Scheme 1. Previously reported nickel-catalysed hydroboration of alkenes. 16b,c In parallel, efforts from our group and others towards the diversification of pseudo-trigonal [Ni(η 5 -C5R5)L(NHC)] (+) species and the study of their catalytic activity in a large array of reactions, have allowed their establishment as a highly versatile platform for important organic transformations. 17 Most notably, interesting activities have been obtained for the hydrosilylation of carbonyl and imine 19 derivatives. Thus, the half-sandwich nickel complex, [Ni(Cp)Cl(IMes)] (1a), was shown to efficiently catalyse the hydrosilylation of a vast array of aldehydes, ketones, aldimines and ketimines (over 60 examples!) under mild conditions in the presence of a catalytic amount of sodium borohydride. 18a,19 This precedent, together with (i) the scarcity of nickel-catalysed alkene hydroborations, 16 (ii) the electronic and structural proximity of [Ni(Cp)Cl(PPh3)] and 1a, (iii) the renewed interest in the fascinating chemistry of nickel, 20 and (iv) the necessity to gain a better understanding of its reaction paths in order to be able to tame its reactivity, prompted us to the enter this challenging field with a series of [Ni(Cp † )L(NHC)] (+) complexes (Cp † = Cp, Cp* (η 5 -C5Me5); L = Cl -, NCMe, NHC = IMes, IPr (1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene), ICy (1,3dicyclohexylimidazol-2-ylidene) and to try to unravel the mechanism at play.

Herein, we show that the Cp* complex, [Ni(Cp*)Cl(IMes)] (2a), efficiently catalyses the anti-Markovnikov hydroboration of alkenes with HBCat (catecholborane) in the presence of a catalytic amount of KOt-Bu. Notably, the selectivity can be reversed for some styrene derivatives by using HBPin (pinacolborane) instead of HBCat. A combination of experiments gives some insight to the mechanism. The latter is proposed to go through a Ni(I)/Ni(III) cycle after initial reduction of the nickel(II) precursor by a tert-butoxo-catecholato-borohydride species.

Results and Discussion

Initial studies focussed on the hydroboration of styrene with HBPin in CH2Cl2 at room temperature, conditions under which the complex [Ni(Cp)Cl(PPh3)] (1 mol%) had been reported to quantitatively convert styrene to 1-phenyl-2-pinacolborylethane in 18 h (Scheme 1, eq. 1). 16b Disappointingly however, absolutely no conversion was observed under these conditions with the closely related complexes, 1a and [Ni(Cp)Cl(IPr)] (1b). Intrigued by this negative result, we reinvestigated the reaction with [Ni(Cp)Cl(PPh3)] and were unable to reproduce Srebnik's result.

We thus decided to investigate the reaction in toluene at 60 °C in the presence of 3 mol% of nickel precatalyst and 6 mol% of KOt-Bu. 16c Under these conditions, 1a proved again to be unreactive (Fig. 1). A first hit was observed with 1b with 30% conversion after 18 h and 64% Markovnikov selectivity, but the Cp derivatives proved to be globally poorly reactive, as was also illustrated by the very poor conversion observed with [Ni(Cp)Cl(ICy)] (1c). A net gain of activity was observed with the electron-richer Cp* derivatives, notably with 2a that allowed 75% conversion with a 40:60 anti-Markonikov/Markovnikov (AM/M) ratio. The IPr derivative 2b proved a little less active in this case, with 59% conversion after 18 h, and showed no selectivity. Complex 2a was thus chosen for the optimization study of the reaction conditions (Table 1). The promising result obtained in 15 h at 60 °C (entry 1) prompted us to decrease the temperature to 25 °C. This resulted in a much better selectivity with a AM/M ratio of 26:74, but the conversion decreased to 62 % (entry 2). Running the reaction for 24 h at 25 °C allowed us to almost reach the conversion observed after 15 h at 60 °C (entry 3). Carrying out the same Conv.

(%) [b] AM/M (%) [b] 1 catalysis at 60 °C led to full conversion, but still with a low selectivity (entry 4). Finally, increasing the number of equivalents of HBPin to 2.5 allowed us to observe full conversion as well, but surprisingly with an even better AM/M ratio of 20:80 than at room temperature (entry 5). However, the impossibility, at least in our hands, to cleanly separate the isomers by column chromatography led us to explore the reaction with HBCat. A similar result to HBPin (1.2 equiv.) at 60 °C (entry 1) was observed with HBCat, with 73 % conversion and a 50:50 AM/M ratio after 15 h reaction (entry 6). Increasing the number of equivalents to 1.8 and 2.5 allowed the conversion to reach 83 and 100%, respectively, but with a reverse regioselectivity (entries 7 and 8), as was recently observed with an aryloxy-tethered NHC-Fe(II) catalyst. 21 Indeed, in this case, the linear isomer was the major product, and a 85:15 AM/M ratio was obtained when 2.5 equiv. of HBCat were used (entry 8). In light of the increased selectivity observed at room temperature with 1.2 equiv. of borane (entries 2 and 3), we then decided to check if a similar tendency could be observed in the presence of 2.5 equiv. of borane. The anti-Markovnikov isomer was the sole product formed under these conditions, but the conversion was only 33 % after 15 h (entry 9). Finally, the replacement of potassium tert-butoxide by sodium or lithium tert-butoxide had no consequence on the reaction (entry 10).

With these optimized conditions in hand (2.5 equiv. borane, Table 2. Hydroboration of alkenes with HBCat catalysed by 2a [a] Entry Substrate Conv.

(%) [b] AM/M (%) [b] Yield (%) [c] 1 100 85:15 80 2 [d] 100 toluene, 60 °C, 15 h), we then examined the scope of the hydroboration of alkenes catalysed by 2a with HBCat (Table 2).

Electron-neutral and electron-rich styrenes were all fully converted with high linear selectivity (entries 1-5). In the cases of 4-methoxystyrene and 2-methylstyrene, the linear isomer was actually the sole product (entries 3 and5). Electron-poor styrenes were hydroborated with conversions ranging from moderate in the case 4-trifluoromethylstyrene (entry 9) to full with 2-bromo-and 4fluoro-styrenes (entries 7 and8). With the exception of 2bromostyrene, which was converted with a poor selectivity, the other substrates all gave good to excellent linear selectivity. In all cases where the branched isomer was formed as a minor product, it decomposed on the silica column, 22 allowing the isolation of the pure AM isomer in yields ranging from 38 % in the case of 4trifluoromethylstyrene (entry 9) to 86 % for 4-methylstyrene (entry 4). The reaction is not limited to styrenes, and vinylcyclohexane was fully converted to the linear isomer, allowing its isolation with 60 % yield (entry 10). Cis-stilbene was also hydroborated with good efficiency (80 %), but the resulting product decomposed on the column (Fig. 2). In addition, a number of substrates performed either poorly or non-selectively. Thus, trans-stilbene and 1phenylcyclohexene gave no conversion at all, and 1methylstyrene, 4-cyanostyrene, cinnamyl chloride and 1heptene [START_REF]A possible explanation for the observation of a mixture of products with 1-heptene may be due alkene chain isomerization[END_REF] gave complicated mixtures despite good to excellent conversions (Fig. 2).

Figure 2.

Substrates that performed either poorly or non-selectively with HBCat or that gave a product that decomposed.

As catalysts that can enable different regioselectivities are invaluable for the construction of a diverse set of value-added compounds, [START_REF] Obligacion | [END_REF] we next also briefly investigated the reaction scope with HBPin to check if the observed reversal of selectivity with styrene was general (Table 3). Electron-rich and electron-poor styrenes such as 4-methoxystyrene, 2-bromostyrene and 4trifluoromethylstyrene were also converted to the branched isomer with good to high regioselectivities (entries 2-4). Surprisingly however, 2-and 4-methylstyrene gave the linear isomer quasi-exclusively (entry 5 and 6). Although substrate dependent regioselectivity is observed every now and then, [9][10][11][12] and had, in particular, been noted with the closely related [Ni(Cp)Cl(PPh3)] for styrene and 4-octene (Scheme 1), 16b it is to our knowledge rarely observed within a series of simple styrene derivatives. We therefore did not pursue our substrate scope investigation with HBPin any further. 3. Hydroboration of styrenes with HBPin catalysed by 2a [a] Entry Substrate Conv.

(%) [b] AM/M (%) [b] Yield (%) 1 100 20:80 -2 100 26:74 51 [c] 3 100 - [d] 94 4 100 13:87 95 [c] 5 100 99:1 22 [e] 6 100 99:1 36 [e] [a] Reaction conditions: alkene (1.0 mmol), HBPin (2.5 mmol), 2a (3 mol%), KOt-Bu (6 mol%) in toluene (2 mL) at 60 °C for 15 h. [b] Conversions and AM/M ratio determined by 1 H NMR spectroscopy with 1,3,5trimethoxybenzene as an internal standard; average values of at least two runs. [c] Isolated yield of the AM/M mixture. [d] The AM:M ratio was difficult to evaluate due to the possible presence of debromoborylation products. [START_REF] Bheeter | [END_REF] [e] The low isolated yields are due to massive decomposition during the chromatographic purification.

To gain insight to the mechanism, we first checked whether the hydroboration process was the result of a true homogeneous catalysis by conducting the hydroboration of styrene with HBCat in the presence of 190 equivalents of Hg relative to 2a. No inhibition was observed, nor a change of selectivity, and thus a process catalysed by nickel particles is unlikely. 25 Owing to the propensity of nickel to easily access multiple oxidation states, 20 we then checked whether the reaction involves a radical by performing control experiments in the presence of radical scavengers. The addition of 1 equiv. of TEMPO or galvinoxyl (relative to styrene) at t0 completely inhibited the reaction (Scheme 2). Furthermore, the addition of 3 mol% TEMPO limited the conversion to 55 % after 15h reaction, and the addition of 1 equiv. TEMPO after 7 h reaction to 76 % (vs. 100% without radical scavenger). This suggested the presence of radicals not only during the activation phase but during the catalytic cycle itself. This point being established, we then recorded a cyclic voltammogram of the pre-catalyst (1 mM) in CH3CN (0.1 M NBu4PF6). As the latter tended to generate a deposit at the surface of the carbon electrode that altered the reproducibility of the measurements, the electrochemical characterization was carried out with the cationic derivative, [Ni(Cp*)(NCMe)(IMes)]PF6 (3a), of 2a that shows a similar catalytic activity (Table 2,entry 2). At a scan rate of 200 mV.s -1 , a quasi-reversible oxidation process can be observed at E1/2ox = 0.16 V vs. Fc/Fc + (DEp = 154 mV), and a quasi-reversible reduction peak is seen at E1/2red = -1.75 V (DEp = 190 mV) (Fig. 3). By analogy with the electrochemical data reported by Crabtree for the Cp derivative 1a (E1/2ox = 0.03 V and E1/2red = -1.51 V vs. Fc/Fc + , v = 100 mV.s -1 ), 26 these values most likely correspond to Ni(II)/Ni(III) oxidation and Ni(II)/Ni(I) reduction processes, while the irreversible oxidation and reduction peaks observed at -0.25 V and -1.48 V (vs. Fc/Fc + ) potential would correspond to ligand-centred redox processes. Nevertheless, the Cp* ligand has been described 'to act as an electron density buffer', 27 which might mean that neither of these redox processes are purely metal centred, and could explain the counter-intuitive observation of a higher Ni(II)/Ni(III) oxidation potential for 3a with respect to 1a. In parallel, stoichiometric studies were conducted to try to identify reaction intermediates. Thus, we monitored the reaction of 2a with 1 equiv. of HBcat and 1 equiv. of KOt-Bu by 1 H NMR in C6D6 from 253 to 298 K. At 293 K, signals belonging to a Cp*-Ni(IMes) species were unambiguously identified (Fig. 4). Though a little broad, these signals are found at the same chemical shifts as those of 2a, suggesting that most of the starting complex is unreacted under these conditions. Nevertheless, a singlet observed at 4.47 ppm is assigned to molecular H2. Furthermore, another singlet, observed at -22.9 ppm and BPin H 2.5 equiv.

+ BPin

BPin

+ AM M toluene / 60 °C / 15 h 2a (3 mol%) KO t Bu (6 mol%) R R R MeO Br F 3 C
TEMPO (1 equiv.) at t = 0: < 1% conv.

Galvinoxyl (1 equiv.) at t = 0: < 1% conv.

No radical scavenger: 100% conv.

TEMPO (3 mol%) at t = 0: 55% conv. TEMPO (1 equiv.) at t = 7 h: 76% conv. present in a ratio of ca. 5% when compared to 2a, is assigned to a nickel-hydride species by analogy with the corresponding signal of [Ni(Cp)H(IMes)]. 18a Albeit to a low extent, this suggests that a reaction has well occurred at the nickel centre. In addition to these observations, the proton of the starting HBCat is only observed in very minor proportions, and the singlet belonging to the tertbutoxide anion has shifted from 1.05 to 1.28 ppm, which corresponds to the shift observed when the latter is reacted with HBCat in the absence of nickel pre-catalyst, and thus suggests that the main reaction occurred between these two species under these conditions.

To gain further insight into this reaction, we next recorded the 11 B NMR spectrum of a 1:1 mixture of HBCat and KOt-Bu in C6D6 at room temperature (Fig. 5). In agreement with the data reported by the groups of S. P. Thomas 28 and T. B. Clark 29 for the reaction of HBPin with NaOt-Bu, several peaks are observed, which can be attributed to BH3 (or more likely to B2H6) at -13.3 ppm, to (t-BuO)BCat at 25.8 ppm, and most importantly to an alkoxo-catecholato-borohydride species, [H(t-BuO)BCat] -(4), at 8.2 ppm, that has been described to act as a universal reducing agent for a number of iron, cobalt, nickel and manganese complexes in various hydrofunctionalization reactions. 12b,28,30 The addition of this preformed mixture (6 mol%) and HBCat (2.5 equiv.) to 2a (3 mol%), previously solubilized in toluene in the presence of styrene, resulted in a catalytic system that demonstrated similar activity to when 4 is generated in situ (Scheme 3), thus suggesting that this species could also act as the pre-catalyst activator here. We were intrigued by the observation of a strong effervescence at the beginning of each catalytic reaction, i.e.: immediately after the addition of HBCat, and this independently of the presence or absence of styrene (the latter being indifferently added before or after HBCat). So, we next conducted a piezometric study in order to quantify the amount of H2 produced. For that purpose, we used a thermostatically controlled doublewalled Schlenk tube that is connected to a pressure transducer, which allows the real-time monitoring of the pressure inside the Schlenk (Fig. S1). 31 Using this experimental set-up, we measured the production of 1 equiv. of H2 per mole of nickel when 83 equiv. HBCat were added to a 1:2 mixture of 2a and KOt-Bu in toluene at room temperature. Similarly, the reaction of 2a with 2 equiv. of KOt-Bu and 2 equiv. of HBCat produced 1 of H2 per mole of nickel. In contrast, the reaction of 1 equiv. 2a with 1 equiv. KOt-Bu and 1 equiv. of HBCat only produced 0.5 equiv. of H2 per mole of nickel.

In view of these data, we considered the possible implication of the nickel hydride complex [NiCp*H(IMes)] (5a), that would act as the true catalytic precursor 18a and would be reduced with concomitant H2 release to the Ni(I) active species, [NiCp*(IMes)] (6a), whose IPr analogues (Cp and Cp*) have already been isolated 32 and shown to be catalytically competent in Suzuki couplings. 33 To try to validate this possibility, we then prepared 6a by reacting 2a with KC8 (or Na/Hg) in THF at -78 °C (Scheme 4). The generation of 6a was established by 1 H NMR spectroscopy. Two highly deshielded singlets at 107 and 31 ppm, integrating for 15 and 2 protons respectively (Fig. S2), could indeed be assigned to the protons of the Cp* group and of the imidazolylidene ring by comparison with the reported data for [NiCp*(IPr)]. 32 However, despite repeated attempts, we have been unable to isolate it pure due to its high sensitivity. Nevertheless, we still attempted the hydroboration of styrene with HBCat in the presence of impure 6a (3 mol%). 70 % conversion was observed after 15 h at 60 °C, with a 60:40 AM/M ratio (Scheme 5). Despite the reduced performance and selectivity compared to 2a and KOt-Bu, which may be attributed to the presence of impurities, this result, observed in the absence of KOt-Bu, tended to confirm the implication of 6a in the catalytic cycle. From that point on, different mechanisms for the hydroboration of styrene by HBCat catalysed by 6a were then considered. Density functional theory (DFT) computations at the PBE-D3(BJ)/def2-TZVPP level of theory were carried out to discriminate between the different hypothesis that were formulated. For this study, only reaction intermediates (local energy minima) were considered, and no transition state (energy saddle-points) were searched for. Thus, only the thermodynamics of the reactions were studied, with no insight into the kinetics. Reaction Gibbs energies computed at this level of theory are compiled in Table 1 at both temperatures considered in the experimental part of this work, i.e.: at 25 °C and 60 °C.

Two reactions were considered for the formation of the Ni(I) catalyst 6a from the Ni(II) hydride 6a: (i) one where 5a would react with an equivalent of 4, which would act as a reductant, to generate 6a and one equivalent of H2; (ii) the other where two molecules of 5a would react together to generate two equivalents of 6a and one of H2. In these two reactions, both reactants should lose a hydrogen radical. Both of these reactions have been found to be endergonic with the former being largely less favourable than the latter at both 25 °C (ΔrG 0 = 53.66 vs. 13.82 kcal/mol) and 60 °C (ΔrG 0 = 52.38 vs. 13.12 kcal/mol). Examples of reactions similar to the latter have been -24.77 -24.87 known for more than 40 years, 34 and even if this reaction is endergonic, the equilibrium should be shifted toward the formation of 6a as a consequence of the law of mass action since H2 mostly escapes the solution as a gas, as observed experimentally.

M • + 5a → 6a + M
The first step of the catalytic cycle was initially supposed to be an oxidative addition of 6a to HBCat to generate a Ni(III) intermediate, as CV measurements suggested a Ni(I)/Ni(III) catalytic cycle could be amenable. However, all DFT attempts to optimize such a structure led instead to the "agostic" adduct 7a where the H-B bond of the HBCat attractively interacts with the Ni(I) centre in an exergonic fashion at both 25 °C (ΔrG 0 = -6.77 kcal/mol) and 60 °C (ΔrG 0 = -4.94 kcal.mol). Mapping the spin density of this adduct revealed an important single electron population on the hydrogen involved in the agostic interaction, giving it a strong hydrogen radical character (Fig. S3).

From this agostic intermediate, two mechanisms have been considered: (i) a first one where the B-H bond would undergo an homolytic fission to generate a boron radical that will then be captured by the styrene forming a carbon radical and 5a (Scheme 6); or (ii) a second one where an adduct 8a between 7a and a styrene (Fig. S4) would homolytically break the B-H bond and release the same carbon radical as in the previous mechanism and 5a (Scheme 7). In both cases, the carbon radical would then react with 5a to regenerate 6a and yield the final hydroboration product. Figure 6 shows the Gibbs energy profiles for both considered mechanisms of the catalytic cycle, leading to both products (AM and M) at 25 °C and 60 °C. The second step of both these mechanism, the only one by which they differ, is endergonic but the reaction Gibbs energy is more than twofold larger in mechanism 1 than in mechanism 2, at both considered temperatures. The final product AM is predicted to be more stable than its regioisomer M, in agreement with the experimental results. The difference of stability between AM and generate the corresponding nickel hydride species 5a, possibly through a σ-bond metathesis mechanism, which contrasts with the commonly proposed formation of a putative nickel alkoxide intermediate that would then react with the borane 16c (or silane 18b35 ) to generate the Ni(II) hydride intermediate. Then, in an initiation step, two 5a could react together to generate the catalytically active Ni(I) species 6a (Scheme 8). 36 The latter would then form an "agostic" intermediate 7a, where the B-H bond of the borane interacts with the Ni(I). Styrene would then form an adduct 8a with 7a before homolytically breaking the B-H bond, hence regenerating 5a and releasing a carbon radical. This carbon radical would finally react with 5a, to yield the hydroboration product and regenerating 6a (Scheme 7). Importantly, this mechanism is in agreement with our previous observation, in the related hydrosilylation reaction of carbonyl derivatives catalysed by 1a, 18a where we noted that although a Ni(II)-H species is most probably the true catalytic precursor, there is no insertion of the C=O or C=C double bond into this Ni(II)-H bond.

Conclusions

In conclusion, we have shown that the [NiCp*Cl(IMes)] complex 2a is an efficient precatalyst for the hydroboration of alkenes in the presence of a catalytic amount of KOt-Bu. With HBCat, about ten styrene derivatives and vinylcyclohexane have been converted to the linear product with yields ranging from moderate to very good depending on the substrate. The use of HBPin instead of HBCat allowed to obtain the branched product as the major one in the cases of styrene, 4-trifluoromethylstyrene and 4methoxystyrene.

Mechanistic studies strongly suggest the initial generation of a the nickel-hydride complex 5a by reaction of 2a with the in situ formed tert-butoxo-catecholato-borohydride species [H(t-BuO)BCat] -( 4), as evidenced by 1 H and 11 B NMR spectroscopy. Two molecules of the latter would then react together to generate two equivalents of the Ni(I) species 6a and one equivalent of H2, as suggested by combined experimental and theoretical studies. The latter would then act as the active species through a catalytic cycle involving an agostic intermediate 7a and a styrene adduct 8a that would homolytically break the B-H bond of the borane. These unprecedented studies with only the fourth nickel hydroboration catalyst reported to date 16 shed light both on the potential of nickel as another base metal for this important transformation, and on the fact that appropriately designed nickel(I) species may be an interesting lead for future research in this domain.

Experimental Section

General comments

All reactions were carried out using a glovebox or standard Schlenk techniques under an atmosphere of dry argon. Solvents were distilled from appropriate drying agents under argon. Unless otherwise specified, solution NMR spectra were recorded at 298 K on Bruker Avance I 300 MHz, or Bruker Avance III HD 400 MHz spectrometers operating at 300.13 or 400.13 MHz for 1 H, at 75.47 or 100.61 MHz for 13 C and at 96.29 or 128.38 MHz for 11 B. The chemical shifts are referenced to the residual deuterated or 13 C solvent peaks. Chemical shifts (δ) and coupling constants (J) are expressed in ppm and Hz respectively. The catalyst precursors 1a, 37 1b, 17b 1c, 38 2a, 39 2b, 39 and 3a 17f were prepared according to published procedures.

General procedure for the hydroboration of alkenes

In an oven dried Schlenk tube were added 2a (17 mg, 0.0318 mmol, 3 mol%), KOt-Bu (7 mg, 0.0624 mmol, 6 mol%), and toluene (2 mL), containing the internal standard (trimethoxybenzene, 0.6 mg/mL). This was followed by the addition of HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.) or HBPin (0.363 mL, 2.50 mmol, 2.5 equiv.) and of the desired alkene (1.00 mmol, 1 equiv.). The resulting reaction mixture was stirred in a preheated oil bath at 60 °C for 15 h. The solution was then cooled to room temperature and an aliquot of the mixture was removed by syringe in order to determine the conversion and the isomeric ratio by 1 H NMR spectroscopy. The rest of the solution was filtered through a plug of silica that was washed with CH2Cl2. The resulting filtrate was dried under high vacuum to give a pale orange solid (with HBCat) or a pale yellow oil (with HBPin). The purification procedure was performed rapidly because of the fast degradation of the desired products, especially with HBCat. 22 The branched isomer, in particular, was found to be very unstable upon filtration over the silica plug, allowing the isolation of the pure linear isomer with HBCat and generating (in some cases) an increase of the linear/branched ratio with HBPin.

Synthesis of [NiCp*(IMes)] (6a)

In a glovebox, a Schlenk tube was charged with a stirring bar and 2a (300 mg, 0.562 mmol). The Schlenk tube was then laid in a horizontal position in order to introduce sparkling orange shavings of KC8 (84 mg, 0.621 mmol) without touching the purple powder. In this position, the Schlenk tube was closed and taken out of the glovebox. 2a was then dissolved in THF without touching KC8 (10 mL). While putting the Schlenk tube in a vertical position, the system was plunged into a bath at -78 °C. The temperature was allowed to slowly increase to room temperature and the heterogeneous purple medium was stirred for 4 days. After this time, the resulting dark yellow suspension was filtered through a thin pad of Celite under argon and the filtrate was concentrated under vacuum. The resulting brown powder was dissolved in a small amount of pentane and stored at -28 °C overnight. The mother liquor was removed with a syringe to yield 6a as a shining yellow-brown powder (234 mg, 0.470 mmol, 84 %) that was dried under high vacuum. 1 H NMR (C6D6,400.13 MHz): δ 107.31 (s v.br.,15H,C5Me5),30.95 (s br.,2H,NCH),5.29 (s br.,4H,3.57 (s,6H,2.21 (s,6H,1.42 (s,6H,.

Control experiments

Investigation of the mercury effect: An oven dried Schlenk tube was charged with 2a (17 mg, 0.0318 mmol, 3 mol%), KOt-Bu (7 mg, 0.0624 mmol, 6 mol%), and toluene (2 mL), containing the internal standard (trimethoxybenzene, 0.6 mg/mL). This was followed by the addition of Hg (1.2 g, 5.98 mmol, 188 equiv.), HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.), and styrene (0.115 mL, 1.00 mmol, 1 equiv.). The resulting reaction mixture was stirred in a preheated oil bath at 60 °C for 15 h. The solution was then cooled to room temperature and an aliquot of the mixture was removed by syringe in order to determine the conversion and the isomeric ratio by 1 H NMR spectroscopy. A conversion of 100 % with a 90:10 AM/M ratio was observed.

Investigation of the radical scavenger effect: An oven dried Schlenk tube was charged with 2a (17 mg, 0.0318 mmol, 3 mol%), KOt-Bu (7 mg, 0.0624 mmol, 6 mol%), and toluene (2 mL), containing the internal standard (trimethoxybenzene, 0.6 mg/mL). This was followed by the addition of HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.), styrene (0.115 mL, 1.00 mmol, 1 equiv.) and TEMPO (156 mg, 1.00 mmol, 1 equiv. (at t = 0 or 7 h) or 5 mg, 0.0320 mmol, 3 mol%) or galvinoxyl (422 mg, 1.00 mmol, 1 equiv.). The resulting reaction mixture was stirred in a preheated oil bath at 60 °C for 15 h. The solution was then cooled to room temperature and an aliquot of the mixture was removed by syringe in order to determine the conversion by 1 H NMR spectroscopy. Complete inhibition of the reaction was observed when 1. 0 equiv. TEMPO or galvinoxyl was added at t = 0 h. 55% conversion was observed when 3 mol% TEMPO was added at t = 0 h. 76% conversion was observed when 1.0 equiv. TEMPO was added at t = 7h.

1 H NMR follow-up of a 1:1:1 mixture of 2a/HBCat/KOt-Bu: An oven dried Schlenk tube was charged with 2a (32 mg, 0.0599 mmol), KOt-Bu (7 mg, 0.0624 mmol) and freeze-pump-thaw degassed C6D6 (2 mL). The resulting mixture was cooled -25 °C before the addition of HBCat (6.4 μL, 0.0598 mmol). The reaction mixture was then vigorously stirred for several seconds prior to the transfer of an aliquot of 0.6 mL into an NMR tube placed in a bath at -25 °C. 1 H NMR spectra were then recorded from 253 to 298 K.

11 B NMR analysis of a 1:1 mixture of HBCat/KOt-Bu: An oven dried Schlenk tube was charged with KOt-Bu (42 mg, 0.375 mmol) and freezepump-thaw degassed C6D6 (2 mL). The resulting mixture was cooled -25 °C before the addition of HBCat (40 μL, 0.375 mmol). The reaction mixture was then vigorously stirred for several seconds prior to the transfer of an aliquot of 0.6 mL into an NMR tube placed in a bath at -25 °C. 11 B NMR spectra were then recorded from 253 to 298 K.

Hydroboration of styrene catalysed by 2a with 4:

To an oven dried Schlenk tube were added HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.) and KOt-Bu (7 mg, 0.0624 mmol, 6 mol%) in toluene (2 mL), containing the internal standard (trimethoxybenzene, 0.6 mg/mL) in order to form 4. This mixture was stirred for 5 min at room temperature before adding 2a (17 mg, 0.0318 mmol, 3 mol%) and styrene (0.115 mL, 1.00 mmol). The resulting reaction mixture was then stirred in a preheated oil bath at 60 °C for 15 h. The solution was finally cooled to room temperature and an aliquot of the mixture was removed by syringe in order to determine the conversion and the isomeric ratio by 1 H NMR spectroscopy. A conversion of 100 % with a 89:11 AM/M ratio was observed.

Hydroboration of styrene catalysed by 6a:

In an oven dried Schlenk tube were added 6a (15 mg, 0,0301 mmol, 3 mol%) and toluene (2 mL), containing the internal standard (trimethoxybenzene, 0.6 mg/mL). This was followed by the addition of HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.) and styrene (0.115 mL, 1.00 mmol, 1 equiv.). The resulting reaction mixture was stirred in a preheated oil bath at 60 °C for 15 h. The solution was then cooled to room temperature and an aliquot of the mixture was removed by syringe in order to determine the conversion and the isomeric ratio by 1 H NMR spectroscopy. A conversion of 70 % with a 60:40 AM/M ratio was observed.

Cyclic voltammetry

The cyclic voltammogram of complex 3a (1 mM in CH3CN) was recorded with a Voltalab 50 potentiostat/galvanostat (Radiometer Analytical MDE150 polarographic stand, PST050 analytical voltammetry and CTV101 speed control unit) controlled by the Voltamaster 4 software. A conventional three-electrode cell (10 cm 3 ) was used in our experiments with a glassy carbon disk (s = 0.07 cm 2 ) set into a Teflon rotating tube as a working electrode, a Pt wire as a counter electrode, and a KCl (3 M)/Ag/AgCl reference electrode (+210 mV vs. NHE). Prior to each measurement, the surface of the glassy carbon electrode was carefully polished with 0.3 𝜇 m aluminum oxide suspension (Escil) on a silicon carbide abrasive sheet of grit 800/2400. Thereafter, the glassy carbon electrode was copiously washed with water and dried with paper towel and argon gas. The electrode was installed into the voltammetry cell along with a platinum wire counter electrode and the reference electrode. The 1 mM solution of 3a was vigorously stirred and purged with O2-free (Sigma Oxiclear cartridge) Ar for 15 min before the voltammetry experiment was initiated, and maintained under an Ar atmosphere during the measurement procedure. The voltammogram was recorded at 23(1) °C with 0.1 M NBu4PF6 as supporting and inert electrolyte. The voltage sweep rate was set at 200 mV.s -1 , and the cyclic voltammogram was recorded from +0.5 V to -2.2 V vs. Fc/Fc + .

Piezometric studies 31

2a, KOt-Bu and toluene were introduced in a double-walled Schlenk vessel of 28 mL, thermostated at 20 °C under Ar. After the resulting mixture was stirred for a few minutes, a Mykrolis A332984-007 absolute pressure-tovoltage transducer was air-tightly screwed to the neck of the glass vessel and connected to a Pico-Log analog-logic converter/data-logger board interfaced to a computer. This allowed the measurement of the pressure of hydrogen (computed from a linear absolute pressure-output voltage relationship) developed in the known overhead volume of the sealed reactor as a function of time. The absence of leaks was preliminarily checked by applying a moderate pressure of argon gas. HBCat was swiftly injected into the vessel via a lateral glass valve, which was tightly closed immediately upon injection to avoid gas leaks. Several experiments have been performed with varying amounts of reagents: (i) 2a (48 mg, 0.0899 mmol), KOt-Bu (20 mg, 0.178 mmol, 2 equiv.) and HBCat (0.799 mL, 7.496 mmol, 83 equiv.) in toluene (6 mL); (ii) 2a (144 mg, 0.270 mmol), KOt-Bu (61 mg, 0.544 mmol, 2 equiv.) and HBCat (60 μL, 0.563 mmol, 2 equiv.) in toluene (12 mL); (iii) 2a (144 mg, 0.270 mmol), KOt-Bu (30 mg, 0.267 mmol) and HBCat (29 μL, 0.272 mmol) in toluene (12 mL).

Computational details

Geometry optimizations of the different species thought to be involved in the reaction mechanism were performed at the density functional theory (DFT) level of theory, using the ORCA program system 40 version 4.2.0. The Perdew-Burke-Ernzerhof (PBE) functional 41 augmented with Grimme's D3 dispersion correction with a Becke-Johnson (BJ) damping function 42 was used for all calculations. All computations were carried out using Karlsruhe's second-generation default triple-ζ valence with two sets of polarization functions (def2-TZVPP) basis set 43 . Solvent effects were accounted for in all computations by using the conductor-like polarizable continuum model (CPCM), 44 simulating toluene (ε=2.4, n=1.497) as a solvent. Geometry optimizations by the quasi Newton update procedure with the BFGS update were carried out in all cases with integration grid accuracy 5 to 7, an energy gradient convergence criterion of 10 -4 Eh/a0, and a tight SCF convergence criterion. All thermodynamic data have been evaluated at 298.15 K and 333.15 K. 3D molecular structures and isosurfaces were produced using Jmol.

Figure 1 .

 1 Figure 1. Pre-catalysts' screening for the hydroboration of styrene with HBPin.

Scheme 2 .

 2 Scheme 2. Effect of radical inhibitors.

Figure 3 .

 3 Figure 3. Cyclic voltammogram of 3a: 1 mM 3a in a 0.1 M NBu4PF6 solution in CH3CN with a glassy C working electrode, a Pt wire counter electrode, and a Ag/AgCl, KCl 3 M reference electrode. Referenced to a 2mM Fc/Fc + external standard: E1/2 = 0.48 V vs. AgAgCl, KCl 3 M. Scan rate = 200 mV.s -1 .

Figure 4 .

 4 Figure 4. 1 H NMR (C6D6, 293 K) spectrum of a 2a / HBCat / KOt-Bu (1:1:1) mixture.

Figure 5 .

 5 Figure 5. 11 B NMR (C6D6, 298 K) spectrum of a HBCat / KOt-Bu (1:1) mixture.

Scheme 3 .

 3 Scheme 3. Addition of preformed 4 to a catalytic reaction mixture.

Scheme 4 .Scheme 5 .

 45 Scheme 4. Synthesis of 6a.

Scheme 7 .

 7 Scheme 6. Proposed catalytic mechanism 1 (outer-sphere transfer), through the release of a boron radical.

Figure 6 .

 6 Figure 6. Gibbs energy profiles for the catalytic cycle following mechanism 1 in orange (synthesis of M) and green (synthesis of AM), or mechanism 2 in red (synthesis of M) and blue (synthesis of AM) at 25 °C (solid lines) and 60 °C (dashed lines).

Scheme 8 .

 8 Scheme 8. Proposed mechanism for the generation of the Ni(I) active species 6a.

Table 1 .

 1 Hydroboration of styrene with HBPin or HBCat catalysed by 2a

	Entry	Borane	Temp.	Time
		(equiv.)	(°C)	(h)

Table 4 .

 4 Reaction Gibbs energies computed at the PBE-D3(BJ)/def2-TZVPP level of theory

	Reaction	ΔrG 0 at 25 °C	ΔrG 0 at 60 °C
		(kcal/mol)	(kcal/mol)
	5a + HBCatOt-Bu -→ 6a + BCatOt-Bu •-	53.66	52.38
	+ H2
	5a → 6a + ½ H2	13.82	13.12
	6a + HBcat →7a	-6.77	-4.94
	7a → 5a + Bcat •	43.85	42.14
	7a + styrene → 8a	17.80	19.74
	Bcat • + styrene → AM •	-41.96	-40.29
	Bcat • + styrene → M •	-26.72	-25.08
	8a → 5a + AM •	-14.84	-16.79
	8a → 5a + M •	0.40	-1.58
	AM • + 5a → 6a + AM	-12.82	-12.95

Acknowledgements

MJC and VR are grateful to the Université de Strasbourg and the CNRS for their financial help. The Institut Universitaire de France is acknowledged for its support to VR, and the Région Grand Est for the doctoral fellowship of F. Ulm. Dr. Mourad Elhabiri is gratefully acknowledged for his help with the cyclic voltammetry measurements. YC and JPD thank the Centre de Calcul de l'Université de Strasbourg for providing access to the HPC facilities for the computational part of this work.


J Gauthier
email: jules.gauthier@polymtl.ca

K Wu

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1

Keywords: Dielectric transmission line, laser micromachining, probe technology, silicon assembling, substrate integrated image guide (SIIG)

We propose, investigate and demonstrate a pure and simple dielectric probe operating in the sub-THz range with an example band of interest 190-220 GHz. High-resistivity silicon technology is deployed for this non-TEM mode probe development as it presents a viable low-loss material. A T-shape alignment dowel was used to integrate the silicon probe with the waveguide block. The probe can be used to measure and characterize the low permittivity-based dielectric waveguide transmission line and its building components free from the use of any transition structures and TEM-mode probes. For demonstration, the silicon probe was used to measure a substrate integrated image guide and its components in the range of 190-220 GHz. A substrate integrated image guide (SIIG) based TRL calibration kit was developed, and a two-tier TRL calibration was used to characterize the SIIG.

could be a daunting task as the resulting measurement set up may not be compatible with current measurement standards. There are two major methods to test the dielectric waveguide, namely the non-contact and contact methods. For the non-contact method, a quasi-optical circuit design is always required [START_REF] Zhu | Determination of propagation constant of terahertz dielectric ridge waveguide using non-contact measurement approach[END_REF]- [START_REF] Zhu | Design, fabrication and measurement of the low-loss SOI-based dielectric microstrip line and its components[END_REF]. Generally, the final bulky quasi-optical circuit needs a precise adjustment [START_REF] Goldsmith | Quasioptical Systems: Gaussian Beam Quasioptical[END_REF]. For the contact method, one simple strategy is to insert the dielectric waveguide into a rectangular waveguide, which causes inconvenience for taking measurements, especially at high frequencies [START_REF] Zhu | Low-cost narrowed dielectric microstrip line-a three dielectric waveguide based on PCB technology for millimeter-wave applications[END_REF]. Another alternative method is using a probe to carry out the test.

A probe station is a traditional commercial method to measure RF circuits. Probes are known as a very important RF measurement tool in characterizing the performances of active/passive components, circuits and systems; quite often parameter extractions and performance evaluations are conducted in this process. Generally, the development of probes becomes more difficult as frequency increases. The reason is related to the mechanical precision and electrical reliability as well as transmission loss, which are all critical for sub-THz and beyond. Recently, attempts have been made for accurate measurements and characterization of sub-THz components and circuits. Based on coaxial cable techniques, industrial products can provide a high performance probe at up to 1.1 THz. For example, the company GGB Inc. provides commercial CPW-based THz probe [START_REF]GSG Model 220[END_REF]. Fig. 1 shows the side view of a metal-based GGB ground-signal-ground (GSG) probe. Once touching the surface of a circuit, the probe will reshape its structure and extend approximate 30 µm under a simple pressing. Nevertheless, such standard GSG probes are designed through a CPW structure with TEM mode input conditions. Moreover, in 2011, the group of Robert M. Weikle, II and N. Scott Barker proposed a set of performance-enhanced micro-machined probes for sub-THz applications [START_REF] Reck | Micromachined probes for submillimeter-wave on-wafer measurements-part I: mechanical design and characterization[END_REF], [START_REF] Reck | Micromachined probes for submillimeter-wave on-wafer measurements-part II: RF design and characterization[END_REF].

Generally, two transitions are needed to measure a dielectric waveguide operating over the sub-THz band, where one transition is designed for coupling the TE 10 mode of a frequency extender output to the TEM mode of a GSG probe and the other transition is set for coupling the TEM mode of a GSG probe to the 𝐸 "" # or 𝐸 "" $ mode of the dielectric waveguide [START_REF] Dolatsha | Millimeter-wave chip-to-chip transmission using an insulated image guide excited by an on-chip dipole antenna at 90 GHz[END_REF]- [START_REF] Yu | Ring-resonator-based sub-THz dielectric sensor[END_REF]. Therefore, it is inconvenient to measure the dielectric waveguide or its constituent components.

Silicon Probe Measurement and Characterization in Sub-THz Range Haotian Zhu, Member, IEEE, Jules Gauthier, and Ke Wu, Fellow, IEEE T Fig. 1. Traditional GGB GSG probe; the probe bends in measurement and 30 µm shift in z-axis, (side view).

In this work, a highly original scheme of dielectric probe is proposed and demonstrated, which can be used to measure and characterize sub-THz dielectric waveguides and components. Without transferring to the TEM mode or intermediate modal transition, the TE 10 mode in the metal rectangular waveguide can be converted to the 𝐸 "" $ mode in the dielectric waveguide directly. Compared with non-contact methods [START_REF] Zhu | Determination of propagation constant of terahertz dielectric ridge waveguide using non-contact measurement approach[END_REF], [START_REF] Zhu | Design, fabrication and measurement of the low-loss SOI-based dielectric microstrip line and its components[END_REF], the dielectric probe method does not need to conduct a stringent quasi-optical circuit alignment. To the authors' knowledge, this is the first time the concept of dielectric probe is demonstrated. In this work, silicon techniques are used as examples for our demonstration as they are popular and low-loss. The substrate integrated image guide (SIIG) is a popular dielectric waveguide in the sub-THz range [START_REF] Wu | Substrate integrated image guide (SIIG) -a planar dielectric waveguide technology for millimeter-wave applications[END_REF]. The advantages of SIIG are its low-cost and easy PCB fabrication and integration. In this paper, the silicon probe is used to measure the SIIG and its components.

II. CONCEPT OF SILICON PROBE

In this work, a dielectric probe concept is devised, designed and demonstrated for probing measurements and characterization over the low-end of sub-THz, which can be extended to the high-end of the sub-THz range, depending on the topology and applications of a dielectric probe of choice. As shown in Fig. 2(a), a detailed graphic illustration is presented for possible measurement and characterization scenarios envisioned in this work. A block of waveguide is connected to a sub-THz source. A silicon-based dielectric probe (blue part) is set in contact with a dielectric substrate (green part).

The proposed probe consists of a coupling tip, E-plane bend, T-junction, and waveguide transition. With the wave number marked in Fig. 2(a), the coupling tip can couple the EM energy from the silicon-based dielectric waveguide (SiDW) into another dielectric waveguide, which is the SIIG in this paper. The principle of coupling can approximatively be explained using geometric optics. In the demonstration model, the cross-sectional size of the SiDW of the coupling tip is 0.915 mm × 0.500 mm. The main material of the SIIG is RO3035 PCB board. The cross-sectional size of the SIIG is 1.1 mm × 0.254 mm. The permittivity of RO3035 and high-resistivity silicon in 140-220 GHz are 3.75 and 11.86, respectively. The values were measured using our own sub-THz substrate permittivity measurement platform [START_REF] Zhu | Complex permittivity measurement of dielectric substrate in sub-THz range[END_REF].

As shown in Fig. 2(a), before being guided into the SIIG, the EM wave undergoes several reflections along the E-plane bend and coupling tip. Most of those reflections are total reflections. Finally, when the angle of incidence (𝜃 +, ) into the silicon probe is less than the critical angle between the SiDW and free space (𝜃 ./ = 19.2° at 200 GHz), the EM can also be leaked out of the probe and transferred into free space. The behavior of 𝜃 +, is described using the following geometric relationship

𝜃 +, = 90°-𝑛 • 𝜃 " -𝜃 : , 𝑛 = 1, 3, 5 ⋯ (1)
Snell's law can be used to describe the refraction occurring on the boundary between the silicon probe and the SIIG in the coupling tip part

sin 𝜃 +, sin 𝜃 BCD = 𝛽 FGGH 𝛽 F+IJ = 𝛽 FGGH 𝑘 / 𝛽 F+IJ 𝑘 / , (2)
where 𝜃 BCD , 𝛽 FGGH 𝑘 / and 𝛽 F+IJ 𝑘 / are the angles of refraction in the SIIG, SIIG normalized phase constant and SiDW normalized phase constant, respectively. According to simulated results at 200 GHz, 𝛽 FGGH 𝑘 / and 𝛽 F+IJ 𝑘 / are 1.42 and 3.04, respectively. When the incident angle into the silicon zone is less than the critical angle (𝜃 ." = 27.8° at 200 GHz), the wave propagates into the SIIG. To satisfy this requirement, 𝑛 is equal to 5, 𝜃 +, is equal to 20° and 𝜃 BCD is equal to 47° at 200 GHz. On the boundary between the substrate and the ground, which is made of an aluminum plate, the EM is subject to the total reflection. On the boundary between the substrate and free space, the EM also undergoes total reflection. Since 𝜃 BCD is larger than the critical angle between the SIIG and free space (𝜃 .: = 44.8° at 200 GHz), the EM wave propagating in the SIIG is along the boundary between the free space and the substrate as a forced surface wave. The propagation mode of the SIIG is the fundamental 𝐸 "" $ mode [START_REF] Wu | Substrate integrated image guide (SIIG) -a planar dielectric waveguide technology for millimeter-wave applications[END_REF]. For any dielectric probe design, the critical issue of the design is to choose a suitable material according to the two critical angles 𝜃 ." and 𝜃 .: . The design emphasis is then on the choice of geometric sizes or dimensions of the dielectric waveguide for the probe to satisfy (1) and [START_REF] Wiedner | A proposed heterodyne receiver for the origins space telscope[END_REF]. Considering the equivalent permittivity or 𝛽 F+IJ 𝑘 / changes in the coupling tip, the actual optical model of the silicon probe is more complex. Commercial full-wave electromagnetic simulation software Ansoft HFSS simulates the coupling performance of the probe and SIIG. The relationship between the performance of the coupling tip and the inclination angle (𝜃 " ) is shown in Fig. 2(b). The length of the silicon probe is fixed to be 6 mm and 𝜃 " changes from 10° to 45° with a step of 2.5°. The coupling is stronger and return loss is better at lower inclination angle. However, if 𝜃 " is less than 10°, the contact area will be much larger and the silicon probe will be much longer. Therefore, 𝜃 " = 10° is chosen for the final design.

The cross-sectional interface sizes of the E-plane bend are 0.915 mm × 0.50 mm and 0.915 mm × 0.64 mm, and their normalized phase constants are shown in Fig. 3(a). The coupling tip and the E-plane bend model of the silicon probe are shown in Fig. 3(b). As shown in Fig. 3(c), the simulated performance is similar to the analyzed results using geometric optics. In the experiments, there will be an intersection angle 𝜃 Q between the tip and the surface of a dielectric transmission line under test. The simulated performance of the coupling tip and E-plane bend part with 𝜃 Q = 5° is shown in Fig. 3(d). As shown in Fig. 3(b), the EM energy radiates into the free space and leaks into the reserve propagation direction of the SIIG when the intersection angle exists. The return loss of port 1, the insertion loss and return loss of port 2 of the structure in Fig. The cross-sectional interface sizes of the silicon-based probe waveguide and the standard WR 5.1 rectangular waveguide are 0.915 mm × 0.64 mm and 1.295 mm × 0.648 mm, respectively. To assemble the SiDW into the waveguide block, a special waveguide transition is designed, as shown in Fig. 4. The silicon probe is packaged by two waveguide block parts. A T-junction provides an arm in support of the contacting force during measurements, as described in Fig. 2(a). The resolution of contacting force is 𝐹 1 and 𝐹 2 . 𝐹 1 supports the main silicon probe beam via the T-junction, and 𝐹 2 can shift the silicon T-shape alignment dowel along the groove of the waveguide block slightly. The maximum shift caused by 𝐹 2 is 50 µm along the silicon T-shape alignment dowel, as marked in Fig. 4(b). Meanwhile, the isolation of the T-junction is more than 25 dB and the insertion loss of the T-junction is less than 0.5 dB, which almost barely affects the transmission performance. The working principle of the T-junction is similar to the crossover in [START_REF] Zhu | Design, fabrication and measurement of the low-loss SOI-based dielectric microstrip line and its components[END_REF]. The E-plane 20° bend structure is used for reducing the length of the silicon probe. The insertion loss of the E-plane bend is less than 1.4 dB. After laser cutting, the silicon-based probe can be released from a 0.915 mm-thickness high-resistivity silicon wafer. The laser beamwidth is around 90 µm . The aluminum housing waveguide block parts are fabricated by our own CNC technology in the Poly-Grames Research Center. Finally, using the silicon T-shape alignment dowel, the housing block and the silicon-based probe were neatly assembled, as shown in Figs. 4(c) and(d). More simulated results of the silicon probe and SIIG structure can be found in Part C, Section III.

III. SIIG AND ITS COMPONENTS DESIGN

As shown in Fig. 6, the SIIG structure is made of two parts: one is the 3.175 mm thick single-side polished aluminum plate, which provides an excellent ground for the SIIG, and the other one is the 0.254 mm thick RO3035 PCB board. The width of the SIIG is 1.1 mm. For the PCB board, one-layer cladding copper layer was removed. The laser micromachining equipment in the Poly-Grames Research Center was used to create the designed pattern on the PCB board. An epoxy adhesive was used to bond the PCB board and the aluminum plate. The thickness of the epoxy adhesive is ~5 µm which is far less than the working wavelength of 1.5 mm.

A. Modal Analysis and Coupling Characteristics

B. Absorber and H-plane Bend of SIIG

The designed 2D absorber of the SIIG is shown in Fig. 8. The saw tooth and the H-plane taper are used to guide the EM waves to the free space. The return loss is less than 30 dB. To construct SIIG-based components, a SIIG-based bend is necessary. A SIIG-based H-plane bend with a 10.16 mm (400 mil) radius was designed. As shown in Fig. 9, it can be seen that the insertion loss and return loss of the H-bend are better than 2.7 dB and 40 dB, respectively.

C. Probe Performance and Short Circuit

The contact probe model of the SIIG and the silicon probe in the housing waveguide block are shown in Fig. 10(a). This model is the error box for TRL calibration. Fig. 10(a) shows the S-parameters. The return losses of the SIIG port for the 𝐸 "" $ mode and WR5.1 waveguide port are both better than 17 dB.

The insertion losses for the 𝐸 "" $ mode and 𝐸 :" $ mode of the one-arm silicon probe and SIIG are approximately 5.8 dB and 61 dB, respectively. Therefore, the 𝐸 :" $ mode can be neglected.

The details of the insertion loss of the one-arm silicon probe and SIIG are shown in Table I. When the SIIG input port (port 2) is shorted, the return loss of the port 1 is approximately 10.8 dB, as shown in Fig. 10(a). The reason is that part of the EM energy propagates along the SIIG to the absorber after being reflected by the short circuit of the SIIG input port.

D. SIIG-based Coupler Design

The top view of the SIIG-based coupler structure is shown in Fig. 11, which is similar to the DML-based coupler [START_REF] Zhu | Design, fabrication and measurement of the low-loss SOI-based dielectric microstrip line and its components[END_REF]. As shown in Fig. 11, the insertion loss between the input port 1 and the through port 3 is smaller than 2.18 dB. The coupling factor is around -14.42 dB. The return loss of port 1 is larger than 23.35 dB. The isolation between port 1 and port 4 is larger than 24.46 dB.

E. SIIG Layout Design

The final layout of the SIIG and its components' design are shown in Fig. 12. The layout consists of alignment graduation markings, reference, coupler, and thru, reflect and line (TRL) kit, absorbers, and two adjacent SIIGs. The coupler consists of two reference SIIGs with a 45° angle. Two alignment pins were used to mount the RO 3035 PCB layer onto the aluminum plate. Alignment graduation markings were plotted on the front side of the SIIG by laser micromachining, as shown in Fig. 12. The fabricated line width is 48.82 ± 4.35 µm. The TRL kit consists of two parts: the upper is for thru and line (T/L), and the lower is for reflection (R) [START_REF] Pozar | Microwave Engineering[END_REF]. As shown in Fig. 15(b), the lengths of T and L are 360 mil and 375 mil, respectively. The phase delay between T and L is 129.5° at 200 GHz. As shown in the inset in Fig. 12, an aluminum box (8.13 mm×8.08 mm×3.05 mm or 320 mil×318 mil×120 mil) was fabricated to realize the short circuit reflection. The distance between the silicon probe and the sidewall of the aluminum box is 180 mil, as shown in Fig. 15(c). Moreover, two dowel pins are used for alignment, and one screw is used for mounting the aluminum box onto the aluminum plate.

IV. PROBE CHARACTERIZATION AND SIIG MEASUREMENT Fig. 13 shows the whole silicon probe measurement system. In this set-up, two VDI WR 5.1 band extenders and one Keysight PNA-X N5274A vector network analyzer (VNA) are used to provide the sources and vector detectors in the measurement system over 190-220 GHz. For each probe arm, one CERNEX 1-inch straight waveguide and 180° E-bend WR5.1 band waveguide are used to connect the VDI WR5.1 extender and the waveguide-based probe block. The Suss microtech semi-automatic probing station is used to place the VDI WR5.1 extenders via two home-made fixtures, as shown in Fig. 16(a). To do the probe contact, the side view of the silicon probe and the SIIG should be seen in detail. A 50° tilt mirror was designed to reflect the side view into the microscope. The simple optical circuit is shown in Fig. 14.

A. Experimental Procedure

Step 1: Waveguide calibration. Set the measurement system as shown in Fig. 15(a) and calibrate it to the A-A' plane of the interface of the two CERNEX 1-inch straight waveguides with the TRL method. Then, connect the two silicon probes to the WR5.1 waveguide flanges as described in Fig. 16(b).

Step 2: Probe alignment and contact. First, as shown in Fig. 17(a), the alignment graduation markings are applied to find the x-axis location of the probe tip. Then, under "align mode" (200 µm height distance between the silicon probe tip and the SIIG surface), the probe is moved to the upward side of the SIIG sample under test. As shown in Fig. 17 (b), the silicon probe is on the middle of the dielectric strip of the SIIG, pictured by microscope from the top view. The probe under the align mode, pictured by microscope from the mirror reflection, is clearly shown in Fig. 17(c). Finally, under the contact mode, the tip of the probe is onto the surface of the SIIG, as shown in Fig. 17(d).

Step 3: Measure with waveguide calibration. Use the probes to contact a 9.14-mm (360-mil) length SIIG, as shown in Fig. 16(c). Measure S-parameters.

Step 4: Probe TRL calibration. The measurement system needs to be calibrated to the B-B' plane, which is the middle of a 9.14-mm (360-mil) length SIIG, as shown in Fig. 15(b). For the thru standard, the two probes are respectively moved to -4.57 mm (-180 mil) and 4.57 mm (180 mil) positions, as shown in Fig. 15(b). For the line standard, keep the left probe in -4.57 mm (-180 mil) position and shift the right probe to 4.95 mm (195 mil) position. For the reflect standard, mount the short block onto the aluminum plate and shift the probes to 4.57 mm (180 mil) away of the block edge, as shown in Fig. 15(c). We used the Keysight PNA-X N5274A VNA to build the TRL calibration.

Step 5: Measure with probe calibration. Move two probes to -12.7 mm ( -500 mi l) and 12.7 mm (500 mil) positions, respectively, and measure the S-parameters of the T/L SIIG, the coupler, the reference and the two adjacent SIIGs.

B. Post-simulation and Measured Results

As shown in Fig. 17(d), the intersection angle 𝜃 Q between the silicon probe tip and the SIIG top surface is approximately 5° ( 𝜃 Q ≈ 15°-𝜃 " = 15°-10°= 5°), which affects the performance of the probe. Therefore, part of the EM energy will leak out into the air space from the intersection angle. A post-simulation based on 𝜃 Q = 5° was conducted. The post-simulated S-parameters of the back-to-back silicon probes and the 9.14-mm (360-mil) length SIIG match well with the measured S-parameters of the Step 3, as shown in Fig. 18. The measured return loss of the silicon probe is better than 15 dB. After removing the loss of the SIIG, the measured insertion loss of each probe is around 9.94 dB. Compared with the ideal scenario 𝜃 Q = 0°, the insertion loss is added approximately 5 dB for each silicon probe and SIIG. The reason of the intersection angle will be discussed in the next part. The measured and simulated phases of the offset short SIIG with different lengths are plotted in Fig. 19.

After the probe TRL calibration, the attenuation constant and the phase constant of the SIIG can be calculated by

𝛼 = - 𝑆 :" \ 𝐿 \ -𝐿 ^ 𝛽 = - ∠𝑆 :" \ -2𝜋 • 𝑁 𝐿 \ -𝐿 ^, (3)
where 𝑆 :" \ and ∠𝑆 :" \ are the magnitude and phase of 𝑆 :" of the measured SIIG, respectively; 𝐿 \ is the length of the measured SIIG; 𝐿 ^ is the length of the thru of the SIIG TRL kit; and 𝑁 is the minima integer of 𝐿 \ -𝐿 ^𝜆c . Here, 𝐿 \ and 𝐿 ^ are 25.40 mm (1000 mil) and 9.14 mm (360 mil), respectively. The measured attenuation constant and phase constant of the SIIG are plotted in Figs. 20(a) and (b), respectively. The measured attenuation constant is 0.241 ± 0.057 dB/mm over the range of 190-220 GHz. The measured normalized phase constant ranges between 1.35 and 1.46, which is lower than its simulated counterpart with 1.5% relative difference. The reason for this difference is related to the substrate in the SIIG, and the substrate in the measurement of permittivity comes from a different batch of the substrate RO3035 from Rogers. As shown in Fig. 20(b), the measured normalized phase constant of the two adjacent SIIGs from port 3 to port 4 is almost the same as the single SIIG results. Therefore, there is almost no coupling between the two adjacent SIIGs with a 2.54 mm distance.

The reference SIIG and SIIG-based coupler were also measured. The parameter 𝑆 Q" of the coupler measurement setup is shown in Fig. 13. The SIIG-based coupler through port 1 was excited with the left silicon probe, and the energy of port 3 is coupled to the right silicon probe. Meanwhile, port 2 and port 4 are matched by the absorber directly. The measured results of the reference and coupler are shown in Fig. 21(a). The reference structure can be used to calculate the normalized 𝑆 :" , as follows, Normalized 𝑆 j" 𝑑𝐵 = 𝑆 j" 𝑑𝐵 -𝑆 :",lmn. 𝑑𝐵 , [START_REF] Reck | A silicon micromachined eight-pixel transceiver array for submillimeter-wave radar[END_REF] where 𝑆 j" is the measured result of the coupler (𝑖 = 2, 3), and 𝑆 :",l is 𝑆 :" of the reference SIIG. As shown in Fig. 21(b), the normalized results and the simulated results basically agree with each other.

C. Error Analysis

The random error and the systematic error are the two main sources of error in the measurements.

The random error is caused by the alignment uncertainty. We align the probe position using the alignment graduation markings, as shown in Fig. 17 For the systematic error, the entire probe measurement system is placed on the Suss probing station. After setting all the components of the measurement system, the intersection angle exists ( 𝜃 Q = 5°) between the silicon probe tip and the SIIG top surface. The intersection angle is the systematic error, because the probing station cannot tune its elevation angle. Moreover, there is a 0.3° mechanical vibration which is caused by the cooling fan of the VDI WR5.1 extender. The vibration direction is along the y axis in Fig. 13. To evaluate the effect of the intersection angle, we studied the relationship between 𝜃 Q and the error box. The signal flow graph of back-to-back error boxes is shown in Fig. 22(a). The structure model and the full-wave EM simulated results (direct simulated results) of the error are shown in Fig. 10(a). We also performed the full-wave EM simulation of the TRL model shown in Fig. 15, and carried out the TRL calibration. The calibrated results are called the "TRL simulated results". When 𝜃 Q is equal to 0°, the TRL simulated and direct simulated S 11 and S 21 of the error box are matched well, as shown in Fig. 22(b). The TRL simulated and direct simulated S 22 of the error box are generally matched. When 𝜃 Q is equal to 5°, the measured, TRL simulated and direct simulated S 11 and S 21 of the error box are generally matched, as shown in Fig. 22(c). However, the measured and TRL simulated S 22 do not match with the direct simulated S 22 . The reason is that the coupling tip is sensitive to the change of 𝜃 Q , as shown in Fig. 3. Therefore, when 𝜃 Q is equal to 5°, the silicon probe can only characterize S 21 of the device under test.

To eliminate the intersection angle, the best method is mounting the VDI WR5.1 extender onto an inclined surface, which can tune its elevation angle. Another solution is to use the mini VDI WR5.1 extenders.

D. Discussions

Compared with the contact type GSG probe, such as the GGB probe [START_REF]GSG Model 220[END_REF] and micro-machined probe [START_REF] Reck | Micromachined probes for submillimeter-wave on-wafer measurements-part I: mechanical design and characterization[END_REF], the proposed silicon probe does not need an extra transition between the GSG probe and the dielectric waveguide under measured. Compared with the non-contact type probe, such as the slot array coupling-based probe [START_REF] Moallem | A non-contact submillimeter-wave S-parameters measurement technique for multiport micro-machined devices[END_REF], the quasi-optical path and planar antenna-based probe [START_REF] Caglayan | Non-contact probes for on-wafer characterization of sub-millimeter-wave devices and integrated circuits[END_REF], and the H-plane dielectric horn-based probe [START_REF] Zhu | Determination of propagation constant of terahertz dielectric ridge waveguide using non-contact measurement approach[END_REF], the proposed probe does not need to design any antenna or quasi-optical path. The probes of [START_REF]GSG Model 220[END_REF], [START_REF] Reck | Micromachined probes for submillimeter-wave on-wafer measurements-part I: mechanical design and characterization[END_REF] and [START_REF] Caglayan | Non-contact probes for on-wafer characterization of sub-millimeter-wave devices and integrated circuits[END_REF] have an excellent measurement accuracy.

V. CONCLUSION

We introduced and presented a dielectric probe technique concept for measuring and characterizing dielectric waveguide and its components in 190-220 GHz. The work principle of the probe is explained in this paper. The silicon probes and waveguide blocks were successfully fabricated and assembled. The mirror was applied to see the side view of the silicon probe and SIIG under a microscope. The propagation constant of the SIIG was successfully characterized over the band of 190-220 GHz. The measured results of the SIIG-based coupler are presented, and they agreed with simulated results. Future work will be set to develop higher frequency prototypes, improve

Fig. 2 .

 2 (a) Silicon-based dielectric probe structure and its working principle at 200 GHz in side view (𝜃 " = 10° and 𝜃 : = 20°). (b) Coupling tip structure and parameter study of the 𝜃 " at 200 GHz.

Fig. 3 .Fig. 4 .

 34 3(b), with 𝜃 Q ranging from 0° to 10° are shown in Figs. 3(e), (f) Coupling tip and E-plane bend parts. (a) Normalized phase constant of SiDW. (b) Model in side view. (c) E-field complex magnitude with 𝜃 Q = 0° and (c) 𝜃 Q = 5° at 200 GHz. (e) Simulated S11, (f) simulated S21 and (g) simulated S22 with 𝜃 Q ranges from 0° to 10°. Assembly technology. (a) Exploded view of the assembling structure of silicon probe block, (b) details of the silicon T-shape alignment dowel, (c) picture of the silicon T-shape alignment dowel, and (d) assembled silicon probe block.

  and (g), respectively. The insertion loss is almost less than 4 dB at 𝜃 Q = 0° in the frequency band of 190-260 GHz. The return loss of port 1 at 𝜃 Q = 0° is better than 20 dB over the range of 190-260 GHz (WR 5.1 band). The insertion loss increases when 𝜃 Q increases.

Fig. 5 (

 5 Fig. 5(a) shows the normalized phase constant (𝛽 𝑘 / ) for the first three modes of the SIIG, which are 𝐸 "" $ , 𝐸 :" $ 𝐸 Q" $ modes. It is difficult to excite the odd symmetry 𝐸 :" $ modes by the even symmetry TE 10 mode in the WR 5.1 waveguide. Considering the attenuation loss of the SIIG for the 𝐸 "" $ mode, a low attenuation loss starts from 190 GHz, as shown in Fig. 5(b). Considering the 𝐸 Q" $ mode appearing at 237 GHz, the SIIG can support the 𝐸 "" $ mode propagation from 190-237 GHz. However, 220 GHz is the frequency point which is the highest frequency of the WR 5.1 band (140-220 GHz) and the lowest frequency of the WR3.4 band (220-330 GHz), as shown in Fig. 3(f). Therefore, to simplify the intended experimental validation, the designed band is 190-220 GHz.The structure of two adjacent SIIGs is shown in Fig.6. To analyze the coupling of the proposed SIIG, two adjacent SIIGs are compared with a single SIIG. Fig.7presents the normalized phase constants of the 𝐸 "" $ mode of the SIIG and the even-odd

Fig. 6 .

 6 Fig. 6. Two SIIG cross-section views (not to scale).

Fig. 7 .

 7 Fig. 7. Normalized dispersions of 𝐸 "" $ mode of single SIIG and even-odd modes of two adjacent SIIGs.

Fig. 10 (

 10 b) shows the simulated E-field magnitude distribution of the contact model without WR 5.1 E-bend.

Fig. 8 SIIG

 8 Fig. 8 SIIG 2D absorber and its S11.

Fig. 9 .Fig. 10 .

 910 Fig. 9. SIIG-based H-bend S-parameter (R = 10.16 mm)

Fig. 13 .

 13 Fig. 13. Silicon probe measurement system in 3D view.

Fig. 14 .

 14 Fig. 14. Optical circuit of probe, mirror and microscope in side view from M-M' plane (not to scale).

Fig. 15 .

 15 Fig. 15. Calibration Methods (a) WR 5.1 TRL kit for A-A' plane in side view, (b) SIIG-based T/L kit for B-B' plane, and (c) SIIG-based R kit in top view.

Fig. 11 .

 11 Fig. 11. SIIG-based coupler: top view and simulated S-parameter and phase difference between coupler port and through port.

Fig. 12 .

 12 Fig. 12. Details of SIIG layout, top view.

  . (a) Whole measurement set up, (b) side view of measurement setup and (c) measurement setup of the 360 mil-length SIIG. (a) (b) (c) (d) Fig. 17. Top view of probe with (a) alignment graduation markings and (b)SIIG. Side view of probe (c) align mode (200 µm height distance) and (d) contact mode.

Fig. 18 .

 18 Fig. 18. Simulated and measured results of back-to-back silicon probes and 360-mil length SIIG. S-Parameters measured with the calibrated plane of A-A' plane.

Fig. 19 .

 19 Fig. 19. Simulated and measured phases of offset short lines with calibrated silicon probes.

  Fig. 21. (a) Measured results of the coupler and (b) normalized results of the coupler.

  (a). The maximum line width of the fabricated alignment graduation markings is 53.17 µm , which indicates the alignment uncertainty is 26.6 µm . Considering the propagation constant of the SIIG in Fig.20, the magnitude uncertainty and phase uncertainty are 0.008 dB and 9°, respectively.

  Fig. 22. S-parameter of error box. (a) Signal flow graph of the thru for TRL calibration. (b) TRL simulated and direct simulated results at 𝜃 Q = 0°. (c) Measured results, TRL simulated results and direct simulated results at 𝜃 Q = 5°. probe performances, and apply the probe to feed a SIIG-based leaky-wave antenna array. MTT-S/AP-S/LEOS and then the restructured IEEE MTT-S Montreal Chapter, Canada. He has served IEEE MTT-S and Administrative Committee (AdCom) as Chair of the IEEE MTT-S Transnational Committee, Member and Geographic Activities (MGA) Committee, Technical Coordinating Committee (TCC) and 2016 IEEE MTT-S President among many other AdCom functions. Currently, he is the Chair of the IEEE MTT-S Inter-Society Committee. He was an IEEE MTT-S Distinguished Microwave Lecturer (2009-2011). Dr. Wu was the inaugural representative of North America as a Member of the European Microwave Association (EuMA) General Assembly. He was the recipient of many awards and prizes including the first IEEE MTT-S Outstanding Young Engineer Award, the 2004 Fessenden Medal of the IEEE Canada, the 2009 Thomas W. Eadie Medal of the Royal Society of Canada, the Queen Elizabeth II Diamond Jubilee Medal in 2013, the 2013 FCCP Education Foundation Award of Merit, the 2014 IEEE MTT-S Microwave Application Award, the 2014 Marie-Victorin Prize (Prix du Quebec -the highest distinction of Québec in the natural sciences and engineering), the 2015 Prix d'Excellence en Recherche et Innovation of Polytechnique Montréal, the 2015 IEEE Montreal Section Gold Medal of Achievement, and the 2019 IEEE MTT-S Microwave Prize.

TABLE I LOSSES

 I IN SILICON PROBE AT 200 GHZ

	Structure for 𝐸 "" $ mode	Value (dB)
	Waveguide block	0.8
	WR5.1 and SiDW transition	1.6
	E-plane bend	1.4
	Coupling tip	2
	Total	5.8

ACKNOWLEDGEMENT

The authors wish to acknowledge the technical support of several staff members in the Poly-Grames Research Center: Dr. David Dousset, Steve Dube and Traian Antonescu. The authors would also like to thank the Rogers Corporation for providing free samples for fabricating the SIIG.

XXXX. This work was supported by the NSERC-Huawei Industrial Research Chair Program.

H.-T.

He is also an adjunct professor with the School of Information Science and Engineering, Ningbo University. He has held guest, visiting, and honorary professorships with many universities around the world. He has authored or co-authored over 1300 referred papers and a number of books/book chapters. He has filed more than 50 patents. His current research interests involve substrate integrated circuits and systems, antenna arrays, field theory and joint field/circuit modeling, ultra-fast interconnects, wireless power transfer and harvesting, and MHz-through-THz technologies and transceivers for wireless sensors and systems as well as biomedical applications. He is also interested in the modeling and design of microwave and terahertz photonic circuits and systems.

Dr. Wu is a Fellow of the Canadian Academy of Engineering (CAE) and of the Royal Society of Canada (The Canadian Academy of the Sciences and Humanities). He is a Member of Electromagnetics Academy, Sigma Xi, URSI, and IEEE-Eta Kappa Nu (IEEE-HKN). He has held key positions in and has served on various panels and international committees including the chair of Technical Program Committees, International Steering Committees, and international conferences/symposia. In particular, he was the General Chair of the 2012 IEEE Microwave Theory and Techniques (IEEE MTT-S) International Microwave Symposium (IMS). He has served on the Editorial/Review Boards of many technical journals, transactions, proceedings and letters as well as scientific encyclopedia including Editor and Guest Editor. He was the Chair of the joint IEEE Montreal chapters of


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